foreland shortening and crustal balancing in the andes at 30 s latitude excellent surface exposures known benioff zone geometry a dynamic morphology and the availability of industry seismic reflection data all make the andes at 30 s an excellent transect for investigating crustal scale balanced sections 150 170 km of horizontal shortening has occurred in three major belts located between the trench and the foreland the thin skinned east verging precordillera of western argentina accounts for 60 75 of the total shortening and formed mostly since major volcanism ceased at 10 ma industry seismic reflection data show that the decollement of the precordillera belt is located anomalously deep at 15 km the belt is dominated by fault propagation folds and contains several prominent out of sequence thrust faults seismic stratigraphie analysis shows that miocene strata in the iglesia valley located between the precordillera and the crest of the andes accumulated in a piggy back basin onlap relations on the western side indicate that the high cordillera was uplifted as a major fault bend fold over a buried ramp thrusting in the two western belts both in the high cordillera of chile formed during the waning stages of arc volcanism 11 16 ma and account for 25 40 of the shortening the observed shortening is probably greater than can be accounted for with reasonable crustal thicknesses indicating the possibility of continental truncation or erosion along the plate margin or an anomalously thick root held down by the nearly flat subducted nazca plate our preferred crustal geometry puts the ramp between upper and lower crustal deformation west of the high topography requiring crustal scale tectonic wedging to thicken the crust beneath the crest of the andes this non unique model provides a simple explanation of the first order morphology of the andes at this latitude

global upper mantle structure from finite frequency surface wave tomography 1 we report global shear wave velocity structure and radial anisotropy in the upper mantle obtained using finite frequency surface wave tomography based upon complete three dimensional born sensitivity kernels because wavefront healing effects are properly taken into account finite frequency surface wave tomography improves the resolution of small scale mantle heterogeneities especially for deep anomalies that are constrained by the longest period surface waves in our finite frequency model ffsw1 the globally averaged radial anisotropy shows a transition from positive sh sv to negative anisotropy sv sh at about 220 km consistent with a change in the dominant mantle circulation pattern from predominantly horizontal flow at shallow depths to vertical flow at greater depths the radial anisotropy beneath cratons and the old pacific plate agrees well with previous studies however our model exhibits a strong negative radial anisotropy at depths greater than 120 km beneath mid ocean ridges a feature that is not present in previous upper mantle models more interestingly the depth extent of the ridge anomalies is distinctly different beneath fast and slow spreading centers anomalies beneath fast spreading centers are stronger but the strength decreases rapidly below 250 km in contrast beneath slow spreading centers such as the northern mid atlantic ridge and the red sea anomalies extend down at least to the top of the transition zone the different depth extent of the ridge anomalies suggests that the primary driving force of slow spreading seafloor may be different from that of fast spreading seafloor and that active upwelling beneath slow spreading ridges may play a major role in the opening of the seafloor

lithospheric structure of the chaco and parana basins of south america from surface wave inversion surface wave data from a portable broadband array have been used to invert for the velocity structure of the crust and upper mantle beneath the chaco and parana basins of central south america the upper mantle velocity structure beneath the parana basin is cratonic in character whereas that beneath the chaco basin is tectonic or asthenospheric in character the surface wave analysis used broadband recordings from a subset of a 14 station array deployed in a roughly east west sawtooth arrangement along 20 s latitude with a total e w aperture of 1 400 km results from receiver function analysis as well as direct p wave regional travel time data were used in the inversions to help constrain moho depths and crust and upper mantle velocities s wave structure for the intracratonic parana basin was determined using interstation phase and group velocities for rayleigh waves fundamental and first higher mode and love waves fundamental mode only based on seven events with paths which traverse the eastern parana basin and one event with a path across the western parana basin the average moho depth in the eastern parana basin is 42 km the high velocity upper mantle lid has a maximum s wave velocity of 4 7 km s with no resolvable low velocity zone to at least 200 km depth this cratonic velocity structure indicates the presence of a lithospheric root beneath the parana basin despite emplacement of the parana plume the limited data from the western parana basin are consistent with a homogeneous upper mantle structure throughout the parana basin waveform inversion of fundamental mode and first higher mode rayleigh waves from a single subandean event was used to obtain estimates for pure path dispersion along propagation paths through the chaco basin and the western half of the parana basin the data were partitioned to isolate the partial path contribution of the phase and group velocities for the chaco basin the phase and group velocities from this somewhat sparse data set were inverted to obtain a velocity depth model for the chaco basin the distinguishing features of the chaco model consist of a rather shallow moho depth 32 km and low asthenospheric upper mantle s wave velocities about 4 2 km s with velocity increasing only slightly to about 4 3km s at 150 km depth

low electrical resistivity associated with plunging of the nazca flat slab beneath argentina beneath much of the andes oceanic lithosphere descends eastward into the mantle at an angle of about 30 ref 1 a partially molten region is thought to form in a wedge between this descending slab and the overlying continental lithosphere as volatiles given off by the slab lower the melting temperature of mantle material2 this wedge is the ultimate source for magma erupted at the active volcanoes that characterize the andean margin but between 28 and 33 s the subducted nazca plate appears to be anomalously buoyant3 4 as it levels out at about 100 km depth and extends nearly horizontally under the continent1 5 6 above this flat slab volcanic activity in the main andean cordillera terminated about 9 million years ago as the flattening slab presumably squeezed out the mantle wedge5 6 but it is unknown where slab volatiles go once this happens and why the flat slab finally rolls over to descend steeply into the mantle 600 km further eastward here we present results from a magnetotelluric profile in central argentina from which we infer enhanced electrical conductivity along the eastern side of the plunging slab indicative of the presence of partial melt this conductivity structure may imply that partial melting occurs to at least 250 km and perhaps to more than 400 km depth or that melt is supplied from the 410 km discontinuity consistent with the transition zone water filter model of bercovici and karato7

combining satellite and seismic images to analyse the shallow structure of the dead sea transform near the desert transect the left lateral dead sea transform dst in the middle east is one of the largest continental strike slip faults of the world the southern segment of the dst in the arava araba valley between the dead sea and the red sea called arava araba fault af has been studied in detail in the multidisciplinary desert dead sea rift transect project based on these results here the interpretations of multi spectral aster satellite images and seismic reflection studies have been combined to analyse geologic structures whereas satellite images reveal neotectonic activity in shallow young sediments reflection seismic image deep faults that are possibly inactive at present the combination of the two methods allows putting some age constraint on the activity of individual fault strands although the af is clearly the main active fault segment of the southern dst we propose that it has accommodated only a limited up to 60 km part of the overall 105 km of sinistral plate motion since miocene times there is evidence for sinistral displacement along other faults based on geological studies including satellite image interpretation furthermore a subsurface fault is revealed 4 km west of the af on two e w running seismic reflection profiles whereas these seismic data show a flower structure typical for strike slip faults on the satellite image this fault is not expressed in the post miocene sediments implying that it has been inactive for the last few million years about 1 km to the east of the af another now buried fault was detected in seismic magnetotelluric and gravity studies of desert taking together various evidences we suggest that at the beginning of transform motion deformation occurred in a rather wide belt possibly with the reactivation of older n s striking structures later deformation became concentrated in the region of today s arava valley till 5 ma ago there might have been other now inactive fault traces in the vicinity of the present day af that took up lateral motion together with a rearrangement of plates 5 ma ago the main fault trace shifted then to the position of today s af

lithospheric structure of the central andes based on surface wave dispersion 1 shear wave velocity is very sensitive to temperature anomalies and partial melt and can provide important insights on the state of the lithosphere with that aim in mind phase velocities of rayleigh and love waves have been inverted for the regionalized shear wave velocity structure of the lithosphere across the central andes this inversion reveals strong lateral variations of vs both across the range and along strike in the altiplano crust in the upper crust the main features of our models are prominent low velocity anomalies probably related to partial melt below the los frailes ignimbrite complex and the southern altiplano at lower crustal level the altiplano is characterized by lower vs than the surrounding regions we find that the transition from the altiplano to the puna is associated with a 7 km thickening of the crust at mantle depths the nazca plate is found to be overlaid by a dipping low velocity zone with decreasing intensity with depth our results favor the idea of a cold mantle lid underlying the whole central andes

cuyania an exotic block to gondwana review of a historical success and the present problems abstract a review of the early history of the cuyania terrane and the numerous pioneering works of the past century provides the present robust framework of evidence supporting a derivation from laurentia travel towards gondwana as an isolated microcontinent and final amalgamation to the protomargin of western gondwana in middle to late ordovician times the major remaining uncertainties and inconsistencies such as the time of deformation and collision with gondwana the lack of evidence of famatinian derived zircons the effects of strike slip displacements proposed along the suture as well as the potential sutures defined by ophiolite assemblages are discussed the precise boundary along the northern and southern limits is not yet well defined the most suitable hypothesis based on present data is that cuyania originated as a conjugate margin of the ouachita embayment south of the appalachian platform during early cambrian times the subsequent travel toward the gondwana protomargin is clearly depicted by the changing faunal assemblages in the carbonate platform new geochemical and age data on k bentonites presented by several authors reinforce the strong connection between cuyania ash fall tuffs and famatina volcanics by 468 470 ma indicating cuyania and gondwana were in close proximity at that time extension related to flexural subsidence preceded by the drowning of the carbonate platform in early llanvirnian times is recorded by abrupt facies changes in the sedimentary cover during late llanvirnian and early caradocian times this episode marked the beginning of contact between cuyania and gondwana the subsequent evolution of the foreland basin indicates that deformation lasted until latest silurian early devonian times the time of collision is tracked by the cessation of arc related magmatic activity in the upper plate gondwana protomargin at about 465 ma in western sierras pampeanas and ages around 454 ma corresponding to syncollisional and postcollisional magmatism the age of the collision is also preserved in the lower plate cuyania where both angular unconformities in the sedimentary cover and the ages of peak of regional metamorphism in the basement rocks point to 460 ma as the most probable age for the beginning of the collision evidence from the upper plate is essentially identical with an age of 463 ma thermal gradients along this suture vary from 13 c km in the lower plate to 18 c km in the fore arc upper plate reaching more than 30 c km along the famatinian arc based on these data a llandelian caradocian age for the collision can be postulated on firm grounds deformation continued through most of the early paleozoic until amalgamation of the chilenia terrane by the late devonian

outer rise stress changes related to the subduction of the juan fernandez ridge central chile 1 although outer rise seismicity is less common than interplate seismicity in subduction zones a significant level of seismicity has occurred between the nazca trench and juan fernandez ridge in central chile during the past 20 years we first study the 9 april 2001 mw 7 0 event and determine its focal mechanism depth and source time function by body waveform inversion from teleseismic broadband data the results indicate tensional faulting in the upper part of the mechanical lithosphere its strike 41 is similar to those observed in events down dip of the slab at about 100 km depth which could indicate that these earthquakes occur in preexisting structures formed at the trench compressive outer rise events have also occurred during the 1980s in front of the rupture zone of the 1985 mw 7 8 valparaiso earthquake to understand their relation with the state of stress of the lithosphere we construct yield stress envelopes of the oceanic lithosphere including static and dynamic stresses dynamic stresses are due either to slab pull ridge push resistive and drag forces we explain the sequence of compressive and tensional events by the accumulation of stress prior to 1985 when the subduction is assumed to be locked and after by the unlockage of the subduction by the valparaiso interplate event the yield stress envelope analysis enables us to quantify the accumulation of compressive forces before 1985 and the tensional force after

pocho volcanic rocks and the melting of depleted continental lithosphere above a shallowly dipping subduction zone in the central andes the late miocene 7 9 to 4 5 ma pocho volcanic field in argentina occurs 700 km east of the chile trench over the modern shallowly dipping andean wadati benioff zone near 32 s latitude in argentina the field is located in the sierra de cordoba which is the easternmost laramide style block faulted range in the sierras pampeanas pampean ranges the arrival of the shallowly dipping slab initiated both volcanism and the uplift of the sierra de cordoba pocho rocks 52 to 68 sio2 feo mgo 2 2 comprise an older 7 5 0 5 ma high k and a younger 5 3 0 7 ma shoshonitic series mineralogic data and fractionation models show that crystallization occurred under hydrous oxidizing conditions which were most extreme in the high k series an unusual pattern of successively lower ree at higher sio2 concentrations can be modeled by sphene apatite and amphibole removal an arc like trace element signature attributed to an arc component is strongest in the younger shoshonitic series an important depleted lower crustal mantle lithospheric source component in both series is indicated by non radiogenic sr and pb isotopic ratios at nd 0 to 2 low rb sr ratios and low u and th concentrations this depleted signature contrasts with the enriched one in potassic back arc central volcanic zone cvz lavas over the steeper subduction zone to the north and is attributed to several processes in the shallow subduction zone first deep crustal mash processes in the nearly normal thickness crust beneath pocho incorporated depleted proterozoic basement components and not complexly mixed structurally thickened crustal components as in the cvz second the association of pocho volcanism with the arrival of the slab allowed little time for modification of the mantle by subduction components third miocene shallowing of the subduction zone beneath the flat slab required thinning of both the astenosphere and the subcontinental lithosphere thus an important subcrustal component could be from blocks removed from the base of the lithosphere to the west and recycled into the asthenosphere similar magmatic sources would have existed during laramide shallow subduction in western north america

gravity isostasy and andean crustal shortening between latitudes 30 and 35 s we started the work by analyzing an e w crustal section at 33 s latitude which extends from the western chilean trench crosses the andean belt and goes inland into argentina a two layer gravity seismic model was prepared here it provided a maximum moho depth of 62 km beneath the andean axis this model is consistent with a one layer standard crustal model with normal thickness t 33 km and densities of 2 9 g cm3 and 3 3 g cm3 for the crust and the upper mantle respectively then we analyzed the andean isostatic behaviour on four andean gravimetric sections at latitudes of 30 32 33 and 35 s based on one layer simple models these results show antiroots near the chilean trench and crustal thickenings beneath the andean belt which locate the moho discontinuity at depths of 70 km at 30 s 65 km at 32 and 33 s and 57 km at 35 s from the crustal gravimetric models the following provisional charts for the sector were prepared and compared 1 topography 2 bouguer gravity 3 isostatic corrections 4 moho contours these charts are consistent with each other and they point out that the compensation of the andean belt trends approximately to isostasy in the airy concept other results also confirm this 1 the remarkable decrease of the isostatic anomalies in each of the analyzed sections as regards the bouguer anomalies and 2 the relationships between the exceeding areas and the corresponding crustal root areas which are consistent with the airy numerical relations as to the perfect isostatic equilibrium thus the bouguer anomalies are mainly controlled by the m discontinuity finally shortenings in the andean sector were fixed based on the crustal thicknesses found from gravity inversion these are 150 km at 30 s decreasing southwards and reaching only 90 km at 35 s these shortenings explain the cenozoic andean uplift without requiring other mechanisms such as magmatic intrusions

large scale crustal conductivity pattern in central europe and its correlation to deep tectonic structures the results of magnetovariational mv soundings are usually presented in the form of induction arrows however many examples show that the horizontal magnetic tensor hmt is more informative the distribution of some hmt invariants directly traces the location of well conducting rocks in the crust and upper mantle the hmt determination requires simultaneous observations in an entire region which is a substantial disadvantage yet it is possible to apply techniques capable of restoring all the magnetic field components necessary for hmt estimation from tipper data arrays alone these techniques exploit the spatial relationships between electromagnetic field components in a non conducting atmosphere for central europe a large data set of induction arrows has been collected by the effort of many groups during the last 50 years based on these data hmt values were calculated and the results are very significant the spatial behavior of certain hmt invariants demonstrates the presence of deeply seated well conducting rocks in the crust anomaly maximums display an arc shaped trend that may be genetically linked with the caledonian and or variscan margin thrust belts which developed following the collision between baltica and avalonia and or gondwana derived terranes respectively this is an important finding because the position of these deformation fronts in relation to the edge of the east european platform is still controversial

an evaluation of proposed mechanisms of slab flattening in central mexico central mexico is the site of an enigmatic zone of flat subduction the general geometry of the subducting slab has been known for some time and is characterized by a horizontal zone bounded on either side by two moderately dipping sections we systematically evaluate proposed hypotheses for shallow subduction in mexico based on the spatial and temporal evidence and we find no simple or obvious explanation for the shallow subduction in mexico we are unable to locate an oceanic lithosphere impactor or the conjugate of an impactor that is most often called upon to explain shallow subduction zones as in south america japan and laramide deformation in the us the only bathymetric feature that is of the right age and in the correct position on the conjugate plate is a set of unnamed seamounts that are too small to have a significant effect on the buoyancy of the slab the only candidate that we cannot dismiss is a change in the dynamics of subduction through a change in wedge viscosity possibly caused by water brought in by the slab

stress in the contorted nazca plate beneath southern peru from local earthquakes we study earthquake focal mechanisms in a region of highly contorted subducting lithosphere to identify dominant sources of stress in the subduction process we observe a stress pattern in the contorted nazca plate beneath southern peru from an analysis of hypocentral trend and focal mechanisms of intermediate depth earthquakes expanding on previous studies we examine the hypocentral trend using 1673 of 2178 well located local events from the nine station arequipa network the dip of the plate beneath southern peru averages 25 30 from 25 to 100 km depth below this depth there is an 80 to 100 km wide contortion between a zone of increasing dip convex to the southeast and a flat lying concave zone to the northwest using more than 6000 p wave first motions of events deeper than 50 km we derive stress orientations from a moving average of composite focal mechanisms across a 200 by 350 km region including the contortion the in plate distribution of tension t and compression p axes reveals a coherent stress pattern the trend is most clear beneath south central peru nw section and below 100 km depth in southernmost peru se section both t and p axes tend to be dominantly in plate especially below 100 km depth t axes orient toward the contortion in a fan shaped trend which suggests that the deepest part of the seismic zone within the convex se section is sinking and pulling the more buoyant nw section we conclude that from 50 to 200 km depth slab pull forces are dominant in the observed stress our results suggest that a significant amount of plate extension occurs in this region of intermediate depth subduction

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

crustal thickness estimation beneath the southern central andes at 30 s and 36 s from s wave receiver function analysis summary we have used the s wave receiver function srf technique to investigate the crustal thickness beneath two seismic profiles from the charge project in the southern central andes a previous study employing the p wave receiver function method has observed the moho interface beneath much of the profiles they found however that the amplitude of the p to s conversion was diminished in the western part of the profiles and have attributed it to a reduction of the impedance contrast at the moho due to lower crustal ecologitization with srf we have successfully detected s to p converted waves from the moho as well as possible conversions from other lithospheric boundaries the continental south american crust reaches its maximum thickness of 70 km along 30 s between 70 w and 68 5 w beneath the principal cordillera and the famatina system and becomes thinner towards the sierras pampeanas with a thickness of 40 km negative phases possibly related to the base of the continental and oceanic lithosphere can be recognized in the summation traces at different depths by comparing our results with data obtained from previous investigations we are able to further constrain the thickness of the crust and lithosphere beneath the central andes

crustal deformation in the south central andes backarc terranes as viewed from regional broad band seismic waveform modelling summary the convergence between the nazca and south america tectonic plates generates a seismically active backarc region near 31 s earthquake locations define the subhorizontal subducted oceanic nazca plate at depths of 90 120 km another seismic region is located within the continental upper plate with events at depths 1 80 th 45 55 km than those of the eastern sierras pampeanas vp 6 0 6 2 km s 1 vp vs 1 70 th 27 35 km in addition we observed an apparent distribution of reverse crustal earthquakes along the suture that connects those terranes finally we estimated average p and t axes over the charge period the entire region showed p and t axis orientations of 275 and 90 plunging 6 and 84 respectively

andean flat slab subduction through time the analysis of magmatic distribution basin formation tectonic evolution and structural styles of different segments of the andes shows that most of the andes have experienced a stage of flat subduction evidence is presented here for a wide range of regions throughout the andes including the three present flat slab segments pampean peruvian bucaramanga three incipient flat slab segments carnegie guanacos tehuantepec three older and no longer active cenozoic flat slab segments altiplano puna payenia and an inferred palaeozoic flatslab segment early permian san rafael based on the present characteristics of the pampean flat slab combined with the peruvian and bucaramanga segments a pattern of geological processes can be attributed to slab shallowing and steepening this pattern permits recognition of other older cenozoic subhorizontal subduction zones throughout the andes based on crustal thickness two different settings of slab steepening are proposed slab steepening under thick crust leads to delamination basaltic underplating lower crustal melting extension and widespread rhyolitic volcanism as seen in the caldera formation and huge ignimbritic fields of the altiplano and puna segments on the other hand when steepening affects thin crust extension and extensive within plate basaltic flows reach the surface forming large volcanic provinces such as payenia in the southern andes this last case has very limited crustal melt along the axial part of the andean roots which shows incipient delamination based on these cases a palaeozoic flat slab is proposed with its subsequent steepening and widespread rhyolitic volcanism the geological evolution of the andes indicates that shallowing and steepening of the subduction zone are thus frequent processes which can be recognized throughout the entire system

crustal structure of the south central andes cordillera and backarc region from regional waveform modelling summary we investigate the crustal structure in the andes cordillera and its backarc region using regional broadband waveforms from crustal earthquakes we consider seismic waveforms recorded at regional distances by the chile argentina geophysical experiment charge during 2000 2002 and utilize previous seismic moment tensor inversion results for each single station earthquake pair we fixed the source parameters and performed forward waveform modelling using ray paths that sample the crust of the highest elevation cordillera and the accreted terranes in the backarc region our investigation indicates that synthetic seismograms for our earthquake station geometry are most sensitive to crustal parameters and less sensitive to mantle parameters we performed a grid search around crustal thickness p wave seismic velocity vp and p t os wave seismic velocity ratio vp vs fixing mantle parameters we evaluated this waveform analysis by estimating an average correlation coefficient between observed and synthetic data over the three broadband components we identified all acceptable crustal models that correspond to high correlation coefficients that provide best overall seismogram fits for the data and synthetic waveforms filtered mainly between 10 and 80 s our results indicate along strike variations in the crustal structure for the north south high cordillera with higher p wave velocity and thickness in the northern segment north of 33 s and persistently high vp vs ratio 1 85 in both segments this is consistent with a colder mafic composition for the northern segment and a region of crustal thickening above the flat slab region in contrast the results for the current volcanic arc south of 33 s agree with a warmer crust consistent with partial melt related to quaternary volcanism presumably of an intermediate to mafic composition a distinctive feature in the backarc region is the marked contrast between the seismic properties of the cuyania and pampia terranes that correlates with their heterogeneous crustal composition the cuyania terrane composed of mafic ultramafic rocks exhibits high vp high vp vs and a thicker layered crust versus the thinner more quartzrich crust of the eastern sierras pampeanas associated with low vp and low vp vs these differences may have some effect on the mechanism that unevenly generates crustal seismicity in the upper 30 km in this active compressional region in particular the seismic properties of the cuyania terrane which shows evidence for a high vp and high vp vs crust may be reflecting the complex tectonic evolution history of this terrane including accretion rifting and re accretion processes since the palaeozoic that promote a high level of crustal seismicity in the upper 30 km enhanced by the flat slab subduction in the segment between 31 s and 32 s another possible mechanism could be directly related to the presence of a strong lower crust above the flat slab that efficiently transfer stresses from the slab to the upper crust generating higher seismicity in the cuyania terrane between 30 s and 34 s

shear wave velocities in the pampean flat slab region from rayleigh wave tomography implications for slab and upper mantle hydration 1 the pampean flat slab region located in central argentina and chile between 29 and 34 s is considered a modern analog for laramide flat slab subduction within western north america regionally flat slab subduction is characterized by the nazca slab descending to 100 km depth flattening out for 300 km laterally before resuming a more normal angle of subduction flat slab subduction correlates spatially with the track of the juan fernandez ridge and is associated with the inboard migration of deformation and the cessation of volcanism within the region to better understand flat slab subduction we combine ambient noise tomography and earthquake generated surface wave measurements to calculate a regional 3d shear velocity model for the region shear wave velocity variations largely relate to changes in lithology within the crust with basins and bedrock exposures clearly defined as low and high velocity regions respectively we argue that subduction related hydration plays a significant role in controlling shear wave velocities within the upper mantle in the southern part of the study area where normal angle subduction is occurring the slab is visible as a high velocity body with a low velocity mantle wedge above it extending eastward from the active arc where flat slab subduction is occurring slab velocities increase to the east while velocities in the overlying lithosphere decrease consistent with the slab dewatering and gradually hydrating the overlying mantle the hydration of the slab may be contributing to the excess buoyancy of the subducting oceanic lithosphere helping to drive flat slab subduction

thermochemistry and phase equilibria of hydrous phases in the system mgo sio2 h2o implications for volatile transport to the mantle calorimetric phase equilibrium and in situ equation of state data for antigorite talc and dense hydrous magnesium silicate dhms phases have been obtained to construct a self consistent thermodynamic data set that ensures internal consistency well behaved extrapolation and physically reasonable values of h s v k k and for talc antigorite and hydrous phase a used in conjunction with temperature profiles of subducting oceanic lithosphere the calculated phase equilibria show that there is no barrier to subducting substantial amounts of water to depth of 400 600 km in colder slabs since the slab can remain in the stability field of hydrous phases throughout its descent once water is brought to these depths it can probably diffuse into hotter mantle regions via hydrous mg2sio4 or by other mechanisms indicating that the transition zone is unlikely to be dry

upper mantle structure in the south central chilean subduction zone 30 to 36 s 1 regional p and s wave travel time data were used to obtain three dimensional seismic tomography models for vp vs and vp vs above the subducting slab in central chile and argentina in this region there is an abrupt change from a normal subduction geometry south of 33 s to a flat subduction geometry to the north we find low vp low vs and high vp vs ratios in the southern half of our study area directly beneath the modern active volcanic arc which we interpret as localized pockets of melt in the northern half of our study area above where the subducting nazca plate flattens at 100 km depth we find low vp high vs and low vp vs ratios these unusual results point to a lack of melt or hydrated mineralogies such as serpentine both of which are characterized by high vp vs values the only mantle rocks that have low vp vs and high vs are mg rich compositions such as dehydrated serpentinite or orthopyroxenite we suggest that significant portions of the mantle overlying the flat slab consist of orthopyroxenite formed by a transient fluxing of silica rich fluids such fluids may have come from sediments that were subducted during the initiation of flat subduction at this latitude at 10 ma this would imply that the hydration of mantle material above a flat slab can be a transient phenomenon which leaves little residual free water behind but significantly alters the mantle chemistry

depleted lithosphere cold trapped asthenosphere and frozen melt puddles above the flat slab in central chile and argentina recent studies of the upper mantle above the flat slab in central chile and argentina indicate the seismic velocity structures in this area are very different from those found in subduction zones with normal steeper slab geometries the mantle above the horizontal section of the flat slab is characterized by low p wave velocities high s wave velocities and low vp vs ratios as the slab begins to transition to a more normal dip to the south the mantle above it changes as well above this transition zone the mantle is characterizedbyhighp wavevelocities highs wavevelocities andgenerallyhighvp vsratios abovethishighvelocityanomaly in the corner between the south dipping slab of the transition zone and the east dipping slab of the normal subduction zone lies a small shallow anomaly distinguished by its very low p and s wave velocities and its moderately high vp vs ratio we interpret these anomalies to represent very depleted cold lithosphere cooled trapped asthenosphere and frozen pooled melt respectively recent research looking at the elastic properties of cratonic xenoliths indicate that a low vp high vs and low vp vs signature is indicative of cratonic lithosphere this suggests that the material above the flat slab may be ancient laurentian lithosphere or other melt depleted dry lithospheric material the presence of frozen asthenosphere to the south of the flat slab may indicate that large volumes of asthenosphere were displaced in advance of the eastward progression of the flattening slab between 10 and 2ma forcing a change in flow patterns from trench normal to ssw around the south eastern corner of the flattening slab as flattening progressed this asthenosphere became trapped above the transition zone eventually temperatures dropped sufficiently to freeze the last remaining pocket of extracted melt which had collected in the corner of the descending slab

controls of subduction geometry location of magmatic arcs and tectonics of arc and back arc regions most variation in geometry and angle of inclination of subducted oceanic lithosphere is caused by four interdependent factors combinations of 1 rapid absolute upper plate motion toward the trench and active overriding of the subducted plate 2 rapid relative plate convergence and 3 subduction of intraplate island seamount chains aseismic ridges and oceanic plateaus anomalously low density oceanic lithosphere cause low angle subduction under conditions of low angle subduction the upper surface of the subducted plate is in contact with the base of the overlying plate the wedge of low density asthenosphere is replaced by subducted lithosphere and the width of the arc trench gap either is significantly increased or a magmatic arc is not developed within the overlying plate the fourth factor is age of the subducting lithosphere subduction of young lithosphere produces two opposing tendencies 1 low angle subduction and increased arc trench distance owing to its low density and 2 decreased arc trench distance owing to its higher temperature two factors of secondary importance contribute to variation in subduction zone geometry and arc trench distance accretion of sediment in trenches depresses the upper portion of the subducting oceanic plate and causes the trench axis to migrate seaward prolonged subduction thickens the upper plate depresses the isotherms in the subducted plate and may create a broader arc both factors increase the arc trench gap the four primary factors also control development of other tectonic elements such as regional subsidence for example the amazon basin and a portion of the cretaceous interior seaway of western united states intra arc extension for example the basin and range province foreland fold and thrust belts and laramide style tectonics

plate reconstructions aseismic ridges and low angle subduction beneath the andes low angle subduction segments beneath peru and chile are believed to represent interaction of the subduction zone with the aseismic nazca and juan fernandez ridges on the basis of symmetric sea floor spreading models and plate hotspot reconstructions predicted continuations of the relatively buoyant ridges correspond well with the limits of the low angle subduction segments further the history of interaction of the aseismic ridges with the subduction zone as predicted by plate reconstructions is not inconsistent with available information on volcanic episodes of the andes if the contemporary correlation of volcanic gaps and low angle subduction is also applicable to the rest of the late cenozoic the occurrence of deep seismicity beneath the eastern flanks of the andes is also believed to reflect the effects of low angle subduction the deep seismic segments are adjacent to segments which are presently or were recently experiencing low angle subduction thus the deep seismicity is interpreted as representative of remnants of normal oceanic lithosphere subducted before interaction of aseismic ridges with the subduction zone other geologic phenomena are associated with low angle subduction beneath south america including landward shift in loci of tectonism surficial subsidence and laramide style deformation the aseismic ridge buoyancy hypothesis appears to be a valuable predictive tool for the interpretation of such geologic effects

the early paleozoic evolution of the argentine precordillera as a laurentian rifted drifted and collided terrane a geodynamic model paleontologic and stratigraphic evidence points to the early paleozoic precordilleran terrane of western argentina as being the conjugate rift pair of the appalachians stratigraphic similarities of the cambrian and early arenig carbonate series and very strong affinities among trilobite conodont and brachiopod faunas show their close relationship the most probable provenance areas are the appalachian ouachita rifted margin and the newfoundland appalachians although the former fits better with geometric and drifting paths suggested by faunal affinities increasing celtic and baltic brachiopod genera and divergent stratigraphy since the arenig indicate the drifting of the precordilleran terrane collisional foredeeps developed on collapsed former platform carbonates as flexural subsidence progressed the collision of the precordillera with western gondwana occurred during the mid llanvirn to llandeilo a magmatic arc related to eastward subduction present coordinates was active in the famatina range east of the precordillera this region of celtic affinity shows faunal exchange with the precordilleran terrane since the late arenig and may represent accreted intra iapetus volcanic island arc complexes the rifting and drifting stages are consistent with paleoclimatic and paleomagnetic data that show the migration of the precordilleran terrane from periequatorial to peripolar latitudes between the cambrian and latest ordovician the deep ocean to the west of the precordillera started to close by the late ordovician with the eastward drift of the chilenian terrane absence of volcanic or pyroclastic arc derived rocks in the precordillera indicate west dipping subduction as chilenia approached the continental margin a new forebulge was established on the former collided precordilleran terrane developing an erosional unconformity in central precordillera talacasto tambolar arch a gondwanic signature was fully developed by the middle silurian when the malvinokaffric clarkeia fauna flourished before then the late ordovician glacial record and associated hirnantia fauna were the first clear tie to gondwana during the silurian the marginal basin behaved as a foreland with lithosphere rheology and eustasy governing the sequence stratigraphy wrench faulting along its eastern boundary displaced the precordillera toward the south continued shortening during closure with the chilenian terrane in the mid devonian produced thrust loading of the basin and generated a thick graywacke succession final accretion of chilenia late devonian generated a regional angular unconformity between the lower and upper paleozoic new eastward subduction was initiated west of the accreted chilenian terrane during the late permian triassic as indicated by the choiyoi volcanic complex which presently outcrops in the frontal cordillera

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

antigorite high pressure stability in the system mgosio2h2o msh abstract the breakdown reactions of antigorite 1 forming talc forsterite water at low pressures and 2 forming forsterite clinoenstatite water at high pressures were determined in reversed equilibrium experiments results on reaction 1 were found to be in good agreement with former experimental determinations by both johannes johannes w 1975 zur synthese und thermischen stabilitat von antigorit fortschr mineral beih 53 36 and evans et al evans b w johannes w oterdoom h trommsdorff v 1976 stability of crysotile and antigorite in the serpentinite multisystem schweiz mineral petrogr mitt 56 79 93 from our experiments the invariant point i1 interconnecting the two reactions can be located at about 15 kbar 650 c this is consistent with the thermodynamic calculations using the dataset of berman berman r g 1988 internally consistent thermodynamic data for minerals in the system j petrol 29 445 522 however it is in contrast to recent experimental studies of ulmer and trommsdorff ulmer p trommsdorff v 1995a serpentine stability to mantle depths and subduction related magmatism science 268 858 861 who determined i1 at 21 kbar 730 c our pt conditions for i1 could be confirmed by equilibrium experiments on reaction 10 talc forsterite clinoenstatite water which is generated at i1 as well up to about 25 kbar the breakdown reaction 2 is nearly pressure independent towards still higher pressures the dp dt slope of reaction 2 bends and becomes negative schreinemakers analysis as well as thermodynamic calculations of the upper pressure stability of antigorite show that the possible antigorite breakdown reaction 3 antigorite clinoenstatite brucite water and reaction 4 brucite clinoenstatite forsterite water could originate at a new invariant point i2 provided that the reactions 2 and 11 brucite antigorite forsterite water intersect bracketing equilibrium 4 and combining these results with those on reaction 2 i2 was located at only about 51 kbar 490 c compared to 77 kbar 680 c according to berman s data however when taking into account the dense hydrous magnesium silicate dhms phase a mg7si2o8 oh 6 the phase relations of antigorite are changed resulting i in the metastability of i2 and reaction 3 and ii in a new invariant point i7 at about 44 kbar and 580 c generating the new antigorite breakdown reaction 16 antigorite phase a clinoenstatite water on the basis of these new data on the stability of antigorite earlier conclusions about dehydration depths in subducted serpentine bearing oceanic lithosphere have to be reconsidered the maximum pressure stability of antigorite according to reaction 16 extends between 44 and 55 kbar that is between about 130 and 160 km depths as opposed to about 75 kbar 220 km following ulmer and trommsdorff see above because many different thermal regimes are possible in subduction zones no specific dehydration depth can be expected but rather more continuous dehydration fronts in space and time

seismic investigations of the o higgins seamount group and juan fernandez ridge aseismic ridge emplacement and lithosphere hydration the o higgins seamount group is a cluster of volcanic domes located 120 km west of the central chilean trench on the crest of the juan fernandez ridge this aseismic hot spot track is subducting under south america triggering a belt of intraslab earthquake hypocenters extending about 700 km inland the juan fernandez ridge marks the southern boundary of a shallow subduction segment subduction of oceanic basement relief has been suggested as a cause for the flat slab segments characterizing the andean trench system the juan fernandez ridge however shows only moderate crustal thickening inadequate to cause significant buoyancy in 2001 wide angle seismic data were collected along two perpendicular profiles crossing the o higgins group we present tomographic images of the volcanic edifices and adjacent outer rise trench environment which indicate a magmatic origin of the seamounts dominated by extrusive processes high resolution bathymetric data yield a detailed image of a network of syngenetic structures reactivated in the outer rise setting a pervasive fault pattern restricted to the hot spot modified lithosphere coincides with anomalous low upper mantle velocities gained from a tomographic inversion of seismic mantle phases reduced uppermost mantle velocities are solely found underneath the juan fernandez ridge and may indicate mineral alterations enhanced buoyancy due to crustal and upper mantle hydration may contribute an additional mechanism for shallow subduction which prevails to the north after the southward migration of the juan fernandez ridge

subduction of the nazca plate beneath peru as determined from seismic observations a detailed reinvestigation of seismicity in the southern half of peru using data from a local seismic network and a focal mechanism study using p wave first motions reveal that for the first 100 km of descent the slab as defined by this seismicity enters at a normal dip angle near 30 and that below this depth it is bent to a nearly horizontal angle this horizontal slab extends eastward for about 300 km and then dips steeply below the thick continental lithosphere scsp observations are found to be consistent with the profile of the subducting plate as determined from the local seismicity beneath the region from southern peru to northern chile the nazca plate descends with an almost constant dip angle 30 down to at least 300 km depth between the 30 dipping plate in southern peru and the horizontal profile beneath central peru there is a contortion over a 100 km wide continuous lateral section of the subducting plate we infer that at least in the upper 150 km depth range the descending nazca plate is contorted rather than torn

lithospheric and upper mantle structure of central chile and argentina summary the tectonics of central chile and argentina have been greatly affected by the shallow dips of the subducting nazca plate which controlled patterns of magmatism and deformation nearly 1000 km away from the plate boundary we calculate receiver functions from data recorded by the charge array which transected the andes and sierras pampeanas in central chile and argentina to better constrain the crustal structure of this region beneath the northern transect of the charge array where the nazca slab flattens near 100 km we find the crust is over 60 km thick beneath the andes and thin to the east the thick crust however extends 200 km to the east of the high elevations estimates of vp vs obtained from receiver functions vary along ancient terrane boundaries exhibiting higher values to the west interestingly we observe that the amplitude of the phase corresponding to the moho on receiver functions diminishes to the west complicating our images of crustal structure we proposed that the observations presented here of thickened crust within a region of low elevations diminished receiver function arrivals and reports of high shear wave speeds atop of the mantle wedge overlying the shallowly subducted nazca slab can be explained by partial eclogitization of the lower crust the moho appears simpler across the southern transect where it can be identified near 50 km depth at its deepest point beneath the andes and shallows eastwards volcanism remains active near the latitudes of our southern transect and we observe multiple crustal lowvelocity zones indicative of regions of partial melt near the centres of volcanism signals related to the nazca slab remain more elusive suggestive of a small impedance contrast between the slab and overlying mantle

broadband ambient noise surface wave tomography across the united states 1 this study presents surface wave dispersion maps across the contiguous united states determined using seismic ambient noise two years of ambient noise data are used from march 2003 through february 2005 observed at 203 broadband seismic stations in the us southern canada and northern mexico cross correlations are computed between all station pairs to produce empirical green functions at most azimuths across the us coherent rayleigh wave signals exist in the empirical green functions implying that ambient noise in the frequency band of this study 5 100 s period is sufficiently isotropically distributed in azimuth to yield largely unbiased dispersion measurements rayleigh and love wave group and phase velocity curves are measured together with associated uncertainties determined from the temporal variability of the measurements a sufficient number of measurements 2000 is obtained between 8 and 25 s period for love waves and 8 and 70 s period for rayleigh waves to produce tomographic dispersion maps both phase and group velocity maps are presented in these period bands resolution is estimated to be better than 100 km across much of the us from 8 40 s period for rayleigh waves and 8 20 s period for love waves which is unprecedented in a study at this spatial scale at longer and shorter periods resolution degrades as the number of coherent signals diminishes the dispersion maps agree well with each other and with known geological and tectonic features and in addition provide new information about structures in the crust and uppermost mantle beneath much of the us

evolution of oceanic upper mantle structure abstract love and rayleigh wave phase velocities increase systematically with increasing age of oceanic lithosphere up to 150 ma the rates of increase differ between oceans and vary for different age intervals modeling of lithospheric age phase velocity relations indicates that the high velocity seismic lid thickens and the velocities in the sub lithospheric low velocity zone lvz increase with age at different rates between oceans the initial thickness of the lithosphere near mid ocean ridges as averaged by long period surface waves varies between 10 and 45 km and 100 ma lithosphere has thickness from 83 to 110 km for models with isotropic shear velocities hotspots in oceanic areas appear to modify oceanic lithosphere evolution producing deviations from average patterns with increasing age

spatial gaps in arc volcanism the effect of collision or subduction of oceanic plateaus abstract one of the major processes in the formation and deformation of continental lithosphere is the process of arc volcanism the plate tectonic theory predicts that a continuous chain of arc volcanoes lies parallel to any continuous subduction zone however the map pattern of active volcanoes shows at least 24 areas where there are major spatial gaps in the volcanic chains 200 km a significant proportion 30 of oceanic crust is subducted at these gaps all but three of these gaps coincide with the collision or subduction of a large aseismic plateau or ridge the idea that the collision of such features may have a major tectonic impact on the arc lithosphere including cessation of volcanism is not new however it is not clear how the collision or subduction of an oceanic plateau perturbs the system to the extent of inhibiting arc volcanism three main factors necessary for arc volcanism are 1 source materials for the volcanics either volatiles or melt from the subducting slab and or melt from the overlying asthenospheric wedge 2 a heat source either for the dehydration or the melting of the slab or the melting within the asthenosphere and 3 a favorable state of stress in the overlying lithosphere the absence of any one of these features may cause a volcanic gap to form there are several ways in which the collision or subduction of an oceanic plateau may affect arc volcanism the clearest and most common cases considered are those where the feature completely resists subduction causing local plate boundaries to reorganize this includes the formation of new plate bounding transform faults or a flip in subduction polarity in these cases subduction has slowed down or stopped and the lack of source material has created a volcanic gap there are a few cases most notably in peru chile and the nankai trough where the dip of subduction is so shallow that effectively no asthenospheric wedge exists to produce source material for volcanism the shallow dip of the slab may be a buoyant effect of the plateau imbedded in the oceanic lithosphere the cases which are the most enigmatic are those where subduction is continuous the oceanic plateau is subducted along with the slab and the dip of the slab is clearly steep enough to allow arc volcanism yet a volcanic gap exists in these areas the subducted plateau may have a fundamental effect on the physical process of arc volcanism itself the presence of a large topographic feature on the subducting plate may affect the stress state in the are by increasing the amount of decoupling between the two plates alternatively the subduction of the plateau may change the chemical processes at depth if either the water rich top of the plateau with accompanying sediments are scraped off during subduction or if the ridge is compositionally different

deep focus earthquakes and recycling of water into the earth s mantle for more than 50 years observations of earthquakes to depths of 100 to 650 kilometers inside earth have been enigmatic at these depths rocks are expected to deform by ductile flow rather than brittle fracturing or frictional sliding on fault surfaces laboratory experiments and detailed calculations of the pressures and temperatures in seismically active subduction zones indicate that this deep focus seismicity could originate from dehydration and high pressure structural instabilities occurring in the hydrated part of the lithosphere that sinks into the upper mantle thus seismologists may be mapping the recirculation of water from the oceans back into the deep interior of our planet

on the geometry of the nazca plate subducted under central chile 32 34 5 s as inferred from microseismic data abstract results obtained from the distribution of local seismicity recorded with a temporarily expanded network in central chile during two months in 1986 are presented data from the bulletin of regional seismicity for south america sisra between 1965 and 1981 and for depths over 50 km are added to extend the spatial covering all of this information evidences the geometry of the subducted plate in ten e w oriented cross sections 33 km wide the passage from subhorizontal subduction north of 33 s to normal subduction is well established as a continuous transition the geometry of the subducted lithosphere beneath the chilean territory remains unchanged throughout with a dip of 25 a difference between segments may be seen to the east of the high andes an almost horizontal seismic zone 300 km wide in the northern part narrows gradually as we move south and disappears completely near 33 s this correlates very well with the beginning of active volcanism it is shown that the vanishing of the horizontal part of the subducted lithosphere represents the transition from subhorizontal to normal subduction no clear activity is observed deeper than 150 km

are the lower planes of double seismic zones caused by serpentine dehydration in subducting oceanic mantle in a number of subduction zones earthquakes at 50 200 km depth define two dipping planes separated by 20 40 km that appear to merge downdip upper plane earthquakes are inferred to occur within the subducting oceanic crust whereas lower plane earthquakes occur in the subducting oceanic mantle beneath northeast japan the results of a new finite element heat transfer model suggest that the lower seismic plane cuts across isotherms at a shallow angle lower plane earthquakes occur at 550 800 c at 100 km depth and at 350 600 c at 160 km depth these conditions coincide with the dehydration reaction antigorite serpentine forsterite enstatite h 2 o which suggests that lower plane earthquakes may be triggered by dehydration embrittlement which in turn suggests that the subducting oceanic mantle is partially hydrated serpentinization may occur in the trench outer rise region where faulting may promote infiltration of seawater several tens of kilometers into the oceanic lithosphere if this hypothesis is correct current subduction zone h 2 o budgets may significantly underestimate the amount of bound h 2 o entering the subduction factory

geometry and brittle deformation of the subducting nazca plate central chile and argentina summary we use data from the chile argentina geophysical experiment charge broad band seismic deployment to refine past observations of the geometry and deformation within the subducting slab in the south american subduction zone between 30 s and 36 s this region contains a zone of flat slab subduction where the subducting nazca plate flattens at a depth of 100 km and extends 300 km eastward before continuing its descent into the mantle we use a grid search multiple event earthquake relocation technique to relocate 1098 events within the subducting slab and generate contours of the wadati benioff zone these contours reflect slab geometries from previous studies of intermediate depth seismicity in this region with some small but important deviations our hypocentres indicate that the shallowest portion of the flat slab is a

magnetic anomaly interpretation across the southern central andes 32 34 s the role of the juan fernandez ridge in the late tertiary evolution of the margin marine and terrestrial magnetic surveys have been integrated to study the tectonic structure of the convergent margin of chile between 32 34 s three magnetic domains have been identified oceanic continental margin and subaerial the oceanic domain has seafloor spreading anomalies 16 37 ma to 18 39 5 ma disturbed by anomalies of the juan fernandez hot spot chain in the continental margin the most prominent fabric are e w anomalies in the upper slope corresponding to onshore e w anomalies of large intrusive bodies onshore a n s lineament of short wavelength anomalies defines the roots of a cretaceous volcanic arc a resembling lineament offshore indicates a submerged older volcanic arc and that continental basement extends to 50 km landward of the trench absolute cenozoic plate motion for nazca and south american plates and dating of the juan fernandez chain provide a kinematic model of ridge continent collision the reconstruction indicates rapid southward migration of the collision point along 1400 km of the margin from 20 to 11 ma 20 cm yr 1 from 11 ma to present the collison point has migrated at a slower rate along 375 km of the margin 3 5 cm yr 1 the predicted location of the subducted portion of the juan fernandez chain coincides with the south edge of the southward migrating flat slab segment of the subducted lithosphere and with a cluster of deep earthquakes indicating a causal relationship in the last 10 myr the ridge has separated a sediment starved trench to the north where subduction erosion may dominate from a sediment filled trench to the south where recent sediment accretion dominates these observations indicate that subduction of the juan fernandez chain plays a major role in arc forearc tectonics

s wave velocity structure beneath the high lava plains oregon from rayleigh wave dispersion inversion abstract the high lava plains hlp hotspot track is a prominent volcanic lineament that extends from the southeast corner of oregon in the northern great basin to newberry volcano in the eastern cascades with the age of silicic volcanism decreasing along track to the northwest the hlp and newberry volcano are a rough mirror image to the eastern snake river plain and yellowstone but in the case of the hlp at an orientation strongly oblique to north american plate motion since this orientation is incompatible with plate motion over a fixed hotspot other proposed origins for the hlp such as asthenospheric inflow around a steepening slab residual effects of a columbia river steens plume backarc spreading and basin and range extension relate it to various tectonic features of the pacific northwest to begin distinguishing between these hypotheses we image upper mantle structure beneath the hlp and adjacent tectonic provinces with fundamental mode rayleigh waves recorded by stations of the usarray transportable array the recently initiated hlp seismic experiment the united states national seismograph network and the berkeley seismic network we estimate phase velocities along nearly 300 two station propagation paths that lie within and adjacent to the hlp and cross the region along two azimuths parallel to and perpendicular to the hlp track the dispersion curves which typically give robust results over the period range 16 171 seconds are grouped by tectonic region and the composite curves are inverted for s wave velocity as a function of depth we also fit a single plane wave to phase delays across the region to identify laterally homogeneous subregions the resulting variations in upper mantle structure correlate with variations in surface volcanism and tectonics the lowest velocities 4 1 km s occur at 50 km depth in the se corner of oregon where there has been extensive basaltic volcanism in the past 2 5 kyr and suggest uppermost mantle temperatures sufficient to produce basaltic partial melting while the seismic velocities of the uppermost mantle beneath the volcanic high lava plains are low relative to the standard tectonic north america tna model they are only slightly lower than those found for the adjacent northern great basin and they appear to be significantly higher than upper mantle velocities beneath the eastern snake river plain our results provide no evidence for a residual plume signature beneath the hlp region leaving open questions as to the origin of the hlp volcanic track itself

the role of hydraulic fractures and intermediate depth earthquakes in generating subduction zone magmatism the presence of magmatism and intermediate depth 70 300 km deep seismicity at subduction zones is at first sight surprising magmatism is unexpected because the subduction of cool oceanic lithosphere makes these regions the coldest in the mantle the current model for subduction zone magmatism is that water released from the subducting slab enters the relatively warm mantle wedge leading to a reduction in melting temperature and magmatism1 2 3 4 but there is a problem with this scheme because it is thought that water cannot leave the slab by porous flow to enter the wedge the occurrence of intermediate depth earthquakes is surprising because of the inhibitory effect of the very high frictional stress on faults expected from the high pressure at these depths one proposal put forward to explain intermediate depth seismicity is that high pore pressure might facilitate faulting by decreasing the friction5 6 7 the hypothesis presented here is that non percolating water provides the high pore pressure that the consequent faulting temporarily interconnects the water pores and when a sufficient vertical height of water is interconnected a hydrofracture is produced which transports the water out into the mantle wedge thereby generating subduction zone magmatism

three dimensional density model of the nazca plate and the andean continental margin 1 we forward modeled the bouguer anomaly in a region encompassing the pacific ocean 85 w and the andean margin 60 w between northern peru 5 s and patagonia 45 s the three dimensional density model that reproduces the gravity field is a continental scale representation of density structure to 410 km depth that characterizes the mantle and crust of the oceanic nazca plate subducted slab and continental margin with a minimum number of bodies we predefined the density of each body after studying the dependency of density on composition of crustal and mantle materials and pressure temperature conditions appropriate for the andean setting a database of independent geophysical information constrains the geometry of the top of the subducted slab locally the moho of the oceanic and continental crusts and indirectly the lithosphere asthenosphere boundary underneath the continental plate other boundaries notably the intracrustal density discontinuity separating upper from lower crust below the continent were not constrained and their geometry is the result of fitting the observed and calculated bouguer anomaly during forward modeling this contribution presents the model to the andean geoscientific community and contains some tools like a sensitivity analysis that helps potential users of the model to interpret its results we describe and discuss some of these results in order to illustrate the application of the model to the study of a wide range of phenomena e g modification of oceanic plate structure by hot spots shape of the subducted slab thermal structure of the continental lithosphere compensation mechanism and formation of orogenic relieve causes of andean segmentation

andean tectonics related to geometry of subducted nazca plate seismological and geological data show that tectonic segmentation of the andes coincides with segmentation of the subducted nazca plate which has nearly horizontal segments and 30 east dipping segments andean tectonics above a flat subducting segment between 28 s to 33 s are characterized by from west to east 1 a steady topographic rise from the coast to the crest of the andes 2 no significant quaternary and possibly neogene magmatism 3 a narrow belt of eastward migrating apparently thin skinned neogene to quaternary shortening of the andes and 4 plio pleistocene uplift of the crystalline basement on reverse faults in the pampeanas ranges from about 15 to 24 s over a 30 dipping subducted plate a west to east andes cross section includes 1 a longitudinal valley east of coastal mountains 2 an active neogene and holocene andesitic volcanic axis 3 the altiplano puna high plateau 4 a high neogene but inactive thrust belt eastern cordillera and 5 an active eastward migrating subandean thin skinned thrust belt tectonics above a steeply subducting segment south of 33 s are similar west of the volcanic axis but quite different to the east early cenozoic tectonics of western north america were quite similar to the neogene andes however duration of segmentation was longer and the width of deformation was greater in the western united states patterns of crustal seismicity are systematically related to plio quaternary structural provinces implying that current deformational processes have persisted since at least the pliocene horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench above flat subducting segments crustal seismicity occurs over a broad region whereas over steep segments it is confined to the narrow thrust belt strain patterns in the forearc region are complex and perhaps extensional and a broad region of the altiplano puna and eastern cordillera appears to be aseismic

variations of slab dip and overriding plate tectonics during subduction insights from analogue modelling we present small scale laboratory models of oceanic subduction in which plates motion is imposed by lateral boundary conditions the oceanic plate moves trenchward at constant speed and subducts below a fixed overriding plate in this configuration the long term process of subduction is not steady state slab interaction with the upper mantle lower mantle boundary results in periods of slab flattening during which the dip of the slab diminishes followed by periods of slab steepening the overriding plate tectonic regime is influenced by the dynamics of subduction slab anchoring favouring trench perpendicular shortening when the slab is anchored slab flattening further favours shortening while slab steepening favours extension or smaller shortening rates non steady state long term subduction may explain part of the variability of slab geometries evidenced by statistical analyses of present day subduction zones experiments suggest that despite boundary conditions applied on the converging plates do not change tectonics pulses within the overriding plate may be caused by this non steady state behaviour

the subduction of young lithosphere studies using a variety of techniques of the subduction beneath western south america and the southwest honshu kyushu region of japan indicate that volcano free segments occur where the subducted and continental lithospheres remain in contact without intervening asthenosphere the subduction is initiated at normal dip angles but the plate deforms at some depth 100 km under central peru to travel horizontally immediately beneath continental lithosphere the most plausible reason for this geometry is that the subducted plate is buoyant a model is developed constrained by age of the plate bathymetry and heat flow estimates of the density of oceanic plates as a function of age show that younger ocean floor may be less dense than the asthenosphere into which it subducts if the high density tranformation of crustal basalt to eclogite is retarded by low temperatures the plate can remain buoyant to considerable depth for long periods of time heat flow data from western south america are consistent with this model

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

age constraints on the tectonic evolution and provenance of the pie de palo complex cuyania composite terrane and the famatinian orogeny in the sierra de pie de palo san juan argentina abstract new u pb age determinations confirm earlier interpretations that the strongly deformed and metamorphosed mafic and intermediate igneous rocks of the pie de palo complex represent a mesoproterozoic fragment of suprasubduction zone oceanic crust a gabbroic pegmatite interpreted to have formed during arc rifting or subsequent back arc spreading yielded a u pb age of 1204 5 3 4 7 ma highly tectonized ultramafic mafic cumulates occurring at the structural base of the pie de palo complex and previously interpreted to represent remnants of a primitive arc phase prior to rifting and back arc spreading could not be dated but should be older than 1204 ma if these inferences are correct tabular sill like bodies of leucogabbro diorite and calc alkaline tonalite granodiorite sills yielded ages of 1174 43 and 1169 8 7 ma respectively they may represent a younger more evolved arc phase established after arc rifting or a younger tectonically unrelated mesoproterozoic arc shrimp analysis of metamorphic zircon rims with low th u ratios in vvl 110 gave a 206pb 238u age of 455 10 ma similar to lower intercept dates determined by discordia lines combined these data indicate that the bulk of the amphibolite facies metamorphism present in the pie de palo complex was generated during the famatinian orogeny analysis of six single detrital zircon grains in a metasedimentary quartzofeldspathic garnet mica schist tectonically interleaved with the igneous rocks of the pie de palo complex and tentatively correlated with the difunta correa metasedimentary sequence of other workers yielded three age populations 1150 1160 ma 1050 1080 ma and 665 ma indicating that these sedimentary rocks were deposited during the late neoproterozoic or early paleozoic in addition they confirm structural evidence that intercalation of rocks of the pie de palo complex with isolated slivers of these sedimentary rocks is due to tectonic imbrications these ages are also consistent with a laurentian provenance and earlier interpretations that these rocks once represented a sedimentary cover to the pie de palo complex the zircon population of 1050 1080 ma could be derived from grenville age felsic plutons identified elsewhere in the pie de palo complex by other workers however no evidence has been found in our samples for a grenville age orogenic event invoked previously to explain accretion of the oceanic pie de palo complex to laurentia prior to the late neoproterozoic early cambrian rifting and drift of cuyania

crustal thickness beneath the andes and sierras pampeanas at 30 s inferred from pn apparent phase velocities 1 we obtained a 2d crustal model beneath central chile and western argentina using apparent pn phase velocities recorded along an ew trending transect at 30 s this model is characterized by a thick 65 km crustal root beneath the high cordillera a 60 km thick crust beneath the precordillera and a gradual shallowing of moho depths beneath the sierras pampeanas from 55 km in the west to 40 km in the east beneath the sierras pampeanas where very little crustal shortening is observed the thick crust must be explained by means other than simple tectonic shortening isostatically balanced crustal thickness is consistently shallower than our model shows suggesting the existence of dynamic forces possibly linked to the flat slab

curie point depths beneath precordillera cuyana and sierras pampeanas obtained from spectral analysis of magnetic anomalies abstract a gravity and magnetic survey has been carried out with the purpose of investigating geophysical features of the crusts beneath three geological provinces in western argentina cuyo precordillera the sierras pampeanas of san juan and la rioja and famatina system the results of which are displayed in three maps bouguer anomaly total field magnetic anomaly and total field reduced to the pole the top and bottom boundaries of the magnetized crust were calculated from power density spectra of the total field anomalies from our terrestrial database in 90 2d windows the depths obtained for the bottom of magnetized crust are assumed to correspond to curie point depths the values thus obtained for the precordillera range between 29 and 40 km whereas for the sierras pampeanas in the sierra de pie de palo and other mountain chains along the bermejo desaguadero lineament or valle fertil lineament such depth ranges between 20 and 35 km these results are consistent with curie point depths determined on different continental regions in the world a map of regional heat flow has been prepared in the present work based on the depth of the curie point isotherm which shows that heat flow patterns in precordillera are different from those found in sierras pampeanas a significant heat flow extending in northeast southwest direction seems to be directly related to juan fernandez ridge trace the results of the present investigations also point out a possible relationship between the base of the magnetized crust and the boundary separating the brittle from the ductile crustal regime

thermal modelling of the laramide orogeny testing the flat slab subduction hypothesis abstract the laramide orogeny is the late cretaceous to palaeocene 80 55 ma orogenic event that gave rise to the rocky mountain fold and thrust belt in canada the laramide block uplifts in the usa and the sierra madre oriental fold and thrust belt in mexico the leading model for driving laramide orogenesis in the usa is flat slab subduction whereby stress coupling of a subhorizontal oceanic slab to the upper plate transmitted stresses eastwards producing basement cored block uplifts and arc magmatism in the foreland the thermal models presented here indicate that arc magma generation at significant distances inboard of the trench 600 km during flat slab subduction is problematic this conclusion is consistent with the coincidence of volcanic gaps and flat slab subduction at modern convergent margins lawsonite eclogite xenoliths erupted through the colorado plateau in oligocene time are inferred to originate from the subducted farallon slab and indicate that the laramide flat slab subduction zone was characterised by a cold thermal regime thermal modelling indicates that this regime can be produced by flat slab subduction of old 50 myr oceanic lithosphere at high convergence rates in the canadian and mexican portions of the laramide orogen the coeval development of a magmatic arc within 300 km of the trench refutes the existence of flat slab subduction in these regions it is proposed that subduction of an oceanic plateau aseismic ridge may have overcome the negative buoyancy inherent in old oceanic lithosphere and resulted in a spatially restricted zone of flat slab subduction in the usa these findings cast doubt on the flat slab model as a primary means of driving laramide orogenesis along its entire length and instead point to the need for an alternative mechanism for cordilleran wide laramide orogenesis

crustal wedging triggering recent deformation in the andean thrust front between 31 s and 33 s sierras pampeanas precordillera interaction 1 we document a new model of crustal structure of the andean front in argentina where numerous historic earthquakes destroyed the cities of mendoza in 1861 ms 7 and san juan in 1944 mw 7 0 the cerro salinas anticline is formed above the west directed cerro salinas thrust an east facing monocline with an amplitude of about 2 km folds the cerro salinas thrust and overlying neogene succession this monocline is formed above a blind crustal thrust in the basement its dip of 14 west is inferred from fold geometry this thick skinned east directed blind thrust and the thin skinned west directed cerro salinas thrust define a tectonic wedge the wedge tip occurs at a depth of 5 4 km growth of the monocline after 8 5 ma is revealed on multichannel seismic msc profile 31017 repsol ypf rates of cerro salinas thrust displacement are of the order of 1 mm yr whereas vertical uplift of 0 45 mm yr results from the combined displacement on the cerro salinas thrust and growth of east facing monocline the lateral extent of the east directed crustal blind ramp corresponds with the along strike extent of the eastern precordillera when combined with the low displacement rate a long earthquake recurrence interval is implied smaller magnitude earthquakes however indicate that segments of the blind thrust ramps ruptured in historic events if all the segments of the blind thrust ruptured together the fault area is 7000 km 2 and could produce a mw 7 7 earthquake the crustal wedge model provides new constraints on the origin and potential size of earthquakes that threaten the densely populated region

continental and oceanic crustal structure of the pampean flat slab region western argentina using receiver function analysis new high resolution results summary the pampean flat slab of central chile and argentina 30 32 s has strongly influenced cenozoic tectonics in western argentina which contains both the thick skinned basement cored uplifts of the sierras pampeanas and the thin skinned andean precordillera fold and thrust belt in this region of south america the nazca plate is subducting nearly horizontally beneath the south american plate at 100 km depth to gain a better understanding of the deeper structure of this region including the transition from flat to normal subduction to the south three iris passcal arrays of broad band seismic stations have been deployed in central argentina using the dense siembra array combined with the broader charge and esp arrays the flat slab is imaged for the first time in 3 d detail using receiver function rf analysis a distinct pair of rf arrivals consisting of a negative pulse that marks the top of the oceanic crust followed by a positive pulse which indicates the base of the oceanic crust can be used to map the slab s structure depths to moho and oceanic crustal thicknesses estimated from rf results provide new more detailed regional maps an improved depth to continental moho map shows depths of more than 70 km in the main cordillera and 50 km in the western sierras pampeanas that shallow to 35 km in the eastern sierras pampeanas depth to moho contours roughly follow terrane boundaries offshore the hotspot seamount chain of the juan fernandez ridge jfr is thought to create overthickened oceanic crust providing a mechanism for flat slab subduction by comparing synthetic rfs based on various structures to the observed rf signal we determine that the thickness of the oceanic crust at the top of the slab averages at least 13 19 km supporting the idea of a moderately overthickened crust to provide the additional buoyancy for the slab to remain flat the overthickened region is broader than the area directly aligned with the path of the jfr however and indicates along with the slab earthquake locations that the flat slab area is wider than the jfr volcanic chain observed in the offshore bathymetry further rfs indicate that the subducted oceanic crust in the region directly along the path of the subducted ridge is broken by trench parallel faults one explanation for these faults is that they are older structures within the oceanic crust that were created when the slab subducted alternatively it is possible that faults formed recently from tectonic underplating caused by increased interplate coupling in the flat slab region