crustal deformation of the eastern tibetan plateau revealed by magnetotelluric imaging the ongoing collision of the indian and asian continents has created the himalaya and tibetan plateau through a range of deformation processes these include crustal thickening detachment of the lower lithosphere from the plate delamination and flow in a weakened lower crust 1 6 debate continues as to which of these processes are most significant 7 in eastern tibet large scale motion of the surface occurs but the nature of deformation at depth remains unresolved a large scale crustal flow channel has been proposed as an explanation for regional uplift in eastern tibet 6 8 9 but existing geophysical data 10 11 do not constrain the pattern of flow magnetotellurics uses naturally occurring electromagnetic waves to image the earth s subsurface here we present magnetotelluric data that image two major zones or channels of high electrical conductivity at a depth of 20 40 km the channels extend horizontally more than 800 km from the tibetan plateau into southwest china the electrical properties of the channels imply an elevated fluid content consistent with a weak crust 12 13 that permits flow on a geological timescale these findings support the hypothesis that crustal flow can occur in orogenic belts and contribute to uplift of plateaux our results reveal the previously unknown complexities of these patterns of crustal flow many previous studies of the indiaasia

do double ss precursors mean double discontinuities summary stacks of ss precursors have been widely used in the past two decades to investigate the existence and characteristics of upper mantle discontinuities on a global scale as well as in several regional cases here we present observations of ss precursors from an mb 6 7 earthquake recorded at the us transportable array in 2010 in this particular case the s660s precursors on the transverse component are strong enough to be identified on individual seismograms across the array without any stacking procedures two s660s precursors are observed seeming to suggest double discontinuities around 660 km depth in the bounce point region through careful analysis of 1 d and 3 d synthetic seismograms we however discover that although they have arrival times and slownesses that are very close to the theoretical values of ss precursors the apparent double precursors are artefacts because of mantle heterogeneity in the upper mantle near the receivers away from the bounce point region this suggests that caution must be taken about appropriate azimuthal coverage at the ss bounce point before interpreting double ss or pp precursors in terms of complex mineralogical transitions

characterization of the san jacinto fault zone near anza california by fault zone trapped waves we installed three 350 m long seismic arrays each array consisting of 12 three component stations across the coyote creek fault ccf clark valley fault cvf and buck ridge fault brf of the san jacinto fault zone sjfz near anza california to record fault zone trapped waves from microearthquakes we observed trapped waves with relatively large amplitudes and long duration at stations close to the fault traces for earthquakes occurring within the fault zone the coda normalized amplitude spectra of trapped waves showed peaks at 4 7 hz which decreased sharply with the distance from the fault trace observations and three dimensional finite difference simulations of trapped waves revealed low velocity and low q waveguides on these active faults with the width of 75 100 m in which shear velocities are reduced by 25 30 from wall rock velocities and q values are 40 90 at depths between the surface and 18 km the locations of earthquakes for which we observed trapped waves delineate the most seismically active fault strands of the sjfz in a region with complicated slip planes near anza the low velocity waveguides inferred from trapped waves extend 15 to 20 km in the length on these active faults and are segmented by the fault discontinuities the waveguide on the brf dips south westward to connect the waveguide on the cvf which dips northeastward this waveguide extends at the seismogenic depth through anza slip gap to another low velocity waveguide on the casa loma fault clf which has been delineated in our previous study of the sjfz using trapped waves li et al 1997 the waveguide on the ccf in coyote mountain is nearly vertical and disconnected from the clf at the south edge of anza gap we interpret the low velocity waveguides on these active strands to partly result from recent prehistoric significant earthquakes on them and evaluate the future earthquake in the anza region

gplates building a virtual earth through deep time gplates is an open source cross platform plate tectonic geographic information system enabling the interactive manipulation of plate tectonic reconstructions and the visualization of geodata through geological time gplates allows the building of topological plate models representing the mosaic of evolving plate boundary networks through time useful for computing plate velocity fields as surface boundary conditions for mantle convection models and for investigating physical and chemical exchanges of material between the surface and the deep earth along tectonic plate boundaries the ability of gplates to visualize subsurface 3 d scalar fields together with traditional geological surface data enables researchers to analyze their relationships through geological time in a common plate tectonic reference frame to achieve this a hierarchical cube map framework is used for rendering reconstructed surface raster data to support the rendering of subsurface 3 d scalar fields using graphics hardware accelerated ray tracing techniques gplates enables the construction of plate deformation zones regions combining extension compression and shearing that accommodate the relative motion between rigid blocks users can explore how strain rates stretching shortening factors and crustal thickness evolve through space and time and interactively update the kinematics associated with deformation where data sets described by geometries points lines or polygons fall within deformation regions the deformation can be applied to these geometries together these tools allow users to build virtual earth models that quantitatively describe continental assembly fragmentation and dispersal and are interoperable with many other mapping and modeling tools enabling applications in tectonics geodynamics basin evolution orogenesis deep earth resource exploration paleobiology paleoceanography and paleoclimate

experiments in migration processing of ss precursor data to image upper mantle discontinuity structure long period ss precursors result from underside reflections off upper mantle discontinuities by grouping and stacking global seismic data by ss bounce point location it is possible to map lateral variations in depths to the 410 and 660 km discontinuities a process analogous to common midpoint cmp stacking in reflection seismology because this method assumes horizontal reflectors energy arriving from dipping or intermittent reflectors may not be correctly imaged to address this possibility we experiment with techniques based on migration processing of shallow seismic reflection data the problem is complicated by the uneven distribution of sources and receivers for the ss precursor observations but the data are sufficiently dense beneath the northwest pacific ocean that reasonably good coverage can be obtained for this region we parameterize the model as a grid of point scatterers in latitude longitude and depth from the surface to 1000 km depth and compute travel times from each grid point to the source and receiver locations these times are used to construct a matrix equation that yields predicted ss precursor waveforms from the assumed scatterers to recover the model we experiment with both simple back projection and full inversions using a conjugate gradient method tests on noise free synthetic data generated using the same source receiver distribution as the actual data suggest that detailed resolution of discontinuity structure is possible at horizontal scales much smaller than the fresnel zone however the real data do not produce coherent results unless some degree of horizontal smoothing is imposed at least partially defeating the purpose of this approach results for the northwest pacific find structure on the 410 and 660 km discontinuities and hints of intermittent reflectors at other depths random resampling tests however suggest that most of these features are not reliably resolved with the exception of a depression on the 660 km discontinuity seen in the northwest pacific our experiments show that it is unlikely that small scale structure on the 660 km discontinuity near subducting slabs causes significant bias in maps of the large scale 660 km topography derived from long period ss precursor observations

intracrystalline nucleation during the post garnet transformation under large overpressure conditions in deep subducting slabs 1 the mechanism of the post garnet transformation in natural single crystalline pyropic garnet has been examined under large overpressure conditions of 31 43 gpa using the multianvil apparatus with sintered diamond anvils intracrystalline nucleation was found to be dominant above 35 38 gpa while only grain boundary nucleation is responsible at lower pressures in addition natural pyropic garnet transformed to a single phase of perovskite without decomposing to multiple phases above 38 gpa both intracrystalline nucleation and polymorphic growth of the post garnet transformation under large overpressure conditions would strongly enhance the transformation kinetics consequently the post garnet transformation may pwroceed rapidly in the subducting slab at depths near 950 km which would cause substantial rheological weakening of the subducting slab the metastable post garnet transformation may also cause the seismic discontinuities at a depth of 900 1080 km depth which is indeed observed beneath some subduction zones

on the deep mantle origin of the deccan traps the deccan traps in west central india constitute one of earth s largest continental flood basalt provinces whose eruption played a role in the cretaceous paleogene extinction event the unknown mantle structure under the indian ocean at the start of the cenozoic presents a challenge for connecting the event to a deep mantle origin we used a back and forth iterative method for time reversed convection modeling which incorporates tomography based present day mantle heterogeneity to reconstruct mantle structure at the start of the cenozoic we show a very low density deep seated upwelling that ascends beneath the reunion hot spot at the time of the deccan eruptions we found a second active upwelling below the comores hot spot that likely contributed to the region of partial melt feeding the massive eruption

seismic and geodetic evidence for extensive long lived fault damage zones during earthquakes slip is often localized on preexisting faults but it is not well understood how the structure of crustal faults may contribute to slip localization and energetics growing evidence suggests that the crust along active faults undergoes anomalous strain and damage during large earthquakes seismic and geodetic data from the calico fault in the eastern california shear zone reveal a wide zone of reduced seismic velocities and effective elastic moduli using seismic traveltimes trapped waves and interferometric synthetic aperture radar observations we document seismic velocities reduced by 40 50 and shear moduli reduced by 65 compared to wall rock in a 1 5 km wide zone along the calico fault observed velocity reductions likely represent the cumulative mechanical damage from past earthquake ruptures no large earthquake has broken the calico fault in historic time implying that fault damage persists for hundreds or perhaps thousands of years these fi ndings indicate that faults can affect rock properties at substantial distances from primary fault slip surfaces and throughout much of the seismogenic zone a result with implications for the amount of energy expended during rupture to drive cracking and yielding of rock and development of fault systems

properties of seismic fault zone waves and their utility for imaging low velocity structures a two dimensional solution for the scalar wave equation in a model of two vertical layers between two quarter spaces is used to study properties of seismic waves in a laterally heterogeneous low velocity structure the waves referred to as seismic fault zone waves include head waves internal fault zone reflections and trapped waves the analysis aims to clarify the dependency of the waves on media velocities media attenuation coefficients layer widths and source receiver geometry additional calculations with extreme low velocity layers provide examples that may be relevant for volcanic and geothermal domains the interference patterns controlling seismic fault zone waves change with the number of internal reflections in the low velocity structure this number increases with propagation distance along the structure decreases with fault zone width and increases for given length scales with the velocity contrast the relative lateral position of the source within the low velocity layer modifies die length scales associated with internal reflections and influences the resulting interference pattern low values of q affect considerably the dominant period and overall duration of the waves thus there are significant tradeoffs between propagation distance along the structure fault zone width velocity contrast source location within the fault zone and q the lateral and depth receiver coordinates determine the particular point where the interference pattern is sampled and observed motion is a strong function of both coordinates the zone connecting sources generating fault zone waves and observation points with appreciable wave amplitude can be over an order of magnitude larger than the fault zone width calculations for cases with layer p wave velocity of 200 m s 1 modeling a vertical dike or crack with fluid and gas show conspicuous persisting oscillations the results resemble aspects of seismic data in volcanic domains if these waves exist in observed records their explicit recognition and modeling will help to separate source and structural effects and aid in the interpretation of volcano seismology signals although the tradeoffs in parameters governing seismic fault zone waves are significant each variable has its own signature and the parameters may be constrained by additional geophysical data simultaneous modeling of many waveforms with an appropriate solution can resolve the various parameters and provide a high resolution structural image

present day crustal motion within the tibetan plateau inferred from gps measurements 1 using the measurements of 726 gps stations around the tibetan plateau we determine the rigid rotation of the entire plateau in a eurasia fixed reference frame which can be best described by an euler vector of 24 38 0 42 n 102 37 0 42 e 0 7096 0 0206 ma the rigid rotational component accommodates at least 50 of the northeastward thrust from india and dominates the eastward extrusion of the northern plateau after removing the rigid rotation to highlight the interior deformation within the plateau we find that the most remarkable interior deformation of the plateau is a glacier like flow zone which starts at somewhere between the middle and western plateau goes clockwise around the eastern himalayan syntaxis ehs and ends at the southeast corner of the plateau with a fan like front the deformation feature of the southern plateau especially the emergence of the flow zone could be attributed to an eastward escape of highly plastic upper crustal material driven by a lower crust viscous channel flow generated by lateral compression and gravitational buoyancy at the later developmental stage of the plateau the first order feature of crustal deformation of the northeastern plateau can be well explained by a three dimensional elastic half space dislocation model with rates of dislocation segments comparable to the ones from geological observations in the eastern plateau although gps data show no significant convergence between the eastern margin of the plateau and the sichuan basin a small but significant compressional strain rate component of 10 5 2 8 nstrain yr exists in a relatively narrow region around the eastern margin in addition a large part of the eastern plateau northeast of the ehs is not undergoing shortening along the northeastward convergence direction of the ehs but is stretching

constructing the longmen shan eastern tibetan plateau margin insights from low temperature thermochronology 1 contrasting models of upper crustal shortening versus lower crustal flow have been proposed to explain the formation of thickened crust in the longmen shan lms eastern tibetan plateau tp margin these models require different structural kinematics along the lms whose structural geometry is defined by three parallel nw dipping fault zones from foreland southeast to hinterland northwest they are the guanxian anxian fault yingxiu beichuan fault ybf and wenchuan maowen fault wmf newly derived and previously published low temperature thermochronology data from the lms were synthesized to constrain the spatial exhumation and test previous models the results show that 1 exhumation increases abruptly across the range bounding ybf suggesting the fault being the main thrust boundary between the lms and the sichuan basin to the east 2 younger late cenozoic cooling ages are found on the hinterland wmf where a dichotomy of ages on the hanging wall versus footwall suggests late cenozoic thrust activity and 3 toward the hinterland to the west exhumation rates decrease twofold over a distance of 30 40 km this exhumation pattern indicates a westward decrease of tectonic uplift providing the regional topography approached a steady state whereby exhumation is in balance with tectonic uplift the observed exhumation estimates support an upper crustal configuration where thrusts in the lms merge gradually into a gentle detachment seated at a depth of 20 30 km results of this study support a revised upper crustal thrusting model

seismic structures of the calico fault zone inferred from local earthquake travel time modelling author posting the authors 2011 this article is posted here by permission of john wiley sons for personal use not for redistribution the definitive version was published in geophysical journal international 186 2011 760 770 doi 10 1111 j 1365 246x 2011 05055 x

oceanic plateau of the hawaiian mantle plume head subducted to the uppermost lower mantle the hawaiian emperor seamount chain that includes the hawaiian volcanoes was created by the hawaiian mantle plume although the mantle plume hypothesis predicts an oceanic plateau produced by massive decompression melting during the initiation stage of the hawaiian hot spot the fate of this plateau is unclear we discovered a megameter scale portion of thickened oceanic crust in the uppermost lower mantle west of the sea of okhotsk by stacking seismic waveforms of ss precursors we propose that this thick crust represents a major part of the oceanic plateau that was created by the hawaiian plume head 100 million years ago and subducted 20 million to 30 million years ago our discovery provides temporal and spatial clues of the early history of the hawaiian plume for future plate reconstructions

ocean basin evolution and global scale plate reorganization events since pangea breakup we present a revised global plate motion model with continuously closing plate boundaries ranging from the triassic at 230 ma to the present day assess differences among alternative absolute plate motion models and review global tectonic events relatively high mean absolute plate motion rates of approximately 9 10 cm yr 1 between 140 and 120 ma may be related to transient plate motion accelerations driven by the successive emplacement of a sequence of large igneous provinces during that time an event at 100 ma is most clearly expressed in the indian ocean and may reflect the initiation of andean style subduction along southern continental eurasia whereas an acceleration at 80 ma of mean rates from 6 to 8 cm yr 1 reflects the initial northward acceleration of india and simultaneous speedups of plates in the pacific an event at 50 ma expressed in relative and some absolute plate motion changes around the globe and in a reduction of global mean plate speeds from about 6 to 4 5 cm yr 1 indicates that an increase in collisional forces such as the india eurasia collision and ridge subduction events in the pacific such as the izanagi pacific ridge play a significant role in modulating plate velocities

radiometric ages for basement rocks from the emperor seamounts odp leg 197 1 the hawaiian emperor seamount chain is the type example of an age progressive hot spot generated intraplate volcanic lineament however our current knowledge of the age distribution within this province is based largely on radiometric ages determined several decades ago improvements in instrumentation sample preparation methods and new material obtained by recent drilling warrant a reexamination of the age relations among the older hawaiian volcanoes we report new age determinations 40ar 39ar incremental heating method on whole rocks and feldspar separates from detroit sites 1203 and 1204 nintoku site 1205 and koko site 1206 seamounts ocean drilling program odp leg 197 and meiji seamount deep sea drilling project dsdp leg 19 site 192 plateaus in incremental heating age spectra for site 1203 lava flows give a mean age of 75 8 0 6 2 ma which is consistent with the normal magnetic polarity directions observed and biostratigraphic age assignments site 1204 lavas produced discordant spectra indicating ar loss by reheating and k mobilization six plateau ages from lava flows at site 1205 give a mean age of 55 6 0 2 ma corresponding to chron 24r drilling at site 1206 intersected a n r n magnetic polarity sequence of lava flows from which six plateau ages give a mean age of 49 1 0 2 ma corresponding to the chron 21n 22r 22n sequence plateau ages from two feldspar separates and one lava from dsdp site 192 range from 34 to 41 ma significantly younger than the cretaceous age of overlying sediments which we relate to postcrystallization k mobilization combined with new dating results from suiko seamount dsdp site 433 and volcanoes near the prominent bend in the lineament sharp and clague 2002 the overall trend is increasing volcano age from south to north along the emperor seamounts consistent with the hot spot model however there appear to be important departures from the earlier modeled simple linear age progression which we relate to changes in pacific plate motion and the rate of southward motion of the hawaiian hot spot

characterization of fault zones there are currently three major competing views on the essential geometrical mechanical and mathematical nature of faults the standard view is that faults are possibly segmented and heterogeneous euclidean zones in a continuum solid the continuum euclidean view is supported by seismic gravity and electromagnetic imaging studies by successful modeling of observed seismic radiation geodetic data and changes in seismicity patterns by detailed field studies of earthquake rupture zones and exhumed faults and by recent high resolution hypocenter distributions along several faults the second view focuses on granular aspects of fault structures and deformation fields the granular view is supported by observations of rock particles in fault zone gouge by studies of block rotations and the mosaic structure of the lithosphere which includes the overall geometry of plate tectonics by concentration of deformation signals along block boundaries by correlation of seismicity patterns on scales several times larger than those compatible with a continuum framework and by strongly heterogeneous wave propagation effects on the earth s surface the third view is that faults are fractal objects with rough surfaces and branching geometry the fractal view is supported by some statistical analysis of regional hypocenter locations by long range correlation of various measurements in geophysical boreholes by the fact that observed power law statistics of earthquakes are compatible with an underlying scale invariant geometrical structure by geometrical analysis of fault traces at the earth s surface and by measurements of joint and fault surfaces topography

quantitative analysis of seismic fault zone waves in the rupture zone of the 1992 landers california earthquake evidence for a shallow trapping structure summary we analyse quantitatively a waveform data set of 238 earthquakes recorded by a dense seismic array across and along the rupture zone of the 1992 landers earthquake a grid search method with station delay corrections is used to locate events that do not have catalogue locations the quality of fault zone trapped waves generated by each event is determined from the ratios of seismic energy in time windows corresponding to trapped waves and direct s waves at stations close to and off the fault zone approximately 70 per cent of the events with s p times of less than 2 s including many clearly off the fault produce considerable trapped wave energy this distribution is in marked contrast with previous claims that trapped waves are generated only by sources close to or inside the landers rupture zone the time difference between the s arrival and trapped waves group does not grow systematically with increasing hypocentral distance and depth the dispersion measured from the trapped waves is weak these results imply that the seismic trapping structure at the landers rupture zone is shallow and does not extend continuously along strike by more than a few kilometres synthetic waveform modelling indicates that the fault zone waveguide has depth of approximately 2 4 km a width of approximately 200 m an s wave velocity reduction relative to the host rock of approximately 30 40 per cent and an s wave attenuation coefficient of approximately 20 30 the fault zone waveguide north of the array appears to be shallower and weaker than that south of the array the waveform modelling also indicates that the seismic trapping structure below the array is centred approximately 100 m east of the surface break

subducted lithosphere beneath the kuriles from migration of pp precursors abstract we seismically image both thermal and chemical heterogeneity of the mantle beneath the kurile subduction zone using p wave energy reflected from the underside of discontinuities arriving as precursors to the seismic phase pp we take advantage of new broadband seismic data provided by the high lava plains seismic experiment and earthscope s usarray collecting a dataset of 31 high quality sumatran earthquakes sampling beneath the kuriles we employ high resolution array analysis techniques including migration and vespagrams to identify precursory arrivals and study lateral variations in discontinuity depth sharpness and impedance of the mantle transition zone we find the 410 km boundary is at 395 km near the subducting kurile slab though the boundary is 410 425 km deep elsewhere in regions away from subduction we do not detect a laterally continuous underside reflection of p waves from the 660 km discontinuity however in the vicinity of the subducting kurile slab we detect robust p660p reflections from interfaces near 620 670 km depth signifying an increase in the impedance contrast at 660 km depth we also detect deeper reflectors down to 720 km depth beneath the kurile slab in a localized area cold aluminum depleted harzburgitic lithosphere residing at the base of the transition zone best explains the local enhancement of the 660 km discontinuity p wave impedance contrast our new discontinuity measurements support the hypothesis of cold depleted lithosphere stagnating at the 660 km discontinuity beneath the kuriles subduction zone and imply the 660 km boundary can locally impede mantle flow and produce chemical heterogeneity within the transition zone

seismic trapped modes in the oroville and san andreas fault zones three component borehole seismic profiling of the recently active oroville california normal fault and microearthquake event recording with a near fault three component borehole seismometer on the san andreas fault at parkfield california have shown numerous instances of pronounced dispersive wave trains following the shear wave arrivals these wave trains are interpreted as fault zone trapped seismic modes parkfield earthquakes exciting trapped modes have been located as deep as 10 kilometers as shallow as 4 kilometers and extend 12 kilometers along the fault on either side of the recording station selected oroville and parkfield wave forms are modeled as the fundamental and first higher trapped sh modes of a narrow low velocity layer at the fault modeling results suggest that the oroville fault zone is 18 meters wide at depth and has a shear wave velocity of 1 kilometer per second whereas at parkfield the fault gouge is 100 to 150 meters wide and has a shear wave velocity of 1 1 to 1 8 kilometers per second these low velocity layers are probably the rupture planes on which earthquakes occur

characteristics of the fault related rocks fault zones and the principal slip zone in the wenchuan earthquake fault scientific drilling project hole 1 wfsd 1 abstract scientific drilling in active faults after a large earthquake is ideal to study earthquake mechanisms the wenchuan earthquake fault scientific drilling project wfsd is an extremely rapid response to the 2008 ms 8 0 wenchuan earthquake which happened along the longmenshan fault eastern margin of the tibetan plateau in order to better understand the fault mechanism and the physical and chemical characteristics of the rocks the wfsd project will eventually drill 5 boreholes along the two main faults this paper focuses on the first hole wfsd 1 which started just 178 days after the earthquake down to a final depth of 1201 15 m petrological and structural analyses of the cores allowed the identification of fault related rocks in the yingxiu beichuan fault fault gouge cataclasite and fault breccia and the principle slip zone psz location of the wenchuan earthquake was determined we found 12 fault zones in the entire core profile with at least 10 including the yingxiu beichuan fault zone with a multiple cores structure and minimum width of 100 m the co seismic slip plane of the wenchuan earthquake at depth corresponding to the yingxiu beichuan fault zone at the outcrop as well as its psz was expected to be located at the bottom of the fault zone at 759 m depth instead it was found at 590 m depth with 1 cm wide fresh fault gouge as determined by logging data such as temperature natural gamma ray p wave velocity and resistivity combined with the fresh appearance magnetic susceptibility and microstructure of the gouge the wenchuan earthquake slip plane has a dip angle of 65 showing the high angle thrust feature the distribution of fault gouge with several meters thick the location of the wenchuan earthquake s psz and the thickness of fresh gouge all imply a correlation between the width of the fault zone and the number of seismic events

rheological control of oceanic crust separation in the transition zone mineral physics observations suggest that distinct density and rheological differences exist between the crustal component of oceanic lithosphere and the underlying mantle we have conducted numerical experiments to investigate the influence of both density and viscosity on the effectiveness of recycling of oceanic crust into the lower mantle confirming previous results the density inversion at 670 km depth alone is not sufficient to prevent crustal recycling however a soft layer may exist between the strong garnet crust and cold slab interior models employing a simplified newtonian sandwich model show that this thin weak layer can effectively decouple the crust and slab once entrained into the lower mantle the then lighter crust can rise sufficiently fast as a rayleigh taylor instability to avoid further entrainment these results suggest that the crustal component of slabs may be trapped at 670 km depth leading to a garnet enriched transition zone

co seismic thrusting rupture and slip distribution produced by the 2008 mw 7 9 wenchuan earthquake china field investigations reveal that the mw 7 9 wenchuan china earthquake of 12 may 2008 produced a 285 km long surface rupture zone with dominantly thrusting slip accompanied by a right lateral component along the central northern segments of the zone and left lateral component along the southern segment along the longmen shan thrust belt eastern margin of the tibetan plateau the co seismic ruptures mainly occurred along the pre existing yingxiu beichuan guanxian anxian and qingchuan faults which are the main faults of the longmen shan thrust belt the displacements measured in the field are approximately 0 5 6 5 m in the vertical typically 1 3 m accompanied by an average left lateral component of 2 m along the 50 km long southernmost segment of the rupture zone and an average right lateral component of 1 m along the 150 km long central northern segments the maximum thrust slip amount is estimated to be 10 m accompanied by 9 m of shortening across the rupture zone this finding is consistent with estimates based on seismic data the rupture length and maximum vertical displacement are the largest among all intracontinental thrust type earthquakes reported to date our findings demonstrate that i the wenchuan earthquake occurred upon pre existing active faults of the longmen shan thrust belt thereby controlling the spatial distribution of co seismic surface rupture and displacement and the rupture processes of the earthquake ii the long rupture length and large thrusting slip resulted from compressive stress associated with eastward extrusion of the tibet plateau as it accommodates the ongoing penetration of the indian plate into the eurasian plate and iii present day shortening strain upon the eastern margin of the tibetan plateau is mostly released by seismic slip along thrust faults within the longmen shan thrust belt

rupture of deep faults in the 2008 wenchuan earthquake and uplift of the longmen shan the longmen shan mountains rise up 6 000 m over a distance of just 100 km but the mechanisms driving formation of this striking topography are debated analyses of crustal movements during the 2008 wenchuan earthquake suggest that fault induced uplift plays a role in building the high topography

crustal structure beneath the eastern margin of the tibetan plateau and its tectonic implications 1 two crustal cross sections through the eastern margin of the tibetan plateau are jointly determined from deep seismic sounding the e w trending line aa passes through the western sichuan plateau including the songpan garze terrane and the longmenshan fault belt and ends in the sichuan basin a part of the yangtze craton line bb has a trend of nne and crosses the songpan garze terrane two dimensional crustal structures along the profiles were jointly determined by the additional use of existing deep seismic sounding data our seismic velocity models indicate that the western sichuan plateau and the sichuan basin have crustal thicknesses of 62 and 43 km average crustal p wave velocities of 6 27 and 6 45 km s and lower crustal vp 6 5 km s thicknesses of 27 and 15 km respectively density models constructed from the seismic velocity models are consistent with observed bouguer gravity anomalies we infer that collision between the tibetan plateau and the yangtze craton has caused thickening of the lower crust and uplift of the western sichuan plateau we detect a low velocity layer in the upper crust of the western sichuan plateau but observe no equivalence in the sichuan basin west dipping thrusts may detach into this low velocity layer the seismic phase pmp in the western sichuan plateau has low amplitude suggesting high attenuation in the lower crust qp of 100 300 we suggest that the high attenuation is a consequence of lower crustal flow caused by the large lower crustal thickness beneath the western sichuan plateau

kinetics of the post garnet transformation implications for density and rheology of subducting slabs abstract mechanisms and kinetics of the post garnet transformation in natural pyrope were examined at 28 1 31 3 gpa and 1320 2150 k by time resolved x ray diffraction measurements using synchrotron radiation combined with microstructural observations of the recovered samples the water content of samples recovered was estimated to be approximately 400 wt ppm h 2 o from infrared spectroscopic measurements the transformation occurs by the grain boundary reaction and reaction rims are formed along the grain boundaries of parental garnet grains decomposed post garnet assemblages consisting of mg and ca perovskites aluminous phase and stishovite show very fine grained symplectic texture analyses of kinetic data obtained have revealed that growth rates in the post garnet transformation are strongly time dependent extrapolations of the present kinetic data suggest that the post garnet transformation cannot complete at less than ca 1730 k even under geological timescales because of the slow growth kinetics due to the presence of the metastable garnet subducting slabs become buoyant in the lower mantle at less than ca 1730 k for the basaltic crust and at ca 1400 1600 k for the underlying peridotite layer rheological weakening of the basaltic crust is expected in the lower mantle through grain size reduction by the metastable post garnet transformation occurring under large overpressure conditions the kinetics of the post garnet transformation significantly affect both the density and rheology of subducting slabs in the lower mantle and possibly has important roles in the mixing and survival of chemically differentiated slab materials

seismic evidence for a 920 km discontinuity in the mantle analysis of hundreds of seismograms from a seismic array in the japanese islands reveals a seismic discontinuity at a depth of about 920 km in the mantle beneath the tonga subduction zone this discontinuity is also evident beneath subduction zones in the japan and flores seas the discontinuity probably represents either a change in mantle chemical composition or the signature of the subducted slab s garnet layer

phase relations of harzburgite and morb up to the uppermost lower mantle conditions precise comparison with pyrolite by multisample cell high pressure experiments with implication to dynamics of subducted slabs

low velocity zones along the san jacinto fault southern california from body waves recorded in dense linear arrays we derive high resolution information on low velocity fault zone fz structures along the san jacinto fault zone sjfz southern california using waveforms of local earthquakes that are recorded at multiple linear cross fault arrays we observe clear across fault delays of direct p and s waves indicating damage zones at different segments of the sjfz we then compute synthetic traveltimes and waveforms using generalized ray theory and perform forward modeling to constrain the fz parameters at the southern section near the trifurcation area the low velocity zone lvz of the clark branch has a width of 200 m 30 45 reduction in vp and 50 reduction in vs from array data across the anza seismic gap we find a lvz with 200 m width and 50 reduction in both vp and vs nearly as prominent as that on the southern section we only find prominent lvzs beneath three out of the five arrays indicating along strike variations of the fault damage fz reflected phases are considerably less clear than those observed above the rupture zone of the 1992 landers earthquake shortly after the event this may reflect partially healed lvzs with less sharp boundaries at the sjfz given the relatively long lapse time from the last large surface rupturing event alternatively the lack of observed fz reflected phases could be partially due to the relatively small aperture of the arrays nevertheless the clear signatures of damage zones at anza and other locations indicate very slow healing process at least in the top few kilometers of the crust

low velocity damaged structure of the san andreas fault at parkfield from fault zone trapped waves 1 we used dense linear seismic arrays across and along the san andreas fault saf at parkfield california to record fault zone trapped waves generated by explosions and microearthquakes in 2002 prominent trapped waves appeared at stations close to the saf main fault trace while some energy was trapped in the north strand at the array site observations and 3 d finite difference simulations of trapped waves at 2 5 hz show evidence of a damaged core zone on the main saf the zone from the surface to seismogenic depths is marked by a low velocity waveguide 150 m wide in which q is 10 50 and shear velocities are reduced by 30 40 from wall rock velocities with the greatest velocity reduction at shallow depth we interpret that this distinct low velocity zone on the main saf is a remanent of damage due to past large earthquakes on the principal fault plane at parkfield a less developed low velocity zone may be evident on the north strand that experienced minor breaks in the 1966 m6 event

plate tectonic reconstructions predict part of the hawaiian hotspot track to be preserved in the bering sea we use plate reconstructions to show that parts of the hawaiian hotspot track of ca 80 90 ma age could be preserved in the bering sea based on these reconstructions the hawaiian hotspot was beneath the izanagi plate before ca 83 ma around that time the part of the plate carrying the hotspot track was transferred to the kula plate after 75 80 ma the hawaiian hotspot underlay the pacifi c plate circa 40 55 ma subduction initiated in the aleutian trench part of the kula plate was attached to the north american plate and is preserved as the oceanic part of the bering sea we show that for a number of different plate reconstructions and a variety of assumptions covering hotspot motion part of the hotspot track should be preserved in the bering sea the predicted age of the track depends on the age of aleutian subduction initiation we speculate that bowers and shirshov ridges were formed by paleohawaiian hotspot magmatism

a sporadic low velocity layer atop the 410 km discontinuity beneath the pacific ocean waveforms of ss precursors recorded by global stations are analyzed to investigate lateral heterogeneities of upper mantle discontinuities on a global scale a sporadic low velocity layer immediately above the 410 km discontinuity lvl 410 is observed worldwide including east asia western north america eastern south america the pacific ocean and possibly the indian ocean our best data coverage is for the pacific ocean where the lvl 410 covers 33 50 of the resolved region lateral variations of our lvl 410 observations show no geographical correlation with 410 km discontinuity topography or tomographic models of seismic velocity suggesting that the lvl 410 is not caused by regional thermal anomalies we interpret the lvl 410 as partial melting due to dehydration of ascending mantle across the 410 km discontinuity which is predicted by the transition zone water filter hypothesis given the low vertical resolution of ss precursors it is possible that the regions without a clear lvl 410 detection also have a thin layer therefore the strong lateral heterogeneity of the lvl 410 in our observations suggests partial melting with varying intensities across the pacific and further provides indirect evidence of a hydrous mantle transition zone with laterally varying water content

ontong java plateau and late neogene changes in pacific plate motion aided by a new geometrical technique called hot spotting we recently proposed a new model for the absolute motion of the pacific plate this model called wk97 assumed the hawaiian hotspot is under loihi and that there was a change in plate motion at 3 6 ma the model located the louisville hotspot close to the hollister ridge a shallow volcanic ridge associated with a geoid high seismicity and recent volcanism most of the tectonic implications of wk97 however were not addressed furthermore although several recent studies suggest that the geochemistry of hollister lavas may indicate a mixture between louisville melt material and pacific antarctic ridge mid ocean ridge basalt these studies strongly disagree with wk97 and consider it seriously flawed here we discuss numerous tectonic predictions of wk97 and present a large amount of observational evidence in support of it we find that a late neogene collision between the ontong java plateau and the northern margin of the australia plate appears to have altered the motion of the pacific plate as inferred from hotspot volcanism by forcing it to rotate counter clockwise this rotation about a low latitude pole appears to have induced right lateral shear stress along the entire pacific plate divergent boundary which resulted in the formation of extensional transform faults microplates and propagating ridges this change in motion also appears to have triggered concomitant circum pacific tectonism including trench migration and back arc rifting the wk97 suggests that long lived plume fed volcanism on the pacific plate appears very limited perhaps only to five to six hotspots hawaii louisville cobb caroline and perhaps marquesas and bowie the geometry of these chains are consistent with our model despite some evidence for volcanic age progressions most other pacific neogene hotspots are most likely crack spots i e sites of extensional volcanism at preexisting zones of weakness reactivated by plate stresses indeed several volcanic ridges such as hollister ridge near the louisville hotspot appear to have formed in response to stresses following the change in absolute plate motion wk97 also obviates the need for simultaneous different directions of mantle flow in the south central pacific transpression at the san andreas and alpine strike slip faults and aleutian arc explosivity since 6 ma also appear to be consequences of the changes in pacific plate motion furthermore both the wk97 model and observed relative plate motions suggest that adjoining plates have changed their absolute motions as well and that large changes in absolute plate motion do not necessarily imply large changes in relative plate motion our findings support slab pull as the dominant plate tectonic driving force highlight the rheological difference between strong oceanic lithosphere and weak continental crust and suggest that wk97 presently provides a unifying albeit preliminary framework for relating both intraplate and circum pacific tectonism and volcanism in the late neogene

mid cretaceous hawaiian tholeiites preserved in kamchatka we report geochemical data on a peculiar group of albian cenomanian 120 93 ma basalts preserved in ophiolites on the kamchatsky mys peninsula kamchatka russia that share trace element and isotopic compositions with enriched tholeiites from the detroit and meiji seamounts in the hawaiian emperor seamount chain melt inclusions in chromium spinel from these rocks representative of melt composition unaffected by post magmatic alteration exhibit hawaiian type th ba n 0 25 0 77 i e distinctively low compared to the majority of oceanic island basalts and mid oceanic ridge basalts low 208 pb 206 pb of 0 93 in rocks and high nb la n 1 1 4 6 in melt inclusions suggest the presence of a distinctive kea type component in their source we propose that the ophiolitic basalts represent older early to middle cretaceous products of the hawaiian hotspot older than preserved on the northwest pacifi c seafl oor that were accreted to the forearc of kamchatka the presence of similar compositional components in modern and cretaceous hawaiian hotspot lavas suggests a persistent yet heterogeneous composition of the mantle plume which may have sampled 15 of the core mantle boundary layer over the past 100 m y

ambient noise rayleigh wave tomography in western sichuan and eastern tibet abstract we perform rayleigh wave tomography in western sichuan and eastern tibet by applying the ambient noise method to broadband data recorded at 49 stations from the sichuan digital seismic network cross correlations of vertical component ambient noise data are computed in one hour segments and stacked over 7 months from june to december 2007 then rayleigh wave group dispersion curves from 6 to 48 s were measured using the multiple filter analysis method group velocity maps at periods from 8 to 44 s were inverted the resulting group velocity maps generally show good correlations with tectonic features the sichuan basin is clearly resolved with lower group velocities at the short periods 8 to 20 s due to thick sedimentary layers and a prominent low velocity zone exhibits in the middle crust beneath the eastern margin of the tibetan plateau the longmen shan thrust belt marks not only the rapid lateral velocity variation between the topographically high tibetan plateau and low elevation sichuan basin but also the transition from the thick crust beneath the tibetan plateau to the normal continental crust beneath the sichuan basin

linking metamorphic textures to u pb monazite in situ geochronology to determine the age and nature of aluminosilicate forming reactions in the northern monashee mountains british columbia abstract the monashee mountains of the canadian cordillera are thought to expose a classic barrovian facies series of isograds the timing of aluminosilicate growth in the region was determined for four pelitic schist samples by combining textural relationships with monazite compositional zoning and monazite u pb geochronology conducted directly on thin sections by the laser ablation method three distinct phases of kyanite growth are recorded in the kyanite zone at c 153 ma between 122 and 94 ma and between 76 and 58 ma for each phase monazite and garnet grew synchronously with kyanite probably by a reaction involving the breakdown of staurolite in contrast sillimanite growth by muscovite dehydration melting occurred at or before c 104 ma in the sillimanite zone and retrograde sillimanite grew in schists previously metamorphosed at the kyanite grade during the first two phases by the influx of hot acidic fluids during top to the east shearing at ca 71 ma these results indicate that rocks metamorphosed at different places and different times in the orogen were juxtaposed prior to being overprinted at the sillimanite grade in the late cretaceous early paleocene during the influx of hot fluids in a structurally coherent body deforming by easterly directed shearing this study also provides new insight into monazite petrogenesis and suggests that at least in some circumstances monazite formation is linked to the staurolite out reaction that produces kyanite

crustal structure across the san andreas fault southern california from teleseismic converted waves crustal structure along a 140 km long profile across the san andreas fault saf in southern california was imaged by stacking teleseismic p s converted phases recorded by a dense short period seismic array the crust mantle discontinuity moho is visible as a continuous feature at a depth around 30 km but is offset 6 to 8 km beneath the saf a small moho disruption can also be seen under the eastern california shear zone ecsz these results suggest that the saf and ecsz extend through the entire crust the moho upwarp under the san gabriel mountains indicates that the mountain ranges were lifted en masse as a result of crustal buckling under horizontal compression

unusual mantle poisson s ratio subduction and crustal structure in central alaska 1 subduction zones alter the upper plate in a variety of ways including metasomatism via slab derived fluids and accretion of exotic terranes these processes should produce distinctive seismic signatures as seismic velocities are sensitive to compositional variations a new method is developed to estimates depths to interfaces and poisson s ratio vp vs ratio for dipping layered structures by stacking teleseismic receiver functions it also estimates crustal vp vs and thickness using this method we analyze a broadband data set from the central alaska subduction zone to seek evidence for mantle wedge composition crustal thickness varies from 27 to 45 km and crustal vp vs indicates an intermediate to mafic composition expected for these accreted island arc terranes inversions for mantle structure confirm the presence of a 15 20 km thick low velocity zone atop of the downgoing plate to 130 km depth perhaps subducted crust of an exotic terrane vp vs in the mantle wedge allows for 15 15 serpentinization where the slab is 80 km deep and the wedge is cold where the slab is deeper vp vs for the mantle wedge is unusually low 1 7 this value is lower than predicted for any common mantle mineral and may indicate significant quantities of quartz provided that past physical properties measurements are not grossly in error these results suggest that the mantle wedge in subduction zones can occasionally differ substantially in bulk composition from normal subcrustal mantle perhaps because collisions of exotic terranes cause tectonic mixing of crustal material into the mantle

high resolution structures of the landers fault zone inferred from aftershock waveform data summary high frequency body waves recorded by a temporary seismic array across the surface rupture trace of the 1992 landers california earthquake were used to determine fault zone structures down to the seismogenic depth we first developed a technique to use generalized ray theory to compute synthetic seismograms for arbitrarily oriented tabular low velocity fault zone models we then generated synthetic waveform record sections of a linear array across a vertical fault zone they show that both arrival times and waveforms of p and s waves vary systematically across the fault due to transmissions and reflections from boundaries of the low velocity fault zone the waveform characteristics and arrival time patterns in the record sections allow us to locate the boundaries of the fault zone and to determine its p and s wave velocities independently as well as its depth extent therefore the trade off between the fault zone width and velocities can be avoided applying the method to the landers waveform data reveals a low velocity zone with a width of 270 360 m and a 35 60 per cent reduction in p and s velocities relative to the host rock the analysis suggests that the low velocity zone extends to a depth of 7 km the western boundary of the low velocity zone coincides with the observed main surface rupture trace

influence of geometry and eclogitization on oceanic plateau subduction abstract subduction is driven by the negative buoyancy of the slab which depends on both the temperature and composition of the lithospheric plate in the case of subduction of an oceanic plateau larger thickness of the lower density crust and harzburgite layers can locally decrease the negative buoyancy available to drive subduction however the ability of a plateau to have an impact on subduction depends on the total buoyancy of the slab and therefore also depends on size of the plateau and the basalt to eclogite transition which substantially increases the density of the crust using 2 d numerical finite element models of subduction we investigate the role of eclogitization and oceanic plateau size thickness and length in the process of oceanic plateau subduction model results show that eclogitization of the crust substantially increases the chances that a plateau will be subducted because the extra buoyancy of larger plateaus is lost through the phase transition for kinematically driven models all plateaus are subducted regardless of the thickness or width extrapolation of these results to 3 d geometry shows that sufficient slab buoyancy is available to sustain subduction for narrow plateaus with slab widths 6 7 times the plateau width depending on plateau thickness and length in dynamically driven models subduction of oceanic plateaus of various sizes can substantially slow subduction and large plateaus 200 km long and 25 km thick can lead to break off of the slab and underplating of the plateau

continental and oceanic crustal structure of the pampean flat slab region western argentina using receiver function analysis new high resolution results summary the pampean flat slab of central chile and argentina 30 32 s has strongly influenced cenozoic tectonics in western argentina which contains both the thick skinned basement cored uplifts of the sierras pampeanas and the thin skinned andean precordillera fold and thrust belt in this region of south america the nazca plate is subducting nearly horizontally beneath the south american plate at 100 km depth to gain a better understanding of the deeper structure of this region including the transition from flat to normal subduction to the south three iris passcal arrays of broad band seismic stations have been deployed in central argentina using the dense siembra array combined with the broader charge and esp arrays the flat slab is imaged for the first time in 3 d detail using receiver function rf analysis a distinct pair of rf arrivals consisting of a negative pulse that marks the top of the oceanic crust followed by a positive pulse which indicates the base of the oceanic crust can be used to map the slab s structure depths to moho and oceanic crustal thicknesses estimated from rf results provide new more detailed regional maps an improved depth to continental moho map shows depths of more than 70 km in the main cordillera and 50 km in the western sierras pampeanas that shallow to 35 km in the eastern sierras pampeanas depth to moho contours roughly follow terrane boundaries offshore the hotspot seamount chain of the juan fernandez ridge jfr is thought to create overthickened oceanic crust providing a mechanism for flat slab subduction by comparing synthetic rfs based on various structures to the observed rf signal we determine that the thickness of the oceanic crust at the top of the slab averages at least 13 19 km supporting the idea of a moderately overthickened crust to provide the additional buoyancy for the slab to remain flat the overthickened region is broader than the area directly aligned with the path of the jfr however and indicates along with the slab earthquake locations that the flat slab area is wider than the jfr volcanic chain observed in the offshore bathymetry further rfs indicate that the subducted oceanic crust in the region directly along the path of the subducted ridge is broken by trench parallel faults one explanation for these faults is that they are older structures within the oceanic crust that were created when the slab subducted alternatively it is possible that faults formed recently from tectonic underplating caused by increased interplate coupling in the flat slab region

fault zone structure at depth from differential dispersion of seismic guided waves evidence for a deep waveguide on the san andreas fault summary seismic guided wave dispersion can be used to image fault zone structure at seismogenic depth a two station differential group velocity technique previously used for surface waves was adapted to solve for local fault zone structure between two stations this method was extended to solve for fault zone structure between two earthquakes using differential group arrival times at a single station the method was tested with finite difference synthetic data for an inhomogeneous fault as well as with a pair of shallow earthquakes recorded in the san andreas fault observatory at depth safod borehole station results from a pair of deep earthquakes recorded in the safod borehole station indicate that the low velocity waveguide of the san andreas fault extends to 10 km depth the waveguide at 10 12 km depth is 120 190 m wide and the velocity contrast is 20 per cent similar to the values in the shallow subsurface multiple earthquakes and receivers could be used to map fault zone structure at seismogenic depth as a function of depth and strike

the role of oceanic plateau subduction in the laramide orogeny the cause of the laramide phase of mountain building remains uncertain conceptual models implicate the subduction of either ocean ridges or conjugates of the buoyant hess or shatsky oceanic plateaux independent verification of these models has remained elusive because the putative ridges or plateaux are no longer at the earth s surface inverse convection models have identified two prominent seismic anomalies on the recovered farallon plate here we combine inverse convection models with reconstructions of plate motions to show that these seismic anomalies coincide palaeogeographically with the restored positions of the shatsky and hess conjugate plateaux as they subducted beneath north america specifically the distribution of laramide crustal shortening events tracked the passage of the shatsky conjugate beneath north america whereas the effects of the hess conjugate subduction were restricted to the northern mexico foreland belt we propose that continued subduction caused the oceanic crust to undergo the basalt eclogite phase transformation during which the shatsky conjugate lost its extra buoyancy and was effectively removed increases in slab density and coupling between the overriding and subducting plates initially dragged the surface downward followed by regional scale surface rebound we conclude that laramide uplift resulted from the removal rather than emplacement of the shatsky conjugate

finite frequency whole mantle p wave tomography improvement of subducted slab images received 23 july 2013 revised 14 october 2013 accepted 17 october 2013 1 we present a new whole mantle p wave tomographic model gap p4 we used two data groups short period data of more than 10 million picked up onset times and long period data of more than 20 thousand differential travel times measured by waveform cross correlation finite frequency kernels were calculated at the corresponding frequency bands for both long and short period data with respect to an earlier model gap p2 we find important improvements especially in the transition zone and uppermost lower mantle beneath the south china sea and the southern philippine sea owing to broadband ocean bottom seismometers bbobss deployed in the western pacific ocean where station coverage is poor this new model is different from a model in which the full data set is interpreted with classical ray theory bbobs observations should be more useful to sharpen images of subducted slabs than expected from simple raypath coverage arguments citation obayashi m j yoshimitsu g nolet y fukao h shiobara h sugioka h miyamachi and y gao 2013 finite frequency whole mantle p wave tomography improvement of subducted slab images geophys res lett 40 doi 10 1002 2013gl057401

can seismic waves be trapped inside an inactive fault zone the case study of nocera umbra central italy in march 1998 a small aperture 350 m linear array of five broadband stations was installed across a pliocene strike slip fault crossing the town of nocera umbra central italy three component seismograms of local earthquakes recorded during the umbria marche seismic sequence have shown that the degradation of mechanical properties in damaged rock causes a significant amplification of ground motion within the fault zone marra et al 2000 during the operation of the array an m w 5 3 subcrustal earthquake occurred at a depth of 48 km and an epicentral distance of 10 km from nocera umbra the near vertical incidence of this event offers a unique opportunity for constraining the 2d geometry of the laterally heterogeneous low velocity structure responsible for the large amplification across the fault based on the observed symmetry of the amplification pattern a 2d wedge shaped model of the fault zone is used for the computation of antiplane sh synthetic waveforms to be compared with observations along the array two parameters of the wedge model can be constrained by independent geophysical and geological data 1 shear wave velocity in pristine rock outside the fault zone v s 2200 m sec and 2 the lateral extent of the damaged rock zone l 160 m the other two model parameters namely the wedge depth h and shear wave velocity of the fault zone v f s are varied computer simulations show that trapped waves which have been observed so far for earthquakes occurring inside the fault zone can develop inside a 2d wedge model when struck by external vertically incident seismic radiation results of numerical modeling indicate that in spite of its extreme simplicity such an idealized model is successful in reproducing many of the essential characters of ground motion within the fault zone of nocera umbra even though the best fit solution is not unique and the model parameters trade off each other a study of parameter sensitivity comparing synthetics with recorded data indicates that the variability of h and v f s can be restricted to a limited range of values the quality of fit is optimal when the shear velocity reduction lies in the range of 40 50 and the depth of the trapping zone is comprised between 1 and 2 km

dynamic uplift during slab flattening subduction exerts a strong control on surface topography and is the main cause of large vertical motions in continents including past events of large scale marine flooding and tilting the mechanism is dynamic deflection the sinking of dense subducted lithosphere gives rise to stresses that directly pull down the surface here we show that subduction does not always lead to downward deflections of the earth s surface subduction of young lithosphere at shallow angles flat subduction leaves it neutrally or even positively buoyant with respect to underlying mantle because the lithosphere is relatively warm compared with older lithosphere and because the thickened and hence drier oceanic crust does not undergo the transformation of basalt to denser eclogite accounting for neutrally buoyant flat segments along with large variations in slab morphology in the south american subduction zone explains along strike and temporal changes in dynamic topography observed in the geologic record since the beginning of the cenozoic our results show that the transition from normal subduction to slab flattening generates dynamic uplift preventing back arc marine flooding