temporal changes in fault strike to 90 and extension directions during multiple episodes of extension an example from eastern nevada in areas with prolonged extensional histories or multiple episodes of extension temporally distinct extensional fault systems including dip slip normal oblique slip and or strike slip faults may form these fault systems may be spatially distinct overlapped or completely superimposed and different fault systems may have different fault strikes and extension directions although fault strike is not a very sensitive indicator of principal stress directions changes through time in fault strike of as much as 90 exceed the uncertainty overprinted extensional fault systems of different ages with significantly different to 90 fault strikes and extension directions provide strong evidence of changes in the regional stress field with time tens of millions of years of extension occurred in the basin and range province of the united states and many temporally distinct fault sets formed through that time we document at least four extensional episodes and the fault kinematics of each where possible in the central hiko range eastern nevada the four cenozoic extensional events are prevolcanic synvolcanic postvolcanic miocene pliocene and postvolcanic pliocene quaternary the prevolcanic faults strike north south and are interpreted as footwall faults to the regional eocene oligocene snake stampede extensional system which caused approximately east west extension the synvolcanic faults are late oligocene miocene age and strike east west although they are rare and have minor offset they accommodated north south directed extension postvolcanic faults part of the timpahute lineament are approximately east west striking normal and northeast striking oblique slip faults they also accommodated approximately north south extension during miocene time the northeast striking left normal oblique slip fault set formed as new faults and records a change between north south and east west extension in miocene time later east west extension occurred along the pliocene to quaternary hiko fault zone these four overprinted extensional systems show two separate changes in normal fault strike of 90 from north south to east west and from east west back to north south consequently the extension direction similarly changed through time we suggest that causes for the changes in direction of upper crustal extension include a change from stresses created by 1 plate interactions which generated approximately east west extension to 2 mantle motions which generated approximately north south extension and 3 back again the extension related to mantle motions is associated with a band of volcanism that passed across the northern basin and range province moving generally from north to south during the protracted period of extension therefore a change from plate boundary or interaction derived stresses to mantle or volcanism related stresses and back again is possible

conodont color and textural alteration an index to regional metamorphism contact metamorphism and hydrothermal alteration experimental and field data are used to extend the utility of conodonts as semi quantitative thermal indices into the regimes of regional and contact metamorphism as well as hydrothermal alteration an arrhenius plot of data from induced conodont color alteration by pyrolysis in air at 1 atm was used to generate the geologic temperatures for conodont color alteration indices cai above 300 c that is for cai values of 5 through 8 such cai values occur in very low to medium grade regionally metamorphosed contact metamorphosed and hydrothermally altered rocks the uniformity or variability of cai values within a sample together with conodont texture can help to distinguish grades and environments of metamorphism particularly in metacarbonate sequences induced cai by pyrolysis in a water methane mixture at kbar results in retardation of cai and in a disparate mixture of both low and high cai values within each experimental sample in this system color alteration processes above a cai of 2 to 3 seem to change from predominantly carbonization to predominantly loss of organic matter presumably by oxidation and volatilization of oxides these experiments approximate the type of cai mixture characteristically found in conodonts recovered from hydrothermally altered rocks these data indicate that cai values of 6 to 8 cannot be used to assess precise temperatures of hydrothermally altered rocks but may serve as useful indicators of potential mineralization

correlation of plate motions with continental tectonics laramide to basin range some of the major tectonic and magmatic events of the last 150 ma in the western cordillera can be correlated with a new model for the displacement histories between western north america and adjacent oceanic plates sierra nevada plutonism ended and laramide compression began during increasingly rapid convergence of the farallon plate with north america and during a moderate increase in the westward motion of north america in the hotspot reference frame the end of the laramide and beginning of widespread arc magmatism and extension correlates with slowing of both of these motions the spectacular slowing of faralion north america convergence is attributed to the decreasing age of the farallon plate entering the trench and thus to a change from negative to positive buoyancy the transition from widespread arc related magmatism and rapid extension to basaltic volcanism and moderate extension in the basin and range province finds no ready explanation in the plate motions of the pacific basin a change to oblique spreading in the basin and range province accompanied growth in length of the san andreas fault as the mendocino triple junction progressed northward

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

kinematics and timing of three superposed extensional systems east central idaho evidence for an eocene tectonic transition cenozoic crustal extension in east central idaho began about 50 ma and continues at present three distinct episodes characterize one of the longest intervals of cenozoic extension yet documented in the north america cordillera crosscutting relationships between ne striking normal faults and volcanic rocks regional dike trends and slickenline data indicate nw se extension during peak eocene volcanism about 49 48 ma episode 1 ne striking normal faults with at most a few kilometers of offset formed in an intraarc setting during rapid ne subduction of oceanic plates under the pacific northwest north to nnw striking and west dipping normal faults with offsets up to 10 15 km formed during a younger middle eocene to oligocene basin forming event episode 2 this newly documented episode was the most important extensional event in east central idaho and began during the waning phases of challis volcanism wsw ene to sw ne extension during episode 2 was nearly perpendicular to the extension direction during episode 1 and perpendicular to the grain of the idaho montana fold and thrust belt the flip in extension direction between episode 1 and episode 2 is tightly constrained by 40ar 39ar age determinations to have taken place at the end of eocene challis magmatism about 46 48 ma i infer that plate boundary forces controlled the geometry of normal faults and dikes during episode 1 whereas internal stresses within previously thickened crust drove major sw to wsw directed extension during episode 2 a drop in convergence rates between the north american and farallon plates between 59 ma and 42 ma stock and molnar 1988 may coincide with the onset of gravitational spreading during episode 2 and may also explain the abrupt end of eocene magmatism in the pacific northwest miocene and younger sw dipping basin and range faults episode 3 extended the region in a ne sw direction although faults formed during episode 2 and episode 3 are not parallel slickenlines indicate only small changes in slip vector trends suggesting little rotation of the extension direction in east central idaho since 46 ma

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

postmylonitic deformation in the raft river metamorphic core complex northwestern utah evidence of a rolling hinge much of the recent debate over low angle normal faults exposed in metamorphic core complexes has centered on the rolling hinge model the model predicts tilting of seismogenic high angle normal faults to lower dips by footwall deformation in response to isostatic forces caused by footwall exhumation this shallow brittle deformation should visibly overprint the mylonitic fabric in the footwall of a metamorphic core complex the predicted style and magnitude of rolling hinge strain depends upon the macroscopic mechanism by which the footwall deforms two end members have been proposed subvertical simple shear and flexural failure each mechanism should generate a distinctive pattern of structures that strike perpendicular to the regional extension direction subvertical simple shear svss should generate subvertical faults and kink bands with a shear sense antithetic to the detachment for an svss hinge the hinge related strain magnitude should depend only on initial fault dip rolling hinge structures should shorten the mylonitic foliation by 13 for an initial fault dip of 30 in flexural failure the footwall behaves as a flexed elastic beam that partially fails in response to bending stresses resulting structures include conjugate faults and kink bands that both extend and contract the mylonitic foliation extensional sets could predominate as a result of superposition of far field and flexural stresses strain magnitudes do not depend on fault dip but depend on the thickness and radius of curvature of the flexed footwall beam and vary with location within that beam postmylonitic structures were examined in the footwall of the raft river metamorphic core complex in northwestern utah to test these predictions observed structures strike perpendicular to the regional extension direction and include joints normal faults tension gash arrays and both extensional and contractional kink bands aside from the subvertical joints the extensional structures dip moderately to steeply and are mainly either synthetic to the detachment or form conjugate sets range wide the extensional structures accomplish about 4 elongation of the mylonitic foliation contractional structures dip steeply mainly record shear antithetic to the detachment and accomplish 1 contraction of the foliation these observations are consistent with the presence of a rolling hinge in the raft river mountains but a rolling hinge that reoriented a high angle normal fault by svss is excluded the pattern and magnitudes of strain favor hinge related deformation mainly by flexural failure with a subordinate component of svss

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

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

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

k ar and 40ar 39ar whole rock ages of slate phyllite from allochthonous basement and cover in the tectonic windows of finnmark norway evaluating the extent and timing of caledonian tectonothermal activity slate phyllite samples collected within allochthonous basement rocks raipas super group exposed in the komagfjord and alfa kvaenangen tectonic windows in finnmark northern norway display variably discordant 40 ar 39 ar age spectra in which apparent ages systematically increase from ca 400 425 ma recorded at low experimental temperatures to dates as old as ca 1750 1800 ma in some high temperature increments the apparent age variations are not matched by fluctuations in apparent k ca ratios and are therefore interpreted to reflect intracrystalline gradients in the concentration of 40 ar within constituent fine grained white mica the gradients are suggested to have developed as a result of a variable caledonian thermal rejuvenation of white mica argon systems which had initially cooled through argon retention temperatures at ca 1700 1800 ma following the sveco karelian orogeny the extent of whole rock spectra discordancy is related to the proximity of the basal thrust of the structurally overlying kalak nappe complex whole rock sample suites were also collected structurally downward 0 6 20 m from the kalak thrust within penetratively cleaved allochthonous cover sequences lomvatn formation exposed along the northern edge of the komagfjord window these display discordant 40 ar 39 ar age spectra with most intermediate temperature gas fractions recording mutually similar apparent 40 ar 39 ar ages of 410 430 m y observed variations in k ar and 40 ar 39 ar ages suggest that a late caledonian tectonothermal event ca 420 425 ma variably affected the allochthonous basement and cover units exposed in the two finnmark windows this thermal overprint is interpreted to have developed during emplacement of the structurally overlying kalak nappe complex while it was maintained at post scandian metamorphic temperatures in excess of at least 350 c although the effects of a late cambrian early ordovician orogenic event have been described from portions of the kalak nappe complex no clear record of correlative tectonothermal activity has been documented within structurally underlying tectonic units it is therefore likely that most of the paleozoic metamorphism and deformation observed along the eastern margin of the caledonide orogen in northern scandinavia developed during eastward transport of previously assembled composite scandian nappes into their present structural position on the baltoscandian platform between ca 400 and 425 ma

age trends in garnet hosted monazite inclusions from upper amphibolite facies schist in the northern grouse creek mountains utah we performed in situ th pb dating of monazite in upper amphibolite facies pelitic schist from the grouse creek mountains in northwest utah sixty six ages from inclusions in four garnet grains range from 37 to 72 ma and decrease with radial distance from garnet cores the age range of 30 matrix monazite grains overlaps and extends to younger ages than inclusions 25 58 ma the monazite grains are not intersected by cracks in the garnets through which dissolution reprecipitation or pb loss might occur and are generally too small 20 lm to allow for more than one age determination on any one grain processes that might explain inclusion ages that decrease with radial distance from garnet cores include 1 pb diffusion in monazite 2 dissolution and reprecipitation of monazite and 3 co crystallization of monazite and garnet after consideration of these possibilities it is concluded that the co crystallization of monazite and garnet is the most plausible with monazite neoblasts deriving rees from the breakdown of muscovite garnet ages derived by regression of the inclusion ages and assuming a constant rate of volume increase during garnet growth yield model ages with a maximum difference between core and rim of 22 m y

tracking changes in crustal thickness during orogenic evolution with sr y an example from the north american cordillera global compilations indicate that the geochemistry of arc magmatism is sensitive to moho depth magmatic products are prevalent throughout the history of cordilleran orogenesis and can be employed to constrain the timing of changes in crustal thickness as well as the magnitude of those changes we investigate temporal variations in crustal thickness in the united states cordillera using sr y from intermediate continental arc magmas our results suggest that crustal thickening began during the late jurassic to early cretaceous and culminated with 55 65 km thick crust at 85 95 ma crustal thicknesses remained elevated until the mid eocene to oligocene after which time crustal thicknesses decreased to 30 40 km in the miocene the results are consistent with independent geologic constraints and suggest that sr y is a viable method for reconstructing crustal thickness through time in convergent orogenic systems

focal depths and fault plane solutions of earthquakes under the tibetan plateau we compare synthetic and recorded p wave forms to place constraints on the focal depths and fault plane solutions of 16 crustal earthquakes beneath the highest parts 4000 m of the tibetan plateau fault plane solutions for all 16 events show combinations of normal and strike slip faulting with t axes oriented approximately east west none of these solutions show thrust faulting thus the data corroborate previous inferences that the active tectonics are dominated by east west extension focal depths for all 16 events are less than 15 km and appear to be between 5 and 10 km this style of deformation and these depths of faulting are similar to those in the basin and range province of the western united states two intermediate depth events below the crust of southern tibet also show primarily normal faulting with east west t axes the solution for one discussed by chen et al 1981 is unambiguous the solution for the other the event of august 1 1973 27 59 n 89 17 e 85 10 km mb 4 9 is less certain both apparently occurred in the mantle beneath a thick aseismic lower crust and their occurrence suggests that brittle deformation occurs there in response to a stress field similar to that operating at shallow depths beneath tibet

kinematics of compressional and extensional ductile shearing deformation in a metamorphic core complex of the northeastern basin and range abstract analysis of shear criteria enables the kinematics of two main ductile shearing events d 1 and d 2 to be established in the raft river grouse creek and albion metamorphic core complex the first event d 1 is a nne thrusting and corresponds to mesozoic shortening a well developed non coaxial ductile deformation d 2 of cenozoic age is marked by the occurrence of opposing eastward in raft river and westward shear criteria in albion grouse creek these characterize an arch structure where the shear strain increases outwards in the axial zone of the complex d 2 seems coaxial cenozoic extension is considered to be related to gravitational instability induced by mesozoic overthickening of the crust involving uplift erosion and abnormal heating brittle extension occurs in the upper part of the uplifted domain it is transformed laterally above undeformed basement towards stretched domains of the middle and lower crust through the ductile shear zones localized at the precambrian paleozoic interface this extension of the middle and lower crust occurred in the vicinity of the root zones of the former mesozoic thrusts which may thus have been reactivated as ductile normal faults during cenozoic extension

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

time of initial thrusting in the sevier orogenic belt idaho wyoming and utah reexamination of the distribution of fossils found in the earliest preserved synorogenic conglomerates within the sevier thrust belt suggests that initial thrust movement may be no older than aptian age this interpretation is corroborated by subsidence analyses of sedimentary sequences lying within and east of the idaho wyoming and utah thrust belts a major episode of middle cretaceous aptian cenomanian subsidence is interpreted as recording the initiation of thrust loading deformation in the adjacent sevier orogenic belt an earlier subsidence event took place during middle jurassic time more than 30 m y prior to the cretaceous event and may be the result of tectonic events to the west that are unrelated to thrust deformation in the idaho wyoming and utah thrust belts we find no verifiable evidence to support previous interpretations that sevier belt deformation and uplift began in late jurassic time

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

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

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

the blake magnetic polarity episode in cores from the mediterranean sea abstract palaeomagnetic and oxygen isotope studies on samples taken from five cores in the mediterranean area are described directions of the stable remanent magnetisation show the presence of an anomalous reversed zone during the transition between the oxygen isotope substages 5e and 5d this position corresponds to the age of about 117 kyr b p and thus to the blake polarity episode the duration of this episode is estimated to be about 6000 years the study of the anisotropy of the magnetic susceptibility shows no indication of sedimentological disturbances in recorded sections the blake episode therefore represents a stratigraphic marker which may be used to link the continental and marine palaeoclimatic and sedimentary records

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

pressure temperature paths from garnet zoning evidence for multiple episodes of thrust burial in the hinterland of the sevier orogenic belt garnets are present in two horizons of the schist of stevens spring from the basin creek area of the grouse creek mountains in northwest utah the two horizons possess different bulk compositions which resulted in garnet growth by different reactions along the same pressure temperature p t path garnet in the upper horizon grew from the breakdown of chlorite at upper greenschist facies conditions and garnet in the lower horizon grew from the breakdown of staurolite at upper amphibolite facies conditions from the upper horizon five garnets from three samples were analyzed all display growth zoning ragged morphologies associated with secondary rim consumption and yield garnet biotite geothermometry temperatures of 460 490 c from the lower horizon one garnet from each of two samples was analyzed these also display growth zoning but differ from garnet in the upper horizon in that they are dominantly idioblastic and yield garnet biotite geothermometry temperatures of 635 c garnet biotite geothermometry calculated for every point along detailed compositional traverses across the garnets revealed localized reequilibration along rims cracks and inclusions in both generations of garnet garnets from the upper horizon display prograde reequilibration and the garnets from the lower horizon display retrograde reequilibration numerical simulations of garnet growth using the gibbs method with duhem s theorem were carried out to determine p t paths the p t path defined by the modeling of five garnets from the upper horizon is an isothermal pressure increase of 1 7 kbar the p t path defined by the modeling of two garnets from the lower horizon has a steep p t trajectory d p d t 32 bars c and a total pressure change of 0 9 kbar both paths are indicative of thrust burial however the two paths cannot be reconciled as products of a single thrust episode these data are interpreted to indicate two different episodes of thrust burial during the sevier orogeny separated by 150 c of heating and partial exhumation