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

vertical averages of rheology of the continental lithosphere relation to thin sheet parameters abstract we calculate vertically averaged rheologies for continental lithosphere subject to biaxial states of stress corresponding to extensional compressional and strike slip regimes we assume that brittle deformation occurs by byerlee s law and that ductile deformation occurs by steady state creep mechanisms under these conditions the vertically averaged rheology may be approximated by a single power law over the range of strain rates from 10 13 to 10 17 s 1 this is true not only when either brittle or ductile behavior dominates the strength of the lithospere but also when the contributions of brittle and ductile behavior are comparable the value of the power law exponent n and the strength of the lithosphere depend on the depth and stress difference at the brittle ductile transition the temperature at the moho and the geothermal gradient in the range of vertically integrated stress between 1012 and 1013 n m 1n reaches a minimum of between 5 and 10 under other circumstancesn increases rapidly as brittle behavior dominates over ductile behavior the strength of the lithosphere depends most strongly on the thermal state of the upper mantle but also is influenced by the stress supported in brittle portions of the lithosphere

effect of subduction of the mendocino fracture zone on tertiary sedimentation in southern california abstract the age and paleogeographic setting of middle to late tertiary fluvial deposits in southern california are consistent with the theory that subduction of the mendocino fracture zone produced a north facing north moving slope and zone of deformation in the north american plate the mendocino fracture zone was a 1 km high north facing scarp in the farallon plate that moved northward at about 3 cm yr 1 subduction of this scarp produced isostatic stress in the overlying north american plate if this stress were expressed in the crust as a north facing slope then a given area should have experienced the following sedimentary evolution 1 normal probably oceanward drainage when the slope was far to the south 2 reorientation to northward drainage as the slope moved to a position immediately south of the area 3 deposition of coarse sedimentary units possibly accompanied by down to the north faulting when the slope reached the area 4 uplift deformation and erosion of previously deposited sediments as the area was uplifted along the slope and 5 reestablishment of normal drainage compilation of data from sedimentary sequences in southern california supports this model in particular the time of fracture zone passage was marked by strong dominance of northward stream flow these relationships suggest that middle tertiary fluvial sedimentation in the southern california area was strongly influenced by passage of the fracture zone

tectonics and topography for a lithosphere containing density heterogeneities the purpose of this paper is to clarify the dynamic role of lithospheric density heterogeneities in particular with respect to mountain building and other processes of intraplate deformation density anomalies within or just beneath the lithosphere constitute major sources for tectonic stress fields the product of their magnitude by their depth is shown to characterize their ability to induce deformation this rule of the density moment directly yields the lithospheric thickening or thinning rate when applied to structures of large lateral extent for anomalies of lateral extent that is small in comparison with their depth the deformation is vertically inhomogeneous and has been computed with the help of simple physical models of a stratified viscous newtonian lithosphere the analytical treatment is based on fourier transform continent continent collision thickens not only the crust but the entire lithosphere the cold root underlying a mountain chain induces strong regional compressive stresses able to sustain the mountain bulding process without further help from forces transmitted from far away thus the continental lithosphere is in a somewhat metastable mechanical state adiabatic i e rapid thickening or thinning tends to grow further once initiated tectonic phases of strong compression correspond to the climax of such instabilities the response of models with cold lithospheric roots of various intensities has been computed both in two and three dimensions they yield velocity distributions and stress fields instructive comparisons are made with earthquake focal mechanisms and in situ stress measurements in the alpine and appalachian regions in the presence of lateral variations of the mechanical properties of the lithosphere the tectonic style is not only function of the local topography and of the nature of its compensation deformations in neighbouring provinces are coupled as shown by 3 dimensional models for example thickening sustained by a cold lithospheric root may generate extension in peripheral zones of weakness these last results illustrate the point that the computation of regional tectonic stresses requires the knowledge of the density anomalies within the lithosphere on the one hand and of geometrical constraints related to lateral mechanical heterogeneities on the other

regional correlation of extension directions in cordilleran metamorphic core complexes cordilleran metamorphic core complexes exhibit tertiary extensional deformation expressed either as detachment faults or detachment faults plus ductile mylonitic shear zones extension directions as indicated by stretching lineations within mylonite and striations along detachment faults fall into regional groups in which the directions are similar in trend throughout each group asymmetric fabrics on both small and large scale give a sense of shear and indicate that tectonic vergence within groups is directed outward from a central axis the regional consistency of extension directions implies a regional control of extension in metamorphic core complexes

additional paleomagnetic results from the sierra nevada further constraints on basin and range extension and northward displacement in the western united states modal analysis of new paleomagnetic results that have a skewed distribution from four plutons in the kings canyon national park area central sierra nevada produces a pole position concordant with the results of frei and others 1984 from the yosemite area these studies suggest a that stable magnetization resides in the more mafic plutons and inclusions in the sierra nevada and b that the amount of internal deformation within the central sierra nevada does not exceed a few degrees at the 95 confidence level the mean pole of both studies limits the relative displacement between the sierra nevada and cratonal north america to 6 8 of clockwise rotation and 6 6 of poleward displacement the clockwise rotation agrees well with the model of frei and others 1984 for basin and range extension which predicts 2 of clockwise rotation for the sierra nevada the poleward displacement suggests right lateral shear of 600 km within the basin and range between 90 and 35 ma

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

internal tides around the hawaiian ridge estimated from multisatellite altimetry 1 satellite altimetric sea surface height anomaly ssha data from geosat follow on gfo and european remote sensing ers as well as topex poseidon t p are merged to estimate m2 internal tides around the hawaiian ridge with higher spatial resolution than possible with single satellite altimetry the new estimates are compared with numerical model runs along track analyses show that m2 internal tides can be resolved from both 8 years of gfo and 15 5 years of ers ssha data comparisons at crossover points reveal that the m2 estimates from t p gfo and ers agree well multisatellite altimetry improves spatial resolution due to its denser ground tracks thus m2 internal tides can be plane wave fitted in 120 km 120 km regions compared to previous single satellite estimates in 4 lon 3 lat or 250 km 250 km regions in such small fitting regions the weaker and smaller scale mode 2 m2 internal tides can also be estimated the higher spatial resolution leads to a clearer view of the m2 internal tide field around the hawaiian ridge discrete generation sites and internal tidal beams are clearly distinguishable and consistent with the numerical model runs more importantly multisatellite altimetry produces larger m2 internal tidal energy fluxes which agree better with model results than previous single satellite estimates this study confirms that previous altimetric underestimates are partly due to the more widely spaced ground tracks and consequently larger fitting region multisatellite altimetry largely overcomes this limitation

relative motions between oceanic plates of the pacific basin appendix tables are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b84 012 2 50 payment must accompany order relative motion poles describing the displacement histories between the pacific plate and once adjacent oceanic plates farallon kula izanagi i izanagi ii and phoenix were derived for the late mesozoic and cenozoic eras because fracture zone and magnetic anomaly data are generally available from the pacific plate but not from adjacent plates a new method of analysis for onesided data was required this analysis produced stage poles and rates of relative plate motion and estimates of their confidence regions the following are the main conclusions drawn from our analysis 1 for time intervals of the order of 107 years termed stages relative motion poles for plate pairs remained nearly fixed between stages shifts in poles were commonly both large and abrupt within stages rates of plate motion were commonly observed to change markedly indicating that plates changed speed more frequently than they changed direction 2 the relative motions of all of the plates analyzed changed at about chron m11 135 ma chron 34 85 ma and chron 25 56 ma 3 during the early cretaceous there were five oceanic plates in the pacific basin rather than the four recognized by previous workers 4 to determine the number of farallon plates that existed to the east of the pacific plate during the time interval from chron 34 85 ma to chron 25 56 ma fracture zones and magnetic anomalies that record pacific farallon spreading from the northern central and southern pacific plate were analyzed separately and collectively the analysis shows that a single pacific farallon relative motion pole and a single rate are consistent with all of the data 5 spreading rates along the pacific kula ridge decreased markedly between chrons 32b and 25 72 56 ma probably in response to the arrival of buoyant kula lithosphere at a subduction zone northwest of the bering sea 6 soon after chron 25 56 ma a major reorganization is recorded along the pacific farallon boundary the kula farallon boundary is also thought to have changed during this reorganization 7 kula pacific anomalies north of anomaly 25 are modeled by assuming continued kula pacific spreading after chron 25 56 ma 8 probably by chron 18 43 ma pacific kula spreading had ceased and the kula farallon ridge system had evolved into alignment with the pacific farallon spreading direction however the timing of these events is uncertain because pacific kula data younger than chron 25 56 ma are sparse

upper crustal structure and neogene tectonic development of the california continental borderland multichannel seismic reflection data sonobuoy seismic refraction data and regional geology are used to define the upper crustal structure of the southern california continental borderland and to delineate the characteristics of the main lithotectonic belts of the region the catalina schist belt is separated on its west side from the gently deformed nicolas forearc belt by faults that have steep west dips and pronounced normal separations on its east side the schist belt is bounded by a large detachment fault that dips gently to the east beneath the west edge of the peninsular ranges belt at the coastline near oceanside the catalina schist was uplifted from middle crustal depths and exposed during a major event of extensional tectonism that started in early miocene time in conjunction with about 10 of clockwise rotation of the western transverse ranges belt part of the uplift of the catalina schist could have occurred on the detachment fault but it is thought to have mostly occurred on the steep faults that bound the west edge of the schist belt a large amount of uplift is required and it probably involved strong footwall flexural deformation in the wake of the translating and rotating western transverse ranges and nicolas forearc belts extension accompanied by probable large amounts of right slip continued in the borderland region during and after middle miocene time the later stage of extension was accompanied by rapid clockwise rotation of the western transverse ranges of at least 90 most of the borderland including the belt of schist that was uplifted in early miocene time was further deformed into numerous basins and ridges during this stage of oblique extension the primary driving force for the deformation is thought to have been derived from the rapid northwest motion of the pacific plate after it had become coupled to the farallon plate system which had previously been subducted beneath the borderland

intracrustal subduction and gravity currents in the deep crust sm nd ar ar and thermobarometric constraints from the skagit gneiss complex washington isotopic and thermobarometric pressure temperature p t data on a gneissic quartz diorite combined with previous u pb ages and p t data on wall rock metapelites define a precise p t time series for a portion of the skagit gneiss complex a deep crustal crystalline complex within a zone of intracontinental collision concordant u pb and sm nd ages on zircon indicate crystallization at 68 ma which preceded metamorphism of nearby pelitic wall rocks garnet core assemblages and the pluton plagioclase hornblende coronas on garnet at 800 900 mpa 700 800 c following nearly isothermal decompression and sillimanite cordierite metamorphism of wall rocks garnet rims plus matrix at 300 500 mpa 650 700 c a sm nd mineral isochron including two garnet separates records an initial phase of cooling of the complex at 60 ma slow cooling 10 c m y extends from approximately 60 ma to 50 ma hornblende and biotite ar ar ages of 47 ma and 45 ma respectively define a second unroofing event cooling rates during this period were approximately 100 c m y late cretaceous deep burial followed by two distinct unroofing episodes is also observed in some cordilleran metamorphic core complexes and may suggest a common origin we propose that regional shortening in the cordilleran interior is accommodated by a process of intracrustal subduction i e the upper crust is forced downward into a crustal asthenosphere resulting in a cold buoyant root heating and weakening of the root cause the formation of gravity currents effective viscosity being in the range of 10 16 10 19 pa s spreading outward below nonfluid upper crust spreading of the gravity current which does not necessarily contribute to crustal thinning is driven by the small density contrast between the root and its mid crustal surroundings hence unroofing is neither a response to overthick crust nor controlled by the orientation of principal stress axes in the upper crust and upper mantle the root then cools slowly until final unroofing to near surface levels which may occur via either extension or shortening with consequent erosion

mesozoic and cenozoic extension recorded by metamorphic rocks in the funeral mountains california the funeral mountains metamorphic core complex like so many of these complexes in the north american cordillera contains early ductile structures related to mesozoic compressional tectonics that have been overprinted by structures related to later extension the core bounding detachment system formed in late miocene time and ductile extensional fabrics in its footwall have been viewed as evidence of a higher temperature stage in the same event integrated structural analysis and geochronology however indicate that ductile fabrics with a transport direction similar to the northwest direction of movement on the detachment are part of an older extensional event of late cretaceous age this event produced mylonitic fabrics a dominant foliation and well developed mineral and stretching lineations mesoscopic folds and a widespread crenulation lineation that developed during a late stage in this event also are consistent with northwest directed shear late cretaceous mylonitic fabrics are related to extensional shear zones within the core complex that have small stratigraphic offsets but may represent significant structural and metamorphic discontinuities rocks in the footwall of the shear zones experienced mid crustal depths that cannot be explained by the overburden available if the funeral mountains core rocks represent an intact block of mesozoic crust instead it appears that a significant section of mid crustal material may have been excised in late cretaceous time by the combined monarch canyon and chloride cliff shear zones as the result of gravitational instability of an overthickened crust in our interpretation this early extension was made possible by a waning of compression within the mesozoic thrust belt combined with a high regional heat flow due to the sierran arc this early extension took place 50 m y before development of the core bounding detachment system movement on which took place at low enough temperatures that deformation was predominantly brittle with some calc mylonite fabric development the youngest structures in the core are normal faults that cut the detachment and indicate a southwest transport direction orthogonal to those of both the earlier extensional events

triple junction instability as cause for late cenozoic extension and fragmentation of the western united states regional extension in the western united states began soon after 30 m y b p the time of first interaction between the pacific and north american plates previous explanations for extension include lithospheric active processes and asthenospheric active processes the former are favored because of coincidence of timing with lithospheric interactions relative timing of volcanism and deformation and coincidence with pre existing structures although extension has occurred in part in a back arc setting it cannot be attributed to age of subducted crust or azimuth of subduction because both attributes favor back arc contraction characteristic behavior before 40 m y b p the mendocino triple junction has been unstable since its inception the continental margin was relatively straight before 30 m y b p and it has become more convex westward as the triple junction has migrated northward the continental margin and arc have been anchored to the subducting slab farallon juan de fuca whereas the triple junction must move parallel to the san andreas transform the combined result has been the northwestward and clockwise movement of coastal blocks relative to the continental interior and the eastward stepping of the san andreas transform relative to the coast these effects result from the unstable geometry of the mendocino triple junction lack of a subducted slab slab window beneath the extended lithosphere enables asthenospheric rise but does not cause extension

palaeotemperature reconstruction during the last glacial from δ18o of earthworm calcite granules from nussloch loess sequence germany abstract the nussloch loess palaeosol sequence rhine valley germany is considered to be one of the most complete records of the last glacial period in western europe due to its very high sedimentation rate and its good chronological control this sequence is therefore a good framework in which to develop new proxies for palaeoenvironmental reconstructions in this study we explore for the first time the potential of earthworm calcite granules as a new bio indicator and climatic proxy of absolute air and soil temperature in the context of last glacial loess these granules are composed of rhomboedric calcite crystals organized in a radial crystalline structure as these granules are individually generated by earthworms at a relative fast rate they are expected to record intra annual variations in the available sources of oxygen percolating waters of meteoric origin we extracted thirty earthworm calcite granules from 11 of 5 cm layers thick from tundra gley and brown soil horizons previously dated at 45 to 23 ka oxygen isotope ratios were measured on each individual granule the 18o of calcite granules and interlinked transfer functions between water cycle air and soil temperatures allowed us to estimate air temperatures ranging from 10 to 12 4 c which most likely reflect the warm periods of the year when earthworms were the most active

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

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

on the gravitational potential of the earth s lithosphere the mean potential energy of the lithosphere is useful for defining the tectonic reference state trs of the earth and can be used to constrain the ambient state of stress in the plates in the absence of external forces applied at the base or along plate boundaries a lithospheric column with the potential energy of the trs would remain undeformed thus the difference between the potential energy of a lithospheric column and the trs determines whether the column is in an extensional joeutral or compressional state of stress we evaluate and intraplate variations about this mean using a simple first order lithospheric density model this model assumed that the continental geotherm is linear and density variations below a depth of 125 km have negligible influence on and is consistent with observed geoid anomalies across continental margins is estimated to be 2 379 1014 n m 1 which is equivalent to the potential energy of both near sea level continental lithosphere 160 to 220 m for an assumed crustal density c in the range 2800 2700 kg m 3 and cooling oceanic lithosphere at a depth of 4 3 km with the exception of eurasia which has anomalously high mean potential energy 2 383 1014 n m 1 the mean potential energies of the continental plates are nearly identical to the global mean the mean potential of the oceanic plates was found to be a strong function of the mean age of the oceanic lithosphere both the global and plate mean potential energies are relatively insensitive to a wide range in c the potential of the mid ocean ridges 2 391 1014 n m 1 is greater than the global mean which is consistent with the divergent nature of the ridges elevated continental lithosphere with a height of about 70 m has an equivalent potential energy to suggesting that in the absence of external forces continental regions will be in a slightly extensional state of stress the importance of our potential energy formulation is substantiated by the strong correlation between the torque poles associated with the potential energy distributions and the observed plate velocity poles for the south american nazca indo australian and pacific plates

tectonics of mid tertiary extension along a transect through west central arizona large magnitude miocene extension in west central arizona occurred primarily along three imbricate northeast dipping normal faults the structurally highest of these faults the gently dipping buckskin rawhide detachment fault accommodated approximately 66 km of crustal extension whereas the two structurally lower faults accommodated a total of about 20 km extension due to this large magnitude extension an area at the earth s surface that was 10 to 20 km wide is now over a 100 km wide and crystalline rocks with mid tertiary mylonitic fabrics uncovered by detachment faulting are exposed over roughly 2000 km in the harcuvar metamorphic core complex most of the upper plate of the buckskin rawhide detachment fault was largely undeformed by internal extension only the thin tapered end of the upper plate was highly extended during extension the lower plate must have flexed to conform to the listric underside of the upper plate and to have flattened to its present subhorizontal form as it was uncovered grooves on the underside of the upper plate were apparently imposed on the pliable lower plate as it was denuded forming extension parallel folds in the lower plate low flexural strength characterized the lower plate during denudation and a highly mobile low viscosity deeper crust must have effectively decoupled the upper crust from the mantle lithosphere

deformation of the central andes 15 27 s derived from a flow model of subduction zones a simple viscous flow model of a subduction zone is used to calculate the deformation within continental lithosphere above a subducting slab our formulation accounts for two forces that dominate the deformation in the overriding lithosphere tectonic forces and buoyancy forces the tectonic forces arise from the subduction of oceanic lithosphere and produce minor extension in the lithosphere near the trench and compression farther inland buoyancy forces can induce horizontal pressure gradients due to variations in crustal thickness which cause lithospheric extension where the crust is thick numerical solutions obtained by using a finite element technique are compared with observations from the central andes 15 27 s the model predicts the observed deformation pattern of extension in the forearc compression in the western monocline corresponding to magmatic activity extension in the altiplano compression in the eastern monocline and subandes and no deformation in the brazilian shield by comparing the calculated solutions with the large scale tectonic observations we are able to evaluate the forces that govern the deformation in the central andes the approximately constant subduction velocity in the past 26 m y suggests that the rate of crustal shortening in the andes has decreased with time due to the thickening of the crust

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