metamorphism melting and extension age constraints from the high himalayan slab of southeast zanskar and northwest lahaul abstract we present evidence for two distinct stages of tertiary metamorphism m1 and m2 in the high himalayan slab of southeast zanskar and northwest lahaul as well as evidence for an older pre himalayan metamorphism pre m1 the m1 was a regional barrovian type event related to crustal shortening and thickening of the indian plate margin while m2 was associated with crustal melting and the emplacement of the gumburanjon leucogranite into the zanskar shear zone at the top of the slab u pb dating of metamorphic and magmatic accessory phases constrains the timing of m1 between 30 and 37 ma and the crystallization and emplacement age of the gumburanjon leucogranite at 21 22 ma inherited accessory phases in metamorphic and magmatic samples suggest that the protoliths of the slab are at least lower paleozoic in age and that they experienced a major pre m1 thermal perturbation at ca 450 500 ma whether this was associated with a regional barrovian type metamorphism or whether it was a thermal event rel

incremental 40ar 39ar thermochronology of mylonitic rocks from the northern snake range nevada a 40ar 39ar thermochronologic study of muscovite biotite and potassium feldspar from footwall rocks of the northern snake range decollement nevada was undertaken in order to constrain the timing and temperatures of mylonitic deformation during extension and the subsequent cooling uplift history of these mylonitic rocks mylonitic deformation has been bracketed between 37 ma and about 24 ma and minimum temperatures during ductile thinning were about 300 c mineral cooling ages along the east flank of the range suggest a minimum tmax gradient of about 55 c ma this steep thermal gradient is the result of a localized heat source and or shearing of colder rocks with older cooling histories over hotter rocks with younger cooling histories although mylonitic deformation may have spanned a 13 m y period 37 ma to 24 ma deformation was not synchronous throughout the range on the west flank of the range age spectra from muscovite and microcline samples indicate that rocks were at temperatures well below 325 c and deformation had ceased by 30 ma while on the east flank of the range lower plate rocks were at temperatures above 325 c and ductile deformation was still ongoing this is also reflected in chrontours of published k ar muscovite ages along with 40ar 39ar ages which monotonically decrease in age from west to east these data provide evidence for a temperature difference across the range within the same structural horizon from about 30 to 24 ma these relations suggest a lateral thermal gradient the east flank of the range cooling more slowly than the west flank alternatively the data suggest either an eastward dip of lower plate units prior to the onset of tertiary mylonitic deformation or that layering dipped eastward due to deformation in an east dipping top to the east shear zone both cases result in differential cooling of lower plate units as they pass through a series of subhorizontal isotherms the 40ar 39ar data along with published finite strain structural and quartz petrofabric data provide constraints on the formation and geometric evolution of metamorphic core complex detachment faults

quaternary channel focused rapid incision in the phung chu arun river in central himalaya implications for a quaternary capture event abstract the phung chu arun river one of the few major trans himalaya rivers consists of a large drainage area captured in the northern slope of the himalaya and provides a key to understand tectonic surface processes and their interplay in topographic development of the himalaya in this study we carried out thermochronologic and topographic analysis along the phung chu arun river in central himalaya to reveal its incision development apatites from a downstream section along its major tributary yield very young fission track and u th he ages of 0 8 1 5 ma these cooling ages combined with previously reported data for higher elevations reveal an abrupt change of the slope in the age elevation plot suggesting an accelerated channel incision initiated at between 1 5 and 2 5 ma the age elevation relationship yields a high apparent exhumation rate aer of 2 6 3 8 mm a during quaternary which is spatially focused along the mainstream of the kharta chentang reaches and the associated nearby tributaries topographic analysis suggests that this channel focused incision acceleration was probably facilitated by a quaternary capture event in upper drainage of the palaeo arun river this capture event greatly enhanced the effective discharge draining southward causing an accelerated incision wave along the mainstream and tributaries of the palaeo arun river furthermore thrusting at physiographic transition zone is also required to sustain high channel gradient and the long term rapid incision channel morphology presenting common knick points in major tributaries suggests that the drainage basin in ama drime range area likely remain in pre steady state adjustment of this capture event

the south tibetan detachment system in the rongbuk valley everest region deformation features and geological implications abstract structural and microstructural investigations into the south tibetan detachment system in the rongbuk valley southern tibet pointed to the presence of a complex extensional setting with the development of two major fault zones with movement in a top to ne direction the lower one is an at least 1000 m wide ductile shear zone that brings into contact the overlying north col formation subject to low grade metamorphism and the high grade micaschist and gneiss of the higher himalayan crystallines characterized by polymetamorphism from barrovian to low pressure high temperature conditions the upper low angle fault zone is characterized by a metric thick level of cataclasites and brings into contact the slightly metamorphosed ordovician limestones and the north col formation the lower ductile shear zone is thought to accommodate at least 5 km of vertical displacement whereas the upper low angle normal fault zone is thought to accommodate at least 1600 m of vertical displacement the lower ductile shear zone is thus to be considered the main extensional feature of the stds in the rongbuk valley

shisha pangma leucogranite south tibetan himalaya field relations geochemistry age origin and emplacement the shisha pangma pluton forming most of the xixabangma 8027 m massif in south tibet is one of the 20 larger leucogranite intrusives that mark the highest structural levels of the himalayan metamorphic core the pluton occurs immediately below the shisha pangma detachment a strand of the south tibetan detachment std system a low angle 30 north dipping normal fault placing paleozoic black slates atop sillimanite grade pelites and calc silicate rocks k feldspar augen gneisses containing fibrolite and sillimanite paragneisses along the footwall show strong internal s c fabrics indicative of down to the north extension the shisha pangma leucogranite is a heterogeneous polyphase intrusion with an earlier foliated biotite rich phase and a later tourmaline muscovite rich phase typically containing the assemblage kfs pl qtz ms tur gt bt sil ap the highly peraluminous granites have high 87sr 86sr ratios 0 738 0 750 typical of pelite derived anatectites nd depleted mantle model

slow exhumation of the greater himalaya in the yadong region the transition between the central and eastern himalaya during the late neogene the yadong area at the geographic boundary between the central and eastern himalaya contains the largest along strike structural discontinuity in the himalaya we conducted zircon fission track zft and apatite u th he ahe dating along a 50 km transect of the greater himalaya sequence ghs to investigate the cooling and exhumation of this structural discontinuity new zft 14 0 8 2 ma and ahe 11 1 4 2 ma data suggest fast tectonic exhumation of the ghs in the middle miocene after this pulse of rapid exhumation the area remained in a long term slower and steady state of exhumation c 0 32 km ma since c 7 7 ma new stream topographic analysis also confirms this slow surface erosion in the hinterlands whereas the frontal range underwent enhanced erosion possibly due to southward upthrusting along the main boundary thrust and main frontal thrust our data underline distinctive exhumation patterns between the eastern and central himalaya and suggest that exhumation of the himalaya was primarily driven by tectonics associated with the underlying main himalaya thrust mht the long term slow and steady exhumation in the yadong area and the eastern himalaya hinterland since the late neogene is consistent with the gentle dip of the mht supporting a slab tear of the subducting indian plate

lake sediment geochemistry reveals 1400 years of evolving extractive metallurgy at cerro de pasco peruvian andes the geochemical record preserved in lake sediments is a potentially powerful tool in archaeometallurgy here sediments from llamacocha a small lake in the central peruvian andes are used to reconstruct a 1400 year legacy of metal extraction from cerro de pasco once the largest silver mine in the world the earliest evidence for anthropogenic lead pb enrichment occurs ca a d 600 and is confi rmed by pb stable isotope ratios that match those of cerro de pasco ores early pb pollution is attributed to precolonial smelting for silver production which relied on galena based fl uxes following colonial control of the mine ca a d 1600 the switch to mercury hg amalgamation for winning silver resulted in atmospheric hg emissions as registered in llamacocha sediments both pb and hg deposition increased through the twentieth century attaining peak values in a d 1968 and 1942 respectively principal components analysis pca identifi es a gradient that differentiates anthropogenic from natural metals within the record confi rming that early smelting led to the volatilization of trace metals associated with local ore mineralogy these results represent the fi rst evidence for a major precolonial mining industry at cerro de pasco provide a chronological framework for evolving extractive technologies and are the fi rst to document widespread hg pollution associated with colonial hg amalgamation 80 w 70 w

orogen parallel ductile extension and extrusion of the greater himalaya in the late oligocene and miocene 1 predominant stretching structures in the greater himalayan crystalline complex ghc trend perpendicular to the belt and are linked to the southward exhumation or emplacement of the ghc between the south tibet detachment std and the main central thrust however our field investigations in southern tibet reveal the widespread presence of gently dipping shear zones with a penetrative orogen parallel stretching lineation which separates the tethyan himalayan sequence and the underlying ghc the shear zones are well preserved in the upper part of the ghc south to and structurally lower than the std field criteria microstructures and quartz fabrics indicate top to the east shearing in the yadong shear zone eastern ghc coexistence of top to the east and top to the west shearing in the nyalam shear zone central ghc but top to the west shearing in the pulan shear zone western ghc characteristic microstructures and slip systems of quartz in the high grade ghc rocks resulted from the lateral flow under upper amphibolite up to 650 700 c to greenschist facies conditions u pb ages of metamorphic zircon rims by sensitive high resolution ion microprobe shrimp and laser ablation multi collector inductively coupled plasma mass spectrometry la mc icp ms analyses yield 28 26 ma for the initiation of the yadong and nyalam shear zones and 22 15 ma for the activation of the pulan shear zone in addition 40ar 39ar cooling ages of biotite and muscovite suggest cessation of ductile sharing at 13 11 ma on the yadong shear zone which is coeval with the activation of the std combined with previous studies we propose that initiation of orogen parallel extension marks the transition from burial crustal thickening to exhumation of the ghc due to lateral crustal thickness gradients in a thickened crust orogen parallel gravitational collapse occurred within the convergent himalayan orogen in the late oligocene miocene this tectonic denudation triggered and enhanced partial melting and ductile extrusion of the ghc in the miocene

fission track and 40ar39ar evidence for episodic denudation of the gangotri granites in the garhwal higher himalaya india abstract in order to document quantitatively the cooling and denudation history of the higher himalayan granites bordering the tethyan sedimentary zone fission track ft apatite and 40 ar 39 ar mica ages have been determined on the gangotri leucogranites and biotite granites in the garhwal region of india gangotri is the source area of the ganges river and lies in the midst of the highest himalayan peaks in india a total of 15 apatite ages from a vertical profile 2580 4370 m on the gangotri granites yields ft ages in the range of 1 5 0 6 to 2 4 0 5 ma indicating that the rock column with a relief of 1800 m cooled through 130 10 c within only 1 million years during the late pliocene an average denudation rate of 2 mm yr is estimated for the past 2 4 million years from the gangotri granites we also report a muscovite 40 ar 39 ar age of 17 9 0 1 ma and a biotite age of 18 0 0 1 ma these reflect cooling of the rocks through 300 350 c probably related to an early miocene pulse of denudation caused by a basement cover detachment the martoli normal fault above the leucogranites time temperature pathways indicate that the cooling of the rocks in the late pliocene quaternary was five to six times the magnitude of cooling between 18 and 2 ma indicating a distinct pulse of rapid denudation in the late pliocene quaternary we interpret these young apatite ages and fast denudation as a geomorphic response increased erosion and cooling of the rocks to a major pulse of tectonic uplift in the higher himalaya shortly before 2 4 ma the effect of climatic cooling on this denudation is considered secondary to the role of tectonic forcing and indeed produced a positive feedback to the primary cause although our study is confined to the garhwal region it is probable that other granitic bodies of the higher himalaya bordering the tethyan sedimentary rocks and forming the loftiest summits in the himalaya have also experienced episodic denudation one major pulse in the early miocene which was mainly tectonic denudation and another in the late pliocene quaternary which was mainly erosional the latter is well recorded by apatite ft data and is consistent with the hypothesis that rapid uplift and denudation of the himalayan rocks may have influenced the initiation of the ice ages in the northern hemisphere

elevated mercury accumulation in a peat bog of the magellanic moorlands chile 53 s an anthropogenic signal from the southern hemisphere increasing mercury deposition rates in the northern hemisphere recorded in natural archives such as peat bogs or lake sediments have been documented in numerous studies however data on atmospheric hg deposition in the southern hemisphere dating back to pre industrial times are rare here we provide a continuous record of atmospheric hg deposition in the southern hemisphere recorded by an ombrotrophic peat bog of the magellanic moorlands chile 53 s extending back 3000 yr pre industrial mercury accumulation rates range between 2 5 and 3 9 wg m 2 yr in the past 100 yr hg accumulation rates increased 18 fold from about 3 wg m 2 yr to a maximum of 62 5 wg m 2 yr if hg accumulation rates were normalized to peat accumulation rates maximum rates were 7 9 wg m 2 yr which is only 2 5 times the pre industrial rates thus hg accumulation rates normalized to peat accumulation rates are more comparable to the three fold net increase in atmospheric hg concentrations estimated for the same period we suggest that the increase in hg accumulation rates in the magellanic moorlands within the past 100 yr is at least partly attributed to global dispersion of hg derived from anthropogenic sources in the northern hemisphere the finding that no increase of atmospheric deposition of pb could be observed in the bog indicates the extraordinary long range transport and ubiquitous dispersion of anthropogenic derived gaseous hg compared to other metals 2002 elsevier science b v all rights reserved

records of the evolution of the himalayan orogen from in situ th pb ion microprobe dating of monazite eastern nepal and western garhwal abstract in situ th pb monazite ages from rocks collected along two transects the dudh kosi everest eastern nepal and the bhagirathi river garhwal himalaya india perpendicular to the main central thrust mct suggest a striking continuity of tectonic events across the himalaya the youngest age reported in this study 5 9 0 2 ma mswd 0 4 from matrix monazite grains collected beneath the mct in the garhwal region is consistent with several age data from rocks at similar structural levels in central nepal providing support for widespread late miocene mct activity the lateral parallelism of orogenic events is further manifested by the 20 7 0 1 ma age of a high himalayan leucogranite from an injection complex that outcrops along the dudh kosi everest transect resembling ages of these bodies reported elsewhere the youngest monazite grain analyzed along the dudh kosi everest transect is 10 3 0 8 ma the absence of 7 3 ma monazite ages in eastern nepal may reflect a different nappe structure which obscures the reactivated ramp equivalent exposed in the garhwal and central nepal garnets from the mct hanging wall greater himalayan crystallines and footwall lesser himalaya display different major element zoning and the patterns are useful for constraining the location of the thrust system that separates the two lithologies pressure temperature paths for two upper lesser himalayan garnets that contain monazite inclusions indicate the utility of an in situ methodology to constrain the metamorphic evolution of the shear zone along the dudh kosi everest transect upper lesser himalayan monazite grains from three rocks record a clear signal at 14 5 0 1 ma mswd 8 4 and the 23 ma age that characterizes the hanging wall is notably absent monazites collected within a large scale greater himalayan crystallines fold yield the 14 ma age consistent with the structure forming due to mct related compression paleo mesoproterozoic 1407 35 ma matrix monazite grains are found within an augen gneiss unit located beneath the mct whereas cambro ordovician 436 8 548 17 ma inclusions are preserved within garnets of the greater himalayan crystallines the presence of 45 2 2 1 ma grains from lower structural levels of the greater himalayan crystallines indicates the unit realized conditions conducive for monazite growth during the eocene

a geochemical and lead isotopic record from a small pond in a remote equatorial island fernando de noronha brazil a 72 cm long core was collected from lagoa da viracao lv a small pond in the fernando de noronha island northern brazil sediments from the lower section of the core 20 72 cm depth contain essen tially mineral matter while in the upper section 0 20 cm depth mineral matter is mixed with organic matter lithogenic conservative elements si al fe ti co cr cu ba ga hf nb ni y v zn zr and ree exhibit remarkablyconstantvalues throughout the core with concentrationssimilarto those of the bedrock the vertical distribution of soluble elements ca mg na k p mn and sr is also homogeneous but these ele ments are systematically depleted in relation to the bedrock loi toc br se hg and pb although showing nearly constant values in the lower section of the core are significantly enriched in the upper section the con centration profiles of br and se suggest that they may be accounted for by natural processes related to the slight affinity of these elements for organic matter on the other hand the elevated levels of hg and pb in recent sed iments may be explained by their long range atmospheric transport and deposition furthermore the isotopic composition of pb clearly indicates that anthropogenic sources contributed to the pb burden in the uppermost pond sediments

geoacoustic inversion using combustive sound source signals in the summer of 2006 combustive sound sources css were deployed in shallow waters off new jersey during the shallow water experiment sw 06 the depth of these sources were 26 m in water depths of the order of 100 m css are low frequency broadband sound sources and suitable for modal dispersion based inversion css data collected on various receivers single hydrophone receiving units shru vertical line array etc will be used for the inversions the inversions are based on an iterative scheme using a new time frequency analysis technique dispersion based short time fourier transform in which each time frequency tiling is adaptively rotated in the time frequency plane depending on the local wave dispersion data from various receivers will be used to investigate the spatial variation in compressional wave speed compressional wave attenuation will also be estimated using modal amplitude ratios inversion results will be validated by comparing the data with model predictions work supported by the office of naval research

tomographic inversion for sediment parameters in shallow water this article discusses inversions for bottom geoacoustic properties using broadband acoustic signals obtained from explosive sources the experimental data used for the inversions are sus charge explosions acquired on a vertical hydrophone array during the shelf break primer experiment conducted south of new england in the middle atlantic bight in august 1996 the sus signals were analyzed for their time frequency behavior using wavelets the group speed dispersion curves were obtained from the wavelet scalogram of the sus signals a genetic algorithm ga was used for the inversion of sound speeds in the water column and compressional wave speeds in the sediment layers the variations in the sound speeds in the water column were represented using empirical orthogonal functions eofs a range independent normal mode routine was used to construct the replica fields corresponding to the parameters comparison of group speeds for modes 1 to 9 and for a range of frequencies 8 to 200 hz was used to arrive at the best parameter fit an efficient hybrid optimization scheme using the ga and a levenberg marquardt algorithm is presented linear perturbation methods were also used to fine tune the inversions and to obtain resolution and variance estimates analysis was also done to compute the degree of convergence of each of the parameters by explicitly calculating the hessian matrices numerically a posteriori estimation of mean and covariance was also done to obtain error estimates group speeds for the inverted sound speed fields provide an excellent match to the experimental data the inverted sediment compressional speed profile compares well with in situ measurements

a review of glacial and holocene paleoclimate records from southernmost patagonia 49 55 s southern south america is the only landmass intersecting the southern westerly wind belt sww that influences the large scale oceanography and controls for example the outgassing of co2 in the southern ocean therefore paleo reconstructions from southernmost patagonia are of global interest and an increasing number of paleoclimate records have been published during the last decades we provide an overview on the different records mostly covering the holocene but partly extending into the late glacial based on a large variety of archives and proxies we particularly discuss possible reasons for regionally diverging palaeoclimatic interpretations and summarize potential climate forcing mechanisms the deglacial and holocene temperature evolution of the region including the adjacent pacific ocean indicates antarctic pattern and timing consistent with glacier re advances during the antarctic cold reversal some records indicate a significant accumulation control on the glacier fluctuations related to changes in sww strength and or position reconstructions of holocene changes in the sww behaviour provide partly inconsistent and controversially discussed pattern while records from the hyperhumid side point to a stronger or southward displaced sww core during the early holocene thermal maximum records from the lee side of the andes show either no long term trend or the opposite suggesting enhanced westerlies during the late holocene neoglacial likewise centennial scale global or hemispheric cold intervals such as the little ice age have been interpreted in terms of enhanced and reduced sww strength some sww variations can be linked to changes in the el nino southern oscillation enso consistent with instrumental climate data sets and might be ultimately forced by solar variability resolving these inconsistencies in southernmost patagonian sww records is a prerequisite for improving hemispheric comparisons and links to atmospheric co2 changes

porphyroclast systems as kinematic indicators porphyroclasts of relatively strong minerals in mylonites commonly have an internal monoclinic shape symmetry defined by tails of dynamically recrystallized material the geometry of a porphyroclast and its tails called a porhyroclast system can serve as a valuable indicator of the sense of vorticity porphyroclast systems have been divided into and types on the basis of the geometry of the tails types have wedge shaped recrystallized tails whose median lines lie on opposite sides of a reference plane parallel to the tails and containing the symmetry axis for the system types are further subdivided into a a types in which the porphyroclast is isolated in a relatively homogeneous matrix and b types in which the porphyroclast system is associated with a shear band foliation in the matrix types typically have narrow recrystallized tails whose median lines cross the reference plane adjacent to the porphyroclast consequently embayments of matrix material occur adjacent to the porphyroclasts and the tails display characteristic bends a porphyroclast system in a mylonite develops when the relatively weak dynamically recrystallized grain aggregate in the porphyroclast mantle changes its shape due to non coaxial flow in the adjacent matrix this behaviour has been modelled in shear box experiments passive marker lines around rigid cylinders embedded in silicone putty were subjected to simple shear the experiments were modified to simulate a change in recrystallization rate r with respect to rate of deformation by decreasing the diameter of the rigid cylinder during deformation at variable rates the ratio r appears to be one of the most important factors in determining which porphyroclast system will develop at high r values flow of recrystallized material away from the porphyroclast is continuously appended by the production of new grains and wedge shaped a type tails develop at low r values relatively few new grains are added to the tails which become thinned and deflected by drag due to the spinning motion of the porphyroclast in addition most porphyroclast systems at low shear strains are of a type or lack monoclinic symmetry whereas types are only developed at high shear strain values complex porphyroclast systems characterized by two generations of tails are observed in many of the natural and model shear zones studied and may form due to fluctuating r conditions that allow isolated a and type porphyroclast systems to be used as sense of vorticity indicators are the systems should have a monoclinic shape symmetry matrix grain size should be small with respect to porphyroclast size matrix fabric should be homogeneous deformation history should be simple and observations should be made on sections normal to the inferred bulk vorticity vector for the mylonite

high resolution 210 pb dating of lake constance sediments stable lead in lake constance two sediment cores bo90 13b and bo90 17b from lake constance were investigated by spectrometry for210pb 134cs 137cs 241am 234th and other members of the238u decay chain the sediments were dated using the constant flux model for210pb and accumulation rates were determined these range from 0 04 to 0 65g cm 2 yr bo90 13b and 0 04 to 0 8g cm 2 yr bo90 17b respectively the mean accumulation rate amounts to 0 16g cm 2 yr for both cores the cores had already been dated by lamination counting and reconstruction of high water events at the limnological institute at constance so that a very precise time scale was available both ages derived are in agreement within statistical error up to 1900 which means dating with the constant flux model for210pb was confirmed up to that age the position of the maxima of bomb cesium and americium confirm the stratigraphic and210pb datings with241am a further radioactive isotope is available which can due to the half life of241pu t 1 2 14 4yr be detected now by spectrometry and can serve as an additional time indicator the maximum being dated at 1963 by applying the various time scales the depth profiles of stable lead and zinc of core bo90 13b were dated both heavy metals show a very significant maximum located beneath the layer of the maxima of bomb cesium and americium showing that these maxima are older than those of the bomb isotopes it is remarkable in this context that the maximum of zinc concentration occurs a little later than that of stable lead similar concentration profiles are observable in core bo90 17b and other older sediment cores cs6 cs10 on a transect across the lake in contrast to a former assumption the depth profile of stable lead in lake constance sediments does not reflect the anthropogenic gasoline lead emissions into the atmosphere for germany their maximum being dated at 1971

an empirical evaluation of the argon diffusion geometry in muscovite the temperature time significance of 40ar 39ar muscovite ages is difficult to evaluate quantitatively because of ambiguities in the choice of an appropriate model diffusion geometry the effective diffusion dimension and large uncertainties in experimentally determined diffusion parameters laser 40ar 39ar studies of natural 40ar concentration gradients suggest that argon loss in muscovite occurs primarily via cylindrical volume diffusion commonly with an effective diffusion dimension equal to the crystal diameter the present study tests the cylinder model geometry and the control of physical grain size on the effective diffusion dimension by measuring the anisotropy of a natural40ar diffusion gradient parallel and perpendicular to 001 in a single euhedral muscovite phenocryst the new data combined with previous studies clearly indicate loss of 40ar via cylindrical volume diffusion and strongly suggest that loss of 40ar across the 001 sheets slab diffusion geometry is minimal in undeformed crystals

north south extension within the convergent himalayan region recent work by burg et al 1984 indicates the presence of east west striking gently north dipping normal faults in the higher himalayas and southern tibet that formed during postcollisional convergence of india and tibet these faults extend for at least 800 km along strike we interpret these normal faults as probable late miocene extensional features with perhaps several tens of kilometres of down to the north displacement a simple elastic model suggests that these normal faults may have formed during gravitational collapse of the miocene topographic front between india and tibet in this interpretation gravitational collapse occurred by southward motion relative to india and tibet of a wedge of crustal rocks bounded above by gently north dipping normal faults and below by thrust faults that probably dip north north south extension produced in this way is probably confined to upper crustal levels only and does not reflect regional extension of the entire lithosphere

seabed characterization on the new jersey middle and outer shelf correlatability and spatial variability of seafloor sediment properties abstract nearly 100 collocated grab samples and in situ 65 khz acoustic measurements were collected on the new jersey middle and outer shelf within an area that had previously been mapped with multibeam backscatter and bathymetry data and more recently with chirp seismic reflection profiling eighteen short cores were also collected and probed for resistivity based porosity measurements the combined data set provides a basis for empirically exploring the relationship among the remotely sensed data such as backscatter and reflection coefficients and directly measured seabed properties such as grain size distribution velocity attenuation and porosity we also investigate the spatial variability of these properties through semi variogram analysis to facilitate acoustic modeling of natural environmental variability grain size distributions on the new jersey shelf are commonly multi modal leading us to separately characterize coarse 4 mm fine

u th pb systematics and ages of himalayan leucogranites south tibet abstract the age and origin of five leucogranites from the high and tethys himalaya and two country rock gneisses were investigated by u pb dating of zircon fractions and single grains and fractions of monazite additionally th u concentrations in whole rock powders and isotopic compositions of pb in leached k feldspars were determined monazites yield ages of 16 8 0 6 m y for the nialam migmatite granite 15 1 0 5 m y for the lhagoi kangri granite 14 3 0 6 m y for a granite from mt everest and 9 8 0 7 m y and 9 2 0 9 m y for two varieties of the maja granite these data together with monazite ages of 21 9 0 2 and 24 0 0 4 m y determined earlier on the makalu granite 1 substantiate a period of intracontinental granite emplacements from 24 to 9 m y ago i e from uppermost oligocene to late miocene times such a period of plutonic activity is consistent with the view that all these granites result from intracrustal melting following the collision of india with eurasia furthermore the individual ages together with structural relationships between granites and country rocks suggest that granite formation and tectono metamorphism occurred as alternating and strongly related processes with a periodicity of 7 to 9 m y inherited lead components present in all granite zircons point to large proportions of precambrian material in the magma source regions up to 2200 m y old th u systematics between monazite and country rocks indicate that u has been leached from most of the granites after crystallisation of monazite zircon dating of the kangmar granite gneiss which occurs in a window through the tethys himalayan sediments shows that this pluton transformed to a gneiss during the alpine orogeny crystallised in lowermost palaeozoic times 562 4 m y ago

episodic exhumation of the greater himalayan sequence since the miocene constrained by fission track thermochronology in nyalam central himalaya abstract the greater himalayan sequence ghs which makes up the core of the himalayan orogen has an uppermost tectonic contact defined by the south tibetan detachment system stds and a lower tectonic contact defined by the main central thrust mct the ghs occurs as one of the most important tectostratigraphic units for deciphering processes related to tectonic and climatic exhumation across the orogen zircon and apatite fission track zft aft dating were carried out along a transect in nyalam central himalaya in southern tibet to constrain cooling driven by orogenic process since the middle miocene the hanging wall of the stds yields an essentially unreset jurassic zft age in the jurassic strata however below the stds within the ghs there is a clear and distinct thermal signal of cooling related to exhumation in the footwall and within the ghs the rocks have zft ages of middle miocene to pliocene and aft ages of late miocene to quaternary that get younger downward and away from the stds in combination with thermal structure modeling a two part episodic model which is widely compatible with existing thermochronological data is proposed for cooling and exhumation of the ghs since the middle miocene 1 middle miocene and 2 pliocene to quaternary recent the middle miocene cooling is suggested to have resulted from a rapid tectonic unroofing by down to the north slip on the stds the tectonic exhumation was also recorded by several other thermochronological systems e g biotite 40ar 39ar with concordant middle miocene cooling ages in different structural positions across the ghs post middle miocene zft and aft cooling ages in the lower part of the ghs suggest accelerated cooling by climate enhanced erosional exhumation which was initiated in the late miocene to pliocene and was dramatic in the quaternary to recent thermochronological data and modeling further imply that the present himalayan topographic front may have been shaped essentially by surface erosion since the late miocene when the himalayan divide might have been some 20 30 km to the south of its present position however these data do not preclude the possibility that the intense erosional exhumation may have triggered rock uplift to approach and or maintain a steady topography in the ghs

late holocene sea level change in arctic norway abstract relative sea level data from the pre industrial era are required for validating geophysical models of glacio isostatic adjustment as well as for testing models used to make sea level predictions based on future climate change scenarios we present the first late holocene past 3300 years relative sea level reconstruction for northwestern norway based on investigations in south hinnoya in the vesteralen lofoton archipelago sea level changes are reconstructed from analyses of salt marsh and estuarine sediments and the micro organisms foraminifera and testate amoebae preserved within the indicative meaning of the microfauna is established from their modern distributions records are dated by radiocarbon 201pb 137cs and chemostratigraphical analyses our results show a continuous relative sea level decline of 0 7 0 9 mm yr 1 for south hinnoya during the late holocene the reconstruction extends the relative sea level trend recorded by local tide gauge data which is only available for the past 25 years our reconstruction demonstrates that existing models of shoreline elevations and gia overpredict sea level positions during the late holocene we suggest that models might be adjusted in order to reconcile modelled and reconstructed sea level changes and ultimately improve understanding of gia in fennoscandia

ductile extrusion of the higher himalayan crystalline in bhutan evidence from quartz microfabrics quartz textures measured from deformed quartz tectonites within the lesser himalaya and higher himalaya crystalline of bhutan show similar patterns orientation and distribution of the quartz crystallographic axes were used to confirm the regional shear sense the asymmetry of c axis and a axis patterns consistently indicates top to the south shearing the obliquity of the texture and the inferred finite strain plane strain to moderately constrictional suggest the strain regime had a combination of rotational and irrotational strain path in most of the samples from the bhutan himalaya the inferred deformation mechanisms suggest moderate to high temperature conditions of deformation that produced the observed crystallographic preferred orientation much higher temperature of deformation is indicated in the quartz veins from a leucogranite the observed ductile deformation is pervasively developed in the rocks throughout the investigated area the intensity of deformation increases only slightly in the vicinity of the main central thrust simultaneous southward shearing within a large part of the higher himalaya crystalline near and above the main central thrust and normal faulting across the south tibetan detachment is explained by the tectonically induced extrusion of a ductily deforming wedge the process of extrusive flow suggested here can be approximated quantitatively by channel flow models that have been used to describe subduction zone processes channel flow accounts for some observed phenomena in the himalayan orogen such as inverted metamorphic sequences near the main central thrust not related to an inversion of isotherms and the syntectonic emplacement of leucogranites into the extruding wedge locally leading to an inversion of isotherms due to heat advection

holocene denudation rates from the superhumid southernmost chilean patagonian andes 53 s deduced from lake sediment budgets abstract holocene denudation rates and their regional variations in the superhumid and temperate climate of the southernmost patagonian andes are poorly surveyed therefore we have investigated denudation in five small lake catchments 0 11 1 62 km 2 across a precipitation gradient from 600 to 9000 mm yr 1 in southernmost chile at 53 s variations in denudation rates can be defined most precisely by using small catchments and short time spans holocene thus can be related to variable bedrock types vegetation cover elevation and climate minimum physical denudation rates were determined from sediment budgets based on sediment echosounder data and physical properties of sediment cores the holocene denudation rates vary from 0 08 mm kyr 1 in the highly elevated bare granitic rock catchments to 9 01 mm kyr 1 in the low and vegetated catchments with a basement of sedimentary rocks these values are comparable with those from similar areas in norway sweden and nw iceland the high annual precipitation in the study area has only a minor effect on denudation enhanced geomorphic activity during paraglacial conditions caused a faster denudation after the glacier retreat until either the clastic glacial detritus was removed or the catchment was covered with vegetation chemical comparisons between basement rocks and lake sediments indicate a pronounced chemical denudation in catchments with peaty soils and low soil water ph values of 3 to 5 the holocene surface lowering rates are on average 200 fold lower than cenozoic surface lowering rates calculated from fission track data for the southern andes this discrepancy can be explained by significantly higher erosion rates during glacial periods

simultaneous miocene extension and shortening in the himalayan orogen the south tibetan detachment system separates the high grade metamorphic core of the himalayan orogen from its weakly metamorphosed suprastructure it is thought to have developed in response to differences in gravitational potential energy produced by crustal thickening across the mountain front geochronologic data from the rongbuk valley north of qomolangma mount everest in southern tibet demonstrate that at least one segment of the detachment system was active between 19 and 22 million years ago an interval characterized by large scale crustal thickening at lower structural levels these data suggest that decoupling between an extending upper crust and a converging lower crust was an important aspect of himalayan tectonics in miocene time

when did the roof collapse late miocene north south extension in the high himalaya revealed by th pb monazite dating of the khula kangri granite th pb ion microprobe measurements made on 12 monazite grains from the khula kangri granite tibet bhutan frontier are interpreted to indicate that crystallization occurred at 12 5 0 4 ma the leucogranite is cut by the gonto la detachment part of the southern tibet detachment system that has allowed upper level north directed extension of the himalayan orogen significant orogen normal extension in southern tibet appears to have continued 8 10 m y later than previously recognized this is the first reported crystallization age for a leucogranite east of the yadong cross structure an apparent 70 km offset of the high himalaya and southern tibet detachment west of the yadong cross structure reliable ages for high himalaya events major main central thrust slip granite generation and emplacement attainment of critical topography and major detachment extension group between ca 24 and 19 ma we interpret the west to east change across the yadong cross structure to be due to either 1 an abrupt 10 m y younging of principal high himalayan events or 2 a deeper thus younger exposed part of the footwall of the southern tibet detachment near khula kangri the southern tibet detachment is cut by the highly oblique yadong gulu rift a manifestation of tibet plateau east west extension integrated estimates of magnitude and rate of detachment displacement suggest that the observed postcrystallization north directed extension lasted for 1 3 m y after which time the yadong gulu rift formed this interpretation is consistent with initiation of east west extension of tibet at ca 8 ma

shortening of analogue models of the continental lithosphere new hypothesis for the formation of the tibetan plateau initial stages of compression produce periodic buckling of models analogous to a four layer continental lithosphere with further shortening amplification of the buckles occurs by thrust faulting at inflection points in both brittle layers of the upper crust and upper mantle the rescaled results suggest that a continental lithosphere under compression will buckle with a first order wavelength of about 200 km and a second order wavelength of 20 30 km continued shortening results in the amplification of undulations troughs sink and become compressional basins while deformation keeps going on major thrusts within the upper mantle thus giving rise to an irregular topography of the moho fifty percent shortening results in a 60 km thick continental crust we argue that this model history may explain the elevation and deep crustal structure of the tibetan plateau

the metamorphic signature of contemporaneous extension and shortening in the central himalayan orogen data from the nyalam transect southern tibet abstract geological relationships and geochronological data suggest that in miocene time the metamorphic core of the central himalayan orogen was a wedge shaped body bounded below by the n dipping main central thrust system and above the n dipping south tibetan detachment system we infer that synchronous movement on these fault systems expelled the metamorphic core southward toward the indian foreland thereby moderating the extreme topographic gradient at the southern margin of the tibetan plateau reaction textures thermobarometric data and thermodynamic modelling of pelitic schists and gneisses from the nyalam transect in southern tibet 28 n 86 e imply that gravitational collapse of the orogen produced a complex thermal structure in the metamorphic core amphibolite facies metamorphism and anatexis at temperatures of 950 k and depths of at least 30 km accompanied the early stages of displacement on the main central thrust system our findings suggest that the late metamorphic history of these rocks was characterized by high t decompression associated with roughly 15 km of unroofing by movement on the south tibetan detachment system in the middle of the metamorphic core roughly 7 8 km below the basal detachment of the south tibetan system the decompression was essentially isothermal near the base of the metamorphic core roughly 4 6 km above the main central thrust the decompression was accompanied by about 150 k of cooling we attribute the disparity between the p t paths of these two structural levels to cooling of the lower part of the metamorphic core as a consequence of continued and probably accelerated underthrusting of cooler rocks in the footwall of the main central thrust at the same time as movement on the south tibetan detachment system

argon 39 argon 40 dating of metamorphic and plutonic events in the north and high himalaya belts southern tibet china new 39ar 40ar radiometric data are presented they concern the metasedimentary series and the orthogneiss pluton of kangmar ages as young as 13 ma are found which may be related either to the metamorphism or to the resetting of the minerals during the emplacement of the numerous intrusive bodies into the north himalaya belt inherited excess argon is found in micas from the orthogneiss previously dated at 562 ma these bodies are constituted by leucogranitic magmas the biotites and the muscovites of which the granites intrude lhagoi kangri massif same range of ages is found in the high himalaya belt for micas from leucogranites ages as young as 4 ma are found on micas from the southern part of the cross section between nyalam and zham the recent uplifts and or the movement of recent normal faults may be related to these young ages

constraints on unroofing rates in the high himalaya eastern nepal thermobarometric data for samples across the main central thrust zone in eastern nepal show an inversion in temperature but not in pressure these data have been interpreted to represent a portion of the paleogeotherm at the time of main central thrust deformation a 40ar 39ar age on hornblende closure temperature tc 500 50 c constrains the timing of this deformation to be 21 0 2 ma the 40ar 39ar ages of other minerals muscovite tc 350 c age t 12 0 0 2 ma k feldspar tc 220 c t 8 0 0 2 ma from the same location further constrain the cooling history of this region together the geochronologic and thermobarometric data yield an average unroofing rate of 1 2 0 6 mm yr for the high himalaya of eastern nepal simple thermal models show that these geochronologic and thermobarometric data are consistent with a wide range of different initial geotherms applied boundary conditions and magnitude of radiogenic heat production the variation through time of the unroofing rates can only be poorly constrained however the unroofing histories were found to be largely insensitive to the details of the assumed initial geotherm fairly sensitive to the magnitude of radiogenic heat production and extremely sensitive to the nature of the boundary conditions applied below the fault zone this study underscores the difficulty in constraining uplift histories on the basis of cooling rates even when thermobarometric data are available to supplement geochronologic constraints on the cooling history of the region

deformation of leucogranites of the crystalline main central sheet in southern tibet china abstract towards the top of the main central sheet in southern tibet china leucogranites intrude metamorphic rocks related to rocks of the main central thrust in the himalayas our study shows that the leucogranites suffered a northward shear deformation the significance of this event is discussed in terms of gravity driven decollement of the 10 000 m thick tethyan sedimentary pile over its crystalline basement

the significance of chemical isotopic and detrital components in three coeval stalagmites from the superhumid southernmost andes 53 s as high resolution palaeo climate proxies abstract stalagmites are important palaeo climatic archives since their chemical and isotopic signatures have the potential to record high resolution changes in temperature and precipitation over thousands of years we present three u th dated records of stalagmites ma1 ma3 in the superhumid southern andes chile 53 s they grew simultaneously during the last five thousand years ka bp in a cave that developed in schist and granodiorite major and trace elements as well as the c and o isotope compositions of the stalagmites were analysed at high spatial and temporal resolution as proxies for palaeo temperature and palaeo precipitation calibrations are based on data from five years of monitoring the climate and hydrology inside and outside the cave and on data from 100 years of regional weather station records water insoluble elements such as y and hree in the stalagmites indicate the amount of incorporated siliciclastic detritus monitoring shows that the quantity of detritus is controlled by the drip water rate once a threshold level has been exceeded in general drip rate variations of the stalagmites depend on the amount of rainfall however different drip water pathways above each drip location gave rise to individual drip rate levels only one of the three stalagmites ma1 had sufficiently high drip rates to record detrital proxies over its complete length carbonate compatible element contents e g u sr mg which were measured up to sub annual resolution document changes in meteoric precipitation and related drip water dilution in addition these soluble elements are controlled by leaching during weathering of the host rock and soils depending on the ph of acidic pore waters in the peaty soils of the cave s catchment area in general higher rainfall resulted in a lower concentration of these elements and vice versa the mg ca record of stalagmite ma1 was calibrated against meteoric precipitation records for the last 100 years from two regional weather stations carbonate compatible soluble elements show similar patterns in the three stalagmites with generally high values when drip rates and detrital tracers were low and vice versa 13c and 18o values are highly correlated in each stalagmite suggesting a predominantly drip rate dependent kinetic control by evaporation and or outgassing only c and o isotopes from stalagmite ma1 that received the highest drip rates show a good correlation between detrital proxy elements and carbonate compatible elements a temperature related change in rainwater isotope values modified the ma1 record during the little ice age 0 7 0 1 ka bp that was 1 5 c colder than today the isotopic composition of the stalagmites ma2 and ma3 that formed at lower drip rates shows a poor correlation with stalagmite ma1 and all other chemical proxies of ma1 hendy tests indicate that the degassing controlled isotope fractionation of ma2 and ma3 had already started at the cave roof especially when drip rates were low changing pathways and residence times of the seepage water caused a non climatically controlled isotope fractionation which may be generally important in ventilated caves during phases of low drip rates our proxies indicate that the neoglacial cold phases from 3 5 to 2 5 and from 0 7 to 0 1 ka bp were characterised by 30 lower precipitation compared with the medieval warm period from 1 2 to 0 8 ka bp which was extremely humid in this region

quantitative kinematic flow analysis from the main central thrust zone nw himalaya india implications for a decelerating strain path and the extrusion of orogenic wedges quantitative kinematic indicators from the main central thrust zone mctz in the nw himalaya have been used to characterize the type of flow during deformation different generations of tension gashes have been rotated by variable angles with respect to the mylonitic foliation forming associated fringe folds these record the late stage brittle ductile flow and reveal that a strong pure shear component of deformation occurred throughout the mctz to characterize earlier deformation increments fabrics from highly deformed quartz ribbons were analyzed well developed shape and lattice preferred orientation patterns show a systematic change of the glide systems and suggest inverted palaeotemperatures within the mctz investigations of the c axes patterns reveal a strong asymmetry at the top of the mctz whereas the samples from the base of the mctz show almost perfectly symmetrical type i crossed girdles deformation within the mctz probably started close to simple shear flow at higher temperatures which progressively became a more general shear during cooling and ended in a pure shear dominated flow during the final stages of brittle ductile deformation i e a decelerating strain path using the higher himalaya crystalline as an example a kinematic model for the extrusion of crustal wedges above major thrust zones is suggested

40ar 39ar age constraints on deformation and metamorphism in the main central thrust zone and tibetan slab eastern nepal himalaya in eastern nepal structural and petrologic observations suggest that movement on the main central thrust mct the development of an inverted metamorphic sequence and leucogranite plutonism were genetically related samples from across the mct zone and its hanging wall the tibetan slab were collected from four locations in the everest region for 40ar 39ar studies 1 the mct zone 2 the lower tibetan slab 3 the upper tibetan slab away from intrusive rocks and 4 the upper tibetan slab adjacent to and including leucogranitic material within the mct zone muscovite and hornblende yield cooling ages of 12 0 0 2 ma and 20 9 0 2 ma respectively k feldspar gives a minimum age of 8 0 0 2 ma closure temperature tc 220 15 c in the lower tibetan slab a sample from a sheared pegmatite yields muscovite and biotite isochron ages of 7 7 0 4 ma and 7 5 0 6 ma and a k feldspar minimum age of 6 4 0 8 ma tc 225 20 c an adjacent gneiss yields a 9 1 0 2 ma biotite isochron and a k feldspar minimum age of 3 6 0 2 ma tc 210 50 c in the upper tibetan slab samples collected 200 m from visible intrusives yield a biotite isochron age of 20 2 0 2 ma and a complex hornblende age spectrum suggestive of excess argon at the fourth location an augen gneiss a pegmatite and a tourmaline bearing leucogranite all in mutual contact yield indistinguishable mineral ages the average biotite and muscovite isochron ages for these samples are 17 0 1 4 and 16 6 0 2 ma respectively the minimum age for k feldspar from the leucogranite is 15 5 1 8 ma as thermobarometric data from the mct zone in this area suggest synmetamorphic deformation at temperatures of 500 550 c the mct hornblende tc 500 c dates this event at 21 ma diffusion experiments on hornblendes from the mct zone provide data which support a maximum duration of peak metamorphic temperatures of 2 ma biotite ages as old as 20 ma from the upper tibetan slab near leucogranites indicate that leucogranite intrusion was essentially coeval with deformation and metamorphism in the mct zone very young ages in a ductile shear zone in the lower tibetan slab suggest that there has been deformation within the tibetan slab that postdates major movement on the mct

40ar 39ar dating of the inner carpathians variscan basement and alpine mylonitic overprinting abstract the 40ar 39ar dating method has been applied to mylonitic rocks from the carpathian mountains these are variscan metamorphic and magmatic rocks that were overprinted during the alpine orogeny the inner western carpathians are conventionally described as a nappe pile formed by northward movement and then subjected to intense strike slip and normal faulting resulting in the formation of mylonitic rocks precise stratigraphie ages provide clear constraints for the geodynamic interpretation related to the succession of alpine events such as the pre gosauian phase during the turonian 90 ma the 40ar 39ar method allows to define the age of the ancient variscan basement and the ages of the alpine overprinting in the high and low tatras mineral ages of around 340 350 ma are yielded by gneisses micaschists and amphibolites providing a younger limit for the variscan metamorphism the emplacement cooling of the variscan granites is defined between 305 and 335 ma these rocks are overprinted by several alpine tectonothermal events the major events are dated at 65 and 86 88 ma during which the mylonites were generated an older event around 140 ma earlier defined by the rb sr method is now confirmed by the 40ar 39ar method dating of phengites extracted from high pressure metamorphic triassic rocks from the kosice area yields new data for the age of this event at 155 ma

late pleistocene to holocene marine transgression and thermohaline control on sediment transport in the western magellanes fjord system of chile 53 s abstract in the western strait of magellan in southernmost chile marine transgression occurred between 14 500 and 13 500 cal bp this is indicated by strongly increased accumulation of biogenic carbonate and first appearance of foraminifers in sediment records from that time until 11 500 cal bp sedimentation in the western fjords became predominant autochthonous due to higher salinity and clay flocculation and late glacial glacier retreat present day thermohaline zonation pattern extensively representative for the holocene and sedimentation rates indicate that westerlies hampered westward outflow of superficial 0 30 m water depth glacial clay rich freshwater from glaciated areas during the holocene isostatic uplift of the andes overcompensated sea level rise in areas with high glacial glacier loading this led to shallowing fjord sills and restricted exchange with marine water especially since high freshwater inflow produced strong pycnoclines and preserved old saline water in fjord bottoms to the east of the climate divide the seno skyring fjord system shows a year round stable stratification despite a strong wind induced eastward superficial current in the upper 30 50 m of the water column such currents enabled significant late glacial eastward transport of glacial clay sediment cores from this area indicate that east ward sediment flux slowed down during the holocene probably due to less intense westerlies investigated present day thermohaline characteristics of the fjord system across the superhumid climate divide of the southern andes indicate details of the exchange between marine and freshwater which are fundamental for evaluation of sediment transport pathways biogenic productivity and interpretation of paleoclimate records in this area

enhanced mercury deposition by amazonian orographic precipitation evidence from high elevation holocene records of the lake titicaca region bolivia abstract very few studies have focused on historical mercury hg deposition in the southern hemisphere especially for the pre mining period therefore the respective contributions of hg emissions from anthropogenic and natural sources are not fully constrained and the long term influence of climate variability is still in debate in this study we reconstructed hg depositions over the last 13 500 years based on two sedimentary records located at the fringe between amazonia and the altiplano multiproxy analyses carried on the same sedimentary archives i e hg accumulation rates ar major elements total organic carbon 13c and pollen analyses show evidence that hg deposition has risen according to amazonian orographic moisture and precipitation during the holocene except for the last 200 years for which modern industry and goldmining overwhelmed the natural signal comparisons with existing hg records indicated that metallurgy and volcanic emissions were not perceptible due to the eastern down wind position of the sites altogether it argues that the historical variability of hg deposition in the central eastern andes has been primarily driven by changes in amazonian moisture at least six main phases of higher hg ar 2 6 1 4 g m 2 y 1 compared to baseline values 0 9 0 3 g m 2 y 1 were identified over the holocene during the early and late holocene phases of higher hg ar coincided with wet episodes recorded in lake titicaca and sajama records during the middle holocene arid phase reconstructed wetter episodes by hg together with vegetation dynamics were attributed to the set up of moisture niches in the eastern flanks of the andes comparisons to other paleoclimatic records supports the role of north atlantic sea surface temperature in forcing precipitation during the late holocene period while other mechanisms allowing moisture niches by orography are discussed for the mid holocene dry phase in the eastern andes region

himalayan metamorphic sequence as an orogenic channel insight from bhutan the bhutan himalayas differ from the rest of the himalayas in two major ways i low grade sedimentary rocks lie above the greater himalayan sequence as klippen i e erosional remains of the south tibetan detachment and ii an out of sequence thrust the kakhtang thrust lies structurally above the klippen and it doubles the exposed thickness of the greater himalayan sequence our field observations and geochronological data constrain the main kinematic events in the bhutan himalayas crystallisation ages of leucogranite dykes deformed by the main central thrust and the south tibetan detachment indicate that these two structures operated together between 16 and 22 ma the out of sequence kakhtang thrust was active at 10 14 ma and was concurrent with reactivation of the south tibetan detachment restoration of the bhutan himalayas prior to the out of sequence thrusting shows that the greater himalayan sequence was the core of a long low viscosity crustal channel extending under the tibetan plateau we propose that the gravity driven southward extrusion of the channel material from underneath the tibetan plateau caused the inverted metamorphic sequence in the lesser himalayan sequence and in the greater himalayan sequence this process also led to occurrences of present day surface rocks that were derived from variable distances down dip but from similar crustal depths such an exhumation pattern can explain the similar peak pressures for the greater himalayan sequence along the length of the himalayas

synorogenic extension quantitative constraints on the age and displacement of the zanskar shear zone northwest himalaya structural thermobarometric and geochronological data place limits on the age and tectonic displacement along the zanskar shear zone a major north dipping synorogenic extensional structure separating the high grade metamorphic sequence of the high himalayan crystalline sequence from the overlying low grade sedimentary rocks of the tethyan himalaya a complete barrovian metamorphic succession from kyanite to biotite zone mineral assemblages occurs within the 1 km thick zanskar shear zone thermobarometric data indicate a difference in equilibration depths of 12 3 km between the lower kyanite zone and the garnet zone which is interpreted as a minimum estimate for the finite vertical displacement accommodated by the zanskar shear zone for the present day dip of the structure 20 a simple geometrical model shows that a net slip of 35 9 km is required to regroup these samples to the same structural level because the kyanite to garnet zone rocks represent only part of the zanskar shear zone and because its original dip may have been less than the present day dip these estimates for the finite displacement represent minimum values field relations and petrographic data suggest that migmatization and associated leucogranite intrusion in the footwall of the zanskar shear zone occurred as a continuous process starting at the barrovian metamorphic peak and lasting throughout the subsequent extension induced exhumation geochronological dating of various leucogranitic plutons and dikes in the zanskar shear zone footwall indicates that the main ductile shearing along the structure ended by 19 8 ma and that extension most likely initiated shortly before 22 2 ma

tectonics and structural zonation of southern tibet china the yarlung zangbo suture zone is composed of several thrust sheets with distinct stratigraphies states of deformation and metamorphic grades successive deformations have been produced by persistent north south shortening since obduction of the ophiolites by late cretaceous palaeocene times a possible decoupling of the crust may be associated with the intracontinental thrust system which has formed in the subducted indian plate this article emphasizes the role of shear deformations in the building of a collision belt

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

two thousand years of atmospheric arsenic antimony and lead deposition recorded in an ombrotrophic peat bog profile jura mountains switzerland a peat core from a swiss bog reveals significant enrichments of as sb and pb extending back to roman times indicating that the anthropogenic fluxes of these metals have exceeded the natural fluxes for more than 2000 years the isotopic composition of pb provides no evidence of vertical downward pb migration suggesting that the bog has faithfully preserved the historical record of atmospheric pb deposition age dating using210pb verified independently using pollen markers provides the chronology of changing metal concentrations during the past 150 years present day enrichment factors relative to the metal sc ratios of typical crustal rocks are of the order of 20 times as 70 sb and 130 pb given the potential toxicity of as and sb these new findings suggest that the environmental significance of these and perhaps other less common trace elements deserve more attention

modelling the effect of arbitrary p t t histories on argon diffusion in minerals using the macargon program for the apple macintosh argon diffusion in mineral grains has been numerically modelled using p t t histories that may be relevant to multiply metamorphosed orogenic terranes and for rocks that have resided at high ambient temperatures in the earth s crust for long durations the macargon program1 generates argon concentration profiles in minerals assuming argon loss occurs via volume diffusion it can be run on an apple macintosh computer with arbitrary p t t histories used as input finite difference equations are used in the calculation of 40ar concentration profiles across individual diffusion domains the associated macspectrometer generates model spectra after a p t t history has been specified the form of model 40ar39ar apparent age spectra suggests that considerable caution needs to be exercised in the use of the closure temperature concept and in the interpretation of the significance of plateaux observed in many 40ar39ar apparent age spectra particularly in cases involving metamorphic rocks where complex p t t histories might apply although modelled spectra cannot be directly compared to experimentally determined 40ar39ar age spectra especially when hydrous phases are involved or in cases where loss of argon has not occurred via volume diffusion they do provide insight into theoretically expected age spectra for samples that have experienced complex p t t histories

emplacement of himalayan leucogranites by magma injection along giant sill complexes examples from the cho oyu gyachung kang and everest leucogranites nepal himalaya abstract the upper part of the high himalayan slab in north central nepal is comprised of a thick layer parallel sheet of biotite muscovite tourmaline garnet sillimanite cordierite leucogranite up to 3 4 km thick and dipping north at 5 20 these strongly peraluminous magmas were emplaced into high temperature low pressure sillimanite and cordierite bearing gneisses calc silicates and rare amphibolites which were metamorphosed at temperatures of 600 650 c some time during the oligocene early miocene parallel stringers of black xenolithic gneisses within the leucogranites suggest passive magmatic intrusion along fractures parallel to the schistosity in the country rocks in the mountains of cho oyu gyachung kang pumori lingtren and the base of the everest massif these leucogranites form part of a single structural horizon bounded at the top by the lhotse detachment the lower of two n dipping normal faults of the south tibetan detachment std system and below by the khumbu thrust kt an out of sequence fault which was partly responsible for the uplift erosion and exhumation of the leucogranites a model for the emplacement of these leucogranites is proposed where they represent viscous minimum melts produced by melting of a pelitic protolith similar to the underlying sillimanite grade gneisses through muscovite breakdown either during fluid absent melting at

vertical concentration profiles of lead in the central pacific at 15 n and 20 s concentrations of lead were measured in a surface transect and at two vertical profile stations 15 n and 20 s in the central pacific these measurements complement similar measurements made earlier in the north pacific at 33 n and in the northwest atlantic at 34 n as well as recent measurements of eolian lead input fluxes near each of these locations the new transect of surface water concentrations of lead corroborates previous measurements which decrease from 13 ng kg at 30 n to 4 ng kg at 17 s in the central pacific this transect gradient is shown to overlie a similar geographic gradient of subsurface maximum concentrations of lead in the three pacific vertical profile stations decreasing from 14 ng kg at 33 n to 11 ng kg at 14 n to 2 5 ng kg at 20 s lead concentrations at each of those locations exhibit maxima at 400 m decreasing concentrations to 2500 m and approximately concentrations of 0 8 1 1 ng kg below that depth the subsurface maximum at the northwest atlantic profile station 36 ng kg at 34 n is also congruent with surface water lead concentrations which decrease from 806 ng kg to 32 ng kg in an offshore transect from rhode island to 34 n 66 w and the shape of the atlantic profile is congruent with those in the pacific there is a positive correlation between the magnitudes of eolian lead input fluxes and the magnitudes of the upper water maxima in lead concentration profiles at corresponding locations as follows south pacific easterlies 3 ng cm 2 yr vs 2 5 ng kg north pacific easterlies 6 ng cm 2 yr vs 11 ng kg north pacific westerlies 50 ng cm 2 yr vs 14 ng kg and north atlantic westerlies 170 ng cm 2 yr vs 36 ng kg this relationship enables one to view the anthropogenic perturbations of the marine lead cycle on a global scale since the industrial origin of eolian and seawater lead has been established by correlations between geographic patterns of industrial lead emissions to the atmosphere and isotopic ratios of industrial leads and by geographic patterns of pb silicate dust ratios and lead isotopic ratios in ocean surface waters these new data coupled with earlier biogeochemical data indicate that surface water concentrations of lead in the north pacific and north atlantic are now conservatively estimated to be 8 to 20 fold greater and those in the south pacific are 2 fold greater than natural concentrations because of industrial emissions of lead to the atmosphere

yadong cross structure and south tibetan detachment in the east central himalaya 89 90 e the yadong cross structure ycs occurring at 89 30 east longitude in the himalaya is the largest across strike discontinuity in the geologic structure and topography of the high himalaya between the himalayan syntaxes it is manifest by a plan view left offset of the topographic crest of the range and a coincident apparent left strike offset of the south tibetan detachment system stds of about 70 km new field mapping indicates that the stds intersects the west side of the ycs at zherger la and further suggests that the ycs is the surface expression of a large north northeast striking west facing lateral ramp in the himalayan thrust system the greater himalayan allochthon is apparently draped across this lateral ramp resulting in a north northeast striking monoclinal flexure of the allochthon manifest in the chomolhari range superimposed steep west northwest directed normal shear appears to have obliterated earlier northerly directed stds shear fabric along along the ycs segment of the tethyan belt greater himalayan belt contact recent seismicity suggests that the greater himalayan allochthon is still moving southward along the lateral ramp u pb monazite dates on leucogranites in the footwall of the stds on opposite sides of the ycs are suggestive of a south to north decrease in the crystallization age of these granites the age pattern is consistent with southward extrusion of the greater himalayan allochthon and suggests the possibility of determining an average slip rate for the stds by dating granites in the immediate footwall of the stds that are substantially separated in north south section an average slip rate of about 7 mm yr between 23 and 12 ma is suggested by the data presented here

episodic rapid uplift in the himalaya revealed by 40ar 39ar analysis of detrital k feldspar and muscovite bengal fan detrital k feldspar and muscovite samples from ocean drilling program leg 116 cores have been dated by the sup 40 ar sup 39 ar technique and have depositional ages from 0 to 18 ma from 4 to 13 individual k feldspars and 1 to 12 individual muscovites have been dated from 7 stratigraphic levels in every level at least one k feldspar and one muscovite yielded a minimum age identical within uncertainty to the age of deposition these results indicate that a significant portion of the material in the bengal fan is first cycle detritus derived from the himalaya therefore the substantial amount of sediment deposited in the distal fan in early to middle miocene time can be ascribed to a significant pulse of uplift and erosion in the collision zone at this time moreover these data indicate that throughout the neogene some part of the himalayan orogen was undergoing rapid erosion 1 to 10 mm yr this erosion must have been less than or equal to uplift relative to sea level the lack of granulite facies rocks in the eastern himalaya and tibetan plateau suggests to us that very rapid uplift must have been distributed in brief pulses over different parts of the mountain belt these datamore are incompatible with tectonic models in which the himalaya and tibetan plateau are uplifted either uniformly over the past 40 m y or mostly within the past 2 to 5 m y less

shear heating in continental strike slip shear zones model and field examples summary a two layer crust and upper mantle finite diverence steady state thermomechanical model of a long lived several million years lithospheric strike slip fault is presented and its predictions compared with field observations from various major fault zones in order to estimate the maximum amount of shear heating all mechanical energy is assumed to be dissipated in heat in ductile as well as in brittle layers deformation follows a friction law in the brittle layer s and a power flow law in the ductile one s variations of several independent parameters and their influence on the thermomechanical state of the fault zone and on shear heating are systematically explored shear heating is found to be more important in fault zones avecting an initially cold lithosphere and increases with slip rate friction coeycient and stivness of materials in extreme cases slip rate of 10 cm yr 1 stiv lithosphere shear heating could lead to temperature increases close to 590 c at the moho and 475 c at 20 km depth for more common cases shear heating leads to smaller temperature increases but can still explain high grade metamorphic conditions encountered in strike slip shear zones however modelled temperature conditions often fall short of those observed this could be due to heat transport by mechanisms more eycient than conduction common syntectonic emplacement of granitic melts in ductile strike slip shear zones can be explained by lower crust partial melting induced by shear heating in the upper mantle besides slip rate the possibility of such melting depends mostly on the upper mantle rheology and on the fertility of the lower crust for hard upper mantle and highly fertile lower crust partial melting could occur at rates of 1 cm yr 1 while in most cases it would result from the breakdown of micas for slip rates over 3 cm yr 1 as a result of shear heating partial melting of the upper mantle could occur in the presence of small amounts of fluids rise of magmas and or hot fluids in the shear zone will further enhance the temperature increase in shallower parts of the fault zone in nature shear heating would inevitably cause strain localization in the deeper parts of strike slip faults as is often observed in the field for crustal shear zones

geoacoustic inversion in a dispersive waveguide using warping operators this paper presents a single receiver geoacoustic inversion method adapted for low frequency impulsive sources it is applied to light bulb data collected during the shallow water 2006 experiment the inversion is carried out by extracting dispersion curves from the received signal and comparing them to simulated replicas to achieve dispersion curve estimation in the time frequency domain modal separability is improved using a signal processing method called warping the inversion scheme allows for a reliable estimation of the new jersey shelf sediment properties compressional sound speed and density it also provides an accurate estimation of the source receiver range