the mechanics of frontal imbrication a first order analysis the detachment and imbrication of thrust slices at the front of a thrust wedge is one of the principle modes by which such wedges grow collapse of the frontal ramp under longitudinal compressional stress cannot explain the regular formation of new slices of finite length unless there are regularly spaced heterogeneities in the footwall layer or the underlying basement surface advance of the thrust wedge over the frontal ramp however increases both the vertical load on the ramp and the traction on the upper flat this will in general produce a peak deviatoric stress in the footwall layer below the leading edge of the thrust wedge failure will occur at this point when the thrust wedge has advanced a distance l such that the deviatoric stress in the footwall layer exceeds its strength l is a function of a rock density b ramp angle c the resistances to motion on the basal detachment the ramp and the upper flat and d the strength and thickness of the footwall layer these mechanical parameters can therefore control the formation of new thrust slices of regular length in the absence of footwall heterogeneities

implications of the north and south panama thrust belts for the origin of the panama orocline high resolution imaging of the north nptb and south panama thrust belts using the seamarc ii seafloor mapping system and digital seismic reflection profiling provides an excellent data base with which to determine the kinematics of oroclinal deformation of panama remarkable continuity of structural orientations over 300 km along the nptb argues for widespread uniformity in direction of thrusting between panama and the caribbean plate westward decrease in the width and height of the accretionary wedge indicates a westward decrease in the total amount of convergence two major cross faults cut the wedge in its eastern part forming two minor blocks with different relative motions with respect to the caribbean frontal thrust orientations change across these faults major faults have been proposed to cut the isthmus of panama but simple cross isthmus faulting alone cannot explain the orientations of the major thrusts of the nptb bending of a flexible beam can explain much of the observed structure both directions and rates though the scale of the orocline precludes considering this as an elastic phenomenon plastic deformation of panama may be a useful mechanical explanation at least for eastern panama but consideration of plastic slip lines must include the northern andes as well changing from more rigid blocklike offsets on faults in western panama to a continuum of strain in eastern panama may be the best explanation for observations at present

coseismic gravity changes of the 2010 earthquake in central chile from satellite gravimetry 1 the first map of coseismic changes in gravity and geoid height has been drawn using the data from the gravity recovery and climate experiment grace satellites for the 2004 sumatra andaman earthquake here we present the second case of coseismic gravity change observation by satellite gravimetry i e the change caused by an interplate thrust earthquake that occurred on 27 february 2010 in central chile mw 8 8 gravity showed a negative jump with the largest drop of 5 gal on the back arc side the observed changes agree with those calculated assuming the realistic earth and fault parameters inferred from coseismic displacements of global positioning system gps stations gravity in this area shows large seasonal and inter annual variability and postseismic gravity changes could be isolated only by carefully removing hydrological signals

crustal dilatation observed by grace after the 2004 sumatra andaman earthquake we report the detection of an earthquake by a space based measurement the gravity recovery and climate experiment grace satellites observed a 15 microgalileo gravity change induced by the great december 2004 sumatra andaman earthquake coseismic deformation produces sudden changes in the gravity field by vertical displacement of earth s layered density structure and by changing the densities of the crust and mantle grace s sensitivity to the long spatial wavelength of gravity changes resulted in roughly equal contributions of vertical displacement and dilatation effects in the gravity measurements the grace observations provide evidence of crustal dilatation resulting from an undersea earthquake

palaeomagnetism of some late mesozoic dolerite sills of east central spitsbergen svalbard archipelago summary two late mesozoic dolerite sills situated near agardhbukta on the east coast of vestspitsbergen and dated radiometrically at 100 4 myr bp have been sampled in five localities and subjected to detailed mineralogical and rock magnetic studies to determine the direction and origin of their magnetization although the sills lie outside the tertiary orogenic belt one locality no 4 has undergone strong hydrothermal alteration and a small part of another locality no 3 has also been affected a conventional procedure based on examination of zijderveld diagrams applied to specimens demagnetized by alternating fields and thermally yielded similar remanence directions at all five localities except at the altered part of locality 3 using a least squares computer method of analysis of step demagnetization data comparable directions were isolated from all localities including the altered part of locality 3 except in this last case all directions were reversed the adjusted mean direction obtained from this analysis is d 159 0 i 62 2 95 9 0 yielding a palaeomagnetic pole situated at 225 0 54 3 n comparable with pole positions obtained from other late mesozoic igneous rocks on spitsbergen and distinct from palaeopoles derived from mesozoic rocks in north america and eurasia this suggests that during the late mesozoic svalbard existed as a semi independent microplate

offscraping and underthrusting of sediment at the deformation front of the barbados ridge deep sea drilling project leg 78a on leg 78a we drilled sites 541 and 542 into the seaward edge of the barbados ridge complex and site 543 into the adjacent oceanic crust the calcareous ooze marls and muds at sites 541 and 542 are lithologically and paleontologically similar to the upper strata at site 543 and are apparently offscraped from the down going plate a repetition of miocene over pliocene sediments at site 541 documents major thrust or reverse faulting during offscraping the hemipelagic to pelagic deposits offscraped in the leg 78a area include no terrigenous sand beds but they contain numerous neogene ash layers derived from the lesser antilles arc hence this sequence is quite unlike the siliciclastic dominated terranes on land that are inferred to be accretionary complexes the structural fabric of the offscraped deposits at sites 541 and 542 is disharmonic probably along a decollement with an underlying acoustically layered sequence suggesting selective underthrusting of the latter the acoustically layered sequence correlates seismically with pelagic strata cored at site 543 on the incoming oceanic plate cores recovered from the possible decollement surface at both sites 541 and 542 show scaly foliation and stratal disruption approximately lithostatic fluid pressure measured in the possible decollement zone probably facilitates the underthrusting of the pelagic sediments beneath the offscraped deposits in the incoming section a transition from smectitic to radiolarian mud with associated increases in density and strength probably controls the structural break between offscraped and underthrust strata in the leg 78a area the underthrust pelagic section can be traced seismically at least 30 km arcward of the deformation front beneath the barbados ridge complex

seismicity and tectonics of southern central america and adjacent regions with special attention to the surroundings of panama abstract ten new focal mechanisms are derived for earthquakes in southern central america and its adjacent regions these are combined with a study of seismicity and data of previous workers to delineate the position and nature of the plate boundaries in this complex region the middle america subduction zone may be divided into four or five distinct seismic segments the plate boundary between north america and the caribbean near the trench might be located more towards the south than previously suspected subduction has basically stopped south of the underthrusting cocos ridge there is not much evidence for a seismically active strike slip fault south of panama but its existence cannot be ruled out more activity reveals the zone north of panama which is identified as a subduction zone with normal fault events shallow seismicity induced by the interaction of the nazca plate extends from the colombia panama border south along the pacific coast to meet a high angle continental thrust fault system subduction with a pronounced slab starts only south of that point near a hot region which offsets the seismic trend at the trench the carnegie ridge and or the change of direction of subduction in ecuador produce a highly active zone of seismicity mainly at the depth of 200 km the area in the pacific displays a termination of activity at a propagating rift west of the galapagos islands the main eastern boundary of the cocos plate the panama fracture zone is offset towards the west at the southern end of the malpelo ridge its northern end consists of two active branches as defined by large earthquakes a strike slip mechanism near the southeastern flank of the cocos ridge was previously believed to be the site of an extended fracture zone this paper proposes submarine volcanic activity as an alternative explanation

irreversible mass loss of canadian arctic archipelago glaciers 1 the canadian arctic archipelago caa contains the largest volume of glacier ice on earth outside of antarctica and greenland in the absence of significant calving caa glacier mass balance is governed by the difference between surface snow accumulation and meltwater runoff surface mass balance here we use a coupled atmosphere snow model to simulate present day and 21st century caa glacier surface mass balance through comparison with gravity recovery and climate experiment mass anomalies and in situ observations we show that the model is capable of representing present day caa glacier mass loss as well as the dynamics of the seasonal snow cover on the caa tundra next we force this model until 2100 with a moderate climate warming scenario ar5 rcp4 5 we show that enhanced meltwater runoff from caa glaciers is not sufficiently compensated by increased snowfall extrapolation of these results toward an ar5 multimodel ensemble results in sustained 21st century caa glacier mass loss in the vast majority 99 of the 7000 temperature realizations

tectonics and hydrogeology of accretionary prisms role of the decollement zone abstract at convergent margins the decollement zone comprises the plate boundary and is marked by profound structural disharmony by changes in stress orientation and by a discontinuity in plate velocity the decollement zone initiates in a weak sediment layer typically a low permeability hemipelagic mud lying below a more rapidly deposited stronger more permeable trench turbidite sequence at the deformation front trench turbidites tend to be offscraped whereas the finer grained hemipelagic and pelagic sediments are more likely to be underthrust offscraped materials may undergo only limited burial whereas underthrust deposits can be deeply buried undergo high pressure metamorphism and are more likely to be preserved in the stratigraphic record burial rates associated with underthrusting are high exceeding 20 km my with high rates of underthrusting sediment descending below the decollement zone is probably buried faster than it can dewater resulting in emplacement of relatively high porosity deposits at depth fluids flowing from modern decollement zones have migrated substantial distances laterally and tapped deep sources the episodicity of flow suggests pumping of fluid by cycles of dilation and flow to the decollement zone followed by failure and fluid expansion along the decollement zone possible pressure gradients below the decollement zone allow flow upward into it while maintaining minimum effective stress along it the thickness of sediment underthrust beneath the decollement zone determines whether these deposits are emplaced by diffusive underplating with stratal disruption and efficient dewatering or by coherent underplating with the formation of macroscopic duplexes and transfer of fluid to the base of the accretionary prism deformation mechanisms affecting accreted sediment depend on depth of entrainment into the decollement zone and duration of residence

grace measurements of mass variability in the earth system monthly gravity field estimates made by the twin gravity recovery and climate experiment grace satellites have a geoid height accuracy of 2 to 3 millimeters at a spatial resolution as small as 400 kilometers the annual cycle in the geoid variations up to 10 millimeters in some regions peaked predominantly in the spring and fall seasons geoid variations observed over south america that can be largely attributed to surface water and groundwater changes show a clear separation between the large amazon watershed and the smaller watersheds to the north such observations will help hydrologists to connect processes at traditional length scales tens of kilometers or less to those at regional and global scales

the margin between senja and spitsbergen fracture zones implications from plate tectonics abstract analysis of multichannel seismic data from the continental margin off svalbard between the senja and spitsbergen fracture zones suggests that the transition between continental and oceanic crust is located at or close to the hornsund fault zone in the late paleocene early eoeene 57 m y the region between svalbard and northeast greenland was subjected to regional shear movements associated with a transform system between the young lofoten greenland basin and the arctic ocean approximately 50 m y ago the spreading axis migrated to the northeast creating a deep basin north of the greenland senja fracture zone forming the passive margin between bear island and 76 5 n north of 76 5 n the regional transform was maintained at the time of the main reorganization of relative plate motion 36 m y the northern margin evolved a continental fragment was possibly cut off from the svalbard margin forming a small microcontinent the microcontinent appears as the submarine ridge which has been associated with the hovgaard fracture zone it is suggested that the sediments west of the hornsund fault zone are not older than eocene in the south and mid oligocene in the north the position of the spreading axis has greatly influenced the margin sedimentation

time variable gravity observations of ice sheet mass balance precision and limitations of the grace satellite data time variable gravity data from the gravity recovery and climate experiment grace mission have been available since 2002 to estimate the mass balance of the greenland and antarctic ice sheets we analyze current progress and uncertainties in grace estimates of ice sheet mass balance we discuss the impacts of errors associated with spherical harmonic truncation spatial averaging temporal sampling and leakage from other time dependent signals e g glacial isostatic adjustment gia the largest sources of error for antarctica are the gia correction the omission of l 1 terms nontidal changes in ocean mass and measurement errors for greenland the errors come mostly from the uncertainty in the scaling factor using release 5 0 rl05 grace fields for january 2003 through november 2012 we find a mass change of 258 41 gt yr for greenland with an acceleration of 31 6 gt yr2 and a loss that migrated clockwise around the ice sheet margin to progressively affect the entire periphery for antarctica we report changes of 83 49 and 147 80 gt yr for two gia models with an acceleration of 12 9 gt yr2 and a dominance from the southeast pacific sector of west antarctica and the antarctic peninsula

dewatering and extensional deformation of the shikoku basin hemipelagic sediments in the nankai trough the shikoku basin hemipelagic sequence which underlies the nankai trough wedge s w japan thins by 50 between the outer edge of the trench wedge and dsdp site 582 14 km arcward a sedimentation model which incorporates changes in sedimentation rates with time and with distance from the trench wedge toe indicates that 57 of the total thinning occurs as a result of temporally varying sedimentation rates and a time transgressive facies boundary between the trench wedge turbidites and the underlying hemipelagites burial induced consolidation beneath the wedgeshaped turbiditic overburden accounts for the remaining 43 of arcward thinning within the hemipelagic unit rapid dewatering modeled as one dimensional consolidation suggests that the excess pore water pressures are quite low during this progressive dewatering thus high pore water pressures should not be assumed to occur universally in convergent margin settings normal faults and vein structures in the hemipelagites suggest near horizontal extension in addition to vertical consolidation the estimates of excess pore water pressures together with fault geometries and horizontal extensional strains determined from the geometry of the subducting oceanic plate can be used to constrain the stress conditions at failure the expulsion of hot water from the rapidly dewatering sediments in the nankai trough may help to explain anomalously high heat flow in the central part of the trough

ellipsoidal correction in grace surface mass change estimation the earth s shape is much closer to an ellipsoid than a sphere the commonly used spherical approximation in mass change inversion is expected to cause bias by the spherical harmonic sh solutions from gravity recovery and climate experiment grace especially in high latitude regions where significant present day ice losses occur this bias or ellipsoidal correction reaches up to 8 from the evaluation by simulations based on synthetic mass change rate models further evaluation using 14 plus years of grace monthly sh solutions from april 2002 to december 2016 indicates that the ellipsoidal correction is also noticeable in the total mass change time series over polar regions before and after the ellipsoidal correction the estimated linear rates from mass change time series differ by 4 3 4 7 5 2 5 7 and 6 6 for five selected regions over greenland antarctic peninsula ap amundsen sea embayment ase alaska glacier and svalbard islands respectively although with amplitudes likely below the current grace s uncertainty level these differences are consistently negative over the five regions this indicates that the spherical approximation leads to systematic underestimation for polar mass change rates thus the ellipsoidal correction needs to be considered for more precise mass recovery with grace sh solutions it also depends on spatial scales of mass change signals the smaller the spatial scale the larger the correction to more reliably estimate high latitude surface mass changes by grace sh solutions the ellipsoidal correction is recommended especially for ice loss signals over polar regions

synthetic seismogram and surface wave constraints on crustal models of spitsbergen abstract seismic refraction data collected on spitsbergen in 1978 are used to obtain a crustal model assuming plane horizontal layering the observed travel times and wave forms are compared with those of synthetic seismograms computed for various published crustal models the more detailed models adequately explain some but not all of the features of the synthetics these models are adjusted utilizing travel times and wave form amplitudes until a satisfactory fit is achieved the best fitting model consists of a 4 layer crust having thicknesses of 4 1 10 0 7 4 and 5 8 km and compressional velocities of 4 65 6 21 6 30 and 6 65 km sec with increasing depth the uppermost mantle has a velocity of 7 90 km sec a comparison of observed and synthetic p n waveforms supports the existence of a thin low velocity zone beginning at a depth of about 5 km beneath the moho boundary an inversion of seismic surface wave group velocity data yields a shear wave model which is compatible with the compressional wave model

integrated paleontologic and paleomagnetic stratigraphy of the upper neogene deposits around limon costa rica a coastal emergence record of the central american isthmus a chronostratigraphic study of mixed carbonate and siliciclastic sediments limon group along the caribbean side of the central american isthmus was conducted to provide refined depositional ages on the uplifted nearshore marine sequence exposed near limon costa rica these upper neogene sediments provide a rich faunal history that spanned the closure of the isthmus and cessation of marine circulation between the pacific ocean and caribbean sea this faunal archive provides a critical link in regional assessment of evolutionary changes that resulted from the development of the central american isthmus results from this study have shown that integrated biostratigraphic and magnetostratigraphic techniques can be successfully combined to provide refined age dating in shallow marine sediments even where some microfossil reworking has occurred we propose the new quebrada chocolate formation to represent latest early early late pliocene mixed reefal and siliciclastic deposits that overlie the early pliocene rio banano formation the superjacent moin formation is expanded to include a second reefal interval deposited near the late pliocene pleistocene boundary as well as fine grained mollusk rich deposits in the forereef and lagoon the style of mixed system reef geometry between the two units is distinct the quebrada chocolate formation reefs consist of a lower interval of alternating reefal units buried by siliciclastic sediment and an upper unit comprised of stacked reefal buildups with relatively minor siliciclastic matrix these stacked reefs formed on a rising sea level in contrast the moin formation reefs are isolated corals and patch reefs within a siliciclastic matrix that were deposited during the peak maximum flooding of the sea level highstand our refined age model for the limon group sediments allows reassessment of pleistocene uplift rates we calculate a rate of about 50 m m y slightly less than previous rate estimates attributed to the shallow subduction in the pacific of the cocos ridge beneath the central american island arc system development of similar high resolution age models from other marine basins should provide the chronostratigraphic control necessary to assess environmental events and evolutionary trends in shallow marine faunas separated and isolated by the central american isthmus

rates and mechanics of rapid frontal accretion along the very obliquely convergent southern hikurangi margin new zealand analysis of seismic reflection profiles swath bathymetry side scan sonar imagery and sediment samples reveal the three dimensional structure morphology and stratigraphic evolution of the central to southern hikurangi margin accretionary wedge which is developing in response to thick trench fill sediment and oblique convergence between the australian and pacific plates a seismic stratigraphy of the trench fill turbidites and frontal part of the wedge is constrained by seismic correlations to an already established stratigraphic succession nearby by coccolith and foraminifera biostratigraphy of three core and dredge samples and by estimates of stratigraphic thicknesses and rates of accumulation of compacted sediment structural and stratigraphic analyses of the frontal part of the wedge yield quantitative data on the timing of inception of thrust faults and folds on the growth and mechanics of frontal accretion under variable convergence obliquity and on the amounts and rates of horizontal shortening the data place constraints on the partitioning of geological strain across the entire southern hikurangi margin the principal deformation front at the toe of the wedge is discontinuous and represented by right stepping thrust faulted and folded ridges up to 1 km high which develop initially from discontinuous protothrusts in the central part of the margin near 41 s where the convergence obliquity is 50 orthogonal convergence rate is slow 27 mm yr and about 75 of the total 4 km of sediment on the pacific plate is accreted frontally the seismically resolvable structures within 30 km of the deformation front accommodate about 6 km of horizontal shortening at least 80 of this shortening has occurred within the last 0 4 0 1 m y at an average rate of 12 3 mm yr this rate indicates that the frontal 30 km of the wedge accounts for about 33 55 of the predicted orthogonal contraction across the entire plate boundary zone despite plate convergence obliquity of 50 rapid frontal accretion has occurred during the late quaternary with the principal deformation front migrating seaward up to 50 km within the last 0 5 m y i e at a rate of 100 km m y the structural response to this accretion rate has been a reduction in wedge taper and consequently internal deformation behind the present deformation front near the southwestern termination of the wedge where there is an along the margin transition to continental transpressional tectonics the convergence obliquity increases to 56 and the orthogonal convergence rate decreases to 22 mm yr the wedge narrows to 13 km and is characterized simply by two frontal backthrusts and landward verging folds these structures have accommodated not more than 0 5 km of horizontal shortening at a rate of 1 mm yr which represents 5 of the predicted orthogonal shortening across the entire plate boundary in southern north island the landward vergent structural domain may represent a transition zone from rapid frontal accretion associated with low basal friction and high pore pressure ratio in the central part of the margin to the northern south island region where the upper and lower plates are locked or at least very strongly coupled

regional surface mass anomalies from grace kbr measurements application of l curve regularization anda priori hydrological knowledge 1 we present a method of directly estimating surface mass anomalies at regional scales using satellite to satellite k band ranging kbr data from the gravity recovery and climate experiment grace twin satellite mission geopotential differences based primarily on kbr measurement are derived using a modified energy integral method with an improved method to calibrate accelerometer measurements surface mass anomalies are computed based on a downward continuation process with optimal regularization parameters estimated using the l curve criterion method we derive the covariance functions in both space and space time domains and use them as light constraints in the regional gravity estimation process in the amazon basin study region the space time covariance function has a time correlation distance of 1 27 months which is evident that observations between neighboring months are correlated and the correlation should be taken into account however most of the current grace solutions did not consider such temporal correlations in our study the artifact in the regional gravity solution is mitigated by using the covariance functions the averaged commission errors are estimated to be only 6 86 and 5 85 for the solutions based on the space covariance function scf and the space time covariance function stcf respectively our regional gravity solution in the amazon basin study region which requires no further post processing shows enhanced regional gravity signatures reduced gravity artifacts and the gravity solution agrees with nasa gsfc s grace mascon solution to about 1 cm rms in terms of water thickness change over the amazon basin study region the regional gravity solution also retains the maximum signal energy while suppressing the short wavelength errors

time variable ice loss in asian high mountains from satellite gravimetry abstract substantial amount of glacial ice is considered to be melting in the asian high mountains gravimetry by grace satellite during 2003 2009 suggests the average ice loss rate in this region of 47 12 gigaton gt yr 1 equivalent to 0 13 0 04 mm yr 1 sea level rise this is twice as fast as the average rate over 40 years before the studied period and agrees with the global tendency of accelerating glacial loss such ice loss rate varies both in time and space mass loss in himalaya is slightly decelerating while those in northwestern glaciers show clear acceleration uncertainty still remains in the groundwater decline in northern india and proportion of almost isostatic e g tectonic uplift and non isostatic e g glacial isostatic adjustment portions in the current uplift rate of the tibetan plateau if gravity increase associated with ongoing glacial isostatic adjustment partially canceled the negative gravity trend the corrected ice loss rate could reach 61 gt yr 1

improved methods for observing earth s time variable mass distribution with grace using spherical cap mascons we discuss several classes of improvements to gravity solutions from the gravity recovery and climate experiment grace mission these include both improvements in background geophysical models and orbital parameterization leading to the unconstrained spherical harmonic solution jpl rl05 and an alternate jpl rl05m mass concentration mascon solution benefitting from those same improvements but derived in surface spherical cap mascons the mascon basis functions allow for convenient application of a priori information derived from near global geophysical models to prevent striping in the solutions the resulting mass flux solutions are shown to suffer less from leakage errors than harmonic solutions and do not necessitate empirical filters to remove north south stripes lowering the dependence on using scale factors the global mean scale factor decreases by 0 17 to gain accurate mass estimates ocean bottom pressure obp time series derived from the mascon solutions are shown to have greater correlation with in situ data than do spherical harmonic solutions increase in correlation coefficient of 0 08 globally particularly in low latitude regions with small signal power increase in correlation coefficient of 0 35 regionally in addition to reducing the error rms with respect to the in situ data reduction of 0 37 cm globally and as much as 1 cm regionally greenland and antarctica mass balance estimates derived from the mascon solutions agree within formal uncertainties with previously published results computing basin averages for hydrology applications shows general agreement between harmonic and mascon solutions for large basins however mascon solutions typically have greater resolution for smaller spatial regions in particular when studying secular signals

time variability of the earth s gravity field hydrological and oceanic effects and their possible detection using grace the grace satellite mission scheduled for launch in 2001 is designed to map out the earth s gravity field to high accuracy every 2 4 weeks over a nominal lifetime of 5 years changes in the gravity field are caused by the redistribution of mass within the earth and on or above its surface grace will thus be able to constrain processes that involve mass redistribution in this paper we use output from hydrological oceanographic and atmospheric models to estimate the variability in the gravity field i e in the geoid due to those sources we develop a method for constructing surface mass estimates from the grace gravity coefficients we show the results of simulations where we use synthetic grace gravity data constructed by combining estimated geophysical signals and simulated grace measurement errors to attempt to recover hydrological and oceanographic signals we show that grace may be able to recover changes in continental water storage and in seafloor pressure at scales of a few hundred kilometers and larger and at timescales of a few weeks and longer with accuracies approaching 2 mm in water thickness over land and 0 1 mbar or better in seafloor pressure

variation in deformation of the south panama accretionary prism response to oblique subduction and trench sediment variation migrated single channel seismic lines and sea marc ii side scan and bathymetry data document an active accretionary prism along the obliquely convergent margin of south panama a region previously believed to be a transform margin the eastern flank of the coiba ridge is being subducted in this region creating the requisite geometry for along strike variation in trench sediment thickness and type the regional east dip of the downgoing plate causes the depth of the oceanic crust along the trench to drop approximately 1800 m from west to east in the western region of the study area the incoming sedimentary section consists of approximately 600 m of pelagic and hemipelagic sediments a wedge of trench turbidites that overlies the incoming hemipelagic sediments thickens from less than 100 m in the western region to more than 900 m in the eastern region of the study area the eastward increase in sediment thickness correlates with the following changes in the accretionary prism 1 decrease in initial surface slope 2 broadening of the inner trench slope 3 increase in thrust spacing 4 steepening of frontal thrusts and 5 fold development each of the responses reflects in varying degrees the increase in volume and changing physical properties of the accreted material as well as the changing shear strength of the decollement the increase in overburden pressure and compaction in the deeper sediments together with the change in lithology from oceanic to trenchfill sediments should produce an eastward increase in sediment shear strength both within the wedge and along its base because the decollement will tend to form where the pore fluid pressure overburden ratio is at a maximum the shear strength within the wedge should increase relative to basal shear strength decreased surface slopes will result from the increasing contrast in shear strength between the wedge and its base

topographic effects on coseismic gravity change for the 2011 tohoku oki earthquake and comparison with grace we estimate gravity changes due to coseismic horizontal deformation and inclined bathymetry referred to as topographic effects in this study associated with the 2011 tohoku oki earthquake using a layered half space dislocation model as an approximation we represent inclined topography with ramp body models in the near field of the earthquake and calculate topographic effects from both an inclined seafloor and moho the moho effect partly compensates that from the seafloor an additional contribution comes from seawater column variations as the sloped seafloor moves trenchward topographic effects are first order contributions to coseismic gravity changes for the 2011 tohoku oki earthquake after 300 km gaussian smoothing comparable to the spatial resolution of the gravity recovery and climate experiment grace topographic effects create a dominantly negative gravity pattern with amplitudes up to 1 1 gal for oceanic areas surrounding the epicenter there are discrepancies between model predictions 10 4 to 6 9 gal and grace observations 10 1 to 1 7 gal errors in the adopted fault slip model and difficulties in separating coseismic and postseismic deformation may contribute to these but topographic effects currently neglected in dislocation models are also responsible topographic effects ought to be included in coseismic fault slip and related gravity modeling for the 2011 tohoku oki earthquake and similar events in regions of steep topography

time variable gravity from grace first results eleven monthly grace gravity field solutions are now available for analyses we show those fields can be used to recover monthly changes in water storage both on land and in the ocean to accuracies of 1 5 cm of water thickness when smoothed over 1000 km the amplitude of the annually varying signal can be determined to 1 0 cm results are 30 better for a 1500 km smoothing radius and 40 worse for a 750 km radius we estimate the annually varying component of water storage for three large drainage basins the mississippi the amazon and a region draining into the bay of bengal to accuracies of 1 0 1 5 cm

seismicity and present day tectonics of the svalbard region summary data collected by temporary seismic networks and individual stations over a 7 yr period in the svalbard archipelago are integrated and used to study the seismicity and present day tectonics of the archipelago and surrounding regions most of the continental seismic activity occurs in three concentrated zones one in heer land near the eastern coast of the island of spitsbergen and two in close proximity to one another on the island of nordaustlandet all three zones are in regions which have been devoid of major orogenic activity since the late devonian the heer land zone and at least one of the zones in nordaustlandet define active faulting which has not been identified by surface mapping the other zone in nordaustlandet occurs in the region of a mapped system of faults all branches of which are relatively minor a number of smaller concentrations and individual earthquakes occur throughout much of the archipelago and surrounding continental shelves seismicity is currently low in the region of tertiary orogenic activity in western spitsbergen a roughly linear pattern of minor activity in southern spitsbergen occurs west of the heer land zone and extends at least 50km southward the major n s faults in spitsbergen which are thought to have accomodated large displacements in devonian time are currently inactive except perhaps at the southern termini of the billefjorden and lomfjorden fault zones where they approach the heer land seismic zone earthquakes located near the western continental margin of spitsbergen may occur on segments of rifting and shearing which reflect the opening of the greenland sea during the tertiary the limited extent of the currently active zones of activity their spatial stability over several years and the lack of activity on the major faults of svalbard argue against the existence of plate boundaries along which motion is currently occurring in the svalbard region all fault plane solutions on svalbard are consistent with a stress field characterized by e w compression whereas previously published fault plane solutions for the ridge system west of svalbard indicate e w extension the magnitude of the compressive stress increases from small values near the ridge system to values sufficiently large to dominate the regional seismicity of svalbard over distances of 200 300 km

mechanisms of subduction accretion along the central aleutian trench we have used migrated 24 fold seismic records to study mechanisms of subduction accretion along the central aleutian trench two major structural units underlie the landward trench slope and are separated by an acoustically defined decollement surface that can be traced at least 20 km landward of the trench axis the upper unit consists of three to four structural blocks of deformed trench fill each block possesses a consistent internal structure and is bound by major landward dipping thrust faults the structures within the blocks include monoclinal sequences antiforms synforms and faulted folds beneath the decollement nearly undeformed trench fill 1 km thick is being under thrust along with the underlying oceanic lithosphere this basal section is an intervening unit between the offscraped sediment and the rough igneous oceanic basement and may prevent the oceanic crust from being incorporated into the subduction complex at least within 20 km of the front of the subduction zone owing to bending stresses the oceanic crust fails along normal faults beneath and at least 15 km landward of the trench axis these normal faults although poorly constrained are believed to propagate upwards producing offsets within both the subducting and the accreting sediment in addition normal faulting enhances the irregular surface of the oceanic crust structural associations imaged on all of the seismic reflection profiles although possibly fortuitous suggest that the subduction of these irregular basement highs results in the temporary displacement of the overlying fault bound sediment packets and promotes the development of the structures found within the accretionary prism tectonic riffling of the accreted sediment by the passage of the basement relief may also provide a mechanism by which underthrusting sediments are decoupled from the subducting lithosphere and underplated to the base of the aleutian accretionary complex

high pore pressure generation in sediments in front of the barbados ridge complex dsdp data for the barbados ridge complex show the existence of high pore pressures in sediments both oceanward and arcward of oceanic trenches we model the evolution of pore pressure by using a finite element procedure three consecutive stages are recognized 1 in the oceanic basin sedimentation rate is low there is little excess pore pressure 2 approaching the trench but before being incorporated into the accretionary prism sediments are subjected to increasing tectonic compression as well as greater sediment overburden thus high pore pressure can be created an important result is that the lowest effective stress occurs within the sedimentary layer instead of at its base and a decollement may develop along this weakened zone 3 arcward of the trench sediments above the decollement are offscraped while those beneath it are subducted fast sedimentation combined with tectonic compression can create pore pressures so high as to cause porosity to increase with depth this facilitates the formation of mud volcanoes even oceanward of the deformation front

antarctica greenland and gulf of alaska land ice evolution from an iterated grace global mascon solution we have determined the ice mass evolution of the antarctica and greenland ice sheets ais and gis and gulf of alaska goa glaciers from a new grace global solution of equal area surface mass concentration parcels mascons in equivalent height of water the mascons were estimated directly from the reduction of the inter satellite k band range rate kbrr observations taking into account the full noise covariance and formally iterating the solution the new solution increases signal recovery while reducing the grace kbrr observation residuals the mascons were estimated with 10 day and 1 arc degree equal area sampling applying anisotropic constraints an ensemble empirical mode decomposition adaptive filter was applied to the mascon time series to compute annual mass balances the details and causes of the spatial and temporal variability of the land ice regions studied are discussed the estimated mass trend over the total gis ais and goa glaciers for the time period 1 december 2003 to 1 december 2010 is 380 plus or minus 31 gt a exp 1 equivalent to 1 05 plus or minus 0 09 mma exp 1 sea level rise over the same time period we estimate the mass acceleration to be 41 plus or minus 27 gt a exp 2 equivalent to a 0 11 plus or minus 0 08 mm a exp 2 rate of change in sea level the trends and accelerations are dependent on significant seasonal and annual balance anomalies

intraplate earthquakes in northern svalbard abstract a temporary network of microearthquake seismographs deployed in the northern part of svalbard recorded numerous earthquakes during the summer of 1982 most of the earthquakes occurred in concentrated zones in restricted portions of three fault complexes in central nordaustlandet but minor activity also occurred to the east and west of these complexes within the uncertainty of the epicentral locations most earthquakes which occurred on land in regions not covered by glaciers could be associated with segments of mapped faults there are numerous fault segments and even entire faults however along which no recorded earthquake occurred the concentrated spatial locations of these earthquakes along preferred portions of mapped faults and the absence of throughgoing lines of activity suggest that these are intraplate earthquakes occurring along existing faults in response to an applied stress field composite fault plane solutions for the three most active regions each include one nodal plane which can be associated at least roughly with a trend of seismicity and trend of a mapped fault the directions of greatest principle stress for the three fault plane solutions vary between n90 w and n55 w directions which agree approximately with maximum stress directions determined for a region about 200 km to the south the consistency of the directions of maximum principle stress obtained for earthquakes throughout a broad region of svalbard suggests that the earthquakes there occur in response to a stress field which is of plate tectonic origin

satellite gravity measurements confirm accelerated melting of greenland ice sheet using time variable gravity measurements from the gravity recovery and climate experiment grace satellite mission we estimate ice mass changes over greenland during the period april 2002 to november 2005 after correcting for the effects of spatial filtering and limited resolution of grace data the estimated total ice melting rate over greenland is 239 23 cubic kilometers per year mostly from east greenland this estimate agrees remarkably well with a recent assessment of 224 41 cubic kilometers per year based on satellite radar interferometry data grace estimates in southeast greenland suggest accelerated melting since the summer of 2004 consistent with the latest remote sensing measurements

alaskan mountain glacial melting observed by satellite gravimetry abstract we use satellite gravity measurements from the gravity recovery and climate experiment grace as an indication of mass change to study potential long term mountain glacial melting in southern alaska and west canada the first 3 5 yr of grace monthly gravity data spanning april 2002 november 2005 show a prominent glacial melting trend in the mountain regions around the gulf of alaska goa grace observed surface mass changes correlate remarkably well with available mass balance data at gulkana and wolverine two benchmark glaciers of the u s geological survey usgs although the grace signals are smaller in magnitude in addition terrestrial water storage tws changes estimated from an advanced land surface model show significant mass loss in this region during the same period after correcting for the leakage errors and removing tws contributions using model estimates we conclude that grace observed glacial melting in the goa mountain region is equivalent to 101 22 km 3 yr which agrees quite well with the assessment of 96 35 km 3 yr based on airborne laser altimetry data and is consistent with an earlier estimate based on the first 2 yr of grace data this study demonstrates the significant potentials of satellite gravity measurements for monitoring mountain glacial melting and regional climate change

late miocene to recent plate tectonic history of the southern central america convergent margin new plate reconstructions constrain the tectonic evolution of the subducting cocos and nazca plates across the southern central american subduction zone from late miocene to recent because of the strong relationships between lower and upper caribbean plate dynamics along this margin these constraints have wide ranging implications for the timing and growth of upper plate deformation and volcanism in southern central america the reconstructions outline three important events in the neogene history of this margin 1 the coeval development of the panama triple junction with the initiation of oblique subduction of the nazca plate at 8 5 ma 2 the initiation of seamount and rough crust subduction beginning at 3 4 ma and 3 cocos ridge subduction from 2 to 3 ma a comparison of these events with independent geologic geomorphic volcanic and stratigraphic data sets reveals that the timing rates and origin of subducting crust directly impacted the neogene growth of upper plate deformation and volcanism in southern central america these analyses constrain the timing geometry and causes of a number of significant tectonic and volcanic processes including rapid plio quaternary arc fore arc contraction due to cocos ridge subduction the detachment of the panama microplate at 1 3 ma and the late miocene cessation of mantle wedge derived volcanism across 300 km of the subduction zone

noncohesive critical coulomb wedges an exact solution active fold and thrust belts or submarine accretionary complexes can be modeled as critically tapered wedges of material on the verge of coulomb failure everywhere overlying a basal decollement where frictional sliding is occurring ignoring cohesion the four strength parameters needed to describe a critical coulomb wedge are its internal and basal coefficients of friction and b and its internal and basal hubbert rubey fluid pressure ratios and b an exact relation between surface slope and basal dip of a noncohesive critical wedge with uniform properties is derived the state of stress within such a wedge has the same orientation everywhere and is constant if is and vice versa a coefficient of internal friction 1 1 is consistent with the known surface slope basal dip and pore fluid pressures in the active fold and thrust belt of western taiwan assuming that byerlee s law b 0 85 is valid on the base the wide variety of tectonic styles observed to occur along convergent margins including subduction erosion active accretion subduction without accretion and even extension and normal faulting may be controlled by relatively small spatial or temporal variations in either b or b

seismicity and seismotectonics of norway and nearby continental shelf areas the earthquake activity of norway and nearby offshore areas is low to intermediate with few events above magnitude 5 recent significant improvements in instrumental coverage in parallel with a better utilization of older including historical data have shown that the seismicity in the south is predominantly confined to the coastal areas and to the viking graben while from the northern north sea to svalbard the earthquakes in a broad sense follow the continental margin fifty one focal mechanism solutions from these areas about half of them new reveal stress directions that clearly indicate a connection to the plate tectonic ridge push force at least for the areas at a minimum distance from the continental margin along the margin stress directions also indicate a possible connection to postglacial uplift as well as to lithospheric loading effects a dominance of normal faulting on the landward side and reverse faulting on the oceanic side agrees with this interpretation on a regional level the seismicity in these areas correlate quite well with geologic features such as grabens fault zones fault complexes fracture zones and the margin itself indicating that these structures act in a general sense as weakness zones in the presence of a regionally more stable stress field in the northern north sea however an area with quite anomalous stress orientations with strike slip faulting is found in a region transitional between normal and reverse faulting most of the earthquake foci are confined to the presumably brittle parts of the crust but many events are also located quite close to and on both sides of the moho discontinuity

computations of the viscoelastic response of a 3 d compressible earth to surface loading an application to glacial isostatic adjustment in antarctica and canada s u m m a r y we develop a 3 d finite element model to study the viscoelastic response of a compressible earth to surface loads the effects of centre of mass motion polar wander feedback and selfconsistent ocean loading are implemented to assess the model s accuracy we benchmark the numerical results against a semi analytic solution for spherically symmetric structure we force our model with the ice 5g global ice loading history to study the effects of laterally varying viscosity structure on several glacial isostatic adjustment gia observables including relative sea level rsl measurements incanada and present day time variable gravity and uplift rates in antarctica canadian rsl observations have been used to determine the earth s globally averaged viscosity profile antarcticgps uplift rates have been used to constrainantarcticgia models and gia time variable gravity and uplift signals are error sources for grace and altimeter estimates of present day antarctic ice mass loss and must be modelled and removed from those estimates computinggia results for a 3 d viscosity profile derived from a realistic seismic tomography model and comparing with results computed for 1 d averages of that 3 d profile we conclude that 1 a gia viscosity model based on canadian relative sea level data is more likely to represent a canadian average than a true global average 2 the effects of 3 d viscosity structure on grace estimates of present day antarctic mass loss are probably smaller than the difference between gia models based on different antarctic deglaciation histories and 3 the effects of 3 d viscosity structure on antarctic gps observations of present day uplift rate can be significant and can complicate efforts to use gps observations to constrain 1 d gia models

effects of subducting seafloor roughness on upper plate vertical tectonism osa peninsula costa rica 1 subduction of seamounts and ridges along thinly sedimented convergent margins results in deformation of the overriding forearc exposures of newly recognized late pleistocene shallow water deposits i e marenco formation record intervals of rapid subsidence and uplift across the costa rican forearc inboard of the subducting cocos ridge in general the marenco formation is a fining upward fossiliferious late pleistocene marine sand disconformably overlying beveled surfaces cut across the competent osa melange basement the 50 to 27 ka age of the marenco formation is constrained by 12 accelerator mass spectrometry and two conventional 14c dates obtained on marine macrofossils the deposition of this sequence coincident with a general fall in sea level during oxygen isotope stage 3 requires 6 mm yr 1 subsidence inboard of the northwest flank of the subducting cocos ridge presently exposures of the marenco formation are found at 75 m above sea level requiring uplift rates in excess of 6 mm yr 1 we interpret the down and up history of vertical tectonism recorded by the marenco formation as the response of the upper plate to variations in the elevation of the subducting cocos ridge on the basis of a model where the upper plate deforms through bends because of roughness on a rigid downgoing plate the rate duration and spatial distribution of vertical tectonism across the forearc are determined by the magnitude of the orthogonal component of the relative convergence vector and the bathymetry of the underthrusting plate application of this model to bathymetric data for the cocos plate offshore yields a broad agreement between predicted future rates of subsidence and uplift and rates over the last 50 kyr recorded by the marenco formation furthermore analysis suggests that the arrival of the blunt tipped leading edge of the cocos ridge 0 5 3 ma resulted in an initial period of very rapid 30 mm yr 1 uplift

space geodesy constrains ice age terminal deglaciation the global ice 6g_c vm5a model a new model of the last deglaciation event of the late quaternary ice age is here described and denoted as ice 6g c vm5a it differs from previously published models in this sequence in that it has been explicitly refined by applying all of the available global positioning system gps measurements of vertical motion of the crust that may be brought to bear to constrain the thickness of local ice cover as well as the timing of its removal additional space geodetic constraints have also been applied to specify the reference frame within which the gps data are described the focus of the paper is upon the three main regions of last glacial maximum ice cover namely north america northwestern europe eurasia and antarctica although greenland and the british isles will also be included if peripherally in the discussion in each of the three major regions the model predictions of the time rate of change of the gravitational field are also compared to that being measured by the gravity recovery and climate experiment satellites as an independent means of verifying the improvement of the model achieved by applying the gps constraints several aspects of the global characteristics of this new model are also discussed including the nature of relative sea level history predictions at far field locations in particular the caribbean island of barbados from which especially high quality records of postglacial sea level change are available but which records were not employed in the development of the model although ice 6g c vm5a is a significant improvement insofar as the most recently available gps observations are concerned comparison of model predictions with such far field relative sea level histories enables us to identify a series of additional improvements that should follow from a further stage of model iteration

seismicity of the caribbean nazca boundary constraints on microplate tectonics of the panama region there are at least three kinds of models that have been used to explain plate motions and boundaries in the panama region these models suggest that the major caribbean nazca boundary is 1 a thrust fault along which the panama deformed belt has developed 2 a left lateral strike slip fault along the southern margin of panama or 3 a diffuse array of mostly thrust faults which cause internal deformation to occur throughout panama in this paper we use joint epicenter location methods to obtain improved locations for earthquakes that occurred in this region between 1964 and 1985 and we use waveform modeling to obtain more precise depths for five events hypocenters of 13 events beneath the panama deformed belt suggest that overthrusting of panama upon the caribbean plate causes active deformation which supports model 1 but does not rule out models 2 and 3 the nature of seismic faulting along the panama deformed belt is complex and supports the premise that slow north south convergence loads the cold thick caribbean lithosphere and causes it to fail seismically epicenters of 64 events along the panama fracture zone indicate that this north south trending rightlateral transform fault is splayed north of 6 n overall the seismicity throughout suggests that the panama arc can be treated as a microplate in accordance with models 1 or 2 similarly a possible second microplate may exist south of panama but if so it does not affect regional plate motions significantly the strongest evidence for model 3 occurs in the easternmost and westernmost portions of panama where the diffuse seismicity prevents us from resolving specific microplate triple junction boundaries

sediment accretion subduction and dewatering at the base of the trench slope off costa rica a seismic reflection view of the decollement sea beam bathymetric and water gun seismic reflection data off the west coast of costa rica image the base of the trench slope its underlying structure and the behavior of a decollement surface in the presence of relief on the subducting plate a low angle landward dipping reflection marks the master decollement above which only about 80 m of a 350 m oceanic sedimentary section is offscraped and accreted to the lower trench slope a small horst block at least 9 km long 1 5 km wide and 300 m high is being obliquely subducted beneath the landward trench slope on five seismic lines the decollement rises over the horst and then descends into the next incoming graben as the fault zone maintains a specific stratigraphic level the undulating geometry of the decollement implies that the subducting grabens are not sites for sediment entrapment and subduction and that the horsts are not causing tectonic erosion of the landward trench wall substantial dewatering within the subducting sediments is documented within about 4 km of the deformation front the dewatering may favor development of bedding parallel zones of low shear strength within the subducting sediments and the master decollement may seek one of these zones

rifting in the northern norwegian greenland sea thermal tests of asymmetric spreading the analysis of heat flow seismic and topographic data collected in the northern norwegian greenland sea reveals an asymmetric evolution of the eurasian and north american plates these data are compared to predictions from three kinematic models of extension which produce asymmetry about the knipovich ridge 1 uniform asymmetric pure shear 2 lithospheric simple shear and 3 rift ridge jumping the data are consistent with a range of deformation scenarios from one ridge jump occurring at about 25 m y after the initiation of spreading to continuous asymmetric extension the simple shear model can match the data only when a detachment fault dips more steeply than 45 under svalbard tectonic and heat flow evidence suggests that asymmetry may have evolved from a combination of all three models when the mohns ridge propagating to the east encountered the preexisting northward trending spitsbergen shear zone the direction of ridge propagation shifted to the north being influenced by the abrupt change in the regional stress field at the shear zone ridge axis intersection as a result the nascent knipovich ridge entered into and propagated along the shear zone consequently formerly active shear faults became the new detachment surfaces along which new crust is minted and asymmetrically extended the high level of deviatoric stress about the mohns and knipovich ridge intersection may cause a gradual eastward migration of the knipovich ridge resulting in multiple zones of magma intrusion off axial bands of high heat flow and volcanism located along the barents sea and svalbard margin along the yermak plateau and on continental svalbard just southeast of the yermak plateau may be evidence for this migration propagation from the nansen ridge may have entered the same shear zone from the north explaining the creation of the small molloy ridge and off axis volcanism on the yermak plateau

measurements of time variable gravity show mass loss in antarctica using measurements of time variable gravity from the gravity recovery and climate experiment satellites we determined mass variations of the antarctic ice sheet during 2002 2005 we found that the mass of the ice sheet decreased significantly at a rate of 152 80 cubic kilometers of ice per year which is equivalent to 0 4 0 2 millimeters of global sea level rise per year most of this mass loss came from the west antarctic ice sheet

time variable gravity from space and present day mass redistribution in theearth system abstract since 2002 the us german grace gravity recovery and climate experiment mission has been providing a precise survey of earth s time variable gravity field with unprecedented temporal and spatial sampling grace time variable gravity fields provide a means of measuring temporal and spatial variations of mass redistribution within the earth system the grace mission has launched a new era in studying a series of geophysical problems ranging from deep earth structure to tracking mass redistribution on and near the surface of the earth grace has greatly improved understanding of mass redistribution in various compartments of the climate system atmosphere oceans terrestrial water and cryosphere in this review we use examples to show how grace has fundamentally enriched a number of fields including but not limited to the global water cycle and land hydrology mass balance of polar ice sheets and mountain glaciers ocean mass and global sea level change and solid earth geophysics

improved source parameter constraints for five undersea earthquakes from north component of grace gravity and gravity gradient change measurements abstract the innovative processing of gravity recovery and climate experiment grace data using only the north component of gravity change and its corresponding gravity gradient changes allows the enhancement of the spatial resolution for coseismic deformation signals here we report the study of five undersea earthquakes using this technique the 2004 sumatra andaman earthquake the 2007 bengkulu earthquake the 2010 maule chile earthquake the 2011 tohoku earthquake and the 2012 indian ocean earthquakes by using the high spherical harmonic degree up to degree 96 data products and the associated grace data processing techniques the retrieved north component of gravity change is up to 34 1 4 gal for the 2004 sumatra andaman earthquake which illustrates by far the highest amplitude of the coseismic signal retrieved from satellite gravimetry among previous studies we creatively apply the localized spectral analysis as an efficient method to empirically determine the practical spherical harmonic truncation degree by combining least squares adjustment with the simulated annealing algorithm point source parameters are estimated which demonstrates the unique constraint on source model from grace data compared to other data sources for the 2004 sumatra andaman earthquake grace data produce a shallower centroid depth 9 1 km as compared to the depth 28 3 km from gps data for the 2011 tohoku earthquake the grace estimated centroid location is southwest of the gps seismic solutions and the slip orientation is about 10 clockwise from the published gps seismic slip models we concluded that these differences demonstrate the additional and critical offshore constraint by grace on source parameters as compared to gps seismic data

extensive underthrusting of undeformed sediment beneath the accretionary complex of the lesser antilles subduction zone along the eastern margin of the caribbean plate which overrides the atlantic oceanic lithosphere at 20 km myr 1 in an easterly direction1 east of the lesser antilles island arc lies a wide accretionary complex barbados ridge complex 260 km wide at the latitude of barbados and 20 km thick beneath barbados3 4 it increases in width and thickness from north to south2 fig 1 the style of deformation varies considerably along the eastern margin of the complex2 3 5 11 and is related to the thickness and type of sediment on the atlantic ocean floor a common feature is that a proportion of undeformed sediment from the ocean floor is brought beneath the deformed sediments of the accretionary complex and in some areas a clearly defined seismic reflector separates the deformed and undeformed sediments which has been interpreted as a decollement surface5 12 13 a seismic section from an investigation of the accretionary complex conducted from rss discovery in 1980 using 12 channel seismic reflection and long range side scan sonar gloria shows undeformed sedimentary rocks carried horizontally 74 km beneath deformed rocks forming the accretionary wedge we report here that the low shear stresses required on the decollement between the deformed and undeformed rocks are made possible by porewater at pressures close to that of the lithostatic load

structure of the nankai trough accretionary zone from multichannel seismic reflection data new multichannel seismic reflection data collected over the nankai trough image the accretionary complex in two areas the international program of ocean drilling leg 87 transect area western area and the region of upcoming ocean drilling program leg 131 eastern area the incoming shikoku basin sedimentary section consists of hemipelagic clays and thin terrigenous turbidites the basin section is overlain by a trench wedge that is 12 16 km wide and 350 750 m thick at the thrust front accretionary deformation begins in a protothrust zone that is characterized by thickening and seaward tilting of the trench wedge the zone in the western area is 4 5 km wide and is characterized by kink folds the zone in the eastern area is only 2 5 km wide and does not exhibit such folds the frontal thrusts in each area are imaged as fault plane reflections and ramp upward from within the basin hemipelagic section the overthrusting sediments form fault bend folds over these ramps thrust spacing at the toe of the slope is 1 5 2 5 km the second thrust cuts up from an inferred decollement within the shikoku basin sedimentary section in the eastern area a reflection marking the top of the basin pelagic sediment section changes from normal to reversed polarity about 6 3 km seaward of the thrust front and underlies the entire protothrust zone this reflector continues with reversed polarity under the accretionary complex and is at the level of the basal decollement the underlying basin pelagic section is apparently thrust undisturbed beneath the accretionary prism the reversal of polarity indicates a change in reflection coefficient that is due to a combination of decreasing seismic velocity and density across the interface this decrease in velocity and density may indicate that the decollement is a zone of high porosity due to fluid expulsion from deeper within the accretionary prism the reflections from the first and second thrusts are also reversed polarity possibly indicating that they also are pathways of fluid expulsion the critical wedge taper of the western area is greater than that of the eastern area an observation that is consistent with the existence of an overpressured decollement in the eastern area

active thrusting in the inner forearc of an erosive convergent margin pacific coast costa rica 1 structural and geomorphic analyses of the fila costena thrust belt in southwest costa rica indicate active thrusting within the inner forearc the fila costena exposes three major thrust faults that imbricate the late tertiary forearc basin sequence of the terraba basin the frontal thrust of the fila costena marks the boundary between an uplifting inner forearc and a subsiding outer forearc with only local uplift astride the indenting cocos ridge on the basis of surface constraints a cross section across the thrust belt suggests that all three thrusts flatten into parallelism with a low angle decollement horizon near the contact between the basement and the cover sequence of the terraba basin this decollement lies at a depth of 4 km the minimum shortening recorded by restoration of fault related folds is 17 km or 45 observations of late tertiary marine sediments tilted and faulted late quaternary fluvial terraces and raised holocene marine terraces indicate that fila costena uplift was likely initiated in the quaternary and is ongoing given that the coastal mountains are separated from the talamanca range by a valley the decollement that delaminates the forearc basin from the underthrusting forearc must continue as a flat beneath the valley but must link with the plate boundary along a crustal scale ramp system a structural geometry that has resulted in uplift of the talamanca range the highest peaks in central america the dichotomy between uplift in the inner forearc and subsidence in the outer forearc is explained in terms of the response of an arcward thickening wedge to rough subducting crust index terms 8150 tectonophysics plate boundary general 3040 8123 tectonophysics dynamics seismotectonics 8010 structural geology fractures and faults 8015 structural geology local crustal structure 8005 structural geology folds and folding

origin of convex accretionary wedges evidence from barbados the surface slopes of submarine accretionary wedges generally decrease away from the toe or defor mation front toward the arc this paper presents evidence that this characteristic convex cross sectional shape is a direct mechanical response to the lithification of sediments during accretion the recently accreted sediments near the toe which are typically unconsolidated and therefore very weak must deform until a critical taper is attained at which they are stable in the presence of the horizontal tectonic compression this critical wedge taper and thus the bathymetric slope of the accretionary prism is relatively high for the porous weak sediments near the toe in contrast the compacted sediments farther back in the wedge are stronger so they require a relatively modest critical taper and therefore a lower bathymetric slope to be stable under the same tectonic compression the result is a convex cross sectional wedge shape we investigate several possible causes of the increase in sediment strength within the wedge and conclude that the primary cause is an increase in cohesive strength with decreasing porosity a straightforward generalization of the exact critical taper model is combined with empirical cohesion porosity velocity relations to infer the distribution of porosity and p wave velocity within the barbados accretionary wedge we estimate that the porosity decreases from its near toe value of 70 to approximately 30 at a depth of 3 km below the seafloor and a distance of 60 km arcward from the front of the wedge

seamount ridge and transform subduction in southern central america understanding the factors that control subduction zone processes is a first order goal in the study of convergent margins in southern central america a growing body of research reveals strong links between the character of the subducting slab and the mechanics of important processes that include subduction erosion fluid flow deformation and seismogenesis in this paper i evaluate the role that seamount ridge and transform subduction have in the development of upper plate deformation and volcanism by summarizing previous work across a 500 km long region of central america where each of these three scenarios are present along strike the data show that the subduction of short wavelength bathymetry e g seamounts and faults on the seafloor produces short wavelength deformation that persists for relatively short timescales 104 105 years whereas the subduction of longer wavelength bathymetry e g the aseismic cocos ridge results in longer wavelength deformation that endures over a longer time scale 106 years the timing and distribution of upper plate deformation are consistent with subhorizontal cocos ridge subduction driving upper plate deformation and the increased crustal thickness 20 km of the subducting cocos ridge is likely one of the most important factors in the production of upper plate contraction and crustal thickening the data illustrate a fundamental connection between lower plate properties and upper plate deformation and highlight the profound influence that bathymetry and crustal thickness have in the localization and kinematics of upper plate strain and volcanism in middle america

deceleration in the earth s oblateness 1 for over three decades satellite laser ranging slr has recorded the global nature of the long wavelength mass change within the earth system analysis of the most recent time series of 30 day slr based estimates of earth s dynamical oblateness characterized by the gravitational degree 2 zonal spherical harmonic j2 indicates that the long term variation of j2 appears to be more quadratic than linear in nature the superposition of a quadratic and an 18 6 year variation leads to the unknown decadal variation reported by cheng and tapley 2004 although the primary trend is expected to be linear due to global isostatic adjustment there is an evident deceleration j 2 18 1 10 13 yr2 in the rate of the decrease in j2 during the last few decades likely due to changes in the rate of the global mass redistribution from melting of the glaciers and ice sheets as well as mass changes in the atmosphere and ocean

the geochemistry of young volcanism throughout western panama and southeastern costa rica an overview oblique aseismic subduction below western panama and southeastern costa rica has produced recent arc related volcanism the aseismicity is probably related to the subduction of relatively hot oceanic lithosphere the volcanism throughout this region over the past 2 ma has been quite distinct consisting of felsic magmas andesites to rhyolites but mainly dacites with geochemical signatures suggesting a metamorphosed basaltic source it is believed that the subduction of young oceanic crust sets up conditions under which the slab melts rather than the overlying mantle wedge rocks with slab melt geochemistries and associated with young subducted crust have been termed adakites elsewhere the young adakite melts are sometimes associated with a few rare young high nb basalts but there is no obvious genetic link between them through differentiation high nb basalts may also be derived from the partial melting of the subducted oceanic crust high nb basalt migmatites have been found with pegmatites of adakite compositions in the exposed subduction terrain of the catalina schist california alternatively the high nb basalts may be partial melts of phlogopite rich mantle that has previously reacted with adakite magmas eruption of adakites and high nb basalts was preceded by a 2 3 ma period of relative quiescence prior to this there was a 7 ma period of calc alkaline volcanism typical of the present day magmatism associated with a distinct benioff zone found throughout the central american arc the abrupt transition in volcanism with time from an early calc alkaline sequence to a later adakite high nb basalt sequence may record a change in the tectonic setting of western panama and southeastern costa rica over the past 12 ma

the new central american seismic hazard zonation mutual consensus based on up to day seismotectonic framework abstract central america is one of the most active seismic zones in the world due to the interaction of five tectonic plates north america caribbean coco nazca and south america and its internal deformation which generates almost one destructive earthquakes 5 4 mw 8 1 every year a new seismological zonation for central america is proposed based on seismotectonic framework a geological context tectonic and geological maps geophysical and geodetic evidence gravimetric maps magnetometric gps observations and previous works as a main source of data a depurated earthquake catalog was collected covering the period from 1522 to 2015 this catalog was homogenized to a moment magnitude scale mw after a careful analysis of all the integrated geological and seismological information the seismogenic zones were established into seismic areas defined by similar patterns of faulting seismicity and rupture mechanism the tectonic environment has required considering seismic zones in two particular seismological regimes a crustal faulting including local faults major fracture zones of plate boundary limits and thrust fault of deformed belts and b subduction taking into account the change in the subduction angle along the trench and the type and location of the rupture the seismicity in the subduction zone is divided into interplate and intraplate inslab seismicity the regional seismic zonation proposed for the whole of central america include local seismic zonations avoiding discontinuities at the national boundaries because of a consensus between the 7 countries based on the cooperative work of specialists on central american seismotectonics and related topics

coseismic gravity changes of the 2011 tohoku oki earthquake from satellite gravimetry 1 the massive tohoku oki earthquake of a moment magnitude mw of 9 0 occurred on 11 march 2011 off the pacific coast of the northeastern japan the mass redistribution in and around the focal region associated with this earthquake was studied using the gravity changes detected by gravity recovery and climate experiment grace satellite after the 2004 sumatra andaman and the 2010 central chile maule earthquakes the present study presents the third case of clear detection of coseismic gravity changes by grace the observed gravity changes were dominated by decrease over the back arc region of 7 gal or less this reflects to a large extent coseismic crustal dilatation of the landward plate they agree well with the changes calculated with the green s function for the realistic earth using fault parameters inferred from coseismic crustal movements the spatial patterns of the gravity changes of these earthquakes are very similar because they are all shallow angle reverse faulting at convergent plate boundaries we found linear relationship between gravity decreases and seismic moments

continental margin tectonics forearc processes recent studies of convergent plate margins and the structural development of forearc terranes are summarized in a critical review of u s research from the period 1987 1990 topics addressed include the geometry of accretionary prisms coulomb wedge taper and vertical motion in response to tectonic processes offscraping vs underplating or subduction the response to oblique convergence fluids in forearc settings the thermal framework and the effects of fluid advection and serpentinite seamounts also included is a comprehensive bibliography for the period

contribution of ice sheet and mountain glacier melt to recent sea level rise altimeter data suggest that sea level rose by about 2 4 mm per year from 2005 to 2011 but estimates of the relative contributions of ocean warming and increased ocean mass are equivocal an analysis of ocean temperature and satellite gravity data suggests that the delivery of meltwater from ice sheets and mountain glaciers contributed 75 of the observed sea level rise

geodynamics of the northern andes subductions and intracontinental deformation colombia new regional seismological data acquired in colombia during 1993 to 1996 and tectonic field data from the eastern cordillera ec permit a reexamination of the complex geodynamics of northwestern south america the effect of the accretion of the baudo panama oceanic arc which began 12 myr ago is highlighted in connection with mountain building in the ec the istmina and ibague faults in the south and the santa marta bucaramanga fault to the northeast limit an e se moving continental wedge progressive indentation of the wedge is absorbed along reverse faults located in the foothills of the cordilleras northward of 5 n and transpressive deformation in the santander massif crustal seismicity in colombia is accurately correlated with active faults showing neotectonic morphological evidences intermediate seismicity allows to identify a n ne trending subduction segment beneath the ec which plunges toward the e se this subduction is interpreted as a remnant of the paleo caribbean plateau pcp as suggested by geological and tomographic profiles the pcp shows a low angle subduction northward of 5 2 n and is limited southward by a major e w transpressive shear zone normal oceanic subduction of the nazca plate np ends abruptly at the southern limit of the baudo range northward the np subducts beneath the choco block overlapping the southern part of the pcp cenozoic shortening in the ec estimated from a balanced section is 120 km stress analysis of fault slip data in the ec northward of 4 n indicates an e se orientation of 1 in agreement with the pcp subduction direction northward near bucaramanga two stress solutions were observed 1 a late andean n80 e compression and 2 an early andean nw se compression

multibeam bathymetric survey and high resolution seismic investigations on the barbados ridge complex eastern caribbean a key to the knowledge and interpretation of an accretionary wedge abstract an important problem of active margin processes is the way accretionary wedges form in response to oceanic crust subduction new data have been obtained along the barbados ridge in front of the lesser antilles tertiary island arc they include high resolution multichannel seismic data and accurate sea beam bathymetric maps both have been used to propose a 3 dimensional interpretation of tectonic and sedimentary processes related to subduction such as oceanic off scraping ductile folding relative uplift and subsidence fore arc basin evolution and mud diapirism they all improved our knowledge of the evolution of accretionary prisms and more specifically of the barbados ridge development in plio quaternary time previous tectonic events are deduced from recently published geological data from barbados and the lesser antilles islands

rough crust subduction forearc kinematics and quaternary uplift rates costa rican segment of the middle american trench orthogonal subduction of bathymetrically rough oceanic lithosphere along the northwestern flank of the cocos ridge imprints a distinctive style of deformation on the overriding costa rican forearc we divide the costa rican forearc into three 100 160 km long deformational domains based on the bathymetric roughness and thickness of the cocos plate entering the middle american trench the dip of the subducting plate the variation in surface uplift rates of late quaternary coastal deposits and the orientations and types of faults deforming paleogene and neogene sedimentary rocks in the 100 km long nicoya domain coastal deposits show localized surface uplift and arcward tilting above the downdip projections of the fossil trace of the cocos nazca panama co nz pa triple junction and the fisher seamount and ridge in the 120 km long central pacific forearc domain between the nicoya peninsula and quepos shallower 60 subduction of seamounts and plateaus is accompanied by trench perpendicular late quaternary normal faults steeply dipping northeast striking margin perpendicular faults accommodate differential uplift associated with seamount subduction uplift and faulting differ between the segments of the forearc facing subducting seamounts and ridges inner forearc uplift along the seamount dominated segment is greatest inboard of the largest furrows across the lower slope localized uplift and arcward tilting of coastal deposits is present adjacent to subducting seamounts in contrast inboard of the underthrusting aseismic cocos ridge along the 160 km long fila costena domain between quepos and the burica peninsula mesoscale fault populations record active shortening related to the 100 km long fila costena fold and thrust belt the observed patterns of faulting and permanent uplift are best explained by crustal thickening the uplifted terraces provide a first order estimate of permanent strain along the forearc in costa rica the permanent strain recorded by uplift of these quaternary surfaces exceeds the predicted rebound of stored elastic strain released during subduction zone earthquakes

holocene forearc block rotation in response to seamount subduction southeastern peninsula de nicoya costa rica the southeastern tip of the peninsula de nicoya costa rica on the caribbean plate margin lies inboard of the rough bathy metric terrain on the subducting cocos plate and along the land ward projection of the convergence vector for the fisher seamount group the southern tip of the peninsula has nearly orthogonal coastlines and extensive well preserved holocene marine terraces

concentrated earthquake zones in svalbard abstract data from a network of seven telemetered and three portable seismic stations operating in the summer of 1979 delineate two concentrated east west trending zones of seismicity in the svalbard archipelago one zone in heer land spitsbergen which has been described previously shows left lateral strike slip movements along a vertical wnw ese striking fault another zone in nordaustlandet reported here for the first time has similar strike direction and dimensions as the heer land zone but differs significantly with regard to its recurrence pattern zones of concentrated intraplate earthquakes appear to be prevalent on the landward side of continental margins in arctic regions the seismicity in these zones can be explained as the interaction of a regional stress field of plate tectonic origin with local zones of weakness

patagonia icefield melting observed by gravity recovery and climate experiment grace 1 using recently released reprocessed gravity solutions from the gravity recovery and climate experiment grace we estimate the ice loss rate for the patagonia icefield pif of south america for the period april 2002 through december 2006 after postglacial rebound and hydrological effects are corrected the estimated rate is 27 9 11 km 3 year equivalent to an average loss of 1 6 m year ice thickness change if evenly distributed over the entire pif area the estimated contribution to global sea level rise is 0 078 0 031 mm year this is an independent confirmation of relatively large melting rate estimates from earlier studies employing topographic and cartographic data

accelerated antarctic ice loss from satellite gravity measurements accurate quantification of antarctic ice sheet mass balance and its contribution to global sea level rise remains challenging gravity recovery and climate experiment data spanning the period april 2002 to january 2009 confirm earlier estimates of ice loss for antarctica and indicate that east antarctica started losing mass in about 2006

central costa rica deformed belt kinematics of diffuse faulting across the western panama block fault kinematics seismicity and geodetic data across central costa rica reveal a diffuse fault zone here named the central costa rica deformed belt ccrdb the ccrdb defines the western margin of the panama block and links the north panama deformed belt npdb along the caribbean coast with the middle america trench mat along the pacific coast the junction of the ccrdb and the mat coincides with an abrupt transition from smooth to rough crust on the subducting cocos plate rough smooth boundary shallow subduction of rough thickened oceanic crust associated with the cocos ridge shifts active shortening into the volcanic arc along faults of the ccrdb variable fault kinematics along this zone may reflect three combined deformation mechanisms horizontal shortening and shear from oceanic ridge indentation basal traction from shallow subduction and localized block uplift from subducting seamount roughness within the forearc domain 1 mesoscale faults express transtension where steep ne striking regional scale faults intersect the pacific coast across the volcanic arc domain 2 mesoscale faults exhibit mostly sinistral and dextral slip on ne and nw striking conjugate faults respectively approaching the npdb in the back arc domain 3 transcurrent faulting is modified by transpression and crustal thickening fault kinematics are consistent with earthquake focal mechanisms and global positioning system gps measurements radiometric age constraints confirm that faulting postdates the late neogene onset of shallow subduction the ensuing deformation front has propagated northward into the volcanic arc to its present position along the seismically active ccrdb within the forearc the effect of shallow subduction is overprinted by local uplift related to underthrusting seamounts

subduction of the eastern panama basin and seismotectonics of northwestern south america spreading along the cocos nazca plate boundary since the breakup of the farallon plate in the miocene has resulted in the formation of the panama basin and a complex interaction of plates in and near northwestern south america current plate boundaries have been defined and segments of subducted lithosphere identified through selection of hypocentral locations of earthquakes considering only well located events and through focal mechanism determinations the existence of relict plate boundaries bathymetric features and the panamanian isthmus has affected the subduction process of the nazca plate beneath south america and determined the present day configuration of the subducting lithospheric plate there is no single triple junction separating the caribbean south american and nazca plates instead the panamanian isthmus and surrounding areas are accommodating east west compression and a lesser degree of north south compression along a series of thrust faults striking nw to ne and the andean ranges of ecuador colombia and venezeula are moving as a block nne relative to the rest of the south american plates along a system of faults following the front of the eastern cordillera the subducted portions of the panama basin and old farallon plate have become segmented into three pieces recognized in this study from north to south they are 1 a bucaramanga segment continuous with the caribbean seafloor northwest of colombia 2 a cauca segment continuous with oceanic crust nazca plate currently being subducted beneath south america at the colombia ecuador trench and 3 an ecuador segment at the northern end of the subducted lithospheric plate which is dipping at a small angle to the east beneath northern peru the segmentation of the subducted plate can be explained by the buoyancy of bathymetric features which have been partially subducted

inferences on sediment strength and fault friction from structures at the aleutian trench the structural relations of conjugate thrust faults and the master decollement imaged in a research level processed multichannel seismic reflection record across the eastern aleutian trench allow quantitative estimates of the sediment strength and friction across thrust boundaries of the subduction zone these estimates are derived from geometric relations between faults and from the overall geometry of accreted prisms they suggest the presence of high pore fluid pressures that result in low levels of friction along a master decollement and on the main bounding thrust faults in the accreted sediment therefore coupling in the immediate trench area apparently does not contribute significantly to the generation of horizontal compression upslope

changes in total ocean mass derived from grace gps and ocean modeling with weekly resolution 1 we derive changes in ocean bottom pressure obp and ocean mass by combining modeled ocean bottom pressure weekly grace derived models of gravity change and large scale deformation patterns sensed by a global network of gps stations in a joint least squares inversion the weekly combination allows a consistent estimation of geocenter motion loading mass harmonics up to degree 30 and a spatially uniform mass correction term which serves as a correction for forcing of the ocean model we provide maps and time series of ocean mass and bottom pressure variations furthermore we discuss the estimated geocenter motion and the estimated model correction our results indicate that the total ocean mass change is predominantly annual with a maximum amplitude corresponding to 7 4 mm in october which is in line with earlier work the mean ocean bottom pressure i e ocean plus atmospheric mass shows an annual amplitude of 8 7 mm and is shifted forward by about 1 5 months in addition the solution exhibits typical autocorrelation times of about 2 weeks a comparison with in situ bottom pressure time series in the southern indian ocean shows a good agreement with correlations of 0 7 0 8 based on these comparisons we see that our results monitor realistic submonthly variations which are strongest at high latitudes the addition of grace data in the inversion is found to improve these high latitude variations and enables better separability of the geocenter motion from other unknowns increasing the obp model error from 3 cm to 4 8 cm affects mainly the higher degree coefficients