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

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

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

the lost inca plateau cause of flat subduction beneath peru abstract since flat subduction of the nazca plate beneath peru was first recognized in the 1970s and 1980s a satisfactory explanation has eluded researchers we present evidence that a lost oceanic plateau inca plateau has subducted beneath northern peru and propose that the combined buoyancy of inca plateau and nazca ridge in southern peru supports a 1500 km long segment of the downgoing slab and shuts off arc volcanism this conclusion is based on an analysis of the seismicity of the subducting nazca plate the structure and geochemistry of the marquesas plateau as well as tectonic reconstructions of the pacific farallon spreading center 34 to 43 ma these restore three sub parallel pacific oceanic plateaus the austral tuamotu and marquesas to two farallon plate counterparts the iquique and nazca ridges inca plateau is apparently the sixth and missing piece in an ensemble of v shaped hotspot tracks formed at on axis positions we argue the mirror image of the inca plateau the marquesas plateau is an ancient edifice overprinted by recent volcanism in disagreement with the widely accepted young

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

sediment accretion against a buttress beneath the peruvian continental margin at 12 s as simulated with sandbox modeling reflection seismic data from the peruvian continental margin at 12 s clearly reveal an accretionary wedge and buttress sandbox experiments applying the physical concept of the coulomb theory allow the systematic investigation of the growth and deformation of such an accretionary structure the style of deformation of the buttress and the internal structure of the wedge is observed in the sandbox models the possibility of underplating material beneath the buttress and the amount of tectonic erosion depend on the physical properties of the materials mainly internal friction cohesion and basal friction boundary conditions such as the height of the subduction gate and the thickness of incoming sand also constrain the style of growth of the model accretionary structure the configurations of two experiments were closely scaled to reflection seismic depth sections across the peruvian margin a deformable buttress constructed of compacted rock powder is introduced to replicate the basement rock which allows deformation similar to that in the seismic data with the sandbox models it is possible to verify a proposed accretionary history derived from seismic and borehole data the models also help in understanding the mechanisms which control the amount of accretion subduction and underplating as a function of physical properties boundary conditions and the duration of convergence

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

vertical tectonics and the origins of bsrs along the peru margin abstract the distribution of bottom simulating reflections bsrs which mark the base of the gas hydrate zone on marine seismic records across the peru margin is examined with respect to tectonic uplift sedimentation and carbon content of sediment where accretion of only deep ocean pelagic sediment containing less than 1 carbon is seismically imaged no bsr is observed no bsr is apparent on seismic records of carbon rich 4 8 sediment of rapidly subsiding basins bsrs are abundant on records of carbon rich sediment affected by tectonic uplift and or rapid sedimentation in areas of uplift the bottom simulating reflectors contain small amounts of free gas in contact with sediment containing small amounts of gas hydrate during uplift free gas is released from gas hydrates by dissociation as the hydrate phase boundary shifts upward with respect to sea level the strong effects of free gas in reducing seismic velocity and gas hydrate in increasing velocity can explain the relatively small total quantities of gas and hydrate indicated by observations of bsrs off peru

the petroleum geology of the unst basin north sea abstract the unst basin is situated in the northern north sea between the east shetland basin and the shetland isles the basin is essentially a three armed permo triassic fault controlled basin containing up to 3600 m of red beds this is overlain by a westerly thickening jurassic and early cretaceous sequence the stratigraphy of which is very similar to that of the east shetland basin in particular the brent group 140 m humber group 685 m and cromer knoll group 300 m are well represented as a result of laramide uplift of the area the thick upper cretaceous and palaeocene strata of the east shetland basin are absent from the unst basin this uplift resulted in substantial erosion within the unst basin providing the major source for palaeocene sands in the viking graben and the faeroes basin late palaeocene and younger tertiary strata transgress westwards across this erosion surface petroleum exploration within the basin culminated in the drilling of two exploration wells these wells encountered potential reservoir and source rocks in the jurassic section however geochemical analyses indicate these source rocks are immature for hydrocarbon generation within the unst basin it is concluded that the unst basin has a low petroleum potential

horizontal shortening versus vertical loading in accretionary prisms 1 the analysis of two phased deformation of subducted turbidites within the shimanto belt on kyushu japan shows strongly contrasted kinematics between an earlier noncoxial fold and thrust type stage and a later metamorphic coaxial vertical shortening stage the deep deformation is characterized by the development of a subhorizontal muscovite chlorite metamorphic foliation associated with pure shear deformation which overprinted the structures inherited from the earlier stage we associate the metamorphic deformation stage with the lower levels of accretionary prism landward segment inner wedge which was therefore deformed by vertical loading and horizontal extension much in contrast with prism trenchward segment outer wedge which often shows fold and thrust structures indicative of horizontal shortening a possible explanation based on wang and hu s 2006 model for these longitudinal variations in deformation kinematics of the prism may lie in the contrasted rheology of its basal surface strain weakening in the seismic portion below the inner wedge while strain strengthening in the aseismic portion below the outer wedge the vertical shortening active within the inner wedge revealed by this study possibly represents a large contribution to the exhumation of the deeper levels of accretionary prisms

the migration history of the nazca ridge along the peruvian active margin a re evaluation abstract the collision zone of the 200 km wide and 1 5 km high nazca ridge and the peruvian segment of the convergent south american margin between 14 s and 17 s is characterized by deformation of the upper plate and several hundred meters of uplift of the forearc this is evident by a narrowing of the shelf a westward shift of the coastline and the presence of marine terraces as the nazca ridge is oblique with respect to both trench and convergence direction of the nazca plate it migrates southward along the active plate boundary for reconstructing the migration history of the nazca ridge this study uses updated plate motion data resulting from a revision of the geomagnetic time scale the new model suggests that the ridge crest moved laterally parallel to the margin at a decreasing velocity of 75 mm a before 10 8 ma 61 mm a 10 8 4 9 ma and 43 mm a 4 9 ma to present intra plate deformation associated with mountain building in the peruvian andes since the miocene reduces the relative convergence rate between nazca plate and peruvian forearc taking an intra plate deformation at a rate of 10 mm a estimated from space geodetic and geological data into account does not significantly reduce these lateral migration velocities constraining the length of the original nazca ridge by its conjugate feature on the pacific plate yields a length of 900 km for the subducted portion of the ridge using this constraint ridge subduction began 11 2 ma ago at 11 s therefore the nazca ridge did not affect the northern sites of ocean drilling program odp leg 112 located at 9 s this is supported by benthic foraminiferal assemblages in odp leg 112 cores indicating more than 1000 m of subsidence since at least middle miocene time and by continuous shale deposition on the shelf from 18 to 7 ma recorded in the ballena industrial well at 11 5 s the model predicts the passage of the ridge crest 9 5 ma ago this agrees with the sedimentary facies and benthic foraminiferal stratigraphy of odp leg 112 cores which argue for deposition on the shelf in the middle and late miocene with subsequent subsidence of a minimum of several hundred meters onshore at 12 s the sedimentary record shows at least 500 m uplift prior to the end of the miocene also in agreement with the model

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

subduction of the nazca ridge and the inca plateau insights into the formation of ore deposits in peru a large number of ore deposits that formed in the peruvian andes during the miocene 15 5 ma are related to the subduction of the nazea plate beneath the south american plate here we show that the spatial and temporal distribution of these deposits correspond with the arrival of relatively buoyant topographic anomalies namely the nazca ridge in central peru and the now consumed inca plateau in northern peru at the subduction zone plate reconstruction shows a rapid metallogenic response to the arrival of the topographic anomalies at the subduction trench this is indicated by clusters of ore deposits situated within the proximity of the laterally migrating zones of ridge subduction it is accordingly suggested that tectonic changes associated with impingement of the aseismic ridge into the subduction zone may trigger the formation of ore deposits in metallogenically fertile suprasubduction environments c 2005 elsevier b v all rights reserved

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

mechanical decoupling and basal duplex formation observed in sandbox experiments with application to the western mediterranean ridge accretionary complex sandbox experiments of accretionary wedges were performed incorporating a thin weak layer of micro glass beads the impact of heterogeneous sedimentary input on wedge mechanics evolution and mass transfer was investigated we report the first experimentally documented growth of basal duplexes these occurred for high basal friction conditions with restricted output of the lower section the upper and lower sections were completely decoupled due to the intervening layer of glass beads with frontal accretion occurring in the upper section simultaneously with basal duplex formation and underplating of subsequent generations of duplexes imerse multichannel seismic reflection data from the western mediterranean ridge wmr image tertiary clastics beneath a thick section of messinian evaporites the base of the evaporites is identified as the primary decollement for deformation in the frontal part of the accretionary complex constriction of the channel of subducting tertiary sediments as well as internal deformation observed as arcward dipping reflectors argue for basal underplating and or two different active decollements we propose an evolution of the wmr in accordance with the sandbox experimental results a weak mid level detachment base of evaporites combined with a strong basal detachment produce mechanical decoupling and basal accretion of toeward verging duplexes

coulomb theory applied to accretionary and nonaccretionary wedges possible causes for tectonic erosion and or frontal accretion based on observations from both modem convergent margins and sandbox modeling we examine the possible conditions favoring frontal accretion and or frontal and basal tectonic erosion mean characteristic parameters t t and are used to discuss the mechanical stability of 28 transects across the frontal part of convergent margins where the coulomb theory is applicable natural observations reveal that typical accretionary wedges are characterized by low tapers with smooth surface slope and subducting plate low convergence rates and thick trench sediment while nonaccretionary wedges display large tapers with irregular surface slopes and rough subducting plate high convergence rates and almost no trench fill sandbox experiments were performed to illustrate the effects of seamounts ridges in the subduction zone on the deformation of an accretionary wedge these experiments show that a wedge of sand is first trapped and pushed in front of the seamount which acts as a moving bulldozer this is followed by a tunnelling effect of the subducting seamount through the frontal wedge material which results in considerable sand reworking at an advanced subduction stage the d6collement jumps back from a high level in the wedge to its former basal position we conclude that a high trench sedimentation rate relative to the convergence rate leads to frontal accretion in contrast several conditions may favor tectonic erosion of the upper plate first oceanic features such as grabens seamounts or ridges may trap upper plate material during their subduction process second destabilization of the upper plate material by internal fluid overpressuring causing hydrofracturing is probably another important mechanism

velocity structure of a bottom simulating reflector offshore peru results from full waveform inversion abstract much of our knowledge of the worldwide distribution of submarine gas hydrates comes from seismic observations of bottom simulating reflectors bsrs full waveform inversion has proven to be a reliable technique for studying the fine structure of bsrs using the compressional wave velocity we applied a non linear full waveform inversion technique to a bsr at a location offshore peru we first determined the large scale features of seismic velocity variations using a statistical inversion technique to maximise coherent energy along travel time curves these velocities were used for a starting velocity model for the full waveform inversion which yielded a detailed velocity depth model in the vicinity of the bsr we found that the data are best fit by a model in which the bsr consists of a thin low velocity layer the compressional wave velocity drops from 2 15 km s down to an average of 1 70 km s in an 18 m thick interval with a minimum velocity of 1 62 km s in a 6 m interval the resulting compressional wave velocity was used to estimate gas content in the sediments our results suggest that the low velocity layer is a 6 18 m thick zone containing a few percent of free gas in the pore space the presence of the bsr coincides with a region of vertical uplift therefore we suggest that gas at this bsr is formed by a dissociation of hydrates at the base of the hydrate stability zone due to uplift and subsequently a decrease in pressure

mechanisms of subduction accretion along the central aleutian trench we have used migrated 24 fold seismic records to study mechanisms of subduction accretion along the central aleutian trench two major structural units underlie the landward trench slope and are separated by an acoustically defined decollement surface that can be traced at least 20 km landward of the trench axis the upper unit consists of three to four structural blocks of deformed trench fill each block possesses a consistent internal structure and is bound by major landward dipping thrust faults the structures within the blocks include monoclinal sequences antiforms synforms and faulted folds beneath the decollement nearly undeformed trench fill 1 km thick is being under thrust along with the underlying oceanic lithosphere this basal section is an intervening unit between the offscraped sediment and the rough igneous oceanic basement and may prevent the oceanic crust from being incorporated into the subduction complex at least within 20 km of the front of the subduction zone owing to bending stresses the oceanic crust fails along normal faults beneath and at least 15 km landward of the trench axis these normal faults although poorly constrained are believed to propagate upwards producing offsets within both the subducting and the accreting sediment in addition normal faulting enhances the irregular surface of the oceanic crust structural associations imaged on all of the seismic reflection profiles although possibly fortuitous suggest that the subduction of these irregular basement highs results in the temporary displacement of the overlying fault bound sediment packets and promotes the development of the structures found within the accretionary prism tectonic riffling of the accreted sediment by the passage of the basement relief may also provide a mechanism by which underthrusting sediments are decoupled from the subducting lithosphere and underplated to the base of the aleutian accretionary complex

crustal structure beneath the central oregon convergent margin from potential field modeling evidence for a buried basement ridge in local contact with a seaward dipping backstop models of magnetic and gravity anomalies along two e w transects offshore central oregon one of which is coincident with a detailed velocity model provide quantitative limits on the structure of the subducting oceanic crust and the crystalline backstop the models indicate that the backstop forming western edge of the siletz terrane an oceanic plateau that was accreted to north america 50 million years ago has a seaward dip of less than 60o3 seismic magnetic and gravity data are compatible with no more than 2 km of subducted sediments between the siletz terrane and the underlying crystalline crust of the juan de fuca plate the data also suggest the presence of a n s trending 200 km long basaltic ridge buried beneath the accretionary com plex from about 43on to 45on although the height and width of this ridge probably vary along strike it may be up to 4 km high and several kilometers wide in places and appears to be locally in contact with the siletz terrane beneath heceta bank several models for the origin of this ridge are discussed these include a sliver of siletz terrane detached from the main siletz terrane dur ing a late eocene episode of strike slip faulting imbrication and thickening of subducted oceanic crust in place an aseismic ridge rafted in on the subducting oceanic crust during the past 1 2 mil lion years and a series of ridges and or seamounts rafted in over a longer period of time and trans ferred from the subducting plate to the overlying plate the last model is the most consistent with the complicated history of local uplift subsidence and slope instability recorded in the ridges basins and banks of this part of the margin we speculate that the massive seaward dip ping western edge of the siletz terrane in this region inhibits subduction of seamounts and sedi ments resulting in fomation of buried ridge as the accumulated flotsam and jetsom of subduction this process may also be responsible for thickening of lower accretionary complex material over steepening of slopes leading to massive slumping and north south extension through strike slip faulting in the accretionary complex to the west of the buried ridge regardless of its origin the ridge may currently be acting as an asperity inhibiting subduction

habitat heterogeneity of hadal trenches considerations and implications for future studies abstract the hadal zone largely comprises a series of subduction trenches that do not form part of the continental shelf slope rise to abyssal plain continuum instead they form geographically isolated clusters of deep sea 6000 11 000 m water depth environments there is a growing realization in hadal science that ecological patterns and processes are not driven solely by responses to hydrostatic pressure with comparable levels of habitat heterogeneity as observed in other marine biozones furthermore this heterogeneity can be expressed at multiple scales from inter trench levels degrees of geographical isolation and biochemical province to intra trench levels variation between trench flanks and axis topographical features within the trench interior sedimentary basins ridges escarpments deeps seamounts to the substrate of the trench floor seabed sediment composition mass movement deposits bedrock outcrop using best available bathymetry data combined with the largest lander derived imaging dataset that spans the full depth range of three hadal trenches including adjacent slopes the mariana kermadec and new hebrides trenches the topographic variability fine scale habitat heterogeneity and distribution of seabed sediments of these three trenches have been assessed for the first time as well as serving as the first descriptive study of habitat heterogeneity at hadal depths this study also provides guidance for future hadal sampling campaigns taking into account geographic isolation total trench particulate organic matter flux maximum water depth and area

plate reconstructions aseismic ridges and low angle subduction beneath the andes low angle subduction segments beneath peru and chile are believed to represent interaction of the subduction zone with the aseismic nazca and juan fernandez ridges on the basis of symmetric sea floor spreading models and plate hotspot reconstructions predicted continuations of the relatively buoyant ridges correspond well with the limits of the low angle subduction segments further the history of interaction of the aseismic ridges with the subduction zone as predicted by plate reconstructions is not inconsistent with available information on volcanic episodes of the andes if the contemporary correlation of volcanic gaps and low angle subduction is also applicable to the rest of the late cenozoic the occurrence of deep seismicity beneath the eastern flanks of the andes is also believed to reflect the effects of low angle subduction the deep seismic segments are adjacent to segments which are presently or were recently experiencing low angle subduction thus the deep seismicity is interpreted as representative of remnants of normal oceanic lithosphere subducted before interaction of aseismic ridges with the subduction zone other geologic phenomena are associated with low angle subduction beneath south america including landward shift in loci of tectonism surficial subsidence and laramide style deformation the aseismic ridge buoyancy hypothesis appears to be a valuable predictive tool for the interpretation of such geologic effects

the fixed hotspot hypothesis and origin of the easter sala y gomez nazca trace reconstructions of the pacific and nazca plates suggest that both the nazca and tuamotu ridges originated from a melting anomaly which existed beneath the pacific farallon ridge during the time interval between anomalies 19 and 11 as would be predicted from the fixed hotspot hypothesis the hotspot hypothesis interpreted according to morphology of the easter sala y gomez nazca trace would further suggest that the hotspot was located entirely beneath the nazca plate during the time interval between anomalies 11 and 2 at the end of this interval the eastern boundary of the easter plate formed centered above the hotspot according to this scheme the present location of the hotspot is about 200 km west of easter island however the easter model is incompatible with the hotspot model for the hawiian emperior chain if the hotspots are fixed relative to one another as the age of the point on the tuamotu ridge which corresponds with the hawaiian emperor bend 53 to 55 m y is significantly greater than the age of the bend 43 m y the fixed hostpot hypothesis can be satisfied by the two traces only if one or a combination of the following conditions is met the kinematic age of the hawaiian emperor bend is substantially greater than the isotopically dated age the magnetic anomaly time scale is in significant error and or magnetic anomaly identifications in the vicinity of the tuamotu ridge are erroneous it might appear that other mechanisms for the origin of hotspot traces are preferable to the hotspot hypothesis the most promising of these involves propagating fractures resulting from intraplate stress this hypothesis has the advantage of accommodating anomalies in some hotspot traces such as persistence of igneous activity long after passage of the trace over the hotspot

geochronology of pediment surfaces in southern peru implications for quaternary deformation of the andean forearc abstract the geomorphology of the andean forearc has historically been viewed as an old remnant of a late miocene planar landscape with no significant active structures accommodating quaternary deformation however the well preserved sequence of planation surfaces and strath terraces developed within the forearc of southern peru provide evidence of recent uplift along the western margin previously the abandonment of these surfaces was attributed to uplift of late miocene age however abrupt changes in topography and drainage incision within the pediment surfaces indicate recent deformation we use in situ produced 10be to determine the exposure ages of these abandoned surfaces in order to derive the spatial and temporal extent of this tectonic activity our new results in contrast to previous work yield pediment surface ages of 119 ka 203 ka 278 ka 549 ka and 1003 ka indicating a youthful morphology additionally these surfaces are affected by steeply dipping active faults producing localized deformation which can be quantified incision rates based on the exposure ages of abandoned strath surfaces are on the order of 0 04 0 3 mm yr the data set presented here not only suggests significant active deformation within the forearc but also highlights a sharp contrast between the style of deformation observed on the eastern and western margins of the andes while the general notion is that active deformation is localized in the subandean fold and thrust belt our data support an emerging view where active deformation is occurring in the western margin as well

andean coastal uplift and active tectonics in southern peru 10be surface exposure dating of differentially uplifted marine terrace sequences san juan de marcona 15 4 s article i nfo along the san juan de marcona bay of southern peru two spectacular sequences of preserved marine terraces record net quaternary uplift previous geomorphic analysis of these paleo shorelines has revealed evidence of upper plate deformation and regional uplift however in the absence of a robust absolute dating method these studies contain substantial uncertainties concerning the numerical dating of these marine markers and thus the corresponding calculated surface uplift rates however field mapping surveying the neotectonic features and 10 be dating of abraded surfaces contained within two sequences of marine terraces along this margin should allow for the robust calculation of pleistocene uplift rates the san juan de marcona bay lies on the southern flank of the subducting and south migrating aseismic nazca ridge where the maximum rates of coastal uplift are expected in this locality we measure high uplift rates ranging from 0 4 m ka to 0 9 m ka during the pleistocene margin parallel normal faults displace several marine terraces and influence the development of bays thereby contributing to the configuration of paleo and present day coastlines the faults have relatively low slip rates b0 1 m ka over 400 ka and have been inactive for the last 80 ka the presence and activity of these normal faults can be directly linked to subduction zone processes with the release and accommodation of short term coseismic compression during megathrust subduction zone events in contrast the regional permanent uplift is probably controlled by post seismic and or interseismic strain accumulation over longer time scales due to inelastic behavior of the upper plate since at least the latest pliocene the san juan de marcona area has experienced long term regional tectonic uplift that has increased since about 800 ka due to the southward migration of the subducting nazca ridge based on migration velocity and geometry of the ridge the influence of the nazca ridge on the uplift of the forearc should account for 0 4 m ka within 145 km south of the ridge axis hence most of the post 400 ka mean uplift rate of the san juan de marcona area could be explained by the nazca ridge subduction

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

drop size distributions and radar observations of convective and stratiform rain over the equatorial indian and west pacific oceans abstracttwo dimensional video disdrometer 2dvd data were analyzed from two equatorial indian gan and west pacific ocean manus islands where precipitation is primarily organized by the intertropical convergence zone and the madden julian oscillation mjo the 18 3 5 months of 2dvd data from manus gan island show that 1 the two sites have similar drop size distribution dsd spectra of liquid water content median diameter rain rate r radar reflectivity z normalized gamma number concentration nw and other integral rain parameters 2 there is a robust nw based separation between convective c and stratiform s dsds at both sites that produces consistent separation in other parameter spaces the 2dvd data indicate an equatorial maritime average c s rainfall accumulation fraction frequency of 81 19 41 59 at these locations it is hypothesized that convective fraction and frequency estimates are slightly higher than previous radar based studies because the ubiquitous weak shallow convec

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

great earthquakes and subduction along the peru trench subduction along the peru trench between 9 and 15 s involves both large interplate underthrusting earthquakes and intraplate normal fault earthquakes the four largest earthquakes along the peru trench are from north to south the 1970 m 7 9 intraplate normal fault earthquake and the interplate underthrusting earthquakes in 1966 m 8 0 1940 m 8 and 1974 m 8 0 we have studied the rupture process of these earthquakes and can locate spatial concentrations of moment release through directivity analysis of source time functions deconvolved from long period p wave seismograms the 1966 earthquake has a source duration of 45 s with most of the moment release concentrated near the epicenter two intraplate normal fault events occurred in 1963 m 6 7 and 7 0 at the down dip edge of the 1966 dominant asperity the 1940 earthquake is an underthrusting event with a simple source time function of 30 s duration that represents the rupture of a single asperity near the epicenter the 1974 earthquake has a source duration of 45 50s and two pulses of moment release this earthquake has a bilateral rupture with the first pulse of moment release located northwest of the epicenter and the second pulse of moment release located southeast of the epicenter both pulses of moment release occur on the northern half of the aftershock area the 1970 earthquake is one of the largest intraplate normal fault earthquakes to occur in a subduction zone and has a moment release comparable with many large underthrusting events the aftershocks for the 1970 earthquake form two distinct clusters the smaller cluster near the epicenter has focal mechanisms characterized by down dip tension but the second aftershock cluster located 80 km southeast of the epicenter has focal mechanisms characterized by down dip compression the p waves for the main shock can be modeled as two sources with different focal mechanisms and depths similar to the two clusters of aftershocks the first event has a down dip tensional focal mechanism and is followed 40 s later by a distinct second event located 80 km southeast of the epicenter with a down dip compressional focal mechanism and a somewhat shallower depth than the first event the observable directivity indicates that the second source is located at the second cluster of aftershocks that have down dip compressional focal mechanisms the occurrence of both down dip tensional and compressional focal mechanisms may be explained by extreme unbending stresses associated with the anomalous slab geometry the unusually large size of the 1970 earthquake may also be related to the subduction of the mendana fracture zone the historic earthquake record along the peru trench indicates that the previous event in 1746 was much larger than any of the three underthrusting earthquakes this century the 1746 earthquake may have ruptured the entire segment in a multiple asperity earthquake thus the mode of rupture along the peru coast has changed between successive earthquake cycles

seismic structure in southern peru evidence for a smooth contortion between flat and normal subduction of the nazca plate rapid changes in slab geometry are typically associated with fragmentation of the subducted plate however continuous curvature of the slab is also possible the transition from flat to normal subduction in southern peru is one such geometrical change the morphology of the subducted nazca plate along this transition is explored using intraslab earthquakes recorded by temporary regional seismic arrays observations of a gradual increase in slab dip coupled with a lack of any gaps or vertical offsets in the intraslab seismicity suggest warping of the slab concentrations of focal mechanisms at orientations which are indicative of slab bending are also observed along the change in slab geometry the presence of a thin ultra slow velocity layer usl atop the horizontal nazca slab is identified and located the lateral extent of this usl is coincident with the margin of the projected linear continuation of the subducting nazca ridge implying a causal relationship wherein increased hydration of the ridge results in the formation of the usl downdip waveform modelling of the 2 d structure in southern peru using a finite difference algorithm provides constraints on the velocity and geometry of the slab s seismic structure and confirms the absence of any tears in the slab the seismicity and structural evidence suggests smooth contortion of the nazca plate along the transition from flat to normal subduction the slab is estimated to have experienced 10 per cent strain in the along strike direction across this transition

structure of the nankai trough accretionary zone from multichannel seismic reflection data new multichannel seismic reflection data collected over the nankai trough image the accretionary complex in two areas the international program of ocean drilling leg 87 transect area western area and the region of upcoming ocean drilling program leg 131 eastern area the incoming shikoku basin sedimentary section consists of hemipelagic clays and thin terrigenous turbidites the basin section is overlain by a trench wedge that is 12 16 km wide and 350 750 m thick at the thrust front accretionary deformation begins in a protothrust zone that is characterized by thickening and seaward tilting of the trench wedge the zone in the western area is 4 5 km wide and is characterized by kink folds the zone in the eastern area is only 2 5 km wide and does not exhibit such folds the frontal thrusts in each area are imaged as fault plane reflections and ramp upward from within the basin hemipelagic section the overthrusting sediments form fault bend folds over these ramps thrust spacing at the toe of the slope is 1 5 2 5 km the second thrust cuts up from an inferred decollement within the shikoku basin sedimentary section in the eastern area a reflection marking the top of the basin pelagic sediment section changes from normal to reversed polarity about 6 3 km seaward of the thrust front and underlies the entire protothrust zone this reflector continues with reversed polarity under the accretionary complex and is at the level of the basal decollement the underlying basin pelagic section is apparently thrust undisturbed beneath the accretionary prism the reversal of polarity indicates a change in reflection coefficient that is due to a combination of decreasing seismic velocity and density across the interface this decrease in velocity and density may indicate that the decollement is a zone of high porosity due to fluid expulsion from deeper within the accretionary prism the reflections from the first and second thrusts are also reversed polarity possibly indicating that they also are pathways of fluid expulsion the critical wedge taper of the western area is greater than that of the eastern area an observation that is consistent with the existence of an overpressured decollement in the eastern area

active detachment faulting above the peruvian flat slab the cordillera blanca detachment fault in the peruvian andes is to our knowledge the first active detachment to be documented above a modern flat slab crustal detachment has unroofed the ca 8 ma cordillera blanca batholith now the backbone of the highest mountain range in peru large magnitude slip along the fault was thermally enhanced by emplacement of the batholith the penultimate magmatic event prior to flattening of the nazca slab however extensional models based on arc magmatism and crustal thickening alone do not adequately explain the scale or structural asymmetry of a series of young deep seated west dipping normal faults across peru here we show that the onset of detachment faulting coincided with subduction of the aseismic nazca ridge and consequent flattening of the nazca slab we propose that slab buoyancy from ridge subduction triggered extensional collapse of the prethickened continental crust and that this buoyancy drove footwall uplift that exceeds basin subsidence the west dipping asymmetry of late cenozoic extensional faults in peru may be controlled by a preexisting crustal anisotropy older thrusts and or formation of riedel like shears kinematically linked to the flat nazca slab

differential preservation in the geologic record of intraoceanic arc sedimentary and tectonic processes abstract records of ancient intraoceanic arc activity now preserved in continental suture zones are commonly used to reconstruct paleogeography and plate motion and to understand how continental crust is formed recycled and maintained through time however interpreting tectonic and sedimentary records from ancient terranes after arc continent collision is complicated by preferential preservation of evidence for some arc processes and loss of evidence for others in this synthesis we examine what is lost and what is preserved in the translation from modern processes to the ancient record of intraoceanic arcs composition of accreted arc terranes differs as a function of arc continent collision geometry forward facing collision can accrete an oceanic arc on to either a passive or an active continental margin with the arc facing the continent and colliding trench and forearc side first in a backward facing collision involving two subduction zones with similar polarity the arc collides backarc first with an active continental margin the preservation of evidence for contemporary sedimentary and tectonic arc processes in the geologic record depends greatly on how well the various parts of the arc survive collision and orogeny in each case preservation of arc terranes likely is biased towards those that were in a state of tectonic accretion for tens of millions of years before collision rather than tectonic erosion the prevalence of tectonic erosion in modern intraoceanic arcs implies that valuable records of arc processes are commonly destroyed even before the arc collides with a continent arc systems are most likely to undergo tectonic accretion shortly before forward facing collision with a continent and thus most forearc and accretionary prism material in ancient arc terranes likely is temporally biased toward the final stages of arc activity when sediment flux to the trench was greatest and tectonic accretion prevailed collision geometry and tectonic erosion vs accretion are important controls on the ultimate survival of material from the trench forearc arc massif intra arc basins and backarc basins and thus on how well an ancient arc terrane preserves evidence for tectonic processes such as subduction of aseismic ridges and seamounts oblique plate convergence and arc rifting forward facing collision involves substantial recycling melting and fractionation of continent derived material during and after collision and so produces melts rich in silica and incompatible trace elements as a result forward facing collision can drive the composition of accreted arc crust toward that of average continental crust

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

the effect of compliant prisms on subduction zone earthquakes and tsunamis abstract earthquakes generate tsunamis by coseismically deforming the seafloor and that deformation is largely controlled by the shallow rupture process therefore in order to better understand how earthquakes generate tsunamis one must consider the material structure and frictional properties of the shallowest part of the subduction zone where ruptures often encounter compliant sedimentary prisms compliant prisms have been associated with enhanced shallow slip seafloor deformation and tsunami heights particularly in the context of tsunami earthquakes to rigorously quantify the role compliant prisms play in generating tsunamis we perform a series of numerical simulations that directly couple dynamic rupture on a dipping thrust fault to the elastodynamic response of the earth and the acoustic response of the ocean gravity is included in our simulations in the context of a linearized eulerian description of the ocean which allows us to model tsunami generation and propagation including dispersion and related nonhydrostatic effects our simulations span a three dimensional parameter space of prism size prism compliance and sub prism friction specifically the rate and state parameter b a that determines velocity weakening or velocity strengthening behavior we find that compliant prisms generally slow rupture velocity and for larger prisms generate tsunamis more efficiently than subduction zones without prisms in most but not all cases larger more compliant prisms cause greater amounts of shallow slip and larger tsunamis furthermore shallow friction is also quite important in determining overall slip increasing sub prism b a enhances slip everywhere along the fault counterintuitively we find that in simulations with large prisms and velocity strengthening friction at the base of the prism increasing prism compliance reduces rather than enhances shallow slip and tsunami wave height

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

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

invited review paper seismicity along the south american subduction zone review of large earthquakes tsunamis and subduction zone complexity abstract earthquakes along the shallow south american subduction zone exhibit heterogeneous rupture characteristics going back several centuries of the earthquake record in this area this heterogeneity is manifest in several ways such as changes in rupture mode from magnitude 8 events during one century followed by smaller ones in other time periods as well as unusual tsunami events there is also an apparent interaction between earthquake rupture and subducting plate complexity in this region significant complexity exists on the subducting nazca plate including fracture zones and ridges such as the large nazca ridge several large magnitude earthquakes have occurred in the region of ridge subduction but no earthquake rupture has ruptured through these features into adjacent regions suggesting that these subducting features act to segment the margin other features such as fracture zones and variable sediment thickness on the nazca plate appear to influence earthquake behavior over a wide range of magnitude scales upper plate features such as crustal faults also lead to heterogeneous earthquake behavior here i provide an overview of seismicity along the margin since 1850 in the context of the subduction zone structures this includes great earthquakes such as the 1906 ecuador and 1960 chile events i also present results showing increased rupture complexity in moderate magnitude earthquakes that can also be linked to certain nazca plate features

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

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

shallow subduction zone earthquakes and their tsunamigenic potential summary we have examined the source spectra of all shallow subduction zone earthquakes from 1992 to 1996 with moment magnitude 7 0 or greater as well as some other interesting events in the period range 1 20 s by computing moment rate functions of teleseismic p waves after comparing the source spectral characteristics of tsunami earthquakes earthquakes that are followed by tsunamis greater than would be expected from their moment magnitude with regular events we identified a subclass of this group slow tsunami earthquakes this subclass consists of the 1992 nicaragua the 1994 java and the february 1996 peru earthquakes we found that these events have an anomalously low energy release in the 1 20 s frequency band with respect to their moment magnitude although their spectral drop ov is comparable to those of the other earthquakes from an investigation of the centroid and body wave locations it appears that most earthquakes in this study conformed to a simple model in which the earthquake nucleates in a zone of compacted and dehydrated sediments and ruptures up dip until the stable sliding friction regime of unconsolidated sediments stops the propagation sedimentstarved trenches e g near jalisco can produce very shallow slip because the fault material supports unstable sliding the slow tsunami earthquakes also ruptured up dip however their centroid is located unusually close to the trench axis the subduction zones in which these events occurred all have a small accretionary prism and a thin layer of subducting sediment ocean surveys show that in these regions the ocean floor close to the trench is highly faulted we suggest that the horst and graben structure of a rough subducting oceanic plate will cause contact zones with the overriding plate making shallow earthquake nucleation and up dip propagation to the ocean floor possible the rupture partly propagates in sediments making the earthquake source process slow two factors have to be considered in the high tsunami generating potential of these events first the slip propagates to shallow depths in low rigidity material causing great deformation and displacement of a large volume of water second the measured seismic moment may not represent the true earthquake displacement because the elastic constants of the source region are not taken into account in the standard cmt determination

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

tectonic structure across the accretionary and erosional parts of the japan trench margin structure sections across the japan trench from prestack depth migrated seismic data define the accretionary prism a wedge shaped seaward end of the continental framework and a tectonized middle slope between them the 10 km wide accretionary prism is underthrust by sediment remaining with the subducted oceanic crust structure beneath the steep middle slope more nearly resembles the adjacent continental margin than the accreted mass this deformed middle slope domain forms a buffer between the compliant accretionary wedge and a more rigid older continental framework along the plate boundary subducted strata thicken beneath the middle and upper slopes and where last imaged are four times the input thickness beneath the accretionary prism this interplate stratified layer appears to control interplate friction because the layer is imaged 45 km landward from the trench axis to 12 km depth and in that expanse few upper plate earthquakes are recorded erosion of the base of the upper plate beneath the middle continental slope is indicated by margin subsidence and material flux this erosion is concurrent with accretion at the front of the margin

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

dolomite formation within the methanogenic zone induced by tectonically driven fluids in the peru accretionary prism early diagenetic dolomite formation in methanogenic marine sediments is enigmatic because acidifi cation by co2 a by product of methanogenesis should lead to carbonate dissolution and not precipitation however petrographic relationships indicate that dolomite breccia layers with 13c values of 15 recovered from the lower slope of the peru continental margin ocean drilling program site 1230 formed deep in the methanogenic zone during tectonic activity of a decollement based on radiogenic sr isotope ratios 87sr 86sr 0 711 and positive 18o values 6 we present evidence that the dolomite breccias mainly formed from fl uids originating from deep sedimentary units within the accretionary prism where they interacted with continental crust and or siliciclastic rocks of continental affi nity due to silicate alteration and dehydration such fl uids are likely alkaline and thus have the potential to neutralize the acidifi cation imposed by the high dissolved co2 concentrations this scenario provides a potential mechanism by which dolomite formation can be induced deep in a highly active methanogenic zone

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

material transfer in accretionary wedges from analysis of a systematic series of analog experiments the structural evolution of accretionary wedges was modeled in a series of analog experiments with systematically varying initial boundary conditions the two principle parameters investigated were basal friction and thickness of sediment input relative to subducted sediment output material transfer in terms of degree and location of accretion and erosion is quantified as a function of these two parameters low basal friction produces long thin wedges by continuous frontal accretion of imbricate thrusts simultaneously substantial basal erosion may occur with little influence on the frontal dynamics the formation of an outer arc high with an accompanying forearc basin depends upon the relative amount of basal erosion the opening of the subduction window spacing of thrust faults is proportional to the layer thickness confirming the principle of self similar wedge growth from critical wedge theory high basal friction typically generates steep wedges marked by an erosional mass wasting frontal slope in the cases i o this is due to underthrusting and or subduction of the entire sedimentary section the case i o also features the late development of normal faulting and an extensional forearc basin for both low and high basal friction the material balance affects the wedge slope erosive conditions i o for low basal friction the primary modes of material transfer are frontal accretion and basal erosion for high basal friction the primary modes of material transfer are underthrusting underplating and frontal erosion applying the experimental results to active convergent margins parallels may be drawn thus the barbados ridge complex is interpreted to be a good example of a low basal friction wedge with an accretionary material balance while the japan and mariana margins provide examples of an erosive material balance

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

crustal structure of the peruvian continental margin from wide angle seismic studies keine zusammenfassung in englischer sprache vorhanden no summary in english language