do transcurrent faults guide volcano growth the case of nw bicol volcanic arc luzon philippines this paper shows that although volcanoes can be aligned along transcurrent faults their growth can in reality be controlled by secondary structures that have a different orientation and kinematics assessing the relationships between local structures and volcano growth can be very useful for the evaluation of volcanic hazards and for economic purposes such as geothermal and hydrogeological exploration in the north western bicol volcanic arc luzon philippines the quaternary labo and caayunan volcanoes are aligned with the nw striking transcurrent philippine fault system they are exceptional examples of composite volcanoes mostly comprising lava domes where regional tectonic structures and hydraulic radial fracturing have alternated to control volcano growth field data and analogue modelling demonstrate that apical depressions affecting the domes reflect the geometry of magma feeding fractures these data together with alignments of domes and hot springs and other structural information suggest that magma rose at depth along the nw striking transcurrent faults but that in the uppermost crust magma rose along ne striking fractures parallel to the greatest principal stress

bounds on the viscosity coefficient of continental lithosphere from removal of mantle lithosphere beneath the altiplano and eastern cordillera 1 the rapid rise of the central andean plateau between 10 and 6 8 ma implies that mantle lithosphere including eclogitized lower crust was removed from beneath the region in that time interval we infer from that removal that the average viscosity coefficient of mantle lithosphere was quite low when removal occurred using scaling laws for the growth of perturbations to the thickness of a dense layer over an inviscid substratum rayleigh taylor instability we place bounds on the average viscosity coefficient for central andean lithosphere when compared with laboratory measurements of flow laws for olivine and eclogite the allowed range of viscosity coefficients yields bounds on the temperature of 500 800 c at the moho beneath this region and suggests that mean stresses across mantle lithosphere during continental deformation are less than 50 mpa this range of temperature is comparable with if a slightly lower than we might expect for lithosphere approximately doubled in thickness and not yet equilibrated with the doubled crustal radioactivity the mean deviatoric stress is comparable to that associated with stresses that drive plates and hence shows that lithospheric material is not too strong to prevent removal of its mantle part

tectonomagmatic characteristics of the back arc portion of the calama olacapato el toro fault zone central andes 1 post 20 ma magmatism in the central andes is either localized in the magmatic arc or distributed east of it on the altiplano puna plateau here there is a distinct concentration of magmatic centers on nw se trending lineaments such as the calama olacapato el toro cot that extends into the eastern cordillera to the east of the puna understanding the possible genetic relationship between prominent structures and magmatic centers on these lineaments is important to elucidate the tectonomagmatic evolution of the central andes we investigated the back arc area of the cot using remote sensing geological structural and petrochemical data our study demonstrates that this portion of the cot consists of nw se striking faults formed under overall left lateral transtension that decreases in activity toward the cot termini deformation on the cot occurred during and after activity of prominent n s striking transpressive fault systems and is coeval with magmatism which is focused on the central cot the most evolved magmatic rocks with an upper crustal imprint are exposed on the central cot whereas more primitive mantle derived mafic to moderately evolved magmatic rocks are found toward the cot termini this points to a genetic relationship between upper crustal deformation and magmatic activity that led to enhanced magma storage in the central cot cot magmas may result either from slab steepening or episodic delamination of the asthenospheric mantle

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

displacement geometry in the volume containing a single normal fault fault displacements measured in coal mines and from seismic data are used to develop a model describing the near field displacements associated with an ideal single normal fault displacement on a fault surface ranges from a maximum at the center of the fault to zero at the edge or tip line the tip line is elliptical with the shorter axis of the ellipse parallel to the displacement direction contours of equal displacement form concentric ellipses centered on the point of maximum displacement displacement gradients vary with fault size and with mechanical properties of host rock fault radius to maximum displacement ratios range from 5 to 500 plotting of displacement contour diagrams and knowledge of displacement gradients are useful in interpreting seismic reflectio data both for quality control of interpretations and for quantitative extrapolation of limited data displacements associated with faulting decrease systematically with increasing distance along the normal to the fault surface this decrease is seen as reverse drag in both hanging wall and footwall hanging wall rollover and tilting of the reflectors cannot be used to distinguish listric from planar normal faults even where fault block rotation can be demonstrated neither listric fault geometry nor a flat detachment surface is geometrically necessary because faulting is accommodated by ductile deformation rigid fault bounded blocks cannot exist except in some special circumstances related to a free surface the displacements within the rock volume affected by a single fault are not simply related to regional extension apparent horizontal extension by faulting varies from one layer to another and a significant proportion of the extension in a basin may be due to ductile deformation

thickening the altiplano crust by gravity driven crustal channel flow 1 in the central andes crustal thickness is not well correlated to upper crustal shortening only little shortening is documented in the upper crust of the 60 65 km thick altiplano plateau whose thickening and uplift were delayed with respect to earlier and greater thickening in the adjacent western and eastern cordilleras because crustal thickness variations induce horizontal stress gradients and cause crustal flow a thickness dependent channel flow is modeled here and applied to the central andes in situ thickening is assumed for both cordilleras while the altiplano crustal thickening is generated by lateral flow from these overthickened adjacent domains a 8 1018 pa s viscosity channel is predicted for crustal thicknesses exceeding 45 50 km to match the estimated topographic evolution of the central andes thickening in the cordilleras was sufficient to generate a flow of 6 109 m3 per unit length toward the initially 30 35 km thick altiplano

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

crustal lineament control on magmatism and mineralization in northwestern argentina geological geophysical and remote sensing evidence abstract the relationship between long lived deep crustal lineaments and the locations of magmatic centers and associated mineral deposits has been investigated in the puna region of northwestern argentina through the analysis of regional aeromagnetic surveys landsat images and geological information the good exposure and excellent preservation of basement and supracrustal geology in this region makes it particularly suitable for such a study at a regional scale several contrasting magnetic domains are recognized which correlate with crustal geology two basement domains are separated by a nne trending boundary which is believed to correlate with a paleozoic suture zone between the pampia to the southeast and arequipa antofalla terranes to the northwest locally overlying these basement terranes is the cenozoic magmatic domain which is best developed in the n s trending volcanic arc at the western edge of the puna the cordillera occidental in addition four southeast trending volcanic zones extend for several hundred kilometers across the puna many important mineral deposits and areas of hydrothermal alteration are associated with these volcanic breakouts and we have selected three such areas for more detailed study bajo de la alumbrera argentina s largest porphyry copper deposit cerro galan the largest ignimbrite caldera in argentina with associated hydrothermal alteration zones and el queva a historic polymetallic district located within a major volcanic range a comparison of lineament maps generated from aeromagnetic and landsat tm images reveals broad correlation between these different remote sensing techniques which respectively highlight subsurface magnetic and surface geological features in addition the locations of magmatic and hydrothermal centers can be related to the interpreted structural framework and are seen to occur near the intersections of major lineament zones it is suggested that in three dimensions such intersection zones form trans lithospheric columns of low strength and high permeability during transpressional or transtensional tectonic strain and may thereby serve as conduits for magma ascent to the shallow crust pooling of large volumes of deeply derived magma in shallow crustal magma chambers may then result in voluminous devolatilization and the formation of hydrothermal mineral deposits it is important to note that in this model structural intersections serve as facilitators for magma ascent and volatile exsolution but do not in themselves cause this process other factors such as magma supply rate and tectonic stress are essential primary ingredients and local magmatic and volcanic processes affect the ultimate potential for ore formation nevertheless we suggest that lineament analysis provides a valuable framework for guiding the early stages of mineral exploration other regional and local geological considerations must then be applied to identify priority targets within this framework

structural architecture of the colima volcanic complex 1 the colima volcanic complex cvc is currently the most active mexican volcano and is located in the western sector of the trans mexican volcanic belt inside the active colima rift a regional n s striking extensional structure the colima rift is filled by a 1 km thick sequence of quaternary lacustrine sediments alluvium and colluvium mostly underling the about 3000 m thick volcanic pile of the cvc in this work we present the results of a detailed morphostructural and field study of quaternary faults and fractures in the cvc and the surrounding area including the regional structures of the colima rift we also present a geometrical modeling of the faults inside the volcano and a numerical model of the gravity induced stress and strain fields of the cvc the study attempts to characterize the geometry kinematics and dynamics of the deformation features of the cvc and relate it with the volcano structure the geology of the substratum and the geodynamic setting of the region our model considers that the observed deformation of the cvc and the surroundings results from the interplay between the active n s trending regional extensional tectonics and the southward spreading of the volcano over its basement forming an e w oriented volcanotectonic graben the interaction between regional tectonics and previously unrecognized volcanic spreading can control magma migration and flank instability in an area where eruptions and sector failures represent a potential high risk for more than 500 000 people

shallowing and steepening subduction zones continental lithospheric loss magmatism and crustal flow under the central andean altiplano puna plateau

bending of the bolivian orocline and growth of the central andean plateau paleomagnetic and structural constraints from the eastern cordillera 22 24 s nw argentina 1 we report new paleomagnetic and structural data from late cretaceous to mio pliocene continental sandy silty sedimentary rocks from the eastern cordillera central andes here n s to nne ssw ridges hosting paleozoic basement and upper cretaceous continental red beds overthrust thick adjacent cenozoic basins pretilting and likely primary reliable directions gathered at 15 sites document 45 9 9 4 30 1 23 9 and 15 4 19 3 clockwise cw rotations with respect to south america occurring after the late cretaceous 80 ma oligo miocene 20 30 ma and late miocene pliocene 5 10 ma respectively conversely four upper cretaceous sites from the walls of a n s left lateral strike slip fault yavi abra pampa fault yield a null rotation about 20 km to the west flower structures and subvertical syntectonic strata dated at 14 26 0 19 ma are exposed along the subparallel abra moreta left lateral strike slip fault relying on data from the literature on the period when deformation began we suggest that since eo oligocene times 30 40 ma the eastern cordillera has undergone a regional cw rotation of 40 50 synchronous with crustal shortening and large scale bending of the andean salient the cw rotation is possibly still active today as documented by regional gps data from the andes since 15 ma ago the activity of n s left lateral strike slip faults induced counterclockwise rotations along the fault zone locally annulling the regional cw rotation in agreement with a previous model we speculate that mid miocene strike slip activity accommodated the progressive southward spreading of the altiplano puna high altitude plateau laterally migrating from the overthickened crustal region of the salient apex

oligocene range uplift and development of plateau morphology in the southern central andes 1 the puna altiplano plateau in south america is a high elevation low internal relief landform that is characterized by internal drainage and hyperaridity thermochronologic and sedimentologic observations from the sierra de calalaste region in the southwestern puna plateau argentina place new constraints on early plateau evolution by resolving the timing of uplift of mountain ranges that bound present day basins and the filling pattern of these basins during late eocene miocene time paleocurrent indicators sedimentary provenance analyses and apatite fission track thermochronology indicate that the original paleodrainage setting was disrupted by exhumation and uplift of the sierra de calalaste range between 24 and 29 ma this event was responsible for basin reorganization and the disruption of the regional fluvial system that has ultimately led to the formation of internal drainage conditions which in the salar de antofalla were established not later than late miocene upper eocene oligocene sedimentary rocks flanking the range contain features that suggest an arid environment existed prior to and during its uplift provenance data indicate a common similar source located to the west for both the southern puna and the altiplano of bolivia during the late eoceneoligocene with sporadic local sources this suggests the existence of an extensive longitudinally oriented foreland basin along the central andes during this time citation carrapa b d adelmann g e hilley e mortimer e r sobel and m r strecker 2005 oligocene range uplift and development of plateau morphology in the southern central andes tectonics 24 tc4011 doi 10 1029 2004tc001762

cenozoic dextral transpression and basin development at the southern edge of the puna plateau northwestern argentina abstract in northwestern argentina the southern edge of the altiplano puna is the most remarkable positive topographic anomaly average altitude 3700 m of the andean chain to the south are the sierras pampeanas a less elevated domain made of alternating compressive basins and ranges developed during the cenozoic at about 27 s the transition between the puna and the sierras pampeanas coincides with a substantial narrowing of the high chain and with a change in the dip of the pacific subduction zone nazca plate from 30 e below the plateau to subhorizontal below the sierras pampeanas we argue that this structural transition which can be identified at the scale of the andean topography is a major dextral transpressional transfer zone the tucuman transfer zone in this region ene wsw to e w directed subhorizontal shortening related to bulk convergence between the nazca and south american plates interferes with nw se directed subhorizontal shortening strains due to lateral influence of changes in the amount of crustal thickening on intraplate kinematics structural observations digital mapping spot satellite images and a microtectonic analysis of fault populations provide general insights into the neogene kinematic evolution of transpressional basins and ranges at the edge of the puna

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

the calama olacapato el toro fault system in the puna plateau central andes geodynamic implications and stratovolcanoes emplacement abstract the structural evolution of the puna plateau is characterized by the activity of both orogen parallel and orogen oblique faults understanding the possible relationship between these two structural styles their geodynamic implications and the influence on the migration of magmas is important to get insights into the tectonic and magmatic evolution of the central andes in this study we present a structural analysis of the orogen oblique calama olacapato el toro fault system and the surrounding orogen parallel thrust faults in the central eastern puna plateau morphostructural analysis and field mapping reveal the geometry kinematics and dynamics of the tectonic features in the studied area we propose a three dimensional geometrical reconstruction of the main fault planes showing their attitude and intersections at depth the study indicates that the crust underwent simultaneous deformation along both the vertical transcurrent calama olacapato el toro fault system and the low angle thrust faults and that the back arc portion of the calama olacapato el toro fault system developed as a transfer zone among the main n striking thrusts our model considers that both orogen parallel and orogen oblique fault systems should be regarded as parts of the same tectonic system accommodating crustal shortening of a thickened crust the study suggests that the tectonic control on the magma and fluid circulation in the crust is mainly related to the geometry of the fault planes and the orientation of the stress field with a previously unrecognized important role played by the orogen parallel thrust faults on the emplacement of the stratovolcanoes

spatially variable exhumation rates in orogenic belts an andean example abstract the cordillera real of the bolivian andes is a large tectonically active mountain range that dominates sediment influx into the amazon basin but rates of exhumation in the orogen are poorly known we present 20 new apatite fission track ages from two valleys in the cordillera real to constrain patterns of mountain range exhumation over 106 107 yr we interpret these and previously published data from a third valley using a 2 d thermal model that accounts for topographic and advective influences on measured cooling ages exhumation rates in the cordillera real are 0 2 0 6 mm yr comparable to rates in parts of denali the washington cascades the olympic mountains and the european alps and an order of magnitude slower than rates in taiwan nanga parbat the greater himalaya of nepal and the southern alps of new zealand three to fourfold cooling age variations in the cordillera real imply at least twofold exhumation rate variations within and between valleys over distances of only tens of ki

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

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

aster watches the world s volcanoes a new paradigm for volcanological observations from orbit onboard nasa terra spacecraft is one of the most sophisticated earth observing instruments ever flown the advanced spaceborne thermal emission and reflection radiometer aster created as part of a joint project between the japanese ministry of economy trade and industry and nasa this instrument consists of three independent bore sighted telescopes with 14 channels in the visible near infrared and thermal infrared regions capable of being pointed up to 22 5 off nadir in addition there is an additional backward looking visible channel telescope for stereo photogrammetric observations the spatial resolutions are 15 m pixel in the visible 30 m pixel in the near ir and 90 m pixel at thermal wavelengths and the swath width is 60 km aster instrument offers unprecedented opportunities for observing volcanoes and their activity from low earth orbit it is the purpose of this short paper to introduce aster to the volcanological community highlight its capabilities advantages and limitations and to show some examples of aster data and related analyses we will also explain how users can readily access aster data

puncoviscana folded belt in northwestern argentina testimony of late proterozoic rodinia fragmentation and pre gondwana collisional episodes stratigraphic correlations and tectonic analysis suggest that the puncoviscana fold belt of northwestern argentina was an intracontinental basin with bimodal igneous suites that formed in connection with the breakup of the rodinia supercontinent at 800 ma several lines of evidences point to an initial lithosphere rupture possibly induced by a rising mantle plume the earliest synrift igneous products are represented by ultra potassic dykes and alkaline lava flows of high lree hree and low zr nb y nb ratios the dyke emplacements and the initiation of rifting were probably synchronous they pass laterally and upwards middle part of the puncoviscana succession into basalts of alkaline transitional character oib like source the distinctive chemical feature of these lavas are very similar to the source of oceanic island basalts thus they are thought to represent a magmatism associated with the rift and rift drift transition stage during this stage of rifting probably true oceanic crust was formed the upper part of the puncoviscana sequence late precambrian lower cambrian in age comprises a thick and monotonous sequence of pillow lavas massive basaltic flows and minor volcanic breccias and hyaloclastites these lavas exhibit morb trace element characteristics with high feot and tio2 low k2o and p2o5 flat light ree spectra little or no depletion in nb and ta this volcanism consists of the major and latest effusive episode from the puncoviscana basin which was slightly modified by subduction processes the geodynamical model proposed for the generation of these volcanic rocks could have been developed in two stages in the first stage the volcanic event is compatible with a progressive opening of a continental rift leading to formation of a mature oceanic basin in contrast the second stage shows the effects of a completed wilson cycle including a primitive volcanic arc which continued until the accreted cuyania arequipa belen antofalla caba terrane against the proto gondwana western borderland of the amazonian shield 535 ma

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

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

kinematics of tertiary to quaternary intracontinental deformation of upper crust in the eastern cordillera southern central andes nw argentina 1 to elucidate factors controlling the geometry of and kinematics associated with prominent upper crustal structures in the eastern cordillera of nw argentina structural analyses of key areas were complemented by fault slip analysis remote sensing and 3d representation of prominent faults the analyses revealed that deformation during tertiary to quaternary times was accomplished mostly by prominent orogen parallel reverse faults the geometry of which was significantly influenced by paleozoic and cretaceous planar structures locally this deformation was preceded by kilometer scale doming of upper crustal rocks analysis of 767 brittle faults at 67 stations in the studied areas indicates that local upper crustal doming and orogen parallel reverse faults formed chiefly under nw se and e w shortening shortening directions inferred from fault slip data portray the kinematics of first order faults and folds more specifically shortening directions are mostly uniform with respect to first order structures age and kinematics of deformation inferred from brittle fault analysis in the study areas is consistent with equivalent data compiled from other parts of the eastern cordillera puna and pampean ranges collectively the data is at variance with hypotheses relating late tertiary to quaternary deformation in the eastern cordillera to plate boundary kinematics the kinematics of intracontinental deformation in the southern central andes points to deformation partitioning of upper crust as a consequence of bulk e w shortening we therefore caution the usage of brittle fault kinematics in continental interiors as an indication for plate kinematic changes

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

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