paragenesis of sodic pyroxene bearing quartz schists implications for the p t history of the sanbagawa belt sodic pyroxene jadeite content x jd 0 1 0 3 occurs locally as small inclusions within albite porphyroblasts and in the matrix of hematite bearing quartz schists in the sanbagawa sambagawa metamorphic belt central shikoku japan the sodic pyroxene bearing samples are characteristically free from chlorite and their typical mineral assemblage is sodic pyroxene subcalcic or sodic amphibole phengitic mica albite quartz hematite titanite epidote spessartine rich garnet occurs in mn rich samples sodic pyroxene in epidote bearing samples tends to be poorer in acmite content average x acm 0 26 0 50 than that in the epidote free samples x acm 0 45 0 47 x jd shows no systematic relationship to metamorphic grade and is different among the three sampling regions saruta gawa asemi gawa and bessi besshi the average x jd of the saruta gawa samples 0 21 0 29 is higher than that of the asemi gawa 0 13 0 17 and bessi 0 14 0 23 the p t conditions of the asemi gawa and bessi regions are estimated at 5 5 6 5 kbar 360 c in the chlorite zone 7 8 5 kbar 440 15 c in the garnet zone and 8 9 5 kbar 520 25 c in the albite biotite zone metamorphic pressure of the saruta gawa region is systematically 1 1 5 kbar higher than that of the asemi gawa and bessi regions and materials of the saruta gawa region have been subducted to a level 3 5 km deeper than materials that underwent metamorphism at equivalent temperatures and are now exposed in the asemi gawa and bessi regions pressure slightly increases toward the north structurally high levels through the sanbagawa belt of central shikoku two types of zonal structure were observed in relatively coarse grained sodic pyroxenes in the matrix one type is characterized by increasing x jd from core to rim the other type by decreasing x jd from core to rim both types of zoned pyroxenes show an increase in x fe 2 fe2 fe2 mg from core to rim the first type of zoning was observed in a sample from the chlorite zone of lowest grade whereas the latter occurs in the garnet and albite biotite zones of higher grade the contrast in zonal structure implies that dp dt during prograde metamorphism decreased with increasing metamorphic grade and may have been negative in some samples from the higher grade zones the estimated dp dt of the prograde stage of the chlorite zone is 3 2 kbar 100 c and that of the garnet and albite biotite zones is 1 8 to 0 9 kbar 100 c the variation of dp dt at shallow and deep levels of a subduction system probably reflects the difference of heating duration and or change in thermal gradient of the subduction zone by continuous cooling of the surrounding mantle

viscous flow and deformation of regional metamorphic belts at convergent plate boundaries 1 viscous flow and deformation of a newtonian fluid between rigid boundaries have been modeled for investigating deformation of a forearc wedge and a regional metamorphic belt as a weak deforming zone between the plates at convergent boundaries three types of analytic formulations together with numerical formulations for a newtonian fluid with a constant viscosity are used for investigating various flows and deformation 1 flows induced by squeezing between two parallel boundaries 2 flows induced by squeezing and heterogeneous mass influx in the wedge between two oblique boundaries e g accretionary wedge over the subducting plate with the continental plate as a backstop and 3 flows induced by dragging along the boundaries and by homogeneous mass influx in the wedge as in type 2 all cases include strike slip movements of the boundaries which allows us to investigate three dimensional 3 d deformation e g 3 d corner flow the corresponding flow deformation of infinitesimal and finite elements and the timescale of flow are calculated for each configuration with various mechanical boundary conditions the model results show that configuration and mechanism of the flow can be inferred from the spatial variation of finite deformation e g deformed radiolarians as a strain marker and geometry of folding as finite deformation of a large block in particular prolate strain nearly parallel to the strike of subduction zone together with a large scale folding can be a good indicator for deformation in the forearc wedge associated with 3 d corner flow induced by oblique subduction deformation observed in the cretaceous regional metamorphic belts in southwest japan can be explained by this mechanism

regional variation in exhumation and strain rate of the high pressure sambagawa metamorphic rocks in central shikoku south west japan regional variation in the p t path of the sambagawa metamorphic rocks central shikoku japan has been inferred from compositional zoning of metamorphic amphibole rocks constituting the northern part saruta river area exhibit a hairpin type p t path where winchite actinolite grew at the prograde stage the peak metamorphism was recorded by the growth of barroisite to hornblende and sodic amphibole to winchite actinolite grew at the retrograde stage in the southern part asemi river area rocks exhibit a clockwise type p t path where barroisite to hornblende core is rimmed by winchite to actinolite the difference in p t path could suggest a faster exhumation rate i e more rapid decompression in the southern than in the northern part on the other hand physical conditions of deformation during the exhumation stage have been independently inferred from microstructures in deformed quartz recrystallized quartz grains in rocks from the low grade chlorite and garnet zones are much more stretched in the southern part aspect ratio 4 0 than in the northern part aspect ratio 4 0 indicating a higher strain rate in the former than in the latter these facts may indicate that the exhumation and strain rates are correlated i e the exhumation rate increases with increasing the strain rate the difference in the exhumation rate inferred from amphibole zoning between the northern and southern parts could be explained by an extensional model involving normal faulting where the lower plate can be exhumed faster than the upper plate due to the displacement along the fault furthermore the model may explain the positive correlation between the exhumation and strain rates because the lower plate tended to support more stress than the upper plate

climatic control on rapid exhumation along the southern himalayan front abstract along the southern himalayan front shf areas with concentrated precipitation coincide with rapid exhumation as indicated by young mineral cooling ages twenty new young 80 of the annual precipitation aft cooling ages reveal a coincidence between rapid erosion and exhumation that is focused in a 50 70 km wide sector of the himalaya rather than encompassing the entire orogen assuming simplified constant exhumation rates the rocks of two age vs elevation transects were exhumed at 1 4 0 2 and 1 1 0 4 mm a with an average cooling rate of 40 50 c ma during pliocene quarternary time following other recently published hypotheses regarding the relation between tectonics and climate in the himalaya we suggest that this concentrated loss of material was accommodated by motion along a back stepping thrust to the south and a normal fault zone to the north as part of an extruding wedge climatically controlled erosional processes focus on this wedge and suggest that climatically controlled surface processes determine tectonic deformation in the internal part of the himalaya

brittle normal faulting in the highest grade sambagawa metamorphic rocks of central shikoku southwest japan indication of the exhumation into the upper crustal level abstract analysis of fault system in the high p t type sambagawa metamorphic rocks of central shikoku southwest japan shows that conjugate normal faults pervasively developed in the highest grade biotite zone upper structural level in three study areas asemi river oriu and niihama areas these conjugate normal faults consist of ne sw to e w striking and moderately north dipping set a and nnw sse striking and moderately east dipping set b faults the fault set a is dominant compared to the fault set b and hence most of deformation is accommodated by the fault set a leading to non coaxial deformation the sense of shear is inferred to be a top to the wnw to nnw based on the orientations of striation or quartz slickenfibre and dominant north side down normal displacement these transport direction by normal faulting is significantly different from that at d1 penetrative ductile flow i e top to the w to wnw it has also been found that these conjugate normal faults are openly folded during the d3 phase about the axes trending nw se to e w and plunging west at low angles or horizontally indicating that normal faulting occurred at the d2 phase d2 normal faults along which actinolite breccia derived from serpentinite by metasomatism sometimes occurs perhaps formed under subgreenschist conditions ca 250 c in relation to the final exhumation of sambagawa metamorphic rocks into the upper crustal level the pervasive development of d2 normal faults in the upper structural level suggests that the final exhumation of sambagawa metamorphic rocks could be caused by distributed extension and normal faulting removal of overburden in the upper crust

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

intermediate high pressure exhumation of the northern segment of the sanbagawa belt saruta gawa area central shikoku japan abstract the retrograde chemical zonal structure of amphibole in hematite bearing basic and quartz schists from the higher grade zone in the saruta gawa area of the sanbagawa belt was studied to investigate the relationships between the prograde and retrograde p t paths of the sanbagawa metamorphism this amphibole coexists with chlorite epidote muscovite albite quartz and hematite and is composed of al rich core and al poor mantle the core is fairly homogeneous and has a barroisitic composition in the mantle part b na increases with decreasing 4 al towards the margins which have winchite magnesioriebeckite compositions the barroisite winchite magnesioriebeckite composite crystal is sometimes rimmed by actinolite and or winchite with low 4 al and b na the al rich core and al poor mantle are regarded as prograde and retrograde products respectively the retrograde mantle in the saruta gawa area 1 is systematically richer in b na 0 40 1 73 per formula unit pfu for o 23 than that from the same grade zone in the asemi gawa area 0 19 0 78 pfu about 8 km south of the studied area 2 tends to be b na poorer less than 1 73 pfu than prograde sodic amphibole up to 1 93 b na pfu produced in the peak temperature stage from the lower grade zone in the same and other areas and 3 extends its compositional range towards higher b na and lower 4 al than prograde formed amphibole from the same grade zone in the same area these zonal characteristics imply that 1 the saruta gawa samples experienced retrograde metamorphism under higher p t conditions than the asemi gawa samples 2 the retrograde p t path of the saruta gawa area passes on the lower pressure side of the metamorphic field gradient and 3 the saruta gawa samples underwent retrograde metamorphism under higher p t conditions than the prograde metamorphism the higher p t conditions of the retrograde metamorphism suggests an increasing dp dt of the geotherm during exhumation retrograde p t conditions during the formation of magnesioriebeckite can be roughly estimated at 7 8 kbar 400 450 c based on semi quantitative phase relations of actinolite winchite magnesioriebeckite barroisite series associated with chlorite epidote muscovite albite quartz and hematite

unpairing metamorphic belts p t paths and a tectonic model for the ryoke belt southwest japan southwest japan is divided into outer and inner zones by the median tectonic line mtl a major transcurrent fault the outer zone is composed of the sambagawa high pressure intermediate or high p t type metamorphism chichibu and shimanto belts in the inner zone the ryoke belt andalusite sillimanite or low p t type metamorphism was developed mainly within a jurassic accretionary complex this spatial relationship between high p t type and low p t type metamorphic belts led miyashiro to the idea that metamorphic belts were developed as paired systems textural relationships and petrogenetically significant mineral assemblages in pelites from the ryoke belt imply peak p t conditions of 5 kbar and up to 850 c in migmatitic garnet cordierite rocks from the highest grade metamorphic zone it is likely that the thermal anomaly responsible for metamorphism of the ryoke belt was related to a segment of the farallon izanagi ridge as it subducted under the eastern margin of the asian continent during the cretaceous the sequence of mineral assemblages developed in pelites implies a metamorphic field gradient with shallow dp dt slope inferred to have been generated by a nested set of hairpin like clockwise p t paths these p t paths are characterized by limited prograde thickening minor decompression at peak t and near isobaric cooling features that may be typical of p t paths in low p t type metamorphic belts caused by ridge subduction a ridge subduction model for the ryoke belt implies that juxtaposition of the high p t metamorphic rocks of the sambagawa belt against it was a result of terrane amalgamation belt parallel ductile stretching recorded as syn metamorphic predominantly constrictional strain in both ryoke and sambagawa belt rocks and substantial sinistral displacement on the mtl are consistent with left lateral oblique convergence diachroneity in fast cooling of the ryoke belt is implied by extant thermochronological data and is inferred to relate to progressive sw to ne docking of the sambagawa belt thus an alternative interpretation of paired metamorphic belts in japan is that they represent laterally contemporaneous terranes rather than outboard and inboard components of a trench arc paired system amalgamation of laterally contemporaneous terranes during large translations of forearcs along continental margins may explain other examples of paired metamorphic belts in the geological record

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

effect of ocean gateways on the global ocean circulation in the late oligocene and early miocene 1 we investigate the effect of changes in the tectonic boundary conditions on global ocean circulation patterns using a fully coupled climate model in an idealized setup we compare situations corresponding to the late oligocene the early miocene and present day the model results show the existence of a flow reversal through the panama seaway between the oligocene and miocene this flow reversal is induced by global tectonic changes related to the widening of the southern ocean passages and the closing of the tethys seaway it mainly involves the wind driven ocean circulation in agreement with previous model studies the global thermohaline circulation in the oligocene and miocene simulations is significantly different from the present day conveyor circulation as there is deepwater formation in both the north atlantic and the north pacific oceans in particular in the oligocene simulation the salinity contrast between the atlantic and pacific oceans is reduced because of water mass exchange through the low latitude connections between the two oceans

global geological mapping of ganymede abstract we have compiled a global geological map of ganymede that represents the most recent understanding of the satellite based on galileo mission results this contribution builds on important previous accomplishments in the study of ganymede utilizing voyager data and incorporates the many new discoveries that were brought about by examination of galileo data we discuss the material properties of geological units defined utilizing a global mosaic of the surface with a nominal resolution of 1 km pixel assembled by the usgs with the best available voyager and galileo regional coverage and high resolution imagery 100 200 m pixel of characteristic features and terrain types obtained by the galileo spacecraft we also use crater density measurements obtained from our mapping efforts to examine age relationships amongst the various defined units these efforts have resulted in a more complete understanding of the major geological processes operating on ganymede especially the roles of cryovolcanic and tectonic processes in the formation of might materials they have also clarified the characteristics of the geological units that comprise the satellite s surface the stratigraphic relationships of those geological units and structures and the geological history inferred from those relationships for instance the characteristics and stratigraphic relationships of dark lineated material and reticulate material suggest they represent an intermediate stage between dark cratered material and light material units

late stage exhumation of hp metamorphic rocks progressive localization of strain and changes in transport direction sambagawa belt japan abstract new geologic mapping delimits major structures associated with the later stages of exhumation of high pressure hp metamorphic rocks of the sambagawa belt of central shikoku the main sets of structures define an inclined n dipping wedge bounded by s vergent thrust faults below and n vergent normal faults above such faults bound every metamorphic mineral zone of the sambagawa belt faults also occur within metamorphic mineral zones such as the asemigawa detachment which is entirely within the oligoclase biotite zone the highest grade metamorphic zone this fault is the structurally lowest of the n vergent normal faults these d2 structures accommodate the exhumation of the higher grade rocks relative to the lower grade rocks within the sambagawa belt rather than relative to units bounding the belt so they are termed an internal extrusional wedge the extrusional wedge is truncated to the east against footwall chlorite zone rocks by the hamegano out of sequence thrust oost the oost divides the traditional chlorite zone into an upper unit consisting of varicolored schist l tectonite and a lower unit consisting of regionally extensive pelitic schist to the north the metamorphic mineral zones are stacked over the oost forming the tomisato duplex and these structures are further modified by the oboke anticline and tsuneyama syncline the evolution of exhumation structures of the sambagawa suggests progressive localization of strain as exhumation proceeded as well as a change in movement direction from d1 oblique orogen subparallel exhumation to the d2 structures with dip parallel orogen normal transport that we describe in detail the evolution of exhumation structures in the sambagawa belt differs from that of the hp rocks of the franciscan complex of california in that the exhumation of the latter is accommodated entirely by discrete faults and the franciscan lacks internal normal faults the internal extrusional wedge structure of the sambagawa hp rocks resembles that of hp rocks of leti island of eastern indonesia

the high p t sambagawa extrusional wedge japan abstract a metamorphic inversion along the asemigawa section of the besshi unit in the sambagawa high p t metamorphic zone has been explained previously by s vergent major recumbent folding or thrusting through careful structural field investigations however we recognize the d2 asemigawa detachment fault at the boundary of the highest grade oligoclase biotite zone thermal culmination in the belt we have also discovered a series of normal faults in the hanging wall of the major detachment in the upper domain and a series of thrust faults bounding every metamorphic mineral zone of the footwall in the lower domain outcrop scale d2 asymmetric folds are closely associated with axial planar pressure solution cleavages and are genetically related to the above d2 normal faulting and thrusting as fault propagation folds northward fold vergence is found only in the upper domain and southward vergence is found only in the lower domain exceptions to this rule occur only for parasitic folds in the overturned limbs of mesoscopic folds which indicates flexural slip as a likely fold mechanism we propose an extrusion wedge model for the inverted metamorphism and accordingly this mechanism may also account for a large part of the exhumation of the sambagawa metamorphic rocks although westward shear at the time of d1 stretching lineation has partly contributed to the exhumation by counter flow in the accretionary wedge we infer that m1 with its retrograde actinolite schists represents a d1 ductile shear zone but it contributes as much as a component of the d2 asemigawa detachment other normal and thrust faults and asymmetric folds as reactivated brittle shear zones thus shear direction had changed orthogonally and drastically between ductile d1 stretching and ductile brittle transition to brittle d2 extrusion we infer that this transition reflects the collision of an actively spreading mid ocean ridge and is a consequence of increased coupling of plates and orogen normal stress followed by oblique subduction d0 and d 1 record eastward down flow during the oblique underthrusting of oceanic plate before peak metamorphism m1

microtectonic evidence for eastward ductile shear in the jurassic orogen of sw japan abstract two jurassic deformation phases are responsible for the thrust and nappe structures in the infrastructure of the outer belt of the mesozoic chain of sw japan the first phase was synmetamorphic and took place under hp lt conditions microtectonic analysis shows that the penetrative deformation corresponded to a ductile shear directed from w to e parallel to the stretching and mineral lineation at the regional scale the first phase corresponded to the obduction of the greenschist nappes upon the kurosegawa continent during the second phase the greenschist nappes were sliced leading to an apparent reverse metamorphic zoning the first phase structures were then locally reworked by folds giving rise to an apparent westwards sense of shear

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

thermal effects of ridge subduction and its implications for the origin of granitic batholith and paired metamorphic belts distribution of temperature and h2o in subduction zones during the course of ridge subduction are investigated numerically in order to constrain the timing and extent of metamorphism and melting within the subducting oceanic crust and the overlying arc crust the model assumes that after ridge subduction the two plates move together without creating a slab window the results suggest that before ridge subduction temperature along the subducting slab starts to rise when the slab is younger than 10 myr a drastic thermal impact occurs associated with subduction of the ridge crest followed by a diffusion stage of the impact the temperature elevation is limited within a few tens of kilometers from the slab crust interface and within 30 myr after ridge subduction although it will last for a longer period if a slab window is formed during the peak stage slab melting occurs and generates a significant amount of granitic melt mainly at depths shallower than 40 km at p t conditions corresponding to low p t metamorphism high p t metamorphism with a significant water flux is also achieved near the interface when a subducting slab is less than 10 myr old before ridge subduction or 20 myr old after ridge subduction consequently the low and high p t metamorphism accompanied by a significant amount of granitic melt is associated closely in time and space with the ridge subduction this model can explain the observed characteristics of the granitic batholith and the paired metamorphic belts through these processes arcs and continents can grow within the short period of time associated with the episodic subduction of ridges

pressure temperature trajectory of the sanbagawa metamorphism deduced from garnet zoning abstract most garnets in a small volume of the pelitic sanbagawa schists nucleated simultaneously grew and then ceased with their growth maintaining near equilibrium with coexisting minerals such as chlorite and biotite during the prograde metamorphism chemical trends of zoned garnets systematically shift from the pyrope poor to pyrope rich portions in a ternary mn fe mg system in order of ascending metamorphic grade a ternary mn fe mg model system suggests that the higher grade garnets started to grow at lower pressures and higher temperatures than the lower grade ones during prograde metamorphism it follows that high grade metamorphism took place at lower pressures in the garnet to oligoclase biotite zones of the sanbagawa metamorphic terrain in central shikoku

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

imaging spectroscopy earth and planetary remote sensing with the usgs tetracorder and expert systems 1 imaging spectroscopy is a tool that can be used to spectrally identify and spatially map materials based on their specific chemical bonds spectroscopic analysis requires significantly more sophistication than has been employed in conventional broadband remote sensing analysis we describe a new system that is effective at material identification and mapping a set of algorithms within an expert system decision making framework that we call tetracorder the expertise in the system has been derived from scientific knowledge of spectral identification the expert system rules are implemented in a decision tree where multiple algorithms are applied to spectral analysis additional expert rules and algorithms can be applied based on initial results and more decisions are made until spectral analysis is complete because certain spectral features are indicative of specific chemical bonds in materials the system can accurately identify and map those materials in this paper we describe the framework of the decision making process used for spectral identification describe specific spectral feature analysis algorithms and give examples of what analyses and types of maps are possible with imaging spectroscopy data we also present the expert system rules that describe which diagnostic spectral features are used in the decision making process for a set of spectra of minerals and other common materials we demonstrate the applications of tetracorder to identify and map surface minerals to detect sources of acid rock drainage and to map vegetation species ice melting snow water and water pollution all with one set of expert system rules mineral mapping can aid in geologic mapping and fault detection and can provide a better understanding of weathering mineralization hydrothermal alteration and other geologic processes environmental site assessment such as mapping source areas of acid mine drainage has resulted in the acceleration of site cleanup saving millions of dollars and years in cleanup time imaging spectroscopy data and tetracorder analysis can be used to study both terrestrial and planetary science problems imaging spectroscopy can be used to probe planetary systems including their atmospheres oceans and land surfaces

the detrital record of orogenesis a review of approaches and techniques used in the himalayan sedimentary basins the sediment archive of material eroded from an active tectonic region and stored in adjacent basins can provide a valuable record of hinterland tectonism especially when information in the source region itself is obscured by later metamorphism or removed by tectonism or erosion using the sediment record to document tectonism is a well established approach but more recently there has been a burgeoning of the number of isotopic techniques which can be applied to detrital material in particular single grain analyses thus the scope for application of detrital studies to a number of different tectonic problems has widened considerably in this review the example of sediments eroded from the himalayan orogen and preserved in the suture zone basin foreland basin remnant ocean basins and deep sea fans is used to illustrate the approach techniques as diverse as petrography heavy mineral xrf and sr nd studies single grain dating by ar ar u pb and fission track methodologies and single grain sm nd and pb isotopic analyses are described the paper documents how the sediment record can be used to determine the thermal and tectonic evolution of the orogen constrain mechanisms of continental deformation exhumation rates and palaeodrainage

exhumation of the sanbagawa blueschist belt sw japan by lateral flow and extrusion evidence from structural kinematics and retrograde p t t paths

dione s spectral and geological properties abstract we present a detailed analysis of the variations in spectral properties across the surface of saturn s satellite dione using cassini vims data and their relationships to geological and or morphological characteristics as seen in the cassini iss images this analysis focuses on a local region on dione s anti saturnian hemisphere that was observed by vims with high spatial resolution during orbit 16 in october 2005 the results are incorporated into a global context provided by vims data acquired within cassini s first 50 orbits our results show that dione s surface is dominated by at least one global process bombardment by magnetospheric particles is consistent with the concentration of dark material and enhanced co 2 absorption on the trailing hemisphere of dione independent of the geology local regions within this terrain indicate a special kind of resurfacing that probably is related to large scale impact process in contrast the enhanced ice signature on the leading side is associated with the extended ejecta of the fresh impact crater creusa 49 n 76 w although no geologically active regions could be identified dione s tectonized regions observed with high spatial resolution partly show some clean h 2 o ice implying that tectonic processes could have continued into more recent times

h2o ice particle size variations across ganymede s and callisto s surface abstract the detailed investigation of the variations in the h2o ice spectral signatures across ganymede s and callisto s surfaces derived from the nims near infrared mapping spectrometer data of the galileo mission have shown that in the case of areal mixing of h2o ice and dark components the associated band depth ratios bdrs of the individual h2o ice absorptions can be reliably used to qualitatively infer variations in the h2o ice particle size and even to semi quantitatively estimate particle size ranges on the satellites surfaces when largely unaffected by hydrated non ice materials or varying illumination conditions this modeling approach implies h2o ice particle radii ranging between 1 m and 1 mm on ganymede and up to 2 mm on callisto our analyses point to a continuous decrease of h2o ice particle sizes toward the poles on both satellites these variations may be caused by similar surface processes the smooth latitudinal trend may be related to the surface temperatures and possible thermal migration of h2o vapor to colder region in higher latitudes spencer 1987 and particle welding at lower latitudes larger particles in the equatorial region of callisto compared to ganymede could be due to its higher maximum surface temperature and a longer callistoan day resulting in more extensive particle welding

dynamic analysis of deformed quartz grains from the folded middle miocene momonoki subgroup of central japan origin of healed microcracks abstract dynamic analyses have been performed on quartz detrital grains from the sandstones of folded middle miocene strata momonoki subgroup found along the itoigawa shizuoka tectonic line istl central japan these quartz grains contain numerous deformation lamellae kink bands and healed microcracks in this study in addition to the dynamic analysis using deformation lamellae the arrow method of carter and friedman the c axis preferred orientations of kinked and unkinked grains and the preferred orientations of healed microcracks in individual grains are analyzed in order to test if these fabrics can be used to determine the palaeostress field in the case of basal a slip being dominant which probably holds for the analyzed quartz the principal stress axes can be inferred from the c axis preferred orientations of kinked and unkinked grains referred to as the kink method here because the preferred orientations indicate the favourable and unfavourable orientations respectively under stress for basal a slip results of the dynamic analyses of deformed quartz by the arrow method and by the kink method are consistent with each other and presumably indicate the palaeostress field associated with the macroscopic folding of the momonoki subgroup although the origin and timing of healed microcracking in the deformed quartz are not completely resolved the preferred orientation of poles to healed microcracks seems to consist of two or three sets of orthogonally disposed maxima whose directions coincide with the principal stress axes responsible for the crystal plasticity i e the lamellae and kinks in the quartz it is suggested that the successive release of residual tectonic stresses in the principal stress directions led to this orthogonal orientation pattern of healed microcracks in the present study then combined dynamic analyses of deformed quartz using deformation lamellae kink bands and healed microcracks provide a more complete quantitative picture of the bulk stress field during deformation particularly the combination of the arrow method with the kink method the latter applied to naturally deformed quartz for the first time in this study provides an indication of the relative magnitude of principal stresses finally in the context of the influence of fluids on the deformation processes in rocks i would like to stress that microcracking in crystalline or consolidated rocks is essential for the incorporation of enough fluid into the constituent crystals to activate fluid assisted deformation processes such as solution precipitation creep and that the regional development of microcracks may be controlled by the stress field associated with the fold development in layered consolidated rocks

crustal channel flows 1 numerical models with applications to the tectonics of the himalayan tibetan orogen 1 plane strain thermal mechanical numerical models are used to examine the development of midcrustal channel flows in large hot orogens in the models radioactive self heating reduces the viscosity of tectonically thickened crust and increases its susceptibility to large scale horizontal flow channels can be exhumed and exposed by denudation focused on the high relief transition between plateau and foreland we interpret the himalaya to have evolved in this manner channel flows are poorly developed if the channel has a ductile rheology based on wet quartz flow laws and well developed if there is an additional reduction in viscosity to 1019 pa s this reduction occurs from 700 c to 750 c in the models and is attributed to a small percentage of in situ partial melt melt weakening model ht1 provides an internally consistent explanation for the tectonic evolution of many features of the himalayan tibetan orogenic system erosional exhumation exposes the migmatitic channel equivalent to the greater himalayan sequence ghs between coeval normal and thrust sense ductile shear zones corresponding to the south tibetan detachment and the main central thrust systems outward flow of unstable upper crust rotates these shears to low dip angles in the model both the ghs and the lesser himalayan sequence are derived from indian crust with the latter from much farther south similar models exhibit a range of tectonic styles including the formation of domes resembling north himalayan gneiss domes model results are relatively insensitive to channel heterogeneities and to variations in the behavior of the mantle lithosphere beneath the model plateau

thermal structure in subducted units from continental moho depths in a paleo subduction zone the asemigawa region of the sanbagawa metamorphic belt sw japan

palaeoclimatic changes in northeastern qinghai tibetan plateau revealed by magnetostratigraphy and magnetic susceptibility analysis of thick loess deposits reconstruction of a complete quaternary record of climatic changes in the northeastern qinghai tibetan plateau is not well obtained because of high relief and extensive surface erosion in this study two long cores obtained from thick loess deposits in the region both contain clear alternations of loess and paleosols indicating distinct climate changes during the quaternary the palaeomagnetic stratigraphy and optically stimulated luminescence dating indicate that the loess deposition began approximately 2 0 ma ago with continuous accumulation until the holocene dust accumulation rates in this region are much higher than those in the central chinese loess plateau suggesting an extended dust source and or robust transport agent variations of magnetic susceptibility of the loess are a good proxy index of warm wet and cold dry alternations and are correlated with the intensity of pedogensis the magnetic susceptibility record reveals that a relatively cold dry climate dominated the northeastern qinghai tibetan plateau in the quaternary punctuated by warm wet phases a stepwise strengthening of the plateau summer monsoon with a significant strengthening at around 1200 1000 ka and at least 7 phases of strengthening of the plateau summer monsoon in the past 800 ka are interpreted from the core data the cores provide evidence that strengthened warm wet climates occurred at around 80 130 190 250 290 340 385 420 500 625 690 720 and 755 780 ka which may correlate to warm wet phases in the qinghai tibetan plateau the palaeoclimate changes probably were regulated by the glacial interglacial alternations

40ar 39ar mineral ages within metamorphic clasts from the kuma group eocene central shikoku japan implications for tectonic development of the sambagawa accretionary prism abstract the sambagawa metamorphic belt exposed in central shikoku records a high p t metamorphism it is comprised by the oboke nappe and the structurally overlying internally imbricated besshi nappe complex the besshi nappe complex locally is unconformably overlain by the kuma group eocene which consists of conglomerates and subordinate sandstones and mudstones clasts within kuma conglomerates exposed in the kamegamori district include low grade schists derived from the unconformably underlying sambagawa complex and high grade metamorphic rocks of uncertain provenance including garnet amphibolite and oligoclase bearing pelitic schist 36 ar 40 ar vs 39 ar 40 ar isotope correlation ages recorded by hornblende from two amphibolite clasts are 131 1 4 9 ma and 156 8 4 3 ma these are interpreted to date post metamorphic cooling through temperatures required for intracrystalline retention of argon and are older than cooling ages previously reported from any presently exposed segments of the sambagawa terrane plateau ages of 108 8 0 7 ma and 115 7 0 6 ma are recorded by muscovite from two clasts of schist muscovite within a proximal basement exposure of unconformably underlying sambagawa pelitic schist records a plateau age of 78 7 0 5 ma combined with isotopic ages previously reported for the sambagawa terrane 40 ar 39 ar data from clasts in the kuma group suggest that metamorphic culmination within a high grade source terrane occured at c 185 145 ma sectors of this complex cooled through c 500 c at c 150 ma other portions cooled through c 500 c at c 130 ma contrasts in the cooling ages likely reflect internal imbrication within the source terrane the besshi nappe complex reached peak metamorphic conditions at c 100 90 ma and experienced relatively rapid uplift and cooling at c 85 75 ma the besshi nappe complex was structurally emplaced onto the oboke nappe which attained peak metamorphic conditions at c 75 ma subsequently the entire sambagawa section was exhumed and most high grade metamorphic rocks were removed the eocene kuma group was unconformably deposited over the sambagawa terrane at c 50 ma

prograde amphiboles in hematite bearing basic and quartz schists in the sanbagawa belt central shikoku relationship between metamorphic field gradient and p t paths of individual rocks the prograde amphibole that coexists with chlorite epidote muscovite albite quartz and hematite in sanbagawa schists was examined to investigate the relationship between the prograde p t paths of individual rocks and the metamorphic field gradient in the sanbagawa metamorphic belt central shikoku the amphibole changes from actinolite through ferri winchite and crossite to barroisite and hornblende with increasing grade along the metamorphic field gradient however the sequence of prograde amphibole compositions in each sample varies in different mineral zones the general scheme can be summarized as magnesioriebeckite riebeckite crossite in the upper chlorite zone of lower grade rocks crossite or glaucophane barroisite in the garnet zone of medium grade rocks and actinolite or winchite barroisite hornblende in the albite biotite zone of higher grade rocks changes of amphibole composition indicate that the prograde p t path recorded in the higher grade rocks was situated on the higher temperature side of that of the lower grade rocks and on the lower pressure side of the metamorphic field gradient the systematic change of p t paths implies an increasing dp dt during continuous subduction these features can be interpreted as documenting prograde metamorphism within a young subduction zone that has a non steady state geotherm

channel flow and ductile extrusion of the high himalayan slab the kangchenjunga darjeeling profile sikkim himalaya abstract the geology of the kangchenjunga darjeeling profile in west sikkim and north bengal is consistent with the interpretation of the high himalaya metamorphic sequence as a ductile channel approximately 15 20 km thick extruding southwards bounded by major ductile shear zones above south tibetan detachment and below main central thrust zone in the yoksam to kangchenjunga section the entire slab is composed of migmatites or leucogranite sheets massive sills or sub horizontal to gently north dipping sheets of leucogranite make up the entire kangchenjunga massif and extend west at least as far as jannu shear sense indicators show both pure shear flattening fabrics and non coaxial south directed simple shear fabrics the carapace of the melt filled channel is seen around the southern margin of the singallila ridge and the darjeeling klippe where inverted metamorphic isograds from sillimanite k feldspar grade down through kyanite and staurolite to garnet biotite grade have been well documented we present an interpretation of the crustal structure of the sikkim himalaya based on field structural observations and utilizing the indepth seismic profile to constrain sub surface structure to the north we suggest that widespread melting within sillimanite grade gneisses triggered sudden and rapid ductile extrusion during the miocene by critically affecting the rheology of the middle crust under south tibet leucogranite melts lubricated shear zones facilitating rapid transport of heat towards the surface when the leucogranite melts began to cool ductile extrusion ended as thrusting propagated down section into the lesser himalaya since leucogranite expulsion was largely horizontal rather than vertical and since the std normal sense shear zone is very low angle dipping to the north decompression melting cannot be invoked as an origin for the leucogranites the heat source for such widespread melting at such shallow levels in the crust can only be explained by having a highly radioactive protolith of indian plate proterozoic sediments the overall map distribution and geometry of leucogranites migmatites and metamorphic isograds is compatible with both the everest makalu section to the west and the bhutan section east of the yadong gulu rift we conclude that the channel flow model is a viable explanation for this section of the himalaya

distribution of icy particles across enceladus surface as derived from cassini vims measurements abstract the surface of enceladus consists almost completely of water ice as the band depths of water ice absorptions are sensitive to the size of particles absorptions can be used to map variations of icy particles across the surface the visual and infrared mapping spectrometer vims observed enceladus with a high spatial resolution during three cassini flybys in 2005 orbits en 003 en 004 and en 011 based on these data we measured the band depths of water ice absorptions at 1 04 1 25 1 5 and 2 m these band depths were compared to water ice models that represent theoretically calculated reflectance spectra for a range of particle diameters between 2 m and 1 mm the agreement between the experimental vims and model values supports the assumption that pure water ice characterizes the surface of enceladus and therefore that variations in band depth correspond to variations in water ice particle diameters our measurements show that the particle diameter of water ice increases toward younger tectonically altered surface units with the largest particles exposed in relatively fresh surface material the smallest particles were generally found in old densely cratered terrains the largest particles 0 2 mm are concentrated in the so called tiger stripes at the south pole in general the particle diameters are strongly correlated with geologic features and surface ages indicating a stratigraphic evolution of the surface that is caused by cryovolcanic resurfacing and impact gardening

the first u th he thermochronology of pseudotachylyte from the median tectonic line southwest japan abstract to constrain the thermal and its spatial conditions of pseudotachylyte formation and cooling we dated zircons from a pseudotachylyte in the ryoke belt obtained about 100 m north from the median tectonic line in the taki area southwest japan and its host rock the hatai tonalite by the u th he method the ages obtained are 60 3 2 7 and 55 5 4 5 1se ma respectively which are essentially the same within the error this is the first u th he age directly measured for a pseudotachylyte the coincidence between these ages indicates that the pseudotachylyte formed at ca 60 ma under the ambient temperature of about 180 c the closure temperature of zircon u th he method the fish canyon tuff zircon one of the most well known geological age standards was also dated to confirm the validity of our analyses which yielded the weighted mean age of 29 6 1 5 1se ma consistent with other reported ages the average cooling rates and denudation rates of the host rock were calculated based on these ages reported zircon fission track ages ca 70 ma and apatite fission track ages ca 40 ma as about 12 c myr and about 0 6 mm yr from 70 to 60 ma and about 4 c myr and about 0 2 mm yr from 60 to 40 ma respectively assuming that the closure temperatures of zircon and apatite ft methods are 300 c and 110 c respectively and a geothermal gradient of 20 c km one possible reason of this slow down of denudation rates is the decrease of convergent rate and change in the movement direction of the pacific plate relative to the eurasian plate along the southwest japan in the paleogene

early orogenic normal faults and their reactivation during thrust belt evolution the gubbio fault case study umbria marche apennines italy foreland early orogenic and hinterland late orogenic extensional domains have been widely documented in the northern apennines where they are synchronous with contraction in the active part of the fold and thrust belt the progressive eastward migration of the contractional front and the associated hinterland extensional field implies that early orogenic extensional structures developed in the foreland domains may experience reactivation inversion we present new field data integrated with seismic evidence from the gubbio normal fault a 22 km long presently active fault showing a evidence for early orogenic extension since the lower miocene time b successive positive inversion during the upper miocene contraction and c renewed late orogenic extension during the quaternary age field data allowed two systems of mesoscale normal faults respectively interpreted as early and late orogenic structures to be recognised stress fields associated with contractional and early late orogenic extensional tectonic regimes are characterised by an overall coaxiality with directions of compression and tension consistently aligned ne sw the symmetry between the successive deformation stages is interpreted as a likely cause for the repeated reactivation of the gubbio fault since the miocene our analysis in the gubbio area shows that normal faults play a key role during the evolution of a fold and thrust belt from the early orogenic stages when they influence the geometry and evolution of foredeep basins to the late orogenic stages when quaternary activity due to extension reactivation controls the evolution of the intramountain basins

nappe structure of the sambagawa belt the thermal structure of the sambagawa belt which is reflected by minerals produced during the highest temperature of metamorphism generally is characterized by the occurrence of the highest grade schists in the middle of the structural pile the origin of this structure has been analysed in the upper horizon of the sambagawa schist sequence in central shikoku as an example for the structure of the whole belt the upper horizon of the sambagawa schist sequence in central shikoku consists of three nappes the saruta nappe ii the saruta nappe i and the fuyunose nappe in descending order of structural level the saruta nappe ii shows a downward increase of metamorphic grade from the garnet zone through the albite biotite zone to the oligoclase biotite zone the saruta nappe i consists mainly of rocks in the albite biotite zone and partly in the oligoclase biotite zone but the direction of the increase of metamorphic grade is not clear the fuyunose nappe shows an upward increase in metamorphic grade which changes from the glaucophane zone to the barroisite zone it is concluded that the saruta nappe ii and saruta nappe i were overturned and then mechanically coupled with the fuyunose nappe the sambagawa metamorphic field which is of the highest temperature phase of metamorphism appears to have had an inverted thermal gradient and a thermal structure comparable with that expected in the deeper parts of a subduction complex

the himalayas organic carbon burial and climate in the miocene cooling ages of rock in the himalayas imply that rapid exhumation between the main central thrust system and the south tibetan detachment system occurred between 21 and 17 ma the generation of relief and enhanced weathering which followed this event may have resulted in a pronounced increase in the delivery of dissolved strontium carbon phosphorus and other chemical weathering products to the ocean richter et al 1992 the increased supply of nutrients stimulated productivity in oceanic upwelling zones and expansion of the oxygen minimum zone leading to enhanced burial and preservation of organic matter in the monterey formation and other deposits from this interval a downdraw of atmospheric co2 associated with enhanced chemical weathering rates and organic matter burial may have led to global cooling and the expansion of the antarctic ice sheet by 15 ma the above scenario differs from the monterey hypothesis of vincent and berger in that co2 downdraw is primarily via silicate weathering rather than organic carbon burial and that organic carbon burial is driven by increased delivery of nutrients to the ocean rather than by stronger upwelling a carbon mass balance calculation which assumes that river fluxes have been increasing over the last 40 ma predicts that absolute organic carbon burial increased over this interval while at the same time the fraction of carbon buried as organic matter versus carbonate decreased this implies that the organic carbon cycle has acted as a net source of co2 to the atmosphere over the late cenozoic

cassini s geological and compositional view of tethys abstract the saturnian satellite tethys exhibits geological and spectral properties whose appearance nature and spatial distribution partly mirror those identified on the neighboring satellites dione and rhea or fit to the picture how spectral surface properties are expected to change from one satellite to the other within the inner saturnian system however we also identified spectral variations that are unique in the saturnian system whereas geologically young surface features are characterized by pure h2o ice composition with relatively large particles which match the particle sizes measured for fresh surface features also on dione and rhea geologically old weathered regions are dominated by submicron sized ice particles our investigations confirm that the odysseus impact event did not cause the formation of tethys extended graben system ithaca chasma on the contrary odysseus might be responsible for the n s trending icy bands that mark tethys surface in the center of its leading and trailing hemisphere

monsoon evolution and tectonics climate linkage in asia an introduction peter d clift1 ryuji tada2 hongbo zheng3 school of geosciences meston building university of aberdeen aberdeen ab24 3ue uk department of earth and planetary science university of tokyo science building 1 7 3 1 hongo tokyo 113 0033 japan institute of surficial geochemistry school of earth science and engineering nanjing university 22 hankou road nanjing people s republic of china

plastic anisotropy and geometrical hardening in quartzites abstract lattice rotation in crystals which creates a characteristic preferred orientation in deformed tectonites takes place whenever slip dislocation glide is involved in the deformation process the crystal lattice either rotates to an orientation favorable for further slip or to one that is unfavorable resulting in an overall hardening or softening of the polycrystal during deformation this effect of plastic anisotropy which arises due to changes in the crystal orientation distribution is analyzed quantitatively for quartz polycrystals using the taylor model the results indicate that in easy basal a slip region low temperature quartzites become softer in flattening with increasing strain whereas in easy prism a slip region high temperature quartzites become softer in elongation with increasing strain accordingly deformation is facilitated by flattening at low temperature and by elongation at high temperature if it can be heterogeneous to a certain degree this is borne out by geological observations which indicate that at low metamorphic grade flattening to plane strain is the most frequently observed strain mode e g moine thrust whereas at high metamorphic grade constriction is favored e g sambagawa belt the strain distribution in a large geological body may depend on internal anisotropic plastic properties such as preferred slip systems as well as imposed external boundary conditions heterogeneities are also observed on the hand specimen scale grains in orientations where easier slip systems are difficult to activate remain relatively undeformed in a ductile matrix consisting of grains in which strain is accommodated mostly by easier slip systems some aspects of the taylor model for creep rate sensitivity are discussed

effect of orogeny plate motion and land sea distribution on eurasian climate change over the past 30 million years the eurasian climates of today 10 million and 3o million years ago are simulated using an atmospheric general circulation model that incorporates realistic continental geography and epicontinental sea distributions the resulting climates compare well with various palaeoclimate records the retreat of the paratethys an epicontinental sea shifts the central asian climate from temperate to continental conditions and plays as important a role as uplift of the himalayan tibetan plateau in driving the asian monsoon changes

lithium isotope evidence for enhanced weathering during oceanic anoxic event 2 the ocean anoxic event 2 oae2 about 93 5 million years ago was marked by high atmospheric co2 concentration rapid global warming and marine anoxia and euxinia the event lasted for about 440 000 years and led to habitat loss and mass extinction the marine anoxia is thought to be linked to enhanced biological productivity but it is unclear what triggered the increased production and what allowed the subsequent rapid climate recovery here we use lithium isotope measurements from carbonates spanning the interval including oae2 to assess the role of silicate weathering we find the lightest values of the li isotope ratio 7li during oae2 indicating high levels of weathering and therefore atmospheric co2 removal which we attribute to an enhanced hydrological cycle we use a geochemical model to simulate the evolution of 7li and the ca sr and os isotope tracers our simulations suggest a scenario in which the eruption of a large igneous province led to high atmospheric co2 concentrations and rapid global warming which initiated oae2 the simulated warming was accompanied by a roughly 200 000 year pulse of accelerated weathering of mafic silicate rocks which removed co2 from the atmosphere the weathering also delivered nutrients to the oceans that stimulated primary productivity we suggest that this process together with the burial of organic carbon allowed the rapid recovery and stabilization from the greenhouse state

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

c axis fabrics and microstructures in quartz schist from the sambagawa metamorphic belt central shikoku japan an integrated study on c axis fabric and microstructure of plastically deformed and dynamically recrystallized quartz in quartz schist from the sambagawa metamorphic belt in central shikoku japan has revealed a correlation between c axis fabrics and three dimensional grain shape recrystallized quartz grains with prolate k 1 oblate 0 2 k 1 and uniaxially oblate k 0 2 shapes exhibit c axis fabric patterns of cleft girdles around the principal finite elongation direction x type i crossed girdles and small circle girdles around the principal finite shortening direction z respectively fabric intensity calculated from the eigenvalues of orientation tensors of the c axis fabrics increases with increasing magnitude of the apparent finite strain inferred from the shape of recrystallized quartz grains these results indicate that the development of recrystallized grain shape as well as c axis fabrics is primarily controlled by finite strain during progressive deformation however the finite strain estimated from three dimensional grain shape is systematically deviated toward the flattening field compared to that estimated from the c axis fabrics we investigated several possible causes for the discrepancy and propose that a scenario of coupled deformation and dynamic recrystallization in the quartz schist is responsible for the formation of these microstructures

p t d path of eclogite from the sambagawa belt deduced from combination of petrological and microstructural analyses both structural and petrological data can be used to constrain the introduction p t path of an eclogitic schist unit the seba basic schist in the metamorphism of the sambagawa belt of sw japan was sambagawa belt of sw japan the relationships between these classified by miyashiro 1973 as an intermediate hightwo sets of data are well defined by porphyroblastic and other pressure facies series petrological chronological and microstructures the derived p t path for the seba basic schist structural data suggest that the evolution of the sambahas an overall clockwise trajectory with the decompression or gawa belt is in general compatible with the predictions exhumation related path taking place under a lower p t gradient made for metamorphism in the deeper parts of an acthan the burial or subduction related path the clockwise nature cretionary wedge e g banno sakai 1989 enami et of the p t path is qualitatively supported by chemical zoning of al 1994 takasu et al 1994 wallis 1998 the belt is amphibole coexisting with eclogitic minerals the significant feature composed mainly of metamorphosed sediments that have of the p t path is the presence of two temperature maxima the pelitic basic or siliceous compositions the mineral asfirst in the eclogite facies and the second in the epidote amphibolite semblages of most rocks are characteristic of metafacies the existence of two temperature maxima gives a simple morphism under greenschist blueschist or epidoteexplanation for the observation that metamorphic zonal boundaries amphibolite facies conditions however eclogite facies postdating the eclogite facies metamorphism cross cut the distribution mineral assemblages have also been reported e g takasu of the main eclogite bodies in the sambagawa belt estimates of 1989 which occur dominantly within metagabbro bodmetamorphic pressure using the jadeite content of clinopyroxene in ies most of these eclogite bearing bodies are located in the seba area demonstrate the existence of a tectonic discontinuity the besshi district fig 1 the estimated p t paths between the eclogitic schist and surrounding non eclogitic schist for the eclogite bearing bodies are distinct from those structural studies show that although these two units have experienced estimated for the surrounding epidote amphibolite grade very different peak metamorphic conditions they became juxtaposed rocks and show considerable variation amongst them

chronology of sanbagawa metamorphism by collating age data based on the fossil age of the protoliths radiometric dating of the metamorphic minerals and sedimentary records of erosion at the earth s surface the history of the sanbagawa metamorphism can be summarized as follows 1 the pre metamorphic sedimentary rocks carboniferous jurassic early cretaceous became mixed and formed a thickened packet in the vicinity of an ancient trench through a variety of subduction related tectono sedimentary processes probably in early cretaceous time c 130 120 ma 2 the subducted protoliths underwent progressive metamorphism reaching a maximum depth of c 30 km in late early cretaceous time c 116 10 ma 3 the high p t metamorphic rocks began to rise toward the surface during the interval 110 50 ma with minimum estimates for the average cooling rate around 9 12 c ma and an average uplift rate around 0 4 0 5 mm year 4 finally at some stage after reaching the erosional surface the high p t metamorphic rocks were covered unconformably by the middle eocene c 50 42 ma kuma group on the basis of the present chronological summary of the sanbagawa metamorphism the areal extent of the sanbagawa metamorphism is also discussed with respect to the weakly metamorphosed subduction accretion complex of the next tectonic belt to the south the northern chichibu belt

coupled micro faulting and pressure solution creep overprinted on quartz schist deformed by intracrystalline plasticity during exhumation of the sambagawa metamorphic rocks southwest japan abstract in the sambagawa schist southwest japan while ductile deformation pervasively occurred at d 1 phase during exhumation low angle normal faulting was locally intensive at d 2 phase under the conditions of frictional viscous transition of quartz c 300 c during further exhumation into the upper crustal level accordingly the formation of d 2 shear bands was overprinted on type i crossed girdle quartz c axis fabrics and microstructures formed by intracrystalline plasticity at d 1 phase in some quartz schists the quartz c axis fabrics became weak and finally random with increasing shear accompanied by the decreasing degree of undulation of recrystallized quartz grain boundaries which resulted from the increasing portion of straight grain boundaries coinciding with the interfaces between newly precipitated quartz and mica we interpreted these facts as caused by increasing activity of pressure solution the quartz grains were dissolved mostly at platy quartz mica interface and precipitated with random orientation and pinned by mica thus having led to the obliteration of existing quartz c axis fabrics in the sheared quartz schist the strength became reduced by the enhanced pressure solution creep not only due to the reduction of diffusion path length caused by increasing number of shear bands but also to enhanced dissolution at the interphase boundaries

shrimp u pb zircon dating of quartz bearing eclogite from the sanbagawa belt south west japan implications for metamorphic evolution of subducted protolith in order to decipher the origin of eclogite in the high p t sanbagawa metamorphic belt shrimp u pb ages of zircons from quartz bearing eclogite and associated quartz rich rock metasandstone were determined one zircon core of the quartz rich rock yields an extremely old provenance age of 1899 79 ma suggesting that the core is of detrital origin eight other core ages are in the 148 134 ma range and are older than the estimated age for trench sedimentation as indicated by the youngest radiolarian fossil age of 139 135 ma from the sanbagawa schists ages of metamorphic zircon rims 132 112 ma from the quartz rich rock are consistent with metamorphic zircon ages from the quartz bearing eclogite indicating that eclogite facies metamorphism peaked at 120 110 ma these new data are consistent with both the iratsu eclogite body and surrounding highest grade sanbagawa schists undergoing coeval subduction zone metamorphism and subsequent re equilibration under epidote amphibolite facies conditions during exhumation

a mechanism for syn collisional rock exhumation and associated normal faulting results from physical modelling abstract properly scaled physical modelling of the subduction of continental lithosphere is performed with a three layer lithospheric model the model includes a strong upper crust a weak ductile lower crust and a strong mantle part the lithosphere is underlain by a low viscosity asthenosphere subduction is produced by a piston push force and the pull force from the mantle lithospheric layer which is slightly denser than the asthenosphere the results of the modelling are tested on the example of the himalayas in the experiments all lithospheric layers subduct into the mantle to a depth corresponding to 200 300 km in nature until the upper crustal layer fails in front of the subduction zone forming a first major thrust fault the mct in our interpretation of the himalayas underthrusting along this fault increases relief which upon reaching a height corresponding to several kilometres in nature is removed mechanically by a blade the blade is moved horizontally back and forth throughout the experiment this erosional unloading mechanism causes the previously subducted segment of the buoyant upper crust to slide upward over the weak lower crust at a rate comparable to the subduction rate the rise of the crustal slice accelerates displacement along the crustal thrust while producing a normal sense motion and exhumation of the material from great depth up to 40 50 km depending on the model parameters along the upper surface of the slice the rapid uplift of the crust then ceases and the orogenesis occurs further due to continuing underthrusting along the first major thrust at a certain stage the crust fails again producing a new major crustal thrust the mbt in the himalayas

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 higher grade metamorphic zonation of the sambagawa metamorphic belt in central shikoku japan the higher grade metamorphic zonation of the sambagawa sanbagawa belt is established for the first time for the whole area of central shikoku as discontinuous reactions to define the isograd are absent the metamorphic grade is primarily determined by the mg fe partitioning between garnet and chlorite along representative traverses however for regional mapping mineralogical features of the pelitic schists such as using mineral assemblages of more than divariant equilibrium the modal garnet to chlorite ratio and the optical properties of chlorite are employed as auxiliary criteria the presence of the highest grade mineral zone in the middle of the structural level is confirmed but its spatial distribution is far more complex than hitherto accepted thermal axes are now confirmed at three different structural levels a model is presented in which the stacking of thrust sheets of different grade took place while metamorphic reactions were in progress thermal readjustment brought a continuous metamorphic temperature gradient across and within the thrust sheets tectonic blocks of metagabbro and ultramafic rock were emplaced synchronously with thinning and subsequently also re equilibrated local anomalies of metamorphic grade represented by mixing of schists of different metamorphic grade exist but they are due to a later stage event

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

muscovite k ar ages of the sanbagawa schists japan and argon depletion during cooling and deformation seventy muscovites from schists in the sanbagawa terrain in central shikoku were dated by the k ar method the muscovite ages are consistently older with increasing metamorphic grade within the same zone the ages are significantly younger in schists which have been more severely deformed these k ar age variations could be due to systematic argon depletion during deformation i e to the dynamic recrystallization of muscovites during ductile deformation that formed a large scale recumbent fold during the uplift and cooling argon loss was greater in schists that were more extensively deformed and in the lower grade zone that experienced a longer period of low temperature deformation than the higher grade zone the relationships between age and grain size in a pelitic schist suggest that coarse grained muscovites lost more argon than the finegrained ones there was no significant resetting of ages in the vicinity of major strike slip faults such as the median tecotonic line or near thrust faults the combination of geochronological and geological data constrains the cooling rate of the sanbagawa schists to 9 12 c ma in the oligoclase biotite zone in central shikoku japan

evolution and variability of the east asian summer monsoon during the pliocene evidence from clay mineral records of the south china sea the late pliocene is thought to be characterized by the simultaneous intensification of both the east asian winter monsoon eawm and east asian summer monsoon easm however the evolution of the easm during the pliocene remains still controversial and only little is known about the dynamics of the easm during the pliocene on orbital time scales here we use clay mineral assemblages in sediments from ocean drilling program odp site 1143 in the southern south china sea scs to obtain proxy records of past changes in the easm climate during the pliocene provenance analysis suggests that illite chlorite and kaolinite originated mainly from the mekong river drainage area smectite was derived mainly from the indonesian islands the kaolinite illite ratio and the chemical index of alteration cia of siliciclastic sediments allowed us to reconstruct the history of chemical weathering and physical erosion of the mekong river drainage area and thus the evolution of the easm during the pliocene our clay minerals proxy data suggests a stronger easm during the early pliocene than during the late pliocene we propose that the long term evolution of the easm has been driven by global cooling rather than the uplift of the tibetan plateau spectral analysis of kaolinite illite ratio displays a set of strong periodicities at 100 ka 30 ka 28 ka 25 ka and 22 ka with no clear obliquityrelated signal our study suggests that the pliocene easm intensity on orbital time scales is not only controlled by the northern hemisphere summer insolation but also strongly influenced by equatorial pacific enso like ocean atmosphere dynamics c 2010 elsevier b v all rights reserved

evolution of the sambagawa metamorphic belt japan abstract the high pressure sambagawa metamorphic belt evolved within a mesozoic accretionary prism that developed in the convergent margin along the east of the eurasian continent although extensively researched recent studies in this region have revealed a number of important features that allow more complete reconstruction of the tectonic history i the grouping of radiometric ages suggests the existence of three major structural units with distinct metamorphic histories one of these has since been completely eroded away ii the difference of ages recorded by different minerals is in many cases small suggesting rapid exhumation after reaching peak metamorphic conditions iii the dominant ductile deformation of the sanbagawa belt representing major orogen parallel flow is commonly related to retrograde metamorphic reactions this shows that the associated fabric developed during exhumation not subduction iv kinematic studies suggest that the main ductile deformation caused major ductile thinning of the region which was probably a significant factor in causing exhumation of the region

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

40ar 39ar mineral age constraints for the tectonothermal evolution of the sambagawa metamorphic belt central shikoku japan a cretaceous accretionary prism abstract the sambagawa metamorphic belt exposed in central shikoku japan is tectonostratigraphically represented by the oboke and structurally overlying besshi nappe complexes oboke units consist of metamorphic rocks largely derived from clastic protoliths sandstone and conglomerate whereas besshi units are dominated by rocks with oceanic protolith affinities shale chert and greenstone whole rock schist samples from low grade oboke units record intermediate temperature 40 ar 39 ar plateau ages of 70 77 ma which are interpreted to closely date attainment of maximum metamorphic conditions whole rock schist samples from besshi units chlorite zone record plateau ages of 85 94 ma suggesting an earlier metamorphic climax within this structural level amphibole within the albite biotite zone in besshi records 36 ar 40 ar vs 39 ar 40 ar plateau isotope correlation ages of 84 87 ma which are interpreted as dating the post metamorphic cooling through temperatures required for intracrystalline retention of argon a slightly older isotope correlation age 94 ma is recorded by amphibole from the higher grade oligioclase biotite zone contrasts in amphibole cooling ages are interpreted as reflecting thrust related metamorphic inversion and resultant earlier cooling of higher grade structural units muscovite from all structural levels of the besshi nappe complex records 40 ar 39 ar plateau ages of 76 89 ma these are interpreted to date post metamorphic cooling through appropriate closure temperatures following structural assembly of the besshi nappe complex protoliths of the besshi units originated at approx 130 ma and attained maximum metamorphic conditions during entrainment in an accretionary complex at approx 90 100 ma higher grade sequences developed at deeper levels along the hanging wall of the accretionary complex during subsequent structural uplift to shallower crustal levels the besshi nappe complex was tectonically emplaced over oboke units

exhuming the sanbagawa metamorphic belt the importance of tectonic discontinuities tectonic processes that have been proposed to explain the transport to the surface of regional metamorphic belts can be broadly divided into two types i corner flow within a convergent margin bounded by two essentially rigid plates associated with extension at shallow levels this type of model assumes deformation is distributed throughout the margin and that any discontinuities are of secondary importance ii expulsion or extrusion of coherent metamorphic nappes in this second idea tectonic discontinuities are fundamental in the transport to the surface of metamorphic rocks the wealth of geological data available from a variety of studies in the sanbagawa metamorphic belt southwest japan makes it well suited for studying the relative importance of continuous vs discontinuous deformation in the process of exhumation in the sanbagawa belt a sudden decrease in metamorphic pressure going down section of several kilobars suggests the presence of a major tectonic contact separating two major regional nappes an overlying higher pressure besshi nappe and an underlying lower pressure oboke nappe major tectonic discontinuities have also been proposed within the besshi nappe however indicators of metamorphic temperature the results of radiometric age dating and microstructural studies all suggest that post metamorphic discontinuities are minor and that this nappe formed and remained as an essentially coherent unit lithological associations and petrological studies suggest the following positions for the two nappes the besshi nappe formed deep within the former accretionary wedge adjacent to the overlying mantle wedge and with a dip of roughly 30 c in contrast the oboke nappe formed at moderate depths within the accretionary wedge was distant from the mantle wedge and was roughly horizontal penetrative deformation that post dates the peak of metamorphism has affected nearly all of the sanbagawa belt and has played an important role in its exhumation however the presence of a broad coherent besshi nappe overlying the lower pressure oboke nappe suggests that some process such as buoyancy driven extrusion was also important in the exhumation process and in forming the structure of the sanbagawa metamorphic belt

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

hydrated salt minerals on ganymede s surface evidence of an ocean below reflectance spectra from galileo s near infrared mapping spectrometer nims suggests that the surface of ganymede the largest satellite of jupiter contains hydrated materials these materials are interpreted to be similar to those found on europa that is mostly frozen magnesium sulfate brines that are derived from a subsurface briny layer of fluid

structural and petrological constraints on the tectonic evolution of the garnet lherzolite facies higashi akaishi peridotite body sanbagawa belt sw japan 1 the higashi akaishi peridotite body in the sanbagawa metamorphic belt is a unique example of a kilometer scale body including garnet lherzolite facies rocks from an oceanic subduction type orogen in this body four distinct deformational phases d1 d2 d3 and d4 can be defined the tectonic significance of d1 is unclear microstructural observations and application of garnet orthopyroxene geothermobarometry suggest that d2 took place during high temperature subduction up to depths of 100 km from 2 3 gpa to above 2 8 gpa at temperature of 700 800 c or more d3 represents a major phase of exhumation after exhumation the higashi akaishi body became juxtaposed with the adjacent besshi and eclogite units at a depth of around 35 km 1 gpa the common history of the two units after d3 is shown by the intertectonic growth of tremolite between d3 and d4 that corresponds to a stage of intertectonic growth of plagioclase in the besshi and eclogite units both the spatial distribution and the estimated pressure temperature p t conditions of this intertectonic phase of mineral growth in the different units are in excellent agreement the development of an antigorite schistosity during the late stage of d2 indicates an anticlockwise p t path with cooling at depths of around 100 km the association of the antigorite bearing fabric and the onset of exhumation after the d2 stage suggest that the density decrease due to antigorite formation may have been sufficient to trigger the exhumation of dense ultramafic body from uhp conditions in subduction zones syn d2 water introduction and the anticlockwise p t path are expected when convection in the mantle wedge brings peridotite toward a subduction boundary through mantle wedge or when peridotite is dragged down along a subduction boundary that is progressively cooling immediately after initiation of subduction

effects of mountain uplift on east asian summer climate investigated by a coupled atmosphere ocean gcm abstract to study the effects of progressive mountain uplift on east asian summer climate a series of coupled general circulation model cgcm experiments were performed eight different mountain heights were used 0 no mountain 20 40 60 80 100 control run 120 and 140 the land sea distribution is the same for all experiments and mountain heights are varied uniformly over the entire globe systematic changes in precipitation pattern and circulation fields as well as sea surface temperature sst appeared with progressive mountain uplift in summertime precipitation area moves inland on the asian continent with mountain uplift while the pacific subtropical anticyclone and associated trade winds become stronger the mountain uplift resulted in an sst increase over the western tropical pacific and the maritime continent and an sst decrease over the western indian ocean and the central subtropical pacific there is a drastic change in the east asian circulations with the threshold value

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

role of panama uplift on oceanic freshwater balance comparison between planktic foraminiferal oxygen isotope records from the caribbean sea ocean drilling program odp site 999 and the equatorial east pacific odp site 851 suggests an increase in caribbean surface water salinity between 4 7 and 4 2 ma the modern atlantic pacific salinity contrast of about 1 became fully established at 4 2 ma as reflected by a 0 5 planktic foraminifera 18o enrichment in the caribbean sea this is interpreted as the result of restricted surface water exchange between the tropical atlantic and pacific in response to the shoaling of the central american seaway as a consequence the atlantic and pacific surface ocean circulation regime changed as did the freshwater balance between the major ocean basins simultaneous shifts in benthic carbon isotope records in the caribbean sea suggest an intensification in north atlantic thermohaline circulation these results indicate that the panamanian isthmus formation caused several new ocean atmosphere feedback mechanisms that have affected climate since the early pliocene

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