structural geology and regional tectonic significance of the ramgarh thrust himalayan fold thrust belt of nepal 1 the ramgarh thrust is one of the major fault systems of the himalayan thrust belt in nepal and northern india the ramgarh thrust sheet is 0 2 2 0 km thick and can be traced along strike the entire length of the himalaya in nepal the fault generally places the oldest paleoproterozoic rocks in the lesser himalayan series upon younger lesser himalayan rocks or lower miocene foreland basin deposits regional balanced cross sections suggest that the ramgarh thrust had at least 120 km of initial south vergent displacement subsequently the frontal part of the thrust experienced further slip as the roof thrust for a large duplex in underlying lesser himalayan rocks ramgarh hanging wall strata are greenschist grade phyllite quartzite and augen gneiss all of which locally exhibit phyllonitic and mylonitic fabrics that indicate a top to the south sense of shear structural fabrics in the ramgarh thrust sheet are generally parallel to the fabrics in rocks above and below the thrust sheet regional and local mapping of the ramgarh thrust in nepal demonstrates that the fault always places a hanging wall flat upon a footwall flat except where local lateral ramps complicate its geometry similarly the structurally overlying main central thrust always places a hanging wall flat in greater himalayan series rocks upon the regionally flat ramgarh thrust sheet these geometric relationships preclude kinematic and thermal models that elevate greater himalayan and lower lesser himalayan rocks along high angle thrust ramps in the vicinity of the present traces of the ramgarh and main central thrust faults instead the corresponding footwall ramps for these thrusts must be located more than 100 km north of the current trace of the main central thrust the present steep dips of the ramgarh and main central thrust sheets can be attributed to tilting during emplacement of structurally lower thrust sheets within a large antiformal duplex that occupies most of the lesser himalayan zone the ramgarh thrust sheet overlaps a bed length of at least 100 km in lower miocene foreland basin deposits indicating that a significant amount of displacement on the thrust must have occurred after 15 ma growth of the lesser himalayan duplex and additional slip on the frontal part of the ramgarh thrust occurred from 12 to 5 ma the presence of a major greenschist grade metasedimentary thrust sheet composed of lesser himalayan rocks directly below the main central thrust suggests that the famous inverted metamorphism in this region is a result of structural inversion similarly the concept of a broad zone of intense shear strain related exclusively to emplacement of the main central thrust sheet is probably invalid in nepal

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

back folds in the core of the himalayan orogen an alternative interpretation the hanging wall of the south tibetan detachment system in the central nepal himalaya is characterized by regional scale northeast verging folds classically interpreted as gravity induced structures developed during down to the north extensional shearing along the detachment system new structural observations and balanced cross sections of the tethyan sedimentary sequence in the kali gandaki area and new u pb geochronology support an alternative interpretation the northeast verging folds developed before ductile extensional shearing along the detachment system thereby recording some of the earliest contraction of this part of the orogen we propose a new model in which the northeast verging folds of the kali gandaki area represent the northern part of a late eocene to oligocene contractional fan structure

mapping granite and gneiss in domes along the north himalayan antiform with aster swir band ratios a series of five gneiss domes that are exposed along the axis of the north himalayan antiform 50 km south of the indus tsangpo suture southern tibet are characterized by cores of cambrian gneiss and neoproterozoic paleozoic schists and migmatites often intruded by tertiary granites and mantled by metamorphosed equivalents of tethyan sedimentary successions images from the swir short wave infrared bands recorded by the aster advanced spaceborn thermal emission and reflection radiometer sensor on the earth observation system terra satellite reveal with unprecedented clarity the nature of the cores of the domes and the extent of the surrounding mantles of metamorphic rock using band ratio methods we distinguish tertiary granite from cambrian gneiss on the basis of the presence of 5 10 muscovite in the granite this methodology is ideally suited to the mapping of a relatively inaccessible area with excellent exposure but limited ground truth with the exception of the well studied kangmar dome our images of the cores of the domes and their associated mantles bear little resemblance to the published geological maps of the region and reveal hitherto unrecognized exposures of granites the true disposition of the domes within the north himalayan antiform have now been established revealing a systematic westward increase in the granite component of himalayan age within the domes across the study area

fabric transitions in the saxony granulite terrain abstract quartz fabric transitions reported from the saxony granulite terrain and from the nearby ore mountains are interesting because of the changes in the pattern types and the variation in the opening angles of the fabric skeletons a short summary is presented of salient features in research documented in the east german literature between 1961 and 1978 the changes in fabric skeleton with decreasing metamorphic grade are proposed as mainly consequent to the deactivation of prism c glide systems as temperature decreases fabrics indicative of the appearance of the c burgers vector might be diagnostic for deformation in regimes of comparatively high temperatures or low strainrates or for deformation under conditions such that there is a relatively high kinetically effective hydroxyl content in the quartz structure

middle late miocene 10 ma formation of the main boundary thrust in the western himalaya three independent data sets from northwestern india and pakistan suggest initial displacement along 1000 km of the main boundary thrust prior to 10 ma at least 5 m y earlier than previously reported regionally extensive changes in the depositional characteristics and rates of the foreland basinfill between 11 and 9 5 ma are interpreted to reflect new hinterland loading due to the formation of the main boundary thrust sediment accumulation rates sandstone siltstone ratios and thickness and amalgamation of individual sandstone bodies all substantially increase after 11 ma in well dated stratigraphic sections from pakistan to nepal across the indo gangetic foreland basin in the himachal pradesh reentrant of northwestern india a newly discovered 8 7 ma conglomerate derived from the hanging wall of the main boundary thrust indicates that source area uplift and denudation must have occurred prior to 9 ma and probably prior to 10 ma assuming a gravelprogradationrateof3cm yr threeapatitefission trackages fromstructuresattheleadingedgeofthemainboundarythrustin the kohat region of northwest pakistan indicate that rapid cooling below 105 c1 2 m y earlier these data indicate that the main boundary thrust in the western himalaya formed synchronously along strike in the middle late miocene has a displacement rate of 10 mm yr and has a displacement history that is coeval with late displacement on the main central thrust

tectonometamorphic evolution of the himalayan metamorphic core between the annapurna and dhaulagiri central nepal the metamorphic core of the himalaya in the kali gandaki valley of central nepal corresponds to a 5 km thick sequence of upper amphibolite facies metasedimentary rocks this greater himalayan sequence ghs thrusts over the greenschist to lower amphibolite facies lesser himalayan sequence lhs along the lower miocene main central thrust mct and it is separated from the overlying low grade tethyan zone tz by the annapurna detachment structural petrographic geothermobaro metric and thermochronological data demonstrate that two major tectonometamorphic events charac terize the evolution of the ghs the first eohimalayan episode included prograde kyanite grade metamorphism during which the ghs was buried at depths greater than c 35 km a nappe structure in the lowermost tz suggests that the eohimalayan phase was associated with underthrusting of the ghs below the tz a c 37 ma 40ar 39ar hornblende date indicates a late eocene age for this phase the second neohimalayan event corresponded to a retrograde phase of kyanite grade recrystallization related to thrust emplacement of the ghs on the lhs prograde mineral assemblages in the mct zone equilibrated at average t 880 k 610 c and p 940 mpa 35 km probably close to peak of meta morphic conditions slightly higher in the ghs final equilibration of retrograde assemblages occurred at average t 810 k 540 c and p 650 mpa 24 km indicating re equilibration during exhumation controlled by thrusting along the mct and extension along the annapurna detachment these results suggest an earlier equilibration in the mct zone compared with higher levels as a consequence of a higher cooling rate in the basal part of the ghs during its thrusting on the colder lhs the annapurna detachment is considered to be a neohimalayan synmetamorphic structure representing extensional reactivation of the eohimalayan thrust along which the ghs initially underthrust the tz within the upper ghs a metamorphic discontinuity across a mylonitic shear zone testifies to significant late to post metamorphic out of sequence thrusting the entire ghs cooled homogeneously below 600 700 k 330 430 c between 15 and 13 ma middle miocene suggesting a rapid tectonic exhumation by movement on late extensional structures at higher structural levels

out of sequence deformation and expansion of the himalayan orogenic wedge insight from the changgo culmination south central tibet 1 the changgo culmination one of the north himalayan domes in south central tibet consists of a multiphase granite core surrounded by a deformed metasedimentary carapace the granitic core records general non coaxial shear with a top to the south sense shear component the contact between the core and the carapace is a shear zone characterized by general non coaxial shear with a top to the north shear sense interpreted to be the northern continuation of the south tibetan detachment system stds the shear zone contains lenses of leucogranite dated at 35 4 ma this is interpreted to reflect eocene crustal thickening coeval with the earliest shortening event recorded in the carapace the main phase of the changgo granite crystallized at 23 5 ma while undeformed aplite dikes the youngest phase observed in the granite were intruded at 22 1 ma aplite dikes crosscut the main deformation fabric within the changgo granite therefore that deformation and associated south directed shearing must have ended between 23 5 ma and 22 1 ma the dikes are strained within the stds indicating that final displacement along the stds must post date 22 ma yet be older than 18 4 ma the cooling age of muscovite in the shear zone it is proposed that the exhumation of the changgo culmination is related to tectonically driven erosion in response to crustal thickening and rebuilding of the orogenic critical taper wedge subsequently deformation in the wedge migrated toward the foreland expanding the orogenic wedge laterally and moving the locus of displacement from the main central thrust structurally downward to the main boundary thrust

first report of lower permian basalts in south tibet tholeiitic magmatism during break up and incipient opening of neotethys abstract the neo tethys ocean began to form at early permian times when continental flood basalts were emplaced in various areas of the newly formed indian passive margin exposed today in the so called tibetan sedimentary zone of the himalaya lower permian mafic volcanic rocks which have long been known from various himalayan localities from kashmir to arunachal pradesh are here for the first time reported to occur also in south tibet bhote kosi basalts of the gyirong county the basalts unconformably overlie lowermost permian diamictites with locally intervening black shales and debris flow deposits and are followed in turn by chert bearing quartzarenites and silty to phosphatic marls yielding brachiopods of roadian wordian age the age of the lavas can thus be bracketed as late early permian post sakmarian and pre roadian the geochemistry of these subalkalic tholeiites akin to morbs testifies to their similarity not only with the adjacent nar tsum spilites of central nepal but also with the panjal traps and abor volcanics of the western and eastern himalayas respectively the geochemical signature of lower permian volcanic rocks is in fact uniform all along the himalayan range and markedly different from that of basaltic rhyolitic alkalic products sporadically emplaced during the previous rifting stage rift volcanism in the tethys himalaya began in the early carboniferous and came to an end in sakmarian times in the early permian initial submergence of the rift shoulders and sediment starvation were followed by tholeiitic magmatism which is therefore interpreted as following break up and incipient sea floor spreading in the neotethys ocean roughly contemporaneous emplacement of continental flood basalts of similar geochemical signature along a 2000 km long rift axis would in fact suggest extensive mantle melting at the transition from continental rifting to break up and opening of the neotethys between northern gondwana and the peri gondwanian blocks

middle crustal ductile deformation patterns in southern tibet insights from vorticity studies in mabja dome kinematic kinematic vorticity wm and deformation temperature analyses were performed to test the hypothesis that mid crustal rocks exposed in mabja dome southern tibet were penetratively deformed within a southward flowing mid crustal channel during the late eocene early oligocene to early miocene outcrop and thin section kinematic indicators show a downward transition from mixed top n and top s shear in chloritoid and garnet zone rocks through dominantly top s shear in garnet and kyanite zone rocks to solely top s shear in staurolite zone and deeper rocks along mineral elongation lineation parallel transects wm in schists and orthogneisses decreases with structural depth from w0 80 w40 pure shear to w0 55 w63 pure shear deformation temperature increases from w450 ci n

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

structural evolution of the tethyan sedimentary sequence in the annapurna area central nepal himalaya the tethyan sedimentary sequence of central nepal has been affected by three phases of folding d1 d 2 and d4 with contrasting vergence interspersed by two major extensional events d3 and d5 the north verging second phase folds f2 are the dominant deformation features preserved in the studied area these d2 structures indicate a vertical structural zonation that is interpreted to mark a preserved d2 strain gradient increasing with depth to a presumed north verging decollement plane at an approximate depth of 10 km palinspastic bed length restoration of the sequence implies a minimum 43 shortening and a minimum 150 vertical thickening due to d2 deformation the d4 event overprints the d3 annapurna detachment and manifests itself as post peak metamorphic south verging kink folds and spaced crenulation cleavage in the structurally lowest level observed d4 is associated with thrust sense meter scale fault zones the youngest extensional event d5 marks the initiation and eventual southward propagation of the thakkhola graben a newly named northeast dipping normal fault the lupra fault is described in the early stages of extension the lupra fault may have acted as a southern termination of the graben system dropping down cretaceous rocks and high level structural features in its hanging wall it is suggested that as the extension progressed southward the lupra fault was itself down faulted in the graben and the extent of the thakkhola graben reached the present position of the annapurna detachment q 2003 elsevier ltd all rights reserved

an empirical evaluation of the argon diffusion geometry in muscovite the temperature time significance of 40ar 39ar muscovite ages is difficult to evaluate quantitatively because of ambiguities in the choice of an appropriate model diffusion geometry the effective diffusion dimension and large uncertainties in experimentally determined diffusion parameters laser 40ar 39ar studies of natural 40ar concentration gradients suggest that argon loss in muscovite occurs primarily via cylindrical volume diffusion commonly with an effective diffusion dimension equal to the crystal diameter the present study tests the cylinder model geometry and the control of physical grain size on the effective diffusion dimension by measuring the anisotropy of a natural40ar diffusion gradient parallel and perpendicular to 001 in a single euhedral muscovite phenocryst the new data combined with previous studies clearly indicate loss of 40ar via cylindrical volume diffusion and strongly suggest that loss of 40ar across the 001 sheets slab diffusion geometry is minimal in undeformed crystals

oligocene miocene middle crustal flow in southern tibet geochronology of mabja dome new u pb zircon monazite 40 ar 39 ar and apatite fission track ages provide constraints on the timing of formation and exhumation of the mabja dome southern tibet shed light on how this gneiss dome formed and provide important clues on the tectonic evolution of middle crustal rocks in southern tibet zircons from a deformed leucocratic dyke swarm yield a u pb age of 23 1 0 8 ma providing the first age constraint on the timing of middle crustal ductile horizontal extension in the north himalayan gneiss domes zircons and monazite from a post tectonic two mica granite yield ages of 14 2 0 2 ma and 14 5 0 1 respectively indicating that vertical thinning and subhorizontal stretching had ceased by the middle miocene mica 40 ar 39 ar ages from schists and orthogneisses increase structurally down section from 12 85 0 13 ma to 17 0 0 19 ma and then decrease at the deepest structural levels to 13 29 0 09 ma micas from the leucocratic dyke swarm and post tectonic two mica granites yield similar 40 ar 39 ar cooling ages of 13 48 0 13 to 12 84 0 08 ma the low temperature steps of potassium feldspar 40 ar 39 ar spectra yield ages of c 11 0 12 5 ma and apatite fission track analyses indicate the dome uniformly cooled below c 115 c at 9 5 0 6 ma based on these data calculated average cooling rates across the dome range from c 40 60 c million years in schist and orthogneiss and following emplacement of the leucocratic dyke swarm to c 350 c million years following emplacement of the two mica granites the mylonitic foliation peak metamorphic isograds and mica 40 ar 39 ar chrontours are domed whereas the low temperature step potassium feldspar 40 ar 39 ar and apatite fission track chrontours are not suggesting that doming occurred between 13 0 and 12 5 ma and at temperatures between 370 and 200 c our new ages along with field structural and metamorphic data indicate that the domal geometry observed at mabja developed by middle miocene southward directed thrust faulting upward and southward along a north dipping ramp above cold tethyan sediments the structural metamorphic and geochronologic histories documented at mabja dome are similar to those of kangmar dome suggesting a common mode of occurrence of these events throughout southern tibet vertical thinning and horizontal stretching metamorphism generation of migmatites and emplacement of leucogranites in the domes of southern tibet are synchronous with similar events in the greater himalayan sequence that underlie the high himalaya these relations are consistent with previously proposed models for a ductile middle crustal channel bounded above by the south tibetan detachment system and below by the main central thrust in the high himalaya that extended northward beneath southern tibet

contact metamorphism in the malashan dome north himalayan gneiss domes southern tibet an example of shallow extensional tectonics in the tethys himalaya combined petrographic structural and geochronological study of the malashan dome one of the north himalayan gneiss domes reveals that it is cored by a miocene granite the malashan granite that intruded into the jurassic sedimentary rocks of tethys himalaya two other granites in the area are referred to as the paiku and cuobu granites new zircon shrimp u pb and muscovite and biotite 40ar 39ar dating show that the paiku granite was emplaced during 22 2 16 2 ma average 19 3 3 9 ma and cooled rapidly to 350 400 c at around 15 9 ma whole rock granite chemistry suggests the original granitic magma may have formed by muscovite dehydration melting of a protolith chemically similar to the high himalayan crystalline sequence abundant calcareous metasedimentary rocks and minor garnet staurolite biotite muscovite andalusite schists record contact metamorphism by three granites that intruded intermittently into the jurassic sediments between 18 5 and 15 3 ma two stages of widespread penetrative ductile deformation d1 and d2 can be defined microstructural studies of metapelites combined with geothermobarometry and pseudosection analyses yield p t conditions of 4 8 0 8 kbar at 550 50 c during a non deformational stage between d1 and d2 and 3 1 4 1 kbar at 530 575 c during syn to post d2 the pressure estimates for the syn to post d2 growth of andalusite suggest relatively shallow depth of 15 2 km extensional ductile deformation that took place within a shear zone of the south tibetan detachment system close temporal association between intrusion of the malashan granite and onset of d2 suggests extension may have been triggered by the intrusion of the malashan granite

active formation of chaos terrain over shallow subsurface water on europa the galileo spacecraft revealed a number of chaos regions on jupiter s moon europa where the surface terrain appears to have been disrupted from below in many places the surface contains sharp edged blocks or rafts of ice that have at some point been flipped or rotated some characteristics of these regions have been hard to explain such as the fact that the archetypal conamara chaos stands above its surroundings and contains matrix domes schmidt et al apply lessons learned from analogous processes within earth s subglacial volcanoes and ice shelves to an analysis of archival data that suggests chaos terrain forms above liquid water lenses that are perched only 3 kilometres deep within the ice shell the data suggest that ice water interactions and freeze out give rise to the varied morphology of chaos terrains implying that more water is involved than has been previously appreciated for instance the sunken topography of thera macula a large chaos area may indicate that europa is actively resurfacing over a lens comparable in volume to north america s great lakes europa the innermost icy satellite of jupiter has a tortured young surface1 2 3 4 and sustains a liquid water ocean1 2 3 4 5 6 below an ice shell of highly debated thickness1 2 3 4 5 7 8 9 10 quasi circular areas of ice disruption called chaos terrains are unique to europa and both their formation and the ice shell thickness depend on europa s thermal state1 2 3 4 5 7 8 9 10 11 12 13 14 15 16 17 no model so far has been able to explain why features such as conamara chaos stand above surrounding terrain and contain matrix domes10 18 melt through of a thin few kilometre shell3 7 8 is thermodynamically improbable and cannot raise the ice10 18 the buoyancy of material rising as either plumes of warm pure ice called diapirs1 9 10 11 12 13 14 15 or convective cells16 17 in a thick 10 kilometres shell is insufficient to produce the observed chaos heights and no single plume can create matrix domes10 18 here we report an analysis of archival data from europa guided by processes observed within earth s subglacial volcanoes and ice shelves the data suggest that chaos terrains form above liquid water lenses perched within the ice shell as shallow as 3 kilometres our results suggest that ice water interactions and freeze out give rise to the diverse morphologies and topography of chaos terrains the sunken topography of thera macula indicates that europa is actively resurfacing over a lens comparable in volume to the great lakes in north america

sedimentary history of the tethyan basin in the tibetan himalayas after an epicontinental phase the sedimentary rocks in the tibetan himalayas document a complete wilson cycle of the neo tethyan tethys ill evolution between the gondwana supercontinent and its northward drifting margin lhasa block from the late permian to the eocene during the triassic rift stage the basin was filled with a huge clastic dominated sediment wedge with up to 5 000 m of flysch in the northern zone widespread deltaic clastics and shallow water carbonates of late norian to earliest jurassic age in the southern zone mark in conjunction with decreasing tectonic subsidence the transition to the drift stage some 4 500 m of jurassic and early cretaceous shallow water carbonates and siliciclastics accumulated on the tethyan indian passive margin deepening upward sequences with condensed beds at their tops alternate with repeated progradational packages of shelf sediments extensive abyssal sediments with basaltic volcanics in the northern deep water zone reflect continued ocean spreading and thermal subsidence paleomagnetic data gained separately for the northern indian plate and the lhasa block indicate that the neo tethys reached its maximum width about 110 ma ago with a spreading rate of 4 8 cm year before it commenced to close again during the remnant basin stage in the late cretaceous and paleogene a shallowing upward megasequence capped by a carbonate platform developed in the southern inner shelf realm in the northern slope basin plain zone turbidites and chaotic sediments derived from both the acretionary wedge and the steepening slope of the passive margin accumulated the depositional center of the remnant basin shifted southward as a result of flexural subsidence and southward overthrusting the sediments from the triassic to the paleogene are tentatively subdivided into five mega sequences which are controlled mainly by regional tectonics climatic influence e g carbonate deposition due to northward plate motion is partially subdued by terrigenous input and or increased water depth during the oligocene and miocene crustal shortening led to rapid uplift and the deposition of fluvial molasse in limited basins

u th pb systematics and ages of himalayan leucogranites south tibet abstract the age and origin of five leucogranites from the high and tethys himalaya and two country rock gneisses were investigated by u pb dating of zircon fractions and single grains and fractions of monazite additionally th u concentrations in whole rock powders and isotopic compositions of pb in leached k feldspars were determined monazites yield ages of 16 8 0 6 m y for the nialam migmatite granite 15 1 0 5 m y for the lhagoi kangri granite 14 3 0 6 m y for a granite from mt everest and 9 8 0 7 m y and 9 2 0 9 m y for two varieties of the maja granite these data together with monazite ages of 21 9 0 2 and 24 0 0 4 m y determined earlier on the makalu granite 1 substantiate a period of intracontinental granite emplacements from 24 to 9 m y ago i e from uppermost oligocene to late miocene times such a period of plutonic activity is consistent with the view that all these granites result from intracrustal melting following the collision of india with eurasia furthermore the individual ages together with structural relationships between granites and country rocks suggest that granite formation and tectono metamorphism occurred as alternating and strongly related processes with a periodicity of 7 to 9 m y inherited lead components present in all granite zircons point to large proportions of precambrian material in the magma source regions up to 2200 m y old th u systematics between monazite and country rocks indicate that u has been leached from most of the granites after crystallisation of monazite zircon dating of the kangmar granite gneiss which occurs in a window through the tethys himalayan sediments shows that this pluton transformed to a gneiss during the alpine orogeny crystallised in lowermost palaeozoic times 562 4 m y ago

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

causes and consequences of protracted melting of the mid crust exposed in the north himalayan antiform gneiss domes exposed by the north himalayan antiform in southern tibet provide evidence for protracted melting of the mid crust during convergent tectonics anatexis was initiated during underthrusting and recurred during the southwards extrusion of a low viscosity crustal sheet sr nd isotope systematics establish that the cores of the domes dominated by cambrian gneiss form a window of the high himalayan crystalline series hhcs indian plate combined trace element and isotopic analyses from 4 two mica granites that intrude both core and the metasedimentary mantle of the domes indicate an origin from melting of the high himalayan crystalline series by the incongruent breakdown of muscovite and to a lesser extent biotite at temperatures from 680 to 760 c u pb dating of accessory phases extends the known period of granite magmatism from the late oligocene to mid miocene a ca rich garnet bearing leucogranite of oligocene age 27 5 ma provides a rare insight into the pre miocene history of the underthrust indian plate reflecting prograde melting at greater depths during a period of crustal thickening the origin of the miocene granites 15 10 ma can be modeled from the exhumation of an extruding channel at a rate of ca 1 mm year constrained by ar isotope data melting over a protracted time scale along the leading edge of the indian plate less than 50 km from the suture accelerated horizontal shortening within the collision zone and provided a positive feedback mechanism between magmagenesis and deformation

ductile extrusion of the higher himalayan crystalline in bhutan evidence from quartz microfabrics quartz textures measured from deformed quartz tectonites within the lesser himalaya and higher himalaya crystalline of bhutan show similar patterns orientation and distribution of the quartz crystallographic axes were used to confirm the regional shear sense the asymmetry of c axis and a axis patterns consistently indicates top to the south shearing the obliquity of the texture and the inferred finite strain plane strain to moderately constrictional suggest the strain regime had a combination of rotational and irrotational strain path in most of the samples from the bhutan himalaya the inferred deformation mechanisms suggest moderate to high temperature conditions of deformation that produced the observed crystallographic preferred orientation much higher temperature of deformation is indicated in the quartz veins from a leucogranite the observed ductile deformation is pervasively developed in the rocks throughout the investigated area the intensity of deformation increases only slightly in the vicinity of the main central thrust simultaneous southward shearing within a large part of the higher himalaya crystalline near and above the main central thrust and normal faulting across the south tibetan detachment is explained by the tectonically induced extrusion of a ductily deforming wedge the process of extrusive flow suggested here can be approximated quantitatively by channel flow models that have been used to describe subduction zone processes channel flow accounts for some observed phenomena in the himalayan orogen such as inverted metamorphic sequences near the main central thrust not related to an inversion of isotherms and the syntectonic emplacement of leucogranites into the extruding wedge locally leading to an inversion of isotherms due to heat advection

evolution of north himalayan gneiss domes structural and metamorphic studies in mabja dome southern tibet field structural and metamorphic petrology investigations of mabja gneiss dome southern tibet suggest that contractional extensional and diapiric processes contributed to the structural evolution and formation of the domal geometry the dome is cored by migmatites overlain by sillimanite zone metasedimentary rocks and orthogneiss metamorphic grade diminishes upsection and is defined by a series of concentric isograds evidence for three major deformational events two older penetrative contractional and extensional events and a younger doming event is preserved metamorphism migmitization and emplacement of a leucocratic dike swarm were syntectonic with the extensional event at mid crustal levels metamorphic temperatures and pressures range from 500 8c and 150 450 mpa in chloritoid zone rocks to 705 65 8c and 820 100 mpa in sillimanite zone rocks we suggest that adiabatic decompression during extensional collapse contributed to development of migmatites diapiric rise of low density migmatites was the driving force at least in part for the development of the domal geometry the structural and metamorphic histories documented in mabja dome are similar to kangmar dome suggesting widespread occurrence of these events throughout southern tibet q 2004 elsevier ltd all rights reserved

the chako antiform a folded segment of the greater himalayan sequence nar valley central nepal himalaya recent and previously published mapping in the nar valley north of the annapurna massif in central nepal recognised an enigmatic metamorphic culmination of the greater himalayan sequence surrounded by rocks correlated with the tethyan sedimentary sequence the map area is re interpreted as two structural levels within the greater himalayan sequence the lower level is correlated to the well studied greater himalayan sequence of the annapurna region based on characteristic rock types high strain zones with south verging shear sense indicators and high grade metamorphism the rocks of the upper level previously mapped as the sub greenschist or zeolite facies tethyan sedimentary sequence are garnet bearing schists petrography and garnet biotite thermometry suggest the upper level equilibrated at amphibolite facies 500 650 8c these results support the recent contention that the upper level rocks are a component of the greater himalayan sequence and thus indicate that components of the himalayan metamorphic core vary considerably along strike structural metamorphic and age constraints are integrated into a cohesive regional tectonometamorphic model the lower and upper levels both experienced d1 deformation and peak metamorphism before w20 ma the lower and upper levels were juxtaposed along the synmetamorphic chame detachment after 20 ma during retrograde metamorphism after w19 ma the phu detachment placed the unmetamorphosed tethyan sedimentary sequence above the lower and upper levels the entire package was then subsequently folded after 19 ma by a non cylindrical antiform synform pair with a w25 km wavelength creating an apparent dome the proposed tectonometamorphic model reconciles previously contradictory interpretations of the transition between metamorphic core and overlying sediments in the nar valley q 2005 elsevier ltd all rights reserved

simultaneous miocene extension and shortening in the himalayan orogen the south tibetan detachment system separates the high grade metamorphic core of the himalayan orogen from its weakly metamorphosed suprastructure it is thought to have developed in response to differences in gravitational potential energy produced by crustal thickening across the mountain front geochronologic data from the rongbuk valley north of qomolangma mount everest in southern tibet demonstrate that at least one segment of the detachment system was active between 19 and 22 million years ago an interval characterized by large scale crustal thickening at lower structural levels these data suggest that decoupling between an extending upper crust and a converging lower crust was an important aspect of himalayan tectonics in miocene time

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

first field evidence of southward ductile flow of asian crust beneath southern tibet there is lively debate on whether asian plate material was involved in southward flow of mid lower crust in a ductile channel beneath southern tibet one argument against such involvement is the apparent absence of material derived from asian lithosphere within the high himalayan series indian plate that could represent the putative channel a north south trending mid miocene dike swarm that intrudes the tethyan sedimentary cover of the sakya gneiss dome indian plate yields new sr nd isotopic data 87sr 86sr 0 7071 0 7079 nd 4 to 6 indicating that these melts share the same source as miocene dacitic dikes from north of the indus tsangpo suture moreover dikes on both sides of this suture represent crustal melts derived largely from mid lower crust of the asian plate exposed today as the nyainqentanglha gneisses that underlie the gangdese batholith we infer that melting of the asian lithosphere extended south of the surface trace of the suture requiring southward propagation of anatectic asian middle crustal material during the miocene the emplacement ages of the southern dike swarm 12 9 ma thus delimit the timing of active southward ductile flow of asian material

40ar 39ar thermochronology of the kampa dome southern tibet implications for tectonic evolution of the north himalayan gneiss domes structural and thermochronological studies of the kampa dome provide constraints on timing and mechanisms of gneiss dome formation in southern tibet the core of kampa dome contains the kampa granite a cambrian orthogneiss that was deformed under high temperature sub solidus conditions during himalayan orogenesis the kampa granite is intruded by syn tectonic leucogranite dikes and sills of probable oligocene to miocene age overlying paleozoic to mesozoic metasedimentary rocks decrease in peak metamorphic grade from kyanite staurolite grade at the base of the sequence to unmetamorphosed at the top the kampa shear zone traverses the kampa granite metasediment contact and contains evidence for high temperature to lowtemperature ductile deformation and brittle faulting the shear zone is interpreted to represent an exhumed portion of the south tibetan detachment system biotite and muscovite 40 ar 39 ar thermochronology from the metasedimentary sequence yields disturbed spectra with 14 22 0 18 to 15 54 0 39 ma cooling ages and concordant spectra with 14 64 0 15 to 14 68 0 07 ma cooling ages petrographic investigations suggest disturbed samples are associated with excess argon intracrystalline deformation mineral and fluid inclusions and or chloritization that led to variations in argon systematics we conclude that the entire metasedimentary sequence cooled rapidly through mica closure temperatures at 14 6 ma the kampa granite yields the youngest biotite 40 ar 39 ar ages of 13 7 ma immediately below the granite metasediment contact we suggest that this age variation reflects either varying mica closure temperatures re heating of the kampa granite biotites above closure temperatures between 14 6 ma and 13 7 ma or juxtaposition of rocks with different thermal histories our data do not corroborate the inverse mica cooling gradient observed in adjacent north himalayan gneiss domes instead we infer that mica cooling occurred in response to exhumation and conduction related to top to north normal faulting in the overlying sequence top to south thrusting at depth and coeval surface denudation

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

north south extension in the tibetan crust triggered by granite emplacement we combine zircon sensitive high resolution ion microprobe u pb spot dating and mica 4 0 ar 3 9 ar plateau ages with field geological and geochemical constraints from the malashan area of southern tibet to show that the deformed granite core of the north himalayan metamorphic domes in this area is not indian basement but was intruded and deformed during the himalayan orogeny microstructural observations reveal that a transition from top to the south thrust related to top to the north extension related deformation occurred during granite intrusion and related metamorphism this suggests that intrusion triggered the onset of extensional tectonics in the tibetan middle to upper crust expected positive feedback mechanisms between decompression melting leading to more intrusion and more extensional deformation suggest that this mechanism may have been important on a regional scale

40ar 39ar age constraints on deformation and metamorphism in the main central thrust zone and tibetan slab eastern nepal himalaya in eastern nepal structural and petrologic observations suggest that movement on the main central thrust mct the development of an inverted metamorphic sequence and leucogranite plutonism were genetically related samples from across the mct zone and its hanging wall the tibetan slab were collected from four locations in the everest region for 40ar 39ar studies 1 the mct zone 2 the lower tibetan slab 3 the upper tibetan slab away from intrusive rocks and 4 the upper tibetan slab adjacent to and including leucogranitic material within the mct zone muscovite and hornblende yield cooling ages of 12 0 0 2 ma and 20 9 0 2 ma respectively k feldspar gives a minimum age of 8 0 0 2 ma closure temperature tc 220 15 c in the lower tibetan slab a sample from a sheared pegmatite yields muscovite and biotite isochron ages of 7 7 0 4 ma and 7 5 0 6 ma and a k feldspar minimum age of 6 4 0 8 ma tc 225 20 c an adjacent gneiss yields a 9 1 0 2 ma biotite isochron and a k feldspar minimum age of 3 6 0 2 ma tc 210 50 c in the upper tibetan slab samples collected 200 m from visible intrusives yield a biotite isochron age of 20 2 0 2 ma and a complex hornblende age spectrum suggestive of excess argon at the fourth location an augen gneiss a pegmatite and a tourmaline bearing leucogranite all in mutual contact yield indistinguishable mineral ages the average biotite and muscovite isochron ages for these samples are 17 0 1 4 and 16 6 0 2 ma respectively the minimum age for k feldspar from the leucogranite is 15 5 1 8 ma as thermobarometric data from the mct zone in this area suggest synmetamorphic deformation at temperatures of 500 550 c the mct hornblende tc 500 c dates this event at 21 ma diffusion experiments on hornblendes from the mct zone provide data which support a maximum duration of peak metamorphic temperatures of 2 ma biotite ages as old as 20 ma from the upper tibetan slab near leucogranites indicate that leucogranite intrusion was essentially coeval with deformation and metamorphism in the mct zone very young ages in a ductile shear zone in the lower tibetan slab suggest that there has been deformation within the tibetan slab that postdates major movement on the mct

kinematics of the crust around the ama drime massif southern tibet constraints from paleomagnetic results abstract secondary magnetic remanences are used to study late orogenic rotation and tilting on a e w transect in southern tibet at 28 5 n sampling was performed on both sides of the ama drime massif adm remanent magnetization resides in pyrrhotite formed during metamorphism and is related to the last cooling event through 325 c curie temperature of pyrrhotite the existence of antipodal ptrms demonstrates that the chrm is of thermoremanent origin and was acquired during cooling of the tethyan metasediments penetrative deformation was still going on in ronghbuk area until ca 250 c whereas it was already completed in dinggye area significant departures from the expected field direction during the age of remanence acquisition cannot be interpreted by consistent vertical axis rotation of blocks antagonist deviations on both sides of the adm are explained as tiltings around an horizontal axis striking n20 these tiltings are estimated to be younger than 13 12 ma and they are related to the adm exhumation initiated since 33 ma

a new type of shelf margin deposit rigid microbial sheets and unconsolidated grainstones riddled with meter scale cavities middle cambrian microbial limestone contains a network of unusual predominantly horizontal cavities up to 2 m in length and 0 5 m in height the microbialite experienced rapid syndepositional lithification but adjacent grainstone sediments remained unlithified during deposition this juxtaposition contributed to sediment instability resulting in fracturing and brecciation of the lithified microbialite while unconsolidated grainstones underwent slumping and injection into some cavities remaining space within the cavities was colonized by a series of encrustations thin crusts 2 8 mm of laminated algal mats followed by several generations of calcified renalcis like cyanobacteria up to 45 mm thick remaining void space was partially filled by internal sediment and then sequentially occluded by banded radiaxial fibrous calcite herringbone calcite and finally saddle dolomite cements the radiaxial and herringbone calcite cements precipitated from porewaters derived from seawater that became anoxic through the breakdown of organic matter in the microbialite noteworthy is the presence of herringbone calcite cement not as a seafloor precipitate but as an early cavity fill we propose that the unusual bedding parallel fractures were caused by gravity collapse along a shallow platform margin coeval foreslope sediments show syndepositional slumping faulting and mass flow deposits these redeposited sediments contain boulders of microbialite and grainstone of platform margin provenance 1999 elsevier science b v all rights reserved

locking of southward extrusion in favour of rapid crustal scale buckling of the greater himalayan sequence nar valley central nepal the south tibetan detachment system stds bounds the upper limit of the greater himalayan sequence ghs which consists of the exhumed middle crust of the himalaya in the annapurna range of central nepal the ghs comprises a sequence of amphibolite grade augen gneisses with a 3 5 km thick leucogranite at the higher structural levels manaslu granite two major low angle normal sense shear zones have been mapped the chame detachment has similar grade metamorphic rocks above and below and is interpreted as a ductile shear zone wholly within the ghs the phu detachment is a ductile brittle normal fault which wraps around the top of the manaslu leucogranite and defines the uppermost youngest strand of the stds placing folded unmetamorphosed palaeozoic rocks of the tethyan sedimentary sequence above the ghs our data indicate that ductile flow and southward extrusion of the ghs terminated with cessation of movement on the brittle upper strand of the phu detachment at c 19 ma which was followed almost immediately by crustal scale buckling argon thermochronology reveals that the bulk of the metamorphic rocks and lower portions of the tethyan sedimentary sequence in the nar valley cooled through the hornblende biotite and muscovite closure temperatures at c 16 ma suggesting very rapid cooling rates the thermochronology results indicate that this cooling occurred 2 3 million years earlier than in the frontal part of the extruded ghs although the extrusion in the frontal part of the ghs must have locked at the same time as in the nar valley the exhumation there was slower and most probably only assisted by erosion rather than by rapid folding as is the case in the nar valley this buckling indicates a step northward in deformation within the himalayan belt which may be a response to a lower deforming taper geometry in the foreland

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

the mechanical paradox of large overthrusts the mechanical paradox of large overthrusts originates with a fallacious assumption in the conceptual model upon which mechanical analyses of overthrust faulting have been based the notion that the maximum width of a sheet of rock that may be displaced along an overthrust fault can be determined by using the mohr coulomb failure criterion to compare the strength of the rocks with the stress required to overcome the total frictional resistance to slip and the total cohesion or adhesion over the entire fault surface tacitly assumes that slip is initiated and occurs simultaneously over the entire fault surface this unstated and untenable assumption which is incompatible with what is known about real faults is the real explanation for the mechanical paradox a realistic model for overthrust faulting must be compatible with the nature of real overthrust faults and particularly with the way in which displacements including earthquake displacements occur on real faults most large overthrusts are discrete shear surfaces within a coherent mass of rock that is physically continuous around their ends total displacement clearly varies from place to place over these discrete shear surfaces and moreover must be the cumulative result of many relatively small incremental displacements that also varied from place to place over the finite part of the fault surface on which they occurred each increment began as a local shear failure and propagated along the fault as a smeared out somigliana dislocation at a velocity that was small considering the time required for propagation over the total area of the fault during any individual displacement event whether a megathrust earthquake or a creep event only a small part of the fault surface was in relative motion at any one time fault displacements may appear to occur simultaneously across laboratory specimens that are very small relative to the velocity of propagation of displacement because the transit time is so short when the linear scale is expanded over six orders of magnitude to encompass an entire surface of a large overthrust fault however the transit time is much longer because the velocity of propagation is very small relative to the area of the fault surface and the displacement clearly involves only a very small part of the fault surface at any one time the shape and size of overthrust faults result from the propagation of dislocations and this process is controlled by the strength heterogeneity and anisotropy of the rock mass as well as by variations in the regional stress field but not by the frictional resistance to sliding integrated over the entire fault surface

structural insights into the early stages of exhumation along an orogen scale detachment the south tibetan detachment system dzakaa chu section eastern himalaya abstract structural transects through the south tibetan detachment system stds in the dzakaa chu valley tibet reveal a 1000 m thick low angle

tectonic evolution of the central annapurna range nepalese himalayas the metamorphic core of the himalayan orogen or greater himalayan sequence is a northward tapering prism bound at the bottom by a n dipping family of thrust faults the main central thrust system and at the top by a n dipping family of normal faults the south tibetan detachment system research in the central annapurna range of nepal demonstrates a close temporal and spatial association between contractional and extensional deformation on these bounding fault systems and within the metamorphic core throughout much of the early miocene the main central thrust system is represented here by a 2 to 3 km thick zone of high strain that developed during two or more episodes of movement most of its displacement was concentrated along the chomrong thrust a sharp late metamorphic discontinuity that places middle amphibolite facies rocks of the greater himalayan sequence on top of lower amphibolite facies rocks of the lesser himalayan sequence the earliest demonstrable movement on this thrust system occurred 22 5 ma the most recent movement may be as young as pliocene the oldest element of the south tibetan detachment system in this area is the deorali detachment which appears to have been active at the same time as the earliest shortening structures of the main central thrust system fabrics related to the deorali detachment are disrupted by a previously unrecognized sw vergent thrust structure the modi khola shear zone the effect of this structure which is constrained to be between 22 5 and 18 5 ma was to shorten rock packages that had been extended previously during movement on the deorali detachment transition back to a local extensional regime after 18 5 ma was marked by development of the machhupuchhare detachment and related splays geologic evidence for rapid two way transitions between contraction and extension in the annapurna range indicates that extensional deformation in convergent settings does not only represent gravitational collapse at the end of an orogenic cycle it also appears to be an important factor in mountain range development

prism and basal plane parallel subgrain boundaries in quartz a microstructural geothermobarometer empirical data on quartz subgrain patterns from various metamorphic rocks show that at least up to 10 kbar in the stability field of low quartz prismatic subgrain boundaries are dominant whereas basal subgrain boundaries are not developed in the high quartz stability field both prismatic and basal subgrain boundaries occur and form typical rectangular chessboard patterns the likely reason behind the different occurrence of these subgrain patterns is that in high quartz prismatic glide becomes as easy as or probably even easier than basal glide the two types of subgrain patterns can be clearly distinguished by optical microscopy consequently the occurrence of chessboard subgrain patterns in quartz represents a practicable geothermobarometer the possibilities of its application are far reaching and include the specification of deformation conditions at high grade metamorphism the recognition of syntectonic intrusions and the distinction between pluton emplacement at lower and at higher crustal levels

onset of mid crustal extensional flow in southern tibet evidence from u pb zircon ages new ion microprobe u pb dates from zircon in deformed orthogneiss and migmatite and an undeformed granite in mabja dome are the first to constrain the timing of peak metamorphism and onset and duration of mid crustal ductile extension in southern tibet at 35 0 0 8 ma and 12 19 million years the structural metamorphic and intrusive his tories in mid crustal rocks exposed in these north himalayan gneiss domes are similar to those recorded in the greater himalayan sequence suggesting that middle crust was continuous from beneath southern tibet southward to the high himalaya strain compatibility indicates that 35 ma ductile extension in mid crustal rocks of southern tibet was accommodated to the south at shallow crustal levels via normal slip along the southern tibetan detachment system the oldest age estimate for slip along this normal fault zone and extrusion of its footwall if gravitational collapse is an additional important process driving extension then southernmost tibet may have been at or near maximum elevation by late eocene early oligocene time

relationships between displacement and distortion in orogens linking the himalayan foreland and hinterland in central nepal greater himalayan sequence rocks exposed in the manaslu himal chuli himalaya can be separated into distinct upper and lower parts deformation recorded in both parts occurred at temperatures ranging between 450 c and 640 c and is characterized by almost equal coaxial and noncoaxial components across the upper greater himalayan sequence peak metamorphic temperatures are essentially isothermal whereas corresponding metamorphic pressure estimates across the same section decrease downward with an apparent gradient of 620 bars km in the lower greater himalayan sequence however both metamorphic pressure and temperature decrease with structural depth the abnormal pressure gradient in the upper greater himalayan sequence is attributed to 50 vertical thinning during southward displacement while the inverted gradient in the lower portion is interpreted to be the result of coeval exhumation and downward expansion of the main central thrust shear zone and the progressive incorporation of more rock into the greater himalayan sequence deformation in the upper portion of the greater himalayan sequence was characterized by extending flow i e extension in the direction of flow whereas deformation in its lower portion was characterized by compressing flow i e compression in the direction of flow extending flow is a distinctive feature of displacement and distortion in deep orogenic hinterlands while compressing flow is emblematic of displacement and distortion in orogenic foreland regions the transition between the upper and lower parts of the greater himalayan sequence therefore represents a fundamental transition between hinterland style deformation involving processes such as lateral midcrustal flow and foreland style deformation involving critical taper thrust fold wedge development

strain deformation temperatures and vorticity of flow at the top of the greater himalayan slab everest massif tibet this paper presents quantitative data on strain deformation temperatures and vorticity of flow at the top of the greater himalayan slab the data were collected from the tibetan side of the everest massif where two low angle normal faults bound the upper surface of the greater himalayan slab the earlier and structurally lower lhotse detachment and the later and structurally higher qomolangma detachment greenschist to sillimanite grade quartz rich metasedimentary rocks exposed in the rongbuk to north col region of the everest massif are characterized by cross girdle quartz c axis fabrics indicating approximate plane strain conditions fabric opening angles progressively increase with depth beneath the overlying lhotse detachment and indicate progressively rising deformation temperatures of 525 625 50 c at depths of 300 600 m beneath the detachment deformation temperatures of c 450 c are indicated by fabric opening angles in epidote amphibolite facies mylonites located closer to the overlying detachment a top down to the north normal shear sense is indicated by the asymmetry of microstructures and c axis fabrics but the degree of asymmetry is low at distances greater than 400 m beneath the detachment and sillimanite grains are drawn into adjacent conjugate shear bands but still appear pristine indicating that deformation occurred at close to peak metamorphic temperatures these quenched fabrics and microstructures indicate rapid exhumation in agreement with previous isotopic dating studies mean kinematic vorticity numbers w m were independently calculated by three analytical methods calculated w m values range between 0 67 and 0 98 and indicate that although a simple shear component is generally dominant particularly in greenschist facies mylonites located between the lhotse and overlying qomolangma detachments there is also a major component of pure shear in samples located at 400 600 m beneath the lhotse detachment pure and simple shear make equal contributions at w k 0 71 our integrated strain and vorticity data indicate a shortening of 10 30 perpendicular to the upper surface of the greater himalayan slab and confirm that the upper surface of the slab is a stretching fault with estimated down dip stretches of 10 40 assuming plane strain deformation measured parallel to the flow plane transport direction

the malashan gneiss dome in south tibet comparative study with the kangmar dome with special reference to kinematics of deformation and origin of associated granites

tectonic evolution of the himalayan thrust belt in western nepal implications for channel flow models we present a new geologic map of western nepal and three balanced regional cross sections in the himalayan thrust belt the minimum shortening between the south tibetan detachment and the main frontal thrust is 485 743 km and suggests that total himalayan shortening may exceed 900 km all rocks involved in the thrust belt are of upper crustal affinity implying that a comparable length of indian lower crust and mantle lithosphere was subducted beneath tibet major structural features are the subhimalayan thrust system lesser himalayan imbricate zone dadeldhura thrust sheet lesser himalayan duplex ramgarh thrust sheet main central thrust sheet and a north dipping normal sense shear zone possibly related to the south tibetan detachment these structures are continuous along the entire nepalese portion of the himalayan thrust belt new 40 ar 39 ar ages from the ramgarh thrust zone greater himalayan rocks and the lower part of the tethyan sequence support a kinematic model in which major thrust systems in nepal propagated southward from early miocene time onward the geometry and kinematic history of the thrust belt in western nepal are not compatible with recent models for southward ductile extrusion of greater himalayan rocks in a mid crustal channel instead the thrust belt in western nepal behaved like a typical forward propagating thrust system involving unmetamorphosed brittlely deformed rocks in its frontal part and ductilely deformed higher grade metamorphic rocks in its hinterland region although our results do not support published versions of the channel flow model they provide additional geological and geo chronological data that will assist future attempts to develop geodynamic models for the himalayan tibetan orogenic system

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred

fabric development during shear deformation in the main central thrust zone nw himalaya india abstract quartz microfabrics and associated microstructures have been studied on a crustal shear zone the main central thrust mct of the himalaya sampling has been done along six traverses across the mct zone in the kumaun and garhwal sectors of the indian himalaya the mct is a moderately north dipping shear zone formed as a result of the southward emplacement of a part of the deeply rooted crust that now constitutes the central crystalline zone of the higher himalaya over the less metamorphosed sedimentary belt of the lesser himalaya on the basis of quartz c and a axis fabric patterns supported by the relevant microstructures within the mct zone two major kinematic domains have been distinguished a noncoaxial deformation domain is indicated by the intensely deformed rocks in the vicinity of the mct plane this domain includes ductilely deformed and fine grained mylonitic rocks which contain a strong stretching lineation and are composed of low grade mineral assemblages muscovite chlorite and quartz these rocks are characterized by highly asymmetric structures microstructures and quartz c and a axis fabrics that indicate a top to the south sense that is compatible with south directed thrusting for the mct zone an apparently coaxial deformation domain on the other hand is indicated by the rocks occurring in a rather narrow belt fringing and structurally above the noncoaxial deformation domain the rocks are highly feldspathic and coarse grained gneisses and do not possess any common lineation trend and the effects of simple shear deformation are weak the quartz c axis fabrics are symmetrical with respect to foliation and lineation moreover these rocks contain conjugate and mutually interfering shear bands feldspar quartz porphyroclasts with long axes parallel to the macrosopic foliation and the related structures microstructures suggesting deformation under an approximate coaxial strain path on moving towards the mct the quartz c and a axis fabrics become progressively stronger the c axis fabric gradually changes from random to orthorhombic and then to monoclinic in addition the coaxial strain path gradually changes to the noncoaxial strain path all this progressive evolution of quartz fabrics suggests more activation of the basal rhomb and a slip systems at all structural levels across the mct