neotectonic reactivation of the western section of the malargue fold and thrust belt tromen volcanic plateau southern central andes abstract this study examines the neotectonic deformation and development of the tromen massif a quaternary retroarc volcanic field located in the western section of the malargue fold and thrust belt in the southern central andes the linkages between neotectonic deformation in the intra arc zone and the recent retroarc structures of the tromen volcanic plateau are not clearly understood these retroarc deformations affect the mid section of the fold and thrust belt leaving to the east a 200 km wide deformed zone that can be considered inactive over the last 12 10 ma this out of sequence deformation west of the orogenic front area has not been previously addressed in detail in this study exhaustive mapping is used to describe and discriminate structures with a neotectonic component from those fossilized by pleistocene strata two balanced cross sections are constructed showing the distribution of the youngest deformations and their relation to pre miocene structures an important means for evaluating this is the morphometric and morphological analyses that allowed identification of perturbations in the fluvial network associated with active structures in a broader perspective neotectonic activity in the fold and thrust belt is discussed and inferred to be caused by local mechanical weakening of the retroarc zone due to injection of asthenospheric material evidenced by magnetotelluric surveys thus deformation imposed by the oblique convergence between south american and nazca plates while to the south being limited to the liquine ofqui fault system that runs through the arc zone in the retroarc area is located at the site of magmatic emplacement presumably in association with a thermally weakened upper crust this control exemplifies the relationship that exists between surficial processes magmatic emplacement and upper asthenospheric dynamics in the southern central andes

fluid and melt pathways in the central chilean subduction zone near the 2010 maule earthquake 35 36 s as inferred from magnetotelluric data

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

plume overriding triggers shallow subduction and orogeny in the southern central andes abstract plate tectonic theory implies that mantle plumes may be eventually overridden by lithosphere during continental drift these events have particular tectonomagmatic consequences for active margins and hence related orogenic processes since the first documentation of plume overriding and the definition of the plume modified orogeny concept only few examples have been recognized in the geologic record in this study we analyze the neogene tectonic evolution of the southern central andes between 35 and 38 s and its potential relation to the subduction of the payenia plume as a recent analogue of this process through a series of tectonic reconstructions we show that progressive payenia plume overriding correlates with neogene arc front migrations linked to slab shallowing fold belt reactivation in the main cordillera and intraplate contraction in the san rafael block additionally nazca slab tear determined from tomographic analyses and subsequent diachronous steepening of the subducted plate may also be an aftermath of plume subduction as often described in the final stages of plume modified orogeny finally we propose a modern analogue for processes previously described dating back to the mesoproterozoic which provides further insights into these complex settings

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

provenance and time constraints on the formation of the first bend of the yangtze river 1 the upper yangtze river flows southward on the southeastern tibet characterizing by uniquely low and continuous relief the river makes a sharp turn at shigu heading northeast and forms the first bend of the yangtze river many previous studies assumed southward flow of the ancestral yangtze river from shigu to the south china sea however field evidence of southward flow of the paleo yangtze is lacking in this paper we report our identification based on detrital zircon u pb age distributions of a range of fluvial sands left by the paleo yangtze in tongdian madeng and nanjian basins cosmogenic10be and 26al burial dating provides burial ages for these fluvial sands from 1 7 to over 8 7 ma rerouting of the yangtze river therefore occurred within the last 1 7 ma postdating the major uplift of the central tibet we attribute the rerouting of the yangtze river to response to activation of the dali fault system and in a larger scale initiation of crustal deformation by clockwise rotation around eastern himalayan syntaxis 2 4 ma ago reorganization of the yangtze drainage pattern most likely reflects regional uplift and displacement due to lower crust flowing beneath major faults in the southeastern tibet and yunnan

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

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

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

observations of high frequency internal waves in the coastal ocean dynamics region current meter data from the second coastal ocean dynamics experiment code ii for july 1982 are analyzed for internal waves in the 6 to 40 cycles per day cpd frequency band it is found that the wave field is anisotropic and that the current ellipses are oriented in approximately the cross isobath direction the squares of the ratio of the major to minor axes of the current ellipses the ellipticity are consistent with a continuum of internal waves propagating onshore but are not consistent with a single wave propagating onshore the reduction of internal wave energy across the shelf is consistent with propagation from the deep ocean or shelf break as is the correlation between vertical velocities and velocities parallel to the minor axis however there is evidence for the generation of additional internal wave energy on the shelf in the evolution of the current ellipses across the shelf and in the bluing of the internal wave spectra across the shelf internal wave energy levels are elevated by a factor of 1 5 to 5 above garrett and munk 1972 levels at the moorings on the 130 and 365 m isobaths the first vertical mode dominates at the 130 m isobath the only mooring for which the vertical modal analysis was done

holocene yardangs in volcanic terrains in the southern andes argentina yardangs of different sizes were developed in the payun matru volcanic field a semiarid area east of the southern andes mountains yardangs from volcanic terrains have not been described previously from earth although studies from mars interpreted linear ridges found by mariner and viking images as yardangs the payun matru volcanic field is an extensive plateau at 2000 m a s l covered by basaltic lava and ignimbrite flows strong westerly winds affect the extensive plateau micro and mesoyardangs are formed on the ignimbrite rock blanket and macroforms or megayardangs several kilometres in length are developed in the basaltic lava flows as long parallel troughs they all have a distinctive 320 azimuth which is the prevailing wind direction no yardang features are noted in the more recent lava flows younger than 1000 years indicating that their formation needed a longer time or they developed in earlier periods with stronger winds the yardang development is explained by the strong unidirectional winds the poor vegetational cover due to the aridity of the region the available quartz sand and volcanic ash particles as abrasive agents and the volcanic lithology texture and flow structure copyright 2001 john wiley sons ltd

filling the north european early middle eocene ypresian lutetian boundary gap insights from the pyrenean continental to deep marine record abstract the early middle eocene ypresian lutetian transition is represented by a hiatus in many north european sections including those in which the classic stratotypes were originally defined however the global stratotype section and point of the lutetian stage which is still pending definition should be placed at a globally correlatable event included within that unrepresented interval the pyrenean eocene outcrops display sedimentary successions that offer the rare opportunity to analyse the ypresian lutetian boundary interval in almost continuous sections and in very different settings seven reference stratigraphic sections were selected on the basis of their quality and correlated by means of biomagnetostratigraphic data this correlation framework casts light on the sequence of chronostratigraphic events that characterize the ypresian lutetian boundary interval which may prove useful in defining the main correlation criterion of the base of the lutetian all of the pyrenean sections show a similar sedimentary evolution despite being up to 350 km apart from each other containing deposits of different origins intrabasinal carbonate sediments siliciclastic sediments sourced from the iberian plate and terrigenous sediments sourced from the uplifting pyrenees and despite having been accumulated in different sedimentary environments from continental to deep marine and in different geodynamic settings piggy back basin foreland basin and cratonic margin this common evolution can be readily interpreted in terms of a sea level driven depositional sequence whose lowstand and transgressive systems tracts are included within the ypresian lutetian boundary interval the pyrenean ypresian lutetian depositional sequence can reasonably be correlated with depositional sequences from classic north european areas shedding light on the palaeoenvironmental history which in those areas has not been recorded furthermore these depositional sequences may possibly correlate with others from the antarctic ocean and from new jersey as well as with oceanic temperature variations suggesting that they might be the result of climatically driven glacioeustatic sea level changes should this hypothesis prove correct it would confirm previous suggestions that the onset of antarctic glaciations needs to be backshifted to the late ypresian at least

a geophysical perspective on mantle water content and melting inverting electromagnetic sounding data using laboratory based electrical conductivity profiles abstract this paper applies electromagnetic sounding methods for earth s mantle to constrain its thermal state chemical composition and water content we consider long period inductive response functions in the form of c responses from four stations distributed across the earth europe north america asia and australia covering a period range from 3 9 to 95 2 days and sensitivity to 1200 km depth we invert c responses directly for thermo chemical state using a self consistent thermodynamic method that computes phase equilibria as functions of pressure temperature and composition in the na 2 o cao feo mgo al 2 o 3 sio 2 model system computed mineral modes are combined with recent laboratory based electrical conductivity models from independent experimental research groups yoshino 2010 and karato 2011 to compute bulk conductivity structure beneath each of the four stations from which c responses are estimated to reliably allocate water between the various mineral phases we include laboratory measured water partition coefficients for major upper mantle and transition zone minerals this scheme is interfaced with a sampling based algorithm to solve the resulting non linear inverse problem this approach has two advantages 1 it anchors temperatures composition electrical conductivities and discontinuities that are in laboratory based forward models and 2 at the same time it permits the use of geophysical inverse methods to optimize conductivity profiles to match geophysical data the results show lateral variations in upper mantle temperatures beneath the four stations that appear to persist throughout the upper mantle and parts of the transition zone calculated mantle temperatures at 410 and 660 km depth lie in the range 1250 1650 c and 1500 1750 c respectively and generally agree with the experimentally determined temperatures at which the measured phase reactions olivine spinel and spinel ferropericlase perovskite occur the retrieved conductivity structures beneath the various stations tend to follow trends observed for temperature with the strongest lateral variations in the uppermost mantle for depths 300 km conductivities appear to depend less on the particular conductivity database conductivities at 410 km and at 660 km depth are found to agree overall with purely geophysically derived global and semi global one dimensional conductivity models both electrical conductivity databases point to 2 o in the upper mantle for transition zone minerals results from the laboratory database of yoshino 2010 suggest that a much higher water content up to 2 wt h 2 o is required than in the other database karato 2011 which favors a relatively dry transition zone 2 o incorporating laboratory measurements of hydrous silicate melting relations and available conductivity data allows us to consider the possibility of hydration melting and a high conductivity melt layer above the 410 km discontinuity the latter appears to be 1 regionally localized and 2 principally a feature from the yoshino 2010 database further there is evidence of lateral heterogeneity the mantle beneath southwestern north america and central china appears wetter than that beneath central europe or australia

partial melt or aqueous fluid in the mid crust of southern tibet constraints from indepth magnetotelluric data summary the indepth project has applied modern geophysical techniques to the study of the crustal structure and tectonic evolution of the tibetan plateau in the lhasa block seismic reflection surveys in 1994 detected a number of bright spots at 15 20 km depths that indicate zones of crustal fluids aqueous fluids or partial melt coincident magnetotelluric mt data collected in 1995 detected a major zone of high electrical conductivity at the same depth as the brightspots using constrained inversion the mt data require a minimum crustal conductance of 6000 s this abnormally high electrical conductance can be best explained by a layered model with fluids partial melt aqueous fluids or a combination of partial melt and aqueous fluids the non uniqueness of the mt method means that a wide range of melt fraction thickness combinations for the above models could all explain the 6000 s conductance to distinguish between these three models other geophysical and geological data are required reflection seismic data suggest that a high fluid content 15 per cent is present at the top of the layer the amplitude versus offset data suggest that the top of this layer may be aqueous fluids rather than partial melt passive seismic data imaged a 20 km thick layer of lower fluid content that is probably partial melt petrological studies suggest that concentrations of aqueous fluids above 0 1 per cent at mid crustal depth cannot be sustained taken together these data show that the high conductivity in southern tibet is most probably the result of a relatively thin layer of aqueous fluids 100 200 m overlying a thicker zone of partial melt 10 km

crustal and mantle structure beneath the southern payenia volcanic province using gravity and magnetic data the auca mahuida volcanic field lies on the southernmost payenia volcanic province one of the broadest retroarc volcanic plateaux in the southern central andes 38 s this voluminous basaltic flooding of quaternary age was originated from a deep asthenospheric source interpreted as a mantle plume product of changing slab dynamics the geometry of this source is deduced from magnetotelluric data but the limited spatial coverage of this array does not allow a detailed resolution of this anomaly in order to present a detailed geometry of the conductive anomaly and related crustal magmatic bodies we used multiple data sources we combined magnetic and bouguer anomalies curie isotherm depth t c elastic thickness t e and moho depth derived from the global earth magnetic anomaly grid emag2 and terrestrial gravity measurements all together in a holistic geophysical analysis the magnetic data depict a nearly 200 km in diameter circular anomaly that would correspond to a dense body according to the bouguer anomaly geoid data from the gravity field model eigen 6c4 have been filtered in order to isolate deeper mass influences and visualize the asthenospheric upwelling previously described from magnetotelluric data moho inversion yields a crustal attenuation at 36 to 32 km depth coinciding with t e below 20 km depth and a shallow t c 15 km depth at the site where geoid positive undulation was calculated finally surface analysis allowed defining a topographic swell compatible with the dimensions of the identified magnetic anomaly where the main rivers deviated potentially due to a recent base level change 2018 american geophysical union all rights reserved

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

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

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

cyclicity and basin axis shift in a piggyback basin towards modelling of the eocene tremp ager basin south pyrenees spain

integrating magnetotelluric and seismic images of silicic magma systems a case study from the laguna del maule volcanic field central chile

stagnant slab wet plumes and cenozoic volcanism in east asia abstract data from seismology and mineral physics indicate the existence of low velocity zones in the deep part of the upper mantle wedge and the mantle transition zone under east asia zhao and ohtani 2009 zhao et al 2009 it suggests a link between the deep subduction and stagnation and deep dehydration of the pacific slab below this area and the cenozoic volcanism observed at the surface here we present a numerical model describing the dynamics of a hydrated layer covering a slab that is stagnating in the earth tm s mantle transition zone specifically our model includes the effects of water on density and viscosity of the mantle minerals we conclude that the presence of water is likely to induce rayleigh taylor type instabilities wet plumes that are able to transport water up to the base of the surficial lithosphere in a relatively short time scale a few million years under initial conditions representative of the subducting pacific slab under east asia melting is induced where wet plumes reach the base of the lithosphere according to our results and analysis of the known composition of cenozoic basalts from southwest japan iwamori 1991 iwamori 1992 wet plumes and their melting induced by water related to the stagnant slab provides an appealing hypothesis to explain cenozoic volcanism in east asia

a new middle to late eocene continental chronostratigraphy from ne spain abstract the calibration of the european paleogene mammal biochronology with the international reference geochronological scale is unconstrained due to the lack of direct marine continental correlations and insufficient robust magnetostratigraphic data for mammal fossil bearing sequences the currently accepted european continental biochronology argues for a correlation of the paleogene mp14 to mp16 mammal reference levels with the bartonian stage and of the mp17 to mp20 levels with the priabonian however new magnetostratigraphic data of the continental sediments of the south central pyrenees substantially challenge this chronology our new results from the la pobla de segur basin and sierra de sis have been integrated into the framework of previous marine continental stratigraphic correlations and magnetostratigraphic data of the south central pyrenees in this new chronology mp14 and mp15 reference levels correspond to the lutetian whereas mp16 and early mp17 correlate with the early bartonian this calibration represents a significant revision of currently accepted marine continental correlations in europe but is not in conflict with the available biostratigraphic data of the south pyrenean ebro foreland basin and other western european reference basins

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

retroarc volcanism in the northern san rafael block 34 35 30 s southern central andes occurrence age and tectonic setting article i nfo one of the major retroarc volcanic provinces in the southern central andes 34 and 37 s is developed in the andean foothills of the san rafael region between the orogenic front and foreland basement uplifts of late miocene age here we present the first comprehensive geochronological study of the quaternary volcanism previously dated mainly on the basis of stratigraphy the new unspiked k ar radiometric and two radiocarbon determinations encompass many volcanic centers most of them monogenetic and of basaltic composition exposed between 34 and 35 30 s the data constrains the basaltic volcanism to between 1 8 ma and the holocene the spatiotemporal distribution of the ages indicates that eruption in the retroarc was episodic with some distinct patterns the orogenic front of the san rafael block is associated with 1 8 0 7 ma volcanic eruptions while the malargue fold and thrust belt front in the andean foothills is related to younger eruptions produced at 0 1 0 01 ma both areas are associated with late cenozoic normal faults that dismembered an uplifted a late miocene peneplain as indicated by younger over older fault relationships between paleozoic rocks and tertiary strata this linkage indicates a major relationship between pleistocene holocene retroarc eruptions of the basaltic centers and extensional collapse of the foreland region that shows a migration of the last volcanic activity towards the trench

partial melting below the magmatic arc in the central andes deduced from geoelectromagnetic field experiments and laboratory data abstract magnetotelluric and geomagnetic deep soudings in northern chile revealed a pronounced high conductivity zone hcz below the western cordillera which constitutes the present magmatic arc with active volcanism of the south american continental margin conductivities in the range of 1 s m are observed the anomalously high conductivities in a broad depth range from approximately 20 km to at least 60 km are interpreted in terms of partial melting other geophysical observations such as a zone of low seismic velocities lvz at similar depths high heat flow values 100 mw m2 and a pronounced negative anomaly in the residual gravity field are also considered impedance spectroscopic laboratory experiments up to and in the temperature range of partial melting were performed under controlled oxygen fugacities at sub solidus temperatures electrical behavior is described by defect electrons with an activation energy of 1 34 ev and a conductivity of 2 5 ms m at 900 c model calculations using a modified brick layer model mbl were compared with experimental observations a good agreement between calculations and experiments is achieved with an electrical resistivity of the melt phase of 7 s m at 1250 c assuming an activation energy of 1 ev the same mbl model is used to calculate melt proportions beneath the western cordillera between 14 and 27 vol of interconnected melt are necessary to explain the observed hcz the stability of the melt rich crust is explained by a dynamic melting crystallisation behavior during crustal anatexis and by magma filled dikes

a geodynamic model linking cretaceous orogeny arc migration foreland dynamic subsidence and marine ingression in southern south america abstract this study synthesizes the tectonomagmatic evolution of the andes between 35 30 s to 48 s with the aim to spotlight early contractional phases on andean orogenic building and to analyze their potential driving processes we examine early tectonic stages of the different fold thrust belts that compose this andean segment additionally we analyzed the spatio temporal magmatic arc evolution as a proxy of dynamic changes in andean subduction during critical tectonic stages of orogenic construction this revision proposes a hypothesis related the existence of a continuous large scale flat subduction setting in cretaceous times with a similar size to the present largest flat slab setting on earth this potential process would have initiated diachronically in the late early cretaceous and achieved full development in late cretaceous to earliest paleocene times constructing a series of fold thrust belts on the retro arc zone from 35 30 s to 48 s moreover we assess major paleogeographic changes that took place during flat slab full development in maastrichtian danian times at this moment an enigmatic atlantic derived marine flooding covered the patagonian foreland reaching as far as the andean foothills based on flexural and dynamic topography analyses we suggest that focused dynamic subsidence at the edge of the flat slab may explain sudden marine ingression previously linked to continental tilting and orogenic loading during a high sea level global stage finally flat subduction destabilization could have triggered massive outpouring of synextensional intraplate volcanic rocks in southern south america and the arc retraction in late paleogene to early neogene times

three dimensional electrical conductivity in the mantle beneath the payun matru volcanic field in the andean backarc of argentina near 36 5 s evidence for decapitation of a mantle plume by resurgent upper mantle shear during slab steepening

electrical conductivity of the pampean shallow subduction region of argentina near 33 s evidence for a slab window 1 we present a three dimensional 3 d interpretation of 117 long period 20 4096 s magnetotelluric mt sites between 31 s and 35 s in western argentina they cover the most horizontal part of the pampean shallow angle subduction of the nazca plate and extend south into the more steeply dipping region sixty two 3 d inversions using various smoothing parameters and data misfit goals were done with a nonlinear conjugate gradient nlcg algorithm a dominant feature of the mantle structure east of the horizontal slab is a conductive plume rising from near the top of the mantle transition zone at 410 km to the probable base of the lithosphere at 100 km depth the subducted slab is known to descend to 190 km just west of the plume but the wadati benioff zone cannot be traced deeper if the slab is extrapolated downdip it slices through the plume at 250 km depth removal of portions of the plume or blocking vertical current flow at 250 km depth significantly changes the predicted responses this argues that the plume is not an artifact and that it is continuous the simplest explanation is that there is a wedge shaped slab window that has torn laterally and opens down to the east with its apex at the plume location stress within the slab and seismic tomography support this shape its northern edge likely explains why there is no deep seismicity south of 29 s

the temporal evolution of back arc magmas from the auca mahuida shield volcano payenia volcanic province argentina in order to better constrain the temporal volcanic activity of the back arc context in payenia volcanic province pvp argentina we present new k ar dating petrographic data major and trace elements from 23 samples collected on the auca mahuida shield volcano our new data coupled with published data show that this volcano was built from about 1 8 to 1 0 ma during five volcanic phases and that auca mahuida magmas were extracted from at least two slightly different oib type mantle sources with a low partial melting rate the first one containing more garnet was located deeper in the mantle while the second contains more spinel and was thus shallower the high mgo basalts or primitive basalts and the low mgo basalts or evolved basalts produced from the deeper and shallower lherzolite mantle sources respectively are found within each volcanic phase suggesting that both magmatic reservoirs were sampled during the 1 myr lifetime of the auca mahuida volcano however a slight increase of the proportion of low mgo basalts as well as of magmas sampled from the shallowest source can be observed through time similar overall petrological characteristics found in the pleistocene holocene basaltic rocks from los volcanes and auca mahuida volcano suggest that they originated from the same magmatic source consequently it can be proposed that the thermal asthenospheric anomaly is probably still present beneath the pvp finally our data further support the hypothesis that the injection of hot asthenosphere with an oib mantle source signature which was triggered by the steepening of the nazca subducting plate induced the production of a large volume of lavas within the pvp since 2 ma

mantle subducting slab structure in the region of the 2010 m8 8 maule earthquake 30 40 s chile we present a new tomographic model of the mantle in the area of the 2010 m8 8 maule earthquake and surrounding regions increased ray coverage provided by the aftershock data allows us to image the detailed subducting slab structure in the mantle from the region of flat slab subduction north of the maule rupture to the area of overlapping rupture between the 1960 m9 5 and the 2010 m8 8 events to the south we have combined teleseismic primary and depth phase arrivals with available local arrivals to better constrain the teleseismic earthquake locations in the region which we use to conduct nested regional global tomography the new model reveals the detailed structure of the flat slab and its transition to a more moderately dipping slab in the maule region south of the maule region a steeply dipping relic slab is imaged from 200 to 1000 km depth that is distinct from the moderately dipping slab above it and from the more northerly slab at similar depths we interpret the images as revealing both horizontal and vertical tearing of the slab at 38 s to explain the imaged pattern of slab anomalies in the southern portion of the model in contrast the transition from a horizontal to moderately subducting slab in the northern portion of the model is imaged as a continuous slab bend we speculate that the tearing was most likely facilitated by a fracture zone in the downgoing plate or alternatively by a continental scale terrane boundary in the overriding plate

the late jurassic panjeh submarine volcano in the northern sanandaj sirjan zone northwest iran mantle plume or active margin abstract the tectonic setting in which jurassic igneous rocks of the sanandaj sirjan zone sasz of iran formed is controversial sasz igneous rocks are mainly intrusive granodiorite to gabbroic bodies which intrude early to middle jurassic metamorphic basement jurassic volcanic rocks are rare here we report the age and petrology of volcanic rocks from the panjeh basaltic andesitic rocks complex in the northern sasz southwest of ghorveh city the panjeh magmatic complex consists of pillowed and massive basalts andesites and microdioritic dykes and is associated with intrusive gabbros the overall sequence and relations with surrounding sediments indicate that this is an unusually well preserved submarine volcanic complex igneous rocks belong to a metaluminous sub alkaline medium k to high k calc alkaline mafic suite characterized by moderate al2o3 13 7 17 6 wt and variable fe2o3 6 0 12 6 wt and mgo 0 9 11 1 wt contents zircon u pb ages 145 149 ma define a late jurassic tithonian age for magma crystallization and emplacement whole rock compositions are enriched in th u and light rare earth elements lrees and are slightly depleted in nb ta and ti the initial ratios of 87sr 86sr 0 7039 0 7076 and end t values 1 8 to 4 3 lie along the mantle array in the field of ocean island basalts and subcontinental metasomatized mantle immobile trace element ti v zr y nb yb th and co behavior suggests that the mantle source was enriched by fluids released from a subducting slab i e deep crustal recycling with some contribution from continental crust for andesitic rocks based the chemical composition of panjeh mafic and intermediate rocks in combination with data for other gabbroic to dioritic bodies in the ghorveh area we offer two interpretations for these and other jurassic igneous rocks of the sasz as reflecting melts from a subduction modified oib type source above a neo tethys subduction zone or b plume or rift tectonics involving upwelling metasomatized mantle mostly reflecting the 550 ma cadomian crust forming event