erosional control on the geometry and kinematics of thrust belt development in the central andes long term erosion rates in the central andes may have influenced not only mean elevation and relief but also the regional geometry and kinematic history of the orogenic belt a drastic along strike erosional gradient exists in the modern central andes from a high erosion region directly north of the 17 5 s bend in the andes to a low erosion region south of the bend this gradient has existed since 10 15 ma based on fission track analyses of middle miocene to holocene denudation and qualitative evaluations of the preservation potential of middle late tertiary volcanic edifices and synorogenic sediment global positioning system velocity data indicate conflicting patterns of active surface shortening north and south of the 17 5 s bend whereas present day shortening in the thrust belt north of the bend is distributed over much of the width of the belt south of the bend it is concentrated near the eastern frontal margin structural data suggest a similar kinematic situation during late miocene to holocene shortening an out of sequence chronology of thrusting in the narrow 200 km wide thrust belt to the north versus a forward breaking sequence of thrusting in the relatively wide 350 km wide thrust belt to the south the long term internal deformation and limited width of the thrust belt north of the bend are attributed to prolonged subcritical thrust wedge conditions induced by rapid erosion rates since 10 15 ma such conditions inhibit thrust front advance and favor distributed deformation within the thrust belt interior in the thrust belt south of the bend a progressive eastward migration of thrusting is interpreted to be the result of long term critical thrust wedge conditions promoted by extremely low rates of denudation since middle miocene time

reconstructing the kinematic evolution of curved mountain belts internal strain patterns in the wyoming salient sevier thrust belt u s a analyses of mesoscopic structural and strain patterns in red beds of the triassic ankareh formation and limestones of the jurassic twin creek formation in concert with complementary paleomagnetic studies constrain the three dimensional kinematic evolution of curved fold and thrust systems in the wyoming salient of the sevier thrust belt spaced cleavage fracture and vein networks minor folds and minor faults in limestones and red beds accommodated early layer parallel shortening lps concentrated in front of the growing thrust wedge along with minor strike parallel extension and wrench shear related to the development of orogenic curvature strain estimated using mass balance relations for cleavage seams crinoid ossicles in bioclastic limestone and reduction spots in red beds displays systematic regional patterns principal shortening directions are subperpendicular to structural trend around the salient reflecting a combination of primary curvature and secondary rotation of early lps fabrics lps magnitudes vary from 5 in central parts of the frontal hogsback thrust system where cleavage is absent to 10 30 in the more interior crawford thrust system where cleavage intensity is moderate to strong strain also increases toward the salient ends internal strain is a significant component of total deformation and should be considered when restoring cross sections strain patterns are consistent with a kinematic model involving curved fault slip and differential shortening that produced progressive secondary curvature during thrusting

migration of dynamic subsidence across the late cretaceous united states western interior basin in response to farallon plate subduction backstripped cross sections of the late cretaceous succession across central utah colorado and southern wyoming in the western interior basin united states reveal a component of continuously evolving long wavelength residual subsidence in addition to subsidence driven by the sevier thrust belt and associated sediment loads the loci of maximum rates of this residual subsidence moved eastward from ca 98 to 74 ma in phase with the west to east passage of the farallon slab as reconstructed from tomography based on quantitative inverse models these new subsidence data allow testing of existing subduction models and confi rm the dynamic subsidence origin of the western interior basin furthermore regional variations in subsidence rates suggest a possible defi cit of negative buoyancy mantle loading inside the slab beneath colorado supporting the hypothesis that the thickened slab represents a subducted oceanic plateau this paper documents how the cretaceous stratigraphy records the timing patterns and position of underlying mantle processes during farallon slab subduction the new data also reveal patterns indicative of the commencement of the laramide orogeny in the western united states

ten million year history of a thrust sheet the final deformed state of a fold and thrust belt may be reached by an infinite number of kinematic paths two end member kinematic paths are due to continuous or discontinuous rates of deformation we have used a new magnetostratigraphic section from the spanish pyrenees to calibrate the emplacement history over 10 m y of a major thrust sheet the sierras marginales thrust sheet and the deformation of both its hanging wall and footwall six time windows from before 36 5 ma until after 24 7 ma were recognized on the basis of structural and stratigraphic relationships between syntectonic strata and major structures footwall deformation of the sierras marginales thrust sheet occurred continuously on a detachment within the foreland basin sequence 4 5 km net shortening shortening rates steadily increased from 0 14 to 1 5 mm yr with time although the detachments at the base of the foreland and the detachment at the base of the sierras marginales thrust sheet were active coevally the rate of displacement on the sierras marginales detachment decayed with time and shows considerable variability emplacement of the thrust sheet may be divided into three distinct periods a rapid 13 8 km translation from 37 0 to 36 5 ma 27 6 mm yr a gradual climb of the toe of the thrust sheet up a 4 3 km long ramp across the foreland basin succession from 36 5 to 32 0 ma 0 95 mm yr and a final 8 9 km translation from 32 0 to 29 5 ma 3 56 mm yr internal deformation of the thrust sheet occurred only after it reached its present position at some time before 29 5 ma shortening rates steadily decreased from 0 6 mm yr between 29 5 and 27 8 ma to 0 26 mm yr from 27 8 until after 24 7 ma only 1 km of shortening accumulated during each of the two periods the last shortening localized on the most northerly thrust in the study area folding and subsequent faulting above the detachment beneath the foreland suggest that slip was transmitted to its tip point continuously throughout the deformation in contrast translation followed by internal deformation on the hinterland side of the toe of the sierras marginales thrust sheet indicates a successive deactivation of the southern parts of the detachment with time rate of deformation on both detachments was discontinuous and shows substantial variability about the mean in general the spatial and temporal pattern of deformation was distributed and continuous in detail however shortening was spatially and temporally discontinuous above each detachment and structures related to each one display distinctly different deformational patterns rates and styles

watershed reconstruction of a paleocene eocene lake basin using sr isotopes in carbonate rocks provenance studies have used sr isotopes 87 sr 86 sr of silicate source rocks as a link to their eroded basinal equivalents this technique however cannot identify the proportional inputs from different watersheds or define more precisely sedimentation events due to tectonic or climatic change erosion of carbonate rocks dominates the sr input within basin drainage and potentially can be used through 87 sr 86 sr ratios to reconstruct paleohydrology of the entire basin and trace watershed inputs and depositional patterns in continental basins the sr isotopic ratios from waters of the source area allowing for the mixing of shallow groundwater and surface water along the transport path are homogenized in the basinal carbonate sediments mineralogy and diagenesis of carbonate rocks generally do not affect the sr isotopic signal in a near surface system lacking external influence by volcanism eolian dust or deep geothermal waters the 87 sr 86 sr ratios from the source area are directly comparable to those in the receiving continental basin the sr isotopic signal of the paleocene eocene flagstaff formation central utah a carbonate lake deposit in a foreland basin is compared to that of projected source waters draining its thrust front the sevier fold thrust belt freshwater carbonates compose a large portion of the lowermost ferron mountain and uppermost musinia peak members of the formation whereas gypsum and carbonates predominate in the middle cove mountain member previous research had attributed gypsum deposition to the deposition of the middle cove mountain member to either climatic change or unroofing of diapirs of jurassic gypsiferous carbonates to examine more closely the influence of climate versus tectonics on flagstaff sedimentation as well as the efficacy of provenance studies using carbonates we collected rock samples from the three members of the formation on the wasatch plateau of central utah in addition to sampling stream water associated with pennsylvanian permian and jurassic carbonate terrains from the nearby thrust front the 87 sr 86 sr ratios in carbonates and gyprock belonging to the flagstaff formation remained unchanged during deposition the average sr isotope composition of the flagstaff rocks being identical to that of sampled waters draining the projected provenance area there was little change in source rock weathering as the thrust front evolved deposition of gypsum occurred in the basinal lake only during the deposition of the middle cove mountain member despite its constant exposure in the drainage area suggesting a changing balance of tectonic and climatic controls during lake sedimentation the 87 sr 86 sr isotopic studies targeting carbonate rocks and their presumed source waters are a simple but accurate method for reconstructing the paleohydrology of lake basins

widespread basement erosion during the late paleocene early eocene in the laramide rocky mountains inferred from 87sr 86sr ratios of freshwater bivalve fossils reconstruction of the timing and pattern of exhumation of basement involved uplifts during the laramide is essential to the understanding of the mechanisms of crustal shortening and thickening in the region we use reconstructions of the 87 sr 86 sr ratios of late cretaceous early cenozoic river water from fossil shells in six basins of the rocky mountains contextualized by modern river water sr chemistry studies to trace the erosion of precambrian basement cores in the laramide ranges the 87 sr 86 sr ratios and sr concentration of modern river water in the rocky mountains are controlled by river bedrock lithology weathering of precambrian silicate rocks in the cores of laramide ranges produces high 87 sr 86 sr ratios in highland rivers weathering of paleozoic and mesozoic carbonates along the basin margins reduces the 87 sr 86 sr ratios and increases sr concentration of rivers as they flow basinward lowland rivers that have headwaters in precambrian basement mostly have 87 sr 86 sr ratios 0 711 whereas rivers confined to or with very long reaches in basins have 87 sr 86 sr ratios between 0 709 and 0 711 river water 18 o values do not change in response to changes in catchment elevation our results from fossil shells show that carbonate weathering dominated the river water sr chemistry in the alberta foreland williston and crazy mountains basins and that proterozoic low grade metamorphic carbonates in the belt purcell supergroup were not exposed in the canadian rocky mountains during late cretaceous early paleocene time silicate weathering influenced the sr chemistry of surface waters in the powder river basin during the late paleocene and the washakie basin during the early eocene suggesting precambrian silicate basement rock was extensively exhumed and eroded the observation that precambrian basement was eroded earlier around the powder river basin than around the washakie basin is consistent with a previous study that suggests laramide ranges in northeast wyoming reached high elevation earlier than the ranges adjacent to the sevier thrust belt widespread basement erosion in late paleocene early eocene time was controlled by tectonic exhumation and may have been intensified by the wet and warm global climate

spatial and temporal evolution of coastal depositional systems and regional depositional process regimes campanian western interior seaway u s a abstract this paper provides a critical review and regional synthesis of late cretaceous shallow marine deposits along part of the western margin of the western interior seaway of north america which contains the most extensively documented outcrop based studies of siliciclastic coastal depositional systems in the world the results of this synthesis are presented in the form of paleogeographic maps covering present day new mexico utah colorado and wyoming usa for five timeslices in the campanian these maps are used to evaluate the spatial and temporal evolution of regional depositional process regimes along a large 1000 m stretch of coastline the evolution of regional depositional process regimes is linked to tectonic and paleoceanographic controls on the western interior seaway which enables the results of this synthesis to be applied to prediction of depositional process regimes in other less intensively studied basins six gross depositional environments have been mapped for each timeslice 1 alluvial to coastal plain sandstones 2 coastal plain coals mudstones and sandstones 3 shoreline sandstones 4 marine mudstones 5 gravity flow siltstones and sandstones and 6 marine marls and chalk shoreline sandstones in each timeslice are interpreted in further detail using documented evidence for the three principal classes of depositional process wave tidal and fluvial and published reconstructions of coastal morphology which is widely considered to reflect depositional process regime based on these interpretations shoreline sandstones are assigned to five categories of depositional process regime 1 regressive wave dominated shorefaces and delta fronts 2 regressive river dominated delta fronts 3 regressive mixed tide and wave influenced delta fronts 4 regressive tide dominated delta fronts and 5 transgressive barrier islands back barrier lagoons and estuaries the accuracy of and uncertainty in classification of depositional process regime are critically evaluated additionally stratal thickness and sediment routing pathways have been interpreted in order to assess the impact of tectonic and paleoceanographic controls on spatial and temporal changes in all of the evaluated shallow marine successions thin tide influenced intervals were deposited in back barrier and lagoon systems associated with net transgressive shorefaces and estuaries however it is notable that all preserved tide dominated and tide influenced regressive deltaic systems are located along the northern margin of the utah bight a tectonically related embayment that had a pronounced expression during the middle campanian the southern margin of the embayment and coastlines to the north are conspicuously more wave dominated which supports their exposure to a larger wave fetch in contrast the northern margin of the embayment was relatively wave protected it is concluded that tidal range was amplified due to resonance of the principal semidiurnal tide in the strongly embayed geometry of the middle campanian utah bight

diapiric ascent of silicic magma beneath the bolivian altiplano 1 the vertical transport of large volumes of silicic magma which drives volcanic eruptions and the long term compositional evolution of the continental crust is a highly debated problem in recent years dyking has been favored as the main ascent mechanism but the structural connection between a distributed configuration of melt filled pores in the source region and shallow magma reservoirs remains unsolved in the central andes inversion of a new high resolution bouguer anomaly data over the altiplano puna magma body apmb reveals 15 km wide vertically elongated low density 3d structures rooted at the top of the apmb at 20 km depth we integrate our gravity inversion with the available geophysical geological and petrological observations and in agreement with petrological mechanical considerations propose that in this region of the andes partially molten granitic bodies ascend diapirically through the hot ductile mid upper crust

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

protolith age of the swakane gneiss north cascades washington evidence of rapid underthrusting of sediments beneath an arc 1 the metamorphic core of the north cascades largely comprises island arc and oceanic terranes juxtaposed prior to circa 96 ma magmatism however the tectonic affinity of the structurally deepest terrane the 9 12 kbar swakane gneiss is distinctly different from other terranes in the core it is not intruded by arc related plutons and contains abundant precambrian zircons new u pb analyses of detrital zircons from the swakane gneiss yield dates from 73 ma to 1610 ma with a dominant late cretaceous population these data indicate that the swakane protolith was deposited as late as 72 5 0 6 ma the 206pb 238u date of the youngest detrital grain following deposition the gneiss was intruded by peraluminous leucogranite sheets that may represent partial melt derived from the gneiss at near to postpeak p t conditions one sheet yielded a u pb crystallization age of 68 36 0 07 ma which indicates that the gneiss was deeply buried within 5 myr of deposition two possible mechanisms considered for this rapid burial 7 mm yr are overthrusting of a forearc or back arc basin by older crystalline rock or underthrusting of trench sediments during low angle subduction the model involving overthrusting of a forearc or back arc basin is most consistent with thermobarometric and isotopic data and the regional geologic setting of the cascades core rapid burial of the swakane protolith is coincident with burial of sediments that formed the pelona orocopia and rand schists of southern california however differences in peak metamorphic temperatures indicate that conditions of burial must have varied along the plate margin

structural configuration of the early cretaceous cordilleran foreland basin system and sevier thrust belt utah and colorado abstract stratigraphic and provenance data from lower cretaceous rocks in utah and colorado and structural evidence from the sevier belt in west central utah allow recognition of the ties between early thrust belt evolution and foreland basin system development regional isopach patterns of lower cretaceous strata define foreland basin system depozones lower cretaceous strata in west central utah that thicken westward from 100 m to 1 2 km were deposited within the flexurally subsiding foredeep depozone correlative units in eastern utah and western colorado 100 m thick were deposited in the back bulge depozone these thickness trends indicate the lower cretaceous foreland basin system consisted of a foredeep 100 180 km wide and forebulge 260 460 km wide a computer generated flexural model of the foreland basin system produces a reasonable match of the observed basin geometry using a 400 km wide l

dynamic versus flexural controls of late cretaceous western interior basin usa the united states cretaceous western interior basin has long been considered a foreland basin driven by the sevier thrust and associated basin sediment loads however flexural studies demonstrate that this effect exists only within a narrow band in front of the thrust belt most of the basin appears to be due to mantle flow induced dynamic subsidence associated with farallon plate subduction here we show how the components of evolving long wavelength dynamic subsidence and flexural subsidence created the accommodation space and controlled the stratigraphy across the western united states based on a correlated stratigraphic section across central utah and colorado these backstripped subsidence data reveal the dynamic topography driven nature of the western interior basin the results seem to support that the depocenters track the trough of dynamic subsidence with ca 18 myr cycles through time and space and the stratigraphic patterns of large scale progradation eastward migration of depocenter and regional clinoform like downlap are related with the dynamic subsidence interpretation of these data also provides more insights into the repeated ca 2 to 6 myr cycles of thrust induced subsidence in front of the thrust belt which control the local eastward progradation of the sand bodies from the thrust belt the dynamic flexural subsidence and eustatic sea level changes interacted and controlled the timing and distribution of unconformities our work shows how the stratigraphy precisely records the timing patterns and position of dynamic versus flexural subsidences and that combination of such data leads to important geophysical discoveries and supplies strict constraints for geodynamic modeling

fold thrust belt evolution expressed in an internal thrust sheet sevier orogen the role of cataclastic flow large hinterland thrust sheets are the dominant structures in retroarc fold thrust belt ftb wedges these internal sheets remain active throughout the history of an ftb and strongly influence the kinematics of the external portions of the ftb and the overall behavior of the entire tectonic wedge their emplacement and deformation account for a significant portion of the total mechanical energy budget in ftb evolution based on estimates using deformation features preserved within such sheets moreover the deformation within dominant internal sheets can be used as a means to evaluate the tectonic history of the wedge as a whole the principal deformation mechanisms in different parts of an ftb wedge are strongly controlled by variations in the geothermal gradient retroarc ftbs typically show a steeper geothermal gradient in the hinterland than in the foreland resulting in a foreland ward slope of the brittl ductile transition therefore a change from plastic to elastico frictional ef mechanisms is expected not only with decreasing depth but also across strike as a dominant internal thrust sheet is transported toward the foreland and undergoes exhumation by synorogenic erosion however evidence for cataclasis overprinting early plastic deformation features is often not preserved in ancient ftbs because of extensive erosion of the wedge top even where evidence for lat stage cataclastic behavior exists in either ancient or modern i e active orogens it is often not recognized or studied our work in the canyon range central utah segment of the sevier ftb shows that unraveling the ef mechanisms such as lat stage cataclastic flow is critical to understanding the total deformation history of an internal thrust sheet after initial emplacement under plastic deformation conditions the cr thrust sheet was continuously reactivated under ef conditions thereby helping to maintain critical taper of the ftb wedge during its development structures preserved in the cr thrust sheet therefore record a significant portion of the deformation history of the ftb as a whole the canyon range cr syncline part of the cr sheet underwent fold tightening within the ef regime and the cataclasized rocks are well preserved the geometry and progressive deformation patterns of the cr syncline reveal four distinct to partly overlapping deformation stages the history of the rocks in the cr thrust sheet has been tracked by incrementally retro deforming the ftb and by unraveling details of the deformation within the cr syncline at various scales for each stage we use the cr syncline as a type example to show that unraveling ef mechanisms is critical to 1 understanding the state of a ftb wedge during its evolution 2 calculating the energy involved in ftb emplacement and 3 developing accurate restorations placing constraints on the depth of the rocks for successive stages of deformation is essential for developing retrodeformable balanced cross sections and in turn geometric and kinematic models for wedge evolution

the use of finite strain data in constructing a retrodeformable cross section of the meade thrust sheet southeastern idaho u s a abstract a method for drawing a balanced cross section is presented which uses the constraint of both the interpretation of the geometry and the analysis of the internal deformation within the meade thrust sheet southeastern idaho u s a earliest deformation associated with the meade thrust sheet is layer parallel shortening which is indicated by a spaced solution cleavage and deformed fossils in the jurassic twin creek formation this early layer parallel shortening fabric is rotated by folding in front of the main meade thrust sheet these folds are then truncated by the meade thrust locally finite strain markers indicate fault parallel shear

crustal scale recycling in caldera complexes and rift zones along the yellowstone hotspot track o and hf isotopic evidence in diverse zircons from voluminous rhyolites of the picabo volcanic field idaho abstract rhyolites of the picabo volcanic field 10 4 6 6 ma in eastern idaho are preserved as thick ignimbrites and lavas along the margins of the snake river plain srp and within a deep 3 km borehole near the central axis of the yellowstone hotspot track in this study we present new o and hf isotope data and u pb geochronology for individual zircons o isotope data for major phenocrysts quartz plagioclase and pyroxene whole rock sr and nd isotope ratios and whole rock geochemistry for a suite of picabo rhyolites we synthesize our new datasets with published ar ar geochronology to establish the eruptive framework of the picabo volcanic field and interpret its petrogenetic history in the context of other well studied caldera complexes in the srp caldera complex evolution at picabo began with eruption of the 10 44 0 27 ma u pb tuff of arbon valley tav a chemically zoned and normal 18o 18o magma 7 9 unit with high zoned 87sr 86sri 0 71488 0 72520 and low e nd 0 18 and e hf 0 28 the tav and an associated post caldera lava flow possess the lowest end 0 23 indicating 40 60 derivation from the archean upper crust normal 18o rhyolites were followed by a series of lower 18o eruptions with more typical lower crustal sr nd hf isotope ratios and whole rock chemistry the voluminous 8 25 0 26 ma west pocatello rhyolite has the lowest 18o value 18omelt 3 3 and we correlate it to a 1 000 m thick intracaldera tuff present in the inel 1 borehole with published zircon ages 8 04 8 35 ma and similarly low 18o zircon values the significant 4 5 decrease in magmatic 18o values in picabo rhyolites is accompanied by an increase in zircon 18o heterogeneity from 1 variation in the tav to 5 variation in the late stage low 18o rhyolites a trend similar to what is characteristic of heise and yellowstone and which indicates remelting of variably hydrothermally altered tuffs followed by rapid batch assembly prior to eruption however due to the greater abundance of low 18o rhyolites at picabo the eruptive framework may reflect an intertwined history of caldera collapse and coeval basin and range rifting and hydrothermal alteration we speculate that the source rocks with pre existing low 18o alteration may be related to 1 deeply buried and unexposed older deposits of picabo age or twin falls age low 18o volcanics and or 2 regionally abundant late eocene challis volcanics which were hydrothermally altered near the surface prior to or during peak picabo magmatism basin and range extension specifically the formation of metamorphic core complexes exposed in the region could have facilitated the generation of low 18o magmas by exhuming heated rocks and creating the large water rock ratios necessary for shallow hydrothermal alteration of tectonically rift zones and volcanically calderas buried volcanic rocks these interpretations highlight the major processes by which supereruptive volumes of magma are generated in the srp mechanisms applicable to producing rhyolites worldwide that are facilitated by plume driven volcanism and extensional tectonics

provenance of the liuqu conglomerate in southern tibet a paleogene erosional record of the himalayan tibetan orogen abstract the liuqu conglomerate exposed along the yarlung zangbo suture zone in southern tibet has been interpreted as a paleogene molasse deposited during india asia collision or the product of collision between india and an intra oceanic arc detailed petrographic studies and analyses of u pb and hf isotopes of detrital zircon indicate multiple sources for the conglomerate including the sedimentary succession within the xigaze forearc basin to the north of the suture the yarlung zangbo ophiolite within the suture itself and the langjiexue group and tethyan himalayan sedimentary rocks to the south of the suture the occurrence of clasts of both indian and asian origin suggests that the conglomerate was deposited after india asia collision the present data suggest that the onset of himalayan tibetan orogenic erosion occurred as early as the middle eocene when the yarlung zangbo suture zone and xigaze forearc succession were uplifted and became sources for the liuqu conglomerate thus the results are inconsistent with the proposal that the provenance and age of the liuqu conglomerate support the hypothesis that india asia collision did not occur until the oligocene it is further suggested that eocene neotethyan slab breakoff which may result in accelerated exhumation and uplift of the orogen provided a deep level dynamic control on the deposition of the liuqu conglomerate

quantifying vertical axis rotation in curved orogens correlating multiple data sets with a refined weighted least squares strike test 1 map scale curvature is a fundamental feature of most contractional orogenic belts and is central to understanding the kinematic and dynamic evolution of mountain systems paleomagnetic analysis combined with detailed structural studies is the most robust means of quantifying vertical axis rotations that produce curvature over a range of temporal and spatial scales this paper explores how vertical axis rotations can best be evaluated for multiple data sets by applying a weighted least squares method to the classic strike test this refined method provides measures of best fit slope confidence interval and goodness of fit between map scale structural trend paleomagnetic rotations and deformation fabric orientations structural trend is estimated by averaging fold axial trace formation contact and bed strike data over kilometer scale areas paleomagnetic and deformation fabric site mean orientations and measurement uncertainties are estimated using vector and bootstrap statistics weighting factors are estimated from combined measurement uncertainty structural noise related to small scale block rotation and stress strain refraction and variations in restoration paths the number of sites needed to obtain a significant confidence interval in strike test slope is a function of combined uncertainty in paleomagnetic or deformation fabric directions and the total range in structural trend around a curved orogen improved estimates of strike test slope and rotation thus require systematic sampling with a wide distribution of sites evaluation of appropriate weighting factors and statistical analysis a case study is presented that highlights application of this refined method to paleomagnetic and deformation fabric data sets from the wyoming salient of the sevier thrust belt paleomagnetic data yield a strike test slope of 0 76 0 11 indicating that the wyoming salient is a progressive arc with 3 4 secondary curvature related to vertical axis rotation synchronous with large scale thrusting and 1 4 initial curvature finite strain anisotropy of magnetic susceptibility and mesoscopic structural orientations which are related to early layer parallel shortening all yield strike test slopes of 0 9 0 1 comparing these slopes with paleomagnetic results indicates that deformation fabrics had initial curvature and thus cannot be used alone to accurately estimate rotations by integrating systematic paleomagnetic and structural data using statistical analysis curvature models for the wyoming salient are closely constrained

tectonically induced change in lake evolution recorded by seismites in the eocene green river formation wyoming seismogenic sedimentary structures seismites provide an opportunity to understand the effects of syndepositional tectonics on lacustrine sedimentation the lowermost wilkins peak member eocene green river formation wyoming usa contains laterally extensive intervals displaying folds load structures mixed brittle ductile deformation microfaults breccias and sedimentary dykes which formed intrastratally in a low energy sublittoral lacustrine environment they developed in situ by liquefaction and plastic or brittle behaviour of semi lithified sediment or by remobilization and injection of granular material their morphological attributes and lateral extent zonation in deformation intensity and the depositional setting all imply deformation triggered by earthquake induced stresses in shallow buried sediment their stratigraphic distribution coincides with a shift in hydrological conditions from a balanced filled to an underfilled lake type independent of climate change this study is the first to ascribe seismites to a pulse of tectonic activity that caused a synchronous change in lacustrine evolution

structural and microstructural evolution of the rattlesnake mountain anticline wyoming usa new insights into the sevier and laramide orogenic stress build up in the bighorn basin the rocky mountains in western us provide among the best examples of thick skinned tectonics following a period of thin skinned tectonics related to the sevier orogeny the compressional reactivation of basement faults gave birth to the so called laramide uplifts arches the bighorn basin located in wyoming is therefore a key place to study the transition from thin to thick skinned tectonics in orogenic forelands especially in terms of microstructural and stress strain evolution our study focuses on a classic laramide structure the rattlesnake mountain anticline rma wyoming usa a basement cored anticline located in the western part of the bighorn basin stress and strain evolution analysis in folded sedimentary layers and underlying faulted basement rocks were performed on the basis of combined analyses of fractures fault slip data and calcite twinning paleopiezometry most of the fractures are related to three main tectonic events the sevier thin skinned contraction the laramide thick skinned contraction and the basin and range extension serial balanced cross sections of rma and displacement profiles suggest that all thrust faults were coeval evidencing strain distribution in the basement during faulting the comparison of rma with another structure located in the eastern edge of the bighorn basin i e the sheep mountain anticline sma allows to propose a conceptual model for the geometric and kinematic evolution of laramide related basement cored anticlines finally the stress evolution is reconstructed at both the fold scale and the basin scale we show that the evolution of stress trends and magnitudes was quite similar in both structures rma and sma during laramide times thick skinned tectonics in spite of different stress regimes during sevier thin skinned tectonics and post laramide times stress trends and fracture patterns were different in these two structures these results suggest that the distance to the orogenic front influenced the fracture patterns but not the foreland stress magnitudes which were likely controlled by the structural style

magnetostratigraphic chronology of cretaceous to eocene thrust belt evolution central utah usa the axhandle basin in central utah is a piggyback basin that formed above the gunnison thrust system the basin contains 1 km of upper cretaceous to eocene strata of the north horn and lower flagstaff limestone formations a significantly more exact chronology based on magnetic polarity stratigraphy has been developed for the basin filling strata the new data provide a detailed temporal framework for the interpretation of the depositional and tectonic evolution of the basin during chrons 33 21 75 49 ma tectonic subsidence curves and stratigraphie relations along the eastern basin margin define two episodes of motion on the gunnison thrust system the majority of tectonic shortening on the system occurred between 83 75 ma and caused widespread non deposition within the axhandle basin a second episode of motion in detail comprising of four separate periods of backthrusting occurred between 70 60 ma the canyon range pavant and nebo thrust sheets were emplaced 30 50 km to the west of the axhandl

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

hyperconcentrated flows and gastroliths sedimentology of diamictites and wackes of the upper cloverly formation lower cretaceous wyoming u s a abstract diamictites and matrix supported wackes of the upper cloverly formation lower cretaceous represent rivers laden with volcaniclastic sediment hyperconcentrated flows the cloverly formation of central wyoming is dominantly fluvial lacustrine and playa deposits the sediments accumulated east of the cordilleran foredeep during the early stages of the sevier orogeny the diamictites and wackes are decimeters thick and stacked vertically forming rock bodies up to 11 m thick pebbles and cobbles within the diamictites and wackes occur as isolated clasts or in poorly defined layers many such clasts exhibit some degree of polish clast lithologies can be correlated to rocks that were exposed in the mountain belt to the west provenance data are consistent with early cretaceous movement on the meade laketown paris willard thrust system the clay fraction of the diamictites and wackes is dominantly illite smectite and smectite characteristic of altered volcanic ash diamictites and matrix supported wackes lack primary sedimentary structures e g cross stratification but do exhibit soft sediment deformation features indicative of dewatering erosional bases and bedding geometries are indicative of channelized flow the diamictites and matrix supported wackes resemble debris flow deposits however maintaining debris flows that behave as bingham plastics on low depositional slopes 200 400 km from the mountain front is improbable rather the diamictites and wackes represent hyperconcentrated flows deposition either en masse by a newtonian or nearly newtonian flow that was turbulent throughout or by progressive sedimentation from a stratified flow with a basal incipient granular mass flow overlain by a turbulent suspension is more consistent with the field data paleohydraulic calculations indicate that grain sizes up to small pebbles could have been transported in suspension with larger clasts transported as bedload consistent with sedimentological evidence much of the finer grained suspended sediment was remobilized volcanic ash likely derived from the idaho batholith to the west northwest the polished extraformational pebbles and cobbles long regarded as dinosaur gastroliths or stomach stones are simply clasts associated with the hyperconcentrated flow deposits the polish exhibited by many of the clasts is attributable to transport in ash laden flows the occurrence of polished extraformational stones in cloverly diamictites and wackes may have implications for presumed gastroliths in other fluvial rocks of the western interior e g the cedar mountain and morrison formations

high resolution stratigraphy of an underfilled lake basin wilkins peak member eocene green river formation wyoming u s a abstract lakes tend to respond noticeably to minor changes in sediment and water balance driven by climatic tectonic or geomorphic processes this unique behavior of lacustrine basins can provide a high resolution record of geologic processes within the continental setting far from the globally averaged record of marine strata the wilkins peak member of the eocene green river formation in wyoming usa is dominated by aggradation of repetitive sedimentary facies successions recording distinct lacustrine expansions and contractions these lacustrine cycles consist of up to four successive facies associations littoral profundal sublittoral palustrine and salt pan because they comprise disparate facies that may never have been simultaneously deposited in the basin wilkins peak member cycles are non waltherian successions that do not readily equate to any established sequence stratigraphic unit the completeness of the stratigraphic record in the wilkins peak member varies continuously across the basin at least 126 cycles are present in the erda white mountain 1 core near the basin depocenter whereas only about one third as many are recognizable 53 km north nearer to the basin margin cycle boundaries terminate northward by gradual amalgamation into palustrine facies reflecting the interplay between varying lake levels and a south dipping deposition gradient evidence for complete desiccation and hiatuses is commonplace even near the basin center a truly complete record may not be present anywhere based on recent 40ar 39ar geochronology of tuffs the average cycle duration in the white mountain 1 core is approximately 10 000 years however the true average duration is shorter due to the presence of lacunae and the time required for deposition of the thinnest cycles may have been less than 1000 years no external driving mechanism is presently known for eocene cycles of such short duration raising the possibility that they are instead autogenic cycles related to geomorphic instability of the surrounding landscape evaporite deposition corresponded closely in time and space to maximum differential subsidence suggesting that tectonic influences on drainage patterns and basin accommodation exerted a primary control differential accommodation was most pronounced during deposition of the lower wilkins peak member and became progressively less so after that time based on structural and stratigraphic observations a period of increased tectonic uplift commenced concurrent with deposition of the lower wilkins peak member

variable shortening rates in the eastern himalayan thrust belt bhutan insights from multiple thermochronologic and geochronologic data sets tied to kinematic reconstructions 1 we present data on the burial displacement and exhumation history of the himalayan fold thrust belt in eastern bhutan these data document the magnitude and timing of displacement of large discrete structures and highlight temporal variability in shortening rates eight new40ar 39ar ages from white mica 32 new zircon u th he ages 7 new apatite fission track ages and 1 new u pb zircon la mc icp ms metamorphic rim growth age are combined with published cooling ages and deformation temperatures and incremental shortening magnitudes from restorations of two published balanced cross sections to illustrate the kinematic and temporal development of the bhutan thrust belt integrating these data from 23 ma to the present illustrates rapid horizontal shortening rates 28 35 mm yr between 23 20 ma and 15 10 ma separated by more moderate rates 10 23 mm yr shortening rates decrease significantly to 7 10 mm yr and possibly as low as 3 4 mm yr from 10 to 0 ma this decrease is interpreted to represent the onset of strain partitioning in the eastern part of the himalayan tibetan orogenic system between shortening in the bhutan thrust belt uplift of the shillong plateau and deformation and outward growth of the northern and eastern tibetan plateau within estimated error horizontal shortening rates during emplacement of the main central thrust sheet and during construction of the upper lesser himalayan duplex approached india asia tectonic velocities thus for periods of time between 23 20 ma and 15 10 ma the bhutan thrust belt may have absorbed nearly all india asian convergence at this longitude

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

a magnetostratigraphic age constraint for the proximal synorogenic conglomerates of the late cretaceous cordilleran foreland basin northeast utah usa

basement involved thin skinned and thick skinned tectonics in the alps the deformation of continental crust during continental collision by folding and thrusting follows three types of structural styles 1 in a true thin skinned style only cover rocks are involved 2 in the case of a thin skinned basement involved style thin slabs of crystalline basement rocks are piled up into a nappe stack 3 in a true thick skinned style the entire upper crust is involved in the deformation in the alps all three styles can be recognized the helvetic nappes and parts of the penninic nappes exhibit true thin skinned style tectonics triassic evaporites jurassic shales and cretaceous marls acted as detachment horizons basement involved thin skinned tectonics is typical for the penninic nappes in the core of the orogen the thickness of the basement thrust sheets is controlled by the effects of mesozoic rifting by deep burial and heating of the subducting crust and by the presence of late palaeozoic structures thick skinned style is observed in the more external parts of the orogen the external massifs and the southalpine nappe system it occurred in the late phase of collision and involved the entire upper crust the basal detachment occurred possibly along phyllonites generated by the breakdown of load bearing feldspar considering the alpine orogen as a whole the lower crust deformed seemingly independently from the upper crust the detachment of the cover units by thin skinned tectonics occurred prior to thrusting related to basement involved thin skinned tectonics thrust faults of both types were overprinted by post nappe folding

geostatistics based regional characterization of groundwater chemistry in a sedimentary rock area with faulted setting sophisticated modeling of a hydrogeological structure and the environment are crucial for underground construction this study characterizes hydrochemical properties of groundwater in a sedimentary rock area clarifying their spatial distribution and correlation with geologic structure and interpreting the groundwater source and chemical evolution water samples geological columns and well logs to 1 km depth were taken at 10 sites in horonobe of northern japan toward the objectives a 3d model of cl concentration was produced in conjunction with resistivity logging data through kriging estimation and sequential gaussian cosimulation variography shows that the dip of the main omagari fault is a control on the spatial correlation structure of cl concentration the 3d model shows that this fault and its auxiliary constitute a clear boundary between high and low saline waters and that cl concentrations tend to change in accord with sedimentary layer structures the integration of stable isotope analysis suggests that deep saline water with heavy d and 18o originated from fossilized seawater whereas shallow freshwater with light values is of meteoric water origin dilute saline water in the deep part of study area is partially attributable to dehydration of silica minerals vertical and lateral groundwater flows are estimated to prevail near the omagari fault and be general in other zones respectively difference in the depth of transition zones may be caused by the dominant flow among downward ascending and lateral consequently geostatistical techniques and data integration are useful to depict regional groundwater systems with a data set of water investigation limited by quantity and location

sequence stratigraphy and marine to nonmarine facies architecture of foreland basin strata book cliffs utah u s a the desert member of the blackhawk formation and the castlegate sandstone in the book cliffs of utah and colorado were analyzed to develop a model of sequence stratigraphy and facies architecture in foreland basins stratal architecture of these rocks is controlled by two regionally extensive surfaces of incision and subaerial exposure interpreted as major sequence boundaries and at least six surfaces of incision and subaerial exposure with more limited lateral extent interpreted as minor sequence boundaries each sequence boundary is a regionally correlatable single surface major sequence boundaries can be traced from marine into proximal alluvial plain environments sequence boundaries have different physical expressions depending upon where in the basin they are observed strata beneath sequence boundaries are unrelated to strata above and time lines cannot be carried across these boundaries flooding surfaces called parasequence or parasequence set boundaries also exert an important control on facies architecture based on parasequence stacking patterns position of sequence boundaries and facies associations the desert member and castlegate sandstone have been divided into sequences with durations of 200 000 to 300 000 yr there are an order of magnitude more sequences in this interval than are predicted by the global sea level chart of haq et al 1988 the major sequences have been divided into the following systems tracts or sequence sets 1 grassy highstand sequence set below the desert sequence boundary named for the grassy member of the blackhawk formation 2 desert lowstand transgressive and highstand systems tracts between the desert and castlegate sequence boundaries 3 castlegate lowstand sequence set resting on the castlegate sequence boundary and 4 castlegate transgressive systems tract parasequence stacking patterns suggest that the grassy highstand sequence set and the desert highstand systems tract were deposited during a relative rise in baselevel the grassy highstand sequence set is punctuated by minor relative falls in baselevel both of these stratal units contain thick

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

late cretaceous paleocene synorogenic sedimentation and kinematic history of the sevier thrust belt northeast utah and southwest wyoming integration of cross cutting structural relationships overlapping sedimentary units new conglomerate provenance data and radiometric and palynological dates provides a basis for reinterpretation of the distribution and timing of late cretaceous through paleocene thrust faulting in the northeast utah southwest wyoming part of the sevier thrust belt these data indicate a general eastward progression of deformation that was punctuated by local out of sequence and hinterlandward verging events provenance data delimit a sequential restoration of a regional cross section the principal thrust systems in northeast utah and southwest wyoming are the willard ogden crawford absaroka and hogsback thrusts the willard is the westernmost structurally highest and oldest of the thrusts it carries a unique section of thick proterozoic sedimentary rocks as well as paleozoic rocks and was folded during displacement on younger thrust systems the next youngest thrust system is the ogden which comprises several basement rooted imbricate thrusts that together form a large antiformal stack with a structural culmination in the wasatch range growth of the wasatch culmination took place during coniacian through paleocene time contemporaneous with sequential displacement on the frontal crawford absaroka and hogsback thrusts total shortening in this part of the thrust belt was 100 km and structural relief of 25 km developed in the area of the wasatch culmination during coniacian through paleocene time thrusting followed an overall eastward progression that was interrupted by local out of sequence and hinterlandward verging events several episodes of synchronous displacement on two or more thrusts can be demonstrated shortening occurred in three main episodes the first episode 89 84 ma involved 33 km of shortening on the crawford thrust and its footwall imbricates approximately 19 km of structural relief on the basement cover contact developed in the area of the wasatch culmination the second episode 84 62 ma with a break between 75 69 ma involved 30 km of shortening mainly on the absaroka thrust and development of an additional 6 km of structural relief in the culmination the third episode of shortening 56 50 ma took place on the hogsback thrust involved 21 km of horizontal shortening and produced no significant increase in structural relief in the culmination long term rates of shortening ranged between 3 0 mm yr and 6 6 mm yr these three episodes of shortening produced three large accumulations of synorogenic conglomerate totaling 3 km in thickness the henefer formation and echo canyon and weber canyon conglomerates were deposited during crawford thrusting the evanston formation was deposited during and after absaroka thrusting and the lower conglomeratic part of the wasatch formation was deposited during and after hogsback thrusting most of the sediment in these synorogenic units however was derived from repeated uplift of the willard thrust sheet and from the eastern flank of the wasatch culmination in the rear part of the thrust belt only local minor accumulations were derived from the frontal ramp anticlines sediment accumulation and structural deformation were generally out of phase periods of regional shortening and uplift were marked by development of unconformities and sediment bypassing to distal parts of the foreland basin periods of structural inactivity were marked by accumulation of aerially widespread braided river conglomerate on top of the thrust belt one exception to this pattern is the henefer echo canyon weber canyon conglomerate deposit which contains evidence of progressive deformation in close proximity to the tip of the crawford thrust comparison of the sequential restoration with the late cretaceous subsidence history and isopach patterns in the distal foreland basin of western wyoming demonstrates that the principal controls on regional subsidence and sediment supply were the growth and erosion of the wasatch culmination growth of the duplex beneath the culmination may have been a means of maintaining critical taper in the thrust wedge

reconstructing the kinematic evolution of curved mountain belts a paleomagnetic study of triassic red beds from the wyoming salient sevier thrust belt u s a determining the kinematic history and mechanics of curved fold and thrust belts is fundamental to understanding the tectonic evolution of mountain systems to better understand the development of a classic curved fold and thrust belt we completed an integrated paleomagnetic and strain study of the wyoming salient paleomagnetic data are reported here from 154 sites collected from red beds of the triassic ankareh formation in the salient and nine sites collected from the relatively stable foreland red beds display three components with distinctly different magnetic behaviors 1 a near primary triassic magnetization carried by hematite that is stable up to 680 c tr component 91 sites 2 a cretaceous chemical remagnetization carried partly by magnetite k component 32 sites and 3 a recent viscous magnetization that is mostly removed by 350 c site mean vectors for the tr and k components show a high degree of scatter from expected triassic and cretaceous reference directions suggesting significant tilt and rotation subsequent to magnetization acquisition restoration of tilt and folding for individual site means results in well clustered shallow and moderate inclinations for the tr and k components respectively and in variable declinations related to systematic vertical axis rotations statistical analysis of declinations for both components indicates that 75 of present day salient curvature resulted from secondary rotation and 25 of primary curvature was likely related to sedimentary basin architecture analysis of individual thrust systems indicates a slightly greater component of rotation in more internal sheets 80 compared to the frontal thrust sheets 65 suggesting that rotations were concentrated near the leading edge of the propagating fold and thrust wedge with only minor additional rotation of internal sheets transfer zones oblique ramps and more deformed overturned fold limbs display locally more complex patterns which can be understood through careful structural analysis when combined with internal strain data and regional structural relations paleomagnetic data support a kinematic model of a progressive arc with curved thrust slip paths and differential shortening that rotated early layer parallel shortening fabrics and produced minor strike parallel extension this kinematic history likely reflects a combination of processes including greater initial stratigraphic thickness and subsequent shortening and wedge propagation in the central part of the salient presence of a weak basal detachment and fault zone weakening that favored lower taper and buttressing by laramide foreland uplifts that formed along basement promontories at the north and south ends of the salient

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