deep seismic structure of the seismogenic plate boundary in the off sanriku region northeastern japan we determined the deep seismic structure of the seismogenic plate boundary zone of off sanriku the northern part of the japan trench inner slope area travel time data from aftershocks of the 1994 far off sanriku earthquake m 7 5 recorded both by ocean bottom seismographs and the local land seismic network enable us to image the three dimensional seismic velocity structure of the region along the plate boundary there exists a p wave low velocity layer of ca 7 km s 1 and with thickness 10 km as the topmost layer of the subducting pacific plate the present analysis indicates that this layer interpreted as the subducting oceanic crust continues without any velocity variations at least down to ca 40 km depth in the overriding plate we found a zone of considerable high p wave velocity ca 8 km s 1 between the pacific coastal line of the northeastern japan arc at the longitude of 142 5 e this manifests that the trenchward toe of the mantle wedge has significantly higher velocity than that beneath the land area of the arc the landward limit of the high p wave velocity toe corresponds to the down dip limit of the seismogenic plate boundary further to the east the p wave velocity is ca 6 km s 1 the structure image obtained indicates that the nature of the overriding plate is significantly different along the plate boundary in the eastern half of the seismogenic zone the island arc crust and subducting oceanic crust are in contact with each other whereas the high velocity mantle wedge makes up the hanging wall of the deeper part of the seismogenic zone

effects of lateral friction on the structural evolution of fold and thrust belts insights from sandbox experiments with implications for the origin of landward vergent thrust wedges in cascadia various factors are inferred to influence the structural evolution of fold and thrust belts or thrust wedges but the effects of lateral friction such as that exerted by the opposite block of a strike slip fault in nature or by sidewalls in analogue models have rarely been given sufficient attention and hence are still far from being fully understood up to now seaward vergent and dually vergent thrust wedges have been satisfactorily explained by the classic coulomb wedge theory and can be well reproduced in analogue and numerical models in contrast landward vergent thrust wedges which are uncommon in nature e g those occurring in the cascadia subduction zone have not yet been adequately explained by the classic coulomb wedge theory and can only be reproduced in a few analogue models under particular conditions in order to look closer at the effects of lateral friction on the structural evolution of fold and thrust belts and to explore the origin of landward vergent thrust wedges we conducted a series of sandbox models under different conditions of lateral friction and various ratios of initial average lateral frictional stress to basal shear stress s d results of our models revealed that in the presence of a weak or ductile basal decollement the direction of lateral friction can dictate not only the deformation propagation direction and the surface slope direction but also the structural vergence of fold and thrust belts fold and thrust belts normally show a structural vergence in the same direction as the direction of lateral friction but a deformation propagation direction and surface slope direction opposite to the direction of lateral friction according to the results of models and the relationships between structural vergence and lateral friction direction occurring both in cascadia and the analogue models we propose that an extremely high s d ratio due to an extremely weak basal decollement and landward directed lateral friction exerted by the opposite blocks of basement involved strike slip faults are probably the two key factors responsible for the origin of landward vergent thrust wedges in cascadia this work illustrates that effects of lateral friction should be properly taken into account both in the kinematic analysis and designs of models to simulate fold and thrust belts in the presence of weak or ductile basal decollements

equatorial undercurrents associated with indian ocean dipole events during contrasting summer monsoons 1 positive indian ocean dipole iod events generally tend to be accompanied by intensified summer monsoon flows and above normal precipitation over the sub continent although strong monsoons have not always coincided with positive iod events numerical simulation experiments and supplementary data diagnostics are performed to understand the iod sub surface dynamics in relation to the monsoon strength the model experiments when forced by iod winds during strong monsoon periods generated a strong subsurface zonal pressure gradient along the equator and an eastward equatorial undercurrent euc leading to supply of cold subsurface waters to the upwelling regions in the eastern equatorial indian ocean however the simulations when forced with iod winds during weak monsoon periods were unable to set up the subsurface pressure gradients and the euc thus resulting in weak sst cooling in the east these results raise the possibility that iod monsoon interactions during strong monsoon years might favor intensification of the iod

geodynamic models of crustal scale episodic tectonic accretion and underplating in subduction zones we investigate the dynamics of crustal scale episodic accretion at convergent margins using a finite element model of subduction the numerical model incorporates effects from flexure subduction loading and large scale variations in material properties the model is used to assess the influence of these parameters on the discrete transfer of units from the lower to the upper plate during subduction the primary controls investigated are spatial variations in density material strength and thickness of material layers these controls correspond to natural examples where episodic accretion results from the thermal structure of subducting lithosphere giving rise to areas of thermally activated ductile creep attempted subduction of small crustal blocks density frictional and or ductile strength variations interaction between well attached basement blocks and overlying detachable sediments and spatial changes in thickness of sediment and crust in the experiments accretion of small continental terranes within a model subduction zone can cause crustal scale fold nappes and shear zones to develop with accompanying tectonic underplating and or frontal accretion the model results may provide insight into real cases of episodic accretion for example the emplacement of the upper penninic nappe stack of the central alps

history of metamorphic fluids along outcrop scale faults in a paleogene accretionary prism sw japan implications for prism scale hydrology 1 synkinematic quartz crystals collected from late stage syn to post cleavage faults in the paleogene shimanto accretionary complex contain fluids that provide constraints on fluid flow within the accretionary wedge in particular the trapped fluids provide a glimpse of intraprism flow during a phase of widespread prism thickening carbonic ch4 co2 rich and aqueous h2o rich inclusions occur intermixed within single crystals that nucleated primarily in releasing stepovers along faults that deform well indurated and or cleaved wallrock invariably the carbonic inclusions contain an aqueous liquid as a perimeter phase and the aqueous inclusions contain a carbonic vapor phase suggesting coeval trapping of unmixed phases from a miscible parent fluid these observations are interpreted to reflect crystal growth following deformation driven dilitation and consequent unmixing of the parent fluid at one location cape oyama the occurrence of ch4 and h2o in the absence of co2 enables explicit constraints to be placed on pressure and temperature conditions during crystal growth these conditions suggest geothermal gradients of 50 c km during late stage faulting our results suggest local intraprism flow that was driven by deformation the scale of flow that is suggested is quite restricted and this leads us to hypothesize that the paleogene shimanto accretionary prism acted largely as a barrier to fluid flow during late stage deformation as a consequence the paleogene rocks probably served to focus fluid escape elsewhere most likely trenchward and arcward the combination of the elevated geothermal gradient which would have contributed to increasing the relative strength of this package of rocks and the lack of fluid pathways within it would have favored the development of new out of sequence thrust faults outboard of the paleogene prism or the reactivation of older more landward structures

subsurface equatorial zonal current in the eastern indian ocean 1 variations of subsurface zonal current in the eastern equatorial indian ocean are investigated by examining 6 year data december 2000 november 2006 from acoustic doppler current profiler adcp mooring at 0 s 90 e the analysis indicates the presence of an eastward equatorial subsurface current between 90 and 170 m depths during both boreal winter and summer during boreal winter the generation of eastward pressure gradient which drives an eastward flow in the thermocline is caused primarily by upwelling equatorial kelvin waves excited by prevailing easterly winds on the other hand the downwelling rossby waves generated by the reflection of the spring downwelling kelvin waves in the eastern boundary as well as the upwelling equatorial kelvin waves triggered by easterlies create an oceanic state that favors the generation of the eastward pressure gradient during boreal summer the subsurface current reveals a distinct seasonal asymmetry the maximum eastward speed of 63 cm s 1 is observed in april and secondary maximum of 49 cm s 1 is seen in october the zonal transport per unit width within depth of the subsurface current exhibits similar variations reaching maximum eastward transport of 35 m2 s 1 in april and secondary maximum of 29 m2 s 1 in october moreover the subsurface current during boreal summer undergoes significant interannual variations it was absent in 2003 but it was anomalously strong during 2006

high precision u th dating of the rock paintings at mt huashan guangxi southern china the rock art and the associated natural scenery at 38 sites located in the zuojiang river valley in the southwest of guangxi zhuang autonomous region southern china were inscribed recently on unesco s world heritage list the painted panel at the site of mt huashan is probably the largest known rock art panel in the world consisting of approximately 1900 identifiable figures and occupying an area of approximately 8000 m 2 to determine a precise age on the rock art at mt huashan 56 secondary carbonate layers above and below the paintings were studied for their mineralogy oxygen and carbon isotopic compositions and dated by the 230 th u method the 230 th u dating results demonstrate that ages of the rock paintings can be bracketed between 1856 16 and 1728 41yr bp corresponding to the middle to the end of the eastern han dynasty ad 25 to 220 the results imply that the rock painting practices at mt huashan probably lasted more than a century and the zuojiang rock art is younger than that at baiyunwan and cangyuan in yunnan province by 1 to 10 centuries

laser ablation u series analysis of fossil bones and teeth abstract over the past decade we have applied laser ablation icp mass spectrometry u series analysis to a large number of bones and teeth the method is fast and provides high resolution data of u series isotopes which give insights into the complexity of uranium migration into within and out of bones in this paper we present our laser ablation approach in detail from the experimental set up to data reduction and uranium diffusion modelling laser ablation analysis can now be applied with minimum damage complete diffusion data sets can be obtained by laser drilling which leaves a hole of around 200 m in diameter and 1 2 mm depth if the natural u diffusion follows a simple single stage process valuable age information can be obtained in other cases highly complex u migrations have been observed which make any age assessment impossible two examples from wadjak and callao illustrate the potential of nearly non destructive laser ablation u series analysis of human skeletal tissues the results demonstrate the physical presence of modern humans in south east asia at a time when our species just started to migrate into europe

uranium series age estimates for rock art in southwest china we report the first uranium series age estimates for rock art in china calcite bracketing a paint layer was used to constrain the age of a naturalistic outline hunter gatherer painting in the jinsha river area of northwest yunnan province southwest china the rock paintings in this region are unique in style and content compared with other bodies of rock art in china which are dominated by neolithic subject matter the minimum and maximum ages were determined using isochron techniques on multiple samples of calcite from above and beneath the paint layer a large painted deer head was dated to between 5738 and 2050 years this painting and underlying flowstone are superimposed on older paintings that suggest the older paintings are at least 3400 years old if not older than 5738 years the results indicate for the first time that jinsha river rock art is older than other forms of rock art in the region and show that rock art likely extends back to at least the transition from the palaeolithic to neolithic in this part of china

observed interannual variability of zonal currents in the equatorial indian ocean thermocline and their relation to indian ocean dipole interannual variability of zonal currents in the eastern equatorial indian ocean thermocline is significantly correlated with sea surface temperature sst in the eastern pole of the indian ocean dipole iod this relationship is more significant than for zonal currents and the dipole mode index the latter of which measures zonal sst anomaly gradient the variability of zonal currents in the thermocline is consistent with the variations of eastward pressure gradient force in the thermocline and the equatorward geostrophic thermocline flows associated with the shallow meridional overturning circulations in the eastern tropical indian ocean our analysis suggests a positive feedback between zonal currents in the thermocline and sst in the eastern pole of iod the combination of equatorial wind and off equatorial wind stress curls associated with sst anomalies in the eastern pole of the iod drives interannual variability in equatorial zonal thermocline flows and its covariability with the shallow meridional overturning cells

impacts of the iod associated temperature and salinity anomalies on the intermittent equatorial undercurrent anomalies the study of equatorial undercurrent euc has attracted a broad attention in recent years due to its strong response and feedback to the indian ocean dipole in this paper we first produce a high quality simulation of three dimensional temperature salinity and zonal current simulation from 1982 to 2014 using a high resolution ocean general circulation model on this basis with two sensitivity experiments we investigate the role of temperature and salinity anomalies in driving and enhancing the euc during the positive iod events by examining the variation of the euc seasonal cycle and diagnosing the zonal momentum budget along the equatorial indian ocean our results show that during january march the euc can appear along the entire equatorial indian ocean in all years but during august november the euc can appear and reach the eastern indian ocean only during the positive iod events the zonal momentum budget analysis indicates that the pressure gradient force contributes most to the variation of the eastward acceleration of zonal currents in the subsurface during the positive iod events strong negative subsurface temperature anomalies exist in the eastern indian ocean with negative surface salinity anomalies in the central and eastern indian ocean resulting in a large pressure gradient force to drive euc during the august november further the results of two sensitivity experiments indicate that the temperature anomalies significantly impact the pressure gradient force playing a leading role in driving the euc while the surface salinity anomalies can secondarily help to intensify the eastward euc through increasing the zonal density gradient in the eastern indian ocean and impacting the vertical momentum advection in the subsurface

sea surface salinity variability during the indian ocean dipole and enso events in the tropical indian ocean an ocean reanalysis that covers the period from 1871 2008 is used to analyze interannual variability of sea surface salinity sss in the tropical indian ocean the reanalysis sss and the sss anomaly patterns during indian ocean dipole iod and el nino southern oscillation enso events are compared with patterns from argo sss data the mean seasonal sss variation is large in the northern bay of bengal compared to variations in the arabian sea and equatorial indian ocean during a positive iod event positive sss anomalies are found along the sumatra coast due to the combination of wind driven upwelling of subsurface high salinity waters enhanced evaporation and anomalous surface circulation the opposite is true to a lesser extent during negative iod events a dipole mode index for salinity dmis based on sss data and a new index based on the average of salinity in a region off the coast of sumatra is introduced to monitor sss variability during iod and enso events the impact of concomitant el nino events on a positive iod event is large with freshening a negative sss anomaly in the equatorial indian ocean and salting positive sss anomaly off the southern sumatra coast the impact of intense freshening reaches into the southwestern tropical indian ocean the impact of concomitant la nina with negative iod is also large with an intense freshening in the southeastern arabian sea and salting off the northern sumatra coast

the pattern and variability of winter kuroshio intrusion northeast of taiwan the variations of the kuroshio path and velocity northeast of taiwan are analyzed based on along track satellite altimeter data as well as high resolution model experiments observations reveal that in winter the kuroshio intrusion into the east china sea ecs at this location is manifested by a secondary maximum current core smcc shoreward of the kuroshio s main path the smcc varies significantly on interannual time scale and its variability is strikingly out of phase with that of the kuroshio entering the ecs meaning that the stronger the kuroshio the weaker the smcc and vice versa model experiments corroborate the observational results and more importantly indicate that the kuroshio intrusion here follows two primary routes a large anticyclonic loop that separates from the kuroshio at the northern end of taiwan and moves forward to form the smcc and a straight northward path onto the shelf when the kuroshio turns sharply eastward along the continental slope of the ecs the intrusion is controlled by both local forcing and remote effect with its pattern and variability depending mostly on the local heat flux and the inertia of the kuroshio current

landward vergence and oblique structural trends in the oregon margin accretionary prism implications and effect on fluid flow abstract the central oregon margin spans a regional transition in accretionary structures from seaward verging in the south to landward verging in the north new multichannel seismic mcs data image both landward and seaward vergent provinces along the central and northern oregon margin landward vergence is characterized by a deep decollement with deformation distributed across a broad lower continental slope in a coherent structural style in the landward vergent area virtually all 4 km of incoming trench sediments overthrust the preceding thrust sheet forming fault bend folds and a distinctive ridge trough morphology this style of landward vergence is not explained by existing models in contrast seaward vergence correlates with a shallower decollement approximately 1 4 km above the oceanic crust and a more intense style of deformation within a narrower slope initial thickening of the trench sediments occurs across a well developed protothrust zone the frontal thrust forms a ramp anticline that is cut by a prominent backthrust previously observed seafloor vent sites in both regions correlate with thrusts that exhibit high amplitude reversed polarity reflections suggestive of enhanced porosity along the faults potential fluid sources and migration paths are strongly influenced by changes in the level of the decollement and vergence along the margin abrupt changes in structural style occur both along strike and updip and are bounded by two sets of oblique slip faults three nw striking left lateral faults are imaged in both mcs and seabeam data plunging anticlines developed along the nw striking faults are venting fluids and were previously interpreted as mud volcanoes the deformation front is locally disrupted where these faults intersect the prism but they appear to have limited influence on the structural evolution of the prism in contrast the ne striking right lateral faults are confined to the deforming sediments of the upper plate these faults interact with the thrusts within the prism forming a rhomboidal pattern of three dimensional deformation

shortening of analogue models with contractive substrata insights into the origin of purely landward vergent thrusting wedge along the cascadia subduction zone and the deformation evolution of himalayan tibetan orogen abstract origin of certain uncommon fold and thrust belts such as the purely landward vergent rearward vergent thrusting wedge along the cascadia subduction zone and the wide flat toped himalayan tibetan orogen were proposed to be closely related to their ductile substrata results of a series of sandbox models with contractive substrata of various contracting velocities and different contracting directions designed to study the influences of horizontal contraction of ductile substrata on deformation evolution and structural style of fold and thrust belts show that purely rearward vergent thrusting wedges can be reproduced under the condition of syn directionally horizontal contraction of the ductile substrata without other requirements of forward dipping backstops landward dipping detachments and strong wedges relative to weak bases therefore syn directionally horizontal contraction of the ductile substrata can be considered an important cause for the formation of purely landward vergent thrusting wedge along the cascadia subduction zone our modeling results together with the results of some other previous physical and numerical models reinforce the opinion that the deformation evolution of the himalayan tibetan orogen were likely governed by its weak lower crust and suggest that the north and south edges of the himalayan tibetan orogen may have undergone almost synchronous cenozoic deformation evolution owing to the existence of weak lower crust it is also proposed that 1 development of thrusts in the himalayan tibetan orogen may not necessarily occur in a frontward sequence 2 much later uplift deduced from cenozoic sedimentary records in the basins along the northeastern margin of the himalayan tibetan orogen does not necessarily imply much later initiation of uplift and deformation at the northeastern margin of the orogen

forearc structure and morphology along the sumatra andaman subduction zone the sunda subduction margin the location of recent magnitude 8 9 megathrust earthquakes is characterized by major changes in prism and forearc morphology and structure along its 5000 km length on the sumatra andaman section measurements of prism width and surface slope indicate along strike segments inclu1ding abrupt changes of prism morphology 1 a narrow and steep prism between burma and the andamans 2 broad with an averaged gentle slope in the andamans nicobars and north sumatra 3 steep and narrow in central sumatra and 4 wider and less steep offshore south sumatra decreasing in width to west java prism width varies from 90 to 180 km and average surface slope from 1 to 3 with a strong inverse correlation between width and slope also observed globally the prism deviates from typical taper geometry in parts of the margin notably offshore north sumatra where it is characterized by a steep toe and broad plateau along strike changes in morphology are strongly linked to input sediment thickness sections of the prism toe represent key global examples of unusual landward vergent thrusting these sections correspond to a thick sediment input and to a wide prism with shallow surface slope a low basal shear stress or backstop mechanism may drive this style of faulting prism morphology and structure appear to be driven predominantly by input sediment thickness linked to oceanic basement topography with sediment properties plate smoothness and orthogonal subduction rate and obliquity also contributing and no clear role of plate age or dip

volume transports of the wyrtki jets and their relationship to the indian ocean dipole the equatorial indian ocean is characterized by strong eastward flows in the upper 80 100 m during boreal spring and fall referred to as the wyrtki jets these jets are driven by westerly winds during the transition seasons between the southwest and northeast monsoons and represent a major conduit for mass and heat transfer between the eastern and western sides of the basin since their discovery over 40 years ago there have been very few estimates from direct observations of the volume transports associated with these currents in this paper we describe seasonal to interannual time scale variations in volume transports based on 5 years of unique measurements from an array of acoustic doppler current profilers in the central equatorial indian ocean the array was centered at 0 80 5 e and spanned latitudes between 2 5 n and 4 s from august 2008 to december 2013 analysis of these data indicates that the spring jet peaks in may at 14 9 2 9 sv and the fall jet peaks in november at 19 7 2 4 sv around which there are year to year transport variations of 5 10 sv the relationship of the interannual transport variations to zonal wind stress forcing sea surface temperature sea surface height and surface current variations associated with the indian ocean dipole iod are further highlighted we also illustrate the role of wind forced equatorial waves in affecting transport variations of the fall wyrtki jet during the peak season of the iod

what 2 d multichannel seismic and multibeam bathymetric data tell us about the north sumatra wedge structure and coseismic response recent large earthquakes have prompted studies to reevaluate seismicity and rupture propagation behavior along the world s major subduction margins our study area covers the entire fore arc from northwest of northern sumatra to west of simeulue island the southern portion of the 2004 tsunamigenic earthquake rupture zone the accretionary prism width is up to 180 km water depths between 4 5 km near the sunda trench and 1 km on fore arc high the wedge consists of a steep outer slope 5 12 a plateau 100 120 km wide with anticlinal folds spaced 2 15 km apart and a steep inner slope adjacent to the aceh basin analysis of seismic profiles and bathymetry reveal three main structural zones consistent along strike from the trench landward 1 predominantly landward vergent folds 2 mixed vergent folds and 3 predominantly seaward vergent folds this paper uses those zones to propose a geometry of an underlying rigid backstop this backstop is seaward dipping and extends from under the aceh basin to beneath the mixed vergence zone a dynamic backstop possibly exists seaward of the rigid backstop and is responsible for the steep slope of the outer prism indurated accreted sediments form the landward vergence zone along with the possible dynamic backstop beneath the outer wedge and the rigid backstop in the inner wedge all behave as a solid block coseismically this block allows great earthquake rupture to propagate farther seaward toward the sunda trench with resultant hazardous tsunamigenic potential

correspondence on erosion rates and weathering history of rock surfaces associated with aboriginal rock art engravings petroglyphs on burrup peninsula western australia from cosmogenic nuclide measurements by brad pillans and keith fifield quaternary science reviews 69 98 106

recent advances and future prospects of the u th and u pb chronometers applicable to archaeology abstract the u th chronometer has revolutionized quaternary science in the last few decades and can now be considered a well established mature technique with growing application in archaeology as an alternative to 14 c dating and osl reaching well beyond the cutoff for both these methods applying the u pb technique to material within a time frame of interest to archaeology is a challenge only recently overcome with enormous potential for future development and use in terms of range u th can generally date material of between a few years and as much as 600 ka while u pb is best suited to material of 1 million years and older u th dating is typically precise with error margins of better than 1 routinely achievable for material of the last interglacial age or younger such precision is possible with u pb dating in the mid quaternary but most ages have uncertainties in the 5 10 range carbonates are the most desirable u series target material with closed system behaviour being particularly important for u th dating making inorganic deposits such as speleothems secondary cave carbonates tufa and calcrete ideal some biogenic carbonates such as coral and eggshell are often amenable to u series dating but less ideal material such as fossil teeth bone and mollusc shell usually continue to gain uranium after deposition meaning accurate ages cannot be calculated in all cases the importance of a thorough understanding of the context of dated material cannot be overstated in this contribution we review u th and u pb dating through an archaeological lens focussing on the methods themselves how to best interpret published data and how best to avoid common pitfalls we highlight the major successes through a number of case studies and provide an overview of what we believe to be the future directions of this field

anomalous surface currents in the tropical indian ocean an anomalous climate event occurred in 1997 in the indian ocean with severe consequences for the surrounding continental areas in response to an intensification of the trade winds a westward gradient of sst and an anomalous reversal of the eastward surface currents with peak velocity anomalies exceeding 1 m s were evident in boreal autumn a similar but weaker event took place in 1994 in this study we examine the observational record during the 1990s including surface drifter velocities sst and altimeter sea level to confirm these dramatic changes we examine the key momentum balance between wind induced momentum flux and the pressure gradient force as well as the important role of horizontal temperature advection in the mixed layer heat response

experimental modelling of orogenic wedges a review abstract experimental modelling applied to the study of orogenic wedge dynamics has been a subject of fruitful research for more than 30 years although the technique dates back as far as the early xix th century on one hand several first order parameters controlling the structural evolution of mountain belts have been intensively investigated using the classic tectonic sandbox models the main parameters are the properties of the basal decollement the deforming material the backstop and fluxes kinematics and surface processes on the other hand the morphological evolution of a mountain relief subjected to changing tectonic or climatic forcing has been addressed using another kind of approach called geomorphic models nowadays the literature is extremely rich particularly for the sandbox technique so that it becomes difficult to have an exhaustive view of the effects of the above parameters on mountain evolution in this article we propose a detailed review of the main results obtained using both tectonic and geomorphic approaches our goal is to provide an almost complete state of the art in the experimental study of relief dynamics to guide present and future researchers in their understanding of mountain belt evolution

growth of the south pyrenean orogenic wedge a six step reconstruction of the south pyrenean foreland fold and thrust belt in spain delineates the topographic slope basal dtcollement angie internal deformation and thrust front advance from the early eocene until the end of contractional deformation in the late oligocene style of thrust front advance dip of the basal dtcollement slope of the upper surface and internal deformation are decoupled and not simply related internal deformation increased decreased and maintained surface slope angle at different stages from the onset to the cessation of deformation the basal dtcollement angle decreased overall suggesting translation of the thrust belt onto stronger crust with time taper angle of the pyrenean thrust wedge was fundamentally controlled by the flexural rigidity of the lower plate the relative rate of creation of structural relief in the rear versus the front of the wedge the extent of deposition of eroded material within the deforming wedge and the taper of the pretectonic stratigraphic wedge

uranium series dating rock art in east timor at many sites throughout the world rock art paintings have been covered by naturally deposited calcite laminations which we demonstrate can be individually dated by recently improved uranium series methods here we report the application of multi collector inductively coupled plasma mass spectrometry to measure the ages for carbonate coatings that bracket red pigment at lene hara cave east timor which could be evidence of human painting these analyses establish the feasibility of dating milligram samples of finely layered calcite deposits associated with archaeological evidence of human occupation in addition to confirming an age of less then 6300 years for the visible red paintings on the carbonate surface we also report a substantially older age of 24 000 to 29 300 years for a similar older red pigment lamination providing possible evidence for an earlier painting episode

geoarchaeological finds below liang bua flores indonesia a split level cave system for homo floresiensis we report on new geoarchaeological finds in a recently discovered cave chamber liang bawah cave under neath positioned below liang bua on the island of flores indonesia where the type specimen for homo floresiensis was recovered from late pleistocene sediment at the rear of liang bua a 23 m long shaft inclined at 60 leads to a lower chamber measuring 23 m x 24 m x 5 m high about half the size of liang bua stone artefacts and bones were found shallowly buried in rubble at the base of the shaft and around a 5 m high mud mound that fills the northwest sector of liang bawah we recovered 17 stone artefacts made from chert and volcanics and more than 220 well preserved bone elements belonging to endemic giant rats pigs primates small murid rodents bats and introduced species multi collector inductively coupled plasma mass spectrometer mc icpms analysis of uranium and thorium in carbonate coatings on four bones yielded ages of similar to 240 180 ka endemic giant rat femur similar to 110 60 ka unidentified phalanx similar to 33 23 ka pig skull fragment and similar to 7 3 ka giant rat femur which overlap with the similar to 95 to 17 ka occupation of liang bua by h floresiensis the similar to 33 23 ka age of the pig skull fragment indicates that sus sp may have dispersed into island southeast asia earlier than previously recognised the passageway at the rear of liang bawah and a currently buried front entrance represent two possible transport paths for cultural and faunal material to the cave chamber analysis of the geomorphic evolution of liang bawah shows that it may have been a late pleistocene depocentre for material transported from the occupation chamber of liang bua and a repository for human subsistence refuse or pit fall trap via the rear passage these physical attributes and the antiquity of the faunal remains found thus far indicate that liang bawah could contain an archive of the late pleistocene and potentially remains of h floresiensis c 2015 elsevier b v all rights reserved

fault kinematics and past plate motions at a convergent plate boundary tertiary shimanto belt southwest japan early and late stage outcrop scale faults have been documented in paleogene strata of the muroto peninsula of southwest japan the former were active early in the consolidation history of the sediment and are crosscut by the latter which were active after regional scale imbrication folding and penetrative deformation i e regional spaced pressure solution cleavage orientation distributions of striae and kinematic axes indicate that the two generations of faults reflect distinct kinematic regimes the orientations and crosscutting relations indicate a counterclockwise viewed downward rotation of the principal shortening direction between early and late stage faulting existing fossil and radiometric age data suggest this transition occurred during eocene oligocene time the fault kinematic data are interpreted to reflect a change in relative plate motions between eurasia and an oceanic plate subducting beneath eurasia in paleogene time these results coupled with existing paleothermal and paleopressure data are consistent with the presence of a spreading ridge in the western pacific basin during this time

mechanical role of backstops in the growth of forearcs we use numerical and laboratory modeling to analyze the mechanical role of backstops in the overriding plate at subduction zones a backstop is defined as a region within a forearc that has significantly greater shear strength than the sediment lying farther trenchward it can be thought of as the bulldozer behind an accretionary wedge we calculate the stress and displacement fields within forearcs for various backstop models using the finite element method and we simulate deformation over a backstop using a small scale laboratory model in this way we model the effects of the mechanical properties and geometry of a backstop on forearc structures we find that the growth of an outer arc high the development of an inner deformation belt with landward vergence and the seemingly paradoxical presence of an undeformed forearc basin within an otherwise highly deformed forearc can be explained by assuming the existence of a geometrically simple backstop with geologically reasonable contrasts in mechanical properties compared to the sediments just trenchward of it the structures produced in our numerical models are quite insensitive to the rheology boundary conditions and exact mesh geometry employed the general types of structures observed in the laboratory models depend only weakly upon the strength and the geometry of the backstop these results suggest that a detailed picture of an underlying backstop cannot be determined from surface information alone backstops in which the contact with the accretionary wedge dips arcward rather than trenchward however should produce only slightly different forearc structures with less development of both the outer arc high and the landward vergent inner deformation belt although natural forearcs are far more complex than our simple models they exhibit many of the same features indicating that relatively simple backstop mechanics may be a very important factor in the overall growth of forearcs

mid holocene age obtained for nested diamond pattern petroglyph in the billasurgam cave complex kurnool district southern india india has one of the world s largest and most significant bodies of rock paintings and engravings yet not a single rock art site or image has been directly and accurately dated using radiometric techniques here we report on results from the billasurgam cave complex near kurnool in southern india although this cave complex has been investigated archaeologically since the late 1800s it was not until 2008 that a large petroglyph consisting of the remains of three nested diamond designs on a stalactite was noted in order to determine if this petroglyph had been made recently flowstone was sampled from on top of and below the engraving radiocarbon dating revealed a mid holocene age of about 5000 cal bp for the petroglyph but we cannot rule out the possibility that the engraving is several centuries younger similar nested diamond designs at some rock painting sites and on a chert core elsewhere in india have been assumed to be mesolithic our result is consistent with this hypothesis although we note that it also consistent with the creation of the petroglyph in the early neolithic we conclude that the billasurgam engraved diamond design was probably made by mesolithic foragers of the kurnool region and is the oldest surviving form of rock art yet directly dated in southern india

effect of overburden thickness on thrust belt geometry and development laboratory sand models simulating development of thrust belts indicate that 1 distance between the backstop and the first formed thrust is a linear function of initial sand layer thickness 2 spacing between the first two thrusts is most likely a nonlinear function of initial sand layer thickness 3 initial dip of successive ramps decreases progressively toward the foreland because of variations in principal stress directions with depth and or changes of mechanical properties of sand with increasing overburden 4 faults to the hinterland steepen and become less active as hinterland thrust sheets undergo penetrative shear during sand wedge growth and 5 new ramp initiation occurs to the hinterland of the basal detachment tip line these observations can be interpreted both in context of area balance considerations and soil mechanics theory if initial sand layer thicknesses in our models represent syntectonic overburden in natural thrust belts i e distance between the ground surface and the detachment then our results suggest that locations of some thrust belt salients are controlled by along strike variations in stratigraphic thicknesses and that initial thrust spacing decreases progressively to the foreland if the thrust belt involves a foreland tapering basin furthermore our sand model observations suggest that the concept of a fixed rigid backstop in the hinterland of thrust belts may be misleading as a consequence of thickening the hinterland portion of the thrust belt becomes stabilized and effectively becomes the backstop itself

introduction to u series geochemistry during the last century the earth sciences underwent two major revolutions in understanding the first was the recognition of the great antiquity of the earth and the second was the development of plate tectonic theory these leaps in knowledge moved geology from its largely descriptive origins and established the modern quantitative earth sciences for any science and particularly for the earth sciences time scales are of central importance until recently however the study of time scales in the earth sciences was largely restricted to the unraveling of the ancient history of our planet for several decades earth scientists have used a variety of isotope chronometers to unravel the long term evolution of the planet a fuller understanding of the physical and chemical processes driving this evolution often remained elusive because such processes occur on time scales 1 105 years which are simply not resolvable by most conventional chronometers the u series isotopes however do provide tools with sufficient time resolution to study these earth processes during the last decade the earth sciences have become increasingly focused on fundamental processes and u series geochemistry has witnessed a renaissance with widespread application in disciplines as diverse as modern oceanography and igneous petrology the uranium and thorium decay series contain radioactive isotopes of many elements in particular u th pa ra and rn the varied geochemical properties of these elements cause nuclides within the chain to be fractionated in different geological environments while the varied half lives of the nuclides allows investigation of processes occurring on time scales from days to 105 years u series measurements have therefore revolutionized the earth sciences by offering some of the only quantitative constraints on time scales applicable to the physical processes that take place on the earth the application of u series geochemistry to the earth sciences was thoroughly summarized in 1982

petrology and provenance of the cretaceous kodiak formation alaska petrographic analysis of 30 sandstone samples from the cretaceous kodiak formation indicates that they consist mainly of intermediate to basic volcanic and plutonic detritus derived from a source area east of kodiak island this source area had extensive volcanic cover with rare exposure and weathering of cogenetic plutons the petrology of the kodiak formation is similar to that of related rocks in the alaskan peninsula and is indicative of the longevity of the volcano plutonic source area

earthquakes along the ryukyu kyushu arc strain segmentation lateral compression and the thermomechanical state of the plate interface we systematically determined the focal depths and mechanisms of 49 large to moderate sized earthquakes mb 5 4 that occurred along the ryukyu kyushu arc since 1963 by inverting the waveforms and amplitudes of p and sh wave trains at teleseismic distances the results are sufficiently precise to delineate seismogenic structures near the plate interface in detail revealing features not predicted by plate kinematics in contrast to previous studies of this arc and those along other subduction zones shallow seismicity along the plate interface is systematic showing two distinct layers of activities the second layer of seismicity is delineated by a few earthquakes that occurred at depths between 50 and 65 km some 10 20 km directly beneath the seismogenic portion of the interplate thrust zone these earthquakes indicate lateral compressional strain within the subducted slab as their p axes are subparallel to the local strike of the arc not perpendicular to the arc as one would expect from the direction of plate convergence the occurrence of these events cannot be accounted for by membrane stress due to the geometry of the subducted slab to our knowledge similar earthquakes occurred only beneath the northern indoburman ranges and along the banda arc where subducted slabs as part of the indian plate are being dragged northward with their northern edges bumping into east west trending collision zones nearby by drawing an analogy between the tectonic settings of these three regions we interpret events beneath the plate interface along the ryukyu kyushu arc as a consequence of ongoing collision between the philippine sea plate and eurasia near taiwan the interplate thrust zone is largely aseismic down to a depth of approximately 30 km a large number of earthquakes showing low angle thrust faulting commence at this depth and are accompanied by two events that show antithetic thrust faulting at a slightly shallower depth of 20 25 km these observations suggest that the strength of the plate interface increases significantly below the depth of proximately 25 5 km based upon available heat flow measurements between the trench axis and the volcanic arc we estimated the temperature field and magnitude of shear traction along the interplate thrust zone the deepest interplate earthquakes 40 50 km correspond to temperatures of approximately 730 980 c comparable to the limiting temperature of intraplate mantle earthquakes 800 c and that of intermediate and deep focus earthquakes at wadati benioff zones potential temperature 900 k therefore the cessation of seismicity in general is probably controlled by similar temperature conditions regardless of the tectonic settings of source regions the magnitude of shear traction along the seismogenic portion 40 50 km depth of the interplate thrust zone seems to be of the order of 100 mpa which in turn implies a low average coefficient of friction of only 0 10 0 05 considerably less than those observed for laboratory specimens the inception of intermediate focus earthquakes within the subducted slab is at a depth of about 100 km whereas shallow earthquakes show no apparent variations along the entire arc intermediate depth earthquakes indicate downdip extension along the northern end of the arc near kyushu but abruptly change to downdip compression along the rest of the arc all available evidence indicates that this is a spatial pattern unrelated to earthquake cycles at the moment tectonic interpretation of such a distinct strain segmentation along the arc is enigmatic without the presence of clear discontinuities in the age of the subducted slab the sudden switch in the strain field of the slab is difficult to explain by the continuously varying rate of subduction along the arc in any case this pattern of strain segmentation requires the presence of a major accommodation structure such as a tear fault in the subducted slab near the tokara channel appendix is available with entire article on microfiche order from american geophysical union 2000 florida ave n w washington dc 20009 document b91 004 2 50 payment must accompany order

episodic accretion and strain partitioning in a model sand wedge we model led thrust wedge accretion and deformation partitioning in a passively layered sand detached and shortened above a smooth rigid decollement the thrust wedge accretes in piggyback style during forward advance of a rear wall the passively layered sand accommodates the shortening penetratively by kinking ramp folding and by imbrication in single layers the style of compression changes with time and or stratigraphic position within the sand prism initial penetrative layer shortening above a slipped zone of decollement is succeeded by formation of monoclinal or conjugate shear bands at temporary terminations of decollement the shear bands subsequently narrow down and lock as thrust ramps the model sand wedge accrets episodically rather than steadily episodicity is controlled by the stick slip mode of decollement propagation and by volume loss and compaction of the wedge material in response to convergence this results in formation of a wedge with a convex upwards cross sectional topography

identification of the static backstop and its influence on the evolution of the accretionary prism in the nankai trough abstract to reveal the origin of a backstop and its influence on the evolution of the accretionary prism we analyzed reflection seismic data acquired in the nankai trough off the kii peninsula the deformation features of the forearc basin sequence show that the landward accretionary prism close to the coast was not deformed after the development of the forearc basin about 2 4 ma the surface of the landward prism can be identified as strong amplitude reflector indicating that the landward prism has higher seismic velocity therefore the landward accretionary prism inferred to be of higher strength constitutes a static backstop based on seismic and geologic observations we interpret that the backstop was generated due to the large age differences of accreted material resulting from an inferred hiatus in subduction between 13 and 6 ma the time dependent processes such as the igneous activity in middle miocene further contribute to the development of the backstop a ridge structure beneath the forearc basin located trenchward of this backstop and running roughly parallel to it appears to reflect activity on an ancient splay fault the strike of the ancient splay fault runs parallel to the backstop identified in this study and oblique to the current trench this geometry suggests that location and mechanical behavior of this splay fault system is influenced by the backstop and its distribution could be related to the coseismic rupture area

loci and maximum size of thrust earthquakes and the mechanics of the shallow region of subduction zones earthquakes do not extend updip along the plate interface at most subduction zones all of the way to the plate boundary at the trench axis or deformation front rather the shallowest part of that interface moves mainly through stable aseismic slip that part of the plate boundary referred to here as the aseismic zone occurs along the base of an accreted wedge of young sediments the probable primary cause for the existence of this aseismic zone is the stable slip properties of the unconsolidated and semiconsolidated sediments in that zone subducted sediment is progressively dewatered and underplated to the base of the overriding plate through this process more consolidated rocks eventually come into contact at depth across both sides of the plate boundary from the point where sufficiently hard rock is found across the plate interface that interface will change its slip behavior to unstable stick slip sliding which is characteristic of consolidated material under most conditions this type of motion is accommodated seismically as episodic slip in large earthquakes the location of this transition to seismic behavior referred to as the seismic front marks the deep end of the aseismic zone and the top of the seismogenic zone i e that part of the plate interface that moves primarily in thrust earthquakes several convergent plate margins are discussed to illustrate the seismic front and the aseismic zone we find that the seismic front defined by smaller earthquakes that occur during the interval between large events is nearly the same as that for large events as inferred from the locations of their aftershocks the location of the seismic front is important for making estimates of the maximum possible size of thrust earthquakes along the plate boundary because it delimits the trenchward limit of the potential rupture area of these events the size of large thrust earthquakes is proportional to rupture area and to at least the cube of the downdip width w of that area thus by defining the location of the seismic front for convergent margins it is possible to make better estimates of w and hence to deduce the maximum size of future interplate earthquakes the average repeat time of such events is also related to w and so better estimates of average repeat time are possible from improved knowledge of w the hypothesis that the location of the seismic front is related to the maximum depth of subduction of unconsolidated sediment has implications for forearc mechanics we use a mechanical analysis laboratory modeling and multichannel seismic information to develop a simple model explaining the growth of forearcs the outer arc high or trench slope break that occurs arcward of most accretionary wedges represents an abrupt change in the critical taper of the accretionary wedge we argue that that change is caused by a large arcward increase in the strength of the material within the overriding plate that stronger material is called the backstop our laboratory modeling experiments indicate that a backstop with a trenchward dipping upper surface results in the development of an overlying structure that includes all of the primary morphologic features observed in modern forearcs in our modeling an accretionary wedge develops trenchward of the backstop an outer arc high develops above the trenchward toe of the backstop and farther arcward a passive forearc basin forms above the stronger material of the backstop this model is consistent with the observation that earthquakes do not extend updip along the plate interface all the way to the trench axis we hypothesize that plate motion is accommodated seismically along the base of the consolidated backstop and mostly or entirely aseismically trenchward of the toe of the backstop along the base of the accretionary wedge at several margins the seismic front is approximately coincident with the outer arc high supporting this interpretation

impact of atmospheric intraseasonal variability in the indian ocean low frequency rectification in equatorial surface current and transport an ocean general circulation model ogcm is used to investigate the low frequency period longer than 90 days rectification of atmospheric intraseasonal variability 10 90 day periods in zonal surface current and transport of the equatorial indian ocean a hierarchy of ogcm solutions is found in an actual tropical indian ocean basin for the period of 1988 2001 to help to identify and isolate nonlinear processes a linear continuously stratified model and a 4 layer intermediate ocean model are also used results from the ogcm solutions suggest 1 2 that intraseasonal atmospheric forcing acts to weaken the equatorial seasonal surface currents amplitudes of the spring and autumn eastward surface jets the wyrtki jets wj and the westward surface current during january march are reduced by as much as 15 25 cm s21 by intraseasonal atmospheric forcing and strengths of the rectification exhibit a significant interannual variability important processes that cause the low frequency rectification are asymmetric response of mixed layer depth to easterly and westerly winds entrainment and upwelling of momentum during spring and autumn the westerly easterly phase of an intraseasonal event enhances weakens or even reverses the seasonal westerly winds increases decreases equatorial convergence and entrainment and thus deepens thins the mixed layer a net westward current is generated over an event mean because easterly wind acts on a thinner surface mixed layer whereas westerly wind acts on a thicker one in contrast during january march when the seasonal winds are equatorial easterlies surface currents are westward and equatorial undercurrents euc develop the rectified surface currents are eastward which reduces the westward surface flow this eastward rectification results largely from the vertical advection and entrainment of the euc the seasonal to interannual variability of the rectified surface flow is determined primarily from the seasonal cycle and interannual variability of the background state seasonal to interannual variability of the intraseasonal wind forcing also contributes the rectified low frequency zonal volume heat transports integrated over the entire water column along the indian ocean equator are persistently eastward with an amplitude of 0 15 3 106 m3 s 21 0 1 2 pw this is because westerly winds generate equatorial downwelling advecting the surface eastward momentum downward and giving an eastward subsurface current easterly winds cause equatorial upwelling and produce an eastward pressure gradient force that drives an eastward subsurface current this eastward subsurface current is advected upward by upwelling the mean effect over an intraseasonal event at the equator is to increase the eastward transport in the water column in the layers above the thermocline the rectified zonal volume heat transports are in the same direction as the rectified surface currents results from this paper may have important implications for understanding climate variability because modification of wj strength and transports can affect the sst and heat storage in the equatorial indian ocean warm pool

a review of rock art dating in the kimberley western australia this paper critically reviews the various approaches used to estimate the age of the rock art in the kimberley region of western australia they include i the relative superimposition of styles ii the use of diagnostic subject matter depictions of extinct animals stone tool technology introduced european and asian objects and animals iii the recovery of a painted slab from a dated archaeological unit iv radiocarbon dating of beeswax figures charcoal pigments organic matter in overlying mineral deposits and accreted paint layers oxalate rich crusts and amorphous silica skin pollen grains from an overlaying mud wasp nest and v optically stimulated luminescence osl dating of quartz grains from overlying mud wasp nests future directions for rock art dating in the kimberley include uranium series dating of overlying and underlying mineral deposits

characteristics of pleistocene megafauna extinctions in southeast asia the extinction of large bodied taxa from the pleistocene in southeast asia is examined although the chronological resolution of these extinctions is poor and number of excavations in the region relatively few broad characteristics of these extinctions can be described many taxa which became extinct appear to have been endemic to regions within southeast asia while some taxa which experienced extinction or severe range reduction occurred in several regions members of the latter group include proboscideans stegodon and palaeloxodon the pygmy hippopotamus hexaprotodon the orangutan pongo hyenas crocuta and hyaena the giant panda ailuropoda tapirs tapirus and megatapirus rhinoceroses rhinoceros and the giant asian ape gigantopithecus the loss of these species cannot be assigned to a single cause rather their disappearance is likely tied to both climatic and human agents unlike other regions which experienced megafauna extinctions eustatic changes in sea level in southeast asia seems to have been an important factor

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

basement slivers within the new britain accretionary wedge implications for the emplacement of some ophiolitic slivers using multichannel seismic data from the western solomon sea we have imaged several 2 5 to 3 km thick oceanic basement slivers within the new britain accretionary wedge which may serve as a modern analogue for the detachment of some ophiolitic slivers the most recently accreted basement sliver is located just over 5 km from the thrust front an approximate area balance indicates the subjacent section of the downgoing plate from which this sliver originated is between 11 km and 15 km arcward of its present position though we have not imaged the basement ramp sedimentary strata beneath the basement sliver pinch out between 9 and 12 km from its seaward edge suggesting that detachment occurred in that vicinity normal faults reactivated in response to the flexural bending of the downgoing oceanic plate are a prominent feature of this region significantly almost all basement normal faults near the recently accreted basement sliver including those imaged beneath the toe of the wedge dip arcward based on these data we suggest that small 1 3 km thick basement slivers are decoupled along favorably oriented preexisting zones of weakness formed by normal fault detachments within the oceanic basement

heat flow and thermal structure of the washington oregon accretionary prism a study of the lower slope heat flow measurements across the washington oregon convergent margin show several characteristic features a well defined peak at the foot of the overthrust a gradual decrease landward across the lower slope reaching a minimum at the ridge and a gradual rise further landward constrained by geological and seismic evidence on the rate of stacking and the dimension of stacked thrust sheets and by drilling results on sediment properties we use a two dimensional time dependent finite element procedure to model the stacking of overthrust slices of sediments and to calculate the thermal structure beneath the lower slope of the accretionary prism the calculated heat flow profile is in good agreement with the measurements advective heat transfer due to stacking of overthrust sheets and topographic changes are shown to play a dominant role in controlling the surface heat flow and the thermal state of the accretionary prism

deepwater fold and thrust belt classification tectonics structure and hydrocarbon prospectivity a review abstract deepwater fold and thrust belts dwftbs are classified into near field stress driven type 1 systems confined to the sedimentary section and type 2 systems deformed by either far field stresses alone or mixed near and far field stresses dwftbs can occur at all stages of the wilson cycle up to early stage continent continent collision type 1 systems have either weak shale or salt detachments they occur predominantly on passive margins but can also be found in convergent related areas such as the mediterranean and n borneo examples include the niger and nile deltas the west coast of africa and the gulf of mexico type 2 systems are subdivided on a tectonic setting basis into continent convergence zones and active margin dwftbs continent convergence zones cover dwftbs developed during continent arc or continent continent collision and those in a deepwater intracontinental setting e g w sulawesi makassar straits active margins include accretionary prisms and transform margins the greatest variability in dwftb structural style occurs between salt and shale detachments and not between tectonic settings changes in fold amplitude and wavelength appear to be more related to thickness of the sedimentary section than to dwftb type in comparison with shale salt detachment dwftbs display a lower critical wedge taper more detachment folds long and episodic duration of deformation and more variation in vergence structures unique to salt include canopies and nappes accretionary prisms also standout from other dwftbs due to their relatively long continuous duration rapid offshore propagation of the thrust front and large amount of shortening in terms of petroleum systems many similar issues affect all dwftbs these include the oceanward decrease in heat flow offshore increase in age of mature source rock and causes of trap failure e g leaky oblique and frontal thrust faults breach of top seal by fluid pipes one major difference between type 1 and type 2 systems is reservoir rock high quality continent derived quartz rich sandstones are generally prevalent in type 1 systems more diagenetically reactive minerals derived from igneous and ophiolitic sources are commonly present in type 2 systems or many are simply poor in well developed turbidite sandstone units however some type 2 systems particularly those adjacent to active orogenic belts are partially sourced by high quality continent derived sandstones e g nw borneo s caspian sea columbus basin in some cases very high rates of deposition in accretionary prisms adjacent to orogenic belts coupled with uplift due to collision results in accretionary prism related fold belts that pass laterally from sub aerial to deepwater conditions e g s caspian sea indo burma ranges the six major hydrocarbon producing regions of dwftbs worldwide gulf of mexico niger delta nw borneo brazil west africa s caspian sea stand out as differing from most other dwftbs in certain fundamental ways particularly the very large volume of sediment deposited in the basins and or the great thickness and extent of salt or overpressured shale sdetachments

seasonal to interannual time scale dynamics of the equatorial undercurrent in the indian ocean abstractthis paper investigates the structure and dynamics of the equatorial undercurrent euc of the indian ocean by analyzing in situ observations and reanalysis data and performing ocean model experiments using an ocean general circulation model and a linear continuously stratified ocean model the results show that the euc regularly occurs in each boreal winter and spring particularly during february and april consistent with existing studies the euc generally has a core depth near the 20 c isotherm and can be present across the equatorial basin the euc reappears during summer fall of most years with core depth located at different longitudes and depths in the western basin the euc results primarily from equatorial kelvin and rossby waves directly forced by equatorial easterly winds in the central and eastern basin however reflected rossby waves from the eastern boundary play a crucial role while the first two baroclinic modes make the largest contribution intermediate modes 3 8 are also

landward vergence in accretionary prisms the role of the backstop and thermal history the accretionary histories recorded along the convergent margins in southwest alaska and southwest japan are punctuated by two anomalous events near trench magmatism closely associated in time and space with landward structural vergence in both areas the near trench magmatism is believed to be related to the subduction of an active spreading center it is postulated that these anomalous thermal events caused the backstop i e mechanical boundary within the prism to be reoriented to a seaward instead of a landward dip this seaward dipping backstop resulted in the development of landward verging structures for the only time in the almost 100 m y history of these two margins a modern analog may be the washington convergent margin these examples illustrate the potential importance of thermal history and the dip of the backstop in controlling the style of accretion in submarine wedges

radiocarbon dating of prehistoric rock paintings by selective oxidation of organic carbon dating of prehistoric rock paintings pictographs has traditionally relied on indirect evidence this includes inferences based on the archaeological context such as superpositions of pictorial styles1 and the depiction of images that constrain their ages1 2 as well as dating of deposits that either cover the art in situ3 4 or contain separated fragments of the painted surface5 migration of ions between the bulk rock and the natural coatings that form on a newly exposed surface has also been exploited to date petroglyphs rock carvings in desert regions6 13 until recently14 17 however direct dating by radiocarbon techniques of pictographs has not been possible18 19 mainly because of the problem of separating inorganic carbon from the organic material in the pigments here we report on a new technique which allows this separation to be effected by using a low temperature low pressure oxygen plasma to oxidize selectively the organic component this may then be analysed using standard14c methods we have applied this technique to a portion of a pictograph from the lower pecos region of southwest texas fig 1 the date obtained 3 865 100 yr bp before present is consistent with that expected on the basis of archaeological inference20 as organic carbon is a ubiquitous component of pictograph paints this technique should be applicable to rock paintings throughout the world

variability of zonal currents in the eastern equatorial indian ocean on seasonal to interannual time scales this study examines equatorial zonal current variations in the upper layers of eastern indian ocean in relation to variations in the indian ocean dipole iod the analysis utilizes data from the research moored array for african asian australian monsoon analysis and prediction rama and the european centre for medium range weather forecasts ocean reanalysis system 4 ecmwf oras4 surface currents are characterized by semiannual eastward flowing wyrtki jets along the equator in boreal spring and fall forced by westerly monsoon transition winds the fall jet intensifies during negative iod niod events when westerlies are anomalously strong but significantly weakens during positive iod piod events when westerlies are anomalously weak as zonal wind stress weakens during piod events sea surface height becomes unusually low in the eastern basin and high in the west setting up an anomalous pressure force that drives increased eastward transport in the thermocline opposite tendencies are evident during niod events in response to intensified equatorial westerlies current transport adjustments to anomalous zonal wind forcing during iod events extend into the following year consistent with the cycling of equatorial wave energy around the basin a surface layer mass budget calculation for the eastern sea surface temperature sst pole of the iod indicates upwelling of 2 9 0 7 sv during normal periods increasing by 40 50 during piod events and reducing effectively to zero during niod events iod related variations in wyrtki jet and thermocline transports are major influences on these upwelling rates and associated water mass transformations which vary consistently with sst changes

evidence for fluid circulation overpressure and tectonic style along the southern chilean margin the southern chilean convergent margin between 50 and 57 s is shaped by the interaction of the three main plates antarctic south america and scotia north of 53 s the convergence between antarctic and south america plates is close to orthogonal to the continental margin strike here the deformational style of the accretionary prism is mainly characterized by seaward verging thrusts and locally by normal faults and fractures a very limited lateral extension of prism a very shallow dip 6 decollement and subduction of a thick and relatively undeformed trench sedimentary sequence south of 53 s convergence is oblique to the margin locally the trench sediments are proto deformed by double vergence thrusting and the front of the prism grows through landward verging thrusting the decollement is sub horizontal and deep involving most of the sediment over the oceanic crust in the accretionary process building a comparatively wide and thicker prism a bottom simulating reflector is present across the whole prism to the abyssal plan suggesting the presence of gas in the sediments the analysis of p and s wave velocity reflectivity sections derived by amplitude versus offset technique avo detailed velocity information and the velocity derived sediment porosity have been integrated with the structural analysis of the accretionary prism of two selected pre stack depth migrated seismic lines aiming to explain the relation between fluid circulation and tectonics accretion along double vergence thrust faults may be associated with the presence of overpressured fluid which decreases the effective shear stress coefficient along the main decollement and within the sediments and modify the rheolgical properties of rocks the presence of an adequate drainage network represented by interconnected faults and fractures affecting the entire sedimentary sequence can favour the escape of pore fluid toward the sea bottom while less permeable and not faulted sediments can favour fluid accumulations gravitational and tectonic dewatering and stratigraphy could control the consolidation and the pore overpressure of sediments involved in subduction along the trench the results of our analysis suggest the existence of a feedback between tectonic style and fluid circulation

non coulomb wedges wrong way thrusting and natural hazards in cascadia landward vergence in accretionary wedges is an uncommon phenomenon not readily explained by classical mohr coulomb critical wedge theory predominantly landward vergent thrust faults are observed along the cascadia convergent margin from 45 n to 48 n we present depth migrated multichannel seismic images of the internal structure of the accretionary wedge offshore washington collected during the orwell project in 1996 these reveal a high p wave velocity 4 km s basal layer that thickens landward and serves as a decollement for a series of overlying landward vergent thrust faults analog modeling using a ductile basal layer consisting of silicone putty produces an array of trenchward propagating landward vergent thrusts and offers a plausible mechanical model for the evolution of these structures the rheological properties of a basal calcareous mudstone layer offshore cascadia are discussed in relation to the mechanics of landward vergence and to rapid loading due to the prograding nitinat and astoria deep sea fans a viscoelastic layer beneath the accretionary wedge is considered to be capable of rupturing during great interplate earthquakes and thus represents an increased risk for both the maximum size of such an event and for the generation of tsunamis

upward delamination of cascadia basin sediment infill with landward frontal accretion thrusting caused by rapid glacial age material flux the cascadia convergent margin is a first order research target to study the impact of rapid sedimentation processes on the mechanics of frontal subduction zone accretion the near trench part of the accretionary prism offshore washington is affected by strongly increased glacial age sedimentation and fan formation that led to an outstanding quaternary growth rate with landward vergent thrust faulting that is rarely observed elsewhere in accretionary wedges multichannel seismic reflection data acquired on the orwell project allows us to study the structure and dynamics of the atypical frontal accretion processes we performed a kinematical and mechanical analysis of the frontal accretion structures and developed a dynamic coulomb wedge model for the landward verging backthrust formation backthrusting results from heterogeneous diffuse strain accumulation in the mechanically heterogeneous cascadia basin sediment succession entering the subduction zone and strain partitioning along a midlevel detachment that is activated by gravitational loading caused by rapid glacial age sedimentation these complex deformation processes cause the passive upward delamination of the upper turbidite beds from the basal pelagic carbonate section similar to triangle zone formation and passive backthrust wedging in foreland thrust belts caused by rapid burial beneath syntectonic sediment deposits the deformation mechanism at the tectonic front of the cascadia margin is an immediate response to the strongly increased late pleistocene sediment flux rather than to atypical physical boundary conditions as generally thought

synchronous and velocity partitioned thrusting and thrust polarity reversal in experimentally produced doubly vergent thrust wedges implications for natural orogens a sandbox analog modeling research program was used to study the detailed evolution of doubly vergent thrust wedges high resolution multilayers of thin alternating sand and mica layers were used in contractional coulomb wedge experiments to simulate deformation of anisotropic brittle upper crustal strata in doubly vergent orogens experiments incorporating syntectonic sedimentation in foreland basins and in piggyback basins were also carried out our laboratory models evolved in two main stages 1 initial high velocity thrusting in the retrowedge and high frequency together with low displacement folding and thrusting in the prowedge and 2 low frequency high displacement synchronous thrusting in the prowedge and low velocity thrusting in the retrowedge transition from stage i to stage ii occurred when the growing wedges reached the critical height at which they behaved as a backstop for further prowedge accretion addition of syntectonic sediments increased the persistence of stage i and triggered out of sequence thrusting in the axial zone of the experimental orogens thrust motion was stick slip retrovergent thrusting occurred along a long lived ramp whose lower tip was located at the subduction slot provergent kink bands nucleated at the subduction slot in stage i in contrast during the second stage of wedge evolution kink bands nucleated in a piggyback fashion in the foreland far from the subduction slot and then evolved into high displacement faults that remained active up until the end of the experiments at progressively decreasing rates of thrusting the axial zones of the model wedges were characterized by fast uplift rates during stage i due to backward translation of the belt along the retrovergent long lived ramp and due to the localization of deformation close to the subduction slot outward migration of the deformation front in the prowedge region during stage ii caused the progressive decrease in the rate of the wedge uplift until it eventually stopped analytical models to quantify the coulomb behavior in the wedges validated the reversal of thrust polarity with increasing shortening triggered by the buildup of the topographic load during deformation this thrust polarity reversal highlights problems with the classic concept of back thrusting as a reflection of a significant change in the deformation regime our results compare well with the kinematic evolution of natural accretionary prisms and of thrust and fold belts such as the lesser antilles arc system the mediterranean ridge and the pyrenees