correlation of permian and triassic deformations in the...

For permission to copy, contact [email protected] 2002 Geological Society of America1210

GSA Bulletin; October 2002; v. 114; no. 10; p. 1210–1221; 8 figures; 1 table.

Correlation of Permian and Triassic deformations in the western GreatBasin and eastern Sierra Nevada: Evidence from the northern Inyo

Mountains near Tinemaha Reservoir, east-central California

Calvin H. Stevens*Department of Geology, San Jose State University, San Jose, California 95192, USA

Paul StoneU.S. Geological Survey, Menlo Park, California 94025, USA

ABSTRACT

Geologic relationships exposed near Ti-nemaha Reservoir southeast of Big Pine,east-central California, provide key chro-nological and structural constraints forlinking Permian and Triassic deformationalevents recognized in the White and InyoMountains in the western Great Basin withthose in the eastern Sierra Nevada, partic-ularly the Mount Morrison and SaddlebagLake pendants. Permian to earliest Triassicdeformation in the Tinemaha Reservoirarea produced a large north- to northwest-trending, east-vergent, originally recum-bent syncline (Mule Spring syncline) cut byan overriding thrust (Strange Hill thrust).We correlate this deformation with a mid-dle Permian to earliest Triassic contraction-al deformation recognized in the southernInyo Mountains, and with a major episodeof folding and thrust faulting in the easternSierra Nevada. After a period of tectonicquiescence and marine sedimentation in theEarly Triassic, rocks in the Tinemaha Res-ervoir area were refolded twice, producingdistinctive sets of steeply plunging folds.Similar structures in the Mount Morrisonpendant that formed prior to intrusion of a225 6 16 Ma dike along the Laurel-Convictfault are correlated with those in the Ti-nemaha Reservoir area. The timing of thesefolding events may be similar to that of dis-placement on the Golconda and/or LundyCanyon thrusts in the Saddlebag Lakependant.

Close structural and stratigraphic tiessuggest that rocks in the northeastern part

*E-mail: [email protected].

of the Mount Morrison pendant and Tine-maha Reservoir area, now separated by;65 km of dextral displacement along thecryptic Tinemaha fault, originally lay ad-jacent to one another. This offset postdatesTriassic folding and is inferred to predateemplacement of the latest Triassic WheelerCrest Granodiorite, which crops out acrossthe projected fault trace.

Keywords: deformation, Great Basin,Permian–Triassic, Sierra Nevada, tectonics.

INTRODUCTION

The region encompassing the White andInyo Mountains and eastern Sierra Nevada ineast-central California (Fig. 1) has emerged asa key area for the study of middle Paleozoicto middle Mesozoic evolution of the Cordil-leran continental margin (Stevens et al.,1997). Rocks and structures exposed in thisregion reflect the change from a dominantlypassive to a dominantly convergent marginbetween middle Paleozoic and middle Meso-zoic time; the most significant tectonism oc-curred during the Permian and Triassic.

Permian and Triassic tectonic events in theWhite and Inyo Mountains have been sum-marized by Stevens et al. (1997), and those inthe Sierra Nevada have been discussed bySchweickert and Lahren (1987, 1993) and Ste-vens and Greene (1999, 2000). In spite of con-siderable work on dating rocks and structuresin both regions, the paucity of clear strati-graphic and structural ties has made it difficultto evaluate possible correlations of tectonicevents.

Rocks in the vicinity of Tinemaha Reser-voir at the western foot of the northern Inyo

Mountains in Owens Valley (Fig. 1), however,appear to provide such ties. In this area wehave identified or reinterpreted critical rocksand structures that can be employed for cor-relation of tectonic events represented in theWhite and Inyo Mountains with those in theeastern Sierra Nevada. The purpose of this pa-per is to show the evidence for these proposedstratigraphic and structural correlations and touse them as a basis for a regional chronologyof deformational events that affected thesouthern part of the North American continen-tal margin during Permian and Triassic time.

OVERVIEW

The primary area of this study includes thebasal slopes of the Inyo Mountains east of Ti-nemaha Reservoir and a few isolated hillswest of the mountain front (Fig. 2). The ge-ology of this area, first mapped by Nelson(1966), was later remapped and reinterpretedby Olson (1970) and Stevens and Olson(1972). The geology shown in Figure 2 ismodified from Stevens and Olson (1972) onthe basis of our more recent mapping. Majordifferences between this and previouslypublished maps are due to the facts that (1)the structure is very complex, (2) some ofthe rocks (especially the cherts) retain littleof their original structure, and (3) until re-cently, the ages of several units had beenmisinterpreted.

Outcrops of particular importance underliethe informally named ‘‘Big Hill’’ along themountain front and ‘‘Strange Hill,’’ which isseparated from the mountain front by Quater-nary alluvium (Fig. 2). An interpretative crosssection through these two hills is shown inFigure 3.

Geological Society of America Bulletin, October 2002 1211

PERMIAN AND TRIASSIC DEFORMATIONS IN THE WESTERN GREAT BASIN AND EASTERN SIERRA NEVADA

Figure 1. Map showing location of the Tinemaha Reservoir area and other places men-tioned in the text. AR—Arcularius Ranch, Bp—pendant southwest of Bishop, HMF—Hunter Mountain fault, ICT—Inyo Crest thrust, NMC—northern Mazourka Canyon,SL—Saddlebag Lake, UVS—Upland Valley syncline. Geology after Jennings (1977).

All the rocks of interest along the base ofthe mountains west of a high-angle range-front fault are separated into two structuralplates by the Big Hill fault (Fig. 3). This low-angle fault is continuously exposed for a dis-tance of several kilometers, and remnants ofits upper plate overlie the eastern limb of amajor syncline, here called the Mule Springsyncline. The partially overturned westernlimb of this syncline, in turn, is cut by a fault,here called the Strange Hill thrust (Figs. 2, 3,4). The complex geology of Strange Hill(Figs. 4, 5) is highly significant for the inter-pretations made here and therefore wasmapped in greater detail than other parts ofthe area.

STRATIGRAPHY

The stratigraphy of the Tinemaha Reservoirarea (Table 1) can be pieced together fromseveral isolated outcrops. The upper plate ofthe Big Hill fault (Figs. 2, 3) comprises awest-dipping sequence of Cambrian to Silu-rian rocks. The lower plate of the Big Hillfault contains Devonian separated from Mis-sissippian and Pennsylvanian rocks by theStrange Hill thrust, which is overlapped byLower Triassic rocks.

Most of the units recognized in the Tine-maha Reservoir area are similar to those ex-posed farther south in northern MazourkaCanyon (Fig. 1) (e.g., Ross, 1965); these and

other units exposed in the area are describedbriefly in Table 1. Four units, the Ely SpringsDolomite, Mount Morrison Sandstone,Squares Tunnel Formation, and Union WashFormation are of particular stratigraphic andstructural significance and therefore are de-scribed here in greater detail.

Ely Springs Dolomite (Ordovician–Silurian)

The Ely Springs Dolomite consists primar-ily of dark gray chert and chert breccia, withrelatively minor dolomite. On Big Hill, wherethe section is most complete, the formation isdivided into three members (Table 1): a lowermember composed primarily of thin-bedded,medium to dark gray chert alternating withmedium to dark gray dolomite; a middlemember consisting of light gray, fine-graineddolomite; and an upper member consisting ofthinly bedded, dark gray chert, much of whichis highly brecciated. The two lower membersbear a general lithologic similarity to expo-sures of Ely Springs Dolomite in MazourkaCanyon. The upper member, however, differsfrom rocks assigned to the Ely Springs Do-lomite elsewhere in the Inyo Mountains inconsisting largely of chert breccia having ajigsaw fabric with a matrix formed of com-minuted chert clasts and earlier voids nowfilled with coarse-grained quartz (Haughy etal., 2000). The chert that forms the brecciaapparently was deposited as part of the ElySprings Dolomite in Ordovician to Early Si-lurian time, but the age and origin of the brec-ciation have been uncertain. In northern Ma-zourka Canyon (Fig. 1), ;13 km southeast ofTinemaha Reservoir, on the other hand, thehighest part of the Ely Springs Dolomite con-sists of moderately disturbed, thin-beddedblack chert ;10 m thick, similar to, but notas highly disturbed as, the brecciated chertnear Tinemaha Reservoir. This thin-beddedchert unit is not present at the top of the verythin Ely Springs Dolomite 4–5 km farthernorthwest. We speculate that the brecciatedchert in the Tinemaha Reservoir area wasoriginally deposited between Mazourka Can-yon and Tinemaha Reservoir and that some-time after lithification (probably in the Silu-rian) these rocks became unstable and weretransported downslope (northwestward) in theform of rockslides and rock avalanches to pileup to a thickness of at least 300 m at the baseof a slope near Tinemaha Reservoir.

A chert breccia identical in composition andfabric to the upper member of the Ely SpringsDolomite overlies yellow shale of the TriassicUnion Wash Formation near the summit of

1212 Geological Society of America Bulletin, October 2002

STEVENS and STONE

Figure 2. Generalized geologic map of theTinemaha Reservoir area modified fromStevens and Olson (1972).

Strange Hill (Figs. 4, 5). This chert brecciapreviously was mapped as a thrust klippe ofEly Springs Dolomite (Olson, 1970; Stevensand Olson, 1972). Here it is reinterpreted aspart of the Union Wash Formation becausedikes of yellow sedimentary rock extend up-ward into it from the underlying yellow shale.This breccia is interpreted as a Lower Triassicslide block derived from the previously brec-ciated upper member of the Ely SpringsDolomite.

Mount Morrison Sandstone and SquaresTunnel Formation (Devonian)

Rocks of Devonian age, not recognized inthe Tinemaha Reservoir area until recently(Stevens and Greene, 1999), consist of anoverturned, lithologically distinct sequence ofpebbly limestone, sandy conglomerate, andphosphatic chert herein assigned to the upperMiddle Devonian Mount Morrison Sandstoneand Upper Devonian Squares Tunnel Forma-tion. The base of this sequence is not exposedin the Tinemaha Reservoir area, and the topof the sequence is faulted; therefore, the strati-graphic context is unknown. These rocks,however, which are confined to the upper plateof the Strange Hill thrust, provide critical ev-idence for structural interpretations of the Ti-nemaha Reservoir area.

Rocks assigned to the Mount MorrisonSandstone at Strange Hill have an aggregateexposed thickness of 25–30 m (Table 1). Thelowest beds consist of pebbly limestone tur-bidites that have yielded late Middle Devonian(Givetian) conodonts. This age is compatiblewith the probable age of the lower part of theMount Morrison Sandstone in the MountMorrison pendant (Stevens and Greene,1999). The upper 11–14 m consists of con-glomerate containing dark gray chert clastsand fragments of several types of Devoniantabulate corals in a matrix of coarse-grainedquartz sandstone. This unit is lithologicallyidentical to a conglomerate in the lower partof the Mount Morrison Sandstone at McGeeMountain in the eastern Sierra Nevada (Fig.1), the only other area where the conglomer-atic facies of this formation is known (Stevensand Greene, 1999).

The Mount Morrison Sandstone has beeninterpreted to represent a major submarine-fancomplex derived from the shelf in the easternInyo Mountains during the Middle Devonian



Figure 3. Interpretive cross section drawn across the southern parts of Strange Hill and Big Hill (see Fig. 2 for locations of sections).Map symbols as in Figure 2 except D (Devonian), which includes both Squares Tunnel Formation and Mount Morrison Sandstone.Quaternary deposits not shown. Short-dashed lines represent bedding within Mk and D; arrows indicate facing direction.

(Stevens and Greene, 1999). Exposures of theconglomeratic facies of the Mount MorrisonFormation at McGee Mountain, where it fillsa channel cut deeply into older rocks, and atStrange Hill are considered to represent thesame major feeder channel to that submarinefan (Stevens and Greene, 1999) because oftheir unique sedimentologic and lithologicsimilarities.

The Squares Tunnel Formation at StrangeHill consists primarily of thin-bedded to lam-inated black chert and shale, some with lightgray phosphatic blebs and lenses. These rocksconcordantly overlie the Mount MorrisonSandstone and closely resemble the Upper De-vonian Squares Tunnel Formation in (1) thenorthern Mazourka Canyon area, where itoverlies either the Silurian to Lower DevonianSunday Canyon Formation of Ross (1966) orthe Ely Springs Dolomite, and (2) the easternSierra Nevada, where it overlies the Mount

Morrison Sandstone (Stevens and Greene,1999).

Union Wash Formation (Triassic)

The Union Wash Formation, exposed onlyat Strange Hill (Figs. 3, 5), consists of a basalchert-clast conglomerate overlain by yellowshale and bluish-gray limestone. These rocks,which are structurally complex, contain onlyrare reworked fossils, and unconformablyoverlie Devonian and Mississippian rocks,were previously mapped as Pennsylvanian andPermian units (e.g., Nelson, 1966; Stevensand Olson, 1972).

This unit is assigned to the Union WashFormation because it is similar to that for-mation exposed in the southernmost InyoMountains and near Darwin (Fig. 1). There,conglomerates composed of locally derivedclasts are common at the base of the unit

(Stone and Stevens, 1988; Stone et al., 1989).Overlying the basal conglomerates both thereand in the Tinemaha Reservoir area is yellowshale interbedded with fine-grained, pure, bluish-gray limestone. Especially diagnostic is athick succession of dark bluish-gray, very finegrained, thin-bedded limestone containingabundant lenticular nodules of black chert ex-posed on the hill west of Strange Hill. Theserocks are lithologically identical to subunit 1of the upper member of the Union Wash For-mation in the Darwin area (Stone et al., 1991)and are dissimilar to any other unit known inthe region. These lithologic comparisons leavelittle doubt that this sequence of rocks does,in fact, represent the Union Wash Formation.

Evidently, the Ely Springs Dolomitecropped out in the immediate area during Ear-ly Triassic time. The chert breccia at the sum-mit of Strange Hill, herein included in theUnion Wash Formation as previously noted, is


STEVENS and STONE

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identical to the rocks of the upper member ofthe Ely Springs Dolomite exposed along thebase of the Inyo Mountains, as are clasts inconglomerates near the base of the UnionWash Formation. In addition, the only fossilsrecovered from the Union Wash Formation inthe area of this study include the conodontWalliserodus of Middle Ordovician to Silurianage (Bruce Wardlaw, 1998, personal com-mun.), a fossil almost surely reworked fromthe Ely Springs Dolomite.

The unconformity at the base of the UnionWash Formation is exposed in several places.On the west side of Strange Hill, upright bedsof the Union Wash Formation unconformablyoverlie the inverted Mount Morrison Sand-stone (Fig. 5). On the south side of the hill,the basal conglomerate of the Union WashFormation overlies the inverted Squares Tun-nel Formation. On the northeastern part ofStrange Hill, a massive chert, thought to rep-resent the thoroughly altered basal conglom-erate of the Union Wash Formation, is over-lain by yellow shale and fine-grainedbluish-gray limestone, typical of that forma-tion. This chert overlies Mississippian rocks(Fig. 5), but although the contact is well ex-posed, its nature is not certain. Because thereis no evidence of faulting along the contact,and because the unconformity at the base ofthe Union Wash Formation cuts uniformlyacross the stratigraphic section and structuresin the previously deformed rocks on StrangeHill, as shown in Figures 5 and 6, we interpretthis contact as depositional. Similarly, we in-terpret the contact between Mississippianrocks and massive chert on the southeasternside of Strange Hill (Fig. 5) as depositional.

The Union Wash Formation is of Early andMiddle(?) Triassic age and therefore is ap-proximately coeval with the Candelaria For-mation (Speed, 1984), which has been recog-nized in the Saddlebag Lake pendant (Fig. 1)in the eastern Sierra Nevada by Schweickertand Lahren (1987). The Candelaria Formationis distinct from the Union Wash Formation,however, in lacking limestone and, in the up-per part, containing volcanic and graniticclasts indicating a very different provenance.

STRUCTURAL GEOLOGY

The structure of the Tinemaha Reservoir areahas been problematic and controversial. Olson(1970), Stevens and Olson (1972), and Stevens(1978) interpreted the dominant structures in thearea as a product of regionally significant Mes-ozoic thrust faulting, whereas Dunne and Gul-liver (1978), in keeping with the earlier mappingof Nelson (1966), considered the same structures



Figure 5. Geologic sketch map of Strange Hill. Truc includes both chert-pebble conglomerate and massive chert of uncertainorigin.

to be a product of Cenozoic normal faultingalong the front of the Inyo Mountains. The pres-ent study has led to a reinterpretation of thestructural framework of the Tinemaha Reservoirarea, largely on the basis of the improved knowl-edge of the stratigraphy.

Big Hill and Range-Front Faults

The low-angle Big Hill fault (Figs. 2, 3)and a high-angle range-front fault east of Ti-

nemaha Reservoir have been mapped and in-terpreted in several different ways. Nelson(1966) mapped the range-front fault as a sim-ple, steeply west-dipping normal fault. Olson(1970), and later Stevens (1978) and Corbett(1989), interpreted the Big Hill fault as partof a regionally significant, east-vergent Mes-ozoic thrust fault (Inyo thrust) underlyingmost of the northern Inyo Mountains andemerging on the east side of the range at

Jackass Flats (Fig. 1) as the Last Chancethrust of Stewart et al. (1966). Dunne andGulliver (1978) reinterpreted the Inyo thrustand range-front fault as a zone of Cenozoicextensional faults, which together with as-sociated westward-emplaced slide masses,resulted in a jumble of down-dropped blocks.It now seems clear that the Big Hill fault isa low-angle normal fault and that the high-angle range-front fault cuts it as shown in


STEVENS and STONE

TABLE 1. STRATIGRAPHY OF THE TINEMAHA RESERVOIR AREA

Figures 2 and 3. This reinterpretation of theBig Hill fault is based on the geometry of thetwo plates and the lack of evidence of ductiledeformation along the fault surface. Therange-front fault is steep and, as pointed out

by Dunne and Gulliver (1978), evidently cutsthe Big Hill fault where the two faults inter-sect. This interpretation does not require anyfault in the area to underlie the Inyo Moun-tains as once thought.

Structure of Strange Hill

Our work indicates that the Paleozoic se-quence on Strange Hill includes an invertedsection of Devonian rocks that has been em-



Figure 6. Diagrammatic cross section showing the shape of the Mule Spring syncline andStrange Hill thrust prior to deposition of the Triassic Union Wash Formation. Symbolsas in Figure 3.

Figure 7. Equal-area stereo plots of foldhinge lines in (A) the Strange Hill area nearTinemaha Reservoir and (B) on the north-east side of the Laurel-Convict fault in theMount Morrison pendant (from Wise,1996).

placed above overturned Mississippian rockson the Strange Hill thrust. All of these Paleo-zoic rocks in turn are unconformably overlainby the Lower to Middle(?) Triassic UnionWash Formation. In addition, all rocks onStrange Hill are folded at several differentscales. Two large synclines, both involvingDevonian and Triassic rocks, have fold widthsof at least 100–200 m. One of these, whichtrends about N108W, plunges subvertically,opens northward, and is exposed at the south-ern end of Strange Hill; the other, whichtrends about N308W and plunges 408 to508NW, is exposed on the west side of the hilland is paired with an anticline on its southwestflank (Fig. 5). On the isolated hill northwestof Strange Hill, a syncline several metersacross trends S308E and plunges ;108SE. Onthe southeastern part of Strange Hill, smaller,subvertically plunging folds having wave-lengths ranging from centimeters to meters arepresent in both the Mississippian Kearsargeand Triassic Union Wash formations, showingthat the two units were deformed together. Ax-ial planes of most of these small folds dip verysteeply. An equal-area stereonet plot of foldhinge lines on Strange Hill and the next hill tothe west (Fig. 7A) shows that these fold hingelines are dispersed along a subvertical planestriking about N288W. We interpret this patternto suggest a sequence of two folding eventsinvolving rocks as young as Triassic; one ofthese events produced northwest-trending foldsand the other produced northeast-trending folds.At present we have no data to determine whichfold set is the older.

A prominent subvertical cleavage strikingabout N258W cuts Mississippian rocksthroughout the area; on Strange Hill thiscleavage also cuts the Triassic rocks and theStrange Hill thrust. This cleavage apparentlyis axial planar to the post-Triassic, northwest-trending folds and clearly postdates all the

rocks on Strange Hill as well as the StrangeHill thrust.

Strange Hill Thrust

The Strange Hill thrust separates the over-turned Devonian rocks of the Mount MorrisonSandstone and Squares Tunnel Formationfrom overturned Mississippian rocks, mostlyassignable to the Kearsarge Formation (Fig.5). We here interpret this contact to be a faultrather than an overturned depositional contactbecause (1) the Devonian rocks comprise astratigraphic sequence unknown elsewhere inthe Inyo Mountains, (2) chert-pebble con-glomerate like that which characterizes thelowest exposed part of the Mississippian sec-tion south of Big Hill and the base of the sec-tion in northern Mazourka Canyon is missing,and (3) different Mississippian facies are incontact with the Devonian rocks at differentplaces on Strange Hill. For instance, along thenorthern part of the contact, Mississippian ar-gillite with thin limestone interbeds (Kear-sarge Formation) is present, whereas to thesouth, a thick black shale (possibly an outcropof Rest Spring Shale) is present at the contact.

The amount of displacement on the StrangeHill thrust is unknown. The stratigraphicthrow is small, but a moderate to a rather largeamount of displacement is implied by thepresence of units typical of the Sierra Nevadain the upper plate.

Mule Spring Syncline

Recent mapping demonstrates the existenceof a major syncline in the Tinemaha Reservoirarea (Figs. 2, 3) largely covered by Quater-nary alluvium. It is revealed by opposing fac-ing directions in outcrops of the MississippianKearsarge Formation between the Inyo Moun-tains front and the hills to the west and by asynclinal axis exposed in the Keeler CanyonFormation near the southern margin of thearea (Fig. 2). Dips in both limbs of the struc-ture generally are steep, and some are over-turned. Beds in the east limb may have beenoverturned during emplacement of the upperplate of the Big Hill fault. The trough of thesyncline exposed in the southern part of thearea (Fig. 2) is inferred to bend northward,passing between Strange Hill and the base ofthe Inyo Mountains to the east. The inverted,east-facing Mississippian rocks exposed onStrange Hill are interpreted to lie in the west-ern limb of the Mule Spring syncline, whichappears much thinner than the generally up-right eastern limb (Figs. 3, 6).

The Mule Spring syncline formed after de-


STEVENS and STONE

position of the Keeler Canyon Formation andprior to accumulation of the Lower to Mid-dle(?) Triassic Union Wash Formation. Res-toration of the Union Wash Formation to hor-izontal indicates that the syncline wasoriginally recumbent (Fig. 6).

Summary of Deformational History ofTinemaha Reservoir Area

Two major pre-Cenozoic deformationalevents are recorded in this area. The first re-sulted in formation of the Strange Hill thrustand the Mule Spring syncline, which involverocks as young as the Pennsylvanian–EarlyPermian Keeler Canyon Formation. Thesestructures are overlapped by the Lower andMiddle(?) Triassic Union Wash Formation.During the second episode the Union WashFormation was folded twice; one event pro-duced northwest-trending folds that appear totrend slightly more northwestward than theolder Mule Spring syncline, and the other re-sulted in northeast-trending folds. These twofolding episodes produced many folds withsubvertical plunges. The minimum age ofthese deformations cannot be determined fromrelationships exposed near Tinemaha Reser-voir, but can be by relationships exposed else-where (see subsequent discussion).

PERMIAN AND TRIASSIC EVENTS INNEARBY AREAS

The Tinemaha Reservoir area is critical forinterpretation of the Permian and Triassic his-tory of east-central California because (1) it issituated in a geographically significant posi-tion relative to both the main mass of the InyoMountains and the eastern Sierra Nevada, (2)it contains key elements of the stratigraphy ofboth regions, and (3) evidence for major de-formational events, which can be correlatedwith similar events in both the Inyo Moun-tains and the Sierra Nevada, is present. Herewe consider the sequence of Permian and Tri-assic events represented in the SaddlebagLake and Mount Morrison pendants in the Si-erra Nevada and in the southern Inyo Moun-tains; this description is then followed by ourregional correlations.

Saddlebag Lake Pendant

The geology of this pendant has beenworked out by Schweickert and Lahren (1987,1993), who assigned the sedimentary se-quence overlying highly deformed lower Pa-leozoic rocks to the Permian Diablo and Low-er Triassic Candelaria Formations. They also

identified two major faults, the Golconda andLundy Canyon thrusts. The Golconda thrustwas active during or after deposition of theCandelaria Formation, which includes strataas young as late Early Triassic in west-centralNevada (Speed, 1984), and is cut by the Lun-dy Canyon thrust, which also cuts volcanicrocks dated at 222 6 4 Ma. The Lundy Can-yon thrust in turn is cut by a pluton dated at219 6 2 Ma (Schweickert and Lahren (1987).This thrust, therefore, is early to middle LateTriassic in age, and the older Golconda thrustmust be late Early to early Late Triassic inage.

Mount Morrison Pendant

The northeastern part of this pendant (Fig.1) is characterized by thrust faults with large,north-northwest–striking footwall synclinesthat commonly plunge subvertically and openin opposite directions (Stevens and Greene,2000). Russell and Nokleberg (1977) recog-nized three sets of northwest-trending folds.According to Russell and Nokleberg, the thirdset is only sporadically developed and isyounger than any of the structures discussedherein. Therefore, it will not be discussedfurther.

Russell and Nokleberg’s (1977) earliest foldset included the map-scale, north- to northwest-striking synclines, as well as small-scale foldshaving an average trend of N88W. One of theassociated thrust faults (Nevahbe thrust) in-volves rocks assigned to the Pennsylvanian toEarly Permian Mount Baldwin Marble, estab-lishing a maximum age for this deformationalevent called the Morrison orogeny by Stevensand Greene (2000).

Russell and Nokleberg’s (1977) secondnorthwest-trending fold set in the MountMorrison pendant consists of folds withsmall amplitudes and an average trend ofN238W; Wise (1996) showed a N288W trendfor folds presumably belonging to this set(Fig. 7B). In addition to these fold sets, Ste-vens (1998) and Stevens and Greene (2000)postulated a northeast-trending fold set thatresulted in formation of the steep plunges inthe north- to northwest-trending, map-scalesynclines. One of the few folds of this set thathas been identified is a moderately large,northeast-trending anticline that crosses thenorthwest-trending Sevahah Cliff syncline(Stevens and Greene, 2000). A second verylarge syncline with a kink-fold geometry isinferred at and northwest of McGee Mountain(Fig. 1) to explain the existence of subverticalfolds that face one another at opposite ends ofthe outcrop belt (Stevens and Greene, 2000).

The origin of these steeply plunging foldsis enigmatic. Stevens (1998) and Stevensand Greene (2000) postulated that the north-west-trending folds were refolded by north-east-trending folds formed along a regionalrestraining bend in the dextral proto–Laurel-Convict fault and that later sinistral displace-ment along the Laurel-Convict fault (Greeneet al., 1997) brought those rocks against otherslacking northeast-trending folds. Regardless,all sets of structures discussed here are cut bythe Laurel-Convict fault, which in turn is in-truded by a Middle to Late Triassic (225 616 Ma) dike (Greene et al., 1997). Thus, allof these structures evidently are Permian toTriassic in age.

Southern Inyo Mountains

A complex history of Permian to earliestTriassic contraction has been documented inthe southern Inyo Mountains where strati-graphic and structural relationships provideevidence for Early Permian development ofthe Fishhook and Lee Flat thrusts (of Stevensand Stone, 1988). Work in progress has led usto modify our earlier regional interpretation ofthese structures as reported by Stevens et al.(1997); we (Stevens and Stone, 2000) nowconsider them to be the frontal part of the LastChance thrust system, as first suggested bySnow (1992). A second major deformationalevent in the region was marked by develop-ment of the Inyo Crest thrust and Upland Val-ley footwall syncline of Swanson (1996),which we now interpret to extend at least 70km from the southern Inyo Mountains into theLast Chance Range (Fig. 1). These structurescut across or involve the Last Chance thrustin the southern Inyo Mountains, Dry Moun-tain, and the Last Chance Range. The UplandValley syncline involves Leonardian and pos-sibly younger Permian rocks and is uncon-formably overlapped by the Lower and Mid-dle(?) Triassic Union Wash Formation,relationships that demonstrate a middle Perm-ian to earliest Triassic age of deformation.This tectonic episode was followed by a pe-riod of relative quiescence during which theUnion Wash Formation was deposited.

REGIONAL CORRELATION OFPERMIAN AND TRIASSIC EVENTS

The structural and age relationships just dis-cussed permit recognition of four major Perm-ian and Triassic events prior to developmentof the right-lateral Tinemaha fault and intru-sion of plutonic rocks (Fig. 8). These includeEarly Permian and middle Permian–earliest



Figure 8. Correlation of major Permian and Triassic geologic events in east-central California.

Triassic contractional events separated fromcomplex Middle and/or early Late Triassiccontractional events by an Early to Middle(?)Triassic period of marine sedimentation.These four events occurred prior to plutonismand the development of the right-lateral Ti-nemaha fault.

Early Permian and Middle Permian–Earliest Triassic Deformations

Two deformational events of Early Permianand middle Permian–earliest Triassic age, re-sulting in development of the Last Chance andInyo Crest thrusts, respectively, are clearlydifferentiated in the southern Inyo Mountainswhere they are dated by the ages of the de-formed rocks and the overlapping sequences.The Strange Hill thrust and associated MuleSpring syncline in the Tinemaha Reservoirarea were formed between Pennsylvanian andEarly Triassic time and thus could correlatewith either deformational event in the south-ern Inyo Mountains. The structural style,scale, and orientation of the Strange Hill thrustand Mule Spring syncline, however, are muchmore similar to those of the Inyo Crest thrustand Upland Valley syncline than to those ofthe Last Chance thrust and related structures.On this basis we consider these structures atTinemaha Reservoir to correlate with the mid-dle Permian–earliest Triassic event (the InyoCrest thrust event).

The first-generation folds of Russell andNokleberg (1977) in the Mount Morrison pen-dant, including the map-scale folds interpretedto be footwall synclines by Stevens andGreene (2000), are dated only as Permian or

Triassic. However, they also are very similarin structural style, scale, and orientation to theMule Spring syncline at Tinemaha Reservoirand the Upland Valley syncline in the southernInyo Mountains. All of these synclines are as-sociated with thrust faults of apparently sim-ilar character. Therefore, all these structuresare considered to belong to the same middlePermian–earliest Triassic deformational event.

Early to Middle(?) Triassic Sedimentation

Contractional deformation during Permian toearliest Triassic time was followed by a periodof rapid subsidence throughout the region. Inthe southern Inyo Mountains and TinemahaReservoir area, the primarily fine-grainedUnion Wash Formation was deposited. In theSaddlebag Lake pendant in the eastern SierraNevada (Schweickert and Lahren, 1987) and inwestern Nevada (Speed, 1977, 1984), the es-sentially coeval Candelaria Formation accu-mulated. These two Triassic formations repre-sent deposition in marine environments but insignificantly different tectonic settings on thesubsiding continental margin.

Middle and/or Late Triassic Contraction

Two sets of folds are represented in the Lowerand Middle(?) Triassic Union Wash Formationnear Tinemaha Reservoir. The minimum age isuncertain, but can be inferred from relationshipsin the Mount Morrison pendant where two sim-ilar sets of folds also are present. The folds atMount Morrison include the second generationof northwest-trending folds of Russell and Nok-leberg (1977) and the northeast-trending set of

Stevens and Greene (2000). We suggest thatthese folds in the two areas are correlative be-cause of their substantial geometric similarities,especially in the fold hinge lines measured byus near Tinemaha Reservoir (Fig. 7A) and thosemeasured by Wise (1996) in the northeasternpart of the Mount Morrison pendant (Fig. 7B).Because none of the folds in the northeasternpart of the Mount Morrison pendant discussedhere is younger than the latest movement on theMiddle to Late Triassic Laurel-Convict fault,these proposed correlations, if valid, also limitthe age of the folds near Tinemaha Reservoir(Fig. 8).

In the Saddlebag Lake pendant, two faults,the Golconda and Lundy Canyon thrusts, de-veloped during Middle to Late Triassic time.One or both of these thrusts may be coevalwith development of the second-generationnorthwest-trending folds in the Tinemaha Res-ervoir area and the Mount Morrison pendant

Middle to Late Triassic contractional defor-mation correlative with that recognized nearTinemaha Reservoir and in the eastern SierraNevada has not been recognized with any cer-tainty elsewhere in east-central California. Twosets of folds have been recognized in the cen-tral White Mountains, a northeast-trending sethaving formed prior to a northwest-trending set(Morgan and Law, 1998), but it is not knownif or how these folds correlate with those nearTinemaha Reservoir and in the Mount Morri-son pendant.

Tinemaha Fault (Middle or Early LateTriassic?)

The Tinemaha fault, inferred to underlieOwens Valley (Fig. 1), is a cryptic structure


STEVENS and STONE

along which Stevens et al. (1997) proposedthat the rocks of the Tinemaha Reservoir areaare displaced ;65 km in a right-lateral sensefrom rocks in the northeastern part of theMount Morrison pendant. The existence ofthis fault is based on the apparent offset of (1)a major Devonian submarine channel, (2) sim-ilar structural features (including the onlyknown areas with vertically plunging folds),and (3) the initial 87Sr/86Sr 5 0.706 isopleth(Kistler, 1993). Because the Tinemaha faultapparently displaces folds that involve theUnion Wash Formation in the Tinemaha Res-ervoir area, it can be no older than Early toMiddle(?) Triassic. Its precise trace and upperage limit are less certain. The fault must lieeast of the eastern Sierran pendants and prob-ably east of rocks in small outcrops ;7.5 kmsouthwest of Bishop and at Arcularius Ranch(Bp and AR, respectively, in Fig. 1), whichare lithologically similar to rocks in the Bish-op Creek pendant. The fault also probably lieseast of the Poverty Hills because the LatePennsylvanian rocks exposed there are verydifferent from those of the same age in theMount Morrison pendant, which would restoreadjacent to the Poverty Hills if the fault laywest of those hills. This inferred trace of theTinemaha fault north of Tinemaha Reservoir(Fig. 1) apparently is crossed by the latest Tri-assic (ca. 210 Ma) Wheeler Crest Granodioriteof Bateman (1992). On the basis of these re-lationships, we interpret the Tinemaha faultto be Middle or early Late Triassic in age,although we realize that it could be youngerbecause we cannot conclusively demonstratethat it is intruded by the Wheeler CrestGranodiorite.

The Tinemaha fault is similar in concept tothe late Mesozoic intrabatholithic break 3(IBB3) of Kistler (1993), which was proposedto account for the dextral offset of the initial87Sr/86Sr 5 0.706 isopleth in the eastern SierraNevada. IBB3, however, was interpreted tocoincide with the Laurel-Convict fault nowknown to be Triassic in age. We suggest in-stead that the major displacement occurred onthe Tinemaha fault east of the Mount Morri-son pendant, a location that also accounts forthe apparent offset between rocks of that pen-dant and those in the Tinemaha Reservoirarea.

DISCUSSION AND SUMMARY

The complexly deformed sedimentary rocksin the Tinemaha Reservoir area, ranging fromLate Cambrian to Early and Middle(?) Triassicin age, include local structures that havecaused considerable confusion and structures

of regional significance that provide a basisfor comparison of tectonic events between theInyo Mountains and the eastern SierraNevada.

A prominent low-angle fault exposed in thearea (here called the Big Hill fault), previouslyinterpreted as part of a large-displacementthrust fault of presumed Mesozoic age (Inyothrust), is reinterpreted as a probable Cenozoiclow-angle normal fault of only local impor-tance. The Inyo thrust, as originally con-ceived, is no longer considered to exist, al-though at least one fault originally included inthe Inyo thrust zone (here called the StrangeHill thrust) is indeed a thrust fault.

Correlation of Permian and Triassic struc-tures between the Tinemaha Reservoir area,the main mass of the Inyo Mountains to theeast and southeast, and the eastern Sierra Ne-vada to the west and northwest provides thebasis for a regional chronology of tectonicevents. The earliest of these events resulted indevelopment of the Early Permian LastChance thrust. This fault probably underliesall of the area considered here, but it and as-sociated deformational features apparently areexposed only in the eastern part of the region.

Effects of a second deformation, the Mor-rison orogeny as defined by Stevens andGreene (2000) in the eastern Sierra Nevada,produced east-vergent thrust faults and largefootwall synclines. This orogenic event, datedas middle Permian to earliest Triassic and rec-ognized throughout the region, is approxi-mately coeval with the Sonoma orogeny asoriginally defined by Silberling and Roberts(1962) in northwestern Nevada.

The Morrison orogeny was succeeded by aperiod of quiescence during which the UnionWash and Candelaria Formations accumulat-ed. Later, prior to the end of the Triassic, thewestern part of the region was deformed bytwo episodes of folding and displacement onthe Golconda and Lundy Canyon thrusts. Thisdeformation was followed by dextral displace-ment on the cryptic Tinemaha fault.

We speculate that these intense and tecton-ically diverse Permian and pre–middle LateTriassic events were the result of changingstress regimes associated with changing plateinteractions that culminated in establishmentof an east-dipping subduction zone and an as-sociated magmatic arc in the latter part of theLate Triassic. Because of the constraintsplaced on the timing of deformational eventsin the Tinemaha Reservoir area, several tec-tonic events previously recognized in only theSierra Nevada or southern Inyo Mountains cannow be related and viewed as geographicallywidespread and regionally significant.

ACKNOWLEDGMENTS

We are grateful to Robert Olson, who first rec-ognized the significance of the Tinemaha Reservoirarea; David Greene and Robert Miller, who visitedthe area and helped Stevens understand some of thecomplicated relationships; and Charles Sandberg,whose conodont work provided the ages needed tosolve the structural problems. George Dunne andRobert Miller have always been helpful in pointingout alternative scenarios, inspiring us to continuestudy of this important region. We are also gratefulfor their critical reading of the manuscript and forsuggestions provided by careful reading of the man-uscript by Andrew Barth, Allen Glazner, RichardSchweickert, Jack Stewart, and Douglas Walker. Wealso acknowledge the late Tina Pelley, who helpedwith the field mapping.

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