origin and emplacement of mariana forearc seamounts large seamounts occur on the outer half of the mariana forearc they represent a new class of seamounts consisting of horsts and diapirs of metamorphosed forearc material the degree of metamorphism in this material depends on the amount of water available and the pressure temperature regime of the forearc wedge the major source for the water involved in the metamorphism is most likely the descending slab theoretical models for thermal regimes in convergent margins suggest that the lower grade metamorphic facies will be restricted to the outermost part of a forearc zeolite and chlorite facies rocks predominate in dredge hauls from horsts on the outer 50 km of the mariana forearc thermal models indicate that higher grade greenschist facies should occur farther from the trench seamounts that were probably formed in response to diapiric emplacement of serpentinite predominate from 50 to 120 km from the trench uplift of the horsts and emplacement of the serpentinite diapirs were probably facilitated by vertical tectonic movement in response to subduction of plate seamounts and by fracturing of the mariana forearc

the mechanics of frontal imbrication a first order analysis the detachment and imbrication of thrust slices at the front of a thrust wedge is one of the principle modes by which such wedges grow collapse of the frontal ramp under longitudinal compressional stress cannot explain the regular formation of new slices of finite length unless there are regularly spaced heterogeneities in the footwall layer or the underlying basement surface advance of the thrust wedge over the frontal ramp however increases both the vertical load on the ramp and the traction on the upper flat this will in general produce a peak deviatoric stress in the footwall layer below the leading edge of the thrust wedge failure will occur at this point when the thrust wedge has advanced a distance l such that the deviatoric stress in the footwall layer exceeds its strength l is a function of a rock density b ramp angle c the resistances to motion on the basal detachment the ramp and the upper flat and d the strength and thickness of the footwall layer these mechanical parameters can therefore control the formation of new thrust slices of regular length in the absence of footwall heterogeneities

initiation of subduction zones along transform and accreting plate boundaries triple junction evolution and forearc spreading centres implications for ophiolitic geology and obduction

geological manifestations of ridge collision evidence from the golfo de penas taitao basin southern chile recent geological and geophysical studies in the southern andes adjacent to the intersection of the chile rise with the peru chile trench ant naz sam triple junction have revealed a number of features and a neogene geologic history that are unique along the pacific margin of south america this history includes 1 development of a tertiary quaternary marine basin with up to 3 km of sediment infill golfo de penas taitao basin gtb 2 disruption of the region by a series of faults with both normal and strike slip movements and 3 localization of silicic near trench volcanism and epizonal plutonism and related hydrothermal activity the northern portion of the gtb began to subside in the late miocene possibly earlier and has subsequently been deformed uplifted and exposed gravity and seismic reflection data suggest that the basin continues offshore where it is still actively subsiding today golfo de penas subsidence and uplift have thus occurred diachronously in the region although it is unclear when subsidence began in the golfo de penas tectonic disruption of the region is likely related to the liquine ofqui fault lof a major ns trending crustal shear zone that curves westward and terminates in the golfo de penas the lof has both down to the west and right lateral offset and separates the main andean cordillera on the east from a large crustal block the chiloe block on the west we hypothesize that the gtb has developed as a pull apart basin in response to northward movement of the chiloe block along the lof we propose a dynamic model whereby a stress gradient that decreases longitudinally away from the chile rise peru chile trench intersection is set up because the youngest most buoyant oceanic lithosphere is being subducted at the triple junction the chile rise is viewed as a type of indenter which is acting to drive the chiloe block northward in front of the northward migrating triple junction this model explains the unique set of geologic features found in the region and suggests that ridge trench interactions may be an important factor in orogenesis at active continental margins

large scale sediment underplating in the makran accretionary prism southwest pakistan field relations in the emergent part of the makran accretionary prism show that a mid miocene to early pliocene slope and shelf sedimentary sequence was deposited directly on abyssal plain turbidites without any detectable stratigraphic or structural discordance sedimentological evidence for rapid shoaling however indicates that the underlying sediment column was tectonically thickened by a factor of between two and three during this period this can be explained by large scale underthrusting and underplating of sediment a mode of accretion that is also favored by mass balance considerations the visible deformation in the coastal makran occurred when the region was 70 100 km north of the contemporary prism front

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

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

ophiolites as indicators of the geodynamic evolution of the tethyan ocean abstract reviewing the ophiolites of the alpine chain from the western mediterranean to the indian ocean we subdivide them into ten groups of outcrops the geological environment through time the internal composition and history are given for each emphasis is put on the variegated and successive geodynamic processes which can be reconstructed for the different ophiolitic belts since their birth as an ocean like crust to their incorporation through accretion to a continent one can distinguish low versus high tholeiitic partial melting fairly or poorly established ridges simple ridge type crust versus complex ridge ensimatic arc and even more complex type long lived versus short lived oceanic crust stages abrupt large scale pure obduction versus progressive collision related obduction or simple progressive juxtaposition to continent the major events through time in the tethyan ophiolitic belts can be listed as follows spreading ridge activity during the jurassic liguria dinaro hellenides lesser caucasus and the cretaceous peri arabic nain sabzewar with a jump of ridge in between repeated change from ridge to ensimatic arc dinaro hellenic lesser caucasus peri arabic west indian during the latest jurassic and cretaceous upper cretaceous tertiary collision related obductions pontic lesser caucasus liguria squeezing out the elevated portions of these basins lack of oceanic magmatism during the cenozoic but progressive accretion along accretionary wedges of the elevated portions of the remaining mesozoic crust van nain sabzewar zahedan mekran

structural development of an accretionary prism by thrust and strike slip faulting shumagin region aleutian trench a combination of seabeam swath bathymetric data closely spaced single channel seismic reflection profiles and multichannel seismic reflection profiles from the shumagin islands region indicates that the large scale deformation of the accretionary prism in this segment of the aleutian trench occurs progressively through several stages 1 folding along axes perpendicular to the plate convergence direction in the region 2 thrust faulting in the direction of plate convergence and 3 oblique strike slip faulting along conjugate west northwest trending right lateral and north northeast trending left lateral faults strike slip faults occur in distinct domains with right lateral oblique faults present west of a major transverse fault at long 161 w and left lateral oblique faults prevalent east of 161 w the structural development of the modern aleutian trench fore arc in this region is probably affected by pre eocene inherited structures that are still active in the present day stress regime a major domain boundary fault located at long 161 w is probably part of a strike slip fault system that also bounds sanak basin farther landward on the shumagin shelf this fault zone is nearly coincident with the mapped position of the western edge of the shumagin seismic gap these transverse structures in the shumagin fore arc probably influence the spatial distribution of stress buildup related to subduction and hence the locations of rupture zones of great interplate earthquakes along this segment of the aleutian trench

new isotopic ages and the timing of orogenic events in the cordillera darwin southernmost chilean andes abstract the cordillera darwin a structural culmination in the andes of tierra del fuego exposes an orogenic core zone that has undergone polyphase deformation and metamorphism some of the classic problems of orogenic zones have remained unanswered in the cordillera darwin the age of deformed plutonic rocks the distinction of structurally reactivated basement and metamorphosed cover rocks and the timing of orogenic events this study addresses and partially answers these questions a well constrained rb sr isochron age of157 8m y and an initial 87 sr 86 sr ratio of 0 7087 obtained from a pre tectonic granitic suite suggest a genetic relation between this suite and upper jurassic silicic volcanic rocks in the cover sequence tobifera formation and also suggest involvement of continental crust in formation of these magmas a poorly constrained rb sr isochron age of240 40m y obtained from supposed basement schists is consistent with field relations in the area which suggest a late paleozoic early mesozoic metamorphism for these pre late jurassic rocks however because of scatter in the data and the uncertainties involved in dating metasedimentary rocks the significance of the isotopic age is dubious compilation of previously published ages in the area 9 with new mineral ages reported here indicate that early andean orogenic events occurred between 100 and 84 m y ago and that subduction related magmatism has contributed probably discontinuously to the crustal evolution of the region throughout the mesozoic

edge deep seismic reflection transect of the eastern aleutian arc trench layered lower crust reveals underplating and continental growth an edge deep crustal seismic reflection transect of the eastern aleutian arc trench traces oceanic crust and moho more than 200 km beneath the accretionary prism to depths of more than 30 km these horizons project beneath a prominent sequence of layered reflectors that extends from about 9 to 35 km beneath the mesozoic core of the prism earthquake hypocenters imply continuity of the downgoing lithosphere from the base of the layered reflectors to beneath and beyond the active augustine volcano rapid lateral growth of the prism in eocene oligocene time coincided with uplift of the mesozoic core of the prism during lateral growth maintenance of critical taper requires thickening either by internal deformation or underplating because exposed rocks show only modest postemplacement shortening thickening most likely occurred by underplating probably of the layered reflectors the overall geometry of the layered reflectors is reminiscent of nappe structures and their emplacement may represent crustal scale duplexing associated with underplating the edge reflection data and borehole results indicate that the self edge is marked by an active out of sequence thrust that separates the paleogene and neogene prisms this thrust apparently developed in response to the prism s need to maintain critical taper and demonstrates that contrasts inmore lithology can result from mechanisms other than terrane emplacement less

mechanisms determining the atmospheric response to midlatitude sst anomalies abstract previous gcm experiments demonstrated that a model atmosphere produces two different responses to a midlatitude warm sst anomaly over the pacific under perpetual january and february conditions to elucidate the mechanisms responsible for the different gcm responses and their dependence on the background flow experiments with two idealized models are conducted experiments with a linear baroclinic model reveal that the gcm responses at equilibrium are primarily maintained by the anomalous eddy forcing the anomalous flow induced directly by an idealized initial heat source exhibits little sensitivity to the background flow eddy feedbacks on the heating induced anomalous flow are examined using a linear storm track model the anomalous eddy forcing produced by the storm track model is sensitive to the basic state the eddy forcing in january acts to shift the heating induced upper level ridge toward the northeast of the gulf of alaska while in february it acts to reinforce the ridge this sugge

evaluation of longitudinal dispersivity estimates from simulated forced and natural gradient tracer tests in heterogeneous aquifers 1 we simulate three types of forced gradient tracer tests converging radial flow unequal strength two well and equal strength two well and natural gradient tracer tests in multiple realizations of heterogeneous two dimensional aquifers with a hydraulic conductivity distribution characterized by a spherical variogram we determine longitudinal dispersivities l by analysis of forced gradient test breakthrough curves at the pumped well and by spatial moment analysis of tracer concentrations during the natural gradient tests results show that among the forced gradient tests a converging radial flow test tends to yield the smallest l an equal strength two well test tends to yield the largest l and an unequal strength two well test tends to yield an intermediate value this finding is qualitatively explained by considering the aquifer area sampled by a particular test a converging radial flow test samples a small area and thus the tracer undergoes a low degree of spreading and mixing an equal strength two well test samples a much larger area so the tracer is spread and mixed to a greater degree results also suggest that if the distance between the tracer source well and the pumped well is short relative to the lengths over which velocity is correlated then the l estimate can be highly dependent on local heterogeneities in the vicinity of the wells finally results indicate that l estimated from forced gradient tracer tests can significantly underestimate the l needed to characterize solute dispersion under natural gradient flow only a two well tracer test with a large well separation in an aquifer with a low degree of heterogeneity can yield a value of l that characterizes natural gradient tracer spreading this suggests that a two well test with a large well separation is the preferred forced gradient test for characterizing solute dispersion under natural gradient flow

late cenozoic tectonics of the southern chile trench marine geophysical data from off shore of southern chile are used to define the interaction between the chile ridge and chile trench during late cenozoic time we identify three distinct ridge trench collision events between 14 and 10 ma a 700 km long nearly continuous section of the chile ridge was sub ducted between 55 s and 48 s shorter sections of the ridge offset by large transform faults were subducted at 6 and 3 ma between 48 s and 47 s at the present day triple junction the subduction of the ridge has a strong influence on the chile trench in this region the landward trench slope has undergone a recent episode of subduction driven tectonic erosion the trench slope is narrower and steeper than along other sectors of the margin and the trench axis has migrated shoreward evidence exists for late neogene and quaternary uplift and plutonism on the adjacent continental margin south of 48 s where collision took place between 10 and 14 ma the effects of collision are much less pronounced in particular the landward trench slope does not appear to have been subjected to extensive tectonic erosion we conclude that the configuration of the spreading centers and transform faults on the chile rise is the chief factor controlling ridge trench tectonic interaction tectonic erosion of the landward trench slope and tectonic activity in the adjacent continental margin are much greater when short sections of ridge offset by large transform faults are subducted a major sub marine channel leads along the trench axis south ward from the triple junction this channel cuts across the outer trench rise and carries sediment westward to the mornington abyssal plain the paleogene tectonic history of the southern chile trench includes a southward migrating ridge trench collision involving the farallon aluk spreading center in eocene time

tectonic erosion at the front of the japan trench convergent margin abstract the imaging of a multichannel seismic record was improved by reprocessing using pre stack techniques the reprocessed record shows structures that indicate tectonic erosion and gravity collapse at the front of the japan trench margin much of the lower slope appears to be underlain by a detached coherent block of continental crust the lower slope has failed by mass wasting and the resulting apron of slump debris at the base of the slope has become involved in thrust faulting at the front of the subduction zone slumping continues as long as debris is removed from the front of the margin by subduction and the apron cannot build up sufficiently to stabilize the failing lower slope truncated beds at the base of the upper plate indicate subcrustal erosion as well this probably being the main cause of massive subsidence of the margin subsidence was the cause of oversteepening destabilization and subsequent gravity collapse of the leading edge of the upper plate

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

cordilleran slab windows the geometry and geologic implications of subducted spreading ridges are topics that have bedeviled earth scientists ever since the recognition of plate tectonics as a consequence of subduction of the kula farallon and east pacific rises slab windows formed and migrated beneath the north american cordillera the probable shape and extent of these windows which represent the asthenosphere filled gaps between two separating subducting oceanic plates are depicted from the late cretaceous to the present possible effects of the existence and migration of slab windows on the cordillera at various times include cessation of arc volcanism and replacement by rift or plate edge volcanism lithospheric uplift attenuation and extension and increased intensity of compressional tectonism eocene extensional tectonism and alkaline magmatism in southern british columbia and the northwestern united states were facilitated by slab window development

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

evolution of structures and fabrics in the barbados accretionary prism insights from leg 110 of the ocean drilling program the microstructures and crystal fabrics associated with the development of an amphibolite facies quartzo feldspathic mylonitic shear zone torridon nw scotland have been investigated using sem electron channelling our results illustrate a variety of microstructures and fabrics which attest to a complex shear zone deformation history microstructural variation is particularly pronounced at low shear strains significant intragranular deformation occurs via a domino faulting style process whilst mechanical incompatibilities between individual grains result in characteristic grain boundary deformation accommodation microstructures a sudden reduction in grain size defines the transition to medium shear strains but many of the boundaries inherited from the original and low shear strain regions can still be recognized and define distinctive bands oriented at low angles to the shear zone margin grains within these bands have somewhat steeper preferred dimensional orientations these domains persist into the high shear strain mylonitic region where they are oriented subparallel to the shear zone margin and consist of sub 20 m grains the microstructures suggest that the principal deformation mechanism was intracrystalline plasticity with contributions from grain size reduction via dynamic recrystallization grain boundary migration and grain boundary sliding crystal fabrics measured from the shear zone vary with position depending on the shear strain involved and are consistent with the operation of several crystal slip systems e g prism basal rhomb and acute rhomb planes in a consistent direction probably parallel to a and or m they also reveal the presence of dauphine twinning and suggest that this may be a significant process in quartz deformation a single crystal fabric evolution path linking the shear zone margin fabric with the mylonitic fabric was not observed rather the mylonitic fabric reflects the instantaneous fabric which developed at a particular location for a particular shear strain and original parental grain orientation the mature shear zone therefore consists of a series of deformed original grains stacked on top of each other in a manner which preserves original grain boundaries and intragranular features which develop during shear zone evolution the stability of some microstructures to higher shear strains the exploitation of others at lower shear strains and a continuously evolving crystal fabric mean that the strain gradient observed across many shear zones is unlikely to be equivalent to a time gradient

structural and magmatic responses to spreading ridge subduction an example from southwest japan subduction of a spreading ridge is expected to leave a geological signature upon the overriding accretionary prism distinct from that of conventional convergent plate boundaries the late oligocene early miocene rocks of the shimanto accretionary prism at cape muroto shikoku island record a tectonomagmatic fabric that is anomalous with respect to typical accretionary prisms these accreted strata have been imprinted with an unusual late stage event that involved 1 regional cusp like flexing of structural trends in the prism 2 near trench magmatism and 3 pervasive faulting this event overlaps in time with the opening of the shikoku back arc basin circa 26 14 ma the shikoku basin spreading ridge trends perpendicular to the strike of the accretionary prism and is now located immediately offshore of cape muroto the regional flexing of structural trends in the prism affects rocks as old as cretaceous although the flexure is most tightly developed in the late oligocene early miocene portion of the prism where it is steeply plunging kinematic analysis of the flexure and the pervasive fault system indicates that the flexure most likely resulted from indentation of the prism by a rigid feature at cape muroto the core of the flexure forms a locus for mafic dikes that are petrochemically equivalent to some mid ocean ridge basalts morb from the shikoku back arc basin preliminary paleomagnetic data suggest that the dikes were intruded during the later stages of flexure development these data combined with plate reconstructions for the area at 15 ma indicate that indentation of the prism most likely resulted from early miocene orthogonal subduction of the topographically elevated shikoku basin spreading ridge in the muroto area morb magmatic rocks in the core of the flexure appear to have emanated from the active spreading ridge during subduction

the tectonic fabric of the barbados ridge accretionary complex abstract a map showing near surface structure and other surficial geological features on the barbados ridge accretionary complex has been constructed from gloria long range side scan sonographs detailed bathymetric surveys using seabeam and single and multichannel seismic reflection sections the map shows major variations in the size orientation and style of structures across the accretionary complex that are principally related to the increasing thickness of the complex and the sediment drape upon it and variations along the complex that derive from the northward thinning of sediment accreted from the ocean floor and the effects of ridges in the oceanic basement the development of crossfaulting is mainly associated with basement ridges mud diapirism is abundant in the southern part of the complex and is characteristic of that part of the complex into which turbidites from the orinoco submarine fan are accreted slumping on a variety of scales is another feature of the complex the map is presented at a scale of 1 1063415 4 inches per degree of longitude and also shows bathymetry at 200 m contour intervals

submarine fan development in the southern chile trench a dynamic interplay of tectonics and sedimentation submarine fans are well developed in the southern chile trench from 33 s to 41 s latitude seamarc ii side scan sonar and seismic reflection records image steep erosional escarpments as much as 400 m in relief extending seaward across the trench basin from the mouths of submarine canyons the scarps bisect trench fans into paired lobes of contrasting morphology where gravity flows either follow or oppose the gradient of the axial trough fanlobes are depositional and constructional up gradient south from the canyon mouths they are composed of aggraded channel levee complexes smooth and conformable sediment drapes and crescentic levees rimming the headwalls of erosional scarps fanlobes are carved and dissected by erosional processes down gradient north from the canyon mouths they exhibit amalgamated lag pavements composite sediment lobes longitudinal furrows braided channels and canyon mouth bars thick massive to laminated sand and gravel with abundant scour surfaces were sampled from the erosional fanlobes whereas fine grained turbidites with expanded hemipelagic intervals typify the depositional fanlobes the texture and composition of the sediment supply the onshore climate and the tectonic perturbations of the axial gradient affect the morphologic development of trench fans stratigraphic intervals recording periods of intensified fan erosion and progradation of the axial channel are manifested by channeled high amplitude seismic facies reflective lenticular bodies truncation and scour surfaces planar amalgamation and sigmoidaccretion structures the severe erosional dissection of down gradient fanlobes and the northerly encroachment of the axial channel are best developed in the surficial strata of the trench basin extensive sediment remobilization and efficient longitudinal transport sculpted the present fan surface and are correlated with the last glacial maximum the trench fans may have been deposited on progressively steeper trench gradients because the buoyant chile rise migrates northward as it subducts beneath the andean continental margin fan distributary channels axial channels slump scars and erosional gullies are largely localized along structural features normal faults propagate through the sedimentary cover and create elongate depressions on the sea floor that capture the high velocity mainstreams of turbidity currents orthogonal fault sets within the deposits of the trench basin which parallel the spreading and transform structures of the extinct pacific farallon rise and the chile rise are evidently reactivated during subduction by flexure of the oceanic basement along the outer wall of the trench basin uplifted thrust ridges generally restricted to a narrow zone along the base of the deformation front are dissected by distributary channels and channel courses are locally deflected seaward of these propagating structures transform oriented basement ridges associated with strike displacements of the axial channel and vertical faults in the trench basin may accommodate renewed strike slip motion as they enter the subduction zone and thereby influence the debouchment points of submarine canyons to the trench basin

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

evidence and mechanisms for forearc extension at the accretionary costa rica convergent margin seismic reflection data across the upper trench slope off the nicoya peninsula costa rica reveal a wide zone of nearly trench parallel normal faults although work in the last decade has shown that normal faults are present at many convergent margins most examples e g japan peru chile and guatemala have been associated with margins experiencing subduction erosion or non accretion in contrast extension in the costa rica study area apparently is coeval with frontal accretion and underplating the normal faults across the costa rica forearc are striking in seismic section due to the well layered 2 km thick upper slope apron fault plane reflections and reflector terminations show that the faults extend through the sedimentary apron and apparently into the underlying accretionary prism indicating a deep seated deformation process the zone of extension is from the midslope area to within 10 km of the shelf edge a minimum width of about 20 km the estimated extension across the zone is at least 1 5 to 3 km within the apron section spacing between the faults is generally 200 500 m and nominal fault dip is 20 40 and predominantly landward activity on the normal faults appears to have occurred over a significant period of time based on increased displacement with depth and on fault controlled sedimentary thickening at least some of the faults may be presently active shallow reflectors and possibly the seafloor are displaced by faulting contemporary sediment accretion is documented by the same seismic reflection profiles showing offscraping and underplating near the toe of the wedge and out of sequence thrusting primarily below the midslope area the consistent landward normal fault dip may be influenced by structural anisotropy in the prism and possible extensional reactivation of earlier thrust faults associated with accretion processes with the available data it is not possible to conclusively determine the cause of the stress field leading to the upper prism and apron extension however the three most likely causes are underplating changes in basal shear stress or a brief episode of subduction erosion

geochemical evidence for formation of the bay of islands ophiolite above a subduction zone the bay of islands boi complex in western newfoundland is one of the world s best known ophiolites and has generally been interpreted as a fragment of ocean floor emplaced on the eastern margin of north america1 2 the similarities of the boi to litho sphere formed at mid ocean ridges has led many investigators to use the boi as an onland laboratory in which to study the geochemical tectonic and geophysical properties of the oceanic lithosphere1 9 there have been nonetheless a few suggestions that the boi ophiolite formed above a subduction zone10 12 as have most other ophiolites13 15 here i report the existence of a substantial tantalum depletion in boi magmas jenner et al 16 demonstrate that boi magmas have a niobium depletion as well these depletions are characteristic of magmas erupted above subduction zones but not at mid ocean ridges indicating that the boi is not after all a good analogue for lithosphere formed at mid ocean ridges

crustal accretion and tectonic setting of the troodos ophiolite cyprus abstract the troodos ophiolite is composed of two distinct magmatic suites an early relatively high tio 2 basaltic suite constitutes the upper gabbros the sheeted dike complex and the andesitic to rhyodacitic lower pillow lavas a late low tio 2 basaltic andesite suite constitutes the lower ultramafic to gabbroic cumulates and the upper pillow lavas parental magmas to the early suite originated as partial melts from hydrous relatively undepleted peridotite diapirs rising in a mantle wedge in response to dehydration of a subducting crust the late basaltic andesite suite is highly depleted in incompatible elements and show similarities with second stage or refractory basalts from oceanic spreading centers the troodos basaltic andesites and associated cumulates may thus have formed in response to continued adiabatic upwelling as second stage melts from the hydrous water undersaturated peridotite diapirs from which the earlier spreading related magmas were extracted the basaltic andesites formed in a relatively slow spreading or non spreading tectonic environment and dominantly crystallized in magma chambers underplating the troodos ophiolite the extrusion of the basaltic andesites of the upper pillow lavas occurred simultaneously with still active evolving volcanic centers belonging to the earlier magmatic suite an alternative possibility would be that the basaltic andesites separated from renewed diapirs rising as a consequence of re initiation of subduction and liberation of fluids from the subducting slab into the overlying depleted peridotite the existence of lree and lil enriched lavas among the upper pillow lavas support this possibility the apparent conflict between a spreading related geology and the arc petrology and geochemistry of the troodos ophiolite may be an artifact of lack of detailed studies of the geology and chemistry of oceanic and back arc rifting environments however it is proposed that the available data for the troodos ophiolite are consistent with its formation in a short lived spreading basin formed by splitting of an immature submarine arc

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

tectonics and geology of spreading ridge subduction at the chile triple junction a synthesis of results from leg 141 of the ocean drilling program an active oceanic spreading ridge is being subducted beneath the south american continent at the chile triple junction this process has played a major part in the evolution of most of the continental margins that border the pacific ocean basin a combination of high resolution swath bathymetric maps seismic reflection profiles and drillhole and core data from five sites drilled during ocean drilling program odp leg 141 provide important data that define the tectonic structural and stratigraphic effects of this modern example of spreading ridge subduction a change from subduction accretion to subduction erosion occurs along strike of the south american forearc this change is prominently expressed by normal faulting forearc subsidence oversteepening of topographic slopes and intensive sedimentary mass wasting overprinted on older signatures of sediment accretion overthrusting and uplift processes in the forearc data from drill sites north of the triple junction sites 859 861 show that after an important phase of forearc building in the early to late pliocene subduction accretion had ceased in the late pliocene since that time sediment on the downgoing oceanic nazca plate has been subducted site 863 was drilled into the forearc in the immediate vicinity of the triple junction above the subducted spreading ridge axis here thick and intensely folded and faulted trench slope sediments of pleistocene age are currently involved in the frontal deformation of the forearc early faults with thrust and reverse kinematics are overprinted by later normal faults the chile triple junction is also the site of apparent ophiolite emplacement into the south american forearc drilling at site 862 on the taitao ridge revealed an offshore volcanic sequence of plio pleistocene age associated with the taitao fracture zone adjacent to exposures of the pliocene aged taitao ophiolite onshore despite the large scale loss of material from the forearc at the triple junction ophiolite emplacement produces a large topographic promontory in the forearc immediately after ridge subduction and represents the first stage of forearc rebuilding

interaction between the chile ridge and chile trench geophysical and geothermal evidence geophysical and geothermal data are examined from the three southernmost sections of the chile ridge starting at 44 s and continuing south to the triple junction of the nazca antarctic and south america plates at 47 s these sections represent three progressively younger stages in a ridge trench collision event corresponding to 3 m y before the collision 1 m y before the collision and culminating in an ongoing collision at the triple junction magnetic and bathymetric data across the chile rise indicate that there is little change in the configuration of the spreading center as the ridge approaches and then collides with the trench in the collision zone a normal looking rift valley can be traced for 40 km before it disappears beneath the toe of the landward trench slope there is no evidence for the complex pattern of ridge jumps and spreading center rotations that occurred when the pacific farallon spreading center collided with north america in contrast the overriding plate is greatly affected by the ridge collision the landward trench slope steepens and narrows as the collision zone is approached at the southern end of the collision zone a ridge associated with the taitao fracture zone is colliding with the trench and may be in the process of being obducted onto the landward trench slope geothermal measurements were made along three transects of the margin corresponding to the time 3 m y before the collision during the collision and 6 m y after the collision the heat flow measurements in the collision zone document a large pulse of heat associated with the subduction of the ridge values as high as 345 mw m2 were recorded on the lower trench slope a prominent bottom simulating reflector bsr observed over a wide area of the landward trench slope north of the triple junction and more locally south of the triple junction is used to expand the grid of heat flow observations excellent agreement is found between measured heat flow and estimates of heat flow based on the depth to the bsr the heat flow measurements compare favorably with a theoretical model assuming conductive heat flow we estimate that the accretionary prism has been substantially removed in the collision zone and conclude that the landward trench slope is undergoing an episode of rapid tectonic erosion periodic ridge collisions in the past may account for the apparent truncation of the andean forearc region

the structure of the western part of the southern uplands of scotland the structures of two areas one in the northern belt and one in the central belt of the southern uplands are described and compared the main structural histories are the same both show a main deformation phase related to se directed thrusting with associated fold and cleavage formation both areas exhibit late minor nw directed thrusting with related refolding and crenulation cleavage development in the case of the central belt both areas also show evidence of sinistral strike slip or transpressive deformation which reactivates steepened thrust faults when these deformation histories are reviewed in the context of geophysical radiometric and biostratigraphical data from the region a forearc accretionary setting for the whole of the southern uplands appears the most likely the strike slip deformation has internally disrupted an accretionary prism but has not juxtaposed originally unrelated terranes the effects of the late silurian closure of iapetus and subsequent events are also represented in the southern uplands