a decade of progress in observing and modelling antarctic subglacial water systems in the decade since the discovery of active antarctic subglacial water systems by detection of subtle surface displacements much progress has been made in our understanding of these dynamic systems here we present some of the key results of observations derived from icesat laser altimetry cryosat 2 radar altimetry operation icebridge airborne laser altimetry satellite image differencing and ground based continuous global positioning system gps experiments deployed in hydrologically active regions these observations provide us with an increased understanding of various lake systems in antarctica whillans mercer ice streams crane glacier recovery ice stream byrd glacier and eastern wilkes land in several cases subglacial water systems are shown to control ice flux through the glacier system for some lake systems we have been able to construct more than a decade of continuous lake activity revealing internal variability on time scales ranging from days to years this variability indicates that continuous accurate time series of altimetry data are critical to understanding these systems on whillans ice stream our results from a 5 year continuous gps record demonstrate that subglacial lake flood events significantly change the regional ice dynamics we also show how models for subglacial water flow have evolved since the availability of observations of lake volume change from regional scale models of water routeing to process models of channels carved into the subglacial sediment instead of the overlying ice we show that progress in understanding the processes governing lake drainage now allows us to create simulated lake volume time series that reproduce time series from satellite observations this transformational decade in antarctic subglacial water research has moved us significantly closer to understanding the processes of water transfer sufficiently for inclusion in continental scale ice sheet models

quaternary sediments in the dien bien phu fault zone nw vietnam a record of young tectonic processes in the light of osl sar dating results abstract the dien bien phu fault zone dbp orientated nne to n is one of the most seismically active zones in indochina in nw vietnam this zone is 160 km long and 6 10 km wide cutting sedimentary and metamorphic rocks of the late proterozoic palaeozoic and mesozoic age as well as palaeozoic and late triassic granitoids along the dbp relatively small narrow pull apart basins occur the three largest of which chan nua lai chau and dien bien phu have been studied in detail all of them are bounded by sinistral and sinistral normal faults responsible for offset and deflected drainage presence of numerous shutter ridges and displaced terraces and alluvial fans the normal component of motion is testified to by well preserved triangular facets on fault scarps highly elevated straths in river watergaps overhanging tributary valleys as well as high and uneven river bed gradients our observations indicate a minimum recent sinistral offset ranging from 6 8 to 150 m for holocene valleys to 1 2 9 75 km for middle late pleistocene valleys in different fault segments the thickness of quaternary sediments varies from 5 25 m in the lai chau area to some 130 m in the dien bien phu basin in the lai chau basin the middle terrace 23 m alluvia of nam na river at muong te bridge have been optically stimulated luminescence single aliquot regenerative dose technique osl sar dated at 23 40 to 13 ka these sediments were normal faulted by some 11 m after 13 ka and mantled by vari coloured slope loams 8 12 m thick containing colluvial wedges composed of angular debris these wedges were probably formed due to at least three palaeoseismic events postdating 6 ka in the dien bien phu basin in turn alluvium of the upper holocene terraces has been osl sar dated to 6 5 7 and 1 7 1 0 ka whereas the younger sub recent terrace sediments give ages of 0 5 0 2 ka displaced terraces and alluvial fans allow us to suppose that the sinistral and sinistral normal faults bounding narrow pull apart basins in the southern portion of the dbp fault reveal minimum rates of left lateral strike slip ranging from 0 6 to 2 mm year in holocene and 0 5 3 8 mm year in pleistocene times whereas rates of holocene uplift tend to attain 1 mm year north of lai chau and 0 4 0 6 mm year west of dien bien phu more precise estimations however are difficult to obtain due to poor age control of the displaced drainage rates of quaternary strike slip are comparable with those of the red river fault the sense of movement being however opposite taking into account the presence of two phases of late cenozoic strike slip of contrasting sense of motion as well as the geometry of the two fault zones we hypothesize that the red river and dien bien phu faults are conjugate faults capable of generating relatively strong earthquakes in the future

the red river fault system in the tonkin gulf vietnam abstract the red river fault system in the tonkin gulf offshore haiphong was studied using seismic profiles calibrated by deep wells well characterised left lateral strike slip occurred continuously within a narrow 30 km wide zone southwest of the vinh minh fault between 30 ma and 5 5 ma however the corresponding amount of offset probably does not exceed a few tens of kilometres no sign of post 5 5 ma right lateral motion can be detected prior to 30 ma there is a widespread extension in a wider 100 km wide zone which could be related to a significant amount of left lateral motion the motion of the fault splays to the northeast of the vinh minh fault prior to 30 ma was absorbed in the rifting of the gulf of beibu a 15 5 ma unconformity separates the transtensional regime from a later transpresional regime this 15 5 ma date coincides with the cessation of sea floor spreading in the south china sea

erosion by tectonic carving in the concordia subglacial fault zone east antarctica abstract in this work we present the analysis of the footwall morphology of the concordia subglacial extensional fault in the east antarctic craton the concordia fault is a regional fault zone that extends for almost 200 km the displacement up to 1800 m and the listric geometry were recognized by numerical modeling of the resulting hangingwall bedrock morphology and is responsible for the marked asymmetry that characterizes the corresponding scarp in the concordia subglacial trench the portion of the footwall in the proximity of the master fault exhibits an excavated morphology about 500 m deep and up to 5 km wide showing strong correlation with the master fault displacement we excluded a predominant glacial and fluvial origin of this morphology considering i the sharp topography of the concordia fault suggesting that the fault activity started after the onset of the ice sheet ii the ice sheet bedrock contact is characterized by a general negligible erosion deposition rates still allowing clast removal iii the lack of significant deposits in the concordia trench we hence explored the possibility that this morphology may result from the combined action of fault induced fracturing and passive clast removal and scattering by flow and plastic deformation within the ice sheet we introduced the term tectonic carving for this process our modeling shows that tectonic carving relates to the relative fracture intensity in the concordia fracture zone that corresponds to the envelope of master and secondary fault damage zones fracture intensity depends on the frequency and the displacement of secondary faulting and can be approximated by a normal distribution using a monte carlo modeling approach we selected the set of parameters that best fits the data set with the carving theoretical curve the final results of the monte carlo analysis show a root mean square of about 50 meters comparable with the data resolution this analysis demonstrates a method to unravel the presence of fracture zones in similar weak erosional environments

precambrian geology of the kazakh uplands and tien shan an overview abstract a comprehensive review of new data on geology and geochronology of precambrian terranes in the western central asian orogenic belt reveals new insights into its evolution at the present surface these terranes mostly consist of meso to neoproterozoic sedimentary magmatic and metamorphic assemblages with insignificant paleoproterozoic rocks archean material is represented exclusively by detrital and xenocrystic zircons in younger strata meso to neoproterozoic felsic magmatic rocks were mostly sourced from neoarchean and paleoproterozoic continental crust indicating its reworking and potential wider presence at deeper crustal levels most meso to neoproterozoic assemblages are of intraplate origin the supra subduction assemblages of neoproterozoic and mesoproterozoic ages are of limited extent we propose to recognize the issedonian and ulutau moyunkum groups of terranes separated by early paleozoic z shaped ophiolitic suture based on their different tectono magmatic evolution in the mesoproterozoic and neoproterozoic distinctly different are the mesoproterozoic and early neoproterozoic assemblages with lithological variations at the beginning of the late neoproterozoic and practically no differences at the end of the neoproterozoic the issedonian group of terranes could be part of a mesoproterozoic ca 1100 ma orogen between the siberian north china and laurentian cratons the pre mesoproterozoic crust of these terranes was completely reworked during the younger events the ulutau moyunkum group of terranes appear to be lithologically and geochronologically similar to the tarim craton both the issedonian and ulutau moyunkum groups of terranes were metamorphosed during the ulutau moyunkum event at 700 25 ma the breakup into currently mappable precambrian terranes took place during end ediacaran to early paleozoic times after opening of oceanic basins whose relics are preserved in numerous paleozoic ophiolitic sutures

emplacement and cooling of the dien bien phu granitic complex implications for the tectonic evolution of the dien bien phu fault truong son belt nw vietnam the dien bien phu dbp fault strikes nearly perpendicular to the general tectonic framework separating northwest vietnam into two compartments the dbp and muong lay granites are exposed respectively east and west of the fault the u pb age of 277 2 ma obtained for the muong lay granite is similar to the age of the chieng khuong complex exposed within the song ma structure east of the dbp fault these two magmatic intrusions are considered as belonging to a same tectonic structure related to the subduction of the song ma ocean under the indochina block during the indosinian orogeny the dbp fault with a total post permian dextral offset of 30 40 km shifts the song ma suture it could then be connected to the ailao shan shuanggou suture to the north the undeformed dbp granite gave a u pb age of 230 1 ma sealing the indosinian deformation on the fault the ladinian carnian activity of the fault is confirmed by the 40ar 39ar biotite age of 228 4 ma obtained from the paragneiss forming the fault zone a geodynamic model is proposed to describe the various tectonic phases recorded by on the dbp fault it associates field data stratigraphic observations and geochronology analyses u pb ar ar fission tracks following the late triassic exhumation of the granite and the end of the indosinian orogeny marked by the norian unconformity the granite was buried under jurassic and lower cretaceous sediments a strong cooling event during the early late cretaceous 100 ma is associated with the reactivation of the dbp fault and the accommodation of a southward thrusting of the song da terrane onto the truong son belt this newly recognised tectonic phase could be associated to the subduction of the paleo pacific plate under the indochina block no data are available yet to constrain the tertiary evolution of the dbp system however the tertiary deformation is likely similar to the cretaceous one

res signatures of ice bottom near to dome c antarctica geophysical surveys have played a fundamental role in glaciology climatology and geology studies conducted at and around the site of the epica dome c ice core antarctica analysis of radio echo sounding res data collected during the 2009 and 2011 italian antarctic expeditions in the immediate vicinity 2 km of the core site reveals the presence of small scale bedrock structures data acquired during the 2011 campaign which applied the latest updates of our instrument show double echoes from the ice bottom separated by very small time intervals these reflections are distributed close to the topographic dome and do not always correspond to particular features of the bedrock structure but instead may correspond to small scale subglacial water storage the unprecedented spatial resolution of the basal environment for dome c from these recent surveys allows us to identify this and other features that have been missed by previous larger scale res surveys of this area

tectonic history of the kyrgyz south tien shan atbashi inylchek suture zone the role of inherited structures during deformation propagation 1 multimethod chronology was applied on intrusives bordering the kyrgyz south tien shan suture stss to decipher the timing of 1 formation and amalgamation of the suturing units and 2 intracontinental deformation that built the bordering mountain ranges zircon u pb data indicate similarities between the tien shan and tarim precambrian crust caledonian 440 410 ma and hercynian 310 280 ma zircon u pb ages were found at the edge of the stss related to subduction and closure of the turkestan ocean and the formation of the suture itself permian triassic 280 210 ma titanite fission track and zircon u th he data record the first signs of exhumation when the stss evolved into a shear zone and the adjacent tarim basin started to subside low temperature thermochronological apatite fission track zircon and apatite u th he analyses reveal three distinct cooling phases becoming younger toward the stss center 1 jurassic cretaceous cooling ages provide evidence that a mesozoic south tien shan orogen formed as a response to the cimmerian orogeny 2 early paleogene 60 45 ma data indicate a renewed pulse of stss reactivation during the early cenozoic 3 neogene ages constrain the onset of the modern tien shan mountain building to the late oligocene 30 25 ma which intensified during the miocene 10 8 ma and pliocene 3 2 ma the cenozoic signals may reflect renewed responses to collisions at the southern eurasian border i e the kohistan dras and india eurasia collisions this progressive rejuvenation of the stss demonstrates that deformation has not migrated steadily into the forelands but was focused on pre existing basement structures

ar ar and fission track ages in the song chay massif early triassic and cenozoic tectonics in northern vietnam the song chay massif is the northeasternmost metamorphic complex in vietnam to the east of the red river shear zone it shows a large antiformal structure involving orthogneisses and migmatites overlain on its northern flank by muscovite bearing marbles an e w striking fault bounds the dome to the south kinematic indicators along a s n section reveal top to the n shear sense along the interface between the orthogneissic core and the overlying metasediments radiometric ages were obtained by the 40ar 39ar method using purified mica separates across the dome ages range from 236 ma at the southern edge to 160 ma in the core attesting to a strong imprint in the early triassic time a clear difference is seen between these mesozoic ages and the eocene to miocene ages from 40 to 24 ma that obtained in the nearby red river shear zone using the same method these data show that the song chay massif was already high in the crust when the high temperature deformation of the red river shear zone took place the final exhumation of the song chay orthogneiss constrained by fission track analysis on samples along the same transect occurred during the early miocene and could be interpreted as the consequence of a first normal sense of motion along the fault which bounds the massif to the south timing is similar to that of exhumation in the red river shear zone

quantifying subglacial bed roughness in antarctica implications for ice sheet dynamics and history abstract glaciated landscapes consist of complex assemblages of landforms resulting from ice flow dynamic regimes and ice sheet history superimposed over and in turn modifying preglacial topography lithology and geological structure insights into the formation of glaciated landscapes can in principle be obtained by analysing modern ice sheet beds but terrain analyses beneath modern ice sheets are restricted by the inaccessibility of the bed it is however possible to quantify roughness the vertical variation of the subglacial interface with horizontal distance along two dimensional images of the bed obtained from radio echo sounding res here we collate several case studies from antarctica where roughness calculations have been used as a glaciological tool to infer basal processes and ice sheet history over large 500 km 2 areas we present two examples from west antarctica which demonstrate the utility of bed roughness in determining the presence and extent of subglacial sediments glacial dynamics and former ice sheet size we also present two examples from east antarctica which illustrate how roughness provides knowledge of ice sheet dynamics in the interior and pre quaternary ice sheet histories in modern ice sheet settings characterising bed roughness along res tracks has the twin advantages of being relatively simple to calculate while producing informative subsurface data and is especially powerful at furthering understanding when coupled with knowledge of ice flow from field satellite and modelling investigations the technique also offers significant potential for the comparison of modern and former ice sheet terrains contributing to an improved understanding of the formation and evolution of glaciated landscapes

evidence of a hydrological connection between the ice divide and ice sheet margin in the aurora subglacial basin east antarctica subglacial hydrology in east antarctica is poorly understood yet may be critical to the manner in which ice flows data from a new regional airborne geophysical survey icecap have transformed our understanding of the topography and glaciology associated with the 287 000 km2 aurora subglacial basin in east antarctica using these data in conjunction with numerical ice sheet modeling we present a suite of analyses that demonstrate the potential of the 1000 km long basin as a route for subglacial water drainage from the ice sheet interior to the ice sheet margin we present results from our analysis of basal topography bed roughness and radar power reflectance and from our modeling of ice sheet flow and basal ice temperatures although no clear cut subglacial lakes are found within the aurora basin itself dozens of lake like reflectors are observed that in conjunction with other results reported here support the hypothesis that the basin acts as a pathway allowing discharge from subglacial lakes near the dome c ice divide to reach the coast via the totten glacier

thermochronological evidence for the movement of the ailao shan red river shear zone a perspective from vietnam in order to explore the timing of strike slip movement along the ailao shan red river shear zone an 40ar 39ar thermochronological study of the day nui con voi metamorphic massif in northern vietnam was undertaken the massif exposed in the southeastern segment of the ailao shan red river shear zone reveals a rapid cooling in the early miocene 25 21 ma after a very slow cooling ca 34 25 ma the slow cooling period most likely resulted from a geothermal perturbation in the lithosphere owing to the late paleogene 40 30 ma extension in eastern tibet and western yunnan china the rapid cooling consistent with evidence of a wider rapid cooling span from 27 to 17 ma compiled from the entire ailao shan red river shear zone constrains the duration of left lateral shearing which eventually offset indochina from south china by about 600 km the time constraints we have established reinforce the argument that the onset of the ailao shan red river shear zone postdates the opening of the south china sea that began ca 30 ma this result highlights the necessity for reevaluating the cenozoic tectonic models of southeast asia

ice cover landscape setting and geological framework of lake vostok east antarctica abstract lake vostok located beneath more than 4 km of ice in the middle of east antarctica is a unique subglacial habitat and may contain microorganisms with distinct adaptations to such an extreme environment melting and freezing at the base of the ice sheet which slowly flows across the lake controls the flux of water biota and sediment particles through the lake the influx of thermal energy however is limited to contributions from below thus the geological origin of lake vostok is a critical boundary condition for the subglacial ecosystem we present the first comprehensive maps of ice surface ice thickness and subglacial topography around lake vostok the ice flow across the lake and the landscape setting are closely linked to the geological origin of lake vostok our data show that lake vostok is located along a major geological boundary magnetic and gravity data are distinct east and west of the lake as is the roughness of the subglacial topography the physiographic setting of the lake has important consequences for the ice flow and thus the melting and freezing pattern and the lake s circulation lake vostok is a tectonically controlled subglacial lake the tectonic processes provided the space for a unique habitat and recent minor tectonic activity could have the potential to introduce small but significant amounts of thermal energy into the lake

kinematics topography shortening and extrusion in the india eurasia collision we examine the problem of partitioning between shortening and extrusion in the india asia collision since 45 ma we compute the amount of shortening expected from the kinematics of india s motion with respect to eurasia using the reconstruction at collision time to put bounds on the possible amounts of surface loss within greater india and within eurasia we then compute the amounts of surface loss corresponding to the thickened crust of tibet and of the himalayas assuming conservation of continental crust the spatial distribution of the topography reveals a large systematic deficit of crustal thickening distributed rather uniformly west of the eastern syntaxis but an excess of shortening east of it this distribution indicates an important eastward crustal mass transfer however the excess mass east of the eastern syntaxis does not account for more than one third to one half of the deficit west of the eastern syntaxis the deficit may be accounted either by loss of lower crust into the mantle for example through massive eclogitization or by lateral extrusion of nonthickened crust a mass budget of the crust of the himalayas indicates that lower crust has not been conserved there but the deficit is so large that local loss in the mantle is unlikely to be the unique cause of the deficit alternatively following zhao and morgan 1985 lower crust may have been transferred below the tibetan crust we conclude that a combination of possible transfer of lower crust to the mantle by eclogitization and lateral extrusion has to account for a minimum of one third and a maximum of one half of the total amount of shortening between india and asia since 45 ma this conclusion leaves open the possibility that the partitioning between extrusion and loss of lower crust into the mantle on the one hand and shortening on the other hand has significantly changed during the 45 m y history of the collision

partial melting of mantle wedge and evolution of island arc crust evolution of the crust in the northeast honshu arc is discussed on the basis of the melting experiments of a peridotite under hydrous conditions and the crustal compositions estimated from deep crustal xenoliths and seismic velocity data the chemical compositions of melts formed by partial melting of a hydrous peridotite within the pressure temperature conditions for shallow parts of the mantle wedge 1 2 2 0 gpa have been experimentally determined with the sandwich experiment technique the melts range from olivine tholeiitic to magnesian andesitic compositions sio2 48 56 wt mgo 9 14 wt beneath the volcanic front in the northeast honshu arc and are slightly less silicic beneath the coast of the sea of japan the crustal compositions of the volcanic zone in the northeast honshu arc estimated from deep seated crustal inclusions in volcanic rocks and the velocity structures of the crust are andesitic in the central zone and high alumina basaltic near the coast of the sea of japan the compositions of possible primary melts formed in the mantle wedge are significantly more mafic than these crustal compositions consequently if the major part of the crustal materials in the volcanic zone was formed by magmas generated in the mantle wedge thick up to 30 km ultramafic cumulates including olivine pyroxenite must have been formed beneath moho during fractional crystallization and would constitute the upper part of the mantle wedge the volume of the mantle wedge consisting entirely of even fertile peridotite was not sufficient to produce crustal materials and cumulates in the volcanic zone by such processes in the northeast honshu arc a static mantle wedge model is therefore not satisfactory for the growth of the crust in northeast honshu arc a dynamic model is suggested in which the mantle materials have been supplied into the mantle wedge from the back arc side by a convective flow

ice flow physical processes derived from the ers 1 high resolution map of the antarctica and greenland ice sheets accepted 1999 july 7 received 1999 june 7 in original form 1998 june 24 summary the ers 1 satellite launched in 1991 has provided altimetric observations of the greenland ice sheet and 80 per cent of the antarctica ice sheet north of 82 s it was placed in a geodetic 168 day repeat orbit between april 1994 and march 1995 yielding a 1 5 km across track spacing at latitude 70 with a higher along track sampling of 350 m we have analysed the waveform altimetric data from this period to compute maps with a 1 30 grid size data processing consists of correcting for environmental factors and editing and retracking the waveforms a further step consists of reducing the radial orbit error through crossover analysis and correcting the slope error to second order the high resolution topography of both ice sheets reveals numerous details a kilometre scale surface roughness running at 45 from the flow direction is the dominant topographic characteristic of both continents antarctica also exhibits many scars due to local flow anomalies several physical processes can be identified abrupt transitions from deformation to sliding and vice versa and impressive strike slip phenomena inducing en echelon folds

rapid cenozoic glaciation of antarctica induced by declining atmospheric co2 the sudden widespread glaciation of antarctica and the associated shift towards colder temperatures at the eocene oligocene boundary 34 million years ago refs 1 4 is one of the most fundamental reorganizations of global climate known in the geologic record the glaciation of antarctica has hitherto been thought to result from the tectonic opening of southern ocean gateways which enabled the formation of the antarctic circumpolar current and the subsequent thermal isolation of the antarctic continent5 here we simulate the glacial inception and early growth of the east antarctic ice sheet using a general circulation model with coupled components for atmosphere ocean ice sheet and sediment and which incorporates palaeogeography greenhouse gas changing orbital parameters and varying ocean heat transport in our model declining cenozoic co2 first leads to the formation of small highly dynamic ice caps on high antarctic plateaux at a later time a co2 threshold is crossed initiating ice sheet height mass balance feedbacks that cause the ice caps to expand rapidly with large orbital variations eventually coalescing into a continental scale east antarctic ice sheet according to our simulation the opening of southern ocean gateways plays a secondary role in this transition relative to co2 concentration

theoretical framework and diagnostic criteria for the identification of palaeo subglacial lakes abstract the antarctic ice sheet is underlain by numerous subglacial lakes which comprise a significant and active component of its hydrological network these lakes are widespread and occur at a range of scales under a variety of conditions at present much glaciological research is concerned with the role of modern subglacial lake systems in antarctica another approach to the exploration of subglacial lakes involves identification of the geological record of subglacial lakes that once existed beneath former ice sheets this is challenging both conceptually in identifying whether and where subglacial lakes may have formed and also distinguishing the signature of former subglacial lakes in the geological record in this work we provide a synthesis of subglacial lake types that have been identified or may theoretically exist beneath contemporary or palaeo ice sheets this includes a discussion of the formative mechanisms that could trigger onset of or drain subglacial lakes these concepts provide a framework for discussing the probability that subglacial lakes exist ed beneath other palaeo ice sheets indeed we conclude that the former mid latitude ice sheets are likely to have hosted subglacial lakes although the spatial distribution frequency and type of lakes may have differed from today s ice sheets and between palaeo ice sheets given this possibility we propose diagnostic criteria for identifying palaeo subglacial lakes in the geological record these criteria are derived from contemporary observations hydrological theory and process analogues and provide an observational template for detailed field investigations

origin of the adventure subglacial trench linked to cenozoic extension in the east antarctic craton abstract the antarctic plate occupies a unique geodynamic setting being mostly surrounded by divergent or transform margins major intracontinental basins and highlands characterize its bedrock buried under the 34 ma east antarctic ice sheet eais their formation atop of the cratonic lithosphere in the interior of east antarctica remains a major open question post mesozoic intraplate extensional tectonic activity has been proposed for their development and is supported by this work here we focus on the adventure subglacial trench ast whose origin is poorly constrained and controversial as currently available geophysical models suggest either extensional or compressional tectonic origin the ast is an over 250 km long 60 km wide subglacial trough elongated in the nnw sse direction adjacent to the westernmost flank of the wilkes subglacial basin and is parallel to regional scale alignments of magnetic and gravimetric anomalies geophysical campaigns allowed better definition of the ast physiography showing its typical half graben geometry the rounded morphology of the western flank of the ast was simulated through tectonic numerical modelling we consider the subglacial landscape to primarily reflect a preserved relict of the tectonic processes affecting the interior of east antarctica in the cenozoic due to the negligible erosion deposition capability of the eais the bedrock morphology was replicated through the activity of the listric adventure fault characterized by a basal detachment at the base of the crust 34 km and a vertical displacement of 2 5 km this length suggests its regional crustal importance the predicted displacement is interpreted either as a newly formed fault or as the partial reactivation of a weaker zone along a major precambrian crustal scale tectonic boundary the extensional regime in the ast is part of a more extensive 800 km long intraplate corridor characterized by nearly along strike extension in cenozoic times with a left lateral transpressional component this corridor may represent the effect of far field stresses induced by plate motions

using radar sounding data to identify the distribution and sources of subglacial water application to dome c east antarctica basal radar reflectivity is the most important measurement for the detection of subglacial water however dielectric loss in the overlying ice column complicates the determination of basal reflectivity dielectric attenuation is a function of ice temperature and impurity concentration temperature distribution is a function of climate history basal heat flow and vertical strain rate all of which can be partially inferred from the structure of dated internal layers using 11 dated layers isotope records from dome c east antarctica and a model of the spatial variation of geothermal flux we calculate the vertical strain rate and accumulation rate history allowing identification of areas where the basal melt rate exceeds 1 5 mm a 1 the accumulation rate history and vertical strain rates are then used as inputs for a transient temperature model the model outputs for the present day temperature distribution are then combined with depth dependent ionic concentrations to model dielectric loss and infer basal reflectivity the resulting reflection coefficients are consistent 5 db across a variety of subglacial water bodies we also identify a high reflectivity 15 db in concordia trench and along suspected subglacial water flow routes in vincennes basin highland areas tend to have highly variable reflection coefficients near 30 db consistent with an ice bedrock interface this combined model also identifies three areas of enhanced basal melting along concordia ridge concordia subglacial lake and vincennes basin melt at concordia subglacial lake exceeds 5 mm a 1 the inferred basal melt at these locations is not possible without enhanced geothermal flux we demonstrate how radar sounding data can provide both input and verification for a self consistent model of vertical strain vertical temperature distribution and meltwater distribution

origin and fate of lake vostok water frozen to the base of the east antarctic ice sheet the subglacial lake vostok may be a unique reservoir of genetic material and it may contain organisms with distinct adaptations but it has yet to be explored directly the lake and the overlying ice sheet are closely linked as the ice sheet thickness drives the lake circulation while melting and freezing at the ice sheet base will control the flux of water biota and sediment through the lake here we present a reconstruction of the ice flow trajectories for the vostok core site using ice penetrating radar data and global positioning system gps measurements of surface ice velocity we find that the ice sheet has a significant along lake flow component persistent since the last glacial maximum the rates at which ice is frozen accreted to the base of the ice sheet are greatest at the shorelines and the accreted ice layer is subsequently transported out of the lake using these new flow field and velocity measurements we estimate the time for ice to traverse lake vostok to be 16 000 20 000 years we infer that most vostok ice analysed to date was accreted to the ice sheet close to the western shoreline and is therefore not representative of open lake conditions from the amount of accreted lake water we estimate to be exported along the southern shoreline the lake water residence time is about 13 300 years

the dimensions and topographic setting of antarctic subglacial lakes and implications for large scale water storage beneath continental ice sheets radio echo sounding res at 60 mhz has penetrated to the ice sheet bed on about 400 000 km of flight track covering more than 50 of the 13 5 x 106 km2 antarctic ice sheet about 70 subglacial lakes have been identified by characteristic strong mirror like and very flat res reflectors most are located in the ice sheet interior where numerical modeling predicts basal melting and 33 are within 100 km of the ice crest mean ice thickness above the lakes is about 3000 m about 75 of lakes have radio echo lengths of 30 km the largest is vostok lake which is 230 km long 14 000 km2 in area and has a water volume of about 2000 km3 more than 60 of lakes have maximum local bedrock elevations of 400 m and bed gradients of 0 1 adjacent to their margins extrapolating the observed side slope topography implies water depths from tens to several hundred meters in a few cases about 20 000 km2 of subglacial lakes are inferred from res which can be doubled for the entire ice sheet to yield an estimated total area of about 40 000 km2 excluding vostok lake assuming average lake depths between 50 and 250 m and including the 2000 km3 of water stored within vostok lake we calculate an envelope of about 4000 to 12 000 km3 for the likely volume of water stored in modern antarctic subglacial lakes equivalent to 10 35 mm of global sea level rise in these volume calculations it is assumed that no subglacial lakes the size of lake vostok remain to be discovered and that res length measurements along single transects are representative of overall lake dimensions our estimates of the total water volume of antarctic subglacial lakes which may be a suitable analogue for full glacial quaternary ice sheets do not necessarily conflict with the possible significance of large meltwater floods during the deglaciation of former midlatitude ice sheets

hairpin river loops and slip sense inversion on southeast asian strike slip faults in the golden triangle region of southeast asia northern thailand laos and burma southern yunnan the mekong salween and neighboring rivers show hairpin geometries where they cross active strike slip faults restoration of young left lateral offsets of these rivers leaves residual right lateral bends of many kilometers we interpret these hairpins as evidence of late cenozoic slip sense inversion on these faults about 5 to 20 ma near the red river fault stress field and slip sense inversion occurred ca 5 ma this implies that the present course of these large rivers has existed for at least several million years pliocene quaternary slip rates possibly on the order of 1 mm yr are inferred on each of the strike slip faults of the golden triangle

duration of strike slip movements in large shear zones the red river belt china abstract sixty four size fractions of zircon monazite xenotime and titanite were dated by the u pb method to temporally and spatially constrain left lateral movements in the red river shear zone between the depths of 15 and 20 km the shearing led to the formation of the high grade metamorphic gneisses that form the bulk of the 4200 m high diancang shan and 3100 m high ailao shan mountain ranges the omnipresence of melt layers and pockets in these gneisses shows that temperatures associated with tectonometamorphism lie at and above the solidus of continental crust and deformation patterns are very homogeneous over the entire length of the belt north of the gneisses subvolcanic bodies intrude unmetamorphosed cover rocks in particular eocene red beds along the projected trend of the red river shear zone for a leucogranitic layer in the diancang shan gneisses an age of 24 2 0 2 ma 2 is obtained for monazite and 22 4 0 2 ma is obtained for xenotime whereas zircon formed through two growth stages at 24 23 ma these ages record solidification of the anatectic melts over a period of at least 1 8 0 4 ma during which left lateral shear was continuously active in the ailao shan gneisses zircon and titanite ages are 26 3 0 3 and 26 1 0 2 ma for two monzonitic bodies and 24 1 0 2 and 22 4 0 2 ma for two pegmatitic layers that occur within the monzonitic intrusions these results show that zircon monazite titanite and probably also xenotime are closed systems for u and pb under melting conditions 650 c therefore the ages date mineral formation in the melt rather than cooling below certain closure temperatures a significantly older age of 35 0 0 1 ma is obtained for a monzonitic subvolcanic intrusion north of the diancang shan gneisses because these intrusions follow the trend of the shear zone they most likely represent the surface expression of early activity along that zone indicating that strike slip movement had already occurred in latest eocene time the total set of u pb ages shows that magmatic and metamorphic activity occurred from 35 to 20 ma strengthening the idea that the red river shear zone absorbed most of the intraplate movements required to compensate the opening of the south china sea between 32 and 16 ma the existence of monzonitic syenitic intrusions along the red river zone implies that melting affected source rocks other than the diancang shan and ailao shan gneiss protoliths potential sources to produce such rocks lie in the deep crust possibly including melts from the mantle consequently the red river shear zone must be a deep rooted structure along which melts migrated to the surface moreover the contemporaneity of shear motion and magma generation suggests that melting occurred in direct relation to strike slip activity

crustal contamination of late neogene basalts in the dien bien phu basin nw vietnam some insights from petrological and geochronological studies early pliocene zanclean basalts in the dien bien phu pull apart basin in nw vietnam associated with the presently sinistral dien bien phu fault zone have been dated by the k ar method at 4 4 4 9 and 5 4 5 2 ma rapid migration of basaltic magma to the surface in the dien bien phu fault zone may be due to pliocene transtension of the crust in this region resulting from asthenospheric upwelling induced by lateral displacement of the mantle the basalts are moderately phyric 10 and consist of olivine hyalosiderite plagioclase bytownite labradorite and orthopyroxene bytownite labradorite phenocrysts and a fine grained crystalline matrix olivine hortonolite plagioclase labradorite clinopyroxene pigeonite and augite k feldspar the presence of fe rich olivine and orthopyroxene phenocrysts indicates that the basalts are sio2 saturated oversaturated olivine tholeiites which formed under water undersaturated conditions the dien bien phu basalts contain both mantle derived pyroxenites dunites gabbros and crustal sillimanite mullite mg fe spinel wallrock xenoliths indicative of crustal contamination during the ascent of the basaltic magma the basalts show selective enrichment in some mobile elements k rb sr and th a feature considered to be a result of metasomatism these rocks classified on the basis of their normative composition as quartz tholeiites could represent primary olivine tholeiites basalts in which the geochemical signatures were modified by the processes of contamination 2006 elsevier ltd all rights reserved

assessment of electromagnetic absorption of ice from ice core measurements ice core drillings have been performed in various zones in antarctica and greenland to obtain climatological information study ice properties or analyze air and dust encapsulated in the ice during the quaternary period during these procedures a set of measurements to characterize the ice and to evaluate its physical and chemical properties are usually performed in situ in particular using known temperature and dielectric profiles dep measurements it is possible to evaluate the ice electromagnetic power absorption profile valid at the drilling site in the last decades bedrock characterization through radio echo sounding surveys has been improved by the analysis of the power of radar echoes this way analysis of the electromagnetic properties of bedrock interfaces makes it possible to assess the physical characteristics and to distinguish between wet and dry conditions power variation of the received echoes also depends on ice absorption and on bedrock reflectivity due to specific physical conditions of the ice in this paper the propagation of electromagnetic waves through the ice sheet is examined and in particular a new method for establishing the electromagnetic absorption profile for ice from core drilling measurements is proposed and discussed variation in the ice absorption is deduced starting from the analysis of ice core data from the european project for ice coring in antarctica epica at the concordia station antarctica and from the greenland ice core project grip site greenland this direct method of measurement is proposed with the aim of defining common characteristics of the ice absorption rate that are valid both in antarctica and in greenland

cenozoic extension along the reactivated aurora fault system in the east antarctic craton abstract the east antarctic craton is characterized by major intracontinental basins and highlands buried under the 34 ma east antarctic ice sheet their formation remains a major open question paleozoic to cenozoic intraplate extensional tectonic activity has been proposed for their development and in this work the latter hypothesis is supported here we focus on the aurora trench at within the aurora subglacial basin latitude 75 77 s longitude 117 118 e whose origin is still poorly constrained the at is an over 150 km long 25 km wide subglacial trough elongated in the nnw sse direction geophysical campaigns allowed better definition of the at physiography showing typical half graben geometry the rounded morphology of the western flank of the at was simulated through tectonic numerical modelling we consider the subglacial landscape to primarily reflect the locally preserved relict morphology of the tectonic processes affecting the interior of east antarctica in the cenozoic the bedrock morphology was replicated through the activity of the listric aurora trench fault characterized by a basal detachment at 34 km considered the base of the crust according to available geophysical interpretations and vertical displacements ranging between 700 and 300 m the predicted displacement is interpreted as the partial reactivation of a weaker zone along a major precambrian crustal scale tectonic boundary we propose that the aurora trench fault is the southern continuation of the 1000 km long aurora fault independently recognized by previous studies together they form the aurora fault system a long lived tectonic boundary with poly phased tectonic history within the eac that bounds the eastern side of the aurora subglacial basin the younger cenozoic reactivation of the investigated segment of the aurora fault system relates to the intraplate propagation of far field stresses associated to the plate scale kinematics in the southern ocean

bedrock features and ice flow near the epica ice core site dome c antarctica the precise bedrock topography near the epica ice core site at dome c in antarctica exhibits impressive bedrock mountains bowls and a clear network of elongated valleys some of these features are probably tectonic and are likely to be buried pre ice sheet landforms the elongated bedrock valleys are clearly detectable in the surface topography suggesting a large scale network the analysis of the relationship between basal and surface elongated structures indicates a damping factor decreasing from 120 near the dome to 80 at few tens of kilometer there is evidence of presence of subglacial water and of erosed bed that can play a role of the ice flow a complete flow modeling taking into account longitudinal stresses should be performed around epica site in order to improve dating of the ice

spatial variations in heat at the base of the antarctic ice sheet from analysis of the thermal regime above subglacial lakes antarctic subglacial lakes provide an important boundary condition for thermal analysis of the ice sheet in that the basal ice temperature over lakes may be assumed to be at the pressure melting point we have used a one dimensional vertical heat transfer equation to determine theoretical temperature values for the ice sheet base above 77 subglacial lakes identified from airborne radio echo sounding data covering 50 of antarctica variations in our temperature results to below the pressure melting temperature over lakes are due to either our estimate of the geothermal heat flux or a neglect of heat derived from a internal ice deformation and b basal sliding in the thermal model our results indicate that when the geothermal heat flux is set at 54 mw m 2 the ice sheet base above 70 of the known antarctic subglacial lakes is calculated to be at the pressure melting value these lakes are located mainly around dome c ridge b and vostok station for the ice sheet above subglacial lakes located hundreds of kilometres from the ice divide using the same thermal model loss of heat due to vertical advection is calculated to be relatively high in such regions if the ice sheet base is at the pressure melting point heat lost due to vertical advection must be supplemented by heat from other sources for the three lakes beneath terre adelie and george v land for instance the basal thermal gradient calculated to produce pressure melting at the ice sheet base is equivalent to 1 5 2 times the value obtained when 54 mw m 2 of geothermal heat is used as the sole basal thermal component we suggest that as distance from the ice divide increases so too does the amount of heat due to internal ice deformation and basal sliding moreover by considering the ice sheet basal thermal characteristics above subglacial lakes which lie on the same ice flowline we demonstrate empirically that the heat due to these horizontal ice motion terms varies pseudo exponentially with distance from the ice divide the location along a flowline where a rapid increase in the basal heat gradient is calculated may correspond to the onset of large scale basal sliding

zircon response to diamond pressure metamorphism in the kokchetav massif ussr ultrahigh pressure metamorphism of the diamond bearing gneisses in the kokchetav massif ussr has been dated at 530 7 ma 2 sgr by shrimp ion microprobe analysis of metamorphic zircons zircon xenocrysts as old as 2000 ma have retained original isotopic compositions through the extreme metamorphic conditions and indicate that the protolith that was buried to diamond forming pressures included early proterozoic components highly variable reset ting of u pb isotopic systems in zircon xenocrysts at the time of metamorphism suggests that diffusive lead loss had commenced but was incomplete a disequilibrium observation consistent with relatively transient exposure of the rocks to extreme crustal depths

electromagnetic ice absorption rate at dome c antarctica radio echo sounding res is a radar technique widely employed in antarctica and greenland to define bedrock topography but over the last decade it has also played an important role in subglacial lake exploration and hydrogeological studies at the bedrock ice interface in recent studies bedrock characterization has been improved through analysis of radar power echoes to evaluate the electromagnetic em properties of the interface and allow the distinction between wet and dry interfaces the res received signal power depends on ice absorption and bedrock reflectivity which is closely linked to the specific physical condition of the bedrock in this paper an evaluation of em ice absorption was conducted starting from res measurements collected over subglacial lakes in antarctica the idea was to calculate ice absorption starting from the radar equation in the case of subglacial lakes where the em reflectivity value is considered a known constant these values were compared with those obtained from analysis of ice core dielectric profiles from epica ice core drilling data our analysis reveals that the ice absorption rate calculated from res measurements has an average value of 7 2dbkm 1 and it appears constant independent of the subglacial lake depth in different zones of the dome c area

the early triassic indosinian orogeny in vietnam truong son belt and kontum massif implications for the geodynamic evolution of indochina new structural field data at various scale and 40ar 39ar geochronological results from the basement rocks in the truong son belt and kontum massif of vietnam confirm that ductile deformation and high temperature metamorphism were caused by the early triassic event of the indosinian orogeny in the range of 250 240 ma a compilation of isotopic data obtained in other countries along the sibumasu indochina boundary broadly indicates same interval of ages this tectonothermal event is interpreted as the result of a synchronous oblique collision of indochina with both sibumasu and south china inducing dextral and sinistral shearing along e w to nw se and n s fault zones respectively the collision along song ma follows the northwards subduction of indochina beneath south china and the subsequent development of the song da zone which in turn was affected by the late triassic indosinian phase of shortening within the indochina plate internal collisions occurred coevally in the early triassic as along the poko suture at the western border of the kontum massif

antarctic subglacial lakes abstract antarctic subglacial lakes were first identified by robin et al 1970 after airborne radio echo sounding res investigations of the ice sheet interior recently satellite altimetry was used to measure anomalous near flat regions on the ice sheet surface that represent a manifestation of the subglacial lake beneath using res and satellite altimetry the location and extent of antarctic subglacial lakes can be identified the largest subglacial lake exists beneath vostok station and is 14 000 km2 in area the combined area of additional subglacial lakes beneath dome c is 15 000 km2 and at least 15 000 km2 of lake surface lies beneath the remainder of the ice sheet the water depth of subglacial lakes can be estimated through seismic investigations although data exist only for lake vostok and consideration of the bedrock slopes that border subglacial lakes the depths of many subglacial lakes are of the order of 10 s 100 s of metres the total volume of water held beneath the ice sheet is estimated between 4000 and 12 000 km3 to date there are six known examples of radio echo reflections from the lake floors at a depth of no more than 20 m since e m attenuation through water is related to the salinity these data indicate that subglacial water is very pure and fresh some near flat surface regions that usually occur over lakes have been observed where no lakes exist such features are may be caused by water saturated basal sediments rather than subglacial lakes finally the spatial variation in geothermal heat flux around the central regions of antarctica can be established estimated by employing a simple thermal model of the ice sheet under an assumption that the basal ice temperature above subglacial lakes is equal to the pressure melting value calculations indicate that the geothermal heat flux varies spatially over the antarctic plate between 37 and 65 mw m 2

the difference between oceanic and continental tholeiites a fluid dynamic explanation continental tholeiites have higher sio2 k2o and light rare earth element contents and more evolved isotopic characteristics than their oceanic counterparts these differences can be explained if the compositions of the parent magmas to both types of tholeiites are similar but if continental magmas assimilate significant amounts of continental crust en route to the surface although there is little doubt that most continental tholeiites have assimilated crustal material the lcoation and mechanism of assimilation remain uncertain longhi 1981 has argued that magmas derived directly from the mantle should crystallize little orthopyroxene the abundance of orthopyroxene in most continental layered intrusions suggests that they have crystallized from magmas which have assimilated continental crust since orthopyroxene is an early crystallizing phase in layered intrusions this assimilation must occur early before the magma enters the chamber assimilation can occur at the margins of the dykes which feed magma chambers depending on the nature of the flow if the flow is turbulent the high temperatures at the centre of the dyke will extend to the margins and the magma will erode the dyke walls if the flow is laminar a conductive profile develops at the margin and the flowing magma chills against the walls protecting them from thermal erosion the nature of flow in a dyke can be predicted from the reynolds number the criteria for turbulence reynolds number calculations suggest that the flow of primitive magmas in continental dykes will be fully turbulent and if this is the case assimilation of low melting point components in the walls of the dyke is inevitable it is therefore suggested that many of the geochemical characteristics of continental tholeiites result from melting at the walls of dykes as primitive magmas ascend through the crust

physiography and tectonic setting of the subglacial lake district between vostok and belgica subglacial highlands antarctica summary we present the interpretation of 11 radio echo sounding res missions carried out over the vostok dome concordia region during the italian antarctic expeditions in the period 1995 2001 the extension and the density of the radar data in the surveyed area allowed to reconstruct a reliable subglacial morphology and to identify four relevant morphological structures namely the aurora trench the concordia trench the concordia ridge and the south hills these structures show evidence compatible with the presence of tectonic features morphological considerations indicate their development in cenozoic time hybrid cellular automata hca based numerical modelling allowed to justify a possible role played by the tectonics of the aurora and concordia trench evolution this was accomplished by matching the bed profiles along opportunely projected sections with the modelled surfaces as derived by the activity of normal faults with variable surfaces within the continental crust the vostok dome c region is characterized by a large number of subglacial lakes from the analysis of basal reflected power echo we identified 14 new lakes and obtained information about their physiography as well as their possible relations with tectonics we propose a grouping of subglacial lakes on the base of their physiography and geological setting namely relief lakes basin lakes and trench lakes relief lakes located in the belgica subglacial highlands and are characterized by sharp and steep symmetric edges suggesting a maximum water depth of the order of 100 m their origin may well relate to localized positive geothermal flux anomalies basin lakes located in the vincennes subglacial basin and are characterized by wider dimension that allow the development of well defined flat ice surface anomalies trench lakes characterize the aurora and concordia trenches as the possible effect of normal fault activity

subglacial lake cecs discovery and in situ survey of a privileged research site in west antarctica we report the discovery and on the ground radar mapping of a subglacial lake in antarctica that we have named lake cecs centro de estudios cientificos in honor of the institute we belong to it is located in the central part of the west antarctic ice sheet right underneath the institute ice stream and minnesota glacier ice divide and has not experienced surface elevation changes during the last 10 years the ratio between the area of the subglacial lake and that of its feeding basin is larger than those for either subglacial lakes ellsworth or whillans and it has a depth comparable to that of ellsworth and greater than that of whillans its ice thickness is 600 m less than that over ellsworth the lake is very likely a system with long water residence time the recent finding of microbial life in lake whillans emphasizes the potential of subglacial lake cecs for biological exploration

basal roughness of the east antarctic ice sheet in relation to flow speed and basal thermal state basal motion of ice sheets depends in part on the roughness and material properties of the subglacial bed and the occurrence of water to date basal motion represents one of the largest uncertainties in ice flow models it is that component of the total flow velocity that can change most rapidly and can therefore facilitate rapid variations in dynamic behaviour in this study we investigate the subglacial properties of the east antarctic ice sheet by statistically analysing the roughness of the bed topography inferred from radio echo sounding measurements we analyse two sets of roughness parameters one derived in the spatial and the other in the spectral domain with two roughness parameters each this enables us to compare the suitability of the four roughness parameters to classify the subglacial landscapes below the ice sheet we further investigate the relationship of the roughness parameters with observed surface flow velocity and modelled basal temperatures of the ice sheet we find that one of the roughness parameters the hurst exponent derived in the spatial domain coincides with the thermal condition at the base of the ice sheet for slow flow velocities and varies with flow velocity

airborne radar survey above vostok region east central antarctica ice thickness and lake vostok geometry during the 1999 2000 italian expedition an airborne radar survey was performed along 12 transects across lake vostok antarctica and its western and eastern margins ice thickness subglacial elevation and the precise location of lake boundaries were determined radar data confirm the geometry derived from previous surveys but with some slight differences we measured a length of up to 260 km a maximum width of 81 km and an area of roughly 14 000 km 2 along the major axis from north to south the ice thickness varies from 3800 to 4250 m with a decreasing gradient from west to east the ice thickness is fairly constant except for two narrow strips located on the western and eastern margins where it increases with high thickening rate over the lake the surface elevation increases from 3476 m a s l south to 3525 north with a decreasing gradient while the lake surface elevation decreases from 315 to 750 m a s l with a decreasing gradient absolute value the ice surface and lake ceiling slopes suggest that the lake is in a state of hydrostatic equilibrium

a little spin in the indian ocean plate circuit abstract divergence of the australian and east antarctic plates is well understood from the late jurassic onset of half graben development on the australian continental shelf and from post mid eocene chron 20 45 ma seafloor spreading isochrons further offshore relative plate motion between these times is less confidently interpretable from magnetic reversal anomalies landwards of isochron 20 and localised evidence for mid to late cretaceous subsidence and growth strata from the continental shelf and rise south of australia a new test of this history examines it within the post 34y 84 ma indian ocean plate circuit built using seafloor spreading data from the wharton and central indian ocean basins the australian antarctic jurassic onset rift system is interpreted to have been abandoned before 34y because motion in the circuit is inconsistent with an active plate boundary during 34y 26y 84 58 ma starting 26y the model depicts plate divergence distances and azimuths that after 25y 57 ma can be independently confirmed by reassessment of the pre chron 20 magnetic reversal anomaly pattern previous studies have identified evidence for mantle exhumation and focused magmatism in basement marginwards of these anomalies these processes are not directly or confidently dated but mantle exhumation is inconsistent with the circuit model s fast plate divergence at 26y 25y hence plate motion during the immediate build up to post 57 ma seafloor spreading may have been accommodated by focused magmatism whilst mantle exhumation may mark the conclusion of the jurassic onset rift phase during a slower pre 84 ma period of plate divergence using the new model to make tectonic reconstructions results in a large overlap between tasmania and victoria land that can be explained with reference to eocene strike slip faulting and transtension in recently discovered subglacial basins of wilkes and george v lands and terre adelie

paleotethyan evolution of the indochina block as deduced from granites in northern laos abstract the paleotethyan evolution of the southeast asia has become better understood in recent years questions remain however over the role of the dien bien phu suture zone in the evolution of the indochina block and whether the song ma suture represents the boundary between the indochina block and the south china block granitoid geochronological and geochemical data obtained in northern laos provide new information vis a vis these arguments zircon u pb ages together with whole rock trace and rare earth element data from 27 granitic rocks from five complexes allow us to conclude that these granites are typical of i type indosinian volcanic arc granites however the 234 256 ma i type granites mismatch the initiation age obtained from the ductile shear zone of the dien bien phu fault thus repudiating the existence of the dien bien phu suture zone this then implies that the qamdo simao and indochina blocks were united the geochemical and geochronological data further suggest that the main crust in the indochina block formed in the late paleoproterozoic to early mesoproterozoic much later than the archean crustal formation age identified east of the song ma suture moreover the 440 404 ma and 234 256 ma i type granites suggest that the boundary between indochina and south china should be the jinsha river suture song ma suture kontum massif instead of the jinsha river suture song chay suture finally the emeishan basalt and granite complexes both form part of the south china tectonic units subducting westward under the qamdo simao and indochina blocks

self determining numerical modeling of compressional fault bend folding we present the application of the hca hybrid cellular automata numerical modeling technique to the evolution of compressional fault bend anticlines involving layers that have competency contrasts and a mechanical stratigraphy the new modeling results show that for competent hanging wall layers 1 the fault style is not preserved in the overlying fold 2 fold development is not self similar and rotational growth wedges develop in the forelimb when syntectonic sedimentation occurs and 3 balanced fault bend anticlines can develop for ramp cutoff angles as high as 60 these results compare favorably with field examples and laboratory analogues and dramatically broaden the applicability of fault bend folding to model the evolution of thrust related anticlines

the supply of subglacial meltwater to the grounding line of the siple coast west antarctica recent satellite studies have shown that active subglacial lakes exist under the antarctic ice streams and persist almost to their grounding lines when the lowest lying lakes flood the water crosses the grounding line and enters the sub ice shelf cavity modeling results suggest that this additional freshwater influx may significantly enhance melting at the ice shelf base we examine the spatial and temporal variability in subglacial water supply to the grounding lines of the siple coast ice streams by combining estimates for lake volume change derived from ice cloud and land elevation satellite icesat data with a model for subglacial water transport our results suggest that subglacial outflow tends to concentrate towards six embayments in the siple coast grounding line although mean grounding line outflow is 60m3 s 1 for the entire siple coast maximum local grounding line outflow may temporarily exceed 300 m3 s 1 during the synchronized flooding of multiple lakes in a hydrologic basin variability in subglacial outflow due to subglacial lake drainage may account for a substantial portion of the observed variability in freshwater flux out of the ross ice shelf cavity the temporal variability in grounding line outflow results in a net reduction in long term average melt rate but temporary peak melting rates may exceed the long term average by a factor of three

advances in modelling subglacial lakes and their interaction with the antarctic ice sheet subglacial lakes have long been considered hydraulically isolated water bodies underneath ice sheets this view changed radically with the advent of repeat pass satellite altimetry and the discover

penetration of antarctic subglacial lakes by vhf electromagnetic pulses information on the depth and electrical conductivity of basal water bodies owing to the high level of absorption of very high frequency radio waves in water previous investigators of airborne radio echo sounding res data from antarctica have assumed that the depth of subglacial lakes cannot be measured directly by this method however we have identified a number of res returns from beneath the ice water interface at the surface of eight subglacial lakes that we have interpreted as being reflected from the lake floor the returns allow us to measure the depth of subglacial lakes since the velocity of radio waves in water 33 4 m s 1 is relatively unaffected by electrical conductivity attenuation of radio waves within water is controlled largely by its electrical conductivity consequently by examining the decay of the radio wave amplitude with depth we can gain information about the conductivity of subglacial water bodies our results indicate that the minimum water depths of eight subglacial lakes vary between 8 and 21 m the lakes from which our depth measurements were taken are distributed widely around the ice sheet thus it may be concluded for the first time that antarctic subglacial water bodies are generally at least several meters in depth by examining the attenuation of radio waves through subglacial water the electrical conductivity of the water is estimated to be extremely low i e fresh pure water

geological setting of the concordia trench lake system in east antarctica summary this study presents the interpretation of radio echo sounding res data collected during the 2003 geophysical campaign of pnra italian national research project in antarctica which focused on the exploration of the concordia trench lake system in east antarctica the data allow us to identify a new lake itl 28 at the southern edge of the concordia trench and a series of n s trending subglacial troughs cutting through the belgica highlands we have mapped the bedrock morphology at 3 km resolution which led to an improved geographical and geomorphological characterization of the concordia trench concordia ridge concordia lake and south hills improved knowledge of the concordia trench allowed us to model the 3 d geometry of the concordia fault suggesting that it played a role in governing the morpho tectonic evolution of the bedrock in the dome c region and to propose a cenozoic age for its activity we recognize the importance of catchment basin morphology in hosting subglacial lakes and discuss the role played by tectonics glacial scouring and volcanism in the origin of the trench lakes basin lakes and relief lakes respectively

refraction correction for radio echo sounding of ice overlain by firn expression du trajet du signal radar permettant d eviter les 10 d erreurs obtenues si on assimile le neve a la glace

petrogenesis and tectonic significance of a mesozoic granite syenite gabbro association from inland south china article i nfo a granite syenite gabbro association from inland south china has been studied for zircon u pb ages and hf isotopic compositions as well as whole rock elemental and sr nd isotopic compositions to constrain their sources and tectonic settings these rocks have distinctive crystallization ages the longyuanba biotite granites have a mean age of 240 ma the longyuanba two mica granites pitou alkali feldspar granites and amphibole bearing alkali feldspar granites tabei and huangbu syenites and chebu and chenglong gabbros have a similar age of 178 ma whereas the dafengnao syenite gives a mean age of 165 ma the longyuanba biotite granites and two mica granites exhibit s type characteristics probably derived from a neoproterozoic crustal source the chenglong gabbro has oib like trace element and highly depleted isotope compositions ehf t 10 0 1 3 end t 5 2 suggesting its parental melt to be of asthenospheric origin with insignificant crustal assimilation however crustal contamination is required to explain the isotopic compositions of the chebu gabbro the syenites are shoshonitic in composition and have depleted sr nd hf isotopic signatures which we interpret to have resulted from a mixed source of asthenospheric mantle and metasomatized lithospheric mantle as expected the signals of crustal assimilation are conspicuous in the petrogenesis of the more evolved syenites the pitou alkali feldspar granite and amphibole bearing alkali feldspar granite exhibit i type and a type characteristics respectively they are isotopically more depleted than the s type longyuanba granites we suggest that they may have formed through magma mixing of predominantly mantle derived melts with the neoproterozoic crust the jurassic granite syenite gabbro association was the product of asthenosphere lithosphere crust interactions which records the primary role of asthenospheric mantle upwelling in magma generation both in the mantle and in the crust in the early yanshanian time in south china they probably occurred in an intraplate rift like environment as a tectonic response to far field stress at plate margins during the early stage of the paleo pacific plate subduction the tectonic transition from the tethys orogenic regime to the paleo pacific regime was accomplished in the early jurassic and the early yanshanian magmatism should be genetically associated with the paleo pacific tectonic regime

dry wet bedrock interface detection by radio echo sounding measurements in this paper a method to distinguish a wet or dry bedrock ice interface is proposed it is based on the analysis of radio echo sounding res measurements a widely employed method for determining bedrock topography in antarctica in particular the res system has played an important role in subglacial lake exploration and hydrogeological studies at the bedrock ice interface recently bedrock characterization has been improved through the analysis of the power of radar echoes signal power depends on bedrock reflectivity and its specific physical condition in this paper a linear model describing the loss term internal ice absorption is proposed this model together with other known quantities contributes toward an assessment of power variation of bedrock reflectivity in order to determinate wet and dry bedrock interfaces in the dome c region in antarctica

oligo miocene midcrustal subhorizontal shear zone in indochina new structural observations of oligo miocene deformation in north vietnam along the red river shear zone suggest that left lateral strike slip shear was restricted to the upper and middle crust above a horizontal shear zone left lateral shear deformation is associated with low pressure low temperature parageneses high temperature deformation is restricted to zones where the foliation has a low dip in the core of the dai nui con voi antiformal dome these observations complete those made earlier in the bu khang extensional dome farther south above the horizontal shear zone left lateral transpression was active during the first stage and changed to transtension some 33 myr ago purely extensional metamorphic domes bu khang or transtensional domes dai nui con voi were exhumed during this younger stage our observations plead for caution when interpreting strike slip structures at lithospheric scale