thrust belts of the southern central andes along strike variations in shortening topography crustal geometry and denudation the andean fold and thrust belts of west central argentina 33 s and 36 s above the normal subduction segment present important along strike variations in mean topographic uplift structural elevation amount and rate of shortening and crustal root geometry to analyze the controlling factors of these latitudinal changes we compare these parameters and the chronology of deformation along 11 balanced crustal cross sections across the thrust belts between 70 w and 69 w where the majority of the upper crustal deformation is concentrated and reconstruct the moho geometry along the transects we propose two models of crustal deformation a 33 40 s model where the locus of upper crustal shortening is aligned with respect to the maximum crustal thickness and a 35 40 s model where the upper crustal shortening is uncoupled from the lower crustal deformation and thickening this degree of coupling between brittle upper crust and ductile lower crust deformation has strong influence on mean topographic elevation in the northern sector of the study area an initial thick and felsic crust favors the coupling model while in the southern sector a thin and mafic lower crust allows the uncoupling model our results indicate that interplate dynamics may control the overall pattern of tectonic shortening however local variations in mean topographic elevation deformation styles and crustal root geometry are not fully explained and are more likely to be due to upper plate lithospheric strength variations

lithospheric structure and along strike segmentation of the central andean plateau seismic q magmatism flexure topography and tectonics the central andean plateau is a 300 km wide nearly 4 km high plateau which is situated above a 30 e dipping segment of the subducted nazca plate systematic changes in the topography the upper mantle seismic attenuation q structure the lithospheric flexural rigidity and the distribution and chemistry of back arc lavas indicate that the upper mantle structure changes along strike beneath the plateau south of 23 s the upper mantle beneath the plateau becomes hotter and the lithosphere becomes thinner and weaker this lateral variation in upper mantle structure coincides with variations in the tectonic style and timing of deformation in two physiographically distinct segments of the plateau and its adjacent foreland thrust belt to the east the bolivian altiplano and subandean ranges in the north and the argentine puna and santa barbara system in the south the segments different mantle lid thicknesses result in an altiplano supported mainly by crustal thickening and a puna supported by a combination of a crustal root and a thermal mantle root a thicker stronger foreland lithosphere in the north favors thin skinned underthrusting of the foreland beneath the plateau while to the south the thinner weaker lithosphere allows more basement involved shortening within the foreland and less underthrusting beneath the plateau a weaker lithosphere results in a longer period of compressional deformation and greater local and structural relief in the puna than in the relatively stable altiplano to the north ultimately this observed n s directed segmentation on the plateau and in the foreland may reflect the different responses of strong and weak continental lithosphere to an andean type subduction margin

material balance in alpine orogeny recent geophysical data are combined with older information for an updated picture of alpine kinematics using semiquantitative considerations of material balance in the alps a conventional pile of thin peel nappes is disrupted by a central longitudinal system of steep faults and steeply limbed folds both affecting disharmoniously the entire edifice of otherwise flat lying nappes it is particularly from late tertiary phases of deformation that structures of the second type the insubric system with the insubric line and the belt of late alpine windows acquired prominence for the present aspect of the alps these late structures formed as a dextrally transpressive intracontinental branch of the africa europa plate boundary mostly between areas of extension early motions in the late cretaceous to the late eocene were probably responsible for at least half of the 300 km dextral displacement required for the palinspastic restoration of the inner west carpathians and the dinarides during oligocene extension the late alpine batholiths intruded in a belt roughly along the insubric line while deep crustal and lithospheric roots were destroyed in the latest oligocene to middle miocene the adriatic plate moved dextrally to the west along the insubric transfer fault from its frontal insubric indenter a lower crust upper mantle flake probably first obducted in the cretaceous was detached and wedged into the penninic nappes of the western alps bird s head of the ivrea body both the northern and the southern transfer faults of the insubric indenter were disrupted and inactivated by late miocene to later events the giudicarie and neo north apennine events instead of the inactivated insubric fault the windows belt of en echelon folds assumed the role of dextral transpression all of these successive zones of motion are entangled in the ligurian knot largely hidden under the sea or young sediments a large part of the pre alpine crust and practically the whole mantle were subducted only in some cases were high pressure rocks of shallow continental origin re obducted the lithospheric root of the neo alps is largely contained in a vertical slab under the alps but some parts seem to have been removed and others disharmonically displaced with respect to the surface structures

evidence for lithospheric detachment in the central andes from local earthquake tomography abstract data from three temporary seismic networks were merged for tomographic inversion although the deployments did not coincide in time spatial overlap was achieved by re occupying existing sites travel times and t operators of about 1600 earthquakes were inverted for 3d models of p p s and p wave attenuation q p 1 all three attributes provide a consistent image of the entire subduction zone on a lithospheric scale the tomographic images reveal low velocities and high attenuation in the crust and mantle underlying the western cordillera and most of the puna plateau indicative of weak rheology and mostly asthenospheric mantle in contrast forearc and eastern foreland are characterized by high q p values corresponding to cold temperatures in accordance with thermal models in the backarc between 23 s and 24 s a high velocity high q p structure beneath the eastern cordillera and eastern puna is interpreted as detaching continental lithosphere that has been thickened in the orogenic process south of this structure the mantle is characterized by low velocities high p s ratios and low q p values here it is believed that lithosphere originally underlying andean crust has already been removed this is supported by new estimates of crustal thickness and volcanic activity

comment on magnetostratigraphy of the hettangian langmoos section adnet austria evidence for time delayed phases of magnetization by y gallet d vandamme and l krystyn the recent paper by gallet vandamme krystyn 1993 deals with the age of magnetization of lower jurassic nodular limestones including the adnet limestone formation exposed near the village of adnet austria the authors conclude that for the red variety of the limestones at adnet the growth of haematite in an early diagenetic process is questionable three sites of these limestones from the adnet locality were studied by channel1 et al 1992 the three site mean directions are slightly more dispersed after tilt correction n 3 kjk 2 33 suggesting a post tilt magnetization however the fold test is not significant in a region where the beds dip fairly uniformly to the sw at about 10 we wish to point out that the results of a conglomerate test for the red variety of the adnet limestone at adnet indicate that the highest blocking temperature haematite magnetization component is indeed early diagenetic in origin a conglomerate test carried out on the blue gray variety of the adnet limestone indicates that the characteristic magnetization in this variety is post mesozoic and probably neogene in age as pointed out by gallet et al 1993 the lower jurassic nodular limestones exposed near adnet show colour variations from red to blue gray and the colour variation is seen to cut bedding surfaces indicating that either the blue gray or the red colouration of the limestones is secondary in origin we have not seen these colour

receiver function images from the moho and the slab beneath the altiplano and puna plateaus in the central andes summary teleseismic data recorded during one and a half years are investigated with the receiver function technique to determine the crustal and upper mantle structures underneath the highly elevated altiplano and puna plateaus in the central andes a series of converting interfaces are determined along two profiles at 21 s and 25 5 s respectively with a station spacing of approximately 10 km the data provide the highest resolution gained from a passive project in this area so far the oceanic nazca plate is detected down to 120 km beneath the altiplano whereas beneath the puna the slab can unexpectedly be traced down to 200 km depth at longer periods a shallow crustal low velocity zone is determined beneath both plateaus exhibiting segmentation in the case of the altiplano the segments present vertical offsets and are separated by inclined interfaces which coincide with major fault systems at the surface an average depth to moho of about 70 km is determined for the altiplano plateau a strong negative velocity anomaly located directly below the moho along with local crustal thinning is interpreted beneath the volcanic arc of the altiplano plateau between 67 w and 68 5 w a deep section of the puna profile reveals thinning of the mantle transition zone although poorly resolved the detected anomaly may suggest the presence of a mantle plume which may constitute the origin of the anomalous temperatures at the depth of the upper mantle discontinuities

miocene quaternary structural evolution of the uyuni atacama region andes of chile and bolivia abstract we describe the miocene quaternary geological structural evolution of the region between the salar de uyuni and de atacama andes of chile and bolivia we recognized four main tectonic events based on fold geometry fault kinematics and stratigraphic data the oldest event of miocene age is characterized by folding and reverse faulting of the sedimentary successions with an e w direction of shortening in the northern part of the studied area and a wnw ese shortening in the southern part the following two events of pliocene age are characterized by lower shortening amounts they occurred first by reverse faulting with a nw se trending greatest principal stress 1 computed with striated fault planes and a vertical least principal stress 3 followed by pervasive strike slip faulting with the same nw se trending 1 and a horizontal ne sw 3 the fourth event dating to the late pliocene quaternary is characterized by normal faulting the 3 still trends ne sw whereas the intermediate principal stress 2 exchanged with 1 volcanism accompanied both the contractional transcurrent and extensional tectonic phases the mio pliocene compression appears directly linked to a rapid convergence and an apparently important coupling between the continental and oceanic plates the e w to wnw ese direction of shortening of the miocene structures and the nw se 1 of the pliocene structures seem to be more linked to an intra andean re orientation of structures following the wnw directed absolute motion of the south american plate the extensional deformations can be interpreted as related to gravity forces affecting the highest parts of the volcanic belt in a sort of asymmetrical sw ward collapse of the belt

attenuation of high frequency seismic waves beneath the central andean plateau observed patterns of high frequency seismic wave attenuation suggest that near 22 s the upper mantle structure beneath the central andean plateau changes along strike to the south contrasting regions of high and low upper mantle seismic wave attenuation beneath the plateau are identified based on striking azimuthal variations in the character and frequency content of p and s waves propagating beneath the plateau to a portable seismic network deployed in jujuy province argentina 24 s 65 w ray paths from intermediate depth earthquakes located north and northwest of the network transmit seismic waves with a higher frequency content than ray paths from earthquakes at similar depths and distances but located west and south of the network the estimated apparent q values fall into two categories qp 500 and qs 350 for the high q paths and qp 350 and qs 200 for the low q paths in addition sn phases from regional crustal earthquakes in the subandean foreland fold thrust belt to the north propagate efficiently to the jujuy network while sn is not observed from foreland earthquakes located at similar distances to the south of the network these observations combined with data from previously reported wave propagation studies suggest that south of about 22 s the upper mantle beneath the plateau and its adjacent foreland thrust belt is more highly attenuating than the upper mantle farther north forward modeling of the q measurements made at jujuy indicates that the observations can be explained either by a variable thickness high q upper plate beneath the plateau or by a thin variable width very low q zone in the asthenospheric wedge above the subducted slab this lateral variation in upper mantle structure coincides with two physiographically distinct segments of the central andean plateau and its adjacent foreland thrust belt to the east the bolivian altiplano and subandean ranges in the north and the argentine puna and santa barbara system in the south we interpret the north south change in upper mantle attenuation and the corresponding changes in physiography topography and tectonic style at the surface to be due to a mantle lid that is thicker beneath the altiplano and the subandean belt than beneath the puna and the santa barbara ranges

revised deformation history of the central andes inferences from cenozoic foredeep and intermontane basins of the eastern cordillera bolivia 1 investigation of cenozoic deformation sediment accumulation and provenance in the eastern cordillera of bolivia at 17 21 s indicates major shortening 60 140 km and foredeep development followed by limited internal shortening and intermontane basin development contrasting histories of shortening deposition sediment dispersal and detrital composition distinguish a formerly extensive paleogene foredeep exposed along an eastern belt of synclines from a zone of principally neogene intermontane basins in the central eastern cordillera new 40ar 39ar ages of 25 17 ma for interbedded tuffs reveal latest oligocene early miocene accumulation in intermontane basins at rates 80 m myr several times lower than andean foredeeps although poorly dated foredeep evolution probably coincided with middle eocene oligocene deformation and denudation in adjoining regions to the west a mid cenozoic transition from foredeep to intermontane conditions may be attributable to emplacement of a basement involved tectonic wedge beneath the eastern cordillera in this interpretation wedge emplacement drove flexural foredeep subsidence from roughly 40 to 25 ma whereas subsequent accumulation occurred in localized internally drained basins in elevated intermontane areas regardless of the regional subsurface structural geometry 40ar 39ar ages define a 25 21 ma onset of intermontane sedimentation that signifies the termination of major upper crustal shortening over large parts of the eastern cordillera and possibly the age of initial shortening in the interandean and subandean zones to the east in contrast to several popular models the revised tectonic history presented here suggests significant pre neogene shortening in the eastern cordillera and underscores the large uncertainties in estimates of long term shortening rates in the andes

hypocentral trend surface analysis probing the geometry of benioff zones a hypocentral trend surface is a continuous function of latitude and longtitude fitted by least squares to a set of hypocenters so that it predicts depth to the middle of a benioff zone in this paper we take a relatively simple approach to hypocentral trend surface analysis the hypocentral trend surface is constructed from a spherical surface harmonic expansion whose coefficients are selected so as to minimize the standard vertical deviation between hypocenters and trend surface the vertical deviation of a hypocenter from the hypocentral trend surface is called its residual consideration of a hypocentral trend surface cannot be divorced from consideration of the associated residuals and their spatial distribution any analysis of benioff zone geometry trades off presumed thickness of the zone with its presumed shape this trade off can be investigated by examining suites of candidate hypocentral trend surfaces together with their associated residual distributions generated by varying the number of degrees of freedom available to the trend surface hypocentral trend surfaces are generated for three high quality sets of hypocenters obtained by local seismic networks in honshu japan cook inlet alaska and south peru hypocenters beneath honshu generate a bimodal distribution of residuals about their trend surface the vertical separation of the upper and lower sheets of this double benioff zone averages about 33 km no double benioff zone configuration is discovered below cook inlet or south peru regional trend surfaces are established for intermediate depth teleseismic data from south and middle america in south america 0 40 s teleseismic data suggest that along strike transitions between flat and moderately steeply dipping sections of the benioff zone are achieved by flexure of a coherent slab rather than by fragmentation of the slab into tear bound flaps or separate tongues of lithosphere hypocentral trend surface analysis of teleseismic data from south peru suggests a geometry similar to that indicated by hypocentral trend surface analysis of the local data of hasegawa and sacks an eastward shallowing benioff zone beneath southwest mexico is closely associated with the geometry of the trans mexican volcanic arc the benioff zone beneath central america is more steeply dipping the flat slab steep slab transition near 96 w may relate to the difference in age of oceanic lithosphere on either side of the tehuantepec ridge while the strong flexure of the subducted nazca plate beneath ecuador is closely associated with the carnegie ridge the nazca ridge extropolates slightly north of the flexure in subducted lithosphere beneath south peru the flexure appears instead more closely associated with the pronounced inflection in the map view shape of the leading edge of the overriding south american plate

adsorption and desorption of phosphate on limestone in experiments simulating seawater intrusion abstract this study investigates the potential release of po 4 3 from carbonate aquifers exposed to seawater intrusion adsorption and desorption of po 4 3 in the presence of deionized water diw and seawater were conducted on a large block of pleistocene age limestone to simulate the effects of seawater intrusion into a coastal carbonate aquifer at the laboratory scale the limestone showed strong adsorption of po 4 3 in diw while adsorption was significantly less in the presence of seawater dissolution of caco3 was found to prevent po 4 3 adsorption at salinities less than 30 psu adsorption of po 4 3 was limited at higher salinities 30 33 psu due to competition with hco 3 ions for adsorption sites at a salinity po 4 3 absorption occurred as caco3 precipitated concentrations of po 4 3 between 2 and 5 mol l were released by desorption when the limestone was exposed to seawater the results of this study suggest that as seawater intrudes into an originally freshwater coastal aquifer adsorbed po 4 3 may be released into the groundwater consequently adsorbed po 4 3 is expected to be released from coastal carbonate aquifers world wide as sea level continues to rise exposing more of the freshwater aquifer to seawater

style and history of andean deformation puna plateau northwestern argentina topographically the puna plateau of northwestern argentina is the southern continuation of the bolivian altiplano its thickening and consecutive uplift result from the andean orogeny to better constrain the structural style and its progressive development we have studied field data topographic and satellite imagery balanced cross sections seismic reflection data kinematic analysis of fault slip data anisotropy of magnetic susceptibility ams paleomagnetic data and apatite fission track aft data across the puna plateau precambrian and paleozoic basement ranges bounded by high angle reverse faults dips 60 alternate with cenozoic intermontane basins major thrusts trend nne ssw and do not show a preferred vergence intermontane basins have various degrees of symmetry depending on the geometries and attitudes of associated thrusts as well as on the magnitudes of their offsets there is a close correlation between the surface expression of a basin and the amount of internal deformation a line balanced cross section of the puna at 25 s has yielded a cenozoic shortening of 10 15 in a direction subperpendicular to the orogen by kinematic analysis of cenozoic fault slip data we have obtained principal directions of strain rate across the puna shortening axes are subhorizontal and trend on average wnw ese n110 stretching axes are subvertical and intermediate axes are subhorizontal and trend on average nne ssw strain ellipsoids are dominantly of plane strain type and they represent dip slip thrusting from paleomagnetic and ams data shortening axes form a radial pattern around the eastern edge of the central andes the pattern is attributed to an inhomogeneous stress field reflecting the eastward convex shape of the central andean thrust front from the history of burial and uplift andean shortening reached the northeastern part of the puna in the late eocene and the adjacent eastern cordillera in the late eocene or early oligocene this shortening was presumably due to the incaic phase of the andean orogeny in the eastern part of the orogen the onset of shortening was probably guided by preexisting paleozoic and mesozoic structures so that andean deformation propagated unevenly eastward

formation of internally drained contractional basins by aridity limited bedrock incision 1 low internal relief aridity and internal drainage characterize the puna altiplano plateau and the tibetan plateau and the tarim basin structurally these areas are reverse fault bounded terrains with intervening wedge top basins that store thick accumulations of sediment for millions of years orographic barriers along the margins of these basins are oriented normal to moisture bearing winds resulting in regional aridity the combination of this aridity rapid shortening and uplift rates and resistant exposed lithologies coincides with the location of these internally drained basins we hypothesize that these large internally drained areas persist indefinitely when uplift overwhelms the fluvial systems and defeats the channel network to test this hypothesis we developed models of channel defeat to examine the threshold conditions required to fragment the channel network the results of these models suggest that low erodibility rocks moderate to high uplift rates and low precipitation favor defeat of orogen traversing channels most coupled tectonic landscape development models prevent the development of internal drainage and so may overestimate the voracity of erosion in regimes that favor internal drainage profoundly influencing predicted orogenic growth patterns

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

strain and rotation rate from gps in tibet anatolia and the altiplano 1 deformation measured by regional gps networks in continental plateaus reflects the geologic and tectonic variability of the plateaus for two collisional plateaus tibet and anatolia and one noncollisional the altiplano we analyze the regional strain and rotation rate by inverting gps velocities to calculate the full two dimensional velocity gradient tensor to test the method we use gridded velocities determined from an elastic block model for the eastern mediterranean middle east region and show that to a first order the deformation calculated directly from the gps vectors provides an accurate description of regional deformation patterns principal shortening and extension rate axes vertical axis rotation and two dimensional 2 d volume strain dilatation are very consistent with long term geological features over large areas indicating that the gps velocity fields reflect processes responsible for the recent geologic evolution of the plateaus differences between geological and gps descriptions of deformation can be attributed either to gps networks that are too sparse to capture local interseismic deformation or to permanent deformation that accrues during strong earthquakes the altiplano has higher internal shortening magnitudes than the other two plateaus and negative 2 d dilatation everywhere vertical axis rotation changes sign across the topographic symmetry axis and is due to distributed deformation throughout the plateau in contrast the collisional plateaus have large regions of quasi rigid body rotation bounded by strike slip faults with the opposite rotation sense from the rotating blocks tibet and anatolia are the mirror images of each other both have regions of positive dilatation on the outboard sides of the rotating blocks positive dilatation in the aegean correlates with a region of crustal thinning whereas that in eastern tibet and yunnan province in china is associated with an area of vertical uplift rollback of the hellenic trench clearly facilitates the rotation of anatolia rollback of the sumatra burma trench probably also enables rotation about the eastern syntaxis of tibet

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

cenozoic stratigraphy and basin tectonics of the andes mountains 20 28 south latitude clastic sedimentary basins have evolved during the past 40 m y in the central andes lat 20 28 s in response to shifting patterns of magmatism and deformation the distribution of these basins and their genetic relations to uplifted areas are analogous to the basins and mountain belts of the north american rocky mountains during the late cretaceous and early cenozoic petroleum exploration has focused on zones underlying the upper cenozoic strata along the eastern margin of the andes mountain belt between about 40 and 25 ma a nonmarine basin extended across the region that is now the andes mountains between about 25 and 10 ma the western part of the former basin became the site of a volcanic arc sediment accumulation continued in the east where marine intercalations demonstrate the low elevation of the basin after 10 ma the volcanic arc remained active and locally widened and crustal shortening caused regionally important thrust and reverse faulted ranges during the past 10 m y up to 4 000 m of coarse clastic debris accumulated in a foreland basin on the eastern flank of the mountains meanwhile in the interior of the mountains over 4 000 m of fine grained strata and evaporites accumulated in local depocenters

high level surfaces plateau uplift and foreland development bolivian central andes newly dated tertiary strata in the bolivian central andean plateau and synthesis of the tertiary record in the adjacent subandean fold thrust belt constrain the age of deformation in both regions age of deformation within the plateau is determined by dated crosscutting relations associated with a regionally extensive high level surface known as the san juan del oro surface new 40 ar 39 ar dates on undeformed strata above the high level surface preclude significant upper crustal shortening within the eastern cordillera after 10 ma tertiary strata within the adjacent subandean region demonstrate that formation of the fold thrust belt occurred after 10 ma on the basis of these data we propose a two stage model of late cenozoic andean growth that links plateau uplift to the development of the fold thrust belt in the first stage early plateau uplift occurred in response to widespread compressional deformation of the plateau eastern cordillera and altiplano during the second stage beginning after 10 ma upper crustal deformation within the plateau terminated and the subandean fold thrust belt developed crustal scale eastward thrusting along the eastern margin of the eastern cordillera drove subandean folding and thrusting the eastern cordillera served as the bulldozer for the deforming subandean wedge

non steady state subduction and trench parallel flow induced by overriding plate structure abstract the direction of plate tectonic motion and the direction of mantle flow as inferred from observations of seismic anisotropy measurements show a good global correlation far from subduction zones however this correlation is poor near subduction zones where below the slab seismic anisotropy is aligned parallel to the trench and above the slab has a complex pattern which has not been fully explained here we present time dependent three dimensional 3d fully dynamic simulations of subduction to study the effect of overriding plate structure on the evolution of slab geometry and induced mantle flow we find that along strike variation in thermal thickness of the overriding plate causes increased hydrodynamic suction and shallower slab dip beneath the colder portion of the overriding plate the variation in slab geometry drives strong trench parallel flow beneath the slab and a complex flow pattern above the slab this new mechanism for driving trench parallel flow provides a good explanation for seismic anisotropy observations from the middle and south america subduction zones where both slab dip and overriding plate thermal state are strongly variable and correlated and thus may be an important mechanism in other subduction zones the location and strength of trench parallel flow vary with the time dependent evolution of the slab suggesting that the global variability in seismic anisotropy observations in subduction zones is in part due to the non steady state behavior of these systems

relative motion of the nazca farallon and south american plates since late cretaceous time by combining reconstructions of the south american and african plates the african and antarctic plates the antarctic and pacific plates and the pacific and nazca plates we calculated the relative positions and history of convergence of the nazca and south american plates despite variations in convergence rates along the andes periods of rapid convergence averaging more than 100 mm a between the times of anomalies 21 49 5 ma and 18 42 ma and since anomaly 7 26 ma coincide with two phases of relatively intense tectonic activity in the peruvian andes known as the late eocene incaic and mio pliocene quechua phases the periods of relatively slow convergence 50 to 55 30 mm a at the latitude of peru and less farther south between the times of anomalies 30 31 68 5 ma and 21 and between those of anomalies 13 36 ma and 7 correlate with periods during which tectonic activity was relatively quiescent thus these reconstructions provide quantitative evidence for a correlation of the intensity of tectonic activity in the overriding plate at subduction zones with variations in the convergence rate

uplift of the western altiplano plateau evidence from the precordillera between 20 and 21 s northern chile 1 we present a quantitative reconstruction of uplift of the western flank of the altiplano plateau central andes one of the largest monoclines on the earth on the basis of an analysis of tectonic structures syntectonic deposits and geophysical data uplift occurred on a west vergent slowly propagating system of high angle reverse faults merging into a joint detachment that ramps down to midcrustal levels below the plateau edge the upper ramps determine local fold geometries while the lower ramp controls large scale surface tilting and uplift at 20 s this fault system was active between 30 ma and 5 10 ma with maximum shortening rates of 0 22 mm yr between 17 and 10 ma it generated some 2600 m of surface uplift with only minor shortening of 3000 m its activity was largely synchronous to eruption of large volume ignimbrites from a midcrustal source geophysical data indicate that the fault system localized deformation at the boundary between a cool strong forearc crust and a presumably fluid rich and or partially molten zone underneath the plateau area the systematic relation between crustal melting and shortening with uplift at the western plateau margin can be traced along most of the plateau flank with a stepwise decrease in age of deformation and magmatism toward the south indicating discontinuous addition of plateau segments crustal thickening to as much as 70 km from westward underthrusting in the back arc parts of the plateau isostatically compensated the tectonic surface uplift and monocline formation with respect to a stable forearc which only reacted with minor tilting

recent rapid uplift in the bolivian andes evidence from fission track dating apatite and zircon fission track cooling ages constrain the tertiary cooling and uplift history of the eastern cordillera and altiplano of bolivia fission track data are from two triassic plutons and surrounding paleozoic metasedimentary rocks in the eastern andes north of la paz zircon cooling ages indicate that the roof of the huayna potosi pluton was emplaced in the zircon partial annealing zone and that the zongo pluton was emplaced entirely in the zircon total annealing zone apatite cooling ages for both plutons record uplift in the past 5 15 m y and zircon cooling ages from the zongo pluton reflect uplift in the past 25 45 m y uplift rates calculated by these apatite and zircon cooling ages suggest that uplift rates were 0 1 0 2 mm yr between 20 and 40 ma and increased significantly between 10 and 15 ma by 3 ma uplift rates may have been as high as 0 7 mm yr

paleogeography and andean structural geometry northwest argentina neogene tectonic segmentation of the andean cordillera in northwestern argentina is a function of both the dip of the subducted plate and of major paleogeographic structures and sedimentary basins vergence fault dip and lateral continuity of neogene structures can be related to the character and pre existing geometries and structures of the materials in which they form south of 27 s the thick skinned sierras pampeanas which occur mostly over a flat segment of the subducted nazca plate formed where little sedimentary cover overlay crystalline basement north of 24 s the thin skinned subandean belt overlying an inclined slab segment formed within a thick elongate eastward tapering trough of paleozoic and mesozoic sediments that may represent a complex of pre andean foreland basin deposits the transitional region between 24 27 s the santa barbara system and cordillera oriental is characterized paleogeographically by complex cross trending structures and irregular basin geometries and the neogene structures are equally complex and variable the paleogeographic features of nw argentina are a product of compressional tectonism during much of the paleozoic that resulted in an orogenic belt oriented obliquely to the modern andes this earlier orogen was disrupted by several zones of transverse structures and by cretaceous extensional or pull apart basins these features clearly influence the geometry of the superposed andean structures more intriguing however is the possibility that preexisting structures of the continental lithosphere may in some way help to modify the geometry of the subducted plate

late cenozoic tectonism and landscape development in the foreland of the andes northern sierras pampeanas 26 28 s argentina the argentine sierras pampeanas are reverse fault bounded mountain blocks of precambrian to paleozoic basement rocks in the foreland of the central andes uplift in the northernmost sierras pampeanas fault blocks of sierra de quilmes sierra cumbres calchaquies and sierra aconquija started about 7 ma and became pronounced between 4 and 3 4 ma the movements culminated after 2 9 ma when the conformable mio pliocene santa maria group was overthrusted faulted and folded in the course of principal basement uplift these movements created climate and base level conditions that resulted in the formation of five pediment levels in the deformed basin strata between 2 5 and 0 3 ma tectonically induced base level changes in the piedmont resulted in three tectonism related pediment surfaces between 2 5 and 0 6 ma which attest to the neotectonic activity in the andean foreland the chronology of young tectonic uplift contradicts models in which early uplift between 20 and 13 ma is due to the interaction between high topography on the subducting nazca plate and the south american craton in conjunction with the principal uplift of the adjacent puna plateau the northernmost sierras pampeanas are better explained by e w compressional stresses superposed on a thinned lithosphere and inherited zones of structural weakness that facilitated uplift

andean age structure of eastern cordillera province of la paz bolivia a moho root beneath the bolivian andes 40 km deep is consistent with 230 km of overlap of neogene age on a single trenchward dipping transcrustal thrust fault with 10 of finite ramp cutoff main andean thrust mat only 10 of the andean crustal volume is ascribable to magmatic addition the mat is located within south american crust of full thickness it intersects the basement top 450 km inland from the neogene crustal margin it is not a collision suture as shown by persistent pre neogene facies continuity thrusting is not accompanied by terrane accretion the present bilaterally symmetrical thrust belt responds to elastic line loading and to coulomb rheology in the hanging wall of the mat a deep high stress wedge base builds a steep critical slope in the footwall the foredeep response is fast subcritical growth by progradation and blind thrusting on a low stress decollement interaction is maintained by out of sequence renewal of movement on the mat

tectonic inversion in the santa barbara system of the central andean foreland thrust belt northwestern argentina 1 the santa barbara system sbs of northern argentina is a 400 km long segment of the subandean foreland thrust belt it is characterized by predominantly west verging relatively high angle thrust faults many of these faults are reactivated normal faults from one branch of a complex cretaceous to paleogene rift system heteroaxial folding in parts of the sbs is probably due to a slight component of range parallel dextral strike slip motion acting on preexisting faults striking north and ne during inversion regional balanced cross sections along two transects across the northern sbs indicate that the major faults flatten into a detachment in the basement at about 10 km depth neogene e w contraction in the sbs is of the order of 21 26 km rift extension is not very well constrained but was probably less than 10 km the structural style of the sbs differs from the thin skinned subandean thrust belt to the north and from the large wavelength sierras pampeanas basement uplifts to the south the changes between the different styles are sharp and coincide with the northern and southern boundaries of the rift in the foreland suggesting that crustal or lithospheric heterogeneities exert an overriding control on foreland structural style

the late miocene quaternary antofalla volcanic complex southern puna nw argentina protracted history diverse petrology and economic potential abstract the antofalla volcanic complex avc is located on the southern puna plateau of nw argentina 100 km east of the main axis of the cordillera occidental volcanic arc it lies on the nw se archibarca lineament one of several transverse structures that cut across the andes from chile and divert arc magmatism in se trending fingers across the puna 40ar 39ar dating of volcanic products from the avc reveals a protracted magmatic history from 10 9 to 1 6 ma initial volcanism was characterized by eruption of voluminous rhyolitic ignimbrites deposited directly onto exposed crystalline basement and supracrustal sedimentary rocks these rhyolites record significant geochemical evidence for crustal contamination or derivation by crustal melting which decreases upwards through the pyroclastic sequence they were followed by extrusion of potassic lavas of the shoshonite latite trachydacite suite from 10 1 to 9 5 ma subsequent magmatism 9 1 1 6 ma was of basaltic andesite andesite dacite composition with localized smaller volume dacitic and rhyolitic ignimbrites monogenetic basaltic andesite cinder cones and lava flows were erupted in the quaternary in response to a change to transtensional tectonics in the puna the volcanic sequence is interpreted to record the early development of a lower crustal mash zone at 11 ma where mantle derived arc magmas interacted extensively with felsic lower crustal rocks to produce evolved shoshonitic compositions and crustal melts that erupted to form rhyolitic ignimbrites common at this time throughout the puna after establishment of a steady state mash zone by 9 ma subsequent basaltic andesite andesite magmas show less evidence of felsic crustal contamination and may have developed largely by interaction with previous ultramafic mafic fractionates of early magmas in the lower crust more felsic dacitic magmas appear to have evolved largely by fractional crystallization from basaltic andesites and andesites with widespread evidence of mixing and mingling between magmas of different degrees of differentiation some contamination by upper crustal components is also evident in these dacites but is shown more clearly by small volume rhyolites some of which may again represent predominantly crustal melts epithermal deposits of au and pb zn ag occur within the avc apparently associated with the formation of rhyolite domes the mineralization style is typical of other andean volcanic centres and may be underlain by deeper seated porphyry styles of mineralization extensive blanketing by younger lavas and limited erosion restricts exposure of these altered and mineralized sequences however although the potential for further discoveries beneath cover or at depth in this system appears high

topography and subduction geometry in the central andes clues to the mechanics of a noncollisional orogen the central andean orogen between 12 and 32 s latitude exhibits a high degree of spatial order principally an extraordinary bilateral symmetry that is common to the earth s surface the underlying wadati benioff zone and the nazca south america plate kinematics which has been stable since the mid tertiary this spatial order must reflect the physical mechanisms of mountain building in this noncollisional orogen the shapes of the topography and subduction zone can be reduced to symmetric and antisymmeric components relative to any vertical symmetry plane the particular plane which minimizes the antisymmetry and maximizes the symmetry is well resolved and is essentially coincident with the stable euler equator of nazca south america relative motion since the mid tertiary that the topography subduction geometry and persistent mid tertiary plate kinematics share common spatial and geometric elements suggests that the distribution of topography in this orogen depends strongly on the dynamics of subduction other factors that might affect the topography and underlying tectonics such as climate and inherited structural fabric which have different spatial characteristics must be of less significance at a continental scale furthermore the small components of asymmetry among the various elements of the orogen appear to be mutually related in a simple way it is possible that this coupled asymmetry is associated with a late tertiary change in plate kinematics these observations suggest that there is a close connection between plate tectonics and the form of the earth s surface in this noncollisional setting it follows that the distribution of topography near convergent plate boundaries may provide a powerful constraint for understanding the dynamics of subduction

a tectonostratigraphic synthesis of the sub andean basins implications for the geotectonic segmentation of the andean belt the tectonic structural and depositional history of 19 sub andean foreland and eight southern caribbean basins has been synthesized and is summarized by regional geohistory charts these charts recapitulate the subsidence regimes recognized and have been used to evaluate correlations between genetically related stratigraphic sequences based on spatial and temporal changes in palaeo depositional setting the sub andean region can be subdivided longitudinally into several tectonostratigraphic domains the differential amount of subsidence between two adjacent tectonostratigraphic provinces or sub provinces relies on the presence of a transverse zone of structural accommodation the location of these transfer zones correlates with a number of known and several newly identified zones of intracontinental deformation combined with changes in the structural geometry of the andean fold and thrust belt and the relative dominance of basement fault systems along the length of the andean foreland it is suggested that the andean belt can be separated into five tectonic domains which are themselves bounded by transverse structural accommodation zones representing the multiphase reactivation of pre existing basement fault systems the influence of these broad zones of transcontinental deformation is most apparent during the triassic jurassic period suggesting that they were intimately associated with accommodating intraplate stresses during the breakup of the gondwana supercontinent

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

temporal variation in climate and tectonic coupling in the central andes analog and numerical models predict a coupling between climate and tectonics whereby erosion infl uences the deformation of orogens a testable prediction from modeling studies is the decrease in width of mountain ranges as a result of increased precipitation here we evaluate the effect of climate on a critically tapered orogen the central andes using sequentially restored balanced cross sections through wet 15 16 s and dry 21 s regions of the orogen in these regions tectonics basin geometry and style of deformation are similar allowing us to use variations in propagation or changes in percent shortening to evaluate whether alongstrike changes in width and morphology are climate driven in the north results indicate similar total percent shortening along the northern 40 and southern 37 sections suggesting that a wetter climate has not limited the width propagation in the north however comparison of early 45 25 ma and recent ca 20 0 ma shortening indicates that early deformation produced 45 2 shortening of both sections while recent deformation produced 41 2 north versus 32 2 south in the actively deforming subandes the latter suggests a coupling between climate and tectonics that began between ca 19 and 8 ma and continues to 0 ma potentially limiting the width of the northern subandes by 40 km

evolution of shallow and deep structures along the maipo tunuyan transect 33 40 s from the pacific coast to the andean foreland fil giambiagi laura beatriz consejo nacional de investigaciones cientificas y tecnicas cientifico tecnologico mendoza instituto argentino de nivologia glaciologia y ciencias ambientales argentina

timing of colorado plateau uplift initial constraints from vesicular basalt derived paleoelevations we have developed a technique for measuring paleoelevation on the basis of the vesicularity of basaltic lava flows we are currently applying this technique to determine the timing and extent of cenozoic uplift of the colorado plateau because the technique measures paleoatmospheric pressure it is not subject to uncertainties stemming from the use of proxies that depend on environmental factors other than elevation alone vesicular lavas preserve a record of paleopressure at the time and place of lava emplacement because the difference in internal pressure in bubbles at the base and top of a lava flow depends on atmospheric pressure and lava flow thickness the modal size of the vesicle bubble population is larger at the top than at the bottom this leads directly to paleoatmospheric pressure and thus elevation because the thickness of the flow can easily be measured in the field and the vesicle sizes can be measured in the laboratory on the colorado plateau volcanic fields are generally found around the margins of the plateau basaltic lavas range in age from 0 to 23 ma present elevations range from 1 7 km to 3 3 km samples were collected from lava flows in the areas of marysvale springerville grand mesa and the san juan mountains and analyzed to determine the timing and extent of uplift the analysis of the samples using x ray computed tomography imaging and our numerical techniques for determining vesicle population statistics indicate uplift of 350 2200 m depending on age there is a clear relationship between age and amount of uplift original elevation subtracted from present elevation the results indicate slow uplift of 40 m m y between 25 ma and 5 ma only 800 m of uplift during that time and rapid uplift of 220 m m y since 5 ma 1100 m during that time these results reconcile the long standing controversy between interpretations of ancient versus recent uplift by providing an uplift history curve for the colorado plateau

exhumation history of the southern altiplano plateau southern bolivia constrained by apatite fission track thermochronology 1 although the structural geometry of the cenozoic altiplano puna plateau in the central andes is well defined the temporal evolution of this contractile deformation is poorly constrained to address this shortcoming we used apatite fission track thermochronology aft to quantify the cooling and exhumation history along a transect at 21 s in southern bolivia through the deformed intermontane altiplano basin the doubly vergent thrust belt of the eastern cordillera and the inner foreland thrust belt east of the plateau interandean zone thermal history modeling combined with published balanced cross sections and stratigraphic data constrain exhumation histories exhumation started during the late eocene 40 36 ma in the central eastern cordillera possibly due to bivergent thrusting and cretaceous rift structure inversion during the early oligocene 33 27 ma exhumation spread across the study area as the current boundary thrusts of the eastern cordillera were activated the inner west vergent thrust system became active in irregular order until circa 20 ma whereas the east vergent interandean thrust belt formed by eastward propagating deformation since circa 30 ma plateau exhumation continued at 0 2 mm yr until shortening terminated by 11 7 ma shortening within the plateau since circa 30 ma did not evolve by lateral accretion of thrust wedges the propagating deformation style is spatially confined to the foreland thrust belt which initiated coeval to plateau deformation interandean zone but propagated mainly after circa 10 ma subandean zone early oligocene plateau wide tectonically driven exhumation suggests that subduction related processes had already thermally weakened the continental lithosphere prior to the 27 25 ma onset of volcanic activity

basement composition and basin geometry controls on upper crustal deformation in the southern central andes 30 36 s deformation and uplift in the andes are a result of the subduction of the nazca plate below south america the deformation shows variations in structural style and shortening along and across the strike of the orogen as a result of the dynamics of the subduction system and the features of the upper plate in this work we analyse the development of thin skinned and thick skinned fold and thrust belts in the southern central andes 30 36 s the pre andean history of the area determined the formation of different basement domains with distinct lithological compositions as a result of terrane accretions during palaeozoic time the development of a widespread permo triassic magmatic province and long lasting arc activity basin development during palaeozoic and mesozoic times produced thick sedimentary successions in different parts of the study area based on estimations of strength for the different basement and sedimentary rocks calculated using geophysical estimates of rock physical properties we propose that the contrast in strength between basement and cover is the main control on structural style thin v thick skinned and across strike localization of shortening in the study area

composition and structural control of crustal domains in the central andes 1 present day ratios of pb isotopes 324 published samples 435 new and nd sr isotopes 150 published 180 new on proterozoic to holocene igneous metamorphic and sedimentary rocks define at high spatial resolution distinct isotopic domains of the crust in the central andes these domains correlate with the internal compositional structure of the crust as revealed by a three dimensional density model pb nd isotopic boundaries thus correspond to variations in crustal compositional structure and reflect proterozoic mafic dominated and paleozoic felsic dominated crustal lithologies age and composition mafic versus felsic of these domains have controlled the rheology of the andean crust have influenced crustal deformation patterns and correlate with the central andean plateau segmentation

contrasting styles of superposed deformation in the southernmost andes brittle fault arrays regional structural patterns and marine multichannel seismic reflection profiles across the magallanes basin near 53 s latitude show contrasting styles of superposed extensional contractional and transtensional deformation below the northeastern end of a ne sw transect of the basin seismic reflection profiles show deepening levels of pre late jurassic basement and upper jurassic to lower cretaceous volcanic horizons toward the southwest this trend is associated with apparent normal fault offsets that may be attributed to a widespread late jurassic early cretaceous extensional event contrastingly below the southwestern end of magallanes basin subsurface depths of basement and volcanic horizons shallow to the southwest integration of seismic reflection data with published seismic refraction data geologic mapping and satellite gravity anomaly data suggest that this shallowing trend is controlled structurally by craton vergent basement involved contractional deformation this style of shortening contrasts with thin skinned styles of thrusting in the northern and central parts of the basin on tierra del fuego structural relationships stratigraphic data and previously published metamorphic data indicate that basement involved contraction occurred during latest cretaceous to early tertiary time overprinting all contractional structures steep tertiary strike slip and oblique slip faults in the central and southern parts of the magallanes basin display oblique normal mostly down to the southwest offsets across wnw ese trending segments of the south american scotia transform plate boundary on the south american continent newly discovered features along the plate boundary segment include sediment filled grabens and half grabens up to 7 km wide that occur between subparallel fault splays in the magallanes fault zone kinematic analyses of fault slip data indicate that sinistral transtensional motion occurred within this fault zone between the eastern arm of the straits of magellan and central tierra del fuego together these data indicate that 1 the southernmost andes were under compression during rapid cooling and exhumation of unique high pressure 8 kbars basement rocks now exposed in the darwin metamorphic complex dmc near 55 s latitude and 2 final exhumation of basement rocks in the dmc occurred during a shift away from compression toward transtension during late tertiary crustal relaxation we suggest that preexisting structural anisotropies formed during basement involved contractional deformation and the initiation of seafloor spreading south of south america during the oligocene controlled late tertiary sinistral transtensional patterns in the magallanes fault zone

thermochronometer record of central andean plateau growth bolivia 19 5 s 1 quantifying the timing magnitude and rates of exhumation and deformation across the central andes is a prerequisite for understanding the history of plateau rise we present 23 new apatite and zircon fission track thermochronometer samples to chronicle the exhumation and deformation across the entire 500 km andean fold thrust belt at 19 5 s in bolivia exhumation and deformation are constrained with inverse thermal modeling of the thermochronometer data regional stratigraphy geothermal gradients and mass deficits inferred from a balanced section results suggest the following 1 initial exhumation of the eastern cordillera ec fore thrust and back thrust belts began in the late eocene to early oligocene 27 36 ma and continued in a distributed manner in the late oligocene to early miocene 19 25 ma interandean zone ia exhumation began 19 22 ma followed by a third pulse of exhumation 11 16 ma in the ec back thrust belt and initial cooling in the westernmost subandes sa 8 20 ma finally exhumation propagated eastward across the sa during the late mio pliocene 2 8 ma 2 exhumation magnitudes are spatially variable and range from maximums of 8 km in the ec fore thrust belt to average values of 4 7 km across the ec 2 5 3 km in the altiplano 4 6 km in the ia and 3 km in the sa 3 exhumation rates range from 0 1 to 0 2 mm a in the ec from 0 1 to 0 6 mm a in the ia and from 0 1 to 0 4 mm a to locally 1 4 mm a or more in the eastern sa we synthesize similar constraints with sediments throughout bolivia and characterize plateau development by 1 distributed deformation throughout the altiplano and ec regions from 20 to 40 ma with minor deformation continuing until 10 ma 2 contemporaneous cessation of most ec deformation and exhumation of the ia 20 ma implying establishment of the modern plateau width with significant but unknown crustal thickness and elevation shortly thereafter by 15 20 ma and 3 dominantly eastward propagation of deformation from the ia since 20 ma with minor out of sequence deformation in the central to eastern sa

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

large scale trench normal mantle flow beneath central south america abstract we investigate the anisotropic properties of the fore arc region of the central andean margin between 17 25 s by analyzing shear wave splitting from teleseismic and local earthquakes from the nazca slab with partly over ten years of recording time the data set is uniquely suited to address the long standing debate about the mantle flow field at the south american margin and in particular whether the flow field beneath the slab is parallel or perpendicular to the trench our measurements suggest two anisotropic layers located within the crust and mantle beneath the stations respectively the teleseismic measurements show a moderate change of fast polarizations from north to south along the trench ranging from parallel to subparallel to the absolute plate motion and are oriented mostly perpendicular to the trench shear wave splitting measurements from local earthquakes show fast polarizations roughly aligned trench parallel but exhibit short scale variations which are indicative of a relatively shallow origin comparisons between fast polarization directions from local earthquakes and the strike of the local fault systems yield a good agreement to infer the parameters of the lower anisotropic layer we employ an inversion of the teleseismic waveforms based on two layer models where the anisotropy of the upper crustal layer is constrained by the results from the local splitting the waveform inversion yields a mantle layer that is best characterized by a fast axis parallel to the absolute plate motion which is more or less perpendicular to the trench this orientation is likely caused by a combination of the fossil crystallographic preferred orientation of olivine within the slab and entrained mantle flow beneath the slab the anisotropy within the crust of the overriding continental plate is explained by the shape preferred orientation of micro cracks in relation to local fault zones which are oriented parallel to the overall strike of the andean range our results do not provide any evidence for a significant contribution of trench parallel mantle flow beneath the subducting slab

characteristics and emplacement mechanisms of the coranzuli ignimbrites central andes abstract we present a detailed stratigraphy of the coranzuli caldera forming deposits this caldera located in the altiplano puna volcanic complex central andes generated four ignimbrite deposits with similar field characteristics and facies that differ from each other in above all the nature of the lithic fragments they contain three different lithofacies fine grained cross stratified facies massive lithic breccia facies and massive ignimbrite facies are found in all the ignimbrite deposits which occasionally also contain a lenticular lithic rich facies and or a pumice rich facies these field characteristics and in particular local deposit thicknesses were used to develop a theoretical model of the dynamics and emplacement mode of the coranzuli pyroclastic flows our results show that these ignimbrites were emplaced by dense pyroclastic density currents subjected to high accumulation rates and velocities thereby indicating rapid en masse emplacement that was also influenced by local paleotopography as deduced from facies analysis

the relationships between tectonics climate and exhumation in the central andes 18 36 s evidence from low temperature thermochronology abstract the central andes between 18 s and 36 s latitude strike north south for 2000 km along the chilean subduction margin cross several climate zones from hyperarid to humid and exhibit mean elevations in excess of 4000 m a s l here we investigate the relationships between tectonics climate and exhumation by inverting low temperature thermochronological data compiled from the literature 824 ages from 549 samples and new data 238 ages from 146 samples to quantify the exhumation rate history of the central andes since 80 ma our inferred exhumation rates west of the drainage divide and between 18 and 32 s did not exceed 0 25 km ma such low exhumation rates are consistent with low shortening rates and arid conditions in this region local pulses of exhumation occurred only during the eocene as response to active deformation and during the miocene probably as response to uplift of the western andean slope east of the drainage divide between 18 and 28 s the observed exhumation pattern reflects the onset and eastward propagation of deformation here exhumation occurred locally since the middle to late eocene in the eastern cordillera and the altiplano puna and subsequently affected larger parts of these regions and the north western sierra pampeanas during the oligocene in the early miocene 20 ma the interandean zone started exhuming and at 12 10 ma exhumation propagated into the subandean zone enhanced shortening rates and intensified precipitation along the eastern deformation front associated with the onset of the south american monsoon led to increased exhumation rates in the eastern interandean and the subandean zones in the plio pleistocene 0 6 km ma higher pleistocene exhumation rates are also observed in the northern sierra pampeanas 1 5 km ma that can be related to rock uplift along steep reverse faults coupled with high precipitation south of 32 s on the western side exhumation rates in the principal cordillera increased from ca 0 25 km ma in the miocene to rates locally exceeding 2 km ma in the pleistocene whereas the tectonic regime in the southern principal cordillera remained unchanged since the late miocene these higher rates are likely associated with enhanced erosion resulting from intensified pleistocene precipitation and glacial growth in this region reinforced by isostatic rock uplift and active tectonics our study shows that the onset of exhumation correlates mainly with the initiation of horizontal shortening and crustal thickening whereas the magnitude of exhumation is largely set by the amount of precipitation and glacial erosion and by the style of deformation which is controlled by inherited structures and the amount of sediments in the foreland

plateau style accumulation of deformation southern altiplano 1 employing surface mapping of syntectonic sediments interpretation of industry reflection seismic profiles gravity data and isotopic age dating we reconstruct the tectonic evolution of the southern altiplano 20 22 s between the cordilleras defining its margins the southern altiplano crust was deformed between the late oligocene and the late miocene with two main shortening stages in the oligocene 33 27 ma and middle late miocene 19 8 ma that succeeded eocene onset of shortening at the protoplateau margins shortening rates in the southern altiplano ranged between 1 and 4 7 mm yr with maximum rates in the late miocene summing rates for the southern altiplano and the eastern cordillera we observe an increase from eocene times to the late oligocene to some 8 mm yr followed by fluctuation around this value during the miocene prior to shutoff of deformation at 7 8 ma and transfer of active shortening to the sub andean fold and thrust belt shortening inverted early tertiary graben and half graben systems and was partitioned in three fault systems in the western central and eastern altiplano respectively the east vergent fault systems of the western and central altiplano were synchronously active with the west vergent altiplano west flank fault system from these data and from section balancing we infer a kinematically linked western altiplano thrust belt that accumulated a minimum of 65 km shortening evolution of this belt contrasts with the eastern cordillera which reached peak shortening rates 8 mm yr in between the above two stages hence local shortening rates fluctuated across the plateau superimposed on a general trend of increasing bulk rate with no trend of lateral propagation this observation is repeated at the shorter length and time scales of individual growth structures that show evidence for periods of enhanced local rates at a timescale of 1 3 myr we interpret this irregular pattern of deformation to reflect a plateau style of shortening related to a self organized state of a weak crust in the central south american back arc with a fault network that fluctuated around the critical state of mechanical failure tuning of this state may have occurred by changes in plate kinematics during the paleogene initially reactivating crustal weak zones and by thermal weakening of the crust with active magmatism mainly in the neogene stage

cenozoic foreland basin system in the central andes of northwestern argentina implications for andean geodynamics and modes of deformation 1 cenozoic strata in the central andes of northwestern argentina record the development and migration of a regional foreland basin system analogous to the modern chaco parana alluvial plain paleocene lower eocene fluvial and lacustrine deposits are overlain by middle upper eocene hypermature paleosols or an erosional disconformity representing 10 15 myr this supersol disconformity zone is traceable over a 200 000 km2 area in the andean thrust belt and is overlain by 2 6 km of upward coarsening eastward thinning upper eocene through lower miocene fluvial and eolian deposits middle miocene pliocene fluvial lacustrine and alluvial fan deposits occupy local depocenters with contractional growth structures paleocurrent and petrographic data demonstrate westerly provenance of quartzolithic and feldspatholithic sediments detrital zircon ages from cenozoic sandstones cluster at 470 491 522 544 555 994 and 1024 1096 ma proterozoic mesozoic clastic and igneous rocks in the puna and cordillera oriental yield similar age clusters and served as sources of the zircons in the cenozoic deposits arc derived zircons become prominent in oligo miocene deposits and provide new chronostratigraphic constraints sediment accumulation rate increased from 20 m myr during paleocene eocene time to 200 600 m myr during the middle to late miocene the new data suggest that a flexural foreland basin formed during paleocene time and migrated at least 600 km eastward at an unsteady pace dictated by periods of abrupt eastward propagation of the orogenic strain front despite differences in deformation style between bolivia and northwestern argentina lithosphere in these two regions flexed similarly in response to eastward encroachment of a comparable orogenic load beginning during late paleocene time

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

coincident anomalies of seismic attenuation and electrical resistivity beneath the southern bolivian altiplano plateau 1 reassessment of local earthquake data from the ancorp seismological network allowed the calculation of 3d attenuation qp tomographic images of crust and upper mantle beneath the southern bolivian altiplano around 21 s the images reveal a low qp middle and lower crust and a moderate qp upper mantle beneath the southern altiplano beneath the recent magmatic arc qp is not significantly decreased at this latitude the distribution of crustal qp coincides with the variation of electrical resistivity thus limiting the possible mechanisms causing the anomalies our findings support the hypothesis that partial melts in middle and lower crust beneath the altiplano are present on a large scale we see no evidence for a shallow asthenosphere beneath the southern altiplano

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

what drives orogeny in the andes the andes the world9s second highest orogenic belt were generated by the cenozoic tectonic shortening of the south american plate margin overriding the subducting nazca plate we use a coupled thermomechanical numerical modeling technique to identify factors controlling the intensity of the tectonic shortening from the modeling we infer that the most important factor was accelerated westward drift of the south american plate changes in the subduction rate were less important other important factors are crustal structure of the overriding plate and shear coupling at the plates9 interface the model with a thick 40 45 km at 30 ma south american crust and relatively high friction coefficient 0 05 at the nazca south american interface generates 300 km of tectonic shortening during 30 35 m y and replicates the crustal structure and evolution of the high central andes the model with an initially thinner

palaeogeographic consequences of conservational models in the south atlantic ocean conservational models like simple shear pure shear or polyphase models that exclude exchanges between the lower continental crust and upper mantle are usually proposed to explain the lithospheric stretching and consequent crustal thinning of passive continental margins these models need large amounts of horizontal movement and have therefore important implications for plate kinematic reconstructions and intraplate deformation in this paper we propose to show these implications in the central segment of the south atlantic ocean in the angola brazilian system these models imply about 240 km of horizontal movement this movement can be com pensated by two end member mechanisms 1 an intraplate deformation located in africa and 2 an intraplate deformation located in south america we detail for each solution the strong geological and geodynamical implications and discuss the consequences for the genesis of passive continental margins

lower bound on the amount of crustal shortening in the central bolivian andes balanced cross sections across the cordillera oriental and subandean zone of the central bolivian andes indicate that crustal shortening probably has played the dominant role in orogeny in this convergent margin setting a minimum amount of shortening 210 km is documented which can account for two thirds of the present day crustal cross sectional area along a transect spanning the entire mountain range substantial crustal shortening may also require loss of the lower lithosphere to the asthenosphere a large minimum amount of crustal shortening in the bolivian andes shows contrary to common assumptions about orogeny that 1 magmatic addition may be volumetrically less important in orogeny in andean type margins and 2 crustal shortening is not uniquely associated with continental or island arc collision

paleomagnetic study of upper miocene rocks from northern chile implications for the origin of late miocene recent tectonic rotations in the southern central andes paleomagnetic studies in the southern central andes have shown the widespread presence of clockwise vertical axis rotations rock units sampled in these studies however are heterogeneously distributed in stratigraphic age with most paleomagnetic data from northern chile being from mesozoic and lower tertiary rocks whereas most data in the southern altiplano puna and cordillera oriental are from upper tertiary rocks in this paper we present the results of a paleomagnetic study on upper miocene sedimentary rocks and ignimbrites from the precordillera of northern chile at 22 s these rocks are coeval with the initiation of crustal shortening in the eastern foreland fold thrust belt which some tectonic models relate to oroclinal rotation of northern chile primary magnetizations in rocks from widely distributed sites in two ignimbrites indicate that no relative rotations have occurred between sites suggesting the study area has acted as a single coherent block with respect to vertical axis rotational deformation although minor inadequate sampling of paleosecular variation can affect our data set its time averaged paleomagnetic direction is indistinguishable from the expected late miocene reference direction indicating no paleomagnetically significant rotation of the study area since circa 11 ma this suggests that late miocene recent oroclinal rotation of the northern chilean forearc if present must be either very low or nonuniform a direct implication of this result is that much of the unquestionable tectonic rotations detected from upper miocene rocks in the southern central andes is of local origin available structural data permit us to relate several of these rotations with shear in dextral transfer zones in the foreland thrust belt on the east side of the andes

strong crustal heterogeneity in the bolivian altiplano as suggested by attenuation of lg waves using the seismological data of four temporary networks installed across the andes of bolivia and chile between 1990 and 1995 we study the amplitudes of the lg phase for 45 regional events located at crustal depths this analysis reveals that lg is strongly attenuated for most of the ray paths crossing the altiplano independent of the azimuth of propagation to map the anomalies and evaluate their frequency dependence we conduct a regionalization of the crustal average qs factor using a damped least squares inversion technique in the frequency range 0 6 5 hz the resulting maps delineate regions with contrasting values of apparent attenuation the altiplano is characterized by strong attenuation with an average qs of 100 at 1 hz whereas the cordilleras are regions of weaker attenuation the frequency dependence of qs is investigated in an attempt to separate the source of the apparent attenuation into scattering and anelastic absorption within the hypothesis of a weak frequency dependence of the intrinsic q we show that at 1 hz scattering plays a major part in the attenuation of lg waves whereas at 4 hz both mechanisms are of equal importance these results indicate that the attenuation of lg waves in the altiplano is due more to scattering by small scale heterogeneities than to a high percentage 1 of partial melt involving a large part of the crust

central andean crustal structure from receiver function analysis abstract the central andean plateau 15 27 s is a high plateau in excess of 3 km elevation associated with thickened crust along the western edge of the south america plate in the convergent margin between the subducting nazca plate and the brazilian craton we have calculated receiver functions using seismic data from a recent portable deployment of broadband seismometers in the bolivian orocline 12 21 s region and combined them with waveforms from 38 other stations in the region to investigate crustal thickness and crust and mantle structures results from the receiver functions provide a more detailed map of crustal thickness than previously existed and highlight mid crustal features that match well with prior studies the active volcanic arc and altiplano have thick crust with moho depths increasing from the central altiplano 65 km to the northern altiplano 75 km the eastern cordillera shows large along strike variations in crustal thickness along a densely sampled sw ne profile through the bolivian orocline there is a small region of thin crust beneath the high peaks of the cordillera real where the average elevations are near 4 km and the moho depth varies from 55 to 60 km implying the crust is undercompensated by 5 km in comparison a broader region of high elevations in the eastern cordillera to the southeast near 20 s has a deeper moho at 65 70 km and appears close to isostatic equilibrium at the moho assuming the modern day pattern of high precipitation on the flanks of the andean plateau has existed since the late miocene we suggest that climate induced exhumation can explain some of the variations in present day crustal structure across the bolivian orocline we also suggest that south of the orocline at 20 s the thicker and isostatically compensated crust is due to the absence of erosional exhumation and the occurrence of lithospheric delamination

late miocene early pliocene onset of n s extension along the southern margin of the central andean puna plateau evidence from magmatic geochronological and structural observations abstract extensional faulting typifies many of the earth s cenozoic orogenic plateaus the altiplano puna plateau in the southern central andes is the world s second largest orogenic plateau it contains a rich array of extensional fractures and normal faults with closely associated mafic volcanic flows dykes and cones this is in stark contrast to the foreland areas which record protracted shortening and continuous eastward growth of the mountain belt within the puna area a kinematic change over from shortening to extension has previously been inferred to have occurred during the quaternary however our research in the southern puna together with earlier petrologic investigations in the south central part of the plateau indicates an earlier onset for the extensional tectonism our analysis has focused on the eastern portion of the cordillera de san buenaventura in the southernmost part of the plateau where we mapped two upper miocene pliocene volcanic units associated with extensional faulting the aguada alumbrera ignimbrite and the la hoyada volcanic complex the structural inventory of these units records n s extensional deformation in the puna associated with the development of e w oriented fractures dykes normal and strike slip faults and folds new 40ar 39ar dating of magmatic rocks together with their relationships with the mapped structures reveal that widespread extension in the southern puna had already commenced by 5 ma and possibly as early as 7 ma although the extensional structures on the puna are typically associated with mafic volcanism the upper miocene pliocene la hoyada volcanic complex hosts dykes of intermediate compositions mafic to intermediate magmatism and extensional tectonism in this region is thus contemporaneous with and structurally akin to that in the central and northern parts of the plateau the ubiquitous cogenetic relationship between mafic volcanism and extensional tectonism both within the puna plateau and along its eastern flanks clearly indicates that the normal faulting is a regional phenomenon that is broadly coeval with an inferred rapid and large scale uplift of the altiplano puna plateau and a decrease in plate convergence rates these observations are compatible with a mantle delamination scenario in which the delamination of lithospheric mantle may have produced an ensuing weakening of the crust and regional uplift which may in turn have generated the conditions necessary to trigger gravitational extensional collapse beginning in the late miocene to early pliocene

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

response of intracontinental deformation in the central andes to late cenozoic reorganization of south american plate motions new stratigraphic chronologic and fault kinematic data from the quebrada del toro in northwestern argentina provide a detailed view of late cenozoic kinematic reorganization in the southern central andes the quebrada del toro is a thrust bounded basin filled with 2 km of synorogenic clastic deposits ranging in age from early miocene to holocene as observed elsewhere in northwestern argentina faults in the quebrada del toro record an early phase of horizontal nw se contraction beginning between early and late miocene and ending after 0 98 ma and a later phase of horizontal ne sw contraction beginning between late miocene and 4 17 ma and still active these results require both kinematic regimes to have been active between 4 17 and 0 98 ma compatible with timing and kinematic data from the adjacent central andes and best explained by a temporal pattern of serial recurrence of the kinematic regimes a hypothesis that intraplate deformation in the central andes is driven by the absolute motion of the south american plate with respect to the hotspot reference frame is tested by comparing quebrada del toro kinematics to plate reconstructions and space based geodetic results nazca south america relative motion and absolute nazca plate motion show consistent azimuths throughout the past 20 5 myr in contrast absolute south american plate motion azimuths mimic the changing contraction directions determined from intraplate andean faults although the timing of the south american plate motion change is poorly resolved by current data it is compatible with the quebrada del toro data absolute upper plate motion might control intraplate kinematics along many of the earth s convergent margins

the structure of the precambrian and lower paleozoic basement of the central andes between 22 and 32 s lat three major diastrophic cycles defined by their structural style and their spatial distribution are recognized in the andean basement of this region the oldest structures are related to the panamerican orogeny 500 to 700 m a which produced in the central craton multiply deformed complexes of schists gneisses and granites that are covered discordantly by unmetamorphosed cambrian and ordovician beds west of the central craton ordovician sedimentary beds are folded with a simple structural style and intruded by granites both the sedimentry beds and the granites are covered discordantly by undeformed devonian sequences the folding of the ordovician is attributed to the ocloyic phase of the caledonian movements west of the ocloyic belt is another foldbelt consisting of strongly folded devonian beds attributed to the chanic phase hercynian the chanic belt is intruded by carboniferous and permian granites and covered discordantly by carboniferous and permian sequences

end member models for andean plateau uplift abstract diverse techniques have been applied over the past decade to quantify the uplift history of the central andean plateau ap in this study opposing models for surface uplift are evaluated including a rapid rise of 2 5 km 10 6 ma and a slow and steady rise since 40 ma these end member models are evaluated by synthesizing observations of the ap lithosphere and the history of deformation sedimentation exhumation magmatism uplift and fluvial incision structural and geophysical studies estimate variable shortening magnitudes 530 150 km involving cover to basement rocks an isostatically compensated thick crust 80 65 km high heat flow and zones of variable velocity and attenuation in the crust and mantle these observations have invoked interpretations such as a hot weak lithosphere partial melt crustal flow and perhaps current localized delamination but do not provide strong support for massive delamination required by the rapid uplift model deformation and associated exhumation began 60 40 ma and generally migrated eastward with consistent long term average shortening rates 12 8 mm yr in bolivia favoring the slow uplift model volcanic and helium isotope evidence show an ap wide zone of shallow mantle melting and thin lithosphere that has existed since 25 ma which is inconsistent with the rapid rise model that suggests lithospheric thinning occurred 10 6 ma paleoaltimetry data suggest a rapid 2 5 km elevation gain 10 to 6 ma but are equally consistent within error with a linear rise since 25 ma widespread fluvial incision 2 5 1 km occurred along the western flank since 11 8 ma and may be associated with surface uplift as proposed by the rapid rise model however the paleoaltimetry and incision data can also be explained by regional climate change associated with plateau uplift implications of these results for reconstructions of ap evolution are that 1 substantial deformation of a weak lithosphere is essential 2 ap growth has taken significantly longer 40 myr and was more uniform along strike 1500 km than previously appreciated and 3 the slow and steady uplift model is most consistent with available constraints we conclude that the rapid uplift model may be an overestimate and that a more protracted cenozoic uplift history is tenable

initial plate geometry shortening variations and evolution of the bolivian orocline comparisons of newly published cross sections across the bolivian andes with existing cross sections through argentina emphasize significant along strike changes in crustal shortening a sharp decrease in the magnitude of crustal shortening from 530 km to 150 km north to south occurs at 23 s a 20 40 m y difference in the ages at which deformation was initiated accompanies the abrupt decrease in the magnitude of shortening extending the western margin of south america to account for 530 km of shortening in the bolivian andes and 150 km of shortening in argentina produces a central andean salient that is perpendicular to the nazca plate shortening direction from 60 to 26 ma during this same time interval the chilean coast south of 23 s was in an orientation sufficiently oblique to oceanic convergence to allow for predominantly strike slip offset and backarc extension deformation within the andean mountain chain may be a function of plate convergence where the oblique nature of convergence south of 23 s inhibited mountain building whereas north of 23 s normal convergence to a central andean salient facilitated contractional deformation the magnitude of deformation north of 23 s is a consequence of both plate convergence direction providing a longer period of contractional deformation from ca 70 ma to the present and a thick phanerozoic sedimentary package that permitted large magnitudes of thin skinned deformation significant along strike changes in the shape of the south american margin allowing for convergence to change from compression to extension along the strike of the orogen may help explain the dramatic differences in timing amount and style of deformation in the andes

growth of the central andean plateau by tectonic segmentation is controlled by the gradient in crustal shortening abstract crustal deformation orocline formation and growth of the central andean plateau as well as the mutual relations of these processes with climate are a matter of international debate earthquake focal mechanism fault kinematic paleomagnetic and cross section balancing data are paramount for understanding the morphotectonic evolution of this noncollisional orogen but collectively await an explanation in terms of a comprehensive kinematic model we offer such a model that approximates simple shear in plan view and that predicts the kinematics of first order fault zones in the southern central andes the model indicates that segmentation of the upper crust into rhomb shaped domains confined by transpressive ranges resulted from the decrease in crustal shortening with distance to the central bend of the andes an anastomosing network of kinematically coupled reactivated orogen parallel and newly formed ne trending deformation zones encompassing components of sinistral and dextral strike slip res

kinematics of neogene to recent upper crustal deformation in the southern central andes 23 28 s inferred from fault slip analysis evidence for gravitational spreading of the puna plateau abstract elucidating the tectono morphologic evolution of the central andes is important for understanding the mechanical behaviour of non collisional orogens at convergent plate boundaries fault kinematic data from the southern central andes are generally interpreted in terms of changes in plate boundary kinematics or vertical axis rotation associated with the formation of the andean orocline however none of these hypotheses accounts adequately for neogene to recent horizontal extension and vertical shortening of upper crust documented from the puna plateau and the adjacent eastern cordillera based on new and compiled fault kinematic data in total 4746 brittle faults at 317 stations we propose that strain axis configurations in the southern central andes are controlled by 1 local deformation kinematics 2 the far field deformation caused either by the orogen normal shortening component of plate convergence or by removal of lithospheric mantle and 3 differences in lithospheric thickness and respective surface elevation i e gravitational potential energy specifically the puna plateau and eastern cordillera have been undergoing horizontal shortening and gravitational spreading of the upper crust whereas the topographically lower pampean ranges continue to undergo horizontal shortening only although less apparent in the fault kinematic data than the e w and nw se extension orogen parallel extension is inherent to the puna plateau and eastern cordillera and compensates likely for overall wnw ese shortening and enhanced surface elevation the importance of orogen parallel extension in the deformation kinematics of topographically elevated parts of the southern central andes is underscored by the strike slip components on prominent reverse and thrust faults the data supports geodynamic interpretations in which the evolution of topography and lithospheric thickness influence significantly changes in upper crustal kinematics of the southern central andes

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

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

new geochemical and isotopic insights to evaluate the geothermal resource of the hydrothermal system of rosario de la frontera salta northern argentina abstract in this study the chemical and isotopic composition of thermo mineral springs from the rosario de la frontera hydrothermal system was used to construct a conceptual model describing the source regions the thermal fluids and the chemical physical processes controlling the chemistry of waters and dissolved gases during their underground circulation the main hydrothermal reservoir hosted within the cretaceous pirgua subgroup deposits is fed by meteoric water and shows a na hco 3 composition produced by water rock interactions involving sedimentary formations mostly consisting of conglomerates and sandstones which are interbedded with alkaline volcanic rocks and shales and limestone deposits this aquifer also receives significant contributions of crustal co 2 and he from mantle degassing the latter being likely favored by the regional tectonic assessment that is characterized by a deep detachment at about 10 km depth in the basement of the santa barbara thick skinned thrust system and a thinned lithosphere the uprising thermal fluids mix with a relatively high salinity na cl dominated aquifer produced by the interaction of meteoric water with the tertiary anta formation evaporite the temperatures of the hydrothermal reservoir estimated with water geothermometers are up to 130 c which are consistent with the thickness of the hydrothermal circuit 2700 3000 m and the relatively high local geothermal gradient 40 c km these results suggest that the heat stored in the fluid phase of rfhs is up to 1 10 18 j a value significantly higher 20 than that previously estimated assuming an average reservoir temperature of 90 c

velocities and propagation characteristics of pn and sn beneath the himalayan arc and tibetan plateau possible evidence for underthrusting of indian continental lithosphere beneath tibet new seismological observations on velocities and propagation characteristics of pn and sn waves beneath tibet can be interpreted although not uniquely to indicate the shallow angle underthrusting of the greater indian continental lithosphere beneath the tibetan plateau this is inferred from available geological and geophysical data and is now supported by new seismological observations the most significant observation is that high frequency sn waves shear waves that travel in the mantle lithosphere propagate efficiently in the uppermost mantle beneath the tibetan plateau except beneath the north central part of tibet the chang thang terrane furthermore sn propagates efficiently across the stable block of the tarim basin along the tien shan and himalayan mountains and across the indian shield apparent velocities of pn and sn waves that traverse the uppermost mantle beneath the tibetan plateau are 8 42 and 4 73 km s respectively these velocities are very similar to those beneath the himalayan mountains and the indian shield the efficient propagation and the velocities of pn and sn waves beneath tibet are also similar to those commonly observed beneath shield and stable continental regions these results are consistent with the underthrusting model and do not favor the class of alternative models in which the tibetan plateau is formed by shortening and thickening of hot crustal and uppermost mantle material in response to the convergence of india and asia

lithospheric flexure and bending of the central andes gravity anomaly and topography data are used to define the effective elastic thickness of the lithosphere t in the bend region of the central andes values of z increase from nearly zero north and south of the bend to values greater than 50 km at the bend in bolivia there is a close correlation between t and the style and magnitude of the shortening in the central andes since the late miocene in the bend region where the lithosphere is flexurally strong with large t foreland deformation is concentrated into a thin skinned fold and thrust belt above a basal decollement which has absorbed more than 100 km of shortening further north and south where the lithosphere is flexurally weak with low t foreland deformation is more complex involves basement and has absorbed less shortening the along strike gradients in foreland shortening have accommodated both clockwise and anticlockwise rotations about a vertical axis of almost the entire width of the bolivian andes we speculate that the observed variation in t is related to the proximity of the brazilian shield and has been an important factor in controlling the nature and amount of foreland deformation and hence bending of the central andes the flexural properties of the lithosphere may play an important role in determining the large scale evolution of mountain belts

geometry and kinematic evolution of passive roof duplexes deduced from cross section balancing example from the foreland thrust system of the southern bolivian subandean zone the subandean zone of bolivia is a foreland fold and thrust belt which forms the eastern edge of the central andes mountains between 19 s and 22 s latitude the construction of five balanced cross sections shows that the n s trending subandean zone is characterized by the existence of passive roof duplexes these complex structures can be distinguished by the lithotectonic unit within which duplexing occurs the five balanced cross sections permit the geometric and kinematic analyses of these passive roof duplexes the sequential restorations of certain cross sections reveal a possible development of a piggy back sequence of three passive roof duplexes apparently these passive roof duplexes propagated toward the foreland from deeper and deeper lithotectonic units while a passive roof duplex was developing the sole thrust stuck and the major horizontal displacements were then transferred either to out of sequence thrusts or to a new sole thrust in a deeper detachment horizon therefore each passive roof duplex would correspond to the orogenic front of the andean range at one very particular time in the history of the subandean zone of southern bolivia from south to north the quantitative analysis by cross section balancing shows a transfer of displacement from the hinterland structures to the passive roof duplex that forms the present orogenic front available data do not permit us to explain completely this phenomenom

seismicity and tectonics in jujuy province northwestern argentina the portable array for numerical digital analysis panda network a digitally recorded seismic array operated for nine months in jujuy province of northwestern argentina the network was deployed along the eastern edge of the altiplano puna plateau in a major n s thrust belt that is transitional in style between the thin skinned deformation of the bolivian foreland to the north and basement involved deformation of the pampean region to the south teleseismic locations of crustal earthquakes in the region indicate that seismicity is associated with compressional structures found near the eastern deformation front no crustal seismicity was detected beneath the puna plateau to the west peak seismicity levels beneath the foreland occurred between 20 and 25 km depth a sharp decrease in seismicity was observed below 25 km an estimate of 42 km for the thickness of the jujuy foreland crust was inferred from wide angle moho reflections observed on the digital seismograms the highest concentration of crustal seismicity was located beneath sierra de zapla a broad anticline immediately east of san salvador de jujuy many of the earthquakes in the 20 25 km depth range have a shallow west dipping nodal plane as does the focal mechanism solution for a moderately large 1973 earthquake inversion of focal mechanism data for the orientation of principal stresses shows that maximum compression is oriented at azimuth 74 closely paralleling both the current nazca south america convergence direction and the shortening direction derived from regional quaternary fault slip data we interpret the earthquakes as occurring on planes of weakness first produced during cretaceous rifting and later reactivated by neogene compressive stresses crustal seismicity patterns and fault plane solutions suggest the presence of a midcrustal detachment along which significant late cenozoic e w shortening has occurred

apatite fission track dating of erosion in the eastern andes bolivia abstract three samples from a triassic age batholith in the eastern andes northeast of la paz bolivia yield apatite fission track ages of 11 13 ma interpreting these young ages as due to uplift and erosion requires approximately 2 5 5 0 km of erosion in the past 12 ma an amount which is consistent with the known geology and which is typical of many active mountain ranges

did shortening in thick crust cause rapid late cenozoic uplift in the northern bolivian andes abstract this study explores the role of crustal shortening as a mechanism for late cenozoic surface uplift in the northern bolivian andes between c 16 s and c 19 s based on new geological cross sections across the corque corocoro basin in the altiplano c 60 km thick crust today and published radiometric ages of volcanic rocks in the context of a simple 2d crustal balancing model deformation since c 10 ma mainly between 10 and 6 ma involved 28 7 km of shortening across the corque corocoro basin with a wider zone of lower crustal strain and a total of c 70 km of underthrusting in the subandes the uplift model is supported by dynamical analyses and is identical within 2 error to published palaeo elevation proxies with 2 5 0 5 km of surface uplift in the corque corocoro basin since c 10 ma 1 8 0 5 km between 10 and 6 ma and 1 6 0 5 km outside the basin in this way the late cenozoic palaeo elevation record in the northern bolivian andes can be explained within error entirely in terms of the late cenozoic crustal thickening expected for the observed history of crustal shortening

tectonic development southeastern border of the puna plateau northwestern argentine andes the puna altiplano a 500 000 km2 area at an average elevation of 3 7 km in the central andes occurs in a non collisional plate setting over a 30 east dipping segment of the benioff zone and is bounded to the north and south by nearly flat segments of the subducted nazca plate a 120 km long portion of the southeastern boundary of the puna in northwestern argentina was studied to gain insight into the structures associated with uplift of the plateau and into the nature of structures over the transition from flat to steeper subduction the structural geometry and kinematic evolution of the region was determined from geologic mapping and kinematic analysis of minor fault arrays the present southeastern margin of the puna was uplifted between 5 10 and 2 ma northwest southeast horizontal shortening and vertical extension during uplift were produced by northeast striking en echelon thrust faults dipping 17 55 northwest four to seven en echelon structural blocks each 20 30 km long constitute the border of the puna their structural geometry precludes large scale overthrusting as an important mechanism of plateau uplift along the studied segment in the past 2 m y there has been a 45 change in the direction of shortening and locally a 90 change in the extension direction indicating that the structures accompanying uplift are no longer active this change could be due to a change either in the geometry of the subducted plate or in convergence directions although neither of these hypotheses is testable given presently available data