numerical comparison of different convergent plate contacts subduction channel and subduction fault summary at convergent plate boundaries the properties of the actual plate contact are important for the overall dynamics convergent plate boundaries both mechanically decouple and link tectonic plates and accommodate large amounts of strain we investigate two fundamental physical states of the subduction contact one based on a fault and the other based on a subduction channel using a finite element method we determine the specific signatures of both states of the subduction contact we pay particular attention to the overriding plate in a tectonic setting of converging plates where the subducting plate is freely moving the subduction channel reduces compression relative to the fault model in a land locked basin setting where the relative motion between the far field of the plates is zero the subduction channel model produces tensile stress regime in the overriding plate even though the amount of slab roll back is small the fault model shows a stronger development of slab roll back and a compressive stress regime in the upper plate based on a consistent comparison of fault and channel numerical models we find that the nature of the plate contact is one of the controlling factors in developing or not of backarc extension we conclude that the type of plate contact plays a decisive role in controlling the backarc state of stress to obtain backarc extension roll back is required as an underling geodynamic process but it is not always a sufficient condition

a double layered seismic zone in arica northern chile a double layered seismic zone is determined in arica northern chile using locally recorded events at depths 100 km two planes of seismicity can be observed one dipping at 30 e with 10 km of thickness and a second parallel plane 20 25 km deeper with the same average thickness fault plane solutions for both layers show a wide variability even between nearby events the genesis of the arica double seismic zone seems to be independent of the age the relative convergence rate and direction of the nazca plate because all of these parameters are almost the same along the whole northern chile and a clear separation of the seismicity into two layers is only observed around the arica elbow moreover it cannot be responsible of the double layered seismic zone because no similar significant changes are observed in the other well studied double seismic zones

an s receiver function analysis of the lithospheric structure in south america 1 we make use of s receiver function to investigate the structure and thickness of the crust and mantle lithosphere in the south american continent and adjacent areas the moho discontinuity has been detected at all stations and goes from 18 km beneath the coast of the continent to 80 km in the andean region the depth of the lithosphere asthenosphere boundary can be clearly detected below those stations that are located on stable areas from 50 km to 160 km the identification of this phase becomes more difficult when the stations are located near subduction zones we also observed the base of the subducted oceanic lithosphere down to depths of 220 km

asperities and barriers on the seismogenic zone in north chile state of the art after the 2007 mw 7 7 tocopilla earthquake inferred by gps and insar data the mw 7 7 2007 november 14 earthquake had an epicentre located close to the city of tocopilla at the southern end of a known seismic gap in north chile through modelling of global positioning system gps and radar interferometry insar data we show that this event ruptured the deeper part of the seismogenic interface 30 50 km and did not reach the surface the earthquake initiated at the hypocentre and was arrested 150 km south beneath the mejillones peninsula an area already identified as an important structural barrier between two segments of the peru chile subduction zone our preferred models for the tocopilla main shock show slip concentrated in two main asperities consistent with previous inversions of seismological data slip appears to have propagated towards relatively shallow depths at its southern extremity under the mejillones peninsula our analysis of post seismic deformation suggests that small but still significant post seismic slip occurred within the first 10 d after the main shock and that it was mostly concentrated at the southern end of the rupture the post seismic deformation occurring in this period represents 12 19 per cent of the coseismic deformation of which 30 55 per cent has been released aseismically post seismic slip appears to concentrate within regions that exhibit low coseismic slip suggesting that the afterslip distribution during the first month of the post seismic interval complements the coseismic slip the 2007 tocopilla earthquake released only 2 5 per cent of the moment deficit accumulated on the interface during the past 130 yr and may be regarded as a possible precursor of a larger subduction earthquake rupturing partially or completely the 500 km long north chile seismic gap

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

shear stresses on megathrusts implications for mountain building behind subduction zones 1 shear stresses on a subduction megathrust play an important role in determining the forces available for mountain building adjacent to a subduction zone in this study the temperatures and shear stresses on megathrusts in 11 subduction zones around the pacific rim hikurangi tonga izu ogasawara western nankai northeastern japan aleutians western alaska cascadia northern chile southern chile and se asia northern sumatra have been determined the main constraint is that vertical normal stresses beneath the highlands behind the subduction zone are nearly equal to horizontal normal stresses in the plane of a trench or arc normal section for a typical brittle and ductile megathrust rheology frictional shear stress gz for depth z and ductile shear stress a exp b rt at temperature t where a b are rheological parameters treated as constants rheological constants common to all the megathrusts crust mantle b are determined by simultaneously solving for the force balance in the overlying wedge and megathrust thermal structure using a simplex minimization algorithm taking account of the induced mantle corner flow at depth 65 15 km 2 and constant radiogenic heating 0 65 0 3 w m 3 2 throughout the crust the a constants are solved individually for each subduction zone assuming that the maximum depth of interplate slip earthquakes marks the brittle ductile transition the best fit solution shows two groupings of megathrusts with most subduction zones having a low mean shear stress in the range 7 15 mpa crust 0 032 0 006 mantle 0 019 0 004 and unable to support elevations 2 5 km for a typical frictional sliding coefficient 0 5 the low effective coefficients of friction suggest high pore fluid pressures at 95 lithostatic pressure tonga and northern chile require higher shear stresses with crust 0 095 0 024 mantle 0 026 0 007 suggesting slightly lower pore fluid pressures at 81 lithostatic ductile shear in the crust is poorly resolved but in the mantle appears to show a strong power law dependency with b 36 18 kj mol 1 amantle values are sensitive to the precise value of b but are in the range 1 20 kpa the power law exponent n for mantle flow is poorly constrained but is likely to be large n 4 the brittle ductile transition in the crust occurs at temperatures in the range 370 c 512 c usually close to the base of the crust and in the mantle at much lower temperatures 180 c 300 c possibly reflecting a marked change in pore fluid pressure or quasi ductile and subfrictional properties in subduction zones where the subducted slab is older than 50 ma a significant proportion of the integrated shear force on the megathrust is taken up where it cuts the mantle and temperatures are 300 c in much younger subduction zones the stress transmission is confined mainly to the crust the shear stresses particularly in the crust may be kept low by some sort of lubricant such as abundant water rich trench fill which lowers the frictional sliding coefficient or effective viscosity and or raises pore fluid pressure the unusual high stress subduction zone in northern chile lacks significant trench fill and may be poorly lubricated with a mean shear stress 37 mpa required to support elevations 4 km in the high andes however where the crust is thin in sediment starved and poorly lubricated subduction zones such as tonga the mean shear stress will still be low sediment may lubricate megathrusts accommodating underthrusting of continental crust such as in the himalayas or eastern central andes which have a low mean shear stress 15 mpa

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

reflection mapping across the convergent margin of western canada summary five marine multichannel seismic reflection profiles totalling 520 km were recorded across the western canada convergent margin where the juan de fuca plate is subducting beneath north america the data extend the results of lithoprobe on vancouver island the primary objectives are definition of the offshore accretionary structures and clarification of the convergent interaction between the two plates the main features of this preliminary interpretation are 1 the subduction deformation front is complex with evidence of sediments being accreted and subducted 2 the top of the oceanic crust and the mono are imaged below the deep water sedimentary basin 3 the top of the subducting plate is clearly imaged below the shelf 4 beneath the inner shelf one band of high reflectivity underlain by a zone of lesser reflectivity lies above the plate 5 alternative interpretations place the present zone of decoupling at the base of the reflective band or the top of the plate 6 the san juan and leech river faults that bound small accreted terranes are imaged as thrusts that merge at depth

active deformation in the bolivian andes south america a method is described for determining the horizontal field of velocity and velocity gradients in the bolivian andes in the vicinity of the bolivian orocline using the following data 1 space geodetic measurements of crustal motions 2 the style distribution and rate of late miocene quaternary folding and faulting and 3 paleomagnetic measurements of rigid body rotations about a vertical axis these data were analyzed in a network of 26 triangles which spanned the bolivian andes by solving the velocity gradient compatibility equations between adjacent triangles subject to the input data constraints this yielded 120 140 linear equations to constrain 108 unknowns making the problem moderately overdetermined a series of experiments were carried out with different combinations of the input data to determine the best fit in the least squares sense field of quaternary horizontal velocities and velocity gradients the azimuths of velocity vectors in the high andes are subparallel to both the relative plate convergence vector and the symmetry axis of the bolivian orocline with magnitudes relative to stable south america in the range 7 17 mm yr 1 accommodating up to 30 of the relative plate convergence however the azimuths of velocities along the range front are nearly orthogonal to the range with a component of distributed range parallel dextral or sinistral shear within the eastern cordillera and altiplano the horizontal velocity field requires bending about a vertical axis of both limbs of the bolivian orocline the field of horizontal dilatation assuming homogeneous deformation of the entire crustal thickness requires vertical velocities of material points up to 2 5 mm yr in the sub andean zone decreasing to 0 1 mm yr farther east in the eastern cordillera and altiplano the direction of maximum gradients of buoyancy stress determined from the field of gradients of appropriate combinations of velocity gradients using a thin sheet model of viscous flow are broadly parallel to the direction of maximum topographic gradients in the eastern cordillera providing support for the idea that the bulk behaviour of the bolivian lithosphere is fluid like a comparison of these gradients with buoyancy stress gradients calculated from crustal thickness contrasts suggest that the bulk lithospheric viscosity is in the range 0 5 1 0 1022 pa s gradients of velocity gradients along the eastern margin of the bolivian andes can be interpreted in terms of marked gradients of viscosity with viscosity increasing toward the brazilan shield in the cratonic core of south america buoyancy stresses calculated from crustal thickness contrasts calibrated with the crustal thicknesses at the changeovers from thrust to strike slip or normal faulting suggest that the average lithosphericscale deviatoric stresses are in the range 7 15 mpa

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

sedimentology and paleontology of a carboniferous log jam a localized sandstone split in the mary lee coal early pennsylvanian langsettian of the black warrior basin alabama has a channel form geometry and preserves a concentrated log and gravel pebbles cobbles and boulders assemblage at the top of the fill sequence gravel lithotypes within and amongst rooting structures of lycopsid cordaitean and calamitean trees are indicative of an appalachian orogenic provenance and support an allochthonous origin for some of the logs in addition a skolithos ichnological assemblage within the channel is indicative of opportunist colonization during channel fill a low sinuosity geometry characterizes the overall channel form belt exhibiting a general northwesterly trend paleocurrent measurements from the cross bedded sandstone at the margin of the channel belt indicate flow was to the northwest log orientations at these sites are subperpendicular to perpendicular to the overall sediment transport direction and are interpreted to represent an ancient log jam the genesis of the fluvial channel the introduction and emplacement of the concentrated log assemblage the effects of this floating log accumulation on sedimentation within the channel and the relationship between paleocurrent and log orientation are discussed the red river located in the southern united states is used as a modern analog for comparative purposes

uplift of the central andean plateau and bending of the bolivian orocline the topography of the central andes can be considered the primary tectonic signal of late cenozoic mountain building in an arid region where the effects of uplift and magmatism are little obscured by denudation the spatial coverage of the topographic signal is more complete than that for sparsely sampled geological and geophysical data a color coded image of digitized topography between 12 s and 37 s highlights the altiplano puna one of the world s most remarkable plateaus and reveals important physiographic clues about the formation of that major feature the topographic data combined with information on structure magmatism seismicity and paleomagnetism support a simple kinematical model for the late cenozoic evolution of the central andes the model does not require collisional effects or enormous volumes of intrusive additions to the crust but instead calls upon plausible amounts of crustal shortening and lithospheric thinning the model interrelates andean uplift a changing geometry of the subducted nazca plate and a changing outline in map view of the leading edge of the south american plate crustal shortening has accommodated convergence between the chilean peruvian forearc and the south american foreland the altiplano puna plateau can be constructed by a combination of crustal shortening and thickening and lithospheric thinning above a shallow dipping 20 30 subducted plate the seawardly concave bend of western south america the bolivian orocline was enhanced but not completely produced by an along strike variation in the amount of late cenozoic shortening maximum shortening in bolivia both produced the widest part of the plateau and increased the seaward concavity of the bolivian orocline the along strike variations of shortening are hypothesized to result from corresponding along strike variations in the width of a weakened zone in the overriding plate weakening occurs above the wedge of asthenosphere located between the subducted and overriding plates hence the width of the zone of weakening depends upon the dip of the subducted plate two types of shortening are recognized 1 a widespread basin and range laramide like shortening that characterizes modem activity in the sierras pampeanas and late miocene deformation of the altiplano puna and 2 on the eastern side of the cordilleras and plateau an east verging foreland fold thrust belt in which the underthrust foreland compresses and thickens the ductile lower crust and produces a plateau uplift of the upper crust the second type of shortening can be applied to plio quaternary deformations throughout the central andes but with a substantial narrowing of the region of plateau uplift in peru and south of 28 s a proposed monoclinal flexure of the upper crust on the western side of the plateau uplift explains the remarkably simple and regular morphology of the main western slope of the central andes the monocline is located above the tip of the asthenospheric wedge between the converging plates it is postulated to occur above the western limit of lower crustal thickening in the regions of horizontal subduction the monocline can be associated with a late miocene asthenospheric wedge tip

structure of the crust and the lithosphere beneath the southern puna plateau from teleseismic receiver functions abstract we present a teleseismic p and s receiver function study using data from a temporary passive source seismic array in the southernmost puna plateau and adjacent regions the p receiver function images show the distribution of crustal thickness and v p v s ratio for this area over much of the southern puna plateau the crustal thickness is 50 55 km whereas to the west a thicker crust 60 to 75 km is observed beneath much of the andean volcanic arc region from the puna southward there is little obvious change in the crustal thickness across the border of the plateau south of 28 s the crust is seen to progressively thin towards the east in the pampean ranges where it is 35 40 km thick the southern puna plateau is characterized overall by a low crustal v p v s ratio less than 1 70 implying a felsic crustal composition an anomalously high v p v s ratio of 1 87 is observed beneath the cerro galan volcanic center in the region where a prominent crustal low velocity zone identified below 10 km depth probably extends into the lower crust the crustal thickness determined under the cerro galan area 59 km is close to that of the rest of the southern puna the prominent high v p v s ratio and low velocity zone beneath the galan region implies the presence of a zone of partial melt or a magma chamber is consistent with hypothesis calling for lithospheric delamination beneath the galan caldera a widespread crustal low velocity layer observed beneath much of the southern puna correlates well with crustal low velocity anomalies observed by teleseismic tomography the lithosphere asthenosphere boundary beneath the array can be clearly observed by both p and s receiver functions at depths of 70 90 km in agreement with previous studies suggesting a thin lithosphere beneath the high elevated plateau the mantle transition zone discontinuities appear at expected depths

seismic imaging of a megathrust splay fault in the north chilean subduction zone central andes abstract prominent trench parallel fault systems in the arc and fore arc of the chilean subduction zone can be traced for several thousand kilometers in north south direction these fault systems possibly crosscut the entire crust above the subduction megathrust and are expected to have a close relationship to transient processes of the subduction earthquake cycles with the motivation to image and characterize the structural inventory and the processes that occur in the vicinity of these large scale fault zones we re processed the ancorp 96 controlled source seismic data set to provide images of the faults at depth and to allow linking geological information at the surface to subsurface structures the correlation of the imaging results with observed hypocenter locations around these fault systems reveals the origin and the nature of the seismicity bound to these fault systems active and passive seismic data together yield a picture of a megathrust splay fault beneath the longitudinal valley at mid crustal level which can be observed from the top of the subduction plate interface and which seems to be connected to the precordilleran fault system pfs known at the surface this result supports a previously proposed tectonic model where a megathrust splay fault defines the western altiplano as a crustal scale fault bend fold furthermore we clearly imaged two branches of the uyuni kenayani fault ukf in a depth range between 0 and 20 km in summary imaging of these faults is important for a profound understanding of the tectonic evaluation and characterization of the subduction zone environment for which the results of this study provide a reliable basis

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

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

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

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

velocity structure in northern chile evidence of subducted oceanic crust in the nazca plate summary 2 d p wave velocity models were determined for the subduction zones near iquique and antofagasta in northern chile simultaneously inverting p and s wave arrival times from locally recorded earthquakes for velocity structure and hypocentral locations a 2 d parametrization was used because of the paucity of data but is justified by the lack of significant variations along the strike of the subduction zone observed from both refraction profiles and simple 3 d inversions the crust and upper mantle are parameterized by constant velocity regions of irregular shape with the size and boundaries of these regions governed by prior information about the structure and by the ability of the data to resolve p wave velocities beneath the antofagasta region there is evidencc of an approximately 10 km thick layer of oceanic crust attached to the top of the subducting nazca plate this crust has a p wave velocity of 7 3 f 0 1 km s and is observed down to a depth of 60 f 10 km this depth also corresponds to the maximum depth of seismogenic coupling in the chilean subduction zone the subducted oceanic crust overlies an oceanic upper mantle with a p wave velocity of 8 0 f 0 1 km s apparently oceanic crust is being subducted beneath iquique as well however this feature is less constrained by the data available from this region

seismic reflection imaging of the subducting juan de fuca plate mckenzie and parker s early suggestion1 that the juan de fuca plate is underthrusting north america has since been confirmed by numerous studies evidence for plate convergence along this junction includes a sediment filled trench2 7 intense compressional deformation of the continental slope and shelf sediments2 7 seafloor magnetic anomalies that extend beneath the continental slope2 4 inland andesitic volcanoes8 paired low high heat flow and gravity anomalies9 10 active onshore deformation11 12 onshore crustal earthquakes with compressional axes parallel to the direction of plate convergence13 sinistral strike slip earthquakes along the nootka fault fig 1 14 and a well defined benioff wadati zone6 15 16 here we report the results of a seismic reflection survey that has delineated two slabs of oceanic lithosphere underlying vancouver island one that is currently being subducted and one that is underplated these findings lead us to speculate that successive underplating of oceanic lithosphere may be an important process in the evolution and growth of continents

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

along strike segmentation of the andean foreland causes and consequences abstract thrust belts of tertiary age characterize the east flank of the andes and part of the adjacent foreland from venezuela to the southern tip of south america this foreland deformation comprises three basically different structural styles 1 thin skinned thrust belts detached within the sedimentary cover 2 thick skinned thrust belts with an inferred basal detachment in the basement at 10 20 km depth and 3 foreland basement thrusts which possibly affect the entire crust alternating foreland structural styles along the orogen often with sharp boundaries produce a segmentation of the andean foreland the changes in structural style are interpreted to be primarily controlled by inherited stratigraphic and structural features of the south american plate for instance thin skinned thrust belts appear to require a more or less conformable sedimentary cover at least some 3 km thick over a metamorphic crystalline basement unaffected by mesozoic extension in areas of mesozoic rifting normal faults have been reactivated during tertiary contraction resulting in the formation of thick skinned thrust belts the basement becomes involved even if potential decollement levels are present deep seated foreland basement thrusts tend to form over basement arches with a thin sedimentary cover the three basic structural styles are unequally efficient in terms of shortening thin skinned belts are typically shortened by 40 70 thick skinned belts by 20 35 and areas of foreland thrusts by less than 10 the structural segmentation of the foreland therefore implies strong along strike variations in the amount of shortening available estimates of shortening along the central andes actually suggest that shortening does not vary steadily but has several maxima and minima strongly and weakly shortened foreland segments correlate poorly with the segmentation of the subducted plate in steeper and subhorizontal segments flat slabs but closely with lithospheric structure the most strongly shortened segment coincides with thick mantle lithosphere suggesting that shortening of the edge of the south american plate has resulted in substantial underthrusting of lithospheric mantle beneath part of the andes this underthrusting of thick lithosphere may have caused steepening of a formerly flat slab in the arica bend region to the normal 25 30 dip presently observed

testing a cellular modelling approach to simulating late holocene sediment and water transfer from catchment to lake in the french alps since 1826 this paper describes the application of a hydrogeomorphological numerical model caesar to simulate at hourly time steps changes in the hydrological and sediment regime of the petit lac d annecy catch ment the outputs of the model were validated in three ways in the short term 5 years water discharge out puts were compared against observed instrumental data over the longer term modelled sediment discharge ad 1825 2005 was compared with proxies for detrital sediment influx environmental magnetism and accumu lation rates discerned from a 210 pb chronology for the lake sediments finally spatial validation of the modelled erosion and deposition of sediment was undertaken by comparison with a field and remotely sensed survey of catchment geomorphology the results suggest that while minor perturbations in forest cover during the last 180 years have partially conditioned the response of the sediment system the bulk of modelled sediment discharge and particularly the peaks in sediment discharge were controlled by flood duration and magnitude which in turn is driven by precipitation storms floods and snowmelt basin geometry and geomorphology of each sub catchment ire and tamie were also important in producing differences in the modelled sediment discharge in essence these differences were a function of sediment accommodation space and the ability of each system to store and release sediments modelled sediment discharge and para lake sediments display similar histories and thus are both interpreted as reflecting variations in detrital sediment supply intriguingly the style of mod elled sediment discharge from the ire a confined mountain torrent displays a greater similarity to and perhaps dominates the lake sediment record these results provide partial validation of the caesar model and indi cate that perhaps in the future it may be used as an exploratory and predictive tool in determining the impact of changes in climate meteorology and land use on lake catchment systems

devonian permian magmatic pulses in the northern vosges mountains ne france result of continuous subduction of the rhenohercynian ocean and avalonian passive margin

sensitivity studies applied to a two dimensional resistivity model from the central andes summary long period magnetotelluric mt data have been collected along an e w profile across the central andes in northern chile and southern bolivia a 2 d resistivity model explaining these data was found by inverse modelling the model includes a prominent conductivity anomaly in the backarc of the south american subduction zone below the bolivian altiplano another comparatively localized anomaly occurs beneath the precordillera in the forearc the enhanced conductivity in the backarc correlates with reflective zones low seismic velocities and high seismic attenuation in contrast no correlation can be found in the forearc a strong seismic reflector in context with the down going slab and another reflector in the middle crust below the pre and western cordillera do not coincide with anomalous conductivity other 2 d models may also be consistent with the data we applied different techniques to find the range of relevant models one simple but useful approach is the direct use of the sensitivity matrix containing the partial derivatives of the data with respect to the model parameters the sensitivity of the model parameters to the data or data subsets can be visualized by calculating columnwise sums from the sensitivity matrix these sensitivities may be used to indicate model parameters that are less resolved by the data and thus should not form part of an interpretation another approach is to perform systematic studies to establish the validity of geologically important features of the model these studies comprise forward modelling and the use of a priori information combining all approaches generates a more complete assessment of the principal model features we conclude that a minimum depth extent of the altiplano conductor can be specified as approximately 50 km low sensitivity in the forearc beneath the longitudinal valley indicates a limited structure resolution and therefore may explain the absence of a conductive slab in the model the existence of the localized conductor below the precordillera can be established by high sensitivity finally we included the two seismic reflectors in the forearc as a priori information in the inversion process as a result the absence of a conductive slab in the model is probably a result of low sensitivity but cannot be totally excluded a conductive zone coinciding with the strong reflector below the pre and western cordillera is not manifested by the mt data

tectonic shortening and crustal thickness in the central andes how good is the correlation exceptional thickening of continental crust beneath the central andes is believed to be mainly a result of tectonic shortening of the south american plate in neogene time this shortening has been estimated to have contributed as much as 70 80 of the present crustal volume a compilation of published shortening values and our own estimates based on balanced cross sections from the central andes between lat 3 s and 40 s suggest that 70 80 is a maximum rather than an average value for this part of the andes tectonic shortening and the crustal cross section area are only loosely correlated variations in tectonic shortening are more abrupt than those of crustal areas particularly near the northern and southern ends of the altiplano puna high plateau where thick crust is associated with relatively small amounts of shortening shortening there may account for no more than about 30 of the present crustal cross section area the processes that created the remaining crustal area are not clear but are likely to involve poorly constrained pre neogene tectonic shortening moderate magmatic additions to the crust tectonic underplating of material derived from the forearc and possible flow of ductile lower crust along strike

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

gps derived deformation of the central andes including the 1995 antofagasta mw 8 0 earthquake in order to study both the interplate seismic loading cycle and the distribution of intraplate deformation of the andes a 215 site gps network covering chile and the western part of argentina was selected monumented and observed in 1993 and 1994 a dense part of the network in northern chile and northwest argentina comprising some 70 sites was re observed after two years in october november 1995 the m w 8 0 antofagasta north chile earthquake of 30th july 1995 took place between the two observations the city of antofagasta shifted 80 cm westwards by this event and the displacement still reached 10 cm at locations 300 km from the trench three different deformation processes have been considered for modeling the measured displacements 1 interseismic accumulation of elastic strain due to subduction coupling 2 coseismic strain release during the antofagasta earthquake and 3 crustal shortening in the sub andes eastward displacement of the sites to the north and to the south of the area affected by the earthquake is due to the interseismic accumulation of elastic deformation assuming a uniform slip model of interseismic coupling the observed displacements at the coast require a fully locked subduction interface and a depth of seismic coupling of 50 km the geodetically derived fault plane parameters of the antofagasta earthquake are consistent with results derived from wave form modeling of seismolog ical data the coseismic slip predicted by the variable slip model reaches values of 3 2 m in the dip slip and 1 4 m in the strike slip directions the derived rake is 66 our geodetic results suggest that the oblique nazca south american plate convergence is accommodated by oblique earthquake slip with no slip partitioning the observed displacements in the back arc indicate a present day crustal shortening rate of 3 4 mm year which is significantly slower than the average of 10 mm year experienced during the evolution of the andean plateau

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

distribution of hydrous minerals in the subduction system beneath mexico abstract teleseismic converted phases are used to probe the composition of the downgoing oceanic crust as a function of depth along the cocos slab in central and southern mexico previously modeling of the receiver function rf conversion amplitude of the flat cocos slab beneath central mexico at 45 km depth revealed a thin low velocity upper oceanic crust of a thickness of 4 1 km which has much lower seismic velocities 20 30 reduction in shear wave velocities than normal lower crust high vp vs ratio 2 0 also suggested a large concentration of hydrous minerals such as talc in combination of high pore fluid pressure in the horizontal segment we extend this previous effort to examine seismic properties of both the steeply subducting cocos oceanic crust beneath the trans mexican volcanic belt tmvb in central mexico and the shallowly dipping crust beneath southern mexico inverted seismic velocities using the converted amplitudes at the top and bottom of the dipping oceanic crust are compared with experimentally constrained seismic velocities of candidate mineral phases in a range of likely pressures and temperatures the composition of the oceanic crust downdip in the steep part of slab beneath the tmvb includes the minerals such as lawsonite and zoisite at 60 100 km depth and the eclogitization occurs around 100 km depth this is related to arc volcanism in the tmvb directly above the slab as well as the slab rollback in contrast the dominant mineral phase in the upper oceanic crust of southern mexico beneath the isthmus of tehauntepec is amphibole on top of unaltered gabbroic oceanic crust the difference in mineral assemblages of the subducted oceanic crust may help explain the difference in slab geometries between central and southern mexico

palaeoclimate sedimentation and continental accretion climate has a pervasive effect on sedimentation today and the same climatic patterns are reflected in the distribution of lithofacies through the palaeozoic as the continents migrate beneath the climatic zones the low latitude hot wet zone is represented by thick elastics coals and carbonates and is best developed along east coasts where prevailing winds bring moisture and heated surface waters toward the continent the desert zones occur on the west sides of continents centred at 20 north and south and these dry belts are represented in the geological record by evaporites tillites thick elastics and coals occur in the temperate rainy belts especially on the windward west sides of continents above 40 latitude continental accretion occurs where subduction zones coincide with rainy zones such that the products of erosion are transported to the trench and thus thrust back extending the margin of the continent the opposite process of tectonic erosion wherein the descending oceanic slab continually rasps away the margin of the continental crust may occur in areas where rainfall and surface run off is insufficient to provide trench sediments this process has been operating adjacent to the atacama desert in south america during the past 200 ma to judge by the eastward migration of the calc alkaline intrusive foci about 250 km of the margin of south america have been transported down the subduction zone during this period

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

recent crustal deformation in the antofagasta region northern chile and the subduction process new neotectonic observations along with a detailed aerial photograph analysis allow a new interpretation of the recent tectonic behaviour of the outer forearc in northern chile between 22 5 s and 24 5 s antofagasta region both the coastal cordillera and the mejillones peninsula are under e w extension normal faults dipping east with an almost n s orientation are predominant large scale neogene to recent deformation is characterized by vertical uplift and subsidence related to normal faulting recent kinematics along the atacama fault system including the atacama fault itself are controlled by the regional extensional stress regime and the predominant component of recent displacement along the atacama fault is vertical normal however strike slip components are also observed but with moderate offsets no larger than one to two tens of metres the sense of shear and the ratio between vertical and horizontal offsets vary coherently with the fault azimuth compatible with e w extension oblique slip faulting with normal and left lateral offsets of similar amplitude is observed on the most northeasterly oriented segment of the atacama fault this small left lateral component of displacement and the strike slip motion predicted by models of strain partitioning of the convergence along the atacama fault are in opposite senses this occurs because the stress regime in the coastal region is not compressional but extensional very fresh recent ruptures are numerous especially between 23 s and 24 s and provide clear indications of the occurrence of moderate to large continental earthquakes during the late quaternary as pointed out by several authors indirect lines of evidence suggest active subduction erosion and underplating along the margin of northern chile we propose that part of the long term extension is due to the broad scale flexure of the outer forearc this flexure may be controlled by offshore subsidence caused by subduction erosion near the trench and by onshore uplift related to the underplating of eroded low density material beneath the coastal cordillera neotectonic observations show that the state of stress in the outer forearc remains extensional in a deviatoric sense in the long term however it is proposed that the state of stress has a strong time dependent component which is intimately linked to the subduction seismic cycle the mw 8 0 antofagasta earthquake of 1995 july 30 showed that large subduction earthquakes produce e w extension in the coastal region the overall e w deviatoric extension in the outer forearc should be reduced by interseismic contraction in the period separating two subduction earthquakes this reduction means that continental faults will remain locked and aseismic in the interseismic period of the subduction cycle in agreement with microseismic observations which indicate an absence of shallow crustal seismicity we assume that the amount of extension produced by subduction earthquakes in the coastal area is larger than the cumulative interseismic contraction this occurs because the displacement field generated by subduction earthquakes is concentrated in the outer forearc the e w extension should grow over prepeated subduction seismic cycles and lead to large continental earthquakes occurring within the overriding block simultaneous with large subduction events we show new surface ruptures associated with normal faulting along the atacama fault which were most probably triggered by the 1995 antofagasta subduction earthquake

the central andean altiplano puna magma body receiver function analysis of 14 teleseismic events recorded by 6 temporary passcal broadband stations within the altiplano puna volcanic complex apvc shows a consistent 2 s negative polarity p to s conversion for all stations for all available azimuths forward modeling of the largest amplitudes suggests that this conversion is produced by the top of a very low velocity zone at a depth of 19 km with a vs 0 5 km s and a thickness of 750 810 m we interpret the characteristics of the low velocity zone low vs areal extent and flatness to be consistent with a sill like magma body on the basis of additional data from the german ancorp experiment the altiplano puna magma body appears to underlie much of the apvc and it may therefore be the largest known active continental crustal magma body

stress distribution and geometry of the subducting nazca plate in northern chile using teleseismically recorded earthquakes summary the stress distribution along the subducting nazca plate in northern chile is analysed using focal mechanism solutions obtained from the inversion of long period p sv and sh waveforms of 15 earthquakes mb 5 5 and from 212 events with reported focal mechanisms which occurred between 1962 and 1993 a joint hypocentral determination was carried out to control the depth of 261 events mb 5 0 recorded at teleseismic distances a change from tensional to compressional stress field along the upper part of the subducting slab is associated with the maximum depth extent of the coupled zone this change occurs in northern chile at 200 250 km from the trench at depths of 60 10 km this depth is larger than the maximum depth observed for the thrusting interplate events 40 10 km probably meaning that at depths of between 40 and 60 km large low dip angle thrust events do not nucleate seismic slip however probably extends down to 40 km in depth the shallow dip angle up to 60 km in depth of the wadati benioff zone does not show variations along the strike of the trench however a gradual southward flattening of the slab is observed at distances greater than 200 250 km from the trench this change observed from about 21 s could be associated with a younger and probably more buoyant lithosphere than that observed to the north of this latitude there are two gaps located between the three main clusters of seismicity these gaps are clearly not related to detachments in the descending litosphere the first cluster is located in and beneath the seismogenic interplate contact and is characterized by reverse and thrust faulting events over a scarce tensional activity in the second cluster the compressional seismicity is scarce for teleseismic events and is located beneath the normal faulting events the third cluster corresponds to tensional events therefore these gaps in seismicity could be associated with alternating changes from compressional to tensional stress field in the subducting slab

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

the footwall ramp of the subandean decollement northernmost argentina from extended correlation of seismic reflection data abstract the link between thin skinned foreland shortening and thickening of the lower crust beneath the hinterlands of mountain belts has seldom been observed we have reprocessed yacimientos petroliferos fiscales ypf s a seismic reflection data from the eastern cordillera of the central andes at 22 s latitude using vibroseis extended correlation the result we tentatively interpret as an image of the crustal scale footwall ramp of the subandean belt decollement three crossing lines extended from 5 to 15 s 45 km provide excellent 3d control a band of strong mid crustal reflections between 4 5 and 6 s 9 to 13 km displays imbricate thrust geometry it may be a shallow zone of decollement but is 5 10 km too shallow for the subandean belt decollement at 7 8 s 16 to 20 km a band of reflections dips steeply nw at about 35 migrated and strikes parallel to the andean structure underlying the camargo syncline the dipping band correlates closely with the westward projection of the subandean decollement east of the dipping event scattered horizontal reflections are present to 12 s 32 km reflections above the dipping event have geometries similar to hanging wall anticlines and may truncate against it west of the band reflections occur to 15 s 41 km none of these lines have sufficient penetration to image moho the footwall ramp of the subandean belt apparently extends to near the base of the crust and would mark the western limit of under thrusting of undeformed craton beneath the orogen based on lateral variation in surface geology existing balanced cross sections and the geometry imaged on the reprocessed seismic lines we suggest that the subandean belt decollement steepens its ramp from north to south as shortening dies out southward at 24 s foreland structures are locally compensated and have seismicity to 25 km

subduction interface processes recorded by eclogite facies shear zones monviso w alps abstract the monviso ophiolite lago superiore unit constitutes a well preserved almost continuous upper fragment of oceanic lithosphere subducted at c 80 km depth thereby providing a unique opportunity to study mechanical coupling processes and meter scale fluid rock interactions occurring at such depths in present day subduction zones it is made of i a variably thick 50 500 m section of eclogitized basaltic crust associated with minor calcschist lenses overlying a 100 400 m thick metagabbroic body and of ii a c 1 km thick serpentinite sole we herein focus on the three major eclogite facies shear zones found at the top of the unit at the boundary between basalts and gabbros and between gabbros and serpentinites respectively strain localization occurred at lithological interfaces irrespective of material strength while ductile deformation dominates along the shear zones local brittle behavior is demonstrated by the existence of numerous eclogite breccias of fe ti metagabbros and widespread garnet fractures possibly linked with intermediate depth eclogite facies micro seismicity these m to hm sized fragments of fe ti metagabbros were later sheared and disseminated within serpentinite schists along the gabbro serpentinite boundary lower shear zone lsz pervasive and focused fluid flow is attested in the lsz by significant alteration of bulk rock compositions weakening of the rocks and widespread crystallization of hydrous parageneses by contrast the intermediate shear zone isz shows evidence for more restricted short range fluid flow the activity of both the isz and lsz ceased during early lawsonite eclogite facies exhumation when deformation localized deeper within the serpentinite sole allowing for the detachment and preservation of this large ophiolitic fragment

episodic out of sequence deformation promoted by cenozoic fault reactivation in nw argentina abstract the central andes are the product of contractional deformation related to subduction of the nazca plate beneath the south american continent in this geodynamic setting thrust belt propagation in the central andean retroarc in nw argentina and southern bolivia has been linked with the critical wedge model implying progressive eastward migration of the deformation front during the cenozoic however eocene unconformities and growth strata within the orogenic hinterland of the eastern cordillera and puna plateau suggest selective reactivation of pre andean and andean faults revealing a pattern of diachronous and spatially disparate range uplifts the calchaqui valley in nw argentina is characterized by basin and range morphology which is intimately linked with the heterogeneous neoproterozoic basement cretaceous rift structures and the cenozoic foreland in this study we analyze this complex deformation by reconstructing cenozoic fault reactivation and sedimentation patterns in the eastern cordillera detailed structural mapping along the eastern calchaqui border tonco area reveals five cenozoic unconformities in the former foreland deposits related to multiple episodes of fault reactivation during the eocene to pleistocene a refined chronostratigraphy u pb zircon new ahe thermochronological data apatite u th sm he and existing evidence for deformation indicate episodic out of sequence deformation and increased eo oligocene puna plateau to miocene eastern cordillera fault displacement apparently controlled by pre cenozoic heterogeneities the coeval eastward propagation of deformation and continuing deformation in the west generally challenge the applicability of the critical wedge model