tectonic implications of rapid cooling of lower plate rocks from the buckskin rawhide metamorphic core complex west central arizona lower plate rocks exposed in the buckskin rawhide metamorphic core complex west central arizona underwent a pronounced increase in cooling rate 7 12 m y after the onset of extensional tectonic denudation at ca 27 ma a detailed 40 ar 39 ar study of lower plate rocks exposed near planet peak in the southwest part of the core complex indicates that the highest level mylonitic rocks exposed in the complex cooled below 350 c prior to 20 1 ma at an average rate of 15 c m y thereafter the cooling rate of these rocks increased progressively peaking at 280 80 c m y prior to initial exposure of the core between 15 1 and 13 3 ma published k ar 40 ar 39 ar and fission track ages for the most recently unroofed lower plate rocks exposed at the northeastern end of the core complex are consistent with a similar cooling history although the abrupt increase in cooling rate occurred as many as 5 m y later in this area because the increase in the cooling rate appears to have occurred progressively later toward the northeast it is unlikely to reflect changes in slip rate it appears to have coincided with cooling below 350 300 c and thus is interpreted to reflect steepening of the detachment system above the brittle ductile transition thermochronological data from lower plate rocks in the planet peak area together with constraints on their initial exposure at the surface are consistent with an increase in the dip of the detachment from le 5 at midcrustal depths where it was represented by a shear zone to at temperatures below 200 150 c lower plate rocks at both ends of the core complex probably cooled at rates 130 c m y which can not be reconciled with the average dip le 25 and slip rate 6 12 mm yr for the detachment fault given the likely geothermal gradient 25 c km during the early stages of extension this implies that cooling did not keep pace with denudation at upper crustal levels the very high cooling rates are interpreted to have primarily reflected the development of a pronounced thermal discontinuity across the detachment fault possibly 100 c as hot lower plate rocks were transported to shallow crustal levels rapid cooling of the lower plate at these levels may have been enhanced by interaction with fluids derived from the upper plate consistent with models that link the shallow level

mechanics of fold and thrust belts and accretionary wedges cohesive coulomb theory a critically tapered fold and thrust belt or submarine accretionary wedge is one that is on the verge of coulomb failure everywhere including its base where frictional sliding along a decollement is assumed to be occurring cohesion within a wedge can add significantly to the overall strength near the toe the effect of this is to decrease the near toe taper leading to a critical topographic profile that is concave upward if the decollement is planar we obtain an approximate self consistent solution for the state of stress within a thin skinned cohesive critical coulomb wedge and determine the relationship between the wedge taper and its strength and basal friction the theory is then applied to the presently deforming fold and thrust belt of western taiwan fitting of theoretical critical wedge shapes to topographic profiles and measurements of the step up angles of thrust faults from the basal decollement are used to constrain the taiwan wedge strength parameters an attractive assertion fully consistent with all the observations is that the mechanics of fold and thrust belts and accretionary wedges is governed by normal frictional and fracture strengths of rocks measured in the laboratory in particular if byerlee s law b 0 85 is adopted as the coefficient of sliding friction on the base we find a coefficient of internal friction 0 9 1 0 in the wedge and a wedge cohesion so 5 20 mpa other solutions having strengths and ambient stresses up to 4 times lower than this can also however satisfy the data

basin centered asperities in great subduction zone earthquakes a link between slip subsidence and subduction erosion 1 published areas of high coseismic slip or asperities for 29 of the largest circum pacific megathrust earthquakes are compared to forearc structure revealed by satellite free air gravity bathymetry and seismic profiling on average 71 of an earthquake s seismic moment and 79 of its asperity area occur beneath the prominent gravity low outlining the deep sea terrace 57 of an earthquake s asperity area on average occurs beneath the forearc basins that lie within the deep sea terrace in sw japan slip in the 1923 1944 1946 and 1968 earthquakes was largely centered beneath five forearc basins whose landward edge overlies the 350 c isotherm on the plate boundary the inferred downdip limit of the locked zone basin centered coseismic slip also occurred along the aleutian mexico peru and chile subduction zones but was ambiguous for the great 1964 alaska earthquake beneath intrabasin structural highs seismic slip tends to be lower possibly due to higher temperatures and fluid pressures kilometers of late cenozoic subsidence and crustal thinning above some of the source zones are indicated by seismic profiling and drilling and are thought to be caused by basal subduction erosion the deep sea terraces and basins may evolve not just by growth of the outer arc high but also by interseismic subsidence not recovered during earthquakes basin centered asperities could indicate a link between subsidence subduction erosion and seismogenesis whatever the cause forearc basins may be useful indicators of long term seismic moment release the source zone for cascadia s 1700 a d earthquake contains five large basin centered gravity lows that may indicate potential asperities at depth the gravity gradient marking the inferred downdip limit to large coseismic slip lies offshore except in northwestern washington where the low extends landward beneath the coast transverse gravity highs between the basins suggest that the margin is seismically segmented and could produce a variety of large earthquakes

coupled basin detachment systems as paleoaltimetry archives of the western north american cordillera abstract stable isotope paleoaltimetry data from the snake range metamorphic core complex mcc and sacramento pass basin nv usa document that extensional mylonite zones and kinematically linked syntectonic basins reliably record paleotopography in the continental interior of western north america when compared to a sea level reference here we show that this basin mcc pair tracks meteoric fluid flow at different levels of actively extending crust in a high topography region during oligo miocene extension of the basin and range province for paleoaltimetry purposes we compare multi proxy oxygen 18 o and hydrogen d isotope data as well as geochronological information from the snake range mcc to a time equivalent ca 20 ma stable isotopic proxy record from the buckskin mountains mcc az usa which developed next to the pacific coast near miocene sea level we complement this paleoaltimetry study by comparing the buckskin mountains mcc data with older 35 ma lacustrine stable isotope and paleofloral records from the nearby house range ut usa whose paleoelevation has been determined independently through paleobotanical analysis each of the investigated compartments of the paleohydrologic system within the snake range mcc depicts a coherent scenario of low oligo miocene 18 o and d values of meteoric water that reflect precipitation sourced at high elevation a 77 difference in d water between the snake range d water 113 and the buckskin mountains d water 36 is consistent with minimum mean paleoelevation of the snake range of about 3850 650 m above miocene sea level additional support for such elevations comes from a comparison between the buckskin mountains mcc and the eocene house range basin ut usa where differences in 18 o water values are consistent with 2300 500 m minimum paleoelevation of the house range based on the presence of brecciated rock avalanche deposits within the sacramento pass basin we conclude that the snake range was a topographic high and locus of significant relief during regional scale extension within the cordilleran hinterland

formation of forearc basins and their influence on subduction zone earthquakes recent observations of an association between forearc basins and slip during subduction thrust earthquakes suggest a link between processes controlling upper plate structure and seismic coupling on the subduction zone thrust fault we present a mechanism for the formation of these basins where sedimentation occurs on landward dipping segments of the subduction wedge which itself is actively growing through the accretion of material from the subducting plate our numerical simulations demonstrate that sedimentation stabilizes the underlying wedge preventing internal deformation beneath the basin maximum slip during great thrust earthquakes tends to occur where sedimentary basins stabilize the overlaying wedge the lack of deformation in these stable regions increases the likelihood of thermal pressurization of the subduction thrust allows the fault to load faster and allows greater healing of the fault between rupture events these effects link deformation of the subduction wedge to the seismic coupling of the subduction thrust

shortening in the upper plate of the buckskin rawhide extensional detachment fault southwestern u s and implications for stress conditions during extension commonwealth post graduate research award u s geological survey national cooperative geologic mapping program under statemap g11ac20455

diachronous uplift and cooling history of the menderes core complex western anatolia turkey based on new zircon u th he ages abstract new u th he thermochronology data from the syn extensional granitoids in the central part of the menderes massif in western turkey reveal a minimum slip rate of 12 5 km myr along the alasehir detachment 14 dip angle and denudation rates between 1 75 km myr and 3 25 km myr between 4 ma and 2 ma these values suggest relatively fast exhumation of the central sub massif associated with cooling rates between 53 c myr and 128 c myr which are higher than the estimated footwall cooling rates 60 c myr to 120 c myr from the northern sub massif based on the initial crystallization ages of the syn extensional granitoid intrusions and their exhumation related cooling ages our thermochronological findings suggest that the central sub massif in menderes underwent accelerated uplift and faster exhumation in the latest cenozoic than the northern and southern sub massifs this latest doming and rapid extension of the central sub massif was associated with the asthenospheric upwelling beneath the region and the related na alkaline kula volcanism our results indicate that the menderes massif has had a diachronous uplift and cooling history during its extensional tectonic evolution in the late cenozoic thermal weakening of the young orogenic crust in western anatolia via both lithospheric and asthenospheric melting episodes and magmatism produced higher than normal geothermal gradients and played a significant role in core complex formation

hematite and mn oxide u th he dates from the buckskin rawhide detachment system western arizona gaining insights into hematite u th he systematics

very high slip rates on continental extensional faults new evidence from u th he thermochronometry of the buckskin mountains arizona abstract new u th he in apatite ages suggest rapid slip rates on the tertiary buckskin rawhide detachment fault which crops out in the buckskin and rawhide mountains of western arizona single crystal u th he in apatite ages were determined for a suite of crystalline rock samples from the footwall to this detachment the samples were collected from a transect parallel to the fault slip direction and proximal to the fault zone they show a linear decrease in age with distance from 14 to 11 ma over 15 km the age versus distance relationship is interpreted to show a slip rate of 4 2 1 2 0 8 mm yr continuing for a period of 3 myr this rate is more or less in agreement with but more precise than previously published slip rates from this and other large offset extensional detachments such slip rates are alarmingly high being roughly 5 10 times faster than rates commonly observed on active structures at the earth s surface these rapid slip rates suggest that significant seismic hazard might be posed by some extensional structures and they raise questions regarding the relationship between such rapidly slipping large offset structures and the slower smaller offset faults that are more commonly observed at the earth s surface

crustal structure of the sunda forearc region west of central sumatra from gravity data free air gravity anomalies near nias island include a 100 mgal low centered 10 20 km landward of the trench axis and a 80 mgal high over the outer arc ridge but also show a few large anomalies unrelated to topography a 90 mgal low is found on the structural high between the banyak islands and sumatra north of nias an 80 mgal rise at a gradient of 8 mgal km is modeled by a near surface body of high density material oceanic crust 4 km wide and tens of kilometers long this slab may be exposed on the southwest coast of nias where ultramafic bodies have been mapped and its northerly trend compared with northwesterly bathymetric trends may be related to other obliquely trending structures near here to a proposed strike slip fault which crosses nias and or to the basement high topped by the batu islands a 30 mgal free air low over the forearc basin can be modeled best if pre miocene melange or continental crust underlies the basin and it seems unlikely that the basin can be underlain by a large thickness of oceanic crust

strain variations in an ancient accretionary complex implications for forearc evolution strain magnitudes and orientations and incremental strain histories are characterized throughout the kodiak formation a portion of a late cretaceous early tertiary accretionary prism the results of these analyses are used to evaluate the history of underplating penetrative strain and unroofing in the forearc of the southwest alaskan convergent margin five major deformation events are recognized in the kodiak formation 1 layer parallel extension and layer parallel shear d1 related to underthrusting 2 seaward verging fold and thrust deformation d2 with development of both a slaty cleavage s2 and a downdip stretching lineation l2 related to underplating and early penetrative strain within the wedge 3 development of a regional scale antiform d3 4 development of a shear zone along the northwest limb of the d3 antiform d4 and 5 right lateral strike slip faulting and penetrative right lateral simple shear in the seaward half of the kodiak formation d5 based on sharp differences in structural style and the magnitude and orientation of strain associated with d2 the kodiak formation has been subdivided into three structural units informally called the landward central and seaward belts the structural variations observed in these three belts correspond to differences in structural relief of 1 5 km associated with the d3 antiform the lowest d2 structural level is in the core of the antiform central belt and progressively higher structural levels are exposed on both limbs landward and seaward belts the regional distribution of incremental strain histories associated with d2 d3 d4 and d5 has been quantified by using syntectonic fibers in pressure shadows around spherical pyrite framboids from the highest to the lowest structural level d2 strain is characterized by 1 an increase in the magnitude of finite strain 2 a change in the orientation of finite extension from nearly vertical to gently northwest plunging 3 a change in the incremental extension direction from nearly vertical to moderately northwest plunging and finally 4 a change in the strain history from coaxial to noncoaxial with a sense of rotation consistent with southeast directed thrusting we interpret these d2 strain patterns to reflect a strain field that was fixed relative to the basal decollement with coaxial subhorizontal shortening in the interior of the prism and noncoaxial strain probably simple shear near the base subsequent variations in the orientation of incremental extension associated with d3 are consistent with relatively rapid vertical translation of the accreted sequences relative to the d2 strain field we specifically propose two periods of wedge thickening in response to a combination of underplating and penetrative strain these two wedge thickening events correspond with episodes of subaerial exposure and erosional unroofing in the late cretaceous and middle tertiary

implications of middle eocene epizonal plutonism for the unroofing history of the bitterroot metamorphic core complex idaho montana petrologic and geochronologic limits on the progression of east directed extensional unroofing of the bitterroot metamorphic core complex provide a means to compare rates of denudation across the footwall during the earlystages of extensional denudation u pb and 4 0 ar 3 9 ar data from epizonal plutons in the northwestern and southwestern bitterroot metamorphic core complex demonstrate that much of the western footwall was exhumed to shallow levels 1 5 km by ca 49 ma while eastern footwall rocks remained at high temperatures following an episode of isothermal decompression u pb ages of epizonal plutons along the western margins of the footwall suggest that as much as 13 km of unroofing occurred between anatectic melting of belt supergroup metasedimentary rocks in the western footwall no later than 56 ma and emplacement of the adjacent epizonal lolo hot springs batholith at ca 49 ma in the northwestern bitterroot range tectonic denudation during this interval removed roughly the same amount of overburden from eastern footwall rocks suggesting that the initial stages of unroofing may have been accommodated by a planar moderately east dipping 30 50 shear zone 4 0 ar 3 9 ar cooling data show that cooling in the western footwall was synchronous with unroofing in contrast we suggest that a significant delay in cooling of eastern footwall rocks may have resulted from a slight increase in geothermal gradient following this unroofing stage exposure of a west dipping mylonitic shear zone along the western limb of the bitterroot complex may provide a key for interpreting the postmylonitic structural evolution of the core complex if this shear zone can be correlated to the bitterroot mylonite zone to the east then strong evidence for flexural uplift consistent with postmylonitic passage of a rolling hinge emerges

a zero damage model for fission track annealing in zircon a zircon fission track annealing model is calculated on the basis of annealing experiments from the literature with induced tracks in decay event damage free zircon samples empirically derived parallel and fanning equations for this zero damage model yield an excellent fit to the data with the fanning model providing slightly better statistical parameters a comparison between annealing models with fanning iso annealing lines but different decay event damage densities reveals that annealing temperatures and closure temperatures for the estimated partial annealing zone are highest for the zero damage model compilations of existing geologic constraints on the zircon partial annealing zone on one hand and the zircon closure temperature on the other show that these constraints do not or only partly overlap with curves of proposed models for the zircon partial annealing zone and closure temperature this finding is consistent with the fact that the annealing behavior of zircon from long duration temperature evolutions is increasingly influenced by the accumulated decay event damage zircon samples of young age or low u content show a behavior closest to the predictions of the zero damage model and are in the predicted range of published models with low decay event damage density for thermal events of more than 10 myr duration however constraints from field studies show marked differences from proposed partial annealing zone boundaries of the zero or low damage models the applicability of the zero damage model is threefold 1 it predicts correct closure temperatures in the case of very rapid cooling across the partial annealing zone where basically no decay event damage is accumulated 2 it predicts an uppermost boundary for complete annealing of a mixture of zircon components of different age as found in sedimentary samples and in this case may be used as a thermometer 3 it represents an important reference for the establishment of a more comprehensive model of zircon fission track annealing that also includes the influence of decay event damage for such a model two different equations are discussed however additional detailed experimental and field data are needed for a more robust annealing model that includes the influence of decay event damage annealing

coseismic slip and afterslip of the great mw 9 15 sumatra andaman earthquake of 2004 we determine coseismic and the first month postseismic deformation associated with the sumatra andaman earthquake of 26 december 2004 from near field global positioning system gps surveys in northwestern sumatra and along the nicobar andaman islands continuous and campaign gps measurements from thailand and malaysia and in situ and remotely sensed observations of the vertical motion of coral reefs the coseismic model shows that the sunda subduction mega thrust ruptured over a distance of about 1500 km and a width of less than 150 km releasing a total moment of 6 7 7 0 10 22 n m equivalent to a magnitude m w 9 15 the latitudinal distribution of released moment in our model has three distinct peaks at about 4 n 7 n and 9 n which compares well to the latitudinal variations seen in the seismic inversion and of the analysis of radiated t waves our coseismic model is also consistent with interpretation of normal modes and with the amplitude of very long period surface waves the tsunami predicted from this model fits rela tively well the altimetric measurements made by the jason and topex satellites neither slow nor delayed slip is needed to explain the normal modes and the tsunami wave the near field geodetic data that encompass both coseismic deformation and up to 40 days of postseismic deformation require that slip must have continued on the plate interface after the 500 sec long seismic rupture the postseismic geodetic moment of about 2 4 10 22 nm mw 8 8 is equal to about 30 5 of the coseismic moment release evolution of postseismic deformation is consistent with rate strengthening frictional afterslip online material summary of geodetic data used in this study

rupture process of the 2004 sumatra andaman earthquake the 26 december 2004 sumatra andaman earthquake initiated slowly with small slip and a slow rupture speed for the first 40 to 60 seconds then the rupture expanded at a speed of about 2 5 kilometers per second toward the north northwest extending 1200 to 1300 kilometers along the andaman trough peak displacements reached 15 meters along a 600 kilometer segment of the plate boundary offshore of northwestern sumatra and the southern nicobar islands slip was less in the northern 400 to 500 kilometers of the aftershock zone and at least some slip in that region may have occurred on a time scale beyond the seismic band

opposing gradients of permanent strain in the aseismic zone and elastic strain across the seismogenic zone of the kodiak shelf and slope alaska an industry acquired seismic reflection line from the aleutian arc to the alaska trench was prestack depth migrated and further constrained with wide angle ocean bottom hydrophone seismic velocity data and swath mapping from the seismic reflection image individual thrust slices the backstop and the forearc basin were resolved sufficiently well to balance the section tectonic shortening and age estimates provided rates of permanent contraction in five discrete segments over 150 km during the past 3 myr about 70 of the plate convergence is measured as contraction across the 40 km wide neogene accretionary prism which composes less than 10 of the forearc permanent contraction is minimal above the 1964 alaska earthquake aftershock zone indicating dominently elastic strain since elastic strain is poorly recorded in the geological record it was approximated from the historical record of the 1964 alaska earthquake the 1964 coseismic vertical deformation displays an inverse pattern to the permanent strain consistent with the earthquake cycle concept elastic deformation is dominant beneath the shelf and upper slope rapid permanent deformation is concentrated beneath the lower slope and a transition between them occurs beneath the middle slope

the great sumatra andaman earthquake of 26 december 2004 the two largest earthquakes of the past 40 years ruptured a 1600 kilometer long portion of the fault boundary between the indo australian and southeastern eurasian plates on 26 december 2004 seismic moment magnitude mw 9 1 to 9 3 and 28 march 2005 mw 8 6 the first event generated a tsunami that caused more than 283 000 deaths fault slip of up to 15 meters occurred near banda aceh sumatra but to the north along the nicobar and andaman islands rapid slip was much smaller tsunami and geodetic observations indicate that additional slow slip occurred in the north over a time scale of 50 minutes or longer

spatial arrangement and size distribution of normal faults buckskin detachment upper plate western arizona abstract fault arrays typically include a wide range of fault sizes and those faults may be randomly located clustered together or regularly or periodically located in a rock volume here we investigate size distribution and spatial arrangement of normal faults using rigorous size scaling methods and normalized correlation count ncc outcrop data from miocene sedimentary rocks in the immediate upper plate of the regional buckskin detachment low angle normal fault have differing patterns of spatial arrangement as a function of displacement offset using lower size thresholds of 1 0 1 0 01 and 0 001 m displacements range over 5 orders of magnitude and have power law frequency distributions spanning four orders of magnitude from less than 0 001 m to more than 100 m with exponents of 0 6 and 0 9 the largest faults with 1 m displacement have a shallower size distribution slope and regular spacing of about 20 m in contrast smaller faults have steep size distribution slopes and irregular spacing with ncc plateau patterns indicating imposed clustering cluster widths are 15 m for the 0 1 m threshold 14 m for 0 01 m and 1 m for 0 001 m displacement threshold faults results demonstrate normalized correlation count effectively characterizes the spatial arrangement patterns of these faults our example from a high strain fault pattern above a detachment is compatible with size and spatial organization that was influenced primarily by boundary conditions such as fault shape mechanical unit thickness and internal stratigraphy on a range of scales rather than purely by interaction among faults during their propagation

accretionary prisms in subduction earthquake cycles the theory of dynamic coulomb wedge 1 we expand the theory of critically tapered coulomb wedge for accretionary prisms by considering stress changes in subduction earthquake cycles building on the coulomb plasticity of the classical theory we assume an elastic perfectly coulomb plastic rheology and derive exact stress solutions for stable and critical wedges the new theory postulates that the actively deforming most seaward part of an accretionary prism the outer wedge overlies the updip velocity strengthening part of the subduction fault and the less deformed inner wedge overlies the velocity weakening part the seismogenic zone during great earthquakes the outer wedge is pushed into a compressively critical state with an increase in basal and internal stresses and pore fluid pressure after the earthquake the outer wedge returns to a stable state the outer wedge geometry is controlled by the peak stress of the updip velocity strengthening part of the subduction fault achieved in largest earthquakes the inner wedge generally stays in the stable regime throughout earthquake cycles acting as an apparent backstop and providing a stable environment for the formation of forearc basins the new theory has important implications for the studies of the updip limit of the seismogenic zone the evolution of accretionary prisms and forearc basins activation of splay faults and tsunami generation evolution of the fluid regime and mechanics of frontal prisms at margins dominated by tectonic erosion

development of extension parallel corrugations in the buckskin rawhide metamorphic core complex west central arizona extension parallel detachment fault corrugations are one of the defining features of metamorphic core complexes yet the origin of these corrugations remains disputed structural data from the buckskin rawhide metamorphic core complex in west central arizona demonstrate that prominent ne trending detachment fault corrugations are folds produced by extension perpendicular nw se shortening during the middle to late stages of core complex extension ca 18 10 ma kinematic data indicate that dominant ne directed slip on the buckskin rawhide detachment fault was locally overprinted by nw and se directed slip associated with corrugation folding the last motion recorded along several nw and se dipping flanks of the detachment fault is reverse slip directed toward the corrugation antiform hinges orientation patterns of upper plate bedding across the corrugations are compatible with folding about a ne trending axis extension perpendicular shortening in the lower plate is recorded by synmylonitic constriction and folding constriction and grain scale y axis shortening were most important during the later stages of mylonitization in the synextensional swansea plutonic suite which typically form l s tectonites upright meter and kilometer scale lower plate folds parallel the detachment fault corrugations and are best developed in well layered pre tertiary rocks most lower plate folding occurred by postmylonitic flexural slip that was coeval with detachment faulting the total amount of nw se shortening across the lower plate is 10 but the amount of nw se shortening recorded by the younger detachment fault is only 1 the relatively late stage development of corrugations in the buckskin rawhide metamorphic core complex suggests that the constriction and corrugation folding developed primarily in response to crustal thinning and tectonic denudation

tectonic features of the southern sumatra western java forearc of indonesia 1 multichannel reflection seismic profiles along the active sunda arc where the indo australian plate subducts under the overriding eurasian margin revealed two accretionary wedges the inner wedge i is of assumed paleogene age and the outer wedge ii is of neogene to recent age the inner wedge i is composed of tectonic flakes stretching from southeast sumatra across the sunda strait to northwest java implying a similar plate tectonic regime in these areas at the time of flake development during upper oligocene today wedge i forms the outer arc high and the backstop for the younger outer wedge ii the missing outer arc high of the southern sunda strait is explained by a combination of neogene transtension due to a clockwise rotation of sumatra with respect to java and by arc parallel strike slip movements the rotation created transtensional pull apart basins along the western sunda strait semangka graben as opposed to transpression and inversion on the eastern sunda strait within the new detected krakatau basin the arc parallel transpressional mentawai strike slip fault zone mfz was correlated from the sumatra forearc basin to the northwest java forearc basin off the sunda strait northward bending branches of the mfz are connected with the right lateral sumatra fault zone sfz along the volcanic arc segment on sumatra it is speculated that the sfz was attached to the cimandiri pelabuhan ratu strike slip fault of java prior to the presumed rotation of sumatra and that since the late lower miocene the main slip movement shifted from the volcanic arc position to the forearc basin area due to increasingly oblique plate convergence

the transition from large magnitude extension to distributed dextral faulting in the buckskin rawhide metamorphic core complex west central arizona brittle fault data from the buckskin rawhide metamorphic core complex west central arizona document the miocene transition from large magnitude ne directed extension to distributed e w extension and dextral faulting the buckskin rawhide detachment fault locally records a clockwise rotation of the slip direction from dominant top ne directed slip to ene and e directed slip during the last stages of activity postmylonitic brittle deformation is dominated by normal and dextral slip along moderately to steeply ne dipping faults relative timing relationships indicate that most postmylonitic normal faulting was coeval with middle miocene detachment fault slip whereas dextral and oblique dextral faulting postdates detachment fault slip post 11 ma based on the persistent sw dip of lower plate fabrics and ne dip of abundant normal faults the amount of ne sw brittle extension across the mylonitic lower plate is estimated to be 20 30 in several areas the end of detachment faulting in the buckskin rawhide metamorphic core complex was marked by a transition to dominantly e w extension and associated dextral normal faulting at least 10 ne dipping postdetachment faults record 0 1 1 km dextral or oblique dextral displacement the cumulative amount of dextral shear across the core complex is probably 7 9 km which is the amount needed to restore the topographic trend of lower plate corrugations into alignment with the dominant extension direction postdetachment dextral transtensional faulting across the core complex reflects the increasing influence of the diffuse pacific north america transform plate boundary toward the end of the middle miocene

active deformation across the sumatran forearc over the december 2004 mw9 2 rupture a 220 km long single channel seismic reflection profile crosses the northern sumatra margin and presumed rupture zone of the december 2004 m w 9 2 tsunamigenic earthquake and images active deformation across the forearc at the largest wavelength tens of kilometers the forearc surface is defined by a steep 55 km wide outer slope a 110 km wide upper slope forming a broad depression between two forearc highs and a 25 km wide steep inner slope between the landward high and forearc basin superimposed on these prism wide variations are anticlinal ridges spaced 13 km apart the inner and outer slopes are characterized by landward and seaward fold vergence respectively between anticlines growth strata deposited in slope basins are folded at 2 3 km wavelengths these small folds deform the seafloor and increase in amplitude with depth verging toward anticlinal hinges we suggest that long wavelength variations are consistent with variations in strength across the forearc the 13 km anticline spacing implies deformation of a slope apron that deforms independently of a stronger wedge interior growth strata geometries indicate ongoing deformation within individual basins our model for prism architecture suggests that the wedge interior advances during great earthquakes like the 2004 m w 9 2 event peeling up shallower and less competent trench fill deforming the toe and the upper slope of the forearc and producing seabottom uplift responsible for the tsunami

late stage evolution of the chemehuevi and sacramento detachment faults from apatite u th he thermochronometry evidence for mid miocene accelerated slip the unique low temperature sensitivity of the apatite u th he ahe technique provides valuable new information relating to the late stage thermal evolution of metamorphic core complexes in the southern basin and range province in all the ranges studied the data show greater complexity than previously seen from other thermochronometers in the chemehuevi and northern sacramento mountains a trend of decreasing ages toward the northeast applies only for the structurally shallowest parts of the footwall this is followed by essentially age invariant segments of 15 1 ma reflecting an increase in the rate of detachment fault movement the timing of the change suggested by the data agrees with recently published ahe results from the nearby harcuvar mountains located 125 km to the southeast suggesting that an underlying regional mechanism triggered this change plate reconstructions indicate that the pioneer mendocino fracture zone migrated northward through the area at this time leaving a slab window in its wake we suggest that the additional heat applied to the base of the lithosphere from the slab window resulted in mechanical weakening of the area and that this combined with the extensional strain regime present resulted in an increased rate of slip within the actively extending metamorphic core complexes this agrees well with timing constraints for extension onset in the central basin and range province of southern nevada and suggests that weakening of the southern basin and range province at 15 1 ma was partially responsible for the onset of extension in the central basin and range these findings demonstrate that improved constraints on changes in extension rate within the basin and range province should be possible through the application of the ahe technique to other metamorphic core complex footwalls

large trench parallel gravity variations predict seismogenic behavior in subduction zones we demonstrate that great earthquakes occur predominantly in regions with a strongly negative trench parallel gravity anomaly tpga whereas regions with strongly positive tpga are relatively aseismic these observations suggest that over time scales up to at least 1 million years spatial variations of seismogenic behavior within a given subduction zone are stationary and linked to the geological structure of the fore arc the correlations we observe are consistent with a model in which spatial variations in frictional properties on the plate interface control trench parellel variations in fore arc topography gravity and seismogenic behavior

origin of convex accretionary wedges evidence from barbados the surface slopes of submarine accretionary wedges generally decrease away from the toe or defor mation front toward the arc this paper presents evidence that this characteristic convex cross sectional shape is a direct mechanical response to the lithification of sediments during accretion the recently accreted sediments near the toe which are typically unconsolidated and therefore very weak must deform until a critical taper is attained at which they are stable in the presence of the horizontal tectonic compression this critical wedge taper and thus the bathymetric slope of the accretionary prism is relatively high for the porous weak sediments near the toe in contrast the compacted sediments farther back in the wedge are stronger so they require a relatively modest critical taper and therefore a lower bathymetric slope to be stable under the same tectonic compression the result is a convex cross sectional wedge shape we investigate several possible causes of the increase in sediment strength within the wedge and conclude that the primary cause is an increase in cohesive strength with decreasing porosity a straightforward generalization of the exact critical taper model is combined with empirical cohesion porosity velocity relations to infer the distribution of porosity and p wave velocity within the barbados accretionary wedge we estimate that the porosity decreases from its near toe value of 70 to approximately 30 at a depth of 3 km below the seafloor and a distance of 60 km arcward from the front of the wedge

seafloor morphology of the sumatran subduction zone surface rupture during megathrust earthquakes high resolution multibeam bathymetry data from the sumatran subduction zone reveal the regional and local morphology including small scale fault related features and landslides that may be linked to earthquakes in the recent geological past the accretionary prism is steeply sloped and pervasively eroded with evidence of unusual landward vergence seaward fault dip of the frontal thrusts small scale 5 100 m height fault scarps folds and troughs are common along the seaward edge of the frontal thrust at the deformation front a model of back thrust fault slip or bending moment folding during plate boundary slip such as during the 2004 m9 2 megathrust earthquake can explain the position of these features on the seaward fold limb of a seaward dipping thrust we infer that in major sumatran and other similar settings plate boundary earthquakes coseismic surface rupture may occur at the prism toe

lipid biomarker signatures in a hypersaline lake on isabel island eastern pacific as a proxy for past rainfall anomaly 1942 2006ad isabel lake is a hypersaline crater lake on isabel island mexico situated in the eastern tropical pacific an area highly sensitive to hydrological changes today annual rainfall mostly occurs during the wet season from june to october when the northern edge of the intertropical convergence zone itcz extends over the island in order to evaluate the potential of sedimentary lipid biomarker signatures as a proxy of past hydroclimatic variability we have performed a calibration analysis comparing changes in biomarker distribution in the upper 16 cm of the sediment core with a regional instrumental data set annual laminations present in the sediment sequence allow for precise chronological control 1942 2006 more than 80 different lipid compounds were identified in the sediment and could be assigned to three major groups of source organisms 1 algal populations 2 a mixed community of ciliates bacteria and cyanobacteria and 3 photosynthetic sulfur bacteria we found that the observed changes in the relative contribution of the different lipid biomarkers to the sediment record were determined by the regional rainfall variability over the last 65 years the planktonic community of isabel lake was highly sensitive to salinity fluctuations related to rainfall variability seasonal precipitation results in freshwater input into the lake driving an annual algal bloom and a relative decrease in the abundance of the more halotolerant populations of cyano bacteria and ciliates consequently the concentration ratio between the two most abundant biomarkers in the isabel lake sediments n alkyl diols and tetrahymanol which we define as the dite index representing algal and ciliate planktonic populations respectively was significantly correlated with the seasonal rainfall anomaly r 0 68 p 0 01 we propose that the dite index is a proxy of changes in the aquatic ecosystem of isabel lake and by extension regional hydrological changes in a sensitive climatic area of the eastern tropical pacific

metamorphic core complexes windows into the mechanics and rheology of the crust metamorphic core complexes are products of normal fault displacements sufficient to exhume rocks from below the brittle ductile transition these faults detachments may initiate within the brittle crust at steep angles but they sole into the ductile middle crust and during displacement rotate to gentler dips due to hanging wall extension the exhumed footwall commonly adopts an arched or domed geometry owing to flexural isostatic readjustment and may be overlain by strongly extended upper crustal rocks that slipped on gently dipping low friction shallow segments of the detachment metamorphic rocks exhumed beneath the detachment record progressively increasing flow stress strain localization and strain rate with decreasing temperature providing a window into physical conditions and deformational processes in the mid crust the metamorphic and deformational history of the footwall rocks may reflect tectonic processes that predate formation of the detachment fault in addition to those accompanying exhumation these processes may include diapiric emplacement of gneiss domes or exhumation in a subduction channel and may not be directly related to formation of the core complex factors favouring core complex formation are high gravitational potential energy of the extending crust weak rheology and a change in the tectonic boundary conditions such as a cessation or slowing of plate convergence

the structure of mountain fronts commonly the part of a mountain front which is visible at the surface consists of foreland dipping thrust belt rocks elevated above their regional structural position and overlain more or less conformably by molasse several explanations for their geometry are possible 1 major detachments exist within or beneath the molasse resulting in transport of the foreland basin examples of this geometry come from the swiss molasse plain the southern pyrenees and the mackenzie mountains of canada 2 displacement is lost on major backthrusts beneath the frontal monocline examples cited here are the rockies of alberta the sulaiman ranges of pakistan the mackenzies and the andes in peru 3 thrust sheets travelled over an old land surface and syntectonic molasse contemporaneously offlaps the topographic high of the thrust front this phenomenon occurs along the alpine thrust front in haute provence 4 the frontal fold represents deformation above a large scale thrust tip no unequivocal example of tip line strain at this scale has been recorded although this type of deformation may occur in the brooks range of alaska in many areas mountain fronts show a combination of these idealized geometries

mylonitization in the lower plate of the buckskin rawhide detachment fault west central arizona implications for the geometric evolution of metamorphic core complexes abstract in metamorphic core complexes it is commonly unclear whether lower plate mylonites formed as the down dip continuation of a detachment fault or whether they represent a subhorizontal shear zone that was captured by a more steeply dipping detachment fault detailed microstructural fabric and strain data from mylonites in the buckskin rawhide metamorphic core complex west central arizona constrain the structural development of the lower plate shear zone widespread exposures of 22 21 ma granitoids of the swansea plutonic suite enable us to separate miocene strain coeval with core complex extension from older deformation mylonites across the lower plate consistently record top to the ne directed shear miocene quartz and feldspar deformation recrystallization mechanisms indicate 450 500 c mylonitization temperatures that were relatively uniform across a distance of 35 km in the extension direction quartz dynamically recrystallized grain sizes do not systematically vary in the extension direction strain recorded in the swansea plutonic suite is also relatively uniform in the extension direction which is incompatible with models in which lower plate mylonites form as the ductile root of a major detachment fault altogether these data suggest the mylonitic shear zone initiated with a 4 dip and was unroofed by a more steeply dipping detachment fault system lower plate mylonites in the buckskin rawhide metamorphic core complex thus represent a captured subhorizontal shear zone rather than the down dip continuation of a detachment fault

timing rate and magnitude of slip on the buckskin rawhide detachment fault west central arizona we present thermochronologic and geochronologic data that constrain the slip history of the buckskin rawhide detachment fault in west central arizona one of the largest extensional fault systems in the north american cordillera u th he zircon and apatite thermochronology integrated with 40ar 39ar geochronology of postdetachment volcanic rocks indicate that large magnitude extension associated with the detachment fault initiated at 21 20 ma and continued until 12 11 ma in the southwestern portion of the buckskin rawhide metamorphic core complex u th he footwall cooling ages from the breakaway zone in the western bouse hills to upper greenschist facies mylonites in the southern buckskin mountains indicate that the slip rate on the detachment fault was 3 1 5 1 km myr during the early miocene space time patterns of hanging wall tilting suggest that at 17 16 ma a secondary detachment fault breakaway developed 12 km northeast of the primary detachment fault breakaway proximal conglomerates deposited in a supradetachment basin adjacent to the secondary breakaway scarp were displaced 6 11 km northeast in the middle miocene by the buckskin rawhide detachment fault at a slip rate of 1 2 2 7 km myr the total displacement across the detachment fault in the southwestern portion of the core complex is 24 10 km well short of the previous estimate of 66 8 km across the entire core complex based on these data and new observations we propose that total displacement on the buckskin rawhide detachment fault system increases in the slip direction to 40 50 km at the northeastern end of the exposed footwall corresponding to time averaged slip rates that ranged from 2 km myr to 6 km myr across the entire core complex

possible origin and significance of extension parallel drainages in arizona s metamorphic core complexes the corrugated form of the harcuvar south mountains and catalina metamorphic core complexes in arizona reflects the shape of the middle tertiary extensional detachment fault that projects over each complex corrugation axes are approximately parallel to the fault displacement direction and to the footwall mylonitic lineation the core complexes are locally incised by enigmatic linear drainages that parallel corrugation axes and the inferred extension direction and are especially conspicuous on the crests of antiformal corrugations these drainages have been attributed to erosional incision on a freshly denuded planar inclined fault ramp followed by folding that elevated and preserved some drainages on the crests of rising antiforms according to this hypothesis corrugations were produced by folding after subaerial exposure of detachment fault footwalls an alternative hypothesis proposed here is as follows in a setting where preexisting drainages cross an active normal fault each fault slip event will cut each drainage into two segments separated by a freshly denuded fault ramp the upper and lower drainage segments will remain hydraulically linked after each fault slip event if the drainage in the hanging wall block is incised even if the stream is on the flank of an antiformal corrugation and there is a large component of strike slip fault movement maintenance of hydraulic linkage during sequential fault slip events will guide the lengthening stream down the fault ramp as the ramp is uncovered and stream incision will form a progressively lengthening extension parallel linear drainage segment this mechanism for linear drainage genesis is compatible with corrugations as original irregularities of the detachment fault and does not require folding after early to middle miocene footwall exhumation this is desirable because many drainages are incised into nonmylonitic crystalline footwall rocks that were probably not folded under low temperature surface conditions an alternative hypothesis that drainages were localized by small fault grooves as footwalls were uncovered is not supported by analysis of a down plunge fault projection for the southern rincon mountains that shows a linear drainage aligned with the crest of a small antiformal groove on the detachment fault but this process could have been effective elsewhere lineation parallel drainages now plunge gently southwestward on the southwest ends of antiformal corrugations in the south and buckskin mountains but these drainages must have originally plunged northeastward if they formed by either of the two alternative processes proposed here footwall exhumation and incision by northeast flowing streams was apparently followed by core complex arching and drainage reversal

upward delamination of cascadia basin sediment infill with landward frontal accretion thrusting caused by rapid glacial age material flux the cascadia convergent margin is a first order research target to study the impact of rapid sedimentation processes on the mechanics of frontal subduction zone accretion the near trench part of the accretionary prism offshore washington is affected by strongly increased glacial age sedimentation and fan formation that led to an outstanding quaternary growth rate with landward vergent thrust faulting that is rarely observed elsewhere in accretionary wedges multichannel seismic reflection data acquired on the orwell project allows us to study the structure and dynamics of the atypical frontal accretion processes we performed a kinematical and mechanical analysis of the frontal accretion structures and developed a dynamic coulomb wedge model for the landward verging backthrust formation backthrusting results from heterogeneous diffuse strain accumulation in the mechanically heterogeneous cascadia basin sediment succession entering the subduction zone and strain partitioning along a midlevel detachment that is activated by gravitational loading caused by rapid glacial age sedimentation these complex deformation processes cause the passive upward delamination of the upper turbidite beds from the basal pelagic carbonate section similar to triangle zone formation and passive backthrust wedging in foreland thrust belts caused by rapid burial beneath syntectonic sediment deposits the deformation mechanism at the tectonic front of the cascadia margin is an immediate response to the strongly increased late pleistocene sediment flux rather than to atypical physical boundary conditions as generally thought

passive roof duplex geometry in the frontal structures of the kirthar and sulaiman mountain belts pakistan abstract exploration for hydrocarbons over the past few years has greatly improved our understanding of the geometry of frontal mountain belt structures in this study we introduce and discuss the concept of the passive roof duplex using as the main example the kirthar and sulaiman ranges in the baluchistan province of pakistan structures similar to those described here have been recognized previously in other mountain belts and they appear to exist as a common feature in many more frontal regions of mountain belts our example of a passive roof duplex which we describe from pakistan is compared briefly with similar structures reported by others the passive roof duplex is here defined as a duplex whose roof thrust has backthrust sense passive roof thrust and whose roof sequence those rocks lying above the roof thrust remains relatively stationary during foreland directed piggy back style propagation of horses within the duplex