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

surface deformation and lower crustal flow in eastern tibet field observations and satellite geodesy indicate that little crustal shortening has occurred along the central to southern margin of the eastern tibetan plateau since about 4 million years ago instead central eastern tibet has been nearly stationary relative to southeastern china southeastern tibet has rotated clockwise without major crustal shortening and the crust along portions of the eastern plateau margin has been extended modeling suggests that these phenomena are the result of continental convergence where the lower crust is so weak that upper crustal deformation is decoupled from the motion of the underlying mantle this model also predicts east west extension on the high plateau without convective removal of tibetan lithosphere and without eastward movement of the crust east of the plateau

lyttelton volcano banks peninsula new zealand primary volcanic landforms and eruptive centre identification abstract lyttelton volcano banks peninsula new zealand has historically been viewed as a simple volcanic cone this paper uses digital terrain models dtm and primary volcanic landforms to reinterpret lyttelton volcano as having multiple eruptive centres primary volcanic landforms are features produced during active volcanism classified as constructional hypabyssal and erosional volcanic features constructional volcanic features are lava flows scoria cones and domes hypabyssal volcanic features are dykes and sills and erosional volcanic features are valley and ridge patterns and orientations lava flow trends are recognised from aerial photograph analysis and supported by field observations highlighting radiating lava trends around specific locations within lyttelton harbour scoria cones and domes occur on the outer flanks of volcanic cones and are used as such in the identification of remnant cone surfaces dyke orientations are plotted and then projected to the interior of the volcano defining 13 zones of convergence the projected arrays of these orientations highlight defined regions along the erosional crater rim each indicating a radial dyke swarm from which the projected trends of the associated dykes indicate an eruptive centre valley and ridge orientations are projected from the longest valley or ridge segment towards the inner harbour radiating erosive patterns are incepted during the growth and degradation of a volcanic cone with the resulting trends orienting to the summit zones of convergence eruptive centres are identified from lava flow orientations onlapping lava sequences scoria cones and intrusive locations the summit of a volcanic edifice can be identified from the orientations of valleys and ridges while radial dyke systems determine whether this summit was a volcanic centre or simply a local topographic high volcanic landforms are used to identify cone sectors the preserved sector associated with a particular eruptive centre cone sector limits are defined by a basal footprint and an erosional crater rim with similar arcuate features remnant cone features being exposed in the interior of the volcano lyttelton volcano comprises fifteen volcanic cones with vent locations controlled by underlying fault lineaments the growth and erosion of each volcanic cone is reflected in primary volcanic landforms with the preserved features of cones confined to cone sectors and cone artefacts

contemporary crustal deformation around the southeast borderland of the tibetan plateau 1 we derive a detailed horizontal velocity field for the southeast borderland of the tibetan plateau using gps data collected from the crustal motion observation network of china between 1998 and 2004 our results reveal a complex deformation field that indicates that the crust is fragmented into tectonic blocks of various sizes separated by strike slip and transtensional faults most notably the regional deformation includes 10 11 mm yr left slip across the xianshuihe fault 7 mm yr left slip across the anninghe zemuhe xiaojiang fault zone 2 mm yr right slip across a shear zone trending northwest near the southern segment of the lancang river fault and 3 mm yr left slip across the lijiang fault deformation along the southern segment of the red river fault appears not significant at present time the region south and west of the xianshuihexiaojiang fault system whose eastward motion is resisted by the stable south china block to the east turns from eastward to southward motion with respect to south china resulting in clockwise rotation of its internal subblocks active deformation is detected across two previously unknown deformation zones one is located 150 km northwest of and in parallel with the longmenshan fault with 4 6 mm yr right slip and another is continued south southwestward from the xiaojiang fault abutting the red river fault with 7 mm yr left slip while both of these zones are seismically active the exact locations of faults responsible for such deformation are yet to be mapped by field geology comparing our gps results with predictions of various models proposed for tibetan plateau deformation we find that the relatively small sizes of the inferred microblocks and their rotation pattern lend support to a model with a mechanically weak lower crust experiencing distributed deformation underlying a stronger highly fragmented upper crust

paleomagnetic study of cretaceous rocks from pu er western yunnan china evidence of internal deformation of the indochina block abstract cretaceous red sandstones and siltstones were collected at 31 sties for paleomagnetic investigation from the jingxing and nanxin formation around pu er 23 0 n 101 0 e western part of yunnan province china in order to examine quantitative amount of tectonic deformation of the simao terrane due to collision of india with asia after stepwise thermal demagnetization the high temperature component magnetization with unblocking temperature of around 680 c is isolated positive field tests show that the high temperature component magnetization from 25 sites is primary for the cretaceous formations a tilt corrected mean direction is d 59 9 i 45 2 with 95 5 1 corresponding to a paleopole at 35 8 n 173 1 e with a 95 5 6 comparison with cretaceous paleomagnetic pole of the whole china block indicates that amount of clockwise rotation in the pu er area is evaluated to be 46 9 with respect to the whole china block which is consistent with large amount of rotation in the mengla and jinggu areas 47 and 66 in the central part of the simao terrane clockwise rotation of the central part is larger by more than 20 compared to the yunlong and yongping areas of northern part of the simao terrane suggesting that the simao terrane experienced internal deformation the southward displacement of the pu er area expected from inclination data is 7 5 6 6 with respect to the chuan dian fragment which is a similar estimate to other areas in the simao terrane and the indochina block while the simao terrane displaced southward by 800 km together with clockwise rotation of 30 as a whole further clockwise rotation occurred in the central part

stratigraphy structure and tectonic evolution of the himalayan fold thrust belt in western nepal regional mapping stratigraphic study and 40ar 39ar geochronology provide the basis for an incremental restoration of the himalayan fold thrust belt in western nepal tectonostratigraphic zonation developed in other regions of the himalaya is applicable with minor modifications in western nepal from south to north the major structural features are 1 the main frontal thrust system comprising the main frontal thrust and two to three thrust sheets of neogene foreland basin deposits 2 the main boundary thrust sheet which consists of proterozoic to early miocene lesser himalayan metasedimentary rocks 3 the ramgarh thrust sheet composed of paleoproterozoic low grade metasedimentary rocks 4 the dadeldhura thrust sheet which consists of medium grade metamorphic rocks cambrian ordovician granite and granitic mylonite and early paleozoic tethyan rocks 5 the lesser himalayan duplex which is a large composite antiformal stack and hinterland dipping duplex and 6 the main central thrust zone a broad ductile shear zone the major structures formed in a general southward progression beginning with the main central thrust in late early miocene time eocene oligocene thrusting in the tibetan himalaya north of the study area is inferred from the detrital unroofing record on the basis of 40ar 39ar cooling ages and provenance data from synorogenic sediments emplacement of the dadeldhura thrust sheet took place in early miocene time the ramgarh thrust sheet was emplaced between 15 and 10 ma the lesser himalayan duplex began to grow by 10 ma simultaneously folding the north limb of the dadeldhura synform the main boundary thrust became active in latest miocene pliocene time transport of its hanging wall rocks over an 8 km high footwall ramp folded the south limb of the dadeldhura synform thrusts in the subhimalayan zone became active in pliocene time the minimum total shortening in this portion of the himalayan fold thrust belt since early miocene time excluding the tibetan zone is 418 493 km the variation depending on the actual amounts of shortening accommodated by the main central and dadeldhura thrusts the rate of shortening ranges between 19 and 22 mm yr for this period of time when previous estimates of shortening in the tibetan himalaya are included the minimum total amount of shortening in the foldthrust belt amounts to 628 667 km this estimate neglects shortening accommodated by small scale structures and internal strain and is therefore likely to fall significantly below the actual amount of total shortening

geochemical characteristics of the abor volcanic rocks ne himalaya india nature and early eocene magmatism the abor volcanic rocks exposed in the siang window ne himalaya occur as dismembered thrust hound packages interbanded with fossiliferous shelf sediments of upper palaeocenc to middle eocene age their age is broadly contemporaneous with the age of collision between the indian and tibetan continents they were therefore erupted in a convergent setting through channel ways in the leading edge of the indian continent the volcanic rocks occur beneath the lesser himalayan thrust sheets and were transported with them during their southward propagation the abor volcanic rocks were erupted in an early palaeogene shallow basin located within marginal parts of the himalayan fold thrust belt south of the collision zone they may thus represent foreland basin magmatism the abundances of some major and large ion lithophile elements have been affected by alteration of the samples ti and some other trace elements remain unaffected however and their abundances indicate that the volcanic rocks form a chemically coherent group of tholciites and alkalinc basalts the tholeiitic and alkaline basalts reflect different degrees of melting and low pressure fractional crystallization involving olivine and plagioclase has played a significant role in the evolution of the basalts on the basis of ratios of strongly incompatible elements the two basalt types appear to have been generated from sources having similar character this source is inferred to he enriched sub continental mantle

clockwise rotation of the red river fault inferred from paleomagnetic study of cretaceous rocks in the shan thai malay block of western yunnan china more than 150 samples were collected at 23 sites from the lower cretaceous jingxing formation near the city of yongping 25 5 n 99 5 e which is located on the west side of the red river fault sixteen sites have characteristic directions with a high temperature component above 500 c the high temperature component of magnetization from twelve sites is of pre fold origin and reveals clockwise deflection in declination d 42 0 i 51 1 and 95 15 7 the easterly declination of more than 40 at yongping is consistent with the cretaceous paleomagnetic direction d 45 of chuxiong 25 n 101 5 e which is located on the east side of the red river fault both areas were subjected to about 25 clockwise rotation with respect to the eastern part of the yangtze block since cretaceous time we conclude that the red river fault was rotated through25 16 with respect to the eastern part of the yangtze block associated with rotation of the yongping and chuxiong areas removal of the rotation from the red river fault indicates that the latter and the jinsha suture formed a straight line along n55 w s55 e within the asian continent from vietnam to the tibetan plateau in the present coordinate system during the cretaceous the linear feature of the red river fault and jinsha suture was probably deformed to the present curved shape as a result of collision of the indian continent with the asian continent

paleomagnetic study of cretaceous rocks from the yangtze block central yunnan china implications for the india asia collision abstract more than 200 samples for this study were collected at 30 sites from the cretaceous matoushan and puchanghe formations around the city of chuxiong 25 n 101 5 e twenty four sites have characteristic directions with a high temperature component above 350 c paleomagnetic directions from 21 sites of the cretaceous formations reveal clockwise deflection in declination d 44 6 i 41 3 and 95 10 7 the reliability of the direction is ascertained through a positive fold test and the presence of normal and reversed polarities comparison with paleomagnetic data from other areas indicates that the chuxiong area was rotated clockwise through26 17 with respect to the eastern part of the yangtze block whereas it was subjected to more than 44 of clockwise rotation with respect to the neighboring blocks of lhasa 56 21 and western sichuan 44 16 since cretaceous time the clockwise rotation of chuxiong is a reflection of rotational motion of the large region including the southeastern qiangtang terrane and the jinsha suture red river fault paleomagnetic data and tectonic features suggest that the rotation occurred prior to the folding in the central yunnan province probably during the paleogene this large rotation is a spectacular example of the deformation in the southeastern part of the asian continent due to the collision of india

new cretaceous and early tertiary paleomagnetic results from xining lanzhou basin kunlun and qiangtang blocks china implications on the geodynamic evolution of asia we present the results of a paleomagnetic study of 360 cores drilled at three different areas from the tibetan plateau 13 sites from lower cretaceous red beds in the xining lanzhou basin around xining and lanzhou cities southwest of the qilian mountains 36 2 n 103 5 e 13 sites from cretaceous red beds in the kunlun block near maqin 34 5 n 100 1 and 9 sites from paleocene lower eocene red beds in the qiangtang block near fenghuoshan 34 5 n 92 8 e thermal demagnetization of the samples allowed us to isolate a high temperature component which passes both positive reversal and fold tests for all formations the corresponding paleopoles lie at 50 3 n 195 5 e a95 4 6 for xining lanzhou 80 1 n 281 2 e dp 7 8 dm 12 7 for maqin and 62 6 n 210 5 e dp 3 9 dm 6 8 for fenghuoshan we discuss these in the frame of a new paleogeographic reconstruction of the cretaceous paleoposition of the blocks forming the asian mosaic we conclude that the xining lanzhou area could not be part of the north china block but rather was associated with the tarim qaidam assemblage paleomagnetic data argue in favor of a qaidam kunlun tarim junggar assemblage in the cretaceous significantly to the south of its current position with respect to the asian continent siberia mongolia and north china the large n s convergence 800 500 km implied since the cretaceous appears to be far larger than could be absorbed in the altay range to the north and qilian shan to the east of the order of 300 km part of this motion could have occurred along a large left lateral strike slip fault system which may connect with the mongol okhotsk suture to the northeast verifying this hypothesis will require new geologic and paleomagnetic data from these remote regions

thrust geometries and deep structure of the outer and lesser himalaya kumaon and garhwal india implications for evolution of the himalayan fold and thrust belt the kumaon garhwal region of the himalaya lies near the center of the himalayan fold and thrust belt we have drawn two balanced cross sections 100 km apart through the outer and lesser himalaya the cross sections incorporate all the surface well log and earthquake seismic data currently available from the region two branch line maps showing trailing and leading branch lines and cutoff lines of the major thrusts in the region are also drawn the three dimensional deep structure of the outer and lesser himalaya is interpreted based on the balanced cross sections and the branch line maps deep structure of the higher and tethyan himalaya is extrapolated based on surface geology and is subject to revision as more surface and seismic data become available from these areas a sequential evolutionary model for the kumaon himalaya along the eastern pindari section is proposed according to this model the kumaon himalaya evolved by an overall forelandward progression of thrusting with some reactivation along the munsiari thrust mt the main boundary thrust mbt and the main central thrust mct we use structural stratigraphic and radiometric criteria to place time constraints on the motion of these thrusts earliest motion along the mbt may have occurred in early middle paleocene but the main episode probably started in late eocene and may still be continuing emplacement of the mt had occurred by middle late eocene whereas the mct shows activity around 20 ma thus exhibiting break back thrusting with respect to the mt shortening estimates are obtained from the pindari section minimum shortening in the sedimentary thrust sheets of the outer and lesser himalaya is 161 km or 65 as a first approximation we have also restored the crystalline sheets in order to obtain shortening estimates for the entire himalaya minimum shortening for the himalaya after restoring the mct sheet varies from 354 76 to 421 km 79 these estimates were further combined with published data from the area between the mct sheet and the indus tsangpo suture zone itsz minimum shortening between the indo ganga foreland and the itsz thus obtained lies in the range 687 754 km or 69 72 we compare our shortening estimates with those available from the pakistan and nepal himalaya

investigation of multifrequency multipolarization radar signatures of rain cells over the ocean using sir c x sar data radar signatures of rain cells are investigated using multifrequency multipolarization synthetic aperture radar sar images acquired from the space shuttle endeavour during the spaceborne imaging radar c x band sar sir c x sar missions in april and october 1994 in sir c x sar images radar signatures of rain cells over the ocean usually consist of irregularly shaped bright and dark patches that strongly depend on radar frequency and polarization the radar signatures of rain cells observed in sir c x sar imagery of the ocean originate from 1 the scattering and attenuation of the microwaves by raindrops and ice particles in the atmosphere and 2 the modification of the sea surface roughness induced by the impact of raindrops and by wind gusts associated with rain cells raindrops impinging on the sea surface generate ring waves which enhance the sea surface roughness but they also generate turbulence in the upper water layer which reduces the sea surface roughness depending on the radar wavelength ocean areas struck by rain can have higher or lower normalized radar cross section nrcs than the surrounding rain free area in ocean areas where heavy rain is impinging on the sea surface the x and c band nrcs is usually enhanced and the l band nrcs is reduced from the phase difference between the horizontally and vertically copolarized signals estimates of the rain rate are obtained the present analysis shows further that the presently used wind speed retrieval algorithms for the scatterometers aboard the ers and adeos satellites may yield biased wind fields if several rain cells lie within a scatterometer resolution cell

continuous deformation of the tibetan plateau from global positioning system data global positioning system velocities from 553 control points within the tibetan plateau and on its margins show that the present day tectonics in the plateau is best described as deformation of a continuous medium at least when averaged over distances of 100 km deformation occurs throughout the plateau interior by ese wnw extension and slightly slower nne ssw shortening relative to eurasia material within the plateau interior moves roughly eastward with speeds that increase toward the east and then flows southward around the eastern end of the himalaya crustal thickening on the northeast ern and eastern margins of the plateau occurs over a zone 400 km wide and cannot be the result of elastic strain on a single major thrust fault shortening there accommodates much of india s penetration into eurasia a description in terms of movements of rigid blocks with elastic strain associated with slip on faults between them cannot match the velocity field

the role of crustal strength variations in shaping orogenic plateaus with application to tibet 1 the tibetan plateau is the type example of a large orogenic plateau formed as a result of continent continent collision the morphology of the plateau and its margins suggests that preexisting variations in crustal strength have influenced the growth of the plateau we have developed a three dimensional numerical model of deformation in a viscous crust in order to investigate the effects of lateral heterogeneities on plateau growth the model includes a two layer crust and allows for lateral variation of viscosity in both the upper and lower layers model results indicate that crustal strength variations have a dramatic effect on the morphology and dynamics of a developing plateau a region of strong crust is characterized by a very steep plateau margin that propagates extremely slowly does not accommodate significant shortening strain and is subparallel to local upper and lower crustal velocities a weak crustal region develops a gently sloping margin uplift propagates rapidly across the weak zone and crustal material within the plateau is diverted toward the low strength region with a relatively simple distribution of strength variations corresponding to strong tarim and sichuan basin crust and a weak southeastern corner our model produces a plateau with many similarities to the tibetan plateau including the overall morphology rotation around the eastern syntaxis and e w extension analysis of model results suggests that e w extension of the central plateau may be related to the rapid eastward flow of crustal material into a weak zone in the se corner without significant change in plateau elevation

evolution of the himalayan paleogene foreland basin influence of its litho packet on the formation of thrust related domes and windows in the eastern himalayas a review abstract eocene oligocene foreland basin was formed in response to the india asia collision and ensuing himalayan orogenesis the initial collision during late paleocene to early eocene was broadly contemporaneous laterally the paleogene sediments although discontinuously exposed are remarkably similar in their character and organization in the eastern himalayas it is virtually concealed tectonically but is exposed close to and beneath the main boundary thrust mbt as narrow but laterally extensive thrust slivers of fossiliferous eocene sediments lateral continuity and similar marine fauna in them establish the presence of thicker eocene sediments beneath mbt further north several domes and windows in the eastern himalayan frontal belt have evolved with similar geometry the largest siang window exposes duplex antiform of paleogene sediments and volcanics at the core which have arched up the mbt roof thrust the latter underlies passively domed up late plaeozoic and proterozoic himalayan thrust packets the cores of other windows mainly expose low grade proterozoic rocks with or without the late paleozoic rocks a similar sequence of pre tertiary thrust packet is exposed to the south in the frontal belt which override the neogene sediments across mbt a buried basement indenture of the indian continent continues beneath the siang window located at the eastern himalayan syntaxis the cores of other windows are possibly arched up by the duplexes of pre tertiary and tertiary sediments during the process of southward movement of the crystalline and foreland sheets

large scale crustal deformation of the tibetan plateau the topography velocity and strain fields calculated from a three dimensional newtonian viscous model for large scale crustal deformation are generally in good agreement with results from geological geodetic and earthquake studies in and around the tibetan plateau provided that the model rheology incorporates a weak zone within the deep crust beneath the plateau equivalent to a viscosity of 1012 pa s within a 250 m thick channel or 1018 pa s within a 15 km thick channel model studies and observations show a plateau at 5 km elevation with steep topographic gradients across the southern and northern plateau margins and more gentle gradients across the southeastern and northeastern margins rapid shortening strain is concentrated along the lower portions of the northern and southern plateau margins at rates 20 mm yr model results show north south shortening 10 mm yr in reasonable agreement with gps data 5 8 mm yr of north south shortening across the northern two thirds of the plateau and east west stretching 10 15 mm yr across the eastern half of the high plateau in reasonable agreement with seismic geologic and gps data upper crustal material moves eastward from the plateau proper into a lobe of elevated topography that extends to the south and east clockwise rotation of material around the east himalayan syntaxis at rates up to 10 mm yr occurs partly as a result of dextral shear between indian and asian mantle at depth and partly as a result of gravitational spreading from the high plateau to the south and east there is little difference in model surface deformation for assumptions of moderately weak or extremely weak lower crust except in southern and northern tibet where margin perpendicular extension occurs only for the case of an extremely weak lower crust our results suggest that the tibetan plateau is likely to have gone through a two stage development the first stage produced a long narrow high orogen whose height may have been comparable to the modern plateau the second stage produced a plateau that grew progressively to the north and east east west stretching eastward plateau growth and dextral rotation around the east himalayan syntaxis probably did not begin until well into the second stage of plateau growth perhaps becoming significant after 20 m y of convergence

paleomagnetic study of middle cretaceous rocks from yunlong western yunnan china evidence of southward displacement of indochina middle cretaceous red sandstones and siltstones were collected at 20 sites for paleomagnetic study from the nanxin formation around yunlong 25 8 n 99 4 e western part of yunnan province china after stepwise thermal demagnetization high temperature component magnetization with unblocking temperature of about 675 c is isolated a positive fold test at the 99 confidence limit shows that the high temperature component magnetization is primary for the middle cretaceous nanxin formation a tilt corrected formation mean direction is d 40 2 i 49 9 with 95 3 9 corresponding to a paleopole at 54 6 n 171 8 e with a95 4 4 comparison with previously reported cretaceous paleomagnetic directions indicates that the yunlong area has been subjected to southward displacement by 11 9 7 5 corresponding to 1300 800 km with respect to the stable yangtze craton since cretaceous time as well as clockwise rotation through 36 3 13 6 this study elucidates that indochina was squeezed out of the asian continent due to collision of india accompanied with clockwise rotation

paleomagnetism of cretaceous rocks from eastern qiangtang terrane of tibet thermal demagnetization has successfully isolated high unblocking temperature characteristic directions from paleomagnetic samples of lower and middle cretaceous redbed formations from eastern qiangtang terrane tibet the tilt corrected mean direction for the lower cretaceous is d i 57 7 50 2 with 95 10 9 and n 12 sites and that for the middle cretaceous is d i 39 7 51 6 with 95 8 5 and n 11 sites both formations pass the fold test and the latter possesses two polarities the magnetization predates the eocene and was probably acquired during cretaceous time comparison of the mean directions with expected directions from the stable interior of eurasia indicates significant clockwise rotation but little paleolatitudinal change for the sampling area deformation and rotation of these rocks are presumably related to the collision of india with asia and its subsequent northward indentation into asia existing paleomagnetic data from tibet and its environs seems to be compatible with a crustal shortening thickening model dewey et al 1988 in that the continental crust directly north of the indian indenter was substantially shortened north south whereas in the vicinity of the western and eastern syntaxes latitudinal displacement was minimal but tectonic rotation was very important more data are needed to further differentiate between this model and the lateral eastward crustal extrusion model molnar and tapponier 1975

geology and tectonic history of the lohit valley eastern arunachal pradesh india abstract the lohit river section of eastern arunachal pradesh comprises four tectonic units from sw to ne these are the lesser himalayan rocks the mishmi crystallines the tidding suture zone and the lohit plutonic complex the mishmi thrust underlies the basal lesser himalayan unit while the mishmi crystallines are thrust over the lesser himalayan unit along the main central thrust the grade of metamorphism in the mishmi crystallines increases up the structural section from chlorite to staurolite kyanite zones exhibiting inverted metamorphism the relationship between deformation and metamorphism shows that the metamorphic peak was syn to post tectonic in relation to the main ductile shearing event continued deformation after the metamorphic peak was accommodated along millimetre scale shear zones developed throughout the sequence parallel to the regional schistosity movement along these shear zones has resulted in inversion of the metamorphic zones the rocks of the tidding suture represent an ophiolitic melange thrust over the mishmi crystallines which in turn are overthrust by the lohit plutonic complex along the lohit thrust the lohit plutonic complex is subdivided into western and eastern belts separated by the walong thrust the western belt consists of deformed quartz diorite diorite gabbro and trondhjemite intruded by basic and acid dykes the eastern belt comprises garnet sillimanite gneiss intercalated with crystalline marble bands followed by a complex zone of leucogranites aplites and pegmatites which intrude the early foliated quartz diorite soda rich granite and microdiorite the rocks of the eastern belt are the northward continuation of the mogok gneissic belt of central burma the occurrence of intrusive rocks in the eastern belt suggests that the magmatism related to subduction extended to the east far from the subduction zone the peraluminous leucogranites aplites and pegmatites are the products of crustal melting induced by crustal thickening related to the intracontinental walong thrust subsequent to metamorphism and shearing the whole sequence was folded into an antiform forming the eastern syntaxis and this deformation steeply tilted the earlier low angle thrusts and foliations later compression partitioned into right lateral strike slip motion producing a superimposed sub horizontal lineation observed mostly in the lohit plutonic complex

discrepant cretaceous paleomagnetic poles between eastern china and indochina a consequence of the extrusion of indochina abstract a paleomagnetic study of cretaceous and paleocene red beds has been carried out in the yunlong area yunnan province of china of the northern indochina block a high temperature component with dual polarities was isolated from both lower cretaceous and paleocene rocks and of solely polarity for upper early cretaceous and lower late cretaceous rocks thirteen magnetic polarity zones are found along the pijiang section the primary nature of cretaceous magnetic remanence is ascertained by positive and reversal fold tests these results combined with previous cretaceous results from the yunlong and yongping areas of the lanping basin indicate that these areas behaved as a relatively rigid block and rotated 37 2 17 clockwise relative to eurasia since the paleocene comparing available early cretaceous paleomagnetic results obtained both from indochina and eastern china a statistically significant paleolatitude difference of 9 6 5 7 is inferred this difference is consistent with an estimated 1000 400 km extrusion of indochina relative to south china through a counter clockwise rotation of 14 of indochina around a finite rotation pole inferred from the stage poles of rotation derived from the magnetic isochrons in the south china sea the reconstruction is fully consistent between the cretaceous pole obtained from the khorat basin of the stable core of the indochina block and the coeval pole of eastern china

geodynamics of the southeastern tibetan plateau from seismic anisotropy and geodesy ongoing plate convergence between india and eurasia provides a natural laboratory for studying the dynamics of continental collision a fi rst order process in the evolution of continents regional climate and natural hazards in southeastern tibet the fast directions of seismic anisotropy determined using shear wave splitting analysis correlate with the surfi cial geology including major sutures and shear zones and with the surface strain derived from the global positioning system velocity fi eld these observations are consistent with a clockwise rotation of material around the eastern himalayan syntaxis and suggest coherent distributed lithospheric deformation beneath much of southeastern tibet at the southeastern edge of the tibetan plateau we observe a sharp transition in mantle anisotropy with a change in fast directions to a consistent e w direction and a clockwise rotation of the surface velocity surface strain fi eld and fault network toward burma around the eastern himalayan syntaxis the coincidence between structural crustal features surface strain and mantle anisotropy suggests that the deformation in the lithosphere is mechanically coupled across the crust mantle interface and that the lower crust is suffi ciently strong to transmit stress at the southeastern margin of the plateau in yunnan province a change in orientation between mantle anisotropy and surface strain suggests a change in the relationship between crustal and mantle deformation lateral variations in boundary conditions and rheological properties of the lithosphere play an important role in the geodynamic evolution of the himalayan orogen and tibetan plateau and require the development of three dimensional models that incorporate lateral heterogeneity

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

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

indication for clockwise rotation in the siang window south of the eastern himalayan syntaxis and new geochronological constraints for the area palaeomagnetic rock magnetic and geochronological investigations were carried out on the abor volcanics of arunachal pradesh ne india a late palaeozoic formation age for part of the abor volcanics cannot be excluded based on k ar whole rock dating low temperature thermochronometers zircon u th he and fission track analyses yield a maximum burial temperature of c 150 170 8c during late miocene zft thermochronology of the yinkiong and miri fms indicates a post paleocene and post jurassic deposition age respectively this infers that the volcanic rocks intercalating or intruding them are not part of the late palaeozoic sequence but represent one or more latest cretaceous to tertiary event s therefore the abor volcanics are connected to at least two separate events of volcanism from palaeomagnetic sites two characteristic magnetic remanence components were separated a low coercivity component demagnetized below 20 mt and a high coercivity component demagnetized between 15 and 100 mt fold tests support a secondary origin of both components thermochronological and rock magnetic analyses indicate a low grade overprint event between india asia collision and miocene which probably represents the time of remanence acquisition the high coercivitycomponent shows a trend of clockwise declinations which is likely related to vertical axis rotations of the eastern himalayas due to eastward extrusion of the tibetan plateau the himalayan mountain range created by collision of the indian and eurasian continents during early eocene provides a unique opportunity for studying processes of continent continent collision yin harrison 2000 the himalaya extends between the nanga parbat syntaxis at the western end and the namche barwa syntaxis at the eastern end the area around the namche barwa syntaxis plays a key role in understanding geodynamic processes related to the eastward extrusion of the tibetan plateau ongoing indentation of india into eurasia has led to considerable tectonic deformation of the eurasian continent molnar tapponnier 1975 clockwise tectonic rotations in the eastern part of the himalayas are predicted by several numerical models and laboratory experiments in which the rigid indian plate penetrates into the continuum of asia e g houseman england 1993 beaumont et al 2001 cook royden 2008 palaeomagnetic investigations on cretaceous miocene rocks in the wider region north and west of the namche barwa syntaxis supported these clockwise rotations e g otofuji et al 1990 1998 funahara et al 1992 1993 huang opdyke 1993 sato et al 1999 2007 yang et al 2001 in addition velocity vectors obtained from global positioning system gps clearly display a clockwise rotational pattern around the namche barwa syntaxis for the present aseismic surface movements e g chen et al 2000 mattauer 2002 zhang et al 2004 shen et al 2005 sol et al 2007 most of the previous studies were performed north or east of the namche barwa syntaxis due to political and infrastructural restrictions and very dense vegetation the area south of the syntaxis is relatively unexplored the present study provides new from gloaguen r ratschbacher l eds growth and collapse of the tibetan plateau geological society london special publications 353 71 97 doi 10 1144 sp353 5 0305 8719 11 15 00 the geological society of london 2011 palaeomagnetic and geochronological data on the abor volcanic and adjacent sedimentary rocks of the siang window in arunachal pradesh ne india located just south of the namche barwa syntaxis the age of the abor volcanic rocks is highly controversial e g gansser 1964 jain thakur 1978 tripathi chowdhury 1983 bhat 1984 acharyya 1994 2007 kumar 1997 we present new geochronological and palaeomagnetic results that offer insight in the regional structure of the eastern himalayan syntaxis area and the extrusion of the tibetan plateau

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

palaeomagnetic results from the tibetan plateau palaeomagnetic measurements were carried out on 1325 oriented samples collected from 246 sites on a traverse of the tibetan plateau from lhasa to golmud in 1985 crossing the lhasa terrane qiangtang terrane and kunlun terrane high blocking temperature high coercivity statistically grouped magnetizations were isolated from the following units lhasa terrane cretaceous takena formation mid cretaceous nagqu volcanics mid cretaceous qelico volcanics qiangtang terrane norian batang group volcanics kimmeridgian yanshiping group paleocene to eocene fenghuoshan group kunlun terrane visean to namurian dagangou formation dykes of the triassic igneous province the triassic data from the kunlun terrane triassic and lower tertiary data from the qiangtang terrane and the cretaceous data from the lhasa terrane indicate palaeolatitudes ca 20 s of their present position within the eurasian frame of reference a possible interpretation is that the terranes successively accreted to eurasia and remained in the southern position until the convergence of india drove them northward via a process of tectonic shortening and or displacement of continental crust the carboniferous data from the kunlun terrane are consistent with moderate southern hemisphere latitude well separated from eurasia which was in the northern hemisphere at this time implying the existence of ocean crust between these blocks during the carboniferous

abor volcanics further evidence for the birth of the tethys ocean in the himalayan segment the himalayan region has a record of several phases of igneous activity which indicate a continued n s orientated tensional regime since at least late precambrian time though the earlier volcanic eruptions were restricted both in time and space the late carboniferous triassic activity was widespread in terms of both the time interval and area covered the abor volcanic activity in the eastern syntaxial bend of the himalayan fold belt is coeval with the initiation of late carboniferous triassic panjal trap volcanism of the western syntaxial bend and eruption of the sikkim lavas chemical characters of the abor volcanics indicate 1 a within plate rift tectonic setting of the lava eruptions 2 a transitional nature between tholeiitic and alkalic compositions 3 relative enrichment in incompatible elements and 4 high fe ti values similar to many such basalts erupted in the regions of active lithospheric rifting the olivine free nature a linear trend for mgo and low co contents of the studied rocks are suggestive of high pressure olivine fractionation for the generation of the bulk chemistry however large variations in the incompatible element abundances and ratios of rocks at a similar fractionation stage discount the evolution of the lava chemistry exclusively by simple fractionation or partial melting processes instead these characters together with the low zr nb ratio and within plate rift tectonic setting suggests the modification of an upper mantle source region possibly by fluid phase co 2 immediately before the partial melting and lithospheric rifting combining geology of the region with the geochemistry and inferred tectonic setting of these rocks the lava eruptions are suggested to have been associated with the 9rift valley stage9 of the tethys ocean formation in the present himalayan region

resolving the problem of shallow magnetizations of tertiary age in asia insights from paleomagnetic data from the qiangtang kunlun and qaidam blocks tibet china and a new hypothesis we present new paleomagnetic results obtained at 39 sampling sites from five sections of tertiary red bed formations two eocene formations from the qiangtang block of tibet xialaxiu locality 32 8 n 96 6 e and the xining basin of qaidam xining locality 36 5 n 102 0 e and three neogene formations from the xining basin jungong locality 34 7 n 100 7 e and the kunlun block tuoluo lake and west yushu localities 35 3 n 98 6 e and 33 2 n 96 7 e respectively thermal demagnetization of the rocks isolated a high temperature component that we interpret as the primary magnetization in four localities the paleopoles lie at 52 6 n 352 e dp dm 6 0 10 7 for xialaxiu 61 6 n 211 3 e dp dm 9 7 16 1 for xining 66 0 n 228 6 e dp dm 3 6 6 9 for jungong and 53 9 n 205 4 e dp dm 5 6 10 0 for west yushu as in previous studies of tertiary formations from asia the inclinations we obtained are shallower by 18 to 26 than the magnetic field computed from the eurasian apparent polar wander path apwp at 10 and 20 ma for neogene rocks and at 40 and 60 ma for eocene rocks on the basis of a compilation of eocene data from the south china block tibet central asia and kyrgyzstan we conclude that this inclination anomaly reflects erroneous predictions of positions of the siberian craton when based on the apwp of eurasia the main reason for this discrepancy might be nonrigid behavior of the eurasian plate in the tertiary combination of this with intracontinental shortening of asia under the penetration of india provides a full explanation for the anomaly verification of this new interpretation of the inclination anomaly will require new geologic and paleomagnetic data from the northern parts of these remote regions in mongolia and siberia

cenozoic structural and metamorphic evolution of the eastern himalayan syntaxis namche barwa combined geological and geochronological investigations of the eastern himalayan syntaxis in the namche barwa region of tibet reveal the first order elements of its cenozoic tectonic evolution the syntaxis is characterized by a northeast plunging antiform and is bounded by two northeast striking strike slip shear zones a left slip shear zone on the western side and a right slip shear zone on the eastern side these strike slip shear zones are linked by east westtrending thrusts and served either as 1 a roof thrust to a large duplex system or 2 transfer faults to a south directed thrust system that accommodated northward indentation of a folded indian plate an east west trending pop up structure in the core of the antiform juxtaposes a granulite bearing complex over sillimanite bearing gneisses of gangdese affinity to the north and of indian affinity to the south previous studies suggest that mafic granulites in the complex record at least two episodes of metamorphism at v800 c the first at high pressures 14 15 kbar followed by a second event at 8 10 kbar zircons from mafic granulites yield four populations of concordant u pb ion microprobe ages two groups are at v65 ma and v160 ma and likely crystallized during andean type gangdese magmatism prior to the indo asian collision a third cluster at v40 ma exhibits very low th u ratios and is interpreted to have crystallized in the presence of fluids associated with a high pressure granulite facies metamorphic event during the early stages of the indo asian collision subsequent to high pressure metamorphism in the western himalaya syntaxis between v50 and 43 ma a fourth cluster of zircons yields ages between 11 and 25 ma and th u ratios that decrease systematically with decreasing age we interpret the youngest zircon age v11 ma to represent the timing of moderatepressure high grade metamorphism with the older ages and higher th u ratios being a result of mixing with a restitic igneous component this interpretation coupled with a v 8m a 40 ar 39 ar age on hornblende from a metadiorite within the core of the antiform suggests that the namche barwa syntaxis has been characterized by rapid cooling and exhumation since at least late miocene time despite its contrasting structural setting the miocene and younger metamorphism and cooling history in the namche barwa syntaxis are strikingly similar to those of the nanga parbat syntaxis of the western himalaya fl 2001 elsevier science b v all rights reserved