arar a software tool to promote the robust comparison of k ar and 40ar 39ar dates published using different decay isotopic and monitor age parameters the comparison of k ar and 40ar 39ar geochronologic data published by different laboratories is markedly hindered by the inconsistent use of the parameters necessary to convert isotopic analyses to dates this problem is particularly acute when we try to evaluate the significance of datasets obtained prior to the development of community consensus values for basic decay constants isotopic abundances and the ages of common monitor minerals unfortunately the effect of using different parameters for the same dataset can sometimes exceed the quoted analytical precision of derived dates we created the argon age recalculator or arar to help researchers account for such discrepancies in a simple efficient manner allowing for more robust comparisons among datasets and more effective compilation of existing datasets using self consistent parameter sets arar is freely available as a platform independent executable application at http group18software asu edu

building the pamirs the view from the underside the pamir mountains are an outstanding example of extreme crustal shortening during continental collision that may have been accommodated by formation of a thick crust much thicker than is currently thought and or by continental subduction we pre sent new petrologic data and radiometric ages from xenoliths in miocene volcanic rocks in the southeastern pamir mountains that suggest that gondwanan igneous and sedimentary assemblages were underthrust northward buried to 50 80 km during the ear ly stage of the india asia collision and then heated and partly melted during subsequent thermal relaxation before finally being blasted to the surface these xenoliths the deepest crustal samples recovered from under any active collisional belt provide direct evidence for early cenozoic thickening of the pamirs and lower crustal melting during collision the xenoliths also suggest that the present mountain range was a steady state elevated plateau for most of the cenozoic

probing the basement of southern tibet evidence from crustal xenoliths entrained in a miocene ultrapotassic dyke abstract a variety of felsic and mafic granulites and ultramafic rocks occur as xenoliths within a 12 7 ma ultrapotassic dyke intruding xigaze flysch immediately to the north of the yarlung tsangpo suture zone in southern tibet garnet clinopyroxene plagioclase quartz thermobarometry on mafic granulite xenoliths gives temperatures of 1130 1330 c and pressures between 22 and 26 kbar indicating equilibration in the high pressure and ultrahigh temperature granulite field and defining a geotherm of c 16 c km 1 ultramafic xenoliths consist mainly of hornblende and biotite probably of restitic crustal rather than mantle origin and attained peak metamorphic conditions of 920 1130 c and 17 24 kbar whereas felsic granulites equilibrated at 870 900 c at an inferred pressure of 17 kbar in situ u th pb laser ablation inductively coupled plasma mass spectrometry dating of zircons shows that protoliths may include proterozoic basement rocks late cretaceous calc alkaline tonalites of the gangdese batholith root and or remnants of a neo tethyan oceanic arc certain zircons from a felsic granulite and an ultramafic xenolith have mean 206 pb 238 u ages of 16 8 0 9 ma and 15 6 0 6 ma respectively and monazites from a micaceous xenolith yielded a mean 208 pb 232 th age of 14 4 0 4 ma these results show that the southern tibet basement reached a thickness of c 80 km by 17 14 ma at the latest and has remained unchanged until the present day supplementary material mineral analyses used for pressure temperature estimations and results of 40 ar 30 ar laser single grain fusion experiments are available at http www geolsoc org uk sup18325

miocene volcaniclastic sequence within the xiyu formation from source to sink implications for drainage development and tectonic evolution in eastern pamir nw tibetan plateau

high heat flux on ancient mars evidence from rift flank uplift at coracis fossae 1 we have identified flexural uplift at the flanks of the coracis fossae rift valley modeling the topography the elastic thickness is constrained to 10 3 12 5 km the corresponding mechanical thickness and thermal gradient are 15 19 km and 27 33 k km 1 respectively the time of rifting is dated by evaluating the crater size frequency distribution of key surface units related to the rifting thus the heat flux between 3 5 and 3 9 ga is calculated to be 54 66 mw m 2 furthermore the stresses supported by the rift bounding faults are estimated to be below 5 mpa we speculate that this weakness may be connected to the presence of water below the surface

delamination of eclogitized lower crust control on the crust mantle boundary in the central fennoscandian shield abstract the crustal structure of the central part of the fennoscandian shield is dominated by a high velocity lower crust hvlc vp 7 3 7 5 km s at the depth of 40 63 km in an area of ca 400 km 600 km formed in the palaeoproterozoic svecofennian orogen at 1 91 1 87 ga in the area of the hvlc the moho is deeper than 50 km and attains depths of up to 63 km in eastern finland at the archaean proterozoic boundary zone we suggest that the present moho dates back to late svecofennian time and was determined by delamination of high density eclogitized lower crust after tectonic collisional thickening of crust densities deduced from seismic and gravity data suggest a very small or completely lacking density contrast between lower crust and uppermost mantle new geological evidence from kimberlite hosted garnet xenocrysts supports the presence of a garnet population with compositions in agreement with crustal eclogitic garnets in addition small clinopyroxene garnet mineral aggregates microxenoliths show omphacitic clinopyroxene compositions therefore the presence of eclogites in the high velocity lower crust is implied although the eclogite proportion as estimated from seismic velocity data is only a few percent in the present hvlc with results on near incidence and wide angle seismic studies and geological and geochemical data on heat producing elements we modeled the svecofennian collision and the thermal evolution of the crust to provide temperature constraints for the delamination model the constructed thermal model simulates the genesis of svecofennian synkinematic at 1 89 1 88 ga and postkinematic 1 88 1 87 ga granitoids as well as the general metamorphic grade of the surface rocks and lower crustal xenoliths moreover the narrow distribution of the ages of the svecofennian mafic ultramafic magmatism ca 1 89 1 87 ga dispersed around the hvlc area is explained by the homogeneous warming of the lithosphere the conditions of lower crustal delamination were estimated with a rayleigh taylor convective instability model constrained with lower crustal and upper mantle rheological data delamination of a lower crustal eclogite layer 5 30 km thick takes place in about 7 50 ma if the rock dynamic viscosity is 1 1021 pa s it requires temperatures of 1000 c in the lower crust and upper mantle in the svecofennian crust such temperatures were attained at about 1860 1850 ma i e after the orogenic peak we conclude that the moho of the hvlc area was controlled by delamination of a ca 5 10 km thick layer of high density lowermost crust into the mantle such a density controlled process may have controlled the moho depth also in other proterozoic shield areas

tajik basin and southwestern tian shan northwestern india asia collision zone 2 timing of basin inversion tian shan mountain building and relation to pamir plateau advance and deep india asia indentation the tajik basin and southwestern tian shan constitute the northwestern tip of the india asia collision zone basin inversion formed the thinskinned tajik fold thrust belt outlined by westward convex fold trains underlain by a decollement in jurassic evaporites the belt s leading edge the uzbek gissar and its transpressional northern lateral margin the tajik gissar constitute the thickskinned foreland buttresses apatite fission track data indicate 40 to 15 ma reheating by sediment burial in the tian shan in the gissar and the tajik fold thrust belt apatite fission track and u th he ages date the major phase of shortening erosion between 12 and 1 ma with exhumation to 2 to 3 km crustal depths within a few myr after onset of shortening shortening spread immediately across the fold thrust belt typical for belts floored by a detachment in ductile rocks and into the foreland buttresses reactivation concentrated in the internal eastern fold thrust belt with the thickest evaporates the youngest ages 6 6 1 6 ma occur along the vakhsh thrust the active erosional front of the fold thrust belt in the northeastern tajik basin where it narrows between the converging tian shan and pamir our study links major events in the pamir hinterland with the tajik basin and tian shan foreland in the late eocene early miocene the advancing pamir plateau crust loaded the foreland inducing subsidence reheating and early shortening basin inversion and major shortening transpression in the foreland buttresses from 12 ma onward were synchronous with the subcrustal indentation of indian lithosphere into the tajik tarim basin lithosphere and the onset of its rollback beneath the pamir

mechanical modeling of thrust faults in the thaumasia region mars and implications for the noachian heat flux abstract insight into the state of the early martian lithosphere is gained by modeling the topography above surface breaking thrust faults in the southern thaumasia region crater counts of key surface units associated with the faulting indicate a scarp emplacement in the late noachian early hesperian periods between 4 0 and 3 7 gyr the seismogenic layer thickness at the time of faulting is constrained to 27 35 km and 21 28 km for the two scarps investigated implying paleo geothermal gradients of 12 18 and 15 23 k km 1 corresponding to heat flows of 24 36 and 30 46 mw m 2 the heat flow values obtained in this study are considerably lower than those derived from rift flank uplift at the close by coracis fossae for a similar time period indicating that surface heat flow is a strong function of regional setting if viewed as representative for magmatically active and inactive regions the thermal gradients at rifts and scarps span the range of admissible global mean values this implies d t d z 17 32 k km 1 with the true value probably being closer to the lower bound

thermal convection in a volumetrically heated infinite prandtl number fluid with strongly temperature dependent viscosity implications for planetary thermal evolution parameterized models of the thermal evolution of planets are usually based on the assumption that the lithosphere convecting mantle boundary can be defined by an isotherm at a temperature below which viscosity is infinite on geologic timescales recent experimental results argue against this assumption we have investigated both the definition of the lithosphere convecting mantle boundary and the power law relation describing convecting heat transfer based on numerical experiments of thermal convection in a volumetrically heated fluid with temperature dependent viscosity other recent studies have treated only the heating from below but volumetric heating is likely to be the dominant mode of heating in planetary mantles either as a consequence of radioactive heating or as a proxy for secular cooling convection can occur either in the whole box or be located under a stagnant lid in the lid regime convection is driven by a temperature contrast depending on the rheology of the fluid and the interior temperature this result in agreement with experimental studies indicates that boundary between the stagnant lid and the convecting layer similar to the lithosphere convecting mantle boundary cannot be defined as a fixed isotherm during thermal evolution of planets the viscosity contrast in the convecting mantle remains constant not the temperature at the bottom of the lithosphere we present an example showing that the evolution of planets is strongly dependent on the criterion chosen to define the lithosphere convecting mantle boundary for reasonable values of the activation energy for thermally activated creep the temperature defining the lithosphere convecting mantle boundary the mantle temperature and the thickness of the lithosphere could be larger than expected from previous models which treat the base of the lithosphere as a fixed isotherm

forward mechanical modeling of the amenthes rupes thrust fault on mars amenthes rupes overlies one of the largest thrust faults on mars with structural topography accurately measured by mola forward mechanical models successfully predict the surface topography across this large contractional structure the best fitting fault parameters are fault dip angle 25 30 depth of faulting 25 30 km and fault offset 1 5 km these first order results are insensitive to either the offset distribution along the fault or crustal material properties both very shallow e g 15 km and very deep 45 km depths of faulting can be ruled out as can significantly different dips and offset magnitudes amenthes rupes may span the ancient seismogenic lithosphere in the martian noachian highlands implying maximum values at the time of faulting for heat flux of 54 70 mw m 2 and paleogeothermal gradient of 15 20 k km 1

zircon geochronology and ca 400 ma exhumation of norwegian ultrahigh pressure rocks an ion microprobe and chemical abrasion study understanding the formation and exhumation of the remarkable ultrahigh pressure uhp rocks of the western gneiss region norway hinges on precise determination of the time of eclogite recrystallization we conducted detailed thermal ionization mass spectrometry chemical abrasion analysis and sensitive high resolution ion microprobe analysis of zircons from four ultrahigh and high pressure hp rocks ion microprobe analyses from the flatraket eclogite yielded a broad range of apparently concordant caledonian ages suggesting long term growth in contrast higher precision thermal ionization mass spectrometry analysis of zircon subject to combined thermal annealing and multi step chemical abrasion yielded moderate pb loss from the first lowest temperature abrasion step possible minor pb loss or minor growth at 400 ma from the second step and a 407 404 ma cluster of slightly discordant 206pb 238u ages most likely free from pb loss from the remaining abrasion steps we interpret the latter to reflect zircon crystallization at 405 400 ma with minor discordance from inherited cores zircon crystallization occurred at eclogite facies possibly post peak conditions based on compositions of garnet inclusions in zircon as well as nearly flat hree profiles and lack of eu anomalies in zircon fractions subjected to chemical abrasion these ages are significantly younger than the 425 ma age often cited for western norway eclogite recrystallization implying faster rates of exhumation 2 5 8 5 km myr and coeval formation of eclogites across the uhp portion of the western gneiss region

prolonged 100 ma ultrahigh temperature metamorphism in the napier complex east antarctica a petrochronological investigation of earth s hottest crust

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

building the pamir tibetan plateau crustal stacking extensional collapse and lateral extrusion in the central pamir 1 geometry and kinematics asian deep crust exposed in the pamir permits determination of the amount sequence and interaction of shortening extension and lateral extrusion over 30 km of crustal section during the india asia collision in the central pamir gneiss domes and their hanging walls record paleogene tripling of the 7 10 km thick phanerozoic upper crustal strata total crustal thickness may have amounted to 90 km two thrust sheets comprising cambro ordovician respectively carboniferous to paleogene strata straddle the domes amphibolite facies metamorphic rocks within the domes equivalent to lower grade rocks outside the domes form fold nappes with dome scale wavelengths e w stretching occurred contemporaneously with top to n imbrication and folding at 22 12 ma bivergent top to n and top to s normal sense shear zones exhumed the crystalline rocks most of the extension occurred along the northern dome margins shortening resumed at 12 ma with opposite sense thrusting and folding focused along the dome margins throughout the building of the central and south pamir dominant n s shortening interacted with e w extension along mostly dextral shear fault zones in the neogene shear is concentrated along a dextral wrench corridor south of the domes we interpret the paleogene shortening to record thickening and northward growth of the pamir tibetan plateau and short lived miocene crustal extension as gravitational adjustment i e collapse of the thickened asian crust to indian slab breakoff synconvergent paleogene lateral extrusion thickened the afghan hindu kush crust west of the india asia collision and the miocene recent dextral shear and e w extension have accommodated collapse of the pamir plateau into the tajik depression

a petrogenetic model for the origin and compositional variation of the martian basaltic meteorites the major element trace element and isotopic compositional ranges of the martian basaltic meteorite source regions have been modeled assuming that planetary differentiation resulted from crystallization of a magma ocean the models are based on low to high pressure phase relationships estimated from experimental runs and estimates of the composition of silicate mars from the literature these models attempt to constrain the mechanisms by which the martian meteorites obtained their superchondritic cao al2o3 ratios and their source regions obtained their parent daughter 87 rb 86 sr 147 sm 144 nd and 176 lu 177 hf ratios calculated from the initial sr nd and hf isotopic compositions of the meteorites high pressure experiments suggest that majoritic garnet is the liquidus phase for mars relevant compositions at or above 12 gpa early crystallization of this phase from a martian magma ocean yields a liquid characterized by an elevated cao al2o3 ratio and a high mg olivine pyroxene garnet dominated cumulates that crystallize subsequently will also be characterized by superchondritic cao al2o3 ratios melting of these cumulates yields liquids with major element compositions that are similar to calculated parental melts of the martian meteorites furthermore crystallization models demonstrate that some of these cumulates have parent daughter ratios that are similar to those calculated for the most incompatible element depleted source region i e that of the meteorite queen alexandra que 94201 the incompatible element abundances of the most depleted que 94201 like source region have also been calculated and provide an estimate of the composition of depleted martian mantle the incompatible element pattern of depleted martian mantle calculated here is very similar to the pattern estimated for depleted earth s mantle melting the depleted martian mantle composition reproduces the abundances of many incompatible elements in the parental melt of que 94201 e g ba th k p hf zr and heavy rare earth elements fairly well but does not reproduce the abundances of rb u ta and light rare earth elements the source regions for meteorites such as shergotty are successfully modeled as mixtures of depleted martian mantle and a late stage liquid trapped in the magma ocean cumulate pile melting of this hybrid source yields liquids with major element abundances and incompatible element patterns that are very similar to the shergotty bulk rock

deep burial of asian continental crust beneath the pamir imaged with local earthquake tomography an inclined zone of intermediate depth seismicity beneath the pamir orogen in central asia has been interpreted as southward subduction of a slab of asian lithosphere however it is not known whether asian lithosphere subducts intact or only partially we used arrival times of shallow and intermediate depth earthquakes recorded with a temporary 2008 2010 seismic network in this region to invert for 3d models of seismic velocities in an attempt to answer this question with local seismicity reaching depths of up to 240 km the deep structure of the pamir could be illuminated with high resolution the resulting velocity models show a north south contrast in crustal seismic velocities in the pamir with very low p velocities 5 7 5 9 km s at 15 30 km depth coupled with relatively low vp vsvp vs 1 70 at mid crustal levels in the southern part of the orogen at sub moho depths we image an arcuate high velocity 8 2 8 6 km s slab dipping south in the eastern pamir and east in the pamir s southwest underlying the intermediate depth earthquakes on top of the high velocity slab and just above the onset of deep seismicity between a depth of 60 to 100 km very low compressional wavespeeds around 7 1 km s and high vp vsvp vs ratios 1 80 attest to subducted crustal rocks additionally we carried out 2d numerical thermomechanical modeling of the continental collision in the pamir focusing on the fate of the crust and mantle lithosphere of the asian and indian plates seismic velocities were computed from the modeling results and the resulting images were compared with the velocity distributions obtained from seismic traveltimes combining tomography and modeling results we infer that a substantial amount of crustal material is pulled down beneath the pamir by cold mantle lithosphere to depths of at least 80 100 km from there on only lower crust and mantle lithosphere continue their subduction and earthquakes occur inside the lower crustal layer probably due to metamorphic reactions the buoyant asian upper and middle crust does not penetrate deeper into the mantle but pools at this depth level from where it might eventually exhume or relaminate

building the pamir tibetan plateau crustal stacking extensional collapse and lateral extrusion in the central pamir 2 timing and rates geothermochronologic data outline the temperature deformation time evolution of the muskol and shatput gneiss domes and their hanging walls in the central pamir prograde metamorphism started before 35 ma and peaked at 23 20 ma reflecting top to n thrust sheet and fold nappe emplacement that tripled the thickness of the upper 7 10 km of the asian crust multimethod thermochronology traces cooling through 700 100 c between 22 and 12 ma due to exhumation along dome bounding normal sense shear zones synkinematic minerals date normal sense shear zone deformation at 22 17 ma age versus elevation relationships and paleoisotherm spacing imply exhumation at 3 km myr south of the domes mesozoic granitoids record slow cooling and or constant temperature throughout the paleogene and enhanced cooling 7 31 c myr starting between 23 and 12 ma and continuing today integrating the central pamir data with those of the east chinese pamir kongur shan and muztaghata domes and with the south pamir shakhdara dome implies i regionally distributed paleogene crustal thickening ii pamir wide gravitational collapse of thickened crust starting at 23 21 ma during ongoing india asia convergence and iii termination of doming and resumption of shortening following northward propagating underthrusting of the indian cratonic lithosphere at 12 ma westward lateral extrusion of pamir plateau crust into the hindu kush and the tajik depression accompanied all stages deep seated processes e g slab breakoff crustal foundering and underthrusting of buoyant lithosphere governed transitional phases in the pamir and likely the tibet crust

the lithosphere asthenosphere boundary in the tien shan karakoram region from s receiver functions evidence for continental subduction 1 significant s to p converted wave energy from the crust mantle and lithosphere asthenosphere boundaries the moho and the lab respectively has been observed in the tien shan and karakoram orogenic belts and used for the determination of the thickness of the crust and the mantle lithosphere the moho lies at depths of 45 65 km in the tien shan and adjacent regions and deepens to 70 km depth in pamir and karakoram the thickness of the lithosphere varies between 90 and 120 km underneath the tien shan and increases to a value of 160 km beneath the tarim basin to the south of the tarim basin the lower boundary of the asian lithosphere can be clearly followed to a depth of 270 km beneath central pamir and karakoram while the indian lithosphere is observed beneath the karakoram dipping from 130 km to 170 km towards the north the observations indicate a scenario of continental collision and subduction consistent with seismicity and the surface wave tomography results

plateau versus fissure ridge travertines from quaternary geothermal springs of italy and turkey interactions and feedbacks between fluid discharge paleoclimate and tectonics abstract morphologically different deposits of thermal travertines are known worldwide but what factors controlled their morphology volume and growth for tens of thousands of years is only partially understood two main morphotypes of quaternary thermal travertines are reconsidered here to understand the reasons for their differential growth the fissure ridge travertines of denizli basin western turkey and the travertine plateau of tivoli central italy for comparable longevities and average vertical deposition rates the main differences between the studied travertines are as follows 1 volume of the travertine plateau is about one hundred times larger than each fissure ridge 2 despite a larger volume the travertine plateau does not produce relief whereas the fissure ridges produce a characteristic prominent topography 3 the travertine plateau grew primarily through lateral progradation whereas the fissure ridges through vertical aggradation 4 travertine deposition occurred in different environments principally low energy flat or shallow environments at tivoli and high energy inclined environments at denizli 5 the growth of the tivoli plateau occurred in a subsiding basin whereas the fissure ridges were not influenced by significant subsidence 6 c and o isotope signatures from the two studied travertines are different 7 despite similar annual precipitations the present water discharge in the tivoli area is about ten times greater than that of the denizli basin u series ages from the two deposits are correlated with paleoclimate oscillations at regional and global scales geological field evidence together with paleoclimate correlations suggest that in both the study cases the main body of travertine deposits the bedded travertine grew preferentially when the water table was high warm and or humid periods conversely when the water table was depressed cold and or dry periods the tivoli travertine underwent partial erosion and the denizli ridges were cut by axial veins and lateral sill like structures filled by banded sparitic travertine a comparative model is proposed where the main factor driving the difference in the morphostratigraphic architecture of fissure ridges and travertine plateaus is the volume of water discharge a high discharge rate resulted in the precipitation of caco3 far away from the springs hence driving the lateral progradation of the tivoli plateau a reduced discharge rate caused travertine precipitation close to the springs thus causing the vertical aggradation of the denizli fissure ridges paleoclimate oscillations must have controlled the amount of fluid discharge which in turn must have influenced the opening of the feeding fractures by an increased pore pressure

geodynamic regimes of subduction under an active margin effects of rheological weakening by fluids and melts the dynamics of subduction under an active margin are analysed by using a 2d coupled petrological thermomechanical numerical model of an oceanic continental subduction system the model includes spontaneous slab bending dehydration of the subducted crust aqueous fluid transport partial melting of both crustal and mantle rocks as well as melt extraction processes resulting in magmatic arc crust growth based on our models the following five geodynamic regimes of subduction which may potentially form at active margins are identified i stable subduction with no backarc spreading centre and without plumes in the mantle wedge ii retreating subduction with a focused backarc spreading centre and without plumes iii retreating subduction with distributed intra arc extension and trans lithospheric sedimentary plumes iv advancing subduction with underplating laterally extending sub lithospheric plumes and v stable to advancing subduction with stationary laterally limited sub lithospheric plumes transitions between these different regimes are mainly caused by the concurrence of rheological weakening effects of i aqueous fluids percolating from the subducting slab into the mantle wedge and ii melts propagating from the partially molten areas formed in the mantle wedge towards the surface the aqueous fluids mainly affect the forearc region strong fluid related weakening promotes plate decoupling and reduces subduction drag and thus results in stacking of sediments in the accretion prism in contrast reduced weakening by fluids results in strong coupling of the plates and leads to advancing collision like subduction with enhanced subduction erosion thickening of the overriding plate and large sedimentary plumes in the mantle wedge are the consequences on the other hand melts extracted from the hot regions above the slab rheologically weaken the lithosphere below the arc which thus controls overriding plate extension and shortening strong rheological weakening by melts in combination with weak plate coupling trigger retreating subduction with a pronounced backarc spreading centre also weakening of the continental lithosphere by melts extracted from trans lithospheric sedimentary plumes generate a weak channel through which these structures may be emplaced into subarc crust if there is insufficient melt related weakening plumes cannot ascend but extend horizontally and thus underplate the lithosphere

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

sub micron scale trace element distributions in natural zircons of known provenance implications for ti in zircon thermometry nanosims measurements of ti p y ce and hf in zircon separates from the youngest toba tuff the bishop tuff the quottoon igneous complex the sierra nevada batholith and an adirondack migmatite show that micron scale oscillatory zoning of ti is common the zircons we have studied typically display banded concentration gradients having between 1 and 7 peaks in ti abundance over length scales of 10 20 m the beam diameter commonly used for sims trace element zircon analyses with amplitudes of up to 4 5 ppm ti baseline values are 1 4 ppm and widths measured at half height of between 0 2 and 4 m spatial correlations between concentrations of ti and other trace elements p y and ce are also common but variable in character ranging from oscillatory co variation of ti p y and ce to cases where only a subset of peaks for a given element is spatially correlated with peaks in ti there are also longer length scale generally positive correlations among concentrations of ti p y and ce i e gradients on which narrower peaks are superimposed in contrast hf concentrations are either uncorrelated or inversely correlated with these longer length scale variations in ti concentrations the wide range in ti concentrations over distances of less than 1 m and the various correlations between ti concentrations and those of the other analyzed elements suggest that on the micron scale and at temperatures between 700 and 800 c zircon liquid ti partitioning is not controlled by bulk or lattice equilibrium treating our nanosims ti ion maps as though they were conventional sims analyses i e generating an average ti concentration for each map we evaluate the hypothesis that while micron scale variations in ti concentrations might be kinetically controlled when averaged over 100 400 m2 such variations capture the thermal state of the growing zircon using these average ti concentrations independent petrologic pressure and temperature constraints estimates of silica and titania activities based on phase assemblages as well as calculations using rhyolite melts we show that crystallization temperatures predicted by ti in zircon geothermometry generally do not agree with the independently constrained temperatures for the samples

elysium planitia lava flows crater count chronology and geological implications mars global surveyor mgs and viking images allow analysis of chronological and geological relations among lava flows in southern elysium planitia based on crater populations mgs and viking images clearly show morphological features of lava flows with some extremely sparsely cratered young flow units atop somewhat older surfaces there is no evidence of substantial dust mantling on the young flows and hence we infer that the crater populations date not sediment accumulation but the lava flows themselves the youngest flows have some of the lowest crater densities that we have seen on mars some 1 of lunar mare values at crater diameters below about 180 to 500 m we infer that thin scattered volcanic flows some tens of meters thick have been emplaced atop stratigraphic units within the last 100 myr and possibly within the last 10 myr volcanism is thus a continuing process in the recent geologic history of mars and this must constrain geophysical models of the planet

elastic thickness estimates for the northern lowlands of mars abstract the northern lowlands cover approximately one third of mars surface and are a fundamental part of the geologic evolution of mars we examine the admittance signature the ratio of gravity to topography data in the spectral domain for four regions in the northern lowlands that are both well resolved in the gravity data and have well constrained lithospheric parameters these regions also have topographic power spectra similar to many highlands regions a cartesian multitaper method is used to calculate admittance we compare the observed admittance signatures to those predicted from models of lithospheric flexure on the basis of these comparisons we estimate the thickness of the martian elastic lithosphere t e required to support the observed topographic load since the time of loading we use both top and bottom loading models to derive values of elastic thickness and crustal thickness or apparent depth of compensation although all four regions are best fit by bottom loading models we obtain best fit elastic thickness estimates between 10 and 25 km with an acceptable error range of 0 to 45 km these low elastic thickness estimates are consistent with the formation in the noachian when heat flow was high the consistency in t e estimates between the noachian highlands and lowlands basement suggests that both regions of the crust formed at similar times the paucity of crustal magnetization in the lowlands is thus more likely a result of demagnetization than formation following shut down of the dynamo

on the valency state of radiogenic lead in zircon and its consequences in zircon u pb systematics extreme robustness up to the temperatures of granulite facies and anatexis contrasts with apparently easy loss of radiogenic pb at low temperatures often without any metamorphic event being in evidence here we propose that this paradoxical behaviour can be understood with the hypothesis that radiogenic pb in zircon is tetravalent we review data and arguments in favour of this hypothesis diffusion profiles calculated for pb sup 2 in a 25 micro m radius zircon xenocryst in a melt at 1000 c combined with the incompatibility of pb sup 2 or for a zircon core inside a younger zircon rim at this temperature show age effects that should have been observed in sims dating further in zircon evaporation as well as in leaching experiments common pb is generally released preferentially to radiogenic pb after removal of less radiogenic pb the evaporation record generally shows pure radiogenic pb during the final evaporation steps the distribution of residual pb in a leached titanite revealed by pixe is similar to that of ti lastly xanes spectra of a 1 ga old titanite predominantly radiogenic pb and an alpine one predominantly common pb are significantly different although the former does not resemble that ofmore pbo sub 2 the arguments why radiogenic pb should be tetravalent are based on analogies with studies relating to the tetravalent state of sup 234 th and the hexavalent state of sup 234 u which show that alpha recoil in silicates generates a strongly oxidizing environment at the site where the recoiling nucleus comes to rest further a zircon grain being small should remain highly oxidizing in its interior by the constant loss of beta particles maintaining the 4 state of radiogenic pb from its effective ion radius similar to that of zr sup 4 and its charge pb sup 4 has to be compatible in the zircon lattice also by analogy with u sup 4 th sup 4 and hf sup 4 its diffusivity should be several orders of magnitude lower than that measured for pb sup 2 in zircon these factors can account for the extreme retentivity of u pb clocks even at high temperatures on the other hand radiogenic pb situated in alpha recoil damaged sites could be leached out by any electrolyte solution that reduces it to the divalent state making it both incompatible and soluble thus discordia can be generated in weathering the curious observation that discordant archaean zircon suites generally define trends to lower intercepts at up to 800 ma instead of zero appears to reflect a c 14 to 28 better average accessibility to leaching of radiogenic sup 207 pb compared to sup 206 pb in accord with the somewhat higher recoil energy of the last branched alpha decays in the sup 235 u chain less

modeling the evolution of continental subduction processes in the pamir hindu kush region several geological and geophysical studies suggest the presence of two converging subduction zones in the western syntaxis of the india eurasia collision zone with steep northward subduction of indian lithosphere beneath the hindu kush and southward subduction of asian lithosphere under the pamir we investigate the geometry and the timing of these subduction processes seismic tomography images are used to constrain the geometry of the two slabs under the pamir and the hindu kush by measuring the tomographically inferred length of the indian slab under the hindu kush region and by comparison with paleomagnetic reconstructions we estimate that the process of indian slab break off most likely occurred at the early stage of the collision at ca 44 48 ma we infer that after slab break off western india continued its northward motion until its northern boundary reached the present hindu kush region where it began to subduct we estimate an age for the initiation of subduction in this region of about 8 ma we apply 2d thermo kinematic and rheological numerical modeling to compute the temperature distribution and brittle field along two vertical sections across the hindu kush and pamir regions for the hindu kush we introduce parameters reproducing fast and near vertical subduction whereas in the pamir region we use the results from published studies to reproduce slower subduction modeling results indicate that faster subduction under the hindu kush results in a deeper brittle region which is consistent with observed deeper seismicity than in the pamir region

variations of elastic plate thickness at continental thrust belts after examining 15 reliable estimates of the elastic thickness of the lithosphere where it underthrusts mountain belts in europe asia and the americas we conclude that the strength of continental lithosphere as a function of depth must be very different from that of oceanic lithosphere depending on the state of stress continental lithosphere may be either very much stronger or very much weaker than would be predicted for oceanic lithosphere of the same thermal age we explain this observation as the result of two competing effects a thicker thermal plate beneath the continents which allows a deeper elastic ductile transition for plates more than 100 million years old and a low activation energy for ductile creep in the continental crust which leads to massive failure of the upper elastic lithosphere at high bending stresses the 100 km values for elastic plate thickness of billion year old lithosphere require a thermal plate thickness of at least 250 km the magnitude of apparent thinning of the elastic plate at high stress is consistent with that calculated from a simple yield envelope which includes a ductile zone in the lower crust the low yield strength of continental materials leads to correlations between effective elastic plate thickness and all of the following dip of the underthrust plate radius of curvature of the thrust front in map view and total length of the mountain belt using insight derived from laboratory experiments on the buckling of spherical shells and drawing analogies with the geophysical characteristics of oceanic subduction zones we arrive at the following sequence of causes and effects the primary variable is the dip of the downgoing plate presumably prescribed by its own excess mass and pressure exerted by dynamic flow in the mantle where this dip is steep the rigidity of the plate is weakened due to inelastic yielding the weakened plate can then buckle at small wavelengths into short arcuate mountain belts thus we believe that the mantle ultimately controls the pattern of surface tectonics but that the nonelastic rheology of the lithosphere is the mechanism by which that control is imposed

an enhanced image of the pamir hindu kush seismic zone from relocated earthquake hypocentres summary we determine the shape of the seismic zone in the pamir hindu kush region de ned by 30 42 0 n 68 78 0 e by obtaining improved hypocentral locations with 90 per cent con dence limits of less than 30 km the depth error bar for most of the earthquakes of about 6000 shallow and intermediate depth earthquakes available s and depth phase arrival times are also used together with the p wave arrival times in the joint hypocentre determination technique to obtain the best possible hypocentral locations the study region is divided into three depth ranges 0 60 60 160 and 160 km the 0 60 km depth zone is then subdivided laterally into 19 blocks with the deeper regions divided into two blocks each the improved delineation of the seismic zone obtained by using the relocated hypocentres implies that the intermediate depth seismicity in the pamir hindu kush region is most simply explained by a single s shaped seismic zone 700 km long and no more than 30 km wide and with most activity concentrated at 100 300 km depth the main features observed are 1 the eastward steepening of the north dipping hindu kush seismic zone through to its overturning at its eastern end beneath the pamirs where it dips to the southeast 2 the curvature and forking of the subducting slab at depths greater than 200 km within the eastern part of the hindu kush seismic zone 3 the very abrupt cut oi in intermediate depth seismicity at 90 110 km depth with no extension to shallower depths under the pamirs and with a persistent gap between the intermediate and shallow seismicity in the northern pamirs and 4 the unusual horizontal t axes for intermediate depth earthquakes of the pamir seismic zone which align with its curvature this study shows that the seismic zone under the hindu kush has stress axes which follow the classical pattern for subducting slabs controlled by gravity whereas the pamir region has horizontal t axes that follow the trend of the contorted seismic zone this suggests that the pamir seismic zone is a slab deformed due to ow in the upper mantle

implications of large elastic thicknesses for the composition and current thermal state of mars abstract the martian elastic lithosphere thickness t e has recently been constrained by modeling the geodynamical response to loading at the martian polar caps and t e was found to exceed 300 km at the north pole today geological evidence suggests that mars has been volcanically active in the recent past and we have reinvestigated the martian thermal evolution identifying models which are consistent with t e 300 km and the observed recent magmatic activity we find that although models satisfying both constraints can be constructed special assumptions regarding the concentration and distribution of radioactive elements the style of mantle convection and or the mantle s volatile content need to be made if a dry mantle rheology is assumed strong plumes caused by e g a strongly pressure dependent mantle viscosity or endothermic phase transitions near the core mantle boundary are required to allow for decompression melting in the heads of mantle plumes for a wet mantle large mantle water contents of the order of 1000 ppm are required to allow for partial mantle melting also for a moderate crustal enrichment of heat producing elements the planet s bulk composition needs to be 25 and 50 sub chondritic for dry and wet mantle rheologies respectively even then models resulting in a globally averaged elastic thicknesses of t e 300 km are difficult to reconcile with most elastic thickness estimates available for the hesperian and amazonian periods it therefore seems likely that large elastic thicknesses in excess of 300 km are not representative for the bulk of the planet and that t e possibly shows a large degree of spatial heterogeneity

interaction of the indian and asian plates under the pamir and hindu kush regions insights from 3 d shear wave velocity and anisotropic structures

the giant shakhdara migmatitic gneiss dome pamir india asia collision zone 2 timing of dome formation 1 cenozoic gneiss domes exposing middle lower crustal rocks cover 30 of the surface exposure of the pamir western india asia collision zone they allow an unparalleled view into the deep crust of the asian plate we use titanite monazite and zircon u th pb mica rb sr and 40ar 39ar zircon and apatite fission track and zircon u th he ages to constrain the exhumation history of the 350 90 km shakhdara alichur dome southwestern pamir doming started at 21 20 ma along the gunt top to n normal shear zone of the northern shakhdara dome the bulk of the exhumation occurred by nnw ward extrusion of the footwall of the crustal scale south pamir normal shear zone along the southern shakhdara dome boundary footwall extrusion was active from 18 15 ma to 2 ma at 10 mm yr slip and with vertical exhumation rates of 1 3 mm yr it resulted in up to 90 km n s extension coeval with n s convergence between india and asia erosion rates were 0 3 0 5 mm yr within the domes and 0 1 0 3 mm yr in the horst separating the shakhdara and alichur domes and in the southeastern pamir plateau rates were highest along the dome axis in the southern part of the shakhdara dome incision along the major drainages was up to 1 0 mm yr thermal modeling suggests geothermal gradients as high as 60 c km along the trace of the south pamir shear zone and their strong n s variation across the dome the gradients relaxed to 40 45 c km since the end of doming

evidence for recent volcanism on mars from crater counts impact craters help characterize the age of a planetary surface because they accumulate with time they also provide useful constraints on the importance of surface erosion as such processes will preferentially remove the smaller craters earlier studies of martian crater populations revealed that erosion and dust deposition are important processes on mars they disagreed however on the age of the youngest volcanism these earlier studies were limited by image resolution to craters larger than a few hundred metres in diameter here we report an analysis using new images obtained by the mars global surveyor spacecraft of crater populations that extend the size distribution down to about 16 m our results indicate a wide range of surface ages with one region lava flows within the arsia mons caldera that we estimate to be no older than 40 100 million years we suggest that volcanism is a continuing process on mars

synchronous oligocene miocene metamorphism of the pamir and the north himalaya driven by plate scale dynamics gneiss domes in the pamir central asia and the himalaya provide key data on mid to deep crustal processes operating during the india asia collision laser ablation split stream inductively coupled plasma mass spectrometry lass icp ms data from monazite in these domes yield a time record from u th pb dates and a petrologic record from rare earth element ree abundances seven samples from the pamir and six samples from the north himalayan gneiss domes yield almost identical monazite dates of ca 28 15 ma most monazite has invariant heavy ree hree abundances two samples however have older monazite that records progressive hree depletion and two samples have younger monazite that records progressive hree enrichment these variations in hree are compatible with increasing garnet abundance prograde metamorphism until ca 20 ma and decreasing garnet abundance thereafter the change from hree depletion to enrichment may record a transition from crustal thickening and heating to dome exhumation and cooling this documentation of synchronous barrovian metamorphism within domes of indian crust along the margin of the orogen himalaya and within domes of asian crust within the core of the orogen pamir is best explained by a plate scale driving force rather than by local events we propose that widespread synchronous thickening was initiated by the resumption of indian subduction following slab breakoff and then terminated by a second slab tearing event both plate scale events inferred from tomography

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

core formation in the earth and shergottite parent body spb chemical evidence from basalts constraints on processes of core formation in terrestrial planets may be inferred from the abundances of siderophile and chalcophile elements in their mantles of particular interest is a comparison of processes of core formation in the earth and in the shergottite parent body spb a terrestrial planet tentatively inferred to be mars to this end we i present new inaa and rnaa analyses of the non antarctic snc meteorites ii infer the composition of the spb mantle from the compositions of the snc meteorites iii infer the composition of the earth s mantle from the compositions of terrestrial basalts and iv deconvolve the effects of volatile depletion core formation and mineral melt fractionation on the abundances of siderophile and chalcophile elements in the spb and the earth element abundances in the mantles of the spb and the earth are estimated from element element correlations observed among basalt samples in basalts from the spb the snc meteorites four groups of covariant elements are observed highly incompatible moderately incompatible indifferent and compatible from correlations within these groups the spb mantle is found to be depleted in volatile elements and strongly depleted in siderophile and chalcophile elements for the earth the element element correlation method gives a mantle composition similar to previous estimates compared to the earth the spb mantle is richer in moderately siderophile elements e g w p consistent with its inferred higher oxidation state chalcophile elements in the spb mantle are more depleted than in the earth s mantle particularly when compared to estimates of the original abundances of volatile chalcophile elements in the two planets in the spb mantle the nico ratio is nonchondritic in contrast to the chondritic ratio in the earth s mantle abundances of siderophile and chalcophile elements in the spb mantle may be modelled by equilibrium with solid metal and metallic sulfide liquid with some metal and sulfide trapped in the mantle i e homogeneous accretion and inefficient core formation neither this model nor a heterogeneous accretion model is satisfactory in explaining element abundances in the earth s mantle particularly the abundances of ni co mo and w nevertheless it appears that core formation in the spb and the earth left quite different chemical signatures in their planetary mantles

zooming into the hindu kush slab break off a rare glimpse on the terminal stage of subduction abstract the terminal stage of subduction sets in when the continental margin arrives at the trench and the opposite forces of the sinking slab and buoyant continent extend and ultimately sever the subducted lithosphere this process although common in geological history is short lived and therefore rarely observed the deep seismicity under the hindu kush central asia including the 2015 m w 7 5 event is a rare case that testifies to this process here we use new seismological data to create a high resolution picture of slab break off and infer its dynamics high precision earthquake locations and tomographic images show subduction of continental crust down to 180 km a large dataset of source mechanisms indicates sub vertical extension in the entire slab but a strain rate analysis showed that the deeper seismogenic portion of the slab below the subducted crust extends at higher rates 40 km ma most m w 7 earthquakes between 1983 2015 relocated relative to our new well constrained earthquake catalog cluster in a small volume below 180 km and indicate shearing on an overturned interface a slip model for the latest 2015 m w 7 5 event suggests that it ruptured into a seismic gap on this interface from this configuration we conclude that a horizontal slab tear develops along strike of the hindu kush seismic zone at the base of the subducted continental crust below the subducted crust the deepest and also largest earthquakes 180 265 km are likely associated with deformation in the mantle lithosphere from the seismicity distribution and the rupture mechanisms we further deduce that the dominant deformation mechanism in this deeper portion of the slab changes along strike from simple to pure shear the fastest detachment rates and largest earthquakes occur during the simple shear dominated stage earthquakes in the upper part 60 180 km above the rapidly extending slab might be triggered by processes related to the subduction of crustal rocks

metamorphism and microstructures along a high temperature metamorphic field gradient the north eastern boundary of the kralovsky hvozd unit bohemian massif czech republic a metamorphic field gradient has been investigated in the moldanubian zone of the central european variscides encompassing from base to the top a staurolite kyanite zone a muscovite sillimanite zone a k feldspar sillimanite zone and a k feldspar cordierite zone respectively the observed reaction textures in the anatectic metapsammopelites of the higher grade zones are fully compatible with experimental data and petrogenetic grids that are based on fluid absent melting reactions from structural and microstructural observations it can be concluded that the boundary between the kyanite staurolite zone and the muscovite and k feldspar sillimanite zones coincides with an important switch in deformation mechanism s besides minor syn anatectic shearing melt enhanced deformation microstructural criteria point a to a switch in deformation mechanism from rotation recrystallization climb accommodated dislocation creep to prism slip and high temperature fast grain boundary migration in quartz b to the activity of diffusion creep in quartz feldspar layers and c to accommodation of strain by intense shearing in fibrolite biotite layers it is suggested that any combination of these deformation mechanisms will profoundly affect the rheological characteristics of high grade metamorphic rocks and significantly lower rock strength hence the boundary between these zones marks a major rheological barrier in the investigated cross section and probably also in other low to medium pressure high temperature areas at still higher metamorphic grades k feldspar cordierite zone where the rheologically critical melt percentage is reached rock rheology is mainly governed by the melt and other deformation mechanisms are of minor importance in the study area the switch in deformation mechanism s is responsible for large scale strain partitioning and concentration of deformation within the higher temperature hanging wall during top to the s thrusting thus preserving a more complete petrostructural record within the rocks of the footwall including indications for a devonian high to medium pressure medium temperature metamorphic event thrusting is accompanied by diapiric ascent of diatexites of the k feldspar cordierite zone and infolding of the footwall suggesting local crustal overturn in this part of the moldanubian zone

intercalibration of 40ar 39ar age standards nl 25 hb3gr hornblende ga1550 sb 3 hd b1 biotite and bmus 2 muscovite abstract 40 ar 39 ar dating requires precise and accurate age data of standard fluence monitors besides primary calibration of k and 40 ar concentrations i e the ages are calculated with precise k ar dating intercalibration of the different monitors in use provides important cross checks of the validity of standard ages and implicitly reliability and uncertainty of chronological studies that rely on these standards here we report intercalibration of 5 primary international 40 ar 39 ar standards and one internal laboratory monitor a major goal of this study was to check the ages of the nl 25 hornblende standard and of the muscovite laboratory standard bmus 2 against the international standards hd b1 ga1550 sb 3 and hb3gr in replicate measurements of three irradiation series 40 ar 39 ar ages for the fluence monitors were found which agree within 1 error limits with the reported age values of 2660 9 ma for nl 25 hb3gr 1073 6 4 6 ma bmus 2 328 5 1 1 ma sb 3 162 9 0 8 ga1550 98 79 0 54 and 24 21 0 32 for hd b1 for the hornblende nl 25 fluence monitor a new slightly different 40 ar 39 ar age of 2657 4 ma is suggested for the age of the muscovite bmus 2 laboratory standard we recommend 328 5 1 1 ma and for biotite hd b1 a more precise value of 24 18 0 09 ma

tio2 activity in metamorphosed pelitic and basic rocks principles and applications to metamorphism in southeastern canadian cordillera the activity of tio2 can be precisely defined as a function of pressure temperature and activities of other components for common mineral assemblages in metapelites ilmenite quartz garnet plagioclase al2sio5 and in metabasites plagioclase sphene ilmenite quartzgarnet these mineral assemblages can be modelled by the equilibria 1 3ilmenite al2sio5 2quartz almandine 3tio2 2 anorthite 2sphene grossular 2tio2 quartz 3 3anorthite 3quartz 6ilmenite grossular 6tio2 2almandine these mineral assemblages can be used at a text tio text 2 1 rutile saturation and a given t to get maximum pressure limits of some metapelites and metabasites when electron microprobe analyses of mineral grains adjacent to ti bearing phases are made these data give maximum pressure estimates in reasonable agreement with other geobarometers the activity of tio2 in many metapelites is very near rutile saturation but for metabasites the activity of tio2 in some sillimanite zone rocks is as low as 0 6 the solubility of tio2 in biotite hornblende and garnet is a complex function of t p the activities of components in coexisting minerals and crystal chemical constraints in these minerals at a given p and t the solubility of tio2 in biotite and hornblende does not appear to be strongly dependent upon a text tio text 2 1 for sphene and ilmenite versus rutile bearing assemblages

missing oligocene crust of the izu bonin arc consumed or rejuvenated during collision the 50 myr old izu bonin mariana ibm arc consists mostly of oligocene middle and lower crust that underlies the upper crust these units are in turn covered by quaternary volcanic rocks seismic imaging forearc geology ocean drilling program drilling and magnetic anomalies suggest that most ibm arc crust was created in eocene oligocene times however remnants of this old crust have never been found at the northern end of the arc where it is colliding with the honshu arc izu collision zone two batholiths in this collision zone thetanzawa tonalites and the kofu granitic complex were emplaced during the miocene 4 17 ma major elements zr y rare earth element ratios and normalized abundance patterns and sr nd isotopic data indicate that these plutonic bodies are compositionally similar to the oligocene ibm volcanic rocks and that they are dissimilar to the miocene pliocene and quaternary ibm lavas and volcaniclastic rocks we suggest that the miocene plutonic rocks in the izu collision zone were derived from partially melted oligocene middle crust a model is proposed in which ibm arc middle crust in the collision zone was partially melted during the collision and then intruded into the overlying upper crust of the honshu and ibm arcs this resulted in the complete loss of chronological information related to their original source

a view into crustal evolution at mantle depths abstract crustal foundering is an important mechanism in the differentiation and recycling of continental crust nevertheless little is known about the dynamics of the lower crust the temporal scale of foundering and its role in the dynamics of active margins and orogens this particularly applies to active settings where the lower crust is typically still buried and direct access is not possible crustal xenoliths derived from mantle depth in the pamir provide a unique exception to this the rocks are well preserved and comprise a diverse set of lithologies many of which re equilibrated at high pressure conditions before being erupted in their ultrapotassic host lavas in this study we explore the petrological and chronological record of eclogite and felsic granulite xenoliths we utilized accessory minerals zircon monazite and rutile for coupled in situ trace element analysis and u th pb chronology by laser ablation split stream inductively coupled plasma mass spectrometry each integrated analysis was done on single mineral zones and was performed in situ in thin section to maintain textural context and the ability to interpret the data in this framework rutile thermo chronology exclusively reflects eruption 11 17 0 06 ma which demonstrates the reliability of the u pb rutile thermo chronometer and its ability to date magmatic processes conversely zircon and monazite reveal a series of discrete age clusters between 55 11 ma with the youngest being identical to the age of eruption matching age populations between samples despite a lack of overlapping ages for different chronometers within samples exhibit the effectiveness of our multi mineral approach the ree systematics and age data for zircon and monazite and ti in zircon data together track the history of the rocks at a million year resolution the data reveal that the rocks resided at 30 40 km depth along a stable continental geotherm at 720 750 c until 24 20 ma and were subsequently melted densified and buried to 80 90 km depth 20 km deeper than the present day moho at 930 35 c the material descended rapidly accelerating from 0 9 1 7 mm yr 1 to 4 7 5 8 mm yr 1 within 10 12 myr and continued descending after reaching mantle depth at 14 13 ma the data reflect the foundering of differentiated deep crustal fragments 2 9 3 5 g cm 3 into a metasomatized and less dense mantle wedge through our new approach in constraining the burial history of rocks we provided the first time resolved record of this crustal recycling process foundering introduced vestiges of old evolved crust into the mantle wedge over a relatively short period c 10 myr the recycling process could explain the variability in the degree of crustal contamination of mantle derived magmatic rocks in the pamir and neighboring tibet during the cenozoic without requiring a change in plate dynamics or source region

observations of guided waves from the pamir seismic zone provide additional evidence for the existence of subducted continental lower crust abstract as part of the tipage tien shan pamir geodynamic program project passive seismological observations were made along an approximately n s profile crossing the pamir seismic zone for about one year from these observations guided waves were recognized these guided waves occur as a single continuous secondary compressional p wave phase behind the first p wave arrivals an equivalent phase in the shear s wavefield is hardly recognizable modelling of the phase shows that an approximately 10 km thick low velocity zone lvz between the moho and about 160 km depth reproduces the guided waves as a single continuous phase much better than a 15 20 km thick lvz modelling of the arrival times of the guided waves reveals that a model with a p wave velocity of 6 3 km s above about 100 km depth and a velocity of 7 6 km s between this depth and the deep cluster of earthquakes at about 150 km depth provides the best fit to the observed travel time data one plausible way to explain the low velocity of 6 3 km s is to invoke the presence of melts in the lvz then taking a velocity of 6 9 km s for the lower crust being subducted about 10 13 melt is required to obtain a velocity of about 6 3 km s in the lvz between the moho and about 100 km depth this would be in keeping with the estimated burial depths from xenoliths of gondwana terrane affinity brought to the surface in the southeastern pamir around 11 million yr ago the present day lvz is interpreted to comprise continental lower crust although guided waves are known to exist associated with subducted oceanic crust or fault zones this is the first time to the knowledge of the authors that guided waves have been observed resulting from a lvz associated with subducted continental lower crust

pb diffusion in zircon abstract chemical diffusion of ti under anhydrous conditions at 1 atm and under fluid present elevated pressure 1 1 1 2 gpa conditions has been measured in natural zircon the source of diffusant for 1 atm experiments was a zro2 tio2 zircon mixture with experiments run in crimped pt capsules diffusion experiments done in the presence of h2o co2 fluid were run in a piston cylinder apparatus using a source of ground tio2 zrsio4 and sio2 with oxalic acid added to produce h2o co2 vapor and partially melt the solid source material yielding an assemblage of rutile zircon melt vapor nuclear reaction analysis nra with the resonant nuclear reaction 48ti p 49v was used to measure diffusion profiles for both sets of experiments the following arrhenius parameters were obtained for ti diffusion normal to cover the temperature range 1350 1550 c at 1 atm d ti 3 33 10 2 exp 754 56 kj mol 1 r t m 2 s 1 ti diffusivities were found to be similar for experiments run under fluid present conditions a fit to all of the data yields the arrhenius relation d 1 34 102 exp 741 46 kj mol 1 rt m2 s 1 these data suggest that zircon should be extremely retentive of ti chemical signatures indicating that the recently developed ti in zircon crystallization geothermometer watson e b harrison t m 2005 zircon thermometer reveals minimum melting conditions on earliest earth science 308 841 844 will be quite robust in preserving temperatures of zircon crystallization titanium diffuses somewhat faster in zircon than larger tetravalent cations u th and hf but considerably more slowly than pb the ree and oxygen hence ti crystallization temperatures may be retained under circumstances when radiometric ages or other types of geochemical information are lost

experimental determination of ree partition coefficients between zircon garnet and melt a key to understanding high t crustal processes the partitioning of rare earth elements ree between zircon garnet and silicate melt was determined using synthetic compositions designed to represent partial melts formed in the lower crust during anatexis the experiments performed using internally heated gas pressure vessels at 7 kbar and 900 1000 c represent equilibrium partitioning of the middle to heavy ree between zircon and garnet during high grade metamorphism in the mid to lower crust the dree zircon garnet values show a clear partitioning signature close to unity from gd to lu because the light ree have low concentrations in both minerals values are calculated from strain modelling of the middle to heavy ree experimental data these results show that zircon is favoured over garnet by up to two orders of magnitude the resulting general concave up shape to the partitioning pattern across the ree reflects the preferential incorporation of middle ree into garnet with dgd zircon garnet ranging from 0 7 to 1 1 dho zircon garnet from 0 4 to 0 7 and dlu zircon garnet from 0 6 to 1 3 there is no significant temperature dependence in the zircon garnet ree partitioning at 7 kbar and 900 1000 c suggesting that these values can be applied to the interpretation of zircon garnet equilibrium and timing relationships in the ultrahigh t metamorphism of low ca pelitic and aluminous granulites

high resolution 40ar 39ar dating using a mechanical sample transfer system combined with a high temperature cell for step heating experiments and a multicollector argus noble gas mass spectrometer 40ar 39ar dating of young 5 10 16 mol 36ar is better than 0 5 1 0 1 n 4 8 we illustrate the system performance by 40ar 39ar dating of whole rock samples and mineral separates from the oman ophiolite as well as from the siebengebirge heldburg and rhon volcanic provinces in central germany

assembly of the pamirs age and origin of magmatic belts from the southern tien shan to the southern pamirs and their relation to tibet 1 magmatic rocks and depositional setting of associated volcaniclastic strata along a north south traverse spanning the southern tien shan and eastern pamirs of kyrgyzstan and tajikistan constrain the tectonics of the pamirs and tibet the northern pamirs and northwestern tibet contain the north facing kunlun suture the south facing jinsha suture and the intervening carboniferous to triassic karakul mazar subduction accretion system the latter is correlated with the songpan garze hoh xi system of tibet the kunlun arc is a composite early paleozoic to late paleozoic triassic arc arc formation in the pamirs is characterized by 370 320 ma volcanism that probably continued until the triassic the cryptic tanymas suture of the southern northern pamirs is part of the jinsha suture a massive 227 ma batholith stitches the karakul mazar complex in the pamirs there are striking similarities between the qiangtang block in the pamirs and tibet like tibet the regional structure of the pamirs is an anticlinorium that includes the muskol and sares domes like tibet the metamorphic rocks in these domes are equivalents to the karakul mazar songpan garze system granitoids intruding the qiangtang block yield 200 230 ma ages in the pamirs and in central tibet the stratigraphy of the eastern pshart area in the pamirs is similar to the bangong nujiang suture zone in the amdo region of eastern central tibet but a triassic ocean basin sequence is preserved in the pamirs arc type granitoids that intruded into the eastern pshart oceanic basin arc sequence 190 160 ma and granitoids that cut the southern qiangtang block 170 160 ma constitute the rushan pshart arc cretaceous plutons that intruded the central and southern pamirs record a long lasting magmatic history their zircons and those from late miocene xenoliths show that the most distinct magmatic events were cambro ordovician 410 575 ma triassic 210 250 ma likely due to subduction along the jinsha suture middle jurassic 147 195 ma subduction along rushan pshart suture and mainly cretaceous middle and late cretaceous magmatism may reflect arc activity in asia prior to the accretion of the karakoram block and flat slab subduction along the shyok suture north of the kohistan ladakh arc respectively before india and asia collided the pamir region from the indus yarlung to the jinsha suture was an andean style plate margin our analysis suggests a relatively simple crustal structure for the pamirs and tibet from the kunlun arc in the north to the southern qiangtang block in the south the pamirs and tibet likely have a dominantly sedimentary crust characterized by karakul mazar songpan garze accretionary wedge rocks the crust south of the southern qiangtang block is likely of granodioritic composition reflecting long lived subduction arc formation and cretaceous cenozoic underthrusting

geochemical evidence for magmatic water within mars from pyroxenes in the shergotty meteorite observations of martian surface morphology have been used to argue that an ancient ocean once existed on mars1 it has been thought that significant quantities of such water could have been supplied to the martian surface through volcanic outgassing but this suggestion is contradicted by the low magmatic water content that is generally inferred from chemical analyses of igneous martian meteorites2 here however we report the distributions of trace elements within pyroxenes of the shergotty meteorite a basalt body ejected 175 million years ago from mars3 as well as hydrous and anhydrous crystallization experiments that together imply that water contents of pre eruptive magma on mars could have been up to 1 8 we found that in the shergotty meteorite the inner cores of pyroxene minerals which formed at depth in the martian crust are enriched in soluble trace elements when compared to the outer rims which crystallized on or near to the martian surface this implies that water was present in pyroxenes at depth but was largely lost as pyroxenes were carried to the surface during magma ascent we conclude that ascending magmas possibly delivered significant quantities of water to the martian surface in recent times reconciling geologic and petrologic constraints on the outgassing history of mars

subduction and slab breakoff controls on asian indentation tectonics and himalayan western syntaxis formation 1 we test the link between large scale asian continent deformations and indian slab subduction and breakoff during convergence by means of three dimensional numerical models of the subducting upper plates system we find that the subduction of the buoyant continent results in the reduction of convergence velocity comparable to that observed in the indian motions yet the upper plate deformation remains accommodated in a narrow belt along a straight margin comparable rates are measured when the subducting slab breakoff is modeled although the convergent margin deforms and curves markedly with large underplating contiguous to ongoing subduction similar to what observed along the himalayan range the models support the interpretation of the himalayan western syntaxis evolution the progressive curvature of the indian margin and the underthrusting as a consequence of the indian slab breakoff the modeled slab detachment is followed by short lived large stresses in the upper plate interiors propagating at large distance from the margin with a trend similar to several major asian lithospheric faults such localized stress has likely provided the conditions for the formation of the central asian intracontinental faulting the bangong red river and the altyn tagh faults that followed successive indian slab breakoff episodes continent subduction and breakoff during india asia convergence offer an explanation for the different deformation mechanisms as the long lived understhrusting and the episodic lithospheric faulting in the asian continent and their link to deep processes

small scale convective instability and upper mantle viscosity under california thermal calculations and convection analysis constrained by seismic tomography results suggest that a small scale convective instability developed in the upper 200 kilometers of the mantle under california after the upwelling and cooling of asthenosphere into the slab window associated with the formation of the san andreas transform boundary the upper bound for the upper mantle viscosity in the slab window 5 x 10 19 pascal seconds is similar to independent estimates for the asthenosphere beneath young oceanic and tectonically active continental regions these model calculations suggest that many tectonically active continental regions characterized by low upper mantle seismic velocities may be affected by time dependent small scale convection that can generate localized areas of uplift and subsidence

crustal and uppermost mantle velocity structure along a profile across the pamir and southern tien shan as derived from project tipage wide angle seismic data summary utilizing seismic refraction wide angle reflection data from 11 approximately in line earthquakes 2 d p and s velocity models and a poisson s ratio model of the crust and uppermost mantle beneath the southern tien shan and the pamir have been derived along the 400 km long main profile of the tipage tien shan pamir geodynamic program project these models show that the crustal thickness varies from about 65 5 km close to the southern end of the profile beneath the south pamir through about 73 6 km under lake karakul in the north pamir to about 57 7 km 50 km south of the northern end of the profile in the southern tien shan average crustal p velocities are low with respect to the global average varying from 6 26 to 6 30 km s 1 the average crustal s velocity varies from 3 54 to 3 70 km s 1 along the profile and thus average crustal poisson s ratio varies from 0 23 beneath the central pamir in the south central part of the profile to 0 265 towards the northern end of the profile beneath the southern tien shan the main layer of the upper crust extending from about 2 km below the earth s surface to 27 km depth below sea level b s l has average p velocities of about 6 05 6 1 km s 1 except beneath the south central part of the profile where they decrease to around 5 95 km s 1 this is in contrast to the s velocities which range from 3 4 to 3 6 km s 1 and exhibit the highest values of 3 55 3 6 km s 1 where the p velocity is lowest thus for the main layer of the upper crust is 0 26 beneath the profile except beneath the south central part of the profile where it decreases to 0 22 the low value of 0 22 for under the central pamir the along strike equivalent of the qiangtang terrane in tibet is similar to that within the corresponding layer beneath the northern lhasa and southern qiangtang terranes in central tibet and is indicative of felsic rocks rich in quartz in the state the lower crust below 27 km b s l has p velocities ranging from 6 1 km s 1 at the top to 7 1 km s 1 at the base further for this layer is 0 27 0 28 towards the northern end of the profile but is low at about 0 24 beneath the central and southern parts of the profile which is similar to the situation found in the northeast tibetan plateau the low values can be explained by felsic schists and gneisses in the upper part of the lower crust transitioning to granulite facies and possibly also eclogite facies metapelites in the lower part within the uppermost mantle the average p velocity is about 8 10 8 15 km s 1 and is about 0 26 assuming an isotropic situation then a relatively cool 700 800 c uppermost mantle beneath the profile is indicated this would in turn indicate an intact mantle lid beneath the profile an upper mantle reflector dipping from 104 km b s l 120 km from the southern end of the profile to 86 km b s l 155 km from the northern end of the profile has also been identified the preferred model presented here for the crustal and lithospheric mantle structure beneath the pamir calls for nearly horizontal underthrusting of relatively cool indian mantle lithosphere the leading edge of which is outlined by the pamir seismic zone this cool indian mantle lithosphere is overlain by significantly shortening warm asian crust the moho trough that is a feature seen beneath some other orogenic belts for example the alps and the urals beneath the northern pamir may mark the southern tip of the actively underthrusting tien shan crust along the main pamir thrust

the giant shakhdara migmatitic gneiss dome pamir india asia collision zone 1 geometry and kinematics 1 cenozoic gneiss domes comprise one third of the surface exposure of the pamir and provide a window into the deep crustal processes of the india asia collision the largest of these are the doubly vergent composite shakhdara alichur domes of the southwestern pamir tajikistan and afghanistan they are separated by a low strain horst top to sse noncoaxial pervasive flow over the up to 4 km thick south pamir shear zone exhumed crust from 30 40 km depth in the 250 80 km shakhdara dome the top to nne alichur shear zone exposed upper crustal rocks in the 125 25 km alichur dome the gunt shear zone bounds the shakhdara dome in the north and records alternations of normal shear and dextral transpression it contributed little to bulk exhumation footwall exhumation along two low angle normal sense detachments resulted in up to 90 km syn orogenic n s extension extension in the southwestern pamir opposes shortening in a fold thrust belt north of the domes and in particular in the tajik depression where an evaporitic decollement facilitated upper crustal shortening gravitational collapse of the pamir plateau margin drove core complex formation in the southwestern pamir and shortening of the weak foreland adjacent to the plateau overall this geometry defines a vertical extrusion scenario comprising frontal and basal underthrusting and thickening and hanging gravitationally driven normal shear in contrast to the himalayan vertical extrusion scenario erosion in the pamir was minor preserving most of the extruded deep crust including the top of the south pamir shear zone at peak elevations throughout the dome

influence of a variable thermal conductivity on the thermochemical evolution of mars 1 the influences of variations in thermal conductivity due to composition temperature and pressure in mantle and crust on the thermochemical evolution of mars have been examined with parameterized convection models we use a temperature and pressure dependent thermal conductivity in the mantle and column averaged values between 1 5 and 4 wm 1k 1 for the crust with 2 wm 1k 1 as the representative value for a basaltic crust the major characteristic of the models is a slower cooling of the upper mantle with a long existing global partial melt zone and a stronger crustal production late in the evolution compared to earlier models which assume a constant thermal conductivity of 4 wm 1k 1 to explain the observed crustal thickness and the early crustal formation on mars the formation of an enriched primordial crust is required recent volcanism can be explained by lateral temperature variations that are caused by thickness variations of the low conducting crust these melt zones which occur underneath thicker than average crust may be the source region for the observed longstanding volcanism in tharsis and elysium instead of plumes rising from the core mantle boundary the evolution of the elastic lithosphere is characterized by a transition at which the elastic strength is first provided by the crust and later also by the upper mantle this finding may explain the estimated elastic lithosphere thickness of less than 16 km in the noachian and more than 100 km in the hesperian and amazonian

an integrated microtextural and chemical approach to zircon geochronology refining the archaean history of the napier complex east antarctica integrated textural and chemical characterisation of zircon is used to refine the u pb geochronology of the archaean ultra high temperature napier complex east antarctica scanning electron microscope characterisation of zircon and the rare earth element compositions of zircon garnet and orthopyroxene are integrated to place zircon growth in an assemblage context thereby providing tighter constraints on the timing of magmatic and metamorphic events data indicate that magmatism occurred in the central and northern napier complex at ca 2 990 ma a regional relatively low pressure metamorphic event occurred at ca 2 850 2 840 ma mineral ree data from garnet bearing orthogneiss indicate that ca 2 490 2 485 ma u pb zircon ages provide an absolute minimum age for the ultrahigh temperature uht foliation preserved in this rock internal zircon zoning relationships and estimated zircon garnet dree values from paragneiss suggest that an absolute minimum age of ultra high temperature metamorphism is ca 2 510 ma but that it is more likely to be older than ca 2 545 ma we suggest that the high proportion of published zircon u pb data with ages between ca 2 490 2 450 ma reflects late post peak zircon growth and does not date the timing of peak uht metamorphism

foundering triggered by the collision of india and asia captured in xenoliths xenoliths that erupted in the se pamir of tajikistan from 1000 to 1050 c and 90 km depth illuminate what happens when crust founders into the mantle 40ar 39ar dating of minerals from the xenoliths and volcanic host rocks of the shoshonitic dunkeldik pipe and dike field indicates eruption at 11 2 0 2 ma u pb and trace element laser ablation split stream inductively coupled plasma mass spectrometry of zircon shows that the igneous and metasedimentary xenoliths were likely derived from the crustal section into which they were intruded the jurassic cretaceous andean style magmatic arc and its proterozoic mesozoic host rocks along the southern margin of asia recrystallization of these zircons was extensive yielding a range of dates down to 11 ma the zircons show distinct changes in eu anomaly lu gd ratio and ti concentrations compatible with garnet growth and minimal heating at 22 20 ma and then 200 300 c of heating 25 km of burial and alkali carbonate melt injection at 14 11 ma these changes are interpreted to coincide with i heat input due to indian slab breakoff at 22 20 ma and ii rapid thickening and foundering of the pamir lithosphere at 14 11 ma prior to and synchronous with collision between deep indian and asian lithospheres beneath the pamir

seismic imaging of subducting continental lower crust beneath the pamir abstract exhumation of ultra high pressure metamorphic rocks testifies that the continental crust can subduct to significant depth into the mantle despite its buoyancy however direct observation of ongoing subduction of continental crust is rare the pamir is regarded as a possible place of active continental subduction because of the intermediate depth seismicity crustal xenoliths and estimates of crustal shortening versus convergence rates here we present for the first time receiver function images from a passive source seismic array traversing the tien shan and the pamir plateau showing southward subduction of eurasian continental crust in the eastern pamir we observe a southerly dipping 10 15 km thick low velocity zone lvz that extends from 50 km depth near the base of the crust to more than 150 km depth with a dip angle increasing to subvertical while the upper and mid crustal material seems to be shortened and incorporated into the pamir the lower eurasian crust detaches and subducts in its deeper part 80 km the lvz envelopes the intermediate depth earthquakes our observations imply that the complete arcuate intermediate depth seismic zone beneath the pamir traces a slab of subducting eurasian continental lower crust

acceleration and deceleration of india asia convergence since the cretaceous roles of mantle plumes and continental collision 1 a strong 50 35 ma decrease in india asia convergence is generally ascribed to continent continent collision however a convergence rate increase of similar magnitude occurred between 65 50 ma an earlier increase occurred at 90 ma both episodes of accelerated convergence followed upon arrival of a mantle plume below and emplacement of a large igneous province lip on the indian plate we here first confirm these convergence rate trends reassessing the indo atlantic plate circuits then using two different numerical models we assess whether plume head arrival and its lateral asthenospheric flow may explain the plate velocity increases and whether decreased plume flux and increasing continent plume distance may explain deceleration even without continental collision the results show that plume head arrival can indeed lead to absolute indian plate motion accelerations on the order of several cm yr followed by decelerations on timescales similar to the reconstructed fluctuations the 90 ma increase could potentially be explained as response to the morondova mantle plume alone the 65 50 ma convergence rate increase however is larger than can be explained by plume head spreading alone we concur with previous hypotheses that plume induced weakening of the indian continental lithosphere asthenosphere coupling and an increased slab pull and ridge push efficiency are the most likely explanations for the large convergence rate increase the post 50 ma decrease is best explained by orogeny related increased trench resistivity decreased slab pull due to continental subduction and possibly restrengthening of lithosphere asthenosphere coupling upon plume demise

rapid emplacement of the oman ophiolite thermal and geochronologic constraints in understanding of ophiolite emplacement requires knowledge of the elapsed time between igneous crystallization and intraoceanic thrusting and the rate and duration of that thrusting hornblende 4oar 39ar ages demonstrate that the igneous oceanic crust in oman crystallized and cooled to 825 k in 1 2 m y hornblende ages from metamorphic rocks and from cross cutting dikes require that the basal metamorphic thrust fault beneath the ophiolite also cooled below 825 k in 1 2 m y motion along the sole thrust accounted for 200 km of displacement at a rate of 100 200 mm yr on the basis of age relationships and thermal considerations we favor a two stage model for the initial stages of samall ophiolite emplacement intraoceanic thrusting over 2 m y old lithosphere at 150 km m y parallel to a spreading ridge for 1 2 m y followed by equally rapid and brief thrusting over cold and old lithosphere preservation of the samail ophiolite is a direct result of its young age and positive buoyancy at the time of ocean closure and we propose that all ophiolites that originated near spreading centers and were emplaced onto continents were young at the time of intraoceanic thrusting

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

the composition and thickness of the crust of mars estimated from rare earth elements and neodymium isotopic compositions of martian meteorites isotopic and trace element compositions of martian meteorites show that early differentiation of mars produced complementary crustal and mantle reservoirs that were sampled by later magmatic events this paper describes a mass balance model that estimates the rare earth element ree content and thickness of the crust of mars from the compositions of shergottites the diverse ree and nd isotopic compositions of shergottites are most easily explained by variable addition of light rare earth element lree enriched crust to basaltic magmas derived from lree depleted mantle source regions antarctic shergottites eet 79001 alh 77005 lew 88516 and que 94201 all have strongly lree depleted patterns and positive initial 143nd isotopic compositions which is consistent with the generation of these magmas from depleted mantle sources and little or no interaction with enriched crust in contrast shergotty and zagami have negative initial 143nd isotopic compositions and less pronounced depletions of the lree which have been explained by incorporation of enriched crustal components into mantle derived magmas jones 1989 longhi 1991 borg et al 1997 the mass balance model presented here derives the ree composition of the crustal component in shergotty by assuming it represents a mixture between a mantle derived magma similar in composition to eet 79001a and a lree enriched crustal component the amount of crust in shergotty is constrained by mixing relations based on nd isotopic compositions which allows the ree pattern of the crustal component to be calculated by mass balance the effectiveness of this model is demonstrated by the successful recovery of important characteristics of the earth s continental crust from terrestrial columbia river basalts self consistent results for nd isotopic compositions and ree abundances are obtained if shergotty contains 10 30 of lree enriched crust with 10 ppm nd this crustal component would have moderately enriched lree sm nd 0 25 0 27 147sm 144nd 0 15 0 17 la yb 2 7 3 8 relatively unfractionated heavy rare earth elements hree and no eu anomaly crust with these characteristics can be produced from a primitive lherzolitic martian mantle by modest amounts 2 8 of partial melting and it would have a globally averaged thickness of 45 km which is consistent with geophysical estimates mars may serve as a laboratory to investigate planetary differentiation by extraction of a primary basaltic crust

deep india meets deep asia lithospheric indentation delamination and break off under pamir and hindu kush central asia abstract subduction of buoyant continental lithosphere is one of the least understood plate tectonic processes yet under the pamir hindu kush at the northwestern margin of the india asia collision zone unusual deep earthquakes and seismic velocity anomalies suggest subduction of asian and indian lithosphere here we report new precise earthquake hypocenters detailed tomographic images and earthquake source mechanisms which allow distinguishing a narrow sliver of indian lithosphere beneath the deepest hindu kush earthquakes and a broad arcuate slab of asian lithosphere beneath the pamir we suggest that this double subduction zone arises by contrasting modes of convergence under the pamir and hindu kush imposed by the different mechanical properties of the three types of lithosphere involved while the buoyant northwestern salient of cratonic india bulldozes into cratonic asia forcing delamination and rollback of its lithosphere india s thinned western continental margin separates from cratonic india and subducts beneath asia this torn off narrow plate sliver forms a prominent high velocity anomaly down to the mantle transition zone our images show that its uppermost section is thinned or already severed and that intermediate depth earthquakes cluster at the neck connecting it to the deeper slab providing a rare glimpse at the ephemeral process of slab break off

subduction ultrahigh pressure metamorphism and regurgitation of buoyant crustal slices implications for arcs and continental growth abstract intracontinental collisional belts that retain mineralogic relics of phanerozoic ultrahigh pressure uhp metamorphism are increasingly being recognized adjacent regions generally lack evidence of coeval calcalkaline volcanism plutonism following consumption of intervening oceanic lithosphere each orogen marks the site of a deeply subducted microcontinental promontory fragments of which later returned to midcrustal levels mafic and ultramafic rocks are volumetrically minor pressures in uhp complexes approached or exceeded 2 8 gpa at 600 900 c subduction zones involve low t prograde gradients and constitute the only plate tectonic realm where such conditions exist although internal portions of descending plates may have even lower geothermal gradients crustal upper margins are typified by p t trajectories of 5 10 c km attending underflow clinoamphibole rich metabasaltic rocks dehydrate at pressures 2 2 2 5 gpa this accounts for the kinetically favorable solid state production of mafic eclogites and partial fusion of morbs to form hydrous andesitic melts devolatilization of partly serpentinized mantle lithosphere also introduces h 2 o to the magmagenic zone as large masses of continental crust enter deeper portions of subduction zones mineralogic transformations slow or cease because micas the major oh bearing phases in such rocks retain volatile contents to pressures 4 gpa persistence of hydroxyl bearing layer silicates may explain the metastable preservation of low density assemblages in continental crust subjected to uhp conditions and the dearth of calcalkaline magmatism in most collision belts exhumed uhp terranes are intensely deformed and disaggregated all consist dominantly of quartzofeldspathic compositions mafic eclogites garnet peridotites constitute only a few volume percent eventual decoupling of the relatively low density subducted crust promotes buoyancy propelled transport to shallow depths along the subduction channel with the hanging wall lithosphere acting as a stress guide extensional collapse and erosional unroofing of rising masses result in the continued ascent of deeply subducted but buoyant crustal material although equidimensional massifs must return surfaceward large volume to surface areas render the contained uhp phase assemblages virtually unquenchable surviving uhp terranes thus consist of relatively thin slabs of continental crust allochthons that retain uhp relics apparently lost heat by conduction across both upper normal fault and lower reverse fault contacts based on present exposures estimated thicknesses of uhp sheets are qinling dabie sulu belt 5 km kokchetav complex 1 3 km dora maira massif 1 2 km western gneiss region 1 km slices evidently rose to midcrustal levels at exhumation rates approaching 10 mm per year back reaction attending decompression in all cases was nearly complete where uhp mineral inclusions persisted retrogression apparently was limited by the relative impermeability of the rocks to catalytic aqueous fluids and by declining temperatures the coarse grain size of host phases also disfavored complete retrogression