gravity anomalies and segmentation of the continental margin offshore west africa the free air edge effect anomaly at passive continental margins provides constraints on the thermal and mechanical properties of the lithosphere in extensional regions like many margins the edge effect high offshore gabon west africa is located at the shelf break in the region of maximum sediment thickness gabon differs however from other margins in that the edge effect low is displaced from over the continental rise to the slope the origin of the low has been investigated by backstripping the sediments at the margin we have used the backstrip to calculate the amount of crustal thinning assuming airy isostasy such a model implies zero strength during rifting alternatively this amount of crustal thinning can be explained by a strong margin and a depth of necking that exactly balances the upward and downward forces that act on the crust during rifting we have developed a method which takes into account variations in the strength during rifting and the depth of necking to compute the thinning directly from the sediment backstrip the backstrip and the crustal thinning have been used to calculate the rifting gravity anomaly differences between the backstrip and the present day water depth give the geometry of the sediment load and hence the contribution to the gaavity anomaly of sedimentation by comparing the combined rifting and sedimentation anomaly to the observed free air gravity anomaly we have been able to constrain the flexural rigidity and equivalent elastic thickness te of the lithosphere during and after rifting the best fitting model is one in which the low is caused by a 125 km wide zone of relatively thin i e 15 km continental crust which was weak i e 0 te 10k during rifting and has remained so since then we speculate that the weak zone extends for 350 400 km along strike of the gabon margin and that passive margins may be highly segmented as regards their long term strength

hatton bank northwest u k continental margin structure summary the continent ocean transition near hatton bank was studied using a dense grid of single ship and two ship multichannel seismic mcs profiles extensive oceanward dipping reflectors in a sequence of igneous rocks are developed in the upper crust across the entire margin at the landward shallowest end the dipping reflectors overlie continental crust while at the seaward end they are formed above oceanic crust beneath the central and lower part of the margin is a mid crustal layer approximately 5 km thick that could be either stretched and thinned continental crust or maybe newly formed igneous crust generated at the same time as the dipping reflector sequence beneath this mid crustal layer and above a well defined seismic moho which rises from 27 km continental end to 15 km oceanic end across the margin the present lower crust comprises a 10 15 km thick lens of material with a seismic velocity of 7 3 to 7 4 kds we interpret the present lower crustal lens as underplated igneous rocks left after extraction of the extruded basaltic lavas a considerable quantity of new material has been added to the crust under the rifted margin the present moho is a new boundary formed during creation of the margin and cannot therefore be used to determine the amount of thinning

throughgoing crustal faults along the northern margin of the south china sea and their role in crustal extension a study of the rifled northern margin of the south china sea based on new single ship and two ship multichannel seismic data has revealed the presence of highly reflective steeply dipping normal faults which cut through the entire crust these faults with dips of 25 40 are associated with localized crustal thinning of 5 15 km and represent horizontal extension of the order of tens of kilometers a range of 5 20 km of horizontal extension has been measured along individual faults although the relative amount of horizontal extension taken up by these faults is modest compared to the total extension across the margin only about 15 or less the amount of localized crustal thinning which takes place along these fault surfaces is quite significant at one fault location the crustal thickness is reduced by 40 although the faults described have been recognized at three separate locations along the margin their trends appear to differ at each in two examples the faults are oblique to the inferred general north south extension direction of the margin this characteristic taken with the fact that some of the crustal faults also appear to serve as abrupt boundaries between broad regions of differing crustal thickness and reflectivity of the lower crust suggests that at least some of these faults were present prior to rifting such faults are postulated to have been former thrust faults perhaps associated with a triassic suture zone and or cretaceous subduction margin which have been reactivated as normal faults during subsequent rifting alternatively some of the throughgoing faults may represent crustal detachment surfaces that first formed as the result of a simple shear response to crustal extension

the importance of small scale faulting in regional extension a recurring observation in many studies of extensional basins has been that the amount of extension visible on normal faults for example on seismic reflection profiles is significantly less than the amount of extension indicated by crustal thickness and thermal subsidence1 5 one mechanism suggested to account for this discrepancy is small scale faulting with offsets too small to be resolved seismically6 7 but earthquake studies8 10 indicate that small faults are responsible for only a small fraction of the total seismic moment in an active area scholz and cowie11 have recently attempted to extend this approach to the total strain at the end of a finite deformation interval by combining scaling laws describing the distributions of fault lengths and displacements here we present fault displacement data that directly conflict with scholz and cowie s conclusions and imply that up to 40 of the extension may be missed by summing fault offsets on basin profiles the fault population at the end of a long deformation interval may differ substantially from that responsible for the earthquake population at any one time

pangaean divergent margins historical perspective abstract pangaean divergent margins caused by the breakup by rifting of that supercontinent display three distinct evolutionary stages in their youth or rift stage divergent margins display a synsediment fault style that has considerable tectonic instability during the mature or drift stage seafloor spreading continental margins are relatively stable and are dominated by thermal subsidence these two stages rifting and drifting are separated in many areas by an unconformity coeval with the oldest associated oceanic crust the breakup unconformity in some areas the onset of seafloor spreading is marked by intense magmatic activity sediment facies the rifting stage range from continental clastics landward of the continental basement hinge and along the flanks of the basin to saline deposits and more open water carbonates in the axial zone of the rift lithologies emplaced during the rift or mature stage of margin development range from shallow clastics carbonates to deep water carbonate siliceous oozes turbidites and contourites some of which were deposited in an anoxic environment deposition then is controlled by changes in configuration of basins changes in sea level and plate migration through several climatic belts during the final or old age of margin development divergence gives way to convergence this stage which generally terminates with continental collision is characterized by tectonism magmatic activity and crustal shortening mesozoic divergent margins along the east west trending former tethys ocean have reached this stage in their evolution

nonseasonal fluctuations of the arctic ocean mass observed by the grace satellites 1 time variable gravity observations from the grace satellites reveal strong nonseasonal fluctuations of bottom pressure in the arctic ocean on the time scales from 2 to 6 months and a record high bottom pressure anomaly in february of 2011 here we examine the nature and driving forces behind those fluctuations we find that the nonseasonal variability of the arctic ocean mass is strongly coupled to wind forcing the zonal wind pattern is correlated with a dipole pattern of arctic ocean mass changes westerly wind intensification over the north atlantic at about 60 n as well as over the russian arctic continental shelf break cause the ocean mass to decrease in the nordic seas and in the central arctic and to increase over the russian arctic shelf basin wide arctic ocean mass fluctuations are correlated with northward wind anomalies over the northeastern north atlantic and nordic seas and over the bering sea we show that positive negative arctic ocean mass anomalies are associated with anticyclonic cyclonic anomalies of the large scale ocean circulation pattern based on ocean model simulations we conclude that the observed nonseasonal arctic ocean mass variability is mostly explained by the net horizontal wind driven transports and the contribution of fresh water fluxes is negligible we demonstrate that transport anomalies across both the atlantic and pacific gateways were equally important for generating large arctic ocean mass anomalies in 2011

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

interannual bottom pressure signals in the australian antarctic and bellingshausen basins abstractanalyses of large scale 750 km ocean bottom pressure pb fields derived from the gravity recovery and climate experiment grace and from an estimating the circulation climate of the ocean ecco state estimate reveal enhanced interannual variability partially connected to the antarctic oscillation in regions of the australian antarctic basin and the bellingshausen basin with pb magnitudes comparable to those of sea level and good correlation between the grace and ecco pb series consistent with the theory of gill and niiler the patterns of stronger pb variability are partly related to enhanced local wind curl forcing and weakened gradients in h f where h is ocean depth and f is the coriolis parameter despite weaker h f gradients motions against them are sufficiently strong to play a role in balancing the local wind input topographic effects are as or more important than changes in f additionally and contrary to the dominance of barotropic processes at subannual time scales barocl

cenozoic subsidence and uplift of continents from time varying dynamic topography time varying global dynamic topography has been computed for the entire cenozoic the spherical viscous flow model is driven by a distribution of density heterogeneity based on mesozoic and cenozoic subduction by using the calculated topography the uplift and subsidence histories of five regions fixed to the north american indian australian indonesian and south american continents are computed predicted vertical motions are in qualitative agreement with observed values except for india and south america for australia and north america the predicted periods of uplift and subsidence are out of phase with respect to observations but display the correct overall trend the latest phase of uplift for north america is directly related to the cessation of kula plate subduction after 48 ma dynamic topography depends on the viscosity structure of the mantle a lower mantle that is 50 times more viscous than the upper mantle yields the best agreement between modeled and predicted vertical motions the presence of a low viscosity channel beneath the lithosphere has only a small influence on our results compared to dynamic topography the time varying geoid is insignificant in controlling relative sea level

large igneous province on the us atlantic margin and implications for magmatism during continental breakup rifted continental margins commonly include sections of igneous rock more than twice as thick as normal oceanic crust explanations for this voluminous magmatic accretion during rifting include plume models1 3 which require a deep seated thermal or chemical anomaly in upwelling mantle and non plume models4 7 which call on broad shallow thermal anomalies and or rapid upwelling of mantle through the melting zone new seismic models from two transects across the continent ocean transition on the us atlantic margin8 10 confirm the presence of a 20 25 km thick igneous section here we argue that the similarity of the crustal structure on these and two previous transects spanning 1 000 km of the margin and the association of thick igneous crust with the east coast magnetic anomaly11 imply that the thick igneous section extends along the entire margin and may have a volume of as much as 3 2 106 km3 the distribution of volcanic and plutonic rocks details of the seismic structure and lack of independent evidence for a hotspot are difficult to reconcile with plume models and suggest that non plume processes created the thick igneous crust

formation and evolution of strike slip faults rifts and basins during the india asia collision an experimental approach the processes which have governed the formation and evolution of large tertiary strike slip faults during the penetration of india into eastern asia are investigated by means of plane strain indentation experiments on layered plasticine models the steady state deformation of plasticine obeys a power creep flow law e c t n the stress exponent n is between 6 and 9 at 25 c uniaxial plane strain tests on cubic specimens show that the growth of faults in layered plasticine results from strain softening a process observed for strain rates ranging from 3 5 10 5 to 3 6 10 3 s 1 fault or shear zones form after only 7 10 bulk strain subsequent deformation is controlled by the geometry of the fault pattern rather than the physical properties of the plasticine a series of nine plane strain indentation experiments shows the influence of boundary conditions as well as that of the internal structure of the plasticine model on the faulting sequence the ubiquity of strain softening in experimental deformation of a variety of rocks as well as the widespread occurrence of shear zones in nature suggest that long term deformation of the continental lithosphere may also be primarily influenced by the geometry of large faults which rapidly develop with increasing strain the deformation and faulting sequence observed in the plasticine indentation experiments may thus be compared to collision induced strikeslip faulting in asia particularly to total offsets and rates of movements on the faults the experiments simulate the evolution of the western ends of the strike slip faults which have probably been analogous to trench fault fault triple junctions the experiments also illustrate mechanisms for the formation of extensional basins such as the south china sea north china basin and andaman sea near active continental margins the basins which appear to absorb terminal offsets along major strike slip faults near such margins may result from mismatch between the sharply angular shape of the deformed continental edge and the more regularly curved trench along which the smoothly flexed oceanic lithosphere subducts the existence of distinct phases of strike slip extrusion corroborates the idea that the discontinuities in time which typify intracontinental tectonics and orogenic cycles may often result from strain localization and the ensuing discontinuous non steady state deformation of the continental lithosphere

crustal structure of the goban spur rifted continental margin ne atlantic summary we present results that constrain the crustal structure across a typical non volcanic rifted continental margin the goban spur in the ne atlantic traveltime and amplitude modelling of wide angle ocean bottom seismograph profiles shows that the continental crust thins in a seaward direction to about 7 km before it breaks to allow a new oceanic spereading centre to develop the crustal velocity structure the stretching factors and the density distribution inferred from gravity anomalies are consistent with bulk pure shear stretching for the development of the rifted continental margin the seismic velocity structure of the thinned continental crust suggests that thinning was accompanied by only limited igneous intrusion and extrusion in the upper crust the adjacent oceanic crust is abnormally thin 5 4km this is explained most readily by accretion at a very slow spreading centre approx 11 mm yr full rate although additional cooling by conductive heat loss from the parent mantle as it rose slowly beneath the stretched and thinned continental lithosphere together with minor igneous intrusion into and surface flows onto the adjacent continental crust may also have reduced the oceanic igneous thickness immediately adjacent to the continent ocean transition

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

mechanisms of decadal sea level variability in the eastern north atlantic and the mediterranean sea decadal sea level variations from tide gauge records along the western european coast and in the mediterranean sea commencing in the late 19th and early 20th centuries are examined relative to large scale atmospheric forcing recent studies have provided evidence for a link between sea level in the eastern north atlantic and atmospheric forcing however the nature of this relationship is still unclear here the outputs of a regional barotropic model and a nearly global baroclinic model are used in conjunction with wind stress and heat flux data to explore the physical mechanisms responsible for the observed sea level variability all tide gauge records show significant decadal variability up to 15 cm and are highly correlated with the nao and among themselves at decadal periods there is a coherent sea level signal that affects the eastern boundary of the north atlantic northward of 25 n and is limited to a narrow band of the order of a few hundred kilometers along the coast this band tends to become narrower towards higher latitudes we find that longshore wind and wave propagation along the boundary are the major contributors to coastal sea level variability but no significant contribution from mass redistribution linked to changes in the strength of the subtropical gyre is observed the mass component dominates sea level in the mediterranean and is mainly driven by mass exchanges with the atlantic which explains the correlation between both regions southward of 25 n sea level changes are mainly driven by heat advection through ekman fluxes

human impact around settlement sites a phytolith and mineralogical study for assessing site boundaries phytolith preservation and implications for spatial reconstructions using plant remains abstract defining the extent of human activity around settlement sites is of particular significance in archaeology as it may define peripheral activity areas and thus the site s boundary in near eastern archaeology site boundaries are usually defined by the presence of architectural and other macroscopic archaeological remains here we use the phytolith concentrations and morphotype assemblages as well as changes in the mineralogical composition of the sediments in and around the small iron age site of izbet sartah in central israel to determine the site boundaries the site has a shallow stratigraphy and highly bioturbated sediments coincidental changes in the clay quartz ratio and phytolith concentrations define the boundary between high and low impact anthropogenic activities this boundary is generally some 20 m away from the architectural remains in addition we note that the phytoliths in the site s core show clear evidence of having been affected by chemical dissolution i e diagenesis while those in the vicinity of the site s boundary have undergone severe diagenesis these observations indicate that phytolith diagenesis will affect site boundaries determination as well as phytolith based reconstructions of activity areas we propose that phytolith preservation depends on the initial amount of available silica the depth of burial with respect to the active root area of modern vegetation and the presence of fresh phytoliths in the soil

geochemistry of coeval mesozoic plutonic and volcanic suites in hong kong middle jurassic to lower cretaceous intermediate to silicic plutonic and volcanic rocks of hong kong record a transition from calc alkaline through high k calc alkaline to transitional shoshonitic compositions with time close spatial and temporal associations among comagmatic volcanic plutonic pairs indicate that magmatism occurred in discrete episodes mostly of less than one million years duration synchronous high k calc alkaline and transitional shoshonitic magmatic activity during at least one pulse suggests a relatively rapid transition from a subduction related to an extension related tectonic setting geochemical signatures indicate that the magmatic suites have a mantle origin with a decreasing crustal contribution from two distinct sources the earliest mantle derived magmas interacted strongly with a dominantly archaean crustal protolith younger magmas show evidence for interaction with a dominantly proterozoic crustal protolith the strongest mantle influence is shown by magmas which were intruded along the boundary between the two dominant crustal sources this interface marks a deep crustal discontinuity which promoted the passage of magmas to the surface

understanding the spatial variation of sea level rise in the north sea using satellite altimetry this paper examines the spatial variation of sea surface height trends in the north sea basin as seen by satellite altimetry and assesses its underlying causes changes in the potential temperature and salinity of the north sea are transposed into corresponding changes in sea surface height and regional anomalies of linear sea level trend calculated the same is carried out for the meteorological processes which act on the sea surface the steric and meteorological regional sea level rise anomalies are summed with those from contributions from land ice and compared against the values seen by satellite altimetry over the period 1993 2014 results show that there is good agreement between the observations and the reconstruction the local meteorological contribution appears to be most important in describing regional variation in linear sea level rise and is reinforced with a local halosteric contribution which shows a similar spatial pattern

comment on flank uplift and topography at the central baikal rift se siberia a test of kinematic models for continental extension by peter van der beek

annual cycle in coastal sea level from tide gauges and altimetry 1 tide gauges provide a unique data set extending many decades back in time but coverage is restricted to continental boundaries and a few oceanic islands and the extent to which the tide gauge records can be used to infer low frequency large scale sea level behavior remains unclear since 1992 satellite altimetry provides near global coverage of sea level variability including coastal regions we compare variability at 345 continental and island tide gauge coastal locations and adjacent shallow and deep oceans as inferred from altimetry initial focus is on the dominant annual cycle on average annual amplitudes in tide gauges are comparable to but larger than those in the nearby shallow ocean 200 m substantial differences are found in areas adjacent to strong river outflows and narrow coastal currents the annual cycle in shallow areas is usually enhanced relative to the open ocean apart from areas with strong western boundary currents offshore differences of 1 2 months in annual phases for coastal shallow and deep ocean are typical our analysis points to the presence of considerable spatial variability in the annual cycle across deep shallow and coastal regions and to the importance of both tide gauge and altimeter measurements for proper resolution and interpretation of such variability

insight into the nature of the ocean continent transition off west iberia from a deep multichannel seismic reflection profile a high quality multichannel seismic reflection profile across the west iberia margin confirms the existence of a 130 km wide transition zone between tilted continental fault blocks and unequivocal oceanic crust the zone is characterized by a 1 0 2 5 km thick seismically unreflective upper basement layer lying over reflective basement that contains major landward dipping reflectors that mostly truncate against the base of the upper layer and penetrate to depths of 10 19 km below the basement surface and minor seaward dipping reflectors moho reflections are absent in this zone we suggest that the unreflective layer represents tectonically exposed upper mantle that has undergone extensive serpentinization enhanced by vigorous seawater circulation about 40 km farther north the transition zone narrows to 35 km and the unreflective layer is less continuous and widespread

grace derived ice mass variations over greenland by accounting for leakage effects 1 after more than 6 years in full operational mode the gravity recovery and climate experiment grace satellite mission provides the opportunity to derive global secular mass changes from space geodetic observations crucial for a reliable estimate of secular mass changes is the ability to correct for spectral and spatial leakage effects in order to account for any leakage signal we present and apply a four step procedure including a validation step based on forward gravity modeling most notably our method is characterized by the separation and quantification of individual leakage sources we test and apply our procedure to the greenland area which exhibits the strongest secular trend signal on the basis of simulation studies we demonstrate that leakage out effects are dominant for the greenland area and if not accounted for mass change rates will be underestimated analyzing time variable grace gravity fields covering 6 whole years august 2002 to july 2008 inclusive we estimate the ice volume loss over greenland to be 177 12 km3 a 1 this value is the average derived from monthly gravity field models provided by csr gfz and jpl with individual contributions of 242 14 km3 a 1 194 24 km3 a 1 and 96 23 km3 a 1 respectively we highlight that without taking leakage effects into account mass change amplitudes over greenland are reduced by a factor of 2 despite the rather large spread of the results among grace processing centers our results are in better agreement with the findings from alternative grace analysis methods and insar observations

satellite derived interannual ocean bottom pressure variability and its relation to sea level 1 knowledge of the relationship between bottom pressure pb and sea level is important for understanding ocean circulation and climate we use recent gravity recovery and climate experiment grace release 05 data along with altimetry to investigate the relationship between and pb over long periods 1 year and large scales 750 km elevated pb signals are observed over deep extratropical regions e g southern ocean basins and shallow or semi enclosed areas e g indonesian and nordic seas in these places considerable variance is explained by pb variance correlation between and pb is significant in many regions including instances of significant negative correlation suggestive of active baroclinic processes results exemplify the good quality of grace release 05 data and demonstrate that contemporary regional variability cannot always be interpreted in terms of steric changes alone

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

lower crustal extension across the northern carnarvon basin australia evidence for an eastward dipping detachment asymmetry and strain partitioning along conjugate margins are often explained in terms of detachment faults nevertheless cogent evidence for their existence remains limited furthermore even when inferred detachment faults are imaged seismically it is difficult to demonstrate that differential displacement has occurred across these surfaces we present tectonic and stratigraphic evidence from the northern carnarvon basin northwest australia that documents the existence of an intracrustal eastward dipping detachment by integrating sequence stratigraphy with kinematic and isostatic models of basin development we conclude that the northern carnarvon basin was formed as a consequence of four extension events 1 a broadly distributed late permian event 2 a predominantly localized rhaetian event responsible for the inception of the barrow and dampier subbasins 3 a localized callovian fault reactivation within the barrow dampier subbasins and 4 a tithonian valanginian event that generated large post valanginian regional subsidence across the northern carnarvon basin with only minor accompanying brittle deformation and erosional truncation the regional distribution and amplitude of the post valanginian subsidence are not consistent with the minor amounts of tithonian valanginian brittle upper crustal extension observed on the margin large portions of the platform were emergent or at very shallow water depths prior to the tithonian valanginian extension to match the distribution and magnitude of the post valanginian thermal type subsidence requires significant lower crustal and mantle extension across the northern carnarvon basin such a distribution of extension implies the existence of an eastward dipping intracrustal detachment having a ramp flat ramp geometry that effectively thinned the lower crust and lithospheric mantle the detachment breached the surface close to the position of the continent ocean boundary west of the exmouth plateau the flat component of the detachment occurred at midcrustal depths 15 km across the plateau and ramped beneath the australian continent lower crustal ductile extension provides a viable mechanism to generate large regional subsidence with little attendant brittle deformation which may explain the paradox that both sides of many conjugate margins appear to be the upper plate

the formation of passive margins constraints from the crustal structure and segmentation of the deep galicia margin spain the crustal structure of the mesozoic deep galicia margin and adjacent ocean continent boundary ocb was investigated by seismic reflection including pre stack depth migration and attenuation of seismic waves with time the seismic data were calibrated using numerous geological samples recovered by drilling and or by diving with submersible the n s trending margin and ocb are divided in two distinct segments by ne sw synrift transverse faults locally reactivated and inverted by cenozoic tectonics the transverse faulting and ocb segmentation result from crustal stretching probably in a ne sw direction during the rifting stage of the margin in early cretaceous times the cenozoic tectonics are related to iberia eurasia convergence in palaeogene times pyrenean event in both segments of the deep margin the seismic crust is made of four horizontal layers 1 two sedimentary layers corresponding to post and syn rift sequences where velocity ranges from 1 9 to 3 5 km s 1 and where the q factor is low the two sedimentary layers being separated by a strong reflector marking the break up unconformity 2 a faulted layer where velocity ranges from 4 0 to 5 2 km s 1 and where the q factor is high this layer corresponds to the margin tilted blocks where continental basement and lithified pre rift sediments were sampled 3 the lower seismic crust where the velocity 7 km s 1 and more and the q factor are the highest this layer probably made of partly serpentinized peridotite is roofed by a strong s s seismic reflector and resting on a scattering poorly reflective moho a composite model based both on analogue modelling of lithosphere stretching and on available structural data accounts for the present structure of the margin and ocb stretching and thinning of the lithosphere are accommodated by boudinage of the brittle levels upper crust and uppermost mantle and by simple shear in the ductile levels lower crust and upper lithospheric mantle two main conjugate shear zones may account for the observations and seismic data one sz1 located in the lower ductile continental crust is synthetic to the tilting sense of the margin crustal blocks another sz2 located in the ductile mantle accounts for the deformation of mantle terranes and their final unroofing and exposure at the continental rift axis now the ocb the s s reflector is interpreted as the seismic signature of the tectonic contact between crustal terranes and mantle rocks partly transformed into serpentinite by syn rift hydrothermal activity it is probably related to both shear zones sz1 and sz2 the seismic moho is lower within the lithosphere at the fresh serpentinized peridotite boundary

re evaluation of extension across the pearl river mouth basin south china sea implications for continental lithosphere deformation mechanisms abstract the tertiary pearl river mouth basin in the south china sea is one of relatively few localities where independent estimates of extension from upper crustal normal faulting crustal thinning and subsidence have been compared previous work indicates that these methods all support extension factor 1 8 for this basin however the estimate from faulting used the controversial rigid domino method whereas summing normal fault heaves gives only 1 3 instead this study pursues the possibility that the true value for this basin is 1 3 and the published estimates of extension from subsidence and crustal thinning are exaggerated because they do not take account of outflow of lower crustal material from beneath this basin such outward channel flow will occur when the lithostatic pressure at the base of the brittle upper crust is greater beneath a basin than beneath its surroundings in response to such a pressure gradient material lost from beneath this basin has probably flowed to beneath the adjacent chinese landmass where it isostatically compensates the erosion that has supplied much of the basin s sediment

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

annual sea level changes on the north american northeast coast influence of local winds and barotropic motions abstractunderstanding the relationship between coastal sea level and the variable ocean circulation is crucial for interpreting tide gauge records and projecting sea level rise in this study annual sea level records adjusted for the inverted barometer effect from tide gauges along the north american northeast coast over 1980 2010 are compared to a set of data assimilating ocean reanalysis products as well as a global barotropic model solution forced with wind stress and barometric pressure correspondence between models and data depends strongly on model and location at sites north of cape hatteras the barotropic model shows as much if not more skill than ocean reanalyses explaining about 50 of the variance in the adjusted annual tide gauge sea level records additional numerical experiments show that annual sea level changes along this coast from the barotropic model are driven by local wind stress over the continental shelf and slope this result is interpreted in the light of a simple dynamic

global evaluation of new grace mascon products for hydrologic applications recent developments in mascon mass concentration solutions for grace gravity recovery and climate experiment satellite data have significantly increased the spatial localization and amplitude of recovered terrestrial total water storage anomalies twsa however land hydrology applications have been limited here we compare twsa from april 2002 through march 2015 from 1 newly released grace mascons from the center for space research csr m with 2 nasa jpl mascons jpl m and with 3 csr tellus gridded spherical harmonics rescaled sf csrt gsh sf in 176 river basins 60 of the global land area time series in twsa mascons csr m and jpl m and spherical harmonics are highly correlated rank correlation coefficients mostly 0 9 the signal from long term trends up to 20 mm yr is much less than that from seasonal amplitudes up to 250 mm net long term trends summed over all 176 basins are similar for csr and jpl mascons 66 69 km3 yr but are lower for spherical harmonics 14 km3 yr long term twsa declines are found mostly in irrigated basins 41 to 69 km3 yr seasonal amplitudes agree among grace solutions increasing confidence in grace based seasonal fluctuations rescaling spherical harmonics significantly increases agreement with mascons for seasonal fluctuations but less for long term trends mascons provide advantages relative to spherical harmonics including 1 reduced leakage from land to ocean increasing signal amplitude and 2 application of geophysical data constraints during processing with little empirical postprocessing requirements making it easier for nongeodetic users results of this product intercomparison should allow hydrologists to better select suitable grace solutions for hydrologic applications

ocean mass from grace and glacial isostatic adjustment 1 we examine geoid rates and ocean mass corrections from two published global glacial isostatic adjustment gia models both of which have been used in previous studies to estimate ocean mass trends from gravity recovery and climate experiment grace satellite gravity data these two models are different implementations of the same ice loading history and use similar mantle viscosity profiles the model results are compared with each other and with geoid rates determined from grace during august 2002 to november 2009 when averaged over the global ocean the two models have rates that differ by nearly 1 mm yr 1 of ocean mass with the first model giving a correction closer to 2 mm yr 1 and the second closer to 1 mm yr 1 by comparing the two models we have discovered that 50 of the difference is caused by a global land ocean mean in the first model while it is appropriate to include this mean when subtracting gia effects from measurements of sea level change measured by tide gauges or satellite altimetry the mean should not be included when subtracting gia effects from ocean mass variations derived from satellite gravity data when this mean is removed the ocean mass corrections from the two models still disagree by 0 4 mm yr 1 we trace the residual difference to the fact that the first model also has large trends over the ocean related to large rates in its predicted degree 2 order 1 geoid coefficients such oceanic trends are not observed by grace nor are they predicted by the second model and they are shown to be inconsistent with the polar wander rates predicted by the first model itself if these two problems are corrected we find that the two model predictions agree at the 3 level on the basis of this analysis we conclude that the ocean mass correction for grace is closer to 1 mm yr 1 than 2 mm yr 1 although significant uncertainties remain

deep penetration seismic soundings across the northern margin of the south china sea twenty reversed two ship expanding spread profiles esps with maximum source receiver offsets of 100 km were collected in three transects across the rifted northern margin of the south china sea source receiver offset versus two way travel time x t data were mapped into the intercept time versus ray parameter p domain and velocity depth solutions were obtained by a combination of sum inversion in the p domain and ray tracing in both the p and x t domains arrivals from the moho were detected on 17 of the esps the depths to moho determined for individual esp interpretations have reproducibilities of 0 1 km to 3 km in most cases the moho depth has been determined to within 1 5 km moho depths determined in this investigation represent a significant improvement over previous estimates of moho along the margin from gravity data variations in present day crustal thickness measured from top of prerift basement to moho are one measure of the amount and nature of the crustal thinning associated with the rifting of continental crust preceding the formation of the adjacent south china sea basin the esp interpretations reveal that across the eastern portion of the south china margin the crust appears to thin more or less continuously toward the continent ocean boundary in the west esp interpretations also show a general trend of seaward crustal thinning but in addition indicate at least two instances of focused localized crustal thinning crustal velocities and the relative proportion of upper crust vp 6 4 km s are used to identify areas of the south china margin with similar and contrasting crustal structures variations in these properties are believed to result primarily from contrasting prerift crustal structure across the margin however magmatic underplating during rifting depth dependent extension and pleistocene igneous intrusions may also have contributed to the variations in present crustal structure reliable information about variations in crustal thickness and velocity structure across and along the south china margin is an important prerequisite to understanding better the nature of the spatially variable rifting processes which dominated the formation of this margin

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

u pb dating of mesozoic igneous rocks from hong kong twenty two new zircon and monazite u pb ages have been obtained for mesozoic plutonic and volcanic rocks of hong kong these have revealed the existence of four distinct periods of mid jurassic to lower cretaceous volcano plutonism 164 6 0 2 to 159 3 0 3 ma 146 6 0 2 to 146 2 0 2 ma 143 7 0 3 to 142 5 0 3 ma and 140 9 0 2 to 140 4 0 2 ma a concordant monazite age of 236 3 0 8 ma has also been obtained from one intrusive body representing the first known occurrence of a mid triassic granitoid in the territory among these rocks discordant u pb data projected to the concordia curve have given a broad range of inheritance ages from early phanerozoic to late archaean a xenocrystic zircon from one sample has yielded a concordant age of 1872 3 ma the new u pb ages have refined the results of recent rb sr whole rock age dating of the granitoid rocks and have considerably improved correlation between intrusive and extrusive units the occurrence of inherited components in many of the analysed rocks indicates that they were derived from or erupted through older crust containing a wide variety of age components either as primary igneous or detrital material

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

the crustal structure of the rockall trough differential stretching without underplating the crustal structure along the axis of the rockall trough in the north atlantic has been studied along a 600 km refraction wide angle reflection transect containing three lines each 200 to 250 km long using explosives and ocean bottom seismometers one dimensional inversions of each section were made using the p method and forward modeling of the observed travel times in the next stage travel times and amplitudes were modeled using ray tracing techniques through two dimensional heterogeneous structures the results indicate that there are three sedimentary layers with velocities ranging from 2 km s to 4 5 km s the whole sedimentary section is up to 6 km thick and interpreted as late paleozoic to tertiary in age a two layer continental crust 5 to 7 km thick occurs along the length of the profile the upper crust 6 0 6 3 km s is circa 2 km thick and the lower crust 6 6 6 9 km s is circa 3 km thick a moho transition zone approximately 1 km thick lies at the base of the crust velocities in this transition zone increase from 6 9 km s up to 7 8 km s along the profile the underlying upper mantle has a laterally variable velocity between 7 6 and 7 8 km s unstretched crust onshore in ireland comprises a three layered crust with each layer approximately 10 km thick and a moho transition zone which is about 3 km thick the two upper layers in the onshore region are interpreted as corresponding to the upper crust in the rockall trough and indicate a stretching factor of 8 10 the velocity pattern in the lower crust in the rockall trough and under ireland are similar suggesting significantly less stretching 2 3 the differential stretching model is supported by the presence of the moho transition zone which is stretched by a similar factor to the lower crust the bulk stretching factor for the crust as a whole is in the range of 4 6 if this represents the lithospheric stretching factor significant underplating would be expected however if the stretching factor for the lower crust in the differential stretching model is more representative of overall lithospheric stretching little or no underplating is predicted the velocity patterns observed in the rockall trough indicate the absence of any significant underplating at the base of the crust such as that observed at the continental margin west of the hatton basin

effects of a temperature dependent rheology on large scale continental extension the influence of a temperature dependent rheology on large scale continental extension is investigated using a thin viscous sheet model a vertically averaged rheology is used that is consistent with laboratory experiments on power law creep of olivine and that depends exponentially on temperature the behavior of the model depends principally on two parameters the peclet number which describes the relative rates of advection and diffusion of heat and a dimensionless activation energy which controls the temperature dependence of the rheology numerical calculations of such a thin sheet subjected to an extensional boundary condition show the following results at short times following the beginning of extension deformation occurs with negligible change in temperature so that only small changes in lithospheric strength occur due to attenuation of the lithosphere and maximum rates of deformation are located close to the extensional boundary however after a certain time interval thermal diffusion results in lowered temperatures in the lithosphere strongly increasing lithospheric strength and slowing the rate of extension this critical time depends principally on the peclet number and is short compared with the thermal time constant of the lithosphere the changes in strength cause the locus of high extensional strain rates to shift with time from regions of high strain to regions of low strain results of the calculations are compared with observations from the aegean where maximum extensional strains are found in the south near crete but the largest present day extensional strain rates occur about 300 km further north the comparisons support the hypothesis that the observed separation of the loci of maximum strain and maximum present day strain rates in the aegean may be the consequence of changes in lithospheric strength arising from the temperature dependent mechanical properties of lithospheric materials

tectono stratigraphic terranes and detachment faulting of the south china sea and sulu sea abstract geoscientific data acquired from the south china sea and sulu sea areas by bgr since 1977 are compiled and interpreted as comprehensive data sets together with available information of commercial wells and drillsites of the ocean drilling program odp generalized stratigraphy and lithology are discussed and correlated with the interpreted seismic profiles on the basis of the stratigraphic and structural analysis 5 main tectono stratigraphic terranes are defined for the south china sea and 4 main terranes for the sulu sea respectively the dangerous grounds reed bank palawan nw borneo trough and palawan island represent terranes of continental origin which developed on the proto china margin by uniform sense low angle simple shearing within the crust in late cretaceous to early paleocene this rift was abandoned and a new westward propagating rift system developed north of the dangerous grounds reed bank area from the middle eocene to the oligocene early miocene together with seafloor spreading in the south china sea basin the leading edge of the southward drifting continental terranes collided with the late cretaceous to early eocene subduction complex of the northernmost terrane of the proto sulu sea continuous convergence of these terranes back arc spreading creating the se sulu sea terrane and the assumed anti clockwise rotation of borneo are responsible for the complex collision compression structures of the sulu sea terranes including the formation of splinters of oceanic crust major right lateral wrench systems are nnw sse running up to 500 km long and cut across most of the identified terranes wrenching presumably ceased 12 to 16 myr ago

thermal subsidence and sedimentary processes in the south china sea basin abstract we coupled analysis of regional reflection seismic data with core data from international ocean discovery program iodp expedition 349 and found differences in both depositional features and calculated subsidence between the east and southwest subbasins we calculated the original decompacted thicknesses of sediment for different time slices the depositional features of the basin show that the sedimentation is controlled by both climate and tectonics decoupling of sedimentation rates and fluxes is often observed between the continental margin and deep basin an abrupt increase in sedimentation rates since the pliocene suggests that glacial interglacial climate variability impacted erosion rates the sedimentation rate peaked during the pliocene in the southwest subbasin but peaked during the quaternary in the east subbasin showing strong influence of surrounding tectonic events such as the buildup of taiwan orogen on erosion rates we examine thermal subsidence of the two subbasins by fitting the isostatically balanced basement depths from the seismic profiles with the half space cooling model the estimated thermal diffusivity of the lithosphere in the southwest subbasin is much higher than in the east subbasin these contrasting observations suggest that lithospheric rock compositions may contribute to different tectonic subsidence curves between the east and the southwest subbasins

global glacial isostasy and the surface of the ice age earth the ice 5g vm2 model and grace abstract the 100 kyr quasiperiodic variation of continental ice cover which has been a persistent feature of climate system evolution throughout the most recent 900 kyr of earth history has occurred as a consequence of changes in the seasonal insolation regime forced by the influence of gravitational n body effects in the solar system on the geometry of earth s orbit around the sun the impacts of the changing surface ice load upon both earth s shape and gravitational field as well as upon sea level history have come to be measurable using a variety of geological and geophysical techniques these observations are invertible to obtain useful information on both the internal viscoelastic structure of the solid earth and on the detailed spatiotemporal characteristics of glaciation history this review focuses upon the most recent advances that have been achieved in each of these areas advances that have proven to be central to the construction of the refined model of the global process of glacial isos

extension of continental lithosphere a model for two scales of basin and range deformation we develop a model for deformation in an extending continental lithosphere that is stratified in density and strength assuming a rheology consistent with seismic focal depths and experimental flow laws the model demonstrates that necking instabilities at two wavelengths will arise due to the presence of a strong upper crust and upper mantle separated by a weak lower crust the magnitudes of the instabilities are directly related to strength contrasts within the lithosphere while the dominant wavelengths of necking are controlled mainly by the thicknesses of the strong layers the results are applied to the basin and range province of the western united states where two scales of deformation can be recognized one corresponding to the spacing of ranges and the other to the width of tilt domains a bouguer gravity anomaly and associated regional topography with a wavelength comparable to the width of tilt domains has also been recognized for plausible density and strength stratifications our results show that the horizontal scale of short wavelength necking is consistent with the spacing of individual basins and ranges while that of the longer wavelength necking is consistent with the width of tilt domains we thus suggest that basin and range deformation may be controlled by two scales of extensional instability extension in the weak lower crust in this model is laterally displaced from regions of upper crustal extension the resultant horizontal shearing in the lower crust may be a mechanism for the initiation of low angle extensional detachments