quaternary tectonic response to intensified glacial erosion in an orogenic wedge active orogens are thought to behave as internally deforming critical taper wedges that are in rough long term equilibrium with tectonic influx and erosional outflux spatial and temporal variations in climate are therefore hypothesized to have a significant influence on denudation topography and deformation of orogens thereby affecting wedge taper however the impact of the most severe transition in northern hemisphere climate during the cenozoic era the onset of glaciation has hitherto not been empirically documented here we analyse the spatial patterns of denudation and deformation and their temporal variations in the heavily glaciated st elias orogen in southern alaska low temperature thermochronometry thermokinematic modelling and offshore seismic reflection and borehole data suggest that the global scale intensification of glaciation in the middle pleistocene epoch enhanced glacier growth and caused ice streams to advance to the edge of the continental shelf this led to focused denudation across the subaerial reaches of the orogen and burial of the actively deforming wedge toe by the eroded sediment we propose that this climatically driven mass redistribution forced a structural reorganization of the orogen to maintain critical taper our empirical results thus support decades of numerical model predictions of orogenesis and provide compelling field evidence for the significant impact of climate change on tectonics intense glaciation during the middle pleistocene epoch led to focused denudation and mass redistribution within the st elias orogen in southern alaska and resulted in structural reorganization of the orogen the tectonic response of this orogen to climate change is consistent with predictions of numerical models

climatic control on rapid exhumation along the southern himalayan front abstract along the southern himalayan front shf areas with concentrated precipitation coincide with rapid exhumation as indicated by young mineral cooling ages twenty new young 80 of the annual precipitation aft cooling ages reveal a coincidence between rapid erosion and exhumation that is focused in a 50 70 km wide sector of the himalaya rather than encompassing the entire orogen assuming simplified constant exhumation rates the rocks of two age vs elevation transects were exhumed at 1 4 0 2 and 1 1 0 4 mm a with an average cooling rate of 40 50 c ma during pliocene quarternary time following other recently published hypotheses regarding the relation between tectonics and climate in the himalaya we suggest that this concentrated loss of material was accommodated by motion along a back stepping thrust to the south and a normal fault zone to the north as part of an extruding wedge climatically controlled erosional processes focus on this wedge and suggest that climatically controlled surface processes determine tectonic deformation in the internal part of the himalaya

denudation and deformation in a glaciated orogenic wedge the st elias orogen alaska apatite u th he dating from the st elias orogen in southern alaska illustrates a potential association between long term denudation and glacier sliding cooling ages as young as 0 4 ma exhumation 4 5 mm yr are concentrated in a narrow band near the glacier equilibrium line altitude ela front where mean quaternary ela intersects the windward flank of the orogen this band of denudation is not correlated with individual faults structural trends or known concentrations of precipitation and we propose that it is produced by focused glacier sliding at or near the ela front this implies that long term glacial erosion is a maximum at ela which corroborates model predictions that glaciers and climate can control the pattern of crust removal from orogens denudation rates do not covary with fluctuations in glacier size along the ela front suggesting that small glaciers are capable of keeping pace with incision by larger ones and tectonic rock uplift ice discharge may thus play a critical but complex role in excavating glaciated orogens

heinrich 0 on the east canadian margin source distribution and timing the last deglaciation was marked by intervals of rapid climatic fluctuations accompanied by glacial advances and retreats along the eastern margin of the laurentide ice sheet one of these climatic events the younger dryas cold reversal was accompanied by a detrital carbonate event referred to as heinrich event 0 h0 that deposited ice rafted debris and especially detrital carbonate rich dc sediment layers in the western and southern labrador sea the precise age duration source and geographical distribution of the h0 dc event however are not entirely clear a high sedimentation rate sequence cored off southern newfoundland yielded an age of 11 5 11 3 calibrated kyr b p for this layer thus pointing to its deposition directly following the younger dryas termination likely associated with rapid ice retreat and warming at the onset of the holocene at the study site the h0 layer contains increased concentrations of detrital carbonates calcite dolomite ratio 2 5 1 and a mature biomarker composition similar to that found in heinrich layers 1 2 4 and 5 which together suggest an origin from hudson strait grain size analysis indicates that the h0 sediments were transported to the study site mostly through shallow plumes along the labrador shelf with some hyperpycnal dispersal deeper along the slope and rise our data thus point to a large meltwater release caused by retreat of the ice margin in the hudson strait area the relvatively short duration of the h0 layer and its timing right after a major climatic transition combined with its ubiquitous regional occurrence makes it valuable for correlating different sediment records from the western labrador sea

modeling of large scale accretionary wedge deformation instantaneous stress and velocity fields within an active accretionary wedge can be calculated using plastic slip line theory given the assumptions of a perfectly plastic rheology and plane strain conditions after selecting a plastic yield stress and an average density a stress field slip line field can be computed for a wedge of known cross sectional configuration from known stress boundary conditions on the upper surface basal surface tractions are derived without basal boundary conditions the geometry of the computed slip line field can then be used to constrain the lower limit of the yield stress the known velocity boundary conditions are insufficient for calculation of a velocity field because the coupling between the wedge and the subducting plate is unknown consequently basal velocity distributions must be assumed to permit the construction of velocity fields these assumed distributions are constrained by known rates of surface uplift and rates of tilt as well as inferred patterns of instantaneous strain rates within a wedge the sunda accretionary wedge west of central sumatra is used to illustrate the plasticity approach a minimum average yield stress of 20 30 mpa and a weaker basal layer of variable effective strength are indicated for this example surface uplift rates may be affected significantly by regions of high basal strain rate finite velocity discontinuities and underplating the known pattern of uplift rates near nias island located on the outer arc ridge is consistent with the constraint that 90 of the total incremental shortening of the wedge is concentrated within 20 km of the trench given a sufficient number of uplift and tilt rate observations across a particular wedge the relative influence of underplating tectonic erosion and basal strain rate variations could be assessed using the perfectly plastic model

frontal accretion an internal clock for bivergent wedge deformation and surface uplift 1 we propose a conceptual model of frontal accretion within bivergent wedges which is based on two dimensional sandbox simulations and the analysis of particle displacement fields each frontal accretion cycle consists of a thrust initiation an underthrusting phase and a reactivation phase the location and magnitude of deformation within a bivergent wedge and its associated surface uplift vary systematically with the phase of the frontal accretion cycle and are thus predictable therefore the frontal accretion cycle can be considered as an internal clock for wedge scaled deformation and surface uplift we further demonstrate that the geometry of the deformation front and the spatial distribution of surface uplift can be used to infer the currently active phase within a frontal accretion cycle surface uplift of the axial zone and the retrowedge may reach up to half of the thickness of the incoming sedimentary layer during a frontal accretion cycle we estimate cycle duration to range between 104 and 105 years which is thus similar to the time frame of climatic cycles the proposed conceptual model provides an explanation for the commonly observed transience of deformation and surface uplift and for the discrepancy between geodetic paleoseismologic and geologic estimates of fault slip

fore arc motion and cocos ridge collision in central america 1 we present the first regional surface velocity field for central america showing crustal response to interaction of the cocos and caribbean plates elastic half space models for interseismic strain accumulation on the dipping subduction plate boundary fit the gps data well and show strain accumulation offshore and beneath the nicoya and osa peninsulas in costa rica but not in nicaragua since large subduction zone earthquakes occur in nicaragua we suggest that interseismic locking in nicaragua and some other parts of central america occurs but is mainly shallow 20 km depth too far offshore to be detected by our on land gps measurements our data also show significant trench parallel motion for most of the region generally interpreted as due to oblique convergence and strong mechanical coupling between subducting and overriding plates however trench parallel motion is also observed in central costa rica where plate convergence is normal to the trench and in the nicaraguan fore arc where trench parallel motion is fast up to 9 mm a 1 but mechanical coupling is low a finite element model of collision as opposed to subduction involving the aseismic cocos ridge also fits the gps surface velocity field most significantly reproducing the pattern of trench parallel motion we infer that buoyant thickened cns 2 cocos ridge crust resists normal subduction and instead acts as an indenter to the caribbean plate driving crustal shortening in southern costa rica and contributing to trench parallel fore arc motion in costa rica and perhaps nicaragua as a type of tectonic escape

irregular earthquake cycle along the southern tianshan front aksu area china 1 a long deformation record 16 5 ka showing nonsteady earthquake recurrence is established for the aksu thrust tianshan china on the basis of cosmogenic 10be dating of faulted surfaces topographic leveling across the aksu fault scarp indicates up to 10 m uplift across a moraine abandoned 16 5 ka and similar uplift of 6 m across two inset surfaces yielding ages of 12 5 and 5 ka successively smaller uplifts of terraces younger than 5 ka indicate that three or more major earthquakes occurred during this period these data show that the aksu thrust fault was quiet for at least 7500 years and active in the last 5000 years and probably in the interval 16 5 12 5 ka the seismic cycle along the aksu thrust fault over the last 16 500 years was thus strongly irregular showing long quiescence and clustering which is a challenge for paleoseismology and hazard assessment this irregularity also makes it more difficult to estimate long term shortening rates nevertheless we are able to constrain a minimum total shortening rate 7 mm yr across the southern tianshan front for the last 12 500 years that is about a third of the total geodetic rate for the western tianshan 20 mm yr

persistent tremor within the northern costa rica seismogenic zone 1 we identify tremor using a spectral detection method and characterize its occurrence over a period of four years 2006 2009 in the vicinity of the nicoya peninsula costa rica although a few major tremor events accompanied by geodetic slow slip occur much of the tremor record consists of minor episodes with short duration and no detectable geodetic slip its persistent occurrence suggests that some portion of the interface is experiencing slow slip nearly continuously driving small patches to fail in accelerated slip locations indicate that much of the tremor occurs at shallow depth in freely slipping regions of the seismogenic zone this result is significant in that locations of slow slip and tremor at other subduction zones are largely limited to the downdip frictional transition tremor locations may help to refine the heterogeneous distribution of locked and freely slipping patches within the costa rica seismogenic zone

erosion and tectonics at the margins of continental plateaus the topography across the eastern margin of the central andean plateau north of 18 s beni region bears a strong resemblance to the topography of the southern margin of the tibetan plateau nepal himalaya with both regions featuring a steep frontal slope and high peaks at the plateau edge in contrast the topography of the eastern margin of the central andean plateau south of 18 s pilcomayo region tapers toward the foreland more gently and has no line of high peaks at the margin both the himalayan and the beni regions have been the sites for large amounts of denudation and in both regions geologic evidence suggests that erosion has been sufficiently vigorous for the physiographic plateau margin to have retreated toward the plateau interior during the neogene we hypothesize that the steep frontal slope and high peaks of the beni region and himalayan front largely reflect the high orographic precipitation and high erosion rates occurring in these regions and that the more gentle topography of the semiarid pilcomayo region reflects a tectonic landform only slightly modified by erosion we propose that orographic precipitation impinging on a plateau margin will generally tend to drop moisture low on the slope eroding back the plateau while enhancing or maintaining the steep long wavelength slope a numerical model coupling orographic precipitation erosion and tectonic uplift demonstrates the plausibility of this hypothesis the erosional efflux in both the beni and nepal himalaya have been considerable and simple mass balance calculations for the himalaya suggest that during the neogene the erosional mass efflux has generally outpaced the tectonic mass influx this contrasts with the apparent prior domination of tectonic influx and may reflect a decrease in the rate of tectonic addition during the same period and or increased late cenozoic erosion rates

active detachment of taiwan illuminated by small earthquakes and its control of first order topography we use 110 000 small earthquakes to locate and map active faults in three dimensions within the taiwan arc continent collision the structure is dominated by a nearly horizontal band of small earthquakes at 10 km depth that is interpreted to seismically illuminate the main detachment zone of the mountain belt with other illuminated fault zones abutting the detachment zone the zone steepens below eastern taiwan to 30 90 and reaches depths of 30 60 km the three dimensional shape of the detachment zone in relation to topography allows a new test of critical taper wedge mechanics and suggests that the reversal of topographic slope across taiwan is controlled by the shape of the detachment

a midcrustal ramp fault structure beneath the taiwan tectonic wedge illuminated by the 2013 nantou earthquake series 1 taiwan s active mountain belt is a spotlight for orogenic studies and was first used to test the critical taper wedge mechanics the concept of an orogenic wedge above a shallow detachment surface has been highly influential on current understanding of orogenic processes in taiwan however the recent ml 6 2 and ml 6 5 2013 nantou reverse faulting earthquakes in central taiwan have nucleated below the proposed detachment indicating that active mountain building is occurring below the orogenic wedge we estimate the coseismic slip distributions and fault geometry using the uniform stress drop slip inversions the earthquakes occur on essentially the same 30 dipping fault plane ramping up from 20km depth near a cluster of 1999 chi chi earthquake aftershocks to the shallow detachment and the chi chi fault plane the fault could be a deep extension of a mature shallow fault or a newly developed deep ramp fault that is not reflected in the surface geology

the march 25 1990 mw 7 0 ml 6 8 earthquake at the entrance of the nicoya gulf costa rica its prior activity foreshocks aftershocks and triggered seismicity on march 25 1990 a large earthquake mw 7 0 ml 6 8 occurred at the entrance of the nicoya gulf costa rica at 1322 55 6 utc producing considerable damage in central costa rica and generating much interest about whether or not the nicoya seismic gap nishenko 1989 had broken the local country wide seismographic network recorded 6 years of activity prior to this large earthquake 16 hours of foreshocks the mainshock and its aftershocks this network is operated jointly by the costa rica volcanological and seismological observatory at the national university ovsicori una and the charles f richter seismological laboratory at the university of california santa cruz cfrsl ucsc we obtained high resolution locations from this network and located the mainshock at 9 38 5 n 84 55 6 w depth is 20 0 km and the largest foreshock mw 6 0 march 25 1990 at 1316 05 8 utc at 9 36 4 n 84 57 1 w depth is 22 4 km we find that the aftershock zone abuts the southeast boundary of the nicoya seismic gap suggesting that the seismic gap did not rupture since the installation of the local network in april 1984 to march 24 1990 nearly 1900 earthquakes with magnitudes from 1 7 to 4 8 318 with magnitude 3 0 or larger have been located at the entrance of the nicoya gulf one of the most active regions in costa rica the march 25 earthquake occurred at the northwest edge of this region where a sequence of foreshocks began 16 hours prior to the mainshock the spatial temporal distribution of aftershocks and directivity analysis of the mainshock rupture process using teleseismic records both indicate a southeast propagating rupture the mainshock ruptured an asperity of approximately 600 km2 of area with this area expanding to 4000 km2 after 7 days we present evidence that suggests that the ruptured asperity is produced by the subduction of a seamount inversion of teleseismic broadband and long period p and sh waves yields a thrust faulting mechanism with the shallow plane striking 292 dipping 26 and with a rake of 88 in agreement with the subduction of the cocos plate under the caribbean plate local first motions for the largest foreshock and the mainshock agree with this solution we also present evidence suggesting that the march 25 1990 earthquake triggered and reactivated several seismic swarms in central costa rica and temporally decreased the activity in the epicentral area of the july 3 1983 ms 6 2 perez zeledon earthquake

a thermomechanical wedge model of taiwan constrained by fission track thermochronometry abstract we present 31 new apatite fission track aft ages for the island of taiwan that when combined with existing aft and zircon fission track zft data provide regional spatial coverage of the island with respect to low temperature thermochronometry the overall pattern of zft and aft ages in taiwan exhibits unreset ages in the southern and western portions of the island and reset ages predominantly in the central range and eastern taiwan this pattern supports interpretations of the orogen kinematics as reflecting a crustal scale wedge with a southward propagating collision zone in this model new material is accreted to the wedge from the west and is transferred to the east with the greatest exhumation occurring along the eastern margin as recorded in the reset ages in the east and unreset ages in the west the southward propagating collision is consistent with reset ages in the north where erosional exhumation has been ongoing for longer and unreset ages in the south where the younger collision implies less time for erosional exhumation despite the variation in the age of the collision along the strike of the island the widths of the aft and zft reset age zones remain nearly constant between 23 00 n to 24 00 n and 23 20 n to 24 00 n respectively suggesting that the orogen is in an exhumational steady state over these regions with respect to the aft and zft thermochronometers we use the fission track data in conjunction with observations of crustal structure crustal fabric and heat flow measurements to constrain a time dependent two dimensional thermomechanical model of orogen evolution by accounting for the heat transfer tectonic and erosion processes needed to predict aft and zft ages we are able to investigate the relationship between the measured ages and the tectonic characteristics of the orogen with our model we conclude that 1 roughly half of the material accretion in taiwan occurs through underplating over an approximately 40 km wide region 2 current average erosion rates are 3 3 mm yr in the eastern central range and 2 3 mm yr over the whole island 3 the collision has been propagating southward at a rate between 20 and 51 km ma over the past 2 3 ma and 4 central taiwan is in a topographic thermal and exhumational steady state

quaternary transfer faulting and belt front deformation at pakuashan western taiwan the arcuate pakuashan anticline is located in the outermost front units of the western foothills of taiwan this oblique feature of the deformation front is investigated in terms of combined morphostructural analysis based on imagery and digital elevation model as well as microtectonic analysis of fault slip data a subsurface structural study based on available seismic and well data was also carried out resulting in improved mapping of the neogene series and associated structures this mapping allowed construction of several along strike cross sections such combined analyses revealed that the transverse pakuashan fold is located above a major transfer fault zone this active fault zone accommodates differential westward propagation of thrust units its kinematic evolution is principally controlled by the geometry of the foreland peikang high behaving as a buttress for the west verging thrust sheets a preliminary analytical model of the oblique thrusting at pakuashan is based on similar cases studied by apotria et al 1992 it involves quaternary transfer faulting accommodating the motion of connected thrust sheets moving over oblique ramps linked to a preexisting major basement boundary the hinge fault of the peikang high this analytical modeling accounts for the occurrence of local extension at the intersection of oblique ramps in the southern pakuashan numerous complementary structural and tectonic evidences led us to establish a complete deformation model involving local rotation in southern pakuashan which caused differential slips in northern pakuashan resulting in tear faulting these evidences include large extension at the intersection of oblique ramps distributed extension in the transverse zone regional wrench deformation absence of major reorientation of local stress inside the transverse zone along strike variation of structural styles coupled with low to high uplift rate from the northern to the southern part of the pakuashan fold thus a synthetic reconstruction of the pakua transfer fault zone evolution is proposed as a typical example of active transfer faulting evolving gradually from a primary tear fault with a slight curvature to the left lateral tear fault or transfer fault that offsets two distinct frontal thrust and fold sheets

afterslip and aftershocks in the rate and state friction law istituto nazionale di geofisica e vulcanologia italy ingv centre national de la recherche scientifique cnrs exxonmobil upstream research company

extension and gold mineralisation in the hanging walls of active convergent continental shear zones abstract orogenic gold bearing quartz veins form in mountain belts adjacent to convergent tectonic boundaries the vein systems hosted in extensional structures within compressively deformed rocks are a widespread feature of these orogens in many cases the extensional structures that host gold bearing veins have been superimposed on and locally controlled by compressional structures formed within the convergent orogen exploring these observations within the context of a three dimensional mechanical model allows prediction of mechanisms and locations of extensional zones within convergent orogens our models explore the effect of convergence angle and mid crustal strength on stress states and compare them to the southern alps and taiwan the dilatation zones coincide with the highest mountains in the hanging walls of major plate boundary faults and can extend as deep as the brittle ductile transition extensional deformation is favoured in the topographic divide region of oblique orogens with mid lower crustal rheology that promotes localisation rather than diffuse deformation in the near surface topography influences the stress state to a depth approximately equal to the topographic relief bringing the rock closer to failure and rotating 1 to near vertical the distribution of gold bearing extensional veins may indicate the general position of the topographic divide within exhumed ancient orogens

slow and delayed deformation and uplift of the outermost subduction prism following ets and seismogenic slip events beneath nicoya peninsula costa rica abstract two odp cork ocean drilling program circulation obviation retrofit kit borehole hydrologic observatory sites deployed in 2002 at the toe of the subduction prism off nicoya peninsula costa rica were visited in december 2013 the five years of seafloor and formation fluid pressure data collected since the previous visit include clear signals associated with an episodic tremor and slip ets event off the coast of nicoya peninsula in 2009 and a mw 7 6 subduction thrust earthquake beneath the peninsula in 2012 formation pressure anomalies associated with the ets event are similar to ones observed following ets events observed previously here as well as ones following very low frequency earthquake swarms within the nankai accretionary prism off southwestern japan positive and negative impulsive transients in the hanging wall and foot wall of the subduction thrust respectively suggest contractional and dilatational strain generated by local slip propagating up the thrust fault beneath the outermost prism in the case of the 2009 event the transients occurred roughly two weeks after the initiation of slip observed at gps sites along the adjacent coast at the same time a decrease in seafloor pressure at the prism site relative to the subducting plate was observed indicating concurrent uplift of the prism of 1 2 cm other events at the prism toe following ets events closer to the coast are seen in 2006 2007 2008 2010 and 2011 the time between the initiation of ets slip constrained by gps and the onset of the prism toe transients suggest up dip rupture propagation along the seaward part of the subduction thrust at rates of a few km day in the case of the 2009 event the slip at the prism toe c 11 cm estimated from the 1 2 cm uplift and the local dip on the decollement 6 is roughly a factor of 5 greater than the slip further landward estimated from gps data by dixon et al in press in other cases slip at the toe is less or unresolvable similar uplift and formation fluid pressure transients occurred two days after the 2012 nicoya thrust earthquake then episodically unrelated to aftershocks every 1 3 weeks over the next few months slip rates at the prism toe estimated from the magnitude and duration of these and ets related uplift events are roughly 5 cm day such rates are much too slow for tsunamigenesis or seismogenesis total slip of the outermost prism after the 2012 earthquake determined from the uplift accumulated until the end of the observation period and the estimated local fault dip is roughly 75 cm a significant fraction of the seismogenic slip of the mw 7 6 earthquake itself a maximum of over 3 m negative steps in formation pressure are observed at both the prism and subducting plate sites simultaneous with the 2012 thrust earthquake the sign of these steps is inconsistent with the contractional deformation predicted by an elastic fault dislocation model with slip limited to the seismogenic zone highly compliant low shear modulus sediment below the decollement or very low stress deformation along splay faults may be responsible for the co seismic dilatation observed in the outermost prism and incoming plate

variations of ice bed coupling beneath and beyond ice streams the force balance 1 a geometrical force balance that links stresses to ice bed coupling along a flow band of an ice sheet was developed in 1988 for longitudinal tension in ice streams and published 4 years later it remains a work in progress now gravitational forces balanced by forces producing tensile compressive basal shear and side shear stresses are all linked to ice bed coupling by the floating fraction of ice that produces the concave surface of ice streams these lead inexorably to a simple formula showing how varies along these flow bands where surface and bed topography are known ho hi with ho being ice thickness hi at x 0 for x horizontal and positive upslope from grounded ice margins this captures the basic fact in glaciology the height of ice depends on how strongly ice couples to the bed it shows how far a high convex ice sheet 0 has gone in collapsing into a low flat ice shelf 1 here captures ice bed coupling under an ice stream and ho captures ice bed coupling beyond ice streams

asymmetric growth of the pyrenees revealed through measurement and modeling of orogenic fluxes orogenic asymmetry is ubiquitous in terms of topography structure and metamorphism but the relative influences of climate and tectonics in determining such asymmetry remain contentious this study demonstrates that for the case of the european pyrenees orogenic asymmetry is not defined by inherited structural asymmetry nor by orographic asymmetry of precipitation but is the simple consequence of the inherent tectonic growth of a doubly vergent orogen the pyrenees record approximately 60 myr of upper crustal accretion erosion and sedimentation that developed from late cretaceous to early miocene times approximately 165 km of plate convergence was accommodated by four phases of orogenic growth that can be summarized in terms of their generic orogenic development as follows 1 early inversion of passive margin extensional faults initiating the pro wedge 2 growth by a combination of frontal accretion and underplating to the pro wedge symmetrical and steady record of erosion and foreland basin sedimentation 3 cessation of pro wedge frontal accretion with propagation of the retro wedge deformation front 4 accelerated growth of pro wedge antiformal stack induced by underplating accelerated erosion over antiformal stack and increase in sediment discharge to pro foreland basin stabilization of outer pro wedge by sediment blanketing discrete element model experiments are used to explore the natural tendency of the system towards asymmetry model output demonstrates that during active frontal accretion to the pro wedge of an orogen the maxima in erosional denudation is offset towards the pro wedge relative to the maximum depths of exhumation this asymmetry is fundamentally driven by the progressive reduction in advection velocities of material within the orogen from the pro wedge into the retro wedge hence for an efficient erosional regime a dynamic coupling between erosion and structural uplift of the pro wedge will reduce activity of the retro wedge and hence drive asymmetry in terms of low temperature thermochronometry erosion and sedimentation the models also demonstrate the tendency for pro wedge growth through frontal accretion to slow however the cessation of frontal accretion as recorded by the south pyrenean thrust belt phases 3 and 4 requires additional forcing it is proposed that the thick wedge top sedimentation recorded at this time increased taper angles and hence stabilized the wedge accretionary influx into the orogen at this time was accomplished through underplating these processes governing asymmetry do not require a spatially variable climate or a laterally heterogeneous underthrust plate

subduction of asian lithospheric mantle beneath tibet inferred from models of continental collision the relative northward motion of the indian subcontinent that followed the onset of continental collision with asia has produced extensive deformation of the earth s crust giving rise to the world s highest mountains in the himalayan chain and the world s largest high elevation region the tibetan plateau the formation of the broad mountain belt implies that contrary to the original tenets of plate tectonics the lithospheric plates have experienced widespread deformation far from the plate boundary1 several models have been proposed2 6 to explain the manner in which this post collisional deformation is distributed within the continental lithosphere of the indian and asian plates here we propose an alternative model in which subduction of the asian lithospheric mantle develops following the collision of india our model is supported by numerical calculations of crustal deformation and thickening and is consistent with available geological and geophysical data7 9 this picture suggests that lithospheric mantle is not deformed along with the crust and would imply that continental collision zones are more analogous to oceanic subduction zones than was previously believed

a tremor and slip event on the cocos caribbean subduction zone as measured by a global positioning system gps and seismic network on the nicoya peninsula costa rica in may 2007 a network of global positioning systems gps and seismic stations on the nicoya peninsula of northern costa rica recorded a slow slip event accompanied by seismic tremor the close proximity of the nicoya peninsula to the seismogenic part of the cocos caribbean subduction plate boundary makes it a good location to study such events several centimeters of southwest motion were recorded by the gps stations over a period of several days to several weeks and the seismic stations recorded three distinct episodes of tremor during the same time span inversion of the surface displacement data for the depth and pattern of slip on the plate interface shows peak slip at a depth of 25 30 km downdip of the main seismogenic zone estimated temperatures here are 250 300 c lower than in other subduction zones where events of this nature have been previously identified there may also be a shallower patch of slip at 6 km depth these results are significant in that they are the first to suggest that slow slip can occur at the updip transition from stick slip to stable sliding and that a critical temperature threshold is not required for slow slip tremor and low frequency earthquake locations are more difficult to determine our results suggest they occur on or near the plate interface at the same depth range as the deep slow slip but not spatially colocated

from tectonically to erosionally controlled development of the himalayan orogen whether variations in the spatial distribution of erosion influ ence the location style and magnitude of deformation within the himalayan orogen is a matter of debate we report new 40 ar 39 ar white mica and apatite fission track aft ages that measure the vertical component of exhumation rates along an 120 km wide ne sw transect spanning the greater sutlej region of northwest india the 40 ar 39 ar data indicate that first the high himalayan crystalline units cooled below their closing temperature during the early to middle miocene subsequently lesser himalayan crys talline nappes cooled rapidly indicating southward propagation of the orogen during late miocene to pliocene time the aft data in contrast imply synchronous exhumation of a ne sw oriented 80 3 40 km region spanning both crystalline nappes during the pliocene quaternary the locus of pronounced exhumation de fined by the aft data correlates with a region of high precipita tion discharge and sediment flux rates during the holocene this correlation suggests that although tectonic processes exerted the dominant control on the denudation pattern before and until the middle miocene erosion may have been the most important factor since the pliocene

structural geology of a classic thrust belt earthquake the 1999 chi chi earthquake taiwan mw 7 6 abstract we document the structural context of the 1999 chi chi earthquake m w 7 6 in western taiwan which is one of the best instrumented thrust belt earthquakes the main surface break and large slip 3 10 m is on two segments of the shallow otherwise aseismic bedding parallel chelungpu sanyi thrust system which shows nearly classic ramp flat geometry with shallow detachments 1 6 km in the pliocene chinshui shale and mio pliocene kueichulin tungkeng formations however rupture is complex involving at least six faults including a previously unknown deeper thrust 8 10 km on which the rupture began we compare the coseismic displacements with a new 3d map of the chelungpu sanyi system the displacements are spatially and temporally heterogeneous and well correlated with discrete geometric segments of the 3d shape of the fault system geodetic displacement vectors are statistically parallel to the nearest adjacent fault segment and are parallel to large scale oblique fault corrugations the displacement magnitudes are heterogeneous at several scales which requires in the long term other non chi chi events or significant aseismic deformation the chelungpu thrust has a total displacement of 14 km but the area of largest chi chi slip 10 m is on a newly propagated north chelungpu chinshui detachment 0 3 km total slip which shows abnormally smooth rupture dynamics

episodic folding inferred from syntectonic carbonate sedimentation the santaren anticline bahamas foreland abstract sedimentation coeval with growth of the santaren anticline provides an excellent opportunity to study the relationships between sedimentation and anticline uplift through time the santaren anticline is a kilometre scale nw se trending fold offshore of cuba in the bahamas foreland of the cuban fold and thrust belt the growth strata associated with this anticline consist of a thick package of carbonate sediments that were deposited without major interruptions from neogene and perhaps before to present day the excellent seismic resolution and age control of a number of seismic horizons within the growth strata allowed us to define 25 growth beds each of them representing between 0 1 and 3 2 ma an analysis of the thickness of these beds allowed us to determine accurate quantitative values of cumulative decompacted thickness and crestal structural relief at the time of their deposition in addition for the same periods sedimentation and fold uplift rates were calculated moreover some information on relationships between sedimentation and fold uplift rates was inferred from the growth stratal geometry growth beds that overlap the fold crest and thin over it indicate that sedimentation rates outpaced fold growth rates during their deposition some overlapping beds have constant thickness indicating that no fold uplift occurred during their sedimentation the rest of the growth beds exhibit onlap offlap geometries that do not indicate a unique sedimentation fold uplift rate relationship only in those cases in which the geometry of the underlying bed at the end of its deposition is known is it possible to infer a specific sedimentation fold uplift rate relationship as a result of this analysis we have been able to 1 illustrate that the geometry of the growth strata associated with the santaren anticline results from competition between sedimentation and tectonic fold uplift 2 document the episodic and non steady nature of fold growth and 3 show that short term rates at the scale of hundreds of thousands years provide much insight into the interplay between sedimentation and tectonic fold uplift that control the growth stratal patterns

exhumation history of the southern altiplano plateau southern bolivia constrained by apatite fission track thermochronology 1 although the structural geometry of the cenozoic altiplano puna plateau in the central andes is well defined the temporal evolution of this contractile deformation is poorly constrained to address this shortcoming we used apatite fission track thermochronology aft to quantify the cooling and exhumation history along a transect at 21 s in southern bolivia through the deformed intermontane altiplano basin the doubly vergent thrust belt of the eastern cordillera and the inner foreland thrust belt east of the plateau interandean zone thermal history modeling combined with published balanced cross sections and stratigraphic data constrain exhumation histories exhumation started during the late eocene 40 36 ma in the central eastern cordillera possibly due to bivergent thrusting and cretaceous rift structure inversion during the early oligocene 33 27 ma exhumation spread across the study area as the current boundary thrusts of the eastern cordillera were activated the inner west vergent thrust system became active in irregular order until circa 20 ma whereas the east vergent interandean thrust belt formed by eastward propagating deformation since circa 30 ma plateau exhumation continued at 0 2 mm yr until shortening terminated by 11 7 ma shortening within the plateau since circa 30 ma did not evolve by lateral accretion of thrust wedges the propagating deformation style is spatially confined to the foreland thrust belt which initiated coeval to plateau deformation interandean zone but propagated mainly after circa 10 ma subandean zone early oligocene plateau wide tectonically driven exhumation suggests that subduction related processes had already thermally weakened the continental lithosphere prior to the 27 25 ma onset of volcanic activity

hydrologic control of forearc strength and seismicity in the costa rican subduction zone the subduction zone beneath costa rica experiences infrequent large earthquakes in its northwestern part whereas slow slip dominates in the southeast seismic data reveal a disparity in fluid accumulation in the overriding continental crust that correlates with this change in seismic behaviour implying that spatial gradients in fluid content may control subduction zone seismicity

investigating the kinematics of mountain building in taiwan from the spatiotemporal evolution of the foreland basin and western foothills the taiwanese range has resulted from the collision between the luzon volcanic arc and the chinese continental margin which started about 6 5 myr ago in the north and has since propagated southward the building of the range has been recorded in the spatiotemporal evolution of the foreland basin we analyze this sedimentary record to place some constraints on the kinematics of crustal deformation the flexure of the foreland under the load of the growing wedge started with a 1 5 myr long phase of rapid subsidence and sedimentation which has migrated southward over the last 3 5 myr at a rate of 31 10 5 mm yr reflecting the structural evolution of the range and the growth of the topography during the oblique collision isopachs from the toukoshan 0 to 1 1 ma and cholan 1 1 to 3 3 ma formations as well as the sedimentation rates retrieved from a well on the pakuashan anticline indicate that the foreland basement has been moving toward the center of mass of the orogen by 45 50 mm yr during the development of the basin from there we estimate the long term shortening rate across the range to 39 5 44 5 mm yr by considering available data on the thrust faults of the foothills of central taiwan we show that most if not all the shortening across the range is accommodated by the most frontal structures with little if any internal shortening within the wedge the range growth appears therefore to have been essentially sustained by underplating rather than by frontal accretion in addition only the upper 7 to 9 km of the underthrusted crust participates to the growth of the orogen this requires that a significant amount of the chinese passive margin crust is subducted beneath the philippine sea plate

punctuated thrust deformation in the context of doubly vergent thrust wedges implications for the localization of uplift and exhumation understanding uplift and exhumation in orogenesis requires integrating the effects of both climatic and tectonic forcing punctuated thrust activity is widely documented from field studies in a range of mountain belts the potential for climatic forcing of thrust activity has been a subject of recent debate here we use a modeling approach to analyze the behavior of individual thrust units in the context of asymmetric doubly vergent thrust wedges the model predicts that rates of surface uplift frontal accretion and exhumation should be punctuated on a time scale linked to thrust sheet geometry and convergence rates this time scale ranges from 0 1 to 5 m y for various settings and should be calculated before external forcings such as climate are invoked

linking taiwan s subcritical hsuehshan range topography and foreland basin architecture of 10 1 kilometers and 10 4 10 5 years causal links between the structural and synorogenic stratigraphic architecture of the foreland basin and coincidence of the puli topographic embayment provide a valuable case study of the effects of changing boundary conditions e g variable erodibility or strength of rocks along strike controlling the evolution of critically tapered thin skinned orogens deeper incision of river networks into a thicker sequence of unconsolidated synorogenic sediments in the central western foreland may affect the onset of a topographically subcritical state

spatially variable exhumation rates in orogenic belts an andean example abstract the cordillera real of the bolivian andes is a large tectonically active mountain range that dominates sediment influx into the amazon basin but rates of exhumation in the orogen are poorly known we present 20 new apatite fission track ages from two valleys in the cordillera real to constrain patterns of mountain range exhumation over 106 107 yr we interpret these and previously published data from a third valley using a 2 d thermal model that accounts for topographic and advective influences on measured cooling ages exhumation rates in the cordillera real are 0 2 0 6 mm yr comparable to rates in parts of denali the washington cascades the olympic mountains and the european alps and an order of magnitude slower than rates in taiwan nanga parbat the greater himalaya of nepal and the southern alps of new zealand three to fourfold cooling age variations in the cordillera real imply at least twofold exhumation rate variations within and between valleys over distances of only tens of ki

kinematic analysis of the pakuashan fault tip fold west central taiwan shortening rate and age of folding inception the pakuashan anticline is an active fault tip fold that constitutes the frontalmost zone of deformation along the western piedmont of the taiwan range assessing seismic hazards associated with this fold and its contribution to crustal shortening across central taiwan requires some understanding of the fold structure and growth rate to address this we surveyed the geometry of several deformed strata and geomorphic surfaces which recorded different cumulative amounts of shortening these units were dated to ages 2 ranging from 19 ka to 340 ka using optical stimulated luminescence osl we collected shallow seismic profiles and used previously published seismic profiles to constrain the deep structure of the fold these data show that the anticline has formed as a result of pure shear with subsequent limb rotation the cumulative shortening along the direction of tectonic transport n118e is estimated to be 1010 160 m an analytical fold model derived from a sandbox experiment bernard et al in press is used to model growth strata this yields a shortening rate of 16 3 4 1 mm yr and constrains the time of initiation of deformation to 62 2 9 6 ka in addition the kinematic model of pakuashan is used to assess how tectonics sedimentation and erosion have sculpted the present day fold topography and morphology the fold model applied here for the first time on a natural example appears promising in determining the kinematics of fault tip folds in similar contexts and therefore in assessing seismic hazards on blind thrust faults

mechanical model for the tectonics of doubly vergent compressional orogens a mechanical model of crustal shortening and deformation driven by the relative convergence of rigid underlying mantle plates explains many features of convergent orogens results based on numerical models and supported by sandbox models show that a coulomb crustal layer subject to basal velocity boundary conditions corresponding to asymmetric detachment and subduction of the underlying mantle passes through three stages of orogenic growth 1 block uplift bounded by step up shear zones 2 development of a low taper wedge over the underthrusting mantle plate and 3 development of a low taperwedge overlying the overthrusting mantle plate and verging in the opposite direction when modified by isostasy basal viscous flow surface erosion and denudation and sedimentation the resultant model orogens exhibit a variety of styles with characteristics in common with small rapidly denuded orogens large orogens with plateaus and extensional characteristics and active subduction margins with doubly vergent accretionary wedges and deformed fore arc basins

do subducting seamounts generate or stop large earthquakes seamount subduction is a common process in subduction zone tectonics contradicting a widely held expectation that subducting seamounts generate large earthquakes seamounts subduct largely aseismically producing numerous small earthquakes on rare occasions when they do produce relatively large events the ruptures tend to be complex suggesting multiple rupture patches or faults we explain that the seismogenic behavior of these seamounts is controlled by the development and evolution of an adjacent fracture network during subduction and cannot be described using the frictional behavior of a single fault the complex structure and heterogeneous stresses of this network provide a favorable condition for aseismic creep and small earthquakes but an unfavorable condition for the generation and propagation of large ruptures

the geologic history of the caribbean cocos plate boundary with special reference to the nicoya ophiolite complex costa rica and d s d p results legs 67 and 84 off guatemala a synthesis abstract the pre upper senonian basement of costa rica crops out in the santa elena and nicoya peninsulas from south to north and from base to top the basement includes the esperanza matapalo and santa elena units the esperanza unit is albian santonian in age and consists mainly of pillow basalt and massive basalt flows the matapalo unit includes callovian to cenomanian radiolarite and includes massive basalt flows basalt and dolerite basement the santa elena unit contains ultramafic and mafic rocks in which harzburgite is the major component the most important tectonic features of the nicoya complex are the large santa elena and matapalo nappes nappe emplacement was from north to south during upper santonian time the sedimentary cover of the nicoya complex comprises 1 1 the campanian el viejo formation that consists of shallow water sediments in the north santa elena peninsula and the campanian maastrichtian sabana grande formation of deep water origin in the south nicoya peninsula 2 2 paleocene strata indicating deposition in a deep water environment comprises the rivas las palmas and samara formations 3 3 a post upper eocene sequence that consists of the shallow water barra honda and montezuma formations two unconformities are significant geological features of the upper senonian to tertiary history of costa rica the lower one is at the base of the sabana grande formation and marks a major change in the geologic conditions basalt is scarce in the campanian tertiary series the upper unconformity at the base of the barra honda and montezuma formations is not as major as the lower one during post campanian time normal faulting occurred in two stages separated by a strong erosional phase the geology of the landward slope of the adjacent middle america trench is outlined by interpreting multifold seismic reflection records off the west coast of costa rica and the dsdp legs 67 and 84 transects off guatemala the western caribbean plate boundary may have been under extensional stress for the last 75 m y the strong landward dipping reflectors of the middle america trench landward slope off guatemala could be equivalent to the on land pre campanian overthrusts of costa rica the available data are consistent with the convergent extensional margin concept

slow slip events in costa rica detected by continuous gps observations 2002 2011 1 a network of continuously recording gps stations has operated in the nicoya peninsula of northern costa rica since 2002 we processed all available data from this network for the period of 2002 2011 to investigate the occurrence of slow slip events sse on the subduction interface between the cocos and caribbean plates in order to overcome signal masking by high levels of tropospheric noise we developed a new technique that facilitates detection of transient events in the presence of noise we identified five significant sses during the 2002 2011 period with event middle times in 2003 2005 2007 2009 and 2011 with an average recurrence interval of 21 6 months time series analysis shows that transient deformation imparts a signature similar to random walk removal of the sses and regional common mode errors from the time series reduced velocity uncertainty by nearly an order of magnitude limited available data for the 2003 2005 and 2011 events preclude detailed characterization of these events however good spatiotemporal coverage of the 2007 and 2009 events suggest that both events had irregular duration and distribution in the 2007 event slow slip started in the northwest coastal area and migrated southeastward over a period of 1 month the 2009 event had a significantly longer event duration and larger surface displacement stations in the northwest area observed two separate sses in 2008 6 and 2009 4 correlating well with the tremor episodes offshore indicating a shallow sse slip patch with shorter recurrence interval significant differences between the 2009 and 2007 events lead us to question the simple recurrence interval model for the sse in nicoya

monte carlo slip inversion in the frequency domain application to the 1992 nicaragua slow earthquake the spatio temporal complexity of moment release of the 1992 nicaragua slow earthquake mw 7 7 is estimated by simulated annealing contrary to linearized inversions monte carlo techniques do not make assumptions about a priori features of the model in particular large variations in rupture velocities can be imaged relative spectra between the main shock and an empirical green s function were obtained from 4 44 mhz using first and second orbit fundamental mode rayleigh waves the spectra were inverted for the moment distribution on a 2d horizontal fault under the physical constraints of slip positivity and causality the non linear inversion revealed two main regions of slip one centered 20 km to the nw of the epicenter the other 160 km to the se but delayed by 70 s rupture velocities in the slipping patches were found to be low around 1 km s large amount of slip occurred trench ward of the epicenter accounting for the large tsunami excitation analysis of the slip distribution suggests that the slow and smooth nature of the 1992 nicaragua event may be related to the frictional properties at the plate interface

crustal reworking at nanga parbat pakistan metamorphic consequences of thermal mechanical coupling facilitated by erosion within the syntaxial bends of the india asia collision the himalaya terminate abruptly in a pair of metamorphic massifs nanga parbat in the west and namche barwa in the east are actively deforming antiformal domes which expose quaternary metamorphic rocks and granites the massifs are transected by major himalayan rivers indus and tsangpo and are loci of deep and rapid exhumation on the basis of velocity and attenuation tomography and microseismic magnetotelluric geochronological petrological structural and geomorphic data we have collected at nanga parbat we propose a model in which this intense metamorphic and structural reworking of crustal lithosphere is a consequence of strain focusing caused by significant erosion within deep gorges cut by the indus and tsangpo as these rivers turn sharply toward the foreland and exit their host syntaxes the localization of this phenomenon at the terminations of the himalayan arc owes its origin to both regional and local feedbacks between erosion and tectonics

extension and partitioning in an oblique subduction zone new zealand constraints from three dimensional numerical modeling 1 contraction strike slip and extension displacements along the hikurangi margin northeast of the north island of new zealand coincide with large lateral gradients in material properties we use a finite difference code utilizing elastic and elastic plastic rheologies to build large scale three dimensional numerical models which investigate the influence of material properties on velocity partitioning within oblique subduction zones rheological variation in the oblique models is constrained by seismic velocity and attenuation information available for the hikurangi margin we compare the effect of weakly versus strongly coupled subduction interfaces on the development of extension and the partitioning of velocity components for orthogonal and oblique convergence and include the effect of ponded sediments beneath the raukumara peninsula extension and velocity partitioning occur if the subduction interface is weak but neither develops if the subduction interface is strong the simple mechanical model incorporating rheological variation based on seismic observations produces kinematics that closely match those published from the hikurangi margin these include extension within the taupo volcanic zone uplift over ponded sediments and dextral contraction to the south

the 5 september 2012 nicoya costa rica mw 7 6 earthquake rupture process from joint inversion of high rate gps strong motion and teleseismic p wave data and its relationship to adjacent plate boundary interface properties 1 on 5 september 2012 a large thrust earthquake mw 7 6 ruptured a densely instrumented seismic gap on the shallow dipping plate boundary beneath the nicoya peninsula costa rica ground motion recordings directly above the rupture zone provide a unique opportunity to study the detailed source process of a large shallow megathrust earthquake using very nearby land observations hypocenter relocation using local seismic network data indicates that the event initiated with small emergent seismic waves from a hypocenter 10 km offshore 13 km deep on the megathrust a joint finite fault inversion using high rate gps strong motion ground velocity recordings gps static offsets and teleseismic p waves reveals that the primary slip zone slip 1 m is located beneath the peninsula the rupture propagated downdip from the hypocenter with a rupture velocity of 3 0 km s the primary slip zone extends 70 km along strike and 30 km along dip with an average slip of 2 m the associated static stress drop is 3 mpa the seismic moment is 3 5 1020 nm giving mw 7 6 the coseismic large slip patch directly overlaps an onshore interseismic locked region indicated by geodetic observations and extends downdip to the intersection with the upper plate moho at deeper depths below the upper plate moho seismic tremor and low frequency earthquakes have been observed most tremor locates in adjacent areas of the megathrust that have little coseismic slip a region of prior slow slip deformation to the southeast also has no significant coseismic slip or aftershocks an offshore locked patch indicated by geodetic observations does not appear to have experienced coseismic slip and aftershocks do not overlap this region allowing the potential for a comparable size rupture offshore in the future

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

a new seismically constrained subduction interface model for central america we provide a detailed seismically defined three dimensional model for the subducting plate interface along the middle america trench between northern nicaragua through to southern costa rica the model uses data from a weighted catalog of about 30 000 earthquake hypocenters compiled from nine catalogs to constrain the interface through a process we term the maximum seismicity method the method determines the average position of the largest cluster of microseismicity beneath an a priori functional surface above the interface this technique is applied to all seismicity above 40 km depth the approximate intersection of the hanging wall mohorovicic discontinuity where seismicity likely lies along the plate interface below this depth an envelope above 90 of seismicity approximates the slab surface because of station proximity to the interface this model provides highest precision along the interface beneath the nicoya peninsula of costa rica an area where marked geometric changes coincide with crustal transitions and topography observed seaward of the trench the new interface is useful for a number of geophysical studies that aim to understand subduction zone earthquake behavior geodynamic and tectonic development of convergent plate boundaries

mechanical model for subduction collision tectonics of alpine type compressional orogens alpine type orogens are characterized by three distinct convergent tectonic phases subduction with deformation that has primarily single vergence a transition from subduction to collision and continental collision with double vergence although the cenozoic history of the european alps has additional complexities a mechanical explanation for these three phases would provide the necessary crustal scale framework in which to develop an understanding of the smaller scale processes we present results from a simple numerical model which explain the mechanics of these three phases as a consequence of the changing buoyancy of the lithosphere subducted beneath the orogen the development and exhumation of a subduction complex suture zone and basement nappe stack piemont suture penninic nappes the presence of a crustal scale back fold and thrust insubric line and uplift of basement on the pro european side of the orogen external basement massifs may be explained as a simple consequence of changing dynamics during the transition from subduction to collision

orogeny and orography the effects of erosion on the structure of mountain belts a numerical model of the coupled processes of tectonic deformation and surface erosion in convergent orogens is developed to investigate the nature of the interaction between these processes crustal deformation is calculated by a two dimensional finite element model of deformation in response to subduction and accretion of continental crust erosion operates on the uplifted surface of this model through fluvial incision which is taken to be proportional to stream power the relative importance of the tectonic and erosion processes is given by a dimensionless erosion number relating convergence velocity rock erodibility and precipitation rate this number determines the time required for a system to reach steady state and the final topographic shape and size of a mountain belt fundamental characteristics of the model orogens include asymmetric topography with shallower slopes facing the subducting plate and an asymmetric pattern of exhumation with the deepest levels of exhumation opposite to subduction these characteristics are modified when the regional climate exhibits a dominant wind direction and orographically enhanced precipitation on one side of the mountain belt the two possible cases are dominant wind in the direction of motion of the subducting plate and dominant wind direction in the opposite direction of the subducting plate velocity models of the former case predict a broad zone of exhumation with maximum exhumation in the orogen interior models of the latter case predict a focused zone of exhumation at the margin of the orogen and at high erosion number a reversal in the topographic asymmetry natural examples of these two cases are presented the southern alps of new zealand exhibits the climate and exhumation asymmetry characteristic of wind in the direction opposite to motion of the subducting plate the asymmetry of topography suggests that erosion is not efficient enough to have reversed the topographic asymmetry the contrasting example of dominant wind in the direction of subduction motion is provided by the olympic mountains of washington state in this case exhumation of deep levels of the cascadia accretionary wedge shows a broad domal pattern consistent with the observed orographic precipitation

the synchronous occurrence of shallow tremor and very low frequency earthquakes offshore of the nicoya peninsula costa rica 1 the occurrence of transient shallow slow slip at seismogenic zones has important implications for earthquake and tsunami hazards here we provide evidence that a tremor and slow slip event occurred at shallow depth offshore of the nicoya peninsula costa rica in august 2008 the temporal coincidence of offshore tremor very low frequency earthquakes vlfes motions consistent with slow slip on the plate interface on western coastal gps stations and a pressure transient in an iodp borehole all indicate slow slip occurring at shallow depths large ocean loading stresses on the shallow plate interface modulate tremor activity with the peak coulomb stress forced by semi diurnal ocean tides correlating with tremor productivity based on beamforming data we constrain that the vlfe activity occurs in the same region as the tremor and slow slip the presence of slow slip at shallow depth has important implications for the up dip extent of earthquake rupture the proximity of the 5 september 2012 mw 7 6 megathrust earthquake to slow slip tremor and vlfe activity in the 2008 event suggests abrupt frictional transitions from locked to conditionally stable behavior on the plate interface offshore of the nicoya peninsula

topography as a major factor in the development of arcuate thrust belts insights from sandbox experiments abstract we have used sandbox experiments to investigate and to illustrate the effects of topography upon the development of arcuate thrust belts in experiments where a sand pack shortened and thickened in front of an advancing rectilinear piston the geometry of the developing thrust wedge was highly sensitive to variations in surface topography in the absence of erosion and sedimentation the surface slope tended to become uniform as predicted by the theory of critical taper under these conditions the wedge propagated by sequential accretion of new thrust slices in contrast where erosion or sedimentation caused the topographic profile to become irregular thrusts developed out of sequence for example erosion throughout a hinterland caused underlying thrusts to remain active and inhibited the development of new thrusts in the foreland where initial topography was irregular in plan view accreting thrusts tended to be arcuate they were convex towards the foreland around an initially high area concave towards the foreland around an initially low area initial plateaux tended to behave rigidly while arcuate thrust slices accreted to them thrust motions were radial with respect to each plateau within transfer zones to each side fault blocks rotated about vertical axes and thrust motions were oblique slip at late stages of deformation the surface slope of the thrust wedge tended towards a uniform value initial mountains of conical shape representing volcanoes also escaped deformation except at depth where they detached arcuate thrust slices accreted to front and back where a developing thrust wedge was subject to local incision accreting thrust slices dipped towards surrounding areas of high topography forming vs across valleys arcuate structural patterns are to be found around the three highest plateaux on earth tibet pamirs and altiplano and around the tromen volcanic ridge in the neuquen basin of northern patagonia we infer that these areas behaved in quasi rigid fashion protected as they were by their high topography

influence of exhumation on the structural evolution of transpressional plate boundaries an example from the southern alps new zealand concentration of erosional activity along transpressional plate boundaries can significantly alter the pattern of mechanical behavior through the influence of exhumation on crustal strength three dimensional numerical modeling of an obliquely convergent orogen shows that a single oblique plate bounding structure is stable if asymmetric erosion patterns such as those observed in orographic mountain belts pertain and if earth9s crust has a strong on weak rheology in early stages of oblique convergence of an initially laterally homogeneous material lateral boundary parallel strain is accommodated along a near vertical structure and convergent boundary normal strain is concentrated on structures dipping at moderate angles into the orogen exhumation of deep crustal material along the convergent structure results in thermal weakening along this dipping structure when the upper crust beneath the orogen is significantly weakened by exhumation lateral strain abandons the vertical structure and shifts to the dipping structure combining with the convergent strain to form a single oblique fault that accommodates the plate motion in the upper crust as is the case along the alpine fault new zealand the process of thermal thinning is controlled by advection and occurs on time frames of 1 2 m y the two components of strain remain separate in the lower crust during active convergence exhumation of lower crustal material occurs only along those structures accommodating convergent strain consequently material exhumed from lower regions of ductile deformation as is the case along the alpine fault contains lineations that indicate a greater component of convergence than predicted from the total plate motion postorogenic exhumation of the roots of an oblique plate boundary will expose two parallel shear zones one dominantly convergent and one dominantly strike slip widely reported orogen parallel transport in the late stages of ancient oblique convergence may represent not a change in plate vector but the exhumation of the lateral transport zone

noncohesive critical coulomb wedges an exact solution active fold and thrust belts or submarine accretionary complexes can be modeled as critically tapered wedges of material on the verge of coulomb failure everywhere overlying a basal decollement where frictional sliding is occurring ignoring cohesion the four strength parameters needed to describe a critical coulomb wedge are its internal and basal coefficients of friction and b and its internal and basal hubbert rubey fluid pressure ratios and b an exact relation between surface slope and basal dip of a noncohesive critical wedge with uniform properties is derived the state of stress within such a wedge has the same orientation everywhere and is constant if is and vice versa a coefficient of internal friction 1 1 is consistent with the known surface slope basal dip and pore fluid pressures in the active fold and thrust belt of western taiwan assuming that byerlee s law b 0 85 is valid on the base the wide variety of tectonic styles observed to occur along convergent margins including subduction erosion active accretion subduction without accretion and even extension and normal faulting may be controlled by relatively small spatial or temporal variations in either b or b

long term glacial erosion of active mountain belts example of the chugach st elias range alaska an emerging paradigm that equates glaciers to buzz saws of exceptional erosional efficiency has been strengthened by short term 10 2 yr sediment yields from southern alaska new low temperature cooling ages from this area the glaciated chugach st elias mountains constrain long term 10 6 yr exhumation rates vertically averaged exhumation rates reach 3 mm yr but are an order of magnitude lower than rates based on short term sediment yields whereas these exhumation rates are not exceptional for oro genic belts denudation patterns are strongly correlated with the distribution of glaciers and erosion appears to keep pace with con vergence and uplift these findings imply a coupling between gla cially dominated erosion and tectonics

sediment flux from a mountain belt derived by landslide mapping in humid uplands landsliding is the dominant mass wasting process in the western southern alps of new zealand landslides are scale invariant and have a power law magnitude frequency distribution independent studies from other regions suggest that this is a general property of landsliding this observation is of critical importance to the evaluation of the impact of events of different length scales over different time intervals on landscape evolution it is particularly useful when estimating regional geomorphic rates because it constrains the frequency and overall significance of extreme events which cannot otherwise be evaluated by integrating the complete response of the system we estimate the regional denudation rate due to landsliding to be 9 4 mm yr 1 sediment discharge from the western southern alps is dominated by landslide derived material

the 1992 nicaragua earthquake a slow tsunami earthquake associated with subducted sediments the 1992 nicaragua earthquake was a tsunami earthquake that is it generated tsunamis disproportionately large for its surface wave magnitude m s the moment magnitude m w determined from long period 250 s surface waves was 7 6 significantly larger than the 20 s m s of 7 this m s m w disparity is also characteristic of tsunami earthquakes the nicaragua earthquake is the first tsunami earthquake to be captured by modern broadband seismic networks allowing us to present here seismograms of sufficiently high quality to make inferences about the rupture mechanisms we conclude that the nicaragua earthquake was a slow thrust earthquake which occurred on the subduction interface between the cocos and north american plates and because of the absence of sediments on the trench floor offshore of nicaragua the slip propagated up dip all the way to the ocean bottom exciting large tsunamis the occurrence of slip on a plate interface filled with soft subducted sediments caused the rupture process to be slower than in ordinary subduction zone thrust earthquakes our results reinforce the idea that tsunami warning systems using long period 100 s waves are necessary to reduce the hazard from this type of earthquake

crustal motion in the zone of the 1960 chile earthquake detangling earthquake cycle deformation and forearc sliver translation temporary deformation in great earthquake cycles and permanent shear deformation associated with oblique plate convergence both provide critical clues for understanding geodynamics and earthquake hazard at subduction zones in the region affected by the mw 9 5 great chile earthquake of 1960 we have obtained gps observations that provide information on both types of deformation our velocity solutions for the first time span the entire latitudinal range of the 1960 earthquake the new observations revealed a pattern of opposing roughly arc normal motion of coastal and inland sites consistent with what was reported earlier for the northern part of this region this finding supports the model of prolonged postseismic deformation as a result of viscoelastic stress relaxation in the mantle the new observations also provide the first geodetic evidence for the dextral motion of an intravolcanic arc fault system and the consequent northward translation of a forearc sliver the sliver motion can be modeled using a rate of 6 5 mm a accommodating about 75 of the margin parallel component of nazca south america relative plate motion with the rate diminishing to the north furthermore the new gps observations show a southward decrease in margin normal velocities of the coastal area we prefer explaining the southward decrease in terms of changes in the width or frictional properties of the megathrust seismogenic zone because of the much younger age of the subducting plate and warmer thermal regime in the south the currently locked portion of the plate interface may be narrower using a three dimensional viscoelastic finite element model of postseismic and interseismic deformation following the 1960 earthquake we demonstrate that this explanation although not unique is consistent with the gps observations to the first order

exhumation rates in the st elias mountains alaska abstract reprocessing and interpretation of seismic profiles from the gulf of alaska in the vicinity of the st elias mountains indicate that over 3000 km 3 of sediment were deposited on the ocean shelf since the end of the last glacial epoch approximately 10 000 years ago this estimate of the sediment volume does not include material moved out of the region by long shore currents transportation from the shelf to deeper waters or material trapped in fiords and basins on and near land thus an estimated sedimentation rate offshore of 7 9 mm year and the average erosion rate of 5 1 mm year from the st elias mountains derived from the estimated sedimentation rate represent minimum estimates we infer that these estimated rates are not typical of the approximately 5 m y history of mountain building in the area but rather they represent the result of efficient erosion by glaciers during this interglacial period

far field triggering of foreshocks near the nucleation zone of the 5 september 2012 m w 7 6 nicoya peninsula costa rica earthquake abstract on 5 september 2012 a moment magnitude mw 7 6 earthquake occurred directly beneath the nicoya peninsula an area with dense seismic and geodetic network coverage the mainshock ruptured a portion of a previously identified locked patch that was recognized due to a decade long effort to delineate the megathrust seismic and aseismic processes in this area here we conduct a comprehensive study of the seismicity prior to this event utilizing a matched filter analysis that allows us to decrease the magnitude of catalog completeness by 1 unit we observe a statistically significant increase in seismicity rate below the nicoya peninsula following the 27 august 2012 mw 7 3 el salvador earthquake about 450 km to the northwest and 9 days prior to the nicoya earthquake additionally we identify a cluster of small magnitude

slip distribution from the 1 april 2007 solomon islands earthquake a unique image of near trench rupture 1 we estimate the slip distribution from the mw 8 1 solomon islands earthquake in 2007 from two post seismic surveys measuring uplifted coral and submerged coastal features the occurrence of islands extremely proximal to the trench and nucleation of rupture allowed for the collection of unprecedented coseismic deformation dataset along a large megathrust earthquake using data from the two surveys along the southeastern half of the slip zone within five weeks of the event we model the elastic dislocation to identify the optimal 29 and alternate 20 dip and distribution of thrust along the southern rupture the vertical deformation which includes both coseismic and early postseismic deformation has highly variable and large slip within 25 km of the trench and straddling ranongga island the shallow focus of slip in the near trench area may explain the locally high tsunami run up on portions of simbo island however the aseismic contribution of afterslip remains unknown

impact of backstop thickness lateral variations on the tectonic architecture of orogens insights from sandbox analogue modeling and application to the pyrenees 1 we performed sandbox analogue models to investigate the influence of along strike tapering of the backstop on the three dimensional architecture of natural doubly vergent thrust fold belts applying the results to a natural example as the pyrenees available geological and geophysical data indicate that the european crust which provided the backstop in the pyrenean orogen has a westward tapering on both the upper westward thickening of the mesozoic cover rocks and the lower westward rising of the moho envelope surfaces accordingly we performed laboratory models characterized by backstops with an along strike tapering of either the upper or the lower surface our results indicate a significant impact of backstop lateral tapering in thrust wedge architecture in terms of the outward propagation of the deformation fronts the width and elevation of the axial zone and the timing of thrust polarity reversal along strike when compared to the pyrenees our results support the inferred westward tapering of the european basement and its commonly accepted backstop role in the pyrenean orogen moreover comparison between the map pattern of the pyrenees and of laboratory experiments suggests that the along strike tapering of the bottom envelope surface of the backstop played a most important role on the three dimensional thrust wedge architecture than the tapering of the upper envelope surface

slip rates on the chelungpu and chushiang thrust faults inferred from a deformed strath terrace along the dungpuna river west central taiwan the chelungpu fault produced the september 1999 m w 7 6 chi chi earthquake central taiwan the shortening rate accommodated by this structure integrated over several seismic cycles and its contribution to crustal shortening across the taiwanese range have remained unresolved to address the issues we focus our study on the chelungpu and chushiang thrust faults within the southernmost portion of the chi chi rupture area structural measurements and available seismic profiles are used to infer the subsurface geometry of structures the chushiang and chelungpu faults appear as two splay faults branching onto a common ramp that further north connects only to the chelungpu surface trace we survey a deformed strath terrace along the dungpuna river buried under a 11 540 309 years old fill deposit given this age the dip angles of the faults and the vertical throw determined from the offset of the strath terrace across the surface fault traces we estimate slip rates of 12 9 4 8 and 2 9 1 6 mm yr on the chelungpu and chushiang faults respectively these yield a total shortening rate of 15 8 5 1 mm yr to be absorbed on their common decollement at depth this total value is an upper bound for the slip rate on the chelungpu fault further north where the chushiang fault disappears and transfers shortening to adjacent faults combining these results with the recently constrained shortening rate on the changhua blind thrust reveals that all these frontal faults presently absorb most of the long term horizontal shortening across the taiwanese range they thus stand as the major sources of seismic hazards in this heavily populated area the return period of earthquakes similar to the chi chi event over a 80 km long stretch of the western foothills is estimated to be 64 years this value is an underestimate because it assumes that all the faults locked during the interseismic period slip only during such large events comparison with historical seismicity suggests that episodic aseismic deformation might also play a major role in accommodating shortening

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

local earthquake tomography of central costa rica transition from seamount to ridge subduction summary the structure and seismicity of the subduction zone of central costa rica have been investigated with local earthquake tomography down to ca 50 km depth seismic traveltime data sets of three on and offshore seismic networks were combined for a simultaneous inversion of hypocentre locations 3 d structure of p wave velocity and v p v s ratio using about 2000 highquality events the seismicity and slab geometry as well as v p and v p v s show significant lateral variation along the subduction zone corresponding to the changes of the incoming plate which consists of serpentinized oceanic lithosphere in the northwest a seamount province in the centre and the subducting cocos ridge in the southeast of the investigation area three prominent features can be identified in the v p and v p v s tomograms a high velocity zone with a perturbation of 4 10 per cent representing the subducting slab a low velocity zone 10 20 per cent in the forearc crust probably caused by deformation fluid release and hydration and a low velocity zone below the volcanic arc related to upwelling fluids and magma unlike previously suggested the dip of the subducting slab does not decrease to the south instead an average steepening of the plate interface from 30 to 45 is observed from north to south and a transition from a plane to a step shaped plate interface this is connected with a change in the deformation style of the overriding plate where roughly planar partly conjugated clusters of seismicity of regionally varying dip are observed it can be shown that the central costa rica deformation belt represents a deep crustal transition zone extending from the surface down to 40 km depth this transition zone indicates the lateral termination of the active part of the volcanic chain and seems to be related to the changing structure of the incoming plate as well

thermal regime of the costa rican convergent margin 2 thermal models of the shallow middle america subduction zone offshore costa rica at the costa rica margin along the middle america trench along strike variations in heat flow are well mapped these variations can be understood in terms of either ventilated fluid flow where exposed basement allows fluids to freely advect heat between the crustal aquifer and ocean or insulated fluid flow where continuous sediment cover restricts heat advection to within the crustal aquifer we model fluid flow within the subducting aquifer using nusselt number approximations coupled with finite element models of subduction and explore its effect on temperatures along the subduction thrust the sensitivity of these models to the initial thermal state of the plate and styles of fluid flow either ventilated or insulated is explored heat flow measurements on cool crust accreted at the east pacific rise are consistent with ventilated hydrothermal cooling that continues with subduction these models yield much cooler temperatures than predicted from simulations initialized with conductive predictions and without hydrothermal circulation heat flow transects on warm crust accreted at the cocos nazca spreading center are consistent with models of insulated hydrothermal circulation that advects heat updip within the subducting crustal aquifer near the trench these models are warmer than conductive predictions and cooler than conductive predictions downdip of the trench comparisons between microseismicity and modeled isotherms suggest that the updip limit of microseismicity occurs at temperatures warmer than 100 c and that the downdip extent of microseismicity is bounded by the intersection of the subduction thrust with the base of the overriding crust

tectonic erosion and consequent collapse of the pacific margin of costa rica combined implications from odp leg 170 seismic offshore data and regional geology of the nicoya peninsula the convergent margin off the pacific coast of the nicoya peninsula of costa rica exhibits evidence for subduction erosion caused by the underthrusting cocos plate critical evidence for efficacy of this process was recovered at the ocean drilling program odp drilling site 1042 leg 170 positioned 7 km landward of the middle america trench axis off the nicoya peninsula the primary drilling objective at this site was to identify the age and origin of a regionally extensive and prominent seismic discontinuity the so called base of slope sediment boss horizon or surface the boss horizon which can be traced landward from near the trench to the nicoya coastal area and parallel to it for hundreds of kilometers separates a low velocity 2 0 2 5 km s 1 sequence of slope sediment from an underlying sequence of much higher velocity 4 4 5 km s 1 rock site 1042 reached the acoustically defined boss horizon at a below sea level depth of 3900 m and yielded a carbonate cemented calcarenitic breccia of early middle miocene age sedimentological geochemical paleontological and cement paragenesis data document that the breccia accumulated in a shallow water depositional environment on the basis of coastal exposures the boss horizon as a margin wide geologic interface can be temporally and lithostratigraphically correlated to a regional angular unconformity this unconformity known as the mal pais unconformity separates neogene and younger shelf to littoral beds from the underlying mafic units of the mesozoic nicoya complex and cretaceous and early tertiary sedimentary sequences at site 1042 it is inferred that tectonism caused the vertical subsidence of the early neogene breccia from a shallow to a deep water setting the mal pais unconformity of the boss horizon thus connects the rock fabric of the outermost part of margin to that of coastal nicoya and implies that in the early neogene the nicoya shelf extended seaward to near the present trench axis this circumstance requires that the early neogene trench axis was at least 50 km seaward of where it is now located the long term effects of subduction erosion similar to those described for the scientifically drilled japan tonga and peru margins best account for offshore and onshore evidence for a post paleogene history of crustal thinning and landward trench migration of costa rica s pacific margin during the past 16 17 myr the calculated mass removal and landward migration rates are 34 36 km myr 1 km 1 of margin and 3 km myr 1 respectively these values are similar to those found for other pacific margins dominated by nonaccretionary subduction zone processes

large and primarily updip afterslip following the 2012 mw 7 6 nicoya costa rica earthquake we present detailed surface measurements of the first 3 5 years of postseismic deformation following the 5 september 2012 moment magnitude mw 7 6 nicoya costa rica earthquake the dominant signal in the first 2 5 years is uniform horizontal trenchward motion totaling 7 26 cm across 40 stations trenchward velocity is strongly diminished by mid 2014 and appears by 2016 to have begun reversing we invert the first 2 5 years to determine the corresponding afterslip on a detailed 3d interface results show significant afterslip both up and down dip of the main coseismic rupture zone with as much as 1 7 m of offset in two patches at 15 20 km depth and immediately up dip of the maximum coseismic slip this updip slip represents an important mechanism to address unrelieved interseismic locking although sufficient strain energy remains to generate up to a mw 7 1 event near the coastline the afterslip patches are anticorrelated with strongly clustered aftershocks at the same depth which is indicative of varying frictional behavior along strike an additional patch of slip is colocated with reoccurring slow slip events beneath the gulf of nicoya the magnitude of the observed slip however cannot be sufficiently explained by the known slow slip events ongoing measurements will be crucial to understanding the relocking process in nicoya

thermal models of the middle america trench at the nicoya peninsula costa rica 1 we present thermal models of the 24 ma cocos plate subducting under the caribbean plate at the nicoya peninsula costa rica our modeling incorporates the effect of hydrothermal cooling of the seafloor and the absence of an accretionary prism initial geotherms of the incoming plate representing different effective cooling depths of hydrothermal circulation affect the thermal structure and temperatures along the main subduction thrust excluding the deforming prism results in a higher heat flow immediately landward of the trench than previous models and therefore significant frictional heating is not required conductive plate cooling over predicts heat flow across the frontal forearc with no or very low frictional heating models with hydrothermal cooling to depths of 2 km under predicts the heat flow comparisons with recently determined hypocenter locations indicate that the updip limit of seismicity is consistent with temperatures between 100 150 c but there are large along strike variations in both the seismicity and thermal regime

has focused denudation sustained active thrusting at the himalayan topographic front the geomorphic character of major river drainages in the himalayan foothills of central nepal suggests the existence of a discrete west northwest trending break in rock uplift rates that does not correspond to previously mapped faults the 40 ar 39 ar thermochronologic data from detrital muscovites with provenance from both sides of the discontinuity indicate that this geomorphic break also corresponds to a major discontinuity in cooling ages samples to the south are proterozoic to paleozoic whereas those to the north are miocene and younger combined these observations virtually require recent pliocene holocene motion on a thrust sense shear zone in the central nepal himalaya 20 30 km south of the main central thrust field observations are consistent with motion on a broad shear zone subparallel to the fabric of the lesser himalayan lithotectonic sequence the results suggest that motion on thrusts in the toe of the himalayan wedge may be synchronous with deeper exhumation on more hinterland structures in central nepal we speculate that this continued exhumation in the hinterland may be related to intense sustained erosion driven by focused orographic precipitation at the foot of the high himalaya

climatic forcing of erosion landscape and tectonics in the bhutan himalayas a fundamental objective in studies of climate erosion tectonics coupling is to document convincing correlation between observable indicators of these processes on the scale of a mountain range the eastern himalayas are a unique range to quantify the contribution of tectonics and climate to long term erosion rates because uniform and steady tectonics have persisted for several million years while monsoonal precipitation patterns have varied in space and time specifically the rise of the shillong plateau the only orographic barrier in the himalayan foreland has reduced the mean annual precipitation downwind in the eastern bhutan himalaya at the miocene pliocene transition apatite fission track aft analyses of 45 bedrock samples from an e w transect along bhutan indicate faster long term erosion rates outside of the rain shadow in the west 1 0 1 8 mm yr than inside of it in the east 0 55 0 85 mm yr furthermore an aft vertical profile in the latter segment reveals a deceleration in erosion rates sometime after 5 9 ma in this drier segment of bhutan there are remnants of a relict landscape formed under a wetter climate that has not yet equilibrated to the present climatic conditions uplift and preservation of the paleolandscape are a result of a climate induced decrease in erosion rates rather than of an increase in rock uplift rate this study documents not only a compelling spatial correlation between long term erosion and precipitation rates but also a climatically driven erosion rate change on the scale of the eastern himalayas a change that in turn likely influences that region s recent tectonic evolution

three dimensional numerical models with varied material properties and erosion rates implications for the mechanics and kinematics of compressive wedges received 19 march 2008 revised 29 january 2009 accepted 13 february 2009 published 21 april 2009 1 we develop three dimensional mechanical models of a compressive wedge and investigate how the form and kinematics of the outboard wedge are affected by variation in initial topography material properties and erosion rate inclusion into the wedge of weaker less dense material affects the form of the wedge producing a region of steeper average slopes and higher topography while depriving the region further inboard of material enhanced erosion has a similar effect the wedge attempts to replace the eroded material by focusing deformation the result is a stepped region of lower topography within the outboard of the orogen we observe that uplift velocities at three points in the orogen vary cyclically from near zero to 3 times the average uplift rate over cycles lasting on the order of 15 200 000 model years our models along with analog models and some well dated examples from active orogens suggest that transient accommodation of strain may be common the cycles observed responded rapidly to changes in the amount of erosion imposed our models suggest that orogens may be driven by strong coupling between erosion and strain on temporal scales of 10 4 10 5 years and spatial scales comparable to the scale of the erosional perturbation we compare our models to the puli embayment of west central taiwan a region of anomalous low topography and suggest that it presence may reflect the presence of weaker and more erodible sediments than those present along strike in the orogen

decoupling of erosion and precipitation in the himalayas the hypothesis that abrupt spatial gradients in erosion can cause high strain rates in active orogens has been supported by numerical models that couple erosional processes with lithospheric deformation via gravitational feedbacks1 2 3 most such models invoke a stream power rule in which either increased discharge or steeper channel slopes cause higher erosion rates spatial variations in precipitation and slopes are therefore predicted to correlate with gradients in both erosion rates and crustal strain here we combine observations from a meteorological network across the greater himalaya nepal along with estimates of erosion rates at geologic timescales greater than 100 000 yr from low temperature thermochronometry across a zone of about 20 km length spanning the himalayan crest and encompassing a more than fivefold difference in monsoon precipitation significant spatial variations in geologic erosion rates are not detectable decreased rainfall is not balanced by steeper channels instead additional factors that influence river incision rates such as channel width and sediment concentrations must compensate for decreasing precipitation overall spatially constant erosion is a response to uniform upward tectonic transport of greater himalayan rock above a crustal ramp

scaling of small repeating earthquakes explained by interaction of seismic and aseismic slip in a rate and state fault model because of short recurrence times and known locations small repeating earthquakes present a rare predictable opportunity for detailed field observations they are used to study fault creeping velocities earthquake nucleation stress drops and other aspects of tectonophysics earthquake mechanics and seismology an intriguing observation about repeating earthquakes is their scaling of recurrence time with seismic moment which is significantly different from the scaling based on a simple conceptual model of circular ruptures with stress drop independent of seismic moment and no aseismic slip here we show that a model of repeating earthquakes based on laboratory derived rate and state friction laws reproduces the observed scaling in the model a small fault patch governed by steady state velocity weakening friction is surrounded by a much larger velocity strengthening region long term slip behavior of the fault is simulated using a methodology that fully accounts for both aseismic slip and inertial effects of occasional seismic events the model results in repeating earthquakes with typical stress drops and sizes comparable with observations for a fixed set of friction parameters the observed scaling is reproduced by varying the size of the velocity weakening patch in simulations a significant part of slip on the velocity weakening patches is accumulated aseismically even though the patches also produce seismic events the proposed model supplies a laboratory based framework for interpreting the wealth of observations about repeating earthquakes provides indirect evidence that rate and state friction acts on natural faults and has important implications for possible scenarios of slip partition into seismic and aseismic parts

three dimensional viscoelastic finite element model for postseismic deformation of the great 1960 chile earthquake 2 5 10 19 pa s for the continental upper mantle the model well explains the gps observations numerical tests suggest that the continental mantle viscosity value is reasonably well constrained the model shows the prolonged postseismic seaward motion of the inland area to be a unique feature of earthquakes with very long rupture along strike and large coseismic fault slip for short rupture and small coseismic slip the motion will stop very quickly after the earthquake explaining why this phenomenon is not more commonly observed with an oceanic mantle viscosity of 10 20 pa s the model also provides an explanation for tide gauge constrained postseismic uplift 200 km from the trench that had previously been explained using a model of prolonged afterslip of a deep segment of the chile subduction fault index terms 1236 geodesy and gravity rheology of the lithosphere and mantle 8160 1242 geodesy and gravity seismic deformations 7205 3230 mathematical geophysics numerical solutions 7209 seismology earthquake dynamics and mechanics 7260 seismology theory and modeling keywords postseismic deformation viscoelastic stress relaxation gps finite element modeling chile subduction zone upper mantle viscosity

changing mechanical response during continental collision active examples from the foreland thrust belts of pakistan abstract we have used data from teleseismic seismic reflection and field geologic studies along with both geomechanical and gravity modeling to contrast the tectonics of four active orogenic wedges in pakistan the kashmir himalaya the salt range potwar plateau foldbelt the sulaiman range and the makran accretionary wedge in makran oceanic crust is still being subducted and a thick pile of sediments is being accreted and underplated undercompaction and excess pore pressures can explain the narrow cross sectional taper and frontal aseismicity of this wedge beneath the sulaiman wedge continental crust is just starting to be underthrust indirect evidence suggests that fine grained carbonate rocks found in abundance deep in the stratigraphic section may be deforming ductilely at the base of the sulaiman wedge and provide a zone of ductile detachment the collision has proceeded to a much more mature stage in the salt range potwar plateau foldbelt and the kashmir himalaya isostatic response to underthrusting of continental crust has kept the sedimentary pile quite thin in both of these wedges so in that respect the two foldbelts are similar however thick eocambrian salt beneath the salt range and potwar plateau permits that foldbelt to be much wider in map view with a thinner cross sectional taper and a mixture of thrust vergence directions a major normal fault in basement causes the salt range to rise in front of the mildly deformed molasse basin of the southern potwar plateau much of the diversity among these mountain belts can be understood in terms of differences in the maturity of the collision process in each area the resulting thickness of the sedimentary pile encountered at the deformation front and the presence or absence of large contrasts in strength between the various layers of the stratigraphic section and basement relief

deformation cycles of subduction earthquakes in a viscoelastic earth subduction zones produce the largest earthquakes over the past two decades space geodesy has revolutionized our view of crustal deformation between consecutive earthquakes the short time span of modern measurements necessitates comparative studies of subduction zones that are at different stages of the deformation cycle piecing together geodetic snapshots from different subduction zones leads to a unifying picture in which the deformation is controlled by both the short term years and long term decades and centuries viscous behaviour of the mantle traditional views based on elastic models such as coseismic deformation being a mirror image of interseismic deformation are being thoroughly revised snapshots of subduction zones using space geodesy reveal that the viscous behaviour of the mantle controls crustal deformation requiring the revision of traditional elastic models for earthquake risk assessment space geodesy the use of satellites to monitor earth has revolutionized our view of crustal deformation between consecutive earthquakes however the brevity of such measurements means that studies must be done by comparing multiple subduction zones at different stages of the earthquake cycle by piecing together geodetic snapshots from the sumatra chile and cascadia subduction zones this review presents a unifying picture in which deformation is controlled by the viscous behaviour of the mantle in both the short term years and the long term decades to centuries

erosion as a driving mechanism of intracontinental mountain growth in nature mountains can grow and remain as localized tectonic features over long periods of time 10 m y by contrast according to current knowledge of lithospheric rheology and neglecting surface processes any intracontinental range with a width that exceeds that which can be supported by the strength of the lithosphere should collapse within a few tens of millions of years for example assuming a quartz dominated crustal rheology the relief of a range initially 3 km high and 300 400 km wide is reduced by half in about 15 m y as a result of lateral spreading of its crustal root we suggest that surface processes might actually prevent such a subsurface collapse removal of material from topographic heights and deposition in the foreland oppose spreading of the crustal root and could eventually drive a net influx of material toward the orogeny we performed a set of numerical experiments in order to validate this hypothesis a section of a lithosphere with a brittle elasto ductile rheology initially loaded by a mountain range is submitted to horizontal shortening and to surface processes if erosion is intense material is removed more rapidly than it can be supplied by crustal thickening below the range and the topography is rapidly smoothed for example a feature 3 km high and 300 400 km wide is halved in height in about 15 m y for an erosion coefficient k 10 3 m 2 yr the erosion rate is of the order of a few 0 1 mm yr this regime might be called erosional collapse if erosion is not active enough the crustal root spreads out laterally and subsurface collapse occurs in the third intermediate regime removal of the material by erosion is dynamically compensated by isostatic rebound and inward flow in the lower crust so that the range can grow in this mountain growth regime the range evolves toward a characteristic graded shape that primarily depends on the erosion law the erosion rate may be high e g 0 5 0 9 mm yr close to the rate of tectonic uplift e g 0 7 1 1 mm yr and few times higher than the rate of topographic uplift 0 15 0 2 mm yr these experiments show that surface processes can favor localized crustal shortening and participate in the development of an intracontinental mountain surface processes must therefore be taken into account in the interpretation and modeling of long term deformation of continental lithosphere conversely the mechanical response of the lithosphere must be accounted for when large scale topographic features are interpreted and modeled in terms of geomorphologic processes

incision and channel morphology across active structures along the peikang river central taiwan implications for the importance of channel width river morphology and dynamics are strongly influenced by active tectonics we report channel dynamics for the peikang river which flows through the hsuehshan range in central taiwan using a digital elevation model and field surveys we constrain channel morphology for an 90 km stretch of river to calculate unit stream power and boundary shear stress along the river path incision rates are estimated with optically stimulated luminescence dating of sand deposited on strath terraces we find a strong correlation between unit stream power shear stress and incision rate but only if variation in channel width is considered a calibrated river incision rule implies river incision rates of 9 13 5 mm yr upstream of the meiyuan and tili faults and suggests that one or both of these structures are presently active our results indicate that the shuilikeng fault is also actively deforming as incision rates increase to 6 10 mm yr across it compared to 1 4 mm yr in adjacent reaches prominent narrowing across the shuilikeng fault and the absence of significant gradient variation indicate that channel width is a first order morphological adjustment to differential incision only when the channel width to depth ratio reaches a minimum does the channel slope significantly adjust to local changes in base level as is the case upstream of the meiyuan and tili faults

brittle frictional mountain building 1 deformation and mechanical energy budget an active fold and thrust belt is analogous to a wedge of soil or snow that forms in front of a moving bulldozer such wedges exhibit a critical taper and a regional state of stress that is everywhere on the verge of coulomb failure the width of such a critically tapered fold and thrust belt does not depend on its brittle strength or frictional properties but rather on the accretionary influx rate of fresh material at its toe and on the rate of erosion a steady state fold and thrust belt is one in which the accretionary influx is balanced by the erosive efflux rocks are accreted at the toe and then horizontally shortened as they are transported toward the rear those that enter lower in the accreted section are more deeply buried before being uplifted by erosion mass balance and isotropy constrain the kinematics of this large scale deformation enabling us to infer the trajectories residence times and stress strain histories of rocks incorporated into eroding fold and thrust belts a typical rock resides in the steady state taiwan wedge for 2 3 m y before it is uplifted and eroded during its motion through the wedge it experiences strain rates in the range 10 13 to 10 14 s 1 the mechanical energy budget of brittle frictional mountain building is described by the equation where is the rate of work performed on the base and front of the fold and thrust belt by the subducting plate is the rate at which energy is dissipated against friction on the decollement fault is the rate at which energy is dissipated by internal frictional processes within the deforming brittle wedge and is the rate of work performed against gravitational body forces in a reference frame attached to the overriding plate the total mechanical power being supplied by the subducting eurasian plate to the active fold and thrust belt in taiwan is slightly over 3 gw approximately 60 of this work of steady state mountain building is being dissipated against friction on the decollement fault and about another 25 is being dissipated against internal friction this leaves only 15 or roughly half a gigawatt available to do useful work against gravity in general fold and thrust belts with moderate pore fluid pressures are dominated by work done against friction on the decollement fault however those with nearly lithostatic pore fluid pressures may be dominated by work done against gravity internal frictional dissipation is always less than basal frictional dissipation as it is in taiwan an alternative and equivalent description of the mechanical energy balance of a steady state fold and thrust belt is provided by the equation in this version the quantity on the left is the rate at which work is performed on the back of the wedge by the overriding plate and is the rate of work performed against gravity in a reference frame attached to the subducting plate the latter quantity is always positive for any critically tapered fold and thrust belt whose decollement fault dips toward its rear in contradiction to the central premise of the gravity spreading theory of fold and thrust tectonics

fault slip rates at depth from recurrence intervals of repeating microearthquakes unique attributes in sequences of recurring similar microearthquakes at parkfield california provide a means for inferring slip rate at depth throughout the active fault surface from the time intervals between sequence events application of the method using an 11 year microseismicity record revealed systematic spatial and temporal changes in the slip rate that were synchronous with earthquake activity and other independent measures of fault zone slip if this phenomenon is found to be generally common behavior in active faults it forms the basis for a method to monitor the changing strain field throughout a seismogenic fault zone

two sided orogen collision and erosion from the sandbox to the southern alps new zealand in continental collision zones the indentor is of approximately the same vertical dimension as the material being deformed thereby allowing material to move over the indentor and out of the deforming orogen this simple constraint on the height of the indentor differs from that imposed by the bulldozer model of wedge deformation and results in the formation of an orogen with two opposite dipping topographic slopes these two wedges are mechanically coupled and the mechanics are strongly influenced by the erosion conditions imposed on the wedge where the growing mountains perturb prevailing moisture laden winds the erosion pattern is highly asymmetric erosional work being concentrated at the toe of the steep wedge facing the indentor the inboard wedge concentration of erosional energy results in concentration of mechanical energy in the same region and produces the distinctive geological and geophysical patterns associated with continental collision these include rapid uplift and exposure of deep crustal assemblages adjacent to the indentor accompanied by high crustal heat flow and anomalously low seismicity the outcrop pattern along the inboard wedge evolves from that of a crustal cross section currently exposed in the southern alps to one dominated by repetitive nappe structures consisting predominantly of lower crustal units as in the western alps of europe the other wedge the outboard wedge undergoes relatively little erosion is broad with a more gentle surface slope and maintains an outcrop pattern consisting of upper crustal material

steady state erosion of critical coulomb wedges with applications to taiwan and the himalaya 1 orogenic structure appears to be partially controlled by the addition to and removal of material from the mountain belt by tectonic accretion and geomorphic erosion respectively we developed a coupled erosion deformation model for orogenic wedges that are in erosional steady state and deform at their coulomb failure limit erosional steady state is reached when all material introduced into the wedge is removed by erosion that is limited by the rate at which rivers erode through bedrock we found that the ultimate form of a wedge is controlled by the wedge mechanical properties sole out depth of the basal decollement erosional exponents basin geometry and the ratio of the added material flux to the erosional constant as this latter ratio is increased wedge width and surface slopes increase we applied these models to the taiwan and himalayan orogenic wedges and found that despite a higher flux of material entering the former the inferred ratio was larger for the latter calculated values for the erodibility of each wedge showed at least an order of magnitude lower value for the himalaya relative to taiwan these values are consistent with the lower precipitation regime in the himalaya relative to taiwan and the exposure of crystalline rocks within the himalayan orogenic wedge independently determined rock erodibility estimates are consistent with the accretionary wedge sediments and metasediments and the crystalline and high grade metamorphic rocks exposed within taiwan and the himalaya respectively therefore differences in rock type and climate apparently lead to key differences in the erosion and hence orogenic structure of these two mountain belts

erosion rates and orogenic wedge kinematics in taiwan inferred from fission track thermochronometry new apatite and zircon fission track ages and previously published thermochronometric data are used to evaluate erosion rates and particle paths within the active taiwan arc continent collision we present 20 new apatite fission track ages and 6 new zircon fission track ages apatite and zircon ages are all reset in the northern and eastern parts of taiwan although the region of reset apatite ages is larger we interpret this pattern as resulting from crustal accretion at the western margin of the orogenic wedge combined with southward propagation of the collision zone a onedimensional thermal model including erosion provides prediction of the fission track ages the distribution of reset ages is best explained with an erosion rate of 4 6 mm yr given a propagation velocity of 60 mm yr this erosion rate implies that nearly 25 km of material has been eroded from northern taiwan the lack of reset 40 ar 39 ar ages from muscovite and biotite suggests that rockparticle paths have a large horizontal component a result consistent with an eroding orogenic wedge model