the flat to normal subduction transition study to obtain the nazca plate morphology using high resolution seismicity data from the nazca plate in central chile abstract data from 45 seismological stations mostly temporary were used to obtain an accurate data set of intraslab seismicity of the nazca subducted plate between 33 s and 35 s the interest zone located in the transition section where the nazca plate changes from flat slab north of 33 to normal slab south of that latitude in addition the study region is located where the active volcanic arc appears from a set of earthquakes which were relocated using a grid search multiple events algorithm we obtained the plate geometry from latitudes of 33 s to 34 5 s and from 60 km to 120 km in depth the obtained morphology shows notable similarity in its structure to maipo orocline revealing some possible strong connection between the overriding plate and the subducting plate we suggest that the subducted plate at the trench has been deformed in its shape consistently with the maipo orocline pattern and its deformation is observed below the interface zone our results are consistent with van keken et al 2011 models and based on this the seismicity located between 70 and 120 km is probably related with dehydration processes rather than mechanical processes from our precise earthquake locations we observed a complete lack of intraslab seismicity below 120 km depth this valuable finding can be used to better constrain thermal models for the subduction region of central chile

seismic velocity structure of the slab and continental plate in the region of the 1960 valdivia chile slip maximum insights into fluid release and plate coupling the south central chilean subduction zone has witnessed some of the largest earthquakes in history making this region particularly important for understanding plate coupling here we present the results of a local earthquake tomography study from a temporary local seismic network in the villarrica region between 39 and 40 s where the largest coseismic displacement of the 1960 valdivia earthquake occurred a lowvelocity anomaly and high vp vs values occur under the coastal region indicating mantle serpentinisation and or underthrusting of forearc material further east a high velocity anomaly is observed interpreted as normal high velocity mantle under the active volcanic arc a low velocity anomaly together with high vp vs ratios 1 8 and higher likely images fluid ascent beneath the volcanoes close to the subducting valdivia fracture zone the coastal low velocity anomaly extends further inland where it interrupts and shifts the high velocity anomalies associated with normal fast mantle velocities this may indicate enhanced fluid presence along this part of the margin probably caused by a stronger hydration of the incoming plate along the valdivia fracture zone this is consistent with geochemical fluid proxies u th pb ce ba nb in young volcanic rocks displaying peak values along the volcanic front at llaima and villarrica volcanoes and with recent gps measurements which suggested a local reduction in plate coupling in this region the shift in the high velocity anomaly underlying the central part may be caused by a north to south decrease in plate age and hydration across the valdivia fracture zone and may explain why a central valley is absent in this segment of the margin the low la yb ratios in the volcanic rocks from villarrica and llaima suggest that the high slab derived fluid flux causes elevated degrees of melting beneath these volcanoes providing an explanation as to why these are amongst the most active volcanoes in south america

morphology and geology of the continental shelf and upper slope of southern central chile 33 s 43 s the continental shelf and slope of southern central chile have been subject to a number of international as well as chilean research campaigns over the last 30 years this work summarizes the geologic setting of the southern central chilean continental shelf 33 s 43 s using recently published geophysical seismological sedimentological and bio geochemical data additionally unpublished data such as reflection seismic profiles swath bathymetry and observations on biota that allow further insights into the evolution of this continental platform are integrated the outcome is an overview of the current knowledge about the geology of the southern central chilean shelf and upper slope we observe both patches of reduced as well as high recent sedimentation on the shelf and upper slope due to local redistribution of fluvial input mainly governed by bottom currents and submarine canyons and highly productive upwelling zones shelf basins show highly variable thickness of oligocene quaternary sedimentary units that are dissected by the marine continuations of upper plate faults known from land seismic velocity studies indicate that a paleo accretionary complex that is sandwiched between the present relatively small active accretionary prism and the continental crust forms the bulk of the continental margin of southern central chile

late pleistocene landscape evolution in south central chile constrained by luminescence and stable cosmogenic nuclide dating landscapes in tectonically active regions develop in response to a complex interplay between rapid tectonic deformation and surface processes in order to understand the evolution of these environments and distinguish between the different factors it is fundamental to identify and quantify geomorphic processes and rates on different time scales the forearc of south central chile provides an ideal setting to elucidate the response of surface processes to climatic and tectonic forcing however in contrast to northern chile this region has attracted little attention due to difficulties in quantifying the age of geomorphic surfaces we use luminescence and stable cosmogenic nuclides 3 he 21 ne on conglomeratic units in the central depression and fluvial valley fills in the coastal cordillera to constrain exposure and burial ages of these units that form major landscape elements in the southern chilean forearc our data indicate that disturbances in the drainage network are controlled by local uplift which may be caused by deformation above an active blind thrust in the coastal cordillera local short term uplift rates reach a minimum of 0 27 mm a 1 and are on the same order as late quaternary minimum incision rates of 0 15 mm a 1 to 0 43 mm a 1 the valley fills in the coastal cordillera have depositional ages of 80 ka and 255 ka respectively exposure ages of the extensive alluvial surfaces in the central depression cluster between 135 ka and 175 ka and between 240 ka and 280 ka hence the deposition of these surfaces broadly coincides with marine isotope stages mis 6 and 8 and appears to be climatically driven

seismotectonic features from accurate hypocentre locations in southern central andes western argentina abstract a local seismic network over a five year period recorded about 450 earthquakes in western argentina in this region the geodynamics is controlled by the subduction of the nazca plate beneath the south american lithosphere which is characterized here by a sub horizontal path before reassuming its downward descent as accurate earthquake locations are of primary importance when studying the seismicity of a given area events recorded by the local seismic network enable in depth investigations into seismo tectonic patterns allowing to improve the earthquake source characterization and knowledge on the ongoing seismo tectonics of the region to this end we performed a simultaneous 1 d inversion of both the velocity structure and the hypocentre location the minimum 1 d model obtained is complemented by station corrections which lead to a first insight into the deeper 3 d structure in addition stability tests were performed to verify the robustness of our earthquake location results they reveal a fairly stable hypocentre determination demonstrating that the locations obtained by the inversion process are not systematically biased the results show that sierra pie de palo is characterized by a crustal seismogenic structure dipping west and extending from its eastern boundary to about 30 km of depth the study also provided new constraints on the geometry of the subducted slab we noted a great concentration of shallower seismicity compared to that of the surrounding areas of the wadati benioff zone at the expected position of the juan fernandez ridge jfr our hypocentres indicate that jfr certainly influences the subduction style along its strike leading to the formation of a bend in the slab geometry

seismic investigation of the continental margin off and onshore valparaiso chile abstract at the latitude of valparaiso chile a fundamental change in the configuration of the benioff zone volcanic activity and the structure of the continental margin occurs opposite the subducting juan fernandez ridge three legs of the german r v sonne cruises so101 so103 and so104 surveyed the continental margin and oceanic plate offshore valparaiso aiming at studying the crustal structure and investigating possible causes for the change in slab configuration sonne cruise so101 investigated the tectonic setting with swath mapping bathymetry magnetics and high resolution seismics following these investigations cruise so103 collected land sea wide angle seismic data and coincident deep seismic reflection data were acquired during cruise so104 coincident near vertical and wide angle seismic measurements were made along two profiles profile 1 located at the south of the study area away from the influence of the subducting ridge crosses the margin where thick trench sediment and an accretionary wedge near the trench is observed profile 2 located in the north runs from the juan fernandez ridge to the chilean coast the crustal velocity models obtained for the two profiles show that the continental crust extends to the middle lower slope boundary which is also reflected in morphology in addition they show that the crustal structure of the oceanic plate is rather similar but the plate seems to be slightly more inclined along the northern profile 13 versus 10 in the south the two profiles are only about 70 km apart but their structures differ significantly no straightforward correlation exists between the two profiles that can be attributed to ridge collision the data support that the 1985 central chile earthquake ruptured the plate boundary in the area that includes the segment boundary and mainly where continental crust forms the upper plate

evidence of quaternary tectonics along rio grande valley southern malargue fold and thrust belt mendoza argentina abstract the malargue fold and thrust belt is developed in the argentinian andes between 34 and 37 s through the tectonic inversion of upper triassic and lower jurassic depocenters of the neuquen basin with an uplift history since the cretaceous evidence of quaternary deformation has been described in the northern part of it 34 34 5 s potentially coeval to neotectonic activity along the eastern edge of san rafael block to the south compressional and extensional structures active during the quaternary were found in the dorso de los chihuidos along the agrio fold and thrust belt front 37 5 38 s contrastingly the southern segment of the malargue fold and thrust belt between these two areas with described neotectonic activity is partially covered by quaternary products of the payun matru volcanic field that may hide evidence of recent deformation in this 300 km gap of neotectonic information the landscape imprint of two individual structures aligned in the mountain front through the rio grande valley was analyzed new evidence of neotectonic deformation were recognized in particular over the western slope of the cara cura range expressed by faulting and folding of quaternary deposits and lava flows an 40ar 39ar age from a deformed lava flow at the flanks of an anticline in the foothills of the cara cura range may suggest at least an upper pleistocene compressional tectonic activity longitudinal river profile analysis revealed anomalies that show some correlation with the neotectonic structures described especially knickpoints and concavity index changes meanwhile normalized steepness index values showed a moderate response to recent deformation a proposed schematic geomorphic evolution for this segment of rio grande river is discussed to put the neotectonic activity into the context of landscape formation all together this evidence supports the idea of an active front through the rio grande valley during the quaternary coetaneous to an active broken foreland to the east in the southern central andes

3d simultaneous seismic refraction and reflection tomography of wide angle data from the central chilean margin we present an application of three dimensional 3d simultaneous seismic refraction and reflection tomography for velocity and interface structure the inversion technique and method for developing the starting model are specifically designed for relatively sparse wide angle data acquired across strongly varying structure the data were recorded in a region of seamount subduction on the chilean margin and consist of seven receivers and ten intersecting airgun profiles over a 90 90 km area providing constraint to 25 km depth the tomographic method and the final model are assessed through a comparison with the large scale geologic features of the margin and a resolution test the 3d model shows the valparaiso forearc basin the accretionary wedge the subducting plate and possibly a subducted seamount our results show the potential of relatively sparse 3d wide angle data

seismic velocities anisotropy and shear wave splitting of antigorite serpentinites and tectonic implications for subduction zones 1 antigorite the high temperature ht form of serpentinite is believed to play a critical role in various geological processes of subduction zones we have measured p and s wave velocities vp and vs anisotropy and shear wave splitting of 17 serpentinite samples containing 90 antigorite at pressures up to 650 mpa the new results combined with data for low temperature lt lizardite and or chrysolite reveal distinct effects of lt and ht serpentinization on the seismic properties of mantle rocks at 600 mpa vp 5 10 and 6 68 km s vs 2 32 and 3 67 km s and vp vs 2 15 and 1 81 for pure lt and ht serpentinites respectively above the crack closure pressure 150 mpa the velocity ratio of antigorite serpentinites displays little dependence on pressure or temperature serpentine contents within subduction zones and forearc mantle wedges where temperature is 300 c should be at least twice that of previous estimates based on lt serpentinization the presence of seismic anisotropy high pressure fluids or partial melt is also needed to interpret ht serpentinized mantle with vp 1 81 the intrinsic anisotropy of the serpentinites 3 8 16 9 with an average value of 10 5 for vp and 3 6 18 3 with an average value of 10 4 for vs is caused by dislocation creep induced lattice preferred orientation of antigorite three distinct patterns of seismic anisotropy correspond to three types of antigorite fabrics s l and ls tectonites formed by three categories of strain geometry i e coaxial flattening coaxial constriction and simple shear respectively our results are thought to provide a new explanation for various anisotropic patterns of subduction systems observed worldwide

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

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

correlations between so2 flux seismicity and outgassing activity at the open vent of villarrica volcano chile 1 the characteristics of the open vent activity of villarrica volcano chile were studied in detail by integrating visual observations of the lava lake analysis of the seismic tremor and measurements of so2 flux the outgassing activity comprises a persistent gas plume emission from the bottom of the crater as well as frequent explosive events three main styles of bubble bursting were identified at the surface of the active lava lake seething magma small short lived lava fountains and strombolian explosions seething magma consists of continual burst of relatively small bubbles a few meters in diameter with varying strength over the entire surface of the lava lake small lava fountains seen as a vigorous extension of seething magma commonly have durations of 20 120 s and reach 10 40 m high above the lava lake correlations between seismicity and visual observations indicate that the seismic tremor is mostly caused by the explosive outgassing activity furthermore for different periods between 2000 and 2006 during which the activity remained comparable the real time seismic amplitude measurement system rsam and so2 emission rates show a very good correlation higher so2 emissions appeared to be related to higher levels of the lava lake stronger bubble bursting activity and changes in the morphology and texture of the crater floor background low levels of activity correspond to a lava lake located 80 m below the crater rim small and or blocky morphology of the roof seismic amplitude rsam lower than 25 units few volcano tectonic earthquakes and daily averages of so2 emissions lower than 600 mg d

the importance of blueschist eclogite dehydration reactions in subducting oceanic crust the metamorphic evolution and dehydration of subducting oceanic crust may be predicted by combining calculated pressure temperature p t paths with a model of metabasalt phase equilibria in steady state subduction zones with high rates of shear heating the upper parts of the subducting oceanic crust progress through the greenschist amphibolite granulite eclogite facies whereas lower parts of the subducting oceanic crust progress through the blueschist eclogite facies in steady state subduction zones with moderate rates of shear heating most of the subducting oceanic crust passes through the blueschist eclogite transition in steady state subduction zones with low rates of shear heating the entire subducting oceanic crust lies within the blueschist facies to depths greater than 70 km for oceanic crust containing 1 2 wt h 2 o dehydration will not begin until the onset of eclogite or amphibolite facies metamorphism depending on the p t path for many subduction zones the most important dehydration reactions in the subducting oceanic crust occur at the blueschist eclogite facies transition associated with the breakdown of lawsonite or clinozoisite glaucophane and chlorite large amounts of h 2 o released by blueschist eclogite dehydration reactions could trigger partial melting in the overlying mantle wedge and may play a crucial role in the generation of arc magmas

contrasting nature of deformation along an intra arc shear zone the liquine ofqui fault zone southern chilean andes abstract the liquine ofquifault zone lofz cuts the patagonian batholith and the modern volcanic arc of southern chile for ca 1000 km the rock fabric along three transects of the lofz combined with new and published geochronological data suggest marked differences in the nature and timing of deformation along strike in the liquine transect 39 s a 1 km wide northeast striking subvertical mylonitic zone shows north plunging stretching lineations this mylonitic zone has been juxtaposed by high angle reverse faulting against a nearly undeformed miocene granitoid metamorphic assemblages and microstructures in the mylonites indicate greenschist facies conditions and sinistral reverse displacement deformation pre dates a 100 2 ma undeformed porphyritic dike hornblende 40ar 39ar mean age in the reloncavi transect 41 42 s deformation in cretaceous and miocene plutons is predominantly brittle kinematic analysis of two fault populations yields compressional and dextral strike slip stress regimes interpreted as late miocene in age in the hornopiren transect 42 43 s a 4 km wide mylonitic zone developed in plutons and wallrocks shows subhorizontal stretching lineations and dextral displacement a single fault population overprinting the mylonites supports a dextral strike slip stress regime available u pb k ar and 40ar 39ar dates on deformed cenozoic plutons and wallrock range from 9 to 13 ma on hornblende and from 6 to 3 ma on biotite microstructures and mineral assemblages indicate that the youngest ductile fabrics in the plutons formed at greenschist facies similar to the biotite ar closure temperature subparallel magmatic and solid state fabrics combined with geochronology suggest that dextral displacement was continuous during emplacement and cooling of the plutons dextral oblique subduction of the nazca plate beneath western south america has driven long term intra arc deformation at the southern andes plate boundary zone ridge collision in turn has favored dextral displacement at the leading edge of the continent since the pliocene

quantifying offshore fore arc deformation and splay fault slip using drowned pleistocene shorelines arauco bay chile most of the deformation associated with the seismic cycle in subduction zones occurs offshore and has been therefore difficult to quantify with direct observations at millennial time scales here we study millennial deformation associated with an active splay fault system in the arauco bay area off south central chile we describe hitherto unrecognized drowned shorelines using high resolution multibeam bathymetry geomorphic sedimentologic and paleontologic observations and quantify uplift rates using a landscape evolution model along a margin normal profile uplift rates are 1 3 m ka near the edge of the continental shelf 1 5 m ka at the emerged santa maria island 0 1 m ka at the center of the arauco bay and 0 3 m ka in the mainland the bathymetry images a complex pattern of folds and faults representing the surface expression of the crustal scale santa maria splay fault system we modeled surface deformation using two different structural scenarios deep reaching normal faults and deep reaching reverse faults with shallow extensional structures our preferred model comprises a blind reverse fault extending from 3 km depth down to the plate interface at 16 km that slips at a rate between 3 0 and 3 7 m ka if all the splay fault slip occurs during every great megathrust earthquake with a recurrence of 150 200 years the fault would slip 0 5 m per event equivalent to a magnitude 6 4 earthquake however if the splay fault slips only with a megathrust earthquake every 1000 years the fault would slip 3 7 m per event equivalent to a magnitude 7 5 earthquake

trace metal evidence for changes in the redox environment associated with the transition from terrigenous clay to diatomaceous sediment saanich inlet bc abstract we measured the concentrations of redox sensitive trace metals mn v mo u cd and re in sediments from odp leg 169s hole 1033b in saanich inlet british columbia to determine changes in redox conditions associated with the onset of laminated sediments at 12 5 kyr the most striking result is a large peak in authigenic re along with detrital levels of mo at the glacial terrigenous clay diatomaceous sediment transition in contrast the underlying glacial terrigenous clay which extends throughout the bottom section of the core is chemically similar to detrital concentrations either cowichan river particulates or average shale values these data suggest a period of oxic bottom waters but reducing pore waters this could be due to the dramatic transformation of saanich inlet during the late deglaciation from an open bay to an inlet which restricted circulation and slowed bottom water oxygen renewal a peak and gradual increase in authigenic mn in younger sediments subsequent to the re peak suggests that increasingly oxic conditions followed the authigenic enrichment in re these conditions could be connected to the younger dryas cooling period which was coincident with an increase in well oxygenated upwelled waters on the west coast of north america that form the bottom waters of saanich inlet metal concentrations in a gray clay bed 11 kyr are similar to their concentrations in the glacial terrigenous clay implying that they have a common source authigenic enrichments of re with little authigenic mo and cd suggest that before the deposition of this bed bottom waters were oxic and pore water oxygen was consumed in the top centimeter or less laminations above the clay layer suggest anoxic conditions which are also indicated by higher authigenic mo and cd and slightly lower re mo ratios in these sediments the basin remained mostly anoxic after the gray clay was emplaced as seen by continuous authigenic enrichment of the redox sensitive trace metals these results are consistent with increased stratification of the water column brought about by an influx of fresh water to the basin by a large flood

the cretaceous and younger thermal history of the ktb site germany apatite fission track data from the vorbohrung a detailed fission track analysis was carried out on 41 apatite samples from the 4001 m deep pilot hole of the german continental deep drilling program ktb the investigations include dating by the population method and length measurements of horizontal confined tracks age and length information indicate that all samples above the present day partial annealing zone cooled from temperatures above 120 c in the cretaceous the top two kilometres of the profile show less variation of age with depth than was expected modelling of the profile indicates that the distribution of fission track ages with depth requires that the profile was disturbed during or after cooling through the partial annealing zone modelling of individual samples reveals that at least the upper 2 km experienced reverse faulting effectively thickening the upper part of the rock column by up to 1000 m in the tertiary a present day partial annealing zone for apatite was observed between 2000 and 4000 m 60 110 c this confirms earlier in situ observations of the position of the partial annealing zone

paleomagnetic rotation pattern of the southern chile fore arc sliver 38 s 42 s a new tool to evaluate plate locking along subduction zones the chile fore arc at 37 s 47 s represents the coseismic deformation zone of the 1960 mw 9 5 valdivia earthquake here we report on the paleomagnetism of 43 oligocene pleistocene volcanic sites from the fore arc sliver between 38 s and 42 s sites were gathered west of the 1000 km long liquine ofqui dextral fault zone lofz that represents the eastern fore arc sliver boundary nineteen reliable sites reveal that the fore arc is characterized by counterclockwise ccw rotations of variable magnitude except at 40 s 41 s where ultrafast 50 myr clockwise cw rotations occur within a 30 km wide zone adjacent to the lofz ccw rotation variability even at close sites and rapidity 10 myr suggest that the observed block rotation pattern is related to nw se seismically active sinistral faults crosscutting the whole fore arc according to previously published data cw rotations up to 170 also occur east of the lofz and have been related to ongoing lofz shear we suggest that the occurrence and width of the eastern fore arc sliver undergoing cw rotations is a function of plate coupling along the subduction zone interface zones of high coupling enhance stress normal to the lofz induce high lofz strength and yield a wide deformation zone characterized by cw rotations conversely low coupling imply a weak lofz a lack of cw rotations and a fore arc entirely dominated by ccw rotations related to sinistral fault kinematics our locking inferences are in good agreement with those recently derived by gps analysis and indicate that seismotectonic segment coupling has remained virtually unchanged during the last 5 ma

the southern andes between 36 and 40 s latitude seismicity and average seismic velocities abstract the project issa 2000 integrated seismological experiment in the southern andes consists of a temporary seismological network and a seismic refraction profile a network of 62 seismological stations was deployed across the southern andes at 38 s three hundred thirty three local seismic events were observed in a 3 month period p and s arrival times of a subset of high quality data were inverted simultaneously for 1 d velocity structure hypocentral coordinates and station delays seismic refraction data along a transect at 39 s provide further constraints on the crustal structure low crustal velocities beneath the forearc may be either due to subducted trench sediments or serpentinized mantle material of the continental lithosphere the continental moho is not clearly observed in this region average velocities of the crust beneath the arc are higher than those beneath the forearc crustal thickness is about 40 km crustal seismicity concentrates in the forearc region along the bio bio and gastre fault zones the area between these two prominent fault zones seems to be nearly devoid of crustal seismicity but shows highest uplift and topography in the forearc region benioff seismicity is observed down to 150 km depth resulting in the first accurate image of the benioff zone in the southern andes a maximum of seismicity at 60 km depth may be caused by dehydration embrittlement

active tectonics of the south chilean marine fore arc 35 s 40 s the south chilean marine fore arc 35 s 40 s is separated into four tectonic segments concepcion north concepcion south nahuelbuta and tolten from north to south these are each characterized by their individual tectonic geomorphology and reflect different ways of mechanical and kinematic interaction of the convergent nazca and south american plates splay faults that cut through continental framework rock are seismically imaged in both concepcion segments and the tolten segment additionally the concepcion south segment exhibits prominent upper plate normal faults normal faults apparently relate to uplift caused by sediment underthrusting at depth this has led to oversteepening and gravitational collapse of the marine fore arc there is also evidence for sediment underthrusting and basal accretion to the overriding plate in the tolten segment there uplift of the continental slope has created a landward inclined seafloor over a latitudinal distance of 50 km in the nahuelbuta segment transpressive upper plate faults aligned oblique to the direction of plate motion control the seafloor morphology based on a unique acoustic data set including 90 of bathymetric coverage of the continental slope we are able to reveal an along strike heterogeneity of a complexly deformed marine fore arc which had escaped attention in previous studies that only considered the structure along transects normal to the plate margin

splay fault slip during the mw 8 8 2010 maule chile earthquake splay faults are thrusts that emerge from the plate boundaries of subduction zones such structures have been mapped at several convergent margins and their activity commonly ascribed to large megathrust earthquakes however the behavior of splay faults during the earthquake cycle is poorly constrained because typically these structures are located offshore and are difficult to access here we use geologic mapping combined with space and land geodesy as well as offshore sonar data to document surface fault ruptures and coastal uplift at isla santa maria in south central chile 37 s caused by the 27 february 2010 maule earthquake m w 8 8 during the earthquake the island was tilted parallel to the margin and normal faults ruptured the surface and adjacent ocean bottom we associate tilt and crestal normal faulting with growth of an anticline above a blind reverse fault rooted in the nazca south america plate boundary which slipped during the maule earthquake the splay fault system has formed in an area of reduced coseismic plate boundary slip suggesting that anelastic deformation in the upper plate may have restrained the 2010 megathrust rupture surface fault breaks were accompanied by prominent discharge of fluids our field observations support the notion that splay faulting may frequently complement and influence the rupture of subduction zone earthquakes

segmentation of the 2010 maule chile earthquake rupture from a joint analysis of uplifted marine terraces and seismic cycle deformation patterns abstract the segmentation of major fault systems in subduction zones controls earthquake magnitude and location but the causes for the existence of segment boundaries and the relationships between long term deformation and the extent of earthquake rupture are poorly understood we compare permanent and seismic cycle deformation patterns along the rupture zone of the 2010 maule earthquake m8 8 which ruptured 500 km of the chile subduction margin we analyzed the morphology of mis 5 marine terraces using lidar topography and established their chronology and coeval origin with twelve luminescence ages stratigraphy and geomorphic correlation obtaining a virtually continuous distribution of uplift rates along the entire rupture zone the mean uplift rate for these terraces is 0 5 m ka this value is exceeded in three areas which have experienced rapid emergence of up to 1 6 m ka they are located at the northern central and southern sectors of the rupture zone referred to as topocalma carranza and arauco respectively the three sectors correlate with boundaries of eight great earthquakes dating back to 1730 the topocalma and arauco sectors located at the boundaries of the 2010 rupture consist of broad zones of crustal warping with wavelengths of 60 and 90 km respectively these two regions coincide with the axes of oroclinal bending of the entire andean margin and correlate with changes in curvature of the plate interface rapid uplift at carranza in turn is of shorter wavelength and associated with footwall flexure of three crustal scale normal faults the uplift rate at carranza is inversely correlated with plate coupling as well as with coseismic slip suggesting permanent deformation may accumulate interseismically we propose that the zones of upwarping at arauco and topocalma reflect changes in frictional properties of the megathrust resulting in barriers to the propagation of great earthquakes slip during the 1960 m9 5 and 2010 events overlapped with the 90 km long zone of rapid uplift at arauco similarly slip in 2010 and 1906 extended across the 60 km long section of the megathrust at topocalma but this area was completely breached by the 1730 m 9 event which propagated southward until carranza both arauco and topocalma show evidence of sustained rapid uplift since at least the middle pleistocene these two sectors might thus constitute discrete seismotectonic boundaries restraining most but not all great earthquake ruptures based on our observations such barriers might be breached during multi segment super cycle events

moho topography and subducting oceanic slab of the chilean continental margin in the maximum slip segment of the 1960 mw 9 5 valdivia chile earthquake from p receiver functions the maximum slip area of the 1960 mw 9 5 valdivia chile earthquake has been investigated down to 100 km depth by a receiver function survey along two trench perpendicular profiles at 39 3 and 39 5 s the data are compared with seismicity and local earthquake tomography we find a slab of ca 35 dip at depths similar to what is observed to the north and south of the study area the slab and the overlying mantle wedge show a local decrease in seismic velocities possibly caused by hydration or underplating of sediments the presence of hydrated or sedimentary rocks would reduce plate coupling in this region in agreement with recent gps measurements the moho is found at depths of 30 to 40 km deepening from the coast to the andes the moho topography is close to isostatic equilibrium under the volcanic centre of villarrica quetrupillan and lanin the moho conversion occurs about 10 km shallower than expected for a standard isostatic model this can be explained by a density reduction in the lower crust due to magmatic processes or by non compensated stress

geological and geophysical variations along the western margin of chile near lat 33 to 36 s and their relation to nazca plate subduction

two dimensional velocity models of the nazca plate subduction zone between 19 5 s and 25 s from wide angle seismic measurements during the cinca95 project the interpretation of seismic refraction wide angle reflection data from the 1995 crustal investigations off and on shore nazca central andes cinca95 project has resulted in the derivation of nine e w two dimensional 2 d velocity cross sections for the region between the peru chile trench and the coast between 19 5 s and 25 s with three of the cross sections extending a farther 100 200 km inland these sections define the major lithospheric structures of the upper south american plate and the lower nazca plate down to uppermost mantle depths of 30 60 km beneath this part of the present day forearc region in addition to showing the nazca plate subducting at an increasing angle of 9 25 down to 30 50 km depth near the coast these cross sections show a portion of the moho dipping eastward from 43 50 km near the coast to 55 64 km up to about 240 km inland owing to a gap of almost 50 km in the data coverage of the moho near the coast it is uncertain from this data set alone whether the moho defined east of the coast should be correlated with the lower oceanic nazca plate or the upper continental south american plate however a comparison with other seismological data suggests that the moho identified here east of the coast defines the base of the continental crust of the upper south american plate immediately behind the downgoing nazca plate thus the hypothesis of delouis et al 1996 that the zone of seismic coupling between the plates correlates with the region where continental crust is in contact with the plate boundary is supported the cross sections also show between 20 s and 22 5 s a boundary extending from upper crustal levels down to 25 30 km depth subparallel to and 5 10 km above the top surface of the subducting nazca plate the discussion focuses on either that the boundary is a fossil extensional structure or preferably that the wedge below the boundary represents gabbroic material from layer 3 of the oceanic crust tectonically underplated onto the lower part of the upper plate

active faulting and heterogeneous deformation across a megathrust segment boundary from gps data south central chile 36 39 s 1 this study focuses on the present day deformation mechanisms of the south central chile margin at the transition zone between two megathrust earthquake segments defined from historical data the valdivia and concepcion sectors new gps data and finite element models with complex geometries constrained by geophysical data are presented to gain insight into forearc kinematics and to address the role of upper plate faults on contemporary deformation gps vectors are heterogeneously distributed in two domains that follow these two earthquake segments we find that models which simulate only interseismic locking on the plate interface fail to reproduce surface deformation in the entire study area in the concepcion domain models that include a crustal scale fault in the upper plate better reproduce the gps observations in the valdivia domain gps data show regional scale vertical axis rotations which could reflect postseismic deformation processes at the edge of the mw 9 5 earthquake that ruptured in 1960 and or activity of another crustal fault related to motion of a forearc sliver our study suggests that upper plate faults in addition to earthquake cycle transients may exert an important control on the surface velocity of subduction zone forearcs

interseismic coupling segmentation and mechanical behavior of the central chile subduction zone global positioning system gps measurements carried out in chile over the last two decades showed that an entire portion of the nazca south america subduction zone 38 s 24 s was locked over this period of time the induced accumulation of elastic deformation in the upper plate was not released until the recent maule earthquake of 27 february 2010 mw 8 8 that ruptured the southern part of this section locking or coupling between the two plates varies both with depth and along strike here we use our own gps data an updated solution of our extended network in central chile combined with other published data sets to quantify the spatial variations of the coupling that prevailed before the maule earthquake using a simple elastic model based on the back slip assumption we show that coupling variations on the subduction plane are sufficient to explain the observed surface deformation with no need of a sliver in central chile we identify four segments characterized by higher coupling and separated by narrow areas of lower coupling this segmentation is in good agreement with historical and recent seismicity in chile in particular the narrow zones of lower coupling seem to have stopped most large seismic ruptures including maule s these zones are often associated with irregular bathymetric or coastal features fracture zones or peninsulas finally coseismic and early post seismic slip distribution of the maule earthquake occurring either in previously highly or weakly coupled zones map a complex distribution of velocity weakening and velocity strengthening patches on the subduction interface

space geodetic estimation of the nazca south america euler vector abstract gps data from four sites in the nazca plate easter island galapagos robinson crusoe and san felix islands and from five sites in the stable core of the south american plate enabled us to estimate the euler vector of the nazca plate with respect to south america the observed velocities of easter island 6 6 cm yr at 102 3 galapagos 5 1 cm yr at 90 0 robinson crusoe 6 6 cm yr at 80 1 and san felix 6 0 cm yr at 82 1 are significantly slower than the global plate model nuvel 1a velocites for those four sites our estimated euler pole is located at 48 8 n 91 7 w with a rate of 0 59 m y the nuvel 1a and earlier space geodetic studies give rotation rates that are 20 faster

impact of megathrust geometry on inversion of coseismic slip from geodetic data application to the 1960 chile earthquake 1 we analyze the role of megathrust geometry on slip estimation using the 1960 chile earthquake mw 9 5 as an example a variable slip distribution for this earthquake has been derived by barrientos and ward 1990 applying an elastic dislocation model with a planar fault geometry their model shows slip patches at 80 110 km depth isolated from the seismogenic zone interpreted as aseismic slip we invert the same geodetic data set using a finite element model fem with precise geometry derived from geophysical data isoparametric fem is implemented to constrain the slip distribution of curve shaped elements slip resolved by our precise geometry model is limited to the shallow region of the plate interface suggesting that the deep patches of moment were most likely an artifact of the planar geometry our study emphasizes the importance of fault geometry on slip estimation of large earthquakes

fissural volcanism polygenetic volcanic fields and crustal thickness in the payen volcanic complex on the central andes foreland mendoza argentina shield volcanoes caldera bearing stratovolcanoes and monogenetic cones compose the large fissural payen volcanic complex located in the andes foreland between latitude 35 s and 38 s the late pliocene pleistocene and recent volcanic activity along e w trending eruptive fissures produced basaltic lavas showing a within plate geochemical signature the spatial distribution of fractures and monogenetic vents is characterized by self similar clustering with well defined power law distributions vents have average spacing of 1 27 km and fractal exponent d 1 33 defined in the range 0 7 49 3 km the fractal exponent of fractures is 1 62 in the range 1 5 48 1 km the upper cutoffs of fractures and vent fractal distributions about 48 49 km scale to the crustal thickness in the area as derived from geophysical data this analysis determines fractured media crust thickness associated with basaltic retroarc eruptions we propose that the payen volcanic complex was and is still active under an e w crustal shortening regime

coseismic slip distribution of the february 27 2010 mw 8 8 maule chile earthquake 1 static offsets produced by the february 27 2010 mw 8 8 maule chile earthquake as measured by gps and insar constrain coseismic slip along a section of the andean megathrust of dimensions 650 km in length 180 km in width gps data have been collected from both campaign and continuous sites sampling both the near field and far field alos palsar data from several ascending and descending tracks constrain the near field crustal deformation inversions of the geodetic data for distributed slip on the megathrust reveal a pronounced slip maximum of order 15 m at 15 25 km depth on the megathrust offshore lloca indicating that seismic slip was greatest north of the epicenter of the bilaterally propagating rupture a secondary slip maximum appears at depth 25 km on the megathrust just west of concepcion coseismic slip is negligible below 35 km depth estimates of the seismic moment based on different datasets and modeling approaches vary from 1 8 to 2 6 1022 n m our study is the first to model the static displacement field using a layered spherical earth model allowing us to incorporate both near field and far field static displacements in a consistent manner the obtained seismic moment of 1 97 1022 n m corresponding to a moment magnitude of 8 8 is similar to that obtained by previous seismic and geodetic inversions

formation of new continental crust in western british columbia during transpression and transtension crustal growth in the coast mountains along the leading edge of the canadian cordillera was the result of processes associated with horizontal flow of material during transpression and subsequent transtension and the vertical accretion of mantle derived melts from 85 to 58 ma as exotic terranes were translated northward during transpression the crust was thickened to about 55 km and melt that originated from a mix of mantle derived basalt with partial melt of the thickened crust intruded into crustal scale transpressive shear zones when the orogen extended between 58 and 50 ma there was large scale decompression melting in the mantle and dehydration melting in the lower crust voluminous emplacement of sub horizontal sills facilitated by ductile flow of the gneissic country rocks partially filled space created as the crust was pulled apart and as 15 to 20 km of tectonic exhumation occurred across low angle normal ductile shear zones by 50 ma the final crustal thickness of the new continental crust was about 34 km comparison of seismic data with other crustal sections suggests that the crust forming processes identified in western british columbia have general applicability to models for the formation of continental crust

comparative mass balance of volcanic edifices at the southern volcanic zone of the andes between 33 s and 46 s volcano edifice volume calculations are presented for 65 volcanoes of the 1400 km long chilean southern volcanic zone svz as a basic step in subduction zone mass budgets volume calculations are performed in a geographical information system that integrates digital elevation models based on of shuttle radar topography mission as well as aster gdem topographic data landsat satellite images and geological maps the method of volume calculation is straightforward for isolated morphologically well defined stratovolcanoes uncertainties increase for volcanic edifices that formed on pre existing rugged terrain for multi phase eruptive centers as well as for eroded edifices a revised segmentation of the arc is used to describe the spatial volume distribution of extruded magma along the svz and to discuss controlling tectonic factors peak volumes between arc and back arc are offset by 400 km the total volcanic extrusion is in the range of 10 13 km3 km ma major differences between the svz and the central american subduction system are notable with regard to volcano density and maximum volumes

first seismic record for intra arc strike slip tectonics along the liquine ofqui fault zone at the obliquely convergent plate margin of the southern andes a temporal seismic network recorded local seismicity along a 130 km long segment of the transpressional dextral strike slip liquine ofqui fault zone lofz in southern chile seventy five shallow crustal events with magnitudes up to m tief w 3 8 and depths shallower than 25 km were observed in an 11 month period mainly occurring in different clusters those clusters are spatially related to the lofz to the volcanoes chaiten michinmahuida and corcovado and to active faulting on secondary faults further activity along the lofz is indicated by individual events located in direct vicinity of the surface expression of the lofz focal mechanisms were calculated using deviatoric moment tensor inversion of body wave amplitude spectra which mostly yield strike slip mechanisms indicating a ne sw direction of the p axis for the lofz at this latitude the seismic activity reveals the present day activity of the fault zone the recent m tief w 6 2 event near puerto aysen southern chile at 45 4 s on april 21 2007 shows that the lofz is also capable of producing large magnitude earthquakes and therefore imposing significant seismic hazard to this region

volcanic tremor and local earthquakes at copahue volcanic complex southern andes argentina abstract in the present paper we describe the results of a seismic field survey carried out at copahue volcano southern andes argentina using a small aperture dense seismic antenna copahue volcano is an active volcano that exhibited a few phreatic eruptions in the last 20 years the aim of this experiment was to record and classify the background seismic activity of this volcanic area and locate the sources of local earthquakes and volcanic tremor data consist of several volcano tectonic vt earthquakes and many samples of back ground seismic noise we use both ordinary spectral and multi spectral techniques to measure the spectral content and an array technique zero lag cross correlation technique to measure the back azimuth and apparent slowness of the signals propagating across the array we locate vt earthquakes using a procedure based on the estimate of slowness vector components and s p time vt events are located mainly along the border of the caviahue caldera lake positioned at the south east of copahue volcano in a depth interval of 1 3 km below the surface the background noise shows the presence of many transients with high correlation among the array stations in the frequency band centered at 2 5 hz these transients are superimposed to an uncorrelated background seismic signal array solutions for these transients show a predominant slowness vector pointing to the exploited geothermal field of las maquinitas and copahue village located about 6 km north of the array site we interpret this coherent signal as a tremor generated by the activity of the geothermal field

payenia volcanic province in the southern andes an appraisal of an exceptional quaternary tectonic setting abstract the southern volcanic zone of the andes has a quaternary basaltic province along the retroarc which has a unique tectonic setting the payenia volcanic province covers an area larger than 40 000 km2 between 33 30 and 38 south latitudes with an estimated volcanic volume of about 8387 km3 erupted through more than 800 volcanic centers in the last 2 ma the mainly basaltic province developed above the san rafael block is subdivided in three segments characterized by the cerro nevado llancanelo payun matru tromen and auca mahuida volcanic fields together with hundreds of minor monogenetic basaltic centers the analysis of the different segments shows the formation of a common basalt plateau with intraplate signature from south to north between 2 0 and 1 7 ma which reached the 35 s to the north above this plateau monogenetic centers as nihuil vn 1 433 ma and cerro chato at 1 352 ma are developed followed by the large polygenetic center of cerro nevado 3980 m a s l at 1 320 ma this plateau was broken by a series of normal faults that produced volcanic cone alignments such as the nnw trending mancha jarilla lineament in the central part at about 1 0 ma extension shifted to the eastern margin of the san rafael block which concentrates tens of monogenetic centers between 0 9 and 0 7 ma extension then migrated towards the foothills in the west where many monogenetic cones were erupted through nw trending normal faults between 0 5 and 0 435 ma the collapse of the large diamante caldera at 0 445 ma coincides with that period subsequent volcanism was concentrated in 1 the payun matru volcanic field with the eruption of cerro payen between 0 272 and 0 261 ma the payun matru shield volcano with polygenetic eruptions at least since the last 0 233 ma and with the caldera formation bracketed between 0 168 0 004 ma and 0 082 0 001 ma followed by several eruptions until 7000 yrs and even historical ones and in 2 the tromen volcano where younger than 0 2 ma eruptions took place and historical eruptions were reported the understanding of these eruptions in time and space combined with geophysical data indicates the geometry of an important crustal attenuation beneath payenia associated with a hot sublithosphere the late miocene uplifted san rafael block collapsed in the early pleistocene as a consequence of the steepening of the subducted slab and the injection of hot asthenosphere produced the quaternary payenia volcanic province melts of the lower crust along the principal cordillera at these latitudes are responsible for the quaternary calderas ignimbritic flows and rhyolitic volcanism that express the crustal delamination of the andes the payun matru volcanic field concentrates this asthenospheric flow in the present

coastal deformation and great subduction earthquakes isla santa maria chile 37 s isla santa maria at the active margin of south central chile is the result of earthquake related uplift and deformation in the forearc since at least late pleistocene time field mapping dating of key depositional horizons and analysis of seismic refl ection profi les reveal ongoing deformation in this sector of the chilean forearc the 30 km 2 island is located 12 km above the interplate seismogenic zone and 75 km landward of the trench it is situated near the southern termination of the concepcion earthquake rupture segment where charles darwin measured 3 m of coseismic uplift during a m 8 megathrust earthquake in 1835 permanent postearthquake deformation from this earthquake and an earlier event in 1751 is registered by emerged landward tilted abrasion surfaces uplift at 2 m k y and tilting at 0 025 k y of the island have been fairly constant throughout the late quaternary and have resulted in emergence of the island above sea level 31 k y ago the island is composed of a late pleistocene upper tilted surface with two asymmetric tilt domains and holocene lowlands characterized by uplifted and tilted strandlines industry offshore seismic refl ection profi les covering an area of 1800 km 2 and crustal seismicity reveal active reversefault cored anticlines surrounding isla santa maria the principal fault apparently roots in the plate interface thrust these reverse faults in the upper plate result from inversion of late cretaceous to early pliocene normal faults and rift structure of the arauco forearc basin positive inversion of these inherited structures started between 3 6 and 2 5 ma and resulted in continuous shortening rates of 0 8 mm yr the seismic refl ection profi les show that the asymmetric tilt domains and progressive syntectonic sedimentation are linked to the position of the island in the forelimbs of two converging anticlines whereas their backlimbs have been removed by cliff retreat the 2 m uplift contour of the 1835 earthquake is parallel to the strike of active faults and antiforms in the arauco concepcion region the close relation among the asymmetric uplift and tilt of the island modern deformation patterns and reverse faults rooted in the plate interface suggests that slip on the plate interface thrust infl uences localizes and segments surface deformation during large interplate earthquakes furthermore the link between positive inversion of pre existing structures uplift and tilt patterns in the forearc emphasizes the importance of inherited structural fabrics in guiding plate boundary deformation

seismic interferometry applied to regional and teleseismic events recorded at planchon peteroa volcanic complex argentina chile abstract the planchon peteroa volcanic complex ppvc is located in the central andes argentina chile even though this active volcanic system is considered one of the most dangerous volcanoes in the region with more than twenty modest vei seismic interferometry si is a high resolution technique based on analyses of the interference of the propagation seismic energy at one or many stations si can be used to characterize the subsurface properties of a target area in particular previous si studies performed in the area of the ppvc describe specific ranges of depth therefore more information is required for a thorough description of the subsurface features in the area and for a better understanding of the ppvc dynamics we apply si based on autocorrelations of selected regional and teleseismic events to image the subsurface structures below stations located in argentina and chile during 2012 the selection of the events is performed according to their location magnitude angle of incidence of p wave seismic energy and signal to noise ratio in the records for each station we extract time windows and we process them using two ranges of frequency which are sensitive to different depth ranges this work describes depths and zones previously not analyzed in the area using si methods the results not only complement the available geological geochemical and geophysical information but present new information for depths between 10 and 750 km depth increasing the general knowledge of the subsurface features in the ppvc finally we also propose a model for the subsurface down to the moho which indicates the crustal structure and the likely distribution of magma bodies in depth

understanding kinematics of intra arc transcurrent deformation paleomagnetic evidence from the liquine ofqui fault zone chile 38 41 s the liquine ofqui fault zone lofz is a major 1000 km long dextral shear zone of southern chile likely related to strain partitioning of nazca plate oblique convergence with south america to understand block rotation pattern along the lofz we paleomagnetically sampled 55 sites 553 samples between 38 s and 41 s we gathered oligocene to pleistocene volcanics and miocene granites at a maximum distance of 20 km from the lofz and at both sides of it rotations with respect to south america evaluated for 36 successful sites show that crust around the lofz is fragmented in small blocks 1 to 10 km in size while some blocks at both fault edges undergo very large 150 170 rotations others do not rotate even adjacent to fault walls we infer that rotations affected equidimensional blocks while elongated crust slivers were translated subparallel to the lofz without rotating rotation pattern across the lofz is markedly asymmetric east of the fault and adjacent to it rotations are up to 150 170 clockwise and fade out 10 km east of fault these data support a quasi continuous crust kinematics characterized by small rigid blocks drag by the underlying ductile crust flow and imply 120 km of total fault offset conversely crust west of the lofz is cut by seismically active nw se sinistral antithetic faults and yields counterclockwise rotations up to 170 at 8 10 km from lofz besides the unrotated blocks further data from the chile fore arc are needed to understand block rotation kinematics and plate dynamics west of the lofz

sub slab mantle anisotropy beneath south central chile knowledge of mantle flow in convergent margins is crucial to unravelling both the contemporary geodynamics and the past evolution of subduction zones by analysing shear wave splitting in both teleseismic and local arrivals we can determine the relative contribution from different parts of the subduction zone to the total observed sks splitting providing us with a depth constraint on anisotropy we use this methodology to determine the location orientation and strength of seismic anisotropy in the south central chile subduction zone data come from the tipteq network deployed on the forearc during 2004 2005 we obtain 110 teleseismic sks and 116 local good quality shear wave splitting measurements sks average delay times are 1 3 s and local s delay times are only 0 2 s weak shear wave splitting from local phases is consistent with a shape preferred orientation spo source in the upper crust we infer that the bulk of shear wave splitting is sourced either within or below the subducting nazca slab sks splitting measurements exhibit an average north easterly fast direction with a strong degree of variation further investigation suggests a relationship between the measurement s fast direction and the incoming ray s back azimuth finite element geodynamic modelling is used to investigate the strain rate field and predicted lpo characteristics in the subduction zone these models highlight a thick region of high strain rate and strong s wave anisotropy with plunging olivine a axes in the sub slab asthenosphere we forward model the sub slab sourced splitting with a strongly anisotropic layer of thick asthenosphere comprising an olivine a axis oriented parallel to the direction of subduction the subducting lithosphere is not thick enough to cause 1 2 s of splitting therefore our results and subsequent models show that the nazca slab is entraining the underlying asthenosphere its flow causes it to be strongly anisotropic our observation has important implications for the controlling factors on sub slab mantle flow and the movement of asthenospheric material within the earth

the 2010 mw 8 8 maule megathrust earthquake of central chile monitored by gps large earthquakes produce crustal deformation that can be quantified by geodetic measurements allowing for the determination of the slip distribution on the fault we used data from global positioning system gps networks in central chile to infer the static deformation and the kinematics of the 2010 moment magnitude m w 8 8 maule megathrust earthquake from elastic modeling we found a total rupture length of 500 kilometers where slip up to 15 meters concentrated on two main asperities situated on both sides of the epicenter we found that rupture reached shallow depths probably extending up to the trench resolvable afterslip occurred in regions of low coseismic slip the low frequency hypocenter is relocated 40 kilometers southwest of initial estimates rupture propagated bilaterally at about 3 1 kilometers per second with possible but not fully resolved velocity variations

crustal model of the southern central andes derived from ambient seismic noise rayleigh wave tomography abstract the distribution and the interconnection of magmatic bodies beneath volcanic arcs is key to asses volcanic hazard and eruptive processes taking place at convergent margins we use ambient seismic noise rayleigh wave tomography to investigate the three dimensional shear wave crustal velocity structure of the southern central andes between latitudes 33 s and 38 s for the upper 30 km we investigate the occurrence of magmatic reservoirs in the upper crust and show how their geometry is affected by local tectonics the first prominent feature to be observed is the shear wave velocity contrast between the volcanic arc and the faster fore arc region we further identify areas of low shear wave velocity from 3 km to 10 km depth beneath the volcanic arc striking n s that we interpret as zones of fluid rich crust possibly characterised by confined regions of partial melts our results allow us to derive a model for the crustal structure of the southern central andes we propose that partial melts marked by lower shear wave velocity anomalies are connected to shallower structural levels and reservoirs by brittle regions where dikes and exsolved fluids may propagate

eruptive sequence and seismic activity of llaima volcano chile during the 2007 2009 eruptive period inferences of the magmatic feeding system abstract llaima volcano one of the most active volcanoes in south america has experienced intense activity during the last 100 years the most recent eruptive activity occurred during the period 2007 2009 with at least six energetic eruptions the january 1st 2008 vei 3 being the strongest episode most of the paroxysmal activity was characterized by a rapid increase in seismic energy minutes to hours and the absence of precursory signals as well as by an accelerated drop of the seismic energy and eruption intensity at the end moreover subtle changes in the increase of low energy long period lp seismicity and the occurrence of minor explosive activity six months before the onset of the eruptive cycle were the only remarkable changes observed in advance this is the first study that includes a detailed description of the 2007 2009 eruptive period and is based on a temporal analysis of the seismic records technical reports from the monitoring network and 2000 photos this set allows a characterization of the coeval eruptive styles including strombolian and hawaiian activity morphological changes of the active vents inside the main crater and two fractures located in the sw and se flanks ten phases were identified based on the eruptive style volcanic explosivity index vei and the salient features of the seismic events the seismicity that accompanied the eruptive phases was characterized by the predominance of a continuous tremor tr and discrete lp events and a remarkable absence of volcano tectonics vt earthquakes a waveform cross correlation analysis of lp events showed an overall low similarity between them which suggests multiple sources our observations and analysis suggest that the plumbing system is composed of multiple independent structures some of which reached the surface during this eruptive period indicating that the upper part of the cone could be an unstable and essentially weak zone the activity and seismicity observed are characteristic of an open vent system in which the magma can ascend rapidly from several km depth with little to no clear precursory activity these results are in agreement with petrological studies of the products of these eruptions

towards understanding the lithospheric structure of the southern chilean subduction zone 36 s 42 s and its role in the gravity field summary the southern andes differ significantly from the central andes with respect to topography and crustal structures and are from a geophysical point of view less well known in order to provide insight into the along strike segmentation of the andean mountain belt an integrated 3 d density model was developed for the area between latitudes 36 s and 42 s the model is based on geophysical and geological data acquired in the region over the past years and was constructed using forward density modelling in general the gravity field of the south american margin is characterized by a relatively continuous positive anomaly along the coastline and the forearc region and by negative anomalies along the trench and the volcanic arc however in the forearc region of the central part of the study area located just to the south of the epicentre of the largest ever recorded earthquake valdivia 1960 the trench parallel positive anomaly is disrupted the forearc gravity anomaly differences thus allow the study area to be

tectonic and magmatic evolution of the andes of northern argentina and chile abstract two orogenic cycles both with different evolution are developed in the western margin of the south american continent in northern argentina and chile the paleozoic hercynic cycle and the meso cenozoic andean cycle the hercynic cycle a wide marine basin extending westward of the cordillera oriental which developed in cambrian ordovician times marks the beginning of this cycle in contrast to peru and bolivia where this basin developed between two precambrian blocks the western margin of this basin in northern argentina and chile is still unknown the ordovician sedimentation and accompanied volcanism ends with the ocloyic deformation phase and its synkinematic granitic plutonism two basins were developed in the silurian devonian separated by the puna arc uplifted during this ocloyic phase the shallow water marine terrigeneous sediments which were deposited in them were deformed by a new tectono magmatic associated phase chanic phase carboniferous to lower permian marine carbonates were deposited west of the puna arc and red beds east of it later on during the permian to triassic a magmatic belt was developed along the cordillera occidental the rhyolites ignimbrites and the granitic to granodioritic related intrusives are well represented in chile although the overall geologic history of this period is known many problems concerning its origin and relations to plate tectonics are still unsolved the andean cycle during this cycle a series of magmatic arc systems related to the subduction of the pacific crust was built up along the western margin of south america huge volumes of calc alkaline lavas and related plutons were emplaced since the jurassic along belts parallel to the present coastline showing a general eastward migration trend up to the lower cretaceous an ensialic back arc basin was formed east of this magmatic arc thousands of meters of marine and continental sediments were deposited in it this basin disappeared during the middle cretaceous probably as a result of the final opening of the atlantic and the active westward movement of the south american plate since middle cretaceous times the magmatic arc has been the fundamental paleogeographic element the arc progressively migrated stepwise eastward each step marked by a tectonic phase subduction related crustal erosion could explain the lack of fore arc petrotectonic assemblages the eastward magmatic polarity which characterizes this section of the andes could also be explained by such a mechanism

tertiary tectonics of the patagonian andes retro arc area between 38 15 and 40 s latitude the structure and tectonics of the retro arc area of the andes between 38 degrees 30 and 40 degrees s remains poorly understood because of the scarcity of previous structural studies this paper analyzes the styles distribution and timing of the deformation in the eastern flank of the andes between these latitudes with focus in the tertiary tectonics of the area this andean section can be divided into an internal and an external domain based on its distinctive structural evolution the boundary between them coincides with the bio bio and alumine valleys which separate sectors of the fold and thrust belt with opposite vergence and different structural styles structures developed in the external domain during the andean cycle are associated with a west verging fold and thrust belt which was controlled geometrically by pre existing normal faults of the mesozoic rift phase we also differentiate three segments along the external domain of the inner retro arc zone at these latitudes each segment is associated with a tertiary depocenter related to the western border of a structural high copahue pino hachado block and cordilleras de catan lil and chachil which constitutes an outstanding feature within the study area the recognition and interpretation of the main structures and the spatial and temporal distribution of the tertiary sequences allowed us to establish a tectonic model in which the uplift of this nnw trending block during upper miocene times originated a series of intermontane depocenters well represented between 38 and 40 degrees s as a result of that syntectonic and synorogenic deposits accumulated in a compressive regime these depocenters can be integrated in a narrow deposition zone associated with the western slope of the copahue pino hachado block and its southward prolongation in the cordillera de catan lil and cordillera del chachil on the basis of tectonostratigraphic controls we define the last andean contractional phase of the inner retro arc area at these latitudes between 11 and 5 67 ma for the northern segment between 10 and 6 2 ma for the central segment and between 13 to 8 and 43 ma for the southern segment after this short lived compressional stage deformation migrated to the west associated with the onset of the liquine ofqui strike slip fault zone along the intra arc zone c 2010 elsevier b v all rights reserved

structural controls of volcanism in transversal chains resheared faults and neotectonics in the cordon caulle puyehue area 40 5 s southern andes abstract the nature of the relationship between tectonics and volcanism in a specific area is the result of the interaction between the crustal structures of the basement and ongoing regional stress field in the southern andes different kinematic environments represented by two nearly orthogonal active fissure systems show apparently incompatible arrays of volcanic chains moreover a local quaternary compressive regime is inferred from basement faults however these mutually orthogonal chains which are also oblique to the volcanic front have contrasting magma evolution and eruptive styles that can be related with their tectonic setting whereas northeast trending monogenetic cones carran los venados can be interpreted as resulting from magma upwelling along regional tension cracks in a quaternary dextral transpressive compressive domain northwest striking fissure system cordon caulle volcanic complex can be the result of extensional fault reactivation and long magma residence in the upper crust different kinematic environments cause contrasting magma evolution and eruptive styles a two way coupling mechanism is suggested for to explain local depart of the regional stress field geometric array of extensional features and final magmatic evolution in these volcanic domains of the southern andes

seismic activity of the nevados de chillan volcanic complex after the 2010 mw8 8 maule chile earthquake abstract couplings between large magnitude subduction zone earthquakes and the subsequent response of their respective volcanic arcs are generally accepted but the mechanisms driving this coupling are not known the 2010 maule earthquake mw8 8 ruptured approximately 500 km along strike in the south central part of chile and provides an opportunity to investigate earthquake volcano interactions in an exploratory campaign we deployed four broadband seismometers atop and surrounding the nevados de chillan volcanic complex because it is located directly behind one of the two primary slip patches of the maule earthquake the data recorded from december 2011 to april 2012 shows significant seismic activity characterized by numerous volcano tectonic events and tremor episodes occurring within the volcanic complex we recorded two strong aftershocks of the maule earthquake mw 6 1 in january 2012 and mw 7 1 in april 2012 and investigated the response of the volcano to the incoming seismic energy we find that volcanic tremor increased significantly upon arrival of the seismic waves from the mw 6 1 event which was then followed a few hours later by a significant increase in volcano tectonic events when tremors subsided this delay between tremor and volcano tectonic events suggests that fluid and or magma migration manifested as tremor readjusted the local stress state that then induced the volcano tectonic events the increased activity persisted during the subsequent two weeks in contrast the mw 7 1 event which occurred at a similar epicentral distance from the volcano did not produce any significant seismic response of the volcanic complex analysis of the particle velocity records of the two events shows that the volcanic system was perturbed in different ways because of the incidence angle of the incoming energy inducing a back elliptical vertical and fault parallel motion for the mw 6 1 and no clear directional dependence for the mw 7 1 this suggests dilatation induced fluid migration within the complex and a kinematic mechanism of the perturbation rather than the perturbation amplitude our results demonstrate the importance for continued monitoring of the arc behind maule with an increased seismometer and gps array density to determine the style of deformation currently occurring in the arc

structural electrical anisotropy in the crust at the south central chilean continental margin as inferred from geomagnetic transfer functions abstract induction vectors as a visualization of geomagnetic deep sounding transfer functions display an unique pattern at the south chilean continental margin between latitudes 38 41 s and longitudes 71 74 w at long periods of approx 3000 s their real parts are uniformly deflected from the w e direction which would be expected due to the coast effect and or anomalies beneath the roughly n s striking andean mountain chain to the ne attempts to model this behavior with simple and geologically realistic 3 d models failed but a reasonable data fit was obtained by employing 2 d models with a structurally anisotropic lower crust this anisotropy hints at a deeply fractured fluid rich crust with a major strike direction of 40 50 sw ne oblique to the continental margin and in accordance with the regional stress field in the region of the volcanic arc a surprising result is that the anisotropy persists in the forearc and may even reach until the continental slope near the trench

incipient axial collapse of the main cordillera and strain partitioning gradient between the central and patagonian andes lago laja chile 1 lago laja is a late quaternary volcanic dammed lake located near the drainage divide of the south central andes field observations lake reflection seismic profiles bathymetry and remote sensing data reveal an active fault system that runs parallel to the volcanic arc along the axis of the main cordillera the lago laja fault system llfs normal faults of this extensional system cut late pleistocene volcanics 2 7 mm yr since 7 1 ka the llfs accounts for 0 7 of arc normal extension at an average minimum rate of 1 2 mm yr and strain rate of 10 14 s 1 seismites and surface ruptures evidence m 6 paleoearthquakes the main cordillera at 37 s is a large scale pop up structure uplifted by thrusting along its foothills in this light we interpret extension in the axial and highest part of the andes as incipient synorogenic gravitational collapse in response to uplift and crustal thickening thermal weakening due to elevated heat flow and postglacial lithospheric rebound and unbending have probably contributed to the arc limited collapse and holocene acceleration of deformation rates the lack of significant strike slip offsets along the llfs as well as along both foothills thrust systems at 37 s contrasts with the intra arc dextral fault zone south of 38 s regional structural data indicates that north of 38 s diffusely distributed strain reflects low partitioning of oblique subduction while to the south deformation is localized in a discrete strike slip fault zone along the volcanic arc reflecting a higher degree of partitioning we relate this strain partitioning gradient to favorable fault orientations in the fore arc north of the arauco peninsula a major seismotectonic boundary

intra arc crustal seismicity seismotectonic implications for the southern andes volcanic zone chile we examine the intra arc crustal seismicity of the southern andes volcanic zone svz our aim is to resolve inter seismic deformation in an active magmatic arc dominated by both margin parallel liquine ofqui fault system lofs and andean transverse faults atf crustal seismicity provides information about the schizosphere tectonic state delineating the geometry and kinematics of high strain domains driven by oblique subduction here we present local seismicity based on 16 months data collected from 34 seismometers monitoring a 200 km long section of the southern volcanic zone including the lonquimay and villarrica volcanoes we located 356 crustal events with magnitudes between mw 0 6 and mw 3 6 local seismicity occurs at depths down to 40 km in the forearc and consistently shallower than 12 km beneath the volcanic chain suggesting a convex shape of the crustal seismogenic layer bottom focal mechanisms indicate strike slip faulting consistent with ene wsw shortening in line with the long term deformation history revealed by structural geology studies however we find regional to local scale variations in the shortening axes orientation as revealed by the nature and spatial distribution of microseismicity within three distinctive latitudinal domains in the northernmost domain seismicity is consistent with splay faulting at the northern termination of the lofs in the central domain seismicity distributes along ene and wnw striking discrete faults spatially associated with hitherto seismic atf the southernmost domain in turn is characterized by activity focused along a n15 e striking master branch of the lofs these observations indicate a complex strain compartmentalization pattern within the intra arc crust where variable strike slip faulting dominates over dip slip movements

compressional and transpressional stress pattern for pliocene and quaternary brittle deformation in fore arc and intra arc zones andes of central and southern chile kinematic analysis of fault slip data for stress determination was carried out on late miocene to quaternary rocks from the fore arc and intra arc regions of the chilean andes between 33 and 46 south latitudes studies of neogene and quaternary infilling the central depression as well as plutonic rocks of the north patagonian batholith along the liquine ofqui fault zone have revealed various compressional and or transpressional states of stress in the pliocene the maximum compressional stress 1 was generally oriented east west during the quaternary the deformation was partitioned into two coeval distinctive states of stress in the fore arc zone the state of stress was compressional with 1 oriented in a n s to nne ssw direction in the intra arc zone the state of stress was transpressional with 1 striking ne sw along the coast in one site 37 30 s the quaternary strain deformation is extensional with an e w direction which can be explained by a co seismic crustal bending readjustment

three dimensional electrical resistivity image of the south central chilean subduction zone abstract based on isotropic 3 d inversion we re interpret long period magnetotelluric data collected across the geotectonic structures of the south central chilean continental margin at latitudes 38 41 s and summarize results of long period magnetotelluric mt investigations performed between 2000 and 2005 the new 3 d conductivity image of the south central chilean subduction zone basically confirms former 2 d inversion models along three profiles and complete the previous results the models show good electrical conductors in the tip of the continental crustal beneath the pacific ocean the frequently observed forearc conductor at mid crustal levels a highly conductive zone at similar levels slightly offset from the volcanic arc and a not well resolved conductor in the argentinian backarc the subducted nazca plate generally appears as a resistive but discontinuous feature unlike before we are now able to resolve upper crustal conductors interpreted as magma reservoirs beneath active lonquimay villarrica and llaima volcanoes which were obscured in 2 d inversion data fit is rather satisfactory but not perfect we attribute this to large scale crustal anisotropy particularly beneath the coastal cordillera which we cannot include into our solution for the time being

seismicity and geometry of the south chilean subduction zone 41 5 s 43 5 s implications for controlling parameters in 2005 an amphibious seismic network was deployed on the chilean forearc between 41 75 s and 43 25 s 364 local events were observed in a 11 month period a subset of the p and s arrival times were inverted for hypocentral coordinates 1 d velocity structure and station delays main seismic activity occurred predominantly in a belt parallel to the coast of chiloe island in a depth range of 12 30 km presumably related to the plate interface the 30 inclination of the shallow part of the wadati benioff zone is similar to observations further north indicating that oceanic plate age is not controlling the subduction angle of the shallower part for the chilean subduction zone the down dip termination of abundant intermediate depth seismicity at approximately 70 km depth seems to be related to the young age and high temperature of the oceanic plate crustal seismicity is associated with the liquine ofqui fault zone and active volcanoes

water input and water release from the subducting nazca plate along southern central chile 33 s 46 s the age of the subducting nazca plate off chile increases northward from 0 ma at the chile triple junction 46 s to 37 ma at the latitude of valparaiso 32 s age related variations in the thermal state of the subducting plate impact on a the water influx to the subduction zone as well as on b the volumes of water that are released under the continental fore arc or alternatively carried beyond the arc southern central chile is an ideal setting to study this effect because other factors for the subduction zone water budget appear constant we determine the water influx by calculating the crustal water uptake and by modeling the upper mantle serpentinization at the outer rise of the chile trench the water release under fore arc and arc is determined by coupling fem thermal models of the subducting plate with stability fields of water releasing mineral reactions for upper and lower crust and hydrated mantle results show that both the influx of water stored in and the outflux of water released from upper crust lower crust and mantle vary drastically over segment boundaries in particular the oldest and coldest segments carry roughly twice as much water into the subduction zone as the youngest and hottest segments but their release flux to the fore arc is only about one fourth of the latter this high variability over a subduction zone of 1500 km length shows that it is insufficient to consider subduction zones as uniform entities in global estimates of subduction zone fluxes

the 1960 chilean earthquakes an unusual foreshock sequence that began with an earthquake of moment 2 0 1021 n m and rupture length of about 150 km preceded the great 1960 chilean earthquake earthquake relocations of the sequence show a progression of seismic activity toward the initiation of the main shock the rupture length of the great 1960 chilean earthquake is estimated from the distribution of aftershocks and crustal deformation to be 920 100 km the source mechanisms of a foreshock and two aftershocks are determined and used to constrain the source mechanism of the main shock the first event in the sequence ruptured the segment of the peru chile trench between 37 03 s and 38 74 s whereas the great 1960 chilean main shock ruptured the adjoining segment to the south the great 1960 chilean main shock initiated within the rupture area of the first event and terminated near the intersection of the chile ridge with the peru chile trench 46 5 s

magmatic arc tectonics in the central andes between 21 and 25 s abstract in northern chile four magmatic arc systems each displaced towards the east in relation to the previous one developed in sequence since the lias this migration of the magmatic arc from the coastal cordillera to the western cordillera probably was a consequence of tectonic erosion of the continental margin within each of these systems tectonic deformation took place contemporaneously to magmatic activity as the continental crust of the upper plate has been weakened in the area of the magmatic arc by the igneous processes it is relatively sensitive to the stresses introduced by plate convergence a very important factor for the deformation in the magmatic arc is the angle of convergence obliqueness which may cause shortening if obliqueness is small

neogene sedimentary and mass wasting processes on the continental margin off south central chile inferred from dredge samples the active continental margin off south central chile 36 to 40 s is transitional between the tectonically erosive empty trench margin north of juan fernandez ridge and the accretionary trench filled margin south of the chile triple junction the small width of the presently active accretionary wedge maximum width of 25 to 50 km argues for past phases of tectonic erosion at present this sector shows indications of contemporaneous accretion subduction and underplating of sediment as well as readjustment of the slope by various mass wasting processes in this context this study aims to examine the neogene sedimentary processes on the continental margin from dredge samples recovered during r v sonne cruise so161 5 within this transitional domain using lithology sandstone petrology shale mineralogy and analysis of sedimentary structures our results yield that the principal transport of material occurs in high energy turbidity currents and debris flows via submarine canyons deeply cutting the continental slope whereas sediment on the shelf is transported by strong coast parallel bottom currents and trapped by submarine canyons cutting into the shelf a wide range of mass wasting processes including slumping debris flows evolving to low density turbidity currents and mud flows rework the slope sediments in contrast thick undisturbed sequences of mostly hemipelagic sediments accumulate in active slope basins which are largely protected from mass movements xrd analyses revealed early diagenetic lithification and overall burial depths of up to 230 mbsf suggesting a shallow subsurface cycle of sedimentation subsidence diagenesis uplift erosion and resedimentation the composition of sandstones is dominated by volcanic rock fragments of andean provenance along strike modal changes reflect a southward increase in glacially denudation and rainfall the combination of which caused more intense erosion of volcanic rocks and exposure weathering and as a result increased fluvial transport of metamorphic and plutonic rocks to the sea

remarkably consistent thermal state of the south central chile subduction zone from 36 s to 45 s delineating rupture areas of subduction zone earthquakes is necessary for understanding the controls on seismic and aseismic slip for the largest recorded earthquake the 1960 chile event with moment magnitude 9 5 the rupture area is only loosely defined due to limitations in the global seismic network at the time the rupture extends 900 km along strike coastal deformation is consistent with either a constant rupture width of 180 200 km along the entire length or a narrower 115 km rupture in the southern half a southward narrowing of the seismogenic zone has been hypothesized to result from warming of the subduction zone to the south where the subducting plate is younger we present results of thermal models at 36 s 38 s 43 s and 45 s to examine potential along strike changes in thermal state models most consistent with observed surface heat flux include fluid circulation in the oceanic crust that advects heat to the ocean this ventilated hydrothermal circulation preferentially cools transects with young subducting lithosphere frictional heating preferentially warms transects with older subducting lithosphere the combined effects of frictional heating and hydrothermal circulation increase decollement temperatures in the 36 s and 38 s transects by up to 155 c and decrease temperatures in the 45 s transect by up to 150 c in our preferred models decollement temperatures 200 km landward of the trench in all four transects are 350 400 c this is consistent with a constant 200 km wide seismogenic zone for the 1960 mw 9 5 rupture with decreasing slip magnitude in the southern half of the rupture

segmentation of megathrust rupture zones from fore arc deformation patterns over hundreds to millions of years arauco peninsula chile 1 this work explores the control of fore arc structure on segmentation of megathrust earthquake ruptures using coastal geomorphic markers the arauco nahuelbuta region at the south central chile margin constitutes an anomalous fore arc sector in terms of topography geology and exhumation located within the overlap between the concepcion and valdivia megathrust segments this boundary however is only based on 500 years of historical records we integrate deformed marine terraces dated by cosmogenic nuclides syntectonic sediments published fission track data seismic reflection profiles and microseismicity to analyze this earthquake boundary over 10 2 10 6 years rapid exhumation of nahuelbuta s dome like core started at 4 1 2 ma coeval with inversion of the adjacent arauco basin resulting in emergence of the arauco peninsula here similarities between topography spatiotemporal trends in fission track ages pliocene pleistocene growth strata and folded marine terraces suggest that margin parallel shortening has dominated since pliocene time this shortening likely results from translation of a fore arc sliver or microplate decoupled from south america by an intra arc strike slip fault microplate collision against a buttress leads to localized uplift at arauco accrued by deep seated reverse faults as well as incipient oroclinal bending the extent of the valdivia segment which ruptured last in 1960 with an mw 9 5 event equals the inferred microplate we propose that mechanical homogeneity of the fore arc microplate delimits the valdivia segment and that a marked discontinuity in the continental basement at arauco acts as an inhomogeneous barrier controlling nucleation and propagation of 1960 type ruptures as microplate related deformation occurs since the pliocene we propose that this earthquake boundary and the extent of the valdivia segment are spatially stable seismotectonic features at million year scale

constructing forearc architecture over megathrust seismic cycles geological snapshots from the maule earthquake region chile we present new field structural data from the chilean coastal cordillera located above the northern and central parts of the interplate contact ruptured by the a d 2010 m w 8 8 maule earthquake the northern study area contains the northwest striking pichilemu normal fault an intraplate structure reactivated after the megathrust event by crustal earthquakes up to m w 7 0 the structural style of this region is dominated by kilometer scale normal faults that have been active at least throughout the quaternary the orientations of these main faults define three structural systems 1 northeast and 2 northwest striking margin oblique faults and 3 north to north northeast striking margin parallel faults from north to south these three systems vary in their predominant occurrence starting with bimodal orientations of groups 1 and 2 followed by predominantly single north to north northeast orientations of group 3 reverse faults coexist in time and space with the normal structures but are scarce and display variable apparently random orientations the shallow crustal normal faults including the pichilemu fault show a persistent kinematic history probably spanning thousands of subduction seismic cycles though historically smaller in magnitude than those of the triggered normal faults interseismic forearc thrust events were recorded above the rupture area prior to the maule earthquake the quaternary reverse faults identified in our study regions may be preserving interseismic slow strain rate permanent deformation signature in the structural grain analogous observations along the a d 2011 tohoku earthquake rupture in japan imply that such a link between the short and long term deformation patterns of the forearc is not exclusive of the maule earthquake region

splay fault slip during the mw 8 8 2010 maule chile earthquake comment over the past decade the isla santa maria south central chile has provided one of the most exquisitely detailed records of deformation and tectonically controlled sea level change anywhere along the andean margin bookhagen et al 2006 1 melnick et al 2006 2 2009 3 2012 4

giant earthquakes in south central chile revealed by holocene mass wasting events in lake puyehue abstract very high resolution reflection seismic profiling 3 5 khz revealed nine holocene mass wasting events in lake puyehue south central chile these events are made up of numerous coeval mass wasting deposits and some homogenites which are radiocarbon dated the two youngest mass wasting events could be attributed to the giant ad 1960 and ad 1575 valdivia earthquakes the most extensive event took place around 1660 cal yr bp evaluation of all possible slope failure processes led us to infer that giant earthquakes roughly comparable to the ad 1960 valdivia earthquake mw 9 5 are the most likely trigger mechanisms of all mass wasting events in lake puyehue these occurred with a mean recurrence rate of 1000 yr although relatively aperiodically ranging between 500 and 2000 yr quantitative comparison of mass wasting events related to historically reported earthquakes ad 1960 and ad 1575 showed significant differences although these earthquakes are assumed to have had a comparable strength a lowered background sedimentation rate could be responsible for this variable earthquake recording which highlights the importance of a thorough assessment of the depositional history before using lacustrine records for quantitative paleoseismic analysis

magmatic evolution of the puyehue cordon caulle volcanic complex 40 s southern andean volcanic zone from shield to unusual rhyolitic fissure volcanism abstract magmas erupted from quaternary volcanoes of southern andes between 37 and 46 s latitude are mainly basaltic to andesitic however pccvc 40 s shows a singular magmatic evolution due to the abnormal evacuation of rhyolites especially in the last 100 ka in addition pccvc is the result of juxtaposing products from the nw trending alignment of cordillera nevada caldera cordon caulle fissure volcano and the puyehue stratocone using 40 ar 39 ar and 14 c geochronology it can be established that they evolved since ca 500 ka as coeval but separated vents with a first stage of shield volcanism followed by repeated collapses that formed an internal nw elongated graben from ca 100 ka volcanic activity occurred in both a fissure system cordon caulle and a central volcano puyehue holocene explosive eruptions mainly in the puyehue crater accompanied the dome growing along a nw trending fissure system last historical eruptions were in 1921 and 1960 when nw fissures of cordon caulle fed rhyodacitic lava flows in 1960 the fissure eruption was triggered by a remote mw 9 5 thrust earthquake cordillera nevada caldera presents a reduced compositional range 52 63 sio 2 and geochemical features of low pressure magma mixing and assimilation instead cordon caulle and puyehue volcanoes have a wide silica range 48 71 sio 2 and an outstanding affinity which can be modelled with initial high pressure fractional crystallization moderate magma mixing and subsequent low pressure fractional crystallization from a common parental source the exceptional magmatic evolution and eruptive style of pccvc in southern andes could be related with the physics of the plumbing system which in turn can be controlled by external factors as the structure of the continental crust and the ongoing stress regime

seismic potential for large and great interplate earthquakes along the chilean and southern peruvian margins of south america a quantitative reappraisal the seismic potential of the chilean and southern peruvian margins of south america is reevaluated to delineate those areas or segments of the margin that may be expected to experience large or great interplate earthquakes within the next 20 years 1984 2004 long term estimates of seismic potential or the conditional probability of recurrence within a specified period of time are based on 1 statistical analysis of historic repeat time data using weibull distributions and 2 deterministic estimates of recurrence times based on the time predictable model of earthquake recurrence both methods emphasize the periodic nature of large and great earthquake recurrence and are compared with estimates of probability based on the assumption of poisson type behavior the estimates of seismic potential presented in this study are long term forecasts only as the temporal resolution or standard deviation of both methods is taken to range from 15 to 25 of the average or estimated repeat time at present the valparaiso region of central chile 32 35 s has a high potential or probability of recurrence in the next 20 years coseismic uplift data associated with previous shocks in 1822 and 1906 suggest that this area may have already started to rerupture in 1971 1973 average repeat times also suggest this area is due for a great shock within the next 20 years flanking segments of the chilean margin coquimbo illapel 30 32 s and talca concepcion 35 38 s presently have poorly constrained but possibly quite high potentials for a series of large or great shocks within the next 20 years in contrast the rupture zone of the great 1960 earthquake 37 46 s has the lowest potential along the margin and is not expected to rerupture in a great earthquake within the next 100 years in the north the seismic potentials of the mollendo arica 17 18 s and arica antofagasta 18 24 s segments which last ruptured during great earthquakes in 1868 and 1877 are also high but poorly constrained

seismicity near the slip maximum of the 1960 mw 9 5 valdivia earthquake chile plate interface lock and reactivation of the subducted valdivia fracture zone understanding the processes behind subduction related hazards is an important responsibility and major challenge for the earth sciences few areas demonstrate this as clearly as south central chile where some of the largest earthquakes in human history have occurred we present the first observation of local seismicity in the villarrica region 39 40 s based on a temporary local network of 55 stations installed from the chilean coast into the argentinian back arc for one year while consistent with the chilean national catalog ssn our results allow us to observe smaller magnitudes with a completeness of about 2 0 and image the geometry of the wadati benioff zone from the chile trench down to 200 km offshore a gap in interplate seismicity is observed in the region of the 1960 valdivia earthquake slip above the interface two offshore seismicity clusters possibly indicate ongoing stress relaxation in the subducting nazca plate we find a prominent seismicity cluster along the extrapolated trace of the oceanic valdivia fracture zone vfz the seismicity cluster is observed between 70 and 130 km depth and comprises mainly strike slip events it indicates weakening and reactivation of the major vfz by dehydration of oceanic crust and mantle interpreting the subducted vfz section as a localized reservoir of potential fluid release offers an explanation for the villarrica volcanic complex that is located above the reactivated vfz and shows the highest volcanic activity in south america crustal seismicity is observed near puyehue volcano which recently started to erupt june 2011 key points we observe local seismicity near the 1960 valdivia maximum slip region the subducted valdivia fracture zone is reactivated by fluid release the seismogenic zone is free of seismicity events occur at faults and volcanoes

crustal thickness and mantle wedge structure from receiver functions in the chilean maule region at 35 s a temporary passive seismic network of 21 broad band stations was deployed in central chile between 35 s to 36 s the network recorded data prior to the magnitude mw 8 8 2010 maule earthquake at a latitude of the major slip and surface deformation the experiment was conducted to survey crustal and mantle structures and to assess the state of hydration of the mantle wedge we present results of a teleseismic p receiver function study supporting a continental moho at approx 38 km depth phase conversion at this boundary could be observed continuously from the intersection of the subducting slab with the continental moho towards the andes the character of receiver functions indicated little evidence for infiltration of water from the subducting plate into the overlying mantle wedge suggesting that only a small amount of water is released from the subducting plate aftershocks of the maule earthquake and post seismic slip reached depths of 50 km and hence slip spread down dip of the continental moho in the post seismic phase co seismic rupture however occurred updip of the continental moho spare aftershock seismicity is observed at the intersection of the continental moho with the subducting slab

morphotectonic segmentation of an active forearc 37 41 s chile abstract active forearc regions are characterized by seismo and morphotectonic segments that record recurrent seismic activity and subduction related tectonic deformation on different timescales however it is not well known on which timescales morphotectonic forearc segments retain their tectono geomorphic identity how they are related to earthquake rupture zones and what governs their long term evolution the forearc of the active convergent margin of south central chile 37 41 s is located within the rupture zone of the 1960 chile mw 9 6 megathrust earthquake in this region we combine geomorphological sedimentologic and morphometric dem analysis with compiled structural and geophysical data to reconstruct the morphotectonic evolution of the forearc the southern chile forearc comprises three segments nahuelbuta tolten and bueno segments that record a differential structural and geomorphic evolution whereas the tolten segment appears to be quasi stable the other two segments record pronounced ongoing quaternary uplift the nahuelbuta and bueno segments contain the n s oriented coastal cordillera characterized by numerous tectonically controlled drainage anomalies and evidence of sediment rerouting the morphotectonic segmentation appears to have persisted over timescales of 105 to 106 years importantly these segments do not correlate with earthquake rupture zones that exhibit a short term variability over 102 to 103 years thus earthquake rupture zones are transient second order phenomena which do not determine the long term morphotectonic forearc segmentation overall the differential forearc evolution and segmentation is influenced by a complex interplay of inherited upper plate structures and physical properties of the subducting oceanic nazca plate