Stratigraphy and Structure of the PelonaSchist in the North Fork Lytle Creek Drainage

of the San Gabriel Mountains, California —Implications for Palinspastic Reconstruction

By

Brent V. NorumDepartment of Geological Sciences

California State Polytechnic University, PomonaJune 12, 1997

A Senior Thesis Submitted in Partial Fulfillmentof the Requirements for the Bachelor of Science Degree in Geology

ABSTRACT

The basement rocks of the eastern San Gabriel Mountains have been the subject of

many studies in the last several decades. The Pelona Schist in this region is disrupted by both

NE-striking and NW-striking fault systems that intersect one another. The nature of

intersections of such faults in the eastern San Gabriel Mountains is enigmatic. Does one set of

faults turn and blend into the other, or does one set of faults truncate and post-date the other?

Detailed geologic mapping was conducted at one such intersection in the North Fork of Lytle

Creek where three faults are in proximity to the Pelona Schist and Tertiary granite in the

hopes of constraining motion along the faults. Structure and stratigraphy of the Pelona Schist

and the intrusive contact of Tertiary granite were used to palinspastically reconstruct a crude,

Late Miocene paleogeology with respect to these mapped rock units. Since 5-2 Ma motion

along the San Antonio Canyon fault was constrained to 2.9 km based on alignment of a

metabasalt layer and two post-metamorphic arches within the Pelona Schist, as well as

alignment of the San Gabriel and Icehouse Canyon faults. The NE-striking, left-lateral motion

of San Antonio Canyon fault is hypothesized to predate movement on the NW-striking, right-

lateral Scotland, San Jacinto, and Glen Helen faults, where Quaternary slip was constrained to

1.9 km, 5.9-8.1 km, and 4.4 km, respectively. Testing of the hypotheses embodied in the

reconstruction is possible by further field mapping and investigation.

Brent measures an attitude on an outcrop of Pelona Schist metagraywacke in the NWdomain of Lytle Creek, California.

ACKNOWLEDGMENTS

I would like to thank Jonathan Nourse for recommending this project to me,

accompanying me in the field for strenuous geologic mapping on occasion, and offering expert

advice with field work and manuscript preparation. Had it not been for his support and

enthusiasm, I probably would have chosen a more accessible study area and denied myself the

challenges I feel privileged to have met. Many thanks to the USDA Forest Service, Lytle

Creek Ranger District for allowing easy access to the study area by furnishing a key to the

locked gate and for granting permission to conduct field studies on Forest Service land.

TABLE OF CONTENTS

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

DATA SECTIONLOGISTICS AND PHYSIOGRAPHY . . . . . . . . . . . . . . . . . . . 3GEOLOGIC MAP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4LITHOLOGIC UNITS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5GEOLOGIC STRUCTURES . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Foliations and large-scale folds . . . . . . . . . . . . . . . . . . . . . . . . . . 9Stratigraphy deduced from gross foliation patterns . . . . . . . . . . . 11Small-scale folds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Igneous structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

DISCUSSION AND INTERPRETATIONSUMMARY OF EXISTING REGIONAL MAPPING . . . . . . 15

Structure of the Pelona Schist . . . . . . . . . . . . . . . . . . . . . . . . . . 15Folding styles within the Pelona Schist . . . . . . . . . . . . . . . . . . . 16Metamorphism and the Vincent Thrust . . . . . . . . . . . . . . . . . . . 17Late Cenozoic faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Cucamonga fault zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18San Antonio Canyon and other NE-striking faults . . . . . . . . . . . . . . 18San Andreas, San Jacinto, and related fault zones . . . . . . . . . . . . . . 19

PALINSPASTIC RECONSTRUCTION . . . . . . . . . . . . . . . . 21Methods/Current Position . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21Model 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Restoration of right-lateral faults . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Restoration of left-lateral faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Model 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25Problems in the reconstructions . . . . . . . . . . . . . . . . . . . . . . . . . 28Implications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

CONCLUSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30REFERENCES CITED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33APPENDIX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Field foliation data used in stereonet plot . . . . . . . . . . . . . . . . . . 35

1

Figure 1. Location map (modified from Morton and Matti, 1993).

INTRODUCTION

The Pelona Schist is a dominant rock type of the eastern San Gabriel Mountains that

has been the subject of many studies in the last few decades. Geologic mapping by Dibblee

(1971), Morton (1981), Ehlig (1957, 1975), Nourse (unpublished mapping, 1986-97),

Jacobson (1983), LaMascus (unpublished mapping, 1991), and Jones (unpublished mapping,

1993) has contributed much to the basic understanding of the origin, structure, and

stratigraphy of the Pelona Schist and associated geologic features, such as faults and other

cross-cutting intrusions. The Pelona Schist consists of oceanic rocks that were

metamorphosed during Late Cretaceous-early Tertiary time in a subduction zone, later to be

called the Vincent thrust fault, that since has been exhumed (Jacobson et al., 1996). In the

2

eastern San Gabriel Mountains the Pelona Schist is disrupted by two Late Cenozoic fault

systems that intersect one another. One system is east-to northeast striking with left-lateral

slip and contains the San Antonio and associated faults. A second is the northwest striking,

right-lateral slip system of faults of the San Andreas, San Gabriel, and San Jacinto fault

zones. The location of this study is at one of these intersections.

The study site is a 1.6 km by 4.8 km area of Pelona Schist (Fig.1) bordered on three

sides by faults: the San Antonio Canyon fault to the west-northwest, the San Jacinto fault

zone to the north-northeast, and the Scotland fault to the south-southwest. The left-lateral

motion of the San Antonio Canyon fault appears to be truncated by large, right-lateral offset

on the San Jacinto and San Andreas fault system. As shown on the 1:250,000 San Bernardino

Quadrangle, this area was previously unmapped in detail (perhaps due to the dense vegetation

overgrowth, or that for many years, the surrounding area was a shooting range!). Thus,

medium-to large-scale geologic mapping (1:12,000) of Pelona Schist structure and

stratigraphy was the primary objective of this study. A second, and possibly more important,

objective of this project is what the study area can tell us about regional fault reconstruction.

That is, do the northeast striking faults turn and blend into the northwest striking faults (as

suggested by Morton and Matti, 1993)? Or, do the northwest striking faults truncate the

northeast striking faults, or vice versa? Study of potential piercing points in this area gives

insight to the kinematic evolution of such fault interaction.

3

DATA SECTION

LOGISTICS AND PHYSIOGRAPHY

The study area is located on the southern side of the North Fork Lytle Creek

Drainage. Field work was conducted over sixteen days between May, 1996 and March, 1997.

The drainage has a paved access road that gave access to a public shooting range located

about a mile up the road where the pavement ends. The shooting range was closed by the

Forest Service a couple of years ago in the interest of public safety. As the area is located on

Forest Service land, the Lytle Creek Ranger District granted permission to conduct field

studies here and a key was issued to open the locked gate restricting access.

The mapped area consists of two main blocks or domains of Pelona Schist (Fig. 7).

The northwestern domain is separated from the southeastern domain by a large, northeast

trending drainage. The terrain is steep, rugged, and covered with areas of dense vegetation, all

of which made mapping difficult, and sometimes, impossible in certain areas. This vegetation

included manzanita and sticker bushes with heights of up to 6 m. Mapping traverses had to be

planned that maximized mappable outcrop while avoiding impenetrable vegetation. Such

traverses covered distances of up to 5 km (one-way), vertical climbs of up to 1 km, and slopes

with gradients up to 31 degrees. Mapping was accomplished during cool temperatures of the

year. Occasionally, the ground was covered with snow, but that did not prevent mapping as

the majority of the mapping was of distinct outcrops. Other than the vegetation problem, the

only other irritation was the increased presence of ticks after the thawing of snow in the

Spring.

4

Figure 2. Geologic map of the North Fork of Lytle Creek.

5

LITHOLOGIC UNITS

Rock type symbols are shown in parentheses as identified in Figure 2 and Plate 1,

Geologic Map of North Fork Lytle Creek, California.

The Pelona Schist is the dominant rock type of the study area. Mapped subunits of the

Pelona Schist are named for their likely protolith as used by other workers. Pelona Schist is a

Late Cretaceous-early Tertiary (Ehlig, 1968) well-foliated, greenschist facies, metamorphic

rock. The most abundant subunit in the area is a gray, well-foliated, albite-quartz-muscovite

schist (mg) with common accessory minerals of chlorite, epidote, and graphite. The albite is

usually porphyroblastic with grain sizes up to 2 mm. Gray schist comprises about 60 percent

of the map area. The suspected protolith is a marine turbidite assemblage of immature

sandstone, siltstone, and shale. Finer-grained protoliths form muscovite-chlorite-graphite

schists or phyllites. A weakly foliated, greenish gray quartz-feldspar meta-sandstone with

microscopic muscovite is also present in the area near gray schist rocks. This is a sandstone

form of graywacke.

Metabasalt (mb) is a greenschist rock that accounts for about 10 percent of the study

area. This metabasalt is a foliated, green to black chlorite-epidote-actinolite-albite schist.

The presence of fine-grained chlorite, epidote, and actinolite gives the rock its dark green

color and contributes to the schistose texture. Rounded albite grains, 1-4 mm in size, occur as

porphyroblasts.

Minor amounts of metachert (mc) are found in the area, usually in association with

metabasalt. This rock is ferromanganiferous in nature and ranges from almost a pure, non-

foliated quartzite to a well-foliated quartz schist. In both cases a high quartz content (60-90

percent) is common. It can be a dark gray rock with lighter white or gray bands that represent

6

original sedimentary textures. Bands may occur as 1-5 mm laminations. The

ferromanganiferous minerals may give it a red, brown, or purple color. Metachert is thought

to have originated from impure cherts (Jacobson, 1983).

A single, 1-2 m thick, rock layer of a foliated, calcium carbonate-rich schist was also

found interlayered with metagraywacke and metabasalt. Foliation or layering occurs in

millimeter to 1 cm bands. I am calling the rock a reddish brown, calc-silicate schist (cs).

Mylonitic rocks (myl) occur in the upper plate above the Vincent thrust fault. In the

map area, these rock are a dirty yellow to brown, cataclastically deformed, quartz diorite

gneiss. Quartz and plagioclase crystals, 2-5 mm in size, occur as stretched and sheared

porphyroblasts. Structural characteristics of this unit were studied by LaMascus (1991) and

Nourse (1991).

Middle Tertiary granitic rock (Tgr) occurs as a pluton and as intrusive dikes and sills

into country rock, usually Pelona Schist, but also the mylonitic rock of the Vincent thrust

fault. These granitic rocks range in composition from quartz monzonite through granite to

granodiorite and in texture from a medium-grained granite to a rhyolite porphyry. The granite

is a fine- to medium-grained, leucocratic hornblende granite with grain sizes of 1-4 mm. The

rock has been referred to as Telegraph Peak granite by other workers, probably due to its

abundance in proximity to Telegraph Peak. This rock has been dated at 26 Ma (Walker and

May, 1986). The rhyolite or quartz latite porphyry is a gray to gray-green rock with quartz

and feldspar phenocrysts up to 5 mm in size and minor amounts of fine-grained hornblende

crystals. Granitic rocks account for up to 30 percent of the study area.

Middle Miocene mafic intrusive rock (Ta) of various andesitic and dioritic compositions

and textures occurs as dike or sill intrusions into older rock. Andesite compositions include

7

three varieties: a dark gray, fine-grained andesite with no apparent phenocrysts, a rock with a

light gray groundmass and small, acicular phenocrysts of hornblende 1-5 mm long (hornblende

andesite), and a blue-gray groundmass with plagioclase phenocrysts 1-5 mm long (plagioclase

andesite). Minor amounts of quartz diorite with equant grains, 1-3 mm in size, of quartz,

plagioclase, and hornblende are present. The naming of andesite versus quartz diorite is largely

by textural comparison as both names represent the same chemical compositions. These rocks

invariably intrude the Late Oligocene Telegraph Peak granite and associated sills and are brittly

faulted by various strands of the San Antonio fault system (Hazelton and Nourse, 1994).

Quaternary rock types mapped in the study area include alluvium, talus, and landslide

materials. Quaternary talus (Qt) covers much of the area of the lower slopes below outcrops.

Most of the talus mapped in the area originates from cobble-sized, angular fragments of Pelona

Schist, granitic rocks, and mafic intrusive rocks (in order from most to least abundant,

respectively). Much of the talus shows fresh fracturing and a lack of lichen to indicate recent

deposition. Quaternary landslide deposits (Qls) shown on the northwest corner of Plate 1 are

inferred to be of Pelona Schist origin (compiled from mapping by Jones, 1993).

Quaternary alluvium (Qal) fills the drainage channels of the mapped area to varying

degrees. This alluvium is up to 800 m wide and 150 m thick where Lytle Creek goes

underground. The rounded particles range in size from silt to boulders, but the vast majority

lies in the cobble to small boulder sizes. All of the rock types of the area are represented in the

alluvium, but each drainage branch contains different rock types depending on the composition

of parent rock material being eroded. The alluvium of Pelona Schist origin is more elongate and

flat-shaped than alluvium of granitic origin which is more spherical.

8

Figure 3. Stereonet plot of poles to axial-plane foliation -- entire map area.

GEOLOGIC STRUCTURES

Field mapping concentrated on the dominant structures of the Pelona Schist in the map

area. As the Pelona Schist has likely undergone several generations of folding and

transposition during metamorphism, primary bedding plane features are seldom seen in the

field. However, axial-planar schistosity is well developed and easily seen in the Pelona Schist.

Thus, primary field data included 280 axial-plane foliation attitudes--115 attitudes from the

9

Figure 4. Density contour plot of poles to axial-plane foliation -- entire map area.

southeastern domain and 165 attitudes from the northwestern domain. Spatial variations in

these attitudes define several(?) large-wavelength open folds that record late stage, post-

metamorphic deformation. Domains of uniformly dipping schistosity are also helpful in

constraining stratigraphic thickness and locating major faults.

Foliations and large-scale folds

Poles to axial-plane foliation were plotted on a stereogram using an equal-area

stereographic net that included both domains of Pelona Schist (Fig. 3). Contouring of these

data points reveals a broad, evenly distributed great-circle girdle (p-girdle) with two maxima

10

Figure 6.. Pole and density contour plot -- southeasterndomain.

Figure 5. Pole and density contour plot -- northwesterndomain.

(Fig. 4). The maxima represent two generalized fold-limb planes with an interlimb angle of

90 degrees. A theoretical fold axis (p-axis) is found by taking the pole to the p-girdle. This p-

axis is horizontal and trends N52oW. Thus, these foliation data points represent a normal,

horizontal, nonplunging, slightly asymmetric, upright fold with a broad, rounded fold hinge

with an interlimb angle of 90 degrees (Marshak and Mitra, 1988).

In looking at stereonet

plots of southeastern and

northwestern foliation domains

individually, one can see the

origin of the composite stereonet

plot of above. Contour density

plots of both domains show

distinct point maximum distributions. The northwestern domain has a southwestern pole

maximum (limb) that dips 40 degrees with the hint of a slight great circle girdle to the

northeast (Fig. 5). This is due to the presence of both northeast- and southwest-dipping

foliation planes within the domain. The southeastern domain has a northeastern pole

maximum (limb) that dips 50

degrees (Fig. 6). Thus, the

northwestern domain contains

primarily northeastern and

secondarily southwestern axial-

plane foliation attitudes. The

southeastern domain contains

11

Figure 7. Gross metamorphic foliation of the Pelona Schist within the map area.

almost exclusively southwest-dipping axial-plane foliations. Schistose structures of the Pelona

Schist may be shown as gross or average foliations within the mapped domains (Fig. 7) as

derived from Plate 1 (or Fig. 2), Geologic Map of North Fork Lytle Creek.

Stratigraphy deduced from gross foliation patterns

Pelona Schist in the map area is exposed in section thickness up to 1900 m in the

northwestern domain and up to 1000 m in the southeastern domain. In the southeastern

domain Pelona Schist is mostly composed of sections 50 m to hundreds of meters thick of

gray schist and quartzite layers centimeters to a few meters thick with minor layers of

12

subordinate green schist up to 3 m thick. Green schist seems to become more abundant

higher in the section. The northwestern domain north of the Scotland fault contains similar

stratigraphy to the eastern domain with a few differences. There is a 1-2 m thick band of a

limestone-like, foliated calcium-silicate rock (cs) found interbedded with gray schist and green

schist about halfway up the section north of the Scotland fault. Also, there is more greenish

metasandstone (gray schist variant) that contains less or very fine-grained muscovite. This

metasandstone has a weak schistosity with a crenulated texture in some spots. South of the

Scotland fault the northwestern domain begins with 5-50 m of gray schist with minor,

interbedded quartzite layers 1-2 m thick. A transition zone where green schist rock is

interbedded with occasional gray schist layers 1-2 m thick gives way to a large section of

green schist up to 700 m thick interbedded with minor quartzite and gray schist layers

centimeters to 1-2 m thick. The section returns to gray schist with a few beds of green schist

and quartzite 50-200 m below the Vincent thrust fault. A section of folded, mylonitic rock

was traversed at the edge of the map area before going down section into gray schist again.

Small-scale folds

Geologic field mapping revealed the basic structure and stratigraphy of the Pelona

Schist. Various types of small-scale folds were seen in the Pelona Schist. Isoclinal folding

was especially dominant in the gray schists (mg) and quartzites (mc). Limbs on such folds

often were nearly parallel and separated by 2 to 30 cm. Folds were best seen where muscovite

was in high contrast to darker, more pelitic, minerals, or to lighter minerals such as quartz and

albite. Deep red to purple metacherts isoclinally folded with gray schist also gave a good

contrast. Parasitic folding of quartz or albite on a millimeter to centimeter scale sometimes

occurred above or below isoclinal folding. Chevron or kink folding on a centimeter scale

usually was seen only in gray schists. In many cases where schistosity was poorly developed

13

in the gray schist (usually due to less muscovite), small 1-5 mm crenulations were observed.

Open, larger-scale (0.5-2 m) folding as well as boudinage structures were occasionally present

in the gray schists.

Igneous structures

Common to the entire study area is the intrusion of both felsic and mafic dikes and

sills. While it appears that the majority of the granitic intrusions are sills and most of the

mafic intrusions are dikes, this is not always the case. Most granitic and mafic rocks occur as

1-5 m wide intrusions, whether sill- or dike-like. Granitic sills are found in both domains

interbedded with both green schist and gray schist, often in alternating bands. In the eastern

domain, midway up the section, a large body of granitic rock intrudes gray schist to form

alternating light and dark bands. This intrusion appears to carry across a drainage that cleaves

this domain into two sub-equal blocks. Mafic dikes and sills are only seen cutting across the

granitic rock and even older Pelona Schist. Thus, the mafic rocks are younger than the

granitic rocks which in turn are younger than the Pelona Schist and mylonitic rocks of the

Vincent thrust fault.

Faults

Several instances of recent faulting were mapped in the study area, mostly in the

western domain. A smooth fault plane striking N73oW and dipping 78o in granitic rock in the

western block of the eastern domain was noted with possible striae along strike. This was the

only fault mapped in the eastern domain. Several faults were mapped in the western domain.

On the north side two small faults in gray schist with attitudes of S85oW/77o and S65oW/77o

show slickenside lineations raking 10o NW and 38o NW, respectively. These faults are 2 m

apart and probably are part of a fault splay. In the same general area a large, vertical fault

14

Figure 8. Right-lateral offset of quartz vein in metagraywacke atfault in NW domain.

plane (3 m tall by 6 m wide) of gray schist striking N14oE has well-developed slickenside

lineations raking 15o NW. Quartz veins (3 cm wide) in the area show right lateral offset

(along strike) of 15-30 cm. Thus, this fault has apparent right lateral separation. The most

interesting fault(s) mapped were three northeast-striking faults with attitudes of S85oW/64o

(lin. raking 55o NE), S66oW/90o (lin. raking 14o NE), and S39oW/65o. These three faults all

strike approximately along the same line of sight and cross the Scotland fault. The

northernmost fault displays 3 cm of right lateral offset of a quartz vein (Fig. 8) although it is

possible that this may be

slope creep. In the same

line of sight, a 10-20 m

wide, vertical vegetation

band has a strong color

contrast to the adjacent

vegetation and soil,

giving more evidence for

the presence of a fault

here. A crushed rock,

fracture zone of granite

occurs in the southernmost drainage channel bordering the contact between the granite and

adjacent green schist. The fracture zone, small fault surfaces, and talus fragments all look

"fresh", i.e. rock edges and fracture surfaces are sharp, angular, and have little lichen build-

up. The area appears to be geologically active during Holocene time. Numerous rock slides

were heard while in the upper confines of the southern area near the Vincent thrust fault.

15

DISCUSSION AND INTERPRETATION

SUMMARY OF EXISTING REGIONAL MAPPING

Structure of the Pelona Schist

Geologic mapping by several workers has contributed much to the understanding of

the basement terrane of the central and eastern San Gabriel Mountains. On the 1:250,000

scale San Bernardino Quadrangle, a 90 to 130 square-km area of Pelona Schist is exposed

southwest of the San Andreas fault and east to northeast of three segments of the Vincent

thrust fault. These segments generally strike northwest and are offset left-laterally by

northeast striking faults. Within the Pelona Schist on this map, two northwest-trending

antiforms are shown separated by the Punchbowl fault. Recent, unpublished mapping by Jones

(1993) and Nourse et al., (1994) shows the presence of two, northwest-trending antiforms

separated by a synform in the region southwest of the San Jacinto and Punchbowl faults and

west of the San Antonio faults. Additionally, a possible antiform is shown further north but

east of San Antonio fault near its possible intersection with the inferred continuation of the

Scotland fault.

Ehlig and Jacobson have been particularly prolific in their collection and interpretation

of data regarding the Vincent thrust and associated mylonitic rocks of the upper plate and the

underlying lower plate rocks of the Pelona Schist. Widespread distribution of several bodies

of the Pelona, Rand, and Orocopia Schists exists in the vicinity of the San Andreas and

Garlock faults from the Rand Mountains in the north to the Chocolate Mountains near the

Salton Trough in the south (Ehlig, 1968).

16

Figure 9. Diagrammatic cross section near the Narrows ofthe East Fork of San Gabriel River (Jacobson, 1983).

Folding styles within the Pelona Schist

The Pelona Schist is presently exposed in section up to 4 km thick and contains several

types of metamorphic folding. Jacobson has separated metamorphic folding into three groups

based on style. Style 1 folds occur throughout the section and include early, synmetamorphic,

tight isoclinal folds as well as later, isoclinally refolded folds. Such folds are easiest to see in

metagraywacke and metachert

with a well-developed axial-

planar schistosity. Style 2

folds occur in the upper 700 m

below the Vincent thrust fault

and are exemplified by the

Narrows Synform and

associated minor folding that overprint style 1 folding. Style 2 folding is defined by open

folding of both compositional layering and schistosity of style 1 folds. The "Narrows

Synform" is a macroscopic feature whose bedding and axial-planar schistosity poles lie on a

great circle with an axis that trends N58oW and plunges 5o (Fig. 9). Style 3 folds are

characterized by kink folds and a broad, post-metamorphic arch of Pelona Schist and the

overlying mylonites. Style 3 folds may be important in that they may show that the

orientations of style 1 and style 2 folds have changed over time. These styles of folding do

not imply discrete pulses of metamorphism that predate one another. Rather, it is suggested

that style 1 and style 2 folds occurred as a continuous process during thrusting. Such varied

styles of folding reinforce the idea that the Pelona Schist has undergone a complex structural

history. (Jacobson, 1983).

17

Metamorphism and the Vincent thrust

Metamorphism of the Pelona Schist is associated with movement on the Vincent thrust

fault on the basis of several structural features in the San Gabriel Mountains. Orientations of

bedding-plane schistosity within the Pelona Schist are approximately parallel to the base of the

Vincent thrust and the foliation of the overlying (upper plate) mylonitic rocks. Upper plate

rocks near the thrust have undergone retrograde metamorphism to similar mineral

assemblages as the lower plate, prograde-metamorphosed Pelona Schist. Metamorphic

deformation is stronger and metamorphic grain size is larger in the Pelona Schist close to the

thrust than it is further away (down section). Based on these points, metamorphism of the

Pelona Schist is thought to have occurred at the same time as movement on the Vincent thrust

fault. It is suggested that this movement took place during Paleocene time (Ehlig, 1982).

The Vincent thrust fault is the most significant feature exposed in the San Gabriel

Mountains west of the San Andreas fault. The thrust (Fig. 9) is marked by a thick zone of

deformed mylonitic rocks that are overlain by gneissic-plutonic basement rocks of

Precambrian to Mesozoic age and underlain by the Pelona Schist of probable late Mesozoic

age (Ehlig, 1982). Although it is generally accepted that the Vincent thrust fault represents a

paleo-subduction zone, there has been a long-standing controversy on the direction of

thrusting. Northeast movement of the upper plate is suggested by overturning of the Narrows

Synform and the assumption that movement was perpendicular to fold axes of the upper and

lower plate rocks (Ehlig, 1982). Such conditions may be accommodated by a collision model

where the protolith of the Pelona Schist was deposited in an oceanic, back-arc basin followed

by thrusting of a outboard, continental arc across the basin. This model proposes a

southwest-dipping subduction zone and requires a suture zone to the east of the Pelona

18

Schist, for which there is no evidence (Jacobson, Oyarzabal, and Haxel, 1996). A competing

model, proposed by Burchfield and Davis (1981) and supported by many others, suggests that

the Pelona Schist is an extension of the Franciscan terrane that was subducted eastward

beneath the continental margin, and that northeast transport of the upper plate occurred

during late deformation and does not represent the original subduction direction but rather the

exhumation direction (Jacobson, Oyarzabal, and Haxel, 1996). The origin and ramifications

of the Vincent thrust have been and may continue to be enigmatic.

Late Cenozoic Faults

Cucamonga fault zone

The eastern San Gabriel Mountains are largely dissected and bordered by Late

Cenozoic faults. The Cucamonga fault zone is the eastern part of the frontal fault system and

borders the southern edge of the San Gabriel Mountains. The Cucamonga fault zone is made

up mostly of east-striking, north-dipping thrust faults that separate crystalline basement rocks

from alluvium of the Santa Ana Valley to the south (Morton and Matti, 1993). Faulting has

occurred since Quaternary time and continues actively today. Compression rates are

estimated to be 3-5 mm per year. Morton and Matti hypothesize that the Cucamonga fault

zone merges with the San Andreas fault zone 13 km downdip to the north.

San Antonio Canyon and other NE-striking, left-lateral faults

As mentioned earlier, the Vincent thrust fault is dissected into three segments by two

northeast-striking, left-lateral faults, the Weber fault to the west and the San Antonio Canyon

fault (SACF) to the east. The Weber fault is a nearly straight, left-lateral, strike-slip fault that

apparently offsets the Vincent thrust fault by 5.1 km as shown on the San Bernardino (S.B.)

Quad 2o sheet. This offset may be mostly erosional as Jones has demonstrated offset of only

19

200 m on a continuous, post-metamorphic arch within the Pelona Schist that crosses the

Weber fault (Jones, unpublished mapping, 1993).

The SACF appears to left-laterally displace the Vincent thrust by 5.2 km as seen on

the S.B. Quad 2o sheet. This offset also may be exaggerated due to subsequent erosion of the

fault surface. Jones argues that offset is only 2.8-3.0 km on the basis of metabasalt layers and

style 3 arches mapped within the Pelona Schist. The Icehouse Canyon fault (IHCF) is a

continuation of the Late Miocene-Early Pliocene San Gabriel fault (SGF) that predates

movement on the SACF. A young episode of movement on the SACF constrains offset of

IHCF and SGF to 3.5 km (Nourse et al., 1994). However, Nourse speculates that a block of

Pelona Schist is missing and has been translated northeastward as shown by a misalignment of

basement terranes. Thus, to accommodate this disparity of lithologies requires 6.5 km of

offset on the SACF during Middle Miocene time, totalling 10 km of offset for both episodes

of movement.

San Andreas and San Jacinto fault zones and related NW-striking faults

The San Andreas fault zone (SAFZ) is an 1100-km long, right-lateral, transform fault

system that plays a key role in the relative motion between the Pacific and North American

plates. By 1981 a classic, plate-tectonic model of the San Andreas fault had three main

points: (1) ~300 km of right-lateral displacement across the entire San Andreas fault system,

(2) slip is distributed in southern California into ~240 km along San Andreas fault proper and

~60 km across the San Gabriel fault, (3) total displacement equates to ~300 km of rifting since

5 Ma in the Gulf of California (Powell and Weldon, 1992). Powell and Weldon feel that the

classic model inadequately describes the complex nature of the San Andreas fault system.

Their model proposes that movement on the fault began at 17 to 20 Ma and evolved in three

20

phases of faulting activity including development and abandonment of ancestral faults of the

San Andreas system, the last of which is the modern fault that emerged at 4-5 Ma with slip

rates of 20-35 mm per year. It is worth noting that the San Gabriel fault is thought to be an

ancestral San Andreas fault that had accumulated 42-45 km of displacement between 13 and 4

Ma at a slip rate of 5-10 mm per year (Powell and Weldon, 1992).

The San Jacinto fault zone (SJFZ) is an active member of the larger San Andreas fault

system in southern California (Thatcher et al., 1975). It differs from the continuous break of

the San Andreas fault in that it is comprised of a series of en echelon faults which strike

N45oW throughout most of its length from Borrego Valley to the east side of the San Gabriel

Mountains (Sharp, 1975). It appears to lose its identity as a mappable fault as it enters the

alluvium of the Lytle Creek drainage. The SJFZ is often shown as joining the SAFZ or

merging with the inactive Punchbowl fault in the North Fork of Lytle Creek (Morton and

Matti, 1993). The S.B. Quad 2o sheet shows a southern branch of two SJFZ splays within the

North Fork of Lytle Creek. This branch is shown as turning to the left between the NW and

SE domains of the study area and dies out or is truncated by the Scotland fault to the south.

Dibblee (1982) feels that the SJFZ continues its NW-strike until it dies out, while Morton and

Matti (1993) suggest that the SJFZ coalesces with three north-dipping faults, each within the

South, Middle, and North Forks of Lytle Creek, respectively. At this junction, these faults

progressively rotate or bend counter-clockwise until they are northeast-striking faults.

Morton and Matti argue that northwest-striking fault separation changes from oblique-right-

reverse to northeast-striking faults with oblique-left-reverse, and also that most east-striking

faults appear to be thrust (e.g. the Cucamonga fault). Slip rates of 20 mm per year are

estimated since SJFZ activation at 1.5 Ma. Accelerated uplift of the eastern San Gabriel

21

Mountains in the last 1.5 m.y. is hypothesized by transference of slip from the SAFZ to the

SJFZ at a structural knot in the San Gorgonio Pass area (Morton and Matti, 1993). The Glen

Helen and Lytle Creek faults are right-lateral faults north and south of the SJFZ, respectively,

that parallel the latter.

The Scotland fault is a right-lateral fault that displaces Tertiary granite and Pelona

Schist and lies between the Middle and North Forks of Lytle Creek. It appears to be a branch

of the SJFZ northwest of the confluence of the South, Middle, and North Forks of Lytle

Creek. Displacement measurements of Tertiary granite on the S.B. Quad 2o sheet reveal a

possible, maximum right-lateral separation of 3.1 km on the Scotland fault. It is speculated

that this is a Quaternary fault associated with the San Jacinto fault zone.

PALINSPASTIC RECONSTRUCTION

Methods/Current Position

The obvious question arises as to how the study area fits into the geologic setting and

complex system of faults mentioned above. As a starting point I will attempt to use known

fault ages and displacements to reconstruct a simplified paleogeology with respect to the rock

types of the mapped area, particularly the Pelona Schist and Tertiary granite. Using new data

acquired in the field (Fig. 10), I will see if new constraints on local fault displacements are

warranted or possible, or if this data simply reinforces existing fault-interaction models of

other workers. I offer two main models in the reconstruction with some alternatives, but

other fault solutions may exist. These reconstructions are supported but not required by the

data.

22

Figure 10. Present position of main rock types with new data added from study area(Adapted from S.B. Quad 2o sheet).

Model 1

Restoration of right-lateral faults

The NW domain of the study area lies approximately at the junction of the left-lateral

SACF and the right-lateral SJFZ. Restoration of the right-lateral, NW-striking faults will

proceed first as it is assumed that they post-date movement on the left-lateral, NE-striking

faults. Tertiary granite contacts with Pelona Schist are the primary constraints for movement

of these faults. Reversing 4.4 km of right-lateral slip along the Glen Helen fault (GHF) is

proposed to restore contacts of a small body of schist and granite to the southeast. I propose

that the GHF deflects left to merge with the SJFZ. Restoration of right slip along the SJFZ in

the North Fork of Lytle Creek is constrained to a minimum of 5.5 km and a maximum of 8.1

km in order to approximately line up the intrusive contact between the Tertiary granite and

23

Figure 11. Model 1 -- Restoration of right-lateral faults.

Pelona Schist on opposite sides of the fault. Likewise, right slip on the Scotland fault is

constrained to a minimum of 1.5 km and maximum of 4 km to line up the intrusive contacts.

With restored right slip of 4.4 km along the GHF, 5.9 km along the SJFZ, and 1.9 km along

the Scotland fault, the intrusive contacts of the granite across the three faults are loosely lined

up to form an elongate, intrusive granite body in contact with Pelona Schist. Additionally, the

NE-trending drainage between the NW and SE mapped domains of Pelona Schist line up with

drainages to the north and to the south and is coincident with the SACF. Thus, I am lining up

drainages north of the Scotland fault with the SACF to the south. Stratigraphy and structure

of the Pelona Schist seem to match as well. The SE domain is now stratigraphically above the

block of Pelona Schist to the north that has also a SW-dipping gross foliation. The

restoration of the Scotland fault places the thick layer of metabasalt of the NW domain of

Pelona Schist stratigraphically above the metagraywacke of the SE domain as well as allowing

24

Figure 12. Model 1 -- Restoration of 2.9 km on the San Antonio Canyon fault.

for an inferred style 3 synform between the domains (Fig. 11). After restoration of right slip

along the GHF, SJFZ, and the Scotland fault, a coherent block of folded Pelona Schist is

reconstructed adjacent to a southwest-trending, elongate intrusion of Tertiary granite. I

assume that the Punchbowl fault has since been abandoned. Thus, motion of the GHF deflects

left to the SJFZ for a short distance which then branches left to a through-going Scotland

fault. This requires at least 12.2 km of cumulative slip along a through-going Scotland fault

that parallels the Punchbowl fault.

Restoration of left-lateral faults

Restoration of left-lateral slip along the SACF is more straightforward. Restoration of

2.9 km left slip along SACF with minor clockwise rotation moves the entire block of Pelona

Schist and intruded granite mentioned above as a single body to a point that is compatible

25

with Jones' constraints of 2.8 to 3.0 km of slip, i.e. to a point where the large metabasalt layer

is in stratigraphic continuity across the SACF. While this is not as much as 3.5 km

recommended by Nourse (1994), erosion may make up the deficit to still line up the SGF with

IHCF. Two inferred style 3 arches east of SACF, an antiform and a synform, line up

approximately with the southern arches to the west of SACF as mapped by Jones (1993).

Jones used a different, more northerly antiform for constraining movement on the SACF. My

mapping data and restoration of faults do not support the presence of this antiform east of

SACF. Restoration of 2.9 km left slip brings together a coherent block of Pelona Schist

across the SACF, the Vincent thrust fault trace (after erosional factors are considered), and

upper plate mylonitic rocks as well as lining up the San Gabriel and Icehouse Canyon faults.

Two inferred style 3 arches offer potential piercing points for the 2.9 km of movement along

the SACF (Fig. 12).

Model 2

Restoration of 4.4 km of right slip along the GHF is as in Model 1. The GHF deflects

left to the SJFZ which becomes a through-going fault that joins or reactivates late-phase

movement on the Punchbowl fault. Movement along the SJFZ is increased from the 5.9 km of

Model 1 to 8.1 km with movement deflecting around the granite pocket leaving it part of the

main granite body. This increased movement will be necessary to allow SACF to pass on the

west side on the NW domain of Pelona Schist. At this point 12.5 km has been restored on the

reactivated San Jacinto/Punchbowl (SJ/PB) fault. I restore a total of 1.9 km along the

Scotland fault east of SACF. To avoid placement of a through-going Scotland fault west of

SACF, I propose that the Scotland fault steps right to the SJFZ as 2.2 km of rift basins are

closed during restoration of motion along Scotland and the SJFZ west of SACF (Fig. 13).

26

Figure 13. Model 2 -- Restoration of right-lateral faults along a reactivated SanJacinto/Punchbowl fault.

Total slip on the reactivated SJ/PB fault is 16.6 km.

Restoration of 2.9 km along the SACF follows as in Model 1 with some minor

differences. There is no clockwise rotation during restoration of left-lateral slip. This creates

more of a faulting void along the SACF (Fig. 14). Since more slip on the SJFZ is proposed,

the SACF passes the NW domain on the west side instead of the east side as in Model 1. This

through-going SACF deflects left after the SJFZ causing pull-apart basins during movement.

An alternative model resolves motion of the SACF into two fault splays. The main fault is

west of the NW domain as above, but a minor fault splits the NW domain into two blocks that

offset one another 0.5-1 km left-laterally (Fig. 15). This alternative allows for a better

alignment of style 3 arches.

27

Figure 15. Model 2 alternative -- left-lateral slip is distributed along splay of SanAntonio Canyon fault that cleaves the NW domain.

Figure 14. Model 2 -- Restoration of 2.9 km along San Antonio Canyon fault.

28

Problems in the reconstructions

An intriguing problem lies in the nature and sequence of fault displacements. In Model

1, I have proposed restoration of 4.4 km along the GHF, 5.9 km along the SJFZ, and 1.9 km

along a through-going Scotland fault west of SACF and south of the inactive Punchbowl fault.

That is fine for keeping the Punchbowl fault dormant, but that amounts to 12.2 km of slip on a

Scotland fault west of SACF for which there is no direct field evidence within the Pelona

Schist. Model 1 also results in an anomalous block granite to the northwest above the GHF.

The NW domain of Pelona Schist ends up next to the SE domain resulting in an apparent

stratigraphic and structural discontinuity. Stratigraphically, it places NE-dipping schist next to

SW-dipping schist to the east resulting in a discontinuity of gross foliations. Structurally, the

above placement of rocks disrupts continuity of style 3 arches across the SACF.

Model 2 helps correct the above problems. The anomalous block of granite is not

separated by the SJFZ; it remains attached to the adjacent, southern block of granite during

movement of the SJFZ. The lack of a through-going Scotland fault is accommodated by the

Scotland fault stepping right to a reactivated SJ/PB fault that also accommodates movement

of the GHF. The closure of rift basins during restoration and opening of the same basins

during activation of the Scotland fault helps deal with the hypothesis that the Scotland fault

begins to die out as it steps right to the SJFZ. The structural discontinuity introduced by the

NW domain is mitigated by break up of the block by a branch of the SACF in the alternative

to Model 2. This branch fault restores structural continuity of style 3 arches across the SACF,

but the sense of motion required to offset the domain may be in the wrong direction (if, in

fact, this fault is right-lateral).

These reconstructions are based on the assumption that movement on the S.J. and

29

G.H. faults post-date movement on the SACF. If these faults have been active coevally during

Quaternary time, one might not expect the blocks of granite and Pelona Schist to end up as a

coherent body in the way they do as mentioned above. That is, as one simultaneously reverses

motion on these NW- and NE-striking sets of faults, the blocks might tend to disperse or

scatter to the south. Upon activation of faults, do the scattered blocks condense as they move

northward? Or, should fault restoration and activation resolve into a north-south motion

without any scattering or condensing of rock units? If so, how does one account for the

preferred right-lateral separation of Tertiary granite and Pelona Schist present today in the

eastern San Gabriel Mountains? Complicating the issue further, the faults may have been

active coevally, but the faults "took turns" each moving in spurts alternating between NW-

and NE-striking faults.

I have not considered rigorously the conservation of rock mass, nor rock area in these

reconstructions. Restoration likely would have created extensional basins while fault

activation would have tended to close these basins during compressional events and uplift. If

there were basins before fault activation, what happened to the sediments deposited in them?

Were they eroded first due to their sedimentary nature when compared to metamorphic and

igneous basement rocks? I have assumed largely strike-slip displacements when these faults

are thought to have oblique components to them as well, however minor they may be.

Erosional offsets were considered only for the SACF as suggested by other workers (Nourse

and Jones). I was not able substantiate the missing block of Pelona Schist nor the added 6.5

km of offset along the SACF as suggested by Nourse (1994). This motion probably predates

fault movement that I am considering in my reconstructions.

Implications

30

The implications of my field work and paleo-reconstruction of the Pelona Schist are

several. I concur with Jones and Nourse that total left-lateral slip on the SACF since Late

Miocene-Early Pliocene time (5 Ma) is loosely constrained to 2.9 km based on metabasalt

stratigraphy and style 3 arches in the Pelona Schist that come together after restoration. I

argue that 1.9 km right-lateral offset on the Scotland fault largely pre-dates 5.9 km right-

lateral offset on the San Jacinto fault zone and 4.4 km right slip on the Glen Helen fault (since

1.5 Ma as proposed by Morton and Matti, 1993). Continued movement on the Scotland fault

stepped right as it joined the SJFZ. I hypothesize that the GHF deflects left to join a

reactivated SJ/PB fault west of San Antonio Canyon and that this fault should have 16.6 km

of right-lateral displacement since 5 Ma. Lastly, I feel that Morton and Matti's hypothesis that

northeast-striking faults turn and blend into northwest-striking faults is poorly justified. I

propose that the truncation of northeast-striking faults by northwest-striking faults is more

plausible due to the reconstruction of Pelona Schist and granite in the mapped area. To test

this hypothesis, one needs to look outside the map area for better piercing points. The SACF

is old, not seismically active, and offset by seismically active faults, the SJFZ and GHF. NE-

trending valleys north of the SJFZ and GHF and north of the San Andreas fault should have

large (2-3 km) left-lateral offsets created by the SACF.

CONCLUSION

During this field and research project I learned many things about myself and geology.

First and foremost, I learned that diligence and independence when doing field work are highly

rewarding experiences. I remember some mornings when I found it difficult to get up and get

31

motivated to go to the field and map geologic structures through vegetation and topography

that seemed to zap one's strength. But, now I am glad that I did. It felt good to be in the field

and to exert oneself on a physical level while collecting data for an intellectual endeavor on a

mental level. On the academic side, I learned that field studies and the acquisition and

recording of geologic data are a permanent record with many uses. With objectivity, good

data may be collected that may continue to be of use through time. Inferences can be made

based on this data to support scientific models.

On a geologic note, I learned many things. I learned good techniques for geologic

field mapping. I learned how detailed mapping in a limited study area may be extended to

regional geology. For example, in the study area I recorded about 300 attitudes of foliation

planes of the Pelona Schist as well as rock type contacts, stratigraphy, and cross-cutting

relationships. I was able to approximate gross foliations of mapped Pelona Schist in the area.

By comparing these foliations and mapped contacts to existing, regional geologic mapping, I

was able to reconstruct a paleogeology of a limited domain of Pelona Schist and a Tertiary

granite body that intrudes it. This was accomplished by restoration of 4.4 km of right slip

along the Glen Helen fault, 8.1 km right slip along the Early Pleistocene age San Jacinto fault

zone, 1.9 km or right slip along the Pre-Early Pleistocene age Scotland fault, and 2.9 km of

left slip along a Post-Late Miocene-Early Pliocene phase of movement along the San Antonio

Canyon fault.

Although a crude paleogeology was reconstructed, it was by no means a rigorous

reconstruction, and may be better constrained by further field work and analysis. Specifically,

the exact nature and position of the contact between the Pelona Schist and Tertiary granite

could use some refinement. However, the present, irregular contact may be explained by

32

intrusion of a thick sill of granite along previously folded schist. Mapping of felsic and mafic

dikes and sills within the Pelona Schist of the study area and outside it may help define precise

piercing points for regional fault reconstruction. Detailed mapping of small-scale faults in the

study area as well as extension or continuation of faults outside the area may offer more

evidence to help answer the question regarding interaction of right-lateral, NW-striking and

left-lateral, NE-striking faults.

33

REFERENCES CITED

Ehlig, P. L., 1968, Causes of distribution of Pelona, Rand, and Orocopia Schists along the SanAndreas and Garlock faults, in Dickinson, W. R., and Grantz, A., eds., Conference onGeologic Problems of San Andreas Fault System, Proceedings: Stanford UniversityPublications, Geological Sciences, v. 11, p. 294-306.

Ehlig, P. L., 1975, Basement rocks of the San Gabriel Mountains, south of the San AndreasFault, southern California, in Crowell, J. C., ed., San Andreas Fault in Southern California, AGuide to San Andreas Fault from Mexico to Carrizo Plain: Sacramento, California, CaliforniaDivision of Mines and Geology Special Report 118, p. 177-186.

Ehlig, P. L., 1982, The Vincent thrust: its nature, paleogeographic reconstruction across theSan Andreas fault and bearing on the evolution of the Transverse Ranges: Geology andMineral Wealth of the California Transverse Ranges, South Coast Geological Society, p. 370-379.

Jacobson, C. E., 1983, Complex refolding history of the Pelona, Orocopia, and Rand Schists,southern California: Geology, v. 11, p. 583-586.

Jacobson, C. E., 1983, Relationship of deformation and metamorphism of the Pelona Schist tomovement on the Vincent thrust, San Gabriel Mountains, southern California: AmericanJournal of Science, v. 283, p. 587-604.

Jacobson, C. E., 1983, Structural geology of the Pelona Schist and Vincent thrust, SanGabriel Mountains, California: Geological Society of America Bulletin, v. 94, p. 753-767.

Jacobson, C. E., Dawson, M. R., and Postlethwaite, C. E., 1988, Structure, metamorphism,and tectonic significance of the Pelona, Orocopia, and Rand Schists, southern California, inErnst, W. G., ed., Metamorphism and crustal evolution of the western United States (RubeyVolume VII): Englewood Cliffs, New Jersey, Prentice-Hall, p. 976-997.

Jacobson, C. E., Oyarzabal, F. R., and Haxel, G. B., 1996, Subduction and exhumation of thePelona-Orocopia-Rand schists, southern California: Geology, v. 24, p. 547-550.

Marshak, S., and Mitra, G., 1988, Basic Methods of Structural Geology: Englewood Cliffs,New Jersey, Prentice-Hall, 446 p.

Morton, D. M., 1975, Synopsis of the geology of the eastern San Gabriel Mountains, southernCalifornia, in Crowell, J. C., ed., San Andreas Fault in Southern California, A Guide to SanAndreas Fault from Mexico to Carrizo Plain: Sacramento, California, California Division ofMines and Geology Special Report 118, p. 170-176.

Morton, D. M., and Matti, J. C., 1993, Extension and contraction within an evolving

34

divergent strike-slip fault complex: The San Andreas and San Jacinto fault zones at theirconvergence in southern California, in Powell, R. E., Weldon, R. J., and Matti, J. C., eds.,The San Andreas Fault System: Displacement, Palinspastic Reconstruction, and GeologicEvolution: Boulder, Colorado, Geological Society of America Memoir 178, p. 217-230.

Nourse, J. A., Hazelton, G. B., and Jones, R. K., 1994, Evidence of two phases of LateCenozoic sinistral displacement on the San Antonio Canyon fault, eastern San GabrielMountains, California: Geological Society of America Abstracts with Programs, CordilleranSection, p. 77-78.

Powell, R. E., and Weldon, R. J. II, 1992, Evolution of the San Andreas Fault: AnnualReview of Earth Planetary Science, v. 20, p. 431-468.

Sharp, R. V., 1975, En echelon fault patterns of the San Jacinto fault zone, in Crowell, J. C.,ed., San Andreas Fault in Southern California, A Guide to San Andreas Fault from Mexico toCarrizo Plain: Sacramento, California, California Division of Mines and Geology SpecialReport 118, p. 147-152.

Thatcher, W., Hileman, J. A., and Hanks, T. C., 1975, Seismic slip distribution along the SanJacinto fault zone, southern California, and its implications: Geological Society of AmericaBulletin, v. 86, p. 1140-1146.

35

APPENDIX

Field Foliation Data used in Stereonet Plot(Strike azimuth obeys right-hand rule, strike and dip units are degrees)

Ref Num Strike Dir Dip Dir Dip Date Rock Type Location1 185 275 23 5/9/96 mg SE Domain2 145 235 30 5/9/96 mg SE Domain3 135 225 30 5/9/96 mg SE Domain4 135 225 30 5/9/96 mg SE Domain5 123 213 31 5/9/96 mg SE Domain6 155 245 31 5/9/96 mg SE Domain7 100 190 30 5/9/96 mg SE Domain8 125 215 30 5/9/96 mg SE Domain9 84 174 30 5/9/96 mg SE Domain10 102 192 35 5/9/96 mg SE Domain11 100 190 56 5/9/96 mg SE Domain12 150 240 22 9/19/96 mg SE Domain13 175 265 27 9/19/96 mg SE Domain14 145 235 25 9/19/96 mg SE Domain15 285 15 49 9/19/96 mg SE Domain16 300 30 35 9/22/96 mg NW Domain17 317 47 41 9/22/96 mg NW Domain18 304 34 50 9/22/96 mg NW Domain19 322 52 35 9/22/96 mg NW Domain20 312 42 56 9/22/96 mg NW Domain21 280 10 62 9/22/96 mg NW Domain22 300 30 65 9/22/96 mg NW Domain23 110 200 80 12/15/96 mg NW Domain24 110 200 85 12/15/96 mg NW Domain25 20 110 45 12/15/96 mg NW Domain26 280 10 25 12/15/96 mg NW Domain27 288 18 35 12/15/96 mg NW Domain28 263 353 30 12/15/96 mg NW Domain29 328 58 55 12/15/96 mg NW Domain30 3 93 70 12/15/96 mg NW Domain31 310 40 33 12/15/96 mg NW Domain32 320 50 35 12/15/96 mg NW Domain33 308 38 37 12/15/96 mg NW Domain34 290 20 60 12/15/96 mg NW Domain35 190 280 32 12/15/96 mg NW Domain36 208 298 24 12/15/96 mg NW Domain37 160 250 18 12/15/96 mg NW Domain38 10 100 30 12/15/96 mg NW Domain39 333 63 50 12/15/96 mg NW Domain40 320 50 54 12/15/96 mg NW Domain41 275 5 85 12/15/96 mg NW Domain42 300 30 21 12/15/96 mg NW Domain43 295 25 85 12/15/96 mg NW Domain44 300 30 50 12/15/96 mg NW Domain45 275 5 31 12/15/96 mg NW Domain46 315 45 42 1/1/97 mg NW Domain47 275 5 50 1/1/97 mg NW Domain

36

Ref Num Strike Dir Dip Dir Dip Date Rock Type Location48 115 205 17 1/1/97 mg NW Domain49 20 110 30 1/1/97 mg NW Domain50 315 45 57 1/1/97 mg NW Domain51 260 350 64 1/1/97 mg NW Domain52 280 10 55 1/1/97 mg NW Domain53 15 105 25 1/1/97 mb NW Domain54 350 80 55 1/1/97 mg NW Domain55 155 245 35 1/1/97 mg NW Domain56 115 205 60 1/1/97 mg NW Domain57 340 70 88 1/1/97 mg NW Domain58 342 72 77 1/1/97 mg NW Domain59 115 205 60 1/1/97 mg NW Domain60 342 72 50 1/11/97 mg NW Domain61 340 70 60 1/11/97 mg NW Domain62 330 60 62 1/11/97 mg NW Domain63 338 68 50 1/11/97 mg NW Domain64 320 50 64 1/11/97 mg NW Domain65 330 60 74 1/11/97 mg NW Domain66 298 28 72 1/11/97 mg NW Domain67 320 50 47 1/11/97 ls NW Domain68 323 53 60 1/11/97 mb NW Domain69 340 70 78 1/11/97 mg NW Domain70 334 64 50 1/11/97 mg NW Domain71 321 51 49 1/11/97 mg NW Domain72 290 20 50 1/11/97 mg NW Domain73 292 22 48 1/11/97 mg NW Domain74 306 36 51 1/11/97 mg NW Domain75 303 33 57 1/11/97 mg NW Domain76 314 44 24 1/11/97 mg NW Domain77 105 195 54 1/17/97 mg NW Domain78 250 340 10 1/17/97 mg NW Domain79 300 30 16 1/17/97 mg NW Domain80 340 70 10 1/17/97 mg NW Domain81 280 10 29 1/17/97 mg NW Domain82 315 45 28 1/17/97 mg NW Domain83 330 60 48 1/17/97 mg NW Domain84 323 53 50 1/17/97 mg NW Domain85 350 80 40 1/17/97 mg NW Domain86 345 75 38 1/17/97 mg NW Domain87 111 201 80 1/19/97 mg NW Domain88 295 25 85 1/19/97 mg NW Domain89 138 228 58 1/19/97 mg NW Domain90 135 225 60 1/19/97 mg NW Domain91 133 223 52 1/19/97 mg NW Domain92 147 237 50 1/19/97 mg NW Domain93 120 210 57 1/19/97 mg NW Domain94 125 215 65 1/19/97 mg NW Domain95 121 211 57 1/19/97 mg NW Domain96 125 215 51 1/19/97 mg NW Domain97 136 226 55 1/19/97 mg NW Domain98 136 226 84 1/19/97 mg NW Domain99 118 208 73 1/19/97 mg NW Domain100 118 208 53 1/19/97 mg NW Domain

37

101 137 227 35 1/19/97 mg NW DomainRef Num Strike Dir Dip Dir Dip Date Rock Type Location102 135 225 25 1/19/97 mg NW Domain103 173 263 12 1/19/97 mg NW Domain104 170 260 23 1/19/97 mg NW Domain105 154 244 14 1/19/97 mg NW Domain106 147 237 56 1/19/97 mg NW Domain107 139 229 29 1/19/97 mg NW Domain108 160 250 35 1/19/97 mg NW Domain109 178 268 22 1/19/97 mg NW Domain110 144 234 18 1/19/97 mg NW Domain111 169 259 27 1/19/97 mg NW Domain112 115 205 38 1/28/97 mg NW Domain113 170 260 14 1/28/97 mg NW Domain114 95 185 14 1/28/97 mg NW Domain115 125 215 38 1/28/97 mg NW Domain116 110 200 18 1/28/97 mg NW Domain117 100 190 32 1/28/97 mg NW Domain118 90 180 6 1/28/97 mg NW Domain119 295 25 54 1/28/97 mb NW Domain120 343 73 32 1/28/97 mb NW Domain121 338 68 24 1/28/97 mb NW Domain122 331 61 34 1/28/97 mb NW Domain123 306 36 19 1/28/97 mb NW Domain124 326 56 42 1/28/97 mb NW Domain125 297 27 30 1/28/97 mb NW Domain126 275 5 26 1/28/97 mb NW Domain127 299 29 26 1/28/97 mb NW Domain128 281 11 35 1/28/97 mb NW Domain129 285 15 55 1/28/97 mb NW Domain130 219 309 40 1/28/97 mb NW Domain131 188 278 18 1/28/97 mb NW Domain132 210 300 15 1/28/97 mb NW Domain133 327 57 23 1/28/97 mb NW Domain134 340 70 24 1/28/97 mb NW Domain135 324 54 25 1/28/97 mb NW Domain136 327 57 41 1/28/97 mb NW Domain137 276 6 15 1/28/97 mb NW Domain138 330 60 19 1/28/97 mb NW Domain139 289 19 33 1/28/97 mb NW Domain140 280 10 6 1/28/97 mg+mc NW Domain141 148 238 17 1/28/97 mg NW Domain142 299 29 65 1/28/97 mg+mb NW Domain143 332 62 46 1/28/97 mg NW Domain144 344 74 33 1/28/97 mg NW Domain145 205 295 25 1/28/97 myl NW Domain146 313 43 29 1/28/97 mg NW Domain147 320 50 28 1/28/97 mb NW Domain148 284 14 19 1/28/97 mb NW Domain149 287 17 19 1/28/97 mb NW Domain150 294 24 42 1/28/97 mb NW Domain151 299 29 57 1/28/97 mb NW Domain152 274 4 31 1/28/97 mb NW Domain153 270 0 39 1/28/97 mb NW Domain

38

154 256 346 29 1/28/97 mg NW Domain155 285 15 25 1/30/97 mg NW DomainRef Num Strike Dir Dip Dir Dip Date Rock Type Location156 308 38 53 1/30/97 mg NW Domain157 316 46 38 1/30/97 mg NW Domain158 323 53 50 1/30/97 mg NW Domain159 107 197 5 1/30/97 mg NW Domain160 324 54 33 1/30/97 mg NW Domain161 273 3 61 1/30/97 mg NW Domain162 20 110 17 1/30/97 mg NW Domain163 13 103 25 1/30/97 mg NW Domain164 335 65 30 1/30/97 mg NW Domain165 355 85 27 1/30/97 mb NW Domain166 153 243 8 1/30/97 mg NW Domain167 250 340 4 1/30/97 mg NW Domain168 257 347 18 1/30/97 mg NW Domain169 200 290 18 1/30/97 mg NW Domain170 253 343 55 1/30/97 mb NW Domain171 288 18 83 1/30/97 mb NW Domain172 20 110 36 1/30/97 mg NW Domain173 21 111 41 1/30/97 mg NW Domain174 5 95 55 1/30/97 mg NW Domain175 333 63 62 1/30/97 mg NW Domain176 314 44 39 1/30/97 mg NW Domain177 323 53 53 1/30/97 mg NW Domain178 321 51 59 1/30/97 mg NW Domain179 325 55 34 1/30/97 mg NW Domain180 312 42 73 1/30/97 mg NW Domain181 288 18 56 1/30/97 mg NW Domain182 108 198 23 2/8/97 mg SE Domain183 133 223 30 2/8/97 mg SE Domain184 142 232 22 2/8/97 mg SE Domain185 115 205 31 2/8/97 mg SE Domain186 113 203 12 2/8/97 mg SE Domain187 93 183 34 2/8/97 mg SE Domain188 95 185 44 2/8/97 mg SE Domain189 236 326 13 2/8/97 mg SE Domain190 177 267 17 2/8/97 mg SE Domain191 117 207 24 2/8/97 mg SE Domain192 124 214 41 2/8/97 mg SE Domain193 12 102 18 2/8/97 mb SE Domain194 126 216 20 2/8/97 mb SE Domain195 136 226 34 2/8/97 mg SE Domain196 127 217 33 2/17/97 mg SE Domain197 115 205 33 2/17/97 mg SE Domain198 137 227 17 2/17/97 mg SE Domain199 142 232 18 2/17/97 mg SE Domain200 141 231 19 2/17/97 mg SE Domain201 132 222 50 2/17/97 mg SE Domain202 175 265 50 2/17/97 mg SE Domain203 112 202 23 2/17/97 mg SE Domain204 146 236 21 2/17/97 mg SE Domain205 131 221 41 2/17/97 mg SE Domain206 128 218 30 2/17/97 mg SE Domain

39

207 119 209 54 2/17/97 mg SE Domain208 128 218 24 2/17/97 mg SE Domain209 141 231 31 2/17/97 mg SE DomainRef Num Strike Dir Dip Dir Dip Date Rock Type Location210 126 216 34 2/17/97 mg SE Domain211 164 254 26 2/17/97 mg SE Domain212 129 219 43 2/17/97 mg SE Domain213 120 210 56 2/17/97 mg SE Domain214 106 196 38 2/17/97 mg SE Domain215 126 216 31 2/17/97 mg SE Domain216 116 206 43 2/19/97 mg SE Domain217 120 210 36 2/19/97 mg SE Domain218 126 216 51 2/19/97 mg SE Domain219 134 224 39 2/19/97 mg SE Domain220 135 225 40 2/19/97 mg SE Domain221 130 220 41 2/19/97 mg SE Domain222 130 220 53 2/19/97 mg SE Domain223 135 225 42 2/19/97 mg SE Domain224 214 304 12 2/19/97 mg SE Domain225 212 302 30 2/19/97 mg SE Domain226 138 228 50 2/19/97 mg SE Domain227 129 219 39 2/19/97 mg SE Domain228 142 232 47 2/19/97 mg SE Domain229 121 211 40 2/19/97 mg SE Domain230 117 207 32 2/19/97 mg SE Domain231 113 203 51 2/19/97 mg SE Domain232 114 204 67 2/19/97 mg SE Domain233 114 204 59 2/19/97 mg SE Domain234 126 216 61 2/19/97 mg SE Domain235 130 220 30 2/19/97 mg SE Domain236 117 207 34 2/19/97 mg SE Domain237 124 214 35 2/19/97 mg SE Domain238 113 203 36 2/20/97 mg SE Domain239 71 161 52 2/20/97 mg SE Domain240 77 167 43 2/20/97 mg SE Domain241 73 163 44 2/20/97 mg SE Domain242 93 183 37 2/20/97 mg SE Domain243 277 7 47 2/20/97 mg SE Domain244 287 17 45 2/20/97 mg SE Domain245 288 18 50 2/20/97 mg SE Domain246 71 161 30 2/20/97 mg SE Domain247 71 161 22 2/20/97 mg SE Domain248 75 165 43 2/20/97 mg SE Domain249 46 136 42 2/20/97 mg SE Domain250 130 220 25 2/20/97 mg SE Domain251 138 228 32 2/20/97 mg SE Domain252 75 165 13 2/20/97 mg SE Domain253 175 265 14 2/20/97 mg SE Domain254 85 175 18 2/20/97 mg SE Domain255 117 207 46 2/20/97 mg SE Domain256 124 214 24 2/20/97 mg SE Domain257 125 215 24 2/20/97 mg SE Domain258 113 203 34 2/20/97 mg SE Domain259 142 232 18 2/20/97 mg SE Domain

40

260 77 167 40 2/20/97 mg SE Domain261 140 230 40 3/2/97 mb SE Domain262 146 236 31 3/2/97 mb SE Domain263 154 244 18 3/2/97 mb SE DomainRef Num Strike Dir Dip Dir Dip Date Rock Type Location264 265 355 64 3/2/97 mb SE Domain265 170 260 38 3/2/97 mc SE Domain266 172 262 37 3/2/97 mb SE Domain267 129 219 21 3/2/97 mb SE Domain268 189 279 32 3/2/97 mb SE Domain269 190 280 15 3/2/97 mb SE Domain270 151 241 36 3/2/97 mb SE Domain271 150 240 20 3/2/97 mb SE Domain272 157 247 13 3/2/97 mb SE Domain273 174 264 41 3/2/97 mb SE Domain274 189 279 20 3/2/97 mb SE Domain275 207 297 10 3/2/97 mb SE Domain276 162 252 20 3/2/97 mb SE Domain277 147 237 19 3/2/97 mc SE Domain278 97 187 18 3/2/97 mb SE Domain279 154 244 29 3/2/97 mc SE Domain280 120 210 18 3/2/97 mb SE Domain281 111 201 41 3/2/97 mg SE Domain