seismic stratigraphic framework of the forearc basin off central sumatra sunda arc abstract new multichannel seismic reflection data provide information on the stratigraphic framework and geologic history of the forearc basin west of central sumatra we recognize six seismic stratigraphic sequences that reflect the cenozoic history and development of the outer continental shelf and forearc basin southeast of nias island these sequences indicate several episodes of uplift of the subduction complex and filling of the forearc basin early in the development of this margin paleogene slope deposits prograded onto the adjacent basin floor onlapping this assemblage are two units interpreted as younger paleogene trough deposits uplift associated with rejuvenation of subduction in the late oligocene led to erosion of the sumatra shelf and formation of a regional unconformity the early miocene was a period of significant progradation a miocene limestone unit partly downlaps and partly onlaps the older paleogene deposits it is characterized by shallow shelf and oblique progradational facies passing into basin floor facies a buried reef zone occurs near the shelf edge the cutting of an erosional unconformity on the shelf and slope in late miocene early pliocene time culminated this episode of deposition in the late pliocene a large flexure developed at the western boundary of the basin displacing the outer arc ridge upward relative to the basin over 1 km of pliocene to recent sediment was deposited as a wedge in the deep western portion of the basin landward of the outer arc ridge these deposits are characterized by flat lying high amplitude continuous reflections that overstep the late miocene unconformity up to 800 m of shallow water limestone have been deposited on the shelf since mid pliocene time

backthrusting in accretionary wedges backthrusting of the rear of the accretionary wedge arcward over the rocks of the forearc basin is a common aspect of accretionary tectonics an older assumption that backstops of accretionary wedges dip back toward the arc and that accreted material is emplaced below such backstops is contradicted by observations from many active forearcs that indicate 1 backthrusting is common 2 forearc basins are nearly ubiquitous associates of accretionary wedges and 3 forearc basement where imaged appears to diverge from the sedimentary package dipping under the wedge while the overlying sediments are often lifted up against it backthrusting may be favored where relief is high between the crest of the wedge and the surface of the forearc basin because the relief must be supported by shear stress along the backthrust we find ratios of lt lb averaging 2 64 0 8 where lt is the horizontal distance from the crest of the wedge to the front of the toe and lb is the distance from the crest of the wedge to the surface projection of the backthrust this ratio is predicted to increase with total convergence and with sedimentation because the toe undergoes higher deformation rates than the backthrust and sedimentation in the trench favors outbuilding of the toe whereas sedimentation in the forearc basin tends to cancel the gravitational component of the driving stress for the backthrust

the evolution of the western ligurian flysch units and the role of mud diapirism in ancient accretionary prisms maritime alps northwestern italy the western ligurian flysch units represent an upper cretaceous to paleogene accretionary prism overthrust onto the paleo european continental margin during the collisional stages of the alpine orogeny eocene their precollisional evolution has been reconsidered according to the data collected through extensive field mapping the study of lowgrade metamorphism and by considering more strictly the processes documented in modern convergent zones

possible evidence for subducted sedimentary materials beneath central california seismic reflection and refraction data from the coast ranges of central california west of the san andreas fault suggest the presence of a low velocity 4 5 5 0 km s wedge at a depth of about 15 22 km that dips east beneath rocks of both the franciscan assemblage and the salinian block we propose that the wedge represents sedimentary materials that were subducted in an offshore trench within the past 23 m y the refraction data also suggest the presence of a second low velocity zone that extends at least to mid crustal depths beneath the san andreas fault zone

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

rate of outward growth of the mediterranean ridge accretionary complex abstract the position as a function time of the deformation front on the southwest flank of the mediterranean ridge accretionary complex is constrained as follows 1 a the deformation front is now active 2 b the site of core ban84 05gc was still near the abyssal plain when displaced shallow water benthic foraminifera of inferred african provenance were redeposited within an upper pliocene age unit 3 c the site of core ban84 05gc on the outer flank of the mediterranean ridge was already within the topographically rugged accretionary complex when a pliocene debris flow was emplaced 4 d dsdp site 125 had already been uplifted into a topographically elevated position when lower pliocene pelagic ooze was deposited 5 e a gypsum bearing breccia in dsdp site 125 requires that the site was either on the abyssal plain or within the tectonically active outer perimeter of the accretionary complex during the messinian salinity crisis 6 f dsdp site 377 had already been uplifted into a topographically elevated position when middle miocene age pelagic marl was deposited 7 g dsdp site 377 was still on or near the abyssal plain when early to lower middle miocene age smectite bearing turbidites of inferred african provenance were deposited and 8 h the mediterranean ridge began to grow by offscraping against a backstop formed by the alpine nappes of the hellenic arc at the time that subduction began 33 ma together these constraints define a range of potential growth curves for the mediterranean ridge with a rate of outward growth of approximately 0 5 to 2 cm yr this growth rate is faster than that inferred for most other modern accretionary prisms both as an absolute value and as a fraction of the subduction velocity an unusually thick incoming section and or an unusually weak evaporitic decollement may contribute to the rapid growth rate the inferred age of accretion does not increase linearly with distance from the deformation front rather there is an apparent acceleration of the rate of outward growth through time this observation may reflect a true acceleration of growth rate in response to the onset of back arc extension in the overriding plate aegean sea and or in response to the development of a weak basal decollement following the introduction of halite into the incoming stratigraphic section alternatively the non linearity of the age vs distance curve may indicate that shortening of the offscraped sediments continues across the breadth of the mediterranean ridge rather than being confined to the outer flank

accretion and subduction tectonics in the chugach mountains and copper river basin alaska initial results of the trans alaska crustal transect geologic seismic gravity and magnetic data from the northern chugach mountains and southern copper river basin alaska indicate that the chugach terrane cgt and the composite peninsular wrangellia terrane pet wrt are thin 10 km rootless sheets bounded on the south by north dipping thrust faults that sole into a shallow horizontal low velocity zone the cgt has been thrust at least 40 km beneath the pet wrt along the border ranges fault system brfs adjacent to the brfs uplift and erosion of 30 40 km since jurassic time have exposed blueschist facies rocks in the cgt and mafic and ultramafic cumulate rocks in the pet wrt four paired north dipping layers of low and high seismic velocities extend beneath the northern cgt and southern pet wrt and may be slices of subducted oceanic crust and upper mantle the upper two pairs may now be joined to the continental plate 15 references 5 figures

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

great earthquake surface ruptures along backthrust of the janauri anticline nw himalaya abstract results of a paleoseismic trenching investigation on a backthrust in the northern margin of janauri hill at mehandpur 31 18 11 37 n 76 18 31 47 e sub himalayan range of the himachal himalaya are presented the active backthrust revealed the last event took place after 0 8 0 03 ka ago i e post ce 1200 the age of the last earthquake with a minimum fault slip 1 m along f1 fault and presence of single colluvium on the back thrust nearly matches with an earthquake event previously documented on the bhatpur forethrust of the himalayan frontal thrust system uplifted and truncated fluvial terraces due to displacement on the backthrust are preserved along several north flowing river valley sections in the northern limb of janauri anticline the long term vertical uplift rate calculated through dividing difference between current elevation and elevation of current river grade is 1 08 0 08 mm yr field evidence shows fore and backthrusts are active and they form simultaneously but displacements are episodic suggesting the backthrust develops either due to locking of forethrust or to accommodate a large amount of fault slip due to magnitude of earthquake event along the forethrust further the backthrust may be used as an indirect method of inferring the age of the last event that took place along the forethrust

paleothermal structure of the point san luis slab of central california effects of late cretaceous underplating out of sequence thrusting and late cenozoic dextral offset late cretaceous shale and sandstone turbidites of the point san luis slab are isoclinally folded locally sheared and faulted but their severity of stratal disruption is relatively mild when compared to adjacent polymictic melange of the franciscan complex we tested the interpretation of a trench slope basin origin for these strata by documenting their paleothermal structure including contacts between turbidites and melange values of mean random vitrinite reflectance rm from turbidites are 0 9 1 7 estimates of maximum paleotemperature are 135 200 c melange matrix samples yield rm values of 1 1 2 5 with an average of 1 5 and peak temperatures between 160 and 240 c the turbidite over melange contact is locally cooler over warmer and was folded after peak heating the relatively high paleotemperatures cast doubt on a shallow slope basin model i e 1 2 km burial depth we suggest instead that thermal maturation of the point san luis slab occurred much deeper in an accretionary prism 10 15 km where offscraped trench wedge deposits were faulted against underplated melange the paleothermal structure was offset and tilted after peak heating by two out of sequence faults late oligocene to pliocene strata rest unconformably above the franciscan and there is a significant gap in thermal maturity across this unconformity with no evidence to show that franciscan rocks were reset thermally following the main episode of uplift and erosion three dimensional orientations of isoreflectance surfaces on opposite sides of the san gregorio san simeon hosgri fault system also can be used to test interpretations of strike slip neotectonics the failure to match these geometries among suspected piercing points at point san luis cambria and point sur favors a suggestion that differential post thermal peak dextral offset of franciscan basement has not exceeded 10 15 km

life cycle of the east carpathian orogen erosion history of a doubly vergent critical wedge assessed by fission track thermochronology apatite fission track b t thermochronology applied to the east carpathian fold and thrust belt in romania constrains the interaction between the erosion history and the tectonic evo lution of the orogen the long lived constant asymmetric topography of the orogen and the erosion pattern as inferred from apatite ft data are in agreement with the style of evolution predicted by doubly vergent critical coulomb wedge models during the miocene up to 4 km of erosion took place over the actively deforming orogen and the erosion patterns forms a mirrored image of the subsurface deforming wedge the erosion products are deposited in flanking molasse basins which subsided contemporaneously due to the growing topographic weight of the wedge deformation of the wedge was presumably active from the early to late miocene but apatite ft ages between 14 and 9 ma and track length modeling predict an acceleration of erosion rates 0 5 0 3 mm yr ini tially in the middle to late miocene 3 2 ma the onset of erosion coincides with the last phase of convergence and a climax in the deformation history these features are interpreted to have been caused by underplating of the european continental margin convergence ceased before the pliocene according to the activity of thrust faults during the pliocene and quaternary up to 2 km of overburden was removed by erosion leading to isostatic rock uplift of the entire region the use of apatite fission track minimum ages supports the general idea that the orogen has a di achronous evolution from north to south the orogen in the southern bend zone has a more juve nile tectonic character and the main erosion phase took place during the pliocene and quaternary 5 0 ma the integrated approach of large scale morphology and fission track analysis confirms the applicability of the theoretical coulomb wedge mechanics to a natural compressive fold and thrust belt the study complements the geometric features of the orogenic wedge with quantified erosion histories and illustrates the effects of orogen evolution on vertical movements in the entire convergent setting

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

the evolution of forearc structures along an oblique convergent margin central aleutian arc multichannel seismic reflection data were used to determine the evolutionary history of the forearc region of the central aleutian ridge since at least late miocene time this sector of the ridge has been obliquely underthrust 30 west of orthogonal convergence by the northwestward converging pacific plate at a rate of 80 90 km m y our data indicate that prior to late eocene time the forearc region was composed of rocks of the arc massif thinly mantled by slope deposits the forearc region probably lacked both major depositional basins and a tectonically attached accretionary prism of offscraped oceanic deposits beginning in latest miocene or earliest pliocene time a zone of outer arc structural highs and a forearc basin began to form formation of these companion intraarc structures may be linked to the late neogene growth of an accretionary wedge that formed as the result of the deposition of a thick turbidite wedge in the aleutian trench initial structures of the zone of outer arc highs formed as the thickening wedge underran compressively deformed and uplifted the seaward edge of the arc massif above a landward dipping backstop thrust forearc basin strata ponded arcward of the elevating zone of outer arc highs however most younger structures of the zone of outer arc highs cannot be ascribed simply to the orthogonal effects of an underrunning wedge oblique convergence created a major right lateral shear zone the hawley ridge shear zone that longitudinally disrupted the zone of outer arc highs truncating the seaward flank of the forearc basin and shearing the southern limb of hawley ridge an exceptionally large antiformal outer arc high structure slivers of forearc basement rocks and overlying strata have been transported along the shear zone that is flanked by differentially elevated structures attributed to localized transpressive and transtensional processes uplift of hawley ridge may be related to the thickening of the arc massif by westward directed basement duplexes in addition the forearc is disrupted by structures transverse to the margin that occur where unusually high stress accumulations have resulted in the rupture of repeated great earthquakes it is likely that many ancient active margins evolved in tectonic and depositional settings similar to those of the central aleutian ridge great structural complexity including the close juxtaposition of coeval structures recording compression extension differential vertical movements and strike slip displacement should be expected even within areas of generally kindred tectonostratigraphic terranes

implications of the north and south panama thrust belts for the origin of the panama orocline high resolution imaging of the north nptb and south panama thrust belts using the seamarc ii seafloor mapping system and digital seismic reflection profiling provides an excellent data base with which to determine the kinematics of oroclinal deformation of panama remarkable continuity of structural orientations over 300 km along the nptb argues for widespread uniformity in direction of thrusting between panama and the caribbean plate westward decrease in the width and height of the accretionary wedge indicates a westward decrease in the total amount of convergence two major cross faults cut the wedge in its eastern part forming two minor blocks with different relative motions with respect to the caribbean frontal thrust orientations change across these faults major faults have been proposed to cut the isthmus of panama but simple cross isthmus faulting alone cannot explain the orientations of the major thrusts of the nptb bending of a flexible beam can explain much of the observed structure both directions and rates though the scale of the orocline precludes considering this as an elastic phenomenon plastic deformation of panama may be a useful mechanical explanation at least for eastern panama but consideration of plastic slip lines must include the northern andes as well changing from more rigid blocklike offsets on faults in western panama to a continuum of strain in eastern panama may be the best explanation for observations at present

dynamics of the polish and eastern slovakian parts of the carpathian accretionary wedge insights from palaeostress analyses the arcuate outer carpathian accretionary wedge formed in front of the east alpine carpathian pannonian alcapa megablock during the eocene sarmatian the wedge accreted sediments of the subducting remnants of the carpathian flysch basin a large oceanic tract left in front of the alpine orogen the palaeostress data for the orogenic hinterland particularly the data related to the early miocene extension that was expanding towards the ne combined with coeval subduction related volcanism that was expanding towards the ne indicate that the uneven roll back of the subduction zone was the main mechanism controlling the development of the northern west carpathian arc the palaeostress data for the tertiary accretionary wedge from the same time period are characterized by outward fanning 1 trajectories that changed gradually during the wedge development in contrast the palaeostress data for the hinterland are characterized by preferred directional stress events that changed abruptly during the wedge development these palaeostress results are in accordance with the behaviour of the wedge and the hinterland as the wedge behaved as a weak continuum and the hinterland behaved as a block mosaic with weak boundaries the fault traces of the northern west carpathian arc converge to both ends of the arc and suggest that the pre existing basin was the factor that controlled the arc location these fault trace patterns are asymmetric indicating a slightly oblique overall convergence in a ne sw direction in accordance with this convergence the palaeostress data for the accretionary wedge indicate that the western part of the wedge which is characterized by nw se oriented maximum principal compressional stress 1 was undergoing sinistral transpression meanwhile the eastern part which is characterized by ne sw oriented 1 was undergoing compression apparently the dynamics of the accretionary wedge was further influenced by the shape of an elongated ne sw trending alcapa megablock which was located behind the wedge and advanced in the direction of the general early miocene convergence during the most pronounced stages of the wedge development this megablock served as the local indenter as its strength surpassed that of the accretionary wedge located to its front further dynamic complexities were added because of the complex shape of the magura unit which was located in the most proximal portion of the wedge and was stronger than the units in front of it wedge outcrops indicate that the large scale shortening which is characterized by the development of detachments and ramps was preceded by an initial layer parallel shortening this is indicated by scaly fabrics and minor reverse faults that rotated into locked positions during the later accretion several outcrops with a wedge detachment fault indicate that there was a relatively low amount of friction during its development the decollement zone is several hundred metres thick and shows evidence of transient fluid flow that was driven by pressure gradients this is documented by frequent hydrofracturing sandstone dykes and fibrous veins that opened against the weight of the whole wedge all of which indicate cycles of higher pore fluid pressures that lowered the basal friction

tectonics and hydrogeology of accretionary prisms role of the decollement zone abstract at convergent margins the decollement zone comprises the plate boundary and is marked by profound structural disharmony by changes in stress orientation and by a discontinuity in plate velocity the decollement zone initiates in a weak sediment layer typically a low permeability hemipelagic mud lying below a more rapidly deposited stronger more permeable trench turbidite sequence at the deformation front trench turbidites tend to be offscraped whereas the finer grained hemipelagic and pelagic sediments are more likely to be underthrust offscraped materials may undergo only limited burial whereas underthrust deposits can be deeply buried undergo high pressure metamorphism and are more likely to be preserved in the stratigraphic record burial rates associated with underthrusting are high exceeding 20 km my with high rates of underthrusting sediment descending below the decollement zone is probably buried faster than it can dewater resulting in emplacement of relatively high porosity deposits at depth fluids flowing from modern decollement zones have migrated substantial distances laterally and tapped deep sources the episodicity of flow suggests pumping of fluid by cycles of dilation and flow to the decollement zone followed by failure and fluid expansion along the decollement zone possible pressure gradients below the decollement zone allow flow upward into it while maintaining minimum effective stress along it the thickness of sediment underthrust beneath the decollement zone determines whether these deposits are emplaced by diffusive underplating with stratal disruption and efficient dewatering or by coherent underplating with the formation of macroscopic duplexes and transfer of fluid to the base of the accretionary prism deformation mechanisms affecting accreted sediment depend on depth of entrainment into the decollement zone and duration of residence

the interaction of two indenters in analogue experiments and implications for curved fold and thrust belts abstract scaled analogue experiments are used to investigate the indentation of two basement blocks into a sedimentary basin and the formation of a fold and thrust belt the experimental set up has two adjacent indenters moving in parallel with a velocity difference the slow indenter moves with a relative velocity ranging from 40 to 80 of that of the fast one in a first experimental series quartz sand and low friction glass beads represent the sediment stack and its basal detachment respectively silicone oil simulates a viscous detachment in the second series of experiments the surface evolution and the spatio temporal strain distribution are derived from particle image velocimetry piv together with the 3d finite structure derived by cutting the models at the end of each experiment this allows the analysis of the structural evolution of the experimental wedges thrusting wedge development depends crucially on the relative velocity when the slow indenter moves with a velocity of more than 55 of the fast indenter a single curved thrust wedge develops the wedge becomes decoupled along strike slip zone at large velocity differences i e when the slow indenter moves slower than 55 of the faster indenter consequently the thrust front is strongly curved at high 55 60 and smoothly curved at low velocity differences 70 80 in all cases curvature increases during indentation along the most strongly curved portion of the thrust wedge the transfer zone rises particle rotation and material transport oblique to the indentation direction occur directed toward the front of the slow indenter thrusting cycles are timed by the fast indenter and influence thrusting in front of the slow indenter thrusts nucleate in front of the fast indenter wedge and propagate laterally to the slow indenter front this implies distant effects of wedge growth active hundreds of kilometres along strike of orogens silicone oil as a detachment induces a low angle wedge which is less curved than in the glass beads experiments and shows variable thrust vergence with fore and back thrusts we compare our experimental results with the curved fold and thrust belts of the tajik basin in front of the hindu kush and pamir indenters our results are able to explain several first order features

frontal accretion an internal clock for bivergent wedge deformation and surface uplift 1 we propose a conceptual model of frontal accretion within bivergent wedges which is based on two dimensional sandbox simulations and the analysis of particle displacement fields each frontal accretion cycle consists of a thrust initiation an underthrusting phase and a reactivation phase the location and magnitude of deformation within a bivergent wedge and its associated surface uplift vary systematically with the phase of the frontal accretion cycle and are thus predictable therefore the frontal accretion cycle can be considered as an internal clock for wedge scaled deformation and surface uplift we further demonstrate that the geometry of the deformation front and the spatial distribution of surface uplift can be used to infer the currently active phase within a frontal accretion cycle surface uplift of the axial zone and the retrowedge may reach up to half of the thickness of the incoming sedimentary layer during a frontal accretion cycle we estimate cycle duration to range between 104 and 105 years which is thus similar to the time frame of climatic cycles the proposed conceptual model provides an explanation for the commonly observed transience of deformation and surface uplift and for the discrepancy between geodetic paleoseismologic and geologic estimates of fault slip

controls on the shape and kinematics of the central andean plateau flanks insights from numerical modeling abstract the flat surface of orogenic plateaus requires a weak detachment at the base of the crust that prevents the formation of large wavelength topography in the centre of the orogen and decouples the deformation of crust and mantle this study uses numerical distinct element simulations of plateau orogeny to show that the mass flux across the lateral tips of this detachment controls the shape and kinematics of the plateau flanks if the mass flux is directed towards the plateau e g foreland crust moves into the detached section it forms a low slope small taper pro flank if the mass flux is directed outward thrusting towards the foreland generates a steep slope large taper retro flank the mass flux itself is controlled by the shear strength distribution at the base of the crust the two general types of orogenic flanks from three combinations that can be used to classify the evolution of plateau orogens in general application of this concept to the andes allows to decode the lower crustal shear strength distribution during the formation of the altiplano puna plateau their western slope is a retro flank localized by the underlying oceanic subduction zone on the eastern side a pro flank accretes foreland crust which is progressively decoupled from the moving mantle below

orogeny and orography the effects of erosion on the structure of mountain belts a numerical model of the coupled processes of tectonic deformation and surface erosion in convergent orogens is developed to investigate the nature of the interaction between these processes crustal deformation is calculated by a two dimensional finite element model of deformation in response to subduction and accretion of continental crust erosion operates on the uplifted surface of this model through fluvial incision which is taken to be proportional to stream power the relative importance of the tectonic and erosion processes is given by a dimensionless erosion number relating convergence velocity rock erodibility and precipitation rate this number determines the time required for a system to reach steady state and the final topographic shape and size of a mountain belt fundamental characteristics of the model orogens include asymmetric topography with shallower slopes facing the subducting plate and an asymmetric pattern of exhumation with the deepest levels of exhumation opposite to subduction these characteristics are modified when the regional climate exhibits a dominant wind direction and orographically enhanced precipitation on one side of the mountain belt the two possible cases are dominant wind in the direction of motion of the subducting plate and dominant wind direction in the opposite direction of the subducting plate velocity models of the former case predict a broad zone of exhumation with maximum exhumation in the orogen interior models of the latter case predict a focused zone of exhumation at the margin of the orogen and at high erosion number a reversal in the topographic asymmetry natural examples of these two cases are presented the southern alps of new zealand exhibits the climate and exhumation asymmetry characteristic of wind in the direction opposite to motion of the subducting plate the asymmetry of topography suggests that erosion is not efficient enough to have reversed the topographic asymmetry the contrasting example of dominant wind in the direction of subduction motion is provided by the olympic mountains of washington state in this case exhumation of deep levels of the cascadia accretionary wedge shows a broad domal pattern consistent with the observed orographic precipitation

predicting orogenic wedge styles as a function of analogue erosion law and material softening 1 the evolution of a compressive frictional wedge on a weak frictional and planar decollement subjected to frontal accretion is predicted with a two step method called sequential limit analysis the first step consists in finding with the kinematic approach of limit analysis the length of the active decollement and the dips of the emerging ramp and of the conjugate shear plane composing the emerging thrust fold the second step leads to a modification of the geometry first because of the thrust fold development due to compression and second because of erosion erosion consists in removing periodically any material above a fictitious line at a selected slope as done in analogue experiments this application of sequential limit analysis generalizes the critical coulomb wedge theory since it follows the internal deformation development with constant frictional properties the deformation is mostly diffuse a succession of thrust folds being activated so that the topographic slope reaches exactly the theoretical critical value frictional weakening on the ramps results in a deformation style composed of thrust sheets and horses applying an erosion slope at the critical topographic value leads to exhumation in the frontal central or rear region of the wedge depending on the erosion period and the weakening erosion at slopes slightly above or below the critical value results in exhumation toward the foreland or the hinterland respectively regardless of the erosion period exhumation is associated with duplexes imbricate fans antiformal stacks and major backthrusting comparisons with sandbox experiments confirm that the thickness dips vergence and exhumation of thrust sheets can be reproduced with friction and erosion parameters within realistic ranges of values

landward vergence and oblique structural trends in the oregon margin accretionary prism implications and effect on fluid flow abstract the central oregon margin spans a regional transition in accretionary structures from seaward verging in the south to landward verging in the north new multichannel seismic mcs data image both landward and seaward vergent provinces along the central and northern oregon margin landward vergence is characterized by a deep decollement with deformation distributed across a broad lower continental slope in a coherent structural style in the landward vergent area virtually all 4 km of incoming trench sediments overthrust the preceding thrust sheet forming fault bend folds and a distinctive ridge trough morphology this style of landward vergence is not explained by existing models in contrast seaward vergence correlates with a shallower decollement approximately 1 4 km above the oceanic crust and a more intense style of deformation within a narrower slope initial thickening of the trench sediments occurs across a well developed protothrust zone the frontal thrust forms a ramp anticline that is cut by a prominent backthrust previously observed seafloor vent sites in both regions correlate with thrusts that exhibit high amplitude reversed polarity reflections suggestive of enhanced porosity along the faults potential fluid sources and migration paths are strongly influenced by changes in the level of the decollement and vergence along the margin abrupt changes in structural style occur both along strike and updip and are bounded by two sets of oblique slip faults three nw striking left lateral faults are imaged in both mcs and seabeam data plunging anticlines developed along the nw striking faults are venting fluids and were previously interpreted as mud volcanoes the deformation front is locally disrupted where these faults intersect the prism but they appear to have limited influence on the structural evolution of the prism in contrast the ne striking right lateral faults are confined to the deforming sediments of the upper plate these faults interact with the thrusts within the prism forming a rhomboidal pattern of three dimensional deformation

slow senonian and fast palaeocene early eocene uplift of the granitoids in the central eastern pontides turkey apatite fission track results abstract dating and forward modelling of the fission track data of apatite samples from the dereli ebinkarahisar region south of giresun in the eastern turkish pontides provides quantitative data on the regional tectonics resulting from the closure of neo thetys and the collision of eurasia and gondwana the age vs elevation profiles identified senonian 80 7 3 2 to 62 4 2 5 ma slow uplift and denudation interpreted as the result of the diapiric ascent of subduction related plutons above the neo tethyan subduction zone beneath the eurasian continent this was followed by rapid differential uplift during the palaeocene early eocene 57 4 2 4 to 47 8 2 4 ma which juxtaposed granitoid units of different ages compositions and emplacement levels in the crust and is thought to be related to the collision between the pontide eurasian and anatolide gondwana basements the modelling results must be interpreted with caution but appear to indicate a period of mio pliocene ca 5 ma reheating related to volcanism associated with the westward escape of the anatolian plate and uplift from the pliocene ca 3 5 ma up to the present

the rise and fall of periodic drumbeat seismicity at tungurahua volcano ecuador abstract highly periodic drumbeat long period lp earthquakes have been described from several andesitic and dacitic volcanoes commonly accompanying incremental ascent and effusion of viscous magma however the processes controlling the occurrence and characteristics of drumbeat and lp earthquakes more generally remain contested here we use new quantitative tools to describe the emergence evolution and degradation of drumbeat lp seismicity at the andesitic tungurahua volcano ecuador in april 2015 the signals were recorded during an episode of minor explosive activity and ash emission without lava effusion and are the first to be reported at tungurahua during the ongoing 17 yrs of eruption following four days of high levels of continuous and pulsed tremor highly periodic lp earthquakes first appear on 10 april over the next four days inter event times and event amplitudes evolve through a series of step wise transitions between stable behaviors each involving a decrease in the degree of periodicity families of similar waveforms persist before during and after drumbeat activity but the activity levels of different families change coincidentally with transitions in event rate amplitude and periodicity a complex micro seismicity initiation sequence shows pulse like and stepwise changes in inter event times and amplitudes in the hours preceding the onset of drumbeat activity that indicate a partial de coupling between event size and rate the observations increase the phenomenology of drumbeat lp earthquakes and suggest that at tungurahua they result from gas flux and rapid depressurization controlled by shear failure of the margins of the ascending magma column

reply to comment on formation and thrusting in some great valley rocks near the franciscan complex california and implications for the tectonic wedging hypothesis

tectonic wedging along the rear of the offshore taiwan accretionary prism abstract the structural geometry kinematics and density structure along the rear of the offshore taiwan accretionary prism were studied using seismic reflection profiling and gravity modeling deformation between the offshore prism and forearc basin at the point of incipient collision and southward into the region of subduction has been interpreted as a tectonic wedge similar to those observed along the front of mountain ranges this tectonic wedge is bounded by an east dipping roof thrust and a blind west dipping floor thrust an east dipping sequence of forearc basin strata in the hanging wall of the roof thrust reaches a thickness in excess of 4 km near the tip of the interpreted tectonic wedge section restoration of the roof sequence yields an estimate of 4 km of shortening which is small compared with that inferred in the collision area to the north based on the variation in distance between the apex of the prism and the island arc previous studies propose that either high angle normal faulting or backfolding has exhumed the metamorphic rocks along the eastern flank of the central range in the collision zone on land to better constrain the initial crustal configuration we tested 350 crustal models to fit the free air gravity anomaly data in the offshore region to study the density structure along the rear of the accretionary prism in the subduction and initial collision zones before the structures become more complex in the collision zone on land the gravity anomaly observed in the region of subduction 20 2 n can be modeled with the arc basement forming a trenchward dipping backstop that is overlain by materials with densities in the range of sedimentary rocks near the point of incipient collision 20 9 n however the free air gravity anomaly over the rear of the prism is approximately 40 mgal higher compared with the region of subduction and requires a significant component of high density crustal rocks within the tectonic wedge these results suggest that the forearc basement may be deformed along the rear of the prism associated with the onset of collision but not in the subduction region further to the south

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

the wetar back arc thrust belt eastern indonesia the effect of accretion against an irregularly shaped arc the 350 km long east west trending wetar back arc thrust belt in eastern indonesia is a result of the collision of australia with the indonesian island arc along the northwest margin of wetar island a short 50 km section of the thrust belt trends northeast coincident with the offset of the indonesian arc by the newly discovered left lateral wetar atauro fault which runs along the shelf region and trends northeast the wetar atauro fault may be viewed as a large scale lateral ramp or wrench fault separating the eastern wetar thrust belt from the western wetar thrust belt the interaction of strike slip faulting and back arc thrust faulting creates several arc blocks whose geometry strongly affects the structure of the deformed wedge of sediment accreting in the wetar back arc thrust belt the varying orientations of the arc backstop make the wetar back arc thrust belt a perfect laboratory for the study of oblique convergence along most of its length the wetar thrust belt parallels the arc slope however along the northeast trending offset in the thrust belt seamarc ii side scan sonar plus seismic reflection data show that the trend of the thrust belt is not parallel to the trend of the arc slope rather it is intermediate between the trend of the arc slope and the perpendicular to the expected convergence direction the side scan images allow us to map the geometry of four main thrust faults the wetar liran atauro and alor faults the wetar liran and atauro faults trend northeast parallel to the thrust front and do not have an en echelon relationship the westernmost fault in the survey area the alor fault trends almost east west the orientation of the wetar atauro fault is consistent with a maximum principal stress within the arc oriented between 8 and 23 west of north depending on the internal coefficient of friction this agrees well with the p axes of nearby earthquake focal mechanisms which trend consistently west of north suggesting that the regional principal tectonic compression is oriented west of north because the thrust belt is oblique to the arc slope we infer the structural directions are influenced by both the arc backstop and by the regional tectonic stress the tectonic compression due to arc continent collision in this region may be modified by the arc geometry in several ways 1 the stress necessary to support the arc topography may be significant close to the arc 2 the change in thickness of the elastic part of the lithosphere may cause a concentration of tectonic stress and 3 the difference in material properties between the relatively rigid arc bulldozer and the weak basin sedimentary fill may be an important factor in producing a thrust belt that conforms roughly to the shape of the arc the structure of the wetar back arc thrust zone demonstrates that the development of small rigid blocks within a major collision zone may produce complex structural patterns and local directions of shortening that are highly oblique to the main direction of plate convergence

active back thrust in the eastern taiwan suture revealed by the 2013 rueisuei earthquake evidence for a doubly vergent orogenic wedge rapid exhumation of 3 10 mm yr of the taiwan metamorphic range is often explained as the unroofing of the retrowedge of a doubly vergent mountain belt yet to date the central range fault forming the boundary of the retrowedge has displayed no definitive evidence for recent seismic activity and no unambiguous geomorphic expression over much of the fault the 2013 m6 4 rueisuei reverse faulting earthquake nucleated at the eastern boundary of the retrowedge and appears to illuminate the west dipping central range fault we estimate the fault geometry and coseismic slip distribution using a uniform stress drop slip inversion and surface displacements derived from gps and strong motion data we identify a 42 dipping blind reverse fault consistent with the previously proposed buried central range fault beneath the highly active longitudinal valley fault this earthquake may be the first indication that rapid exhumation and uplift occur along a distinct fault structure bounding the eastern margin of the taiwan retrowedge

sandbox model simulation of forearc evolution and noncritical wedges convergent plate boundaries exhibit a wide range of behaviors ranging from sediment accretion and underplating to large scale erosion of the forearc we have used sandbox modeling to simulate the growth of thin skinned contractional wedges over a strong backstop at an accretionary margin the backstop in our model causes the growth of two distinct wedges with quite different growth histories analogous to the accretionary wedge and the inner deformation belt retrowedge found in at least some forearcs after starting with an initially steep slope the surface flattens as the accretionary wedge grows by frontal accretion until the wedge attains and maintains a minimum possible critical taper the retrowedge driven by uplift of the outer arc high increases its surface slope as it grows until a kinematic steady state is attained it exhibits a stable taper with slope larger than the minimum critical taper but never larger than the theoretical maximum critical taper when a basal decollement is present above the backstop in models in which the retrowedge has not developed a decollement it has a taper with a slope at the angle of repose when a basal decollement is present above the backstop the main body of the retrowedge acts as a bulldozer or a backstop pushing the sediments in front of it which then form an additional wedge with a small minimum critical taper a broad outer arc high stands above the toe of the backstop where maximum principal stresses rotate to meet the stress conditions in the two contractional belts on either side of it as these belts grow in opposite directions the accretionary wedge accretes large quantities of sediment at its front but the retrowedge does not because the retrowedge can exhibit a non critical stable taper in the absence of rapid erosion or accretion it is misleading to assume that active thin skinned wedges are necessarily at maximum or minimum critical taper

sunda banda arc transition incipient continent island arc collision northwest australia the eastern sunda arc represents one of the few regions globally where the early stages of continent arc collision can be studied we studied along the western limit of the collision zone at the sunda banda arc transition where the australian margin collides with the banda island arc causing widespread back arc thrusting we present integrated results of a refraction wide angle reflection tomography gravity modeling and multichannel reflection seismic imaging using data acquired in 2006 southeast of sumba island the composite structural model reveals the previously unresolved deep geometry of the collision zone changes in crustal structure encompass the 10 12 km thick australian basement in the south and the 22 24 kmthick sumba ridge in the north where backthrusting of the 130 km wide accretionary prism is documented the structural diversity along this transect could be characteristic of young collisional systems at the transition from oceanic subduction to continent arc collision citation shulgin a h kopp c mueller e lueschen l planert m engels e r flueh a krabbenhoeft and y djajadihardja 2009 sunda banda arc transition incipient continent island arc collision northwest australia geophys res lett 36 l10304 doi 10 1029 2009gl037533

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

noncohesive critical coulomb wedges an exact solution active fold and thrust belts or submarine accretionary complexes can be modeled as critically tapered wedges of material on the verge of coulomb failure everywhere overlying a basal decollement where frictional sliding is occurring ignoring cohesion the four strength parameters needed to describe a critical coulomb wedge are its internal and basal coefficients of friction and b and its internal and basal hubbert rubey fluid pressure ratios and b an exact relation between surface slope and basal dip of a noncohesive critical wedge with uniform properties is derived the state of stress within such a wedge has the same orientation everywhere and is constant if is and vice versa a coefficient of internal friction 1 1 is consistent with the known surface slope basal dip and pore fluid pressures in the active fold and thrust belt of western taiwan assuming that byerlee s law b 0 85 is valid on the base the wide variety of tectonic styles observed to occur along convergent margins including subduction erosion active accretion subduction without accretion and even extension and normal faulting may be controlled by relatively small spatial or temporal variations in either b or b

accretion and collision related deformation in the southern uplands accretionary wedge southwestern scotland recent models for the tectonic development of the southern scottish caledonides are reviewed in terms of the deformation sequence observed within the southern uplands of scotland the early stages of this history are consistent with deformation within an accretionary prism formed above a northwest directed subduction zone later stages show a reversal in transport direction and also evidence for sinistral shear a deformational history for the area is proposed in terms of recent data on the deep structure of the southern uplands and adjacent regions the southern uplands is considered to be a large scale popup structure between a forethrust the iapetus suture and a backthrust which partially obducted it toward the northwest

tectonic relationships between forearc basin strata and the accretionary complex at bath barbados the island of barbados exposes the crestal zone of the accretionary prism of the lesser antilles forearc tertiary rocks at bath barbados include four lithic suites 1 inter bedded volcanogenic and mainly calcareous pelagic rocks the oceanic beds 2 radiolarite 3 siliceous hemipelagic rocks and 4 melange suites 1 3 are in structurally discrete groups of fault packets melange occurs in apparently diapiric crosscutting bodies the oceanic beds occupy a pre pleistocene nappe complex that underlies much of barbados radiolarite and hemipelagic rocks belong to a thick accretionary basal complex and constitute the relative autochthon to the oceanic nappes fault rocks of the sub oceanic fault zone developed during emplacement of the oceanic nappes the lower portion of the tectonic stack is deformed in a major south verging fold couple with wavelength of 250 m lower early emplaced oceanic nappes are restricted to the core of the major synform and are increasingly deformed with depth structures suggest easterly transport of oceanic nappes and upward younging of nappe emplacement indicating that the deformation of oceanic beds propagated westward the oceanic nappes provided a seal to rising fluids transport of nappes may have been facilitated by elevated fluid pressure beneath their sole thrust pre and post nappe folds record north south shortening about shallow west plunging axes consistent with folded accretionary strata elsewhere on barbados deformation occurred under shallow nonmetamorphic conditions the oceanic beds are interpreted as being trapped outer forearc basin strata that have been tectonically shouldered by the arcward migrating prism flank or crest the hemipelagic and radiolarite suites may be offscraped abyssal plain and trench wedge accumulations paleogene lower miocene oceanic strata were deposited at depths between 2 0 and 4 8 km middle miocene oceanic beds accumulated at shallower depths 1 1 5 km thus early miocene was the probable time of inception of major uplift and arcward shouldering in the crestal zone of the accretionary prism accretion of the basal complex of barbados probably occurred in late eocene time

parallel thrust and normal faulting in peru and constraints on the state of stress abstract we present evidence for recent normal faulting in the cordillera blanca of peru along planes trending approximately n40 w parallel to the chain this type of faulting suggests that there is an important component of extension perpendicular to the chain and contrasts markedly with the underthrusting of the nazca plate on the west side of peru and with the folding and thrusting observed in the sub andes on the eastern flank of the chain these contrasting styles of deformation and associated stress fields can be explained by buoyancy forces due to the high mountains and crustal roots in so far as this explanation is correct the difference in elevation between zones of normal and thrust faulting can be used to place an upper bound of 500 bars on the average stress difference in the crust

a dual doubly vergent orogen in the banda arc continent arc collision zone as observed on deep seismic reflection profiles new deep seismic reflection profiles across the banda arc of indonesia reveal reflectors in the uppermost 50 km of the lithosphere combined with existing earthquake hypocenter locations and focal mechanisms the new structural geometries inferred from the reflectors yield a more complete analysis of deformation during the past 10 m y and provide insights into how strain is partitioned across an orogenic belt in which volcanic arcs and continents converge a clearly defined wadati benioff zone and recent deformation of shelf sediments observed on older shallow seismic profiles indicated to previous workers that substantial convergence occurred at the timor trough a few focal mechanisms seafloor escarpments and recent geodetic surveying indicate that convergence at 7 cm yr 1 currently last 200 kyr occurs at the northern margin of the now inactive volcanic arc the wetar thrust zone reflectors on the new seismic profiles are interpreted as thrust faults and folds that occur throughout the crust and within the uppermost mantle between the timor trough and wetar thrust specifically basement reflectors beneath the toe of the accretionary complex have reverse sense offsets that imply blind thrusts the whole crust is horizontally shortened not only the sedimentary cover rocks that previously deformed into duplexes above a decollement reflectors dipping away from both margins of the forearc basin and at the northern margin of the volcanic arc are interpreted as evidence of thrusting thus each arc represents a doubly vergent fold and thrust belt but only the northern one is currently active crustal thicknesses inferred from seismic velocities reflectors and gravity anomalies are consistent with the merging of a thinned continental shelf margin with oceanic lithosphere to form an orogenic belt with at present 3 4 km of topographic relief in the region of eastern timor

late cenozoic structural deformation and evolution of the central southern longmen shan fold and thrust belt china insights from numerical simulations abstract since the late cenozoic the central southern longmen shan lms region is characterized by a two decollement thrust fold system with a localized upper evaporite weaker layer just beneath the sichuan basin and the lower stronger decollement beneath the lms to date the role of upper decollement and erosion in shaping the structural deformation of the lms remains unknown the investigation of this question may shed lights on the assessment of the structural evolution and the further discrimination of mountain building mechanism in this work a 2d finite difference method is applied with regards to the variations of viscosity friction angle and lengths of upper decollement under different erosion rates two types of numerical models are setup with purely lower upper frictional ff decollements and lower frictional upper viscous fv decollements tectonic evolution with ff models exhibits large similarity with that of fv models meanwhile our results underline the presence of in sequence fault propagation procedures during most of the stages the development of the boundary ramp the range frontal blind fault is strongly induced by a prerequisite with upper decollement of necessary length the activation of beichuan pengguan faults or blind ramp piercing to the surface is extremely mechanically sensitive to the viscosity of the salt decollement erosion rate and the length of the upper decollement out of sequence thrusting shallow and possible basal accretion processes beneath the sichuan basin are found when deformation transfers from the thick skinned lms to the thin skinned sichuan basin at the very late stage supporting upper crustal shortening dominates the mountain building mechanism

crustal structure of the ancestral northwestern california forearc region from seismic reflection imaging implications for convergent margin tectonics abstract analysis of a suite of 2 d seismic reflection profiles reveals that the northwestern sacramento valley and eastern coast range foothills northern california are underlain by a system of blind west dipping thrust faults homoclinally east dipping and folded mesozoic marine forearc strata exposed along the western valley margin define the forelimbs of northeast vergent fault propagation folds developed in the hanging walls of the thrusts exhumed coherent blueschists of the accretionary complex and attenuated remnants of the ophiolitic forearc basement presently exposed in the eastern coast ranges are in the hanging wall of the blind thrust system and have been displaced from their roots in the footwall deep east dipping magnetic reflectors in the footwall of the thrust system may be fragments of sheared serpentinized and attenuated ophiolitic basement restoration of slip on the thrusts suggests that the coast range fault which is the exposed structural contact between the coherent blueschists and attenuated ophiolite originally dipped east and is associated with the east dipping magnetic reflectors in the footwall this interpretation of the reflection data is consistent with previous inferences about the deep structure in this region and supports a two stage model for blueschist exposure in the eastern coast ranges 1 blueschist exhumation relative to the forearc basin by attenuation of the ophiolitic basement along the east dipping coast range fault system in late cretaceous 2 blueschists attenuated ophiolite and forearc strata all were subsequently uplifted and folded in the hanging wall of the blind thrust system beginning in latest cretaceous early tertiary the blind thrust system probably rooted in and was antithetic to the east dipping subduction zone beneath the forearc region active transpressional plate motion in western california is locally accommodated in part by reactivation of blind thrust faults that originally developed during the convergent regime

dynamic rupture activation of backthrust fault branching we perform dynamic rupture simulations to investigate the possible reactivation of backthrust branches triggered by ruptures along a main thrust fault simulations with slip weakening fault friction and uniform initial stress show that fast propagation speed or long propagation distance of the main rupture promotes reactivation of backthrust over a range of branch angles the latter condition may occur separately from the former if rupture speed is limited by an increasing slip weakening distance towards the junction direction the results suggest a trade off between the amplitude and duration of the dynamic stress near the main rupture front for backthrust reactivation termination of the main rupture by a barrier can provide enhanced loading amplitude and duration along a backthrust rooted near the barrier facilitating its reactivation especially with a high frictional resistance the free surface and depth dependent initial stress can have several additional effects the sign of the triggered motion along the backthrust can be reversed from thrust to normal if a deeply nucleated main rupture breaks the free surface while it is preserved as thrust if the main rupture is terminated by a barrier at depth the numerical results are discussed in relation to several recent megathrust earthquakes in sumatra chile and japan and related topics such as branch feedbacks to the main fault the dynamic view on backthrust fault branching provided by the study fills a gap not covered by quasi static models or observations a specific examined case of antithetic fault branching may be useful for indicating a barrier like behavior along the main fault

cenozoic tectonics of central hispaniola and adjacent caribbean sea seismic reflection profiles across the island margin south of the dominican republic indicate the presence of geologic structures similar to structures in the forearc regions seaward of pacific island arcs the thick mildly deformed tertiary section in san pedro basin south of santo domingo is separated from the highly deformed sediments of the north slope of the muertos trench by an anticlinal ridge san pedro basin is homologous to numerous forearc basins bounded seaward by structurally high trench slope breaks the transition landward from subsiding san pedro basin to rising llanos costeros del seibo is homologous to the upper slope discontinuity present in many pacific arcs structures within the sediment wedge beneath the north slope of the muertos trench that have omologs in pacific trench inner slopes include trench floor turbidite fill landward tilted ponded terraces landward dipping internal reflections and a landward continuation of ocean crust beneath the sediment wedge the northwest southeast trending basins and ridges parallel with the cordillera central in central hispaniola may be caused by compression between the beata ridge and the bahama platform these compressional basin and ridge structures die out eastward in hispaniola where thinner ocean crust apparently under thrusts hispaniola from the south right lateral offset along the northeast trending beata ridge lineament may explain the pattern of central hispaniola mountain ridges terminating eastward against subsiding basins and vertical offset along the same lineament may explain the topographic and structural rise from the venezuela basin westward up to the east flank of the beata ridge the basin and ridge structure of hispaniola has probably developed in a manner similar to the basin and ridge province of southern california which has formed as a consequence of shear along the san andreas fault

seismic evidence for olivine phase changes at the 410 and 660 kilometer discontinuities the view that the seismic discontinuities bounding the mantle transition zone at 410 and 660 kilometer depths are caused by isochemical phase transformations of the olivine structure is debated combining converted wave measurements in east asia and australia with seismic velocities from regional tomography studies we observe a correlation of the thickness of and wavespeed variations within the transition zone that is consistent with olivine structural transformations moreover the seismologically inferred clapeyron slopes are in agreement with the mineralogical clapeyron slopes of the mg fe 2sio4 spinel and postspinel transformations

rates of neogene and quaternary tectonic movements in the southern banda arc based on micropalaeontology it is generally considered that the outer banda arc islands are composed in part of strongly deformed sediments that accumulated at the north australian continental margin the onset and completion of folding nappe emplacement imbrication and uplift that produced the islands of the outer banda arc have been dated by means of planktonic foraminifera the dating of the onset of uplift of timor from a deep submarine position at the end of the neogene nappe emplacement to mountains now reaching nearly 3 km high indicates the rate of post collision uplift to have been initially 3 mm yr then it slowed to about 1 5mm yr where the australian continental margin meets the eastern end of the present java trench there are indications that the australian margin has overridden the trench in the timor region by 240 km micropalaeontological dating of the neogene collision between the australian continental slope proximal rise and the trench provides constraints for checking the overriding hypothesis by calculating rates of convergence derived from plate motions 7 5cm yr against the rates based on the tectonic model 7 4cm yr after nappe emplacement possibly faster than 6 25 cm yr shortening of the continental crust appears to have migrated towards the australian continent so that the shelf itself has become involved in the imbrication with shortening of the cover rocks between the nappes and the present shelf edge amounting to about 40 km during the last 2 ma

mechanical role of backstops in the growth of forearcs we use numerical and laboratory modeling to analyze the mechanical role of backstops in the overriding plate at subduction zones a backstop is defined as a region within a forearc that has significantly greater shear strength than the sediment lying farther trenchward it can be thought of as the bulldozer behind an accretionary wedge we calculate the stress and displacement fields within forearcs for various backstop models using the finite element method and we simulate deformation over a backstop using a small scale laboratory model in this way we model the effects of the mechanical properties and geometry of a backstop on forearc structures we find that the growth of an outer arc high the development of an inner deformation belt with landward vergence and the seemingly paradoxical presence of an undeformed forearc basin within an otherwise highly deformed forearc can be explained by assuming the existence of a geometrically simple backstop with geologically reasonable contrasts in mechanical properties compared to the sediments just trenchward of it the structures produced in our numerical models are quite insensitive to the rheology boundary conditions and exact mesh geometry employed the general types of structures observed in the laboratory models depend only weakly upon the strength and the geometry of the backstop these results suggest that a detailed picture of an underlying backstop cannot be determined from surface information alone backstops in which the contact with the accretionary wedge dips arcward rather than trenchward however should produce only slightly different forearc structures with less development of both the outer arc high and the landward vergent inner deformation belt although natural forearcs are far more complex than our simple models they exhibit many of the same features indicating that relatively simple backstop mechanics may be a very important factor in the overall growth of forearcs

structural evolution of backthrusting in the mentawai fault zone offshore sumatran forearc 1 we present the interpretation of newly acquired high quality industry standard deep seismic reflection and swath bathymetry data to provide insight into the structural style and evolution of the mentawai fault zone mfz the mfz lies along the boundary between the accretionary wedge and the proposed continental backstop this zone exhibits arcuate ridges on the seafloor convex toward the east beneath these ridges the structures developed as landward vergent imbricated backthrusts in the inner part of the accretionary wedge and higher angle backthrusts that deformed the forearc basin sediments in the forearc high anticlines were developed due to the seaward vergent forearc high thrusts originating in the accretionary wedge the imbricated backthrusts may have initiated during the early middle miocene contemporaneously with the slide and back rotation of forearc high thrusts in the late miocene the higher angle backthrusts were initiated continuous contraction induced the frontal higher angle backthrusts and formed a fold thrust belt toward the east during the pliocene the folds and thrusts were disturbed by diapirs and mud volcanoes backthrusting and fold thrust belts developed in the mfz may explain the compressional features observed at the boundary between the accretionary wedge and continental backstop along the southern sumatra margin the backthrusts along the mfz are waning in activity and hence the risk of a large earthquake and associated tsunami at the present time should be small

mechanical model for the tectonics of doubly vergent compressional orogens a mechanical model of crustal shortening and deformation driven by the relative convergence of rigid underlying mantle plates explains many features of convergent orogens results based on numerical models and supported by sandbox models show that a coulomb crustal layer subject to basal velocity boundary conditions corresponding to asymmetric detachment and subduction of the underlying mantle passes through three stages of orogenic growth 1 block uplift bounded by step up shear zones 2 development of a low taper wedge over the underthrusting mantle plate and 3 development of a low taperwedge overlying the overthrusting mantle plate and verging in the opposite direction when modified by isostasy basal viscous flow surface erosion and denudation and sedimentation the resultant model orogens exhibit a variety of styles with characteristics in common with small rapidly denuded orogens large orogens with plateaus and extensional characteristics and active subduction margins with doubly vergent accretionary wedges and deformed fore arc basins

comparison of deep structure along three transects of the western north american continental margin similarities in geology and potential field data that have in the past been noted among the regions of southern alaska southern vancouver island and central california are now seen to be accompanied by similarities in deep crustal structure a number of tectonic elements have been identified in the deep structure along transects in these three regions although not all elements are present along each transect these elements are a an actively subducting oceanic plate and b an overriding continental plate that consists of 1 a cenozoic accretionary prism 2 a mesozoic accretionary prism 3 a backstop to the mesozoic prism 4 a tectonically underplated body of oceanic rocks and 5 a crustal root the mesozoic prism is in some cases an underthrust body type 2a but in other cases forms the principal component of a landward verging tectonic wedge type 2b the technically underplated body of oceanic rocks extends landward from the fault contact between the cenozoic and mesozoic prisms to a point beneath the backstop the crustal root lies beneath the backstop and landward of the underplated body all of these elements are interpreted to be present along the alaskan and vancouver island transects in alaska the underplated body is interpreted to be fragments of the kula plate the same may be true at vancouver island these two transects appear to differ in that in alaska the mesozoic prism in one interpretation is the principal component of a tectonic wedge type 2b whereas at vancouver island it is an underthrust body type 2a along the central california transect active subduction is no longer taking place and the san andreas fault has removed the cenozoic prism from this region of the north american plate on the north american plate i e east of the san andreas fault the mesozoic prism interpreted as the main component of a tectonic wedge type 2b and the backstop to the mesozoic prism are present there is however no clear evidence of tectonically underplated oceanic rocks and the crust is thin no root in both alaska and vancouver island the mesozoic prisms above the underplated bodies experienced low pressure high temperature metamorphism at about the time of tectonic underplating no such metamorphism is currently exposed in california the metamorphism may have been caused by the underplating of young hot oceanic crust or alternatively by subduction of an oceanic ridge the presence of a tectonic wedge type 2b in alaska and california and the absence of such a wedge at vancouver island could arise either from the fact that in the former two locations the mesozoic prisms were more voluminous owing to either more rapid trench sedimentation or more rapid convergence or to the possibility that at the latter location the mesozoic prism was juxtaposed with the backstop primarily by strike slip faulting

earthquakes along the ryukyu kyushu arc strain segmentation lateral compression and the thermomechanical state of the plate interface we systematically determined the focal depths and mechanisms of 49 large to moderate sized earthquakes mb 5 4 that occurred along the ryukyu kyushu arc since 1963 by inverting the waveforms and amplitudes of p and sh wave trains at teleseismic distances the results are sufficiently precise to delineate seismogenic structures near the plate interface in detail revealing features not predicted by plate kinematics in contrast to previous studies of this arc and those along other subduction zones shallow seismicity along the plate interface is systematic showing two distinct layers of activities the second layer of seismicity is delineated by a few earthquakes that occurred at depths between 50 and 65 km some 10 20 km directly beneath the seismogenic portion of the interplate thrust zone these earthquakes indicate lateral compressional strain within the subducted slab as their p axes are subparallel to the local strike of the arc not perpendicular to the arc as one would expect from the direction of plate convergence the occurrence of these events cannot be accounted for by membrane stress due to the geometry of the subducted slab to our knowledge similar earthquakes occurred only beneath the northern indoburman ranges and along the banda arc where subducted slabs as part of the indian plate are being dragged northward with their northern edges bumping into east west trending collision zones nearby by drawing an analogy between the tectonic settings of these three regions we interpret events beneath the plate interface along the ryukyu kyushu arc as a consequence of ongoing collision between the philippine sea plate and eurasia near taiwan the interplate thrust zone is largely aseismic down to a depth of approximately 30 km a large number of earthquakes showing low angle thrust faulting commence at this depth and are accompanied by two events that show antithetic thrust faulting at a slightly shallower depth of 20 25 km these observations suggest that the strength of the plate interface increases significantly below the depth of proximately 25 5 km based upon available heat flow measurements between the trench axis and the volcanic arc we estimated the temperature field and magnitude of shear traction along the interplate thrust zone the deepest interplate earthquakes 40 50 km correspond to temperatures of approximately 730 980 c comparable to the limiting temperature of intraplate mantle earthquakes 800 c and that of intermediate and deep focus earthquakes at wadati benioff zones potential temperature 900 k therefore the cessation of seismicity in general is probably controlled by similar temperature conditions regardless of the tectonic settings of source regions the magnitude of shear traction along the seismogenic portion 40 50 km depth of the interplate thrust zone seems to be of the order of 100 mpa which in turn implies a low average coefficient of friction of only 0 10 0 05 considerably less than those observed for laboratory specimens the inception of intermediate focus earthquakes within the subducted slab is at a depth of about 100 km whereas shallow earthquakes show no apparent variations along the entire arc intermediate depth earthquakes indicate downdip extension along the northern end of the arc near kyushu but abruptly change to downdip compression along the rest of the arc all available evidence indicates that this is a spatial pattern unrelated to earthquake cycles at the moment tectonic interpretation of such a distinct strain segmentation along the arc is enigmatic without the presence of clear discontinuities in the age of the subducted slab the sudden switch in the strain field of the slab is difficult to explain by the continuously varying rate of subduction along the arc in any case this pattern of strain segmentation requires the presence of a major accommodation structure such as a tear fault in the subducted slab near the tokara channel appendix is available with entire article on microfiche order from american geophysical union 2000 florida ave n w washington dc 20009 document b91 004 2 50 payment must accompany order

mesozoic and late tertiary submarine fan sequences and their tectonic significance sumba indonesia abstract the indonesian island of sumba occupies a critical zone which separates a normal subduction complex java trench system from a continent island arc collision zone banda arc sumba is considered by some authors to be a microcontinental fragment although in this context its pre fragmentation origin is uncertain detailed studies of the stratigraphy of sumba however throw light on the origin and subsequent geological history of the island and surrounding areas on available evidence the oldest outcropping rocks are of cretaceous age and locally contain a tropical tethyan marine fauna they comprise dark coloured sometimes carbonaceous and often volcanogenic mudstones sandstones gravels and diamictites pervasively intruded by andesitic and dacitic dykes and locally intruded by granodiorite plutons these sediments are siliclastic and appear to have had a continental provenance the cretaceous rocks in many areas compromise part of a major submarine fan complex which at least in south central sumba prograded towards the southwest and south shallow marine and non marine paleogene sediments and volcanic agglomerates lie unconformably above the uplifted tilted and eroded cretaceous strata these are overlain in turn by the widely outcropping miocene sumba formation an oceanic sequence with island arc affinities accessible outcrops of the sumba formation comprise platform carbonates with abundant reef developments in west sumba and a major submarine fan pelagic chalk complex in east sumba the fan complex which extends beneath the southern savu sea north of sumba is dominated by large scale slumps olistostromes and thin and thick bedded turbidites turbidite sands and gravels invariably are volcaniclastic containing components derived from a typical inner volcanic arc complex paleocurrent indicators are difficult to find in the sumba formation thus it is uncertain at this stage whether the volcaniclastic turbidites were derived from the present volcanic arc or from an undetermined source which may since have subsided the portion of the fan complex which underlies east sumba however has undergone vertical uplift since its emplacement of between one and several kilometres uplift is actively in progress at present possibly due to underthrusting by northward moving australian continental crust in this context the island of sumba comprises an uplifted north tilted and subaerially exposed part of the forearc basin of the indonesian arc this uplift can be related to initial stages of continental collision

loci and maximum size of thrust earthquakes and the mechanics of the shallow region of subduction zones earthquakes do not extend updip along the plate interface at most subduction zones all of the way to the plate boundary at the trench axis or deformation front rather the shallowest part of that interface moves mainly through stable aseismic slip that part of the plate boundary referred to here as the aseismic zone occurs along the base of an accreted wedge of young sediments the probable primary cause for the existence of this aseismic zone is the stable slip properties of the unconsolidated and semiconsolidated sediments in that zone subducted sediment is progressively dewatered and underplated to the base of the overriding plate through this process more consolidated rocks eventually come into contact at depth across both sides of the plate boundary from the point where sufficiently hard rock is found across the plate interface that interface will change its slip behavior to unstable stick slip sliding which is characteristic of consolidated material under most conditions this type of motion is accommodated seismically as episodic slip in large earthquakes the location of this transition to seismic behavior referred to as the seismic front marks the deep end of the aseismic zone and the top of the seismogenic zone i e that part of the plate interface that moves primarily in thrust earthquakes several convergent plate margins are discussed to illustrate the seismic front and the aseismic zone we find that the seismic front defined by smaller earthquakes that occur during the interval between large events is nearly the same as that for large events as inferred from the locations of their aftershocks the location of the seismic front is important for making estimates of the maximum possible size of thrust earthquakes along the plate boundary because it delimits the trenchward limit of the potential rupture area of these events the size of large thrust earthquakes is proportional to rupture area and to at least the cube of the downdip width w of that area thus by defining the location of the seismic front for convergent margins it is possible to make better estimates of w and hence to deduce the maximum size of future interplate earthquakes the average repeat time of such events is also related to w and so better estimates of average repeat time are possible from improved knowledge of w the hypothesis that the location of the seismic front is related to the maximum depth of subduction of unconsolidated sediment has implications for forearc mechanics we use a mechanical analysis laboratory modeling and multichannel seismic information to develop a simple model explaining the growth of forearcs the outer arc high or trench slope break that occurs arcward of most accretionary wedges represents an abrupt change in the critical taper of the accretionary wedge we argue that that change is caused by a large arcward increase in the strength of the material within the overriding plate that stronger material is called the backstop our laboratory modeling experiments indicate that a backstop with a trenchward dipping upper surface results in the development of an overlying structure that includes all of the primary morphologic features observed in modern forearcs in our modeling an accretionary wedge develops trenchward of the backstop an outer arc high develops above the trenchward toe of the backstop and farther arcward a passive forearc basin forms above the stronger material of the backstop this model is consistent with the observation that earthquakes do not extend updip along the plate interface all the way to the trench axis we hypothesize that plate motion is accommodated seismically along the base of the consolidated backstop and mostly or entirely aseismically trenchward of the toe of the backstop along the base of the accretionary wedge at several margins the seismic front is approximately coincident with the outer arc high supporting this interpretation

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

the influence of backstop dip and convergence velocity in the growth of viscous doubly vergent orogenic wedges insights from thermomechanical laboratory experiments abstract we investigate the effects of convergence velocity and backstop geometry on the evolution of experimental viscous doubly vergent orogenic wedges experiments are performed in a thermomechanical shortening box using the newtonian properties of 52 54 en type paraffin as analogue to the natural strength distribution in the continental crust a velocity discontinuity is imposed at the subduction slot where the base plate is pulled below a rigid backstop the mobile base plate dips 5 two different backstop dips 60 and 30 are used in combination with constant convergence velocities of 2 and 10 cm h 1 results point out that backstop geometry exerts a first order control on the evolution of doubly vergent experimental wedges steeply dipping backstops cause the stationary uplift of the wedge axial region above the subduction slot whereas shallow dipping backstops allow its retroward translation in concomitance with a more pronounced deformation in the retrowedge convergence velocity controls the uplift in the wedge axial region and the amount of the outward migration of the deformation front in both the prowedge and retrowedge regions for fast convergence velocity and shallow dipping backstop deformation in the retrowedge is favoured an increase in the backstop dip and or a decrease in the convergence velocity favour deformation in the prowedge both prowedge and retrowedge tend to grow in self similar conditions with an aspect ratio depending upon the backstop dip and convergence velocity these results may have important consequences for the evolution and deformation style of ductile dominated orogenic wedges suggesting that location of the axial region and deformation progression in natural orogens can be strongly controlled by boundary and kinematic conditions

continental margin tectonics forearc processes recent studies of convergent plate margins and the structural development of forearc terranes are summarized in a critical review of u s research from the period 1987 1990 topics addressed include the geometry of accretionary prisms coulomb wedge taper and vertical motion in response to tectonic processes offscraping vs underplating or subduction the response to oblique convergence fluids in forearc settings the thermal framework and the effects of fluid advection and serpentinite seamounts also included is a comprehensive bibliography for the period

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

lithospheric deformation within the molucca sea arc arc collision evidence from shallow and intermediate earthquake activity this paper presents the results of a local earthquake survey in the molucca sea arc arc collision zone of eastern indonesia although intermediate depth earthquakes define zones dipping to the west beneath the sangihe arc and to the east beneath the halmahera volcanic arc only shallow activity less than 60 km depth occurred beneath the central part of the collision zone where the majority of activity is observed the concentration of earthquake foci in the 10 to 50 km depth range in a limited region beneath the talaud mayu ridge suggests that convergence between the arcs proceeds by shortening within basement of the intervening molucca sea plate rather than by slip along shallow dipping planes between the arcs and the subducted slabs published focal mechanism solutions suggest that high angle reverse faulting beneath the talaud mayu ridge generates almost all of the seismic energy release within the collision zone the predominance of shallow reverse and strike slip faulting at the axis of the bilaterally subducting molucca sea lithosphere suggests that stresses are not due to bending alone and have a large horizontal regional component perpendicular to the island arcs the geometry and state of stress within the molucca sea lithosphere requires an external driving force for the convergence between the two arcs and a strong degree of coupling between the island arcs and the subducting molucca sea plate at least at the present stage of collision

seismic imaging of forearc backthrusts at northern sumatra subduction zone forearc tectonics at accretionary convergent margins has variously been studied using analogue and numerical modelling techniques numerous geophysical investigations have targeted the subsurface structure of active forearc settings at convergent margins however several critical details of the structure mode of tectonic evolution and the role forearcs play in the subduction seismic cycle remain to be further understood especially for large accretionary margins in this study we present a high resolution deep seismic reflection image of the northern sumatran subduction forearc near the 2004 december 26 sumatra earthquake epicentral region the profile clearly demarcates the backthrust branches at the seaward edge of the aceh forearc basin along which the inner forearc continues to evolve sharp bathymetric features at the seafloor suggest that the imaged backthrusts are active coincident wide angle seismic tomographic image of the sumatra forearc allows us to image the geometry of the seaward dipping backstop buttress with which the imaged backthrust branches are associated the presence of forearc backthrusting confirms model predictions for the development of backthrusting over seaward dipping backstops the west andaman fault at the seaward edge of aceh basin appears to be a shallow tributary of the backthrust and sheds light on the complex deformation of the forearc uplifting along the backthrust branches may explain the presence of forearc islands observed all along sumatran margin and help further constrain the tectonic models for their evolution moreover if these backthrusts slip coseismically they would contribute to tsunamigenesis and seismic risk in the region

juxtaposed sequence stratigraphy temporal spatial variations of sedimentation and development of modern forming forearc lichi melange in north luzon trough forearc basin onshore and offshore eastern taiwan an overview abstract the south china sea oceanic lithosphere has been subducting eastward beneath the huatung basin philippine sea plate since the early miocene 18 ma the subduction is followed by the oblique collision between the luzon arc and the subducting eurasian plate from 6 5 ma the north luzon trough forearc strata and the luzon arc are then obducted northwestward as the coastal range eastern taiwan in the last 1 ma the collision propagates southward and is presently active in the region offshore se taiwan integrating seismic surveys offshore and a detailed forearc stratigraphy study onshore the coastal range this paper overviews the characteristics of forearc deformation dynamic sequence stratigraphy temporal spatial variations of forearc sedimentation and stratigraphic correlation onshore and offshore forearc sequences in response to the oblique convergent tectonics north of 20 n combining onshore and offshore forearc geology together allows us to reconstruct a structural evolution of the north luzon trough forearc basin from subduction through collision to obduction and to discuss the mechanism and processes responsible for developments of the modern forming forearc lichi melange during the active taiwan orogeny seismic surveys offshore show that forearc deformation in the subduction zone is primarily caused by increase of rear prism slope and west vergent thrusting of forearc strata along the prism top since the early forearc sedimentation east vergent backthrusting occurs during the late forearc sedimentation and propagates arcward when the volcanic arc collides with the accretionary prism in the collision zone bivergent thrusting leads to a development of the forearc huatung ridge popup as a bathymetric high which further controls the sedimentation of the younger forearc sequence in the collision zone in response to the syn sedimentation deformation forearc depocenter shifts progressively eastward forearc stratigraphy thus changes from two mega sequences bounded by an unconformity in the subduction zone to three mega sequences juxtaposed from west to east unconformably in the collision zone as a consequence the forearc deformation and stratigraphy in the oblique collision zone off se taiwan show a characteristic temporal spatial pattern that the lowest mega sequence with the most intensive deformation occurs restrictedly in the west whiles the middle and the upper mega sequences with mild deformation crop out in the center and the eastern part of the forearc basin respectively detailed biostratigraphy study using planktonic foraminifera and calcareous nannoplanktons indicates that the forearc strata onshore the coastal range are also composed of three sequences lower unit s 1 6 5 5 8 ma middle unit s 2 5 8 3 0 ma and upper unit s 3 a structure reconstruction reveals that the north luzon trough forearc strata have experienced multiple stages of thrust deformation from subduction through collision to obduction these deformations account for the mechanism and processes to develop the modern forming highly sheared ssz bearing forearc lichi melange tectonically in the western coastal range during the last 1 ma

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

change in motion of pacific plate at 5 myr bp alignments of volcanoes such as the hawaiian seamount chain record the passage of oceanic plates over mantle plumes or hot spots and changes in the orientation of a seamount chain may indicate changes in the direction of motion of the plate within the hotspot frame of reference1 this hypothesis can be tested for the pacific plate by comparing data from the hawaiian volcanoes with tectonic movements inferred from the geology of adjacent plates we have now found that the motion of the pacific plate changed to a more northerly direction 5 myr ago probably as a result of the detachment of a piece of pacific sea floor which had been subducted beneath the fiji plateau during the tertiary

structure of the lesser antilles subduction zone backstop and its role in a large accretionary system 1 the role of a backstop in subduction zones has been the subject of numerous laboratory and numerical modeling studies however few field observations exist revealing how backstops control deformation in subduction zones and accretionary wedge construction a seismic reflection and refraction survey acquired in 1998 with the r v maurice ewing reveals the geometry of the forearc igneous crust accretionary wedge and forearc basin structure of the northern guadeloupe area of the lesser antilles forearc an accreted block of buoyant crust accreted in the late miocene forms the toe of the overriding arc crust and forms the backstop we imaged the top of this surface beneath the forearc basin to its seaward edge where it meets the subducting oceanic crust the toe of the backstop was thrust upward and forms a steep buttress in contact with the lower half of the accretionary wedge the steep buttress produces a narrow inner deformation zone with minimal backthrusting of the accretionary complex landward over the backstop and a narrow 10 km transition between accreted and forearc basin sediment seismic reflections from the subducting crust and the decollement appear beneath the entire accretionary wedge and below the backstop toe separating the decollement and the subducting crust is an interval usually between 500 and 750 m of underthrust sediment carried underneath the accretionary wedge and subducted 15 km landward and beneath the toe of the backstop we speculate that the upturned geometry of the toe of the backstop and a weak fluid rich decollement may facilitate sediment subduction beneath the backstop and potentially into the mantle

dynamics of sediment subduction accretion at convergent margins short term modes long term deformation and tectonic implications a conceptual model is proposed for the short term operation of subduction accretion systems at convergent margins the operation is described in terms of the mass fluxes among the four system components namely the accretionary or prowedge wedge p an uplifted plug u a retrowedge r and the subduction zone the latter consists of a conduit c of slowly moving and deforming material and an underlying subduction channel where material moves and deforms more rapidly a broad range of convergent margin types can be accommodated within the conceptual model framework with differing short term modes that depend on which of the p u r c components that are active finite element geodynamical model experiments are used to investigate whether these modes are dynamically feasible these experiments determine excitation of modes under controls that include model entry and exit mass fluxes flexural loading by accreted material and slab pull forces boundary velocities corresponding to subduction and subduction zone advance retreat distribution thickness and density of accreted sediment and internal frictional properties of the coulomb sediments that all modes with the exception of p in which only a prowedge is created are seen in the models provides support for the concepts and allows conditions that favor particular modes to be recognized in addition the finite deformation predicted by the models is used to infer the long term tectonic styles associated with both single mode evolutions and those that exhibit mode switching a steady state mass balance can occur for modes 0 pure subduction p c i e where components p and c are active or other modes when the surface mass denudation plus the subduction flux balances accretion plus tectonic underplating the p c mode is favored by subduction zone retreat and or an increasing subduction load and creates an apparent landward dipping backstop p u c is predicted to be the most common mode and it creates an apparent seaward dipping backstop p u r c is an inefficient mode but is favored by easily detached retrocrust or sediment and or a decreasing subduction load

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

speculations on the petroleum geology of the accretionary body an example from the central aleutians abstract in the 300 km wide adak amlia sector of the central aleutian trench 36 000 km 3 of offscraped trench fill makes up the wedge shaped mass of the aleutian accretionary body within this wedge seismic reflection profiles reveal an abundance of potential hydrocarbon trapping structures these structures include antiforms thrust and normal faults and stratigraphic pinchouts maximum closure on these features is 2 km in addition the silt and possibly sand size sediment within the offscraped turbidite deposits and the porous diatomaceous pelagic deposits interbedded with and at the base of the wedge may define suitable reservoirs for the entrapment of hydrocarbons potential seals for these reservoirs include diagenetically altered and produced siliceous and carbonate sediment the organic carbon input into the central aleutian trench based on carbon analyses of dsdp legs 18 and 19 core samples suggests that the average organic carbon content within the accretionary body is approximately 0 3 0 6 heat flow across the aleutian terrace indicates that at present the oil generation window lies at a depth of 3 6 5 km at depths of 8 km which corresponds to the maximum depth the offscraped sediment has been seismically resolved beneath the lower trench slope the probable high 170 180 c temperatures prohibit all but gas generation the dewatering of trench sediment and subducted oceanic crust should produce an abundance of fluids circulating within the accretionary body these fluids and gases can conduct hydrocarbons to any of the abundant trapping geometries or be lost from the system through sea floor seepage in the aleutian accretionary body all the conditions necessary for the formation of oil and gas deposits exist the size and ultimate preservation of these deposits however are dependent on the deformational history of the prism both during accretion and after the accretion process has been superceded by subsequent tectonic regimes

synchronous and velocity partitioned thrusting and thrust polarity reversal in experimentally produced doubly vergent thrust wedges implications for natural orogens a sandbox analog modeling research program was used to study the detailed evolution of doubly vergent thrust wedges high resolution multilayers of thin alternating sand and mica layers were used in contractional coulomb wedge experiments to simulate deformation of anisotropic brittle upper crustal strata in doubly vergent orogens experiments incorporating syntectonic sedimentation in foreland basins and in piggyback basins were also carried out our laboratory models evolved in two main stages 1 initial high velocity thrusting in the retrowedge and high frequency together with low displacement folding and thrusting in the prowedge and 2 low frequency high displacement synchronous thrusting in the prowedge and low velocity thrusting in the retrowedge transition from stage i to stage ii occurred when the growing wedges reached the critical height at which they behaved as a backstop for further prowedge accretion addition of syntectonic sediments increased the persistence of stage i and triggered out of sequence thrusting in the axial zone of the experimental orogens thrust motion was stick slip retrovergent thrusting occurred along a long lived ramp whose lower tip was located at the subduction slot provergent kink bands nucleated at the subduction slot in stage i in contrast during the second stage of wedge evolution kink bands nucleated in a piggyback fashion in the foreland far from the subduction slot and then evolved into high displacement faults that remained active up until the end of the experiments at progressively decreasing rates of thrusting the axial zones of the model wedges were characterized by fast uplift rates during stage i due to backward translation of the belt along the retrovergent long lived ramp and due to the localization of deformation close to the subduction slot outward migration of the deformation front in the prowedge region during stage ii caused the progressive decrease in the rate of the wedge uplift until it eventually stopped analytical models to quantify the coulomb behavior in the wedges validated the reversal of thrust polarity with increasing shortening triggered by the buildup of the topographic load during deformation this thrust polarity reversal highlights problems with the classic concept of back thrusting as a reflection of a significant change in the deformation regime our results compare well with the kinematic evolution of natural accretionary prisms and of thrust and fold belts such as the lesser antilles arc system the mediterranean ridge and the pyrenees