extensional collapse of the tibetan plateau results of three dimensional finite element modeling 1 following their initial collision 50 70 myr ago the indian and eurasian plates have been continuously converging toward each other whereas the regional stress field is predominately compressive the late cenozoic tectonics within the tibetan plateau features widespread crustal extension numerous causes of the extension have been proposed but their relative roles remain in debate we have investigated the major factors contributing to the tibetan extension in a three dimensional viscoelastic model that includes both lateral and vertical variations of lithospheric rheology and relevant boundary conditions constrained by the present topography and gps velocity field the model predicted predominately extensional stress states within the plateau crust resulting from mechanical balance between the gravitational buoyancy force of the plateau and the tectonic compressive stresses the predicted stress pattern is consistent with the earthquake data that indicate roughly e w extension in most of tibet and nearly n s extension near the eastern margin of the tibetan plateau we explored the parameter space and boundary conditions to examine the stress evolution during the uplift of the tibetan plateau when the plateau was lower than 50 of its present elevation strike slip and reverse faults were predominate over the entire plateau and no e w crustal extension was predicted significant crustal extension occurs only when the plateau has reached 75 of its present elevation basal shear associated with underthrusting of the indian plate beneath tibet may have enhanced crustal extension in southern tibet and the himalayas and a stronger basal shear during the miocene may help to explain the development of the south tibetan detachment system

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

control of detachment geometry on lateral variations in exhumation rates in the himalaya insights from low temperature thermochronology and numerical modeling 1 the himalayan range is commonly presented as largely laterally uniform from west to east however geological structures topography precipitation rate convergence rates and low temperature thermochronological ages all vary significantly along strike here we focus on the interpretation of thermochronological data sets in terms of along strike variations in geometry and kinematics of the main crustal detachment underlying the himalaya the main himalayan thrust mht we report new apatite fission track aft ages collected along north south transects in western and eastern central nepal at the latitudes of the annapurna and langtang massifs respectively aft ages are consistently young 3 ma along both n s transects in the high relief zone of the higher himalaya and increase 4 to 6 ma toward the south in the lesser himalaya we compare our new data to published low temperature thermochronological data sets for nepal and the bhutan himalaya we use the full data set to perform both forward and inverse thermal kinematic modeling with a modified version of the pecube code in order to constrain potential along strike variations in the kinematics of the himalayan range our results show that lateral variations in the geometry of the mht in particular the presence or absence of a major crustal scale ramp strongly control the kinematics and exhumation history of the orogen

palynological evidence for pennsylvanian extra basinal vegetation in atlantic canada abstract pollen preserved in latest westphalian and early stephanian clastic deposits of the sydney coalfield cape breton island suggest that there were zones of different vegetational habitats surrounding the wetlands where the coal forming peat was formed on the margins of the wetlands was a narrow band with large cordaitanthacean trees this was surrounded by occasionally dry9 habitats supporting schuetzia plants ruflorioid plants and aethophyllum like herbaceous conifers around this were mesic or seasonally dry habitats dominated by walchian conifers and disaccate producing peltasperms these in turn were surrounded by xeric or seasonally wet habitats dominated by ullmanniacean like conifers cycads and monosaccate producing peltasperms during latest westphalian and early stephanian times the mesic and xeric habitats encroached nearer the wetlands although the vegetation of the wetlands themselves remained essentially unaffected this vegetation change was not a response to climate change but probably to changing drainage patterns and topography in the surrounding hinterland the florinites producing cordaitanthaceans appear not to have been upland trees as previously suggested but occupied mainly coastal habitats or riparian habitats on the margins of the wetlands

channel flow extrusion model to constrain dynamic viscosity and prandtl number of the higher himalayan shear zone constraining magnitudes of mechanical and thermo mechanical parameters of rocks and shear zones are the important goals in structural geology and tectonics talbot in j struct geol 21 949 957 1999 such parameters aid dynamic scaling of analogue tectonic models ramberg in gravity deformation and the earth s crust in theory experiments and geological applications 2nd edn academic press london 1981 which are useful to unravel tectonics in further details schultz ela and walsh in j struct geol 24 247 275 2002 the channel flow extrusion of the higher himalayan shear zone hhsz higher himalaya can be explained by a top to s sw simple shear i e the d2 deformation in combination with a pressure gradient induced flow against gravity presuming its newtonian incompressible rheology with parallel inclined boundaries the viscosity of this shear zone along a part of the himalayan chain through india nepal and bhutan is estimated to vary widely between 1016 and 1023 pa s and its prandtl number pr within 1021 1028 the estimates utilized ranges of known thickness 6 58 km of the hhsz that of its top subzone of ductile shear of normal shear sense stdsu 0 35 9 4 km total rate of slip of its two boundaries 0 7 131 mm year 1 pressure gradient 0 02 6 kb km 1 density 2 2 3 1 g cm 3 and thermal diffusivity 0 5 10 6 2 1 10 6 m s 2 along the orogenic trend considering most of the parameters specifically for the sutlej section india the calculated viscosity and the prandtl number pr of the hhsz are deduced to be 1017 1023 pa s and pr 1022 1028 the upper limits of the estimated viscosity ranges are broadly in conformity with a strong tibetan mid crust from where a part of the hhsz rocks extruded on the other hand their complete ranges match with those for its constituent main rock types and partly with those for the superstructure and the infrastructure the estimated mechanical and thermo mechanical parameters of the hhsz will help to build dynamically scaled analogue models for the himalayan deformation of the d2 phase

thermal characteristics of the main himalaya thrust and the indian lower crust with implications for crustal rheology and partial melting in the himalaya orogen abstract the main himalaya thrust mht is the current tectonic boundary between the subducting indian lithosphere and the overlying himalayan orogenic prism and the tibetan crust we present thermo kinematic calculations and metamorphic p t t paths of the indian lower crust ilc that constrain the thermal structure of the mht and the southern tibetan crust lhasa block and explain the origin of a thin seismic low velocity zone that was revealed by the recent hi climb experiment from receiver functions of teleseismic waves northward of the himalayas the low velocity zone occurs within the ductile regime of the crust and is thought to extend along the mht into the lhasa block in the lhasa block the low velocity zone occurs directly above the ilc predicted evolution of mineralogy of the ilc along its subduction p t t path shows that its dehydration can potentially induce wet melting within the orogenic prism above the inclined portion of the mht however north of the yarlung tsangpo suture yts below the southern lhasa block where subduction of the ilc is flat the ilc is predicted to be anhydrous eclogite and therefore it cannot supply h 2 o to the overlying crust the seismic low velocity zone above this portion of the ilc is best explained by dehydration melting due to strain heating the mht there appears to be localized by the rheological contrast between the ductile lower lhasa block and the strong eclogitic ilc southward thrusting of the himalaya orogenic prism which contains accreted indian upper crust causes advection of hot middle crustal rocks to shallower levels thereby producing a shallow ductile regime between the himalayas and the yts the shallow ductile regime is evident in the limit of upper crustal earthquake foci to shallow depths in this region

magnetic reversal history of pleistocene sediments at old crow northwestern yukon territory the banks of the porcupine river near old crow in the unglaciated part of the northern yukon consist of about 55 m of interbedded fluvial and lacustrine gravels sands silts and clays this sequence was sampled closely for palaeomagnetic analysis and the sediments were found to possess a natural remanent magnetization nrm that is stable in direction with mean destructive fields of 200 300 oe 16 24 ka m therefore good palaeomagnetic data can be obtained from sediments that have been subjected to permafrost conditions although some samples show disturbance of nrm directions by cryoturbation magnetic polarity transitions are centred at 17 and 26 m above river level and the 15 samples between them define an interval of reversed polarity of considerable duration the behaviour of nrm during the transition periods is complex with declination switching abruptly by 180 but with the inclination displaying very erratic variation over a considerable time this may be caused by mixing in single samples of c

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

thrust wedges and fluid overpressures sandbox models involving pore fluids the well known model for the critical taper of an accretionary wedge includes overpressure as a first order parameter fluid overpressures reduce frictional resistance at the base of a wedge but they also act as body forces on all material particles of the wedge in addition to that of gravity by means of sandbox modeling many workers have tried to verify the predictions of the critical taper model but few of them have so far incorporated true fluid pressures we have used scaled experiments in which compressed air flows through sand packs so as to model the deformation of overpressured wedges a new apparatus provides for a horizontally varying fluid pressure for example a linear variation as in the critical taper model we have done three series of experiments involving horizontal shortening of homogeneous or multilayered sand models for various gradients of fluid pressure as predicted by the critical taper model the apical angle of the resulting wedge depends on the overpressure gradient in homogeneous sand at a high overpressure gradient deformation becomes diffuse and looks ductile in multilayered models detachments form beneath layers of low permeability so that thrusts propagate rapidly toward the undeformed foreland the efficiency of a detachment and its ability to propagate depend not only on the fluid pressure but also on the permeability ratios between the various layers

the boundary between the indian and asian tectonic plates below tibet the fate of the colliding indian and asian tectonic plates below the tibetan high plateau may be visualized by in addition to seismic tomography mapping the deep seismic discontinuities like the crust mantle boundary moho the lithosphere asthenosphere boundary lab or the discontinuities at 410 and 660 km depth we herein present observations of seismic discontinuities with the p and s receiver function techniques beneath central and western tibet along two new profiles and discuss the results in connection with results from earlier profiles which did observe the lab the lab of the indian and asian plates is well imaged by several profiles and suggests a changing mode of india asia collision in the east west direction from eastern himalayan syntaxis to the western edge of the tarim basin the indian lithosphere is underthrusting tibet at an increasingly shallower angle and reaching progressively further to the north a particular lithospheric region was formed in northern and eastern tibet as a crush zone between the two colliding plates the existence of which is marked by high temperature low mantle seismic wavespeed correlating with late arriving signals from the 410 discontinuity poor sn propagation east and southeast oriented global positioning system displacements and strikingly larger seismic sks anisotropy

modelling the geometry of coulomb thrust wedges internal geometry and detachment of coulomb thrust wedges is examined in a combined gravity and pressure force field under various boundary conditions and coulomb parameters frontally buttressed sand wedges accreted with narrow tapers above soles with negligible friction in these models serial imbricates stepped up in conjugate sets which operated synchronously or serially with increases in sole friction model coulomb wedges accreted in piggyback style with enhanced taper but narrower spacing of imbricates in this mode flat topped box fold anticlines nucleated at the frontal tip of a sole thrust prior to the fore limb failing as a ramp the initial ramp thrusts subsequently rotated and steepened to acquire concave upwards listric geometry when cohesion was a significant component of strength and the sand grains interlocked thoroughly ramp climb during piggyback thrusting occurred through the generation of a stack of backthrusts the taper of cohesionless sand wedges which lack any length scale was consistent with the critical cohesionless coulomb wedge taper in contrast the taper of experimental cohesive sand wedges which are scale dependent was consistent with the theoretical cohesive coulomb narrow taper only when the cohesive length scale was lr sr rar 10 6 this was constrained by comparing geometry of cohesive sand wedges shortened at different body force per unit mass

rheological dependence of extension in wedge models of convergent orogens abstract although several mechanical models of extension in contractional orogens have been proposed in many cases these models have been based on viscous constitutive laws lack of more complicated rheological models has primarily been due to the technical difficulty of solving for deformation with complex rheological behavior and boundary conditions a finite element model is presented that allows for solution of the wedge mechanical problem with viscous non linear viscous and coulomb plastic rheologies this model is used to investigate extension in a doubly vergent wedge model in which deformation is primarily contractional driven by a reversal of basal traction proposed models showing extension in the crust at shallow crustal levels in spite of overall contraction and contraction at depth are confirmed but only for high argand number low crustal viscosity at low argand number little or no extension is observed non linear stress dependence power law flow also suppresses extension but not entirely some extension is observed at high effective argand number however with a coulomb plastic rheology no extension is observed for a wide range of parameter values no extension is observed for models with viscous deformation at depth and plastic deformation near the surface this suggests that the extension is a function of the rheologic model not the effective strength of the crust coeval extension and contraction in convergent orogens with steady external forcing therefore requires viscous deformation to very shallow depths

crustal channel flows 1 numerical models with applications to the tectonics of the himalayan tibetan orogen 1 plane strain thermal mechanical numerical models are used to examine the development of midcrustal channel flows in large hot orogens in the models radioactive self heating reduces the viscosity of tectonically thickened crust and increases its susceptibility to large scale horizontal flow channels can be exhumed and exposed by denudation focused on the high relief transition between plateau and foreland we interpret the himalaya to have evolved in this manner channel flows are poorly developed if the channel has a ductile rheology based on wet quartz flow laws and well developed if there is an additional reduction in viscosity to 1019 pa s this reduction occurs from 700 c to 750 c in the models and is attributed to a small percentage of in situ partial melt melt weakening model ht1 provides an internally consistent explanation for the tectonic evolution of many features of the himalayan tibetan orogenic system erosional exhumation exposes the migmatitic channel equivalent to the greater himalayan sequence ghs between coeval normal and thrust sense ductile shear zones corresponding to the south tibetan detachment and the main central thrust systems outward flow of unstable upper crust rotates these shears to low dip angles in the model both the ghs and the lesser himalayan sequence are derived from indian crust with the latter from much farther south similar models exhibit a range of tectonic styles including the formation of domes resembling north himalayan gneiss domes model results are relatively insensitive to channel heterogeneities and to variations in the behavior of the mantle lithosphere beneath the model plateau

ductile extrusion of the higher himalayan crystalline in bhutan evidence from quartz microfabrics quartz textures measured from deformed quartz tectonites within the lesser himalaya and higher himalaya crystalline of bhutan show similar patterns orientation and distribution of the quartz crystallographic axes were used to confirm the regional shear sense the asymmetry of c axis and a axis patterns consistently indicates top to the south shearing the obliquity of the texture and the inferred finite strain plane strain to moderately constrictional suggest the strain regime had a combination of rotational and irrotational strain path in most of the samples from the bhutan himalaya the inferred deformation mechanisms suggest moderate to high temperature conditions of deformation that produced the observed crystallographic preferred orientation much higher temperature of deformation is indicated in the quartz veins from a leucogranite the observed ductile deformation is pervasively developed in the rocks throughout the investigated area the intensity of deformation increases only slightly in the vicinity of the main central thrust simultaneous southward shearing within a large part of the higher himalaya crystalline near and above the main central thrust and normal faulting across the south tibetan detachment is explained by the tectonically induced extrusion of a ductily deforming wedge the process of extrusive flow suggested here can be approximated quantitatively by channel flow models that have been used to describe subduction zone processes channel flow accounts for some observed phenomena in the himalayan orogen such as inverted metamorphic sequences near the main central thrust not related to an inversion of isotherms and the syntectonic emplacement of leucogranites into the extruding wedge locally leading to an inversion of isotherms due to heat advection

bedrock incision rock uplift and threshold hillslopes in the northwestern himalayas the topography of tectonically active mountain ranges reflects a poorly understood competition between bedrock uplift and erosion dating of abandoned river cut surfaces in the northwestern himalayas reveals that the indus river incises through the bedrock at extremely high rates 2 12 mm yr 1 in the surrounding mountains the average angles of hillslopes are steep and essentially independent of erosion rate suggesting control by a common threshold process in this rapidly deforming region an equilibrium is maintained between bedrock uplift and river incision with landsliding allowing hillslopes to adjust efficiently to rapid river down cutting

metamorphism transient mid crustal rheology strain localization and the exhumation of high grade metamorphic rocks 1 we present a series of three dimensional numerical models investigating the effects of metamorphic strengthening and weakening on the geodynamic evolution of convergent orogens that are constrained by observations from an exposed mid crustal section in the new england appalachians the natural mid crustal section records evidence for spatially and temporally variable mid crustal strength as a function of metamorphic grade during prograde polymetamorphism our models address changes in strain rate partitioning and topographic uplift as a function of strengthening weakening in the middle crust as well as the resultant changes in deformation kinematics and potential exhumation patterns of high grade metamorphic rock results suggest that strengthening leads to strain rate partitioning around the zone and suppressed topographic uplift rates whereas weakening leads to strain rate partitioning into the zone and enhanced topographic uplift rates deformation kinematics recorded in the orogen are also affected by strengthening weakening with complete reversals in shear sense occurring as a function of strengthening weakening without changes in plate boundary kinematics

topographically driven crustal flow and its implication to the development of pinned oroclines pinned oroclines a type of curved orogen which results from lateral pinning of a growing fold thrust belt tend to resemble parabolic newtonian curvature modified by different degrees of flattening at the flow front we propose that such curves can be generated by newtonian crustal flow driven by topographic variations in our model regional topographic differences create a regional flow which produces a parabolic flow front upon interaction with lateral bounding obstacles local topographic variations modify the parabolic curves and yield more flat crested non newtonian like curvatures the degree of modification depends on the relative strength of the local driving potential which in turn is dependent on rock type and fluid pressure on the basis of a finite difference thin skin tectonic simulation we demonstrate that both newtonian like and non newtonian like curved orogens can be produced within a newtonian crust in effect the shape of curved orogens in plan may provide insight into the rheology of the earth s upper crust over geological time scales

an early india asia contact paleomagnetic constraints from ninetyeast ridge odp leg 121 new paleomagnetic results from sedimentary rock and basement of the ninetyeast ridge ocean drilling program leg 121 sites 756 758 detail the northward movement of the indian plate for the past 80 m y analysis of the combined paleolatitude age profile indicates a distinct reduction in india9s northward movement rate at 55 ma interpreted as completion of suturing of greater india and asia india9s northward motion slowed from 18 19 5 cm yr to 4 5 cm yr for the location of site 758 comparison of this profile with paleomagnetic data from the wider himalayan region indicates that initial contact between northwestern greater india and southern asia was already established by cretaceous tertiary time this supports a possible causal link between the india asia convergence and the deccan traps extrusion

evolutionary model of the himalaya tibet system geopoem based on new modelling geological and geophysical data a two dimensional thermo mechanical laboratory modelling of continental subduction was performed the subducting continental lithosphere includes a strong brittle upper crust a weak ductile lower crust and a strong upper mantle the lithosphere is underlain by a low viscosity asthenosphere subduction is produced by a piston push force and the pull force from the mantle lithospheric layer which is denser than the asthenosphere the lithospheric layers are composed of material whose strength is sensitive to and inversely proportional to temperature throughout the experiment the model surface was maintained under relatively low temperature and the model base at higher temperature the subduction rate satisfied the peclet criterion modelling confirms that the continental crust can be deeply subducted and shows that slab break off delamination and tectonic underplating are fundamental events with drastic consequences on the subsequent evolution of the convergent system combining these results with previous purely mechanical modelling we elaborate a new evolutionary model for the himalaya tibet convergent system the principal successive stages are 1 subduction of the indian continental lithosphere to 200 250 km depth following subduction of the tethys oceanic lithosphere 2 failure and rapid buoyancy driven uplift of the subducted continental crust from ca 100 km depth to some depth that varies along the mountain belt 20 30 km on average 3 break off of the indian subducted lithospheric mantle with the attached oceanic lithosphere 4 subduction underplating of the indian lithosphere under asia over a few to several hundred kilometers 5 delamination roll back and break off of the indian lithospheric mantle 6 failure of the indian crust in front of the mountain belt formation of the main central thrust and underthrusting of the next portion of indian lithosphere beneath tibet for a few hundred kilometers at the beginning of stage 6 the crustal slice corresponding to the crystalline himalayas undergoes erosion activated uplift and exhumation fl 2000 elsevier science b v all rights reserved

a two layer lithospheric compressional model for the tertiary uplift of the southern united kingdom reflection seismic records show that many of the mesozoic sedimentary basins of the southern united kingdom were compressed and uplifted inverted during the late cretaceous tertiary recent quantitative studies based on apatite fission track analysis and chalk porosity data show that uplift and erosion although concentrated in the inverted basins was regional and not limited to the recognized areas of crustal shortening inversion a two layer or heterogeneous model of lithospheric compression similar to two layer models of lithospheric extension may account for this pattern of uplift upper lithospheric shortening is accommodated by reverse movement on major reactivated extensional detachments causing inversion of pre existing basins pure shear shortening of the lower lithosphere is decoupled and may be laterally displaced from that in the upper lithosphere lower lithospheric shortening and thickening displaced from that in the upper lithosphere causes initial subsidence due to the submersion of cold dense lithosphere into the surrounding asthenosphere subsequent thermal re equilibration of the lower lithosphere may have generated the uplift without attendant crustal shortening witnessed by apatite fission track and chalk porosity data

implications of channel flow analogue models for extrusion of the higher himalayan shear zone with special reference to the out of sequence thrusting the higher himalayan shear zone hhsz contains a ductile top to n ne shear zone the south tibetan detachment system lower stdsl and an out of sequence thrust oost as well as a top to n ne normal shear at its northern boundary and ubiquitously distributed compressional top to s sw shear throughout the shear zone the oost that was active between 22 ma and the holocene varies in thickness from 50 m to 6 km and in throw from 1 4 to 20 km channel flow analogue models of this structural geology were performed in this work a newtonian viscous polymer pdms was pushed through a horizontal channel leading to an inclined channel with parallel and upward diverging boundaries analogous to the hhsz and allowed to extrude to the free surface a top to n ne shear zone equivalent to the stdsu developed spontaneously this also indirectly connotes an independent flow confined to the southern part of the hhsz gave rise to the stdsl the pdms originally inside the horizontal channel extruded at a faster rate through the upper part of the inclined channel the lower boundary of this faster pdms defined the oost the model oost originated at the corner and reached the vent at positions similar to the natural prototype some time after the channel flow began the genesis of the oost seems to be unrelated to any rheologic contrast or climatic effects profound variations in the flow parameters along the hhsz and the extrusive force probably resulted in variations in the timing location thickness and slip parameters of the oost variation in pressure gradient within the model horizontal channel however could not be matched with the natural prototype channel flow alone presumably did not result in southward propagation of deformation in the himalaya

what drives orogeny in the andes the andes the world9s second highest orogenic belt were generated by the cenozoic tectonic shortening of the south american plate margin overriding the subducting nazca plate we use a coupled thermomechanical numerical modeling technique to identify factors controlling the intensity of the tectonic shortening from the modeling we infer that the most important factor was accelerated westward drift of the south american plate changes in the subduction rate were less important other important factors are crustal structure of the overriding plate and shear coupling at the plates9 interface the model with a thick 40 45 km at 30 ma south american crust and relatively high friction coefficient 0 05 at the nazca south american interface generates 300 km of tectonic shortening during 30 35 m y and replicates the crustal structure and evolution of the high central andes the model with an initially thinner

present day kinematics of asia derived from geologic fault rates active fault geometry available quaternary rates on major faults and the far field plate motion are used to determine crustal kinematics in the collision zone between india and asia using a finite element formalism to construct a spherical shell model of the asian continent with embedded faults we seek a velocity solution approaching the motion of rigid blocks by minimizing the elastic strain energy in the fault bounded blocks by doing so we test the assumption that long term deformation within continents is mostly localized into major faults in the solution fault motion accounts for more than 80 of the deformation allowing us to describe our velocity model in terms of quasi rigid block rotations on the sphere south china is rotating clockwise about a pole located southwest of borneo implying an e ese velocity vector of 11 mm yr for a point at shanghai in agreement with the velocity vector determined by very long baseline interferometry heki et al 1995 the eastward movement of south china is accommodated by oblique extension along the red river fault at a rate of 10 5 mm yr in the south and by the combination of left lateral strike slip motion on the qinling dabie shan fault and the counterclockwise rotation of the ordos and adjacent blocks in the north the tarim rotates clockwise with respect to dzungaria about a pole located at 44 n 92 e consistent with increasing crustal shortening toward the west throughout the tien shan assuming incompressibility a crustal volume budget over a domain encompassing the tertiary mountain ranges in asia shows that over the last 10 000 years 73 4 of the north south shortening between india and asia has been absorbed by thickening of the lithosphere and 27 has been accommodated by lateral extrusion of continental blocks the present day predominance of thickening in asia results from the relatively slow eastward motion of south china controlled by strike slip faulting in the qinling shan and oblique extension in northeastern china

time dependent subduction dynamics driven by the instability of stagnant slabs in the transition zone abstract seismic tomographic images indicate that subducted slabs are commonly deflected within the transition zone and that accumulated slab material may be subject to localized flux into the lower mantle these observations are consistent with numerical models of mantle flow in which slab flux through phase transition at 660 km depth is impeded by the combination of rheological contrast and thermodynamic effects we use a suite of 2d numerical experiments to explore the impact of slab avalanches on plate boundary dynamics the experiments are initiated at the onset of the avalanche and two distinct classes of model geometry are considered in the first the stagnant deflected slab within the transition zone is connected to subduction at the surface of the model domain in this case we find that the location of the avalanche relative to the trench governs whether the slab dip steepens or shallows in response to the instability at 660 km depth these experiments show a relatively modest impact on plate dynamics at the surface in the second class of models the stagnant slab material is not connected to the shallow subduction and the former is interpreted as a remnant of either a relic subduction event or an accumulation of material from a distant zone of convergence in this case the impact on plate boundary dynamics is more profound for example when the avalanche is positioned under or forward of the island arc it drives a significant retreat of the trench in contrast an avalanche that initiates under the back arc leads to a thrusting of the island arc over the retro plate and a reversal in the polarity of subduction we test the influence of various boundary conditions on the evolution of these models and consider several different rheological parameterizations for the lithosphere and mantle the models are intended as general templates for the impact of slab avalanches however we discuss the potential connection of these results to various zones of convergence along the western pacific margin

characteristics and consequences of flow in the lower crust in some places there is strong evidence that the lower continental crust has flowed so as to smooth out variations in crustal thickness caused by differential crustal extension or shortening in order to better understand the processes involved we investigate the behavior of a fluid layer over a fluid half space to see how such a system responds to the deformation of its upper and lower boundaries this simple system can be used to study both the decay of crustal thickness contrasts and the behavior of a thin lithospheric sheet the changing response of the system to variations in density and viscosity contrasts and to different boundary conditions imposed on the fluid interface can easily be studied analytically the most important results are that variations in crustal thickness on a wavelength of a few times the thickness of the flowing channel will decay quickest and that large lateral variations in crustal thickness cause the fluid to develop a steep front which may cause a topographic step above it at the earth s surface deformation within the channel will be principally by simple shear the clear association of lower crustal flow with regions of thickened crust and magmatic activity suggests that both can reduce the viscosity of the lower crust to levels at which flow can occur the smoothing of crustal thickness contrasts leads to differential vertical motions and is thus a method by which substantial tilting can occur without faulting this differential uplift may be responsible for rotating and exhuming some of the detachment faults in metamorphic core complexes in the basin and range province of the western united states it is also a method of causing structural inversion in basins that does not require the reactivation of normal faults as thrusts or reverse faults

an unstable kinematic state of the himalayan tectonic wedge evidence from experimental thrust spacing patterns abstract the frontal tectonic wedge in the nw himalayas shows a sequence of imbricate thrusts with increasing spacing in the foreland direction we used analogue model experiments to analyze the thrust pattern in relation to the kinematics of wedge evolution experimental findings reveal two kinematic states of the foreland ward propagation of a wedge unstable state and stable state the unstable state is characterized by continuous vertical growth of the wedge with progressive horizontal contraction on the other hand in the stable state the wedge ceases to grow vertically but propagates laterally in the foreland direction in the experimental runs thrust wedges remained in the unstable state even after large horizontal shortening 50 when the basal friction was high 0 46 our analysis suggests that successive thrusting occurs always with increasing spacing in unstable wedges and the rate of increase is larger for larger basal friction it can maintain uniform spacing only when the wedges turn into the stable state using finite element models we determined the stress distribution in the deforming wedge to find the potential locations of new thrusts in front of a wedge which also show larger spatial distances with increasing vertical thickness of the wedge both physical and numerical models suggest that the tectonic wedge in the nw himalayan frontal belt has evolved in an unstable state

exhumation history of a section of the sikkim himalayas india records in the metamorphic mineral equilibria and compositional zoning of garnet the exhumation history of pelitic migmatite samples from the high himalayan crystalline complex hhc near the south tibetian detachment system in the sikkim darjeeling section has been determined on the basis of thermo barometric analyses retrograde breakdown reactions and compositional zoning of garnet the peak metamorphic condition is estimated to be 10 4 kbar 800 c from thermo barometric and phase equilibrium constraints the observed retrograde breakdown of garnet to spinel and cordierite requires near isothermal and hence extremely rapid 15 mm yr exhumation up to the depth of 15 km numerical modeling suggests that the initial rapid exhumation must have been followed by a much slower process 2 mm yr up to at least 5 km depth to lead to the development of the observed compositional zoning of garnet the dramatic change of the exhumation velocity vz might reflect a process of tectonic thinning followed by erosion and or horizontal flow at shallow depth assuming that the vz did not change significantly below 5 km depth these results suggest that the hhc samples studied in this work exhumed from a depth of 34 km within 8 ma

estimating the viscosity and prandtl number of the tso morari crystalline gneiss dome indian western himalaya the tso morari crystalline tmc gneiss dome in the indian himalaya extruded from a depth of 120 km through an inclined subduction channel of sub elliptical cross section at the leading edge of the indian plate the velocity profile of this gneiss dome is derived after 1 presuming its incompressible newtonian rheology 2 finding the best fit of the outcrop of the gneiss dome to an ellipse 3 taking into account different lithologies to have existed at the top of the extruding gneiss body 4 considering the extrusion to have been driven by the buoyant push of the denser mantle beneath the lighter gneiss and 5 assigning a range of plausible densities for different litho units fitting the known rates of extrusion from a few centimetres up to about one hundredth of a millimetre per year from 53 ma onwards of this gneiss dome to its velocity profile constrains its maximum possible viscosity to 7 5 1022 pa s this magnitude is 102 104 times higher than previous estimates for gneisses and granites alternative explanations of our data are the following 1 there was a fall in extrusion rates of the tmc gneiss from 53 to 30 ma because of an increase in the estimated maximum viscosity from 6 2 1020 to 7 5 1022 pa s possibly indicating a fall in temperature and or compositional change of the tmc gneiss 2 lower the extrusion rates higher are the estimated viscosities 3 the tmc gneiss was more viscous probably due to its eclogite content 4 the estimated maximum viscosity is 102 times higher than that in collision zones and 102 104 times than that in the tibetan lower crust but broadly conforms to that for the crustal channel and average lithospheric and asthenospheric values the high magnitude of maximum possible prandtl number of 1028 of the tmc gneiss might be related to isothermal decompression of the gneiss during its extrusion

three dimensional architecture of the nankai accretionary prism s imbricate thrust zone off cape muroto japan prism reconstruction via en echelon thrust propagation 1 a 9 km wide 92 km long three dimensional 3 d seismic reflection volume acquired off shikoku island japan images the seaward portion of the subduction of the philippine sea plate at the nankai trough and nankai accretionary prism detailed interpretation of the imbricate thrust and protothrust zones the portions of the prism between the deformation front and the first out of sequence thrust shows a high degree of variability in the thrust faults that all parallel the frontal thrust but are arranged in en echelon patterns along strike and frequently include complications such as piggyback faults and fault splays interestingly the sinuous seafloor morphology of the prism does not accurately reflect the en echelon 3 d architecture of the primary prism thrusts seafloor morphology appears to average across several thrusts along strike and is further modified by near surface thrust splays and backthrusts suggesting that care must be taken in interpreting seafloor relief in terms of lateral continuity or thrust fault geometry subduction of the kinan seamounts 20 km northeast of the center of the muroto 3 d volume generated a scallop shaped embayment in the prism the rebuilding process appears to influence the northeastern portion of the 3 d volume where a 625 m landward step in the position of the frontal thrust and numerous changes in prism architecture are observed these observations imply that accretionary prisms may reattain equilibrium following seamount subduction by lateral en echelon fault propagation into damaged zones that facilitate an increase accretion rate until a laterally continuous deformation front is reestablished

exhumation of high pressure rocks a review of concepts and processes the exhumation of high pressure metamorphic rocks requires either the removal of the overburden that caused the high pressures or the transport of the metamorphic rocks through the overburden exhumation cannot be achieved simply by thrusting or strike slip faulting it may be caused by erosion of shortened and thickened crust but this is unlikely to be the only mechanism for exhuming rocks from depths greater than about 20 km one or more of the following additional mechanisms may be involved 1 corner flow of low viscosity material trapped between the upper and lower plates in a subduction zone can cause upward flow of deeply buried rock and may explain some occurrences of high pressure tectonic blocks in melange this process does not however appear to be adequate to explain the exhumation of regional high pressure terrains 2 buoyancy forces acting directly on metamorphic rock bodies may cause them to rise relative to more dense surroundings this is likely to be the most important mechanism of exhumation of crustal rocks subducted into the mantle but cannot explain the emplacement of coherent tracts of high density metamorphic rock into shallow crustal levels some high pressure blocks emplaced at shallow levels in accretionary terrains may have been entrained in diapiric intrusions of low density mud or serpentinite 3 extension driven by the forces associated with contrasts in surface elevation may explain the exhumation and structural setting of many high pressure terrains extension may occur in the upper part of an accretionary wedge thickened by underplating or it may affect the whole lithosphere in a region of intracontinental convergence if surface elevation has been increased by the removal of a lithospheric root in the second case extension may be accompanied by magmatism and an evolution towards higher temperature during decompression of the metamorphic terrain