recent and pleistocene beach dune sequences western australia abstract wave dominated sandy shores occur along much of the coast of western australia despite local variations there is a characteristic distribution of lithofacies corresponding to different geomorphic zones five lithofacies are recognised 1 trough bedded sand gravel 2 laminated sand 3 laminated bubble sand 4 laminated disrupted sand and 5 aeolian cross stratified sand the trough bedded sand gravel lithofacies is being deposited in the shallow shoreface below lwl the laminated sand and laminated bubble sand lithofacies are sands with gravel layers being deposited on the foreshore swash zone extensive bubble or vesicular sand is common towards hwl especially in berms the laminated disrupted sand lithofacies is being deposited on the backshore between hwl and storm water levels and consists of horizontally layered to homogeneous sands with storm debris especially wood weed and floatable skeletons e g sepla and spirula the aeolian cross stratified sand lithofacies is forming in beach ridge dune areas and consists of fine sands with large scale generally landward dipping forests soils and rootlets are common recognition of these lithofacies within a sedimentary sequence enables reconstruction of gross shoreline conditions in terms of wave and eolian environments tidal and storm heights and palaeogeography each of these lithofacies with their characteristic features is recognised in pleistocene sequences in perth basin the pleistocene sequences fit a model of coastal progradation with the trough bedded sand gravel lithofacies at the base and the aeolian sand lithofacies at the top the value of such a stratigraphic sequence however extends beyond the pleistocene

deformation textures and mechanical behavior of a hydrated amorphous silica formed along an experimentally produced fault in chert 1 we conducted a series of friction experiments on chert at intermediate to high slip velocities v 0 87 104 mm s and at low normal stress n 1 5 mpa to better understand the process of fault weakening by silica gel formation as identified in previous friction experiments on quartz rocks fault weakening in chert samples occurred in association with the formation of a 0 1 mm thick fault gouge layer that contains a thin foliated layer of fine grained material and a cataclastically fragmented zone characterized by clast matrix texture mft ir and xrd analyses reveal that the fault gouge consists of a mixture of hydrated amorphous silica and quartz grains the mechanical behavior of the fault gouge at a small magnitude of strain 0 008 as examined independently of the friction experiments using a rheometer is characterized by a negative dependence of shear stress on strain rate with a notable hysteresis behavior

revision of the late pleistocene stratigraphy of bermuda the quaternary stratigraphy of bermuda is one of the world s most complete sedimentary records of interglacial highstands representing at least the past million years yet in terms of the last interglacial rocky bay formation marine isotope substage mis 5e only scanty deposits are preserved in contrast mis 7 generally regarded as a diminutive interglaciation exposes widespread emergent subtidal deposits of the belmont formation indicating a prolonged sea level highstand at ca 2 5 0 3 m this belmont paradox has prompted a reexamination of the geology of key sites along the south shore of bermuda this revision of the late quaternary stratigraphy of bermuda is based on geological field observations primarily the questionable origin of red soil like deposits it is concluded here that there is no physical evidence of the exposure of the upper belmont surface for a full glacial cycle 70 100 ka and that interbedded reddish soil like lenticular deposits are the result of colluvial activity during mid mis 5e reexamination of previously published uranium series u th ages indicates that several 3 m notches contain a mix of ca 125 ka and older coral ages contrary to previous works this study favors an early mis 5e interpretation of the coral ages from the 2 5 m belmont shoreline which is further supported by amino acid racemization aar ratios on whole rock poecilozonites glycymeris and brachiodontes samples from shore and notch deposits an updated aar kinetic model demonstrates that the bulk of epimerization occurred during warm interglacial intervals between 130 and 80 ka and the last 10 ka of the holocene this revised stratigraphy of bermuda now falls into accordance with a more general view of middle and late pleistocene sea level history

boulder deposits from large waves during the last interglaciation on north eleuthera island bahamas abstract seven boulders measuring 100 to 1000 m 3 are scattered along the coastal ridge of north eleuthera some are situated on ridge crests up to 20 m above present sea level the boulders were probably deposited during oxygen isotope substage 5e or 5d as shown by their stratigraphic setting and by amino acid racemization ratios d alloisoleucine l isoleucine ratios were determined for land snails and oolite of both marine and eolian origin was associated with the boulders like the boulders the probable source rocks exposed in the adjacent cliffs are composed of marine and eolian limestone of oolitic and peloidal composition the source beds are correlated with stage 9 or 11 the largest boulder is about 10 times the size of the largest holocene ones moved by waves in the area tsunamis are a reasonable possibility as a transporting mechanism of the pleistocene boulders however if deposited instead by storms during the last interglaciation the storms were of much greater intensity than those occurring in the region during the late holocene

chevron ridges and runup deposits in the bahamas from storms late in oxygen isotope substage 5e abstract landward pointing v shaped sand ridges several kilometers long are common along the windward margin of the bahama islands their axes share a northeast southwest trend internally the ridges contain low angle oolitic beds with few erosional truncations commonly interbedded are tabular fenestrae rich beds such as those formed by the sheet flow of water over dry sand defined here as chevron ridges these landforms appear to have originated in the rapid remobilization of bank margin ooid bodies by the action of long period waves from a northeasterly source deposits along adjacent coastlines also preserve evidence of the impact of large waves reworked eolian sand bodies preserve beach fenestrae and hydraulic scour traces up to 40 m on older ridges on cliffed coasts 1000 ton boulders have been thrown well inland recording the impact of large waves amino acid ratios confirm a correlation of the ridges across the archipelago while stratigraphy spacing and cross cutting relationships indicate emplacement as sea level fell rapidly from the substage 5e maximum at or above 6 m

development of the flow field during the onset of the somali current 1979 abstract during the spring and summer of 1979 a multi ship survey studied the changes in currents along the east african coast in response to the transition from the northeast to the southwest monsoon the somali current in late april and early may flowed southwestward along the coast from northern somalia to about the equator surface currents were 50 100 cm s 1 and the transport was 3 106 m3 s 1 south of the equator the east african coast current eacc flowed northward the two currents met in the vicinity of the equator and turned offshore to the southeast surface velocities in the eacc were about 200 cm s 1 and its transport in the top 100 m was 15 106 m3 s 1 with the initial onset of southerly winds in early may the somali current reversed by mid may at 3 n surface speeds of 200 cm s 1 were observed the flow did not continue up the coast but turned offshore by 4 n the second onset of the monsoon took place in mid june shortly after this surface currents 350 cm s 1 were observed at the tur

processes of 30 90 days sea surface temperature variability in the northern indian ocean during boreal summer during summer the northern indian ocean exhibits significant atmospheric intraseasonal variability associated with active and break phases of the monsoon in the 30 90 days band in this paper we investigate mechanisms of the sea surface temperature sst signature of this atmospheric variability using a combination of observational datasets and ocean general circulation model sensitivity experiments in addition to the previously reported intraseasonal sst signature in the bay of bengal observations show clear sst signals in the arabian sea related to the active break cycle of the monsoon as the atmospheric intraseasonal oscillation moves northward sst variations appear first at the southern tip of india day 0 then in the somali upwelling region day 10 northern bay of bengal day 19 and finally in the oman upwelling region day 23 the bay of bengal and oman signals are most clearly associated with the monsoon active break index whereas the relationship with signals near somali upwelling and the southern tip of india is weaker in agreement with previous studies we find that heat flux variations drive most of the intraseasonal sst variability in the bay of bengal both in our model regression coefficient 0 9 against 0 25 for wind stress and in observations 0 8 regression coefficient 60 of the heat flux variation is due do shortwave radiation and 40 due to latent heat flux on the other hand both observations and model results indicate a prominent role of dynamical oceanic processes in the arabian sea wind stress variations force about 70 100 of sst intraseasonal variations in the arabian sea through modulation of oceanic processes entrainment mixing ekman pumping lateral advection our 100 km resolution model suggests that internal oceanic variability i e eddies contributes substantially to intraseasonal variability at small scale in the somali upwelling region but does not contribute to large scale intraseasonal sst variability due to its small spatial scale and random phase relation to the active break monsoon cycle the effect of oceanic eddies however remains to be explored at a higher spatial resolution

runaway slip to the trench due to rupture of highly pressurized megathrust beneath the middle trench slope the tsunamigenesis of the 2011 tohoku earthquake off the east coast of northern japan abstract the gigantic 2011 march 11 mw 9 tohoku earthquake is examined from the viewpoint of the pre seismic forearc structure the seismic reflection properties of a megathrust around the usual up dip limit of the seismogenic zone the thermal state of a shallow subduction zone and the dehydration of underthrust sediments at the japan trench the pacific plate is subducting westward beneath northeast japan at a dip angle of 4 6 the middle and lower slopes of the landward side dip eastward at angles of 2 5 and 8 0 respectively the forearc prism beneath the middle and lower slopes is inferred to be in extensionally and compressively critical states respectively based on the presence of clear internal deformation features and on the occurrence of aftershock earthquakes the rapid uplift of the forearc that caused the 2011 tohoku tsunami may have been associated with this internal deformation of the prism the critical state of the prism indicates that the effective basal friction b of the plate boundary megathrust is 0 08 for the lower prism the megathrust especially under the middle slope is characterized by a prominent reflector with negative polarity one of multiple possible explanations is that the megathrust hosts highly pressurized fluids underthrust sediments in this part of the japan trench are dominated by pelagic and siliceous vitric diatomaceous silt with clay the dehydration kinetics of opal a to quartz the clay transformation of smectite illite and the thermal structure of the japan trench suggest that maximum dehydration of the sediments would take place at 50 60 km horizontally from the deformation front where the temperature along the megathrust is 100 120 c the zone of maximum dehydration coincides with the prominent seismic reflector that has negative polarity we hypothesize a possible free slip along this portion of the megathrust during the 2011 tohoku earthquake caused by anomalously high fluid pressure resulting from fluid accumulation over centuries

microstructural evolution and deformation mechanisms of khao kho fault thailand abstract lithologies and microstructures of fault rocks from the khao kho fault in thailand are investigated to understand fault behavior and underlying deformation mechanisms fault gouges in the shear zone are mainly composed of kaolinite illite mica and illite smectite and exhibit weak preferred orientation ranging from 1 2 to 2 3 m r d these fault cores are characterized by high localized strain and intense deformation due to cataclasis and frictional granular flow mechanisms which produce transgranular fractures rounded quartz grains and an ultrafine clay matrix in contrast wall rocks show stronger clay preferred orientation up to 3 9 m r d despite having less clay content than the fault gouges much larger fault related bent and sheared clay grains are present in the wall rocks suggesting less severe deformation the discrepancy between fault cores and wall rocks indicate that the development of fabrics is related to faulting and authigenic processes weak fabrics in the slip zone are attributed to the comminution of grains and neomineralization allowing clay platelets to grow and rotate in many directions in addition the abundance of metal oxides indicates extensive multiple fluid alteration processes which further influence microstructures and fault properties

summer monsoon response of the northern somali current 1995 preliminary results on the development of the northern somali current regime and great whirl during the summer monsoon of 1995 are reported they are based on the water mass and current profiling observations from three shipboard surveys of r v meteor and on the time series from a moored current meter and adcp array the monsoon response of the gw was deep reaching to more than 1000m involving large deep transports the northern somali current was found to be disconnected from the interior arabian sea in latitude range 4 n 12 n in both water mass properties and current fields instead communication dominantly occurs through the passages between socotra and the african continent from moored stations in the main passage a northward throughflow from the somali current to the gulf of aden of about 5 sv was determined for the summer monsoon of 1995

low frictional strength of quartz rocks at subseismic slip rates 1 laboratory experiments on rocks at low sliding velocity typically yield values of the friction coefficient of 0 6 0 85 here we demonstrate that an extraordinary decrease in friction coefficient accompanies sliding of room dry quartz rocks at rates faster than in most laboratory experiments but slower than seismic slip rates in some cases the friction coefficient decreases from low speed values by more than a factor of 3 this extraordinary weakening likely results from the formation of finely comminuted amorphous wet material on the sliding surface

constructing constitutive relationships for seismic and aseismic fault slip for the purpose of modeling natural fault slip a useful result from an experimental fault mechanics study would be a physically based constitutive relation that well characterizes all the relevant observations this report describes an approach for constructing such equations where possible the construction intends to identify or at least attribute physical processes and contact scale physics to the observations such that the resulting relations can be extrapolated in conditions and scale between the laboratory and the earth the approach is developed as an alternative but is based on ruina 1983 and is illustrated initially by constructing a couple of relations from that study in addition two example constitutive relationships are constructed these describe laboratory observations not well modeled by ruina s equations the unexpected shear induced weakening of silica rich rocks at high slip speed goldsby and tullis 2002 and fault strength in the brittle ductile transition zone shimamoto 1986 the examples provided as illustration may also be useful for quantitative modeling

pin stripe lamination a distinctive feature of modern and ancient eolian sediments abstract pin stripe laminations are a distinctive feature of modern and ancient eolian sediments in sets of eolian ripple or translatent strata they represent deposition of silt and very fine sand in the troughs of the advancing wind ripples in sets of avalanche strata they probably result from the downward settling of fine sand and silt within the moving avalanche to the interface of moving and unmoving sands wind tunnel experiments suggest that pin stripe laminations can also form in grainfall deposits the textural segregation associated with deposition of the fine layers in most cases leads to early cementation along and near the finest sand and silt comprising the pin stripe lamination the pin stripe effect seen in outcrops is usually due to resistance to weathering along such cemented zones the cementation of the pin stripe laminations can occur early in the history of diagenesis and thus may provide clues to the post depositional history of the rock pin stripe laminations in many instances represent the sequestering of the small population of ultrafine sediment present in most eolian depositional systems they may prove useful in the recognition of ancient eolian sediments

palaeoclimatic significance of co occurring wind and water induced sedimentary structures in the last interglacial coastal deposits from bermuda and the bahamas abstract the late pleistocene stratigraphic record from the bahamas and bermuda archipelagoes includes peculiar v shaped coastal ridges and wedges of seaward dipping planar beds showing fenestral porosity up to 40 m above present sea level judging from this porosity these landforms were till now interpreted as resulting from the action of giant waves during a period of climatic instability at the end of the last interglacial period the occurrence of widespread mm thick laminae of aeolian origin subcritically climbing translatent stratification throughout these deposits does not agree with this hypothesis it rather suggests that the v shaped ridges and seaward dipping beds represent fossil parabolic and climbing aeolian dunes respectively the occurrence of fenestrae high above the intertidal zone could be related to both wave splashing and rainfall action parabolic dunes are restricted to the nw bahamas they were probably formed during a time interval ca 500 5000 yr when regional climatic conditions were dryer than today and characterized by persistently blowing ne trade winds

significance of interdune deposits and bounding surfaces in aeolian dune sands bounding surfaces and interdune deposits provide keys for detailed interpretations of the development shape type wavelength and angle of climb of aeolian bedforms as well as overall sand sea conditions current alternate interpretations of bounding surfaces require very different but testable models for sand sea deposition two perpendicular traverses of jurassic entrada sandstone utah reveal relations among cross strata first order bounding surfaces and horizontal strata these field relations seem explicable only as the deposits of downwind migrating climbing enclosed interdune basins horizontal strata and dune bodies consisting of superimposed smaller crescentic dunes cross stratified deposits a 1 7 km traverse parallel to the palaeowind direction provides a time transgressive view showing continuous cosets of cross strata first order bounding surfaces and interdune deposits climbing downwind at an angle of a few tenths of a degree changes occur in the angle of climb cross strata structure and interdune deposits these reflect changes in depositional conditions through time a 1 5 km traverse perpendicular to the palaeowind direction provides a view at an instant in geological time showing first order bounding surfaces and interdune deposits forming flat laterally discontinuous lenticular bodies the distribution of interdune sedimentary structures in this traverse is very similar to that of some modern interdune basins such as those on padre island texas hierarchies of bounding surfaces in an aeolian deposit reflect the bedform development on an erg the presence of three orders of bounding surfaces indicates dune bodies consisting of smaller super imposed dunes the geometry of first order bounding surfaces is a reflection of the shape of the inter dune basins second order bounding surfaces originate by the migration of the superimposed dunes over the larger dune body and reflect individual dune shape and type third order bounding surfaces are reactivation surfaces showing stages in the advance of individual dunes the presence of only two orders of bounding surfaces indicates simple dunes modern and entrada interdune deposits show a wide variety of sediment types and structures reflecting deposition under wet damp and dry conditions interdune deposits are probably the best indicators of overall erg conditions and commonly show complex vertical sequences reflecting changes in specific depositional conditions

chevrons origin and relevance for the reconstruction of past wind regimes abstract since its first use in the late 80 s the term chevron has been employed in numerous studies to describe large u and v shaped ridges found in or near shorelines worldwide most studies have so far focused on bahamian chevrons that are exclusively of late pleistocene age and on the supposed holocene chevrons found in s madagascar and australia in the bahamas these deposits have been interpreted as the product of extreme storms at the end of the last interglacial lig warm period in contrast the extensive chevrons complex exposed in s madagascar and on the western coast of australia have been associated with a tsunami induced by a meteorite impact finally several authors have also proposed a non catastrophic i e eolian origin based on the recognized importance of wind related processes in these coastal areas and term such deposits parabolic dunes in this paper we collect and synthesize existing data on the morphology sedimentology and age of these chevrons and review the different interpretations proposed in the literature with the aim to lay out a consistent database to assist further investigations on these important coastal morphologies in addition we generated a synthesis of wind data at the three study areas which highlights the relationship between present wind regimes and chevrons morphologies the ubiquity of chevrons likely actually parabolic dunes in coastal areas around the globe and their relationship with coastal processes makes them crucial archives for reconstructing past wind regimes

scalloped bank margins beginning of the end for carbonate platforms bathymetric seismic reflection and satellite data have revealed the common existence of a new type of carbonate bank margin scalloped margin throughout the gulf of mexico and the caribbean region these large up to tens of kilometres across irregular convexbankward embayments are interpreted as predominantly erosional features produced by large scale failure of platform margins in tectonically active areas they may be triggered by earthquake shocks whereas in stable regions they may more likely be the result of undercutting by dissolution and or deep water currents scalloped margins are common in the south eastern bahamas this suggests that these relatively small isolated bahama banks may be erosional remnants of once larger platforms the processes that create scalloped margins may result in erosional deep water carbonate basins and potentially the demise and disintegration of carbonate platforms

the great whirl observations of its seasonal development and interannual variability 1 in situ measurements are used together with sea surface height data to study the development and variability of the great whirl gw a large quasi stationary anticyclone that appears off the coast of somalia during the southwest monsoon season we find that anticyclonic circulation indicative of the gw appears on average in april almost two months before the onset of the southwest monsoon winds this early initiation is coincident with the arrival of annual rossby waves at the western boundary with the onset of the monsoon winds in early june the gw proper intensifies quickly remaining at its peak throughout july august and september and dissipating about one month after the winds have died the gw is present for an average 166 30 days per year and the position of its northern flank close to 9 n coincides with the latitude of zero wind stress curl the intraseasonal variability of the gw is intense as a result of mutual advection with one to three flanking cyclones which accompany the gw 70 of the time and tend to circulate clockwise around it there is no consistent seasonal pattern for the development or dissipation of the gw although movement to the southwest is common toward the end of the season the gw of 1995 deepened from 200 m in june to over 1000 m in september and strengthened from a swirl transport of 10 to 60 sv cool waters in its core resulted from advection via the somali current and some local vertical mixing

influence of paleo trade winds on facies patterns of the cambrian shanganning carbonate platform north china abstract the shanganning carbonate platform scp of the north china craton ncc was located in the trade winds belt during the cambrian which influenced its facies development in recent years numerous drillcores have been recovered from these units but the depositional environments and facies architecture of this large 2 0 105 km2 carbonate platform system have not yet been reported in detail in this study we integrate sedimentological and magnetic fabric data collected from outcrops and drillcores in order to document variation in microfacies and facies associations within the scp to infer paleowind directions and constrain the paleogeographic location of the scp and to evaluate eolian controls on platform development this analysis documented numerous carbonate microfacies belonging to six fabric types mudstone wackestone packstone grainstone framestone and dolostone along with rarer siliciclastic sediments these microfacies belong to facies associations representing tidal flat open platform platform margin slope and deep basinal depositional environments anisotropy of magnetic susceptibility ams measurements yielded mean paleocurrent directions for the early middle and late cambrian of 114 63 and 83 30 from paleo north respectively that were relatively consistent for 12 out of 13 platform margin sites all vectors being corrected for a post cambrian 150 clockwise rotation of the ncc these wind orientations are consistent with the scp shifting from south of the paleo equator in the early cambrian to north of it by the middle cambrian paleo trade winds influenced facies patterns across the platform producing a distinct windward leeward zonation the windward margin is characterized by well sorted oolitic grainstone as well as favositids coral and dasycladacean algal sponge framestone and the leeward margin by poorly sorted oolitic wackestone grainstone and fine bioclastic sediments the present study is innovative in 1 integrating microfacies and ams analyses in order to establish paleowind patterns and influences on an ancient carbonate platform 2 documenting windward leeward variation in quantifiable sediment properties across the scp and 3 constraining the paleo hemispheric location of the ncc during the cambrian

sedimentology and petrography of dredged carbonate sands from stocking island bahamas implications for meteoric diagenesis and aeolianite formation abstract piles of carbonate sand dredged onto stocking island exuma chain bahamas during world war ii are now relatively well lithified down to a depth of about 80 cm and may contain up to 8 of interstitial calcite cement a volume comparable to that measured from 1000 yr old beach ridges elsewhere in the bahamas rates of meteoric diagenetic changes are thus more rapid than previously assessed in this area 600 cm3 of calcite cement produced by m3 of rock per year preservation of hydrodynamic sedimentary structures footprints and the lack of aeolian bedforms e g dunes ripples on the surface of these sediments further show they have not been reworked by the wind at all therefore the carbonate aeolianites of holocene and pleistocene ages forming the bulk of the islands of bermuda and the bahamas were likely deposited during relatively dry periods when the effects of meteoric diagenesis were limited reduced moisture along with sediment availability and wind energy appears as a key player in carbonate aeolianite genesis

strong velocity weakening and powder lubrication of simulated carbonate faults at seismic slip rates 1 high velocity friction tests were conducted on solid and hollow cylinders of carrara calcite marble dolomite marble silicate bearing calcite marble and calcite gouge to investigate the strength of carbonate faults during seismic slip the experiments performed at normal stresses of 0 6 14 7 mpa slip rates of 0 03 1 60 m s and room temperature in a rotary shear friction testing machine yielded an extraordinarily low steady state friction coefficient 0 1 at slip rates of 1 1 1 2 m s the slip weakening distance of 4 28 m became shorter at higher normal stress or frictional work rate strong velocity weakening was observed not only in steady state but also in nonsteady state friction while the slip rate was changing thus slip deceleration was accompanied by fault strength recovery large rapid temperature rises in narrow shear localization zones less than a few micrometers induced carbonate decomposition such as the breakdown of calcite into aggregates of cao nanograins and co2 in carrara marble scanning electron microscope observation revealed that the shear localization zone in the highly porous decomposition product was a layer of scattered small grains mostly 1 m in diameter these microstructures and the measured high permeability 10 14 m2 of the decomposed marble indicate that the dominant weakening mechanism in our experiments was possibly powder lubrication powder rheology at high slip rates is not yet well understood but the frictional behavior of nanograins appears to be strongly velocity dependent if decarbonation occurs during seismic slip in natural carbonate faults powder lubrication may make the faults slippery even under fluid drained conditions

high velocity frictional properties of clay rich fault gouge in a megasplay fault zone nankai subduction zone 1 we conducted high velocity friction experiments on clay rich fault gouge taken from the megasplay fault zone in the nankai subduction zone under dry and wet conditions in the dry tests dehydration of clay minerals occurred by frictional heating and slip weakening is related to thermal pressurization associated with water vaporization resulting in a random distribution of clay clast aggregates in the gouge matrix in the wet tests slip weakening is caused by pore fluid pressurization via shear enhanced compaction and frictional heating and there is a very weak dependence of the steady state shear stress on the normal stress the resulting microstructure reflects the grain size segregation in a granular fluid shear flow at high shear rates these results suggest that earthquake rupture propagates easily through clay rich fault gouge by high velocity weakening potentially leaving the microstructures resulting from the frictional heating or the flow sorting at high slip rates

development of near surface flow pattern and water mass distribution in the somali basin in response to the southwest monsoon of 1979 abstract near surface observations of temperature salinity and current are used to describe the seasonal reversal of the somali current during 1979 in response to the onset of the southwest monsoon winds during april prior to the reversal of the winds north of the equator the northward flowing east african coastal current eacc and the southward flowing somali current sc converged near the equator the eacc was characterized by surface waters with salinities less than 35 1 and the sc by salinities greater than 35 3 the winds reversed north of the equator during the first week of may and the boundary current intruded in the form of an anticyclonic gyre to 2 5 n most of the low salinity water was recirculated back south of the equator by the offshore limb of the gyre it did not flow continuously at the surface into the eastward equatorial jet which was present farther offshore during may and june that current was fed by high salinity water from the region to the north of the low latitude gy

currents and transports of the great whirl socotra gyre system during the summer monsoon august 1993 from august 11 to 22 1993 a conductivity temperature depth acoustic doppler current profiler survey was carried out in the somali socotra region to investigate currents and transports associated with the great whirl and socotra gyre circulation during the height of the summer monsoon the monsoon circulation was confined to the upper 300 m depth with intense surface currents up to 2 2 m s 1 in the great whirl and up to 1 4 m s 1 in the socotra gyre deeper reaching flow was found in the northwestern part of the somali basin and in the passage between the shelf of somalia and abd al kuri the great whirl transport was 58 sv of which nearly 25 were due to ageostrophic flow components the northern part of the great whirl thereby appeared as a closed circulation cell in which the offshore transport was balanced by a southward transport of the same magnitude upwelled water was advected from the cold wedge of the upwelling regime at the somali coast along the edge of the gyre the water in the center of the gyre had the characteristics of indian equatorial water iew the socotra gyre carried 23 sv of modified arabian sea water asw with the transports in the two anticyclonic gyres nearly balanced the exchange of water masses between the somali basin west of the carlsberg ridge and the arabian sea occurred in two areas about 16 sv of warm and saline surface water of southern offshore origin entered the northern somali basin within a 120 km wide swift current between the great whirl and the socotra gyre the other key region for the exchange of water masses was the passage between somalia and abd al kuri there the total northward transport was 13 sv with contributions of iew of upwelled water close to the surface and asw underneath

growth of molten zone as a mechanism of slip weakening of simulated faults in gabbro during frictional melting 1 to understand how frictional melting affects fault instability we performed a series of high velocity friction experiments on gabbro at slip rates of 0 85 1 49 m s 1 at normal stresses of 1 2 2 4 mpa and with displacements up to 124 m experiments have revealed two stages of slip weakening one following the initial slip and the other immediately after the second peak friction the first weakening is associated with flash heating and the second weakening is due to the formation and growth of a molten layer along a simulated fault the two stages of weakening are separated by a marked strengthening regime in which melt patches grow into a thin continuous molten layer at the second peak friction the frictional coefficient decays exponentially from 0 8 1 1 to 0 6 during the second weakening the host rocks are separated completely by a molten layer during this weakening so that the shear resistance is determined by the gross viscosity and shear strain rate of the molten layer melt viscosity increases notably soon after a molten layer forms however a fault weakens despite the increase in melt viscosity and the second weakening is caused by the growth of molten layer resulting in the reduction in shear strain rate of the molten layer very thin melt cannot be squeezed out easily from a fault zone so that the rate of melting would be the most critical factor in controlling the slip weakening distance effect of frictional melting on fault motion can be predicted by solving a stefan problem dealing with moving host rock molten zone boundaries

abyssal circulation in the somali basin the bottom and deep circulation in the somali basin are investigated on the basis of hydrographic and direct velocity profiles from three shipboard surveys carried out during the southwest monsoon in 1995 and of velocity time series from the woce mooring array icm7 the inflow of bottom water into the somali basin through the amirante passage drives a thermohaline circulation which may be modulated by the monsoon wind forcing details of the abyssal circulation have been discussed controversially deep velocity records from the mooring array in the northern somali basin are dominated by fluctuations with periods of 30 50 days and amplitudes above full size image 1 k despite this strong variability annual record averages indicate the existence of a deep western boundary current dwbc below full size image 1 k at the base of the continental slope south of socotra island as part of a cyclonic bottom circulation the southwestward dwbc transport off socotra island is estimated to full size image 1 k the bottom and deep water exchange between the somali and arabian basin north of 7 n is estimated from two cross basin geostrophic velocity sections referenced by vertically averaged ladcp currents for the bottom water an eastward transport into the arabian basin of full size image 1 k and full size image 1 k was determined in june and august respectively while for the deep water layer above full size image 1 k eastward transports of full size image 1 k in june and full size image 1 k in august were obtained

semi brittle flow of granitoid fault rocks in experiments field studies and seismic data show that semi brittle flow of fault rocks probably is the dominant deformation mechanism at the base of the seismogenic zone at the so called frictional viscous transition to understand the physical and chemical processes accommodating semi brittle flow we have performed an experimental study on synthetic granitoid fault rocks exploring a broad parameter space temperature t 300 400 500 and 600 c confining pressure pc 300 500 1000 and 1500 mpa shear strain rate 10 3 10 4 10 5 and 10 6 s 1 to finite shear strains 0 5 the experiments have been carried out using a granular material with grain size smaller than 200 m with a little h2o added 0 2wt only two experiments performed at the fastest strain rates and lowest temperatures have failed abruptly right after reaching peak strength 1400 mpa all other samples reach high shear stresses 570 1600 mpa then weaken slightly by 10 190 mpa and continue to deform at a more or less steady state stress level clear temperature dependence and a weak strain rate dependence of the peak as well as steady state stress levels is observed in order to express this relationship the strain rate stress sensitivity has been fit with a stress exponent assuming n and yields high stress exponents n 10 140 which decrease with increasing temperature the microstructures show widespread comminution strain partitioning and localization into slip zones containing at first nanocrystalline and partly amorphous material later during continued deformation fully amorphous material in slip zones despite the mechanical steady state conditions the fabrics in the slip zones and outside continue to evolve and do not reach a steady state microstructure below 5 within the slip zones the fault rock material progressively transforms from a crystalline solid to an amorphous material we present and interpret the experimental results both in terms of sliding friction and viscous flow and we discuss the possible effect that the formation of nanocrystalline and amorphous layers may have on earthquake nucleation

disappearing ink how pseudotachylytes are lost from the rock record abstract melt origin pseudotachylytes are the most widely accepted feature recording earthquake slip in the fault rock record however reports of pseudotachylytes are rare compared to the frequency and distribution of earthquakes in active faults suggesting melting occurs only under exceptional circumstances and therefore that pseudotachylytes are rarely formed in this paper we document the processes whereby pseudotachylytes are overprinted destroyed and otherwise removed from the rock record we present examples of recrystallized altered and cataclastically and crystal plastically deformed pseudotachylytes from a variety of ancient faults based on these observations we identify characteristics of pseudotachylytes that are resistant to change over geologic time and develop criteria to allow recognition of relict pseudotachylytes our results imply that pseudotachylytes are vastly under reported due to their vulnerability to destruction and the resulting difficulty in identification as a consequence the significance of frictional melting is underestimated the criteria we propose to distinguish relict pseudotachylytes can help to reconcile the observed frequency of earthquakes with the difficulty of demonstrating ancient seismic slip in the rock record

friction evolution of granitic faults heating controlled transition from powder lubrication to frictional melt the experimental shearing of granitic faults revealed complex evolution of strength and microstructure we use a rotary shear apparatus to shear three types of granitic rocks at moderate slip velocity of 0 01 0 11 m s normal stress of 1 0 6 8 mpa and slip distance up to 60 m three stages of strength evolution with slip distance were systematically observed in stage i faults exhibited initial weakening in which the friction coefficient dropped to a steady state level of 0 3 this weakening is associated with powder lubrication and the development of cohesive gouge layers stage ii included strengthening to 0 5 associated with volumetric expansion melting of fault patches and viscous braking at these patches in stage iii fault weakening is due to melt lubrication when the melted patches reach a critical fraction of the fault surface area we show that this complex weakening strengthening weakening evolution is controlled by thermally activated deformation processes that can explain the friction behavior of igneous rocks at seismic velocity range

somali current rings in the eastern gulf of aden 1 new satellite based observations reveal that westward translating anticyclonic rings are generated as a portion of the somali current accelerates northward through the socotra passage near the mouth of the gulf of aden rings thus formed exhibit azimuthal geostrophic velocities exceeding 50 cm s are comparable in overall diameter to the width of the gulf of aden 250 km and translate westward into the gulf at 5 8 cm s ring generation is most notable in satellite ocean color imagery in november immediately following the transition between southwest boreal summer and northeast winter monsoon regimes the observed rings contain anomalous fluid within their core which reflects their origin in the equator crossing somali current system estimates of socotra passage flow variability derived from satellite altimetry provide evidence for a similar ring generation process in may following the winter to summer monsoon transition cyclonic recirculation eddies are observed to spin up on the eastern flank of newly formed rings with the resulting vortex pair translating westward together recent shipboard and lagrangian observations indicate that vortices of both sign have substantial vertical extent and may dominate the lateral circulation at all depths in the eastern gulf of aden

distinct influence of air sea interactions mediated by mesoscale sea surface temperature and surface current in the arabian sea abstractduring the southwest monsoons the arabian sea as develops highly energetic mesoscale variability associated with the somali current sc great whirl gw and cold filaments cf the resultant high amplitude anomalies and gradients of sea surface temperature sst and surface currents modify the wind stress triggering the so called mesoscale coupled feedbacks this study uses a high resolution regional coupled model with a novel coupling procedure that separates spatial scales of the air sea coupling to show that sst and surface currents are coupled to the atmosphere at distinct spatial scales exerting distinct dynamic influences the effect of mesoscale sst wind interaction is manifested most strongly in wind work and ekman pumping over the gw primarily affecting the position of gw and the separation latitude of the sc if this effect is suppressed enhanced wind work and a weakened ekman pumping dipole cause the gw to extend northeastward delaying the sc separation by 1 current wind i

underplated units in an accretionary complex melange of the shimanto belt of eastern shikoku southwest japan imbricated thrust stacks composed of melange and oceanic slabs occur in the shimanto belt eastern shikoku sw japan the melanges are divided into two types melange i consists of lenticular sandstone blocks surrounded by scaly shale matrix and melange ii is composed of blocks of oceanic material such as basalts cherts and red shale in a scaly matrix of shale melange ii is observed only near the boundaries between melange i and oceanic slabs the fabric of melange i indicates progressive deformation during underthrusting of originally coherent trench fill turbidites layer parallel extension characterizes early deformation and is divided into two stages an early stage formed during normal faulting and a subseguent stage formed perpendicular to the early extension the early extension is interpreted in terms of riedel shear associated with layer parallel shearing and tectonically induced loading normal to the layering the second extension is due to sticking thrust movement late deformation of melange i is represented by folding in association with layer parallel shearing this appears to be related to compression just before or during underplating of melange i beneath the accretionary prism layer parallel shear extends downward due to strain hardening of sheared sediments and then reaches to the base of the sediments subsequently the shear zone penetrates pelagic sediments and finally oceanic basement at this time melange ii is formed by the shearing an imbricated stack of melanges i ii and oceanic slabs forms duplexes during the underplating process the weak metamorphism of the melange complex in the study area suggests that underplating occurs at shallow depths where trench fill is thin

intraseasonal variability in the southwestern arabian sea and its relation to the seasonal circulation an analysis of topex poseidon altimeter data and in situ current and temperature data obtained between april 1995 and october 1996 from a moored array shows strong intraseasonal fluctuations in the southwestern arabian sea an oceanic region where the great whirl gw a predominantly wind generated very energetic anticyclone is present during the southwest monsoon fluctuation periods between 30 and 50 days up to 100 days during some years are observed in the 8 year altimetric dataset mostly during late summer and fall these fluctuations are largest in a 1000 km wide region off the somali omani and yemeni coasts north of 5 n suggesting a local generation mechanism the in situ data at different moorings show strong and coherent fluctuations that are characterized by southwestward phase propagation and northward energy propagation their periods range from 30 to 60 days and increase steadily from july 1995 to january 1996 in the first stage these periods are at and below the cut off period of freely propagating first baroclinic mode rossby waves but approach this theoretical limit later in the year instabilities of the flow in the transition region between the southern gyre and the gw are likely sources of these fluctuations

holocene sea level fluctuations and coastal evolution in the central algarve southern portugal in armacao de pera bay southern portugal environmental changes during the holocene can be interpreted based on the morphological and sedimentological similarities between older geomorphic features cemented beach and dune rocks and present coastal features using knowledge of the present beach and dune processes we propose a two step model for the evolution of armacao de pera bay first during the rapid sea level rise between about 8 800 and 6 600 yr cal bp the bay changed from a positive to a negative budget littoral cell and transgressive dunes formed favoured by drought conditions at about 5 000 yr cal bp during a sea level maximum beach width was less than the critical fetch and dunes stabilized and underwent cementation during the wetter atlantic climatic event the second phase of dune accumulation started at about 3 200 yr cal bp due to a regression of sea level during which the bay changed back to a positive budget littoral cell in which beach width was greater than the critical fetch currently the beach width is less than the critical fetch dunes are inactive and the sedimentary budget is negative due to sediment storage in local river systems

transient water adsorption on newly formed fault gouge and its relation to frictional heating determination of the amount of frictional heating on faults during slips provides insight into the mechanics of faulting fault slips cause frictional heating as well as production of gouge materials newly formed gouge is mechanochemically stimulated to gain a transient water adsorption ability we showed that the synthetic gouge from friction tests on rocks at a 1 mm s slip rate without frictional heating adsorbed a large amount of water comparable to that adsorbed by clay minerals the amount of adsorbed water decreased significantly with increasing slip rate to 100 mm s where the fault temperature increased above 400 c we confirmed there was no additional adsorption on the heated gouges after termination of the slip we conclude that the adsorption state can be considered a new indicator of a temperature rise of 400 c by frictional heating associated with recent near surface faulting

controls on localization and densification of a modern decollement northern barbados accretionary prism in the northern barbados ridge area the decollement zone beneath the accretionary prism lies in a lower miocene radiolarian claystone seismic data allow recognition of this continuous stratigraphic horizon seaward of the accretionary prism logging while drilling lwd data show that the radiolarian claystone is of low density relative to surrounding sediments and indicate that the interval densified heterogeneously with progressive deformation the low density interval is relatively weak and possibly overpressured which together foster decollement zone propagation characterization of densification processes in this interval by image analysis shows that reorientation of clays by shear enhanced consolidation and tectonic overburden is the dominant densification process the correlation between densification and clay reorientation suggests that predeformational conditions play a deciding role in decollement propagation 3d seismic reflection data show that the topographic relief of the decollement zone arcward of the deformation front is greater than that in the protodecollement zone and is apparently controlled by basement structure thus it is unlikely that the processes forming the decollement zone are truncating relief in the protodecollement surface but rather sliding over it along a weak stratigraphic interval although localization of the decollement may be originally due to low density and weak sediments strain softening in a smectite rich lithology and perhaps development of a low permeability cap may contribute to the continued deformation in this stratigraphic interval

rapid postseismic strength recovery of pingxi fault gouge from the longmenshan fault system experiments and implications for the mechanisms of high velocity weakening of faults 1 slide hold slide tests were conducted on gray blackish gouge gbg and yellowish gouge yg from pingxi fault zone to see how rapidly the strength of longmenshan fault recovers after the 2008 wenchuan earthquake twenty dry runs were made at a normal stress of 0 8 mpa at a seismic slip rate of 1 4 m s and with hold time th ranging 0 3 105 s results exhibit very rapid healing by more than 0 4 in friction coefficient in less than 5 10 s followed by gradual healing in proportion to log th healing rates increase in log th during rapid and slow healing are 0 188 and 0 015 for gbg and 0 154 and 0 016 for yg respectively the average temperature in the outer half of a 5 m thick slip zone decreases from 260 300 c to 110 170 c in 5 10 s and hence temperature drop appears to be correlated with the rapid healing previously reported rapid healing at subseismic slip rates 85 90 mm s begins to occur in 10 300 s after the stop of sliding and this cannot be explained by the cooling of gouge the difference in healing between subseismic slip rates delayed and rapid healing and seismic slip rates immediate and rapid healing suggests that the dominant weakening mechanism shifts from tribochemical processes at subseismic slip rates to frictional heating at seismic slip rates slip zone structures are too complex and variable from run to run to reveal microscopic mechanisms for the strength recovery rapid healing following seismic slip can be a cause for reduced aftershocks along major coseismic faults

variability of the somali current system during the onset of the southwest monsoon 1979 abstract an array of six current meter moorings and several coastal temperature recorders was deployed on the shelf and continental slope off northern somalia from march to july 1979 a seventh mooring was placed near 2 s in addition four deep sea moorings were deployed for a period of one month in may june farther offshore already during the late northeast monsoon in march the somali current north of 5 n was flowing northeastward in the top 150 m underneath in the depth range 150 400 m a narrow southward undercurrent was observed from march to june after the first onset of the southwest monsoon which occurred around 5 may when winds shifted from easterly to southwesterly parallel to the coast the near surface temperatures on the shelf decreased immediately with no detectable phase difference between 6 and 10 n but no change was observed in the offshore circulation pattern the final monsoon onset around 10 june was characterized by a drastic increase in wind speeds and the establishment of a st

silica gel in a fault slip surface field evidence for palaeo earthquakes high velocity friction experiments have shown an almost complete loss of strength associated with silica gel formation on slip surfaces the identi cation of frictional silica gel products in palaeo seismic faults is however problematic because there are multiple natural sources of silica gel and recrystallization of gel to quartz complicates preservation the importance of gel formation on natural faults is therefore unknown here we report a structurally distinct and semi continuous 0 5 10 mm thick layer of microcrystalline quartz along a major carbonate hosted fault the olive fault in the naukluft nappe complex namibia the quartz layer is distinguished by ow banding like textures and unusual cathodoluminescence characteristics the layer consists of 2 20 m hexagonal quartz crystals which include distinct crystalline pore bearing micro to nano spheres separated by pore geometries indicative of volumetric contraction and with grain boundaries enriched in aluminium we interpret these features to indicate that the quartz crystals formed from recrystallization and dehydration of a silica gel because it is found in a carbonate hosted fault and crosscuts lithological layering the silica source is not from comminution of local wall rocks rather the gel likely formed from reshear of a quartz coated fault surface or incremental shear slip associated with precipitation of silica driven by co seismic pressure drops this example of fault related silica gel may have formed by a different mechanism than the gels produced in high velocity friction experiments but once formed may have comparable rheological effects

sedimentation et fracturation de haute energie en milieu recifal tsunamis ouragans et cyclones et leurs effets sur la sedimentologie et la geomorphologie d un atoll motu et hoa a rangiroa tuamotu pacifique se abstract a longitudinal festoon fracture of the reef flat widened by slow biological and mechanical erosion transverse along the grooves divides the nw coral rim of rangiroa tuamotu se pacific into colossal blocks 15 10 5 m 1500 t the elaboration of these blocks can be attributed to the dislocation of the top of the reef flat by catastrophic events tsunamis or hurricanes typhoons in the sw and s pacific the shifting of these blocks mega boulders would necessitate a considerable amount of energy and at present we cannot decide which one of these phenomena was responsible various examples observed in the pacific are cited to illustrate abrupt changes in the sea level these could be attributed either to meteorological events hurricanes cyclones or typhoons or to seismic events of very high energy ground sea or tsunamis the reader is reminded of the different characteristics that these phenomena produce this study is mainly concerned with tsunamis especially the frequency of their occurrence energy and effects on the coasts of small oceanic islands hurricanes cyclones and typhoons roughly originate from the n ne and e whereas tsunamis originate from all directions except from se s and sw angle these very high energy events hurricanes and tsunamis seem then to be responsible for 1 the decreasing grain size of the coral rim sediments from the ocean to the lagoon through the range of colossal blocks smaller boulders scree gravels coarse and fine sands 2 the sedimentary accretion of the motu itself at each very high energy so called catastrophic event they are also responsible for the preservation of the two characteristic geomorphic forms of an atoll motu and hoa 3 finally the alternation motu hoa motu etc on the coral rim this alternation seems to result from the interaction between oceanic disturbances hurricanes or tsunamis and the water volume which must flow over the coral rim the transported material the reef flat and the angle of incidence between the swell and the coast during these catastrophic events

energy partition for grain crushing in quartz gouge during subseismic to seismic fault motion an experimental study abstract brunauer emmett teller bet surface area of quartz gouge was measured before and after shearing deformation at subseismic to seismic slip rates to determine how much of frictional work and fracture energy was consumed during grain crushing measurement of bet surface area provides more direct data on surface area change than grain size measurement does twenty four experiments on quartz gouge composed of ottawa sands or crushed quartz were conducted dry with room humidity or wet with added water at slip rates of 0 006 1 3 m s and normal stresses of 0 76 3 37 mpa using a high velocity friction apparatus and a low to high velocity friction apparatus about 1 mm thick gouge was placed between cylindrical specimens of belfast dolerite of 25 mm in diameter results show that grain crushing absorbed only 0 02 0 22 of frictional work and only 0 05 1 06 of fracture energy under dry and wet conditions thus grain comminution is unlikely to be an important energy sink at least for mature faults with well developed slip zone surface area of gouge decreases with deformation at seismic slip rates this and sem observations suggest partial sintering due to frictional heating thus a simple scenario of grain crushing as an energy sink does not hold for estimating the energy budget during earthquakes

palaeoclimatic significance of co occurring wind and water induced sedimentary structures in last interglacial coastal deposits from bermuda and the bahamas response to hearty et al s comment abstract we respond to hearty et al s objections to our interpretation of upper pleistocene coastal landforms from bermuda and the bahamas as aeolian dunes rather than wave induced deposits their remarks concern the origin of fine scale laminae and fenestral pores observed in these landforms the apparent lack of steeply dipping foresets the inference that carbonate coastal dunes can migrate the vicinity of wave deposited boulders that we did not mention and finally the palaeoclimatic reconstruction we derived from our interpretation although we do not exclude and never did that occasional storm generated waves washed up and over these dunes and created some scours we maintain that these controversial deposits have an aeolian origin

high velocity frictional properties of gabbro high velocity friction experiments have been performed on a pair of hollow cylindrical specimens of gabbro initially at room temperature at slip rates from 7 5 mm s to 1 8 m s with total circumferential displacements of 125 to 174 m and at normal stresses to 5 mpa using a rotary shear high speed friction testing machine steady state friction increases slightly with increasing slip rate at slip rates to about 100 mm s velocity strengthening and it decreases markedly with increasing slip rate at higher velocities velocity weakening steady state friction in the velocity weakening regime is lower for the non melting case than the frictional melting case due perhaps to severe thermal fracturing a very large peak friction is always recognized upon the initiation of visible frictional melting presumably owing to the welding of fault surfaces upon the solidification of melt patches frictional properties thus change dramatically with increasing displacement at high velocities and such a non linear effect must be incorporated into the analysis of earthquake initiation processes

birdseyes fenestrae shrinkage pores and loferites a reevaluation abstract birdseyes birdseye limestone fenestrae fenestral fabric shrinkage pores and loferites are considered similar if not synonymous when occurring in lime mudstone or syndepositional dolomite especially in association with mudcracks and stromatolites compaction experiments indicate however that in the absence of early cementation these rugs can be completely obliterated whereas mudcracks and stromatolites remain relatively unchanged lumping of burrows and root tubes under the general terms birdseyes or fenestrae such as for example tubular fenestrae burrow fenestrae or root tube fenestrae is discouraged they should be called burrows or root tubes because the birdseyes fenestrae and shrinkage pores are so intimately associated with tidal flats in the minds of most geologists submarine cementation of pelletal and oolitic botryoidal grainstone under 5 to 6 m of water on the bahama banks has resulted in rugs provocatively similar to many birdseyes and fenestrae generally attributed to peritidal conditions in the literature birdseyes and fenestrae in ancient grainstones cannot therefore be reliably identified without consideration of the sedimentary sequence and assoc ated sedimentary structures

the response of the surface circulation of the arabian sea to monsoonal forcing abstracttwo decades of drifter and satellite data allow the authors to describe the seasonal evolution of the surface circulation of the arabian sea which reverses annually with the indian monsoon winds this study finds several features that advance current understanding most significantly northward flow appears along the length of the western boundary together with a weak anticyclone at 6 n a precursor to the great whirl as early as march or april one or two months before the southwest monsoon winds this circulation is driven by planetary waves which are initiated by wind curl forcing during the previous southwest monsoon leading the authors to speculate that there is an oceanic mechanism through which one monsoon may precondition the next second the authors find that the eastward south equatorial counter current secc is present year round fed by the northward east african coastal current eacc during the southwest monsoon the eacc overshoots the equator and splits feeding both northwa

significance of aeolian limestone lenses in quartz sand formations an interdigitation of coastal and continental facies perth basin southwestern australia abstract in the quaternary of the perth basin of southwestern australia the transition from coastal dune ridges to continental siliciclastic sand hinterland is a zone of calcareous sand or limestone lenses on or within siliciclastic sand in the holocene sequences the transition is a zone of landward extended and detached parabolic dunes which in section appear as lenses of calcareous sand resting on pleistocene quartz sand and limestone in the pleistocene sequences aeolian limestone lenses are common in thick sections of yellow quartz sand using the holocene coast to hinterland transition as an analogue the limestone lenses are interpreted to be buried outliers of attenuated parabolic calcareous dunes that migrated inland over an undulating plain of yellow quartz sand from a pleistocene coastal zone these dunes were later buried by influxes of aeolian yellow quartz sand during glacial age desert phases of the pleistocene in section these buried dunes form a zone of limestone lenses within aeolian quartz sand formations thus in a given time interval limestone lenses represent the transition between coastal and hinterland continental desert environments and record the extent that coastal dunes penetrated inland the pleistocene stratigraphic model of aeolian limestone lenses set within a sequence of aeolian siliciclastic sand is a distinctive style of coastal to continental transition which can be used to unravel similar sequences in the stratigraphic record where coastal aeolian calcareous facies interdigitate with desert aeolian siliciclastic facies

origin of intraseasonal variability of circulation in the tropical central indian ocean observed upper ocean currents south of sri lanka exhibit large irregular fluctuations with periods of days to weeks an ocean model driven by daily surface winds is able to reproduce the observed fluctuations we find from model experiments that low frequency 30 50 day intraseasonal variability isv arises when rossby waves radiated from the eastern boundary are amplified by hydrodynamic instability in the eastern and central indian ocean high frequency 10 15 day isv is forced directly by isv of the wind field in the eastern indian ocean in spite of the contribution from instability the ocean circulation south of sri lanka is a deterministic response to wind forcing

chevrons are not mega tsunami deposits a sedimentologic assessment since the introduction of the term chevron for large v or u shaped bed forms in egypt and the bahamas others have adopted the term to describe large scale coastal bed forms in australia madagascar and elsewhere these authors interpret chevron bed forms as deposits of mega tsunamis resulting from holocene oceanic asteroid impacts we reason that chevron type bed forms are common and are present far enough from the coast to preclude tsunami genesis moreover we argue that chevrons are not mega tsunami deposits by modeling tsunami behavior and evaluating sediment transport conditions under which such features formed we model the southern madagascar case with an impact source in the indian ocean and show that a modeled wave approach is inconsistent with chevron orientation we then evaluate sediment transport conditions under which these chevron bed forms could persist i e bed load transport in our analysis no conditions specified generate pure bed load transport and most result in pure suspended load transport

compaction and dewatering processes of the oceanic sediments in the costa rica and barbados subduction zones estimates from in situ physical property measurements abstract during ocean drilling program legs 170 and 171a logging while drilling lwd tools were deployed on the costa rica and barbados subduction margins high quality density resistivity and natural ray logs were acquired across the decollement zones on both margins based on a new method of layer by layer correlation of the logs changes in thickness and volume between incoming and subducted or accreted sediments are determined with 15 m resolution and 1 accuracy the change in sediment thickness and volume generally decreases with depth however this change strongly depends on the lithology siliceous layers such as diatomaceous and radiolarian clay tend to be fluid bearing and the stratigraphic position of such zones is a critical factor in the fate of the subducted sediment section on the costa rica margin the sediment section on the cocos plate is underthrust intact beneath the toe of the caribbean plate with no frontal offscraping where a siliceous fluid bearing zone is present only in the upper part of the section on the barbados margin a layer of radiolarian clay exists providing a narrow zone of mechanical weakness and anomalously high dewatering in the middle of the sediment section this layer divides the sediments that are subducted from those that are accreted accreted and subducted sediments show different compaction styles accreted sediments are characterized by rapid compaction with vertical thickening whereas subducted sediments are characterized by slow compaction with vertical flattening the vertical thickening of the accreted sediments is due to horizontal tectonic compaction and contributes to the vertical thickening of the accretionary prism as a whole in the early stages of deformation dewatering flux is calculated by the volume change of the sediment sequences across the trench the dewatering flux computed from the lwd data provides an estimate of the minimum fluid flux in the subduction zone and is significantly greater than the flux estimated from laboratory experiments because of meter scale fluid conduits which influence the downhole logs but not the centimeter scale sample measurements

distinctions and uses of stratification types in the interpretation of eolian sand abstract eolian and subaqueous cross strata cannot be distinguished reliably by many commonly cited criteria they can however generally be distinguished by the characteristics of their component types of stratification which represent processes of sorting and transport of the grain population on dunes eolian dune stratification types consist of grainflow cross strata grainfall laminae and wind ripple generated climbing translatent strata some of these types especially translatent strata have characteristics unique to the eolian realm these same stratification types are found to compose some ancient cross strata and their occurrence confirms the eolian interpretations of pans of the entrada jurassic navajo triassic jurassic and galesville cambrian formations a well as revealing emergent islands in the curtis formation jurassic previously considered to be totally marine in origin stratification types show a characteristic distribution on modern eolian dunes and differ in their relative abundances and structure on dunes of differing size and kind these same relations allow some estimates of the type shape and original height of ancient dune deposits as well as influencing the occurrences of surface features such as tracks and tipple forms the geological record of stratification types and other dune features is greatly affected however by the extent of the post depositional truncation of dunes

silica gel formation during fault slip evidence from the rock record dynamic reduction of fault strength is a key process during earthquake rupture many mechanisms for causing coseismic weakening have been proposed based on theory and laboratory experiments including silica gel lubrication however few have been observed in nature here we report on the fi rst documented occurrence of a natural silica gel coating a fault surface the corona heights fault slickenside in san francisco california is covered by a shiny layer of translucent silica microstructures in this layer show fl ow banding armored clasts and extreme comminution compared to adjacent cataclasites the layer is composed of 100 nm to 1 m grains of quartz hydrous crystalline silica and amorphous silica with 10 100 nm inclusions of fe oxides and ellipsoidal silica colloids kinematic indicators and mixing with adjacent cataclasites suggest the shiny layer was fl uid during fault slip the layer therefore represents a relict silica gel that formed during fault motion and which could have resulted in frictional instability these observations confi rm that the silica gels formed in rock friction experiments do occur in natural faults and therefore that silica gel formation can act as a dynamic weakening mechanism in faults at shallow crustal conditions

quasi equilibrium melting of quartzite upon extreme friction quartz minerals in earth s crust are thought to melt at high temperatures laboratory friction experiments however show that metastable melting of quartz on a fault surface can occur at lower temperatures and could lead to large earthquakes