extension during continental convergence with application to the tibetan plateau the tibetan plateau is the product of crustal thickening caused by the collision between india and asia and is the largest active example of extensional tectonics in a zone of continental collision throughout most of the tertiary the tectonics of the plateau were dominated by north south shortening a significant proportion of which took place on east west striking thrust faults for the last 5 m y or so the plateau has been thinning by the mechanism of extension on north south trending normal faults previous investigations of the collision have been able to account for the large scale features of the tertiary deformation but have failed to explain the transition in the late tertiary to quaternary strain rate field of the plateau from north south compression to east west extension the transition could in principle be effected by a reduction in the rate of convergence between india and asia or by a uniform reduction in strength of the whole continental lithosphere of asia it could not however be effected by a reduction in strength of the elevated region alone this produces increased compressional strain rates in the weakened zone an alternative explanation for the transition to extension comes from considering the thermal evolution of thickened continental lithosphere the lower part of the lithosphere consists of a thermal boundary layer which when thickened by horizontal shortening is colder and denser than its surroundings convective instability of the thickened thermal boundary layer and its replacement by hot asthenosphere would rapidly raise the surface elevation and gravitational potential energy of the overlying part of the lithosphere the convective instability would happen in a time brief compared with the collision time scale 50 m y in the case of india and asia but would only occur after there had already been substantial thickening of the lithosphere numerical experiments show that for a range of lithospheric parameters the increase in surface height as much as 2 km and of potential energy 5 to 10 1012 n m 1 resulting from convective instability of the lower lithosphere are sufficient for east west extension to replace north south compression as the dominant feature of the stress field of the tibetan plateau

transition from a singly vergent to doubly vergent wedge in a young orogen the greater caucasus the greater caucasus mountains due to their youth 5 ma provide an opportunity for insight into the early stages of orogen development during continent continent collision however their recent tectonic evolution and first order architecture remain unclear here we investigate the evolution of the orogen by integrating new observations of the fluvial geomorphology and neotectonics of the range with prior work on seismicity geodetic strain bedrock geology and foreland basin structure we find that the range contains four zones along strike that differ in structural architecture topography and first order tectonic history in particular two south directed singly vergent zones at the western and eastern tips of the orogen are separated by both a central doubly vergent zone that is dominated by north directed deformation and an eastern doubly vergent zone in which south directed thrusting dominates we hypothesize that the along strike changes in vergence and locus of deformation reflects different stages in the development of a doubly vergent orogen with the tips of the range preserving an early singly vergent form and the center recording a more advanced orogen the differences between the two doubly vergent zones seem to be driven by the initial stages of collision between the structurally thickened crust of the greater and lesser caucasus orogens which initiated at 5 ma

the 2013 wyoming upper mantle earthquakes tomography and tectonic implications on 21 september 2013 two earthquakes m 4 8 and 3 0 focal depths 70 km occurred in the lithospheric mantle beneath central wyoming which provide a rare opportunity to investigate the lithosphere rheology and dynamics we determined a detailed 3 d p wave tomography in the source area of the wyoming earthquakes using a large number of high quality travel time data of local earthquakes and teleseismic events our results show that the 2013 wyoming earthquakes occurred in an isolated high velocity high v anomaly which extends down to a depth of 160 km we propose two possible scenarios about the cause of the 2013 wyoming earthquakes one is that the high v anomaly reflects a remnant of the subducted oceanic slab which fractured under long term negative buoyancy and heating by the infilling asthenosphere the other is removing of a dense mantle lithospheric root the present results shed new light on the rheology and seismotectonics of the continental lithosphere

lithospheric deformation during the early stages of continental collision numerical experiments and comparison with south island new zealand 1 the nature of lithospheric deformation during continental plate collision still remains unresolved while it has often been proposed that the mantle lithosphere is accommodated by distributed thickening of a viscous root an alternate hypothesis suggests that significant portions of convergent mantle lithosphere essentially undergo underthrusting or subduction to further consider this issue we model the thermochemical evolution of the lithosphere mantle system using arbitrary lagrangian eulerian finite element techniques we incorporate a mix of viscous thermally activated power law creep and plastic frictional coulomb with strain softening rheologies in the numerical experiments to treat disparate composition in the crust and mantle a range of rheological and mechanical parameters is explored to determine controls on the style of lithospheric deformation the models suggest that during the initial stages of plate collision the mantle lithosphere is characterized by plate like behavior and underthrusting subduction of the upper region in conjunction with distributed thickening and rayleigh taylor type viscous instability of the lower portion depending on the material rheology temperature regime and imposed convergence velocity the deforming mantle lithosphere demonstrates various combinations of these end member behavioral modes the modeling results are interpreted in the context of observed lithospheric deformation across south island new zealand a combined style of underthrusting and distributed thickening is consistent with the observed crustal structure in the young collisional orogen as well as seismic imaging of the geometry of the underlying lithosphere

the surface tectonics of mantle lithosphere delamination following ocean lithosphere subduction insights from physical scaled analogue experiments many postulated lithospheric removal events occur in regions with an earlier history of subduction but the relationship between the two processes has not been explored in this work we use physical scaled analogue experiments to investigate the evolution from ocean lithosphere subduction to collision and possible delamination of the mantle lithosphere from the crust we test how varying the magnitude of plate convergence alters the behavior of the subduction delamination model our experiments show that a retreating ocean proplate can evolve to continental mantle lithosphere delamination negative surface topography is supported at the delamination hinge and this migrates back with the peeling lithosphere with high plate convergence delamination is suppressed rather the crust and mantle lithosphere split at the collision zone in a form of flake tectonics as oncoming procrust is accreted on top of the retroplate and the promantle lithosphere subducts below localized high topography develops at this zone of crustal accretion and thickening the results suggest that delamination may be a continental continuation of plate retreat and that lithospheric removal is triggered by the transition from one process to another

a flow law for dislocation creep of quartz aggregates determined with the molten salt cell we have used the molten salt cell to conduct an experimental study on the rheology of a natural quartzite containing 0 15 wt water co axial deformation experiments were conducted at constant piston displacement rates approximating constant strain rates at low strain the strengths of our natural quartzite measured in the molten salt cell are approximately half those measured at the same conditions in solid media because unlike solid confining media molten salt does not contribute to the strength of the sample it reduces the friction on the moving piston and it allows clear identification of the hit point we have limited the experimental conditions to those required for dislocation creep and have used only steady state flow stresses measured during climb accommodated dislocation creep to calculate the flow law parameters two flow laws were determined one for samples containing minor amounts of melt 1 2 and one for melt free samples in both cases the power law stress exponent n is 4 0 0 9 which is greater than that previously reported in flow laws for dislocation creep of quartz aggregates determined in solid media the activation energy q is 137 34 kj mol 1 for samples with melt and 223 56 kj mol 1 for those without within the range of previously determined values for quartz aggregates containing 0 1 wt water the pre exponential term a is 1 1 10 4 2 mpa ns 1 for samples without melt and 1 8 10 8 2 mpa ns 1 for those with melt the lower strengths measured in the molten salt cell indicate that previous piezometer relations for quartz experimentally determined in solid media are not correct extrapolation of the flow law for melt free aggregates to natural strain rates predicts higher strengths than most previous quartz flow laws however accurate extrapolation requires determining the dependence of flow stress on fh2o and or ah

near surface diagnostics of dripping or delaminating lithosphere 1 in various geological regions it has been postulated that the mantle lithosphere has been thinned or completely removed two of the primary removal mechanisms that have been put forward include 1 delamination a wholesale peeling away of a coherent block of the mantle lithosphere and 2 lithospheric dripping viscous rayleigh taylor instability of the mantle lithosphere using computational models we investigate several near surface observables to determine if these may be diagnostic of either often ambiguous removal mechanism surface topography associated with delamination has a broad region of uplift above the lithospheric gap and a localized and mobile zone of subsidence at the delaminating hinge with dripping lithosphere the topographic expression is symmetric and fixed above the downwelling delamination of mantle lithosphere is more efficient than dripping for thermal heating of the crust the onset is more rapid and the elevated temperatures persist longer the resultant crustal p t t paths show modest pressure variations and high temperature increases with large scale delamination or dripping delamination also causes contraction directly above the migrating hinge and distal extension dripping lithosphere induces superimposed contraction and extension above and symmetric about the viscous instability for all the observables if only a portion of the mantle lithosphere is removed by viscous instability delamination inherently removes all of the mantle lithosphere the differences between the two removal mechanisms are even more pronounced with only partial removal of the mantle lithosphere uppermost mantle lithosphere remains well coupled to the crust leading to lower surface temperature variations and broad zones of crustal deformation thickening

convective instability of thickening mantle lithosphere mantle lithosphere being colder and therefore denser than the underlying mantle is prone to convective instability that can be induced by horizontal shortening numerical experiments on a cold layer with imposed horizontal shortening are carried out to examine the relative importance of mechanical thickening thermal diffusion and gravitational instability in deforming the layer this analysis is then used to develop a method for determining which of these styles dominates for a layer thickening at a given rate if viscosity is non newtonian the imposition of shortening decreases the lithospheric strength which causes perturbations to the lithosphere s temperature structure to grow exponentially with time once these perturbations become sufficiently large they then grow super exponentially with time eventually removing the lithospheric base because lithospheric viscosity is highly temperature dependent at most only the lower 30 per cent of the lithosphere participates in the downwelling associated with this initial super exponential growth event after this event however a downwelling develops that removes material advected into the region of downwelling by horizontal shortening the magnitude of this persistent downwelling depends on the rate and duration of shortening if the total amount of shortening does not exceed 50 per cent doubling of crustal thickness then this downwelling extends to a depth three to four times the thickness of undeformed lithosphere and forms a sheet significantly thinner than the width of the region undergoing shortening once shortening stops this downwelling is no longer replenished by the shortening process and should then detach due to its inherent gravitational instability the hottest 60 per cent of the mantle lithosphere could be removed in such an event which would be followed by an influx of hot buoyant asthenosphere that causes rapid surface uplift because more cold material is removed after the cessation of shortening than by the initial gravitational instability the former has a potentially greater influence on surface uplift the tibetan interior is thought to have been shortened by about 50 per cent in 30myr and afterwards at 8ma experienced a period of rapid uplift that may have resulted from the removal of a large downwelling finger of cold lithosphere generated by shortening

global lithium resources relative importance of pegmatite brine and other deposits abstract previous studies of the availability of lithium for use in batteries to power electric vehicles evs have reached the generally encouraging conclusion that resources are sufficient to meet growing demand for the remainder of the 21st century however these surveys have not looked past estimates of lithium resource to the geological constraints on deposit size and composition that will allow the resources to be converted to reserves from which lithium can be produced economically in this survey we review the relevant geological features of the best characterized pegmatite brine and other types of lithium deposits and compare their potential for large scale long term production the comparison shows that the average brine deposit 1 45 mt li is more than an order of magnitude larger than the average pegmatite deposit 0 11 mt li and that brine deposits especially the large atacama chile and uyuni bolivia deposits have a much larger total lithium resource 21 6 mt li brine deposits clearly have a much greater capacity for large scale long term production than do pegmatite deposits however individual brine deposits differ considerably and successful production from one deposit does not necessarily assure success at others conversion of brine resources to reserves will depend on the degree of compartmentalization of brine reservoirs the extent to which brine can be recovered without dilution from recharge waters and degradation of salt bearing reservoir rock and whether other constituents of the brines including potassium magnesium bromine and boron restrict processing or the capacity of production to respond to changes in demand for lithium despite their smaller size and total estimated resource 3 9 mt pegmatites will remain of interest because of their wider geographic distribution and consequently lesser susceptibility to supply disruptions and their more lithium dominant compositions which might allow more flexible response to market changes some deposits will be amenable to large scale mining but significant production might also come from selective underground mining of small deposits in areas with low labor costs even the largest pegmatite deposits such as those at greenbushes australia and manono kitolo democratic republic of congo have estimated resources that are similar to only an average brine deposit for this reason continuous long term production from pegmatites will require extensive exploration and discovery of new pegmatite deposits and districts two other types of lithium deposits unusual rocks and unusual fluids have also received attention deposits consisting of rocks and minerals that are enriched in lithium by interaction with brines and hydrothermal solutions including the large jadar serbia and king valley usa deposits have estimated resources of about 3 4 mt li unusual brines in oilfields and the salton sea geothermal field usa contain an estimated 2 mt li both of these deposit types will require new processing methods but are likely to become important because of their lithium dominant compositions and relatively large size the total lithium resource in these deposits 31 1 mt li far exceeds estimated lithium demand of up to 20 mt li including consideration for recycling in all markets through 2100 the difference between these two estimates should be vied largely as a challenge rather than a comfort evaluation of the economic extractability of these resources will require large investments and result in only partial success if prices provide sufficient encouragement and exploration is sustained however lithium resources should be available for the next century

coupled crust mantle dynamics and intraplate tectonics two dimensional numerical and three dimensional analogue modeling 1 tectonic deformation of some intraplate regions may be caused by the rayleigh taylor rt instability of dense subcrustal lithosphere mantle lithosphere as it descends into the mantle we report on a series of scaled three dimensional 3 d analogue and 2 d numerical experiments of coupled crust mantle dynamics that study such a process the models investigate the effects of two geometries of rt instability linear versus axisymmetric and use different rheological stratifications of the crust the growth of the rt instability is strongly influenced by crustal rheology while rt growth rates are not greatly influenced by the geometry of the mantle lithosphere instability itself the surface is experiments having a viscous only crust are characterized by significant crustal contraction and thickening above the mantle downwelling and extension and thinning in adjacent regions in experiments with a brittle upper crust the surface deformation is much more subdued however strong mantle flow induced deformation still occurs in the ductile lower crust beneath the quiescent brittle upper crust when the lower crust is relatively strong localized styles of extensional and contractional structure develop in the brittle upper crust owing to the greater degree of coupling between upper crust and the mantle instability in all the experiments a portion of the ductile lower crustal material is entrained within the downgoing rt instability deep into the mantle the interplay between dynamic topography and crustal thickening thinning induced by the underlying mantle flow governs various time dependent phases of subsidence and uplift of the model surface the experiments help to account for a number of first order tectonic behaviors of the lithospheres of earth and other terrestrial planets for example intraplate orogens and basins complex localized tectonic structure in intraplate settings and deep crustal seismic fabrics may arise as the variable crustal response to underlying mantle lithosphere rt instabilities

teleseismic tomography of the mantle in the carpathian pannonian region of central europe summary subducted slab roll back lithospheric instability and asthenospheric extrusion have all been proposed as mechanisms that explain the evolution of the extensional pannonian basin within the convergent arc of the alpine carpathian mountain system in central europe we determine the p and s wave velocity structure of the mantle to depths of 850 km beneath this region using tomographic inversion of relative arrival time residuals from 225 p waves and 124 s waves teleseismic earthquakes recorded by 56 stations of the carpathian basins project cbp temporary seismic network 16 month duration and 44 permanent seismic stations the observed median p wave relative arrival time residuals vary between 1 13 s early in the alps and 1 12 s late at the western end of the carpathians s wave relative arrival time residuals are about twice as large 2 13 s and 3 39 s we tested the effect of deterministic corrections on our relative arrival time residuals using crustal velocity models from controlled source experiments but show that the use of station terms in the inversion provides a robust method of correcting for near surface crustal variation our tomographic models reduce the p wave rms residual by 71 per cent to 0 130 s and our s wave rms residual by 59 per cent to 0 624 s at shallow sublithospheric depths we image several localized lower velocity regions correlated with higher heat flow and interpreted as upwelling asthenosphere we image a high velocity structure down to depths of about 350 km beneath the eastern alps further east beneath the pannonian basin a deeper continuation of the eastern alps fast anomaly is imaged trending e w from 300 km depth and extending into the mantle transition zone mtz in the mtz we image a fast anomaly extending outwards as far as the carpathians the dinarides and the eastern alps this higher velocity mantle material is interpreted as being produced by a mantle downwelling whose detachment from the lithosphere above may have triggered the extension of the pannonian basin

increasing long wavelength relief across the southeastern flank of the sierra nevada california abstract a high degree of correlation between present day relative rock uplift measured using continuous gps geodesy and spatially averaged surface elevations suggests that long wavelength topographic relief is presently increasing along the southeastern flank of the sierra nevada range and within an adjacent portion of the northern basin and range province current estimates for erosion rate are an order of magnitude smaller than the relative rates determined by geodesy thus although the uplift serves to enhance long wavelength relief it cannot be explained entirely as an isostatic response to erosion if uplift rates have been constant through time the duration over which the uplift could have been active

vertical mantle flow associated with a lithospheric drip beneath the great basin rapid surface uplift or subsidence and voluminous magmatic activity have often been ascribed to regional scale downwelling of lithospheric mantle however because lithospheric drips sinking plumes of cold and dense lithosphere are relatively small and transient features direct evidence of their existence has been difficult to obtain moreover the significant vertical mantle flow that should be associated with such structures has not been detected here we integrate seismic anisotropy data with tomographic images to identify and describe a lithospheric drip beneath the great basin region of the western united states the feature is characterized by a localized cylindrical core of cooler material with fast seismic velocities and mantle flow that rapidly shifts from horizontal to vertical our numerical experiments suggest that the drip can be generated by gravitational instability resulting from a density anomaly of as little as 1 and a localized temperature increase of 10 the drip tilts to the northeast opposite to the motion of the north american plate in the hotspot reference frame and thereby indicates northeast directed regional mantle flow seismic anisotropy data for the great basin region of the western united states coupled with tomographic images help delineate a northeast dipping lithospheric drip numerical experiments suggest that the drip could have formed owing to gravitational instability triggered by a density increase of about 1 and a temperature increase of about 10

source receiver interferometric redatuming using sparse buried receivers to address complex near surface environments a case study of seismic imaging quality and time lapse repeatability

supplemental material zircon and monazite geochronology in the palmer zone of transpression south central new england usa constraints on timing of deformation high grade metamorphism and lithospheric foundering during late paleozoic oblique collision in the northern appalachian orogen analytical methods for zircon u pb and monazite th pb sims geochronology zircon u pb la icp ms geochronology data tables and supporting previous work

aegean tectonics strain localisation slab tearing and trench retreat abstract we review the geodynamic evolution of the aegean anatolia region and discuss strain localisation there over geological times from late eocene to present crustal deformation in the aegean backarc has localised progressively during slab retreat extension started with the formation of the rhodope metamorphic core complex eocene and migrated to the cyclades and the northern menderes massif oligocene and miocene accommodated by crustal scale detachments and a first series of core complexes mccs extension then localised in western turkey the corinth rift and the external hellenic arc after messinian times while the north anatolian fault penetrated the aegean sea through time the direction and style of extension have not changed significantly except in terms of localisation the contributions of progressive slab retreat and tearing basal drag extrusion tectonics and tectonic inheritance are discussed and we favour a model 1 where slab retreat is the main driving engine 2 successive slab tearing episodes are the main causes of this stepwise strain localisation and 3 the inherited heterogeneity of the crust is a major factor for localising detachments the continental crust has an inherited strong heterogeneity and crustal scale contacts such as major thrust planes act as weak zones or as zones of contrast of resistance and viscosity that can localise later deformation the dynamics of slabs at depth and the asthenospheric flow due to slab retreat also have influence strain localisation in the upper plate successive slab ruptures from the middle miocene to the late miocene have isolated a narrow strip of lithosphere still attached to the african lithosphere below crete the formation of the north anatolian fault is partly a consequence of this evolution the extrusion of anatolia and the aegean extension are partly driven from below asthenospheric flow and from above extrusion of a lid of rigid crust

topographic response to mantle lithosphere removal in the southern sierra nevada region california geological studies of mantle xenoliths entrained in late neogene quaternary lavas from the southern sierra nevada region and regional geophysical studies suggest that the highdensity mantle lithosphere that formed beneath the sierra nevada batholith in conjunction with arc magmatism is being convectively removed as a drip structure this structure as imaged seismically is roughly cylindrical in shape with a diameter of 100 km and extends to 225 km depth centered above this structure is a region 120 km in diameter that is undergoing active subsidence relative to adjacent regions such subsidence is seen in the active fluvial alluvial sediment flooding of mountainous topography of the southwestern sierra and in the development of the adjacent tulare lake basin of the san joaquin valley dynamic modeling of such upper mantle drip structures predicts a phase of overlying surface subsidence during the most vigorous phase of drip formation the southern sierra upper mantle drip and the overlying crust appear to be in this phase of their dynamically coupled evolution

magma generation by slab steepening and breakoff beneath a subduction accretion complex an alternative model for collision related volcanism in eastern anatolia turkey 1 eastern anatolia is regarded as one of the best examples of continental collision represented by a 2 km high plateau it displays shallow and diffuse seismicity indicating that the crust is still being actively deformed perhaps the most striking aspect of eastern anatolia is the volume and compositional variability of collision related volcanic products erupted during the neogene and quaternary time a number of models have been proposed for the genesis of this volcanism in the region however recent geophysical studies have revealed that a mantle lithosphere is almost completely absent beneath a greater portion of the region which makes us question the validity of the previous models in this paper we propose a new model for magma genesis slab steepening and breakoff beneath a large subduction accretion complex this model holds that a northward subducting oceanic lithosphere beneath the eastern anatolia accretionary prism gets steepened and eventually detached from the continental lithosphere of the bitlis poturge massif following the continent accretionary complex collision this brings the asthenospheric mantle in contact with the accretionary complex at shallow depths e g around 50 km resulting in rapid regional uplift and extensive melting which leads to collisional volcanism

erosion driven uplift of the modern central alps abstract we present a compilation of data of modern tectono geomorphic processes in the central european alps which suggest that observed rock uplift is a response to climate driven denudation this interpretation is predominantly based on the recent quantification of basin averaged late holocene denudation rates that are so similar to the pattern and rates of rock uplift rates as determined by geodetic leveling furthermore a gps data based synthesis of adriatic microplate kinematics suggests that the central alps are currently not in a state of active convergence finally we illustrate that the central alps have acted as a closed system for holocene redistribution of sediment in which the peri alpine lakes have operated as a sink for the erosional products of the inner central alps while various hypotheses have been put forward to explain central alpine rock uplift e g lithospheric forcing by convergence mantle processes or ice melting we show with an elastic model of lithospheric deformation that the correlation between erosion and rock uplift rates reflects a positive feedback between denudation and the associated isostatic response to unloading thus erosion does not passively respond to advection of crustal material as might be the case in actively converging orogens rather we suggest that the geomorphic response of the alpine topography to glacial and fluvial erosion and the resulting disequilibrium for modern channelized and associated hillslope processes explains much of the pattern of modern denudation and hence rock uplift therefore in a non convergent orogen such as the central european alps the observed vertical rock uplift is primarily a consequence of passive unloading due to erosion

nonuniform surface uplift of the andean plateau revealed by deuterium isotopes in miocene volcanic glass from southern peru abstract proposals for rapid late miocene surface uplift driven by large scale lithospheric removal beneath the central andean plateau have been based largely on temperature sensitive paleoaltimeters both the magnitude and mechanism of this proposed pulse of uplift have been challenged first climatic general circulation models support protracted uplift with predicted temperature and isotopic shifts enhanced by attainment of threshold elevations second tectonic models in which surface elevations are compensated by regional contraction and crustal thickening question the need for lithospheric removal and predict broadly coeval uplift of the entire plateau we present hydrogen isotope data using a novel temperature insensitive volcanic glass proxy from continuous well dated lower to middle miocene basin fill in the western cordillera of the northern plateau that show a rapid decrease in d values 62 8 at 19 16 ma with extremely negative values continuing into the pliocene we propose that the basin reached its current elevation by 16 ma 6 myr earlier than proposed for the central plateau the rapid decrease in d values is consistent with punctuated surface uplift of 2 2 3 7 km between 19 and 16 ma whereas the 3 7 km upper estimate assumes a static climate similar to modern the 2 2 km lower estimate conservatively incorporates modeled changes in the isotopic composition of precipitation associated with elevation change comparison of these results to existing paleoelevation estimates from the andean plateau facilitates a tentative reconstruction of earliest middle miocene paleotopography showing a central depression with flanking hinterland and thrust belt highlands this apparent pattern of nonuniform plateau uplift contradicts shortening proportional topographic growth over tens of myr we propose that temporally and spatially irregular surface uplift may be linked to pronounced local variability in crustal shortening and or piecemeal removal of dense mantle lithosphere insofar as these irregularities reflect the scale of heterogeneity for surface uplift processes available paleoelevation estimates point to a principal uplift mechanism that varies over relatively short

tectonics and melting in intra continental settings abstract most of the geodynamic theories of deformation as well as metamorphism and melting of continental lithosphere are concentrated on plate boundaries and are dominated by the effects of subduction upon deformation of the margins of continental lithospheric blocks however it is becoming increasingly apparent that suture zones or mobile belts presumably representing fossil subduction zones or other types of pre deformation which occur far from present plate boundaries play a key role in intra continental deformation in such zones the crust is strongly sheared and the mantle lithosphere is metasomatized reworking of such settings reveals a surprisingly large range of instabilities that develop in compressed lithosphere with lateral heterogeneities inherited from previous deformational processes structural complexity arises which is sensitive to lithospheric age and tectonic setting this complexity influences localization of deformation topographic evolution melt generation and melt generation ascent and emplacement in this paper using fully coupled petrological thermomechanical modeling various tectonic responses are correlated with magmatic events in intra continental settings and are compared to observed intra cratonic orogenies and magmatic events

lithospheric instability beneath the transverse ranges of california recent high resolution seismic experiments reveal that the crust beneath the san gabriel mountains portion of the transverse ranges thickens by 10 15 km contrary to earlier studies associated with the transverse ranges there is an anomalous ridge of seismically fast upper mantle material extending at least 200 km into the mantle this high velocity anomaly has previously been interpreted as a lithospheric downwelling both lithospheric downwelling and crustal thickening are associated with the oblique convergence of pacific and north america plates across the san andreas fault though it seems likely that the lithospheric downwelling is driven at least partly by gravitational instability of the cold lithospheric mantle we show by means of numerical experiment that the balance between buoyancy forces that drive deformation and viscous stresses that resist deformation determines the geometry of crustal thickening and mantle downwelling we use a simple two layered lithospheric model in which dense lithospheric mantle overlies relatively inviscid and less dense asthenosphere and is overlain by buoyant crust external plate motion drives convergence which is constrained by boundary conditions to occur within a central convergent zone of specified width a fundamental transition in the geometry of downwelling is revealed by our experiments for slow convergence or low crustal viscosity downwelling occurs as multiple sheets on the margins of the convergent zone for fast convergence or crust that is stronger than mantle lithosphere a single downwelling occurs beneath the center of the convergent zone this complexity in the evolution of the system is attributed to the interaction of crustal buoyancy with the evolving gravitational instability in order for a narrow downwelling slab to have formed beneath the transverse ranges within the last 5 myr the effective lithospheric viscosity of the convergent region is at most about 10 20 pa s

shear wave tomography of china using joint inversion of body and surface wave constraints 1 the india eurasia collision and the decratonization of the north china craton have drawn much attention from the scientific community here we provide the first large scale s wave velocity model for china ch11 s based on constraints from both teleseismic surface and body waves we take advantage of the recent deployment of the 140 permanent stations of the chinese digital seismic network and temporary network deployments to resolve both the lithospheric and deeper mantle structure slow velocities are widespread in the crust and upper mantle deeply rooted fast anomalies are located beneath the stable yangtze craton and the western ordos block of the north china craton an upper mantle fast anomaly is observed beneath the eastern block of the north china craton and could represent thermally eroded or delaminated precambrian lithosphere another flat and fast feature appears beneath the tibetan plateau from 50 to 250 km depth this may represent the indian slab stalled in the mantle due to its buoyancy or a lithospheric instability triggered by the india eurasia collision a large fast anomaly apparently stagnant in the transition zone is observed beneath the yangtze craton and may play a role in the stability of this block in contrast on both sides of the south china block active and reactivated areas coincide with oceanic slab material that has already sunk into the lower mantle and that may have enhanced tectonic activity by forcing convection finally the upper mantle beneath tibet seems almost completely surrounded by adjacent high velocity and presumably strong blocks

thermo mechanical controls of flat subduction insights from numerical modeling this study was supported by national basic research program of china 2014cb440901 the strategic priority research program b of cas xdb18020104 national science foundation of china 41190073 41372198 and 41304071 and nerc grant ne j021822 1

continental delamination insights from laboratory models 1 one of the major issues of the evolution of continental lithospheres is the detachment of the lithospheric mantle that may occur under certain conditions and its impact on the surface in order to investigate the dynamics of continental delamination we performed a parametric study using physically scaled laboratory models the adopted setup is composed of a three layers visco elastic body analog for upper crust lower crust lithospheric mantle locally thickened thinned to simulate a density anomaly lithospheric root and an adjacent weak zone lying on a low viscosity material simulating the asthenosphere the results emphasize the interplay between mantle flow deformation surface topography and plate motion during a three phases process 1 a slow initiation phase controlled by coupling and bending associated with contraction and dynamic subsidence 2 lateral propagation of the delamination alongside with extension and a complex topographic signal controlled by coupling and buoyancy while poloidal mantle flow develops around the tip of the delaminating lithospheric mantle and 3 a late phase characterized by a counterflow that triggers retroward motion of the whole model a semiquantitative study allows us to determine empirically two parameters 1 an initiation parameter that constrains the propensity of the delamination to occur and correlates with the duration of the first stage 2 a buoyancy parameter characterizing the delamination velocity during late stages and therefore its propensity to cease finally we point out similarities and differences with the sierra nevada california usa in terms of topography deformation and timing of delamination

influence of viscosity pressure dependence on deep lithospheric tectonics during continental collision 1 previous geodynamic models of continental collision show that the behavior of the continental lithosphere is strongly influenced by its rheology we build on previous work by quantitatively investigating with numerical experiments the influence of the pressure dependence of viscosity on the process of tectonic deformation during collision the models demonstrate how the inclusion of viscosity pressure dependence can quite substantially alter the style of continental mantle lithosphere deformation at low activation volumes high convergence rates and low to moderate initial moho temperatures the subduction style of mantle lithosphere deformation is dominant increasing the activation volume of mantle material allows the subduction style of deformation to occur at all convergence rates studied in the experiments at the expense of the subduction drip and ablative drip styles of deformation at low activation volumes high convergence rates and high initial moho temperatures the distributed pure shear style of deformation occurs with these same conditions increasing the activation volume of mantle material produces an ablative subduction style of mantle lithosphere deformation at low activation volumes low convergence rate and moderate to high initial moho temperatures the mantle lithosphere prefers a convective removal style of deformation increasing the activation volume here yields an ablative drip and distributed pure shear styles of deformation the results demonstrate that inclusion of the pressure dependence of viscosity quite often neglected in lithosphere scale geodynamic models can be significant in modulating deformation of the deforming lithosphere

on the origin of orogens in order to understand how orogens work a quantitative approach demonstrating proof of concept is essential our goal is to reconcile the diverse array of tectonic features observed in natural orogens in the context of working numerical models that are consistent with both the underlying physics and first order geological constraints we present a simple conceptual temperature magnitude t m framework for orogenesis in terms of the progression from small cold to large hot orogens and we use forward numerical models to test hypotheses corresponding to specific stages along the t m spectrum small cold orogens are analyzed using crustal scale singularity s point models in which suborogenic mantle lithosphere is kinematically subducted beneath crust that deforms by critical wedge mechanics the transition from oceanic subduction to continental collision and the subsequent evolution of large hot orogens has been investigated using both crustal and upper mantle scale models the latter including dynamic subduction of suborogenic mantle lithosphere large hot orogens with thick crust are characterized by elevated plateaus with a strong superstructure underlain by hot weak lower crustal infrastructure beneath plateaus tectonic processes are dominated by ductile flow of weak crust in response to differential pressure while plateau flanks form external thrust sense wedges we discuss four topical issues in orogenic tectonics including the response of the suborogenic mantle lithosphere to convergence the interaction of climate and tectonics the current debate concerning wedge versus channel flow models to explain the himalayan tibetan system and the interpretation of metamorphic architecture in terms of orogenic processes we conclude that collisional orogenesis is driven largely by subduction and accretion of material at convergent margins accompanied by shortening thickening and heating of deformed crust

time dependent surface topography in a coupled crust mantle convection model summary recent geodynamic research has shown that convective flow in the mantle may have an important role in the development of long wavelength surface topography this flow induced dynamic topography is usually derived from mantle convection models by computing the vertical component of hydrodynamic stress at the top of the model and assuming the stress is compensated by deflection of the surface however these models have generally ignored the presence of an overlying buoyant crust and its deformational response to the mantle flow we consider the effects of horizontal convective forcing on the crust and investigate how this crust lithosphere deformation interacts with the vertical component of mantle flow induced subsidence uplift at the surface in particular we test the response of various rheologies of the crust and mantle lithosphere to an episode of mantle downwelling the evolution of crustal thickness and topography is tracked using thermomechanical numerical models of the crust mantle system with a free surface upper boundary a strong crust c 2 5 10 25 pa s does not experience significant internal deformation and subsides above a mantle downwelling for a weaker crust c 10 23 pa s descending mantle flow initially induces subsidence but there is an inversion to surface uplift as crustal thickening induced by convergent mantle flow overcomes the dynamic subsidence the presence of a strong mantle lithosphere however may effectively shield a weak crust from deformation imposed by the underlying horizontal mantle convective stresses with temperature dependent rheologies in the model the interplay of the vertical horizontal mantle forcings with the thermal evolution of the crust results in a highly time dependent signal of topography subsequent to crustal thickening and uplift the system may undergo rapid topographic subsidence and crustal thinning by localized channel flow in the hot and weak lower crust these results suggest an alternative interpretation for the development of mantle flow induced topography in certain tectonic environments

intraplate volcanism due to convective instability of stagnant slabs in the mantle transition zone the study of volcanism can further our understanding of earth s mantle processes and composition continental intraplate volcanism commonly occurs above subducted slabs that stagnate in the mantle transition zone mtz such as in europe eastern china and western north america here we use two dimensional numerical models to explore the evolution of stagnant slabs in the mtz and their potential to sustain mantle upwellings that can support volcanism we find that weak slabs may go convectively unstable within tens of million years upwellings rise out of the relatively warm underbelly of the slab are entrained by ambient mantle flow and reach the base of the lithosphere the first and most vigorous upwellings rise adjacent to lateral heterogeneity within the slab ultimately convective instability also acts to separate the compositional components of the slab harzburgite and eclogite from each other with harzburgite rising into the upper mantle and eclogite sinking into the lower mantle such a physical filtering process may sustain a long term compositional gradient across the mtz

delamination in collisional orogens thermomechanical modeling 1 modes of mantle delamination in collision zones are discussed in the light of 2d numerical modeling freely evolving thermomechanical models take into account phase changes in the mantle and melting in the crust distinct modes are identified in which orogens undergo mantle delamination concurrently or after collision delamination propagates along the moho of the subducted plate together with the retreating trench provided that slab pull is sufficient and that the meta stability of the crust lithospheric mantle interface is overcome topography is an instantaneous response to delamination and migrates with the focused and localized separation between crust and lithospheric mantle delamination front early exhumation of high pressure rocks is followed by exhumation of high temperature partially molten rocks convective stabilization of delamination outlasts slab break offs and impedes renewed build up of mechanically strong lithospheric mantle by static cooling parameter studies investigate the sensitivity of delamination to the age of the subducted oceanic plate the collision rate the mantle rheology and explore the effects of melting and sedimentation mantle flow patterns that form around delamination and subduction are sensitive to and often limited by the semi permeable 670 km spinel perovskite transition demonstrating that phase changes are essential factors delamination may account for the formation of complex boundary zones between continents such as between africa and europe in the mediterranean where transient compression and extension the lack of mantle lithosphere and the exhumation of high grade metamorphic core complexes are features also obtained in our modeling results

syn convergent extension observed using the retreat gps network northern apennines italy 1 we present crustal deformation results from a geodetic experiment retreating trench extension and accretion tectonics retreat focused on the northern apennines orogen in italy the experiment centers on 33 benchmarks measured with gps annually or more frequently between 2003 and 2007 supplemented by data from an additional older set of 6 campaign observations from stations in northern croatia and 187 continuous gps stations within and around northern italy in an attempt to achieve the best possible estimates for rates and their uncertainties we estimate and filter common mode signals and noise components using the continuous stations and apply these corrections to the entire data set including the more temporally limited campaign time series the filtered coordinate time series data are used to estimate site velocity we also estimate spatially variable seasonal site motions for stations with sufficient data the rms scatter of residual time series are generally near 1 mm and 4 mm horizontal and vertical respectively for continuous and most of the new campaign stations but scatter is slightly higher for some of the older campaign data velocity uncertainties are below 1 mm yr for all but one of the stations maximum rates of site motion within the orogen exceed 3 mm yr directed ne relative to stable eurasia this motion is accommodated by extension within the southwestern and central portions of the orogen and shortening across the foreland thrust belt to the northeast of the range the data set is consistent with contemporaneous extension and shortening at nearly equal rates the northern apennines block moves northeast faster than the northern adria microplate convergence between the northern apennines block and the northern adria microplate is accommodated across a narrow zone that coincides with the northeastern apennines range front extension occurs directly above an intact vertically dipping slab inferred by previous authors from seismic tomography the observed crustal deformation is consistent with a buried dislocation model for crustal faulting but associations between crustal motion and seismically imaged mantle structure may also provide new insights on mantle dynamics

upper mantle tomography and dynamics beneath the north china craton 1 a high resolution tomographic model of the upper mantle beneath the north china craton ncc is determined using a large number of precisely hand picked teleseismic p wave arrival times the results are generally consistent with previous results but high quality arrivals provide new insights into the dynamics beneath the ncc obviously north south trending low velocity low v zones are revealed down to 300 400 km depth under the shanxi rift and tanlu fault zone while a north south trending high velocity high v zone representing the remainder of detached lithosphere is visible down to 200 km depth under the western portion of eastern ncc high v anomalies representing the detached lithosphere are detected at 200 400 km depth under central and eastern ncc under the ordos block high v anomalies are visible above 400 km depth indicating intact lithosphere broad high v anomalies representing the stagnant pacific slab are imaged with a low v anomaly from datong volcano to the edge of bohai sea in the mantle transition zone beneath eastern and central ncc suggesting that the pacific slab has subducted to central ncc but with a gap a continuously y shaped low v structure is clearly imaged under datong volcano and bohai sea from the lower mantle through this gap in the mantle transition zone to the upper mantle indicating the existence of a lower mantle plume these results suggest that in addition to the subduction of the pacific plate the plume has also played an important role in lithospheric destruction by thermal erosion of the asthenosphere and detachment of the lithosphere beneath the ncc

rifting and subsidence following lithospheric removal in continental back arcs it has been suggested that post orogenic lithospheric removal in continental back arcs promotes extension and surface subsidence however the surface response of this process and its primary difference from classical back arc opening have remained uncertain here the back arc extension process with varying continental mantle lithosphere thickness and thermal heterogeneities is studied by using thermomechanical subduction experiments the experiments illustrate that models with only slab retreat result in minor surface subsidence and extension in the back arc region alternatively there is notable extension due to the slab retreat and a localized high temperature zone in the back arc with uniform lithospheric thickness models with advecting mantle after lithospheric removal in the extending back arc predict rifting stretching factor b 2 and surface subsidence 1 5 km in the center of the basin the results of this work suggest that lithospheric removal may be an important trigger for continental back arc development rather than slab retreat alone causing lithospheric extension and subsidence the findings help explain rift formation and subsidence in the aegean sea west anatolia and possibly other mediterranean back arcs such as the alboran sea and the pannonian basin

lithosphere asthenosphere interaction beneath the western united states from the joint inversion of body wave traveltimes and surface wave phase velocities summary the relation between the complex geological history of the western margin of the north american plate and the processes in the mantle is still not fully documented and understood several pre usarray local seismic studies showed how the characteristics of key geological features such as the colorado plateau and the yellowstone snake river plains are linked to their deep mantle structure recent body wave models based on the deployment of the high density large aperture usarray have provided far more details on the mantle structure while surface wave tomography ballistic waves and noise correlations informs us on the shallow structure here we combine constraints from these two data sets to image and study the link between the geology of the western united states the shallow structure of the earth and the convective processes in mantle our multiphase dna10 s model provides new constraints on the extent of the archean lithosphere imaged as a large deeply rooted fast body that encompasses the stable great plains and a large portion of the northern and central rocky mountains widespread slow anomalies are found in the lower crust and upper mantle suggesting that low density rocks isostatically sustain part of the high topography of the western united states the yellowstone anomaly is imaged as a large slow body rising from the lower mantle intruding the overlying lithosphere and controlling locally the seismicity and the topography the large e w extent of the usarray used in this study allows imaging the slab graveyard a sequence of farallon fragments aligned with the currently subducting juan de fuca slab north of the mendocino triple junction the lithospheric root of the colorado plateau has apparently been weakened and partly removed through dripping the distribution of the slower regions around the colorado plateau and other rigid blocks follows closely the trend of cenozoic volcanic fields and ancient lithospheric sutures suggesting that the later exert a control on the locus of magmato tectonic activity today the dna velocity models are available for download and slicing at http dna berkeley edu

continental delamination and mantle dynamics drive topography extension and fluid discharge in the apennines the apennines belt of italy undergoes a northeast trending extension at a rate of a few millimeters per year that generates moderate to large normal faulting earthquakes in this paper we show that seismicity large earthquakes strong gas emission and belt topography all correlate with a broad low vp anomaly in the uppermost mantle we propose that a thermal fluid anomaly in the mantle associated with sub lithospheric mantle replacement after delamination of the adria lithosphere supports the topography of the belt and drives the extensional tectonics the mantle anomaly is likely caused by deep fluids coming from the dehydration of the material subducted during the europe adria collision and the delamination of adria beneath the belt co 2 rich fluids are accumulated and occasionally discharged during large normal faulting earthquakes after the replacement of sub lithospheric mantle the temperature at the base of the crust increases causing crustal stretching anatexis and strong degassing

hinterland drainage closure and lake formation in response to middle eocene farallon slab removal nevada u s a abstract hinterland basins can accumulate high resolution archives of orogenic processes and continental climate but are challenging to reconstruct due to tectonic overprinting and the inherent complexity of their lithofacies assemblages the cordilleran hinterland of northeast nevada has been interpreted to have overlain a flattened farallon slab from the late cretaceous to eocene slab removal and advection of asthenospheric mantle beneath nevada have been invoked to explain a southwestward migrating wave of eocene to oligocene volcanism and proposed as a driver for topographic uplift however the timing of slab removal and possible subsequent delamination of north american lithospheric mantle can only ambiguously be related to the surface record subsequent neogene extension and basin filling has complicated the correlation and interpretation of strata that record these events here we apply single crystal sanidine 40ar 39ar geochronology to 26 ash beds in northeast nevada to reconstruct paleogene geographic and hydrologic evolution we use these ages and legacy geochronology to compare lithofacies and isotope proxy records of meteoric waters to regional tectonics and global climate and assess competing tectonic interpretations for lake basin formation lakes formed locally prior to ca 48 7 ma in northeast nevada coeval with foreland lakes of the green river formation the most expansive phase of lacustrine deposition resulted in onlap onto locally derived fluvial deposits and folded paleozoic bedrock and occurred between ca 43 4 and ca 40 8 ma elko formation strata exhibit a basin wide transition from fluvial lacustrine to fluctuating profundal lithofacies at ca 42 7 ma suggesting a shift towards regional hydrologic closure the stromatolitic upper elko formation is intercalated with ash fall tuffs and several partially welded to unwelded ignimbrites from increasingly proximal volcanism elko formation deposition ended by ca 40 4 ma 40ar 39ar ages for seven ash beds in the dead horse formation at copper basin in northern elko county indicate intermittent ash bed deposition between 45 2 ma and 38 6 ma and an episode of lacustrine deposition between 39 8 ma and 38 6 ma that post dates the main phase of lake elko d values of volcanic glass sampled from dated ash beds reflect changes in the hydrogen isotope compositions of local eocene waters and systematically vary by 80 102 according to their depositional environment the elko formation and overlying volcanic strata are overlain regionally by a pronounced unconformity of 20 m y in the copper basin area deposition continued locally into the oligocene in the hanging wall of a ductile detachment the geochronologic and isotopic framework presented here permits reanalysis of the pinon range carbonate proxy record that was previously interpreted to record both regional uplift and the middle eocene climatic optimum new data suggest instead that isotope values of hydration waters within the elko formation were strongly influenced by evaporation and a change from lacustrine to non lacustrine conditions can account for the 18 o shift that was interpreted to reflect regional uplift moreover the end of elko formation deposition predated the middle eocene climatic optimum we interpret the overall record of drainage ponding and paleovalley inundation progressively more evaporative lacustrine conditions increasingly proximal volcanism and subsequent prolonged unconformity to reflect the surface effects of progressive ne to sw removal of the farallon slab

postcollisional lithospheric evolution of the southeast carpathians comparison of geodynamical models and observations seismic evidence and thermal and topographic transients have led to the interpretation of lithospheric removal beneath the southeast carpathians region a series of numerical geodynamic experiments in the context of the tectonic evolution of the region are conducted to test the surface crustal response to lithosphere delamination and slab break off the results show that a delamination type removal plate like migrating instability causes a characteristic pattern of surface uplift subsidence and crustal extension shortening to occur due to the lithospheric deformation and dynamic thermal forcing of the sublithospheric mantle these features migrate with the progressive removal of the underlying lithosphere model results for delamination are comparable with observables related to the geodynamic evolution of the southeast carpathians since 10 ma the mantle structure inferred by seismic tomography migrating patterns of uplift 1 5 km and subsidence 2 km in the region crustal thinning in the carpathian hinterland and thickening at the focsani depression and regional extension in the carpathian corner e g opening of brasov basin correlating with volcanism e g harghita and persani volcanics in the last 3 myr

effects of multiple phase transitions in a three dimensional spherical model of convection in earth s mantle numerical models of mantle convection that incorporate the major mantle phase changes of the transition zone reveal an inherently three dimensional flow pattern with cylindrical features and linear features that behave differently in their ability to penetrate the 670 km discontinuity the dynamics are dominated by accumulation of cold linear downwellings into rounded pools above the endothermic phase change at 670 km depth resulting in frequent avalanches of upper mantle material into the lower mantle the effect of the exothermic phase transition at 400 km depth is to reduce the overall degree of layering by pushing material through the 670 km phase change resulting in smaller and more frequent avalanches and a wider range of morphologies large quantities of avalanched cold material accumulate above the coremantle boundary cmb resulting in a region of strongly depressed mean temperature at the base of the mantle the 670 km phase change has a strong effect on the temperature field with three distinct regions being visible 1 the upper mantle containing linear downwellings and pools of cold material in the transition zone and characterized by a high amplitude long wavelength spectrum 2 the midmantle containing quasi cylindrical avalanche conduits and characterized by a low amplitude broad spectrum and 3 the deep mantle containing large pools of cold avalanched material and characterized by a high amplitude ultra red i e long wavelength spectrum the effect on the velocity field is very different flow penetration across the 670 km phase change is strongly wavelength dependent with easy penetration at long wavelengths but strong inhibition at short wavelengths thus when comparing numerical models with long wavelength seismic tomography diagnostics based on the density field such as the radial correlation function are much more sensitive to the effects of phase transitions than those based on the velocity field the amplitude of the geoid is not significantly affected by the partial layering because the contribution from the strong heterogeneity in the transition zone is almost perfectly balanced by the contribution from deflection of the 670 km discontinuity avalanches are associated with geoid lows however a more complex viscosity structure is required to correctly match the sign of the geoid over slabs in earth

using tracer thermochronology to measure modern relief change in the sierra nevada california abstract we examine relief change in two major sierra nevada drainages the kings and the san joaquin the modern distribution of bedrock cooling ages is derived by fitting isochrones to a dense distribution of existing cooling ages this analysis indicates southwest tilting of the range by 3 4 0 8 which is consistent with other geologic evidence for tilting after 5 ma the detrital cooling ages from the kings show an excess of young ages relative to what would be expected from uniform erosion whereas the san joaquin drainage shows excess old ages these results indicate increasing relief in the kings the results in the san joaquin are more difficult to interpret due to the dense network of dams which trap sediment however we analyze the effect of storage behind the dams and suggest steady or declining relief in the san joaquin the residence time of sediment in these drainages indicates that these relief estimates are averaged over the last 100 to 10 ka we suggest that relief change is related to localized surface uplift in the southern sierra nevada including the kings drainage caused by ongoing lower lithosphere removal the san joaquin drainage may be north of the lithospheric anomaly which is consistent with stagnating or decaying relief there

sensitivity analysis of rift induced delamination with application to rwenzori mountains summary rift induced delamination rid was favoured as a geodynamic process explaining the special situation of an extremely elevated old crustal block located in a continental rift system the rwenzori mountains the concept of rid is an upwelling asthenosphere encircling stiff lithosphere if temperature and stress dependent viscosity of the undermost lower crust is reduced and strength fails the detachment of cold and dense mantle lithosphere root is trigged surface expressions are propagating rift tips surrounding an uplifting less dense crustal block this study was performed to test the scope of application of rid hypothesis characteristic features of the numerical models are kept constant whereas parameters controlling the initial temperature anomaly rheological properties and the background geotherm are varied systematically the system answer is a bifurcation either the anomaly cools down or the central mantle lithosphere detaches and sinks down or rather an unsuccessful and successful rid the excess temperature of the prescribed initial anomaly is most sensitive and serves at the transition from cooling to delamination as a critical temperature delamination works only in certain ranges the anomaly full width ranges between 20 and 50km the offset of the symmetric anomalies lies between 64 and 180km and the top anomaly has to reach into the lower crust the corresponding geometric characteristics of the rwenzori region always lies inside the found parameter ranges for the delamination regime even near the optimum quantitative parameters concerning rheology are uncertain but qualitatively conform to the available findings thus we establish rid as a possible mechanism coinciding roughly with the local observations such as an old crustal horst inside a young rift system a missing mountain root the high seismicity low seismic velocities in the lower crust minor volcanism and extreme topography if rid is valid for the very special situation of the rwenzoris and by now the sole example one ought to generalize it and the parameter regimes explored can be used as a pattern for searching similar geological situations

tectonics of pliocene removal of lithosphere of the sierra nevada california pliocene ca 3 5 ma removal of dense eclogitic material under the sierra nevada has been proposed from variations in the petrology and geochemistry of neogene volcanic rocks and their entrained xenoliths from the southern sierra the replacement of eclogite by buoyant warm asthenosphere is consistent with present day seismologic and magnetotelluric observations made in the southern sierra a necessary consequence of replacing eclogite with peridotite is that mean surface elevations and gravitational potential energy both increase an increase in potential energy should increase extensional strain rates in the area if these forces are insuffi cient to signifi cantly alter pacifi c north american plate motion then increased extensional strain rates in the vicinity of the sierra must be accompanied by changes in the rate and style of deformation elsewhere changes in deformation in california and westernmost nevada agree well with these predictions existing geologic evidence indicates that a period of rapid uplift along the sierran crest of more than 1 km occurred between 8 and 3 ma most likely as a consequence of removal of lower lithosphere about this same time extensional deformation was initiated within 50 km of the eastern side of the sierra 5 3 ma and regional shortening began to produce the california coast ranges 5 3 ma we suggest that these events were induced by the 1 2 10 12 n m increase of gravitational potential energy generated by the sierran uplift evidence for pliocene uplift adjoining crustal extension and shortening in directly opposing parts of the coast ranges is found along nearly the entire length of the sierra nevada and implies that lithosphere was removed beneath all of the presentday mountain range the uplifted area lies between two large upper mantle high pwave velocity bodies under the south end of the san joaquin valley and the north end of the sacramento valley these high velocity bodies plausibly represent the present position of material removed from the base of the crust lithospheric removal may also be responsible for shifting of the distribution of transform slip from the san andreas fault system to the eastern california shear zone a prediction that awaits better defi ned slip histories on both faults overall the late cenozoic deformational history of the sierra nevada and vicinity illustrates that locally derived forces can infl uence deformation kinematics within plate boundary zones

relationship between observed upper mantle structures and recent tectonic activity across the southeastern united states the lithospheric structure of the southeastern united states is a product of earlier episodes of continental collision and breakup the region is located in the interior of the north american plate away from active plate margins however there is ongoing tectonism in the region with multiple zones of seismicity uplifting arches and cenozoic intraplate volcanism the mechanisms controlling this activity and the state of stress remain enigmatic two important factors are plate strength and preexisting inherited structures here we present new tomographic images of the upper mantle beneath the southeastern united states revealing large scale structural variations in the upper mantle examples include the relatively thick lithospheric mantle of stable north america that abruptly thins beneath the paleozoic appalachian orogeny and the slow upper mantle of the proterozoic reelfoot rift our results also indicate fast seismic velocity patterns that can be interpreted as ongoing lithospheric foundering this provides a viable explanation for seismicity uplifting and young intraplate volcanism we postulate that not only tectonic inheritance but also continuing lithospheric foundering may control the ongoing activity of the region long after it became a passive margin based on distinct variations in the geometry and thickness of the lithospheric mantle and foundered lithosphere we propose that piecemeal delamination has occurred beneath the region throughout the cenozoic removing a significant amount of reworked deformed mantle lithosphere ongoing lithospheric foundering beneath the eastern margin of stable north america explains significant variations in thickness of lithospheric mantle across the former grenville deformation front

more thoughts on convergent crustal plateau formation and mantle dynamics with regard to tibet evidence suggests that some 10 m y ago a large portion of thickened mantle lithosphere below the tibetan plateau was rapidly removed removal appears to have been abrupt and delayed from the time of substantial thickening fluid dynamical models of convective thermal boundary layer instability have had difficulties explaining abrupt lithospheric removal long after initial thickening such models have implied that either removal should be synchronous with thickening or should not occur at all due to boundary layer stabilization resulting from high lithospheric viscosity we present simple thermal chemical boundary layer convection models that suggest a compromise between previous end member results the models allow for delayed instability of a thickened thermal boundary layer under geologically reasonable parameter conditions instability delay relies on high viscosity of thickened mantle lithosphere stabilizing it against free convective removal and on lateral extent of thickened crust exceeding that of mantle lithosphere the latter condition leads to a peripheral region of thickened crust and relatively unthickened mantle lithosphere and an interior plateau region of both thickened crust and mantle lithosphere if the thermal resistance and or the rate of internal heat production of crust is greater than that of mantle then mantle lithosphere at the peripheral region can be thermally eroded to the point that the base of the crust comes into contact with bulk mantle this creates a peripheral weak zone that allows thickened mantle lithosphere of the interior plateau region to become mechanically detached and sink into bulk mantle the instability is akin to delamination as originally defined and its delay time is roughly the thermal diffusion time across the thinnest portion of mantle lithosphere

convective instability of a boundary layer with temperature and strain rate dependent viscosity in terms of available buoyancy summary cold mantle lithosphere is gravitationally unstable with respect to the hotter buoyant asthenosphere beneath it leading to the possibility that the lower part of the mantle lithosphere could sink into the mantle in convective downwelling such instabilities are driven by the negative thermal buoyancy of the cold lithosphere and retarded largely by viscous stress in the lithosphere because of the temperature dependence of viscosity the coldest and therefore densest parts of the lithosphere are unavailable for driving the instability because of their strength by comparing theory and the results of a finite element representation of a cooling lithosphere we show that for a newtonian fluid the rate of exponential growth of an instability should be approximately proportional to the integral over the depth of the lithosphere of the ratio of thermal buoyancy to viscosity both of which are functions of temperature and thus depth we term this quantity available buoyancy because it quantifies the buoyancy of material sufficiently weak to flow and therefore available for driving convective downwelling for non newtonian viscosity with power law exponent n and temperature dependent pre exponential factor b the instabilities grow superexponentially as described by houseman molnar 1997 and the appropriate timescale is given by the integral of the nth power of the ratio of the thermal buoyancy to b the scaling by the available buoyancy thus offers a method of determining the timescale for the growth of perturbations to an arbitrary temperature profile and a given dependence of viscosity on both temperature and strain rate this timescale can be compared to the one relevant for the smoothing of temperature perturbations by the diffusion of heat allowing us to define a parameter similar to a rayleigh number that describes a given temperature profile s tendency toward convective instability like the rayleigh number this parameter depends on the cube of the thickness of a potentially unstable layer therefore mechanical thickening of a layer should substantially increase its degree of convective instability and could cause stable lithosphere to become convectively unstable on short cales we estimate that convective erosion will in 10 myr reduce a layer thickened by a factor of two to a thickness only 20 to 50 per cent greater than its pre thickened value thickening followed by convective instability may lead to a net thinningof a layer if thickening also enhances the amplitude of perturbations to the layer s lateral temperature structure for the mantle lithosphere the resulting influx of hot asthenosphere could result in rapid surface uplift and volcanism

geodynamic models of cordilleran orogens gravitational instability of magmatic arc roots cordilleran orogens such as the central andes form above subduction zones and their evolution depends on both continental shortening and oceanic plate subduction processes including arc magmatism and granitoid batholith formation arc and batholith magma compositions are consistent with partial melting of continental lithosphere and magmatic differentiation whereby felsic melts rise upward through the crust leaving a high density pyroxenite root in the deep lithosphere we study gravitational removal of this root using two dimensional thermal mechanical numerical models of subduction below a continent the volcanic arc position is determined dynamically based on thermal structure and formation of a batholith root complex is simulated by changing the density of the arc lithosphere over time for the model lithosphere structure magmatic roots with even a small density increase are readily removed for a wide range of root strengths and subduction rates the dynamics of removal depend on the relative rates of downward gravitational growth and lateral shearing by subduction induced mantle fl ow gravitational growth dominates for high root densifi cation rates high root viscosities and low subduction rates resulting in drip like removal as a single downwelling over 1 2 5 m y at lower growth rates the root is removed over 3 m y through shear entrainment as it is carried sideways by mantle fl ow and then subducted in all models 80 of the root is removed making this an effective way to thin orogenic mantle lithosphere this can help resolve the mass problem in the central andes where observations indicate a thin mantle lithosphere despite signifi cant crustal shortening and thickening currie c a ducea m n decelles p g and beaumont c 2015 geodynamic models of cordilleran orogens gravitational instability of magmatic arc roots in decelles p g ducea m n carrapa b and kapp p a eds geodynamics of a cordilleran orogenic system the central andes of argentina and northern chile geological society of america memoir 212 p 1 22 doi 10 1130 2015 1212 01 for permission to copy contact editing geosociety org 2014 the geological society of america all rights reserved on september 10 2014 memoirs gsapubs org downloaded from

recent slow normal and strike slip faulting in the pasto ventura region of the southern puna plateau nw argentina 1 recent normal and strike slip faulting on the puna plateau of nw argentina has been linked to lithospheric foundering gravitational spreading plate boundary forces and a decrease in crustal shortening from north to south however the timing kinematics and rate of extension remain poorly constrained we focus on the pasto ventura region nw argentina located on the southern puna plateau and recent deformation 1 ma field mapping and kinematic analysis across offset volcanic cinder cones show that the overall extension direction is subhorizontal is oriented ne sw to nne ssw and occurs at a slow time integrated rate of 0 02 to 0 08 mm yr since at least 0 8 0 5 ma a regional compilation from this study and existing data shows that recent extension across the puna plateau is subhorizontal but varies in azimuthal orientation dramatically data from the pasto ventura region are consistent with a number of models to explain normal and strike slip faulting on the puna plateau all of which likely influence the region some role for lower lithospheric foundering through dripping appears to be seen based on the regional extension directions and ages of mafic volcanism in the southern puna plateau

deep and near surface consequences of root removal by asymmetric continental delamination abstract the occurrence of continental delamination has been proposed for a number of areas characterized by highly variable geodynamic settings in this study we present results of numerical simulations considering different initial setups representative for geodynamic scenarios where delamination could potentially develop to mimic a post collisional orogenic scenario we have designed an initial state characterized by the presence of an area of orogenic lithosphere with both crustal and lithospheric roots in a second setup we have considered a lithospheric root representative of a remnant slab with a flat overlying crust we focus on predicted evolution of surface and near surface observables namely the crustal structure surface heat flow and isostatic and dynamic topography evolution our results show that a high density orogenic lower crust likely related to the presence of eclogite significantly accelerates the sinking of the lithospheric mantle the pattern of local isostatic elevation is characterized by laterally migrating surface uplift subsidence this pattern is shown to be little sensitive to lower crust density variations in contrast predicted dynamic topography is more sensitive to these changes and shows surface subsidence adjacent to the delaminating lithospheric mantle for the model with a high density lower crust and surface uplift above the slab for a model with a less dense lower crust the reason for uplift in this second model is that the effect of the positive buoyancy of the thickened crust overwhelms the effect of negative buoyancy of the slowly sinking lithospheric mantle we infer from our modeling that there is not a specific characteristic pattern of topography changes associated with delamination but it depends on the interplay between highly variable factors as slab sinking velocity asthenospheric upwelling and changes in crustal thickness

continuity of the alpine slab unraveled by high resolution p wave tomography the question of lateral and or vertical continuity of subducted slabs in active orogens is a hot topic partly due to poorly resolved tomographic data the complex slab structure beneath the alpine region is only partly resolved by available geophysical data leaving many geological and geodynamical issues widely open based upon a finite frequency kernel method we present a new high resolution tomography model using p wave data from 527 broadband seismic stations both from permanent networks and temporary experiments this model provides an improved image of the slab structure in the alpine region and fundamental pinpoints for the analysis of cenozoic magmatism u hp metamorphism and alpine topography our results document the lateral continuity of the european slab from the western alps to the central alps and the downdip slab continuity beneath the central alps ruling out the hypothesis of slab break off to explain cenozoic alpine magmatism a low velocity anomaly is observed in the upper mantle beneath the core of the western alps pointing to dynamic topography effects a ne dipping adriatic slab consistent with dinaric subduction is possibly observed beneath the eastern alps whereas the laterally continuous adriatic slab of the northern apennines shows major gaps at the boundary with the southern apennines and becomes near vertical in the alps apennines transition zone tear faults accommodating opposite dipping subductions during alpine convergence may represent reactivated lithospheric faults inherited from tethyan extension our results suggest that the interpretations of previous tomography results that include successive slab break offs along the alpine zagros himalaya orogenic belt might be proficiently reconsidered

the interaction of a subducting lithospheric slab with a chemical or phase boundary we have used a finite element model of time dependent convection to determine the conditions for penetration of the subducted plate into the lower mantle a temperature dependent and non newtonian rheology is applied to achieve platelike behavior of the upper and sinking thermal boundary layer of convection the 650 km discontinuity is taken as either a chemical or phase boundary or as a combination of both it is represented by a marker chain which effects additional buoyancy when distorted out of its equilibrium position when the compositional density contrast is greater than about 5 the descending slab is deflected sidewards at the boundary and two layer convection prevails a resulting depression of the boundary in the range of 50 200 km should be detectable with seismic methods below 5 density difference the slab plunges several hundred kilometers into the lower mantle and below 2 it will probably not stop before reaching the core mantle boundary and extensive mixing would be expected with a pure phase change a negative clapeyron slope of about 6 mpa k 60 bar k is required to establish a type of leaky double layer convection a more moderate slope can aid a small compositional density difference to prevent slab penetration into the lower mantle with the present uncertainties about the physical nature of the 650 km discontinuity a variety of convective styles appears possible on dynamical grounds

recent seismicity of italy active tectonics of the central mediterranean region and seismicity rate changes after the mw 6 3 l aquila earthquake abstract in this paper we present a new image of the instrumental seismicity of italy obtained by refining hypocentral determinations for about 100 000 earthquakes that occurred in the period 2005 2012 the improved locations yield new constraints on active tectonics of the central mediterranean area where prolonged interaction between nested plates and continental slivers led to the development of the alpine and apennines systems intermediate depth and deep earthquakes define a lateral heterogeneous process of delamination and sinking of the continental lithosphere active beneath the mountain belts shallow seismicity prevalently occurs beneath elevated topography and correlates with low velocity mantle anomalies suggesting a superposition of gravity related forces to the eurasia africa plate convergence the delamination process drives a paired system of compression and extension that stretches the mountain range while shortening the external side of the belts the updated seismic catalog permits us to resolve a sharp variation of seismic rates that occurred in recent years timely after the 2009 mw 6 3 l aquila earthquake the increase of seismic rates is reasonably due to regional scale perturbation of the stress field induced by fluid flow and pore pressure variations within the crust probably related to deep dehydration processes active beneath the mountain belt

crustal structure and gravity anomalies beneath the rif northern morocco implications for the current tectonics of the alboran region we analyse bouguer anomaly data and previously published moho depths estimated from receiver functions in order to determine the amount of isostatic compensation or uncompensation of the rif topography in northern morocco we use moho depth variations extracted from receiver function analyses to predict synthetic bouguer anomalies that are then compared to observed bouguer anomaly we find that moho depth variations due to isostatic compensation of topographic and or intracrustal loads do not match moho depth estimates obtained from receiver function analyses the isostatic misfit map evidences excess crustal root as large as 10 km in the western part of the study area whereas a missing crustal root of 5 km appears east of 4 3 e this excess root missing topography correlates with the presence of a dense mantle lid the noticeable southwestward drift of the western rif area and with a current surface uplift we propose that a delaminated mantle lid progressively detaching westward or southwestward from the overlying crust is responsible for viscous flow of the ductile lower crust beneath the rif area this gives rise to isostatic uplift and westward drift due to viscous coupling at the upper lower crust boundary at the same time the presence of this dense sinking mantle lid causes a negative dynamic topography which explains why the observed topography is too low compared to the crustal thickness

topographic evolution of the sierra nevada measured directly by inversion of low temperature thermochronology at present there are at least two competing ideas for the topographic evolution of the sierra nevada one idea is that the sierra nevada was formed as a monocline in the cretaceous marking the transition from the great valley forearc basin to the west and a high nevadaplano plateau to the east similar to the west flank of the modern altiplano of the andes both the thermochronologic imprint of local relief and the stable isotopic evidence of a topographic rain shadow support this hypothesis a second idea supported by geomorphic observations suggests that the sierra gained a large fraction of its present elevation as recently as the pliocene this recent surface uplift could have been driven by convective removal of the lower part of the lithosphere the isostatic response to basin and range faulting and or by changes in dynamic topography associated with deep subduction of the farallon plate here we present the first comprehensive analysis of low temperature thermochronology in the sierra nevada which indicates that both ideas are likely correct we show that thermochronology can resolve surface uplift as well as exhumation by measuring long wavelength topographic evolution relative to sea level uplift measurements are possible whenever there is sufficient constraint on vertical rock velocity the tilting of isochrones defined as surfaces of equal cooling age provide this constraint in the sierra nevada three new factors further distinguish our analysis the first is that we allow for the local relief and the long wavelength topography to evolve independently second we use al in hb barometry to constrain the initial depth of emplacement for the sierra nevada plutons third our analysis is tied to a sea level datum by using the paleo bathymetric record of the great valley basin where it transitions to the sierra nevada batholith results indicate that westward tilting of the sierra nevada accounts for 2 km of uplift since 20 ma topographic relief increased by a factor of 2 these findings suggest that the sierra nevada lost elevation through most of the tertiary but regained much of its initial elevation following the onset of surface uplift in the miocene

geodynamic models of mature continental collision evolution of an orogen from lithospheric subduction to continental retreat delamination 1 the behavior of deep lithospheric processes in continental collision are frequently speculated on but poorly understood in this study we build on previous work to explore the styles of continental lithosphere deformation during mature collision where 1800 km of convergence has been accommodated by horizontal shortening we conducted a suite of numerical geodynamic experiments that test the sensitivity of mature continent collision to varying mantle lithosphere density mantle lithosphere yield stress lower crustal strength and to the presence of phase change related density changes in the lower crust i e eclogitization of mafic lower crust the models suggest that the early stages of collision are accommodated by subduction of lower crust and mantle lithosphere along a discrete shear zone beneath the overriding plate following this initial stage of subduction the subducting lower crust and mantle lithosphere can retreat from the collision zone permitting the sub lithospheric mantle to upwell and intrude the overriding plate as a result the lower crust and mantle lithosphere of the overriding plate delaminate from the overlying crust with local isostatic adjustment subduction and delamination driven crustal processes plateau like uplift occurs the sub crustal evolution also causes bands of syn convergent crustal extension to develop in models with a rheologically weaker lower crust surface crustal response to the deep lithosphere dynamics becomes more diffuse as an example the numerical experiments satisfy a number of surface observables of the himalayan tibetan orogen namely 1 the current general mantle lithosphere architecture as defined by seismic analyses 2 the long wavelength plateau uplift of the western tibetan plateau 3 the surface heat flow measurements in india the tethyan himalaya qiangtang and in qaidam basin and 4 the anomalous syn convergent extension in the southern portion of the orogen

thermal effects of continental collisions thickening a variable viscosity lithosphere abstract based on data that indicate that the lithosphere and the crust are anomalously hot in some convergence zones simple calculations of the thermal effect of crustal thickening are presented the results indicate that post tectonic melting of the lower crust requires high levels of mantle heat flux it has been suggested that this high heat flux can be explained by the convective removal of thermal boundary layers thickened by convergence to test this hypothesis a series of numerical experiments were carried out on the removal of thickened thermal boundary layers whose viscosity depends on temperature and pressure it is found that the boundary layers can be removed quickly 10 m y in some cases however these layers are too thin to greatly alter the thickness of the lithosphere or the heat flux at the top of the mantle a set of calculations was done with internal heat sources which give equilibrium lithospheric thicknesses half that of the thickened lithospheres these indicate that the time scale for convective thinning by one half is extremely long at least 100 m y for initial lithospheric thicknesses of 100 km therefore the lithosphere must be anomalously hot and thin before deformation for extensive crustal melting to occur after thickening in observed times 40 m y for tibet finally it is suggested that subduction induced convection may raise the general mantle temperatures over a broad region behind a convergence zone this effect can inhibit the cooling of the lithosphere for more than 50 m y after the subduction induced convection has ceased

thermal evolution rate of exhumation and tectonic significance of metamorphic rocks from the floor of the alboran extensional basin western mediterranean high grade metamorphic rocks drilled at ocean drilling program site 976 in the alboran sea show a pt path characterized by decompression from about 1050 mpa 40 km depth to 350 mpa 13 km depth accompanied by an increase in temperature from about 550 50 c to 675 25 c the ar ar dating on muscovite and apatite fission track analysis indicate that the final stage of exhumation and cooling occurred very rapidly in the interval 20 5 18 ma which coincides with the initiation of sedimentation in the alboran sea basin the alboran sea formed by miocene extension on the site of a late cretaceous to paleogene contractional orogen and extension coincided with thrusting in the peripheral parts of the betic rif arc which surrounds the basin on three sides thermal modeling of the pt path was carried out with the aim of constraining geodynamic models for the formation of the basin variables considered in the modeling included 1 the thickness and thermal gradient of the postorogenic lithosphere 2 the radiogenic heat production in the thickened crust 3 the time gap pause between the end of contractional tectonics and the start of extension 4 removal of lithospheric mantle below 125 75 or 62 5 km and 5 the rate of extension the only combinations of variables that produce modeled pt paths with the observed characteristics involve high radiogenic heat production combined with a significant postcontractional pause to produce high temperatures in rocks initially at 40 km depth removal of lithosphere below 62 5 km to produce further heating during decompression extension by a factor of 3 in 6 m y to delay the attainment of the maximum temperature until the rocks reached shallow depths and final exhumation and cooling in about 3 3 m y to satisfy radiometric and petrological constraints this gives a maximum of about 9 m y for exhumation from 40 km depth to the surface lithospheric stretching in response to plate boundary forces such as trench rollback without removal of lithosphere cannot explain the late onset of heating and the high temperatures reached by these rocks removal of lithosphere at depths significantly greater than 62 5 km cannot explain the combination of high temperatures reached by these rocks and the shallow depth at which they attained the maximum temperature only a combination of significant postcollisional radiogenic heating then wholesale removal of lithospheric mantle below the orogenic crust followed by rapid stretching can explain the observed pt path these results appear to support some form of lithospheric delamination as the primary cause for the formation of the alboran sea basin

numerical models of the magmatic processes induced by slab breakoff abstract after the onset of continental collision magmatism often persists for tens of millions of years albeit with a different composition in reduced volumes and with a more episodic nature and more widespread spatial distribution compared to normal arc magmatism kinematic modelling studies have suggested that slab breakoff can account for this post collisional magmatism through the formation of a slab window and subsequent heating of the overriding plate and decompression melting of upwelling asthenosphere particularly if breakoff occurs at depths shallower than the overriding plate to constrain the nature of any melting and the geodynamic conditions required we numerically model the collision of two continental plates following a period of oceanic subduction a thermodynamic database is used to determine the de hydration reactions and occurrence of melt throughout this process we investigate melting conditions within a parameter space designed to generate a wide range of breakoff depths timings and collisional styles under most circumstances slab breakoff occurs deeper than the depth extent of the overriding plate too deep to generate any decompressional melting of dry upwelling asthenosphere or thermal perturbation within the overriding plate even if slab breakoff is very shallow the hot mantle inflow into the slab window is not sustained long enough to sufficiently heat the hydrated overriding plate to cause significant magmatism instead for relatively fast shallow breakoff we observe melting of asthenosphere above the detached slab through the release of water from the tip of the heating detached slab melting of the subducted continental crust during necking and breakoff is a more common feature and may be a more reliable indicator of the occurrence of breakoff we suggest that magmatism from slab breakoff alone is unable to explain several of the characteristics of post collisional magmatism and that additional geodynamical processes need to be considered when interpreting magmatic observations

relative timing of crustal plateau magmatism and tectonism at tellus regio venus crustal plateaus subcontinent sized regions of thickened crust that stand 1 4 km above surrounding low lying plains define a major physiographic province of venus thus their mode of emplacement plays a fundamental part in the evolution of the entire planet local topographic lows up to several hundred kilometers across filled with radar dark volcanic flows termed intratessera floodlava basins itbs are common within crustal plateaus itbs record abundant crustal plateau volcanism and have implications for crustal plateau emplacement and evolution we ask the fundamental question did itbs develop before during or after crustal plateau development to address this question we mapped in detail the geology a particular itb in tellus regio to determine temporal relationships between itb volcanic flows and local crustal plateau tectonism mechanical arguments and crosscutting relationships indicate that itb volcanism occurred during crustal plateau formation extending the timing relationships resolved at this basin we suggest that itb magmatism played a major role in crustal plateau genesis the geologic history discerned is consistent with the upwelling model for crustal plateau evolution in which early surface extension and volcanism would be expected in a hotspot environment conversely early widespread extension and volcanism would not be expected if crustal plateaus formed above a mantle downwelling because 1 predicted horizontal and radial convergence would obstruct early surface extension and 2 lower than normal geothermal gradients would impede rather than enhance volcanism

the nature of orogenic crust in the central andes 1 the central andes 16 22 s are part of an active continental margin mountain belt and the result of shortening of the weak western edge of south america between the strong lithospheres of the subducting nazca plate and the underthrusting brazilian shield we have combined receiver function and surface wave dispersion results from the banjo seda project with other geophysical studies to characterize the nature of the continental crust and mantle lithospheric structure the major results are as follows 1 the crust supporting the high elevations is thick and has a felsic to intermediate bulk composition 2 the relatively strong brazilian lithosphere is underthrusting as far west 65 5 w as the high elevations of the western part of the eastern cordillera ec but does not underthrust the entire altiplano 3 the subcrustal lithosphere is delaminating piecemeal under the altiplano ec boundary but is not completely removed beneath the central altiplano the altiplano crust is characterized by a brittle upper crust decoupled from a very weak lower crust that is dominated by ductile deformation leading to lower crustal flow and flat topography in contrast in the high relief inland sloping regions of the ec and sub andean zone the upper crust is still strongly coupled across the basal thrust of the fold thrust belt to the underthrusting brazilian shield lithosphere subcrustal shortening between the altiplano and brazilian lithosphere appears to be accommodated by delamination near the altiplano ec boundary our study suggests that orogenic reworking may be an important part of the felsification of continental crust

from underplating to delamination retreat in the northern apennines abstract recordings of teleseismic earthquakes from a dense set of temporary and permanent broadband seismic stations reveal the lithospheric structure of the northern apennines and support the scenario of a retreating detachment within the mid crust lithospheric delamination appears crucial to the formation and evolution of the apennines orogen receiver function rf stacks outline a continuous west dipping ps converted phase from a positive velocity jump that we interpret as the top of the lower crust and mantle of the adria continental lithosphere which is descending into the shallow mantle the correlation of seismicity with two rf profiles across the northern apennines suggests distinct stages of lithospheric delamination active penetration of the detachment into the adria lithosphere seems evident in the south east with induced shallow mantle flow facilitated by slab dehydration penetration of the detachment in the north west seems to have arrested and is possibly marked by crustal underplating this layer atop the apennines slab is visible only down to 80 km depth and suspends above an oppositely dipping paired positive negative ps converted phase in stacked receiver functions the break in the west dipping adria lithosphere conflicts with a westward subduction scenario continuous from the oligocene lateral changes of deep structure and seismicity along the northern apennines suggest that underplating of crustal material and delamination retreat are distinct mechanisms active today in the western and eastern sectors respectively of the northern apennines negative ps pulses at 100 120 km depth help to define a seismic lithosphere asthenosphere boundary lab but cross cut a volume of high velocity mantle rock as inferred from tomographic models we hypothesize that this seismic lab is a rheological discontinuity that affects the frequency band of seismic body waves but not the long term viscous response that governs the evolution and eventual detachment of the continental slab

timing of volcanism in the sierra nevada of california evidence for pliocene delamination of the batholithic root recent seismic experiments across the southern sierra nevada california show that the range lacks a thick crustal root xenolith studies indicate that delamination and loss of much of the lower crust may have occurred between 10 and 3 ma we estimate that delamination occurred ca 3 5 ma on the basis of a sudden pulse of mafic potassic magmatism within and just east of the sierra nevada from 4 to 3 ma this pulse interrupted the low level of volcanism that had prevailed regionally since 13 ma delamination could have promoted magmatism via decompression melting heating of the shallow lithosphere by upwelling asthenosphere and foundering of crustal rocks into the mantle and could have provided dynamic support to the range foundering of the lower crust may have occurred spontaneously owing to negative buoyancy or may have been triggered by extension between the sierra nevada and death valley

lateral migration of a foundering high density root insights from numerical modeling applied to the southern sierra nevada abstract the southern sierra nevada is a geodynamically complex region where several models have been proposed to explain the rapid removal of lithospheric mantle occurring sometime between 8 0 and 3 5 ma tomographic studies show the presence of an east dipping slab shaped fast seismic anomaly reaching to about 300 km depth below the western sierras and great valley and receiver function studies indicate a broad region of lithosphere removal this work presents thermo mechanical modeling of asymmetric foundering of a high density batholithic root with lateral intrusion of asthenospheric material the predicted evolution is controlled by a the upwelling of buoyant asthenosphere facilitated by the presence of a weakened lithospheric mantle adjacent to a dense batholitic root b the westward inflow enabled by a low viscosity lower crust and c negative buoyancy of a batholithic dense root the dynamics of these models can be characterized as a slowly migrating lithosphere foundering process driven by the density anomaly of the ultramafic root but controlled by the magnitude of the lower crustal viscosity which determines the rate at which asthenospheric material can flow into the opening lower crustal gap final model predicted upper mantle structure is compatible with existing tomographic images and the observed v shape geometry of the moho below the western margin of the southern sierra nevada model predicted topography is also generally consistent with observations and shows a monotonous uplift of the high region since 7 ma and presently ongoing and an area of maximum subsidence west of the area of the v shaped moho due to the pull exerted by the sinking of the high density root

mantle instability beneath the sierra nevada mountains in california and death valley extension the southern sierra nevada mountain range rapidly uplifted at 3 5 ma simultaneously with a pulse of basaltic volcanism xenoliths recovered from volcanics indicate that the range lost a dense crustal root after the miocene the vertical motions and removal of the root have been linked to a fast seismic velocity anomaly that extends 200 km into the mantle but is offset to the west of the range with visco elasto plastic thermo mechanical numerical models we have tested the influence of crustal strength on the kinematics of removal and on the amount of associated uplift we find that delamination of the dense root is the most likely mechanism for gravitational instability to occur the model satisfies the plio quaternary vertical motions the shift of the mantle anomaly to the west of the range and intense miocene extension to the east based on those results we propose the existence of a dynamic link between the sierra nevada mantle instability and death valley rifting

geometry of the pamir hindu kush intermediate depth earthquake zone from local seismic data 1 we present new seismicity images based on a two year seismic deployment in the pamir and sw tien shan a total of 9532 earthquakes were detected located and rigorously assessed in a multistage automatic procedure utilizing state of the art picking algorithms waveform cross correlation and multi event relocation the obtained catalog provides new information on crustal seismicity and reveals the geometry and internal structure of the pamir hindu kush intermediate depth seismic zone with improved detail and resolution the relocated seismicity clearly defines at least two distinct planes one beneath the pamir and the other beneath the hindu kush separated by a gap across which strike and dip directions change abruptly the pamir seismic zone forms a thin approximately 10 km width curviplanar arc that strikes east west and dips south at its eastern end and then progressively turns by 90 to reach a north south strike and a due eastward dip at its southwestern termination pamir deep seismicity outlines several streaks at depths between 70 and 240 km with the deepest events occurring at its southwestern end intermediate depth earthquakes are clearly separated from shallow crustal seismicity which is confined to the uppermost 20 25 km the hindu kush seismic zone extends from 40 to 250 km depth and generally strikes east west yet bends northeast toward the pamir at its eastern end it may be divided vertically into upper and lower parts separated by a gap at approximately 150 km depth in the upper part events form a plane that is 15 25 km thick in cross section and dips sub vertically north to northwest seismic activity is more virile in the lower part where several distinct clusters form a complex pattern of sub parallel planes the observed geometry could be reconciled either with a model of two sided subduction of eurasian and previously underthrusted indian continental lithosphere or by a purely eurasian origin of both pamir and hindu kush seismic zones which necessitates a contortion and oversteepening of the latter

an arbitrary lagrangian eulerian formulation for creeping flows and its application in tectonic models summary this paper presents and analyses numerical techniques developed to investigate viscoplastic stokes flows within a model of lithospheric deformation in particular the techniques are related to a subduction model of compressional orogens the driving mechanism in the model corresponds to the near rigid convergence and subduction of one mantle lithosphere beneath another in plane strain and this boundary condition forces flow in an overlying viscoplastic model crust the numerical techniques use the arbitrary eulerian lagrangian formulation in which flows with free surfaces and large deformation are computed on an evolving eulerian finite element grid that conforms to the material domain a regridding algorithm allows the associated lagrangian motion and fields to be followed and in addition coupled back to the eulerian calculation of the flow mass flux boundary conditions are used so that the effects of erosion and deposition by surface processes and mass loss by subduction can be included in the model calculation the evolving model crustal layer is flexurally compensated using a general elastic beam formulation the applicability of the numerical techniques to problems ranging from accretionary wedges to crustal and lithospheric scale deformation is discussed simple flows a linear viscous subduction model a whirl flow and a quasi convection model are used to show that the mass conservation regridding and surface tracking errors are small the broader applicability of the modelling techniques is reviewed

mantle earthquakes crustal structure and gravitational instability beneath western north island new zealand a cluster of 30 52 km deep earthquakes and a 7 10 km step in the moho beneath western north island new zealand are both interpreted as manifestations of active delamination of the continental lower crust and mantle lithosphere these phenomena occur in the back arc region beneath the east west oriented taranaki ruapehu tr line which strikes at a high angle to the present day plate boundary through new zealand across the line there is an abrupt change in crustal and mantle lid thickness and in upper mantle seismic attenuation q p 1 showing that the mantle lithosphere has been highly thinned on the north side we show from a receiver function profile that nearly all of the deep earthquakes reside in the uppermost mantle on the northern side of the tr line a sum of earthquake moment tensors suggests strike slip motion either parallel or orthogonal to the tr line resulting in northwest southeast oriented horizontal extension active normal faults oriented northwest southeast and north south are seen at the surface on each side of the tr line but the surface is uplifting here at 0 4 mm yr this requires the mantle lithosphere to be thinning at a higher rate than the overlying crust consistent with a delamination process

numerical simulations of the mantle lithosphere delamination 1 sudden uplift extension and increased igneous activity are often explained by rapid mechanical thinning of the lithospheric mantle two main thinning mechanisms have been proposed convective removal of a thickened lithospheric root and delamination of the mantle lithosphere along the moho in the latter case the whole mantle lithosphere peels away from the crust by the propagation of a localized shear zone and sinks into the mantle to study this mechanism we perform two dimensional 2 d numerical simulations of convection using a viscoplastic rheology with an effective viscosity depending strongly on temperature depth composition crust mantle and stress the simulations develop in four steps 1 we first obtain classical sublithospheric convection for a long time period 300 myr yielding a slightly heterogeneous lithospheric temperature structure 2 at some time in some simulations a strong thinning of the mantle occurs progressively in a small area 100 km wide this process puts the asthenosphere in direct contact with the lower crust 3 large pieces of mantle lithosphere then quickly sink into the mantle by the horizontal propagation of a detachment level away from the asthenospheric conduit or by progressive erosion on the flanks of the delaminated area 4 delamination pauses or stops when the lithospheric mantle part detaches or when small scale convection on the flanks of the delaminated area is counterbalanced by heat diffusion we determine the parameters crustal thicknesses activation energies and friction coefficients leading to delamination initiation step 2 we find that delamination initiates where the moho temperature is the highest as soon as the crust and mantle viscosities are sufficiently low delamination should occur on earth when the moho temperature exceeds 800 c this condition can be reached by thermal relaxation in a thickened crust in orogenic setting or by corner flow lithospheric erosion in the overriding lithosphere of subduction zones