the closing of tethys and the tectonics of the himalaya recent geological and geophysical data from southern tibet allow refinement of models for the closing of southern neo tethys and formation of the himalaya shelf sediments of the indian passive continental margin which pass northward into deep sea tethyan sediments of the indus tsangpo suture zone were deposited in the late cretaceous an andean type margin with a 2 500 km long trans himalayan kohistan ladakh gangdese granitoid batholith formed parallel to the southern margin of the lhasa block together with extensive andesites rhyolites and ignimbrites lingzizong formation the southern part of the lhasa block was uplifted deformed and eroded between the cenomanian and the eocene in the western himalaya the kohistan island arc became accreted to the northern plate at this time the northern part of the lhasa block was affected by jurassic metamorphism and plutonism associated with the mid jurassic closure of the bangong nujiang suture zone to the north the timing of collision between the two continental plates ca 50 40 ma marking the closing of tethys is shown by 1 the change from marine flysch like to continental molasse like sedimentation in the indus tsangpo suture zone 2 the end of gangdese i type granitoid injection 3 eocene s type anatectic granites and migmatites in the lhasa block and 4 the start of compressional tectonics in the tibetan tethys and indus tsangpo suture zone south facing folds south directed thrusts after the eocene closure of tethys deformation spread southward across the tibetan tethys zone to the high himalaya deep crustal thrusting barrovian metamorphism migmatization and generation of oligocene miocene leucogranites were accompanied by south verging recumbent nappes inverting metamorphic isograds and by south directed intracontinental shear zones associated with the main central thrust continued convergence in the late tertiary resulted in large scale north directed backthrusting along the indus tsangpo suture zone more than 500 km shortening is recorded in the foreland thrust zones of the indian plate south of the suture and 150 km shortening is recorded across the indian shelf zanskar range and the indus suture in ladakh there was also large scale shortening of the karakoram and tibetan microplates north of the suture as much as 1 000 km shortening occurred in tibet the more recent deformation however involved the spreading of this thickened crust and the lateral motion of the tibetan block along major approximately east west trending strike slip fault zones

late cenozoic geodynamic evolution of eastern indonesia this paper presents an internally and globally consistent model of plate evolution in eastern indonesia from middle miocene to present time it is centered on the banda sea region located in the triple junction area between the pacific philippine australia and south east asia plates the geological and geophysical data available from indonesia were until recently insufficient to define a unique plate tectonic model in this paper the new data taken into account clearly restrict the possible interpretations owing to a great number of geological geophysical and geochemical studies the major plate boundaries the sunda banda subduction zone to the south the tarera aiduna fault zone and the seram thrust to the east and the sorong fault zone and molucca sea collision zone to the north are now clearly identified the age of the major tectonic structures is also better known geodetic measurements well constrain the present time plate kinematics we also consider the deformation history within eastern indonesia where numerous short lived microplates and their related microcontinents successively accreted to the asiatic margin moreover magnetic anomalies identification of the north and south banda sea basins allows a precise kinematic reconstruction of the back arc opening we used the plates software to test the coherency of our model presented as a series of 4 plate reconstruction maps from 13 ma to the present finally the origin of oceanic domains restored by our reconstruction is discussed

crustal structure of the java margin from seismic wide angle and multichannel reflection data seismic investigations across the convergent sunda margin off indonesia provide a detailed image of the crustal architecture of the sunda plate boundary the combined analysis and interpretation of wide angle and reflection seismic data along two coincident profiles across the subduction zone are complemented by additional lines within the forearc domain which yield some three dimensional 3 d constraints on the velocity depth structure across the margin a detailed cross section of the subduction zone is presented which is confirmed by supplementary gravity modeling the sunda convergence zone is a prime example of an accretionary margin where sediment accretion has led to the formation of a massive accretionary prism with a total width of 110 km between the trench and the forearc basin it is composed of a frontal wedge which documents ongoing accretion and a fossil part behind the present backstop structure which constitutes the outer high moderate seismic velocities derived from wide angle modeling indicate a sedimentary composition of the outer high the subducting oceanic slab is traced to a depth of almost 30 km underneath the accretionary prism the adjacent forearc domain is characterized by a pronounced morphological basin which is underlain by a layer of increased seismic velocities and a shallow upper plate moho at 16 km depth we speculate that remnant fragments of oceanic crust might be involved in the formation of this oceanic type crust found at the leading edge of the upper plate beneath the forearc basin

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

kinematic model of active deformation in central asia the velocity field of present day deformation in central asia is modelled using a set of four rotating blocks siberia tarim tibet india on a spherical earth a best fit is inverted on the basis of estimated shortening rates across the main thrust zones himalayas tien shan and measured slip rates along the principal strike slip faults altyn tagh and karakorum separating those blocks the fit to the data implies that nearly all the present convergence between india and asia can be accounted for by slip partitioning on these four zones with as much as 50 absorbed by northeastwards extrusion of tibet this suggests that localised deformation governs the present mechanical behaviour of the central asian lithosphere

active thrusting and folding along the northern tien shan and late cenozoic rotation of the tarim relative to dzungaria and kazakhstan we have studied geometries and rates of late cenozoic thrust faulting and folding along the northern piedmont of the tien shan mountain belt west of urumqi where the m 8 3 manas earthquake occurred on december 23 1906 the northern range of the tien shan rising above 5000 m overthrusts a flexural foredeep filled with up to 11 000 m of sediment of the dzungarian basement our fieldwork reveals that the active thrust reaches the surface 30 km north of the range front within a 200 km long zone of neogene quaternary anticlines fault scarps are clearest across inset terraces within narrow valleys incised through the anticlines by large rivers flowing down from the range in all the valleys the scarps offset vertically the highest terrace surface by the same amount 10 2 0 7 m inferring an early holocene age 10 2 kyr for this terrace which is continuous with the largest recent fans of the piedmont yields a rate of vertical throw of 1 0 0 3mm yr on the main active thrust at the surface a quantitative morphological analysis of the degradation of terrace edges that are offset by the thrust corroborates such a rate and yields a mass diffusivity of 5 5 2 5 m 2 kyr a rather fresh surface scarp 0 8 0 15 m high that is unlikely to result from shallow earthquakes with 6 m 7 in the last 230 years is visible at the extremities of the main fold zone we associate this scarp with the 1906 manas earthquake and infer that a structure comprising a deep basement ramp under the range gently dipping flats in the foreland and shallow ramps responsible for the formation of the active fault propagation anticlines could have been activated by that earthquake if so the return period of a 1906 type event would be 850 380 years the small size of the scarp for an earthquake of this magnitude suggests that a large fraction of the slip at depth 2 3 is taken up by incremental folding near the surface comparable earthquakes might activate flat detachments and ramp anticlines at a distance from the front of other rising quaternary ranges such as the san gabriel mountains in california or the mont blanc aar massifs in the alps we estimate the finite cenozoic shortening of the folded dzungarian sediments to be of the order of 30 km and the cenozoic shortening rate to have been 3 1 5 mm yr assuming comparable shortening along the tarim piedmont and minor additional active thrusting within the mountain belt we infer the rate of shortening across the tien shan to be at least 6 3 mm yr at the longitude of manas 85 5 e a total shortening of 125 30 km is estimated from crustal thickening assuming local airy isostatic equilibrium under the same assumption serial n s sections imply that cenozoic shortening across the belt increases westwards to 203 50 km at the longitude of kashgar 76 e as reflected by the westward increase of the width of the belt this strain gradient implies a clockwise rotation of tarim relative to dzungaria and kazakhstan of 7 2 5 around a pole located near the eastern extremity of the tien shan west of hami 96 e 43 5 n comparable to that revealed by paleomagnetism between tarim and dzungaria 8 6 8 7 a 6 mm yr rate of shortening at the longitude of manas would imply a rate of rotation of 0 45 m y and would be consistent with a shortening rate of 12 mm yr north of kashgar taking such values to be representative of late cenozoic rates would place the onset of reactivation of the tien shan by the india asia collision in the early to middle miocene 16 22 9 m y in accord with the existence of particularly thick late neogene and quaternary deposits such reactivation would thus have started much later than the collision roughly at the time of the great mid miocene changes in tectonic regimes denudation and sedimentation rates observed in southeast asia the himalayas and the bay of bengal and of the correlative rapid change in seawater sr isotopic ratio 20 to 15 ma like these other changes the rise of the tien shan might be a distant consequence of the end of indochina s escape

sequence stratigraphy and the evolution of the ganges brahmaputra delta complex 1 a seismic stratigraphic analysis of 2600 km of data covering part of the ganges brahmaputra delta has shown that it evolved in three stages 1 the proto ganges delta developed immediately after the breakup of gondwana at approximately 126 ma the indian plate then moved rapidly northward ultimately crossing more than 50 degrees of latitude because the catchment area was small sedimentation was relatively restricted and the early delta consists of relatively small scale prograding clastic bodies alternating with mixed carbonate clastic and carbonate platform assemblages 2 the transitional delta developed following a major eustatic sea level low at 49 5 ma initially sedimentation was relatively restricted but at about 40 ma the flux of water and clastic sedime ts dramatically increased and the delta grew rapidly with the development of major prograding clastic depositional sequences abundant evidence of lobe switching suggests that the delta was stream dominated at that time the abrupt change in the morphology of the delta appears to be a direct response to the collision of the indian plate with the eurasian plate and the rise of the himalayas the delta remained stream dominated some time after the major plate collision and until the bay of bengal became a well developed deep embayment at about 21 ma the bengal fan began to accumulate during this interval as turbidity currents carried the increased sediment load into deeper water 3 the modern delta began to appear in its present form following a major eustatic sea level low and erosio at 10 5 ma by this time the delta had become tidally dominated and had prograded well beyond the study area into the now deeply embayed bay of bengal

thermal evolution of the gangdese batholith southern tibet a history of episodic unroofing the gangdese batholith southern tibet was part of an andean type arc at the southern margin of asia prior to the collision of india and asia at approximately 50 to 40 ma fission track and 40ar 39ar analyses of 28 rocks from 10 gangdese granitoid plutons along an 250 km length of the batholith in the lhasa region provide a detailed understanding of the age and the postcrystallization erosional and tectonic history of these rocks these data suggest a range of ages for these plutons of 94 to 42 ma with the majority being of tertiary age the postcrystallization cooling histories of all of these plutons are characterized by marked discontinuities we conclude that most of these discontinuities and all of them after 40 ma reflect tectonic changes that produced brief pulses of rapid erosion which were distributed in both space and time in addition to the initial cooling of hot magma against cold country rock all of the rocks we studied showed evidence for at least one subsequent episode of rapid cooling dropping many tens of degrees in a few million years conversely these plutons all experienced intervals during which they cooled very slowly or not at all these slow cooling intervals lasted from 5 to 50 million years our data indicate that since the collision between india and asia began response to continued convergence has been quite variable in even this relatively small area the data reported here are consistent with a recently proposed model of oligo miocene crustal shortening along the gangdese thrust system in this area

fault plane solutions of earthquakes and active tectonics of the tibetan plateau and its margins summary fault plane solutions of earthquakes within and on the margins of the tibetan plateau show diverse styles of faulting and deformation with thrust faulting and crustal shortening normal to the margins of the plateau and with normal and strike slip faulting resulting in roughly east west crustal extension within the plateau the direction of overthrusting of the himalaya onto the indian shield is radially outward varying from southwest in the western himalaya to south southeast in the east assuming that the indian shield behaves rigidly this requires a west northwest divergence of western tibet from southeastern tibet at a rate of 18 9 mm yr 1 comparable with the rate of convergence at the himalaya fault plane solutions of earthquakes in the southern portion of the tibet plateau consistently show large components of normal faulting on roughly north striking planes and corroborate such extension within the high plateau where elevations exceed 5000 m normal and strike slip faulting occur so that an overall east southeast west northwest extension of the region at about 10 mm yr 1 is partitioned into roughly equal parts of crustal thinning and north northeast south southwest crustal shortening about 5 mm yr 1 in general strike slip faulting characterizes solutions for earthquakes within eastern tibet where mean elevations drop below 4500 5000 m but the orientations of the strike slip faults vary across the region in central tibet left lateral slip occurs on planes trending roughly northeast but for earthquakes farther east the orientations of that plane become progressively east west and then southeast this variation in orientation implies a rotation of material along curved left lateral shear zones thus the eastward extrusion of tibet appears to be facilitated not only by rapid left lateral shear but also by large clockwise rotations of the material in eastern tibet the rate of eastward extrusion of material in eastern tibet relative to the tarim basin to its north is roughly 30 40 mm yr 1 fault plane solutions of earthquakes in the northern and eastern margins of tibet show large components of thrust faulting with the p axes oriented radially outward from the plateau and approximately perpendicular to the regional topographic contours of the plateau the orientation of this crustal shortening is northeast southwest on the northeastern margin east west on the eastern margin and northwest southeast in the longmenshan on the southeastern margin thus at least some of the extrusion of eastern tibet out of india s northward path into asia is absorbed by crustal shortening on the margins of the plateau the variation from normal faulting in the high tibetan plateau where elevations exceed 5000 m to dominantly strike slip faulting farther east where elevations are lower and then to thrust faulting on the margins of the plateau where elevations drop below 3000 m surely results at least in part from a decrease in the value of the vertical stress the magnitude of the east west compressive stress need not vary across the plateau

structural petrological and thermal evolution of a tertiary ductile strike slip shear zone diancang shan yunnan the diancang shan a horst massif within the red river fault zone in yunnan people s republic of china preserves a structural petrological and thermal record of two distinct phases of tectonic activity a left lateral ductile shear that terminated between 20 and 17 ma and a ductile to brittle phase of normal faulting which began at 4 7 ma and remains active mylonitic rocks in the core of the range display an early steep high temperature ht schistosity and a horizontal stretching lineation that are both parallel to the trend of the belt kinematic indicators indicate that shear was left lateral the complex shape of the ht schistosity at the southern termination of the massif likely results from a large scale oblique left lateral c shear plane that dismembered the shear zone and separated the ailao shan and the diancang shan as left lateral deformation terminated thermochronological and thermobarometric results suggest that the gneisses were partially unroofed during this event along the eastern edge of the diancang shan the ht fabrics were overprinted by low temperature structures during activation of east dipping normal faults cooling associated with this normal right lateral faulting along the diancang shan and perhaps activation of the right lateral normal movement on the range front fault farther south along the ailao shan began at 4 7 0 1 ma these results tend to support the view that extrusion of indochina occurred along the left lateral red river shear zone between 35 and 19 17 ma initiation of right lateral normal slip during the late miocene may relate to eastward extensional collapse of the thickened tibetan crust or more probably to initiation of the second phase of extrusion

seismic reflection evidence for a dangerous grounds miniplate no extrusion origin for the south china sea the collision of india and asia has caused large strike slip faults to form in east asia resulting in the extrusion of crustal blocks toward the southeast since the eocene as a result of the indentation of rigid india into asia it has been suggested that the south china sea opened as a result of relative motion between a rigid indochina sundaland block and china alternative models propose that rifting and seafloor spreading were driven by trench forces to the south we test these competing models by analysis of seismic reflection profiles across the boundary between sundaland and the southern rifted margin known as the dangerous grounds we show that the southern boundary of the dangerous grounds is a subduction zone that jammed in the middle miocene to the west the dangerous grounds are bounded by a strike slip zone also active until similar to 16 ma that becomes diffuse south of the now inactive south china sea seafloor spreading center we place the western edge of the dangerous grounds just to the east of the natuna arch lupar line the west baram line is confirmed as originating as a major strike slip fault within the dangerous grounds and is continuous with the red river fault zone because the dangerous grounds were independent of sundaland until similar to 6 ma its motion cannot have been constrained by motion of this block making extrusion impossible as a mechanism to rift the south china sea se motion by both the dangerous grounds and sundaland suggests subduction forces were the primary trigger for plate motions our reconstruction places a similar to 280 km upper limit on the motion on the red river fault and a similar to 1400 km width to the paleo south china sea

formation and evolution of strike slip faults rifts and basins during the india asia collision an experimental approach the processes which have governed the formation and evolution of large tertiary strike slip faults during the penetration of india into eastern asia are investigated by means of plane strain indentation experiments on layered plasticine models the steady state deformation of plasticine obeys a power creep flow law e c t n the stress exponent n is between 6 and 9 at 25 c uniaxial plane strain tests on cubic specimens show that the growth of faults in layered plasticine results from strain softening a process observed for strain rates ranging from 3 5 10 5 to 3 6 10 3 s 1 fault or shear zones form after only 7 10 bulk strain subsequent deformation is controlled by the geometry of the fault pattern rather than the physical properties of the plasticine a series of nine plane strain indentation experiments shows the influence of boundary conditions as well as that of the internal structure of the plasticine model on the faulting sequence the ubiquity of strain softening in experimental deformation of a variety of rocks as well as the widespread occurrence of shear zones in nature suggest that long term deformation of the continental lithosphere may also be primarily influenced by the geometry of large faults which rapidly develop with increasing strain the deformation and faulting sequence observed in the plasticine indentation experiments may thus be compared to collision induced strikeslip faulting in asia particularly to total offsets and rates of movements on the faults the experiments simulate the evolution of the western ends of the strike slip faults which have probably been analogous to trench fault fault triple junctions the experiments also illustrate mechanisms for the formation of extensional basins such as the south china sea north china basin and andaman sea near active continental margins the basins which appear to absorb terminal offsets along major strike slip faults near such margins may result from mismatch between the sharply angular shape of the deformed continental edge and the more regularly curved trench along which the smoothly flexed oceanic lithosphere subducts the existence of distinct phases of strike slip extrusion corroborates the idea that the discontinuities in time which typify intracontinental tectonics and orogenic cycles may often result from strain localization and the ensuing discontinuous non steady state deformation of the continental lithosphere

paleomagnetic study of permian and mesozoic sedimentary rocks from northern thailand supports the extrusion model for indochina abstract the mode of deformation of southeast asia resulting from the indian collision is still controversial is convergence between stable india and siberia absorbed principally by crustal shortening and thickening and minor strike slip faulting or by a mechanism of extrusion involving crustal thickening and lateral escape of blocks along major strike slip zones in order to test these models we have carried out a paleomagnetic study of sedimentary rocks of jurassic to cretaceous age on the khorat plateau in thailand a large discrepancy between the poles of indochina presented in this paper and a compilation of south china block pole positions both at the triassic jurassic and the jurassic cretaceous boundary imply some 1500 800 km of post middle cretaceous left lateral slip along the red river and xian shui he fault zones and approximately 14 7 of clockwise rotation for the indochina block relative to the south china block such relative motions are in agreement with a model of lateral extrusion of indochina during the india asia collision additional data of permian upper triassic and lower jurassic age collected in indochina indicate a post late permian remagnetization for the upper permian limestones a comparison of these data with those from yunnan south china the north china block and the south china block shows that these blocks have probably been in contact at least since the late triassic

evolution of the eastern indian ocean since the late cretaceous constraints from geosat altimetry we propose a new model for the tectonic evolution of the eastern indian ocean from the late cretaceous to the present two types of data are used to improve previously published reconstructions first recent reinterpretations of seafloor magnetic anomalies between australia and antarctica and in the wharton basin provide new constraints on spreading rates and the timing of major reorganizations second vertical deflection profiles i e horizontal gravity anomaly derived from 22 repeat cycles of geosat altimeter data reveal the tectonic fabric associated with fracture zones these new geosat data provide tight constraints on paleospreading directions for example three prominent fracture zones can be traced from south of tasmania to the george v basin antarctica providing an important constraint on the relative motions of australia and antarctica through the late eocene in addition the geosat profiles are used to locate the conjugate continental margins and continent ocean boundaries of australia and antarctica as well as the conjugate rifted margins of kerguelen plateau and broken ridge based on a compilation of magnetic anomaly data from the crozet basin the central indian basin the wharton basin and the australian antarctic basin ten plate tectonic reconstructions are proposed reconstructions at chrons 5 11 ma 6 21 ma 13 36 ma and 18 43 ma confirm that the southeast indian ridge behaved as a single plate boundary since chron 18 the constraints from the geosat data provide an improvement in the fit of the kerguelen plateau and broken ridge at chron 20 46 ma to avoid overlaps between broken ridge and the kerguelen plateau prior to their breakup our model includes relative motions between the northern and southern provinces of the kerguelen plateau finally we examine the implications of our model for the relative motions of india australia and antarctica on the tectonic evolution of the kerguelen plateau and broken ridge and the adjacent labuan basin and diamantina zone as well as the emplacement of the ninetyeast ridge and the kerguelen plateau over a fixed hot spot

oblique plate convergence in the indo burma myanmar subduction region the indo burma myanmar subduction boundary is highly oblique to the direction of relative velocity of the indian tectonic plate with respect to the eurasian plate the area includes features of active subduction zones such as a wadati benioff zone of earthquakes a magmatic arc thrust and fold belts it also has features of oblique subduction such as an arc parallel strike slip fault sagaing fault that takes up a large fraction of the northward component of motion and a buttress the mishmi block that resists the motion of the fore arc sliver

structure of the papuan fold belt papua new guinea 1 the structure of the papuan fold belt differs substantially from the classical thin skinned structures of the canadian rockies mainly due to the weak and broken papuan lithosphere and incompetent clastic section compared to the strong cratonic lithosphere in canada overlain by limestones and quartzites the papuan margin was dissected by mesozoic extensional faults in part reactivated during neogene compression which created basement involved anticlines or elevated plateaus in the papuan fold belt the sedimentary section in the fold belt is deformed into large ramp anticlines in the southwest smaller miocene and mesozoic duplexes in the center and fault propagation folds in the northeast hydrocarbon discoveries to date have occurred in the southwestern anticlines which are interpreted as reactivated extensional structures the precursor rollover anticlines trapped oil generated in the late cretaceous such that present accumulations probably result from preservation of that oil during neogene thrusting mesozoic duplex structures are likely to be prospective but difficult to define and fault propagation folds rely upon unproven upper cretaceous sandstones as a reservoir

neogene basin history and paleomagnetism of a rifted and inverted forearc region on and offshore sumba eastern indonesia the island of sumba is located in the arc trench gap at the junction of the sunda and banda arcs to elucidate its enigmatic forearc position stratigraphic and paleomagnetic data were collected along three transects in central and east sumba sumba represents the emerged part of a terrane of se asian origin results of stratigraphic and paleomagnetic investigations on central and east sumba are presented and discussed in order to identify patterns of horizontal and vertical crustal movements a synthetic seismogram of the onshore record enables correlation with the offshore neogene e of sumba in order to interpret the data in a regional context the neogene basin history of sumba started when overall subsidence took place around the transition from early to middle miocene on east sumba above a basal condensed interval up to 600 m of middle late miocene volcaniclastic submarine fan deposits occur the sediments reflect island arc volcanism and are interpreted as parts of a prograding submarine fan system on a north dipping paleoslope fed by a volcanic source to the south of sumba biostratigraphic data indicate a high volcanic supply during the middle miocene and early middle tortonian a waning supply during the late tortonian and renewed supply during the messinian these patterns probably reflect variation in volcanic activity during the messinian calcilutites and foraminiferal chalks became dominant and deposition of these pelagic sediments continued into the early pliocene using the pattern of magnetic polarity changes in a selected interval the position of the mioceneepliocene boundary is better constrained seismic studies in the offshore area e of sumba van der werff 1995a b indicate the presence of an extensive tract of n prograding turbidites on sumba ridge savu basin the proposed correlation with the onshore record suggests lateral continuity from the island into the offshore area where the volcaniclastic submarine fan system presumably had its largest development subsidence proceeded so fast i m ka on east sumba that before the beginning of the late miocene sediments were deposited below the ccd this pattern suggests that the sumba region underwent strong crustal attenuation due to rifting as this was coupled with the arrival of volcaniclastics we envisage intra arc rifting coupled with the growth of a short lived volcanic arc further south its initiation at the transition of middle late miocene suggests interaction with the then approaching banda arc subduction system the paleomagnetic data indicate that during this process sumba was at a paleolatitude of 12 south and underwent a counter clockw ise rotation of 5 the reappearance of planktonic foraminiferal assemblages during the latest miocene and their present elevation above sea level suggest an average rate of post miocene uplift of 0 7 mm a emergence of sumba probably did not take place before 3 ma this inverted motion of the sumba terrane is probably coupled with the change to the present tectonic situation in which volcanic activity was resumed in the present inner arc after a non volcanic interval during the spreading stage 1 1997 elsevier science ltd

indian plate motion and shape constraints on the geometry of the himalayan orogen abstract sea floor palaeomagnetic data that reflect variations in rate and vector of indian plate movement and rotation suggest that initial collision between india and asia occurred at about 50 55 ma ago as the pre collisional indian plate was diamond shaped with the northern margin comprised of two oblique boundaries collision was earliest where these boundaries meet or in what is now the northwest himalaya oblique convergence along each of these two boundaries would generate rotation of thrust sheets as they climb on to the indian plate the oroclinal shape of the main himalayan chain to the east of the northwest himalayan syntaxes reflects a combination of the effects of oblique convergence post collisional anticlockwise rotation of the indian plate and the pinning of the main thrusts at their northwestern terminations by crust thickened during the earliest collisional stage the indian plate rotation enhances a strike slip component of movement along the western oblique margin with the transpressively sinistral chaman fault zone now acting as a continental escape structure

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

source to sink constraints on tectonic and sedimentary evolution of the western central range and cenderawasih bay indonesia abstract the island of new guinea is the result of continent arc collision that began building the island s central range during the late miocene recent studies have shown that rapid subduction uplift and exhumation events took place in response to rapid oblique convergence between the pacific and the australian plates the tectonic and sedimentary evolution of cenderawasih bay in the northwestern part of the new guinea island is still poorly understood this bay links a major structural block the kepala burung block to the island s central ranges previous studies have shown that cenderawasih bay contains a thick 8 km sequence of undated sediments one hypothesis claims that the embayment resulted from a 3 ma opening created by anticlockwise rotation of the kepala burung block with respect to the northern rim of the australian plate alternatively the current configuration of cenderawasih bay could have resulted from the southwest drift of a slice of volcanics and oceanic crust between 8 and 6 ma we test these hypotheses using i a geomorphologic analysis of the drainage network dynamics ii a reassessment of available thermochronological data and iii seismic lines interpretation we suggest that sediments started to accumulate in cenderawasih bay and onshore in the waipoga basin in the late miocene since the inception of growth of the central range beginning at 12 ma resulting in sediment accumulation of up to 12 200 m this evidence is more consistent with the second hypothesis and the volume of sediment accumulated means it is unlikely that the embayment was the result of recent 2 3 ma rotation of structural blocks at first order we predict that infilling is mainly composed of siliciclastics sourced in the graphite bearing ruffaer metamorphic belt and its equivalent in the weyland overthrust ophiolites volcanic arc rocks and diorites contribute minor proportions from the unroofing paths in the central range we deduce two rates of solid phase accumulation spar since 12 ma the first one at a mean spar ranging between 0 12 and 0 25 mm a with a maximum spar of 0 23 0 58 mm a and the second during the last 3 ma at a mean spar ranging between 0 93 and 1 62 mm a and with a maximum spar between 2 13 and 3 17 mm a i e 6700 10 000 m of plio pleistocene sediment accumulation local transtensional tectonics may explain these unusually high rates of sedimentation in an overall sinistral oblique convergence setting

cretaceous cenozoic tectonic events in the western myanmar burma assam region correlation of late mesozoic mafic and ophiolitic rocks suggests that in the late lower cretaceous a northeasterly facing mafic arc was emplaced onto southeast borneo western sumatra western myanmar burma then 1100 km south of its present position the mogok belt of myanmar and the denqen bangong co ophiolite zone in tibet all then situated on the southwestern margin of asia reversal in tectonic polarity and eastward subduction generated a late cretaceous magmatic arc in western myanmar and sumatra and its continuation through the mogok belt into the gandise batholith of tibet magmatism interrupted in the latest cretaceous probably resumed in the western myanmar tibet and sumatra arc segments in the early palaeogene it was again interrupted in the mid eocene when an east vergent ophiolite nappe overrode the indo burman ranges deforming lower eocene turbidites derived from the magmatic arc to the east following renewed northward and eastward subduction of ocean floor in the latest eocene india collided with northern myanmar and with tibet along a suture south of the jurassic triassic flysch and adjacent yarlung ophiolite in the mogok segment of the late cretaceous arc crust thickened in the early cretaceous orogeny underwent extension to form a metamorphic core complex as myanmar west of the sagaing fault moved northward more than half of this northward movement preceded the well documented 450 km of post early miocene dextral displacement on the sagaing fault and related spreading in the andaman sea generation of late cretaceous to early eocene i and s type granites in and east of the mogok belt was related either to crustal shortening or more probably to subsequent extension in the mogok belt

mesozoic anomalies in the bay of bengal abstract the analysis of 8200 line km of total magnetic intensity data in the bay of bengal northeastern indian ocean revealed the presence of approximately n30 e trending seafloor spreading type magnetic anomalies these anomalies resemble the mesozoic anomaly series m11 m0 reported elsewhere the oldest anomaly m11 132 5 ma identified close to the east coast of india is followed by the younger series of mesozoic anomalies towards the offshore some of the anomalies are offset by 60 80 km the configuration of the offsets of the isochrons allowed us to propose approximately n120 e trending oceanic fracture zones the mesozoic crust of 132 5 118 ma is estimated to evolve with an average half spreading rate of 3 5 cm yr except for the ocean floor between the m9 and m4 magnetic isochrons this part of the crust appears to be affected by the buried subsurface 85 e ridge

a complete magnetic polarity stratigraphy of the miocene continental deposits of mae moh basin northern thailand and a reassessment of the age of hominoid bearing localities in northern thailand northern thailand comprises more than 40 tertiary intermontane basins these basins tectonically interpreted as grabens or half grabens have yielded hominoid fossils and rich neogene mammal faunas relative ages provided by biochronological studies are controversial and neither absolute ages nor correlations with marine deposits are available a precise temporal framework provided by magnetic polarity stratigraphy is therefore crucial to understand the evolution of these southeast asian mammalian faunas to estimate the timing and duration of basin formation and finally to allow correlations with other neogene hominoid localities of the old world a complete magnetostratigraphic study which significantly extends previous paleomagnetic investigations has been conducted in the miocene sequence of mae moh basin lampang province northern thailand based on the biostratigraphic constraints the 15 polarity zones recognized from the composite section have been correlated with chrons c5acr c5r 3r between 14 1 and 12 0 ma the present study provides a high resolution magnetostratigraphic reference for the continental middle miocene of all southeast asia by correlation with the miocene sequence of the nearby basin of chiang muan the large bodied hominoid khoratpithecus chiangmuanensis bearing levels can be precisely dated between 12 4 and 12 2 ma

the sedimentary and tectonic evolution of the yinggehai song hong basin and the southern hainan margin south china sea implications for tibetan uplift and monsoon intensification 1 the yinggehai song hong basin is one of the world s largest pull apart basins lying along the trace of the red river fault zone in the south china sea south of hainan island this basin crosscuts the rifted margin of the northern south china sea in this paper we present for the first time a regional compilation of multichannel seismic reflection data from both the strike slip and rifted margins the basins started to open after similar to 45 ma especially after similar to 34 ma the yinggehai basin was folded and inverted in the middle miocene after 21 ma in the north and 14 ma in the south before rapidly subsiding again after similar to 5 ma because of continued tectonism this subsidence has caused shale diapirism especially driven by associated sedimentation in the late pliocene 2 6 2 0 ma extension along the adjacent south hainan margin shows preferential lower crustal extension suggestive of lower crustal flow increasing toward the continent ocean transition during breakup sediment supply is reconstructed to peak in the middle miocene then falls between 14 and 10 3 ma to reach a low in the late miocene however rates rose again in the pliocene pleistocene the red river sediment budget is incompatible with climate models that propose stronger monsoon rains starting at 8 ma or with large scale river capture away from the red river after similar to 10 ma both lines of evidence point to major uplift in the red river drainage being middle miocene or older the recent preindustrial red river carried much more sediment than the average pleistocene accumulation rate indicating modest sediment buffering onshore at least in recent geologic time

the origin and evolution of complex transfer zones graben shifts in conjugate fault systems around the funan field pattani basin gulf of thailand abstract changes in dip province associated with conjugate fault sets produce complex transfer zones locally known as graben shifts between sets of convergent conjugate faults in the pattani basin 3d seismic data over a well developed graben shift geometry in the funan field area of the pattani basin has revealed details of the transfer zone development in map view the graben shift is seen as west dipping faults forming a wedge shaped incursion into a zone of east dipping faults the wedge boundaries trend ne sw and nw se at a high angle to the n s striking normal faults within the transfer zones faults only slightly overlap with their neighbours with individual faults tending to widen upwards the narrow regions of overlap indicate displacement transfer must occur between more widely separated faults not neighbouring faults the initial oligocene early miocene syn rift fault pattern appears to have been strongly influenced by pre existing basement trends as indicated by 1 the restriction of west dipping secondary faults to the present day area of the graben shift 2 the line of the ne sw boundary to the graben shift coinciding with fault segment linkage geometries in two major syn rift faults and 3 curvature of minor fault tips into the transfer zone the subsequent conjugate fault system developed during thermal subsidence shows that the dip asymmetry and map boundaries of the fault dip panels are strongly influenced by the syn rift fault geometry there is no indication of active strike slip faulting affecting the graben shift geometry fault displacement diagrams commonly show linkage between deeper and shallower displacement maxima indicating early late oligocene early miocene minor west dipping normal faults influenced the location and width of higher middle late miocene conjugate faults hence two stages of structural inheritance led to the middle late miocene graben shift geometry

pre breakup continental extension in east gondwanaland and the early opening of the eastern indian ocean abstract extension of continental crust by up to 360 km on a north northeast azimuth in the great australian bight occurred prior to the middle cretaceous 96 ma onset of seafloor spreading between australia and antarctica this large amount of continental extension is constrained to the late jurassic 160 ma middle cretaceous interval about a pole estimated to lie about 40 southeastward of southeastern australia using bathymetric data combined with seismic and magnetic determinations of the continent ocean boundaries off australia india and antarctica we determine a revised fit of east gondwanaland prior to continental extension and at various stages thereafter in the first stage from 160 to 132 5 ma m11 india australia rotated from antarctica with continental crustal extension between australia and antarctica and largely transform motion between the coromandel coast margin of india and the kron prins olav kyst margin of antarctica in the second stage from 132 5 to 96 ma india rotated northwestward away from australia antarctica producing m10 and younger magnetic anomalies along the western margin of australia continental extension was greatest between australia and antarctica during this stage the newly determined 132 5 96 ma rotation between india and australia provides an improved fit between the orientation of observed m10 m0 magnetic anomalies and the southward decrease in spreading rate off the western australian margin together with bathymetry the new rotation parameters lead to identification of the abandoned 96 ma spreading ridge along the lost dutchmen and dirck hartog ridges the naturaliste fracture zone is found to lie nearly parallel to the early cretaceous transform direction between india and antarctica suggesting that slow seafloor spreading required between australia and antarctica to accommodate the continental extension further east occurred between naturaliste fracture zone and the m4 m0 anomalies adjacent to the naturaliste plateau the revised fit of india in east gondwanaland reduces the eastward extent of greater india to 95 e but does not change its extent as far north as the cape range fracture zone the new pole determined for the early cretaceous rotation between india and australia antarctica lies within 15 of the southern tip of india significantly closer than in previous determinations thereby reducing the amount of seafloor spreading generated between africa and antarctica by the india antarctica rotation and thus offering a possible solution to the perceived spreading rate problems between m11 and m0 in the somali mozambique and antarctic basins

a plate tectonic model of the eastern indonesia collision zone eastern indonesia is unique in the present global plate configuration being situated at the t junction between the world s two main active orogenic belts the alpine himalayan and circum pacific belts consequently the tectonics of this region must be understood as a zone of interaction between three major plates whereas most other orogenic areas can be modelled as deformation between just two major plates with numerous intervening microplates or terranes previous models to explain the tectonics of eastern indonesia have generally started from small scale geological observations and attempted to work upwards to an overall plate synthesis in this paper i reverse the process suggesting what might be expected from one particular collision model in a three plate system and comparing this with what is actually seen in the region s geological record

combined escape tectonics and subduction rollback back arc extension a model for the evolution of tertiary rift basins in thailand malaysia and laos the tertiary rift basins of thailand and adjacent countries show considerable variability in the timing of rift initiation termination the timing and magnitude of thermal subsidence and the timing and intensity of inversion episodes the rift basins developed on continental blocks that were extruded southeastwards hence their development must be tied into himalayan extrusion tectonics current tectonic models propose that the tertiary basins opened up as pull apart basins associated with strike slip faults published geochronology of strike slip fault zone rocks mapping of fault patterns in the tertiary basins and mapped releasing restraining bend geometries all indicate that in thailand major sinistral strike slip motion ceased at about 30 ma prior to the formation of most rift basins the thailand the effects of later dextral slip were minor and probably a result of reactivation during episodes of inversion during nw se to ne sw himalayan compression dextral slip was not responsible for opening most of the rift basins in thailand an alternative mechanism to open the rift basins is subduction rollback of the indian plate to the west of thailand it is proposed that subuction rollback can help explain some of the characteristics of the rift basins such as non uniform lithospheric extension deep sag basins and the diachronous onset and termination of rifting

early cretaceous strata of the nepal himalayas conjugate margins and rift volcanism during gondwanan breakup early cretaceous sandstones shales and marlstones chukh tangbe and muding formations at least 625 m thick crop out north of the main central thrust in central nepal they were strongly deformed during himalayan collision with telescoping of facies transitions as a result of crustal shortening the sediments show northerly palaeoflow and were deposited on the steadily subsiding northern tethyan margin of gondwana berriasian deltaic deposits pass upward into valanginian albian storm dominated shelf deposits and into latest albian pelagic slope carbonates an unconformity probably of valanginian age separates the chukh and tangbe formations locally and is overlain by volcanic conglomerates the gondwanan margin succession matches well with that observed on the formerly conjugate margin of nw australia which separated from greater india during the valanginian to hauterivian volcanic detritus where suitable for analysis shows within plate geochemical affinity and reflects gondwanan fragmentation abundant volcanic detritus in aptian strata may have been derived from extensions of the rajmahal traps of northeast india however volcanism was active near central nepal in the valanginian and volcanic detritus possibly far travelled reached the area during the berriasian early cretaceous strata1 successions and relative sea level changes show a first order relationship to tectonism associated with gondwanan break up

processes of allochthonous terrane evolution mindoro island philippines on mindoro two tectonostratigraphic terranes the north palawan block and the mindoro block are separated by a steeply dipping shear zone the north palawan block was rifted off southern china in the oligocene and comprises most of the shallow southern end of the south china sea the eastern part of this terrane exposed on mindoro includes jurassic and paleogene strata probable rift basalts of oligocene age and oligo miocene graben fill deposits in the late oligocene and early miocene the eastern north palawan block underwent crustal stretching with grabens oriented at a high angle to synchronous spreading anomalies in the south china basin we hypothesize that grabens formed in an overall transtensional environment near a transform boundary at the eastern margin of the opening south china basin the mindoro block consists of a pre upper cretaceous low grade metamorphic basement overlain by upper cretaceous and upper eocene strata the mindoro block records several episodes of intense deformation of mesozoic and tertiary age and contrasts with the north palawan block both in terms of stratigraphy and structural evolution the mindoro suture which separates the mindoro and north palawan blocks is a complex structural boundary comprising slices of diverse lithologies separated by steeply dipping anastomosing faults deformed tectonic slices in the suture zone include serpentinized ultramafic rock amphibolite and rocks derived from both of the bounding terranes we interpret this suture as a zone of mostly transcurrent faulting that was active through much of mid tertiary time paleogeographic considerations for early and middle miocene time support the premise that the west side of the central philippine archipelago was a left lateral transform boundary the mindoro and north palawan blocks were initially juxtaposed along the transcurrent mindoro suture by late miocene time subsequent deformation on mindoro includes crustal shortening associated with the southern end of the manila trench which projects onshore on southwest mindoro the onset of shortening here probably reflects a late miocene change in philippine eurasian plate motion the mindoro suture zone is now locked and it acts as a rigid buttress against which strata of the north palawan block have been structurally stacked the mindoro suture zone is similar to other terrane boundaries in the northern philippines the history of terrane transport in this region was apparently dominated by strike parallel shuffling of terranes along major transcurrent boundaries within the philippine arc transcurrent motion has taken place in extensional and compressional regimes and it is ongoing today this may be a dominant process by which complex island arc systems are disrupted prior to accretion onto a continental margin

tectonic discrimination of basalts with classification trees traditionally geochemical classification of basaltic rocks of unknown tectonic affinity has been performed by discrimination diagrams although easy to use this method is fairly inaccurate because it only uses bi or trivariate data furthermore many popular discrimination diagrams are statistically not very rigorous because the decision boundaries are drawn by eye and they ignore closure thus violating the rules of compositional data analysis classification trees approximate the data space by a stepwise constant function and are a more rigorous and potentially more effective way to determine tectonic affinity trees allow the simultaneous use of an unlimited number of geochemical features while still permitting visualization by an easy to use two dimensional graph two classification trees are presented for the discrimination of basalts of mid ocean ridge ocean island and island arc affinities the first tree uses 51 major minor and trace elements and isotopic ratios and should be used for the classification of fresh basalt samples a second tree only uses high field strength element analyses and isotopic ratios and can also be used for basalts that have undergone alteration the probability of successful classification is 89 for the first and 84 for the second tree as determined by 10 fold cross validation even though the trees presented in this paper use many geochemical features it is not a problem if some of these are missing in the unknown sample classification trees solve this problem with surrogate variables which give more or less the same decision as the primary variables the advantages of the classification tree approach over discrimination diagrams are illustrated by a comparative test on a sample dataset of known tectonic affinities although arguably better than discrimination diagrams classification trees are not perfect and the limitations of the method are illustrated on a published dataset of basalts from the pindos basin greece

indochina peninsula and the collision of india and eurasia the collision of india with asia results in a rapidly changing stress pattern around both himalayan syntaxes the maximum compressive stress which is north south north of the indenter fans radially through nearly 180 around the syntaxes to accommodate the necessary strike slip motion on the edges of the indenter thus the whole indochina peninsula was swept by the migrating stress and associated strain pattern as the eastern syntaxis moved to the north as a result the maximum horizontal component of the stress field is predicted to have been first oriented east west then north south we propose that this changing stress field controlled the opening of the sedimentary basins and of the south china sea as well as the direction of motion of the major strike slip faults

north palawan block philippines its relation to asian mainland and role in evolution of south china sea the island of mindoro the northern part of palawan island and the reed bank area southwestern philippines together constitute a continental fragment the north palawan block lying within an island arc oceanic setting the permian to paleogene rocks of these areas indicate a geologic origin and history for the block contrasting with that of the rest of the philippine archipelago these rocks also suggest that the north palawan block once occupied a pre drift position contiguous with the south china mainland four prominent pre neogene regional unconformities are recognized both onshore and offshore the china mainland in taiwan and in the north palawan block the synchrony of these unconformities and the facies relation of the unconformity bounded sedimentary units strongly suggest a common pre neogene history for all these areas by contrast an important regional unconformity occurring at the top of the middle miocene throughout the palawan area is absent from the asian mainland extension in the south china sea basin since the mesozoic which has separated the north palawan block from the asian mainland has been approximately uniform from west to east however there is strong evidence to suggest that this extension has been achieved by temporally separated phases of continental crustal attenuation and more recent sea floor spreading from the foregoing observations and using the most recent magnetic spreading anomaly data for the south china sea a suite of palinspastic reconstructions has been prepared which shows the evolution of the south china sea area from the late triassic to the pliocene the reconstructions illustrate 1 the convergent continental margin setting of the north palawan block during much of the jurassic and cretaceous 2 the late cretaceous inception of the philippine island arc system 3 the subsequent counter clockwise rotation of the arc system from the late eocene onward 4 the paleocene to middle miocene opening of the south china sea and 5 the early to middle miocene collision between the north palawan block and the palawan subduction system

the configuration of asia prior to the collision of india cretaceous paleomagnetic constraints paleomagnetic data from central asia show that 1700 610 km of shortening of southern asia since cretaceous time have been absorbed by distributed deformation between southern tibet and the siberia craton this result is based on a compilation of cretaceous poles from the junggar tarim tibet indochina south china north china and mongolia blocks complementing the recent compilation of enkin et al 1992a we propose a paleogeographic reconstruction of asia in the cretaceous in which the position of siberia is derived from the synthetic apparent polar wander path of besse and courtillot 1991 the resulting map which likely represents asia as it remained throughout the cretaceous until the collision with india began features an unbent tibet with an east west trending andean margin at tropical latitudes and a rather continuous belt of continental red bed basins extending from sichuan to tarim through tibet the map allows one to estimate continental shortening and rotations between the blocks which are attributed to the collision despite large uncertainties these have amounts and senses which are in all cases compatible with some recent kinematic models such as that of avouac 1991 appendix is available with entire paper on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b93 007 2 50 payment must accompany order

tectonic geomorphology of australia the australian continent is actively deforming in response to far field stresses generated by plate boundary interactions and buoyancy forces associated with mantle dynamics on the largest scale several 10 3 km the submergence of the northern continental shelf is driven by dynamic topography caused by mantle downwelling along the indo pacific subduction system and accentuated by a regionally elevated geoid the emergence of the southern shelf is attributed to the progressive movement of australia away from a dynamic topography low on the intermediate scale several 10 2 km low amplitude c 100 200 m long wavelength c 100 300 km topographic undulations are driven by 1 anomalous smaller scale upper mantle convection and or 2 lithospheric scale buckling associated with plate boundary tectonic forcing on the smallest scale 10 1 km fault related deformation driven by partitioning of far field stresses has modified surface topography at rates of up to c 170 m ma 21 generated more than 30 50 of the contemporary topographic relief between some of australia s highlands and adjacent piedmonts and exerted a first order control on long term 10 4 10 6 a bedrock erosion although australia is often regarded as tectonically and geomorphologically quiescent neogene to recent tectonically induced landscape evolution has occurred across the continent with geomor phological expressions ranging from mild to dramatic australia is one of the lowest flattest most arid and most slowly eroding continents on earth the average elevation of the continent is only c 330 m above sea level asl maximum local topographic relief is everywhere 1500 m defined by elevation ranges with 100 km radii and two thirds of the con tinent is semi arid to arid with the exception of localized upland areas in the flinders and mt lofty ranges and the eastern highlands bedrock

revised and synthetic apparent polar wander paths of the african eurasian north american and indian plates and true polar wander since 200 ma we have reviewed paleomagnetic data available for the eurasian african north american and indian plates over the last 200 ma selection criteria are those generally accepted with an emphasis on evidence for lack of remagnetization accurate dating and proper structural analysis this results in 23 35 51 and 2 poles for eurasia africa north america and india respectively we believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander apw paths than averaging of larger data sets involving less stringent selection criteria we propose and describe revised apw paths but more importantly we next use relative motion models to transfer all data in a common reference frame we find good agreement between transferred data when they are averaged separately for each plate in independent 20 ma windows this is a check on consistency of paleomagnetic data kinematic models and of the geocentric dipole hypothesis transferred data from all plates are averaged in 20 ma windows to generate synthetic apw paths for all plates studied these synthetic paths are in agreement with the original revised apw paths that use only data from a single plate moreover both geographic and time resolution are improved spatial confidence intervals are of the order of 5 the synthetic paths display interesting features such as a previously ill recognized apw loop for eurasia paleomagnetic and hotspot apw are next compared and a determination of true polar wander tpw is derived we find significant tpw amounting to over 20 in the last 200 ma tpw appears to be episodic with a standstill between 180 and 110 ma there is general agreement with a previous study of livermore et al 1984 the tpw standstill appears to correlate with a time of decreasing reversal frequency ending with the cretaceous long normal superchron other periods of fast tpw would seem to correspond to increasing reversal frequency however it is suggested that the major tpw hairpin at 50 ma might correspond to the collision of india with eurasia tentative correlations with core or mantle indicators can be understood as a result of couplings between the core mantle and lithosphere we believe primarily related to episodic evolution of the d layer

the receiver structure beneath the china digital seismograph network stations abstract six new crustal models for the region beneath the china digital seismograph network stations are obtained by inversion and forward modeling broadband teleseismic p wave receiver functions the seismic phases modeled are primarily p to s conversions and reverberations generated at the two highest impedance interfaces the moho and the sediment basement contact these are the best constrained features of the crustal models receiver functions from all of the stations show some indication of laterally varying structure but relatively simple 1d models can represent the crustal structure beneath the three stations in northeast china and the two stations on the margin of the tibetan plateau the station on the northern flank of the tien shan mountains is on a complex upper crustal structure that we represent with a 2d crustal model crustal velocity models and moho depths from the receiver function analysis are compared with other compilations of chinese crustal structure these closely agree for the more uniform crustal region in northeastern china but the differences are greater in southern and western china where the crustal structure is more complex

a seismological study of the eastern new guinea and the western solomon sea regions and its tectonic implications summary we use new and pre existing seismological data to study the neotectonics and the state of stress in the seismic zones of eastern papua new guinea and the western solomon sea regions earthquake hypocentres for the period 1964 90 were relocated using the joint hypocentre determination jhd technique we show that the t axes of earthquakes in the 100 250km depth range beneath eastern papua new guinea are aligned parallel to the seismic zone with no seismicity below 250 km this intermediate depth seismic zone forms an inverted u shaped zone with limbs dipping to the north and south and is interpreted to be seismicity along the subducted solomon sea plate the northern and southern limbs of the seismic zone are connected by a flattened zone of seismicity which is marked by predominantly strike slip and a few normal fault mechanisms we suggest that the solomon sea plate at depth beneath the finisterre mountains is no longer influenced by the tectonic forces acting at the surface but breaking up and sinking under its own gravitational forces a significant north dipping seismic zone is imaged above the deeper seismic zone this feature characterized by thrust mechanisms extrapolates to the surface along the ramu markham valley and is attributed to the ramu markham fault rmf the rmf has an average dip of about 40 to the north east but its detailed geometry would appear to be complex the rmf is interpreted to ramp steeply in the uppermost 10 20 km flattening out sharply to a dip of 15 at a depth of about 20km the rmf marks the suture between the finisterre terrane and the australia new guinea plate and may extend to a depth of 90km beneath the western limit of the finisterre mountains there is a clear gap in seismicity between the rmf and the deeper solomon sea plate seismic zone beneath the finisterre mountains which together with the alignment of the t axes of earthquakes along the seismic zone at depths 100km indicates that these are indeed two distinct features the seismicity in the depth range 50 80 km along the new britain arc follows the trend of the 2000m bathymetric contour of the new britain trench and the finsch deep far more closely than it does the trend of the surface plate boundary indicating that the increase in the distance between the solomon sea south bismarck plate boundary and the active volcanos along the new britain arc west of 6 83 150 o e is only a shallow phenomenon and does not reflect the geometry of the solomon sea plate at depths greater than 50 km

cenozoic motion of the philippine sea plate palaeomagnetic evidence from eastern indonesia the history of motion of the philippine sea plate is poorly known because it is isolated from the oceanic ridge system interpretation of palaeomagnetic results from the plate has been controversial because declination data have been obtained only from the eastern margin where subduction related tectonic processes may have caused local rather than plate wide rotations new palaeomagnetic data relevant to the problem have been obtained from 34 sites north of the sorong fault and 29 sites within the sorong fault system these sites record south ward movement during the eocene and northward movement during the neogene sites within the sorong fault system record both counterclockwise and clockwise rotations interpreted as the result of neogene block movements at the southern boundary of the philippine sea plate north of the sorong fault all sites record clockwise declinations neogene rocks have small deflections consistent with rotation about the present day eurasia philippine sea plate pole oligocene middle eocene rocks show consistent clockwise declination deflections of 40 o declinations of lower eocene rocks indicate 90 o of clockwise rotation we propose that the entire area north of the sorong fault in east indonesia has always been part of the philippine sea plate and that the whole plate has rotated clockwise in a discontinuous manner by approximately 90 o since the early eocene the new data from north of the sorong fault provide a basis for determining rotation poles which satisfy all the palaeomagnetic data from the philippine sea plate and permit its reconstruction

paleomagnetic results from palau west caroline islands a constraint on philippine sea plate motion paleomagnetic results from the palau islands indicate that the islands have undergone 60 70 of clockwise rotation since mid oligocene time this post mid oligocene rotation is interpreted to represent the motion of the philippine sea plate and not a local rotation for the following reasons 1 lack of morphologic expression of the differential rotation along the palau kyushu ridge 2 similarity to predicted rotations obtained by using phase shifted magnetic anomaly data and 3 ubiquitous clockwise rotations seen in paleomagnetic data from around the philippine sea plate this strong clockwise rotation of the philippine sea plate provides a mechanism for oblique subduction and related transcurrent motion along the margin of the philippine archipelago

evolution of the boundary between the philippine sea plate and australia palaeomagnetic evidence from eastern indonesia abstract the boundary between the philippine sea and australian plates is the left lateral sorong fault system of eastern indonesia until recently modelling this boundary for the period before about 5 ma was difficult the tertiary motion of the philippine sea plate was uncertain and palaeomagnetic data from areas adjacent to the fault were lacking recent geological and palaeomagnetic studies of the area north of the sorong fault have elucidated the tertiary motion history of the philippine sea plate providing a reference for examining movements within the fault system we report new palaeomagnetic data from within the sorong fault zone from the islands of taliabu and obi taliabu is part of the sula platform and is considered to be derived from australia pelagic limestones from the upper cretaceous tanamu formation of taliabu yielded a direction of d 329 1 i 34 9 implying counter clockwise rotation and a formation latitude of 19 5 s sula and misool are postulated to be part of a single microcontinent which had a different late cretaceous mid tertiary movement history from australia the sula platform was transported to its present position by movement along the sorong fault system in the late miocene obi includes rocks of philippine sea and australian origin all the new sites are in rocks of philippine sea plate origin since the early neogene the philippine sea plate which includes all islands north of the sorong fault has rotated 40 clockwise and moved 10 15 northwards philippine sea plate rocks within the sorong fault zone record similar latitude shifts but different rotations in north obi the upper oligocene anggai river formation and the middle miocene woi formation record 60 and 30 counter clockwise rotations respectively the sense of rotation is consistent with motion within a left lateral fault system with the philippine sea and australian plates providing the shear couple in contrast the woi formation in southeast obi records 15 20 clockwise rotation this area is separated from the zone of counter clockwise movement in north obi by a strand of the sorong fault arc volcaniclastic rocks from the upper cretaceous leleobasso formation of northwest obi have a primary magnetisation with a mean direction of d 357 1 i 21 9 these rocks formed at 11 n or 11 s depending on the interpreted rotation history and indicate a pacific rather than indian ocean origin a volcanic arc at the southern edge of the philippine sea plate collided with eastern new guinea at 25 ma the philippine sea australia plate boundary then changed from subduction to strike slip as the philippine sea plate began its neogene rotation initiating the sorong fault system we suggest that many of the arc fragments in the new guinea orogenic belt originated in the southern philippine sea plate arc which has subsequently been dismembered by strike slip faulting

diverse ages and origins of basement complexes luzon philippines geological field investigations and 40ar 39ar ages from two basement complexes in southeast luzon document the first known occurrences of pre late cretaceous age rocks in the eastern philippines however individual components within the two complexes vary in age from late jurassic caramoan basement complex to early cretaceous and early miocene camarines norte calaguas islands basement complex these and other data show that southeast luzon basement complexes are genetically diverse and they indicate that the concept of an old autochthonous basement in the philippines is open to question this supports the hypothesis that the philippine archipelago is an amalgamation of allochthonous mesozoic and cenozoic island arc ocean basin and continental fragments that were assembled during the tertiary

cenozoic global plate motions cenozoic global plate motions relative to the hot spots are investigated and compared to plate motions in a mean lithosphere reference frame plate motions were analyzed over six time intervals divided by ages 10 25 43 48 and 56 ma chosen as much as possible to coincide with key plate reorganizations alternative motion circuits and rotational parameters were considered and evaluated with paleomagnetic data from the pacific and north american plates the circuit found to be in best agreement with the paleomagnetic data is one in which the hot spots in the atlantic region are assumed to be fixed relative to the hot spots in the pacific region throughout the cenozoic the hot spot and mean lithosphere reference frames have been in continual slow relative motion the rate of motion is nonuniform however most of the motion having occurred during the middle cenozoic the net cenozoic rotation of the lithosphere relative to the hot spots is described by a right handed rotation of 7 about a euler pole at 46 s 87 e which yields a 5 displacement of the north poles of the two reference frames this motion is small enough that inferences drawn about plate speeds in one reference frame should be valid in the other analysis of the global motions resulting from our preferred model showed that many characteristics of current plate motions have persisted throughout the cenozoic plate speeds correlate with latitude plates moving faster near the equator than near the poles throughout the cenozoic as at present continental plates except for the indian plate moved slower than oceanic plates throughout the cenozoic even the structure of the velocity fields as revealed in a contour of root mean square velocities in equatorial bands persists throughout the cenozoic the migration of the paleomagnetic axis over time is also compared to the hot spot and mean lithosphere reference frames the paleomagnetic axis has shifted 5 10 relative to the hot spot frame and a lesser amount relative to the mean lithosphere frame

denudation history of onshore central vietnam constraints on the cenozoic evolution of the western margin of the south china sea abstract apatite fission track analysis is used to monitor the timing and rate of upper crustal cooling and denudation across central vietnam in response to rifting of the south china sea results show regional denudation in the syn and post rift phases of the south china sea occurred at a similar rate 40 10 m myr consistent with the sedimentation record in adjacent offshore basins this suggests that extension was not the dominant factor controlling cenozoic denudation probably because the study area was sufficiently distant from the actively rifted continental margin a significant change in regional cooling rate during the upper miocene inferred denudation rates also shift from 34 m myr to 390 500 m myr is linked to enhanced erosion and deposition of prograding sediments in adjacent offshore basins this shift in denudation is associated and contemporaneous with initiation of regional basaltic magmatism and development of topography

paleomagnetic results from cretaceous and jurassic rocks of south and southwest yunnan evidence for large clockwise rotations in the indochina and shan thai malay terranes abstract cretaceous and jurassic redbeds have been sampled from south and southwest yunnan china for paleomagnetic study the paleomagnetic directions resolved from the cretaceous rocks indicate that the xiaguan area which is located on the red river fault has not been rotated with respect to stable eurasia since at least late cretaceous times and that the jinggu mengla area 200 400 km farther south has been rotated by 46 65 clockwise the jurassic results reveal the same rotation pattern at face value the differences between the observed cretaceous paleolatitudes and those predicted from the eurasian apparent polar wander path apwp indicate a southward extrusion of the region but the amount of extrusion is not large enough to be resolved by paleomagnetic methods comparison with mesozoic paleomagnetic data from central yunnan northern thailand and northwestern borneo suggests that the red river fault is not a demarcation between unrotated and significantly rotated regions and that sundaland did not respond to the india asia collision as a single coherent unit the existing paleomagnetic data from east asia appear to indicate that the crust in central tibet and tien shan north of tarim directly in front of the indian indentor has been substantially shortened and thickened whereas the crust in the syntaxial areas has probably been largely squeezed out to further evaluate the relative importance of crustal thickening and extrusion in the tectonic evolution of the tibetan plateau paleomagnetic investigation should be intensified in the bounding zones to the indian indentor and be expanded into the cenozoic sedimentary rocks in tibet and surrounding regions

simulation of borehole sonic waveforms in dipping anisotropic and invaded formations a numerical simulation study has been made of borehole sonic measurements that examined shoulder bed anisotropy and mud filtrate invasion effects on frequency dispersion curves of flexural and stoneley waves numerical simulations were considered for a range of models for fast and slow formations computations are performed with a cartesian 3d finite difference time domain code simulations show that presence of transverse isotropy ti alters the dispersion of flexural and stoneley waves in slow formations the flexural wave becomes less dispersive when the shear modulus c44 governing wave propagation parallel to the ti symmetry axis is lower than the shear modulus c66 governing wave propagation normal to the ti symmetry axis conversely the flexural wave becomes more dispersive when c44 c66 dispersion decreases by as much as 30 at higher frequencies for the considered case where c44 c66 and decrease

the pearl river mouth basin a rift basin and its geodynamic relationship with the southeastern eurasian margin abstract based on multichannel seismic reflection profiles exploration well data and gravity studies the tectonic development of the pearl river mouth basin is discussed in terms of the geodynamic evolution of the southeastern eurasian margin the area around the pearl river mouth basin has been dominated by two episodes of rifting of the attenuated continental crust along the south china margin from the late cretaceous to the early oligocene the rifting brought about ruptures of various magnitudes in various segments of the basin regional normal faults have been produced and resulted in the formation of three major subbasins zhu 1 zhu 2 and zhu 3 which are separated by local uplifts and are characterized by horst and graben structures regional subsidence prevailed throughout the late oligocene recent period and resulted in a tilted continental shelf upon which rests a unified rift basin the pearl river mouth basin a three stage model involving doming rifting and drifting is proposed for the tectonic evolution of the pearl river mouth basin the uplifting and rifting of the continental crust along the south china margin are related to the late episodes of the yanshan orogeny during the late cretaceous early oligocene period when the intensity of rifting decreased the thermal subsidence of the continental margin of south china prevailed during the late oligocene recent period of the himalaya orogeny

tectonic setting of western pacific marginal basins abstract global kinematics as well as magnetic anomalies of marginal basins with continental geology and paleomagnetic data as additional constraints are used to present a set of reconstructions of the western pacific marginal basins between 56 ma and the present at key periods 56 43 32 20 12 and 3 ma our model accounts for the rapid motion of exotic terranes along the whole of the western pacific convergent zone marginal basins appear to open in a great variety of tectonic settings the two extreme examples being the mariana trough where trench suction may be the predominant driving force and the south china sea where intracontinental deformation appears to be the major mechanism the study of marginal basins is a 3 d problem which must take into account the whole tectonic context subduction related tectonics in cross section and upper plate deformation in map view and not only a 2 d problem the classical trench arc back arc problem

active tectonics of the eastern sunda and banda arcs the tectonics of the eastern sunda and banda arcs of eastern indonesia are inferred from the centroid depths seismic moments and fault plane solutions of 63 large earthquakes that occurred between 1962 and 1984 source parameters are estimated by inverting long period teleseismic p and sh waves earthquakes show that the collision of the australian continent with the banda arc shortens the upper plate beneath the forearc and back arc basin over the broad area between the timor and seram troughs along the eastern end of the sunda arc and along the southern banda arc earthquakes reveal that the arc and forearc respond to collision by shortening in the direction of convergence by elongating in the direction perpendicular to convergence and by thrusting over the back arc basin shallow thrust and strike slip earthquakes beneath the banda basin the back arc basin demonstrate that deformation within the back arc accommodates some of the northward motion of the australian continent the rate of north south shortening of the entire upper plate at the longitude of timor calculated from the seismic moments of the earthquakes over the 22 year period is roughly 20 of the predicted convergence rate between australia and southeast asia strike slip faulting within the banda basin results also in eastward motion of the banda arc with respect to western indonesia and australia eastward extrusion of the banda arc results in thrusting at the aru trough earthquakes due to thrusting of the back arc basin beneath the volcanic arc are too shallow to corroborate that the arc s polarity has reversed although the wetar back arc thrust zone at present has a high seismic slip rate earthquakes and their mechanisms provide evidence that the lithosphere of the weber basin a 7500 m deep forearc basin rests on the subducting lithosphere without intervening asthenosphere so that the weber basin subsides in response to sinking of the subducting lithosphere closure of the banda basin is geometrically incompatible with the australian plate subducting simultaneously beneath both the timor and seram troughs instead the bird s head which subducts beneath seram is decoupled from the australian plate in western new guinea and probably moves west or southwest with respect to australia in the eastern indonesian example strike slip faulting plays a major role in the evolution of the collision zone and clearly demonstrates the complexity in three dimensions of continental accretion and mountain building by arc continent collision

cenozoic carbonates in southeast asia implications for equatorial carbonate development cenozoic equatorial carbonate production was at its most extensive and diverse in the seas of southeast asia yet the carbonates of this region remain poorly documented this paper presents the first published review of carbonate deposition during the cenozoic collating sedimentological data for carbonate successions throughout the region current models of warm water carbonate platform evolution sedimentation and facies distribution inspired by studies of the classic sub tropical areas provide inadequate analogues for the evaluation of modern or cenozoic equatorial carbonates in southeast asia the equatorial carbonates differ in being dominated by bioclasts coated grains and associations with evaporites or dolomites are rare the carbonate systems occur in a range of depositional settings which were often affected by coeval tectonism siliciclastic input or volcanism an understanding of the carbonate depositional environments spatial facies distributions and controls on deposition and diagenesis is essential in order to characterize equatorial carbonate development and to evaluate their considerable economic potential

origin of the sumba forearc basement abstract an interpretation of acoustic basement structures in the eastern sunda western banda forearc region suggests structural continuity between sumba and northern timor the area between sumba and timor is delineated as a zone of continuous structures without major strike slip offsets these structures trend east west in the west but gradually change towards the east into ne sw trending structures a major ne sw trending discontinuity west of sumba between 117 30 and 118 30 e marks the transition between an intraoceanic volcanic arc system in the west and a volcanic arc continent collision zone in the east the extent of the sumba basement suggests either a common late jurassic rift drift history for both sumba and northern timor or an early miocene magmatic welding of two continental fragments of different origin resulting in structural continuity between two microplates the first explanation implies that sumba originated from the north west australian continental margin south of tethys from which it drifted away in the late jurassic the second hypothesis which is favoured proposes a northern provenance of sumba the island would then originate from the south east asian continent north of tethys from which it was detached in the eocene

late oligocene recent stress evolution in rift basins of northern and central thailand implications for escape tectonics abstract the tertiary rift basins of thailand are known through sub surface seismic and borehole data collected for hydrocarbon and coal exploration outcrops in open cast coal mines and a few natural outcrops these data provide mixed perspectives on how the basins evolved some providing data on ages and structural geometries others palaeostress data in general the region evolved under approximately e w extension although the extension direction probably changed periodically within basins extension was episodically interrupted by inversion events associated with sub horizontal nw se to ne sw 1 direction there is no evidence to suggest that the string of rift basins running from n thailand and laos into the gulf of thailand evolved in a similar way on the contrary the evolution of each basin seems different although certain regional trends along the rift system are apparent as follows 1 oligocene lower miocene extension is widespread in the region 2 in central and northern thailand middle miocene extension is also important and extension persisted into the upper miocene pliocene 3 in the extreme south of the area w natuna basin penyu basin malay basin extension ceased in the earliest miocene 4 in the northern gulf of thailand extension ceased in the middle miocene 5 thermal subsidence is greatest up to 4 km in the south malay pattani w natuna and penyu basins and least in northern thailand a few tens to hundreds of metres 6 inversion in the southern gulf of thailand was intense and occurred during the lower and middle miocene in the northern gulf of thailand inversion is very mild and occurred during the lower and middle miocene onshore inversion is patchy during the miocene but is strongest in the northwestern rift basins particularly the li basin the most widespread inversion event that affects the north occurred during the plio pleistocene it might be associated with the change from left to right lateral motion on the red river fault the relationship between strike slip faults and rift basins in terms of timing of extensional and inversion events and palaeostress orientation and evolution is more complex than can be explained by simple escape tectonic models

bounds on strain in large tertiary shear zones of se asia from boudinage restoration we have used surface balanced restoration of stretched boudinaged layers to estimate minimum amounts of finite strain in the mylonitic gneisses of the oligo miocene red river ailao shan shear zone yunnan china and of the wang chao shear zone thailand the layer parallel extension values thus obtained range between 250 and 870 we discuss how to use such extension values to place bounds on amounts of finite shear strain in these large crustal shear zones assuming simple shear these values imply minimum total and late shear strains of respectively 33 6 and 7 3 at several sites along the red river ailao shan shear zone for the wang chao shear zone a minimum shear strain of 7 4 is deduced assuming homogeneous shear would imply that minimum strike slip displacements along these two left lateral shear zones which have been interpreted to result from the india asia collision have been of the order of 330 60 km red river ailao shan and 35 20 km wang chao

active deformation in the new guinea fold and thrust belt seismological evidence for strike slip faulting and basement involved thrusting the new guinea fold and thrust belt forms a 1000 km long mountain chain running the length of new guinea but its trend is oblique to the predicted convergence direction a large component of left lateral shear across the mountain belt is expected but geological evidence for such motion is ambiguous surface structures in the fold and thrust belt show mountain building by folding and thrusting supposedly thin skinned of pre pliocene continental shelf sediments although some evidence indicates that active faulting penetrates the basement we examine the active deformation of this mountain belt by using teleseismic long period p and sh waveforms short period p waveforms and p wave first motions to determine reliable focal mechanisms and source depths for the 18 largest earthquakes in the new guinea fold and thrust belt since 1964 eight thrust events with seismic moments m0 between 1017 and 2 1018 n m are distributed throughout the thrust belt at depths ranging from 0 5 to 45 km and have steeply dipping fault planes that strike parallel to surface structural trends six of the eight events are between 11 and 25 km deep the steep dips of nodal planes the depths of these earthquakes and the known thicknesses of the sedimentary section show that thrust faulting is not exclusively thin skinned i e confined above a shallow detachment surface but penetrates the crystalline basement at high angles most earthquakes in the western half of the thrust belt including the three largest earthquakes examined m0 5 1018 n m show strike slip faulting with the probable fault plane oriented east west so that displacement is left lateral the nine strike slip earthquakes account for almost 90 of the observed seismic moment in the period 1964 1985 and indicate a seismic slip rate of 5 25 mm a all but one of these strike slip earthquakes are shallower than the thrust earthquakes suggesting that east northeast convergence across the fold and thrust belt is accommodated by a decoupled system of strike slip and dip slip faults while the great height of the mountain belt is an obvious indication that crustal thickening is an important consequence of plate motion in new guinea seismological evidence suggests that translation by strike slip faulting may play a much greater role than previously recognized by analogy in older collisional orogens whose motions can be inferred only from remnant structures and not from seismicity strike slip faulting may have been equally important in accommodating convergence we estimate from seismic moments of earthquakes and from the volume of the mountains that deformation of the new guinea highlands may account for 5 20 of the total convergence between the pacific plate and australia in new guinea

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