wave field decomposition of ocean bottom seismograms seismograms recorded on the seafloor are affected by reverberations in the overlying ocean layer we examine the feasibility of removing these reverberations through a 1d wave field decomposition based on a reflection transmission formulation of the problem two decomposition schemes are presented the first scheme involves a simple manipulation of the fundamental matrix relating stress and displacement of plane harmonic waves to upgoing and downgoing wavevector coefficients and requires measurement of both ocean bottom displacement and pressure the second approach involves only displacement recordings and knowledge of the water column depth this latter quantity will generally be known a priori from deployment logs or alternatively it may be estimated using vertical displacement and pressure seismograms in a preprocessing step both approaches require prior information on seabed properties in the case of p wave incidence the decomposition depends primarily on the seabed s velocity this quantity can be determined by examining trial decompositions over a range of s and selecting the value that minimizes the energy of the upgoing s wave component at the arrival time of the incident wave we apply this approach to synthetic seismograms for a simple earth structure and to recordings of two large events from the central oregon locked zone array colza on the continental margin of oregon the real data indicate that the wave field decompositions are largely successful at lower frequencies 0 1 hz but that 3d scattering likely originating near the sediment basement contact is manifested at higher frequencies

seismic evidence for sharp lithosphere asthenosphere boundaries of oceanic plates the mobility of the lithosphere over a weaker asthenosphere constitutes the essential element of plate tectonics and thus the understanding of the processes at the lithosphere asthenosphere boundary lab is fundamental to understand how our planet works it is especially so for oceanic plates because their relatively simple creation and evolution should enable easy elucidation of the lab data from borehole broadband ocean bottom seismometers show that the lab beneath the pacific and philippine sea plates is sharp and age dependent the observed large shear wave velocity reduction at the lab requires a partially molten asthenosphere consisting of horizontal melt rich layers embedded in meltless mantle which accounts for the large viscosity contrast at the lab that facilitates horizontal plate motions

formation of clay minerals and exhumation of lower crustal rocks at atlantis massif mid atlantic ridge 1 low temperature alteration products in gabbros from the ocean floor have significant implications for incipient processes of seawater rock interaction and exhumation tectonics of the lower crustal rocks in this paper we report mode of occurrence and mineralogical characteristics of clay minerals in gabbroic rocks recovered from integrated ocean drilling program iodp hole u1309d at an oceanic core complex atlantis massif mid atlantic ridge at 30 n the clay minerals were identified by optical microscope electron microprobe raman spectrometer and transmission electron microscope as mainly composed of mixed layer saponite talc saponite and vermiculite they are characteristically rich in iron that is significantly oxidized and distributed into the tetrahedral site suggesting a relatively high temperature condition for oxidation they are restricted to domains near the contacts between olivine and talc or form pseudomorphs after olivine near microcracks filled with zeolite or clay minerals these facts suggest the infiltration of oxidative seawater and reactions to variable fluid rock ratios at variable temperatures close association of vermiculite with microcracks radiated from serpentinized olivine suggests that the deep infiltration of seawater at an off axis region was caused by fracturing resulting from serpentinization and enhanced by relatively abundant olivine rich lithology at atlantis massif compared with gabbroic rocks of an oceanic core complex at ultraslow spreading ridge odp hole 735b those of atlantis massif substantially lack mixed layer smectite chlorite mixed layer smectite chlorite is a product of prehnite actinolite to greenschist facies alteration and looks to preserve a record of ambient thermal structure through which the massif passed on rising to a shallow level the absence of pervasive formation of mixed layer smectite chlorite under relatively reducing conditions suggests low permeability and or limited fluid rock reactions on the way to shallow levels from the observation and consideration of the characteristics of clay minerals sequence and distribution of static alteration related to fracturing original lithology and tectonic settings of the oceanic core complexes we conclude that atlantis massif was more rapidly exhumed to the oxidative subseafloor environment than atlantis bank the difference of exhumation rate possibly reflected either the disparity in spreading rate between the whole ridge systems or regional variation of exhumation tectonics between the two oceanic core complexes

stratigraphy and sedimentology of the paleoproterozoic thluicho lake group southwestern rae province canada alluvial basin development in the hinterland of the taltson orogen abstract the thluicho lake group is a middle paleoproterozoic succession of greenschist facies continental clastic rocks that was deposited on variably mylonitized crystalline basement rocks in the southwestern rae province of the canadian shield near the end of 2 02 1 92 ga thelon taltson orogenesis two informal formations are proposed for this fining upward succession the powder lake formation comprises predominantly coarse grained rocks including conglomerate gulo lake member pebbly sandstone camel lake member and sandstone wellington lake member whereas the overlying camsell portage formation consists of sandstone to siltstone falls member rhythmic sandstone and argillite waterloo lake member and argillite slate island member the two informal formations represent two principal facies associations a lower conglomerate sandstone facies association representing an alluvial fan to distal braided plain that underwent episodic flash flooding events and an upper sandstone siltstone muds

mid eocene alluvial lacustrine succession at gebel el goza el hamra shabrawet area ne eastern desert egypt facies analysis sequence stratigraphy and paleoclimatic implications abstract this study aims to provide the depositional facies sequence stratigraphic and paleoclimatic characteristics of the mid eocene bartonian continental succession exposed at gebel el goza el hamra shabrawet area ne eastern desert egypt the studied succession consists of siliciclastic rocks followed upward by carbonate rocks detailed field observation and petrographic investigation indicate accumulation in floodplain dominated alluvial and shallow lacustrine systems the floodplain dominated alluvial facies 45 m thick is composed mainly of carbonate nodules bearing mottled mudrock with subordinate sandstone and conglomerate beds the conglomerate and pebbly sandstone bodies interpreted as ephemeral braided channel deposits the massive laminated planner cross bedded fine to medium grained sandstone bodies interlayered within mudstone reflect sheet flood deposits the mudrocks associated with paleosols represent distal floodplain deposits the shallow lacustrine facies 15 m thick is made up of an alternation of marlstone micritic limestone dolostone and mudrock beds with charophytes and small gastropods both the alluvial and lacustrine facies show evidence of macro and micro pedogenic features pollen assemblages stable 18 o and 13 c isotopes and paleopedogenic features reflect prevalence of arid to semi arid climatic conditions during the bartonian the sequence stratigraphic framework shows an overall fining upward depositional sequence consisting of low and high accommodation systems tracts last hast and is bounded by two sequence boundaries sb 1 sb 2 conglomerate and pebbly sandstone deposits braided channel and sheet flood deposits of the lower part of the alluvial facies reflect a last mudrock and silty claystone facies distal floodplain deposits of the upper part of alluvial facies and its overlying lacustrine facies correspond to a hast the last hast and sb were formed during different accommodation to sediment supply a s ratio phases the variation in a s ratios was mainly controlled by sea level change as well as by local tectonic subsidence and uplift of the basin coincident with the reactivation of the syrian arc system during the bartonian

discussion on structural development and stratigraphy of the kyokpo pull apart basin south korea and tectonic implications for inverted extensional basins author s reply

unusually large shear wave anisotropy for chlorite in subduction zone settings using first principle simulations we calculated the elasticity of chlorite at a density 2 60 g cm 3 the elastic constant tensor reveals significant elastic anisotropy vp 27 vs1 56 and vs2 43 the shear anisotropy is exceptionally large for chlorite and enhances upon compression upon compression the shear elastic constant component c44 and c55 decreases whereas c66 shear component stiffens the softening in c44 and c55 is reflected in shear modulus g and the shear wave velocity vs our results on elastic anisotropy at conditions relevant to the mantle wedge indicates that a 10 20 km layer of hydrated peridotite with serpentine and chlorite could account for the observed shear polarization anisotropy and associated large delay times of 1 2 s observed in some subduction zone settings in addition chlorite could also explain the low vp vs ratios that have been observed in recent high resolution seismological studies

a spreading drop model for plumes on venus many of the large scale plume related features on venus can be modeled by a buoyant viscous drop or plume head as it rises and spreads laterally below a free fluid surface the drop has arbitrary density and viscosity contrast and begins as a sphere below the surface of a fluid half space the boundary integral method is used to solve for the motion of the plume head and for the topography geoid and stress at the fluid surface as the plume approaches the surface stresses in the fluid above it cause it to spread and become thin below the surface during the spreading the surface swell above evolves through various stages whose morphologies resemble several different plume related features observed on venus when the plume head first approaches the surface a high broad topographic dome develops with a large geoid and radial extensional deformation patterns at later stages the topography subsides and becomes plateau like the geoid to topography ratio gtr decreases and the dominant stress pattern consists of a band of concentric extension surrounded by a band of concentric compression we find that a low viscosity model plume head viscosity that is 0 1 times the mantle viscosity produces maximum topography that is 20 lower and swell features which evolve faster than for an isoviscous plume we compare model results with both the large scale highland swells and smaller scale features such as coronae and novae the dome shaped highlands with large gtrs such as beta atla and western eistla regiones may be the result of early stage plume motion while the flatter highlands such as ovda and thetis regiones which have lower gtrs may be later stage features comparison of model results with gtr data indicates that the highlands result from plume heads with initial diameters of about 1000 km on a smaller scale an evolutionary sequence may begin with novae domes having radial extensional deformation followed by features with radial and concentric deformation such as arachnoids and end with coronae with mostly concentric deformation the model predicts that the highlands evolve on a timescale of order 10 ma and the smaller scale features evolve in a 100 ma timescale

three dimensional configuration of a pull apart basin from high resolution magnetotelluric profiling eumsung basin cretaceous korea abstract a new high resolution magnetotelluric mt survey in a pull apart basin cretaceous eumsung basin korea provides integrated images of internal architecture two cross basinal mt profiles show an asymmetric form with a subbasin in the southeastern part and these characteristics are reflected well in paleoflow directions and facies transitions of the surface lithofacies units the results suggest that the basin experienced pull apart opening with rapid subsidence of the central blocks combined with the outcrop data on asymmetric lithofacies distribution facies transitions and paleoflow directions of the alluvio lacustrine systems the mt data help explain basin fill processes during basin formation the high frequency mt technique is a useful substitute for the costly burden of a seismic reflection survey

seismic imaging of hotspot related crustal underplating beneath the marquesas islands volcanism in active continental rift zones1 2or on rifted continental margins3is frequently associated with unusually high lower crustal seismic velocities indicating the presence of large igneous intrusions at the base of the crust the only comprehensive investigation of crustal underplating at an oceanic hotspot beneath hawaii4 6 has yielded controversial results7 here we report the results of seismic refraction experiments across the marquesas islands hotspot trace which show that the island chain is underlain by crust 15 17 km thick and that a large lower crustal region 275 km wide and 2 8 km thick has seismic velocities 7 3 7 75 km s 1 indicative of crustal underplating wide angle reflections occur near the base of the normal lower crustal velocities and at the base of the underplating complex we interpret these reflectors as the relict pre hotspot moho and the current post hotspot moho respectively the high velocity material may be purely intrusive or may consist of a mixture of intrusive and preexisting rocks the volume of the high velocity body is impressive amounting to nearly twice the volume of volcanics erupted at the surface

high k alkali basalts of the western snake river plain idaho abrupt transition from tholeiitic to mildly alkaline plume derived basalts abstract basaltic volcanism in the western snake river plain underwent an abrupt change circa 700 ka to 900 ka from low k tholeiitic basalt and ferrobasalt to high k transitional alkali basalt the low k tholeiitic basalts share major element trace element and isotopic characteristics with olivine tholeiites of the eastern snake river plain and must have been derived by similar processes from similar sources in contrast the younger high k alkali basalts share major element trace element and isotopic characteristics with plume derived alkali basalts of ocean islands suites like hawaii we conclude that this abrupt transition reflects either or both the erosion of pre existing mantle lithosphere in the wake of the yellowstone snake river plume or the depletion of this lithosphere in fusible components so that it no longer contributed to the overall mass flux of magma the abruptness of the transition implies that it may have a catastrophic origin such as lithospheric delamination caused by a rayleigh taylor instability beneath the idaho batholith

contrasting development patterns of crevasse channel deposits in cretaceous alluvial successions korea abstract the eumsung and pyonghae basins both of cretaceous age in korea comprise a thick alluvial sequence of conglomerate sandstone and purplish siltstone laterally impersistent gravelly sandstone beds can be classified into two types based on bed geometry internal structure and occurrence a vertically stacked sheetlike bodies facies 1 and b migratory ribbon shaped bodies facies 2 facies 1 is characterized by symmetrical channel geometry with a large ratio of width to thickness of sandstone beds which are mostly stratified and bounded by relatively distinct lower and upper boundaries they are frequently found in aggradational style facies 2 shows asymmetric geometry with a relatively small ratio of width to thickness of sandstone beds it comprises massive cross stratified units bounded by sharp basal contacts and gradational upper boundaries which resulted from laterally shifting mode both types represent crevasse distributary channels developed in floodplain environments facies 1 is suggestive of gradual migration of crevasse distributary channels under the conditions of overall aggradation whereas facies 2 represents discrete abrupt diversion of crevasse channels these features are due to differences in floodplain gradients paleohydrology and trunk channel behaviour

lithofacies and depositional environments of miocene deposits from tectonically controlled basins red river fault zone northern vietnam abstract the lao cai yen bai and bao yen basins connected with the red river fault zone rrfz are filled with over 1000 m thick neogene miocene clastic deposits of different type and origin coarse grained alluvial deposits predominate along the south western margin of the lao cai and yen bai basins while different types of conglomerates and sandstones even siltstones occur in their central and along their north eastern parts such asymmetry in the cross section basin fill points to the south western active margin of these basins three main facies associations composed of 12 lithofacies have been recognized in the miocene succession filling the studied basins 1 debris flow dominated alluvial fan association 2 sheetflood dominated alluvial fan association and 3 lacustrine association both alluvial fan facies associations are broadly spread in the studied basins especially along their south western margins the debris flow dominated deposits are represented by decimetre to several meters thick sand and mud supported disorganized pebble to cobble size occasionally boulder breccia and conglomerate beds the sheetflood dominated deposits are represented by alternating planar bedded pebble and cobble gravel couplets the occurrence of conglomerate bodies within and overlapping fine deposits of the lacustrine facies association may suggest deposition in a debris flow and sheetflood dominated alluvial fan setting associated with narrow elongated basins that could be occupied by lakes and bordered by active fault scarps the facies pattern of the particular basins is connected with the evolution of the rrfz the origin of these basins may be correlated with a sinistral transtensional regime which occurred in the early to middle miocene moreover the high grade metamorphic core of the day nui con voi massif presently the main structural element of the rrfz was probably not exposed during sedimentation of most of the strata in the studied basins

sediments and processes on a small stream flow dominated devonian alluvial fan shetland islands abstract the main conglomerate type of a small devonian alluvial fan in shetland northern britain is an inverse to normally graded framework supported gravel the sedimentological details of these beds and fundamental considerations of the mechanics of movement of highly concentrated flows suggests that neither debris flow nor grain flow were responsible for the deposition of these conglomerates on the contrary these inverse to normally graded conglomerates were deposited by water as thick gravel sheets with little topographic relief but broadly analogous to longitudinal bars they were deposited under high aggradation conditions first on the rising and then on the falling flood the high concentration of material in transport on the rising and peak flood was responsible for the polymodal and unstratified nature of the conglomerates as the flood waned normal grading was developed and a preferred horizontal fabric was produced under low concentrations of sediment other conglomerate types comparatively of lesser importance were deposited from highly concentrated clast dispersions and are notably richer in matrix and locally possess inverse grading the hydrological environment was one of flashy discharges of short duration but of high velocity flow was dissipated rapidly perhaps due to extreme water loss this example from the devonian of shetland provides an interesting alternative to the mass transport dominated fan models currently in vogue

subaerial debris flow deposition in the upper paleozoic cutler formation western colorado abstract outcrops of the lower part of the cutler formation near gateway colorado contain proximal alluvial fan deposits of two principal lithofacies the diamictite lithofacies d consists of a variety of sharply bounded massive muddy beds which are interpreted as deposits of subaerial debris flows the gravelly sandstone lithofacies s consists of amalgamated beds of laminated and cross bedded conglomeratic sandstones which are interpreted as water deposited sediments clasts in s are rarely larger than 0 5 m in maximum diameter but clasts in d are commonly much larger up to 8 m both normal and inverse grading are visible in d but grading is generally not well developed in either lithofacies imbrication of clasts although more pronounced and more consistent in s is fairly commo in both lithofacies four subfacies of diamictite are distinguishable on the basis of clast packing and grading massive matrix supported dram graded matrix supported dmg inversely graded clast supported dci and massive clast supported dcm given the hydrodynamic and rheologic properties of debris flows as reported by previous workers four debris flow regimes are postulated on the basis of varying water content and clast interaction subfacies dram and drag typify deposits of flows having viscous clast interactions subfacies dci represents deposits of flows having inertial or colliding clast interactions subfacies dram comprises deposits of flows with demonstrably high yield strengths subfacies dcm represents deposits of flows in which yield strength was insufficient to support most clasts this classification specifically recognizes turbulence selective deposition and bedload transport of clasts as essential characteristics of certain dilute debris flows it also accommodates as debris flow deposits some beds which have features usually associated with aqueous deposition including clast frameworks channelization imbrication and normal grading

the cretaceous pyonghae basin southeast korea sequential development of crevasse splay and avulsion in a terminal alluvial fan abstract the pyonghae basin early cretaceous southeast korea comprises a thick sequence of purplish siltstone with subordinate gravelstone and gravelly sandstone deposited in a terminal alluvial fan environment based on bed geometry and facies assemblage the gravelly sandstone bodies are divided into four types type a deposit consists of disorganized gravel patches and massive sandstone lenticules a few decimetres thick and a few metres wide most likely deposited from overflows type 2 deposit is characterized by crudely stratified and massive sandstone beds up to 2 m thick 20 m wide and comprises lower disorganized gravel layers and upper stratified gravelly sandstone beds forming a fining upward trend it probably represents crevasse channel distributary channel fills type 4 deposit 3 5 m thick 20 m wide consists of stratified cross stratified gravelly sandstone beds which are interpreted as main braided channel fills aggradation of the terminal fan was associated with frequent shift of channels which range from crevasse splay channel type 2 through crevasse channel distributary channel type 3 to main channel type 4 the crevassing or avulsion was preceded by overflows type 1 the avulsion processes in the terminal fan were achieved through stepwise enlargement of crevasse channels which reflect different channel responses to floods of different magnitude and frequence

the deposits internal structure and geometry in six alluvial fan fan delta bodies devonian norway a study in the significance of bedding sequence in conglomerates discussion and reply

structural and sedimentary characteristics of the liangjia wangchang strike slip region yitong basin china abstract the yitong basin a typical strike slip basin is part of the northern segment of the world famous tanlu strike slip fault area in china research is needed to understand how the strike slip tectonic environment affects the development of structures and depositional systems within the basin the aim of this paper is to analyze how the right lateral strike slip movement of the northwestern boundary faults in this region controls extensional faulting and to analyze differences in two main sedimentary sequences termed sysq1 and sysq2 that result from strike slip movement the study is based on geological and geophysical data the following conclusions are drawn 1 the two periods of strike slip movement of the boundary fault generated two episodes of extensional fault inside the basin the extensional faults have a characteristically episodic activity related to the episodic strike slip movement the extensional faults have small fault throw and their activity lags behind the strike slip movement of the northwestern boundary faults 2 the strike slip movement of the boundary faults generated the extensional faults inside the basin and changed the locus of depocenters with complimentary changes in the paleogeomorphology and in the distribution of depositional facies it is this strike slip movement that caused the differences in sedimentary and structural characteristics between sequences sysq1 and sysq2

late miocene nonmarine sedimentation and formation of magnesites in the acigol basin southwestern anatolia turkey abstract the late miocene lacustrine acigol basin sw turkey formed as an orogen top extensional half graben with the subaqueous accommodation controlled by the lake level and the bulk accommodation provided by active subsidence along a wsw trending normal fault at the basin s southern margin the basin fill sedimentary succession consists of terminal alluvial fan facies overlain by ephemeral lake margin facies and perennial lake facies with widespread fluvial facies at the top the distal alluvial fan facies include massive to stratified sandstones and massive mudstones with intervening nodular dolostones and incipient pedogenic horizons the lake margin facies are micritic magnesites passing laterally into peloidal irregularly laminated magnesites towards the palaeolake margin and overlain by marlstones and dolostones all with abundant evidence of episodic subaerial exposure desiccation cracks pedogenic features and tepee structures the perennial lake facies are micritic magnesites passing upwards into clayey dolostones and dolomitic or clayey marlstones the fluvial facies capping the succession include planar cross stratified conglomerates channel fill deposits planar parallel stratified planar cross stratified and rippled cross laminated sandstones crevasse fill and crevasse splay deposits and assemblages of mudstones intercalated with thin sandstone beds overbank floodplain deposits the sedimentological mineralogical and geochemical data reveal large variations in the basin s hydrological regime including short term oscillations and bulk rise of the lake level periodical changes in the mg ca ratio and terrigenous mud supply and a negative covariance of 18 o and 13 c fluctuations the composition of terrigenous sediment and the chemistry of water supplied to the lake were controlled by the weathering chemical leaching and erosion of the ultramafic dolomitic bedrock in the catchment area the bedrock yielded mg rich carbonate solutions that caused the deposition of mg carbonates in the lake despite short term lake level fluctuations the lake s net water budget remained positive it is suggested that the region s present day climate and mg rich alkaline lakes can serve as an analogue for the climatic and hydrological conditions in the late miocene acigol basin

dynamic control on serpentine crystallization in veins constraints on hydration processes in oceanic peridotites deformation and hydration processes are intimately linked in the oceanic lithosphere but the feedbacks between them are still poorly understood especially in ultramafic rocks where serpentinization results in a decrease of rock density that implies a volume increase and or mass transfer serpentinization is accompanied by abundant veining marked by different generations of vein filling serpentines with a high variety of morphologies and textures that correspond to different mechanisms and conditions of formation we use these veins to constrain the role of deformation and mass transfer processes during hydration of oceanic peridotites at slow spreading ridges we have selected a representative set of veins from ocean floor serpentinites of the mid atlantic ridge near kane transform fault 23 n and characterized these in detail for their microstructures and chemistry by coupling optical and electron microscopy sem tem with electron microprobe analyses four main veining episodes v1 to v4 accompany the serpentinization the first episode identified as vein generation v1 is interpreted as the tectonically controlled penetration of early seawater dominated fluid within peridotites enhancing thermal cracking and mesh texture initiation at 3 4 km up to 8 km depth and at t 300 350 c the two following vein stages v2 and v3 formed in a closed diffusive system and accommodate volume expansion required to reach almost 50 serpentinization of the protolith the cracks exploited by these veins were caused by the progressive unroofing at depths of 4 to 2 km along a detachment fault degree and rate of serpentinization seem to be controlled by the capacity of the system to create space and to drive the mass transfer needed for ongoing serpentinization and this capacity is in turn linked to the exhumation rate and local tectonics during this period water consumed by hydration may prevent the establishment of convective hydrothermal cells the onset of an open hydrothermal system in the shallow lithosphere 2 km where brittle fracturing and advective transfer dominate and enable the completion of serpentinization is marked by the last vein generation v4 these results show a complete history of alteration with the crystallization of different types of serpentine recording different tectonic events chemical conditions and modes of hydrothermal alteration of the lithosphere

unraveling the sequence of serpentinization reactions petrography mineral chemistry and petrophysics of serpentinites from mar 15 n odp leg 209 site 1274 1 the results of detailed textural mineral chemical and petrophysical studies shed new light on the poorly constrained fluid rock reaction pathways during retrograde serpentinization at mid ocean ridges uniformly depleted harzburgites and dunites from the mid atlantic ridge at 15 n show variable extents of static serpentinization they reveal a simple sequence of reactions serpentinization of olivine and development of a typical mesh texture with serpentine brucite mesh rims followed by replacement of olivine mesh centers by serpentine and brucite the serpentine mesh rims on relic olivine are devoid of magnetite conversely domains in the rock that are completely serpentinized show abundant magnetite we propose that low fluid flux serpentinization of olivine to serpentine and ferroan brucite is followed by later stages of serpentinization under more open system conditions and formation of magnetite by the breakdown of ferroan brucite modeling of this sequence of reactions can account for covariations in magnetic susceptibility and grain density of the rocks citation bach w h paulick c j garrido b ildefonse w p meurer and s e humphris 2006 unraveling the sequence of serpentinization reactions petrography mineral chemistry and petrophysics of serpentinites from mar 15 n odp leg 209 site 1274 geophys res lett 33 l13306

the morphology and evolution of coronae on venus coronae on venus are large circular to ovoidal surface features that have distinctive tectonic volcanic and topographic expressions they range in diameter from less than 200 km to at least 1000 km new data from the magellan spacecraft have shown coronae to be among the dominant tectonic forms on the planet and have revealed their morphology in unprecedented detail typical coronae are dominated by concentric tectonic features and have a raised rim a central region higher than the surounding plains but in many instances lower than the rim and commonly a peripheral depression or moat some coronae also show significant amounts of radial tectonic structure and in most cases this predates the concentric features in addition there are other features on venus recognized for the first time in magellan data that consist of domical rises with intense radial tectonic patterns and little or no concentric structure all of these features commonly are associated with moderate to large quantities of volcanism in fact some radially fractured domes have undergone so much volcanism that volcanic construction appears to have played a significant role in establishing their topography we explore a model of corona formation that links these forms into a genetic sequence the model begins with the ascent of a mantle diapir upward mantle flow driven by its ascent forces the lithosphere above the diapir upward producing a gentle dome with a radiating pattern of extensional fractures as the diapir impinges on the underside of the lithosphere it flattens and spreads transforming the uplift to a more flat topped shape in this flattened near surface configuration the diapir can cool rapidly with the resultant loss of buoyancy the raised plateau can relax to form a central sag a raised rim and a depressed moat concentric tectonic features develop primarily during the latter stages of corona formation and hence are best preserved on mature coronae volcanism takes place during all phases of the uplift and may diminish as the relaxation occurs our analyses to date suggest that this scenario is broadly consistent with many of the coronae on venus however there is enormous diversity in corona morphology and features are present that require substantial deviations from this simple model in particular some circular depressions appear corona like in synthetic aperature radar images but may in fact be large calderas some of the variations observed in corona morphology may ultimately be interpretable in terms of variations in the behavior of individual diapirs and in the local properties of the venusian lithosphere

are the lower planes of double seismic zones caused by serpentine dehydration in subducting oceanic mantle in a number of subduction zones earthquakes at 50 200 km depth define two dipping planes separated by 20 40 km that appear to merge downdip upper plane earthquakes are inferred to occur within the subducting oceanic crust whereas lower plane earthquakes occur in the subducting oceanic mantle beneath northeast japan the results of a new finite element heat transfer model suggest that the lower seismic plane cuts across isotherms at a shallow angle lower plane earthquakes occur at 550 800 c at 100 km depth and at 350 600 c at 160 km depth these conditions coincide with the dehydration reaction antigorite serpentine forsterite enstatite h 2 o which suggests that lower plane earthquakes may be triggered by dehydration embrittlement which in turn suggests that the subducting oceanic mantle is partially hydrated serpentinization may occur in the trench outer rise region where faulting may promote infiltration of seawater several tens of kilometers into the oceanic lithosphere if this hypothesis is correct current subduction zone h 2 o budgets may significantly underestimate the amount of bound h 2 o entering the subduction factory

seismic evidence for hydrothermally altered mantle beneath old crust in the tydeman fracture zone abstract to study the structure of old atlantic fracture zones seismic refraction profiles were obtained using ocean bottom seismometers and two ship multichannel expanding spread experiments on two inactive sections of the tydeman fracture zone the results indicate that a crustal seismic velocities are low compared with normal oceanic crust b there is a marked absence of an identifiable layer 3 c the depth to the moho is shallow though not exceptionally so and d anomalously low seismic velocities of 7 2 7 5 km s 1 are observed over ranges at which upper mantle velocities are normally detected s wave arrivals corresponding to both crustal and mantle refractions are also observed which enable the first estimates of poisson s ratio within a fracture zone to be made using these results and those from other surveys we show that the seismic velocity in the upper mantle beneath fracture zones decreases systematically with increasing age these findings suggest that hydrothermal circulation which can lower the upper mantle velocity by serpentinization persists under fracture zones for at least 70 ma and occurs at greater depths than for normal oceanic crust the serpentinization process is not sufficiently rapid to obscure the location of the original crust mantle boundary

oyster patch reefs as indicators of fossil hydrocarbon seeps induced by synsedimentary faults abstract the late jurassic deposits of the boulonnais area n france represent the proximal lateral equivalent of the kimmeridge clay formation they accumulated on a clastic dominated ramp subject to synsedimentary faulting as a result of the atlantic ocean rifting in the gris nez cape area i e close to the northern border fault zone of the jurassic basin the late jurassic sequence contains small dimensioned oyster patch reefs nanogyra nana embedded in a microsparitic calcite matrix 13 c measurements carried out within both the matrix and the shells display significantly lower values in the matrix compared to the oyster shells which suggests that the carbonate matrix precipitation was involving a carbon source different from marine dissolved inorganic carbon most probably related to sulfate reduction which is evidenced by light 34 s in pyrites similarities but also differences with lucinid rich bioconstructions namely the late jurassic pseudo bioherms of beauvoisin se france suggest that the patch reefs developed at hydrocarbon seeps are related to synsedimentary faults the extensional block faulting segmentation of the northern margin of the boulonnais basin in late jurassic times is thus believed to have induced a sort of small dimension hydrocarbon seepage field recorded by the patch reef distribution

lithofacies variations within sandur deposits the role of runoff regime flow dynamics and sediment supply characteristics abstract the icelandic sandur plains have for long acted as classic sites for models of braided river behaviour and of associated lithofacies assemblages however many of the icelandic sandurs are subject to repeated catastrophic flood events or jokulhlaups which are likely to have played a significant role in sandur construction established lithofacies models based on the icelandic sandurs have failed to take account of the varying runoff regimes which characterise the different sandur environments in addition sediment inputs to sandur channel systems are highly variable fluctuating in response both to varying runoff patterns and to advance and retreat of the source glacier that affect sediment availability within the proglacial zone this paper describes the main lithofacies types and vertical profiles that characterize the sandurs of south iceland and demonstrates that they can largely be attributed to extreme runoff events with few deposits typical of braided river facies occurring within the sedimentary record a preliminary qualitative model is developed that allows interpretation and prediction of vertical lithofacies profiles derived from glacial outwash in terms of i the drainage mechanism which in turn controls the dominant runoff population ii sediment availability and iii the spatial variability in flow dynamics

sedimentary records of rift to pull apart tectonics in the miocene eoil basin se korea the miocene eoil basin se korea is a pull apart basin formed by dextral crustal deformation during the backarc opening of the east sea the gampo conglomerate the lowermost formation of the basin provides significant information on how a pull apart basin initiates and is filled by sedimentary strata the formation consists of nonmarine conglomerates sandstones mudstones and a variety of volcaniclastic deposits that were deposited in alluvial fans delta slopes braided streams mouth bars and lakes these deposits are organized into several stratal packages sp with contrasting geometries sp i at the base consists of mouth bar to delta front and volcaniclastic deposits and shows rapid facies variations along the southeastern normal fault margins sp i suggests that the early basin was bounded by multiple incipiently developed normal faults with variable displacement and accommodation creation sp ii consists of thick alluvial fan conglomerates and overlies sp i abruptly suggesting sudden input of coarse grained sediment from the uplifted footwalls it is thickest near the central part of the southeastern normal fault margins suggesting development of a large alluvial fan near a transfer zone sp iii consists mostly of mouth bar sandstones indicating abrupt subsidence of the basin and the transition from subaerial to subaqueous environments sp iv consists of braided stream conglomerates suggesting gradual progradation of coarse grained fluvial systems toward the basin when the extension and subsidence of the basin was slowed sp v shows upward deepening facies changes from braided stream to mouth bar deposits suggesting resumption of basin subsidence notably the cumulative thickness of sp iii to sp v is thickest near the northeastern and southwestern strike slip margins suggesting migration of the loci of maximum subsidence from the normal fault margins towards the strike slip margins after deposition of sp ii the change in stratal architecture suggests that the activity of the normal faults was dominant in the early stage of the basin evolution whereas the activity of the strike slip faults became more pronounced afterwards overall the stratal architecture of the gampo conglomerate appears to record the transition tectonics from rift to pull apart during the incipient stage of a pull apart basin development the eoil basin is thus interpreted to have been hybrid in nature influenced by the dynamic changes of the basin margin faults which were either normal or strike slip

discrete alternating hotspot islands formed by interaction of magma transport and lithospheric flexure the large scale geometry and age progression of many hotspot island chains such as the hawaiian emperor chain are well explained by the steady movement of tectonic plates over stationary hotspots but on a smaller scale hotspot tracks are composed of discrete volcanic islands whose spacing correlates with lithospheric thickness1 moreover the volcanic shields themselves are often not positioned along single lines but in more complicated patterns such as the dual line known as the kea and loa trends of the hawaiian islands2 3 here we make use of the hypothesis that island spacing is controlled by lithospheric flexure1 to develop a simple nonlinear model coupling magma flow which feeds volcanic growth to the flexure caused by volcanic loads on the underlying plate for a steady source of melt underneath a moving lithospheric plate magma is found to reach the surface and build a chain of separate volcanic edifices with realistic spacing if a volcano is introduced away from the axis of the chain as might occur following a change in the direction of plate motion the model perpetuates the asymmetry for long distances and times thereby producing an alternating series of edifices similar to that observed in the kea and loa trends of the hawaiian island chain

alluvial to lacustrine systems in a pull apart margin southwestern eumsung basin cretaceous korea abstract in the pull apart margin of pull apart basins sedimentation patterns are poorly known compared with those in rift basins the rhomb shaped eumsung basin cretaceous mid west korea was formed in an overstep of the kongju fault system and provides significant information on how sedimentary bodies dynamically filled the pull apart margin according to a detailed analysis of sedimentary facies palaeoflow directions clast compositions and downstream changes in clast size the southwestern basin fills comprise three facies associations representing distinctive depositional environments 1 facies association f a i volcaniclastics dominated alluvial fan 2 f a ii alluvial plain and 3 f a iii floodplain lake the matrix supported and clast supported conglomerate f a i is indicative of deposition in alluvial fans the hollow fill or sheetlike deposits encased in purple siltstone f a ii are suggestive of channel or sheetflood deposition in an alluvial plain the dominant purple and green mudstone f a iii indicates deposition in floodplain and lacustrine environments the distribution of facies associations represents a basinward change in depositional environments from alluvial fan to lacustrine systems the succession is divisible into three large scale lateral and partly vertical units the southernmost ochang 1 5 km thick the medial baeti 1 km thick and the northernmost kugokri units 0 5 km thick on the basis of basinal structure and changes in clast composition and lithofacies the ochang baeti transition reflects synchronous development of the alluvial to lacustrine systems in a pull apart margin this succession development is attributable to the basinal faults progressed during the basin development the baeti kugokri transition recorded the changes in clast composition and lithofacies which was due to the progressive shifting in basinal setting from a pull apart margin to a strike slip margin unlike that of the strike slip margins clast composition of the pull apart marginal fills reflects adjacent source rock lithology in the pull apart margin alluvial to lacustrine systems show a transverse to margin channel network 060 135 converging into the southwestern depocentre although the basin is of strike slip origin sedimentation patterns in the pull apart margin are suggestive of relatively synchronous developments of the alluvial to lacustrine systems reflecting the lithology of the adjacent source rock and subsurface structure of normal slip dominant faults

alluvial deposits from the strike slip fault lo river basin oligocene miocene red river fault zone north western vietnam abstract the lo river basin lrb is one of several narrow sedimentary basins associated with the main faults of the red river fault zone separating the south china and indochina microplates the basin is located on the ne boundary of the high grade metamorphic con voi massif and the sedimentary and metasedimentary viet bac fold zone in north eastern vietnam the lrb is filled with over 6000 m of oligocene miocene alluvial deposits the source area was probably located on the ne margin of the basin and was composed mostly of low grade metamorphic rocks with a minor component of sedimentary rocks three alluvial systems are recognised the oldest system was a proximal braided river system with the minor occurrence of alluvial fans the younger systems record changes in clast composition and lithofacies which suggests a transition from a distal braided river to a distal braidplain system the lrb fill shows a range of features characteristic of strike slip fault basins the origin of the lrb is correlated with the left lateral transtensional regime the present shape of the basin is a result of post sedimentation tectonic activity

sequential development of alluvial lacustrine system southeastern eumsung basin cretaceous korea abstract in a strike slip setting depositional history of marginal basins depends on the fault movements in order to constrain controlling factors for the sequence changes of alluvial lacustrine systems in the southeastern part of cretaceous eumsung basin a pull apart basin southwestern korea we made a detailed analysis of sedimentary facies paleoflow directions clast compositions and downstream changes in clast size the conglomeratic sequence 2 2 km thick in the study area can be organized into five facies associations representing distinctive depositional environments 1 facies association fa i debris flow dominated alluvial fan 2 fa ii stream dominated alluvial fan 3 fa iii alluvial fan fringe 4 fa iv alluvial plain and 5 fa v floodplain lake the entire succession can be divided into two stratigraphic units the dootasan sequence below and the upper berjae above on the basis of facies associations and stratigraphic architecture of channel fill and floodplain deposits the dootasan sequence 1 2 km thick is characterized by a basinward change from matrix supported onglomerate lower part to ribbon shaped channel fills and sheetlike beds encased in purple siltstone with a thinning upward trend upper part the matrix supported conglomerate facies is indicative of deposition in a debris flow dominated alluvial fan the hollow fill or sheetlike deposits in purple siltstone are suggestive of channel or sheetflood deposition on an alluvial plain the berjae sequence 1 km thick is transitional from superimposed channel fill sheetflood deposits with infrequent intercalation of purple siltstone wedges to conglomerate beds encased in siltstone the stacked channel fill or sheetflood deposits with rare siltstone beds are indicative of deposition in stream dominated alluvial fan each sequence shows a basinward spatial change in facies between the two sequences green siltstone beds max 100 m thick with conglomerates are intercalated suggesting deposition in a lacustrine environment the sequence transition from the dootasan to the berjae sequence along the basin margin fault was most likely due to the changes in fault movements the dootasan sequence was formed during sinistral fault movement with a relatively high rate of fault slip early stage of fault activation whereas the berjae sequence was deposited in the late stage with a relatively low rate of fault slip

a theoretical model of hotspot volcanism control on volcanic spacing and patterns via magma dynamics and lithospheric stresses many linear island chains are thought to be the result of the steady motion of the lithospheric plates over stationary hotspots the occurrence of discrete nearly regularly spaced volcanoes rather than continuous ridges are assumed to be caused by the interaction of flexural stresses due to the volcanic loads with magma percolation a parameter study is performed on a simple model that incorporates these effects in addition to dike wall erosion it is demonstrated that the strength of the erosional feedback determines whether the model generates discrete volcanoes or a continuous ridge the intervolcanic spacing depends not only on the elastic thickness of the lithosphere but also on the magma pressure at the base of the lithosphere the size of the eruptive region of the individual volcanoes is controlled by the elastic response of the lithosphere to magma overpressurization if an initial off axis edifice is introduced the model is able to preserve this asymmetry and produce an alternating series of volcanoes a small initial perturbation grows over time resulting in double lines or wider patterns depending on the width of the magma source region single lines of volcanoes therefore indicate very narrow magma source regions