quaternary tephrochronology helps define conditioning factors and triggering mechanisms of rock avalanches in nw argentina abstract ten previously defined quaternary tephra layers from northwestern argentina are described here in terms of electron microprobe analyses of glass shards in addition xrf analyses of three tephras alemania buey muerto and el paso ash were performed the alemania ash and buey muerto ash occur within 1 m of each other stratigraphically in several outcrops and exhibit identical glass chemistry as well xrf analyses show that both tephras have identical composition based on stratigraphic relations and age determinations these tephras can clearly be separated as two individual layers tephra ages were defined relatively by ams 14c dates of organic material by 10be surface exposure ages of landslides in stratigraphic association with these tephra layers or directly by 40ar 39ar ages of mineral separates or fission track dating of zircons ages of these tephra layers constrain the temporal distribution of large landslides rock avalanche type in northwestern argentina temporal distribution shows that these large collapses concentrated in deeply incised valleys during climate periods characterized by enhanced precipitation and run off in addition in the tonco valley tephra characteristics also suggest earthquake triggering of coeval landsliding in this valley four landslides one with a 10be age of 7820 830 yr lie in direct contact atop un redeposited paranilla ash showing the coeval age of these landslides the age of the dated landslide deposit coincides with the age of soft sediment deformation structures in nearby lake deposits with an ams 14c age of 7430 7570 cal yr bp

bajo de la alumbrera copper gold deposit stable isotope evidence for a porphyry related hydrothermal system dominated by magmatic aqueous fluids alteration zones at the gold rich bajo de la alumbrera porphyry copper deposit in northwestern argentina are centered on several porphyritic intrusions they are zoned from a central copper iron sulfide and gold mineralized potassic biotite k feldspar quartz core outward to propylitic chlorite illite epidote calcite assemblages a mineralized intermediate argillic alteration assemblage chlorite illite pyrite has overprinted the potassic alteration zone across the top and sides of the deposit and is itself zoned outward into phyllic quartzinuscovite illite pyrite alteration this study contributes new data to previously reported delta 18 o and delta d compositions of fluids responsible for the alteration at bajo de la alumbrera and the data are used to infer likely ore forming processes measured and calculated delta 18 o and delta d values of fluids 8 3 to 10 2 and 33 to 81 parts per thousand respectively confirm a primary magmatic origin for the earliest potassic alteration phase lower temperature potassic alteration formed from magmatic fluids with lower delta d values down to 123 parts per thousand these depleted compositions are distinct from meteoric water and consistent with degassing and volatile exsolution of magmatic fluids derived from an underlying magma variability in the calculated composition of fluid associated with potassic alteration is explained in terms of phase separation or boiling if copper iron sulfide deposition occurred during cooling as proposed elsewhere this cooling was largely a result of phase separation magmatic water was directly involved in the formation of overprinting intermediate argillic alteration assemblages at bajo de la alumbrera calculated delta 18 o and delta d values of fluids associated with this alteration range from 4 8 to 8 1 and 31 to 71 per mil respectively compositions determined for fluids associated with phyllic alteration 0 8 to 10 2 and 31 to 119 parts per thousand overlap with the values determined for the intermediate argillic alteration we infer that phyllic alteration assemblages developed during two stages the first was a high temperature 400 degrees 300 degrees c stage with d depleted water delta d 66 to 119 parts per thousand this compositional range may have resulted from magma degassing and or the injection of new magmatic water into a compositionally evolved hydrothermal system the isotopic variations also can be explained by increased fluid rock interaction the second stage of phyllic alteration occurred at a lower temperature similar to 200 degrees c and variations in the modeled isotopic compositions imply mixing of magmatic and meteoric waters ore deposition that occurred late in the evolution of the hydrothermal system was probably associated with further cooling of the magmatic fluid in part caused by fluid rock interaction and phase separation changing ph and or oxygen fuoracity may have caused additional ore deposition the ingress of meteoric water appears to postdate the bulk of mineralization and occurred as the system at bajo de la alumbrera waned

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

tephrochronologic constraints on temporal distribution of large landslides in northwest argentina abstract two morphologic settings in the northwestern argentine prone to giant mountain front collapse deeply incised narrow valleys and steep range fronts bordering broad piedmonts were analyzed through detailed investigations of fossil landslides and related fluvio lacustrine sediments nine different rhyodactic tephra layers were defined by geochemical fingerprinting of glass morphology of pumice stratigraphic relationships and mineralogy the age of three tephra could be determined either directly by 40ar 39ar dating or relatively by 14c dating of associated sediments paranilla ash documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont decl

geomorphic signal of active faulting at the northern edge of lut block insights on the kinematic scenario of central iran recent works documented neogene to quaternary dextral strike slip tectonics along the kuh e sarhangi and kuh e faghan intraplate strike slip faults at the northern edge of the lut block of central iran previously thought to be dominated by sinistral strike slip deformation this work focuses on the evidence of quaternary activity of one of these fault systems in order to provide new spatiotemporal constraints on their role in the active regional kinematic scenario through geomorphological and structural investigation integrated with optically stimulated luminescence dating of three generations of alluvial fans and fluvial terraces at 53 25 and 6 ka this study documents i the topographic inheritance of the long term myr punctuated history of fault nucleation propagation and exhumation along the northern edge of lut block ii the tectonic control on drainage network evolution pediment formation fluvial terraces and alluvial fan architecture iii the minimum holocene age of quaternary dextral strike slip faulting and iv the evidence of late quaternary fault related uplift localized along the different fault strands the documented spatial and temporal constraints on the active dextral strike slip tectonics at the northern edge of lut block provide new insights on the kinematic model for active faulting in central iran which has been reinterpreted in an escape tectonic scenario

accurate source depths and focal mechanisms of shallow earthquakes in western south america and in the new hebrides island arc synthetic seismograms are matched to long period p waveforms in order to obtain accurate depths of shallow earthquakes with known focal mechanisms accurate depths are obtained for a large sample of moderate sized 6 ms 7 events which produced relatively simple p waveforms and do not require intensive analysis the estimated error in the determination of depth is about 5 km or less for most of the events considered the procedure is applied to two suites of events one in south america 83 events and one in the new hebrides 61 events in these two areas of contrasting styles of subduction the accurate depths provide new information on the bending of descending plates near trenches the seismicity and tectonics of the upper plate and the geometry and structure of the zone of contact between the upper and descending plates depths of suboceanic earthquakes which occurred near the trench and which have either tensional and compressional horizontal stress axes agree well with chapple and forsyth s 1979 preferred model of a bending elastic perfectly plastic plate however an unusually deep event occurred beneath the fiji plateau in an area of young ocean floor supposed to have been formed by sea floor spreading processes only since late miocene time the calculated depth of the event 48 km places it below the lithosphere asthenosphere boundary derived from thermal models vertical cross sections through the shallow parts of the subduction zones show that most of the earthquakes with thrust faulting focal mechanisms can be interpreted to be interplate events located in a thin 10 km thick curved zone of contact dipping arcward or landward from the trench axis however a few events with focal mechanisms similar to the interplate events do not fit on the inferred thin zone of contact these events would be accommodated by an interplate zone 15 km thick in both subduction zones interplate events occur at depths between about 15 and 50 km however the downdip width of the south american interplate zone is 2 to 3 times larger than that in the new hebrides most of the intraplate events near the interplate boundary are located in the descending plate in the upper plate seismicity is more pronounced in the back arc regions of the two subduction zones thus our results support the generality of a relatively aseismic zone in the upper plate located trenchward of the magmatic arc depths of shallow earthquakes in the eastern andes and sub andean regions of south america range between about 9 and 36 km indicating that both the upper and middle crust are seismically active about 75 of the events occurred in the upper 25 km furthermore the focal mechanisms indicate that from ecuador to argentina the crust of the eastern side of the andean cordilleran is in a state of east west compression oriented nearly parallel to the direction of convergence between the nazca and south american plates the depths the orientations of the earthquake nodal planes and the close relationships to quaternary tectonics imply basement involved crustal shortening in much of the eastern andes and sub andean zones of peru and argentina

the state of stress in an overriding plate situated above a flat slab the andes of central peru in the andes of central peru that are situated above a flat dipping slab analysis of both structural data collected in the field and available focal mechanisms show that the quaternary and recent state of stress is characterized by n s trending extension in the western cordillera and along the pacific coast whereas e w trending compression prevails in the sub andean lowlands and at the contact between the nazca and south american plates the eastern cordillera deformations suggest an intermediate state of stress i e strike slip faulting characterized by e w trending compression and n s trending extension thus major fault zones striking nw se i e parallel to the central peruvian andes are normal with a sinistral component in the western cordillera e g cordillera bianca and reverse sinistral in the eastern cordillera e g cordillera de huaytapallana this state of stress displays similarities with that of southern peru and may be also interpreted as an effect of compensated high topography in this model the vertical stress zz increases with the topography and hmax is considered as fairly constant trending e w roughly parallel to the convergence on both edges of the andes tectonics being compressional zz is 3 and hmax is 1 in the high andes zz becomes 1 then hmax is 2 allowing hmin striking n s to be 3 along the pacific coast tectonics is not compressional as expected and a nearly neutral state of stress may be due to a topographical effect related to the nearby deep peru chile trench however a conspicuous difference occurs in the andes of central peru in respect to the southern peru andes compressional strike slip tectonics is observed in the eastern cordillera indeed the mean elevation of the eastern cordillera is 3 7 km whereas that of the western cordillera is 4 2 km allowing an intermediate state of stress to occur therefore in central peru a lower mean andean topography and a stronger coupling consequence of the flat subduction seems to explain that compressional tectonics is distributed in a more widespread area than in southern peru superficial deformations related to the subduction of the aseismic nazca ridge are restricted to the coastal domain opposite to this ridge they support the fact that submarine topography of large extension may subduct without producing compressional deformation in the overriding plate which can be reminiscent of a collisional setting

tectonics and climate of the southern central andes the history of the southern central andes including the world s second largest plateau and adjacent intermontane basins and ranges of the eastern cordillera and the northern sierras pampeanas of argentina and bolivia impressively documents the effects of tectonics and topography on atmospheric circulation patterns the development of orographic barriers and their influence on erosion and landscape evolution at various timescales protracted aridity in the orogen interior has facilitated the creation and maintenance of the puna altiplano plateau contraction and range uplift filling of basins and possibly wholesale uplift of the plateau increased gravitational stresses in the orogen interior which caused the eastward migration of deformation into the foreland and successive aridification the uplift of the andean orogen has also had a far reaching influence on atmospheric and moisture transport patterns in south america this is documented by the onset of humid climate conditions on the eastern side of the andes in late miocene time which was coupled with the establishment of dramatic precipitation gradients perpendicular to the orogen and changes in tectonic processes in the andean orogenic wedge

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

environmental control on the end of the dolomia principale hauptdolomit depositional system in the central alps coupling sea level and climate changes abstract the norian in the western tethys is characterised by the deposition of early dolomitised inner platform facies dolomia principale hauptdolomit dp hd bordered on the landward side by terrigenous coastal deposits keuper and on the seaward side by calcareous backreef and reefal facies dachstein limestone passing basinward to open sea sediments hallstatt facies the inner carbonate platform is locally lombardy basin carnic alps central austroalpine dissected by normal faults leading to the development of intraplatform troughs close to the norian rhaetian boundary sedimentation records an abrupt environmental change both on platform top and basins all over the western tethys e g western carpathians transdanubian range alps central apennine the top of the dolomia principale locally emerged reflecting a major eustatic sea level fall emersion is recorded in favourable settings by the development of polycyclic paleosols up to 30 m thick in the norian intraplatform basins the succession is capped by 4 to 8 m of thin bedded fine grained limestones yielding abundant remnants of fishes and terrestrial reptiles fossil concentration as well as sedimentological features is indicative of reduced sedimentation rates due to decreased carbonate production induced by the emersion of the platform top the sea level fall was followed by deposition of mixed fine grained siliciclastic carbonate successions e g riva di solto shale kossen beds rhaetavicula contorta beds fatra formation stratigraphic evidence indicates a dry climate in the western tethys during the norian as indicated by the presence of evaporites burano apennine and arid to semi arid coastal to playa settings upper keuper germany in contrast the basal layers of the basinal shales show evidence of wet climate the end of the norian depositional system records two different phenomena 1 an important sea level fall was responsible for the emersion of the platform top and deposition of a condensed horizon in the basins and 2 transition from dry to humid climate the observed evolution is explained with a global cooling which caused the rapid sea level fall responsible for the abrupt end of the dp hd depositional system and the shift of the boundary between arid and temperate climate belts which modified the distribution and amount of rainfall triggering the deposition of shales along the western tethys margin

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

dynamics of deformation and sedimentation in the northern sierras pampeanas an integrated study of the neogene fiambala basin nw argentina reply the thick skinned sierras pampeanas morphotectonic domain of western and northwestern argentina 27 s 33 s is characterized by reverse fault bounded basement blocks that delimit internally deformed neogene sedimentary basins foreland basin evolution in this part of the andes is still not very well understood for example challenging questions exist as to how thick skinned deformation develops if there are distinct spatiotemporal trends in deformation and exhumation how such deformation styles influence sedimentation patterns and whether or not broken foreland basins are related to regional plate tectonic processes such as flat slab subduction the fiambala basin of the northwestern sierras pampeanas is the largest of several intermontane basins in the transition to the southern margin of the puna plateau this basin preserves a thick continental neogene sequence that provides information on the dynamics of thick skinned deformation and resulting sedimentation the fiambala basin contains 4 km of fluvial alluvial sedimentary rocks that comprise the tamberia guanchin and punaschotter formations u pb geochronology of ashes intercalated within the fiambala stratigraphic sequence demonstrates that these sedimentary rocks are late miocene to pliocene 8 2 0 3 ma to 3 05 0 4 ma in age sedimentology and provenance data indicate that the source of the tamberia formation was located to the west of the modern western basin bounding range the guanchin and punaschotter formations record input from local sources including the modern basin bounding range to the west and the southern puna plateau to the north suggesting reorganization of the catchment area at ca 5 5 ma the coarsening upward trends recorded by the fluvial tamberia and guanchin formations indicate enhanced tectonics and relief during sedimentation the punaschot ter conglomerates record alluvial fan sedimentation and local sources fault kinematic data document a contractional regime characterized by e w and ne sw shortening active throughout the middle late miocene and pliocene furthermore a comparison between the fiambala basin and similar sedimentary basins in the sierras pampeanas e g bermejo foreland basin and the eastern cordillera leads us to propose that the study area originally constituted an integral part of a continuous and more extensive foreland basin system thin skinned for much of its early history our data suggest coeval intrabasin deformation along strike from the bermejo region northward to the eastern cordillera the coeval change at ca 6 ma from a regional to more compartmentalized thick skinned tectono sedimentary environment in the regions adjacent to the eastern cordillera the southern puna margin and other sectors within the sierras pampeanas domain may thus reflect a regional tectonic process related to flat subduction our data combined with existing sedimentological and petrological evidence imply that the passage from steep to flat subduction occurred synchronously from 30 s to 26 s

the role of inherited structures in a foreland basin evolution the metan basin in nw argentina the foreland basin of the central andes in nw argentina is formed by partially unconnected basins limited by uneven high ranges the metan basin is located in the foothills of the central andes in nw argentina between the eastern cordillera the sierras pampeanas and the santa barbara system this basin resulted from a cretaceous to paleogene rifting event and from two neogene to quaternary foreland basin stages for this study field data log well seismic reflection and satellite images have been interpreted the propagation of deformation in this sector of the andes is influenced by inherited structures e g cretaceous extensional faults which have been reactivated during andean compression as high angle and oblique reverse faults deformation does not advance regularly throughout the foreland in a normal forward sequence but jumps across inherited faults fault reactivation has resulted in uplift of basement cored ranges and hanging wall anticlines that divided the foreland into small basins e g the metan basin

cenozoic plate kinematics subduction and magmatism south american andes revised plate reconstructions and enlarged isotopic age data sets strongly support three mechanisms for controlling the configuration and associated volcanism and tectonism of the andean subduction zone these include age of subducted plate convergence rate and interaction with hotspot traces from 50 to 25 ma in the andes the age of oceanic plate subducted progressively decreased apparently resulting in a9gradual decrease in subduction angle and consequent eastward shift in the loci of magmatic activity a major reorganization of plate motions at 25 ma resulted in an increased convergence rate normal to the chilean andes and was accompanied by subsequent broadening of the locus of magmatic activity into adjacent portions of bolivia and argentina convergence rates changed only slightly across the peruvian margin and no clear effect on subsequent magmatism is indicated interaction of the andean subduction zone with the juan fernandez and easter nazca hotspot traces apparently resulted in lower angle subduction and consequent gaps in magmatic activity as predicted continuations of the traces correspond with regions lacking dated volcanic rocks the current gaps in igneous activity correspond with contemporary subduction of the two traces and this association apparently occurred in the past as well

late pleistocene climate change and erosion in the santa maria basin nw argentina present erosion and sediment flux in the semiarid intra montane santa maria basin nw argentina are compared with con ditions during a period of wetter and more variable climate at about 30 000 14 c yr ago the results suggest that the influence of climate change on the overall erosional sediment budget is significant mainly because of a change in the erosion regime coupled with an increase in mass movements the most effective mechanism to increase landslide activity in this environment is a highly variable climate on interannual timescales in contrast quaternary changes in erosional budgets due to variations in moisture regimes is small in the santa maria basin because the magnitude of a potential increase in background erosion as well as enhanced landsliding is smaller than typical levels of uncer tainty of erosional budgets for such large basins it is not likely that climate driven erosional unloading can influence tectonic style and rates in this semiarid environment on time scales of several thousands to a few tens of thousands of years

the puncoviscana trough a large basin of late precambrian to early cambrian age on the pacific edge of the brazilian shield during the late precambrian to early cambrian a large basin with a homogeneous psammitic pelitic sediment fill existed in the area of the nw argentine andes it is now exposed in different tectonic levels this basin of meridional elongation was situated on a stable continental margin at the western edge of the brazilian shield it was underlain by segmented older continental crust according to the modal and chemical composition of the greywackes and subgreywackes a predominant metasedimentary source may be supposed four clastic facies types indicate a transport by gravity currents within a submarine fan system while massive red pelites represent cut off periods and rare carbonates deposition on rises the gravity currents derived from easterly directions the sedimentation age is proved by a rich ichnofauna and some medusoid impressions

non explosive magma water interaction in a continental setting miocene examples from the eastern cordillera central andes nw argentina the middle upper miocene las burras almagro el toro bat igneous complex within the eastern cordillera of the central andes 24 s nw argentina has revealed evidence of non explosive interaction of andesitic magma with water or wet clastic sediments in a continental setting including peperite generation we describe and interpret lithofacies and emplacement mechanisms in three case studies the las cuevas member 11 8 ma comprises facies related to i andesite extruded in a subaqueous setting and generating lobe hyaloclastite lava and ii marginal parts of subaerial andesite lava dome s in contact with surface water comprising fluidal lava lobes hyaloclastite and juvenile clasts with glassy rims the lampazar member 7 8 ma is represented by a syn volcanic andesite intrusion and related peperite that formed within unconsolidated water saturated coarse grained volcaniclastic conglomerate and breccia the andesite intrusion is finger shaped and grades into intrusive pillows pillows are up to 2 m wide tightly packed near the intrusion fingers and gradually become dispersed in the host sediment 50 m from the parent intrusion the almagro a member 7 2 ma shows evidence of mingling between water saturated coarse grained volcaniclastic alluvial breccia and intruding andesite magma the resulting intrusive pillows are characterized by ellipsoidal and tubular shape and concentric structure the high level penetration of magma in this coarse sediment was unconfined and irregular magma was detached in apophyses and lobes with sharp contacts and fluidal shapes and without quench fragmentation and formation of a hyaloclastite envelope the presence of peperite and magma water contact facies in the bat volcanic sequence indicates the possible availability of water in the system between 11 7 ma and suggests a depositional setting in this part of the foreland basin of the central andes characterized by an overall topographically low coastal floodplain that included extensive wetlands

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

average pleistocene climatic patterns in the southern central andes controls on mountain glaciation and paleoclimate implications abstract despite elevations of 5000 6800 m modern glaciers occur along the southern puna plateau and the northern sierras pampeanas in the southern central andes the modern snowline rises from 5100 m in sierra aconquija to 5800 m in the puna as a result of a westward decrease in precipitation from 450 to less than 100 mm yr during the pleistocene these arid highlands experienced multiple cirque and valley glaciation that likely postdate the last interglacial period although lack of age control prevents an absolute chronology glaciation in the puna and along the eastern puna edge produced a 300 m pleistocene snowline psl depression while in the sierras pampeanas the psl depression was at least 900 m the greater psl depression in the sierras pampeanas is best explained by a combination of cooling and increase of easterly moisture whereas the psl depression in the puna appears more sensitive to moisture increases than temperature previously glaciations in this region have been explained by increa

the late cenozoic uplift climate change paradox the geologic evidence for worldwide uplift of mountain ranges in the neogene is ambiguous estimates of paleoelevation vary according to whether they are based on the characteristics of fossil floras on the masses and grain sizes of eroded sediments or on calculations of increased thickness of the lithosphere as a result of faulting detrital erosion rates can be increased both by increased relief in the drainage basin and by a change to more seasonal rainfall patterns the geologic record provides no clear answer to the question whether uplift caused the climatic deterioration of the neogene or whether the changing climate affected the erosion system in such a way as to create an illusion of uplift we suggest that the spread of c4 plants in the late miocene may have altered both the erosion and climate systems these changes are responsible for the apparent contradictions between data supporting uplift and those supporting high elevations in the past

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

multiple landslide clusters record quaternary climate changes in the northwestern argentine andes the chronology of multiple landslide deposits and related lake sediments in the semi arid eastern argentine cordillera suggests that major mass movements cluster in two time periods during the quaternary i e between 40 and 25 and after 5 14 c kyr bp these clusters may correspond to the minchin maximum at around 28 27 14 c kyr bp and titicaca wet periods after 3 9 14 c kyr bp the more humid conditions apparently caused enhanced landsliding in this environment in contrast no landslide related damming and associated lake sediments occurred during the coipasa 11 5 10 14 c yr bp and tauca wet periods 14 5 11 14 c yr bp the two clusters at 40 25 and after 5 14 c kyr bp may correspond to periods where the el nin o southern oscillation enso and tropical atlantic sea surface temperature variability tav were active this however was not the case during the coipasa and tauca wet periods lake balance modelling of a landslide dammed lake suggests a 10 15 increase in precipitation and a 3 4 c decrease in temperature at v30 14 c kyr bp as compared to the present in addition time series analysis reveals a strong enso and tav during that time the landslide clusters in northwestern argentina are therefore best explained by periods of more humid and more variable climates 2003 elsevier science b v all rights reserved

geology and geochemistry of the ojos del salado volcanic region chile calc alkaline volcanic rocks ranging in age from miocene to recent are encountered in the ojos del salado area 27 09s 68 459w they lie at the southernmost end of the central volcanic zone coinciding with a major morphological seismic and volcanic discontinuity it has previously been suggested that this ojos del salado lineament may represent a continental extension of the chain of islands and seamounts referred to as the 9easter hot line9 the volcanic rocks of the area have been sub divided into five groups based on k ar dates the first three groups all of miocene age show a progressive younging eastwards with a concomitant increase in k 2 o the formation of wheelwright and other caldera structures appears to date from the close of this phase 8 6 ma during the pliocene the group 4 volcanoes were constructed mainly along a line trending n65 e but a number of other cones developed around wheelwright caldera which was partially filled by their products clockwise rotation of the volcanic axis continued in pleistocene recent times with the eruption of the group 5 volcanoes of ojos del salado extensive pumice flows were associated with this new volcanic chain and there is generally a shift to more acidic compositions especially dacite apart from the increase in k and related elements from west to east in groups 1 to 3 there is no evidence for any systematic compositional change in particular the youngest group of volcanoes with an east west trend is geochemically indistinguishable from its predecessors on the dominantly north south axes trace element ratios show similar trends the rare earth element patterns are lree enriched and the 86 sr 87 sr initial ratios are all approximately 0 706 the ojos del salado volcanics are thus devoid of any compositional signature to link them with the easter 9hot line9

normal faulting along the southern margin of the puna plateau northwest argentina 1 though orogen parallel shortening and vertical extension have dominated the tectonic evolution of the central andes a significant kinematic shift from horizontal contraction to extension appears to have occurred within the high puna altiplano plateau with the establishment of extension oblique to the orogen since late miocene time we present data from the southern margin of the puna plateau nw argentina where new normal faults have been documented in the fiambala punta negra and la quebrada areas the unifying characteristics of these areas are that young normal faults reactivate or crosscut older thrust and reverse faults the relationship between the faults and the late miocene pliocene punaschotter conglomerate suggests that the extensional faulting must be younger than 3 5 to 7 ma existing data are incomplete but indicate that similar horizontal extension has occurred in many regions throughout the puna altiplano plateau while shortening continues along the plateau margins given the spatial and temporal distribution of this late miocene to pliocene kinematic shift both lithospheric loss in the puna plateau and plateau wide gravitational extensional spreading enhanced by slowing of plate convergence rate could be responsible the young disorganized horizontal extension in the andes today may be the precursor to more pronounced extension such as observed on the tibetan plateau since mid miocene time

timing of past glaciation at the sierra de aconquija northwestern argentina and throughout the central andes abstract advances in cosmogenic nuclide exposure dating have made moraines valuable terrestrial recorders of palaeoclimate a growing number of moraine chronologies reported from the central andes show that tropical glaciers responded sensitively to past changes in precipitation and temperature over timescales ranging from 103 to 105 years however the causes of past glaciation in the central andes remain uncertain explanations have invoked insolation modulated variability in the strength of the south american summer monsoon teleconnections with the north atlantic ocean and or cooling in the southern hemisphere the driver for these past climate changes is difficult to identify partly due to a lack of dated moraine records especially in climatically sensitive areas of the southern central andes moreover new constraints are needed on precisely where and when glaciers advanced we use cosmogenic 10be produced in situ to determine exposure ages for three generations of moraines at the sierra de aconquija situated at 27 s on the eastern flank of the southern central andes these moraines record glacier advances at approximately 22 ka and 40 ka coincident with summer insolation maxima in the sub tropics of the southern hemisphere as well as at 12 5 ka and 13 5 ka during the younger dryas and the antarctic cold reversal respectively we also identify minor glaciation during bond event 5 also known as the 8 2 ka event these moraines register past climate changes with high fidelity and currently constitute the southernmost dated record of glaciation on the eastern flank of the central andes to contextualise these results we compile 10be data reported from 144 moraines in the eastern central andes that represent past glacier advances we re calculate exposure ages from these data using an updated reference production rate and we re interpret the moraine ages by taking the oldest clustered boulder age after the exclusion of outliers attributed to nuclide inheritance as closest to the timing of glacier advance an approach for which we provide empirical justification this compilation reveals that central andean glaciers have responded to changes in temperature and precipitation we identify cross latitude advances in phase with insolation cycles the last global glacial maximum and episodes of strengthened monsoonal moisture transport including the younger dryas and heinrich stadials 1 and 2 our results from the sierra de aconquija allow us to constrain the southerly limit of enhanced precipitation associated with heinrich stadials at 25 s more broadly our findings demonstrate at both local and regional scales that moraines record past climate variability with a fine spatial and temporal resolution

structure and development of detrital reef deposits in turbid nearshore environments inhaca island mozambique coral communities are intermittently developed along the seaward margins of wide up to 500 m intertidal flats that fringe the western margins of inhaca island southern mozambique the coral communities occur in an environment that can be considered as marginal for coral growth in the sense that they are subject to high turbidity levels which results in rapid light attenuation and to the effects of periodic smothering and burial associated with a mobile siliciclastic dominated sediment substrate these constraints limit coral community development both spatially and bathymetrically in addition historical evidence suggests that the main sites of coral growth shift over decadal timescales due to periodic sediment remobilisation such rapid rates of turnover restrict the development of rigid framework deposits and in contrast the main depositional product of coral growth is an abundance of branched mainly acropora sp coral rubble rubble production is aided both by intense internal bioerosion sponge bivalve and worm traces and by periodic storm damage much of this rubble accumulates along the channel margins and provides a substrate for subsequent coral recruitment the deposits can be distinguished from those associated with clear water reef environments by 1 the lack of a rigid reef framework either comprising of in situ coral or encrusted coral rubble 2 the distinct lack of rubble encrustation syn depositional cementation and 3 the predominance of a siliciclastic rich sediment matrix the deposits indicate however a long term history of coral community occurrence in sites dominated by siliciclastic sedimentation cores recovered from a transect across the tidal flats identify a coral rubble floatstone facies extensively developed along the western margins of the island it comprises mainly branched coral rubble often heavily bioeroded but sparsely encrusted along with spheroidal rhodoliths and molluscan fragments set within a poorly sorted mixed siliciclastic carbonate sediment matrix 14c dating indicates that most of the rubble deposits date from the period 6500 4700 years bp which corresponds to a period of regional sea level highstand a detailed growth chronology cannot be established due to the lack of in situ coral but rapid rubble deposition and reef flat progradation around 350 m can be inferred for this period equivalent to an average progradation rate of around 0 2 m year 1 over the subsequent 4700 years progradation of only around 25 m has occurred this may be attributed to reworking and erosion of the seaward margin of the deposits during periods of fluctuating sea level between 4700 and 900 years bp or to a lack of seaward accommodation space

paleomagnetic study along the southeastern edge of the altiplano puna plateau neogene tectonic rotations in northwestern argentina the transition zone between the puna and the sierras pampeanas was deformed in late miocene and pliocene time structural observations suggest that local clockwise block rotations might be observed associated with dextral transpression this study contributes new paleomagnetic results from four sites in cretaceous rocks and 19 sites in neogene sedimentary sequences when compared with reference paleomagnetic data for stable continental south america the results show an average inclination shallowing error of 10 5 and a pattern of clockwise rotations up to 29 study of the anisotropy of magnetic susceptibility ams on 278 specimens provided for most sites ams tensors with an oblate shape controlled by sedimentation and compaction explaining the observed inclination shallowing a slight ams lineation was measured and we argue that this magnetic lineation reflects late miocene pliocene compression a correlation observed between magnetic lineations deduced from the ams analysis and rotation estimates from remanent magnetizations demonstrates that the inferred shortening directions have also been rotated clockwise

thick skinned tectonics and basement involved fold thrust belts insights from selected cenozoic orogens defining the structural style of fold thrust belts and understanding the controlling factors are necessary steps towards prediction of their long term and short term dynamics including seismic hazard and to assess their potential in terms of hydrocarbon exploration while the thin skinned structural style has long been a fashionable view for outer parts of orogens worldwide a wealth of new geological and geophysical studies has pointed out that a description in terms of thick skinned deformation is in many cases more appropriate this paper aims at providing a review of what we know about basement involved shortening in foreland fold thrust belts on the basis of the examination of selected cenozoic orogens after describing how structural interpretations of fold thrust belts have evolved through time this paper addresses how and the extent to which basement tectonics influence their geometry and their kinematics and emphasizes the key control exerted by lithosphere rheology including structural and thermal inheritance and local regional boundary conditions on the occurrence of thick skinned tectonics in the outer parts of orogens

probing the evolving andean lithosphere mid late tertiary magmatism in chile 29 30 30 s over the modern zone of subhorizontal subduction in the mid late tertiary 27 6 m y ago volcanism diminished then ceased in the main andean cordillera in chile between 29 and 30 30 s over the same time span the subduction angle shallowed under this region which today lies in the modern shallowly dipping nonvolcanic segment of the south central andes 28 33 s the trace and major element chemistry and mineralogy of the volcanic rocks in this region record the detailed pattern of lithospheric cooling and crustal thickening that accompany this major tectonic transition volcanic changes include in addition to a decreased rate of volcanism a decrease in the range of compositions from basaltic andesites to rhyolites in the oldest rocks to only rhyolite in the youngest rocks a change in source and fractionating mineralogy with an increased role for amphibole and garnet in the younger rocks and an increase in crustal assimilation these volcanic changes also correlate in time with deformational events that help to account for crustal thickening and the space time pattern of deformation and volcanism in the foreland region although the model stands alone support for the tectonic setting and magmatic source region of these tertiary volcanic rocks can be found in comparisons with recent volcanic rocks erupting over more steeply dipping modern andean segments the older dona ana formation 27 18 m y is similar to volcanic rocks in the southern volcanic zone while the younger cerro de las tortolas 14 10 m y and vallecito 6 m y formations are similar to volcanic rocks over regions of thicker crust in the central volcanic zone and the northern part of the southern volcanic zone

late miocene earliest pliocene glaciation in southern argentina implications for global ice sheet history abstract in south america at latitude 47 s till was deposited sometime between 7 m y and 4 6 m y ago at a time when the local climate was colder than today s during this same interval glaciers extended to sea level in southeast alaska and widespread cooling of the ocean surface in middle latitudes worldwide marine regression and change in the oxygen isotopic composition of ocean water occurred from the last three events major late miocene expansion of the antarctic ice sheet has been inferred on the assumption that the history of north atlantic ice rafting precludes the existence of northern hemisphere ice sheets until 3 m y ago this is disputed first because precipitation in antarctica would probably have decreased at temperatures below today s second because the antarctic ice sheet cannot expand appreciably until buildup of northern hemisphere ice sheets has lowered sea level third because virtually no late miocene sediments are present at the labrador sea dsdp sites that are critical to the reconstruction of north atlantic ice rafting history and fourth because the scale of late miocene glaciation in alaska is at least permissive for simultaneous buildup of ice at similar latitudes further east if global ice sheet volume at the end of the miocene was greater than it is today by an amount that would have lowered sea level by several tens of meters the excess ice did not accumulate in antarctica but in the northern hemisphere chiefly in north america away from the atlantic coast

influence of attenuated lithosphere and sediment loading on flexure of the deep water magallanes retroarc foreland basin southern andes flexural subsidence in foreland basins is controlled by applied loads such as topography water sediment and subcrustal forces and the mechanical properties of the lithosphere we investigate the controls on subsidence observed within the upper cretaceous magallanes retroarc foreland basin of southern south america to evaluate the impact of lateral variations in flexural rigidity due to late jurassic extension conventional elastic models cannot explain the observed basin deflection and thick accumulation of deep water cenomanian turonian basin strata however models in which the lithosphere has been previously thinned and deflects under topographic and sedimentary loads successfully reproduce regional subsidence patterns results satisfy paleobathymetric observations in the magallanes basin and suggest that lithospheric thinning is necessary to produce both long wavelength and deep subsidence during late cretaceous basin evolution results indicate that elastic thickness decreases westward from 45 25 km in the distal foreland to 37 15 km beneath the foredeep these findings are consistent with a westward reduction in crustal thickness associated with the jurassic extensional history of the patagonian lithosphere our results also show that sediment loading exerts an important control on regional deflection patterns and promotes a wider region of subsidence and reduced forebulge uplift we propose that lateral variations in mechanical properties and large sediment loads restrict depocenter migration and may cause the foredeep to remain fixed for prolonged periods of time these findings confirm that loading of thinned lithosphere imposes different mechanical controls on the flexural profile and have potential implications for other retroarc foreland basins characterized by earlier extensional histories

chronology of neogene sedimentation stratigraphy and tectonism in the campo de talampaya region la rioja province argentina abstract chronostratigraphic paleoenvironmental and tectonic analysis of a 2320 m thick neogene continental section was conducted along the southern limit of the campo de talampaya magnetic polarity stratigraphy and fission track ages were used to develop a relatively precise geochronology the strata record a period of mixed evaporitic and terrigenous elastic sedimentation 20 6 1 ma that persisted in the distal bermejo basin during eastward migration of the precordillera fold thrust belt a critical transition of depositional environments and a change from western to eastern source areas occurred between 12 9 and 12 3 ma the saline mud flat environment of the rio manero formation derived from a western source and characterized by fine grained sandstones mudstones and evaporites yielded to a short period of playa lake deposition this was soon overwhelmed by deposition of the desencuentro formation composed of sandstones and mudstones with an eastern source area and deposited in a sand flat mud flat environment we propose that this stratigraphic transition resulted from initiation of uplift of the sierra de velasco a range of the sierras pampeanas located 65 km to the east northeast of the section basement clasts probably derived from the sierra de velasco are found in conglomeratic lenses beginning in the lower part of the desencuentro formation the modern campo de talampaya basin formed after deposition of the top of the alto de san nicolas paleomagnetic section 6 1 ma when west verging reverse faults uplifted the sierra morada and sierra de los tarjados all strata in the section were tilted eastward during these culminating uplift events

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

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

fragmentation of a foreland basin in response to out of sequence basement uplifts and structural reactivation el cajon campo del arenal basin nw argentina the style and mechanisms by which a foreland region is incorporated into an orogen depends on the tectonic style effectiveness of uplift and dynamic subsidence classical foreland basin models reflect a self similar propagation of deformation into the foreland in a thin skinned thrust belt setting governed by wedge mechanics thick skinned foreland regions which are characterized by high angle reverse fault bounded basement uplifts and intervening basins however do not fit this idealized model unlike thin skinned tectonic provinces deformation and uplift in these regions may be highly variable in time and space furthermore deformation patterns may be complicated by the presence of preexisting structures particularly those which lie at orientations that enable them to be reactivated and utilized to accommodated deformation under compression the neogene el cajon campo del are nal basin is one of a series of basins located along the eastern margin of the puna plateau within the sierras pampeanas a region that is composed of a thick skinned foreland fragmented by reverse fault bounded basement uplifts that regionally characterize an eastward younging trend this region is superimposed onto the cretaceous salta rift province which provides a series of pre existing structures that may potentially be reactivated the basin is located along the eastern margin of the puna plateau an integral component of the andean orogen which includes several filled uplifted and internally drained cenozoic intraplateau basins structural and sedimentological similarities exist between basins along the margin of the puna and those within it understanding the evolution of foreland basins such as the el cajon campo del arenal basin provides possible mechanisms for the development and incorporation of marginal basins into orogenic belts and in the case of the andean orogen the potential for these basins to be incorporated into the plateau our analysis which integrates seismic sedimentary and thermochronological data characterizes the evolution of this basin and surrounding ranges the appearance in the sedimentary section of a distinct grain age population derived from the basement erosion surface constrains the uplift and erosion of an out of sequence intrabasin high to ca 6 ma the basin fill therefore records an evolution from an undeformed foreland to one that is compartmentalized by basement uplifts and that is incorporated into the greater orogenic structure the data reveal the importance of the reactivation of preexisting structures along the basin margin in creating east dipping structures in a generally west dipping domain these opposing faults on the basin margin consequently caused the out of sequence uplift of the intrabasin range the sierra de quilmes the sierra de quilmes fragments the foreland and because its position is locked by loads to the west and east creates increased deformation within the basin basin fill uplift and incorporation into the orogen unlike basins within the plateau however the el cajon campo del arenal basin has been re excavated and integrated once more into the foreland drainage network

coupled tectonic evolution of andean orogeny and global climate abstract the largest tectonic relief breaking the earth s surface 13 km vertically is at the subduction margin of the andes which generates routinely megathrust earthquakes mw 8 5 and drives the paradigmatic andean orogen here we present key geologic evidence to reassess first order features of geomorphology and tectonics across the central andes where the orogen includes the altiplano plateau and attains its maximum integrated height and width the andean subduction margin has a stepped morphology dominated by the low relief atacama bench which is similar to a giant uplifted terrace slopes gently over a width of 60 100 km from the andes to the pacific and runs over more than 1000 km of coastal length we find that the genesis of stepped morphology at the andean seaboard is due to concomitant development of large west vergent thrusts parallel to the subduction interface and increasing aridity in the atacama desert which keeps an unprecedented large scale record of interplaying tectonics and cenozoic climate change incorporating our results with published geological knowledge demonstrates that andean orogeny is characterized by trench perpendicular bivergent and trench parallel bilateral growth over the past 50 myr associated with positive trench velocity toward the continent trench advance and subduction of a wide slab under south america we hypothesize that a global plate tectonic reorganization involving long lasting viscous mantle flow has probably forced both andean orogeny and global climate cooling since 50 ma in contrast two important stepwise pulses of increasing aridity and trench perpendicular andean growth appear to be results of changes in erosion rates due to global late eocene and middle miocene cooling events

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

mio pliocene volcaniclastic deposits in the famatina ranges southern central andes a case of volcanic controls on sedimentation in broken foreland basins abstract a well constrained record of miocene pliocene explosive volcanism is preserved within the broken foreland of western argentina along the famatina ranges this paper focuses on the volcaniclastic record known as the rio blanco member of the el durazno formation three facies can be recognized in the study area 1 massive tuffs 2 volcaniclastic conglomerates and 3 pumiceous sandstones these facies are interpreted as primary pyroclastic flow deposits ignimbrites and reworked volcanogenic deposits within interacting volcanic fluvial depositional systems alternation between ignimbrites and volcanogenic sandstones and conglomerates suggest a recurrent pattern of sedimentation related to recurrent volcanic activity considering the facies mosaic and relative thicknesses of facies short periods of syn eruption sedimentation volcaniclastic deposits seem to have been separated by longer inter eruption periods where normal stream flow processes were dominant the volcaniclastic component decreases up section suggesting a gradual reduction in volcanic activity the mean sedimentation rate of the rio blanco member is higher 0 44 mm year than those obtained for the underlying and overlying units this increase cannot be fully explained by foreland basement deformation and tectonic loading hence we propose subsidence associated with volcanic activity as the causal mechanism volcanism would have triggered additional accommodation space through coeval pyroclastic deposition modification of the stream equilibrium profile flexural loading of volcanoes and thermal processes these mechanisms may have favored the preservation of volcaniclastic beds in the high gradient foreland system of famatina during the mio pliocene thus the rio blanco member records the response of fluvial systems to large volcanism induced sediment loads

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

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

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