structure of the vakhsh fold and thrust belt tadjik ssr geologic mapping on a landsat image base an enhanced landsat image of the northeastern tadjik depression in the soviet union was combined with field data to produce a 1 250 000 geologic map of the vakhsh fold and thrust belt and adjacent autochthon mapping of the relatively continuous geologic structure in this region first required recognizing marker beds and contacts at or near the resolution of the landsat sensor and then defining mappable units satellite image mapping was complemented by field study which was necessary in this recently deformed area both to verify the image mapping and to obtain more accurate structural dips deformation of the cretaceous and tertiary strata of the fold and thrust belt is in the western half of the mapped area conspicuously absent north of a buried basement fault that marks the hinge zone of the late mesozoic passive margin in the eastern half of the mapped area however the major thrusts displacing rocks northward have moved coherent sheets over the block fault these sheets now lie flat atop the autochthon to the north tectonically these observations indicate that 1 the crustal structure inherited from the mesozoic extensional phase has strongly influenced the late cenozoic pattern of deformation 2 the thrust sheets that are displaced over the paleo hinge zone actively load the basement rocks to the north and 3 the development of the fold and thrust belt which is markedly asymmetric has included the progressive overlapping of thrusts the later thrusts apparently formed internal to the older thrusts and subsequently overrode them

propagating rift pseudofaults zones of crustal underplating imaged by multichannel seismic reflection data two multichannel seismic reflection lines recorded over the northern juan de fuca plate off western canada provide images of the crustal structure of three pseudofaults the traces of propagating rifts anomalous reflectivity including both subhorizontal and dipping events at times below the normal crust mantle transition is prevalent in the region of the pseudofaults although the crustal thickness elsewhere on the lines varies by only about 10 crust associated with the pseudofaults is as much as 30 thicker than average this may be due to an increase in thickness of the interpreted oceanic crustal layer 3b or crustal underplating the results indicate that magma supply at propagating rifts may at times be large perhaps due to the effects of nearby hotspots or convective partial melting associated with the initiation of rifting

seismic structure of the mid atlantic ridge 8 9 s 1 the mid atlantic ridge at 8 9 s is characterized by a transition from axial valley to axial high and recent episodes of ridge jumping and ridge propagation we present constraints on the structure of 0 4 ma crust in this region on the basis of the analysis of wide angle seismic data from a grid of profiles across and parallel to the current and abandoned spreading centers a 350 800 m thick oceanic layer 2a interpreted as high porosity extrusive basalts is underlain by a 2 0 2 5 km layer 2b with velocities which increase with age and decrease in the vicinity of the pseudofaults layer 3 velocities are uniform across the area except for a possible localized anomaly at the ridge axis the crustal thickness varies from 6 7 km near the pseudofaults formed by ridge propagation to 9 10 km at the segment center of the recently 0 3 ma abandoned spreading center seismically determined crustal thickness and density variations and age related lithospheric cooling can plausibly account for all observed variations in gravity across the area and there is no requirement for the thicker crust at the segment center to be underlain by hot mantle the transition from axial valley to axial high occurs at a crustal thickness of 8 km

estimates of heat flow derived from gas hydrates a gas hydrate phase boundary in marine sediments is often observed as an anomalous reflector in seismic profiling data from the depth of the reflectors heat flow values can be estimated using the phase relation of the gas hydrate system and inferred density and thermal conductivity of sediments this method is applied to the data in the nankai trough around central america and along the blake outer ridge the estimated values consistent with those measured by conventional means suggest that derivation of heat flow by this method is feasible on the landward slope of the nankai trough we find that heat flow increases downslope toward the floor of the trough

strain distribution over the east mediterranean ridge a synthesis incorporating new sea beam data abstract we present the results of several multi narrow beam sea beam traverses of the mediterranean ridge on board r v jean charcot these results can be considered to be complementary in resolution and coverage to the previously published long range side scan sonar gloria results a characteristic of the mediterranean ridge is the presence over about half of its surface of an extensive fold system affecting the messinian and plio quaternary cover which is dissected by conjugate strike slip faults the resolution of sea beam was used to map the main characteristics of folding and faulting this analysis together with previously available data enables us to define the strain pattern over the mediterranean ridge we thus now have an overall view of the strain pattern from a marginal basin aegea through the hellenic trench and adjacent sea floor to the opposite continent the bottom of the hellenic trench is a boundary between an extensional domain to the north and a compressional one to the south the southern domain is strongly affected by the presence of the lybian promontary it is related to the formation of an accretionary ridge due to the inability of the upper 3 4 km of sediments to subduct this ridge is similar to the accretionary prism but it is much wider with respect to its height probably because of the existence of a decollement over a level of overpressure the hellenic trench then is in the situation of a fore arc basin its origin is due to the fact that the aegean continental slope is not vertical but rather has a small 4 dip as a result a sedimentary wedge symmetric but shorter than the main accretionary wedge develops over the buried portion of the slope it is characterized by landward thrusts such an origin probably applies to fore arc basins in general

deformation and sedimentation along a developing terrane suture eastern sunda forearc indonesia the collision of the eastern sunda arc with northwest australia has resulted in the development of a suture between the sumba ridge and sawu timor terranes along a zone of intraforearc convergence the developing suture varies from the low angle sawu thrust with attendant mud diapirs in the sumba basin to high angle reverse faults near a basement high of the underthrust sumba ridge terrane bottom currents associated with the flow of pacific ocean deep water into the indian ocean have eroded the terranes and subsequently deposited the detritus in an assemblage of contourites along the suture this study reveals the high structural variability of a terrane suture and the oceanographic influence on the deposition of overlap assemblages

evolution of abyssal lavas along propagating segments of the galapagos spreading center abstract the unusual petrological diversity of abyssal lavas erupted along some segments of the galapagos spreading center is a direct consequence of the propagation elongation of these segments into older oceanic crust with increasing distance behind propagating rift tips relatively unfractionated morb erupted close to the tips are joined first by feti basalts bimodal assemblage and then by a wide range of basaltic and siliceous lavas further behind propagating rift tips this broad range diminishes again approaching the narrow compositional range of adjacent normal ridge segments these compositional variations reflect the evolution of the subaxial magmatic system beneath the newly forming spreading center as it propagates through a pre existing plate we envisage this evolution as proceeding from small isolated ephemeral magma chambers through increasing numbers of larger increasingly interconnected chambers to the steady state buffered system of a normal ridge throughout this evolution magma supply rates gradually increase and cooling rates of crustal magma bodies decrease high degrees of crystal fractionation are favored only when a delicate balance between cooling rate and resupply rate of primitive magma is achieved at other propagating and non propagating ridge transform intersections the degree to which the balance is achieved and the length of ridge over which it evolves control the distribution of fractionated lavas these effects may be evaluated provided a number of tectonic variables including transform length spreading and propagation rates are taken into account

structural grain mud volcanoes and other features on the barbados ridge complex revealed by gloria long range side scan sonar abstract a long range side scan sonar survey of the broad band of deformed sediments east of the antilles volcanic arc has revealed the detailed structural trends of folds and reverse faults or thrusts in plan view for the first time the prevalent trend lies substantially parallel with the arc and is simplest near to the deformation front at the eastern edge of these deformed sediments these trends are interrupted locally by more or less transverse zones of disturbance associated with ridges in the oceanic basement that are obstructing the relative eastward movement of the deformation front local rough surfaced mounds interpreted as mud volcanoes underlain by diapirs are common and occur much further to the north than the mud diapir field already described near trinidad only one large sediment slide has been identified

mechanics of fold and thrust belts and accretionary wedges cohesive coulomb theory a critically tapered fold and thrust belt or submarine accretionary wedge is one that is on the verge of coulomb failure everywhere including its base where frictional sliding along a decollement is assumed to be occurring cohesion within a wedge can add significantly to the overall strength near the toe the effect of this is to decrease the near toe taper leading to a critical topographic profile that is concave upward if the decollement is planar we obtain an approximate self consistent solution for the state of stress within a thin skinned cohesive critical coulomb wedge and determine the relationship between the wedge taper and its strength and basal friction the theory is then applied to the presently deforming fold and thrust belt of western taiwan fitting of theoretical critical wedge shapes to topographic profiles and measurements of the step up angles of thrust faults from the basal decollement are used to constrain the taiwan wedge strength parameters an attractive assertion fully consistent with all the observations is that the mechanics of fold and thrust belts and accretionary wedges is governed by normal frictional and fracture strengths of rocks measured in the laboratory in particular if byerlee s law b 0 85 is adopted as the coefficient of sliding friction on the base we find a coefficient of internal friction 0 9 1 0 in the wedge and a wedge cohesion so 5 20 mpa other solutions having strengths and ambient stresses up to 4 times lower than this can also however satisfy the data

melange diapirs into the cambria slab a franciscan trench slope basin near cambria california near cambria california a franciscan trench slope basin of late cretaceous age is intruded by two diapirs of blueschist block bearing mud matrix melange the rise of the diapirs caused major upturning of the bedded strata within a few hundred meters of the contact and complex interfingering of mud matrix melange with basin sediments near the contacts at one locality a fragment of basin sandstone is observed partially detached from the wall rock the mud matrix melange is highly mobile and the intrusive character during diapirism is clearly analogous to that of magma in igneous intrusions diapirism may be responsible for large scale intercalation of slope and slope basin sediments with subduction generated melange at actively convergent plate margins

earthquake triggered soft sediment deformation structures seismites in travertine deposits abstract earthquakes related soft sediment deformations seismites are relevant structures to help reconstructing the palaeoseismicity in a region nevertheless although seismites have been described for a large variety of palaeo environments and stratigraphic settings they are rare and in some cases ambiguous structures in this paper we document for the first time impressive seismites affecting a latest quaternary travertine deposit located in southern tuscany serre di rapolano italy these structures are extremely relevant as seismites have been never described for this kind of terrestrial deposits the seismites developed within a 2 5 m thick stratigraphic succession made of alternating terrigenous and carbonate sediments embedded within the travertine deposits earthquake produced liquefaction and fluidization of some terrigenous levels and the injection of fluidized silty sediments dykes and sills into carbonate and terrigenous levels slumped levels and associated decollement surfaces produced highly non cylindrical folds overall induced by the loss of brittle shear strength along the basin slopes the occurrence of these soft sediment deformations implies an important seismic event that took place in a time spam encompassed between 84 8 and 48 92 5 08 ka in an area characterized by low seismicity and scarce knowledge of its seismotectonic setting their spectacular exposition and preservation in saw cuts of an abandoned quarry cava oliviera quarry highlights their relevance and importance

accumulations of melt at the base of young oceanic crust as hot solid rock ascends through the mantle the pressure around it decreases while its temperature decreases approximately adiabatically during the ascent the melting temperature decreases faster than the adiabat temperature and if the initial temperature of the rock is high enough the rock will begin to melt at the depth where the adiabat crosses the melting temperature curve models of upwelling beneath mid ocean ridges predict significant melt production over a zone several tens of kilometres wider than the ridge axis region suggesting that some of the ascending melt must arrive away from the ridge axis here i report ocean bottom seismograph observations of an uncommon and previously unreported marine seismic phase which provide strong evidence that melt accumulates in sills at the base of young oceanic crust the phase results from the conversion of compressional p waves to shear s waves on interaction with the crust mantle boundary the moho this phase is rare and very difficult to model but when visible it is strikingly prominent and usually appears at unexpectedly great source to receiver ranges the observations imply the presence of bodies of extremely low rigidity presumably melt sills at the moho close to the receiver melt may be fairly mobile at the base of the crust allowing ridges to accumulate melt through a lateral plumbing system at the locally uplifted boundary between the crust and the underlying thermally expanded mantle escape of this melt to the sea floor may be responsible for the formation of off axis seamounts

detailed tectonics near the tip of the galapagos 95 5 w propagator how the lithosphere tears and a spreading axis develops a model for propagation of an active spreading axis into preexisting lithosphere has been developed based on intensive detailed studies using seamarc ii sea beam deep tow and alvin near the tip of the propagator growing westward at 52 km m y along the east west trending galapagos spreading axis near 95 5 w initial lithospheric rifting appears to be accommodated along reactivated roughly east west abyssal hill faults at the tectonic tip extension leads to downdropping of a keystone block which is pervasively cut by normal faults and fissures the initial volcanism initial volcanic tip occurs about 6 5 km behind the tectonic tip as basalt erupts through these fissures covering most of the keystone block the neovolcanic axial tip occurs another 4 5 km back at the western termination of a well defined axial pillow ridge along which volcanism is localized spreading along the propagator axis accelerates to the full rate 29 mm yr half rate at the full rate tip interpreted to be 10 km behind the neovolcanic axial tip or 21 km behind the tectonic tip a complex pattern of strain in the tip area results from the interaction of plate spreading transform shear stress crack propagation dynamic depression of the western end of the axial valley and the transform zone and variations in the strength thickness heterogeneity and anisotropy of the lithosphere southwesterly curvature of the neovolcanic axis near its tip is inferred to result primarily from the response of young thin weak relatively homogeneous lithosphere to counterclockwise rotation of the maximum tensional stress direction relative to the spreading direction due to the influence of shear stress from the transform zone similar stress interactions likely occur at the tectonic tip but are inferred to be accommodated in this older thick strong heterogeneous and anisotropic lithosphere by oblique slip along a set of left stepping reactivated preexisting east west abyssal hill structures although on a time scale of 106 years the overall process of propagation is mostly continuous rifting at the tectonic tip appears discontinuous at a scale of 1500 m or 30 000 years based on the east west spacing of grabens formed as rifting begins the scale of discontinuity of propagation of the neovolcanic axis is 3 5 km or 50 000 100 000 years based on the distribution of northeast trending features interpreted as beheaded western terminations of paleoneovolcanic axes the extension involved in rifting is 10 20 based on summing the horizontal normal separation of all normal faults and fissures integrated and averaged over the zone of rifting these values are approximately doubled if simple shear listric faulting is dominant rather than pure shear normal faulting

backscattered coherent noise and seismic reflection imaging of the oceanic crust an example from the rift valley of the mid atlantic ridge at 23 n seismic reflection surveys shot over oceanic crust are contaminated by source generated coherent noise that limits the identification and interpretation of subbasement reflections coherent noise arrivals are generated around the seismic line by scattering from the upper surface of the basaltic layer and by backscattering of turning waves from the underside of the same interface both types of arrival may be enhanced by conventional common midpoint cmp stacking some turning arrivals also exhibit negative moveout in a cmp gather due to the increase of crustal velocity with depth in unsedimented areas coherent arrivals scattered from the top of the seafloor can be aligned by dip moveout with or without a subsequent zero offset migration prestack and suppressed but turning arrivals will usually remain turning arrivals generated by linear features in the seafloor such as fault scarps appear on a stack section as nearly linear coherent arrivals the apparent dip of which is related to the features angle to the seismic line after migration these turning wave arrivals can extend close to the topography that produced them and continue to late times leading to the possibility that such arrivals might be incorrectly interpreted as intrusions that crosscut but do not offset the moho careful identification of crustal reflections on a seismic profile along the mark area rift valley indicates that the snake pit hydrothermal field marks the northern limit of identifiable crustal reflections at least some of which appear to be related to magma injection the recent extensive volcanism of the northern mark area may well be a result of faulting caused by the stong northward growth of the southern spreading center permitting melt to rise more easily to shallow crustal levels

anomalous seismic crustal structure of oceanic fracture zones summary the seismic structure of crust found within fracture zones falls outside the range of velocity structures observed for normal oceanic crust in the north atlantic the crust in fracture zones is frequently very thin and is characterized by low crustal velocities and by the conspicuous absence of a refractor with a velocity typical of oceanic layer 3 anomalous crust is present in both large and small offset fracture zones since they are among the most common tectonic features in the ocean basins and are particularly closely spaced on slow spreading ridges fracture zones represent a major source of seismic crustal heterogeneity we interpret the anomalous crust as a thin intensely fractured faulted and hydrothermally altered basaltic and gabbroic section overlying ultramafics that in places are extensively serpentinized the unusually thin crust found within fracture zones and the gradual crustal thinning over a distance of several tens of kilometres on either side of the fracture zones can be explained by two main processes firstly the cold lithosphere edge opposite the spreading centre at the ridgetransform intersection modifies the normal intrusive and extrusive processes of the spreading centre leading to the accretion of an anomalous and thin igneous section and secondly each spreading ridge segment is fed from a separate subcrustal magma supply point so as the magma flows laterally down the spreading centre it generates a crustal section of decreasing thickness culminating in the very thin crust of the fracture zones at either end of the ridge segment

deep biological communities in the subduction zone of japan from bottom photographs taken during nautile dives in the kaiko project abstract twenty seven dives of the submersible nautile in the subduction zone around japan conducted in the french japanese project kaiko proved that fairly luxuriant benthic communities dominated by deep sea giant clams of the genus calyptogena family vesicomyidae were consistently present on the accretionary prism at abyssal depths benthic communities characterized by three hitherto undescribed bivalves of the genus calyptogena were found between depths of about 3800 and 4020 m at the mouth of tenryu canyon and at the top of basement swell of the zenisu ridge both situated in the eastern nankai subduction zone sporadic but discrete patches of organisms characterized by one more undescribed bivalve belonging to the genus calyptogena were observed and collected between depths of 5130 and 5960 m on the landward wall of the japan and kouriles trenches photographic inventories were prepared semiquantitatively using each series of bottom photographs taken in these areas with bow cameras of the submersible nautile observations on the sporadic but dense distribution of the clams and other characteristic associated organisms match well with the scheme that communities sustained by chemosynthetic energy sources can be present at connate water seepages in subduction zones these are to date the deepest record of benthic communities supposedly associated with chemosynthetic processes

subduction zones an introduction to comparative subductology abstract the subduction model for trench arc back arc systems was deduced more or less as a logical consequence of sea floor spreading many features in these systems such as thrust type inter plate earthquakes and deep earthquakes along wadati benioff zones are readily explained by the subduction model but other features such as extensional spreading and high heat flow in back arc regions and arc volcanism are difficult to explain an approach to solve these problems may be to recognize the existence of the two basically different modes of subduction one chilean type causing a compressional stress regime in the arc and back arc regions the other mariana type causing a tensional stress regime implications of the two types of the mode of subduction for some tectonic problems such as vertical movement and sediment accretion at trenches are discussed with special reference to the results of recent dsdp active margin drilling in discussing the possible causes of the two modes it is suggested that the large scale thermal and stress regimes in the back arc areas may not be the direct products of simple subduction but of some additional factors involving much greater energy such as mantle flow associated with the motion of major plates finally the possible great importance of various types of collision accretion and erosion at subduction zones is emphasized

hot spot ridge crest convergence in the northeast pacific evolution of the juan de fuca ridge during the past 7 m y has been reconstructed taking into account both the propagating rift history and migration of the spreading center in the absolute fixed hot spot reference frame northwestward migration of the spreading center at a rate of 30 km m y has resulted in progressive encroachment of the ridge axis on the cobb hot spot and westward jumping of the central third of the ridge axis more recently than 0 5 ma seamounts in the cobb eickelberg chain are predicted to display systematic variations in morphology and petrology and a reduction in the age contrast between the edifice and underlying crust as a result of the ridge axis approach relative seamount volumes also indicate that magmatic output of the hot spot varied during this interval with a reduction in activity between 2 5 and 4 5 ma compared with relatively more robust activity before and after this period spatial relationships determined in this reconstruction allow hypotheses relating hot spot activity and rift propagation to be evaluated in most cases rift propagation has been directed away from the hot spot during the time period considered individual propagators show some reduction in propagation rate as separation between themore propagating rift tip and hot spot increases but cross comparison of multiple propagators does not uniformly display the same relationship no obvious correlation exists between propagation rate and increasing proximity of the hot spot to the ridge axis or increasing hot spot output taken together these observations do not offer compelling support for the concept of hot spot driven rift propagation however short term reversals in propagation direction at the cobb offset coincide with activity of the heckle melting anomaly suggesting that local propagation effects may be related to excess magma supply at the ridge axis less

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

structural styles of an accretionary wedge south of the island of sumba indonesia revealed by seamarc ii side scan sonar the accretionary wedge south of sumba island indonesia is in the early stages of continent island arc collision australian continental shelf sediments are being actively accreted to the sunda arc at the timor trough we have correlated seamarc ii side scan images and bathymetry with 3 5 khz and seismic reflection profiles to construct a structural interpretation of this zone deformation is concentrated on the lower slope of the accretionary wedge within 15 to 25 km of the thrust front above which the strain rate appears to decrease abruptly three distinct structural styles are developed in the survey area the characteristics of the easternmost region are 1 deformation in a fold and thrust geometry 2 presence of mud volcanoes and mud ridges parallel to the thrust front indicating elevated fluid pressure and 3 the development of a long decollement extending 10 to 15 km from the thrust front in contrast the central region shows a more homogeneous thickening resulting mainly from closely spaced faulting seaward verging thrust faults and conjugate strike slip faults are the main structures in this segment of the accretionary wedge mud volcanoes and a very long decollement are absent in the central region we present a hypothesis in which structural style is controlled by the length of the decollement developed in front of the thrust front short thick thrust sheets may favor uniform shortening whereas long thin sheets may be unstable to buckling and deform by folding high fluid pressure may facilitate propagation of the decollement in contrast to the accretion occurring in the other two regions the western part of the accretionary wedge is in the process of being indented and redeformed by a basement ridge

crustal structure of northern juan de fuca plate from multichannel reflection data the northern juan de fuca plate forms the basement of a young 8 ma ocean basin off western canada two subparallel multichannel seismic reflection lines totaling 230 km in length and trending obliquely to the spreading direction were recorded across the basin and provide high quality images of crustal structure the processing sequence applied to the data includes a prestack inside trace mute of common midpoint cmp gathers to reduce noise levels on the deep data cmp stack poststack dip filtering f k migration and band pass filtering coherency filtered stacks are helpful in tracing weaker reflectors the stacked sections reveal a horizontally layered sedimentary sequence overlying a rugged and prominent basement reflector which dips slightly landward topography of the basement was formed at or near the spreading ridge there is no evidence for subsequent deformation a strong fairly continuous reflection from the base of the crust oceanic moho at 2 0 2 2 s two way travel time below the basement surface generally mimics the basement topography and shows the characteristic multicyclic waveform seen in other surveys estimates of reflection coefficients for the moho at several locations indicate that in general the crust mantle transition is unlikely to occur as a discrete first order interface although much of the crust appears acoustically transparent a number of discontinuous intracrustal reflectors are observed over 40 km at the eastern end of the northern line some of these are interpreted to arise from the oceanic layer 2 3 and layer 3a 3b boundaries the noncontinuous nature of these reflectors is an indication of the complexity of the processes producing intracrustal reflectivity and the lateral variability of crustal formation however the interpreted crustal thickness varies by only about 10 away from the distal ends of spreading segments indicating that while the process of crustal formation may be locally variable it produces a broadly uniform result

californian blueschists subduction and the significance of tectonostratigraphic terranes glaucophane and related schists are present as tectonic fragments in ophiolitic suture zones and as discrete lithotectonic belts along the accreted mesozoic tertiary californian margin occurrences include parts of the klamath mountains the western sierran foothills the coast ranges faulted marginal segments of the mojave desert the transverse ranges and the southern california borderland these high pressure low temperature blueschist assemblages reflect the thermal regime of subduction zone environments considerable underflow accompanied drifting and the assembly of far traveled tectonostratigraphic terranes as documented by sea floor magnetic anomaly patterns and age relationships of the oceanic crust capped lithosphere the eastern limbs of paleo pacific plates especially the farallon cocos have been extensively or completely overridden by the westward encroaching north american plate 7000 km since early cretaceous time and nearly 10 000 km since jurassic time subduction is attested to by remnant high pressure mineral assemblages scattered throughout california by construction of related roughly contemporaneous calc alkaline volcanic plutonic belts and forearc basin deposits and by the stranding of ophiolitic complexes although substantial northward drift transported exotic oceanic and continental materials to the growing californian crust and caused extensive dislocation of the post paleozoic continental margin much of the plate motion evidently involved a large component of convergence and eastward underflow terrane shuffling has complicated the picture but the dominant mechanism of continental growth at the californian margin during mesozoic and paleogene time was subduction

a three dimensional gravity study of the 95 5 w propagating rift in the galapagos spreading center abstract seafloor at the galapagos 95 5 w propagating rift pr has a varied morphological expression that can be spatially correlated with the predicted kinematic history of the pr a median valley like depression occurs near the tip of the growing ridge axis to test if this bathymetry is a dynamic feature supported by mantle or lithosphere strength or if it is due to isostatically compensated crustal thickness variations we use three dimensional gravity modelling to constrain the crustal structure in this region from data collected by hey in 1979 and 1982 the gravity anomaly at the pr tip depression suggests that the tip depression is not caused by crustal thinning the data are consistent with a stress supported pr tip depression caused by asthenospheric along axis flow into the growing ridge axis phipps morgan and parmentier 1 in contrast to the tip depression seafloor in the sheared zone of material transferred through transform migration from the cocos to nazca plate is anomalously shallow and has a pronounced regional 300 400 m tilt towards the growing ridge axis over the 20 km width of the sheared zone the gravity data also suggest that the sheared zone is not compensated by crustal thickening

heat flow determination in three dsdp boreholes near the japan trench the first deep borehole determinations of temperature gradients and heat flow of the landward wall of the japan trench and forearc were made on ipod dsdp leg 57 these heat flow values are based on temperature logs corrected to equilibrium using a detailed model of the drilling disturbance heat flow values on a deeply submerged terrace landward of the trench slope break are 28 and 32 mw m sup 2 a measurement in the midslope terrace basin on the landward wall of the trench yielded a value of 22 mw m sup 2 the results are in good agreement with earlier seafloor measurements and indicate that most of the forearc area is characterized by heat flow about one half of that over oceanic lithosphere seaward of the trench our observations indicate only a small increase of heat flow from the trench to the volcanic arc in agreement with thermal models which suggests that the subduction of the relatively cold oceanic plate continues to dominate the temperature structure for distances of up to 250 km landward of the trench the temperature profile in the borehole on the midslope terrace indicates possible vertical flow of pore waters hundreds of conductivity determinations were made usingmore a new technique less

a fluid model for the shape of accretionary wedges in this paper the accretionary wedges landward of most oceanic trenches are modelled assuming a newtonian fluid rheology it is shown that lubrication theory is applicable and that the addition of material to the wedges can be neglected in determining the shape of the wedges the predicted shape of the wedges is in good agreement with bathymetric profiles across the kurile ryukyu and aleutian wedges from the shape of the above wedges it is deduced that accretionary sediments have viscosities between 1018 and 1019 poise

deep seated flow as a mechanism for the uplift of broad forearc ridges and its role in the exposure of high p t metamorphic terranes forearc systems evolve through the interplay of numerous processes but aside from lateral movements much of the observed variability in morphology can be related to evolution of ductility in the deeper levels of the subduction complex in the eastern aleutian lesser antilles makran and cascade arc trench systems a 50 to 200 km wide forearc ridge is developed along the inboard edge of the subduction complex these forearc ridges did not form until subduction accretion had expanded the arc trench gap to 275 300 km and the near vertical rise of the ridge occurred 50 km or more from the trench slope break the forearc high apparently maintained by accretion at the trench thus these broad forearc ridges are not emergent trench slope breaks the process responsible for uplift is occurring within the subduction complex and the size of the accreted mass is apparently a major factor rheological models assuming frictional sliding laws at shallow levels and power law rheologies at depth predict different behaviors for various parts of a typical large subduction complex the older forearc basement beneath which the inboard part of the subduction complex is emplaced represents a rigid brittle massif that extends 100 150 km from the volcanic arc a prediction supported by seismicity studies in modern convergent margins because of the wet quartz rich lithologies of the subduction complexes however the brittle to ductile transition apparently occurs at depths of 10 20 km within the accreted mass thus in large subduction complexes where the downgoing slab is 40 50 km beneath the inboard edge a significant region at the deeper levels of the subduction complex is apparently capable of flow the predicted zone of ductile deformation is directly beneath the area in which a forearc ridge is generally developed hence we conclude that the broad structural highs of large subduction complexes are dynamically maintained by deep seated flow the character of this flow is uncertain yet it is likely that the stripping of low viscosity partially subducted sediments plays a role and the volume of sediments added by underplating may be the controlling factor in the dynamic uplift of broad forearc ridges kinematically the flow could be a pure shear deformation induced by subhorizontal loading but we prefer the familiar corner flow mechanism because it provides the simplest explanation for the observations the interaction between offscraping at the trench and underplating affects the distribution of mass on the downgoing slab and probably controls uplift rates in broad forearc ridges because of the complexity of the interaction and the observation that modern forearc ridges appear to be rising slowly we conclude that high pressure rocks are only exposed in unusual forearc systems e g systems with either a prolonged period of uplift or large influxes of deep underplated sediment the evolution of ductility plays a major role in the development of forearc morphologies when the subduction complex is small brittle deformation and processes related to accretion at the trench dominate and the classic growth models are applicable however as continued growth of the subduction complex depresses the downgoing lithosphere and increases the arc trench gap the subduction complex becomes ductile at depth and ultimately deep seated flow leads to abrupt uplift along the inboard edge of the subduction complex

high pore pressure generation in sediments in front of the barbados ridge complex dsdp data for the barbados ridge complex show the existence of high pore pressures in sediments both oceanward and arcward of oceanic trenches we model the evolution of pore pressure by using a finite element procedure three consecutive stages are recognized 1 in the oceanic basin sedimentation rate is low there is little excess pore pressure 2 approaching the trench but before being incorporated into the accretionary prism sediments are subjected to increasing tectonic compression as well as greater sediment overburden thus high pore pressure can be created an important result is that the lowest effective stress occurs within the sedimentary layer instead of at its base and a decollement may develop along this weakened zone 3 arcward of the trench sediments above the decollement are offscraped while those beneath it are subducted fast sedimentation combined with tectonic compression can create pore pressures so high as to cause porosity to increase with depth this facilitates the formation of mud volcanoes even oceanward of the deformation front

evidence for variations in magma production along oceanic spreading centers a critical appraisal recent geologic geophysical and geochemical studies of the oceanic lithosphere near fracture zones have resulted in the proposal that the magma budget defined as the amount of magma delivered to magma chambers or conduits beneath a spreading center for a given amount of spreading decreases as fracture zones are approached seismic reflection and refraction studies indicate that the crust becomes anomalously thin near fracture zones but reference must be made to the boundary between residual upper mantle peridotites and overlying cumulate rocks in order to assess potential variations in the magma budget the position of this interface however generally is not constrained by geophysical studies geochemical variations in basaltic glasses collected near fracture zones are consistent with a decrease in partial melting as fracture zones are approached but they could also be produced by variations in open system mag matic processes with no change in the extent of partial melting although a decrease in the magma budget as fracture zones are approached is consistent with these data so are alternative models that incorporate a constant magma budget

collision rotation and the initiation of subduction in the evolution of sulawesi indonesia the island of sulawesi indonesia has been shaped and deformed as a result of collision with the sula platform a sliver of continental material from the northern margin of australia new guinea the collision has resulted in rotation of the north volcanic arm of sulawesi and the development of the accretionary wedge of the north sulawesi trench the north sulawesi trench changes laterally from a zone of no active deformation in the eastern part to a wide accretionary wedge in the west early stages of thrusting produce a steep frontal slope 8 16 indicative of relatively high basal shear stress whereas the more advanced western zone of thrusting produces a gentle 2 slope consistent with low basal shear stress reported paleomagnetic data suggest post late eocene counter clockwise rotation of the north arm and the offshore geophysics are explained by a pivot of the north arm with respect to the celebes basin about the eastern end of the arc convergence between the north banda basin and southeast sulawesi is documented by the presence of the tolo thrust its outcrop is strongly arcuate and its accretionary wedge varies in width from a minimum of a few kilometers at each end to a maximum of 30 40 km in the central part the northern end transforms to the leftlateral matano fault with a reported offset of 20 km the southern end of the thrust projects toward the deformed rocks of buton but the structural relations there are not clear the matano fault zone appears to connect westward with the palu fault which forms the western transform of the north sulawesi trench the palu matano fault system acts as a trench trench transform between the north sulawesi trench and the tolo thrust and this system is described by the same rotation pole as that for the sulawesi north arm

evidence for northward thrusting south west of the rhodes basin models of convergence between the major african and european plates involve a stretching aegean plate whose present motion relative to africa is south west1 3 this motion has resulted in subduction of eastern mediterranean sea floor as shown by seismicity volcanism and morphology the southern boundary of the aegean plate is formed by the hellenic trench intensive study culminating in the use of submersibles and multibeam echo sounding as part of the hellenic arc and trench program4 has outlined the complexity of this boundary detailed reflection profiling which we carried out in autumn 1980 of the eastern portion of the hellenic trench system shows arcward verging thrusting south west of the rhodes basin we conclude that collision of the african continental margin with the hellenic arc is now in progress causing overriding by the mediterranean ridge along the southeastern branch of the hellenic trench system

oceanic rift propagation a cause of crustal underplating and seamount volcanism we present the first seismic reflection data across a pseudofault zone the trace of a propagating rift away from a spreading axis strong reflections from the crust mantle transition are discontinuous across the pseudofault deeper reflections which originate near the base of the crust formed at the propagator tip dip beneath the older oceanic crust and become subhorizontal they are interpreted to represent the lower limit of an underplated subcrustal plutonic complex an anomalously smooth basement surface indicative of massive lava flows and a 1200 m high seamount are above the underplated zone the sill complex extends tens of kilometres ahead of and off axis from the former location of the propagating rift indicating that the magmatic supply to the propagator tip was unusually vigorous other seamounts in the northeast pacific lie close to pseudofaults and may have formed as a result of rift propagation

new seismic images of oceanic crustal structure remarkable new images of the internal structure of oceanic crust have been recorded from a two ship multichannel seismic survey in the western north atlantic results include strong planar dipping reflectors in the lower crust evidence of whole crustal failure generating reflectors from the top of the basement to the moho and banded layering in the lower crust and in some places in the upper mantle anomalous crust in the blake spur fracture zone is documented from both normal incidence and wide angle seismic profiles the internal structure of the crust records the magmatic and tectonic history of the crust as it was generated at and then moved away from the spreading center

distribution and origin of igneous rocks from the landward slopes of the mariana trench implications for its structure and evolution the landward slope of the mariana trench is composed largely of igneous rocks serpentinites and serpentinized ultramafic rocks occur at nearly all structural levels on the slope from depths of 8000 to 1200 m seamountlike features on the trench slope break are the surface expression of serpentinite diapirs cumulate and massive gabbros are found several varieties of volcanic rocks are common including boninites altered and metamorphosed basalts andesites and dacites the chemical characteristics of the volcanic rocks indicate that nearly all are products of island arc volcanism together with the gabbros these volcanic rocks represent what is probably a late eocene arc complex these rocks were probably the first volcanic products to result from the subduction of the pacific plate beneath the phillippine sea plate their exposure on the trench slope today implies a significant amount of tectonic erosion of the landward slope since eocene time most of this removal of material appears to have occurred during the early stages of subduction there are isolated occurrences on the landward slope of rock assemblages including alkalic basalts chert hyaloclastites upper cretaceous siliceous sediments and shallow water limestones these assemblages are very similar to rocks dredged from seamounts on the offshore flankmore of the trench and their presence on the landward slope suggests that since the cessation of vigorous tectonic erosion there has been episodic accretion of seamount fragments to the landward slope less

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

cleavage duplexes in the marcellus shale of the appalachian foreland abstract cleavage duplexes are zones of platy spaced cleavage that either parallel bedding or ramp through shale sections examples are 2 60 m thick the cleavage within the zones is generally subperpendicular to the zone boundaries but sigmoidally dragged against floor roof and internal thrusts between the well developed floor thrust and the more obscure roof thrusts of the duplexes the shale has been shortened 30 perpendicular to cleavage the thrusts are sharp strain discontinuities because both overlying and underlying shale is uncleaved and less deformed

origin of vein structures in the slope deposits of modern accretionary prisms vein structures that have been described from the slope apron sediments of modern accretionary prisms represent brittle failure caused by an effective tensile stress the effective stress arises from abnormal pore water pressure acting in opposition to the compressional stresses on the sediment thus even though 3 the minimum stress may be compressional if the pore water pressure is of greater magnitude than 3 the resultant effective stress will be tensional and failure opening of the veins will occur in the 1 2 plane as the veins open the pore water can escape into the veins and eventually out of the sediment along more permeable coarser beds the initial movement of pore water into the veins may entrain finer grains from the host rock near the vein walls resulting in the observed finer grained vein filling then as the veins close as pore water escapes the elongate and platy grains may be rotated into near parallelism with the vein walls the tributaries that are observed at the ends of some veins may be a result of continued dewatering after the veins have closed as the pore water is forced to diffuse back into the sediment after being concentrated into an efficient flow by the parallel preferred orientation of the platy mineral grains in the vein

geology and tectonic evolution of a juvenile accretionary terrane along a truncated convergent margin synthesis of results from leg 66 of the deep sea drilling project southern mexico drilling results from the pacific margin of southern mexico indicate that this region is characterized by a neogene accretionary wedge progressively emplaced against older tectonically truncated continental crust accretion has occurred by both offscraping of sediments at the base of the trench slope and underplating of sediments at depth beneath the accretionary wedge and the continental crust mass balance and incremental uplift studies suggest that about one third of the incoming sediment is subducted beneath the leading edge of the continental crust piston and drill cores indicate that the trench is sand dominated and flanked by slopes covered principally by mud a large submarine canyon bypasses sediment past the shelf and inner trench slope the volume of sediment bypassed to the trench and adjacent lower and outer slope equals 5 to 6 times that deposited on the shelf upper slope and mid slope the muddy inner slope is characterized by foraminiferan free mud below the calcite compensation depth ccd and foraminiferan bearing mud above the ccd the upper slope accumulates laminated mud within the oxygen minimum zone the shelf is covered by sand and mud quartzofeldspathic sand compositions in the leg 66 area reflect sources in the crystalline basement complex exposed along the coast structural fabrics of leg 66 cores from offscraped and overlying slope deposits show zones of inconsistent dip stratal disruption and scaly mudstone characteristics of many melange wedges exposed on land deformation transgresses the boundary between the offscraped and slope deposits demonstrating tectonic incorporation of the slope sediments into the accretionary wedge the rate of deformation of the slope deposits diminishes rapidly landward from the trench deposits overlying the continental crust show dip patterns due to mesoscopic folding as well as local spaced cleavage and faulting but they show no stratal disruption or scaly mudstone oblique slip faulting predominates between the accreted wedge and the continental crust and may reflect decoupling of these two basement types frozen sediment probably bearing gas hydrate was recovered above a bottom simulating reflector at two sites

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

terrane motion by strike slip faulting of forearc slivers forearc slivers bounded by a trench and an active strike slip fault occur in about 50 of modern subduction zones analysis of earthquake slip vectors indicates that modern sliver terranes typically migrate at rates of 1 2 cm yr tertiary transport of some forearc slivers by 1000 km or more is therefore expected as a consequence of normal subduction active arc parallel strike slip faulting occurs whenever interplate coupling is strong and convergence is somewhat oblique strike slip rate increases with greater convergence obliquity

geotechnical properties of lower trench inner slope sediments abstract geotechnical property analyses on sediments recovered by dsdp drilling within select convergent margins were integrated to determine the variability in these properties along the trench inner slope and to delineate the effects of convergence on these properties consolidation states range from very underconsolidated to highly overconsolidated with preconsolidation pressures exceeding 46 000 kpa in quaternary sediments underconsolidated sediments are attributed to 1 1 high sedimentation rates 2 2 low sediment permeability relative to the length of the drainage path 3 laterally applied stresses and 4 induced pore water pressures resulting from the subduction of pelagic sediments with high water content factors contributing to the state of overconsolidation include 1 tectonically induced overpressures 2 removal of overburden by mass movement processes 3 3 low sediment accumulation rates 4 4 age vertical gradients of index properties also vary greatly with the maximum gradient associated with overconsolidated sediments values of porosity generally exceed 30 at all margins studied suggesting this porosity represents the minimum attainable solely by the effects of convergence geotechnical property results and site specific parameters suggest two end members should be considered in the general geotechnical property model for convergent margin sediments clastic dominated margins form the basis for the initial geotechnical property model these margins are characterized by thick trench sediment sequences which are folded and faulted and become progressively more deformed upslope convergence rates are low and no well developed faults or hrst and graben structures are evident on the downbending oceanic plate highly overconsolidated sediments and index property values which change rapidly with depth result from the slow progressive deformation of trench and lower slope sediments examples of clastic dominated margins include the nankai trough aleutian trench and washington continental margin pelagic dominated margins have significantly different geotechnical properties and site specific parameters these margins are characterized by thin clastic trench sediment sequences overlying pelagic sediments on a rapidly converging oceanic plate well developed faults and or hrst and graben structures are evident on the downbending oceanic plate sediment deformation along the trench and lower slope appears limited these sediments are underconsolidated and index property values change gradually but often irregularly with depth japan trench and middle america trench guatemala margins are considered pelagic dominated middle america trench mexico appears to represent an intermediate case having characteristics of both margin types

structure of the accretionary complex of barbados i chalky mount the island of barbados is underlain by a basal tectonic complex that may be representative of rocks and structures of the structural high of the accretionary prism of the lesser antilles arc at chalky mount the complex consists of six fault bounded packets of east northeast strike and steep dips the packets are composed of deformed but generally coherent cenozoic sedimentary rocks terrigenous turbidite in five of the packets and hemipelagic rocks in the sixth there is no evident stratigraphic affiliation among beds in any of the six packets rocks in fault packets underwent sequential deformations most of which can be timed as early contemporaneous or late with respect to cessation of interpacket motions early structures in packet 1 consist of a main fold train with easterly striking axial planes and related thrusts broken formation and homoaxially refolded folds packet 1 also contains later open folds with northeast strikes packets 2 4 are relatively thin slices within a fault complex the chalky mount fault zone in which late left oblique slip occurred on two of the four faults of the zone packet 5 contains three phases of deformation an early near recumbent major fold marginal belts of fault related second folds and open post fault third folds packet 6 includes early folds that are tightened and reoriented toward its northern fault boundary where it is locally a thrust early structures in each packet are the most pervasive and record the major stratal shortening shortening in each packet is approximately normal to packet boundaries and early axes lie in the plane of the boundaries packet boundaries are interpreted as surfaces of accretion and early structures as deformations generated during transfer of layer successions from an ocean floor to the front of the accretionary prism contemporaneous and post fault structures are local and are due to displacements during reactivation of faults mainly inter packet within the accretionary prism early structures at chalky mount developed in the brittle state apparently with displacements taken up by sliding between grains and along layer boundaries all structures record distinctly nonmetamorphic environments

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

structural styles in mesozoic and cenozoic melanges in the western cordillera of north america the term melange is currently used to describe several different kinds of mudstone rich rocks that are broadly characterized by an obscure stratigraphy stratal disruption or a chaotic block in matrix fabric four types of melange which can be defined in outcrop on the basis of mesoscopic fabric and lithologic composition are particularly widespread and distinctive type i includes sequences of originally interbedded sandstone and mudstone that record incipient to thorough disruption and fragmentation of strata accomplished largely by layer parallel extension type ii consists of similarly deformed thin layers of green tuff radiolarian ribbon chert and minor sandstone originally interbedded with black mudstone disruption in both types i and ii which probably occurred while the sediments were incompletely consolidated has been ascribed to either imbricate faulting in accretionary wedges or gravitationally driven deformation type iii comprises inclusions of diverse shapes sizes and compositions enveloped in a locally scaly pelitic matrix the ultimate source of fragments is obscure because the majority were not derived by either the progressive disruption of interbedded sediments or in situ tectonic plucking and abrasion of adjacent rocks although some type iii melanges may have originated deep within accretionary prisms final emplacement as olistostromes muddy debris flow deposits or mud diapirs seems likely type iii melanges are mechanically analogous to scaly sheared serpentinites many probably have been tectonically remobilized or even intruded into shallow level fault zones type iv consists of lenticular inclusions bounded by an anastomosing network of subparallel faults their fabric records progressive slicing in brittle fault zones each of the four types of melange described here could in theory have formed in a variety of settings on or within an accretionary wedge at an active convergent margin none can yet be singled out as a uniquely diagnostic subduction melange

gravity and seismic study of crustal structure along the juan de fuca ridge axis and across pseudofaults on the ridge flanks variations in topography and seismic structure are observed along the juan de fuca jdf ridge axis and in the vicinity of pseudofaults on the ridge flanks left by former episodes of ridge propagation here we analyze gravity data coregistered with multichannel seismic data from the jdf ridge and flanks in order to better understand the origin of crustal structure variations in this area the data were collected along the ridge axis and along three ridge perpendicular transects at the endeavor northern symmetric and cleft segments negative mantle bouguer anomalies of 21 to 28 mgal are observed at the axis of the three segments thicker crust at the endeavor and cleft segments is inferred from seismic data and can account for the small differences in axial gravity anomalies 3 7 mgal additional low densities elevated temperatures within and or below the axial crust are required to explain the remaining axial mba low at all segments gravity models indicate that the region of low densities is wider beneath the cleft segment gravity models for pseudofaults crossed along the three transects support the presence of thinner and denser crust within the pseudofault zones that we attribute to iron enriched crust on the young crust side of the pseudofaults a 10 20 km wide zone of thicker crust is found reflection events interpreted as subcrustal sills underlie the zones of thicker crust and are the presumed source for the iron enrichment

convective partial melting 1 a model for the formation of thick basaltic sequences during the initiation of spreading stretching and thinning of continental lithosphere during rifting and initiation of seafloor spreading cause upwelling of mantle rocks into a pressure temperature environment where partial melting occurs the amount of melt liberated is however insufficient to account for the thickness of igneous crust emplaced near the continent ocean boundary at some passive margins these igneous structures are known from seismic reflection and deep refraction studies on numerous passive margins worldwide seismic studies also suggest that the transition from continental to oceanic crust on these volcanic margins is relatively abrupt their evolution apparently does not include a prolonged extensional phase preceding the inception of spreading to account for these observations we propose a model for the generation of partial melt in which the lateral temperature contrasts evoked by rifting and asthenospheric upwelling drive convection in the upper mantle deep hot mantle material is transported upward by the convection and hence much larger volumes flow through the region where partial melting occurs than during passive upwelling numerical calculations show that convective partial melting can provide substantial additions to melting due to passive upwelling alone using this model we propose an evolutionary scheme for volcanic passive margins emphasize the contrast with nonvolcanic margins and describe conditions which might lead to the development of both types

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

duplex accretion and underplating in an ancient accretionary complex kodiak islands alaska the landward belt of the kodiak formation represents a sequence of lower maastrichtian turbidites that was underplated to the kodiak accretionary complex by duplex accretion evidence for duplexes exists on the mesoscopic and map scale at the microscopic scale incremental strain histories are consistent with early shear during underthrusting abrupt landward rotation of individual thrust slices over fault ramps and locking of thrust packets in steeper orientations duplex geometries at the mesoscopic scale are represented by high angle reverse faults bounded either above or below by low angle southeast verging detachments at the map scale different structural belts can be related to a low angle detachment zone central belt flooring a more landward duplex landward belt duplex underplating can result in the uplift and thickening of accretionary prisms without necessarily deforming the overlying previously accreted material and slope sediments

geophysical survey of a mud volcano seaward of the barbados ridge accretionary complex a detailed geophysical investigation that included multibeam bathymetry 3 5 khz channel and single channel seismic profiles 20 heat flow measurements and four piston cores was made of a mud volcano located about 8 km seaward of the toe of the barbados ridge accretionary complex in the northern guiana basin the material erupted from the volcano covers an oval area about 3 9 km with its long axis oriented e w roughly perpendicular to the deformation front a flat topped mound on the western side of the feature is the currently active center acoustic stratigraphy suggests that mud volcanism began at the site about 200 000 ma a piston core taken on the flank of the active mound sampled deposits erupted 20 000 30 000 ma that contain late miocene nannofossils heat flow on the active mound is 4 times that in the surrounding basin one dimensional modeling of this anomaly supports a low level continuing flux of mud and water at rates of 2 5 cm yr the mud volcanism appears to have arisen from the combined action of overpressured water that comes from beneath the accretionary prism and existing overpressures in the oceanic section large east west trending basement escarpments underlie the two known mud volcanoes seaward of the barbados ridge accretionary complex and they probably play an important role in developing vertical pathways for fluid to escape and in determining the location where mud volcanism starts

multibeam study of the flores backarc thrust belt indonesia using the seamarc ii seafloor mapping tool in conjunction with closely spaced seismic reflection profiles we have mapped a segment of the flores back arc thrust zone structural irregularities along the deformation front of the thrust zone result from changing stratigraphy and basement structure of the lower plate ne trending faults cutting the outer slope of the flores basin are easily mapped because they truncate dense drainage patterns obliquely mud diapirs probably indicating elevated fluid pressures have formed throughout the accretionary wedge but appear to be concentrated as do back arc thrust earthquakes at the ends of thrust faults the overall orientation of the deformation front of the accretionary wedge is 100 suggesting a nne sense of thrust motion and supporting an origin of the thrust zone by collision of the arc with australia rather than by magmatic forcing or gravitational sliding or spreading orientations of faults and folds close to the arc are not consistent with the trends of the frontal thrusts however and the difference may be due to either different initial orientations or to later rotations of structural features as the accretionary wedge grew

the structure of mountain fronts commonly the part of a mountain front which is visible at the surface consists of foreland dipping thrust belt rocks elevated above their regional structural position and overlain more or less conformably by molasse several explanations for their geometry are possible 1 major detachments exist within or beneath the molasse resulting in transport of the foreland basin examples of this geometry come from the swiss molasse plain the southern pyrenees and the mackenzie mountains of canada 2 displacement is lost on major backthrusts beneath the frontal monocline examples cited here are the rockies of alberta the sulaiman ranges of pakistan the mackenzies and the andes in peru 3 thrust sheets travelled over an old land surface and syntectonic molasse contemporaneously offlaps the topographic high of the thrust front this phenomenon occurs along the alpine thrust front in haute provence 4 the frontal fold represents deformation above a large scale thrust tip no unequivocal example of tip line strain at this scale has been recorded although this type of deformation may occur in the brooks range of alaska in many areas mountain fronts show a combination of these idealized geometries

tectonics of the hellenic trench a synthesis of sea beam and submersible observations abstract detailed bathymetric investigations were performed in four zones of the hellenic trench system in 1978 using the multi narrow beam echo sounder sea beam on r v jean charcot in 1979 a field study using the 3000 m submersible cyana was conducted in three of the four sites covered by sea beam we summarize here the principal tectonic results of these studies and publish the corresponding sea beam maps it is shown that compressional structures affect the sedimentary cover of the outer wall and trench whereas extensional tectonics is present on the inner wall in addition there is evidence that most of the upper sedimentary cover is not subducted but rather piles up in front of the trench to form the mediterranean ridge we suggest that the surface limit of the zone of mechanical decoupling occurs near the base of the inner wall we show that the gross structure of the trench as well as the strain pattern observed is controlled by the kinematics of subduction and we discuss its evolution since its initiation about 13 m y ago

major element constraints on melting differentiation and mixing of magmas from the galapagos 95 5 w propagating rift system projections in the pseudo quaternary system diopside plagioclase olivine quartz di pl ol q are used in this paper as a base on which to plot additional compositional petrologic or tectonic variables used in this manner the projections provide a sensitive means of displaying and evaluating the chemical variability of mid ocean ridge basalt morb suites such as that of the galapagos 95 5 w propagating rift system and allow mantle controls on the chemical variability of morb to be distinguished from the predominant effects of shallow level crystal fractionation and mixing primitive lavas mg 63 from the 95 5 w region form a broadly linear array within the di pl ol q tetrahedron parallel to the di ol join in terms of two recent high pressure studies of morb petrogenesis stolper 1980 takahashi and kushiro 1983 the primitive liquids in this array are not primary but have evolved by less than 10 olivine fractionation from an array of primary liquids which separated from residual mantle over a range of pressures between 10 20 kb with appropriate assumptions as to residual lithology and mg of residual olivine the nature and origin of 95 5 w primary magmas can be deduced 1 lavas of the normal rift are derived from a broad range of primary magmas that separated from compositionally uniform in terms of major elements mantle over a pressure range of 10 20 kb 2 beneath the newly forming propagating rift melting is initiated perhaps by pressure release at shallow depth 9 11 kb degree of melting and source composition appear similar to those for the normal rift within a few km of the propagating rift tip this shallow source is joined by a second deeper 11 12 kb more iron rich source both sources persist for at least 100 km along the propagating rift but the deeper source appears to become dominant farther behind the tip maximum pressure for melt separation anywhere beneath the propagating rift is less than 15 kb in addition to these source effects the projections allow a more complete description of the shallow level fractionation and mixing effects along the propagating rift primitive lavas of restricted composition erupted near the rift tip give way within 2 3 km to an increasingly diverse suite of lavas characterized by variable extents of fractionation and by numerous single mixing events consistent with magmatic evolution in small only occasionally interconnected magma bodies beyond 15 km the diversity gradually declines toward a restricted population of cpx saturated lavas which characterizes the propagating rift beyond 75 km and is consistent with the presence of a large well mixed magma chamber in this region

andean tectonics related to geometry of subducted nazca plate seismological and geological data show that tectonic segmentation of the andes coincides with segmentation of the subducted nazca plate which has nearly horizontal segments and 30 east dipping segments andean tectonics above a flat subducting segment between 28 s to 33 s are characterized by from west to east 1 a steady topographic rise from the coast to the crest of the andes 2 no significant quaternary and possibly neogene magmatism 3 a narrow belt of eastward migrating apparently thin skinned neogene to quaternary shortening of the andes and 4 plio pleistocene uplift of the crystalline basement on reverse faults in the pampeanas ranges from about 15 to 24 s over a 30 dipping subducted plate a west to east andes cross section includes 1 a longitudinal valley east of coastal mountains 2 an active neogene and holocene andesitic volcanic axis 3 the altiplano puna high plateau 4 a high neogene but inactive thrust belt eastern cordillera and 5 an active eastward migrating subandean thin skinned thrust belt tectonics above a steeply subducting segment south of 33 s are similar west of the volcanic axis but quite different to the east early cenozoic tectonics of western north america were quite similar to the neogene andes however duration of segmentation was longer and the width of deformation was greater in the western united states patterns of crustal seismicity are systematically related to plio quaternary structural provinces implying that current deformational processes have persisted since at least the pliocene horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench above flat subducting segments crustal seismicity occurs over a broad region whereas over steep segments it is confined to the narrow thrust belt strain patterns in the forearc region are complex and perhaps extensional and a broad region of the altiplano puna and eastern cordillera appears to be aseismic

multibeam bathymetric survey and high resolution seismic investigations on the barbados ridge complex eastern caribbean a key to the knowledge and interpretation of an accretionary wedge abstract an important problem of active margin processes is the way accretionary wedges form in response to oceanic crust subduction new data have been obtained along the barbados ridge in front of the lesser antilles tertiary island arc they include high resolution multichannel seismic data and accurate sea beam bathymetric maps both have been used to propose a 3 dimensional interpretation of tectonic and sedimentary processes related to subduction such as oceanic off scraping ductile folding relative uplift and subsidence fore arc basin evolution and mud diapirism they all improved our knowledge of the evolution of accretionary prisms and more specifically of the barbados ridge development in plio quaternary time previous tectonic events are deduced from recently published geological data from barbados and the lesser antilles islands

seismic crustal structure and morphotectonic features associated with the chain fracture zone and their role in the evolution of the equatorial atlantic region oceanic transform faults and fracture zones fzs represent major bathymetric features that keep the records of past and present strike slip motion along conservative plate boundaries although they play an important role in ridge segmentation and evolution of the lithosphere their structural characteristics and their variation in space and time are poorly understood to address some of the unknowns we conducted interdisciplinary geophysical studies in the equatorial atlantic ocean the region where some of the most prominent transform discontinuities have been developing here we present the results of the data analysis in the vicinity of the chain fz on the south american plate the crustal structure across the chain fz at the contact between 10 and 24 ma oceanic lithosphere is sampled along seismic reflection and refraction profiles we observe that the crustal thickness within and across the chain fz ranges from 4 6 5 9 km which compares with the observations reported for slow slipping transform discontinuities globally we attribute this presence of close to normal oceanic crustal thickness within fzs to the mechanism of lateral dike propagation previously considered to be valid only in fast slipping environments furthermore the combination of our results with other data sets enabled us to extend the observations to morphotectonic characteristics on a regional scale our broader view suggests that the formation of the transverse ridge is closely associated with a global plate reorientation that was also responsible for the propagation and for shaping lower order mid atlantic ridge segmentation around the equator

propagation as a mechanism of reorientation of the juan de fuca ridge we present a revised model of the tectonic evolution of the juan de fuca ridge by propagating rifting the new model has three different relative rotation poles covering the time intervals 17 0 8 5 ma 8 5 5 0 ma and 5 0 ma to the present the rotation pole shifts at 8 5 and 5 0 ma imply clockwise shifts in the direction of relative motion of 10 to 15 at each of these shifts the pattern of propagation reorganizes and the new ridges formed by propagation are at an orientation closer to orthogonal to the new direction of motion than the orientation of the preexisting ridges the model containing a total of seven propagation sequences shows excellent agreement with the isochrons inferred from the magnetic anomaly data except in areas complicated by the separate explorer and gorda plates the agreement between model and data near the explorer plate breaks down abruptly at an age of about 5 ma indicating that the probable cause of the rotation pole shift at that time was the separation of the explorer plate from the juan de fuca plate

anomalous lithospheric structure of northern juan de fuca plate a consequence of oceanic rift propagation abstract anomalous crustal and upper mantle structure of northern juan de fuca plate is revealed from wide angle seismic and gravity modelling a 2 d velocity model is produced for refraction line ii of the 1980 vancouver island seismic project visp80 the refraction data were recorded on three ocean bottom seismometers obss deployed at the ends and middle of a 110 km line oriented parallel to the north american continental margin the velocity model is constructed via ray tracing and conforms to first arrival amplitude observations and travel time picks of direct converted and reflected phases between sub sediment depths of 3 to 11 km depths normally associated with the lower crust and upper oceanic mantle the final model shows that compressional wave velocities decrease significantly from southeast to northwest along the profile at sub sediment depths of 11 km at the northwestern end of the profile p wave velocities are as low as 7 2 km s a complementary 2 d gravity model using the geometry of the velocity model and velocity density relationships characteristic of oceanic crust is produced the high densities required to match the gravity field indicate the presence of peridotites containing 25 30 serpentine by volume rather than excess gabbroic crust within the deep low velocity zone anomalous travel time delays and unusual reflection characteristics observed from proximal seismic refraction and reflection experiments suggest a broader zone of partially serpentinized peridotites coincident with the trace of a pseudofault we propose that partial serpentinization of the upper mantle is a consequence of slow spreading at the tip of a propagating rift

transition from frontal accretion to underplating in a part of the nankai trough accretionary complex off shikoku sw japan and extensional features on the lower trench slope abstract migrated multichannel seismic reflection profiles and bathymetry from a 200 120 km area of the nankai trough inner slope reveal three physiographic tectonic domains on the lowermost slope linear ridges demarking laterally continuous hangingwall anticlines above ramps in a relatively simple imbricate stack of trench turbidites characterize the western domain an imbricate fan underlies a relatively flat structural terrace in the east between these two domains lies a compound knoll minami muroto knoll some 40 km long opposite which the thrust front pushes some 10 km further seaward than is the case in the domains to east and west in the western linear ridge domain previous dsdp drilling penetrated turbiditic trench fill uplifted in the lowermost thrust fold terrace above a decollement within the underthrusting shikoku basin oceanic plate sequence the shikoku basin sequence in the western domain is divided into an upper poorly reflective hemipelagic claystone unit and a lower strongly reflective unit comprising pliocene turbidites the lower unit is traceable intact up to c 20 km landward below the lower trench slope and in the better resolved profiles the decollement lies along the base of the claystone unit a similar decollement within the shikoku basin sequence in the eastern domain is traceable up to c 22 km landward a critical seismic record crossing the western part of minami muroto knoll shows that the decollement is traceable only 8 km landward to a point under the steep slope at the front of the knoll landward of which the subducting shikoku basin sequence is apparently thickened by as much as twice this thickening occuring as it does immediately along strike from a simple imbricate fan to the east of the knoll and a relatively simple imbricate stack to the west both evidently involving no strata from the lower shikoku basin unit we ascribe to underplating by formation of duplexes of shikoku basin strata strike parallel extension akin to that postulated for high structural levels in certain thrust belts is caused by uplift of the knoll as a result either of the underplating or segmentation of the subducting oceanic crust or both a normal fault throws to the west off the west flank of the knoll it bounds a transverse trough like slope basin with at least 900 m of fill upslope from the knoll broadly slope parallel normal faults cut and pond recent slope sediments the most impressive is a listric growth fault which dips trenchward alternative explanations for these involve extensional collapse of this part of the prism resulting from the subduction of a topographic high or a zone of selective underplating below the trenchward portion of minami murato knoll

comments on the growth of accretionary wedges uplift within accretionary foreland wedges has been explained by the development of duplexes which provides a mechanism for underplating in submarine accretionary wedges with little deformation in the upper part of the wedge we interpret duplex development beneath the costa rica forearc a zone widely considered to be a prime example of nonaccretion because of the apparent absence of an imbricate fan of thrusts at the toe of the wedge large scale structural geometry of accretionary wedges is compatible with low angle trenchward dipping backstops in centrist to reverse angle backstops used commonly in models of return flow within wedges which have been invoked to explain the presence of high p t metamorphic rocks we suggest that emplacement of high p t metamorphic rocks are better explained in a collisional setting where known uplift rates are high than by flow within noncollisional accretionary wedges

back arc thrusting in the eastern sunda arc indonesia a consequence of arc continent collision the structure of the eastern sunda back arc region is dominated by two large north directed thrusts the wetar and flores thrusts and one or more minor thrusts which may represent early stages of subduction polarity reversal of the arc the relationships between thrusting and magmatic activity surface slopes cross arc faulting uplift forearc structure and collision by the australian continental margin provide constraints on the likely driving mechanisms of thrust formation driving mechanisms are considered in two broad categories one set of possible mechanisms originates within the upper plate and includes gravity sliding gravity spreading and magmatic intrusion the other set focuses on stress propagation between the upper and lower plates and for this arc system the major mechanism is that of continental collision while no one mechanism explains the range of observations we see in the back arc region all of these aspects play some role in the development of the thrusts we can eliminate upper plate mechanisms as primary driving forces because slope effects are similar in areas of both thrusting and nonthrusting in addition the wetar thrust shows a negative relation between thrusting and volcanism and side looking airborn radar images show no evidence of postvolcanic rifting that could accompany intrusion without volcanism collision between the arc and the australian continent provides the clearest driving mechanism but it does not explain the discontinuous nature or the particular location of the thrusts we suggest a sequence that appears to account for the data and to provide insight into the controversies often associated with other such thrust belts thrusting was initiated in those areas where the crust of the forearc region was thick a factor that facilitated stress propagation across the arc and where the crust of the back arc was thin which facilitated back arc thrusting thrusting concentrated at the base of the slope where the slope stress is maximum the early thrusting may have been helped also by thermal weakening of the crust due to volcanism as convergence proceeded volcanism waned then ceased in the eastern part of the arc the crust then strengthened and thrusting required higher stress to maintain and propagate this later stage should show higher seismic energy release which appears to be the case for the wetar thrust region displacement on the flores thrust appears to be matched by deflection of the volcanic arc we have estimated shortening in the back arc from interpretation of reflection profiles and for the flores thrust the value of approximately 30 km is consistent with that estimated for deflection of the arc compressional deformation continues west of the collision zone across the bali basin and into java it is reported in ne sumatra as well this deformation involves much less convergence than the eastern sunda thrusts and it may be related to magmatic processes to subduction of oceanic plateaus such as the roo rise or in the case of the bali basin to efficient lateral propagation of the flores thrust

inferences on sediment strength and fault friction from structures at the aleutian trench the structural relations of conjugate thrust faults and the master decollement imaged in a research level processed multichannel seismic reflection record across the eastern aleutian trench allow quantitative estimates of the sediment strength and friction across thrust boundaries of the subduction zone these estimates are derived from geometric relations between faults and from the overall geometry of accreted prisms they suggest the presence of high pore fluid pressures that result in low levels of friction along a master decollement and on the main bounding thrust faults in the accreted sediment therefore coupling in the immediate trench area apparently does not contribute significantly to the generation of horizontal compression upslope

construction and destruction of volcanic knobs at the cocos nazca spreading system near 95 w characterization of volcanic knobs observed in sea beam bathymetry near the 95 w galapagos propagator system on the cocos nazca spreading axis provides insight into volcanic and tectonic processes at propagators and mid ocean ridges despite evidence suggesting a higher magma supply rate at the propagator axis crust accreted there contains fewer knobs than crust created at the failing or doomed spreading axes fissure fed flows rather than seamount construction are more important along the propagator the process of transferring lithosphere from one plate to another as this ridge offset migrates through a region destroys about half of the preexisting knobs data tentatively suggest that on axis volcanic cones increase in size and contribution to crustal construction but decrease in abundance with decreasing spreading rate possibly reflecting the magma plumbing systems in differing thermal regimes

passive roof duplex geometry in the frontal structures of the kirthar and sulaiman mountain belts pakistan abstract exploration for hydrocarbons over the past few years has greatly improved our understanding of the geometry of frontal mountain belt structures in this study we introduce and discuss the concept of the passive roof duplex using as the main example the kirthar and sulaiman ranges in the baluchistan province of pakistan structures similar to those described here have been recognized previously in other mountain belts and they appear to exist as a common feature in many more frontal regions of mountain belts our example of a passive roof duplex which we describe from pakistan is compared briefly with similar structures reported by others the passive roof duplex is here defined as a duplex whose roof thrust has backthrust sense passive roof thrust and whose roof sequence those rocks lying above the roof thrust remains relatively stationary during foreland directed piggy back style propagation of horses within the duplex