mesozoic tectonics and metamorphism in the pequop mountains and wood hills region northeast nevada implications for the architecture and evolution of the sevier orogen discussion and reply the pequop mountains wood hills east humboldt range region northeast nevada exposes a nearly continuous cross section of precambrian to mesozoic strata representing middle to upper crustal levels of the mesozoic hinterland of the sevier orogen these rocks preserve the transition from unmetamorphosed mesozoic upper crust to partially melted middle crust integration of new structural metamorphic and u pb thermochronologic data from the wood hills and pequop mountains coupled with a regional tectonic reconstruction reveals substantial cretaceous metamorphism contraction and extension in the sevier hinterland in northeast nevada we report two phases of contraction not previously recognized that are accommodated by top to the southeast thrust faults the windermere and independence thrusts contraction was succeeded by two phases of extension along west rooted normal faults the late cretaceous pequop fault and tertiary mary9s river fault system the earliest phase of thrust faulting resulted in as much as 30 km of crustal thickening and an estimated minimum of 69 km of shortening along an inferred fault called the windermere thrust the timing of this thrusting event is bracketed between late jurassic ca 153 ma and late cretaceous 84 ma relaxation of crustal isotherms following and perhaps during thrusting resulted in barrovian style metamorphism of footwall rocks and partial melting of metapelite at deep levels peak metamorphism was attained ca 84 ma and by this time hinterland crustal thickening had reached a maximum during 84 75 ma another minor pulse of shortening and thickening along the independence thrust was followed by partial exhumation of the metamorphic rocks and as much as 10 km of crustal thinning along the pequop fault thus the interval from 84 to 75 ma in northeast nevada marks a fundamental and apparently permanent change from horizontal contraction to extension in the upper to middle crust in the hinterland final exhumation of the metamorphic rocks was accomplished by the tertiary mary9s river fault system our data indicate that much of the metamorphism and some of the contraction in the sevier hinterland in northeast nevada which was previously thought to be largely late jurassic is actually cretaceous in age furthermore the data indicate that widespread metamorphism of the middle crust is a byproduct of tectonic burial and that hinterland and foreland thrust faulting were coeval suggesting that thrust faults in the sevier orogen do not form a simple foreland younging sequence

allochthonous archean basement in the northern east humboldt range nevada archean basement rocks occur in the core of a fold nappe in the northern east humboldt range nevada the archean rocks consist primarily of plutonic orthogneiss and metasedimentary paragneiss both units contain numerous amphibolitic bodies interpreted as mafic intrusions u pb zircon studies indicate that the orthogneiss unit is at least 2520 110 ma and therefore is an example of an archean granitoid these data extend the known distribution of precambrian basement rocks into northeastern nevada and provide new constraints on the delineation of the archean wyoming province the basement rocks are surrounded by upper amphibolite facies metasedimentary rocks correlated with late precambrian to devonian rocks of the eastern great basin miogeoclinal sequence the contact between the archean basement rocks and the metasedimentary mio geoclinal sequence is interpreted as a premetamorphic and prefolding low angle fault the age and prefolding geometry of this low angle fault are currently poorly constrained but it may record important mesozoic crustal shortening in the hinterland of the sevier orogenic belt the age of nappe formation is not yet fully established but at least some isoclinal folds in the area were formed in the tertiary and are kinematically linked to a major extensional deformation

rapid extension in an eocene volcanic arc structure and paleogeography of an intra arc half graben in central idaho a study of extension volcanism and sedimentation in the middle eocene panther creek half graben in central idaho shows that it formed rapidly during an episode of voluminous volcanism the east southeast tilted panther creek half graben developed across the northeast edge of the largest cauldron complex of the challis volcanic field and along the northeast trending trans challis fault zone two normal fault systems bound the east side of the half graben one fault system strikes northeast parallel to the trans challis fault zone and the other strikes north to northwest the geometry of the basin fill deposits shows that movement on these two normal fault systems was synchronous and that both faults controlled the development of the panther creek half graben strikes of the synextension volcanic and sedimentary rocks are similar throughout the half graben whereas dips decrease incrementally upsection from as much as 60 to less than 10 previous k ar dates and a new 40 ar 39 ar plateau date from the youngest widespread tuff in the basin suggest that most of basin formation spanned 3 m y between about 47 7 ma and 44 5 ma as much as 6 5 km of volcanic and sedimentary rocks were deposited during that time although rates of extension and subsidence were very high intense volcanic activity continually filled the basin with ash flow tuffs outpacing subsidence and sedimentation until the end of basin development after the abrupt end of challis volcanism locally derived pebble to boulder conglomerate and massive reworked ash accumulated in the half graben these sedimentary rocks make up a small part of the basin fill in the panther creek half graben and were derived mainly from proterozoic metasedimentary rocks uplifted in the footwall of the basin the east southeast tilt of the sedimentary rocks their provenance and coarse grain size and the presence of a gravity slide block derived from tilted volcanic rocks in the hanging wall attest to continued tectonism during conglomerate deposition provenance data from the sedimentary rocks imply that the highland in the footwall of the panther creek half graben was never thickly blanketed by synextension volcanic rocks despite intense volcanic activity analysis of the panther creek half graben and other intra arc rift basins supports previous interpretations that relative rates of volcanism and subsidence control the proportion of volcanic rocks deposited in intra arc rifts

crustal velocity structure beneath the eastern flank of the ruby mountains metamorphic core complex results from normal incidence to wide angle seismic data abstract p wave velocity structure along the eastern side of the ruby mountains metamorphic core complex determined using 2 d ray inversion modeling shows that 1 the crust can be divided into three layers corresponding to the upper middle and lower crust 2 from east of ruby dome to the northern end of the seismic profile velocities of the upper crust range from 6 12 0 1 to 6 20 0 1 km s and near surface velocities range from 1 90 0 1 to 4 80 0 1 km s from this point to the southern end of the profile upper crustal velocities range from 5 80 0 1 to 6 25 0 1 km s and near surface velocities range from 3 01 0 1 to 4 80 0 1 km s 3 middle crustal velocities range from 6 35 0 15 to 6 45 0 15 km s 4 lower crustal velocities range from 6 60 0 15 to 6 80 0 15 km s and 5 depths to the moho vary irregularly between 30 5 and 33 5 km from interpretation of these results we conclude that 1 the transition in metamorphic grade from deep upper crustal rocks south of harrison pass to middle crustal rocks north of harrison pass does not correlate with an increase in seismic velocity near surface basement seismic velocities increase in the central part of the range near ruby dome well north of the major metamorphic transition 2 depths to the moho do not reflect local surface relief isostatic balance is achieved predominantly in the crust rather than by formation of a local crustal root and 3 the crust has been modified during extension by intracrustal processes rather than by large scale magmatic underplating

thermochronologic evidence of major tectonic denudation associated with detachment faulting northern ruby mountains east humboldt range nevada fission track ft thermochronology studies on igneous and metamorphic rocks from the ruby mountains east humboldt range metamorphic core complex provide important constraints on the timing and nature of major mid tertiary extension in northeast nevada rocks from within the ruby east humboldt detachment brittle ductile normal sense shear zone were analyzed the dominant lithology studied was variably mylonitic mafic orthogneiss nonmylonitic amphibolite from the top of the structurally lower migmatitic core and porphyritic biotite granodiorite from the oligocene circa 36 ma harrison pass pluton were also studied ft ages on apatite zircon and sphene except one 18 4 ma apatite from the basal portion of the detachment zone are concordant and range in age from 26 5 ma to 23 6 ma all ages overlap at 1 between 25 4 ma and 23 4 ma these data suggest that rocks of the upper and middle portions of the detachment zone cooled rapidly from temperatures above 285 c sphene closure temperature to below 70 c lower temperature limit of track stability in apatite near the beginning of the miocene minimum cooling rate of 40 c ma the length distribution of confined fission tracks in apatites from these exhumed middle crustal rocks is strikingly similar to those of rocks known to have cooled rapidly fish canyon ash flow tuff the lower part of the detachment zone as well as the underlying migmatitic core also cooled through sphene and zircon closure temperatures during this time interval but apparently only cooled to a temperature within the zone of partial annealing of apatite 70 130 c residence of these rocks in this zone led to a partial retention of fission tracks thus resulting in a reduced age confined track length distributions from apatite provide independent evidence of very rapid cooling but also indicate that the rocks of the lower detachment zone experienced a more protracted cooling history remained hotter longer than structurally higher levels of the zone ft data firmly establish the lower limit on the timing of mylonitization during detachment faulting in this area as 23 4 ma rapid cooling of the region is considered to reflect large scale tectonic denudation intracrustal thinning the vertical complement to crustal extension rocks originating in the middle crust 10 15 km were quickly brought near the surface along the ruby east humboldt detachment fault brittle ductile simple shear zone and juxtaposed against brittlely extended rocks deformed under upper crustal conditions

cretaceous to early tertiary paleogeography in the hinterland of the sevier thrust belt east central nevada the cretaceous to early tertiary hinterland of the sevier thrust belt in east central nevada underwent two periods of sedimentary basin development the older early cretaceous episode is characterized by basins that formed coeval with deformation within a belt of contractional structures basins were characterized by periods of through flowing fluvial drainage the younger late cretaceous to eocene episode of basin development was characterized by internally drained alluvial basins that subsequently expanded into more regional lacustrine basins basal alluvial detritus indicates that basin inception occurred as a result of rapid development of topographic relief syn sedimentary tectonism was characterized by horizontal extension due to attainment of equilibrium topography in response to crustal thickening

rapid eocene extension in the robinson district white pine county nevada constraints from 40ar 39ar dating precise 40ar 39ar ages on pretectonic syntectonic and posttectonic rhyolites from the robinson mining district which is within a highly extended domain in east central nevada indicate that extreme extension 400 occurred in fewer than 900 k y during the eocene the duration of extension is tightly delimited by an eocene lacustrine unit that was horizontal prior to extension and by younger rhyolitic units the rhyolites represent at least five eruptive episodes including the preextensional and synextensional rhyolite of white hill emplaced between 37 6 and 37 4 ma and the postextensional rhyolite of garnet hill emplaced at 36 7 ma although some of it is perhaps as old as 37 1 ma thus the duration of extreme extension at robinson was between 0 9 and 0 3 m y comparable to or shorter than other highly extended areas in western north america the extensional domain in east central nevada is a composite feature produced by localized episodic rapid extension that began in early eocene time and continued to at least middle miocene time

cretaceous early tertiary paleolatitude migration in the sanshui basin of south china and the evolution of the south china sea abstract the evolutionary model of the south china sea scs is discussed based on data from the sanshui basin which indicates the paleolatitude migration of south china the results show that south china played a prevailing role in the evolution of the scs which has been basically formed by the southward migration and later reversal of the south china continent in the course of extension faulting and breaking up in the south of south china during the late cretaceous to miocene the microcontinent fragments which are found scattered in the scs today were separated there when south china migrated back the philippine arc migrated northwards with philippine sea plate until the pliocene to form the east boundary of the scs the indochina block only played a transitional boundary role

differential stress strain rate and temperatures of mylonitization in the ruby mountains nevada implications for the rate and duration of uplift kno ledge of the m agnitude of the differential stress during the formation of mylonitic rocks provides constramts on mechanical and thermal models for the exhumation of the metamorphosed footwalls of major low angle detachment faults we have analyzed the differential flow stress during the mylonitization of quartzose rocks in the ruby mountains nevada using grain size piezometers and kinetic laws for grain g owth quartzites from mylonitic shear zones in lamoille canyon and secret creek gorge have grain sizes of 91 151 m and 42 64 m respectively the peak temperature during mylonitization was 630 i 50 c and analysis of grain growth kinetics indicates that mylonitization continued during cooling to temperatures s450 c quartz grain size piezometers suggest that the mylonitization occurred under differential stresses crl cr3 of 38 64 mpa or maximum shear stresses of 19 32 mpa extrapolation of quartzite flow laws indicates that the mylonitization occurred at strain rates between 10 10 and 10 13 1 arguments presented m the paper suggest that the likely range of strain rates is 10 11 to 10 12 1 these strain rates are compatible with displacement rates of the order of 23 mm yr l along a 1 5 km thick simple shear zone such a shear zone dipping 15 would produce an uplift rate of 5 8 km m y 1 and a horizontal extensioo rate of 22 km m y l this uplift rate indicates that midcrustal mylonitic rocks could have been lifted up along a 1 5 km thick simple shear zone dipping 15 in 2 6 m y

the mesozoic cenozoic tectonothermal evolution of the ruby mountains east humboldt range nevada a cordilleran metamorphic core complex a combined k ar and 40ar 39ar geochronological study of the ruby mountains east humboldt range nevada a cordilleran metamorphic core complex has revealed a complex tectonothermal history the three structural subdivisions of the core complex migmatitic core mylonitic zone and cover share magmatic and deformational histories in part but also display important contrasts in structural style metamorphic paragenesis and thermal history during the late jurassic ca 160 ma miogeoclinal rocks were polyphase deformed metamorphosed under amphibolite facies conditions and pervasively intruded by peraluminous granitic magmas thereby forming an igneous and metamorphic complex late jurassic muscovite granite porphyry plugs also intruded unmetamorphosed cover rocks late cretaceous cooling of the igneous and metamorphic complex is locally suggested by poorly resolved cretaceous incremental release 40ar 39ar hornblende spectra and by u pb monazite ages from pelitic schist exposed in the northern ruby mountains between ca 45 ma and 20 ma the igneous and metamorphic rocks experienced an episode of complex extensional tectonism which involved the development of low angle normal sense simple shear zones as a result rocks metamorphosed at depth in the mesozoic were translated to higher crustal levels which resulted in cooling through temperatures required for the intracrystalline retention of argon within minerals near concordance of hornblende and biotite plateau ages from the migmatitic core of the complex suggest relatively rapid exhumation however it is not certain if this cooling followed a prolonged maintenance at elevated temperatures after the ca 160 ma metamorphism or if distinct locally superposed late mesozoic and or middle tertiary thermal overprints had affected the cooling history a major extensional simple shear zone is marked by an approximately 1 kilometer thick mylonite zone that experienced late kinematic superposed brittle deformation a geometric consequence of the development of low angle extensional fault zones is the eventual translation of colder rocks over hotter rocks such a history is supported by the 40ar 39ar data for example mylonitic mafic orthogneisses from a major allochthon record 40ar 39ar hornblende plateau ages between 44 and 48 ma and biotite plateau ages between 32 and 33 ma and are structurally positioned above mylonitic rocks of the northern ruby mountains which record biotite cooling ages of ca 22 24 ma such relations suggest either multiple episodes of tertiary mylonitization or a protracted multiphase history of mylonitization however after ca 20 ma all of the presently exposed mylonitic rocks were maintained below 100 c and have only experienced upper crustal brittle deformation this article contains supplementary material

petrology of upper amphibolite facies marbles from the east humboldt range nevada usa evidence for high temperature retrograde hydrous volatile fluxes at mid crustal levels

crustal structure in the elko carlin region nevada during eocene gold mineralization ruby east humboldt metamorphic core complex as a guide to the deep crust the deep crustal rocks exposed in the ruby east humboldt metamorphic core complex northeastern nevada provide a guide for reconstructing eocene crustal structure 50 km to the west near the carlin trend of gold deposits the deep crustal rocks in the footwall of a west dipping normal sense shear system may have underlain the pinon and adobe ranges about 50 km to the west before tertiary extension close to or under part of the carlin trend eocene lakes formed on the hanging wall of the fault system during an early phase of extension and may have been linked to a fluid reservoir for hydrothermal circulation the magnitude and timing of paleogene extension remain indistinct but dikes and tilt axes in the upper crust indicate that spreading was east west to northwest southeast perpendicular to a paleozoic and mesozoic orogen that the spreading overprinted high geothermal gradients associated with eocene or older crustal thinning may have contributed to hydrothermal circulation in the upper crust late eocene eruptions upper crustal dike intrusion and gold mineralization approximately coincided temporally with deep intrusion of eocene sills of granite and quartz diorite and shallower intrusion of the harrison pass pluton into the core complex rocks stacked mesozoic nappes of metamorphosed paleozoic and precambrian rocks in the core complex lay at least 13 to 20 km deep in eocene time on the basis of geobarometry studies in the northern part of the complex the presently exposed rocks had been even deeper in the late mesozoic to 30 km depths before losing part of their cover by eocene time nappes in the core plunge northward beneath the originally thicker mesozoic tectonic cover in the north part of the core complex mesozoic nappes and tectonic wedging likely occupied the thickened midlevel crustal section between the deep crustal core complex intrusions and nappes and the overlying upper crust these structures as well as the subsequent large displacement cenozoic extensional faulting and flow in the deep crust would be expected to blur the expression of any regional structural roots that could correlate with mineral belts structural mismatch of the mineralized upper crust and the tectonically complex middle crust suggests that the carlin trend relates not to subjacent deeply penetrating rooted structures but to favorable upper crustal host rocks aligned within a relatively coherent regional block of upper crust

thermal evolution of a portion of the sevier hinterland the northern ruby mountains east humboldt range and wood hills northeastern nevada conventional thermobarometry and gibbs method modeling have been used to obtain new information pertaining to the mesozoic tertiary thermal evolution of the ruby mountains east humboldt range and wood hills metamorphic complexes northeastern nevada kyanite grade assemblages from clover hill in the extreme northeastern part of the east humboldt range were produced at temperatures of 780 810 k and depths in excess of 35 km previously published geochronologic data suggest an early cretaceous or late jurassic age for this event the clover hill area subsequently experienced a major unroofing interval prior to late cretaceous time during which it was brought roughly 20 km closer to the surface the lack of stratigraphic evidence for substantial cretaceous erosion in the region and the topology of the pressure temperature paths for the clover hill samples suggest that the unroofing mechanism may have been tectonic denudation related to the relaxation of large topographic gradients produced by crustal thickening in the hinterland of the sevier orogen metamorphic rocks at clover hill and within the wood hills core complex equilibrated at 820 900 k and 500 640 mpa 18 24 km in cretaceous time prior to 115 ma other parts of the ruby mountains east humboldt range metamorphic complex presently structurally separated from the clover hill sequence were intruded extensively by late cretaceous granitic magmas and attained substantially higher temperatures than the clover hill wood hills block sillimanite grade assemblages from these areas apparently underwent roughly 10 km of unroofing and 130 k of cooling prior to final equilibration at 820 920 k and 360 430 mpa 13 16 km during oligocene extension

mechanical stability of the triangle zone the backthrust wedge the triangle zone arrangement of faults that commonly develops at the foreland margin of fold and thrust belts is one of the few places in this structural setting that backthrusts of significant dimensions are observed even in this setting the backthrust may be a relatively ephemeral feature as it can be cannibalized by the foreland advance of thrusting in an active fold and thrust system one of the factors that determines the longevity of a specific backthrust and triangle zone in an active system is the mechanical stability of the backthrust wedge the wedge of rock that forms the hangingwall to the backthrust upper and lower stability limits for this backthrust wedge can be determined using the critical wedge concept for a cohesive coulomb material the configuration of the backthrust wedge developed in triangle zones along the southern and central alberta portion of the canadian cordillera agrees well with the stability limits derived for reasonable rock properties and normal fluid pressures in contrast a low taper triangle zone interpretation for northeastern british columbia does not fall within the calculated stability limits even when abnormally high fluid pressures and extreme rock properties are assumed however a similar low taper geometry may be achieved by the shingling of stable backthrust wedges upper crustal delamination and passive roof duplexing are both processes that serve to sustain a mechanically stable backthrust wedge configuration suggesting that they are both viable paths for the progressive advance of the mountain front triangle zone

mesozoic evolution hinterland of the sevier orogenic belt west of the mesozoic foreland sevier orogenic belt in northern utah and southern idaho a regional detachment of inferred mid jurassic age occurs in upper mississippian shale and can be traced westward to the margin of the coeval magmatic arc represented by spaced calc alkalic intrusions the detachment may be the oldest and structurally highest intrusions the detachment may be the oldest and structurally highest thrust of the idaho wyoming thrust belt if so it bridges the apparent spatial and temporal gap between magmatism and thrusting thus suggesting a direct genetic link thin skinned deformation is interpreted here to be driven from the heated magmatic arc toward the colder craton by lateral tectonic compression created by lateral volume increase due to intrusion elevated crustal geotherms and possible thickening in the arc the metamorphic complexes occur at the crustal interface between these two thermal tectonic regimes and may themselves be allochthonous

late paleozoic extension in the great basin western united states geologic mapping in the toiyabe range in central nevada has revealed the existence of normal faults of probable mid mississippian to early permian age that strike roughly east west and dip northward additional evidence of uplift and erosion followed by mafic volcanism and subsidence suggests that much of the central and southern toiyabe range was affected by late paleozoic extension similar patterns of late paleozoic uplift and subsidence together with local basaltic volcanism are widespread in the western united states suggesting that the continental margin was dominated by extension or transtension in mississippian to permian time this extension was coeval with convergence between north america and south america across the ouachita and marathon belts and the dynamic interaction of these two margins may by analogy with the cenozoic tectonics of asia have given rise to complex late paleozoic deformation in the ancestral rocky mountains and adjacent areas of the interior western united states

mesozoic structural and metamorphic history of the central ruby mountains metamorphic core complex nevada the central ruby mountains experienced relatively little deformation during tertiary uplift of the ruby mountains east humboldt range metamorphic core complex and so mesozoic fabrics are well preserved three penetrative deformational events and two amphibolite facies metamorphic events have been identified all of which are interpreted to have occurred during emplacement of a suite of two mica granites dated at 153 1 ma late jurassic phase equilibria indicate that there was an increase in both temperature and pressure between m 1 410 580 c 2 6 3 75 kb and m 2 540 660 c 3 5 4 7 kb peak m 2 pressures suggest a jurassic paleodepth of 13 18 km 3 8 km more than the known thickness of the overlying stratigraphic column the late jurassic therefore appears to have been a time of major crustal shortening and thickening in northeastern nevada in contrast there are no cretaceous penetrative fabrics in the central ruby mountains suggesting that the sevier orogeny was not a significant event in the area this lack of cretaceous deformation in the mid crustal rocks of the ruby mountains argues against recent models that link shortening in the sevier belt to emplacement of the sierra nevada batholith via a regional mid crustal shear zone extending across the hinterland

evidence for large scale transport on the bullard detachment fault west central arizona the bullard detachment fault is a gently to steeply dipping normal fault that flanks the harcuvar and harquahala mountains of the basin and range province in west central arizona the stratigraphy of upper plate miocene conglomerates and the regional distribution of upper and lower plate pre tertiary units indicate that upper plate rocks were displaced about 50 km to the northeast with respect to the lower plate during middle to late tertiary time normal slip of this magnitude on the regionally northeast dipping bullard fault indicates that deep seated tertiary cretaceous mylonitic gneisses and mesozoic thrust faults of the lower plate were drawn out from beneath precambrian rocks along the margin of the transition zone of central arizona during middle to late tertiary crustal extension the transition zone was therefore affected by deep seated tectonism in both mesozoic and tertiary time