40ar 39ar geochronological constraints on the age and evolution of the permo triassic emeishan volcanic province southwest china abstract the biostratigraphically constrained permo triassic emeishan volcanic province evp extending over wide areas in southwest china has been recently considered as a large igneous province contemporaneous with the siberian traps and the siliceous tuffs at the p t boundary in south china we report the first 40ar 39ar ages on this igneous province minimum ages have been obtained on phenocrystic plagioclase of the emeishan basalt which has undergone a pervasive metamorphism most likely during subsequent tectonization as a consequence of terrane amalgamation comparison between the ar release spectra obtained on clear vs cloudy plagioclase indicates a 40 30 ma sericite resetting time a minimum apparent age of 246 4 ma for plagioclase from a plagiogranite a late differentiate of the panzhihua layered complex and an age of 254 5 ma for phlogopite from a pyroxenite near lake erhai provide the first absolute age constraint on this igneous province additional ar ar age measurements on post emeishan alkaline and mafic magmatism yielded 104 2 and 100 2 ma for an alkaline complex near panzhihua and 42 1 ma for a gabbro sill emplaced near the ertan dam further study is still needed to determine the age of the emeishan volcanic emission accurately and to test the validity of the assumed short duration of the eruption

structural variations in carbonates there has been considerable recent interest in the stabilities of carbonate minerals within the earth calcite is the dominant c bearing phase in the earth s crust and acts as a buffer for the long term cycling of co2 between the atmosphere oceans and the solid earth berner 1994 bickle 1996 it has been shown that aragonite and dolomite kraft et al 1991 or calcite biellman et al 1992 are stable at pressures of 30 gpa and temperatures of the order of 2000 k the controls on the stabilities of the carbonates are of course defined by the way that their crystal structures respond to varying temperature pressure and composition much of the literature in this area is already reviewed in the excellent chapters of volume 11 of reviews in mineralogy carbonates mineralogy and chemistry edited by r j reeder 1983 here therefore i focus on aspects of carbonate crystal chemistry that have been of particular interest since the publication of that volume and that have seen significant advances in understanding below i discuss the recent work into the nature of polymorphism in the rhombohedral carbonates in particular the high temperature disordering transition in calcite and the high pressure low temperature metastable modifications of the calcite structure transitions in the aragonite related orthorhombic carbonates at very high pressures are then surveyed followed by a brief review of recent work on the breakdown of dolomite at high pressures and temperatures the penultimate topic dealt with here is that of the compression characteristics of the rhombohedral carbonates and i end with an indication of the breadth of work ongoing into new mineral structures that employ the carbonate ion as a structural building block i refer the reader to the reviews of reeder 1983 and speer 1983 for an introduction to the underlying crystal chemistry of the rhombohedral

intercalibration of 40ar39ar dating standards a number of standards used for k ar and 40ar39ar dating have been intercalibrated multiple splits 15 40 mg each of mmhb 1 sb 3 biotite lp 6 biotite 40 60 ga1550 biotite fish canyon tuff 3 biotite taylor creek rhyolite sanidine bern 4 muscovite bern 4 biotite and pennsylvania state university orthoclase 1a were irradiated in capsules fused and analyzed in replicate analyses most of these standards proved to be homogeneous in 40ar 39ark ratio age k ca and 36ar contents mmhb 1 is inhomogeneous in age at the 15 mg level indicating it is unsuitable for use as an interlaboratory standard using material taken straight from the bottle the orthoclase sample is also inhomogeneous at the 30 mg level quoted relative to an age of 162 9 ma for the sb 3 biotite standard total fusion ages are lp 6 biotite 40 60 128 1 ma ga1550 biotite 97 8 ma fish canyon tuff 3 biotite 27 95 ma taylor creek rhyolite sanidine 28 0 ma bern 4 muscovite 18 51 ma bern 4 biotite 17 21 ma these ages are in good agreement with the absolute k ar ages of these minerals different preparations of biotite and sanidine standards from a welded tuff such as the fish canyon tuff may not be of identical ages tests on submilligram splits reveals that sb 3 biotite ga1550 biotite fish canyon tuff 3 biotite and taylor creek rhyolite sanidine are suitable for use as monitors in laser work the sanidine specimen appears to be the most suitable for use as the primary monitor for 40ar39ar dating

the influence of climate on the tectonic evolution of mountain belts simple physical arguments analogue experiments and numerical experiments all suggest that the internal dynamics of actively deforming collisional mountain ranges are influenced by climate however obtaining definitive field evidence of a significant impact of climate on mountain building has proved challenging spatial correlations between intense precipitation or glaciation and zones of rapid rock uplift have indeed been documented in numerous mountain ranges and are consistent with model predictions more compelling evidence such as tectonic changes in response to rather than just coincident with climate change has however rarely been documented triggered by a climate driven increase in erosion rate friction dominated mountain ranges are expected to show a number of simultaneous responses a decrease in the width of the range a temporary increase in sediment yield a persistent increase in the rate of rock uplift and a reduction in the subsidence rate of surrounding basins the most convincing field evidence for such a coordinated response of a mountain range to climate change comes from the european alps and the st elias range of alaska field studies and experimental research during the past two decades have provided considerable evidence for a significant influence of climate on tectonics recent advances suggest that model predictions can guide future fieldwork aimed at substantiating this view

erosional control on the geometry and kinematics of thrust belt development in the central andes long term erosion rates in the central andes may have influenced not only mean elevation and relief but also the regional geometry and kinematic history of the orogenic belt a drastic along strike erosional gradient exists in the modern central andes from a high erosion region directly north of the 17 5 s bend in the andes to a low erosion region south of the bend this gradient has existed since 10 15 ma based on fission track analyses of middle miocene to holocene denudation and qualitative evaluations of the preservation potential of middle late tertiary volcanic edifices and synorogenic sediment global positioning system velocity data indicate conflicting patterns of active surface shortening north and south of the 17 5 s bend whereas present day shortening in the thrust belt north of the bend is distributed over much of the width of the belt south of the bend it is concentrated near the eastern frontal margin structural data suggest a similar kinematic situation during late miocene to holocene shortening an out of sequence chronology of thrusting in the narrow 200 km wide thrust belt to the north versus a forward breaking sequence of thrusting in the relatively wide 350 km wide thrust belt to the south the long term internal deformation and limited width of the thrust belt north of the bend are attributed to prolonged subcritical thrust wedge conditions induced by rapid erosion rates since 10 15 ma such conditions inhibit thrust front advance and favor distributed deformation within the thrust belt interior in the thrust belt south of the bend a progressive eastward migration of thrusting is interpreted to be the result of long term critical thrust wedge conditions promoted by extremely low rates of denudation since middle miocene time

climate tectonics and the morphology of the andes large scale topographic analyses show that hemisphere scale climate variations are a first order control on the morphology of the andes zonal atmospheric circulation in the southern hemisphere creates strong latitudinal precipitation gradients that when incorporated in a generalized index of erosion intensity predict strong gradients in erosion rates both along and across the andes cross range asymmetry width hypsometry and maximum elevation reflect gradients in both the erosion index and the relative dominance of fluvial glacial and tectonic processes and show that major morphologic features correlate with climatic regimes latitudinal gradients in inferred crustal thickening and structural shortening correspond to variations in predicted erosion potential indicating that like tectonics nonuniform erosion due to large scale climate patterns is a first order control on the topographic evolution of the andes

ultrahigh pressure metamorphism in the magnesite aragonite stability field evidence from two impure marbles from the dabie sulu uhpm belt two magnesite bearing impure dolomitic marbles from the dabie sulu ultrahigh pressure uhp region have been investigated to clarify if they had actually attained p t conditions outside the dolomite stability field limited by the reaction dolomite aragonite magnesite and to test their potential for recording u hp conditions in both cases the silicate mineral assemblage records conditions around the terminal amphibole breakdown reaction amphibole aragonite quartz clinopyrox ene talc which is a good geobarometer between at least 2 0 and 2 6 gpa at higher pressures the terminal breakdown of talc to clinopyroxene coesite is the last p t milestone below 4 gpa recordable by the silicate assemblage but evidence of former coesite is ambiguous the dolomite dissociation curve becomes strongly divariant in fe bearing marbles and may thus be attainable during cold subduction near the 5 ck m 1 geotherm at least one of the samples from xinyan village near taihu dabie shan preserved relicts of both magnesite and aragonite and most likely attained conditions within the aragonite magnesite stability field for the second sample from sanqingge village in the sulu terrane no certain evidence has been found in this study impure dolomitic marbles have considerable potential to preserve ultra high pressure relicts particularly massive impure marbles that have not been pervasively infiltrated by fluids during exhumation are considered to preserve high pressure relicts better than metapelites paragneisses and orthogneisses and perhaps almost as well as eclogites or metaperidotites the inconspicuous mineral assemblage clinopyroxene talc or quartz after former coesite may in fact record uhp conditions

thermochronology geochronology and upper crustal structure of the cordillera real implications for cenozoic exhumation of the central andean plateau 1 structural mapping 40ar 39ar and fission track thermochronology u pb geochronology and basin analysis reveal rapid cooling during middle eocene late oligocene and late miocene pliocene exhumation in the central andean plateau of bolivia in the 4 6 km high cordillera real numerous granites and sw directed fold thrust structures define the central andean backthrust belt along the altiplano eastern cordillera boundary u pb zircon analyses indicate permo triassic granitic magmatism with less extensive magmatism of late oligocene age mapping reveals low magnitudes of slip 5 km depth the 26 ma quimsa cruz granite postdated most thrust structures suggesting that upper crustal shortening in the cordillera real had largely ceased by late oligocene time results of 40ar 39ar and fission track modeling help constrain the moderate to low temperature 350 c cooling history revealing two phases of rapid cooling from 45 40 ma to 26 ma and from 11 ma onward initial cooling coincided with middle eocene late oligocene deformation in the backthrust belt and associated deposition of coarse clastic sediments in the altiplano basin eocene oligocene exhumation of 7 5 km of upper crust is estimated on the basis of thermochronologic data rapid late miocene and younger cooling involved an estimated 3 5 km of exhumation and occurred in the apparent absence of upper crustal shortening these findings suggest that crustal shortening and resultant exhumation of middle eocene late oligocene age played a major role in construction of the central andes however for late miocene exhumation the importance of alternative nonshortening mechanisms is difficult to ascertain due to a poor understanding of subsurface structures we speculate that greater precipitation on the eastern edge of the central andean plateau north of 17 5 s was a key factor in driving rapid youthful exhumation of the cordillera real

magnetostratigraphic investigations on an emeishan basalt section in western guizhou province china abstract a magnetostratigraphic investigation has been conducted on a late permian emeishan basalt section at duge in western guizhou province china the characteristic remanent magnetization chrm directions resolved at high temperatures from the basalts pass the fold test with dual polarity and define a normal magnetozone of five sixths of the total thickness in the lower part and a reversed magnetozone of one sixth of the thickness in the top part of the section the chrm directions from the basalt clasts of pyroclastic rocks fail the conglomerate test but the direction and polarity of the chrm of the clasts from the upper reversed zone are consistent with those of the basaltic lavas in the reversed zone and the normal directions also dominate the chrm of the clasts taken from the lower normal zone this suggests that either the original remanence of the clasts was reset by cementing basaltic lavas or the chrm of the basalts was acquired late but not beyond a field reversal the magnetostratigraphic pattern n r observed at duge is as simple as that observed from some major continental flood basalt provinces in the world where very few magnetic polarity zones were observed from volcanic sequences of great thickness the emplacement of the emeishan basalts in western guizhou is estimated to be less than two million years the normal polarity magnetozone observed indicates that the extrusion of the emeishan basalts in western guizhou postdates the permo carboniferous reversed superchron pcrs the apparent shortness of the reversed magnetozone explains why far fewer reverse directions than normal ones have been so far reported from the emeishan basalts

structural petrological and thermal evolution of a tertiary ductile strike slip shear zone diancang shan yunnan the diancang shan a horst massif within the red river fault zone in yunnan people s republic of china preserves a structural petrological and thermal record of two distinct phases of tectonic activity a left lateral ductile shear that terminated between 20 and 17 ma and a ductile to brittle phase of normal faulting which began at 4 7 ma and remains active mylonitic rocks in the core of the range display an early steep high temperature ht schistosity and a horizontal stretching lineation that are both parallel to the trend of the belt kinematic indicators indicate that shear was left lateral the complex shape of the ht schistosity at the southern termination of the massif likely results from a large scale oblique left lateral c shear plane that dismembered the shear zone and separated the ailao shan and the diancang shan as left lateral deformation terminated thermochronological and thermobarometric results suggest that the gneisses were partially unroofed during this event along the eastern edge of the diancang shan the ht fabrics were overprinted by low temperature structures during activation of east dipping normal faults cooling associated with this normal right lateral faulting along the diancang shan and perhaps activation of the right lateral normal movement on the range front fault farther south along the ailao shan began at 4 7 0 1 ma these results tend to support the view that extrusion of indochina occurred along the left lateral red river shear zone between 35 and 19 17 ma initiation of right lateral normal slip during the late miocene may relate to eastward extensional collapse of the thickened tibetan crust or more probably to initiation of the second phase of extrusion

stability of magnesite mgco3 at mantle pressure and temperature conditions a raman spectroscopic study the stability of magnesite mgco3 has been studied by raman spectroscopy at high pressure and high temperature using a diamond anvil cell heated by a co2 laser raman spectra up to 32 gpa at room temperature unambiguously confirm that under quasi hydrostatic conditions unlike caco3 magnesite does not undergo phase transitions raman spectra recorded at simultaneous high pressure 26 gpa and high temperature 1200 200 k show that under these conditions magnesite retains its ambient r3cbar structure optical observations during heating as well as raman spectra recorded at 30 gpa after laser heating up to 2000 2500 k show that no decarbonation has occurred and that magnesite is stable all these results are in agreement with other experimental data and show that magnesite can act as a host for c storage down to at least 1000 km and can also be the carrier for c in subducting plates

greenhouse to icehouse a biostratigraphic review of latest devonian mississippian glaciations and their global effects the latest devonian mississippian interval records the long term transition from devonian greenhouse conditions into the late palaeozoic ice age lpia this transition was punctuated by three short glaciation events in the latest famennian mid tournaisian and visean stages respectively primary evidence for glaciation is based on diamictite deposits and striated pavements in south america appalachia and africa the aim of this review is to assess the primary biostratigraphic and sedimentological data constraining diamictite deposits through this transition these data are then compared to the wider record of eustasy mass extinction and isotope stratigraphy in the lower palaeolatitudes precise age determinations are vital to integrate high and low palaeolatitude datasets and to understand the glacial control on wider global changes palynological techniques currently provide the best biostratigraphic tool to date these glacial deposits and to correlate the effects of glaciation globally this review highlights a high degree of uncertainty in the known history of early lpia glaciation as much of the primary stratigraphic data are limited and or unpublished future high resolution stratigraphic studies are needed to constrain the history of glaciation both spatially and temporally through the latest devonian and mississippian

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

controls on timing of exhumation and deformation in the northern peruvian eastern andean wedge as inferred from low temperature thermochronology and balanced cross section in northern peru a 500 km long regional balanced section has been constructed across the eastern andean wedge using fieldwork industrial seismic sections and wells the structure is characterized by a thin skinned thrust system involving the eastern cordillera ec the sub andean zone saz and the maranon foredeep in the saz and the easternmost foredeep the development of the thrust system has been driven by the combination of two structural events permian thrust faults had been reactivated to form a basement duplex underlying the saz and the foredeep at the same time a triassic jurassic extensional basin has been transported as a crustal ramp anticline on to the duplex roof fault giving rise to the ec the impingement of the ec was responsible for the deformation of the saz and the propagation of the thrust wedge the minimum shortening calculated is 142 km representing a shortening strain of 28 a sequential restoration calibrated by u th he and fission track dating on apatites and vitrinite reflectance values shows that shortening rates vary from 7 1 mm yr 1 between 17 and 8 ma to 3 6 mm yr 1 between 8 ma and today and suggests that the thrust wedge commenced propagation between 30 and 24 ma when compared with other andean thrust wedges we suggest that the timing of the thrust wedge propagation is not a simple function of the distance to the hinge of the bolivian orocline and the propagation is not controlled by the precipitation regime we rather suggest that reactivated basement faults favored thrust wedge propagation

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

the kokchetav massif kazakhstan type locality of diamond bearing uhp metamorphic rocks abstract after the discovery of metamorphic coesite in crustal rocks from the western alps italy and the western gneiss region norway in the mid 1980s of the last century metamorphic diamond was observed only a few years later in situ in the kokchetav massif kazakhstan findings of such coesite and diamond bearing ultrahigh pressure metamorphic uhp rocks with protoliths formed or embedded in crustal levels and subsequently experienced pt conditions within or even higher than the coesite stability field have dramatically changed our geodynamic view of orogenetic processes these occurrences provide evidence that crustal rocks were subducted into mantle depths and exhumed to the surface recent studies even suggest continental subduction to depths exceeding 300 km these rocks have been extensively studied and many new and important observations have been made thus far more than 350 papers have been published on various aspects of kokchetav uhp rocks the kokchetav massif of northern kazakhstan is part of one of the largest suture zones in central asia and contains slices of hp and uhp metamorphic rocks classical uhp rocks mainly occur in the kumdy kol barchi kol and kulet areas and include a large variety of lithologies such as calcsilicate rocks eclogite gneisses schists marbles of various compositions garnet pyroxene quartz rocks and garnet peridotite most of them contain microdiamonds some of which reach a grain size of 200 m most diamond grains show cuboid shapes but in rare cases diamonds within clinozoisite gneiss from barchi kol occur as octahhedral form microdiamonds contain highly potassic fluid inclusions as well as solid inclusions like carbonates silicates and metal sulfides which favour the idea of diamond formation from a c o h bearing fluid nitrogen isotope data and negative 13c values of kokchetav diamonds indicate a metasedimentary origin pt estimates of kokchetav uhp rocks yield peak metamorphic conditions of at least 43 kbar at temperatures of about 950 1000 c some zircon separates show inherited proterozoic cores and 537 530 ma uhp metamorphic mantle zones several ar ar ages on micas scatter around 529 528 and 521 517 ma and reflect different stages of the exhumation history migmatization occurred during exhumation at about 526 520 ma isotopic studies on calcsilicate rocks confirm a metasedimentary origin 18o values of garnet and clinopyroxene of a layered calcsilicate rock rule out the possibility having a primitive mantle protolith similar studies on eclogites indicate their basaltic protolith having experienced water rock interaction prior to uhp metamorphism a number of unique mineralogical findings have been made on kokchetav uhp rocks k feldspar exsolutions in clinopyroxene demonstrate that potassium can be incorporated into the cpx structure under upper mantle pressures other significant observations are coesite exsolutions in titanite quartz rods in cpx the discovery of k tourmaline as well as new minerals like kokchetavite a hexagonal polymorph of k feldspar and kumdykolite an orthorhombic polymorph of albite the kokchetav uhp rocks represent a unique and challenging stomping ground for geoscientists of various disciplines from crystallography petrology and geochemistry to geophysics and geodynamics geotectonics it concerns all who are interested in the diverse metamorphic processes under upper mantle conditions

orogeny and orography the effects of erosion on the structure of mountain belts a numerical model of the coupled processes of tectonic deformation and surface erosion in convergent orogens is developed to investigate the nature of the interaction between these processes crustal deformation is calculated by a two dimensional finite element model of deformation in response to subduction and accretion of continental crust erosion operates on the uplifted surface of this model through fluvial incision which is taken to be proportional to stream power the relative importance of the tectonic and erosion processes is given by a dimensionless erosion number relating convergence velocity rock erodibility and precipitation rate this number determines the time required for a system to reach steady state and the final topographic shape and size of a mountain belt fundamental characteristics of the model orogens include asymmetric topography with shallower slopes facing the subducting plate and an asymmetric pattern of exhumation with the deepest levels of exhumation opposite to subduction these characteristics are modified when the regional climate exhibits a dominant wind direction and orographically enhanced precipitation on one side of the mountain belt the two possible cases are dominant wind in the direction of motion of the subducting plate and dominant wind direction in the opposite direction of the subducting plate velocity models of the former case predict a broad zone of exhumation with maximum exhumation in the orogen interior models of the latter case predict a focused zone of exhumation at the margin of the orogen and at high erosion number a reversal in the topographic asymmetry natural examples of these two cases are presented the southern alps of new zealand exhibits the climate and exhumation asymmetry characteristic of wind in the direction opposite to motion of the subducting plate the asymmetry of topography suggests that erosion is not efficient enough to have reversed the topographic asymmetry the contrasting example of dominant wind in the direction of subduction motion is provided by the olympic mountains of washington state in this case exhumation of deep levels of the cascadia accretionary wedge shows a broad domal pattern consistent with the observed orographic precipitation

tectonic evolution of the central andean plateau and implications for the growth of plateaus current end member models for the geodynamic evolution of orogenic plateaus predict a slow and steady rise during crustal shortening and ablative subduction i e continuous removal of the lower lithosphere or b rapid surface uplift following shortening which is associated with punctuated removal of dense lower lithosphere and or lower crustal flow this review integrates results from recent studies of the modern lithospheric structure geologic evolution and surface uplift history of the central andean plateau to evaluate the geodynamic processes involved in forming it comparison of the timing magnitude and distribution of shortening and surface uplift in combination with other geologic evidence highlights the pulsed nature of plateau growth we discuss specific regions and time periods that show evidence for end member geodynamic processes including middle late miocene surface uplift of the southern eastern cordillera and altiplano associated with shortening and ablative subduction latest

thermochronological evidence for the movement of the ailao shan red river shear zone a perspective from vietnam in order to explore the timing of strike slip movement along the ailao shan red river shear zone an 40ar 39ar thermochronological study of the day nui con voi metamorphic massif in northern vietnam was undertaken the massif exposed in the southeastern segment of the ailao shan red river shear zone reveals a rapid cooling in the early miocene 25 21 ma after a very slow cooling ca 34 25 ma the slow cooling period most likely resulted from a geothermal perturbation in the lithosphere owing to the late paleogene 40 30 ma extension in eastern tibet and western yunnan china the rapid cooling consistent with evidence of a wider rapid cooling span from 27 to 17 ma compiled from the entire ailao shan red river shear zone constrains the duration of left lateral shearing which eventually offset indochina from south china by about 600 km the time constraints we have established reinforce the argument that the onset of the ailao shan red river shear zone postdates the opening of the south china sea that began ca 30 ma this result highlights the necessity for reevaluating the cenozoic tectonic models of southeast asia

exhumation of the dabie shan ultra high pressure rocks and accumulation of the songpan ganzi flysch sequence central china the presence of coesite and diamond bearing ultra high pressure uhp metamorphic rocks in the dabie and sulu regions central china suggests that a 100 km thick crustal section 4 x 106 km6 in volume has been denuded this volume is comparable to that represented by the 10 15 km thick middle to upper triassic flysch rocks in the songpan ganzi region regional geology and radiometric dates are compatible with an interpretation that the majority of these sedimentary rocks were derived from denudation of the orogenic belt between north and south china blocks following their latest paleozoic to triassic collision this correlation implies a high denudation rate of 4 mm yr that may have been in part promoted by tropical precipitation and extreme topographic relief for as long as 25 m y

kinematic flexural and thermal modelling in the central andes unravelling age and signal of deformation exhumation and uplift abstract quantifying age rate and lateral variation of deformation and exhumation in convergent systems relies on integration of geologic map patterns ag and locations of reset thermochronometer systems and synorogenic sediment distribution the central bolivian andes provide an ideal location to examine the influence of variations in shortening and stratigraphic architecture on the structural evolution of the mountain range due to differential age and rates of shortening and distinct sedimentary basin geometries along strike to quantify age and rate of shortening we link thermokinematic modelling of sequentially deformed forward modelled balanced cross sections to synorogenic and thermochronologic histories the preserved basin history in the altiplano and eastern cordillera argues for an early fold and thrust belt located in the now western cordillera with subsequent propagation of shortening eastward around 40 50 ma flexural modelling incorporating isostasy and erosion requires multiple basement thrust sheets with 35 97 km of displacement a temporally evolving effective elastic thickness as well as imposed subsidence in the foreland and uplift in the hinterland is required to reproduce the surface geology increase subandean foreland basin depth limit altiplano sedimentation and facilitate altiplano uplift to modern elevation thermokinematic modelling is compatible with initiation of deformation at 50 40 ma in a marked increase in subandean velocities from 5 5 to 8 10 mm yr from 12 10 ma to present out of sequence thrusting at the westernmost limit of the subandes is required to match measured young and partially reset zircon helium ages out of sequence faulting is supported by high ksn values indicative of active uplift and was likely promoted by the abrupt eastern edge of the paleozoic basin rocks which limited forward propagation of structures and or increased erosion due to focused precipitation our results highlight the importance of incorporating detailed structural modelling in differentiating the geometry kinematics and timing of deformation to reproduce thermochronologic ages and basin histories

simulating foreland basin response to mountain belt kinematics and climate change in the eastern cordillera and subandes an analysis of the chaco foreland basin in southern bolivia

tectonic and lithological controls on denudation rates in the central bolivian andes abstract the topographic signature of a mountain belt depends on the interplay of tectonic climatic and erosional processes whose relative importance changes over times while quantifying these processes and their rates at specific times remains a challenge the eastern andes of central bolivia offer a natural laboratory in which such interplay has been debated here we investigate the rio grande catchment which crosses orthogonally the eastern andes orogen from the eastern cordillera into the subandean zone exhibiting a catchment relief of up to 5000 m despite an enhanced tectonic activity in the subandes local relief mean and modal slopes and channel steepness indices are largely similar compared to the eastern cordillera and the intervening interandean zone nevertheless a dataset of 57 new cosmogenic 10 be and 26 al catchment wide denudation rates from the rio grande catchment reveals up to one order of magnitude higher denudation rates in the subandean zone mean 0 8 mm yr compared to the upstream physiographic regions we infer that tectonic activity in the thrusting dominated subandean belt causes higher denudation rates based on cumulative rock uplift investigations and due to the absence of a pronounced climate gradient furthermore the lower rock strength of the subandean sedimentary units correlates with mean slopes similar to the ones of the eastern cordillera and interandean zone highlighting the fact that lithology and rock strength can control high denudation rates at low slopes low denudation rates measured at the outlet of the rio grande catchment abapo are interpreted to be a result of a biased cosmogenic nuclide mixing that is dominated by headwater signals from the eastern cordillera and the interandean zone and limited catchment sediment connectivity in the lower river reaches therefore comparisons of short i e sediment yield and millennial denudation rates require caution when postulating tectonic and or climatic forcing without detailed studies

retroarc deformation and exhumation near the end of the andes southern patagonia abstract the southern patagonian andes constitute the narrow high latitude end of the andean orogen belt in south america where inherited basin paleogeography subduction processes retroarc crustal thickening and late cenozoic glaciation have collectively influenced their unusual tectonic and physiographic evolution new zircon and apatite u th he thermochronology from the patagonian andes between 50 30 and 51 30 s suggest concentrated exhumation across the retroarc leeward side of the orogen since early miocene time zircon u th he zhe ages range from 44 to 10 ma oldest ages are recorded in the patagonian batholith and along the far eastern frontal foreland monocline regionally uniform zhe ages between 22 and 18 ma located across a 75 km wide zone of the patagonian retroarc thrust belt indicate widespread early miocene cooling through the zhe partial retention zone mesozoic sedimentary and volcanic rocks in this region have been exhumed from at least 5 to 6 km depths early miocene denudation of the thrust belt deformation and increased foreland sedimentation rates coincided with opening of the scotia sea suggesting a causal response of the foreland to changes in plate dynamics the apatite u th he ahe ages from a similar region range from 11 to 3 ma the youngest ages 6 3 ma are spatially clustered within the more deeply exhumed central thrust domain we interpret these ahe ages to record 1 2 km of erosional denudation associated with late cenozoic glaciation and fluvioglacial processes in patagonia beginning ca 7 ma zircon results indicate that since ca 22 ma long term exhumation rates have been highest across the western and central thrust domains 0 22 0 37 mm yr 1 and significantly lower along the eastern thrust front 0 10 0 17 mm yr 1 since 7 ma apatite results from these same regions suggest comparable and slightly higher denudation rates 0 14 0 46 mm yr 1 particularly within the eastern thrust domain consistent with efficient erosional processes acting in the retroarc region these results from the patagonian retroarc region particularly the predominance of

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

spatial and temporal distribution of deformation at the front of the andean orogenic wedge in southern bolivia new observations from an active orogenic wedge help link the seismotectonic behavior of individual faults to wedge deformation rates and patterns over multiple timescales we provide the first detailed constraints on the distribution and timing of deformation at the front of the andean orogenic wedge in southern bolivia where a recent study suggests that great mw 8 earthquakes could rupture the master fault underlying the wedge we use stratigraphic relationships across fault related folds and elastic dislocation modeling of seismic reflection horizons to obtain probabilistic estimates of wedge front fault ages and slip rates our analyses reveal that at least half of the previously determined gps based wedge loading and quaternary whole wedge shortening rates are absorbed across a 20 40 km wide wedge front zone consisting of 1 4 en echelon and partially to fully overlapping faults and folds associated with blind thrust faults the difference between our slip rates and the geodetic geologic observations combined with evidence for activity across internal wedge structures supports the notion that nonsteady state mass balance conditions coupled with elevated erosional efficiency result in distributed wedge deformation the orogenic wedge in southern bolivia behaves in a similar fashion to the taiwanese and himalayan ranges slip accumulates at downdip locations along the master fault and is released incrementally by earthquakes that rupture the wedge front fault zone the faults and folds comprising this zone pose a major source of seismic hazard accumulating slip is also released in the wedge interior and older internal wedge faults must be considered in any future assessment of regional earthquake risk

the influence of bedrock orientation on the landscape evolution surface morphology and denudation 10be at the niesen switzerland landscape evolution and surface morphology in mountainous settings are a function of the relative importance between sediment transport processes acting on hillslopes and in channels modulated by climate variables the niesen nappe in the swiss penninic prealps presents a unique setting in which opposite facing flanks host basins underlain by identical lithologies but contrasting litho tectonic architectures where lithologies either dip parallel to the topographic slope or in the opposite direction i e dip slope and non dip slope the north western facing diemtigen flank represents such a dip slope situation and is characterized by a gentle topography low hillslope gradients poorly dissected channels and it hosts large landslides in contrast the south eastern facing frutigen side can be described as non dip slope flank with deeply incised bedrock channels high mean hillslope gradients and high relief topography results from morphometric analysis reveal that noticeable differences in morphometric parameters can be related to the contrasts in the relative importance of the internal hillslope channel system between both valley flanks while the contrasting dip orientations of the underlying flysch bedrock has promoted hillslope and channelized processes to contrasting extents and particularly the occurrence of large landslides on the dip slope flank the flank averaged beryllium 10 10be derived denudation rates are very similar and range between 0 20 and 0 26 mm yr 1 in addition our denudation rates offer no direct relationship to basin s slope area steepness or concavity index but reveal a positive correlation to mean basin elevation that we interpret as having been controlled by climatically driven factors such as frost induced processes and orographic precipitation our findings illustrate that while the landscape properties in this part of the northern alpine border can mainly be related to the tectonic architecture of the underlying bedrock the denudation rates have a strong orographic control through elevation dependent mean annual temperature and precipitation

the effects of lateral density gradients slopes and buoyancy on channel flow 1d analytical solutions and applications to the se canadian cordillera abstract we present 1d analytical solutions for channel flow in orogens driven by various types of pressure gradients our calculations demonstrate that lateral density gradients in the upper crust such as would occur across a suture zone separating arc rocks from pericratonic sediments provide a driving force for poiseuille flow as large as topographic gradients observed in modern mountain belts for cases for which the gradients are external topographic and lateral density gradients and internal e g partial melting of channel material to the channel inclination decreases and increases the poiseuille component of the average flow velocity within the channel by the cosine and sine of the slope respectively the magnitude of the pressure gradient consequent upon the buoyancy generated by partial melting of metapelites in a channel with a 30 slope such as would occur above an underthrusting basement ramp is similar to that of topographic or lateral density gradients channel flow up a ramp could thus constitute an important exhumation mechanism in large hot orogens our calculations indicate that mid crustal channel flow was a highly likely process in the late cretaceous paleocene setting of the southeastern canadian cordillera the flow was first driven by the lateral density contrast between pericratonic sediments and the arc related intermontane terrane then by combined effect of topographic gradient and melt induced buoyancy of the lower selkirk allochthon part of the shuswap complex flow up the underthrusting basement ramp resulted in exhumation from mid to upper crustal levels channel flow then migrated downward to involve basement and overlying cover sequence rocks our results indicate that syn convergent channel flow was a viable and very likely process in the southeastern canadian cordillera

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

late miocene to early pliocene paleohydrology and landscape evolution of northern chile 19 to 20 s abstract northern chile s forearc lies in the hyperarid atacama desert the northern portion 150 km of the forearc basin known as the central depression contains perennial streams that drain to the pacific ocean through canyons that are deeply incised within a pediplain however during the late miocene and early pliocene a lake existed in the western part of the central depression between 19 3 s and 19 7 s indicating a forearc geomorphic landscape and paleohydrologic regime distinct from the present one for approximately 2 ma this lake was large 375 km2 and deep up to 120 m the catchment area sourcing the lake covered an area of 2900 5200 km2 new 40ar 39ar ages of volcanic ashes intercalated with the lake deposits and the overlying fluvial deposits indicate that lacustrine diatomite rich deposition had begun by 10 86 0 04 ma and the final stage of incision of a canyon through the lake deposits is younger than 3 04 0 03 ma additional published age constraints demonstrate that the expanse of the lake shrunk shortly after 9 ma yet the water balance in the western sector of the basin permitted lacustrine diatomite deposition until more recently than 6 4 ma even after the sediment hydrology balance shifted from balanced fill to overfilled a near surface groundwater table led to accumulation of salt pan salar evaporites until more recently than 3 5 ma the existence of a large deep lake is consistent with but not conclusive evidence for a wetter than modern climate in the catchment region during the late miocene the ultimate demise of the paleolake system is attributed to a kilometer scale decline of the base level of drainages downstream of the paleolake this study takes advantage of the unique characteristic of the drainage profiles in the study region a spatial separation between areas of deep incision east and west of a prominent knickzone to evaluate three end member scenarios of landscape evolution that could explain the base level fall event s the sedimentological stratigraphic and geomorphological data best fit the interpretation that forearc tectonic uplift and tectonically driven coastal retreat both contributed to base level fall

onset of convective rainfall during gradual late miocene rise of the central andes a decrease in the ratio of 18o to 16o 18o of sedimentary carbonate from the bolivian altiplano has been interpreted to indicate rapid surface uplift of the late miocene andean plateau ap here we report on paleoclimate simulations of andean surface uplift with an atmospheric general circulation model gcm that tracks oxygen isotopes in vapor the gcm predicts changes in atmospheric circulation and rainfall that influence ap isotopic source and amount effects on eastern ap slopes summer convective precipitation increases by up to 6 millimeters per day 500 for plateau elevations that are greater than about 2000 meters high precipitation rates enhance the isotope amount effect leading to a decrease in precipitation 18o at high elevations and an increase in 18o lapse rate our results indicate that late miocene 18o depletion reflects initiation and intensification of convective rainfall

the kinematic history of the central andean fold thrust belt bolivia implications for building a high plateau this paper presents a model for the kinematic evolution of the central andean plateau based on balanced cross sections across the bolivian andes the proposed model links the formation of the andean plateau to the development of the andean fold thrust belt through the creation and propagation of two large basement mega thrusts support for large basement involved thrust sheets is found in significant steps in both the topography and the exposed structural elevation of the andean fold thrust belt the structurally highest basement thrust raised folds and faults in predominantly lower paleozoic rocks of the eastern cordillera with respect to tertiary rocks in the broad internally drained basin of the altiplano to the west and east verging folds and faults in upper paleozoic rocks of the interandean zone to the east the interandean zone was in turn raised both structurally and topographically with respect to the frontal folds and faults of the fold thrust belt the subandean zone by a second structurally lower basement thrust sheet thus these two megathrusts divide the andean fold thrust belt into four areas of markedly different structural elevations the eastern cordillera can be further subdivided into two zones of west and east vergent folds and thrusts shortening accommodated by the fold thrust belt can be divided among these tectono structural zones and linked to shortening accommodated by the inferred basement megathrusts the proposed kinematic model suggests that the eastward propagation of the structurally highest basement thrust fed 105 km of slip into the eastern cordillera along east vergent and west vergent faults this structure also fed 90 km of eastward slip into the interandean zone the initiation and eastward propagation of a lower basement thrust structurally elevated the interandean zone with respect to the foreland while feeding 65 km of slip into the subandean zone out of sequence basement thrusting to the west is proposed to have elevated the western edge of the plateau and accommodated 40 km of shortening within the altiplano total cumulative shortening within the cover rocks of the andean fold thrust belt 300 330 km can be balanced by an equivalent amount of shortening along two basement megathrusts to the first order the eastern margin of the central andean plateau defined by the 3 km topographic contour is contiguous with the leading edge of the upper basement megathrust this relationship between the basement highs and the physiographic boundaries of the andean plateau suggests that extensive megathrust sheets involving strong rocks such as crystalline basement or quartzite play an important role in the formation of the central andean plateau and a similar link between megathrust sheets and plateaus may be found in other orogens

new constraints on the chronology magnitude and distribution of deformation within the central andean orocline 1 constructing an accurate kinematic model for crustal thickening in the central andes is hampered by the inability of 2 d balanced cross sections to resolve the 3 d displacement field within the poorly studied orocline core this study presents new structural and thermochronometer data from the orocline axis in the central andes that constrains the magnitude and timing of deformation through 1 a balanced cross section quantifying shortening perpendicular to strike 2 field observations that constrain translational displacements parallel to strike and 3 thermochronology that brackets the age of exhumation and deformation mapping results show that north directed right lateral motion has been accommodated on at least one major strike slip fault in the eastern cordillera ec shortening perpendicular to fault strike is 179km 41 for the ec and inter andean zone ia and 86km 39 for the sub andes sa apatite fission track cooling ages indicate rapid ec ia exhumation from 46 to 18ma and 15 to 0ma in the sa zircon and apatite u th he ages show that rapid exhumation began 10 5ma in the sa these results are integrated with existing paleomagnetic and gps rotation data in map view reconstructions to quantify the magnitude of translational faulting related to oroclinal bending the reconstructions suggest that at least 85km of north directed translational displacement has been accommodated within the orocline core orogen parallel displacement and vertical axis rotations have focused crustal thickening at the orocline core and facilitated development of a high elevation plateau suggesting a genetic link between large scale oroclines and plateaus

erosion and tectonics at the margins of continental plateaus the topography across the eastern margin of the central andean plateau north of 18 s beni region bears a strong resemblance to the topography of the southern margin of the tibetan plateau nepal himalaya with both regions featuring a steep frontal slope and high peaks at the plateau edge in contrast the topography of the eastern margin of the central andean plateau south of 18 s pilcomayo region tapers toward the foreland more gently and has no line of high peaks at the margin both the himalayan and the beni regions have been the sites for large amounts of denudation and in both regions geologic evidence suggests that erosion has been sufficiently vigorous for the physiographic plateau margin to have retreated toward the plateau interior during the neogene we hypothesize that the steep frontal slope and high peaks of the beni region and himalayan front largely reflect the high orographic precipitation and high erosion rates occurring in these regions and that the more gentle topography of the semiarid pilcomayo region reflects a tectonic landform only slightly modified by erosion we propose that orographic precipitation impinging on a plateau margin will generally tend to drop moisture low on the slope eroding back the plateau while enhancing or maintaining the steep long wavelength slope a numerical model coupling orographic precipitation erosion and tectonic uplift demonstrates the plausibility of this hypothesis the erosional efflux in both the beni and nepal himalaya have been considerable and simple mass balance calculations for the himalaya suggest that during the neogene the erosional mass efflux has generally outpaced the tectonic mass influx this contrasts with the apparent prior domination of tectonic influx and may reflect a decrease in the rate of tectonic addition during the same period and or increased late cenozoic erosion rates

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

high pressure phase transformations of carbonates in the system caomgosio2co2 the phase behaviour of the carbonates in the system mgocaosio2co2 have been studied in a diamond anvil press employing yag laser heating from about 40 to 260 kbar at 1000 c it is already known that at the caco3mgco3 join calcite caco3 is stable only at relatively low pressures 30 kbar that aragonite caco3 is stable between 30 and 400 kbar and that magnesite mgco3 is stable between 7 and 550 kbar at 1000 c in the present study it has been found that the intermediate mineral dolomite caco3 mgco3 decomposes into the mixture aragonite magnesite in the pressure range 60 70 kbar at 1000 c another intermediate mineral huntite caco3 3mgco3 transforms to a new phase at pressures below 40 kbar and then decomposes to the mixture dolomite magnesite at pressures greater than 60 kbar the new phase with the huntite composition possesses an orthorhombic cell with ao 9 933 0 002 bo 6 712 0 001 and co 24 06 0 02 a at ambient conditions there are only two well known silicate carbonate minerals in the caosio2co2 system these are spurrite and tilleyite both were found to decompose into their component silicates and carbonates at pressures below 40 kbar at 1000 c on the basis of the present study it can be concluded that aragonite and magnesite are the only two carbonates in the entire caomgosio2co2 system which are stable at pressures greater than about 70 kbar at 1000 c if no interaction occurs between carbonates and silicates at still higher pressures

massive middle miocene gypsic paleosols in the atacama desert and the formation of the central andean rain shadow abstract the central andes of south america are one of the highest and broadest mountain ranges on earth and have a profound influence on climate in the southern hemisphere yet the timing of andean uplift and its subsequent impact on atmospheric circulation is not well known here we use fossil soils from along an 800 km transect 19 25 s in the atacama desert of northern chile to identify the initiation of extreme aridity along the west coast of south america and infer a minimum age for the development of the central andean rain shadow paleosols record the onset of extreme aridity at the boundary between the middle and late miocene 12 10 ma lower miocene 24 20 ma soils are chemically weathered and contain soil carbonate indicative of a vegetated landscape and semi arid climate in contrast gypsum dominates massive 2 5 m thick upper middle miocene soils that formed on a hyperarid landscape devoid of vegetation the development of extreme hyperaridity is linked to the formation of the central andean rain shadow through the divergence of oxygen isotope values oxygen isotope values of lower miocene soil carbonates in the atacama desert 5 0 1 1 vpdb and central andes 7 8 1 are similar indicating no or a limited rain shadow by 15 mya however 18 o values of soil carbonate differ by 8 with values of soil carbonate in altiplano paleosols getting lower due to uplift while soil carbonate values in the atacama desert get higher due to aridity therefore by 15 mya there is clear evidence for a rain shadow from the central andes by the latest middle miocene ca 12 10 ma paleosols in the altiplano decreased to 11 8 1 0 while soils in the atacama desert increase to 1 5 4 5 this almost 14 divergence in the 18 o values of soil carbonate for soils only 250 km apart is evidence of a strong central andes rain shadow effect in place during the late middle miocene 12 10 ma

new late jurassic palaeomagnetic data from the northern sichuan basin implications for the deformation of the yangtze craton summary upper jurassic red sandstones and red siltstones were collected from 67 layers at 12 localities in the penglaizhen formation this formation is in the north of bazhong county 31 8 n 106 7 e in the sichuan basin which is located in the northern part of the yangtze craton thermal demagnetization isolated a high temperature magnetic component with a maximum unblocking temperature of about 690 c from 45 layers the primary nature of the magnetization acquisition is ascertained through the presence of magnetostratigraphic sequences with normal and reversed polarities as well as positive fold and reversal tests at the 95 per cent confidence level the tilt corrected mean direction of 36 layers is d 20 0 i 28 8 with a 95 5 8 a late jurassic palaeomagentic pole at 64 7 n 236 0 e with a 95 7 0 is calculated from the palaeomagnetic directions of 11 localities this pole position agrees with the two other late jurassic poles from the northern part of the yangtze craton a characteristic late jurassic pole is calculated from the three poles 68 6 n 236 0 e with a 95 8 0 for the northern part of the yangtze craton this pole position is significantly diverent from that for the southern part of the yangtze craton this suggests that the southern part of the yangtze craton was subjected to southward extrusion by 1700 1000 km with respect to the northern part intracraton deformation occurred within the yangtze craton

exhumation of the precordillera and northern sierras pampeanas and along strike correlation of the andean orogenic front northwestern argentina

linking orography climate and exhumation across the central andes quantifying interactions between uplift climate deformation and exhumation remains difficult in many cases due to a paucity of data relevant to all processes we synthesize new and existing data to understand the orogen scale orographic changes across the central andes bolivia we use a regional climate model and geo thermochronologic data to identify the correlations between changes in precipitation due to surface uplift and spatiotemporal patterns of deformation and erosional exhumation mean orographic rainfall patterns do not reach near present day gradients and values until the topography grows to 75 modern elevations new fission track data near the orocline apex indicate that rapid exhumation moved eastward beginning in the eastern cordillera ca 50 15 ma the interandean zone ca 18 6 ma and in the subandes ca 7 3 ma throughout bolivia exhumation is consistent with deformation until ca 15 11 ma after which the pattern corresponds better with the increased rainfall toward modern values these linked observations suggest that ca 15 11 ma regional elevations reached threshold values 75 modern necessary to generate near present day enhanced rainfall gradients these gradients have resulted in variable exhumation implied by the structural level of rocks exposed across the thrust belt and confirmed by fission tracks in apatite the main insight is that the climate induced middle miocene recent exhumation varies over scales of a few hundred kilometers across bolivia and implies that high mean rainfall 3 m yr and long time scales 10 m y may be necessary for climate to induce orographically driven exhumation patterns recorded by fission tracks

three dimensional 3 d finite strain at the central andean orocline and implications for grain scale shortening in orogens three dimensional 3 d finite strain analyses from across the central andes are used to document the contribution of grain scale strain in quartzites and sandstones to the total shortening budget the results are compared to thermal stratigraphic and strain data from other fold and thrust belts to determine the influence of lithologic strength and deformation temperature on strain accommodation during orogenic evolution in the central andes 3 d best fit ellipsoids are inconsistently oriented relative to structural trends have short axes at high angles to bedding z mean plunge 78 21 and have bedding parallel long axes x mean plunge 6 24 ellipsoid shapes are dominantly oblate x y z indicate low natural octahedral shear strains e s 0 03 0 19 and have axial ratios that range from 1 02 1 0 81 e s 0 19 to 1 02 1 0 97 e s 0 03 highly variable r f data r f 1 0 5 0 fluctuations exceeding 100 indicate detrital grain shapes may overwhelm any measurable tectonic strain fabric recorded by grain geometry the best fit ellipsoids may reflect either weak compaction strain or they may be related to a depositional fabric at a minimum granular strain was insufficient to reset the detrital grain fabric and therefore grain scale strain in quartzites and sandstones is not a significant factor in deformation we suggest that the nonstrained nature of these stiffer lithologies indicates a lack of regional penetrative strain in the central andes like that quantified in similar lithologies in other orogens the regional lack of strain may be due to deformation temperatures

response of meteoric δ18o to surface uplift implications for cenozoic andean plateau growth abstract the timing and magnitude of surface uplift provide important constraints on geodynamic models of orogen formation oxygen isotope 18 o and mass 47 isotopolog 47 compositions from terrestrial carbonate sediments have been used with modern isotope and temperature lapse rates to infer past surface elevations of the andes however these paleoaltimetry interpretations are contentious because variations in the oxygen isotope composition in meteoric water 18 o p are caused by changes in elevation orographic and regional climate here we use a limited domain isotope tracking general circulation model to simulate changes in 18 o p and isotopic lapse rates in response to andean surface uplift and to re evaluate 18 o and 47 changes in late miocene carbonates previously associated with rapid andean growth results indicate that andean surface uplift leads to changes in low level atmospheric circulation and an increase in precipitation along the eastern andean flank which influences isotopic source and amount effects simulated changes in andean 18 o p are not systematic with an increase in surface elevation but are instead a function of orographic thresholds that abruptly change regional climate a 18 o p decrease of 5 over the central andes and an increase in isotopic lapse rates up to 0 8 km 1 coincide with andean surface uplift from 75 to 100 of modern elevation these changes in the isotopic signature could account for the entire 3 4 18 o depletion in late miocene carbonate nodules and suggest an andean paleoelevation of 3000 m 75 of modern elevations before 10 ma

assessing uncertainties in balanced cross sections abstract balanced structural cross sections are models that are fit to incomplete data the models are under constrained with respect to any particular two dimensional line length model but enough data generally exists to yield a well constrained area balance solution furthermore the area balance encompasses all possible line length solutions therefore where the primary objective of section balancing is the determination of horizontal shortening magnitude area balancing provides an analytical solution we use this analytical solution to develop a comprehensive robust analysis of the uncertainty in shortening estimates resulting from cross section balancing the analytical solution allows us to propagate errors formally on the input parameters stratigraphic thicknesses depth to decollement eroded hanging wall cutoffs through the equations and produce the resulting uncertainty on the magnitude of shortening balanced cross sections from the subandean belt of the central andes are used to demonstrate the relative importance of stratigraphy and eroded hanging wall cutoffs in the contribution to the overall error

emeishan basalt ar ar overprint ages define several tectonic events that affected the western yangtze platform in the mesozoic and cenozoic abstract ar ar whole rock dating was carried out as part of a detailed stratigraphical investigation of the emeishan basalt large igneous province lip in the stratotype area sichuan china thirteen from twenty one specimens from three sections yielded reliable reversed isochron and plateau ages maximum 1 errors of 2 1 and 1 6 m y respectively and form two clusters centered on the late jurassic early cretaceous and early late cretaceous with a tail spanning 82 40 ma however all are appreciably younger than the magnetobiostratigraphically constrained late middle permian 255 260 ma age of the basalts reflecting varying degrees of thermal resetting during the mesozoic and cenozoic recently middle jurassic early cretaceous ages reported from elsewhere in the lip have been used to infer a single 40 m y long tectonic episode that affected the western yangtze platform developing this idea the new information have been combined with data we hold for other parts of the terrain and results from three emeishan lip ar ar dating studies published during 2002 to give 32 reliable age dates three 10 12 m y events appear to be recorded in different parts of the province middle jurassic late jurassic early cretaceous and early late cretaceous with a fourth shorter middle eocene episode the 40 m y mesozoic episode appears to be a sampling artifact following a review of the regional deformation tectonic features it is argued that activity related to various phases of deformation in the longmen shan thrust belt is the most likely cause of resetting the final suturing of the north and south china blocks may have also been responsible for the middle jurassic event however the oligocene present indentation of india into asia appears not to have had an impact possibly due to the large scale related strike slip faults that have effectively shielded the lip sichuan basin

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

late miocene climate variability and surface elevation in the central andes abstract temporal and spatial variations in topography and oxygen stable isotope ratios in precipitation in the central andes have stimulated widespread discussion about the competing roles of mantle and crustal processes and their feedbacks with global scale climatic change in uplifting and shaping the central andes in general one of the major obstacles in assessing the relative contributions of long term 10 5 10 6 a tectonic processes and precipitation as a proxy for climate to the uplift history of the andean orogen is the lack of integrated data sets that record late miocene patterns of uplift and climate radiogenic 87 sr 86 sr sedimentologic and stable isotope 18 o data from subandean foreland deposits of the chaco basin bolivia show a rapid 18 o and 87 sr 86 sr values at 8 5 ma that we interpret to reflect a change in precipitation patterns along the eastern cordillera and the subandean fold thrust belt in agreement with 13 c studies on paleosol carbonates we attribute this change to a southward deflection of the south american low level jet llj that currently exerts the dominant control over the seasonality and amount of precipitation along the eastern flanks of the andes deflection of the llj occurred most likely as the combined effects of readjustment of relief and topography within the eastern cordillera at 20 22 s and possibly associated surface uplift of the altiplano contemporaneous rapid positive shifts in 18 o and 87 sr 86 sr of pedogenic carbonate in fluvial foreland deposits are consistent with a transition to more seasonal precipitation conditions and critical threshold elevations being attained that affected south american atmospheric circulation patterns a four fold increase in sedimentation rates in the foreland together with a shift to strongly radiogenic 87 sr 86 sr ratios in paleo river water and sediment load as well as river incision into the well preserved san juan del oro paleo surface directly reflects a rapid change in moisture distribution along the eastern flank of the central andes at 8 5 ma and associated rearrangement of the drainage systems collectively the data presented here strongly suggest that between 12 and 8 ma the amount oxygen isotopic composition spatial distribution and seasonality of precipitation along the eastern flank of the bolivian andes changed dramatically which in turn require changes in trans andean moisture flux and isotopic composition of precipitation on the uplifting plateau region

a scenario for late neogene andean shortening transfer in the camisea subandean zone peru 12 s implications for growth of the northern andean plateau precise knowledge of the timing of deformation in the subandean zone of the andean plateau is a prerequisite for deciphering the late neogene growth of the andean plateau in this paper we report new apatite fission track aft and vitrinite reflectance ro data for a regional balanced cross section of the camisea basin in the central peruvian subandean zone adjacent to the northern andean plateau the balanced cross section shows that the structure of this basin is characterized by a broad internal passive roof duplex and external thrust related anticlines the balanced cross section restoration shows 53 km 39 of total horizontal shortening we sampled paleozoic to cenozoic sedimentary strata for aft and ro analyses along the 4 km thick vertical profile of the mainique back thrust passive roof thrust the innermost preserved subandean structure young components of aft ages are spread between ca 6 ma and ca 24 ma a break in the slope in the aft ages determines the geometry of the miocene partial annealing zone and the exhumation of the mainique back thrust at ca 6 ma sequential restoration calibrated by aft and ro data indicates that the last 23 km horizontal shortening were accommodated by the camisea thrust system over the past 6 m y giving a mean shortening rate of 3 8 mm yr using this shortening rate for the first 30 km horizontal shortening we calculate that the andean shortening transfer into the peruvian subandean zone initially started at ca 14 ma this result suggests that the transfer of shortening from the northern andean plateau to the subandean zone occurred prior to the removal of dense lithosphere previously reported to have occurred between ca 10 ma and ca 7 ma we rather propose that the late neogene growth of the northern andean plateau mostly resulted from a continuous crustal shortening combined with lower crustal flow

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

andean shortening inversion and exhumation associated with thin and thick skinned deformation in southern peru a balanced cross section spanning the eastern cordillera and subandean zone of southern peru 13 15 s constrains 130 km 38 of cenozoic orogen normal sw ne andean deformation accommodated by thick and thin skinned retro arc fold thrust belt shortening that overprinted pre andean triassic normal faults zircon and apatite u th he ages demonstrate continuous oligocene to miocene cooling of the permo triassic coasa pluton in the eastern cordillera zircon u th he ages 34 18 ma are reset and define a steep age versus elevation relationship apatite u th he results reveal reset ages that define two spatially separated groups with ages of 30 26 ma and 17 11 ma detrital zircon u pb geochronologic results from cretaceous cenozoic siliciclastic rocks from the altiplano eastern cordillera record andean fold thrust belt and magmatic arc sediment sources correlative subandean zone rocks preserve a cratonic sediment contribution with minor andean sediment appearing in some cenozoic rocks we propose that earliest andean deformation and structural compartmentalization of the eastern cordillera was linked to selective inversion of inherited permo triassic basement involved normal faults that guided subsequent thick and thin skinned deformation provenance variations between the hinterland and foreland depocentres reveal competing eastern and western sediment sources reflecting an axial zone in the eastern cordillera that coincided with the inherited triassic graben and impeded sediment source mixing our zircon and apatite u th he ages are consistent with published constraints along strike and support pulses of eocene to late miocene exhumation that were likely driven by normal fault reactivation and protracted eastern cordillera deformation

exploring landscape sensitivity to the pacific trade wind inversion on the subsiding island of hawaii the interaction of the subsiding limb of the hadley circulation and the easterly north pacific trade winds establishes a persistent thermal inversion at about 2000 m above sea level in the subtropical pacific the inversion restricts convective rainfall to the lower elevations of the windward flank of the island of hawaii creating an order of magnitude vertical rainfall gradient as well as high interbasin variability in precipitation in the high rainfall zone streams are incised tens to hundreds of meters below the surface of the volcanic shield we use a digital elevation model and 1 d numerical modeling to assess whether deep incision on the flank of mauna kea is tied to the elevation of the trade wind inversion the 83 channels examined can be well fit in aggregate by all models that account for differences in precipitation between basins specifically the maximum depth of incision in each drainage is a power function of precipitation weighted drainage area with an exponent of individual longitudinal stream profiles are generally better fit by models that acknowledge both along channel precipitation variability and the subsidence of the island through that gradient but this relationship is not clearly demonstrable for many channels we argue that island subsidence through the dry to wet precipitation gradient has resulted in less cumulative discharge in the lower reaches of the drainages over their lifetime relative to a nonsubsiding case this reduces differential erosion between the wet and dry reaches by 20 30 over 300 kyr making the longitudinal profiles less sensitive to the very strong climate gradient

strain accumulation controls failure of a plate boundary zone linking deformation of the central andes and lithosphere mechanics 1 we make use of observations on orogenic strain accumulation and deformation partitioning in the central andes to explore the backarc strength evolution at the lithospheric scale in plan view the altiplano puna plateaux experienced rapid initial increase of surface area undergoing active deformation during the cenozoic beyond the maximum lateral extent reached around 10 15 ma 40 50 of entire proto andes undergoing deformation at 10 20 total strain rapid localization initiated at the eastern flank of the altiplano inter and subandean thrust belt but not at the puna latitude localization was associated with a significant increase in bulk shortening rate average fault slip rates equally increased by an order of magnitude following a protracted period of stable average rates estimates of strength evolution based on force balance calculations and critical wedge analysis suggest significant backarc weakening driving this change after the middle miocene strain accumulation led to localization and weakening with development of a detachment propagating through crust and upper mantle we find that lithosphere scale failure resulting from strain weakening beyond a critical strain threshold c 20 and fault coalescence with formation of a weak detachment in shales effective coefficient of friction 0 1 plays a key role in the evolution of the andes strain related lithosphere weakening appears to dominate over the impact of external forcing mechanisms such as variations of plate convergence mantle assisted processes or erosion comparison of these orogen scale observations with experimental rock rheology indicates substantial similarity of deformation behavior with similar weakening thresholds across a wide range of scales

the jinshajiang ailaoshan suture zone china tectonostratigraphy age and evolution abstract the jinshajiang suture zone is important for enhancing our understanding of the evolution of the paleo tethys and its age tectonic setting and relationship to the ailaoshan suture zone have long been controversial based on integrated tectonic biostratigraphic chemostratigraphic and isotope geochronological studies four tectono stratigraphic units can be recognized in the jinshajiang suture zone the eaqing complex the jinshajiang ophiolitic melange the gajinxueshan group and the zhongxinrong group isotope geochronology indicates that the redefined eaqing complex composed of high grade metamorphic rocks might represent the metamorphic basement of the jinshajiang area or a remnant micro continental fragment eaqing complex protolith rocks are pre devonian and probably of early middle proterozoic age and are correlated with those of the ailaoshan complex two zircon u pb ages of 340 3 and 294 3 ma separately dated from the shusong and xuitui plagiogranites within the ophiolitic assemblage indicate that the jinshajiang oceanic lithosphere formed in latest devonian to earliest carboniferous times the oceanic lithosphere was formed in association with the opening and spreading of the jinshajiang oceanic basin and was contiguous and equivalent to the ailaoshan oceanic lithosphere preserved in the shuanggou ophiolitic melange in the ailaoshan suture zone the latter yielded a u pb age of 362 41 ma from plagiogranite the re defined gajinxueshan and zhongxinrong groups are dated as carboniferous to permian and latest permian to middle triassic respectively on the basis of fossils and u pb dating of basic volcanic interbeds the gajinxueshan group formed in bathyal slope to neritic shelf environments and the zhongxinrong group as bathyal to abyssal turbidites in the jinshajiang ailaoshan back arc basin latest permian earliest middle triassic synorogenic granitoids with ages of 238 18 and 227 5 255 8 ma respectively and an upper triassic overlap molasse sequence indicate a middle triassic age for the jinshajiang ailaoshan suture formed by collision of the changdu simao block with south china

influence of the andes mountains on south american moisture transport convection and precipitation mountain ranges are known to have a first order control on mid latitude climate but previous studies have shown that the andes have little effect on the large scale circulation over south america we use a limited domain general circulation model regcm3 to evaluate the effect of the andes on regional scale atmospheric dynamics and precipitation we present experiments in which andean heights are specified at 250 m and 25 50 75 and 100 of their modern values our experiments indicate that the andes have a significant influence on moisture transport between the amazon basin and the central andes deep convective processes and precipitation over much of south america through mechanical forcing of the south american low level jet llj and topographic blocking of westerly flow from the pacific ocean when the andes are absent the llj is absent and moisture transport over the central andes is mainly northeastward as a result deep convection is suppressed and precipitation is low along the andes above 50 of the modern elevation a southward flowing llj develops along the eastern andean flanks and transports moisture from the tropics to the subtropics moisture drawn from the amazon basin provides the latent energy required to drive convection and precipitation along the andean front large northerly moisture flux and reduced low level convergence over the amazon basin leads to a reduction in precipitation over much of the basin our model results are largely consistent with proxy evidence of andean climate change and have implications for the timing and rate of andean surface uplift

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

orographic rainfall hot spots in the andes amazon transition according to the trmm precipitation radar and in situ data the andes amazon transition along the eastern peruvian andes features hotspots with strong precipitation using 15 years of trmm pr data we established a robust relation between terrain elevation and mean surface precipitation with the latter peaking around 1000 masl coinciding with the moisture flux peak of the south american low level jet sallj there is strong diurnal variability with afternoon 13 18 lt convection in the amazon plains while on the eastern slopes 1000 2000 masl after the forcing associated with the thermal heating of the andes subsides convection grows during the night and surface precipitation peaks around 01 06 lt and organizes into mesoscale convective systems mcss these then displace downslope to an terrain elevation of 700 masl with stratiform regions spreading upslope and downslope and then decay during the remainder of the morning the large mcss contribute with at least 50 of daily rainfall 40 of the 01 06 lt rainfall on synoptic scales the large mcss are more common in stronger sallj conditions although subtropical cold surges are responsible for 16 of the cases

eocene to recent variations in erosion across the central andean fold thrust belt northern bolivia implications for plateau evolution abstract quantifying the erosional and kinematic evolution of orogenic plateaus has been limited by insufficient age constraints on their deformation and erosion histories palinspastic restorations suggest the central andean fold thrust belt and plateau evolved concurrently in bolivia we present an analysis that synthesizes 19 new and 32 previous apatite and zircon fission track and u th he mineral cooling ages along a 200 km traverse across the plateau margin and entire thrust belt in northern bolivia the new apatite fission track data are interpreted using a grain age deconvolution algorithm with inverse thermal modeling of track lengths grain ages and mineral composition proxy data results suggest 1 eo oligocene 40 25 ma initial rapid erosion of the plateau margin 2 accelerated distributed erosion across the entire thrust belt since the early to mid miocene 15 ma and 3 the magnitude of erosion decreases eastward from 10 to 4 km we compare these results with two end member models of the central andes that contrast in duration and magnitude of deformation the rapid eo oligocene 40 25 ma erosion is only consistent with the end member that emphasizes the long duration and large magnitude deformation controlled by the sequential stacking of basement thrust sheets however the distributed miocene 15 ma to recent erosion is consistent with both end members because the recorded cooling could have resulted from active deformation protracted erosion or both if the long duration model is correct the time between the two phases of accelerated cooling brackets the cessation of the first basement thrust sheet and implies the early development of the andean plateau analogous to its modern width but unknown elevation by the early miocene 20 ma

retroarc basin reorganization and aridification during paleogene uplift of the southern central andes u s national science foundation ear 1049605 robert r shrock foundation at indiana university nsf ear award 1338583 project pdts unsj e985

impacts of cenozoic global cooling surface uplift and an inland seaway on south american paleoclimate and precipitation δ18o stable isotope records of precipitation 18 o 18 o prec have been used as paleoclimate and paleoelevation archives of orogens however interpretation of these records is limited by knowledge of how 18 o prec responds to changes in global and regional climate during mountain building events in this study the influence of atmospheric co 2 levels the extent of the antarctic ice sheet changes in andean surface elevation and the presence of the south american inland seaway on climate and 18 o prec in south america are quantified using the genesis v3 atmospheric general circulation model with isotope tracking capabilities results are presented in the context of cenozoic south american climate and 18 o prec changes more specifically we find 1 precipitation rates in the andes are sensitive to andean surface elevation the seaway and to a lesser extent co 2 levels increasing andean elevations and the presence of a seaway both cause large increases in precipitation but in different parts of the andes the growth of the antarctic ice sheet is found to have a small influence on south american precipitation 2 the stable isotopic composition of precipitation is sensitive to all of the parameters investigated an increase in 18 o prec of up to 8 is found in simulations with higher atmospheric co 2 in agreement with previous studies 18 o prec decreases with increasing andean elevation by an amount greater than that predicted by the modern adiabatic lapse rate furthermore the presence of an inland seaway causes a decrease in 18 o prec of 1 8 in the northern and central andes the amount of depletion is dependent on the isotopic composition of the seaway simulations without the antarctic ice sheet result in 18 o prec that is 0 3 lower than the modern finally time specific simulations for the miocene and eocene show that 18 o prec has decreased during the cenozoic and that local geographical gradients of 18 o prec have increased particularly in regions of high modern elevation we demonstrate that in addition to andean uplift and associated climate change co 2 levels and an inland seaway are likely to have influenced 18 o carb records from south america consideration of these global and paleogeographic changes is necessary when interpreting paleoclimate or paleoelevation from stable isotope records of 18 o prec

isolating active orogenic wedge deformation in the southern subandes of bolivia a new gps derived surface velocity field for the central andean backarc permits an assessment of orogenic wedge deformation across the southern subandes of bolivia where recent studies suggest that great earthquakes mw 8 are possible we find that the backarc is not isolated from the main plate boundary seismic cycle rather signals from subduction zone earthquakes contaminate the velocity field at distances greater than 800 km from the chile trench two new wedge crossing velocity profiles corrected for seasonal and earthquake affects reveal distinct regions that reflect 1 locking of the main plate boundary across the high andes 2 the location of and loading rate at the back of orogenic wedge and 3 an east flank velocity gradient indicative of decollement locking beneath the subandes modeling of the subandean portions of the profiles indicates along strike variations in the decollement locked width wl and wedge loading rate the northern wedge decollement has a wl of 100 km while accumulating slip at a rate of 14 mm yr whereas the southern wedge has a wl of 61 km and a slip rate of 7 mm yr when compared to quaternary estimates of geologic shortening and evidence for holocene internal wedge deformation the new gps derived wedge loading rates may indicate that the southern wedge is experiencing a phase of thickening via reactivation of preexisting internal structures in contrast we suspect that the northern wedge is undergoing an accretion or widening phase primarily via slip on relatively young thrust front faults

geometric kinematic and erosional history of the central andean plateau bolivia 15 17 s 1 latitudinal changes in topography climate and thrust belt geometry in the central andes have led to conflicting hypotheses that climate or tectonics exert a first order control on orogen evolution the relative roles of climate and tectonics in the evolution of the andean orogen are difficult to quantify because of a lack of detailed observations for both the long term deformation and erosion history of the andean fold thrust belt we contribute to the resolution of this problem by presenting a sequentially restored balanced cross section based on new mapping across the northern bolivia portion of the thrust belt 15 17 s the timing and magnitude of exhumation across the cross section are determined by synthesizing 10 new and 70 previously published mineral cooling ages once balanced and restored the section was sequentially forward modeled using stratigraphic and cooling age constraints results indicate the eastern cordillera ec records the highest magnitudes of shortening 123 km or 55 the interandean zone ia has shortened 48 km or 30 in both the ec and ia individual thrust sheets are tightly folded and have minor offsets of 1 5 km the subandes sa has multiple levels of detachments allowing for thrust sheets with relatively large offsets 6 17 km total shortening in the sa is 66 km or 40 total magnitude of shortening for the entire fold thrust belt in this region is 276 km 40 new apatite and zircon fission track cooling ages in conjunction with published ages indicate two phases of rapid exhumation an earlier phase from 40 to 25 ma in the ec and one prior to 25 ma in the ia followed by distributed exhumation of the entire fold thrust belt from 15 0 ma combined exhumation estimates from the balanced cross section and thermochronology suggest 9 11 km of exhumation in the ec 5 9 km in the ia and 3 4 km in the sa long term shortening rates are 7 mm a for the ec and ia and 4 8 mm a for the sa the sa shortening rates are based on a 15 0 ma or 8 0 ma deformation window by linking cooling ages to location and magnitude of shortening we suggest an 10 17 ma pause or a dramatic deceleration in the rate of deformation and propagation of the fold thrust belt between 25 and 15 or 8 ma

incision into the eastern andean plateau during pliocene cooling canyon incision into mountain topography is commonly used as a proxy for surface uplift driven by tectonic or geodynamic processes but climatic changes can also instigate incision the 1250 kilometer km long eastern margin of the andean plateau hosts a series of 1 5 to 2 5 km deep canyons that cross major deformation zones using u th he thermochronology we document a transition from miocene faulting to pliocene canyon incision across the northeastern plateau margin regionally widespread pliocene incision into the eastern plateau margin is concurrent with a shift in global climate from early pliocene warmth to late pliocene cooling enhanced moisture transport onto the andean plateau driven by sea surface temperature changes during cooling is the likely pacemaker for canyon incision

experimental investigation on dolomite dissociation into aragonite magnesite up to 8 5 gpa we carried out high pressure and high temperature experiments on the composition camg co3 2 to determine the equilibrium boundary of dolomite aragonite magnesite up to 8 5 gpa and 1200 c using a 6 8 type multianvil high pressure apparatus employing a quenching technique dolomite dissociates into aragonite and magnesite under high p t conditions and the phase boundary of dissociation is described by a linear equation p gpa 1 75 0 0061t c at p 6 3 gpa and t 800 c the equilibrium boundary was established by reverse and backward experiments to grow dolomite from mixtures of caco3 and mgco3 with or without dolomite respectively this study suggests that the equilibrium line of the dolomite aragonite magnesite reaction could be very useful in addition to phase transformation of quartz coesite and graphite diamond to constrain p t conditions for the metamorphism of dolomite bearing ultrahigh pressure rocks

genesis of diamonds in the lower mantle the forbidden assemblage ferropericlase enstatite as inclusions in diamonds has been taken as evidence to imply that these inclusions and their host diamonds formed initially in the lower mantle magnesite is probably the only stable carbonate at depths greater than 220 km like dehydration reactions the reaction boundary for the decarbonation of magnesite has a positive dt dp slope at lower pressures which becomes negative at higher pressures if no other phase intervenes this reaction boundary probably intersects the geotherm between 900 and 1100 km below which magnesite decomposes into an assemblage periclase diamond oxygen thus ferropericlase is the most likely inclusion in diamond formed in the lower mantle the high frequency of sole occurrence of ferropericlase in diamonds from sao luiz brazil seems to substantiate the present speculation

orographic barriers high resolution trmm rainfall and relief variations along the eastern andes 1 quantifying the degree to which tectonic and erosive processes shape landscapes is key to understanding the evolution of tectonically active mountain belts here we explore the interplay of these two processes along the humid eastern flank of the south american andes we use high resolution tropical rainfall measurement mission trmm and srtm data to characterize elevation relief and hillslope angle of peak rainfall at orographic barriers over a distance of more than 3500 km along the eastern flanks we find that peak rainfall 3 5 m yr occurs at a mean elevation of 1 3 0 17 km a mean relief of 0 95 0 08 km and at moderate mean hillslope angles of 18 3 1 7 we suggest that topographic relief is the best first order rainfall predictor and we demonstrate how relief changes along strike south of the andean orocline alter rainfall distribution changes in climatic sedimentary and tectonic processes prevent the formation of high relief amounts at the mountain front and these areas are not characterized by pronounced rainfall peak

petrofabric of forsterite marbles and related rocks from a low pressure metamorphic terrain almaden de la plata massif ossa morena zone sw spain and its kinematic interpretation abstract a fabric study of forsterite marble and calc silicate rock paragenetic minerals is presented for samples collected in the almaden de la plata massif of sw spain that experienced variscan low pressure metamorphism related to the tectonic evolution of the suture between the ossa morena and south portuguese zones of the iberian massif two tectonically stacked metamorphic units contain marble metacarbonate and variably impure dolomitic rocks that record contrasting thermal conditions of the upper greenschist structuraly upper unit and the upper amphibolite granulite facies structuraly lower unit different reactional paths lead to formation of forsterite marbles in both units dolomite and calcite petrofabrics disclose their contrasting rheological behaviour during syn metamorphic ductile deformation under temperatures between 500 and 700 c dolomite formed harder porphyroclast systems and boudinaged bands whereas weaker calcite underwent dynamic recrystallization in both cases deformation was facilitated by activation of various intracrystalline slip systems complemented by other deformation mechanisms the fabrics of other paragenetic metamorphic minerals also denote crystal plasticity and exhibit close geometrical relationships with respect to the macroscopic foliation and mineral stretching lineation including a systematic asymmetry suggestive of non coaxial deformation components foliations and lineations are interpreted as solid state flow planes and directions bearing a kinematic significance in particular regional sinistral shear at mid deep crustal realms coeval with low pressure metamorphism explains the imbrication of metamorphic units and their exhumation concomitant with the overall wrench tectonic framework ruling the tectonic evolution of the ossa morena south portuguese suture between the late devonian 360 ma and the latest carboniferous 300 ma

drainage reversal of the amazon river due to the coupling of surface and lithospheric processes abstract the uplift of central and northern andes occurred concomitantly with an important reorganization of the drainage pattern in the northern south america in early miocene the fluvial systems that flowed from the amazonian craton towards the sub andean basins and northward to the caribbean were replaced by a megawetland the pebas system covering more than 10 6 km 2 in western amazonia by late miocene the pebas system progressively disappeared and gave place to the transcontinental amazon river connecting the andes and the equatorial atlantic margin a previous work suggested that the reversal of the amazon river and the disappearance of the pebas system were driven by dynamic topography promoted by mantle convection based on a three dimensional numerical model that couple surface processes flexural isostasy and crustal thickening due to orogeny here i propose that the response of the surface processes to the uplift of the central and northern andes along with the flexural isostasy of the lithosphere can explain the drainage reversal of the amazon river during miocene time with no need to invoke dynamic topography induced by mantle convection moreover according to this new numerical model the timing of the drainage reversal in the northern south america is mainly controlled by the efficiency of the sediment transport within the drainage basins and the rate of the andean uplift also the present numerical experiments were able to predict the increase in sedimentation rate in the amazon fan after the drainage reversal of the amazon river as observed in late miocene pliocene sedimentary record however the proposed model fails to fully reproduce the spacial and temporal evolution of the pebas system as observed in geological data further investigation is therefore needed in order to understand the dynamic interaction between surface and tectonic processes and their implications on the development of a megawetland that preceded the reversal of amazon river drainage nevertheless this work can give new insights into the landscape and stratigraphic evolution of northern south america during andean orogeny providing quantitative constraints for the paleogeography reconstructions of amazonia

unsteady evolution of the bolivian subandean thrust belt the role of enhanced erosion and clastic wedge progradation abstract the subandean fold and thrust belt of bolivia constitutes the easternmost part of the andean orogen that reflects thin skinned shortening and eastward propagation of the andean deformation front the exact interplay of tectonics climate and erosion in the deposition of up to 7 5 km of late cenozoic strata exposed in the subandes remains unclear to better constrain these relationships we use four w e industry seismic reflection profiles eight new zircon u pb ages from mio pliocene sedimentary strata and cross section balancing to evaluate the rates of thrust propagation shortening and deposition pinch out migration eastward thrusting arrived in the subandean belt at 12 4 0 5 ma and propagated rapidly toward the foreland unit approximately 6 ma this was followed by out of sequence deformation from ca 4 to 2 1 ma and by renewed eastward propagation thereafter our results show that the thrust front propagation and deposition pinch out migration rates mimic the sediment accumulation rate the rates of deposition pinch out migration and thrust propagation increased three and two fold respectively 8 mm a 3 3 mm a at 8 6 ma the three fold increase in deposition pinch out migration rate at this time is an indication of enhanced erosional efficiency in the hinterland probably coupled with flexural rebound of the basin following the pulse of pinch out migration the subandean belt witnessed rapid 80 km eastward propagation of thrusting to the la vertiente structure at 6 ma as there is no evidence for this event of thrust front migration being linked to an increase in shortening rate the enhanced frontal accretion suggests a shift to supercritical wedge taper conditions we propose that the supercritical state was due to a drop in basal strength caused by sediment loading and pore fluid overpressure this scenario implies that climate controlled variation in erosional efficiency was the driver of late miocene mass redistribution which induced flexural rebound of the subandean thrust belt spreading of a large clastic wedge across the basin and subsequent thrust front propagation

kinematics exhumation and sedimentation of the north central andes bolivia an integrated thermochronometer and thermokinematic modeling approach quantifying mountain building processes in convergent orogens requires determination of the timing and rate of deformation in the overriding plate in the central andes large discrepancies in both timing and rate of deformation prevent evaluating the shortening history in light of internal or external forcing factors geologic map patterns age and location of reset thermochronometer systems and synorogenic sediment distribution are all a function of the geometry kinematics and rate of deformation in a fold thrust belt foreland basin ftb fb system to determine the timing and rate of deformation in the northern bolivian andes we link thermokinematic modeling to a sequentially forward modeled balanced cross section isostatically accounting for thrust loads and erosion displacement vectors in 10 km increments are assigned variable ages to create velocity fields in a thermokinematic model for predicting thermochronometer ages we match both the pattern of predicted cooling ages with the across strike pattern of measured zircon fission track apatite fission track and apatite u th he cooling ages as well as the modeled age of fb formations to published sedimentary sections results indicate that northern bolivian ftb deformation started at 50 ma and may have begun as early as 55 ma acceptable rates of shortening permit either a constant rate of shortening 4 5 mm yr or varying shortening rates with faster rates 7 10 mm yr at 45 50 ma and 12 8 ma significantly slower rates 2 4 mm yr from 35 to 15 ma and indicate the northern bolivian subandes started deforming between 19 and 14 ma

large along strike variations in the onset of subandean exhumation implications for central andean orogenic growth abstract plate tectonics drives mountain building in general but the space time pattern and style of deformation is influenced by how climate geodynamics and basement structure modify the orogenic wedge growth of the subandean thrust belt which lies at the boundary between the arid high elevation central andean plateau and its humid low elevation eastern foreland figures prominently into debates of orogenic wedge evolution we integrate new apatite and zircon u th he thermochronometer data with previously published apatite fission track data from samples collected along four subandean structural cross sections in bolivia between 15 and 20 s we interpret cooling ages vs structural depth to indicate the onset of subandean exhumation and signify the forward propagation of deformation we find that subandean growth is diachronous south 11 3 ma vs north 6 2 ma of the bolivian orocline and that subandean exhumation magnitudes vary by more than a factor of two similar north south contrasts are present in foreland deposition hinterland erosion and paleoclimate these observations both corroborate diachronous orogenic growth and illuminate potential propagation mechanisms of particular interest is an abrupt shift to cooler more arid conditions in the altiplano hinterland that is diachronous in southern bolivia 16 13 ma vs northern bolivia 10 7 ma and precedes the timing of subandean propagation in each region others have interpreted the paleoclimate shift to reflect either rapid surface uplift due to lithosphere removal or an abrupt change in climate dynamics once orographic threshold elevations were exceeded these mechanisms are not mutually exclusive and both would drive forward propagation of the orogenic wedge by augmenting the hinterland backstop either through surface uplift or spatially variable erosion in summary we suggest that diachronous subandean exhumation was driven by piecemeal hinterland uplift orography and the outward propagation of deformation

differential exhumation in response to episodic thrusting along the eastern margin of the tibetan plateau abstract thermochronological data from the songpan ganze fold belt and longmen mountains thrust nappe belt on the eastern margin of the tibetan plateau in central china reveal several phases of differential cooling across major listric thrust faults since early cretaceous times differential cooling indicated by distinct breaks in age data across discrete compressional structures was superimposed upon a regional cooling pattern following the late triassic indosinian orogeny 40ar 39ar data from muscovite from the central and southern longmen mountains thrust nappe belt suggest a phase of differential cooling across the wenchuan maouwen shear zone during the early cretaceous the zircon fission track data also indicate differential cooling across a zone of brittle re activation on the eastern margin of the wenchuan maouwen shear zone during the mid tertiary between 38 and 10 ma apatite fission track data from the central and southern longmen mountains thrust nappe belt reveal differential cooling across the yingxiu beichuan and erwangmiao faults during the miocene forward modelling of apatite fission track data from the northern longmen mountains thrust nappe belt suggests relatively slow regional cooling through the mesozoic and early tertiary followed by accelerated cooling during the miocene beginning at ca 20 ma to present day regional cooling is attributed to erosion during exhumation of the evolving longmen mountains thrust nappe belt lmtnb following the indosinian orogeny differential cooling across the wenchuan maouwen shear zone and the yingxiu beichuan and erwangmiao faults is attributed to exhumation of the hanging walls of active listric thrust faults thermochronological data from the longmen mountains thrust nappe belt reveal a greater amount of differential exhumation across thrust faults from north to south this observation is in accord with the prevalence of proterozoic and sinian basement in the hanging walls of thrust faults in the central and southern longmen mountains the two most recent phases of reactivation occurred following the initial collision of india with eurasia suggesting that lateral extrusion of crustal material in response to this collision was focused along discrete structures in the lmtnb

climatic forcing on channel profiles in the eastern cordillera of the coroico region bolivia abstractorographic precipitation has a large impact on channel morphology and rock uplift via a positive feedback to erosion we show that in the eastern cordillera of bolivia channel concavities reach their highest values where annual precipitation increases in the downstream direction exceeding 3000 mm the steepest channels are upstream of this zone of high concavity where precipitation rates are 1000 mm yr 1 channels exhibit graded forms both upstream and downstream of this transient reach we conclude that the prolonged effect of orographic erosion and related tectonic uplift is the preservation of channels with extreme concavities in the eastern cordillera

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