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

effect of predeformational basin geometry in the kinematic evolution of a thin skinned orogenic wedge insights from three dimensional finite element modeling of the provo salient sevier fold thrust belt utah 1 in fold thrust belts sedimentary cover rocks are detached from undeformed basement and undergo crustal scale shortening and internal deformation we have investigated a three dimensional 3 d nonlinear elastic plastic finite element model using the restored provo salient of the sevier belt as our initial configuration in the model the deformed sedimentary prism displays large scale geometries that are seen in many natural fold thrust belts e g arcuate salient wedge shaped cross section and kinematics that are compatible with observations in the internal and external portions of the provo salient these suggest that the model can be used to predict geologic information that is generally not available from detailed observational studies in natural fold thrust belts e g strain history material displacements stress conditions the model results indicate symmetric noncoaxial plane strain paths with consistent stress and strain orientations and material displacement directions in the middle of the 3 d wedge and fully 3 d nonsymmetric noncoaxial nonplane strain paths with out of transport material displacements over the lateral boundaries the results from test runs further suggest that oblique ramps with strike direction less than 20 from the regional transport direction behave like lateral ramps and those with strike direction greater than 80 from the regional transport direction behave similar to frontal ramps oblique ramps with dips greater than 60 behave like tear faults these variations in different parts of the wedge are caused mainly by interaction between the transport parallel motion of the moving wedge and the preexisting footwall template of ramps and flats that the wedge has to ride over during its evolution

carbon isotope systematics of a mantle hotspot a comparison of loihi seamount and morb glasses the carbon isotope geochemistry of glasses from loihi seamount has been compared with that of morb glasses stepped heating shows two carbon components in both sample suites 1 isotopically light carbon avg 13c 26 3 released 600 c regarded as indigenous the high temperature component in morb samples varied from 52 to 169 ppm c average 13c 6 6 consistent with previous studies overall mord average 13c 6 4 0 9 and new results for indian ocean glasses are similar to atlantic and pacific ocean samples carbon release profiles produced by stepped heating may be typical of locality but there are no significant differences in 13c values between morb samples from different areas lower yields 17 110 ppm c correlated with depth in the loihi samples suggest that they are partially degassed this degassing has not affected 13c values significantly avg 5 8 loihi tholeiites have higher 13c avg 5 6 than the alkali basalts avg 7 1 carbon abundances correlate well with he concentration data comparison of the 13c values with trace element and he sr nd and pb isotope data from the literature suggests that the loihi samples with highest 13c have high3he 4he and possibly the least depleted143nd 144nd and87sr 86sr the carbon isotope data are consistent with previous models for loihi involving several mantle sources lithospheric contamination and mixing the slightly higher 13c of loihi tholeiites suggests that the undegassed plume component manifested by high3he 4he values might have 13c about 1 higher than the morb average

seismogenic sources potentially responsible for earthquakes with m 6 in the eastern southern alps thiene udine sector ne italy summary new geomorphological and structural data permitted to define the active faulting framework of the eastern southern alps ne italy all the active faults detected in the investigated area are thrust segments of the complex thrust system which has been responsible for the latest building of the eastern southalpine chain esc geomorphological investigations were performed to identify the surficial traces of recent fault activity generally represented by gentle scarps connecting uplifted palaeolandscapes of quaternary age with the flat and lower areas of the venetian and friulian plains surficial and subsurficial data the latter from reflection seismic profiles available for the investigated faults indicate that the thrusts have been responsible for the displacement of the entire wedge of quaternary deposits in the western sector of the investigated area the six recognized fault segments represent portions of a 100 km long thrust system at the boundary between the alpine relief and the plain areas in the eastern sector active tectonics is the result of parallel thrust segments located both in the alpine mountainous area and in the friulian plain the 3 d geometry of the active thrust segments has been derived from new structural surficial surveys and the interpretation of reflection seismic profiles for a total length of 1700 km on this basis we defined the geometry of 10 seismogenic sources whose dimensions are consistent with the occurrence of earthquakes with m 6 the comparison between the source geometry and the highest intensity data point distribution of large historical earthquakes has permitted to make hypotheses on the association of past seismic events to specific seismogenic sources this procedure indicated that no large historical events can be attributed to three sources montello conegliano arba ragogna medea this may indicate an elapsed time since the last activation of more than eight centuries based on the completeness of the historical catalogues the available data define therefore sources and related areas for which a high level of seismic hazard may be invoked

controls on the strength of coupling among climate erosion and deformation in two sided frictional orogenic wedges at steady state 1 many important insights regarding the coupling among climate erosion and tectonics have come from numerical simulations using coupled tectonic and surface process models however analyses to date have left the strength of the coupling between climate and tectonics uncertain and many questions unanswered we present an approximate analytical solution for two sided orogenic wedges obeying a frictional rheology and in a condition of flux steady state that makes explicit the nature and sensitivity of the coupling between climate and deformation we make the simplifying assumption that the wedge grows in a self similar fashion consistent with airy isostasy such that topographic taper is invariant with orogen width tectonic influx rate and climate we illustrate first how and why the form of the erosion rule matters to orogen evolution and then derive a physically based orogen scale erosion rule we show that steady state orogen width crest elevation and crustal thickness are controlled by the ratio of accretionary flux to erosional efficiency to a power dictated by the erosion process remarkably we show that for most combinations of parameters in the erosion law rock uplift rate is more strongly controlled by erosional efficiency than it is by the accretionary flux further assuming frontal accretion with no underplating the spatial distribution of erosional efficiency dictates the relative rock uplift rates on the pro wedge and retro wedge and the time averaged trajectories of rocks through the orogen the restriction to invariant frictional properties is conservative in these respects systems subject to positive feedback between erosion and rheology will exhibit even stronger coupling among climate erosion and deformation than shown here

three dimensional kinematic history at an oblique ramp leamington zone sevier belt utah abstract differences in kinematics in adjoining salients in fold thrust belts are typically accommodated along bounding transverse zones the leamington zone is an oblique ramp that accommodates slip between structures in the provo salient and the central utah segment of the sevier fold thrust belt the leamington zone consists of the leamington canyon thrust associated second order asymmetric folds and an out of syncline reverse fault the leamington canyon thrust is a rotated se dipping thrust fault with top to southeast motion it represents an oblique ramp of the folded canyon range thrust fold tightening of the syncline se of the folded leamington canyon thrust trace during later emplacement of underlying structures caused out of syncline reverse faulting in the fold core analysis of the finite strain minor fault populations and conjugate fracture sets were used in interpreting the 3 d kinematic history along the leamington zone restored maximum stretching directions of finite strain ellipsoids trend eastward overall inferred motion planes from populations of slickenlines on minor faults are consistent with overall southeastward motion acute bisectors of conjugate fracture sets may reflect more southward directions later in the deformation history these superimposed deformations with local relative chronologies of plastic and brittle structures using cross cutting relationships indicate that the leamington zone area shows overall clockwise changes in kinematic directions from e to ese to sse during successive pulses of deformation the temporal changes in kinematic directions along the leamington zone most likely reflect variations in kinematics over the oblique ramp as the overall easterly displaced 3 d fold thrust belt wedge interacted with the pre existing oblique ramp structure with small magnitudes of superimposed local vertical axis rotations during later folding and fold tightening of the leamington canyon thrust

structural and sedimentological development of footwall growth synclines along an intraforeland uplift east central bighorn mountains wyoming structural sedimentological and provenance data from paleogene synorogenic deposits of the east central flank of the bighorn mountains provide new information about the development of footwall growth synclines the evolution of fault related folds and the erosional unroofing history of intraforeland uplifts three conglomerate units the upper conglomerate member of the fort union formation and the kingsbury and moncrief members of the wasatch formation are incorporated within an asymmetric east verging growth syncline in the footwall of the main range bounding thrust system three stages of footwall deformation are recorded within these conglomerates analysis of mapped progressive unconformities retrodeformed balanced cross sections and conglomerate clast composition data define these stages as part of a continuum of deformation associated with the development of footwall growth synclines development of an anticline syncline pair marked the earliest stage of growth syncline formation stage i rotation of the shared fold limb resulted in amplification of the growth syncline fine grained synorogenic sediment derived from easily eroded mesozoic mudstone bypassed the growth syncline during this stage by the end of lebo shale deposition an average of 12 1 of shortening and 6 46 km of uplift had occurred along the range margin continued growth syncline development was marked by the deposition of the kingsbury conglomerate the kingsbury conglomerate was derived from resistant middle and lower paleozoic carbonate strata in the uplifted source terrane intraformational unconformities recording as much as 55 of bed rotation were developed within the kingsbury conglomerate as fold limb rotation occurred coeval with deposition cross sections indicate that during this early stage of fault related folding an average of 16 9 shortening and 8 12 km of uplift occurred along the eastern flank of the bighorn mountains end of stage i the intermediate stage stage ii of footwall growth syncline development involved partial truncation of the growth syncline by the advancing thrust faults and deposition of the moncrief conglomerate the lower portion of the moncrief conglomerate was rotated basinward in the developing growth syncline the final stage of deformation stage iii was dominated by the thrust faulting of middle and lower paleozoic strata eastward over steeply dipping mesozoic strata and rotated eocene synorogenic conglomerate during this stage of deformation the moncrief conglomerate was deformed as the initially blind thrusts propagated into the near surface conglomerate deposits truncated the entire footwall syncline and overrode the synorogenic conglomerate package cross sections in areas where this final stage of deformation is well developed indicate that an average of 24 1 shortening and 9 7 km of uplift had occurred along the eastern margin of the bighorn mountains the caliber of synorogenic deposition in the powder river basin was linked directly to the lithologic composition of the bighorn mountains approximately half of the 3 6 km thick source stratigraphic section of the eastern bighorn mountains was eroded prior to accumulation of conglomerate the majority of this eroded material was derived from mesozoic mudstone and poorly indurated sandstone that were incapable of generating coarse detritus the first paleogene conglomerates deposited along the east central bighorn mountains therefore do not represent the initiation of laramide uplift but instead represent the exposure of coarse clast forming rock types from the lower half of the hanging wall stratigraphic section i e the mississippian madison limestone and ordovician bighorn dolomite

comment on carbon isotope systematics of a mantle hotspot a comparison of loihi seamount and morb glasses by r a exley d p mattey d a clague and c t pillinger commentaire sur l article de r a exley d p mattey d a clague et c t pillinger paru dans earth planet sci letters nld 1986 78 189 199

late cenozoic clockwise rotations in the westernmost part of the arcuate qiulitage fold and thrust belt of southern tian shan foreland and its tectonic implications curvature of fold and thrust belts ftbs is a rather common feature in foreland basins the problem of how curves in ftbs originate is essential for understanding the propagation of deformation and tectonic history of orogens in this study we carried out systematic paleomagnetic studies in the westernmost part of the qiulitage ftb southern tian shan foreland where thrusting strike slip fault and tectonic boundary coexist our new results suggest that the study area has been subjected to similar to 20 degrees clockwise rotations after similar to 5ma oroclinal test of the paleomagnetic data across the ftb suggests that oroclinal bending caused by the formation of the curved qiulitage ftb is the dominate reason for these tectonic rotations the kalayuergun dextral strike slip transfer fault delimiting the western boundary of the qiulitage ftb is a thin skinned structure accommodating the discrepancy in horizontal displacement on both sides of it these results also suggest that the formation of the curved qiulitage ftb should not be older than similar to 5ma indicating that the southern tian shan foreland has experienced significant tectonic shortening since the latest miocene to early pliocene our new results together with previous studies on deformation history estimates of crustal shortening gps observations and earthquake records suggest that the southern tian shan foreland has been subjected to significant deformation during the past similar to 5million years and the crustal shortening is still ongoing

structural development and stress evolution of an arcuate fold and thrust system southwestern greater caucasus republic of georgia abstract the southern front of the greater caucasus is quite rectilinear in plan view with the exception of part of the rioni basin where marine and continental deposits of cretaceous neogene age were locally folded and uplifted this resulted in the formation of an arcuate fold and thrust system that extends 45 km into the foreland although previous studies suggested that this system has developed only since miocene times our new detailed and systematic field measurements of brittle and ductile structures show a very complex history consisting in four main phases of brittle deformation and folding dated from eocene to quaternary times we collected microtectonic data at 248 faults and calculated the related paleostress tensors the first two phases which we document here predated folding and were characterised by dominant transcurrent faulting and subordinate reverse motions the greatest principal stress 1 was perpendicular and later parallel to the mountain belt afterwards nw se e w and ne sw trending south vergent asymmetrical folds started to form in the western sector of the study area folds are sinuous in plan view whereas to the east they show a left stepping en echelon geometry another two brittle deformation phases took place after the folding due to the activity of a set of right lateral strike slip faults that strike nw se and ne sw respectively as well as by left lateral strike slip faults mostly striking nw se ne sw and nne ssw these two additional phases were produced by a ne sw to n s trending 1 the arcuate belt is marked by along strike variations in the tectonic regime and deformation geometry plus belt parallel stretching based on our field data integrated with published analogue models we suggest a possible explanation for the rioni structure in terms of the oblique asymmetric indentation of an upper crustal blocks moving to the ssw

origin of the uinta recess sevier fold thrust belt utah influence of basin architecture on fold thrust belt geometry abstract structural trends in the sevier fold thrust belt define a pronounced concave to the foreland map view curve the uinta recess in north central utah this recess separates two convex to the foreland curves the provo salient on the south and the wyoming salient on the north the two limbs of the recess comprise transverse zones fault zones at a high angle to the regional trend of the orogen that border the flanks of the east west trending uinta cottonwood arch our structural analysis indicates that the transverse zones formed during the sevier orogeny and that they differ markedly from each other in structural style the charleston transverse zone ctz on the south side of the arch initiated as a complex sinistral strike slip fault system that defines the abrupt northern boundary of the provo salient the mount raymond transverse zone mrtz on the north side of the arch represents the region in which the southeast verging southern limb of the gently curving wyoming salient was tilted northwards during the laramide phase of uplift of the uinta cottonwood arch in effect the mrtz represents an oblique cross section through a thrust belt the contrasting architecture of these transverse zones demonstrates how pre deformation basin geometry influences the geometry of a fold thrust belt analysis of isopach maps indicates that at the time the sevier fold thrust belt formed the area just north of the present site of the uinta cottonwood arch was a basement high with a gently dipping north flank and a steeply dipping south flank thus predeformational sediment thickened abruptly to the south of the high and thickened gradually to the north of the high as illustrated by sandbox models the distance that a fold thrust belt propagates into the foreland depends on the thickness of the sedimentary layer being deformed so the shape of the salient mimics the longitudinal cross sectional shape of the sedimentary basin where basins taper gradually along strike the thrust belt curves gently but where basins taper abruptly along strike the thrust belt curves so tightly that it disarticulates and becomes bounded laterally by a strike slip accommodation zone the geometry of the uinta recess provides a field example of this concept differential movement of sevier thrusts led to formation of gradually curving thrusts on the north side of the high because of the gradual slope of the high s north flank but led to the along strike disarticulation of thrusts on the south side of the high because of the steep slope of the high s south flank in effect therefore thrust belt map view geometry provides insight into predeformational basin geometry

basin scale stratigraphic architecture and potential paleocene distributive fluvial systems of the cordilleran foreland basin alberta canada abstract the paleocene paskapoo formation in western canada is composed of several hundred meters of fluvial strata that were deposited in an actively subsiding retro arc foreland basin adjacent to the southern canadian cordillera here we utilize newly available subsurface data from down hole gamma ray well logs to reconstruct the large scale fluvial stratigraphy and depositional history of the paskapoo formation in the central portion of the cordilleran foreland basin in alberta paleocene fluvial strata in this region contain several notable characteristics including variable degrees of channel sandstone amalgamation regionally variable stratigraphic architecture and a large fan shaped sandstone unit that emanated from the adjacent fold thrust belt fluvial channel sandstone bodies are abundant and amalgamated near the base of the formation in the southern portion of the basin but become less apparent in the northernmost 100 km of the basin the middle interval of the paskapoo formation characterized by abundant mudstone with isolated channel sandstone bodies is prevalent across the entire study area the upper portion of paskapoo formation is dominated by a large 5000 km2 north northwest trending laterally continuous sandstone in the northern portion of the basin that abuts against the fold thrust belt and fans outward into the basin this feature is interpreted as either a distributive fluvial system or a fixed outlet river system that experienced repeated avulsions this area of the fold thrust belt may be the site of a long lived fluvial outlet the variable degree of sandstone amalgamation in the paskapoo formation suggests two different modes in the relative ratio of accommodation to sediment supply periods of rapid subsidence likely associated with thrust emplacement in the adjacent fold thrust belt resulted in mudstone dominated successions with isolated sandstone bodies whereas tectonically inactive periods associated with less subsidence resulted in amalgamated sandstone units the principal stratigraphic features reconstructed from the data in this area can be used to better understand and predict fluvial stratigraphy within other foreland basins

active deformation and plio pleistocene fluvial reorganization of the western kura fold thrust belt georgia implications for the evolution of the greater caucasus mountains since the plio pleistocene southward migration of shortening in the eastern part of the greater caucasus gc into the kura foreland basin has progressively formed the kura fold thrust belt kftb and alazani piggyback basin which separates the kftb from the gc previous work argued for an eastward propagation of the kftb implying that the western portion in georgia is the oldest but this hypothesis was based on coarse geologic maps and speculative ages for units within the kftb here we investigate this hypothesis and focus on the gombori range gr which defines nw edge of the belt previous work divided the sediments of northern flank of the range into three facies the rock types in the older and middle facies suggest a gc source provenance despite the modern drainage network in the ne gr which is dominated by ne flowing rivers paleocurrent analyses of the alluvial conglomerates of the oldest and youngest syntectonic units indicate a switch from dominantly sw directed paleocurrents in the oldest unit to paleocurrents more similar to the modern drainage network in the youngest unit a single successful 26al 10be burial date indicates these syntectonic sediments are 1 1 ma which while not a precise age is consistent with original mapping suggesting these sediments are akchagyl apsheron 2 7 0 88 ma age tectonic geomorphologic analyses indicate that western gr is the most active given its close proximity to the capital city of tbilisi this suggests that active structures within the gombori range pose seismic hazard to this city of 1 2 million people

anomalous clastic wedge development during the sevier laramide transition north american cordilleran foreland basin usa new high resolution regional correlation and isopach maps provide evidence that 1 laramide style deformation began as early as ca 77 ma in central utah and 2 rapid 208 km m y 1 and extensive 400 km progradation of a clastic wedge was facilitated by reduced subsidence during the transition from sevier to laramide style deformation this study defi nes three campanian alluvialto marine clastic wedges that traversed 200 400 km eastward across the utah colorado segment of the cordilleran foreland basin wedges a and c are thicker successions with rising trajectory shoreline stacking patterns blackhawk formation and lower castlegate sandstone bluecastle tongue and rollins sandstone that refl ect relatively slow overall progradation 50 81 km m y 1 of narrow 10 20 km wide wave dominated shorelines in contrast wedge b consists of lower volume successions with a fl at to falling shoreline stacking pattern middle castlegate sandstone sego sandstone neslen formation corcoran and cozzette members of the iles formation that suggests rapid progradation 208 km m y 1 of embayed 60 80 km wide mixed energy wave and tide infl uenced shore lines wedges a and c prograded 200 250 km in more than 3 m y whereas wedge b prograded 340 400 km in 2 m y 170 km m y 1 the anomalously extensive wedge b is unique in the utah colorado segment because of its long extent rapid progradation rate dominance of tidally infl uenced facies long transit transgressions regressions and low accommodation nested sequence architecture stratigraphic relationships indicate development of wedge b coeval with both sevier and laramide style deformation in utah assuming a constant sediment supply the extensive 300 400 km sheet like amalgamated wedge wedge b may have been caused by 1 reduced subsidence driven by a fl exural interference pattern whereby two short wavelength 200 km fl exural profi les are superimposed during the uplift of both basement cored and thin skinned thrust belt 2 an increase in or eastward migration of dynamic subsidence during tectonic transitions associated with slab fl attening or rollback i e long wavelength fl exure or 3 reduced subsidence due to short wavelength fl exural interference augmented by the reduction or migration of long wavelength fl

the cenozoic thrusting sequence of the longmen shan fold and thrust belt eastern margin of the tibetan plateau insights from low temperature thermochronology abstract the 2008 mw 7 8 wenchuan earthquake in china led to intensive studies on the development and dynamic mechanisms of the longmen shan fold and thrust belt to construct the uplift history of the longmen shan previous works had undertaken numerous low temperature thermochronological studies in mountainous area in particular apatite and zircon u th he dating which provided valuable evidence for understanding phases of uplift in the longmen shan during the cenozoic however few u th he ages have been obtained from the sichuan basin which leaves a blank in our understanding of the link between the wenchuan earthquake and the out of sequence longmen shan in this study we carried out u th he dating on apatite and zircon separated from samples collected in the sichuan basin and the frontal southern longmen shan together with previous low temperature thermochronological studies we reconstructed the thrusting sequence of the longmen shan the results of dating of apatite suggest that thrusting in the longmen shan is temporally and spatially variable after continuous thrust propagation into the sichuan basin over several to tens of myr deformation migrated back into the mountain range which in this case termed out of sequence style of deformation

late cretaceous fluvial megafan and axial river systems in the southern cordilleran foreland basin drip tank member of straight cliffs formation and adjacent strata southern utah u s a abstract the upper cretaceous lower campanian drip tank member of the straight cliffs formation in the grand staircase region of southern utah is an upward coarsening sandstone rich low sinuosity fluvial deposit in the cordilleran foreland basin the drip tank member thins from 75 m to 30 m eastward across the basin foredeep approximately parallel to its dominant eastward sediment dispersal direction and thins southward along the foredeep to as little as 17 m near its southern outcrop extent along modern cliff lines it overlies and underlies mudstone rich fluvial strata of the john henry member of the straight cliffs formation and lower member of the wahweap formation respectively both of which were deposited by northeast flowing rivers the john henry drip tank contact lies in an interfingering progradational stratigraphic transition from medium grained lenticular sandstone bodies to tabular coarse grained amalgamated sandstone bodies the contact with the overlying wahweap formation is locally a prominent surface of erosion or sediment reworking sandstone petrography and detrital zircon age populations of the strata indicate detrital sources in basement south of the foreland basin thrust sheets to the west and magmatic arc rocks to the southwest the uppermost part of the john henry member contains as much as 10 feldspar and abundant 1 84 1 60 ga detrital zircons derived from yavapai mazatzal basement rocks in the mogollon highlands the drip tank member ranges from quartzarenite to chert bearing sublitharenite with grenville 1 2 1 1 ga neoproterozoic and early paleozoic detrital zircons derived from proterozoic through jurassic strata exposed in the sevier orogenic belt lower member wahweap litharenites contain abundant thrust belt derived detrital carbonate grains absent from compositionally mature drip tank sandstones as well as a broad range of zircon ages the drip tank member constitutes deposits of a sand rich fluvial megafan or distributive fluvial system that displaced a subjacent but partly time equivalent axial fluvial system eastward as the megafan driven in part by seasonally high discharge fed by monsoonal precipitation prograded across the basin foredeep a thick amalgamated fan axis succession present in northern exposures contrasts with a thinner fan flank succession in southern localities tabular sixth order lithosomes each containing channels on the order of 7 m deep define three individual fluvial fans or fan lobes in the megafan system younger wahweap fluvial channels eroded uppermost drip tank deposits of the fan axis and fan flank transverse fan progradation across the foredeep recorded falling accommodation sediment supply in an aggradational to progradational systems tract erosion and reworking of the upper fan surface record decreased accommodation sediment bypass and erosion prior to reoccupation of the foredeep by the wahweap axial fluvial system the upper contact of the drip tank member is thus the primary sequence boundary in this multi component fluvial system and the lower wahweap represents transitional systems tract deposits accumulated as accommodation sediment supply increased

computation of t phase coda t phases result from the conversion of seismic to acoustic energy at the seafloor in the vicinity of an earthquake epicenter the time frequency characteristics of t phase coda are observed to be strongly dependent upon the bathymetry of the ocean floor in the source region thus earthquakes from the same geographic area generate similar t phase coda in this study detailed bathymetry and sound speeds computed from the levitus atlas are used to simulate the t phase amplitudes which would be recorded at distances of several thousand kilometers from these natural sound sources it is hypothesized that seismic acoustic coupling occurs over a distributed region of the ocean floor with seismic energy being scattered into low order modes in shallow water comparison with observed coda indicates that t phase onset times and durations are modeled quite accurately with this simple method the feasibility of using this method to determine accurate source times and locations for small suboceanic earthquakes given

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

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

tectonic controls on drainage evolution and development of terminal alluvial fans southern pyrenees spain an understanding of how drainage patterns respond to tectonics can provide an insight into past deformational events within mountain belts and where sediment flux is supplied to depositional basins the transverse rivers draining the spanish pyrenees show sudden diversions to axial courses and the capture of lateral systems producing large trunk rivers that break through the thrust front at structurally controlled points the drainage reorganization from the initial regularly spaced pattern in the late eocene caused by the growth of thrust controlled topography influenced the location of outlets into the ebro basin the headward capture and merger of rivers as a result of structural diversion formed two large terminal fan systems during the oligo miocene along the pyrenean thrust front the early structural topographic controls on drainage evolution will have long term effects on sedimentation and stratigraphic architecture of foreland basins this will only be maintained as long as there is tectonic uplift and the river systems strive to re attain a regular drainage spacing across the orogenic belt as partly achieved in the pyrenees

kinematic stratigraphic evolution of a growth syncline and its implications for tectonic development of the proximal foreland basin southeastern ebro basin catalunya spain in the southeastern ebro basin of spain analysis of sequential fold history in combination with stacking patterns of conglomerate deposited in a growing syncline demonstrates the impact of local uplift attendant upon fold growth on nonmarine depositional systems in a foreland basin the penyagalera syncline is part of a belt of asymmetric folds developed in proximal paleogene foreland basin deposits by foreland directed propagation of a detachment beneath the basin foreland basin strata form a progradational succession of interbedded siltstone sandstone and evaporite overlain by conglomerate stratal stacking patterns and fold history indicate that conglomerate of the syncline marks the transition within the foreland basin succession from foredeep to wedge top depositional zones the conglomerate is divided into three informal members separated by unconformities the members are divided in turn into lithosomes separated by prominent bounding surfaces lithosomes represent depositional elements deposited by episodic progradation of alluvial fan lobes older conglomerate members are more tightly folded than younger members demonstrating that folding accompanied conglomerate deposition the uplift of opposing limbs of the penyagalera syncline was diachronous the asymmetric syncline comprises an overturned interior southeastern limb and a gently dipping exterior northwestern limb the interior limb rotated to a steep to slightly overturned attitude before significant rotation and uplift of the exterior limb took place rotation of the interior fold limb was episodic and punctuated by periods of erosion of older conglomerate uplift rates during rotation events along the interior limb generally were more rapid than sediment accumulation rates and caused sediment bypass to the foreland basin however sediment accumulation rates overtook uplift rates during late limb rotation causing alluvial fan lithosomes to onlap previously formed unconformities offlap signifying a slight excess of uplift over sediment accumulation rates is not observed in the syncline on the exterior limb sedimentation rates exceeded early limb uplift rates but the relationship of later folding and deposition is not preserved most synclinal closure postdated the youngest preserved lithosomes in the fold young lithosomes occupy a paleocanyon incised deeply into the interior limb of the syncline and mesozoic strata of the thrust belt the paleocanyon was backfilled during late aggradation of the ebro basin an event independent of local folding

dynamic fluvial systems and gravel progradation in the himalayan foreland although the large scale stratigraphy of many terrestrial foreland basins is punctuated by major episodes of gravel progradation the relationships of such facies to hinterland tectonism and climate change are often unclear structural reentrants provide windows into older and more proximal parts of the foreland than are usually exposed and thus provide key insights to earlier phases of foreland evolution our magnetostratigraphic studies show that although the major lithofacies preserved within the himachal pradesh structural reentrant in northwestern india resemble neogene facies in pakistan they have a much greater temporal and spatial variability from 11 5 to 7 ma major facies boundaries in himachal pradesh vary by as much as 2 3 m y across distances of 20 30 km and are controlled by the interference between a major southeastward flowing axial river and a major southwestward flowing transverse river a thick but highly confined middle to late miocene conglomerate facies includes the oldest extensive siwalik conglomerates yet dated 10 ma and implies the development of significant erosional topography along the main boundary thrust prior to 11 ma our studies document extensive syntectonic gravel progradation with conglomerates extending tens of kilometers into the undeformed foreland during a period of increased subsidence rate and within 1 2 m y of major thrust initiation overall gravel progradation is modulated by the interplay among subsidence sediment supply and the proportion of gravels in rivers entering the foreland

growth of the south pyrenean orogenic wedge a six step reconstruction of the south pyrenean foreland fold and thrust belt in spain delineates the topographic slope basal dtcollement angie internal deformation and thrust front advance from the early eocene until the end of contractional deformation in the late oligocene style of thrust front advance dip of the basal dtcollement slope of the upper surface and internal deformation are decoupled and not simply related internal deformation increased decreased and maintained surface slope angle at different stages from the onset to the cessation of deformation the basal dtcollement angle decreased overall suggesting translation of the thrust belt onto stronger crust with time taper angle of the pyrenean thrust wedge was fundamentally controlled by the flexural rigidity of the lower plate the relative rate of creation of structural relief in the rear versus the front of the wedge the extent of deposition of eroded material within the deforming wedge and the taper of the pretectonic stratigraphic wedge

reconstruction of tertiary palaeovalleys in the south alpine foreland basin of france eocene oligocene of the castellane arc abstract the dynamics of depositional environments and the spatial deformation of drainage networks in foreland basins reflect the tectonic and erosional dynamics associated with the development of mountain belts the spatial and temporal organization of the eocene oligocene 40 25 ma sedimentation in the external part of the south alpine foreland basin of france was reconstructed using an integrated cartographic sedimentological and petrographic analysis of the tertiary sedimentary successions the depositional geometries and variations in facies and thickness of the palaeogene nummulitic succession as well as the observed flow directions in various continental and marine sediments suggest that the barreme blieux and taulanne synclines were present as palaeovalleys since the eocene the sedimentological analysis of the nummulitic succession allows the identification of three depositional sequences separated by transgressive surfaces that are recognized in the barreme blieux and taulanne synclines correlation of these sequences between the three synclines suggests that these palaeovalleys were connected by a local valley network that recorded the same sea level fluctuations during the marine nummulitic sedimentation the palaeovalley network was structurally controlled by the east west axes of the blieux and taulanne synclines and the north south axis of the barreme syncline formed during the pyrenean provencal late cretaceous middle eocene shortening and the first stage of the alpine history middle eocene respectively later on the westward alpine compression since the early oligocene induced local depocenter migration and reversal in flow direction however compared to the modern river pattern the palaeovalley orientation highlights a geometrical stability since their formation about 40 ma suggesting a long term stability of the early structures in the foreland basin this constancy can be explained by the location of the study area in a piggy back basin transported at the top of the provencal thrust sheet that facilitated the preservation of the overall axis orientation of the palaeovalleys

carbonaceous matter in peridotites and basalts studied by xps sali and leed abstract carbonaceous matter in peridotite xenoliths and basalt from the hualalai volcano in a basalt glass collected directly from an active lava lake on the east rift of kilauea in garnet and diopside megacrysts from the jagersfontein kimberlite and in gabbros from the stillwater and bushveld complexes has been studied by x ray photoelectron spectroscopy xps thermal desorption surface analysis by laser ionization sali and low energy electron diffraction leed the basalt and two of the four xenoliths from hualalai and both jagersfontein megacrysts yielded trace quantities 10 nanomoles of organic compounds on heating to 700 c organics were not detected in the rocks from the layered intrusions and neither carbonaceous matter nor organics were detected in the glass from the lava lake where detected organics appear to be associated with carbonaceous films on microcrack surfaces carbonaceous matter exists as films less than a few nm thick and particles up to 20 m across both of which contain elements expected to be present in significant quantities in magmatic vapors namely si alkalis halogens n and transition metals leed studies suggest that the carbonaceous films are amorphous the data suggest two possible mechanisms for the formation of the organics one is that they are a product of abiotic heterogeneous catalysis of volcanic gas on new chemically active mineral surfaces formed by fracturing during cooling alternatively organics may have been assimilated into the volcanic gases prior to eruption and then deposited on cracks formed during eruption and cooling in any case there is no evidence to suggest that the organics represent laboratory or field biogenic contamination

controls on the deposition and preservation of the cretaceous mowry shale and frontier formation and equivalents rocky mountain region colorado utah and wyoming regional variations in thickness and facies of clastic sediments are controlled by geographic location within a foreland basin preservation of facies is dependent on the original accommodation space available during deposition and ultimately by tectonic modification of the foreland in its postthrusting stages the preservation of facies within the foreland basin and during the modification stage affects the kinds of hydrocarbon reservoirs that are present this is the case for the cretaceous mowry shale and frontier formation and equivalent strata in the rocky mountain region of colorado utah and wyoming biostratigraphically constrained isopach maps of three intervals within these formations provide a control on eustatic variations in sea level which allow depositional patterns across dip and along strike to be interpreted in terms of relationship to thrust progression and depositional topography the most highly subsiding parts of the rocky mountain foreland basin near the fold and thrust belt to the west typically contain a low number of coarse grained sandstone channels but limited sandstone reservoirs however where subsidence is greater than sediment supply the foredeep contains stacked deltaic sandstones coal and preserved transgressive marine shales in mainly conformable successions the main exploration play in this area is currently coalbed gas but the enhanced coal thickness combined with a mowry marine shale source rock indicates that a low permeability basin centered play may exist somewhere along strike in a deep part of the basin in the slower subsiding parts of the foreland basin marginal marine and fluvial sandstones are amalgamated and compartmentalized by unconformities providing conditions for the development of stratigraphic and combination traps especially in areas of repeated reactivation areas of medium accommodation in the most distal parts of the foreland contain isolated marginal marine shoreface and deltaic sandstones that were deposited at or near sea level lowstand and were reworked landward by ravinement and longshore currents by storms creating stratigraphic or combination traps enclosed with marine shale seals paleogeographic reconstructions are used to show exploration fairways of the different play types present in the laramide modified cretaceous foreland basin existing oil and gas fields from these plays show a relatively consistent volume of hydrocarbons which results from the partitioning of facies within the different parts of the foreland basin

the strontium isotopic composition of ordovician and silurian brachiopods and conodonts relationships to geological events and implications for coeval seawater utilized for strontium isotope measurements the results document a gradual decrease in 87 sr 86 sr from 0 7091 to 0 7087 from tremadoc to llandeilo a sharp decline to 0 7078 during the late llandeilo early caradoc little change during caradoc and the ashgil and a steady rise to 0 7087 through the silurian these long term 10 7 yr variations with magnitudes in the range of 10 23 are interpreted to be controlled primarily by continental collisional tectonics and its associated erosion and weathering the gradual decrease in 87 sr 86 sr ratio during the early ordovician may record the reduction in uplift and weathering rates due to waning of the pan african orogenies the rapid decline near the llandeilo caradoc boundary suggests a strong hydrothermal flux likely due to increased sea floor spreading and a possible superplume event the latter may have caused the prominent transgressive phase the largest in the phanerozoic which would have muted continental flux input although the caradoc was the main interval for the taconic orogeny its impact on the sr continental flux may have been delayed until the early llandovery this effect complemented by reworking of glacial deposits near the ordovician silurian boundary and enhanced by phases of the silurian salinic orogeny may have combined to give the progressive increase in the strontium isotope ratio through the silurian the scale and directionality of these changes makes the strontium isotope curve valuable for dating and correlation purposes copyright 1998 elsevier science ltd

long distance longitudinal transport of gravel across the cordilleran thrust belt of montana and idaho two newly identified middle eocene paleovalleys 100 km long preserved on top of the southwest montana reentrant of the cordilleran fold and thrust belt indicate long lived longitudinal flow across the thrust belt and resolve a long standing debate about the source of the voluminous quartzite debris in the upper cretaceous to lower tertiary divide harebell and pinyon conglomerates of montana idaho and wyoming geologic mapping stratigraphic provenance and geochronologic studies revealed that eocene volcanic and sedimentary rocks in the paleovalleys are as thick as 2 km onlap preexisting bedrock and interfinger with well rounded conglomerate derived from formations exposed only to the west the middle eocene paleovalleys are the youngest expression of a major paleoriver system that transported sediment toward the foreland during the sevier orogeny an eocene subcrop map shows that the headwaters of the eocene paleovalleys coincided with structural culminations in the thrust belt that supplied sediment to the divide conglomerate of the upper cretaceous to lower tertiary beaverhead group ultimately the lemhi pass and hawley creek paleovalleys provided several thousand cubic kilometers of quartzite debris to the pinyon and harebell conglomerates of northwest wyoming 200 350 km away and formed the northwest half of a giant longitudinal drainage system sevier contraction not the rising idaho batholith first uplifted vast culminations beneath the headwaters of this river system

provenance of upper cretaceous paleogene sandstones in the foreland basin system of the sierra madre oriental northeastern mexico and its bearing on fluvial dispersal systems of the mexican laramide province sandstone petrography and detrital zircon u pb analysis of upper cretaceous paleogene sandstones in the foreland basin of the sierra madre oriental in northeastern mexico indicate long distance sediment transport from arc basement and thrust belt sources lying to the west northwest and south the basin fill termed the difunta group consists of sublitharenites litharenites feldspathic litharenites and lithic arkoses derived from mixed sources that included sedimentary rocks magmatic arc rocks and subordinate basement rocks six age populations comprise the detrital zircon content of the sandstones proterozoic 1900 900 ma early paleozoic 500 400 ma late paleozoic early triassic 288 235 ma jurassic 180 151 ma early cretaceous 150 111 ma and late cretaceous paleogene 110 54 ma these grains were derived from several arc terranes ranging in age from permian to paleogene in western mexico and the southwestern united states from sedimentary rocks and possibly interbedded tuffs of the sierra madre oriental orogen and from basement sources or their derivative sandstones of the southwestern united states the petrographic and geochronologic provenance data corroborate existing models for derivation of much foreland detritus from arc sources to the west identify the sierra madre orogen itself as an important source for sediment and these data modify the late cretaceous paleogene paleogeography of mexico to include a long orogen parallel fluvial system with headwaters in the southwestern united states the difference in average ages of the youngest grains in the sandstones and their inferred depositional ages is 10 5 m y indicating that the initial coarse fill of the foreland basin was derived from early laramide uplift and eastward arc migration in northwestern mexico and the southwestern united states

structural evolution of the piqiang fault zone nw tarim basin china abstract the piqiang fault is a prominent strike slip tear fault that laterally partitions the keping shan thrust belt in the nw tarim basin china in satellite images the piqiang fault appears as a sharp nw trending lineament that can be traced for more than 70 km it is oblique to the general structural trend of the thrust belt and subparallel to the thrust transport direction this paper presents a structural analysis of the piqiang fault based on satellite image interpretation and field data a net loss of late paleozoic sediment across the fault zone implies that it was initiated as a major normal fault during the early permian and corresponds to widespread extension and magmatism during this period differential erosion across the fault resulted in the subsequent removal of sediment from the east relative to the west during the middle to late cenozoic contraction of the nw tarim basin and the formation of the keping shan thrust belt resulted in reactivation of the piqiang fault as a strike slip tear fault the fault has accommodated lateral differences in thrust density and spacing which have arisen due to the abrupt pre existing change in stratigraphic thickness across it the piqiang fault provides an insight into the formation of oblique strike slip tear faults in contractional belts and demonstrates the importance of inherited basement structures in such settings

co 2 13 c 12 c and h 2 o variability in natural basaltic glasses a study comparing stepped heating and ftir spectroscopic techniques abstract a comparison of two independent techniques was used to assess the homogeneity of co 2 and h 2 o concentrations in a number of natural basaltic glasses variations in carbon concentration and isotopic ratio were determined by comparison of stepped heating data obtained in two different laboratories dissolved volatile concentrations were also obtained by stepped heating and fourier transform infrared ftir spectroscopy replicate stepped heating analyses of a mid ocean ridge basaltic glass show that the concentration and 13 c 12 c of bulk magmatic and dissolved co 2 vary by less than 10 and 0 5 respectively a similar degree of correlation is observed for replicate stepped heating analyses of mariana trough glasses conducted in two different laboratories dissolved co 2 concentrations determined by stepped heating also correlate well with concentrations measured by ftir spectroscopy the correspondence of results obtained in these experiments provide an upper limit to the degree of natural variation in concentrations and isotopic ratios of these volatiles in basaltic glasses and suggest that intrinsic magmatic carbon has a relatively homogeneous distribution in these glasses water concentrations determined through extraction by heating and ftir also show excellent agreement

influence of inherited taper on structural variability and conglomerate distribution cordilleran fold and thrust belt western united states north south changes in thickness of the prethrust stratigraphy of the north american cordillera within the united states controlled kinematic development of the late mesozoic sevier fold and thrust belt variation in stratigraphic thickness determined the initial slope o of the basal decollement beneath the thrust wedge as predicted by critical wedge theory segments of the sevier thrust belt with initially thick precambrian and paleozoic stratigraphic sections are structurally simple and are composed of few thrust sheets segments with thin prethrust stratigraphy and comparably low values of o are intensely imbricated the difference in structural style resulted because wedge segments with higher values of o required less internal deformation to build and maintain the critical taper required for thrust wedge advance transverse structural zones across which kinematic style differed formed at the terminations of thrust belt segments and controlled the locations of long lived antecedent drainage systems that transported gravel to the foreland therefore along strike changes in prethrust stratigraphic thickness resulted in an observed disjunct distribution of conglomerate in synorogenic deposits of the thrust wedge and foreland basin

structure of the mount raymond transverse zone at the southern end of the wyoming salient sevier fold thrust belt utah abstract the mount raymond transverse zone mrtz forms the east west trending boundary between the wyoming salient of the sevier fold thrust belt and the uinta cottonwood arch in north central utah major faults in the zone dip 40 to 45 north our structural analysis indicates that the mrtz contains both contractional and extensional structures the contractional structures thrusts and related folds initially formed as part of a southeast verging northeast trending thrust system this system gradually curved and merged to the north with the east verging absaroka thrust system in the apex of the wyoming salient the contrast in trend between structures in the mrtz and those in the apex of the wyoming salient reflects the initial curvature of the salient this curve formed because the stratigraphic sequence involved in thrusting thinned gradually southward toward the axis of a proto uinta arch the present east west trend of the mrtz developed during laramide uplift of the unita cottonwood arch an event which tilted the faults into their present position cenozoic crustal extension subsequently reactivated segments of the mrtz in sum the evolution of the mrtz illustrates how post thrusting processes can affect the map view geometry of thrust belts to create transverse zones

cascading carbonate lakes of the mayran basin system northeast mexico the interplay of inherited structural geometry bedrock lithology and climate we investigated the mayran formation late neogene exposed in northeast mexico to determine the ways in which antecedent topography climate sediment input bedrock geology and primary production controlled facies architectures in this lacustrine dominated succession the succession was deposited in four broadly coeval lake subbasins mayran basin system developed at different elevations the subbasins are separated by cretaceous bedrock ridges and infilled by differing proportions of siliciclastic and carbonate deposits adjacent subbasins are connected by spillover points to investigate how these factors controlled overall facies architectures we generated a structural and stratigraphic framework for each subbasin in the most proximal subbasin i alluvial sediments consisting of nonchannelized and channelized conglomerates pebbly sandstones and calcareous sandy and siliciclastic mudstones change laterally into horizontally bedded lacustrine limestones calcareous mudstones and evaporites vertically these lithofacies are organized into shallowing upward cycles the more distal subbasins ii iv are predominantly filled by lacustrine limestones striking aggradational and progradational tufa topsets and clinoforms are present at the spillover points waterfalls between subbasins these tufas interfinger up and downdip with the lacustrine limestones the distribution of clastic facies was mainly controlled by their proximity to the major sediment input in the south and to the cretaceous bedrock ridges primary production within each subbasin was controlled by a combination of lake level accommodation availability and solute inputs overall while autogenic processes likely controlled facies distribution within the basin climate was the overall driver of large scale facies variability

layer parallel shortening across the sevier fold thrust belt and laramide foreland of wyoming spatial and temporal evolution of a complex geodynamic system abstract varying patterns of layer parallel shortening lps and vertical axis rotations from the thin skin sevier fold thrust belt to the thick skin laramide foreland of wyoming are quantified from integrated structural anisotropy of magnetic susceptibility ams and paleomagnetic analyses within the sevier belt widespread early lps was accommodated by spaced cleavage fracture sets minor folds and minor faults lps directions are subperpendicular to structural trends of systematically curved thrust sheets of the wyoming salient reflecting a combination of primary dispersion and secondary rotation during thrusting within the laramide foreland limited lps was accommodated mostly by minor faults with conjugate wedge and strike slip geometries lps directions in gentler fold limbs vary from perpendicular to acute with structural trends of variably oriented anastomosing basement cored arches steep forelimbs display more complex relations including younger fault sets that developed during evolving stress states and localized vertical axis rotations although internal strain is limited weak ams lineations defined by kinked and rotated phyllosilicates are widely developed and consistently oriented perpendicular to measured lps directions palinspastically restored lps directions corrected for paleomagnetically determined vertical axis rotations vary on average from w e in the sevier belt to wsw ene in the laramide foreland in detail lps directions display deflections related to primary sedimentary wedge geometry and basement fabrics lps in the sevier belt is interpreted to partly reflect stress transmitted from the hinterland through the growing orogenic wedge and topographic stress along the front of the wedge lps in the laramide foreland is interpreted to partly reflect basal traction during flat slab subduction beneath thick cratonic lithosphere with spatial temporal variations in stress trajectories related to basement heterogeneities and evolving fault systems evidence for significant late n s laramide shortening as proposed by multi stage shortening models is not observed

a comparison of the effectiveness of clast based finite strain analysis techniques to ams in sandstones from the sevier thrust belt wyoming abstract geological strain analysis of sedimentary rocks is commonly carried out using clast based techniques which rely on assumptions regarding clast shapes and orientations as well as distances between clast centres to evaluate the accuracy of these methods in low strain regimes we have compared strain analysis estimates to the anisotropy of magnetic susceptibility ams in sandstone samples from the wyoming salient the cordilleran mountain belt of north america is one of the world s classic foreland fold and thrust belts the sevier belt represents the thin skinned front of this orogenic event characterized by thrust faults and folds that shortened and translated sequences of devonian to cretaceous strata eastward deformation increases westward providing an ideal laboratory and geological setting to explore the potential correlation of anisotropy of magnetic susceptibility ams to clast based strain analyses studies attempting to define the relationship between ams and finite strain have been in vogue since the link between layer parallel shortening and ams was first established this relationship despite proven strong correlations between the ams tensors and tectonic directions is generally complicated by competing sub fabrics as well as the various magnetic anisotropy properties of the minerals contributing to the ams fabric even with these complications we conclude that ams is much more sensitive to incipient tectonic fabrics than clast based methods

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