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

nature and origin of seismic reflection fabric ruby east humboldt metamorphic core complex nevada seismic reflection profiling across exposed upper and middle crustal rocks of the ruby east humboldt metamorphic core complex delineates important characteristics of the crustal structure developed during tertiary extensional deformation the goals of this study were to trace a tertiary extensional shear zone from mylonitic surface outcrops into a seismic section and to characterize the deeper crustal fabric associated with the polyphase deformational history of the complex reflections in the shallow surface correlate with a plastic to brittle shear zone that formed during the tectonic unroofing of the middle crustal rocks constructive interference from strong planar layering in the mylonitic shear zone is considered chiefly responsible for generating the reflections east and southwest dipping reflectors in the seismic section appear to correlate with exposed opposing dipping mylonitic foliation domains the opposing dips of the mylonitic layering may reflect warping of the normal sense shear zone during tectonic exhumation coupled with an overall anastomosing character deeper in the section a heterogeneous reflection character correlates with increases in velocity shown from wide angle measurements in this region and is interpreted as penetrative fabric originating from extensional flow facilitated by broad scale pure shear and localized simple shear lower crustal rocks apparently achieved granulite facies metamorphism during extension a maximum of 6 8 km of mafic material could have been added to the crust during cenozoic extension the moho reflection has such a high amplitude that it may be caused by partially molten rocks interleaved with peridotite reflections from the moho show no significant upwarping of the base of the crust beneath the core complex this suggests that the lower crustal configuration is at least as young as the period of extensional activity that lasted from approximately 40 to 20 ma which was responsible for the exhumation of the metamorphic core complex

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

smnd isotopic and geochemical characterisation of the paleoproterozoic torngat orogen labrador canada abstract the torngat orogen is a paleoproterozoic granulite facies belt that welds the archean components of the nain and rae provinces in eastern labrador sm nd isotopic and supporting bulk rock major trace and rare earth element data help elucidate the sources of the main paleoproterozoic lithologies that comprise the orogen in the foreland paleoproterozoic ramah group metasediments intercalated with reworked nain province gneiss show variable trace element and nd 1 9 ga from 2 9 to 12 0 ranging between nain province crust and paleoproterozoic mugford group basalts compositions the ramah group data suggest derivation of ramah detritus from archean nain province gneisses and from the mugford group in the lac lomier complex rae province hinterland metasediments with variable trace element distribution patterns akin to rae province crust and nd 1 9 ga from 2 9 to 10 5 suggest derivation from rae province crust and from an unspecified juvenile component in the core of the orogen pelitic and psammitic protoliths of the tasiuyak gneiss nd 1 9 ga from 1 9 to 2 6 were derived from crustal sources unknown in both nain and rae provinces and may have been deposited in a trench environment starved of adjacent cratonic debris dioritic to granitic eastern and western metaplutonic suites intruding the tasiuyak gneiss and the lac lomier complex display contrasting geochemical and sm nd isotopic compositions the eastern metaplutonic suite has a tendency toward lower sio2 total alkalis less fractionated ree and higher mgo cao and tio2 relative to the western metaplutonic suite the eastern suite yields nd 1 9 ga from 3 8 to 6 7 and negative f sm nd suggesting igneous precursors from rae crust and a a lree enriched juvenile component the western suite yields nd 1 9 ga from 1 5 to 8 5 suggesting an important contribution from rae crustal sources in the petrogenesis of the igneous protoliths the presence of rae province nd isotopic compositions and the absence of nain province signatures in these intrusions favors the eastern rae province as the active margin during nain rae convergence

evolving geographic patterns of cenozoic magmatism in the north american cordillera the temporal and spatial association of magmatism and metamorphic core complexes four maps are presented here that show the location and extent of magmatic fields between eastern alaska and northern mexico during the successive time intervals of 55 40 40 25 25 10 and 10 0 ma and four others show the distribution of metamorphic core complexes during the same cenozoic time intervals the maps are based on u s geological survey and canadian cordilleran data bases contining about 6000 isotopic dates and extensive literature review for nearly 60 ma the development of metamorphic core complexes has coincided with the locus of a really extensive and voluminous intermediate composition magmatic fields the association is suggestive of a close link between magmatism and core complex formation namely that magma directly and indirectly lowers the strength of the crust magmatism thus controls the location and timing of core complex formation the stresses responsible may be inherited from mesozoic crustal thickening locally created by uplift and magmatic thickening of the crust and imposed by the global pattern of plate motions and driving forces since the miocene rates of magmatism extension and core complex formation have declined the modern basin and range province is not a suitable model for the situation that existed during major magmatic culminations the singular event of early miocene time the merging of two large magmatic fields extinguishing the laramide magmatic gap explains several disconnected observations the hyperextension episode of the colorado river corridor rapid reorientation of stress patterns across much of western north america and subsequent rapid tectonic movements in california magma triggered breakup of western north america lithosphere coincided with development of the san andreas transform system thermal destruction of the laramide magmatic gap created a california microplate about 22 ma ago that moved rapidly away from north america thus two plate tectonic processes thermal destruction of the lithosphere bridge and northward growth of a transform system interacted to produce miocene and later tectonic patterns and events

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

two dimensional travel time inversion for the crustal p and s wave velocity structure of the ruby mountains metamorphic core complex ne nevada problems about the nature of extended crust in the basin and range include the role of plastic flow during extension the possibility of fluids in the crust and the amount of mantle derived material added to the crust to address these problems the university of wyoming conducted a multicomponent wide angle seismic experiment in the ruby mountains of the basin and range the northern ruby mountains expose upper and middle crustal rocks of a metamorphic core complex whereas the southern ruby mountains consist of low grade miogeoclinal rocks the wide angle experiment consisted of a 95 km long n s profile that extended from the southern to the northern ruby mountains travel time inversion of the wide angle reflection data reveals relatively high seismic velocities in the midcrust to lower crust which suggest that fluid filled pores if they exist do not reduce the seismic velocities significantly therefore the midcrustal to lower crustal porosity is probably much smaller than 1 2 vol the average p wave velocity for the crust is 6 4 6 5 km s in the north and 6 1 6 2 km s in the south this difference suggests that larger amounts of mantle derived material were added during the tertiary to the core complex crust of the northern ruby mountains than to the normal basin and range crust of the southern ruby mountains the velocity profile is consistent with a maximum of 6 7 km of mantle derived material in the south and a maximum of 12 15 km of mantle derived material in the north thus volumetric mafic intrusions probably accommodated a maximum of 7 km of core complex exhumation shear wave splitting indicates that the upper to middle crust is seismically anisotropic probably due to superposed tertiary plastic flow patterns doming and inflation of a midcrustal layer in the seismic model are compatible with tertiary flow of material from the south to the north that compensated about 2 5 km of core complex exhumation precritical moho reflections 0 70 km offset 10 11 s normal two way travel time are weaker and less continuous than midcrustal reflections 7 9 s normal two way travel time strong and continuous moho reflections occur at postcritical 80 km offsets only modeling shows that the moho depth increases from about 32 5 km in the south to about 34 5 km in the north

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

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

carlin gold deposits nevada origin in a deep zone of mixing between normally pressured and overpressured fluids gold mineralization at carlin is clearly younger than hydrocarbon maturation pre cretaceous and felsic dike intrusion cretaceous and older than deep oxidation late tertiary within the episode of gold mineralization the main gold ore mgo stage and late gold ore ego stage are distinguished paragenetically with a variety of vein and mineralization types in each mgo stage fluids contained 5 to 10 mole percent co 2 appreciable h 2 s and 3 or 1 wt percent nacl equiv at least portions of mgo stage mineralization were characterized by two phase boiling co 2 exsolution at 215 degrees or 30 degrees c and 800 or 400 bars in contrast ego stage fluids were gas poor with salinities 18 o sub h 2 o values of 5 to 9 per mil whereas ego stage fluids resembled unevolved meteoric water with delta 18 o sub h 2 o values 18 o values shifted from near whole rock values of 12 or 3 per mil to around 0 or 1 per mil as ego stage fluids flooded the system jasperoids also record a large range 9 22 in delta 18 o sub h 2 o values these data indicate the involvement of two very different fluids in ore deposition because mgo and ego stage features are closely associated spatially with each other and with au as sb hg and other ore elements both fluids are believed to have both been present during most stages of ore deposition at pressures of 80 to 85 percent lithostatic depths of 3 8 or 1 9 km are required to accommodate the 800 or 400 bars of pressure recorded in mgo stage fluid inclusions carlin therefore is not an epithermal or hot spring deposit carbon dioxide in gas rich mgo stage fluids may have originated either directly from buried intrusions or their contact aureoles or from low grade regional metamorphism at depth the water may have been originally meteoric and au may be magmatic or derived from leaching of deep metamorphic or sedimentary rocks ore deposition appears to have occurred in zones of throttling at a pressure seal between normally pressured and overpressured regimes where fluids experienced a change from near lithostatic to hydrostatic conditions such pressure seals are common in deep sedimentary basins and may be a key to highly localized gold deposition mixing of two fluids and interaction with host rocks along thin permeable bioclastic horizons are believed to have been the major factors in depositing ore

mid crustal flow during tertiary extension in the ruby mountains core complex nevada structural analysis and geochronologic data indicate a nearly orthogonal late eocene oligocene flow pattern in migmatitic infrastructure immediately beneath the kilometer thick extensional mylonitic shear zone of the ruby mountains metamorphic core complex nevada new u pb radiometric dating indicates that the development of a northward trending lineation in the infrastructure is partly coeval with the development of a pervasive west northwest trending lineation in the mylonitic shear zone u pb monazite data from the leucogranite orthogneiss of thorpe creek indicate a crystallization age of ca 36 39 ma zircon fractions from a biotite monzogranite dike yield an age of ca 29 ma the three dated samples from these units exhibit a penetrative approximately north south trending elongation lineation this lineation is commonly defined by oriented bundles of sillimanite and or elongated aggregates of quartz and feldspar indicating a synmetamorphic and syndeformational origin the elongation lineation can be interpreted as a slip line in the flow plane of the migmatitic nonmylonitic infrastructural core of the northern ruby mountains a portion of this midcrustal flow is coeval with the well documented west northwest sense of slip in the structurally overlying kilometer thick mid tertiary mylonitic shear zone lineations in the mylonitic zone are orthogonal to those in the deeper infrastructure suggesting fundamental plastic decoupling between structural levels in this core complex furthermore the infrastructure is characterized by overlapping oppositely verging fold nappes which are rooted to the east and west one of the nappes may be synkinematic with the intrusion of the late eocene orthogneiss of thorpe creek in addition the penetrative elongation lineation in the infrastructure is subparallel to hinge lines of parasitic folds developed synchronous with the fold nappes suggesting a kinematically related evolution the area is evaluated in terms of a whole crust extension model magmatic underplating in the lower crust stimulated the production of late eocene early oligocene granitic magmas which invaded metasedimentary and mesozoic granitic rocks of the middle crust the midcrustal rocks weakened by the magmatic heat influx acted as a low viscosity compensating material decoupled from an extending upper crust the fold nappes and lineation trends suggest large scale flow of the weakened crust into the study area the inflow pattern in the migmatitic infrastructure can be interpreted as a manifestation of midcrustal migration into an area beneath a domain of highly extended upper crustal rocks at present the inferred eocene early oligocene phase of upper crust extension remains unknown but available data on relative and geochronologic timing are not inconsistent with our model of return flow into an area already undergoing large scale upper crustal extension

a special issue devoted to gold deposits in northern nevada part 2 carlin type deposits this is the second of two special issues of economic geology devoted to gold deposits in northern nevada readers interested in a general overview of these deposits their economic significance their context within the tectonic evolution of the region and synoptic references on each gold deposit type are directed to the preface of the first special issue john et al 2003 volume 98 issue 2 contains five papers that address regional aspects important to the genesis of gold deposits in northern nevada and five papers that present detailed studies of epithermal deposits and districts all of the regional papers are pertinent to carlin type gold deposits because they address the age of mineralization arehart et al 2003 origin and evolutionary history of the northwest striking mineral belts that localize many deposits grauch et al 2003 nature of the middle and lower crust below these mineral belts howard 2003 district to deposit scale stream sediment and lithogeochemical anomalies theodore et al 2003 and stratigraphy and structure of a district located along a northeast striking lineament peters et al 2003 the nine papers in this second special issue focus on an array of problems pertinent to genetic and exploration models for carlin type deposits in northern nevada fig 1 these investigations sort out and characterize the sequence of deformational igneous and hydrothermal events in mining districts constrain the age of mineralization map paleothermal gradients identify structures and lithologies that are preferentially mineralized ascertain processes of ore formation determine sources of ore fluid components and define fluid flow paths a common theme among these papers is inheritance whereby older features in the mineral belts influence ore formation in subsequent carlin type hydrothermal systems three types of inheritance are inferred by one or more of these investigations 1 structural inheritance where older faults are reactivated during subsequent contractional and or

magnetic structural and geochronologic evidence bearing on volcanic sources and oligocene deformation of ash flow tuffs northeast nevada magnetic properties of mid tertiary volcanic rocks west of jiggs in northeast nevada were investigated for the purposes of interpreting igneous structural and tectonic processes in this part of the basin and range province anisotropy of magnetic susceptibility ams patterns reflect flow fabrics and suggest previously unknown sources for these ash flow tuffs paleomagnetic and structural evidence suggest counterclockwise relative rotation of the southern part of the area with respect to the north new stratigraphic chemical and k ar isotopic data support these interpretations dacite to rhyolite ash flow tuffs of the indian well formation were subdivided into two main units the lower and predominant tuff of jiggs 35 8 37 0 ma and the unconformably overlying but areally restricted tuff of hackwood 30 8 ma the jiggs unit has two polarities whereas the hackwood has only a reversed polarity remanence together these units with tilt correction define a pole 92 e 87 n dp 6 dm 8 for approximately 30 37 ma this pole is concordant with coeval north american reference poles indicating that this zone of approximately 30 km ns extent has not undergone significant vertical axis rotation relative to the north american reference andesite lavas of normal magnetic polarity and of 32 5 ma age characterize the diamond hills immediately to the south we interpret this region from both structural evidence and discordant paleomagnetic direction to have rotated approximately 25 counterclockwise relative to the indian well volcanic units to the north the apparent rotation of the diamond hills is possibly the result of drag on the left lateral garcia fault which limits the diamond hills on the southwest analysis of ams data suggests by patterns of the k1 axes two distinct sources for the jiggs unit a northern buried source and a central partially buried source lithologic evidence consistent with proximal vent facies is found near the latter source an unusual pattern in the ams k3 axes suggested as resulting from the tilt correction applied to initial dip is consistent with the interpreted flow directions tight age constraints can be placed on the deformation of the main ash flows of the jiggs unit in the 35 8 to 30 8 ma interval because the diamond hills unit is slightly younger and appears to have been affected by the same deformative episode as the jiggs the regional deformation affecting both units can be more narrowly confined to the 32 5 30 8 ma interval this deformation is inferred to coincide with the major early tertiary episode of crustal extension in this region of the basin and range

structural and thermal constraints on the initiation angle of detachment faulting in the southern basin and range the chemehuevi mountains case study the cenozoic normal fault system exposed in the chemehuevi mountains of the southern cordillera provides constraints on the initiation angle and geometry of an extensional fault system that has accommodated extreme crustal stretching there three stacked brittle low angle normal faults that formed at depths as great as 10 12 km cut gently down section northeastward through deformed proterozoic and mesozoic crystalline basement hanging wall blocks are displaced relatively northeastward the upper crust above the chemehuevi detachment fault was pulled apart along high angle normal faults that rotated to more gentle dips through time in contrast rocks of originally mid crustal affinity in the footwall were only gently rotated and accommodated minor extension application of 40 ar 39 ar and fission track thermochronology to rocks in the footwall of the chemehuevi detachment fault system provides further constraints on the timing and initiation angle of regional detachment faulting at the onset of extension between 22 and 24 ma granitic rocks exposed in the southwestern and northeastern portions of the footwall were at 200 c and 350 400 c respectively separated by a distance of some 23 km down the known slip direction this gradual increase in temperature with original depth is attributed to the gentle southwest tilting of broadly planar pre extension isothermal surfaces and constrains the exposed part of the chemehuevi detachment fault to have had a regional dip initially of about 15 to 30 the fault system apparently cut gently down through the upper crust to a minimum depth of 10 12 km the deepest exposed parts of the system today and was domed from midcrustal depths and locally denuded during continued slip together the structural and thermochronologic data confirm the suggestion that faults accommodating large magnitude slip can be initiated and move within the seismogenic regime at moderate to low angles that is 30

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

eocene magmatism the heat source for carlin type gold deposits of northern nevada the origin of carlin type or sediment hosted disseminated gold deposits of the great basin the major source of gold in the united states is poorly understood we propose that eocene magmatism was the heat source that drove the hydrothermal systems that generated these deposits in the carlin trend and independence mountains in northern nevada this interpretation is based on a strong spatial and temporal association of eocene intrusive volcanic centers with the gold deposits of this region our new work and published 40 ar 39 ar dates indicate that magmatism was particularly intense between 39 and 40 ma throughout northeastern nevada especially in and around the area of gold deposits carlin type deposits may have formed preferentially during eocene magmatism because it was 1 more intense in the area than other magmatic episodes 2 somehow compositionally distinct or 3 accompanied by extension that promoted hydrothermal flow however large scale extension does not appear to have been a factor in generating carlin type deposits

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

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

geophysical and isotopic constraints on crustal structure related to mineral trends in north central nevada and implications for tectonic history we combined information from sr and pb isotope data and magnetotelluric models to develop a new magnetic and gravity interpretation of the crustal structure of north central nevada to better understand the origin of mineral trends the new interpretation suggests a crustal structure that is composed of precambrian continental crust transitional crust and primarily oceanic crust that are separated by northwest and northeast striking fault zones the magnetic expression of the buried precambrian continental crust is recognized for the first time low magnetic values primarily reflect magnetite poor crystalline crust rather than elevated temperatures at depth northwest and northeast striking crustal boundaries are defined by isotopic data and abrupt gradients in gravity and magnetic data the carlin and battle mountain eureka mineral trends are associated with two of three northwest striking boundaries the carlin boundary is primarily defined by a change in density and isotopic character of the lower to middle crust the battle mountain eureka boundary coincides with a density contrast in the upper crust and a change in isotopic character in the lower to middle crust magnetotelluric models suggest that the battle mountain eureka boundary represents a crustal fault zone for most of its extent but that deep rooted faulting is more complex near and northwest of battle mountain crustal fault zones inferred from the magnetotelluric models near the carlin trend are oblique to it suggesting that they may not have been controlled by the deep boundary seen in the gravity and isotopic data the third northwest trending boundary is related to the western edge of the buried precambrian continent in west central nevada but lacks an associated mineral trend a northeast striking boundary forms the northern limit of precambrian continental and transitional crust the boundaries may have originated as rift or transform faults during precambrian breakup of rodinia or as faults accommodating lateral movements or accretion during later paleozoic tectonic events comparing the crustal structure to tectonic elements produced by successively younger events shows that it had a profound influence on subsequent sedimentation deformation magmatism extension and most important mineralization

laser probe 40 ar 39 ar dating of strain fringes mid cretaceous synconvergent orogen parallel extension in the interior of the sevier orogen 1 uv and co2 laser probe 40ar 39ar in situ analyses of phlogopite and muscovite in fibrous strain fringes from greenschist facies metamorphic rocks document mica growth ages at temperatures lower than their closure temperatures and therefore directly date deformation the new dates resolve the age of the earliest ductile fabric recorded in the raft river albion grouse creek metamorphic core complex of utah and idaho phlogopite was dated in quartz calcite phlogopite strain fringes around pyrite in pennsylvanian permian rocks from the grouse creek mountains utah using both the uv and co2 laser probe muscovite was dated in quartz muscovite strain fringes around pyrite in deformed jurassic sills from the black pine mountains idaho using the co2 laser probe phlogopite 40ar 39ar ages for individual strain fringes grouse creek mountains range from 92 ma to 110 ma with the most reliable ages ranging from 101 ma to 110 ma mean age 105 0 5 8 ma muscovite 40ar 39ar ages for individual strain fringes black pine mountains range from 97 ma to 112 ma mean age 104 7 5 8 ma strain fringes are associated with a subhorizontal foliation and a generally n trending elongation lineation exhibiting components of top to the north simple shear and coaxial strain accommodating n s extension and subvertical shortening midcrustal northward flow at 105 6 ma within the interior of the sevier orogen coeval with east directed shortening in the foreland and with plate convergence records orogen parallel synconvergent extension we favor gravitational relaxation of structural culminations resulting from focused crustal shortening as a driving mechanism for orogen parallel flow

evidence for large magnitude post eocene extension in the northern shoshone range nevada and its implications for the structural setting of carlin type gold deposits in the lower plate of the roberts mountains allochthon the northern shoshone and toiyabe ranges in north central nevada expose numerous areas of mineralized paleozoic rock including major carlin type gold deposits at pipeline and cortez paleozoic rocks in these areas were previously interpreted to have undergone negligible postmineralization extension and tilting but here we present new data that suggest major post eocene extension along west dipping normal faults tertiary rocks in the northern shoshone range crop out in two w nw trending belts that locally overlie and intrude highly deformed lower paleozoic rocks of the roberts mountains allochthon tertiary exposures in the more extensive northern belt were interpreted as subvertical breccia pipes intrusions but new field data indicate that these pipes consist of a 35 8 ma densely welded dacitic ash flow tuff informally named the tuff of mount lewis interbedded with sandstones and coarse volcaniclastic deposits both tuff and sedimentary rocks strike n s and dip 30 to 70 e the steeply dipping compaction foliation in the tuffs was interpreted as subvertical flow foliation in breccia pipes the southern belt along mill creek previously mapped as undivided welded tuff includes the tuff of cove mine 34 4 ma and unit b of the bates mountain tuff 30 6 ma these tuffs dip 30 to 50 east suggesting that their west dipping contacts with underlying paleozoic rocks previously mapped as depositional are normal faults tertiary rocks in both belts were deposited on paleozoic basement and none appear to be breccia pipes we infer that their present east tilt is due to extension on west dipping normal faults some of these faults may be the northern strands of middle miocene ca 16 ma faults that cut and tilted the 34 0 ma caetano caldera 40 east in the central shoshone range 5 km south of mill creek but further mapping is necessary to trace the faults through the highly deformed paleozoic rocks that surround the isolated tertiary outcrops significant post eocene extensional faulting in the northern shoshone range may have important implications for both the structure of the roberts mountains allochthon and the exposure of potentially mineralized rocks in its lower plate both of which were likely east tilted and repeated by west dipping faults together with overlying tertiary rocks

rheological control on the initial geometry of the raft river detachment fault and shear zone western united states the strain exhumation history and field orientation of a well exposed shear zone and detachment fault in the raft river mountains of northwestern utah a cordilleran metamorphic core complex have been studied to determine the kinematics of ductile shearing and initial orientations of the shear zone and detachment fault mapping and strain and kinematic analysis indicate that the top to the east raft river shear zone initially developed parallel to an unconformity separating archean rocks from overlying proterozoic quartzite and schist for at least 24 km in the shear direction experimental rock deformation data from lithologies similar to the archean and proterozoic rocks suggest the unconformity represented a significant rheological boundary at the deformation temperatures the base of the shear zone was localized along the boundary between relatively weak quartzite above and stronger monzogranite below an extensive thermochronological database is used to reconstruct the position of the basement unconformity in temperature lateral distance coordinates the initial average dip of the shear zone and basement unconformity is estimated between 7 and 30 assuming subhorizontal isotherms and geothermal gradients of 20 40 c km the east dip of the unconformity at the onset of miocene extension is interpreted to have resulted from late eocene unroofing and flexure beneath a top to the wnw extensional shear zone in the western raft river grouse creek and albion mountains the observations from the raft river shear zone suggest that the orientation of some midcrustal shear zones may not reflect the predicted orientation for ductile faults according to ductile failure criteria but rather the orientation of rheological boundaries along which deformation is localized furthermore detachment faults that are superimposed on mylonite during progressive displacement and footwall unroofing may use an inherited mechanical anisotropy from the mylonite and their orientations may not reflect the predicted orientation of shear fractures in isotropic rock the common parallelism between detachment faults and mylonitic foliation may indicate a mechanical and kinematic preference for localization of throughgoing brittle faults parallel to preexisting mylonitic foliation because of this preference studies restricted to detachment faults which lack footwall mylonite or restricted to structural levels between the breakaway and mylonitic front have more bearing on the question of the initial dip of normal sense shear fractures faults within the seismogenic crust

age constraints on jerritt canyon and other carlin type gold deposits in the western united states relationship to mid tertiary extension and magmatism carlin type gold deposits are difficult to date and a wide range of ages has been reported for individual deposits therefore several methods were employed to constrain the age of the gold deposits in the jerritt canyon district dated igneous rocks with well documented crosscutting relationships to ore provided the most reliable constraints k ar and 40 ar 39 ar dates on igneous rocks are as follows andesite dikes 324 ma sericitic alteration in andesite dikes 118 ma basalt dikes 40 8 ma quartz monzonite dikes 39 2 ma and calc alkaline ignimbrites 43 1 to 40 1 ma of these only the andesite and basalt dikes are clearly altered and mineralized the gold deposits are therefore younger than the 40 8 ma basalt dikes the sericitic alteration in the andesite dikes is unrelated to the gold deposits a number of dating techniques did not work k ar and 40 ar 39 ar dates on mica from mineralized ordovician to devonian sedimentary rocks gave misleading results the youngest date of 149 ma from the smallest 187 os 188 os initial 0 9 3 6 but did not yield a meaningful isochron due to very low re and os concentrations and large analytical uncertainties paleomagnetic dating techniques failed because the hydrothermal fluids sulfidized nearly all of the iron in the host rocks leaving no remnant magnetism when published isotopic dates from other carlin type deposits in nevada and utah are subject to the rigorous evaluation developed for the jerritt canyon study most deposits can be shown to have formed between 42 and 30 ma k ar and 40 ar 39 ar dates on the youngest preore igneous rocks range from 41 to 32 ma whereas the oldest postore igneous rocks range from 35 to 33 ma hydrothermal adularia from the twin creeks deposit yields similar 40 ar 39 ar dates of 42 ma k ar dates on supergene alunite range from 4 to 30 ma k ar and 40 ar 39 ar dates on micas separated from sedimentary 395 43 ma and igneous 145 38 ma rocks are usually much older than the gold deposits and most are suspect because they are from incompletely reset preore micas or from mixtures of preore and ore stage mica fission track dates on zircons are also generally older than the deposits 169 35 ma and are not completely reset by mineralization apatites are likley to be reset by the hydrothermal systems and by younger thermal events and yield dates 83 22 ma that are younger than those from zircon independent support of a mid tertiary age is provided by the low delta d sub h 2 o values of hydrothermal fluids from 10 widely separated carlin type deposits 134 or 23ppm the low delta d sub h 2 o values are consistent with a mid tertiary age when the climate was cool but are inconsistent with the cretaceous and jurassic ages favored by some workers when the climate was warm the age and distribution of carlin type deposits coincides with areas that underwent extension and calc alkaline magmatism over the same time period despite this relationship the deposits lack clear spatial or genetic relationships to mid tertiary epizonal plutons rather many deposits are localized along preexisting crustal fault zones in the getchell trend structures tapped deep sourced metamorphic or magmatic fluids and subsequently variably exchanged meteoric water in the other trends and districts only variably exchanged meteoric water has been detected these relationships suggest either that all of the deposits formed from deep sourced fluids and were subsequently flooded by meteoric water or that fluids from different sources evolved to produce deposits with similar charateristics

timing of cenozoic volcanism and basin and range extension in northwestern nevada new constraints from the northern pine forest range eocene middle miocene volcanic rocks in the northern pine forest range nevada are ideally situated for reconstructing the timing and style of volcanism and extensional faulting in the northwesternmost part of the basin and range province a conformable sequence of cenozoic volcanic and sedimentary strata in the northern pine forest range dips 30 w and 11 new 40 ar 39 ar ages from this sequence defi ne 3 major episodes of volcanic activity pre tertiary basement and older ca 38 ma tertiary intrusive rocks are overlain unconformably by oligocene ca 30 23 ma basalt fl ows and dacitic to rhyolitic ash fl ow tuffs interbedded with fi ne grained tuffaceous sedimentary rocks oligocene rocks are overlain by 550 m of ca 17 16 ma basalt fl ows equivalent to the steens basalt in southern oregon and basalt fl ows are capped by a thin 16 3 ma ignimbrite that likely is correlative with either the idaho canyon tuff or the tuff of oregon canyon the northern pine forest range is bounded to the east by a major down to the east normal fault that dips 40 e with well developed fault striations indicative of dip slip motion this fault initiated at an angle of 70 and was rotated 30 during uplift of the range a suite of 17 apatite fi ssion track ages from the pine forest footwall block demonstrates that exhumation uplift and slip on the range bounding fault began ca 12 11 ma and continued until at least 7 ma with moderate slip since then the pine forest range did not undergo signifi cant extension before or during peak oligocene and miocene volcanism and similar geologic relationships in nearby ranges suggest that a larger region of northwestern nevada was also little extended during this interval basin and range faulting in northwestern nevada appears to have begun no earlier than 12 ma making it distinctly younger than deformation in much of central and southern nevada where peak extension occurred in the middle miocene or earlier

tertiary structural development of selected valleys based on seismic data basin and range province northeastern nevada reflexion seismic data in railroad diamond mary s river and goshute valleys provide information on their structural development that cannot be deduced solely from outcrop and well data these valleys contain tertiary sediments that in dip section define an asymmetrical basin bounded along the eastern flank by a major listric normal fault with about 3 0 4 5 km of displacement the west flank is defined by a gentle east dipping ramp seismically the trace of the listric fault is interpreted to dip westward and sole into the palaeozoic section exploiting regionally recognized mesozoic decollement surfaces the tertiary depocentre adjacent to this fault shifted from west to east with continued slippage through time the greatest movement occurring in the miocene and post miocene in the strike direction the valleys are separated into at least two subbasins by an east west structurally high axis the axis is postulated to be the result of a tear fault associated with movement along the listric normal fault tertiary stratigraphy varies between valleys and between sub basins in a given valley all the valleys contain miocene and younger rocks however not all sub basins contain the pre miocene section suggesting a complex scheme of structural development

thrust emplacement of the schoonover sequence northern independence mountains nevada preliminary paleontologic data from the northern independence mountains nevada suggest that at least part of the schoonover sequence is time equivalent to the lithologically and structurally similar upper paleozoic havallah sequence the base of the schoonover is a major thrust fault that juxtaposes paleogeographically distant but in part time equivalent rocks along the west side of the range the schoonover sequence structurally overlies late mississippian nonmarine to shallow marine rocks that unconformably overlie deformed ordovician strata of the roberts mountains allochthon this nonmarine to shallow marine sequence is correlative to overlap assemblage sequences elsewhere in nevada along the east side of the range the schoonover is thrust over shallow marine to slope facies permian rocks that unconformably overlie deformed lower paleozoic miogeoclinal rocks as well as the roberts mountains allochthon the allochthonous schoonover sequence is a heterogeneous assemblage of deformed deep water sedimentary rocks and greenstone units isoclinally folded chert in the structurally lower half of the schoonover yields ages based on radiolaria that range from earliest mississippian to possibly latest devonian at the very base of the sequence to as young as early pennsylvanian in structurally higher parts of the sequence clastic rocks within the lower part of the sequence contain debris derived from erosion of roberts mountains allochthon type rocks rare volcaniclastic and volcanogenic units in the schoonover indicate an andesitic dacitic source the structurally higher part of the allochthon contains a thick succession of silty limestone turbidite deposits that represent yet a third carbonate source terrane for the schoonover sequence structural data indicate southeast directed thrusting of the schoonover sequence thrusting postdates deposition of late permian rocks and is inferred to predate intrusion of late jurassic plutons present to the north of the map area similarity of structural style and constraints on the timing of thrusting in the independence mountains strongly suggest that the schoonover sequence may represent the northeasternmost exposures of the golconda allochthon the age of at least some of the pelagic sediments in the schoonover sequence may overlap the age of the youngest rocks known in the roberts mountains allochthon as well as the age of the antler orogeny this suggests that the schoonover sequence was deposited at least in part in a location that was paleogeographically distant from the edge of the nevadan portion of the north american shelf during the antler orogeny no volcanism occurred on the continental shelf during the emplacement of the roberts mountains allochthon therefore the presence of tuffaceous turbidites within the schoonover also suggests deposition far away from the continental margin on the other hand the composition of coarse clastic rocks within the schoonover is compatible with their derivation from the eroding roberts mountains allochthon suggesting proximity to the north american shelf if sedimentary rocks from dual sources volcanic arc and roberts mountains allochthon are indeed interbedded in the schoonover rather than tectonically interleaved then this might argue in favor of deposition of the schoonover havallah sequences in a back arc basin setting

relations between hinterland and foreland shortening sevier orogeny central north american cordillera the tectonic relations between foreland and hinterland deformation in noncollisional orogens are critical to understanding the overall development of orogens the classic central cordilleran foreland fold and thrust belt in the united states late jurassic to early tertiary sevier belt and the more internal zones to the west central nevada thrust belt provide data critical to understanding the developmere of internal and external parts of orogens the garden valley thrust system part of the central nevada thnkst belt crops out in south central nevada within a region generally considered to be the hinterland of the jurassic to eocene sevier thrust belt the thrust system consists of at least four principal thrust plates composed of strata as young as pennsylvanian in age that are unconformably overlain by rocks as old as oligocene suggesting that contraction occurred between those times new u pb dates on intrusions that postdate contraction combined with new paleomagnetic data showing significant tilting of one area prior to intrusion suggest that regionally these thrusts were active before 85 100 ma the thrust faults are characterized by long relatively steeply dipping ramps and associated folds that are broad and open to close upright and overturned although now fragmented by cenozoic crustal extension individual thrusts can be correlated from range to range for tens to hundreds of kilometers along strike we correlate the structurally lowest thrust of the garden valley thrust system the golden gate mount irish thrust southward with the gass peak thrust of southern nevada this correlation carries the following regional implications at least some of the slip across jurassic to mid cretaceous foreland thrusts in southern nevada continues northward along the central nevada thrust belt rather than noaheastward into utah this continuation is consistent with age relations which indicate that thrusts in the type sevier belt in central utah are synchronous with or younger than the youngest thrusts in southern nevada this in turn implies that geometrically similar sevier belt thrusts in utah must die out southward before they reach nevada that slip along the southem nevada thrusts is partitioned

denudation of metamorphic core complexes and the reconstruction of the transition zone west central arizona constraints from apatite fission track thermochronology apatite fission track thermochronologic results from transects across the basin and range and transition zone provinces in west central arizona provide constraints on the denudational history and structural framework of the region apatite fission track ages decrease from 21 to 14 ma in the harcuvar mountains and from 16 to 13 ma in the buckskin rawhide mountains in the slip direction sw ne of detachment faults in the lower plates of the metamorphic core complexes mean lengths of confined fission tracks from the core complexes are all 14 m indicating that the apparent apatite ages record rapid cooling through the apatite partial annealing zone 40 c m y for lower plate rocks apparent apatite ages in the transition zone province generally increase from 25 ma to 100 ma away from the basin and range province this trend of increasing apatite age is disrupted by faulting as many as seven times at the fronts of major mountain ranges and within valleys between the weaver mountains and the colorado plateau gradients of apatite fission track age and confined track length with elevation in the mountain ranges and in the phillips kirkland drill hole reveal parts of denuded mesozoic and cenozoic apatite partial annealing zones these paleopartial annealing zone profiles provide a reference datum for preextension reconstructions of fault blocks the reconstructions indicate that the major faults in the transition zone province have relative displacements of 1 km and that offset on them occurred mostly after 25 ma these data also indicate that most of the rocks now exposed in the transition zone of west central arizona were not exposed until miocene time

carlin style gold deposits youjiang basin china tectono thermal and structural analogues of the carlin type gold deposits nevada usa the triassic gold deposits of the youjiang basin southern china have been variously correlated to carlin style and orogenic gold deposits or classified as a new intermediate deposit type however in terms of a multi scale mineral system approach they show remarkable similarities to the tertiary carlin type deposits of nevada and distinct contrasts to orogenic gold deposits both the nevada and youjiang deposit groups formed in a continent scale post orogenic extension event on fragmented continental crust underlain by metasomatized lithosphere both form roughly orthogonal deposit trends that subparallel near orthogonal margins of a continental crustal block with deposits controlled by gentle anticlines monoclines or half horsts and extensional faults not tight locked up anticlines and shear zones the mineralogy and ore geochemistry of the two groups are similar with differences consistent with slightly deeper and higher temperature of formation of the older chinese deposits commensurate with deeper erosional levels the youjiang gold deposits should be classified as carlin type rather than carlin like or other terminologies with their lower gold endowment probably related to a more distal thermal and fluid source than the nevada carlin type deposits

regional surficial geochemistry of the northern great basin the regional distribution of arsenic and 20 other elements in stream sediment samples in northern nevada and southeastern oregon was studied in order to gain new insights about the geologic framework and patterns of hydrothermal mineralization in the area data were used from 10 261 samples that were originally collected during the national uranium resource evaluation nure hydrogeochemical and stream sediment reconnaissance hssr program in the 1970s the data are available as u s geological survey open file report 02 0227 the data were analyzed using traditional dot maps and interpolation between data points to construct highresolution raster images which were correlated with geographic and geologic information using a geographic information system gis wavelength filters were also used to deconvolute the geochemical images into various textural components in order to study features with dimensions of a few kilometers to dimensions of hundreds of kilometers the distribution of arsenic antimony gold and silver is different from distributions of the other elements in that they show a distinctive high background in the southeast part of the area generally in areas underlain by the pre mesozoic craton arsenic is an extremely mobile element and can be used to delineate structures that served as conduits for the circulation of metal bearing fluids it was used to delineate large crustal structures and is particularly good for delineation of the battle mountain eureka mineral trend and the steens lineament which corresponds to a post miocene fault zone arsenic distribution patterns also delineated the black rock structural boundary northwest of which the basement apparently consists entirely of miocene and younger crust arsenic is also useful to locate district sized hydrothermal systems and clusters of systems most important types of hydrothermal mineral deposit in the northern great basin appear to be strongly associated with arsenic this is less so for low sulfidation epithermal deposits in addition to individual elements the distribution of factor scores that resulted from principal component studies of the data was used the strongest factor is characterized by fe ti v cu ni and zn and is used to map the distribution of distinctive basalts that are high in cu ni and zn and that appear to be related to the steens basalt the other important factor is related to hydrothermal precious metal mineralization and is characterized by sb ag as pb au and zn the map of the distribution of this factor is similar in appearance to the one for arsenic and we used wavelength filters to remove regional variations in the background for this factor score the resulting residual map shows a very strong association with the most significant precious metal deposits and districts in the region this residual map also shows a number of areas that are not associated with known mineral deposits illustrating the utility of the method as a regional exploration tool a number of these prospective areas are distant from known significant mineral deposits the deconvolution of the spatial wavelength structure of geochemical maps combined with the use of large regional geochemical data sets and gis permits new possibilities for the use of stream sediment geochemistry in the study of large scale crustal features as well as the isolation of mineral district scale anomalies

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

the 40ar 39ar ages and tectonic setting of the middle eocene northeast nevada volcanic field widespread middle to late eocene calc alkalic volcanism which formed the northeast nevada volcanic field marks the earliest tertiary volcanism in the northern basin and range the central part of this major field in northeast nevada and adjacent utah is herein defined by 23 40ar 39ar ages that range from 42 6 to 39 0 ma rock chemistry from 12 localities stratigraphic position of the volcanic rocks above a regional middle eocene unconformity volcanic setting and lithology the type area is at nanny creek in the northern pequop mountains nevada where rhyolite ash flow tuffs are overlain by a thick section of intercalated andesitic to dacitic flows and flow breccias and rhyolite ash flow tuffs the intermediate composition rocks are locally derived throughout the volcanic field whereas the sources for rhyolite ash flow tuffs are unknown the uniform and widespread occurrence of the andesitic and dacitic flows and flow breccias strongly suggests that the upper crust was perforated by intermediate composition magma across the entire region in the central part of the field the middle eocene volcanic rocks rest with depositional angular discordance on deformed middle paleozoic to triassic strata ostracode bearing limestone probably of early eocene age is locally present below the volcanic rocks in the western and southeastern parts of the field these middle eocene volcanic rocks rest with depositional angular discordance on lower eocene lacustrine strata of the elko and white sage formations respectively this angular discordance documents a middle eocene deformational event previously unrecognized in the region

flexural rotation of normal faults a conceptual model is proposed for the generation of low angle normal faults in metamorphic core complexes the model is based on three assumptions 1 the isostatic response to normal fault motion is of regional extent 2 when a fault segment is significantly rotated from the optimum angle of slip relative to the crustal stress field it is replaced by a new planar fault oriented in the optimum direction and 3 the fault cuts the entire upper crust and fault motion always nucleates in the same region at the base of the upper crust the stress field is considered to be uniform through the crust and the regional isostatic response of the crust to loads is computed using the thin plate flexure approximations active low angle normal faults are difficult to reconcile with rock mechanics theories earthquake focal mechanism studies and geochronologic results indicating rapid cooling of core complex rocks the model does not require active fault slip on low angle faults the flexural response to normal faulting is shown to be significantly affected by anelastic behavior of the crust and by loading due to sedimentation the anelastic response to large bending stresses results in significant reduction in the effective elastic rigidity of the upper crust this can explain the observed short wavelength of topographic response to normal fault loads the model results in 1 a nearly flat lying abandoned normal fault or detachment below slices of upper plate rocks and sedimentary infill 2 a strong contrast in metamorphic grade across the abandoned detachment 3 rapid movement of lower plate rocks from midcrustal depths to shallower depths these results are qualitatively in agreement with geologic observations for core complexes

relationship between mesozoic and cenozoic tectonic features in west central arizona and adjacent southeastern california the maria fold and thrust belt mftb is a narrow belt of mesozoic crustal shortening that trends east west across west central arizona and adjacent southeastern california it is characterized by generally south vergent folds and thrusts that commonly displace proterozoic crystalline rocks over deformed and metamorphosed paleozoic and mesozoic strata the mftb is cut by a south to southeast trending belt of mid tertiary extensional deformation extension was characterized by large displacements on low angle normal faults known as detachment faults and by isostatic uplift of mylonitic midcrustal rocks now exposed in metamorphic core complexes the geometry and style of extensional deformation change along the extensional belt and reveal the influence of the mftb several extensional features are spatially coincident with the root zone of mftb thrusts 1 an areally extensive west northwest trending belt of denuded tertiary mylonitic fabrics in the whipple and harcuvar metamorphic core complexes 2 a style of extension characterized by minimum extensional dismemberment of the upper plate and maximum denudation and uplift of deep seated lower plate rocks 3 an abrupt bend in the belt of arched uplifted rocks below detachment faults and 4 an abrupt bend in the trend of the breakaway zone of the detachment faults and an associated lateral ramp in the detachment fault system the first two features may be the result of isostatic uplift of a crustal root downward protruding moho bulge inherited from crustal thickening within the mftb and the other two features may reflect at the time of detachment fault initiation the influence of stresses in the crust associated with flexural support of the buoyant mesozoic crustal root

geologic and geophysical evidence for the influence of deep crustal structures on paleozoic tectonics and the alignment of world class gold deposits north central nevada usa abstract geologic data concur with geophysical and isotopic data that suggest the presence of deep crustal fault zones along the battle mountain eureka bme trend and elsewhere in nevada the fault zones may have originated during proterozoic rifting of the continent and were likely substantially reactivated and modified during paleozoic tectonism five distinct paleozoic structural and stratigraphic domains are defined that demonstrate the complexity of paleozoic tectonic events and also lead to hypotheses about ways in which the margin could have been modified the current locations of these domains adjacent to the geophysically and isotopically defined indicators of the buried continent edge corroborate their interactions with the continental margin during the tertiary preexisting crustal fault zones were intersected and reopened during episodes of extension and served as the conduits for deep sourced gold rich fluids which were disseminated into paleozoic slope facies sedimentary rocks forming sediment hosted carlin type and other deposits multiple factors including the locations of these deep seated structures the original configuration of the lower paleozoic continental margin of nevada and its subsequent reactivation during the paleozoic all were fundamental controls on the location of younger mineral deposits a clearer understanding of the original configuration of the margin and of the effects of subsequent paleozoic and mesozoic tectonic events on the margin would provide insight into the locations of these and other prospective mineral belts

cainozoic evolution of the state of stress and style of tectonism of the basin and range province of the western united states cainozoic evolution of the modern plate boundary along the western united states from subduction to a predominantly transform boundary coincided with a change from compressional to extensional deformation in the western united states extensions tectonism responsible for the modern basin and range province appears to represent a unique late stage episode of a much longer period of extension initiated in an in tra arc setting contemporaneously with calc alkaline magmatism basin range extension is distinguished from early extension on the basis of angular unconformities differences in fault trends and spacing and associated magmatism basaltic prebasin range extension i e extension preceding the break up of the region into ranges resembling the modern ones was under way locally by at least 30 ma and is now recognized by faulted and highly tilted strata exposed in uplifted range blocks by large regions of the crust underlain by passively emplaced subvolcanic batholiths and by the thickness and distribution of stratigraphic units locally high strain rates that accompanied early extensions of as much as 50 100 are implied data on preferentially orientated dyke swarms and fault slip vectors indicate a strikingly uniform wsw ene least principal stress orientation in the period ca 20 10 ma during this early extension the change from early extension to basin range style faulting of the upper 15 km of crust which resulted in broadly spaced ranges 25 35 km crest crest spacing was time transgressive and probably not abrupt locally both types occurred concurrently southern basin and range block faulting occurred largely in the period 13 10 ma in response to a stress field orientated similarly to that responsible for the early extension in contrast northern basin and range block faulting developed after 10 ma and continues to the present in response to a stress field orientated approximately 45 clockwise to the earlier stress field this modern stress field with a wnw ese to e w directed least principal stress characterizes the entire modern basin and range province and rio grande rift region the 45 change in least principal stress orientation is consistent with superposition of dextral shear associated with the development of the san andreas transform fault inclusion of pre basin range extension may help resolve the discrepancy between estimates of 15 30 for basin range block faulting and total extension estimates of 100 300 for the basin and range province

the style of late cenozoic deformation at the eastern front of the california coast ranges the 1983 coalinga earthquake occurred at the eastern boundary of the california coast ranges in response to northeast directed thrusting such movements over the past 2 ma have produced coalinga anticline by folding above the blind eastern tip of the coalinga thrust zone the 600 km length of the coast ranges boundary shares a common structural setting that involves westward upturn of cenozoic and cretaceous strata at the eastern front of the coast ranges and a major southwest facing step in the basement surface beneath the western great valley like coalinga anticline pliocene and quaternary folding and faulting along the rest of the boundary also result from northeast southwest compression acting nearly perpendicular to the strike of the san andreas fault we suggest that much of this deformation is related to active thrusts beneath the eastern coast ranges the step in the basement surface beneath the great valley seems to have controlled the distribution of this deformation and the shape of the coast ranges boundary

petrology and geochemistry of the late eocene harrison pass pluton ruby mountains core complex northeastern nevada which suggest a single mixing event before emplacement in the upper the late eocene harrison pass pluton was emplaced in the crust late stage activity consisted of three pulses of monzogranitic transition zone between the infrastructure and suprastructure of the magma plus sparse mafic dikes the largest and youngest of these ruby mountains core complex emplacement was at 3 kbar pulses the two mica monzogranite of green mountain creek is pressure and was in two stages early stage tonalitic to monzogranitic distinct from all other units in the presence of restitic enclaves low magmas followed by late stage monzogranites and mafic dikes nd and high initial sr sr pod like bodies of leucocratic biotite the early stage began with emplacement of biotite hornblende amphibole monzogranite and sheets and dikes of leucocratic granodiorite of toyn creek followed by the biotite monzogranite of two mica monzogranite make up the other late stage granites these corral creek quenched equivalents of these units are preserved as rocks display deep negative eu anomalies and show wide nonporphyritic dikes in the roof leucocratic monzogranite that forms systematic concentrations of high field strength elements however cupolas in the roof zone represents the product of fractional crystheir isotopic compositions are identical to those of the early stage tallization of the early magmas al in hornblende barometry and rocks they are thought to be small melt fractions of the lower to the presence of magmatic epidote suggest that early stage magmas middle crust their elemental compositions are thought to result resided at a pressure of 5 6 kbar before emplacement in the from the effects of residual plagioclase and accessory minerals the upper crust compositional variation in the toyn creek and corral variable and non systematic isotopic compositions of all granitic creek units is essentially linear and can be explained by mixing units in the pluton suggest a heterogeneous source region such as of a tonalitic end member with a monzogranitic end member such the proterozoic mojave province as evolved samples of the monzogranite of corral creek the tonalitic end member was itself a hybrid that formed by interaction

omissional faulting during mesozoic regional contraction at carlin canyon nevada faults at low angles to bedding that emplace younger over older rocks are widespread in the hinterland of the sevier orogenic belt near the border between nevada and utah and are attributed to horizontal extension during latest jurassic cretaceous or mid tertiary time at carlin canyon approximately 150 km west of the border we document a low angle fault of probable mesozoic age between upper pennsylvanian rocks and lower pennsylvanian strata a contact that previously was mapped as a locally angular segment of a regional middle pennsylvanian unconformity and was considered evidence for upper pennsylvanian orogeny humboldt orogeny loss of section in carlin canyon was in response to top northwest layer parallel shear along a backthrust during northwest southeast regional contraction deformation is later than permian and earlier than miocene but is probably latest jurassic to cretaceous structures in the upper paleozoic rocks of carlin canyon may record shallow thin skinned contraction in the hinterland of the sevier orogenic belt above deep seated metamorphism and deformation exposed nearby in the ruby mountains east humboldt range the fault at carlin canyon indicates that the humboldt orogeny reflected primarily vertical not horizontal displacements

gold butte crustal section south virgin mountains nevada the gold butte block consists of a complex of proterozoic medium to high grade metamorphic and plutonic rocks unconformably overlain by steeply east dipping paleozoic sedimentary rocks on its east side this complex is one of the largest continuous exposures of crystalline rocks in the southwestern united states that lie structurally beneath a steeply tilted section of phanerozoic strata and it may represent part of an intact proterozoic miocene crustal section reaching paleodepths of approximately 15 km the section was exposed during tectonic denudation in miocene time at about 15 ma and lies directly adjacent to the colorado plateau to the east proterozoic and tertiary geological features support the hypothesis that the block represents a relatively intact crustal section a large exposure of proterozoic rapakivi granite the gold butte granite circa 1 45 ga crops out over most of the crystalline complex and displays strong zoning in phenocryst abundance which increases toward the top of a 10 to 12 km thick magma chamber the wallrocks of the granite are predominantly garnet bearing gneisses circa 1 7 ga which display increasing pressures of final equilibration westward away from the basal phanerozoic unconformity thermobarometry of gneisses immediately beneath the unconformity indicate pressures of 2 3 kbar whereas those 12 13 km west of the unconformity yield pressures of 5 6 kbar suggesting the crustal section was established by 1 7 ga the style of deformation displayed by denudation structures also varies from east to west with brittle faulting only in eastern portions of the block and retrograde metamorphism mylonitization and chlorite brecciation of the garnet gneiss observed in western areas

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

on the causes of 18 o depletion and 18 o 16 o homogenization during regional metamorphism the east humboldt range core complex nevada previous stable isotope studies at lizzies basin revealed that metasedimentary rocks are 18o depleted relative to protolith values particularly in the lower parts of the section lower zone where the rocks are also isotopically homogeneous on a scale of hundreds of meters quartz 18o 9 0 to 9 6 per mil in contrast metasedimentary rocks at higher levels at lizzies basin upper zone are less 18o depleted and more heterogeneous in 18o in order to understand more fully the isotopic evolution of this terrane a series of detailed meter scale traverses across various metamorphic and igneous lithologies were completed at lizzies basin and at the structurally higher angel lake locality traverses in the lizzies basin lower zone and in the lower parts of angel lake angel lake lower sequence across various silicate lithologies including abundant granitoids reveal similar degrees of homogeneity although the average 18o values are higher at angel lake in contrast traverses which include substantial thicknesses of marble and calc silicate gneiss and very little granitoid have more heterogeneous quartz 18o values 11 9 to 13 4 per mil and also have a higher average 18o 12 9 per mil than observed elsewhere the scale of 18o 16o homogeneity in quartz observed at lizzies basin and angel lake meters to hundreds of meters requires fluid mediated isotope exchange which accompanied tertiary metamorphism there is a correlation between the degree of 18o depletion in metasedimentary rocks 18o 16o homogenization between lithologies and the proportion of granitoids leucogranites in particular within any part of the section and a corresponding anticor relation with the proportion of marble this points to a causal relationship whereby the leucogranites as well as the tertiary hornblende diorite and biotite monzogranite acted as both a relatively low 18o reservoir and a source of fluids to enhance exchange while the marbles hindered isotope deplction and homogenization by acting as relatively high 18o reservoirs and impermeable layers material balance calculations help delineate the plausible mechanisms of exchange between granitoids and metasediments single pass infiltration of magmatic fluids from the granitoids is not capable of reproducing all of the observations fluidmediated exchange by convective recirculation of magmatic fluids on a scale of meters is the mechanism which explains all of the observations the generalized model for the isotopic evolution of the east humboldt range core complex provides and excellent opportunity to establish the main causes and controlling factors of 18o depletion and 18o 16o homogenization during regional metamorphism

magnetic fabric flow directions and source area of the lower miocene peach springs tuff in arizona california and nevada we have used anisotropy of magnetic susceptibility ams to define the flow fabric and possible source area of the peach springs tuff a widespread rhyolitic ash flow tuff in the mojave desert and great basin of california arizona and nevada the tuff is an important stratigraphic marker from the colorado plateau to barstow california a distance of 350 km however the location of its source caldera is unknown dated at 18 5 ma by 40ar 39 ar the tuff erupted during the early stages of miocene extension along the lower colorado river the thicker accumulations 100 m occur at kingman arizona and in the piute mountains california on opposite sides of the colorado river extensional corridor our ams studies produced well defined magnetic lineations in 30 of 42 sites distributed throughout the tuff typical ratios of the principal ams axes are 1 01 for the magnetic lineation kmax kint and 1 02 for the foliation kint kmin the bulk magnetic susceptibility of the peach springs tuff averages 2 0 10 3 in the si unit system the subhorizontal lineations which presumably parallel the flow directions form a pattern radiating outward from the approximate center of the outcrop area magnetic foliations define an imbrication that generally dips away from the distal margins and toward the center of the outcrop of the tuff the lineation and imbrication indicate a source region near the southern tip of nevada defining the best intersection of the ams lineations required restoration of major extension strike slip faulting and associated tectonic rotation in the disrupted tuff the optimum intersection of magnetic lineations lies in the southern black mountains of arizona on the eastern side of the colorado river extensional corridor no caldera structures are known from that area but the area contains thick sections of the peach springs tuff above a silicic volcanic center the caldera may be buried under younger deposits in the mohave valley of arizona tertiary granite in the newberry mountains may represent a deeper level of the peach springs tuff vent that has been exhumed by detachment faulting

petrogenesis of garnet two mica granites in the ruby mountains nevada garnet two mica granites of jurassic 160 3 m y and cretaceous 83 1 3 m y ages intrude amphibolite facies metasedimentary rocks of precambrian age and lower paleozoic sedimentary rocks in the northern part of the ruby mountains nevada high initial 87sr 86sr and high values of 18o for minerals suggest a dominantly sedimentary source for the magmas total rare earth contents are low and less fractionated cen ybn 10 than granitic rocks of similar composition in the sierra nevada and show a negative eu anomaly eu eu 0 57 these data are consistent with melting in the stability field of plagioclase with little contribution to the melt from refractory minerals such as zircon sphene and apatite minor euhedral garnets are manganese rich and magnesium poor al60sp34py3 6gr2 8 compared to garnet al79sp6py13gr2 in the intruded metamorphic rocks and show an increase in manganese and a decrease in calcium in the outer few microns fractionation of magnesium iron between garnet and biotite and of sodium potassium between k feldspar and plagioclase in the granites suggest submagmatic equilibration temperatures in the range of 365 to 505 c whereas oxygen isotope equilibration temperatures for two granites are higher at 480 and 570 c the compositions of the granites plot near minimum melting compositions in the water saturated quartz albite orthoclase system comparison of muscovite plus quartz stability the water saturated granite solidus and experimental garnet melt equilibria suggests pressures of crystallization no lower than about 3 5 kbar this pressure is compatible with that estimated from garnet plagioclase sillimanite quartz equilibria for the intruded metasedimentary rocks

eocene extension of early eocene lacustrine strata in a complexly deformed sevier laramide hinterland northwest utah and northeast nevada new data from a small enclave that did not undergo significant neogene tectonism in northwest utah document eocene tilting normal faulting land folding of the early eocene lacustrine white sage formation this deformation was part of a complexly deformed eocene sevier laramide hinterland that underwent east west contraction and extension in different areas and ultimately gave way to widespread oligocene extension in the eastern great basin such mixed mode supracrustal eocene tectonism may reflect local adjustments to maintain a state of regional gravitational equilibrium or may record responses to changing plate convergence rates at the west edge of the continent

part 1 regional studies and epithermal deposits northern nevada is one of the earth s premier gold and silver producing regions and gold and silver are mined from a wide range of deposit types table 1 current production largely comes from carlin type sedimentary rock hosted disseminated gold deposits christensen 1995 teal and jackson 1997 bettles 2002 many of which lie along linear trends fig 1 roberts 1960 1966 berger and bagby 1991 the small area of the carlin trend approximately 8 x 65 km is north america s most prolific gold mining district producing nearly 125 t 4 moz of gold annually since large scale mining began in 1965 with the opening of the carlin mine the carlin trend has produced more than 1555 t 50 moz of gold during 2001 gold production from the entire state of nevada was nearly 253 t 8 13 moz which accounted for approximately 76 percent of the united states and 10 percent of the world production price and meeuwig 2002 in 2001 only the countries of south africa and australia produced more gold than the state of nevada in addition nevada produced 543 t 17 4 moz of silver in 2001 about 31 percent of the united states production and 3 percent of the world production in 1996 the mineral resources program of the u s geological survey began a multidisciplinary project to investigate the origin of gold deposits in northern nevada this project was undertaken in collaboration with the nevada bureau of mines and geology the ralph j roberts center for research in economic geology at the university of nevada reno several other universities and numerous mining companies active in the region in addition several universities and mining companies were conducting research on the origin of gold deposits in northern nevada this research built on more than a century of studies by the

reappraisal of the relationship between the northern nevada rift and miocene extension in the northern basin and range province the northern nevada rift is a prominent mafic dike swarm and magnetic anomaly in north central nevada inferred to record the middle miocene 16 5 15 0 ma extension direction in the northern basin and range province in the western united states from the 245 250 rift direction basin and range extension is inferred to have shifted 45 clockwise to a modern direction of 290 300 during the late miocene the region surrounding the northern nevada rift was actively extending while the rift formed and these domains are all characterized by extension oriented 280 300 this direction is distinctly different from the rift direction and nearly identical to the modern basin and range direction although the rate structural style and distribution of basin and range extension appear to have undergone a significant change in the late miocene ca 10 ma the overall spreading direction does not middle miocene extension was directed perpendicular to the axis of the thickest crust formed during mesozoic shortening and this orientation may reflect gravitational collapse of this thick crust orientation of northern nevada rift dikes may reflect a short lived regional stress field related to the onset of yellowstone hotspot volcanism

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

western united states extension how the west was widened abstract cenozoic extension in the western united states presents a complex interrelation of extension volcanism and plate boundary tectonics that defeats simple notions of active or passive rifting forces driving extension can originate at plate boundaries through basal traction basal normal forces or from buoyancy forces internal to the crust and lithospheric mantle the latter two are most responsible for driving extension where it is observed in the basin and range the complex evolution of the northern basin and range probably represents removal or alteration of mantle lithosphere interacting with buoyancy stored in the crust in contrast crustal buoyancy forces combined with a divergent plate boundary between about 28 and 16 ma to drive extension in the southern basin and range the central basin and range most likely extended as a result of boundary forces external to itself but arising from buoyancy forces elsewhere in the western united states

role of crustal thickening and extensional collapse in the tectonic evolution of the sevier laramide orogeny western united states the effects of crustal thickening and extensional collapse of continental crust combined with variations in plate convergence forces provide a coherent explanation for evolution of the cretaceous early tertiary sevier laramide orogeny crustal thickening of the sevier laramide hinterland region was triggered by the combination of compressive plate convergence forces and subduction induced conductive heating of the crust eastward progradation of the locus of sevier laramide deformation through time reflects the progressive widening of the region of crustal thickening the region of maximum crustal thickening migrated outward until about 80 to 75 ma when it encountered excessively strong lithosphere of the colorado plateau the locus of deformation was then transmitted laterally across both a hinterland that had attained maximum crustal thickness and the rigid colorado plateau into the laramide rocky mountain foreland an episode of vigorous extensional collapse of orogenically thickened crust affected the southern cordillera between about 75 and 35 ma early tertiary waning of plate convergence rates and continued subduction induced conductive heating of thickened crust allowed extensional collapse along the southern cordillera region to drive the colorado plateau block northward the result was the late laramide phase of rocky mountain foreland deformation because of this compressive deformation in the central and southern laramide rocky mountain region continued through eocene time rather than ceasing at 56 ma as it did in the northern cordillera

an amagmatic origin of carlin type gold deposits carlin type deposits are major sources of gold yet their origins are enigmatic suggested genetic models make connections to magmatism regional metamorphism or regional extension depositional mechanisms are uncertain as well we propose on the basis of geologic physical and chemical reasoning a genetic model in which meteoric fluids were circulated by heat released during crustal extension these fluids interacted at depth with the sedimentary rock pile and scavenged gold upon upwelling these fluids interacted with various lithologies and or other fluids and produced the characteristic alteration and metal suites of these deposits to test the viability of this amagmatic model we have investigated certain physical and chemical constraints implicit to the model heat balance calculations indicate that ample surface waters could be heated to appropriate temperatures and circulated during rapid crustal extension mass balance calculations indicate that solution transport efficiencies of 10 percent can account for several times the amount of known alteration and mineralization in contrast magma driven or metamorphic fluids appear to be less adequate sources a two step source to deposit approach was used to simulate fluid rock interaction and mass transport implied in the amagmatic model fluids with a fixed sigma cl were first equilibrated with various potential source rocks arkose graywacke and pelite each with various f o2 and f s2 buffer assemblages at 300 degrees c and 1 kbar to simulate reaction at depth perhaps near the brittle ductile transition these fluids were then reequilibrated under conditions indicated for deposit formation 225 degrees 150 degrees c 0 5 kbars to investigate processes including cooling wall rock reaction and mixing which could be important to ore deposition gold transport was favored for low chlorinity intermediate oxidation state below pyrrhotite pyrite magnetite slightly acid fluids from sulfide bearing sources i e equilibration with arkosic rocks these conditions are not conducive to base metal transport and explain the rarity of base metals in carlin type systems when reacted under deposit level conditions these calculated ore fluids leach carbonate and deposit quartz the volume ratio of carbonate dissolved to quartz precipitated is approximately 0 2 to 2 5 and increases when mixing with a second fluid is considered arkose equilibrated source fluids can precipitate gold by combinations of wall rock reaction with a decarbonated host mixing with various sulfide poor fluids and reaction with iron rich host rocks cooling alone is ineffective over this temperature interval although most deposits are hosted by decarbonated marls geochemical calculations suggest that other depositional environments are feasible consistent with observation although here applied to the amagmatic hypothesis these chemical calculations are not mechanism specific and might also be appropriate to metamorphic or magma driven hydrothermal systems these results demonstrate that carlin type deposits can result from the coincidence of an effective driving force rapid major crustal extension a source of dilute fluids and chemically appropriate sets of both source and host rocks the aqueous chemistry involved is unexceptional and shares many features with other types of moderate to low temperature gold bearing hydrothermal systems it is the geologic setting and perhaps the fluid driving force that is unusual an amagmatic model is consistent with both deposit scale and regional scale observations it rationalizes carlin type systems in the framework of southwestern north american metallogeny and it implies multiple possible mechanisms for gold mineralization detachment type gold systems may be the high oxidation state high salinity analogues of carlin type systems formed by major extension but from contrasting lithologic and fluid sources

imbricate low angle faulting in uppermost franciscan rocks south yolla bolly area northern california the south yolla bolly area of northern california contains the intersection of the franciscan complex the klamath mountains and the great valley sequence four distinct lithic units among them the south fork mountain schist as well as several subunits have been discerned in the franciscan rocks of this area these units are composed mostly of coherent or semi coherent layered sedimentary and mafic igneous rocks and their metamorphic equivalents rather than melange and are arranged in largely upright parallel imbricate fault slices four major low angle faults have been clearly recognized paleontologic data indicate that at least one fault places older material over younger lithic units exhibit various degrees of differences in metamorphic mineral assemblages textural grade and amount of internal deformation across the faults with more intensely deformed and metamorphosed material occupying successively higher structural positions these contrasts indicate that some of these faults served as surfaces by which deformed rocks were emplaced differentially upward during and or after metamorphism original bedding and compositional layering in all of the coherent units generally parallel the thrust faults two fault bounded units are each composed of a coherent sequence of basalt at the base overlain by bedded chert and further overlain by mudstone and graywacke or their metamorphic equivalents the faults which underlie these two sequences seem to have originated as decollements localized at or near a sedimentary rock mafic crust interface perhaps as discrete underthrust slices the distinct contrasts in original rock composition among these units plus the aforementioned characteristics seem to indicate that these faults had an early history in the accretionary prism apart from their later use as convenient surfaces for differential upward emplacement of metamorphosed material

tertiary magmatism and mylonitization in the ruby east humboldt metamorphic core complex northeastern nevada u pb geochronology and sr nd and pb isotope geochemistry u pb analyses of 48 zircon and or monazite fractions from ten intrusive rocks that range in composition from quartz diorite to leucogranite yield crystallization ages between 40 and 29 ma the new data indicate that tertiary intrusive rocks constitute a widespread and significant magmatic component of the ruby east humboldt metamorphic core complex some of the intrusive rocks are gneissic to mylonitic and have been penetratively deformed in a shear zone that can be traced for 100 km along the west flank of the core complex these data when coupled with previously published geochronologic data indicate that an important episode of mylonitization occurred between 29 and 23 ma late oligocene some field relations and geochronologic data however also suggest an earlier history of mylonitization in parts of the core complex this earlier mylonitic history may have occurred in the eocene or even late mesozoic time although more detailed geochronologic data are needed to clearly define the exact time interval sr nd and pb isotopic data from the dated intrusive units indicate that a relatively sharp approximately east west striking crustal boundary separating predominantly archean basement in the north from proterozoic basement in the south transects the complex we speculate that the crustal boundary represents the continuation of the cheyenne belt into this part of northeastern nevada these new data cast doubt on the existence of the proposed 2 0 2 3 ga crustal province in this part of the northern great basin the isotopic data from the plutonic rocks coupled with published data from at least in part coeval volcanic rocks indicate that mantle derived magmas were important in their genesis collectively these data indicate that significant crustal growth via the addition of mantle derived magmas occurred during large magnitude extension in this part of the northern great basin

rapid extension recorded by cooling age patterns and brittle deformation naxos greece the metamorphic core complex exposed as the island of naxos in the aegean sea records an unusually complete sequence of structures developed as a result of continental extension the structures formed during miocene rise and cooling from ductile upper amphibolite facies and anatectic conditions to brittle near surface conditions beneath the naxos detachment fault top to the north ductile fabrics in the footwall which initially developed during amphibolite facies prograde metamorphism were overprinted by a succession of north directed normal sense lower temperature brittle structures as the footwall was tectonically unloaded and unroofed pseudotachylite and cataclasite formation brittle faulting alteration and erosion of the footwall occurred during continued slip and tectonic denudation neogene conglomerate and megabreccia in part derived from exhumation of this footwall lie structurally above the peripheral naxos fault published k ar and 40ar 39ar ages for hornblende white mica and biotite in the footwall decrease northwestward apparent ages 17 50 ma in the southeast correspond to areas of low metamorphic grade where preextension argon was partially retained published ages 16 10 ma in higher grade rocks of the domal core in the north are cooling ages that for each of the three minerals show a component of younging in the nne direction of extension assuming this is the direction of unroofing we interpret the rate of this younging as the fault slip rate as the footwall rocks moved 20 km ssw relative to their hanging wall along the base of the naxos detachment fault the calculated rates of slip average 5 8 mm yr comparable to maximum rates reported in the basin and range province

geochemical and geochronological constraints on mineralization within the hilltop lewis and bullion mining districts battle mountain eureka trend nevada the hilltop lewis and bullion mining districts located within the northern shoshone range lander county nevada are part of the greater battle mountain eureka trend historic production of mostly ag with au cu pb and zn from all three districts exploited 3 m wide quartz veins hosted mainly within upper plate ordovician devonian siliceous and siliciclastic rocks of the roberts mountains allochthon eocene felsic intrusive rocks located both proximal and distal to the vein deposits were emplaced throughout the northern shoshone range along a west northwest trend molybdenite mineralization is associated with some intrusive rocks and some intrusive rocks host vein type mineralization new and previously published radiometric ages for primary igneous minerals from unaltered rocks within and or proximal to mineralized areas are nearly coincident with published re os ages for several northern shoshone range molybdenite occurrences 40 1 0 6 ma avg supporting a relationship between pluton emplacement and molybdenite mineralization age differences between quartz vein hosted gold base metal mineralization e g 38 3 0 1 ma gray eagle mine bullion district and nearby intrusive rocks e g 39 0 0 1 ma granite mountain imply that vein mineralization probably formed during prolonged hydrothermal activity synchronous with or subsequent to igneous rock emplacement or from heat associated with a second slightly younger phase of intrusive igneous activity primary fluid inclusion salinity 0 6 4 wt nacl equiv and th 115 425 c data from ore bearing vein quartz in conjunction with measured 18ovsmow data 0 97 to 17 3 indicate that vein forming fluids contained variable mixtures of magmatic and meteoric components calculated 18ovsmow values of 16 0 to 13 3 and spanned the epithermal porphyry temperature continuum new and previously published stable isotope c o and s data from vein hosted ore and gangue minerals unaltered and altered siliceous upper plate host rocks and unaltered carbonate lower plate and upper plate rocks indicate mixed and variable sources of vein hosted ore and gangue components and variable interaction between some vein forming fluids and nonsiliceous rock types carbonates bedded barite that underlie the northern shoshone range large vein widths high vein densities and lack of evidence supporting fluid boiling in primary fluid inclusions suggest that vein deposits of the northern shoshone range probably formed at shallow depths 1 km under lithostatic conditions geochronology and stable isotope data support a temporal and genetic link between eocene magmatism quartz vein hosted precious and base metal mineralization and molybdenite porphyry mineralization within the hilltop lewis and bullion mining districts

crustal structure of the ruby mountains and southern carlin trend region nevada from magnetotelluric data abstract we have collected about 150 magnetotelluric mt soundings in northeastern nevada in the region of the ruby mountains metamorphic core complex uplift and southern carlin mineral trend in an effort to illuminate controls on core complex evolution and deposition of world class gold deposits the region has experienced a broad range of tectonic events including several periods of compressional and extensional deformation which have contributed to the total expression of electrical resistivity most of the soundings reside in three east west profiles across increasing degrees of core uplift to the north bald mountain harrison pass and secret pass latitudes one short cross line was also taken to assess an east west structure to the north of the northern profile model resistivity cross sections were derived from the mt data using a 2 d inversion algorithm which damps departures of model parameters from an a priori structure geological interpretation of the resistivity combines previous seismic potential field and isotope models structural and petrological models for regional compression and extension and detailed structural stratigraphic interpretations incorporating drilling for petroleum and mineral exploration to first order the resistivity structure is one of a moderately conductive phanerozoic sedimentary section fundamentally disrupted by intrusion and uplift of resistive crystalline rocks late devonian and early mississippian shales of the pilot and chainman formations together form an important conductive marker sequence in the stratigraphy and show pronounced increases in conductance conductivity thickness product from east to west these increases are attributed to graphitization caused by elko sevier era compressional shear deformation and possibly by intrusive heating the resistive crystalline central massifs adjoin the host stratigraphy across crustal scale steeply dipping fault zones the zones provide pathways to the lower crust for heterogeneous upper crustal induced electric current flow resistive core complex crust appears steeply bounded under the middle of the neighboring grabens and not to deepen at a shallow angle to arbitrary distances to the west the numerous crustal breaks imaged with mt may contribute to the low effective elastic thickness t e estimated regionally for the great basin and exemplify the mid crustal steeply dipping slip zones in which major earthquakes nucleate an east west oriented conductor in the crystalline upper crust spans the east humboldt range and northern ruby mountains the conductor may be related to nearby graphitic metasediments with possible alteration by middle tertiary magmatism lower crustal resistivity everywhere under the profiles is low and appears quasi one dimensional it is consistent with a low rock porosity

transition from contraction to extension in the northeastern basin and range new evidence from the copper mountains nevada abstract new mapping structural analysis and 40ar 39ar dating reveal an unusually well constrained history of late eocene extension in the copper mountains of the northern basin and range province in this area the northeast trending copper creek normal fault juxtaposes a distinctive sequence of metacarbonate and granitoid rocks against a footwall of upper precambrian to lower cambrian quartzite and phyllite correlation of the hanging wall with footwall rocks to the northwest provides an approximate piercing point that requires 8 12 km displacement in an ese direction this displaced fault slice is itself bounded above by another normal fault the meadow fork fault which brings down a hanging wall of dacitic to rhyolitic tuff that grades conformably upward into conglomerate these relationships record the formation of a fault bounded basin between 41 3 and 37 4 ma the results are consistent with a regional pattern in which volcanism and extension swept southward from british columbia to southern ne

machine learning predicts reach scale channel types from coarse scale geospatial data in a large river basin author s guillon herva byrne colin f lane belize a sandoval solis samuel pasternack gregory b

homogenization and lowering of 18o 16o in mid crustal rocks during extension related magmatism in eastern nevada the oxygen isotope systematics of tertiary volcanic rocks of east central nevada and of plutonic and metamorphic rocks of the ruby mountains east humboldt range core complex provide complementary evidence for major18o depletion and 18o 16o homogenization of mid crustal rocks during metamorphism and magmatism the 18o value of crustal source material for silicic volcanic rocks decreased from between 9 and 11 to between 7 and 8 over 5 ma mid crustal metasedimentary and granitic rocks in the east humboldt range have 18o values very similar to the volcanic rocks and values are lower and more homogeneous at deeper structural levels exchange with deep seated mantle derived igneous rocks or fluids derived therefrom is the most plausible18o depletion mechanism intrusion of these mafic magmas promoted crustal melting and fluid migration homogenization of 18o 16o resulted from migration of high temperature fluids and melts at mid crustal levels and was less effective at higher structural levels where the crust was dominated by less permeable carbonate rocks

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

deep burial of the footwall of the northern snake range decollement nevada metapelitic eocambrian strata from eastern exposures of the footwall of the northern snake range decollement experienced mesozoic amphibolite facies metamorphism and greenschist facies overprint accompanying extreme ductile thinning samples from the hampton creek area where the overprint is weakest contain the assemblage quartz muscovite biotite garnet plagioclase opaque oxides staurolite kyanite tourmaline apatite textural relations and consistency of rim analyses among multiple domains in each sample indicate equilibrium rim pressure temperature estimates were obtained using the garnet biotite fe mg exchange thermometer and the garnet muscovite plagioclase biotite and garnet aluminosilicate plagioclase barometers the results suggest final equilibration at 610 50 c and 810 70 mpa or a depth of 30 3 km about a factor of 3 greater than the stratigraphic depth to the west in the schell creek range correlative strata are contiguous with unmetamorphosed upper paleozoic and tertiary strata from west to east exposures of eocambrian rocks in the two ranges collectively show increasing intensity of crystal plastic deformation metamorphic recrystallization and metamorphic grade from subgreenschist to amphibolite facies and a monotonic decrease in 40 ar 39 ar muscovite ages from mesozoic age to about 23 ma in light of these relations the pressure temperature data suggest that eocambrian strata were inclined eastward during mesozoic metamorphism with 15 20 km of structural relief we interpret the eastward tilting and burial to be the result of west directed thrusting as expressed by folding to the north in the deep creek range unroofing of the deeply buried strata may have occurred partly in cretaceous or early cenozoic time with final unroofing in oligocene and miocene time along the northern snake range decollement these results exclude the hypothesis that the northern snake range decollement initiated as a brittle ductile transition zone within cambrian strata of a little deformed miogeoclinal section and support the hypothesis that it is a major low angle extensional shear zone

thermal and unroofing history of a thick tilted basin and range crustal section in the tortilla mountains arizona we estimate here a geothermal gradient of only 17 5 c km 1 for the tilted grayback fault block in southeastern arizona when extension began 25 ma this gradient is lower than preextension gradients estimated elsewhere in the basin and range is only about 50 of typical gradients in the basin and range today and needs to be accounted for in models of continental extension the grayback block exposes a 12 km thick crustal section of proterozoic and cretaceous granitoids which was tilted 90 during extension between 25 and 15 ma zircon fission track ages decrease structurally downward westward across the block and were all within a zone of partial track annealing prior to tilting and quenching the zircon age gradient suggests that the 220 240 c isotherm migrated downward 5 6 km during paleogene erosion and regional cooling apatite fission track ages decrease westward from 83 ma in the structurally highest crystalline rocks to 24 ma at 6 km paleodepth and then to 15 ma another 6 km farther west track length analysis confirms that apatites above the break in slope in age at 5 7 km paleodepth resided in a zone of partial annealing prior to tilting and deeper apatites record rapid cooling upon tilting and unroofing beginning 25 ma at that time the 110 10 c isotherm determined by the depth at which tracks in apatite were fully erased was at a basement paleodepth of 5 7 km and the 220 30 c isotherm as estimated from zircon data resided at a pretilting basement depth of 12 15 km from consistent values of paleogeothermal gradient for two depth intervals we estimate the pretilt gradient was 17 5 c km 1 from 25 to 15 ma the rotating grayback block cooled rapidly as higher westward moving blocks unroofed it tectonically at a rate of 1 km m y 1

post laramide removal of the farallon slab western united states i propose that post laramide removal of the subhorizontally subducting farallon slab occurred by buckling downward along an approximately east northeast trending axis this process was accomplished by a tearing or necking separation of the subducted slab near the northern and southern boundaries of the united states and propagation of the separated edges toward the central axis of downwelling accompanied by aesthenosphic upwelling behind the trailing edges initial buckling probably began near 50 ma and slab removal was complete by 20 ma this model is based primarily on the space time evolution of the ignimbrite flare up a major mid tertiary igneous event of mantle origin which involved two propagating fronts of initiation of volcanism that followed the proposed motions of the separated slab edges as they converged on central nevada from the north and southeast post laramide uplift extension establishment of the cascadia subduction zone and active magmatism may be consequences of lithosphere scale modifications caused by the laramide removal of the slab and the resulting asthenospheric upwelling

kinematic role of transverse structures in continental extension an example from the las vegas valley shear zone nevada analysis of the slip budget of the las vegas valley shear zone southern nevada leads to a model that explains many of the kinematic complexities that characterize transverse structures in continental extensional settings including 1 abrupt terminations of these structures along strike 2 large lateral translations across relatively short faults 3 apparent lack of strike slip offset or contradictory evidence of shear sense along some transverse structures 4 significant variation in slip magnitude along strike and 5 juxtaposition of differentially extended terranes on opposite sides of faults many major transverse structures may function largely as transfer faults that either link or form passive boundaries between differentially extended terranes

stress magnitude strain rate and rheology of extended middle continental crust inferred from quartz grain sizes in the whipple mountains california knowledge of the magnitude of differential stress and strain rate during the formation of mylonitic shear zones in metamorphic core complexes provides constraints on the mechanical behavior of the middle continental crust during extension we analyzed the differential flow stress during the mylonitization of quartzofeldspathic rocks in the whipple mountains california using grain size piezometers and kinetic laws for grain growth mylonitic gneisses collected from two widely separated transects have grain sizes that cluster in a range from 32 to 61 m analysis of grain growth kinetics indicates that mylonitization of the gneisses continued during cooling to temperatures 500 c compatible with estimates from two feldspar thermometry quartz grain size piezometers suggest that the mylonitization occurred under differential stresses 1 3 of 40 150 mpa or maximum shear stresses of 20 75 mpa extrapolation of quartzite flow laws to 500 c indicates that the mylonitization occurred at strain rates faster than 10 14 s 1 these estimates suggest that the mylonitic zone within the whipple mountains had an effective viscosity of the order of 1018 4 1020 4 pa s these low viscosities and rapid strain rates combined with seismic reflection data showing that continental crust is layered suggest that more realistic physical models of extension of the continental lithosphere should treat the lithosphere as a heterogeneous distribution of high viscosity regions separated by low viscosity zones

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

evaluation of radioisotope dating of carlin type deposits in the great basin western north america and implications for deposit genesis a variety of techniques have been used in attempts to date the mineralization in carlin type deposits in the great basin with highly variable results these techniques and results are reviewed in this paper along with presentation of two new dates for the rodeo deposit on the carlin trend and for the barneys canyon deposit in utah complete resetting of sericite by hydrothermal fluids of the temperature and duration of hydrothermal activity that form carlin type deposits is considered highly unlikely therefore dates from sericite are generally of questionable value unless that sericite can be shown to have formed during the same hydrothermal event during which au was deposited for the deposits that have been investigated to date sericite dates rarely if ever record the age of au mineralization however sericite ages do appear to record pre au events in some districts such events may have contributed to ground preparation and to a much lesser extent to the tenor of the ore fission track and u th he techniques provide important age constraints on mineralization in some districts but also suffer from a less than clear association with au rb sr dating of galkhaite an hg sulfosalt provides the only direct age of mineralization but galkhaite has been recognized in only a few locations and dated in only two deposits in a similar manner adularia is contemporaneous with au at twin creeks but twin creeks is the only carlin type deposit where adularia has been reported several other techniques u pb reos sm nd have been used in attempts to date the deposits but with limited success the best available ages are those from preore and postore igneous events and those from galkhaite galkhaite from getchell indicates a mineralization age of 39 ma and that from rodeo carlin trend indicates a mineralization age of 40 ma a new u th he age 34 ma for apatite at barneys canyon suggests contemporaneity with the nearby bingham porphyry cu system the most reliable data from several deposits and districts in the great basin indicate that carlin type mineralization occurred in a relatively narrow time interval between 33 and 42 ma this interval coincides with the transition from a compressional to an extensional tectonic environment in the region although similar tectonic environments have existed at different times and places in western north america no other carlin type mineralization is known to exist in the region therefore although the age and tectonic environment of these deposits is now reasonably well constrained it is clear that tectonic environment alone is insufficient to explain the genesis of carlin type deposits additional perhaps unique factors such as host rock geochemistry must have contributed to the development of these exceptional deposits

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

chronology of polyphase extension in the windermere hills northeast nevada fission track and 40 ar 39 ar dating and chemical correlation of volcanic strata exposed in the windermere hills and northern pequop mountains northeast nevada indicate a protracted polyphase history of tertiary late eocene late miocene extension along the northern margin of a major cordilleran metamorphic core complex early extension is recorded by a west tilted half graben filled with early oligocene 34 79 0 18 39 18 0 12 ma sedimentary rocks in the eastern windermere hills above the lowangle black mountain detachment fault the early oligocene half graben conformably overlies a widespread suite of late eocene 39 18 0 12 40 38 0 06 ma calc alkaline volcanic rocks reflecting a temporal link between early extension at a high structural level and the end of the ignimbrite flare up these strata are cut by east west striking normal faults which are exposed along and parallel to the northern margin of the metamorphic complex available age data e g between 14 93 0 08 and 34 79 0 18 ma permit the interpretation that the east west striking faults formed at the same time as or after large magnitude unroofing of highgrade rocks we interpret the east west striking faults to accommodate differential uplift of greenschist grade metamorphic rocks in the upper crust above a lateral ramp in a west northwest directed mylonitic shear zone subsequent extension in the windermere hills is defined by deep rapidly filled half grabens of middle miocene 7 42 2 0 to 14 93 0 08 ma age that unconformably overlie older faults and synextensional deposits these are the youngest half grabens in the region and are inferred to be initiated by extensional stresses imparted to the base of the lithosphere by a laterally spreading mantle plume e g the yellowstone hotspot located in southeastern oregon at this time

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

late paleogene extensional collapse of the cordilleran foreland fold and thrust belt the cordilleran foreland fold and thrust belt collapsed and spread to the west during a middle eocene to early miocene ca 49 20 ma episode of crustal extension the sedimentary and structural record of this event is preserved in a network of half grabens that extends from southern canada to central utah extensional structures superposed on this allochthonous terrain are rooted to the physical stratigraphy structural relief and sole faults of preexisting thrust fold structures the sole faults dip 3 6 west above an undeformed precambrian crystalline basement and accommodated tectonic transport of a thick up to 20 km eastward tapering hanging wall during regimes of crustal shortening and extension the chronology of tectonism for the foreland fold and thrust belt is established here by dating latest thrusting and initial normal faulting and is best defined where thrusts and normal faults are linked by common detachment surfaces dated movement on two extensionally reactivated thrusts the lewis thrust of northwest montana and southeast british columbia and the medicine butte thrust of southwest wyoming and northeast utah suggests that the hiatus between the end of crustal shortening in the early or early middle eocene and the start of extension in the early middle eocene was brief lateral spreading and extensional basin formation in the cordilleran foreland fold and thrust belt were partly concurrent with formation of metamorphic core complexes and regional magmatism conceptually linking extensional processes that were simultaneously deforming both the hinterland and foreland of the late paleogene cordilleran orogenic wedge is accomplished by applying the extensional wedge coulomb critical taper model the rapid drop in north america pacific plate convergence rate and or steepening of the subducted oceanic slab at ca 50 ma resulted in a large reduction in east west horizontal compressive stress in the cordillera as a result the cordilleran orogenic wedge was left unsupported and it gravitationally collapsed and horizontally spread west until a new equilibrium was established at ca 20 ma subsequently crustal extension and magmatism during the basin and range event ca 17 0 ma overprinted much of this earlier phase of extension

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