segmentation of the nazca and south american plates along the ecuador subduction zone from wide angle seismic profiles we describe the deep structure of the south colombian northern ecuador convergent margin using travel time inversion of wide angle seismic data recently collected offshore the margin appears segmented into three contrasting zones in the north zone affected by four great subduction earthquakes during the 20th century normal oceanic crust subducts beneath the oceanic cretaceous substratum of the margin underlined by seismic velocities as high as 6 0 6 5 km s in the central zone the subducting oceanic crust is over thickened beneath the carnegie ridge a steeper slope and a well developed high velocity cretaceous oceanic basement characterizes the margin wedge this area coincides with a gap in significant subduction earthquake activity in the south zone the subducting oceanic crust is normal the fore arc is characterized by large sedimentary basins suggesting significant subsidence velocities in the margin wedge are significantly lower and denote a different nature or a higher degree of fracturing even if the distance between the three profiles exceeds 150 km the structural segmentation obtained along the ecuadorian margin correlates well with the distribution of seismic activity and the neotectonic zonation

influence of the subduction of the carnegie volcanic ridge on ecuadorian geology reality and fiction the proposed ages for the collision of the carnegie ridge with the south america trench offshore ecuador range from 1 to 15 ma in this time frame many geological features of ecuador are commonly linked to the subduction of the carnegie ridge 1 after the ridge collided with the trench at ca 15 ma the subsequent interplate coupling produced high exhumation rates of volcanic materials at ca 9 ma 2 the oblique convergence of the carnegie ridge would have caused the northward drift of the north andean block and the opening of the gulf of guayaquil 3 during the late miocene the subduction of the carnegie ridge would have triggered a regional tectonic inversion along the forearc 4 along the collision front of the ridge with the trench subduction related erosion is occurring and the ecuadorian continental margin is being uplifted in the present day 5 the chemistry of the active volcanic arc is explained as resulting from the arrival of the carnegie ridge into the trench for instance the adakitic signal which appears at 1 5 ma is thought to be ridge induced 6 the buoyancy of the subducted carnegie ridge would explain the flatness of the slab beneath ecuador in this paper we review the geological evolution of the northern andes in order to establish which of these geological events may be related to the subduction of the carnegie ridge this review suggests that there is no clear deformation linked with the subduction of the carnegie ridge or with its landward prolongation postulated at depth

mode and timing of terrane accretion in the forearc of the andes in ecuador the volcanic basement of the ecuadorian western cordillera pallatanga formation and san juan unit is made up of mafi c and ultramafi c rocks that once formed an oceanic plateau radiometric ages from these rocks overlap with a hornblende 40 ar 39 ar plateau age of 88 1 6 ma age obtained for oceanic plateau basement rocks of the pinon formation in coastal ecuador and with ca 92 88 ma ages reported for oceanic plateau sequences in the caribbean and western colombia these results suggest that the oceanic plateau rocks of the western cordillera and fl at forearc in ecuador are derived from the late cretaceous caribbean colombia oceanic plateau intraoceanic island arc sequences rio cala group overlie the plateau in the western cordillera and yield crystallization ages that range between ca 85 and 72 ma the geochemistry and radiometric ages of island arc lavas from the rio cala group combined with the age range and geochemistry of their turbiditic volcaniclastic products indicate that the arc was initiated by westward subduction beneath the caribbean plateau they are coeval with island arc rocks of coastal ecuador las orquideas san lorenzo and cayo formations and colombia ricaurte arc these island arc

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

origin and cretaceous tectonic history of the coastal ecuadorian forearc between 1 n and 3 s paleomagnetic radiometric and fossil evidence the presence of at least one block pallatanga block derived from the caribbean colombian oceanic plateau has been proposed in the western cordillera of ecuador new 40ar 39ar dating of the basement of the coastal blocks 90 87 ma and fossil ages in the overlying calentura fm 89 84 ma argue for late turonian coniacian extrusion of the basement of the pinon block paleomagnetic inclinations indicate that the mafic basement of the pinon and san lorenzo blocks extruded at equatorial low southern latitudes no subsequent significant latitudinal drift can be observed during the cretaceous rapid changes in paleomagnetic declination between 73 and 70 ma in both the pinon and the san lorenzo blocks indicate that the large igneous province which originally hosted the coastal blocks experienced a major tectonic event during late campanian times similar late turonian coniacian basement ages and late campanian tectonic events are reported in the pallatanga block of the western cordillera we propose that the pinon san lorenzo and pallatanga blocks all derive from a common large igneous province the caribbean colombian oceanic plateau and were incorporated into the south american margin by collision during the late campanian 73 70 ma

simple bouguer gravity anomaly field and the inferred crustal structure of continental ecuador the simple bouguer gravity anomaly field of continental ecuador corresponds closely to the physiographic provinces of the country the sierra which includes the andes and their foothills is characterized by a pronounced low with values to 292 mgal which reflects the deep andean root bouguer anomalies over the oriente become less negative away from the sierra chiefly in response to progressive thinning of continental crust eastward the costa between the sierra and the pacific shore in the north has the most positive on land bouguer anomalies 162 mgal so far known in the western hemisphere this part of the costa is underlain by an ancient oceanic plate now welded to the northwestern corner of the otherwise continental south american plate

crust and mantle structure across the basin and range colorado plateau boundary at 37 n latitude and implications for cenozoic extensional mechanism we present new evidence on the seismic velocity and density of the crust and upper mantle along a 200 km long transect across the eastern basin and range and western colorado plateau at 37 n latitude receiver functions computed from the p waveforms recorded with 10 portable broadband stations deployed along the transect were used to estimate crustal thickness variations the crust is 30 35 km thick within the eastern basin and range and increases over a distance of 100 km at the western edge of the colorado plateau reaching a maximum of approximately 45 km east of the hurricane fault the timing of crustal multiples within the reciever functions were used to estimate the v p v s of the crust along the profile and we found that the western colorado plateau crust is characterized by a high poisson s ratio 0 28 0 29 indicative of a crust with an average mafic composition we estimated the upper mantle lid thickness along our profile based on teleseismic p wave travel times and constraints provided by gravity data our data and available geophysical constraints are most consistent with a lithosphere that thickens from an average thickness of 60 km beneath the basin and range to 100 km beneath the western colorado plateau although the basin and range lithosphere may have significant thickness variations the thick strong mafic crust and thicker mantle lid under the colorado plateau can account for the relative geologic stability and subdued magmatism of the plateau during laramide compression and cenozoic extension compared to surrounding regions the crustal and lithospheric thinning across the tectonic boundary occurs over a short distance 100 km suggesting it is a geologically young feature produced by a predominantly mechanical response to late cenozoic extension our new lithosphere model at 37 n latitude is consistent with the existence in early cenozoic time of a flat subducted slab at 100 km depth and a relict sevier laramide 50 60 km thick crustal welt and 60 100 pure shear extension values of 1 6 2 0 during the late cenozoic

thickness and composition of the bonanza arc crustal section vancouver island canada the composition and origin of continental crust is in part constrained by evidence preserved in exhumed arc crustal sections the bonanza arc on vancouver island canada is a jurassic aged arc crust section exhumed in the eocene we use al in hornblende geobarometry of felsic plutons in the bonanza arc to assess the depths of crystallization of its exposed arc crust plutons of the unfoliated island plutonic suite are emplaced as sheets at depths of 2 10 km within a stratigraphy of devonian to triassic supracrustal rocks the west coast complex is more generally strained and intrudes below the pre jurassic supracrustals at depths of 10 18 km previous work shows that rare ultramafic units within the west coast complex crystallized at depths of 14 26 km the bonanza arc crustal section has a present structural thickness of at least 15 km the average composition for each arc component volcanic and plutonics when apportioned into the crustal section derived by stratigraphy and hornblende barometry shows the bulk composition of the entire arc is basaltic andesite 56 sio 2 mg mg fe mg 50 not unlike some estimates for bulk continental crust although only vestiges of ultramafic rocks occur in the bonanza arc today a significant component 6 20 km of this rock type must have been present in the section for the bulk arc to have been a melt in equilibrium with mantle olivine and may have been lost by foundering or tectonic thinning

motion of continental slivers and creeping subduction in the northern andes regions of intense continental deformation termed continental slivers have been identified in chile bolivia and ecuador analyses of gps data now identify another large sliver in peru the inca sliver that is moving away from a neighbouring sliver in ecuador implying that moving continental slivers control the deformation of almost the entire andean mountain range

hydrologic control of forearc strength and seismicity in the costa rican subduction zone the subduction zone beneath costa rica experiences infrequent large earthquakes in its northwestern part whereas slow slip dominates in the southeast seismic data reveal a disparity in fluid accumulation in the overriding continental crust that correlates with this change in seismic behaviour implying that spatial gradients in fluid content may control subduction zone seismicity

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

accreted oceanic terranes in ecuador southern edge of the caribbean plate the western part of ecuador is made of several oceanic terranes which comprise two oceanic plateaus of early 120 ma and late cretaceous age 90 ma respectively the older oceanic plateau was accreted to the andean margin in the late campanian 75 ma fragments of the turonian coniacian plateau were accreted to the ecuadorian margin in the late maastrichtian 68 ma guaranda terrane and late paleocene 58 ma pinon naranjal terrane the guaranda terrane received either fine grained oceanic sediments of coniacian maastrichtian age or island arc back arc volcanic suites of middle campanian middle maastrichtian age the pinon naranjal terrane recorded a comparable history completed in the maastrichtian paleocene either by pelagic cherts or by island arc products macuchi arc the late cretaceous plateau of ecuador is interpreted as part of the caribbean oceanic plateau cop because their evolutions are comparable if so the cop was not formed by the galapagos hotspot but on the farallon oceanic plate south of ecuador and close to the south american margin the cop belonged to the farallon plate until a subduction zone separated both plates in the middle campanian giving way to a well developed mid campanian mid maastrichtian island arc accretion in the late maastrichtian triggered a change in the subduction system and the development of a new arc system of late maastrichtian late paleocene age which crosscut the south america cop plate boundary the last accretion occurred in the late paleocene

interplate patchiness and subduction erosion mechanisms evidence from depth migrated seismic images at the central ecuador convergent margin prestack depth migrated seismic lines provide accurate images of the convergent ecuadorian margin where the southern flank of the carnegie ridge subducts the margin is fronted by a 5 7 km wide compressional sediment prism beneath the slope margin thinning is accomplished by gradual thinning of the lower part of the basement accompanied by pervasive seaward dipping normal faulting indicating basal subduction erosion at a low friction plate interface along the plate boundary a thin subduction channel locally thickens to form low velocity 600 m thick lenses of underthrusted fluid rich sediment supplied by slope mass wasting the contrast between the fluid rich sediment and the surrounding thinner and drier sections of the subduction channel creates a three dimensional patchiness across the plate boundary implying variations in mechanical interplate coupling the subduction channel patchiness modulates subduction erosion processes by alternately favoring margin basement weakening and material removal basement weakening would occur 1 at the base of the upper plate where enhanced reflectivity indicates basement damage by overpressured fluids from the subduction channel and 2 at the basement apex where basement breakup is caused by superposition of compressional and extensional fault systems resulting from time variable interplate mechanical coupling the detachment of rock debris in the higher coupled sections and the subsequent dragging of the fragments into the subduction channel cause tectonic erosion deeper in the subduction the subduction channel patchiness may influence processes like earthquake nucleation and rupture propagation and material recycling in the mantle

foundering of lower island arc crust as an explanation for the origin of the continental moho a long standing theory for the genesis of continental crust is that it is formed in subduction zones however the observed seismic properties of lower crust and upper mantle in oceanic island arcs differ significantly from those in the continental crust accordingly significant modifications of lower arc crust must occur if continental crust is indeed formed from island arcs here we investigate how the seismic characteristics of arc crust are transformed into those of the continental crust by calculating the density and seismic structure of two exposed sections of island arc kohistan and talkeetna the kohistan crustal section is negatively buoyant with respect to the underlying depleted upper mantle at depths exceeding 40 kilometres and is characterized by a steady increase in seismic velocity similar to that observed in active arcs in contrast the lower talkeetna crust is density sorted preserving only relicts about ten to a hundred metres thick of rock with density exceeding that of the underlying mantle specifically the foundering of the lower talkeetna crust resulted in the replacement of dense mafic and ultramafic cumulates by residual upper mantle producing a sharp seismic discontinuity at depths of around 38 to 42 kilometres characteristic of the continental mohorovicic discontinuity the moho dynamic calculations indicate that foundering is an episodic process that occurs in most arcs with a periodicity of half a million to five million years moreover because foundering will continue after arc magmatism ceases this process ultimately results in the formation of the continental moho

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

cretaceous and tertiary terrane accretion in the cordillera occidental of the andes of ecuador new field geochronological geochemical and biostratigraphical data indicate that the central and northern parts of the cordillera occidental of the andes of ecuador comprise two terranes the older pallatanga terrane consists of an early to late cretaceous oceanic plateau suite late cretaceous marine turbidites derived from an unknown basaltic to andesitic volcanic source and a tectonic melange of probable late cretaceous age the younger macuchi terrane consists of a volcanosedimentary island arc sequence derived from a basaltic to andesitic source a previously unidentified regionally important dextral shear zone named the chimbo toachi shear zone separates the two terranes regional evidence suggests that the pallatanga terrane was accreted to the continental margin the already accreted cordillera real in campanian times producing a tectonic melange in the suture zone the macuchi terrane was accreted to the pallatanga terrane along the chimbo toachi shear zone during the late eocene probably in a dextral shear regime the correlation of cretaceous rocks and accretionary events in the cordillera occidental of ecuador and colombia remains problematical but the late eocene event is recognised along the northern andean margin

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

a new approach to obtaining a 3d shear wave velocity model of the crust and upper mantle an application to eastern turkey article i nfo we present a new approach to the joint inversion of surface wave dispersion data and receiver functions by utilizing common conversion point ccp stacking to reconcile the different sampling domains of the two datasets utilizing ccp stacking allows us to suppress noise in the data by waveform stacking and correct for backazimuthal variations and complex crustal structure by mapping receiver functions back to their theoretical location when applied to eastern turkey this approach leads to a higher resolution image of the subsurface and clearly delineates different tectonic features in eastern turkey that were not apparent using other approaches we observe that theslow seismic velocities near the karliova triple junction correlate to moderate strain rates and high heat flow which leads to a rheologically weak crust that has allowed for the upward propagation of miocene and younger volcanics near the triple junction we find seismically fast presumably rigid blocks located in the southeastern anatolian plate and arabian plate are separated by a band of low shear wave velocities that correspond to the east anatolian fault zone which is consistent with the presence of fluids in the fault zone we observe that the arabian plate has underthrust the eurasian plate as far as the northern boundary of the bitlis massif which can explain the high exhumation rates in the bitlis massif as a result of slab break off of the arabian oceanic lithosphere we also find a shallow 33 km anomaly beneath eastern turkey that we interpret as a localized wedge of mantle that was underthrust by a crustal fragment during the collision of arabia and eurasia these observations are possible because of the high resolution images obtained by combining common conversion point receiver function stacks with ambient noise dispersion data to create a data driven three dimensional shear wave velocity model

the nature and provenance of accreted oceanic terranes in western ecuador geochemical and tectonic constraints western ecuador consists of a complex tectonic melange of oceanic terranes accreted to the continental margin from late cretaceous to eocene time new geochemical data from these accreted terranes arising from a 5 year british geological survey mapping programme indicate that they comprise rocks from a variety of oceanic tectonic settings from thickened and relatively unsubductable oceanic plateau basalts through island arc tholeiites with occasional more calc alkaline lavas to back arc basin basalt sequences this study has enabled us to construct a new geodynamic model for the cretaceous tertiary evolution of the northern andes and has placed important new constraints on the extent of oceanic plateau sequences in colombia and around the caribbean the age and nature of sediments combined with evidence for the age of peak metamorphism suggests that a prolonged 15 20 ma accretionary event occurred in late cretaceous time and involved the collision of an oceanic plateau represented by the pallatanga unit with the continental margin this accreted unit can be correlated with similar oceanic plateau sequences from the western cordillera of colombia and those within and around the caribbean region the naranjal and macuchi island arcs and the associated la portada back arc basin developed along the accreted margin from late campanian to eocene time and these arcs accreted to the continental margin along with oceanic plateau material represented by the pinon unit and pedernales esmeraldas sequences during eocene time the development of island arcs which separate the two accretionary events implies that the most westerly coastal oceanic plateau sequences both in ecuador pinon and pedernales esmeraldas and in colombia gorgona and serrania de baudo cannot belong to the caribbean colombian oceanic plateau ccop it therefore appears that at least two different oceanic plateaux are preserved within the accreted oceanic terranes of the northern andes it is possible that the ccop formed over the galapagos hotspot as previously proposed but the more westerly coastal plateau was derived from a more southerly hotspot source region such as sala y gomez in the se pacific

tectonic accretion and underplating of mafic terranes in the late eocene intraoceanic fore arc of new caledonia southwest pacific geodynamic implications this paper deals with the tectonic events that result in the accretion of mafic terranes in the fore arc region and a close juxtaposition of ultramafic rocks low grade and high grade mafic terranes in many collisional orogens the example is taken from new caledonia where tectonic accretion subduction underplating and obduction of mafic terranes took place during the late eocene in an intra oceanic forearc setting the late eocene tectonic complex comprised three major terranes an overlying ultramafic mainly harzburgitic allochthon named the ophiolitic nappe an intermediate mafic mainly basaltic off scraped melange composed of kilometre scale slices of oceanic upper crust called the poya terrane parts of which have been metamorphosed into an eclogite blueschist facies complex the pouebo terrane and a lower continental basement formed by the norkolk ridge terranes based upon exhaustive sampling of the mafic terranes and field surveys our tectonic micropaleontologic and geochemical data reveal that poya and pouebo terranes rocks originally formed within one single campanian to late paleocene oceanic basin floored by tholeiitic basalt associated with some minor seamount related intraplate alkali basalt the tholeiitic basalt displays a continuous range of compositions spanning between undepleted and depleted end members the former being volumetrically predominant the overall geochemical and isotopic features indicate an origin from a prominently heterogeneous mantle source during the opening of a marginal basin the south loyalty basin which almost completely disappeared during eocene convergence the opening of this basin originally located to the east of the norfolk ridge was synchronous with that of tasman sea basin as a consequence of oceanward migration of the west dipping pacific subduction zone establishing the origin of the ultramafic ophiolitic nappe is beyond the scope of this paper however it appears to be genetically unrelated to the mafic poya and pouebo terranes although it was located in the late eocene fore arc the ophiolitic nappe and the corresponding oceanic lithosphere originated before the late cretaceous to the east of the south loyalty basin in a back arc setting or alternatively in a much older trapped basin for reasons that remain unclear a new east dipping subduction started in the eocene and consumed most of the south loyalty basin forming the intra oceanic loyalty arc due to a changing subduction regime underplating of the diahot terrane the mafic slices that now form the poya terrane were tectonically accreted in the loyalty fore arc region and remained under low pressure low temperature conditions possibly at the subsurface until the norfolk ridge reached the subduction zone diachronously this resulted in the final obduction of the fore arc area the two step obduction involved first the mafic complex forming the poya terrane and thereafter the lithospheric mantle that now forms the ophiolitic nappe in contrast pieces of the accretionnary complex were dragged down into the subduction zone underplated at depth ca 70 km and metamorphosed into high temperature eclogite to form the pouebo terrane metamorphics that display the same geochemical features as the poya terrane basalt a mid to late eocene syntectonic piggy back sedimentary basin the nepoui flysch basin mainly filled with mafic clastic material and shallow water carbonates that record the progressive uplift of the fore arc region due to the accretion and underplating of mafic ocean related and other material in contrast a slightly younger foreland basin located upon the norfolk ridge the priabonian bourail flysch basin received a massive input of detrital material derived from the norfolk ridge itself and a time increasing amount of mafic poya derived material that recorded the first step of obduction thereafter the bourail flysch was overthrust by the poya terrane and finally by the ophiolitic nappe at the same time buoyancy driven uplift and exhumation of the high pressure metamorphic complex occurred as a consequence of the diachronous blocking of the subduction zone finally for a short time a new subduction started along the west coast of new caledonia and generated small amounts of active margin related magmas these events have resulted in the close juxtaposition of unmetamorphosed and highly metamorphosed mafic and ultramafic terranes that may represent a good pre collision analogue of mafic ultramafic belts found in many collisional mountain ranges

seismicity patterns along the ecuadorian subduction zone new constraints from earthquake location in a 3 d a priori velocity model to improve earthquake location we create a 3 d a priori p wave velocity model 3 dvm that approximates the large velocity variations of the ecuadorian subduction system the 3 dvm is constructed from the integration of geophysical and geological data that depend on the structural geometry and velocity properties of the crust and the upper mantle in addition specific station selection is carried out to compensate for the high station density on the andean chain 3 d synthetic experiments are then designed to evaluate the network capacity to recover the event position using only p arrivals and the maxi technique three synthetic earthquake location experiments are proposed 1 noise free and 2 noisy arrivals used in the 3 dvm and 3 noise free arrivals used in a 1 dvm synthetic results indicate that under the best conditions exact arrival data set and 3 dvm the spatiotemporal configuration of the ecuadorian network can accurately locate 70 per cent of events in the frontal part of the subduction zone average azimuthal gap is 289 44 noisy p arrivals up to 0 3 s can accurately located 50 per cent of earthquakes processing earthquake location within a 1 dvm almost never allows accurate hypocentre position for offshore earthquakes 15 per cent which highlights the role of using a 3 dvm in subduction zone for the application to real data the seismicity distribution from the 3 d maxi catalogue is also compared to the determinations obtained in a 1 d layered vm in addition to good quality location uncertainties the clustering and the depth distribution confirm the 3 d maxi catalogue reliability the pattern of the seismicity distribution a 13 yr record during the inter seismic period of the seismic cycle is compared to the pattern of rupture zone and asperity of the mw 7 9 1942 and the mw 7 7 1958 events the mw 8 8 1906 asperity patch is not defined we observe that the nucleation of 1942 1958 and 1906 events coincides with areas of positive simple bouguer anomalies and areas where marine terraces are still preserved on the coastal morphology from north to south 1 the 1958 rupture zone is almost aseismic and is attributed to a zone of high coupling 2 south of the galera alignment perpendicular to the trench the 1942 rupture zone presents moderate seismicity deeper on the seismogenic interplate zone and abutting on the jama cluster to the south this cluster is facing the cabo pasado cap and positive bouguer anomalies on the overriding margin we suspect that this cluster reflects a zone of local asperity partial coupling south of the jama cluster the spherical aseismic zone in the bahia area is interpreted as having a low seismic coupling steady creep motion or slow slip events we suspect that the site that generated the three m 7 events 1896 1956 and 1998 correspond to a small patch of strong coupling to the south in the manta puerto lopez zone the seismicity is mainly organized in earthquake swarms 1998 2002 2005 although slow slip events have been observed in the area vallee et al submitted we infer from the coastline shape the marine terraces and the high positive bouguer anomalies that the seismicity here might reveal a significant amount of seismic coupling

distribution of discrete seismic asperities and aseismic slip along the ecuadorian megathrust abstract a dense gps network deployed in ecuador reveals a highly heterogeneous pattern of interseismic coupling confined in the first 35 km depth of the contact between the subducting oceanic nazca plate and the north andean sliver interseismic models indicate that the coupling is weak and very shallow 0 15 km in south ecuador and increases northward with maximum found in the rupture areas of large mw 7 0 megathrust earthquakes that occurred during the 20th century since the great 1906 mw 8 8 colombia ecuador earthquake may have involved the simultaneous rupture of three to six asperities only one or two asperities were reactivated during the large seismic sequence of 1942 mw 7 8 1958 mw 7 7 1979 mw 8 2 and 1998 mw 7 1 the axis of the carnegie ridge which is entering the subduction zone south of the equator coincides well with the location of a 50 km wide creeping corridor that may have acted as persistent barrier to large seismic ruptures south of this creeping region a highly locked asperity is found right below la plata island while this asperity may have the potential to generate an mw 7 0 7 5 earthquake and a local tsunami until now it is unknown to have produced any similar events that region is characterized by the presence of slow slip events that may contribute significantly to reduce the long term moment deficit accumulated there and postpone the failure of that asperity at the actual accumulation rate a characteristic recurrence time for events such as those in 1942 1958 and 1979 is 140 30 yr 90 20 yr 153 80 yr respectively for the great 1906 event we find a recurrence time of at least 575 100 yr making the great 1906 earthquake a rare super cycle event

structure of the ecuadorian forearc from the joint inversion of receiver functions and ambient noise surface waves the ecuadorian forearc is a complex region of accreted terranes with a history of largemegathrust earthquakes most recently amw7 8 megathrust earthquake ruptured the plateboundary offshore of pedernales ecuador on 16 april 2016 following this event an inter national collaboration arranged by the instituto geofisico at the escuela polit ecnica nacionalmobilized a rapid deployment of 65 seismic instruments along the ecuadorian forearc wecombine this new seismic data set with 14 permanent stations from the ecuadorian nationalnetwork to better understand how variations in crustal structure relate to regional seismichazards along the margin here we present receiver function adaptive common conversionpoint stacks and a shear velocity model derived from the joint inversion of receiver functionsand surface wave dispersion data obtained through ambient noise cross correlations for theupper 50 km of the forearc beneath the forearc crust we observe an eastward dipping slowvelocity anomaly we interpret as subducting oceanic crust which shallows near the projectedcentre of the subducting carnegie ridge we also observe a strong shallow positive conversionin the ecuadorian forearc near the borbon basin indicating a major discontinuity at a depth of 7 km this conversion is not ubiquitous and may be the top of the accreted terranes we alsoobserve significant north south changes in shear wave velocity the velocity changes indicatevariations in the accreted terranes and may indicate an increased amount of hydration beneaththe manab i basin this change in structure also correlates geographically with the southernrupture limit of multiple high magnitude megathrust earthquakes the earthquake record alongthe ecuadorian trench shows that no event with amw 7 4 has ruptured south of 0 5 sinsouthern ecuador or northern peru our observations along with previous studies suggestthat variations in the forearc crustal structure and subducting oceanic crust may influance theoccurrence and spatial distribution of high magnitude seismicity in the region

the geochemistry of manganese carbonate in panama basin sediments abstract the mn distribution in panama basin area sediments and interstitial waters is discussed striking surficial mn enrichments produced by a well known diagenetic recycling process characterize the sediments of the region thermodynamic solubility calculations indicate that in at least one core interstitial waters approach saturation with respect to mnco3 a mixed carbonate phase of composition mn48 ca47 mg5 co3 was recovered from an ash band in the same core the association of this material with the coarse volcaniclastic debris is thought to result from facile manganous carbonate precipitation in sediment horizons of coarser mean grain size since sulphate reduction in the upper two metres of panama basin sediments is fairly minor little increase in alkalinity is observed and it is postulated that significant production of carbonate alkalinity is not a prerequisite for manganous carbonate generation in hemipelagic sediments a more important factor appears to be the availability of mn oxides for solution during early diagenesis stable c isotopic analyses indicate that little c of organic origin is used in the precipitation reaction in either panama basin or loch fyne scotland sediments

mapping the subducting pacific slab beneath southwest japan with hi net receiver functions abstract we have used 4th root receiver function stacks and pre stack receiver function depth migrations to study the transition zone discontinuity structure beneath southwestern japan receiver functions were calculated from the quiet short period seismograms recorded by a recently deployed borehole network hi net we found that a relatively broad frequency band can be retrieved from a short period seismogram by a deconvolution of the instrument response the quality of the receiver functions formed from large earthquake recordings is comparable to those from broadband instruments we applied common conversion point gathering to the receiver function data to image the p to s conversion events beneath the network by stacking with a 4th root technique to improve lateral coherence we found that the topographic anomalies of the 410 and 660 km discontinuities beneath southwest japan have very different length scales the former is characterized by a narrow 150 200 km wide topographic high while the latter exhibits a broad 400 km wide moderate topographic low together with a small scale larger amplitude depression a 2 5d pre stack depth migration of the receiver functions shows the transition zone features clearly as well as images of a change of slope in the subducting slab at the 410 discontinuity and flattening of the slab onto the 660 km these observations show that the subducted pacific slab is deflected when it encounters the upper and lower boundaries of the transition zone and is flat lying either above or across the 660 km discontinuity the flat lying slab is however restricted to the bottom of the transition zone and probably experiences much less thickening than is suggested by some global tomographic images in which subhorizontal high velocity anomalies are seen throughout the transition zone between the two discontinuities

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

receiver function study of the cascadia megathrust evidence for localized serpentinization 1 we characterize the crustal and upper mantle structure of the cascadian fore arc using receiver function analysis of data from long running observatories located along the north american margin in oregon and washington we identify coherent sv polarized phases consistent with p s conversion from the top of the subducting juan de fuca plate throughout the area west of the volcanic arc in many instances we also find evidence for deeper features likely associated with the subducted oceanic crust at many locations sh polarized energy in receiver functions is associated with the surface of the subducting plate these sh polarized phases display significant directional variation and show clear polarity reversals consistent with the presence of seismic anisotropy previously a layer of serpentinized material within the megathrust has been proposed to explain an anisotropic low velocity high poisson s ratio layer identified by receiver function analysis at corvallis oregon global seismic network station cor our new results suggest that this feature is widespread in cascadia but the specific receiver function signature varies this anisotropic supraslab layer is supported by receiver function estimates beneath seismic stations which similar to cor are 100 km from the coast and 40 km above the slab surface however the orientation of rock fabric implied by the anisotropy varies greatly from station to station modeling with synthetic seismograms supports the existence of serpentinite beneath station gnw green mountain washington closer to the coast where the slab is shallower an anisotropic layer is likely beneath some stations but the evidence is less conclusive we propose that the anisotropic signature beneath the inland stations is related to serpentinization of the mantle wedge at greater depths best explained by the presence of the polymorph antigorite hypothetically suctioned upward by slab rollback a deformed sliver of serpentinite rich rock above the slab interface would likely display the observed large spatial variation in anisotropic symmetry axis within the layer consistent with our observations

numerical constraints on rates of metamorphism fluid production and fluid flux during regional metamorphism one dimensional numerical models of regional metamorphism were constructed in order to constrain rates of metamorphism rates of fluid production and fluid flux as a function of time and position in thickened continental crust four end member numerical experiments result from combinations of sawtooth single thrust versus homogeneous crustal thickening geometries and continuous versus discontinuous metamorphic reactions that release up to 5 wt volatiles from the lower half of the thickened crust metamorphic reactions consume substantial amounts of heat and retard the thermal evolution of thickened continental crust by several million years peak rates of metamorphism defined as the rate of advance of the 400 c isotherm range from 1 4 to 5 7 mm yr 1 in the saw tooth models and from 0 5 to 0 6 mm yr 1 in the homogeneous models maximum rates of fluid production range from 90 mg fluid m 3 yr 1 in the sawtooth discontinuous reaction model to 6 mg fluid m 3 yr 1 in the homogeneous continuous reaction model fluid production in the sawtooth models produce a maximum fluid flux of 0 2 kg fluid m 2 yr 1 out of the 40 km thick lower plate the maximum fluid flux in the homogeneous models are lower 2 yr 1 fluid fluxes resulting from all models require significant focusing into channels spaced greater than 10 km apart in order to perturb the overlying thermal structure of the continental crust in these numerical experiments most fluid flow infiltrates overlying rocks at temperatures substantially below peak metamorphic temperatures suggesting that strongly focused or multi pass fluid circulation systems may be required to explain reported high fluid rock ratios

an inverted continental moho and serpentinization of the forearc mantle volatiles that are transported by subducting lithospheric plates to depths greater than 100 km are thought to induce partial melting in the overlying mantle wedge resulting in arc magmatism and the addition of significant quantities of material to the overlying lithosphere asthenospheric flow and upwelling within the wedge produce increased lithospheric temperatures in this back arc region but the forearc mantle in the corner of the wedge is thought to be significantly cooler here we explore the structure of the mantle wedge in the southern cascadia subduction zone using scattered teleseismic waves recorded on a dense portable array of broadband seismometers we find very low shear wave velocities in the cold forearc mantle indicated by the exceptional occurrence of an inverted continental moho which reverts to normal polarity seaward of the cascade arc this observation provides compelling evidence for a highly hydrated and serpentinized forearc region consistent with thermal and petrological models of the forearc mantle wedge this serpentinized material is thought to have low strength and may therefore control the down dip rupture limit of great thrust earthquakes as well as the nature of large scale flow in the mantle wedge

intense interface seismicity triggered by a shallow slow slip event in the central ecuador subduction zone we document a 1 week long slow slip event sse with an equivalent moment magnitude of 6 0 6 3 which occurred in august 2010 below la plata island ecuador south of the rupture area of the 1906 mw 8 8 megathrust earthquake gps data reveal that the sse occurred at a depth of about 10 km within the downdip part of a shallow 15 km isolated locked patch along the subduction interface the availability of both broadband seismometer and continuous geodetic station located at the la plata island 10 km above the sse enables a careful analysis of the relationships between slow and rapid processes of stress release along the subduction interface during the slow slip sequence the seismic data show a sharp increase of the local seismicity with more than 650 earthquakes detected among which 50 have a moment magnitude between 1 8 and 4 1 however the cumulative moment released through earthquakes accounts at most for 0 2 of the total moment release estimated from gps displacements most of the largest earthquakes are located along or very close to the subduction interface with focal mechanism consistent with the relative plate motion while the earthquake sizes show a classical distribution gutenberg richter law with a b value close to 1 the space time occurrence presents a specific pattern first the largest earthquakes appear to occur randomly during the slow slip sequence which further evidence that the seismicity is driven by the stress fluctuations related to aseismic slip moreover the seismicity observed during the sse consists in individual events and families of repeating earthquakes these observations indicate that the stress increment induced by the episodic aseismic slip may lead both to sudden seismic moment release and to progressive rupture within small locked patches this study offers an a posteriori interpretation of the seismogenesis in the central ecuador subduction zone where intense seismic swarms have been regularly observed 1977 1998 2002 and 2005 these swarms have likely been triggered by large magnitude slow slip events

collision induced tectonism along the northwestern margin of the indian subcontinent as recorded in the upper paleocene to middle eocene strata of central pakistan kirthar and sulaiman ranges abstract outcrop data from the upper paleocene to middle eocene ghazij formation of central pakistan provide information about the depositional environments source areas and paleogeographic and tectonic settings along the northwestern margin of the indian subcontinent during the closing of the tethys ocean in this region in the lower part of the exposed stratigraphic sequence are various marine carbonate shelf deposits jurassic to upper paleocene overlying these strata is the ghazij which consists of marine mudstone lower part paralic sandstone and mudstone middle part and terrestrial mudstone and conglomerate upper part petrographic examination of sandstone samples from the middle and upper parts reveals that rock fragments of the underlying carbonate shelf deposits are dominant also present are volcanic rock fragments and chromite grains paleocurrent measurements from the middle and upper parts suggest that source areas were located northwest of the study area we postulate that the source areas were uplifted by the collision of the subcontinent with a landmass during the final stages of the closing of the tethys ocean middle eocene carbonate shelf deposits that overlie the ghazij record a return to marine conditions prior to the miocene to pleistocene sediment influx denoting the main collision with eurasia