ocean drilling program leg 112 peru continental margin part 1 tectonic history studies of ocean drilling program leg 112 cores and geophysical data define early eocene quaternary normally faulted continental sediment sequences across the peru margin to the middle slope and a 15 km wide late miocene quaternary accretionary complex beneath the lower slope the eocene shelf and upper slope sea floor were eroded during the oligocene and in middle miocene the seaward part had subsided to lower slope depths this subsidence and the missing paleogene continental slope indicate tectonic erosion during normal plate convergence subduction of the nazca ridge locally accelerated tectonic erosion leaving a scarp but no associated compressional structure nearby late miocene sediment accreted against the erosional scarp local tectonic mechanisms of forearc basin subsidence are suggested by the rapid subsidence of a late miocene forearc basin and stability of the adjacent one despite similar histories of plate convergence

late cretaceous early palaeogene tectonic development of se asia abstract the late cretaceous early palaeogene history of the continental core of se asia sundaland marks the time prior to collision of india with asia when se asia from the tethys in the west to the palaeo pacific in the east lay in the upper plate of subduction zones in myanmar and sumatra subduction was interrupted in the aptian albian by a phase of arc accretion woyla and mawgyi arcs and in java eastern borneo and western sulawesi by collision of continental fragments rifted from northern australia subsequent resumption of subduction in the myanmar thailand sector explains 1 early creation of oceanic crust in the andaman sea in a supra subduction zone setting 95 ma 2 the belt of granite plutons of late cretaceous early palaeogene age starting 88 ma in western thailand and central myanmar and 3 amphibolite grade metamorphism between 70 and 80 ma seen in gneissic outcrops in western and central thailand and 4 accretionary prism development in the western belt of myanmar until glancing collision with the ne corner of greater india promoted ophiolite obduction deformation and exhumation of marine sediments in the early palaeogene the ranong strike slip fault and other less well documented faults were episodically active during the late cretaceous palaeogene time n to nw directed subduction of the palaeo pacific ocean below southern china vietnam and borneo created a major magmatic arc associated with rift basins metamorphic core complexes and strike slip deformation which continued into the late cretaceous the origin and timing of termination of subduction has recently been explained by collision of a large luconia continental fragment either during the late cretaceous or palaeogene evidence for such a collision is absent from the south china sea well and seismic reflection record and here collision is discounted instead relocation of the subducting margin further west possibly in response of back arc extension which created the proto south china sea is preferred lying between the two subduction related arcs the khorat basin is of predominantly late jurassic early cretaceous age but stratigraphic and apatite fission track data also indicates deposition of 1 2 km of late cretaceous sediments the synformal basin geometry probably arose due to the dynamic topography created by converging tethyan and palaeo pacific subduction zones the aptian albian slowing of basin subsidence and onset of evaporite deposition coincides with collision of the mawgyi and woyla island arcs extensive palaeogene deformation and exhumation 3 km in places affected all margins of the khorat plateau deformation includes folds of the phu phan uplift and strike slip faults thrusts and folds on the southern and eastern margins south of the khorat plateau outcrop and seismic reflection data from the ton le sap basin cambodia and the gulf of thailand indicate syn depositional fault controlled subsidence was important during cretaceous deposition the hot thickened crust developed during the late cretaceous palaeogene events follows the weak indosinian crustal scale inthanon and sukhothai zones which persistently guided the location of later structures including cenozoic extensional and post rift basins and influenced the widespread occurrence of low angle normal faults metamorphic core complexes and eastern gulf of thailand super deep post rift basins

seabeam and seismic reflection imaging of the tectonic regime of the andean continental margin off peru 4 s to 10 s abstract marine geophysical surveys employing seabeam multi and single channel seismic reflection gravity and magnetic instruments were conducted at two locations along the continental slope of the peru trench during the seaperc cruise of the r v jean charcot in july 1986 these areas are centered around 5 30 s and 9 30 s off the coastal towns of paita and chimbote respectively these data indicate that 1 the continental slope off peru consists of three distinct morpho structural domains from west to east are the lower middle and upper slopes instead of just two as previously reported 2 the middle slope has the characteristics of a zone of tectonic collapse at the front of a gently flexured upper slope 3 the upper half of the lower slope appears to represent the product of mass wasting 4 thrusting at the foot of the margin produces a continuous morphologic feature representing a deformation front where the products of mass wasting are overprinted by a compressional tectonic fabric 5 a change in the tectonic regime from tensional to compressional occurs at the mid slope lower slope boundary the accretionary prism being restricted to the very base of the lower slope in the paita area the andean margin off peru is an extensional active margin or a collapsing active margin developing a subordinated accretionary complex induced by massive collapse of the middle slope area

tectonic erosion and accretion in steady state trenches abstract the geometry of subduction in a steady state trench is analysed to derive simple equations linking the rate of plate convergence across the trench with the outer slope and trough axial sedimentation rates the dips of the outer slope and the slope trough sequence boundary and the angle of decollement below the inner slope it is concluded that tectonic accretion will generally occur where the decollement is steeper than the outer trench slope and conversely that tectonic erosion will occur if the decollement is shallower than the dip of the outer slope the analysis is applied to the andean trench to explain tectonic erosion in the arica bight and to the timor trough to demonstrate that subduction has ceased

development of the accretionary prism along peru and material flux after subduction of nazca ridge results from ocean drilling program leg 112 indicated tectonic erosion of the peruvian convergent margin during subduction of the nazca ridge followed by renewed accretion against the erosional scar seismic images and swath mapping morphology show a regional geology shaped as subduction of nazca ridge migrated 800 km along the peru trench the accretionary prism is imaged at two stages of development since upper miocene time with depth migrated seismic records in which the back stop is well defined from these data material flux can be quantified after the ridge crest subducted and accretion dominated over erosion the prism grew rapidly to 10 to 15 km width during rapid growth the margin taper was large and about 60 percent of the sediment supply accreted as the prism growth slowed only 30 percent accreted despite an increased trench sediment supply since the convergence rate changed little the inverse relation between sediment supply and prism growth rate suggests that other processes allow most of the sediment input to be subducted the decreased prism growth may be partially controlled by varying the structure of the back stop the force required to activate faults cutting across the back stop is probably greater than the force allowed by basal friction across the plate boundary thus the shear stress transmitted across the plate boundary is insufficient to drive out of sequence thrust faults that would thicken the back stop or raise it beyond a limited height a constant back stop height limits accretion and the subduction window appears to open and accommodate increased trench sediment beneath the back stop rather than inducing a transport of sediment over the top of the back stop nazca ridge subduction affected the convergent margin mass flux for about 8 ma mass balancing indicates an order of magnitude increase in the current sediment volume subducted in the north compared to that near the ridge crest when growth of the accretionary prism slowed a greater terrigenous input through the trench was accommodated by sediment subduction the resulting range of material flux is estimated to increase the sediment interlayer between the plates about 500 m which may affect coupling

50 ma initiation of hawaiian emperor bend records major change in pacific plate motion the hawaiian emperor bend has played a prominent yet controversial role in deciphering past pacific plate motions and the tempo of plate motion change new ages for volcanoes of the central and southern emperor chain define large changes in volcanic migration rate with little associated change in the chain s trend which suggests that the bend did not form by slowing of the hawaiian hot spot initiation of the bend near kimmei seamount about 50 million years ago ma was coincident with realignment of pacific spreading centers and early magmatism in western pacific arcs consistent with formation of the bend by changed pacific plate motion

rb sr sm nd and u pb geochronology of the rocks within the khlong marui shear zone southern thailand abstract in southern thailand the khlong marui shear zone is dominated by a nne ssw striking high topographic lozenge shaped area of ca 40 km long and 6 km wide between the khlong marui fault and the bang kram fault the geology within this strike slip zone consists of strongly deformed layers of mylonitic meta sedimentary rocks associated with orthogneisses mylonitic granites and pegmatitic veins with a steeply dipping foliation the strike slip deformation is characterized by dextral ductile deformation under amphibolite facies and low to medium greenschist facies in situ u pb ages of inherited zircon cores from all zircons in the khlong marui shear zone indicate that they have the same material from the archean late triassic to late cretaceous ages obtained for zircon outer cores of the mylonitic granite are probably related to a period of magmatic activity that was significantly influenced by the west burma and shan thai collision and the subduction along the sunda trench the early dextral ductile deformation phase of the khlong marui shear zone in the early eocene suggested by u pb ages of zircon rims and the later dextral transpressional deformation in the late eocene indicated by mica rb sr ages rb sr sm nd and u pb dating correlation implies that the major exhumation period of the ductile lens was in the eocene this period was tectonically influenced in the se asia region by the early india asia collision

arc continent collisions sediment recycling and the maintenance of the continental crust author posting geological society of london 2009 this is the author s version of the work it is posted here by permission of geological society of london for personal use not for redistribution the definitive version was published in geological society london special publications 318 2009 75 103 doi 10 1144 sp318 3

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

mass transport deposits in the northern ecuador subduction trench result of frontal erosion over multiple seismic cycles article i nfo investigations of mass transport deposits mtds and turbidite deposition in the confined north ecuador subduction trench provide access to paleoseismic information and insights into long term mechanisms for frontal tectonic erosion at a convergent margin the studied trench has been the site of four great subduction earthquakes 7 7 mw 8 8 during the 20th century the trench is isolated from major continental sediment input so that investigated mtds and turbidites are considered of local origin swath bathymetry seismic reflection and chirp data together with sedimentary cores and 14 c dating revealed that seven mtds were emplaced in distinct trench sub basins since 23 kyr and 27 turbidites deposited in the southernmost trench sub basin since 4 9 kyr our analysis shows that six mtds were derived from the margin while a single one stemmed from the outer trench wall temporal correlations between mtds emplaced within trench sub basins separated by a structural saddle indicate that the seven mtds were emplaced during five main events three were triggered locally and tentatively dated 5 8 1 6 kyr and recent whereas four were emplaced in distinct trench sub basins as a result of two regional events at 22 6 and 15 4 kyr none of the mtds occurred during the fast stage of the last sea level rise 13 to 8 kyr however dissociation of gas hydrates during the last 8 kyr stage of slow sea level rise might have contributed to trigger the three youngest mtds the large 1 5 13 5 kyr return time of the mtds contrasts with that of 189 yr of the turbidites the later is consistent with the 73 yr return time of two local mw 8 2 earthquakes implying that turbidites might have been triggered by large earthquakes the very large mtds return time is attributed to long term deformation processes and mechanical weakening of the margin outer wedge in response to repeated variations in basal friction pore pressure and margin extensional contraction strain over multiple earthquake cycles this process contributes to short term frontal erosion the rate of which is estimated to be 8 6 10 3 km 3 kyr km since at least 15 4 kyr

crustal structure and seismicity beneath the forearc off northeastern japan marine seismological and other data in the japan trench area north of 38 5 n were used to infer the relation of subducting plate structure seismicity and focal mechanisms especially at the aseismic to seismogenic zone along the interface between the two interacting plates previous crustal models from ocean bottom seismographic refraction obs surveys were improved by taking into account the relative seismic amplitude characteristics and constraining the shallow structure using multichannel seismic data a wave speed discontinuity in these models is interpreted to be the contact zone of the crust of the overriding plate and the subducting pacific plate crust its dip angle increases to about 7 110 km landward of the trench axis a large increase beneath the deep sea terrace is required to reach the well defined angle of 25 beneath the tohoku east coast a large wave speed gradient within layer 2 commonly observed under normal oceans seems to vanish beneath the inner trench slope at about 10 km depth within the overriding plate apparently brittle material with a p wave speed of 6 km s can be found as near as 35 km from the trench axis the upper limit of the seismogenic zone of interplate low angle thrust events is about 15 km depth from obs seismicity and large event analyses both the strength of the crust of the overriding plate and the characteristics of subducting sediments must be investigated to define the seismic coupling of interacting plates

focused subaerial erosion during ridge subduction impact on the geomorphology in south central peru data obtained from low temperature thermochronometers such as apatite fission track and u th he are combined with morphometric information extracted from digital elevation models this combination shows several geomorphological effects that are caused by the migration of the nazca ridge along the peruvian coastal margin offshore the depth of the deep sea trench decreases by 1500 m where the nazca ridge collides with the continental south american plate onshore the ridge causes an uplift of at least 800 m in the coastal cordillera this uplift results in a westward shift of the coastline thereby focusing and increasing erosion in the uplifted areas at the trailing edge the shelf subsides and the coastline retreats eastwards producing at least part of the indentation observed between paita and pisco the ridge acts therefore like a wave uplifting the andean margin as it traverses inland and southwards leaving a clear fingerprint on the topographic evolution of the peruvian coastal margin

fore arc seafloor unconformities and geology insight from 3 d seismic geomorphology analysis peru new 3d seismic data collected over 4870 km2 in the 3 45 12 30 s peruvian segment of the east pacific subduction system image seafloor erosional surfaces that can be mapped across the forearc basins forearc basins experience various stresses from their base where basal tectonic erosion acts to the seafloor which is influenced by aerial shallow and deep water currents driven by waves or thermohaline oceanic currents previously there has been little interest in stresses on the upper layer and there is a lack of documentation of unconformities and the erosive processes in certain bathymetric domains in forearc basins we address this with the study of examples sourced from 3d seismic reflection surveys of the seafloor offshore peru unconformities occur in two distinctive bathymetric domains associated with the continental shelf and the upper slope of the margin identification and characterization of unconformity surfaces yield estimates of the amount of erosion at the modern seafloor that range from 18 to 100 regional physical oceanography allows us to calibrate potential candidates for these two distinctive domains the first control on erosion is the dynamics of deep to intermediate oceanic currents related to the humboldt peru chile water masses while the second is wave action in the shallower erosional surfaces this study illustrates the unseen landscape of the forearc basins of south america and helps to highlight the importance of erosive surficial processes in subduction landscapes

transfer zones of deformation in thrust wedges an experimental study abstract although numerous transfer zones of deformation have been observed in thrust wedges their mechanisms and conditions of development are poorly understood in this paper three basic questions are addressed 1 1 what are the parameters controlling the genesis of transfer zones 2 2 what are the kinematics of thrust propagation during transfer zone development 3 3 which kinds of geological structure link the two compartments on either side of the transfer zone we attempted to solve these problems using a 3 d experimental approach sand models were used to simulate the foreland a rigid but mobile backstop was used to produced a thrust coulomb wedge which was studied during the various stages of its formation shortening led to the development of transfer zones in the wedge separating two domains of homogenous behaviour and connecting thrust planes in different domains the experimental results show that the development of transfer zones is controlled by mechanical and or geometrical parameters such as 1 1 variations in the sedimentary thickness in the foreland 2 2 the steepness of the backstop 3 3 domains with different basal friction in the foreland 4 4 kinked backstop geometry during deformation of the models two types of thrust sequence were identified alternative thrust propagations on both sides of the transfer zone and synchronous thrust propagations which are anastomosed within the transfer zone 3 d reconstructions revealed that the thrust deformations observed in the transfer zone are oblique ramps the frontal thrust bending of the barbados accretionary complex as well as the imbrication of thrust in the castellane arc and the oblique ramp in the zanskar range are discussed in relation to the mechanisms and structures revealed by the models the results of analogue modelling suggest that the development of transfer zones is directly linked to variations in the boundary conditions mechanical and or geometrical between two adjacent parts of a thrust wedge

continental and marine influences expressed by deep sea sedimentation off japan kaiko project abstract a total of about 100 sediments and sedimentary rocks composed of various clays to muddy silstones of late cenozoic age and limestones and sandstones of early cretaceous age were sampled by the submarine nautile during the kaiko project at 3000 to 6000 m water depths in the trench areas south and east of japan a total of 70 samples were selected and studied mainly by optical and electronic microscopy x ray diffraction atomic absorption spectrometry and microprobe analysis the dominant terrigenous influences allow us to identify two main mineralogical provinces to recognize the sedimentary expression of an early pleistocene tectonic event southeast of japan and to propose an indirect stratigraphic use of clay assemblages in poorly fossiliferous deep sea sediments the cretaceous sedimentary environments in kashima seamount area were submitted to various terrigenous and volcanogenic influences with possible early to late diagenetic modifications only very minor post sedimentary changes oxidations metallic coating barite and zeolite formation opal dissolution affect most of the late cenozoic deposits cropping out on the sea floor included those charaterized by the development of deep sea benthic communities e g clams important early diagenetic changes occur only in volcanic rich sediments of south zenisu ridge and south kuril trench in those peculiar deposits very abundant smectites develop with various shapes and chemical compositions pointing to the local peculiarities of volcanic glass morphology chemistry and alteration of permeability and of fluid migration processes

the possible role of fluids for the structuring of the continental crust abstract some well known seismic reflectivity patterns of continental crusts characterize specific tectonic terranes the role of fluids in enhancing or destroying such structures is studied seismicity is strongly dependent on the different pore pressure of fluids in intraplate settings in strike slip or extensional areas hydrostatic pore pressure seems to be approached in accordance with standard rheological models and standard seismicity depth distributions in suture zones or zones of developing loads possibly also along zones of near horizontal normal faults extremely high pore pressure develops which approaches lithostatic values they are accompanied by a number of weakening processes with enhanced shear and reduced viscosity these values correlate with low seismic velocities v p in accordance with an empirical v p relationship

small scale deformation structures and physical properties related to convergence in japan trench slope sediments diatomaceous mudstones cored on dsdp legs 56 57 off japan contain numerous postdepositional veins healed fractures and microfaults beneath the forearc basin these secondary structures occur only at depths greater than 620 m miocene in a zone of normal faulting beneath the landward slope of the trench however they occur at depths as shallow as 250 m pliocene in hemipelagic sediments which are inferred to blanket tectonically accreted deposits the intensity of deformation in the trench slope deposits increases in a seaward direction bulk densities indicate that the trench slope deposits have not dewatered normally but are above 400 m more consolidated than sediment deposited on the deep sea terrace a decline in sediment bulk density with increasing depth suggests that open fractures may exist in situ perhaps induced and or maintained by excess pore pressure below about 550 m a normal pattern of increasing density with increasing depth may reflect fracture closure and consolidation under lithostatic load we suggest that tectonic stress related to plate convergence has been communicated to the sediment on the landward japan trench slope causing tectonic dewatering within the subduction complex and inducing pervasive but small scale veining fracturing and faulting in near surface 700 m hemipelagic deposits this deformation occurs in sediment which has not been subject to large tectonic movement and does not eradicate larger 50 100 m thick structural depositional units which are apparent on seismic reflection records the brecciation induced by deformation may reflect natural hydrofracturing under abnormally high pore pressures or opening of fractures formed earlier at shallower depths during the dewatering process the overpressures may in turn locally reduce the sediment shear strength at shallow levels in the sediment column and induce downslope mass sediment movement

experimental formation of massive hydrate deposits from accumulation of ch4 gas bubbles within synthetic and natural sediments in order for methane to be economically produced from the seafloor prediction and detection of massive hydrate deposits will be necessary in many cases hydrate samples recovered from seafloor sediments appear as veins or nodules suggesting that there are strong geologic controls on where hydrate is likely to accumulate experiments have been conducted examining massive hydrate accumulation from methane gas bubbles within natural and synthetic sediments in a large volume pressure vessel through temperature and pressure data as well as visual observations observations of hydrate growth suggest that accumulation of gas bubbles within void spaces and at sediment interfaces likely results in the formation of massive hydrate deposits methane hydrate was first observed as a thin film forming at the gas water interface of methane bubbles trapped within sediment void spaces as bubbles accumulated massive hydrate growth occurred these experiments suggest that in systems containing free methane gas bubble pathways and accumulation points likely control the location and habit of massive hydrate deposits

a new type of active margin the convergent extensional margin as exemplified by the middle america trench off guatemala the results of dsdp legs 67 and 84 during which a transect of holes was drilled across the middle america trench off guatemala show no accretion at the front of that subduction zone since early eocene time the tectonic evolution of the trench includes extensional structure on the landward as well as the seaward trench slopes the guatemalan margin is proposed as the model of a new type of active margin the convergent extensional active margin ce margin

late palaeozoic and mesozoic palaeogeography of southeast asia abstract southeast asia comprises a complex assemblage of allochthonous terranes derived directly or indirectly from gondwana land and assembled between the late palaeozoic and cenozoic the continental core of the region is made up of four principal continental blocks south china indochina sibumasu and east malaya south china indochina and east malaya were rifted from northeast gondwanaland during the silurian or devonian and amalgamated to each other during the early carboniferous sibumasu remained attached to northwest australian gondwana land until the late early permian when it rifted away as part of the elongate cimmerian continent collision between sibumasu and indochina east malaya probably occurred in the early triassic forming the continental core of southeast asia other small terranes including mount victoria land sikuleh natal semitau and southwest borneo were accreted to this core during the jurassic and cretaceous to form sundaland a series of palaeogeographic maps are presented showing interpreted palaeopositions of the various southeast asian terranes in the late palaeozoic and mesozoic which are consistent with the current geological and geophysical constraints

comment on a 40ar 39ar and u pb isotopic study of the ilimaussaq complex south greenland implications for the 40k decay constant and the duration of magmatic activity in a peralkaline complex by krumrei et al

subduction of seafloor asperities revealed by a detailed stratigraphic analysis of the active margin shelf sediments of central ecuador the uplift of the coastal cordillera of central ecuador is a likely consequence of the subduction of the carnegie ridge a 400 km long and 2 km high topographic asperity this study aims at analyzing the impact of the subduction of the carnegie ridge on the continental shelf sedimentation during the quaternary we interpret high resolution 50 450 hz frequency multi channel 72 channels and single channel sparker seismic data piston cores sediment profiles 3 5 khz and high resolution multibeam bathymetry acquired during the atacames cruise with the rv l atalante in 2012 in the la plata island cabo san lorenzo peninsula region the results show a full and detailed record of the last ten quaternary sedimentary sequences deposited in intra shelf basins at and just seaward of the shelf break these 100 ka scale sequences are tied to piston core data to well dated quaternary onshore exposures and to a flight of coastal marine terraces on cabo san lorenzo peninsula and la plata island these sequences correlate with the global ice volume and deep marine temperature changes d18o for the last 0 7 ma which points to a strict climatic control on depositional sequence development and preservation however the subsidence of the continental shelf acoustic basement estimated by the stepwise backstripping of the sedimentary record exhibits a complex deformation pattern with uplifting and subsiding regions deep marine seismic data currently under processing show evidence for a subducted seamount beneath la plata island and gps data indicate an important interplate coupling in the same area with the potential to generate a mw 7 7 5 earthquake the pattern of the continental shelf deformation is consistent with the shape of this seamount we tested the hypothesis of a link between the margin deformation and the subduction of the seamount by comparing a stepwise subduction of the seamount to the palinspastic restoration of the deformation of the continental shelf for the last 1 ma this comparison shows that the collision probably started c 500 ka ago together with the syntectonic sedimentation and drastically slowed down by c 50 ka with the sealing of most of the deformation on the shelf

reconstruction of subduction zone paleogeometries and quantification of upper plate material losses caused by tectonic erosion we propose in this paper an original method for quantifying the material losses at subduction zones mainly based on vertical movements of the active margins we first test the method using isostatic and elastic models applying it to 15 different well constrained convergent margins the main conclusion is that most of them are presently close to the isostatic equilibrium consequently we consider as a first hypothesis that such was also the case in the past and that erosion or accretion are natural mechanisms restoring the equilibrium in response to a perturbation then we apply this model to compute the geometry of the palcoslab knowing the palcotopography of the margin the comparison between present and past ocean margin lithospheric wedge geometries provides estimates of the removed material that constituted the initial wedge two cases were studied for palcoreconstructions japan and peru the results show that a considerable amount of material was removed during the neogene along both margins a detailed interpretation of the neogene history of the japan margin is proposed on the basis of our results

subduction erosion and accretion in the solomon sea region abstract the solomon sea region is an area of intense tectonic activity characterized by structural complexity a high level of seismicity and volcanism and rapid evolution of plate boundaries there is little accretion in the eastern new britain trench accretion gradually increases westward with thick accretion in the western new britain trench and in the trobriand subduction system the thick accretion in the western part of the new britain trench may be a result of collision from the north of finisterre huon block with new guinea mainland the present boundary of the collision is along the ram markham fault deformation structures and present day seismicity suggest that the northern block is under compression accretion has occurred in the sediment filled trenches in the solomon sea the scale of the accretionary wedge depends on the amount of trench fill sediment available it is unlikely that there is no sediment supply to the eastern part of the new britain trench where no accretion is observed and subduction erosion may be occurring there are two possible mechanisms for subduction erosion of sediment either a rapid rate of subduction relative to the supply of sediment inhibiting sediment accumulation in the trench or horizontal tensional force superimposed on both the forearc and backarc regions of the arc seafloor spreading in both the manus and woodlark basins is fan like with nearby poles in the western margins of the basins this may be a reflection of a horizontally compressional field in the western part and a tensional field in the eastern part of the solomon sea therefore it is possible to conclude that the consumption of sediment in the eastern new britain trench is related to the horizontal tensional field superimposed on both the forearc and backarc regions of the subduction system imbricated thrust and overthrust faults in the western new britain trench and trobriand trough are not linear over long distance but form wavy patterns in blocks with unit distance of approximately 10 km

rock magnetic and geochemical evidence for authigenic magnetite formation via iron reduction in coal bearing sediments offshore shimokita peninsula japan iodp site c0020 sediments recovered at integrated ocean drilling program iodp site c0020 in a forearc basin offshore shimokita peninsula japan include numerous coal beds 0 3 7 m thick that are associated with a transition from a terrestrial to marine depositional environment within the primary coal bearing unit 2 km depth below seafloor there are sharp increases in magnetic susceptibility in close proximity to the coal beds superimposed on a background of consistently low magnetic susceptibility throughout the remainder of the recovered stratigraphic sequence we investigate the source of the magnetic susceptibility variability and characterize the dominant magnetic assemblage throughout the entire cored record using isothermal remanent magnetization irm thermal demagnetization anhysteretic remanent magnetization arm iron speciation and iron isotopes magnetic mineral assemblages in all samples are dominated by very low coercivity minerals with unblocking temperatures between 350 580 c that are interpreted to be magnetite samples with lower unblocking temperatures 300 400 c higher arm higher frequency dependence and isotopically heavy 56fe across a range of lithologies in the coal bearing unit between 1925 1995 mbsf indicate the presence of fine grained authigenic magnetite we suggest that iron reducing bacteria facilitated the production of fine grained magnetite within the coal bearing unit during burial and interaction with pore waters the coal peat acted as a source of electron donors during burial mediated by humic acids to supply iron reducing bacteria in the surrounding siliciclastic sediments these results indicate that coal bearing sediments may play an important role in iron cycling in subsiding peat environments and if buried deeply through time within the subsequent deep biosphere

continental and marine paleoenvironments reflected by west pacific clay sedimentation various stratigraphic and paleoenvironmental results arise from mineralogical geochemical and micromorpho logical investigations on late cretaceous and cenozoic sediments recovered by the deep sea drilling project in the west pacific stratigraphic correlations the deep pelagic red clays deposited on either side of the shatsky rise contain a variety of clay minerals which point to the importance of eolian detrital supply and allow rough stratigraphic correlations climate and paleocirculation neogene sediments of the lord howe rise area reflect closely the relationship between the evolution of the antarctic ice sheet and related aridification stages and the atmospheric and oceanic circulation pattern since 14 my regional and global tectonics the post eocene deposits around new zealand express a sequence of tectonic events and of relaxation stages associated with changes in the circulation regime along the inner wall of the japan trench clay mineral successions document the existence of an outer continental landmass until the late oligocene its collapse associated with volcanism close to the paleogene neogene boundary and an increased detrital influence from japan since the early middle miocene volcanism diagenesis and detrital supply the aptian to maastrichtian sediments in the mariana basin document a complex sequence of geochemical and geodynamical events proximal volcanism and local supply from subaerial archipelagoes early marine diagenesis during lithospheric subsidence and plate migration and finally the occurrence and evolution of terrigenous input from distant sources distributed by marine currents

a 40ar 39ar and u pb isotopic study of the ilimaussaq complex south greenland implications for the 40k decay constant and for the duration of magmatic activity in a peralkaline complex abstract magmatism in the gardar province south greenland is related to two main rifting events at 1280 ma and 1180 to 1140 ma little is known about the duration of the magmatic activity in a specific complex the ilimaussaq intrusion belongs to the second period of rifting and comprises an extraordinary diversity of granitic and syenitic rock types which intruded and fractionated in three successive magmatic events as the intrusion contains some of the most evolved incompatible element rich rocks on earth it was chosen for a detailed geochronological study to quantify the duration of melt production intrusion fractionation and cooling amphiboles which are abundant in all rock types including pegmatites and late magmatic veins were dated using the 40 ar 39 ar technique since the solidus temperature of the most evolved melts is below the closure temperature of amphibole and no later heating event occurred the 40 ar 39 ar ages reflect the magmatic crystallisation and can be used to determine the duration of igneous differentiation the 40 ar 39 ar plateau ages range between 1142 6 2 2 ma and 1152 3 3 7 ma using the steiger and jager steiger r h and jager e 1977 subcommision on geochronology convention on the use of decay constants in geo and cosmochronology earth planet sci lett 36 359 362 40 k decay constant these ages are younger than an u pb age of 1160 5 ma on baddeleyite from the first magma batch our results indicate that the 40 k decay constant of steiger and jager may be too high a lambda similar to that proposed by kwon et al kwon j min k bickel p and renne p r 2002 statistical methods for jointly estimating decay constant of 40 k and age of a dating standard math geol 34 457 474 is required to make the 40 ar 39 ar match the u pb age

eocene krabi basin southern thailand paleontology and magnetostratigraphy we present the magnetostratigraphy of the late eocene krabi section from the krabi basin thailand the investigated section is about 105 m thick and consists of red and yellow siltstones sandstones gray claystones lignites and limestones we collected 86 samples at 43 different stratigraphic levels from the 105 m thick section the samples yielded a paleomagnetic direction and paleomagnetic analyses give favorable demagnetization behaviors two magnetic components can be isolated rock magnetic experiments show that the natural remanent magnetization generally results from the presence of titanomagnetite the first component has roughly the direction of the present day field the second component displays only reversed polarity with a mean direction declination 192 34 inclination 15 5 n 71 95 4 7 k 14 after bedding correction mammalian biostratigraphy indicates a late eocene age correlation to the geomagnetic polarity time scale relies on biostratigraphic data previously proposed for the fauna found in the same section these correlations have been achieved by comparing variations in the sedimentation rate derived from the alternative correlation the proposed correlation puts the krabi section in chron c12r or chron c13r this suggests sedimentation rates of 4 9 and 9 4 cm k y respectively this correlation suggests that the krabi section is older than 31 ma and younger than 34 ma

tectonic regime of the southern kurile trench as revealed by multichannel seismic lines the analysis of the two multichannel seismic reflection records p 855 and p 856 reveals the complexity of the tectonic structure of the kurile margin the absence of trench fill and the relatively constant thickness of the subducted sediments on top of the oceanic basement shows that little or no accretion is occurring at present the lower slope is characterized by active thrusting within a small accretionary prism the toe of the middle slope is a more consolidated unit that constitutes an efficient buttress a piggy back basin possibly filled by sediments that have drifted down slope has formed on top of this unit a narrow high marks the transition to the edge of the continental terrace furthermore the upper slope reveals an unconformity marking the top of the basement the similarity of this unconformity and its sedimentary cover with the well documented data acquired around dsdp legs 56 57 suggests that northern japan and the southern kurile forearcs may have experienced a similar cenozoic vertical history 1 regional setting the southwestern part of the kurile arc in the vicinity of the so called hokkaido corner underwent a polyphase tectonic evolution during the tertiary kurile subduction is believed to have begun in the early jolivet 1986 or late kimura and tamaki 1985 cretaceous after the accretion of the okhotsk terrane to the continent of siberia both the nemuro peninsula and the is

strong b enrichment and anomalous δ11b in pore fluids from the japan trench forearc abstract interstitial pore waters from ocean drilling program odp sites 1150 and 1151 both penetrating 1200 m of sub seafloor sediment from the upper japan trench forearc were analyzed for b concentrations and 11 b isotopes b concentrations cover a wide range from 329 to 3920 m 0 8 9 3 seawater and vary isotopically from 20 to 46 relative to srm 951 in general b concentrations increase more or less steadily with depth at both sites the b enrichment and isotopic changes seem to be related to a deep fluid influence possibly coupled with alteration of volcanic products silica diagenesis does not affect the b geochemistry of the waters as is evident from intact diatom tests and high opal a contents throughout the holes the b enrichment is more pronounced at site 1150 in the seismic portion of the forearc here a large positive anomaly in 11 b values is observed which corresponds with abundant fracturing and two prominent shear zones at about the same depth range at site 1151 the 11 b values show a negative shift attesting to an enrichment in 10 b in the fluid as a result of b desorption from clay due to burial and tectonic stress we conclude that b release from clay minerals is the major contributor to pore fluid anomalies and that fluid flow through permeable out of sequence thrust like the shear zones at site 1150 may be an efficient mechanism for b backflux from the forearc into the hydrosphere

development of cenozoic basins in thailand abstract there are over 60 cenozoic basins distributed in various parts of thailand both onshore and offshore these basins are mainly n s trending half grabens or grabens and most were initiated in the late oligocene the structural framework of the basins is generally governed by n s trending extensional faults which are spatially related to the movement of the nw se and ne sw trending strike slip faults that have been active since the oligocene the nw se faults are the principal dextral strike slip faults whereas the ne sw faults are the conjugate sinistral strike slip faults the collision of the india craton with southern asia since 40 50 ma caused clockwise rotation of southeast asia and movements on the strike slip faults with the associated development of cenozoic basins in this region four major cenozoic basins mergui pattani malay and phitsanulok contain up to 8000 m of sediments generally the cenozoic fills are continental sediments except in the mergui basin in the andaman sea where the sedimentary fill is mainly marine sediments fluviatile with occasional lacustrine deposits were prominent in the gulf of thailand and onshore basins during the oligocene to early miocene from early to middle miocene widespread perennial lake conditions were well developed in the onshore basins and in the western graben area of the gulf of thailand a change in tectonic and or climatic conditions took place in the middle to late miocene times resulting in the cessation of perennial lake conditions and development of a regional unconformity

comment to distribution and geochronology of the oregon plateau u s a flood basalt volcanism the steens basalt revisited by m e brueseke et al j volcanol geotherm res 161 2007 187 214 abstract the 40 ar 39 ar stepheating results of brueseke et al 2007 are critically evaluated firstly for possible alteration of the samples dated steps passing the alteration index test baksi 2007a can then be used to derive plateau ages which must satisfy rigorous statistical tests of the eleven samples studied by brueseke et al 2007 only four yield plateau values that reflect eruption ages these suggest steens basalt was erupted within a relatively short period of time 0 2 m y considerably less than the 1 m y suggested by the authors the seeming difference between the plateau ages for the steens mountain basalts obtained by these authors and two earlier attempts is resolved by a proper adjustment of ages at various laboratories for interlaboratory calibration differences and by rejecting ages from altered samples the age of the important steens mountain geomagnetic polarity transition is 16 59 0 03 ma

the phuket terrane a late palaeozoic rift at the margin of sibumasu abstract it is widely accepted that sibumasu rifted from gondwana in the late palaeozoic but the rifts themselves have not previously been documented in southeast asia this paper identifies the pre middle permian kaeng krachan group of upper peninsular thailand as the infill of one such rift which is given the name phuket terrane indirect evidence suggests the rift infill is several kilometres thick and glacially influenced diamictites are conspicuous in the succession there are significant similarities with the 3 km thick pre middle permian rift infill of the carnarvon basin of western australia east of the khlong marui fault belt the succession is thinner and diamictites are a minor component a tectono stratigraphic model is proposed involving gondwana glaciers dropping their load at the present western margin of the phuket terrane from where it was re sedimented in the rapidly subsiding marine rift basin it is suggested that the khlong marui fault formed part of the eastern boundary of the rift system the three pagodas fault belt similarly juxtaposes different pre middle permian successions rifting ceased in the early permian and a passive margin formed as the mesotethys ocean widened the upper part of the kaeng krachan group and the overlying ratburi limestone representing the post rift sequence

a summary of cenozoic tectonic history along the ipod japan trench transect along the international program of ocean drilling ipod japan trench transect a mid cenozoic period of tectonism without arc volcanism separated an earlier cretaceous to paleogene and a later neogene period of plate convergence and subduction the cretaceous to early paleogene period of convergence is evidenced by andesitic volcanism associated with a large syncline probably a forearc basin the seaward flank of the basin is a tectonically thickened sediment sequence inferred to be an accretionary complex the arc the syncline and the thickened sediment sequence resemble an arc trench system of the size and structure of the great valley forearc basin and franciscan accretionary complex of california and they were probably continuations of the yezo geosyncline and associated sequences exposed to the north on hokkaido the arc trench system is contemporaneous with the adjacent shimanto arc trench system southwest of tokyo bay in the early paleogene volcanism ceased and part of the thickened sediment sequence of the cretaceous paleogene margin was emergent as a 160 km wide landmass which is recognized in seismic records as a later paleogene sediment source at the end of the paleogene this landmass began to subside and volcanism began again but it was located first briefly on the east and then on the west of the cretaceous arc along the present arc of northern honshu the land mass erosion surface is clearly visible as an angular unconformity in multichannel seismic reflection records the sequence of neogene sediment explosive volcanism and benthic foraminiferal assemblages recording subsidence from subareal conditions to the present bathyal depths have been studied in deep sea drilling project dsdp cores cretaceous subduction appears to have resulted in a very extensive accretionary complex whereas the neogene subduction period appears to have been marked by little net accretion in the forearc area but massive subsidence and some erosion of the front of the convergent margin

middle jurassic turtles from southern thailand a new genus and species of primitive cryptodiran turtle siamochelys peninsularis n g n sp from the middle jurassic mab ching locality in the southern peninsula of thailand is described on the basis of complete shells they are characterized by a combination of primitive features the presence of a pair of mesoplastra meeting on the midline and a series of derived characters loose plastron carapace attachment short diamond shaped entoplastron dorsal thickening of the lateral edge of the second to seventh peripheral plates expanded seventh to eleventh peripheral plates extension of the anal scute to the hyo hypoplastral suture or onto the hypoplastron midline sulcus sinusoidal and thus closely resemble chengyuchelys from the middle jurassic of china and more particularly xinjiangchelys and its relatives from the middle and late jurassic of china and central asia this supports a middle jurassic age for the locality and suggests that the sibumasu shan thai block was in contact with the asian mainland by that time

quantitative kinematic analysis within the khlong marui shear zone southern thailand abstract the nne trending khlong marui shear zone has a strong geomorphic signal with marked fault strike parallel topographic ridges the lithologies within the strike slip zone mainly consist of vertical layers of mylonitic meta sedimentary rocks associated with orthogneisses mylonitic granites and pegmatitic veins the pegmatitic veins concordantly intrude the mylonitic foliation but were sheared at the rims indicating syn kinematic emplacement microstructures and mineral assemblages suggest that the rocks in the area have been metamorphosed at amphibolite facies and low to medium greenschist facies by the first deformation the khlong marui shear zone was deformed under dextral simple shear flow with a small finite strain the ductile to brittle deformation involves a period of exhumation of lenses of higher grade rocks together with low grade fault rocks probably associated with positive flower structures the final stage brittle deformation is reflected by normal faulting and formation of proto cataclasites to cataclasites of the original mylonitic meta sedimentary host rock although clear age constraints are still missing we use regional relationships to speculate that earlier dextral strike slip displacement of the khlong marui shear zone was related to the west burma and shan thai collision and subduction along the sunda trench in the late cretaceous while the major exhumation period of the ductile lens was tectonically influenced by the early india asia collision the changing stress field has responded by switching from dextral strike slip to normal faulting in the khlong marui shear zone and is associated with escape tectonics arising from the overall india asia collision

stratigraphic significance of siliceous microfossils collected during nautiperc dives off peru 5 6 s the geological evolution of the northern peru convergent margin can be traced using samples collected during deep sea dives of the submersible nuutile in the paita area 5 6 s the sedimentary sequence was intensively sampled along the main scarp of the middle slope area it consists of upper miocene 7 9 ma to pleistocene siltstone sandstone and rare dolostone the age distribution of these samples is the basis for a new geologic interpretation of the multichannel seismic line cdp3 siliceous microfossils both diatoms and radiolarians show influence of both cold and temperate waters local species mixed with upwelling ones diatom assemblages studied from the npl 13 and npl 15 dives bear a strong resemblance to assemblages from the pisco formation of southern peru micropaleontological data from siliceous microfossils provide evidence for two main unconformities one is at the base of the quatemary sequence and the other corresponds to a hiatus of 1 myr separating the upper miocene 7 8 ma sediments from uppermost miocene 5 6 ma sediments during the past 400 kyr a wide rollover fold developed in the middle slope area associated with a major seaward dipping detachment fault a catastrophic debris avalanche occurred as the result of an oversteepening of the landward flank of the rollover fold the gravity failure of the slope recognized by seabeam and hydrosweep mapping displaced enough material to produce a destructive tsunami which occurred 13 8 f 2 7 kyr ago

late quaternary geomorphologic evolution of submarine canyons as a marker of active deformation on convergent margins the example of the south colombian margin abstract the morphology of patia and mira canyons on the south colombian convergent margin reflects an interplay between tectonic deformation sea level variation and canyon evolution and provides new insight into the age and location of margin deformation over the last 150 ka multibeam bathymetry seismic and sedimentary data reveal that tectonically active structural highs control canyon incision and the canyon s course the canyons developed across the margin in five major stages first the upper slope was incised by headward and downward erosion during the pleistocene infilling a structurally bounded slope basin the basin periodically spilled over and breached the accretionary prism at 150 ka leading to the development of isolated sediment lobes in the trench the prism was efficiently breached leading to a well developed trench channel levee system at 53 67 ka today the system shows limited activity antecedent streams convex up axial incision profiles and increasing height width ratio indicate an active uplift of the structural highs since at least 150 ka and support localized shortening through the margin accommodated by out of sequence structures thrusts and folds an 80 m high scarp where the canyon crosses a fault on the middle slope further supports active uplift related to a major thrust previous seismostratigraphic studies of the margin have demonstrated that active uplift occurred during the early pliocene here we demonstrate that uplift continued throughout the late pleistocene comparisons with canyons on other convergent margins reveal that features relating to margin deformation and canyon age tortuous path convex up profiles abandoned canyon paths overincision abrupt canyon turns are generally restricted to accretionary margins because of their complex morphology accretionary margins appear more favorable for the occurrence of such features than margins undergoing tectonic erosion with more simple morphology

structure of the frontal part of the andean convergent margin a multichannel seismic reflection record across the central peru margin and trench was improved by processing 24 rather than the 12 channels previously processed and by thorough migrating to reject the strong diffractions that obscured weaker primary reflections the increased resolution clarifies the structure of the 15 km wide frontal accretionary complex and the adjacent truncated continental framework against which the trench sediment was imbricated resolved are individual thrust slices and packets adjacent to the trench axis subducting sediment filled graben in the ocean crust beneath the lower slope and a tertiary stratigraphic section of the upper slope yaquina basin which is cut by normal faults the resolution in this multichannel record provides criteria for reinterpreting single channel data off chile with increased confidence the previously proposed truncation of the south american continent along much of the peru chile trench is confirmed and the development of an accretionary complex in front of the truncated continental crust appears to vary with the amount of sediment seen in the trench axis the andean margin frontal structure is similar to that off central america the aleutian trench and the japan trench which suggests common truncation as well as accretion at the front of convergent margins

cenozoic evolution of the savu basin indonesia forearc basin response to arc continent collision abstract a seismo stratigraphic analysis of the savu basin relates the evolution of a complexly evolved forearc basin to the late miocene recent collision with australia initial early miocene subsidence of the outer forearc basin and the subsequent development of a middle late miocene volcanic proto arc located in the south of the basin both resulted from the late oligocene early miocene eastward propagation of the java timor trench block faulting of the relatively thick continental forearc basement probably determined the rather irregular and scattered pattern of middle late miocene volcanic centres just north of the accretionary prism the forearc basement is flexed down and reshaped into a trenchward dipping backstop that facilitates backthrusting of the accretionary prism the southern forearc basement probably acted as a barrier against compression the thickness of this continental basement is suggested to be a critical factor in the response of the forearc to continent collision owing to its strength the savu basin did not react to collision by the development of northward verging inter arc thrusts in contrast it responded to the underthrusting of continental crust by reactivation of the basement ridges this resulted in the differentiation of the forearc basin into a presently extinct and uplifted miocene south savu basin and a pliocene recent active north savu basin the simultaneous late miocene recent uplift of large segments of the outer forearc and subsidence of the north savu basin is interpreted to result from tectonic underplating of continental material and uplift caused by the buoyancy of the subducted continental crust the relatively recent decoupling of this crust from the oceanic slab and the subsequent isostatic rebound may superimpose the initial effects of post collision extension on arc continent collision

orogeny and relative plate motions example of the japanese islands abstract the paleogeography of the eastern margin of asia near japan for the past 250 ma has been reconstructed on the basis of the relative motions between the oceanic plates and the eurasian plate engebretson et al 1982 and on land geology the relationships between the plate kinematics and orogeny are as follows calc alkaline magmatism is related to the presence of subduction when a plate boundary is a transform fault there is no volcanism very slow convergence rates do not give rise to magmatism migration of rft type triple junctions or ridge subduction produces forearc volcanism near the trench and also a wider volcanic zone uplift of low p t type regional metamorphic belts is related to collision episodes the uplift of the high p t blueschist belts on the contrary is not contemporaneous with them they appear to be related to the start of the subduction of a young plate after the migration of an rft type triple junction because the formation of a voluminous accretionary complex is contemporaneous with this and thrusting of this young accretionary complex beneath the uplifted metamorphic belt is often observed the underplating of the young accretionary complex beneath the older accretionary complex would be the cause of the uplift of the high p t type metamorphic belt the subduction of a young plate would thus be an important factor causing these orogenic events highly oblique convergence also results in formation of a serpentinite melange zone with fragments of blueschists presumably along a mechanical plate boundary in the forearc

accretion and erosion in subduction zones the role of fluids convergence occurs today along about 60 000 km of plate boundaries parsons 1981 in this paper we consider frontal subduction which occurs along about 40 000 km of convergent plate boundaries using the kine matic solution of de mets et al 1990 and including 0 5 km 2 yr1 of marginal basin openings otsuki 1989 we find that 2 95 km 2 per year disappear in frontal subduction at an average rate of 7 3 cm yr the upper overriding plate is significantly modified not only through the addition of a large amount of magmatic material markhinin 1968 but also through tectonic processes which result either in outgrowth of the margin through addit ion of sediments silver 1969 or in recession through frontal and subcrustal erosion which appears to be linked to sediment subduction coats 1962 gilluly 1963 the various phenomena observed must in some way be related to variations in mechanical coupling between the two plates uyeda kanamori 1979 had proposed that the transition from erosion to accretion results from increasing mechanical coupling the marianas trench was presented as typical of an crosional margin and the chile trench of an accreting margin ruff kanamori 1980 and kana mori 1986 showed that the magnitude of the subduction earthquakes increases with the convergence velocity and the inverse of the age of the subducted lithosphere this correlation was qualitatively explained by the control of age on the d ip of the platc and of convergence velocity on the

paleo environments and tectonic setting of the mesozoic thung yai group in peninsular thailand with a new record of parvamussium donaiense mansuy 1914 abstract the thung yai group extends over a large area of peninsular thailand along the eastern margin of the shan thai block bound by angular unconformities 300 m thick dominantly detritic brackish to non marine deposits with few intercalated limestone beds between triassic marine and tertiary non marine sediments represent the thung yai group that comprises four formations khlong min lam thap sam chom and phun phin formations in the ao luk plai phraya alpp area the khlong min and lam thap formations yield marine brackish water and non marine fossil assemblages these include trace fossils and for the first time in peninsular southern thailand the bivalve parvamussium donaiense mansuy 1914 based on fossil determinations the thung yai group has a late early jurassic to early cretaceous age our new observations help unravel the tectonic history of mesozoic peninsular thailand after the complete closure of the paleotethys in the late triassic renewed inundation from the late early jurassic to the early middle jurassic brought a regime of shallow to open marine and lagoon sedimentation over northwestern western and southern peninsular thailand in the eastern part of sundaland bordering the mesotethys to the west

the timing of strike slip shear along the ranong and khlong marui faults thailand 1 the timing of shear along many important strike slip faults in southeast asia such as the ailao shan red river mae ping and three pagodas faults is poorly understood we present 40ar 39ar u pb shrimp and microstructural data from the ranong and khlong marui faults of thailand to show that they experienced a major period of ductile dextral shear during the middle eocene 48 40 ma centered on 44 ma which followed two phases of dextral shear along the ranong fault before the late cretaceous 81 ma and between the late paleocene and early eocene 59 49 ma many of the sheared rocks were part of a pre kinematic crystalline basement complex which partially melted and was intruded by late cretaceous 81 71 ma and early eocene 48 ma tin bearing granites middle eocene dextral shear at temperatures of 300 500 c formed extensive mylonite belts through these rocks and was synchronous with granitoid vein emplacement dextral shear along the ranong and khlong marui faults occurred at the same time as sinistral shear along the mae ping and three pagodas faults of northern thailand a result of india burma coupling in advance of india asia collision in the late eocene 37 ma the ranong and khlong marui faults were reactivated as curved sinistral branches of the mae ping and three pagodas faults which were accommodating lateral extrusion during india asia collision and himalayan orogenesis

continental margin tectonics submarine accretionary prisms submarine accretionary prisms are sites of incipient mountain building where sediments and rocks of the deep ocean are uplifted deformed and ultimately transformed to continental crust accretionary prisms develop at sites of lithospheric convergence that is subduction zones commonly accretionary prisms form along continental margins although accretionary prisms may also occur adjacent to island arcs which are ultimately coalesced into continental landmasses here we review the studies of submarine accretionary prisms during the past four years with primary focus on the contributions of geoscientists from the united states

first dinosaur from the shan thai block of se asia a jurassic sauropod from the southern peninsula of thailand a vertebra collected from the jurassic non marine khlong min formation of southern thailand is referred to the family euhelopodidae a group of sauropod dinosaurs that apparently was endemic to eastern asia during the jurassic and early cretaceous at a time when that part of the world was isolated from other land masses the occurrence of a euhelopodid in the jurassic of the shan thai block supports the idea of a collision of the shan thai block with the indochina block thus establishing connections with mainland asia early in the mesozoic probably before the jurassic

nonlithostatic pressure during sediment subduction and the development and exhumation of high pressure metamorphic rocks a subduction shear zone can be modeled as a long narrow channel with the flow of subducted metasedimentary rocks in the channel driven by two sets of forces the downward shearing force exerted by the subducting slab and the gradient in the hydraulic potential which combines the effect of both pressure and buoyancy if the channel walls are effectively rigid very slight narrowing or broadening of the channel convergence angles 2 gpa in the channel at only 40 km depth the model is consistent with a horizontal balance of forces across the plates and with a reasonable value for the thickness of subducted sediment 650 m the practical limit for overpressures attainable in subduction zones is determined by the strength and permeability of the channel walls at 40 km depth the channel is effectively confined on both sides by cold lithospheric mantle which should be strong enough to support a significant tectonic overpressure episodic failure of the upper plate to produce great earthquakes at 30 40 km focal depth could vent overpressured fluid from the channel allowing a cyclical buildup and release of both rock and fluid pressure topography on the subducting plate e g seamounts and thinned continental crust may lead to an anvil like jamming of the channel and local high overpressures tectonic erosion by topography on the lower plate of slivers from overlying continental crust and the compression of these slivers between the topography and the narrowing channel walls could produce high overpressures in continental rocks a decrease in the convergence rate or cessation of subduction with a consequent general warming within the channel and associated viscosity decrease promotes exhumation by buoyant reverse flow the most rapid reverse flow occurs in the region of previously greatest overpressure since the exhumation distance is shorter than for a simple lithostatic pressure distribution and any increase in temperature is coupled with a strong increase in the rate of exhumation preservation of high pressure assemblages at the surface in fossil subduction zones is promoted for such a model

implications of measured thermal gradients for water movement through the northeast japan accretionary prism thermal gradients of 24 36 c km have been found in the slope cover sediments of the accretionary prism off northeast japan deep sea drilling project dsdp sites 438a 439 and 440 these gradients lead to the prediction of temperatures in excess of 400 c at depths of 15 km using an increase in thermal conductivity with depth due to the decrease in porosity such temperatures conflict with the predictions of temperatures of 150 c at even greater depths from conductive thermal models and petrologic studies of low temperature blueschists at similar ancient margins one dimensional steady state conduction and advection via darcian water flow of heat up through the prism was simulated numerically the measured thermal gradients can be reconciled with temperatures less than 125 c at the prism base by water flowing upward at rates of the order of 10 10 m s such velocities imply that the total volume of water available from compaction of sediments subducted beneath the northeast japan prism would flow up through a 10 km width of the prism perpendicular to the trench and would require the dsdp sites to have been located fortuitously over the localized zone s of water flow smaller velocities will reproduce the measured gradients and thus greater widths through which water flow occurs are predicted if the temperature at the prism base is higher and or if the prism is thinner and a subduction channel several kilometers high separates the accreted sediments from the descending plate where water flow is significant as likely wherever abundant water rich sediments are subducted thermal gradients below a few kilometers in accretionary prisms may be much lower than near surface values this has profound implications for the thermal structure of the prism off northeast japan and elsewhere

along arc structural variation of the plate boundary at the japan trench margin implication of interplate coupling 1 at the japan trench convergent margin many large interplate earthquakes of greater than m7 5 frequently occur their epicenters have uneven distribution mostly located in the northern area to investigate the relationship between this distribution and tectonic structures we have conducted multichannel seismic surveys since 1996 our data show two kinds of interplate sedimentary units a wedge shaped unit and a channel like unit both units have a lower p wave velocity than the basal part of the overriding island arc crust the wedge shaped unit having a velocity of 2 3 km s is widely distributed over the forearc region in the northern area its thickness decreases with depth becoming several hundred meters at a depth of 12 km the channel like unit having a velocity of 3 4 km s is observed in the southern area extending in the downdip direction its thickness reaches 2 km at a depth of 12 km if the low velocity of these units results from the existence of fluid as many authors assume the units being thick implies higher fluid content assuming constant porosity considering that fluid reduces basal friction and with an assumption that fluid available at a specific interface is proportional to the total fluid content in the sediment the thickness variation of the units would cause different degrees of coupling at the plate boundary along the arc this may provide one explanation for the regional disparity in the interplate earthquake occurrence in the margin furthermore we attempt to call attention to the possibility that the channel like sediment works as a shear stress releaser

stratigraphic correlation of upper cretaceous to paleocene forearc basin sediments in northeast japan cyclic sedimentation and basin evolution abstract cretaceous to paleocene sediments are distributed across a 200 km wide and 1400 km long belt in northeast japan to sakhalin twenty six stratigraphic sections including land based and offshore drill holes and surface sections are correlated to delineate the sedimentary history and basin evolution of the yezo forearc basin two second order shallowing upward cycles are recognized in hokkaido but are obscure in north honshu shallow marine to paralic sediments of the first cycle early albian to turonian are represented by the mikasa formation late albian to turonian as the western marginal facies while the eustatic curves e g haq curves reach the highstand maximum at the late cenomanian the campanian to maastrichtian shallow and non marine facies of the second cycle coniacian to maastrichtian are characterized by the hakobuchi group in hokkaido and its correlatives in north honshu this large scale shallowing may have reflected directional changes of the izanagi kula plate motion near the early campanian the uppermost maastrichtian to upper paleocene is possibly absent everywhere in the basin the shallow marine to non marine facies often show several third order depositional sequences with upward coarsening facies successions they may have been controlled by global eustasy even in an active margin setting since the stacking patterns of sequences and the timing of sequence boundaries appear considerably concordant with the oscillation patterns of the haq curves

isotope tracking of pore water freshening in the fore arc basin of the japan trench abstract pore waters of sediments in convergent zones typically show a decrease in chloride cl concentrations with depth that suggests the advection of chloride depleted fluids studies from odp leg 186 indicate that sediments accumulated in the fore arc of the japan trench also exhibit a freshening of pore waters with depth here the oxygen 18 o and hydrogen d isotopic composition of pore waters from site 1150 were determined to investigate the source of cl depleted fluids whereas the upper 100 m of sediments have pore waters exhibiting an increase in both 18 o and d values with depth the remaining 1000 m of studied sediments have pore waters exhibiting an overall decreasing trend in isotope values with depth dissociation of gas hydrates is a likely explanation for the elevated isotope values for pore waters in the upper 100 m in contrast the overall decreasing trend in isotope values probably resulted from the advection of cl depleted fluids exhibiting low 18 o and d values depletion of cl 18 o and d likely resulted from the diagenetic transformation of hydrous minerals and the subsequent precipitation of carbonates or other 18 o enriched minerals generation of cl depleted fluids did not occur within the studied depth interval but rather at deeper possible over pressured sites thereby pointing to the significance of deep seated fluid flow in fore arc basins

discussion of tectonic models for cenozoic strike slip fault affected continental margins of mainland se asia abstract understanding the roles of cenozoic strike slip faults in se asia observed in outcrop onshore with their offshore continuation has produced a variety of structural models particularly pull apart vs oblique extension escape tectonics vs slab pull driven extension to explain their relationships to sedimentary basins key problems with interpreting the offshore significance of major strike slip faults are 1 reconciling conflicting palaeomagnetic data 2 discriminating extensional and oblique extensional fault geometries from strike slip geometries on 2d seismic reflection data and 3 estimating strike slip displacements from seismic reflection data focus on basic strike slip fault geometries such as restraining vs releasing bends and strongly splaying geometries approach the gulfs of thailand and tonkin suggest major strike slip faults probably do not extend far offshore splays covering areas 10 000 s km2 in extent are characteristic of the southern portions of the sagaing mae ping three pagodas and ailao shan red river faults and are indicative of major faults dying out the areas of the fault tips associated with faults of potentially 100 km displacement scale appropriately with global examples of strike slip faults on log log displacement vs tip area plots the fault geometries in the song hong yinggehai basin are inappropriate for a sinistral pull apart geometry and instead the southern fault strands of the ailao shan red river fault are interpreted to die out within the nw part of the song hong yinggehai basin hence the fault zone does not transfer displacement onto the south china seas spreading centre the strike slip faults are replaced by more extensional oblique extensional fault systems offshore to the south the sagaing fault is also superimposed on an older paleogene early miocene oblique extensional rift system the sagaing fault geometry is complex and one branch of the offshore fault zone transfers displacement onto the pliocene recent andaman spreading centre and links with the west andaman and related faults to form a very large pull apart basin

the geologic history of the caribbean cocos plate boundary with special reference to the nicoya ophiolite complex costa rica and d s d p results legs 67 and 84 off guatemala a synthesis abstract the pre upper senonian basement of costa rica crops out in the santa elena and nicoya peninsulas from south to north and from base to top the basement includes the esperanza matapalo and santa elena units the esperanza unit is albian santonian in age and consists mainly of pillow basalt and massive basalt flows the matapalo unit includes callovian to cenomanian radiolarite and includes massive basalt flows basalt and dolerite basement the santa elena unit contains ultramafic and mafic rocks in which harzburgite is the major component the most important tectonic features of the nicoya complex are the large santa elena and matapalo nappes nappe emplacement was from north to south during upper santonian time the sedimentary cover of the nicoya complex comprises 1 1 the campanian el viejo formation that consists of shallow water sediments in the north santa elena peninsula and the campanian maastrichtian sabana grande formation of deep water origin in the south nicoya peninsula 2 2 paleocene strata indicating deposition in a deep water environment comprises the rivas las palmas and samara formations 3 3 a post upper eocene sequence that consists of the shallow water barra honda and montezuma formations two unconformities are significant geological features of the upper senonian to tertiary history of costa rica the lower one is at the base of the sabana grande formation and marks a major change in the geologic conditions basalt is scarce in the campanian tertiary series the upper unconformity at the base of the barra honda and montezuma formations is not as major as the lower one during post campanian time normal faulting occurred in two stages separated by a strong erosional phase the geology of the landward slope of the adjacent middle america trench is outlined by interpreting multifold seismic reflection records off the west coast of costa rica and the dsdp legs 67 and 84 transects off guatemala the western caribbean plate boundary may have been under extensional stress for the last 75 m y the strong landward dipping reflectors of the middle america trench landward slope off guatemala could be equivalent to the on land pre campanian overthrusts of costa rica the available data are consistent with the convergent extensional margin concept

tectonic history of the northern peru convergent margin during the past 400 ka the late pleistocene tectonic history of the northern peru convergent margin can be retraced using data collected during deep sea dives of the submersible nautile combined with a recent hydrosweep survey conducted off peru at lat 5 6 s by the r v sonne during the past 400 ka a broad rollover fold formed in the middle slope area in association with a major seaward dipping detachment fault a catastrophic debris avalanche occurred as the result of oversteepening of the landward flank of the rollover fold the gravity failure of the slope previously recognized by seabeam mapping occurred at 13 8 2 7 ka and produced a destructive tsunami

slope instabilities at an active continental margin large scale polyphase submarine slides along the northern peruvian margin between 5 s and 6 s a mega submarine slide was evidenced along the peruvian margin during a seabeam survey of the r v charcot bourgois et al 1986 the debris slide was initially interpreted as the result of the slope failure occurring along a seaward curved scarp and extending from east to west along the line of maximum slope complementary bathymetrie data obtained from the r v sonne survey conducted to the north and to the south of the previous surveyed area has resulted in the identification of large scale polyphase submarine slides involving a total surface of about 1000 km2 between latitudes 5 15 s and 6 05 s using seabeam and hydrosweep multibeam echosounder data in combination with deep sea submersible observations three distinct slope failure types related to three main stages have been revealed the three sliding phases occurred roughly along the same trend orientated n230 and are therefore mainly controlled by the n80 orientation of the subducting nazca plate 1 the first phase of slope failure is documented by a debris avalanche deposit which extends from the lower slope down to 5 km within the trench floor the deposit originates from the northern wall of a wide valley located along the upper slope 2 the second phase of slope failure is characterized by a debris avalanche with a crescent shaped scar located along the middle slope and a hummocky deposit covering the lower slope and extending up to 10 km across the trench the volume of rock involved in this event is estimated to some 250 km3 the slope failure is assumed to be related to an oversteepening of the middle slope induced by a rollover deformation 3 the third phase of slope failure corresponds to a translational sliding block and a toppling block with volumes of 6 km3 and 13 km3 respectively the seismic energy produced during the seismic cycle has greatly increased facturation and fluids buildup along the area previously weakened by a rollover fold a restricted n s folding is observed in the vicinity of the trench to the north of the two debris avalanche deposits it may have formed in relation to the local compression limited to the subduction of the nazca plate sliding and folding thus document the paradox between the compressive regime in the lower plate and the extensional regime in the upper part of the upper plate

cenozoic increase in subduction erosion during plate convergence variability along the convergent margin off trujillo peru abstract subduction erosion is a long term large scale geological process that dominates the structural inventory of a large number of convergent margins along the peruvian convergent margin this process has controlled the structural deformation of the forearc as a consequence of spatio temporal variations during the cenozoic however the pace at which tectonic erosion occurred has not been linked to regional subduction related events the aim of this study is to show that subduction erosion along the central peruvian convergent margin off trujillo 7 s 9 s was influenced by plate convergence variability since the late cretaceous drilling data multi channel seismic mcs data and subsidence analyses are used to investigate the long term effect of subduction erosion on the tectonic evolution of the peru margin our study shows that subduction erosion 1 did not occur between the cretaceous and early eocene 110 56 ma 2 increased and fluctuated between the eocene and pliocene 56 5 ma with a maximum average trench retreat of 3 km myr 1 between the middle miocene and late miocene 20 5 ma and 3 has decreased since the pliocene 5 ma 1 km myr 1 of average trench retreat a remarkable finding is that shorter periods of faster subduction erosion coincide with or follow major plate reorganization events that were marked by changes in the velocity and direction of relative plate convergence as well as with well known phases of andean orogeny based on our analyses we therefore conclude that convergence variability off peru is a primary mechanism controlling the pace at which tectonic erosion occurred along the trujillo basin during the cenozoic

reply to comment on precise k ar 40ar 39ar rb sr and u pb mineral ages from the 27 5 ma fish canyon tuff reference standard by m a lanphere and h baadsgaard

late oligocene early miocene major tectonic crisis and related basins in bolivia recent advances in biochronology geochronology magnetostratigraphy structural geology and basin analysis make untenable the traditional correlation of the first major andeanage deformational episode in bolivia with the middle eocene incaic tectonic phase defined in central peru and instead demonstrate that the episode took place in late oligocene and early miocene time this major tectonic crisis resulted in contemporary development of the subandean external foreland basin and altiplano intermontane basin which were separated by the initiation of thrusting in the present cordillera oriental area the deformation suggests that the bolivian orocline began to develop at that time it is likely that this tectonic upheaval is genetically linked to the marked increase in rate of plate convergence produced by the contemporaneous breakup of the farallon plate

tectonic framework and phanerozoic evolution of sundaland abstract sundaland comprises a heterogeneous collage of continental blocks derived from the india australian margin of eastern gondwana and assembled by the closure of multiple tethyan and back arc ocean basins now represented by suture zones the continental core of sundaland comprises a western sibumasu block and an eastern indochina east malaya block with an island arc terrane the sukhothai island arc system comprising the linchang sukhothai and chanthaburi blocks sandwiched between this island arc formed on the margin of indochina east malaya and then separated by back arc spreading in the permian the jinghong nan uttaradit and sra kaeo sutures represent this closed back arc basin the palaeo tethys is represented to the west by the changning menglian chiang mai inthanon and bentong raub suture zones the west sumatra block and possibly the west burma block rifted and separated from gondwana along with indochina and east malaya in the devonian and were accreted to the sundaland core in the triassic west burma is now considered to be probably cathaysian in nature and similar to west sumatra from which it was separated by opening of the andaman sea basin south west borneo and or east java west sulawesi are now tentatively identified as the missing argoland which must have separated from nw australia in the jurassic and these were accreted to se sundaland in the cretaceous revised palaeogeographic reconstructions illustrating the tectonic and palaeogeographic evolution of sundaland and adjacent regions are presented

do trench sediments affect great earthquake occurrence in subduction zones seismic energy release is dominated by the underthrusting earthquakes in subduction zones and this energy release is further concentrated in a few subduction zones while some subduction zones are characterized by the occurrence of great earthquakes others are relatively aseismic this variation in maximum earthquake size between subduction zones is one of the most important features of global seismicity previous work has shown that the variation in maximum earthquake size is correlated with the variation in two other subduction zone properties age of the subducting lithosphere and convergence rate these two properties do not explain all the variance in maximum earthquake size i propose that a third subduction zone property trench sediments explains part of the remaining variance in maximum earthquake size subduction zones are divided into two groups 1 those with excess trench sediments and 2 those with horst and graben structure at the trench thirteen of the 19 largest subduction zone events including the three largest occur in zones with excess trench sediments about half the zones with excess trench sediments are characterized by great earthquake occurrence most of the other zones with excess trench sediments but without great earthquakes are predicted to have small earthquakes by the age rate correlation two notable exceptions are the oregon washington and middle america zones overall the presence of excess trench sediments appears to enhance great earthquake occurrence one speculative physical mechanism that connects trench sediments and earthquake size is that excess trench sediments are associated with the subduction of a coherent sedimentary layer which at elevated temperature and pressure forms a homogeneous and strong contact zone between the plates

fluvial architecture and sequence stratigraphy of the eocene to oligocene iwaki formation northeast japan channel fills related to the sea level change abstract the iwaki formation the lowermost part of the eocene to oligocene shiramizu group northeast japan consists of conglomeratic braided drainage sediments the formation displays fining upward succession that reflects an overall transgressive sequence with two other overlying formations of the group although the clast supported conglomerates are by far the major components of the formation they also intercalate with many concave up channel sediments filled up with sandy deposits the lower part of the formation mainly consists of multistorey large scale channel sandstones with minor amount of small scale single storey channel sandstones which are related to low rates of base level rise in contrast the middle to upper part of the formation comprises single storey strata with relatively small scale channels which were probably associated with more rapid rates of base level rise the field evidence shows that the change of scale and internal structures of channel sandstones within the conglomerates most likely reflect the rate of changes of sea level rise

extensional tectonics in the honshu fore arc japan integrated results of dsdp legs 57 87 and reprocessed multichannel seismic reflection profiles

subduction zone recycling processes and the rock record of crustal suture zonesthis article is one of a series of papers published in this special issue on the theme lithoprobe parameters processes and the evolution of a continent offshore observations at modern ocean margin subduction zones omszs reveal that bodies of accreted material are commonly volumetrically small or missing that crustal thinning and subsidence 3 5 km has occurred and that most trench axes lie close 5 30 km to the seaward tapering edge of coastal basement rock onshore mapping commonly documents missing or only narrow terranes of former forearc rock and the inboard migration of the arc magmatic front these observations are evidence that subduction is accompanied by the removal of sediment and crustal material from the submerged forearc by the kindred tectonic processes respectively of sediment subduction and subduction erosion subduction erosion truncates the margin migrates the trench inboard at 2 5 km ma onshore observations at ancient crust suturing subduction zones csszs imply that collisional suturing is accompanied by sediment subduction and truncation of both upper and lower plates during a protracted period of suturing 20 50 millio

tectonic record of strain buildup and abrupt coseismic stress release across the northwestern peru coastal plain shelf and continental slope during the past 200 kyr 200 kyr both segments uplifted at high rates of 10 to 20 mm yr 1 through tectonic pulses coeval with the eustatic deglacial sea level rises of isotope stage 1 and warm isotope substage 5e respectively the uplift and related extensive emersion of the coastal plain require high coupling along the subduction zone and or underplating at depth offshore industry acquired reflection seismic lines combined with em12 bathymetric data allow us to investigate the tectonic regime and deformation of the continental margin and shelf major dipping seaward detachments control the long term subsidence of this area these main tectonic features define a tectonic segmentation the talara paita and sechura segments are identified from north to south no clear tectonic correlation in time exists between the onshore and the continental margin segmentations or in space either the longterm subsidence of the offshore indicative of subduction erosion working at depth requires low coupling along the subduction channel at depth the distribution of permanent deformation along the northern peru forearc area includes long term uplift along the coastal plain and long term subsidence along the continental margin the neutral line being located within the 10 km seaward from the present coastline an extensive sequence of raised marine cliffs and associated notches evidences that the most recent uplift step 20 23 ka to present along the cabo blanco segment is related to a sequence of major earthquakes we infer that eustacy exerts important feedback coupling to the seismogenic behavior of the north peru subduction zone we speculate that during sea level fall pore fluid pressure diminishes along the subduction channel inducing a possible seaward migration of the locked zone i e migration of the updip limit reaching a maximum by the end of the eustatic low stand during eustatic sea level rise pore fluid pressure increases along the subduction channel this in turn is capable of weakening the previously locked zone along the plate interface beginning an earthquake sequence earth s orbital variations are a potential external cause that may control the physical processes at work along plate interface

seabeam and deep sea submersible nautile surveys in the chiclayo canyon off peru 7 s subsidence and subduction erosion of an andean type convergent margin since pliocene times abstract a reconstruction of the subsidence history of the chiclayo canyon area off northern peru 7 s is proposed using geophysical data collected during the seaperc cruise in 1986 combined with geological data of a recent deep sea dive cruise with the submersible nautile the main results are 1 the detailed morphology and bathymetry of the chiclayo canyon 2 the recovery of samples from the palaeozoic metamorphic basement of the andean continental margin that crops out along the middle and upper slope areas at depths ranging from 2400 to 4000 m below sea level which means that the andean metamorphic basement extends seaward to a site 19 km landward from the trench axis 3 the main subaerial unconformity which extends across the margin from the upper slope to the lower middle slope boundary is of pliocene age 4 the rates of subsidence of the middle and upper slope areas are 713 and 452 m ma during the past 5 2 ma respectively and 5 the estimated amount of solid material removed by subduction erosion is between 128 and 526 km3 km 1 during the past 5 1 5 3 ma

reevaluation of plate motion models based on hotspot tracks in the atlantic and indian oceans abstract plate motion models based on hotspot tracks in the atlantic and indian oceans predict minimal movement less than a few millimeters per year between these hotspots and their counterparts in the pacific ocean for the past 100 m yr whereas plate circuit exercises indicate relative motions of 20 mm yr hotspot based models also suggest that the rajmahal traps india were located 1000 km away from the kerguelen hotspot at 115 ma and the deccan traps india were located a similar distance from the reunion hotspot at 65 ma this is at odds with conclusions derived from paleomagnetism plate circuits and geochemical parameters that suggest a genetic link between flood basalt provinces in india and hotspots in the indian ocean these divergent views may be explained by plume action 1000 km from its center or errors in the hotspot motion models the latter hypothesis is scrutinized in this article by examination of the radiometric ages for hotspot tracks in the atlantic and indian oceans the

deformation style of the mesozoic sedimentary rocks in southern thailand abstract mesozoic sedimentary rocks in southern thailand are widespread from nne ssw and n s in chumphon and trang provinces the mesozoic stratigraphic units are the marine triassic sai bon formation and the non marine jurassic cretaceous thung yai group the latter subdivided into khlong min lam thap sam chom and phun phin formations these units overlie permian carbonate rocks with an angular unconformity and are overlain unconformably by cenozoic units and the quaternary sediments the mesozoic rocks have been folded to form two huge first ordered syncline or synclinoria the chumphon and surat thani krabi trang synclinoria these synclinoria are elongated in nne ssw to n s direction and incorporate asymmetric lower order parasitic folds the folds have moderately to steeply dipping eastward limbs and more gently dipping westward limbs these folds were transected by brittle fractures in four major directions these geologic structures indicate wnw ese to e w contraction with top to the east simple shear at some time before the deposition of the cenozoic sedimentary units no major deformation has affected the rocks subsequently apart from the formation of the fault controlled cenozoic basins