palaeomagnetic re investigation of early permian rift basalts from the baoshan block sw china constraints on the site of origin of the gondwana derived eastern cimmerian terranes a palaeomagnetic investigation was carried out on a series of rift basalts woniusi formation that accumulated on the baoshan block sw china in the early permian the aim being to provide quantitative palaeogeographical information on the eastern cimmerian terrane as it detached from eastern gondwana reliable data were obtained from four locations 28 individual cooling units and when combined with the findings of an earlier study three locations 19 sites indicate that breakup occurred at 41 9 s with errors 34 2 51 2 s using this information we fit baoshan against gondwana within a narrow longitudinal belt close to where northeast greater india and northwest australia were once in close proximity furthermore we suggest that sibumasu simao burma malyasia sumatra the largest of the eastern cimmerian blocks lay directly to the east offshore of australia qiangtang and lhasa almost certainly sat to the west off northern greater india se arabia but we are uncertain as to their exact configuration our findings are compared with several rather different models that have been published in recent years the new palaeomagnetic constraint highlights the flexibility authors currently have in reconstructing the region principally because of the overall lack of similar high quality data from the various blocks we explain how new data could resolve these ambiguities thereby offering more robust explanations for eastern gondwana s late palaeozoic development

miocene to present activity along the red river fault china in the context of continental extrusion upper crustal rotation and lower crustal flow detailed mapping of field relationships along the red river fault reveals information about the distribution and magnitude of slip along the fault about its interactions with other regional fault systems and the relationship between river incision and growth of the eastern margin of the tibetan plateau the red river fault is complex consisting of up to four strands and is dominated by right lateral strike slip displacement evidence for an extensional component of displacement is strongest along the northern part of the fault and decreases to the southeast to zero southeast of a major bend in the fault results of this study indicate dextral displacement on the red river fault is probably at least 40 km 15 16 km of which predates incision of the red river in pliocene time or later and probably also predates plateau growth and development of other regional fault systems long term average slip rate on the red river fault is a minimum of 5 mm yr however regional active tectonics are characterized by rotation of the upper crust around the eastern himalayan syntaxis bounded to the east by the xianshuihe xiaojiang fault system which deflects but does not cut the red river fault the distributed nature of the xianshuihe xiaojiang fault system as it approaches the red river fault is important for accommodating shearing across the strong crustal anisotropy formed by the red river fault and ailao shan shear zone the red river fault appears to terminate in the extending dali fault system in northwest yunnan pliocene surface uplift river incision and rotation around the eastern himalayan syntaxis are inconsistent with the pre pliocene displacement on the red river fault and lateral extrusion along the ailao shan shear zone advocated by the extrusion model this suggests a significant change in crustal conditions on the southeast margin of the tibetan plateau in pliocene time possibly the result of lower crustal flow

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

mineralogy fluid inclusions and s pb isotope geochemistry study of the tuboh pb zn ag polymetallic deposit lubuklinggau sumatra indonesia abstract the tuboh pb zn ag polymetallic deposit is located in lubuklinggau sumatra indonesia the ore bodies occur within the contact zone between the eocene jangkat quartz monzonite and limestone of the jurassic lower cretaceous rawas formation and are controlled by skarn and ne sw faults in order to reveal the relationship between mineralization and magmatism a comprehensive study was conducted that included detailed field observation petrographic study fluid inclusions and s pb isotope analysis the results include the following findings 1 four mineralization stages were recognized the prograde stage is characterized by garnet and wollastonite the retrograde stage is dominated by magnetite hematite chlorite quartz with minor sulfides the quartz sulfide stage is enriched in sphalerite galena chalcopyrite pyrite pyrrhotite and quartz the quartz calcite stage is typified by quartz and calcite 2 electron probe microanalysis epma results revealed that garnet in the tuboh deposit is rich in andradite adr37 99grs1 59 indicating skarn was formed under relatively oxidized conditions the temperature and s fugacity for sphalerite crystallization were 270 to 392 c and 10 9 7 to 10 7 1 bars respectively the decreasing trend between s fugacity with temperature suggests that ore precipitation occurred in a progressive reducing environment epma analyses demonstrate that the ag is primarily hosted by galena which is present as solid solution with bi via coupled substitution of pb in the galena 3 fluid inclusions trapped in garnet indicates that the ore forming fluid was high in temperature with high to moderate salinity at 380 450 c 9 21 21 4 wt nacl equivalent respectively which indicates ore forming fluid may derive from magmatic fluids fluid inclusions from quartz suggest ore precipitation from cooler and moderate to low salinity fluids at 337 420 c and 6 45 11 1 wt nacl equivalent respectively which is in agreement with the sphalerite crystallization temperature the fluid inclusions in the quartz and calcite reveal that ore forming fluid from the post ore stage was moderate to low in both temperature and salinity at 151 266 c and 0 35 7 45 wt nacl equivalent respectively the positive correlation between homogenization temperature and salinity indicates that the ore forming fluid mixed with meteoric water 4 the 34s values of sulfides range from 2 44 to 2 52 suggesting a magmatic origin of s the pb isotopic ratios of the galena are very uniform and have low radiogenic pb at 206pb 204pb 18 342 18 370 207pb 204pb 15 576 15 599 and 208pb 204pb 38 369 38 430 this indicates that the metals originated from the eocene jangkat quartz monzonite finally in combination of available data suggest that the tuboh deposit is a typical skarn type pb zn ag polymetallic deposit which is the result of the eocene quartz monzonite intruded the jurassic cretaceous rawas formation and ore precipitation is the consequent of the mixture of ore forming fluid with meteoric water and decreases in s fugacity

on microboudin paleopiezometers and their applications to constrain stress variations in tectonites abstract accounting for interfacial shear induced near end fracturing we propose a new microboudin piezometer e c 0 25 l w where is the shear flow stress of the ductile matrix e g polycrystalline quartz c is the tensile fracture strength of a columnar mineral e g tourmaline which ranges generally between e 1000 to e 250 e is young s modulus e is a dimensionless parameter named equivalent shear strength and l w is the mean length width ratio of the microboudins formed by near end fracturing this provides a straightforward method of estimating the magnitude of shear flow stresses responsible for ductile deformation of natural rocks the piezometer is easy to apply because it does not require the use of tem sem ebsd or the identification of prevailing deformation mechanism e g dislocation creep diffusion creep hardening or weakening the samples from the gaoligong metamorphic belt yunnan china yield the equivalent shear flow stresses of 0 145 0 424 36 106 mpa if taking c e 800 or 250 mpa for felsic gneisses mylonites and phyllonites the earlier formed microboudins had consistently a larger mean length width ratio than the later formed ones suggesting that the rocks recorded a tectonic history of continuous hardening prior to the cessation of quartz plasticity

india and sunda plates motion and deformation along their boundary in myanmar determined by gps using a regional gps data set including 190 stations in asia from nepal to eastern indonesia and spanning 11 years we update the present day relative motion between the indian and sundaland plates and discuss the deformation taking place between them in myanmar revisiting measurements acquired on the main boundary thrust in nepal it appears that points in southern nepal exhibit negligible deformation with respect to mainland india including these points using a longer time span than previous studies and making an accurate geodetic mapping in the newest reference frame allows us to refine the present day indian motion our results confirm that the current motion of india is slower than predicted by the nuvel 1a model and in addition our india eurasia motion is significantly 5 mm yr slower than previous geodetic determinations this new indian motion combined with a refined determination of the sundaland motion gives way to a relative india sunda angular velocity of 20 2 n 26 1 e 0 370 myr in itrf2000 predicting a relative motion of 35 mm yr oriented n10 at the latitude of myanmar there the sagaing fault accommodates only 18 mm yr of right lateral strike slip only half of the shear component of motion we present two models addressing how and where the remaining deformation may occur a first model of distributed deformation implies convergence on the arakan subduction the northern continuation of the now famous sumatra andaman trench and wrench faulting in the arakan wedge the second model uses localized deformation where deformation observed west of the sagaing fault is entirely due to elastic loading on a faster and oblique arakan subduction 23 mm yr this latter model predicts that a major earthquake of mw 8 5 may occur every century on this segment of the subduction

large volume silicic volcanism along the proto pacific margin of gondwana lithological and stratigraphical investigations from the antarctic peninsula jurassic magmatism in western gondwana produced the most voluminous episode of continental volcanism in the phanerozoic era during the early to middle jurassic some 2 5 3 million km3 of dominantly basalt and to a lesser extent rhyolite were erupted onto a supercontinent in the early stages of break up the major silicic portion of the gondwana magmatic province is exposed in patagonian south america the volcanic rocks of patagonia have been collectively termed the chon aike province and constitute one of the world s most voluminous silicic provinces the volcanic rocks are predominantly pyroclastic dominated by ignimbrite units of rhyolite composition volcanic rocks crop out sporadically across much of the once contiguous antarctic peninsula and are considered to form an extension of the chon aike province a continuation of the province to include the antarctic peninsula would extend its strike length along the active pacific margin by c 2000 km volcanic rocks exposed along the east coast of the antarctic peninsula defined here as the mapple formation are also dominated by rhyolitic ignimbrite flows with individual units up to 80 m in thickness and a total thickness of c 1 km the ignimbrites vary in degree of welding from high grade rheomorphic ignimbrites with parataxitic textures to unwelded lithic rich ignimbrites rhyolite lava flows air fall horizons debris flow deposits and epiclastic deposits are volumetrically minor occurring as interbedded units within the ignimbrite succession the lithology and stratigraphy of the jurassic volcanic rocks of the mapple formation are presented and comparisons are made to the chon aike province a consistent stratigraphy of permo triassic metasedimentary rocks unconformably overlain by terrestrial mudstone siltstone sequences which are in turn conformably overlain by largely silicic subaerial volcanic rocks is present at several localities along the antarctic peninsula and at localities in the chon aike province precise zircon u pb middle jurassic ages exist for two volcanic formations from the antarctic peninsula and a middle lower jurassic age has been suggested for the underlying sedimentary formations based on fossil flora analysis the antarctic peninsula chronostratigraphy coupled with lithological similarities indicate a close relationship to those sequences of the chon aike province

new insights into the cenozoic lateral extrusion of crustal blocks on the southeastern edge of tibetan plateau evidence from paleomagnetic results from paleogene sedimentary strata of the baoshan terrane in order to characterize the lateral extrusion of crustal material during the cenozoic we conducted a paleomagnetic study of the eocene oligocene zhushan formation and paleocene muguahe formation in the central part of the baoshan terrane bst on the southeastern edge of tibetan plateau a primary magnetic component and a remagentized component with the miocene acquisition age were isolated from the muguahe formation and zhushan formation respectively these data indicate that the bst did not commence clockwise rotation in the period of the paleocene and the oligocene and that the bst experienced 80 clockwise rotation relative to east asia since the miocene combining with the documented history of crustal boundary strike slip faults on the southeastern edge of the tibetan plateau shows that the bst and tengchong terrane tct firstly underwent latitudinal crustal shortening in the oligocene and the eastward extension was the main type of motion of the crustal material during this period which induced the initial strike slip movement of their boundary faults since the miocene the main form of crustal motion of bst tct and simao terrane was gradually transformed into clockwise rotation which possibly indicates that the fold and thrusting fault systems induced crustal shortening and thickening in the southeastern edge of the tibetan plateau have reached extremity and the southeastern part of the tibetan plateau have already been uplifted to the similar elevation with today s elevation in the miocene

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

late cenozoic to recent transtensional deformation across the southern part of the gaoligong shear zone between the indian plate and se margin of the tibetan plateau and its tectonic origin abstract within the southeastern part of the tibetan plateau the gaoligong shan shear zone forms the boundary between the tengchong and baoshan blocks and is one of the main cenozoic strike slip fault systems of the eastern himalayan syntaxis the mountain ranges rise to a maximum elevation of 3623 m with a width of 20 km and a length of 400 km the core of the shear zone consists of a narrow belt of mylonitic rocks with right lateral strike slip fabrics formed during the northward movement of india relative to eurasia in oligocene and miocene time to the south the mountains decreases in elevation and end against the wanding fault the latter is one of a series of ne sw striking active left lateral strike slip faults within the southeastern part of the tibetan plateau the cause and timing for the change in elevation of the gaoligong shan is unknown but based on our recent structural and fission track geochronological work the decrease in elevation is the result of east west extension across the southern part of the gaoligong shear zone inferred from the late cenozoic and active normal faults along its eastern and western boundaries southward the eastern boundary fault system merges with a series of splays of the wanding strike slip fault while the western boundary fault system is bent into parallelism with the wanding fault this geometric relationship suggests that the normal faulting along the eastern and western boundary of the gaoligong shear zone was caused by transfer of the ne sw left lateral movement along the wanding fault into e w extension on faults to the northeast the resultant normal faulting caused the lowering of the topography in the southern part of the shear zone fission track data from 11 samples from the mylonitic rocks in the footwall of the ruili oblique slip fault left lateral thrust slip yielded ages between 8 4 and 0 9 ma coeval with volcanism within the jietou and mangbang basins the left lateral strike slip movement along the wanding and ruili faults is interpreted to have accommodated differential clockwise rotation of the gaoligong shear zone and its adjacent tectonic units during late cenozoic to recent time during the northward movement of the indian plate with respect to the southeastern margin of the tibetan plateau along the right lateral sagaing fault

coseismic slip distribution of the 24 march 2011 tarlay myanmar mw 6 8 earthquake from alos palsar interferometry abstract we investigate coseismic deformation of the 24 march 2011 m w 6 8 tarlay myanmar earthquake using alos palsar data from both descending and ascending passes using high quality synthetic aperture radar interferograms and amplitude offset images the nearly linear surface rupture is well traced along the western end of the nam ma fault and strikes 69 from both descending and ascending pass interferometric synthetic aperture radar data and a rigorous maximum a posteriori probabilistic inversion method we infer that the event involved mostly a pure left lateral strike slip rupture with near vertical geometry our one segment model shows that the maximum slip of 4 1 m occurred at 4 km depth much larger than the slip at the surface both interferograms also reveal a small segment to the east of the main rupture in a densely populated farming area our inversion of a two segment model shows a similar slip distribution on the main fault in addition to 0 1 0 3 m left lateral slip with normal component on a 58 north dipping segment the total seismic moment from the two segment model is 1 95 10 19 n m equivalent to an m w 6 79 earthquake which is comparable to the u s geological survey seismic inversion estimate of 2 10 10 19 n m m w 6 84 the earthquake occurred within a group of east northeast striking left lateral strike slip faults near the myanmar laos border which are seismically active and reflect a system of actively clockwise rotating blocks online material figures of insar observations amplitude offset maps insar deformation decomposition one segment model inversion residuals single data set inversion results and tectonic setting

anatomy of an escape tectonic zone western irian jaya indonesia 1 the western fold and thrust belt of new guinea in irian jaya is presently a complex boundary dominated by the bird s head block escape from the collision between the australian plate and the remnants of volcanic belts carried by the pacific plate the escape rate given by geodetic measurements is of the order of 7 cm yr and movement is accommodated by a broad shear zone we analyze this shear pattern using fault slip analyses performed in the field from 1992 to 1996 combined with focal mechanism inversions moment tensors summations and surface trace of structures inferred from radar and multispectral satellite images time control of the deformation is attained by isotope dating of the recent syntectonic intrusives the geometry of the macro and microstructures occurred in two stages from the early pliocene to the present the first stage 5 to 2 ma is marked by flat and ramps structures guided by n60 e lateral ramps associated with a n60 e cleavage affecting the whole of western irian jaya the second stage 2 ma to present shows the collapse of the western fold and thrust belt and the escape of the bird s head and the lengguru belts along n60 e transtensile faults in the latter stage the strike slip offset is distributed on the n60 e schistosity zone along which some fracture planes are reactivated as left lateral transtensile faults shallow earthquakes moment tensors have been inverted for stress and summed to get strain rates to define contrasting structural provinces the spatial variations in both stress and strain fields from earthquakes and microtectonics show that 1 they are consistent and are assumed to be coeval 2 they reveal that oblique convergence is partitioned and 3 they are influenced by the existence of a free boundary we see no significant rotation of stress axes laterally along the escape zone instead stresses change according to the different orientations of basement structures and thus undergo rapid spatial variations or principal axes permutations eastward in front of the australian craton the oblique convergence is partly distributed on reverse and strike slip faults westward along the nw part of the craton we infer the seram trench is a free boundary which allows material to escape these results show that the evolution of the escape zone depends on the importance of the geometry of the former margin of australia e g tilted blocks which controls the style deformation relative to the regional stress and the boundary conditions which control the variation of the state of stress through time

mica dominated seismic properties of mid crust beneath west yunnan china and geodynamic implications abstract measurements of crystallographic preferred orientations cpo and calculations of p and s wave velocities v p and v s and anisotropy were conducted on three quartz mica schists and one felsic mylonite which are representative of typical metamorphic rocks deformed in the middle crust beneath the southeastern tibetan plateau results show that the schists have v p anisotropy av p ranging from 16 4 to 25 5 and maximum v s anisotropy av s max between 21 6 and 37 8 the mylonite has lower av p and av s max but slightly higher foliation anisotropy which are 13 2 18 5 and 3 07 respectively due to the lower content and cpo strength of mica with increasing mica content the deformed rocks tend to form transverse isotropy ti with fast velocities in the foliation plane and slow velocities normal to the foliation however the presence of prismatic minerals e g amphibole and sillimanite forces the overall symmetry to deviate from ti an increase in feldspar content reduces the bulk anisotropy caused by mica or quartz because the fast axis of feldspar aligns parallel to the slow axis of mica and or quartz the effect of quartz on seismic properties of mica bearing rocks is complex depending on its content and prevailing slip system the greatest shear wave splitting and fastest v p both occur for propagation directions within the foliation plane consistent with the fast pms s wave converted from p wave at the moho polarization directions in the west yunnan where mica amphibole bearing rocks have developed pervasive subvertical foliation and subhorizontal lineation the fast pms directions are perpendicular to the approximately e w orienting fast sks s wave traversing the core as p wave directions indicating a decoupling at the moho interface between the crust and mantle beneath the region the seismic data are inconsistent with the model of crustal channel flow as the latter should produce a subhorizontal foliation where vertically incident shear waves suffer little splitting

experiments on shortening of a 4 layer model of the continental lithosphere davy ph and cobbold p r 1991 experiments on shortening of a 4layer model of the continental lithosphere in p r cobbold editor experimental and numerical modelling of continental deformation tecronophysics 188 l 25 we present an experimental method for modelhng lithospheric deformation the method is based on a simplified profile of lithosphere rheology where only four layers are considered brittle crust ductile crust brittle mantle and ductile mantle the strengths of the four layers calculated from laboratory determined flow laws depend strongly on geothenns and slightly on strain rates and deformation regimes the rheological profiles of the lithosphere have been recreated in experimental models built with analog materials sand silicone putty and golden syrup which flow under their own weight during lithospheric shortening the style of deformation depends on the relative strengths of the four layers especially those in the mantle of special importance is the existence or non existence of a brittle mantle the initial stages of compression often produce flexures or buckles without local isostatic compensation buckling exerts a strong control on the localisation of thrust faults in later stages the thrusts tend to form at the inflexion points of buckles the style of crustal thickening depends on the relative thicknesses and resistances of the lithospheric layers in 2 layer lithospheric models without resistant mantle shortening was concentrated upon a small number of thrusts widely spaced in 3 layer models shortening was concentrated into a large number of reverse faults more closely spaced and more steeply dipping than before in clayer models a single thrust formed in the brittle mantle motion on this thrust dragged the crustal layers down forming an asymmetric crustal root with steep imbricate slices

diachronous cooling along the mogok metamorphic belt shan scarp myanmar the trace of the northward migration of the indian syntaxis abstract we present new observations and new mineral ages eight by the 40 ar 39 ar and nine by the 40 k 40 ar methods from high grade metamorphic and foliated intrusive rocks collected along the mogok metamorphic belt of central myanmar that complete preliminary results we previously published all 30 ages presented here confirm the oligocene to middle miocene age of the mogok metamorphism moreover the ages are young from the gulf of martaban in the south to the china myanmar border in the north showing a clear diachronism these results confirm a northward migration of the tectonic event responsible for the cooling of our samples at a velocity similar to the india relative to indochina velocity at the same period in addition intensive field work along a 800 km long section of the mogok metamorphic belt has revealed the numerous ductile structures that resulted from a strong nnw sse trending extension from the analogy between the present day local extrusion around the eastern indian syntaxis and the extension observed along the mogok metamorphic belt and from the diachronism we document we suggest that the central part of myanmar has recorded the northward passage of the eastern indian syntaxis with respect to indochina from oligocene to middle miocene

a dynamical model of lithosphere extension and sedimentary basin formation convection in the upper mantle can give rise to normal stresses on the base of the continental lithosphere that may cause localized elevation of its surface and extensional deviatoric stresses this possibility is investigated assuming homogeneous biaxial strain in a continental lithosphere whose strength is governed by a combination of brittle failure and power law creep in the crust and by brittle power law or dorn law behavior in the mantle for a given geotherm and for rheologies based on current laboratory data the lithosphere responds to convection induced uplift in one of three ways depending primarily on the magnitude of the driving stresses these regimes are as follows 1 when the driving stress exceeds a threshold value the resulting strain rates are high enough greater than about 3 10 16 s 1 that the lithosphere strains almost isothermally its effective rheology is strain thinning and it undergoes accelerating extension the magnitude of the dynamic uplift required to produce the threshold value of the driving stress depends on the geotherm characterized here by the moho temperature and varies from about 1 km for a moho temperature of 700 c to around 3 km for a moho temperature of 550 c an uncertainty of at least 100 c is attached to these temperatures owing to the inaccuracies in extrapolating laboratory data to geological strain rates 2 at stress levels due to less than about half of the critical uplift extensional strain is negligible 3 at intermediate stress levels moderate uplift produces extension that is bounded by strength increases due to cooling of the ductile portion of the lithosphere during extension

the motion and active deformation of india measurements of surface displacements using gps constrain the motion and deformation of india and india eurasia plate boundary deformation along the himalaya the gps velocities of plate interior sites constrain the pole of the angular velocity vector of india with respect to eurasia to lie at 25 6 1 0 n 11 1 9 0 e approximately 6 west of the nuvel 1a pole of 3 ma plate motion the angular rotation rate of 0 44 0 03 myr 1 is 14 slower than the long term rate of 0 51 myr 1 insignificant velocities between plate interior sites indicate that the exposed indian plate is stable to within 7 10 9 yr 1 the observed contraction vector across the himalaya 20 mm yr veers from n20 e in the northwest himalaya to n25 w in east nepal consistent with east west extension of southern tibet

present day crustal deformation in china constrained by global positioning system measurements global positioning system gps measurements in china indicate that crustal shortening accommodates most of india s penetration into eurasia deformation within the tibetan plateau and its margins the himalaya the altyn tagh and the qilian shan absorbs more than 90 of the relative motion between the indian and eurasian plates internal shortening of the tibetan plateau itself accounts for more than one third of the total convergence however the tibetan plateau south of the kunlun and ganzi mani faults is moving eastward relative to both india and eurasia this movement is accommodated through rotation of material around the eastern syntaxis the north china and south china blocks east of the tibetan plateau move coherently east southeastward at rates of 2 to 8 millimeters per year and 6 to 11 millimeters per year respectively with respect to the stable eurasia

magnitude and symmetry of seismic anisotropy in mica and amphibole bearing metamorphic rocks and implications for tectonic interpretation of seismic data from the southeast tibetan plateau we calibrated the magnitude and symmetry of seismic anisotropy for 132 mica or amphibole bearing metamorphic rocks to constrain their departures from transverse isotropy ti which is usually assumed in the interpretation of seismic data the average bulk vp anisotropy at 600 mpa for the chlorite schists mica schists phyllites sillimanite mica schists and amphibole schists examined is 12 0 12 8 12 8 17 0 and 12 9 respectively most of the schists show vp anisotropy in the foliation plane which averages 2 4 for phyllites 3 3 for mica schists 4 1 for chlorite schists 6 8 for sillimanite mica schists and 5 2 for amphibole schists this departure from ti is due to the presence of amphibole sillimanite and quartz amphibole and sillimanite develop strong crystallographic preferred orientations with the fast c axes parallel to the lineation forming orthorhombic anisotropy with vp x vp y vp z effects of quartz are complicated depending on its volume fraction and prevailing slip system most of the mica or amphibole bearing schists and mylonites are approximately transversely isotropic in terms of s wave velocities and splitting although their p wave properties may display orthorhombic symmetry the results provide insight for the interpretation of seismic data from the southeast tibetan plateau the n s to nw se polarized crustal anisotropy in the sibumasu and indochina blocks is caused by subvertically foliated mica and amphibole bearing rocks deformed by predominantly compressional folding and subordinate strike slip shear these blocks have been rotated clockwise 70 90 around the east himalayan syntaxis without finite eastward or southeastward extrusion in responding to progressive indentation of india into asia

tectonic amalgamation of the gaoligong shear zone and lancangjiang shear zone southeast of eastern himalayan syntaxis abstract the gaoligong shear zone glgsz and lancangjiang shear zone lcjsz are two major large scale shear zones southeast of eastern himalayan syntaxis ehs and play important roles in the accommodation of the intracontinental deformation in southeast asia the two shear zones which are mainly composed of high grade metamorphic rocks and have long been regarded as the precambrian basement of this area are gradually merged northward and become tectonically amalgamated from fugong to gongshan area structural and kinematic analyses reveal that the amalgamation area has experienced strongly partitioned dextral transpression n s trending tight to isoclinal folds sub vertical foliation and sub horizontal lineation are ubiquitously developed both within the shear zones and the carboniferous flysch sediments of baoshan block in between micro and macro kinematic indicators such as cs structures rotated porphyroclasts asymmetric shear folds sheath folds domino and boudin structures suggest that the shear sense of the glgsz and lcjsz is dextral in the amalgamation area comparative analyses of strata reveal that the baoshan block is aligned with south qiangtang block and the lanping simao block was aligned with north qiangtang block prior to the cenozoic large scale tectonic extrusion and rotation the amalgamation area which has progressively switched positions from northeast of ehs to east and southeast of ehs is just located at the neck of the large scale boudin of south qiangtang baoshan block a boudin model is proposed for the tectonic amalgamation of the glgsz and lcjsz from fugong to gongshan area in response to the continued northward convergence between indian plate and eurasian plate since early eocene resistance of south china block southeastward extrusion and clockwise rotation of baoshan block around ehs

a comparison of geodetic and finite strain pattern in the aegean geodynamic implications abstract a comparison of the active strain pattern of the aegean constrained by global positioning system measurements and the oligo miocene finite strain field shows some striking similarities in terms of direction and rates of extension the geometry of extension at crustal scale is also very similar with north dipping normal faults connecting near the brittle ductile transition to shallow north dipping shear zones this pattern of extension is not significantly modified by the recent extrusion of the motion of anatolia and aegea the persistence over more than 25 myr of the same pattern of extension suggests that the cause for extension resides within the aegean lithosphere gravitational collapse allowed by extensional boundary conditions after 30 ma slab retreat is the primary cause for post orogenic extension in the aegean and western anatolia

right lateral shear along the northwest pacific margin and the india eurasia collision right lateral shear along the eastern margin of asia from the eocene to the present has led to the opening of pull apart basins intracontinental such as the bohai basin or oceanic such as the japan sea we suggest in this paper that this right lateral shear is a consequence of indentation of asia by india as in small scale analog experiments we conclude that antithetic wrench faults accommodate the counterclockwise rotation of large domino blocks between two major left lateral shear zones tien shan baikal stanovoy for the northern one and qin ling for the southern one we discuss the compatibility of this mechanism which involves a rather small amount of extrusion with the fast eastward expulsion described for southeast asia we re emphasize the role played in the opening of marginal basins by the pacific subduction as a free boundary to the east

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

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

observing plate motions in s e asia geodetic results of the geodyssea project this paper presents the final geodetic results of the geodyssea project the gps data from a 42 station network observed during two field campaigns 1994 1996 were analyzed by four groups using different software packages and analysis strategies the precision of both campaign coordinate solutions was found to be 4 7 mm for the horizontal and 1 cm for the vertical component the campaign solutions were merged into one unique solution which was accurately mapped into the itrf 96 reference frame the global accuracy of this solution with respect to itrf 96 is 1 cm while the resolution of the relative horizontal velocities is estimated to be at the level of 2 3 mm yr this solution was used as the basis for all scientific interpretations which are published in separate papers the velocity estimates of a part of the network provided the first direct measurement of a relative motion of the sundaland block with respect to eurasian plate

global positioning system measurements from eastern tibet and their implications for india eurasia intercontinental deformation we present and interpret global positioning system gps measurements of crustal motions for the period 1991 1998 for a network encompassing the eastern part of the tibetan plateau and its foreland relative to a eurasian frame defined by minimizing the velocities of 16 gps stations in europe central asia and siberia stations within all parts of the plateau foreland in south china move 6 10 mm yr east southeast indicating that the eastward movement within the plateau is part of a broader eastward movement that involves the plateau and its eastern and northern foreland north of the plateau foreland stations move northeastward at 10 mm yr indicating that the northern boundary of the deformation zone lies north of the plateau with this realization of a eurasian frame the velocity of the gps station at bangalore in southern india implies that the northward motion of india is 5 12 mm yr slower than that predicted from the nuvel 1a plate reconstruction viewed relative to the south china block stations of the northeast plateau bounded on the north by the qilian shan and the altyn tagh fault move nne to ne with velocities ranging from 19 mm yr within the plateau to 5 11 mm yr in its foreland the altyn tagh fault shows left lateral slip of 10 mm yr at 95 e and shortening across the fault of 5 mm yr stations south and west of the xianshuihe xiaojiang fault system define a crustal fragment rotating clockwise at 10 mm yr relative to the south china block around the eastern himalayan syntaxis the gps measurements indicate no significant shortening 3 mm yr within the longmen shan of the central eastern plateau and its adjacent foreland although the longmen shan rise over 6 km in 100 km horizontal distance geological studies indicate that the deformational field was established diachronously in late miocene to pliocene time was characterized by no east west shortening of tibetan crust and has an inhomogeneous style of deformation resulting from a balance of different tectonic processes

uplift of the hengduan mountains on the southeastern margin of the tibetan plateau in the late miocene and its paleoenvironmental impact on hominoid diversity abstract the southeastern margin of the tibetan plateau has a number of well known late miocene localities that have yielded fossil hominoids belonging to lufengpithecus and khoratpithecus these localities provide critical evidence about the evolutionary history of hominoids including their response to environmental changes associated with uplift of the tibetan plateau here we present magnetostratigraphic dating of the baoshan hominoid locality in yunnan southwestern china which based on preliminary biochronological evidence was previously assumed to be the youngest known miocene hominoid locality in eurasia paleomagnetic investigations yield three normal and three reversed magnetozones which can be best correlated in conjunction with biostratigraphic constraints to chrons c3n 4n to c3ar the baoshan hominoid is derived from a horizon correlated with the lowest part of c3r having an estimated age of 6 0 ma this indicates that the baoshan hominoid is slightly younger than l cf lufengensis from zhaotong dated to 6 2 ma and l lufengensis from lufeng dated to 6 9 6 2 ma our results coupled with previously published magnetochronology indicate that hominoids occurred in southeast asia from the latest middle miocene to the terminal miocene 13 6 ma the period after 6 ma coincides with the uplift of the hengduan mountains especially the gaoligong mountain we propose that the late miocene uplift of these n s oriented mountains on the southeastern margin of the tibetan plateau created a barrier to the warm and moist indian monsoon from the west and this led to a cooler and drier climate in the southeastern margin of the tibetan plateau this paleoenvironmental change potentially had a profound impact on the diversity of the hominoid community in southwestern china and southeast asia

characteristics of focal mechanisms and the stress field in the southeastern margin of the tibetan plateau crustal earthquake focal mechanisms are investigated in the southeastern margin of the tibetan plateau where the tibetan plateau and stable south china block merge an updated database of focal mechanisms has been compiled by selecting 132 global centroid moment tensor solutions and by adding the 173 new solutions 3 5 ms 7 4 estimated by waveform inversion in this study a total of 305 mechanisms are included in this database these solutions show regionally specific distributions with dominant strike slip faulting and some normal and reverse faulting focal mechanism solutions have also been inverted for the stress tensor orientation to obtain the principal stress axes over the study region results show that the horizontal maximum principal 1 axes rotate clockwise with a wider range than the geodetically measured surface motion in the east which is not limited to the xianshuihe xiaojiang fault but has some overlap with the zhaotong lianfeng fault localized normal faulting stress regimes are observed in the jinshajiang litang fault areas and the baoshan sub block the minimum principal axes are oriented with a gradually changing trend from north south to northwest southeast from north to south indicating diverse compression stress patterns significant changes in the crustal stress field after the wenchuan earthquake are preliminarily observed in the baoshan sub block where orientations of two principal axes have changed and in the jinggu ximeng sub block areas where the strike slip faulting stress pattern has transformed to normal faulting

dynamics of the tengchong volcanic region in the southeastern tibetan plateau a numerical study abstract the constraints of surface heat flow and lithospheric structure were used to investigate the dynamics of the tengchong volcanic region based on thermal mechanical modeling three different boundary conditions representing three types of tectonic movements were modeled to discuss the thermal state and lithospheric deformation the compressive model shows that the subduction of the indian plate causes the upwelling of asthenosphere beneath the tengchong block in this process upwelling of the asthenosphere causes thinning of the lithosphere beneath the tengchong block however the surface heat flow of this model is not consistent with the observations the regional extension model shows that under the extension the asthenosphere upwells along the preexisting weak suture zone in the southeastern tibetan plateau the thickness of lithosphere is the thinnest along the suture zone and the surface heat flow is the highest however the extension time was found to generally need 10 15 myr to conform to the surface heat flow however this is not consistent with accepted geochronological data when a boundary condition from compression to extension is imposed both the surface heat flow and the lithospheric structure fit well with the observations the results show that the subduction of the indian plate leads to the upwelling of the asthenosphere which causes a thinned lithosphere beneath the tengchong block since the late miocene 8 5 ma the extension or extrusion has been predominant the thermal erosion of the lithosphere caused by upwelling of the asthenosphere renders the lithosphere even thinner along the bangong nujiang suture zone the upwelling of the asthenosphere provides an upward driving force and high temperature environment for the tengchong volcanic region these findings suggest that the origin of the tengchong volcanic region is caused by both the subduction of the indian plate and regional extension across the bangong nujiang suture zone

hairpin river loops and slip sense inversion on southeast asian strike slip faults in the golden triangle region of southeast asia northern thailand laos and burma southern yunnan the mekong salween and neighboring rivers show hairpin geometries where they cross active strike slip faults restoration of young left lateral offsets of these rivers leaves residual right lateral bends of many kilometers we interpret these hairpins as evidence of late cenozoic slip sense inversion on these faults about 5 to 20 ma near the red river fault stress field and slip sense inversion occurred ca 5 ma this implies that the present course of these large rivers has existed for at least several million years pliocene quaternary slip rates possibly on the order of 1 mm yr are inferred on each of the strike slip faults of the golden triangle

further paleomagnetic results for lower permian basalts of the baoshan terrane southwestern china and paleogeographic implications abstract the baoshan terrane of southwestern china is considered to have been part of the cimmerian block during the late paleozoic consequently knowledge of its paleoposition and geological evolution can provide constraints on the permian breakup of northern east gondwana therefore we conducted paleomagnetic and rockmagnetic studies on lower permian basalts from four localities in the baoshan terrane the basalts hold a stable characteristic remanent magnetization chrm at high temperatures 300 680 c that is carried by magnetite maghemite and hematite with both pseudo single and multiple domains to test the reliability of data from these volcanic rocks we analyzed the geomagnetic secular variation gsv and reliability of both the present data and previous paleomagnetic data the results from 23 sites yield a single reversed polarity directed downwards to the southwest giving a site mean direction of d g i g 156 7 56 6 k g 8 0 95 11 4 before tilt correction and d s i s 218 3 60 1 k s 14 1 95 8 4 after tilt correction the result passed the fold test but the gsv was able to be averaged out in only two sections all available data were examined section by section using the angular dispersion s b of virtual geomagnetic poles vgps to ensure that the gsv was completely averaged out because the dispersion in declinations is likely to have been affectedby subsequent tectonic deformation the paleosecular variation psv could not be evaluated from all the data amassed from different sections and the psv was able to be removed from only four combined sections a small circle fit of these vgps gives an averaged paleocolatitude of 51 9 3 7 n 31 sites centered on 24 n 99 e the result indicates that the sampled area of the baoshan terrane was located at a latitude of 38 s 3 7 during the late early permian a comparison of this result with early permian data from gondwanan blocks suggests that the baoshan terrane was situated near the junction of northern india and northwestern australia and that it broke away from western australia after the early permian

finite strain calculations of continental deformation 2 comparison with the india asia collision zone numerical experiments on a thin viscous sheet model for deformation of continental lithosphere subjected to an indenting boundary condition yield distributions of crustal thickness of stress and strain rate and of latitudinal displacements that may be compared with observations in the india asia collision zone a simple indenting boundary condition applied to initially laterally homogeneous sheets obeying a power law rheology produces results that are in broad agreement with the observations provided that the power law exponent is three or greater and the sheet can support vertically integrated stress differences of 2 1013 5 1012 n m 1 in regions in front of the indenter under these conditions the calculated deformation shows accommodation of convergence primarily by crustal thickening to produce a plateau in front of the indenter palaeomagnetic data from india and tibet and the observed distribution of topography suggest that much of the post eocene convergence of india with asia has been taken up by deformation within asia that involved crustal thickening the principal difference between calculation and observation is the absence from the calculated strain rate fields of east west extension of the plateau in front of the indenting boundary the calculations show that once such a plateau is formed the buoyancy force associated with the crustal thickness contrast inhibits further thickening and the plateau strains at less than half the rate of its immediate surroundings seismically determined regional strain rates exhibit a similar distribution with the tibetan plateau straining at about one quarter the rate of the tien shan and ningxia gansu regions calculated principal compressive stress orientations and regional strain rates agree with the seismically determined quantities in the mongolia baikal tien shan tibet and ningxia gansu regions of asia to within the uncertainty of the latter the vertically integrated stresses that are calculated for the viscous sheet are comparable with those that can be supported by a theologically stratified continental lithosphere obeying laboratory determined flow laws we suggest that the thin viscous sheet model described in this paper and its companion gives a simple and physically plausible description of the observed deformation in central asia in this description the predominant mechanism of accommodation of continental convergence is diffuse crustal thickening with shear on vertical planes playing a subsidiary role once large crustal thickness contrasts have been established

tectono magmatic evolution of the gaoligong belt southeastern margin of the tibetan plateau constraints from granitic gneisses and granitoid intrusions abstract the gaoligong belt is located in the southeastern margin of the tibetan plateau and is bound by the tengchong and baoshan blocks this paper presents new data from zircon geochronology geochemistry and whole rock sr nd pb hf isotopes to evaluate the tectonic evolution of the gaoligong belt the major rock types analysed in the present study are granitic gneiss granodiorite and granite they are metaluminous to peraluminous and belong to high k calc alkaline series laser ablation inductively coupled plasma mass spectrometry la icp ms analyses of zircons from nine granitic rocks yielded crystallization ages of 495 487 ma 121 ma 89 ma and 70 63 ma the granitoids can be subdivided into the following four groups 1 early paleozoic granitic gneisses with high e nd t and e hf t values of 1 06 to 3 45 and 1 16 to 2 09 and model ages of 1 16 ga to 1 33 ga and 1 47 ga to 1 63 ga respectively their variable 87 sr 86 sr and pb values resemble the characteristics of the early paleozoic pinghe granite in the baoshan block our data suggest that the rocks were derived from the break off of the proto tethyan oceanic slab between the outboard continent and the baoshan block which induced the partial melting of mesoproterozoic pelitic sources mixed with depleted mantle materials 2 early cretaceous granodiorites with low e nd t and e hf t values of 8 92 and 4 91 with nd and hf model ages of 1 41 ga and 1 49 ga respectively these rocks have high initial 87 sr 86 sr 0 711992 and lower crustal pb values suggesting that they were derived from mesoproterozoic amphibolites with tholeiitic signature leaving behind granulite residue at the lower crust 3 early late cretaceous granites with low e nd t and e hf t values of 9 58 and 4 61 with nd and hf model ages of 1 43 ga and 1 57 ga respectively these rocks have high initial 87 sr 86 sr 0 713045 and lower crustal pb isotopic values these rocks were generated from the partial melting of mesoproterozoic metapelitic sources resulting from the delamination of thickened lithosphere following the closure of the bangong nujiang ocean and collision of the lhasa qiangtang blocks 4 late cretaceous to paleogene granitic gneisses with low e nd t and e hf t values of 4 41 to 10 and 5 95 to 8 71 nd model ages ranging from 1 08 ga to 1 43 ga and hf model ages from 1 53 ga to 1 67 ga respectively these rocks show high initial 87 sr 86 sr 0 713201 and 714662 and lower crustal pb values the data suggest that these rocks are likely related to the eastward subduction of the neo tethyan oceanic slab which induced partial melting of mesoproterozoic lower crustal metagreywacke the results presented in this study from the gaoligong belt offer important insights on the evolution of the proto tethyan bangong nujiang and neo tethyan oceans in the southeastern margin of the tibetan plateau

successive shifts of the india africa transform plate boundary during the late cretaceous paleogene interval implications for ophiolite emplacement along transforms abstract the arabian sea in the nw indian ocean is a place where two major transform boundaries are currently active the owen fracture zone between india and arabia and the owen transform between india and somalia these transform systems result from the fragmentation of the india africa transform boundary which initiated about 90 myrs ago when the india seychelles block separated from madagascar to move towards eurasia therefore the geological record of the arabian sea makes it possible to investigate the sensitivity of a transform system to several major geodynamic changes here we focus on the evolution of the india africa transform system during the 47 90 ma interval we identify the late cretaceous 90 65 ma transform plate boundary along chain ridge in the north somali basin from 65 to 42 47 ma the india africa transform is identified at the chain fracture zone which crossed both the owen basin and the north east oman margin finally the transform system jumped to its present day location in the vicinity of the owen ridge these shifts of the india africa boundary with time provide a consistent paleogeographic framework for the emplacement of the masirah ophiolitic belt which constitutes a case of ophiolite emplaced along a transform boundary the successive locations of the india africa boundary further highlight the origin of the owen basin lithosphere incoming into the makran subduction zone

tectonic deformation around the eastern himalayan syntaxis constraints from the cretaceous palaeomagnetic data of the shan thai block summary lower to middle cretaceous red sandstones were sampled at four localities in the lanpin simao fold belt of the shan thai block to describe its regional deformational features most of the samples revealed a characteristic remanent magnetization with unblocking temperatures around 680 c primary natures of magnetization are ascertained through positive fold test a tilt corrected formation mean direction for the jingdong 24 5 n 100 8 e locality which is located at a distance of 25 km from the ailaoshan red river fault revealed northerly declination with steep inclination dec inc 8 3 48 8 95 7 7 n 13 however mean directions obtained from the zhengyuan 24 0 n 101 1 e west zhengyuan 24 0 n 101 1 e and south mengla 21 4 n 101 6 e localities indicate an easterly deflection in declination such as dec inc 61 8 46 1 95 8 1 n 7 dec inc 324 2 49 4 95 6 4 n 4 and dec inc 51 2 46 4 95 5 6 n 13 respectively the palaeomagnetic directions obtained from these four localities are incorporated into a palaeomagnetic database for the shan thai block when combined with geological geochronological and gps data the processes of deformation in the shan thai block is described as follows subsequent to its rigid block clockwise rotation of about 20 in the early stage of india asia collision the shan thai block experienced a coherent but southward displacement along the red river fault prior to 32 ma this block was then subjected to a north south compressive stresses during the 32 27 ma period which played a key role in shaping the structure of chongshan lancang chiang mai belt following this some local clockwise rotational motion has occurred during the pliocene quaternary time in central part of the shan thai block as a result of internal block movements along the reactivated network of faults

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

cenozoic clockwise rotation of the tengchong block southeastern tibetan plateau a paleomagnetic and geochronologic study abstract paleomagnetic data from 50 35 ma likely 40 ma mafic dykes in yunnan southeastern tibetan plateau cutting 115 ma granitoids record the rotation of the tengchong lhasa block around the east himalayan syntaxis ehs ti rich titanomagnetite and magnetite carry a primary magnetic component group1 a magnetic overprint resides in magnetite group2 likely induced by low grade metamorphism between 30 and 10 ma the tilt corrected overall mean directions are d i 89 8 35 1 for group1 and d i 33 3 41 4 for group2 these data imply a clockwise rotation of 87 87 3 12 5 of the tengchong block since remanence acquisition at 40 ma with respect to stable eurasia the average rotation rate of 2 18 0 31 myr is at the upper limit of the present day rotation rates around the ehs obtained from gps velocities and quaternary strain rates the remagnetized group2 indicates a poorly defined rotation of 31 31 2 32 9 our key results the detection of 87 clockwise rotation with high rotation rates following the india asia collision and a decrease in rotation rates during the miocene suggest that first the tengchong block rotated rapidly around the ehs synchronous with eastward tectonic escape of lithospheric blocks the later slower clockwise rotation similar to those recorded geodetically may be related to viscous flow of tibetan crust

distorted drainage basins as markers of crustal strain east of the himalaya extensive lithospheric deformation on the eastern margin of the india eurasia collision has brought three major rivers into exceptionally close proximity for over 300 km the salween mekong and yangtze rivers are only tens of kilometers apart 10 times closer than rivers of comparable length elsewhere in the world we view deeply incised river valleys as displacement markers that move with the underlying crust over time scales of at least tens of millions of years accordingly the close spacing of these major rivers and their nearly parallel trends reflect the large crustal strains near the eastern himalayan syntaxis these river characteristics are consistent with two primary phases of deformation in this tectonic setting ne sw shortening of 0 5 oriented radially from the eastern syntaxis followed by dextral shear strain of 6 1 along a north trending zone pervasive shearing of this magnitude throughout the 150 km wide region of anomalous drainage would account for 1000 km of northward motion of india relative to southern china directly east of this region another 1000 km of northward motion is likely to have been accommodated by pervasive shearing through a zone hundreds of kilometers wide finally we draw attention to the remarkable accord between the arcuate paths of each of the three great rivers in the broader southeastern tibet region and small circles that curve around the eastern himalayan syntaxis

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

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

finite strain calculations of continental deformation 1 method and general results for convergent zones a finite element method is used to calculate stresses and strain rates in a thin viscous sheet representing the continental lithosphere that is bordered in one part by an indenting boundary we calculate solutions for velocity fields crustal thickness distributions strain rates rotation time integrated vorticity and finite strain ellipsoids to show the dependence of these quantities on the controlling parameters the stress strain exponent n and the strength of the sheet relative to the gravitational forces which may be expressed as a dimensionless number the argand number ar for n 3 and ar between 1 and 10 a plateau of thickened crust is formed in front of the indenter the strain histories of individual elements within the viscous sheet may be quite complex and the finite strain at the end of deformation may bear no obvious relation to the principal stress orientations during the deformation the strain rate fields are interpreted in terms of the style of faulting that would be expected in the brittle upper crust if it were coupled to the ductile but stronger upper mantle in general the length scale of deformation increases with ar and time since the start of the collision and decreases with n for ar 3 the increase in length scale is accompanied by a change in the style of deformation from predominantly crustal thickening with thrust faulting in the brittle layer to predominantly transcurrent deformation strike slip faulting in the brittle layer

seismic structure of the tengchong volcanic area southwest china from local earthquake tomography article i nfo weperformedalocalearthquaketomographytoimagethecrustofthetengchongvolcanicareainsouthwestern china using p wave arrival data from a temporary network and permanent stations the objective is to deter mine the magma sources of the volcanic area and the seismic structure of two m 7 earthquakes in the longling area south of tengchong the result reveals a correlation between the velocity structures and the volcanic activ ities from late miocene to holocene a prominent low velocity zone is observed beneath the central volcanic area that is cored at 10 20 km depths and extends down to 30 km depth with a lateral extent of 20 30 km we infer that this unusual low velocity zone represents a magma source of the heat flows for the volcanic erup tion during pleistocene and holocene close to it a high velocity zone appears beneath the eastern volcanic area and itislikelyto reflect solidifiedmagma intrusionsand high densityremnants withinthe cooled volcanicchan nel prior topleistocene in the m 7 earthquake area theuppercrustalstructureshows evidentvariationsacross the longling fault and the nu river fault bounded by the two fault zones the gaoligong metamorphic belt and the baoshan block are imaged by high velocities that are consistent with their high strength in the structures on the contrary low velocities are imaged in the area between the longling fault and the nu river fault implying a decreaseof the strength probably caused byigneous intrusions itwas thelateral variation intherheologic struc ture that led to the uneven stress accumulations across these fault zones and created tectonic conditions for the seismic ruptures of the two m 7 earthquakes in the longling area

the active tectonics of the eastern himalayan syntaxis and surrounding regions source parameters of 53 moderate sized earthquakes obtained from the joint inversion of regional and teleseismic distance long period body waves provide the data set for an analysis of the style of deformation and kinematics in the region of the eastern himalayan syntaxis focal mechanisms of eastern himalayan events show oblique thrust consistent with the n ne directed movement of the indian plate as it underthrusts a boundary that strikes at an oblique angle to the direction of convergence earthquakes near the sagaing fault show strike slip mechanisms with right lateral slip earthquakes on its northern splays however indicate predominant thrusting evidence that the dextral motion on the sagaing fault which accommodates a portion of the lateral motion between india and southeast asia terminates in a zone of thrust faulting at the eastern himalayan syntaxis remaining motion between india and southeast asia is accommodated in a zone of distributed shear in east burma and yunnan manifested by strike slip and oblique normal faulting east west extension crustal thinning and clockwise rotation of crustal blocks we determined strain rates throughout the region with a moment tensor summation using 25 years modern and 85 years modern and historic of earthquake data we matched the observed strains with a fifth order polynomial function and from this we determined both the velocity field and rotations with respect to a specified region velocities calculated relative to south china stationary show that the entire area extending from 20 n 36 n within deforming asia yunnan western sichuan and east tibet constitutes a distributed dextral shear zone with clockwise rotations up to 1 7 m y maximum in the region of the eastern syntaxis proper integrated strains across this zone relative to south china stationary show 38 mm yr 12mm yr of north directed motion at the himalaya remaining plate motion relative to south china fixed must be taken up by the underthrusting of india beneath the lesser himalaya strike slip motion on the sagaing fault and intraplate ne directed shortening within ne india as well as ne directed shortening within the eastern syntaxis proper 10 mm yr 2 mm yr of relative right lateral motion between india and southeast asia is absorbed in the region between the sagaing and red river faults 94 e 100 e it is the clockwise vorticity relative to south china associated with the deformation in yunnan east tibet and western sichuan that provides the relative north directed motion of 38 12 mm yr at the himalaya not all of the deformation is accommodated in right lateral shear between india and south china and between east tibet and south china velocity gradients exist that are parallel to the trend of the shear zone relative to a point within western sichuan 32 n 100 e the velocity field shows that the yunnan crust is moving s se at rates of 8 10 mm yr relative to south china there is no eastward expulsion of crustal material beyond the eastern margin of the tibetan plateau

large river offsets and plio quaternary dextral slip rate on the red river fault yunnan china using multispectral spot images and 1 100 000 topographic data we present an improved map of the active red river fault zone between midu yunnan china and hanoi vietnam the fault zone is composed of parallel strands one of which the yuanjiang fault was previously undetected there also appears to be a component of extension all along the fault zone such extension increases toward the se from yunnan to the south china sea coast and the vector describing the motion of south china relative to indochina points within the n45 135 e quadrant we attempt to assess the plio quaternary dextral slip rate on the red river fault rrf by restoring large river offsets and searching for the largest plausible one across much of yunnan the fault is perpendicular to local catchments that drain into the red river from precise mapping of the river courses on spot satellite images and on 1 100 000 topographic maps numerous multiple offsets along the fault can be detected and reconstructed the lack of correlation between the apparent offsets and the lengths of the rivers upstream from the fault suggests either that the drainage system was in large part established prior to the onset of dextral slip along the fault or that frequent captures have occurred we thus try to find the best fit between series of river channels upstream and downstream from the fault by progressively restoring the dextral displacement in increments of 500 m up to an offset of 50 km for each increment we measure the misfits root mean squares rms between the upstream and downstream channels the best fit and smallest rms are obtained for an offset of 25 0 5 km that we interpret to represent the clearest large right lateral displacement recorded in the geomorphology along the active red river fault since dextral motion is likely to have started around 5 myr the most probable average plio quaternary slip rate on the fault is of order of 5 mm yr we attribute the apparent lack of seismic activity on a large stretch of the fault to millennial recurrence times between great earthquakes our study shows that relatively small drainage systems can keep a good record of fairly large cumulative fault offsets

use of the stochastic source model to simulate ground motion and response spectra in northern vietnam abstract northern vietnam has experienced large earthquakes in the past but waveforms are not mentioned in the vietnamese specification for bridge design and the acceleration response spectrum in these specifications has not been adequately studied under vietnamese seismic conditions the simulation of future earthquake events based on regional seismicity and a ground motion model is necessary because of the absence of data on strong ground motions this paper summarizes artificial ground motion procedures which were studied using a stochastic point source model simulated waveforms were employed to synthesize seismograms with vn l1 and vn l2 ground motions estimated using a 475 year return period m 5 8 and the largest recorded earthquake events m 7 0 ground motions were simulated using different source parameters and their response spectra were compared with corresponding available data as a result target response spectra are proposed for future earthquake resistant design in vietnam

crustal motion and block behaviour in se asia from gps measurements results acquired using global positioning system gps data taken over a large part of se asia indicate that sundaland i e indochina along with the western and central part of indonesia constitutes a stable tectonic block moving approximately east with respect to eurasia at a velocity of 12 3 mm yr 31 with respect to india and australia this block moves due south significant motion has not been detected along the northern boundary to south china i e along the red river fault whereas nearly 50 mm yr 31 of right lateral motion has to be accommodated between india and sundaland in the andaman burma region fl 2001 elsevier science b v all rights reserved

a tectonic model for the tertiary evolution of strike slip faults and rift basins in se asia abstract models for the tertiary evolution of se asia fall into two main types a pure escape tectonics model with no proto south china sea and subduction of proto south china sea oceanic crust beneath borneo a related problem is which if any of the main strike slip faults mae ping three pagodas and aliao shan red river asrr cross sundaland to the nw borneo margin to facilitate continental extrusion recent results investigating strike slip faults rift basins and metamorphic core complexes are reviewed and a revised tectonic model for se asia proposed key points of the new model include 1 the asrr shear zone was mainly active in the eocene oligocene in order to link with extension in the south china sea the asrr was less active during the miocene tens of kilometres of sinistral displacement with minor amounts of south china sea spreading centre extension transferred to the asrr shear zone 2 at least three important regions of metamorphic core complex development affected indochina from the oligocene miocene mogok gneiss belt doi inthanon and doi suthep around the asrr shear zone hence paleogene crustal thickening buoyancy driven crustal collapse and lower crustal flow are important elements of the tertiary evolution of indochina 3 subduction of a proto south china sea oceanic crust during the eocene early miocene is necessary to explain the geological evolution of nw borneo and must be built into any model for the region 4 the eocene oligocene collision of ne india with burma activated extrusion tectonics along the three pagodas mae ping ranong and klong marui faults and right lateral motion along the sumatran subduction zone 5 the only strike slip fault link to the nw borneo margin occurred along the trend of the asrr fault system which passes along strike into a right lateral transform system including the baram line

paleogeographic and paleotectonic evolution of the himalayan range as reflected by detrital modes of tertiary sandstones and modern sands indus transect india and pakistan detrital modes of sandstones derived from the himalayan suture belt record the history of the mountain range since initial collision between india and asia which began in latest paleocene time tertiary clastic wedges deposited in fore arc foreland and remnant ocean basins and exposed along the indus transect from northernmost india to the arabian sea represent the best opportunity to study sedimentary responses to successive tectonic events during continental collision quartzose continental block and feldspatholithic magmatic arc sandstones were deposited respectively on the passive indian tethys himalayan succession and active asian indus group continental margins during late cretaceous paleocene time closure of the neotethys was marked by sudden arrival of volcanic and ophiolitic detritus on the passive continental margin of the indian plate during deposition of sediments dated at foraminiferal zones p6 pakistan to p8 india starting in early eocene time deposition of chulung la formation and murree supergroup volcanic and ophiolitic to metasedimentary detritus was accumulated in rapidly subsiding piggy back and foreland basins homogeneous petrographic composition within the eocene lower miocene murree supergroup with only slight progressive increase of detrital feldspars suggests erosion of largely supracrustal rocks involved in thrusting in the north in middle miocene time marked enrichment in medium to high grade metamorphic detritus in foreland sandstones siwalik group reflects rapid uplift of a warm wedge of indian crust which was carried southward along the main central thrust this major paleogeographic change was recorded also by quartzolithic remnant ocean turbidites which were fed great distances along transverse fracture zones and later accreted in the coastal makran subduction complex panjgur association and makran group recycled orogen detritus derived from the elevated himalayan chain is still accumulating today in the indus fan enrichment in feldspars with respect to ancient sandstones reflects deep erosion levels into mid crustal rocks along the core of the growing orogen

internal crustal deformation in the northern part of shan thai block new evidence from paleomagnetic results of cretaceous and paleogene redbeds in order to describe the internal deformation of the northeast part of the shan thai block resulting from the penetration of the indian plate into asia a paleomagnetic study was carried out on the cretaceous and paleogene strata in the southern part of the simao terrane high temperature magnetic components with unblocking temperatures around 680 degrees c were isolated and positive fold test shows its primary nature of magnetization the tilt corrected site mean directions of the lower cretaceous and upper cretaceous from the menglun area are d s 46 2 degrees i s 45 9 degrees k 11 0 alpha 95 10 6 degrees and d s 33 2 degrees i s 30 9 degrees k 68 4 alpha 95 8 2 degrees respectively the site mean directions of the upper cretaceous and lower paleogene for the mengban area are d s 50 5 degrees i s 31 0 degrees k 207 6 alpha 95 6 4 degrees and d s 43 5 degrees i s 23 0 degrees k 26 1 alpha 95 13 4 degrees respectively the results of the lower cretaceous and upper paleogene from the mengla area are d s 46 9 degrees i s 42 2 degrees k 27 3 alpha 95 7 7 degrees and d s 41 8 degrees i s 23 8 degrees k 38 9 degrees alpha 95 5 8 degrees respectively in comparison with previous paleomagnetic data of the simao terrane the rotational deformation can be divided into four stages the cretaceous and paleocene sediments of the simao terrane experienced intensive folding before the early oligocene during the early oligocene to early miocene the shan thai and indochina blocks experienced southeastward extrusion with similar to 20 degrees clockwise rotation relative to the south china block during the middle to late miocene the shan thai blocks experienced more 15 degrees clockwise rotation since the late miocene the activity of strike slip faults surrounding the simao terrane enhanced the internal differential rotations of the terrane these deformation processes finally resulted in a complex internal deformation of the simao terrane c 2013 elsevier b v all rights reserved

oroclinal origin of the simao arc in the shan thai block inferred from the cretaceous palaeomagnetic data summary an active oroclinal bending is discovered in the shan thai block to the south of the eastern himalayan syntaxis to investigate the evolution of the simao arc using palaeomagnetic techniques the middle cretaceous red beds of the nanxin formation were sampled at the zhengwan 22 8 n 100 9 e and dadugang 22 4 n 101 0 e localities in the southern simao basin most of the studied samples revealed the presence of characteristic remnant magnetization with unblocking temperatures around 680 c a primary nature for this magnetization is interpreted on the basis of a positive fold and reversal test tilt corrected mean directions calculated for zhengwan and dadugang localities are characterized by large easterly deflected declination d 51 8 i 47 9 ks 45 0 95 6 9 n 11 and d 64 1 i 48 1 ks 36 0 95 7 3 n 12 respectively steep inclination values at both these localities with respect to those expected are in the range previously reported from the shan thai block confirming their southward displacement by 6 2 1 7 as part of the shan thai block combination of the present data two localities with those previously reported from simao basin seven localities reveals a positive palaeomagnetic oroclinal test indicating that the present day arc like geometry of the simao basin was formed by oroclinal bending comparison with recent gps and structural data suggest that formation of the simao curvature started after the early pliocene after 4 ma and continuing until the present origin of the simao arc is ascribed to southwestward movement of the crustal material across the ailao shan red river fault around the eastern himalaya syntaxis which was formed by westward movement of the decollement with progressive eastward deepening of the lanping simao basin decoupling between the upper and the middle lower crusts is a requisite condition for the arc formation on the continent

restoration of cenozoic deformation in asia and the size of greater india a long standing problem in the geological evolution of the india asia collision zone is how and where convergence between india and asia was accommodated since collision proposed collision ages vary from 65 to 35 ma although most data sets are consistent with collision being underway by 50 ma plate reconstructions show that since 50 ma 2400 3200 km west to east of india asia convergence occurred much more than the 450 900 km of documented himalayan shortening current models therefore suggest that most post 50 ma convergence was accommodated north of the indus yarlung suture zone we review kinematic data and construct an updated restoration of cenozoic asian deformation to test this assumption we show that geologic studies have documented 600 750 km of n s cenozoic shortening across and north of the tibetan plateau the pamir hindu kush region accommodated 1050 km of n s convergence geological evidence from tibet is inconsistent with models that propose 750 1250 km of eastward extrusion of indochina approximately 250 km of indochinese extrusion from 30 to 20 ma of indochina suggested by se asia reconstructions can be reconciled by dextral transpression in eastern tibet we use our reconstruction to calculate the required size of greater india as a function of collision age even with a 35 ma collision age the size of greater india is 2 3 times larger than himalayan shortening for a 50 ma collision the size of greater india from west to east is 1350 2600 km consistent with robust paleomagnetic data from upper cretaceous paleocene tethyan himalayan strata these estimates for the size of greater india far exceed documented shortening in the himalaya we conclude that most of greater india was consumed by subduction or underthrusting without leaving a geological record that has been recognized at the surface

the irrawaddy river sediment flux to the indian ocean the original nineteenth century data revisited the irrawaddy ayeyarwady river of myanmar is ranked as having the fifth largest suspended load and the fourthhighest total dissolved load of the world s rivers and the combined irrawaddy and salween thanlwin system is regarded as contributing 20 of the total flux of material from the himalayan tibetan orogen the estimates for the irrawaddy are taken from published quotations of a nineteenth century data set and there are no available published data for the myanmar reaches of the salween apart from our own field studies in 2005 and 2006 no recent research documenting the sediment load of these important large rivers has been conducted although their contribution to biogeochemical cycles and ocean geochemistry is clearly significant we present a reanalysis of the irrawaddy data from the original 550 page report of gordon covering 10 yr of discharge 1869 1879 and 1 yr of sediment concentration measurements 1877 1878 we describe gordon s methodologies evaluate his measurements and calculations and the adjustments he made to his data set and present our revised interpretation of nineteenth century discharge and sediment load with an estimate of uncertainty the 10 yr average of annual suspended sediment load currently cited in the literature is assessed as being underestimated by 27 on the basis of our sediment rating curve of the nineteenthcentury data on the basis of our sampling of suspended load the nineteenth century concentrations are interpreted to be missing about 18 of their total mass which is the proportion of sediment recovered by a 0 45 mm filter the new annual irrawaddy suspended sediment load is mt our revised estimate of the annual sediment load 364 60 from the irrawaddy salween system for the nineteenth century 600 mt represents more than half the present day ganges brahmaputra flux to the indian ocean since major chinese rivers have reduced their load due to damming the irrawaddy is likely the third largest contributor of sediment load in the world

crustal thickening versus lateral expulsion in the indian asian continental collision since the beginning of the continental collision between india and asia there has been about 2500 km of convergence and the northward movement of india has been accommodated by major internal deformation of the asian lithosphere the crustal thickening in and around the tibetan plateau is clearly a direct consequence of this collision but there is considerable debate as to whether a large fraction of the indentation has been accommodated by eastward motion of the lithospheric blocks of southeastern asia and southern china numerical experiments described here test this hypothesis for a range of indentation geometries and rheological models of the lithosphere we employ a thin viscous sheet model of the lithosphere with a depth averaged nonlinear viscous rheology described by a stress strain rate exponent n and including gravitational buoyancy forces scaled by the dimensionless argand number ar the eastern boundary for the collision region is described as a lithostatic boundary the precollision normal stress is determined by static balance and that constant stress is applied throughout the collision the experiments show that during collision the eastern boundary is smoothly displaced to the east at a rate about 1 4 of the indentation rate with only minor variation due to geometry or rheology crustal thickening rates in the region of the plateau are reduced by between 10 and 25 of the corresponding rates determined for similar experiments with a rigid eastern boundary to generalize from the results of these experiments the total north south shortening strain produced by the collision is partitioned between crustal thickening and eastward displacement in the ratio of at least 3 1 and more probably 4 1

gps measurements of present day convergence across the nepal himalaya the high elevations of the himalaya and tibet result from the continuing collision between india and asia which started more than 60 million years ago1 4 from geological and seismic studies of the slip rate of faults in asia5 it is believed that approximately one third of the present day convergence rate between india and asia 58 4mmyr 1 is responsible for the shortening uplift and moderate seismicity of the himalaya great earthquakes also occur infrequently in this region releasing in minutes the elastic strain accumulated near the boundary zone over several centuries and accounting for most of the advance of the himalaya over the plains of india the recurrence time for these great earthquakes is determined by the rate of slip of india beneath tibet which has hitherto been estimated indirectly from global plate motions6 from the slip rates of faults in asia7 8 from seismic productivity9 and from the advance of sediments on the northern ganges plain10 here we report geodetic measurements using the global positioning system gps of the rate of contraction across the himalaya which we find to be 17 52 2 mm yr 1 from the form of the deformation field we estimate the rate of slip of india beneath tibet to be 20 5 2 mmyr 1 strain sufficient to drive one or more great himalayan earthquakes with slip similar to that accompanying the magnitude 8 1 bihar nepal 1934 earthquake may currently be available in western nepal

cenozoic metamorphism along the shan scarp myanmar evidences for ductile shear along the sagaing fault or the northward migration of the eastern himalayan syntaxis the mogok metamorphic belt exposed along the n s trending shan scarp and sagaing fault in the eastern part of myanmar has been regarded as paleozoic to precambrian for a long time new observations in the shan scarp area close to the sagaing fault from thaton in the south to mandalay in the north allowed us to collect samples of high grade metamorphic and intrusive rocks that have been analyzed by 40ar 39ar step heating method the 13 samples we analyzed provide oligocene to lower miocene ages for this metamorphism oriented thin sections and field observations suggest that this metamorphism was caused by a nnw sse to n s ductile extension therefore we suggest that this metamorphism is not directly related to the sagaing fault but could be instead related to the northward migration of the eastern himalayan syntaxis characterized by crustal thinning resulting from the india asia oblique collision

cenozoic deformation in western yunnan china myanmar border in the present paper we describe ductile and brittle deformation styles in western yunnan and ne myanmar using field data and landsat 7 imagery we show that this complex area located at the northern termination of sunda plate three rivers area was wedged during the tertiary between the left lateral ailao shan chong shan metamorphic belts to the east and the right lateral shan scarp gaoligong metamorphic belt in west this triangular region therefore underwent the effects of these continental size ductile strike slip faults separating major blocks with a dominant ew to ene compression since the late miocene date of the reversal of motion along the rrf the incipient eastward motion of the sunda block and the persisting right lateral motion along its western boundary sagaing fault created n s compression and e w to wnw extension underlined by left lateral transtension along the wanding nanting fault zones at the same time the diangcan shan situated along strike the ailao shan metamorphic belt was slightly impinged by the blocks extruded from the syntaxis and exhumed again from the early pliocene in accordance with this late and still active state of stress q 2004 elsevier ltd all rights reserved

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

tertiary paleomagnetic data from northwestern yunnan china further evidence for large clockwise rotation of the indochina block and its tectonic implications abstract eocene red sandstones were collected at 15 sites for paleomagnetic study from two areas lanping and yunlong of the western part of yunnan province china in the northern part of indochina the high temperature magnetization component with an unblocking temperature of around 680 c is isolated after stepwise thermal demagnetization characteristic directions from nine accepted sites reveal a positive fold test at the 99 confidence limit indicating that the high temperature magnetization component is possibly of eocene age a tilt corrected mean direction from nine accepted sites is d 266 1 i 39 8 with 95 11 2 corresponding to a paleopole at 14 5 n 169 7 e with a95 10 9 a comparison with previously reported tertiary paleomagnetic data indicates that the lanping basin of the northern part of the indochina block rotated clockwise through 91 15 with respect to the yangtze craton this large magnitude clockwise rotation is probably caused by the superimposition of regional and local rotations our previous middle cretaceous paleomagnetic results from the yunlong area indicate that about 40 of clockwise rotation out of 90 is explained by tectonic rotation due to a relatively rigid body rotation of the simao terrane during indentation of india the remaining component of rotation is probably ascribed to further eocene post eocene rotational motion of the tectonically independent small blocks within the lanping basin associated with fault movement

varied thermo rheological structure mechanical anisotropy and lithospheric deformation of the southeastern tibetan plateau abstract to investigate the lithospheric structure and deformation of the tibetan plateau we present thermo rheological models for two transects across the southeastern tibetan plateau se tibet showing strong lateral heterogeneity the geodynamic implications are also investigated in relation to gps motions seismic anisotropy seismicity distribution and crustal isostatic state the unmatched pattern between pms and sks splittings which even intersect at right angles to the south of 26 n suggests the crust mantle decoupling constrained by a brittle load bearing layer at the uppermost mantle beneath the indochina block the decoupling may occur below the uppermost mantle the strong crust beneath the south china plate and indochina block implied by the seismicity distribution has two thick brittle load bearing layers indicating a coupled system the crust beneath the emeishan large igneous province elip also has two brittle load bearing layers but the brittle deformation is restricted to the topmost 10 km of the upper and lower crust only one brittle load bearing layer resides in the upper crust in the other part of the chuandian block combining with gps and pms splitting we can infer that the weak crust with seismicity restricted into the depth of

tectonics and paleomagnetic rotation pattern of yunnan 24 n 25 n china gaoligong fault shear versus megablock drift

late cenozoic to holocene deformation in southwestern sichuan and adjacent yunnan china and its role in formation of the southeastern part of the tibetan plateau from at least 2 4 ma to present crust in the southeastern part of the tibetan plateau west of the convex east xianshuihe xiaojiang fault system has deformed internally and rotated clockwise around the eastern himalayan syntaxis the northwest striking ganzi fault zone bounds the rotating crust on the north and has a total left slip of 78 100 km of which 60 km is transferred to the xianshuihe fault zone across a diffuse transfer zone and 22 40 km is absorbed by bending of older structures and crustal shortening crustal shortening is expressed along and east of the eastern end of the ganzi fault zone by mountains capped by permanent glaciers locally rising nearly 1000 m above the average elevation of the tibetan plateau a similar transfer of left slip into shortening occurs farther south across the xianshuihe fault in the high mountains around and east of gongga shan 7556 m the northwest striking convex east left lateral litang fault zone lies southwest of the ganzi xianshuihe xiaojiang fault zone and appears to be less well developed but otherwise similar to the ganzi fault zone the batang chenzhi and other northeast striking right lateral faults of small displacement occur within the rotating crustal fragment together with the left slip faults they accommodate east west shortening northeast of the eastern himalayan syntaxis south of this region of shortening the crust is extending to form grabens within the dali and southern xiaojiang fault systems and in the tengchong volcanic province the progressive change from shortening southward into extension is related to variations in strain that characterize the region from northeast to southeast of the eastern himalayan syntaxis the assemblage of structures in southwestern sichuan geometrically resembles structures of eocene to miocene age in southern yunnan that were positioned northeast of the eastern himalayan syntaxis similar to present day southwestern sichuan at the time of their development the similarity in the structural development in the two areas indicates that crust northeast of the syntaxis underwent a common evolution as the syntaxis migrated northward during the past 50 m y structures in sichuan are less fully developed than older structures in southwestern yunnan and can serve as a guide to reconstruct the progressive tectonic development in the region of the syntaxis deformation in these areas indicates that plateau formation has been complex inhomogeneous and diachronous at scales from 1000 km to less than 100 km

structure kinematics and ages of transpression during strain partitioning in the chongshan shear zone western yunnan china abstract the chongshan shear zone extends from the eastern himalayan syntaxis to the lincang granitic pluton in yunnan province china the structure and kinematics show that the shear zone comprises mainly of mylonitic gneiss migmatite and schist with a dextral dominated strike slip motion in an n s trending northern segment and a sinistral strike slip shear in nw se trending middle and southern segments both were developed under a bulk regional scale sinistral transpression shrimp and la icpms u pb and 40 ar 39 ar dating reveal two tertiary magmatic events along the zone followed by younger sinistral strike slip shear the eocene magmatic event c 55 38 ma followed by metamorphism at c 36 ma happened before the strike slip motion the strike slip shear along the zone began c 32 ma which generated shear heating from 32 to 22 ma the 40 ar 39 ar ages of syn kinematic micas range from 19 to 14 ma and indicate that the strike slip shear continued to this time with coeval transpressional exhumation and uplift of the metamorphic rocks along the biluoxueshan chongshan chain the chongshan zone is thus a cenozoic shear zone which was contemporaneous with motion on the left lateral ailao shan red river shear zone and the right lateral gaoligong shear zone and should be important in accommodating the northwards movement of india during collision during oligocene to miocene times the continental block that was extruded between the ailao shan red river and gaoligong shear zones was dismembered into at least two major fragments by the chongshan shear zone

the impact of the 24 march 2011 myanmar earthquake ms 7 2 on seismic structure of the yunnan region abstract the temporal and spatial variations of the velocity structure 60 days before and after the march 24 2011 myanmar m7 2 earthquake in the yunnan region are studied by using the ambient noise cross correlation method with the continuous seismic waveform records in addition the response ratio of strain energy release is obtained using moderate and small earthquakes and the coulomb stress changes along different faults are calculated using the source parameters of the myanmar earthquake there are three regions where the velocities increased the area from the zemuhe xiaojiang fault zone to the yuanmou lvzhi fault zone the southern segment of the honghe fault zone and the area from the nantinghe fault zone northward extending to the dayingjiang fault zone the seismic activity and the coulomb stress of these regions are also enhanced by contrast there are several regions where velocities decreased such as the area from the nantinghe fault zone southward extending to the simao region the seismic activity and the coulomb stress of these regions are decreased as well the consistency of the variation indicates some relationship between the variation and the myanmar earthquake

exhumation of west sundaland a record of the path of india abstract the indian plate commenced its northward migration towards eurasia in the early cretaceous the lateral effect of this migration on the western edge of the sunda plate in southeast asia still remains equivocal in order to assess this effect we evaluate several key sectors characterized by deep crustal exhumation along a n s transect from the southern malay peninsula to the east himalayan syntaxis the evaluation is aided by a structural analysis of vertical movements and basin development five major metamorphic domes with similar geodynamic evolution maximum p t burial conditions and exhumation are studied exhumation of these domes migrated with time between late cretaceous in the stong complex north malaysia in the south and late miocene in the gaoligong shear zone south china in the north as documented by published work and our new fission track data presented herein deformation is characterized by a n s oriented extension that followed the more regional e w oriented plate tectonic shortening creating local core complexes and syn kinematic magmatism in the footwall of crustal scale detachments which displays a consistent temporal northward migration the n s extension was associated with the onset of hanging wall deposition in the sedimentary basins of western sundaland e g malay sumatra and thai basins during continuous exhumation of the footwall to upper brittle levels our multifaceted analysis of structural and thermochronological data shows a similar succession of tectonic thermal and sedimentary events in west sundaland that was driven by the gradual northward migration of india starting from cretaceous times we infer that the principal mechanism was driven by the subduction of an excess topography of greater india rifted continental margin during its underplating resulting in uplift thermal anomalies extensional exhumation and associated subsidence