Map and Descr ip t ion of the Act ive Par t o f theSlumgul l ion Landsl ide , Hinsdale County, Colorado

ByRobert W. Fleming and Rex L. Baum, U.S. Geological Survey, and Marco Giardino, Consiglio Nazionale Delle Ricerche—Centro Studi

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

Pamphlet to accompanyGEOLOGIC INVESTIGATIONSSERIES MAP I-2672

1999

ii

CONTENTS

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Background and Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Topographic Base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4What is Shown on the Map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

History of Movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Types of Movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7The Landslide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Main Headscarp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Active Landslide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Zone 1—Head . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Zone 2—Broad Bands of Normal Faults and Tension Cracks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Zone 3—The Hopper and Neck . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Zone 4—Pull-Apart Basins Along Both Flanks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Zone 5—Pond Deposits and Emergent Toe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20Zone 6—Region of Shortening and Spreading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Zone 7—Active Toe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Summary of Kinematic Description of the Active Landslide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26Speculations About Future Movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Factors That May Affect Future Movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27Displacement Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27Change in Resistance to Sliding at the Toe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30Change in Supply of Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Future Movement of the Slumgullion Landslide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31References Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

FIGURES

1. Index map showing location of Slumgullion landslide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22. Outline map and aerial view of the Slumgullion landslide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33. Sketch showing typical deformation to trees in two different kinematic settings on landslide surface . . . 94. Simplified interpretive map of deformational features in zone 1, head . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115. Simplified interpretive map of deformational features in zone 2, broad bands of normal faults

and tension cracks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126. Simplified interpretive map of deformational features in zone 3, hopper and neck . . . . . . . . . . . . . . . . . . 157. Idealized plan view and cross sections through two places in the hopper . . . . . . . . . . . . . . . . . . . . . . . . . . 168. Simplified interpretive map of deformational features in zone 4, pull-apart basins along

both flanks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189. Two views of the deepest part of the pull-apart basin looking downhill or westerly . . . . . . . . . . . . . . . . . 19

10. Idealized features in same pull-apart basin as shown in figure 9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2111. Simplified interpretive map of lower part of the active landslide, zones 5, 6, and 7 . . . . . . . . . . . . . . . . . . 2212. View of pond sediments at the location of emergent toe of the active landslide . . . . . . . . . . . . . . . . . . . . . 2313. Sketch of concept of pond formation in conjunction with emergent toe of reactivated

movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2414. View of split tree. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2515. View of north side of active toe (zone 7), where trees are being overridden . . . . . . . . . . . . . . . . . . . . . . . . 2616. Summary maps of principal landslide elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

1

INTRODUCTION

BACKGROUND AND PURPOSE

The Slumgullion landslide is in southwesternColorado about 5 km southeast of the town of LakeCity, Hinsdale County (figs. 1 and 2). The landslideis within the San Juan Mountains, and nearly all ofthe bedrock is of volcanic origin. Materials in thelandslide deposit have been derived from the vol-canic rocks. The Slumgullion landslide has a com-plex history of movement and currently consists of

a large area of inactive landslide deposits and asmaller area that is active (fig. 2A and B). The cur-rently active part of the landslide is itself complex; itconsists of a variety of different materials moving atdifferent rates along irregular boundaries. One ofthe more interesting aspects of the currently activelandslide is that it has been moving for more than100 yr, and observations during the 100 yr indicatethat annual rates of movement might have beenabout constant throughout that time.

Map and Descr ip t ion of the Act ive Par t o f theSlumgul l ion Landsl ide , Hinsdale County, Colorado

By

Robert W. Fleming and Rex L. Baum, U.S. Geological Survey, and

Marco Giardino, Consiglio Nazionale Delle Ricerche—Centro Studi

ABSTRACT

This text accompanies a map of many of the features on the active part of the Slumgullion landslide,Hinsdale County, Colo. Long-term movement creates and destroys a variety of structural features on the surfaceof the landslide including faults, fractures, and folds, as well as basins and ridges.

The Slumgullion landslide consists of a large volume of inactive landslide deposits and a much smaller vol-ume that is actively moving within the deposits of the older landslide. Previously, collapse of the south side ofthe scarp on Mesa Seco produced materials that blocked the Lake Fork of the Gunnison River and created LakeSan Cristobal. The current landslide activity was triggered by a collapse, which apparently extended the pre-existing headscarp toward the north. The loading induced by the deposition of the collapsed materials reactivat-ed some of the older landslide deposits.

Displacement rates in the active part of the landslide range from about 0.2 m/yr at the uppermost fracturesto a maximum of 7.4 m/yr in the narrowest part of the landslide. From this maximum rate, displacement ratedeclines to 2 or less m/yr at the toe. The interplay between different displacement rates, varying width, andcurving boundaries gives rise to the structures within the landslide.

For purposes of description, the landslide has been divided into seven zones: head, zone of stretching, thehopper and neck, zone of pull-apart basins, pond deposits and emergent toe, zone of shortening and spreading,and active toe. Each zone has its characteristic kinematic expression that provides information on the internaldeformation of the landslide. In general, the upper part of the landslide is characterized by features such as nor-mal faults and tension cracks associated with stretching. The lowermost part of the landslide is characterized bythrust faults and other features associated with shortening. In between, features are a result of widening, bend-ing, or narrowing of the landslide. Also, in places where the slope of the landslide is locally steeper than aver-age, small landslides form on the surface of the larger landslide.

On the basis of qualitative observations of changes in the morphology and displacement, we speculate thatthe landslide is unlikely to accelerate and is more likely to stop movement over a time scale of decades. Thisspeculation is based on the observation that driving forces are gradually diminishing and resisting forces areincreasing. Rejuvenation or reactivation probably requires collapse of a new block in the head of the landslide.

2

The Colorado Geological Survey obtained aerialphotographs of the Slumgullion landslide in 1985. In1990, the U.S. Geological Survey began a compre-hensive study of the Slumgullion landslide andobtained a new set of aerial photographs. The 1985and 1990 aerial photographs were used by Smith(1993) to obtain displacement rates of different partsof the landslide from change in position of photo-identifiable points.

The study of the Slumgullion landslide by theU.S. Geological Survey includes geophysical investi-gations, precise surveying and leveling, history ofmovement, displacement rates, landslide kinemat-ics, material properties, and landslide dams. Prelim-inary results of some of these investigations are inVarnes and Savage (1996). This map builds on infor-mation collected during the early part of the projectby two exchange students from Italy (Parise andGuzzi, 1992; and Guzzi and Parise, 1992). Theymade estimates of the volume of the active and inac-tive parts of the landslide and demonstrated thatdetailed mapping of the active part of the landslidecould provide useful information about the kine-matics of movement. Some of the descriptions pre-sented herein were published as part of a guidebookarticle for a field trip of the Geological Society ofAmerica at the 1996 Annual Meeting in Denver,Colo. (Fleming and others, 1996). The map has notbeen previously published.

Total elevation difference from the toe of the inac-tive part at the Lake Fork to the top of the 200-m-high scarp is about 1,000 m (3,400 ft), and theaverage slope of the surface is about 1:6, or 9°-10°.Excluding the height of the scarp, the average slopeis about 7.5°. The length from top of scarp to theinactive toe at the level of Lake Fork is about 6 km(3.6 mi). Part of the inactive toe is under the waterof Lake San Cristobal. When the landslide reachedthe valley floor of the Lake Fork, the toe spread bothupstream and downstream. Maximum depth ofLake San Cristobal at the upstream terminus of thetoe is 29.5 m (Crandell and Varnes, 1961). Thedeposit covers an area of 4.64 km2 and has a volumeestimated to be 170 million m3 (Parise and Guzzi,1992).

The active part of the Slumgullion landslide isnearly completely surrounded by deposits of theinactive landslide (fig. 2). It covers an area of1.46 km2 and has a volume conservatively estimatedat 20 million m3 (Parise and Guzzi, 1992).

The active part of the Slumgullion landslide isshown here on three map sheets that can be joinedalong match lines. This map is principally of

The Slumgullion landslide has been described byseveral investigators, beginning with a short note byEndlich (1876) in the report of the Hayden Survey of1874. The name “Slumgullion,” already ascribed tothe landslide by miners at the time of Endlich’s(1876) description, is an old term referring to a meatstew containing many different ingredients and hav-ing a variegated appearance. The first publishedphotograph of the landslide was by Whitman Cross(pl. XXB in Howe, 1909) and later mentioned inAtwood and Mather (1932) and Burbank (1947).The first investigation of the rates and history ofmovement was by Crandell and Varnes (1960, 1961).Rates of movement were monitored by resurveys oflines of stakes across the active landslide at severallocations. Radiocarbon ages determined on woodfragments exposed at the toe of the inactive land-slide provided an estimate of about 700 yr for theoverall age of movement. Their estimate of age ofthe active part of the landslide, based on analysis oftree rings, was suggested to be about 300 yr.

Lake City

Lake

San

Cri

stob

al

Slumgullion landslide

Campground

149

Mesa Seco4W

D T

RA

IL

Slum

gullio

n Pas

s

Lake

Henson

Creek

Gun

niso

n

38° 00'

107° 15'

COLORADO

MAP LOCALITY

Riv

er

Fork

149

149

0 5 KILOMETERS

0 3 MILES

Figure 1. Index map showing location of theSlumgullion landslide. Shaded area outlines both theactive and inactive parts of landslide.

3

1

1

Lake San

Cristsobal

Lake Forkof the Gunnison

R

SlumgullionCreek

Deposits fromcollapse ofwest part ofmain scarp

Inactive part

Older unit

Spillover

EXPLANATION

Scarp—Hachures point downhill

Toe—Sawteeth on moving ground

Flank—Arrows show relative movement

KILOMETER

MILEcontour interval = 200 ft

High flank rid

ges

Active part

Mesa Seco

Main headscarp

Older unit

Activepart

Activetoe

Inactivetoe

Lake San Cristobal

149

Figure 2. A, Outline map of inactive and activeparts of the Slumgullion landslide. Inactive part oflandslide is made up of materials that blockedSlumgullion Creek and spilled over a local topo-graphic divide farther upslope, subsequentlyblocking the Lake Fork of Gunnison River and cre-ating Lake San Cristobal. A collapse of west side ofmain scarp that provided a surcharge load toupper part of older landslide deposits apparentlytriggered currently active part of landslide. B, Aerial view of Slumgullion landslide, lookingnortheast (photograph by David Varnes).

B

A

4

structural features within the irregular boundariesof the active landslide. We have attempted to drawfeatures such as faults and scarps and to report evi-dence of internal deformation such as split trees inorder to depict the kinematics of movement and tounderstand the behavior of different parts of thelandslide. The map also shows annual displacementat more than 300 places within and along theboundaries of the active landslide. We use the com-bined kinematic and displacement information tospeculate on the future behavior of the landslide.

Fundamental tenets of this study of landslidestructures are that the ground-surface expression offractures and other deformational features are prod-ucts of (1) local variations in boundary geometry,including the shape of the failure surface and of(2) position relative to driving and resisting ele-ments within the landslide. Variations in boundarygeometry produce a variety of structures such aspull-apart basins, grabens, and steps in faults thataccommodate widening or narrowing of the land-slide. These structures may have counterparts inmuch larger geologic structures in tectonic situations(Johnson and others, 1997). The idea of driving andresisting elements within a landslide comes from thelimit-equilibrium method of stability analysis. Theanalysis evaluates driving and resisting forces with-in the landslide and compares resistance againstsliding to forces promoting sliding. We believe thatwe can recognize these elements in the field on thebasis of deformational behavior (in other words,stretching or shortening) of the landslide.

These tenets are important to landslide studies.The boundary geometry, including the position andshape of the failure surface, is an essential quantitytogether with strength parameters of the materialand water-seepage conditions in the analysis of sta-bility. Recognition of driving and resisting elementswithin the landslide may have application inimprovements in methods of stability analysis(Baum and Fleming, 1991). If the properties andgeometry are correctly expressed in a limit-equilibrium analysis, the positions of driving andresisting elements will also be correct. Thus, surfacemapping and measurement of deformation haveapplication to stability analysis as well as processstudies.

TOPOGRAPHIC BASE

Base maps at a scale large enough to produce amap of landslide features at their true scale andposition are generally unavailable and must be spe-

cially made. For other large-scale specialized maps,we typically have prepared our base map by planetable or total-station survey techniques. (See, forexample, Fleming and Johnson, 1989; Johnson andothers, 1994; and Baum and others, 1993). For thisproject, the area was too large to prepare a detailedbase map by traditional survey techniques. Rather,the base map was drawn from aerial photographsusing an analytical stereoplotter that stored contourinformation digitally. Because this method is some-what unusual in landslide studies, we describe thevarious steps involved.

An array of control points outside the boundariesof active movement was precisely surveyed by afield party under the leadership of David Varnes.Plastic panels in the form of crosses were centeredon the control points, and in 1990 aerial photographswere obtained of the landslide area at scales of1:14,000 and 1:6,000. The base map was prepared byJames Messerich in the U.S. Geological SurveyPlotter Lab from the 1:6,000-scale aerial photographsusing a Kern DSR-11 analytical stereoplotter. Datawere digitally stored with reference to the local coor-dinate system established by the Varnes survey. Basemaps can be plotted at any scale from the digitaldata set; the 1:1,000 scale we adopted for mappingof landslide features was adequate given the densityof fractures and quality of detail preserved in mostplaces.

The landslide moved differentially, but as muchas about 18 m, between the time the topography wasmapped and the landslide features were mapped.The base map was made from aerial photographstaken during 1990, and our field mapping was com-pleted during three 2-week intervals during 1992and 1993. Positions of features to be mapped werebased on the detailed form of the contour lines onthe base map, and the topographic forms on movingground were offset a variable amount with respectto nonmoving ground outside the landslide bound-ary. Even though the contour lines were displacedby slide movement, the form of contour lines wasnot visibly changed by movement between 1990 and1993. Features generally could be located within afew meters on the ground. However, accurate loca-tions were impossible in areas lacking distinctivetopographic features and in areas of dense vegeta-tion.

1 Any use of trade, product, or firm names is for descrip-tive purposes only and does not imply endorsement bythe U.S. Government.

5

WHAT IS SHOWN ON THE MAP

Most of the mapped features are indicators ofdeformation in the landslide. We separate direct andindirect indicators of deformation. Features such asfaults and fractures are direct indicators. Thrustfaults, whether within the boundaries of the land-slide or along part of the landslide perimeter, aredrawn with sawteeth on the overriding plate. Thrustfaults are discontinuities that indicate shorteningdeformation, and in the nomenclature of landslidesare called landslide toes. A thrust fault within thebody of the landslide is termed an internal toe andis distinguished from the structure at the distal endof the landslide, the toe. A fault scarp is a purelydescriptive structure that can represent normal fault-ing, strike-slip faulting, or oblique-slip faulting. Inmost cases it was impossible to ascertain the senseof slip along a scarp because of the crumbly natureof the landslide soils. The presence of the scarpscombined with information from nearby displace-ment vectors and (or) indirect indicators of move-ment such as stretched roots or split trees can give aqualitative idea of sense of offset. Faults and scarpswithin the boundaries of the landslide are termedinternal faults or scarps. Tension cracks are irregu-lar, open gashes in the landslide surface. The sur-faces of tension cracks are rough and irregular,indicating that they formed by pure opening. Alsomapped are en echelon cracks that are small tensioncracks whose pattern indicates shear deformation;left-stepping cracks indicate right-lateral shear andvice versa. And, finally, we use fracture as a generalterm to describe a break in the surface that is notidentifiable as a tension crack or a fault. The sense ofmovement on a fracture is undefined unless indicat-ed by an arrow. We believe that most fractures arestrike-slip faults along the landslide boundary andoblique-slip faults within landslide boundaries.However, the crumbly nature of the soils at the sur-face of the landslide contributes to ambiguity in theinterpretation of rupture type.

For all the mapped features, a solid line indicatesthat the feature is believed to be active and is accu-rately located. A dashed line indicates that the fea-ture is apparently inactive, but the location isaccurate. A dotted line indicates that the locationand type of movement are inferred. In all cases, thelocations were drawn with respect to the localtopography on the landslide. The criteria used todistinguish between active and inactive featureswere imprecise. The presence of tiny cracks and evi-dence of active soil or plant disturbance made sim-ple the recognition of active features. Where there

was some question about activity, the presence ofloose and crumbly soil at the ground surface wasused as an indicator of active deformation. The soilsurface on inactive features was typically a little stiffor crusty. There is minor cementation at the surfaceof inactive features that results from evaporation ofwater containing large amounts of dissolved solids.Active deformation tends to destroy this crust.Surfaces that have been undisturbed for relativelylong periods of time contain a scattered covering ofselenite crystals.

We also mapped several indirect indicators ofdeformation. Highly fractured areas, indicated by ahachured pattern, were outlined where individualfractures are too closely spaced to depict separatelyor where deformation is so intense that the surficialsoils are dilated and soft. These highly fracturedareas typically were found at intersections of frac-ture zones and at bends and curves in fracturezones. Areas mapped as extrusion areas are placeswhere clay-rich material from the substrate, typical-ly gray, white, yellow, or purple silty clay or clayeysilt, has been extruded to the ground surface. Theextrusion areas occur in several different kinematicsituations within the active landslide. Boundaries ofboth the extrusion areas and highly fractured areasare indistinct, and thus the map units are subjective.

Other indirect indicators provide specific infor-mation about the kinematics of deformation. Shownon the map are the locations and orientations ofstretched tree roots, split trees, and small bucklefolds. These features are created by local deforma-tion within boundaries of active movement andqualitatively depict the direction of maximum, one-dimensional strain on the landslide surface.

The fractures in the surface of the landslide areso informative that it is unfortunate more of the sur-face was not conducive to their formation andpreservation. In places, particularly in the upperpart of the landslide, there are large areas of cobble-to-boulder-size fragments of columnar-jointed rock.These areas do not show or preserve fractures ofany type. Much of the remainder of the landslide iscovered by a yellow, red, or brown clay-rich silt orsilty clay that is soft and powdery when dry.Different kinds of fractures are preserved to differ-ent degrees in these materials. Movement thatresults in a scarp is generally identifiable; purestrike-slip movement is poorly expressed except onthe landslide flanks where differential displace-ments are very large and materials are more moist.The small structures that we have used to under-stand the kinematics of other large landslides

6

Displacement was also measured around theperimeter of the active landslide with woodenstakes and a tape measure. The locations of thesemeasurement points and annual displacement arealso shown on the map. Small dots indicate theapproximate positions of the wooden stakes. Themeasurement of displacement around the perimeterwas during the interval September 1992 toSeptember 1993. The data obtained by the pho-togrammetric and direct methods were thus fromdifferent times, and the agreement in values seemsremarkable.

At about the narrowest part of the landslide(shown on sheet 2), there is a line of six rebar rodsthat were installed by David Varnes as part of earli-er displacement measurements. We surveyed thepositions of the rods four times beginning in 1991and last in 1996. The first survey was with a planetable and later surveys were with a total station. Notall the points were visible from the instrument sta-tion at the time of each survey, so there are a fewgaps in the data set. We also located a 2 by 2-inchwooden stake near the left flank of the active land-slide that had been installed more than 30 yr earlier(Crandell and Varnes, 1961); we located the end ofthat stake line and were able to obtain a 30-yr rateestimate (sheet 2).

There are a few other notes on the maps thatidentify locations of displacement and (or) deforma-tion measurements. Positions of striped posts thatwere placed by the Bureau of Public Roads areshown on sheet 3. Locations of recording exten-someters that are described in Savage and Fleming(1996) and locations of quadrilaterals that wereplaced in different parts of the landslide to deter-mine local deformation are also shown.

HISTORY OF MOVEMENT

At least three separate events are recognizable indeposits of the Slumgullion landslide. Crandell andVarnes (1960, 1961) recognized an older, inactivedeposit and a younger, active landslide (fig. 2). Ourmapping has determined that the currently activelandslide is a result of loading caused by collapse ofthe north side of the scarp on Mesa Seco. Thus,there are three clearly identifiable events, althoughthe collapse of the north side of the scarp and reacti-vation of part of the old landslide was probably acontinuous process.

Chleborad (1993) and Madole (1996) have stud-ied different parts of the inactive landslide depositand suggested there may be additional episodes of

(Fleming and Johnson, 1989) were preserved only ina few localized places where they formed in pondsediments or alluvium. In part, the presence of splittrees and stretched tree roots have served as indica-tors of local deformation that could be obtainedfrom the fractures in a more competent material.

Bearing in mind that the currently active part ofthe landslide may have been moving for about300 yr, features such as scarps, pull-apart basins andthrust faults are created, transported, and destroyedby the movement. A mapped feature may have beencaused by a condition that no longer exists wherethe feature exists. Or a similar feature may be beingcreated at another place on the landslide. We knowthat some structures, such as listric faults and thrustfaults, formed at specific parts of a pull-apart basinwithin the landslide. Continued movement trans-ported these features downslope to positions wherethey are no longer active but are still recognizable.This aspect of the mapping, while a limitation wherewe did not recognize or understand a particularstructure, allows improved interpretation of land-slide behavior where we do recognize the structure.In the case of the pull-apart basin just mentioned,the recognition of the displaced features of the basinis evidence that the position of the basin is fixed,and structures are created in the basin and trans-ported as inactive features downslope. We were ableto recognize transported and inactive features morethan 100 m downslope from the places where theyhad formed.

The map provides some information about loca-tions of springs (where water seeps or flows onto theground surface from the subsurface) and sinks(where water disappears into the soil or into cracksat the ground surface). Because the locations ofsprings and sinks were not a principal focus of thisstudy, their depiction on the map is incomplete.Most springs having a discharge larger than a fewliters/minute are shown.

We also show average annual landslide displace-ment on the map. Displacement vectors, shown as ared overprint on the map, were obtained by Smith(1993). The displacements were calculated fromreplicate measurements of positions of more than300 identifiable points on aerial photographs thatwere taken in 1985 and 1990. Positions are knownwith an accuracy of about 1 m for the entire 5-yrinterval between photographs. Thus, average annualrates, except for a few places of obvious errors inmeasurement, should typically be within about20 cm/yr.

7

movement. They used radiocarbon dating, differ-ences in degree of weathering and soil formation,and morphology to separate ages of deposits. Thetwo oldest deposits (fig. 2) occur (1) outside of thebasin containing the Slumgullion landslide as aspillover that crossed a topographic divide at eleva-tion 11,200 ft (3,677 m) and (2) as a lobe of depositthat blocked Slumgullion Creek at elevation 10,200 ft(3,349 m). The spillover site (Chleborad, 1993) con-tained wood that provides an estimated age ofemplacement of about 1,000 to 1,300 yr before thepresent. Madole (1996) recognized well-developedsoils formed on the lobe of the landslide thatblocked Slumgullion Creek and along the outermostflank ridge on the south side of the inactive land-slide. Both of these deposits appear to be from theearliest episode of movement. The oldest depositsare principally yellow to reddish-brown clay andseem to best match the exposed clays in the southside of the scarp area between 11,800 and 12,000 ft(3,874 and 3,940 m). Furthermore, the oldestdeposits lack large concentrations of unaltered vol-canic rock fragments that are more common towardthe north side of the scarp.

The deposit at the extreme distal part of the inac-tive landslide along the Lake Fork contains buriedwood that provides an estimate of the age of block-age and creation of Lake San Cristobal. Using resultsof radiocarbon measurements, Madole (1996) esti-mated the blockage at 800 to 900 yr before the pre-sent. The radiometric ages of the oldest depositsdescribed above and the deposits that caused block-age of the Lake Fork could be interpreted as a resultof two events separated in time by 100-500 yr.Differences in degree of soil formation on depositsat the distal end at the Lake Fork and the blockageof Slumgullion Creek suggest a period of inactivity(Madole, 1996). However, the distance between thedeposit that blocked Slumgullion Creek and thedeposit that blocked the Lake Fork is nearly 4 km,and if the landslide were moving at a rate compara-ble to the current rate, 300 to perhaps 500 yr wouldbe required for the landslide to advance over thatdistance. Thus, the idea that the landslide activitymay be divisible into separate episodes may not becorrect. Rather, the sequence we recognize may haveoccurred as a continuum for perhaps the past1,300 yr.

The current active movement of the Slumgullionlandslide is due to collapse of the north part of theheadscarp; movement from that scarp was towardthe south. Remnants of the toe of this collapse arepreserved in the upper part of the active landslide

between elevations 11,200 and 11,400 ft (3,677 and3,743 m). The exposed materials in the north part ofthe headscarp are distinctly less weathered (moregray) and less altered than in the scarp farther to thesouth. The deposits from the collapse cut across fea-tures from the earlier collapse of the south part ofthe scarp, which moved toward the west.

The part of the Slumgullion landslide that wasactive at the time of our mapping had its upper part,or head, in deposits from the collapse of the northend of the headscarp (sheet 1). The collapse trig-gered a reactivation of older landslide deposits andpropagated movement 2.6 km downslope to aboutelevation 10,000 ft (3,283 m). There, an active toeapparently formed and emerging landslide materialslid over the surface of the old landslide to its cur-rent position (sheet 3). The formation and displace-ment of the active toe were discussed by Guzzi andParise (1992). Crandell and Varnes (1961) suggestedthat, on the basis of tree-ring studies, the age of thecurrently active landslide is about 300 yr. Historicaldescriptions of the landslide extend from the late1800’s (Endlich, 1876), and the landslide has appar-ently been moving at a nearly constant rate sincethen.

TYPES OF MOVEMENT

Actively moving landslides are characterized byfractures and other deformational features that are aresult of differential movement at the surface andsubsurface. These features collectively determine thekinematics of the landslide. Parts of a landslide hav-ing different kinematic expression are separable aslandslide elements. If a landslide were displaced asa rigid block, there would be a separation betweenthe sliding block and nonmoving material at thehead, an area at the toe where the sliding blockoverrides nonmoving material, and strike-slip faultson the flanks. Because the block is rigid, there wouldbe no fractures within the sliding block.

Most landslides do not behave as rigid blocks.For most landslides, irregular boundaries, differentinternal forces (driving and resisting), and thedeformable nature of landslide materials lead todeformation within the landslide. Certain types offeatures, as well as their positions and orientations,are typical of different kinds of deformation.Mapping the positions, orientations, and sense ofdeformation of these features describes the kinemat-ic behavior of the landslide (Fleming and Johnson,1989; Baum and Fleming, 1991; Baum and others,1989).

8

In the upper part of an active landslide, groundthat is moving downslope is separating from non-moving ground farther upslope. The features pro-duced are caused by stretching and thinning of theground in the upper part of the slide as well as inground uphill from the slide. The most commonfractures include tension cracks having their longaxes oriented at right angles to the direction ofmovement and normal faults. In the lower part of alandslide, where the toe is resisting sliding, principaldeformation is shortening and thickening. Featuressuch as thrust faults and small buckle folds are typi-cal. Tension cracks are generally oriented parallel tothe direction of movement where deformation issimply shortening; the tension cracks may be atright angles to the direction of movement if theyform along the crest of a shortening structure suchas a buckle fold.

Measurement of displacement at different pointsacross the surface of a landslide produces indepen-dent data on deformational features (Fleming andothers, 1993). Displacement-rate values should pro-gressively increase from the crown of the landslideto a maximum value at some point that is part waydown the surface of a landslide. There, displacementrate should begin to decrease. Along the longitudi-nal axis of the landslide, increasing displacementrate in a downslope direction indicates stretchingand decreasing rate indicates shortening.

On the landslide flanks, strike-slip deformation ispredominant. The features along landslide flanks aretypically more complex than at the head or toe (seeFleming and Johnson, 1989). The simplest structureon a landslide flank is a strike-slip fault where slipoccurs across a single surface that is oriented paral-lel to the direction of slip. A fault segment is similarto a fault in that it is a single surface of slip but isdifferent in that the orientation is misaligned withrespect to the overall direction of slip. Fault seg-ments are typically oriented 10°-20° clockwise withrespect to the slip direction for right-lateral slip and10°-20° counterclockwise for left-lateral slip (Gilbert,1928; Fleming and Johnson, 1989). Even though theyare misaligned, we know that the segments arefaults because their surfaces are smooth and slicken-sided. Continued displacement on the fault seg-ments causes buckles or small thrust faults to formbetween the overlapping ends of the fault segments.The fault zone is the width of the misaligned faultsegments measured normal to the overall trend ofthe zone.

Tension cracks may also be indicators of strike-slip deformation. En echelon tension cracks are as

much as several meters long and inclined fromabout 30° to 45° clockwise with respect to right-lateral faulting and from 30° to 45° counterclockwisewith respect to left-lateral faulting. We know thatthese cracks originated in tension because they haverough surfaces that would be interlocked if theywere closed.

These deformational indicators of strike-slipfaulting on landslide flanks may occur separately ortogether in a zone of shearing. The fracture type thatforms is partly dependent on the rheology of thematerials at the surface of the landslide. Hard, brit-tle surfaces tend to fracture as en echelon tensioncracks. Very soft, deformable clay tends to produce asimple, throughgoing fault. Materials having prop-erties intermediate between these extremes tend toproduce fault segments in strike-slip fault zones.

The simple indicators of strike-slip deformationon the lateral boundary of a landslide become morecomplex where the fault curves or steps. A left stepor bend in a right-lateral fault zone is constrainingand contains deformational features characteristic ofcompression in addition to the lateral shift. A rightstep or bend in a right-lateral fault zone is exten-sional and contains deformational features charac-teristic of stretching. Constraining steps tend tonarrow the width of the landslide; extensional stepstend to widen the landslide. Features associatedwith both narrowing and widening of a landslideare described in Fleming and Johnson (1989).

In addition to fractures that provide a direct indi-cator of landslide kinematics, trees and tree roots areindirect indicators. In the Slumgullion landslide,trees may be tilted and (or) split; tree roots may bestretched or buckled.

Trees are tilted in active landslides by movementover an irregular failure surface (fig. 3) and bydeformation caused by differential rates of displace-ment. A tree that is undisturbed tends to grow verti-cally. Tilt of a tree produces new vertical growth atthe tip, and the tree develops a kink or curve in thetrunk with continued growth. New vertical growthat the tips of tilted trees was used by Fleming andJohnson (1989) to show that a long, linear ridge onthe Twin Lake landslide, Utah, was actively enlarg-ing. Tilted trees were used by Fleming and others(1988) to estimate the shape of the failure surface ofa slump in the crown of the Manti landslide and byBaum and others (1993) to evaluate a bump in thefailure surface of the Aspen Grove landslide, Utah.

The trees on the Slumgullion landslide reflect along history of movement. Some trees have severaldistinct bends produced by tilting in different

9

directions as they were transported downslope bysliding. On the map of the Slumgullion landslide,we use two types of information from trees. First,there are several places where trees have been splitby differential displacement across a fracture. Thedirection of differential movement required to pro-duce the split is used as an indicator of local defor-mation. Likewise, the local deformation is indicatedin the direction of stretching or buckling of treeroots.

In summary, deformational indicators identifythe kinematics of different parts of an active land-slide. Generally, the head of a landslide is character-ized by stretching and thinning, the toe by

shortening and thickening. The flanks contain indi-cators of strike-slip deformation that may be faults,fault segments, or tension cracks where the zone issimple. Fracture patterns become more complexwhere landslide boundaries are irregular.Compressional or extensional bends occur wherethe landslide becomes narrower or wider, respec-tively. Some of the bends are in the form of steps,others simply involve a curving fault zone. We usefracture patterns together with indirect informationfrom split trees, tilted trees, and stretched tree rootsto infer the kinematics of the active part of theSlumgullion landslide.

THE LANDSLIDE

MAIN HEADSCARP

The headscarp of the Slumgullion landslide isnearly 1.3 km long and has a maximum height ofabout 200 m (elevation 3,500-3,700 m); part of thenorth part of the scarp is shown on sheet 1. Theoverall trend of the scarp is about northwest-south-east. The older part of the scarp, the southeast end,contains an abundance of hydrothermally alteredmaterial that matches much of the general appear-ance of the materials in the landslide. Yellow, white,gray, and red silty clays form smooth, roundedhummocks on the unvegetated lower part of thescarp. The upper part of the scarp is tree coveredand may be underlain by less altered rocks (Diehland Schuster, 1996). The average slope of this part ofthe scarp is about 75 percent (35°-40°). From nearthe southeast end for about 700 m, the scarp appar-ently formed by breaking through the crest of MesaSeco, and the top of the scarp is a local topographicdivide. The northwestern part of the scarp is formedin more resistant rocks and is steeper than thesoutheastern part, the slope being nearly vertical inplaces.

The materials exposed in the main headscarp area complex array of hydrothermally altered andunaltered volcanic materials including tuffs andintrusive and extrusive rocks of variable composi-tion (Diehl and Schuster, 1996). The MioceneHinsdale Formation at the top of the scarp is unal-tered vesicular basalt. Below the basalt is theMiocene Sunshine Peak Tuff, a welded ash flow tuff.These Miocene units unconformably overlie thehighly altered Oligocene volcanics of UncompahgrePeak, which consist mainly of andesite flows(Lipman, 1976). The products of hydrothermal alter-ation, including alunite, smectite, and opal, havebeen found in the altered rocks of the main scarp as

Figure 3. Typical deformation to trees in two differentkinematic settings on landslide surface. Measurements ofchange in orientation of tilted trees can provide informa-tion on shape of buried failure surface. A, Initial conditionof upright trees. B, Pattern of tilted trees created by land-slide movement over a bump in slip surface. C, Pattern oftilted trees created by landslide movement over a ramp inslip surface. Amount of displacement is given by length ofarrow and by change in position of numbered trees in Band C compared to A. (From an unpublished figure pro-vided by A.M. Johnson.)

7

65 4 3 2 1

7

65 4 3 2

1

7

65

4 3 2 1

A

B

C

Ramp in slip surface

Bump in slip surface

10

well as in the debris forming the slide (Larsen, 1913;Diehl and Schuster, 1996). Faults, breccia pipes, joint-ing, and alteration contribute to the instability of themain scarp, and rock avalanche chutes coincide withmany of the faults (Diehl and Schuster, 1996).

ACTIVE LANDSLIDE

This description of the active part of theSlumgullion landslide begins at the head and pro-ceeds downslope to the toe. The flanks of the land-slide are defined as looking downslope as the left orright flank; at Slumgullion, the left flank is the southside and the right flank the north side of the land-slide. The active landslide can be divided into sevenzones based on similarity of structural features with-in each part. The uppermost, zone 1, is called thehead. It contains fractures indicating both stretchingand shortening. Downslope from the head is zone 2,a broad zone of normal faults and tension cracksthat is more than 350 m wide. Zone 3, the hopperand neck, is a funnel-shaped stretch where the land-slide width is at its minimum and the displacementrate at its maximum. Downhill from the hopper andneck, the landslide gradually increases in widththrough curves in the flank faults, creating zone 4,an area of pull-apart basins along both flanks. Zone5 is an area of pond deposits and the emergent toe,followed by zone 6, an area of shortening andspreading, and zone 7, the active toe.

For each of the seven zones, we include an inter-pretive cartoon drawing (figs. 4-8, 10, 11) showingsome of the main structural features in that part ofthe landslide. Alphabetically noted points have beenadded to both sides of the cartoon so that the readercan readily locate the part of the landslide beingdescribed. The cartoons contain a small area of over-lap with adjacent zones. In these overlap areas, notall the schematic fractures are shown unless they arewithin the zone under discussion. The reader mayfind it helpful to first locate features on the cartoonbefore attempting to locate them on the larger, moredetailed maps.

Zone 1—Head

The uppermost fractures that we mapped in thehead of the landslide form a 150-m-wide zone on atopographic bench below a large pile of coarse talus(fig. 5, sheet 1). The fractures are tension cracks andnormal faults, both indicating stretching. The land-slide material in the area of the fractures is a pow-dery clayey silt to silty clay that does not preservefractures well. The scarps of the normal faults are

0.1-0.2 m high. Some face uphill, others downhill.Several features that have the appearance of sink-holes and long, linear furrows are interpreted to bedepressions above tension cracks that opened atdepth and propagated to the surface as collapse fea-tures. The band of most active fractures, near A-A’,is about 100 m across the landslide and about 20 mwide in the direction of sliding. The stretching rateacross the zone of fractures is about 0.2 m/yr. Theflanks of the landslide downhill from this zone ofstretching are obscure, but several segments of anactive strike-slip fault are preserved along the leftflank about 70 m downhill. Other than these fewfaults, the flanks were located (fig. 4, sheet 1) by anabrupt change in slope or a small furrow. Movementrates apparently are insufficient to maintain a con-spicuous set of fractures in this area.

The direction of sliding at the head is toward thesouth and is in the same direction as the inferredcollapse of Mesa Seco (fig. 2A). Remnants of the toeof this collapse are preserved as linear ridges out-side the left flank in the vicinity of C’ and within theactive landslide along a discontinuous zone of toesabout 70-90 m downhill from the line C-C’. Uphillfrom the toes, in the area of C-C’, is another zone ofstretch features much like the uppermost zonemapped at A-A’ and consisting mostly of uphill-fac-ing scarps as much as 1.5 m high.

The stretch (lengthening) features produced bynormal faults and tension cracks in the active land-slide contrast sharply with the compressional (short-ening) features produced by collapse of part of themain scarp in the same general area. The uppermoststretch features are at A-A’. Principal shortening fea-tures are along a line between D and C’. Here, azone of discontinuous internal toes and trees thathave been pushed over and partly overridden bylandslide material are diagnostic of shortening.Overlapping the shortening features is a band ofstretching features, principally scarps of normal andoblique-slip faults, that loosely connect with thenorth (right) flank. The stretch faults are probably acontinuation of the zone of normal faults that inter-sect the right flank between D and E. The toe of thesmall landslide between A-A’ and D-C’ is beingpulled apart by more rapid movement along theright flank farther downhill.

Displacement rates in the head of the landslideare about 0.2 m/yr southward. The movement inthe head of the landslide is nearly independent ofthe westward movement farther downslope. Thevalue of 0.2 m/yr at the uppermost fractures is min-imal because measurement points did not extend

N

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EXPLANATION

Fracture—Dotted where inferred

En echelon fractures—Indicate strike-slip faulting

Scarp—Hachures face same direction as scarp

Thrust fault—Sawteeth on overriding block

0.3

0.3

A'

Arrow showing direction and amount of movement for a point within the active landslide—Displacement in m/yr. Data from aerial photographs taken in 1985 and 1990

Arrow showing direction of lateral shift across fault zone— Displacement in m/yr

Reference point for features described in text

Scale (m)

0 100 200

Head

Toe0 500

Scale (m)

INDEX MAP

SLUMGULLION LANDSLIDE

N

Figure 4. Simplified interpretive map of deformational features in zone 1, head.

11

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12

across all the fractures. Farther downslope, at apoint on the left flank about 25 m uphill from line C-C’, movement was only 0.1 m/yr. On the rightflank, a point near line C-D’ was displaced 0.3 mand a point 110 m downhill from there and duesouth of D was displaced 0.1 m/yr. These fewmeasurements indicate the displacement pattern inzone 1, the head, is largely from a separate, discretelandslide that resulted from the collapse of the northside of the main scarp. Indeed, we were unable tofind continuous fracturing along the left or rightflank directly uphill from D-D’, and the interior ofthe landslide in the vicinity of D-D’ has very fewfractures of any type that would connect the smalllandslide between A-A’ and D-C’ with the largeractive landslide farther downslope.

Zone 2—Broad Bands of Normal Faults and Tension Cracks

Fracturing that is more nearly typical of that inthe head of a landslide is found in three broad bandsof normal faults and tension cracks within zone 2(fig. 5, sheet 1). Overall, the zone is nearly 1,000 mlong and as much as 350 m wide. Fractures are simi-lar in each of the bands; they reflect stretching orextension. Included are normal faults with scarpsfacing both uphill and downhill, tension cracks ori-ented approximately at right angles to the directionof sliding, and stretched roots and split trees show-ing the stretch direction parallel to the direction ofsliding. The main structures and annual displace-ment for a few points within and on the boundariesof the landslide are summarized in figure 5.Displacement progressively increases through thezone of stretching from less than 0.1 m/yr to morethan 3 m/yr over the length of the zone.

The uppermost band of stretching features, whichintersects the flank near point C on the north side ofthe landslide, contains normal faults in a 200–m-long band that extends only part way across thelandslide at C-D’ (fig. 4). This band extends throughthe thrusts of the landslide deposit in zone 1. Itoccurs at the abrupt 90° clockwise bend in the land-slide and contains a right-lateral, strike-slip fault onthe north side that carries almost the entire displace-ment on the right flank (fig. 5). The strike-slip faultis loosely connected with the poorly developed nor-mal faults and tension cracks that extend part wayacross the landslide. Boundary fractures farther tothe north appear to be inactive, or the landslide ismoving too slowly there to maintain open fractures.

Displacement rates are 0.3 and 0.7 m/yr on thenorth and south sides of the landslide, respectively.If the movement was purely rotational, we wouldexpect the velocities to be about 0.3 m/yr on thenorth and 0.72 m/yr to perhaps as large as 0.85m/yr on the south side of the bend. Thus, the slidemass in this area is essentially rotating (plan view)through the 90° bend as a rigid body.

The middle band of stretching extends across abroad area between E and E’ (fig. 5). This bandextends entirely across the 350-m-wide landslideand downhill at least 250 m. Individual scarps donot extend all the way across the landslide; rather,they occupy the band as many overlapping normalfaults and tension cracks. Scarps of the active nor-mal faults range to as much as about 2 m in height.Inactive scarp heights locally exceed 5 m.

The band of structures at E-E’ reflects stretchingin this area, which produces an increase of slidingrate. On the left flank, the sliding rate increases from0.8 m/yr near E’ to 1.4 m/yr near F’. On the rightflank, the sliding rate increases from 0.1 m/yr nearD to 1.2 m/yr between E and F.

The difference of 0.4 m/yr in displacement rateon the right flank at E compared to the left flank ofthe landslide at E’ (1.2 m/yr compared to 0.8 m/yr)has kinematic significance. The difference must beaccommodated either by internal distortion or bymovement on one or more fault zones within thebody of the landslide.

As we have proceeded down the landslide, wehave seen two ways that the velocity changes areaccommodated. One is normal faulting and tensioncracking that reflects an increase of sliding velocityfirst across the bend and then near sections E-E’ andF-F’ within the landslide. We also saw rigid blockrotation at D-D’.

A different form of accommodation is visible atF-F’. Here, displacement rates are larger, and thelandslide does not move through the curve in theboundaries by block rotation. The normal faults andtension cracks of zone 2 terminate in a left-lateralstrike-slip fault zone (near the left flank at F’ andtrending from F’ toward D, fig. 5) that carries about0.3 m of the displacement rate difference betweenthe landslide flanks. The internal left-lateral faultnear F’ bounds a block that moves through a 20°bend between F’ and G’ with relatively little internaldistortion because the block is free to move nearlyparallel to the flanks downhill from the bend.

Continuing downslope, a third band of normalfaults (fig. 5) extends from F’ toward H. Here, scarps

13

and fractures are in a band more than 50 m widethat turns and transforms into a right-lateral strike-slip fault where the band narrows midway acrossthe slide near G. The right-lateral strike-slip faultintersects the right flank near H (fig. 5). The dis-placement rate along the right flank is 1.8 m/yr nearG and declines to 1.7 m/yr near H. The displace-ment rate along the left flank increases from 2.9m/yr near G’ to 4.1 m/yr near H’. Thus, thereappears to be a displacement rate of at least 1.1m/yr across the internal right-lateral strike-slipfault. The position of this band also corresponds to asharp bend in the right flank of the landslide; itturns counterclockwise 30° directly upslope from H.The left flank, though, is relatively straight. The areaalong the right flank between G and H was recog-nized as a relatively “dead zone” by Guzzi andParise (1992) because of the minor amount of inter-nal deformation there.

Another band of normal faults beginning at H-H’(fig. 6) extends across most of the width of the land-slide. This group of faults gives the appearance ofan additional zone of stretching. However, a belt ofthrust faults (internal toes) just downhill from thescarps reveals that this structure is a small landslidesuperimposed on the larger landslide in a region ofsteeper slope (fig. 6, sheet 1).

Within zone 2 we recognize three different struc-tures in the region of stretching, two of which are aresult of curving boundaries and one of whichresults from simple stretching. The clockwise bend(fig. 4) between C-D and C’-D’ is an area of rigid-block rotation. The area beginning at D-E’ (fig. 5)and continuing downslope for 200 m is simplestretching across normal faults and tension cracks.Here the flanks of the landslide are relativelystraight and parallel across this zone. The landslideaccommodates bends at G-G’ and H-H’ with internalstrike-slip faults that contain most of the slip differ-ence between the amounts on the landslide flanks.Both of the broad bands of normal faults have anassociated strike-slip fault that joins the band to theflank where it curves (fig. 5). The strike-slip faultsallow internal blocks to slide with very little internaldeformation of the slide material between the faults.

On the basis of our mapping across the zone ofstretching, we can make the following importantobservations:

I. The landslide moves as a series of blocksbounded by zones of deformation. The defor-mation within blocks is very small.

II. Accommodation between blocks occurs byformation of multiple discontinuities in theform of faults and other structures that accom-modate highly localized deformation.III. The two primary modes of movement ofblocks are plan-view rotation and translation.

From the downhill end of zone 2 the appearanceof the landslide changes dramatically. Gone are thescarps and other stretching features that extendacross the landslide. These are replaced by scarpsthat are oblique or longitudinal with respect to thelandslide boundaries, and different kinds of struc-tures are produced.

Zone 3—The Hopper and Neck

The hopper is a 200-m-long, funnel-shaped areain the 400-m-long zone termed “hopper and neck”that accommodates narrowing of the landslide(fig. 6, sheets 1 and 2). The right flank of the land-slide has a major restraining bend while the leftflank is very straight. The width of the landslidedecreases from 230 m at H-H’ to 180 m at I-I’, over ahorizontal distance of 150 m.

The hopper begins where there is an abruptincrease in slope and extends downslope to the nar-rowest part of the landslide, the neck. The materialsentering this narrowing part of the landslide areconstrained laterally, and materials have the appear-ance of entering a hopper.

On both sides of the landslide, 30-50 m downhillfrom point H, the landslide surface has been tiltedtoward the center (indicated by “tilt” with arrowson fig. 6). The tilting extends a little farther down-slope on the south side. The slope of the tilted sur-face on the north side is as much as 50 percentwhereas the slope of the tilted surface on the southside is less, as much as 20 percent. The downslopeend of the tilted surface on the north side is termi-nated by a discontinuous line of scarps that extendsfrom the north flank near point H, obliquely towardpoint I’. The sense of slip is oblique containing bothleft-lateral and normal movement. Small en echelontension cracks in the line of scarps provide the basisfor kinematic interpretations.

The inferred deformation within the hopper isdepicted in two idealized cross sections across thelandslide (fig. 7). Trees on the tilted surface on theside near A and B (fig. 7) are uniformly inclinedtoward the middle of the landslide and are tiltedapproximately normal to the ground surface. Treesare generally absent on the south side of the hopper.

14

N

1.7

2.55-10% slope35% slope

>1.0

H+

3.0

3.0

Tilt

Tilt

Tilt

4.1

Small

lands

lide

within

large

r

lands

lide

4.2

3.93.8 4.1

4.34.7Extrusion zone

5.4

5.4

6.3I'

+

+H'

Left flank

Right flank

Ext

rusi

on z

one

I+

+6.0J

+J'

T-10

4.6

Zone of dist

ributed sh

earing

5.8

5.75.2

5.6

5.65.0

T-9

7.4

Scale (m)

0 100 200

Head

Toe0 500

Scale (m)

INDEX MAP

SLUMGULLION LANDSLIDE

N

Zone 3

Surface tilted down in direction of arrowTilt

T-10

EXPLANATIONFracture—Dotted where inferred

En echelon fractures—Indicate strike-slip faulting

Scarp—Hachures face same direction as scarp

Thrust fault—Sawteeth on overriding block

0.3

0.3

A'

Arrow showing direction and amount of movement for a point within the active landslide—Displacement in m/yr. Data from aerial photographs taken in 1985 and 1990

Arrow showing direction of lateral shift across fault zone— Displacement in m/yr

Reference point for feature described in text

Survey bench mark

Figure 6. Simplified interpretive map of deformational features in zone 3, hopper and neck.

15

The tilted surfaces are inclined toward each other,and the trends of the inclined surfaces form anasymmetrical V with an intersection point near themiddle of the landslide at I-I’ (fig. 6). Along bothflanks of the landslide, from H to I and H’ to I’ (fig.6), the landslide debris is devoid of trees and soft,powdery, white-to-yellow, clay-rich material isexposed. The remaining trees on the surface of thelandslide between points H and I are along the bot-tom of a swale that trends down the middle of thelandslide. Idealized motion on each flank of the hop-per appears to be screw-like (in transverse cross sec-tion) with material being extruded near thelandslide flanks and being folded together along themidline of the landslide (fig. 7). Landslide materialsare extensively remolded in the short stretchbetween H-H’ and I-I’. The hopper-like structureends approximately at I-I’ where two thrust faultshaving traces oriented parallel to the direction ofsliding mark the intersection of the tilted surfaces.

The downslope end of the hopper structure isabruptly broken by a belt of normal faults and ten-sion cracks. The belt begins with a chevron-shapedscarp near point I’. Small en echelon tension cracksin the west-trending zone about 50 m inside thesouth boundary at point I’ indicate that slip isoblique, right-lateral, and normal (down). Most ofthe narrowing of the landslide has been accommo-dated farther upslope, and this structure is apparent-ly formed to accommodate stretching. Maximumdisplacement rate is near the line J-J’.

A line of fractures and scarps trends downslopefrom the end of the chevron-shaped zone of normalfaults (halfway between points I and J, 50 m insidethe north boundary). This line continues downslopeabout parallel to the landslide boundaries. The fea-tures along the line are not well preserved; locally,there are en echelon cracks indicating that the zoneis partly a right-lateral strike-slip fault. Definitiveevidence from split trees or stretched roots is lack-ing, though. The zone may be a relict structure thatwas produced in the hopper and has been transport-ed downhill. As such, it would have limited kine-matic significance for current movement. On theother hand, there is a well-developed, longitudinal,right-lateral fault zone farther downslope that maycarry as much as 0.5 m/yr of differential displace-ment (fig. 8). These longitudinal fractures in the neckcould be the beginning of that fault zone.

The landslide continues to narrow to near the lineof J-J’, where it is about 160 m wide. Maximum dis-placement rate was recognized by Crandell andVarnes (1961) to be near here, and they established a

Extrusion zone,no trees

A'A

B B'

Tilt

Tilt

Ext

rusi

onN

o tr

ees

No

tree

s

Ext

rusi

on

A A'

B B'

Tiltedtrees

Figure 7. Cartoon of an idealized plan view and cross sec-tions through two places in the hopper. As the groundsurface is tilted toward middle of the hopper, fresh materi-al is extruded onto ground surface between landslideflanks and edges of tilted blocks. Width of extrusiondeposits increases downslope. The two thrust faultsshown trending down the axis of the landslide on the planview represent shortening and compression.

16

survey line and took time-lapse photographs of themovement. Note that there is a poorly developedright-lateral fault about 30 m from the south bound-ary at T-10. This fault is apparently the right side ofa small landslide element along the south flank thatis superimposed on the larger Slumgullion land-slide. Displacement rate should be a maximum herewhere the shallow movement is superimposed onthe maximum rate of displacement of the Slum-gullion landslide. The point having the value 7.4,shown at the bottom of figure 6, is believed to benear the point of maximum rate. This value wasfrom a spot measurement and may not be a crediblerepresentative of displacement rate over a longerperiod of time. Near the point of the spot measure-ment, a line of six rebar rods was surveyed severaltimes over a 4-yr period. Movement rates of individ-ual rods ranged from 5.2 to 5.8 m/yr in the mainbody of the landslide and are probably a better indi-cator of maximum landslide velocity (fig. 6).

There are extensive fractures along the strike-slipzone of the left flank in the neck area. These frac-tures are inclined at 30°-50° counterclockwise to themain strike-slip fault zone on the boundary; some ofthe fractures have a low, downhill-facing scarp butmost appear to be tension fractures. The fracturesmake up a zone 5 to 10 m wide and, overall, appearto define a broad zone of left-lateral shearing adja-cent to the left flank (identified as distributed shear-ing on fig. 6, mapped on sheet 2). In most placesalong the landslide flanks, shearing is confined to azone that is less than a meter wide.

The upper part of the landslide was dominatedby movement of large blocks bounded by zones ofintense deformation. That behavior contrastsmarkedly with movement in the hopper and neck,where materials are intensely deformed throughout.Downhill from the hopper the landslide behaviorpartly returns to movement of blocks bounded byzones of intense deformation, partly to flow-likestructures, and partly to development of structuresindicative of a decreasing displacement rate.

Zone 4—Pull-Apart Basins Along Both Flanks2

The landslide widens downhill from the neck atsteps or offsets in both flanks (zone 4, fig. 8, sheet 2).Over a distance of nearly 1 km, the slide widensfrom 160 to 260 m. Over this same distance, the dis-

placement rate declines from about 6 to 4 m/yr .Long, narrow pull-apart basins2 have formed direct-ly downhill from three right-stepping offsets in theright flank and at one small, left-stepping offset inthe left flank (fig. 9). A step in the bounding flankfault forms at a bend in the fault that tends to widenthe landslide (clockwise bend on the right flank,counterclockwise bend on the left flank). Within afew meters downhill of the bend, a topographicdepression forms between the flank fault and theprojection of trend of the fault from uphill of thebend. The landslide material deforms to fill thebasin, and the basin is filled over a distance of sever-al hundred meters. Part of the deformation is by themovement of blocks and part is by flow-like defor-mation. The best expressed basin is on the northside, between points J and K (fig. 10).

The zone of strike-slip faulting on the north(right) flank bifurcates where it changes orientationat point J (fig. 10). One part follows the north flankas a narrow zone of strike-slip faulting on the land-slide boundary; the other part forms a crescent-shaped group of scarps and fractures that graduallyturns parallel to the north flank. A 5-m-deep basinoccupies the area between this crescent-shapedgroup of scarps and the north flank. Within thebasin and parallel to the landslide boundary are sev-eral shortening features, such as small thrust faultsand buckle folds. Rotated blocks bounded by thecrescent-shaped group of scarps and the shorteningfeatures in the basin are smaller landslides withinthe overall larger landslide in which material isbeing transported obliquely into the basin (fig. 10).The crescent-shaped group of scarps is the surfacemanifestation of oblique-slip (right-lateral- and dip-slip) listric faults. The structure appears analo-gous to pull-apart basins in larger tectonic settingswhere movement on listric faults transports materialfrom the flanks of the basin to its center.

About 200 m downhill from point J, the group ofcrescent-shaped scarps turns toward the basin andterminates in the basin in folds and thrust faults(fig. 10). The buckling and thrusting is evidentmostly in the tilting of small trees and bushestoward the deepest part of the basin. Individualfaults are difficult to recognize, so that the deforma-tion resembles flow rather than block deformation.There is a small pond at the deepest part of thebasin that accumulates water-transported sedimentthat helps fill the basin. Pond sediments extenddownslope from the existing pond as the landslidetranslates the materials. The pull-apart structure,though, stays in the same place.

2Pull-apart basins are structural and topographic basinsthat form next to the flanks where they step laterally towiden the landslide (releasing step in a fault zone).

17

+

+

Stee

p

+M

3.3

+N

3.3

3.2

3.7

3.4

3.4

4.3

4.0 4.4

Pul

l-apa

rt

basi

ns

Abr

upt i

ncre

ase

in s

lope

+ M'

N'

+

4.0

Dis

cont

inuo

us li

ne o

f sca

rps

Sm

all

land

slid

e

BenchR

ight

flan

k

Left

flank

Pull-

apar

t bas

in

Poo

rly d

evel

oped

pu

ll-ap

art b

asin

+K'

+L'

4.8

4.3

4.3

4.6

4.0

3.9

3.93.

93.

7H

umm

ocky

grou

ndS

mal

l

land

slid

e

Small lan

dslid

e

5.2

4.7

4.1

+

+

J'

J

T-9

T-1

0

K

L

N

EX

PL

AN

AT

ION

Fra

ctur

e—D

otte

d w

here

infe

rred

En

eche

lon

frac

ture

s—In

dica

te s

trik

e-sl

ip f

aulti

ng

Scar

p—H

achu

res

face

sam

e di

rect

ion

as s

carp

Thr

ust

faul

t—Sa

wte

eth

on o

verr

idin

g bl

ock

0.3

0.3

A'

Arr

ow s

how

ing

dire

ctio

n an

d am

ount

of

mov

emen

t fo

r a

poin

t

wit

hin

the

acti

ve la

ndsl

ide—

Dis

plac

emen

t in

m/y

r.

Dat

a fr

om a

eria

l pho

togr

aphs

take

n in

198

5 an

d 19

90

Arr

ow s

how

ing

dire

ctio

n of

late

ral s

hift

acr

oss

faul

t zo

ne—

D

ispl

acem

ent i

n m

/yr

Ref

eren

ce p

oint

for

feat

ure

desc

ribe

d in

tex

t

T-10

Surv

ey b

ench

mar

k

Hea

d

Toe

050

0

Sca

le (

m)

IND

EX

MA

P

SL

UM

GU

LL

ION

LA

ND

SL

IDE

N

Zon

e 4

Sca

le (

m)

010

020

0

Figu

re 8

. Si

mpl

ified

inte

rpre

tive

map

of

def

orm

atio

nal f

eatu

res

in z

one

4, p

ull-

apar

t bas

ins

alon

g bo

th fl

anks

.

18

Figure 9. Two views of the deepest part of the pull-apart basin looking downhill or westerly. A, Pond sediments contribute to filling the deepest part of the basin. Small spruce trees to left ofpond are tilted to the right and curved in response to a component of thrusting in that direction.The jumbled aspen trees in the right foreground are the result of a component of movement fromleft to right along an oblique-slip fault. B, Closer view of pond and pond sediments in the deepestpart of the basin. The bounding flank fault is visible in the small troughs to the left of and downhillfrom the tripod. The dark-colored material in the left foreground that is surrounded by pond sedi-ments is a compressional structure produced by a component of movement into the basin. Overalldisplacement rate at this part of the landslide is about 4 m/yr. The deformational features withinthe pull-apart basin are the result of a small component of the overall displacement rate.

A

B

19

Downhill 50 m from point K (fig. 10), the basinhas been completely filled. Farther downhill fromthe basin there are numerous inactive scarps andhummocky topography, shown on the sketch (fig. 8)as “hummocky ground.” These inactive featuresapparently formed in the pull-apart basin and weredeactivated as they were transported beyond thebasin.

Other basins along the active flanks of theSlumgullion landslide (fig. 8) do not contain all thefeatures shown in figure 10. However, they do generally have slump blocks that are moving fromthe inboard side of the basin toward the flanks. Thetoes of these slumps form compressional featuresincluding folds and thrust faults in the deepest partsof the basins.

A discontinuous line of scarps that begins in themiddle of the landslide at K-K’ trends axially toabout 150 m uphill from M-M’. These scarps arealong a zone of right-lateral shear that apparentlycontains about 0.5 m/yr of displacement. The southside of the landslide is moving 0.5-0.8 m/yr fasterthan the north side.

There is one puzzling structure in this zone(fig. 8). A narrow zone of scarps and tension crackstrends partway across the slide from about point Mtoward M’. This zone generally follows a line ofabrupt increase in slope. The position of this zone isperhaps 100 m downhill from the inferred positionof the buried drainage divide separating the Slum-gullion Creek drainage basin on the south from thebasin of an unnamed creek to the north. These frac-tures appear to be superimposed on other fracturesassociated with a poorly developed pull-apart basinand an axial right-lateral strike-slip fault zone. Thescarps and tension cracks are consistent with arch-ing that could occur as landslide debris passes overa buried topographic bump; and, indeed, the struc-ture forms at a sharp break in slope from a bench toa much steeper section. The scarps and cracks couldalso be due to superficial sliding along the steeperslope just downhill on the active landslide.

Zone 5—Pond Deposits and Emergent Toe

Zone 5 contains an area of thrust faults andextensive pond deposits. The area of thrust faultsuphill from the pond deposits is centered approxi-mately on O-O’ and is the first indication of deep-seated shortening on the landslide (fig. 11, sheet 3).Other thrust faults uphill from this area are relatedto pull-apart basins or represent internal toes ofsmall landslides superimposed on the larger land-

slide. Across the zone of thrust faults at O-O’, thedisplacement rate diminishes from about 4 m/yr toabout 2.5 m/yr. The position of this zone of shorten-ing coincides with a large, inactive segment of land-slide toe to the north of the active landslide flank atpoint O. This inactive part of the toe is labeled, butits outline is not shown on the sketch (fig. 11). Eventhough the toe segment to the north is inactive, itoverlies older, inactive deposits of the Slumgullionlandslide and is topographically contiguous with theactive landslide. Thus, the toe segment is a depositthat formed and became inactive during creation ofthe current landslide toe. As such, this deposit is lessthan 300 yr old and overlies landslide deposits thatare between 900 and 1,300 yr old.

Pond deposits are recognizable in many places onthe active landslide by their uniform tan color andsilty texture. About 2.8 km downhill from the activelandslide head and about 0.6 km uphill from thefront of the active toe, there is an extensive area(80 m by 80 m) of pond deposits (fig. 11). Thesedeposits begin near line P-P’, directly downslopefrom hummocky topography with thrust faults. Thearea contains a small shallow pond. The surface hasbeen tilted northward to the point that a smallstream that formerly fed the pond now flows acrossthe north side of the pond deposits and continuesdownhill. A few dead and broken spruce trees pro-trude from the sediment where they were partlyburied. Small aspen and spruce trees have sproutedin the pond sediments near the downhill edge of thedepression (fig. 12). The downhill edge is at a broadbreak in slope that separates surfaces that are tilteduphill from those tilted downhill.

The pond sediments extend downslope at least250 m beyond the edge of the pond as defined bythe break in slope (Van Horn, written commun.,1994). Apparently, the location of the pond hasremained fixed while the landslide material hasbeen displaced across the pond site. Fixed featureshave also been observed at other landslides(Fleming and others, 1988; Baum and others, 1993).Given the current rate of movement at the pond site,the position of the pond apparently has remainedfixed for about 100 yr and perhaps longer (VanHorn, written commun., 1994).

Parise and Guzzi (1992) suggested that the pondsediments and thrust faults mark the initial positionof the toe of the reactivated Slumgullion landslide.Observations at the tip of the currently active toeindicate that it is moving along the surface of the oldlandslide deposits in bulldozer fashion, topplingand overriding trees as it goes. Upslope of the area

20

TIL

T

Bas

infil

led

Nor

thfla

nko

fact

ive

land

slid

e

Dee

pest

par

t of

bas

inB

asin

cre

ated

Str

ike-

slip

faul

t bi

furc

ates

Sca

le (

m)

050

100

Rot

ated

slu

mp

bloc

ks in

to b

asin

+K

+J

Faul

ts tr

ansf

er m

ater

ial t

o fil

l bas

in

Cha

nge

in o

rient

atio

n of

land

slid

e bo

unda

ry—

Faul

ts o

bliq

ue to

land

slide

flan

ks

Pul

l-apa

rt b

asin

Blo

cks

slid

ing

into

bas

in

N

E

XP

LA

NA

TIO

N

Fra

ctur

e—A

rrow

sho

ws

dire

ctio

n of

slip

(st

rike

-slip

fau

lt)

Thr

ust

faul

t—Sa

wte

eth

on o

verr

idin

g pl

ate

Scar

p—H

achu

res

face

dow

nslo

pe

(

norm

al o

r ob

lique

-slip

fau

lt)

Ref

eren

ce p

oint

for

feat

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desc

ribe

d in

tex

t+

K

Ant

iclin

e

Arr

ow p

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dow

n in

dir

ecti

on o

f ti

lt

Figu

re 1

0.Id

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ed f

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res

in s

ame

pull-

apar

t bas

in a

s sh

own

in fi

gure

9 (

betw

een

poin

ts J

and

K).

The

bas

in f

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s w

here

the

land

slid

e be

com

es w

ider

thro

ugh

a be

nd o

r st

ep in

bou

ndin

g st

rike

-slip

fau

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res

crea

ted

wit

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mov

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land

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rve

to tr

ansl

ate

mat

eria

l int

o th

e ba

sin.

In

uppe

r pa

rt m

ater

ial i

sm

oved

into

the

basi

n by

a s

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s of

rot

atio

nal s

lum

ps. F

arth

er d

owns

lope

mat

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l is

tran

slat

ed in

to th

e ba

sin

alon

g cu

rvin

g st

rike

-slip

fau

lts

that

term

inat

e in

the

basi

n in

buc

kle

fold

s an

d th

rust

fau

lts.

At d

owns

lope

end

of

the

basi

n a

zone

of

obliq

ue s

lip f

ault

s tr

ansl

ates

mat

eria

l acr

oss

land

slid

e an

d c

ompl

etes

filli

ng o

fth

e ba

sin.

21

++

+

+

N O'

N'

O

2.8

2.6

2.9

2.7

2.2

2.4

2.4

Pon

d/em

erge

ntto

e

Tilt

Def

orm

ed

pond

sed

imen

ts

1.5

Rel

ativ

e di

spla

cem

ent

2.6

2.0

2.2

2.0

0.6

Zon

e of

spr

eadi

ng

Rig

ht-la

tera

l fau

lt zo

ne

+ P'

1.9

1.9

1.3

++

Q'

R'

0.4

1.9

1.9

2.0

2.1

0.8

1.6

1.1

0.3

+

+

Q

P

N

Inac

tive

depo

sit

Hea

d

Toe

050

0

Sca

le (

m)

IND

EX

MA

P

SL

UM

GU

LL

ION

LA

ND

SL

IDE

NZ

one

7Zon

e 6

Zon

e 5

Sca

le (

m)

010

020

0

Surf

ace

tilt

ed d

own

in d

irec

tion

of

arro

w

EX

PL

AN

AT

ION

Fra

ctur

e—D

otte

d w

here

infe

rred

, arr

ows

indi

cate

sen

se o

f di

spla

cem

ent

En

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lon

frac

ture

s—In

dica

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trik

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ng

Scar

p—H

achu

res

face

sam

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rect

ion

as s

carp

Thr

ust

faul

t—Sa

wte

eth

on o

verr

idin

g bl

ock

0.3

0.3

A'

Arr

ow s

how

ing

dire

ctio

n an

d am

ount

of

mov

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t fo

r a

poin

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ide—

Dis

plac

emen

t in

m/y

r.

Dat

a fr

om a

eria

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togr

aphs

take

n in

198

5 an

d 19

90

Arr

ow s

how

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late

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hift

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oss

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D

ispl

acem

ent i

n m

/yr

Ref

eren

ce p

oint

for

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ribe

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tex

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Figu

re 1

1.Si

mpl

ified

inte

rpre

tive

map

of

low

er p

art o

f th

e ac

tive

land

slid

e, z

ones

5, 6

, and

7. F

eatu

res

show

n in

clud

e th

rust

fau

lts

and

tens

ion

crac

ks o

f zo

ne 5

(pon

d d

epos

its

and

em

erge

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e), t

hrus

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obl

ique

-slip

fau

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of z

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egio

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ead

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, and

zon

e 7

(act

ive

toe)

.

22

of thrust faults (uphill of N-N’), the reactivatedlandslide appears to be formed within the olderlandslide deposits. At the pond and farther down-hill, the reactivated landslide materials are slidingon top of the older deposits.

The concept of the processes of formation andpreservation of the pond is outlined in figure 13,which is a sketch showing the influence of the shapeof the failure surface on the morphology of the land-slide. The response to the collapse of the north sideof the headscarp and loading in the head of theinactive landslide apparently triggered the reactiva-tion. Fractures propagated downslope through theinactive landslide material to the approximate posi-tion of the pond. There, the fracture along the basalfailure surface emerged to the ground surface (fig.13). From this point on, the reactivated landslideadvanced over the surface of the old, inactive land-slide. While this explanation for the positions of thethrust faults and the pond deposits is consistentwith observations, the mechanism has not been doc-umented.

Zone 6—Region of Shortening and Spreading

At N-N’ (fig. 11) the active landslide is about260 m wide. Downslope 500 m the landslide is

430 m wide. This increase in width is a result ofspreading along the north flank. The zone of spread-ing on the north side contrasts with a zone of short-ening on the south side. The different kinematicexpressions of the two parts of the landslide are sep-arated by a major right-lateral strike-slip fault thatbounds the north flank of the landslide at point Nand continues internally through the landslidebeginning near point O (fig. 11, sheet 3). This majordividing fault intersects the downslope end of theactive landslide about 150 m northwest of point R’.North of this fault, the landslide is advancing andspreading. South of this fault, the landslide isadvancing but at progressively reduced ratesbecause of internal shortening within the landslide.

The deformation structures on the north side ofthe landslide consist of strike-slip and oblique-slip(right-lateral slip and opening or spreading) faults.Pure strike-slip faults are difficult to identify in thispart of the landslide because the crumbly landslidedebris does not preserve fractures well. In a fewplaces along the trends of some strike-slip faults,there are scarps that enable the fault positions to beaccurately located. Where pure strike-slip faultschange orientation a scarp is produced and stretchedroots or split trees (fig. 14) show that the fault

young treestension cracks

old, dead trees

Figure 12. View (looking downslope) of pond sediments on back-tilted surface at the location of emer-gent toe of active landslide. Note young spruce and aspen trees growing along divide at downslopeend of pond. Note also that older spruce trees in foreground have been buried by pond sediments.Evidence such as this indicates that location of pond has remained fixed while trees have been trans-ported into pond and beyond.

23

about 80 m wide, begins near point N’ and extendsobliquely across the landslide. Within this band, thethrust faults consist of individual, overlapping faultsegments. Based on the change in annual displace-ment (fig. 11), there is more than 1 m/yr of shorten-ing across this internal toe.

accommodates oblique slip indicative of spreadingof the toe.

The major strike-slip fault bounding the northflank at point N divides near point O into twoparts. One part curves northwesterly (obliquely)down to the active toe. The main fault continuessouthwesterly to between points P and Q, where itdivides again, here into three parts. In eachinstance, the curving fault trends toward the land-slide boundary and apparently accommodatesoblique slip. The spreading is accommodated bythese oblique-slip fractures.

The entire north side of the landslide toe iscomposed of oblique-slip structures. Deformationacross these structures consists of right-lateral slip,opening and spreading. This is well illustrated at aspruce tree that was caught in one of theseoblique-slip zones (fig. 14). The fault zone occupiesa small furrow that crosses under the tree. Thereare no open cracks or fractures; the position of thefault is given only by the furrow and the split tree.The right-hand side of the tree is about 1 m fartherdownslope than the left-hand side, and the open-ing normal to the strike-slip direction is about0.8 m. The split tree is definitive evidence of right-lateral slip together with spreading. There arenumerous troughs and furrows in this part of thelandslide, and only in a few places, as shown infigure 14, can they be shown to be locations ofactive movement.

Interpretation of displacement rate of the northpart of zone 6 is difficult. The displacement pro-gressively diminishes along the major dividingstrike-slip fault from 2.8 m/yr near point N to2.5 m/yr just downhill from the first division ofthe fault. Annual displacement rate decreases to1.7 m/yr about 50 m further downhill. This changein displacement rate, however, reflects movementof the south side of the major strike-slip fault rela-tive to the north side. Movement rate of materialon the north side of the fault near point P relativeto the boundary is unknown. Farther downhill,where the major fault divides into three parts,more displacement is transferred to the oblique-slip faults on the north side such that two fault-bounded blocks along the line Q-Q’ and north ofthe major fault are displaced 1.1 and 1.6 m/yr.

The part of the landslide that is south of themajor internal strike-slip fault contains two bandsof internal toes that represent shortening of land-slide material (fig. 11). One band of toes is thethrust fault described above as part of zone 5, thepond sediments and emergent toe. This band,

Propagating fracture

from reactivation

Base of old landslide deposits

Active landslide

Emergent toe

Pond

Base of old landslide deposits

Deformedtrees

Newtrees

Pondsediments

Pond

Dead trees

A

BBase of old landslide deposits

C

Overruntrees

Figure 13. Sketch of concept of pond formation in conjunctionwith emergent toe of reactivated movement. A, Collapse of rimof Mesa Seco triggers a reactivation of movement. B, Failurepropagates to region between O to O’ and P to P’ (fig. 11) wherethe failure surface emerges to the ground surface. C, The featurespresent on the surface of the landslide are a direct result of thegeometry of the buried failure surface.

24

The next band of thrust faults begins along thesouth flank about 80 m downhill from point P’. Thethrusting is confined to a much narrower band, andmany of the shortening features within the bandmay be inactive. The band consists of a northwest-trending thrust fault that apparently connects to thesouth flank and extends continuously into the land-slide body for about 60 m. There it steps along aleft-lateral strike-slip fault to another thrust faultthat extends to the major internal strike-slip faultzone. The amount of shortening across this band issmall. Rate at the downslope toe of the landslidenear point R’ is 1.9 m/yr, and uphill from the inter-nal toe (between Q and Q’) it is 2.1 m/yr. Thus,annual shortening across this internal toe is appar-ently only about 0.2 m/yr.

Zone 7—Active Toe

The active toe is the downhill terminus of theactive landslide beginning near point O and contin-uing all the way around the perimeter, O-P-Q-R’-Q’(fig. 11, sheet 3). Everywhere along the perimeter,the toe is advancing across the surface of the old,inactive landslide. In the process of overriding thelandscape, it pushes trees over and engulfs theminto the base of the landslide (fig. 15).

At the toe the thickness of the active landslide isequal to the height of the landslide above the pro-jected surface of the old, inactive landslide. Pariseand Guzzi (1992) used this observation to estimatethe volume of the active landslide at about 20 x 106

m3. The volume estimation was based on the thick-ness of the active landslide at the toe combined withdisplacement vectors and slope to compute the vol-ume of segments of the landslide uphill from thetoe. The total volume was obtained by summation(Parise and Guzzi, 1992).

The active toe has an irregular trace along itsnorthern half. Superficial sliding along the over-steepened front of the toe has obscured fractures, soit is not possible to observe causes for the irregulari-ties. We assume that the steps and bends along theterminus are a result of a velocity discontinuity. Atmany of the irregularities, one of the oblique-slipfaults can be projected to the general area of theassumed velocity discontinuity.

The annual displacement rate was measured atseven points around the perimeter of the toebetween Q and Q’. All the measurements except thevalue of 1.3 m/yr near the terminus were measuredby taping distances between wooden stakes. All themeasured distances are minimum values because ofpossible differences between measurement andmovement directions. The value of 1.3 m/yrobtained by surveying mentioned above is also aminimum because measurement points extendedonly about 10 m onto actively moving material.

In the place on the active toe where we weremeasuring displacement rate by surveying ratherthan simply taping distances, we were able to deter-mine the displacement vector of the active toe rela-tive to nonmoving ground farther downhill. Weexpected that the displacement would be approxi-mately parallel to the slope because it is advancingover old landslide debris. To our surprise, we foundthat the displacement vector is directed nearly hori-zontally out of the slope. This condition, whichtends to thicken the toe, may be the result of accu-mulation of trees and other debris along the contact

Figure 14. View of split tree. The tree is broken into fourparts by right-lateral and spreading movement. Fault thatcaused rupture of tree has not fractured ground surface,but it does form a linear trough about 1 m deep.

25

between moving and nonmoving ground (fig. 15).Displacement tends to steepen the front of the activetoe. Sloughing of materials on the oversteepenedface gives rise to a churning of material along theactive front, and the advance is similar to the motionof the tread of a bulldozer. Continuing movementresults in continuing thickening of the toe.

SUMMARY OF KINEMATIC DESCRIPTION OFTHE ACTIVE LANDSLIDE

We have divided the active part of the Slum-gullion landslide into seven zones, each having adifferent kinematic expression. Figure 16A is a sum-mary sketch of the principal structures on the land-slide that have been described. Figure 16B shows thesame kinematic elements as figure 16A, but includescontours of displacement rate. Note that the con-tours in figure 16B are discontinuous across themajor structural features on the landslide. It is possi-ble to identify the sense of deformation across agiven fracture by the difference in displacement rateon opposite sides of the fracture. For example, theinternal strike-slip fault in zone 6 (fig. 11) that sepa-rates the slower north side from the faster south sidecan be readily identified as right lateral on the basisof displacement rate differences alone. Likewise,small landslides superimposed on the larger, activelandslide can be identified by closed contours of dis-placement rate in the hopper, zone 3, and at the

downhill end of zone 4. In general, the displace-ment-rate contours support the kinematic descrip-tions in the preceding sections. However, thecontours were drawn after we had completed themapping, and they were not independently drawn.An example of a contour map of displacement ratedrawn without information about internal kinemat-ics of the landslide is in Smith (1993).

The uppermost part of the landslide shown by ashaded pattern on fig. 16A is the block of landslidematerial that remains following collapse of part ofMesa Seco. Within the lower part of this remnantblock there are a series of scarps that face bothuphill and downhill. These scarps represent thehead of the currently active landslide (zone 1). Overthe next 900 m downslope (zone 2), the landslide ischaracterized by scarps oriented perpendicular tothe direction of movement that indicate stretching.The boundaries of the landslide are curved in thiszone, and part of the displacement is carried byinternal strike-slip faults that connect the scarpswith the flanks. The downslope end of this zonemarks the end of simple stretching of the landslidematerials. The annual displacement over this reachincreases from about 0.5 to 3 m/yr.

The form of the landslide changes dramatically atthe hopper. Here the landslide narrows abruptlyfrom 230 to 180 m over a distance of 150 m. Materialalong the flanks is tilted toward the center of thelandslide, and fresh material has been extruded to

Figure 15. View of northside of active toe (zone 7),where trees are being over-ridden by bulldozer-likemovement of landslide.Moving ground on leftpushes trees over andburies them.

26

the ground surface adjacent to the bounding strike-slip faults. Within this zone nearly all the trees onthe surface of the landslide are destroyed.

Downhill from the hopper and neck and point ofmaximum displacement the landslide becomeswider over a zone nearly 900 m long (zone 4). Thewidening is accomplished by the landslide bound-ary curving outward and producing pull-apartbasins. The basins form adjacent to the landslideflanks and are filled by material transportedobliquely to the overall direction of sliding. The fill-ing of the basins is the result of differential move-ments along faults rather than intense internaldeformation such as occurs in the hopper. Over the900-m zone of widening the displacement ratedecreases from about 5 m/yr to 3-3.5 m/yr.

The next zone of distinct kinematic features onthe surface of the active landslide is an expression ofshortening in the lower part of the landslide (zone6). It is basically the counterpart to zone 2, whichcontains structures that accommodate stretching.Here most of the shortening occurs within twozones of internal toes. The upper internal toe in thelandslide is expressed as a thrust fault and absorbsmore than 1 m/yr of shortening. The next internaltoe is less active and absorbs less than 0.2 m/yr ofshortening. Fractures in these internal toes consist ofoverlapping segments of thrust faults that make upa zone several tens of meters wide.

At the lower edge of the upper internal toe is anarea of extensive pond deposits (zone 5). The ponddeposits extend downslope as a trail of pond-laidsediments at least 250 m long as a continuous layerand for as much as 500 m as a broken, discontinu-ous zone (Van Horn, written commun., 1994). Thelocation of the pond has apparently remained fixedin time, and landslide debris has been transportedthrough the pond, receiving a coating of pond sedi-ments in the process. The surface of the landslide istilted upslope where the pond has formed. The sur-face crosses a broad divide and is tilted downslopethe rest of the way to the active toe. The pond isspeculated to be the position of the toe of the activelandslide when it formed in older landslide depositsbetween 200 and 300 yr ago. The toe apparentlyemerged here and advanced farther downslope overthe older landslide surface (fig. 13). The advance ofthis part of the landslide has been bulldozer-like,pushing over trees and engulfing them in the baseof the active landslide as it moves. Resistance tosliding through the zone containing the buried treesmay contribute to an apparent thickening of thelandslide at the front of the active toe. Displacement

vectors were not parallel with the slope at the activetoe, but rather they were about horizontal. Thedownhill face of the landslide is covered with nearlycontinuous superficial slides, adding to the impres-sion that movement at the toe is somewhat analo-gous to movement of the tread of the bulldozer.

The strike-slip fault that bounds the landslide onthe north flank divides into several faults of obliqueslip as it approaches the toe. Material north of thisfault is advancing over old landslide deposits bysliding and spreading. Spreading is accommodatedby numerous oblique-slip faults in the zone. Southof this fault material is advancing by sliding down-hill over the old landslide deposits. Differential displacement within the landslide here is accommo-dated by slip along internal strike-slip faults. Theshape of the active toe (zone 7) is a crude velocityprofile across the distal end of the active landslide.The maximum rate of movement at the toe is about2 m/yr compared to about 0.8 m/yr on the northand south edges.

SPECULATIONS ABOUT FUTURE MOVEMENT

FACTORS THAT MAY AFFECTFUTURE MOVEMENT

The internal deformation and morphology of theactive Slumgullion landslide provide informationthat can be applied to speculate on future behaviorof the landslide. Most of the observations aboutlandslide kinematics indicate that the landslide willstop moving or continue moving at the current rateindefinitely. The kinds of information describedabove are not ordinarily applied to assess futurebehavior. However, we believe that part of the valueof making observations of kinematic behavior is toassess future behavior, and we offer the following asan experimental methodology.

Displacement Rate

Most observations of rate of movement suggestthat the Slumgullion landslide has been moving atan approximately constant rate for at least 30 yr andperhaps more than 100 yr. A wooden stake stillremaining on the landslide from the study ofCrandell and Varnes (1961) was found near the leftflank in zone 4, pull-apart basins (sheet 2). Displace-ment of that stake, averaged over about 30 yr, was4.4 m/yr. Measurements of displacement rate in thesame area by Smith (1993) result in slightly smallervalues in this part of the landslide for the period1985-90. A measurement by us in 1992-93 along the

27

Stri

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p—H

achu

res

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de,

da

shed

whe

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ncer

tain

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—Sa

wte

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pper

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per

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ins

(str

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ecog

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as b

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onto

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of a

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of m

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28

11

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29

landslide flank, a few meters downhill from theCrandell and Varnes stake, gave a larger value ofnearly 4.9 m/yr. For our purposes, these differencesare insignificant because they were measured at dif-ferent places in different ways. The old photographstaken of the active landslide by Cross (Howe, 1909)show the landslide looking much the same morethan 80 yr ago as it does today. On the basis of theseobservations, one could conclude that the annualmovement has been and will continue to be aboutsteady.

More precise measurements over a shorter term,however, indicate that the displacement rate is vari-able. Gomberg and others (1995) measured displace-ment rate with ultra-sensitive creep meters whileattempting to detect and locate sources of bursts ofmicro-seismic energy release from within the active-ly moving landslide. Small triggered-creep eventswere set off by small explosions several tens ofmeters away; the explosions were being used to cali-brate the velocity structure for location of sources.The creep record (Gomberg and others, 1995) indi-cated that the triggered slip is an excursion from asteady rate. The small explosions caused an abruptincrease in slip followed by a few minutes of slower-than-average slip with the net result a steadyrate of creep in a record about 30 min long. Themagnitude of the triggered slip events was a fewtenths of millimeters, and the entire slip recoverylasted about 5 min for each of two tests. Theseresults do not provide information to predict long-term behavior other than to support steady-statemovement.

Measurements by Savage and Fleming (1996)were made using recording extensometers on thesouth flank of the landslide near the point of mostrapid movement in the neck. Here the maximumrate was 11.6 m/yr in early May 1993. The mini-mum, measured in early April 1993 when theground was frozen, was 2.75 m/yr. The average ratefrom April 1993 to March 1994 was 5.84 m/yr.

The observation that the displacement rate variesshows that movement rate is sensitive to seasonalchanges within the landslide. Baum and others(1993) noted the same phenomenon at the AspenGrove landslide in Utah and were able to correlatethe slowing and cessation of movement with adecrease in water levels in borings. These seasonalchanges apparently are due to freezing of theground surface that reduces infiltration and causes aconcurrent change in subsurface water pressures.

Based on previous work (Crandell and Varnes,1961) and before the precise measurement of dis-

placement rate at Slumgullion by Savage andFleming (1996), we believed that the movement ratewas steady at each point on the surface of the land-slide. At that time, we speculated that an increase insubsurface water level would be buffered by anincrease in surface-water discharge and vice versa.We observed through the study period that the levelof water in surface ponds on the landslide hasremained about constant as has the discharge of sur-face water from the toe of the landslide. While thismay be true for part of the year, the prolonged win-ter freeze contributes to a dramatic decrease in rateof movement just before the ground thaws in thespring.

The point of describing these observations is thatwe now recognize that the landslide rate is sensitiveto seasonal change, and movement could reasonablybe expected to change in the event of prolongeddrought or high precipitation. If the feedback mech-anism described above is significant, an increase inprecipitation might be less effective in acceleratingmovement than a drought in slowing movement.Acceleration would be buffered by increased surfacerunoff from the landslide. Slowing could occur if theamount of surface water on the landslide dimin-ished to the point that the subsurface water couldnot be replenished.

While the displacement data are interesting, theyhave very little value in predicting the future behav-ior of the landslide. Probably the most significantobservation is that the displacement rate doeschange seasonally and, thus, is subject to changeannually, given a change in one or more of the fac-tors that control the rate of movement.

Change in Resistance to Sliding at the Toe

Four features associated with the active landslidetoe indicate that resistance to sliding is increasing.First, the displacement vector along the front of theactive toe is horizontal rather than parallel to theslope. Horizontal displacement implies thickening ofthe toe and added resistance to sliding. Second, thetoe is divisible into two kinematic elements separat-ed by the major internal strike-slip fault described inthe section on spreading. The part of the toe that isnorth of the strike-slip fault is spreading laterally aswell as advancing downslope; displacement is alongoblique-slip faults. Spreading also adds resistance.Third, downhill from the active toe the active land-slide must override the surface of older landslidedeposits from an earlier landslide episode. The slopeof this surface contains local benches and ramps but

30

overall is becoming flatter. For the next 100 m ormore downhill from the active toe there is a localbench that produces a flatter slope than average.And, fourth, the shape in plan view of the toe to thesouth of the major internal strike-slip fault is becom-ing more point shaped because of differences in dis-placement rate around the perimeter. At some timethis part of the toe will become so thick and so nar-row that it will also begin to spread laterally.

Change in Supply of Materials

Beginning in the upper part of the landslide nearE-E’ (fig. 5) and extending 2 km downhill, the sur-face of the active landslide is topographically lowerthan the inactive deposits that bound the active partof the landslide. This topographic trough that con-tains the active landslide extends downslope on theleft flank to a point about 50 m uphill from point L’and on the right flank to a point about 100 m uphillfrom point M (fig. 8). The surface of the active land-slide uphill from these points is typically about 3 mlower than the top of the inactive flank ridges thatare adjacent to the moving ground. Downhill fromthese points, the active landslide is topographicallyelevated with respect to nonmoving ground adjacentto its flanks. The amount of positive elevationchange is particularly large on the south flankbetween points O’ and Q’ (fig. 11) where the land-slide is constrained by the valley of SlumgullionCreek. The height of landslide deposits with respectto the valley floor is about 12 m. The continuedincrease of resisting forces on the downhill side ofthe line between points M and L’ (fig. 8) should, overthe long term, halt the movement. Duncan and oth-ers (1986) noted a similar depletion of materials inthe upper part and accumulation of materials in thelower part of the Thistle landslide in Utah. There theupper part of the landslide was bounded by a scarpnearly 30 m high.

Local changes in the surface of the landslide werestudied with sequential aerial photography. Thepull-apart basin on the north flank between points Jand K (fig. 8), was examined to further study theapparent redistribution of material from the upper tothe lower part of the landslide. Loss of material inthe upper part represents a decrease in driving force,and gain of material in the lower part represents anincrease in resisting force. If the pull-apart basin isbecoming deeper or larger, the apparent loss of dri-ving force is continuing. Topographic maps having a2-m contour interval were prepared of the landslidefrom 1985 and 1990 sets of photographs. Contour

data were stored digitally. Philip Powers comparedthe digital data for the topography in the pull-apartbasin. The method involved computing the voidspace from the ground surface in the basin to a pla-nar surface fitted to the upper, topographic perime-ter of the basin. The 1985 and 1990 data werecompared to the same upper planar surface and thevolume computed from a 2-m-spaced grid of pointsover the entire basin. The volume of the void abovethe basin was 39,300 m3 in 1985 and 41,300 m3 in1990; net change was 2000 m3 of basin enlargement.The loss of material from the basin is consistent withour other observations that driving forces are dimin-ishing.

FUTURE MOVEMENT OF THE SLUMGULLION LANDSLIDE

The changes in morphology of the landslide andthe movement record for the toe are evidence thatthe landslide is in the process of stopping ratherthan accelerating. All the kinematic data that webelieve relevant to predict the future movementimply both reduced driving forces and increasedresisting forces.

Even though some evidence supports a steadystate for long-term movement, extensometer mea-surements show that the rate varies seasonally.Thus, future movement rates will be stronglydependent on subsurface water pressures. Very wetor dry periods might change the rates significantlyover the short term, but the changes in morphologythat are leading to an end of movement shoulddominate in the longer term. The time frame for anend to this episode of active movement is even lesscertain than our conclusion that the movement willstop. Over the near term, perhaps decades, weexpect movement to continue.

Another collapse of the rim of Mesa Seco couldrejuvenate the sliding. At present there is no evi-dence for an impending collapse such as the onethat triggered the current activity. Thus, we con-clude that the current episode of movement will runits course without catastrophic effects on Lake Fork.

This study, while a broad overview of displace-ment and kinematics of the active Slumgullion land-slide, left many interesting topics unexplored. In thissection, a few of these unexplained topics are listedwith the hope that they might be of interest tofuture researchers.

The landslide should be rephotographed andremapped. This map provides a base line of dis-placement, kinematic, and structural informationthat might change in interesting ways in the future.

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This map was based on aerial photographs andtopography as of 1990. Many of our inferencesabout fixed positions of structures on the landslidesurface and changes in driving and resisting forcescould readily be tested.

We recognized but have not described areaswhere materials from the subsurface have beenextruded onto the ground surface. The mechanics ofsuch extrusion may be discernable from the kine-matic setting of the extrusions. We observed activeextrusions in the hopper, where landslide materialsare moving into the most narrow part of the land-slide. The extrusions there are along the landslideflanks. The deposits of extrusion were evident inother places where their kinematic setting was notclear.

Likewise, we found tabular dikes of highly plas-tic clay (liquid limit=110, plasticity index=75)intruded into the landslide debris at several loca-tions in the landslide. Similar intruded dikes werereported in the Twin Lake landslide in Utah byFleming and Johnson (1989). This clay is apparentlythe weakest material in the landslide, and intrusionsof clay dikes along fault zones could facilitate move-ment.

Individual structures such as pull-apart basins,flank ridges, shear zones, and different types offaults are amenable to detailed study that wouldreveal how they form. The formation could bederived from precise surveys and measurements ofdeformation using quadrilaterals (Baum and others,1988). Qualitative information on kinematic historyof the landslide could be obtained from a study oftilted trees and stretched roots.

And, finally, there have been no systematic obser-vations about surface and subsurface water on theSlumgullion landslide.

A major advantage to research at the Slumgullionlandslide is its history of continuing movement—day to day, decade to decade. Measurements can bemade to test models of fracturing and distortion indifferent situations that have a good chance of pro-viding a long record of data. While we believe thatthe landslide will stop moving in the long term, it isnot yet clear when that might be.

Acknowledgments—We especially acknowledgethe contributions of Rafaella Guzzi, who tragicallylost her life in an automobile accident in Italy in1994. Rafaella participated in many of the prelimi-nary studies of the Slumgullion landslide includingestablishment of the survey control and mapping ofstructures in the active landslide. Her dedication to

geology, good humor, and charm were a specialreward for everyone who worked with her.

We also appreciate the assistance and efforts ofthe other exchange students sponsored by theItalian Research Council, including Mario Parise,Giovani Crosta, Marta Chiarle, and Guzzi’s andParise’s academic sponsor, Professor MarinoSorriso-Valvo. We also appreciate the assistance ofthe USGS Plotter Lab staff and James Messerich inthe preparation of the base map and development ofthe means to measure displacement from sequentialaerial photographs. Other USGS colleagues thatsupplied data and information useful to this map-ping effort were David and Katherine Varnes,William Savage, William Smith, and RobertSchuster. Philip Powers computed the volumechange in a pull-apart basin. Jack Odum helpedestablish the displacement-monitoring arrays.William Smith provided data on displacement indigital form. Duane Eversol of the NebraskaGeological Survey helped us in the field and withsurveying. Arvid Johnson of Purdue University andJeff Coe provided thorough and critical reviews ofthe map and text. And, finally, Richard Walker ofLake City provided hospitality and help whereverand whenever it was needed. To all these people, weexpress our sincere thanks.

REFERENCES CITED

Atwood, W.W., and Mather, K.F., 1932,Physiography and Quaternary geology of theSan Juan Mountains, Colorado: U.S. GeologicalSurvey Professional Paper 166, 176 p.

Baum, R.L., and Fleming, R.W., 1991, Use of longitu-dinal strain in identifying driving and resistingelements of landslides: Geological Society ofAmerica Bulletin, v. 103, p. 1121-1132.

Baum, R.L., Fleming, R.W., and Ellen, S.D., 1989,Maps showing landslide features and relatedground deformation in the Woodlawn area ofthe Manoa Valley, City and County of Honolulu,Hawaii: U.S. Geological Survey Open-FileReport 89-290, 16 p., 2 pls.

Baum, R.L., Fleming, R.W., and Johnson, A.M., 1993,Kinematics of the Aspen Grove landslide,Ephraim Canyon, central Utah, chap. F ofLandslide processes in Utah—Observation andtheory: U.S. Geological Survey Bulletin 1842,p. F1-F34.

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Baum, R.L., Johnson, A.M., and Fleming, R.W., 1988,Measurement of slope deformation usingquadrilaterals, chap. B of Landslide processes inUtah—Observation and theory: U.S. GeologicalSurvey Bulletin 1842, p. B1-B23.

Burbank, W.S., 1947, Lake City area, HinsdaleCounty, in Mineral resources of Colorado:Colorado Mineral Resources Board, p. 439-443.

Chleborad, A.F., 1993, Description, origin, andimplications of a newly identified Slumgullionlandslide deposit, San Juan Mountains, south-western Colorado: U.S. Geological SurveyOpen-File Report 93-548, 16 p.

Crandell, D.R., and Varnes, D.J., 1960, Slumgullionearthflow and earthslide near Lake City,Colorado [abs.]: Geological Society of AmericaBulletin, v. 71, no. 12, pt. 2, p. 1846.

———1961, Movement of the Slumgullion earthflownear Lake City, Colorado, art. 57 in Short papersin the geologic and hydrologic sciences: U.S.Geological Survey Professional Paper 424-B,p. B136-B139.

Diehl, S.F., and Schuster, R.L., 1996, Preliminary geo-logic map and alteration mineralogy of the mainscarp of the Slumgullion landslide, chap. 3 inVarnes, D.J., and Savage, W.Z., eds., TheSlumgullion earth flow—A large scale naturallaboratory: U.S. Geological Survey Bulletin2130, p. 13-19.

Duncan, J.M., Fleming, R.W., and Patton, F.D., 1986,Report of the Thistle Slide Committee to theState of Utah, Department of Natural Resources,Division of Water Rights: U.S. GeologicalSurvey Open-File Report 86-505, 95 p., 8 pls.

Endlich, F.M., 1876, Report of F.M. Endlich, in U.S.Geological and Geographical Survey (Hayden)of the Territories Annual Report 1874, p. 203.

Fleming, R.W., Baum, R.L., and Johnson, A.M., 1993,Deformation of landslide surfaces as indicatorsof movement processes, in Proceedings of the2nd Seminar on Landslide Hazards, Cosenza,Italy, March 5-6, 1990: Geographica Fisica eDinamica Quaternaria, v. 16, no. 1, p. 9-11.

Fleming, R.W., Baum, R.L., and Savage, W.Z., 1996,The Slumgullion landslide, Hinsdale County,Colorado, in Colorado Geological SurveySpecial Publication 44: Geological Society ofAmerica Guidebook for Field Trips, 108thAnnual Meeting, Denver, Colo., 1996, 23 p.(Available on CD-ROM from the ColoradoGeological Survey, 1313 Sherman Street, Denver,CO.)

Fleming, R.W., and Johnson, A.M., 1989, Structuresassociated with strike-slip faults that boundlandslide elements: Engineering Geology, v. 27,p. 39-114.

Fleming, R.W., Johnson, R.B., and Schuster, R.L.,1988, The reactivation of the Manti landslide,chap. A of The Manti, Utah landslide: U.S.Geological Survey Professional Paper 1311,p. 1–22, 1 pl.

Gilbert, G.K., 1928, Studies of basin-range structure:U.S. Geological Survey Bulletin 324, p. 1-13.

Gomberg, J.S., Bodin, P.W., Savage, W.Z., andJackson, M.E., 1995, Landslide faults and tecton-ic faults, analogs? The Slumgullion earthflow,Colorado: Geology, v. 23, no. 1, p. 41-44.

Guzzi, Rafaella, and Parise, Mario, 1992, Surface fea-tures and kinematics of the Slumgullion land-slide near Lake City, Colorado: U.S. GeologicalSurvey Open-File Report 92-252, 45 p.

Howe, Ernest, 1909, Landslides in the San JuanMountains, Colorado: U.S. Geological SurveyProfessional Paper 67, 45 p.

Johnson, A.M., and Fleming, R.W., 1993, Formationof left-lateral fractures within the Summit Ridgeshear zone, 1989 Loma Prieta, California, earth-quake: Journal of Geophysical Research, v. 98,no. B12, p. 21823-21837.

Johnson, A.M., Fleming, R.W., and Cruikshank,K.M., 1994, Shear zones formed along long,straight traces of fault zones during the 28 June1992 Landers, California, earthquake: Bulletinof the Seismological Society of America, v. 84,p. 499-510.

Johnson, A.M., Fleming, R.W., Cruikshank, K.M.,Martosudarmo, S.Y., Johnson, N.A., Johnson,K.M., and Wei, Wei, 1997, Analecta of structuresformed during the 28 June 1992 Landers-BigBear, California earthquake sequence: U.S.Geological Survey Open-File Report 97-94,7 pls., 59 p.

Larsen, E.E., 1913, Alunite in the San Cristobal quad-rangle, Colorado: U.S. Geological SurveyBulletin 530-F, p. 179-183.

Lipman, P.W., 1976, Geologic map of the Lake Citycaldera area, western San Juan Mountains,southwestern Colorado: U.S. Geological SurveyMiscellaneous Investigations Series Map I-962,scale 1:48,000.

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Madole, R.F., 1996, Preliminary chronology of theSlumgullion landslide, Hinsdale County,Colorado, chap. 1 in Varnes, D.J., and Savage,W.Z., eds., The Slumgullion earth flow—A largescale natural laboratory: U.S. Geological SurveyBulletin 2130, p. 5-8.

Parise, Mario, and Guzzi, Rafaella, 1992, Volumeand shape of the active and inactive parts of theSlumgullion landslide, Hinsdale County,Colorado: U.S. Geological Survey Open-FileReport 92-216, 29 p.

Savage, W.Z., and Fleming, R.W., 1996, Slumgullionlandslide fault creep studies, chap. 12 in Varnes,D.J., and Savage, W.Z., eds., The Slumgullionearth flow—A large scale natural laboratory:U.S. Geological Survey Bulletin 2130, p. 83-87.

Smith, W.K., 1993, Photogrammetric determinationof movement on the Slumgullion slide, HinsdaleCounty, Colorado, 1985-1990: U.S. GeologicalSurvey Open-File Report 93-597, 17 p., 2 pl.

Varnes, D.J., and Savage, W.Z., eds., 1996, TheSlumgullion earth flow—A large scale naturallaboratory: U.S. Geological Survey Bulletin2130, 95 p.

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