electrically conductive crust in southern tibet from indepth magnetotelluric surveying the crust north of the himalaya is generally electrically conductive below depths of 10 to 20 km this conductive zone approaches the surface beneath the kangmar dome dipping north and extends beneath the zangbo suture a profile crossing the northern yadong gulu rift shows that the high conductivity region extends outside the rift and its top within the rift coincides with a bright spot horizon imaged on the indepth cmp common midpoint profiles the high conductivity of the middle crust is atypical of stable continental regions and suggests that there is a regionally interconnected fluid phase in the crust of the region

evidence from earthquake data for a partially molten crustal layer in southern tibet earthquake data collected by the indepth ii passive source experiment show that there is a substantial south to north variation in the velocity structure of the crust beneath southern tibet north of the zangbo suture beneath the southern lhasa block a midcrustal low velocity zone is revealed by inversion of receiver functions rayleigh wave phase velocities and modeling of the radial component of teleseismic p waveforms conversely to the south beneath the tethyan himalaya no low velocity zone was observed the presence of the midcrustal low velocity zone in the north implies that a partially molten layer is in the middle crust beneath the northern yadong gulu rift and possibly much of southern tibet

crustal structure of the india asia collision zone southern tibet from indepth mt investigations abstract project indepth was initiated in 1992 to develop a better understanding of the deep structure and mechanics of the himalaya tibet region and magnetotelluric surveying was added during indepth phase ii in 1995 broadband and long period data were acquired in 1995 along a n s transect crossing the india asia collision zone the yarlung zangbo suture at 90 e longitude however these data failed to penetrate to the deep crustal and mantle depths due to low solar activity and the presence of the thick conductive crust deep crustal and upper mantle information is key to test the model of subcretion to southern tibet by the stiff indian mantle lid in order to try to obtain such information and to test along strike variability of the yarlung zangbo suture zone a magnetotelluric experiment was performed in 2001 roughly along longitude 92 e containing an ultra long period component mt data were acquired at 28 locations along the new profile with long period data at 15 of these and ultra long period data at five of these 15 initial comparisons of the models generated from the 1996 and 2001 datasets indicate little variation in electrical conductivity structure along strike

structural elements of the southern tethyan himalaya crust from wide angle seismic data a deep seismic common midpoint cmp profile shot across the southern margin of the tethyan himalaya as the first stage of project international deep profiling of tibet and the himalaya indepth imaged the top main himalayan thrust or mht and bottom moho of the indian continental crust underthrusting the himalaya we used portable seismographs to record the cmp profile shots at a wide range of offsets up to 155 km our short offset data corroborate the cmp profile data while our large offset data are dominated by a band of reflectivity we interpret the bright onset of this reflective band as the basal detachment of the south tibetan detachment system and a phase at the end of the reflective band as the mht we used the cmp profile first break data to model the uppermost 2 km in detail the depth of young extensional basins of the yadong rift system along the cmp profile was constrained to a maximum of 2 km yielding a throw of 4 6 km across the eastern flank of the rift and a rough estimate of approximately 1 5 east west extension by normal faulting in the tethyan himalaya the wide angle data were used to construct a crustal seismic velocity model down to the mht the south tibetan detachment system stds basal detachment reflector is observed dipping 12 5 nne from a depth of about 6 km beneath the surface under the south end of the cmp profile to a depth of 22 km then flattening to a dip of only 2 5 nne thus our observations suggest that the stds basal detachment is a deep rooted basement fault for the mht we observe a dip of 7 5 nne from 20 km depth below sea level at the crest of the high himalaya crystalline sheet to 36 km below sea level 40 km beneath the surface at a distance of about 70 km south of the indus yarlung suture iys from geometrical arguments we suggest that indian crust underthrusts the surface expression of the iys within the crust but does not form the lower crust of central and northern tibet

mechanics of fold and thrust belts and accretionary wedges cohesive coulomb theory a critically tapered fold and thrust belt or submarine accretionary wedge is one that is on the verge of coulomb failure everywhere including its base where frictional sliding along a decollement is assumed to be occurring cohesion within a wedge can add significantly to the overall strength near the toe the effect of this is to decrease the near toe taper leading to a critical topographic profile that is concave upward if the decollement is planar we obtain an approximate self consistent solution for the state of stress within a thin skinned cohesive critical coulomb wedge and determine the relationship between the wedge taper and its strength and basal friction the theory is then applied to the presently deforming fold and thrust belt of western taiwan fitting of theoretical critical wedge shapes to topographic profiles and measurements of the step up angles of thrust faults from the basal decollement are used to constrain the taiwan wedge strength parameters an attractive assertion fully consistent with all the observations is that the mechanics of fold and thrust belts and accretionary wedges is governed by normal frictional and fracture strengths of rocks measured in the laboratory in particular if byerlee s law b 0 85 is adopted as the coefficient of sliding friction on the base we find a coefficient of internal friction 0 9 1 0 in the wedge and a wedge cohesion so 5 20 mpa other solutions having strengths and ambient stresses up to 4 times lower than this can also however satisfy the data

receiver function imaging of crustal suture steep subduction and mantle wedge in the eastern india tibet continental collision zone abstract to understand the along strike variation of crustal deformation and tectonic processes in the india tibet continental collision zone we deployed a linear array of broadband seismic stations along 92 e to image lithospheric structure our receiver function cross section reveals a prominent negative converter dipping 20 north from 10 55 km depth below sea level almost through the whole crust beneath the southern lhasa terrane we interpret it to be a manifestation of the yarlung zangbo suture zone yzs separating the continental crust of the indian and eurasian plates this implies the hypothesized channel flows of indian middle crust extruding southwards from tibet are limited at this longitude to the southernmost portion of the lhasa terrane a positive converter consistent with previous suggestions of eclogite formation is seen about 10 15 km above the moho and continuing 50 km north of the 20 dipping yzs converter we image this positive converter continuously from 60 km south of the surface trace of the yzs to the vicinity of the jiali fault supporting the interpretation of sub horizontal underplating of tibetan crust by indian crust to 31 n at 85 e on the hi climb transect however we also show a negative mantle converter sub parallel to the crustal yzs converter from the northern limit of the underplating indian lower crust to at least 140 km depth that we interpret as the base of tibetan lithosphere overlying an asthenospheric mantle wedge based on the lithospheric structure observed in this and other studies we infer that indian mantle lithosphere currently detaches from indian lower crust at the mantle suture that is nearly 50 km south of the surface trace of the yzs at 92 e south of the mantle suture suggested by indepth transect beneath the surface trace of the yzs at 90 e and far south of the mantle suture suggested to be at the 31 n northern limit of underthrusting indian lower crust suggested by hi climb transect at 85 e a change from underplating in the west to steep subduction in the east can reconcile all these observations

seismic velocity reduction and accelerated recovery due to earthquakes on the longmenshan fault various studies report on temporal changes of seismic velocities in the crust and attempt to relate the observations to changes of stress and material properties around faults although there are growing numbers of observations on coseismic velocity reductions generally there is a lack of detailed observations of the healing phases here we report on a pronounced coseismic reduction of velocities around two locked sections asperities of the longmenshan fault with a large slip during the 2008 mw 7 9 wenchuan earthquake and subsequent healing of the velocities the healing phase accelerated significantly at the southern asperity right after the nearby 2013 mw 6 6 lushan earthquake the results were obtained by joint inversions of travel time data at four different periods across the wenchuan and lushan earthquakes the rapid acceleration of healing in response to the lushan earthquake provides unique evidence for the high sensitivity of seismic velocities to stress changes we suggest that stress redistribution plays an important role in rebuilding fault strength after a fault ruptures the recovery of its strength may be accelerated by earthquakes that redistribute stress according to an analysis of temporal variation in crustal seismic velocity on the longmenshan fault in china

u pb zircon geochronology of major lithologic units in the eastern himalaya implications for the origin and assembly of himalayan rocks models for the origin and deformation of himalayan rocks are dependent upon geometric and age relationships between major units we present field mapping and u pb dating of igneous and detrital zircons that establish the lithostratigraphic architecture of the eastern himalaya revealing that 1 the south tibet detachment along the bhutan china border is a top to the north ductile shear zone 2 late triassic and early cretaceous sedimentary samples from the northern indian margin show a similar age range of detrital zircons from ca 3500 ma to ca 200 ma but the late triassic rocks are distinguished by a significant age cluster between ca 280 and ca 220 ma and a well defined age peak at ca 570 ma 3 an augen gneiss in the south tibet detachment shear zone in southeast tibet has a cambrian ordovician crystallization age 4 main central thrust hanging wall paragneiss and footwall quartzites from the far western arunachal himalaya share similar provenance and late proterozoic maximum depositional ages and 5 main central thrust footwall metagraywacke from the central western arunachal himalaya has a paleoproterozoic maximum depositional age indicated by a single prominent age peak of ca 1780 ma recent work in the eastern himalaya demonstrates that in the early middle miocene the himalayan crystalline core here was emplaced southward between two subhorizontal shear zones that merge to the south a proposed subsequent middle miocene brittle low angle normal fault accomplishing exhumation of these rocks along the range crest can be precluded because new and existing mapping demonstrates only a ductile shear zone here the ca 280 220 ma detrital zircons of the late triassic strata are derived from an arc developed along the northern margin of the lhasa terrane detritus from this arc was deposited on the northern margin of india during india lhasa rifting along strike heterogeneity in main central thrust footwall chronostratigraphy is indicated by detrital zircon age spectrum differences from central western to far western arunachal nonetheless the late proterozoic rocks in the main central thrust hanging wall and footwall in far western arunachal can be correlated to each other and to previously analyzed rocks in the south tibet detachment hanging wall to the west and in the indian craton to the south these findings are synthesized in a reconstruction showing late triassic india lhasa rifting and cenozoic eastern himalayan construction via in situ thrusting of basement and cover sequences along the north indian margin

seismic reflection imaging of crustal deformation within the eastern yarlung zangbo suture zone abstract various models have been proposed to explain formational mechanisms for the himalayan orogeny but uncertainties remain concerning uplift processes and deeper crustal structure of the orogen imaging of the orogen s deep structure can help resolve these uncertainties this paper describes one migrated deep seismic reflection profile across the eastern yarlung zangbo suture yzs at 92 e which provides a high resolution image of crustal structure that reveals 1 yarlhashampo dome is divided into upper middle and lower tectonic units by two detachments from rim mantle to the core of dome 2 one blind north dipping reverse fault that offsets the south tibet detachment std developed along the northern flank of the yarlhashampo dome 3 a shift of the main himalayan thrust mht dip angle from 18 into relatively gentle dips 6 in the central part of the profile 18 20 s t w t meantime further inferring that 4 north south compression event happened after ca 10 ma in north himalayas 5 multi stage duplexing as the primary mechanism for crustal thickening in the eastern yzs based on interpreted structures

crustal structures revealed from a deep seismic reflection profile across the solonker suture zone of the central asian orogenic belt northern china an integrated interpretation abstract the solonker suture zone is one of the most important tectonic boundaries in the southeastern part of the central asian orogenic belt caob an 630 km long reflection seismic profile across this suture was recently completed by the chinese sinoprobe project the processed seismic data show clear crustal structures and provide new constraints on the tectonic and crustal evolution models the moho is delineated as a relatively flat boundary between a strongly reflective lower crust and a transparent mantle at a depth of 40 45 km 14 5 s two way travel time which is in agreement with the refraction data recorded along the same profile in a broad view the profile images an orogen that appears bivergent with and approximately centered on the solonker suture zone the southern portion of this profile is dominated by a crustal scale cratonward propagating fold and thrust system that formed during the late permian and triassic through collision and subsequent convergence in a post collisional stage the major thrust faults are truncated by mesozoic granitoid plutons in the upper crust and by the moho at the base of the crust this geometry suggests that the moho was formed after the thrusting event the northern portion of the profile although partially obliterated by post collisional magmatic bodies shows major south dipping folding and thrusting bands of layered reflectors immediately overlying the moho are interpreted as basaltic sills derived from the mantle episodic mafic underplating may have occurred in this region giving rise to post collisional magmatic events and renewal of the moho a few mantle reflectors are also visible the overall geometry of these mantle reflectors supports the tectonic models that the southern orogen manchurides experienced south directed subduction and the northern orogen altaids underwent north directed subduction prior to collision along the solonker suture zone

deep seismic reflection evidence for continental underthrusting beneath southern tibet the himalaya and adjacent tibetan plateau constituting earth s largest region of elevated topography and anomalously thick crust formed as a consequence of cenozoic collision between india and asia itself considered the archetypal continent continent collision1 3 here we report the first results from an attempt to image the structure of the crust beneath this region using deep seismic reflection profiling our 100 km long profile acquired in the tethyan himalaya shows a mid crustal reflection that prob ably marks the active thrust fault along which the indian plate is underthrusting southern tibet upper crustal reflections with geo metries suggestive of large scale structural imbrication of the upper crust and moho reflections from the base of the double normal thickness crust underlying the region these results lend substantial support to the view that crustal thickening beneath southernmost tibet was accomplished by wholesale underthrusting of indian continental crust beneath the structurally imbricated upper crust comprising the tethyan himalaya

north south extension within the convergent himalayan region recent work by burg et al 1984 indicates the presence of east west striking gently north dipping normal faults in the higher himalayas and southern tibet that formed during postcollisional convergence of india and tibet these faults extend for at least 800 km along strike we interpret these normal faults as probable late miocene extensional features with perhaps several tens of kilometres of down to the north displacement a simple elastic model suggests that these normal faults may have formed during gravitational collapse of the miocene topographic front between india and tibet in this interpretation gravitational collapse occurred by southward motion relative to india and tibet of a wedge of crustal rocks bounded above by gently north dipping normal faults and below by thrust faults that probably dip north north south extension produced in this way is probably confined to upper crustal levels only and does not reflect regional extension of the entire lithosphere

how does the mid crust accommodate deformation in large hot collisional orogens a review of recent research in the himalayan orogen abstract the presence of hot weak crust is a central component of recent hypotheses that seek to explain the evolution of continent continent collisions and in particular may play an important role in accommodating the 3000 km of convergence within the himalaya tibetan collision over the last 55 myr models that implicate flow of semi viscous midcrustal rocks south toward the front of the himalayan orogen channel flow are able to account for many geologic observations in the himalaya while alternative models of collision particularly thrust wedge taper demonstrate that much of the observed geology could have formed in the absence of a low viscosity mid crustal layer several recent studies synthesized here have prompted a shift from initial assumptions that channel flow and thrust wedge taper processes are by definition mutually exclusive these new studies reveal the presence of several tectonometamorphic discontinuities in the midcrust that appear to reflect a continuum of deformation in which both channel and wedge type processes operate in spatially and temporally distinct domains within the orogen and further that the system may migrate back and forth between these types of behavior this continuum of deformation styles within the collisional system is of crucial importance for explaining the evolution of the himalayan orogen and hence for understanding the evolution of earth s many continent continent collision zones

crustal channel flows 1 numerical models with applications to the tectonics of the himalayan tibetan orogen 1 plane strain thermal mechanical numerical models are used to examine the development of midcrustal channel flows in large hot orogens in the models radioactive self heating reduces the viscosity of tectonically thickened crust and increases its susceptibility to large scale horizontal flow channels can be exhumed and exposed by denudation focused on the high relief transition between plateau and foreland we interpret the himalaya to have evolved in this manner channel flows are poorly developed if the channel has a ductile rheology based on wet quartz flow laws and well developed if there is an additional reduction in viscosity to 1019 pa s this reduction occurs from 700 c to 750 c in the models and is attributed to a small percentage of in situ partial melt melt weakening model ht1 provides an internally consistent explanation for the tectonic evolution of many features of the himalayan tibetan orogenic system erosional exhumation exposes the migmatitic channel equivalent to the greater himalayan sequence ghs between coeval normal and thrust sense ductile shear zones corresponding to the south tibetan detachment and the main central thrust systems outward flow of unstable upper crust rotates these shears to low dip angles in the model both the ghs and the lesser himalayan sequence are derived from indian crust with the latter from much farther south similar models exhibit a range of tectonic styles including the formation of domes resembling north himalayan gneiss domes model results are relatively insensitive to channel heterogeneities and to variations in the behavior of the mantle lithosphere beneath the model plateau

a three dimensional model of the early precambrian crust under the southeastern fennoscandian shield karelia craton and belomorian tectonic province abstract the early precambrian structures of the archaean karelia craton and belomorian tectonic province have been investigated by reflection seismology integration of the existing geological database for these two entities and adjacent tectonic units with the seismic data for the crust and upper mantle along the 1 eu geotraverse and cross traverse 4b totaling c 1300 km allows development of a three dimensional model of the deep structure of the southeastern fennoscandian shield interpretation of the seismic data provides evidence that the early precambrian crust is characterized by inclined layering the crustal architecture is a combination of thrust nappe and thrust underthrust structural assemblages consisting of neoarchaean and palaeoproterozoic rocks that were subsequently further deformed during uplift and emplacement of granite gneiss domes the new model substantially modifies previous conceptions of this part of the shield as being constituted of blocks with sub vertical margins and individual internal layering it is apparent that the surface traces of gently dipping boundaries are defined by the erosion level and consequently these boundaries are not those of particular tectonic blocks in the traditional sense

permian magmatic sequences of the bilihe gold deposit in central inner mongolia china petrogenesis and tectonic significance abstract the bilihe gold deposit is located in the eastern section of the ondor sum yanji suture at the southern margin of the xing an mongolian orogenic belt xmob and the northern margin of the north china craton ncc central inner mongolia the magmatic rocks in the ore district are generally high k calc alkaline enriched in lree zr and hf and depleted in hree nb ta and p the magmatic evolution sequences are norite gabbro granodiorite porphyry granite or norite gabbro andesite dacite porphyry granodiorite which show a trend of decreasing tio2 feo mgo cao and p2o5 with increasing sio2 in the bilihe ore district hydrothermal processes were coeval with granitic magmatism for a period of 17 myr 272 255 ma the ages of the granite granodiorite porphyry granodiorite and dacite porphyry are 271 5 264 1 ma 269 8 255 8 ma 268 3 ma and 268 6 259 4 ma respectively the magmatic rocks contain magmatic hydrothermal and magmatic hydrothermal zircons the magmatic zircons have ce 4 la 2 5 the hydrothermal zircons have ce 3 ppm and smn lan

continental subduction and exhumation of high pressure rocks insights from thermo mechanical laboratory modelling abstract thermo mechanical physical modelling of continental subduction is performed to investigate the exhumation of deeply subducted continental crust the model consists of two lithospheric plates made of new temperature sensitive analogue materials the lithosphere is underlain by liquid asthenosphere the continental lithosphere contains three layers the weak sedimentary layer the crust made of a stronger material and of a still stronger lithospheric mantle the whole model is subjected to a constant vertical thermal gradient causing the strength reduction with depth in each lithospheric layer subduction is driven by both push force and pull force during subduction the subducting lithosphere is heating and the strength of its layers reduces the weakening continental crust reaches maximal depth of about 120 km and cannot subduct deeper because its frontal part starts to flow up the subducted crust undergoes complex deformation including indicated upward ductile flow of the most deeply subducted portions and localised failure of the subducted upper crust at about 50 km depth this failure results in the formation of the first crustal slice which rises up between the plates under the buoyancy force this process is accompanied by the delamination of the crustal and mantle layers of the subducting lithosphere the delamination front propagates upwards into the interplate zone resulting in the formation of two other crustal slices that also rise up between the plates average equivalent exhumation rate of the crustal material during delamination is about 1 cm year the crust asthenosphere boundary near the interplate zone is uplifted the subducted mantle layer then breaks off removing the pull force and thereby stopping the delamination and increasing horizontal compression of the lithosphere the latter produces shortening of the formed orogen and the growth of relief the modelling reveals an interesting burial exhumation evolution of the sedimentary cover during initial stages of continental subduction the sediments of the continental margin are dragged to the overriding plate base and are partially accreted at the deep part of the interplate zone at 60 70 km depth these sediments remain there until the beginning of delamination during which the pressure between the subducted crust and the overriding plate increases this results in squeezing the underplated sediments out part of them is extruded upwards along the interplate zone to about 30 km depth at an equivalent rate of 5 10 cm year

the south tibetan detachment system facilitates ultra rapid cooling of granulite facies rocks in sikkim himalaya 1 the eastern himalaya is characterized by a region of granulites and local granulitized eclogites that have been exhumed via isothermal decompression from lower crustal depths during the india asia collision spatially most of these regions are proximal to the south tibetan detachment system an orogen parallel normal sense detachment system that operated during the miocene suggesting that it played a role in their exhumation here we use geo and thermochronological methods to study the deformation and cooling history of footwall rocks of the south tibetan detachment system in northern sikkim india these data demonstrate that the south tibetan detachment system was active in sikkim between 23 6 and 13 ma and that footwall rocks cooled rapidly from 700 to 120 c between 15 13 ma while active the south tibetan detachment system exhumed rocks from mid crustal depths but an additional heat source such as strain heating advected melt and or crustal thinning is required to explain the observed isothermal decompression cessation of movement on the south tibetan detachment system produced rapid cooling of the footwall as isotherms relaxed a regional comparison of temperature time data for the eastern south tibetan detachment system indicates a lack of synchronicity between the sa er sikkim yadong section and the nw bhutan section to accommodate this requires either strike slip tear faulting or local out of sequence thrusting in the younger segment of the orogen

isotopic characteristics of the gurla mandhata metamorphic core complex implications for the architecture of the himalayan orogen isotopic data from the gurla mandhata metamorphic core complex provide insight on the character of rocks exposed within it and permit possible correlations of these rocks to those exposed in the himalayan thrust belt whole rock sr and nd isotopic analyses show that rock units in the metamorphic core have isotopic signatures that correlate to those of greater and lesser himalayan rocks exposed in the foreland these data are the first to document lesser himalayan rocks north of the higher himalayan physiographic zone when combined with previously published structural reconstructions of the himalayan orogen these data reveal a crustal architecture that requires significant uplift of lesser himalayan rocks in the hinterland with respect to their position in the foreland a greater amount of underthrusting of the lesser himalayan rocks and more shortening in the hinterland within the tethyan himalaya physiographic zone than currently estimated

crustal generation of the himalayan leucogranites abstract detailed studies of the himalayan two mica leucogranites such as the manaslu pluton indicate that they have very uniform mineralogical petrological and structural characteristics one can relate their occurrence to the thickest zones of the underlying tibetan slab in these zones migmatization attains its greatest development and vertical extension the granite is emplaced at first along the main disharmonic plane above the main central thrust mct at the top of the tibetan slab infrastructure ductile deformation of the granite is variable the granite being syn to late kinematic with regard to the functioning of the mct major elements are very homogeneous except for na and k implying that p t conditions of melting were relatively uniform the melted material was of a similar composition over a vast volume and the percentage of melting was small 10 15 trace elements are highly variable some are characteristic for very evolved material ta rb cs u or show the link with the tibetan slab ba sr whilst others are problematic th ree ree and th abundances being much less in the granite than in the tibetan slab imply that they have been extracted during one of the main stages of formation possibly by monazite radiogenic pb sr nd and stable o isotopes are consistent with the origin of the granite from the tibetan slab however the heterogeneous sr isotopic ratios make age dating difficult and imply poor mixing or little fluid interaction during its evolution h isotope data indicate that magmatic compositions of the main body of the manaslu granite have been preserved late or post magmatic alterations are extremely local in the main pluton to the north another belt of two mica granites occurs whose characteristics are very similar to the high himalaya belt they were probably generated in a similar way during this recent intracontinental evolution

the kangmar dome a metamorphic core complex in southern xizang tibet the kangmar metamorphic igneous complex is one of the most accessible examples of an enigmatic group of gneiss domes the north himalayan belt that lies midway between the greater himalaya and the indus tsangpo suture in southern tibet structural analysis suggests that the domal structure formed as a consequence of extensional deformation much like the tertiary metamorphic core complexes in the north american cordillera unlike its north american counterparts the kangmar dome developed in an entirely convergent tectonic setting the documentation of metamorphic core complexes in the himalayan orogen supports the emerging concept that extensional processes may play an important role in the evolution of compressional mountain belts

the leading edge of the greater himalayan crystalline complex revealed in the nw indian himalaya implications for the evolution of the himalayan orogen the three himalayan lithologic units the lesser himalayan sequence the greater himalayan crystalline complex and the tethyan himalayan sequence have a specific structural correlation with the main central thrust and south tibet detachment in the central himalaya there the main central thrust places the greater himalayan crystalline complex over the lesser himalayan sequence and the south tibet detachment places the tethyan himalayan sequence over the greater himalayan crystallines although this division has formed the basis for all himalayan tectonic models it fails to explain aspects of the geology of the western himalaya where the main central thrust places the tethyan himalayan sequence directly above the lesser himalayan sequence our mapping in nw india shows that this relationship results from southward merging of the main central thrust and south tibet detachment this finding in conjunction with observed alternating shear senses on the south tibet detachment is inconsistent with the wedge extrusion and erosion induced channel flow models both require only top to the n motion on the south tibet detachment but is consistent with a tectonic wedging model

a short duration pulse of ductile normal shear on the outer south tibetan detachment in bhutan alternating channel flow and critical taper mechanics of the eastern himalaya in easternmost bhutan the south tibetan detachment std is a ductile shear zone that juxtaposes the radi or sakteng klippe of the tethyan sedimentary series from underlying high grade greater himalayan rocks in situ la icpms u th pb analysis of metamorphic monazite from the immediate footwall and hanging wall of the std within the shear zone at the base of the klippe constrains north vergent normal shear to between 25 and 20 ma coeval thrusting on the main central thrust during this time supports a phase of channel flow viscous wedge model activity lasting only ca 3 ma geochronologic data from the eastern himalaya indicate alternating mechanisms for extrusion of the metamorphic core of the orogen from the late oligocene through to the late miocene switching from channel flow viscous wedge behavior to critical taper frictional wedge behavior each phase lasting approximately only 2 to 5 ma the tectonic evolution of the eastern himalaya is comparable to central and western himalayan tectonics during the early miocene but during the middle miocene metamorphism and magmatism in the eastern himalaya migrated towards the orogenic hinterland a process not widely documented elsewhere in the himalaya thus highlighting the need for an orogenic model in three spatial dimensions

evolutionary model of the himalaya tibet system geopoem based on new modelling geological and geophysical data a two dimensional thermo mechanical laboratory modelling of continental subduction was performed the subducting continental lithosphere includes a strong brittle upper crust a weak ductile lower crust and a strong upper mantle the lithosphere is underlain by a low viscosity asthenosphere subduction is produced by a piston push force and the pull force from the mantle lithospheric layer which is denser than the asthenosphere the lithospheric layers are composed of material whose strength is sensitive to and inversely proportional to temperature throughout the experiment the model surface was maintained under relatively low temperature and the model base at higher temperature the subduction rate satisfied the peclet criterion modelling confirms that the continental crust can be deeply subducted and shows that slab break off delamination and tectonic underplating are fundamental events with drastic consequences on the subsequent evolution of the convergent system combining these results with previous purely mechanical modelling we elaborate a new evolutionary model for the himalaya tibet convergent system the principal successive stages are 1 subduction of the indian continental lithosphere to 200 250 km depth following subduction of the tethys oceanic lithosphere 2 failure and rapid buoyancy driven uplift of the subducted continental crust from ca 100 km depth to some depth that varies along the mountain belt 20 30 km on average 3 break off of the indian subducted lithospheric mantle with the attached oceanic lithosphere 4 subduction underplating of the indian lithosphere under asia over a few to several hundred kilometers 5 delamination roll back and break off of the indian lithospheric mantle 6 failure of the indian crust in front of the mountain belt formation of the main central thrust and underthrusting of the next portion of indian lithosphere beneath tibet for a few hundred kilometers at the beginning of stage 6 the crustal slice corresponding to the crystalline himalayas undergoes erosion activated uplift and exhumation fl 2000 elsevier science b v all rights reserved

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

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

crustal structure of the himalayan orogen at 90 east longitude from project indepth deep reflection profiles international deep profiling of tibet and the himalaya indepth deep seismic reflection profiles show that the indian lithosphere is underthrusting the central himalaya along a gently north dipping decollement that is traceable northward beneath the tethyan belt to 28 6 n and to a depth of about 45 km the decollement carries in its hanging wall a near crustal thickness slice of internally deformed indian continental basement and cover represented in outcrop by the greater himalayan belt and structurally overlying tethyan belt geometric relationships suggest that the decollement probably ramps downward beneath the northern tethyan belt to near the base of the crust and that the hanging wall crustal slice was detached from indian mantle lithosphere that presently underlies southern tibet to the north the north himalayan anticlinorium kangmar dome appears to be a large duplex ramp anticline in the hanging wall of the decollement the south tibetan detachment appears to have been imaged in two areas in the wagye la area it appears to follow the tethyan belt greater himalayan belt contact northward in the subsurface and to have been folded over the kangmar dome in the zherger la area it appears to have been active relatively recently as a 30 north dipping normal fault that cuts deeply into the greater himalayan belt allochthon palinspastic reconstruction indicates a minimum of about 326 km of shortening of the indian basement across the himalaya since the initiation of the main central thrust hence indian continental lithosphere largely stripped of its overlying crust can extend northward beneath the tibetan plateau to at least 32 n where earthquake seismological observations show that the properties of the upper mantle change markedly e g mcnamara et al 1995 taken together the seismic reflection and earthquake seismological observations lend support for the view that the indian mantle lid mechanically underplates roughly the southern half of the tibetan plateau the indepth reflection data do not show that indian crust attached to this lithosphere extends north of southernmost tibet kangmar dome nor do they negate this possibility similarly the reflection data do not yield direct evidence for or against lower crustal subduction beneath the himalaya

the zedong window a record of superposed tertiary convergence in southeastern tibet determining the timing magnitude and location of deformation due to the indo asian collision is widely acknowledged as an important step in understanding how the lithosphere responds during continental collision a puzzling result of geological investigations of the lhasa block over the past 2 decades has been the apparent lack of significant tertiary deformation there perhaps the most important structural feature of the lhasa block is the south directed gangdese thrust system which developed along its southern edge the thrust system which separates the andean type batholith of southern asia from rocks of indian affinity is obscured at most locations across southeastern tibet by back thrusts of the younger north directed renbu zedong thrust system the best documented site where both thrusts are exposed occurs near zedong zedong window systematic geochronologic analyses were conducted in this area u pb zircon dating of three samples of a synkinematically to postkinematically deformed hanging wall granitoid the yaja granodiorite cut by the gangdese thrust indicates a crystallization age of 30 4 0 4 ma 2 thus placing an upper bound on the initiation of the thrust u pb zircon dating of granitoid samples structurally higher in the gangdese hanging wall yields emplacement ages of 42 5 1 0 ma and 48 9 0 8 ma 2 similar to other magmatic complexes within the gangdese arc geochemical results are consistent with these plutons forming in the same environment as precollisional intrusions within the gangdese batholith suggesting a significant post collisional input of juvenile heat the 40ar 39ar thermochronology of samples from a vertical section through the hanging wall within the yaja granodiorite coupled with results of a numerical thermal model indicate an average slip rate along the gangdese thrust of 7 mm yr between 30 and 23 ma and a minimum displacement of 50 km farther east in the zedong window thermal effects produced by later north directed thrust sheets of the renbu zedong system appear to have obscured thermal history signatures in the gangdese hanging wall related to earlier south directed thrusting thermochronological results from this region indicate that thermal overprinting related to the north directed thrusting occurred between 25 and 10 ma consistent with previous estimates

deformation of leucogranites of the crystalline main central sheet in southern tibet china abstract towards the top of the main central sheet in southern tibet china leucogranites intrude metamorphic rocks related to rocks of the main central thrust in the himalayas our study shows that the leucogranites suffered a northward shear deformation the significance of this event is discussed in terms of gravity driven decollement of the 10 000 m thick tethyan sedimentary pile over its crystalline basement

mechanical model for the tectonics of doubly vergent compressional orogens a mechanical model of crustal shortening and deformation driven by the relative convergence of rigid underlying mantle plates explains many features of convergent orogens results based on numerical models and supported by sandbox models show that a coulomb crustal layer subject to basal velocity boundary conditions corresponding to asymmetric detachment and subduction of the underlying mantle passes through three stages of orogenic growth 1 block uplift bounded by step up shear zones 2 development of a low taper wedge over the underthrusting mantle plate and 3 development of a low taperwedge overlying the overthrusting mantle plate and verging in the opposite direction when modified by isostasy basal viscous flow surface erosion and denudation and sedimentation the resultant model orogens exhibit a variety of styles with characteristics in common with small rapidly denuded orogens large orogens with plateaus and extensional characteristics and active subduction margins with doubly vergent accretionary wedges and deformed fore arc basins

effect of overburden thickness on thrust belt geometry and development laboratory sand models simulating development of thrust belts indicate that 1 distance between the backstop and the first formed thrust is a linear function of initial sand layer thickness 2 spacing between the first two thrusts is most likely a nonlinear function of initial sand layer thickness 3 initial dip of successive ramps decreases progressively toward the foreland because of variations in principal stress directions with depth and or changes of mechanical properties of sand with increasing overburden 4 faults to the hinterland steepen and become less active as hinterland thrust sheets undergo penetrative shear during sand wedge growth and 5 new ramp initiation occurs to the hinterland of the basal detachment tip line these observations can be interpreted both in context of area balance considerations and soil mechanics theory if initial sand layer thicknesses in our models represent syntectonic overburden in natural thrust belts i e distance between the ground surface and the detachment then our results suggest that locations of some thrust belt salients are controlled by along strike variations in stratigraphic thicknesses and that initial thrust spacing decreases progressively to the foreland if the thrust belt involves a foreland tapering basin furthermore our sand model observations suggest that the concept of a fixed rigid backstop in the hinterland of thrust belts may be misleading as a consequence of thickening the hinterland portion of the thrust belt becomes stabilized and effectively becomes the backstop itself

indepth wide angle reflection observation of p wave to s wave conversion from crustal bright spots in tibet three component wide angle seismic data acquired in southern tibet during project indepth show strong p to s converted reflections from reflectors that are aligned at a depth of 15 kilometers beneath the northern yadong gulu rift these converted reflections are locally higher in amplitude than the corresponding p wave reflections modeling of reflection mode conversion as a function of incidence angle indicates that this condition obtains for a reflector that is a solid over fluid interface it is not typical of a solid solid interface the likely candidates for a fluid trapped within the crystalline crust of southern tibet are granitic magma and water brine

tectonics of the northern himalaya since the india asia collision abstract the india asia collision resulted in the construction of the vast himalayan orogen the northern himalayan domain began to experience extensional tectonics since eocene oligocene transition eot when the himalayan orogeny was still in progress major structures generated by the extension in the northern himalaya include the south tibet detachment system stds the north himalayan gneiss domes nhgd and the north south trending rifts nstr the earliest emplacement of syn deformational leucogranite at 36 32 ma along the stds marks the initial transformation from thickening to thinning in the northern himalayan domain at eot the thickening before eot caused crustal partial melting which formed the weak root of the thickened orogen or the so called channel flow this channel flow triggered the collapse of the orogen extrusion of the greater himalayan crystalline complex ghc and the onset of extensional tectonics of the stds the similarities in tectono thermal history and geochemistry of rocks between the stds and nhgd suggest that the formation of the nhgd has a direct relationship to the activity of the stds the extension of stds and the resultant thinning caused further partial melting of the crust leading to the larger scale emplacement of leucogranite during miocene 27 13 ma diapirism of these plutons shaped the domes in nhgd exposing the ghc and the shear zones of the stds in the northern himalaya in gyirong another tectonic transform from extension to shortening occurred after 18 7 ma in dinggye region the stds was offset by the nstr culminating the deformation at 13 ma when the nstr began to be active this indicates another tectonic transform in the northern himalaya at 13 ma when the n s extension of the stds ceased and gave way to e w extension of the nstr marking the end of the peak granitic magmatism in northern himalaya multiple episodes of deformation in the major structures such as the stds mct and nstr and superposition of different structures indicate a multiphase orogenic process in the himalayas in which mountain building and collapse occurred alternately with the formation of nstr and conjugate shear zones in tibet this tectonic scenario was possibly controlled by the india asia convergent rate we propose that the extension was an integral part of the orogenic process rather than a simple marker of the culmination of the orogeny

himalayan metamorphism and deformations in the north himalayan belt southern tibet china abstract the kangmar massif which has been attributed to the north himalayan belt of two mica granites in southern tibet china is shown to be an orthogneiss in the core of a dome which displays an intermediate high pressure type metamorphism himalayan in age 40 to 20 ma and above the main central sheet this metamorphism took place during high shear strain deformation and it is not recorded to the west where north himalayan plutons have developed contact metamorphism

underplating in the himalaya tibet collision zone revealed by the hi climb experiment we studied the formation of the himalayan mountain range and the tibetan plateau by investigating their lithospheric structure using an 800 kilometer long densely spaced seismic array we have constructed an image of the crust and upper mantle beneath the himalayas and the southern tibetan plateau the image reveals in a continuous fashion the main himalayan thrust fault as it extends from a shallow depth under nepal to the mid crust under southern tibet indian crust can be traced to 31 n the crust mantle interface beneath tibet is anisotropic indicating shearing during its formation the dipping mantle fabric suggests that the indian mantle is subducting in a diffuse fashion along several evolving subparallel structures

tertiary structural evolution of the gangdese thrust system southeastern tibet structural and thermochronological investigations of southern tibet xizang suggest that intracontinental thrusting has been the dominant cause for formation of thickened crust in the southernmost tibetan plateau since late oligocene two thrust systems are documented in this study the north dipping gangdese system gts and the younger south dipping renbu zedong system rzt west of lhasa the gangdese thrust juxtaposes the late cretaceous forearc basin deposits of the lhasa block the xigaze group over the tethyan sedimentary rocks of the indian plate whereas east of lhasa the fault juxtaposes the late cretaceous eocene andean type arc the gangdese batholith over tethyan sedimentary rocks near zedong 150 km southeast of lhasa the gangdese thrust is marked by a 200 m thick mylonitic shear zone that consists of deformed granite and metasedimentary rocks a major south dipping backthrust in the hanging wall of the gangdese thrust puts the xigaze group over tertiary conglomerates and the gangdese plutonics north of xigaze and west of lhasa a lower age bound for the gangdese thrust of 18 3 0 5 ma is given by crosscutting relationships the timing of slip on the gangdese thrust is estimated to be 27 23 ma from 40 ar 39 ar thermochronology and a displacement of at least 46 9 km is indicated near zedong the age of the gangdese thrust gt is consistent with an upper age limit of 24 ma for the initiation of movement on the main central thrust in places the younger renbu zedong fault is thrust over the trace of the gt obscuring its exposure the rzt appears to have been active at circa 18 ma but had ceased movement by 8 1 ma the suture between india and asia has been complexly modified by development of the gts rzt and locally strike slip and normal fault systems

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

two sided orogen collision and erosion from the sandbox to the southern alps new zealand in continental collision zones the indentor is of approximately the same vertical dimension as the material being deformed thereby allowing material to move over the indentor and out of the deforming orogen this simple constraint on the height of the indentor differs from that imposed by the bulldozer model of wedge deformation and results in the formation of an orogen with two opposite dipping topographic slopes these two wedges are mechanically coupled and the mechanics are strongly influenced by the erosion conditions imposed on the wedge where the growing mountains perturb prevailing moisture laden winds the erosion pattern is highly asymmetric erosional work being concentrated at the toe of the steep wedge facing the indentor the inboard wedge concentration of erosional energy results in concentration of mechanical energy in the same region and produces the distinctive geological and geophysical patterns associated with continental collision these include rapid uplift and exposure of deep crustal assemblages adjacent to the indentor accompanied by high crustal heat flow and anomalously low seismicity the outcrop pattern along the inboard wedge evolves from that of a crustal cross section currently exposed in the southern alps to one dominated by repetitive nappe structures consisting predominantly of lower crustal units as in the western alps of europe the other wedge the outboard wedge undergoes relatively little erosion is broad with a more gentle surface slope and maintains an outcrop pattern consisting of upper crustal material

a mechanism for syn collisional rock exhumation and associated normal faulting results from physical modelling abstract properly scaled physical modelling of the subduction of continental lithosphere is performed with a three layer lithospheric model the model includes a strong upper crust a weak ductile lower crust and a strong mantle part the lithosphere is underlain by a low viscosity asthenosphere subduction is produced by a piston push force and the pull force from the mantle lithospheric layer which is slightly denser than the asthenosphere the results of the modelling are tested on the example of the himalayas in the experiments all lithospheric layers subduct into the mantle to a depth corresponding to 200 300 km in nature until the upper crustal layer fails in front of the subduction zone forming a first major thrust fault the mct in our interpretation of the himalayas underthrusting along this fault increases relief which upon reaching a height corresponding to several kilometres in nature is removed mechanically by a blade the blade is moved horizontally back and forth throughout the experiment this erosional unloading mechanism causes the previously subducted segment of the buoyant upper crust to slide upward over the weak lower crust at a rate comparable to the subduction rate the rise of the crustal slice accelerates displacement along the crustal thrust while producing a normal sense motion and exhumation of the material from great depth up to 40 50 km depending on the model parameters along the upper surface of the slice the rapid uplift of the crust then ceases and the orogenesis occurs further due to continuing underthrusting along the first major thrust at a certain stage the crust fails again producing a new major crustal thrust the mbt in the himalayas

mantle fluids in the karakoram fault helium isotope evidence abstract the karakoram fault kkf is the 1000 km long strike slip fault separating the western himalaya from the tibetan plateau from geologic and geodetic data the kkf is argued either to be a lithospheric scale fault with hundreds of km of offset at several cm a or to be almost inactive with cumulative offset of only a few tens of kilometers and to be just the upper crustal localization of distributed deformation at depth here we show 3he 4he ratios in geothermal springs along a 500 km segment of the kkf are 3 100 times the normal ratio in continental crust providing unequivocal evidence that a component of these hydrologic systems is derived from tectonically active mantle mantle enrichment is absent along the indus yarlung suture zone isz just 35 km southwest of the kkf suggesting that the mantle fluids flow only within the kkf within the last few ma the kkf must have accessed tectonically active tibetan mantle northeast of the mantle suture which we therefore locate vertically beneath the kkf very close to the surface trace of the isz hence in southwestern tibet indian crust may not now be underthrusting substantially north of the isz even though miocene underthrusting may have placed indian crust north of the isz in the lower half of the tibetan plateau crust this is in significant contrast to central and eastern tibet where underthrust indian material not only forms the lower half of the tibetan crust but is also currently underthrusting for 200 km north of the isz our new constraint on kkf penetration to the mantle allows an improved description of the continuously evolving karakoram fault as a tectonically significant yet perhaps geologically ephemeral lithospheric structure