shear zone thickness and the seismicity of chilean and marianas type subduction zones chilean type convergent margins have many large m 7 6 earthquakes whereas marianas type ones do not this dichotomy is enigmatic if the plate interface is viewed as a thin frictional decollement whereas it becomes understandable if it is viewed as a relatively thick sediment filled shear zone which thins or thickens arcward depending on subduction speed and sediment supply chilean type margins have thick trench fills and their shear zones generally thin arcward from inlets as much as several thousand metres high the most pronounced thinning being located near backstops tall up to several kilometres seamounts are subducted essentially intact to relatively great depths and confining pressures before jamming into the roof of the channel and becoming seismogenic asperities their near basal ruptures can generate large thrust type earthquakes mainly concentrated in seismic fronts near backstops marianas type margins in contrast have thin trench fills and their shear zones generally thicken arcward from inlets that can be as little as 300 m high seamounts are truncated near the inlet at low confining pressures and generate only small earthquakes after passing the inlet they do not touch the roof and therefore cannot generate large earthquakes a similar mechanism may explain seismic gaps at sediment poor regions of subduction zones

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

seismicity and structure in central mexico evidence for a possible slab tear in the south cocos plate the morphology of the transition from flat to normal subduction in eastern central mexico is explored using intraslab earthquakes recorded by temporary and permanent regional seismic arrays observations of a sharp transition in slab dip near the abrupt end of the trans mexican volcanic belt tmvb suggest a possible slab tear located within the subducted south cocos plate the eastern lateral extent of a thin ultra slow velocity layer usl imaged atop the cocos slab in recent studies along the meso america subduction experiment array is examined here using additional data we find an end to this usl which is coincident with the western boundary of a zone of decreased seismicity and the end of the tmvb near the sharp transition in slab dip waveform modeling of the 2 d structure in this region using a finite difference algorithm provides constraints on the velocity and geometry of the slab s seismic structure and confirms the location of the usl analysis of intraslab seismicity patterns reveals clustering sudden increase in depth variable focal mechanism orientations and faulting types and alignment of source mechanisms along the sharp transition in slab dip the seismicity and structural evidence suggests a possible tear in the south cocos slab this potential tear together with the tear along the orozco fracture zone to the northwest indicates a slab rollback mechanism in which separate slab segments move independently allowing for mantle flow between the segments

the great tumaco colombia earthquake of 12 december 1979 southwestern colombia and northern ecuador were shaken by a shal low focus earthquake on 12 december 1979 the magnitude 8 shock located near tumaco colombia was the largest in northwestern south america since 1942 and had been forecast to fill a seismic gap thrust faulting occurred on a 280 by 130 kilometer rectangular patch of a subduction zone that dips east beneath the pacific coast of colombia a 200 kilometer stretch of the coast tectonically subsided as much as 1 6 meters uplift occurred offshore on the continental slope a tsunami swept inland immediately after the earthquake ground shaking intensity vi to ix caused many buildings to collapse and generated liquefaction in sand fills and in holocene beach lagoonal and fluvial deposits

geodynamic controls on tertiary arc magmatism in ecuador constraints from u pb zircon geochronology of oligocene miocene intrusions and regional age distribution trends abstract we obtained u pb zircon ages of late tertiary intrusions in the northern andes to provide robust time calibration points for the intrusive geochronologic framework of ecuador which is mostly based on k ar data magmatism associated with intrusion emplacement ranges from 31 to 7 ma with ages mainly pooling in the late oligocene early miocene where both k ar and u pb data exist for a given intrusive system ages obtained by the different methods are usually concordant within 1 4 m y suggesting that k ar ages are accurate indicators for the timing of tertiary arc magmatism on a regional multi m y scale except for a single sample the investigated intrusions completely lack externally inherited zircons in agreement with dominantly zircon poor oceanic crustal basement domains spatio temporal distribution trends of tertiary arc magmatism inferred from screened u pb k ar and zircon fission track geochronologic data allow tracking of the progressive along arc broadening of a flat slab segment below southernmost ecuador in the mid to late miocene and of moderate slab shallowing in northern central ecuador in the late miocene these regional arc migration patterns correlate in time with the subduction of the buoyant inca plateau and the carnegie ridge seamount chain the temporal distribution of tertiary ecuadorian arc magmatism indicates a late oligocene early miocene arc magmatic flare up event comprising widespread ignimbrite eruption and batholith construction initiation of the flare up event coincides in time with accelerating less oblique farallon nazca south america plate convergence suggesting a positive feedback between convergence rates asthenospheric melt production mantle crust melt flux and upper crustal arc magmatic productivity in ecuador

imaging of subducted lithosphere beneath south america tomographic images are produced for the deep structure of the andean subduction zone beneath western south america the data used in the imaging are the delay times of p pp and pwp phases from relocated teleseismic earthquakes in the region regionally structural features larger than about 150 km are resolved by the data presentations of layer anomaly maps and cross sections reveal 1 the nazca slab is probably continuous laterally and at depth over most regions 2 the offset between the north and south deep earthquake zones containing the 1994 deep bolivia main shock and its aftershocks can be modelled by a northwest striking and steeply northeast dipping slab structure and 3 the nazca slab clearly penetrates the lower mantle beneath central south america but is partly deflected in the southern deep zone

collisional model for rapid fore arc block rotations arc curvature and episodic back arc rifting in subduction settings 1 we review geophysical and geological data from 23 active and ancient plate boundaries to document a compelling spatial and temporal relationship between collision plate boundary curvature rapid tectonic rotations and the occurrence of back arc rifting our observations support a conceptual model where the change from subduction to collision causes rapid fore arc rotation when viewed in a reference frame relative to the bounding tectonic plates leading to marked plate boundary curvature our global compilation reveals that most active back arc rifts are associated with rapidly rotating fore arcs and nearby collisions thus we propose that collision and rapid fore arc rotations play a major role in the evolution and kinematics of back arc basins we conduct numerical modeling to better understand the physical processes behind these observed rapid tectonic rotations at convergent margins our results suggest that the presence of an indentor or choke point in the subduction system e g collision can generate rapid rotation of the fore arc about a nearby pole and lead to back arc rifting the rate of fore arc rotation and back arc rifting depends on the incoming indentor velocity and can be greatly enhanced by slab rollback and the presence of a low viscosity back arc where viscosity of the back arc is low fore arc rotation dominates where back arc viscosity is high the formation of strike slip faults and tectonic escape dominates our observational and model derived results illustrate that shallow crustal forces produced by the entry of buoyant features into subduction zones are a fundamental mechanism for the generation of fore arc block rotations plate boundary curvature back arc rift evolution and tectonic escape in subduction systems rollback of the subducting slab within the mantle will certainly enhance the processes we describe but it is not the only driving force

seismicity and shape of the subducted nazca plate an updated compilation of earthquake locations and focal mechanism solutions from the international seismological centre and preliminary determination of earthquakes is the basis of a comprehensive study of the geometry of the wadati benioff zone beneath western south america the new data support previous mapping of a sharp flexure rather than a tear in the subducted nazca plate beneath southern peru and provide evidence for a similar flexure in the southward transition from nearly horizontal subduction to a slab with 30 dip at latitude 33 s in contrast the transition from 30 slab dip beneath bolivia to a nearly horizontal dip in the region between 28 s 32 s is more gradual occurring over several hundred kilometers of along strike distance between 20 s and 32 s this southward flattening corresponds to a broadening of a horizontal benchlike part of the subducted plate formed between 100 and 125 km depth the transition in continental tectonic style near 27 s 28 s from a wide volcanically active plateau to a narrow nonvolcanic cordillera appears not to be associated with the main slab flattening which begins to the north of these latitudes but with a more abrupt change in curvature of the subducted slab from convex upward to concave upward immediately below the plate boundary interface the concept of gaussian curvature is applied to slab bending to explain how subduction geometry is affected by the shape of the south american plate we hypothesize that the polarity of vertical curvature in the subducting slab is governed by the orientation of lateral curvature of the plate margin focal mechanism solutions for intermediate and deep earthquakes are grouped by geographic region and inverted for the orientation and relative magnitudes of the principal stresses results of the inversion indicate that downdip extension dominates in the slab above 350 km while downdip compression dominates at greater depths

tectonic implications of tomographic images of subducted lithosphere beneath northwestern south america we used seismic tomography to investigate the complex structure of the upper mantle below northwestern south america images of slab structure not delineated by previous seismicity studies help us to refine existing tectonic models of subducted caribbean pacific lithosphere beneath the study area beneath western venezuela and colombia we distinguish two slabs a maracaibo and a redefined bucaramanga slab the maracaibo slab coinciding with most of the bucaramanga slab previously defined by w d pennington dips in a direction of 150 at an angle of 17 to a depth of 275 km and correlates to the subducted late cretaceous oceanic plateau of the caribbean plate farther south a second slab dips at an angle of 50 in a direction of 125 to a depth of at least 500 km and correlates to the subducted oceanic crust of the nazca plate and the downdip extension of the panama island arc we refer to this slab as the redefined bucaramanga slab because it is different from the bucaramanga slab segment defined by pennington the area of the south american plate overriding both slabs is characterized by the absence of an active volcanic arc an anomalously wide topographically uplifted and tectonically active area and the northward escape of the maracaibo block along active strike slip faults in support of earlier studies we attribute this to the underthrusting of the caribbean oceanic plateau our shallowly dipping maracaibo slab along the base of the south american lithosphere and to the recent collision of the panama island arc rafted in on more steeply dipping crust of the nazca plate our redefined bucaramanga slab

the late quaternary of the lesotho highlands southern africa preliminary results and future potential of ongoing research at sehonghong shelter abstract previous archaeological and palaeoenvironmental work relating to the late quaternary of the lesotho highlands southern africa is reviewed emphasis is placed upon the region s importance for the investigation of late pleistocene hunter gatherer settlement subsistence systems and of the transition from middle msa to later stone age lsa technologies the paper then reports on the re excavation of sehonghong rock shelter in 1992 and provides an improved radiocarbon chronology for the site and initial results of the analysis of the late pleistocene and holocene assemblages recovered of particular importance is the identification of assemblages transitional between msa and lsa stoneworking techniques but a re assessment of the existing industrial subdivisions of the later stone age of southern africa may also be supported by the sehonghong sequence the palaeoenvironmental potential of the extensive faunal and botanical assemblages recovered is stressed especially given the limited extent of previous palaeoenvironmental work in lesotho the importance of the lesotho highlands for investigating differences in site use and subsistence strategies through the late pleistocene and the holocene is emphasized within an overall aim of testing previously proposed models of resource exploitation under glacial and interglacial conditions

active thrusting landscape evolution and late pleistocene sector collapse of baru volcano above the cocos nazca slab tear southern central america in this paper stratigraphy geochronology and geologic mapping are used to characterize a sequence of quaternary deposits associated with edifice failure of baru volcano which together with balanced cross sections illustrate the upper plate s response to along strike variations in subduction properties that occur across the tear in the subducting slab located at the panama fracture zone the subducting panama fracture zone is an active transform that separates disparate styles of subduction between the thick rapid and flat subduction of the cocos plate to the west and the thinner more oblique and steeper subduction of the nazca plate to the east we focus on the arc forearc region above the cocos nazca slab tear where both the fila costena inner forearc fold and thrust belt and the exhumed cordillera de talamanca terminate along strike to the southeast the fila costena thrust belt imbricates an eocene late miocene forearc basin sequence with up to 40 km of shortening inboard of cocos plate subduction and crosscuts a sequence of pleistocene and younger volcano sedimentary units on the southwestern flanks of the active baru volcano at its southeastern termination these units constrained in age by radiocarbon dating n 11 soil color and 40 ar 39 ar n 5 data include a sequence of late pleistocene to holocene debris avalanche and lahar deposits landscape morphology the areal distribution of units and new shortening estimates from balanced cross sections collectively suggest that the southeastern termination of the fila costena thrust belt actively propagates to the southeast coeval with migration of the panama triple junction the prevailing map pattern suggests that the forearc s response to southeastward migration of the slab tear varies with distance to the trench in contrast to deformation patterns in the outermost forearc plate boundary tractions associated with shallow subduction and cocos caribbean convergence are more influential in the development of arc forearc deformation than either oblique subduction of the 2 km bathymetric scarp or right lateral shear which both occur astride the subducting panama fracture zone

a pliocene quaternary compressional basin in the interandean depression central ecuador summary the segment of the interandean depression of ecuador between ambato and quito is characterized by an uppermost pliocene quaternary basin which is located between two n s trending reverse basement faults the victoria fault to the west and the pisayambo fault to the east the clear evidence of e w shortening for the early pleistocene between 1 85 and 1 21 ma favours a compressional basin interpretation the morphology river deviations landslides folded and flexure structures demonstrates continuous shortening during the late quaternary the late pliocene quaternary shortening reached 3400 f 600 m with a rate of 1 4 f 0 3 mm yr the e w shortening is kinematically consistent with the current right lateral reverse motion along the ne sw trending pallatanga fault the quito ambato zone appears to act as a n s restraining bend in a system of large right lateral strike slip faults the compressive deformation which affects the interandean depression during the pliocene is apparently coeval to the beginning subduction of very young oceanic lithosphere north of the gulf of guayaquil the relatively buoyant new crust may have significantly increased the mechanical coupling in the subduction zone from pliocene to present

plate motions in the north andean region repeated geodetic measurements with the global positioning system gps provide direct measurements of displacements due to plate motions and active crustal deformation in central america and northern south america an area of complex interaction of the nazca cocos caribbean and south american plates the displacement rates for the period 1988 1991 obtained from the results of the first three central and south america casa gps campaigns are in general agreement with the predictions of the nuvel 1 plate motion model but there are differences in detail between the observations and the model the nazca north andes convergence rate vector measured by gps is different from the nuvel 1 vector at 95 confidence the difference implies that the north andes are moving northward relative to south america the measured convergence between the caribbean plate and the north andes suggests that the southern margin of the caribbean plate is located in the south caribbean deformed belt the april 1991 costa rica earthquake and the cocos caribbean convergence rate determined by gps suggest the possibility of significant ongoing deformation between central america and the stable interior of the caribbean plate our gps results are consistent with deformation of the overriding plates at the convergent margins of central and south america and confirm that active convergence is occurring around much of the southern margin of the caribbean plate from colombia west to costa rica costa rica and panama are not part of the stable caribbean plate instead the south caribbean deformed belt and the north panama fold belt probably represent the southern margin of the caribbean plate

roughness characteristics of oceanic seafloor prior to subduction in relation to the seismogenic potential of subduction zones we have developed a new approach to characterize the seafloor roughness seaward of the trenches as a proxy for estimating the roughness of the subduction interface we consider that abrupt elevation changes over given wavelengths play a larger role in the seismogenic behavior of the subduction interface than the amplitude of bathymetric variations alone the new database subrough provides roughness parameters at selected spatial wavelengths here we mainly discuss the spatial distribution of short 12 20 km and long wavelength 80 100 km roughness rsw and rlw respectively along 250 km wide strips of seafloor seaward of the trenches compared with global trend seamounts show distinct roughness signature of much larger amplitudes at both wavelengths whereas aseismic ridges only differ from the global trend at long wavelengths fracture zones cannot be distinguished from the global trend which suggests that their potential effect on rupture dynamics is not the consequence of their roughness at least not at these wavelengths based on rlw amplitude segments along subduction zones can be defined from rough to smooth subduction zones like the solomons or the ryukyus appear dominantly rough whereas others like the andes or cascadia are dominantly smooth the relative contribution of smooth versus rough areas in terms of respective lateral extents probably plays a role in multipatch rupture and thus in the final earthquake magnitude we observe a clear correlation between high seismic coupling and relatively low roughness and conversely between low seismic coupling and relatively high seafloor roughness

cenozoic plate kinematics subduction and magmatism south american andes revised plate reconstructions and enlarged isotopic age data sets strongly support three mechanisms for controlling the configuration and associated volcanism and tectonism of the andean subduction zone these include age of subducted plate convergence rate and interaction with hotspot traces from 50 to 25 ma in the andes the age of oceanic plate subducted progressively decreased apparently resulting in a9gradual decrease in subduction angle and consequent eastward shift in the loci of magmatic activity a major reorganization of plate motions at 25 ma resulted in an increased convergence rate normal to the chilean andes and was accompanied by subsequent broadening of the locus of magmatic activity into adjacent portions of bolivia and argentina convergence rates changed only slightly across the peruvian margin and no clear effect on subsequent magmatism is indicated interaction of the andean subduction zone with the juan fernandez and easter nazca hotspot traces apparently resulted in lower angle subduction and consequent gaps in magmatic activity as predicted continuations of the traces correspond with regions lacking dated volcanic rocks the current gaps in igneous activity correspond with contemporary subduction of the two traces and this association apparently occurred in the past as well

historical 1942 ecuador and 1942 peru subduction earthquakes and earthquake cycles along colombia ecuador and peru subduction segments two large shallow earthquakes occurred in 1942 along the south american subduction zone inclose proximity to subducting oceanic ridges the 14 may event occurred near the subducting carnegie ridge off the coast of ecuador and the 24 august event occurred off the coast of southwestern peru near the southern flank of the subducting nazca ridge source parameters for these for these two historic events have been determined using long periodp waveforms p wave first motions intensities and local tsunami data we have analyzed thep waves for these two earthquakes to constrain the focal mechanism depth source complexity and seismic moment modeling of thep waveform for both events yields a range of acceptable focal mechanisms and depths all of which are consistent with underthrusting of the nazca plate beneath the south american plate the source time function for the 1942 ecuador event has one simple pulse of moment release with a duration of 22 suconds suggesting that most of the moment release occurred near the epicenter the seismic moment determined from thep waves is 6 8 1020n m corresponding ot a moment magnitude of 7 8 7 9 the reported location of the maximum intensities ix for this event is south of the main shock epicenter the relocated aftershcks are in an area that is approximately 200 km by 90 km elongated parallel to the trench with the majority of aftershocks north of the epicenter in contrast the 1942 peru event has a much longer duration and higher degree of complexity than the ecuador earthquake suggesting a heterogeneous rupture seismic moment is released in three distinct pulses over approximately 74 seconds the largest moment release occurs 32 seconds after rupture initiation the seismic moment as determined from thep waves for the 1942 peru event is 10 25 1020n m corresponding to a moment magnitude of 7 9 8 2 aftershock locations reported by the iss occur over a broad area surrounding the main shock the reported locations of the maximum intensities ix are concentrated south of the epicenter suggesting that at least part of the rupture was to the south we have also examined great historic earthquakes along the colombia ecuador and peru segments of the south american subduction zone we find that the size and rupture length of the underthrusting earthquakes vary between successive earthquake cycles this suggests that the segmentation of the plate boundary as defined by earthquakes this century is not constant

the effect of seamount subduction on seismic coupling it has long been speculated that the subduction of seamounts may have profound mechanical consequences particularly on the generation of large subduction zone earthquakes we consider this problem as a special case of the seismic coupling model of scholz and campos the model predicts that the subduction of a large seamount will increase the normal stress across the subduction interface and hence will enhance seismic coupling in the case of coupled arcs this increased coupling will result in a large increase in the recurrence intervals of earthquakes such events will therefore appear rare in the instrumental record as is observed for decoupled arcs the subduction of a large seamount should cause them to become locally coupled three of the latter localities have been found with satellite altimetry data two on the tonga kermadec trench and one on the izu bonin trench the diagnostic feature is a shallowing or obstruction of the trench axis adjacent to a chain or group of large seamounts on the subducting plate in each case the small to moderate plate interface seismicity typical of decoupled seismic zones is almost completely absent in a 100 200 km arc segment indicating that the subduction zone is locally locked as predicted by the model the occurrence of large interplate earthquakes at both tonga sites confirms that they are seismically coupled

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

effects of aseismic ridge subduction on the geochemistry of frontal arc magmas abstract aseismic ridges are variably broad linear topographic features built by submarine volcanic activity that are thicker and more buoyant than the surrounding oceanic crust together with oceanic plateaus they constitute a non trivial 30 of the oceanic floor as such aseismic ridge subduction significantly affects subduction dynamics as well as deformation of the overriding plate indeed it is considered to exert major controls on volcanic activity the generation of juvenile earth s crust and the formation of porphyry cu au deposits previous studies claimed that magma chemistry changes associated with aseismic ridge subduction are mainly due to source processes such as melting of the subducting ridge melting of the edges of the oceanic plate following slab tear or subduction erosion of the continental margin with subsequent partial melting of the scraped and subducted continental material here we explore the role of the subducted aseismic carnegie ridge on modulating frontal arc magma chemistry in the ecuadorian southernmost colombian frontal arc we show symmetric changes of several geochemical indices th nd sm nb th la ba th and evolutionary paths na2o sio2 correlations in frontal volcanic arc rocks with respect to the equatorial latitude which coincides with the projection of the crest of the subducting carnegie ridge at the frontal arc we explore the systematic along arc changes of these geochemical indices through geochemical modelling using a monte carlo approach and conclude that the trends are controlled to a significant extent by intracrustal processes in particular we argue that magmatic systems associated with the central part of the ecuadorian volcanic arc evolve through fractionation partial melting of crustal rocks and mixing at average greater depths than those in the northern and southern sides we speculate about possible links between the systematic along arc geochemical changes and the subduction of the carnegie ridge our findings highlight a new facet of the controls that are exerted by the overriding plate on the geochemical variability of arc magmas

seismic velocity structure at a gas hydrate reflector offshore western colombia from full waveform inversion seismic reflection profiles across many continental margins have imaged bottom simulating reflectors bsrs which have been interpreted as being formed at the base of a methane hydrate stability field such reflectors might arise either from an impedance contrast between high velocity partially hydrated sediments and water saturated sediments or from a contrast with partially gas saturated sediments these alternatives may be hard to distinguish by conventional amplitude versus offset or waveform modeling approaches here we investigate the origin of a high amplitude bsr in the accretionary wedge offshore of western colombia by seismic waveform inversion the inversion procedure consists of three steps firstly determination of root mean square velocities and hence estimates of the interval velocities between major reflectors by a global grid search for maximum normalized energy along elliptical trajectories in the intercept time slowness domain secondly determination of accurate interval velocities between these reflectors by a monte carlo search for maximum energy and thirdly a waveform fit in the frequency slowness domain using differential reflectivity seismograms and a conjugate gradient optimization algorithm to minimize the sample by sample waveform misfit between data and synthetic at two locations near a structural high we find a 30 m thick low velocity zone beneath the bsr with the properties of a partially gas saturated zone while at a third location where the bsr amplitude is lower we find no evidence for anomalously low velocities the preferential development of the bsr in structures that would tend to intercept fluid flow or migrating gas and the presence of free gas beneath the bsr indicate a mechanism of bsr formation in which free methane gas migrates upward into the hydrate stability field or is carried there in advecting pore water

volcano tectonic segmentation of the northern andes data from landsat images published geologic and geophysical reports and field observations are used to document eight major transverse boundaries that divide the northern andes into segments about 125 km long most of the segment boundaries have no apparent relation to structure on the nazca plate being subducted and may instead represent fundamental and long lasting structures of the continent subduction of the carnegie ridge results in increased uplift of the andes widespread and chemically diverse volcanism and major faulting and seismicity subduction of a highly fractured zone in southern ecuador has led to the formation of a separate platelet with a distinct orientation and to a notable change in volcanic style from andesitic stratovolcanoes to rhyolitic ash flow sheets recognition of the segmented structure of the northern andes provides a framework for interpreting details of the regional geology and illustrates that not all segment boundaries are related to 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

the 1999 2000 seismic experiment of macas swarm ecuador in relation with rift inversion in subandean foothills we propose to explain the origin of the double trend in seismicity of the macas swarm in the subandean cordillera of cutucu ecuador and characterize the corresponding active deformation of that region for that purpose seismological and geological data have been used with the deployment of a temporary seismological array with geological field observations and image processing we found that some earthquakes are aligned on a well known nne ssw trend corresponding to the orientation of the nodal planes of the reverse focal mechanism of the mw 7 0 1995 macas earthquake as for its aftershocks

quaternary sanukitoid like andesites generated by intracrustal processes chacana caldera complex ecuador implications for archean sanukitoids high mg diorites enriched in incompatible elements and their extrusive equivalents are rare subduction related rock types that have been found in modern arc settings and in late archean sequences where they are associated with trondhjemite tonalite granodiorite ttg suites archean rocks with these geochemical features are known as sanukitoids and despite their limited abundance are considered to be the indicators of the onset of modern plate tectonics because of their similarities to modern subduction related high mg andesites and diorites understanding the genesis of sanukitoid rocks is thus an essential step towards understanding crustal growth processes the accepted petrogenetic models for modern enriched high mg andesites and their archean equivalents the sanukitoids consist of metasomatic enrichment of the mantle wedge by slab components and its subsequent partial melting or the modification of siliceous slab components through continuous reaction with mantle peridotite during their ascent through the mantle wedge we present new data on the petrography mineral chemistry and whole rock geochemistry major and trace elements and sr nd pb isotopes of andesitic rocks from an 30 ka lava flow yuyos flow from the chacana caldera complex eastern cordillera of ecuador these rocks show a remarkable geochemical affinity with archean sanukitoids including high magnesium numbers 0 58 0 63 accompanied by high contents of incompatible elements e g th 17 23 ppm u 6 7 5 ppm ba 1600 1800 ppm sr 1430 1565 ppm la 74 94 ppm additionally the sanukitoid like andesites of yuyos are associated with predominant silica rich adakite like andesites which are widespread throughout the quaternary arc of ecuador this makes the quaternary ecuadorian magmatic province a close equivalent of the archean ttg sanukitoid association the bulk rock geochemistry petrography and mineral chemistry data indicate that the sanukitoid like features of the andesites of the yuyos flow derive from intracrustal recycling of the felsic intermediate to mafic ultramafic roots of the quaternary volcanic arc of ecuador by normal mantle derived basaltic magmas with the geochemical characteristics of continental arc basalts or high alumina basalts in view of the similarities between the yuyos andesites and archean sanukitoids in terms of geochemistry and lithological association we suggest that genetic models should consider the possibility of intracrustal recycling as a process responsible for the peculiar signatures of both archean sanukitoids and modern enriched high mg andesites

quaternary state of stress in the northern andes and the restraining bend model for the ecuadorian andes inversion of shallow focal mechanisms in the northern andes together with a neotectonic analysis of the ecuadorian andes shows that the state of stress is homogeneous in most of the northern andes e w trending 1 however north of 5 n 1 is roughly nw se trending this difference in state of stress is due to the force balance between the nazca caribbean and south american plates south of 5 n the state of stress appears to be mainly controlled by the nazca south american plate interaction while north of 5 n it appears to be mainly controlled by the caribbean south american plate interaction this state of stress difference from south to north is consistent with dextral motions along faults trending parallel to the chain south of 5 n and sinistral motions along faults trending parallel to the chain north of 5 n this inversion of shallow focal mechanisms also illustrates the incomplete strain partitioning of the oblique convergence between the nazca and south american plates along the ecuadorian southern colombian trench the convergence obliquity that increases northward controls the active transcurrent fault slip rate in the upper plate finally this convergence obliquity is responsible for the location of the seismicity in the upper plate in addition the andean block is considered in this paper as a large triple zone under constriction with one of its borders caribbean behaving partly as a free border

late quaternary kinematics of the pallatanga strike slip fault central ecuador from topographic measurements of displaced morphological features the northeast trending pallatanga right lateral strike slip fault runs across the western cordillera connecting n50 e n70 e trending normal faults in the gulf of guayaquil with n s reverse faults in the interandean depression over most of its length the fault trace has been partly obscured by erosional processes and can be inferred in the topography only at the large scale only the northern fault segment which follows the upper rio pangor valley at elevations above 3600 m is prominent in the morphology valleys and ridges cut and offset by the fault provide an outstanding record of right lateral cumulative fault displacement the fault geometry and kinematics of this particular fault segment can be determined from detailed topographic levellings the fault strikes n30 e and dips 75 to the nw depending on their size and nature transverse morphological features such as tributaries of the rio pangor and intervening ridges reveal right lateral offsets which cluster around 27 11m 41 5 4 m 590 65 m and 960 70 m the slip vector deduced from the short term offsets shows a slight reverse component with a pitch of about 11 5 sw the 41 5 4 m displacements are assumed to be coeval with the last glacial termination yielding a mean holocene slip rate of 2 9 4 6 mm yr 1 assuming a uniform slip rate on the fault in the long term the 27 m offset appears to correlate with an identified middle holocene morphoclimatic event and the long term offsets of 590 m and 960 m coincide with the glacial terminations at the beginning of the last two interglacial periods

ridge collision slab window formation and the flux of pacific asthenosphere into the caribbean realm mantle wedge derived arc volcanism ceased in southern costa rica after ca 8 ma because of subduction of the aseismic cocos ridge beneath the central american arc and the subsequent opening of a slab window geochemical and isotopic compositions of small volumes of adakitic and alkalic backarc lavas erupted between 5 8 and 2 ma identify a source derived from the galapagos plume the presence of this source is explained by an influx of pacific upper mantle into the caribbean mantle wedge through a slab window where the alkalic rocks form by melting of the upwelling mantle and the adakites result from melting of the leading edge of the subducted cocos ridge by using geochemical and isotopic signatures we trace this upper mantle flow beneath central america from southern costa rica northward at a rate of 40 mm yr

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

pre eruptive physical conditions of el reventador volcano ecuador inferred from the petrology of the 2002 and 2004 05 eruptions abstract a petrological study of the eruptive products of el reventador allowed us to infer the magmatic processes related to the 2002 and 2004 05 eruptions of this andesitic stratovolcano on november 3 2002 el reventador experienced a highly explosive event which was followed by emplacement of two lava flows in november december 2002 silica contents range from 62 to 58 wt sio2 for the november 3 pyroclastic deposits to 58 56 and 54 53 wt sio2 for the successive lava flows in november 2004 eruptive activity resumed supplying four new lava flows 56 54 wt sio2 between november 2004 and august 2005 volatile contents in matrix glasses and glass inclusions from the november 3 pyroclastic deposits allow us to estimate the total amount of so2 and hcl released into the atmosphere during the paroxysmal phase i e 80 kt of so2 and 280 kt of hcl pre eruptive pressure temperature conditions of the magmas range from 300 to 150 mpa and 1000 c with high water contents 5 wt we propose the existence of an andesitic magma body located at 7 12 km depth that is frequently intruded by more primitive hydrous magmas from a deeper source the initial crystallization of amphibole from the hydrous primitive magma seems typical of el reventador as well as the historically recurrent and regular periods of eruptive activity lasting several years this eruptive behaviour coupled with the fractionation and mixing processes inferred from the 2002 and 2004 05 petrologic data suggest that deep magmatic recharge at el reventador is frequent and is probably responsible for the high frequency of eruptions

deciphering the late cretaceous cenozoic structural evolution of the north peruvian forearc system the link between plate tectonics and the evolution of active margins is still an ongoing task to challenge since the acceptance of plate tectonic paradigm this paper aims at deciphering the structural architecture and uplift history of the north peruvian forearc system to better understand the evolution and the mechanics that govern the late cretaceous cenozoic building of this active margin in this study we report surface structural geology data interpretation of seismic reflection profiles apatite fission track data and the construction of two offshore onshore crustal scale balanced cross sections the structure of the north peruvian forearc system is dominated by an accretionary style with northwestward propagation of thrust related structural highs involving continental oceanic basement rocks and off scrapped sediments the thrust systems bound thick thrust top forearc depocenters mainly deformed by crustal normal to strike slip faults and thin skinned gravitational instabilities the sequential restoration of the margin calibrated with apatite fission track data suggests a correlation between uplift shortening and plate convergence velocity during late cretaceous and miocene pliocene quaternary shortening and uplift of the coastal zone is rather related to the subduction of asperities during convergence rate decrease the development of crustal normal to strike slip faulting and subsidence zones might be the consequence of slab flexure local basal erosion along subduction fault and or oblique subduction associated with sediment loading control we conclude that the evolution of the north peruvian forearc system was controlled by subduction dynamics strong sediment accumulation and recent ridge subduction and it recorded the orogenic loading evolution of the andes over the cenozoic

subduction of the eastern panama basin and seismotectonics of northwestern south america spreading along the cocos nazca plate boundary since the breakup of the farallon plate in the miocene has resulted in the formation of the panama basin and a complex interaction of plates in and near northwestern south america current plate boundaries have been defined and segments of subducted lithosphere identified through selection of hypocentral locations of earthquakes considering only well located events and through focal mechanism determinations the existence of relict plate boundaries bathymetric features and the panamanian isthmus has affected the subduction process of the nazca plate beneath south america and determined the present day configuration of the subducting lithospheric plate there is no single triple junction separating the caribbean south american and nazca plates instead the panamanian isthmus and surrounding areas are accommodating east west compression and a lesser degree of north south compression along a series of thrust faults striking nw to ne and the andean ranges of ecuador colombia and venezeula are moving as a block nne relative to the rest of the south american plates along a system of faults following the front of the eastern cordillera the subducted portions of the panama basin and old farallon plate have become segmented into three pieces recognized in this study from north to south they are 1 a bucaramanga segment continuous with the caribbean seafloor northwest of colombia 2 a cauca segment continuous with oceanic crust nazca plate currently being subducted beneath south america at the colombia ecuador trench and 3 an ecuador segment at the northern end of the subducted lithospheric plate which is dipping at a small angle to the east beneath northern peru the segmentation of the subducted plate can be explained by the buoyancy of bathymetric features which have been partially subducted

circum pacific seismic potential 1989 1999 the seismic potential for 96 segments of simple plate boundaries around the circum pacific region is presented in terms of the conditional probability for the occurrence of either large or great interplate earthquakes during the next 5 10 and 20 years i e 1989 1994 1989 1999 and 1989 2009 this study represents the first probabilistic summary of seismic potential on this scale and involves the comparison of plate boundary segments that exhibit varying recurrence times magnitudes and tectonic regimes presenting these data in a probabilistic framework provides a basis for the uniform comparison of seismic hazard between these differing fault segments as well as accounting for individual variations in recurrence time along a specific fault segment and uncertainties in the determination of the average recurrence time

subduction controls on the compositions of lavas from the ecuadorian andes three volcanoes of the ecuadorian andes atacazo antisana and sumaco lie in a transect perpendicular to the trench and the main trend of the andean arc each of the volcanoes lies on crust of substantially different age composition and thickness few compositional or isotopic features correspond in a straightforward way to the type of the crust through which the magmas have passed isotopic data limit assimilation to 15 at each of the volcanoes instead a systematic relationship exists between the compositions of the lavas and the depth to the benioff zone suggesting that subduction imparts the principal control on the compositions of the magmas atacazo s lavas have low concentrations of the incompatible trace elements and very large lil hfs ratios sumaco s lavas are strongly enriched in the incompatible trace elements and have small lil hfs ratios antisana s lavas are intermediate in almost every respect these features are consistent with devolatilization of the subducted slab controlling the extent of partial melting of a depleted mantle source a mixing and melting model suggests the volcanic front magmas are made by large extent of partial melting 15 and include a large slab input 1 1 added to the depleted mantle the magmas of the middle belt of volcanoes are made by smaller extent of partial melting 3 induced by moderate amounts of slab derived fluid 0 06 the back arc magmas result from small degrees of melting 2 and small slab input

trench motion slab geometry and viscous stresses in subduction systems summary a semi analytic 3 d model for subduction within a newtonian viscous upper mantle provides a dynamically consistent means of computing viscous stress trench motion and slab geometry in subduction systems although negative slab buoyancy provides the basic driving force for subduction slabs that extend from the surface to the base of the upper mantle are oversupported by viscous stresses in the shallow 100 150 km mantle and undersupported by viscous stresses at greater depth in the upper mantle these deeper parts of the subduction system act as an engine for subduction while shallower parts act as a brake on trench motion trench migration rates and slab geometry reflect a competition between these two effects during steady state subduction trench migration rates vary approximately linearly with slab buoyancy and model rates of trench motion are in good agreement with the range of observed rates for a two layer upper mantle viscosity of 2 10 20 pa s above 300 km and 5 10 20 pa s below steady state slab dip increases as slab density decreases especially for very low density slabs which dip significantly more steeply than high density slabs the horizontal velocity at the top of the lower mantle measured relative to the foreland has a very large effect on trench migration rates rivalling or even exceeding that of slab buoyancy slab width parallel to the trench also has a significant effect on trench migration rates due to the viscous pressure of toroidal flow around the slab the stiffness of the subducting lithosphere does not exert a significant effect on trench migration rates or slab geometries for rigidities compatible with oceanic lithosphere very stiff slabs with elastic plate thicknesses more that 40 km or viscosities in the range of 10 25 10 26 pa s subduct significantly more slowly than weak slabs with trench migration rates in the range of half to a third that of weak slabs large unexpected effects on trench migration rates and slab geometry are exerted by the structure and density of the frontal prism and overriding plate indicating that local geology can exert important constraints on slab dynamics during non steady state subduction rates of trench migration respond rapidly as variably buoyant lithosphere penetrates into the asthenosphere in the absence of other driving forces for convergence trench migration rates can change by a factor of two or more in as little as 2 3 myr for example from 35 to 70 mm yr 1 when an oceanic piece of slab follows a continental one into the subduction system subduction of variable buoyancy lithosphere is accompanied by changes in slab dip with depth and through time

strike slip deformation within the colombian andes the colombian andes are characterized by a dominant ne structural trend which is offset by ene trending right lateral and nw trending left lateral structures ne trending faults are either dip slip or oblique thrusts generated as a result of a transpressive regime active since at least palaeogene times nw trending faults are considered to be reactivated pre cretaceous extensional structures right lateral shear on ene trending faults has resulted from oblique convergence between the nazca plate and the northern andes major changes in the geometry of the oblique plate convergence between the nazca and south american plates have generated the northward escape of the northern andes and stress strain partitioning within the mountain belt these strike slip structures have exerted important controls on sedimentation source rock distribution fluid flow and ore mineralization during cenozoic times the interpretation of the northern andes as a mountain belt affected by strike slip deformation provides a structural context in which to reassess the exploration plays each of these cordilleras has a different composition and evolution as a result of different tectonic processes that have affected the region since mesozoic times the central cordillera originated in response to triassic subduction and consequent volcanic and igneous activity barrero et al 1969 barrero vesga 1976 bartok et al 1981 triassic and jurassic backarc rifting generated a predominantly ne sw structural trend that influenced the sedimentation in the adjacent magdalena valley and eastern cordillera the western cordillera is composed of oceanic crust and deformed deep marine sediments representing an accretionary complex established in the late cretaceous de freitas et al 1997 the eastern cordillera represents the inversion of thick mesozoic and tertiary sedimentary basins see taboada et al 2000 eastward subduction of the nazca plate beneath the northern andes occurs at c 60 mm a 1 trenkamp et al 2002 along the western margin of ecuador and colombia fig 1 additionally north of latitude 8 n the caribbean plate is moving ese at an average velocity of 20 mm a 1 trenkamp et al 2002 as a result of the oblique convergence of both the nazca and the caribbean plates with the northern andes the andean block fig 1 is moving northeastwards relative to the south american plate pennington 1981 kellogg et al 1985 the colombian andes have been interpreted as an assemblage of terraines that have been accreted to south america with a dominant nne structural trend etayo serna et al 1983 lopez barrero 2003 this interpretation has led to the assumption of plane strain deformation orthogonal to the strike of the main structures which in colombia is almost parallel to the continental margin fig 1 for this reason the colombian andes have been described as a classical fold and thrust belt mojica franco 1990 schamel 1991 cooper et al 1995 however oblique ene and nw strike slip deformation has been recognised in the region feininger 1970 boinet et al 1986 diederix et al 1987 cuervo 1995 velandia komuro 1998 montes 2001 ujueta 2001 branquet et al 2002 acosta et al 2004 and the importance of these structures in the evolution of the belt has for the most part been neglected in this paper a variety of geological geophysical geodetic and geodynamic information has been integrated to examine the origin and evolution and of some of these strike slip fault systems within the colombian andes

the rupture process of the great 1979 colombia earthquake evidence for the asperity model we have determined the asperity distribution in the segment of the ecuador colombia subduction zone that was ruptured by the 1979 mw 8 2 and 1958 mw 7 7 earthquakes for the 1979 earthquake we are able to document directivity in the p wave source time functions and consequently have spatially located the moment release by combining results from both p wave and surface wave directivity we have defined three regions 1 from 0 to 56 km northeast of the epicenter with low moment release and a small seismic displacement 2 the asperity from 56 to 116 km northeast of the epicenter with most of the moment release and a displacement of 6 m and 3 a small displacement region from 116 to 180 240 km northeast of the epicenter thus the 1979 rupture zone is composed of an asperity 60 km in length bounded by two weak small displacement regions the displacement at the asperity is equal to the cumulative tectonic displacement between 1979 and 1906 the year of the previous great earthquake in this segment the 1958 earthquake is characterized by the rupture of a small asperity length scale approximately 25 km in the epicentral region the 1958 event continued to rupture past this small epicentral asperity and stopped near the epicenter of the following 1979 event this detailed study of the spatial variation in moment release offers strong support for the asperity model of large earthquake occurrence

seismic evidence for a slab tear at the puerto rico trench 1 the fore arc region of the northeast caribbean plate north of puerto rico and the virgin islands has been the site of numerous seismic swarms since at least 1976 a 6 month deployment of five ocean bottom seismographs recorded two such tightly clustered swarms along with additional events joint analyses of the ocean bottom seismographs and land based seismic data reveal that the swarms are located at depths of 50 150 km focal mechanism solutions found by jointly fitting p wave first motion polarities and s p amplitude ratios indicate that the broadly distributed events outside the swarm generally have strike and dip slip mechanisms at depths of 50 100 km while events at depths of 100 150 km have oblique mechanisms a stress inversion reveals two distinct stress regimes the slab segment east of 65 w longitude is dominated by trench normal tensile stresses at shallower depths 50 100 km and by trench parallel tensile stresses at deeper depths 100 150 km whereas the slab segment west of 65 w longitude has tensile stresses that are consistently trench normal throughout the depth range at which events were observed 50 100 km the simple stress pattern in the western segment implies relatively straightforward subduction of an unimpeded slab while the stress pattern observed in the eastern segment shallow trench normal tension and deeper trench normal compression is consistent with flexure of the slab due to rollback these results support the hypothesis that the subducting north american plate is tearing at or near these swarms the 35 year record of seismic swarms at this location and the recent increase in seismicity suggest that the tear is still propagating

correlations between nazca farallon plate kinematics and forearc basin evolution in ecuador the tertiary evolution of the forearc basins of ecuador shows a close correlation between the changing convergence rate of the farallon and later nazca oceanic plates and continental south america the correlation occurs during the subduction of a relatively young slab and in the late miocene onset of the subduction of the carnegie aseismic ridge the ecuador forearc basins lie on a basement of oceanic crust known as the pinon terrane the accretion of this terrance occurred in the paleocene as the leading edge of the farallon plate the macuchi island arc collided with south america in the middle eocene this forearc terrane was the site of major pull apart basin formation and turbiditic sedimentation coincident with a phase of very rapid convergence between chron 21 and chron 13 48 37 ma this deformation was bounded by the trench and a major dextral strike slip fault zone and resulted in the northward translation of the forearc with respect to continental south america during the oligocene a phase of extension normal to the trend of the active margin occurred coincident with a phase of relatively slow convergence chron 13 to chron 6 37 20 ma this extension was followed in the middle miocene by inversion of most of the forearc basins coincident with a return to relatively fast convergence from chron 6 20 ma to the present day subduction of the carnegie aseismic ridge occurred during this period circa 8 ma to present and may have enhanced the compressive event further northward translation of the forearc silver accompanied this later deformation the relationships outlined for the forearc may be modeled in terms of a dynamic orogenic wedge which responds directly to changes in convergence rate at the subduction zone the convergence rate appears to be an important control on the coupling between the downgoing slab and overriding continental plate

can slab melting be caused by flat subduction slab melting has been suggested as a likely source of adakitic arc magmas i e andesitic and dacitic magmas strongly depleted in y and heavy rare earth elements existing numerical and petrologic models however restrict partial melting to very young 5 ma oceanic crust typically at 60 80 km depth paradoxically most of the known pliocene quaternary adakite occurrences are related to subduction of 10 45 ma lithosphere which should not be able to melt under normal subduction zone thermal gradients we propose an unusual mode of subduction known as flat subduction occurring in 10 of the world s convergent margins that can produce the temperature and pressure conditions necessary for fusion of moderately old oceanic crust of the 10 known flat subduction regions worldwide eight are linked to present or recent 6 ma occurrences of adakitic magmas observations from chile ecuador and costa rica suggest a three stage evolution 1 steep subduction produces a narrow calc alkaline arc typically 300 km from the trench above the asthenospheric wedge 2 once flat subduction begins the lower plate travels several hundred kilometers at nearly the same depth thus remaining in a pressure temperature window allowing slab melting over this broad distance and 3 once flat subduction continues for several million years the asthenospheric wedge disappears and a volcanic gap results as in modern day central chile or peru the proposed hypothesis which reconciles thermal models with geochemical observations has broad implications for the study of arc magmatism and for the thermal evolution of convergent margins

quaternary uplift astride the aseismic cocos ridge pacific coast costa rica the pacific coast of costa rica lies within the central american forearc and magmatic arc region that was created by northeastward subduction of the cocos plate beneath the caribbean plate at the middle america trench from the peninsula de nicoya south eastward toward the peninsula de osa and the peninsula de burica on the panamanian border the middle america trench loses its physiographic expression where it intersects the aseismic cocos ridge interaction between subduction of the buoyant aseismic cocos ridge and the overriding caribbean plate is invoked to explain the variation in rates of vertical crustal uplift along a coastal transect from nicoya to burica the pliocene and pleistocene stratigraphic record and holocene marine terraces and beach ridge complexes indicate that maximum rates of crustal uplift have occurred on the peninsula de osa immediately landward of the aseismic cocos ridge crustal uplift rates decrease northwest toward the peninsula de nicoya and to a lesser extent southwest toward the peninsula de burica the late quaternary stratigraphy on the peninsula de osa is subdivided into two major chronostratigraphic sequences from groupings of radiocarbon dates crustal uplift rates calculated from these sequences systematically decrease from 6 5 to 2 1 m ka north east across the peninsula deformation of the peninsula is modeled as uplifted and down to the northeast tilted fault blocks with an angular rotation rate of 0 03 to 0 06 per thousand years although less well constrained crustal uplift rates on the peninsula de nicoya 200 km to the northwest of the peninsula de osa vary from 1 m ka for pliocene and pleistocene sediments to 2 5 m ka for holocene marine terraces in the quepos region 100 km to the northwest of the peninsula de osa calculated uplift rates derived from incision of late quaternary fluvial terraces range from 0 5 to 3 0 m ka on the peninsula de burica only 60 km to the southwest of the peninsula de osa calculated uplift rates range from 4 7 m ka for a late holocene marine terrace to 1 2 m ka for post late pliocene deep sea sediments the variations in calculated uplift rates on the peninsula de osa constrain a dynamic model for subduction of the cocos ridge and the resulting uplift of the overriding caribbean plate deflection of the caribbean plate is modeled using various effective elastic thicknesses as the response of an elastic plate to the buoyant force of the subducted cocos ridge because the shape of the subducted end of the cocos ridge is unknown two scenarios are evaluated 1 a radially symmetric ridge with a slope similar to the slope of the flanks of the ridge and 2 a ridge where the subducted end was truncated by the panama fracture zone the best fit model utilizes a truncated ridge that has been subducted during the past 0 5 m y 50 km beneath the overriding caribbean plate which has an effective elastic thickness of 5 km the model predicts that the highest uplift rate should be 3 7 m ka and occur on the southwest coast of the peninsula de osa the rate of uplift slows considerably to the northeast and indicates that the peninsula de osa is tilting to the northeast which agrees with observations in that region the predicted uplift rate attributed to aseismic ridge subduction also decreases along the coast both north and south of the peninsula de osa resulting in little uplift that can be attributed to cocos ridge subduction in the northwestern portions of the peninsula de nicoya

lithospheric structure and compensation mechanisms of the galapagos archipelago volcanic islands of the galapagos archipelago are the most recent subaerial expression of the galapagos hotspot these islands and numerous seamounts are constructed mainly upon a broad volcanic platform that overlies very young 10 m y oceanic lithosphere just south of the active galapagos spreading center the 91 w fracture zone crosses the platform and creates an estimated 5 m y age discontinuity in the lithosphere major tectonic features of the galapagos include an unusually broad distribution of volcanic centers pronounced structural trends such as the nw se wolf darwin lineament wdl and a steep escarpment along the western and southern margins of the archipelago we use shipboard gravity and bathymetry data along with geosat geoid data to explain the tectonic and structural evolution of the galapagos region we model the gravity anomalies using a variety of compensation models including airy isostasy continuous elastic flexure of the lithosphere and an elastic plate with embedded weaknesses and we infer significant lithospheric strength variations across the archipelago the outboard parts of the southern and western escarpment are flexurally supported with an effective elastic thickness of 12 km this area includes the large shield volcanoes of fenandina and isabela islands where the lithosphere regionally supports these volcanic loads the central platform is weaker with an elastic thickness of 6 km or less and close to airy isostasy the greatest depths to the moho are located beneath eastern isabela island and the central platform thinner lithosphere in this region may account for the broad distribution of volcanoes the extended period of eruption of the central volcanoes and their reduced size the transition from strong to weak lithosphere along the southern escarpment appears to be abrupt within the resolution of our models and can be best represented by a free end or faultlike discontinuity also modeling the wdl as a lithospheric fault increases the match to the observed gravity anomalies the primary feature is the weak central platform whose existence is a logical convergence of several factors the 91 w transform age offset the possible increase in age of the lithosphere along the southern part of the platform due to ridge jumping and the formation of the central platform at a spreading ridge

along strike variations in the thermal and tectonic response of the continental ecuadorian andes to the collision with heterogeneous oceanic crust oblique to strike geological segmentation in the andean chain has been previously recognised at various scales and is commonly attributed to changes in the convergence vectors of the oceanic and continental plates as well as the upperplate expressions of differing along strike subducted slab age strength and composition we present new white mica and biotite 40 ar 39 ar and zircon and apatite fission track data from several traverses across the cordillera real of ecuador in the northern andes that reveal distinct along strike differences in the timing of accelerated crustal cooling during the cenozoic the data record elevated cooling rates from temperatures of v380 c during v65 55 and v43 30 ma from all sampled regions of the cordillera real and at v15 ma and since v9 ma in the northern cordillera real each cooling period was probably driven by exhumation in response to the accretion and subduction of heterogeneous oceanic crust elevated cooling rates of up to v30 20 c myr were initiated during the palaeocene and eocene early oligocene along the entire contemporaneous margin of ecuador and were driven by the accretion of the oceanic pallatanga terrane and pin on macuchi block respectively onto northwestern south america both of these geological provinces originated at the southern parts of the leading and trailing boundaries of the caribbean plateau and accreted onto the margin during the approximately northeastward migration of the plateau into its current position within ecuador the development of higher topography and elevated cooling rates of up to 50 c myr at v15 ma and since v9 ma are restricted to the region north of 1 30ps and is situated above the postulated subducted flatslab section of the aseismic carnegie ridge plate convergence rate calculations suggest the carnegie ridge collided with the ecuador trench at v15 ma which caused the pre existing coastal provinces to displace to the northeast subsequently driving extension and marine ingressions in southern ecuador and compression and uplift in northern ecuador fl 2001 elsevier science b v all rights reserved

cocos nazca slab window beneath central america abstract integration of petrologic and tectonic data favours a model of slab window formation beneath central america in the pliocene pleistocene central america has been the site of voluminous cenozoic arc volcanism the cocos and nazca plates which are subducting beneath central america are diverging along the east trending cocos nazca spreading ridge since 25 ma the americas have advanced about 1800 km west over the ridge transform system since at least 8 ma plate integrity and the ridge transform configuration have been preserved during convergence resulting in subduction of the spreading ridge and development of a slab window the panama fracture zone an active transform fault is the part of the ridge transform system currently being subducted the ridge transform system formerly adjoining the northern end of the panama fracture zone is likely to have been left stepping we use present day plate motions to design a slab window to fit known variations in igneous composition hypocentre distribution and mantle anisotropy the modeling demonstrates that subduction of ridge segments and resultant slab window development began between 6 and 10 ma cessation of ridge subduction occurred between 1 and 3 ma when subduction of the panama fracture zone is considered to have begun the slab window is continuing to expand and migrate northeastward below the central american volcanic arc the absence of a wadati benioff zone from southeastern costa rica through panama corresponds to the position of the slab window within this region dacitic and rhyolitic volcanic rocks have adakitic compositions and are thought to result from anatexis of the young buoyant crust which forms the trailing edges of the slabs bounding the window basalts in this area were derived from an enriched ocean island type mantle source whereas basalts from the rest of the arc in nicaragua el salvador and guatemala are mainly derived from slab modified depleted mantle characteristic of volcanic arcs the presence of ocean island type mantle beneath southern costa rica and panama is explained by eastward flow of enriched asthenosphere from the galapagos plume head through the slab window and into the volcano source region eastward transfer of asthenosphere is consistent with global plate motion studies and seismic anisotropy in the asthenosphere beneath the nazca and caribbean plates the flow of peridotite is a consequence of progressive shrinkage of the pacific mantle reservoir and concurrent expansion of the atlantic mantle reservoir

seismicity fault plane solutions depth of faulting and active tectonics of the andes of peru ecuador and southern colombia the intracontinental seismicity of the andes of peru ecuador and southern colombia is concentrated along the easternmost flank of the cordillera beneath the western margin of the sub andes the focal depths and fault plane solutions of the largest events were constrained by comparing the observed long period p waves with synthetic waveforms in general the fault plane solutions show reverse faulting on steeply dipping nodal planes striking northwest southeast and reflect crustal shortening perpendicular to the range probably in response to the subduction of the nazca plate to the west earthquakes in the sub andes occur at depths of between 8 and 38 km indicating that much of the crust deforms in a brittle manner the seismicity seems to reflect antithetic underthrusting of the brazilian shield beneath the eastern margin of the andes however the earthquakes are too deep and their nodal planes are too steep to be associated with the thin skinned decollement of the sedimentary cover instead they appear to reflect the tectonic deformation of the underthrust basement west of the zone of active deformation at the surface in the sub andes hence the earthquakes on the east side of the andes may reflect the style of deformation in the hinterland of fold and thrust belts such as in the canadian rockies while much of the decollement farther east occurs aseismically in contrast normal faulting occurs in the high andes on planes parallel to the strike of the mountain belt thus there is a delicate balance between the compressive stress applied to the andes in the direction of subduction which causes thrust faulting in the sub andes and the gravitational body forces acting on the topographically higher parts and the crustal root of the andes which may cause normal faulting at high elevation

complex anisotropy beneath the peruvian flat slab from frequency dependent multiple phase shear wave splitting analysis flat or shallow subduction is a relatively widespread global occurrence but the dynamics remain poorly understood in particular the interaction between flat slabs and the surrounding mantle flow has yet to be studied in detail here we present measurements of seismic anisotropy to investigate mantle flow beneath the peruvian flat slab segment the largest present day region of flat subduction we conduct a detailed shear wave splitting analysis at a long running seismic station nna located near lima peru we present measurements of apparent splitting parameters fast direction and delay time t for sks scs and local s phases from 80 events we observe well defined frequency dependence and backazimuthal variability indicating the likely presence of complex anisotropy forward modeling the observations with two or three layers of anisotropy reveals a likely layer with a trench normal fast direction underlying a layer with a more trench oblique to trench subparallel fast direction in order to further constrain the anisotropic geometry we analyzed the source side splitting from events originating within the slab measured at distant stations beneath the flat slab segment we found trench normal fast splitting directions in the subslab mantle while within the dipping portion of the slab further to the east likely trench subparallel anisotropy within the slab itself this subslab pattern contradicts observations from elsewhere in south america for normal i e more steeply dipping slab conditions it is similar however to inferences from other shallowly dipping subduction zones around the world while there is an apparent link between slab dip and the surrounding mantle flow at least beneath peru the precise nature of the relationship remains to be clarified

mantle subducting slab structure in the region of the 2010 m8 8 maule earthquake 30 40 s chile we present a new tomographic model of the mantle in the area of the 2010 m8 8 maule earthquake and surrounding regions increased ray coverage provided by the aftershock data allows us to image the detailed subducting slab structure in the mantle from the region of flat slab subduction north of the maule rupture to the area of overlapping rupture between the 1960 m9 5 and the 2010 m8 8 events to the south we have combined teleseismic primary and depth phase arrivals with available local arrivals to better constrain the teleseismic earthquake locations in the region which we use to conduct nested regional global tomography the new model reveals the detailed structure of the flat slab and its transition to a more moderately dipping slab in the maule region south of the maule region a steeply dipping relic slab is imaged from 200 to 1000 km depth that is distinct from the moderately dipping slab above it and from the more northerly slab at similar depths we interpret the images as revealing both horizontal and vertical tearing of the slab at 38 s to explain the imaged pattern of slab anomalies in the southern portion of the model in contrast the transition from a horizontal to moderately subducting slab in the northern portion of the model is imaged as a continuous slab bend we speculate that the tearing was most likely facilitated by a fracture zone in the downgoing plate or alternatively by a continental scale terrane boundary in the overriding plate