Deformation in the Paleozoic Stratigraphy of the Piute Mountains in the MojaveDesert Region

K. Kochanski, J.Pershken, K. BrentMIT Department of Earth, Atmosphere and Planetary Science

(Dated: 12 May 2014)

We present a geologic map of the Piute Mountains of the Mojave Desert Region and an accompa-nying structural interpretation. The region is part of a Precambrian metamorphic core complex, andhas a predominantly overturned stratigraphy. Our investigation focused on a deformed sequence ofPaleoizoic sediments within this complex. These sediments have a distinctive stratigraphy whichreveals a series of distinct deformations. We found four distinct generations of ductile events, aswell as recent extensional faulting. The timing of deformation events are constrained by two igneousunits: the intrusive Lazy Daisy pluton (74Ma±3) and the late Tertiary Peach Springs Tuff (18.5Ma).Three ductile events date between the arrival of the pluton and the fall of the tuff, but at least onefold is younger than the tuff, implying that ductile deformation continued in the region for longerthan was previously thought.

I. INTRODUCTION

We present a new map (see Fig. 5) of the Piute Moun-tains focused on a structural analysis of deformation inthe Paleozoic sequence. The Piutes lie a few hundredmiles south of the abrupt truncation of the NorthAmerican Cordillera (see Fig. 1), which is discussed indetail in the next section. The region surrounding thePiute Mountains has a dominantly overturned stratig-raphy, and the majority of the Piutes surface areaconsists of highly metamorphosed Precambrian units.This metamorphic complex is disrupted by the EastPiute and Lazy Daisy plutons which were dated byFletcher & Karlstrom (1990) at 85±7Ma and 74±3Marespectively, and by a kilometre-wide band of highlydeformed Paleozoic units. The structure of this bandis the subject of our study. The Paleozoic units areless highly deformed than the basement and retain adistinctive stratigraphy which offers insight into thesequence of deformation events in the region. Thisstratigraphy has been correlated with the undeformedGrand Canyon Sequence of Arizona (Stone, Howard &Hamilton, 1983). The stratigraphy is also very similarto the Death Valley Sequence of California.

We found that the Piute Mountains have under-gone multiple stages of both compression and exten-sion. The most visible indicator of this is the highlydeformed Paleozoic sequence within the Precambrianmetamorphic core complex. There is evidence of severalstages of folding, including east-west oriented isoclinalfolds. This contrasts with the north-south trendingfolds and thrusts which are found in the bulk of theNorth American Cordillera and observed only a fewhundred kilometres north. In addition, the Tertiarytuff layers show repeated west-dipping normal faultswhich appear to be part of a large-scale detachment fault.

Our mapping was done over the course of a month-longfield course. Satellite imagery was used to confirm some

contacts when the maps were digitalized. Outcrops inthe region are well-exposed. The mapping area can beaccessed by a jeep road running southeast-northwestdown the main wash. Areas not immediately on thewash must be accessed by foot over rough terrain.

FIG. 1. Location of the Piute Mountains in the Mojave DesertRegion. The rock units in the Piutes resemble the Paleozoicstratigraphy of Death Valley and the Grand Canyon, but arehighly deformed.

II. REGIONAL GEOLOGIC SETTING: THENORTH AMERICAN CORDILLERA

The North American Cordillera, which comprises themajor mountain ranges of the United States includingthe Rocky Mountains, the Pacifc Coast Range and theSierra Nevada, truncates abruptly a few hundred milesnorth of the Piute Mountains. The western coast ofNorth America has been an active margin, contiguouswith the circum-Pacifc orogenic belt, for the past 350million years. During this time, a series of accretionand subduction events beginning in the Devonian andcontinuing into the early Mesozoic uplifted the Cordillerawe see today. The geologic features of the Cordillera,including thick foredeep sediments and fold and thrustbelts, generally strike north-south.

2

Rift sequence and formation of the Cordilleran miogeo-cline The eastern part of the Cordillera is underlainby the Laurentian craton, which rifted beginning inwhat is now Western Canada in the Neoproterozoic andcontinuing into the current western United States in thelower Paleozoic (Dickinson, 2004). The rift is clearlydelineated by an elongate belt of marine sediments,the Cordilleran miogeocline, which run north-southfrom Southern California into Canada and truncates allpre-existing strata. These west-dipping sediments reachthicknesses of 8 km and were initially deposited in therift basin in Rodinia, and then along the Laurentianpassive margin after rifting was completed. The riftsequence is associated with thinning and extensionalfaulting of the underlying basement, as well as crustalflexure from the weight of deposited sediments

Accretionary and orogenic events from the Devo-nian through the Pennsylvanian From the late Devonianuntil the end of the Laramide thrust during the Eocene,the Laurentian western mar- gin was active and im-pacted by several major thrusts (Robert MountainsThrust; Sonoma Orogeny; Laramide Thrust) and aseries of accreted island arcs. The resulting strainraised the topography of the Cordillera in a north-southbelt running from Canada to an abrupt truncation insouthern California (Burchfiel, Cowen, & Davis). Theeastern edge of the Cordillera contains the ColoradoPlateau and the Basin and Range Province of Arizona.This region, including the Piute Mountains, is domi-nated by deformations most recently activated by E-Wcompression of the Laramide Thrust in the Eocene,which overprint the NE-SW strain created during theformation of the Ancestral Rocky Mountains in theMississippian. The central Cordillera, covering modernIdaho and eastern Nevada and within our area of studyincluding the Spring Mountains and the Keystoneand Red Springs thrusts, contains repeated oceanicsequences due to overthrusting by oceanic crust afterthe Devonian. Finally, westwards from middle Nevadathe Cordillera is composed of a series of largely volcanicisland arcs of exotic origins, some of which traversedthe ocean in its entirety and bear more resemblancegeologically to what is now Asia than to Laurentia. TheLaurentian margin activated in the late Devonian withthe Roberts Mountain Thrust, which thrust the oceaniccrust over existing passive-margin sediments. This eventcontinued for 25Ma into the beginning of the Mississip-pian (Dickinson, 2004). The oceanic Antler allocthonwas deformed by a series of shallow west-dipping thrustfaults.

The weight of the allochthon flexed the underlyingcontinental margin, creating a basin which filled witheroded sediments, and formed the Antler Foredeep.These sediments dip to the east and contain erodedbasalts and cherts formed from the oceanic sediments

of the Robert Mountains Thrust. The foredeep andallochthon cover much of modern Nevada (see p. 421of Burchfiel, Cowen & Davis) but do not reach as fareast as the Mojave region or our area of study. Howeverthe roughly east-west compressional stress from thethrust, however, was transmitted through the region,until the sequences truncation immediately north ofthe Piute Mountains. Within 110 million years of thedeactivation of the subduction zone, a major tectonicevent occurred to the southeast of the Cordillera: thecreation of the Ancestral Rocky Mountains. This rangeruns NW-SE from Texas to Utah, with sediment nowfilling synclinal basins that trend in the same directionbetween Precambrian sediments which were folded tothe surface. Although the formation of the AncestralRockies is contemporary with the Ouachita Orogenfrom the southeast, the trend of the folds indicates thatthe primary stress was in the NE-SW direction. Thiscould have been caused by an accretionary event inwhat is now Mexico and southern California. If so, thetruncation and deformation of the Cordillera during theLate Paleozoic left little evidence.

Late Paleozoic left little evidence. After the for-mation of Ancestral Rockies a deformational eventlasting 25 million years occurred as the oceanic Sonomaallochthon was thrust over Laurentia from the west.This continental subductionand may be associated witha foredeep covering Nevada (Burchfiel, Cowen, & Davis)although Dickenson (2006) asserts that no forelandbasin has been identified beyond the thrust front.During this period, the far western edge of Laurentiacontinued to accumulate marine sediments. Thesecan be observed the in the Havallah basin in Nevada.Unconformities related to the Sonoma orogeny arefound between the Silurian and the Triassic. After theAntler event, a series of island arcs containing Permianand Devonian volcanics and volcaniclastics (Dickinson,2004) collided with the margin. During this period ofsubduction, the granitic inclusions which now makeup the Klamath Mountains were formed. Dickenson(2006) proposes that these island arcs were responsi-ble for slab rollback and the low angle over- thrustingof oceanic crust during the Antler and Sonoma orogenies.

The area of our detailed study is at the southerntrun- cation of the Cordillera, just to the south of theregion covered by the major fold-trust belt. We observea sharp transition between the area of Nevada to thenorth of Las Vegas, which like much of the centralCordillera contains major west- and northwest-dippingthrust faults such as the Keystone and Red SpringsThrusts, and the generally north-dipping faults of theMaria Fold and Thrust Belt one hundred miles to thesouth (see Spencer & Reynolds, p. 542). The trunca-tion is marked by the abrupt end of the miogeoclinalsediments, although its location is not well constrainedas the full extent of these Paleozoic strata is uncertain,

3

underneath the thrust belts and foredeeps.

Cenozoic subduction and magmatism in the south-ern Cordillera The eastern edge of the Cordillera wasuplifted and deformed into the current Rocky Mountainsin the Laramide Thrust, an orogeny lasting from thelate Cretaceous to the Eocene. Additionally, igneousrocks, including the Sierra Nevada batholith, intrudedthe Cordillera and the region south of its truncationthroughout the Mesozoic and Cenozoic. The Piutes weredeformed by the arrival of the Lazy Daisy pluton at743Ma, and the East Piute pluton at 857Ma (Fletcher &Karlstrom, 1990). In Canada and the northern UnitedStates, the Laramide thrust is associated with arcmagmatism and deformation continuing until 45 Ma;however, in the southern United States, deformationcontinued for the same period with a magmatic lull from75 Ma through the Eocene (Dickinson, 2004). The lackof magmatic activity despite continuing subduction, andthe uplift of the Rocky Mountains hundreds of miles fromthe continental margin, are evidence for shallow-anglesubduction of the Farallon Plate. In the northern UnitedStates subduction of the Juan de Fuca plate continues tothis day, while in the southwest of the United States theorogeny terminated with the complete subduction of theFarallon plate at 45 Ma. As a consequence, magmatismin the Mojave and high-temperature ductile deformationin the Piute Mountains ceased with the crystallization ofthe Lazy Daisy pluton at 74 Ma. Fletcher & Karlstrom(1990) propose that this ended ductile deformation inPiutes.

III. MAJOR ROCK UNITS OF THE PIUTEMOUNTAINS

This section contains rock descriptions for the majorunits in the Piute Mountains. This will be followedin the next section by a stratigraphy for the Paleozoicsequence, which is placed in a regional context using theGrand Canyon and Death Valley sequences.

Precambrian basement Most of the outcrop of thePiute Mountains is a heterogeneous and highly deformedunit which we will refer to as the Precambrian basement.It is distinguished as the only unit in the area tocontain igneous intrusions outside the borders of theEast Piute and Lazy Daisy plutons or any metaba-sites. At the contacts with the other metamorphicunits in the area, the basement typically consists ofa banded orthogneiss, occasionally intruded by whitemetagranite dikes up to 5m wide. Elsewhere in thebasement we observed a dark-red augen gneiss; a30m wide metagabbro intrusion; a zebra basite gneiss;a fine-grained grey quartzite; and a series of paragneisses.

Cross-bedded quartzite The lowest layer in the Pa-leozoic sequence is quartzite. This layer is almost

invariably cliff-forming, and its red-purple weatheringto a dark-brown varnish makes it one of the darkestunits in the area and easily distinguished. The quartzitegrains are closely interlocked and smaller than 1mm.In direct contact with the basement is a layer of basalconglomerate, with rounded quartzite clasts between 0.5and 10cm in size. The conglomerate forms beds up to 1mthick which grade into the bulk of the quartzite over afive to twenty meter section. This section is occasionallyinterbedded with a 0.5 to 5m section of biotite gneiss,which appears most regularly on the northeastern edgeof the Paleozoic section. A second layer of basal con-glomerate is observed above the gneissic layer. Finally,above the basal conglomerate, the quartzite commonlypreserves cross-bedding from the original sandstone inbeds 30 to 100cm thick. Both the basal conglomerateand the cross-bedding distinguish this unit from anyquartzite found within the Precambrian basement.Additionally, they provide reliable paleo-up indicators.

Schist The second-lowest layer in the Paleozoicstratigraphy is a muscovite schist. This schist is usuallydark blue or grey but highly lustrous, though green-greyand pale red colorings also appear. The constituentmineral grains are consistently between 0.2 and 0.5 mmin size. There is a clear foliation throughout the schist,though the direction of foliation varies in places on ascale of as little as half a metre in highly deformedareas. The schist’s metamorphic grade also varies acrossthe mapping area: muscovite is uniformly present,with staurolite, garnet, biotite and chloritoid appearinglocally.

The schist is easily weathered and slope-forming.In the Piutes it commonly occurs in outcrops whichprotrude no more than two inches above the soil. Thebest outcrops are found in gullies or ridge tops. On steepslopes, the schist layer may be nearly masked by soiland colluvium from a more durable quartzite or marblelayer above. In these places it may be identifed by itsappearance in the colluvium, and a marked decrease inthe slope steepness. Pelitic schist is also found in thePrecambrian basement. These are generally of higher-grade than the Paleozoic schists and more durable, andunlike the Paleozoic layer they may contain biotite.However, both the Paleozoic and the Precambrianschists are of highly variable metamorphic grade, and inmany instances they can only be distinguished withinthe context of the surrounding outcrops.

Laminate marble A layer of dark grey marble withthin laminations commonly appears in contact with theschist layer. These laminations are between 0.5 and 10mm thick and differentially weathered. In the north-ernmost part of the mapped area this layer becomesgrey-blue, with light and dark shades alternating acrossthe laminated layers. Original bedding is not visiblein this layer, but the laminations roughly parallel the

4

nearby contacts. This marble is composed of interlocking¡1 mm grains . They have no consistent cleavage, but thesides of individual crystals are subtly reflective on closeinspection. Other minerals are almost non-existent,except for an isolated outcrop of wollastonite-bearingmarble in D7. In this region the marble consists ofnearly 5% modal wollastonite in parallel individualgrains, 4 to 10 mm long, which are evenly distributedthroughout the rock.

White marble A bright white marble, distinctiveby both its color and the coarseness of its grains, isirregularly present between the laminate and massivegray marbles in the Paleozoic sequence. It is a veryself-cohesive unit which forms very little colluvium. It ischaracterized by very large, coarse grains (varying from1 mm to 5 mm in size). The unit is primarily white,although it does contain silicate nodules which are de-formed into tightly folded layers no more than 5cm thick.Thin layers of a pale grey marble, of similar grain size,are occasionally observed and offer the most consistentattitudes in the unit. Massive marble laminated and

FIG. 2. White marble.

shows no differential wreathing, is found either directlyin contact with the grey laminate marble or with thecoarse-grained white marble. It is distinguished by colorvariation within the marble and by brown dolomiticboudins. The color variation is typically between regularalternating planes up to 50cm thick and does notappear to be associated with a change in grain size orcomposition. The boudins are a fine-grained dolomiteranging from regular ovals of 50cm diameter to large10m blocks, and are found intruding into this marblewithin 20m of its contacts with the coarse-grainedwhite marble. The grains of this marble are interlockedwithout regular cleavage, and less than one millimeterin size. Local post-metamorphosis uid deformation hasleft some regions laced with brown-varnished quartz.

This unit contains wide zones of breccia. The ma-

jority of breccia is angular, with homogeneous clastsvarying in size from 0.5 to 20cm embedded in a fine-grained matrix. Its presence may imply that this unitwas more resistant to deformation than the others,and failed with brittle fracture while the other layersfolded. At the peak in D5 is an outcrop of rounded-clastbreccia, picture in Fig. ??. Whereas the other, angularbreccias are likely related to local shear zones, roundedclasts imply that this area was subjected to physicalwear by landslides or fluids. No convincing outcropsof rounded-clast breccia were found elsewhere in themapping area.

Lazy Daisy Granite The pink granite Lazy Daisypluton is found in the southeastern part of our mappingarea. It was intruded in at least two phases: coherentunits of both meta- and paraluminous granite are foundwithin the pluton. The paraluminous granite is 25%quartz in 2-5mm grains interlocked with 50% potassiumfeldspar, 25% plagioclase and small quantities of 1mmmuscovite. The granite weathers into smooth, roundedpeaks which may form very steep slopes. It is period-ically cut by straight pegmatite dikes, and by quartzveins 2 to 10cm thick. Granite dikes and sills extendin thin fingers from the pluton up to 100m into thesurrounding units. This area is indicated by hatching onthe map.

Tuff Lying atop the Paleozoic sequence is the PeachSprings Tuff. Across most of the region, the tuff isrhyolitic. It has with a reddish fine-grained groundmassand porphyritic crystals including 0.5 to 3mm grains ofpotassium feldspar, olivine and orthopyroxene. Elongatevesicles appeared locally. The most abundant inclusionis quartz, with crystals as large as 5mm making up 10%of the rock. This is tuff is often directly in contact withthe older Paleozoic and Precambrian sequences, with aconglomerate holding 0.5 to 30cm clasts at its base. It isnot a very competent layer. In many areas tuff remainsonly in isolated outcrops no more than 10m thick.

Tuff outcrop is widespread in the northwest partof the field area. These thick outcrops reveal multiplegenerations of ash flow. Below red tuff lies a blackcrystalline layer followed by a tan lithic layer. This layeris the least competent, and has substantial variationin grain size: the bulk of the rock is very fine-grainedand weathers into smooth corners and hollows, but isinterspersed with sandy beds with 1-3mm grains. Oneexample of this is shown in Fig. ??.

IV. STRATIGRAPHY OF THE PIUTEMOUNTAINS

Stratigraphy of the Mojave Desert Region The Pre-cambrian basement in the Mojave Desert region isa remnant of the Laurentian craton. It consists of

5

orthogneisses intruded by granitic dikes, which weobserve in the basement in the Piutes. During thePaleozoic, sedimentary strata were deposited on top ofthis basement along the Laurentian passive margin. Thissequence is revealed intact in both the Grand Canyonand Death Valley (see Fig 1 for geographic locations),and has been well-documented. One hundred milesfurther south, the Paleozoic sequences of the MojaveDesert region are the metamorphosed and deformedcounterparts to this sequence. Stone et al. (1983)gives a detailed comparison between the Grand Canyonsequence and the metapelitic sequences which appearin various parts of the Mojave Desert region. We havechosen to compare our stratigraphy to the Death Valleysequence as it bears slightly closer resemblance to thePiutes.

Stratigraphy of the Piute Mountains Firstly, youngestrock units in the area are the two igneous units. ThePeach Springs Tuff unconformably overlies the meta-morphic units in the region, and the Lazy Daisy Plutonintrusively cuts through them. Although these two unitsare never in contact, radioactive dating has found theLazy Daisy pluton to be older. The pluton intrudedduring the Cretaceous, while the Peach Springs Tuffis commonly dated at 18.5Ma (Fletcher & Karlstrom,1990). However, ductile folding in parts of the tuff,which is discussed in detail in the Structures section,suggests that some tuff outcrops may be older than this.

The oldest unit in the region is the PrecambrianBasement, which is highly deformed and unconformablyoverlain by the other rock units. The basement isinhomogenous, but some of its orthogneiss layers havebeen dated. One of these is the Fenner Gneiss whichwas found by Bender (2008) to be 1.6 billion yearsold. The basement’s thickness is greater than thatof any other units in the area. It extends out of ourmapping area to the northeast and southwest ends ofthe Piute Mountains. The remaining metapelites weredeposited conformably with one another. They have notbeen dated, but correlate with other Paleozoic units insouthern California (Stone, Howard, & Hamilton, 1983).The stratigraphy is frequently incomplete due to thefrequent deformations in the area, as units are frequentlysheared out which causes them to vary in thickness byan order of magnitude. (See minimum and maximumthicknesses in attached stratigraphic columns; note thedifferences in scale.)

The next oldest unit is the quartzite, which is in-variably found between the Precambrian basement andthe schist layer. Paleo-up readings from the quartzitescross-bedding show that it is stratigraphically lowerthan the schist. This leaves the three marble units(laminate marble, white marble and massive marble)as the youngest They are not always clearly distinct.The white marble is frequently but not invariably foundbetween the laminate marble and the massive marble.

We are convinced that that the white marble wasdeposited between the other two marbles, as indicatedby the stratigraphy in Fig. 3. This sequence is clearlydisplayed in the large syncline seen at F7, which hasbeen illustrated in Fig. 4.

The white marble unit is unusual in the region:the cross-bedded quartzite, schist, and grey marblelayers correlate fairly closely with both the Death Valleyand Grand Canyon Paleozoic sequences, but neitherof these contains anything which resembles the whitemarble of the Piutes. This observation was also made byStone, Howard & Hamilton (1983). We assume that thecarbonates which formed this marble were deposited lessuniformly than those which formed the other marbles.

Using these rock units, we present our full geologicmap in Fig. 5.

V. DESCRIPTIVE STRUCTURE

Deformation in the tuff The tuff is the youngest layerand has been subject to the fewest deformations. Thenortheast corner of the map- ping area is covered in tuff.The majority of the tuff dips 30-40 degrees west. It formstilted plateaus on top of peaks at D5, C8) and D4. It isclear that at least one normal fault is necessary to explainits abrupt contact with the Paleozoic sequence along C4.

At the very western edge of the mapping area, theattitudes of the tuff become much less consistent, asshown by the stereonets in Fig. 6. The tuff is very thickhere, covering a mountain with 113m relief. The twostereonets to the left were taken from the strikes anddips around A3 on the northwest and southeast sidesof the mountain respectively. The fold axes calculatedfrom the stereonets trend 164/30 and 265/41. This isclear evidence that ductile deformation has occurred inthe Piutes since the tuff fell.

High temperature deformation in the Paleozoic se-quence We can immediately observe that the rock unitsin this area have undergone high to medium grademetamorphosis. This is particularly striking when wecompare them to the undeformed, low-grade rock unitsin the Grand Canyon. The degree of deformation variesthroughout the mapping area. In many areas the se-quence is relatively undisturbed - for example, in C4 andG7 we find that the quartzite shows intact cross-beddingand the schist has no higher-grade minerals than mica.In contrast, the quartzite in D5 is sheared to no morethan a meter thin, and instead of schist running alongit we find a banded pelitic gneiss. These units canbe identified only by tracing them until they rejoinless-deformed sequences.

6

FIG. 3. Stratigraphy of the Piute Mountains. The Paleozoic sequence is highlighted and compared to the Death Valley Sequence(left-most column). Scale changes between columns.

7

FIG. 4. Large syncline revealing the entire Paleozoic stratigraphy. Photo (top) was taken looking northwards across F7, andshows a 350m-long section of the ridge. The lower image is artificially colored to indicate rock units. The units on the ridgelinedip consistently north-northwest: the sequence is inverted, with the basement sitting on top. Rotation along the small faultcutting the ridge line is not completely constrained, but we estimate a displacement of 20m along the slope.

As additional evidence for high-temperature defor-mation, we found wollastonite grains at several locationsin the Piutes. Wollastonite is formed from calcite andquartz, typically at temperatures above 500 ◦C (Ferry,Wing & Rumble, 2001). The largest crystals were foundsurrounding the tuff outcrops at D5. In this regionwollastonite forms irregular splays of crystals up to 2cmlong which cover the nearby marble. The mineral wasnot found more than 7m from the tuff outcrop and mayhave formed by contact metamorphism during the ashfall.

More dispersed wollastonite is found in the marbleat D7. This area is also distinguished by an abruptcontact between laminate marble and the Precam-brian basement. The laminate marble within 50mof the contact contains wollastonite, which occurs inindividual 1cm-long grains dispersed at roughly 5cmintervals throughout the marble. These grains areparallel, trending 013/81. Because of their consistentalignment, this deposit must have been formed inthe last stage of ductile deformation in the area. Inturn, this requires that the last stage of deformationat D7 was accompanied by temperatures reaching 500 ◦C.

Ductile folding In the Piute Mountains, the Paleo-zoic sequence is folded tightly into a narrow bandsurrounded by Precambrian basement. The stratigraphyis overturned on a large scale, and shows many changesin orientation.

The largest fold which is visible on the ground isthe dramatic syncline at F7 (see Fig. 4). The whitemarble layer is clearly pinched out in the nose of theplunging fold. The stratigraphy on top of the ridge isoverturned, indicating that the fold is recumbent andthat it is only a local feature within the large area ofdeformation which has pushed the basement up into ametamorphic core complex in the area.

The entire area is riddled with folds on every scale. Aseries of small folds may be observed across the washfrom the large syncline, at F7. Mining in the area hasobscured details in that area, but we can still see thatthe quartzite and schist layers rotate and move in andout of the hillside. Even within the white marble, whichforms a cohesive block, we can observe isoclinal folds upto 1m in size. These are accompanied by 1m boudins ofbrown dolomitic marble, indicating large-scale foldingthroughout the block.

8

FIG. 5. Geologic map of the Piute mountains.

9

FIG. 6. Stereonets showing orientations measured in the tuffon the northwest (left) and east (right) side of the peak inA3.

Faulting in the Paleozoic sequence After the nor-mal faults in the tuff, the region’s clearest fault is foundin E7. This fault is marked by an extremely visible finof marble breccia. The fin, as well as the plane whichdescribes its contact with the underlying marble unit, liein a place along 120/51. We did not find any evidenceto constrain horizontal motion along the fault. However,it has been identified as a normal fault by an outcrop ofschist and quartzite at E7. This outcrop is small andbadly weathered, leaving little but quartzite bouldersand schist which outcrops mere centimetres above themarble, but the sequence here is overturned and itsorientation parallels that found on top of the ridge tothe north, implying that it was brought down by thefault.

Other faulting in the region is inferred by localbrecciation and by offset bedding planes. We see clearoffsets in the saddle of F6, which has clearly been shiftedby a fault with right-handed transverse motion. Theschist has been offset by 5m measured along the southernslope, and the quartzite by 10m measured along thenorth slope. South of the saddle, the fault appearsto continue in a plane striking approximately 210/60through a colluvium-filled gulley. We have inferred thisfault plane north into the basement. Smaller offsets inthe schist and laminate marble layers along the ridgeindicate parallel faults.

The fault at D5) is also very visible in the field.We observe three distinct layers of quartzite here. Thesouthernmost quartzite layer is 8m thick and cross-bedded, with a basal conglomerate at the north. It is inconformable contact with 17m of biotite gneiss, whichwe attribute to the slaty layers observed in the Zabriskiequartzite. This is followed by another 5m quartzite layerwhich sits conformably on a 20m layer of orthogneiss.All of these layers dip near-vertically, between 330/68and 141/05, and truncate abruptly against white marblein gullies to both the west and southeast. Finally, wehave highly brecciated zone of quartzite just south of thewash. This breccia, along with the unusual tripling of

the quartzite, is strong evidence for a fault. The curveof the brecciated contact around the areas topographyrequires a normal fault with a plane striking roughly140/30.

VI. INTERPRETATIVE STRUCTURE

In Fig. 9 we present a structural interpretation forthe geology of the Piute Mountains. Our aim was toprovide a coherent description of the deformation processwhich brought up the metamorphic core complex, thePrecambrian basement, and folded it into the Paleozoicsequence. Where possible, we have indicated constraintson the timing of deformation events. Our interpretationrequires four generations of folding - three of whichpre-date the arrival of the tuff and the Lazy Daisypluton, and one of which post-dates the tuff. This isfollowed by two normal faulting events: a large-scaledetachment fault system which tilted the tuff, and anormal fault striking 120/51 which created the marblefin referred to above.

Stages of ductile deformation We begin by observingthe previously described syncline (Fig. 4). This fold isisoclinal and overturned and has clearly been deformedby a series of later folds. We refer to this generation offolds as first generation, D1, and can trace it throughoutthe mapping area where it runs roughly from northwestto southeast. Secondly, we observe that the stratigraphyaround this syncline is overturned, and that the entireformation is surrounded by Precambrian basement. Wealso observe the full Paleozoic sequence repeating noless than three times in C2 and again across F7-E8. Toexplain these features, we introduce a second fold gen-eration, referred to as D2, which re-folded the synclineinto an antiform. This structure is clearly visible incross-section C. It runs the length of the mapping area(though the syncline shifts from the north to the southside of the antiform), reappearing at the north end ofmapping area as shown on the left end of cross-section D.

Finally, we observe that the entire Paleozoic se-quence, including fold generations D1 and D2, has beenwrapped around so that it forms a crescent shape onthe map. We call this generation D3. We calculateda fold axis of 040/32 using orientations taken from thelaminate marble in D7, as shown in Fig. 10.

It is worth noting that this fold axis is 28 ◦ fromthe dominant trend of the wollastonite grains in thearea, which does not support the hypothesis that thewollastonite grains were aligned by stresses during thisfold. However, it is clear from the map that the scale ofthis fold is as large as the exposed Paleozoic sequence inthe area. We cannot access the fold limbs to accuratelyconstrain the fold angle.

10

FIG. 7. Cross-section C, showing the antiform and syncline structure of fold generations D1 and D2. Heading 56 degrees.

FIG. 8. Cross-section D. Thick black lines indicate apparent dips (heading 261 degrees). Detailed folding has been inferredfrom the geometry of the layers as they intersect the topography to the west.

We believe that folds in this generation lifted thePaleozoic sequence above the Precambrian basement inG7. This structure is indicated on the right-hand sideof cross-section D. Smaller folds in this generation arefound in F7, where the schist and quartzite layers weavein and out of the hillside on the south edge of the wash.Attitudes in this area were sparse due to mining, butthe fold pattern was in general agreement with the axiscalculated above. Unlike previous generations, this foldlies in a plane perpendicular to the boundary of theLazy Daisy pluton. The folding is also concentratednear the pluton - we did not observe any folds of thisgeneration northwest of D7. We therefore hypothesizethat this stage of deformation was directly associatedwith the intrusion of the Lazy Daisy.

Ductile deformation in the tuff This leaves one latergeneration of folds, D4. This fold trends north-souththrough the tuff in the northwest area of the map (A3).As the Peach Springs tuff was found by Fletcher &Karlstrom (1990) to post-date the arrival of the Lazy

Daisy by 55Ma, we are left with two hypotheses: eitherductile deformation resumed over 55Ma after the LazyDaisy cooled, or some of the tuff in the Piute Mountainswas deposited long before the Peach Springs ash fall.Further work is required to date the tuff in the Piutesand resolve this question.

Detachment faulting in the Piutes As has beenpreviously observed, there are two major directions offaulting in the Piutes: north-south faults running alongthe tuff, and a major northwest-southeast normal faultfound in F7. We observe that the offset elevations of thetuff, as well as its tilt of 30 degrees east (except in thefolded area described in the preceding paragraph), areconsistent across the Piute Mountains and surroundingregion. We therefore assume that these faults are partof a large detachment system. An instance of this faultsystem is presented on the left-hand side of cross-sectionC (Fig. 7).

11

FIG. 9. Structural map of the Paleozoic sequence within the Piute Mountains. Straight lines ABCD indicate locations forcross-sections shown below. Four distinct generations of folds are labelled D1, D2, D3, and D4.

12

FIG. 10. Stereonet of orientations in laminate marble at D7.Yellow line indicates fold plane. Fold axis runs along 040/32.

VII. DISCUSSION

The stratigraphy of the Mojave is overturned ona regional scale. This must have occurred after thedeposition of the Paleozoic sediments, but before thefirst deformations we observed. As can be seen in cross-section C (Fig. 7), fold generations D1 and D2 onlypush the Paleozoic sequence through the basement anddo not seriously interrupt the overturned stratigraphy.

We found four distinct generations of folds withinthe Piutes. At least three generations pre-date the fall ofthe Peach Springs tuff which was dated by Bender (2008)at 18.5Ma. We contrast this hypothesis with previouswork in the region by Fletcher & Karlstrom (1990), whoexplain the structure of the Piutes in terms of a singleevent during which deformation was concentrated intwo major shear zones. We did find varying degrees ofshearing and deformation within the Paleozoic, but wefound clear evidence of multiple generations of folds.

Additionally, Fletcher and Karlstrom assert thatductile deformation in the Piutes ceased after thecooling of the Lazy Daisy pluton. Using apatite fissuretrack retention, they found that the pluton had cooledto 100 ◦C by 72Ma±3. We agree that three generations

of folds appear to pre-date the arrival of the pluton,and that in particular generation D3 appears to havebeen influenced by its arrival. The presence of regularlylineated wollastonite around D7 also indicates hot tem-peratures in the region at the end of ductile deformation.All of this indicates that the Paleozoic sediments wereburied within the basement, then rapidly uplifted anddeformed before 72Ma.

Ductile deformation continued further south intothe Little Maria Mountains, the Maria Fold and ThrustBelt, and the Clark Mountains, where zircon dating ofthe Ivanpah pluton placed an upper age limit of 147Maon deformation of the Precambrian basement (Walker,Burchfiel & Davis, 1995). The Marias were studied bySpencer and Reynolds (1990) who concluded that themetamorphic core complex, the Precambrian basement,could have been rapidly uplifted by isostatic forces. TheMaria Mountains were thickened by the Maria Foldand Thrust Belt. This abnormally thick region couldhave been eroded rapidly, creating isostatic uplift. It ispossible that similar uplift in the Piutes occurred in re-sponse to thickening during the arrival of the Lazy Daisy.

However, we also found tight folds in the tuff inthe Piute mountains. These were not recorded byFletcher and Karlstrom, but they indicate that ductiledeformation in the Piutes continued much longer thanhas been previously thought, or that some of the tuff ismuch older than the Peach Springs ash fall at 18.5Ma.Further work in the region with the objective of datingthe oldest tuff could resolve this question, and leave usto explain this final stage of deformation in the contextof the Mojave Desert Region.

ACKNOWLEDGMENTS

Oliver Jagoutz, Claire Bucholz and Clark Burchfiel forinvaluable instruction both in the field and in the map-room. Allie Anderson and Emily Tencate for assistancein the field and for being photogenic scale indicators formany photos. Daniel Sheehan for ArcGIS lessons andtroubleshooting.

[1] Bender, E.E. (2008) Petrogenesis of the Fenner Gneiss,Piute and Old Woman Mountains, San BernardinoCountry, California, GSA Abstracts with Programs 40(6): 240.

[2] Burchfiel, Cowen & Davis (n.d.) Tectonic overview ofthe Cordilleran orogen in the western United States,in Burchfiel, Lipman & Zoback The Cordilleran Oro-gen: Conterminous U.S. p.407-479. Geological Societyof America, The Geology of North America.

[3] Davis, G. (1973) Relation between the Keystone and RedSpring thrust faults, Eastern Spring Mountains, Nevada,GSA Bulletin 84: 3709-3716.

[4] Dickenson, W. (2006) Geotectonic evolution of the GreatBasin, Geosphere 2(7): 353-368.

[5] Dickenson, W. (2004) Geotectonic evolution of the GreatBasin, Annual Review of Earth and Planetary Science,13-45.

[6] Ferry, Wing, & Rumble III (2001) Formation of Wol-lastonite by chemically reactive fluid flow during contact

13

metamorphism, Mt. Morrison Pendant, Sierra Nevada,California USA, J. Petrology 42 (9): 1705-1728.

[7] Fletcher, J. & Karlstrom, K. (1990) Late Cretaceous duc-tile deformation, metamorphism and plutonism in thePiute Mountains, eastern Mojave Desert, J. GeophysicalResearch, 95: 487-500.

[8] Spencer & Reynolds (1990) Relationship between Meso-zoic and Cenozoic tectonic features in West Central Ari-zona and adjacent Southeastern California, J. Geophysi-cal Research, 95(B1): 539-555.

[9] Stone, P., Howard, K. & Hamilton, W. (1983) Correla-tion of metamorphosed Paleozoic strata of the southeast-ern Mojave Desert region, California and Arizona, GSABulletin, 94: 1135-1147.

[10] Dickenson, W. (1995) New controls on initiation and tim-ing of foreland belt thrusting in the Clark Mountains,southern California, GSA Bulletin, 107: 742-750.