Text and references to accompany NBMG Field Studies Map 19

GEOLOGY OF THE MOUNT MORIAH QUADRANGLE, NEVADA

byJeffrey Lee, Elizabeth L. Miller, Phillip B. Gans, and Carey C. Huggins

INTRODUCTION creeks in the northern Snake Range originate alongthe flanks of this mountain, including the deeply incised

The Snake Range, in eastern White Pine County, Smith Creek, Deadman Creek, anlj Deep CanyonNevada, is a 150-km-long, north-trending mountain Creek drainages, and the headwaters of Hendry's,range in the northern Basin and Range province (fig. Hampton, and Negro Creeks (fig. 2).1). Sacramento Pass divides the range into two main The northern Snake Range includes the Mountparts, the northern and southern Snake Range (fig. 1). Moriah Wilderness Area, which was established inThe Mount Moriah Quadrangle is one of twelve 7.5' 1989. The steep-walled canyons and rugged ridgequadrangles that cover the northern Snake Range and crests of the northern Snake Range provide access tois situated in the central part of the range (fig. 2). This The Table and Mount Moriah, forming some of thequadrangle includes the highest point in the northern most scenic hiking country in the Basin and RangeSnake Range, Mount Moriah at 12,067 feet (3,678 m) province. The broad, arch-like phY:5iography of thein elevation, and below it at about 11,000 feet (3,300 northern Snake Range is shaped by its geology, whichm) is a small plateau called The Table. The major is unique compared to other mountain ranges in the

region. The northern SnakeRange is novo' considered aclassic example of a Cenozoicmetamorphic core complex (forexample, Coney, 1979). Themost prominent structural featureof the range is the northernSnake Range decollement(NSRD), a low-angle fault thatjuxtaposes an upper plate ofcomplexly normal-faulted Paleo-zoic and Tertiary strata against alower plate of ductilely attenuatedmetasedimentary and igneousrocks (figs. 2 and 3). The NSRDdefines a north-trending asym-metric dome with about 5,000feet (1.5 km) of structural relief(fig. 4). The age, origin, andtectonic significance of the NSRDhave been topics of continuingdebate since the fault was firstdescribed by Hazzard and others(1953) and Misch (1960).Although the origin of the NSRDand core complex detachmentfaults in general remain con-troversial, there is commonagreement that these complexesprovide excellent exposure of

..both brittle and ductile structuresFigure 1. Index map of east-central Neva.da and we~t-central ~tah showing 10ca~lon of f d It f I -the Snake Range with respect to surrounding mountain ranges In the northern Basin and orme. as a resu 0, arge

Range province, western United States. magnitude crustal extension.

" ~". ..,

-[9] Quaternary alluvium LOWER PLATE

UPPER PLATE § Upper Cambrian to Ordovician metasedi~entary114°22'30. 114°15'00. D Tertiary sedimentary and volcanic rocks, undifferentiated

rocks ~ Middle Cambrian limestone, undifferentiated~ Undifferentiated Middle Cambrian to ~ .

Permian sedimentary rocks ~ Lower Cambnan Prospect Mountain Quartzitecomplexly faulted' and Pioche Shale

Q '- II!llII!J Proterozoic McCoy Creek Group""'" Northern Snake Range decollement

INTRUSIVE ROCKS" Normal fault

[2J Ter1iary rhyolite dikes

.Tert(?)-Cret(?) pegmatite and aplite

.Cretaceous pegmatite and aplite

D Cretaceous biotite granite

.Jurassic granite114°00'00.

39°30'00.

Q

39°22'30.Spring Mountain (FS-18)39°22'30.

Uttle HorseCanyon(FS-20)

~ QN

I39°15'00. 39°15'00.

012345I I I I I I

kilometers

~ ~ ~ ~ t Q

milesThe Cow (FS-22) 39°07'30.

114°15'00. 114°07'30. 114°00'00.

Figure 2. Index map of the northern Snake Range showing simplified geology and location of mapped 7.S-minute quadrangles.

2

Period Depth Graphic C.olumnar Rock U it(Km) SectIon n

...'-"'.

Penn. Ely LimestoneMiss. 3 ~~:::;::;~:: ~;:~~~~~~~:~:

c:.~ Guilmette Formation5 4>£3 Simonson Dolomite

S DolomiteSil,.,SO dolomite.~ 5 Eureka Quartzite Re,presentative structural.~ thickness of upper plate-e Pogonip after normal faulting0 Group \ [;~~~::g OJ 6 ,,""'::':,,':'::"'" Notch Peak Limestone \\\ ~ ~ ~

§: ,', ,',',',', ,', ',', \ ~ ~=> ~

-~--"- , r-" Dunderberg Shale \

7 \\ Northern Snake Flange decollement\

c: \m Q) ""-- \E '6 ~""~---'- Middle CambrianE :g limestone \m ~() 8 \

\\

Pioche Shale " \\, \

OJ 9 " \~ Prospect Mountain Quartzite , \.3 ' \

" \ Representative structuralOsceolla Argillite ' \\ thickness of lower plate

10 " \ after ductile thinningUnit 1 quartzite , \

" \, Northern Snake Ran e '~~~~~~~~~~~ ~~~~~~~~~~~~~ ~~ ~ ~~ ,, ~:~~:~deco II e m ent

11 :::::::-;::~:::::::~-;:::::~::::- , , c.-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:- Urut 2 quartzite and :J

I 12 tll~IJirt,~~ schist i /////'

Q) () /

§ U . 3 .~ /rut quartzite /

//

13 U ' h./ nit 4 sc 1st //

//

Unit 5 quartzite //14 /

Figure 3. Representative stratigraphic column for the Snake Range and environs. Representative structural thicknesses for

upper plate and lower plate rocks after normal faulting and ductile thinning.

3

",. ",.~"-"

,

I0 miles 5 N ..~

,- -I0 kilometers 5,- -I

.Normal fault, ball on downthrown side

Northern Snake Range decollement,boxes on upper plate

Figure 4. Structure contour map of the northern Snake Range decollement (from Lee, 1990).

4

,.."," ..".." "..~-

GEOLOGIC SETTING AND PREVIOUS WORK propose a model wherein the Snake Range andenvirons represented an uplifted hinterland, where

Both the northern and southern Snake Range are extension was linked to coeval shortening in theunderlain primarily by Late Proterozoic to Permian foreland via a basal detachment fault now exposed asmiogeoclinal shelf strata deposited along the subsiding the NSRD. Armstrong (1972) was the first to suggestcontinental margin of western North America (fig. 3). that many of these faults might be Tertiary rather thanMiogeoclinal strata in the footwall of the NSRD range Mesozoic in age, and therefore unrelated to Mesozoicin age from Late Proterozoic to Ordovician and gener- thrust faulting. Armstrong specifically cited geochrono-ally define a broad, north-trending antiform. These rock logic and stratigraphic relations from the southernunits are relatively unfaulted but record a polyphase Snake Range as some of his principal evidence for ahistory of ductile deformation, metamorphism, and Cenozoic age for the Snake Range decollement.intrusion. The hanging wall or upper plate of the NSRD Coney (1974) suggested that at least some of theincludes Middle Cambrian to Permian miogeoclinal lower plate deformation in the northern Snake Rangerocks, as well as Tertiary sedimentary and volcanic might be related to the Snake Range decollement. Herocks. In striking contrast to the lower plate, these studied folds in marble mylonite beneath the NSRDrocks are little metamorphosed but highly faulted and and proposed "quaquaversal" or radial sliding of uppertilted by multiple generations of normal faults (fig. 3). plate rocks off the northern Snake Range.

In many ways, the evolution of ideas concerning Hose and Blake (1976) compiled a 1 :250,000-the origin of th~ NSRD charts the progress of our scale geologic map of White Pine County whichunderstanding of the extensional history of the included the first compiled geologic map of theCenozoic Basin and Range province as a whole. Until northern Snake Range based largely on reconnais-the early 1970s, most low-angle faults like the NSRD in sance mapping by Hose. They ma~)ped the low-anglethe western United States were mapped as thrust NSRD in its entirety, which they described as separa-faults. The Snake Range decollement was first ting a lower plate of undifferentiate,d Lower Cambriandescribed by Hazzard and others (1953). A more quartzites and pelites and Middle Cambrian marblesdetailed study of the geology of the northern Snake from an upper plate of comple):ly faulted MiddleRange by Misch (1960) and Misch and Hazzard (1962) Cambrian to Permian carbonate rocl<s. Hose and Blakefollowed as part of a regional survey of the geology of (1976) proposed that two metamorphic and defor-eastern Nevada. They noted that th~ major structure in mational events were recorded in lower plate rocks, athe northern Snake Range was a low-angle "decolle- post-mid-Jurassic to pre-early Eoc:ene intrusive andment" that separated an autochthon of strongly meta- high-grade metamorphic event followed closely by amorphosed rocks from an allochthon of faulted and low-grade metamorphic event associated with thefolded, but largely unmetamorphosed, carbonate rocks. development of a strong penetrative foliation and west-Nelson (1966, 1969) mapped the northern end of the northwest-trending mineral elongation lineation. Thisrange as part of a regional mapping project that was followed by post-early Eocene movement alongincluded the Kern Mountains and southern Deep Creek the NSRD and associated faulting in the upper plate.Range. He mapped the decollement of Misch (1960) As part of the Wilderness RARE II study, additionaland identified lower-plate schists and marbles as meta- mapping of the southern part of the northern Snakemorphosed equivalents of Proterozoic and Cambrian- Range was carried out at a scale ,of 1 :62,500 (Hose,Ordovician miogeoclinal units and upper plate rocks as 1981). This publication pointed out the magnitude ofCambrian to Permian carbonate rocks and younger structural thinning of upper plate units by normalTertiary rocks. Nelson proposed that rocks in the lower faulting. In an influential paper by VVernicke (1981) onplate (his "autochthon") recorded three deformational the geometry and kinematic significance of extensionalevents, the youngest of which involved the generation detachment faults, the Snake Range decollement wasof mylonites and formation of folds that he assigned to cited as a key example of a large-displacement,the Mesozoic. He believed that these structures were eastward-rooting low-angle normal fault.associated with east-directed thrusting along the Studies in the northern Snake Range by geologistsdecollement horizon. The position of the Snake Range based at Stanford University began in 1981 and arein the hinterland of the Cretaceous Sevier orogeniG belt still ongoing. Miller and others (1983) and Gans and(fig. 1) led these and later workers to relate the low- Miller (1983) suggested that the basic structuralangle "decollement faulting" in the northern Snake relationships in the northern Snake Range were bestRange to Mesozoic thin-skinned thrust faulting further explained as extensional in origin arId Cenozoic in age.east, where the NSRD represented the basal shearing- For the first time, the Late Proterozoic lower plate unitsoff plane for these thrusts (Misch, 1960; Miller, 1966). ~n th~. central and southe~n part of, the ra~ge w.ereHowever, Hose and Danes (1973) and Hintze (1978) Identified and correlated, bringing to light the Incrediblerecognized that the deformation in the upper plate was tect~nic stretchi~g or attenuation of these rock units ?ydominated by normal faulting and attenuation of the ductile deformational processes. The uppe~ plate unitsstratigraphic section rather than by shortening and ,,!ere shown to ~ave been aff~ct~d by multiple genera-thickening of the rock column. This led them to tlons of predominantly east-dipping normal faults, and

5 I

J " ,..,'"-,""~."'.."

it was pointed out that over much of the range there metamorphosed by a mid-Jurassic plutonic complexappeared to be near stratigraphic continuity between (fig. 2). Structural fabrics associated with this event arethe oldest units present above and youngest units strongly overprinted by superimposed Cretaceous andpresent below the decollement. These and other Cenozoic fabrics.relations led Miller and others (1983) to qu,estion the The second metamorphic event, of Lateneed for significant displacement on the, ~ecollement Cretaceous age, affected a much broader region of theand to propose instead that the NSRD originated as a lower plate. A series of mineral-in isograds mappedsubhorizontal ductile-brittle transition zo~e betwee~ a along the eastern side of the range indicates that thebrittlely extendin~ u~per plate .and a ductllely stretching grade of metamorphism increases from greenschist to

flower plate. This Interpretation was challeng~? by amphibolite facies from south to north and withBartley and Wernicke (1984), who specifically structural depth in the succession (Geving, 1987; Iproposed that the NSRD repr~sented a low-angle Huggins, 1990). A Late Cretaceous pegmatite andnormal fault or shear zone with 60 km or more aplite dike swarm was intruded during this metamor-displacement that brought,lower plat.e rocks up and out phic event, which has been dated at about 82-78 Mafrom under a thrust plate In the: S~vler belt to the east. (Huggins and Wright, 1989; Huggins, 1990). StructuralIn their model, the near continuity or lack of stra~al fabrics associated with this metamorphic event haveomission between upper and lower pl~te rock .unlts also been strongly overprinted by Cenozoic fabrics,cited by Miller and others (1983) was strlc.tly fortulto~s. making their analysis and interpretation difficult.Gans and Miller (1985) responded to this alternative However, on the northwestern flank of the range,interpretation by citing additional region~1 ~tra~igrap~ic Tertiary strain decreases and eventually dies out.and structural relations that created difficulties with Here, west-dipping foliations, minor thrust faults, and aBartley and Wernicke's proposed model. map-scale fold now inferred to be of Cretaceous age

Further studies in the northern Snake Range (Lee, 1990; P. B. Gans, 1992, unpub. data) areexpanded our geologic mapping and utilized structural preserved. Lower to upper greenschist-facies meta-and kinematic analyses, seismic reflection profiling, morphism of Eocene to Miocene age (Lee and Sutter,metamorphic petrology, and extensive geochronology 1991; Lee, .1995) strongly affected mu?h of the. lowerand thermochronology in order to help resolve issues plate, causing retrogression .of older mid-JurassIc andas to the amount of displacement and initial angle of Late Cretace,ous metamorph!c assemblages. .h NSRD II s the age of lower plate The Tertiary metamorphic event was ~ccompan~edt e .' as ,we a. , , , by vertical thinning and horizontal stretching, resulting

de~ormatlon and !ts geometric and klnema.tlc relatlon- in a subhorizontal, bedding-parallel mylonitic foliationship to the evolving NSRD (Rowles, ,1982, Gans and and west-northwest-trending stretching lineation. ThisMiller, 1983; Grier, 1983, 1984; Miller and others, foliation is axial planar to isoclinal, recumbent folds in1983, 1987, 1988, 1989; Gans and others, 1985, 1989; the northern half of the range. A strong gradient in theGeving, 1987; Lee and others, 1987; Huggins, 1990; amount of deformation or strain of lower plate rockLee, 1990, 1995; Lee and Sutter, 1991). Our geologic units developed during this youngest event. Strainmapping at scales of 1: 12,000 and 1 :24,000 over a 12- increases dramatically from west to east across theyear period (1981-92) was the first detailed mapping to range. Mesoscopic, microstructural,. .and petrofabricbe completed in the range. During the first half of this studies on lower plate: rocks ~er? utilized by Lee andproject, mapping was carried out on 1 :16,000 black- others (1987) t? modify the In-situ pure shear modeland-white and 1 :24,000 color aerial photographs and proposed by Miller and o~hers (1983). Lee and others

'I d Pon orthophotoquadrangles because topo- (1987) proposed a strain path wh~reby. pure a~dcompl e u , .simple shear (top to the east) acted In unison and Ingraphic maps, were not yet available for the region. sequence in the lower plate and that this strain was

Our studies have shown that lower ~Iate rocks intimately tied to the evolution of the NSRD, ultimatelyconsist of metamorphosed ~ate Proter?zolc to Lo,":,er leading to slip along this surface in the brittle regime.Cambrian quartzites and pelltes and Middle Cambrian Structural studies by Gaudemer and Tapponnierto Ordovician marbles that correlate in a straightfor- (1987) were used to promote a model whereby lowerward fashion to less deformed and metamorphosed plate deformation occurred entirely by simple shear, Assections in the adjacent Schell Creek, Deep Creek, pointed out by Lee and others (1987), the question ofand southern Snake Ranges. Jurassic and Cretaceous simple versus pure shear remains controversial, asgranitic plutons and Tertiary dike swarms intrude lower most structural and pet~ographic observat!ons use.d toplate units. Lower plate rocks record at least three ~esolve ~hese questl.ons do not. Yield. umq~emetamorphic and deformational events. The first Interpr~tatlons. In sum, Important questions stili remain

, f J .' is best preserved regarding the exact age of development of lower platemetamorphic event, 0 urasslc age, fabrics whether they are developed as a consequencealong the southern fI~nk of the northern ~nake Ran,ge. of pur~ and/or simple shear, and what the exactHere, Late ProterozoIc and Low~r Cambrian quartzites kinematic relation is between these fabrics and theand metapelites have been Intruded and contact evolving NSRD.

6

'" , _.'"'

-

6Km

169% EXTENSION

~ 13.7 Km ~ 600

2.8Km

437% EXTENSIONi

.I~ .27.4 Km ~I,'f 400 'f 200

.2KmNSRD

Figure 5. Schematic diagram illustrating two generations of normal faults and attendant rotation that may be responsiblefor the observed attenuation of the stratigraphic section shown in figure 3 (modified from Miller and others, 1983).

In the overlying upper plate, unmetamorphosed to complex arrays of smaller faults alleviate space prob-weakly metamorphosed Middle Cambrian to Permian lems at the toes of normal faults as they merged withcarbonate rocks have been, in general, attenuated by the NSRD. Figure 5 schematically illlJstrates how thisat least two sets of imbricate normal faults which are faulting history operated. Regardless of the details andplanar down-dip but not so along strike. Parts of the the exact geometry of the faults, it is clear that theupper plate of the NSRD are characterized by a process of faulting led to tremendous attenuation of thesystematic history of east-directed normal faulting that stratigraphic sequence which once formed the hangingresulted in successive northwestward tilting about a wall or upper plate of the NSRD (figs. 3 and 5).common axis in response to WNW-ESE extension, Normal-sense movement along the f'JSRD during theparallel to that recorded by the ductile deformational time span of upper plate faulting is imjicated as only afabrics in the lower plate (Miller and others, 1983). few mapped faults actually cut and offset the NSRD.However, the amount of strain indicated by these faults Motion along the NSRD is believe(j to have beenand the amount of rotation related to faulting varies down-to-the east and resulted in the! juxtaposition ofacross the range, as does the direction of tilting or the less metamorphosed and mostl), younger upperrotation. Despite the extremely complex map pattern of plate rocks down upon the more highly metamor-upper plate normal faults, a generally systematic phosed and generally older lower plate rocks. Anstructural style is evident upon close inspection. important exception to this occurs in the northern partStructural sections of Middle Cambrian to Permian of the range where Middle Cambriian and younger(and locally Tertiary) strata that "young" to the we~t are rocks of the upper plate routinel), overlie Upperrepeated eastward on east-dipping normal faults that Cambrian strata in a lower plate position.merge with, but do not offset the NSRD. Within these Age of movement along thesl3 normal faultstilted sections, older, gently west-dipping faults omit remains poorly constrained but is at least in partunits as well and are interpreted as originating as Eocene(?)-Miocene. For example, in the Sacramentosteep, down-to-the-east normal faults that once Pass area and at the northern end of the northernshallowed into the NSRD. These older faults rotated to Snake Range, Tertiary alluvial fan conglomerate,low angles as they moved until cut by younger faults lacustrine deposits, and volcanic rocks are cut bywhich rotated them past horizontal and into their normal faults that merge along strike with the NSRD,present westward dips. Macroscopically ductile demonstrating a Tertiary age for much if not all of thedeformation such as warping, normal drag, folding, and extensional faulting (Gans and others, 1989).

7

'"" """Co

We previously postulated that most of the move- GEOLOGY AND STRUCTURAL HISTORY OFment on the NSRD was Oligocene to early Miocene in THE MOUNT MORIAH QUADRANGLEage (for example, Gans and Miller, 1983; Miller andothers, 1983; Gans and others, 1985; ~ee and Sutte!, The large topographic relief and deeply incised1991). However, new thermochronologlc and geologic canyons in the Mount Moriah Quadrangle provide~ata su~ges.t that ~ove~ent. along t~e NSRD was excellent three-dimensional exposure of the typicallikely episodiC. Multiple diffusion domain analyses of structural relations of a metamorphic core complex.

potassium feldspar Arrhenius data and 4oAr/39Ar age The dominant structural feature, the NSRD, isspectra together with apatite fission-track studies show spectacularly exposed along the deeply incised canyonthree rapid cooling events: 1) middle Eocene (48-41 walls and beneath Mount Moriah on The Table.Ma), 2) late Oligocene (30-26 Ma) and 3) early to Metamorphosed Late Proterozoic and Lower Cambrianmiddle Miocene (20-15 Ma) (Miller and others, 1989, quartzites and schists, Middle and Upper Cambrian1990; Lee, 1995). These cooling events may indicate marbles and calc-schists, and Cretaceous biotitediachronous exhumation of footwall rocks from be- tonalite orthogneiss and pegmatite-aplite dike swarmsneath the NSRD. The spatial and temporal distribution in the lower plate are well exposed in the drainagesof these cooling events suggest that the NSRD is a and throughout the northern part of the quadranglecomposite structure; the NSRD along the west flank of (see map). These rocks record two metamorphicthe range moved during the Eocene and Oligocene events and possess a well-developed mylonitic(Lee, 1995), and the NSRD along the east flank of the foliation and mineral elongation lineation. Middlerange moved during the Early to Middle Miocene Cambrian to Permian and Tertiary sedimentary rocks(Miller and others, 1989, 1990; Lee, 1995). Field in the upper plate are exposed along steep, ruggedrelations indicate that Miocene or younger sedimentary slopes in the northeastern and southwestern corners ofsequences along the northern, eastern, and southern the quadrangle. These unmetamorphosed rocks areflanks of the range are cut and tilted by a set of normal attenuated along at least two generations of normalfaults that we infer either to cut or to sole into the faults (see map).NSRD in the subsurface supporting the inferred Earlyto Middle Miocene episode of movement along at least The Northern Snake Range Decollernentthese parts of the NSRD (Gans and others, 1989;Miller and others, 1989, 1990). The NSRD is spectacularly exposed on The Table as a

In summary, we now think that the combined data nearly flat, exhumed fault surface at an elevation ofon the deformational history of upper and lower plate about 11,000 feet (3,300 m), the highest exposure ofrocks indicate that the NSRD is a composite structure the NSRD within the northern Snake Range (fig. 4).rather than a single fault; thus it was never simul- Toward Hendry's Creek and Hampton Creek, thetaneously active over its entire mapped extent. East- NSRD dips gently (5-8°) and uniformly to the east.dipping normal faults along the eastern flank of the Toward Negro Creek, the NSRD dips more steeply (12range were likely active in middle Miocene and later to 18°) to the west where it is crosscut and offset by atimes and perhaps originated as steep faults that have west-dipping high-angle normal fault. To the north-since been rotated to present low dips. Because the northeast, the NSRD dips 10 to 15° into the Smitholder parts of the NSRD were dragged into parallelism Creek area, defining a map-scale, west-northwest-with these younger faults, it is not obvious where the trending trough-like depression or mullion, as thetrace of the NSRD ends and the younger faults begin. NSRD drops in elevation from 11 ,000/feet (3,300 m) onIf the high-strain rocks and mylonites of the lower plate The Table to less than 8,000 feet (2,400 m) in therepresent more than one Tertiary event, our studies to Smith Creek area. The axis of the trough is subparalleldate have not unequivocally resolved these with to the inferred WNW-ESE extension direction in bothconfidence. Limiting factors include the resolution of the lower and upper plates, and is located in a regionavailable geochronologic techniques given the complex where the polarity of upper plate normal faultingthermal and deformational histories of the minerals changes across Smith Creek from predominantlyavailable for dating and the inability of structural down-to-the-west to the north to predominantly down-studies to distinguish more than one superimposed to-the-east to the south. The trough may be the relatedevent if these are developed at low angles to one to the formation of a transform fault or zone, no longeranother. Clearly, the NSRD represents the end result exposed, in the upper plate that accommodated theof an involved history of extensional strain at both change in polarity of normal faulting, or it mayductile and brittle levels of the crust and that more than represent a primary structure, such as a fault mullion,one episode of faulting is responsible for the relative that developed parallel to the movement directionuplift and exposure of ductile extensional fabrics in the along the NSRD. On this flank of Mount Moriah, therange. However, more sophisticated studies and NSRD also dips less than the dip of the unit-parallelmodeling of existing data are necessary to fully mylonitic foliation. As a result, the NSRD cuts up struc-understand the kinematic history and mechanics of tural and stratigraphic section, being localized alongdeformation and faulting. the Middle Cambrian-Lower Cambrian contact in the

8

" """",,. "-".." .~d~_~"~_..

southern part of the quadrangle and within the Upper Overprinting this Cretaceous metamorphic event isCambrian Notch Peak Limestone in the northeastern a middle to upper greenschist facies metamorphic andpart of the quadrangle. The lower plate biotite mylonitic deformational event that ductilely thinned andorthogneiss is locally cut by the NSRD, but does not stretched lower plate rocks and collapsed olderappear anywhere in the upper plate, indicating a metamorphic isograds (Geving, 1987; Lee and others,minimum offset along the NSRD of about 8 km, the 1987; Miller and others, 1989; Huggins, 1990). As withdistance from the truncated orthogneiss to the eastern the older event, grade of metamorphism increases withflank of the range where upper plate rocks are covered depth and from south to north and resulted in newby younger basin-filling sediments. growth of coarse-grained plagioclase, chlorite, and

muscovite, and new biotite growth at deepest structuralLower Plate Rocks levels in the Smith Creek region. Older metamorphic

porphyroblasts are partially to completely replaced byLower plate units beneath the NSRD possess a sericite :f: chlorite :f: biotite. New(?) fibrous kyanite waspervasive shallow-dipping mylonitic or gneissic foliation observed along the edges of staurolite and garnetand west-northwest-trending mineral elongation porphyroblasts and is aligned parallel to the predomi-lineation of Tertiary age. A large biotite tonalite nant foliation. The predominant foliation is a myloniticorthogneiss sill defines a west-northwest-trending foliation defined by coarse-grained chlorite, muscovite,outcrop pattern across the central part of the biotite and recrystallized quartz grains in the schistquadrangle (see map). U-Pb analyses of zircons units, dynamically recrystallized, flattened and elongateyielded an intrusive age of 100 :f: 8 Ma (Miller and quartz grains in the quartzites, and flattened andothers, 1988). A deformed pegmatite and aplite dike elongate calcite and color banding in the marbles. Theswarm defines a west-northwest-striking zone in the mylonitic foliation is subparallel to contacts of meta-Smith Creek, Deadman Creek, and Rye Grass Canyon sedimentary units, and in the southern half of the quad-areas. This swarm crosscuts the biotite orthogneiss rangle, subparallel to the overlying NSRD (see map).and yields an intrusive age of 82 Ma (U-Pb on In this part of the quadrangle, the mylonitic foliationmonazite) (Huggins, 1990). Late Proterozoic to Upper dips gently to the east into the headwaters of Hendry'sCambrian metasedimentary rocks, exposed throughout and Hampton Creeks and to the west in the head-most of the quadrangle, record two distinct metamor- waters of Negro Creek. To the northeast of The Table,phic and deformational events. The older event, of Late the mylonitic foliation is subparallel to metasedimentaryCretaceous age, resulted in peak metamorphic condi- contacts but dips more steeply to the northeast thantions of upper greenschist to amphibolite facies. This the NSRD. Within the mylonitic foliation is a west-metamorphic event is responsible for a series of northwest-trending mineral elongation lineation,mineral-in isograds (not shown on map) mapped along defined by stretched quartz and calcite grains, alignedthe eastern flank of the range and indicate an increase older metamorphic porphyroblasts, and pressurein metamorphic grade with depth and from south to shadows on older porphyroblasts. Mesoscopicnorth (Geving, 1987; Huggins, 1990). The highest asymmetric kinematic indicators, such as SIC foliationsgrade assemblages occur in the Smith Creek area, in the orthogneiss, extensional crenulation cleavageswhere peak metamorphic mineral assemblages include in the schist units, and shear bands indicate abiotite + clinozoisite + muscovitelphlogopite + quartz :f: component of top-to-the-east sense of shear duringplagioclase :f: calcite :f: sphene in the Upper Cambrian formation of the mylonitic foliation and stretching

I calc-schists and muscovite + quartz + biotite + li,neation. Str~in associated with the mylonitic deforma-; plagioclase + garnet :f: staurolite :i: kyanite in Unit 2 of tlonal event Increases from west to east, as clearly

the Late Proterozoic McCoy Creek Group. Peak evidenced by the dramatic thinning of lower plate units.metamorphic temperatures of 600 to 650°C, based on The thickness of the Prospect Mountain Quartzitemineral assemblages and garnet-biotite geothermo- decreases from about 20% of its original stratigraphicmetry, were reached in the Smith Creek area (Huggins, thickness in the region of The Table to about 6% at the1990). U-Pb analyses of metamorphic monazite from junction of Deep Canyon and Smith (;reek.the schist units yield ages of 78 Ma, interpreted as the Multiple diffusion domain analyses of potassiumage of metamorphism (Huggins, 1990). Since it is likely feldspar 40Ar/39Ar age spectra from pegmatite in thethat emplacement of the 82-Ma pegmatites was Smith Creek area indicate that lower plate rocks didconcurrent with peak metamorphism, we interpret the not drop below 300°C until 20 Ma, thus providing adata to indicate that peak metamorphic conditions were minimum age for the mylonitic deformation (Lee,reached between 78 and 82 Ma (Huggins, 1990). 1995).Deformation occurred during this metamorphic event, Brittle structures in the lower plate include locallyas evidenced by curved inclusion trails within well-developed, subvertical joint sets that cut allmetamorphic porphyroblasts. However, strong over- penetrative fabrics. Although these joints have manyprinting by a younger Tertiary deformation prevents orientations, the most prominent set strikes northeast-detailed study of the geometry and orientation of these southwest, perpendicular to the lower plate stretchingfabrics. lineation. Other brittle structures include a north-

9

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, ,..."CC- ~ .

striking, moderately to steeply west-dipping normal their present westward dips (fig. 5) (Gans and Miller,fault on the western side of the quadrangle that cuts 1983; Miller and others, 1983).and offsets the mylonitic foliation, lower plate units, and There are no absolute age constraints on normalthe NSRD several tens of meters. faulting in this quadrangle. However, just to the east at

the mouth of Smith Creek, Tertiary alluvial-fanUpper Plate Rocks conglomerates dip into the mapped trace of an east-

dipping normal fault. Since the conglo,merate containsIn the rugged, higher peaks of the northeastern and clasts of lower plate lithologies, the bounding fault mustsouthwestern corners of the quadrangle, complexly be younger than and cut the NSRD mapped higher infaulted Middle Cambrian to Permian and Tertiary the range. Conglomerates are MiocE~ne or younger,sedimentary rocks rest in fault contact above lower since apatite fission-track ages from (~obbles of lowerplate units along the NSRD (see map). A systematic plate lithologies are middle Miocene. To the north offaulting history is evident, but the two areas exhibit the range, the youngest set of normal faults cut anddifferent geometries and kinematics of normal faulting. moderately tilt a 24.0:t0.7 Ma rhyolite tuff (Gans andTwo domains of different geometries and kinematics of others, 1989). As yet, no stratigraphic relations havenormal faulting are best exposed to the east in the been found that place a younger ag'e limit on upperLittle Horse Canyon Quadrangle. Here, the two plate normal faulting.structural domains are separated by an east-trendingboundary approximately coincident with Smith Creek. Correlations of Upper and Lower PI~rte RocksSouth of Smith Creek, upper plate units dip mainly tothe west-northwest and are cut east-dipping normal Prior to Cenozoic faulting, Late Proterozoic tofaults. North of Smith Creek, units dip mainly to the Paleozoic strata in the Mount Moriah Quadranglesoutheast and are cut by predominantly west-dipping formed part of a regionally extensr\le sequence ofnormal faults. The northeastern corner of the Mount miogeoclinal strata deposited on the subsidingMoriah Quadrangle is the western continuation of the continental shelf of western North America (Gans andnorthern (east-tilted) domain of these two structural Miller, 1983). Formational designations, thicknesses,domains (see map). Here, structural sections of Middle and regional facies variations have been described byCambrian to Pennsylvanian and unconformably over- Drewes and Palmer (1957), Whitebread (1969), Hoselying Tertiary strata dip in general to the southeast and and Blake (1976), and Stewart (1980), among others.are cut and offset by northeast-striking, moderate (30- In the Mormon Jack Pass Quadrangle, Paleozoic rocks60°) northwest-dipping faults. Most of these faults have in the upper plate of the northern Snake Rangesmall offsets (stratigraphic throw of 0.1-0.8 km), decollement (NSRD) are complexly faulted and mostalthough one fault places the Devonian Simonson sections are incomplete. Exceptions include the intactDolomite against Upper Cambrian Notch Peak Lime- section of Middle Cambrian to Ordovician strata in thestone, a stratigraphic throw of about 2 km. Locally, eastern edge of the quadrangle. Similarly, Latehigh-angle southeast-dipping faults cut and offset the Proterozoic to Upper Cambrian rocks in the lower platenorthwest-dipping faults. A few south-dipping faults are are strongly deformed and metamor~lhosed, and theiralso exposed in this area and are probably related to present thicknesses are not representative of theirinternal fault block deformation. Faults merge with or original stratigraphic thicknesses. De!scriptions of theare cut by the underlying NSRD. The geometry of unmetamorpbosed and undeformed counterparts ofnormal faults indicates a WNW-ESE direction of these units in the adjacent Schell Creek Range can beextension. found in Young (1960).

The southwestern corner of the quadrangle, along At the scale of the northern Snake Range, unitthe western flank of Mount Moriah and along the Negro descriptions and formational names occasionally vary.Creek drainage in the adjacent quadrangle to the west We have attempted to handle most of these in our(Sixmile Canyon), is part of the southern (west-tilted) legend of map units and stratigraphil~ column (fig. 3).domain. Here, structural sections of Middle Cambrian Specifically, unit descriptions for the Middle Cambrianto Permian strata dip moderately to steeply "to the units vary in different parts of the northern Snakenorthwest and are repeated eastward on gently east- Range so we have further clarified our unitdipping normal faults that merge with, but do not offset, designations and their equivalence in figure 6.

the NSRD (see map). In the Mount Moriah quadrangle,two of these faults are well exposed. Older, gently ACKNOWLEDGMENTSwest-dipping faults within these structural blocks omitunits as well but are interpreted as originating as steep, Geologic mapping of the northern Snake Range at?own-to-the-east normal faults that once shall owed scales of 1:24,000, 1:16,000, and 1:12,000 began inInto the NS~D. These older faults rotat,ed to lower 1981 by the "Stanford Geological Survey," Stanfordangles, domino-style, as they moved until ?ut .by the University's Geology Summer Field program, taught bysteep, down-to-t.he-east younger fault~, which In ~urn Elizabeth L. Miller and Phillip B. Gans until 1987,rotated the earlier faults through horizontal and Into accompanied by studies by Lisa Rowles in the

10

Hampton Creek area (Rowles,1982) S G .. th Upper plate, northern part of Upper plate, soutlhern part of

usan rler In eS 't P (G .northern Snake Ran e northern Snake Rangeacramen 0 ass area rler, .

1984), Carey Huggins in the £n Notch Peak Limestone ---

Smith Creek area (Huggins, £d Dunderber Shale ---£d Dunderber Shale

1990) and Jeffrey Lee in the £m Middle Cambrian Limestone £1 Lincoln Peak Formationentire northern part of the range (undifferentiated) on Limestone(Lee, 1990). We are grateful to base not exposedStanford University's School ofEarth Sciences, the Geology Lower plate, northern part of Lower plate, southern part ofDepartment, and Sohio northern Snake Ran e northern Snake RangePetroleum Company for £d Dunderber Shale top not exposedfinancial support of this £r Raiff Limestoneprogram during these years. £mn Monte Neva Formation £pc Pole Canyon LimestoneMost of all we thank all of the .undergraduate students and £e. EI~orado Limestone ---graduate student teaching £ I Ploche Shale ---Shale

assistants who energetically £ m Pros ect Mountain Quartzite ---ect Mountain Quartziteand enthusiastically contributedto the making of the preliminary Figure.G. Stratigraphic nomenclatu~e, unit designations, and correlations for Middlegeologic maps of this area over Cambrian upper and lower plate units across the northem Snake Range.

these seven years. , .Additional geologic mapping and studies in the Drewes, H., and Palmer, A., 1957, ("am~rlan ro~ks of

Mount Moriah Quadrangle of the northern Snake south,ern Sna~e Range, Nevada: Geological Society of.America Bulletin, v. 69, p. 221-240.Range were carried out by Lee supported by NSF Fritz, W.H., 1968, Geologic map and sections of the southem

Grants EAR- T2-063~9, EAR84-18678, .and. EAR8~- Cherry Creek and northern Egan Ranges, White Pine04814 awarded to Miller at Stanford UnIversity. Lee s County, Nevada: Nevada Bureau of Mines Map 35,mapping and studies, in addition, were supported by 1 :62,500.Stanford University Geology Department's Shell Fund Gans, P.B., and Miller, E.L., 1983, Style of mid-Tertiaryand a GSA Penrose Grant. Final compilation, field extension in east-central Nevada, in Nash, W.P., andchecking, and writing of this report were partially Grugel, K.D., eds., Geologi.cal I~xcursions in thefunded by the Geological Society of Nevada. During overthrust b,elt an? metamorphic c.ore compl~xes of .thethis time, Lee was at the Division of Geological and Intermountal~ reglo~: Utah Geological and Mineralogical

S . C I' f . I t.t t f T h I Survey Special Studies, p. 107-160.Planetary clences, a I orma ns I u e 0 ec no ogy, Gans, P.B., and Miller, E.L., 1985, Comment on "The SnakePasadena and supporte~ by NSF grant EAR-92-96102 Range decollement interpreted as at major extensionalawarded to J. Stock. shear zone" by John M. Bartley and Brian P. Wernicke:

Many thanks to Arthur Snoke (University of Tectonics, v. 4, p. 411-415.Wyoming) Eric Seedorff (Magma Copper Company), Gans, P.B., Mahood, G.A., and Schermer, E., 1989,and Christopher Henry (Nevada Bureau of Mines and Synextensional magmatism in the Basin and RangeGeology) for office reviews, and Eric Seedorff and Province: A case study from the eastern Great Basin:Christopher Henry for field reviews. Geological Society of America Special Paper 233, v. p.

53 p.REFERENCES Gans, P.B., Miller, E.L., McCarthy, J., and Ouldcott, M.L.,

1985, Tertiary extensional faulting and evolving ductile-Armstrong, R.L., 1972, Low-angle (denudational) faults, b~i~l~ transitio.n ~ones in the ~ort~ern Snake Range and

hinterland of the Sevier orogenic belt, eastem Nevada VICInity: New Insights from seismic data: Geology, v. 13,and western Utah: Geological Society of America p.189-193.Bulletin, v. 83, p. 1729-1754. ..Gaudemer, Y., and Tapponnier, P., 1987', Ductile and brittle

Bartley, J.M., and Wernicke, B.P., 1984, The Snake Range deformations in the northern Snake Range, Nevada:decollement interpreted as a major extensional shear Journal of Structural Geology, v. 9, p. 159-180.zone: Tectonics, v. 3, p. 647-657. Geving, R.L., 1987, A study of the metan,10rphic petrology of

Coney, P.J., 1974, Structural analysis of the Snake Range the northem Snake Range, east-central Nevada [M.S.decollement, east-central Nevada: Geological Society of thesis]: Southern Methodist Universit:'/, 75 p.America Bulletin, v. 85, p. 973-978. Grier, S.P., 1983, Tertiary stratigraphy ani:! geologic history of

Coney, P.J., 1979, Tertiary evolution of Cordilleran the Sacramento Pass area, Nevada, in Nash, W.P., andmetamorphic core co.mplexes, in Armetrout, J.W., and Grugel, K.D., eds., Geological ,excursions in theoth.ers, eds., CenozoIc ~aleogeography of the western overthrust belt and metamorphic core complexes of theU~lted States: .Economl~. Geolo~y, Paleont~logy, and Intermountain region: Utah Geological and MineralogicalMineralogy Society, Pacific Section Symposium III, p. S S . I St d. 139 14415-28. urvey pecla U les, p. -.

11

-.."" ~ ",--

Grier, S.P., 1984, Alluvial fan and lacustrine carbonate Miller, E.L., Gans, P.B., and Gleadow, A.J.W., 1989, Upliftdeposits in the Snake Range: A study of Tertiary history of the Snake Range metamorphic core complex,sedimentation and associated tectonism [M.S. thesis]: Basin and Range Province, USA, from apatite fissionStanford University, 60 p. track data: EOS American Geophysical Union!

Hazzard, J.C., Misch, P., Wieses, J.H., and Bishop, W.C., Transactions, v. 70, p. 1309.i 1953, Large-scale thrusting in the northem Snake Miller, E.L., Gans, P.B., and Lee, J., 1987, The Snake Range

Range, White Pine County, northeastern Nevada [abs.]: decollement, eastern Nevada, in Hill, M.L., ed.,Geological Society of America Bulletin, v. 64, p. 1507- Cordilleran Section, Geological Society of America1508. Centennial Field Guide: Geological Society of America,

Hintze, L.F., 1978, Sevier orogenic attenuation faulting in the. p.77-82.Fish Springs and House Ranges, western Utah: Brigham Miller, E.L., Gans: P.B.,. Lee, J.,. and Brown, ~". 1990,Y U .. ty G I St d. 25 t 1 11 24 Geochronologic studies bearing on the ongln and

oung nlversl eo ogy u les, v. , p. ,p. -. I . f I .bH Id M J 1971 St b 'I' t f d I . t d th evo utlon 0 myonltes eneath normal fault systems,

0 away, .., ,allyoanauslean e B . dR P . 7th I . ICf0 asln an ange rovlnce: nternatlona on erencealuminum silicate phase diagram: American Journal of

on Geochronolog C h I d I t.y, osmoc rono ogy an so opeScience, v. 271, p. 97-131. Geology Canberra Australia: Geological Societ f

Hose, R.~., 1981, Geologic map of the Mount Moriah further Australi~ Abstracts, ~o. 27, v. p. 37. Y 0

planning (RARE II) area, eastern Nevada: U.S. Miller, E.L., Gans, P.B., Wright, J.E., and Sutter, J.F., 1988,Geological Survey Miscellaneous Field Studies Map MF- Metamorphic history of the east-central Basin and

1244A, 1 :62,500. Range province: Tectonic setting and relationship toHose, R.K., and Blake, M.C., 1976, Geology and mineral magmatism, in Ernst, W.G., ed., Metamorphism and

resources of White Pine County, Nevada, Part 1, Crustal Evolution of the Western United States, RubeyGeology: Nevada Bureau of Mines and Geology Bulletin Volume VII: Prentice Hall, Englewood Cliffs, New Jersey,85, p. 1-35, map scale 1 :250,000. p. 649-682.

Hose, R.K., and Danes, Z.F., 1973, Development of late Miller, G.M., 1966, Structure and stratigraphy of the southernMesozoic to early Cenozoic structures of the eastern part of the Wah Wah Mountains, southwest Utah:Great Basin, in DeJong, K.A., and Scholten, R., eds., American Association of Petroleum Geologists Bulletin,Gravity and Tectonics: John Wiley and Sons, Inc., New v. 50, p. 858-900.York, p. 429-441. Misch, P., 1960, Regional structural reconnaissance in

Huggins, C.C., 1990, Nature and timing of metamorphism in central-n~rtheast Nev~da an~ adjacent area~:the Smith Creek area, northern Snake Range, Nevada Obse~a~lons and Interpretatlor~s: Intermountain[M.S. thesis]: Stanford University, 92 p. Association of. Petroleum Geologists 11th. Annual

Huggins, C.C., and Wright, J.E., 1989, Superimposed. Conference Guidebook, v. p. 17-42. .Cretaceous and Tertiary metamorphism in the northern Misch, P., an~ Hazzard, J.C., 196.2, Stratlgr~phy andS k R N d G I . I S . ty f A .metamorphism of late Precambrian rocks In central

na e ange, eva a: eo oglca ocle 0 menca rth t N d d d. t Ut h A . .no eas ern eva a an a Jacen a: merlcanAbstracts with Programs, VB' 21, p. A95

C.Association of Petroleum Geologists Bulletin, v. 46, p.Lee, D.E., and Fischer, L. ., 1985, retaceous meta- 289-343.

morphism ~n the northern Snake Range, Nevada, a Nelson, R.B., 1966, Structural development of northernmostmetamorphic core complex: Isochron/West, no. 42, p. 3- Snake Range, Kern Mountains, and Deep Creek Range,

7. 40 39 Nevada and Utah: American Association of PetroleumLee, J., 1990, Structural geology and Ar/ Ar thermo- Geologists Bulletin, v. 50, p. 921-951.

chronology in the northern Snake Range metamorphic Nelson, R.B.,' 1969, Relation and historIJ of structures in acore complex, Nevada [Ph.D. thesis]: Stanford sedimentary succession with deeper metamorphicUniversity, 184 p. structures, eastern Great Basin: American Association of

Lee, J., 1995, Rapid uplift and rotation of mylonites: Insights Petroleum Geologists Bulletin, v. 53, p. 307-339.from potassium feldspar 40Ar/39Ar thermochronology, Rowles, L.D., 1982, Deformational history of the Hamptonnorthern Snake Range, Nevada: Tectonics, v. 14, p. 54- Creek canyon area, northern Snake Range, Nevada77. [M.S. thesis]: Stanford University, 80 p.

Lee, J., and Sutter, J.F., 1991, Incremental 40Ar/39Ar Stewart, J.H., 1980, Geology of Nevada: Nevada Bureau ofthermochronology of mylonitic rocks from the northern Mines and Geology Special Publication 4, 136 p.Snake Range, Nevada: Tectonics, v. 1, p. 77-99. Wernicke, B.P., 1981: Low-angle normal. fa~lts in the Ba~in

Lee, J., Miller, E.L., and Sutter, J.F., 1987, Ductile strain and and Range province: Nappe tectonics In an extendingmetamorphism in an extensional tectonic setting: A case .orogen: Nature, v. 291, p. 6~5-648.stud from the northern Snake Range, Nevada, USA: in Whitebread, D,-H., 1969, Geologic map of the Wheeler Peak

y and Garrison quadrangles, Nevada and Utah: U.S.Coward, M.P., Dewey, J.F., and Hancock, P.L., (eds.), G I . I S M. II I t. t.

M Ieo oglca urvey Isce aneous nves Iga Ions ap-Continental Extensional Tectonics, Geological Society of 578 I 1.48 000 7.,. 9 ,scae., ,po

0 London Special Publication ~o. 28, p. 267-2 8. Young, J.C., 1960, Structure and stratigraphy in the north

Miller, E.L., Gans, P .B., and Garing, J.D.,. 1983,. The Sn~ke central Schell Creek Range: Intermountain Association

Range decollement; an exhumed mid-Tertiary ductlle- of Petroleum Geologists 11th Annual Field Conference,

brittle transition: Tectonics, v. 2, p. 239-263. Guidebook, p. 158-172.

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