evolution of shallow crustal thermal structure from subduction to collision an example from taiwan we study crustal thermal evolution by examining heat flow patterns along a convergent boundary from a young subduction zone to a more structurally mature collision zone more than 8000 km of seismic profiles covering an offshore region of 45 000 km 2 in southern taiwan show widespread bottom simulating reflectors bsrs we derived 1107 bsr based heat flows before combining 42 additional published offshore thermal probe data and 86 on land heat flow data to document the shallow forearc thermal structures from the subduction zone to the collision zone in the subduction zone the geothermal gradient ranges mostly within 30 80 c km and decreases toward the arc due to slab cooling intensive dewatering at the toe sediment blanketing topographic effects and other processes the geothermal gradient ranges mostly from 30 to 90 c km in the collision zone and increases instead of decreases toward the arc possibly caused by exhumation erosion topographically induced ground water circulation and some upper mantle processes related to collision heat flow in the collision zone ranges from 80 to 250 mw m 2 the high heat flow in the collision zone correlates with a shallower seismicity zone and high seismic attenuation while the lower heat flow in the subduction zone might allow the earthquakes to rupture to greater depth the heat flow increases along the topographic high from subduction to collision zone due to increasing geothermal gradients and higher thermal conductivities of the exhumed basement rocks this heat flow variation may generate an artificial exhumation rate pattern if a conventional 30 c km geothermal gradient was used in fission track studies

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

structure of the makran subduction zone from wide angle and reflection seismic data abstract makran is one of the largest accretionary wedges on the globe formed by the convergence between the eurasian and the arabian plates it is characterised by an extremely high sediment input of 7 km and a shallow subduction angle we present seismic velocity models from four wide angle seismic lines that image the wedge sediments and the subducted oceanic crust three strike lines show complex but rather one dimensional structures where the observed seismic phases have been studied with the reflectivity method a 160 km long dip line was surveyed coincident with a mcs line collected by cambridge university in 1986 prestack depth migration of this data with the new wide angle velocities shows improved images compared to earlier results interpretation of the wide angle and the mcs data indicates that a decollement is developed along a bright reflector within the turbiditic sequence more than 3 km of sediment bypasses the first accretionary ridges underthrusting and is transported to greater depth this might help to explain the sparse earthquake activity associated with subduction here in the strike lines low velocity layers occur landward of the deformation front in zones where active thrusting or underplating takes place the oceanic crust is characterised by a strong velocity contrast between layer ii and layer iii

relict subduction zones in canada 1 relict subduction zones that are interpreted on 10 lithoprobe deep seismic reflection profiles in canada provide key observational evidence that lithosphere plate interactions have operated with essentially the same resultant geometry for at least 2 7 gyr subduction zone reflections are generally found beneath known collisional belts and their orientations are often consistent with the locations of exposed arcs sutures and subduction complexes they project from the lower crust into the upper mantle and provide key geometric relationships between the crust moho and upper mantle the interpretation of subduction zone reflections as being syntectonic establishes a minimum age for the overlying lower crust and mantle wedge and precludes inferred formation of the moho and upper mantle by postorogenic delamination in these areas the orientation of subduction zone reflections may be synthetic or antithetic to exposed sutures indicating juxtaposition by subcretion or by delamination respectively the mantle wedge above the subduction zone reflections may exhibit sparse reflectivity subduction zone reflections commonly dip toward the craton indicating a preferred dip of subduction during the last stages of convergence within an orogen or else a higher likelihood of preservation beneath older domains interpreted ages of the canadian examples range from recent to neoarchean 2 7 ga providing observational evidence that plate tectonics or similar processes have been active since at least the neoarchean the locations and geometries of these relict subduction zones provide a snapshot of accretion boundaries in the mantle and establish a framework for the neoarchean recent accretionary architecture of the north american lithosphere

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

incipient subduction of the ontong java plateau along the north solomon trench abstract the ontong java plateau ojp in the western central pacific is the largest and thickest oceanic plateau and one of a few oceanic plateaus converging on an island arc solomon island arc sia to better understand the evolution of the north solomon trench nst active oblique convergence between the ojp and sia and late neogene development of malaita accretionary prism map we present 850 km of multichannel seismic reflection data integrated with 7832 km 2 of izanagi side scan sonar coverage we have focussed the study at the transition area between the well defined northwestern end of the north solomon trench and a diffusely deformed area where the trench is actively propagating in a northwestward direction the deeper structure beneath the survey area is discussed by phinney et al oceanic plateau accretion in the malaita accretionary prism inferred from multi channel seismic reflection data this issue using deeper penetration multichannel seismic reflection lines the serial cross sections provided by multichannel seismic profiling combined with the izanagi backscattering imagery provides a time series evolution for the development of the north solomon trench the main evolutionary stages include 1 the incipient trench in the northern area marked by a diffuse zone of deformation above a broad arch in the crust deeper penetration profiles by phinney et al show the bulge is related to a deeper decollement fault that is propagating upward and seaward through the crust 2 the formation of a continuous thrust front in the central area deeper penetration profiles by phinney et al show this thrust front is surface expression of the same decollement present at depth to the north the boundary between the surface trace of the thrust and the diffuse area of deformation in the northern area is inferred as a vertical high angle tear fault with left lateral offset 3 the formation of a deep elongate trench which controls gravitationally related slumping and sedimentation around the steep edges of the trench fill basin the areas to the southeast are those that have undergone convergence for the longest period of time and therefore show better developed trench structures and a reduced width of the map areas to the northwest have undergone convergence for a shorter period of time and show less developed trench structures and a wide area of the map

neotectonic deformation of the central chile margin deepwater forearc basin formation in response to hot spot ridge and seamount subduction 1 underlying the midslope valparaiso terrace the valparaiso basin constitutes a unique and prominent deepwater forearc basin on the central chile margin the basin contains a 3 3 5 km thick sediment fill of late cenozoic age the seismic stratigraphic architecture of the valparaiso basin fill shows that margin wide subsidence above the underthrusting juan fernandez ridge an aseismic volcanic swell crested with seamounts on the nazca plate characterized the basin evolution contemporaneously with margin subsidence and seaward rotation of the upper slope and shelf the outer part of the forearc was deformed by thrust faulting above the leading flank of underthrusting seamounts that rotated the middle slope to a more horizontal attitude and formed seaward vergent outer forearc highs subsidence of the margin above the underthrusting juan fernandez ridge is ascribed to processes of subduction erosion enhanced by factors related to subduction of the ridge that caused dismemberment of upper plate material along the shallow aseismic part of the plate interface and transfer of material to the downgoing nazca plate accelerated midslope subsidence 55 75 km landward of the trench coincides with the seaward limit of interplate seismicity suggesting that enhanced basal erosion is related to the transition from aseismic to stick slip subduction behavior since subduction of the juan fernandez ridge began near 33 s 10 ma ago the chile trench north of 33 s is inferred to have retreated 3 km myr and the midslope valparaiso basin subsided 0 3 0 5 km myr approximating analogous neogene rates for the peruvian margin

tectonic erosion of the peruvian forearc lima basin by subduction and nazca ridge collision 1 subsidence of lima basin part of the peruvian forearc is controlled by tectonic erosion by the subducting nazca plate multichannel seismic reflection data coupled with age and paleowater depth constraints derived from ocean drilling program odp coring now allow the rates of erosion to be reconstructed through time in trenchward locations the forearc has experienced limited recent relative uplift 700 850 m likely due to preferential basal erosion under the center of lima basin long term subsidence driven by basal tectonic erosion dominates and is fastest closest to the trench since 47 ma eocene up to 148 km of the plate margin have been lost at an average rate of up to 3 1 km myr 1 appoximately 110 km of that total appears to be lost since 11 ma implying much faster average rates of trench retreat 10 km myr 1 since collision of the nazca ridge with the lima basin at 11 ma although there is no clear subsidence event at odp site 679 during the time at which nazca ridge was subducting beneath this part of the forearc 4 11 ma the more trenchward odp sites 682 and 688 show significant deepening after 11 ma indicating that subduction of the ridge accelerates tectonic erosion long term rates of crustal erosion in the region of lima basin are greater than estimates of regional arc magmatic productivity implying that such margins are net sinks of continental crust

model of modern polycystine radiolarian shallow water zoogeography abstract there appear to be about six shallow water polycystine radiolarian provinces in the modern ocean these provinces owe their integrity to the major circulation systems that create packages of water and environments which vary in degrees of oceanographic and temporal stability the subpolar cyclonic gyres are low diversity provinces dominated by the radiolarian groups cenodiscids spongodiscids spongotrochins lithelids and lophophaenins some of these members exhibit the phenomenon of tropical submergence endemism is moderate in the southern subpolar cyclonic gyre this gyre contains such endemic forms as antarctisins the transition province contains a moderate diversity fauna dominated by cold water morphotypes of warm water sphere taxa and a few endemics however this transition province appears to be a region of radiolarian isolation and evolution the subtropical anticyclonic gyre province exhibits the highest radiolarian diversities densities and endemism of any of the provinces this province is dominated by radiolarians that host symbiotic algae and this association might possibly explain why they dominate these oligotrophic regions the equatorial province is very similar to the subtropical anticyclonic gyre province however it does not exhibit as high a diversity density or degree of endemism as does the gyre the eastern tropical province is dominated by symbiont bearing radiolarians as is the subtropical anticyclonic gyre however unlike the gyre there are few endemics and only low to moderate diversities and densities the main reason for few endemics in the eastern tropical province is probably due to the youth of this province it has only existed since the mid pliocene whereas the subtropical anticyclonic gyre province has a history extending into the mid miocene the boundary current provinces are really just tappings of the equatorial subpolar or transition provinces and reflect these parental provinces in their radiolarian characteristics

basin centered asperities in great subduction zone earthquakes a link between slip subsidence and subduction erosion 1 published areas of high coseismic slip or asperities for 29 of the largest circum pacific megathrust earthquakes are compared to forearc structure revealed by satellite free air gravity bathymetry and seismic profiling on average 71 of an earthquake s seismic moment and 79 of its asperity area occur beneath the prominent gravity low outlining the deep sea terrace 57 of an earthquake s asperity area on average occurs beneath the forearc basins that lie within the deep sea terrace in sw japan slip in the 1923 1944 1946 and 1968 earthquakes was largely centered beneath five forearc basins whose landward edge overlies the 350 c isotherm on the plate boundary the inferred downdip limit of the locked zone basin centered coseismic slip also occurred along the aleutian mexico peru and chile subduction zones but was ambiguous for the great 1964 alaska earthquake beneath intrabasin structural highs seismic slip tends to be lower possibly due to higher temperatures and fluid pressures kilometers of late cenozoic subsidence and crustal thinning above some of the source zones are indicated by seismic profiling and drilling and are thought to be caused by basal subduction erosion the deep sea terraces and basins may evolve not just by growth of the outer arc high but also by interseismic subsidence not recovered during earthquakes basin centered asperities could indicate a link between subsidence subduction erosion and seismogenesis whatever the cause forearc basins may be useful indicators of long term seismic moment release the source zone for cascadia s 1700 a d earthquake contains five large basin centered gravity lows that may indicate potential asperities at depth the gravity gradient marking the inferred downdip limit to large coseismic slip lies offshore except in northwestern washington where the low extends landward beneath the coast transverse gravity highs between the basins suggest that the margin is seismically segmented and could produce a variety of large earthquakes

tectonic erosion and consequent collapse of the pacific margin of costa rica combined implications from odp leg 170 seismic offshore data and regional geology of the nicoya peninsula the convergent margin off the pacific coast of the nicoya peninsula of costa rica exhibits evidence for subduction erosion caused by the underthrusting cocos plate critical evidence for efficacy of this process was recovered at the ocean drilling program odp drilling site 1042 leg 170 positioned 7 km landward of the middle america trench axis off the nicoya peninsula the primary drilling objective at this site was to identify the age and origin of a regionally extensive and prominent seismic discontinuity the so called base of slope sediment boss horizon or surface the boss horizon which can be traced landward from near the trench to the nicoya coastal area and parallel to it for hundreds of kilometers separates a low velocity 2 0 2 5 km s 1 sequence of slope sediment from an underlying sequence of much higher velocity 4 4 5 km s 1 rock site 1042 reached the acoustically defined boss horizon at a below sea level depth of 3900 m and yielded a carbonate cemented calcarenitic breccia of early middle miocene age sedimentological geochemical paleontological and cement paragenesis data document that the breccia accumulated in a shallow water depositional environment on the basis of coastal exposures the boss horizon as a margin wide geologic interface can be temporally and lithostratigraphically correlated to a regional angular unconformity this unconformity known as the mal pais unconformity separates neogene and younger shelf to littoral beds from the underlying mafic units of the mesozoic nicoya complex and cretaceous and early tertiary sedimentary sequences at site 1042 it is inferred that tectonism caused the vertical subsidence of the early neogene breccia from a shallow to a deep water setting the mal pais unconformity of the boss horizon thus connects the rock fabric of the outermost part of margin to that of coastal nicoya and implies that in the early neogene the nicoya shelf extended seaward to near the present trench axis this circumstance requires that the early neogene trench axis was at least 50 km seaward of where it is now located the long term effects of subduction erosion similar to those described for the scientifically drilled japan tonga and peru margins best account for offshore and onshore evidence for a post paleogene history of crustal thinning and landward trench migration of costa rica s pacific margin during the past 16 17 myr the calculated mass removal and landward migration rates are 34 36 km myr 1 km 1 of margin and 3 km myr 1 respectively these values are similar to those found for other pacific margins dominated by nonaccretionary subduction zone processes

when seamounts subduct data from an array of seismometers on the sea floor show the complex pattern of earthquakes around subducted seamounts

crustal structure and evolution of the mariana intra oceanic island arc a new high resolution velocity model of the mariana arc backarc system obtained from active source seismic profiling demonstrates velocity variations within the arc middle and lower crusts of intermediate to felsic and mafic compositions the characteristics of the oceanic island arc crust are a middle crust with velocity of 6 km s laterally heterogeneous lower crust with velocities of 7 km s and unusually low mantle velocities petrologic modeling suggests that the volume of the lower crust composed of restites and olivine cumulates after the extraction of the middle crust should be significantly larger than is observed suggesting that a part of the lower crust especially the cumulates is seismically a part of the mantle

global tectonic significance of the solomon islands and ontong java plateau convergent zone abstract oceanic plateaus areas of anomalously thick oceanic crust cover about 3 of the earth s seafloor and are thought to mark the surface location of mantle plume heads hotspot tracks represent continuing magmatism associated with the remaining plume conduit or tail it is presently controversial whether voluminous and mafic oceanic plateau lithosphere is eventually accreted at subduction zones and therefore 1 influences the eventual composition of continental crust and 2 is responsible for significantly higher rates of continental growth than growth only by accretion of island arcs the ontong java plateau ojp of the southwestern pacific ocean is the largest and thickest oceanic plateau on earth and the largest plateau currently converging on an island arc solomon islands for this reason this convergent zone is a key area for understanding the fate of large and thick plateaus on reaching subduction zones this volume consists of a series of four papers that summarize the results of joint us japan marine geophysical studies in 1995 and 1998 of the solomon islands ontong java plateau convergent zone marine geophysical data include single and multi channel seismic reflection ocean bottom seismometer obs refraction gravity magnetic sidescan sonar and earthquake studies objectives of this introductory paper include 1 review of the significance of oceanic plateaus as potential contributors to continental crust 2 review of the current theories on the fate of oceanic plateaus at subduction zones 3 establish the present day and neogene tectonic setting of the solomon islands ontong java plateau convergent zone 4 discuss the controversial sequence and timing of tectonic events surrounding ontong java plateau solomon arc convergence 5 present a series of tectonic reconstructions for the period 20 ma early miocene to the present day in support of our proposed timing of major tectonic events affecting the ontong java plateau solomon islands convergent zone and 6 compare the structural and deformational pattern observed in the solomon islands to ancient oceanic plateaus preserved in precambrian and phanerozoic orogenic belts our main conclusion of this study is that 80 of the crustal thickness of the ontong java plateau is subducted beneath the solomon island arc only the uppermost basaltic and sedimentary part of the crust 7 km is preserved on the overriding plate by subduction accretion processes this observation is consistent with the observed imbricate structural style of plateaus and seamount chains preserved in both precambrian and phanerozoic orogenic belts

autocyclic perturbations of orbitally forced signals in the sedimentary record small scale cyclicity in stratigraphic sequences 1 mm 10 m is often attributed to regular orbital signals commonly however these signals are overprinted by other signals a dynamic forward model for clastic sedimentation in foreland basins suggests that this overprint can be caused by slope instability stress variations in the lithosphere and autocyclicity induced by climate change spectral analyses of predicted sedimentation rates with various external controls demonstrate the occurrence of autocyclic fluctuations in sediment flux on time scales smaller than the time scale of the variation of the original triggering external control this feature probably represents the response of the sedimentary system to external variations depending on the eigenfrequencies of this system because the eigenfrequencies are determined by the basin geometry changing through time a chaotic frequency distribution of sedimentation rates is expected this hypothesis explains the commonly observed absence of a significant number of frequencies of the orbital spectrum in a large number of sediment sequences

tectonic accretion versus erosion along the southern chile trench oblique subduction and margin segmentation 1 the southernmost tip of south america is an active continental margin where oblique convergence between plates transcurrent motion and tectonic rotation on land make the geodynamic setting more complex than that of the central andes a multichannel seismic data set has been used in conjunction with multibeam and altimetry data to clarify the regional architecture of the continental margin from 50 s to 57 s despite the thick sedimentary section in the trench and the slow plate convergence rate peculiar of typical accretionary margins seismic reflection profiles image widely varying frontal wedge morphologies different rates of accretion a high degree of structural diversity and different modes of continental building offscraping underplating tectonic erosion correlation between structural parameters depth of the decollement level width of the wedge accretionary rates moho depth suggests large scale structural control on margin geometry and structural diversity the transition from tectonic accretion to erosion and general structural variations do not show any gradual trend but rather they occur along oblique structural trends that produce a tectonic segmentation of the margin this is mainly related to tectonic processes on the overriding plate block rotations along strike slip faults while local disturbances on the incoming plate seamounts fracture zones and oceanic fabric relative orientation add further structural complexity altimetry data in conjunction with structural analysis suggest that large scale tectonic variations and structural development are related to trench parallel gravity anomaly variations and ultimately to basal friction on the plate interface strong negative gravity anomalies are associated with sedimentary basins wide wedges and accretionary domains while positive gravity anomalies mainly refer to transverse structural highs and narrow wedges

episodic development of a convergent margin inferred from structures and processes along the southern chile margin seismic reflection data acquired in the vicinity of isla mocha across the southern coast of chile image structures formed along the continental margin and reveal an episodic history of accretion nonaccretion and possibly erosion structures formed at the toe of the continental slope suggest frontal accretion of to 1 km of trench fill seismic images also reveal that a small accretionary wedge 20 30 km wide abuts the truncated continental metamorphic basement that extends seaward from beneath the shelf the small size of the accretionary wedge on three profiles examined here is not consistent with a long history of accretion with the current deformational style as current rates of frontal accretion could have accumulated all of the existing accretionary wedge in less than 1 2 m y this is a small fraction of convergence history along this margin and the current accretionary mode has not been consistently maintained in the past the isla mocha region is located between the temperate climate of central chile and the glacial climate of southern chile and climatic conditions in this region have likely fluctuated sufficiently to cause significant variation in trench sediment supply accretionary and nonaccretionary or erosional episodes are probably linked to temporal variations in trench sediment thickness as suggested by observations along the chile margin currently thick trench sediment correlates with accretion along the southern chile margin and thin trench sediment correlates with nonaccretion tectonic erosion as near the chile ridge and from the juan fernandez ridge to northern chile the isla mocha region also lies 900 1000 km north of the chile triple junction and the chile ridge lies approximately 2000 km to the west and has not yet collided and affected the margin near isla mocha this part of the precollision zone provides an excellent reference to examine the effects of chile ridge collision in the development of the chile margin the most apparent effect of subduction of the buoyant young crust of the chile ridge is a shallow trench that is nearly devoid of sediment consequently the triple junction is undergoing nonaccretion or erosion and the accretionary complex near the triple junction remains smaller than to the north or south because the current phase of rapid accretion elsewhere in the trench has bypassed the triple junction region the interplay of subduction zone processes such as trench sedimentation and ridge collision has resulted in an episodic development of the margin and produced a discontinuous record of convergence history within the accretionary wedge

rootless serpentinite seamount on the southern izu bonin forearc implications for basal erosion at convergent plate margins we use multichannel seismic reflection mcs and gravity data from the southern izu bonin convergent plate margin to test for tectonic erosion the hahajima seamount is a serpentinite seamount on the izu bonin forearc this serpentinite body likely originated from the mantle wedge beneath the izu bonin arc but it subsequently detached mcs data and gravity modeling demonstrate that the ogasawara plateau on the incoming pacific plate has been partly subducted beneath the hahajima seamount our analysis indicates that the hahajima seamount is a rootless serpentinite seamount and the root of the serpentinite body was eroded during subduction of the western edge of the ogasawara plateau structural features of the hahajima seamount the ogasawara plateau and the philippine sea plate suggest that tectonic erosion particularly basal erosion has been occurring throughout this area

structure of an active arc continent collision area the aleutian kamchatka junction abstract the plate boundary between the north american and pacific plates along the westernmost portion of the aleutian arc is formed by several active dextral strike slip fracture zones fracturing is expressed by fault escarpments visible in reflection seismic profiles and narrow zones of high seismicity the seismicity changes from the aleutian arc where shallow earthquakes occur to intermediate to deep earthquakes in the kamchatka aleutian junction area and to the south in the vicinity of this junction the distribution of earthquakes follows the traces of major aleutian fracture zones down to a depth of 100 km south of the kronotsky peninsula the distribution of earthquakes changes and is similar to other subduction zones the fracture zones cut the western aleutian arc into individual blocks which move along strike slip faults to the west and collide with kamchatka related transtension and transpression has led to the development of pull apart basins and block tilting the fracture zones splay into numerous strike slip faults to the west where they meet the kamchatka margin the main faults of the aleutian fault system can be traced on the kamchatka mys peninsula where deformed pleistocene sediments high uplift rates recent thrust faulting and changes in the drainage system provide evidence for neotectonic activity the fault pattern onshore and offshore in the junction area supports an indentation tectonic model of the collision of aleutian arc slivers with kamchatka

accretion in the wake of terrane collision the neogene accretionary wedge off kenai peninsula alaska subduction accretion and repeated terrane collision shaped the alaskan convergent margin the yakutat terrane is currently colliding with the continental margin below the central gulf of alaska during the neogene the terrane s western part was subducted after which a sediment wedge accreted along the northeast aleutian trench this wedge incorporates sediment eroded from the continental margin and marine sediments carried into the subduction zone on the pacific plate prestack depth migration was performed on six seismic reflection lines to resolve the structure within this accretionary wedge and its backstop the lateral extent of the structures is constrained by high resolution swath bathymetry and seismic lines collected along strike accretionary structure consists of variably sized thrust slices that were deformed against a backstop during frontal accretion and underplating toward the northeast the lower slope steepens the wedge narrows and the accreted volume decreases notwithstanding a doubling of sediment thickness in the trench in the northeasternmost transect near the area where the terrane s trailing edge subducts no frontal accretion is observed and the slope is eroded the structures imaged along the seismic lines discussed here most likely result from progressive evolution from erosion to accretion as the trailing edge of the yakutat terrane is subducting

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

late cretaceous truncation of the western idaho shear zone in the central north american cordillera the mesozoic boundary separating accreted oceanic rocks and continental north america in west central idaho forms a prominent right angle bend the origin and kinematic significance of which have had a great influence on models for the evolution of the north american cordilleran orogen based on structural and geochronologic relations we conclude that the bend in the steep crustal boundary is formed by the intersection of two shear zones with different ages structural fabrics of the north striking western idaho shear zone are overprinted by penetrative structures of the west northwest striking orofino shear zone u pb sensitive high resolution ion microprobe ages on metaplutonic rocks and late synkinematic leucocratic dikes constrain the age of deformation in the orofino shear zone to ca 90 70 ma distinctly younger than ca 120 90 ma magmatism and deformation within the western idaho shear zone the orofino shear zone is interpreted as a southwest vergent contractional relay structure that linked basement cored foreland thrusts of the middle and southern rocky mountains with the deep crustal duplex of the shuswap complex in the canadian rockies truncation ca 90 ma terminated late cretaceous margin parallel translation of terranes along the western idaho shear zone and required displacement to step outboard to poorly understood structures to the west

coast plutonic complex a mid cretaceous contractional orogen early late cretaceous east vergent thrusts deform much of the eastern margin of the coast plutonic complex in western british columbia similarities in timing and style suggest that the faults represent a system of backthrusts to a west vergent thrust belt on the west side of the complex this geometry and the rarity of mid cretaceous strike slip faults indicate that the coast plutonic complex was a strongly contractional orogen in mid cretaceous time

slow rates of subduction erosion and coastal underplating along the andean margin of chile and peru marine seismic and drilling data from the andean margin of peru and northern chile show trench retreat and tectonic erosion since at least 20 ma however the onshore coastal zone has undergone modest regional uplift since ca 2 ma this argues against subduction erosion of the margin by steady state retreat such as recognized in tonga or south sandwich instead we identify a new style of slow tectonic erosion uplifted forearc sediments indicate underplating of subducted material beneath the base of the coastal forearc synchronous with tectonic erosion under the trenchward part of the forearc these observations require slow rates of forearc crust subduction in the region during this time period averaging no more than 13 km3 m y km since 20 ma as the forearc wedge taper increased isostatic volume estimates suggest that as much as 85 of subducted material was underplated under the forearc since 330 ka and does not contribute to crustal growth under the andes the switch in tectonic style parallels that in the southern andes but cannot be triggered by increased sediment flux to the trench as proposed for that area

tectonic delamination and subcrustal imbrication of the precambrian lithosphere in northwestern canada mapped by lithoprobe seismic reflection profiling of the precambrian lithosphere in northwestern canada has produced images of delamination structures tectonic wedges at crustal scale to about 30 km depth and for the first time at lithospheric scale to about 100 km depth and has further delineated subcrustal structures that were likely caused by imbrication of subducted lithosphere coupled with observations of similar structures in thrust and fold belts delamination and lower plate imbrication are persistent features at many scales

eolian derived siltstone in the upper permian phosphoria formation implications for marine upwelling previously described organic rich shale facies of the meade peak member of the phosphoria formation at soda springs idaho consist of well sorted laminated siltstone containing little or no clay the planar parallel fabric preserved in many of the siltstone beds suggests suspension settling onto the outer shelf or upper slope most likely under low oxygen conditions evidence for transport of silt to the depositional site by sediment gravity flows is absent as are sandstone beds we propose that meade peak siltstone facies record subaqueous deposition of windborne silt that was transported southward from central montana as such they provide the first direct geologic evidence of a wind regime favorable for marine upwelling the process commonly postulated to have localized the deposition of permian phosphatic sediments they may also provide indirect evidence of upwind eolian sand transport at that time

opposing gradients of permanent strain in the aseismic zone and elastic strain across the seismogenic zone of the kodiak shelf and slope alaska an industry acquired seismic reflection line from the aleutian arc to the alaska trench was prestack depth migrated and further constrained with wide angle ocean bottom hydrophone seismic velocity data and swath mapping from the seismic reflection image individual thrust slices the backstop and the forearc basin were resolved sufficiently well to balance the section tectonic shortening and age estimates provided rates of permanent contraction in five discrete segments over 150 km during the past 3 myr about 70 of the plate convergence is measured as contraction across the 40 km wide neogene accretionary prism which composes less than 10 of the forearc permanent contraction is minimal above the 1964 alaska earthquake aftershock zone indicating dominently elastic strain since elastic strain is poorly recorded in the geological record it was approximated from the historical record of the 1964 alaska earthquake the 1964 coseismic vertical deformation displays an inverse pattern to the permanent strain consistent with the earthquake cycle concept elastic deformation is dominant beneath the shelf and upper slope rapid permanent deformation is concentrated beneath the lower slope and a transition between them occurs beneath the middle slope

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

geological record of fluid flow and seismogenesis along an erosive subducting plate boundary a fossil erosive subduction channel the shear zone marking the plate boundary preserved in the northern apennines of italy is identified the fossil zone records the presence of two decollements which seem to have been simultaneously active at the top and base of the subduction channel in modern subduction zones the onset of seismic activity is believed to occur at 150 c however in the fossil channel the onset seems to have occurred at colder temperatures tectonic erosion of the overriding plate by the downgoing slab is believed to occur at half the earth s subduction zones1 2 in situ investigation of the geological processes at active erosive margins is extremely difficult owing to the deep marine environment and the net loss of forearc crust to deeper levels in the subduction zone until now a fossil erosive subduction channel the shear zone marking the plate boundary3 has not been recognized in the field so that seismic observations have provided the only information on plate boundary processes at erosive margins here we show that a fossil erosive margin is preserved in the northern apennines of italy it formed during the tertiary transition from oceanic subduction to continental collision and was preserved by the late deactivation and fossilization of the plate boundary the outcropping erosive subduction channel is 500 m thick it is representative of the first 5 km of depth with its deeper portions reaching 150 c the fossil zone records several surprises two decollements were simultaneously active at the top and base of the subduction channel both deeper basal erosion and near surface frontal erosion occurred at shallow depths extension was a key deformation component within this erosive convergent plate boundary and slip occurred without an observable fluid pressure cycle at depths greater than about 3 km a fluid cycle is clearly shown by the development of veins and the alternation of fast co seismic and slow inter seismic slip in the deepest portions of the outcropping subduction channel extension is finally overprinted by compressional structures in modern subduction zones the onset of seismic activity is believed to occur at 150 c but in the fossil channel the onset occurred at cooler palaeo temperatures

structure of the kuril trench from seismic reflection records three multichannel seismic reflection records across the kuril convergent margin provide the first deep data in this area the records are located across the southern tip of kamchatka the central kuril arc and the northern extension of hokkaido island platform which show three distinct tectonic regimes the lower slope contains a wedge shaped buttress surrounded by low velocity sediments underplating sediment uplifts the buttress as indicated by faults that displace its upper surface the middle slope is a block of acoustic basement which has a rough surface with significant arcward dipping faults the middle slope is separated from the upper slope along a steep arcward dipping reflection the middle slope boundary the upper slope structure off kamchatka is different from that off hokkaido off kamchatka a regular stratified sediment section has been uplifted tilted and dips seaward off hokkaido stronger uplift has tilted regional blocks seaward and arcward along the southern line north of hokkaido well studied nonthrust earthquakes with lateral motions occur beneath the middle slope boundary and on the boundary of the subducting plate thrust earthquakes occur under the middle and upper slope whereas tsunami earthquakes occur under the lower slope high amplitude reflections along the lower boundary of the wedge shaped buttress and along the active decollement indicate high fluid concentrations which reduce the friction along the tectonic units so that through the weak coupling slow rupture may extend up to the seafloor from a tsunami earthquake

origin of a crustal splay fault and its relation to the seismogenic zone and underplating at the erosional north ecuador south colombia oceanic margin 1 splay faults within accretionary complexes are commonly associated with the updip limit of the seismogenic zone prestack depth migration of a multichannel seismic line across the north ecuador south colombia oceanic margin images a crustal splay fault that correlates with the seaward limit of the rupture zone of the 1958 mw 7 7 tsunamogenic subduction earthquake the splay fault separates 5 6 6 km s velocity inner wedge basement rocks which belong to the accreted gorgona oceanic terrane from 4 to 5 km s velocity outer wedge rocks the outer wedge is dominated by basal tectonic erosion despite a 3 km thick trench fill subduction of 2 km high seamount prevented tectonic accretion and promotes basal tectonic erosion the low velocity and poorly reflective subduction channel that underlies the outer wedge is associated with the aseismic decollement thrust subduction channel fluids are expected to migrate upward along splay faults and alter outer wedge rocks conversely duplexes are interpreted to form from and above subducting sediment at 14 to 15 km depths between the overlapping seismogenic part of the splay fault and the underlying aseismic decollement coeval basal erosion of the outer wedge and underplating beneath the apex of inner wedge control the margin mass budget which comes out negative intraoceanic basement fossil listric normal faults and a rift zone inverted in a flower structure reflect the evolution of the gorgona terrane from cretaceous extension to likely eocene oblique compression the splay faults could have resulted from tectonic inversion of listric normal faults thus showing how inherited structures may promote fluid flow across margin basement and control seismogenesis

uplift and subduction erosion in southwestern mexico since the oligocene pluton geobarometry constraints details of the late oligocene to middle miocene uplift and tectonic erosion episodes of the southwestern continental margin of mexico can be inferred using altot geobarometry of igneous hornblendes geochronology and field relations on the basis of such analyses carried out between acapulco and huatulco we find the following 1 calc alkaline batholiths exposed along the coast from acapulco to huatulco mostly in the 35 25 ma age range were emplaced at depths between 13 and 20 km 2 the contact relationships between these plutons and their host rocks and the exposure of volcanic counterparts 70 km from the coastline indicate a landward decrease in the amount of uplift 3 a comparison of the time differences between intrusion and cooling ages of batholiths along the coast suggest that cooling rates were in general higher between acapulco and huatulco than those along the margin between puerto vallarta and manzanillo 700 km northwest of acapulco 4 the uplift of this coastal belt occurred during the late stages of magmatism and after its cessation triggering intensive subaerial erosion of supracrustal rocks and the exposure of midcrustal rocks such as granitic batholiths and amphibolite facies metamorphic assemblages of the xolapa complex these findings in conjunction with the geometry of the present continental margin as well as the offshore tectonic and stratigraphic features support previous interpretations of very active late oligocene to middle miocene subduction erosion after the onset of strike slip tectonics related to the detachment and subsequent eastward displacement of the chortis block subduction erosion involved both trench sediments and crystalline continental framework rocks different rates of continental framework erosion are assessed on the basis of the bathymetric fluctuations of the upper slope trench sediments and the age of the accretionary prism subsidence of the offshore continental basement suggests intense episodes of basal erosion of lower continental crust whereas the construction of the present day accretionary prism and the uplift of the upper slope indicate a decline in the frontal and basal erosion of the continental framework comparing the calculated depths of pluton crystallization with the present depth of the continental crust subducted slab boundary interpreted using previously published seismic refraction and gravity models we conclude that onshore basal erosion played a subordinate role during miocene episodes of subduction erosion major removal of lower crustal sections was probably restricted to offshore regions plate reconstructions of the cocos plate and its predecessors with respect to north america indicate that the uplift and probably the offshore subduction erosion in this region coincided with the initial stages of the subhorizontal trajectory of the guadalupe plate beneath southwestern mexico

deep structures of the ecuador convergent margin and the carnegie ridge possible consequence on great earthquakes recurrence interval 1 the deep structure of the ecuador subduction zone and adjacent carnegie ridge cr was investigated using on shore off shore wide angle seimics a crustal model obtained by 2 d inversion of traveltimes reveals the overthickened 14 km oceanic crust of the cr that underthrusts the high velocity 6 km s basement of the upper plate margin wedge interpreted as part of the accreted oceanic terranes described on shore the plate interface dips 4 to 10 east from the trench to a depth of 15 km shadow zones observed on the margin obs records are interpreted as a low velocity zone consisting of a thin layer of underthrust sediments and the 3 km thick cr layer 2 whose average seismic velocity of 5 1 km s is slower than that of the margin wedge increased interplate coupling related to the subduction of the thick buoyant cr may account for an apparent local increased recurrence interval between great interplate earthquakes

large neogene subsidence event along the middle america trench off mexico 18 n 19 n evidence from submersible observations exposures of pre eocene plutonic rocks located along the inner slope of the middle america trench were sampled during deep sea dives off manzanillo lat 18 n 19 n at depths ranging from 3950 m to 2823 m fission track data show a rapid cooling of these plutonic rocks during the paleocene a subaerial unconformity indicates that the plutonic rocks were exposed after paleocene time the presence of upper middle bathyal upper miocene lower pliocene marine sediments above the unconformity documents a neogene subsidence event at a mean rate of 0 35 mm yr this subsidence indicates subduction erosion along this segment of the margin

the growth and destruction of continental crust during arc continent collision in the southern urals abstract the southern urals of russia contain a well preserved example of a paleozoic arc continent collision in which the laurussia margin was subducted beneath the magnitogorsk island arc in the devonian providing an ideal field area for studying the possible growth and destruction of the continental crust during this process high pressure rocks derived from the leading edge of the continental margin indicate that it was subducted to a depth of between 70 km eclogite assemblages and 120 km micro diamonds the vast majority of the high pressure rocks have a sedimentary protolith with mafic eclogite having been derived from dikes intruding into the sediments high pressure rocks derived from the mafic ganulite that currently makes up the middle and lower crust of the southern urals do not occur in outcrop suggesting that much of the subducted margin remained in the upper mantle however extensive geological geochemical and geophysical data do not unequivocally clarify its subsequent fate crustal delamination and foundering into the mantle can be ruled out and recycling through the volcanic arc system appears to have been minor while the metamorphic products of subducted continental crust can remain in the mantle for long periods of time in the southern urals it has not been imaged by the available geophysical data sets a possible explanation for this is that the progressive metamorphism of mafic granulite to eclogite assemblages would have altered its physical properties to those of typical mantle lithologies making them difficult to detect by geophysical methods it is estimated that the volume of continental crust that was subducted and lost to the mantle was approximately one third of the volume that was added to the laurussia margin by the accretion of the magnitogorsk arc

how does the nazca ridge subduction influence the modern amazonian foreland basin the subduction of an aseismic ridge has important consequences on the dynamics of the overriding upper plate in the central andes the nazca ridge subduction imprint can be tracked on the eastern side of the andes the fitzcarrald arch is the long wavelength topography response of the nazca ridge flat subduction 750 km inboard of the trench this uplift is responsible for the atypical three dimensional shape of the amazonian forelland basin the fitzearrald arch uplift is no older than pliocene as constrained by the study of neogene sediments and geomorphic markers according to the kinematics of the nazca ridge subduction

seismic evidence for broken oceanic crust in the 2004 sumatra earthquake epicentral region the 2004 sumatra earthquake was one of the largest events to occur in a subduction zone in the past 50 years seismic reflection data for this subduction zone reveal thrust faults cutting across the entire oceanic crust this observation coupled with the hypocentres of aftershocks suggests that the megathrust the interface between the indo australian plate and the sunda plate currently lies in the oceanic mantle

deep structure of an island arc backstop lesser antilles subduction zone 1 we present the results from a coincident seismic reflection refraction grid conducted at the lesser antilles subduction zone near 16 n this paper focuses on the seismic refraction data and constraints these data place on the three dimensional structure of the island arc backstop we find that the backstop in this region contains considerable topography in both the strike and dip directions two ridges each 25 35 km in length and 10 km in width rise 1 6 km above the adjacent basement the eastern edge of one of the ridges deepens by 4 6 km over a horizontal distance of 10 km and forms the eastern edge of the backstop in contrast to the complex nature of the backstop the adjacent accretionary wedge displays little lateral variability at large scales this may be a consequence of the spatial scales involved the backstop topography is 10 35 km in width while the accretionary wedge extends 125 km from the deformation front to the backstop the top of the subducting oceanic crust as identified by an increase in velocities to 6 6 5 km s intersects the backstop at a depth of 14 15 km the updip limit of plate boundary seismicity is located 75 100 km west and downdip of the backstop however two earthquake clusters are observed at the intersection of the subducted barracuda ridge and tiburon ridge with the backstop suggesting active deformation associated with the backstop edge at these locations

the great sumatra andaman earthquakes imaging the boundary between the ruptures of the great 2004 and 2005 earthquakes segmentation along convergent margins controls earthquake magnitude and location but the physical causes of segment boundaries and their impact on earthquake rupture dynamics are still poorly understood one aspect of the 2004 and 2005 great sumatra andaman earthquakes is their abrupt termination along a common boundary this has led to speculation on the nature of the boundary its origin and why it was not breached for the first time the boundary has been imaged and with newly acquired marine geophysical data we demonstrate that a ridge on the subducting indo australian oceanic crust may exert a control on margin segmentation this suggests a lower plate influence on margin structure particularly its segmentation the ridge is masked by the sedimentary cover in the trench its most likely trend is nne ssw it is interpreted as a fracture zone on the subducting oceanic plate a ramp or tear along the eastern flank of the subducting fracture zone beneath simeulue island may be considered as an intensification factor in terms of rupture propagation barrier

oceanic ridge subduction vs slab break off plate tectonic evolution along the baja california sur continental margin since 15 ma the interaction of the pacific farallon spreading centers with the north american convergent margin off baja california mexico supposedly ceased at 12 ma when plate convergence and seafloor spreading stopped we propose a new geodynamic evolution based on full bathymetry coverage and magnetic profiles from 23 n to 27 n famex cruise of the r v l9atalante april 2002 the data unveil a major clockwise rotation of the pacific farallon spreading direction starting ca 14 ma that formed a series of short spreading centers that became extinct ca 8 7 ma we suggest that the transcurrent motion between the pacific and north america along baja california was accommodated by seafloor spreading and oblique convergence along the trench this change in spreading direction was followed by a concomitant progressive demise of both pacific farallon seafloor spreading and farallon north america subduction that are attributed to the break off of the farallon slab this also resulted in the opening of a trench parallel slab window beneath baja california

carbon helium neon and argon isotopes in a po basin northern italy natural gas field the abundance and isotopic composition of he ne and ar have been determined in 19 gas samples taken from the dosso degli angeli hydrocarbon gas field between 3000 and 3750 m depth in the po basin carbon isotope ratios in these samples vary between 13c ch4 67 5 and 73 4 indicating a predominantly biogenic source for the methane c2 c2h6 c3h8 concentrations increase systematically from 0 04 to 0 17 by volume with depth suggesting that there is a small additional thermogenic or diagenetic hydrocarbon contribution to these gases ch4 36ar ratios in the dosso degli angeli gases are in the range predicted for a groundwater which has derived its rare gas by equilibration with the atmosphere and is ch4 saturated under reservoir conditions there is also up to 10 excess of ch4 which is not associated with the groundwater these results are consistent with 90 of the ch4 in the dosso degli angeli reservoir originating from ch4 saturated groundwater measured 20ne 36ar ratios depart from the predicted ratio in air equilibrated groundwater this fractionation is consistent with water gas phase equilibration under reservoir conditions and is similar to that observed in gas reservoirs in the vienna and pannonian basins austria and hungary respectively the he in the dosso degli angeli gases is almost entirely radiogenic and derived from the crust the concentration of 4he in the reservoir gases requires that he is mostly sourced from outside the reservoir volume radiogenic 4he 40ar ratios input into the reservoir gas must be much higher than averaged crustal production ratios as no shift from the atmospheric 40ar 36ar ratio is observed preferential release of 4he over 40ar from the sites of their production is the preferred explanation this observation contrasts with previous studies from active extensional basins in which radiogenic 4he and 40ar appear to have been stored released and transported at near production ratio on a regional scale the most obvious difference between these studies is the comparatively low thermal gradient of the po basin

backstop geometry and accretionary mechanics of the sunda margin the convergent sunda margin off indonesia displays all geological features characteristic of an accretion dominated subduction zone a combined interpretation of prestack depth migrated seismic reflection data and velocity information gained from refraction studies is supplemented by high resolution bathymetric data and for the first time allows the exact mapping of backstop regimes initially the outer high evolved as material was pushed against a static rigid arc framework backstop underlying a forearc basin increasing material strength of the outer high due to lithification formed a dynamic backstop which controls accretion today an out of sequence thrust marks the transition from the recent active frontal accretionary prism to the outer high and may be traced in the seismic and bathymetric data over the whole extent of the study area the existence of a static as well as a dynamic backstop controls the forearc geometry and is associated with the segmentation of the forearc which is observed in regimes of frontal as well as of oblique subduction mass balance calculations which account for porosity changes and metamorphism indicate a subduction history dominated by accretionary processes since the late eocene accretion is associated with the low values of basal friction inferred for the sunda margin structural investigations of conjugate fault planes indicate a very weak basal detachment effective stress analyses reveal that intrinsically weak material causes the high strength ratio of the detachment to the overlying sediments whereas overpressuring within the frontal accretionary prism is negligible

are rupture zone limits of great subduction earthquakes controlled by upper plate structures evidence from multichannel seismic reflection data acquired across the northern ecuador southwest colombia margin 1 subduction of the nazca plate beneath the ecuador colombia margin has produced four megathrust earthquakes during the last century the 500 km long rupture zone of the 1906 mw 8 8 event was partially reactivated by three thrust events in 1942 mw 7 8 1958 mw 7 7 and 1979 mw 8 2 whose rupture zones abut one another multichannel seismic reflection and bathymetric data acquired during the sisteur cruise show evidence that the margin wedge is segmented by transverse crustal faults that potentially correlate with the limits of the earthquake coseismic slip zones the paleogene neogene jama quininde and esmeraldas crustal faults define a 200 km long margin crustal block that coincides with the 1942 earthquake rupture zone subduction of the buoyant carnegie ridge is inferred to partially lock the plate interface along central ecuador however coseismic slip during the 1942 and 1906 earthquakes may have terminated against the subducted northern flank of the ridge we report on a newly identified manglares crustal fault that cuts transversally through the margin wedge and correlates with the limit between the 1958 and 1979 rupture zones during the earthquake cycle the fault is associated with high stress concentration on the plate interface an outer basement high which bounds the margin seaward of the 1958 rupture zone may act as a deformable buttress to seaward propagation of coseismic slip along a megathrust splay fault coseismic uplift of the basement high is interpreted as the cause for the 1958 tsunami we propose a model of weak transverse faults which reduce coupling between adjacent margin segments together with a splay fault and an asperity along the plate interface as controlling the seismogenic rupture of the 1958 earthquake

sequence stratigraphy structural style and age of deformation of the malaita accretionary prism solomon arc ontong java plateau convergent zone possibilities for the fate of oceanic plateaus at subduction zones range from complete subduction of the plateau beneath the arc to complete plateau arc accretion and resulting collisional orogenesis deep penetration multi channel seismic reflection mcs data from the northern flank of the solomon islands reveal the sequence stratigraphy structural style and age of deformation of an accretionary prism formed during late neogene 5 0 ma convergence between the 33 km thick crust of the ontong java oceanic plateau and the 15 km thick solomon island arc correlation of mcs data with the satellite derived free air gravity field defines the tectonic boundaries and internal structure of the 800 km long 140 km wide accretionary prism we name this prism the malaita accretionary prism or map after malaita the largest and best studied island exposure of the accretionary prism in the solomon islands mcs data gravity data and stratigraphic correlations to islands and odp sites on the ontong java plateau ojp reveal that the offshore map is composed of folded and thrust faulted sedimentary rocks and upper crystalline crust offscraped from the solomon the subducting ontong java plateau pacific plate and transferred to the solomon arc with the exception of an upper sequence of quaternary island derived terrigenous sediments the deformed stratigraphy of the map is identical to that of the incoming ontong java plateau in the north solomon trench

active thrusting in the inner forearc of an erosive convergent margin pacific coast costa rica 1 structural and geomorphic analyses of the fila costena thrust belt in southwest costa rica indicate active thrusting within the inner forearc the fila costena exposes three major thrust faults that imbricate the late tertiary forearc basin sequence of the terraba basin the frontal thrust of the fila costena marks the boundary between an uplifting inner forearc and a subsiding outer forearc with only local uplift astride the indenting cocos ridge on the basis of surface constraints a cross section across the thrust belt suggests that all three thrusts flatten into parallelism with a low angle decollement horizon near the contact between the basement and the cover sequence of the terraba basin this decollement lies at a depth of 4 km the minimum shortening recorded by restoration of fault related folds is 17 km or 45 observations of late tertiary marine sediments tilted and faulted late quaternary fluvial terraces and raised holocene marine terraces indicate that fila costena uplift was likely initiated in the quaternary and is ongoing given that the coastal mountains are separated from the talamanca range by a valley the decollement that delaminates the forearc basin from the underthrusting forearc must continue as a flat beneath the valley but must link with the plate boundary along a crustal scale ramp system a structural geometry that has resulted in uplift of the talamanca range the highest peaks in central america the dichotomy between uplift in the inner forearc and subsidence in the outer forearc is explained in terms of the response of an arcward thickening wedge to rough subducting crust index terms 8150 tectonophysics plate boundary general 3040 8123 tectonophysics dynamics seismotectonics 8010 structural geology fractures and faults 8015 structural geology local crustal structure 8005 structural geology folds and folding

exposed plate interface in the european alps reveals fabric styles and gradients related to an ancient seismogenic coupling zone 1 we present observations from a continuous exposure of an ancient plate interface in the depth range of its former seismogenic zone in the central alps of europe related to late cretaceous early tertiary subduction and accretion of the south penninic lower plate underneath the adriatic upper plate the material forming the exposed plate interface zone has experienced flow and fracturing over an extended period of time followed by syncollisional exhumation thus reflecting a multistage evolution fabric formation and metamorphism however chiefly record the deformation conditions of the precollisional setting along the plate interface we identify an unstable slip domain from pseudotachylytes occurring in the temperature range between 200 and 300 c this zone coincides with a domain of intense veining in the subduction melange with mineral growth into open cavities indicating fast possibly seismic rupture evidence for transient near lithostatic fluid pressure as well as brittle fractures competing with mylonitic shear zones continues into the region below the occurrence of pseudotachylytes possibly reflecting a zone of conditionally stable slip the zone above the unstable slip area is devoid of veins but displays ample evidence of fluid assisted processes similar to the deeper zone solution precipitation creep and dehydration reactions in the melange matrix hydration and sealing of the base of the upper plate seismic rupture here is possibly expressed by ubiquitous localized deformation zones we hypothesize that trenchward sealing of parts of the plate interface as well as reaction enhanced destruction of upper plate permeability is an important component localizing the unstable slip zone this relation may result from the competition of the pervasive presumably interseismic pressure solution creep destroying permeability and building elevated fluid pressure until the strength threshold is reached with seismic failure

splay fault branching along the nankai subduction zone seismic reflection profiles reveal steeply landward dipping splay faults in the rupture area of the magnitude m 8 1 tonankai earthquake in the nankai subduction zone these splay faults branch upward from the plate boundary interface that is the subduction zone at a depth of approximately 10 kilometers approximately 50 to 55 kilometers landward of the trough axis breaking through the upper crustal plate slip on the active splay fault may be an important mechanism that accommodates the elastic strain caused by relative plate motion

response of the tectonically erosive south peruvian forearc to subduction of the nazca ridge analysis of three dimensional analogue experiments 1 the collision zone of the obliquely subducting nazca ridge and the erosive peruvian forearc is characterized by enhanced tectonic erosion normal faulting forearc uplift and a minor indentation we apply two three dimensional sandbox experiments monitored by an optical particle imaging velocimetry system to decipher the spatial and temporal evolution of forearc deformation during oblique convergence the rigid model ridge causes a wave of uplift followed by subsidence to shift along the forearc by varying the strength of the analogue material we demonstrate that the forearc response to ridge subduction in particular the amount and rate of uplift and wedge indentation is controlled by the mechanical strength of the forearc the model using the high strength material yields results more compatible with the geological record of the peruvian forearc first the modeled amount of uplift scales to 1 2 km in nature which agrees with 1 0 km of uplift recorded by marine terraces second the calibrated model uplift rate of 0 9 km myr is similar to the natural rate of 0 7 km myr third the development of normal faults above the model ridge is in accordance with the style of faulting above the nazca ridge finally oversteepening of the uplifted wedge and local slope failure as observed in the experiment has also been identified off southern peru a comparison between the erosive peruvian and accretive margins emphasizes that the style of ridge induced deformation depends on the forearc strength and crustal structure which in turn are controlled by the long term evolution of the margin s mass transfer regime

palaeoclimate sedimentation and continental accretion climate has a pervasive effect on sedimentation today and the same climatic patterns are reflected in the distribution of lithofacies through the palaeozoic as the continents migrate beneath the climatic zones the low latitude hot wet zone is represented by thick elastics coals and carbonates and is best developed along east coasts where prevailing winds bring moisture and heated surface waters toward the continent the desert zones occur on the west sides of continents centred at 20 north and south and these dry belts are represented in the geological record by evaporites tillites thick elastics and coals occur in the temperate rainy belts especially on the windward west sides of continents above 40 latitude continental accretion occurs where subduction zones coincide with rainy zones such that the products of erosion are transported to the trench and thus thrust back extending the margin of the continent the opposite process of tectonic erosion wherein the descending oceanic slab continually rasps away the margin of the continental crust may occur in areas where rainfall and surface run off is insufficient to provide trench sediments this process has been operating adjacent to the atacama desert in south america during the past 200 ma to judge by the eastward migration of the calc alkaline intrusive foci about 250 km of the margin of south america have been transported down the subduction zone during this period

development of the gulf of guayaquil ecuador during the quaternary as an effect of the north andean block tectonic escape 1 interpretation of industrial multichannel seismic profiles and well data are used to identify the main tectonic features of the gulf of guayaquil gg area these include two e w trending major detachments the posorja and the jambeli detachment systems which represent half grabens with oppositely dipping detachments to the south and to the north respectively the ne sw trending puna santa clara fault system developed as a transfer fault system between the posorja and the jambeli detachments the esperanza and the jambeli basins exhibit 3 4 km of sediment that accumulated during the past 1 6 1 8 myr the puna santa clara fault system bounds the esperanza and the jambeli basins evidencing that the evolution of these basins is tightly controlled by the two detachments at depth to the west the n s trending domito fault system bounding the posorja detachment system and the esperanza basin to the west acted as a transfer zone between the shelf area and the continental slope the pliocene series show no significant variations in thickness throughout the gulf of guayaquil area suggesting that no important tectonic deformation occurred from 5 2 to 1 8 1 6 ma the major period of tectonic deformation in the gulf of guayaquil area occurred during the pleistocene times three main tectonic steps are identified from early pleistocene to 180 ka major subsidence occurred along the esperanza and jambeli basins from 180 to 140 ka most of the gulf of guayaquil area was above sea level during the isotope substage 6 low stand from 140 ka to present tectonic activity is restricted along the normal faults bounding the esperanza basin the tenguel fault and the puna santa clara and domito fault systems a n s trending tensional stress regime characterizes the pleistocene times throughout the northward drifting of the north andean block is proposed to control the tectonic evolution and associated subsidence of the gulf of guayaquil area it is also accepted that the collision of the carnegie ridge with the trench axis has to play a major role in controlling the north andean block northward drift because the carnegie ridge subduction is possibly an ongoing process which began prior to the pliocene we postulate the along strike morphology of the ridge at the origin of interplate coupling variations the subduction of an along strike positive relief of the ridge is proposed at the origin of the major tectonic reorganization of the gg area occurring at 1 8 1 6 ma

the internal structure of oceanic plateaus inferences from obducted cretaceous terranes in western colombia and the caribbean although the structure of mantle plume derived oceanic plateaus has recently been assessed using remote geophysical techniques combined with petrological modelling it is nevertheless desirable to test whether these theoretical rock types dunites gabbros and basalts actually exist and to establish their geochemical nature oceanic plateaus may have initially formed above or near sea level during a short vigorous pulse and thereafter commonly subside to abyssal depths as the lithosphere cools thus making sampling of their deeper levels extremely difficult however the cretaceous age colombian caribbean oceanic plateau was partially accreted against the south american continent so making the imbricated segments available for study during the process of plateau accretion and imbrication it is predominantly the basaltic layers which are obducted but parts of the sequence down to layered and banded gabbros with associated pyroxenites and dunites sometimes foliated can be exposed where the imbricate thrusting brings up deeper levels most of the upper crustal sequence in western colombia is composed of basaltic pillowed and massive flows and sills that are chemically uniform and undepleted relative to normal mid ocean ridge basalts komatiites and more abundant picrites are found at intervals and appear to occur near the base of the sequence in these zones both depleted and moderately enriched basalt and komatiite compositions occur and may result from dynamic partial melting and mixing processes associated with the high temperature part of the plume it is possible to integrate these compositional characteristics into a general model for oceanic plateau structure where the rate of magma supply is in excess of that which can be accommodated by normal spreading processes thus leading to extrusion of flows and the emplacement of sills and high level magma chambers but relatively few dykes the dense and chemically heterogeneous ultramafic magmas intrude the base of the pile and undergo fractionation to form ultramafic cumulates whereas the well mixed basaltic magmas are erupted to form the homogeneous plateau basalts based on these observations a possible structure of the caribbean colombian plateau is proposed which is compatible with geophysical models for other less well exposed oceanic plateaus

the geological evolution of the tibetan plateau the geological evolution of the tibetan plateau is best viewed in a context broader than the india eurasia collision zone after collision about 50 million years ago crust was shortened in western and central tibet while large fragments of lithosphere moved from the collision zone toward areas of trench rollback in the western pacific and indonesia cessation of rapid pacific trench migration 15 to 20 million years ago coincided with a slowing of fragment extrusion beyond the plateau and probably contributed to the onset of rapid surface uplift and crustal thickening in eastern tibet the latter appear to result from rapid eastward flow of the deep crust probably within crustal channels imaged seismically beneath eastern tibet these events mark a transition to the modern structural system that currently accommodates deformation within tibet

ridge subduction at an erosive margin the collision zone of the nazca ridge in southern peru the 1 5 km high obliquely subducting nazca ridge and its collision zone with the peruvian margin have been imaged by wide angle and reflection seismic profiles swath bathymetry and gravity surveying these data reveal that the crust of the ridge at its northeastern tip is 17 km thick and exhibits seismic velocities and densities similar to layers 2 and 3 of typical oceanic crust the lowermost layer contributes 10 12 km to the total crustal thickness of the ridge the sedimentary cover is 300 400 m thick on most parts of the ridge but less than 100 m thick on seamounts and small volcanic ridges at the collision zone of ridge and margin the following observations indicate intense tectonic erosion related to the passage of the ridge the thin sediment layer on the ridge is completely subducted the lower continental slope is steep dipping at 9 and the continental wedge has a high taper of 18 tentative correlation of model layers with stratigraphy derived from ocean drilling program leg 112 cores suggests the presence of eocene shelf deposits near the trench continental basement is located 15 km landward of the trench normal faults on the upper slope and shelf indicate extension a comparison with the peruvian and northern chilean forearc systems currently not affected by ridge subduction suggests that the passage of the nazca ridge along the continental margin induces a temporarily limited phase of enhanced tectonic erosion superposed on a long term erosive regime

slip on an active wedge thrust from geodetic observations of the 8 october 2005 kashmir earthquake by combining global positioning system observations of surface displacements and the locations of aftershocks we infer that the 8 october 2005 kashmir earthquake occurred on multiple fault planes mean slip of 5 1 m occurred on a rupture between bagh and balakot with strike 331 and dip 29 additional slip occurred at depth on a nne dipping fault plane extending wnw from balakot and on an intersecting nearly fiat dislocation at 5 km depth forming an active wedge thrust both the simple fault plane and the blind wedge accommodate convergence between peshawar and leh ladakh accumulating at 7 2 mm yr suggesting a 680 150 yr recurrence interval for kashmir 2005 like events

morphotectonic and morphometric analysis of the nazca plate and the adjacent offshore peruvian continental slope implications for submarine landscape evolution we use new swath bathymetry data acquired during the rv sonne cruise geopeco and complement them with swath data from adjacent regions to analyse the morphotectonics of the peruvian convergent margin the nazca plate is not covered with sediments and therefore has a rough surface along the entire peruvian trench the styles of roughness differ significantly along the margin with linear morphological features trending in various directions most of them oblique to the trench and roughness magnitudes of a few to several hundred meters the lower slope is locally very rough and at the verge of failure throughout the entire peruvian margin as a result of subduction erosion causing the lower slope to over steepen using curvature attributes to quantitatively examine the morphology in the yaquina and mendana areas revealed that the latter shows a larger local roughness both seaward and landward of the trench however the amplitude of morphological roughness is larger in the yaquina area we identified a 125 km2 large slump on the lima middle slope morphometric dating suggests an age of 74500 years within 35 to 40 error estimated incision rates on the upper slope are between 0 1 and 0 3 mm per year suggesting that landscape evolution on the peruvian submarine continental slope is similarly slow than that in the atacama desert

the java margin revisited evidence for subduction erosion off java the eastern sunda margin off indonesia from central java to sumba island remains a little investigated subduction zone contrary to its well studied northwestern segment whereas large portions of the sunda margin are considered a classical accretionary zone subduction characteristics along the central java sector indicate erosive processes as the dominant mode of mass transfer the tectonic framework of the central java margin with a convergence rate of 6 7 cm yr insignificant sediment input and a pronounced seafloor roughness where the oceanic roo rise is subducting underneath java facilitates subduction erosion evidence for erosion comes from newly acquired geophysical data off central java local erosive processes in the wake of seamount subduction are documented by a high resolution bathymetric survey and result in an irregular trend of the deformation front sculpted by seamount collision scars subduction of oceanic basement relief leads to large scale uplift of the forearc as recorded on a reflection seismic profile and to a dismemberment of the previous outer forearc high giving way to isolated topographic elevations the broad retreat of the java trench and deformation front above the leading edge of the roo rise has exposed an area of approximately 25 000 km2 of deeper seafloor formerly covered by the previous frontal prism frontal erosion coincides with a steepening of the lower slope angle in the central java sector compared to the neighbouring segments in global compilations the key geological parameters of the central java margin lie in the erosive regime reflecting the interplay of basement relief subduction negligible sediment supply and a high convergence rate on the evolution of the margin

underplating in the himalaya tibet collision zone revealed by the hi climb experiment we studied the formation of the himalayan mountain range and the tibetan plateau by investigating their lithospheric structure using an 800 kilometer long densely spaced seismic array we have constructed an image of the crust and upper mantle beneath the himalayas and the southern tibetan plateau the image reveals in a continuous fashion the main himalayan thrust fault as it extends from a shallow depth under nepal to the mid crust under southern tibet indian crust can be traced to 31 n the crust mantle interface beneath tibet is anisotropic indicating shearing during its formation the dipping mantle fabric suggests that the indian mantle is subducting in a diffuse fashion along several evolving subparallel structures

anatomy and global context of the andes main geologic features and the andean orogenic cycle the andes make up the largest orogenic system developed by subduction of oceanic crust along a continental margin subduction began soon after the breakup of rodinia in late proterozoic times and since that time it has been intermittently active up to the present the evolution of the pacifi c margin of south america during the paleozoic occurred in the following stages 1 initial proterozoic rifting followed by subduction and fi nal re amalgamation of the margin in early cambrian times as depicted by the puncoviscana and tucavaca basins and related granitoids in southern bolivia and northern argentina 2 a later phase of rifting in the middle cambrian and subsequent collisions in middle ordovician times of parautochthonous terranes derived from gondwana such as paracas arequipa and antofalla and exotic terranes originating in laurentia such as cuyania chilenia and chibcha 3 fi nal permian collision between south america and north america to form pangea during the alleghanides orogeny leaving behind rifted pieces of laurentia as the tahami and tahuin terranes in the northern andes and other poorly known orthogneisses in the cordillera real of ecuador in the late permian early triassic and 4 amalgamation of the mejillonia and patagonia terranes in early permian times representing the last convergence episodes recorded in the margin during the gondwanides orogeny these rifting episodes and subsequent collisions along the continental margin were the result of changes of the absolute motion of gondwana related to global plate reorganizations during proterozoic to paleozoic times generalized rifting during pangea breakup in the triassic concentrated extension in the hanging wall of the sutures that amalgamated the paleozoic terranes the opening of the indian ocean in early jurassic times was associated with a new phase of subduction along the continental margin the northeastward absolute motion of western gondwana produced a negative trench roll back velocity that controlled subduction under an extensional regime until late early cretaceous times the northern andes of venezuela colombia and ecuador record a series of collisions of island arcs and oceanic plateaus from the early cretaceous to the middle miocene as a result of interaction with the caribbean plate the remaining central and southern andes record periods of orogenesis and mountain building alternating with periods of quiescence and absence of deformation as recorded in parts of the oligocene based on the generalized occurrence of fl at slab subduction episodes through time as recorded in most ramos v a 2009 anatomy and global context of the andes main geologic features and the andean orogenic cycle in kay s m ramos v a and dickinson w r eds backbone of the americas shallow subduction plateau uplift and ridge and terrane collision geological society of america memoir 204 p 31 65 doi 10 1130 2009 1204 02 for permission to copy contact editing geosociety org 2009 the geological society of america all rights reserved

low latitude sea surface temperatures for the mid cretaceous and the evolution of planktic foraminifera isotopic data from exceptionally well preserved foraminifera show that the subtropical north atlantic had sea surface temperatures ssts between 30 and 31 c during the late albian and early cenomanian these temperatures were warmer than average modern temperatures in the tropical western atlantic and support previous inferences based on diagenetically suspect material of albian tropical ssts of 30 c this finding is consistent with basic theories of oceanic thermal response in the tropics to the high atmospheric carbon dioxide levels during the middle cretaceous greenhouse however our data do not support the concept of a super tethyan region of hypersaline waters in the western north atlantic during late albian and early cenomanian time because seawater salinities substantially higher than modern ones would require significantly higher ssts than those reported here isotopic analysis of mid cretaceous planktic foraminifera shows that planomalina ticinella and biticinella grew in surface waters and provide isotopic proxies for sst whereas deeper growing species include hedbergella rotalipora and praeglobotruncana none of these taxa display isotopic indications of photosymbiosis suggesting that this ecology did not evolve in planktic foraminifera before the late cretaceous contrary to previous hypotheses none of these species increased their depth habitat during their life cycles unlike many extant planktic foraminifera the rotaliporids may have been preferentially susceptible to extinction during oceanic anoxic events because they lived within or below the thermocline for most of their life cycles

unusual mantle poisson s ratio subduction and crustal structure in central alaska 1 subduction zones alter the upper plate in a variety of ways including metasomatism via slab derived fluids and accretion of exotic terranes these processes should produce distinctive seismic signatures as seismic velocities are sensitive to compositional variations a new method is developed to estimates depths to interfaces and poisson s ratio vp vs ratio for dipping layered structures by stacking teleseismic receiver functions it also estimates crustal vp vs and thickness using this method we analyze a broadband data set from the central alaska subduction zone to seek evidence for mantle wedge composition crustal thickness varies from 27 to 45 km and crustal vp vs indicates an intermediate to mafic composition expected for these accreted island arc terranes inversions for mantle structure confirm the presence of a 15 20 km thick low velocity zone atop of the downgoing plate to 130 km depth perhaps subducted crust of an exotic terrane vp vs in the mantle wedge allows for 15 15 serpentinization where the slab is 80 km deep and the wedge is cold where the slab is deeper vp vs for the mantle wedge is unusually low 1 7 this value is lower than predicted for any common mantle mineral and may indicate significant quantities of quartz provided that past physical properties measurements are not grossly in error these results suggest that the mantle wedge in subduction zones can occasionally differ substantially in bulk composition from normal subcrustal mantle perhaps because collisions of exotic terranes cause tectonic mixing of crustal material into the mantle

subduction and exhumation of the pelona orocopia rand schists southern california the pelona orocopia and rand schists of southern california and southwestern arizona are thought to have formed in either the same east dipping subduction zone as the franciscan complex or in a southwest dipping subduction zone related to collision of an outboard continental fragment with north america the principal justification for the collision model has been the observation that continental rocks overlying the schists locally show transport to the northeast field and petrographic studies now confirm that the northeast movement occurred during exhumation of the schist not during subduction combined with regional geologic relations this evidence makes the collision model unlikely

ghosts of lithospheres past imaging an evolving lithospheric mantle in southern africa group ii 143 117 ma and group i kimberlites 108 74 ma intrude across the southwest boundary of the kaapvaal craton sampling the same volume of the subcontinental lithospheric mantle sclm in two time slices major and trace element analyses of 3699 peridotitic garnet xenocrysts were used to construct paleogeotherms for 17 kimberlite localities and to place each garnet at its depth of origin the ti contents of each garnet and the calculated x mg of its coexisting olivine were projected onto a southwest northeast section across the craton boundary and splines were used to interpolate between the virtual boreholes in each age group the sections show that the cratonic sclm extends at least 75 km southwest of the mapped craton boundary suggesting a dipping contact marked differences between the time slice sections show that between 117 and 108 ma the sclm on both sides of the craton boundary was heated and chemically refertilized by infiltrating asthenosphere derived melts thinning the depleted layer by 40 km the thermal and geochemical changes record a significant tectonothermal event that may be related to changes in the stress field associated with opening of the south atlantic or with mantle upwelling

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

evidence for precollisional subduction erosion in ancient collisional belts the case of the mid european variscides analysis of the fore arc remnants and suture zone rocks in the european variscides using criteria established for the circum pacific margins geometry sedimentary evolution and style of deformation shows that precollisonal orogenic evolution can also be reconstructed for ancient collisional belts the collision zone of the lower plate thrust belt rhenohercynian zone and the upper plate arc and fore arc remnants saxothuringian zone and saar basin indicate subduction erosion of the upper plate by some 50 km length of upper plate lithosphere prior to collision removal started during early convergence in the famennian ca 365 ma and lasted until the late visean onset of collision ca 330 ma part of this material may have been underplated below the upper plate and has since resurfaced as high pressure rocks contributing to the metamorphic core of the orogen

subcrustal tectonic erosion in orogenic belts a model for the late cretaceous subsidence of the northern calcareous alps austria analysis of the upper cretaceous to eocene gosau group of the northern calcareous alps austria indicates rapid subsidence after a short interval of deformation uplift and erosion reconstructions of basement subsidence of 3000 m from the late turonian 89 ma onward show a distinct shift of the deformation and subsidence from the northwest to the southeast together with a tilting of the northern calcareous alps toward the north comparison with recent analogues suggests that subcrustal tectonic erosion due to the subduction of a topographic high of the south penninic ocean may be the cause for this subsidence pattern tectonic erosion along ancient plate margins can be recognized by deformation and widespread erosion followed by regional subsidence not related to extension or flexural loading

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

seamount subduction erosion in the nankai trough and its potential impact on the seismogenic zone seamount subduction along subduction zone plate boundary thrusts has long been implicated as a mechanism for abrasion and tectonic erosion of the base of the overriding plate however tectonic erosion processes have not been examined in detail with high quality three dimensional 3 d seismic reflection imaging in 1999 we acquired 3 d seismic reflection data from the nankai trough subduction zone to image the plate boundary fault and the overlying accretionary wedge structure fortuitously these data reveal a small to 1 km high basement ridge that has subducted to 7 km subseafloor updip from the basement ridge a 1 km thick sequence of sediment from the base of the accretionary wedge appears to be missing we interpret these data as evidence for tectonic erosion of the base of the accretionary wedge following the basement ridge subduction tectonic erosion has removed more than 25 km 3 from the updip edge of the seismogenic zone and carried it down into the seismogenic zone the tectonically eroded sediments are presumed to enhance fault zone fluid content potentially reducing fault zone effective stress and may temporarily inhibit earthquake rupture potential after the passage of the ridge the boundary fault returns to its former position in a period of enhanced underplating

weak interplate coupling by seamounts and repeating m 7 earthquakes subducting seamounts are thought to increase the normal stress between subducting and overriding plates however recent seismic surveys and laboratory experiments suggest that interplate coupling is weak a seismic survey in the japan trench shows that a large seamount is being subducted near a region of repeating earthquakes of magnitude m 7 both observed seismicity and the pattern of rupture propagation during the 1982 m 7 0 event imply that interplate coupling was weak over the seamount a large rupture area with small slip occurred in front of the seamount its northern bound could be determined by a trace of multiple subducted seamounts whereas a subducted seamount itself may not define the rupture area its width may be influenced by that of the seamount

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

megathrust earthquakes can nucleate in the forearc mantle evidence from the 2004 sumatra event current models predict that the seismogenic zone along subduction thrusts where the largest earthquakes nucleate and propagate does not extend to the forearc mantle below the crust of the upper plate stable sliding conditions have been shown to prevail there particularly along several circum pacific margins that underwent great megathrust earthquakes mw 8 5 during the twentieth century based on geophysical investigation we show that the great 2004 sumatra andaman earthquake mw 9 9 3 contradicts these models not only did it propagate downdip along the interface between the forearc mantle and the subducting plate but it actually nucleated along this reportedly aseismic part of the interplate contact petrological models can therefore underestimate the downdip extent of rupture zones to be expected in megathrust earthquakes and need to be revised to account for this observation albeit unusual

subduction of a late cretaceous seamount of the louisville ridge at the tonga trench a model of normal and accelerated tectonic erosion the hotspot generated louisville ridge is a 4000 km chain of seamounts typically 2 2 5 km high and 10 40 km in diameter and an underlying crustal swell 1 5 km high and 100 km wide trending nnw across the southwestern pacific the northwest end of the ridge collides with the north trending tonga trench 26 s which just north of that point is exceptionally deep 10 8 km and lacks both a turbidite wedge and a bordering accretionary complex the collision zone is moving rapidly southward multichannel seismic reflection data in the collision zone show a west dipping subsurface platform 2 3 km beneath the lower western trench slope which is interpreted as the flat summit of a subducted guyot motuku of the louisville chain projected eastward the summit plain passes 1 2 km above the trench floor dredging of the nearby inner trench wall recovered uppermost cretaceous maestrichtian oceanic pelagic sediments interpreted to be fragments of the sedimentary mantle of a subducted louisville seamount the principal effects of hotspot ridge collision with a sediment starved trench are 1 the impacting seamounts are subducted rather than accreted and 2 although some seamount rocks are temporarily accreted the inner trench wall is tectonically eroded arcward at rates possibly as high as 50 km m y accelerated tectonic erosion is related to 1 fracturing shearing and general weakening of arc substrate rocks as they are lifted by the swell penetrated by impacting seamounts and left to collapse as the ridge moves away 2 a more effective removal of weakened rock in underthrusting grabens which are larger in the crustal swell 3 a possible elevation of the subduction decollement to account for the removal of as much as 30 000 km of material from a 400 km sector of the trench in 1 million years and 4 a reduction in supply of arc derived debris resulting from the gap in arc volcanism accompanying subduction of the ridge normal tectonic erosion in the tonga trench is apparently minor and we conclude that the bulk of the 37 000 km of material which fills subducting grabens each million years is arc derived volcanic and pelagic sediment