axial topographic relief associated with ridge transform intersections the axial depths of mid ocean ridges are consistently observed to increase along strike from the central part of a ridge segment to its intersection with a transform fault variations in the magnitude of this axial relief probably reflect variations in the magmatic and tectonic processes that characterize plate accretion and seafloor spreading at each individual segment constrained by observations from 41 ridge transform intersections we investigate the relative effect on axial depths of transform offset segment length regional depth and spreading rate we first solve for a model of ridge segment relief as a function of the age offset of the transform and the segment length both scaled by spreading rate through a series of regression analyses covering a range of spreading rate scalings we determine a model which predicts segment relief at both fast and slow spreading centers to within 240 m standard deviation we find that half segment scale deepening is more than a direct thermal effect of the juxtaposition of a cold lithospheric edge and the end of a ridge at a transform fault a second inversion problem is solved to determine the parameters that contribute to the absolute depth of the ridge axis we can show that the absolute depth of the shallowest section of a ridge segment is strongly dependent on regional depth and that this dependence is more complex at slow spreading ridges reflecting the common but not ubiquitous existence of a median valley there these results in combination with the segment relief analysis clearly indicate that the axial depth at a ridge transform intersection is a function of both long wavelength processes such as may control hot spot distribution and local tectonic factors including but not limited to transform offset

the process of formation of ocean crust ocean crust is the outermost layer of earth under the oceans it is separated from the underlying mantle by a seismic transition zone called the moho a widely held view is that the moho represents a petrologic change from basaltic type rocks to a mantle composed mostly of olivine and pyroxene according to this view crust is formed by a steady segregation of basaltic melt derived from partial melting of the mantle into a crustal magma chamber wherein cooling and crystallization bring about steady state accretion to the continuously spreading plates there is sufficient disagreement between the predictions of this hypothesis and marine geophysical data to cause one to doubt the validity of this formation process at least two other processes are more compatible with the geophysical data in one the crust is formed from the episodic injection of basaltic dikes from a mantle reservoir and the moho is a primary petrologic boundary in the other the crust is treated as a mechanical boundary layer in which thermal contraction results in cracking by comparison in the mantle thermal contraction is accommodated by flow the upper part of the crust is formed from episodic extrusion and intrusion of basaltic melt the lower crust is formed by rapid hydrothermal alteration of mantle that may be continuously or episodically injected by viscous flow at temperatures below the melting temperature

magmatism and metamorphism in the lhasa block tibet a geochronological study u pb systematics in small fractions of zircon monazite and sphene 5 to 20 grains each have been analyzed in order to decipher the age and origin of magmatism in the lhasa block and to date the tectono metamorphic events additionally we analyzed a few minerals for rb sr the results show that the granites and an ignimbrite were formed essentially through anatexis of continental crust the magma source regions of which consisted of palaeozoic to precambrian materials up to 2 b y in age the following episodes can be distinguished 1 an early tertiary plutonism volcanism in the southern lhasa block yangbajing region strongly related to intracontinental tectono metamorphism 2 a late jurassic to early cretaceous plutonism in the central and northern lhasa block and 3 a middle jurassic low grade metamorphism affecting the basement at the northern margin 171 6 m y ago the early tertiary magmatism and metamorphism in the south are probably a direct result of tethys subduction indian plate

structural elements of the southern tethyan himalaya crust from wide angle seismic data a deep seismic common midpoint cmp profile shot across the southern margin of the tethyan himalaya as the first stage of project international deep profiling of tibet and the himalaya indepth imaged the top main himalayan thrust or mht and bottom moho of the indian continental crust underthrusting the himalaya we used portable seismographs to record the cmp profile shots at a wide range of offsets up to 155 km our short offset data corroborate the cmp profile data while our large offset data are dominated by a band of reflectivity we interpret the bright onset of this reflective band as the basal detachment of the south tibetan detachment system and a phase at the end of the reflective band as the mht we used the cmp profile first break data to model the uppermost 2 km in detail the depth of young extensional basins of the yadong rift system along the cmp profile was constrained to a maximum of 2 km yielding a throw of 4 6 km across the eastern flank of the rift and a rough estimate of approximately 1 5 east west extension by normal faulting in the tethyan himalaya the wide angle data were used to construct a crustal seismic velocity model down to the mht the south tibetan detachment system stds basal detachment reflector is observed dipping 12 5 nne from a depth of about 6 km beneath the surface under the south end of the cmp profile to a depth of 22 km then flattening to a dip of only 2 5 nne thus our observations suggest that the stds basal detachment is a deep rooted basement fault for the mht we observe a dip of 7 5 nne from 20 km depth below sea level at the crest of the high himalaya crystalline sheet to 36 km below sea level 40 km beneath the surface at a distance of about 70 km south of the indus yarlung suture iys from geometrical arguments we suggest that indian crust underthrusts the surface expression of the iys within the crust but does not form the lower crust of central and northern tibet

mid ocean ridges fine scale tectonic volcanic and hydrothermal processes within the plate boundary zone a first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics as we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth s surface it is becoming increasingly necessary to study the tectonic volcanic and hydro thermal processes within the spreading center plate boundary zone all ocean ic crust bears the imprint of these processes this review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid ocean ridges with reference to associated volcanic and hydrothermal processes it draws heavily on recent studies that use deeply towed instrument packages multi beam bathymetric mapping ocean bottom instruments and alvin e g the famous amar rise and galapagos expeditions we begin with a review of the large scale structure of spreading centers we then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity as well as the generation of magnetic anomaly stripes and their implications for crustal generation one of our findings is that the initial two dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations

new insights into the origin of two contrasting himalayan granite belts the two parallel belts of miocene granite that extend along much of the himalaya differ in age petrogenesis and emplacement style we suggest that their origin is linked to shear heating on a continuously active decollement that cuts through previously metamorphosed indian supracrustal rocks that were transformed into basement during the initial stages of the indian asian collision numerical simulations assuming a shear stress of 30 mpa indicate that initiation of slip on the himalayan thrust at 24 ma could trigger discontinuous melting reactions leading to formation of the high himalayan granite chain from 24 to 20 ma and the north himalayan belt from 18 to 12 ma this model result agrees well with observation as do model predictions regarding emplacement style and location

timing of tibetan uplift constrained by analysis of volcanic rocks the tibetan plateau has had a central role in the development of recent models for the mechanics of mountain belts1 2 and cenozoic global climate change3 the present elevation and extensional deformation of the plateau probably result from uplift owing to convective thinning of the underlying lithospheric mantle1 2 an age for the uplift would provide a valuable constraint on these models but because recently proposed indicators of uplift are all climate dependent they are equivocal possibly reflecting global cooling rather than regional uplift4 here we present new geochemical data on post mid miocene lavas from the plateau which show that the lavas were derived from the lithospheric mantle simple thermal arguments indicate that the generation of such magmas also necessitates thinning of the lithospheric mantle thus volcanism is coincident with uplift providing a climate independent means of dating the onset of uplift dating by the laser 40ar 39ar technique places the beginning of this volcanism and therefore the time of uplift of the tibetan plateau at 13 myr ago

landslides on charon abstract we investigated five large landslides identified in the serenity chasma region of charon the identification of these landslides involved a search for these features in images taken by cameras onboard the new horizons spacecraft various landslide properties were analyzed based on their morphologies using a digital terrain model of the region we found that landslides are confined to the walls of the large normal fault scarps that make up serenity chasma based on extensive landslide runout lengths l relative to their drop heights h we classified these features as long runout landslides by analyzing their geometries we estimated the friction coefficients of the landslide material h l to be between 0 15 to 0 31 and the runout efficiencies l h to be between 3 2 and 6 8 we also estimated that the specific energy released during landslide motion ranged from 0 8 to 1 3 kj kg 1 these amounts of energy were too low to have generated significant melt around landslide particles

structure and evolution of the himalaya tibet orogenic belt the 1981 french chinese expedition to tibet focused on the lhasa block extending earlier coverage 400 km north of the tsangpo suture the lhasa block stood between 10 and 15 n latitude over most of the upper cretaceous and eocene and if gondwanian in origin had detached from gondwana by early permian seismic profiles reveal a complex moho topography resulting both from multiple continental thrusting and large scale strike slip faulting subduction related granitoids representing mixtures of mantle and crustal components and anatectic granitoids have been analysed and dated this study emphasizes the role of smaller blocks in the accretion of the continental mosaic

thermal and mechanical behaviour of the oceanic lithosphere constrained by geoid and gravity anomalies over the mid atlantic ridge axial valley abstract geoid profiles derived from seasat altimetry data crossing the north atlantic mid oceanic ridge are analysed an obliquity of about 60 does not perturb the analysis much conjointly with bathymetric and gravimetric profiles of the famous campaign these geoid profiles are compared with theoretical ones assumptions made in this study concern the neglect of the variations along the ridge axis the choice of the half space model for the cooling of the plate which is improved by the introduction of a lead term and the use of a flat earth approximation in the computation of geoid and gravity anomalies the choice of a full two dimensional numerical technique allows a good evaluation of these anomalies for the detailed structure close to the axis the models take into account the thermal cooling of the plate the shifting of the isotherms due to the axial valley the contribution of the crust different assumptions are made relative to crustal thinning beneath the valley to a lithospheric necking or to a viscous drag effect of the rising magma eventually connected with the formation of the axial valley isostatic compensation of the axial valley by crustal thinning is clearly confirmed by the gravimetric data alone but not by the altimetric data differences in g and h gravimetry and geoid low respectively are not large enough to really discriminate between the proposed models the theoretical amplitudes appear overestimated for gravity anomalies and underestimated for geoid anomalies if confirmed this fact would give indications to improve the model

mesozoic and cenozoic volcanic rocks from central and southern tibet 39ar 40ar dating petrological characteristics and geodynamical significance field relations radiometric ages and mineralogical and chemical characteristics of the orogenic volcanic rocks from the lhasa block tibet are studied volcanic activity started before the deposition of the albian aptian sedimentary units takena formation and xigaze group volcanism is volumetrically limited in the northern part of the lhasa block basalts andesites and dacites are present while ignimbrites are scarce radiometric ages of these rocks range between 110 and 80 ma conversely in the southern part of the lhasa block volcanism is largely developed gangdese belt there paroxysmal activity occurred later than in the north between 60 and 50 ma lingzizong formation this volcanism is mainly represented by andesites and ignimbrites a younger episode at about 15 10 ma took place in this southern region the mineralogical and geochemical characteristics indicate that as a whole the volcanic rocks from the lhasa block belong to a calc alkaline continental margin series enriched in k20 and other incompatible elements subordinate potassic alkaline lavas are present in the northern margin of the gangdese belt some basalts exhibit a tholeiitic affinity daquin area southern region the geodynamic significance of these cenozoic volcanic rocks lies in the relative motion of india and the lhasa block after accretion of the latter to the northern qantang block a geodynamic model based on changes of the type of subduction of the indian oceanic crust beneath the lhasa block for the period 120 40 ma is proposed to account for the abundance age and magmatic affinity of the volcanic products

hornblende the continent maker evolution of h2o during circum pacific subduction versus continental collision andesitic arcs are sited over convergent lithospheric plate junctions aqueous fluids driven off the downgoing slab are responsible for partial fusion of the warming subducted plate and or the overlying mantle wedge and the ascent of calc alkaline melts various hydrous minerals have been proposed to be the source of this h2o experimental equilibrium studies show that under subduction zone geothermal gradients of 5 7 c km clinoamphibole constitutes a major phase in deep seated 75 km metabasalts other hydrous minerals are absent or are of very minor abundance clinoamphiboles dehydrate at pressures of 2 2 2 4 gpa under equilibrium conditions pressure overstepping is probable so mafic blueschists and amphibolites expel h2o at magmagenic depths partly serpentinized mantle beneath the oceanic crust devolatilizes at comparable to slightly higher pressures however micas remain stable in pelitic and granitic gneisses to pressures far exceeding 4 0 gpa so at subduction depths 100 km micaceous lithologies characterizing the upper and middle sialic crust fail to evolve significant h2o deep underflow of hydrated oceanic lithosphere thus generates most of the volatile flux along and or above a subduction zone prior to continental collision as large masses of quartzofeldspathic material enter a suture zone volatile evolution at deep levels nearly ceases while small amounts of peraluminous s type anatectic melts may be produced i type calc alkaline arcs and the continents owe their formation over geologic time to the sustained underflow of oceanic lithosphere

a fluid model for the shape of accretionary wedges in this paper the accretionary wedges landward of most oceanic trenches are modelled assuming a newtonian fluid rheology it is shown that lubrication theory is applicable and that the addition of material to the wedges can be neglected in determining the shape of the wedges the predicted shape of the wedges is in good agreement with bathymetric profiles across the kurile ryukyu and aleutian wedges from the shape of the above wedges it is deduced that accretionary sediments have viscosities between 1018 and 1019 poise

implications of crustal property variations for models of tibetan plateau evolution shear coupled teleseismic p waves sampling the interior of the tibetan plateau provide evidence of systematic variations in crustal structure the crust thins by up to 20 km from south to north with a concomitant increase in poisson s ratio from normal values in the south to unusually high values in the north this suggests that the crust of the northern plateau is partially melted due to high temperatures these changes imply spatial and perhaps temporal variations in the way the elevation of the high plateau is created and maintained

the east pacific rise and the thermal model although the thermal mode of lithosphere cooling can be used to predict the east pacific rise topography reasonably well it does not adequately predict the observed gravity when the zero temperature boundary condition is at the seafloor the misfit arises primarily because this model predicts thermally lowered densities in the crust at the rise axis which counteract the gravitational effect of the topography generating a theoretical gravity anomaly smaller than that observed it is shown using data at 12 n as an example that improved agreement with the thermal model is obtained if to first order the crust is treated as an isothermal constant thickness constant density layer with variable depth this result is consistent with seismic data and observations of convective cooling of the crust by seawater circulation it implies that convective cooling of the whole crust is very rapid even at the axis of fast spreading ridges the oceanic moho may represent the depth to which cracking and convective cooling extends

stratigraphic evidence for an early collision between northwest india and asia the collision of india with asia1 had a profound influence on late cretaceous and cenozoic oceanography2 climate3 fauna extinctions4 and the motion of at least some of the earth s litho spheric plates5 as the collision ended a period of rapid indo asian convergence6 a precise knowledge of its timing when the crust of the neo tethys ocean was completely subducted7 at some point along the plate boundary is important for understanding its wider consequences but current estimates of the collision age range from 65 to 38 myr before present6 8 11 here we report the results of extensive biostratigraphic analyses from waziristan and kurram in northwest pakistan which show that accretionary prism and trench strata were first thrust onto the northwest indian passive margin after 66 myr but before 55 5 myr after this time volcanic arc fragments the accretionary prism trench material and imbricates of the north indian slope were raised to shallow water depths and overlapped by upper palaeocene shallow water carbonates and shales12 14 indicative of post collision thrusting in this region finally both the suture and the indian craton were overlapped by continuous unconformable upper lower eocene shallow marine strata demonstrating that suturing was complete by 49 myr

constraints on the structure of the east pacific rise from gravity gravity data across the east pacific rise at 9 n and 12 n the rose area are characterized by a high of 12 20 mgal with a width of about 20 km it is shown that this axial gravity high is not adequately explained by uncompensated constant density topography or an isostatically compensated thermally expanded ridge crest it can be adequately explained by airy compensation with a depth of compensation of at least 20 km or by elastic plate compensation with a flexural rigidity between 1018 and 1020 n m the significance of the depth and flexural rigidity parameters is that in both cases they result in a gravity field from the compensation whose amplitude is below the data resolution for wavelengths less than about 50 km that is the gravity data do not require compensation of the rise axis for wavelengths less than about 50 km this suggests that these wavelengths are either supported mechanically or by other forces dynamic the mechanical interpretation is not consistent with partial melt under the axis indicating that dynamic forces may be important in supporting the rise axis the same conclusion is arrived at from the thermal model the thermal model can be made to fit the gravity data if we allow departures from isostasy and the addition of a dike like body about 2 km wide in the crust having a positive density contrast of about 0 25 mg m 3 this geometry is consistent with a seismically determined low velocity zone in the crust in the rose area in this model the nonhydrostatic pressures must be supported dynamically a general conclusion of this study for the portions of the east pacific rise studied here is that the axial topography is not buoyantly supported by low density material in the crust therefore either the crust under the rise axis mechanically supports the axial topography or a mass excess at the thermally expanded rise axis is supported by other forces such as plate driving forces the latter option is favored on the basis of heat flux and seismic data

a thin viscous sheet model for continental deformation summary for the purposes of describing its large scale and long term deformation the continental lithosphere is regarded as a continuum obeying a newtonian or a power law rheology the flow of a thin sheet of power law material overlying an inviscid substrate is studied under the assumption that vertical gradients of the horizontal velocity are negligible a numerical model is used to investigate the deformation of such a sheet under conditions approximating those of continent continent collision the material flows in response to forces applied to its boundaries for example the indenting of one continent by another and to forces in its interior arising from gradients in crustal thickness the horizontal divergence of the flow produces changes in the crustal thickness and hence a time dependent form to the flow itself for a given set of boundary conditions the flow depends on the stress exponent in the power law rheology n and on the argand number which is a measure of the ratio between the stress arising from crustal thickness contrasts and the stress required to deform the material at the ambient strain rates when the effective viscosity of the medium is very high ar o crustal thickness variations do not influence the flow if the material is newtonian n l the deformation associated with an influx of material approximating an indenter is of much greater lateral dimension than the width of the indenter whereas when material has a power law rheology n 3 5 are used the deformation is confined to a region of lateral extent comparable to that of the indenter as ar increases the forces arising from crustal thickness contrasts exert more influence on the flow and the maximum crustal thickness that can be sustained by a given influx of material is related by a simple expression to the effective viscosity of the medium at the ambient strain rates in the limit of a very weak medium ar 10 the lithosphere is unable to sustain appreciable crustal elevation contrasts the results of these numerical experiments show that systems in which the effective viscosities are such that the maximum deviatoric stresses are between 1 kbar

tertiary deformation history of southeastern and southwestern tibet during the indo asian collision geologic mapping and geochronological analysis in southwest kailas area and southeast zedong area tibet reveal two major episodes of tertiary crustal shortening along the classic indus tsangpo suture in the yalu river valley the older event occurred between ca 30 and 24 ma during movement along the north dipping gangdese thrust the development of this thrust caused extensive denudation of the gangdese batholith in its hanging wall and underthrusting of the xigaze forearc strata in its footwall examination of timing of major tectonic events in central asia suggests that the initiation of the gangdese thrust was approximately coeval with the late oligocene initiation and development of north south shortening in the eastern kunlun shan of northern tibet the nan shan at the northeastern end of the altyn tagh fault the western kunlun shan at the southwestern end of the altyn tagh fault and finally the tian shan north of the tarim basin such regionally synchronous initiation of crustal shortening in and around the plateau may have been related to changes in convergence rate and direction between the eurasian plate and the indian and pacific plates the younger thrusting event along the yalu river valley occurred between 19 and 10 ma along the south dipping great counter thrust system equivalent to the locally named renbu zedong thrust in southeastern tibet the backthrust system in south central tibet and the south kailas thrust in southwest tibet the coeval development of the great counter thrust and the north himalayan granite gneiss dome belt is consistent with their development being related to thermal weakening of the north himalayan and south tibetan crust due perhaps to thermal relaxation of an already thickened crust created by the early phase of collision between india and asia or frictional heating along major thrusts such as the main central thrust beneath the himalaya

introduction to special section magmatism and extension the relationship between magmatism and the formation of continental rift zones is the subject of much controversy in particular the cause and effect relationships between magmatism and extension and the mode of generation of magma during the process of extension are still hotly debated this controversy served as the theme of a symposium on cenozoic magmatism in the colorado river extensional corridor and adjacent areas and a field trip held as part of the geological society of america cordilleran rocky mountain section meeting in reno nevada metcalf et al 1993 it was clear from data presented at the symposium that a considerable amount of new information has become available regarding magmatism and extension since the last special section on this topic published by the journal of geophysical research june 1989

stratigraphy of the upper cretaceous and lower tertiary strata in the tethyan himalayas of tibet tingri area china the 1500 m thick marine strata of the tethys himalaya of the zhepure mountain tingri tibet comprise the upper albian to eocene and represent the sedimentary development of the passive northern continental margin of the indian plate investigations of foraminifera have led to a detailed biozonation which is compared with the west tethyan record five stratigraphic units can be distinguished the gamba group upper albian lower santonian represents the development from a basin and slope to an outer shelf environment in the following zhepure shanbei formation lower santonian middle maastrichtian outer shelf deposits continue pebbles in the top layers point to beginning redeposition on a continental slope intensified redeposition continues within the zhepure shanpo formation middle maastrichtian lower paleocene the series is capped by sandstones of the jidula formation danian deposited from a seaward prograding delta plain the overall succession of these units represents a sea level high at the cenomanian turonian boundary followed from the turonian to danian by an overall shallowing upward megasequence this is followed by a final transgression regression cycle during the paleocene and eocene documented in the zhepure shan formation upper danian lutetian and by upper eocene continental deposits the section represents the narrowing and closure of the tethys as a result of the convergence between northward drifting india and eurasia the plate collision started in the lower maastrichtian and caused rapid changes in sedimentation patterns affected by tectonic subsidence and uplift stronger subsidence and deposition took place from the middle maastrichtian to the lower paleocene the final closure of remnant tethys in the tingri area took place in the lutetian

a late miocene pliocene origin for the central himalayan inverted metamorphism abstract perhaps the best known occurrence of an inverted metamorphic sequence is that found immediately beneath the himalayan main central thrust mct generally thought to have been active during the early miocene however in situ 208 pb 232 th dating of monazite inclusions in garnet indicates that peak metamorphic recrystallization of the mct footwall occurred in this portion of the central himalaya at only ca 6 ma the apparent inverted metamorphism appears to have resulted from activation of a broad shear zone beneath the mct which tectonically telescoped the young metamorphic sequence this explanation may resolve some outstanding problems in himalayan tectonics such why the mct and not the more recently initiated thrusts marks the break in slope of the present day mountain range it also renders unnecessary the need for exceptional physical conditions e g high shear stress to explain the generation of the himalayan leucogranites

magmatism and metamorphism in the ladakh himalayas the indus tsangpo suture zone abstract ladakh india provides a complete geological section through the northwestern part of the himalayas from kashmir to tibet within this section the magmatic metamorphic and geotectonic evolution of the northern himalayan orogeny has been studied using petrographic geochemical and isotope analytical techniques the beginning of the himalayan cycle was marked by large basaltic extrusions panjal trap of permian to lower triassic age at the northern margin of the gondwana continent indian shield these continental type tholeiitic basalts were followed by a more alkaline volcanism within the triassic to jurassic lamayuru unit of the gondwana continental margin lower jurassic to cretaceous oceanic crust and sediments ophiolitic melange s s accompany the triassic to cretaceous flysch deposits within the indus tsangpo suture zone the major structural divide between the indian shield high himalaya and the tibetan platform so far no relic of paleozoic oceanic crust has been found subduction of the tethyan oceanic crust during upper jurassic and cretaceous time produced an island arc represented by tholeiitic and calc alkaline volcanic rock series dras volcanics and related intrusives accompanied by volcaniclastic flysch deposits towards the tibetan continental margin subsequent to the subduction of oceanic crust large volumes of calc alkaline plutons trans himalayan or kangdese plutons intruded the tibetan continental margin over a distance of 2000 km and partly the dras island arc in the ladakh region the collision of the indian shield and tibetan platform started during the middle to upper eocene and caused large scale still active intracrustal thrusting as well as the piling up of the himalayan nappes the tectonically highest of these nappes is built up of oceanic crust and huge slices of peridotitic oceanic mantle spongtang klippe in the high himalayas the tectonic activity was accompanied and outlasted by a barrovian type metamorphism that affected triassic sediments of the kashmir nun kun synclinorium up to kyanite staurolite grade and the deeper seated units up to sillimanite grade cooling ages of micas are around 20 m y muscovite and 13 m y biotite towards the indus tsangpo suture zone metamorphism decreases with no obvious discontinuity through greenschist prehnite pumpellyite to zeolite grade remnants of possibly an eo himalayan blueschist metamorphism have been found within thrust zones accompanying ophiolitic melange in the suture zone

age and thermal history of the geysers plutonic complex felsite unit geysers geothermal field california a 40ar 39ar and u pb study abstract sixty nine ion microprobe spot analyses of zircons from four granite samples from the plutonic complex that underlies the geysers geothermal field yield 207pb 206pb vs 238u 206pb concordia ages ranging from 1 13 0 04 ma to 1 25 0 04 1 ma the weighted mean of the u pb model ages is 1 18 0 03 ma the u pb ages coincide closely with 40ar 39ar age spectrum plateau and terminal ages from coexisting k feldspars and with the eruption ages of overlying volcanic rocks the data indicate that the granite crystallized at 1 18 ma and had cooled below 350 c by 0 9 1 0 ma interpretation of the feldspar 40ar 39ar age data using multi diffusion domain theory indicates that post emplacement rapid cooling was succeeded either by slower cooling from 350 to 300 c between 1 0 and 0 4 ma or transitory reheating to 300 350 c at about 0 4 0 6 ma subsequent rapid cooling to below 260 c between 0 4 and 0 2 ma is in agreement with previous proposals that vapor dominated conditions were initiated within the hydrothermal system at this time heat flow calculations constrained with k feldspar thermal histories and the present elevated regional heat flow anomaly demonstrate that appreciable heat input from sources external to the known geysers plutonic complex is required to maintain the geothermal system this requirement is satisfied by either a large underlying convecting magma chamber now solidified emplaced at 1 2 ma or episodic intrusion of smaller bodies from 1 2 to 0 6 ma

a model for the origin of himalayan anatexis and inverted metamorphism the origin of the paired granite belts and inverted metamorphic sequences of the himalaya has generally been ascribed to development of the main central thrust mct although a variety of models have been proposed that link early miocene anatexis with inverted metamorphism recent dating studies indicate that recrystallization of elements of the mct footwall occurred in the central himalaya as recently as 6 ma the recognition that hanging wall magmatism and footwall metamorphism are not spatially and temporally related renders unnecessary the need for exceptional physical conditions to explain generation of the high himalayan leucogranites and north himalayan granites which differ in age petrogenesis and emplacement style we suggest that their origin is linked to shear heating on a continuously active thrust that cuts through indian supracrustal rocks that had previously experienced low degrees of partial melting numerical simulations assuming a shear stress of 30 mpa indicate that continuous slip on the himalayan decollement beginning at 25 ma could trigger partial melting reactions leading to formation of the high himalayan granite chain between 25 and 20 ma and the north himalayan belt between 17 and 8 ma the ramp flat geometry we apply to model the himalayan thrust system requires that the presently exposed rocks of the hanging wall resided at middle crustal levels above the decollement throughout the early and middle miocene late miocene out of sequence thrusting within the broad shear zone beneath the mct provides a mechanism to bring these rocks to the surface in their present location i e well to the north of the present tectonic front and has the additional benefit of explaining how the inverted metamorphic sequences formed beneath the mct we envision that formation of the mct zone involved successive accretion of tectonic slivers of the lesser himalayan formations to the hanging wall and incorporate these effects into the model the model predicts continued anatexis up to 400 km north of the himalayan range consistent with the timing and geochemistry of leucogranites exhumed on the flank of a south tibetan rift

the zedong window a record of superposed tertiary convergence in southeastern tibet determining the timing magnitude and location of deformation due to the indo asian collision is widely acknowledged as an important step in understanding how the lithosphere responds during continental collision a puzzling result of geological investigations of the lhasa block over the past 2 decades has been the apparent lack of significant tertiary deformation there perhaps the most important structural feature of the lhasa block is the south directed gangdese thrust system which developed along its southern edge the thrust system which separates the andean type batholith of southern asia from rocks of indian affinity is obscured at most locations across southeastern tibet by back thrusts of the younger north directed renbu zedong thrust system the best documented site where both thrusts are exposed occurs near zedong zedong window systematic geochronologic analyses were conducted in this area u pb zircon dating of three samples of a synkinematically to postkinematically deformed hanging wall granitoid the yaja granodiorite cut by the gangdese thrust indicates a crystallization age of 30 4 0 4 ma 2 thus placing an upper bound on the initiation of the thrust u pb zircon dating of granitoid samples structurally higher in the gangdese hanging wall yields emplacement ages of 42 5 1 0 ma and 48 9 0 8 ma 2 similar to other magmatic complexes within the gangdese arc geochemical results are consistent with these plutons forming in the same environment as precollisional intrusions within the gangdese batholith suggesting a significant post collisional input of juvenile heat the 40ar 39ar thermochronology of samples from a vertical section through the hanging wall within the yaja granodiorite coupled with results of a numerical thermal model indicate an average slip rate along the gangdese thrust of 7 mm yr between 30 and 23 ma and a minimum displacement of 50 km farther east in the zedong window thermal effects produced by later north directed thrust sheets of the renbu zedong system appear to have obscured thermal history signatures in the gangdese hanging wall related to earlier south directed thrusting thermochronological results from this region indicate that thermal overprinting related to the north directed thrusting occurred between 25 and 10 ma consistent with previous estimates

periodicity in the accretion process on the southeast indian ridge at 27 40 s abstract image processing of a seabeam bathymetric grid on the southeast indian ridge seir at 27 40 s has been used to analyse the morphotectonics of the ridge flanks steep topographic slopes indicating inward facing or outward facing faults and those representing tilted blocks were automatically separated an autocorrelation process allows an estimation of the periodicity of the fault spacing the outward facing faults first appear at 16 18 km from the axial valley are more common on bathymetric highs and correspond to tilted block zones in adjacent deeper areas the tilting of the blocks is 4 the transformation of the rift valley into the smooth undulating relief of the flanks is accomplished by these outward facing faults and tilted blocks inward facing faults appear either as large faults with major throws or as a series of smaller faults with minor throws the minor faults are mainly localized on bathymetric highs and are densely organized the major faults continuous and symmetrically spaced on each side of the axis zone are organized with a periodicity of 8 2 km these faults seem to be old inner walls which have been moved outwards periodically during the spreading process they alternate with positive magnetic lineaments reflecting old neovolcanic zones linked to intense magmatic periods considering a mean half spreading rate of 31 km m y during the past million years this would mean the development of a major fault each 0 26 m y we propose a model for the evolution of the seir at 27 40 s controlled by a periodic succession of magmatic and tectonic episodes 1 1 faulting and volcanism are concentrated in the axial zone above the melt reservoir where the crust is weakened magmatic period outside this zone the external relief is frozen in supported by the finite strength of the plate 2 2 the axial valley is enlarged fills up with lavas and bulges due to thermal effects 3 3 at the end of the magmatic period this bulged zone collapses by thermal subsidence along lines of weakness such as the faults forming the inner walls 4 4 when the melt reservoir is empty tectonic period the processes are no longer concentrated and active faulting extends over tens of kilometres from the axis the lithosphere is thinned a new axial valley is created the flanks are uplifted and outward facing faults and block tilting are induced

the cretaceous tertiary deformation of the lhasa block and its implications for crustal thickening in tibet the strong tertiary deformation of the northern margin of the indian plate and of the tsangpo suture zone is not matched by comparable deformation within the lhasa block yet the southern margin of the block was the site of large scale intrusive activity the gangdese or transhimalayan batholith throughout the late cretaceous and was therefore presumably weaker than its surroundings this poses a problem for any approach that interprets the deformation of tibet in terms of diffuse crustal thickening however there is evidence that the lhasa block was in elevation as well as in magma type an andean margin by the end of the cretaceous and we suggest that the buoyancy force associated with the elevated crust was sufficient to inhibit compressional deformation within the southern lhasa block this suggestion is tested quantitatively by treating the continental lithosphere as a thin viscous sheet containing an isostatically compensated elevation contrast when subjected to boundary conditions representing the collision between india and asia the sheet accommodates convergence by diffuse deformation over a region comparable in size with the tibetan plateau in the absence of an elevation contrast this deformation consists of approximately 100 thickening strain over the region a precollision elevation contrast of 3000 m in the lhasa block would have resulted in its experiencing roughly 40 rather than 100 postcollisional thickening smaller elevation contrasts would have resulted in greater thickening and vice versa these calculations have implications for the mode of crustal thickening in northern tibet as the available evidence suggests that the topographic expression of the earlier mesozoic tectonic activity in southern asia was erased before the india asia collision we expect that there has been widespread and relatively homogeneous crustal shortening migrating northward from the lhasa block during the tertiary

oceanic ridge crest processes oceanic ridge crest processes constitute a vigorous frontier of multidisciplinary research with global ramifications in the earth atmospheric and biologic sciences this deep ocean frontier is gradually opening in response to investigations which are elucidating the role of these processes in the lithosphere hydrosphere atmosphere and biosphere and the application of advanced instrumental and interpretive techniques which are increasing the quality and quantity of data these techniques include crustal drilling with borehole logging deep sea drilling project reports high resolution bathymetry sea beam etc side scan sonar seamarc i and ii gloria etc imagery with video and photography multichannel seismics determination of water column properties ctd with rosette sampler etc and submersible applications

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

tethyan subducted slabs under india abstract tomographic imaging of the mantle under tibet india and the adjacent indian ocean reveals several zones of relatively high p wave velocities at various depths under the hindu kush region in northeastern afghanistan and southern tajikistan a regional northward dipping slab is seen in the entire upper 600 km of the mantle and is apparently still attached to the lithosphere of the indian plate under northern pakistan this same slab shows a roll over structure with the deeper portion overturned and dipping southward as can also be seen in the distribution of earthquake hypocenters farther east southeast e g in the vicinity of nepal a well resolved anomaly below 450 km depth is connected to the slab under the hindu kush but seems to be separated from the lithosphere above 350 km these upper mantle anomalies are interpreted as the remnants of delaminated sub continental lithosphere that went down when greater india continued to converge northward with asia after 45 ma the deeper high velocity anomalies under the indian sub continent appear clearly separated from the shallower ones as well as from each other and are inferred to represent remnants of oceanic lithospheric slabs that have sunk into the lower mantle and were subsequently overridden by the indian plate they occur at depths between 1000 and 2300 km and occasionally descend down to the core mantle boundary the anomalies form three parallel wnw ese striking zones we interpret the two southern zones as remnants of oceanic lithosphere that was subducted when the neo tethys ocean closed between india and tibet in the cretaceous and earliest tertiary the northern deep mantle zone under northern afghanistan the himalayas and the lhasa block in southern tibet may represent the last subducted remnant of the paleo tethys ocean which is thought to have closed before the hauterivian stage of the early cretaceous the middle zone continues southeastward as a rather straight high velocity zone towards sumatra where it becomes convex southward and parallel to the subduction zone under the sunda arc comparison of this straight middle zone near india with the shallower upper 600 1000 km northern zone which displays a cusp like shape near the yunnan sw china syntaxis of the eastern himalayas supports the notion that the shallow northern zone represents later subduction than the deeper middle zone the suggestion of a counterclockwise rotation 20 of the indian plate during tertiary indentation of asia is supported by these features the present day latitudes of 5 35 n of the deep slabs under india and adjacent areas correspond to the approximate paleolatitudes of the cretaceous subduction zones the slab remnants in the middle mantle occur therefore near the ancient locations where they started their downward journey which implies that lateral movements in the deeper mantle were not large

upb geochronology of gangdese transhimalaya plutonism in the lhasa xigaze region tibet abstract a series of different plutons from the lhasa xigaze segment of the gangdese transhimalaya belt has been studied by high resolution u pb analyses of zircon using zircon fractions of 5 100 grains each selected upon specific grain characteristics for two diorites located east of xigaze dazhuka the zircons yield concordant ages of 93 4 1 0 and 94 2 1 0 m y respectively also concordant ages of 41 1 0 4 and 41 7 0 4 m y have been obtained for two granodiorites collected southwest of lhasa qushui the precision on the ages of two granites from the xigaze and lhasa area is limited by two factors the presence of inherited radiogenic lead and the occurrence of subsequent lead loss however some concordant zircons detected in both granites define approximate ages of about 67 and 53 m y respectively the inherited lead components show that melting of precambrian material was involved in magma genesis the u pb ages substantiate a magmatic activity lasting from mid cretceous cenomanian to eocene lutetian time such a period of plutonism at the southern margin of eurasia as well as the occurrence of magma generation from continental crust suggest that the gangdese range results from the subduction of tethys oceanic lithosphere indian plate underneath eurasia eurasian plate if this model is true the collision of india with eurasia along the lhasa xigaze sector postdates the emplacement of the 41 m y old gangdese granodiorites i e the collision occurred after lutetian time

centroid depths of mid ocean ridge earthquakes dependence on spreading rate from inversions of teleseismic p and sh waveforms we have determined the source parameters of 50 large earthquakes that occurred during 1962 1983 on slowly spreading mid ocean ridges all events are characterized by predominantly normal faulting on planes that dip at approximately 45 and strike parallel to the local trend of the ridge axis centroid depths range from 1 to 6 km beneath the seafloor the p waves from these earthquakes show strong water column reverberations suggesting that fault rupture extended to the seafloor under the assumption that the centroid depth marks the mean depth of fault slip earthquake faulting extended to a depth of 2 10 km for these earthquakes the water depths inferred from the predominant periods of the water column reverberations constrain the epicenters of these earthquakes to lie within the relatively deep inner floor of the median valley the maximum centroid depths of ridge crest earthquakes decrease with increasing spreading rate and the maximum seismic moment may also decrease with increasing spreading rate these results indicate a general decrease with spreading rate in maximum thickness of the mechanically strong layer beneath the ridge axis region the concentration of large earthquakes within the deepest parts of the median valley and the depth extent of seismic faulting support lithospheric necking models for the origin of the median valley

did the indo asian collision alone create the tibetan plateau it is widely believed that the tibetan plateau is a late cenozoic feature produced by the indo asian collision however because tibet was the locus of continental accretion and subduction throughout the mesozoic crustal thickening during that time may also have contributed to growth of the plateau this portion of the geologic history was investigated in a traverse through the central lhasa block southern tibet together with earlier studies our mapping and geochronological results show that the lhasa block underwent little north south shortening during the cenozoic rather our mapping shows that 60 crustal shortening perhaps due to the collision between the lhasa and qiangtang blocks occurred during the early cretaceous this observation implies that a significant portion of southern tibet was raised to perhaps 3 4 km elevation prior to the indo asian collision

sequence stratigraphy and sea level history of oligocene strata of the northern aral sea region kazakhstan implications for glacioeustatic reconstructions we have applied sequence stratigraphic techniques to the upper eocene to oligocene strata of the region north of the aral sea and northern usturt plateau western kazakhstan and compared the inferred sea level history with other records derived from different parts of the world during late eocene to early miocene time the entire region of study was within the northeastern part of the paratethys sea clastic and carbonate deposition occurred in a range of environments from fluvial and lacustrine in coastal plains northern priaral to relatively deep marine in the north usturt trough stratal geometries and sedimentation rates indicate that the deposition of the northern priaral sections occurred under tectonically quiet conditions whereas noticeable syndepositional subsidence took place in the central part of the north usturt trough the same oligocene sequences have been identified in these areas despite significant spatial variations in tectonic subsidence and sediment supply rates we therefore conclude that eustasy played a major role in controlling sedimentation in the region of study especially in the northern priaral through sequence stratigraphic analysis we identified four sequence boundaries resulting from regional sea level falls two major sea level falls are identified in the earliest oligocene and at the early late oligocene boundary and two smaller sea level falls took place in late early oligocene and latest oligocene time each of these events can be correlated to peaks in the global 18 o record in addition paleoclimatic data from the region indicate that regional periods of cooling are coincident with the intervals of inferred sea level falls we therefore conclude that global glacioeustasy represents the main controlling force for the sea level record we observe in the northern priaral and north usturt trough this makes the region of study very useful for reconstruction of eustatic history for late eocene oligocene time

stretching of the deep crust at the slow spreading southwest indian ridge abstract the 500 metres of gabbro drilled in the southwest indian ocean during odp leg 118 allow direct observation of early high temperature extensional deformation in the deep crust of a slow spreading rift valley about 30 of the recovered gabbros have been plastically stretched at progressively decreasing temperatures granulite to lower amphibolite facies metamorphic conditions increasing water rock ratio and probably increasing deviatoric stresses the early solid state strain in granulite facies metamorphic conditions produced low dipping foliations and low strain gradients at this stage mylonitic shear zones only formed in oxide rich gabbros strain localization in the deepest and hottest part of the oceanic crust thus appears to have been influenced by the magmatic stratigraphy in upper to lower amphibolite facies metamorphic conditions the ductile strength of the gabbro was probably higher and deformation concentrated in mylonitic shear bands which may have provided access to seawater derived fluids strain localization at this stage was accompanied by brittle failure and boudinage in the mylonitic foliation of individual porphyroclasts or of thick gabbro intervals the influence of such brittle failure on hydrothermal circulations was critical it allowed for extensive crust seawater interactions at high temperatures upper amphibolite facies conditions in a spreading context where continuous magma supply provides heat and new crustal material to the axial region the mechanical behaviour of the ridge axis lithosphere should be controlled by continuous ductile deformation in granulite facies conditions this may lead to the steady state necking configuration described by tapponnier and francheteau 1978 alternatively a discontinuity of the magma supply allowing the lower crust of the axial region to cool below granulite facies temperatures should cause mylonitic shear zones characteristic of amphibolite facies deformation to extend deeper into the axial domain eventually leading to the detachment fault configuration proposed by karson in press as magma supply to slow spreading oceanic ridges is likely to be discontinuous necking and faulting periods should alternate through time

a new analysis of gravity and topography data over the mid atlantic ridge non compensation of the axial valley abstract topographic and associated gravity signals often result from the interference of several phenomena in the present paper an admittance function for the superposition of several physical mechanisms acting in the same waveband is derived if the processes are not phase related the resulting admittance function is the average of the individual admittance functions weighted by the squared amplitude of the topography for each process we use this concept of superposition of different processes to understand the compensation of the atlantic ridge valley classical transfer function studies applied to slow spreading ridges have concluded that the topography was emplaced on an elastic plate about 10 km thick these analyses of gravity and topography seem to contradict the physical models explaining the median rift valley which is thought to be due to dynamical effects and thus to be uncompensated we obtain an average axial valley gravity and topography profile by stacking several profiles perpendicular to the mid atlantic ridge and subtracting a long wavelength thermal effect we find that the gravity over topography spectral ratio of the average axial valley signal is consistent with an uncompensated process our study thus confirms that the mode of formation of the axial valley of a slow spreading ridge involves an uncompensated mechanism the presence of an additional process characterized by low admittance values uncorrelated from one profile to the other is also suggested in order to explain the observed admittance function the study of the long wavelength 300 km gravity and topography signal leads us to invoke the cooling of the upper portion of the crust by water circulation and to exclude the presence of a large amount of partial melt at depth more than 5 over a 20 km thick layer at a mean depth of 60 km

isostasy magma chambers and plate driving forces on the east pacific rise the density distribution within a cooling plate is calculated which incorporates temperature and pressure effects from this density distribution the pressure field within the plate and the gravity field at sea level are computed for various degrees of isostatic compensation in this model the pressure field within the plate has a horizontal gradient at shallow depths away from zero age and a horizontal gradient toward zero age at greater depths caused by the loading of the ocean isostatic equilibrium is approached if one allows the loading due to the water to depress the seafloor and at the same time allows mass conservation by flow at depth toward zero age a viscosity model based on a newtonian rheology which included temperature and pressure effects has a high gradient close to the plane separating positive and negative pressure gradients which would facilitate the return flow and decouple the lithosphere from the asthenosphere addition of a crust to the homogeneous model does not substantially change these conclusions comparison of this model with examples of east pacific rise data suggests that some areas may not be in complete isostatic equilibrium implying the existence of horizontal pressure gradients toward zero age in the asthenosphere this model can be made to fit the general features of the east pacific rise but not the detailed gravity and topography near zero age if one allows convective cooling of the crust by water partial melting of the upper mantle and intrusion of this partial melt into the crust the water depths increase more rapidly near zero age and an increased positive gravity anomaly is produced over the rise axis both of which produce a better fit to east pacific rise data at 12 n this study suggests that crustal magma chambers on the east pacific rise may be associated with anomalous positive gravity anomalies caused by a positive density contrast between magma and fractured porous crustal rocks

on the dynamics of mid ocean ridges most extant flow models for ocean ridges suffer either from unrealistic geometry vertical conduit models or a stress singularity at the ridge axis corner flow models i present a model which avoids these difficulties by incorporating a weak plate boundary zone pbz between diverging surface plates an analytical solution for the flow of a constant viscosity asthenosphere and a coexisting melt phase is obtained in elliptic coordinates and used to predict the dynamically supported topography axial valley on the pbz the force required to pull the plates apart and the rate of melt extraction at the ridge stress in the asthenosphere increases linearly with spreading rate but decreases as the plates become thinner and the dynamics of ridge crests is controlled by the balance between these opposing effects the depth of the axial valley is controlled by the plate thinning effect and is therefore a decreasing function of spreading rate the depth of the axial valley on the mid atlantic ridge can be explained if the viscosity of the asthenosphere is 6 1019 1021 pa s and the lower surface of the plate is the 600 c isotherm the plate separation force is controlled by the spreading rate effect and therefore increases with spreading rate about five times as much force is required to maintain a fast spreading ridge 9 cm yr 1 half rate as a slow spreading ridge 1 cm yr 1 melt extraction can produce 6 km of basaltic crust if the volume fraction of melt present in the asthenosphere total degree of melting ranges from about 2 5 4 0 for a slow spreading ridge to 4 0 6 5 for a fast spreading ridge the dynamics of melt extraction does not depend significantly on the finite width of the pbz

tertiary structural evolution of the gangdese thrust system southeastern tibet structural and thermochronological investigations of southern tibet xizang suggest that intracontinental thrusting has been the dominant cause for formation of thickened crust in the southernmost tibetan plateau since late oligocene two thrust systems are documented in this study the north dipping gangdese system gts and the younger south dipping renbu zedong system rzt west of lhasa the gangdese thrust juxtaposes the late cretaceous forearc basin deposits of the lhasa block the xigaze group over the tethyan sedimentary rocks of the indian plate whereas east of lhasa the fault juxtaposes the late cretaceous eocene andean type arc the gangdese batholith over tethyan sedimentary rocks near zedong 150 km southeast of lhasa the gangdese thrust is marked by a 200 m thick mylonitic shear zone that consists of deformed granite and metasedimentary rocks a major south dipping backthrust in the hanging wall of the gangdese thrust puts the xigaze group over tertiary conglomerates and the gangdese plutonics north of xigaze and west of lhasa a lower age bound for the gangdese thrust of 18 3 0 5 ma is given by crosscutting relationships the timing of slip on the gangdese thrust is estimated to be 27 23 ma from 40 ar 39 ar thermochronology and a displacement of at least 46 9 km is indicated near zedong the age of the gangdese thrust gt is consistent with an upper age limit of 24 ma for the initiation of movement on the main central thrust in places the younger renbu zedong fault is thrust over the trace of the gt obscuring its exposure the rzt appears to have been active at circa 18 ma but had ceased movement by 8 1 ma the suture between india and asia has been complexly modified by development of the gts rzt and locally strike slip and normal fault systems

a simple thermal mechanical model for mid ocean ridge topographic variation summary a simple dynamic model based on the geometry of mantle divergence and thermal parameters controlling equilibrium size of the axial magma chamber explains the variation in topography along mid ocean ridges among morphological characters accounted for are 1 the change from axial valley to axial high type ridge crests with increasing spreading rate 2 the localized occurrence of deeps at ridge transform intersections and 3 the correlation of average transform spacing with spreading rate the model also yields an explanation for anomalous ridge topography associated with oceanic hot spots incorporation of smaller scale bathymetric and ophiolite data into this scheme permits construction of a comprehensive model of ocean crust accretion

rapid early miocene acceleration of uplift in the gangdese belt xizang southern tibet and its bearing on accommodation mechanisms of the india asia collision abstract five samples from a biotite hornblende granodiorite phase of the 42 5 ma quxu pluton gangdese batholith southern tibet have been collected at 250 m vertical intervals biotite from these rocks yields monotonically decreasing40ar 39ar isochron ages with decreasing elevation of 26 8 0 2 23 3 0 5 19 7 0 3 18 4 0 4 and17 8 0 1ma tc 335 c coexisting k feldspars have virtually identical minimum apparent40ar 39ar ages of 17 0 0 4ma tc 285 c these data indicate parts of southern tibet experienced a pulse of uplift in the early miocene with the rate of uplift rising from 0 07 to 4 4 mm year in the interval 20 to 17 ma an apatite fission track age of 9 9 0 9ma from this locality constrains the average uplift rate at this site to about 0 81 mm year between 17 and 9 9 ma and 0 30 mm year from 9 9 ma to present k feldspar from the dagze granite 30 km to the east near lhasa yields a minimum apparent40ar 39ar age of 35 9 0 9ma tc 227 c which indicates an average uplift rate there of 0 21 mm year since then the marked pulse of uplift of the quxu granodiorite and the difference in uplift history between the dagze and quxu plutons suggests southern tibet has experienced discrete pulses of uplift variable in both space and time these data are not consistent with models which require a large proportion of uplift of the tibetan plateau to have occurred in the last 2 ma the data support the suggestion that convergence between india and asia was largely accommodated by tectonic escape during the opening of the south china sea 32 to 17 ma ago and permit distributed shortening as a mechanism for crustal thickening and uplift of this part of the tibetan plateau subsequent to 20 ma

a stochastic model for the creation of abyssal hill topography at a slow spreading center the present study is an attempt to model abyssal hill topography i e the randomly varying topography that is superimposed on the subsidence of the ocean floor due to thermal contraction we assume that abyssal hill topography is created by faulting and that it can be represented as the convolution of a series of stochastic fault offsets with a fault impulse response as a constraint the power spectrum of the model generated topography will have to resemble the power spectrum of a profile of abyssal hill topography across a typical slow spreading ridge such as found in the south atlantic using a series of fault offsets positively correlated in space and fault impulse responses limited to a finite width w profiles that statistically resemble abyssal hill topography have been generated if the fault offsets are constrained to be positive i e to cause uplift the synthetic profiles contain a steady state depression at the axis the fit of the synthetically derived topography to the observed topography is very sensitive to the width w of the fault impulse response our results suggest that the effects of a faulting event must be limited to distances of about 3 5 km and that tectonic activity must take place in the immediate vicinity of the inner wall of the axial valley if faulting is indeed the process that creates abyssal hill topography the best fit flexural impulse response to faulting implies a flexural rigidity of 5 1017 n m if instead constructional volcanism contributes significantly to the topography our study suggests that the emplacement of volcanic constructions must also take place within a narrow width w about the axis

paleontological view of the ages of the deccan traps the cretaceous tertiary boundary and the india asia collision volcanism in the deccan traps of india occurred over at most three magnetic chrons centered on a main reversed chron paleontological data indicate that this reversed chron must coincide with 29r which contains the cretaceous tertiary k t boundary recent 40 ar 39 ar data may therefore provide one of the best estimates of the absolute age of the k t boundary 65 7 2 0 ma moreover the eurasian character of k t boundary terrestrial faunas in india suggests that this was also close to the time of india asia collision this is significantly earlier than generally recognized and implies about 4000 km of continental shortening in the collision zone probably absorbed jointly by crustal thickening and strike slip extrusion

the seismic moment budget of slowly spreading ridges mid ocean ridges with well developed median valleys are commonly the sites of large normal faulting earthquakes1 2 a recent global study3 6 of the centroid depths and source mechanisms of such earthquakes has yielded two important conclusions first that all the largest mid ocean ridge earthquakes occur beneath the inner floor of the median valley and second that the maximum thickness of the seismogenic layer increases with decreasing spreading rate these results lend support to the hypothesis that the median valley arises from the necking of a strong brittle lithosphere under horizontal extension7 9 here we compare the global rate of seismic moment release along slowly spreading mid ocean ridges with the rate predicted if widening of the median valley occurs entirely by seismogenic extension of the brittle lithosphere beneath the inner floor we find that earthquakes account for no more than about 10 20 of the rate of plate separation across the central median valley

the mid ocean ridge axial valley as a steady state neck abstract in this paper the mid ocean ridge axial valley is modelled as a steady state lithospheric neck in which lithospheric stretching balances lithospheric accretion conversely the axial high is a steady state lithospheric bulge the lithosphere is modelled as a thin plate with a newtonian rheology it is shown that an axial valley will occur if the rate of viscosity increase away from the ridge axis is faster than the rate at which accretion decreases an axial high will occur if the opposite condition holds this is consistent with the observation that axial valleys occur at low spreading rates and axial highs at high spreading rates by fitting our model to profiles across the mid atlantic ridge and the east pacific rise and assuming the lithospheric thickness at the ridge axis to be 5 km we find accretion widths of 6 8 km we find the width over which there is a significant increase in lithospheric viscosity to be also 6 8 km