provenance of jurassic accretionary complex mino terrane inner zone of south west japan implications for palaeogeography of eastern asia provenance and tectonic history of the jurassic accretionary complex mino terrane located in the inner zone of south west japan were studied using sandstone framework composition and mudrock geochemistry modal analysis of sandstones shows that the tectonic setting of the source area for the studied mino terrane clastic rocks was uplifted basement largely dominated by high grade metamorphic terrain composed of quartz and feldspar especially plagioclase the textural and mineralogical immaturity extent of alkali and alkaline earth element leaching low chemical index of alteration values and depleted rare earth element ree contents suggest rapid uplift and erosion within the source terrain and a relatively weak weathering intensity factor analysis revealed that grain size effects governed compositional heterogeneity in the studied sediments provenance of the sediments is interpreted as being plagioclase enriched felsic basement rock such as granodiorite within a continental margin and evolved arc tectonic setting rather than active volcanic arc lack of a contribution from active volcanic arcs may have resulted from the cessation of volcanism during the reorganization of the subducting plate system and or erosion of arc volcanics and exposure of basement considering the previous studies on palaeogeography and palaeocurrent reconstruction the north eastern part of the yeongnam massif in the korean peninsula is interpreted as the most probable source area for the studied turbidites the results of mixing calculation for mino terrane sediments suggest that precambrian leucocratic granite and the basement rock of the cretaceous gyeongsang basin shed large amounts of sediments to the mino trench whilst precambrian granitic gneiss and the triassic pluton supplied lesser amounts the results of this study reveal that although active subduction accretion processes were occurring the mino trench was bordered by continental basement rocks this knowledge contributes to enhanced understanding of the jurassic palaeogeography of the east asia continental margin

bromide chloride fractionation in seafloor hydrothermal fluids from 9 10 n east pacific rise hydrothermal fluids collected from 9 10 n epr during 5 cruises between 1991 and 1994 were analyzed for br and cl the chlorinity of these fluids ranges between 32 6 and 860 mmol kg in comparison to local seawater values of 540 mmol kg these very low chlorinity fluids some with temperatures high enough to be within the vapor stability field were collected following a volcanic eruption in 1991 1 although fluids with chlorinities less than 250 mmol kg had been sampled once previously at axial volcano 2 the vent fluid chlorinities at 9 10 n epr were much lower than at axial volcano fluids with cl 250 mmol kg hadbr cl ratios identical to seawater in agreement with previous studies in contrast fluids with cl 250 mmol kg collected from this area hadbr cl ratios as much as 40 less than the seawater value bromide depletions observed in the low chlorinity fluids from 9 10 n epr were probably the result of fractionation of br from cl during subcritical phase separation this fractionation process appeared to be limited to fluids with cl 250 mmol kg and to require open system phase separation to produce vapors with cl 250 mmol kg at pressure and temperature conditions appropriate for 9 10 n epr requires conditions that are close to the critical point where the properties of the vapor and liquid phases are quite similar resulting in lack of fractionation in thebr cl ratio between the two phases

isotopic and geochemical evidence for the origin of plio pleistocene volcanic rocks near the indo eurasian collisional margin at tengchong china abstract in yunnan province china regional extensional stresses associated with the collision between india and asia have formed a series of downfaulted north south trending basins near tengchong in extreme western yunnan close to the burma border one of these basins is characterized by volcanism which began in the pliocene 7m y and has continued to historic times the tengchong volcanic suite ranges in composition from basalt to dacite with most lavas being potassium rich five chemical groups can be recognized based on major element trace element and isotopic compositions these groups are broadly consistent with previous groupings made on the basis of geologic and age data the chemical and isotopic data show that the main series most voluminous lavas were derived by partial melting of a metasomatized and heterogeneous mantle source with crustal and possibly seawater components probably related to prior subduction beneath asia an andesite dacite series with similarities to tibetan calc alkaline lavas shows clear indications of assimilated crust and crystal fractionation there is no evidence in these rocks for the presence of a strongly depleted subcontinental mantle such as has been recognized elsewhere 5 21

paleomagnetic constraints on the late cretaceous and cenozoic tectonics of southeastern asia abstract many features of the cenozoic tectonic history of central and southeastern asia can be understood as direct consequences of the thrust and penetration of india into asia recent indentation experiments with plasticine tapponnier et al 7 have extended this idea and have led to the prediction of a pattern of large rotations and displacements of continental blocks that can be tested by paleomagnetism the available cretaceous and cenozoic paleomagnetic data from this part of the world have been reviewed and a new apwp for eurasia has been constructed for reference the negligible rotation of south china and large clockwise rotation of indochina are consistent with the model i e with an history of large scale left lateral strike slip motion along the altyn tagh and red river faults data from malaya and borneo can be reconciled with the model although in a less straightforward fashion the large counter clockwise rotation of south tibet implies that it rotated in sympathy with india during the collision and suggests that future indentation experiments should include this feature finally a middle cretaceous reconstruction of the south margin of asia is proposed one interesting result is the restored continuity of geological features in tibet and indochina with active subduction of oceanic indian plate crust taking place to the south at subtropical latitudes

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

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

evidence for middle triassic to miocene dual subduction zones beneath the shan thai terrane western thailand from magnetotelluric data abstract kanchanaburi province western thailand is in the middle of shan thai terrane paleomagnetic and geological data support the hypothesis of dual subduction zones beneath the shan thai terrane during the late triassic there was a westward subduction in the east where the lampang chiang rai block subducted beneath the shan thai terrane in addition in the early tertiary the western burma terrane subducted underneath the shan thai in the west resulting in an eastward subduction a pioneer survey of this region was therefore conducted using magnetotelluric mt techniques with the aim of finding deep structures associated with these ancient subduction zones 39 mt stations were deployed to cover most of the province resistivity structures are obtained from the 3 d inversion the near surface resistivity structures correspond well with the surface geology the mid and lower crusts are conductive and this is interpreted as the crusts being composed of mafic granulites of 3 porosity in which the remnant dehydrated fluid was accumulated during the subduction in addition the westward and eastward subduction zones generated mafic ultramafic intrusions which appear as two conductive zones on the east and west respectively these two conductors and the conductive lower crust support the hypotheses of the middle triassic to miocene dual subduction zones

depth of emplacement fluid provenance and metallogeny in granitic terranes a comparison of western thailand with other tin belts the most important tin mineralization in thailand is associated with the late cretaceous to middle tertiary western thai granite belt a variety of deposit types are present in particular pegmatite vein and greisen styles of mineralization a feature common to most of the deposits is that they are associated with granites that were emplaced into the khang krachan group which consists of poorly sorted carbonaceous pelitic metasediments most of the deposits contain low to moderately saline aqueous fluid inclusions and aqueous carbonic inclusions with variable ch4 co2 ratios low salinity aqueous inclusions represent trapped magmatic fluid in at least one case the nong sua pegmatite based on their occurrence as primary inclusions in magmatic garnet aqueous carbonic inclusions are commonly secondary and neither the co2 nor nacl contents of these inclusions decrease in progressively younger inclusions implying that they are not magmatic in origin reduced carbon is depleted in the metasediments adjacent to granites and the d values greisen muscovites are variable but are as low as 134 per mil indicative of fluid interaction with organic graphitic material this suggests that the aqueous carbonic fluid inclusions represent fluids that were produced at least in part during contact metamorphism metasomatism by comparing the western thai belt with other sn w provinces it is evident that there is a strong correlation between fluid composition and pressure in general low to moderately saline aqueous inclusions and aqueous carbonic inclusions are characteristic of mineralization associated with relatively deep plutonic belts mineralized pegmatites are also typically of deeper plutonic belts and pegmatite hosted deposits may contain cassiterite that is magmatic crystallized from granitic melt or is orthomagmatic hydrothermal crystallized from aqueous or aqueous carbonic fluids in origin the magmatic aqueous fluids those that were exsolved from granitic melts are interpreted to have had low salinities as a consequence of the low salinities tin is partitioned in favour of the melt on vapour saturation thus with a high enough degree of fractionation the crystallization of a magmatic cassiterite or different sn phase such as wodginite is inevitable because tin is not partitioned in favour of the vapour phase upon water saturation of the granitic melts it is proposed that relatively deep vein and greisen systems tend to form by remobilization processes in addition many deeper greisen systems are hosted in part by carbonaceous pelitic metasediments and the reduced nature of the metasediments may play a key role in remobilizing tin sub volcanic systems by contrast are characterized by high temperature high salinity fluids owing to the high chlorinity tin is strongly partitioned in favour of the vapour and cassiterite mineralization can form by of orthomagmatic hydrothermal processes similar relationships between the depth of emplacement and fluid composition also appear to apply to other types of granite hosted deposits such as different types of molybdenum deposits

sedimentology of the cretaceous maha sarakham evaporites in the khorat plateau of northeastern thailand abstract evaporites of the cretaceous to early tertiary maha sarakham formation on the khorat plateau of southeast asia thailand and laos are composed of three depositional members that each include evaporitic successions each overlain by non marine clastic red beds and are present in both the khorat and the sakon nakhon sub basins these two basins are presently separated by the northwest trending phu phan anticline the thickness of the formation averages 250 m but is up to 1 1 km thick in some areas in both basins it thickens towards the basin centre suggesting differential basin subsidence preceding or during sedimentation the stratigraphy lithological character and mineralogy of the evaporites and clastics are identical in both basins suggesting that they were probably connected during deposition evaporites include thick successions of halite anhydrite and a considerable accumulation of potassic minerals sylvite and carnallite but contain some tachyhydrite and minor amounts of borates during the deposition of halite the basin was subjected to repeated inflow of fresher marine water that resulted in the formation of anhydrite marker beds sedimentary facies and textures of both halite and anhydrite suggest deposition in a shallow saline pan environment many halite beds however contain a curious sieve like fabric marked by skeletal anhydrite outlines of gypsum precursor crystals and are the product of early diagenetic replacement by halite of primary shallow water gypsum the 34s isotopic values obtained from different types of anhydrite interbedded with halite range from 14 3 to 17 0 cdt suggesting a marine origin for this sulphate bromine concentration in the halite of the lower member begins around 70 ppm and systematically increases upward to 400 ppm below the potash rich zone also suggesting evaporation of largely marine waters in the middle member the initial concentration of bromine in halite is 200 ppm rising to 450 ppm in the upper part of this member the bromine concentration in the upper member exhibits uniform upward increase and ranges from 200 to 300 ppm the presence of tachyhydrite in association with the potassic salts was probably the result of 1 the large volumes of halite replacement of gypsum on a bed by bed basis releasing calcium back into the restricted waters of the basin and 2 early hydrothermal input of calcium chloride rich waters the borates associated with potash rich beds likely resulted from erosion and influx of water from surrounding granitic terrains however hydrothermal influx is also possible interbedded with the evaporites are non marine red beds that are also evaporative with displacive anhydrite nodules and beds and considerable amounts of displacive halite the 34s isotopic values of this anhydrite have non marine values ranging from 6 4 to 10 9 cdt these data indicate that the khorat and sakhon nakhon basins underwent periods of marine influx due to relative world sea level rise but were sporadically isolated from the world ocean

the downward continuation of magsat crustal anomaly field over southeast asia a general method for the local downward continuation at ground level of geopotentials measured by low orbiting satellites at different altitudes is proposed such a continuation is the inverse solution of the external dirichlet problem the stability of the solution with respect to the altitude and the distribution of data points is discussed principal component analysis is shown to perform well and leads to a stable solution the method proposed has the advantage of being directly applicable to measured data points which are not evenly spaced and at different altitude as an illustration it is applied to magsat data over southeast asia two independent maps of the vertical component of the anomaly field at a constant altitude near ground level are derived for dawn descending orbits and dusk ascending orbits data separately the two maps reveal similar anomalies and confirm the correlation of the magsat derived anomaly field with the mosaic structure of the continental crust in southeast asia stable continental blocks are associated with positive anomalies and intervening collision belts correspond to well defined negative anomalies a very prominent low is observed above the himalaya and extends over the tibet plateau confirming an anomalous crustal and thermal structure of the plateau

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

a preliminary upper triassic paleomagnetic pole for the khorat plateau thailand consequences for the accretion of indochina against eurasia we report new paleomagnetic results from the upper triassic norian huai hin lat formation at the base of the khorat group in central thailand five sites of conglomerate sandstone and limestone reveal a stable component of magnetization d 43 i 43 95 7 5 we argue that this component is of primary origin on the basis of a positive fold test observation of both normal and reversed directions and a reported magnetostratigraphy from middle triassic to cretaceous the new triassic pole yields together with published younger poles a mesozoic apparent polar wander path for indochina this unit is found to have remained at tropical northern latitudes since the upper triassic comparison with eurasian poles reveals 1650 850 km of sw ne convergence and 15 10 of counterclockwise rotation between 205 and 160 ma two mechanisms are discussed to account for this motion the first one implies the closure of an oceanic basin to the north of the south china block the second one involves post welding intracontinental deformation

geologic and paleomagnetic evidence for a possible miocene collision in western panay central philippines the palawan metamorphic terrane represented by paleozoic basement on the northwestern tip of panay central philippines collided with the remainder of the island between late oligocene and middle miocene time subduction continued north and south of the collision zone along the manila and negros trenches respectively evidence for the collision is 1 the juxtaposition along a major fault of old melange terrane against highly metamorphosed paleozoic continental sediments similar to those exposed on palawan island itself and 2 miocene thrusting in the melange terrane oriented parallel to the suture paleomagnetic studies show that since the early miocene the island of panay has rotated 20 clockwise declination 20 9 inclination 22 1 for six sites whereas the northern philippines has rotated counterclockwise these contrasting rotations are consistent with a collision of the philippine arc with a northeastward converging palawan block

magnetostratigraphy of the dali basin in yunnan and implications for late neogene rotation of the southeast margin of the tibetan plateau 1 the rotation pattern and fault activity in the southeast margin of the tibetan plateau semtp provide meaningful constraints on the geodynamic evolution of the plateau however the lack of cenozoic paleomagnetic studies and accurate age constraints on neogene sediments prevents a better understanding of the late cenozoic tectonic activity in this area to clarify the tectonic rotation pattern and deformation history of the semtp we report new magnetostratigraphic data from a late neogene sedimentary sequence in the dali basin northwestern yunnan province china rock magnetic analyses indicate that both magnetite and hematite are the main carriers of the characteristic remanent magnetizations chrms magnetostratigraphic results show that the sedimentary profile spans from chron c4n 1r to chron c2n the age of the sedimentary sequence in the dali basin can thus be paleomagnetically constrained to an interval from late miocene to early pleistocene the basal age of the sediments is 7 6 ma which indicates that the unroofing of diancang shan and activation of the dali fault system were initiated at this time the appearance of conglomerates and syntectonic sediments suggests the reactivation of the dali fault system at 2 5 ma moreover the overall mean chrm direction suggests that the dali basin experienced significant 4 4 2 5 but minor post late miocene rotation this indicates that most of the clockwise rotation demonstrated by previous paleomagnetic studies in the semtp occurred prior to late miocene and may be concentrated between eocene and miocene which is contemporaneous with the sinistral slip of the ailao shan red river fault

the himalayan arc large scale continental subduction oroclinal bending and back arc spreading abstract palaeomagnetic results from the himalayan arc and southern tibet compared with simulated apparent polar wander paths for the indian plate show a consistent pattern of rotations of the himalayan arc relative to the indian shield varying gradually from 45 clockwise in the northwestern himalaya to slightly counterclockwise in the lhasa region this pattern is consistent with continental underthrusting of greater india beneath the tibetan plateau since the early miocene over at least 650 km at the longitude of western nepal and oroclinal bending since the latest miocene available palaeomagnetic observations support the steady state model for the formation of the himalayan arc with refinements as follows 1 collision between greater india s northern boundary and southern asia occurred at equatorial latitudes with progressive suturing from palaeocene in the northwestern himalaya until early eocene in the eastern himalaya 2 continuing convergence and indentation of greater india into southern asia over about 2000 km up to the early miocene resulted in southeastward extrusion of indochina and 3 neogene counterclockwise rotational underthrusting of greater india along the main central thrust with pliocene quaternary oroclinal bending of the himalayan arc

thailand s crustal properties from tele seismic receiver function studies abstract thirty three seismic stations throughout thailand were used to infer the crustal thickness vp vs ratio and poisson s ratio of thailand using the receiver function technique the crustal thickness increases from around 31 km in the shan thai terrane stt in the west to around 38 km in the khorat plateau kp ict located in the indochina terrane ict in the east similarly the mean vp vs and poisson s ratios derived here show that vp vs increases from 1 69 to 1 73 from the stt to the kp ict while poisson s ratio increases from 0 23 to 0 25 from the stt to the kp ict our results are well supported by geological evidence found at the surface in addition we incorporate gravity and magnetic data to constrain the crustal properties of thailand the low poisson s ratios show that the crust of thailand is mostly felsic only the lower crust of kp ict is different as it is thicker denser and has higher mafic composition than the other terranes of thailand this special feature in the kp ict is required to explain the lower bouguer anomaly in the kp ict in addition we interpret the crustal evolution and deformation of these terranes and compare our results with the results from the vietnam network of the ict

new paleomagnetic data from the huanan block china and cretaceous tectonics in eastern china abstract a total of 72 grey green fine grained sandstone and siltstone core samples from 10 sites were collected from the early cretaceous jieshan and hekou formations at yongan in the huanan block of southeastern china fujian province after removal of an erratic soft magnetic component in fields of 10 20 mt stable magnetizations were retrieved from both rock units which are carried mainly by single domain pseudo single domain sd psd magnetite or titanomagnetite as suggested by isothermal remanent magnetization coercivity measurements and microscopic observations the hekou formation yields a tilt corrected pole at 66 9 n 221 4 e n 5 sites k 203 3 a 95 5 4 a fold test on this result suggests that the magnetization was acquired before tilting which occurred shortly after deposition between early and late cretaceous this magnetization is related to a primary sedimentary fabric as indicated by the low field anisotropy of magnetic susceptibility ams measurements the positive fold test and occurrence of a primary sedimentary fabric suggest a depositional origin for the stable magnetization the jieshan formation gives a pole at 70 8 n 228 7 e n 5 sites k 47 7 a 95 11 2 in geographic coordinates which is statistically indistinguishable from the tilted pole for the hekou formation the associated stable magnetization corresponds to a secondary fabric these features indicate that the jieshan formation has been remagnetized during cretaceous time the predominance of sd psd magnetite or titanomagnetite and igneous activities in eastern fujian during the cretaceous suggest a viscous thermal remanent magnetization origin for the secondary magnetization the early cretaceous pole from the hekou formation is similar to those for the yangzi block suggesting that the huanan block has been accreted to the yangzi block at least since early cretaceous time a high consistency of cretaceous poles from the sino korean yangzi and huanan blocks as well as hainan island indicates that eastern china has been a coherent tectonic unit since cretaceous time although there might have been a small amount of northward displacement of the sampling area with respect to the yangzi block during the early late cretaceous period as a result of the overthrust of the huanan block onto the yangzi carbonate platform comparison of cretaceous poles for eastern china with the eurasian apparent polar wander path apwp suggests that no paleomagnetically detectable relative motion has taken place since the cretaceous the cretaceous poles of eastern china however display a paleomagnetically significant westward motion about 9 in angular distance which is not observed in the eurasian apwp this difference may be indicative of a younger age corresponding to the cretaceous tertiary boundary for all late cretaceous poles from eastern china or more probably may imply a minor error in the definition of the eurasian apwp

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

tertiary paleomagnetism of north and south china and a reappraisal of late mesozoic paleomagnetic data from eurasia implications for the cenozoic tectonic history of asia abstract to contribute to the understanding of the cenozoic tectonic history of china and eastern eurasia in general we collected paleomagnetic samples from tertiary rocks in both north and south china our miocene paleomagnetic data for northern china are from four widely separated basalt sections in the inner mongolia autonomous region and shandong province both normal and reversed polarities of the characteristic magnetization are present in the stratigraphie sequence and the mean normal and reversed directions are closely antiparallel to each other the fact that the same characteristic direction was carried by both magnetite and hematite suggests that the hematite was probably formed by auto oxidation processes during primary cooling of the basalts the paleomagnetic data from these four areas also pass a regional generalized fold test the corresponding mean miocene paleomagnetic pole for northern china is longitude 261 6 e latitude 88 3 n a 95 6 4 our early tertiary paleomagnetic results for south china are from two sandstone sections around the city of nanning 22 7 n 108 3 e in guangxi zhuang autonomous region high temperature characteristic directions isolated in 31 samples from five sites pass both tilt and reversal tests the paleomagnetic pole position determined from the five sites lies at longitude 228 0 e latitude 83 6 n a 95 8 5 the tertiary paleomagnetic poles for north and south china are statistically indistinguishable from each other and thus confirm that north and south china were fully assembled at least by the tertiary our new tertiary paleomagnetic data are consistent with the hypothesis that the major continental blocks of north and south china were in the same position as today during the tertiary a critical evaluation and selection of paleomagnetic results from the late jurassic onward for siberia europe and north and south china further corroborate this hypothesis most previously existing jurassic and cretaceous paleomagnetic poles from europe siberia are of uncertain reliability and should not be used for tectonic reconstruction using poles derived from transferring reliable data from other continents we show that the previous paleomagnetic inferences of large eastward displacements between these chinese blocks during the cenozoic are no longer warranted the available paleomagnetic data that meet our acceptance criteria are also of interest concerning the boundary conditions of the propagating extrusion model for asian tectonics additional mesozoic and cenozoic paleomagnetic data from siberia and north and south china are still needed to resolve the smaller second order displacements that appear to characterize the cenozoic tectonic history of this part of asia

plates and their motions the last reviews which covered research in plate motions were those of herron and pitman 1975 on marine magnetic anomalies and dewey 1975 on plate tectonics both reporting on research for the period 1971 1974 in the last 12 years there has been considerable refinement of relative motion models as well as further development of global models also major effort has gone into defining reference frames for the study of plate motions and discriminating between them more recently the role of errors in individual plate rotations and the associated uncertainties in reconstructions have received attention in this report i concentrate on work published in the last 4 years 1983 1986 citing a minimum of publications from previous years however some papers in press for early 1987 have been included this report first reviews work on relative plate motions for the present and past and reconstructions and then models for these motions global ones for driving forces and local ones determining subduction history and back arc spreading the problem of reference frames both hotspot and paleomagnetic is discussed next i end with a review of work on the errors in plate motions

collision tectonics in the himalaya as evidenced by the indus and shyok rock assemblages abstract the himalayan orogeny is closely associated with the tectonic developments which occurred along the indus suture zone during the last decade many new data on sediment stratigraphy and volcanic plutonic suites have been obtained these data have been interpreted in terms of plate tectonics the meso tethys was generated following the permo triassic rifting and spreading during the jurassic the tethys ocean is postulated in accordance with geophysical data to have been consumed during northward subduction along the indus suture resulting in the himalayan orogeny through collision between india and eurasia however the nature of tectonic processes and their chronology appear to be different in the western and eastern himalayan segments in the western himalaya subduction in an island arc setting and eocene collision are evident mainly by the occurrence of the island arc dras volcanics the sangeluma group srikantia and rajdhan 1980 trench deposits ophiolitic blocks older 60 m y calc alkaline plutons and blueschist facies rocks the indus group srikantia and rajdhan 1980 continental deposits and the younger

development of the active doublet method for measuring small velocity and attenuation changes in solids the measurement of small changes in elastic wave velocity and attenuation is important to a broad range of problems such as earthquake prediction and early detection of rock failure in mines previous authors proposed a method for estimating small temporal velocity changes in the earth s crust by analyzing progressive relative phase delays between the scattered waves of two signals generated by nearly identical earthquake sources called doublets recorded at different times at the same receivers several improvements have been made to the original method and are presented here the reliability of measured velocity changes has been increased by using active repeatable sources instead of natural earthquakes the robustness of the analysis technique has been improved by eliminating unnecessary intermediate phase regression steps and thus reducing the sensitivity to spurious data finally the phase delay algorithm has been extended to allow measurement of small attenuation changes from relative amplitude

phanerozoic plate boundaries in mainland se asia the himalayas and tibet recently available geological data on tibet yunnan and burma facilitate recognition of phanerozoic subduction systems and two continental fragments of gondwanaland which extend through mainland se asia a cambrian subduction system and 5 mesozoic early caenozoic collision belts are identified in the region indochina eastern thailand and central tibet accreted to china in the early triassic western se asia and s tibet separated from gondwanaland in the permian or early triassic and collided with asia in the late triassic and the western burma island arc system collided with asia in the jurassic upper triassic flysch and schist in the eastern indoburman ranges were accreted to western burma in the jurassic early cretaceous during eastward subduction and formation of a back arc thrust belt following collision of greater india with arc complexes to the n in the cretaceous and with tibet in the early eocene western burma moved northwards about 450 km along the sagaing namyin ridge thrust transform fault formation of the 3 major belts of tin granites and the porphyry copper deposits in the region took place in specific tectonic settings within the subduction systems and collision belts

widespread cenozoic remagnetization in thailand and its implications for the india asia collision abstract paleomagnetic study of 13 sites from the khorat plateau in thailand spanning ages from the devonian to the jurassic and reappraisal of published results from the triassic to the cretaceous show that most of the indochina block suffered complete remagnetization after the triassic and possibly after the cretaceous most likely at the time of the collision of india and eurasia the paleodirection associated with this remagnetization is consistent with the contention that thailand was a part of eurasia at that time and that it has suffered some 20 of clockwise rotation subsequently also consistent is the observed azimuthal difference of mesozoic fold axes on either side of the red river fault as is the early geometry of and sense of motion on this fault re interpretation of late neogene data suggests that rotation may have continued into the neogene the remagnetization event prevents the determination of paleopositions of thailand prior to the cenozoic but carries great interest in the frame of the extrusion tectonics concept

extension tectonics the neogene opening of the north south trending basins of central thailand paleomagnetic samples were collected from late neogene basalt flows from thailand all of these flows are horizontal and are relatively unaltered in thin section these rocks possess a stable magnetization which is believed to be primary samples from 48 lava flows were collected from sites located within the khorat plateau the chao phraya phitsanulok basin and the mountainous terrane west of the chao phraya phitsanulok basin these data were combined with previously reported late neogene data from five flows from western thailand although the average inclination from the 53 sites is indistiguishable from the expected dipole inclination the average declination has a net clockwise rotation of 13 5 5 8 from the geocentric dipole field furthermore the mean declination values from the 29 flows from the khorat plateau are indistinguishable from the present dipole field direction dm 4 3 7 5 and indistinguishable from the mean declination from 28 late neogene volcanic flows from vietnam in contrast the mean declinations from 24 flows collected from central and western thailand are deflected significantly clockwise dm 24 4 7 7 from the geocentric dipole field direction the differential rotation between western and central thailand versus the khorat plateau suggests that indochina is composed of at least two structural blocks which underwent a different rotational history these observations when combined with geologic and geophysical data from the chao phraya phitsanulok basin gulf of thailand and the intermontane basins of western thailand suggest that the rotations are recording a late neogene phase of e w extension of these basins we suggest that the formation of these basins and the related basaltic volcanism developed in reponse to subduction of the indian plate under western burma we envision the tectonics of this region is similar in style to the basin and range region of the western united states last we have observed field relationships from some of the rhyolites located in the central basin although these rhyolites are reported to be mesozoic or paleozoic in age our field observations and a k ar age date show that at least some of these rhyolites are younger than the basalts we suggest that the rhyolites form a bimodal suite with the basaltic rocks which were erupted in the later stages of the extension

39 ar 40 ar dating of the trans himalayan calc alkaline magmatism of southern tibet the age of the trans himalayan calk alkaline magmatism has not been clearly established on the basis of geological control it is possible to infer a pre late cretaceous age for the plutonites at the southern edge of the batholith belt and a late cretaceous or younger age for the lava series on the top and or to the north of the plutons k ar radiometric data previously obtained by chinese groups agree with these geological ages the 39ar 40ar method has been used to analyse more accurately some of the rocks which have been sampled during the joint french chinese field work in southern tibet in the summer of 1980 the incremental degassing of argon gives generally more information about the thermal history of the sample using this method we now show that the earlier basic plutons of the southern edge of the trans himalayan belt have ages of 90 110 myr albian to cenomanian whereas the lavas of the upper unconformable series are clearly younger 60 myr palaeocene the ladakh and kohistan magmatic belts further west are probable equivalents of the trans himalaya north of the indus suture their precise ages are also poorly known orbitolinas of middle cretaceous aptian age have been found in the drosh yasin series which lie unconformably on dioritic plutons in the northern edge of the belt the few radiometric data that are available give ages in fairly good agreement with those we have obtained in the trans himalaya so we conclude that a subduction zone has been active during the cretaceous between india and asia the only outstanding question is whether this active margin was at that time close to india or to asia or somewhere in between

the geodynamic evolution of the eastern eurasian margin in mesozoic times abstract the cenozoic features of eurasia such as marginal seas and intracontinental deformation are unraveled using available paleomagnetic and field data the late cretaceous shape of eurasia is then reconstructed from the examples of sw japan and south russian far east the major features of the mesozoic eurasian margin suture zones marked by ophiolites or high pressure metamorphic rocks olistostromes characterized by the lithology of the olistoliths calc alkaline magmatism and strike slip faults are outlined correlations of the ophiolitic belts and the olistostromes along the eastern eurasian margin from borneo to siberia allow the definition of four microcontinents the west philippines south japan and the anuy and okhotsk sea microblocks from south to north the mesozoic structure of the eurasian margin is due to three processes 1 1 oceanic subduction and the accretion of subduction complexes 2 2 the collision of these microcontinents with eurasia between the late jurassic and the late cretaceous and 3 3 the reactivation of the ophiolitic sutures by left lateral wrench faulting

amount of clockwise rotation of southwest japan fan shape opening of the southwestern part of the japan sea abstract paleomagnetic measurements have been carried out on welded tuffs ranging in age between 58 ma and 112 ma from yamaguchi and go river areas in the central part of southwest japan the new data together with those of younger igneous rocks published previously define the change of paleomagnetic field direction during the late mesozoic cenozoic period for southwest japan the paleomagnetic direction from this area has pointed 56 3 clockwise from the expected field direction estimated from apwp apparent polar wandering path of the whole of eurasia during the period between 100 and 20 ma in comparison with the expected one from the eastern margin of eurasia korea china siberia the cretaceous field direction of southwest japan shows the clockwise deflection by 44 49 these results establish that while the eastern margin of eurasia including southwest japan was rotated more or less with respect to the main part of eurasia during last 100 ma southwest japan was rotated clockwise through more than 40 with respect to the eastern margin of eurasia since 20 ma the large amount of rotation for southwest japan implies that it is rotated by an opening of the southwestern part of the japan sea which widens northeastward fan shape opening the tectonic feature of southwest japan and the japan sea is analogous to that of corso sardinia and the ligurian sea in the mediterranean indicating that the fan shape opening is a specific feature of the rifting of the continental sliver at the continental rim

fragmentation of asia in the permian asia is a composite continent formed by the accretion of crustal blocks1 3 a likely sequence of events is that siberia collided with a kazakhstan block which together then collided with europe along the urals in permo triassic times2 this was followed by the sucessive accretion of various blocks of south and east asia to siberia culminating in the collision of india with asia during the cenozoic palaeomagnetic data have already demonstrated that the kolyma and sikhote alin blocks of eastern asia collided with the main continent along the late mesozoic fold belts of verkhoyansk and sikhote alin4 furthermore results from the malay peninsula5 and japan6 suggest that south east asia and japan were situated near the equator in permian times and were therefore separated from the asian continent we report here some palaeomagnetic results from the permian of the sino korean and yangtze blocks of china that show they were also situated near the equator were not in their present relationship to one another and were widely separated from siberia

tectonically driven changes in the indian ocean circulation over the last 25 ma neodymium isotope evidence abstract changes in ancient oceanic current patterns have been strongly influenced by plate tectonics particularly by the opening or closing of gateways between different oceans such modifications of oceanic circulation have important consequences on climate we studied the nd seawater isotopic composition of indian and pacific ocean cores using nd isotopes which are good paleo oceanographic tracers we focused on the past 25 ma which are marked by the closure of the indonesian gateway as well as the mediterranean connection we show that a strong westerly oceanic surface current which we refer to the miocene indian ocean equatorial jet miojet linked the eastern and western indian ocean from 14 ma to 3 ma and infer that this major change in oceanic circulation probably induced important variations of global climate

gondwana and cathaysian blocks palaeotethys sutures and cenozoic tectonics in south east asia the triassic indosinian orogeny followed extinction of the palaeotethys ocean resulting in suturing of gondwana affinity and cathaysian blocks