late quaternary paleohydrology of the madre de dios river southwestern amazon basin peru abstract late quaternary climatic and hydrologic variability triggered changes in fluvial deposition and erosion along the course of the madre de dios river peru the largest tributary basin of the madeira basin itself the largest tributary basin of the amazon three laterally extensive quaternary age terrace tracts are present within the madre de dios basin analysis of sedimentary facies present in the modern cut banks and terraced sequences along with radiocarbon dates on fossil wood and leaf material preserved in the terraced strata allow reconstruction of the late quaternary depositional history of the sedimentary sequences including determination of the approximate timing of aggradation and downcutting episodes and its relationship to the timing of past climate change in this portion of the amazon basin and beyond the quaternary sediments underlying the terraces most often recorded deposition in a coarse grained meandering fluvial system the t3 terrace the highest terrace is underlain by the miocene ipururi formation which is unconformably overlain by the late miocene pleistocene 48 000 cal yrs bp madre de dios formation a multistory coarse sandy to gravelly channel and point bar complex the latter was downcut before 29 850 100 cal yrs bp this downcut landscape was infilled by meandering fluvial strata characterized by gravelly channel deposits in a sequence dominated by floodplain and lateral accretion deposits these strata were in turn downcut to form the t2 terrace before 11 970 100 cal yrs bp a third episode of aggradation resulted in the deposition of a sand dominated meandering channel complex that infilled the t2 valley and was subsequently downcut after 3780 50 cal yrs bp this most recent terrace is infilled by the modern fluvial sediment which has been actively aggrading since at least 870 50 cal yrs bp importantly the madre de dios fluvial system actively aggraded between 30 000 and 25 000 cal yrs bp and likely much younger as dated samples were thus far only found near the base of the t2 sequence this observation implies that some combination of 1 increased precipitation and decreased temperature 2 decreased evapotranspiration and increased runoff 3 increased andean glacial erosion and increased sediment supply and 4 decreased atmospheric co 2 hence decreased rain forest primary productivity and altered rain forest physiology ecology entering the last glacial maximum period brought about increased floodplain deposition in the southwestern amazon elsewhere in the amazon basin few if any fluvial sediments of this age range have been observed the start of the next major phase of aggradation coincided with the younger dryas and suggested that floodplain sedimentation in the lowlands was again related to cold and wet conditions in the adjacent highlands and perhaps in the lowlands as well and that madre de dios history was also tied to large scale global climate this aggradation may have continued throughout the early and mid holocene until at least 3 780 cal yr bp if so and this is uncertain this episode of sedimentation took place during a dry period

petrology geochemistry and zircon u pb lu hf isotopes of paleoproterozoic granite gneiss from bomdila in the western arunachal himalaya ne india

isotopic constraints on the structural relationships between the lesser himalayan series and the high himalayan crystalline series garhwal himalaya nd and sr isotope systematics may provide important constraints on the location of major thrust systems that separate lithologically similar sedimentary sequences the potential of the technique is illustrated by this isotopic study of the main central thrust system of the himalaya nd isotope data from the garhwal himalaya indicate that metasedimentary rocks from the vaikrita group nd 14 to 19 correlate closely with those from the high himalayan crystalline series which constitutes the hanging wall lithologies of the main central thrust in contrast metasedimentary rocks from the munsiari group nd 23 to 28 show marked similarities to the lesser himalayan series in the footwall of the main central thrust sr isotopes support the correlations in that the vaikrita group shows partial reequilibration at 500 ma whereas the munsiari group has not undergone sr isotope homogenization since 1800 ma thus the vaikrita thrust that juxtaposes these two formations is recognized as the main central thrust in garhwal himalaya the thrust coincides approximately with the location of the kyanite isograd confirming that inverted metamorphism is characteristic of both hanging wall and footwall of the main central thrust along the tons thrust known locally as the srinagar thrust 50 km south of the main central thrust low grade quartzarenites with nd sr isotope and trace element characteristics typical of lesser himalayan formations have been emplaced on phyllites and siltstones with geochemical characteristics of the high himalayan crystalline series the field relationships most probably result from out of sequence thrusting in which lesser himalayan series rocks to the north were emplaced over low grade equivalents of the high himalayan crystalline series preserved in the external part of the orogen this study establishes the value of isotope data for lithostratigraphic correlations within orogenic belts

new age constraints for the proterozoic aravalli delhi successions of india and their implications abstract proterozoic sedimentary successions of india are important archives of both the tectonic history of the indian subcontinent and the geochemical evolution of earth surface processes however the lack of firm age constraints on many of these stratigraphic units limits their current utility here we present new detrital zircon age data from strata of the southern aravalli delhi orogenic belt adob and the rajasthan vindhyan successions the alwar group of the southern delhi supergroup yielded a large population of 1 2 ga detrital zircon grains which refutes the 1 9 1 7 ga age assertion for this unit detrital zircon age distributions from the southern alwar group differ strongly from the alwar group of the north delhi belt demonstrating miscorrelation between these two regions the jhamarkotra formation of the lower aravalli group contains a large population of 1 9 1 7 ga detrital zircon grains therefore the unit cannot be 2 1 ga as traditionally assumed age distributions of the aravalli and delhi supergroups are similar to those of the lower and upper vindhyan successions and we postulate contiguous sediment sources for both regions with strata of the tectonically deformed adob representing the distal margin equivalents of the vindhyan successions additionally a late paleoproterozoic age for the jhamarkotra formation nullifies the hypothesis that the markedly positive carbonate 13c values in this unit are linked to the 2 3 2 0 ga lomagundi jatuli positive isotope excursion the potential of a large late paleoproterozoic ca 1 7 ga positive 13c excursion contrasts with the long held view of a prolonged period of carbon isotope stasis during the so called boring billion

geomorphology and evolution of the late pleistocene to holocene fluvial system in the south eastern llanos de moxos bolivian amazon in the bolivian amazon several paleochannel generations are preserved their wide spectrum of morphologies clearly provides crucial information on the type and magnitude of geomorphic and hydrological changes within the drainage network of the andean foreland therefore in this study we mapped geomorphological characteristics of paleochannels and applied radiocarbon and optically stimulated luminescence dating seven paleochannel generations are identified significant changes in sinuosity channel widths and river pattern are observed for the successive paleochannel generations our results clearly reflect at least three different geomorphic and hydrological periods in the evolution of the fluvial system since the late pleistocene changes in discharge and sediment load may be controlled by combinations of two interrelated mechanisms i spatial changes and re organizations of the drainage network in the upper catchment and or ii climate changes with their associated local to catchment scale modifications in vegetation cover and changes in discharge inundation frequencies and magnitudes which have likely affected the evolution of the fluvial system in the llanos de moxos in summary our study has revealed the enormous potential which geomorphic mapping and analysis combined with luminescence based chronologies hold for the reconstruction of the late pleistocene to recent fluvial system in a large portion of amazonia

the time averaged geomagnetic field global and regional biases for 0 5 ma summary palaeodirectional data from lava flows and marine sediments provide information about the long term structure and variability in the geomagnetic field we present a detailed analysis of the internal consistency and reliability of global compilations of sediment and lava flow data time averaged field models are constructed for normal and reverse polarity periods for the past 5 ma using the combined data sets nonzonal models are required to satisfy the lava flow data but not those from sediments alone this is in part because the sediment data are much noisier than those from lavas but is also a consequence of the site distributions and the way that inclination data sample the geomagnetic field generated in the earth s core different average field configurations for normal and reverse polarity periods are consistent with the palaeomagnetic directions however the differences are insignificant relative to the uncertainty in the average field models thus previous inferences of non antipodal normal and reverse polarity field geometries will need to be re examined using recently collected high quality palaeomagnetic data our new models indicate that current global sediment and lava flow data sets combined do not permit the unambiguous detection of northern hemisphere flux lobes in the 0 5 ma time averaged field highlighting the need for the collection of additional high latitude palaeomagnetic data anomalous time averaged field structure is seen in the pacific hemisphere centred just south of hawaii the location of the anomaly coincides with heterogeneities in the lower mantle inferred from seismological data the seismic observations can be partly explained by lateral temperature variations however they also suggest the presence of lateral compositional variations and or the presence of partial melt the role of such heterogeneities in influencing the geomagnetic field observed at the earth s surface remains an unresolved issue requiring higher resolution time averaged geomagnetic field models along with the integration of future results from seismology mineral physics and numerical simulations

present day south american climate abstract this paper documents the main features of the climate and climate variability over south america on the basis of instrumental observations gathered during the 20th century it should provide a modern reference framework for paleoclimate research in south america targeting high resolution proxies over the past few centuries several datasets suitable for present day climate research are first described highlighting their advantages as well as their limitations we then provide a basic physical understanding of the mean annual cycle of the precipitation and atmospheric circulation over the continent and the adjacent oceans in particular the diversity of precipitation temperature and wind patterns is interpreted in terms of the long meridional extent of the continent and the disruption of the large scale circulation caused by the andes cordillera the contrasting oceanic boundary conditions and the landmass distribution similarly the intensity and timing of the interannual and interdecadal climatic fluctuations exhibit considerable geographical dependence as some regions are more influenced by large scale phenomena rooted in the tropical oceans while others are more influenced by high latitude phenomena the impact of these large scale phenomena over south america is documented by a regression analysis between selected atmospheric indices and the precipitation and temperature fields we have included a discussion on the seasonality and long term stability of such impacts and complemented our general description by an updated review of the literature on climate variability over specific regions

thermotectonic history of the chiplakot crystalline belt in the lesser himalaya kumaon india constraints from apatite fission track thermochronology abstract fission track ages and confined track length distribution of apatite samples separated from the chiplakot crystalline belt ccb of the lesser himalayan crystalline lhc zone located to the south of the main central thrust mct munsiari thrust mt in kumaon india have been determined ages from the ccb along the kali and darma valleys fall in two distinct groups in the northern part of the ccb the ages range from 9 8 0 6 to 7 6 0 6 ma with a weighted mean of 9 6 0 1 ma while in the southern part the ages vary from 17 9 0 9 to 12 9 1 1 ma with a weighted mean of 14 1 0 1 ma the bimodal distribution of track lengths indicates that the ages are mixed ages rather than simple cooling ages the apatite fission track aft ages and already published structural data of the ccb suggest a complex erosional denudation history within the upper 3 4 km of the crust of the ccb the ages further indicate that the ccb was thrust into place earlier than the middle miocene i e at the time of development of the mct since then these rocks have remained within the upper 3 km of crust and were affected by only moderate to slow erosion and exhumation these results have important implications for the tectonic evolution of the lhc zone to the south of the mct mt the exhumation of the lhc zone in different parts of the himalaya was not uniform in the kumaon himalaya it was not controlled as in the himachal himalaya by any major tectonic event since it was thrust over the lesser himalayan meta sedimentary lhms zone and underwent moderate to slow erosion and exhumation

the proterozoic of eastern bolivia and its relationship to the andean mobile belt abstract the previously unmapped bolivian sector of the central brazil shield 220 000 km in area has been studied during an anglo bolivian technical co operation programme the metamorphic basement comprises the lomas maneches granulitic complex the chiquitania gneissic complex and the san ignacio schist supergroup which crops out as metasedimentary belts rb sr dates on some of the granulites suggest a protolith age of 2000 ma but the outcrop pattern of the metamorphic rocks as a whole is due to the 1300 ma san ignacio orogeny which was accompanied by widespread granite emplacement and metamorphism up to granulite facies the sunsas cycle began with the deposition of the supracrustal sunsas vibosi group and closed with the development of the sunsas and aguapei mobile belts at 1000 ma the upper proterozoic brasiliano cycle is represented by a bifurcating sedimentary basin in the se of the area with a closing tectonic event of 520 ma correlation is suggested with units in the central brazil shield the bolivian belts are extrapolated east into brazil and their limits indicated to the west the belts show trends parallel to the phanerozoic andean chain and they reflect the major deflection in the chain the arica elbow furthermore when the belts are traced towards the andes their ages broadly correspond to those of the sub andean precambrian basement this leads to the proposal of an enlarged andean orogenic belt system 1000 km wide comprising a 1300 ma old proto andean orogen which has controlled the trends of subsequent mobile belts

pre himalayan tectono magmatic imprints in the darjeeling sikkim himalaya dsh constrained by 40 ar 39 ar dating of muscovite abstract the lower lesser himalayan sequence l lhs in darjeeling sikkim himalaya dsh displays intensely deformed low grade meta sedimentary rocks frequently intervened by granite intrusives of varied scales the principal motivation of our present study is to constrain the timing of this granitic event using 40ar 39ar geochronology we dated muscovite from pegmatites emplaced along the earliest fabric in the low grade daling phyllite and obtained 1850 ma ar ar muscovite cooling age which is broadly coeval with crystallization ages of lingtse granite protolith e g 1800 1850 ma u pb zircon ages reported from the l lhs we present here field observations to show the imprints tectonic fabrics of multiple ductile deformation episodes in the lhs terrain the earliest penetrative fabric axial planar to n s trending reclined folds suggest a regional tectonic event in the dsh prior to the active phase of indo asia collision based on the age of granitic bodies and their structural correlation with the earliest fabric we propose that the l lhs as a distinct convergent tectono magmatic belt delineating the northern margin of indian craton in the framework of the 1850 ma columbia supercontinent assembly

holocene floodplain soils along the rio mamore northern bolivia and their implications for understanding inundation and depositional patterns in seasonal wetland settings abstract the llanos de moxos lm in the bolivian amazon basin host one of the largest seasonally inundated savannah landscapes on earth very little is known of this area with regard to sedimentary dynamics soil formation or their relationship to longer term climatic and hydrological variability in this setting here we present a detailed study of three floodplain depositional sequences building up the inundation savannah along the mamore river in the central lm pleistocene sands and silts unit 1 are overlain by thick overbank deposits of mostly holocene age unit 2 on both sides of the mamore river and underline the importance of extensive flooding processes for the late quaternary sedimentary and geomorphic evolution of the lm the fine grained distal overbank sediments show signs of strong modification by hydromorphic processes and reflect spatial and temporal variations in flood inundation patterns in addition widespread dark gray to blackish soil and paleosol horizons are intercalated with the overbank sediments but lack evidence for significant weathering these horizons are likely the result of cumulative incorporation of organic material under conditions of particularly low sedimentation rates therefore their formation should be linked to processes such as channel migration longer term meanderbelt evolution avulsive drainage reorganization changes in sediment supply or climatically induced variations in flooding type frequency or magnitude the common occurrence of these floodplain soils in the early to mid holocene may thus reflect the combined effects of hydrological geomorphic and sedimentary changes during a drier mid holocene

was himalayan normal faulting triggered by initiation of the ramgarh munsiari thrust and development of the lesser himalayan duplex the ramgarh munsiari thrust is a major orogen scale fault that extends for more than 1 500 km along strike in the himalayan fold thrust belt the fault can be traced along the himalayan arc from himachal pradesh india in the west to eastern bhutan the fault is located within the lesser himalayan tectonostratigraphic zone and it translated paleoproterozoic lesser himalayan rocks more than 100 km toward the foreland the ramgarh munsiari thrust is always located in the proximal footwall of the main central thrust northern exposures toward the hinterland of the thrust sheet occur in the footwall of the main central thrust at the base of the high himalaya and southern exposures toward the foreland occur between the main boundary thrust and greater himalayan klippen although the metamorphic grade of rocks within the ramgarh munsiari thrust sheet is not significantly different from that of greater himalayan rock in the hanging wall of the overlying main central thrust sheet the tectonostratigraphic origin of the two different thrust sheets is markedly different the ramgarh munsiari thrust became active in early miocene time and acted as the roof thrust for a duplex system within lesser himalayan rocks the process of slip transfer from the main central thrust to the ramgarh munsiari thrust in early miocene time and subsequent development of the lesser himalayan duplex may have played a role in triggering normal faulting along the south tibetan detachment system

holocene precipitation and atmospheric changes inferred from river paleowetlands in the bolivian andes abstract in the southern tropical andes previous studies performed on lacustrine basins display large water level fluctuations due to changes in the precipitation minus evaporation balance during the holocene significant changes in groundwater levels were also inferred from river paleowetland deposits in northern chile and the northern bolivian altiplano but the discrepancies which appeared in the data from paleolakes and paleowetlands are still not understood in this paper we present stratigraphy and diatom analyses in new paleowetland records from non glacial valleys these data compared with previous results show that non stormy precipitation recorded by fine and or organic sedimentation dominated continuously from 11 200 to 1500 cal yr bp and that convective rainfall recorded by strong erosion occurred only episodically in the northern altiplano convective rainfall was similar to the type of precipitation which nowadays occurs during the rainy season austral summer when the tropical easterlies from the atlantic reach the bolivian andes non stormy precipitation is interpreted as a result of an intensification of cold air incursions from southern polar regions in good agreement with data from brazil the maximum intensity in cold air incursions occurred during the early holocene and coincided with a climatic optimum in antarctica and a considerably reduced el nino southern oscillation enso during the middle and late holocene cold air incursions decreased and the enso frequency increased we suggest that the interactions between the southern high and low latitudes by means of cold air incursions and associated changes in the west wind flow have been the main mechanisms involved in climatic changes at the latitudes of bolivia

himalayan metamorphism and its tectonic implications the himalayan range exposes a spectacular assemblage of metamorphic rocks from the mid and deep crust that have fostered numerous models of how the crust responds to continental collisions recent petrogenetically based petrologic and geochronologic studies elucidate processes with unprecedented resolution and critically test models that range from continuum processes to one time events the pronounced metamorphic inversion across the main central thrust reflects continuum thrusting between ca 15 and 20 ma whereas exposure of ultrahigh pressure rocks in northwestern massifs and syntaxis granulites reflects singular early 45 ma and late 10 ma exhumation events multiple mechanisms including wedge collapse and flow of melt weakened midcrust are debated to explain pressure temperature trajectories patterns of thinning and thermal overprinting a geochronologic revolution is under way in which spatially resolved compositions and ages of accessory minerals are combined in a petrogenetically valid co

stratigraphic and tectonic implications of field and isotopic constraints on depositional ages of proterozoic lesser himalayan rocks in central nepal abstract in the himalaya of central nepal uncertainty in the absolute depositional ages and the relative stratigraphic positions of several formations in the proterozoic part of the lesser himalayan series has hindered structural mapping east west correlation of units and structures and interpretations of basin architecture during both the proterozoic and paleozoic at the stratigraphic base of the lesser himalayan series in central nepal 441 new u pb isotopic ages of detrital zircons from the previously defined type kuncha kushma and fagfog formations constrain the maximum possible depositional ages for these clastic units to be c 1900 1770 and 1810 ma respectively 182 detrital zircon u pb isotopic ages confirm the identity of another sample of the fagfog formation and field relations indicate that this unit rests depositionally on the kushma formation with no obvious unconformity or lithologic difference this interpretation combined with the recognition that the term kushma formation historically has been applied incongruously to apparently different units in different parts of nepal leads us to recommend abandoning the term kushma formation and replacing it with lower fagfog formation the overlying part of the fagfog then becomes the upper fagfog formation because the 1770 ma constraint on the maximum depositional age comes from the lower part of the unit the entire fagfog formation must have been deposited after 1770 ma the depositional ages of both the lower and upper parts of the fagfog must be younger than the kuncha formation because it was intruded by a granite at 1878 22 ma 2 sigma thus an unconformity lasting at least 86 m y probably separates the kuncha formation deposited between 1900 and 1856 ma considering uncertainties and the fagfog formation deposited after 1770 ma the lower fagfog formation is present in the kali gandaki region but apparently pinches out to the east because it is not present structurally beneath the kathmandu nappe suggestions about the depositional age of the carbonate dominated malekhu formation at the top of the proterozoic part of the lesser himalayan series have ranged from mesoproterozoic to late paleozoic these age estimates largely stem from correlation with parts of the krol succession some 500 km to the west in northwest india and changing interpretations of the depositional age of the krol rocks 104 new measurements of carbon isotope abundances in carbonate from a 300 m measured section through type exposures of the malekhu formation reveal a narrow range of 13c values between 1 7 and 0 2 vpdb and a mean of 0 9 0 4 1 standard deviation these values are inconsistent with correlation with the krol succession which preserves large amplitude positive and negative excursions in 13c values comparison with known marine carbonate 13c values through time suggests deposition of the malekhu formation prior to c 1300 ma upper and lower bounds for deposition of the proterozoic part of the lesser himalayan series thus are c 1300 and c 1900 ma though actual deposition likely occurred during only a portion of this interval further a profound unconformity representing at least 900 m y separates the malekhu formation from the depositionally overlying carboniferous permian sisne formation no upper mesoproterozoic neoproterozoic or lower paleozoic rocks have been found in the lesser himalayan series in central nepal the carbon isotope stratigraphy of the malekhu formation in central nepal matches that of the buxa formation exposed in the ranjit window of sikkim to the east but does not match the chemostratigraphy of rocks correlated with the buxa formation in eastern bhutan and arunachal pradesh northeast india

zircon u pb ages and hf isotopes of the askot klippe kumaun northwest india implications for paleoproterozoic tectonics basin evolution and associated metallogeny of the northern indian cratonic margin

channel flow extrusion model to constrain dynamic viscosity and prandtl number of the higher himalayan shear zone constraining magnitudes of mechanical and thermo mechanical parameters of rocks and shear zones are the important goals in structural geology and tectonics talbot in j struct geol 21 949 957 1999 such parameters aid dynamic scaling of analogue tectonic models ramberg in gravity deformation and the earth s crust in theory experiments and geological applications 2nd edn academic press london 1981 which are useful to unravel tectonics in further details schultz ela and walsh in j struct geol 24 247 275 2002 the channel flow extrusion of the higher himalayan shear zone hhsz higher himalaya can be explained by a top to s sw simple shear i e the d2 deformation in combination with a pressure gradient induced flow against gravity presuming its newtonian incompressible rheology with parallel inclined boundaries the viscosity of this shear zone along a part of the himalayan chain through india nepal and bhutan is estimated to vary widely between 1016 and 1023 pa s and its prandtl number pr within 1021 1028 the estimates utilized ranges of known thickness 6 58 km of the hhsz that of its top subzone of ductile shear of normal shear sense stdsu 0 35 9 4 km total rate of slip of its two boundaries 0 7 131 mm year 1 pressure gradient 0 02 6 kb km 1 density 2 2 3 1 g cm 3 and thermal diffusivity 0 5 10 6 2 1 10 6 m s 2 along the orogenic trend considering most of the parameters specifically for the sutlej section india the calculated viscosity and the prandtl number pr of the hhsz are deduced to be 1017 1023 pa s and pr 1022 1028 the upper limits of the estimated viscosity ranges are broadly in conformity with a strong tibetan mid crust from where a part of the hhsz rocks extruded on the other hand their complete ranges match with those for its constituent main rock types and partly with those for the superstructure and the infrastructure the estimated mechanical and thermo mechanical parameters of the hhsz will help to build dynamically scaled analogue models for the himalayan deformation of the d2 phase

quaternary fluvial systems of tropics major issues and status of research abstract fluvial systems in tropical regions are not only the lifeline for modern civilizations there but also form an important continental archive for reconstructing quaternary palaeoclimates and palaeoenvironments tropical rivers drain through a variety of geological and geomorphological settings across the world and have been studied for their hydrology sediment transport characteristics facies models stratigraphic development and flood hazards this paper presents an overview of the research on tropical rivers globally and identifies major gaps and unanswered research questions as a part of the igcp 582 on tropical rivers the participants of this project are continuing to tackle many of these issues the collection of papers in this special issue is an outcome of this activity and presents new data on processes and alluvial stratigraphy of the tropical rivers from china india south america and northern australia

geochemistry of precambrian mafic magmatic rocks of the western himalaya india petrogenetic and tectonic implications abstract precambrian sequences of the higher himalayan crystallines vaikrita group and the lesser himalaya chail jutogh and jaunsar groups in the garhwal and himachal regions of the western himalaya include abundant metamorphosed mafic lavas and dykes amphibolites a gabbroic body within the chail group has been dated at 1907 91 ma initial 87 sr 86 sr ratio of 0 7022 0 0008 by the whole rock rb sr method this age is consistent with several age data 1800 2000 ma whole rock rb sr method assigned to the granitoids and gneisses of this region these amphibolites and gabbros exhibit low ti tholeiite characteristics the lesser himalayan samples are enriched in light rare earth elements lree and large ion lithophile element lile with distinct negative nb sr p and ti anomalies conversely the vaikrita group samples are characterized by less enriched lree lile and absence of the above anomalies but have distinct positive sr anomalies the chemical characteristics of the lesser himalayan samples are remarkably similar to the basal aravalli volcanics of the nw indian shield inferred to reflect significant components from enriched mantle sources and crustal contamination samples of the vaikrita group appear to have been influenced by an asthenospheric mantle and are less contaminated as indicated by the absence of negative nb p and ti anomalies these distinct geochemical characteristics are used to demarcate the main central thrust along the vaikrita thrust the chail and jutogh groups are affiliated with the lesser himalaya and the chail group gabbro body may represent portion of one among the numerous magma chambers that fed a large precambrian magmatic province in the south of the main central thrust the similar emplacement ages 1800 2000 ma for the mafic and felsic magmatic rocks suggest that this region experienced a major episode of crustal generation and evolution in a rift environment

fifty thousand year vegetation and climate history of noel kempff mercado national park bolivian amazon abstract pollen and charcoal records from two large shallow lakes reveal that throughout most of the past 50 000 yr noel kempff mercado national park in northeastern lowland bolivia southwestern amazon basin was predominantly covered by savannas and seasonally dry semideciduous forests lowered atmospheric co 2 concentrations in combination with a longer dry season caused expansion of dry forests and savannas during the last glacial period especially at the last glacial maximum these ecosystems persisted until the mid holocene although they underwent significant species reassortment forest communities containing a mixture of evergreen and semideciduous species began to expand between 6000 and 3000 14 c yr b p humid evergreen rain forests expanded to cover most of the area within the past 2000 14 c yr b p coincident with a reduction in fire frequencies comparisons between modern pollen spectra and vegetation reveal that the moraceae dominated rain forest pollen spectra likely have a regional source area at least 2 3 km beyond the lake shore whereas the grass and sedge dominated savanna pollen spectra likely have a predominantly local source area the holocene vegetation changes are consistent with independent paleoprecipitation records from the bolivian altiplano and paleovegetation records from other parts of southwestern amazonia the progressive expansion in rain forests through the holocene can be largely attributed to enhanced convective activity over amazonia due to greater seasonality of insolation in the southern hemisphere tropics driven by the precession cycle according to the milankovitch astronomical theory

unconfined alluvial flow processes recognition and interpretation of their deposits and the significance for palaeogeographic reconstruction abstract palaeogeographic interpretation of the sedimentary rock record depends on correct recognition from the preserved evidence of the processes responsible for transporting and depositing the sediment this in turn depends on robust knowledge transfer from previous workers and the successful exchange of ideas between workers requires consistent use of a well defined vocabulary we have identified serious breakdowns in all these interpretation steps in the case of terrestrial unconfined flow and its deposits and these failures are leading to unreliable environmental and climatic interpretation this is significant because such alluvial deposits commonly form a majority of the rock record of continental environments working from the basic principles of geomorphology and fluid dynamics we have undertaken a wide ranging analysis of the nature of out of channel flow and from this make predictions about the characteristics of its deposits we identify the range of possible locations and conditions that lead to the development of unconfined flow review the processes operating in each case and examine the range of lithological features that can be produced by these processes this allows us to evaluate the reliability of the criteria claimed for identification of out of channel flow deposits and examine how our new insights might alter palaeoclimatic and palaeogeographic reconstructions published previously by others the sedimentary record of unconfined flows is much more diverse and complex than usually portrayed the received wisdom that the record of unconfined flow consists solely of upwards fining thin beds produced from shallow waning flows is shown to be flawed a wide range of lithofacies are possible and the variation in both flow steadiness and uniformity needs to be taken into account the previously published criteria for recognition of flows of this type are not diagnostic of process or location unconfined flow deposits cannot reliably be identified from grain size or bed thickness similar lithofacies may develop in a wide range of geomorphic and climatic settings because the deposits solely reflect the local flow conditions and sediment availability we recommend that the terms sandflat and sheetflood should not be used in sedimentological accounts because there is no longer a safe informal usage for either contradictory application of these terms and lack of robust definitions is leading to significant misunderstanding of palaeogeography and process our analysis should improve reconstruction of past terrestrial environments because it reveals more clearly the true variety of possibilities for the occurrence of unconfined flow and the resultant deposits enhanced understanding of the inherent uncertainties and realisation of the wider range of plausible alternative explanations should help resolve apparent contradictions with independent indicators of climate or geographic position

upper crustal shortening and forward modeling of the himalayan thrust belt along the budhi gandaki river central nepal a balanced cross section along the budhi gandaki river in central nepal between the main central thrust including displacement on that fault and the main frontal thrust reveals a minimum total shortening of 400 km minimum displacement on major orogen scale structures include 116 km on the main central thrust 110 km on the ramgarh thrust 95 km on the trishuli thrust and 56 km in the lesser himalayan duplex the balanced cross section was also incrementally forward modeled assuming a generally forward breaking sequence of thrusting where early faults and hanging wall structures are passively carried from the hinterland toward the foreland the approximate correspondence of the forward modeled result to observe present day geometries suggest that the section interpretation is viable and admissible in the balanced cross section the trishuli thrust is the roof thrust for the lesser himalayan duplex the forward model and reconstruction emphasize that the lesser himalayan duplex grew by incorporating rock from the footwall and transferring it to the hanging wall along the main himalayan thrust as the duplex developed the lesser himalayan ramp migrated southward the movement of lesser himalayan thrust sheets over the ramp pushed the lesser himalayan rock and the overburdens of the greater and tibetan himalayan rock toward the erosional surface this vertical structural movement caused by footwall collapse and duplexing in combination with erosion exhumed the lesser himalaya

a multi proxy palaeoecological record of late holocene forest expansion in lowland bolivia abstract pollen charcoal and bulk sediment stable carbon isotope analysis of a 2 4 m sediment core from laguna yaguaru in eastern bolivia located near the modern ecotone between the humid evergreen amazon rainforest and the seasonally dry semi deciduous chiquitano dry forest provides a 5600 year record of environmental change in an area sensitive to changes in the strength of the south american monsoon the core consists of organic sediment interrupted by a 15 cm mineral facies deposited 5270 yr bp in a low energy flood event in the yaguaru basin pollen data indicate that the basin contained a cattail typha marsh prior to the flood some dry forest elements were present probably growing within small patches of forest within a matrix of generally open vegetation including marsh savanna and savanna woodland above the flood layer typha pollen is absent indicating open water conditions and pollen assemblages are similar to modern pollen spectra from bolivian dry forests around 1200 bp charcoal concentrations decrease carbon isotope ratios become more negative and celtis pollen rises dramatically signaling forest expansion and the development of the modern closed canopy dry forest at laguna yaguaru the wetter conditions implied by this forest expansion suggest the region was affected by the strengthening south american monsoon at this time the timing of the arrival of monsoon moisture at yaguaru is consistent with nearby records allowing a 700 year delay for the migration of the monsoon to reach the more southerly location of yaguaru

spatial and temporal evolution of a terminal fluvial fan system the permian organ rock formation south east utah usa the recognition of terminal fluvial systems otherwise termed terminal fans or distributary fluvial fan systems preserved in the ancient rock record is based primarily on the recognition of facies characteristics indicative of a progressive downstream decrease in i fluvial discharge ii channel depth and width iii lateral and vertical connectivity of channel fill elements and iv evidence for channellized flow and a systematic increase in i evidence for sheetflood deposition ii aeolian and or playa deposits and iii channel bifurcation however despite these criteria having been applied previously to a variety of outcrop successions there is still no unifying facies model that adequately accounts for the complex stratigraphic architectural relationships expected for such systems based on the varied styles of fluvial activity and system interaction known from modern examples moreover few previous studies have given significant consideration to the long term temporal evolution of terminal fluvial fans these issues are addressed by this study of the permian leonardian artinskian organ rock formation of the paradox basin south east utah a detailed stratigraphic framework based on 84 sedimentary logs demonstrates proximal to distal variations in sedimentary style integration of these data with high resolution architectural panels depicting the geometry and facies characteristics of individual fluvial elements has enabled the development of a series of depositional models that account for both the spatial and temporal evolution of the system and which are representative of i initial progradation of the fluvial system into the paradox foreland basin ii retreat of the fluvial system and expansion of a distal aeolian dune system iii the final phase of fluvial progradation following aeolian dune deflation and iv the final retrogradation of the fluvial system back towards the hinterland

contribution of columbia and gondwana supercontinent assembly and growth related magmatism in the evolution of the meghalaya plateau and the mikir hills northeast india constraints from u pb shrimp zircon geochronology and geochemistry abstract the meghalaya plateau and the mikir hills constitute a northeastern extension of the precambrian indian shield they are dominantly composed of proterozoic basement granite gneisses granites migmatites granulites the shillong group metasedimentary cover sequence and mesozoic tertiary igneous and sedimentary rocks medium to coarse grained equigranular to porphyritic cambrian granite plutons intrude the basement granite gneisses and the shillong group u pb shrimp zircon geochronology and geochemistry of the granite gneisses and granites have been carried out in order to understand the nature and timing of granite magmatism supercontinent cycles and crustal growth of the meghalaya plateau and mikir hills zircons from the rongjeng granite gneiss record the oldest magmatism at 1778 37 ma an inherited zircon core has an age of 2566 4 26 9 ma indicating the presence of recycled neoarchaean crust in the basement granite gneisses zircons from the sonsak granite have two ages 523 4 7 9 ma and 1620 8 9 2 ma which indicate partial assimilation of an older granite gneiss by a younger granite melt zircons from the longavalli granite gneiss of the mikir hills has a crystallization age of 1430 4 9 6 ma and a metamorphic age of 514 18 6 ma an inherited core of a zircon from longavalli granite gneiss has an age of 1617 1 14 5 ma zircons from younger granite plutons have cambrian mean ages of 528 7 5 5 ma kaziranga 516 9 0 ma south khasi 512 5 8 7 ma kyrdem and 506 7 7 1 ma and 535 11 ma nongpoh these plutons are products of the global pan african tectonothermal event and their formation markedly coincides with the later stages of east gondwana assembly 570 500 ma kuunga orogen the older inherited zircon cores 2566 4 26 9 ma 1758 1 54 3 ma 1617 1 14 ma imply a significant role for recycled ancient crust in the generation of cambrian granites thus the meghalaya plateau and mikir hills experienced major felsic magmatic episodes at 1800 ma 1600 ma 1400 ma and 500 ma with recycling of neoarchaean crust and later contributions from paleo mesoproterozoic granite gneiss sources a 258 20 ma lower intercept age of the rongjeng granite gneiss perhaps indicates a permo triassic thermal imprint on the meghalaya plateau the granite gneisses and granites have peraluminous to metaluminous compositions and syn orogenic to post collisional affinities we conclude that the orogenic history of the meghalaya plateau and the mikir hills records crustal growth of the columbia and gondwana supercontinents as noted in other pan african indian prydz brasiliano orogens

does a monsoon climate exist over south america the climatology and the basic state of the summertime circulation and rainfall over south america are studied using assimilation products from the data assimilation system of goddard earth observing system 1 geos1 and satellite derived rainfall results indicate the existence of a regional summer monsoon circulation regime induced by strong diabatic heating over the subtropical south american highland centered at the altiplano plateau sensitivity of the results to the assimilation scheme is tested by comparing that with the national centers for environmental prediction ncep reanalysis and with satellite rainfall estimates results show general agreement between the model produced rainfall anomaly and the satellite estimates as well as consistency between the basic circulation features in the geos 1 and the ncep reanalyses a case study of 1989 90 south american summer monsoon sasm reveals the following characteristics 1 in late spring the onset of sasm is signaled by an abrupt merging of the upper tropospheric double westerly jets one in the subtropics and the other in the subpolar region into a single jet in the midlatitudes this is followed by the establishment of a vortex to the southeast of altiplano and occurrence of heavy precipitation over subtropical eastern brazil 2 during the mature phase of sasm the heavy rainfall zone moves over the altiplano plateau and the southernmost brazilian highland the fully established sasm features are the following a an enhancement of equatorial north atlantic trade wind which emanates from the sahara high and crosses the equator over the south american continent b a buildup of strong northwesterlies along the eastern side of the tropical andes and c development of the south atlantic convergence zone in the southernmost position with strong convective activity meanwhile the upper tropospheric return flow emerges from an anticyclone formed over the altiplano plateau crosses the equator and sinks over northwestern africa 3 the withdrawal of sasm in late summer is signaled by the resplitting of the midlatitude westerly jet at the same time the low level northwest monsoon flow diminishes reducing the moisture supply and leading to the termination of heavy precipitation over the subtropical highland results also show that the above mentioned characteristics of sasm are clearly linked to the tropospheric temperature changes over the central south american highland sensible versus latent heating over the highland are bound to play an important role in the evolution of sasm to provide further support of presence of a monsoon climate over south america sasm is compared and contrasted to the classic east asian summer monsoon easm many similar features including evolution characteristics between the two systems have been identified contrasting aspects of the sasm from the easm are also discussed it is pointed out that a number of monsoonal characteristics of the climate of south america such as the seasonal reversal of the low level wind become apparent only when the strong annual mean wind is removed based on the characteristic features and their evolution the authors conclude that a monsoon climate does exist over south america

sm nd age of the garhwal bhowali volcanics western himalayas vestiges of the late archaean rampur flood basalt province of the northern indian craton abstract whole rock sm nd isotope dating of the garhwal and bhowali mafic volcanics of the kumaun lesser himalayan sector yield an age of 2 51 0 08 ga and an initial nd 5 1 0 5 this age and initial nd are the same as that of the rampur mandi volcanics which occur to the west in the lesser himalayas in addition to confirming the widespread presence of the late archaean crustal elements in the himalayas these data constrain the age of the oldest rifting event and deposition of supracrustal sediments in the himalayas and set at rest the long standing controversy about the age and inter relationship of these mafic volcanics despite their present day disparate occurrence as a result of the post eruption deformation the area covered by these mafic volcanics is large enough to classify them as flood basalts the rampur flood basalt rfb province rift related volcanism of the same age is known from the contiguous aravalli region of the nw indian shield the orientation of the rifts in the two regions suggests the possibility of a mantle plume setting for the rfb

neotectonics of the subandes brazilian craton boundary using geomorphological data the maranon and beni basins abstract active and abandoned fluvial traces are used together with neotectonic seismotectonic and subsurface structural data to study the neotectonic evolution of the peruvian and bolivian foreland basins the maranon basin to the north and the beni basin to the south are located near the ends of the peru bolivia andean segment this segment lies above the flat slab subduction of the nazca plate beneath the andes the surface of these basins shows a complex network of present day rivers and fossil river traces a relative chronology of the river traces deduced from morphological criteria allows the identification of successive shifting of fluvial belts up to the present position of the main rivers as it enters the ucamara depression the ucayali river is deflected to the northeast successive shifts of the deflection point are directed upstream along the upper reaches of the river along the foothills simultaneously the maranon river is deflected to the north lining up with the straight ne trending lower reaches of the huallaga river in the central part of the depression the rivers trend northeast southwest parallel to the main strike of the basement faults this regional trend is also parallel to 1 the greatest principal stress observed both to the east and the west of the depression and to 2 the neotectonic faults on the margin of the brazilian craton the beni basin is characterized by a counterclockwise shift of the beni river successive shifts of the river involve the northward migration of the deflection point of the beni river as it enters the basin a fault connected to the foothill margin controls this downstream movement recent erosion of the flood plain surface to the east suggests a tendency of the craton margin to ascend no coaxial stress resulting from either plate motion or from collapse of the andean topography seems to control the directional shift of rivers in the basins

age and geochemistry of the paleoproterozoic bhatwari gneiss of garhwal lesser himalaya nw india implications for the pre himalayan magmatic history of the lesser himalayan basement rocks

structural and thermochronological studies of the almora klippe kumaun nw india implications for crustal thickening and exhumation of the nw himalaya

proterozoic crustal evolution in the nw himalaya india as recorded by circa 1 80 ga mafic and 1 84 ga granitic magmatism abstract single zircon dating of the rampur metabasalts of the larji kullu rampur window in the lesser himalayas yielded an evaporation age of 1800 13 ma the zircon age is considerably younger than the previously published whole rock sm nd age of 2510 90 ma suggesting that the sm nd age may be geologically meaningless and that the sm nd whole rock array may have resulted from mixing in the nw himalaya there is also evidence for extensive silicic melt generation in the paleoproterozoic zircons from a metarhyodacite in the larji kullu rampur window yielded an evaporation age of 1840 16 ma which we interpret as the minimum age of magmatism the main central thrust granitic mylonites are interpreted as the basement of the neoproterozoic haimanta group metasediments together with the granitic rocks from the lesser himalaya they were derived from pre existing continental crust prior to 1 84 ga the nd depleted mantle model ages are in the range of 2 6 2 4 ga suggesting a contribution of archean crust a recycled archean component is also documented by a 2 9 ga domain in one of the zircons

cambrian biostratigraphy of the tal group lesser himalaya india and early tsanglangpuan late early cambrian trilobites from the nigali dhar syncline precise biostratigraphic constraints on the age of the tal group are restricted to 1 a basal level correlative with the anabarites trisulcatus protohertzina anabarica assemblage zone of southwest china 2 a level near the boundary of the lower and upper parts of the tal group correlative with the early tsanglangpuan stage drepanuroides zone and 3 an interval low in the upper part of the tal group correlative with later in the tsanglangpuan stage palaeolenus zone these correlations are based on small shelly fossil and trilobite taxa other chronostratigraphic constraints include the marked negative 13 c isotopic excursion coincident with the transition from the krol group to the tal group this excursion is used as a proxy for the precambrian cambrian boundary in several sections worldwide and if applied to the lesser himalaya indicates that the boundary is at or just above the base of the tal group the upper parts of the tal group may be of middle or late cambrian age and might form proximal equivalents of sections in the zanskar spiti region of the tethyan himalaya both faunal content and lithological succession are comparable to southwest china furthering recent arguments for close geographic proximity between the himalaya and the yangtze block during late neoproterozoic and early cambrian time trilobites from the uppermost parts of the sankholi formation from the nigali dhar syncline are described and referred to three taxa one of which drepanopyge gopeni is a new species they are the oldest trilobites yet described from the himalaya

mid proterozoic pb pb ages for some himalayan base metal deposits and comparison to deposits in rajasthan nw india abstract lead isotope ratios from three base metal deposits rangpo gorubathan in the eastern lesser himalaya and bageswar in the lesser himalaya of kumaun have been studied the first two deposits generally stratiform and stratabound occur within pelitic psammopelitic wacke type metasediments which are locally carbonaceous the bageswar deposit is hosted by silicified dolomitic carbonate rocks the pb isotope ratios appear to be homogenous in two and heterogenous in one of the deposits model pb pb ages obtained for the rangpo and gorubathan deposits are 1800 ma and for the bageswar deposit 1550 1700 ma dispersion in isotopic ratios reflect the geochemical heterogeneity particularly with respect to larger variation in the composition of pb isotope ratios in the bageswar deposit may be explained by the participation of pb from heterogenous sources pb isotope characteristics of the himalayan deposits are compared with those of a number of base metal deposits in the delhi aravalli belt in rajasthan nw india ore lead from the deposits of both the himalaya and of rajasthan are enriched in 206pb and 207pb however lead in all cases plots on the evolution lines corresponding to sources with higher u pb ratios but the sources of lead in the himalayan deposits had a lower th u ratio the geologic observations as well as the pb isotope compositions are consistent with a sedimentary diagenetic origin of the himalayan base metal deposits

ion microprobe 207pb 206pb ages of zircons from the bundelkhand massif northern india implications for crustal evolution of the bundelkhand aravalli protocontinent abstract 207pb 206pb ages of zircon from gneissic rocks and granitoids representing all major lithological units of the bundelkhand massif northern india have been determined using an ion microprobe the presence of early archaean granitic crust in this region is established from these ages ages for the kuraicha and the mahoba gneisses and associated metabasic enclaves suggest the oldest partial and metamorphic melting event in this region occurred at 3 3 ga the presence of younger gneisses with ages of 2 7 and 2 5 ga suggests the existence of multiple protolith age components within the massif three major granitoid suites hornblende biotite leuco granitoids were emplaced at 2 5 ga within a time span of a few tens of millions of years felsic volcanism also occurred around this time an age of 2 49 ga for the youngest phase of leucogranitoid marks the time of overall stabilization of this massif the geochronological history of the bundelkhand massif appears to be remarkably similar to that of the basement complex of the aravalli delhi orogenic belt another prominent archaean crustal block to the west of this massif geochemical features of the different rock types in these two regions are also very similar we propose that these two apparently discrete blocks evolved as a single crustal unit with stabilization taking place at 2 5 ga

nature of magmatism and sedimentation at a columbia active margin insights from combined u pb and lu hf isotope data of detrital zircons from nw india abstract detrital zircons from a palaeoproterozoic quartzite deposited between 1 85 and 1 82 ga in the northern aravalli orogen of nw india show a distinctive age peak of ca 1 85 ga and variable but largely subchondritic ehf1 85 ga between 1 3 and 21 0 corresponding to hafnium model ages of 2 5 to 3 6 ga these data indicate an important period of reworking of ancient eo to neoarchaean strongly heterogeneous continental crust at this time prevalence of ca 1 85 ga subduction related granitoids almost identical u pb age spectra and ehft of detrital zircons in ca 1 85 ga metasedimentary rocks in the aravalli orogen and the inner lesser himalaya indicate similar sediment provenances and thus a geological connection between these two terranes during late palaeoproterozoic all together the data constrain a rapid succession of sedimentation metamorphism and subduction related magmatic activity and support the interpretation of an active geodynamic realm along the entire north indian margin at ca 1 85 ga comparison of detrital zircon data in conjunction with published paleomagnetic data from north india and other crustal blocks of the columbia supercontinent additionally suggest a close affinity of north india with madagascar the cathaysia block of south china and south korea during columbia times

thrust geometries and deep structure of the outer and lesser himalaya kumaon and garhwal india implications for evolution of the himalayan fold and thrust belt the kumaon garhwal region of the himalaya lies near the center of the himalayan fold and thrust belt we have drawn two balanced cross sections 100 km apart through the outer and lesser himalaya the cross sections incorporate all the surface well log and earthquake seismic data currently available from the region two branch line maps showing trailing and leading branch lines and cutoff lines of the major thrusts in the region are also drawn the three dimensional deep structure of the outer and lesser himalaya is interpreted based on the balanced cross sections and the branch line maps deep structure of the higher and tethyan himalaya is extrapolated based on surface geology and is subject to revision as more surface and seismic data become available from these areas a sequential evolutionary model for the kumaon himalaya along the eastern pindari section is proposed according to this model the kumaon himalaya evolved by an overall forelandward progression of thrusting with some reactivation along the munsiari thrust mt the main boundary thrust mbt and the main central thrust mct we use structural stratigraphic and radiometric criteria to place time constraints on the motion of these thrusts earliest motion along the mbt may have occurred in early middle paleocene but the main episode probably started in late eocene and may still be continuing emplacement of the mt had occurred by middle late eocene whereas the mct shows activity around 20 ma thus exhibiting break back thrusting with respect to the mt shortening estimates are obtained from the pindari section minimum shortening in the sedimentary thrust sheets of the outer and lesser himalaya is 161 km or 65 as a first approximation we have also restored the crystalline sheets in order to obtain shortening estimates for the entire himalaya minimum shortening for the himalaya after restoring the mct sheet varies from 354 76 to 421 km 79 these estimates were further combined with published data from the area between the mct sheet and the indus tsangpo suture zone itsz minimum shortening between the indo ganga foreland and the itsz thus obtained lies in the range 687 754 km or 69 72 we compare our shortening estimates with those available from the pakistan and nepal himalaya

himalayan architecture constrained by isotopic tracers from clastic sediments we report the first geochemical traverse to integrate u pb ages and hf data on single detrital zircons with bulk rock sm nd rb sr isotopic measurements across the breadth of the himalayan orogen in northwest india our dataset from key tectonostratigraphic units in the sutlej valley provides detailed insights into both the provenance of the sedimentary formations and their tectonic history whole rock nd data provide information on the mean age of source areas contributing detritus to the sedimentary units in the mountain belt regardless of their current metamorphic grade u pb ages of detrital zircons from the same units identify the age of melt formation in the source rocks while lu hf isotope data from zircons resolve discrete crust formation ages in the multiple eroding terrains complex whole rock sr systematics hint at the extent of pre himalayan thermal events our data confirm the geochemical distinction between the older lesser himalayan formations and those of the depositionally younger units including the high himalayan crystalline series that has been recorded along the strike of the himalaya from the western syntaxis to nepal palaeoproterozoic metasediments of the jutogh group and rampur formation yield end 500 values below 17 whereas neoproterozoic and younger metasediments yield end 500 above 13 the latter group encompasses the vaikrita group gneisses of the high himalayan crystallines and the overlying haimanta group but also includes a number of units in thrust sheets north of the main boundary thrust in the lesser himalayan zone designated the douter lesser himalayat to distinguish them from the palaeoproterozoic dinner lesser himalayat zircon populations from the inner lesser himalaya are characterised by palaeoproterozoic late archaean ages 2 6 1 8 ga whereas the depositionally younger units contain populations both of this age and a younger period meso to neo proterozoic 1 1 0 8 ga detrital zircon ages are younger than their respective hf isotope derived crustal formation ages by 0 7 2 1 ga indicating that the source regions of the detrital zircons consisted of older terranes with considerable amounts of reworking and renewed magmatism the

a 45 kyr palaeoclimate record from the lowland interior of tropical south america we present a well dated high resolution 45 kyr lake sediment record reflecting regional temperature and precipitation change in the continental interior of the southern hemisphere sh tropics of south america the study site is laguna la gaiba llg a large lake 95 km2 hydrologically linked to the pantanal an immense seasonally flooded basin and the world s largest tropical wetland 135 000 km2 lake level changes at llg are therefore reflective of regional precipitation we infer past fluctuations in precipitation at this site through changes in i pollen inferred extent of flood tolerant forest ii relative abundance of terra firme humid tropical forest versus seasonally dry tropical forest pollen types and iii proportions of deep versus shallow water diatoms a probabilistic model based on plant family and genus climatic optima was used to generate quantitative estimates of past temperature from the fossil pollen data our temperature reconstruction demonstrates rising temperature by 4 c at 19 5 kyr bp synchronous with the onset of deglacial warming in the central andes strengthening the evidence that climatic warming in the sh tropics preceded deglacial warming in the northern hemisphere nh by at least 5 kyr we provide unequivocal evidence that the climate at llg was markedly drier during the last glacial period 45 0 12 2 kyr bp than during the holocene contrasting with sh tropical andean and atlantic records that demonstrate a strengthening of the south american summer monsoon during the global last glacial maximum 21 kyr bp in tune with the 20 kyr precession orbital cycle holocene climate conditions occurred as early as 12 8 12 2 kyr bp when increased precipitation in the pantanal catchment caused heightened flooding and rising lake levels in llg in contrast to this strong geographic variation in lgm precipitation across the continent expansion of tropical dry forest between 10 and 3 kyr bp at llg strengthens the body of evidence for widespread early mid holocene drought across tropical south america

the fluvial record of climate change fluvial landforms and sediments can be used to reconstruct past hydrological conditions over different time scales once allowance has been made for tectonic base level and human complications field stratigraphic evidence is explored here at three time scales the later pleistocene the holocene and the historical and instrumental period new data from a range of field studies demonstrate that croll milankovitch forcing dansgaard oeschger and heinrich events enhanced monsoon circulation millennial to centennial scale climate variability within the holocene probably associated with solar forcing and deep ocean circulation and flood event variability in recent centuries can all be discerned in the fluvial record although very significant advances have been made in river system and climate change research in recent years the potential of fluvial palaeohydrology has yet to be fully realized to the detriment of climatology public health resource management and river engineering

geochronology and stratigraphy of late pleistocene lake cycles on the southern bolivian altiplano implications for causes of tropical climate change large paleolakes 33 000 60 000 km2 that once occupied the high altitude poopo coipasa and uyuni basins in southern bolivia 18 22 s provide evidence of major changes in low latitude moisture in these now dry or oligosaline basins extensive natural exposure reveals evidence for two deep lake and several minor lake cycles over the past 120 k y fifty three new u th and 87 new 14c dates provide a chronologic framework for changes in lake level deposits from the ouki deep lake cycle are extensively exposed in the poopo basin but no deep lakes are apparent in the record between 98 and 18 1 ka the ouki lake cycle was 80 m deep and nineteen u th dates place this deep lake cycle between 120 and 98 ka shallow lakes were present in the terminal uyuni basin between 95 and 80 ka salinas lake cycle at ca 46 inca huasi lake cycle and between 24 and 20 5 ka sajsi lake cycle the tauca deep lake cycle occurred between 18 1 and 14 1 ka resulting in the deepest 140 m and largest lake in the basin over the past 120 ka multiple 14c and u th dates constrain the highest stand of lake tauca along a topographically conspicuous shoreline between 16 4 and 14 1 ka a probable post tauca lake cycle the coipasa produced a 55 m deep lake that is tentatively dated between 13 and 11 ka we suggest that paleolakes on the bolivian altiplano expanded in response to increased moisture in the amazon and enhanced transport of that moisture onto the altiplano by strengthened trade winds or southward displacement of the intertropical convergence zone itcz pole to equator sea surface temperature sst and atmospheric gradients may have influenced the position of the itcz affecting moisture balance over the altiplano and at other locations in the amazon basin links between the position of the itcz and the ca 23 ka precessional solar cycle have been postulated march insolation over the altiplano is a relatively good fit to our lake record but no single season or latitude of solar cycling has yet to emerge as the primary driver of climate over the entire amazon basin temperature may influence altiplano lake levels indirectly as potentially dry glacial periods in the amazon basin are linked to dry conditions on the altiplano intensification of the trade winds associated with la nina like conditions currently brings increased precipitation on the altiplano and deep lake development during the tauca lake cycle coincided with apparently intense and persistent la nina like conditions in the central pacific this suggests that sst gradients in the pacific are also a major influence on deep lake development on the altiplano

preliminary stratigraphic and structural architecture of bhutan implications for the along strike architecture of the himalayan system article i nfo preliminary mapping and stratigraphic correlation of lesser himalayan rock in eastern bhutan using field characteristics u pb detrital zircon dating and nd geochemistry define the first order stratigraphic architecture of the indian passive margin sequence in the eastern himalaya we use this new image of the lateral and vertical relationships of the original stratigraphy to determine the structural framework of the eastern himalayan fold thrust belt in bhutan we propose that lesser himalayan rock in bhutan can be divided into lower lesser himalayan rocks with a paleoproterozoic detrital zircon signal and upper lesser himalayan rocks with detrital zircon signals of 1000 500 ma the 500 ma detrital grains are from rocks in the frontal portions of the fold thrust belt north of the main boundary thrust as well as directly in the footwall of the main central thrust in the hinterland our preliminary stratigraphic study coupled with mapping allows us to construct a composite balanced cross section through the kuru chu valley in eastern bhutan which provides the first image of the geometry and amount of shortening through bhutan the main frontal thrust tilts a 6 km of thick section of neogene foreland basin deposits these deposits are separated from folded and faulted upper lesser himalayan rocks by the main boundary thrust north of the main boundary thrust we propose two duplex systems the southernmost duplex contains 9 repeated sections of upper lesser himalayan units including the permian gondwana sequence and the cambrian or younger baxa group the northernmost duplex is located in the footwall of the main central thrust and is comprised of two repeated sections of the proterozoic shumar and daling formations the southern boundary of this northern duplex is the shumar thrust which acts as a roof thrust for the southern duplex system and may correlate to the ramgarh thrust in nepal and india we propose that the development of the duplex systems passively folded overlying tethyan and greater himalayan rocks minimum shortening for this part of the himalayan fold thrust belt is 359 km all of which has occurred from 22 ma to present defining a long term shortening rate of 16 mm yr

mid to late holocene fluvial activity behind pre columbian social complexity in the southwestern amazon basin the scale spatial variability and implications of pre columbian human induced changes in the amazon basin are controversial while some scholars believe that large settlements and complex societie

processes facies and architecture of fluvial distributary system deposits abstract there is evidence from the stratigraphic record of examples of fluvial deposits that were the products of deposition from river systems which had decreasing discharge down flow and transitions from proximal channelised to distal unconfined flow these deposits form fan shaped bodies several tens of kilometres in radius and their stratigraphic architecture is aggradational with no evidence of deep incision driven by base level fall the fluvial systems that generated these deposits formed under conditions for which there is no complete analogue today an endorheic basin with a relatively arid climate adjacent to an uplifted area with higher precipitation a conceptual model for fluvial systems of this type has therefore been built on the basis of outcrop examples and a consideration of the controls on sedimentation proximal areas are characterised by amalgamated coarse pebbly and sandy channel deposits with little preservation of overbank facies channel dimensions are generally smaller in the medial areas but sizes are variable deposits are of braided meandering and simple channels which show varying degrees of lateral migration the channel fills may be mud or sand with overbank flow processes playing an important role in filling channels abandoned on the floodplain after avulsion the proportion of overbank deposits increases distally with sheets of sand deposited as lateral and terminal splays by unconfined flow interconnection of sandstone bodies is poor in the distal areas because channel fill bodies are sparse small and are not deeply incised the radial pattern of the sediment body forms by the repeated avulsion of channels active channels build up lobes on the alluvial plain and rivers switch position to follow courses on lower lying areas the term fluvial distributary system is here used to describe a river system which has a downstream decrease in discharge and has a distal zone which is characterised either by terminal splays on to a dry alluvial plain or a lake delta during periods of lake highstand

thrusting and back thrusting as post emplacement kinematics of the almora klippe insights from low temperature thermochronology abstract crystalline klippen over the lesser himalayan sequence lhs in the kumaon and garhwal regions of nw himalaya are the representative of southern portion of the main central thrust mct hanging wall these were tectonically transported over the juxtaposed thrust sheets berinag tons and ramgarh of the lhs zone along the mct these klippen comprise of nw se trending synformal folded thrust sheet bounded by thrusts in the south and north in the present study the exhumation histories of two well known klippen namely almora and baijnath and the ramgarh thrust sheet in the kumaon and garhwal regions vis a vis himalayan orogeny have been investigated using apatite fission track aft ages along a 60 km long orogen perpendicular transect across the almora klippe and the ramgarh thrust sheet 16 aft cooling ages from the almora klippe and 2 from the ramgarh thrust sheet have been found to range from 3 7 0 8 to 13 2 2 7 ma and 6 3 0 8 to 7 2 1 0 ma respectively from lhs meta sedimentary rocks only a single aft age of 3 6 0 8 ma could be obtained three aft ages from the baijnath klippe range between 4 7 0 5 and 6 6 0 8 ma aft ages and exhumation rates of different klippen show a dynamic coupling between tectonic and erosion processes in the kumaon and garhwal regions of nw himalaya however the tectonic processes play a dominant role in controlling the exhumation thrusting and back thrusting within the almora klippe and ramgarh thrust sheet are the post emplacement kinematics that controlled the exhumation of the almora klippe combining these results with the already published aft ages from the crystalline klippen and the higher himalayan crystalline hhc the kinematics of emplacement of the klippen over the lhs and exhumation pattern across the mct in the kumaon and garhwal regions of nw himalaya have been investigated

tectonics and surface effects of the supercontinent columbia abstract assembly of the supercontinent columbia at about 1 85 1 90 ga coincided with several events that affected the entire earth the oldest worldwide network of orogenic belts formed at the same time although some granite granodiorite gg suites had formed earlier the gg suites became common in the 1 8 1 9 ga orogenic belts these suites succeeded the older tonalite trondhjemite granodiorite ttg suites which were not produced after 1 8 ga changes on the earth s surface at 1 8 1 9 ga include rapid increase in the concentration of oxygen in the atmosphere and oceans and probably the evolution of eukaryotes all of these surface changes occurred as columbia accreted and the assembly of columbia may have contributed to the drastic changes in the earth s surface environment as well as to the evolution of primitive life forms

the kumaun and garwhal lesser himalaya india part 1 structure and stratigraphy our understanding of the geologic evolution of the himalaya remains incomplete particularly in regard to structural and geochronologic details of the proterozoic paleozoic lesser himalayan sequence we conducted an integrated field mapping geochronological study and geochemical analysis of the lesser himalayan sequence strata in the kumaun and garwhal himalaya of nw india 78 00 80 30 e structural observations reveal a systematic change in deformation styles from lower to higher structural levels in the main central thrust footwall in the south and at lower structural levels the main central thrust footwall is characterized by parallel folding and sparse development of axial cleavage quartz microstructures indicate that deformation occurred at temperatures below 350 c in contrast in the north and at higher structural levels close to the main central thrust deformation is characterized by replacement of original bedding the development of penetrative cleavage and schistosity locally folds are tight in places isoclinal and overturned the corresponding quartz microstructures indicate that deformation occurred above 350 c dating of detrital zircons from lesser himalayan sequence metasedimentary units and igneous zircons from schistose gneisses throughout the lesser himalayan sequence allow refinements to the stratigraphic framework first unlike has been observed in western nepal immediately west of our study area u pb zircon dating suggests the absence of paleoproterozoic strata in the ramgarh thrust hanging wall this suggests that the thrust cuts upsection laterally along strike second u pb detrital zircon dating suggests that metasedimentary strata in the ramgarh thrust hanging wall are correlative with basal units of the outer lesser himalayan sequence removing the necessity that the ramgarh thrust hanging wall was allochthonous with respect to its footwall lesser himalayan sequence units schistose gneisses with u pb ages of ca 1850 ma are older than lesser himalayan sequence units in the area suggesting that they are thrust emplaced lesser himalayan sequence basement we conclude that the ramgarh thrust is an out of sequence thrust postdating a folding event in its footwall the earlier proposal that neoproterozoic cambrian lesser himalayan sequence strata are the southern extension of the tethyan himalayan sequence requires that the tons thrust which separates the distal and proximal facies of the lesser himalayan sequence be a major south directed structure with a slip magnitude of 50 100 km

the kumaun and garwhal lesser himalaya india part 2 thermal and deformation histories an integrated petrologic thermochronologic and numerical modeling study constrains the thermal evolution of the lesser himalayan sequence in the kumaun and garwhal regions of india south of the tons thrust peak metamorphic temperatures do not exceed 330 c on the northern side a transition toward higher temperatures adjacent to the main central thrust hanging wall is documented in the immediate footwall of the main central thrust peak temperatures average 550 c an inverted thermal field gradient of 30 c km beneath the vaikrita thrust is calculated an inverted thermal field gradient of 20 c km is also documented beneath a klippe of the main central thrust hanging wall 25 km south of the vaikrita thrust and a peak footwall temperature of 530 c is recorded close to the shear zone at the base of the klippe the mechanism for thermal metamorphism in the lesser himalayan sequence is interpreted to be conduction of heat from the main central thrust fault and or hanging wall to the footwall a localized thermal field gradient is also associated with the tons thrust the 40 ar 39 ar thermochronology reveals that the exposed rock in the kumaun and garwhal lesser himalaya underwent cooling below the closure temperature tc for white mica at different times in the late tertiary the proximal footwall of the munsiari thrust experienced cooling below the white mica tc at ca 8 5 ma south of the munsiari thrust in the proximal footwalls of the chaukori and askot klippen muscovites ages are ca 11 5 ma 3 ma older than samples to the north the age differences in samples from northern and southern exposures are interpreted to document cooling related to thermal relaxation following passage of the main central thrust hanging wall atop the lesser himalayan sequence results of thermal modeling achieve good fits to the data when scenarios involve an early miocene phase of overthrusting of a hot hanging wall over a downgoing footwall followed by the initiation of a duplex within lesser himalayan sequence rocks

luminescence dating of the niederweningen mammoth site switzerland abstract luminescence dating is used to further constrain the so far controversial age of the mammoth site at niederweningen switzerland gamma spectrometric analyses reveal the presence of radioactive disequilibria in the uranium decay chain in most of the samples but these cause only minor uncertainties in the age estimates the major uncertainty is actually related to the problem of estimating the average water content for the whole burial time nevertheless even considering these problems involved with dose rate determination the dating results clearly indicate that the peat is significantly younger than early wurmian in contrast to some previous assumptions together with the results of radiocarbon dating and pollen analyses the luminescence ages point to a correlation of the mammoth peat with interstadial conditions during the early part of the middle wurmian at around 45 ka ago

evaluation of a wide range laser diffraction grain size analyser for use with sediments the qualities of a laser diffraction grain size analyser the coulter ls 100 range claimed by the manufacturer 0 4 900 mum in a single measurement are evaluated on sediments of fluvial and lacustrine origin accuracy and resolution of measurement on standard latex spheres are excellent reproducibility of the results on natural sediments appears to be satisfactory but the method underestimates the fraction of clay particles with an efficiency of detection 36 70 proportional to the clay content determined from pipette analysis this efficiency is somewhat higher than those reported from other instruments of the same generation comparison of the coulter ls 100 with other sizing techniques shows good agreement with the sieving method but some differences appear with the electroresistance particle technique median and mean size values measured by the coulter counter ta1 are systematically lower than those obtained by the coulter ls 100 analyses show good correlation with those of a malvern laser particle analyser but a discrepancy appears with very fine silt and clay sized sediments the coulter ls 100 detects a higher clay content than that measured with the malvern laser sizer 2600 except when precise measurements of clay content are needed the coulter ls 100 produces precise and accurate results in size ranges required for geological and environmental studies

geometry and kinematics of the darjeeling sikkim himalaya india implications for the evolution of the himalayan fold thrust belt abstract the darjeeling sikkim himalaya lies in a zone where the convergence vector between the indian and eurasian plates is almost perpendicular to the himalayan deformation front current outcrop pattern preserves high grade hanging wall rocks of the main central thrust mct and the pelling thrust pt within 5 km of the mountain front u pb shrimp analysis on zircon grains from mct sheet record two 206pb 238u age groups of ca 2575 and ca 943 ma and from the pt 1835 13 ma the lesser himalayan duplex in the footwall of the mct is composed of two duplex systems a structurally higher hinterland dipping duplex and a lower duplex system that varies from hinterland dipping in the north to an antiformal stack in the middle and foreland dipping duplex in the south the foreland dipping horses and reactivation of the roof thrust resulted in large translation of the overlying mct and pelling thrust sheets the duplex also resulted in a plunge culmination forming a broad n s trending anticline a transport parallel restorable balanced cross section reveals a minimum shortening of 450 km 81 south of the south tibetan detachment system and an average long term shortening rate 20 mm yr although the study area lies in a zone of high convergence angle with the deformation front the surface structures do not record the greatest shortening of the orogen the shillong plateau and a tranverse zone lying southeast of the study area have accommodated a part of the convergence related shortening thereby under representing the shortening in the sikkim himalayan fold thrust belt lateral variations in the original width of the lesser himalayan basin may have played a role in partitioning the shortening in the fold thrust belt

geodynamic environment of quaternary morphostructures of the subandean foreland basins of peru and bolivia characteristics and study methods abstract the quaternary evolution of western amazon basins is considered using morphostructural analysis of fluvial belts and lakes meandering rivers from the basin surface are analyzed using three scales of elements the ridge and swale pattern the mosaic pattern and the fluvial belt lake patterns are differentiated according to their shape the hydrological environment and the structural context influences of basement structures and neotectonic deformations when known are considered the morphostructural patterns are discussed in the context of a balance of internal tectonic and external hydraulic and sedimentologic processes special attention is focussed on the effect of neotectonics which is generally neglected on the formation and the location of quaternary morphostructures the adaptation of study methods and sampling to the characteristics of the different morphostructures is considered

landscape changes in the southern amazonian foreland basin during the holocene inferred from lake ginebra beni bolivia our study is located in northern beni and aims to improve knowledge on regional landscape changes from the last 8600 years based on pollen and charcoal analyses from a lacustrine sediment core from lake ginebra our results showed that gallery forest and lacustrine sediment were observed from 8645 until 3360 cal yr bp after a change from a lacustrine to a swamp environment at 1700 cal yr bp the cerrados and the mauritia swamp became installed 1000 years ago on our study site the environmental changes we observed over the last 8600 years in the ginebra record reinforce the evidence of a west east climatic gradient with the persistence of rain forest throughout the holocene on the western side and the presence of the cerrados until the late holocene on the eastern side moreover the persistence of a wet forest in the early to mid holocene in southwestern amazonia highlighted some local responses to the global trend that could be related to the distance from the andes while in the late holocene both an increase in insolation and strengthening of the south american summer monsoon system enabled the installation of a seasonal flooded savanna in northern beni and of the rain forest in eastern beni

soft sediment deformation structures from the paleoproterozoic damtha group of garhwal lesser himalaya india abstract soft sediment deformation ssd structures are observed in the paleoproterozoic damtha group in the garhwal lesser himalaya for a lateral extent of over 110 km in comparable litho sections the ssd structures are primarily observed in medium to fine grained sandstones siltstones and shale at sand mud interfaces that were deposited in an inter tidal to supra tidal setting of the shallow marine platform the ssd structures are conspicuously absent in horizons within facies dominated by a single grain size the ssd structures include load structures namely drops pseudonodules and ball and pillow structures along with contorted beds slumps cusps and water escape structures of varying geometry and dimensions the ssd structures are invariably bounded by undeformed beds and their dimensions depend on the thickness of bed affected by liquefaction the observed ssd structures show distinct similarities with liquefaction induced deformation structures in geological records and those produced in the laboratory under reverse and normal density gradient conditions the recurring nature of ssd structures in the stratigraphic column over large spatial distances points toward a trigger mechanism with recurrent activity that affected the basin on a regional scale such as prolonged rifting of the damtha basin during the paleoproterozoic this would have caused frequent earthquakes and shakes of the basin floor producing recurrent ssd structures in view of the similarity with seismites and absence of a viable alternative rift related seismicity is preferred as the trigger mechanism for the observed ssd structures in the damtha group

holocene paleohydrology and glacial history of the central andes using multiproxy lake sediment studies here we document at century to millennial scale the regional changes of precipitation evaporation from the late pleistocene to present with multiproxy methods on a north south transect of lake sites across the eastern cordillera of the central andes the transect of study sites covers the area from v14 s to 20 s and includes core studies from seven lakes and modern calibration water samples from twenty three watersheds analyzed to constrain the down core interpretations of stable isotopes and diatoms we selected lakes in different hydrologic settings spanning a range of sensitivity to changes in the moisture balance these include 1 lakes directly receiving glacial meltwater 2 overflowing lakes in glaciated watersheds 3 overflowing lakes in watersheds without active glaciers and 4 lakes that become closed basins during the dry season the results of our current work on multiple lakes in the bolivian andes show that while the overall pattern of holocene environmental change is consistent within the region conditions were not always stable over centennial to over millennial timescales and considerable decadal to centuryscale climate variability is evident abbott et al quat res 47 1997 70 80 quat res 47 1997 169 180 quat sci rev 19 2000 1801 1820 polissar master s thesis university of massachusetts 1999 comparison of the paleoclimate record from one well studied site lago taypi chaka kkota ltck with others within the region illustrates a consistent overall pattern of aridity from the late glacial through the middle holocene previous work noted a difference between the timing of water level rise in lake titicaca v5 0 3 5 ka b p abbott et al quat res 47 1997 169 180 cross et al holocene 10 2000 21 32 rowe et al clim change 52 2002 175 199 and the onset of wetter conditions at 2 3 ka b p in ltck a lake that drains into the southern end of lake titicaca abbott et al quat res 47 1997 70 80 sedimentary and oxygen isotope evidence from paco cocha 13 54ps located in the northern reaches of the expansive 57 000 km 2 titicaca watershed which spans v14 s to 17 s indicates that

using isotopic and chronologic data to fingerprint strata challenges and benefits of variable sources to tectonic interpretations the paro formation bhutan himalaya 1 we combine detrital zircons dz and epsilon neodymium nd signatures with field mapping in the paro formation in western bhutan dz age spectra are strongly variable and display signatures that have been used to uniquely identify both greater himalayan gh and lesser himalayan lh strata dz age peaks from six quartzite samples require sources for 0 5 0 8 1 2 1 4 1 7 1 8 and 2 5 ga zircons in the paro formation the youngest 0 5 ga zircons argue for a cambrian maximum deposition age two samples have a youngest 1 8 ga peak typically attributed to paleoproterozoic lh rocks a 450 ma crystallization age from two granite samples constrains the minimum deposition age as ordovician new nd signatures from six detrital samples from the paro formation show significant variation with lithology schists have nd 0 values between 12 0 and 16 9 while quartzite values vary between 18 8 and 24 5 these data imply that the paro formation was derived from both young and old sources with dz and nd values obtained from the same quartzite samples requiring old detritus while the nd values obtained from interbedded schist require younger detritus using published isotopic and chronologic definitions of himalayan strata schist rich layers wouldbeconsideredgh whiletheinterbeddedquartzite

holocene paleohydrology of the tropical andes from lake records two century scale time series in northern bolivia constrain the ages of abrupt changes in the physical geochemical and biological characteristics of sediments obtained from lakes that formed during deglaciation from the late pleistocene glacial maximum the watersheds of laguna viscachani 16 12 s 68 07 w 3780 m and lago taypi chaka kkota 16 13 s 68 21 w 4300 m located on the eastern and western slopes of the cordillera real respectively contain small cirque glaciers a high resolution chronology of the lake sediments is provided by 23 ams14c dates of discrete macrofossils late pleistocene glaciers retreated rapidly exposing the lake basins between 10 700 and 970014c yr b p the sedimentary facies suggest that after 890014c yr b p glaciers were absent from the watersheds and remained so during the middle holocene an increase in the precipitation evaporation balance is indicated above unconformities dated to 230014c yr b p in both lago taypi chaka kkota and laguna viscachani an abrupt increase in sediment accumulation rates after 140014c yr b p signals the onset of neoglaciation a possible link exists between the observed millennial scale shifts in the regional precipitation evaporation balance and seasonal shifts in tropical insolation

comparison of laser grain size analysis with pipette and sieve analysis a solution for the underestimation of the clay fraction classically the grain size of soil and sediment samples is determined by the sieve method for the coarse fractions and by the pipette method based on the stokes sedimentation rates for the fine fractions results from the two methods are compared with results from laser diffraction size analysis which is based on the forward scattering of monochromatic coherent light from a point of view of laboratory efficiency the laser sizing technique is far superior accuracy and reproducibility are shown by measurements on certified materials it appears that laser grain size measurements of certified materials correspond very well with the certificated measurements tests were also done on a set of randomly selected sediments of fluvial aeolian and lacustrine origin except for the 2 m clay fraction there is a coarsening of the mean diameter of one to two size classes 0 25 caused by the non sphericity of the particles the platy form of the clay particles induces considerable differences eight size classes between pipette and laser measurements the 2 m grain size defined by the pipette method corresponds with a grain size of 8 m defined by the laser particle sizer for the studied sediments using a higher grain size level for the clay fraction when laser analysis is applied enables workers in the geological and environmental field to compare classical pipette analysis with a laser sizing technique

structural events and metamorphic consequences in almora nappe during himalayan collision tectonics abstract almora nappe in uttarakhand india is a lesser himalayan representative of the himalayan metamorphic belt that was tectonically transported over the main central thrust mct from higher himalaya the basal shear zone of almora nappe shows complicated structural pattern of polyphase deformation and metamorphism the rocks exposed along the northern and southern margins of this nappe are highly mylonitized while the degree of mylonitization decreases towards the central part where the rocks eventually grade into unmylonitized metamorphics mylonitized rocks near the roof of the basal shear zone show dynamic metamorphism m2 reaching upto greenschist facies 450 c 4 kbar in the central part of nappe the unmylonitized schists and gneisses are affected by regional metamorphism m1 reaching upper amphibolite facies 4 0 7 9 kbar and 500 709 c four zones of regional metamorphism progressing from chlorite biotite to sillimanite k feldspar zone demarcated by specific reaction isograds have been identified these metamorphic zones show a repetition suggesting that the zones are involved in tight f2 folding which has affected the metamorphics south of the almora town the regionally metamorphosed rocks have been intruded by almora granite 560 20 ma resulting in contact metamorphism the contact metamorphic signatures overprint the regional s2 foliation it is inferred that the dominant regional metamorphism in almora nappe is highly likely to be of pre himalayan precambrian age

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

current strain accumulation in the hinterland of the northwest himalaya constrained by landscape analyses basin wide denudation rates and low temperature thermochronology abstract rupture associated with the 25 april 2015 m w 7 8 gorkha nepal earthquake highlighted our incomplete understanding of the structural architecture and seismic cycle processes that lead to himalayan mountain building in central nepal in this paper we investigate the style and kinematics of active mountain building in the himalayan hinterland of northwest india approximately 400 km to the west of the hypocenter of the nepal earthquake via a combination of landscape metrics and long ma and short term ka erosion rate estimates from low temperature thermochronometry and basin wide denudation rate estimates from 10 be concentrations we focus our analysis on the area straddling the pt 2 the physiographic transition between the lesser and high himalaya that has yielded important insights into the nature of hinterland deformation across much of the himalaya our results from northwest india reveal a distinctive pt 2 that separates a lesser himalaya region with moderate relief 1000 m and relatively slow erosion 2 has persisted in the same relative position since at least the past 1 5 ma we interpret these observations to suggest that strain accumulation in this hinterland region throughout at least the past 1 5 ma has been accomplished both by crustal thickening via duplexing and overthrusting along transient emergent faults despite the 400 km distance between them similar spatiotemporal patterns of erosion and deformation observed in northwest india and central nepal suggest both regions experience similar styles of active strain accumulation and both are susceptible to large seismic events

late quaternary climate history of the bolivian altiplano abstract sediment cores and outcrops from the bolivian altiplano are used to interpret late pleistocene holocene paleoclimates and lake evolution based on a multi proxy approach ostracod content palynology sedimentology and radiocarbon dating despite the different sensitivity of the lacustrine basins to environmental changes interpreting the records in terms of paleohydrology climate and more especially timing of events is difficult notwithstanding these problems lake titicaca in the north and lake pocoyu lake poopo salars of coipasa and uyuni in the south reveal a similar evolution in general trends through the course of time during the minchin phase until 30 26 kyr bp the altiplano was wetter than present the last glacial maximum ca 26 14 kyr bp marked by cooler drier conditions was followed by a return to a wetter climate interrupted by short arid events between ca 14 and 10 5 kyr bp another cycle of aridity between 10 5 and 8 kyr bp took place abruptly just after the humid tauca phase the mid holocene is interpreted as climatically unstable with an alternation of humid and dry episodes since 3 9 kyr bp wetter conditions have persisted and intensified to the present the data are interpreted in terms of changes in inter tropical convergence zone extension summer precipitation and in polar air mass intensity winter precipitation

a late quaternary diatom record of tropical climatic history from lake titicaca peru and bolivia abstract a composite high resolution diatom stratigraphy from three piston cores and one box core in the deep sub basin of lake titicaca reveals large moisture variations during the past 30 kyr in the altiplano region diatom sequences indicate orbital and millennial scale variability in water level and salinity the pelagic freshwater diatom species cyclotella andina and cyclotella stelligera dominate glacial age sediments suggesting that the lake was above its present outlet generally wet conditions continued until 11 000 cal yr bp as indicated by high percentages of freshwater planktonic diatoms large pulses of benthic diatom species between about 11 000 and 10 000 cal yr bp suggest brief intervals of large amplitude declines in lake level during the early holocene 10 000 8500 cal yr bp a freshwater diatom assemblage suggests overflowing conditions pelagic freshwater diatoms are replaced ca 8500 cal yr bp by the salinity indifferent species cyclotella meneghiniana and by benthic taxa indicating the beginning of lake regression during the mid holocene 6000 3500 cal yr bp the abundance of the saline taxon chaetoceros muelleri coupled with high abundances of epiphytic and epipelic diatoms indicates maximum salinity and lowest lake levels in the entire 30 000 year record lake transgression began ca 4000 cal yr bp and the lake achieved modern levels by about 1500 cal yr bp these water level changes imply changes in effective moisture most likely resulting from large precipitation changes precipitation was high throughout the last glacial maximum 21 000 18 000 cal yr bp likely due to an enhanced south american summer monsoon during peak summer insolation in the southern hemisphere in contrast the mid holocene transition was dryer than today in association with an austral summer insolation minimum and the subsequent weakening of the summer monsoon

the himalayan passive margin from precambrian to cretaceous times abstract the himalaya passive margin should be separated into the indian himalaya and the pakistan himalaya divided by the nanga parbat haramosh basement uplift in the northwestern syntaxes the indian passive margin shows a complex history involving splitting of microcontinents off the northern gondwana margin from early paleozoic times until the jurassic in contrast the pakistan himalaya formed part of the stable indian shelf until separation from africa started in the jurassic each margin has a distinct stratigraphic history until the mid mesozoic after which they show similar histories until the tertiary the himalaya mountains in india consist of three distinct tectonic units juxtaposed during the neogene by southward thrusting of the northern indian precambrian to mesozoic passive margin the lesser himalaya on the south forms a late precambrian passive margin usually directly overlain by late cretaceous to tertiary clastics its stratigraphy is closest to that of the indian shield in contrast the high himalaya has a thin ordovician to carboniferous shelf sequence deposited after early ordovician deformation and granite intrusion above this thick permian and mesozoic shelf sequences mark the separation of continental blocks off the northern indian margin and the opening of the neotethys ocean the north himalaya formed the slope and basin of this ocean and consists of reactivated paleozoic gneiss domes overlain by thick mesozoic sediments which pass in the northeastern himalaya into an enormously thick sedimentary sequence resting on oceanic crust paleomagnetic and structural evidence indicates at least 500 km of southward thrusting along the main central thrust between the lesser and high himalaya this thrusting together with a further 250 km of erosion at the front of the nappes has removed the entire inner shelf of the himalaya passive margin and can explain the startling contrast between the high and lesser himalaya stratigraphic sequences but there are still discrepancies particularly in the eastern himalaya where precambrian basement is juxtaposed with the north himalaya mesozoic slope these discrepancies can be partly resolved by strike slip movements roughly parallel to the himalaya which have removed parts of the northern indian passive margin such faults of the requisite orientation displacement and age occur in southeast asia where their cumulative displacements can add up to several thousand kilometres similar faults of undoubted pre tertiary age occur in the pakistan himalaya here the passive margin shows a much simpler history of jurassic to miocene subsidence

the history of south american tropical precipitation for the past 25 000 years long sediment cores recovered from the deep portions of lake titicaca are used to reconstruct the precipitation history of tropical south america for the past 25 000 years lake titicaca was a deep fresh and continuously overflowing lake during the last glacial stage from before 25 000 to 15 000 calibrated years before the present cal yr b p signifying that during the last glacial maximum lgm the altiplano of bolivia and peru and much of the amazon basin were wetter than today the lgm in this part of the andes is dated at 21 000 cal yr b p approximately coincident with the global lgm maximum aridity and lowest lake level occurred in the early and middle holocene 8000 to 5500 cal yr b p during a time of low summer insolation today rising levels of lake titicaca and wet conditions in amazonia are correlated with anomalously cold sea surface temperatures in the northern equatorial atlantic likewise during the deglacial and holocene periods there were several millennial scale wet phases on the altiplano and in amazonia that coincided with anomalously cold periods in the equatorial and high latitude north atlantic such as the younger dryas

middle to late holocene paleochannels and migration of the palar river tamil nadu implications of neotectonic activity abstract geomorphic tectonic and sedimentary investigations have been carried out to understand the landscape evolution of the palar river basin southern peninsular india for this purpose satellite data interpretations coupled with field investigations fluvial architecture and optically stimulated luminescence osl dating methods were adopted to infer climatic variations and neotectonic deformation that took place during the mid to late holocene period several paleochannels have been identified in the northern part of the present day palar river the distribution pattern of the palar paleochannels and its present course indicate its migration towards the south leaving behind the old palar river streams as paleochannels significant changes in the morphological characteristics channel width and river pattern in the successive paleochannels reveal evidences of channel movement by avulsion largely controlled by reactivation of pre existing lineaments and intensified ne monsoon based on osl ages two major phases of migration have been identified that occurred between 4 83 ka to 1 88 ka first phase occurred between 3 59 ka to 3 26 ka due to intensified monsoon accompanied with small scale neotectonic activity the second phase took place between 2 42 ka to 1 88 ka which occurred probably due to the rapid upstream avulsion this rapid avulsion is due to the close proximity of mpa and the reactivation of the pre existing lineaments moreover the present palar river basin exhibits evidences of neotectonic activity such as soft sediment deformation structures anomalous drainage deflections and reactivation of lineaments and faults further indicating that the palar river is still draining a tectonically active region thus neotctonics and fluctuations in the intensity of monsoon are largely responsible for sculpting the present landscape

configuration of the late paleoproterozoic supercontinent columbia insights from radiating mafic dyke swarms abstract remnants of 2 1 1 8 ga orogens can be recognized in nearly every craton assembled within reconstructions of the rodinia supercontinent although no particular pattern of laterally extensive orogenic belts emerges these cratons may be the fragments of an older supercontinent formed in response to the collision and accretion of even older cratons during the early earth history paleomagnetic constraints have played a limited role in many previous reconstructions of the early precambrian supercontinent mainly because of the poor age control and a large scatter of the paleomagnetic poles the geometry of giant radiating dyke swarms and orogenic belts provide important constraints for the reconstruction of the pre rodinian supercontinent some early unmetamorphosed and undeformed mafic dyke swarms in north china southern peninsular india and north america share coeval age of 1 85 ga if these continents are assembled the overall pattern exhibited by the mafic dyke swarms appears to constitute a giant radiating swarm with a piercing point between the cuddapah rift in south india and the xiong er aulacogen in north china this suggests that the north china craton indian craton and canadian shield may be fragmented from the same landmass the reconstruction of 1 85 ga giant radiating dyke swarm attempted in this paper suggests that the north china craton india craton and canadian shield were united together to form a landmass within the columbia supercontinent before its extension and break up the 1 90 1 85 ga andean style north orogenic belt suggests that a subduction zone existed on the northern margin of the north china craton wopmay orogenic belt developed in a 1 88 1 84 ga continental volcano plutonic arc which is interpreted as the product of eastward subduction of oceanic lithosphere the transantarctica orogenic belt in east antarctica may be another candidate to be linked with the subduction zone the north orogenic wopmay orogenic and transantarctica orogenic belts could be connected to form a continuous subduction zone the orogenic comparison strengthens the configuration of the columbia supercontinent proposed by the reconstruction of the giant radiating dyke swarm the laurentia west australia and east antarctica were relatively stable from 1 85 ga to 1 20 ga as inferred from the data on dyke swarms magmatism and the paleomagnetism these continents constituted the core of the columbia supercontinent in the late paleoproterozoic time the north china craton and indian craton were however separated from the columbia supercontinent during the mesoproterozoic time

tropical climate changes at millennial and orbital timescales on the bolivian altiplano tropical south america is one of the three main centres of the global zonal overturning circulation of the equatorial atmosphere generally termed the walker circulation1 although this area plays a key role in global climate cycles little is known about south american climate history here we describe sediment cores and down hole logging results of deep drilling in the salar de uyuni on the bolivian altiplano located in the tropical andes we demonstrate that during the past 50 000 years the altiplano underwent important changes in effective moisture at both orbital 20 000 year and millennial timescales long duration wet periods such as the last glacial maximum marked in the drill core by continuous deposition of lacustrine sediments appear to have occurred in phase with summer insolation maxima produced by the earth s precessional cycle short duration millennial events correlate well with north atlantic cold events including heinrich events 1 and 2 as well as the younger dryas episode at both millennial and orbital timescales cold sea surface temperatures in the high latitude north atlantic were coeval with wet conditions in tropical south america suggesting a common forcing

a new estimate of the holocene lowstand level of lake titicaca central andes and implications for tropical palaeohydrology new evidence from piston cores and high resolution seismic reflection data shows that water levels in lake titicaca were as much as 100 m below the present level during the early to mid holocene between 6 and 3 8 14c kyr bp climatological and modelling studies indicate that lake titicaca rainfall depends on convective activity in upwind amazonia the lake level data therefore suggest a drier amazon basin during this time this view is bolstered by an excellent match between the titicaca lake level curve and decreased methane concentrations in greenland ice previously ascribed to drying of low latitude wetlands blunier et al 1995 the postglacial history of lake titicaca fits a global pattern of lake level change in the tropics characterized by opposite phasing between the southern and northern hemispheres this pattern is most likely the result of orbital controls over the intensity of summer insolation

evidence for a far traveled thrust sheet in the greater himalayan thrust system and an alternative model to building the himalaya the galchhi shear zone underlies the kathmandu klippe in central nepal and has emerged as a key structure for discriminating competing models for the formation of the himalayan orogenic wedge new chronologic data from the galchhi area suggest a new structural and orogenic interpretation zircons from quartzites and an orthogneiss restrict protolith deposition to between 467 7 10 ma and 570 ma with metamorphic zircon growth at 23 29 ma comparable data from the greater himalayan sequence ghs at the intra ghs langtang thrust north of galchhi similarly restrict ghs deposition to between 475 7 3 and 660 ma undeformed pegmatites near galchhi constrain movement of the galchhi shear zone to 22 5 2 3 ma long before slip of the main central thrust in the region 17 ma shear sense indicators in the galchhi area indicate both top to the south and top to the north shears the old age of movement neoproterozoic youngest detrital zircons occurrence of top to the south shear sense indicators and intrusive paleozoic granites all suggest that the galchhi shear zone is an intra ghs top to the south thrust rather than either a thrust involving lesser himalayan rocks or a top to the north shear zone that juxtaposed tethyan and ghs rocks during coeval movement of the main central thrust the ghs in nepal was not emplaced as a single body of rock but consists of at least two ductile thrust sheets present in both the hinterland at langtang and toward the foreland at galchhi ghs thrust sheets sequentially underplated during southward propagation of the thrust belt

fluvial fans myths misconceptions and the end of the terminal fan model we propose that the so called terminal fan facies model should be abandoned since it is flawed on several counts and it is leading to misunderstanding and poor communication rivers in drylands may experience excessive down stream discharge reduction such that they terminate subaerially rather than reach the sea or a lake the facies model predicts that the distal reaches of such rivers form a network of bifurcating distributary channels producing a fan shaped sediment body with downstream thinning and fining of sedimentary units ending in sand filled ribbons encased in mud extensive review of modern rivers has failed to turn up convincing examples that fit the model rivers in drylands do not ubiquitously end in fans fan shaped fluvial bodies are common wherever rivers are released from confinement and the discharge conditions promote frequent avulsion channels on such fans generally do not repeatedly bifurcate downstream where they are seen to do so it can usually be shown they are lacustrine deltas inherited from wetter times the term distributary is being used carelessly and is conveying incorrect understanding of sediment geometry and architecture the proposed synonym of fluvial distributary systems is unsatisfactory as it perpetuates the same misunderstandings reliance on planform alone in analogue selection is highly risky the fluvial fan is a composite sediment body resulting from frequent nodal avulsions in a setting without horizontal constraints channels on fans range in planform as much as any other river the resultant sedimentary record differs little from that expected from non fan fluvial systems except having a regionally radiating orientation when viewed over geological time scales contrary to the implications of the facies model there is no distinctive terminal fan sedimentary succession

features and characterization of graphite in almora crystallines and their implication for the graphite formation in lesser himalaya india layers bands lenses and pockets of graphite are confined to mica schists garnet mica schist micaceous quartzites and black carbonaceous phyllites of the almora group forming graphite prospect in indian lesser himalaya the field and microscopic features of graphite reveal that the graphite in almora group was crystallized from metamorphism of the organic material before himalayan thrusting upto 100 m size nodular and 5 m size circular features points to the involvement of biogenic carbonaceous material in graphitization as also substantiated by 13c values and probably by the presence of v and p associated with this graphite impurities consisting si al mg o fe ti v ca and k are common with graphite these are derived from associated minerals such as muscovite quartz biotite garnet etc raman shifts obtained from this graphite suggest that it is an ordered graphite with its fine dust scattered in the host rocks the calculated degree of graphitization varies from 64 to 89 with no systematic spatial variation in the estimated temperatures cf 485 568 c of graphite formation within this range the temperatures for the intensely folded graphite are higher probably promoted by the himalayan tectonics although high end thermal records reported from the almora group are not evident in graphite however these temperatures obtained from graphite match with host rock thermometry implying that the graphite was formed during regional metamorphism

inundations in the mamore basin south western amazon bolivia and sea surface temperature in the pacific and atlantic oceans abstract extensive inundations have been observed in the llanos de mojos mamore basin south western amazon causing social and economic disasters since the beginning of the seventies precipitations have increased in the bolivian lowlands llanos and in the over rainy eastern cordillera and andean foothills yungas and inundations have become more frequent as a result inundations have been investigated in relation to rainfall over the mamore basin in trinidad puerto varador and in order to determine whether they are predictable in relation to sea surface temperature sst in the atlantic and the equatorial pacific oceans the methods are correlation and composite techniques as expected during the 1945 1946 1998 1999 period inundations have been associated with abundant rainfall in the mamore basin mainly in the llanos and yungas the role of rainfall in the inner dry andes and downstream from trinidad is more limited when consecutive floods are observed the ground water storage contributes to the occurrence of the second or third inundation event and rainfall anomaly is generally weaker rainfall in the mamore basin is hardly associated with sea surface temperature anomalies ssta in the pacific and atlantic oceans during the 1952 1953 1998 1999 period however during the nineties the southern atlantic ssta account for 50 percent of rainfall variability inundations are also related to negative ssta differences between the tropical and subtropical southern atlantic two thirds of the 22 inundation events occurred in association with this oceanic anomaly that features a weak ssta gradient in the southern atlantic during the 1988 1989 1998 1999 period a particularly significant relation can be observed between ssta and inundations events despite being associated with major el nino events 1982 1983 1991 1992 inundations are not significantly related to the equatorial pacific ssta

reconstructing the greater indian margin a balanced cross section in central nepal focusing on the lesser himalayan duplex across much of the himalaya rocks in the lesser himalayan duplex are covered by roof thrusts of other paleoproterozoic lesser himalayan rocks or greater himalayan rocks however in central nepal between the main central thrust and the main boundary thrust lesser himalayan rocks are exposed in structurally complex relationships within the lesser himalayan duplex we present two balanced cross sections with different stratigraphy involved in the duplex one with the basal kuncha formation and one without this unit to test stratigraphic assumptions both cross sections have roof thrust sheets consisting of the main central thrust ramgarh munsiari thrust and trishuli thrust folded over the hinterland dipping lesser himalayan duplex dissected by late faults cross section 1 has a shortening estimate from the main central thrust to the main boundary thrust including motion on the main central thrust of 359 km or 78 cross section 2 has a shortening estimate of 349 km or 76 over the same region because the amount and percentage of shortening are not significantly different between the two cross sections the different stratigraphic assumptions did not change the shortening results this similarity suggests that many of the choices made when constructing a cross section may be less important than researchers previously thought

origin of rectangular and aligned lakes in the beni basin of bolivia abstract the origin of aligned rectangular shaped lakes in the beni basin of bolivia has been investigated utilizing landsat imagery shuttle imaging radar and the available scientific literature the lakes tend to line up in two preferred orientations northwest and northeast they occur in cenozoic sediments of the beni basin situated between the brazilian shield and the andean foredeep available evidence suggests that the shapes and orientations of the lakes are primarily controlled by a nearly orthogonal fracture pattern propagated upward from the underlying granitic brazilian shield basement these orientations and shapes may reflect a reactivated ancient global fracture pattern or may be the result of an e w greatest principal compressive stress 1 and subsequent downwarping and uplifting of the beni basin during the cenozoic andean orogeny the lakes themselves may occupy down dropped basement blocks induced in the gently upwarped basement while unusual these lakes are not unique this suggests that transmission of basement structure through overlying sediments may occur in other regions but that special conditions such as little or no lateral compression of the sediments the absence of high trees and vegetation stability of surface material from drifting and a high water table are necessary to preserve and enhance such surface features

alluvial plain dynamics and human occupation in sw amazonia during the holocene a paleosol based reconstruction abstract the present study reconstructs holocene fluvial dynamics in the southern amazonian foreland basin through the analysis of 36 stratigraphic profiles taken along a 300 km long transect across the llanos de moxos lm in the bolivian amazon based on 50 radiocarbon ages from paleosols intercalated with fluvial sediments the most important changes in floodplain dynamics on a millennial scale are reconstructed and the links between pre columbian cultural processes and environmental change in the region explored results show that the frequency of river avulsions and crevasses as inferred from the number and age of the cored paleosols is stable from 8k cal yrs bp to 4k cal yrs bp and increases significantly from 4k to 2k cal yrs bp following the strengthening of el nino la nina cycle and an increase in average precipitation fluvial activity then decreases and reaches its minimum after 2k cal bp a comparison between the stratigraphic record and the archaeological record shows a match between periods of landscape stability in sw amazonia low river activity and periods of pre columbian human occupation the first amazonians lived in the lm until 4k yrs bp when an abrupt increase in the frequency of river avulsions and crevasses forced the abandonment of the region after two thousand years of archaeological hiatus which matches the period of highest river activity in the region agriculturists reoccupied the bolivian amazon

trace element geochemistry in the upper amazon drainage basin bolivia abstract the distribution of trace elements sr mo ba rb u mn ni cu zn and cd was investigated in surface waters and associated particulates in two upper amazonian basins mamore and beni dissolved sr ba mo rb u and to a lesser extent zn and cd correlate with major ions and appear to be predominantly derived from soluble rocks carbonates evaporites or sulfides essentially contained in shales these elements are conservative in waters except in highly alkaline or sulfate rich waters where calcite and barite can precipitate the concentrations of transition element are generally not correlated with major ion chemistry their behaviour can be largely explained by equilibrium reactions in solution and solid liquid exchanges additionally the distribution of mn and cu reflects also the lithological differences between the mamore and beni basins the occurrence of mineralisation in the beni basin explains the high zn cd as well as the concentrations of cu in some rivers of the beni basin the higher content of mn in the mamore basin may reflects the predominance of carbonate rocks the fractionation of trace elements in spm has been attributed to the sorting of feldspars during transport in water and to the mixing between clay minerals and quartz these minerals refractory to weathering and introduced in water rivers by mechanical erosion have not contributed to the dissolved load