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

annually resolved atlantic sea surface temperature variability over the past 2 900 y global warming due to anthropogenic factors can be amplified or dampened by natural climate oscillations especially those involving sea surface temperatures ssts in the north atlantic which vary on a multidecadal scale atlantic multidecadal variability amv because the instrumental record of amv is short long term behavior of amv is unknown but climatic teleconnections to regions beyond the north atlantic offer the prospect of reconstructing amv from high resolution records elsewhere annually resolved titanium from an annually laminated sedimentary record from ellesmere island canada shows that the record is strongly influenced by amv via atmospheric circulation anomalies significant correlations between this high arctic proxy and other highly resolved atlantic sst proxies demonstrate that it shares the multidecadal variability seen in the atlantic our record provides a reconstruction of amv for the past 3 millennia at an unprecedented time resolution indicating north atlantic ssts were coldest from 1400 1800 ce while current ssts are the warmest in the past 2 900 y

constraints on crustal composition beneath a metamorphic core complex results from 3 component wide angle seismic data along the eastern flank of the ruby mountains nevada metamorphic core complexes expose rocks deformed at deep upper to middle crustal levels during extreme crustal extension however mechanisms of crustal extension and exhumation of core complexes remain to be fully understood detailed study of crustal velocity structure and inferences about the composition of the crust beneath core complexes and nearby provide useful constraints on core complex evolution p and s wave velocity structures determined from seismic experiments along the eastern flank of the ruby mountains metamorphic core complex nevada show that the crust can be divided into three main layers corresponding to the upper middle and lower crust we interpreted crustal composition by integrating results of p wave velocities vp s wave velocities vs poisson s ratios seismic anisotropy and reflection character with published geologic maps of the area near surface estimates of vp vs and anisotropy of 1 90 4 8 km s 1 01 2 75 km s 0 25 0 33 and 0 6 2 5 respectively are consistent with surface exposures of unconsolidated to consolidated sedimentary rocks limestone dolomite siltstone sandstone porous sandstone conglomerate and weathered granite results from analysis of reflection responses vp vs and anisotropy also indicate that 1 upper crustal rocks most likely consist of metaquartzite schist granite gneiss and granite granodiorite with vp of 5 80 6 25 km s vs of 3 20 3 72 km s of 0 22 0 25 and anisotropy of 0 6 2 5 2 possible middle crustal rocks are paragranulite felsic granulite felsic amphibolite gneiss granite granodiorite and mica quartz schist with vp of 6 35 6 45 km s vs of 3 70 3 75 km s and of 0 24 and 3 lower crustal rocks most likely consist of granulite rather than amphibolite facies rocks with vp of 6 60 6 80 km s vs of 3 85 3 92 km s of 0 24 0 25 and anisotropy of 3 our principal conclusions are 1 significant addition of gabbroic rocks underplating is unlikely in the lower crust 2 lower crustal rocks were stretched into sub horizontal geometries with aligned minerals during extension creating seismic lamellae in the lower crust 3 present day seismic velocities of highly extended core complex crust and normally extended basin and range crust are similar and 4 orientations of fast shear waves near the surface and in the upper crust are sub parallel to the regional maximum horizontal compressive stress in the nevada part of the basin and range province

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

sedimentologic and tectonic origin of an incised valley fill sequence along an extensional marginal lacustrine system in the basin and range province united states implications for predictive models of the location of incised valleys incised valleys in extensional lacustrine systems should be common and significant petroleum targets yet documentation and analysis of these systems are limited and hence so are predictive models for their location geologic mapping of the miocene pliocene humboldt formation in knoll basin northeastern nevada has revealed a significant incised valley system formed along the lacustrine margins of an extensional basin the valley formed during a relative lake level fall and incised into lacustrine shoreface and offshore sandstone and subsequently was filled with fluvial and eolian sediment as lake level rose the valley s location was tectonically influenced it is situated in the hinge zone of a syncline near the tip of the range bounding fault system folding of the syncline was broadly synchronous with incision and filling and it appears to have localized the valley along the topographically low hinge zone furthermore the large relative lake level change that produced the valley is only recorded in strata in the syncline area suggesting that the location and cause of incision was greatly influenced by tectonics thus the location of similar incised valleys in other extensional basins may be predictable if comparable tectonic features and processes are recognized our study suggests that the best locations to develop and preserve incised valleys are near the tips of normal faults during periods of overall high tectonic subsidence specific areas along basin bounding faults where tectonically influenced incised valleys are more likely to form include fault propagation folds synthetic relay ramps near transfer faults and areas that have large changes in fault slip although lake volume changes caused predominantly by climate change can be an important factor in producing incised valleys tectonically influenced incised valleys are likely to be larger better preserved and more petroleum prone compared to climate controlled incised valleys

alteration associated with gold deposition at the getchell carlin type gold deposit north central nevada wall rock alteration at the getchell underground deposit was examined to determine the effects of au bear ing fluids on host lithologies and the relationship between k bearing alteration minerals and au deposition the major minor and trace element geochemistry of highly altered and mineralized to unmineralized rocks from the getchell deposit was quantified for more than 50 samples collected along 13 transects through calcareous siltstone and carbonaceous limestone and along one transect through a rhyodacite dike each transect in sedimentary rocks was collected along a single homogeneous bed that could be followed from high grade ore to moderately altered rock or waste rock analyses were obtained for 39 elements 10 oxides and loss on ignition using multiple techniques petrographic studies were integrated with geochemistry and x ray diffraction and electron microbeam analyses to identify ore and alteration minerals and to correlate mineralogy with geochemical fluxes geochemical analyses indicate that ti al zr and th behave as immobile elements immobility isocon diagrams show that significant hg sb se te tl and cs are typically added to the wall rocks with au fe is im mobile in some transects added to wall rocks in others and removed from rhyodacite host rocks minor s and w are usually added to the rock whereas minor cu and mo are variably added or removed sio 2 appears to be either added during alteration or remain constant elements that are consistently removed during alteration include ca mn sr and sc and minor mg ba and k na was significantly removed from the rhyodacite au correlates positively with ag hg sb se si te and tl and negatively with ca mn and sc element fluxes correspond with observations of mineral abundances and show that sulfidation and decar bonitization are spatially associated with gold mineralization au hg sb tl cu and possibly te and w were incorporated in trace element rich pyrite as host rock fe was sulfidized fe was added to mineralized rocks in some transects indicating that pyritization occurred in addition to sulfidation cs is a component of galkhaite a trace mineral that is relatively common in some parts of the getchell system decarbonitization removed as much as 95 wt percent calcite from mineralized rocks in some transects and is responsible for loss of ca mn sr and sc although sio 2 as and s are significantly added to carlin type systems they do not appear as major added components in the isocon diagrams this is because the halos for these components are much larger than the areas examined by the sample transects x ray diffraction analyses identified minerals in ore and waste samples and quantified clay minerals including illite and montmorillonite kaolinite was not identified or is present in low abundances in both mineralized and unmineralized rocks montmorillonite does not correlate with gold and textures are consistent with pore filling deposition of montmorillonite following au deposition illite abundance exhibits a positive correlation with au r 2 0 80 intergrowths of illite with ore stage pyrite support a genetic relationship between the two minerals and indicate that illite formed as part of the au related hydrothermal event mineral abundances and textures suggest that illite formed by alteration of k feldspar during au deposition results show that dating of k bearing phases in sedimentary rocks other than illite would provide ages unrelated to gold deposition

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

the long canyon deposit anatomy of a new off trend sedimentary rock hosted gold discovery in northeastern nevada the long canyon deposit is a new sedimentary rock hosted gold discovery located in northeastern nevada over 150 km east of the well explored carlin trend unlike most examples of this type the deposit is hosted in a sequence of cambro ordovician carbonate rocks deposited in a periodically emergent platform to shelf edge environment the strata were strongly deformed and weakly metamorphosed during a mid mesozoic orogenic event that segmented and extended a brittle 80 m thick dolomite layer into a series of elongate n to ne trending boudin blocks while the enclosing limestone underwent ductile deformation this unusual structural architecture focused subsequent contractional deformation as well as tertiary extensional brittle deformation forming pathways for meteoric and hydrothermal fluids which formed and enlarged dissolution cavities and dissolution breccias along boudin margins boudin necks and normal faults gold occurs primarily in zones of polyphase dissolution breccias that are localized in minor faults and fold hinges in the structurally complex areas along and adjacent to dolomite block margins two general phases of brecciation are present including calcareous breccias and later fe as oxide rich breccias which carry the highest gold grades breccias are accompanied by variable pervasive silicification alteration assemblages include zoned dickite white mica hematite in the cores of mineralized zones with peripheral kaolinite sericite and goethite hematite and ultimately more distal illite smectite local preservation of fine grained sulfides encapsulated in calcite and silica suggests that the iron oxide assemblages may represent oxidation of fine grained pyrite marcasite arsenopyrite assemblages as currently defined the long canyon deposit extends for 3 km in a northeasterly direction and consists of several subparallel zones of mineralization focused on boudin block margins boudin necks and incipient boudins it contains a combined measured indicated and inferred estimated resource comprising 2 2 million ounces of gold at an average grade of 2 3 g t gold metallurgical work indicates that the deposit is highly amenable to cyanidation by heap leaching or milling the long canyon deposit is open to further exploration and resource additions and is shallowly buried relatively high grade and thoroughly oxidized the stratigraphic position of the long canyon deposit on the paleozoic shelf and its location well east of the majority of identified major trends indicate significant potential for new discoveries in this underexplored area of eastern nevada

reply to comment by e l miller et al on geodynamics of synconvergent extension and tectonic mode switching constraints from the sevier laramide orogen 1 we welcome the comment from miller et al 2012 and the opportunity to address their concerns and to expand on several aspects of the proposed geodynamic model in wells et al 2012 many of the points raised in the comment concern late cretaceous crustal anatexis and extension aspects of the proposed geodynamic model that were developed in more detail in wells and hoisch 2008 we address each of their three main points below

three dimensional laramide deformation of the colorado plateau competing stresses from the sevier thrust belt and the flat farallon slab 1 kinematic analysis of laramide basement cored uplifts on the northern colorado plateau indicate that four of the uplifts were constructed by northeast directed shortening while at least two were constructed by southeast directed shortening available timing data suggest that all of the uplifts rose at approximately the same time 72 50 ma this paper proposes that the different shortening directions are a predictable result of deformation in a three dimensional stress field where the magnitude of 2 approaches that of 1 the concept is supported by both angelier stereographic analysis and elastic modeling tectonically 1 is may have been the result of coupling between the horizontally subducting farallon slab and the north american plate the elevated values of 2 are attributed to far field stresses generated by the topographically high sevier thrust belt which wraps around the western edge of the colorado plateau and was active at the time of deformation in the plateau

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

quantifying the timing and rate of crustal evolution global compilation of radiometrically dated detrital zircon grains abstractmultiple models steady state episodic and early growth followed by crustal reworking have been postulated to explain the evolution of earth s continental crust an independent assessment of these models is now possible as a result of the massive numbers of detrital zircon grains that have been dated individually over recent years using u pb and hf isotopes a compilation of 200 000 published zircon dates reveals a global age distribution u pb with six prominent statistically significant peaks 3 2 3 0 2 7 2 5 2 0 1 7 1 2 1 0 0 7 0 5 and 0 3 0 1 ga in most cases these peaks are recorded on all seven continents most of the peaks are detectable also in modern sediments a comparison of grains with both u pb ages and hf isotope model ages available for 5100 detrital zircons suggests that the age frequency distribution of detrital grains reflects predominantly episodic crustal recycling rather than crustal growth the zircon age distributions and associated data provide consistent ev

prograde metamorphism strain evolution and collapse of footwalls of thick thrust sheets a case study from the mesozoic sevier hinterland u s a abstract analysis of prograde metamorphic fabrics in the exhumed footwall of the mesozoic windermere thrust northeast nevada reveals fabrics and style of ductile flow developed during tectonic burial metamorphism of a sedimentary footwall of a thick thrust sheet in response to structural burial footwall strata underwent barrovian metamorphism synchronous with development of bedding parallel or near parallel s and s l tectonites footwall rocks range from unmetamorphosed at high structural levels and progressively increase in metamorphic grade to upper amphibolite facies at deep levels attenuation of footwall stratigraphic units accompanied tectonite development whereby the amount of attenuation varies with metamorphic grade from 0 in lower greenschist facies to 30 50 in upper amphibolite facies rocks microstructures in metacarbonate metapsammite and metapelite and analysis of crystallographic preferred orientations of quartz c axes in quartzite suggest a dominantly coaxial strain path with flattening strain predominating at lower metamorphic grade and plane strain more dominant at higher metamorphic grade the microstructural data indicate that dominantly coaxial deformation during metamorphism accommodated extension of the footwall ductile extension most likely records footwall collapse induced by a loss of strength due to relative upward migration of isotherms during and or following burial extension may act as a mechanism that in part facilitates isostatic accommodation or sinking of the overlying load resulting in a reduction of topography attenuation of footwalls of thick thrust sheets and production of predominantly coaxial layer parallel or near parallel fabrics may be an intrinsic process once sufficient structural overburden is developed and thermal relaxation and prograde metamorphism ensues

hinterland drainage closure and lake formation in response to middle eocene farallon slab removal nevada u s a abstract hinterland basins can accumulate high resolution archives of orogenic processes and continental climate but are challenging to reconstruct due to tectonic overprinting and the inherent complexity of their lithofacies assemblages the cordilleran hinterland of northeast nevada has been interpreted to have overlain a flattened farallon slab from the late cretaceous to eocene slab removal and advection of asthenospheric mantle beneath nevada have been invoked to explain a southwestward migrating wave of eocene to oligocene volcanism and proposed as a driver for topographic uplift however the timing of slab removal and possible subsequent delamination of north american lithospheric mantle can only ambiguously be related to the surface record subsequent neogene extension and basin filling has complicated the correlation and interpretation of strata that record these events here we apply single crystal sanidine 40ar 39ar geochronology to 26 ash beds in northeast nevada to reconstruct paleogene geographic and hydrologic evolution we use these ages and legacy geochronology to compare lithofacies and isotope proxy records of meteoric waters to regional tectonics and global climate and assess competing tectonic interpretations for lake basin formation lakes formed locally prior to ca 48 7 ma in northeast nevada coeval with foreland lakes of the green river formation the most expansive phase of lacustrine deposition resulted in onlap onto locally derived fluvial deposits and folded paleozoic bedrock and occurred between ca 43 4 and ca 40 8 ma elko formation strata exhibit a basin wide transition from fluvial lacustrine to fluctuating profundal lithofacies at ca 42 7 ma suggesting a shift towards regional hydrologic closure the stromatolitic upper elko formation is intercalated with ash fall tuffs and several partially welded to unwelded ignimbrites from increasingly proximal volcanism elko formation deposition ended by ca 40 4 ma 40ar 39ar ages for seven ash beds in the dead horse formation at copper basin in northern elko county indicate intermittent ash bed deposition between 45 2 ma and 38 6 ma and an episode of lacustrine deposition between 39 8 ma and 38 6 ma that post dates the main phase of lake elko d values of volcanic glass sampled from dated ash beds reflect changes in the hydrogen isotope compositions of local eocene waters and systematically vary by 80 102 according to their depositional environment the elko formation and overlying volcanic strata are overlain regionally by a pronounced unconformity of 20 m y in the copper basin area deposition continued locally into the oligocene in the hanging wall of a ductile detachment the geochronologic and isotopic framework presented here permits reanalysis of the pinon range carbonate proxy record that was previously interpreted to record both regional uplift and the middle eocene climatic optimum new data suggest instead that isotope values of hydration waters within the elko formation were strongly influenced by evaporation and a change from lacustrine to non lacustrine conditions can account for the 18 o shift that was interpreted to reflect regional uplift moreover the end of elko formation deposition predated the middle eocene climatic optimum we interpret the overall record of drainage ponding and paleovalley inundation progressively more evaporative lacustrine conditions increasingly proximal volcanism and subsequent prolonged unconformity to reflect the surface effects of progressive ne to sw removal of the farallon slab

multiple and single aliquot luminescence dating techniques applied to quartz extracted from middle and upper weichselian loess zemechy czech republic this paper reports the application of optically stimulated luminescence osl dating to relatively coarse grains 100 200 m of quartz extracted from loess two approaches were tested a multiple aliquot technique and a new single aliquot approach which is especially suited to the dating of small quantities of such grains found in some loess the results are compared with infra red stimulated luminescence and thermoluminescence tl measurements on the finer grained 4 11 m polymineral fraction and tl measurements on the relatively coarse grains of quartz the advantages of the new single aliquot osl approach are described and the iterating computer controlled measurement procedure is given the automated iterative single aliquot regeneration aisar method corrects for changes in luminescence sensitivity during the measurement sequence by monitoring the response to a test dose some initial measurements are presented for the loess section at zemechy czech republic and these show that the aisar method gives more precise results than those using a multiple aliquot method copyright 2000 john wiley sons ltd

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

the role of mantle delamination in widespread late cretaceous extension and magmatism in the cordilleran orogen western united states extension during plate convergence and mountain building is widely recognized yet the causes of synconvergent extension remain controversial here we propose that delamination of lithospheric mantle aided by decoupling of the crust from the mantle via a reduction in the viscosity of the lower crust through heating incursion of fl uids and partial melting explains many enigmatic yet prevalent aspects of the metamorphic magmatic and kinematic history of the sevier laramide orogen of the western united states during the late cretaceous extension heating anatexis magmatism and perhaps rock uplift were widespread during a restricted time interval in the late cretaceous 75 67 ma along the axis of maximum crustal thickening within the mojave sector of the sevier orogen and to a lesser extent within the interior of the idaho utah wyoming sector to the north similar processes may have been active in the peninsular range sierran western mojave and salinian segments of the mesozoic cordilleran arc these processes are viewed as predictable consequences of the thermal rheological and dynamic state of the overlying crust following delamination of mantle lithosphere beneath isostatically compensated mountain belts the proposed delamination would have occurred immediately prior to eastward propagation of lowangle subduction of the farallon plate during the inception of the laramide orogeny following delamination extension and anatexis of the north american crust were aided locally by egress of slab derived fl uids from the low angle farallon slab we suggest that lithospheric delamination may have aided in the shallowing of the slab to achieve lowangle subduction geometry delamination has been proposed to be common in areas of thickened continental lithosphere in the terminal phase or late in orogenesis the late cretaceous delamination event proposed here for the sevier laramide orogen occurred during protracted plate convergence and was synchronous with and followed by continued shortening in the external part of the orogen

metamorphic core complexes windows into the mechanics and rheology of the crust metamorphic core complexes are products of normal fault displacements sufficient to exhume rocks from below the brittle ductile transition these faults detachments may initiate within the brittle crust at steep angles but they sole into the ductile middle crust and during displacement rotate to gentler dips due to hanging wall extension the exhumed footwall commonly adopts an arched or domed geometry owing to flexural isostatic readjustment and may be overlain by strongly extended upper crustal rocks that slipped on gently dipping low friction shallow segments of the detachment metamorphic rocks exhumed beneath the detachment record progressively increasing flow stress strain localization and strain rate with decreasing temperature providing a window into physical conditions and deformational processes in the mid crust the metamorphic and deformational history of the footwall rocks may reflect tectonic processes that predate formation of the detachment fault in addition to those accompanying exhumation these processes may include diapiric emplacement of gneiss domes or exhumation in a subduction channel and may not be directly related to formation of the core complex factors favouring core complex formation are high gravitational potential energy of the extending crust weak rheology and a change in the tectonic boundary conditions such as a cessation or slowing of plate convergence

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

timing of exhumation wheeler pass thrust sheet southern nevada and california late jurassic to middle cretaceous evolution of the southern sevier fold and thrust belt zircon u th he zhe thermochronologic data for 46 samples from the wheeler pass thrust sheet which carries thick passive margin strata and is discontinuously exposed across the southern part of the sevier fold and thrust belt record late jurassic cooling related to exhumation during early thrust slip and local middle cretaceous cooling related to footwall basement imbrication within the frontal part of the wheeler pass sheet exposed in the northwest spring mountains zhe ages decrease from ca 160 to 140 ma over a paleodepth interval of 6 to 9 km in devonian to lower cambrian strata interpreted to record exhumation and enhanced cooling above a frontal ramp followed by slow cooling a similar pattern of zhe ages occurs over a more limited paleodepth range exposed within the resting spring range to the south in the nopah range zhe ages of ca 140 ma in lower cambrian strata record early cooling followed by slow cooling until ca 100 ma zhe ages in underlying paleoproterozoic basement rocks decrease from ca 100 to 85 ma over a paleodepth interval of 9 to 12 km interpreted to record uplift exhumation and cooling during footwall basement imbrication as slip was transferred eastward onto the keystone thrust system late jurassic slip on the wheeler pass thrust overlapped with deformation along the nw trending east sierran thrust system early hinterland crustal thickening and growth of the sierra magmatic arc and may have been correlative with early deformation in the central nevada thrust belt slip on the wheeler pass thrust preceded major slip on other thrust sheets that carried thick passive margin strata in more northern parts of the sevier belt probably reflecting the influence of different initial widths and tapers of the passive margin sedimentary wedge

deformation localization in orogens spatiotemporal expression and thermodynamic constraint abstract orogens are spatiotemporal expressions of instabilities in materials under load constrained by thermodynamics and preserved in the cold outer shell of the planet their pressure temperature time histories are consistent with the predictions of differential grade 2 dg 2 materials in pure shear we place the statistically invariant shear localization mechanism of these materials in a coherent thermodynamic context using an analysis of strained elastic materials this prototype system exhibits non classical thermodynamic symmetry breaking where the potentials are all functions of a single variable and the distinction between heat and work fades from view consequently internal energy must be described by a monotonically decreasing function of the entropy in order for heat capacity and absolute temperature to be positive the entropy itself exhibits an inverse dependence on length these constraints are satisfied by the overall shape and slope of the distributed deformation threshold d for dg 2 materials and its noted 1 length correlation with naturally observed folds as a function of thermomechanical competence we predict that temperature in this non linear elastic material will vary in proportion to the slope of d being high at low competence and low at high competence similar constraints apply to a self gravitating body where the energy function varies inversely with radius assigning zero pressure at the surface of the body we also predict that pressure the tensor trace of its stress energy density will vary inversely with radius thus the body force of gravity will be expressed in this elastic self gravitating system through the interplay of elastic and thermal lengths deformation localization in dg 2 materials arises due to the dynamic rescaling of lengths in response to a spike in the intrinsic energy i at while the intrinsic i and localization l thresholds are monotonically decreasing for they exhibit positive slopes at lower competence signaling a return to classical thermodynamics and joule heating in this transitional domain numerous structural and tectonic observations can be correlated using this remarkably simple model beginning with the thickness and mechanical character of the brittle crust and oceanic lithosphere in effect this model projects the kinematic theory of plate tectonics into four dimensional spacetime

the lower cretaceous king lear formation northwest nevada implications for mesozoic orogenesis in the western u s cordillera cretaceous paleogeography of the u s cordillera west of the sevier fold and thrust belt is poorly known due to the scarcity of cretaceous supracrustal rocks in the arc and backarc regions the lower cretaceous king lear formation exposed in the jackson mountains and the krum hills of northwest nevada provides a rare and important record of early cretaceous tectonism and paleogeography our work shows that the king lear formation everywhere overlies deformed and metamorphosed triassic jurassic rocks across a major unconformity the king lear formation is dominated by conglomerates and sandstones deposited in subaerial alluvial fan and gravelly braided river systems and it can be divided into three new members based on differences in clast provenance a lower member derived from local arc and backarc rocks a middle member dominated by externally derived quartzite and chert clasts and an upper member derived largely from an intrabasinal volcanic complex sedimentary features and provenance analysis indicate that clasts in the middle member were derived from nearby exposures of paleozoic rocks such as the roberts mountains and golconda allochthons which must have formed a topographically elevated area in central nevada during the early cretaceous the stratigraphic record provides evidence of deposition in a tectonically active basin in contrast with some prior studies inferring that deposition was synchronous with contractional deformation our new field observations structural data and a shallow seismic reflection profile confirm that the king lear formation was deposited in an active half graben in the jackson mountains these results provide new upper age constraints on the timing of shortening deformation in the arc and backarc regions shortening must have been complete not only before deposition of the king lear formation in the middle early cretaceous but also prior to several kilometers of exhumation of its depositional basement possible driving forces for extension leading to development of the king lear basin include relaxation of thick crust in the hinterland of thrust belts to the east stresses associated with regional tilting and development of dynamic topography and stresses related to dextral strike slip deformation to the west

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

linking thermodynamic modelling lu hf geochronology and trace elements in garnet new p t t paths from the sevier hinterland major element trace element and lu hf geochronological data from amphibolite facies pelitic schist in the raft river and albion mountains of northwest utah and southern idaho indicate that garnet grew during increasing pressure interpreted to be the result of tectonic burial and crustal thickening during sevier orogenesis garnet growth was interrupted by hiatuses interpreted from discontinuities in major element zonation pressure temperature paths were determined from the pre hiatus portions of the garnet chemical zoning profiles and indicate an increase of 2 kbar and 50 c in the western raft river mountains garnet lu hf dates of 150 1 ma in the western raft river mountains and 138 7 0 7 ma and 132 5 ma in the southern albion mountains indicate the timing of garnet growth lutetium garnet zoning profiles indicate that the lu hf ages are biased towards the post hiatus or outer pre hiatus segments indicating that the determined ages likely post date the recorded p t path history or date the tail end of the paths crustal thickening associated with sevier orogene sis in the western raft river mountains thus began slightly before 150 1 ma in the late jurassic this study shows that integrating p t paths determined from garnet growth zoning with lu hf gar net geochronology and in situ garnet trace element analyses is an effective approach for interpreting and dating deformation events in orogenic belts