orogen response to changes in climatic and tectonic forcing despite much progress many questions remain regarding the potential dynamic coupling between atmospheric and lithospheric processes in the long term evolution of mountain belts as a complement to recent efforts to discover the interrelationships among climate topography erosion and rock deformation under conditions of mass flux steady state we explore orogen response to changes in climate and tectonic influx we derive an analytical model that predicts a powerful climatic control on orogen evolution and distinct potentially diagnostic responses to climatic and tectonic perturbations due to isostatic compensation the near surface rock uplift rate during transients is tightly coupled to climate modulated erosional efficiency system response is approximately exponential with a characteristic response timescale that is inversely proportional to the climate and lithology modulated erosional efficiency and is largely insensitive to initial conditions tectonic influx and both the sign and magnitude of perturbations

minimum age of initiation of collision between india and asia north of everest based on the subsidence history of the zhepure mountain section the collision of india with asia one of the most profound tectonic events to have occurred in past 100 m y is thought to have had geological geochemical and climatological consequences of global extent surprisingly the age of initiation of this collision remains poorly constrained estimates range from the late cretaceous 65 ma to latest eocene 40 ma with little consensus in between this paper reviews the stratigraphic section preserved on zhepure mountain on the north flank of everest mount qomolongma and its implied subsidence history zhepure mountain lies about 65 km south of the indus yarlung zangbo suture and contains the most complete and youngest passive margin shelf sediments in the tethyan himalayas on the basis of the subsidence history of the preserved section there is no evidence of acceleration of the subsidence up to the youngest rocks therefore collision related loading and accelerated subsidence must post date the youngest sediments preserved which date from the earl

variations in exhumation level and uplift rate along the obliqu slip alpine fault central southern alps new zealand we use geophysical and geological data from the southern alps to explore the relationship between plate motions and crustal structure on the geomorphology exhumation state and deformation style of rocks uplifted along amajor oblique slip fault a 50 km long segment of the southern alps has a higher uplift rate more relief deeper exhumation and a narrower width than surrounding regions there the delaminated east tilted crust of the pacific plate yields the youngest late cenozoic thermochronometric ages contours for fission track ar ar and k ar ages on several different minerals define an asymmetrically nested pattern of ages that increase away from the western side of the central southern alps eleven new 4 0 ar 3 9 ar samples of hornblende from the hanging wall of the alpine fault indicate that lower crustal rocks exhumed from 500 c in the late cenozoic are confined to a 20 km long culmination at the southern end of the central southern alps ages as low as 3 5 ma imply time integrated vertical exhumation rates as high as 6 9 mm yr this is the only part of this 5 8 ma range that may have achieved exhumational steady state remnant plugs of the original crustal hanging wall ramp are apparently preserved outside the central alps implying 70 km of fault convergence there 4 0 ar 3 9 ar age trends for hornblende near the alpine fault suggest that horizontal surfaces in the lower crust in the pacific plate have been overturned by reverse slip ductile shearing across a zone of distributed deformation that extends 2 km beyond the 1 km thick basal mylonite zone at the broadest orogen scale higher uplift rates throughout the central southern alps may be related to a rheologically controlled increase in the convergent velocity of points to the east of the alpine fault associated with a strengthening of the pacific crust at a more local scale maximum rates of uplift are inferred to occur near franz josef and fox glaciers because the alpine fault steepens at depth structural data suggests that its footwall ramp is curved and that the fault s dip steepens by 15 20 relative to its attitude farther to the south this 10 20 km long restraining bend may enhance local rates of rock uplift near franz josef and fox glaciers contemporary normal stress and shear resistance may also be increased on this part of the alpine fault helping to explain the central region s quiet historic seismicity and apparently strongly locked nature

very high strains recorded in mylonites along the alpine fault new zealand implications for the deep structure of plate boundary faults abstract oblique displacement on the alpine fault which forms the principal structure along the australian pacific plate boundary in south island new zealand has resulted in exhumation of a kilometre wide mylonite zone in the hanging wall adjacent to the current brittle fault trace the mylonites formed under amphibolite facies conditions at depths of ca 25 km and have been uplifted during the past 5 ma a suite of 65 70 ma pegmatite veins in the hanging wall alpine schists has been progressively deformed within the mylonite zone and sheared out over a strike length of ca 100 km measurements of the thickness distribution of the pegmatite veins within the non mylonitised schists and at three localities within the progressively strained mylonites have been used to estimate strain values within the mylonites the thicknesses approximate a log normal distribution with a mean value that is progressively reduced through the protomylonites mylonites and ultramylonites by assuming that the thickness distribution currently observed in the schists was the same for the pegmatites within the mylonites before strain a model of deformation incorporating simple shear and simultaneous pure shear is used to strain the undeformed veins until a fit is obtained with the strained distributions shear strains calculated range from 12 to 22 for the protomylonites 120 to 200 for the mylonites and 180 to 300 for the ultramylonites corresponding to pure shear values of 1 3 in each case these values are compatible with the strains predicted if most of the surface displacement on the fault over the past 5 ma were accommodated within a 1 2 km wide mylonite zone through the middle and lower crusts the results suggest that processes such as erosional focussing of deformation and thermal weakening may cause intense strain localisation within the lower crust with plate boundary deformation restricted to narrow zones rather than becoming increasingly distributed over a widening shear zone with depth

quartz fabrics in the alpine fault mylonites influence of pre existing preferred orientations on fabric development during progressive uplift strong quartz crystallographic preferred orientations cpos were developed during dislocation creep in the mylonite zone exposed in the hanging wall of the dextral reverse alpine fault zone new zealand the cpos have a consistent asymmetry indicating a high ratio of simple to pure shear strain with a shear sense of dextral up to the nw consistent with the mesoscopic shear sense indicators and with slip on the active fault there is a transition from y maxima and asymmetric single girdles in mylonites and ultramylonites within 300 m of the present fault trace to cross girdle fabrics in the protomylonites further from the fault the strong y maxima or single girdle cpos are ascribed to high ductile shear strains under amphibolite facies conditions while the cross girdle patterns found in the protomylonites are interpreted to represent deformation under lower temperature conditions however the observed fabric transition cannot logically be attributed to variations in temperature during the last increment of deformation the highly oriented y maximum fabrics formed at high temperature contain very few grains suitably oriented for basal slip so that the slip systems activated during subsequent shear at lower temperatures during exhumation were prism or rhomb further from the fault where shear strains under high temperature conditions were lower weaker fabrics developed under these conditions were modified at higher levels in the crust into crossed girdle patterns this interpretation implies that intense localisation of shear strain along the fault zone within the lower crust must have occurred in order for the high temperature fabrics to become sufficiently intense to be preserved this in turn implies that deep seated localised shear was taking place early during the evolution of the current oblique slip plate boundary fault the results of this study also indicate that the current model that operation of certain slip systems in quartz is mostly a function of temperature is incorrect future work should consider the effects of deformation history total strain and the presence of other mineral phases

ductile fabrics in the zone of active oblique convergence near the alpine fault new zealand identifying the neotectonic overprint the mid crustal alpine schist in central southern alps new zealand has been exhumed during the past similar to3 m y on the hanging wall of the oblique slip alpine fault these rocks underwent ductile deformation during their passage through the similar to 150 km wide pacific australia plate boundary zone likely to be cretaceous in age peak metamorphism predates the largely pliocene and younger oblique convergence that continues to uplift the southern alps today late cenozoic ductile deformation constructively reinforced a pre existing fabric that was well oriented to accommodate a dextral transpressive overprint quartz microstructures below a recently exhumed brittle ductile transition zone reflect a late cenozoic increment of ductile strain that was distributed across deeper levels of the pacific plate deformation was transpressive including a dextral normal shear component that bends and rotates a delaminated panel of pacific plate crust onto the oblique footwall ramp of the alpine fault progressive ductile shear in mylonites at the base of the pacific plate overprints earlier fabrics in a dextral reverse sense a deformation that accompanies translation of the schists up the alpine fault ductile shear along that structure affects not only the 12 km thick section of alpine mylonites but is distributed across several kilometres of overlying nonmylonitic rocks c 2001 elsevier science ltd all rights reserved

an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

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

distinguishing eustatic and epeirogenic controls on lower middle cambrian boundary successions in west gondwana morocco and iberia by ed landing gerd geyer and wolfram heldmaier published in sedimentology 2006 53 4 899 918 discussion

from symmetric necking to localized asymmetric shearing the role of mechanical layering we study the formation of localized shear zones during the layer parallel extension of viscous multi layers using two dimensional numerical simulations based on the finite difference method for power law viscous layers and a linear viscous embedding medium the extended multi layer develops boudins due to necking for power law viscous layers embedded in a power law viscous medium the extended multi layer develops first distributed necks and subsequently a localized shear zone with a vertical offset with a size of several layer thicknesses along the multi layer during the extension the deformation style switches from distributed and symmetric necking to localized and asymmetric shearing a localized shear zone develops in the viscous multi layer although the rheology is everywhere strain rate hardening power law stress exponent 1 and no material softening and or energy feedback mechanism e g shear heating combined with a temperature dependent viscosity is applied the shear localization is caused by structural softening because the formation of a localized shear zone decreases the bulk resistance and hence the work required to deform the multi layer a localized shear zone forms in the multi layer when the distance between the stiff layers is approximately equal to or less than the layer thickness the shear localization was observed in multi layers with nine and with only three stiff layers

fission track analysis of the late cenozoic vertical kinematics of continental pacific crust south island new zealand during the late cenozoic the pacific plate has been converging obliquely with the australia plate in south island new zealand a result of this convergence has been the growth of a major mountain range the southern alps at the leading edge of the pacific plate the results of fission track analysis of 140 samples from 13 transects across the alps reported here establish the late cenozoic vertical kinematics amount age and rate of rock uplift of the pacific crust underlying the alps the late cenozoic rock uplift of the pacific crust is asymmetrical with respect to the alpine fault being a maximum 19 km immediately east of the central part of the fault with lesser values at the eastern 3 km northern 10 km and southern 8 km extremities of the alps the age of the start of rock uplift varies spatially across the southern alps the earliest indications from fission track analysis being at 8 ma at the southern end of the alps decreasing to 5 ma at the northern end and 3 ma along the southeastern margin this age variation reflects the longer time over which the southern parts of the alps have been in collision the rate of propagation of rock uplift southeastward into the pacific plate has been 30 mm yr nearly 4 times the late cenozoic average rate of convergence normal to the plate boundary late cenozoic mean rock uplift rates range from a maximum of 2 8 mm yr at the alpine fault to a minimum of 1 0 mm yr in the east and have been sustained for periods of 3 8 m y accompanying denudation has exhumed amphibolite grade rocks immediately east of the alpine fault the rock uplift has been controlled by oblique slip displacement on the alpine fault a continental crustal section at least 19 km thick has been uplifted on the alpine fault comparison of the late cenozoic mean rock uplift rates with uplift rates derived from reset zircon data 2 10 mm yr near the alpine fault shows that uplift has accelerated over time but only significantly since 1 3 0 3 ma the amount of mesozoic uplift ranged from minimal amounts north of arthur s pass to 3 km near mount cook to 10 km in the south at lake wanaka

provenance analysis using raman spectroscopy of carbonaceous material a case study in the southern alps of new zealand detrital provenance analyses in orogenic settings in which sediments are collected at the outlet of a catchment have become an important tool to estimate how erosion varies in space and time here we present how raman spectroscopy on carbonaceous material rscm can be used for provenance analysis rscm provides an estimate of the peak temperature rscm t experienced during metamorphism we show that we can infer modern erosion patterns in a catchment by combining new measurements on detrital sands with previously acquired bedrock data we focus on the whataroa catchment in the southern alps of new zealand and exploit the metamorphic gradient that runs parallel to the main drainage direction to account for potential sampling biases we also quantify abrasion properties using flume experiments and measure the total organic carbon content in the bedrock that produced the collected sands finally we integrate these parameters into a mass conservative model our results first demonstrate that rscm t can be used for detrital studies second we find that spatial variations in tracer concentration and erosion have a first order control on the rscm t distributions even though our flume experiments reveal that weak lithologies produce substantially more fine particles than do more durable lithologies this result implies that sand specimens are good proxies for mapping spatial variations in erosion when the bedrock concentration of the target mineral is quantified the modeling suggests that highest present day erosion rates in whataroa catchment are not situated at the range front but around 10 km into the mountain belt

structure and evolution of the himalaya tibet orogenic belt the 1981 french chinese expedition to tibet focused on the lhasa block extending earlier coverage 400 km north of the tsangpo suture the lhasa block stood between 10 and 15 n latitude over most of the upper cretaceous and eocene and if gondwanian in origin had detached from gondwana by early permian seismic profiles reveal a complex moho topography resulting both from multiple continental thrusting and large scale strike slip faulting subduction related granitoids representing mixtures of mantle and crustal components and anatectic granitoids have been analysed and dated this study emphasizes the role of smaller blocks in the accretion of the continental mosaic

porphyroclast systems as kinematic indicators porphyroclasts of relatively strong minerals in mylonites commonly have an internal monoclinic shape symmetry defined by tails of dynamically recrystallized material the geometry of a porphyroclast and its tails called a porhyroclast system can serve as a valuable indicator of the sense of vorticity porphyroclast systems have been divided into and types on the basis of the geometry of the tails types have wedge shaped recrystallized tails whose median lines lie on opposite sides of a reference plane parallel to the tails and containing the symmetry axis for the system types are further subdivided into a a types in which the porphyroclast is isolated in a relatively homogeneous matrix and b types in which the porphyroclast system is associated with a shear band foliation in the matrix types typically have narrow recrystallized tails whose median lines cross the reference plane adjacent to the porphyroclast consequently embayments of matrix material occur adjacent to the porphyroclasts and the tails display characteristic bends a porphyroclast system in a mylonite develops when the relatively weak dynamically recrystallized grain aggregate in the porphyroclast mantle changes its shape due to non coaxial flow in the adjacent matrix this behaviour has been modelled in shear box experiments passive marker lines around rigid cylinders embedded in silicone putty were subjected to simple shear the experiments were modified to simulate a change in recrystallization rate r with respect to rate of deformation by decreasing the diameter of the rigid cylinder during deformation at variable rates the ratio r appears to be one of the most important factors in determining which porphyroclast system will develop at high r values flow of recrystallized material away from the porphyroclast is continuously appended by the production of new grains and wedge shaped a type tails develop at low r values relatively few new grains are added to the tails which become thinned and deflected by drag due to the spinning motion of the porphyroclast in addition most porphyroclast systems at low shear strains are of a type or lack monoclinic symmetry whereas types are only developed at high shear strain values complex porphyroclast systems characterized by two generations of tails are observed in many of the natural and model shear zones studied and may form due to fluctuating r conditions that allow isolated a and type porphyroclast systems to be used as sense of vorticity indicators are the systems should have a monoclinic shape symmetry matrix grain size should be small with respect to porphyroclast size matrix fabric should be homogeneous deformation history should be simple and observations should be made on sections normal to the inferred bulk vorticity vector for the mylonite

precise radiolarian age constraints on the timing of ophiolite generation and sedimentation in the dazhuqu terrane yarlung tsangpo suture zone tibet well preserved abundant radiolarians provide high precision biostratigraphic age constraints on the timing of the eruption of ophiolitic basalts exposed along the yarlung tsangpo suture zone in southern tibet dazhuqu terrane ophiolites were generated in an intra oceanic supra subduction zone setting within a relatively short

the origin and significance of crustal minerals in ophiolitic chromitites and peridotites abstract various combinations of zircon quartz corundum k feldspar plagioclase apatite amphibole rutile titanite almandine garnet kyanite andalusite and coesite have been recovered from podiform chromitites of the luobusa and dongqiao ophiolites in tibet the oman ophiolite and the ray iz ophiolite in the polar urals russia chromitites in all four ophiolites also contain moissanite and the luobusa and ray iz ophiolites contain in situ diamonds most of the recovered zircons are sub rounded grains with complex internal structures indicating polyphase growth trace element contents and a low pressure inclusion assemblage quartz muscovite k feldspar apatite ilmenite rutile indicate a continental crustal origin for the zircons they have sims u pb ages that are generally much older than the host body total range 90 to 2500 ma the presence of numerous crustal minerals particularly zircon suggests derivation from metasedimentary rocks subducted into the mantle their preservation in chromitites and peridotites implies effective isolation from the mafic melts that formed the ophiolites we suggest that most of these minerals were derived from the crustal parts of subducted slabs and were encapsulated into chromite grains precipitated from rising asthenospheric and suprasubduction magmas the chromite grains were carried in melt channels to shallow levels in suprasubduction mantle wedges and then deposited as podiform chromitites near the moho the rise of asthenospheric peridotites into suprasubduction zones was facilitated by subduction initiation and possibly by slab tear allowing mixing of the uhp highly reduced minerals and crustal minerals now found in ophiolitic chromitites and peridotites

a brief review of ophiolites in china abstract ophiolites are widely distributed in western southwestern and northern china where they fall into four principal age groups neoproterozoic early paleozoic late paleozoic and mesozoic cenozoic neoproterozoic ophiolites are known only in the north qinling orogenic belt in ne jiangxi province and in western sichuan province phanerozoic ophiolites are grouped into the paleo asian tethyan and circum pacific series the paleo asian ophiolites crop out in the western and northern parts of china between the siberian and north china blocks and range in age from early to late paleozoic all of these ophiolites are of the franciscan formerly cordilleran type and many are superimposed on one another suggesting repeated accretion of arc assemblages in an environment similar to the present day western pacific ocean mediterranean type formerly tethyan type ophiolites are confined to sw china particularly tibet and yunnan province paleo tethyan ophiolites are characterized by morb type lavas and are typically bounded by old continental lithosphere suggesting that they formed in small intercontinental ocean basins neo tethyan ophiolites contain a range of lava types including morb iat and boninite indicating formation in a variety of suprasubduction zone ssz environments circum pacific ophiolites occur sporadically in taiwan and ne china where they form tectonic melanges composed mainly of metaperidotite gabbro and basalt

thermochronological analysis of the dynamics of the southern alps new zealand the isotopic ages of material exposed close to the alpine fault in the southern alps new zealand vary in a consistent fashion along the orogen for the k ar system muscovite and biotite ages display two distinct spatial trends that meet in the area of fox glacier where a strong relationship is observed between k ar age and altitude excess argon is present in samples between mount kinnaird and haast pass in the southwest of the southern alps giving unrealistically old inconsistent k ar ages across this region incorporation of excess argon appears to be highly variable however and minimization of its influence on dynamical interpretations of thermochronology is possible through considering only the youngest age from a given area such treatment indicates that excess argon contamination does not play an important role in the systematic k ar age variation identified along the southern alps the observed age distribution is consistent with estimates of the modern rapid uplift and exhumation of the southern alps beginning ca 5 ma k ar mica ages older than this south of the copland valley indicate that substantial late cenozoic exhumation occurred along the southern alps prior to the development of the modern tectonic regime but at rates lower than at present total exhumation during the current phase of tectonism in the southern alps decreases from the whataroa river southward north of fox glacier the southern alps have attained a dynamical steady state such that the observed distribution of exhumation will not change significantly with further deformation and denudation under current conditions

crustal structure of the indus tsangpo suture zone and its ophiolites in southern tibet abstract the 2000 km long indus tsangpo suture zone itsz in southern tibet contains remnants of neotethyan oceanic lithosphere ophiolitic melanges flysch units and continental rocks and has been interpreted as the boundary between the india and eurasia plates we have used field based structural analyses and geological mapping in five key ophiolite massifs luobusa zedang xigaze jiding and dongbo and petrofabric studies of core samples recovered through the luobusa scientific drilling project to document the structural architecture and the deformation history of the itsz the structural and kinematic analyses of the suture zone units indicate the occurrence of superposed deformation fabrics which developed first during the ophiolite emplacement and then during the subsequent india asia continental collision the suprasubduction zone ophiolites were emplaced southward onto the northern margin of the proto indian plate in the late cretaceous the continental collision between the ophiolite laden proto indian margin and the lhasa terrane of the eurasian plate in the early paleogene resulted in backthrusting and inversion of the structural order of the itsz the ophiolite massifs ophiolitic melanges and flysch units were thrust northward as nappe sheets over the eocene and younger forearc basin strata the itsz shows therefore a complex structural anatomy characterized by both south and north directed thrust faults and ductile and brittle deformation fabrics

on steady states in mountain belts the dynamic system of tectonics and erosion contains important feedback mechanisms such that orogenic systems tend toward a steady state this concept is often invoked but the nature of the steady state is commonly not specified we identify four types of steady state that characterize the orogenic system and illustrate these cases by using numerical model results and natural examples these types are 1 flux steady state 2 topographic steady state 3 thermal steady state and 4 exhumational steady state they refer to the erosional flux the topography the subsurface temperature field and the spatial pattern of cooling ages respectively models suggest that the topography will reach a steady mean form at the scale of an orogenic belt but perfect topographic steady state is unlikely to be achieved at shorter length scales thermal steady state is a precondition for exhumational steady state and in the case of temperature dependent deformation topographic steady state exhumational steady state is characterized by reset age zones spatially nested according to closure temperature as illustrated in natural systems from new zealand the cascadia accretionary margin and taiwan

kinematic strain localization deformation within a steady state compressional orogen i e where tectonic accretion is on geological time scales balanced by surface erosion can best be described by a stationary velocity field instantaneous deformation results from spatial gradients in the velocity field whereas total accumulated strain results from the integration of this instantaneous deformation along material paths following the flow lines defined by the velocity field we have synthesized the net strain distributions for rocks exposed at the surface of such an orogenic system using simple linear velocity fields corresponding to a simple shear within a dipping shear zone and b pure vertical shear in both cases we demonstrate the development of surface patterns of finite strain accumulation that do not reflect the geometry of the assumed velocity field in a simple manner large gradients in finite strain arise as a consequence of the geographic variation in particle residence time imposed by the surface boundary even for the limiting case where no instantaneous strain gradient exists such patterns of deformation are often recognised in exhumed orogenic systems but have commonly been assumed to reflect more complex velocity fields resulting from nonlinear localizing crustal rheologies we therefore demonstrate that caution should be exercised in interpreting observed strain patterns because a proportion of the observed strain localization must be attributed to this purely kinematic or geometric effect and this proportion may be significant in many systems such kinematic effects should be quantified and subtracted from observed strain distributions before they are used to infer the rheological behavior of crustal rocks we also suggest that in a simple shear thrust setting kinematic strain localization may in fact nucleate strain softening on the side of the deforming region that is stable or fixed with respect to the earth s surface and thus be responsible for the asymmetry that characterizes the large majority of thrust systems

u pb zircon constraints on the tectonic evolution of southeastern tibet namche barwa area the eastern syntaxis of the himalayas is expressed in the crust as a pronounced southward bend in the orogen the change in strike of geologic features coincides with the high topography of the namche barwa region the exposure of granulite grade metamorphic rocks and a 180 degree bend in the yalu tsangpo we have conducted a geochronologic and geochemical investigation of several suites of granitoids collected from the namche barwa massif and subjacent terranes of southeastern tibet ranging from cm scale dikes and sills to larger outcrop scale intrusions u pb shrimp rg zircon ages establish at least five magmatic episodes 400 to 500 ma 120 ma 40 to 70 ma 18 to 25 ma and 3 to 10 ma these episodes broadly correlate to spatial patterns in sample localities as follows 400 to 500 ma ages occur in zircon cores collected from within the massif proper 120 ma granites related to early gangdese arc plutonism are primarily located northeast of namche barwa later 40 70 ma gangdese activity is expressed in granites west of namche barwa 18 to 25 ma granites occur both along the suture zone west of gyala peri and directly north of namche barwa along the area of the jiali fault zone and are attributed both to shearing within the jiali fault zone and to an early miocene gangdese thrust event exceptionally young 10 ma zircon ages are clustered near the core of the massif along the yalu tsangpo gorge trace element geochemical data indicates the presence of both fluid present and fluid absent melts with a fluid absent decompression melting regime dominating near the core of namche barwa

the approach to steady state thermochronological distribution following orogenic development in the southern alps of new zealand a diachronous sequence of isotopic ages along the southern alps of new zealand illustrates details of the development of the modern tectonic regime of this orogen at about 5 ma coupled with the rapid cooling rates experienced in the southern alps which ensure negligible residence time at temperatures allowing partial radiogenic product retention during exhumation and thus effectively instantaneous thermochronological closure this record presents important general insight into the transient physical and thermal effects of changes in tectonic conditions the tectono thermal response of the southern alps to the change in dynamical conditions at 5 ma is resolved into two evolutionary stages that are observed with progressive exhumation the removal by erosion of material that had cooled below the relevant closure temperature prior to the change in dynamics at 5 ma results in an initial decrease in age this is followed by a sharp drop to younger ages as material subjected to thermal re equilibration associated with the modern orogenic regime is exposed culminating in the eventual exposure of time invariant ages reflecting the new steady state dynamics of the region this two step response is a direct result of the relationship between cooling and exhumation and illustrates the care needed in reconstructing physical histories from isotopic ages and cooling rates in tectonically active regions

the obliquely convergent plate boundary in the south island of new zealand implications for ancient collision zones abstract the alpine fault of new zealand forms the western boundary of a zone of distributed deformation formed by the oblique convergence of continental crust belonging to the pacific and australian plates structural and geodetic data from the alpine fault show that a large proportion of the total plate displacement is accommodated by rapid oblique slip on the fault the remaining displacement is distributed over a 200 km wide zone to the east the collision may be modelled as a two sided deforming orogen with the partitioning of deformation being controlled by erosional differences between the narrow high strain inboard and broad low strain outboard sides in ancient collision zones little evidence may remain of the nature and amount of displacement on the inboard side partitioning of deformation among pre existing structures complicates interpretation of the outboard zone radiometric ages may post date collision by several tens of millions of years and indicate slow isostatic uplift and unroofing collision ages if preserved may be recognized by high uplift rates calculated from muscovite biotite pairs

structure and tectonics of the himalaya constraints and implications of geophysical data the himalayan range is a product of continental collision between india and eurasia and for several reasons it is a logical place to examine the physical causes and mechanisms of mountain building the exceptionally high mountains and the great length of the range alone single the himalaya out as a major structural feature the convergence rate between india and eurasia of about 50 mm yr 1 minster jordan 1978 requires rapid rates of slip and the occurrence of four major earthquakes with magnitudes in excess of 8 4 in the last 100 years attests to a high level of seismicity and a rapid rate of active deformation e g richter 1958 seeber armbruster 1981 the exposures of high grade metamorphic rocks in the greater himalaya may suggest large vertical transport of lower crustal rocks e g valdiya 1980a b and the occurrences of young postcollision granites imply recent large perturbations of the temperature structure of the range e g lefort 1981 moreover as sediments derived from the himalaya fill the broad ganga basin south of the range the mountain belt simul taneously seems to override the basin e g g d johnson et al 1979 n m johnson et al 1982 as such the processes occurring now and since the collision began in the early tertiary as well as the present structure of the himalayan belt are probably similar to those at different stages of collision and mountain building in other older orogenic belts such as the appalachians the scottish and scandinavian caledonides the alps and

a thin viscous sheet model for continental deformation summary for the purposes of describing its large scale and long term deformation the continental lithosphere is regarded as a continuum obeying a newtonian or a power law rheology the flow of a thin sheet of power law material overlying an inviscid substrate is studied under the assumption that vertical gradients of the horizontal velocity are negligible a numerical model is used to investigate the deformation of such a sheet under conditions approximating those of continent continent collision the material flows in response to forces applied to its boundaries for example the indenting of one continent by another and to forces in its interior arising from gradients in crustal thickness the horizontal divergence of the flow produces changes in the crustal thickness and hence a time dependent form to the flow itself for a given set of boundary conditions the flow depends on the stress exponent in the power law rheology n and on the argand number which is a measure of the ratio between the stress arising from crustal thickness contrasts and the stress required to deform the material at the ambient strain rates when the effective viscosity of the medium is very high ar o crustal thickness variations do not influence the flow if the material is newtonian n l the deformation associated with an influx of material approximating an indenter is of much greater lateral dimension than the width of the indenter whereas when material has a power law rheology n 3 5 are used the deformation is confined to a region of lateral extent comparable to that of the indenter as ar increases the forces arising from crustal thickness contrasts exert more influence on the flow and the maximum crustal thickness that can be sustained by a given influx of material is related by a simple expression to the effective viscosity of the medium at the ambient strain rates in the limit of a very weak medium ar 10 the lithosphere is unable to sustain appreciable crustal elevation contrasts the results of these numerical experiments show that systems in which the effective viscosities are such that the maximum deviatoric stresses are between 1 kbar

numerical modeling of intraplate deformation simple mechanical models of continental collision we study a model of continental collision in which one of the continents acts as a rigid die indenting the other plate which flows as an incompressible viscoplastic medium we consider two extreme cases of plane deformation 1 plane strain which corresponds to an infinitely thick lithospheric plate and 2 plane stress corresponding to a very thin plate deformation of the lithosphere a thick plate should be intermediate between those extremes we found that the flow in the plane strain case is quite similar to that obtained by slip line theory the plane stress results are quite different since in this case most of the plate shortening is taken up by the thickening of the lithosphere we also explored the role of boundary conditions on the flow in particular the role of the side walls containing the flow of the lithosphere in the case of a free lateral boundary the main feature is a flow of matter toward this free wall and a s like pattern for the horizontal stress field for a rigid wall on the other hand the plane strain and the plane stress results are quite different in the first case there is a large return flow on the sides of the punch the material being extruded along the only free surface available in the plane stress case the return flow disappears and the material displaced by the penetration of the die tends to thicken the plate the role of a nonlinear constitutive relation is studied for power law creep as the power of the flow limit increases the flow retains its general features but the deformation localizes creating sharper contrasts between high and low strain rate areas in plane stress the effect of nonlinearity is to increase the contrasts in vertical motion available data for asia are discussed in the light of the new results

tertiary deformation history of southeastern and southwestern tibet during the indo asian collision geologic mapping and geochronological analysis in southwest kailas area and southeast zedong area tibet reveal two major episodes of tertiary crustal shortening along the classic indus tsangpo suture in the yalu river valley the older event occurred between ca 30 and 24 ma during movement along the north dipping gangdese thrust the development of this thrust caused extensive denudation of the gangdese batholith in its hanging wall and underthrusting of the xigaze forearc strata in its footwall examination of timing of major tectonic events in central asia suggests that the initiation of the gangdese thrust was approximately coeval with the late oligocene initiation and development of north south shortening in the eastern kunlun shan of northern tibet the nan shan at the northeastern end of the altyn tagh fault the western kunlun shan at the southwestern end of the altyn tagh fault and finally the tian shan north of the tarim basin such regionally synchronous initiation of crustal shortening in and around the plateau may have been related to changes in convergence rate and direction between the eurasian plate and the indian and pacific plates the younger thrusting event along the yalu river valley occurred between 19 and 10 ma along the south dipping great counter thrust system equivalent to the locally named renbu zedong thrust in southeastern tibet the backthrust system in south central tibet and the south kailas thrust in southwest tibet the coeval development of the great counter thrust and the north himalayan granite gneiss dome belt is consistent with their development being related to thermal weakening of the north himalayan and south tibetan crust due perhaps to thermal relaxation of an already thickened crust created by the early phase of collision between india and asia or frictional heating along major thrusts such as the main central thrust beneath the himalaya

remnants of a cretaceous intra oceanic subduction system within the yarlung zangbo suture southern tibet extensive field investigations along the yarlung zangbo suture zone in southern tibet reveal the presence of now fragmented remnants of a south facing intra oceanic subduction system this system developed within tethys during the cretaceous the associated arc forearc ophiolite and subduction complex were emplaced onto the leading edge of india at the end of the cretaceous rapid sedimentation in oblique slip basins and disruption of water saturated sediments into melange was widespread and concomitant with ophiolite emplacement we describe the tectonic entities that developed during this previously unrecognized phase of tethys tibet evolution and present a new model for the evolution of this portion of tibet

fault zone reactivation kinematics and mechanisms the kinematics and mechanisms of fault zone reactivation are reviewed reactivation is dependent upon fault zone orientation and the existence of weak mylonites along these zones the recognition of reactivation within mylonite zones and the softening processes that first concentrate deformation into these and secondly provide a weak medium for reactivation are discussed attention is given to the darling mobile zone western australia and the redbank zone see obee white this symposium as examples of reactivated zones

deformed conglomerates and textural zones in the otago schists south island new zealand abstract the prehnite pumpellyite to greenschist facies rocks of the classic low grade otago schist terrane are subdivided into a number of textural zones based on the development of slaty cleavage and the appearance of segregation lamination in metagreywackes in this study conglomerates from fifteen localities have been studied in order to investigate the relationship between ductile strain and the sequence of textural zones strain measurements on lithic pebbles show a clear correlation between strain and textural zone ranging from around 40 shortening in tz iia with the development of slaty cleavage to more than 70 in tz iiia corresponding to the appearence of segregation lamination the much lower and more prolate strains of the quartzose pebbles compared to the corresponding lithic clasts may be explained by differential strain partitioning between clast and matrix microstructural evidence including well developed strain shadows and solution seams indicates that solution transfer was the dominant deformation mechanism the development of quartz segregation laminae in tz iii is due to three related processes the development of mica rich solution seams demarcating quartzo feldspathic microlithons the development of amalgamated and recrystallised strain shadow overgrowths on clastic grains and the formation rotation and disruption of early formed veins the large increase in strain from tz ii to tz iii is responsible both for increased segregation and for the rotation and flattening of segregation features into the foliation thus producing a segregation lamination large strains developed by solution transfer during the low grade metamorphism of greywackes will lead inevitably to segregated schists

distributed deformation in southern and western tibet during and after the india asia collision field and radiometric data are used to describe and date strain and stress states in southern longitude 88 o to 91oe latitude 28 o to 30on and western tibet longitude 79 o to 82oe latitude 30 o to 34on we factorize deformation into syncollisional and postcollisional and we present stretching lineation and displacement orientation maps two sections across the indian shelf sequence and stress orientations calculated from mesoscale fault slip data in southern tibet syncollisional stretching and displacement directions trend 90 46 o and displacement is top to south synkinematic low grade metamorphism is dated at 50 ma at one locality in the indian shelf sequence underlying the main mantle thrust of the indus yarlung suture this implies paleocene onset of continental collision for the investigated section postcollisional structures comprise a backthrust group which includes foreland and hinterland directed thrusts reverse and strike slip faults and folds it dominates postcollisional deformation is concentrated along the indus yarlung suture and portrays n s compression otrend of 8o 17 o o2 of 97o 17o a strike slip group consists of conjugate strike slip faults is concentrated in east trending narrow highly deformed zones and indicates that n s compression is locally compensated by e w extension oof 15o 29 o o3 of 103o 30o synkinematic muscovite dates postcollisional deformation as late early miocene 17 5 ma at one locality at the suture strike slip and oblique normal 03 of 600 23 o oof 144o 21 o and normal o3 114o 16 o faulting dated between late miocene and recent and including active deformation represents dominant e w and minor n s extension due to e w stretching of southern tibet and oroclinal bending along the himalayan arc restoring syncollisional and postcollisional deformation yields a minimum of 67 258 km shortening across the indian shelf sequence incorporating recently published contraction estimates across the eastern himalaya yields minimum shortening between undeformed india and the indus yarlung suture of 66 536 km the himalaya tibet orogenic system south of the indus yarlung suture had an initial width of 811 km in the southern tibetan section in western tibet imbrication of an ophiolite sequence of the bangong nujiang suture is top to south stretching lineation trend of 15o 18o and o3 of active deformation trends ese faulting along the shiquanhe fault zone which transfers displacement from the northern part of the karakorum fault to a system of rifts in western central tibet indicates dextral strike slip alternating with sinistral oblique normal faulting and block rotations around vertical axes during a prolonged shearing history the indian shelf sequence south of mount kailas shows top to south imbrication stretching lineation trend of 52o 60o both indian shelf rocks and oligocene miocene kailas conglo merates record backthrusting and backfolding oof 33 o and recent e w extension 03 of 85o 28o

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

the zedong terrane a late jurassic intra oceanic magmatic arc within the yarlung tsangpo suture zone southeastern tibet an overturned sequence of igneous and volcaniclastic rocks crops out along the yarlung tsangpo suture zone ytsz in southeastern tibet these rocks are remnants of an intra oceanic island arc the zedong terrane that once lay between india and asia 40ar 39ar dating and u pb ion microprobe analyses reported here reveal that this arc was active during the jurassic u pb dating of zircon from a dacite breccia from the middle portion of the zedong terrane yields an age of 161 2 3 ma 1 40ar 39ar dating of hornblende from a cross cutting andesite dyke yields the youngest age of 152 2 3 3 ma 1 40ar 39ar results from hornblende from andesite dykes and an andesite breccia from the upper portion yield a mean age of 156 0 3 ma 1 additional u pb and 40ar 39ar dating of zircon and hornblende from quartz diorite yields a mean age of 156 8 0 8 ma 1 geochronological data reported here and other published work indicate that the intra oceanic subduction system to which the zedong terrane belonged was active from at least late jurassic to early to mid cretaceous

on the thermomechanical evolution of compressional orogens summary we present results from a newly developed fully coupled thermomechanical model of the continental crust in which crustal shortening at a convergent plate boundary is driven by a basal velocity discontinuity which represents delamination and subduction in the underlying mantle this new dynamical model incorporates complex rheologies elasticity thermally activated creep and brittle frictional behaviour allows for extremely large deformation and is coupled along its top boundary to a complex erosion sedimentation model the model is based on the dynamic lagrangian remeshing dlr method which uses lagrangian spatial discretization of the crust and therefore allows for an accurate tracking of rock particles as they travel through the deforming orogen this information is in turn used to produce synthetic ptt paths we have used this model to predict the distribution of apparent ages for a wide range of isotopic systems at the surface of an actively deforming orogen we have also predicted other geophysical and geological observables such as the metamorphic grade of exposed rocks regarded here as a first order approximation for total denudation topography surface heat flux and the thickness of sediment deposited in the adjacent foreland basins results clearly demonstrate that the highest exhumation rates and thus the youngest isotopic ages are found in regions of maximum topography near the centre of the orogen but that the most deeply exhumed rocks are found on the side of the orogen in the vicinity of the retro shear zone a crustal scale fault which accommodates most of the crustal shortening within the orogen it is also in this region that the isotopic systems characterized by the greatest closure temperatures display the youngest ages and the highest grade metamorphic rocks are found these conclusions are derived from the assumption of uniform erosion across the orogen in cases where rainfall and thus erosion is orographically controlled the tectonic style of the orogen is different from the uniform erosion case as is the distribution of isotopic ages and metamorphic grades across the orogen

subduction zone retreat and recent tectonics of the south island of new zealand the central south island of new zealand is a zone of recent oblique continent continent convergence between the australian and pacific plates convergence is localized on the alpine fault and its broader expression has created the southern alps we propose that many of the strike variations in orogenic style of this zone e g physical extent and style of crustal shortening argon release patterns and fission track observations can be explained by differential amounts of subduction zone retreat beneath the orogen the observed patterns suggest greater eastward retreat of the subducting pacific plate in the south than in the north to support this hypothesis we compare geodynamic models having varying amounts of subduction zone retreat with geological and geophysical observations incorporating subduction zone retreat not only creates tectonic variations similar to those observed but also provides a geodynamically consistent explanation for the size shape and location of the broad negative bouguer gravity anomaly associated with the orogen in addition these geodynamic models suggest that subduction zone retreat may result in local topographic lows which may serve as preferential locations for the formation of intermontane basins and associated lakes possibly explaining the chain of south island lakes our preferred interpretation of the geologic history of the central south island convergence has an initial phase of pacific plate lower crust and mantle subduction underthrusting with no retreat followed by a second phase of subduction zone retreat in the southern part of the zone this retreat may be a consequence of oblique convergence which creates interference beneath fiordland between the south east dipping australian plate and the underthrust west dipping pacific plate to the north and forces retreat

bainang terrane yarlung tsangpo suture southern tibet xizang china a record of intra neotethyan subduction accretion processes preserved on the roof of the world the bainang terrane an intra oceanic island arc subduction complex into which tethyan oceanic rocks were accreted during the cretaceous is preserved within the yarlung tsangpo suture zone of tibet the lithostratigraphic succession established from field mapping records a long history of sedimentation in different portions of the central tethyan domain from late triassic to mid cretaceous time these rocks are preserved within a south verging imbricate thrust stack of thin 1 km thick northward younging tectonic slices five lithotectonic units were mapped in the terrane and these units are assigned to two distinct tracts the northern tract which accumulated on the north side of neotethys was probably separated from its southern counterpart by a mid ocean ridge detailed radiolarian biostratigraphy is used to constrain the timing of depositional events within each tract oceanic plate stratigraphy of the northern tract records its northward travel and mid cretaceous late aptian approach towards a south facing intra oceanic subduction zone rocks in the southern tract developed closer to the indian subcontinent and experienced thermotectonic subsidence and mid jurassic basic alkaline intraplate magmatism they were probably accreted late in the cretaceous variations in structural style across the terrane indicate deformation at different depths and vertical growth of the wedge rather than lateral accretion the overall tectonostratigraphy of the terrane reflects its development in a remote intra oceanic setting

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

plate tectonic interpretation of the western himalaya abstract the western himalaya which includes the regions of kumaun garhwal himachal jammu and kashmir in india displays a complete cross section of the himalayan orogenic belt encompassing the principal tectonic zones of outer himalaya lesser himalaya higher himalaya tethys himalaya and trans himalaya the rock formations of the lesser himalaya which are correlatable to those of the northern part of the indian peninsular shield represent deposits on the northern edge of the indian craton the phanerozoic sequence of the tethys himalaya zone is interpreted as representing sedimentary deposits of the continental margin the central crystallines of the higher himalaya zone underlying the phanerozoic sequence of the tethys himalaya zone have been exposed in their present position as a result of uplift and southward movement along the main central thrust the trans himalaya zone consists of the indus suture ladakh magmatic arc shyok suture and karakoram subzones the nidar ophiolite and the shergol and zildat ophiolitic melanges of the indus suture are the tectonised remnants of the tethys ocean floor the ladakh plutonic complex the dras volcanics and other related cretaceous volcanics belong to the plutonic volcanic series of the ladakh magmatic arc the shyok suture is interpreted as a back arc basin and the palaeozoic mesozoic sequence of the karakoram subzone represents continental margin deposits of the karakoram block the molasse sedimentary deposits of the outer himalaya zone are interpreted as representing post collision sedimentation in a trough that developed in front of the rising himalaya the tectonic evolution of northwestern himalaya has been involved in the following sequence of events 1 a subduction of the indian oceanic plate under the karakoram tibet block in the cretaceous giving rise to the ladakh magmatic arc and arc trench gap sedimentation 2 b collision of the indian plate with the ladakh magmatic arc in the middle eocene forming the indus palaeosubduction zone 3 c closure of the shyok back arc basin and collision of the ladakh magmatic arc with the karakoram block 4 d suturing of the lithospheric blocks as a result of collision the penetrative and non penetrative deformation regional metamorphism generation of s type granites in the higher himalaya zone and the formation of nappes and thrusts owe their origin to crustal shortening in the oligocene miocene

the tectonic evolution of the southern alps new zealand insights from fully thermally coupled dynamical modelling summary we present here the results of numerical models of the southern alps in which the thermal and dynamical developments of the orogen are fully coupled to one another this interlinkage allows the use of a wide range of thermal and physical features of the orogen as constraints on the applicability of our models in particular the thermal aspect of this method enables the prediction of the distribution of thermochronological ages at the surface of the model perturbation of the geothermal structure due to rapid uplift and exhumation causes an intrinsic weakening and concentration of strain along the equivalent of the alpine fault zone in these models this thermal weakening of the crust also produces a zone of high strain antithetic to the alpine fault in the upper crust that is comparable to the main divide fault zone within the southern alps the introduction of orographic rainfall and erosional processes into the model leads to the development of high asymmetric topography comparable to that of the southern alps this topographic profile results from the capture of available precipitation by the windward side of the orogen and the resultant rain shadow on the leeward side due to the time required to accumulate suycient topography for this rain capture evect to become significant the establishment of this high asymmetric topography lags the initiation of surface uplift by several million years comparison of the observed thermal history of the southern alps with that seen in models based on divering hypotheses of the tectonic evolution of the south island shows that the present tectonic regime of the orogen most probably developed in a single rapid reorganization of plate motions at approximately 5 ma with relative stability of the tectonic regime since that time the variation in isotopic ages along the southern alps is consistent with that expected from variation in accumulated uplift and exhumation along the orogen arising from the obliquity of convergence of the australian and pacific plates

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

brittle and ductile deformation in a zone of rapid uplift central southern alps new zealand schists exposed in the central southern alps new zealand 1 5 km east of the alpine fault zone have been rapidly uplifted during the late cenozoic a regionally consistent sequence of mesoscopic structures discordant to the schistosity is recognized extending backward in time from recent brittle displacements of deglaciated surfaces through structures exhibiting brittle ductile transitional behaviour to ductile deformational features kinematic analysis of these structures gives consistent principal subhorizontal shortening directions similar to the present day principal horizontal shortening direction indicating their relation to late cenozoic uplift of the southern alps analysis of deformed veins suggests a minimum shortening strain of about 50 perpendicular to the foliation the veins crosscut upright mesoscopic to macroscopic folds which commonly develop high strain zones on their limbs within such zones a strong stretching lineation plunges gently sw approximately perpendicular to the stretching lineation in the mylonites along the alpine fault the folds and high strain zones do not appear to be related to late cenozoic uplift but may have originated during an earlier phase of dominantly strike slip motion fluid inclusion studies give depth temperature estimates of around 3 km and 285 c for the development of the brittle structures and 6 8 km at 310 350 c for the brittle ductile transition the deformed veins show evidence of extensive fluid infiltration during the later phases data from fluid inclusions and metamorphic assemblages give an estimate of 15 20 km and 400 450 c for their deformation these data combined with uplift rates determined by other studies allow the construction of a depth temperature time path which indicates nearly isothermal decompression associated with rapid uplift a shallow brittle ductile transition 6 8 km is consistent with numerical modeling of thermal and mechanical behavior of the crust during rapid uplift associated with continental collision and with the presence of high heat flow in the area because of the small temperature change over a large amount of decompression pressure may be an important factor in controlling the depth of the brittle ductile transition the marked temperature drop above the brittle ductile transition is thought to be enhanced by the influence of convection as an effective cooling mechanism

a model for the origin of himalayan anatexis and inverted metamorphism the origin of the paired granite belts and inverted metamorphic sequences of the himalaya has generally been ascribed to development of the main central thrust mct although a variety of models have been proposed that link early miocene anatexis with inverted metamorphism recent dating studies indicate that recrystallization of elements of the mct footwall occurred in the central himalaya as recently as 6 ma the recognition that hanging wall magmatism and footwall metamorphism are not spatially and temporally related renders unnecessary the need for exceptional physical conditions to explain generation of the high himalayan leucogranites and north himalayan granites which differ in age petrogenesis and emplacement style we suggest that their origin is linked to shear heating on a continuously active thrust that cuts through indian supracrustal rocks that had previously experienced low degrees of partial melting numerical simulations assuming a shear stress of 30 mpa indicate that continuous slip on the himalayan decollement beginning at 25 ma could trigger partial melting reactions leading to formation of the high himalayan granite chain between 25 and 20 ma and the north himalayan belt between 17 and 8 ma the ramp flat geometry we apply to model the himalayan thrust system requires that the presently exposed rocks of the hanging wall resided at middle crustal levels above the decollement throughout the early and middle miocene late miocene out of sequence thrusting within the broad shear zone beneath the mct provides a mechanism to bring these rocks to the surface in their present location i e well to the north of the present tectonic front and has the additional benefit of explaining how the inverted metamorphic sequences formed beneath the mct we envision that formation of the mct zone involved successive accretion of tectonic slivers of the lesser himalayan formations to the hanging wall and incorporate these effects into the model the model predicts continued anatexis up to 400 km north of the himalayan range consistent with the timing and geochemistry of leucogranites exhumed on the flank of a south tibetan rift

recycling of crustal materials through study of ultrahigh pressure minerals in collisional orogens ophiolites and mantle xenoliths a review abstract newly recognized occurrences of ultrahigh pressure uh p minerals including diamonds in ultrahigh temperature uh t felsic granulites of orogenic belts in chromitites associated with ophiolitic complexes and in mantle xenoliths suggest the recycling of crustal materials through deep subduction mantle upwelling and return to the earth s surface this circulation process is supported by crust derived mineral inclusions in deep seated zircons chromites and diamonds from collision type orogens from eclogitic xenoliths in kimberlites and from chromitities of several alpine himalayan and polar ural ophiolites some of these minerals contain low atomic number elements typified by crustal isotopic signatures ophiolite type diamonds in placer deposits and as inclusions in chromitites together with numerous highly reduced minerals and alloys appear to have formed near the mantle transition zone in addition to ringwoodite and inferred stishovite a number of nanometric minerals have been identified as inclusions employing state of the art analytical tools reconstitution of now exsolved precursor uh p phases and recognition of subtle decompression microstructures produced during exhumation reflect earlier uh p conditions for example tibetan chromites containing exsolution lamellae of coesite diopside suggest that the original chromitites formed at p 9 10 gpa at depths of 250 300 km the precursor phase most likely had a ca ferrite or a ca titanite structure both are polymorphs of chromite and at 2000 c would have formed at minimum pressures of p 12 5 or 20 gpa respectively some podiform chromitites and host peridotites contain rare minerals of undoubted crustal origin including zircon feldspars garnet kyanite andalusite quartz and rutile the zircons possess much older u pb ages than the time of ophiolite formation these uh p mineral bearing chromitite hosts evidently had a deep seated evolution prior to extensional mantle upwelling and partial melting at shallow depths to form the overlying ophiolite complexes these new findings together with stable isotopic and inclusion characteristics of diamonds provide compelling evidence for profound underflow of both oceanic and continental lithosphere recycling of surface organic carbon into the lower mantle and ascent to the earth s surface through mantle upwelling intensified study of uh p granulite facies lower crustal basement and ophiolitic chromitites should allow a better understanding of the geodynamics of subduction and crustal cycling

a simple parameterization of strain localization in the ductile regime due to grain size reduction a case study for olivine we propose a simple parameterization of the transition between dislocation creep and grain size sensitive creep under conditions characteristic of the lithospheric mantle and derived from the results of laboratory experiments on olivine rich rocks through numerical modeling and linear stability analysis we determine the conditions under which this transition takes place and potentially leads to strain localization we pay particular attention to the effect of cooling rate and strain rate which are likely to be dominant parameters in actively deforming tectonic areas we conclude that at constant temperature strain localization can only take place if the rheology of the material is nonlinearly related to grain size that strain localization is facilitated by syndeformation cooling that there is only a narrow region in the strain rate versus cooling rate parameter space where localization is likely to take place and that grain growth inhibits strain localization at fast cooling rates but may lead to grain growth localization at low cooling rates we draw attention to the potential consequences of our analysis of strain localization for the style of plate motions at the earth s surface

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

the continental collision zone south island new zealand comparison of geodynamical models and observations the south island zone of oblique continent continent convergence occurs along a 400 km long section of the modern australia pacific plate boundary zone across which about 50 km of shortening has been accommodated since about 10 ma the orogen comprises a central mountain range southern alps flanked on both sides by what are interpreted to be foreland basins two essential features that characterize the orogen are 1 the degree of denudation that accompanied deformation and 2 a fundamental structural asymmetry the architectural asymmetry of the orogen can be explained by plane strain finite element models of continental convergence incorporating mantle subduction comparison of model and orogen polarity implies that pacific plate mantle subducts the models predict two crustal scale dipping shear zones that form above the point where the pacific mantle subducts the localized one more distant from the incoming plate retro step up shear zone corresponds to the alpine fault whereas its conjugate pro step up shear zone corresponds to the distributed strain and thrusting along the eastern margin of the mountain belt parameters that modify the model boundary conditions top surface degree of denudation basal zone subduction load crust mantle velocity discontinuity subduction of lower crust mantle retreat and distributed decrease in mantle velocity and the internal strength of the crust two layer crust with moderate coupling temperature distribution strain weakening are varied in a series of numerical model calculations that establish the combination of material properties and boundary conditions that lead to different cross sectional architectures of the modeled collision zone in turn these are compared with observations about the south island orogen the calculations show how the style and extent of deformation across the whole orogen depend on the rheological properties of the crustal layer and on the balance between its internal strength and the combined effects of the boundary and gravitational stresses north to south along strike differences in the width and two dimensional architecture of the orogen simulated in the experiments by varying the model parameters can be explained by a combination of southward increases in preconvergent crustal thickness geothermal gradient convergence and potentially subduction retreat with the added possibility of a southward decrease in the component of lower crustal subduction

transpressive ductile flow and oblique ramping of lower crust in a two sided orogen insight from quartz grain shape fabrics near the alpine fault new zealand 1 the three dimensional 3 d grain shape fabric gsf of deformed quartz veins was measured in 30 samples from the hanging wall of the oblique slip alpine fault in the central southern alps new zealand these record aspects of the neotectonic strain experienced by lower crustal rocks in the pacific plate as they were transported through a two sided orogen deformation was nonsteady state transpression was followed by near vertical shearing as the rocks negotiated the oblique footwall ramp of the alpine fault at the brittle ductile transition ramping up was accommodated by an escalator like back shearing process this thinned the transpressed crust and boosted dip slip rate on the alpine fault mechanically this suggests a sharp step in the footwall ramp adjacent to which differential stresses were transiently high margin parallel motion was partitioned onto these shears fabric development is simulated using a 3 d kinematic model into which we input constraints on plate motion fault slip rates crustal structure fabrics and geodetic strain in the southern alps this model can explain many aspects of the southern alps fabrics including their nonvertical foliation and lineation similarities between the model predictions and the observed fabrics suggest that lower crustal flow in the pacific plate mimicked the geodetic velocity pattern at the surface today with obliquely convergent deformation being distributed across a wide region to the east of the alpine fault the model shows that the timescale for an exhumational steady state with respect to deformational fabrics is slow 107 years a condition which the 6 m y old southern alps have probably not yet achieved

geochemistry and geochronology of mafic rocks from the luobusa ophiolite south tibet abstract this study presents geochemical compositions of mafic rocks outcropped in the luobusa ophiolite that locates at the eastern part of the yarlung zangbo suture zone yzsz the studied mafic rocks include gabbroic diabase dykes cutting the peridotites and foliated amphibolites embedded within the subophiolitic melange the diabases have low k 2 o contents and display n morb like geochemical characteristics i e with flat ree patterns and weak enrichment in lile e g rb ba th and u the gabbros show lree depleted patterns and variable enrichment in rb and ba foliated amphibolites mainly consist of hornblendes and plagioclases with minor titanites they commonly show lree depleted patterns with strong enrichment in lile three diabases have depleted sr nd hf isotope compositions with initial 87 sr 86 sr ratios of 0 703009 0 703502 e nd t of 5 0 to 8 5 and e hf t of ca 14 two gabbros have similar nd hf isotopes with the diabases but slightly higher initial 87 sr 86 sr ratios i e 0 704820 and 0 704550 compared to both diabases and gabbros the amphibolites have higher initial 87 sr 86 sr ratios i e 0 705131 0 705825 but more depleted nd hf isotope compositions with e nd t of 9 1 to 11 6 and e hf t of 18 2 to 21 9 geochemical compositions of the diabase dykes indicate that they were formed in a mid ocean ridge setting zircon sims u pb dating of the gabbroic dyke cutting the serpentinites yields an age of 128 4 0 9 ma which is identical within uncertainty to the zircon u pb age of the amphibolite i e 131 0 1 2 ma low u and th contents of zircons in the amphibolite support their metamorphic origin titanites in the amphibolites have been dated by la icpms and give u pb ages of 131 134 ma which are similar to the zircon u pb ages of the dated gabbro and amphibolite therefore we suggest that the luobusa ophiolite was generated at the early cretaceous and underwent the intra oceanic emplacement immediately after its formation this implies that ophiolites in the eastern segment of the yzsz were formed roughly synchronously with those in both central and western segments of the yzsz

zoned cretaceous and cenozoic garnet and the timing of high grade metamorphism southern alps new zealand new garnet sm nd and lu hf and existing rb sr 40ar 39ar u pb and sm nd ages and data on deformational fabrics and mineral compositions show for the first time that the garnet growth and ductile deformation in the alpine schist belt and southern alps orogen new zealand are diachronous and partly cenozoic in age the dominant metamorphic isograds in the alpine schist formed during crustal thickening at a previously unsuspected time at c 86 ma immediately prior to the opening of the tasman sea at c 84 82 ma obvious changes in the textures and compositional zoning patterns of garnet are not always reliable indicators of polymetamorphism and fabric elements can be highly diachronous a detailed timing history for the growth of a single garnet is recorded by a sm nd garnet whole rock age of 97 8 8 1 ma for the inmost garnet core zone 1 lu hf ages of 86 2 0 2 ma and 86 3 0 2 ma for overgrowth zones 2 and 3 a step leach sm nd age of 12 37 ma for zone 4 and growth of the garnet rim zone 5 over the alpine fault mylonite foliation during the modern phase of oblique collision that began at c 5 6 ma plate convergence along the new zealand portion of the gondwana margin continued after c 105 ma almost certainly culminating in the oblique collision of a large oceanic plateau hikurangi plateau the metamorphism of the alpine schist at c 86 ma is evidence of that hit the mid to late cretaceous extension that is widespread elsewhere in the new zealand region is attributed to upper plate extension and slab roll back the effects of the collision with the hikurangi plateau may have contributed to the changing plate motions in the region leading up to the opening of the tasman sea at c 82 ma

tertiary structural evolution of the gangdese thrust system southeastern tibet structural and thermochronological investigations of southern tibet xizang suggest that intracontinental thrusting has been the dominant cause for formation of thickened crust in the southernmost tibetan plateau since late oligocene two thrust systems are documented in this study the north dipping gangdese system gts and the younger south dipping renbu zedong system rzt west of lhasa the gangdese thrust juxtaposes the late cretaceous forearc basin deposits of the lhasa block the xigaze group over the tethyan sedimentary rocks of the indian plate whereas east of lhasa the fault juxtaposes the late cretaceous eocene andean type arc the gangdese batholith over tethyan sedimentary rocks near zedong 150 km southeast of lhasa the gangdese thrust is marked by a 200 m thick mylonitic shear zone that consists of deformed granite and metasedimentary rocks a major south dipping backthrust in the hanging wall of the gangdese thrust puts the xigaze group over tertiary conglomerates and the gangdese plutonics north of xigaze and west of lhasa a lower age bound for the gangdese thrust of 18 3 0 5 ma is given by crosscutting relationships the timing of slip on the gangdese thrust is estimated to be 27 23 ma from 40 ar 39 ar thermochronology and a displacement of at least 46 9 km is indicated near zedong the age of the gangdese thrust gt is consistent with an upper age limit of 24 ma for the initiation of movement on the main central thrust in places the younger renbu zedong fault is thrust over the trace of the gt obscuring its exposure the rzt appears to have been active at circa 18 ma but had ceased movement by 8 1 ma the suture between india and asia has been complexly modified by development of the gts rzt and locally strike slip and normal fault systems

how ductile are ductile shear zones three premises of ductile deformation in the middle and lower crust are widely accepted 1 rocks flow with power law viscous rheology 2 localization in ductile shear zones involves strain softening and 3 fluid flows into and is channelized within ductile shear zones ductile viscous shear zones should therefore initially develop along planes of maximum shear stress strain soften and rotate as material planes into the field of progressive extension however the mean stress or pressure within a weak elongate viscous band being stretched is higher than the surrounding matrix and this is difficult to reconcile with premise 3 in contrast mohr coulomb brittle faults always have lower pressure within the fault zone flow of fluid and melt into high temperature shear zones therefore implies that ductile shear zones are not perfectly viscous but have a pressure dependent viscoplastic rheology the continued pressure dependence may reflect significant microcracking on the grain scale even when localized deformation does not produce larger scale discrete fractures

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

imbricate structure of the luobusa ophiolite and surrounding rock units southern tibet abstract the cretaceous luobusa ophiolite is a tectonic slice less than 1 2 km thick the structurally underlying unit is the tertiary luobusa formation and the overlying unit is composed of feebly metamorphosed triassic sedimentary rocks the top and bottom contacts of the ophiolite dip gently to the south the major part of the luobusa ophiolite is mantle peridotite including podiform chromitite this chromitite has received much attention because it contains an unusual mineral assemblage which includes diamond and moissanite a serpentinite melange zone including clasts of basaltic and sedimentary rocks occurs underneath the mantle peridotite mesoscopic microscopic structures were observed in the sheared rocks shear indicating structures c type shear bands and type porphyroclasts in the rocks near the top and bottom boundaries of the luobusa ophiolite show consistent top to the north or northeast reverse displacement the results reveal that the luobusa ophiolite was overturned and intercalated into an imbricate structure the thrust faults on the top and bottom of the ophiolite can be correlated with north vergent back thrusting which was associated with crustal shortening along the main central thrust due to the continued northward movement of india after the welding of india to asia

kinematics of oblique collision and ramping inferred from microstructures and strain in middle crustal rocks central southern alps new zealand the hanging wall of the alpine fault near franz josef glacier has been exhumed during the past similar to2 3 m y providing a sample of the ductilely deformed middle crust of a modem obliquely convergent orogen presently exposed rocks of the pacific plate are inferred to have undergone several phases of ductile deformation as they moved westward above a mid crustal detachment initially they were transpressed across the outboard part of the orogen resulting in oblate fabrics with a down dip stretch later they encountered the alpine fault experiencing an oblique slip backshearing on vertical planes this escalator like deformation tilted and thinned the incoming crust onto that crustal scale oblique ramp this style of hanging wall deformation may affect only the most rapidly uplifting central part of the southern alps because of the low flexural rigidity of the crust in that region and its displacement over a relatively sharp ramp angle at depth a 3d transpressive flow affected mylonites locally near the fault but their shear direction remained parallel to plate motion ruling out ductile extrusion as an important process in this orogen outside the mylonite zone late cenozoic shortening is inferred to be modest 30 40 as measured from deformation of younger biotite grains oblique collision is dominated by translation on the alpine fault and rocks migrate rapidly through the deforming zone preventing the accumulation of large finite strains transpression may play a minor role in oblique collision c 2001 elsevier science ltd all rights reserved

metamorphism and uplift of alpine schist in the franz josef fox glacier area of the southern alps new zealand geothermometry and geobarometry of 10 garnet oligoclase zone schists in the franz josef fox glacier area southern alps new zealand give temperatures ranging from 415 to 625 c and pressures from 5 2 to 9 2 kbar indicating a t p array of about 50 c kbar and inferred peak temperature conditions over a c 15 km thick section at depths between c 20 and 34 km the present day distribution of the schist samples implies that only about one third of the original crustal section is now exposed the garnet oligoclase zone schists represent the deeper part of a metamorphosed and deformed accretionary complex that was associated with late palaeozoic early mesozoic subduction along the gondwana continental margin partial uplift c 0 2 m ma and erosion of the complex during jurassic cretaceous times rangitata uplift was synchronous with d2 deformation and recrystallization as recorded by the p t array cenozoic kaikoura uplift and exhumation of the schist since c 30 ma to form the southern alps was associated with oblique slip movement on the alpine fault the present day position and steep eastward dip of isograds and d2 structures suggest considerable clockwise rotation during uplift associated with ductile attenuation and tectonic thinning by over two thirds of the original schist sequence largely due to simple shear along schistosity planes as the schist generally shows only incipient greenschist facies retrograde recrystallization an apparently complete although contracted prograde mineral sequence has been preserved by rapid uplift 5 km ma of hot rock and the effects of limited shear heating near the alpine fault

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

tectonic assembly of inverted metamorphic sequences inverted metamorphic sequences are characterized by peak metamorphic temperatures that increase structurally upward with isograds that are typically parallel to associated thrust faults a fully coupled thermal mechanical model for convergent orogens shows how an inverted sequence can be tectonically assembled in a crustal scale ductile shear zone at moderate to high rates of synorogenic erosion model inverted sequences form by tectonic juxtaposition of points with widely differing initial positions that reach peak temperatures at different times and in different places within the model orogen and thus do not represent metamorphic or thermal gradients inverted crustal isotherms are not required to produce model inverted isograds model results agree well with metamorphic pressure temperature data from the main central thrust zone of the central nepal himalayas

numerical modelling of strain localisation during extension of the continental lithosphere dipping reflectors in the upper mantle are observed on reflection seismic data and in some cases have been interpreted to represent mantle shear zones it is not clear however how these shear zones develop and under what circumstances previous attempts to model strain localisation by strain softening in the ductile regime have failed to show the development of mantle shear zones we present here results from a numerical model of lithospheric extension the model uses an elasto visco plastic rheology dependent on temperature pressure deviatoric stress and composition strain softening is incorporated into the mechanical model via a simple ad hoc strain viscosity relationship the results demonstrate that strain localisation can dominate lithospheric deformation in an extensional regime we illustrate the effects of varying the parameters which define the strain viscosity relationship on the geometry of lithospheric scale shear zones and the rift basin we also investigate the effects on shear zone development of introducing a variety of pre extension weak zones in the crust and or mantle our results suggest that if dipping reflectors observed on deep seismic data around the world are evidence of the existence of mantle shear zones there is a need to review our conceptual models of strain strength evolution during the development of continental rifts

interplay of orbital forcing and tectonic pulses in the cambrian iberian platform ne spain the stratigraphy of carbonate shale couplets cycles and cycle stacking patterns in a cambrian shallow water platform iberian chains ne spain are related to sea level changes driven by orbital forcing and by tectonic pulses the interplay of both effects can be discriminated in the iberian fault controlled platform in which the tectonic activity can be analysed by accurate and detailed biostratigraphic correlations based on trilobite zonation the stratigraphic hierarchy of rhythmically interbedded limestones and shales in two coeval but structurally separated geodynamic settings yields cycle ratios of 1 44 1 this ratio is supported by time thickness and spectral analysis which is based on a graphic method of analysis the map of grey lines the cycle ratio seems to be evidence for orbital forcing by obliquity and precession cycles predicted for early paleozoic time carbonate shale couplets the smallest rhythmic units recognisable in the field represent short term periodic fluctuations in supply of terrigenous sediments and carbonate productivity of uncertain origin which could be associated with one of several harmonics of the former orbital cycles the pulsating tectonic activity was approximated by using a quantitative analysis of tectonically induced subsidence shaw method recurrence frequencies of tectonic pulses were estimated and dated by biostratigraphy as a result tectonic disturbances in the cambrian iberian platform show an episodic periodicity comparable to that of orbital eccentricity cycles which could mask their recognition

orogeny and metamorphism a model for deformation and pressure temperature time paths with applications to the central and southern appalachians a model is presented for deformation uplift and exhumation in convergent orogens based on the following premises 1 foreland basin subsidence monitors the distribution in time and space of overthrust loads added to an orogen 2 the presence of an inherited rifted margin plays an important role in the subsequent history of an orogen by forming a basement ramp or buttress that must be surmounted in order for thrusts to advance on the craton on the seaward side of this transition up to 20 km of overthrusts can be accumulated without creating topography 3 critical shape theory can be applied to orogens as a whole allowing predictions to be made concerning dynamic balance between mass addition and loss uplift exhumation and growth of topography and 4 pressure temperature time p t t data can be used to reconstruct particle paths within the deforming orogen three states of overthrust orogens can be defined based on the relative rates of mass loss and mass addition as monitored by the foreland rapid exhumation of metamorphic terranes is most likely in the steady state where erosion balances accretion a constructive state where mass addition is greater than mass loss results in crustal thickening and initially little exhumation a destructive state where mass loss outweighs mass addition results in slow exhumation or subsidence of metamorphic rocks in the orogen depending on whether mass is removed by erosion or by extension p t t data on peak metamorphism pluton emplacement uplift exhumation and cooling can be used in conjunction with data from the foreland to distinguish among these various states the topography and strength of the inherited rifted margin play an important role in localizing zones of exhumation and uplift within the orogen application of this model to the central and southern appalachians suggests that during the constructive phases of the taconian and acadian orogenies overthrust loads accumulated mainly on the seaward side of the ocean continent transition as marked by the bouguer gravity gradient between the acadian and alleghanian orogenies the appalachian orogen was probably in a steady state with uplift and exhumation of rocks in the orogen but little subsidence of the foreland or advance of thrusts toward the craton the constructive stage of the alleghanian orogeny achieved the critical topography necessary to drive crystalline thrust sheets over the craton this resulted in widespread uplift and westward transport of older metamorphic rocks post orogenic unroofing of the southern appalachians is compatible with the predictions of flexural models the post orogenic unroofing history of the central appalachians and ouachitas probably reflects mesozoic extension in these areas

three dimensional numerical experiments of strain partitioning at oblique plate boundaries implications for contrasting tectonic styles in the southern coast ranges california and central south island new zealand a three dimensional numerical model is used to investigate the nature and distribution of crustal scale deformation and strain partitioning across obliquely convergent transpressive plate boundaries a uniform elastic plastic crustal layer obeying murrell s extension to three dimensions of the griffith failure criterion was deformed under basal kinematic boundary conditions that correspond to the oblique convergence between two mantle lithospheres one of which detaches and subducts beneath the other the results show deformation concentrated in relatively narrow shear zones the character planar versus curved and dip of these shear zones or crustal scale faults depend on the relative amount of convergence versus transcurrent motion imposed across the plate boundary in models dominated by transcurrent boundary conditions two separate sets of coexisting structures develop 1 a pair of planar relatively shallow dipping thrust zones arranged in a v structure that accommodate convergent motion and 2 a set of curved near vertical strike slip zones arranged in a flower structure that accommodates the transcurrent motion in models dominated by convergence both contractional and transcurrent deformations are accommodated by oblique slip across a single pair of structures that have the same geometry as the thrust shear zones there is a gradational behavior between these end member partitioned and nonpartitioned modes for intermediate boundary conditions in all cases the dip of the shear zones predicted by the numerical model is in good agreement with the dip of ideal faults predicted by mohr s incipient faulting hypothesis the two basic modes are interpreted to underlie the contrasting tectonic styles of the southern coast ranges california and central south island new zealand factors other than obliquity of convergence which play a role in strain partitioning are discussed

u pb zircon ages for yarlung tsangpo suture zone ophiolites southwestern tibet and their tectonic implications ophiolite complexes preserved along the yarlung tsangpo suture zone ytsz and obducted onto the northern continental margin of india in southern tibet represent the remnants of the once extensive permian mesozoic neo tethyan ocean that separated india from asia complete ophiolite successions are preserved near xigaze whereas the rest of the belt is essentially represented by mantle rocks with subordinate disrupted lower crustal rocks u pb zircon la mc icp ms geochronology on two gabbro samples from the luobusa ophiolite yielded concordant data with mean 206pb 238u ages of 149 9 1 4 2 ma and 150 0 5 0 ma these ages are in contrast to a younger age of 131 8 1 0 ma obtained from a pegmatitic gabbro in xigaze five u pb zircon tims ages from gabbroic samples in the western portion of the ophiolite belt reveal that the dangxiong ophiolite formed between 126 7 0 4 ma and 123 4 0 8 ma zircons from the jungbwa ophiolite have similar ages of 123 4 0 8 ma and 123 9 0 9 ma a single zircon analysed from a gabbro in kiogar has an age of 159 7 0 5 ma geochronological data reported here show ytsz ophiolites formed in association with intra oceanic subduction zone systems and are related a significant tectonic episode within the tethyan ocean during late jurassic to early to mid cretaceous times