change from late tertiary compression to quaternary extension in southern tibet during the india asia collision field studies in southern tibet show that subsequent to folding resulting from the india asia collision 50 40 ma reverse faults and backthrusts formed as a result of nne ssw and nnw sse trending compressions they affect conglomerates of oligocene miocene age and are probably contemporaneous with shearing on the himalayan main central thrust and folding of paleogene basins along the northern border of southern tibet 25 16 ma rift systems postdate compressional structures they were active during the pleistocene and probably during the pliocene 5 2 ma change from a compressional to an extensional regime in tibet probably occurred during the late miocene 10 5 ma this change is tentatively correlated with uplift inferred by chinese workers from paleogene to present day variations of the tibetan vegetation recent and active faulting in the high plateau results from a wnw ese striking extension this is in agreement with tibetan focal mechanisms the himalayan lowlands are subjected to a nne ssw trending compression as demonstrated by superficial deformations and seismicity therefore the horizontal maximum compressive stress trajectory is roughly parallel to india asia convergence hmax being 1 in the lowlands and 2 in the high plateau this change results from the increase of the vertical principal stress value due to the weight of the elevated mass zz is 3 in the lowlands and 1 in the high plateau thus the third principal stress axis is 2 in the lowlands and 3 in the high plateau allowing extension to occur in the wnw ese direction extensional tectonics are significantly different in southern and northern tibet southern tibet being constrained between the himalayan syntaxes the e w extensional principal deviatoric stress value 3 is of rather small magnitude and the n s trending 2 value is extensional as demonstrated by analysis of recent faults normal components prevail along all the azimuths in the field this favors formation of rift systems of large axial extension on the contrary north of the himalayan syntaxes northern tibet flows easily eastward the e w extensional principal deviatoric stress value 3 is of higher magnitude and the n s stress 2 is compressional as shown by inversion of focal mechanism data this favors strike slip components on faults which deviate from the n s direction as expressed in the field by shorter n s trending graben linked with normal strike slip faults these faults permit some n s shortening in northern tibet without thickening in response to india asia collision in high plateaus which are subjected to lateral compression such as the andean altiplano and tibet the extensional direction is basically controlled by the convergence direction being roughly orthogonal to the latter

etude petro structurale des chevauchements ductiles himalayens sur la transversale de l everest makalu nepal oriental tectonic and microtectonic data in eastern nepal indicate that the major observed thrusting 100 km on the main central thrust mct postdates the barrovian metamorphism of the high himalaya gneisses this result at variance with the famous reverse metamorphism model better explains the abnormal metamorphic superpositions in the himalayas and accounts for the lack of high pressure assemblages under the thick allochtonous high himalaya tibetan slab pressure and temperature estimates by microprobe analysis on plagioclase biotite garnet kyanite sillimanite and cordierite assemblages are presented for samples collected along the mct shear zone and across the gneiss slab in the everest makalu area since there is very little difference in pressure at the front of the slab kathmandu klippe and its root these estimates support the existence of important late metamorphic thrusting the decrease of pressure towards the top of the gneiss pile combined with a small temperature increase explains the k

a numerical method for determining the state of stress using focal mechanisms of earthquake populations application to tibetan teleseisms and microseismicity of southern peru the numerical method described in this paper enables the study in terms of stress of the kinematics of seismic faults provided by focal mechanisms this method assumes a mean state of stress in the source region and is based on the simple mechanical model used for fault population analysis which supposes slip in the direction of the resolved shear stress acting on the fault plane the proposed algorithm first defines compressional and tensional zones resulting from superimposition of the compressional and tensional quadrants limited by the nodal planes this enables one to test the data homogeneity furthermore this restricts the space where the principal stress axes have to be searched then for each principal stress reference whose location is constrained by above confined zones the r value chosen equal to 2 1 3 1 is computed which fits the slip vector on each nodal plane this permits one to select a set of preferred seismic fault planes from a set of auxiliary planes finally a state of stress is computed from the preferred seismic fault plane set using the non linear simplex method already applied to fault populations this algorithm is constructed so as to avoid two major difficulties misleading estimation of the deviatoric stress tensor which may result from excessive emphasis by minimizing the residuals by a least squares method if some of the data are wrong and a lengthy prospection of the stress references over all the space of directions this calculation does not take into account possible changes of the strike and dip of the nodal planes however we use rotations of nodal planes in agreement with the first arrival data to test the compatibility of these data with the computed state of stress tibetan teleseisms and southern peruvian microseisms are analyzed and results are compared with kinematics of superficial recent and active faults measured in field in the same regions this suggests that major seismic events may give a good evaluation of the regional mean state of stress focal mechanisms of the small events are also modelled by a mean stress deviator but this differs significantly from the regional state of stress in southern peru deduced from quaternary faulting this may be due to imprecise location of the small events however some movements are strongly different from those predicted by the computed mean stress deviator this suggests that small fault kinematics surely depend on the regional state of stress but that local effects may control the small deformations

late weichselian glacier retreat in kongsfjorden west spitsbergen svalbard the chronology of late weichselian to holocene deglaciation of kongsfjorden west spitsbergen has been reconstructed based on the geomorphic and stratigraphic record of ice retreat relative sea level relationships and 14c dating of associated marine organic materials the seaward extent of glacial drift and fjord bathymetry constrain a secure reconstruction for the ice sheet near the mouth of the fjord at ca 13 000 yr b p but do not rule out the possibility that more extensive glaciation was achieved earlier during the late weichselian regional shoreline relations rates of emergence and radiocarbon dating of foraminifera deposited just above till indicate that deglaciation occurred in two steps one beginning during or just prior to the late weichselian marine limit phase at 13 000 12 000 yr b p and another beginning at 10 000 9500 yr b p the fjord was completely deglaciated by 9440 130 yr b p a period of stable relative sea level began 10 700 yr b p and ended between 10 000 and 9500 yr b p which we take to indicate renewed glacial loading during the younger dryas glacier readvance within kongsfjorden at this time was diminutive suggesting that most of the younger dryas ice sheet growth was confined to the eastern part of the archipelago and or to the barents shelf the two step deglaciation of kongsfjorden occurred during intervals of accelerated global ice sheet melting and rapid oceanic and atmospheric warming in more temperate latitudes of the circum atlantic region this coincidence most likely resulted from contemporaneous increases in the poleward transport of oceanic heat

tectonics and topography for a lithosphere containing density heterogeneities the purpose of this paper is to clarify the dynamic role of lithospheric density heterogeneities in particular with respect to mountain building and other processes of intraplate deformation density anomalies within or just beneath the lithosphere constitute major sources for tectonic stress fields the product of their magnitude by their depth is shown to characterize their ability to induce deformation this rule of the density moment directly yields the lithospheric thickening or thinning rate when applied to structures of large lateral extent for anomalies of lateral extent that is small in comparison with their depth the deformation is vertically inhomogeneous and has been computed with the help of simple physical models of a stratified viscous newtonian lithosphere the analytical treatment is based on fourier transform continent continent collision thickens not only the crust but the entire lithosphere the cold root underlying a mountain chain induces strong regional compressive stresses able to sustain the mountain bulding process without further help from forces transmitted from far away thus the continental lithosphere is in a somewhat metastable mechanical state adiabatic i e rapid thickening or thinning tends to grow further once initiated tectonic phases of strong compression correspond to the climax of such instabilities the response of models with cold lithospheric roots of various intensities has been computed both in two and three dimensions they yield velocity distributions and stress fields instructive comparisons are made with earthquake focal mechanisms and in situ stress measurements in the alpine and appalachian regions in the presence of lateral variations of the mechanical properties of the lithosphere the tectonic style is not only function of the local topography and of the nature of its compensation deformations in neighbouring provinces are coupled as shown by 3 dimensional models for example thickening sustained by a cold lithospheric root may generate extension in peripheral zones of weakness these last results illustrate the point that the computation of regional tectonic stresses requires the knowledge of the density anomalies within the lithosphere on the one hand and of geometrical constraints related to lateral mechanical heterogeneities on the other

quaternary normal and reverse faulting and the state of stress in the central andes of south peru field studies in the andes of southern peru show that in the high andes and pacific lowlands quaternary and recent faults are normal this extensional tectonics postdates compressional deformations of pliocene early quaternary age in the sub andes the observed deformations are compressional they affect early quaternary deposits some of the faults separate quaternary deposits from the bedrock and thus are clearly of tectonic origin and not landslide effects striations on the fault planes indicate n s trending extension in the high andes and pacific lowlands the total amount of crustal stretching is small probably of the order of 1 during the last 1 2 m y in the sub andes folds and faults affecting neogene and early quaternary deposits result from n s shortening nevertheless it is supposed that this n s shortening is of early quaternary age the present day compression probably strikes e w judging from focal mechanisms in the sub andes of central peru southern bolivia and northwest argentina data from structural analysis of faults and from earthquake focal mechanisms allow us to surmise the state of stress in the andes of southern peru the high andes and pacific lowlands subjected to n s trending extension are bounded by two zones of e w trending compression the sub andes to the east and the contact between the convergent nazca and south america plates to the west in our model the maximum horizontal compressive stress trajectory hmax is roughly parallel with the e w convergence between the two plates hmax corresponds to 1 in the sub andes and to 2 in the high andes the latter situation is caused by the elevated mass of the high andes where zz the vertical stress is inferred to be 1 thus the third principal stress axis being orthogonal to the other two axes it is oriented n s allowing extension to occur in that direction on the other hand in the sub andes zz is 3 and horizontal e w shortening occurs the state of stress in the andean continental crust above the 30 dipping slab appears to be different from that in the andes of central peru situated above the flat subducting segment in this region compressional deformantion affect a wider part of the cordillera

position of the lhasa block south tibet during the late cretaceous the latitude of the lhasa block xizang south tibet during the late cretaceous early tertiary is of prime importance for understanding the collision of india and eurasia and the mechanism of the formation of the qinghai xizang plateau during the first field expedition of the french chinese collaboration several geological formations have been sampled in southern tibet between lhasa 29 40 n 91 09 e and dingri 28 35 n 86 38 e we present here the first results obtained for middle cretaceous to palaeocene red beds from the lhasa block north of the yarlung zang bo suture zone recognized as the main suture between the indian and the eurasian plates they indicate a palaeolatitude of the lhasa block of 20 n during the late cretaceous while india was situated 30 s

river profiles along the himalayan arc as indicators of active tectonics abstract longitudinal profiles along sixteen major transverse himalayan rivers add important constraints to models of active continental subduction and its evolution these profiles are characterized by a zone of relatively high gradient that cannot be associated with differential resistence to erosion in all cases the base of the zone of increased gradients correlates with 1 the topographic front between the lesser and high himalayas 2 the narrow belt of intermediate magnitude thrust earthquakes 3 the main central thrust zone mct these features define a small circle in the central portion of the himalayan arc these correlations suggest that the discontinuity in the river profiles and the other features are controlled by a major tectonic boundary between the rising high himalayas and the lesser himalayas no sharp increases in gradient are observed near the main boundary thrust mbt except on a few rivers such as the jhelum or kundar where the mbt lies close to both the mct and the seismic belt thus it is unlikely that the mbt is a major tectonic boundary the diversion of river courses along the mbt and around anticlines in the sub himalayas has probably been caused by aggradation near the rosion deposition boundary upstream of uplifts in the mahabharat range and sub himalayas a parallel is drawn between the himalayas and new guinea based on the hypothesis that continent arc collision of the type occurring in northern australia preceded continent continent collision in the himalayas the present sedimentary tectonic phase in new guinea resembles the subathu paleocene eocene phase in the himalayas incipient counterparts of the major himalayan structures including the mct and the mbt are recognized in new guinea the drainage patterns in the himalayas and in new guinea bear a similar relation to major structures this suggests that 1 the tectonic evolution of the himalayas has been rather uniform since early stages of collision and 2 the himalayan drainage was also formed at these early stages and is therefore antecedent to the rise of the high himalayas

recent geomorphologic evolution of a glaciovolcanic active stratovolcano popocatepetl mexico abstract the recent geomorphologic evolution of the statovolcano popocatepetl 19 03 n 98 35 w including the volcanic activity and the evolution of the glacier that still exists on the north face was assessed most of the data were obtained from references in historical sources and aerial photographs dating from 1945 this information was supplemented with data from fieldwork conducted between 1992 and 1995 landform typology shows volcanic and glacial interactions the conclusions affirm that a geomorphologic evolution of the volcano occurred since the maximum glacier advance in the little ice age lia and the formation of a proglacial ramp later fluvioglacial gorges cut into the ramp especially during the 1921 27 eruption once the glacier had shrunk to a small size and was left hanging above the steep slope the streams disappeared and the gorges filled with blocks that fell in snow and ice avalanches

field evidence for active normal faulting in tibet crustal thickening does not now occur in southern tibet field observations made during the chinese french expedition of 1980 support an earlier hypothesis based on a combined analysis of satellite images and fault plane solutions of earthquakes immediately north of the highest peaks of the himalayas the tectonic regime is dominated by east west extension we have mapped a large number of north south normal faults which sharply cut the glacial and post glacial morphology and followed several recent earthquake breaks along faults detected on satellite and aerial photographs microtectonic measurements in the quaternary suggest that the maximum principal stress is vertical

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

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

seismotectonics of the himalayan collision zone geometry of the underthrusting indian plate beneath the himalaya fault plane solutions and well determined focal depths of medium sized earthquakes topography and landsat imagery in conjunction with seismicity maps cross sections and available geological information are used to investigate the present tectonics of the himalayan continental collision zone most of the accurately located epicenters of events along the himalayan arc 78 e 95 e that occurred between 1961 and 1981 are concentrated in a narrow zone about 50 km wide lying between the northerly dipping main boundary mbt and main central mct thrusts most of these events are located just south of the mct though the epicenters of the events are in general well located their depths as determined by teleseismic travel time data are very unreliable events with accurately determined depths obtained from identification of surface reflected phases define a simple planar zone from about 10 km and 20 km depth with an apparent dip of about 15 this result is all the more remarkable considering that the events used were located along about an 1800 km length of the himalyan arc except for one all available focal mechanisms of events within this zone indicate shallow 30 north dipping thrusts this shallow north dipping zone apparently defines a part of the detachment that separates the underthrusting indian plate from the lesser himalayan crustal block the spatial extent and the geometry of this interplate thrust zone strongly indicate that the mbt and nearby subsidiary surface and blind thrusts rather than the mct are currently the most active structures of the himalayan arc we suggest that the great himalayan earthquakes m 8 occur along the same detachment surface as defined by the thrust type medium sized events events located to the south of the mbt and beneath the ganges foredeep show normal faulting with t axes perpendicular to the himalayan trend the above results suggest that the indian continental plate is underthrusting the himalayan crustal blocks in a relatively coherent and simple geometry and that this geometry is not much different from that observed along oceanic subduction zones the november 19 1980 earthquake that occurred near the mct near 88 5 e shows a predominantly strike slip focal mechanism one of the nodal planes of this mechanism is transverse to the himalayan structural grain and moreover this plane has a trend similar to that of the recently mapped yadong gulu rift in the tethyan himalaya and in southern tibet just northeast of the earthquake we interpret this predominantly left lateral strike slip mechanism to indicate a possible genetic relationship between transverse structural features in the underthrusting indian plate the kishangang basement fault and the upper himalayan blocks and tibet

crustal structure and variability of the himalayan border of tibet reversed critical wide angle reflection profiles of the crust mantle boundary south of the yarlung zangbo suture in tibet reveal a deep 70 km moho extending north from the high himalayas whilst to the south the moho is 15 km higher even further south reflections from 35 km depth may correspond to the moho of peninsular india

parallel thrust and normal faulting in peru and constraints on the state of stress abstract we present evidence for recent normal faulting in the cordillera blanca of peru along planes trending approximately n40 w parallel to the chain this type of faulting suggests that there is an important component of extension perpendicular to the chain and contrasts markedly with the underthrusting of the nazca plate on the west side of peru and with the folding and thrusting observed in the sub andes on the eastern flank of the chain these contrasting styles of deformation and associated stress fields can be explained by buoyancy forces due to the high mountains and crustal roots in so far as this explanation is correct the difference in elevation between zones of normal and thrust faulting can be used to place an upper bound of 500 bars on the average stress difference in the crust

paleogeographic and tectonic evolution of southern tibet since middle cretaceous time new paleomagnetic data and synthesis during the second french chinese paleomagnetic campaign in tibet in the summer of 1981 we sampled the middle cretaceous red beds of the takena formation and the cenozoic volcanics of the lingzizong formation a detailed analysis of about 400 specimens of red beds reveals a stable component of normal magnetization n in the north of the lhasa block north of the nyainqentanglha range and two stable components in the south lt ht where the red beds are more strongly folded and are unconformably overlain by the lingzizong volcanics all three components are carried by hematite blocking temperatures range from 400 to 600 c for lt and up to the neel temperature of hematite for both n and ht lt is shown to postdate part of the folding of the takena whereas n and ht both indicate a positive fold test at the 99 probability level in addition a small less than 10 of the natural remanent magnetization antiparallel component is superimposed in most samples and may be consistent with a model of long term acquisition of a chemical remanent magnetization for the red beds analysis of the volcanics reveals a single stable component with mixed polarities carried either by hematite or magnetite the paleomagnetic directions indicate that 1 the lhasa block stood at 12 5 3 n by upper cretaceous and at 13 5 6 5 n in the paleocene eocene thus confirming the limited displacement of the margin of eurasia between albian aptian deposition of the red beds and eocene onset of the india asia collision these values imply crustal shortening by as much as 1900 850 km north of the block since the upper cretaceous and by 1400 1200 km south of the block since the onset of the collision 2 southern tibet formed a single tectonic unit by middle cretaceous time and 3 differential rotations are observed within the block and are related to the collision these rotations are discussed in the frame of a recent propagating extrusion tectonics model this study confirms the close paleogeographic relationship between indochina khorat plateau and southern tibet prior to the india asia collision

the tibetan side of the india eurasia collision results are reported from the 1980 joint french chinese field expedition in tibet the area covered was from the high himalaya in the south to the region of nagqu 250 km north of yangbajain ophiolites in the zangbo valley represent remnants of the crust of an ocean basin which lay adjacent to the gangdise granodiorite belt in the late mesozoic the ophiolites were thrust to the south onto the cretaceous melange and triassic flysch but the age of this event is unclear in the tertiary sediments of the indian margin and the xigaze basin were folded with steeply dipping cleavage which indicates some north south shortening of the crust however the tertiary volcanic sequence on the lhasa block and further north shows comparatively little folding and thrusting in the quaternary deformation of tibet was characterized by east west extension

sequential late cenozoic structural disruption of the northern himalayan foredeep chronologies for the siwalik molasse and intermontane basins along the southern margin of the himalaya and hindu kush ranges constrain the timing and pattern of facies migration and structural disruption of the indo gangetic foredeep this synthesis indicates that quiescent intervals are punctuated by pulses of rapid deformation as thrusting migrates in a stepwise fashion across the foredeep

constraints on the structure of the tibet plateau from pure path phase velocities of love and rayleigh waves pure path phase velocities of love and rayleigh waves across the tibet plateau are determined for the period range 30 90 s and for two opposite directions of propagation along the length of the plateau the method used is a two event method similar in principle to the classical two station method where the roles of stations and events are interchanged the success of this method is due to a favorable geometry whereby events located at each end of the plateau align to within 1 of a great circle path with one wwssn station at teleseismic distance to the east and one to the west body wave modeling is used to constrain depth and source mechanism of the events in order to calculate the corresponding source phases for surface waves phase velocities are determined to within 0 02 km s or better for rayleigh waves and 0 007 km s for love waves the constraints added by these data to the crust and upper mantle structure beneath tibet are discussed in the light of previous studies the best fitting model has a 65 km thick crust with low average crustal velocities and an sn velocity of 4 65 km s the low velocity zone is limited to the depth range 100 150 km the interesting result of this study is that both love and rayleigh wave data can be matched to within 1 in phase velocity by a unique isotropic model this points to a similarity with a stable region upper mantle structure such as that of the canadian shield finally additional rayleigh wave phase velocity data are used to compare propagation on different paths in the vicinity of tibet the tarim basin to the north of tibet stands out as having a distinct crustal structure more similar to an average tectonic region structure moreover evidence is presented for the existence of a very low velocity region at the western end of tibet which correlates well with a pronounced gravity low observed in that area

tectonics and structural zonation of southern tibet china the yarlung zangbo suture zone is composed of several thrust sheets with distinct stratigraphies states of deformation and metamorphic grades successive deformations have been produced by persistent north south shortening since obduction of the ophiolites by late cretaceous palaeocene times a possible decoupling of the crust may be associated with the intracontinental thrust system which has formed in the subducted indian plate this article emphasizes the role of shear deformations in the building of a collision belt

velocities and propagation characteristics of pn and sn beneath the himalayan arc and tibetan plateau possible evidence for underthrusting of indian continental lithosphere beneath tibet new seismological observations on velocities and propagation characteristics of pn and sn waves beneath tibet can be interpreted although not uniquely to indicate the shallow angle underthrusting of the greater indian continental lithosphere beneath the tibetan plateau this is inferred from available geological and geophysical data and is now supported by new seismological observations the most significant observation is that high frequency sn waves shear waves that travel in the mantle lithosphere propagate efficiently in the uppermost mantle beneath the tibetan plateau except beneath the north central part of tibet the chang thang terrane furthermore sn propagates efficiently across the stable block of the tarim basin along the tien shan and himalayan mountains and across the indian shield apparent velocities of pn and sn waves that traverse the uppermost mantle beneath the tibetan plateau are 8 42 and 4 73 km s respectively these velocities are very similar to those beneath the himalayan mountains and the indian shield the efficient propagation and the velocities of pn and sn waves beneath tibet are also similar to those commonly observed beneath shield and stable continental regions these results are consistent with the underthrusting model and do not favor the class of alternative models in which the tibetan plateau is formed by shortening and thickening of hot crustal and uppermost mantle material in response to the convergence of india and asia

uplift of tibetan plateau a history of the elevation and crustal thickness of the tibetan plateau since the continental collision at 45 ma has been computed using a kinematic model based on plate tectonic reconstructions and conservation of crustal volume in our reconstructions the indochina block was attached to the southern margin of eurasia before being extruded to the southeast as evidenced by the opening of the south china sea this opening affected the mass input into tibet so that tibet did not uplift to an elevation higher than 1 km until early miocene when spreading of the south china sea ceased the model then gives a steady rise of tibet from middle miocene to its present elevation

lhasa block and bordering sutures a continuation of a 500 km moho traverse through tibet a north south wide angle fan profile through the yarlung zangbo suture across the lhasa block and through the bangong nujiang suture shows several significant and sharp changes in crustal thickness both sutures appear as vertical 20 km steps in the moho and may have been the loci for eastward strike slip motion of the tibetan lithosphere