the crustal structure of the southern nain and makkovik provinces of labrador derived from refraction seismic data data from a refraction seismic profile parallel to the coast of labrador canada were used to determine the crustal structure across the boundary of the nain and makkovik provinces and to look for evidence for an offshore continuation of the mesoproterozoic nain plutonic suite nps seven seismometers recorded airgun shots along the 283 km long line p and s wave velocity models were developed from forward and inverse modeling of travel times the velocity model distinguishes three distinct zones in the saglek block of the nain province the crust is divided into three layers with p wave velocities between 5 8 and 6 9 km s farther to the south upper crustal velocities increase to 6 3 6 5 km s and the poisson s ratio increases from 0 24 to 0 27 this zone correlates with a gravity low that is interpreted to outline the offshore continuation of the nps the upper crustal velocities are intermediate between anorthositic and granitoid rock samples collected from the nps a lower crustal reflector is li

alkalic to transitional ferrogabbro magma associated with paleohelikian anorthositic plutons in the flowers river area southeastern nain igneous complex labrador fine grained gabbroic chilled margins and crosscutting dikes are associated in space and time with three ca 1400 ma anorthositic plutons in the flowers river area southeastern nain igneous complex both the anorthositic and gabbroic rocks have distinctive compositions compared to rocks of similar age and lithology elsewhere in northcentral labrador the anorthositic rocks contain olivine and augite rather than orthopyroxene and fe ti oxides apatite and orthoclase are unusually abundant cumulus plagioclase is abnormally enriched in incompatible elements most of the gabbroic rocks are uniform in composition although the effects of contamination and fractionation are evident in some places they define a transitional to alkalic ferrogabbro magma that is strongly enriched in k p and incompatible trace elements the chemical characteristics of the ferrogabbro magma imply derivation from enriched mantle or involvement of a significant crustal component a parent daughter relationship between the ferrogabbro magma and anorthositic rocks is suggested by their compositional similarities and the fact that the gabbroic chilled margins and plagioclase rich pluton interiors appear to be completely gradational in composition and texture

the structure of archean ketilidian crust along the continental shelf of southwestern greenland from a seismic refraction profile the velocity structure of the continental crust on the outer shelf of southwestern greenland is determined from dense wide angle reflection refraction data obtained with large air gun sources and ocean bottom seismometers along a 230 km seismic line this line crosses the geological boundary between the archean block and the ketilidian mobile belt although the data have high noise levels p and s wave arrivals from within the upper intermediate and lower crust and at the moho boundary can be consistently identified and correlated with one dimensional wkbj synthetic seismograms in the archean p and s wave velocities in the upper crust are 6 0 and 3 4 km s while in the intermediate crust they are 6 4 and 3 6 km s these velocities match for the upper crust a quartz feldspar gneiss composition and for the intermediate crust an amphibolitized pyroxene granulite in the ketilidian mobile belt p and s wave velocities are 5 6 and 3 3 km s for the upper crust and 6 3 and 3 6 km s for the intermediate

the boron isotope geochemistry of the neogene borate deposits of western turkey abstract we have analyzed the boron isotope composition of 80 borate minerals major minerals borax colemanite and ulexite minor minerals veatchite a tunellite kernite terrugite probertite meyer hofferite inderite inyoite hydroboracite howlite and pandermite from the main deposits kirka bigadic and emet and two smaller deposits kestelek and sultancayir in the western turkish borate deposits forty three samples were also analysed for their sr isotope composition the data span a wide range in 11b values from 1 6 o to 25 3 o the 11b values of the main borate minerals are largely controlled by their mineralogy and the ph of the brines from which they precipitated an inverse correlation between the average 11b and 87 sr 86 sr ratios of colemanite in the different deposits suggests there is some variation in the sources of boron and sr to the deposits emet has the highest contribution from aluminosilicates and kirka the highest contribution from eocene carbonates with bigadic occupying an intermediate position the 11b values of the minor borate minerals distinguish between those which are primary precipitates from the original brines or formed from primary borates without boron loss from the system and those which formed from alteration of preexisting borate minerals with substantial loss of boron from the system

lithospheric structure of the labrador sea from constrained 3 d gravity inversion regional inversions of free air gravity data constrained by bathymetric and sediment thickness information were undertaken over the labrador sea and its margins to generate 3 d density anomaly models to investigate broad scale crustal structural variations across the extinct spreading centre benchmarked against independent seismic moho depth constraints a density anomaly isosurface within the inverted volumes was selected as a moho proxy and regional maps of moho structure were developed inversions using two different sources for depth to basement constraints revealed similar moho structures with a depth to moho of 12 km beneath the labrador sea which deepens to 20 km and greater towards davis strait and beneath the offshore extension of the grenville province density anomaly slices through the models corresponding to seismic lines show good agreement between the inverted moho proxy and the seismic moho with the only exceptions occurring where a high velocity lower crustal zone or underplate has been modelled from wide angle reflection refraction profiling studies the inverted depth to moho estimates were combined with depth to basement constraints to investigate crustal thickness both for the full crust and for individual crustal layers revealing that the crust of the labrador sea is generally 5 10 km thick but thickens to 20 25 km towards davis strait and beneath the offshore extension of the grenville province not taking into account high density underplates or anomalously high density lower crust sediment and crustal thickness variations were investigated to compute stretching factors across the labrador sea and to identify zones which deviate from local isostatic compensation assuming both an initial unstretched crustal thickness of 35 km and using a variable unstretched crustal thickness model much of the labrador sea has experienced 70 90 per cent thinning the derived values suggest that embrittlement of the entire crust and serpentinization of the upper mantle are likely to have occurred for large portions of the central and southern labrador sea inboard of known oceanic crust isostatic considerations reveal sediment excess and deficiency on the labrador and greenland margins respectively possibly reflecting fundamental rheological asymmetry with the greenland margin appearing weaker than the labrador margin a strong gradient from sediment deficiency to excess exclusive to the labrador margin may reveal the southwestern limit of a regional graben bounding listric detachment or of a zone of distributed faults and detachments

paleoenvironmental and archaeological investigations at qinghai lake western china geomorphic and chronometric evidence of lake level history abstract qinghai lake located on the northeastern qinghai tibet plateau qing zang gaoyuan is china s largest extant closed basin lake its position relative to major asian climate systems makes it sensitive to global climate change the lake has been the subject of numerous paleoenvironmental investigations including dating of shoreline features around the lake basin here we report new age estimates of shoreline features geomorphic exposures and archaeological sites that contribute to the development of a lake level history for qinghai lake and a landscape model of the qinghai lake basin lake highstands above 3230 m 36 m above the modern lake level appear to date to late mis 5 70 110 ka the lake has had much more modest highstands since then no evidence of mis 3 lake stands higher than modern were found and early holocene highstands are no more than 12 m above modern if the age of highstands greater than 3230 m is confirmed through future work then the qinghai lake basin hydrologic balance prior to 70 ka was dramatically different than after that time including during the holocene a simple hydrologic balance model provides insights into the combination of precipitation evaporation and runoff generation needed to sustain the lake at 3260 m the highest shoreline observed a range of factors may explain the difference primarily the relative strength of the east asian monsoon the basin was apparently subject to extensive alluviation during mis 3 interrupted by widespread erosion and development of cryogenic features before and during the last glacial maximum lgm loess that presently drapes much of the lower basin landscape began to be deposited after the lgm 16 18 ka the landscape model outlined here has implications for archaeological visibility of early human occupation of the qinghai tibet plateau

early to mid holocene lake high stand sediments at lake donggi cona northeastern tibetan plateau china lake high stand sediments are found in three onshore terraces at lake donggi cona northeastern tibetan plateau and reveal characteristics of hydrological changes on lake shorelines triggered by climate change geomorphological processes and neo tectonic movements the terraces consist of fluvial alluvial to littoral lacustrine facies end member modeling of grain size distributions allowed quantification of sediment transport processes and relative lake levels during times of deposition radiocarbon dating revealed higher than modern lake levels during the early and mid holocene lake levels follow the trend of asian monsoon dynamics and are modified by local non climatic drivers site specific impacts explain fluctuations during the initial lake level rise 11 cal ka bp maximum lake extension reached 9 2 cal ka bp at 16 5 m above present lake level a p l l littoral and lacustrine sediment deposition paused during a phase of fluvial activity and post depositional cryoturbations at 8 5 cal ka bp when the lake level fell to 8 m a p l l after a second maximum at 7 5 cal ka bp lake level declined slightly at 6 8 cal ka bp probably due to a non climatic pulse that caused lake opening the level remained high until a transition towards drier conditions 4 7 cal ka bp though discontinuous high stand sediments provide a unique high resolution archive

variability of southwest indian summer monsoon precipitation during the bolling allerod we have generated a high resolution 20 yr 2 3 0 th dated stalagmite oxygen isotope record from timta cave in the western himalaya in india that documents southwest indian summer monsoon ism precipitation variations during the bolling allerod interstadial from 15 2 to 11 7 ka compared with the glacial and younger dryas ism precipitation was enhanced during the bolling allerod ism precipitation was apparently coupled to variations in the east asian monsoon and north atlantic climate on millennial and multicentennial time scales during the deglaciation analyses of a high growth rate interval 2 5 yr resolution encompassing the late bolling early alterod suggest that multidecadal monsoon variability was an important aspect of ism behavior at that time the frequency spectrum of ism precipitation during this time interval is similar to that of the 1 4 c record and other ism precipitation records during the latest holocene this raises the hypothesis that multidecadal climate dynamics during the late boiling early allerod may have been similar to those that operated during the last several millennia even though the boundary conditions of these two time intervals were very different

proterozoic anorthosite granite nain batholith structure and intrusion processes in an active lithosphere scale fault zone northern labrador we present field evidence of the structure and evolution of a typical proterozoic massif type anorthosite the anorthosite forms a batholith composed of numerous plutons and ring complexes intruded episodically in two main sequences each producing anorthositic rocks and monzonite ferrodiorite followed by granite the batholith is located where a lithosphere scale fault zone 50 km wide and extending over 1000 km from greenland to labrador intersected an older continental suture intermittent episodes of mainly sinistral transtension along this gardar voisey s bay fault zone were associated with the generation of a number of pulses of magmatism and controlled the rise of magmas to middle and upper crustal levels between 1363 and 1289 ma both the large and small scale structures and magma intrusion processes are similar to those of many cordilleran batholiths with tabular plutons associated with ring dykes emplaced by cauldron subsidence and basin shaped plutons associated with cone sheets these

geological record of meltwater events at qinghai lake china from the past 40 ka abstract we report here on a previously unpublished sediment core from qinghai lake china that preserves a continuous record of sedimentation for the past 40 ka a striking feature of the record is a set of distinct meltwater events recorded at 35 19 and 14 ka respectively these events are manifest as distinct pulses of relatively old organic radiocarbon in the sediments we interpret these as a signal of glacial melting in the qinghai lake watershed the meltwater signals are closely correlated to temperature and precipitation records associated with deglaciation the events at 19 ka and 14 ka correspond to well established high latitude melt water pulse mwp events during marine isotope stage mis 2 and the 35 ka event corresponds to a period of pervasive high lake levels in western china during late mis 3 we interpret these anomalous dates as the result of relatively old carbon that was destabilized by the glaciers and released into the lake as the glaciers melted the data indicate that this process takes thousands of years we expect that the approach employed here to identify these events is generally applicable to any lake system with a significant glacial meltwater component

wide angle seismic transect across the torngat orogen northern labrador evidence for a proterozoic crustal root a refraction wide angle reflection seismic transect across the labrador peninsula covers the core zone of the se churchill province the paleoproterozoic torngat orogen and the archean nain province including a portion of the labrador continental margin an airgun array was used as source and 11 ocean bottom seismometers and 16 land stations recorded the shots forward modeling of travel times and amplitudes reveals a deep asymmetric crustal root beneath the torngat orogen with a crustal thickness of 49 km and with p wave velocities of 6 9 7 0 km s the geometry of the velocity model and the root can be best explained by either westward dipping subduction or westward underthrusting of the nain crust gravity modeling suggests a correlation of the crustal root with a gravity low that extends 100 km in an e w direction and 200 km from north to south the preservation of the crustal root is attributed to the lack of postorogenic heating and ductile reworking consistent with the lack of abundant postcollisional magmatism in the se churchill province a discontinuity possibly cutting through the entire crust is interpreted as a zone of major transcurrent shearing associated with the main phase of deformation west of the torngat orogen the crustal thickness in the core zone of the churchill province varies between 35 and 38 km p wave velocities of 5 8 7 0 km s east of the orogen the crystalline crust in the nain province is 38 km thick velocities from 5 8 to 6 9 km s but thins to 9 km in the shelf area of the labrador margin where it is covered with up to 8 km of sediments no high velocity zone was found beneath the thinned continental crust at the margin

mineralogy diagenesis and hydrochemical evolution in a probertite glauberite halite saline lake miocene emet basin turkey abstract the emet basin is one of the neogene basins in western turkey containing significant amounts of borate minerals mainly colemanite the petrologic study of core samples from two exploratory wells in the doganlar sector under optic and electron microscopy reveals a complex mineral association in which probertite glauberite and halite constitute the major primary phases without mineral precursors precipitated in a saline lake placed in a volcano sedimentary context other sulfates anhydrite gypsum thenardite celestite and kalistrontite borates colemanite ulexite hydroboracite tunellite kaliborite and aristarainite and sulfides arsenopyrite realgar and orpiment are attributed to early diagenesis so far the doganlar deposit is the most important deposit of probertite known up to now chemical changes in the groundwater inflow led to the precipitation of ca bearing borates colemanite in the tuff flat environment surrounding the lake while na ca sulfates and borates glauberite and probertite precipitated in the center of the lake fluid inclusion compositions in halite indicate that the advanced brines correspond to the na k cl so 4 type during restricted stages of the saline lake the residual brines seeped through the tuff flat sediments leading to transformations of previous precipitates that resulted in the formation of k bearing minerals the abundance of coccoid like biogenic dolomite colloidal arsenopyrite and the isotopic composition of sulfates are indicative of bacterial sulfate reduction in contrast arsenic sulfides are attributed to acidophilic micro organisms in oxidizing conditions fluctuations of redox conditions in both free and interstitial brines control the biological influence in some diagenetic transformations

shore platform downwearing in eastern canada a 9 14 year micro erosion meter record abstract downwearing rates erosion in the vertical plane were measured with a micro erosion meter mem in eastern canada on an argillacious sub horizontal shore platform at mont louis in eastern quebec and on two sloping basaltic and sandstone platforms at respectively scots bay and burntcoat head in the bay of fundy nova scotia the original data covered a period from 2002 to 2009 this dataset was extended by measurements repeated at surviving mem stations in 2017 producing records ranging over 9 14 years depending on when each station was installed because of rapid surface downwearing many of the original mem stations were inoperable in 2017 especially at burntcoat head nevertheless data were obtained from 19 stations at burntcoat 35 of the 2009 original 25 at mont louis 83 of the original and 38 at scots bay 75 of the original for the stations at mont louis and scots bay that were still functioning in 2017 there were no significant differences in rates of downwearing over the shorter from station installation up to 2009 and extended periods from installation to 2017 mean rates of downwearing calculated from all the stations in each area declined through time however due to the loss of the more rapidly eroding stations a simple procedure which was proposed to compensate for this decrease produced mean downwearing rates that were broadly similar to those reported over the original measurement period there were significant relationships between downwearing rates and elevation r2 0 32 and downwearing rates and rock hardness r2 0 41 in the extended record at scots bay and a small but significant relationship between downwearing rates and rock hardness at mont louis r2 0 17 differences in downwearing rates across the platforms suggest that salt weathering and wetting and drying are dominant weathering mechanisms at scots bay and mont louis chemical weathering of the sandstone cementing agent and the premature removal of weathered grains by wave generated bottom currents may however be more important at burntcoat head

numerical dating of a late quaternary spit shoreline complex at the northern end of silver lake playa mojave desert california a comparison of the applicability of radiocarbon luminescence terrestrial cosmogenic nuclide electron spin resonance u series and amino acid racemization methods a late quaternary spit shoreline complex on the northern shore of pleistocene lake mojave of southeastern california usa was studied with the goal of comparing accelerator mass spectrometry ams radiocarbon luminescence electron spin resonance esr terrestrial cosmogenic radionuclide tcn surface exposure amino acid racemization aar and u series dating methods the pattern of ages obtained by the different methods illustrates the complexity of processes acting in the lakeshore environment and highlights the utility of a multi method approach tcn surface exposure ages mostly 20 30 ka record the initial erosion of shoreline benches whereas radiocarbon ages on shells determined in this and previous studies within the spit supported by aar data record its construction at fluctuating lake levels from 16 to 10 ka luminescence ages on spit sediment 6 7 ka and esr ages on spit shells 4 ka are anomalously young relative to radiocarbon ages of shells within the same deposits the significance of the surprisingly young luminescence ages is not clear the younger esr ages could be a consequence of post mortem enrichment of u in the shells high concentrations of detrital thorium in tufa coating spit gravels inhibited the use of single sample u series dating detailed comparisons such as this provide one of the few means of assessing the accuracy of quaternary dating techniques more such comparisons are needed

evaluating the size and extent of paleolakes in central tibet during the late pleistocene subhorizontal lake shorelines allow a geodynamic test of the size and extent of a hypothesized paleo lake in central tibet the east qiangtang lake eql during the last interglacial period marine isotope stage mis 5e reconstructions based on relict lake deposits suggest that the eql would have been 400 m deep and over 66 000 km2 models of flexural rebound driven by lake recession predict that shorelines near the eql center at the present day location of siling co would have rebounded 60 90 m above their initial elevation new 36cl chronology of the highest relict shorelines around siling co indicates that they reflect lake levels between 110 190 ka these shorelines however are presently 300 m below their predicted elevations implying a substantially smaller water load our results reveal that the expansion of tibetan lakes during mis 5e was relatively limited instead individual lakes were supplied by river networks much as they are today

holocene lake level history of the tangra yumco lake system southern central tibetan plateau massive carbonate banks representing ancient lacustrine deposits are exposed in the catchment of the lake tangra yumco southern central tibetan plateau and nearby lake xuru co nine sediment samples were taken below and above these lacustrine deposits to determine periods of changing lake level using optically stimulated luminescence osl applying a multiple aliquot regeneration mar protocol according to facies and stratigraphy samples below the carbonate banks indicate a rising and samples from above a falling lake level the results indicate that the rising lake level of tangra yumco passed the sampling location at 10 5 and 2 1 ka while a falling lake level passed the sampling location at 0 9 ka the rising lake level of xuru co passed the sampling location at 7 9 and 1 7 ka and a falling lake level at 0 5 ka showing a similar trend as at tangra yumco combining these results with recalculated cosmogenic nuclide ages and previously published feldspar luminescence data allows the establishment of

cosmogenic 10be and 26al dating of paleolake shorelines in tibet abstract we have used in situ produced cosmogenic nuclides 10be and 26al to date lacustrine shorelines around eight lakes in tibet two lakes located south of the yarlung tsangpo river discharged at high lake levels one of them drolung co co is a local term meaning lake became closed its water level fell below its outlet channel at 3 8 ka and lowered over 120 m vertically since then this implies a significant change in intensity of precipitation driven by the indian monsoon over southern tibet in the middle late holocene nam co and dajia co located to the north of the yarlung tsangpo river also drained at high lake levels and became closed 53 36 ka and 48 ka respectively of four other lakes developed closely in the qiangtang basin north of the yarlung tsangpo river tangra yum co and siling co located at 31 32 n show exposure ages for high lake shorelines of over 220 ka the oldest lake shorelines reported for tibet while towards the north and west the high lake of zhari nam co appeared after the last glacial maximum lgm the different pattern of lake development to the north of yarlung tsangpo river may reflect regional climate effects

age constraints on the late quaternary evolution of qinghai lake tibetan plateau dating and geomorphology of shoreline features in the qinghai lake basin of northwestern china suggest that contrary to previous interpretations the lake likely did not reach levels 66 140 m above modern within the past 90 000 yr maximum highstands of 20 66 m above modern probably date to marine isotope stage mis 5 mis 3 highstands are undated and uncertain but may have been at or below post glacial highs the lake probably reached 3202 3206 m 8 12 m during the early holocene but stayed below 3202 m after 8 4 ka this shoreline history implies significantly different hydrologic balances in the qinghai lake basin before 90 ka and after 45 ka possibly the result of a more expansive asian monsoon in mis 5

long term shore platform surface lowering rates revisiting gill and lang after 32 years abstract in 1978 e d gill and j g lang installed 53 micro erosion meter mem sites on shore platforms along the otway coast in south eastern australia surface lowering rates were originally determined from 2 yr of measurements 1979 and 1980 and showed that the mean annual rate of lowering from all sites was 0 37 mm yr 1 we re measured these mem sites 32 yr after the first set of readings were taken in february 1979 we relocated 49 of the original 53 bolt sites and were able to measure 45 of them from the sites we measured the mean annual rate of shore platform lowering was 0 31 mm yr 1 rates of surface lowering on shore platforms are commonly reported from studies of only two or three years and only one other record exceeds 30 yr kaikoura peninsula nz 1 09 mm yr 1 in that case fewer than half of the original bolt sites were functioning because erosion had removed bolts along the otway coast rates of erosion are much slower hence the greater number of still readable sites after 32 yr compared to other micro erosion meter studies the rates reported here are at the lower end of the range of values from around the world but nevertheless consistent with rates from the otway coast and others in similar lithology we found no statistical difference between erosion rates calculated over 2 yr 1979 to 1980 and 32 1979 to 2011 years supporting the view that shore platform lowering rates measured over 2 yr are representative of decadal scales

lake level reconstruction for 12 8 2 3 ka of the ngangla ring tso closed basin lake system southwest tibetan plateau article i nfo we present a shoreline based millennial scale record of lake level changes spanning 12 8 2 3 ka for a large closed basin lake system on the southwestern tibetan plateau fifty three radiocarbon and eight u th series ages of tufa and beach cement provide age control on paleoshorelines ringing the basin supplemented by nineteen ages from shell and aquatic plant material from natural exposures generally recording lake regressions our results show that paleo ngangla ring tso exceeded modern lake level 4727 m asl continuously between 12 8 and 2 3 ka the lake was at its highstand 135 m 4862 m asl above the modern lake from 10 3 ka to 8 6 ka this is similar to other closed basin lakes in western tibet and coincides with peak northern hemisphere summer insolation and peak indian summer monsoon intensity the lake experienced a series of millennial scale oscillations centered on 11 5 10 8 8 3 5 9 and 3 6 ka consistent with weak monsoon events in proxy records of the indian summer mon soon it is unclear whether these events were forced by north atlantic or indian ocean conditions but based on the abrupt lake level regressions recorded for ngangla ring tso they resulted in significant periodic reductions in rainfall over the western tibetan plateau throughout the holocene

late quaternary palaeolake levels in tengger desert nw china comprehensive field investigations and laboratory analyses show that palaeolakes including the megalake tengger and other semi connected and isolated water bodies during late pleistocene covered an area of more than 20 000 km 2 this is an area more than half the size of the tengger desert in nw china stratigraphic correlations and chronological evidence indicate that the palaeolakes started to develop around 42 000 14 c year bp uncalibrated all throughout the paper but their extent was limited until 37 000 14 c year bp based on the chronology of representative lacustrine deposits it can be deduced that the megalake tengger was established around 35 000 14 c year bp and maintained until 22 000 14 c year bp leading to the formation of the baijian hu terraces which are regarded as firm evidence of the existence of palaeolakes the formation mechanism is unclear and the climate situation at the time is still an open question the holocene palaeolakes started to develop around 12 000 14 c year bp the baijian hu terraces indicate that the high water levels in the area occurred around 8500 5400 5100 3500 and 1860 1370 14 c year bp the extent of the holocene palaeolakes primarily migratory lakes was smaller than that of the late pleistocene palaeolakes d 2004 elsevier b v all rights reserved

erosion history of the tibetan plateau since the last interglacial constraints from the first studies of cosmogenic 10be from tibetan bedrock abstract the cosmogenic 10be exposure histories of in situ bedrock surfaces from the tibetan plateau indicate low erosion rates of

hazard from himalayan glacier lake outburst floods sustained glacier melt in the himalayas has gradually spawned more than 5 000 glacier lakes that are dammed by potentially unstable moraines when such dams break glacier lake outburst floods glofs can cause catastrophic societal and geomorphic impacts we present a robust probabilistic estimate of average glofs return periods in the himalayan region drawing on 5 4 billion simulations we find that the 100 y outburst flood has an average volume of 33 5 3 7 3 7 106 m3 posterior mean and 95 highest density interval hdi with a peak discharge of 15 600 2 000 1 800 m3 s 1 our estimated glof hazard is tied to the rate of historic lake outbursts and the number of present lakes which both are highest in the eastern himalayas there the estimated 100 y glof discharge 14 500 m3 s 1 is more than 3 times that of the adjacent nyainqentanglha mountains and at least an order of magnitude higher than in the hindu kush karakoram and western himalayas the glof hazard may increase in these regions that currently have large glaciers but few lakes if future projected ice loss generates more unstable moraine dammed lakes than we recognize today flood peaks from glofs mostly attenuate within himalayan headwaters but can rival monsoon fed discharges in major rivers hundreds to thousands of kilometers downstream projections of future hazard from meteorological floods need to account for the extreme runoffs during lake outbursts given the increasing trends in population infrastructure and hydropower projects in himalayan headwaters

makhavinekh lake pluton labrador canada geological setting subdivisions mode of emplacement and a comparison with finnish rapakivi granites abstract makhavinekh lake pluton is an oval elsonian 1322 ma granitic intrusion within the nain plutonic suite of central coastal labrador the pluton comprises at least five textural mineralogical subdivisions dominated by quartz monzonite and granite and it is surrounded by a well defined pyroxene hornfels thermal aureole fayalite and clinopyroxene are common constituents of the eastern half of the makhavinckh lake pluton hornblende is present throughout the whole pluton and is the dominant mafic mineral in the western half of the intrusion the hornblende rich parts of the intrusion appear to be a consequence of late to post emplacement hydration of originally olivine bearing rocks textural mineralogical and geochemical features are similar to the rapakivi granites of finland the makhavinekh lake pluton has the morphology of a sheath separating an external gneissic country rock complex from an internal basic plutonic suite it is proposed that the makhavinekh lake pluton developed through cumulate processes and was emplaced as a hot dry crystal charged mush via a mechanism of cauldron subsidence block faulting that allowed ascent of the pluton may have been related to crustal bulging from an underlying mafic magma pond

tectonic evolution of southern baffin bay and davis strait results from a seismic refraction transect between canada and greenland 1 wide angle reflection refraction seismic data were acquired on a 450 km long transect in southern baffin bay extending from baffin island to greenland dense air gun shots were recorded on 22 ocean bottom seismometers apwave velocity model was developed from forward and inverse modeling of the observed travel times beneath the baffin island shelf a three layered continental crust is observed with velocities of 5 5 to 6 9 km s typical for transform margins there is a sharp transition between continental and oceanic crust off baffin island 7 km thick oceanic crust is interpreted to lie in a major transform fault identified on the gravity map beneath the deep baffin bay basin 9 km thick oceanic crust is encountered but thins to 6 km within an assumed fracture zone the thicker than normal oceanic crust indicates an ample magma supply possibly related to melt extracted from a mantle plume seaward of the greenland continental crust 20 km thick igneous crust 6 3 to 7 3 km s is encountered in a 25 km wide zone interpreted as a leaky transform fault that can be correlated southward through davis strait the igneous crust is bounded by a 20 km wide basin to the west underlain by 4 km thick crust of unknown affinity this structure is probably associated with transform movements a high velocity lower crustal layer 7 1 km s of 8 km thickness is indicated beneath the greenland crust and can be correlated into the adjacent thick igneous crust both the thick igneous and greenland crust are covered by up to 4 km of paleogene volcanics 5 2 to 5 7 km s

late pleistocene lake and glaciation evolution on the northeastern qinghai tibetan plateau a review late pleistocene paleoclimatic history on northeastern qinghai tibetan plateau qtp has been reconstructed mainly from lake sediments however data regarding dry wet climate changes reported in this region are still not clear and controversial based on shoreline features and highstand lacustrine sediments around lakes on the qtp high lake level histories in this paper were summarized and compared with paleoclimatic records from lake sediments ice core and glaciation evolution surrounding mountains on the ne qtp during late pleistocene the results indicate that periods of high lake level occurred at mis 5 mis 3 and early middle holocene and most likely corresponding to warm and wet climate periods while periods of low lake level existed in intervening intervals corresponding to cold and dry climate periods which most likely coincide with glacial advances surrounding high mountains with an exception no wide glacial advance in study area was found during mis 3 possibly suggesting that effective moisture is lower than that in the other region of ne qtp in this period

petrogenesis of a mid proterozoic anorthosite mangerite charnockite granite amcg complex isotopic and chemical evidence from the nain plutonic suite the nain plutonic suite nps about 1 30 ga underlies some 19 000 km intruding the boundary between middle to late archean rocks in the east nain province and late archean and early proterozoic rocks in the west churchill province nd isotopic compositions in most igneous units of the nps reflect their geographic position relative to the inferred boundary between the nain and churchill provinces rare exceptions are local intrusions of olivine bearing basic rocks with highest epsilon nd 1 3 ga near 3 0 present in both eastern and western sectors of the nps anorthositic rocks have variable and significant contributions of crustal nd but notably lower i sr than granitoid rocks ferrodiorites are similar isotopically to the anorthositic group and have chemical and mineral compositions more akin to the anorthositic group than the granitoids aluminous orthopyroxene megacrysts in nps anorthositic rocks indicate crystallization pressures of 6 2 to 11 0 kbar consistent with lower cru

quaternary ecological responses and impacts of the indian ocean summer monsoon at nam co southern tibetan plateau the transition from the last glacial to the current interglacial the holocene represents an important period with climatic and environmental changes impacting ecosystems in this study we examined the interplay between the indian ocean summer monsoon iosm and the westerlies at lake nam co southern tibet to understand the climatic effects on the ecosystem different organic geochemical proxies n alkanes glycerol dialkyl glycerol tetraethers d 13corg 15n are applied to reconstruct the environmental and hydrological changes on one of the longest available paleorecords at the tibetan plateau based on our paleohydrological d proxies the aquatic signal lags the terrestrial one due to specific ecological thresholds which in addition to climatic changes can influence aquatic organisms the aquatic organisms response strongly depends on temperature and associated lake size as well as ph and nutrient availability because the terrestrial vegetation reacts faster and more sensitively to changes in the monsoonal and climatic system the d of n c29 and the reconstructed inflow water signal represent an appropriate iosm proxy in general the interplay of the different air masses seems to be primarily controlled by solar insolation in the holocene the high insolation generates a large land ocean pressure gradient associated with strong monsoonal winds and the strongest iosm in the last glacial period however the weak insolation promoted the westerlies thereby increasing their influence at the tibetan plateau our results help to elucidate the variable iosm and they illustrate a remarkable shift in the lake system regarding ph 13corg and 15n from the last glacial to the holocene interglacial period

cosmogenic 10be inferred lake level changes in sumxi co basin western tibet abstract most tibetan lakes are surrounded by conspicuous regressive shorelines attesting to high water levels in the past concentrations of the in situ produced cosmogenic radionuclide 10 be in bedrock from the three highest terraces surrounding sumxi co situated in western kunlun indicate that the highest lake level appeared before 10 000 11 500 yr and most likely between 11 000 and 12 800 yr younger ages for the two lower terraces imply regression of sumxi co during the early mid holocene the concurrency of the highest lake level with orbitally induced maximum northern hemisphere summer insolation suggests that the increase in water supply to sumxi co was most likely associated with increased recharge from melting glaciers we conclude that the enhanced indian monsoon during the early holocene did not penetrate the tibetan plateau and affect the northwestern part of tibet significantly

shrinking lakes in tibet linked to the weakening asian monsoon in the past 8 2 ka expansion or shrinkage of closed lakes is a natural response to fluctuations in precipitation and evaporation linked closely to changes in strength or position of atmospheric circulation in tibet there are many such lakes with paleo shorelines that can be used for reconstructions of climate history despite the fact that many paleo shorelines are well preserved in tibet dating them has been seriously hindered by various difficulties here we present the first optical dating chronology for a series of paleo shorelines in zhari namco the third largest inland lake in central tibet our results indicate that the lake level has dropped 128m over the past 8 2ka younger shorelines are found at lower altitudes indicating that the shorelines follow a geomorphic chronological order and a broadly continuous trend of stepwise shrinkage the surface area of zhari namco has shrunk in size from 4605km2 at 8 2ka ago to 996km2 at present 300km3 of water has been lost from this lake such a loss in water implies a significant reduction in precipitation over the past 8 2ka a likely result of a weakening asian monsoon following the decreasing precipitation since the early holocene this area has become increasingly arid

upb geochronology of the northern torngat orogen labrador canada a record of palaeoproterozoic magmatism and deformation abstract the palaeoproterozoic torngat orogen located in northeastern quebec and northern labrador canada records the sinistral transpressional collision of the western margin of the archaean nain province and the eastern margin of the rae province u pb age determinations are presented for twelve samples from the northern part of the orogen and used to evaluate models for its tectonic evolution voluminous granitoid plutonic rocks which in part intrude gneisses interpreted as the northern extension of the nain province and are possibly related to subduction below it have zircon ages of 1910 2 ma 1895 3 ma 1891 2 ma 1890 2 ma 1869 3 2 ma and 1864 2 ma suggesting at least 45 m y of convergence between the rae and nain provinces granulite facies conditions may have existed in the northern part of the area at 1 84 ga these plutonic rocks as well as tonalitic gneisses of the nain province were reworked at amphibolite facies conditions within a north south corridor referred to as the komaktorvik zone zircon age determinations of two granitic veins which were intruded synchronously with ongoing deformation in the komaktorvik zone are 1789 2 ma and 1719 2 ma suggesting that deformation within the zone may have lasted for up to 70 m y metamorphic titanite from samples within the komaktorvik zone have ages of 1776 4 ma 1774 4 ma 1767 4 ma 1710 4 ma and 1632 4 ma and support the interpretation of long lived deformation in the komaktorvik zone palaeoproterozoic subduction beneath the nain province has not been widely recognized in southern parts of the orogen suggesting that a distinct tectonic regime existed in the study area

baddeleyite zro2 and zircon zrsio4 from anorthositic rocks of the laramie anorthosite complex wyoming petrologic consequences and u pb ages

cosmogenic 10be surface exposure dating and glacier reconstruction for the last glacial maximum in the quemuqu valley western nyainqentanglha mountains south tibet the nature and timing of glaciations enable inferences to be made about glacial climates however the paucity of reliable glacial chronologies impedes a full understanding of the last glacial maximum lgm climate in the western nyainqentanglha mountains south tibet in this study we investigated the lgm glacial history in the quemuqu valley western nyainqentanglha using 10be surface exposure dating we used a physically based glacier model to reconstruct the lgm glacial surfaces and estimated the lgm equilibrium line altitude ela using integrated methods combined with previously published 10be exposure ages from the western nyainqentanglha ten 10be exposure ages imply a relatively restricted glacial extent in the lgm but extensive growth in the early last glacial the modeled lgm glaciers had ice areas of 36 3 km2 3 21 km3 in volume and 89 m m in average thickness in the quemuqu valley the reconstructed lgm elas have values of 5463 5583 m asl with a depression of 244 364 m by combining the reconstructed elas with an equation deduced from glacier mass balance we concluded that the temperature dropped by 2 6 4 6 c in the lgm relative to the present with 30 70 lower precipitation than the present global cooling induced the lgm glacial advance in the western nyainqentanglha despite decreased precipitation

summer temperature fluctuations in southwestern china during the end of the lgm and the last deglaciation abstract we present a sub centennial resolution summer temperature record from the se tibetan plateau tiancai lake sw china 3900 m a s l derived from a chironomid stratigraphy covering the last c 19 5 ka the record highlights the interaction between tropical and high latitude climate forcing through the changes in atmospheric circulation during the last deglaciation the scale of the last glacial maximum lgm cooling is consistent with other tropical mountain regions at c 5 c and a rapid recovery of temperatures at 19 0 ka is related to changes in adiabatic lapse rates at the end of the glaciation the overall pattern of change shows that north atlantic deglaciation climate events the heinrich 1 and younger dryas cooling events and the bolling allerod warm period are all recorded but the influence of the events decline as the deglaciation progresses we relate these patterns to north atlantic deep water ventilating the southern ocean and to the consequent movements of the intertropical convergence zone itcz in the indian ocean basin as transmitted through changes in the southern mid latitude circulation

three dimensional structure of the torngat orogen ne canada from active seismic tomography the crustal velocity structure and the moho depth of the proterozoic torngat orogen ne canada is determined by active seismic tomography using travel times of crustal turning rays and moho reflections the orogen developed during oblique convergence of the archean superior and nain provinces which trapped an interior belt of archean crust core zone between them with the torngat orogen evolving between the core zone and the nain province beneath the central orogen a crustal root is found with a preserved depth of 52 km and a width of 100 km to the north the root shallows to 44 km and narrows to a width of 45 km the root correlates with a set of major late orogenic shear zones that accommodated oblique convergence of the superior and nain provinces this correlation suggests that the transpressional shearing focused strain in the region of the root and contributed to the crustal thickening absence of postorogenic magmatic activity prevented reworking or thermal relaxation of the root the lack of late magmatism is probably related to the depleted and refractory nature of the archean lithosphere underlying the orogen upper crustal velocities are lowest in the core zone 5 7 km s at the surface and are compatible with laboratory measurements carried out on gneissic rock samples from that area higher velocities in the nain province 5 9 km s correlate with felsic gneiss and anorthosite rock samples a high velocity region immediately to the north of the crustal root is associated with a moho uplift 34 km this is explained by extension along the ungava transform fault and possibly in hudson strait at 55 ma when rifting in the labrador sea was transferred into baffin bay

wide angle seismic imaging of a mesoproterozoic anorthosite complex the nain plutonic suite in labrador canada the mesoproterozoic nain plutonic suite nps of labrador canada one of the largest anorogenic plutonic terranes was studied by a refraction wide angle seismic experiment four ocean bottom seismometers and 18 land stations were deployed along a 330 km profile and recorded air gun shots from the easternmost 160 km with the nps located in the center of the line at the suture of the nain and churchill provinces p and s wave velocity models were developed by forward modeling of travel times and amplitudes upper and middle crustal p wave velocities outside and beneath the nps range from 5 9 to 6 5 km s lower crustal p wave velocities range from 6 55 to 7 0 km s within the anorthositic rocks velocities are as high as 6 8 km s and reflections define the base of the nps to be 8 km deep in the se churchill province and 11 km in the nain province a variation that may be the result of lateral density changes within the country rocks or the anorthosites the total crustal thickness is 39 km west of the nps but is only 32 34 km beneath the nps some 5 km less than nain province crust distal from the nps the inferred crustal thinning is possibly related to anatexis of the lowermost crust by a thermal plume that generated the plutonism the poisson s ratios are 0 275 within the anorthosite plutons 0 27 in the upper and middle crust and 0 285 in the lower crust these values are some 0 03 higher than in the archean nain crust distal to the nps indicating a higher plagioclase content at all crustal levels as result of the plutonism we postulate that a crustal root similar to the root observed farther north in the torngat orogen was completely removed by anatexis and the silicic and basic magmas probably ascended to midcrustal levels along preexisting zones of weakness at the nain churchill boundary

shore platform abrasion in a para periglacial environment galicia northwestern spain abstract the schmidt rock test hammer was used to study the effect of abrasion on shore platforms in galicia northwestern spain on platforms where tidally induced weathering salt wetting and drying etc is dominant rock strength is significantly lower than in areas where abrasion is or has been active in the recent past this suggests that abrasion removes weathered surface material exposing the stronger less weathered rock below abrasion downwearing measured with a transverse micro erosion meter ranged between 0 13 and 1 8 mm yr 1 over the last year most active abrasion occurs in the upper part of the intertidal zone but weathering is slowly destroying formerly abraded surfaces at lower elevations these abandoned surfaces were abraded by materials supplied by erosion of fluvio nival and periglacial slope deposits that covered or partially covered parts of the galician coast during the middle and late weichselian during the holocene rising sea level and erosion of the slope deposits caused the abrasion zone to gradually migrate up to its present position near the high tidal level the spatial and temporal role of abrasion on this coast is therefore closely associated with the exhumation and inheritance of ancient platform surfaces from beneath weichselian deposits

climatic and topographic controls on the style and timing of late quaternary glaciation throughout tibet and the himalaya defined by 10be cosmogenic radionuclide surface exposure dating abstract temporal and spatial changes in glacier cover throughout the late quaternary in tibet and the bordering mountains are poorly defined because of the inaccessibility and vastness of the region and the lack of numerical dating to help reconstruct the timing and extent of glaciation throughout tibet and the bordering mountains we use geomorphic mapping and 10 be cosmogenic radionuclide crn surface dating in study areas in southeastern gonga shan southern karola pass and central western nyainqentanggulha shan and tanggula shan tibet and we compare these with recently determined numerical chronologies in other parts of the plateau and its borderlands each of the study regions receives its precipitation mainly during the south asian summer monsoon when it falls as snow at high altitudes gonga shan receives the most precipitation 2000 mm a 1 while near the margins of monsoon influence the karola pass receives moderate amounts of precipitation 500 600 mm a 1 and in the interior of the plateau little precipitation falls on the western nyainqentanggulha shan 300 mm a 1 and the tanggula shan 400 700 mm a 1 the higher precipitation values for the tanggula shan are due to strong orographic effects in each region at least three sets of moraines and associated landforms are preserved providing evidence for multiple glaciations the 10 be crn surface exposure dating shows that the formation of moraines in gonga shan occurred during the early mid holocene neoglacial and little ice age on the karola pass during the lateglacial early holocene and neoglacial in the nyainqentanggulha shan date during the early part of the last glacial cycle global last glacial maximum and lateglacial and on the tanggula shan during the penultimate glacial cycle and the early part of the last glacial cycle the oldest moraine succession in each of these regions varies from the early holocene gonga shan lateglacial karola pass early last glacial western nyainqentanggulha shan and penultimate glacial cycle tanggula shan we believe that the regional patterns and timing of glaciation reflect temporal and spatial variability in the south asian monsoon and in particular in regional precipitation gradients in zones of greater aridity the extent of glaciation has become increasingly restricted throughout the late quaternary leading to the preservation of old 100 ka glacial landforms in contrast in regions that are very strongly influenced by the monsoon 1600 mm a 1 the preservation potential of pre lateglacial moraine successions is generally extremely poor this is possibly because lateglacial and holocene glacial advances may have been more extensive than early glaciations and hence may have destroyed any landform or sedimentary evidence of earlier glaciations furthermore the intense denudation mainly by fluvial and mass movement processes which characterize these wetter environments results in rapid erosion and re sedimentation of glacial and associated landforms which also contributes to their poor preservation potential

lithosphere structure of the ukrainian shield and pripyat trough in the region of eurobridge 97 ukraine and belarus from gravity modelling abstract the southern segment of the seismic profile eurobridge eurobridge 97 eb 97 located in belarus and ukraine crosses the suture zone between two main segments of the east european craton fennoscandia and sarmatia as well as sarmatia itself at the initial stage of our study a 3 d density model has been constructed for the crust of the study region including the major part of the osnitsa mikashevichi igneous belt omib superimposed by sediments of the pripyat trough pt and three domains in the ukrainian shield the volhyn domain vd with the anorthosite rapakivi korosten pluton kp the podolian domain pd and the ros tikich domain rtd the model comprises three layers sediments with maximum thickness 6 km in the pt and two heterogeneous layers in the crystalline crust separated at a depth of 15 km 3 d calculations show the main features of the observed gravity field are caused by density heterogeneities in the upper crust allocation of density domains deeper than 15 km is influenced by moho topography fitting the densities here reveals an increase up to 2960 kg m 3 in the modelled bodies accompanied by a moho deepening to 50 km in contrast a moho uplift to a level of 35 37 km below the kp and major part of the pt is associated with domains of reduced densities an important role for the deep odessa gomel tectonic zone dividing the crust into two regions one of basically archean consolidation in the west pd and rtd and one of proterozoic crust in the east kirovograd domain was confirmed 2 d density modelling on the eb 97 profile shows that in the upper crust three main domains of different precambrian evolution the omib with the superimposed pt the vd with the kp and the pd can be distinguished deeper in the middle and lower crust layered structures having no connection to the surface geology are dominant features of the models least thickness of the crust was obtained below the kp greatest crustal thickness more than 50 km was found below the pd characterised also by maximum deviation of velocity density relation in the rocks from a standard one the velocity and density models along the eb 97 profile have been interpreted together with inferred v p v s ratios to estimate crustal composition in terms of sio 2 content in the course of the modelling the status of the pd as a centre of archean granulitic consolidation has been confirmed the crustal structure of the anorthosite rapakivi kp is complex for the first time a complicated structure for the lower crust and lower crust upper mantle transition zone beneath the kp has been determined the peculiarities of the crustal structure of the kp are quite well explained in terms of formation of rapakivi anorthosite massifs as originating from melt chambers in the upper mantle and lower crust an important role for the south pripyat fault spf repeatedly activated during proterozoic palaeozoic times has been ascertained at the subplatform stage of crustal evolution the spf was probably a magma channel facilitating the granitic intrusions of the kp in the palaeozoic the fault was reactivated during rifting in the pt

geophysical characteristics of the continental crust along the lithoprobe eastern canadian shield onshore offshore transect ecsoot a review the eastern canadian shield onshore offshore transect ecsoot of the lithoprobe program included 1200 km of normal incidence seismic profiles and seven wide angle seismic profiles across archean and proterozoic rocks of labrador northern quebec and the surrounding marine areas archean crust is 33 44 km thick p wave velocity increases downwards from 6 0 to 6 9 km s there is moderate crustal reflectivity but the reflection moho is unclear archean crust that stabilized in the proterozoic is similar except for greater reflectivity and a well defined moho proterozoic crust has similar or greater thickness variable lower crustal velocities and strong crustal reflectivity geodynamic processes of paleoproterozoic growth of the canadian shield are similar to those observed in modern collisional orogens the suturing of the archean core zone and superior provinces involved whole crustal shearing top to west in the core zone linked to thin skinned deformation in the new quebec orogen the torngat orog

inversion for crustal structure using reflections from the passcal ouachita experiment an interface inversion has been tested and applied to wide angle reflection data from the 1986 passcal ouachita experiment an interface corrugation with a relief of 10 km and a width of 30 km was successfully imaged in a test of the interface inversion velocity depth curves derived for shot points 14 19 using the tail sum method are similar to an average one dimensional velocity model from forward modeling using travel time correlations from southern profiles 14 19 the similarity of these velocity depth curves positioned over the southern half of the passcal experiment suggests that crustal structure in this region is approximately laterally homogeneous lower crustal velocities are not well resolved due to the recording on most profiles of only small segments of the travel time triplications a two dimensional velocity structure was derived by using the average one dimensional velocities for the deeper crustal layers and formally inverting for depth to interfaces the final inversion model is found to be consistent with previous refraction interpretations south of the ouachita orogenic trend and concurrent interpretations of the passcal data set inversion results for the central and southern portion of the passcal profile indicate a depth of 10 12 km for a midcrustal layer which thins southward from approximately 10 km to about 4 km a lower crustal layer with an average thickness of 12 km and a moho depth of approximately 29 5 km are also determined interface depths are in agreement with a normal moveout stack of the passcal data set in particular the shallowing of the moho to a depth of 30 km over the northern 50 km of the profile matches previous interpretations of the data set and has been interpreted here and in previous studies as the location of the paleozoic continental margin geophysical studies of the modern atlantic continental passive margin provide the simplest comparison to the crustal structure derived here the lower crustal layer found south of the shallowing of the moho to 30 km beneath the passcal profile is analogous in thickness and position to rift stage lower crust however other tectonic interpretations are possible given the uncertainties in velocity and lithology relationships

tropical climate instability the last glacial cycle from a qinghai tibetan ice core an ice core record from the guliya ice cap on the qinghai tibetan plateau provides evidence of regional climatic conditions over the last glacial cycle 36 cl data suggest that the deepest 20 meters of the core may be more than 500 000 years old the 18 o change across termination i is 5 4 per mil similar to that in the huascaran peru and polar ice cores three guliya interstadials stages 3 5a and 5c are marked by increases in 18 o values similar to that of the holocene and eemian 124 000 years ago the similarity of this pattern to that of ch 4 records from polar ice cores indicates that global ch 4 levels and the tropical hydrological cycle are linked the late glacial stage record contains numerous 200 year oscillations in 18 o values and in dust nh 4 and no 3 levels

interpretation of crustal seismic velocities in the san gabriel mojave region southern california the increased concern over seismic hazards in the los angeles region has created interest in the crustal structure and mechanical properties associated with faults like the sierra madre and san andreas this study presents the seismic properties of major lithologies found in the vicinity of the san andreas fault zone in order to correlate with the los angeles region seismic experiment lars e results scientists from the larse group have recently acquired high quality seismic data along three transects thirty nine rock samples were collected along the larse line i which trends ne sw across the central mojave desert and the san gabriel mountains these rock samples represent the following lithologies precambrian gneisses san gabriel anorthosite mesozoic granitic intrusives pelona schist mendenhall granulite gneisses and san andreas fault zone cataclasites the seismic properties were determined by measuring compressional and shear wave velocities as a function of confining pressure average compressional and shear wave velocities at 150 mpa show a wide range of values the mendenhall gneiss and the san gabriel anorthosite complex have high velocities 6 5 km s and 6 4 km s respectively the pelona schist the mojave granitic intrusives and the san andreas cataclasitic rocks have average velocities of 5 8 km s 5 2 km s and 5 1 km s respectively average densities range from 2320 kg m3 for fault zone cataclasites to 3100 kg m3 for a mendenhall granulite gneiss gneisses and schists show significant seismic anisotropy 5 20 due to preferred mineral orientation while most other rocks are nearly isotropic poisson s ratios range from a high of 0 29 in the san gabriel anorthosite to a low of 0 22 in the san gabriel gneiss comparison of the measured seismic properties with the larse velocity model allows interpretation of crustal structure in terms of rock compositions the upper crust southwest of the san andreas fault contains granitic intrusives gneisses and schists while under the mojave desert the upper crust is dominated by gneisses high velocities under the san gabriel mountains represent either lower crustal granulites or anorthosite the velocity decrease beneath the san andreas fault zone originates from minor amounts of cataclasite or the presence of low velocity lithologies such as pelona schist 1998 elsevier science b v all rights reserved

constraining holocene lake level highstands on the tibetan plateau by 10be exposure dating a case study at tangra yumco southern tibet abstract many lakes on the tibetan plateau are surrounded by palaeo shorelines that document former lake level highstands and hence past changes in climate establishing accurate chronologies for these shorelines is crucial to correlate lake level fluctuations with palaeo environmental histories that are commonly inferred from lake sedimentary records here we apply surface exposure dating to constrain the age of two lake level highstands at one of the largest tibetan lakes tangra yumco which has well developed palaeo shorelines located up to 185 m above the current lake for sampling we focussed on wave cut bedrock terraces that vary considerably in width because they were carved into rocks of different erodibility samples from the highest strath terrace 180 185 m above the lake at three different sites yield tightly clustered 10 be ages between 7 39 0 19 ka and 7 87 0 27 ka internal errors which constrain the end of an early holocene lake level highstand at 7 6 0 6 ka external error the consistency of the 10 be ages indicates that bedrock erosion by wave attack reduced the inherited 10 be component to negligible amounts because otherwise samples would show considerable age scatter two bedrock samples from a lower terrace 140 145 m above the lake and one amalgamated clast sample from a beach ridge on the same terrace yield 10 be ages between 4 04 0 14 ka and 4 50 0 15 ka these ages document that the lake level dropped by 40 m between 7 6 ka and 4 3 ka granite boulders from the two investigated terraces yield apparent 10 be ages that are 6 ka and 27 ka older than the ages of the respective terraces indicating that this sample type is not reliable in this setting overall our results demonstrate that exposure dating of bedrock terraces provides a valuable tool to reconstruct lake level histories on the tibetan plateau

sodium sulphate deposits of neogene age the kirmir formation beypazari basin turkey the evaporite member of the kirmir formation was deposited in shallow lacustrine environments during the upper miocene the most soluble minerals of this member can be currently observed in the cayirhan mine the evaporite member which is composed of secondary gypsum at outcrop can be subdivided into a bedded lower unit and a massive upper unit in the bedded lower unit most of the gypsum throughout the basin can be identified as having been transformed from glauberite in the glauberite layers of the cayirhan mine some glauberite textures clear glauberite suggest a primary subaqueous free precipitation on a depositional floor more common however are the glauberite textures indicating an interstitial growth within a clayey magnesitic matrix in the thenardite layers accompanying the glauberite in the cayirhan mine some disruption structures can be assigned to synsedimentary dissolution these structures together with the textures of the thenardite suggest that the original sodium sulphate was mirabilite thenardite being a secondary phase which formed during early to moderate burial diagenesis the massive upper unit in which evidence of sodium bearing minerals is absent is characterized by laminated to banded gypsum and nodular gypsum in the marginal areas of the evaporitic basin whereas thick clast supported gypsum breccias prevail in the northern deeper part of the basin the brecciation of these calcium sulphate layers occurred as a result of synsedimentary gravitative slumping under tectonic control although the sulphur isotopic values d 34 s of the sulphates of the kirmir formation suggest a marine derived brine supply the oxygen isotopic values d 18 o and the strontium ratios 87 sr 86 sr do not support such a supply the origin of the mother brines the glauberite genesis the depositional model of the sodium sulphates and the salinity evolution are discussed d 2002 elsevier science b v all rights reserved

long term east west asymmetry in monsoon rainfall on the tibetan plateau variations in the strength and duration of the asian monsoon affect more than half the population of the planet and there is intense interest in where and how much rainfall amounts will change regionally in response to warmer climate conditions the modern monsoon region is divided into distinct subsystems so the response may be regionally heterogeneous northern hemisphere monsoon systems intensified during the early to mid holocene increasing summer rainfall and creating wetter conditions across much of eastern asia in this study we use the area encompassed by high shorelines of early holocene paleolakes in tibet to reconstruct paleorainfall and hence paleomonsoon patterns during this very wet period we found that the early holocene relative paleolake expansions of 130 closed basin lake systems in the central tibetan plateau display a strong east west gradient paleolake areas expanded by approximately fourfold in the western plateau compared to approximately twofold expansion in the eastern region this early holocene pattern mirrors the modern west east climate division on the plateau rainfall in the west is closely tied to the indian summer monsoon ism subsystem and in the east is closely tied to a probable mix of ism and east asian summer monsoon easm subsystems our results suggest that these modern climate divisions are an enduring feature of the plateau and that ism rainfall increased much more than easm rainfall in response to the same insolation forcing

cosmogenic 10be and 26al exposure dating of nam co lake terraces since mis 5 southern tibetan plateau abstract lake paleo shorelines can be used to reconstruct lake level fluctuations which serve as sensitive indicators of climate change the tibetan plateau tp contains the highest areal proportion of lakes in china and dramatic past lake level changes have led to the formation of numerous relict shorelines here we present 24 10be and 26al exposure ages for five of seven lake terraces situated north of nam co on the southern tp the ages of four high stands marked by terraces t7 t6 t5 and t4 are 81 5 9 0 ka 33 6 1 9 ka 19 9 1 9 ka and 15 2 0 4 ka the topographically highest terrace t7 correlates with periods of enhanced indian summer monsoon ism and supports an mis 5 maximum lake expansion on the tp as opposed to mis 3 ages of two lower terraces t5 and t4 possibly correspond to melt water pulse mwp events we also evaluate different possible sources of uncertainty from geologic processes on 10be exposure ages of lake terraces subaerial erosion following terrace exposure has the most significant influence on 10be exposure ages on ancient high terraces for example 80 ka while inherited 10be from prior exposure and partial shielding by shallow water beach or talus are the most important geological source error of 10be exposure ages for relatively low and young terraces for example

the crustal structure of the guayana shield venezuela from seismic refraction and gravity data abstract we present results from a seismic refraction experiment on the northern margin of the guayana shield performed during june 1998 along nine profiles of up to 320 km length using the daily blasts of the cerro bolivar mines as energy source as well as from gravimetric measurements clear moho arrivals can be observed on the main e w profile on the shield whereas the profiles entering the oriental basin to the north are more noisy the crustal thickness of the shield is unusually high with up to 46 km on the archean segment in the west and 43 km on the proterozoic segment in the east a 20 km thick upper crust with p wave velocities between 6 0 and 6 3 km s can be separated from a lower crust with velocities ranging from 6 5 to 7 2 km s a lower crustal low velocity zone with a velocity reduction to 6 3 km s is observed between 25 and 25 km depth the average crustal velocity is 6 5 km s the changes in the bouguer anomaly positive 30 mgal in the west and negative 20 mgal in the east cannot be explained by the observed seismic crustal features alone lateral variations in the crust or in the upper mantle must be responsible for these observations

some phase equilibrium constraints on the origin of proterozoic massif anorthosites and related rocks experimental data in the range of 1 bar to 13 kbar enable us to introduction map the liquidus equilibria relevant to proterozoic massif anorthere is hardly a consensus on the petrogenesis of thosites and related mafic rocks massif anorthosites are widely proterozoic massif anorthosites but many investigators believed to have formed by accumulation of plagioclase into highbelieve in a petrogenetic scheme that involves at least al basaltic liquids mantle derived basaltic liquids fractionating two major stages 1 extensive crystallization asat pressures sufficiently high 10 13 kbar to crystallize the highly similation of a mantle derived magma ponded at or aluminous orthopyroxene megacrysts typically observed in anorthosite near the base of the crust that produces suspensions of massifs reach plagioclase saturation at low normative silica contents plagioclase in fe rich high al gabbroic liquids followed peritectic like equilibria e g liq opx plag cpx sp by 2 intrusion of these suspensions into the mid upper and a thermal divide on the plagioclase pyroxene liquidus surface crust where they form complex magma chambers e g ensure that mantle derived liquids become nepheline normative with duchesne 1984 emslie 1985 ashwal 1993 wiebe further crystallization and crustal assimilation at depth such 1994 a key component of the scheme is the presence liquids cannot produce the full range of troctolitic noritic to trocin most massifs of high al orthopyroxene megacrysts tolitic gabbroic mineral assemblages observed in anorthosite massifs haom emslie 1975 that appear to preserve a record without extensive low pressure granite assimilation conversely the of the high pressure stage maquil duchesne 1984 array of plausible anorthosite parental liquids not only lies along longhi et al 1993 the structures of massifs vary from the trace of the plagioclase two pyroxene cotectic from 10 to 13 those that are composites of multiple diapiric intrusions kbar but also straddles the thermal divide on the plagioclase harp lake emslie 1980 rogaland duchesne 1984 pyroxene liquidus surface this condition requires mafic source to intrusive bodies with well developed layering of regions such as lower continental crust or foundered mafic plutons leucocratic troctolites and leuconorites or leucogabbros for liquids parental to massif anorthosites and associated mafic overlain by masses of anorthosite michikamau emslie intrusions 1970 laramie frost et al 1993 so there may be

abrupt changes in indian summer monsoon strength during 33 800 to 5500 years b p speleothem proxy records from northeastern ne india reflect seasonal changes in indian summer monsoon strength as well as moisture source and transport paths we have analyzed a new speleothem record from mawmluh cave meghalaya india in order to better understand these processes the data show a strong wet phase 33 500 32 500 years b p followed by a weak dry phase from 26 000 to 23 500 years b p and a very weak phase from 17 000 to 15 000 years b p the record suggests abrupt increase in strength during the bolling allerod and early holocene periods and pronounced weakening during the heinrich and younger dryas cold events we infer that these changes in monsoon strength are driven by changes in temperature gradients which drive changes in winds and moisture transport into northeast india

potential forcing mechanisms of holocene lake level changes at nam co tibetan plateau inferred from the stable isotopic composition of shells of the gastropod radix the timing of lake level fluctuations on the tibetan plateau and their relationship with climatic changes is still under debate and the main reason for this is the lack of suitable archives for reconstructing the paleohydrology and paleoclimatology of the lakes here we present the results of analyses of the shell geochemistry of radix sp from an exposed terrace of nam co lake on the south central tibetan plateau optically stimulated luminescence osl dating reveals that deep water lacustrine sediments formed between ca 4 4 and 2 2 ka suggesting a high and stable lake level significantly above the present the results of sr ca 13c and 18o analyses of the fossil shells of radix sp indicate that during the mid to late holocene lake level variations in nam co were mainly controlled by variations in the indian summer monsoon a trend of decreasing evaporation also played an important role comparison with other results suggests a consistent pattern of mid to late holocene lake level changes acro

crust and upper mantle structure along the dss baltic profile in se finland summary the 430 km long dss baltic profile in se finland runs through the rapakivi intrusion the svecokarelian geosynclinal complex including the ladoga bothnian bay zone and the archean basement complex a brief description of field operations in 1982 is presented record sections of p and s waves were used for the interpretations a starting model was constructed by inverse methods and improved by dynamic ray tracing modelling three blocks are indentified on the cross section from sw to ne these are the southern associated with the rapakivi massif depth to moho m about 40 km the central ladoga bothnian bay zone with m of approximately 55 60 km and the northern archean crystalline basement with m about 40 45 km the average crustal velocity is 6 6 6 7 kms the crustal structure along the baltic profile is compared with the surrounding dss profiles finnish sveka and soviet kern tulos and ladoga profiles the eastern border of the ladoga bothnian bay zone was located and specific features of the crust and uppermost mantle along the profile area are discussed

composition of the crust in the grenville and appalachian provinces of north america inferred from v p v s ratios we use the ratios between p and s wave velocities vp vs derived from seismic refraction data to infer the composition of the crust in the grenville and the appalachian provinces of north america the crust exhibits vp vs increasing with depth from 1 64 to 1 84 there is a clear distinction between the grenville province average vp vs 1 81 and the appalachian province average vp vs 1 73 which persists at all depths the boundary between these provinces is east dipping extending for 100 km east of the champlain thrust in the appalachian province the increase in vp vs ratios with depth from 1 67 to 1 74 0 02 may reflect a normal decrease of silica content in the continental crust in the grenville province beneath the central granulite terrane an anomalous vp vs ratio of 1 82 0 02 is observed extending to a depth of 10 km this correlates with the abundance of ca plagioclase in the marcy anorthosite at greater depth 15 20 km where seismic lamination and high electrical conductivity is observed vp vs is 1 84 0 02 and correlates with the tahawus complex a layered mafic intrusion within the 25 km thick lower crust of the grenville province the vp vs is 1 84 0 02 and p velocity is 7 0 0 1 km s which are typical for plagioclase bearing rocks gabbro norite the high vp vs ratio in the grenville province has not been reported in crust of any other age since the grenville province contains 75 of the world s known anorthosites high vp vs ratio is related to high plagioclase we suggest that the composition of the grenville lower crust was significantly modified by the emplacement of the anorthosites in the mid proterozoic

crustal structure across the nain makkovik boundary on the continental shelf off labrador from seismic refraction data a detailed seismic refraction profile was shot along the continental shelf off labrador across the boundary between the archean nain province to the north and the proterozoic makkovik orogenic zone to the south a large air gun source was used with five ocean bottom seismometers as receivers the data were analysed by forward modelling of traveltimes and amplitudes and provided a well determined seismic velocity structure of the crust along the profile within the nain province thin postrift sediments are underlain by crust with a p wave velocity of 6 1 km s which increases with depth and reaches 6 6 km s at about 8 km moho is at around 28 km and there is no evidence for a high velocity 7 km s lower crust the p and s wave velocity structure is consistent with a gneissic composition for the archean upper crust and with granulites becoming gradually more mafic with depth for the intermediate and lower crust in the makkovik zone the sediments are thicker and a basement layer of p wave velocity

reconstruction of the 30 40 ka bp enhanced indian monsoon climate based on geological records from the tibetan plateau abstract multi proxy records from ice caps lakes and pollen records from the tibetan plateau are used to reconstruct the palaeoclimate during the period 30 40 ka bp lake geomorphological and lacustrine sedimentological studies show that numerous high lake level stands and or freshwater lakes occurred in large areas of western china at this time including over the tibetan plateau where the area contains mostly shallow water and saline hypersaline water lakes under extremely dry conditions of today pollen based reconstruction of tibetan vegetation shows systematic differences from the present alpine steppe forests were shifted ca 400 km further north and the alpine conifer forest extended ca 400 800 km beyond their present western limits variations in 18 o curves from ice cores from the plateau suggest that the temperature was 2 4 c higher and precipitation was 40 to over 100 higher than today all of the evidence consistently suggests the existence of an exceedingly strong summer monsoon climate over the plateau the vigorous evaporation of the tropical ocean surface would also play an important role in promoting the penetration of moisture rich southwest monsoon over the tibetan plateau these climate patterns with strong summer monsoon circulation during the period 30 40 ka bp reflect the 20 ka earth orbital precessional cycle when the tibetan plateau region received greater solar radiation which thus enlarged the thermodynamical contrast between the plateau and the mid south portion of the indian ocean

large variations of oxygen isotopes in precipitation over south central tibet during marine isotope stage 5 oxygen isotope 18 o records of two stalagmites from tianmen cave on the south central tibetan plateau grew during much of marine isotope stage 5 the tianmen record the fi rst cave record from the tibetan plateau characterizes a precipitation 18 o history larger in amplitude but similar in structure to asian monsoon records from the adjacent regions providing essential evidence that the asian monsoon system including the east asian and indian monsoon subsystems responds largely to changes in northern hemisphere summer insolation extremely low 18 o values during marine isotope stages 5a 5c and 5e suggest that precipitation rather than temperature was a major factor controlling 18 o in precipitation on orbital time scales in the south central tibetan plateau the tianmen record may help in the interpretation of regional ice core 18 o records the large range of orbital scale shifts in meteoric 18 o 9 raises important considerations related to reconstructing the uplift history of the plateau

landscape evolution of a bedrock peneplain on the southern tibetan plateau revealed by in situ produced cosmogenic 10 be and 21 ne article i nfo 10 be cosmogenic 21 ne low relief bedrock surfaces that occur at high altitude are a common feature of cenozoic mountain belts and have often been used to infer a significant amount of rock uplift after their generation at low elevation the timescale over which such surfaces can be preserved at high elevation and the rate at which they are modi fied by weathering and erosion are poorly known here we use cosmogenic 10 be and 21 ne to quantify the landscape evolution of a bedrock peneplain in southern tibet that occurs at an altitude of 5300 m the pe neplain is developed in cretaceous granitoids and jurassic metasediments of the northern lhasa block 90 e 31 n and originally had a minimum extent of 150 km east west and 75 km north south it has been dissected by small rivers that generated a few hundred meters of relief and formed additional bedrock sur faces of limited extent at lower elevation local denudation rates for the peneplain and the lower bedrock surfaces based on 10 be concentrations in grus samples and amalgamated quartz clasts cluster between

rapid and punctuated late holocene recession of siling co central tibet this work was supported by a national science foundation grant from the tectonics program ear 0911587 to e k and k p f additional support to e w was provided by grants from the chinese academy of sciences xdb03010500 x s also thanks the support from the earth observatory of singapore nanyang technological university through its funding from the national research foundation singapore and the singapore ministry of education under the research centers of excellence initiative

hydrological variations on the central tibetan plateau since the last glacial maximum and their teleconnection to inter regional and hemispheric climate variations decadal to sub decadal variability of inflow evaporation and biological productivity derived from lake nam co was used to reconstruct hydrological changes for the past ca 24 k cal a bp the timing of these variations corresponds to known climatic shifts on the northern hemisphere after a dry and cold last glacial maximum the lake level of nam co initially rose at ca 20 k cal a bp moist but further cold conditions between ca 16 2 and 14 k cal a bp correspond to heinrich event 1 a warm and moist phase between ca 14 and 13 k cal a bp is expressed as a massive enhancement in inflow and biological productivity and might be associated with a first intensification of the indian ocean summer monsoon coinciding with the bolling allerod complex a two step decrease in inflow and a contemporaneous decline in biological productivity until ca 11 8 k cal a bp points to cool and dry conditions during the younger dryas lake levels peak at ca 9 4 k cal a bp although hydrological conditions remain relatively stable during the holocene with only low amplitude variations observed

crustal scale shortening and extension across the grenville province of western quebec a deep seismic reflection survey shot in 1993 crosses almost the full width of the grenville province in western quebec the seismic transect provides a very clear image of the crust mantle boundary and the most precise definition to date of the various grenvillian terranes the crust is around 44 km thick beneath the grenville front but thins rapidly to 36 km some 60 km to the southeast it is also notable that the greatest crustal thickness of 50 km occurs at the southeast end of the transect far from the inferred location of the main grenvillian collision the grenville front zone in which nw directed thrusting at about 1 ga was followed by se directed extension is defined by discontinuous se dipping reflections which extend down to the moho to the southeast the overlying migmatitic archean parautochthon almost half of the transect is characterised instead by nw dipping reflectors extending into the lower crust these reflectors are in turn truncated to the southeast by a 12 km thick zone of intense se dipping reflections the baskatong crustal ramp the base of the allochthonous terranes allochthon boundary thrust is likely located at this ramp which flattens out at around 30 km depth into the base of the relatively transparent intermediate crust a highly reflective upper crustal deck corresponding to rocks of the mont laurier terrane was thrust over the baskatong crustal ramp and is represented further to the northwest by the klippelike cabonga allochthon the synformal transparent morin anorthosite charnockite complex belongs to the same upper crustal level ramp anticlines and the overriding basal thrust of the upper allochthons demonstrate the nw directed propagation of tectonic transport during the grenvillian orogeny involving deformation and displacement along the baskatong ramp with a relay into the grenville front zone postaccretional extension appears to have been primarily accommodated along the latter two crustal discontinuities thinning the crust immediately south of the grenville front and affecting the crust up to 350 km away from the front

u pb geochronology of deformation and metamorphism across a central transect of the early proterozoic torngat orogen north river map area labrador1 the early proterozoic torngat orogen resulted from the oblique collision of the archean nain and southeastern rae provinces and evolved in four stages 0 deposition of platformal supracrustal assemblages followed by subduction related arc magmatism in the margin of the rae province i crustal thickening and nappe tectonics ii sinistral transpression and formation of the abloviak shear zone iii uplift on steeply dipping east verging mylonites along the eastern orogenic front u pb geochronology on zircon and monazite from major rock units and syntectonic intrusions indicates that arc magmatism at ca 1880 ma was followed by 40 ma of deformation and high grade metamorphism from ca 1860 to 1820 subsequent uplift and final cooling occurred ca 1795 1770 ma several ages of mineral growth that correspond to distinct structural and metamorphic events have been recognized 1 1858 1853 ma zircon and monazite dates are interpreted as the minimum age of stage i and peak metamorphic conditions 2

rapakivi granite anorthosite magmatism a way of thinning and stabilisation of the svecofennian crust baltic sea basin in the palaeoproterozoic a 55 80 km thick layer of crust was formed in the baltic sea region during the svecofennian orogeny at 1 9 1 8 ga today the remaining crustal thickness is 50 65 km in the marginal parts of the 1 000 000 km2 svecofennian domain the moho depth reached 40 75 km which exceeds the 40 km crustal thickness of the southwestern edge of the karelian archaean domain to which svecofennia was accreted the 1 65 1 50 ga rapakivi magmatism of the fennoscandian province was limited to the svecofennian domain the rapakivi igneous structures are confined to areally isolated subprovinces which each have distinct 20 60 ma long age spans of formation the petrologic sequences of the subprovinces are alike although similar petrological events occur at different times in the various subprovinces the internal structure of a subprovince generally consists of a main igneous polyphase unit in a central position with smaller felsic intrusions as well as mafic dike swarms spread over the peripheries of the subprovince the rapakivi magmatism started in juvenile crust which was in a late stage of erosional thinning 150 300 ma after the period of maximal thickening the most extensive rapakivi igneous activities were associated with crustal thinning down to 45 50 km as a result the thinner marginal parts of svecofennia and the large interior rapakivi subprovinces were of similar thicknesses as the crust the primary thickness of the original crust was maintained only in areas void of rapakivi magmatism no major events destroyed the svecofennian and rapakivi related crustal structures subsequent to emplacement thus it can be concluded that the extensive rapakivi igneous activity substantially thinned and stabilised the overthickened portions of the svecofennian crust

the volcanic margins of the northern labrador sea insights to the rifting process 1 a new interpretation of seismic reflection data on the continental margin of northern labrador shows basement structures similar to those observed on other volcanic rifted margins seaward dipping reflections inner flows volcanic plateaus and lava deltas are observed on these data magnetic chron 27n 61 ma is coincident with the volcanic plateaus connecting these features to the paleocene volcanism farther north in davis strait therefore we are able to extend the region of volcanism about 500 km south along the margin similar structures are also observed on the conjugate west greenland margin below the volcanic plateaus gravity and wide angle seismic data show thick igneous crust which is fairly symmetrically distributed across the conjugate margins however the geometry of the thinned continental crust is not symmetrical a narrower zone of thinning is observed off northern labrador the thick igneous crust lies seaward of thinned continental crust which exhibits little or no underplating by the magmatic event this observation is compatible with recent models for the formation of nonvolcanic margins followed by a magmatic event late in the rift evolution in mesozoic time rifting of the cold thick cratonic lithosphere occurred between greenland and north america with minimal volcanic activity then in late cretaceous time the zone of rifting narrowed and localized seaward of the shelf creating a relatively narrow zone of lower lithospheric thinning and mantle upwelling into which the paleocene magmas were eventually channeled this scenario is supported by studies of mesozoic igneous rocks on land in coastal west greenland and labrador

constraints on the late quaternary glaciations in tibet from cosmogenic exposure ages of moraine surfaces abstract this contribution provides new constraints on the timing of tibetan glacial recessions recorded by the abandonment of moraines we present cosmogenic radionuclide 10 be inventories at 17 sites in southern and western tibet 32 crests 249 samples and infer the range of permissible emplacement ages based on these analyses individual large embedded rock and boulder samples were collected from the crests of moraine surfaces and analyzed for 10 be abundance we consider two scenarios to interpret the age of glacial recession leading to the moraine surface formation from these sample exposure ages 1 erosion of the moraine surface is insignificant and so the emplacement age of the moraines is reflected by the mean sample age and 2 erosion progressively exposes large boulders with little prior exposure and so the oldest sample age records the minimum moraine emplacement age we found that depending on the scenario chosen the moraine emplacement age can vary by 50 for 100 ka old samples we consider two scaling models for estimating the production rates of 10 be in tibet which has an important although lesser effect on inferred moraine ages while the data presented herein effectively increase the database of sample exposure ages from tibet by 20 we find that uncertainties related to the interpretation of the 10 be abundance within individual samples in terms of moraine emplacement ages are sufficient to accommodate either a view in which glacial advances are associated with temperature minima or precipitation maxima that are recorded by independent paleoclimate proxies a reanalysis of published data from moraines throughout tibet shows that the variation we observe is not unique to our dataset but rather is a robust feature of the tibetan moraine age database thus when viewed in a similar way with other samples collected from this area uncertainties within moraine exposure ages obscure attribution of tibetan glacial advances to temperature minima or precipitation maxima our work suggests that more reliable chronologies of tibetan glaciations will come from improvements in production rate models for this portion of the world as well as a better understanding of the processes that form and modify these geomorphic surfaces

variations in water level and glacier mass balance in nam co lake nyainqentanglha range tibetan plateau based on icesat data for 2003 09 water level fluctuations of inland lakes are related to regional scale climate changes and reflect variations in evaporation precipitation and glacier meltwater flowing into the lake area in its catchment in this paper ice cloud and land elevation satellite icesat altimeter data and landsat imagery 2002 09 are used to estimate nam co lake nyainqentanglha range tibetan plateau water elevation changes during 2002 09 in 2003 nam co lake covered an area of 1998 8 4 2 km 2 and was situated at 4723 m a s l over such inland water bodies icesat altimeter data offer both wide coverage and spatial and temporal accuracy we combine remote sensing and gis technology to map and reconstruct lake area and increased volume changes during a 7 year time series nam co lake water level increased by 2 4 0 12 m 0 33 m a 1 between 23 february 2003 and 1 october 2009 and lake volume increased by 4 9 0 5 km 3 in the past 7 years nam co lake area has increased from 1998 78 5 4 to 2023 8 3 4 km 2 the glacier covered area has decreased from 832 34 to 821 0 km 2 and the drainage basin area has decreased from 201 1 4 2 to 196 1 2 3 km 2 however the most spectacular feature is the continual water level rise from 2003 to 2009 without an obvious associated increase in precipitation based on digital elevation models dems from shuttle radar topography mission srtm dem data and corrected icesat elevation data significant changes to glacier mass balance in the western nyainqentanglha mountains are indicated nyainqentanglha mountain glacier surface elevations decreased by 8 39 0 45 m during 2003 09 over the same period at least 1 01 km 3 of glacial meltwater flowed into nam co lake assuming a glacial runoff coefficient of 0 6 the mean glacier mass balance value is 490 mm w e over the corresponding period indicating that glacier meltwater in the catchment contributes to lake level rise the contribution rate of glacial meltwater to lake water volume rise is 20 75 the temporal lake level fluctuation correlates with temperature variations over the same time span

age and tectonic implications of paleoproterozoic granitoid intrusions within the nain province near nain labrador u pb zircon age determinations from three granitoid intrusions in the nain region formerly assigned to the well known 1 35 1 29 ga mesoproterozoic nain plutonic suite nps indicate that they

the timing and cause of glacial activity during the last glacial in central tibet based on 10be surface exposure dating east of mount jaggang the xainza range abstract mountain glaciers are sensitive to climate change and can provide valuable information for inferring former climates on the tibetan plateau tp the increasing glacial chronologies indicate that the timing of the local last glacial maximum lgm recorded across the tp is asynchronous implying different local influences of the mid latitude westerlies and asian summer monsoon in triggering glacier advances however the well dated sites are still too few especially in the transition zone between regions controlled by the two climate systems here we present detailed last glacial chronologies for the mount jaggang area in the xainza range central tibet with forty three apparent 10be exposure ages ranging from 12 4 0 8 ka to 61 9 3 8 ka these exposure ages indicate that at least seven glacial episodes occurred during the last glacial cycle east of mount jaggang these include a local lgm that occurred at 61 9 3 8 ka possibly corresponding to marine isotope stage 4 mis 4 subsequent glacial advances at 43 2 2 6 ka and 35 1 2 1 ka during mis 3 one glacial re advance standstill at mis3 2 transition 29 8 1 8 ka and three glacial re advances standstills that occurred following mis 3 at 27 9 1 7 ka 21 8 1 3 ka and 15 1 0 9 ka the timing of these glacial activities is roughly in agreement with north atlantic millennial scale climate oscillations heinrich events suggesting the potential correlations between these abrupt climate changes and glacial fluctuations in the mount jaggang area the successively reduced glacial extent might have resulted from an overall decrease in asian summer monsoon intensity over this timeframe

geophysical interpretation of the torngat orogen along the north river nutak transect labrador a 19 3 mgal 1 gal 1 cm s2 positive bouguer gravity anomaly and a broad aeromagnetic low coincide with the tasiuyak domain of the early paleo proterozoic torngat orogen the domain evolved during the collision of the western margin of the archean nain province with an inferred eastward facing platform to rise sedimentary wedge sited on a proto continent of churchill rae province a density contrast of 0 065 g cm 3 was measured between rocks of the tasiuyak domain and those of the flanking lac lomier complex of the orogen and nain province using this contrast a two and one half dimensional model shows that rocks of tasiuyak domain constitute a triangular prismatic body with maximum thickness of 13 km adjacent to nain province which thins westward to a feather edge the model is compatible qualitatively with the aeromagnetic anomaly and consonant with geological interpretation of an eastward facing and thickening continental slope deposit deep exhumation of the torngat orogen may account for t

modeling wide angle seismic data for crustal structure southeastern grenville province a modeling methodology for obtaining two dimensional 2 d crustal structure from wide angle seismic data is applied to data from the southeastern grenville province pre modeling steps include 1 assignment of arrival pick uncertainties for appropriate data fitting and weighting using an empirical relationship based on signal to noise ratio 2 using a modified form of travel time reciprocity to avoid unreasonable levels of model heterogeneity and 3 identifying data unsuitable for 2 d modeling the goal of the travel time inversion amplitude modeling approach is to obtain a minimum structure and minimum parameter model that takes into account both horizontal and vertical variations in the resolution of typical wide angle data each step of a layer stripping procedure involves a series of inversions in which a one dimensional or simple starting model is improved with additional velocity and or interface nodes until a satisfactory trade off between travel time fit parameter resolution and complete ray coverage of all source receiver pairs is achieved using zero vertical velocity gradient layers and head waves during preliminary first arrival inversion can 1 decrease the number of intermediate models 2 allow greater lateral heterogeneity to be imaged and 3 simplify incorporation of amplitude modeling constraints into the final model using amplitude distance curves allows quantitative modeling of the relative amplitude and offset variations of phases discrepancies between observed and calculated reflection amplitudes are used to infer fundamental non step like velocity changes at layer boundaries later arrivals due to unresolved velocity anomalies are modeled using reflecting segments that float within the model without an associated velocity structure these reflectors provide a spatial image like that obtained from vertical incidence reflection data as opposed to a velocity image the model of grenville crustal structure is more detailed than a model obtained from a previous interpretation of the data and includes elements analogous to those imaged in nearby deep reflection data a crustal scale zone of wide angle reflectors with an average easterly apparent dip of 13 defines a major grenvillian terrane boundary

wide angle seismic imaging of pristine archean crust in the nain province labrador a detailed refraction wide angle reflection seismic experiment was carried out in northern labrador to determine the velocity structure of relatively unaltered archean crust in the nain province six 3 component land seismometers were used to record an airgun source along a profile parallel to the coast forward modeling of traveltimes and amplitudes yields a p and s wave velocity model that shows two crustal blocks separated by a fault magnetic data suggest but do not prove that the fault is the offshore continuation of the handy fault a southwards thickening of the lower crust across the fault indicates that a transcurrent component might have been associated with the faulting the total crustal thickness is 33 km to the north and 38 km to the south of the fault the presence of pms reflections imply a sharp transition at the moho upper crustal velocities of 5 8 6 3 km s and poisson s ratios of 0 20 and 0 24 north and south of the fault respectively are consistent with a gneissic composition

lith5 0 a revised crustal model for canada based on lithoprobe results summary during the last 15 yr numerous seismic refraction experiments have been conducted overthe canadian landmass most as part of the lithoprobe initiative the new results providethe basis for revising existing global models of crustal structure e g crust5 1 mooney et al 1998 which do not incorporate the new lithoprobe data the crustal model discussedhere lith5 0 combines the lithoprobe and crust5 1 data to provide better constraints oncrustal structure in canada the objectives of the study are to 1 compile available new datainto a single consistent database of crustal thickness and velocity including the uppermostmantle pn velocity and 2 to construct a revised 5 5 crustal model of the canadianlandmass in a format that is generally compatible with existing global models the lithoproberesults suggest a 5 5 km average root mean square rms crustal thickness difference betweenlith5 0andcrust5 1 evenwiththeincorporationofthenewdata thegeographiccoveragein north and central canada remains sparse the new model does not explicitly incorporategeological constraints but instead relies on spherical spline interpolation for extrapolatingvalues into regions that lack control to illustrate the implications of this new crustal model we use lith5 0 to correct for the isostatically compensated crustal heterogeneity in order toobtain improved estimates of the dynamic surface topography the latter provides importantconstraints on the density structure of the uppermost mantle