moho topography in the central andes and its geodynamic implications p to s converted waves at the continental moho together with waves multiply reflected between the earth s surface and the moho have been used to estimate the moho depth and average crustal vp vs variations in the central andes our analysis confirms and significantly complements the moho depth estimates previously obtained from wide angle seismic studies and receiver functions the resulting crustal thickness varies from about 35 km in the forearc region to more than 70 km beneath the plateau and thins 30 km further to the east in the chaco plains beneath the andean plateau the moho is deeper in the north altiplano and shallower in the south puna where the plateau attains its maximum elevation a non linear relation exists between crustal thickness and elevation and bouguer gravity suggesting that the crust shallower than 50 55 km is predominately felsic in contrast to a predominately mafic crust below such a relation also implies a 100 km thick thermal lithosphere beneath the altiplano and with a lithospheric thinning of a few tens of kilometers beneath the puna absence of expected increase in lithospheric thickness in regions of almost doubled crust strongly suggests partial removal of the mantle lithosphere beneath the entire plateau in the subandean ranges at 19 20 s the relation between altitude and crustal thickness indicates a thick lithosphere up to 130 150 km and lithospheric flexure beneath a relative topographic low at the salar de atacama a thick crust 67 km suggests that the lithosphere in this region is abnormally cold and dynamically subsided possibly due to coupling with the subducting plate this may be related to the strongest ms 8 0 known intra slab earthquake in the central andes that happened very close to this region in 1950 the average crustal vp vs ratio is about 1 77 for the altiplano puna and it reaches the highest values 1 80 1 85 beneath the volcanic arc indicating high ambient crustal temperatures and wide spread intra crustal melting 2002 elsevier science b v all rights reserved

a density model of the andean subduction zone density changes in basement rock or the deeper crust tend to cause gravity anomalies that dominate the observed data now that oil companies have begun using airborne gravity in their exploration of remote regions of the andes interpreters are building detailed earth models which necessarily involve geologically constrained gravity regional fields to account for the overwhelming signal from deep crustal structure

lithospheric scale structure across the bolivian andes from tomographic images of velocity and attenuation for p and s waves we have developed a three dimensional lithospheric scale model across the bolivian andes at 20 s based on tomographic images of velocity and attenuation for both p and s waves observations of travel time and attenuation for this study are from regional mantle earthquakes in the subducted nazca plate recorded on a portable broadband seismic array broadband andean joint experiment and seismic exploration of the deep andes in bolivia and chile the shallow mantle under the altiplano from 18 s to 21 s is high velocity and moderately high q vp 8 3 vs 4 7 qp 500 and qs 200 suggesting lithospheric mantle high velocity material in the altiplano extends to a depth of 125 150 km the shallow mantle of the western cordillera is characterized by high vp vs 1 83 suggesting a correlation between vp vs and arc volcanism seismic velocity in the western cordillera mantle is on average only slightly reduced from global averages however velocity and attenuation anomalies are locally strong vp 7 8 vs 4 3 qp 200 and qs 100 consistent with partial melt conditions under the los frailes volcanic field in the eastern cordillera shallow mantle velocity and q decrease drastically from the neighboring altiplano vp 7 8 vs 4 3 qp 300 qs 100 however high vp vs is not as pervasive as it is in the western cordillera we believe that slab derived water and perhaps other volatiles strongly influence the western cordillera while the eastern cordillera low velocity region is more affected by partial melt and or compositional changes average velocity and q in the shallow mantle across the bolivian andes where the tomographic images are best resolved are significantly higher than in most mantle wedge environments where corresponding images are available this is likely the result of a compressional back arc setting in the andes this implies that lithospheric shortening and thickening associated with the formation of the andes has profoundly influenced the shallow mantle structure across the range shallow mantle structure is locally influenced by the subduction processes particularly under the western cordillera however the differing volcanism and seismic character under the two cordilleras suggest that the volcanic process in the eastern cordillera may be distinct from arc volcanism tertiary volcanism in the eastern cordillera is located in the region where mantle shortening is suspected to be greatest both the timing and location of volcanism are consistent with upward migration of mantle wedge asthenosphere following the removal of over thickened lithosphere

thermal effects of metamorphic fluids in subduction zones in subduction zones large volumes of fluid are released from the subducting slab during prograde metamorphism of hydrated oceanic crust once released metamorphic fluids tend to migrate upward owing to buoyancy fluid migration can affect the thermal structure of a subduction zone by 1 advecting heat and 2 causing retrograde hydration carbonation reactions in the hanging wall under most p t conditions the volumetric heat capacities of h 2 o and rocks are similar therefore the thermal effects of fluid migration and uplift are comparable calculations suggest that advective heat transport by fluid flow causes relatively minor thermal effects pervasive upward fluid flow results in thermal effects analogous to regional uplift rates on the order of 0 2 mm yr fluid flow channelized along the subduction shear zone translates the local thermal structure of the shear zone updip at 1 mm yr a more important thermal effect results from exothermic retrograde reactions in the hanging wall that release 30 kj mol of h 2 o consumed retrograde reactions significantly retard cooling of the hanging wall just as prograde reactions retard warming of the subducting slab

upper mantle s velocity structure of central and western south america we have used broadband seismograms recorded at portable and permanent seismic stations in south america to image the three dimensional s velocity structure beneath central and western south america we have interactively fitted the waveforms of s and surface wave trains in 500 seismograms from 90 earthquakes and 60 different stations the constraints on upper mantle s velocity structure provided by the waveform fits have been combined with independent estimates of depth to the mohorovicic discontinuity yielding model sa99 the s velocity structure of sa99 reveals a highly heterogeneous upper mantle with a maximum peak to peak difference of 1 km s above 100 km the heterogeneity seems to be correlated with tectonic and geologic domains sa99 shows high lithospheric s velocities for the precambrian cratons the amazon basin is also underlain by high s velocities while three other intraplatform basins are underlain by low s velocities at various depths sa99 shows extremely low velocities in the mantle wedge beneath the volcanically active andes we explain these low s velocities as originating from realistic amounts of volatiles released by the subducting nazca crust and resulting amounts of partial melt within the peridotitic mantle wedge without requiring elevated temperatures while sa99 could be consistent with diffuse influx for corner flow in the mantle wedge it does not support models that require influx of subcontinental upper mantle into the mantle wedge through a laterally uniform low viscosity channel

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

metamorphism isotopic ages and composition of lower crustal granulite xenoliths from the cretaceous salta rift argentina crustal xenoliths from basanitic dikes and necks that intruded into continental sediments of the cretaceous salta rift at quebrada de las conchas provincia salta argentina were investigated to get information about the age and the chemical composition of the lower crust most of the xenoliths have a granitoid composition with quartz plagioclase garnet rutile k feldspar as major minerals the exceedingly rare mafic xenoliths consist of plagioclase clinopyroxene garnet hornblende all xenoliths show a well equilibrated granoblastic fabric and the minerals are compositionally unzoned thermobarometric calculations indicate equilibration of the mafic xenoliths in the granulite facies at temperatures of ca 900 c and pressures of ca 10 kbar the sm nd mineral isochron ages are 95 1 10 4 ma 91 5 13 0 ma 89 0 4 2 ma granitoid xenoliths and 110 7 23 6 ma mafic xenolith these ages are in agreement with the age of basanitic volcanism ca 130 100 and 80 75 ma and are interpreted as minimum ages of metamorphism lower crustal temperature at the time given by the isochrons was above the closure temperature of the sm nd system 600 700 c the sm nd and rb sr isotopic signatures 147sm 144nd 0 1225 0 1608 143nd 144ndt0 0 512000 0 512324 87rb 86sr 0 099 0 172 87sr 86srt0 0 708188 0 7143161 and common lead isotopic signatures 206pb 204pb 18 43 18 48 207pb 204pb 15 62 15 70 208pb 204pb 38 22 38 97 of the granitoid xenoliths are indistinguishable from the isotopic composition of the early paleozoic metamorphic basement from nw argentina apart from the lower 208pb 204pb ratio of the basement the sm nd depleted mantle model ages of ca 1 8 ga from granitoid xenoliths and early paleozoic basement point to a similar proterozoic protolith time constraints the well equilibrated granulite fabric p t conditions and lack of chemical zoning of minerals point to a high temperature in a crust of nearly normal thickness at ca 90 ma and to a prominent thermal anomaly in the lithosphere the composition of the xenoliths is similar to the composition of the early paleozoic basement in the andes of nw argentina and northern chile a thick mafic lower crust seems unlikely considering low abundance of mafic xenoliths and the predominance of granitoid xenoliths

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

light dark cycles in opal rich sediments near the plio pleistocene boundary dsdp site 532 walvis ridge continental terrace diester haass l meyers p a and rothe p 1986 light dark cycles in opal rich sediments near the plio pleistocene boundary dsdp site 532 walvis ridge continental terrace mar geol 73 1 23 samples taken at 10 cm intervals from dsdp core 532b 17 contain variations in carbonate opal organic carbon and terrigenous components that correlate with light dark cycles in sediment color the core site at 1300 m water depth is well above the ccd yet the color variations appear to result largely from cyclical fluctuations in carbonate dissolution which was greater during glacial periods higher concentrations of organic carbon and of terrigenons sediment components correlate with enhanced carbonate dissolution but opal concentrations inversely correlate and suggest that biological productivity at this site diminished during glacial periods a complicated glacial interglacial picture emerges from the data in interglacial times upwelling associated with the benguela current produced abundant opaline material organic matter was fairly well preserved and carbonate was only moderately dissolved in glacial times the upwelling core shifted as sea level fell and winds intensified productivity in the waters over site 532 decreased but lateral supply of oxidizable organic matter enhanced carbonate dissolution giving rise to light dark cycles in these sediments

numerical models of crustal scale convection and partial melting beneath the altiplano puna plateau we employ 2 d thermo mechanical modelling to study possible mechanisms for generating large scale crustal magmas in the altiplano puna region of the central andes the peak of ignimbrite activity in the late miocene and pliocene is associated in space and time with tectonic shortening and plateau uplift a seismic low velocity zone and other geophysical observations indicate that partial melting in the mid crust is still present under the ignimbrite province today we show that neither radiogenic heat production in a thickening crust nor shear heating due to tectonic shortening nor heat brought by intrusions of arc magmas into the mid crust can heat the mid crust to the degree and within the time frame suggested by the geologic petrologic and geophysical observations a viable mechanism to achieve high temperatures within the time constraints is convective heat and mass transfer by partially molten lower crust which itself was heated by enhanced mantle heat flow possibly associated with delamination of the mantle lithosphere during tectonic shortening and intensified magmatic arc activity the thermo mechanical models explain the mid crustal low velocity zone and the high and strongly variable surface heat flow observed in the altiplano convection by bulk flow of the crust appears when the middle and lower crust are mechanically weak quartz dominated rheology the basal heat flow from the mantle is high s 60 mw m 2 and tectonic shortening is active we argue that these conditions are satisfied in the central andes orogen 6 2002 elsevier science b v all rights reserved

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

automatic identification of rockfalls and volcano tectonic earthquakes at the piton de la fournaise volcano using a random forest algorithm abstract monitoring the endogenous seismicity of volcanoes helps to forecast eruptions and prevent their related risks and also provides critical information on the eruptive processes due the high number of events recorded during pre eruptive periods by the seismic monitoring networks cataloging each event can be complex and time consuming if done by human operators automatic seismic signal processing methods are thus essential to build consistent catalogs based on objective criteria we evaluated the performance of the random forests rf machine learning algorithm for classifying seismic signals recorded at the piton de la fournaise volcano la reunion island france we focused on the discrimination of the dominant event types rockfalls and volcano tectonic earthquakes using over 19 000 events covering two time periods 2009 2011 and 2014 2015 we parametrized the seismic signals using 60 attributes that were then given to rf algorithm when the rf classifier was given enough training samples its sensitivity rate of good identification exceeded 99 and its performance remained high above 90 even with few training samples the sensitivity collapsed when using an rf classifier trained with data from 2009 to 2011 to classify data from 2014 to 2015 catalog because the physical characteristics of the rockfalls and hence their seismic signals had evolved between the two time periods the main attribute families waveform spectrum spectrogram or polarization were all found to be useful for event discrimination our work validates the performance of the rf algorithm and suggests it could be implemented at other volcanic observatories to perform automatic near real time classification of seismic events

repeated crustal thickening and recycling during the andean orogeny in north chile 21 26 s posttectonic granitoids 2 lower crustal p wave velocities of 7 3 7 7 km s 1 compatible with underplated mafic crust and 3 results from recent experimental petrologic work showing garnet stability in mafic mineralogies 12 kbar 40 km crustal thickness analogous to older andean magmatic episodes in north chile newly underplated basaltic crust may account for the remaining deficit in neogene crustal thickening similar evolutionary patterns in geochemistry and initial sr and nd isotopic characteristics of andean 200 ma to present magmatic rocks suggest that the andean orogeny in this region evolved by a combination of processes of repeated arc migration tectonic and magmatic crustal thickening and igneous recycling which was controlled by periodically changing plate convergence rates and obliquity and corresponding changes in the rheologic behavior of the continental crust index terms 1020 geochemistry composition of the crust 1030 geochemistry geochemical cycles 0330 8159 evolution of the earth rheology crust and lithosphere 9360 information related to geographic region south america keywords andean orogeny continental crust recycling adakite ttg crustal thickening

high resolution image of the oceanic moho in the subducting nazca plate from p s converted waves short period p s converted waves outline the oceanic moho of the descending nazca plate in northern chile at 24 s comparison with hypocentral locations obtained with temporary local seismic networks show that earthquakes inside the subducting oceanic lithosphere of the nazca plate are located predominantly in the oceanic crust at depths between 70 and 120 km and probably in the upper mantle at depths below 120 km the results suggest that dehydration embrittlement as mechanism for intermediate depth earthquakes is active both in the former oceanic crust and upper mantle of subducting lithosphere

the precordilleran fault system of chuquicamata northern chile evidence for reversals along arc parallel strike slip faults abstract the chilean precordillera situated between the longitudinal valley and the western cordillera of northern chile was the site of the andean magmatic arc from the late cretaceous to the eocene oligocene boundary magmatism came to an end during the incaic tectonic phase 38 ma which caused arc normal shortening and the development of longitudinal dextral strike slip faults precordilleran fault system this magmatic arc tectonism is also related to the formation of the chuqicamata porphyry copper ore deposit as well as of other important deposits of this type in the precordillera structural investigations in and around the chuqicamata open pit mine have shown that wrench tectonics determined the kinematics of the area the n s striking west fissure which separates a 35 ma old non mineralised pluton to the west from a central late paleozoic basement ridge containing the mineralization became a sinistral fault along with other parallel faults in the area the central part is separated from similar paleozoic rocks to the east by the messabi este fault and a narrow faulted and sheared syncline of mesozoic cenozoic sediments this fault bears structures indicating dextral movements which probably are of an age that is similar to the mylonites 34 8 ma in the western pluton the dextral movements preceded the sinistral shear thus the fault system of chuqicamata displays a reversal of arc parallel shear movements according to the orientation of quartz veins in the mineralized body it is presumed that the sense of displacement of these strike slip motions reversed when mineralization started at about 32 ma during this time the stress field must have changed fundamentally the incaic phase dextral transpression is supposed to have been induced by the oblique vector of plate motion the following sinistral transtension corresponds to a time of reduced convergence rate and possibly reduced plate coupling as however the vector of plate motion remained unchanged during that time oblique subduction cannot be used as an argument for arc parallel sinistral shear movements

consistency of geologic and geodetic displacements during andean orogenesis displacement and convergence rates inferred from gps and marine magnetic data suggest that this trend may be continuing at present hence an increasing fraction of convergence is being absorbed by mountain building this change may reflect increased plate coupling due to decreased sediment supply younger subducting lithosphere or increased normal stress at the interface from the effects of uplift index terms 8102 tectonophysics continental contract orogenic belts 8157 evolution of the earth plate motions past 3040 8158 evolution of the earth plate motions present and recent 3040 8099 structural geology general or miscellaneous

indicators of waterdepth in bottom sediments of the continental margin off west africa abstract the coarse fractions of 56 bottom surface samples from 5 east west profiles off west africa 13 19 n from waterdepths of 30 3000 m were analysed to find lithologic and biologic parameters dependent only upon waterdepth rather than upon facies or latitude the following parameters show depth dependent variations sand fraction and quartz 125 m decrease with depth relict material and ratios of benthonic planktonic foraminifers decrease with depth fragmentation of planktonic foraminifers is minimum between 100 and 300 m ratios of planktonic benthonic molluscs is maximum between 100 and 300 m ratio of radiolaria planktonic foraminifers is zero between 30 and 600 m and increases below 600 1000 m echinoderms are minimum between 600 and 1000 m total loss of bivalves and gastropods below 600 1000 m ratios of imperforate perforate benthonic foraminifers shows a regular decrease to zero from 30 to between 600 and 1000 m and subsequent increase at greater depths

late quaternary sedimentation on the eastern walvis ridge se atlantic hpc 532 and four piston cores five sediment cores from the eastern walvis ridge have been investigated by means of a quantitative coarse fraction analysis 40 m numerous biogenous terrigenous and authigenic particles have been distinguished the variations in composition suggest the following conclusions core walda ks 12 from the northern flank of the walvis ridge consists entirely of a slump which contains mainly glacconite sands and biogenous opal foraminiferal skeletons and terrigenous matter the sediments in the other four cores comprise the late quaternary oxygen isotope stages 1 6 there are several levels at the transition from warm to cold intervals where benthos plankton foraminiferal ratios increase from 10 up to 50 90 here imperforate benthonic foraminifera decrease strongly dissolution of planktonic foraminifera can only partly explain this finding lateral supply of benthos or the influence of reduced oxygen content in the bottom water might have played an additional role calcium carbonate dissolution as revealed by the fragmentation of planktonic foraminifera shows intermediate values in coldest and warmest periods dissolution maxima occur at warm cold stage boundaries dissolution minima have been found at the transition from cold to warm periods upwellinginfluence is reflected in varying amounts of biogenous opal in interglacial periods the influence of upwelling is strong in all cores with an increase in intensity to the east core v19 259 it is explained by eddies of upwelled water which drifted westward under the influence of the westward turning benguela current in glacial periods the influence of upwelling at the core sites was very reduced the benguela current problably was close to the continent and turned westward only at 17 s thus it did not affect the core sites climate in the near coastal area has been arid in the latitude of 20 s throughout the investigated time slice accumulation rates reflect the position of the cores hpc 532 and circe gb 211 are in the area of the sediment filled trough which forms the walvis ridge terrace here accumulation rates are 2 to 3 times higher in stages 2 4 and 5 than in v19 259 and walda ks 09 which are from the steeper flank of the terrace outside the trough mean values of calcium carbonate dissolution are similar in all cores lower caco3 content in v19 259 and lower sand fraction percentages are explained by lateral supply of fine grained terrigenous matter in the area of core v19 259 which probably received insoluble material from the kuene shelf area by the n s flowing countercurrent

three dimensional models of p wave velocity and p to s velocity ratio in the southern central andes by simultaneous inversion of local earthquake data the pisco 94 proyecto de investigation sismologica de la cordillera occidental 1994 seismological network of 31 digital broad band and short period three component seismometers was deployed in northern chile between the coastal cordillera and the western cordillera more than 5300 local seismic events were observed in a 100 day period a subset of high quality p and s arrival time data was used to invert simultaneously for hypocenters and velocity structure additional data from two other networks in the region could be included the velocity models show a number of prominent anomalies outlining an extremely thickened crust about 70 km beneath the forearc region an anomalous crustal structure beneath the recent magmatic arc western cordillera characterized by very low velocities and a high velocity slab a region of an increased vp vs ratio has been found directly above the wadati benioff zone which might be caused by hydration processes a zone of lower than average velocities and a high vp vs ratio might correspond to the asthenospheric wedge the upper edge of the wadati benioff zone is sharply defined by intermediate depth hypocenters while evidence for a double seismic zone can hardly be seen crustal events between the precordillera and the western cordillera have been observed for the first time and are mainly located in the vicinity of the salar de atacama down to depths of about 40 km

seismic imaging of a convergent continental margin and plateau in the central andes andean continental research project 1996 ancorp 96 1 a 400 km long seismic reflection profile andean continental research project 1996 ancorp 96 and integrated geophysical experiments wide angle seismology passive seismology gravity and magnetotelluric depth sounding across the central andes 21 s observed subduction of the nazca plate under the south american continent an east dipping reflector nazca reflector is linked to the down going oceanic crust and shows increasing downdip intensity before gradual breakdown below 80 km we interpret parts of the nazca reflector as a fluid trap located at the front of recent hydration and shearing of the mantle the fluids being supplied by dehydration of the oceanic plate patches of bright quebrada blanca bright spot to more diffuse reflectivity underlie the plateau domain at 15 30 km depth this reflectivity is associated with a low velocity zone p to s wave conversions the upper limits of high conductivity and high vp vs ratios and to the occurrence of neogene volcanic rocks at surface we interpret this feature as evidence of widespread partial melting of the plateau crust causing decoupling of the upper and lower crust during neogene shortening and plateau growth the imaging properties of the continental moho beneath the andes indicate a broad transitional character of the crust mantle boundary owing to active processes like hydration of mantle rocks in the cooler parts of the plate margin system magmatic underplating and intraplating under and into the lowermost crust mechanical instability at moho etc hence all first order features appear to be related to fluid assisted processes in a subduction setting

crustal structure of the altiplano from broadband regional waveform modeling implications for the composition of thick continental crust we have modeled the full waveforms from six intermediate depth and two shallow earthquakes recorded at regional distances by the banjo broadband andean joint experiment banjo and seismic exploration of the deep altiplano seda portable seismic networks in the central andes in this study we utilize data from those banjo and seda stations located within the altiplano and eastern cordillera we used reflectivity synthetic seismograms and a grid search to constrain four parameters of the altiplano eastern cordillera lithosphere crustal thickness average crustal velocity vp and crustal and upper mantle poisson s ratios crust and mantle using our grid search we investigated the crustal and upper mantle structure along 36 individual event station paths and applied forward modeling to 56 event station paths robust models for the altiplano that provide the best overall fit between the data and synthetic seismograms are characterized by an average vp of 5 75 6 25 km s crustal thicknesses of 60 65 km crust 0 25 and mantle 0 27 0 29 we find a north south variation in the structure of the altiplano with the crust south of the banjo transect characterized by either lower than average crustal p wave velocities or a slightly higher crust relative to crust north of the banjo transect these results are consistent with a model of crustal thickening caused predominantly by tectonic shortening of felsic crust rather than by underplating or magmatic intrusion from the mantle

the bolivian altiplano conductivity anomaly 1 a long period magnetotelluric study was carried out in the central andes between latitudes 19 5 s and 21 s along two almost parallel profiles of 220 and 380 km length respectively the investigation area extends from the pacific coast to the southern altiplano plateau in the back arc of the south american subduction zone the main geoelectrical structure resolved is a broad and probably deep reaching highly conductive zone in the middle and deeper crust beneath the high plateau although the data show deviations from two dimensionality a two dimensional approach is justified for large parts of the profiles sensitivity studies were carried out in order to constrain the depth extent another electrically conductive structure was resolved in the middle crust of the chilean forearc thought to be connected with the precordillera fault system the andean continental research program ancorp seismic reflection profile carried out along the same line at 21 s revealed highly reflective zones below the altiplano in good correlation with the upper boundary of the altiplano conductor this highly conductive domain also coincides with low seismic velocities and a zone of an elevated vp vs ratio and although not well resolved with low qp seismic quality factors taking into account the enhanced heat flow and a derived temperature model the most probable explanation lies in the assumption of granitic partial melts the good conductor below the volcanic arc which was found in regions farther south at 22 s gradually vanishes toward the north this is consistent with the results of seismic tomography concerning qp values and a gap of recent volcanism

broad depth range seismic imaging of the subducted nazca slab north chile abstract in this paper we present a compilation of modern seismic and seismological methods applied to image the subduction process in north chile south america we use data from active and passive seismic experiments that were acquired within the framework of the german collaborative research center sfb267 deformation processes in the andes the investigation area is located between 20 and 25 s and extends from the trench down to 100 km depth in the depth range between the sea bottom and 15 km we process an offshore seismic reflection profile using a recently developed velocity model independent stacking procedure we find that the upper part of the subducting oceanic lithosphere in this depth range is characterized by a horst and graben structure this structure supports an approximately 3 km thick coupling zone between the plates in the depth range between 15 and 45 km we analyse the spatial distribution of aftershocks of the antofagasta earthquake 1995 the aftershock hypocenters are concentrated in an approximately 3 km thick layer finally in the depth range between 45 and 100 km we apply kirchhoff prestack depth migration to the onshore ancorp profile a double reflection zone is observed between 45 and 60 km depth which may represent the upper and lower boundary of the subducted oceanic crust over the whole range down to more than 80 90 km depth we obtain an image of the subducting slab at that depth the hypocenters of local earthquakes deviate significantly in the direction perpendicular to the slab face from the reflective parts of the slab consequently our results yield a complete seismic image of the downgoing plate and the associated seismic coupling zone

origin of the high plateau in the central andes bolivia south america the bolivian altiplano in the central andes of south america is part of the second largest high plateau on earth it is an elongate region of subdued relief 1 2 105 km2 and 4 km above sea level bounded by the eastern cordillera and volcanic arc western cordillera here the crust is up to 75 km thick we describe the cenozoic geological evolution of this region using a revised chronostratigraphy and an analysis of the crustal and lithospheric structure crustal shortening and magmatic addition and locally sedimentation are the main mechanisms of cenozoic crustal thickening leading to nearly 4 km of surface uplift since the paleocene addition of mafic melts appears to be a first order mechanism of cenozoic crustal growth contributing 40 of the crustal thickening beneath the volcanic arc removal of the basal part of the lithosphere may have caused two episodes of widespread arc and behind arc mafic volcanism at 23 ma and 0 5 ma contributing to the surface uplift the altiplano originated as a sedimentary basin several hundred kilometers wide between the proto western cordillera and a narrow zone of uplift proto eastern cordillera farther east the latter zone formed by inversion of the center of a wide lacustrine or marine cretaceous paleocene basin close to sea level at 45 ma a thickness of 2 4 km of paleogene continental elastics accumulated in the proto altiplano basin subsequently in the oligocene we estimate that this region and the western margin of the eastern cordillera were technically shortened 22 65 km resulting in 9 km of average crustal thickening the altiplano basin was rejuvenated at 25 ma and subsequently flooded with up to 8 km thickness of detritus eroded from the uplifting eastern and western cordilleras between 25 and 5 ma folding and thrusting in the western margin of the eastern cordillera migrated westward into the center of the altiplano basin essentially terminating deposition except in local subbasins and accommodating 13 30 km of shortening and an estimated 7 km of average crustal thickening subsequently there has been strike slip deformation and limited local thrusting 5 km of shortening geomorphological and geochronological evidence for 1 5 2 km of surface uplift of this region since the late miocene suggests 14 km of lower crustal thickening beneath an essentially rigid lid and can be explained by 100 150 km of underthrusting of the brazilian shield and adjacent regions beneath the eastern margin of the central andes the present subdued relief in the altiplano may be a result of ductile flow in the lower crust and sedimentation and erosion in an internal drainage basin

seismicity and average velocities beneath the argentine puna plateau a network of 60 seismographs was deployed across the andes at 23 5 s the array was centered in the backarc atop the puna high plateau in nw argentina p and s arrival times of 426 intermediate depth earthquakes were inverted for 1 d velocity structure and hypocentral coordinates average velocities and v p v s in the crust are low average mantle velocities are high but difficult to interpret because of the presence of a fast velocity slab at depth although the hypocenters sharply define a 35 dipping benioff zone seismicity in the slab is not continuous the spatial clustering of earthquakes is thought to reflect inherited heterogeneties of the subducted oceanic lithosphere additionally 57 crustal earthquakes were located seismicity concentrates in the fold and thrust belt of the foreland and eastern cordillera and along and south of the el toro olacapato calama lineament tocl focal mechanisms of two earthquakes at this structure exhibit left lateral strike slip mechanisms similar to the suggested kinematics of the tocl we believe that the puna north of the tocl behaves like a rigid block with little internal deformation whereas the area south of the tocl is weaker and currently deforming

attenuation tomography in the western central andes a detailed insight into the structure of a magmatic arc high quality data from 1498 local earthquakes recorded by the pisco 94 proyecto de investigation sismologica de la cordillera occidental 1994 and ancorp 96 andean continental research project 1996 temporary seismological networks allowed the detailed determination of the three dimensional 3 d attenuation structure qp 1 beneath the recent magmatic arc in the western central andes 20 to 24 s assuming a frequency independent qp 1 in a frequency band between 1 and 30 hz whole path attenuation t was estimated from the amplitude spectra of the p waves using spectral ratios and a spectral inversion technique the damped least squares inversion tomography of the data reveals a complex attenuation structure crust and mantle of the forearc and subducting slab are generally characterized by low attenuation qp 1000 crust and mantle beneath the magmatic arc show elevated attenuation the strongest anomaly of extremely low qp is found in the crust between 22 and 23 s beneath the recent volcanic arc qp 100 n s variations can be observed the western flank of the crustal attenuation anomaly follows the curved course of the volcanic front north of 21 s the attenuation is less developed in the northern part of the study area the low qp zone penetrates in the forearc mantle down to the subducting slab in the south a deeper zone of high attenuation is resolved between 23 and 24 s directly above the subducting slab low qp in the mantle correlates with earthquake clusters the strong crustal attenuation is confined to the distribution of young ignimbrites and silicic volcanism and is interpreted as a thermally weakened zone with partial melts the attenuation pattern in the upper mantle might reflect the variable extent of the asthenosphere and maps variations of subduction related hydration processes in the mantle wedge from slab derived fluids

crustal thickness variations in the central andes we estimated the crustal thickness along an east west transect across the andes at lat 20 s and along a north south transect along the eastern edge of the altiplano from data recorded on two arrays of portable broadband seismic stations banjo and seda waveforms of deep regional events in the downgoing nazca slab and teleseismic earthquakes were processed to isolate the p to s converted phases from the moho in order to compute the crustal thickness we found crustal thickness variations of nearly 40 km across the andes maximum crustal thicknesses of 70 74 km under the western cordillera and the eastern cordillera thin to 32 38 km 200 km east of the andes in the chaco plain the central altiplano at 20 s has crustal thicknesses of 60 to 65 km the crust also appears to thicken from north 16 s 55 60 km to south 20 s 70 74 km along the eastern cordillera the subandean zone crust has intermediate thicknesses of 43 to 47 km crustal thickness predictions for the andes based on airy type isostatic behavior show remarkable overall correlation with observed crustal thickness in the regions of high elevation in contrast at the boundary between the eastern cordillera and the subandean zone and in the chaco plain the crust is thinner than predicted suggesting that the crust in these regions is supported in part by the flexural rigidity of a strong lithosphere with additional constraints we conclude that the observation of airy type isostasy is consistent with thickening associated with compressional shortening of a weak lithosphere squeezed between the stronger lithosphere of the subducting nazca plate and the cratonic lithosphere of the brazilian craton

measuring the seismic properties of tibetan bright spots evidence for free aqueous fluids in the tibetan middle crust seismic bright spots are commonly interpreted to mark fluid concentrations but their nature melt or aqueous is usually inferred only from circumstantial evidence of the geologic setting a band of bright spot reflections has been imaged by project indepth international deep profiling of tibet and the himalayas at about 15 km depth along 150 km of the northern yadong gulu rift southern tibet we use indepth three component wide angle seismic data to measure seismic velocities at the bright spot reflector and theoretical rock physics bounds to constrain the nature of the fluids merging of data from multiple bright spots allows us to use a one dimensional approximation travel time modeling yields average p and s velocities for the upper crust above the bright spots of 5 3 0 2 and 3 2 0 2 km s 1 respectively reflection amplitude variation with offset avo modeling constrains the p and s velocities of the bright spots to 3 0 0 8 and 1 6 0 8 km s 1 respectively multiple modeling procedures suggest these velocities are not model dependent our results imply that of the order of 10 volume of free aqueous fluids in the tibetan middle crust produces the observed bright spot reflections the presence of relatively large quantities of free aqueous fluids presumably mostly saline supercritical h2o does not preclude the presence of melt but does constrain the maximum temperature at the bright spots to the wet granite solidus about 650 c and thus the maximum surface heat flow to 110 mw m 2 the observed bright spots can alternatively be explained as a result of transient flow of aqueous fluids through a lower temperature and lower heat flow southern tibetan crust

pure and simple shear plateau uplift altiplano puna argentina and bolivia abstract the concepts of pure and simple shear strain provide a meaningful way of discussing the alternative modes of lithospheric shortening which produced uplift of the central andean plateau since 10 ma discrete segments of the plateau exemplify pure or simple shear modes characteristics of the simple shear mode are a flanking thin skinned thrust belt and subdued relief and lack of significant late miocene to recent deformation within the plateau the pure shear mode is characterized by more rugged internal relief and active neotectonic deformation within the plateau and thick skinned foreland deformation involving seismic deformation of the crust to 30 km or more these modes of crustal thickening correlate with changes in lithosphere thickness modes of isostatic compensation broad wavelength topography and back arc magmatism the correlation of the boundary between pure and simple shear segments with major changes in pre existing anisotropies in the foreland of the orogen suggests that events which long pre date the andean orogeny indirectly controlled the along strike variation in the magnitude and style of shortening in the central andes and may have influenced the development of flat subduction

early diagenesis of marine organic matter alteration of the carbon isotopic composition in two distal carbonate turbidites labelled a and e from the madeira abyssal plain oxidation of organic matter om has been accompanied by a reduction in hydrogen index hi and change in the isotopic composition of carbon since the emplacement of turbidite 0 5 ka ago 35 of the om in the upper part has been oxidised by oxygen and nitrate diffusing from sea water into the sediment there is an accompanying decrease of 0 3 in 13c om and a decrease of 25 in hi turbidite e was emplaced about 73 ka and was exposed to bottom water for 12 3 ka before being buried beneath later turbidites during exposure oxygen and nitrate diffused into its upper part and oxidised 50 of the toc without an accompanying change in isotopic composition further oxidation of the more refactory organic components decreased toc by a further 25 and decreased 13c by 2 5 oxidation also reduced hi by 50 from originally low values unless corrected for the isotopic changes accompanying extended oxic and suboxic degradation of organic matter in sediments will give rise to erroneous estimates of marine terrestrial om ratios

neogene shortening contribution to crustal thickening in the back arc of the central andes to illustrate the neogene shortening distribution in the back arc units of the central andes and to estimate the contribution of the shortening to crustal thickening two balanced crustal cross sections have been constructed across the northern and southern branches of the bolivian orocline total neogene shortening which varies from 191 to 231 km is accommodated by a crustal duplex below the cordillera oriental but is insufficient to produce the 70 km of crustal thickness evidenced by geophysical data below the altiplano the best explanation for this anomalous thickening seems to be crustal underplating by material tectonically eroded from the continental margin this process probably caused the altiplano uplift the subduction of oceanic lithosphere coupled with this underplating and a brief episode of gravity spreading of the altiplano constituted the driving forces that produced neogene shortening and development of the central andes