accelerator mass spectrometry radiocarbon dating of rock varnish accelerator mass spectrometry ams 14c dating off rock varnish provides minimum limiting ages for landforms and archaeological artifacts in arid and semiarid lands that are undatable by conventional radiocarbon methods experiments on sample collection the effect of different individuals preparing samples the influence of different chemical extraction procedures the incorporation of carbonate detritus possible contamination from rock underlying varnish the role of bio geochemical erosion of varnish and other influences reveal procedures that yield reproducible results conventional radiocarbon dates from charcoal beneath lava flows of hualalai volcano hawaii and from arid sites in western north america provide controls to test varnish radiocarbon dating ams radio carbon ages on the very bottom layer of varnishes are typically 10 younger than the 14c ages of these controls applications are illustrated here for fluvial aeolian peri glacial hillslope lacustrine and glacial geo morphology as well as rock art research in archaeology

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

crustal structure and earthquake focal depths beneath northeastern india and southern tibet summary we use broad band teleseismic data recorded at eight sites along a north south profile from karimganj 24 84 n 92 34 e south of the eastern shillong plateau to bomdilla 27 27 n 92 41 e in the eastern lesser himalaya to determine the seismic characteristics of the crust in northeastern india we also analyse data from the chinese digital seismic network station at lhasa and indepthii stations located on the southern tibetan plateau north of our profile to extend the seismic images of the crust further northwards although the northeastern indian stations and the tibetan stations do not lie along a linear profile across the himalaya the wellrecognized uniformity of the himalaya along strike make this comparison of the two profiles meaningful receiver functions calculated from these data show that the crust is thinnest 35 38 km beneath the shillong plateau receiver functions at cherrapunji on the southern edge of the shillong plateau have a strong azimuthal dependence those from northern backazimuth events show that the moho beneath the southernmost shillong plateau is at a depth of 38 km while receiver functions from southern backazimuth events indicate that the moho beneath the northernmost bengal basin is at a depth of 44 km receiver functions from sites on the brahmaputra valley demonstrate that the moho is deeper by 5 7 km than below the shillong plateau a result which agrees with the hypothesis that the shillong plateau is supported by shearing stress on two steep faults that cut through the crust further north of the eastern himalayan foredeep the moho dips gently northwards reaching a depth of 48 km beneath bomdilla in the lesser himalaya and 88 km below lhasa in tibet using the crustal velocity models obtained from receiver function inversions we redetermined focal depths of wellrecorded earthquakes across this part of the indo tibetan collision zone and find all of these to occur within the crust hence we find no evidence for bimodal depth distribution of earthquakes beneath this region of northeastern india

models of rock varnish formation constrained by high resolution transmission electron microscopy nanometre scale transmission electron microscope imagery of manganiferous rock varnishes from death valley california peru antarctica and hawaii confirms prior infrared mineralogy studies the building blocks of rock varnish are clay minerals that are cemented to the rock by oxyhydroxides of manganese and iron rock varnish is layered on the scale of nanometres with the basic structure defined by the subparallel alignment of detrital clay minerals although only a few examples of bacteria were found possible cell wall encrustations are ubiquitous and aligned with the clay minerals mn fe oxides appear to be mobilized from bacterial casts and then reprecipitate on clay minerals that weather into monolayers these observations have implications for varnish dating and palaeoenvironmental techniques k ar and uranium series dating of rock varnish oxides can only yield minimum ages however this small spatial scale of mobilization would not alter the signal from micron scale microchemical laminations

two dimensional modeling of the p t t paths of regional metamorphism in simple overthrust terrains quantitative modeling of metamorphic p t t paths in thrust belts has so far been limited to one dimensional simplification we have developed a two dimensional time dependent finite element procedure capable of tracing and updating fault and strata boundaries during faulting for modeling thermal evolution of overthrust terrains we show that the two dimensionality of an overthrust and the speed and duration of thrusting have significant effects on the p t t paths of rocks in orogenic belts the results suggest that petrological studies of p t paths of rocks from different lateral positions in an orogenic belt along with two dimensional modeling procedure can put strong constraints on models of regional tectonic processes

control of detachment geometry on lateral variations in exhumation rates in the himalaya insights from low temperature thermochronology and numerical modeling 1 the himalayan range is commonly presented as largely laterally uniform from west to east however geological structures topography precipitation rate convergence rates and low temperature thermochronological ages all vary significantly along strike here we focus on the interpretation of thermochronological data sets in terms of along strike variations in geometry and kinematics of the main crustal detachment underlying the himalaya the main himalayan thrust mht we report new apatite fission track aft ages collected along north south transects in western and eastern central nepal at the latitudes of the annapurna and langtang massifs respectively aft ages are consistently young 3 ma along both n s transects in the high relief zone of the higher himalaya and increase 4 to 6 ma toward the south in the lesser himalaya we compare our new data to published low temperature thermochronological data sets for nepal and the bhutan himalaya we use the full data set to perform both forward and inverse thermal kinematic modeling with a modified version of the pecube code in order to constrain potential along strike variations in the kinematics of the himalayan range our results show that lateral variations in the geometry of the mht in particular the presence or absence of a major crustal scale ramp strongly control the kinematics and exhumation history of the orogen

bacterial oxidation of manganese and iron in a modern cold spring a marshy cold spring occurs in late pleistocene glacial outwash exposed on the floor of the squalicum creek valley near bellingham washington a 5 m by 25 m zone of black soil with an average depth of 30 cm surrounds the spring this soil contains about 43 mno 2 and 20 to 30 iron oxide calculated as fe 2 o 3 the oxidized material appears amorphous when analyzed by x ray diffraction field observations and laboratory studies indicate mn and fe are accumulated due to bacterial oxidation of trace amounts of these metals supplied by ground water two strains of pseudomonad bacteria isolated from the black soil rapidly oxidize fe and mn when grown on a culture medium containing soil organic matter as a nutrient both bacteria can be cultured on tryptone glycerol agar but are unable to oxidize mn or fe added to this synthetic medium although the composition of the culture medium is important in controlling whether oxidation occurs the reaction is not merely due to the catalytic effect of hydroxy acids contained within the nutrient mixture optical microscopy and x ray fluorescence analysis using a scanning electron microscope equipped with an energy dispersive detector reveal that fe and mn oxides precipitated by the microbes occur as extra cellular deposits and these metals are not accumulated within the bacterial cells although microbiologists have shown that several genera of bacteria are able to oxidize mn in laboratory cultures these studies are of uncertain value when used to explain the role microbes play in sedimentary mn oxide deposits because most studies have involved microbes isolated from ordinary soil rather than from mn rich environments this locality demonstrates the ability of soil bacteria to accumulate high concentrations of fe and mn under natural conditions

generation of abnormal trace element abundances in antarctic eucrites by weathering processes abstract based on rees antarctic eucrites can be divided into two groups those showing normal trace element characteristics e g similar to juvinas and those showing abnormal trace element abundances many antarctic eucrite polymict eucrite and basaltic clast samples show the abnormal trace element abundances with ree patterns exhibiting positive ce anomalies sometimes negative ce anomalies positive eu anomalies and low abundances of the remainder of the rees with the lrees generally being at lower relative abundances than the hrees most samples of crystalline clasts from the polymict eucrites lew85300 lew85302 and lew85303 show the abnormal patterns while the glassy matrixes of these meteorites show normal patterns exterior samples generally show more abnormal patterns larger anomalies greater depletions than interior samples from the same meteorites comparison of all basaltic eucrite literature data combined with our data shows that positive and negative ce anomalies and positive eu anomalies are found in about 61 of antarctic eucrite analyses and are virtually unknown in non antarctic eucrite analyses further positive ce anomalies and positive eu anomalies are commonly associated with antarctic eucrites having low ree concentrations consideration of mineral melt partition coefficients shows that it is unlikely that ce anomalies are magmatic features from the hed parent body cerium anomalies on earth are generally restricted to the weathering zone where the relative ease of oxidizing ce to the 4 state allows for fractionation of ce from the 3 rees we believe the unusual ree patterns of abnormal antarctic eucrites arise from weathering effects generated in or on the antarctic ice our suggested scenario involves formation of melt water and its equilibration with the atmosphere which promotes dissolution of ree rich phosphates and oxidation of ce tetravalent ce can then be fractionated from the trivalent ree in solution the details of the weathering process are unclear and will require detailed chemical and sem investigations of eucrites for their elucidation we predict that rapidly chilled eucrites with glassy rather than crystalline mesostases will be more likely to survive the antarctic environment without alteration of their ree patterns occasional s se and k enrichments are likely due to weathering in the antarctic environment as well but these enrichments are not well correlated with ce anomalies

how fast does rock varnish grow rates of rock varnish accumulation have never been systematically documented owing to the difficulty in accurately determining both varnish thickness and age in this study we quantitatively assess varnish accumulation rates through thin sectioning and microscopic examination of rock varnish from radiometrically dated geomorphic features of late quaternary age in the western united states drylands our data indicate that rock varnish grows at rates ranging from 1 to 40 m k y on subaerially exposed rock surfaces and rarely reaches a thickness exceeding 200 m regardless of age our data also indicate that varnish accumulation rates vary greatly from sample to sample at a given site suggesting that varnish thickness does not correlate with the age of the associated geomorphic feature invalidating the potential use of varnish thickness as a relative age indicator in geomorphology and archaeology nevertheless being the slowest known accumulating terrestrial sedimentary deposit rock varnish constitutes a unique long term microscale sedimentary archive of past environmental changes in deserts

stratigraphy structure and tectonic evolution of the himalayan fold thrust belt in western nepal regional mapping stratigraphic study and 40ar 39ar geochronology provide the basis for an incremental restoration of the himalayan fold thrust belt in western nepal tectonostratigraphic zonation developed in other regions of the himalaya is applicable with minor modifications in western nepal from south to north the major structural features are 1 the main frontal thrust system comprising the main frontal thrust and two to three thrust sheets of neogene foreland basin deposits 2 the main boundary thrust sheet which consists of proterozoic to early miocene lesser himalayan metasedimentary rocks 3 the ramgarh thrust sheet composed of paleoproterozoic low grade metasedimentary rocks 4 the dadeldhura thrust sheet which consists of medium grade metamorphic rocks cambrian ordovician granite and granitic mylonite and early paleozoic tethyan rocks 5 the lesser himalayan duplex which is a large composite antiformal stack and hinterland dipping duplex and 6 the main central thrust zone a broad ductile shear zone the major structures formed in a general southward progression beginning with the main central thrust in late early miocene time eocene oligocene thrusting in the tibetan himalaya north of the study area is inferred from the detrital unroofing record on the basis of 40ar 39ar cooling ages and provenance data from synorogenic sediments emplacement of the dadeldhura thrust sheet took place in early miocene time the ramgarh thrust sheet was emplaced between 15 and 10 ma the lesser himalayan duplex began to grow by 10 ma simultaneously folding the north limb of the dadeldhura synform the main boundary thrust became active in latest miocene pliocene time transport of its hanging wall rocks over an 8 km high footwall ramp folded the south limb of the dadeldhura synform thrusts in the subhimalayan zone became active in pliocene time the minimum total shortening in this portion of the himalayan fold thrust belt since early miocene time excluding the tibetan zone is 418 493 km the variation depending on the actual amounts of shortening accommodated by the main central and dadeldhura thrusts the rate of shortening ranges between 19 and 22 mm yr for this period of time when previous estimates of shortening in the tibetan himalaya are included the minimum total amount of shortening in the foldthrust belt amounts to 628 667 km this estimate neglects shortening accommodated by small scale structures and internal strain and is therefore likely to fall significantly below the actual amount of total shortening

influence of thrust belt geometry and shortening rate on thermochronometer cooling ages insights from thermokinematic and erosion modeling of the bhutan himalaya advancements in thermochronology and numerical modeling offer the potential to associate the age of thermochronometric samples to both exhumational and deformational processes however understanding how these components are related in compressional systems requires linking the geometry and magnitude of fault slip to the distribution and amount of erosion to address this we apply a 2 d thermokinematic model to a forward modeled balanced cross section to quantify the cooling history in fold thrust belt settings the restored cross section provides a kinematic path of rocks and structures necessary to reproduce the surface geology by assigning ages to displacement amounts we produced a range of potential velocity vectors used to calculate heat transport erosion and rock cooling we test the predicted ages against a suite of previously published thermochronometric data from the bhutan himalaya to explore the utility of the data to constrain the timing rate and geometry of fault motion as well as variations in the exhumation rate we evaluate the cooling history associated with a constant rate of shortening of 18 mm yr rates that are 2 0 1 5 0 75 and 0 5 times the constant rate and rates that vary with time to determine which kinematic history best matches the measured cooling ages the combination of relatively old apatite fission track and zircon u th he measured ages and younger 15 9 ma 40ar 39ar ages from white mica is best matched with faster rates relative to constant rates between 11 5 and 8 ma and slower than constant rates from 17 to 11 5 ma and 8 ma to present

baking black opal in the desert sun the importance of silica in desert varnish comment and reply reply we welcome the opportunity to reply to dorn 2007 who outlines the classical issues of desert varnish formation we agree that most previous research has suggested a biological mechanism bacterial enhancement of manganese in manganese rich coatings is intriguing however as we have published

rapid long term erosion in the rain shadow of the shillong plateau eastern himalaya abstract geodynamic models of collisional orogens suggest that precipitation gradients profoundly influence spatial patterns of exhumation and deformation in active collisional mountain ranges a basic tenet of this hypothesis is that in unglaciated areas spatial patterns of long term precipitation erosion and exhumation should be correlated a correlation of this type has been observed in the eastern himalaya where uplift of the shillong plateau by pliocene time drastically reduced monsoonal rainfall in the himalayan range downwind existing apatite fission track data suggest that the resulting precipitation gradient caused a twofold gradient in long term erosion rates across an area with similar geology suggesting a strong influence of climate on the region s geomorphic and tectonic evolution we extend this dataset by presenting 53 new bedrock apatite and zircon fission track ages from deeper within the rain shadow we expected latest miocene to pliocene apatite ages similar to previously published ages from neighboring areas in the rain shadow instead apatites as young as 1 3 0 2 ma and zircons as young as 4 5 1 0 ma 2 demonstrate that spatial gradients in precipitation do not correlate with variations in long term erosion and crustal strain as predicted by geodynamic models thermal kinematic modeling of these data suggests that local exhumation patterns reflect gradients in rock uplift dictated by fault kinematics in this rapidly deforming area despite a dramatic precipitation gradient these findings both highlight the need to better understand how erosive processes scale with precipitation amount and intensity in such settings and suggest a disconnect between the predictions of orogen scale geodynamic models and the relationship between erosion and tectonics at the regional scale

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

quaternary bedrock erosion and landscape evolution in the sor rondane mountains east antarctica reevaluating rates and processes abstract rates and processes of rock weathering soil formation and mountain erosion during the quaternary were evaluated in an inland antarctic cold desert the fieldwork involved investigations of weathering features and soil profiles for different stages after deglaciation laboratory analyses addressed chemistry of rock coatings and soils as well as 10be and 26al exposure ages of the bedrock less resistant gneiss bedrock exposed over 1 ma shows stone pavements underlain by in situ produced silty soils thinner than 40 cm and rich in sulfates which reflect the active layer thickness the absence of cryoturbation and the predominance of salt weathering during the same exposure period more resistant granite bedrock has undergone long lasting cavernous weathering that produces rootless mushroom like boulders with a strongly fe oxidized coating the red coating protects the upper surface from weathering while very slow microcracking progresses by the growth of sulfates geomorphological evidence and cosmogenic exposure ages combine to provide contrasting average erosion rates no erosion during the quaternary is suggested by a striated roche moutonnee exposed more than 2 ma ago differential erosion between granite and gneiss suggests a significant lowering rate of desert pavements in excess of 10 m ma 1 the landscape has been on the whole stable but the erosion rate varies spatially according to microclimate geology and surface composition

critical taper model of fold and thrust belts and accretionary wedges the fold and thrust belts and submarine accretionary wedges that lie along compressive plate boundaries are one of the best understood deformational features of the earth s upper crust although there is considerable natural variation among the many fold and thrust belts and accretionary wedges that have been recognized and explored several features appear to be universal in cross section fold and thrust belts and accretionary wedges occupy a wedge shaped deformed region overlying a basal detachment or decollement fault the rocks or sediments beneath this fault show very little deformation the decollement fault characteristically dips toward the interior of the mountain belt or in the case of a submarine wedge toward the island arc the topography in contrast slopes toward the toe or deformation front of the wedge deformation within the wedge is generally dominated by imbricate thrust faults verging toward the toe and related fault bend folding two north american fold and thrust belts that exhibit these features are shown in figure 1 neither of these two examples is tectonically active today the southern canadian fold and thrust belt was active during the late jurassic and cretaceous 150 100 ma whereas the southern appa lachians were deformed during the late carboniferous to permian alle ghenian orogeny 300 250 ma figure 2 shows two examples that are currently active the taiwan fold and thrust belt produced by the sub duction of the eurasian plate beneath the philippine sea plate suppe 1981 1987 and the barbados accretionary wedge produced by the sub

manganese rich rock varnish does occur in antarctica abstract despite accounts to the contrary we have found that mn rich rock varnish is present in antarctica as others have before it is chemically and texturally similar to many varnishes found in lower latitudes antarctic varnishes show considerable potential as a dating and paleoenvironmental research tool

baking black opal in the desert sun the importance of silica in desert varnish comment and reply comment perry et al 2006 1 contribute a fourth alternative of silica binding to three prior hypotheses of varnish formation 1 abiotic mn fe enhancement e g engel and sharp 1958 2 2 biotic mn fe enhancement e g dorn and oberlander 1982 3 allen et al 2004 4 and 3 a

argon behaviour in an inverted barrovian sequence sikkim himalaya the consequences of temperature and timescale on 40ar 39ar mica geochronology abstract 40 ar 39 ar dating of metamorphic rocks sometimes yields complicated datasets which are difficult to interpret in terms of timescales of the metamorphic cycle single grain fusion and step heating data were obtained for rocks sampled through a major thrust sense shear zone the main central thrust and the associated inverted metamorphic zone in the sikkim region of the eastern himalaya this transect provides a natural laboratory to explore factors influencing apparent 40 ar 39 ar ages in similar lithologies at a variety of metamorphic pressure and temperature p t conditions the 40 ar 39 ar dataset records progressively younger apparent age populations and a decrease in within sample dispersion with increasing temperature through the sequence the white mica populations span 2 9 ma within each sample in the structurally lower levels garnet grade but only 0 3 ma at structurally higher levels kyanite sillimanite grade mean white mica single grain fusion population ages vary from 16 2 3 9 ma 2 to 13 2 1 3 ma 2 from lowest to highest levels white mica step heating data from the same samples yields plateau ages from 14 27 0 13 ma to 12 96 0 05 ma biotite yield older apparent age populations with mean single grain fusion dates varying from 74 7 11 8 ma 2 at the lowest structural levels to 18 6 4 7 ma 2 at the highest structural levels the step heating plateaux are commonly disturbed temperatures 600 c at pressures of 0 4 0 8 gpa sustained over 5 ma appear to be required for white mica and biotite ages to be consistent with diffusive open system cooling at lower temperatures and or over shorter metamorphic timescales more 40 ar is retained than results from simple diffusion models suggest diffusion modelling of ar in white mica from the highest structural levels suggests that the high temperature rocks cooled at a rate of 50 80 c ma 1 consistent with rapid thrusting extrusion and exhumation along the main central thrust during the mid miocene

paleocene eocene foreland basin evolution in the himalaya of southern tibet and nepal implications for the age of initial india asia collision siliciclastic sedimentary rocks derived from the southern lhasa terrane sitting depositionally upon rocks of the northern indian passive continental margin provide an estimate of the age of initial contact between the continental parts of the indian and asian plates we report sedimentological sedimentary petrological and geochronological data from upper cretaceous paleocene strata in the sangdanlin section located along the southern flank of the indus yarlung suture zone in southern tibet this is probably the most proximal and therefore the oldest record of the india asia collision these strata were deposited by high density turbidity currents or concentrated density flows and suspension settling of pelagic biogenic debris in a deep marine setting an abrupt change from quartz arenitic to feldspatholithic sandstone compositions marks the transition from indian to asian sediment provenance the abrupt compositional change is accompanied by changes in u pb ages of detrital zircons diagnostic of a sediment provenance reversal from indian to asian sources the timing of the transition is bracketed between 60 ma and 58 5 0 6 ma by detrital zircon u pb ages and zircon u pb ages from a tuffaceous bed in the upper part of the section in the context of a palinspastically restored regional paleogeographic framework data from the sangdanlin section combined with previously published data from the northern tethyan himalaya and the frontal nepalese lesser himalaya and subhimalaya suggest that a flexural wave migrated 1300 km southward across what is now the himalayan thrust belt from paleocene time to the present

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

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

new insight into the south tibetan detachment system not a single progressive deformation 1 low angle normal faults lanf typically regarded as accommodating crustal or lithospheric extension may also form during lithospheric shortening the best studied system of syn contractional lanfs is the south tibetan detachment system a network of low angle normal sense faults and shear zones that formed coevally with and parallel to south vergent thrusts during lithospheric shortening accompanying development of the himalayan orogen in the eastern himalaya there are several across strike exposures of the south tibetan detachment system we present new structural and thermometry data from the eastern himalaya that demonstrate that the south tibetan detachment system cannot have formed as a single progressive structure we characterize and distinguish two distinct structural and tectonic components within the currently recognized system 1 an extensive diffuse sheared layer that formed the boundary between strong upper crust and weak southward flowing middle crust and 2 a network of brittle ductile lanfs that locally exhume partly excise and overprint the earlier mylonite zone at the topographic break between the himalayan orogen and the tibetan plateau the sheared layer not a lanf formed the boundary between upper and middle crust during ductile flow of the middle crust and is extensively exposed in the himalaya at the base of klippen of upper crustal rocks preserved in bhutan along the crest of the himalaya where it has been excised and exhumed by the brittle ductile extrusion lanfs and bounding the cores of the north himalayan gneiss domes citation kellett d a and d grujic 2012 new insight into the south tibetan detachment system not a single progressive deformation tectonics 31 tc2007 doi 10 1029 2011tc002957

on steady states in mountain belts the dynamic system of tectonics and erosion contains important feedback mechanisms such that orogenic systems tend toward a steady state this concept is often invoked but the nature of the steady state is commonly not specified we identify four types of steady state that characterize the orogenic system and illustrate these cases by using numerical model results and natural examples these types are 1 flux steady state 2 topographic steady state 3 thermal steady state and 4 exhumational steady state they refer to the erosional flux the topography the subsurface temperature field and the spatial pattern of cooling ages respectively models suggest that the topography will reach a steady mean form at the scale of an orogenic belt but perfect topographic steady state is unlikely to be achieved at shorter length scales thermal steady state is a precondition for exhumational steady state and in the case of temperature dependent deformation topographic steady state exhumational steady state is characterized by reset age zones spatially nested according to closure temperature as illustrated in natural systems from new zealand the cascadia accretionary margin and taiwan

how steep are the himalaya characteristics and implications of along strike topographic variations differences in the topography of central nepal and bhutan are characterized by map and profile patterns of elevation slope relief and stream gradients nepal exhibits a narrow hinterland zone of extreme relief steep slopes and channels and deep fluvial dissection and a wide foreland zone of lower relief active shortening on the main frontal thrust occurs far outboard of the steep topography in contrast bhutan has two high relief zones separated by a narrow low relief step in the topographic and river profiles the southern zone has the higher relief rising abruptly from the main frontal thrust to a series of 4000 m peaks whereas the northern zone consists of low relief high elevation plateau like terrain cut by widely spaced steep walled valleys spatial relationships between these geomorphic features and geologic contrasts are used to examine scenarios of tectonic and surface process interactions the observations favor a model of along strike segmentation in which timing differences in the varying balance of uplift and erosion produce spatial differences of topographic and geologic patterns although other interpretations are possible

seismotectonics of the himalayan collision zone geometry of the underthrusting indian plate beneath the himalaya fault plane solutions and well determined focal depths of medium sized earthquakes topography and landsat imagery in conjunction with seismicity maps cross sections and available geological information are used to investigate the present tectonics of the himalayan continental collision zone most of the accurately located epicenters of events along the himalayan arc 78 e 95 e that occurred between 1961 and 1981 are concentrated in a narrow zone about 50 km wide lying between the northerly dipping main boundary mbt and main central mct thrusts most of these events are located just south of the mct though the epicenters of the events are in general well located their depths as determined by teleseismic travel time data are very unreliable events with accurately determined depths obtained from identification of surface reflected phases define a simple planar zone from about 10 km and 20 km depth with an apparent dip of about 15 this result is all the more remarkable considering that the events used were located along about an 1800 km length of the himalyan arc except for one all available focal mechanisms of events within this zone indicate shallow 30 north dipping thrusts this shallow north dipping zone apparently defines a part of the detachment that separates the underthrusting indian plate from the lesser himalayan crustal block the spatial extent and the geometry of this interplate thrust zone strongly indicate that the mbt and nearby subsidiary surface and blind thrusts rather than the mct are currently the most active structures of the himalayan arc we suggest that the great himalayan earthquakes m 8 occur along the same detachment surface as defined by the thrust type medium sized events events located to the south of the mbt and beneath the ganges foredeep show normal faulting with t axes perpendicular to the himalayan trend the above results suggest that the indian continental plate is underthrusting the himalayan crustal blocks in a relatively coherent and simple geometry and that this geometry is not much different from that observed along oceanic subduction zones the november 19 1980 earthquake that occurred near the mct near 88 5 e shows a predominantly strike slip focal mechanism one of the nodal planes of this mechanism is transverse to the himalayan structural grain and moreover this plane has a trend similar to that of the recently mapped yadong gulu rift in the tethyan himalaya and in southern tibet just northeast of the earthquake we interpret this predominantly left lateral strike slip mechanism to indicate a possible genetic relationship between transverse structural features in the underthrusting indian plate the kishangang basement fault and the upper himalayan blocks and tibet

metamorphic history of the south tibetan detachment system mt everest region revealed by rscm thermometry and phase equilibria modelling this study combines microstructural observations with raman spectroscopy on carbonaceous material rscm phase equilibria modelling and u pb dating of titanite to delineate the metamorphic history of a well exposed section through the south tibetan detachment system stds in the dzakaa chu valley of southern tibet in the hanging wall of the stds undeformed tibetan sedimentary series rocks consistently record peak metamorphic temperatures of 340 c temperatures increase down section reaching 650 c at the base of the shear zone defining an apparent metamorphic field gradient of 310 c km 1 across the entire structure u th pb geochronological data indicate that metamorphism and deformation at high temperatures occurred over a protracted period from at least 20 to 13 ma deformation within this 1 km thick zone of distributed top down to the northeast ductile shear included a strong component of vertical shortening and was responsible for significant condensing of palaeo isotherms along the upper margin of the greater himalayan series ghs we interpret the preservation of such a high metamorphic gradient to be the result of a progressive up section migration in the locus of deformation within the zone this segment of the stds provides a detailed thermal and kinematic record of the exhumation of footwall ghs rocks from beneath the southern margin of the tibetan plateau

geochemical insights into the relationship of rock varnish and adjacent mineral dust fractions abstract rock varnishes are m thin dark manganese mn rich crusts that accrete in the order of few m ka on weathering resistant lithologies although these crusts can form in all climates they are best known in arid to semi arid settings aeolian dust is understood as a major contributor to the distinct trace metal and ree enrichments in rock varnish however the exact proportions of abiotic and biotic formation mechanisms that may explain the oxidation reactions of mn2 to mn4 present as mn oxyhydroxides in the varnish are still a matter of ongoing debate we present here the first systematic study of trace element enrichment processes between the uppermost layer of the varnish sequence and their adjacent 50 m dust fractions across a selection of 41 major and trace elements this approach is used to investigate samples from three fully arid deserts the an nafud in saudi arabia the negev in israel and the mojave in the usa which are compared to the significantly different environment of the semi arid knersvlakte in south africa a new in situ trace element analysis protocol was developed to perform femto and nanosecond sector field la icp ms microanalyses at high spatial resolution which allows us to measure the most recent varnish layers for their comparison with recent dust in agreement with previous studies all varnishes are enriched in mn pb ce co ba zn ni and the rare earth elements ree here we demonstrate that fine 50 m has low trace element mass fractions and enrichment patterns plotting distinctly away from varnish and fine dust based on these geochemical patterns our results indicate a general enrichment mechanism from fine dust to varnish previous studies suggested dust to play an integral role in providing the trace elements that are incorporated into the varnish by ph eh fluctuations in short term rain and fog events we amalgamate and refine previous growth models by providing direct evidence that leaching of about 10 of the mn and other trace elements from clay minerals in rain or fog droplets at ph 5 and subsequent scavenging on varnish surfaces at ph 8 leads integrated over time to the distinct enrichment patterns of the varnish while initial mn oxyhydroxide formation is suggested to follow pathways of metal oxide mediated photo catalysis for the semi arid occurrence in the knersvlakte we present a distinct growth model as both environment varnish and dust composition differ significantly from the arid settings here thick metallic looking varnish occurs mainly on the rim of quartz pebbles lacks microlamination and likely has upscaled growth processes among other aspects we suggest a more complex interplay between photo catalysis nocturnal condensation events on quartz pebbles with subsequent water trapping at the rock soil atmosphere interface due to a salic top soil layer to mainly account for these differences

geologic correlation of the himalayan orogen and indian craton part 2 structural geology geochronology and tectonic evolution of the eastern himalaya despite being the largest active collisional orogen on earth the growth mechanism of the himalaya remains uncertain current debate has focused on the role of dynamic interaction between tectonics and climate and mass exchanges between the himalayan and tibetan crust during cenozoic india asia collision a major uncertainty in the debate comes from the lack of geologic information on the eastern segment of the himalayas from 91 e to 97 e which makes up about one quarter of the mountain belt to address this issue we conducted detailed field mapping u pb zircon age dating and 40ar 39ar thermochronology along two geologic traverses at longitudes of 92 e and 94 e across the eastern himalaya our dating indicates the region experienced magmatic events at 1745 1760 ma 825 878 ma 480 520 ma and 28 20 ma the first three events also occurred in the northeastern indian craton while the last is unique to the himalaya correlation of magmatic events and age equivalent lithologic units suggests that the eastern segment of the himalaya was constructed in situ by basement involved thrusting which is inconsistent with the hypothesis of high grade himalaya rocks derived from tibet via channel flow the main central thrust in the eastern himalaya forms the roof of a major thrust duplex its northern part was initiated at ca 13 ma while the southern part was initiated at ca 10 ma as indicated by 40ar 39ar thermochronometry crustal thickening of the main central thrust hanging wall was expressed by discrete ductile thrusting between 12 ma and 7 ma overlapping in time with motion on the main central thrust below restoration of two possible geologic cross sections from one of our geologic traverses where one assumes the existence of pre cenozoic deformation below the himalaya and the other assumes flat lying strata prior to the india asia collision leads to estimated shortening of 775 km 76 strain and 515 km 70 strain respectively we favor the presence of significant basement topography below the eastern himalaya based on projections of early paleozoic structures from the shillong plateau i e the central shillong thrust located 50 km south of our study area since northeastern india and possibly the eastern himalaya both experienced early paleozoic contraction the estimated shortening from this study may have resulted from a combined effect of early paleozoic and cenozoic deformation

a new kinematic evolutionary model for the growth of a duplex an example from the rangit duplex sikkim himalaya india abstract the lesser himalayan duplex lhd is a prominent structure through much of the lesser himalayan fold thrust belt in the darjeeling sikkim himalaya a component of the lhd is exposed in the rangit window as the rangit duplex rd the rd consists of ten horses of the upper lesser himalayan sequence gondwana buxa upper daling the duplex varies from hinterland dipping in the north through an antiformal stack in the middle to foreland dipping in the south the ramgarh thrust rt is the roof thrust and based on a balanced cross section the main himalayan sole thrust is the floor thrust at a depth of 10 km and with a dip of 3 5 n retrodeformation suggests that the rd initiated as a foreland dipping duplex with the early ramgarh thrust as the roof thrust and the rt as the floor thrust the rt became the roof thrust during continued duplexing by a combination of footwall imbrication and concurrent rt reactivation this kinematic history best explains the large translation of the overlying mct sheets the restoration suggests that rd shortening is 125 km and the original gondwana basin extended 142 km northward of its present northernmost exposures within the window

spatial temporal and geographic considerations of the problem of rock varnish diagenesis article i nfo the rock varnish literature hosts an abundance of prima facie contradictory empirical data past and perhaps future empirical contradictions however can be resolved by theoretical considerations of different spatial and temporal scales of varnish diagenesis as well as the geomorphic position of different types of varnishes for example twentieth century contamination by lead and other heavy metals has led to claims of accumulation rates in the last century far more rapid than prior published empirical studies a consideration of spatial scales resolves this contradiction nanoscale processes allow migration of lead into varnish deposited well before the twentieth century time of heavy metal pollution evidence of nanometer scale disequilibrium in three samples led to claims that varnish cannot be used in paleoclimatic research these data rest in contrast to replicable patterns in varnish deposition observed by dr tanzhuo liu and others in over ten thousand micro sedimentary basins this contraction can be resolved by understanding that the types of varnishes studied differ and that processes differ substantially between the nanoscale and the micron scale a lack of evidence of mn oxidizers in genetic analyses contrasts with culturing studies and in situ evidence of mn enhancement by bacteria this contradiction has a likely resolution in vastly different temporal scales dna material analyzed may be no older than 200 years but in situ fossilized remains of bacteria may be preserved for thousands of years recording palaeoecological conditions favoring growth of mn enhancing bacteria

climatic forcing of erosion landscape and tectonics in the bhutan himalayas a fundamental objective in studies of climate erosion tectonics coupling is to document convincing correlation between observable indicators of these processes on the scale of a mountain range the eastern himalayas are a unique range to quantify the contribution of tectonics and climate to long term erosion rates because uniform and steady tectonics have persisted for several million years while monsoonal precipitation patterns have varied in space and time specifically the rise of the shillong plateau the only orographic barrier in the himalayan foreland has reduced the mean annual precipitation downwind in the eastern bhutan himalaya at the miocene pliocene transition apatite fission track aft analyses of 45 bedrock samples from an e w transect along bhutan indicate faster long term erosion rates outside of the rain shadow in the west 1 0 1 8 mm yr than inside of it in the east 0 55 0 85 mm yr furthermore an aft vertical profile in the latter segment reveals a deceleration in erosion rates sometime after 5 9 ma in this drier segment of bhutan there are remnants of a relict landscape formed under a wetter climate that has not yet equilibrated to the present climatic conditions uplift and preservation of the paleolandscape are a result of a climate induced decrease in erosion rates rather than of an increase in rock uplift rate this study documents not only a compelling spatial correlation between long term erosion and precipitation rates but also a climatically driven erosion rate change on the scale of the eastern himalayas a change that in turn likely influences that region s recent tectonic evolution

diversity of rock varnish bacterial communities from black canyon new mexico 1 scientists vigorously debate the degree to which rock varnish is formed through the actions of microorganisms to investigate this enigma we utilized a three pronged approach that combined 1 culture independent molecular methods to characterize bacterial communities associated with varnish that coats the rhyolitic volcanic rocks of black canyon new mexico and rocks with no visible varnish 2 culturing of varnish in media supplemented with reduced forms of manganese and or iron and no or low amounts of carbon to isolate bacteria capable of precipitating iron and or manganese oxides and 3 scanning electron microscopy sem of varnish and nearby rock that lacks macroscopically visible varnish our culture independent studies revealed significant differences between varnish and nonvarnish communities chloroflexi and ktedobacteria dominated one varnish site while the other varnish site was dominated by cyanobacteria the nonvarnish sites were dominated by actinobacteria and to a lesser extent cyanobacteria and were the only samples to contain deinococcus thermus sequences approximately 65 of varnish cultures produced visible manganese precipitates most culture isolates were not closely related to known manganese oxidizers with the exception of bacillus spp sem revealed microbial morphologies and two types of varnish morphologies 1 relatively smooth layers and 2 patches of botryoidal pinnacles which were often associated with increased manganese concentrations bare rock showed evidence of incipient varnish these results have important implications for the detection of life on extraterrestrial planets such as mars where putative varnish coatings have been observed and represent some of the first culture independent characterizations of varnish communities

constraints on the tectonic and landscape evolution of the bhutan himalaya from thermochronometry the observed geomorphology and calculated thermal histories of the bhutan himalaya provide an excellent platform to test ideas regarding the influence of tectonics and climate on the evolution of a convergentmountain range however little consensus has been reached regarding the late cenozoic history of the bhutan himalaya some researchers have argued that observed geologic relationships show slowing deformation rates such that the range is decaying from a geomorphic perspective while others see the range as growing and steepening we suggest that a better understanding is possible through the integrated interpretation of geomorphic and thermochronometric data from the comparison of predictions from models of landscape evolution and thermal kinematic models of orogenic systems new thermochronometric data throughout bhutan aremost consistent with a significant decrease in erosion rates from2 to 3 km ma down to 0 1 0 3 km ma around 6 4ma we interpret this pattern as a decrease in rock uplift rates due to the activation of contractional structures of the shillong plateau an uplifted region approximately 100 km south of bhutan however low relief fluvial landscapes throughout the bhutanese hinterland record a late pulse of surface uplift likely due to a recent increase in rock uplift rates constraints from our youngest thermochronometers suggest that this increase in rock uplift and surface uplift occurred within the last 1 75ma these results imply that the dynamics of the bhutan himalaya and shillong plateau have been linked during the late cenozoic with structural elements of both regions active in variable ways and times over that interval

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

exhumation of the main central thrust from lower crustal depths eastern bhutan himalaya geothermometry and mineral assemblages show an increase of temperature structurally upwards across the main central thrust mct however peak metamorphic pressures are similar across the boundary and correspond to depths of 35 45 km garnet bearing samples from the uppermost lesser himalayan sequence lhs yield metamorphic conditions of 650 675 c and 9 13 kbar staurolite kyanite schists about 30 m above the mct yield p t conditions near 650 c 8 10 kbar kyanite bearing migmatites from the greater himalayan sequence ghs yield pressures of 10 14 kbar at 750 800 c top to the south shearing is synchronous with and postdates peak metamorphic mineral growth metamorphic monazite from a deformed and metamorphosed proterozoic gneiss within the upper lhs yield u pb ages of 20 18 ma staurolite kyanite schists within the ghs a few metres above the mct yield monazite ages of c 22 1 ma we interpret these ages to reflect that prograde meta morphism and deformation within the main central thrust zone mctz was underway by c 23 ma u pb crystallization ages of monazite and xenotime in a deformed kyanite bearing leucogranite and kyanite garnet migmatites about 2 km above the mct suggest crystallization of partial melts at 18 16 ma higher in the hanging wall south verging shear bands filled with leucogranite and pegmatite yield u pb crystallization ages for monazite and xenotime of 14 15 ma and a 1 2 km thick leuco granite sill is 13 4 0 2 ma thus metamorphism plutonism and deformation within the ghs con tinued until at least 13 ma p t conditions at this time are estimated to be 500 600 c and near 5 kbar from these data we infer that the exhumation of the mct zone from 35 to 45 km to around 18 km occurred from 18 to 16 to c 13 ma yielding an average exhumation rate of 3 9 mm year 1 this process of exhumation may reflect the ductile extrusion by channel flow of the mctz from between the overlying tibetan plateau and the underthrusting indian plate coupled with rapid erosion

a search for molecular fossils in the kerogen of precambrian sedimentary rocks abstract hydrous pyrolysis of the kerogen in precambrian sedimentary rocks of a range of ages and thermal histories was used in an attempt to produce normal iso and anteiso isoprenoid sterane and triterpane hydrocarbons whose structures can be related to known biochemical precursors gas chromatography mass spectrometry was used in the selected ion monitoring mode to achieve a sensitivity of one part hydrocarbon in 10 9 of the rock small amounts of some of these hydrocarbon classes were found in certain lower cambrian and upper precambrian sediments with normal alkanes predominating the yields diminished with increasing age even for well preserved rocks where the minerals showed little effect of metamorphism samples with ages greater than 1 6 10 9 years yielded no detectable hydrocarbons spontaneous dehydrogenation of kerogen results in cross linking cyclization and aromatization of organic structures and extensive transformation of molecular fossils little information on the evolution of precambrian life is contained in the organic matter of the rocks studied in this work further advances along this line will depend critically on the discovery of sediments in which the kerogen is better preserved

landscape evolution in antarctica abstract the extreme aridity of antarctica means that very little water is available for weathering and geomorphic processes the landscape everywhere has been shaped by ice but land exposed after ice retreat has been modified by cold desert weathering processes wind is the main agent for removal and redeposition of weathered material while fluvial action is insignificant as an eroding or transporting agent there are few suitable deposits that allow reliable or accurate dating of events but the dates that are available show that landform evolution has been taking place very slowly soil weathering studies have provided a useful means of extrapolation showing relative ages of surfaces where no dates are available the major weathering processes in antarctica involve patterned ground movement frost action wind salt weathering and soil weathering these combine to form distinctive landscape features the complex nature of landform development is illustrated by considering events that have taken place within wright valley one of the largest ice free areas in antarctica within this valley are land surfaces ranging from those found on late pleistocene up valley tills to high altitude extremely fretted and weathered land surfaces that have been ice free possibly since the early miocene

ferromanganese rock varnish in north norway a subglacial origin a thin dark brown rock varnish is described from ice smoothed bedrock on the forefield of a glacier in north norway it occurs only in narrow strips ca 100 mm or less wide which run roughly parallel to the ice front and along the top edges of small treads of a series of bedrock steps the varnish is hard thin 10 m and consists of an iron manganese deposit greatly enriched in these elements compared with the composition of the underlying gabbroic bedrock it is suggested that the varnish formation is due to localized changes in eh ph conditions in subglacial regelation ice and meltwater at the top of the bedrock steps these changes may be due to co2 and or o2 degassing from water held at higher pressures under the ice than in cavities downstream where the ice is decoupled from the bedrock glacier retreat rates indicate that the varnish has remained exposed subaerially at the surface for about twenty years and its restricted occurrence suggests that it is not of biological origin but rather is essentially authigenic possible subglacial origins for associated iron and silica precipitates are also suggested

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

can erosion drive tectonics can climate modulated erosion affect the location rate and style of tectonic deformation in earth s crust and thus seismic hazards particularly in the himalaya there are clear correlations between rugged topography and high rates of rainfall erosion and rock uplift 1 2 but a vigorous cause and effect debate continues with new papers presenting evidence for 1 4 and against 5 7 erosional control of tectonics see the figure although much field data are consistent with predicted associations among topography erosion climate and deformation unambiguous evidence has proven elusive 8 by focusing on a simple diagnostic test wang et al show on page 978 1 of this issue 9 that the zone of rugged topography and extreme erosion rates where the tsangpo brahmaputra river cuts through the namche barwa massif see the figure often argued to reflect a dynamic tectonic response to river incision 2 records instead a passive landscape response to tectonics over the past 2 to 2 5 million years the data threaten to undermine one of the most dramatic examples of a proposed tectonic response to erosion 1 lookup volpage 346 978 iss 6212

rapid formation of rock varnish and other rock coatings on slag deposits near fontana california manganiferous rock varnish silica glaze and iron skins have formed on 20 to 40 year old slag piles near fontana southern california rapid rock varnish formation is associated with an unidentified cocci bacterium that grows rapidly in culturing experiments combined with the likelihood that mn rich solutions flow over slag surfaces a new model is proposed for the formation of silica glaze involving soluble al si complexes and the nature of wetting films iron skins occur where runoff is concentrated and where iron bacteria grow observations indicate i substrate can be important in supplying mn as well as maintaining a stable surface and channelling runoff to microspots where varnish grows ii ignorance of boundary layer ph eh conditions makes chemical physical models of varnish formation speculative iii the balance between cementation of recently nucleated varnishes and erosional shear stresses is poorly understood but is probably an important limiting factor on varnish growth rates and iv there must be at least two steps in biotic varnishing accretion of mn fe in casts followed by cementation of clays by either in situ cast residue or mn fe remobilized from casts varnish accretion may be limited by the rate at which both steps occur rapid development of readily distinguishable manganiferous rock varnishes at this post 1952 site highlights the danger of speculation on the ages of landforms based solely on the appearance of rock varnish coatings

a short duration pulse of ductile normal shear on the outer south tibetan detachment in bhutan alternating channel flow and critical taper mechanics of the eastern himalaya in easternmost bhutan the south tibetan detachment std is a ductile shear zone that juxtaposes the radi or sakteng klippe of the tethyan sedimentary series from underlying high grade greater himalayan rocks in situ la icpms u th pb analysis of metamorphic monazite from the immediate footwall and hanging wall of the std within the shear zone at the base of the klippe constrains north vergent normal shear to between 25 and 20 ma coeval thrusting on the main central thrust during this time supports a phase of channel flow viscous wedge model activity lasting only ca 3 ma geochronologic data from the eastern himalaya indicate alternating mechanisms for extrusion of the metamorphic core of the orogen from the late oligocene through to the late miocene switching from channel flow viscous wedge behavior to critical taper frictional wedge behavior each phase lasting approximately only 2 to 5 ma the tectonic evolution of the eastern himalaya is comparable to central and western himalayan tectonics during the early miocene but during the middle miocene metamorphism and magmatism in the eastern himalaya migrated towards the orogenic hinterland a process not widely documented elsewhere in the himalaya thus highlighting the need for an orogenic model in three spatial dimensions

an early india asia contact paleomagnetic constraints from ninetyeast ridge odp leg 121 new paleomagnetic results from sedimentary rock and basement of the ninetyeast ridge ocean drilling program leg 121 sites 756 758 detail the northward movement of the indian plate for the past 80 m y analysis of the combined paleolatitude age profile indicates a distinct reduction in india9s northward movement rate at 55 ma interpreted as completion of suturing of greater india and asia india9s northward motion slowed from 18 19 5 cm yr to 4 5 cm yr for the location of site 758 comparison of this profile with paleomagnetic data from the wider himalayan region indicates that initial contact between northwestern greater india and southern asia was already established by cretaceous tertiary time this supports a possible causal link between the india asia convergence and the deccan traps extrusion

flattening the bhutan himalaya a detailed thermobarometric transect of 35 samples across the greater and tethyan himalayan sequences in central bhutan demonstrates a tectonostratigraphically intact section with uniform apparent thermal and baric field gradients of 2072 1c km and 0 5770 08 kbar km pressure temperature paths determined from chemically zoned garnets in 6 samples demonstrate that these p t conditions correspond with maximum pressures the super lithostatic baric gradient cannot be explained by pre to syn metamorphic tectonic processes or by extension within an inclined slab instead the data imply 50 post peakmetamorphic flattening of the himalayan metamorphic core accommodated by distributed top to thenorth shear consistent with microstructural analysis orogenic flattening best explains the development of the south tibetan detachment system as a strain incompatibility feature rather than a structure bounding the top of a tectonically inserted wedge and helps reconcile debate attributing first order himalayan structural features to either wedge failure critical taper or pipe flow channel flow

low temperature thermochronometric ages in fold and thrust belts we use a 2 dimensional thermal kinematic thrust fault model to generate low temperature thermochronometric ages in a fold and thrust belt to understand better the rates and timing of thrust deformation recorded by cooling ages our model links the kinematic structural model 2dmove to a finite element advection diffusion thermal model to track the cooling history of material exhumed by erosion during thrust faulting and fault bend folding we use the cooling histories to model fission track annealing and he retention and predict zircon and apatite fission track and apatite u th he thermochronometric ages we model thermochronometric age patterns generated during motion along individual and multiple forward propagating thrust faults and explore the effects of fault slip rate and fault geometry all our models generate a characteristic u shaped thermochronometric age pattern along a transect perpendicular to the fault this pattern is formed by the distribution of reset unreset and partially reset thermochronometric ages above the thrust ramp reset material forms a zone of young thermochronometric ages partially reset and unreset material lies on either side of this zone of young ages although some thermochronometric ages predicted by our fault bend fold model are similar to those predicted using a 1 d thermal model the patterns of ages are markedly different the smooth u shaped pattern is generated in both the individual thrust fault and multiple thrust fault models in the foreland propagating multiple fault models there are no distinct changes in age across individual thrust faults in these multiple fault models the thermochronometric age pattern changes with thrust fault spacing depending on the fault spacing and amount of slip along each fault early faulting can be recorded either in the hinterland or foreland of the thrust belt

the characterisation of the iron oxide phase in desert varnish from antarctica using backscattered conversion electron and x ray mossbauer spectroscopy abstract the iron oxide phase in the desert varnish surface coatings on coarse and fine grained ferrar dolerite aztec siltstone and orthoquartzite sandstone rocks from the upper taylor valley area mcmurdo dry valley region south victoria land antarctica has been characterised using backscattered conversion electron and x ray mossbauer spectroscopy and x ray diffraction methods the x ray diffraction results show that although visually the red brown desert varnish layer extends some several millimetres into the rock no crystalline or hydrous fe oxides could be detected the conversion x ray mossbauer spectra show that at a depth of 20 m the fe 3 fe 2 ratio is increased over that of the bulk rock but there is still a dominant contribution from fe 2 iron in the primary ferromagnesian minerals notably ortho and clinopyroxenes at shallower depths of up to 0 1 m the conversion electron mossbauer spectra show the fe 3 fe 2 ratio is further increased but for the dolerites and siltstone this fe 2 contribution is still significant these results suggest that the colour and extent of the desert varnish layer arises from the formation of a thin coating or staining less than 0 1 m thick of hydrous fe oxides on the primary minerals at or near the surface of the rock on the sandstone where the fe is derived from an external source the oxide coating is precipitated from solution relatively quickly this forms a very poorly ordered hydrated ferric oxide polymer on the fine grained dolerites where the diffusion and evaporation is slower due to the fine pore structure a more ordered hydrous fe oxide polymer resembling ferrihydrite is found on the coarse grained dolerites and also the siltstone the fe oxide phase is a slightly less ordered ferrihydrite like material

late quaternary ice surface fluctuations of hatherton glacier transantarctic mountains abstract former longitudinal profiles of hatherton glacier an outlet through the transantarctic mountains constrain nearby polar plateau elevations and ice shelf grounding in the southwestern ross embayment four gravel drift sheets of late quaternary age beside hatherton glacier are from youngest to oldest hatherton britannia i britannia ii and danum the hatherton drift limit is uniformly 20 to 70 m above the present ice surface the britannia ii drift limit is within 100 m of the present surface of uppermost hatherton glacier but is 450 m above middle hatherton glacier extrapolation of this profile downglacier indicates a surface elevation 1100 m above the present ross ice shelf the britannia i drift limit is parallel to but 50 100 m below britannia ii drift the danum drift limit is parallel to but 50 100 m above the britannia ii profile from correlation with drifts near mcmurdo sound and from local 14 c dates we assign an early holocene age to hatherton drift a late wisconsin age to britannia drifts and an age of marine isotope stage 6 to danum drift by our age model the upper reaches of hatherton glacier and presumably the adjacent polar plateau have not exceeded their current elevations by more than 100 150 m during the last two complete global glacial interglacial cycles whereas the middle and lower reaches of hatherton glacier have thickened considerably during the last two global glaciations late wisconsin and marine isotope stage 6 the effect of ice shelf grounding probably was the major control of these changes of hatherton glacier holocene ice surface lowering probably represents the last pulse of grounding line recession in the southwestern ross embayment

timing of india asia collision geological biostratigraphic and palaeomagnetic constraints 1 a range of ages have been proposed for the timing of india asia collision the range to some extent reflects different definitions of collision and methods used to date it in this paper we discuss three approaches that have been used to constrain the time of collision the time of cessation of marine facies the time of the first arrival of asian detritus on the indian plate and the determination of the relative positions of india and asia through time in the qumiba sedimentary section located south of the yarlung tsangpo suture in tibet a previous work has dated marine facies at middle to late eocene by far the youngest marine sediments recorded in the region by contrast our biostratigraphic data indicate the youngest marine facies preserved at this locality are 50 6 52 8 ma in broad agreement with the timing of cessation of marine facies elsewhere throughout the region double dating of detrital zircons from this formation by u pb and fission track methods indicates an asian contribution to the rocks thus documenting the time of arrival of asian material onto the indian plate at this time and hence constraining the time of india asia collision our reconstruction of the positions of india and asia by using a compilation of published palaeomagnetic data indicates initial contact between the continents in the early eocene we conclude the paper with a discussion on the viability of a recent assertion that collision between india and asia could not have occurred prior to 35 ma

40ar 39ar pressure temperature and fission track constraints on the age and nature of metamorphism around the main central thrust in the eastern bhutan himalaya ar 39 ar data from a profile across the main central thrust in the eastern bhutan himalaya indicate muscovite cooling ages of 14 1 0 2 ma from a sample in the immediate hanging wall of the thrust and 11 2 ma from about 400 m structurally higher in the hanging wall these two ages are repeated by two samples from 2 1 and 4 7 km vertical distance from the thrust within the hanging wall respectively a single apatite fission track age from the immediate hanging wall of the thrust gives an age of 3 1 0 6 ma pressure temperature estimates give temperatures around 6508c and 6 5 kbar for the highest sample collected samples closer to the main central thrust give also temperatures between 600 and 6508c at the same pressure indicating possibly a slight temperature decrease with proximity to the thrust however uncertainties are large and the parageneses are thermodynamically too highly variant to place much significance on their interpretation the 40 ar 39 ar cooling age data are consistent with a repetition of the sequence in the hanging wall of the thrust they confirm the data of davidson et al 1997 metamorphic reactions related to decompression and synkinematic intrusion of leucogranite high himalayan crystallines bhutan journal of metamorphic geology 15 593 612 and are consistent with a more rapid exhumation of deeper levels towards the centre of the high himalayan crystalline complex despite the large uncertainties the pt data shown here are also consistent with this interpretation the apatite fission track results reveal low temperature cooling and final exhumation of the main central thrust at the same time as in nepal q 2001 elsevier science ltd all rights reserved

desert varnish in antarctica a nuclear microprobe and backscattered 57fe mossbauer spectroscopic study abstract nuclear microprobe analysis and backscattered 57 fe mossbauer spectroscopic methods have been used to study the desert varnish surface coatings on samples of dolerite and sandstone from the taylor valley area of the mcmurdo dry valley region south victoria land antarctica with the nuclear microprobe method the oxygen content of the varnish coating and underlying bulk rock was measured directly using the emitted protons from the 16 o d p 1 17 o nuclear reaction and also the 871 kev gamma rays emitted from the excited oxygen nucleus in addition the iron calcium and sulphur contents were measured using their characteristic proton induced fluorescent x rays the concentration profiles of these elements are presented in three and two dimensional diagrammatic representations these show that in the fe containing dolerites the desert varnish is formed by subsurface leaching of the fe from the bulk rock this is transported to the surface and deposited as a coating of hydrous ferric oxide probably ferrihydrite by subsequent oxidation and evaporation in the essentially fe deficient sandstone the fe is derived externally from the overlying dolerite sills and is similarly transported and deposited as a varnish coating on the upper surfaces of the sandstone windblown inorganic salts notably thenardite na 2 so 4 may be incorporated in the outer layers of the varnish the backscattered 57 fe mossbauer spectra recorded using both the conversion electrons and x rays show that at a depth of 0 1 m from the surface the predominant oxidation state of the fe in the dolerite is fe 3 whereas at depths of 10 m or more it is fe 2 for the sandstone it is present only as fe 3

a note on the characteristics and possible origins of desert varnishes from southeast morocco two substrate controlled varnish types were identified from arid southeast morocco 1 patchy varnish interspersed with silica glaze on silicified limestone and 2 a complete varnish cover on partially silicified limestone electron probe microanalysis was used in conjunction with scanning electron microscopy to investigate their chemical composition in both instances varnishes are predominantly silica and iron rich with localized concentrations of manganese inorganic fixation of iron and manganese is the preferred explanation with manganese and iron derived predominantly from external sources possibly wind blown dust some iron and silica may also be derived by outward migration from within the substrate particularly on partially silicified limestone

quantifying internal strain and deformation temperature in the eastern himalaya bhutan implications for the evolution of strain in thrust sheets abstract quartz microstructural analysis together with quantification of 3 d crystal plastic strain of quartz clasts normalized fry and rf methods constrain deformation temperature ranges and internal strain magnitude orientation within himalayan thrust sheets in bhutan lesser himalayan lh thrust sheets display an inverted deformation temperature gradient from 250 to 310 c at the main boundary thrust to 500 670 c at the main central thrust mct attributed primarily to stacking of lh thrust sheets deformed at progressively higher temperatures toward the hinterland the hot iron effect of the mct hanging wall only affects the upper few hundred meters of hinterland lh thrust sheets frontal thrust sheets exhibit 7 layer parallel shortening lps strain all other thrust sheets exhibit layer normal flattening lnf strain with 1 9 1 8 1 0 lh rocks and 2 1 1 8 1 0 greater and tethyan himalayan rocks median ellipsoids x parallel to lineation we propose that lps strain developed foreland ward of the thrust deformation front and that lnf strain resulted from later tectonic loading the lps to lnf transition occurs at minimum temperatures for quartz plasticity ca 250 270 c lnf ellipsoids are folded along with strata within thrust sheets indicating that internal strain preceded thrust imbrication and translation at the scale of 10 s of meters structural distance strain magnitude of hanging wall rocks does not increase near thrusts which supports the existence of discrete faults with large translations as generally depicted in balanced cross sections strain magnitudes in low grade greater and tethyan himalayan rocks in central bhutan indicate that the top to the north component of channel flow extrusion is most likely less than 19 km

geometry and crustal shortening of the himalayan fold thrust belt eastern and central bhutan we present a new geologic map of eastern and central bhutan and four balanced cross sections through the himalayan fold thrust belt major structural features from south to north include 1 a single thrust sheet of subhimalayan rocks above the main frontal thrust 2 the upper lesser himalayan duplex system which repeats horses of the neoproterozoic cambrian baxa group below a roof thrust shumar thrust carrying the paleoproterozoic daling shumar group 3 the lower lesser himalayan duplex system which repeats horses of the daling shumar group and neoproterozoic ordovician jaishidanda formation with the main central thrust mct acting as the roof thrust 4 the structurally lower greater himalayan section above the mct with overlying tethyan himalayan rock in stratigraphic contact in central bhutan and structural contact above the south tibetan detachment in eastern bhutan and 5 the structurally higher greater himalayan section above the kakhtang thrust cross sections show 164 267 km shortening in subhimalayan and lesser himalayan rocks 97 156 km structural overlap across the mct and 31 53 km structural overlap across the kakhtang thrust indicating a total of 344 405 km of minimum crustal shortening 70 75 our data show an eastward continuation of lesser himalayan duplexing identified in northwest india nepal and sikkim which passively folded the overlying greater himalayan and tethyan himalayan sections shortening and percent shortening estimates across the orogen although minima do not show an overall eastward increase which may suggest that shortening variations are controlled more by the original width and geometry of the margin than by external parameters such as erosion and convergence rates

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

variable shortening rates in the eastern himalayan thrust belt bhutan insights from multiple thermochronologic and geochronologic data sets tied to kinematic reconstructions 1 we present data on the burial displacement and exhumation history of the himalayan fold thrust belt in eastern bhutan these data document the magnitude and timing of displacement of large discrete structures and highlight temporal variability in shortening rates eight new40ar 39ar ages from white mica 32 new zircon u th he ages 7 new apatite fission track ages and 1 new u pb zircon la mc icp ms metamorphic rim growth age are combined with published cooling ages and deformation temperatures and incremental shortening magnitudes from restorations of two published balanced cross sections to illustrate the kinematic and temporal development of the bhutan thrust belt integrating these data from 23 ma to the present illustrates rapid horizontal shortening rates 28 35 mm yr between 23 20 ma and 15 10 ma separated by more moderate rates 10 23 mm yr shortening rates decrease significantly to 7 10 mm yr and possibly as low as 3 4 mm yr from 10 to 0 ma this decrease is interpreted to represent the onset of strain partitioning in the eastern part of the himalayan tibetan orogenic system between shortening in the bhutan thrust belt uplift of the shillong plateau and deformation and outward growth of the northern and eastern tibetan plateau within estimated error horizontal shortening rates during emplacement of the main central thrust sheet and during construction of the upper lesser himalayan duplex approached india asia tectonic velocities thus for periods of time between 23 20 ma and 15 10 ma the bhutan thrust belt may have absorbed nearly all india asian convergence at this longitude

implications of shortening in the himalayan fold thrust belt for uplift of the tibetan plateau 1 recent research in the himalayan fold thrust belt provides two new sets of observations that are crucial to understanding the evolution of the himalayan tibetan orogenic system first u pb zircon ages and sm nd isotopic studies demonstrate that 1 greater himalayan medium to high grade metasedimentary rocks are much younger than true indian cratonic basement and 2 these rocks were tectonically mobilized and consolidated with the northern margin of gondwana during early paleozoic orogenic activity these observations require that greater himalayan rocks be treated as supracrustal material in restorations of the himalayan fold thrust belt rather than as indian cratonic basement in turn this implies the existence of greater himalayan lower crust that is not exposed anywhere in the fold thrust belt second a regional compilation of shortening estimates along the himalayan arc from pakistan to sikkim reveals that 1 total minimum shortening in the fold thrust belt is up to 670 km 2 total shortening is greatest in western nepal and northern india near the apex of the himalayan salient and 3 the amount of himalayan shortening is equal to the present width of the tibetan plateau measured in an arc normal direction north of the indus yalu suture zone this information suggests that a slab of greater indian lower crust composed of both indian cratonic lower crust and greater himalayan lower crust with a north south length of 700 km may have been inserted beneath the tibetan crust during the cenozoic orogeny we present a modified version of the crustal underthrusting model for himalayan tibetan orogenesis that integrates surface geological data recent results of mantle tomographic studies and broadband seismic studies of the crust and upper mantle beneath the tibetan plateau previous studies have shown that incremental mesozoic and early cenozoic shortening had probably thickened tibetan crust to 45 55 km before the onset of the main cenozoic orogenic event thus the insertion of a slab of greater indian lower crust with maximum thickness of 20 km tapering northward could explain the cenozoic uplift of the tibetan plateau the need for tibetan crust to stretch laterally as the greater indian lower crust was inserted may explain the widespread east west extension in the southern half of the plateau our reconstruction of the himalayan fold thrust belt suggests that indian cratonic lower crust of presumed mafic composition and high strength should extend approximately halfway across the tibetan plateau to the banggong suture from there northward we predict that the tibetan plateau is underlain by more felsic and therefore weaker lower crust of greater himalayan affinity two slab break off events are predicted by the model the first involved neotethyan oceanic lithosphere that foundered 45 35 ma and the second consisted of greater indian lithosphere most likely composed of greater himalayan material that delaminated and foundered 20 10 ma asthenospheric upwelling associated with the break off events may explain patterns of cenozoic volcanism on the tibetan plateau although the model predicts a northward migrating topographic front due solely to insertion of greater indian lower crust the actual uplift history of the plateau was complicated by early middle tertiary shortening of tibetan crust

characterization and differentiation of rock varnish types from different environments by microanalytical techniques abstract we investigated rock varnishes collected from several locations and environments worldwide by a broad range of microanalytical techniques these techniques were selected to address the challenges posed by the chemical and structural complexity within the micrometer to nanometer sized structures in these geological materials femtosecond laser ablation inductively coupled plasma mass spectrometry fs la icp ms scanning transmission x ray microscopy near edge x ray adsorption fine structure spectroscopy stxm nexafs in combination with scanning electron microscopy sem of focused ion beam fib ultra thin 100 200 nm sections conventional and polarization microscopy as well as electron paramagnetic resonance epr measurements were used to obtain information about these rock varnishes rock varnishes from different environments which cannot readily be distinguished based on their macroscopic appearance differ significantly in their constituent elemental mass fractions e g of mn fe ni co ba and pb and their rare earth element ree patterns structural characteristics such as the particle sizes of embedded dust grains internal structures such as layers of mn fe and ca rich material and structures such as cavities varied between varnishes from different environments and regions in the world the epr spectra were consistent with aged biogenic mn oxides in all samples but showed subtle differences between samples of different origin our observations allow us to separate rock varnishes into different types with differences that might be indicators of distinct geneses five different types of rock varnish could be distinguished type i v of which only type i might be used as potential paleoclimate archive each varnish type has specific characteristics in terms of their elemental composition element distribution and structures the combination of element ratios mn ba al ni mn rey mn ce mn pb lan ybn and ce ce total ree contents and structures can be used to separate the different types of rock varnish from each other

nanometer scale layering in rock varnish implications for genesis and paleoenvironmental interpretation manganiferous rock varnish collected from death valley and antarctica contains the smallest known terrestrial sedimentary deposits with some layers only a few nanometers thick irregularities in these nanometer scale layers are consistent with shrinking cracking and weathering of clay minerals in the death valley rock varnish very different high resolution transmission electron microscope hrtem textures coexist that may be related to climatic change hrtem observations contradict previous microbial models of mn fe enhancement requiring a new three step model of biomineralization and diagenesis for varnish formation

documenting basin scale geometry and provenance through detrital geochemical data lessons from the neoproterozoic to ordovician lesser greater and tethyan himalayan strata of bhutan abstract detrital zircon dz ages augmented with end 0 and 13 c isotopic values from 18 new and 22 published samples collected from lesser himalayan lh greater himalayan gh and tethyan himalayan th rocks in bhutan support deposition of 7 km of sedimentary rock in late cambrian ordovician time and provide a stratigraphic framework for the pre collisional indian margin youngest gh dz grains become younger upsection from 900 ma to 477 ma youngest dz grains in th samples are 490 460 ma both the lh jaishidanda formation fm and the lh baxa group overlie paleoproterozoic lh rocks the jaishidanda fm exhibits distinct populations of youngest dz peaks 475 550 ma and 800 1000 ma the baxa group manas pangsari and phuntsholing formations contains youngest dz peaks at both 500 525 ma and 0 9 1 0 ga however most samples from the baxa group in western bhutan contain no grains younger than 1 8 ga samples from the lh paro fm which sits directly under the mct in western bhutan have youngest dz peaks at 0 5 0 8 1 0 1 7 1 8 ga end values generally match dz spectra with samples that contain old youngest grain populations corresponding to more negative end signatures the paro fm is an exception where end 0 values from quartzite samples are quite negative 19 to 24 whereas the end 0 values from interbedded schist contain younger detritus 12 to 17 13 c values from the jaishidanda paro and manas formations have 13 c values 1 8 to 6 suggestive of deposition over late neoproterozoic to ordovician time 13 c values from the pangsari fm vary from 2 8 to 1 8 compatible with deposition in the early to middle neoproterozoic the young latest cambrian ordovician grains preserved in th gh and lh rocks suggest that the late cambrian ordovician orogeny documented in gh rocks throughout the orogen served as a significant sediment source in bhutan

characteristics and origin of rock varnish from the hyperarid coastal deserts of northern peru abstract the characteristics of a new type of rock varnish from the hyperarid coastal deserts of northern peru combined with laboratory experiments on associated soil materials provide new insights into the formation of rock varnish the peruvian varnish consists of an fe rich mn poor component covering up to 95 of a varnished surface and a fe rich mn rich component found only in pits and along cracks and ridges the alkaline soils plus the catalytic fe oxyhydroxides that coat much of the varnish surfaces make the peruvian situation ideal for physicochemical precipitation of mn however the low mn content of the dominant fe rich mn poor component suggests that such precipitation is minor this plus the presence of abundant bacteria in the mn rich varnish and the recorded presence of mn precipitating bacteria in varnish elsewhere suggests that bacteria are almost solely responsible for mn precipitation in rock varnish a set of experiments involving peruvian soil samples in contact with water co2 solutions indicates that natural fogs or dews release mn but not fe when they come in contact with eolian materials on rock surfaces this mechanism may efficiently provide mn to bacteria on varnishing surfaces the lack of fe in solution suggests that a large but unknown proportion of fe in varnish may be in the form of insoluble fe oxyhydroxides sorbed onto the clay minerals that form the bulk of rock varnish the results of this study do not substantively change r i dorn s paleoenvironmental interpretations of varnish mn fe ratios but they do suggest areas for further inquiry

rock varnish in new york an accelerated snapshot of accretionary processes abstract samples of manganiferous rock varnish collected from fluvial bedrock outcrop and erie barge canal settings in new york state host a variety of diatom fungal and bacterial microbial forms that are enhanced in manganese and iron use of a dual beam focused ion beam scanning electron microscope to manipulate the varnish in situ reveals microbial forms that would not have otherwise been identified the relative abundance of mn fe enriched biotic forms in new york samples is far greater than varnishes collected from warm deserts moisture availability has long been noted as a possible control on varnish growth rates a hypothesis consistent with the greater abundance of mn enhancing bioforms sub micron images of incipient varnish formation reveal that varnishing in new york probably starts with the mortality of microorganisms that enhanced mn on bare mineral surfaces microbial death results in the adsorption of the mn rich sheath onto the rock in the form of filamentous networks clay minerals are then cemented by remobilization of the mn rich material thus the previously unanswered question of what comes first clay mineral deposition or enhancement of mn can be answered in new york because of the faster rate of varnish growth in contrast very slow rates of varnishing seen in warm deserts of microns per thousand years make it less likely that collected samples will reveal varnish accretionary processes than samples collected from fast accreting moist settings

intraplate deformation of the indian subcontinent 1 while deformation at the earth s surface primarily occurs along tectonic plate boundaries major earthquakes have shaken regions deep within continental interiors three of the largest m 7 5 historic intraplate earthquakes occurred within the indian subcontinent suggesting the possibility of significant intraplate deformation we consider surface velocities determined from new gps data collected at 29 continuous gps stations and 41 survey mode gps stations in india between 1995 and 2007 to find a north south shortening rate of 0 3 0 05 nanostrain yr 1 which may be accommodated by 2 1 mm yr of more localized convergence across central india southward motions at 4 7 mm yr of sites on the shillong plateau in northeast india reflect rapid shortening and high earthquake hazard associated with active thrust faults bounding the plateau the width and magnitude of the elastic strain accumulation field across the himalaya varies little from 76 90 longitude but the strain is more broadly distributed and convergence rates are higher along the eastern 200 km of the range

variable exhumation rates and variable displacement rates documenting recent slowing of himalayan shortening in western bhutan abstract we link exhumational variability in space and time to the evolving geometry of the himalayan fold thrust belt in western bhutan by combining new and published geochronologic and thermochronologic data we document the burial age peak temperatures and complete cooling history from 20 ma to the present over an across strike distance of 125 km these integrated cooling curves highlight windows of fast exhumation that vary spatially and temporally we propose that pulses of fast exhumation are a result of structures that facilitate the vertical motion of material illustrated in sequentially restored cross sections due to a range of permissible geometries at depth we explore and evaluate the impact of geometry on kinematics and rates of deformation the linked cooling history and cross sections provide estimates of both magnitude and timing of thrust sheet displacement and highlight temporal variability in potential shortening rates structural and chronologic data illustrate a general north to south progression of himalayan deformation with emplacement of the main central thrust mct paro thrust and shumar thrust by 12 to no later than 9 ma two different geometries and kinematic scenarios for the lesser himalayan duplex are proposed a north to south propagating duplex system requires that the southern portion of that system south of the mct deformed and cooled by 9 ma leaving only the southernmost thrust sheets including the main boundary and main frontal thrusts to deform between 9 and 0 ma this limited post 9 ma shortening would necessitate a marked slowdown in convergence accommodated on the main himalayan thrust a two tiered duplex system which allows for the paro window duplex and the southern baxa duplex to form simultaneously permits duplex formation and accompanying exhumation until 6 ma limited cooling from 200 c to the surface post 6 ma suggests either a decrease in shortening rates from 6 to 0 ma or that duplex formation and exhumation are temporally decoupled our combined cooling curves highlight that the youngest cooling ages may not mark the fastest thrusting rates or the window of fastest exhumation instead temporal variations in exhumation are best viewed through identifying transients in exhumation rate we suggest that the strongest control on exhumation magnitude and variability is fold thrust belt geometry particularly the locations and magnitudes of footwall ramps which can change over 10 s of km distance balanced cross sections predict the location and magnitude of these ramps and how they vary in space and time providing an untapped potential for testing permissible cross section geometries and kinematics against measured cooling histories

fission track and numerical thermal modeling of differential exhumation of the glarus thrust plane switzerland abstract geometry and differences in metamorphic grade along the glarus thrust external european alps can be explained by a restoration of the hanging wall some 10 km toward the south and back rotation of the thrust plane into a ramp flat ramp geometry the apatite fission track ft age pattern from the area suggests that isotherms were parallel to the thrust plane during cooling of the rocks along the thrust plane below the apatite closure temperature two forward modeling programs that both produce time temperature paths are used to constrain the thermal and structural evolution of differential exhumation along the glarus thrust a a two dimensional finite difference thermal model is used to examine differential exhumation of the thrust plane starting from an assumed horizontal position of the middle part of the glarus thrust at some 10 ma when parallel to the paleo isotherms five locations in the crustal section corresponding to locations on a final horizontal and vertical sampling traverse record their time temperature t t history during exhumation along particle paths the near surface temperature distribution is strongly influenced by the development of a simplified step like topographic relief which causes all modeled samples to reach the surface at the end of the modeled time period b the calculated t t paths of these five modeled samples were used to forward model apatite ft data based on an existing annealing model by comparison between synthetically generated ft data and actual measured ft samples the exhumation history of the glarus alps was evaluated with respect to its thermal and temporal brackets several parameters such as the location of the rotation point the thermal gradient and the topography have significant influence on model results best fit models produce results that are consistent with independent geological and geophysical information

late wisconsin and early holocene glacial history inner ross embayment antarctica abstract lateral drift sheets of outlet glaciers that pass through the transantarctic mountains constrain past changes of the huge ross ice drainage system of the antarctic ice sheet drift stratigraphy suggests correlation of reedy iii reedy glacier beardmore beardmore glacier britannia hatherton darwin glaciers ross sea mcmurdo sound and younger terra nova bay drifts radiocarbon dates place the outer limits of ross sea drift in late wisconsin time at 24 000 13 000 yr b p outlet glacier profiles from these drifts constrain late wisconsin ice sheet surface elevations within these constraints we give two extreme late wisconsin reconstructions of the ross ice drainage system both show little elevation change of the polar plateau coincident with extensive ice shelf grounding along the inner ross embayment however in the central ross embayment one reconstruction shows floating shelf ice whereas the other shows a grounded ice sheet massive late wisconsin holocene recession of grounded ice from the western ross embayment which was underway at 13 040 yr b p and completed by 6600 6020 yr b p was accompanied by little change in plateau ice levels inland of the transantarctic mountains sea level and basal melting probably controlled the extent of grounded ice in the ross embayment the interplay between the precipitation low late wisconsin and high holocene values and the influence of grounding on outlet glaciers late wisconsin thickening and late wisconsin holocene thinning with effects dying out inland probably controlled minor elevation changes of the polar plateau

blind testing of rock varnish microstratigraphy as a chronometric indicator results on late quaternary lava flows in the mojave desert california rock varnish is a manganiferous dark coating ubiquitous in desert landscapes to test the validity of varnish microstratigraphy as a chronometric indicator varnish samples were collected from radiometrically dated and undated late quaternary lava flows in amboy cima and pisgah volcanic fields avf cvf pvf in the mojave desert of california western united states varnish microstratigraphies show a replicable layering sequence that appears to record regional climate changes that likely correspond in time to the younger dryas and heinrich events in the north atlantic region microstratigraphic patterns on these volcanic fields match patterns found in varnishes from other western us sites with available radiometric age constraints based on this regional chronology varnishes from the a flow h flow and a stone pavement surface in the cima volcanic field were estimated to be 16 5 24 74 85 and 74 85 ka respectively these ages are consistent with previously published cosmogenic 3 he ages of 18 20 72 74 and 80 85 ka for these geomorphic surfaces varnishes from the i flow at cima yielded a puzzling age estimate of 39 ka which is consistent with an older 3 he age of 37f6 ka reported for the i flow but inconsistent with a younger 3 he age of 31f7 ka and a cosmogenic 36 cl age of 27f1 3 ka for the same flow reinterpretation of the original varnish age data with knowledge of then available field mapping results of the i flow suggests that the i cone is polycyclic and different flow units were probably unintentionally sampled in the field the revised varnish ages of 30 and 39 ka for the i flow thus may be in good agreement with their corresponding 3 he and 36 cl ages in a blind test of the method varnishes from the phase 1 flow at pisgah an unnamed flow called here the ivflow at cima and the amboy flow were estimated to be 24 30 46 60 and 74 85 ka respectively these ages agree well with 36 cl ages of 22 5f1 3 46f2 and 79f5 ka reported for the same flows by phillips geomorphology 2002 these test results provide convincing evidence that varnish microstratigraphy once radiometrically calibrated can be used as a valid dating tool to estimate surface exposure ages of desert landforms in the western us drylands d 2002 elsevier science b v all rights reserved

manganese rich rock coatings from iceland manganese rich rock coatings are widespread in iceland especially in fractures in basalt lavas three common types of coating are described a thick subsurface coating is found on whaleback forms on high plateaux the most common type is thinner and occurs in hydrothermal clay filled fracture systems which are especially well developed in tholeiite lavas a third type occurs in younger rocks and is associated with tephra rich loessic sediment which fills open fractures and vesicles there are differences in the chemistry morphology and mineralogy of these coatings which are interpreted as being due to the different microenvironments in which they form the fracture microenvironment is regarded as being of much more importance to coating thickness and chemistry than simple age and in this respect the hydrothermal clays seem to favour rapid manganese concentration on the other hand there are variations in coating type both laterally and vertically through the lava succession which are related to regional distribution of hydrothermal alteration and rock type the regional patterns are the result of iceland s lateral drift and as a result there is an incidental connection between coating type and rock age there is some evidence to suggest that the manganese coatings may be both forming and being degraded rapidly in the present environment

constraining rates of thrusting and erosion insights from kinematic thermal modeling we present a thermal model of a simple thrust system that can be used to determine thrust rates and erosion rates from low temperature thermochronology unlike previous models this model incorporates the effects of erosion both during and after thrusting in particular we examine the modeled evolving thermal structure and pressure temperature time evolution of hanging wall rocks that undergo fault bend folding due to transport over a blind footwall ramp in all cases rocks cool as they move over the footwall ramp potentially providing a common pinpoint for determining thrust rates in the simplest case low temperature thermochronology of minerals that pass through their closure temperature over the ramp will yield details on thrust kinematics thrust rate timing of initiation and duration of thrusting additional cooling ages of a more comprehensive sample suite can capture cooling due to erosion in these latter cases model results can place limitations on erosion rates

characteristics of desert varnish from nanometer to micrometer scale a photo oxidation model on its formation abstract rock varnish is a widespread mn rich rock coating commonly developed in arid environments but the mechanism for its formation is still under debate in this study rock varnish and adjacent soil dust collected from gobi desert were analyzed for exploring the possible abiotic oxidation mechanism of mn oxides the occurrence of rock varnish shows a direct and close relationship with strong irradiation of sunlight giving the first evidence of photochemical genesis abundant caves and tunnels with average diameter of 1 5 m are observed on varnish surface regions which facilitate the penetration of sunlight and water metal oxyhydr oxides including birnessite hematite goethite rutile and anatase account for the major components of rock varnish which are all solar light responsive semiconducting minerals the trace elements enrichment patterns revealed by la icp ms provide the indication of an aqueous origin the positive ce anomalies in varnish in contrast to the rock substrate as well as the positive correlation between ce and mn suggest a strong oxidizing environment in the genesis of rock varnish therefore the photo generated holes and reactive oxygen species ross on metal oxides in varnish can promote mn ii oxidation which are further demonstrated from thermodynamic and kinetic considerations ross including 1o2 and oh with strong oxidizing capability are detected by epr in varnish suspension providing the direct evidence for mn ii oxidation laboratory experimental simulations show the photocatalysis of metal oxides in rock varnish greatly promote mn ii oxidation by 2 10 7 97 times in comparison with homogenous solution oxidation all these lines of evidence suggest that light induced abiotic process may play important roles in the formation of rock varnish together with other abiotic and biotic pathways which can even provide implications for the evolution of mn oxides on surface of terrestrial planets