the eastern termination of the zagros fold and thrust belt iran structures evolution and relationships between salt plugs folding and faulting 1 the zagros mountains result from the ongoing collision between the arabian and central iran plates the main features of the eastern zagros are 1 numerous emerged or buried salt diapirs made up of late precambrian hormuz salt and 2 the irregular along strike shape of the collision related detachment folds with frequent bending to understand this layout four geological cross sections have been constructed from the persian gulf foreland basin to the inner part of the zagros fold and thrust belt shortening in the deformed parts of the sections is less than 10 and is mainly accommodated by detachment folding we show that late cenozoic folding occurred in a region that was already punctuated by salt domes and diapirs in fact almost continuous halokinesis developed since the earlier paleozoic i e just short time after the deposition of the hormuz salt and continued up to the present these preexisting salt structures and their relevant local thickening strongly influenced both the localization and the direction of folds

lithospheric structure in central eurasia derived from elevation geoid anomaly and thermal analysis we present new crustal and lithospheric thickness maps for central eurasia from the combination of elevation and geoid anomaly data and thermal analysis the results are strongly constrained by numerous previous data based on seismological and seismic experiments tomographic imaging and integrated geophysical studies our results indicate that high topography regions are associated with crustal thickening that is at a maximum below the zagros himalaya tien shan and the tibetan plateau the stiffer continental blocks that remain undeformed within the continental collision areas are characterized by a slightly thickened crust and flat topography lithospheric thickness and crustal thickness show different patterns that highlight an important strain partitioning within the lithosphere the arabia eurasia collision zone is characterized by a thick lithosphere underneath the zagros belt whereas a thin to non existent lithospheric mantle is observed beneath the iranian and anatolian plateaus conversely the india eurasia collision zone is characterized by a very thick lithosphere below its southern part as a consequence of the underplating of the cold and stiff indian lithosphere our new model presents great improvements compared to previous global models available for the region and allows us to discuss major aspects related to the lithospheric structure and acting geodynamic processes in central eurasia

reconciling subduction dynamics during tethys closure with large scale asian tectonics insights from numerical modeling we use three dimensional numerical models to investigate the relation between subduction dynamics and large scale tectonics of continent interiors the models show how the balance between forces at the plate margins such as subduction ridge push and far field forces controls the coupled plate margins and interiors evolution removal of part of the slab by lithospheric break off during subduction destabilizes the convergent margin forcing migration of the subduction zone whereas in the upper plate large scale lateral extrusion rotations and back arc stretching ensue when external forces are modeled such as ridge push and far field forces indentation increases with large collisional margin advance and thickening in the upper plate the balance between margin and external forces leads to similar convergent margin evolutions whereas major differences occur in the upper plate interiors here three strain regimes are found large scale extrusion extrusion and thickening along the collisional margin and thickening only when negligible far field forces ridge push and larger far field forces respectively add to the subduction dynamics the extrusion tectonics develops a strong asymmetry toward the oceanic margin driven by large scale subduction with no need of preexisting heterogeneities in the upper plate because the slab break off perturbation is transient the ensuing plate tectonics is time dependent the modeled deformation and its evolution are remarkably similar to the cenozoic asian tectonics explaining large scale lithospheric faulting and thickening and coupling of indentation extrusion and extension along the asian convergent margin as a result of large scale subduction process

new insights on 3 d plates interaction near taiwan from tomography and tectonic implications abstract recent tomographic results are used to trace the south china sea and philippine sea subducting slabs south and northeast of taiwan along the manila and ryukyu trenches respectively in particular the 3 d plates interaction beneath taiwan is discussed based on a close up view of the tomographic sections together with earthquake hypocenters distribution our study indicates that 1 the east dipping south china sea slab can be followed to the north up to the latitude of hualien 2 the eurasian plate subducts beneath most part of the taiwan island down to the 670 km depth discontinuity 3 the north dipping philippine sea slab can be followed slightly west of the longitude of hualien both plates thus interact beneath northern taiwan where the arc continent collision is paroxysmal 4 slab detachment is envisaged at the northern edge of the subducted eurasian plate beneath the coastal range of taiwan which may facilitate the northwestward motion of the philippine sea plate with respect to eurasia slabs geometries obtained from tomographic sections allow us to reconstruct the late neogene plate kinematics and dynamics in this region our main conclusions are 1 the size of the original south china sea was about twice its present size 2 the subducted part of the west philippine basin i e the largest oceanic basin of the philippine sea plate extends only 400 km north of the ryukyu trench 3 slab detachment might have occurred 3 5 my ago beneath the central and northern ryukyu arc along a weak zone that is aligned with the gagua ridge an ancient plate boundary 4 the ryukyu trench has propagated westward from 126 e of longitude southeast of miyako island to the locus of the present arc continent collision along a major lithospheric tear that cut through the continent ocean boundary first and then through the continental lithosphere as a consequence the southern ryukyu margin developed progressively from east to west as a subduction zone during the last 8 my it has been elaborated onto a passive margin which corresponded to the edge of the eurasian continental shelf bordering the south china sea prior to pliocene 5 the southern okinawa trough rifted during the last 2 1 ma following the propagation of the subduction zone 6 subduction of the eurasian margin probably stopped recently beneath northern taiwan and a flip of subduction from west to east is being initiated near hualien

late cretaceous paleocene formation of the proto zagros foreland basin lurestan province sw iran late cretaceous emplacement of ophiolitic radiolaritic thrust sheets over the arabian passive margin was the first manifestation of the protracted closure of the neotethys ocean which ended with the continental collision between arabia and central iran and the formation of the present zagros fold belt this tectonic stacking produced a flexural basin the amiran basin 400 200 km in size in the northwest zagros that was filled with a 1225 m thick shallowing upward detrital succession made up of the amiran taleh zang and kashkan formations this succession sits unconformably above the late cretaceous gurpi formation and is overlain by the oligocene miocene shahbazan asmari carbonate succession dating of the amiran kashkan succession is based on detailed biostratigraphy using large foraminifera and calcareous nannoplankton the cretaceous tertiary k t boundary is located within the uppermost 25 45 m of the gurpi formation the overlying amiran and taleh zang formations have been dated as paleocene in age however the base of the paleocene within the gurpi formation lacks np1 and np2 zones implying a hiatus of 2 m y at ca 65 5 ma which is inferred to correspond to an early folding phase near the cretaceous paleocene boundary the upper part of the kashkan formation is dated to the earliest eocene by palynostratigraphy a large hiatus or very slow deposition lasting about 15 m y occurs between the kashkan and shahbazan formations in the studied region the base of the prograding shahbazan platform deposits is dated by 87sr 86sr stratigraphy at ca 33 9 ma the upper part of the asmari formation is dated as early middle miocene using foraminifera associations reconstruction of the amiran taleh zang kashkan succession of the amiran basin indicates a thickening of the basin fill from the southern pinch out along the se flank of the kabir kuh anticline to sw of the khorramabad anticline where the flexure is at least 900 m in contrast the ne part of the basin underwent coeval contraction and uplift of 1300 m superimposed smaller undulations onto the large scale flexure are interpreted as late cretaceous paleocene folds regional comparisons se zagros oman and turkey indicate that late cretaceous early tertiary deformation affected the entire ne margin of arabia but that compression was not synchronous being younger in lurestan than in the nw persian gulf where inversion tectonics occurred from turonian to mid campanian times the long sedimentary hiatus spanning most of the middle and late eocene must have been related to deep lithospheric processes linked to the initial events of the protracted closure of the neotethys ocean between arabia and central iran the tectono sedimentary history recorded in the zagros basin may help to understand early foreland basin growth in other orogens in which subsequent continental collision has obliterated these early events

dynamics of retreating slabs 2 insights from three dimensional laboratory experiments 1 a laboratory analogue of a three layer linear viscous slab upper mantle lower mantle system is established in a silicone putty honey and crystallized honey tank experiment the same setup as in the numerical investigation part 1 is used we focus on the interaction of the slab with the induced passive mantle flow by widely varying the mantle volume flux boundary conditions in our numerical experiments the lateral volume flux was set to zero in interpreting the results relative to the real earth the base of the box is taken as the bottom of the mantle convection system while the lateral boundaries may be associated with the presence of other nearby slabs dynamic force equilibrium assessed on the basis of an analytical review of forces is described for four different phases 1 the subduction initiation instability 2 the accelerating dynamic free fall phase of the slab 3 the dynamic interaction with the 660 km discontinuity and 4 a final phase of steady state trench retreat phase 3 is an important feature not observed in the numerical experiments this highly dynamic phase of interrupted trench retreat can therefore be attributed to boundary conditions on mantle volume flux on the basis of integration constants of force equilibrium in phases 2 and 4 we identify two different classes of volume flux one in which the lateral boundary can be considered open and the other class where it is closed closed boundary condition cases are obtained if any of the lateral box boundaries are 600 km away from the slab assuming a one to one relation between trench retreat and back arc spreading enigmatic observations of episodic opening of back arc basins can be explained by our experimental observations

seismic anisotropy in the java banda and philippine subduction zones and its implications for the mantle flow system beneath the sunda plate

high resolution regional crustal models from irregularly distributed data application to asia and adjacent areas we propose a new methodology to obtain crustal models in areas where data is sparse and data spreading is heterogeneous this new method involves both interpolating the depth to the moho discontinuity between observations and estimating a velocity depth curve for the crust at each interpolation location the moho observations are interpolated using a remove compute restore technique used in for instance geodesy observations are corrected first for airy type isostasy the residual observations show less variation than the original observations making interpolation more reliable after interpolation the applied correction is restored to the solution leading to the final estimate of moho depth the crustal velocities have been estimated by fitting a velocity depth curve through available data at each interpolation location uncertainty of the model is assessed both for the moho and the velocity model the method has been applied successfully to asia the resulting crustal model is provided in digital form and can be used in various geophysical applications for instance in assessing rheological properties and strength profiles of the lithosphere the correcting gravity for the crustal effects seismic tomography and geothermal modelling

reconstructing greater india paleogeographic kinematic and geodynamic perspectives abstract key in understanding the geodynamics governing subduction and orogeny is reconstructing the paleogeography of greater india the indian plate lithosphere that subducted since tethyan himalayan continental collision with asia here we discuss this reconstruction from paleogeographic kinematic and geodynamic perspectives and isolate the evolution scenario that is consistent with all three we follow recent constraints advocating a 58 ma initial collision and update a previous kinematic restoration of intra asian shortening with a recently proposed model that reconciles long debated large and small estimates of indochina extrusion our new reconstruction is tested against paleomagnetic data and against seismic tomographic constraints on paleo subduction zone locations the resulting restoration shows 1000 1200 km of post collisional intra asian shortening leaving a 2600 3400 km wide greater india from a paleogeographic sediment provenance perspective eocene sediments in the lesser himalaya and on undeformed india may be derived from tibet suggesting that all greater indian lithosphere was continental but may alternatively be sourced from the contemporaneous western indian orogen unrelated to india asia collision a quantitative kinematic paleomagnetic perspective prefers major cretaceous extension and a greater india basin opening within greater india but data uncertainty may speculatively allow for minimal extension finally from a geodynamic perspective assuming a fully continental greater india would require that subduction rates close to 20 cm yr was driven by a down going lithosphere crust assemblage more buoyant than the mantle which seems physically improbable we conclude that the greater india basin scenario is the only sustainable one from all three perspectives we infer that old pre collisional lithosphere rapidly entered the lower mantle sustaining high subduction rates whilst post collisional continental and young greater india basin lithosphere did not inciting the rapid india asia convergence deceleration 8 myr after collision subsequent absolute northward slab migration and overturning caused flat slab subduction tibetan shortening arc migration and arc volume decrease

dynamics of continental accretion three dimensional dynamic computer models show how accretionary tectonic margins evolve from the initial plate collision state through a period of plate margin instability and then re establish a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region subduction zones where one plate dives under another become congested when they try to accommodate buoyant exotic crust louis moresi et al describe new numerical models of continental accretion that follow the entire process from the initial collision state through a period of plate margin instability to the re establishment of a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region subduction zones become congested when they try to consume buoyant exotic crust the accretionary mountain belts orogens that form at these convergent plate margins have been the principal sites of lateral continental growth through earth s history modern examples of accretionary margins are the north american cordilleras and southwest pacific subduction zones the geologic record contains abundant accretionary orogens such as the tasmanides1 along the eastern margin of the supercontinent gondwana and the altaides which formed on the southern margin of laurasia2 in modern and ancient examples of long lived accretionary orogens the overriding plate is subjected to episodes of crustal extension and back arc basin development often related to subduction rollback3 and transient episodes of orogenesis and crustal shortening4 5 6 7 coincident with accretion of exotic crust here we present three dimensional dynamic models that show how accretionary margins evolve from the initial collision through a period of plate margin instability to re establishment of a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region the complexity of the morphology and the evolution of the system are caused by lateral rollback of a tightly arcuate trench migrating parallel to the plate boundary and orthogonally to the convergence direction we find geological and geophysical evidence for this process in the tasmanides of eastern australia and infer that this is a recurrent and global phenomenon

flexural bending of the zagros foreland basin we constrain and model the geometry of the zagros foreland to assess the equivalent elastic thickness of the northern edge of the arabian plate and the loads that have originated due to the arabia eurasia collision the oligo miocene asmari formation and its equivalents in iraq and syria is used to estimate the post collisional subsidence as they separate passive margin sediments from the younger foreland deposits the depth to these formations is obtained by synthesizing a large database of well logs seismic profiles and structural sections from the mesopotamian basin and the persian gulf the foreland depth varies along strike of the zagros wedge between 1 and 6 km the foreland is deepest beneath the dezful embayment in southwest iran and becomes shallower towards both ends we investigate how the geometry of the foreland relates to the range topography loading based on simple flexural models deflection of the arabian plate is modelled using point load distribution and convolution technique the results show that the foreland depth is well predicted with a flexural model which assumes loading by the basin sedimentary fill and thickened crust of the zagros the model also predicts a moho depth consistent with free air anomalies over the foreland and zagros wedge the equivalent elastic thickness of the flexed arabian lithosphere is estimated to be ca 50 km we conclude that other sources of loading of the lithosphere either related to the density variations e g due to a possible lithospheric root or dynamic origin e g due to sublithospheric mantle flow or lithospheric buckling have a negligible influence on the foreland geometry moho depth and topography of the zagros we calculate the shortening across the zagros assuming conservation of crustal mass during deformation trapping of all the sediments eroded from the range in the foreland and an initial crustal thickness of 38 km this calculation implies a minimum of 126 18 km of crustal shortening due to ophiolite obduction and post collisional shortening

timing of thrust activity in the high zagros fold thrust belt iran from u th he thermochronometry 1 apatite and zircon u th he cooling ages are used to quantify the timing of exhumation associated with thrust faulting in the high zagros fold thrust belt single grain cooling age data were collected from 1 cambrian sandstone in various thrust sheets 2 sandstone and basement clasts derived from structurally controlled salt plugs or fault bounded slices and 3 syntectonic neogene siliciclastics strata in the northwestern kuhrang and central kuh e lajin high zagros apatite u th he ahe ages range from 26 7 to 0 38 ma most cooling and exhumation occurred in the early to middle miocene constrained by ahe ages 19 15 ma from the high zagros thrust sheet localized faults and reset cooling ages from bakhtiyari deposits in the southeastern high zagros kuh e dinar cooling occurred later with ahe ages ranging from 16 5 to 0 79 ma here most cooling and exhumation occurred in the late miocene constrained by ahe ages 12 8 ma from the high zagros fault and exhumed paleozoic clasts in synorogenic strata zircon u th he zhe ages from bedrock samples across the high zagros are reflective of the precollisional thermal history the preservation of precollisional zhe ages limits the pre miocene maximum burial temperature of the exhumed strata to 180 c and indicate 7 9 km of maximum exhumation in the central zagros this study shows that thrust activity in the high zagros and continental suturing along the zagros suture was underway by at least 19 ma and initiated no later than latest oligocene to early miocene time 23 ma

slab breakoff a model for syncollisional magmatism and tectonics in the alps slab breakoff is the buoyancy driven detachment of subducted oceanic lithosphere from the light continental lithosphere that follows it during continental collision in a recent paper davies and von blanckenburg 1994 have assessed the physical conditions leading to breakoff by quantitative thermomechanical modeling and have predicted various consequences in the evolution of mountain belts breakoff will lead to heating of the overriding lithospheric mantle by upwelling asthenosphere melting of its enriched layers and thus to bimodal magmatism breakoff will also lead to thermal weakening of the subducted crustal lithosphere thereby allowing buoyant rise of released crustal slices from mantle depths in this paper we present a test of this model in the tertiary evolution of the european alps in the alps both basaltic and granitoid magmatism occur between 42 and 25 ma following the closure of oceanic basins by subduction and continental collision the granitoids are now well established to result from mixing of basalt with assimilated continental crust to identify the tectonically crucial origin of the partial mantle melts we have compiled all published geochemical and isotopic data of numerous mafic dykes occurring throughout the whole alpine arc their trace element and isotopic composition suggests that they have been formed by low degree melting of the mechanically stable lithospheric mantle we see no evidence for melting of asthenospheric mantle it was thus not decompressed to depths shallower than 50 km once initiated rapid lateral migration of slab breakoff will result in a linear trace of magmatism in locally thermal weakened crust this explains why all alpine magmatic rocks intruded almost synchronously along a strike slip fault the periadriatic lineament a compilation of ages from penninic high pressure rocks subducted to depths of up to 100 km shows that subduction took place at circa 55 40 ma followed by uplift at 40 35 ma from the short time interval between their uplift and the onset of magmatism we infer that both processes have been induced by the breakoff the slab breakoff model fulfills its predictions in the case of the alps and therefore supports the assumptions made in the theoretical model on a geological basis we believe that the characteristic association of magmatic activity with the return of high pressure rocks to the surface allows the identification of this process in the earth s mountain belts

mantle dynamics in the mediterranean the mediterranean offers a unique opportunity to study the driving forces of tectonic deformation within a complex mobile belt lithospheric dynamics are affected by slab rollback and collision of two large slowly moving plates forcing fragments of continental and oceanic lithosphere to interact this paper reviews the rich and growing set of constraints from geological reconstructions geodetic data and crustal and upper mantle heterogeneity imaged by structural seismology we proceed to discuss a conceptual and quantitative framework for the causes of surface deformation exploring existing and newly developed tectonic and numerical geodynamic models we illustrate the role of mantle convection on surface geology a coherent picture emerges which can be outlined by two almost symmetric upper mantle convection cells the downwellings are found in the center of the mediterranean and are associated with the descent of the tyrrhenian and the hellenic slabs during plate convergence these slabs migrated backward with respect to the eurasian upper plate inducing a return flow of the asthenosphere from the backarc regions towards the subduction zones this flow can be found at large distance from the subduction zones and is at present expressed in two upwellings beneath anatolia and eastern iberia this convection system provides an explanation for the general pattern of seismic anisotropy in the mediterranean first order anatolia and adria microplate kinematics and may contribute to the high elevation of scarcely deformed areas such as anatolia and eastern iberia more generally the mediterranean is an illustration of how upper mantle small scale convection leads to intraplate deformation and complex plate boundary reconfiguration at the westernmost terminus of the tethyan collision

subduction and the depth of convection in the mediterranean mantle 1 we tie together geological data paleotectonic reconstruction plate motion and tomographic analysis to unravel the history of subduction and back arc extension of the eastern and central mediterranean in these two regions extensional processes started contemporaneously around 30 myr ago but with marked differences in the eastern region the aegean basin opened slowly 1 cm yr behind a shallow dipping slab 40 45 the corresponding high velocity anomaly extends inside the upper mantle and can be also followed in the midmantle down to a depth of at least 1500 km its descent into the midmantle initiated most probably during the late cretaceous and the trench moved northeastward following the path of the eurasian plate and under the persistent push of the african plate conversely in the central mediterranean region subduction initiated later and the motion of the subducting slab is confined to the upper mantle causing punctuated and rapid episodes of back arc extension provencal and tyrrhenian basins behind a slab that dips steeply 75 we explore the causes that control how the slab subducted and interacted with the lower more viscous part of the mantle

tertiary sequence of deformation in a thin skinned thick skinned collision belt the zagros folded belt fars iran 1 we describe how thin skinned thick skinned deformation in the zagros folded belt interacted in time and space homogeneous fold wavelengths 15 8 5 3 km tectono sedimentary evidence for simultaneous fold growth in the past 5 5 2 5 ma drainage network organization and homogeneous peak differential stresses 40 15 mpa together point to buckling as the dominant process responsible for cover folding basin analysis reveals that basement inversion occurred 20 ma ago as the arabia eurasian plate convergence reduced and accumulation of neogene siliciclastics in foreland basin started by 10 ma ongoing contraction occurred by underplating of arabian crustal units beneath the iranian plate this process represents 75 of the total shortening it is not before 5 ma that the zagros foreland was incorporated into the southward propagating basement thrust wedge folds rejuvenated by 3 2 ma because of uplift driven by basement shortening and erosion since then folds grew at 0 3 0 6 mm yr and forced the rivers to flow axially a total shortening of 65 78 km 16 19 is estimated across the zagros this corresponds to shortening rates of 6 5 8 km ma consistent with current geodetic surveys we point out that although thin skinned deformation in the sedimentary cover may be important basement involved shortening should not be neglected as it requires far less shortening moreover for such foreland folded belts involving basement shortening underplating may be an efficient process accommodating a significant part of the plate convergence

kinematics topography shortening and extrusion in the india eurasia collision we examine the problem of partitioning between shortening and extrusion in the india asia collision since 45 ma we compute the amount of shortening expected from the kinematics of india s motion with respect to eurasia using the reconstruction at collision time to put bounds on the possible amounts of surface loss within greater india and within eurasia we then compute the amounts of surface loss corresponding to the thickened crust of tibet and of the himalayas assuming conservation of continental crust the spatial distribution of the topography reveals a large systematic deficit of crustal thickening distributed rather uniformly west of the eastern syntaxis but an excess of shortening east of it this distribution indicates an important eastward crustal mass transfer however the excess mass east of the eastern syntaxis does not account for more than one third to one half of the deficit west of the eastern syntaxis the deficit may be accounted either by loss of lower crust into the mantle for example through massive eclogitization or by lateral extrusion of nonthickened crust a mass budget of the crust of the himalayas indicates that lower crust has not been conserved there but the deficit is so large that local loss in the mantle is unlikely to be the unique cause of the deficit alternatively following zhao and morgan 1985 lower crust may have been transferred below the tibetan crust we conclude that a combination of possible transfer of lower crust to the mantle by eclogitization and lateral extrusion has to account for a minimum of one third and a maximum of one half of the total amount of shortening between india and asia since 45 ma this conclusion leaves open the possibility that the partitioning between extrusion and loss of lower crust into the mantle on the one hand and shortening on the other hand has significantly changed during the 45 m y history of the collision

reconciling surface plate motions with rapid three dimensional mantle flow around a slab edge a central axiom of the theory of plate tectonics is that the surface plates are coupled to and carried along by convection in the earth s mantle the situation is rather different at subduction zones however margarete jadamec and magali billen now present calculations that quantify this variance they use three dimensional numerical models of buoyancy driven deformation with realistic slab geometry for the alaska subduction transform system to determine the surrounding mantle flow field they show that near a subduction zone edge mantle flow velocities can have magnitudes of greater than ten times the surface plate motions and that the complex mantle flow field is consistent with observations from seismic anisotropy these results imply local rapid transport of geochemical signatures through subduction zones the direction of tectonic plate motion at the earth s surface and the flow field of the mantle inferred from seismic anisotropy are well correlated globally implying large scale coupling between the mantle and surface plates but the fit is poor at subduction zones here three dimensional numerical models of buoyancy driven deformation for the alaska subduction transform system are used to find the origin of the regional decoupling of flow the results imply local rapid transport of geochemical signatures through subduction zones the direction of tectonic plate motion at the earth s surface and the flow field of the mantle inferred from seismic anisotropy are well correlated globally suggesting large scale coupling between the mantle and the surface plates1 2 the fit is typically poor at subduction zones however where regional observations of seismic anisotropy suggest that the direction of mantle flow is not parallel to3 4 5 6 7 and may be several times faster than6 plate motions here we present three dimensional numerical models of buoyancy driven deformation with realistic slab geometry for the alaska subduction transform system and use them to determine the origin of this regional decoupling of flow we find that near a subduction zone edge mantle flow velocities can have magnitudes of more than ten times the surface plate motions whereas surface plate velocities are consistent with plate motions8 and the complex mantle flow field is consistent with observations from seismic anisotropy5 the seismic anisotropy observations constrain the shape of the eastern slab edge and require non newtonian mantle rheology the incorporation of the non newtonian viscosity9 10 results in mantle viscosities of 1017 to 1018 pa s in regions of high strain rate 10 12 s 1 and this low viscosity enables the mantle flow field to decouple partially from the motion of the surface plates these results imply local rapid transport of geochemical signatures through subduction zones and that the internal deformation of slabs decreases the slab pull force available to drive subducting plates

arabia eurasia continental collision insights from late tertiary foreland basin evolution in the alborz mountains northern iran a poorly understood lag time of 15 20 m y exists between the initial arabia eurasia continental collision in late eocene to early oligocene time and the acceleration of tectonic and sedimentary processes across the collision zone in the early to late miocene the late eocene to miocene pliocene clastic and shallow marine sedimentary rocks of the kond eyvanekey and semnan basins in the alborz mountains northern iran offer the possibility to track the evolution of this orogen in the framework of collision processes a transition from volcaniclastic submarine deposits to shallow marine evaporites and terrestrial sediments occurred shortly after 36 ma in association with reversals in sediment provenance strata tilting and erosional unroofing these events followed the termination of subduction arc magmatism and marked a changeover from an extensional to a contractional regime in response to initiation of continental collision with the subduction of stretched arabian lithosphere this early stage of collision produced topographic relief associated with shallow foreland basins suggesting that shortening and tectonic loading occurred at low rates starting from the early miocene 17 5 ma flexural subsidence in response to foreland basin initiation occurred fast sediment accumulation rates and erosional unroofing trends point to acceleration of shortening by the early miocene we suggest that the lag time between the initiation of continental collision 36 ma and the acceleration of regional deformation 20 17 5 ma reflects a two stage collision process involving the soft collision of stretched lithosphere at first and hard collision following the arrival of unstretched arabian continental lithosphere in the subduction zone

subduction zone interaction controls on arcuate belts the interaction of neighboring subducting lithospheres is a characteristic feature of many tectonically complex areas here we use numerical modeling to study the interactions between two oppositely dipping adjacent subducting lithospheres and to understand their impact on the subduction evolution mantle flow and stress propagation through the mantle as slabs subduct rollback and approach they strongly affect each other if plate edges are at distances 600 km in this case the mantle flow around slabs combines into a single large convective cell trench migration is delayed and stress increases progressively with decreasing slabs distance the stress increases at depth as the slabs approach each other and it shallows up to the near surface as the slabs diverge a similar setting with two neighboring slabs migrating and passing each other is found in areas such as the alps apennines europe and the ryukyu manila subduction zones around taiwan where the arcuate trenches suggest the controls of deep slab interactions

conservation and redistribution of crust during the indo asian collision we evaluate the mass balance of the indo asian orogen by reconstructing the indian and asian margins prior to collision using recently published paleomagnetic and surface shortening constraints and subtracting modern crustal volumes derived from gravity inversions and deep seismic soundings results show a 30 deficit between original and modern orogen volumes if the average global crustal thickness of 41 km is assumed prior to collision even once eastward extrusion and crustal flow are considered such a large discrepancy requires crustal recycling of a magnitude that is greater than one half of the modern orogenic mass as others have previously suggested proposals for extensive high elevations prior to or soon after the collision further exacerbate this mismatch and dramatically increase the volume of material necessary to be placed into the mantle however we show that this discrepancy can be eliminated with a 23 29 km thick crust within the orogen prior to collision along with a thick southern tibet margin the lhasa and qiangtang terranes because of the relatively low magnitude of surface shortening in asia an initially thin crust would require underplating of indian crust in southern tibet and displacement of a highly mobile lower crust to the north and east in order to explain modern crustal thicknesses the contrast between a proposed thinner asian interior and older and thicker lithosphere of the north china block may have defined the distal extent of deformation at the time of collision and since

the predictability of extratropical cyclones nonlinear sensitivity to localized potential vorticity perturbations there is a range of phenomena which if misrepresented in a numerical weather prediction model could cause localized finite amplitude errors in the potential vorticity pv examples are convective systems the tropopause pv trough and orographic pv perturbations motivated by this the sensitivity of short range cyclone forecasts up to two days to localized finite amplitude initial pv anomalies is investigated using a method termed pv sensitivity mapping an initial pv anomaly perturbation is applied at each grid point in turn and the corresponding impact in a nonlinear cyclone development is computed in contrast to the adjoint method there is no linear approximation the pv sensitivity map depicts the forecast time impacts in energy and pressure for each initial spatial location of pv perturbation a nonlinear three dimensional quasi geostrophic model is used because it has pv as a prognostic variable is numerically efficient and simulates the essence of cyclogenesis the sensitivity maps are calculated for a dry and a moist cyclone scenario and for a synoptic scale and mesoscale initial pv perturbation the degree of nonlinearity in error growth is significant for the moist case the most sensitive regions for both cases are in the region of the steering level and the incipient cold and warm fronts a physical interpretation using pv thinking is given royal meteorological society 2003 a a white s contribution is crown copyright

crustal scale duplexing beneath the yarlung zangbo suture in the western himalaya the fate of the indian plate during collision with asia is debated seismic images of the western himalaya reveal large scale thrust faults that transfer indian crust upwards into the overriding asian plate

dynamic models of downgoing plate buoyancy driven subduction subduction motions and energy dissipation abstract it is much debated whether the forces associated with the downgoing plate the overriding plate passive or active mantle flow are dominant in controlling the paths of plates into the mantle we investigate the dynamics and energetics for a free subduction system driven solely by downgoing plate buoyancy using a finite element model of a viscoelastic plate with a free surface sinking into a passive unbounded mantle represented by drag forces parameters are varied to study effects of an asthenosphere ridge push and a passive overriding plate for a range of subducting plate viscosities and densities such a single free plate achieves subduction mainly through trench retreat most of the energy dissipation occurs in driving the passive mantle response as a result the slab s sinking velocity is its stokes velocity determined by lithospheric buoyancy and mantle viscosity the total subduction velocity and dip adjust to minimize bending dissipation in the lithosphere and are affected by slab rheology as well as buoyancy a low viscosity asthenosphere and ridge push facilitate plate advance increasing plate dips and lowering subduction velocity while suction and buoyancy of a work free passive overriding plate decreases plate dips thus increasing subduction and rollback velocities however the geometrical relation between the different parameters is the same in all model cases because the slabs sink according to their stokes velocity the free subduction models thus provide a reference that can be used to distinguish the signature of downgoing plate buoyancy from that of other driving forces in global compilations of subduction parameters

variation of structural style and basin evolution in the central zagros izeh zone and dezful embayment iran abstract structural style and its relationship to sedimentary facies and the evolution of sedimentary depocenters since late cretaceous times have been studied on the basis of one regional balanced transect and several updated isopach maps in the izeh zone and the dezful embayment central zagros this study relies on fieldwork data existing geological maps seismic data and well information a new structural classification for part of the zagros sedimentary cover is presented to highlight the different mechanical behavior of the formations in the stratigraphic column it shows the existence of several local decollement levels activated during folding these decollement levels separate lithotectonic units which accommodate shortening in different ways the lower paleozoic is the basal decollement level throughout the studied area triassic evaporites albian shales eocene marls and miocene evaporites can act as intermediate decollement levels and present variable facies in the central zagros lateral facies and thickness variations the sedimentary overburden and the close relationship with inherited fault patterns influenced the wavelength amplitude and style of folding in the study area furthermore surface structures do not necessarily coincide with deeper objectives where these disharmonic levels are involved in folding the evolution of sedimentary depocenters from the late cretaceous obduction episode until early miocene collision is described based on updated isopach maps and tectonic subsidence curves this analysis shows continuous compression and movements along n s and nw se trending faults in the zagros basement which predate the neogene zagros folding and influence sedimentation history it also indicates a southwestward migration of the depocenters through time in addition to the basement involvement below some folds during the zagros orogeny

mountain building in taiwan insights from 3 d geodynamic models taiwan is widely considered to be a typical example of an arc continent collision surrounded by two opposite dipping subduction zones the manner by which the interaction of the two neighboring slabs caused plate collision and mountain building is insufficiently understood various hypotheses have been proposed but the geodynamic feasibility of those remains to be tested here we present 3 d thermomechanical models to study the geodynamic evolution process of a taiwan like setting after an initial transform fault was consumed in our model setup the boundary between the eurasian plate and the south china sea is northeast trending the results show that all simulations result in toroidal mantle flow around the slab edges and that slab breakoff as well as a small scale mountain belt with high topography and crustal exhumation occurs in most cases the eurasian continental crust is exhumed in a dome like manner exposing higher grade metamorphic rocks facilitated by high erosion rates and a weak continental lower crust rheology but inhibited by the presence of a weak arc a high topography within the orogen as well as continental slab detachment can develop for the convergence direction of n307 degrees and large convergence rates our modeling results are thus generally consistent with the eurasian slab tearing model proposed for taiwan based on seismic tomographic studies and we suggest that the main characteristic features in taiwan can be explained by the combined effects of fast erosion a weak lower crust fast convergence and a small convergence azimuth

cenozoic exhumation of the internal zagros first constraints from low temperature thermochronology and implications for the build up of the iranian plateau the iranian plateau is a flat 1 5 2 km high plateau thought to result from the collision between the arabian and eurasian plates since 30 5 ma and may represent a young analogue to the so far better studied tibetan plateau in order to constrain the exhumation history of the internal zagros and of the iranian plateau we herein present apatite u th he ahe and apatite aft and zircon fission track zft data on plutonic rocks from the sanandaj sirjan zone ssz urumieh dokhtar magmatic arc udma central iran and kopet dagh thermochronologic data show that the ssz was exhumed early in the collision process essentially before 25 20 ma with a likely acceleration of cooling during the late eocene from 0 04 to 0 3 mm year results suggest that cooling of the internal zagros migrated from the ssz to the udma during a more mature stage of the continental collision after 17 ma i e coeval with the outward propagation of deformation and topography fronts in the external zagros constant exhumation rates in the udma 0 3 mm year suggest that no significant variation of erosion rates occurred since the onset of continental collision in central iran the overlap of zft aft and ahe ages from gneissic samples points to rapid cooling during the late eocene 42 c myr which is consistent with previous reports on the formation of eocene metamorphic core complexes

zagros orogeny a subduction dominated process this paper presents a synthetic view of the geodynamic evolution of the zagros orogen within the frame of the arabia eurasia collision the zagros orogen and the iranian plateau preserve a record of the long standing convergence history between eurasia and arabia across the neo tethys from subduction obduction processes to present day collision from 150 to 0 ma we herein combine the results obtained on several geodynamic issues namely the location of the oceanic suture zone the age of oceanic closure and collision the magmatic and geochemical evolution of the eurasian upper plate during convergence as testified by the successive sanandaj sirjan kermanshah and urumieh dokhtar magmatic arcs the p t t history of the few zagros blueschists the convergence characteristics across the neo tethys kinematic velocities tomographic constraints subduction zones and obduction processes together with a survey of recent results gathered by others we provide lithospheric scale reconstructions of the zagros orogen from 150 to 0 ma across two sw ne transects the evolution of the zagros orogen is also compared to those of the nearby turkish and himalayan orogens in our geotectonic scenario for the zagros convergence we outline three main periods regimes 1 the mid to late cretaceous 115 85 ma corresponds to a distinctive period of perturbation of subduction processes and interplate mechanical coupling marked by blueschist exhumation and upper plate fragmentation 2 the paleocene eocene 60 40 ma witnesses slab break off major shifts in arc magmatism and distributed extension within the upper plate and 3 from the oligocene onwards 30 0 ma collision develops with a progressive sw migration of deformation and topographic build up sanandaj sirjan zone 20 15 ma high zagros 12 8 ma simply folded belt 5 0 ma and with partial slab tear at depths 10 ma to present our reconstructions underline the key role played by subduction throughout the whole convergence history we finally stress that such a long lasting subduction system with changing boundary conditions also makes the zagros orogen an ideal natural laboratory for subduction processes

predicting trench and plate motion from the dynamics of a strong slab the motion of oceanic plates is commonly related to the subduction of cold and dense oceanic material into the mantle these models predict plate velocities from subduction velocities but the trench motion is not directly included in the computation here using a recent compilation of a global data set we found that the motion of trenches either advancing or retreating with respect to upper plates scales with their corresponding subducting plates motion based on simple experimental tests we found that subduction of strong slabs inside the upper mantle correctly predicts these kinematic relationships we deduce that the motion of the trenches represent the surface manifestation of the resistance encountered by the subducting lithosphere to bend and penetrate within the upper mantle

new results from physical modelling of arc continent collision in taiwan evolutionary model abstract both the analysis of the geological data and the results from 2d and 3d experimental modelling of arc continent collision in taiwan reveal the following major stages in this process 1 failure and thickening of the subducting crust of the chinese continental margin under the frontal part accretionary prism of the philippine sea plate psp since ca 6 7 ma 2 uplift squeezing and exhumation of the subducted crustal slice of the chinese margin and initiation of the central range about 5 ma ago 3 flexural buckling subsidence of the luzon arc fore arc under strong horizontal compression and sedimentary infilling of this area by the material shed from the growing and exhuming central range 4 failure of the psp along the west vergent longitudinal valley fault dipping beneath the arc and closure of the fore arc 5 subduction reversal and stoppage of continental subduction which has already occurred in northern taiwan and propagates to the south the southward propagation of the collision is not stationary currently occurring initial collision in southern taiwan and offshore to the south does not follow the same described above evolution as in central and northern taiwan the evolution of the collision in time and space should be considered in the framework of a 3d model where the psp rupture is preparing in the central and northern taiwan during a long time passing through a number of evolutionary stages once it has occurred the rupture longitudinal valley fault rapidly propagates to the south into the domain which did not undergo the pre rupture evolution the rupture front is currently located near 21 30 n

the 12 november 2017 mw 7 3 ezgeleh sarpolzahab iran earthquake and active tectonics of the lurestan arc

seismological constraints on the crustal structure beneath the zagros mountain belt iran summary the zagros mountain belt of western iran results from the collision of the arabian and central iran continental blocks the stage of the collision is unclear and the crustal structure of the zagros is rather poorly known in this study we investigate the velocity structure of the crust and upper mantle beneath the ghir region located in the central zagros using data collected by a temporary local seismological network including a broad band instrument the structures of the sedimentary cover and the upper crystalline crust are estimated from the inversion of p and s traveltimes of local earthquakes recorded on a dense seismological network the upper crust consists of an 11 km thick sedimentary layer v p 4 70 km s 1 above a 8 km thick upper crystalline crust v p 5 85 km s 1 the velocity of the lower crust and the depth of the moho are found using receiver function analysis of teleseismic earthquakes the lower crystalline crust is unusually slow v p 6 5 km s 1 and 27 km thick the upper bound for the total crustal thickness beneath the ghir region is 46 2 km a comparison of the thickness of the crystalline crust of the zagros with available information for the thickness of the crystalline crust of the arabian platform shows that at present the zagros has a thinner crust the current crustal thickness beneath the zagros is comparable to the pre collision crustal thickness of the arabian platform suggesting that the zagros is now in a very early stage of continental collision

subduction polarity reversal at the junction between the western alps and the northern apennines italy abstract the controversial relationship between the orogenic segments of the western alps and the northern apennines is here explored integrating recently published 3d tomographic models of subduction with new and re interpreted geological observations from the eclogitic domain of the voltri massif ligurian alps italy where the two belts joint each other the voltri massif is here described as an extensional domain accommodating the opposing outward migration of the alpine and apennine thrust fronts since about 30 35 ma using tomographic images of the upper mantle and paleotectonic reconstructions we propose that this extensional setting represents the surface manifestation of an along strike change in polarity of the subducted oceanic slab whose polarity changed laterally in space and in time our tectonic model suggests that the westward shift of the alpine thrust front from the oligocene onward was the consequence of the toroidal asthenospheric flow induced by the retreat of the apenninic slab

crustal mass budget and recycling during the india asia collision abstract the long lasting collision between india and asia has resulted in crustal and lithospheric deformation of both continents providing unequalled opportunities to evaluate their long term mechanisms of deformation we have quantified a crustal mass budget for the collision by comparing the present day and initial crustal volumes of the indian and asian continents involved in the collision initial crustal thickness was estimated from the non deformed parts of the continents whereas their initial extent was mapped from their contours at the onset of collision using seismic tomography we quantify the portion of the initial crustal thickness of both continents stored within the currently thickened crust or redistributed due to extrusion for the indian continent this portion amounts to about 19 km far lower than the mean observed present day crustal thickness of the indian craton of about 38 km we conclude that between 40 and 50 of indian crust has been recycled into the mantle by continental subduction corresponding to a decoupling level at about 15 19 km depth for the asian continent the estimated crustal thickness stored during collision is about 33 km close to the initial asian crustal thickness we estimate that only 3 of the asian crust was recycled into the mantle this corresponds to one episode of continental subduction occurring most probably soon after the initiation of collision along the bangong suture we identify the related slab using seismic tomography the strong contrast between india and asia implies an age dependent capacity of the continental crust to be recycled into the mantle this result has to be taken into account for further analysis of global crustal recycling during earth s history

a re assessment of focal depth distributions in southern iran the tien shan and northern india do earthquakes really occur in the continental mantle summary we investigate the depth distribution of earthquakes within the continental lithosphere of southern iran the tien shan and northern india by using synthetic seismograms to analyse p and sh body waveforms in the zagros mountains of southern iran earthquakes are apparently restricted to the upper crust depths of 20 km whereas in the tien shan and northern india they occur throughout the thickness of the continental crust to depths of 40 45 km we find no convincing evidence for earthquakes in the continental mantle of these regions in spite of previous suggestions to the contrary and question whether seismicity in the continental mantle is important in any part of the world in some regions such as iran the aegean tibet and california seismicity is virtually restricted to the upper continental crust whereas in others including parts of east africa the tien shan and northern india the lower crust is also seismically active although usually less so than the upper crust such variations cannot reliably be demonstrated from published catalogue or bulletin locations even from ones in which depth resolution is generally improved in contrast to the oceanic mantle lithosphere in which earthquakes certainly occur the continental mantle lithosphere is we suggest virtually aseismic and may not be significantly stronger than the lower continental crust these variations in continental seismogenic thickness are broadly correlated with variations in effective elastic thickness suggesting that the strength of the continental lithosphere resides in the crust and require some modification to prevalent views of lithosphere rheology

spontaneous development of arcuate single sided subduction in global 3 d mantle convection models with a free surface we present temporally evolving 3 d global mantle convection models with single sided subduction and a free surface in both 3 d cartesian and fully spherical geometry special focus is given to the spontaneous development of three dimensional structures at the surface and in the upper mantle we find that an arcuate shape is the natural form for trenches and slabs cartesian models are used first to study the dynamic evolution of subduction zones spreading ridges and interconnected transform features these experiments highlight the strong variation of spontaneously developing arcuate slab curvature and subduction polarity along the trench strike the spontaneous development of spreading ridges leads to lateral offsets between separated segments that are characterized by normal transform motion spherical models then allow insights into the evolution of plate tectonics on a sphere investigated are the spontaneous evolution of slab geometry trench motion and subduction induced mantle flow two new dynamical features are discovered back slab spiral flow and slab tunneling

delamination vs break off the fate of continental collision 1 the fate of a convergent continental margin is investigated we perform a set of 2d numerical models to study how and why continental collision can evolve in different scenarios since the rheology of continental lithosphere has a major control on the dynamics of subduction we explore a range of different lithosphere and lower crust viscosity values to understand their sensitivity on the possible scenarios we find that with a rheologically layered crust both delamination and break off are feasible we identify three modes 1 slab detachment in which the lithospheric mantle and the crust are strongly coupled subduction slows down and the slab eventually breaks 2 delamination of the lithospheric mantle that separates from the crust and continue to subduct and 3 an intermediate mode where the lithospheric mantle and the crust remain partially coupled resulting in an initial stage of delamination followed by the slow down and cessation of subduction

retrodeforming the arabia eurasia collision zone age of collision versus magnitude of continental subduction the arabia eurasia collision has been linked to global cooling the slowing of africa mediterranean extension the rifting of the red sea an increase in exhumation and sedimentation on the eurasian plate and the slowing and deformation of the arabian plate collision age estimates range from the late cretaceous to pliocene with most estimates between 35 and 20 ma we assess the consequences of these collision ages on the magnitude and location of continental consumption by compiling all documented shortening within the region and integrating this with plate kinematic reconstructions shortening estimates across the orogen allow for 350 km of neogene upper crustal contraction necessitating collision by 20 ma a 35 ma collision requires additional subduction of 400 600 km of arabian continental crust using the oman ophiolite as an analogue ophiolitic fragments preserved along the zagros suture zone permit 180 km of subduction of the arabian continental margin plus overlying ophiolites wholesale subduction of this more dense continental margin plus ophiolites would reconstruct 400 500 km of postcollisional arabia eurasia convergence consistent with a ca 27 ma initial collision age this younger arabia eurasia collision suggests a noncollisional mechanism for the slowing of africa and associated extension

thermal histories from the central alborz mountains northern iran implications for the spatial and temporal distribution of deformation in northern iran we integrate new and existing thermochronological geochronological and geologic data from the western and central alborz mountains of iran to better constrain the late cenozoic tectonic evolution of northern iran in the context of the arabia eurasia collision new data are presented for two granitic plutons north of the alborz range crest additional new apatite u th he data are also presented for volcanic intrusive and detrital apatite grains from two transects south of the range crest our most definitive results include zircon and apatite u th he and limited k feldspar 40 ar 39 ar thermal history data from the cretaceous ca 98 ma nusha pluton that reveal that the alborz basement underwent generally slow denudation 0 1 km m y as late as 12 ma with more accelerated exhumation 0 45 km m y that likely began shortly after 12 ma the lahijan pluton a late neoproterozoic cambrian basement exposure near the caspian shore records apatite u th he closure at 17 13 ma additional u th he results from detrital apatites sampled along two separate horizontal transects all consistently yielded latest miocene to pliocene apparent ages that imply that even supracrustal cover rocks within the alborz have undergone significant regionally extensive exhumation overall our data are consistent with 5 km of regionally extensive denudation since ca 12 ma the onset of rapid exhumation in the alborz at ca 12 ma appears to be consistent with other timing estimates that place the onset of the arabia eurasia collision between 14 and 10 ma

timing of arabia eurasia collision constraints from restoration of crustal scale cross sections abstract the timing of arabia eurasia collision is crucial to the understanding of evolution of the zagros orogen as well as the himalaya tibet orogeny but it remains an issue of considerable controversy drastically different estimates ranging from 65 to 5 ma have been proposed for this issue although many workers prefer to consider that the arabia eurasia collision initiated at 36 20 ma most of the tectonic responses to the collision occurred after 20 ma in this paper we reconstruct the pre collisional arabian northeastern and eurasian southwestern boundaries based on the restoration of ten crustal scale cross sections across the main body of the arabia eurasia collision system from which together with the positions of arabia relative to eurasia during the cenozoic we infer the timing of arabia eurasia collision our work shows that the arabia eurasia collision started no earlier than 18 3 1 2 ma and that since the collision at least 152 30 km wide arabian crust has underthrust eurasia and eurasian crust has been shortened for at least 37 km this may provide a reasonable explanation for why most of the tectonic responses to the collision occurred after 20 ma

subduction dynamics and the origin of andean orogeny and the bolivian orocline the building of the andes results from the subduction of the oceanic nazca plate underneath the south american continent however how and why the andes and their curvature the bolivian orocline formed in the cenozoic era 65 5 million years myr ago to present despite subduction continuing since the mesozoic era 251 0 65 5 myr ago is still unknown three dimensional numerical subduction models demonstrate that variations in slab thickness arising from the nazca plate s age at the trench produce a cordilleran morphology consistent with that observed the age dependent sinking of the slab in the mantle drives traction towards the trench at the base of the upper plate causing it to thicken thus subducting older nazca plate below the central andes can explain the locally thickened crust and higher elevations here we demonstrate that resultant thickening of the south american plate modifies both shear force gradients and migration rates along the trench to produce a concave margin that matches the bolivian orocline additionally the varying forcing along the margin allows stress belts to form in the upper plate interior explaining the widening of the central andes and the different tectonic styles found on their margins the eastern and western cordilleras the rise of the central andes and orocline formation are directly related to the local increase of nazca plate age and an age distribution along the margin similar to that found today the onset of these conditions only occurred in the eocene epoch this may explain the enigmatic delay of the andean orogeny that is the formation of the modern andes

imaging the indian subcontinent beneath the himalaya new seismic images reveal the geometry of the indian plate as it passes beneath the himalayan plateau several important subsurface features of vanished india are revealed including a shear zone above the base of the indian crust beneath the himalaya and conversion of the deep indian crust into denser rock the rocks of the indian subcontinent are last seen south of the ganges before they plunge beneath the himalaya and the tibetan plateau they are next glimpsed in seismic reflection profiles deep beneath southern tibet1 yet the surface seen there has been modified by processes within the himalaya that have consumed parts of the upper indian crust and converted them into himalayan rocks2 3 the geometry of the partly dismantled indian plate as it passes through the himalayan process zone has hitherto eluded imaging here we report seismic images both of the decollement at the base of the himalaya and of the moho the boundary between crust and mantle at the base of the indian crust a significant finding is that strong seismic anisotropy develops above the decollement in response to shear processes that are taken up as slip in great earthquakes at shallower depths north of the himalaya the lower indian crust is characterized by a high velocity region consistent with the formation of eclogite a high density material whose presence affects the dynamics of the tibetan plateau

building the zagros collisional orogen timing strain distribution and the dynamics of arabia eurasia plate convergence he zagros mountains are the result of the arabia eurasia collision initiated at 35 ma as the rifted arabian lithosphere was underthrusted beneath the iranian plate due to its negative buoyancy the onset of crustal thickening started at 25 ma as recorded by the hinterland exhumation and foreland clastic deposition deformation throughout the arabia eurasia collision zone and the uplift of the iranian plateau occurred after 15 12 ma as a result of shortening thickening of the thin iranian crust we emphasize that only 42 of the post 35 ma convergence is partitioned by shortening within central iran tomographic constraints show ongoing slab steepening or breakoff in the nw zagros whereas underthrusting of the arabian plate is observed beneath central zagros the current subduction dynamics can be explained by the original lateral difference in the buoyancy of the distal margin that promoted slab sinking in nw zagros and underthrusting in central zagros critical wedge approach applied to the zagros favors the hypothesis of strong brittle crust detached above a viscous lower crust in contrast the weak sedimentary cover deforms by buckling of a thick multilayered cover thrust faulting associated with folding occurs in the competent layers and is responsible for most of the earthquakes there is evidence that the role of the slab pull force in driving the arabian plate motion was reduced after 12 ma large scale mantle flow induced by mantle upwelling at the afar plume appears to be the main driver of the arabia plate motion we stress that the main kinematic change in the zagros region occurred at 15 12 ma as the zagros uplifted before the arabian slab detached the zagros appears key to investigate coupling between continental rheology plate driving forces and mountain building in which the role of rift inheritance appears to be central

counterclockwise rotation of the western alps since the oligocene new insights from paleomagnetic data 1 a paleomagnetic study of penninic units in the southern part of the alpine arc has been carried out more than 200 samples 23 sites were collected in brianconnais ammonitico rosso limestones of the high ubaye valley and the ligurian alps a characteristic component of magnetization of reverse polarity was isolated on most of the sites this component does not pass the fold test and is interpreted as a tertiary overprint related to alpine metamorphism mean directions in geographic coordinates are d 121 i 52 95 11 and d 72 i 48 95 15 for the ubaye and the liguria localities respectively indicating large counterclockwise rotations about vertical axis of 68 and 117 relative to stable europe these rotations in agreement with a previous study conducted in the briancon area together with other paleomagnetic data from the western alps show that the internal alps suffered a large but nonhomogeneous counterclockwise rotation since the oligocene the rotations are in agreement with the combination of earlier separately proposed processes the rotation of the adriatic plate accounts for about 25 of rotation remaining rotation and southward gradient would be related to left lateral shear accommodating the displacement between adria and europe at the southern border of the western alps furthermore the southward extrusion of the western alps south of the simplon fault zone may account for up to 10 of rotation rotations appear therefore as a major process accommodating deformation in the western alps since the oligocene

influence of trench width on subduction hinge retreat rates in 3 d models of slab rollback subduction of tectonic plates limited in lateral extent and with a free trailing tail i e free subduction is modeled in a three dimensional 3 d geometry the models use a nonlinear viscoplastic rheology for the subducting plate and exhibit a wide range of behaviors depending on such plate characteristics as strength width and thickness we investigate the time evolution of this progressive rollback subduction measure the accompanying return flow in the upper mantle and quantify the plate kinematics due to the 3 d geometry flow is allowed to accompany slab rollback around the lateral edges of the slab the toroidal component as opposed to 2 d geometry where material is forced to flow underneath the slab tip the poloidal component a simple force balance is provided which relates the speed of backward trench migration to the resistive forces of generating flow and weakening the plate our results indicate most of the gravitational energy of the system i e the negative buoyancy of the subducting slab is converted into a toroidal flow 69 a much smaller amount goes into weakening the plate 18 and the remaining amount goes into driving flow parallel to displacement of the slab 13 for the trench widths w we investigate 1500 km a maximum trench retreat rate occurs for trenches 600 km wide which is attributed to the interaction between a plate of finite width and the induced flow which has a lengthscale in the horizontal direction these numerical results quantitatively agree with comparable 3 d laboratory experiments using analogue models with a purely viscous plate material schellart 2004a 2004b including correlations between increasing retreat rate with increasing plate thickness trench width for maximum retreat rate 500 km and estimated amount of slab buoyancy used to drive rollback induced flow 70 several implications for plate tectonics on earth result from these models such as rollback subduction providing a physical mechanism for ephemeral slab graveyards situated above the more viscous lower mantle and endothermic phase transition prior to a flushing event into the lower mantle mantle avalanche

large scale subduction of continental crust implied by india asia mass balance calculation continental crust is buoyant compared with its oceanic counterpart and resists subduction into the mantle when two continents collide the mass balance for the continental crust is therefore assumed to be maintained here we use estimates of pre collisional crustal thickness and convergence history derived from plate kinematic models to calculate the crustal mass balance in the india asia collisional system using the current best estimates for the timing of the diachronous onset of collision between india and eurasia we find that about 50 of the pre collisional continental crustal mass cannot be accounted for in the crustal reservoir preserved at earth s surface today represented by the mass preserved in the thickened crust that makes up the himalaya tibet and much of adjacent asia as well as southeast asian tectonic escape and exported eroded sediments this implies large scale subduction of continental crust during the collision with a mass equivalent to about 15 of the total oceanic crustal subduction flux since 56 million years ago we suggest that similar contamination of the mantle by direct input of radiogenic continental crustal materials during past continent continent collisions is reflected in some ocean crust and ocean island basalt geochemistry the subduction of continental crust may therefore contribute significantly to the evolution of mantle geochemistry buoyant continental crust is thought to resist subduction calculation of the crustal mass balance during the collision between india and eurasia indicates that about 50 of pre collisional continental crust has been subducted into the mantle

slab break offs in the alpine subduction zone after the onset of plate collision in the alps at 32 34 ma the deep structure of the orogen is inferred to have changed dramatically european plate break offs in various places of the alpine arc as well as a possible reversal of subduction polarity in the eastern alps have been proposed we review different high resolution tomographic studies of the upper mantle and combine shear and body wave models to assess the most reliable geometries of the slabs several hypotheses for the tectonic evolution are presented and tested against the tomographic model interpretations and constraints from geologic and geodetic observations we favor the interpretation of a recent european slab break off under the western alps in the eastern alps we review three published scenarios for the subduction structure and propose a fourth one to reconcile the results from tomography and geology we suggest that the fast slab anomalies are mainly due to european subduction adriatic subduction plays no or only a minor role along the tauern window sections possibly increasing towards the dinarides the apparent northward dip of the slab under the eastern alps may be caused by imaging a combination of adriatic slab from the dinaric subduction system and a deeper lying european one as well as by an overturned retreating european slab

comparisons of the kinematics and deep structures of the zagros and himalaya and of the iranian and tibetan plateaus and geodynamic implications 1 we compare the geologic histories the deep structures and the present day kinematics of deformation of the himalaya and the adjacent tibetan plateau with those of the zagros and iranian plateau to test geodynamic processes of continental collision shortly after india and arabia collided with eurasia horizontal shortening manifested itself by folding and thrust faulting of sedimentary rock detached from india s and arabia s underlying crystalline basement subsequently slip on thrust faults stacked slices of india s basement to build the himalaya on india s northern margin such faulting has not yet developed in the zagros where collision is more recent and arabia penetrates into eurasia more slowly than india does so that postcollision convergence with eurasia is less the greater elevation thicker crust and more marked heterogeneity of the upper mantle beneath the tibetan than beneath the iranian plateau also reflect a more advanced stage of development moreover while thrust or reverse faulting and crustal shortening continue on the margins of both plateaus normal faulting suggesting horizontal extension and crustal thinning occurs within tibet but not in iran hence the balance of forces that built the high tibetan plateau must have changed apparently some time since 15 ma removal of tibetan mantle lithosphere could have altered that balance if mantle lithosphere beneath the iranian plateau has been removed however the change in force balance has been too small to initiate normal faulting low seismic wave speeds in the uppermost mantle just beneath the moho of both plateaus suggest to us that lithosphere beneath both is thin consistent with late cenozoic removal of it but alternative explanations might account for these low speeds despite its apparently thin and hence presumably weak mantle lithosphere much of central iran undergoes little deformation it illustrates how a crustal block can behave rigidly not necessarily because it is strong but because deviatoric stresses can be small whereas differences between the two regions clearly depend on the amount that arabia and india have penetrated into eurasia which scales with both the dates of collision and rates of convergence we see no differences in the operative processes that depend on the present day rates of convergence

early neogene foreland of the zagros implications for the initial closure of the neo tethys and kinematics of crustal shortening we study the transition from passive margin to foreland basin sedimentation now exposed in the high zagros belt to provide chronological constraints on the initial stage of arabia eurasia collision and closure of the neo tethys we performed magnetostratigraphy and strontium isotope stratigraphy along two sections near the zagros suture which expose the oldest preserved foreland deposits the shalamzar section in the west and the dehmoord section in the east the top of the passive margin asmari formation has an age of 28 29 ma in the high zagros and is overlain by foreland deposits with a major basal unconformity representing 7 myr of hiatus the base of the foreland deposits has an age of 21 5 ma at dehmoord and ca 26 ma at shalamzar the sedimentation rate increased from 30 m myr in the passive margin to 247 m myr in the foreland combined with available age constraints across the zagros our results show that the unconformity is diachronous and records the southwestward migration of the flexural bulge within the arabian plate at an average rate of 24 2 mm yr over the last 27 ma the time evolution of sediment accumulation in the zagros foreland follows the prediction from a flexural model as the foreland is thrust beneath the orogenic wedge and loaded by the wedge and basin fill we detect the onset of forebulge formation within the asmari formation around 25 ma we conclude that closure of the neo tethys formed the zagros collisional wedge at 27 2 ma hence the arabia eurasia collision was probably not the main driver of global cooling which started near the eocene oligocene boundary ca 33 7 ma we estimate 650 km of forebulge migration since the onset of the collision which consists of 350 km of shortening across the orogen and 300 km of widening of the wedge and increasing flexural rigidity of arabia we conclude the average rate of shortening across the zagros to be ca 13 mm yr over the last 27 myr a value comparable to the modern rate palinspastic restoration of structural cross sections and crustal volume conservation comprise only ca 200 km of shortening across the zagros and metamorphic sanandaj sirjan belt implying that at least 150 km of the arabian crust was underthrust beneath eurasia without contributing to crustal thickening possibly due to eclogitization

slab rollback orogeny model a test of concept buoyancy forces associated with subducting lithosphere control the dynamics of convergent margins in the post collisional stage these forces are significantly reduced yet mountain building and seismicity are ongoing albeit at lower rates we leverage advances of a newly developed seismo thermo mechanical modeling approach to simulate tectonic and seismicity processes in a self driven subduction and continental collision setting we demonstrate that the rearrangement of forces due to slab breakoff in the post collisional stage causes bending and rollback of the residual slab suction forces and mantle traction at the base of the upper plate while stress coupling transfers to the shallow crust our results provide an explanation for the post collisional evolution of the central alps where the so called slab rollback orogeny models explains the slow yet persistent upper plate advance the height of the mountain range and a seismicity pattern consistent with the different tectonic regimes throughout the orogen

revised age of proximal deposits in the zagros foreland basin and implications for cenozoic evolution of the high zagros the regionally extensive coarse grained bakhtiyari formation represents the youngest synorogenic fill in the zagros foreland basin of iran the bakhtiyari is present throughout the zagros fold thrust belt and consists of conglomerate with subordinate sandstone and marl the formation is up to 3000 m thick and was deposited in foredeep and wedge top depocenters flanked by fold thrust structures although the bakhtiyari concordantly overlies miocene deposits in foreland regions an angular unconformity above tilted paleozoic to miocene rocks is expressed in the hinterland high zagros the bakhtiyari formation has been widely considered to be a regional sheet of pliocene pleistocene conglomerate deposited during and after major late miocene pliocene shortening it is further believed that rapid fold growth and bakhtiyari deposition commenced simultaneously across the fold thrust belt with limited migration from hinterland ne to foreland sw thus the bakhtiyari is generally interpreted as an unmistakable time indicator for shortening and surface uplift across the zagros however new structural and stratigraphic data show that the most proximal bakhtiyari exposures in the high zagros south of shahr kord were deposited during the early miocene and probably oligocene in this locality a coarse grained bakhtiyari succession several hundred meters thick contains gray marl limestone and sandstone with diagnostic marine pelecypod gastropod coral and coralline algae fossils foraminiferal and palynological species indicate deposition during early miocene time however the lower miocene marine interval lies in angular unconformity above 150 m of bakhtiyari conglomerate that in turn unconformably caps an oligocene marine sequence these relationships attest to syndepositional deformation and suggest that the oldest bakhtiyari conglomerate could be oligocene in age the new age information constrains the timing of initial foreland basin development and proximal bakhtiyari deposition in the zagros hinterland these findings reveal that structural evolution of the high zagros was underway by early miocene and probably oligocene time earlier than commonly envisioned the age of the bakhtiyari formation in the high zagros contrasts significantly with the pliocene quaternary bakhtiyari deposits near the modern deformation front suggesting a long term 20 myr advance of deformation toward the foreland

present day crustal deformation and plate kinematics in the middle east constrained by gps measurements in iran and northern oman summary a network of 27 gps sites was implemented in iran and northern oman to measure displacements in this part of the alpine himalayan mountain belt we present and interpret the results of two surveys performed in 1999 september and 2001 october gps sites in oman show northward motion of the arabian plate relative to eurasia slower than the nuvel 1a estimates e g 22 2m m yr 1 at n8 5 e instead of 30 5 mm yr 1 at n6 e at bahrain longitude we define a gps arabia eurasia euler vector of 27 9 0 5 n 19 5 1 4 e 0 41 0 1 myr 1 the arabia eurasia convergence is accommodated differently in eastern and western iran east of 58 e most of the shortening is accommodated by the makran subduction zone 19 5 2m m yr 1 and less by the kopet dag 6 5 2m m yr 1 west of 58 e the deformation is distributed in separate fold and thrust belts at the longitude of tehran the zagros and the alborz mountain ranges accommodate 6 5 2m m yr 1 and 8 2m m yr 1 respectively the right lateral displacement along the main recent fault in the northern zagros is about 3 2m m yr 1 smaller than what was generally expected by contrast large rightlateral displacement takes place in northwestern iran up to 8 mm yr 1 the central iranian block is characterized by coherent plate motion internal deformation 2m m yr 1 sites east of 61 e show very low displacements relative to eurasia the kinematic contrast between eastern and western iran is accommodated by strike slip motions along the lut block to the south the transition zone between zagros and makran is under transpression with right lateral displacements of 11 2m m yr 1

effect of spatial distribution of hormuz salt on deformation style in the zagros fold and thrust belt an analogue modelling approach scaled analogue models of thin skinned simultaneous shortening above adjacent viscous and frictional decollements simulate the effect of hormuz salt on the shortening in the zagros fold and thrust belt the models consisted of sand layers that partly overlay a viscous layer of silicone and were shortened from one end spatial distribution of the viscous decollement varied along strike and dip as occurs in part of the zagros fold and thrust belt in this belt phanerozoic sedimentary cover was shortened partly above the hormuz salt lying on the precambrian crystalline basement behaving as a basal viscous decollement model results display how the nature of the decollement affects the evolution of an orogenic belt using model results we explain the development of deflection zones and discuss strain partitioning formation of different topographic wedges and differential sedimentation along the zagros fold and thrust belt model results suggest the formation of a gentle taper consisting of both foreward and backward thrusts above a viscous decollement and a relatively steeper taper consisting only of forward vergent imbricates above a frictional decollement however in our models the steepest wedge with the highest topography formed where the viscous substrate had a limited extent with a transitional boundary pinch out perpendicular to the shortening direction shortening of this boundary led to development of frontal ramps associated with significant uplift of the area behind the deformation front

arabia eurasia collision and the forcing of mid cenozoic global cooling the end of the eocene greenhouse world was the most dramatic phase in the long term cooling trend of the cenozoic era here we show that the arabia eurasia collision and the closure of the tethys ocean gateway began in the late eocene at 35 ma up to 25 million years earlier than in many reconstructions we suggest that global cooling was forced by processes associated with the initial collision that reduced atmospheric co2 these are 1 waning volcanism across southwest asia 2 increased organic carbon storage in paratethyan basins e g black sea and south caspian 3 increased silicate weathering in the collision zone and 4 a shift towards modern patterns of ocean currents associated with increased vigour in circulation and organic productivity

does subduction polarity changes below the alps inferences from analogue modelling the surface expression of a lateral polarity change of continental mantle lithosphere subduction has been studied by using lithosphere scale physical models key parameters investigated were the degree of lateral coupling between adjacent domains of opposing subduction polarity the width of the zone separating the domains and the lithosphere geometry and rheology the model results illustrate an asymmetric lithospheric structure induced by deformation of the downgoing plates which have been separated by a narrow transition zone a wide and symmetric orogenic wedge overlying a region of thickened mantle lithosphere and hampered subduction characterizes this transition zone in addition interaction between the neighboring subduction domains caused downbending of the upper plates and resulted in the lateral termination of crustal structures and lowering of surface topography the lateral extent of interaction between the domains strongly depends on the degree of coupling between the domains the rheology of the mantle lithosphere and the amount of bulk shortening the modelling results have major implications on the interpretation of seismic and tomographic data from the european alps in terms of the crust and lithosphere geometries it appears that an observed lateral change of subduction polarity at mantle depth can explain the variations of wedge build up between the western central and eastern alps

oligocene uplift of the western greater caucasus an effect of initial arabia eurasia collision the greater caucasus is europe s largest mountain belt significant uncertainties remain over the evolution of the range largely due to a lack of primary field data this work demonstrates that depositional systems within the oligocene early miocene maykop series on either side of the western greater caucasus wgc display a similar provenance and divergent palaeocurrents away from the range constraining a minimum age for the subaerial uplift of the range as early early oligocene an eocene oligocene hiatus basal oligocene olistostromes and a marked increase in nannofossil reworking also point to initial deformation in the earliest oligocene the initial uplift of the wgc occurred during the final assembly of the tethysides to its south uplift commenced after the late eocene final suturing of northern neotethys and during the initial collision of arabia with the southern accreted margin of eurasia this suggests that compressional deformation was rapidly transferred across the collision zone from the indenting arabian plate to its northern margin

the structure and kinematics of the southeastern zagros fold thrust belt iran from thin skinned to thick skinned tectonics 1 we present the first complete balanced cross section across the southeastern zagros fold thrust belt zftb the following main structural features emerge from this section 1 in the south of the zftb the proterozoic to recent sedimentary sequence has been decoupled from its panafrican basement along the ductile basal evaporites and folded into a series of large detachment anticlines ongoing shortening of these structures has resulted in migration of the basal salt layers into the cores of the anticlines and propagation of forelimb thrusts 2 in the north of the zftb deep seated ramps have folded the hanging wall rocks and produced imbrications and duplex structures within the higher levels of the sedimentary sequence 3 out of sequence thrusts linked to major seismogenic basement faults have cut through the structures in the cover of the zftb a three step incremental restoration of the section shows that two main phases of deformation can be separated in the tectonic evolution of the zftb a mio pliocene thin skinned phase in the course of which most of the structures in the cover were generated followed by a pliocene to recent thick skinned phase expressed as out of sequence faulting in the cover which is currently underlined by the seismicity within the basement in plan view the initial structures of the southeastern zftb developed with a curved shape essentially controlled by the shape and thickness of the underlying proterozoic salt basin i e the jura style in the following basement involved phase out of sequence thrusts cut at oblique angles through the preexisting structures of the cover the total shortening absorbed in the cover amounts to at least 45 km corresponding to a ratio of 22

new views on earthquake faulting in the zagros fold and thrust belt of iran the zagros fold and thrust belt is amongst the world s most seismically active mountain ranges and is influential in our understanding of continental collisions the presence of hormuz salt at the base of the 10 km thick folded sedimentary cover together with the general absence of coseismic surface rupturing have led to an assumption that earthquakes are concentrated within the basement here we reinterpret the seismicity of the zagros in light of insar studies that show some recent earthquakes ruptured the competent group of mechanically strong strata in the lower sedimentary cover local network data demonstrate that microseismicity occurs within the basement reaching depths of 20 km and in places 30 km centroid depths of larger mw 5 earthquakes are mostly 4 14 km with rare events up to 20 km and 28 km along the oman line within the simply folded belt most of these events are contained within the competent group and are limited to mw 6 5 earthquakes occur earthquakes within the competent group can account for much of the overall shortening at this level but the seismic strain rate in the basement is much lower it is unclear whether the basement deforms by aseismic creep or folding beneath the sfb or by ductile shortening beneath the high zagros or iranian plateau

orogenic plateau growth expansion of the turkish iranian plateau across the zagros fold and thrust belt 1 this paper shows how the turkish iranian plateau grows laterally by incrementally incorporating adjacent parts of the zagros fold and thrust belt the limit of significant seismogenic thrusting in the zagros mw 5 occurs close to the regional 1250 m elevation contour the seismicity cutoff is not a significant bedrock geology boundary elevations increase northward toward regional plateau elevations of 2 km implying that another process produced the extra elevation between the seismogenic limit of thrusting and the suture this process is a plausibly ductile thickening of the basement suggesting depth dependent strain during compression similar depth dependant crustal strain may explain why the tibetan plateau has regional elevations 1500 m greater than the elevation limit of seismogenic thrusting at its margins we estimate 68 km shortening across the zagros simply folded belt in the fars region and 120 km total shortening of the arabian plate the dezful embayment is a low strain zone in the western zagros deformation is more intense to its northeast in the bakhtyari culmination the orogenic taper across strike topographic gradient across the dezful embayment is 0 0004 and across the bakhtyari culmination 0 022 lateral plateau growth is more pronounced farther east fars where a more uniform structure has a taper of 0 010 up to elevations of 1750 m a 100 km wide region of the zagros further northeast has a taper of 0 002 and is effectively part of the turkish iranian plateau internal drainage enhances plateau development but is not a pre requisite aspects of the seismicity structure and geomorphology of the zagros do not support critical taper models for fold and thrust belts

analog and numerical experiments of double subduction systems with opposite polarity in adjacent segments datasets for this article are available in 655 http hdl handle net 20 500 11850 364963 doi 10 3929 ethz b 000364963

crustal scale cross sections across the nw zagros belt implications for the arabian margin reconstruction quantified balanced and restored crustal cross sections across the nw zagros mountains are presented in this work integrating geological and geophysical local and global datasets the balanced crustal cross section reproduces the surficial folding and thrusting of the thick cover succession including the near top of the sarvak formation 90 ma that forms the top of the restored crustal cross section the base of the arabian crust in the balanced cross section is constrained by recently published seismic receiver function results showing a deepening of the moho from 42 2 km in the undeformed foreland basin to 56 2 km beneath the high zagros the internal parts of the deformed crustal cross section are constrained by new seismic tomographic sections imaging a 50 ne dipping sharp contact between the arabian and iranian crusts these surfaces bound an area of 10800 km 2 that should be kept constant during the zagros orogeny the arabian crustal cross section is restored using six different tectonosedimentary domains according to their sedimentary facies and palaeobathymetries and assuming airy isostasy and area conservation while the two southwestern domains were directly determined from well constrained surface data the reconstruction of the distal domains to the ne was made using the recent margin model of wrobel daveau et al 2010 and fitting the total area calculated in the balanced cross section the arabian continental oceanic boundary at the time corresponding to the near top of the sarvak formation is located 169 km to the ne of the trace of the main recent fault shortening is estimated at 180 km for the cover rocks and 149 km for the arabian basement including all compressional events from late cretaceous to recent time with an average shortening rate of 2 mm yr 1 for the last 90 ma