timing and episodicity of middle eocene volcanism and onset of conglomerate deposition idaho seventeen 40 ar 39 ar incremental release mineral ages from lava flows and tuffs in the lost river and lemhi ranges idaho 1 demonstrate deposition of thick sequences of syntectonic conglomerate in middle eocene and oligocene time and 2 reveal two temporally and compositionally distinct phases of the challis volcanic group thick sections of coarse conglomerates are preserved in two nnw trending half graben these were previously assigned to the pliocene but are reassigned to the eocene and oligocene based on ages from four tuffs interbedded with conglomerates andesitic and dacitic lava flows and tuffs of the challis volcanic group up to 2 5 km thick were deposited between about 49 and 48 ma whereas rhyolite tuffs only tens of meters thick accumulated between about 46 and 45 5 ma the rhyolite tuffs are intercalated within or underlie the syntectonic conglomerates lithologic stratigraphic and geochronologic evidence correlate these tuffs with the tuff of challis creek which erup

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

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

alligator ridge district east central nevada carlin type gold mineralization at shallow depths carlin type deposits in the alligator ridge mining district are present sporadically for 40 km along the north striking mooney basin fault system but are restricted to a 250 m interval of devonian to mississippian strata their age is bracketed between silicified ca 45 ma sedimentary rocks and unaltered 36 5 to 34 ma volcanic rocks the silicification is linked to the deposits by its continuity with ore grade silicification in devonian mississippian strata and by its similar 18 o values 17 and trace element signature as sb tl hg eocene reconstruction indicates that the deposits formed at depths of 300 to 800 m in comparison to most carlin type gold deposits they have lower au ag au grades and contained au more abundant jasperoid and textural evidence for deposition of an amorphous silica precursor in jasperoid these differences most likely result from their shallow depth of formation the peak fluid temperature 230 c and large 18 o h2o value shift from the meteroric water line 20 suggest that ore fluids were derived from depths of 8 km or more a magnetotelluric survey indicates that the mooney basin fault system penetrates to mid crustal depths deep circulation of meteoric water along the mooney basin fault system may have been in response to initial uplift of the east humboldt ruby mountains metamorphic core complex convection also may have been promoted by increased heat flow associated with large magnitude extension in the core complex and regional magmatism ore fluids ascended along the fault system until they encountered impermeable devonian and mississippian shales at which point they moved laterally through permeable strata in the devonian guilmette formation devonian mississippian pilot shale mississippian joana limestone and mississippian chainman shale toward erosional windows where they ascended into eocene fluvial conglomerates and lake sediments most gold precipitated by sulfidation of host rock fe and mixing with local ground water in zones of lateral fluid flow in reactive strata such as the lower devonian mississippian pilot shale

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

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

stratigraphy and structure of metamorphosed upper paleozoic rocks near mountain city nevada upper paleozoic rocks near mountain city nevada consist of a marine sequence of late mississippian age unconformably overlying deformed ordovician rocks of the roberts mountains allochthon this is the oldest and thickest mississippian antler overlap sequence in nevada at its base clastic rocks of the grossman formation are overlain by a fining upward sequence of the meramecian banner formation limestone at the top of the banner formation is overlain by basaltic lava tuff and minor limestone of the nelson formation the nelson formation is in turn overlain by the chesterian chainman shale this upper mississippian sequence records subsidence and rifting or strike slip tectonics that affected the continental margin of western north america soon after thrusting associated with the late devonian early mississippian antler orogeny the occurrence of the chainman shale in the upper part of this antler overlap sequence both near and to the east of mountain city seems to record a westward migration of the antler foredeep basin across a foundered portion of the antler orogenic belt the occurrence of submarine basalt flows of meramecian age in the nelson formation in correlative rocks to the west and in exposures of the golconda allochthon south of mountain city suggests that late mississippian basaltic volcanism was widespread affecting the continental margin of north america as well as the offshore golconda basin block faulting and uplift during pennsylvanian permian time caused disruption and erosion of upper mississippian sequences deposited elsewhere on the antler orogenic belt a metamorphosed folded and thrust faulted assemblage of chert argillite basalt gabbro siliciclastic and limestone turbidites fine grained dolomitic quartz arenite and minor siliceous tuff of late paleozoic age rests in thrust contact above the mississippian overlap sequence these basinal rocks referred to as the reservation hill sequence rhs were probably deposited in deep water on the continental slope and rise of an ocean basin receiving chert rich quartzose and carbonate detritus from north america the rhs is lithologically similar to mississippian permian rocks of the golconda allochthon prior to its southward thrust emplacement during the mesozoic the rhs was probably deposited on oceanic or transitional crust in an east west trending re entrant or embayment of the late paleozoic continental margin in the area of the present snake river plain during the late mesozoic rocks in the vicinity of mountain city were regionally metamorphosed folded and cut by south verging imbricate thrust faults this episode of compressive deformation occurred prior to intrusion of a quartz diorite stock which has a minimum age k ar hornblende of 134 ma other similar dioritic stocks in northern elko county that have k ar ages on biotite of 153 155 ma also post date thrusting and metamorphism of rocks as young as triassic suggesting that the age of deformation is post triassic and pre or syn late jurassic mesozoic folds and thrusts near mountain city represent considerable north south shortening and form part of a belt of south verging folds and thrust faults trending east west across northern elko county this belt may be an eastward deflection or splay of the jurassic and cretaceous winnemucca luning and fencemaker fold and thrust belts recognized farther to the south and is subparallel to other major cordilleran lithotectonic belts in northeastern nevada including the edge of the precambrian sialic crust the wholesale eastward deflection of cordilleran trends in northeastern nevada may reflect either the presence of a long standing indentation in the continental margin of western north america or an oroclinal bend of post triassic and possibly post late cretaceous age

thresholds for paleozoic ice sheet initiation continental drift and atmospheric greenhouse gas concentrations have each in turn been proposed to explain the evolution of paleozoic climate from early era ice free conditions to late era continental scale glaciation despite continually increasing solar luminosity to assess the relative roles of continental configuration and atmospheric p co 2 on the formation of continental scale ice sheets we use a coupled ice sheet climate model to simulate ice sheet initiation at eight different paleozoic time slices using uniform topography for each time slice we simulate the climate at three atmospheric pco 2 levels 560 840 and 1120 ppm and both constant 97 5 of modern and time appropriate solar luminosity values under constant luminosity our results indicate that continental configurations favor ice sheet initiation in the mid paleozoic 400 340 ma after accounting for solar brightening ice sheet initiation is favored in the early paleozoic 480 370 ma simulations neither of these results is consistent with geological evidence of continental scale glaciation changes in atmospheric pco 2 can reconcile these differences sufficiently high 1120 ppm or low 560 ppm pco 2 overcomes paleogeographic and luminosity predispositions to ice free or ice age conditions based on our simulations and geological evidence of glaciation and atmospheric composition we conclude that atmospheric pco 2 was the primary control on paleozoic continental scale glaciation while paleogeographic configurations and solar irradiance were of secondary importance

implications of the mechanical and frictional behavior of serpentinite to seismogenic faulting the widespread occurrence of serpentinite along major strike slip seismogenic faults warrants systematic investigation to determine how its frictional characteristics may affect slip along the fault different locations along the motagua fault zone in guatemala were sampled to investigate the sliding mode as a function of composition and texture confining pressure and displacement rate air dried right circular cylinders 7 1 cm in length and 3 3 cm in diameter with a precut 35 to the cylinder and load axes were deformed at confining pressures up to 200 mpa room temperature and displacement rates of 10 4 and 10 7 cm s compositional analyses of specimens cored from five blocks of serpentinite that were tested show that the serpentinite can be divided into two groups one is a mesh textured serpentinite containing up to 70 serpentine mostly lizardite 11 enstatite phenocrysts 19 oxides and minor amounts of olivine and carbonate undifferentiated the other is a flare textured serpentinite containing up to 84 serpentinte mostly antigorite 10 oxides 6 magnesite and dolomite and almost no enstatite or olivine the flare serpentinite undergoes the transition from stable sliding to stick slip sliding at confining pressures as low as 10 mpa the mesh serpentinite results only in stable sliding up to confining pressures of 200 mpa and a displacement rate of 10 4 cm s at a displacement rate of 10 7 cm s only one of the three mesh blocks exhibits stick slip the shear stress required to initiate sliding for the flare textured serpentinites is given by 0 77 n for the mesh textured serpentinites at a displacement rate of 10 4 cm s it is given by 0 56 n and at a displacement rate of 10 7 cm s by 0 50 n differences in fracture strength between the five specimen blocks correlate with differences in frictional strength differences in the sliding behavior and in the deformation observed along the sliding surfaces can be related to the mineralogical components of each serpentinite type extensive deformation occurs in the mesh serpentinites as a result of hard oxide and enstatite indenters ploughing through a soft lizardite substrate lizardite lizardite contacts prevail and these deform by plastic flow the stick slip behavior in the flare textured serpentinites is explained by brittle failure of antigorite antigorite contacts the smaller grain size of the flare serpentinites relative to the mesh serpentinites may also contribute to the higher frictional strength of the flare serpentinites we discuss the implications of the antigorite rich flare textured serpentinite to seismogenic faulting in particular we consider the case of antigorite forming by shear heating induced progressive metamorphism of lizardite chrysotile serpentinites if serpentine dehydration temperatures are approached talc may form having a lower coefficient of friction than serpentine talc may act as a lubricant along the fault plane

geochronology of intrusive and metamorphic rocks in the pilot range utah and nevada and comparison with regional patterns k ar and u pb isotopic studies demonstrate middle mesozoic metamorphism and plutonism followed by eocene plutonism in the pilot range northeastern great basin combined with field relations the age constraints indicate that local amphibolite facies and widespread greenschist facies metamorphism peaked between 165 and 150 ma and that plutons were emplaced early in the deformational and metamorphic history between 165 and 155 ma cenozoic plutons and widespread dikes that were emplaced at about 40 ma cut folds and faults including a major detachment fault that placed unmetamorphosed on metamorphosed rocks the mesozoic and early cenozoic history of metamorphism and plutonism in the pilot range is consistent with age data and field relations throughout the northeastern great basin that indicate late jurassic plutonism and metamorphism followed by eocene and oligocene plutonism and local metamorphism in many areas rocks that were metamorphosed during late jurassic time were cooled through ar blocking temperatures for micas between 85 and 60 ma the widespread occurrence of these mica ages suggests a regional mechanism for cooling

late cretaceous extension in the hinterland of the sevier thrust belt northwestern utah and southern idaho cover rocks of the raft river metamorphic core complex located in the sevier belt hinterland preserve a structural and metamorphic history that predates the middle tertiary extension of the region in the eastern raft river mountains cambrian permian rocks form two allochthons that occur in the hanging wall of the mid miocene raft river detachment fault dramatically attenuated metamorphosed cambrian pennsylvanian strata of the lower allochthon yield late cretaceous muscovite sup 40 ar sup 39 ar plateau ages ca 82 90 ma that are interpreted to date cooling during final stages of layer parallel extension devonian permian rocks exposed in the black pine mountains underwent east west extension approximately 160 associated with syntectonic growth of white mica along cleavage sup 40 ar sup 39 ar whole rock spectra of associated slates also indicate a late cretaceous metamorphism together field and sup 40 ar sup 39 ar results suggest that late cretaceous extension occurred in the sevier belt hinterland at the same time as shortening in the eastern foreland and at depth in the hinterland sufficient topography must have been present to drive upper crustal extension in the eastern hinterland

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

paleobotanical evidence of eocene and oligocene paleoaltitudes in midlatitude western north america comparisons of the physiognomy of leaves from modern vegetation of known climates with that of eocene and oligocene fossil leaf assemblages from middle latitudes of western north america indicate paleoaltitudes comparable or higher than those at present using canonical correspondence analysis a multivariate statistical approach that includes nonlinear relationships between characters and environmental parameters we relate leaf physiognomy of fossil samples to modern vegetation of known environmental parameters including moist enthalpy of the atmosphere a thermodynamically conserved quantity of the atmosphere that varies with altitude by estimating enthalpy values for both lowland leaf assemblages and coeval interior leaf assemblages altitudes for the interior assemblages can be estimated to 890 m in northeastern washington and southern british columbia the high eocene altitudes indicate subsidence or collapse of an area undergoing crustal extension and may reflect an immediately preceding period of uplift that triggered the extension similarly other areas that have undergone cenozoic crustal extension appear to have been at least as high as they are at present three sites from the rocky mountains also indicate elevations at least as high as at present and therefore suggest subsidence either resulting from cooling of a hot upper mantle or erosion and isostatic compensation of surrounding terrain high altitudes during eocene and oligocene time in western north america appear to have been normal even in areas such as the green river basin and therefore cast doubt on the commonly inferred late cenozoic uplift of that region

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

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

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

miocene extension in the east range nevada a two stage history of normal faulting in the northern basin and range the east range in northwestern nevada is a large east tilted crustal block bounded by west dipping normal faults detailed mapping of tertiary stratigraphic units demonstrates a two phase history of faulting and extension the oldest sedimentary and volcanic rocks in the area record cumulative tilting of 30 45 e whereas younger olivine basalt fl ows indicate only a 15 20 e tilt since ca 17 13 ma cumulative fault slip during these two episodes caused a minimum of 40 extensional strain across the east range and quaternary fault scarps and seismic activity indicate that fault motion has continued to the present day apatite fi ssion track and u th he data presented here show that faulting began in the east range ca 17 15 ma coeval with middle miocene extension that occurred across much of the basin and range this phase of extension occurred contemporaneously with middle miocene volcanism related to the nearby northern nevada rifts suggesting a link between magmatism and extensional stresses in the crust that facilitated normal faulting in the east range younger fault slip although less well constrained began after 10 ma and is synchronous with the onset of low magnitude extension in many parts of northwestern nevada and eastern california these fi ndings imply that rather than migrating west across a discrete boundary late miocene extension in western nevada is a distinct younger period of faulting that is superimposed on the older middle miocene distribution of extended and unextended domains the partitioning of such middle miocene deformation may refl ect the infl uence of localized heterogeneities in crustal structure whereas the more broadly distributed late miocene extension may refl ect a stronger infl uence from regional plate boundary processes that began in the late miocene

lithospheric gravitational potential energy and past orogenesis implications for conditions of initial basin and range and laramide deformation gravitational body forces i e buoyancy forces have come to be seen as critical to the evolution of orogens nevertheless constraining the role of body forces in specific geologic scenarios is made difficult by the substantial number of poorly constrained physical parameters needed to fully relate forces to deformation by separating the calculation of buoyancy forces from the calculation of the resulting deformation models based on relatively simple descriptions of the lithosphere can yield geologically useful constraints among these are the importance of paleoelevation in driving syn and postcontractional extension and in localizing contractional strain although such phenomena have been considered in more complex models of continental deformation the simpler analysis presented here clearly establishes first order limits on lithospheric structures and paleoelevations consistent with buoyancy driven deformation in the early cenozoic great basin of the western united states we show that the low elevations inferred in much of the geologic literature are inconsistent with a body force origin for observed extensional tectonism east of the colorado plateau localization of laramide deformation coincides with pre laramide subsidence of the western interior seaway this subsidence prestressed the lithosphere making the southern rocky mountains the weak link in responding to regional compressional stress

space time patterns and tectonic controls of tertiary extension and magmatism in the great basin of the western united states structural and stratigraphic relations in the great basin indicate widespread pre middle miocene crustal extension that appears to define two north trending belts most extension in these belts was oligocene age but locally it began earlier or lasted into early miocene time the eastern belt straddles the nevada utah border and includes the snake range nevada area with its southern end near 37 5 n and its western edge at the seaman butte mountains breakaway the southern boundary of the eastern belt is occupied by the 26 15 ma caliente caldera complex and approximately coincides with the present east west trending margin of the great basin north of saint george utah crust north of this boundary extended approximately east west before volcanism began at 30 32 ma but to the south extension began after about 15 ma this boundary may have been a rooted zone of left slip faults that allowed the footwall of the stampede detachment to move west relative to unextended terrain to the south the eastern margin of the eastern belt is probably located near the present eastern edge of the great basin but its northern end is poorly defined the western belt runs from the funeral and grapevine mountains california to the ruby mountains nevada and north northeast to the albion range idaho tens of kilometers of crustal extension occurred at least locally in both belts but magnitude of extension is poorly known for large areas of each tertiary volcanism in the great basin began in the north in eocene time with predominantly effusive volcanism and swept southward ending with voluminous oligocene miocene ignimbrite eruptions from calderas in an irregular discontinuous belt between marysvale utah and reno nevada a result of the south ward migration of volcanism is that the onset of extension in both belts was syn or post volcanic in the north but was pre volcanic in the south late paleogene extension and crustal magmatism coincided in both time and space only locally where south migrating magmatism overlapped active north south trending extensional belts most calderas in the southern great basin formed in previously extended belts or on their margins southward migration of ignimbrite sources was apparently blocked by unextended crust to the south in contrast volcanism north of the ignimbrite province was dominated by nonexplosive effusion of lava prior to or during crustal extension this is consistent with observations in the southern basin and range where volcanism and crustal extension were generally synchronous and volcanism was dominantly effusive thus caldera formation may be controlled by the distribution of upper crustal extension although the physical mechanism for this control remains speculative the space time patterns of late paleogene extension in the great basin are consistent with extension being triggered by thermal weakening of subducted oceanic lithosphere rather than by effects transmitted from the plate margin but being driven by gravitational collapse of thick crust space time patterns of tertiary volcanism in the basin and range also appear to conform to patterns of thermal weakening or destruction of the subducted slab both active and passive rifting mechanisms are inapplicable on the scale of the extensional belts because both predict close spatial and temporal association of extension and magmatism which is not generally observed

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

sedimentation and paleogeography of an eocene to oligocene rift zone idaho and montana examination of sedimentary basins in east central idaho and analysis of previous work in western montana indicate that late middle eocene to oligocene extension north of the eastern snake river plain was confined to a north to north northwest trending rift zone 100 25 km wide in east central idaho six remnants of half grabens preserve up to 2 6 km of eocene to oligocene boulder to pebble conglomerate sandstone and intercalated tuff the paleogene basin bounding normal faults strike north to north northwest dip west and accommodated up to 10 km of dip slip clasts at the base of each basin fill sequence were derived almost exclusively from underlying middle eocene volcanic rocks whereas higher in the section the detritus was derived increasingly from pre tertiary formations exposed in the footwall of the basin bounding normal faults the uppermost units of the basin fill deposits typically lack volcanic clasts pebble count data indicate that the basin fill deposits were produced by unroofing of proterozoic to tertiary rocks in the uplifted footwall east of each basin slip on the basin bounding normal faults created substantial relief and spawned gravity emplaced slide blocks and megabreccias that interfinger with thick alluvial fan deposits in the basins in contrast previous studies of the upper eocene to lower miocene renova formation in southwestern montana suggest that relatively low relief and tectonic quiescence characterized the region east of the proposed rift zone i speculate that a river flowing from the rift zone into a basin east of the rift shoulder may have carried fine grained sediment into the renova basin throughout east central idaho initial slip on the north to north northwest striking normal faults was closely associated with rhyolitic volcanism during the waning phases of challis magmatism ca 47 45 ma 40ar 30ar ages however show that sedimentation and extension outlasted volcanism by more than 15 m y and were largely amagmatic gravitational collapse of previously thickened crust probably drove this episode of extension within and west of the sevier fold and thrust belt i interpret the rift zone which can be traced into western montana and southern british columbia as a northward continuation of a west directed belt of late paleogene extension previously documented in the great basin

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

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

alternating contraction and extension in the hinterlands of orogenic belts an example from the raft river mountains utah combined macroscopic to microscopic structural analyses detailed geologic mapping and thermochronology were used to study the structural evolution of midcrustal rocks of the sevier orogenic belt hinterland in northwestern utah these results when combined with previous studies provide new insight into the structural history of this region and suggest alternating tectonic contraction and extension during mesozoic to early cenozoic time two allochthons form the upper plate of the miocene raft river detachment fault in the eastern raft river mountains the lower allochthon comprises neoproterozoic ordovician and pennsylvanian strata and is bounded below by the raft river detachment fault and above by the middle detachment fault strata within the lower allochthon were dramatically attenuated by two episodes of ductile deformation the first deformation d1 took place at metamorphic temperatures of 500 c and resulted in penetrative fabrics throughout these rocks that record combined flattening and top to northeast shearing strains the second deformation d2 resulted in significant stratigraphic attenuation along discrete top to the west shear zones that are generally parallel to lithologic contacts separates of synkinematic muscovite from the penetrative fabric yield 40ar 39ar cooling ages that indicate that d1 deformation occurred prior to cooling ca 90 ma both fabrics were subsequently folded about d3 kilometer scale recumbent folds the areally extensive middle allochthon composed chiefly of pennsylvanian and permian rocks metamorphosed in the greenschist facies was emplaced d4 along the low angle middle detachment fault this fault cuts across various structural levels of the recumbently folded lower allochthon in its footwall and juxtaposes greenschist facies over amphibolite facies metamorphic rocks the lower and middle allochthon were subsequently deformed d5 into open folds with north trending axes neogene extension d6 produced an 200 m thick top to east ductile shear zone in precambrian rocks and formed the younger raft river detachment fault which forms the present upper contact of the ductile shear zone northeast vergent d1 fabrics probably record shortening deformation on the basis of fabric correlations with the grouse creek and albion mountains deformation kinematics and synkinematic prograde metamorphism d2 attenuation faults have been interpreted to record crustal extension of late cretaceous age if d3 recumbent folds developed during extension then the deformation sequence records early shortening followed by protracted extensional unroofing with variable structural styles the favored alternative is that d3 recumbent folds developed during shortening in this case d1 through d4 record episodic alternations of contraction and extension such alternations are consistent with observations from analog and theoretical models of contractional mountain belts and suggest that the hinterland of the sevier orogenic belt underwent dynamic adjustments in crustal thickness and deformation kinematics in response to changes in the boundary conditions of the orogenic belt