contribution of supra permafrost discharge to thermokarst lake water balances on the northeastern qinghai tibet plateau abstract the seasonal hydrological mechanisms of two thermokarst lakes on the northeastern qinghai tibet plateau qtp were characterized by three year intensive field observations and a water balance model in three ice free seasons the supra permafrost discharge contributed a mean ratio of over 170 of the precipitation in the ice cover seasons the supra permafrost discharge contribution varied between 20 and 22 of the water storage change results show that a large portion of the lake water storage change is because of the supra permafrost discharge resulting from precipitation furthermore a precipitation subsurface runoff function is preliminarily identified in which the supra permafrost discharge nonlinearly increased with more precipitation our results show that the recent lake expansion is linked with increasing supra permafrost discharge dominated by precipitation this study also suggests that we need to pay attention to the nonlinear increase of precipitation controlled supra permafrost discharge on the large lake expansion at the catchment scale in the qtp region instead of only looking at the inputs e g precipitation and river discharge as shown in the previous studies

topography relief and trmm derived rainfall variations along the himalaya 1 along the southern himalayan topographic front the indian summer monsoon modulates erosive processes and rates to investigate the influence of topography and relief on rainfall generation and resultant erosion we processed satellite rainfall amounts for the last 8 years 1998 2005 from the tropical rainfall measurement mission trmm based upon a spatial resolution of 5 5 km for the himalaya we identify 1 the spatial distribution of rainfall and 2 the large scale relationships between topography relief and rainfall locations our results show two distinct rainfall maxima along strike in the himalaya the first outer rainfall peak occurs along the southern margin of the lesser himalaya within a narrow band of mean elevation 0 9 0 4 km and mean relief 1 2 0 2 km the second discontinuous inner band typically occurs along the southern flank of the greater himalaya elevation and relief both 2 1 0 3 km

an early pliocene thermal disturbance of the main central thrust central nepal implications for himalayan tectonics since the beginning of the collision between india and asia at about 50 ma the convergence in the himalaya has largely been taken up along major thrust zones in this study samples of the lesser himalaya formations up to 10 km below the main central thrust mct and the greater himalaya sequence up to 12 km above the mct have been analyzed by the 40ar 39ar and u pb methods to investigate the thermal history of the mct the ages can be summarized as follows 1 the youngest ages from muscovites 3 1 ma biotites 3 4 ma and hornblendes 4 1 ma all come from within 1 km of the mct 2 there is a marked asymmetry of ages between the footwall and the hanging wall of the mct the maximum mica age in the hanging wall greater himalaya sequence is 13 ma whereas a muscovite 5 km below the mct in the footwall has an age spectrum with a gradient from 400 to 1400 ma 3 five k feldspars from 6 2 to 11 9 km above the mct all give minimum ages in the range 3 0 6 4 ma and 4 a 206pb 238u age on a brannerite from the greater himalayan sequence is 4 8 ma structural and petrologic observations preclude the possibility that this age distance distribution reflects faulting within the greater himalaya sequence and production of the necessary thermal energy by shear heating requires unrealistically high shear stresses infiltration of hot fluids through the mct zone appears to be the best hypothesis to explain these data simple numerical simulations which account for heat transfer by advection within the fluid flow zone and by conduction outside it indicate that the observed age distribution could have been produced by infiltration of hot fluids through the mct zone at circa 5 to 4 ma within the following range of conditions the fluids heated rocks to temperatures in the range 470 to 510 c for less than 1 million years in a region narrower than the entire mct zone the temperature of the thermal disturbance inferred from the 40ar 39ar data is consistent with petrologic data from albitic alteration assemblages in the greater himalaya sequence and the u pb age of the brannerite is similar to the youngest 40ar 39ar hornblende age this scenario hydrothermal heating culminating at about 4 ma is similar to the model of le fort 1981 for the generation of the high himalayan leucogranites in the late oligocene early miocene by dehydration of the footwall rocks of an active thrust in the younger instance we interpret that fluids derived from the footwall rocks of the main boundary thrust migrated upward through the lesser himalaya formations and were subsequently channeled along the mct producing the thermal disturbance

the role of advective heat transport in talik development beneath lakes and ponds in discontinuous permafrost 1 regions of warm thin discontinuous permafrost have been observed to be experiencing rapid changes in lake and pond dynamics in recent decades even though surface water and groundwater interactions are thought to play a significant role in heat transport in these regions the effect of these interactions on permafrost remains largely unquantified in order to examine the influence of groundwater flow on permafrost dynamics we modeled the development of a sub lake talik under permafrost conditions similar to those observed in the southern central seward peninsula region of alaska using a numerical solution that couples heat transport and groundwater flow including the effect of water phase changes on soil permeability and latent heat content a comparison of model simulations with and without near surface subpermafrost groundwater flow indicates that stable permafrost thicknesses are 2 to 5 times greater in the absence of groundwater flow simulations examining the thermal influence of lakes on underlying permafrost suggest that a through going talik can develop in a matter of decades and that the incorporation of advective heat transport reduces the time to complete loss of ice beneath the lake by half relative to heat transport by conduction alone this work presents the first quantitative assessment of the rates of sub lake permafrost response to thermal disturbances such as talik development in systems with near surface groundwater flow the results highlight the importance of coupled thermal and hydrologic processes on discontinuous permafrost dynamics

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

heat sources for tertiary metamorphism and anatexis in the annapurna manaslu region central nepal following the collision of india with southern tibet crustal rocks of the leading edge of india 1 underwent regional metamorphism to upper amphibolite grade 2 melted locally to produce anatectic granitoids and 3 were sheared and thrust onto lower grade rock along the main central thrust yielding an inverted metamorphic sequence this sequence is exemplified in the annapurna manaslu region we use simple physical calculations to examine the heat sources involved in the different phases of metamorphism the regional metamorphism apparently is due to the burial of the northern edge of india beneath the accretionary prism along the southern edge of tibet the observed temperatures and pressures of this first phase of metamorphism are consistent with the thermal relaxation during the 10 30 m y before slip on the main central thrust began of thickened continental lithosphere whose original surface heat flux was between 50 and 70 mw m 2 the release of water from the footwall of the main central thrust apparently facilitated melting of the overlying crust in the second phase such melting could have occurred in the first million years or so of thrusting if warm 550 650 c crust in the footwall contained the necessary water if melting did not occur in the earliest stages of slip on the main central thrust dissipative heating with shear stresses of 10 to 100 mpa is required for temperatures near the main central thrust to have remained high enough to generate melting above the fault during the underthrusting of cold material the thickness 6 8 km of the zone of inverted isograds associated with the fault if undisrupted and due solely to thermal diffusion implies that the time required to carry the rocks preserving the inverted metamorphism from the surface to depths of 30 40 km was 4 8 m y the apparent inverted temperature gradients about 10 25 c km in this zone can be understood as the combined result of two processes diffusion of heat from hot rock thrust over cold rock expunged the original temperature gradient near the fault and could have created an inverse gradient of 10 20 c km the peak temperatures in such a zone however would not have exceeded about 350 c without an additional source of heat dissipative heating at shear stresses of about 100 mpa can account for peak temperatures in excess of 600 c during slip on the fault and would have contributed as much as 13 c km to the inverse gradient although inversion of metamorphic isograds could have occurred as a result of deformation within the main central thrust zone the high temperatures during slip on this zone still require dissipative heating unless the duration of slip exceeded 25 m y

modern thermokarst lake dynamics in the continuous permafrost zone northern seward peninsula alaska 1 quantifying changes in thermokarst lake extent is of importance for understanding the permafrost related carbon budget including the potential release of carbon via lake expansion or sequestration as peat in drained lake basins we used high spatial resolution remotely sensed imagery from 1950 51 1978 and 2006 07 to quantify changes in thermokarst lakes for a 700 km2 area on the northern seward peninsula alaska the number of water bodies larger than 0 1 ha increased over the entire observation period 666 to 737 or 10 7 however total surface area decreased 5 066 ha to 4 312 ha or 14 9 this pattern can largely be explained by the formation of remnant ponds following partial drainage of larger water bodies thus analysis of large lakes 40 ha shows a decrease of 24 and 26 in number and area respectively differing from lake changes reported from other continuous permafrost regions thermokarst lake expansion rates did not change substantially between 1950 51 and 1978 0 35 m yr and 1978 and 2006 07 0 39 m yr however most lakes that drained did expand as a result of surface permafrost degradation before lateral drainage drainage rates over the observation period were stable 2 2 to 2 3 per year thus analysis of decadal scale high spatial resolution imagery has shown that lake drainage in this region is triggered by lateral breaching and not subterranean infiltration future research should be directed toward better understanding thermokarst lake dynamics at high spatial and temporal resolution as these systems have implications for landscape scale hydrology and carbon budgets in thermokarst lake rich regions in the circum arctic

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

the age and rate of displacement along the main central thrust in the western bhutan himalaya abstract in western bhutan the main central thrust mct is broadly folded creating multiple exposures of the fault surface over a 70 km across strike distance this unusual map pattern presents a unique opportunity to map the mct and document both the magnitude and age of displacement in situ th pb sims and la icp ms geochronology of metamorphic monazite from the immediate hanging wall of the mct indicates that prograde monazite growth in greater himalayan gh rocks continued until 20 8 1 1 ma whereas crystallization of in situ melts characterized by high y monazite overgrowths occurred during cooling from ca 15 10 ma prograde monazite growth at 15 ma in lesser himalaya lh rocks in the immediate footwall requires that lh footwall strata began to be buried at this time and the mct had reached its southernmost exposed extent by combining prograde monazite ages in the immediate hanging wall and footwall the duration of mct displacement is bracketed between 20 8 1 1 and 15 0 2 4 ma immediately north of our study area a published estimate of shearing along the outer south tibetan detachment std argues for displacement between 20 and 15 ma coeval with the age range for mct displacement that we document in this study however retrograde monazite grains as young as 10 ma suggest that gh rocks were cooling until 10 ma 5 myr later than motion on the outer std immediately to the north this cooling was either the result of continued displacement on the mct or growth of a duplex that passively folded the mct using a sequential reconstruction we estimate a total displacement of 230 km which is the sum of displacements on the mct and the structurally lower paro thrust over a duration of 5 8 2 6 myr this indicates a horizontal shortening rate of 4 0 3 2 1 3 cm yr which exceeds present rates estimated from geodetic measurements across the himalaya and mct displacement rates c 2 cm yr inferred from petrologic and thermal models in central nepal but is indistinguishable from plate convergence rates calculated for eastern bhutan between 23 and 20 ma 3 3 0 7 cm yr our study highlights that displacement on the mct alone achieved plate velocity rates in western bhutan and that the age and rate of mct displacement varied significantly across the himalaya

exhumational variability within the himalaya of northwest india in the himalaya of chamba nw india a major orographic barrier in front of the greater himalayan range extracts a high proportion of the monsoonal rainfall along its southern slopes and effectively shields the orogen interior from moisture bearing winds along a 100 km long orogen perpendicular transect 28 new apatite fission track aft and 30 new zircon u th he zhe cooling ages reveal marked variations in age distributions and long term exhumation rates between the humid frontal range and the semi arid orogen interior on the southern topographic front very young elevation invariant aft ages of 4 ma have been obtained that are concentrated in a 30 km wide zone 1 d thermal modeling suggests a plio pleistocene mean erosion rate of 0 8 1 9 mm yr 1 in contrast aft and zhe ages within the orogen interior are older 4 9 and 7 18 ma respectively are positively correlated with sample elevation and yield lower mean erosion rates 0 3 0 9 mm yr 1 protracted low exhumation rates within the orogen interior over the last 15 myr prevailed contemporaneously with overall humid conditions and an effective erosional regime within the southern himalaya this suggests that the frontal dhauladar range was sufficiently high during this time to form an orographic barrier focusing climatically enhanced erosional processes and tectonic deformation there thrusting along the two frontal range bounding thrust the main central thrust and the main boundary thrusts was initiated at least 15 ma ago and has remained localized since then the lack of evidence for localized uplift farther north indicates either a rather flat decollement with no ramp or the absence of active duplex systems beneath the interior of chamba exhumational variability within chamba is best explained as the result of continuous thrusting along a major basal decollement with a flat beneath the slowly exhuming internal compartments and a steep frontal ramp at the rapidly exhuming frontal range the pattern in chamba contrasts with what is observed elsewhere along the himalaya where exhumation is focused in a zone 150 km north of the orogenic front in the nw himalaya preserved high himalayan crystalline nappes and lesser himalayan windows alternate on a relatively small scale of 100 km these alternations are closely correlated with the pattern of exhumation although the spatial distribution of high exhumation zones varies considerably between individual himalayan sectors all of these zones are closely correlated with locally higher rock uplift rates sharp topographic discontinuities and focused orographic precipitation suggesting strong feedbacks between tectonically driven rock uplift orographically enhanced precipitation and erosional processes

lateral and subsurface flows impact arctic coastal plain lake water budgets arctic thaw lakes are an important source of water for aquatic ecosystems wildlife and humans many recent studies have observed changes in arctic surface waters related to climate warming and permafrost thaw however explaining the trends and predicting future responses to warming is difficult without a stronger fundamental understanding of arctic lake water budgets by measuring and simulating surface and subsurface hydrologic fluxes this work quantified the water budgets of three lakes with varying levels of seasonal drainage and tested the hypothesis that lateral and subsurface flows are a major component of the post snowmelt water budgets a water budget focused only on post snowmelt surface water fluxes stream discharge precipitation and evaporation could not close the budget for two of three lakes even when uncertainty in input parameters was rigorously considered using a monte carlo approach the water budgets indicated large positive residuals consistent with up to 70 of mid summer inflows entering lakes from lateral fluxes lateral inflows and outflows were simulated based on three processes supra permafrost subsurface inflows from basin edge polygonal ground and exchange between seasonally drained lakes and their drained margins through runoff and evapotranspiration measurements and simulations indicate that rapid subsurface flow through highly conductive flowpaths in the polygonal ground can explain the majority of the inflow drained lakes were hydrologically connected to marshy areas on the lake margins receiving water from runoff following precipitation and losing up to 38 of lake efflux to drained margin evapotranspiration lateral fluxes can be a major part of arctic thaw lake water budgets and a major control on summertime lake water levels incorporating these dynamics into models will improve our ability to predict lake volume changes solute fluxes and habitat availability in the changing arctic published 2016 this article is a u s government work and is in the public domain in the usa

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

landsat based mapping of thermokarst lake dynamics on the tuktoyaktuk coastal plain northwest territories canada since 1985 abstract several remote sensing studies have documented widespread thermokarst lake expansion in continuous permafrost regions of north america over the past few decades other studies have found no long term trends in water body extents but large intra and inter annual changes driven by precipitation these differences could be due to geographic variability in physical conditions geology climate permafrost and hydrology or in the data and methods used to extract water bodies this study tested water extraction methods over the tuktoyaktuk coastal plain northwest territories nwt canada based on the landsat 5 shortwave infrared swir channel and validated them using water extents obtained from 0 5 m resolution orthophoto imagery methods included applying thresholds to generate binary land water classifications as well as deriving 30 m water fractions from both linear unmixing and a new histogram breakpoint method results indicated that the histogram breakpoint method outperformed other methods underestimating overall water fraction by 0 26 and overestimating the number of pure 30 m water pixels by 0 66 compared to water fractions calculated from orthophotos the breakpoint method was then applied to a stack of 17 near peak of season landsat 5 and landsat 7 images acquired between july 4 and august 18 from 1985 to 2011 to create a water fraction time series for examining both trends and inter annual variation in water extent results showed an overall expansion of lake area along margins by 55 km 2 with isolated lakes experiencing rapid drainage totalling 15 km 2 leading to a net water area increase of 40 km 2 over the 26 year period

surface processes recorded by rocks and soils on meridiani planum mars microscopic imager observations during opportunity s first three extended missions the microscopic imager mi on the mars exploration rover opportunity has returned images of mars with higher resolution than any previous camera system allowing detailed petrographic and sedimentological studies of the rocks and soils at the meridiani planum landing site designed to simulate a geologist s hand lens the mi is mounted on opportunity s instrument arm and can resolve objects 0 1 mm across or larger this paper provides an overview of mi operations data calibration and analysis of mi data returned during the first 900 sols mars days of the opportunity landed mission analyses of opportunity mi data have helped to resolve major questions about the origin of observed textures and features these studies support eolian sediment transport rather than impact surge processes as the dominant depositional mechanism for burns formation strata mi stereo observations of a rock outcrop near the rim of erebus crater support the previous interpretation of similar sedimentary structures in eagle crater as being formed by surficial flow of liquid water well sorted spherules dominate ripple surfaces on the meridiani plains and the size of spherules between ripples decreases by about 1 mm from north to south along opportunity s traverse between endurance and erebus craters

thermal conductivity of hydrate bearing sediments 1 a thorough understanding of the thermal conductivity of hydrate bearing sediments is necessary for evaluating phase transformation processes that would accompany energy production from gas hydrate deposits and for estimating regional heat flow based on the observed depth to the base of the gas hydrate stability zone the coexistence of multiple phases gas hydrate liquid and gas pore fill and solid sediment grains and their complex spatial arrangement hinder the a priori prediction of the thermal conductivity of hydrate bearing sediments previous studies have been unable to capture the full parameter space covered by variations in grain size specific surface degree of saturation nature of pore filling material and effective stress for hydrate bearing samples here we report on systematic measurements of the thermal conductivity of air dry water and tetrohydrofuran thf saturated and thf hydrate saturated sand and clay samples at vertical effective stress of 0 05 to 1 mpa corresponding to depths as great as 100 m below seafloor results reveal that the bulk thermal conductivity of the samples in every case reflects a complex interplay among particle size effective stress porosity and fluid versus hydrate filled pore spaces the thermal conductivity of thf hydrate bearing soils increases upon hydrate formation although the thermal conductivities of thf solution and thf hydrate are almost the same several mechanisms can contribute to this effect including cryogenic suction during hydrate crystal growth and the ensuing porosity reduction in the surrounding sediment increased mean effective stress due to hydrate formation under zero lateral strain conditions and decreased interface thermal impedance as grain liquid interfaces are transformed into grain hydrate interfaces

hydrodynamic and suspended sediment transport controls on river mouth morphology 1 river mouths building into standing bodies of water have strikingly varied growth habits this presents a compelling pattern formation problem that is also of great practical relevance for subsurface prediction and managing coastal wetlands here we present a generalized 2 5 dimensional potential vorticity pv theory that explains sedimentation patterns of a sediment laden stationary jet by coupling an understanding of vorticity with suspended sediment concentration fields we explore the physical meaning of this new sediment pv definition and its impact on outflow depositional patterns by analyzing data from a shallow wall bounded plane jet experiment and by discussing new theoretical insights a key result is that lateral advection and diffusion of suspended sediment are directly proportional to jet vorticity a feature that reveals the mechanistic process that forms elongated channels by focused levee deposition the new pv theory constitutes a more generalized mathematical framework that expands the rouse theory for the equilibrium of suspended sediment

hydrogeomorphic processes of thermokarst lakes with grounded ice and floating ice regimes on the arctic coastal plain alaska thermokarst lakes cover 20 of the landscape throughout much of the alaskan arctic coastal plain acp with shallow lakes freezing solid grounded ice and deeper lakes maintaining perennial liquid water floating ice thus lake depth relative to maximum ice thickness 1 5 2 0 m represents an important threshold that impacts permafrost aquatic habitat and potentially geomorphic and hydrologic behaviour we studied coupled hydrogeomorphic processes of 13 lakes representing a depth gradient across this threshold of maximum ice thickness by analysing remotely sensed water quality and climatic data over a 35 year period shoreline erosion rates due to permafrost degradation ranged from 0 2 m year in very shallow lakes 0 4 m up to 1 8 m year in the deepest lakes 2 6 m this pattern of thermokarst expansion masked detection of lake hydrologic change using remotely sensed imagery except for the shallowest lakes with stable shorelines changes in the surface area of these shallow lakes tracked interannual variation in precipitation minus evaporation p el with periods of full and nearly dry basins shorter term 2004 2008 specific conductance data indicated a drying pattern across lakes of all depths consistent with the long term record for only shallow lakes our analysis suggests that grounded ice lakes are ice free on average 37 days longer than floating ice lakes resulting in a longer period of evaporative loss and more frequent negative p el these results suggest divergent hydrogeomorphic responses to a changing arctic climate depending on the threshold created by water depth relative to maximum ice thickness in acp lakes copyright 2011 john wiley sons ltd

near shore talik development beneath shallow water in expanding thermokarst lakes old crow flats yukon it is generally assumed that permafrost is preserved beneath shallow lakes and ponds in the western north american arctic where water depth is less than about two thirds of the late winter lake ice thickness here we present field observations of talik development beneath water as shallow as 0 2 m despite a lake ice thickness of 1 5 m in old crow flats ocf yt conditions leading to the initiation and development of taliks beneath shallow water were investigated with field measurements of shore erosion rates bathymetry ice thickness snow accumulation and lake bottom temperature near the shores of two expanding lakes in ocf the sensitivity of talik development to variations in lake bottom thermal regime was then investigated numerically where ice reached the lake bottom talik development was controlled by the ratio of freezing degree days to thawing degree days at the lake bottom fddlb tddlb in some cases spatial variations in on ice snow depth had a minimal effect on annual mean lake bottom temperature tlb but caused sufficient variations in fddlb tddlb to influence talik development where tlb was close to but greater than 0 c simulations indicated that the thermal offset allowed permafrost aggradation to occur under certain conditions resulting in irregular near shore talik geometries the results highlight the sensitivity of permafrost to small changes in lake bottom thermal conditions where the water column freezes through in early winter and indicate the occurrence of permafrost degradation beneath very shallow water in the near shore zone of arctic ponds and lakes

leucogranite intruding the south tibetan detachment in western nepal implications for exhumation models in the himalayas the most popular models regarding the exhumation of the greater himalayan sequence ghs such as extrusion channel flow critical taper and wedge extrusion require prolonged activity of the two bounding shear zones and faults the main central thrust mct and the south tibetan detachment std we present the crystallization age of an undeformed leucogranite that intrudes both the ghs and the tethyan himalaya sequence ths zircon and monazite u pb ages in the leucogranite give ages between 23 and 25 ma constraining at that time the end of shearing along the std our results limit the contemporaneous activity of the mct and std to a short period of time 1 2 ma and thus argue against exhumation models requiring prolonged contemporaneous activity of the mct and std

lengthening plus shortening of river to lake connection times in the mackenzie river delta respectively via two global change mechanisms along the arctic coast 1 river deltas along the circumpolar arctic coast are lake rich and poorly understood ecosystems set in a region expected to change rapidly over the past 30 years annual river to lake connection times in the mackenzie delta have lengthened 30 days in the lowest elevation lakes and may have shortened in the highest elevation lakes respectively via sea level rise and declining effects of river ice breakup lengthened connection times indicate summer low water levels in the delta have increased by an amount 0 3 m equivalent to three times local sea level rise 0 1 m over the same period such an amplification effect of recent sea level rise has been completely unexpected and may be a result of enhanced storm surges in response to receding arctic sea ice or coastal backwater effects on the river flow shortened connection times are consistent with other work showing a decline in river ice breakup effects an important control on annual peak water levels

lake ice growth and decay in central alaska usa observations and computer simulations compared abstract the canadian lake ice model climo a one dimensional thermodynamic model withunsteady heat conduction and penetrating solar radiation is used to simulate ice growth and decay onshallow ponds in and near fairbanks central alaska usa simulations are compared with observationsof ice thickness and composition snow ice congelation ice freeze up break up and duration simulations run using us national weather service meteorological data as input variables do not agreewellwith ice thickness measurements the simulations improve significantly when the model is run withmore representative locally measured data forair temperature and depth of snowon the ice the causesof some discrepancies between simulations and observations are discussed and some suggestions formodel improvements are made 1 introduction lake ice growth and decay includes freeze up in theautumn a long period of growth and thickening in winter a short period of ice melting and thinning and finally break up and the complete disappearance of the ice cover inspring the sensitivity of lake ice growth and decay particularly freeze up break up and thus duration some times referred to as ice phenology to climate variabilityandchange has been demonstrated by observation on theground and from space e g palecki and barry 1986 robertson and others 1992 wynne and others 1998 magnuson and others 2000 the sensitivity of icephenology and thickness to climate variability and changehas also been investigated using computer models e g liston and hall 1995b vavrus and others 1996 stefan andfang 1997 zhang and jeffries 2000 an important aspectof computer modelling of lake ice indeed of any modellingexercise is the evaluation of model performance by com parison of simulated results with recent observations andmeasurements heron and woo 1994 liston and hall 1995a fang and others 1996 walsh and others 1998 me nard and others 2002 duguay and others 2003 using the canadian lake ice model climo me nard andothers 2002 duguay and others 2003 morris and others 2005 simulate the effects of the mid 1970s alaska climateshift on lake ice phenology thickness and composition congelation ice snow ice in central alaska usa and thesensitivity of those variables to possible future changes in themagnitude and timing of air temperature and precipitation inthe region prior to performing the climate sensitivity study we assessed the performance of climo by simulating icephenology thickness and composition in ice seasons 2001 02 and 2002 03 and comparing the results with observationsand measurements made at ponds in the vicinity of pokerflat research range pfrr and in fairbanks central alaska this paper describes the model performance assessment which is based on a more comprehensive dataset than wasavailable to duguay and others 2003 for pfrr ice season1999 2000 this assessment includes freeze up and break up data obtained every 1 2days at 11 ponds and weeklydata for ice thickness and the depth and density of the snowon the ice during the winter

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

assessing quaternary reactivation of the main central thrust zone central nepal himalaya new thermochronologic data and numerical modeling we study the recent dynamics of the central nepal himalaya focusing on possible reactivation of the footwall of the main central thrust which is marked by an abrupt topographic transition different tectonic mechanisms such as overthrusting of a major crustal ramp underplating or out of sequence thrusting have been suggested to explain the morphology and exhumation patterns in this area we present 25 new apatite fission track ages collected along a north south transect in central nepal as well as two age elevation profiles ages are consistently younger than 3 ma old in the main central thrust zone and increase continuously to 4 6 ma old in the south no jump in apatite fission track ages is observed across the topographic transition apparent exhumation rates from age elevation relationships vary from 0 46 0 13 0 09 km ma in the palung granite south of kathmandu to 4 4 4 8 1 5 km ma in the main central thrust zone the latter rate is probably overestimated by a factor of two due to topographic effects as shown by a new numerical model these strongly varying exhumation rates can be explained by overthrusting of a crustal ramp which exerts a primary control on age patterns and do not require out of sequence reactivation of thrusts in the main central thrust zone

a new approach to stable isotope based paleoaltimetry implications for paleoaltimetry and paleohypsometry of the high himalaya since the late miocene the change in oxygen isotopic composition of precipitation is modeled using equilibrium fractionation during rayleigh distillation linked to the thermodynamics of atmospheric ascent and water vapor condensation the primary controllers of the vertical variation in isotopic composition with elevation are the low elevation temperature and relative humidity as these control the vertical distribution of condensation an empirical fit of precipitation versus model condensation based on alpine stations is derived this fit is represented in the model as the weighted mean composition of condensation within a 1000 m thick air parcel 1500 500 m above the ground surface and is used for all other regions comparison of model versus observed modern precipitation reveals a close fit particularly of more highly elevated sites comparison of modern waters in the himalayas and southern tibet with model predictions particularly as revealed by comparison of observed and predicted hypsometry provides additional support to the validity of the model finally application of this model to estimates of paleo waters in the himalayas and southern tibet reveals that this region had already achieved its present hypsometry by the late miocene about 10 ma ago fl 2001 published by elsevier science b v

shifting balance of thermokarst lake ice regimes across the arctic coastal plain of northern alaska 1 the balance of thermokarst lakes with bedfast and floating ice regimes across arctic lowlands regulates heat storage permafrost thaw winter water supply and over wintering aquatic habitat using a time series of late winter synthetic aperture radar sar imagery to distinguish lake ice regimes in two regions of the arctic coastal plain of northern alaska from 2003 2011 we found that 18 of the lakes had intermittent ice regimes varying between bedfast ice and floating ice conditions comparing this dataset with a radar based lake classification from 1980 showed that 16 of the bedfast ice lakes had shifted to floating ice regimes a simulated lake ice thinning trend of 1 5 cm yr since 1978 is believed to be the primary factor driving this form of lake change the most profound impacts of this regime shift in arctic lakes may be an increase in the landscape scale thermal offset created by additional lake heat storage and its role in talik development in otherwise continuous permafrost as well as increases in over winter aquatic habitat and winter water supply

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

estimating late winter heat flow to the atmosphere from the lake dominated alaskan north slope the conductive heat flux through the snow cover f a is used as a proxy to examine the hypothesis that there is a significant heat flow from the alaskan north slope to the atmosphere because of the large number of lakes in the region f a is estimated from measurements of snow depth temperature and density on tundra grounded ice and floating ice in mid april 1997 at six lakes near barrow northwestern alaska the mean f a values from tundra grounded ice and floating ice are 1 5 5 4 and 18 6 w m 2 respectively a numerical model of the coupled snow ice water soil system is used to simulate f a and there is good agreement between the simulated and measured fluxes the flux from the tundra is low because the soils have a relatively low thermal conductivity and the active layer cools significantly after freezing completely the previous autumn the flux from the floating ice is high because the ice has a relatively high thermal conductivity and a body of relatively warm water remains below the growing ice at the end of winter the flux from the grounded ice is intermediate between that from the tundra and that from the floating ice and depends on the timing of the contact between the growing ice and the lake sediments and whether or not those sediments freeze completely using the estimated f a values combined with the areal fractions of tundra grounded ice and floating ice derived from synthetic aperture radar images area weighted f a values are calculated for six areas f a values for the ice vary between 9 8 and 13 8 w m 2 and those from the ice plus tundra vary between 3 9 and 5 3 w m 2 the f a values are similar to those observed in the sea ice covered regions of the south and north polar oceans in winter the north slope of alaska may thus make a significant contribution to the regional energy budget in winter

the origin of channels on lower taylor glacier mcmurdo dry valleys antarctica and their implication for water runoff well developed surface channels on taylor glacier mcmurdo dry valleys antarctica begin as medial moraines incised as shallow narrow surface depressions and retain this geometry for tens of km over a distance of 1100m the channel geometry dramatically changes reaching depths 20m and widths 100m after rapidly enlarging the channels appear to evolve toward a new equilibrium geometry compared to the glacier surface the air temperature in the channels is warmer by 1 78c wind speed is reduced by 2 4m s and net shortwave radiation is greater by 14wm the microclimate in the channel shifts the energy balance towards enhanced melt field evidence and energy balance modeling indicate ablation in the deep channels is 4 5 times greater than the local horizontal glacier surface and that melt accounts for 99 of the summer ablation compared to 75 on the adjacent horizontal glacier surface melt in these channels supplies 65 of the unaccounted water discharge into the neighboring lake in large part the channels generate the water they carry rather than merely route water generated elsewhere

an energy balance model of lake ice evolution a physically based mathematical model of the coupled lake lake ice snow and atmosphere system is developed for studying terrestrial atmospheric interactions in high elevation and high latitude regions the ability to model lake ice freeze up break up total ice thickness and ice type offers the potential to describe the effects of climate change in these regions model output is validated against lake ice observations made during the winter of 1992 93 in glacier national park montana u s a the model is driven with observed daily atmospheric forcing of precipitation wind speed and air temperature in addition to simulating complete energy balance components over the annual cycle model output includes ice freeze up and break up dates and the end of season clear ice snow ice and total ice depths for two nearby lakes in glacier national park each in a different topographic setting modeled ice features are found to agree closely with the lake ice observations model simulations illustrate the key role that the wind component of the local climatic regime plays on the growth and decay of lake ice the wind speed affects both the surface temperature and the accumulation of snow on the lake ice surface higher snow accumulations on the lake ice depress the ice surface below the water line causing the snow to become saturated and leading to the formation of snow ice deposits in regions having higher wind speeds significantly less snow accumulates on the lake ice surface thus limiting snow ice formation the ice produced by these two different mechanisms has distinctly different optical and radiative properties and affects the monitoring of frozen lakes using remote sensing techniques

evolution of lakes and basins in northern alaska and discussion of the thaw lake cycle 1 we evaluated the development of lake basins on the central coastal plain of northern alaska on the basis of topographic profiles soil and ground ice surveys radiocarbon dating photogrammetric analysis and regional comparisons our analysis reveals that lake evolution is much more complex and less cyclic than theorized by previous investigations in the area we studied there was insufficient ground ice in the oldest terrain to form thaw lakes the aggradation of ice in the margins of drained lake basins was insufficient to heave the surface up to near original topographic conditions and the process occurs at too slow a rate for a thaw lake cycle to develop within the holocene accordingly we revised the conceptual model of lake and basin development to be consistent with the patterns and process we observed on the extensive sand sheets underlying most of the coastal plain developmental stages include 1 initial flooding of depressions to form primary lakes 2 lateral erosion with sorting and redistribution of sediments 3 lake drainage as the stream network expands 4 differential ice aggradation in silty centers and sandy margins 5 formation of secondary thaw lakes in the heaved centers of ice rich basins and infilling of ponds along the low margins and 6 basin stabilization

ice processes and surface ablation in a shallow thermokarst lake in the central qinghai tibetan plateau the qinghai tibetan plateau qtp is characterized by a cold climate and a large number of lakes the long ice season necessitates study of the widespread ice covers in the region an unprecedented multidisciplinary field campaign was conducted on lake ice processes in the central qtp during the period 2019 13 the study lake generally froze up in late october or early november and broke up in mid or late april with a maximum ice thickness of 50 70 cm the mass balances at both ice surface and bottom were measured continuously significant ice surface sublimation ablation was detected and accounted for up to 40 of the whole ice thickness over the ice season a simple heat transfer model was developed for the surface ice loss the calculated values were in good agreement with the observations they also indicated that atmospheric conditions including low air humidity and prevailing strong winds are the primary drivers of the ice surface sublimation

miocene to holocene exhumation of metamorphic crustal wedges in the nw himalaya evidence for tectonic extrusion coupled to fluvial erosion the himalayan crystalline core zone exposed along the sutlej valley india is composed of two high grade metamorphic gneiss sheets that were successively underthrusted and tectonically extruded as a consequence of the foreland directed propagation of crustal deformation in the indian plate margin the high himalayan crystalline sequence hhcs is composed of amphibolite facies to migmatitic paragneisses metamorphosed at temperatures up to 750 c at 30 km depth between eocene and early miocene during early miocene combined thrusting along the main central thrust mct and extension along the sangla detachment induced the rapid exhumation and cooling of the hhcs whereas exhumation was mainly controlled by erosion since middle miocene the lesser himalayan crystalline sequence lhcs is composed of amphibolite facies para and orthogneisses metamorphosed at temperatures up to 700 c during underthrusting down to 30 km depth beneath the mct the lhcs cooled very rapidly since late miocene as a consequence of exhumation controlled by thrusting along the munsiari thrust and extension in the mct hanging wall this renewed phase of tectonic extrusion at the himalayan front is still active as indicated by the present day regional seismicity and by hydrothermal circulation linked to elevated near surface geothermal gradients in the lhcs as recently evidenced in the himalayan syntaxes active exhumation of deep crustal rocks along the sutlej valley is spatially correlated with the high erosional potential of this major trans himalayan river this correlation supports the emerging view of a positive feedback during continental collision between crustal scale tectono thermal reworking and efficient erosion along major river systems

time dependent morphology of thaw lakes and taliks in deep and shallow ground ice 1 the shape and depth of thaw lake basins depends on lake age on whether the talik thaw bulb is at steady state and on the distribution of ice in the ground to investigate implications of this broad hypothesis we use a numerical model of conductive heat transfer phase change and thaw subsidence of ice rich sediment beneath a lake in cross section for ground thermal properties with lake temperatures and dimensions consistent with measurements modeled talik depth approximately increases with except in lakes in deep ground ice which deepen more rapidly because of consolidation on thawing in deep ground ice environments basins achieve depths of 20 m in 5 ka expanding lakes with disequilibrium taliks have deep basins with broad 100 m wide inclined margins in shallow ground ice settings either original epigenetic ice or in permafrost that has reformed in drained basins lakes are 3 m deep and flat bottomed unaffected by preexisting topography and ground ice variations first generation lakes in deep ground ice are rounder and grow larger in area than later generation lakes these predictions are consistent with gps sonar and remote sensing measurements of bathymetry and plan view shape of first and later generation lakes in substrates with deep syngenetic ground ice pleistocene loess northern seward peninsula alaska and shallow epigenetic ice yukon arctic coastal plain

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

morphometric analysis of ice walled lake plains in northern illinois implications of lake elongation by wind induced dual cycle currents abstract ice walled lake plains iwlps are rounded flat topped mounds that formed in stagnant ice environments along the margins of the laurentide ice sheet we conducted detailed morphometric and statistical analyses of the shape size and orientation of more than 400 iwlps identified from aerial photos aided with lidar data in dekalb county illinois usa lake elongation theories include extraterrestrial impact e g the carolina bays ice flow dynamics and crevasses and wind induced currents that preferentially erode the shorelines perpendicular to the dominant wind direction the results indicate that elliptical iwlps with a perimeter greater than 3050 m have preferred orientations roughly normal to the paleo wind direction as indicated by contemporaneous parabolic dunes located 50 km to the west the orientations of the iwlps with a perimeter less than 1220 m are scattered and show no apparent trend the iwlp orientation is not related to ice flow dynamics or glacial crevasses because no statistically significant relationship exists with regard to the ice flow as proxied by the moraine direction the orientation of large iwlps in dekalb county are consistent with wind induced lake elongation observed in modern permafrost thaw lakes suggesting that the prevailing wind also played an important role in controlling the orientation of iwlps during the last glacial period and led to the preferred orientation we see today

mayotte coral reveals hydrological changes in the western indian ocean between 1881 and 1994 we reconstruct the hydrologic history of the tropical western indian ocean by calculating the 18oseawater from coupled coral sr ca and 18o measurements in a massive porites coral from mayotte comoros between 1881 and 1994 we found that the precipitation evaporation balance varies naturally on time scales of 5 6 years and 18 25 years high low ssts are associated with positive negative 18oseawater implying that atmospheric variability is linked with remote climate modes in the indian ocean and the tropical extratropical pacific ocean warm el nino southern oscillation events are associated with a negative freshwater balance at mayotte this case study demonstrates that a much denser network of 18oseawater reconstructions is crucial for understanding the spatial patterns of hydrological conditions

methods to assess natural and anthropogenic thaw lake drainage on the western arctic coastal plain of northern alaska 1 thousands of lakes are found on the arctic coastal plain of northern alaska and northwestern canada developed atop continuous permafrost these thaw lakes and associated drained thaw lake basins are the dominant landscape elements and together cover 46 of the 34 570 km2 western arctic coastal plain wacp lakes drain by a variety of episodic processes including coastal erosion stream meandering and headward erosion bank overtopping and lake coalescence comparison of landsat multispectral scanner mss imagery from the mid 1970s to landsat 7 enhanced thematic mapper etm imagery from around 2000 shows that 50 lakes completely or partially drained over the approximately 25 year period indicating landscape stability the lake specific drainage mechanism can be inferred in some cases and is partially dependant on geographic settings conducive to active erosion such as riparian and coastal zones in many cases however the cause of drainage is unknown the availability of high resolution aerial photographs for the barrow peninsula extends the record back to circa 1950 mapping spatial time series illustrates the dynamic nature of lake expansion coalescence and drainage analysis of these historical images suggests that humans have intentionally or inadvertently triggered lake drainage near the village of barrow efforts to understand landscape processes and identify events have been enhanced by interviewing inupiaq elders and others practicing traditional subsistence lifestyles they can often identify the year and process by which individual lakes drained thereby providing greater dating precision and accuracy in assessing the causal mechanism indigenous knowledge has provided insights into events landforms and processes not previously identified or considered

fluvial incision and tectonic uplift across the himalayas of central nepal the pattern of fluvial incision across the himalayas of central nepal is estimated from the distribution of holocene and pleistocene terraces and from the geometry of modern channels along major rivers draining across the range the terraces provide good constraints on incision rates across the himalayan frontal folds sub himalaya or siwaliks hills where rivers are forced to cut down into rising anticlines and have abandoned numerous strath terraces farther north and upstream in the lesser himalaya prominent fill terraces were deposited probably during the late pleistocene and were subsequently incised the amount of bedrock incision beneath the fill deposits is generally small suggesting a slow rate of fluvial incision in the lesser himalaya the terrace record is lost in the high range where the rivers are cutting steep gorges to complement the terrace study fluvial incision was also estimated from the modern channel geometries using an estimate of the shear stress exerted by the flowing water at the bottom of the channel as a proxy for river incision rate this approach allows quantification of the effect of variations in channel slope width and discharge on the incision rate of a river the determination of incision rates requires an additional lithological calibration the two approaches are shown to yield consistent results when applied to the same reach or if incision profiles along nearby parallel reaches are compared in the sub himalaya river incision is rapid with values up to 10 15 mm yr it does not exceed a few millimeters per year in the lesser himalaya and rises abruptly at the front of the high range to reach values of 4 8 mm yr within a 50 km wide zone that coincides with the position of the highest himalayan peaks sediment yield derived from the measurement of suspended load in himalayan rivers suggests that fluvial incision drives hillslope denudation of the landscape at the scale of the whole range the observed pattern of erosion is found to closely mimic uplift as predicted by a mechanical model taking into account erosion and slip along the flat ramp flat geometry of the main himalayan thrust fault the morphology of the range reflects a dynamic equilibrium between present day tectonics and surface processes the sharp relief together with the high uplift rates in the higher himalaya reflects thrusting over the midcrustal ramp rather than the isostatic response to reincision of the tibetan plateau driven by late cenozoic climate change or late miocene reactivation of the main central thrust

migrating cretaceous eocene magmatismin the serra do mar alkaline province se brazil melts from the deflected trindade mantle plume department of geological sciences university of durham south road durham dh1 3le uk department of earth sciences university of cambridge downing street cambridge cb2 3eq uk department of physics the university newcastle upon tyne ne1 7ru uk department of geology mcmaster university 1280 main street west hamilton ont canada l8s 4m1 departmento de geoquimica e recursos minerais instituto de geociencias universidade de brasilia

expansion rate and geometry of floating vegetation mats on the margins of thermokarst lakes northern seward peninsula alaska usa investigations on the northern seward peninsula in alaska identified zones of recent 50 years permafrost collapse that led to the formation of floating vegetation mats along thermokarst lake margins the occurrence of floating vegetation mat features indicates rapid degradation of near surface permafrost and lake expansion this paper reports on the recent expansion of these collapse features and their geometry is determined using geophysical and remote sensing measurements the vegetation mats were observed to have an average thickness of 0 57 m and petrophysical modeling indicated that gas content of 1 5 5 enabled floatation above the lake surface furthermore geophysical investigation provides evidence that the mats form by thaw and subsidence of the underlying permafrost rather than terrestrialization the temperature of the water below a vegetation mat was observed to remain above freezing late in the winter analysis of satellite and aerial imagery indicates that these features have expanded at maximum rates of 1 2 m yr 1 over a 56 year period including the spatial coverage of floating thermokarst mats increases estimates of lake area by as much as 4 in some lakes copyright 2011 john wiley sons ltd

tundra lakes and permafrost richards island western arctic coast canada lakes of average size 33 ha occupy a quarter of the surface area of richards island northwest territories most of the lakes have a central pool deeper than the thickness of winter ice and many have prominent shallow littoral terraces the relatively warm lake bottoms cause considerable disturbance to the surrounding continuous permafrost water and lake bottom temperatures the configuration of permafrost and active layer thickness were measured at a tundra lake between 1992 and 1997 the lake is oval 1 6 km long 800 m wide and as deep as 13 m sandy terraces covered by less than 1 m of water extend over 100 m from the shore the terraces are underlain by permafrost which terminates almost vertically at their edge the annual mean temperature measured at lake bottom in the central pool ranged between 1 5 c and 4 8 c depending on depth and between 0 2 c and 5 c on the terraces due to differences in snow cover and proximity to the central pool in consequence the temperature of permafrost at 7 m depth in the terraces also varied from 2 c near shore to 5 c in mid terrace the active layer in the terraces was uniformly 1 4 m deep geothermal modelling of talik configuration indicates that there is no permafrost beneath the central pool of the lake the modelling indicates that under equilibrium conditions about one quarter of the lakes on richards island have taliks that penetrate permafrost and at least 10 15 of the island is underlain by talik short term climatic changes predicted for the region imply a small increase in summer lake water temperature and an extension of the open water season accompanied by thicker snow cover in winter following such changes with longer freeze up and warmer terrace temperatures in winter permafrost may not be sustainable in the lake terraces

kinematics of the dadeldhura klippe shear zones w nepal implications for the foreland evolution of the himalayan metamorphic core mapping combined with structural analyses in the foreland edge of the metamorphic core of the himalayas in sw nepal highlights the existence of two north dipping shear zones with opposite sense of shear here the metamorphic core is mainly affected by non coaxial top to the south sense of shear at temperatures between 450 c and 550 c that switch to a top to the north sense of shear at the top of the metamorphic core we regionally correlate this upper shear zone with the south tibetan detachment system ar dating on white mica indicates that both shear zones operated between 23 ma and 17 ma restoration of the folded south tibetan detachment in far western nepal yields a minimum dip slip distance of 190 km compatible with predictions made by models of extrusion of a weak mid crustal channel our results support an orogenic model in which channel flow in the hinterland coexisted with thrust wedge mechanics in the foreland

new constraints on the age of the manaslu leucogranite evidence for episodic tectonic denudation in the central himalayas the manaslu leucogranite of central nepal transacts one segment of the south tibetan detachment system a major extensional feature that helped to moderate large topographic gradients in the himalayan orogen in middle tertiary time 40 ar 39 ar ages for hornblendes from the northeastern contact aureole of the pluton indicate that intrusion occurred prior to 22 23 ma providing a minimum age for formation of the detachment system in the manaslu region that is at least 2 m y older than previous estimates from other parts of the orogen 40 ar 39 ar mica ages from the aureole similar to previously published mica ages from the upper part of the leucogranite indicate an episode of rapid cooling at 19 16 ma we attribute this cooling event to tectonic denudation of the pluton by movement on structurally higher exten sional faults and we suggest that gravitational collapse of the orogenic front occurred episodically over the late oligocene early miocene interval in the manaslu area

magnetization of exsolution intergrowths of hematite and ilmenite mineral chemistry phase relations and magnetic properties of hemo ilmenite ores with micron to nanometer scale lamellae from allard lake quebec hemo ilmenite ores from allard lake quebec were first studied over 50 years ago interest was renewed in these coarsely exsolved oxides based on the theory of lamellar magnetism as an explanation for the high and stable natural remanent magnetizations nrms 32 to 120 a m reported here to understand the magnetism and evolution of the exsolution lamellae the microstructures and nanostructures were studied using scanning electron microscopy and transmission electron microscopy tem phase chemistry and relations between mineral chemistry and the hematite ilmenite phase diagram cycles of exsolution during slow cooling resulted in lamellae down to 1 2 nm thick combined electron microprobe tem and x ray diffraction xrd results indicate that hematite hosts reached a composition approximately ilmenite ilm 14 4 and ilmenite hosts ilm 98 the bulk of the very stable nrm which shows thermal unblocking 595 620 c was acquired during final exsolution in the two phase region canted antiferromagnetic r3c hematite r3 ilmenite hysteresis measurements show a very strong anisotropy with a stronger coercivity normal to than parallel to the basal plane orientation of the lamellae magnetic saturation m s values are up to 914 a m compared to 564 a m predicted for a modally equivalent spin canted hematite corrected for 15 r 2 tio 3 substitution low temperature hysteresis ac susceptibility measurements and mossbauer results indicate a neel temperature t n of the geikielite substituted ilmenite at 43 k the low temperature hysteresis and ac susceptibility measurements also show a cluster spin glass like transition near 20 k below t n of ilmenite an exchange bias occurs with a 40 mt shift at 10 k

transient electromagnetic surveys for the determination of talik depth and geometry beneath thermokarst lakes thermokarst lakes are prevalent in arctic coastal lowland regions and sublake permafrost degradation and talik development contributes to greenhouse gas emissions by tapping the large permafrost carbon pool whereas lateral thermokarst lake expansion is readily apparent through remote sensing and shoreline measurements sublake thawed sediment conditions and talik growth are difficult to measure here we combine transient electromagnetic surveys with thermal modeling backed up by measured permafrost properties and radiocarbon ages to reveal closed talik geometry associated with a thermokarst lake in continuous permafrost to improve access to talik geometry data we conducted surveys along three transient electromagnetic transects perpendicular to lakeshores with different decadal scale expansion rates of 0 16 0 38 and 0 58 m year we modeled thermal development of the talik using boundary conditions based on field data from the lake surrounding permafrost and a borehole independent of the transient electromagnetics a talik depth of 91 m was determined from analysis of the transient electromagnetic surveys using a lake initiation age of 1400 years before present and available subsurface properties the results from thermal modeling of the lake center arrived at a best estimate talk depth of 80 m which is on the same order of magnitude as the results from the transient electromagnetic survey our approach has provided a noninvasive estimate of talik geometry suitable for comparable settings throughout circum arctic coastal lowland regions

crustal structure restoration and evolution of the greater himalaya in nepal south tibet implications for channel flow and ductile extrusion of the middle crust

the evolution of a thermokarst lake landscape late quaternary permafrost degradation and stabilization in interior alaska thermokarst processes characterize a variety of ice rich permafrost terrains and often lead to lake formation the long term evolution of thermokarst landscapes and the stability and longevity of lakes depend upon climate vegetation and ground conditions including the volume of excess ground ice and its distribution the current lake status of thermokarst lake landscapes and their future trajectories under climatewarming are better understood in the light of their long term development we studied the lake rich southern marginal upland of the yukon flats northern interior alaska using dated lake sediment cores observations of river cut exposures and remotely sensed data the region features thick up to 40 m quaternary deposits mainly loess that contain massive ground ice two of three studied lakes formed 11 000 12 000 cal yr bp through inferred thermokarst processes and fire may have played a role in initiating thermokarst development from 9000 cal yr bp all lakes exhibited steady sedimentation and pollen stratigraphies are consistentwith regional patterns the current lake expansion rates are low 0 to b7 cmyr 1 shoreline retreat compared with other regions 30 cm yr 1 or more this thermokarst lake rich region does not showevidence of extensive landscape lowering by lake drainage nor of multiple lake generations within a basin however lidar images reveal linear corrugations n5 m amplitude deep thermo erosional gullies and features resembling lake drainage channels suggesting that highly dynamic surface processes have previously shaped the landscape evidently widespread early holocene permafrost degradation and thermokarst lake initiation were followed by lake longevity and landscape stabilization the latter possibly related to establishment of dense forest cover partial or complete drainage of three lakes in 2013 reveals that there is some contemporary landscape dynamism holocene landscape evolution in the study area differs from that described from other thermokarst affected regions regional responses to future environmental change may be equally individualistic