channel arrangements and depositional styles in the sao lourenco fluvial megafan brazilian pantanal wetland abstract the brazilian pantanal is an extensive lowland tropical basin characterized by the presence of fluvial megafans and seasonally inundated savanna floodplain wetlands with an area of about 16 000 km 2 the sao lourenco is the second largest megafan in the pantanal three distinct fluvial channel styles that formed at different times during the late quaternary are found here a geomorphological and sedimentary assessment of these depositional patterns provides valuable insight on the environmental context of their evolution new optically stimulated luminescence data indicate that the upper five meters of sediment in the sao lourenco megafan has been accumulating since the late pleistocene ancient fan lobes located in upper and intermediate fan settings consist of medium and coarse grained fluvial sands and exhibit well preserved distributary braided paleochannels on their surfaces as the megafan evolved through time pleistocene lobes were incised by a prominent valley filled with holocene aged meander belt deposits which consist of silts interbedded with very fine sands and clays currently the incised valley is a zone of sediment bypass modern deposition occurs along the distal toe of the megafan system where lobes characterized by distributary channel levee ridges are widespread these features formed by progradation of avulsion belts into a broad swampy floodbasin which caused the lower portion of the meander belt to be abandoned the significant differences observed in intra fan morphology appear to be linked to the variability in effective precipitation fan lobes deposited with braided distributary channels occurred under relatively dry conditions in the late pleistocene by contrast aggradational meander belt deposits and lobes with distributary channel levee ridges formed during fluctuating precipitation conditions of the holocene when the pantanal emerged from deglacial aridity modern lobes form under heavy seasonal flooding and deposition occurs in response to very rapid and common avulsion events these results have implications for interpreting the complexity of megafan facies in similar continental basins

geochemical evolution of the precambrian old rag granite virginia u s a testing a uth exploration model abstract the old rag granite is one of several recognized units within the virginia blue ridge complex generally composed of grenville age gneisses and granitoid intrusive rocks chemical variation in the old rag granite indicates simple orthomagmatic crystallization from the borders to the core of the pluton fractional crystallization of plagioclase oxides apatite and zircon accounts for much of the variation some trace elements however show anomalous behavior in the central part of the pluton y and sr and possibly ba and nb are enriched whereas rb and total ree are depleted in the central part of the pluton late crystallization of monazite could contribute to the observed ree patterns it could not however account for rb depletion and sr enrichment and no clear relationship between ree data and th u ratios are observed the geochemical patterns indicate overall orthomagmatic crystallization overlain by a late stage hydrothermal event an exploration model for uraniferous granites based upon the uranium deposits at rossing and bokan mountain included the precambrian basement of central and northern virginia in a list of proposed target areas geological mineralogical and whole rock chemical characteristics show a close association between the old rag granite and the rossing type deposits the uranium mean 6 9 ppm range 0 1 19 8 ppm and thorium mean 43 ppm range 3 5 114 ppm values are anomalous th u ratios average 7 5 and are widely variable indicating a decoupling of uranium from thorium neither u nor th are lognormally distributed within the old rag granite requiring an explanation beyond simple removal of u by weathering rayleigh fractionation modeling shows that both u and th although probably distributed originally by closed system fractionation have been subsequently mobilized uranium can be lost in an oxidized form by secondary processes such as weathering or hydrothermal alteration thorium however will be affected only by hydrothermal processes it is concluded that although primary fractionation was orthomagmatic the old rag granite was affected by late hydrothermal alteration whereby u was lost from the body and th was redistributed three primary avenues for u migration are possible 1 the rocks of the saddleback mountain intrusive suite which may have been contemporaneous with the old rag granite 2 paleozoic fault zones that cut the old rag pluton and 3 quartz veins pegmatites cupolas and roof pendants of the country rock nellysford gneiss that have been lost to erosion the last option is preferred on geological and geochemical grounds it is possible that the sedimentary uranium deposits of the adjacent triassic culpepper basin were derived from eroded upper levels of the old rag granite and like intrusives of the virginia blue ridge complex

paleogene synorogenic sedimentation in the altiplano plateau and implications for initial mountain building in the central andes sedimentologic data and palynological ages from paleogene clastic deposits of the northern and central altiplano plateau suggest foreland basin development in the central andes by mid paleocene time the nonmarine potoco formation 3000 6500 m thick constitutes the majority of cenozoic basin fill the potoco overlies the santa lucia formation 50 300 m thick previously dated as mid paleocene by mammal fossils and magnetostratigraphy new geochronologic data for the potoco formation include late eocene to oligocene palynomorph assemblages recovered throughout lower to upper stratigraphic levels these ages and published 40 ar 39 ar and k ar ages from overlying upper oligocene lower miocene volcaniclastic rocks indicate 1 nondeposition or greatly reduced deposition average sediment accumulation rates 10 m m y from mid paleocene to middle eocene time top santa lucia to lowermost potoco formation followed by 2 rapid deposition average sediment accumulation rates up to 500 m m y throughout late eocene and oligocene time majority of potoco formation lithofacies and paleocurrent data confirm both depositional phases a 20 100 mthick interval of superimposed paleosols in the basal potoco supports reduced sediment accumulation and low rates of subsidence during mid paleocene to middle eocene time the overlying main body of the potoco contains facies assemblages and stratigraphic architecture dominantly nonero

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

late cretaceous tectonic switch from a western pacific to an andean type continental margin evolution in east asia and a foreland basin development in ne china the early cretaceous structure of ne china was a result of slab rollback driven extensional tectonics characteristic of western pacific type continental margins oblique docking of a microcontinent along the asian active margin in the early late cretaceous induced a compressional stress regime that brought about an andean type continental margin development partitioning of contractional transpressional strain across ne china produced a retroarc foreland basin system comprising from east to west an orogenic wedge a foredeep songliao basin a forebulge great xing an range and a back bulge depozone hailar and erlian basins a sub circular lacustrine depozone in the pre existing songliao basin evolved into a nne trending depocentre near the forebulge and acquired a westward flowing fluvial deltaic drainage system during the campanian development of this retroarc foreland basin system signals a significant tectonic switch from a western pacific type to an andean type continental margin evolution in the geological history of east asia this article is protected by copyright all rights reserved

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

seismic stratigraphic record of transition from mesozoic subduction to continental breakup in the zealandia sector of eastern gondwana abstract the southwest pacific between australia new zealand and new caledonia is a block of continental crust zealandia that moved away from australia and antarctica after a long period of subduction beneath eastern gondwana we use 100 000 line km of seismic reflection profiles to identify intra continental basins related to the gondwana active margin overlain with erosional unconformity by retrogradational strata we interpret this regional scale first order unconformity the eastern gondwana composite surface and seismic stratigraphic megasequence pattern to represent the transition from subduction to continental breakup and separation of eastern gondwana rocks that make up the lower seismic stratigraphic megasequence zealandia 3 permian to early cretaceous have been drilled near new zealand and can be correlated with murihiku supergroup rocks farther north on the lord howe rise we correlate this megasequence with triassic to jurassic rocks of the clarence moreton basin of eastern australia we reinterpret the fairway aotea basin to be underlain by a wedge of zealandia 3 deposits thrust in a retro arc like foreland basin that was active immediately prior to breakup the lower rift and retrogradational megasequence zealandia 2 late cretaceous to eocene we correlate with the pakawau and kapuni groups of taranaki basin new zealand and the formation a charbon and phtanites of new caledonia the boundary with the shallowest megasequence zealandia 1 late eocene to present is the eocene oligocene unconformity which is primarily overlain by pelagic carbonate rocks this regional unconformity likely represents the onset of the modern tonga kermadec subduction system

erosion rates and landscape evolution of the lowlands of the upper paraguay river basin brazil from cosmogenic 10be abstract the importance of earth s low sloping areas in regard to global erosion and sediment fluxes has been widely and vigorously debated it is a crucial area of research to elucidate geologically meaningful rates of land surface change and thus the speed of element cycling on earth however there are large portions of earth where erosion rates have not been well or extensively measured for example the tropical lowlands the cuiabana lowlands are an extensive low altitude and low relief dissected metamorphic terrain situated in the upper paraguay river basin central west brazil besides exposures of highly variable dissected metamorphic rocks flat residual lateritic caps related to a late cenozoic planation surface dominate interfluves of the cuiabana lowlands the timescale over which the lowlands evolved and the planation surface developed and the rate at which they have been modified by erosion are poorly known here we present measurements of in situ produced cosmogenic 10 be in outcropping metamorphic bedrock and clastic lateritic caps to quantify rates of erosion of the surface and associated landforms in order to better understand the quaternary landscape evolution of these lowlands overall slow erosion rates mean 10 m ma suggest a stable tectonic environment in these lowlands erosion rates vary widely between different lithologies range 0 57 to 28 3 m ma consistent with differential erosion driving regional landform evolution the lowest erosion rates are associated with the low relief area irregular plains where clastic laterite mean 0 67 m ma and quartzite mean 2 6 m ma crop out whereas the highest erosion rates are associated with dissection of residual hills dominated by metasandstone mean 11 6 m ma and phyllite mean 27 6 m ma these data imply that the cuiabana lowland is comprised of two dominant landform sets with distinct and different dynamics because the planation surface mostly lowlands is lowering and losing mass more slowly than associated residual hills regional relief is decreasing over time and the landscape is not in steady state the extremely slow erosion rates of the clastic laterite are similar to the slowest outcrop erosion rates reported worldwide these slow rates are due to the material s properties and resistance being comprised of quartzite fragments cemented by an iron rich crust and reflecting long term weathering with iron chemical precipitation and ferricrete formation at least since the middle pleistocene the lateritic caprock appears to be a key factor maintaining hilltop summits of the planation surface over long timescales

structure and cenozoic kinematics of the eastern cordillera southern bolivia 21 s 1 the eastern cordillera is a main morphotectonic unit of the central andes in southern bolivia it is predominantly composed of ordovician sedimentary rocks as thick as 10 km revision of the ordovician stratigraphy and new structural fieldwork resulted in detailed balanced cross sections improved estimates of crustal shortening and a new kinematic model for the eastern cordillera new estimates of total shortening in the eastern cordillera are up to 95 km 41 44 higher than previous estimates and sufficient to thicken the andean crust to its present state between the eastern foreland and the magmatic arc after initial tertiary uplift at 58 ma first thrusts developed in the middle eocene to early oligocene 40 30 ma exhuming the central part of the eastern cordillera tectonic shortening culminated in late oligocene and early miocene time 25 20 ma when west facing thrust systems developed in the central and western part of the eastern cordillera thrusting terminated in the western part by the end of the early miocene and middle miocene tectonic activity was restricted to minor strike slip faulting in the central part thrusting remained active until the late miocene by 10 ma shortening in the eastern cordillera essentially ceased kinematic fault analyses indicate clockwise rotation of the shortening direction from nne ssw or ne sw in eocene early oligocene time over e w to wnw ese or even nw se in late oligocene to middle miocene time nevertheless wnw ese directed shortening and subvertical extension predominate sinistral oblique motion on many thrusts probably reflects plan view flexural slip in the clockwise rotating southern limb of the bolivian orocline it is proposed that space problems in the oroclinal hinge slowed the rotation there and caused decoupling of the still rotating south limb along ne trending transfer zones

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

cretaceous early paleocene drainage shift of amazonian rivers driven by equatorial atlantic ocean opening and andean uplift as deduced from the provenance of northern peruvian sedimentary rocks huallaga basin abstract the northern part of south america has undergone several major changes in its tectonic setting since the triassic evolving from rifting to the development of a retroarc foreland basin on its andean margin leading to margin extension and continental uplift following the breakup between africa and south america so far it is unclear when and how these geodynamic events affected the paleo amazonian drainage in this study we investigate the provenance of triassic eocene sedimentary rocks deposited in the northern peruvian amazonian basin based on their sr nd isotopic compositions and u pb zircon dating the triassic jurassic samples have end 0 values ranging from 7 9 to 10 0 and main u pb zircon peaks at 0 9 1 3 ga 31 33 and 0 5 0 7 ga 21 28 that suggest a mixed clastic supply from the western and eastern cordillera or craton within a rift to post rift setting samples from the albian maastrichtian interval yield much lower end 0 values 16 8 to 18 6 and a dominance of zircon grains derived from terranes in the easternmost brazilian shield ventuari tapajos 2 0 1 82 ga rio negro jurena 1 82 1 54 ga and rondonia san ignacio 1 54 1 3 ga thus indicating a cratonic source for these sedimentary rocks finally the early paleocene eocene sedimentary rocks record the first arrival of andean detritus in the amazonian retroarc foreland with end 0 values ranging between 5 6 to 12 0 and up to 16 of the zircon grains yielding ages younger than 120 ma together with recently published studies these provenance data document the existence of a long lived aptian maastrichtian continent wide cratonic drainage in the northern part of south america this cratonic drainage developed in response to the late cretaceous uplift of the northeastern part of the south america craton likely driven by geodynamic processes related to post rift opening of the atlantic equatorial ocean the formation of a late maastrichtian early paleocene mountain chain in the peruvian andes associated with the onset of provenance from the andean orogenic belt led to a shift of the amazon drainage pattern hence the earliest andean amazonian rivers are no younger than late maastrichtian early paleocene

salar de atacama basin a record of compressional tectonics in the central andes since the mid cretaceous the salar de atacama basin lies in the inner fore arc of northern chile topographically and structurally it is a first order feature of the central andes the sedimentary fill of the basin constrains the timing and extent of crustal deformation since the mid cretaceous we have studied good exposures along the western edge of the basin and have correlated them with seismic reflection sections and data from an exploration well throughout most of its history the basin developed in a foreland setting during periods of thin skinned and thick skinned thrusting growth strata provide evidence for coeval sedimentation and thrust motions during mid cretaceous paleogene and neogene times pre neogene deformation was significant in the basin and in surounding areas of the early central andes models that attempt to explain the current thickness of the central andes should consider late cretaceous and paleogene shortening as well as the more obvious neogene and quaternary shortening

geomorphic evidence for plio quaternary compression in the andean foothills of the southern neuquen basin argentina the southern central andes foothills are made of imbricate thick and thin skinned fold and thrust belts they were formed during cretaceous and miocene compressive periods by successive eastward propagation episodes of the orogenic front we analyze remnant terraces and alluvial deposits of the neuqun river and its tributaries to study the plio quaternary tectonic regime in the southern neuqun basin topographic profiles of remnant terraces show a crustal scale tens of kilometers anomaly above the chihuido south anticline this anomaly is accompanied by sedimentary aggradations downstream and upstream of the anticline another anomaly in the terrace profiles correlated with a knickpoint in the current neuqun river is observed farther east near aelo we interpret these anomalies to be the consequence of a plio quaternary uplift of the chihuidos and aelo anticlines folding and surface faulting of plio quaternary alluvial deposits show that this uplift is concomitant with the reactivation of the miocene orogenic front and of the predominantly thin skinned agrio fold and thrust belt from these data we infer that the southern neuqun basin is still shortening under a compressive stress regime and that the andean orogenic front is migrating farther to the east extensional features in the study area correspond mainly to thin skinned gravitational gliding resulting from crustal uplift thus we argue that the tectonic compressive regime is preserved in the southern neuqun basin throughout the plio quaternary though the slab may have steepened during this period

nonuniform surface uplift of the andean plateau revealed by deuterium isotopes in miocene volcanic glass from southern peru abstract proposals for rapid late miocene surface uplift driven by large scale lithospheric removal beneath the central andean plateau have been based largely on temperature sensitive paleoaltimeters both the magnitude and mechanism of this proposed pulse of uplift have been challenged first climatic general circulation models support protracted uplift with predicted temperature and isotopic shifts enhanced by attainment of threshold elevations second tectonic models in which surface elevations are compensated by regional contraction and crustal thickening question the need for lithospheric removal and predict broadly coeval uplift of the entire plateau we present hydrogen isotope data using a novel temperature insensitive volcanic glass proxy from continuous well dated lower to middle miocene basin fill in the western cordillera of the northern plateau that show a rapid decrease in d values 62 8 at 19 16 ma with extremely negative values continuing into the pliocene we propose that the basin reached its current elevation by 16 ma 6 myr earlier than proposed for the central plateau the rapid decrease in d values is consistent with punctuated surface uplift of 2 2 3 7 km between 19 and 16 ma whereas the 3 7 km upper estimate assumes a static climate similar to modern the 2 2 km lower estimate conservatively incorporates modeled changes in the isotopic composition of precipitation associated with elevation change comparison of these results to existing paleoelevation estimates from the andean plateau facilitates a tentative reconstruction of earliest middle miocene paleotopography showing a central depression with flanking hinterland and thrust belt highlands this apparent pattern of nonuniform plateau uplift contradicts shortening proportional topographic growth over tens of myr we propose that temporally and spatially irregular surface uplift may be linked to pronounced local variability in crustal shortening and or piecemeal removal of dense mantle lithosphere insofar as these irregularities reflect the scale of heterogeneity for surface uplift processes available paleoelevation estimates point to a principal uplift mechanism that varies over relatively short

petrology and petrogenesis of the bokan granite complex southeastern alaska the bokan granite complex is a peralka line ring dike complex emplaced into marine shales volcanic flows and tuffs and plutonic rocks mineralogically and chemically the complex is composed of i type granites aegirine and riebeckite bearing granite aplites porphyries and pegmatites comprise twelve distinct intrusive episodes the aegirine bearing rocks occur in an outer annular zone formed during early crystallization subsequent rocks are riebeckite bearing due to devolatilization of the magma chamber during a collapse ring dike emplacement event early crystallization of alkali feldspar occurred during magma ascension from a lower crustal upper mantle source at shallow depths subsolvus crystallization allowed microcline and albite to dominate local bodies of aegirine syenite were formed during the early collapse ring dike emplacement in response to the magma devolatilization the riebeckite granites reflect lower po2 and possibly declining peralkalinity the granitic rocks at bokan all exhibit na2o contents greater than k2o lithophile elements are concentrated in all the rocks especially in zones where hydrothermal albite and chlorite formed rb sr ratios increase in progressively younger rocks in the complex agpaitic ratios vary from 0 92 to 2 08 for the granitic rocks

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

possible flexural accommodation on the eastern edge of the altiplano in relation to focussed erosion in the rio la paz drainage system it has been proposed that erosional unloading by focused incision can be accommodated by flexural rebound which in turn potentially exerts a positive feedback on erosion this paper presents morphometric data from the la paz drainage basin in the bolivian andes to illustrate the effects of such a feedback mechanism between erosion and crustal bending the rio la paz is a prominent system with the headwaters on the altiplano and a trunk stream segment that cuts across the cordillera real thereby connecting large portions of the altiplano with the amazon basin the flexural feedback model explains why all drainages beyond the watershed disperse their waters to the altiplano it also provides an explanation for the presence of the highest peaks just next to the location where the la paz river cuts into the bedrock across the cordillera real

spatial and temporal evolution of an avulsion on the taquari river distributive fluvial system from satellite image analysis avulsion processes have been observed and analyzed using field based studies however they have rarely been analyzed through use of satellite imagery even though such an approach allows documentation of spatial and temporal variations of the resulting deposits in order to document changes that are occurring at the caronal avulsion on the taquari river brazil a time series of landsat imagery from 1985 to present 2010 was compiled as of july 2010 the imagery shows that the avulsion is not yet complete with discharge being routed to both the parent channel and the avulsion channel in the floodbasin the avulsion was initiated as flow was diverted from the parent channel to the floodbasin through two small crevasses allowing the development and evolution of splays with a single splay stabilizing only since 1999 the avulsion area is marked by the growth of new splays the formation of an avulsion belt and progradation of avulsion deposits into the floodbasin the parent channel downstream of the avulsion site has experienced decreasing width through time associated with the higher volumes of water and sediment being captured by the avulsion channel because this avulsion is not yet complete we expect more morphological changes to occur as the avulsion belt evolves into the primary channel rapid sedimentation caused by this avulsion and the 24 year time series of imagery that captured this change allowed us to construct a 3d block representing the evolution of depositional environments associated with avulsion using time in the vertical dimension thus we could represent the evolution of geomorphological patterns including floodbasin proximal levee distal levee and channel deposits and project their distributions through time this block of evolving sedimentary settings can be used as a guide for future sedimentologic studies by offering visual clues regarding the way which depositional systems track avulsion initiated changes

miocene forebulge development previous to broken foreland partitioning in the southern central andes west central argentina 1 in the distal region of the modern flat slab segment in the southern central andes an unusual stack of middle miocene paleosols together with regional upwarping and normal faulting indicate episodic aggradation and condensed sedimentation contemporaneous with the principal stage of foreland basin development associated with foreland flexure farther to the west these features are consistent with development of a forebulge zone during the early stages of a proposed asymmetric foreland basin system sedimentary thickness farther east and far from the cordilleran tectonic loads suggests accommodation and preservation driven by nonisostatic dynamic subsidence regional overlapping relationships and basin modeling suggest that the modern broken foreland present sierras pampeanas can be interpreted as a reactivation of a formerly partitioned broad forebulge

analysis of the interaction between neotectonic and surface processes in a low land intracratonic setting of south america abstract the quaternary sedimentary record in the continental interiors has generally been studied taking into consideration the climatic forcing however episodes of uplift controlling the development of landforms have often been dismissed the interiors of continents are typically considered to have low deformation rates lower than the rates of exogenous processes deformation in the middle of the pampean plains an intracratonic setting of south america has been observed to have generated an elevated area surrounded by lowland plains this elevated area is susceptible to fluvial erosion from this fact we developed a geomorphologic modelling constrained from geomorphic markers in order to estimate the relative incidence of the different factors that control erosion in lowland landscapes an adapted erosion code was employed which combines fluvial incision equation the stream power law an uplift function spatially distributed and the hillslope diffusion equation a low dem was achieved for restoring an initial non deformed topography affected by low fluvial incision it represents a palaeosurface chrono stratigraphically constrained the estimation of the model parameters was achieved by applying the inversion method the best fit between the numerical solution calculated topography and the measured topography simulated by means a dem was approached by comparison of the stream longitudinal profiles the yield numerical approach suggests that a geomorphological modelling based on the stream power law is possible to be applied in low land landscapes for this a large scale change of the magnitude of the processes modelling the landscape should be expected in comparison with mountain erosional environments remarkably low k empirical values k 5 10 6 6 10 6 yr 1 suggest that the extremely low slopes exert a larger influence on the stream power in low land landscapes also the diffusion coefficient d values resulted largely lower 3 10 4 1 9 10 3 cm 2 yr than presented in tectonically active landscapes 0 03 cm 2 yr regional uplift rates can be higher 0 35 mm yr than it is generally expected in low uplift rate settings 0 1 0 25 mm yr these estimations evidence highly dynamic surface and tectonic processes controlling the landscape evolution over intermediate time scales in an intracratonic setting

geology and uranium thorium mineral deposits of the bokan mountain granite complex southeastern alaska abstract the bokan mountain kendrick bay uranium thorium deposits are associated with a late jurassic peralkaline granite ring dike complex the uranium thorium deposits form pipe like bodies along contacts or occur as pods in en echelon northwest striking shear zones the host intrusive is a multiphased body consisting mainly of riebeckite granite porphyry and subordinately of pegmatite aplite aegirine granite porphyry and eight other lesser rock types the ore is closely associated with an albitized aegirine syenite the ore consists of thorium and rare earth rich rock containing uranothorite uraninite and generally less than 2 of sulfide species about 100 000 tons 89 000 t of ore have been mined at an average grade of about 1 u3o8 and nearly 3 tho2 mainly from the ross adams pipe the pipe itself lies along the contact of aegirine syenite and aegirine granite porphyry it is as much as 24 m across and was mined along strike for over 300 m the ore in shear zones occurs in lenses as much as 3 m thick and 30 m in strike length wallrock alteration within and adjacent to orebodies consists of pervasive hydrothermal albite and lesser amounts of chlorite fluorite calcite quartz sericite and tourmaline hematite is present in the outer distal parts of the ore zones the minerals produced to date occur within the outer annular zone of the bokan granite complex in which aegirine is present as the major alkali ferro magnesian mineral ore emplacement occurred during the first subsidence event in association with devolatilization of the magma chamber the pod like ore zones appear to have formed during regional faulting synchronous with magma crystallization and subsequent hydrothermal events subsequent magma crystallization yielded riebeckite granites that were depleted in u and th filling temperatures in ore stage fluid inclusions within quartz range from 320 to 331 c with pressure corrections increasing the temperature of formation to 420 c or higher depending on dissolved gas content sulfur isotopic analyses on pyrite galena sphalerite and pyrrhotite indicate disequilibrium but 34sh2s is estimated at 7 6 carbon and oxygen isotopic analyses on ore stage calcite yield 18oh2o of 6 8 to 8 1 and 13c c of 4 3 to 7 0 the oxygen and carbon isotopic data support a magmatic origin for the calcite and associated thorium uranium deposits

tertiary provenance history of the northern and central altiplano central andes bolivia a detrital record of plateau margin tectonics abstract the internally drained altiplano plateau in the high elevation hinterland of the central andes contains an extremely thick up to 12 km succession of tertiary nonmarine strata that recorded the topographic evolution of eastern and western plateau margins in this study temporal and spatial variations in detrital composition based on 113 sandstone thin sections and 31 conglomerate clast counts combined with sediment dispersal patterns and regional stratigraphic correlations help delineate the tertiary history of altiplano basin development in relationship to thrust deformation and arc magmatism along plateau margins stratigraphic and provenance data suggest limited subsidence and west directed sediment dispersal during the early tertiary history of the altiplano region by late eocene time development of a rapidly subsiding basin probable foredeep was underway as recorded by extensive east directed fluvial systems of the 3 6 5 km thick potoco formation potoco compositional data are compatible with sediment sources in the incipient western cordillera and regions farther west including 1 a probable thrust belt source terrane composed of paleozoic mesozoic strata and precambrian granitic gneissic basement and 2 a subordinate magmatic arc source terrane composed of tertiary volcanic rocks increased abundances of plagioclase grains and volcanic lithic fragments upsection indicate a progressively greater arc influence due to either enhanced volcanism or eastward arc migration by late oligocene time west directed fluvial and alluvial fan systems of the upper potoco formation and the 3 km thick coniri formation reached the altiplano basin providing a minimum uplift age for the backthrust belt along the western flank of the eastern cordillera paleozoic clast conglomerate and relatively quartz rich sandstone containing sedimentary and metasedimentary lithic fragments show unroofing of predominantly paleozoic strata in the backthrust belt continued late oligocene to quaternary accumulation in the altiplano reveals that the basin persisted as a depositional region during a well documented period of neogene shortening and uplift in the andean thrust belt farther east this intra orogenic hinterland deposition between eastern and western highs during ongoing shortening suggests that the altiplano basin existed as a crustal scale piggyback basin possibly in an internally drained plateau like geomorphic setting since 25 ma provenance data and independent structural and thermochronological evidence for spatially and temporally varying tectonic histories along the eastern and western plateau margins imply that several modes of basin development were active during growth of the altiplano although flexural subsidence due to thrust loading along both plateau margins initially in the west and subsequently in the east may explain most tertiary subsidence possible closed basin conditions due to structural damming could have promoted additional sediment accumulation from late oligocene to quaternary time

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

continent elevation mountains and erosion freeboard implications 1 to the simplest approximation earth s continental crust is a floating aggregate on the planet s surface that is first attracted to subduction zones and upon arrival thickened by mountain building then producing some extension thickened regions are thinned again by erosion a comparison between 65 ma and the present shows that the modern state is significantly more mountainous an estimated average continental elevation increase relative to average ocean floor depth of about 54 m and sea level decrease relative to the ocean floor of about 102 m add up to a 156 m increase of continent elevation over sea level since 65 ma both are affected most strongly by the roughly 1 7 continent surface area decrease caused by cenozoic mountain building this includes contributions from erosion volumes of sediments in deltas and submarine fans indicate an average thickness of 371 m deposited globally in the ocean basins since 65 ma this relatively large change of continent area over a short span of earth history has significant consequences extrapolating if continent area change exceeded 5 in the past either severe erosion or flooded continents occurred if continent elevation freeboard remains at the present value of a few hundred meters the past continent ocean area ratio might have been quite different depending on earlier volumes of continental crust and water we conclude that along with the ages of ocean basins continental crustal thickening exerts a first order control on the global sea level over hundreds of million years

basement reactivation in a sub andean foreland flexural bulge the pantanal wetland sw brazil the role of sub andean foreland peripheral bulges in controlling sedimentary sequence deposition and quaternary geomorphology is discussed in the case of the pantanal wetland sw brazil this shallow 0 5 km and wide 200 km depression was formed because of uplift and flexural extension in the brittle upper crust which mechanically reactivated the neoproterozoic paraguai fold thrust belt depth to the basement and structural information within the pantanal wetland were obtained from gravity reflection seismics drill hole and digital topography data the upper part of the basin is filled with pliocene quaternary unlithified sands the age of the oldest sediments is unknown and normal faults are mostly observed on the western border of the basin a two dimensional flexural model composed of a semi infinite thin elastic plate for the brazilian shield lithosphere with an effective elastic thickness of 125 150 km its western edge at 67 5 w under the altiplano and loaded on a free end by the andes topography predicts the forebulge to be presently located over the pantanal wetland flexural modeling was constrained using gravity data extending from the pacific ocean to the east of the pantanal wetland the width of the bulge is 700 km and its amplitude is 310 m the total amount of extension and subsidence observed in the pantanal basin is interpreted as due to a cumulative strain since late oligocene when the brazilian shield started its westward migration together with compression and uplift of the central andes the main phase of the wetland subsidence is very likely related to the last compressional pulse in the andes during the upper pliocene lower pleistocene 2 5 ma as inferred from field study of fault kinematics of the central andes

partial melting fractional crystallisation liquid immiscibility and hydrothermal mobilisation a recipe for the formation of economic a type granite hosted hfse deposits abstract a number of a type granites host resources of the high field strength elements hfse including the rare earth elements ree that have been considered for economic exploitation the best known of them is the strange lake pluton canada with a resource of 20 mt grading 1 44 wt ree2o3 of which 50 are heavy rare earth oxides in this paper we examine the various processes that are likely to be responsible for the hyper enrichment of the hfse starting with the generation of a parental magma through a very low degree of partial melting of a mantle plume or a metasomatically enriched mantle in both cases below a continental crust continental rift we trace the evolution of this magma olivine nephelinite through fractional crystallisation of olivine and clinopyroxene followed by plagioclase k rich alkali feldspar and arfvedsonite to the final residues of hyper enriched hfse granitic pegmatites in so doing we show that at least two additional steps are essential to the process of hfse enrichment the first of these is the abrupt termination of plagioclase crystallisation prior to the final consumption of ca or assimilation of a ca source and the resulting exsolution of an immiscible ca fluoride melt that sequesters the lree and y the second of these steps is prolonged crystallisation of arfvedsonite which delays saturation of the magma with an aqueous phase and ensures the development of an agpaitic assemblage of na bearing hfse minerals e g na zirconosilicates and na titanosilicates finally we show that saturation of the magma with an aqueous carbonic fluid which evolves to a very acidic composition is an essential ingredient in the recipe for the formation of an economic a type granite hosted hfse deposit

lacustrine records of holocene flood pulse dynamics in the upper paraguay river watershed pantanal wetlands brazil abstract the pantanal is the world s largest tropical wetland and a biodiversity hotspot yet its response to quaternary environmental change is unclear to address this problem sediment cores from shallow lakes connected to the upper paraguay river pr were analyzed and radiocarbon dated to track changes in sedimentary environments stratal relations detrital particle size multiple biogeochemical indicators and sponge spicules suggest fluctuating lake level lowstand conditions between 11 000 and 5300 cal yr bp punctuated by sporadic and in some cases erosive flood flows a hiatus has been recorded from 5300 to 2600 cal yr bp spurred by confinement of the pr within its channel during an episode of profound regional drought sustained pr flooding caused a transgression after 2600 cal yr bp with lake level highstand conditions appearing during the little ice age holocene pr flood pulse dynamics are best explained by variability in effective precipitation likely driven by insolation and tropical sea surface temperature gradients our results provide novel support for hypotheses on 1 stratigraphic discontinuity of floodplain sedimentary archives 2 late holocene methane flux from southern hemisphere wetlands and 3 pre colonial indigenous ceramics traditions in western brazil

100 kyr sedimentary record of marginal gangetic plain implications for forebulge tectonics abstract the gangetic foreland is a consequence of continent continent collision and formation of the himalayan thrust and fold belt in the more distal part of the peripheral gangetic foreland basin the late quaternary sedimentary architecture analysis from 17 stratigraphic successions together with optically stimulated luminescence dating osl divulge interaction of peripheral bulge tectonics and climate over the past 100 kyr these sections exhibit three sedimentary packages formed under different environmental conditions detailed vertical and lateral sedimentary architecture delineates nine lithofacies that are grouped into three facies associations i flood plain facies association ii channel facies association and iii interfluve facies association the basal package i 114 ka old was deposited by sandy meandering channels and overlying to this with a hiatus of 30 ka is package ii 80 54 ka deposited by gravelly rivers the sediments of both the packages i ii are derived from rocks exposed in the peripheral bulge region overlain package iii deposited by small meandering channels consist of the sediments derived from the himalaya the results indicate that the duration of 80 54 ka was a period of forebulge uplift when gravelly fans prograded basinward below the fan sediments lies a peripheral bulge unconformity marked by regionally significant pedogenic horizon this pedogenic horizon qualifies as to be termed as peripheral bulge uncomformity after 54 ka the fine himalayan sediments onlap the cratonic sediments implying rather a stable forebulge tectonics coupled with fluctuating climatic conditions occasionally bringing micaceous gray coloured sand silt of himalayan origin

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

petrology of serpentine sandstone as a key to tectonic development of serpentine belts abstract the petrology of serpentine sandstone which is ubiquitous in the serpentine belt could provide some constraints on petrological characteristics for lost or eroded serpentinite masses the chemistry of detrital chrome spinel and chrome diopside in particular is a useful indicator of the equilibrium temperature and lithology of the source peridotite combined with the petrology of the associated serpentinite masses this could provide the difference in the petrological characteristics if any between the lost serpentine and that now exposed in a serpentine belt and consequently some constraints could be placed on the tectonic history of the serpentine belt itself in the mineoka belt a narrow serpentine belt in central japan the serpentinite mass exposed during the miocene has experienced a lower pyroxene temperature and wider lithology than the serpentinite masses now exposed in the same area this fact possibly indicates that a vertical slice of the upper mantle peridotite has been uplifted and intermittently protruded since the miocene the mineoka belt is probably placed on part of a fossil transcurrent fault which had worked in the opening of the shikoku basin of the northern end of the philippine sea plate along the transcurrent fault the mantle peridotite slices with various petrologic characteristics could have been continuously emplaced in the same way as in the present day oceanic fracture zone on the other hand in the kamuikotan belt of hokkaido northern japan the source of a miocene serpentine sandstone at furenai has the same petrologic characteristics as the now associated sarugawa peridotite mass the sarugawa mass is a dismembered part of an ophiolite suite uplifted during the collision between the eurasia plate and the okhotsk north america plate in miocene times the furenai serpentine sandstone was produced in a small sedimentary basin within the sarugawa peridotite mass during the middle miocene contrary to the mineoka belt large vertical uplift movements of the peridotite mass did not occur in the kamuikotan zone after the miocene

statistical estimation of the position of an apex application to the geological record abstract knowing the position of an apex of a distributive depositional system can provide important spatial constraints on paleogeographic reconstructions and thus can greatly help facies predictions at both a system and a basin scale to date predicting the position of an apex of a sedimentary system is often limited to generalized statements based on facies mapping and qualitative analyses of paleocurrent readings this paper presents a user friendly quantitative methodology based on the von mises distribution and uses the method of maximum likelihood to obtain an estimated apex and associated confidence regions for a dataset the methodology presented has been applied to two modern distributive fluvial systems dfss the taquari dfs situated in southwestern brazil and the gilbert dfs situated in northwestern queensland australia the position of each apex is known for the two systems thus allowing the accuracy of the methodology to be tested a range of datasets within which the amount and spatial distribution of localities were selected independently was analyzed the predicted apices came within encouraging proximity of the true apices ranging in distance from 2 7 km to 40 3 km 1 6 to 23 4 of the total dfs length away with accuracy generally increasing with increasing dataset size and proximity to the apex data collected from the late jurassic salt wash dfs were also analyzed using the code results have helped to give better geographical constraints on the system and apex location as well as on the southern margin of the morrison depositional basin although tested on modern and outcrop based datasets from dfs the methodology can be applied to any dataset subsurface or surface in which dispersion occurs from a point source thus unlocking the potential for better paleogeographic constraint on a broad range of sedimentary environments such as deltas and submarine fans

the early jurassic bokan mountain peralkaline granitic complex southeastern alaska geochemistry petrogenesis and rare metal mineralization abstract the early jurassic ca 177 ma bokan mountain granitic complex located on southern prince of wales island southernmost alaska cross cuts paleozoic igneous and metasedimentary rocks of the alexander terrane of the north american cordillera and was emplaced during a rifting event the complex is a circular body 3 km in diameter of peralkaline granitic composition that has a core of arfvedsonite granite surrounded by aegirine granite all the rock forming minerals typically record a two stage growth history and aegirine and arfvedsonite were the last major phases to crystallize from the magma the bokan granites and related dikes have sio2 from 72 to 78 wt high iron feo tot 3 4 5 wt and alkali 8 10 wt concentrations with high feo tot feo tot mgo ratios typically 0 95 and the molar al2o3 na2o k2o ratio

late quaternary paleosol sediment sequences and landscape evolution along the andean piedmont bolivian chaco abstract as part of the bolivian chaco the andean piedmont is situated in the transitional zone between the tropical humid climate regime to the north amazonia and the subtropical semiarid regime to the south gran chaco in this zone which is sensitive to climatic changes late quaternary landscape and climate history was reconstructed based on detailed sedimentological pedological and geochemical analyses of paleosol sediment sequences along the andean piedmont the determination of characteristic lithofacies allowed correlation between the investigated profiles and their interpretation in terms of paleoenvironmental conditions i e geomorphic stability soil formation or geomorphic activity fluvial or fluvio aeolian sedimentation erosion etc coarse fluvial deposits gravels sands form the base of the investigated profiles and have likely been deposited in a braided river environment with low vegetation cover and intensive discharge events before and during the last glacial maximum well developed paleosols formed in the upper part of these sediments during the lateglacial documenting geomorphic stability high vegetation cover and wet conditions overlying palustrine sediments dated to the early holocene indicate a dramatic change in seasonality i e increased winter precipitation and decreased summer precipitation subsequent erosion followed by accumulation of fluvial and fluvio aeolian sands points to much drier conditions during the mid holocene since 2 9 cal ka bp soil formation indicates a return to geomorphic stability and wetter conditions characteristic of the present day climate interrupted by short arid intervals

holocene stratigraphic evolution of saline lakes in nhecolandia southern pantanal wetlands brazil nhecol ndia is a fossil lobe of the taquari river megafan and a prominent geomorphic subunit of the pantanal wetlands because of the presence of 10 000 small lakes we investigated the stratigraphic records of three saline lakes from nhecol ndia to explore their potential as quaternary hydroclimate archives radiocarbon data indicate that accumulation at two lakes was approximately continuous in the late holocene and chemostratigraphic variability suggests sensitivity to environmental change with multicentennial resolution a basal sandy unit and an upper muddy unit comprise the shallow stratigraphy of each lake a pronounced change in depositional environment from freshwater wetlands to saline lakes at 3300 3200 cal yr bp best explains the lithofacies transition ephemeral freshwater wetlands formed on the abandoned megafan lobe which was molded by deflation in the arid early holocene wind scouring of the megafan lobe generated topographically closed depressions with complex marginal sand ridges which allowed permanent lakes to evolve when rainfall increased in the late holocene the lakes became highly saline and alkaline after 910 cal yr bp which influences biogeochemistry and aquatic ecology the results hold implications for understanding the response of the southern pantanal to climate change as well as the development of pans in tropical megafan settings

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

contrasting fluvial styles of the paraguay river in the northwestern border of the pantanal wetland brazil abstract the upper paraguay drainage basin is situated mainly in west central brazil near the bolivian border flowing from north to south the paraguay is the trunk river of an alluvial depositional tract characterized by complex geomorphologic zonation that resulted from an intricate geologic evolution since the late pleistocene this paper focuses on the geomorphology of the paraguay river at the northwestern border of the pantanal wetland where two broad geomorphologic zones were distinguished north from the pantanal wetland the paraguay river flows in an aggradational fluvial plain 5 km wide and incised into older alluvial deposits the river exhibits a meandering style over most of its course but sinuosity drops from 2 2 to 1 1 near the northwest border of the pantanal wetland where the river has been forming the paraguay fluvial megafan since the late pleistocene the river deflects 90 eastward at the entrance into the pantanal changing its fluvial style because of a progressive loss of confinement downstream of the point where the river reaches lowland plains the river becomes more sinuous adopts a distributary pattern within the wetland and brings about the creation of the modern depositional lobe characterized by higher topographic gradient and active sedimentation likely linked to increased accommodation space allowing progradation fluvial discharge diminishes in the pantanal wetland because of channel overbank flow during the rainy season and frequent levee crevassing avulsion belts and channel levee complex are preserved on the floodplain as relict forms south of the convergence of the two main channels that define the taiama island a loss of gradient marks the base of the depositional lobe further downstream the paraguay river returns to a meandering fluvial style but crossing a large fluvial plain populated by hundreds of small lakes and seasonally flooded that characterizes the pantanal wetland

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

sediment transit time and floodplain storage dynamics in alluvial rivers revealed by meteoric 10be quantifying the time scales of sediment transport and storage through river systems is fundamental for understanding weathering processes biogeochemical cycling and improving watershed management but measuring sediment transit time is challenging here we provide the first systematic test of measuring cosmogenic meteoric beryllium 10 10bem in the sediment load of a large alluvial river to quantify sediment transit times we take advantage of a natural experiment in the rio bermejo a lowland alluvial river traversing the east andean foreland basin in northern argentina this river has no tributaries along its trunk channel for nearly 1 300 km downstream from the mountain front we sampled suspended sediment depth profiles along the channel and measured the concentrations of 10bem in the chemically extracted grain coatings we calculated depth integrated 10bem concentrations using sediment flux data and found that 10bem concentrations increase 230 from upstream to downstream indicating a mean total sediment transit time of 8 4 2 2 kyr bulk sediment budget based estimates of channel belt and fan storage times suggest that the 10bem tracer records mixing of old and young sediment reservoirs on a reach scale 10bem transit times are shorter where the channel is braided and superelevated above the floodplain and longer where the channel is incised and meandering suggesting that transit time is controlled by channel morphodynamics this is the first systematic application of 10bem as a sediment transit time tracer and highlights the method s potential for inferring sediment routing and storage dynamics in large river systems

lithospheric evolution of the andean fold thrust belt bolivia and the origin of the central andean plateau we combine geological and geophysical data to develop a generalized model for the lithospheric evolution of the central andean plateau between 188 and 208 s from late cretaceous to present by integrating geophysical results of upper mantle structure crustal thickness and composition with recently published structural stratigraphic and thermochronologic data we emphasize the importance of both the crust and upper mantle in the evolution of the central andean plateau four key steps in the evolution of the andean plateau are as follows 1 initiation of mountain building by 70 ma suggested by the associated foreland basin depositional history 2 eastward jump of a narrow early fold thrust belt at 40 ma through the eastward propagation of a 200 400 km long basement thrust sheet 3 continued shortening within the eastern cordillera from 40 to 15 ma which thickened the crust and mantle and established the eastern boundary of the modern central andean plateau removal of excess mantle through lithospheric delamination at the eastern cordillera altiplano boundary during the early miocene appears necessary to accommodate underthrusting of the brazilian shield replacement of mantle lithosphere by hot asthenosphere may have provided the heat source for a pulse of mafic volcanism in the eastern cordillera and altiplano at 24 23 ma and further volcanism recorded by 12 7 ma crustal ignimbrites 4 after 20 ma deformation waned in the eastern cordillera and interandean zone and began to be transferred into the subandean zone long term rates of shortening in the fold thrust belt indicate that the average shortening rate has remained fairly constant 8 10 mm year through time with possible slowing 5 7 mm year in the last 15 20 myr we suggest that cenozoic deformation within the mantle lithosphere has been focused at the eastern cordillera altiplano boundary where the mantle most likely continues to be removed through piecemeal delamination d 2005 elsevier b v all rights reserved

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

the modern foreland basin system adjacent to the central andes regional variations in sediment thickness internal structures average elevation and bouguer gravity define a four component foreland basin system adjacent to the central andes in the most proximal part of the foreland basin system the eastern subandean zone and westernmost chaco plain 1 3 km of cenozoic deposits overlies active folds and thrusts of the frontal andean orogenic wedge these wedge top deposits pass cratonward into a foredeep depozone containing a 3 4 km thick sedimentary prism that tapers toward and locally pinches out against a broad wavelength forebulge in the central eastern chaco plain the forebulge is underlain by precambrian mesozoic rocks and is largely covered by a thin veneer of quaternary alluvium east of the forebulge a thin 0 5 km saucer shaped accumulation of sediment beneath the pantanal wetland represents a back bulge depozone ancient counterparts of these four depozones can be identified in the central andes suggesting that modern basin architecture is the result of continuous eastward migration of the coupled orogenic wedge and foreland basin system since the late cretaceous paleocene