reconstructing subducted oceanic lithosphere by reverse engineering slab geometries the northern philippine sea plate subducting slabs commonly acquire complex geometries from the migration of subduction trenches slab mantle interaction slab tearing and collision of slabs at depth although it is possible to construct three dimensional models of subducted slabs using earthquake hypocenter locations and tomographic models it is often not possible to rigorously test their accuracy here we present a methodology for performing such a test by reverse engineering the presubduction configuration of a slab of oceanic lithosphere from interpretations of its present day morphology we illustrate our approach for the ryukyu and shikoku slabs northwest philippine sea having simulated them as viscoelastic sheets that we unfolded and floated to the surface the net strain distribution of the floated mesh indicated which parts of the original slab model were geometrically viable minimal net strain and which parts of the mesh required additional tears and or zones of localized ductile extension to have enabled the slab to deform during subduction in the instance of the ryukyu and shikoku slabs the palau kyushu and gagua ridges are shown to have both acted as planes of weakness that broke into major vertical slab tears these subducted ridges are connected by a trench parallel tear that represented the former contact between the huatung and west philippine basins the fossil spreading center of the shikoku basin formed a separate slab window upon subduction along the nankai trough the methodology presented herein is a powerful tool to evaluate complex slab morphologies infer the locations of slab tears and therefore reconstruct intricate configurations of subducted oceanic lithosphere

a slab in depth three dimensional geometry and evolution of the indo australian plate geoscientists use plate tectonics to explain many aspects of both continental evolution and evolution of the planet as a whole the subduction of material at convergent plate boundaries forms a fundamental component to the theory of plate tectonics plates continents subduction zones and spreading centers all exhibit motion and geometric evolution so to try and resolve the past geometries of the planet geologists have utilized plate tectonic reconstructions here we present a three dimensional image of the subducted indo australian plate below southeast asia and show that the geometry of the subducted slab at depth is intimately related to the geometric evolution of se asia over the past 50 ma including the collision of india with the asian continent we show how the once semicontinuous subducting indo australian plate has been segmented during collision between india australian and the subduction margin to the north thus we have found that the geometry of the subducted plate should form a key component to the interpretation of the evolution of earth s surface as complexities and evolution of the subducted plate are manifest in the evolution of the overriding plate

estimating groundwater exchange with lakes 1 the stable isotope mass balance method groundwater inflow and outflow contributions to the hydrologic budget of lakes can be determined using a stable isotope 18o 16o mass balance method the stable isotope method provides a way of integrating the spatial and temporal complexities of the flow field around a lake thereby offering an appealing alternative to the traditional time and labor intensive methods using seepage meters and an extensive piezometer network in this paper the method is applied to a lake in northern wisconsin demonstrating that it can be successfully applied to lakes in the upper midwest where thousands of similar lakes exist inflow and outflow rates calculated for the wisconsin lake using the isotope mass balance method are 29 and 54 cm yr respectively which compare well to estimates derived independently using a three dimensional groundwater flow and solute transport model of 20 and 50 cm yr such a favorable comparison lends confidence to the use of the stable isotope method to estimate groundwater exchange with lakes in addition utilization of stable isotopes in studies of groundwater lake systems lends insight into mixing processes occurring in the unsaturated zone and in the aquifer surrounding the lake and verifies assumed flow paths based on head measurements in piezometers

seismic tomographic imaging of the eastern mediterranean mantle implications for terminal stage subduction the uplift of anatolia and the development of the north anatolian fault

alternative biological sources for 1 2 3 4 tetramethylbenzene in flash pyrolysates of kerogen pyrolysates of kerogens isolated from indian ocean surface sediments and from black sea unit ii contain abundant 1 2 3 4 tetramethylbenzene 1 2 3 4 tmb suggesting the presence of macromolecularly bound isorenieratane skeletons derived from isorenieratene biosynthesized by green sulphur bacteria cf hartgers et al 1994c however the 13c content of 1 2 3 4 tmb is identical to that of algal lipids excluding an origin from green sulphur bacteria the presence of 1 2 3 4 tmb in pyrolysates of stable residues of several marine algae indicates that it can also have an algal origin therefore the assessment of photic zone anoxia on the basis of the abundant presence of 1 2 3 4 tmb in the pyrolysate is not possible without determination of its stable carbon isotopic composition and comparison with those of algal lipids

segmented african lithosphere beneath the anatolian region inferred from teleseismic p wave tomography summary lithospheric deformation throughout anatolia a part of the alpine himalayan orogenic belt is controlled mainly by collision related tectonic escape of the anatolian plate and subduction roll back along the aegean subduction zone we study the deeper lithosphere and mantle structure of anatolia using teleseismic finite frequency p wave traveltime tomography we use data from several temporary and permanent seismic networks deployed in the region approximately 34 000 p wave relative traveltime residuals measured in multiple frequency bands are inverted using approximate finite frequency sensitivity kernels our tomograms reveal segmented fast seismic anomalies beneath anatolia that corresponds to the subducted portion of the african lithosphere along the cyprean and the aegean trenches we identify these anomalies as the subducted aegean and the cyprus slabs that are separated from each other by a gap as wide as 300 km beneath western anatolia this gap is occupied by slow velocity perturbations that we interpret as hot upwelling asthenosphere the eastern termination of the subducting african lithosphere is located near the transition from central anatolia to the eastern anatolian plateau or arabian eurasian collision front that is underlain by large volumes of hot underplating asthenosphere marked by slow velocity perturbations our tomograms also show fast velocity perturbations at shallow depths beneath northwestern anatolia that sharply terminates towards the south at the north anatolian fault zone nafz the associated velocity contrast across the nafz persists down to a depth of 100 150 km hence our study is the first to investigate and interpret the vertical extent of deformation along this nascent transform plate boundary overall the resolved upper mantle structure of anatolia is directly related with the geology and tectonic features observed at the surface of the anatolian plate and suggest that the segmented nature of the subducted african lithosphere plays an important role in the evolution of anatolia and distribution of its tectonic provinces

the role of ridges in the formation and longevity of flat slabs flat slab subduction occurs when the descending plate becomes horizontal at some depth before resuming its descent into the mantle it is often proposed as a mechanism for the uplifting of deep crustal rocks thick skinned deformation far from plate boundaries and for causing unusual patterns of volcanism as far back as the proterozoic eon for example the formation of the expansive rocky mountains and the subsequent voluminous volcanism across much of the western usa has been attributed to a broad region of flat slab subduction beneath north america that occurred during the laramide orogeny 80 55 million years ago here we study the largest modern flat slab located in peru to better understand the processes controlling the formation and extent of flat slabs we present new data that indicate that the subducting nazca ridge is necessary for the development and continued support of the horizontal plate at a depth of about 90 kilometres by combining constraints from rayleigh wave phase velocities with improved earthquake locations we find that the flat slab is shallowest along the ridge while to the northwest of the ridge the slab is sagging tearing and re initiating normal subduction on the basis of our observations we propose a conceptual model for the temporal evolution of the peruvian flat slab in which the flat slab forms because of the combined effects of trench retreat along the peruvian plate boundary suction and ridge subduction we find that while the ridge is necessary but not sufficient for the formation of the flat slab its removal is sufficient for the flat slab to fail this provides new constraints on our understanding of the processes controlling the beginning and end of the laramide orogeny and other putative episodes of flat slab subduction

anisotropy and mantle flow in the chile argentina subduction zone from shear wave splitting analysis 1 we examine shear wave splitting in teleseismic phases to observe seismic anisotropy in the south american subduction zone data is from the charge network which traversed chile and western argentina across two transects between 30 s and 36 s beneath the southern and northwestern parts of the network fast polarization direction j is consistently trench parallel while in the northeast j is trench normal the transition between these two zones is gradual we infer that anisotropy sampled by teleseismic phases is localized within or below the subducting slab we explain our observations with a model in which eastward nazca entrained asthenospheric flow is deflected by retrograde motion of the subducting nazca plate resulting southward flow through this area produces n s j observed in the south and northwest e w j result from interaction of this flow with the local slab geometry producing eastward mantle flow under the actively flattening part of the slab index terms 7203 seismology body wave propagation 7218 seismology lithosphere and upper mantle 8123 tectonophysics dynamics seismotectonics 8150 tectonophysics plate boundary general 3040 9360 information related to geographic region south america citation anderson m l g zandt e triep m fouch and s beck 2004 anisotropy and mantle flow in the chile argentina subduction zone from shear wave splitting analysis geophys res lett 31 l23608 doi 10 1029 2004gl020906

a new seismically constrained subduction interface model for central america we provide a detailed seismically defined three dimensional model for the subducting plate interface along the middle america trench between northern nicaragua through to southern costa rica the model uses data from a weighted catalog of about 30 000 earthquake hypocenters compiled from nine catalogs to constrain the interface through a process we term the maximum seismicity method the method determines the average position of the largest cluster of microseismicity beneath an a priori functional surface above the interface this technique is applied to all seismicity above 40 km depth the approximate intersection of the hanging wall mohorovicic discontinuity where seismicity likely lies along the plate interface below this depth an envelope above 90 of seismicity approximates the slab surface because of station proximity to the interface this model provides highest precision along the interface beneath the nicoya peninsula of costa rica an area where marked geometric changes coincide with crustal transitions and topography observed seaward of the trench the new interface is useful for a number of geophysical studies that aim to understand subduction zone earthquake behavior geodynamic and tectonic development of convergent plate boundaries

the ups and downs of north america evaluating the role of mantle dynamic topography since the mesozoic the driving force for transient vertical motions of earth s surface remains an outstanding question a main difficulty lies in the uncertain role of the underlying mantle especially during the geological past here i review previous studies on both observational constraints and physical mechanisms of north american topographic evolution since the mesozoic i first summarize the north american vertical motion history using proxies from structural geology geochronology sedimentary stratigraphy and geomorphology based on which i then discuss the published physical models overall there is a progressive consensus on the contribution of mantle dynamic topography due to buoyancy structures associated with the past subduction at the continental scale a largely west to east migrating deformation pattern suggests an eastward translation of mantle dynamic effects consistent with models involving an eastward subduction and sinking of former farallon slabs since the cretaceous among the existing models the inverse model based on an adjoint algorithm and time dependent data constraints provides the most extensive explanations for the temporal changes of north american topography since the mesozoic at regional scales debates still exist on the predicted surface subsidence and uplift within both the western and eastern united states where discrepancies are likely due to differences in model setup e g mantle dynamic properties and boundary conditions and the amount of time dependent observational constraints toward the development of the next generation predictive geodynamic models new research directions may include 1 development of enhanced data assimilation capabilities 2 exploration of multiscale and multiphysics processes and 3 cross disciplinary code coupling

reconstruction of 7500 years of coastal environmental change impacting seagrass ecosystem dynamics in oyster harbour sw australia abstract seagrass ecosystems which have important functions such as coastal protection and blue carbon sequestration are threatened by anthropogenic pressure including climate change long term data series from seagrass sedimentary archives mats can be used to understand natural cycles of environmental change and answer key questions related to contemporary management a 7500 yr sediment record from posidonia australis meadows in oyster harbour albany sw australia was subjected to multiproxy reconstruction by means of pigment analysis uhplc analytical pyrolysis py gc ms carbonate content 13c and 15n stable isotope ratios organic c corg content corg n ratio and glomalin related soil proteins grsp the study revealed a brackish lagoon 7500 7000 cal yr bp that was transformed in an open marine environment 7000 4100 cal yr bp due to holocene transgression earliest evidence of seagrass establishment was detected around 4500 cal yr bp and meadow extension accelerated between 4100 and 3700 cal yr bp the meadow environment was surprisingly resistant against environmental perturbations as the mat composed of p australis seagrass fibres embedded within a siliciclastic mineral matrix containing biogenic carbonates continued to develop steadily until 190 cal yr bp 1830 ce then shifts in several proxies pigments grsp showed evidence of terrestrial runoff triggered eutrophication turbidity likely driven by forest clearance and agricultural activities after european settlement but the seagrass showed resilience no decline of the proportion of seagrass derived corg by contrast since 1930 ce seagrass retreat is evident in the biogeochemical record lighter 13c values lower lignin abundance and shifts in pigment abundance and types affecting the balance between seagrass inputs and alternative sources as was observed in previous studies of the area the findings show that pigment proxies are useful early indicators of shifts in seagrass ecosystem condition while lignocellulose and other pyrolysis products are useful proxies of more profound ecosystem alterations that influence seagrass abundance the record indicates that the climax seagrass ecosystem condition which prevailed for several millennia had been impacted over the last century management of seagrass and coastal ecosystems should aim to avoid crossing ecological thresholds and diminish local impacts aggravating those of global change

seismicity and shape of the subducted nazca plate an updated compilation of earthquake locations and focal mechanism solutions from the international seismological centre and preliminary determination of earthquakes is the basis of a comprehensive study of the geometry of the wadati benioff zone beneath western south america the new data support previous mapping of a sharp flexure rather than a tear in the subducted nazca plate beneath southern peru and provide evidence for a similar flexure in the southward transition from nearly horizontal subduction to a slab with 30 dip at latitude 33 s in contrast the transition from 30 slab dip beneath bolivia to a nearly horizontal dip in the region between 28 s 32 s is more gradual occurring over several hundred kilometers of along strike distance between 20 s and 32 s this southward flattening corresponds to a broadening of a horizontal benchlike part of the subducted plate formed between 100 and 125 km depth the transition in continental tectonic style near 27 s 28 s from a wide volcanically active plateau to a narrow nonvolcanic cordillera appears not to be associated with the main slab flattening which begins to the north of these latitudes but with a more abrupt change in curvature of the subducted slab from convex upward to concave upward immediately below the plate boundary interface the concept of gaussian curvature is applied to slab bending to explain how subduction geometry is affected by the shape of the south american plate we hypothesize that the polarity of vertical curvature in the subducting slab is governed by the orientation of lateral curvature of the plate margin focal mechanism solutions for intermediate and deep earthquakes are grouped by geographic region and inverted for the orientation and relative magnitudes of the principal stresses results of the inversion indicate that downdip extension dominates in the slab above 350 km while downdip compression dominates at greater depths

long term carbon storage and its recent loss in an estuarine posidonia australis meadow albany western australia abstract oyster harbour on the south coast of western australia supports 3 6 3 9 km 2 of seagrass meadows following the loss of approximately 2 8 3 1 km 2 in the 1980s this small area of prevailing meadows hold significant carbon stores accumulated over the past 3000 years in this study we sampled three sediment cores from a posidonia australis meadow and analysed organic matter om organic carbon oc and inorganic carbon ic contents in bulk sediments and 13 c signatures of om the om and oc contents mean se in the cores were 9 07 0 36 and 2 24 0 05 respectively the mean ic content was 3 16 0 17 13 c signatures of the sedimentary om ranged from 10 01 to 13 28 using a bayesian isotopic mixing model it is estimated that 57 67 of the om in the seagrass sediments was derived from p australis detrital matter the total carbon tc stores in 150 cm thick seagrass sediments averaged 27 92 kg tc m 2 10 79 kg oc m 2 and 17 13 kg ic m 2 based on radiocarbon dating the mean sediment accumulation rate was 0 0494 cm yr 1 which led to a long term tc accumulation rate of 8 92 g tc m 2 yr 1 3 45 g oc m 2 yr 1 and 5 47 g ic m 2 yr 1 based on historical seagrass cover 3 6 6 7 km 2 during the 1960s to 1980s the estimated tc stores in 150 cm thick seagrass sediments at oyster harbour would have been 101 187 gg tc the eutrophication driven loss of seagrasses during the 1980s resulted in the absence of oc accumulation capacity amounting to 280 310 mg oc over 29 years the loss of seagrass area could also have resulted in the release of 37 41 gg co 2 assuming that all of the oc in shallow sediment is remineralised after meadow disturbance these results exemplify the importance of seagrasses meadows as important carbon sinks and the potential for losses of carbon stores due to ecosystem degradation

imaging the transition from flat to normal subduction variations in the structure of the nazca slab and upper mantle under southern peru and northwestern bolivia

a regionalized upper mantle rum seismic model seismic velocity heterogeneity in the earth s mantle is strongly concentrated near its top the shallow heterogeneity of the mantle correlates strongly with surface tectonics we use these observations as constraints of a tomographic experiment aimed at building a regionalized upper mantle rum reference model we use a select set of teleseismic travel times to minimize the mapping of mislocation into structure the data selection emphasizes the robustness of individual picks the form of the rum model is a set of velocity profiles as functions of depth through the upper mantle for each of the different tectonic provinces of earth together the profiles constitute a three dimensional model which incorporates considerable structural detail but is described by only 90 parameters and has only about 22 degrees of freedom this is achieved by irregularly sampling a detailed regionalization of the globe by detailed mapping of subducted lithosphere in the mantle as defined by seismicity and by combining these structures in an irregular grid in which bookkeeping is efficiently handled the resulting rum model includes subducting slabs as sharp fast features in the upper mantle old continents are fast young oceans are slow models have been derived for both compressional and shear velocity the rum model is designed to represent as much of upper mantle heterogeneity as seen by body wave travel times as possible with a simple model it can be useful as a reference model for individual tectonic regions travel times are efficiently generated for the rum model mislocations of explosions of known location are significantly reduced when corrections for the rum model are applied to travel time residuals for a spherically symmetrical earth model

simulating the influence of lake position on groundwater fluxes groundwater flow around three hypothetical flow through lakes located in the upper middle and lower portions of a watershed was numerically simulated in cross section under steady state and transient conditions results showed that 1 groundwater fluxes into and out of the lake located in the lowest position were higher and more stable than for lakes farther upgradient and 2 periods of intense groundwater recharge caused flow reversals and the formation of a stagnation point downgradient of the uppermost lake thereby causing a net increase in groundwater inflow these simulation results provide guidance on monitoring network design including the frequency of monitoring and or sampling

mantle provinces under north america from multifrequency p wave tomography this is a survey of mantle provinces large scale seismic anomalies under north america from the surface down to 1500 1800 km depth the underlying p velocity model was obtained by multifrequency tomography a waveform based method that systematically measures and models the frequency dependence of teleseismic body waves a novel kind of three dimensional rendering technique is used to make the considerable structural complexities under north america accessible in the transition zone and below the north american mantle is dominated by seismically fast provinces which represent distinct subduction episodes of the farallon plate i attempt to date and interpret the various slab fragments by reconciling their present positions with paleotrench locations from plate tectonic reconstructions and with major geologic surface episodes differences in vertical sinking velocity have led to large vertical offsets across adjacent coeval slabs some of the mantle provinces have not been discussed much previously including 1 a seismically slow blanket overlying the oldest farallon subduction along the eastern continental margin 2 a transition zone slab coeval with the laramide orogeny ca 80 60 myr which i discuss in analogy to the stagnant slab subduction style commonly found in the western pacific today 3 the lower mantle root of present day cascadia subduction which may have started out as intraoceanic subduction 4 a lower mantle slab under arizona and new mexico the last material to subduct before strike slip motion developed along the san andreas boundary and 5 two narrow plate tears thousands of kilometers long one of which is the subducted conjugate of the mendocino fracture zone

groundwater interaction with a kettle hole lake relation of observations to digital simulations abstract observations made on a small fully penetrating kettle hole lake are used to test predictions made using digital stimulation about the exchange of water between lakes and groundwater systems on the upgradient side of lower nashotah lake groundwater inseepage decreases exponentially away from the shore water exchange on the downgradient side is controlled by the presence and strength of a groundwater mound but that mound and its effects are ephemeral when the mound increases in strength it generates a local groundwater flow cell beneath it which contains water of chemical composition distinct from deeper regional groundwater the growing mound also causes local inseepage of groundwater to the lake in an expanding zone along the downgradient shore all of these phenomena occur as they have been predicted in published studies where digital simulation of relatively simple groundwater lake systems has been carried out the field observations thus confirm the validity of the simulation studies and their usefulness as a tool for understanding water exchanges in unmonitored systems

a numerical study on the influence of fractured regions on lake groundwater interaction the lake kinneret sea of galilee case increased lake salinity is a growing problem in arid and semi arid regions operational management which is based on a reliable hydrological understanding has the potential to reduce the lake salinity this is the case of the salinity in lake kinneret sea of galilee where saline water flows into the lake through on shore and off shore springs here we present a time dependent flow and transport numerical model that successfully reproduces the monitored groundwater level discharge and salinity of the lake springs the model utilizes a continuum approach and describes the flow through a confined saline carbonate aquifer which interacts with the discharge lake through fractures and faults in particular the model investigates the hydrology around two groups of springs fuliya and tabgha along the western shore of the lake based on seasonal characteristics of the springs and measured boundary conditions the two springs groups were defined as lake dominated springs lds fuliya and aquifer dominated springs ads tabgha the models of the two groups differ from each other in the distribution of fractured regions that link between the lake and the underlying aquifers the numerical solution shows how the fractured regions affect groundwater head spring discharge and spring salinity the different behavior of the lds and ads was reproduced with an excellent agreement between measured and calculated patterns differences in salinity within a group of springs are shown to be dependent on the depth of the fractured region it was revealed that in addition to the appropriate distribution of fractured regions some adjustment in the up stream boundary conditions is necessary to fully reproduce the system hydrology the model has improved our understanding and provides a prediction tool for future management strategies

environmental changes recorded in the holocene sedimentary infill of a tropical estuary abstract early to late holocene sedimentary infill of the potengi jundiai estuary brazil was studied in a 31 5 m long sediment core the borehole was drilled in a mangrove area within the central part of the flood tide delta the sedimentary record encompassing the last 10 kyr was studied for sediment texture and for the n alkanes composition of organic matter om sediments rich in om and bivalve shells accumulated during the period of sea level rise mostly in an intertidal environment of mangrove forest mudflat and subtidal estuarine flood delta the characterization of n alkanes c10 c33 and lignin methoxyphenols by means of direct analytical pyrolysis py gc ms technique allowed determining the sources of the extant om downcore fluctuations were observed in n alkanes and in several n alkane based indices such as long chain carbon preference index cpi c24 c33 long to short chain ratio l s and average chain length acl as well as in the relative proportion of lignin derived compounds the n alkanes distribution points to changing sources of om input to the estuarine sediments over most of the holocene period major inputs of terrestrial vegetation alternated with periods of mixed input from aquatic macrophytes as the rate of sea level rise changed over time the n alkanes distribution and the lignin markers were useful for reconstructing paleoenvironmental conditions particularly during the early holocene when the lower potengi river valley now an estuarine area could accommodate terrestrial forest vegetation before switching to halophytic mangroves as sea levels rose

complex subduction and small scale convection revealed by body wave tomography of the western united states upper mantle abstract new high resolution p and s wave tomography of the united states upper mantle from the pacific coast to the great plains reveals strong multi scale heterogeneity closely correlated with tectonic and magmatic activity we invert teleseismic travel time residuals from the earthscope transportable array and more than 1700 additional temporary and permanent stations for 3 d velocity perturbations to a depth of 1000 km the inversion uses recent advances in western u s crust models to better isolate the mantle component of travel time residuals and frequency dependent 3 d sensitivity kernels to map travel time residuals measured in multiple frequency bands into velocity structure in addition to separate v p and v s models we jointly invert the two datasets for v p v s perturbations by imposing a smoothness constraint on the ln v s ln v p field the joint inversion helps us identify regions where partial melt is probable the amplitude of v p v s and v p v s variations is greatest in the upper 200 km of the mantle and the form of velocity anomalies suggests a provincially heterogeneous lithosphere and the occurrence of widespread small scale convection partially molten mantle is inferred beneath yellowstone and the eastern snake river plain srp the salton trough and the clear lake volcanic field the inferred depth extent of partial melt is consistent with a generally hydrated upper mantle and elevated temperatures beneath the eastern srp and yellowstone despite continuous subduction since the cretaceous the distribution of sub lithospheric high velocity anomalies is dissected similar to other recent studies based on our new tomography models western u s geologic history and plate tectonic reconstructions we infer patchy and incomplete removal of the flat subducting laramide slab and slab tearing associated with eocene accretion in the northwestern u s

two stage subduction history under north america inferred from multiple frequency tomography eastward subduction of oceanic tectonic plates has shaped the geologic history of western north america over the past 150 million years the mountain building and volcanism that brought forth the spectacular landscapes of the west are credited to the vast ancient farallon plate which interacted mechanically and chemically with the overlying continent as it plunged back into the mantle here we use finite frequency travel time and amplitude measurements of teleseismic p waves in seven frequency bands to obtain a high resolution tomographic image to approx1 800 km depth we discover several large previously unknown pieces of the plate which show that two distinct stages of whole mantle subduction are present under north america the currently active one descends from the pacific northwest coast to 1 500 km depth beneath the great plains whereas its stalled predecessor occupies the transition zone and lower mantle beneath the eastern half of the continent we argue that the separation between them is linked to the laramide era 70 50 myr ago a time of unusual volcanism and mountain building far inland generally explained by an episode of extremely flat subduction

central andean mantle and crustal seismicity beneath the southern puna plateau and the northern margin of the chilean pampean flat slab earthquake hypocenters recorded in the andean southern puna seismic array 25 28 s 70 65 w provide new constraints on the shape of the subducting nazca plate beneath the puna plateau the transition into the chilean pampean flat slab and the thermal state of the mantle and crust some 270 new mantle hypocenters suggest that the subducting slab under the puna shoals into the flat slab segment more abruptly and farther to the north than previously indicated the revised geometry is consistent with the central volcanic zone incapillo caldera being the southernmost center with pleistocene activity until reaching the southern side of the flat slab region evidence for the revised slab geometry includes three well defined hypocenter clusters in the pipanaco nest 27 5 29 s 68 66 w which are interpreted to reflect slab bending stresses a few low magnitude earthquakes with strongly attenuated s waves in the long recognized antofalla teleseismic gap 25 5 27 5 s support a continuous slab under the southern puna the paucity of gap earthquakes and the presence of mafic magmas are consistent with a hot mantle wedge reflecting recent lithospheric delamination evidence for a hot overlaying puna crust comes from new crustal earthquake hypocenters concentrated at depths shallower than 5 km two notable short duration swarms were recorded under the resurgent dome of the 2 ma back arc cerro galan caldera and the near arc cerro torta dome new crustal earthquake focal mechanisms from 17 events in the array along with two existing mechanisms have strike slip oblique reverse and oblique normal solutions fitting with regional e w compression and n s extension

segmentation of the farallon slab abstract recent tomography images reveal a complex 3d mantle structure beneath western united states with feature morphology varying rapidly with depth by assimilating plate motion history paleo age of sea floor and paleo geography of plate boundaries in a 3 d numerical model we simulate the farallon juan de fuca subduction during the past 40 ma we find that the highly segmented upper mantle structure of western u s is a direct result of the farallon subduction we show that the tilted horseshoe shaped fast seismic anomaly beneath nevada and utah at 300 600 km depth range is in fact a segment of curled slab subducted since 15 ma and the shallower linear slab beneath the cascades is younger than 5 ma the distinct morphology between these two parts of the subduction system indicates the strong influence of the fast trench rollback since 20 ma the northward migrating jf pa na triple junction and the toroidal flow around slab edges the observed mantle structures are used to constrain the rheology of the upper mantle through matching the shape depth and location of modeled subducted slab segments the inferred viscosity for the asthenosphere is 5 10 19 pa s and those for the transition zone and lower mantle are 1 5 10 21 pa s and 2 10 22 pa s respectively the slab is found to be about 2 orders of magnitude stronger than the ambient mantle above 410 km depth but of similar order of magnitude viscosity in the transition zone

philippine sea and east asian plate tectonics since 52 ma constrained by new subducted slab reconstruction methods we reconstructed philippine sea and east asian plate tectonics since 52 ma from 28 slabs mapped in 3 d from global tomography with a subducted area of 25 of present day global oceanic lithosphere slab constraints include subducted parts of existing pacific indian and philippine sea oceans plus wholly subducted proto south china sea and newly discovered east asian sea mapped slabs were unfolded and restored to the earth surface using three methodologies and input to globally consistent plate reconstructions important constraints include the following 1 the ryukyu slab is 1000 km n s too short to account for 20 philippine sea northward motion from paleolatitudes 2 the marianas pacific subduction zone was at its present location 200 km since 48 10 ma based on a 1000 km deep slab wall 3 the 8000 2500 km east asian sea existed between the pacific and indian oceans at 52 ma based on lower mantle flat slabs 4 the caroline back arc basin moved with the pacific based on the overlapping coeval caroline hot spot track these new constraints allow two classes of philippine sea plate models which we compared to paleomagnetic and geologic data our preferred model involves philippine sea nucleation above the manus plume 0 150 e near the pacific east asian sea plate boundary large philippine sea westward motion and post 40 ma maximum 80 clockwise rotation accompanied late eocene oligocene collision with the caroline pacific plate the philippine sea moved northward post 25 ma over the northern east asian sea forming a northern philippine sea arc that collided with the sw japan ryukyu margin in the miocene 20 14 ma

effect of ground water recharge on configuration of the water table beneath sand dunes and on seepage in lakes in the sandhills of nebraska u s a abstract analysis of water level fluctuations in about 30 observation wells and 5 lakes in the crescent lake national wildlife refuge in the sandhills of nebraska indicates water table configuration beneath sand dunes in this area varies considerably depending on the configuration of the topography of the dunes if the topography of an interlake dunal area is hummocky ground water recharge is focused at topographic lows causing formation of water table mounds these mounds prevent ground water movement from topographically high lakes to adjacent lower lakes if a dune ridge is sharp the opportunity for focused recharge does not exist resulting in water table troughs between lakes lakes aligned in descending altitudes parallel to the principal direction of regional ground water movement generally have seepage from higher lakes toward lower lakes

tomography reveals buoyant asthenosphere accumulating beneath the juan de fuca plate the boundary between earth s strong lithospheric plates and the underlying mantle asthenosphere corresponds to an abrupt seismic velocity decrease and electrical conductivity increase with depth perhaps indicating a thin weak layer that may strongly influence plate motion dynamics the behavior of such a layer at subduction zones remains unexplored we present a tomographic model derived from on and offshore seismic experiments that reveals a strong low velocity feature beneath the subducting juan de fuca slab along the entire cascadia subduction zone through simple geodynamic arguments we propose that this low velocity feature is the accumulation of material from a thin weak buoyant layer present beneath the entire oceanic lithosphere the presence of this feature could have major implications for our understanding of the asthenosphere and subduction zone dynamics

systematic variation in the depths of slabs beneath arc volcanoes summary the depths to the tops of the zones of intermediate depth seismicity beneath arc volcanoes are determined using the hypocentral locations of engdahl et al these depths are constant to within a few kilometres within individual arc segments but differ by tens of kilometres from one arc segment to another the range in depths is from 65 km to 130 km inconsistent with the common belief that the volcanoes directly overlie the places where the slabs reach a critical depth that is roughly constant for all arcs the depth to the top of the intermediate depth seismicity beneath volcanoes correlates neither with age of the descending ocean floor nor with the thermal parameter of the slab this depth does however exhibit an inverse correlation with the descent speed of the subducting plate which is the controlling factor both for the thermal structure of the wedge of mantle above the slab and for the temperature at the top of the slab we interpret this result as indicating that the location of arc volcanoes is controlled by a process that depends critically upon the temperature at the top of the slab or in the wedge of mantle immediately below the volcanic arc

a narrowly spaced double seismic zone in the subducting nazca plate 1 high precision relocations of intermediate depth earthquakes 80 130 km below the central andes reveal a fine scale double layered wadati benioff zone wbz upper and lower band of seismicity are separated by about 9 km and occur at the top of the oceanic crust and in the uppermost oceanic mantle respectively analysis of focal mechanisms and waveform similarities indicate that fluid processes are causing the events earthquakes in the oceanic crust occur on pre existing normal faults due to hydraulic embrittlement from metamorphic dehydration and on subvertical faults that connect the two layers in a narrow depth range extensional faulting predominates in both layers indicating that slab pull forces are the dominant stress source superseding possible unbending forces in this segment of the nazca plate

evidence for both crustal and mantle earthquakes in the subducting juan de fuca plate 1 we investigate the relationship between high precision hypocentres in the subducting juan de fuca plate beneath southwest british columbia and the velocity structure from receiver function analysis and reflection seismic data 108 earthquakes at depths of 40 to 65 km were relocated using a double difference algorithm to obtain precise earthquake locations correlation of the relocated seismicity with structural information shows a concentration of earthquakes near the top of the subducting oceanic crust and a deeper layer of seismicity in the uppermost mantle of the subducting juan de fuca plate the strong correlation of the seismicity with the structure of the subducting plate suggests that seismic failure is caused by changes in mechanical strength within the plate supporting the hypotheses that phase transformations and thermal petrological conditions play an important role in the seismogenesis of double seismic zones index terms 7209 seismology earthquake dynamics and mechanics 7218 seismology lithosphere and upper mantle 7230 seismology seismicity and seismotectonics 7220 seismology oceanic crust citation cassidy j f and f waldhauser evidence for both crustal and mantle earthquakes in the subducting juan de fuca plate geophys res lett 30 2 1095 doi 10 1029 2002gl015511 2003

closing the gap between regional and global travel time tomography recent global travel time tomography studies by zhou 1996 and van der hilst et al 1997 have been performed with cell parameterizations of the order of those frequently used in regional tomography studies i e with cell sizes of 1 2 these new global models constitute a considerable improvement over previous results that were obtained with rather coarse parameterizations 5 cells the inferred structures are however of larger scale than is usually obtained in regional models and it is not clear where and if individual cells are actually resolved this study aims at resolving lateral heterogeneity on scales as small as 0 6 in the upper mantle and 1 2 3 in the lower mantle this allows for the adequate mapping of expected small scale structures induced by for example lithosphere subduction deep mantle upwellings and mid ocean ridges there are three major contributions that allow for this advancement first we employ an irregular grid of nonoverlapping cells adapted to the heterogeneous sampling of the earth s mantle by seismic waves spakman and bijwaard 1998 second we exploit the global data set of engdahl et al 1998 which is a reprocessed version of the global data set of the international seismological centre their reprocessing included hypocenter redetermination and phase reidentification finally we combine all data used p pp and pwp phases into nearly 5 million ray bundles with a limited spatial extent such that averaging over large mantle volumes is prevented while the signal to noise ratio is improved in the approximate solution of the huge inverse problem we obtain a variance reduction of 57 1 synthetic sensitivity tests indicate horizontal resolution on the scale of the smallest cells 0 6 or 1 2 in the shallow parts of subduction zones decreasing to approximately 2 3 resolution in well sampled regions in the lower mantle vertical resolution can be worse up to several hundreds of kilometers in subduction zones with rays predominantly pointing along dip important features of the solution are as follows 100 200 km thick high velocity slabs beneath all major subduction zones sometimes flattening in the transition zone and sometimes directly penetrating into the lower mantle large high velocity anomalies in the lower mantle that have been attributed to subduction of the tethys ocean and the farallon plate and low velocity anomalies continuing across the 660 km discontinuity to hotspots at the surface under iceland east africa the canary islands yellowstone and the society islands our findings corroborate that the 660 km boundary may resist but not prevent present day large scale mass transfer from upper to lower mantle or vice versa this observation confirms the results of previous global mantle studies that employed coarser parameterizations

trench motion controlled slab morphology and stress variations implications for the isolated 2015 bonin islands deep earthquake the subducted old and cold pacific plate beneath the young philippine sea plate at the izu bonin trench over the cenozoic hosts regional deep earthquakes we investigate slab morphology and stress regimes under different trench motion histories with mantle convection models viscosity temperature and deviatoric stress are inherently heterogeneous within the slab which we link to the occurrence of isolated earthquakes models expand on previous suggestions that observed slab morphology variations along the izu bonin subduction zone exhibited as shallow slab dip angles in the north and steeper dip angles in the south are mainly due to variations in the rate of trench retreat from the north where it is fast to the south where it is slow geodynamic models consistent with the regional plate tectonics including oceanic plate age plate convergence rate and trench motion history reproduce the seismologically observed principal stress direction and slab morphology we suggest that the isolated 680 km deep 30 may 2015 m w 7 9 bonin islands earthquake which lies east of the well defined benioff zone and has its principal compressional stress direction oriented toward the tip of the previously defined benioff zone can be explained by pacific slab buckling in response to the slow trench retreat

slab1 0 a three dimensional model of global subduction zone geometries 1 we describe and present a new model of global subduction zone geometries called slab1 0 an extension of previous efforts to constrain the two dimensional non planar geometry of subduction zones around the focus of large earthquakes slab1 0 describes the detailed non planar three dimensional geometry of approximately 85 of subduction zones worldwide while the model focuses on the detailed form of each slab from their trenches through the seismogenic zone where it combines data sets from active source and passive seismology it also continues to the limits of their seismic extent in the upper mid mantle providing a uniform approach to the definition of the entire seismically active slab geometry examples are shown for two well constrained global locations models for many other regions are available and can be freely downloaded in several formats from our new slab1 0 website http on doi gov d9arbs we describe improvements in our two dimensional geometry constraint inversion including the use of average active source seismic data profiles in the shallow trench regions where data are otherwise lacking derived from the interpolation between other active source seismic data along strike in the same subduction zone we include several analyses of the uncertainty and robustness of our three dimensional interpolation methods in addition we use the filtered subduction related earthquake data sets compiled to build slab1 0 in a reassessment of previous analyses of the deep limit of the thrust interface seismogenic zone for all subduction zones included in our global model thus far concluding that the width of these seismogenic zones is on average 30 larger than previous studies have suggested

mantle flow through a tear in the nazca slab inferred from shear wave splitting nsf ear 0738935 ear 0739001 ear 1565475 colorado college patricia buster scholarship fund national science foundation through the seismological facilities for the advancement of geoscience and earthscope sage proposal of the national science foundation ear 1261681

an analysis of historic production trends in australian base metal mining abstract the base metal mining sector including copper lead zinc silver and nickel has been a prominent and critical feature of the australian minerals industry the various mines and fields have been producers of world significance including broken hill mt isa mt lyell olympic dam cobar and kambalda the long term production trends in the base metal sector governing these historic fields remain relatively undocumented this includes trends in ore grades mining technique open cut versus underground solid wastes produced tailings and waste rock technology e g milling and known economic resources this paper presents these results for the australian base metals sector arguably the first such systematic compilation undertaken a historical overview is discussed for each major commodity to outline the principal developments and changes for that commodity followed by the presentation of mining and milling trends overall the key trends are declining ore grades versus increasing metal production and ore milled and increased open cut mining and associated waste rock though this latter aspect remains significantly under reported the extent of known economic resources has steadily increased for all commodities analysed principally due to the inclusion of lower grade ores and or difficult to treat ores such as nickel laterites or new deposit discoveries based on present mine plans and proposals future metal production will increasingly shift towards lower ore grades and larger open cut mines to maintain production levels there are sufficient known economic resources for about three decades or more providing a basis to sustain the existing base metal industry but beyond this timeframe is difficult to predict these trends point to the need to accurately report complete data on base metal mining and milling as key inputs into quantifying mineral resource trends as well as the environmental aspects of sustainable mining

spatial and temporal variability in the holocene sea level record of the south australian coastline abstract studies of past sea levels based on radiocarbon dated field samples suffer variously from compilation of information from disparate locations and the imprecise nature of the dated indicators geographic segregation and systematic elimination of poor samples from time depth data sets leads to improved interpretation of holocene sea level history an example is presented from southern australia where this is achieved through selection of higher resolution palaeoenvironmental indicators separation of transgressive from regressive populations and geographic regionalisation of data in southern australia fossil sea level indicators preserved in prograding coastal plain settings include seagrass sandflat mangrove samphire and chenier ridge organo sedimentary remains these provide sea level time depth data points with a variety of elevational and dating errors preserved organic remains at the transition from posidonia seagrass to intertidal sandflat environment and from sandflat to avicennia mangrove environment provide the most precise local data for tracing sea level change time depth plots of 233 dated sea level indicators from south australia generate a broad sea level envelope tracing the southern australian transgression from 10 000 to ca 6000 radiocarbon years bp followed by a more or less consistent level to the present finer details of sea level behaviour are only apparent after systematic selection separation and regionalisation of the data the data indicate a very rapid sea level rise in the early holocene at about 16 mm year reaching present levels at 6400 years bp this was followed by regionally variable regression and emergence of the land of 1 3 m a process that continues to the present the systematic increase from 1 to 3 m in the elevation of the 6400 year bp highstand with distance away from the shelf margin is consistent with a hydro isostatic origin for the emergence as predicted by geophysical models

p and s wave tomography of the mantle beneath the united states mantle seismic structure beneath the united states spanning from the active western plate margin to the passive eastern margin was imaged with teleseismic p and s wave traveltime tomography including usarray data up to may 2014 to mitigate artifacts from crustal structure 5 40 s rayleigh wave phase velocities were used to create a 3 d starting model major features of the final p and s models include two distinct low velocity anomalies at depths of 60 300 km beneath the central and northern appalachians and passive margin the central appalachian low velocity anomaly coincides with eocene basaltic magmatism and the northern anomaly is located along the cretaceous track of the great meteor hot spot at depths of 300 700 km beneath the central and eastern u s large high velocity anomalies are inferred to be remnants of the farallon slab that subducted prior to 40 ma during the laramide orogeny

juan de fuca slab geometry and its relation to wadati benioff zone seismicity 1 a new model of the subducted juan de fuca plate beneath western north america allows first order correlations between the occurrence of wadati benioff zone earthquakes and slab geometry temperature and hydration state the geo referenced 3d model constructed from weighted control points integrates depth information from earthquake locations and regional seismic velocity studies we use the model to separate earthquakes that occur in the cascadia forearc from those that occur within the underlying juan de fuca plate and thereby reveal previously obscured details regarding the spatial distribution of earthquakes seismicity within the slab is most prevalent where the slab is warped beneath northwestern california and western washington suggesting that slab flexure in addition to expected metamorphic dehydration processes promotes earthquake occurrence within the subducted oceanic plate earthquake patterns beneath western vancouver island are consistent with slab dehydration processes conversely the lack of slab earthquakes beneath western oregon is consistent with an anhydrous slab double differenced relocated seismicity resolves a double seismic zone within the slab beneath northwestern california that strongly constrains the location of the plate interface and delineates a cluster of seismicity 10 km above the surface that includes the 1992 m7 1 mendocino earthquake we infer that this earthquake ruptured a surface within the cascadia accretionary margin above the juan de fuca plate we further speculate that this earthquake is associated with a detached fragment of former farallon plate other subsurface tectonic elements within the forearc may have the potential to generate similar damaging earthquakes

the interaction of lakes with variably saturated porous media numerical simulation of variably saturated porous media indicates that groundwater recharge is variable in time and space depending on the thickness of the unsaturated zone through which infiltrating water must move the resulting complex transient groundwater flow systems have significant impact on contiguous surface water in very permeable media small local closed groundwater flow systems can develop and dissipate within a few weeks to several months after major recharge these have a direct effect on contiguous surface water by alternately causing seepage to and seepage from the surface water the transient nature of these flow systems indicates that reversals of the direction of groundwater flow may be common in less permeable media the same complex flow systems may occur but the time for development and dissipation is much greater for example it is conceivable that small local flow systems may exist for many months or years as a result of major recharge therefore directions of flow in such systems are more stable and the effect on contiguous surface water also is more stable the findings of this study indicate that wells and groundwater quality sampling sites need to be carefully located to define accurately water table configuration groundwater recharge direction of seepage through the beds of surface water bodies and complex geochemical processes related to changing directions of groundwater flow

subduction of the philippine sea plate beneath southwestern japan slab geometry and its relationship to arc magmatism 1 we carry out high resolution three dimensional seismic tomography of the crust and upper mantle beneath southwestern sw japan using arrival time data obtained from the nationwide seismograph network the tomographic images provide new insights into the configuration of the philippine sea slab and arc magmatism the results confirm the existence of an aseismic portion of the philippine sea slab at greater depths beyond the seismic portion the philippine sea slab is subducting aseismically down to at least a depth of 200 km in chubu 60 80 km in kinki and 60 km in chugoku and is subducting seismically to depths of 150 200 km in kyushu in the chubu district it is subducting seismically at a shallow dip and then bends downward beneath the volcanic area whereas in the kinki district it is subducting subhorizontally as far as the japan sea the differences in the slab geometry between adjacent regions correlate with the differences in volcanic activity suggesting the importance of the geometry of the philippine sea slab on arc magmatism our tomographic images further imply subduction of the philippine sea slab down to a depth of around 70 km northwest of the izu peninsula the philippine sea slab in that region might have split into western and eastern parts separated by a slab tear and collided with the pacific slab at depths of 150 200 km

simulation of late cenozoic south american flat slab subduction using geodynamic models with data assimilation abstract the formation mechanisms of flat slabs in south america remain unclear to quantitatively evaluate the earlier proposed mechanisms we simulate the post 100 ma subduction history below south america using 4 d geodynamic models by progressively incorporating plate kinematics seafloor ages and key tectonic features including the buoyant oceanic crust continental cratons oceanic plateaus i e the inferred inca plateau subducting nazca ridge and juan fernandez ridge as well as deformable trench profiles according to recent geological reconstructions we find that in the absence of an overriding plate and subducting buoyancy features the seafloor age affects slab dip angle by controlling the slab s mechanical strength i e the resistance to bending and negative buoyancy integrated positive density anomaly that enhances bending our models show that slab strength dominates its buoyancy at age 30 ma and the opposite for younger ages the existence of a thick overriding plate reduces the slab dip by increasing dynamic suction and individual cratonic roots further lead to along trench variations of dip angle reduction while dynamic suction from the overriding plate generates a permanent reduction of the long wavelength slab dip angle it is the final addition of subducting oceanic plateau and aseismic ridges that produces the transient and localized flat slabs as observed these results suggest that all mechanisms except the buoyancy features affect the slab dip only at large spatial scales our best fit model with all the above tectonic features included provides a good match to both the upper mantle benioff zones and the temporal evolution of volcanic arcs since the mid miocene the imperfect match of the peruvian flat slab is likely associated with the uncertain 3 d configuration of the amazonian craton

interaction of subducted slabs with the mantle transition zone a regime diagram from 2 d thermo mechanical models with a mobile trench and an overriding plate transition zone slab deformation influences earth s thermal chemical and tectonic evolution however the mechanisms responsible for the wide range of imaged slab morphologies remain debated here we use 2 d thermo mechanical models with a mobile trench an overriding plate a temperature and stress dependent rheology and a 10 30 or 100 fold increase in lower mantle viscosity to investigate the effect of initial subducting and overriding plate ages on slab transition zone interaction four subduction styles emerge i a vertical folding mode with a quasi stationary trench near vertical subduction and buckling folding at depth vf ii slabs that induce mild trench retreat which are flattened horizontally deflected and stagnate at the upper lower mantle interface hd iii inclined slabs which result from rapid sinking and strong trench retreat isr iv a two stage mode displaying backward bent and subsequently inclined slabs with late trench retreat bir transitions from regime i to iii occur with increasing subducting plate age i e buoyancy and strength regime iv develops for old strong subducting and overriding plates we find that the interplay between trench motion and slab deformation at depth dictate the subduction style both being controlled by slab strength which is consistent with predictions from previous compositional subduction models however due to feedbacks between deformation sinking rate temperature and slab strength the subducting plate buoyancy overriding plate strength and upper lower mantle viscosity jump are also important controls in thermo mechanical subduction for intermediate upper lower mantle viscosity jumps 30 our regimes reproduce the diverse range of seismically imaged slab morphologies

the influence of oxygen exposure time on the composition of macromolecular organic matter as revealed by surface sediments on the murray ridge arabian sea the arabian sea represents a prime example of an open ocean extended oxygen minimum zone omz with low oxygen concentrations down to less than 2 m between 200 and 1000 m water depth the omz impinges on the ocean floor affecting organic matter om mineralization we investigated impact of oxygen depletion on the composition of macromolecular om mom along a transect through the omz on the slopes of the murray ridge this sub marine high in the northern arabian sea with the top at approximately 500 m below sea surface mbss intersects the omz we analyzed sediments deposited in the core of omz suboxic conditions directly below the omz dysoxic conditions and well below the omz fully oxic conditions the upper 18 cm of sediments from three stations recovered at different depths were studied mom was investigated by rock eval and flash pyrolysis techniques the mom was of a predominant marine origin and inferred from their pyrolysis products most biomolecules tetra alkylpyrrole pigments polysaccharides proteins and their transformation products and polyphenols including phlorotannins showed a progressive relative degradation with increasing exposure to oxygen alkylbenzenes and in particular aliphatic macromolecules increased relatively the observed differences in mom composition between sediment deposited under various bottom water oxygen conditions i e in terms of concentration and exposure time was much larger than within sediment cores implying that early diagenetic alteration of organic matter depends largely on bottom water oxygenation rather than subsequent anaerobic degradation within the sediments even at longer time scales

effects of mantle and subduction interface rheologies on slab stagnation and trench rollback abstract trench rollback has been a widely discussed phenomenon in recent years and multiple studies have concentrated on various parameters that may influence trench migration and related aspects of slab deformation in the upper mantle here we concentrate on the effects of rheological description yield stress lower mantle viscosity viscosity of crust in controlling the rollback and associated stagnation of slabs in the transition zone 410 660 km depth we perform numerical simulations of slab evolution in a 2d cartesian model with strongly nonlinear rheology combining diffusion creep dislocation creep and a power law stress limiter we demonstrate that trench retreat develops in most models considered regardless of the subducting plate age or prescribed strength rollback then mostly produces slabs that are horizontally deflected at the 660 km phase boundary and remain subhorizontal at the bottom of the transition zone slab morphologies are in agreement with stagnant horizontally deflected structures reported in the transition zone by seismic tomography furthermore if the strength of the slab is limited to less than 0 5 gpa the slab experiences a significant amount of horizontal buckling the amplitude of the rollback velocity is sensitive to several model parameters as one might expect it increases with the age of the subducting plate thus reflecting its increasingly negative buoyancy on the other hand rollback velocity decreases if we increase the viscosity of the crust and thereby strengthen the coupling between the subducting and overriding plates high friction on the contact between the subducting and overriding plates may even result in slabs penetrating into the lower mantle after a period of temporary stagnation also reducing the additional negative buoyancy imparted by the 410 km exothermic phase transition suppresses trench rollback interpretation of the controls on slab rollback and stagnation may be rather complex in strongly nonlinear rheological models where for example buoyancy effects may be counteracted by associated yield stress weakening

crustal seismic anisotropy in central tibet implications for deformational style and flow in the crust 1 receiver functions obtained at indepth iii stations located near the bangong nujiang suture in central tibet display a weak moho signal and strong p to s conversions within the first 5 s that vary systematically with back azimuth a single station with representative azimuthal variations located at the sharp onset of strong sks splitting is modeled for both dipping layers and seismic anisotropy by using a global minimization technique inversion results indicate strong anisotropy 10 near the surface and in the middle crust separated by a south dipping 25 layer possibly related to the earlier phase of crustal shortening near surface anisotropy has a fabric dipping steeply southward and trending wnw ese that correlates with the suture and younger strike slip faults in contrast mid crustal anisotropy occurs in a low velocity zone and has a fabric dipping gently 18 northward that might be related to a well developed near horizontal rock fabric induced by crustal flow

mantle subducting slab structure in the region of the 2010 m8 8 maule earthquake 30 40 s chile we present a new tomographic model of the mantle in the area of the 2010 m8 8 maule earthquake and surrounding regions increased ray coverage provided by the aftershock data allows us to image the detailed subducting slab structure in the mantle from the region of flat slab subduction north of the maule rupture to the area of overlapping rupture between the 1960 m9 5 and the 2010 m8 8 events to the south we have combined teleseismic primary and depth phase arrivals with available local arrivals to better constrain the teleseismic earthquake locations in the region which we use to conduct nested regional global tomography the new model reveals the detailed structure of the flat slab and its transition to a more moderately dipping slab in the maule region south of the maule region a steeply dipping relic slab is imaged from 200 to 1000 km depth that is distinct from the moderately dipping slab above it and from the more northerly slab at similar depths we interpret the images as revealing both horizontal and vertical tearing of the slab at 38 s to explain the imaged pattern of slab anomalies in the southern portion of the model in contrast the transition from a horizontal to moderately subducting slab in the northern portion of the model is imaged as a continuous slab bend we speculate that the tearing was most likely facilitated by a fracture zone in the downgoing plate or alternatively by a continental scale terrane boundary in the overriding plate

slab2 a comprehensive subduction zone geometry model subduction zones are home to the most seismically active faults on the planet the shallow megathrust interfaces of subduction zones host earth s largest earthquakes and are likely the only faults capable of magnitude 9 ruptures despite these facts our knowledge of subduction zone geometry which likely plays a key role in determining the spatial extent and ultimately the size of subduction zone earthquakes is incomplete we calculated the three dimensional geometries of all seismically active global subduction zones the resulting model called slab2 provides a uniform geometrical analysis of all currently subducting slabs