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

thrust tectonics in the andean retro foreland basin of northern peru permian inheritances and petroleum implications abstract in northern peru the huallaga moyabamba maranon subandean foreland basin system results from the interaction between thin and thick skinned tectonics geophysical data and the construction of two balanced cross sections show that this structural configuration has been controlled by permian inheritances a fossilized west verging middle permian fold and thrust belt which developed during the gondwanide orogeny has been partly reactivated by the andean compression and controlled thick skinned tectonics propagation this west verging thrust system is still active and causes the crustal and damaging seismicity of the moyabamba region late permian salt which has sealed the middle permian fold and thrust belt controlled thin skinned tectonics propagation and the development of the must large overthrust of the peruvian subandean zone the fossilized and partly reactivated middle permian fold and thrust belt constitutes a new petroleum play for the exploration in the northern peruvian subandean basins sub thrust traps of the moyabamba and huallaga wedge top basins are particularly attractive but stay unexplored

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

plio quaternary vertical motions and the subduction of the nazca ridge central coast of peru abstract the coastal region of south central peru displays raised marine terraces indicating that unlike the central peruvian coast it underwent a strong uplift since the late pliocene these recent vertical movements which were probably the most rapid motion experienced along the pacific coast of south america are commonly considered to be closely related to the subduction of the aseismic nazca ridge however several questions regarding the geometry and the precise mechanism s responsible for this deformation have not yet been fully addressed the kinematic constraints of the andean convergence suggest that the deformation of the coastal area between pisco and lomas is characterized by a quite continuous sequence of uplift events progressing southwards in spite of some difficulties encountered in precisely dating the uplifted quaternary terraces and other remnants of pliocene marine platforms it is proposed that uplift rates during the late pleistocene reached maximum values of the order of 700 mm 10 3 y as against a recently proposed estimate of 470 mm 10 3 y the reconstructed finite deformation as recorded by the study area for the last 3 ma presents a nw se oriented asymmetric dome shaped pattern it is emphasized that the strongest uplift has been occurring immediately south of the inland projection of the nazca ridge and not along the ridge axis as envisioned by the previously constructed physical models

review of erosion dynamics along the major n s climatic gradient in chile and perspectives abstract chile is an elongated country running in a north south direction for more than 30 along a subduction zone its climate is progressively wetter and colder from north to south this particular geography has been used positively by a growing number of studies to better understand the relationships between erosion processes and climate land use slope tectonics volcanism etc here we review the erosion rates factors and dynamics over millennial to daily periods reported in the literature in addition 21 new catchment mean erosion rates suspended sediment and 10be are provided and previous suspended sediment derived erosion rates are updated a total of 485 local and catchment mean erosion rates are reported erosion rates vary between some of the smallest values on earth 10 5 mm a to moderate values 0 5 mm a compared to other active ranges this review highlights strong limitations concerning the quantification of local erosion factors because of uncertainties in sampling point location slope and rainfall data for the mean erosion rates e for the millennial and decennial catchments a model of the form e s 1 s 0 6 2 r with 0 3 0 8 accounts for 40 to 70 of the erosion variance confirming a primary role of slope s compared to precipitation rate r over this time scale over the long term this review points to the long 5 to 10 ma response time of rivers to surface uplift in north central arid chile over millennia data provide evidence for the progressive contribution of extreme erosion events to millennial averages for drier climates as well as the link between glacier erosion and glacier sliding velocity in this period of time a discrepancy exists between the long term offshore sedimentological record and continental decennial or millennial erosion data for which no single explanation appears still little information is available concerning the magnitude of variation of millennial erosion rates over centuries data show the variable role of groundwater in the dynamics of suspended load and document a decrease in erosion over hundreds of years probably associated with historical harvesting

deformation of erosive and accretive forearcs during subduction of migrating and non migrating aseismic ridges results from 3 d finite element models and application to the central american peruvian and ryukyu margins subduction of aseismic oceanic ridges causes considerable uplift and deformation of the upper plate and may lead for example to the indentation of the forearc the formation of marine terraces or distinct fault patterns in the upper plate depending on the orientation of the ridge relative to the plate convergence direction the ridge may either be stationary or migrate along the margin here we use three dimensional numerical models to investigate the tectonic evolution of forearcs affected by ridge subduction in different experiments we distinguish between migrating non migrating ridges and accretive erosive margins respectively our results reveal that displacement and strain fields above migrating and non migrating ridges are asymmetric with respect to the ridge axis unless both ridge and plate convergence direction are perpendicular to the trench as the asymmetric deformation pattern shifts along the margin through time uplift caused by the underthrusting ridge is followed by subsidence when the ridge crest passed by and regions initially experiencing shortening may subsequently undergo extension and vice versa if the forearc comprises an accretionary prism the ridge induced reentrant is larger than those in models with erosive forearcs and strain localizes in the frontal part of the wedge additional models with a setup adjusted to the cocos and gagua ridges provide constraints on the onset of their subduction at the central american and ryukyu margins at 2 ma and 1 ma respectively displacement and strain fields from a model for the nazca ridge collision zone show good agreement with geological data from marine terraces and quaternary faulting

ichnology and sedimentology reveal depositional characteristics of bay margin parasequences in the miocene amazonian foreland basin abstract proposed depositional models for miocene amazon foreland basin strata pebas formation peru are controversial recent depositional models include lacustrine and tidally influenced brackish water embayment this paper presents data that support tidally influenced brackish water deposition for at least part of pebas time 10 14 ma two parasequences are presented santa julia and tamshiyacu both crop out along the amazon river in upper amazonia near iquitos peru at these locations abundant evidence of brackish water tidally influenced deposition is documented including marginal marine bioturbation sedimentary couplets semidiurnal couplets preserved in burrows and pinstripe lamination the deposits are locally highly bioturbated at both locations ichnogenera normally associated with marine to brackish water depositional environments are common three normally marginal marine ichnofabrics are reported 1 a chondrites reburrowed planolites ichnofabric resident only in massive appearing muds 2 a scolicia laminites thalassinoides ophiomorpha ichnofabric that is manifested as intensely bioturbated silty sands and 3 a thalassinoides generated ichnofabric that is interpreted to have descended into consolidated substrates and is thus representative of the glossifungites ichnofacies several trace fossils contain laminated infills organized into distinct sedimentary couplets that are best interpreted as resulting from semidiurnal processes six conclusions are arrived at 1 sedimentological and ichnological data consistently indicate that sediment accumulation dominantly occurred in sporadically dysaerobic marine to brackish water under a tidal influence 2 sediment accumulation occurred in bay margin environments that prograded into a shallow quiescent bay 3 a stratified water column is indicated by the ichnofauna 4 low accommodation space repetitive and rapid adjustments of relative sea level shallow wave base and a stratified water column combined to generate an atypical parasequence architecture 5 previously published isotopic data are consistent with sediment accumulation in brackish to marine water and 6 marine incursion into amazonia occurred during the middle miocene

cretaceous early paleocene drainage shift of amazonian rivers driven by equatorial atlantic ocean opening and andean uplift as deduced from the provenance of northern peruvian sedimentary rocks huallaga basin abstract the northern part of south america has undergone several major changes in its tectonic setting since the triassic evolving from rifting to the development of a retroarc foreland basin on its andean margin leading to margin extension and continental uplift following the breakup between africa and south america so far it is unclear when and how these geodynamic events affected the paleo amazonian drainage in this study we investigate the provenance of triassic eocene sedimentary rocks deposited in the northern peruvian amazonian basin based on their sr nd isotopic compositions and u pb zircon dating the triassic jurassic samples have end 0 values ranging from 7 9 to 10 0 and main u pb zircon peaks at 0 9 1 3 ga 31 33 and 0 5 0 7 ga 21 28 that suggest a mixed clastic supply from the western and eastern cordillera or craton within a rift to post rift setting samples from the albian maastrichtian interval yield much lower end 0 values 16 8 to 18 6 and a dominance of zircon grains derived from terranes in the easternmost brazilian shield ventuari tapajos 2 0 1 82 ga rio negro jurena 1 82 1 54 ga and rondonia san ignacio 1 54 1 3 ga thus indicating a cratonic source for these sedimentary rocks finally the early paleocene eocene sedimentary rocks record the first arrival of andean detritus in the amazonian retroarc foreland with end 0 values ranging between 5 6 to 12 0 and up to 16 of the zircon grains yielding ages younger than 120 ma together with recently published studies these provenance data document the existence of a long lived aptian maastrichtian continent wide cratonic drainage in the northern part of south america this cratonic drainage developed in response to the late cretaceous uplift of the northeastern part of the south america craton likely driven by geodynamic processes related to post rift opening of the atlantic equatorial ocean the formation of a late maastrichtian early paleocene mountain chain in the peruvian andes associated with the onset of provenance from the andean orogenic belt led to a shift of the amazon drainage pattern hence the earliest andean amazonian rivers are no younger than late maastrichtian early paleocene

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

response of the mantle to flat slab evolution insights from local s splitting beneath peru the dynamics of flat subduction particularly the interaction between a flat slab and the overriding plate are poorly understood here we study the seismically anisotropic properties and deformational regime of the mantle directly above the peruvian flat slab we analyze shear wave splitting from 370 local s events at 49 stations across southern peru we find that the mantle above the flat slab appears to be anisotropic with modest average delay times 0 28 s that are consistent with 4 anisotropy in a 30 km thick mantle layer the most likely mechanism is the lattice preferred orientation of olivine which suggests that the observed splitting pattern preserves information about the mantle deformation we observe a pronounced change in anisotropy along strike with predominately trench parallel fast directions in the north and more variable orientations in the south which we attribute to the ongoing migration of the nazca ridge through the flat slab system

effect of aseismic ridge subduction on slab geometry and overriding plate deformation insights from analogue modeling abstract we present analogue models simulating the subduction of a buoyant ridge oriented perpendicularly or obliquely with respect to the trench beneath an advancing overriding plate the convergence velocity is imposed by lateral boundary conditions in this experimental set we analyze the three dimensional geometry of the slab the deformation and topography of the overriding plate experiments suggest that ridge subduction diminishes the dip of the slab eventually leading to the appearance of a horizontal slab segment in case boundary conditions impose a rapid convergence this result contrasts with that obtained in free subduction experiments in which ridge subduction diminishes the convergence velocity which in turn increases the dip of the slab beneath the ridge the slab dip decrease is accompanied by the indentation of the overriding plate by the ridge resulting in arc curvature experiments suggest that indentation is larger for small convergence velocity and large slab dip ridge subduction also uplifts the overriding plate uplift first occurs close to the trench fore arc area and is accompanied by the flexural subsidence of the overriding plate behind the uplifted area back arc subsidence the uplifted area migrates within the overriding plate interiors following the appearance of a horizontal slab segment these results are compared with natural examples of ridge subduction in the circum pacific area they explain why ridge subduction may have contrasted effects on the overriding plate dynamics depending on the global conditions that constrain the converging system

geomorphic controls on andean denudation rates abstract to predict erosion rates throughout the andes we conducted a multiple regression analysis of the sediment discharge from 47 drainage basins in the bolivian andes and various topographic climatologic and geologic parameters these mountainous basins are typically large 17 81 000 km2 documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape mathrm mean 11 000 end document km2 often have decades of measurement data on daily water and sediment discharge and display an extraordinary range of denudation 0 01 6 9 mm yr

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

how does the nazca ridge subduction influence the modern amazonian foreland basin comment and reply reply in their comment of our geology paper espurt et al 2007 1 clift and ruiz 2008 2 argue that 1 the flat slab subduction of the nazca ridge is unlikely to have produced uplift of the fitzcarrald arch in the amazonian retroforeland basin using geologic data from the forearc area 2

detrital zircon record of neogene exhumation of the central alaska range a far field upper plate response to flat slab subduction far field deformation in the upper plate of many ancient and modern convergent margins is commonly attributed to flat slab subduction processes recent studies have suggested that the high topography of the central alaska range alaska usa for example is the far field deformational response to neogene shallow subduction processes in southern alaska in this study we analyze six stratigraphically controlled detrital zircon samples from miocene pliocene sandstone of the tanana foreland basin to evaluate the temporal and spatial links between exhumation of the central alaska range and shallow subduction the tanana basin located north of the central alaska range contains 2 km of neogene strata lower miocene strata from the basin have detrital zircon ages with dominant precambrian populations these grain ages are attributed to sediment derivation from the yukon composite terrane source area located at the northern edge and north of the alaska range during the early middle miocene the provenance of the tanana basin switched to a distinctly mesozoic source signal these mesozoic age grains are attributed to sediment derived from source areas within and south of the alaska range the early middle miocene change in provenance from predominantly precambrian to mesozoic sediment sources in the context of previously published basin analysis studies is interpreted to represent the onset of neogene uplift of the central alaska range the timing of exhumation of the central alaska range is important not only for understanding the tectonics of the highest mountain range in north america but additionally because recent modeling studies predict that this range is one of the first expected products of neogene flat slab subduction along the southern alaska convergent plate boundary results of our study provide a needed chronologic framework to fully evaluate the temporal and spatial upper plate responses to shallow subduction processes

flat subduction dynamics and deformation of the south american plate insights from analog modeling received 14 june 2007 revised 13 january 2008 accepted 12 march 2008 published 21 june 2008 1 we present lithospheric scale analog models investigating how the absolute plates motion and subduction of buoyant oceanic plateaus can affect both the kinematics and the geometry of subduction possibly resulting in the appearance of flat slab segments and how it changes the overriding plate tectonic regime experiments suggest that flat subductions only occur if a large amount of a buoyant slab segment is forced into subduction by kinematic boundary conditions part of the buoyant plateau being incorporated in the steep part of the slab to balance the negative buoyancy of the dense oceanic slab slab flattening is a long term process 10 ma which requires the subduction of hundreds of kilometers of buoyant plateau the overriding plate shortening rate increases if the oceanic plateau is large enough to decrease the slab pull effect slab flattening increases the interplate friction force and results in migration of the shortening zone within the interior of the overriding plate the increase of the overriding plate topography close to the trench results from 1 the buoyancy of the plate subducting at trench and 2 the overriding plate shortening experiments are compared to the south american active margin where two major horizontal slab segments had formed since the pliocene along the south american subduction zone flat slab segments below peru and central chile nw argentina appeared at 7 ma following the beginning of buoyant slab segments subduction in northern ecuador and northern chile the process of slab flattening resulting from the carnegie and iquique ridges subductions respectively seems to be active but not completed the formation of flat slab segments below south america from the pliocene may explain the deceleration of the nazca plate trenchward velocity citation espurt n f funiciello j martinod b guillaume v regard c faccenna and s brusset 2008 flat subduction dynamics and deformation of the south american plate insights

quaternary uplift of the peruvian coast related to the subduction of the nazca ridge 13 5 to 15 6 degrees south latitude abstract marine terraces between 13 5 and 15 6 s latitude on the peruvian coast preserve a history of quaternary sea level fluctuations and forearc movement above the subducting nazca ridge field mapping and amino acid racemization epimerization electron spin resonance radiocarbon and uranium series disequilibrium ages from mollusk shells from terrace deposits are used to define a marine terrace chronology terraces on the peruvian coast formed during global high stands of quaternary sea level in a similar way to the marine terraces on the west coast of the united states and baja california mexico a last interglacial marine terrace 125 ka is preserved on nearly every terrace flight along the coast from northeast to southwest each terrace varies in elevation because of variations in the rate of coastal uplift variations in uplift rate from northeast to southwest in the study area are explained by a simple geometric model in which the variables are the rate of plate convergence morphology of the nazca ridge and orientation of the ridge relative to the peruvian coast the oblique northeastward orientation of the ridge relative to the east west direction of plate convergence results in a ca 70 mm yr southeastward migration of the zone of nazca ridge subduction beneath the peruvian coast the coastal area above the northern flank and crest of the nazca ridge is stable or has undergone net subsidence while the coast above the ridge crest is rising at about 0 3 m kyr above the southern flank of the nazca ridge the coast is rising at a rate of ca 0 5 m kyr the fastest rate determined from studies of marine terraces on the coasts of peru and chile south of the influence of the nazca ridge uplift rates are 0 2 m kyr the pattern of faster uplift above the southern flank and slower uplift above the northern flank of the ridge is a predictable consequence of oblique ridge subduction

foreland uplift during flat subduction insights from the peruvian andes and fitzcarrald arch abstract foreland deformation has long been associated with flat slab subduction but the precise mechanism linking these two processes remains unclear one example of foreland deformation corresponding in space and time to flat subduction is the fitzcarrald arch a broad ne sw trending topographically high feature covering an area of 4 105 km2 in the peruvian andean foreland recent imaging of the southern segment of peruvian flat slab shows that the shallowest part of the slab which corresponds to the subducted nazca ridge northeast of the present intersection of the ridge and the peruvian trench extends up to and partly under the southwestern edge of the arch here we evaluate models for the formation of this foreland arch and find that a basal shear model is most consistent with observations we calculate that 5 km of lower crustal thickening would be sufficient to generate the arch s uplift since the late miocene this magnitude is consistent with prior observations of unusually thickened crust in the andes immediately south of the subducted ridge that may also have been induced by flat subduction this suggests that the fitzcarrald arch s formation by the nazca ridge may be one of the clearest examples of upper plate deformation induced through basal shear observed in a flat slab subduction setting we then explore the more general implications of our results for understanding deformation above flat slabs in the geologic past

renewed uplift of the central andes forearc revealed by coastal evolution during the quaternary most of the pacific coast of the central andes between 15 s and 30 s displays a wide a couple of kilometres planar feature gently dipping oceanwards and backed by a cliff this morphology usually of marine orgin is called rasa and argues for a recent and spatially continuous uplift of the margin over the 1 500 km long coastal region we describe the cliff foot is found at a similar elevation 110 m amsl all over the studied area with the exception of peninsulas such as the mejillones peninsula the compilation of published chronological data and the extrapolation of re appraised uplift rates provide evidence for a common cliff foot age of around 400 ka i e marine isotopic stage mis 11 this together with other geological constraints indicates a quaternary renewal of uplift in the central andes forearc after a late pliocene quiescence or subsidence

dynamics of deformation and sedimentation in the northern sierras pampeanas an integrated study of the neogene fiambala basin nw argentina comment and discussion in a recent contribution to the geological society of america bulletin carrapa et al 2008 1 provided an interesting geochronological and thermochronological database for the fiambala basin northern end of the sierras pampeanas which together with stratigraphic sedimentological and

geomorphic evidence for recent uplift of the fitzcarrald arch peru a response to the nazca ridge subduction abstract the 400 000 km 2 wide fitzcarrald arch constitutes a wide topographic high of the amazon basin against the central andes in order to constrain its formation mechanisms and in particular to test its relationships to the nazca ridge subduction a quantitative geomorphology analysis of the arch is performed using hypsometric integrals elongation and azimuths of 7th and 5th order catchments they all express a trend from high maturity to low maturity from nw towards se this maturity gradient coupled with the local drainage direction demonstrate that the fitzcarrald arch is not a classical alluvial fan since its apex is located 100 km east to the subandean thrust front and the corresponding sedimentary pile is lacking nor is the arch the superficial expression of an inherited transfer zone because its geomorphic shape is radial and it does not diverge from a symmetry axis moreover such a reactivated structure is not found at depth on seismic profiles in addition our data show that underlying geomorphic control on catchment initiation and development has progressed from nw to se which in combination with the observation of crustal doming by espurt et al espurt n baby p brusset s roddaz m hermoza w regard v antoine p o salas gismondi r bolanos r 2007 how does the nazca ridge subduction influence the modern amazonian foreland basin geology 35 515 518 suggests that this relief is caused by the eastward sliding of the buoyant nazca ridge beneath the south american lithosphere

paleozoic structural controls on shortening transfer in the subandean foreland thrust system ene and southern ucayali basins peru 1 the neogene evolution of the ene and southern ucayali basins of the subandes has been controlled by two stacked thrust wedges that differ in terms of tectonic styles the lower thrust wedge is formed by deep seated decollements within the basement related to thick skinned foreland structures inherited from an early carboniferous thrust system seismic reflection data show that this paleozoic compressional system has been eroded and unconformably covered by late carboniferous clastic sediments it generated an irregular paleozoic sedimentary architecture controlling the neogene thrust propagation the upper thin skinned thrust wedge developed within this paleozoic sedimentary series and constitutes the subandean zone cross section balancing shows an along strike homogenous horizontal shortening of 56 km 30 across the ene southern ucayali thrust system this amount of shortening was vertically partitioned onto the two stacked thrust wedges the n s thickness variations of the paleozoic sedimentary prism controlled the eastward propagation of the upper thrust wedge the southern thickening of the paleozoic series generated major decollements and the shortening excess is of 7 km 16 in comparison to the north consequently the northern lack of shortening onto the upper thrust wedge was transferred to the early carboniferous compressional structures of the lower thrust wedge we suggest that this vertical partitioning of the shortening was accommodated by a regional oblique ramp the tambo transfer zone this geometrical analysis of the ene southern ucayali thrust system provides new perspectives for future hydrocarbon exploration in this region

steady state exhumation pattern in the central andes se peru the western cordillera of se peru is part of the central andes and is situated to the west of the eastern andes from which it is separated by the northern termination of the altiplano the inter andean valley it is a volcanic volcano detrital chain that developed in the palaeogene and is characterized by a 4000 m high mean altitude whose origin is poorly constrained we selected a vertical profile in the region of abancay to trace the record the evolving uplift and erosion history of the andean orogen fission track data on both apatite and zircon crystals were completed on plutonic rocks of the tertiary andahuaylas yauri batholith ages ranged between 24 and 14 ma and 38 and 30 ma respectively thermal modelling was completed for the whole profile and does not like age altitude relationships show evidence of any clear disruption of the exhumation paths since 38 ma either by sedimentary burial and or rapid exhumation one of the noteworthy aspects of the data is that exhumation was steady at a rate of 0 17 km ma

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

tectonic segmentation of the north andean margin impact of the carnegie ridge collision the north andean convergent margin is a region of intense crustal deformation with six great subduction earthquakes mw 7 8 this century the regional pattern of seismicity and volcanism shows a high degree of segmentation along strike of the andes segments of steep slab subduction alternate with aseismic regions and segments of flat slab subduction this segmentation is related to heterogeneity on the subducting nazca plate in particular the influence of the carnegie ridge collision is investigated four distinct seismotectonic regions can be distinguished region 1 from 6on to 2 5on with steep ese dipping subduction and a narrow volcanic arc region 2 from 2 5on to 1os showing an intermediate depth seismic gap and a broad volcanic arc region 3 from 1os to 2os with steep ne dipping subduction and a narrow volcanic arc region 4 south of 2os with flat subduction and no modern volcanic arc the carnegie ridge has been colliding with the margin since at least 2 ma based on examination of the basement uplift signal along trench parallel transects the subducted prolongation of carnegie ridge may extend up to 500 km from the trench as suggested by the seismic gap and the perturbed broad volcanic arc these findings conflict with previous tectonic models suggesting that the carnegie ridge entered the trench at 1 ma furthermore the anomalous geochemical adakitic signature of the volcanoes in the broad ecuador volcanic arc and the seismicity pattern are proposed to be caused by lithospheric tears separating the buoyant shallowly subducting carnegie ridge from segments of steep subduction in regions 1 and 3 it is further suggested that carnegie ridge supports a local flat slab segment similar to that observed in peru the impact of the carnegie ridge collision on the upper plate causes transpressional deformation extending inboard to beyond the volcanic arc with a modern level of seismicity comparable to the san andreas fault system the pattern of instrumental and historical seismicity indicates 1 great earthquakes on the northern and southern flanks of the colliding ridge 2 a slight reduction in observed seismicity at the trench ridge intersection 3 increased stress far into the continent and 4 a nne displacement of the n andes block to be further effects of the collision 1999 elsevier science b v all rights reserved

from source to sink preserving the cosmogenic 10be derived denudation rate signal of the bolivian andes in sediment of the beni and mamore foreland basins abstract the denudation rate signal of the bolivian andes as measured by cosmogenic 10 be in sediment is preserved in the floodplain of adjacent foreland basins even though these basins store the sediment for thousands of years this conclusion is drawn from comparing published andean source area denudation rates with new cosmogenic 10 be data as measured in the floodplains of the large beni and mamore basins for the entire beni basin including the sediment producing andes and the vast flooded plains of the foreland the cosmogenic nuclide derived denudation rate is 0 45 0 06 mm yr while that of the mamore basin is 0 55 0 19 mm yr by comparison the respective andean source areas erode at averaged rates of 0 37 0 06 mm yr upper beni and at 0 56 0 09 mm yr upper mamore we notice a decrease in variability of denudation rate with increasing spatial scale as small scale processes are averaged out sediment mixing within the floodplain damps scatter of nuclide derived denudation rates picked up in the source area on the temporal scale a remarkable agreement between cosmogenic nuclide derived rates averaging over a few kiloyears with those from fission track dating averaging over millions of years seems to suggest that cosmogenic nuclide derived denudation rates capture the long term erosional characteristics of the mountain belt the sum of these observations suggests that any sample collected along a river traversing a floodplain will yield the denudation rate of the source area this finding opens the unique possibility of constraining cosmogenic nuclide derived paleo sediment budgets for these large basins as the long term spatially averaged denudation rate signal of the sediment producing area is preserved in sedimentary archives

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

a reassessment of the suspended sediment load in the madeira river basin from the andes of peru and bolivia to the amazon river in brazil based on 10 years of data from the hybam monitoring programme abstract the madeira river is the second largest tributary of the amazon river it contributes approximately 13 of the amazon river flow and it may contribute up to 50 of its sediment discharge to the atlantic ocean until now the suspended sediment load of the madeira river was not well known and was estimated in a broad range from 240 to 715 mt yr 1 since 2002 the hybam international network developed a new monitoring programme specially designed to provide more reliable data than in previous intents it is based on the continuous monitoring of a set of 11 gauging stations in the madeira river watershed from the andes piedmont to the confluence with the amazon river and discrete sampling of the suspended sediment concentration every 7 or 10 days this paper presents the results of the suspended sediment data obtained in the madeira drainage basin during 2002 2011 the madeira river suspended sediment load is estimated at 430 mt yr 1 near its confluence with the amazon river the average production of the madeira river andean catchment is estimated at 640 mt yr 1 30 the corresponding sediment yield for the andes is estimated at 3000 t km 2 yr 1 30 and the average denudation rate is estimated at 1 20 mm yr 1 30 contrary to previous results that had mentioned high sedimentation rates in the beni river floodplain we detected no measurable sedimentation process in this part of the basin on the mamore river basin we observed heavy sediment deposition of approximately 210 mt yr 1 that seem to confirm previous studies but while these studies mentioned heavy sedimentation in the floodplain we showed that sediment deposition occurred mainly in the andean piedmont and immediate foreland in rivers parapeti grande pirai yapacani chimore chapare secure maniqui with discharges that are not sufficiently large to transport their sediment load downstream in the lowlands

spatial gaps in arc volcanism the effect of collision or subduction of oceanic plateaus abstract one of the major processes in the formation and deformation of continental lithosphere is the process of arc volcanism the plate tectonic theory predicts that a continuous chain of arc volcanoes lies parallel to any continuous subduction zone however the map pattern of active volcanoes shows at least 24 areas where there are major spatial gaps in the volcanic chains 200 km a significant proportion 30 of oceanic crust is subducted at these gaps all but three of these gaps coincide with the collision or subduction of a large aseismic plateau or ridge the idea that the collision of such features may have a major tectonic impact on the arc lithosphere including cessation of volcanism is not new however it is not clear how the collision or subduction of an oceanic plateau perturbs the system to the extent of inhibiting arc volcanism three main factors necessary for arc volcanism are 1 source materials for the volcanics either volatiles or melt from the subducting slab and or melt from the overlying asthenospheric wedge 2 a heat source either for the dehydration or the melting of the slab or the melting within the asthenosphere and 3 a favorable state of stress in the overlying lithosphere the absence of any one of these features may cause a volcanic gap to form there are several ways in which the collision or subduction of an oceanic plateau may affect arc volcanism the clearest and most common cases considered are those where the feature completely resists subduction causing local plate boundaries to reorganize this includes the formation of new plate bounding transform faults or a flip in subduction polarity in these cases subduction has slowed down or stopped and the lack of source material has created a volcanic gap there are a few cases most notably in peru chile and the nankai trough where the dip of subduction is so shallow that effectively no asthenospheric wedge exists to produce source material for volcanism the shallow dip of the slab may be a buoyant effect of the plateau imbedded in the oceanic lithosphere the cases which are the most enigmatic are those where subduction is continuous the oceanic plateau is subducted along with the slab and the dip of the slab is clearly steep enough to allow arc volcanism yet a volcanic gap exists in these areas the subducted plateau may have a fundamental effect on the physical process of arc volcanism itself the presence of a large topographic feature on the subducting plate may affect the stress state in the are by increasing the amount of decoupling between the two plates alternatively the subduction of the plateau may change the chemical processes at depth if either the water rich top of the plateau with accompanying sediments are scraped off during subduction or if the ridge is compositionally different

tectonic and magmatic responses to the subduction of high bathymetric relief subduction of high bathymetric relief such as aseismic ridges and magmatic plateaus is considered to be responsible for dramatic changes in the dynamics and kinematics of the subduction zone for example the buoyancy of high bathymetric relief is thought to flatten the dip of the subducting slab modifying the structural and magmatic evolution of the overriding plate and terminating arc volcanism in addition the effect of ridge subduction in retreating plate boundaries can inhibit subduction rollback a process that could locally pin the subduction hinge and lead to the development of cusps and slab tearing here we discuss the tectonic response to subduction of high bathymetric relief using examples from the circum pacific subduction systems we demonstrate that flattening of the subduction dip angle is only significant in the eastern pacific where the average slab dip angle is relatively shallow in the western pacific in contrast the average subduction dip angle is steeper and there is no significant flattening of the dip angle in areas of ridge subduction subduction of high bathymetric relief in the circum pacific is commonly associated with reduced arc volcanism and in many cases the area of ridge subduction coincides with a volcanic gap in the overriding plate ridge subduction is associated with pronounced changes in the style of deformation involving uplift reactivation of basement thrusts development of orogen perpendicular tear faults and block rotations leading to oroclinal bending the discussed characteristic patterns associated with ridge subduction provide important guidelines for reconstructing past plate tectonic processes and could help constraining the geodynamics of ancient subduction systems

fluvial perturbance in the western amazon basin regulation by long term sub andean tectonics haffer s refuge theory proposes that during the arid climatic phases of the late pleistocene tropical lowland forests of amazonia were reduced to isolated patches contributing to the high species richness of the present day forest the theory was developed because no obvious historic or modern geomorphic isolation barriers were recorded in amazonia analyses of radar images combined with stratigraphical data show that in the basinal forelands of the tectonically active andes the geological setting causes long term fluvial perturbance this leads to a temporally structured highly complex mosaic of fossil and present floodplains these dynamics have been present with varying activity and geographic range during the tertiary and quaternary providing site turnover that has not been recognized by the biogeographic tradition of the amazon basin

seamount ridge and transform subduction in southern central america understanding the factors that control subduction zone processes is a first order goal in the study of convergent margins in southern central america a growing body of research reveals strong links between the character of the subducting slab and the mechanics of important processes that include subduction erosion fluid flow deformation and seismogenesis in this paper i evaluate the role that seamount ridge and transform subduction have in the development of upper plate deformation and volcanism by summarizing previous work across a 500 km long region of central america where each of these three scenarios are present along strike the data show that the subduction of short wavelength bathymetry e g seamounts and faults on the seafloor produces short wavelength deformation that persists for relatively short timescales 104 105 years whereas the subduction of longer wavelength bathymetry e g the aseismic cocos ridge results in longer wavelength deformation that endures over a longer time scale 106 years the timing and distribution of upper plate deformation are consistent with subhorizontal cocos ridge subduction driving upper plate deformation and the increased crustal thickness 20 km of the subducting cocos ridge is likely one of the most important factors in the production of upper plate contraction and crustal thickening the data illustrate a fundamental connection between lower plate properties and upper plate deformation and highlight the profound influence that bathymetry and crustal thickness have in the localization and kinematics of upper plate strain and volcanism in middle america

widening of the andes an interplay between subduction dynamics and crustal wedge tectonics shortening of the continental lithosphere is generally accommodated by the growth of crustal wedges building above megathrusts in the mantle lithosphere we show that the locus of shortening in the western margin of south america has largely been controlled by the geometry of the slab numerical models confirm that horizontal subduction favors compression far from the trench above the asthenospheric wedge and steeply dipping segment of the subducting slab as a result a second crustal wedge grows in the hinterland of the continent and widens the andes in the bolivian orocline this wedge corresponds to the eastern cordillera whose growth was triggered by a major episode of horizontal subduction when the slab returned to a steeper dip angle shortening and uplift pursued facilitated by the structural and thermo chemical alteration of the continental lithosphere we review the successive episodes of horizontal subduction that have occurred beneath south america at different latitudes and show that they explain the diachronic widening of the andes we infer that the present day segmented physiography of the andes results from the latitudinally variable transient interplay between slab dynamics and upper plate tectonics over the cenozoic we emphasize that slab flattening or absence thereof is a major driving mechanism that sets the width of the andes at any latitude

structural inheritance in mountain belts an alpine apennine perspective abstract the geological structure of continental lithosphere shows complex variety that is inherited into orogenic belts and influences the localization and amplification of contractional structures during mountain building in the alpine apennine sector together with other sectors of the tethyan orogenic system the pre orogenic crustal template can include arrays of extensional faults other faults can form adjacent to the evolving mountain belt and subsequently become incorporated as the thrust belts migrate into their forelands while in some areas these inherited features may simply reactivate under inversion more commonly faults show complex partial reactivation structures in volumes of distributed strain faults may serve to nucleate large scale buckle folds for example along basement cover interfaces these different patterns of basement reactivation may reflect spatially varying strength depth profiles in continental lithosphere that are themselves inherited from spatially distinct geological histories even when not themselves reactivating basement faults can control deformation in the overlying sedimentary cover by offsetting otherwise regionally extensive detachment horizons the 3d form of thrust systems can be strongly compartmentalized by pre existing cross faults such as the oblique lineaments of the apennines on a large scale the distribution of pre existing faults and other weaknesses may affect the propensity for orogenic contraction in basement and therefore directly control larger scale tectonic processes in the central mediterranean the evolution of slab roll back and the related growth of overlying extensional basins e g tyrrhenian sea may be strongly modulated by the distribution of rift related weak zones in the adjacent continental crust the subduction of continental crust will strongly depend on the inherited structure of this crust specifically the distribution of deep crust of basic composition this develops relatively higher densities associated with eclogite metamorphism which act in turn to reduce the buoyancy of thickened continental crust that otherwise serves to inhibit further shortening investigating all these aspects from the scale of bulk crustal compositions to the geometry timing and strength of earlier fault zones preserved in orogenic belts requires the integration of substantial multidisciplinary geological data sets the extent to which continental orogenic belts represent the amplification of inherited geological heterogeneities as opposed to self ordered phenomena modulated by the syntectonic environment remains unclear

the structure of the nazca ridge and sala y gomez seamount chain from the dispersion of rayleigh waves summary we report single station measurements of group velocity for fundamental mode rayleigh waves that propagated along two aseismic ridges in the eastern pacific ocean basin the nazca ridge and the sala y gomez chain we find that the nazca ridge dispersion curves when regionalized to isolate the ridge contribution indicate group velocities of about 3 5 km s for short periods these values are slower than predicted for the geometry of the propagation paths interpreting this velocity heterogeneity is complicated by the recognition that the nazca ridge is about 1000 km longer than previously thought but models of vyv for the ridge derived by inverting the dispersion curves reveal abnormally thick 18 f 3 km oceanic crust along its longitudinal axis this value compares favourably with previously published results over the tuamotu plateau thus strengthening the hypothesis that the nazca ridge and tuamotu plateau are mirror images formed by a hotspot coincident with the farallon pacific spreading centre before 20 ma in contrast the measurements along the sala y gomez chain are consistent with typical oceanic crust and mantle when these values are inverted they reveal no thick crust 6 km along the sala y gomez chain we conclude from this result that the chain was likely formed as a leaky fracture zone not as a simple hotspot track

recent and ancient fluvial deposition systems in the amazonian foreland basin peru still active sub andean foreland deformation is suggested to have syndepositionally modified the fluvial depositional environments in the peruvian amazonian foreland basin throughout neogene quaternary time modern fluvial aggradation continues to proceed on a large scale c 120 000 km 2 in two differing depositional systems firstly various multistoried floodbasin deposits are derived from the meandering and anastomosing rivers within the subsiding intraforeland basins secondly in the northern part of the pastaza maranon basin the largest known holocene alluvial fan like formation c 60 000 km 2 composed of reworked volcaniclastic debris derived from active ecuadorian volcanoes has been identified the widespread poorly known dissected surface alluvium terra firme which covers the main part of the peruvian amazonian foreland basin shows further evidence of long term foreland deformation and terraces indicate both the effects of tectonism and pleistocene climatic oscillations in northern peru the surface alluvium was deposited by a tertiary fluvial system with palaeocurrents to the west and northwest into the andean foreland basin in southern peru the respective surficial alluvium was part of a post miocene fluvial system flowing northeast into the main amazon basin both systems were gradually abandoned when the eastward migrating andean foreland deformation led to the more distinctive partitioning of the intraforeland basins and the modern drainage system was created

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

the origin of oriented lakes evidence from the bolivian amazon the presence of hundreds of rectangular and oriented lakes is one of the most striking characteristics of the llanos de moxos lm landscape in the bolivian amazon oriented lakes also occur in the arctic coastal plains of russia alaska and canada and along the atlantic coastal plain from northeast florida to southeast new jersey and along the coast of northeast brazil many different mechanisms have been proposed for their formation in the lm plafker s 1964 tectonic model in which subsidence results from the propagation of bedrock faults through the foreland sediments is the most accepted however this model has not been verified here we present new results from stratigraphic transects across the borders of three rectangular and oriented lakes in the lm a paleosol buried under mid holocene sediments is used as a stratigraphic marker to assess the vertical displacement of sediments on both sides of the alleged faults our results show that there is no vertical displacement and therefore that plafker s model can be ruled out we suggest that among all the proposed mechanisms behind lake formation the combined action of wind and waves is the most likely the evidence from the lm provides new hints for the formation of oriented lakes worldwide

alluvial plain dynamics and human occupation in sw amazonia during the holocene a paleosol based reconstruction abstract the present study reconstructs holocene fluvial dynamics in the southern amazonian foreland basin through the analysis of 36 stratigraphic profiles taken along a 300 km long transect across the llanos de moxos lm in the bolivian amazon based on 50 radiocarbon ages from paleosols intercalated with fluvial sediments the most important changes in floodplain dynamics on a millennial scale are reconstructed and the links between pre columbian cultural processes and environmental change in the region explored results show that the frequency of river avulsions and crevasses as inferred from the number and age of the cored paleosols is stable from 8k cal yrs bp to 4k cal yrs bp and increases significantly from 4k to 2k cal yrs bp following the strengthening of el nino la nina cycle and an increase in average precipitation fluvial activity then decreases and reaches its minimum after 2k cal bp a comparison between the stratigraphic record and the archaeological record shows a match between periods of landscape stability in sw amazonia low river activity and periods of pre columbian human occupation the first amazonians lived in the lm until 4k yrs bp when an abrupt increase in the frequency of river avulsions and crevasses forced the abandonment of the region after two thousand years of archaeological hiatus which matches the period of highest river activity in the region agriculturists reoccupied the bolivian amazon

from oceanic plateaus to allochthonous terranes numerical modelling abstract large segments of the continental crust are known to have formed through the amalgamation of oceanic plateaus and continental fragments however mechanisms responsible for terrane accretion remain poorly understood we have therefore analysed the interactions of oceanic plateaus with the leading edge of the continental margin using a thermomechanical petrological model of an oceanic continental subduction zone with spontaneously moving plates this model includes partial melting of crustal and mantle lithologies and accounts for complex rheological behaviour including viscous creep and plastic yielding our results indicate that oceanic plateaus may either be lost by subduction or accreted onto continental margins complete subduction of oceanic plateaus is common in models with old 40 ma oceanic lithosphere whereas models with younger lithosphere often result in terrane accretion three distinct modes of terrane accretion were identified depending on the rheological structure of the lower crust and oceanic cooling age frontal plateau accretion basal plateau accretion and underplating plateaus complete plateau subduction is associated with a sharp uplift of the forearc region and the formation of a basin further landward followed by topographic relaxation all crustal material is lost by subduction and crustal growth is solely attributed to partial melting of the mantle frontal plateau accretion leads to crustal thickening and the formation of thrust and fold belts since oceanic plateaus are docked onto the continental margin strong deformation leads to slab break off which eventually terminates subduction shortly after the collisional stage has been reached crustal parts that have been sheared off during detachment melt at depth and modify the composition of the overlying continental crust basal plateau accretion scrapes oceanic plateaus off the downgoing slab enabling the outward migration of the subduction zone new incoming oceanic crust underthrusts the fractured terrane and forms a new subduction zone behind the accreted terrane subsequently hot asthenosphere rises into the newly formed subduction zone and allows for extensive partial melting of crustal rocks located at the slab interface and only minor parts of the former oceanic plateau remain unmodified oceanic plateaus may also underplate the continental crust after being subducted to mantle depth u hp terranes are formed with peak metamorphic temperatures of 400 700 c prior to slab break off and subsequent exhumation rapid and coherent exhumation through the mantle along the former subduction zone at rates comparable to plate tectonic velocities is followed by somewhat slower rates at crustal levels accompanied by crustal flow structural reworking and syndeformational partial melting exhumation of these large crustal volumes leads to a sharp surface uplift

dynamic uplift during slab flattening subduction exerts a strong control on surface topography and is the main cause of large vertical motions in continents including past events of large scale marine flooding and tilting the mechanism is dynamic deflection the sinking of dense subducted lithosphere gives rise to stresses that directly pull down the surface here we show that subduction does not always lead to downward deflections of the earth s surface subduction of young lithosphere at shallow angles flat subduction leaves it neutrally or even positively buoyant with respect to underlying mantle because the lithosphere is relatively warm compared with older lithosphere and because the thickened and hence drier oceanic crust does not undergo the transformation of basalt to denser eclogite accounting for neutrally buoyant flat segments along with large variations in slab morphology in the south american subduction zone explains along strike and temporal changes in dynamic topography observed in the geologic record since the beginning of the cenozoic our results show that the transition from normal subduction to slab flattening generates dynamic uplift preventing back arc marine flooding