structural and geochronological constraints on the early mesozoic north longmen shan thrust belt foreland fold thrust propagation of the sw qinling orogenic belt northeastern tibetan plateau

evidence for normal and deep buried features of the longquan shan fault zone at the eastern margin of the tibetan plateau abstract the longquan shan fault zone lqsf locates in the sichuan basin of china parallel to the well known longmen shan fault zone different models for the lqsf indicate different understanding of the geological evolution of the tibetan plateau in this paper we provide evidences which demonstrate that the lqsf is a normal and deep buried fault zone firstly we find distinct extensional strains around the lqsf based on global positioning system gps observations we also find that the downward gps velocities across the lqsf decrease suddenly from 4 to 5 mm yr at the west to roughly zero at the east secondly we collate focal mechanism solutions to earthquakes of m 3 0 in the sichuan basin and find that 12 out of 21 earthquakes are normal earthquakes the surrounding extensional strains and the high proportion of normal earthquakes indicate that the shallow part of the lqsf is normal at present thirdly based on the migration section obtained from deep seismic reflection profiling we outline the geometric structure of the lqsf it is a deep buried fault zone that reaches the moho with two branches at the top such a fault model corresponds with the realities of the low seismic activities around the longquan shan anticline and the scattered earthquakes beneath the chengdu plain finally we propose a three stage evolutional model about the longquan shan fault system which includes the formation of anticline in late triassic the uplift of the mountain in oligence and the stage of normal characteristics of the fault zone from late miocene

physical connectivity between pulley ridge and dry tortugas coral reefs under the influence of the loop current florida current system abstract the pulley ridge and dry tortugas coral reefs are among the most pristine but also fragile marine ecosystems of the continental united states understanding connectivity processes between them and with surrounding shelf and deep areas is fundamental for their management this study focuses on the physical processes related to the connectivity of these reefs unprecedented in situ time series were used at these specific reef locations together with satellite observations and numerical simulations to investigate the dynamics controlling local circulation on the southwestern florida shelf swfs under oceanic influence the approach of the loop current and florida current lc fc system to the swfs slope can induce 0 5 to 1 m s offshore flows impacting the pulley ridge and dry tortugas reefs on the other hand when the lc fc system retreats from the slope onshore flows can carry open sea waters over the coral reefs local formation of cyclonic eddies is possible near the dry tortugas reefs in the lc approach case and passage of upstream lc frontal eddies is possible in the lc retreat case offshore currents 1 m s over the swfs slope were also found during periods of anticyclonic lc eddy separation a novel finding is the shedding and northward propagation of mesoscale anticyclonic eddies from the core of the lc along the west florida shelf eddy shedding may have a broader effect on the dynamics of the shelf around the study reef areas long periods of lc fc domination over these coral reefs reaching several weeks to months are characterized by strong 1 m s along shelf currents and continuous upwelling processes which may weaken the slope stratification and bring colder deeper waters over the shelf break and toward the shallower shelf region

seismic structure of the longmenshan area in sw china inferred from receiver function analysis implications for future large earthquakes abstract following the 2008 mw 7 9 wenchuan and 2013 ms 7 0 lushan earthquakes the longmenshan thrust fault belt and the songpan ganzi terrane have been the focus of several investigations here we use the h k stacking technique and neighborhood algorithm to investigate the seismic structure of this area based on the presence of felsic lower crust and the mesozoic crustal architecture of the songpan ganzi and longmenshan area we exclude the model on the eastward flow of the middle and lower crust assigned as the cause for the crustal thickening in previous studies in contrast the e w trending cumulative compression induced by the continued northward motion of the indian plate and india asian collision are identified as the dominant factors leading to the crustal thickening in the longmenshan thrust fault region as well as the songpan ganzi terrane particularly the 2008 mw 7 9 wenchuan and 2013 ms 7 0 lushan earthquakes likely indicate the cumulative offsets and the e w continuing compression

propagation of the deformation and growth of the tibetan himalayan orogen a review abstract long standing problems in the geological evolution of the tibetan himalayan orogen include where the india asia convergence was accommodated and how the plateau grew to clarify these problems we review the deformations and their role in the plateau s growth our results show that 1630 km of shortening occurred across the tibetan himalayan orogen since 55 ma with more than 1400 km accommodated by large scale thrust belts these thrust belts display an outward expansion from central tibet and couple with the surficial uplift the development of the tibetan plateau involved three significant steps primitive plateau 90 55 ma proto plateau 55 40 ma and neoteric plateau 40 0 ma several processes have collaborated to produce the proto plateau including the pre existing primitive plateau the india asia collision and subductions of greater india and songpan ganzi beneath the lhasa qiangtang terrane since 40 ma the proto plateau which was dominated by a topographic gradient lower crustal flow and continuous india asia convergence experienced three periods of rapid outward growth 40 23 23 10 and 10 0 ma in general the n s trending rifts were caused by the eastward growth of the plateau dominated by thrusting and crust flow in central tibet while they were the results of intense n s shortening in himalaya

dynamics of the india eurasia collision zone we present simple new dynamic calculations of a vertically averaged deviatoric stress field over a depth average of 100 km for asia from geodetic geologic topographic and seismic data a first estimate of the minimum absolute magnitudes and directions of vertically averaged deviatoric stress is obtained by solving force balance equations for deviatoric stresses associated with gravitational potential energy differences within the lithosphere plus a first order contribution of deviatoric stresses associated with stress boundary conditions this initial estimate of the vertically averaged deviatoric stress field is obtained independent of assumptions about the rheology of the lithosphere absolute magnitudes of vertically averaged deviatoric stresses vary between 5 and 40 mpa assuming bulk viscous behavior for the lithosphere the magnitudes of deviatoric stresses together with the magnitudes of strain rates inferred from quaternary fault slip rate and gps data yield vertically averaged effective viscosities for tibet of 0 5 5 1022 pa s compared with 1 2 5 1023 pa s in more rigid areas elsewhere in the region a forward modeling method that solves force balance equations using velocity boundary conditions allows us to refine our estimates of the vertically averaged effective viscosity distribution and deviatoric stress field the total vertically averaged deviatoric stress and effective viscosity field are consistent with a weak lower crust in tibet they are consistent with some eastward motion of tibet and south china lithosphere relative to eurasia and they confirm that gravitational potential energy differences have a profound effect on the spatially varying style and magnitude of strain rate around the tibetan plateau our results for the vertically averaged deviatoric stress argue for a large portion of the strength of the lithosphere to reside within the seismogenic upper crust to get deviatoric stress magnitudes there to be as high as 100 300 mpa in accord with laboratory and theoretical friction experiments indicating that stress drops in earthquakes are small fractions of the total deviatoric stress

two phase growth of high topography in eastern tibet during the cenozoic high topography in eastern tibet is thought to have formed in response to weak lower crust flowing towards the plateau margin thermochronologic analyses of rocks exposed at the eastern plateau margin record periods of mountain growth early in the indo asian collision implying that crustal flow alone could not have created the high topography

an upwelling case study on florida s west coast using a combination of satellite sea surface temperature imagery in situ currents temperature and sea level data and a numerical circulation model we describe an upwelling event observed along florida s west coast although ubiquitous features of continental shelves descriptions of coastal upwellings or downwellings are generally complicated by the continuum of processes in which they occur for the case study presented the winds were light and the baroclinic background currents were steady for several days prior to the event these conditions allow the evolution of a specific winddriven upwelling event to be viewed as an initial value problem the data and the numerical model analysis show that the route of adjustment by the sea level and currents is a classical ekman geostrophic one sea level drops due to an offshore ekman transport causing a geostrophic alongshore jet and the associated onshore flow within the bottom ekman layer accounts for the appearance of cold water at the surface this surface manifestation occurs as an elongated feature centered about the 25 m isobath that spans roughly half of the florida peninsula from just north of tampa bay to key west the coldest water is observed offshore from tampa bay and we explain this region of locally maximum upwelling on the basis of the coastline and isobath geometries

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

structural and geochronological constraints on the mesozoic cenozoic tectonic evolution of the longmen shan thrust belt eastern tibetan plateau 1 the longmen shan thrust belt in the eastern margin of the tibetan plateau underwent deformation of d1 associated with the late triassic collision between the north and south china blocks followed by deformation of d2 and d3 related to the eastward growth of the tibetan plateau the d1 is marked by moderately tight folds f1 spaced cleavage s1 mineral lineation l1 multistage reactivated faults and top to the south thrusting this deformation initiated before 237 ma and ended at 208 ma the 193 159 ma d2 deformation is top to the southeast directed and normal ductile in nature and was associated with the formation of half graben basins the mesozoic deformation of the longmen shan belt was related to the subduction of the paleo tethys oceanic lithosphere followed by the south china block underneath the songpan ganze terrane the d1 was produced in the pro side of the overriding songpan ganze plate most probably driven by slab corner flow above the retreating south china block the d2 and extensional songpan ganze turbidite basin were produced by roll back and migration of the overriding plate above the retreating subduction zone this model for the deformation from d1 to d2 is similar to a mediterranean style model the d3 is characterized by se ward thrusting with stepwise migration of rapid denudation and dextral strike slip faulting during the late cretaceous to present therefore the longmen shan thrust belt involved mesozoic compressional and extensional events that were overprinted by the late cretaceous to cenozoic compressional event

observations of currents on the west florida shelf break year long measurements by five acoustic doppler current profilers moored across the central west florida shelf wfs and shelf break reveal new information on the local velocity structure and its temporal evolution the moorings were between the 30 and 300 m isobaths at depths greater than about 100 m the currents were sometimes directly affected by the deep ocean during the measurement period the loop current lc directly impacted the central wfs three times amounting to a total of about 13 of the year for the remainder of the year the shelf break currents had equal likelihood of flowing northward or southward northward currents achieved over 70 cm s and the southward currents driven by the lc achieved 120 cm s neighboring instruments recorded strong 30 cm s currents flowing in opposite directions current reversals in one case totally nearly 100 cm s within a few days were also found

timing and rate of exhumation along the litang fault system implication for fault reorganization in southeast tibet the litang fault system that crosses the litang plateau a low relief surface at high elevation 4200 4800 m above sea level that is not affected by regional incision provides the opportunity to study exhumation related to tectonics in the se tibetan plateau independently of regional erosion combining apatite and zircon fission track with apatite u th he thermochronologic data we constrain the cooling history of the litang fault system footwall along two transects apatite fission track ages range from 4 to 16 ma ahe ages from 2 to 6 ma and one zircon fission track age is 99 ma these data imply a tectonic quiet period sustained since at least 100 ma with a slow denudation rate of 0 03 km ma interrupted at 7 to 5 ma by exhumation at a rate between 0 59 and 0 99 km ma we relate that faster exhumation to the onset of motion along the left lateral normal litang fault system that onset is linked to a lower miocene important kinematic reorganization between the xianshuihe and the red river faults with the eastward propagation of the xianshuihe fault along the xiaojiang fault system and the formation of the zhongdian fault such strike slip faults allow the sliding to the east of a wide continental block with the litang fault system accommodating differential motion between rigid blocks the regional evolution appears to be guided by the strike slip faults with different phases of deformation which appears more in agreement with an hidden plate tectonic model rather than with a lower channel flow model

frequency of ring separations from the loop current in the gulf of mexico a revised estimate the most energetic events in the circulation of the gulf of mexico are the separation of large anticyclonic rings from the loop current building on previous work the authors examine all the apparent rings since july 1973 this new dataset includes the satellite altimetry since 1992 providing a set of 34 consecutive ring formations the primary advantage of altimetry is that the data remain available in the summer one finding is that the ambiguity of whether or not a ring has separated is reduced but not eliminated the uncertainty with which separation events can be specified remains approximately 4 weeks even with nearly continuous data primary peaks in the distribution of separation intervals are found at 6 and 11 months with a smaller peak at 9 months if the spectrum is smoothed heavily enough a peak in the distribution can be formed nearer 12 months but this near annual peak is a result more of the smoothing than of the data

constructing the longmen shan eastern tibetan plateau margin insights from low temperature thermochronology 1 contrasting models of upper crustal shortening versus lower crustal flow have been proposed to explain the formation of thickened crust in the longmen shan lms eastern tibetan plateau tp margin these models require different structural kinematics along the lms whose structural geometry is defined by three parallel nw dipping fault zones from foreland southeast to hinterland northwest they are the guanxian anxian fault yingxiu beichuan fault ybf and wenchuan maowen fault wmf newly derived and previously published low temperature thermochronology data from the lms were synthesized to constrain the spatial exhumation and test previous models the results show that 1 exhumation increases abruptly across the range bounding ybf suggesting the fault being the main thrust boundary between the lms and the sichuan basin to the east 2 younger late cenozoic cooling ages are found on the hinterland wmf where a dichotomy of ages on the hanging wall versus footwall suggests late cenozoic thrust activity and 3 toward the hinterland to the west exhumation rates decrease twofold over a distance of 30 40 km this exhumation pattern indicates a westward decrease of tectonic uplift providing the regional topography approached a steady state whereby exhumation is in balance with tectonic uplift the observed exhumation estimates support an upper crustal configuration where thrusts in the lms merge gradually into a gentle detachment seated at a depth of 20 30 km results of this study support a revised upper crustal thrusting model

quantifying landscape differences across the tibetan plateau implications for topographic relief evolution 1 we quantify the bulk topographic characteristics of the tibet qinghai plateau with specific focus on three representative regions northern central and southeastern tibet quantitative landscape information is extracted from shuttle radar topography mission digital elevation models we find that the morphology of the tibetan plateau is nonuniform with systematic regional differences the northern and central parts of the plateau are characterized by what we suggest to call positive topography i e a topography in which elevation is positively correlated with relief and mean slope a major change from the internally drained central part of tibet to the externally drained part of eastern tibet is accompanied by a transition from low to high relief and from positive to negative topography i e a topography where there is an inverse or negative correlation between elevation and relief and between elevation and mean slope relief in eastern tibet is largest along rivers as they cross an ancient eroded plateau margin at high angle to the major strike slip faults the yalong yulong thrust belt implying strong structural control of regional topography we propose that the evolution of river systems and drainage efficiency the ability of rivers to transport sediments out of the orogen coupled with tectonic uplift is the simplest mechanism to explain systematic regional differences in tibetan landscapes basin filling due to inefficient drainage played a major role in smoothing out the tectonically generated structural relief this mode of smoothing started concurrently with tectonic construction of the relief as most clearly illustrated today in the qilian shan qaidam region of the northeastern plateau in the interior of tibet further passive filling due to internal drainage only continued to smooth the local relief millions of years after the cessation of major phases of surface uplift due to crustal shortening thus diachronous beveling at high elevation produced the low relief surface of the high plateau in southeast tibet headward retreat of external drainages brought back in the global ocean base level first disrupting then interrupting the relief reduction process it produced a transitional topography by dissecting the old remnant plateau surface which introduced younger and steeper incision of this hitherto preserved high base level this provides a unifying mechanism for the formation of the low relief plateau interior and for the origin of the high elevation low relief relict surface in southeastern tibet our analysis brings forth the importance of surface processes in particular drainage efficiency in shaping plateau morphology and landscape relief such key processes appear to have been mostly ignored in numerical models of plateau deformation our results also cast doubt on and provide a more realistic alternative to the fashionable contention that a continuous preuplift low relief surface first formed at low elevation extending all the way to the south china sea shore before being warped upward in the late miocene pliocene by lower crustal channel flow

the subglacior drilling probe concept and design in response to the oldest ice challenge initiated by the international partnerships in ice core sciences ipics new rapid access drilling technologies through glacier ice need to be developed these will provide the information needed to qualify potential sites on the antarctic ice sheet where the deepest section could include ice that is 1 ma old and still in good stratigraphic order identifying a suitable site will be a prerequisite for deploying a multi year deep ice core drilling operation to elucidate the cause and mechanisms of the mid pleistocene transition from 40 ka glacial interglacial cycles to 100 ka cycles as part of the ice lasers subglacior projects we have designed an innovative probe subglacior with the aim of perforating the ice sheet down to the bedrock in a single season and continuously measuring in situ the isotopic composition of the melted water and the methane concentration in trapped gases here we present the general concept of the probe as well as the various technological solutions that we have favored so far to reach this goal

local and deep ocean forcing contributions to anomalous water properties on the west florida shelf 1 material property distributions on continental shelves result from the mixing and modifications of estuarine and deep ocean source waters how this occurs depends on the momentum and buoyancy that are input either locally on the shelf or from the deep ocean at the shelf break we address this question of local versus deep ocean forcing for the west florida shelf wfs using in situ data and a numerical circulation model the spring and summer seasons of 1998 and 1999 show distinctively different water properties on the shelf and at the shelf break we account for these differences by a combination of local forcing independent of the adjacent gulf of mexico loop current and interactions of the loop current with the shelf the primary role of the deep ocean is to set the height of material isopleths along the shelf slope whether or not these material isopleths broach the shelf break is then a consequence of local shelf wide wind and buoyancy forcing the subsequent along and across shelf distributions are accomplished through a combination of local and deep ocean effects with the bottom ekman layer being the major conduit for the across shelf transport of ecologically important materials

current meter observations on the continental slope at two sites in the eastern gulf of mexico abstract current meter observations obtained at two sites on the continental slope of the eastern gulf of mexico at nominal positions of 29 n 88 w the mobile site and 27 5 n 85 5 w the tampa site are presented data were collected at three levels at mobile 90 190 and 980 m from july 1977 through august 1978 and at four levels at tampa 150 250 550 and 950 m from june 1978 through june 1979 at 90 and 190 m the flow at mobile was on the average to the east sustained periods of flow to the west were observed during the summer 1977 and spring 1978 during the periods of eastward flow the wind was generally out of the north and during the periods of westward flow the wind was out of the east the flow at the top meter at tampa was on the average to the west in the same direction as the average wind at both sites the motions are perturbed by events associated with the loop current these events make it difficult to define any seasonal variability in the upper layers the flow at the bottom m

subduction of the indian lithosphere beneath the tibetan plateau and burma abstract subduction of the indian lithosphere under eurasia plays an important role in the tectonic evolution of the tibetan plateau and surrounding regions to improve our knowledge of pertinent mantle structures through tomographic imaging we combine p wave arrival time data from temporary arrays in tibet and stations of the chinese seismograph network with reprocessed data from the international seismological centre the new images reveal considerable spatial variations in mantle structure along the collision zone and the horizontal distance over which presumed continental indian lithosphere slides northward beneath the plateau decreases from west where it underlies the himalayas and the entire plateau to east where no indication is found for present day underthrusting beyond the himalayan block and indus tsangpo suture p wavespeed appears low in the shallow mantle beneath much of central and eastern tibet these observations suggest that indian lithosphere underlies only the southwestern part of the plateau and that the central and northeastern part is underlain by lithosphere of asian origin the continental parts of the indian plate that currently underthrust or subduct below the plateau appear generally detached from the oceanic slab fragments that subducted longer ago and that have been detected deeper in the mantle except perhaps in the central part of the collision zone 80 90 e this suggests that ongoing india eurasia collision must increasingly be driven by other forces such as subduction of the indian plate further west and east e g beneath indochina our images reveal structures associated with eastward subduction along the burmese arc southeast of the eastern himalayan syntaxis the tengchong volcanic complex in southwest china is marked by slow wave propagation to 150 km depth suggesting a causal link to subduction along the burmese arc the low velocities beneath the red river fault region extend to greater depth and may be related to upper mantle processes further southeast

a loop current intrusion case study on the west florida shelf the gulf of mexico loop current intruded upon the west florida continental shelf in june 2000 in situ currents and hydrography along with satellite temperature and altimetry measurements are used to describe this event and its effects on the shelf a strong southward current is observed to flow along the shelf slope seaward of the intruded water boundary this current transported cold nutrient rich water from the north thereby producing anomalous hydrographic features near the shelf break 80 m isobath an array of moored velocity profilers reveals that the currents landward of the intruded water are independent of the loop current and primarily driven by local winds a series of idealized numerical model simulations inclusive of forcing by both the loop current and local winds confirm the observational findings that the shelfbreak currents are largely loop current controlled while the shelf currents are largely controlled by the local winds

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

tectonics reanimating eastern tibet the high eastern tibetan plateau was thought to have formed from an inflow of material from the lower crust the cooling histories of rocks exposed at the plateau margin however reveal protracted episodic growth suggesting that faulting also played a role

summertime coastal current reversal opposing offshore forcing and local wind near the middle east coast of korea observation and dynamics a 6 year long current measurement at a buoy station off the middle east coast of korea reveals an equatorward reversal of coastal current in summer opposing poleward local wind stress and offshore boundary current the current reversal extends about 40 km offshore from the coast and is concurrent with warming and freshening of water column estimates of the depth averaged alongshore momentum balance suggest a major balance between the alongshore pressure gradient and the lateral friction sources of the pressure gradient for the summertime current reversal are identified as the alongshore buoyancy gradient driven by the wind curl gradient and the prevalence of warmer and lower salinity water in the north alongshore pressure gradient and velocity induced by the wind curl gradient are quantified which yields the observed seasonal current reversal

viscosity and density of a two phase drilling fluid many ice core drilling projects expect to reach depths greater than 1000m at such depths it is necessary to fill the borehole with a fluid to compensate the ice pressure and avoid the resulting significant hole closure knowledge of the ice chips and drilling fluid circulation parameters e g flow rate pressure drop velocity is essential for understanding the behaviour of the fluid around the drill and will support drill design through a better parameterization in models a characterization of the dynamic viscosity and of the density of the ice chips and drilling fluid mixture is required to calculate these circulation parameters the goal of this study is to propose a method to calculate to a first approximation both these physical properties the relationships presented here have been established after experimentation building on prior assumptions taken from the literature

the role of crustal strength variations in shaping orogenic plateaus with application to tibet 1 the tibetan plateau is the type example of a large orogenic plateau formed as a result of continent continent collision the morphology of the plateau and its margins suggests that preexisting variations in crustal strength have influenced the growth of the plateau we have developed a three dimensional numerical model of deformation in a viscous crust in order to investigate the effects of lateral heterogeneities on plateau growth the model includes a two layer crust and allows for lateral variation of viscosity in both the upper and lower layers model results indicate that crustal strength variations have a dramatic effect on the morphology and dynamics of a developing plateau a region of strong crust is characterized by a very steep plateau margin that propagates extremely slowly does not accommodate significant shortening strain and is subparallel to local upper and lower crustal velocities a weak crustal region develops a gently sloping margin uplift propagates rapidly across the weak zone and crustal material within the plateau is diverted toward the low strength region with a relatively simple distribution of strength variations corresponding to strong tarim and sichuan basin crust and a weak southeastern corner our model produces a plateau with many similarities to the tibetan plateau including the overall morphology rotation around the eastern syntaxis and e w extension analysis of model results suggests that e w extension of the central plateau may be related to the rapid eastward flow of crustal material into a weak zone in the se corner without significant change in plateau elevation

late cenozoic uplift of southeastern tibet the age of surface uplift in southeastern tibet is currently unknown but the initiation of major river incision can be used as a proxy for the timing of initial uplift the topographically high eastern plateau and gently dipping southeastern plateau margin are mantled by an elevated low relief relict landscape that formed at a time of slow erosion at low elevation and low tectonic uplift rates prior to uplift of the eastern tibetan plateau thermochronology from deep river gorges that are cut into the relict landscape shows slow cooling between ca 100 and ca 10 20 ma and a change to rapid cooling after ca 13 ma with initiation of rapid river incision at 0 25 0 5 mm yr between 9 and 13 ma a rapid increase in mean elevation of eastern tibet beginning at this time supports tectonic climate models that correlate the lateral eastern expansion of high topography in tibet with the late miocene intensification of the indian and east asian monsoons

large scale crustal deformation of the tibetan plateau the topography velocity and strain fields calculated from a three dimensional newtonian viscous model for large scale crustal deformation are generally in good agreement with results from geological geodetic and earthquake studies in and around the tibetan plateau provided that the model rheology incorporates a weak zone within the deep crust beneath the plateau equivalent to a viscosity of 1012 pa s within a 250 m thick channel or 1018 pa s within a 15 km thick channel model studies and observations show a plateau at 5 km elevation with steep topographic gradients across the southern and northern plateau margins and more gentle gradients across the southeastern and northeastern margins rapid shortening strain is concentrated along the lower portions of the northern and southern plateau margins at rates 20 mm yr model results show north south shortening 10 mm yr in reasonable agreement with gps data 5 8 mm yr of north south shortening across the northern two thirds of the plateau and east west stretching 10 15 mm yr across the eastern half of the high plateau in reasonable agreement with seismic geologic and gps data upper crustal material moves eastward from the plateau proper into a lobe of elevated topography that extends to the south and east clockwise rotation of material around the east himalayan syntaxis at rates up to 10 mm yr occurs partly as a result of dextral shear between indian and asian mantle at depth and partly as a result of gravitational spreading from the high plateau to the south and east there is little difference in model surface deformation for assumptions of moderately weak or extremely weak lower crust except in southern and northern tibet where margin perpendicular extension occurs only for the case of an extremely weak lower crust our results suggest that the tibetan plateau is likely to have gone through a two stage development the first stage produced a long narrow high orogen whose height may have been comparable to the modern plateau the second stage produced a plateau that grew progressively to the north and east east west stretching eastward plateau growth and dextral rotation around the east himalayan syntaxis probably did not begin until well into the second stage of plateau growth perhaps becoming significant after 20 m y of convergence

the geological evolution of the tibetan plateau the geological evolution of the tibetan plateau is best viewed in a context broader than the india eurasia collision zone after collision about 50 million years ago crust was shortened in western and central tibet while large fragments of lithosphere moved from the collision zone toward areas of trench rollback in the western pacific and indonesia cessation of rapid pacific trench migration 15 to 20 million years ago coincided with a slowing of fragment extrusion beyond the plateau and probably contributed to the onset of rapid surface uplift and crustal thickening in eastern tibet the latter appear to result from rapid eastward flow of the deep crust probably within crustal channels imaged seismically beneath eastern tibet these events mark a transition to the modern structural system that currently accommodates deformation within tibet

early cenozoic faulting of the northern tibetan plateau margin from apatite u th he ages models to explain the distributed nature of continental deformation predict the propagation of strain and high topography away from the plate boundary yet a growing body of evidence in the tibetan orogen suggests that deformation occurred at the far northern extent of the modern plateau early in the orogen s history and thus our current mechanical understanding of orogenic plateau development is incomplete new apatite u th he ages from four elevation transects document periods of rapid exhumation related to erosion pulses in hanging wall rocks of major thrust structures accelerated erosion is used as a proxy of fault timing and is interpreted in a larger context of structural data and sediment accumulation in adjacent foreland basins helium results are synthesized with published geologic thermochronometric and sedimentologic data from which a growing picture of regional compressional deformation in middle to late eocene time in northern tibet emerges we relate the early cenozoic period of deformation to the initiation of collision between india and eurasia despite the fact that the plate boundary was located 3000 km to the south regardless of whether or not high topography was built simultaneously as a result of this deformation early cenozoic strain signifies that the modern limit of the orogen has been relatively stationary since continental collision began and that deformation has not significantly propagated farther away from the plate boundary in time

block rotation tectonic response of the sichuan basin to the southeastward growth of the tibetan plateau along the xianshuihe xiaojiang fault gps field and seismic data show that the southeastern margin of the tibetan plateau is tectonically and seismically active this activity is due to the southeastward extrusion of the chuandian fragment a large crustal block rotating clockwise around the northeastern syntaxis of the himalayas the eastern boundary fault of this fragment is defined by the left lateral xianshuihe xiaojiang fault which abruptly truncates the sichuan basin of the yangtze block our paper presents evidence indicating that the sichuan basin experienced right lateral shear along its margin including the longmen shan fault belt as shown by the presence of a large number of interference deformation features including s shaped and z shaped folds and faults aligned in an en echelon pattern this study hypothesizes that the sichuan basin experienced counterclockwise rotation dragged by the left lateral movement along the xianshuihe fault and it is this rotation that was the underlying cause of the 12 may 2008 wenchuan ms 7 9 earthquake during the rotation the sichuan basin decoupled along a subhorizontal decollement fault zone that developed along triassic gypsum and coal bearing rocks at a mean depth of 5000 m below which the paleozoic rocks experienced much more intense deformation than the overlying mesozoic rocks suggesting that the lower part of the basin experienced a larger scale rotation relative to the uppermost part of the basin based on thermal data from the western margin of the sichuan basin and from along the xianshuihe fault the counterclockwise bending rotation of the sichuan basin initiated in late cenozoic time 13 ma

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

ocean currents and sea surface heights estimated across the west florida shelf abstract the across shelf structures of the ocean circulation and the associated sea surface height ssh variability are examined on the west florida shelf wfs for the 3 yr interval from september 1998 to december 2001 five sets of characteristic circulation patterns are extracted from 2 day low pass filtered data using the self organizing map extreme upwelling and downwelling structures with strong currents asymmetric upwelling and downwelling structures with moderate currents and a set of transitional structures with weak currents the temporal variations of these structures are coherent with the local winds on synoptic weather time scales on seasonal time scales they are related to both the local winds and the water density variations the circulation is predominantly upwelling during autumn to spring months october april and downwelling during summer months june september coastal sea level fluctuations are related to both the dynamical responses of the inner shelf circulation to meteorol

climatic limits on landscape development in the northwestern himalaya the interaction between tectonism and erosion produces rugged landscapes in actively deforming regions in the northwestern himalaya the form of the landscape was found to be largely independent of exhumation rates but regional trends in mean and modal elevations hypsometry frequency distribution of altitude and slope distributions were correlated with the extent of glaciation these observations imply that in mountain belts that intersect the snowline glacial and periglacial processes place an upper limit on altitude relief and the development of topography irrespective of the rate of tectonic processes operating

wind induced sea surface slopes on the west florida shelf abstract tidal and meteorological records at stations in the eastern gulf of mexico have been studied the sea level response is a maximum for winds along the coast and varies symmetrically with angle the coherence is maximum at periods of 4 10 days the horizontal coherence of sea level is high out to 500 km for 4 10 day periods the horizontal coherence for wind measured at coastal stations is high out to at least 500 km the amplitude of the response of sea level to winds is larger by a factor of 4 here on a broad shelf than on the oregon coast which is more narrow by about the same ratio a response of 16 cm is induced by 4 m s 1 wind this response or transfer function is uniform over the spectral range 4 100 days the sea level response to the longshore wind stress is not linear but to the power 0 8 0 1 and is attributed to the relatively low tidal currents in this region the large horizontal coherences of wind and sea level imply broad longshore flows extending 500 km or more along

normal faulting in central tibet since at least 13 5 myr ago tectonic models for the evolution of the tibetan plateau interpret observed east west thinning of the upper crust to be the result of either increased potential energy of elevated crust1 or geodynamic processes that may be unrelated to plateau formation2 3 4 5 6 a key piece of information needed to evaluate these models is the timing of deformation within the plateau the onset of normal faulting has been estimated to have commenced in southern tibet between about 14 myr ago7 and about 8 myr ago8 and in central tibet about 4 myr ago9 here however we report a minimum age of approximately 13 5 myr for the onset of graben formation in central tibet based on mineralization ages determined with rb sr and 40ar 39ar data that post date a major graben bounding normal fault these data along with evidence for prolonged activity of normal faulting in this and other tibetan grabens support models that relate normal faulting to processes occurring beneath the plateau thinning of the upper crust is most plausibly the result of potential energy increases resulting from spatially and temporally heterogeneous changes in thermal structure and density distribution within the crust and upper mantle beneath tibet this is supported by recent geophysical and geological data10 11 12 13 14 15 16 17 which indicate that spatial heterogeneity exists in both the tibetan crust and lithospheric mantle

intracrustal detachment within zones of continental deformation geologic mapping of active to recently active geologic structures in panamint valley western united states and in the haiyuan region and northern tibetan plateau china suggests detachment in the middle and lower crust on a scale of tens of kilometres to at least several hundred kilometres detachment occurs similarly in predominantly extensional panamint valley and in predominantly compressional china environments it involves structures with displacements of more than 10 km and displacement rates of more than 3 mm yr perhaps more than 10 mm yr the steeply dipping strike slip faults present in all three areas hunter mountain fault haiyuan fault and altyn tagh fault zone terminate in zones of extension or compression and geometric relations indicate that all structures including strike slip faults are thin skinned and restricted to the upper crust in panamint valley and in the haiyuan region deformation within these systems can be reconstructed in three dimensions displacement on the strike slip faults is absorbed by extension or compression occurring at the termination of the faults so strike slip displacement is roughly equal to and in the same direction as shortening or extension we propose that left slip on the altyn tagh fault zone in northern tibet is similarly absorbed by shortening southeast of the fault zone within the qaidam basin and the nan shan

observations of a loop current frontal eddy intrusion onto the west florida shelf hydrographic and satellite data from the west florida shelf between april 1 7 1982 showed the intrusion of a loop current frontal eddy onto the shelf data were examined to describe the structure of this feature and study effects of its intrusion on water masses in the outer shelf region a frontal eddy consisting of a warm filament separated from the main current by a region of cooler water propagated southeastward at 30 cm s intruding onto the shelf near 26 n between april 4 and 6 temperature salinity t s properties revealed that water in the filament was loop current water that had been contiguous with 80 m deeper loop current water in the main body of the current water in the cold region was continental edge water a transitional water mass with cooler fresher t s characteristics upwelling of deeper loop current water occurred under this region and elevated nutrient concentrations were found in the upwelled dome under the cold region interleaving occurred along water mass boundaries enabling the exchange of heat and salt this mixing and the supply of cool nutrient rich water to the outer shelf was the major consequence of the intrusion the length scale and speed of this loop current frontal eddy was similar to gulf stream frontal eddies through the upwelling was not as intense as in intrusions of gulf stream frontal eddies

use of a regional relict landscape to measure vertical deformation of the eastern tibetan plateau 1 field work and topography analysis show that remnant local areas of a low relief landscape or erosion surfaces are geographically continuous across the southeastern tibetan plateau margin we correlate these remnant surfaces as a paleolandscape that formed at low elevation remnants of this paleolandscape are preserved because incision of the fluvial system has been largely limited to major rivers and principal tributaries and has not yet progressed throughout the entire fluvial network the incomplete adjustment of the fluvial system signals initiation of rapid bedrock incision into a developing plateau margin and erosional denudation is concentrated in the major river channels this interpretation contradicts earlier notions that low gradient regional topography is the product of regional elevation reduction by intense landscape dissection due to the presence of several large southeast flowing rivers the modern altitude of the reconstructed paleolandscape or relict landscape constrains the vertical displacement of the plateau surface in response to crustal thickening and subsequent erosion during the lateral growth of the tibetan plateau regional preservation of the relict landscape is consistent with minor surface disruption by late cenozoic thrust faults and folds and supports a model of distributed lower crustal thickening because significant erosion is limited to narrow river gorges an increase in the plateau elevation due to isostatic rebound is minor therefore we propose that the modern elevation of the relict landscape reflects isostatically compensated thickening of the lower crust

on the decay of a baroclinic jet flowing along a continental slope a jet flowing in the kelvin wave propagation direction along the seaward side of a continental slope is found to decrease in strength as it leaks mass onto the continental shelf as a result a shelf break jet flowing in the opposite direction is induced on the shelf the combination of these two flows creates a decaying high pressure tongue centered about the shelf break an idealized reduced gravity model is developed to examine the two flow combination in the model a baroclinic open ocean jet flows next to a continental shelf over which the flow is barotropic and dominated by a linear bottom friction proportional to along shelf velocity the far field depth of the thermocline is assumed to be much greater than the depth of the shelf break a solution is found in the limit of small cross slope flow the mathematical problem of finding the flow on the shelf is equivalent to that of heat conduction on a semi infinite rod with a finite heat source placed at the end of the rod a numerical shelf slope junction model is devised to allow the requirement of small cross slope flow to be relaxed the numerical model which incorporates higher order dynamics confirms the qualitative results obtained from the analytic model the motivation for the development of the theory lies in observational evidence of a northward flowing jet along the continental slope off the west florida shelf apparently stemming from the loop current the structure of the flow combination over the shelf break is confirmed with historical hydrographic data and advanced very high resolution radiometer imagery