kinematic model for the main central thrust in nepal we present a kinematic model for the himalayan thrust belt that satisfies structural and metamorphic data and explains recently reported late miocene pliocene geochronologic and thermochronologic ages from rocks in the main central thrust zone in central nepal at its current exposure level the main central thrust juxtaposes a hanging wall flat in greater himalayan rocks with a footwall flat in lesser himalayan rocks of the ramgarh thrust sheet which is the roof thrust of a large lesser himalayan duplex sequential emplacement of the main central early miocene and ramgarh middle miocene thrust sheets was followed by insertion of thrust sheets within the lesser himalayan duplex and folding of the main central and ramgarh thrusts during late miocene pliocene time thorium lead th pb ages of monazite inclusions in garnets from central nepal record the timing of coeval progressive metamorphism of lesser himalayan rocks in the footwall of the main central thrust although this model does not rule out minor late stage reactivation of the main central thrust major late miocene reactivation is not required

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

late miocene environmental change in nepal and the northern indian subcontinent stable isotopic evidence from paleosols neogene sediments belonging to the siwalik group crop out in the himalayan foothills along the length of southern nepal carbon and oxygen isotopic analyses of siwalik paleosols from four long siwalik sections record major ecological changes over the past 11 m y the carbon isotopic composition of both soil carbonate and organic matter shifts dramatically starting ca 7 0 ma marking the displacement of largely c3 vegetation probably semi deciduous forest by c4 grasslands by the beginning of the pliocene all the flood plains of major rivers in this region were dominated by monsoonal grasslands the floral shift away from woody plants is also reflected by the decline and final disappearance of fossil leaves and the decrease in coal logs in the latest miocene a similar carbon isotopic shift has been documented in the paleosol and fossil tooth record of pakistan and in terrigenous organic matter from the bengal fan showing that the floral shift was probably continentwide the latest miocene also witnessed an average change of 4 in the oxygen isotopic composition of soil carbonate as observed previously in pakistan the reasons for this are unclear if not diagenetic a major environmental change is indicated perhaps related to that driving the carbon isotopic shift recently described pollen and leaf fossils from the surai khola section show that evergreen forest was gradually displaced by semi deciduous and dry deciduous forest between 11 and 6 ma the gradual nature of this floral shift which culminated in the rapid expansion of c4 grasses starting 7 0 m y ago is difficult to explain by a decrease in atmospheric pco2 alone cerling et al 1993 but fits well with a gradual onset of monsoonal conditions in the late miocene in the northern indian subcontinent himalayan uplift driving both monsoonal intensification and consumption of co2 through weathering may be the common cause behind major late miocene environmental change globally however the decline of effective moisture associated with monsoon development has probably slowed not increased the rate of consumption of co2 by chemical weathering of himalayan sediments

isotopic constraints on the structural relationships between the lesser himalayan series and the high himalayan crystalline series garhwal himalaya nd and sr isotope systematics may provide important constraints on the location of major thrust systems that separate lithologically similar sedimentary sequences the potential of the technique is illustrated by this isotopic study of the main central thrust system of the himalaya nd isotope data from the garhwal himalaya indicate that metasedimentary rocks from the vaikrita group nd 14 to 19 correlate closely with those from the high himalayan crystalline series which constitutes the hanging wall lithologies of the main central thrust in contrast metasedimentary rocks from the munsiari group nd 23 to 28 show marked similarities to the lesser himalayan series in the footwall of the main central thrust sr isotopes support the correlations in that the vaikrita group shows partial reequilibration at 500 ma whereas the munsiari group has not undergone sr isotope homogenization since 1800 ma thus the vaikrita thrust that juxtaposes these two formations is recognized as the main central thrust in garhwal himalaya the thrust coincides approximately with the location of the kyanite isograd confirming that inverted metamorphism is characteristic of both hanging wall and footwall of the main central thrust along the tons thrust known locally as the srinagar thrust 50 km south of the main central thrust low grade quartzarenites with nd sr isotope and trace element characteristics typical of lesser himalayan formations have been emplaced on phyllites and siltstones with geochemical characteristics of the high himalayan crystalline series the field relationships most probably result from out of sequence thrusting in which lesser himalayan series rocks to the north were emplaced over low grade equivalents of the high himalayan crystalline series preserved in the external part of the orogen this study establishes the value of isotope data for lithostratigraphic correlations within orogenic belts

crustal structure and earthquake focal depths beneath northeastern india and southern tibet summary we use broad band teleseismic data recorded at eight sites along a north south profile from karimganj 24 84 n 92 34 e south of the eastern shillong plateau to bomdilla 27 27 n 92 41 e in the eastern lesser himalaya to determine the seismic characteristics of the crust in northeastern india we also analyse data from the chinese digital seismic network station at lhasa and indepthii stations located on the southern tibetan plateau north of our profile to extend the seismic images of the crust further northwards although the northeastern indian stations and the tibetan stations do not lie along a linear profile across the himalaya the wellrecognized uniformity of the himalaya along strike make this comparison of the two profiles meaningful receiver functions calculated from these data show that the crust is thinnest 35 38 km beneath the shillong plateau receiver functions at cherrapunji on the southern edge of the shillong plateau have a strong azimuthal dependence those from northern backazimuth events show that the moho beneath the southernmost shillong plateau is at a depth of 38 km while receiver functions from southern backazimuth events indicate that the moho beneath the northernmost bengal basin is at a depth of 44 km receiver functions from sites on the brahmaputra valley demonstrate that the moho is deeper by 5 7 km than below the shillong plateau a result which agrees with the hypothesis that the shillong plateau is supported by shearing stress on two steep faults that cut through the crust further north of the eastern himalayan foredeep the moho dips gently northwards reaching a depth of 48 km beneath bomdilla in the lesser himalaya and 88 km below lhasa in tibet using the crustal velocity models obtained from receiver function inversions we redetermined focal depths of wellrecorded earthquakes across this part of the indo tibetan collision zone and find all of these to occur within the crust hence we find no evidence for bimodal depth distribution of earthquakes beneath this region of northeastern india

dynamic recrystallization mechanisms and their transition in the daling thrust dt zone darjeeling sikkim himalaya abstract the daling thrust dt delineates a zone of intense shear localization in the lesser himalayan sequence lhs of the darjeeling sikkim himalaya from microstructural studies of deformed quartzite samples we show a transition in the dynamic recrystallization mechanism with increasing distance from the dt dominated by grain boundary bulging blg recrystallization closest to the dt and progressively replaced by sub grain rotation sgr recrystallization away from the thrust the transition is marked by a characteristic variation in the fractal dimension d of grain boundaries estimated from the area perimeter method for the blg regime d 1 046 which decreases significantly to a value as low as 1 025 for the sgr regime using the available thermal data for blg and sgr recrystallization we infer increasing deformation temperatures away from the dt in the hanging wall based on the quartz piezometer our estimates reveal strong variations in the flow stress 59 00 mpa to 16 00 mpa over a distance of 1 2 km from the dt deformation mechanism maps constructed for different temperatures indicate that the strain rates 10 12 s 1 to 10 14 s 1 comply with the geologically possible range finally we present a mechanical model to provide a possible explanation for the cause of stress intensification along the dt

the tectonostratigraphy and structure of the eastern nepal himalaya six years of geological research in eastern nepal has resulted in a new geological map of the eastern nepal himalaya which includes the region stretching from the sikkhim border in the east to the kathmandu valley in the west and from the summits of the higher himalaya in the north to the ganges plain in the south this research has permitted the determination of the tectonostratigraphy and structure of one section of the central himalayan arc south of the tibetan plateau the eastern nepal himalaya can be divided into three distinct thrust bound tectonic packages 1 the higher himalayan thrust sheet composed of the higher himalayan crystallines 2 the lesser himalayan thrust sheet composed of the lesser himalayan series and 3 the sub himalayan imbricate zone composed of sedimentary rocks belonging to the siwalik group the higher himalayan thrust sheet of eastern nepal has been thrust over the lesser himalayan metasediments a minimum of 140 km and possibly as much as 175 210 km along the main central thrust mct the lesser himalayan thrust sheet is overlain by the mct and is underlain by the main detachment fault mdf and the main boundary thrust mbt out of sequence thrust faults in the hanging wall of the mbt have breached and offset the presently inactive mct the sub himalayan imbricate zone is an emergent imbricate fan bounded by the mbt to the north and the main frontal thrust mft to the south and is underlain by the mdf which lies at a depth of between 5 km and 7 km a balanced cross section constructed across the higher lesser and sub himalaya of eastern nepal shows that the eastern nepal himalayan orogenic wedge has undergone a minimum of between 210 and 280 km of horizontal north south tectonic shortening since the initiation of the mct the lesser and sub himalaya have absorbed 70 km of north south shortening by thrusting along the basal mdf of which the sub himalayan imbricate zone has accommodated 25 km the sun kosi thrust has accommodated about 10 km and the mbt has accommodated the remaining 35 km of shortening since the initiation of the mct between 15 ma and 25 ma shortening across the eastern nepal himalaya has occurred at an average rate of 8 4 18 6 mm per year the structural geometry of the eastern nepal himalaya suggests an overall piggyback sequence of thrusting with motion transferred from the mct to the underlying mdf and its emergent splay thrust the mbt and with the mbt rotated to its present steep orientation by imbricate thrusting within the sub himalaya

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

correlation of lithotectonic units across the eastern himalaya bhutan clastic sediments deposited in foreland basins and offshore fans allow the evolution of an orogen to be reconstructed provided their source regions are properly characterized isotopic data from the lithotectonic units of the eastern himalaya bhutan indicate clear isotopic differences directly comparable with the equivalent units from the central himalaya zircons from metaquartzites of the high himalayan series that range in age from 980 to 1820 ma and an orthogneiss intruded at 825 9 ma bracket deposition to between 816 and 980 ma model ages derived from nd isotopes of associated metapelites range from 1700 to 2200 ma in contrast zircons from a metaquartzite from the lesser himalayan series range from 1850 to 2550 ma a paleoproterozoic deposition age ca 1750 ma is inferred from the age of a metarhyolite associated with the sediments model nd ages of metapelites from the lesser himalayan series range from 2500 to 2600 ma the bhutanese himalayan units can be correlated more than 1000 km westward along strike from catchments of the brahmaputra to the headwaters of the sutlej on the basis of their structural position provenance and isotopes because current discharge from the brahmaputra carries the dominant sedimentary flux into the bengal fan results from this work validate the interpretation of isotopic variations observed in offshore deposits in terms of the unroofing history of the component lithologies of the himalaya

growth history of hydrothermal black smoker chimneys hollow mineral spires known as black smoker chimneys precipitate around 350 c jets of hydrothermal fluid spouting from the sea floor on the east pacific rise epr at 21 n refs 1 2 i describe and discuss here the formation of mineral zoning across the walls of these structures throughout two major growth phases a sulphate dominated stage and a sulphide replacement stage during the sulphide replacement stage at least four distinct sulphide mineral zonation sequences develop across chimney walls from interior to exterior the apparently successive formation of these sulphide zonation sequences is attributed to evolution of radial thermal and chemical gradients accompanying the gradual thickening and sealing of black smoker chimney walls

osmium isotopes in hydrothermal fluids from the juan de fuca ridge we present os data for axial high temperature and off axial low temperature hydrothermal solutions from the juan de fuca ridge the high temperature h s bearing axial fluids have unradiogenic os isotopes pointing to a nearly complete domination of osmium isotopes from the basalts during hydrothermal circulation the ridge axis fluids typically do not show large enrichment in osmium concentration over seawater although one high temperature fluid has an os concentration enhanced by a factor of 4 above seawater it appears that the os concentration of high temperature hydrothermal fluids is typically buffered at roughly the seawater concentration we suggest that subseafloor precipitation of pyrite from the high temperature hydrothermal fluids controls the osmium transportation the axial hydrothermal activity does not supply significant amounts of unradiogenic osmium to the deep oceans in contrast a low temperature off axis fluid is enriched in non radiogenic osmium over seawater by a factor of 9 showing much less precipitation of osmium at low temperature because a large fraction of the cooling of oceanic lithosphere occurs on ridge flanks os from low temperature ridge flank hydrothermal circulation may be a significant contributor to the balance of os in the oceans a detailed balance between os contributions from dissolution of cosmic dust and hydrothermal fluids is still not possible the low temperature hydrothermal sample gives os os 0 110 0 001 this extremely unradiogenic osmium can only come from a source that underwent depletion of re over 2 6 ga ago assuming no contamination during sample collection this result suggests that the convecting upper mantle contains ancient depleted material that imparted unradiogenic osmium to intruding basaltic melts

lateral variation in geometry of thrust planes and its significance as studied in the shumar allochthon lesser himalayas eastern bhutan abstract the features mapped indicate a sse striking displacement vector for the shumar thrust of eastern bhutan sections parallel to this direction show that the frontal flat segment of the thrust plane along with the subthrust and the hanging wall rocks has been folded into a synform a longitudinal section however shows a cuspate lobate shape broad concave upward curves meeting along sharp crests of the thrust plane which laterally cuts up through the subthrust rocks the broad salients and sharp re entrants defining the multiple lobate pattern of the thrust trace is due to the cuspate lobate lateral curvature which is a primary feature of the thrust plane each cusp combined with the forward or backward inclination of the thrust plane imparts a scoop like shape several of which are joined by transverse ridges parallel to the displacement vector the s surfaces transposed parallel to the limbs and axial surfaces of the isoclinal early folds in the allochthon have been progressively reoriented parallel to the curviplanar thrust surface later post thrusting folds superposed on these initially curved surfaces have curved hinges which trend at high angles to the displacement direction as a result the bent hinges are congruent to the laterally curved thrust surface below the evolution of the shumar thrust can be explained by lateral propagation along concave upward trajectories thrust fronts propagating from multiple nuclei subsequently coalesced to form a single thrust surface the ridges along which the thrusts coalesce have the form of lateral ramps

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

late miocene movement within the himalayan main central thrust shear zone sikkim north east india in the sikkim region of north east india the main central thrust mct juxtaposes high grade gneisses of the greater himalayan crystallines over lower grade slates phyllites and schists of the lesser himalaya formation inverted metamorphism characterizes rocks that immediately underlie the thrust and the large scale south tibet detachment system stds bounds the northern side of the greater himalayan crystallines in situ th pb monazite ages indicate that the mct shear zone in the sikkim region was active at c 22 14 15 and 12 10 ma whereas zircon and monazite ages from a slightly deformed horizon of a high himalayan leucogranite within the stds suggest normal slip activity at c 17 and 14 15 ma although average monazite ages decrease towards structurally lower levels of the mct shear zone individual results do not follow a progressive younging pattern lesser himalaya sample kbp1062a records monazite crystallization from 11 5 0 2 to 12 2 0 1 ma and peak conditions of 610 25 c and 7 5 0 5 kbar whereas in the mct shear zone rock chg14103 monazite crystallized from 13 8 0 5 to 11 9 0 3 ma at lower grade conditions of 525 25 c and 6 1 kbar the p t t results indicate that the shear zone experienced a complicated slip history and have implications for the understanding of mid crustal extrusion and the role of out of sequence thrusts in convergent plate tectonic settings

metamorphic reactions related to decompression and synkinematic intrusion of leucogranite high himalayan crystallines bhutan the high himalayan crystallines hhc of bhutan were penetratively deformed intruded by leucogranite and metamorphosed during the collision of the indian and asian plates metamorphic reaction textures in the hhc show that it experienced decompression while maintaining a laterally heterogeneous and locally inverted internal temperature range of c 600 750c this thermal structure was produced by thrusting hot migmatitic rocks over lower grade rocks within the hhc and by the advection of heat from the intrusion of leucogranite dykes and sills during decompression a variable velocity field within the hhc during exhumation and extrusion between india and tibet caused the inversion of top to the south sense of shear present throughout most of the hhc to top down to the north shear near its top

gravity anomalies flexure and deformation of the converging indian lithosphere in nepal and sikkim darjeeling himalayas researchers ubiquitously noted that the common processes of partitioning oblique convergence in response to drag from the trench hanging plate simultaneously produce radial slips along strike translation and extension parallel to the deformation front here we focus on the area between nepal and sikkim darjeeling himalayas and carry out gravity and finite element stress modeling of the strike orthogonal converging indian lithosphere we delineate the geometries of different layers and their interfaces through gravity modeling the optimum model parameters along with rheological parameters of different layers are used for finite element modeling finite element modeling is done with boundary conditions of keeping the upper surface free and rigidly fixing the section of the northern boundary below the main himalayan thrust we impart on its frontal section an amount of 6 1012 n m force equivalent to resistive force of the himalayan tibet system and analyze the maximum and minimum compressive stress fields evolved in the lithosphere we testify our observations with earthquake database and other geophysical and geological studies we note that an increasing flexing of the indian lithosphere beyond the main boundary thrust becomes maxima between the main central thrust and south tibetan detachment in both the areas however more steepening of the moho boundary is identified in the sikkim darjeeling himalaya this abrupt change in lithospheric geometry beneath the greater himalaya is likely correlated with the sharp elevation changes in the topography although the highest seismicity concentration is dominant in this zone the lesser and the tethys himalayas in sikkim darjeeling area also record relatively fair seismic activity more compressive stress field in different layers right within the sharp bending zone supports this observation we thus propose that the sharp bending zone beneath the greater himalaya is suffering maximum deformation and the deformation is continued in the mantle too we also identify both right lateral shear and radial vergence slip which are presumably associated with the general dynamics and kinematics of the himalaya

a new species of zangerlia testudines nanhsiungchelyidae from the upper cretaceous redbeds at bayan mandahu inner mongolia and the relationships of the genus zangerlia neimongolensis sp nov is described on the basis of material from the upper cretaceous redbeds at bayan mandahu in inner mongolia zangerlia neimongolensis is similar to zangerlia testudinimorpha in the proportions of the carapace and plastron and presence of a knob at the posterior end of the neural series but differs from it in the arrangement of scutes covering the bridge the placement of zangerlia in the nanhsiungchelyidae is supported by derived features of the bridge peripherals and plastral scutes shared by z neimongolensis basilemys and nanhsiungchelys these are the presence of ventrally expanded sixth inframarginal scutes humeral scutes that are narrow at the midline and expanded laterally pectoral scutes that are wide at the midline and narrow laterally and large rectangular abdominal scutes the skull of zangerlia is more primitive than that of nanhsiungchelys the only other member of the family for which a skull is known it shows extensive emargination of the temporal and

the onset of india asia continental collision early steep subduction required by the timing of uhp metamorphism in the western himalaya ultrahigh pressure uhp rocks in the nw himalaya are some of the youngest on earth and allow testing of critical questions of uhp formation and exhumation and the timing of the india asia collision initial collision of india with asia is widely cited as being at 55f1 ma based on a paleomagnetically determined slowdown of india s plate velocity and as being at ca 51 ma based on the termination of marine carbonate deposition even relatively small changes in this collision age force large changes in tectonic reconstructions because of the rapid india asia convergence rate of 134 mm a at the time of collision new u pb shrimp dating of zircon shows that indian rocks of the tso morari complex reached uhp depths at 53 3f0 7 ma given the high rate of indian subduction this dating implies that indian continental crust arrived at the asian trench no later than 57f1 ma providing a metamorphic age for comparison with previous paleomagnetic and stratigraphic estimates india s collision with asia may be compared to modern processes in the timor region in which initiation of collision precedes both the slowing of the convergence rate and the termination of marine carbonate deposition the indian uhp rocks must have traveled rapidly along a short hence steep path into the mantle early continental subduction was at a steep angle probably vertical comparable to modern continental subduction in the hindu kush despite evidence for modern day low angle subduction of india beneath tibet oceanic slab break off likely coincided with exhumation of uhp terranes in the western himalaya and led to the initiation of low angle subduction and leucogranite generation d 2005 elsevier b v all rights reserved

early tertiary deformation of the zhongba gyangze thrust in central southern tibet abstract as the boundary thrust between india and asia in southern tibet the zhongba gyangze thrust zgt emplaced the yarlung zangbo suture zone yzsz units in the hanging wall southward onto tethyan himalaya sequences ths of the northern indian continental margin in the footwall detailed field investigation electron backscatter diffraction ebsd analysis detrital zircon u pb geochronology and 40 ar 39 ar thermochronology were conducted to understand the evolution of the zgt in sangsang area central southern tibet the shear zone of zgt is located within the sedimentary matrix melange of yzsz that is mainly composed of matrix of blueschist with meta pelagic hemipelagic siliceous and siliciclastic rocks and blocks of basalt limestone and sandstone penetrative f1 foliation and kink band structure were recorded within the matrix both on outcrop and under microscope strong lattice preferred orientation lpo fabric initiated by the low temperature 350 450 c 010 001 slip system was detected by ebsd in the sodic amphiboles of the blueschist the 40 ar 39 ar ages of the phengites from blueschist and sericites from the phyllite in the shear zone indicate that the activity of zgt occurred between 71 and 60 ma in the ths a newly documented younger unit preserving detrital zircons from the southern asian margin lies above the triassic cretaceous sequences that carries only detrital zircons from the indian continent this unit is dated to be 61 ma by the detrital zircon ages similar to the sangdanlin formation and representing the yarlung zangbo foreland basin yzfb system the zgt had probably been active due to the initial india asia collision and acted as the frontal thrust controlling the development of yzfb

convergence across the northwest himalaya from gps measurements 1 horizontal velocities of 26 global positioning system gps stations in the northwest himalayan region provide new constraints on the partitioning of india eurasia convergence and elastic strain accumulation about the locked main frontal thrust mft the northwest striking karakorum fault slips at 11 4 mm yr and contributes to east west extension of southern tibet and westward motion of the northwest himalaya towards nanga parbat rather than playing a role in eastward extrusion of tibet crustal shortening across the himalaya occurs within a zone centered about 100 km north of the siwalik foothills and the mft model inversions of the gps data indicate that the mft is locked over a width of 100 km comparison with geologic mft slip rate estimates suggests that this zone is building up a slip deficit at a rate of 14 1 mm yr and will eventually fail in future great earthquakes

a new kinematic evolutionary model for the growth of a duplex an example from the rangit duplex sikkim himalaya india abstract the lesser himalayan duplex lhd is a prominent structure through much of the lesser himalayan fold thrust belt in the darjeeling sikkim himalaya a component of the lhd is exposed in the rangit window as the rangit duplex rd the rd consists of ten horses of the upper lesser himalayan sequence gondwana buxa upper daling the duplex varies from hinterland dipping in the north through an antiformal stack in the middle to foreland dipping in the south the ramgarh thrust rt is the roof thrust and based on a balanced cross section the main himalayan sole thrust is the floor thrust at a depth of 10 km and with a dip of 3 5 n retrodeformation suggests that the rd initiated as a foreland dipping duplex with the early ramgarh thrust as the roof thrust and the rt as the floor thrust the rt became the roof thrust during continued duplexing by a combination of footwall imbrication and concurrent rt reactivation this kinematic history best explains the large translation of the overlying mct sheets the restoration suggests that rd shortening is 125 km and the original gondwana basin extended 142 km northward of its present northernmost exposures within the window

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

channel flow and ductile extrusion of the high himalayan slab the kangchenjunga darjeeling profile sikkim himalaya abstract the geology of the kangchenjunga darjeeling profile in west sikkim and north bengal is consistent with the interpretation of the high himalaya metamorphic sequence as a ductile channel approximately 15 20 km thick extruding southwards bounded by major ductile shear zones above south tibetan detachment and below main central thrust zone in the yoksam to kangchenjunga section the entire slab is composed of migmatites or leucogranite sheets massive sills or sub horizontal to gently north dipping sheets of leucogranite make up the entire kangchenjunga massif and extend west at least as far as jannu shear sense indicators show both pure shear flattening fabrics and non coaxial south directed simple shear fabrics the carapace of the melt filled channel is seen around the southern margin of the singallila ridge and the darjeeling klippe where inverted metamorphic isograds from sillimanite k feldspar grade down through kyanite and staurolite to garnet biotite grade have been well documented we present an interpretation of the crustal structure of the sikkim himalaya based on field structural observations and utilizing the indepth seismic profile to constrain sub surface structure to the north we suggest that widespread melting within sillimanite grade gneisses triggered sudden and rapid ductile extrusion during the miocene by critically affecting the rheology of the middle crust under south tibet leucogranite melts lubricated shear zones facilitating rapid transport of heat towards the surface when the leucogranite melts began to cool ductile extrusion ended as thrusting propagated down section into the lesser himalaya since leucogranite expulsion was largely horizontal rather than vertical and since the std normal sense shear zone is very low angle dipping to the north decompression melting cannot be invoked as an origin for the leucogranites the heat source for such widespread melting at such shallow levels in the crust can only be explained by having a highly radioactive protolith of indian plate proterozoic sediments the overall map distribution and geometry of leucogranites migmatites and metamorphic isograds is compatible with both the everest makalu section to the west and the bhutan section east of the yadong gulu rift we conclude that the channel flow model is a viable explanation for this section of the himalaya

isotopic constraints on the age and provenance of the lesser and greater himalayan sequences nepalese himalaya the main central thrust of the central himalaya juxtaposes the lesser himalayan sequence footwall with the greater himalayan sequence hanging wall both of which are regarded as components of the northern passive margin of india considerable uncertainty surrounds their respective ages and sedimentary derivation u pb and sm nd isotopic studies of samples from the langtang area central nepal demonstrate important distinctions between the two rock packages the greater himalayan sequence had a sedimentary provenance that included a major source of 0 8 1 0 ga zircons implying a late proterozoic age the source of the precambrian part of the lesser himalayan sequence contained 1 87 2 60 ga zircons and its depositional age is probably middle proterozoic nd isotopic characteristics of the two sequences are different at 21 ma the greater himalayan rocks had end values between 14 6 and 18 5 while their lesser himalayan counterparts had values between 21 4 and 25 9 giving rise to major differences in their model ages the lesser and greater himalayan sequences at langtang are interpreted as correlative to the lower and upper parts of the vindhyan supergroup currently exposed south of the gangetic plain in northern india on the basis of available age data the exposed hanging wall of the main central thrust in central and eastern nepal contains no archean early proterozoic basement if the fault does root into the basement at depth it must do so farther north than recently published cross sections indicate this implies a total displacement on the fault that is substantially greater than the 140 210 km required by tectonic overlap in the eastern himalayas

eocene early miocene foreland basin development and the history of himalayan thrusting western and central nepal sedimentologic petrographic and u pb detrital zircon ages from middle eocene through early miocene sedimentary rocks in the lesser himalayan zone of western and central nepal indicate that a peripheral foreland basin system had developed in the eastern himalayan collision zone by middle eocene time the shallow marine eocene bhainskati formation accumulated in a back bulge depozone between a southward migrating forebulge and the indian craton migration of the forebulge through this region during eocene oligocene time produced a regional unconformity that spans 15 20 myr by early miocene time the forebulge unconformity was onlapped by the distal fringes of the southward migrating foredeep depozone represented by fluvial deposits of the dumri formation continued southward migration of the foredeep during the neogene accommodated the fluvial siwalik group light mineral provenance data and u pb detrital zircon ages suggest that the bhainskati was derived partly from tethyan sedimentary rocks of the tibetan himalayan zone during initial growth of the himalayan fold thrust belt the dumri was derived from metasedimentary and crystalline rocks of the greater himalayan zone during emplacement of the main central thrust and contemporaneous tectonic unroofing by normal faulting along the south tibetan detachment system the lesser himalayan crystalline thrust sheets were emplaced soon after deposition of the dumri formation 15 10 ma paleocurrent and lithofacies data from the dumri formation indicate deposition by west southwestward flowing rivers that drained into the indus portion of the himalayan foreland basin system during the early miocene thick channel sandstones in the lower dumri may represent the early miocene counterpart of the modern ganges river eastward diversion of the ganges drainage system to near its present location had occurred by 15 ma as the high standing aravalli range on the northern indian shield approached the front of the fold thrust belt assuming reasonable values for the flexural rigidity of indian lithosphere the time span of the forebulge unconformity yields a velocity of 14 33 mm yr for the southward migration of the fold thrust belt relative to india this range of values is consistent with neogene and present day estimates and suggests that only one third to one half of india eurasia convergence has been accommodated by shortening in the himalayan fold thrust belt since the onset of collision

the kinematic evolution of the nepalese himalaya interpreted from nd isotopes neodymium nd isotopes from the himalayan fold thrust belt and its associated foreland basin deposits are useful for distinguishing between himalayan tectonostratigraphic zones and revealing the erosional unroofing history as controlled by the kinematic development of the orogen neodymium isotopic data from the himalayan fold thrust belt in nepal n 35 reveal that the lesser himalayan zone consistently has a more negative nd 0 value than the greater and tibetan himalayan zones our data show the average nd 0 value in the lesser himalayan zone is 21 5 whereas the greater and tibetan himalayan zones have an average nd 0 value of 16 these consistently distinct values throughout nepal enable the use of nd isotopes as a technique for distinguishing between lesser himalayan zone and greater himalayan zone rock the less negative nd 0 values of the greater himalayan rocks support the idea that the greater himalayan zone is not indian basement but rather a terrane that accreted onto india during early paleozoic time neodymium isotopic data from eocene through pliocene foreland basin deposits n 34 show that sediment provenance has been dominated by greater and tibetan himalayan detritus across nepal the nd t values in the synorogenic rocks in western and central nepal generally show an up section shift toward more negative values and record the progressive unroofing of the different tectonostratigraphic zones at 10 ma in khutia khola and 11 ma in surai khola a shift in nd t values from 16 to 18 marks the erosional breaching of a large duplex in the northern part of the lesser himalayan zone this shift is not seen in eastern nepal where the nd t values remain close to 16 throughout miocene time because there has been less erosional unroofing in this region

southward extrusion of tibetan crust and its effect on himalayan tectonics the tibetan plateau is a storehouse of excess gravitational potential energy accumulated through crustal thickening during india asia collision and the contrast in potential energy between the plateau and its surroundings strongly influences the modern tectonics of south asia the distribution of potential energy anomalies across the region derived from geopotential models indicates that the himalayan front is the optimal location for focused dissipation of excess energy stored in the plateau the modern pattern of deformation and erosion in the himalaya provides an efficient mechanism for such dissipation and a review of the neogene geological evolution of southern tibet and the himalaya shows that this mechanism has been operational for at least the past 20 million years this persistence of deformational and erosional style suggests to us that orogens like other complex systems can evolve toward steady state configurations maintained by the continuous flow of energy the capacity of erogenic systems to self organize into temporally persistent structural and erosional patterns suggests that the tectonic history of a mountain range may depend on local energetics as much as it does on far field plate interactions

climatic forcing of erosion landscape and tectonics in the bhutan himalayas a fundamental objective in studies of climate erosion tectonics coupling is to document convincing correlation between observable indicators of these processes on the scale of a mountain range the eastern himalayas are a unique range to quantify the contribution of tectonics and climate to long term erosion rates because uniform and steady tectonics have persisted for several million years while monsoonal precipitation patterns have varied in space and time specifically the rise of the shillong plateau the only orographic barrier in the himalayan foreland has reduced the mean annual precipitation downwind in the eastern bhutan himalaya at the miocene pliocene transition apatite fission track aft analyses of 45 bedrock samples from an e w transect along bhutan indicate faster long term erosion rates outside of the rain shadow in the west 1 0 1 8 mm yr than inside of it in the east 0 55 0 85 mm yr furthermore an aft vertical profile in the latter segment reveals a deceleration in erosion rates sometime after 5 9 ma in this drier segment of bhutan there are remnants of a relict landscape formed under a wetter climate that has not yet equilibrated to the present climatic conditions uplift and preservation of the paleolandscape are a result of a climate induced decrease in erosion rates rather than of an increase in rock uplift rate this study documents not only a compelling spatial correlation between long term erosion and precipitation rates but also a climatically driven erosion rate change on the scale of the eastern himalayas a change that in turn likely influences that region s recent tectonic evolution

distributed deformation in southern and western tibet during and after the india asia collision field and radiometric data are used to describe and date strain and stress states in southern longitude 88 o to 91oe latitude 28 o to 30on and western tibet longitude 79 o to 82oe latitude 30 o to 34on we factorize deformation into syncollisional and postcollisional and we present stretching lineation and displacement orientation maps two sections across the indian shelf sequence and stress orientations calculated from mesoscale fault slip data in southern tibet syncollisional stretching and displacement directions trend 90 46 o and displacement is top to south synkinematic low grade metamorphism is dated at 50 ma at one locality in the indian shelf sequence underlying the main mantle thrust of the indus yarlung suture this implies paleocene onset of continental collision for the investigated section postcollisional structures comprise a backthrust group which includes foreland and hinterland directed thrusts reverse and strike slip faults and folds it dominates postcollisional deformation is concentrated along the indus yarlung suture and portrays n s compression otrend of 8o 17 o o2 of 97o 17o a strike slip group consists of conjugate strike slip faults is concentrated in east trending narrow highly deformed zones and indicates that n s compression is locally compensated by e w extension oof 15o 29 o o3 of 103o 30o synkinematic muscovite dates postcollisional deformation as late early miocene 17 5 ma at one locality at the suture strike slip and oblique normal 03 of 600 23 o oof 144o 21 o and normal o3 114o 16 o faulting dated between late miocene and recent and including active deformation represents dominant e w and minor n s extension due to e w stretching of southern tibet and oroclinal bending along the himalayan arc restoring syncollisional and postcollisional deformation yields a minimum of 67 258 km shortening across the indian shelf sequence incorporating recently published contraction estimates across the eastern himalaya yields minimum shortening between undeformed india and the indus yarlung suture of 66 536 km the himalaya tibet orogenic system south of the indus yarlung suture had an initial width of 811 km in the southern tibetan section in western tibet imbrication of an ophiolite sequence of the bangong nujiang suture is top to south stretching lineation trend of 15o 18o and o3 of active deformation trends ese faulting along the shiquanhe fault zone which transfers displacement from the northern part of the karakorum fault to a system of rifts in western central tibet indicates dextral strike slip alternating with sinistral oblique normal faulting and block rotations around vertical axes during a prolonged shearing history the indian shelf sequence south of mount kailas shows top to south imbrication stretching lineation trend of 52o 60o both indian shelf rocks and oligocene miocene kailas conglo merates record backthrusting and backfolding oof 33 o and recent e w extension 03 of 85o 28o

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

exhumation of the main central thrust from lower crustal depths eastern bhutan himalaya geothermometry and mineral assemblages show an increase of temperature structurally upwards across the main central thrust mct however peak metamorphic pressures are similar across the boundary and correspond to depths of 35 45 km garnet bearing samples from the uppermost lesser himalayan sequence lhs yield metamorphic conditions of 650 675 c and 9 13 kbar staurolite kyanite schists about 30 m above the mct yield p t conditions near 650 c 8 10 kbar kyanite bearing migmatites from the greater himalayan sequence ghs yield pressures of 10 14 kbar at 750 800 c top to the south shearing is synchronous with and postdates peak metamorphic mineral growth metamorphic monazite from a deformed and metamorphosed proterozoic gneiss within the upper lhs yield u pb ages of 20 18 ma staurolite kyanite schists within the ghs a few metres above the mct yield monazite ages of c 22 1 ma we interpret these ages to reflect that prograde meta morphism and deformation within the main central thrust zone mctz was underway by c 23 ma u pb crystallization ages of monazite and xenotime in a deformed kyanite bearing leucogranite and kyanite garnet migmatites about 2 km above the mct suggest crystallization of partial melts at 18 16 ma higher in the hanging wall south verging shear bands filled with leucogranite and pegmatite yield u pb crystallization ages for monazite and xenotime of 14 15 ma and a 1 2 km thick leuco granite sill is 13 4 0 2 ma thus metamorphism plutonism and deformation within the ghs con tinued until at least 13 ma p t conditions at this time are estimated to be 500 600 c and near 5 kbar from these data we infer that the exhumation of the mct zone from 35 to 45 km to around 18 km occurred from 18 to 16 to c 13 ma yielding an average exhumation rate of 3 9 mm year 1 this process of exhumation may reflect the ductile extrusion by channel flow of the mctz from between the overlying tibetan plateau and the underthrusting indian plate coupled with rapid erosion

new garnets for old cautionary tales from young mountain belts major and trace elements in garnet may be used to characterize p t t paths in orogenic belts but they are also often used without age constraints to obtain p t estimates alone during investigations into the prograde phase of the alpine himalayan tertiary orogeny garnets with anomalously old sm nd ages were discovered in three different areas they occurred in identical parageneses to garnets that grew during tertiary prograde metamorphism ages 500 ma were determined for homogenized garnets from the garhwal himalaya while garnets with distinctive growth zonation from the betic cordillera s spain gave an age of 235 1 1 7 ma although the garhwal date coincides with a recognized magmatic and metamorphic event the 235 1 ma age from the betics at first sight appears one of the more unlikely times for garnet growth duplicate analyses precisely date growth of the garnet cores which x ray maps reveal to be entirely distinct from the rims implying a hiatus before resorption and rim development this initial garnet growth may be linked to early crustal extension during pangaea break up these results reaffirm the value of garnet in the integrated study of polymetamorphic orogenic belts at the same time the data show the potential for unravelling the history of these complex zones and strike a cautionary note for thermobarometry

an early india asia contact paleomagnetic constraints from ninetyeast ridge odp leg 121 new paleomagnetic results from sedimentary rock and basement of the ninetyeast ridge ocean drilling program leg 121 sites 756 758 detail the northward movement of the indian plate for the past 80 m y analysis of the combined paleolatitude age profile indicates a distinct reduction in india9s northward movement rate at 55 ma interpreted as completion of suturing of greater india and asia india9s northward motion slowed from 18 19 5 cm yr to 4 5 cm yr for the location of site 758 comparison of this profile with paleomagnetic data from the wider himalayan region indicates that initial contact between northwestern greater india and southern asia was already established by cretaceous tertiary time this supports a possible causal link between the india asia convergence and the deccan traps extrusion

himalayan architecture constrained by isotopic tracers from clastic sediments we report the first geochemical traverse to integrate u pb ages and hf data on single detrital zircons with bulk rock sm nd rb sr isotopic measurements across the breadth of the himalayan orogen in northwest india our dataset from key tectonostratigraphic units in the sutlej valley provides detailed insights into both the provenance of the sedimentary formations and their tectonic history whole rock nd data provide information on the mean age of source areas contributing detritus to the sedimentary units in the mountain belt regardless of their current metamorphic grade u pb ages of detrital zircons from the same units identify the age of melt formation in the source rocks while lu hf isotope data from zircons resolve discrete crust formation ages in the multiple eroding terrains complex whole rock sr systematics hint at the extent of pre himalayan thermal events our data confirm the geochemical distinction between the older lesser himalayan formations and those of the depositionally younger units including the high himalayan crystalline series that has been recorded along the strike of the himalaya from the western syntaxis to nepal palaeoproterozoic metasediments of the jutogh group and rampur formation yield end 500 values below 17 whereas neoproterozoic and younger metasediments yield end 500 above 13 the latter group encompasses the vaikrita group gneisses of the high himalayan crystallines and the overlying haimanta group but also includes a number of units in thrust sheets north of the main boundary thrust in the lesser himalayan zone designated the douter lesser himalayat to distinguish them from the palaeoproterozoic dinner lesser himalayat zircon populations from the inner lesser himalaya are characterised by palaeoproterozoic late archaean ages 2 6 1 8 ga whereas the depositionally younger units contain populations both of this age and a younger period meso to neo proterozoic 1 1 0 8 ga detrital zircon ages are younger than their respective hf isotope derived crustal formation ages by 0 7 2 1 ga indicating that the source regions of the detrital zircons consisted of older terranes with considerable amounts of reworking and renewed magmatism the

preliminary stratigraphic and structural architecture of bhutan implications for the along strike architecture of the himalayan system article i nfo preliminary mapping and stratigraphic correlation of lesser himalayan rock in eastern bhutan using field characteristics u pb detrital zircon dating and nd geochemistry define the first order stratigraphic architecture of the indian passive margin sequence in the eastern himalaya we use this new image of the lateral and vertical relationships of the original stratigraphy to determine the structural framework of the eastern himalayan fold thrust belt in bhutan we propose that lesser himalayan rock in bhutan can be divided into lower lesser himalayan rocks with a paleoproterozoic detrital zircon signal and upper lesser himalayan rocks with detrital zircon signals of 1000 500 ma the 500 ma detrital grains are from rocks in the frontal portions of the fold thrust belt north of the main boundary thrust as well as directly in the footwall of the main central thrust in the hinterland our preliminary stratigraphic study coupled with mapping allows us to construct a composite balanced cross section through the kuru chu valley in eastern bhutan which provides the first image of the geometry and amount of shortening through bhutan the main frontal thrust tilts a 6 km of thick section of neogene foreland basin deposits these deposits are separated from folded and faulted upper lesser himalayan rocks by the main boundary thrust north of the main boundary thrust we propose two duplex systems the southernmost duplex contains 9 repeated sections of upper lesser himalayan units including the permian gondwana sequence and the cambrian or younger baxa group the northernmost duplex is located in the footwall of the main central thrust and is comprised of two repeated sections of the proterozoic shumar and daling formations the southern boundary of this northern duplex is the shumar thrust which acts as a roof thrust for the southern duplex system and may correlate to the ramgarh thrust in nepal and india we propose that the development of the duplex systems passively folded overlying tethyan and greater himalayan rocks minimum shortening for this part of the himalayan fold thrust belt is 359 km all of which has occurred from 22 ma to present defining a long term shortening rate of 16 mm yr

sedimentologic and stratigraphic constraints on the orientation of the late triassic northern indian passive continental margin abstract the orientation and distribution of the northern indian passive continental margin basin during different epochs constrained the geometric characteristics of the hyperextended northern indian edge greater india traditional paleogeographic reconstructions envisage the late triassic northern indian passive continental margin as e w trending whereas paleomagnetic data support an 55 counterclockwise rotation during northward drifting this case study of upper triassic units from the northern indian margin based on facies analysis sandstone petrography heavy mineral assemblages u pb detrital zircon geochronology and stratigraphic correlation from the zanskar range to southern tibet provides new constraints and suggests that the northern indian passive continental margin might have been n s trending in the late triassic given the dominance of quartz sandstones with zircon tourmaline and rutile indicating a provenance from continental blocks and the recycling of older quartz rich sandstones and detrital zircon age spectra characterized by a neoproterozoic detrital zircon u pb age peak 921 13 ma the upper triassic sandstones widespread along the tethyan himalayas may have been partly derived from the eastern ghats mobile belt in eastern india and the rayner orogen in eastern antarctica the upper triassic quartzite series comprises feldspatho quartzose to pure quartzose sandstones with intercalated bioclastic oosparites deposited in backshore foreshore and shoreface coastal environments without considering the subsequent counterclockwise rotation of india the consistent pattern of the upper triassic bidirectional e w paleocurrent observed throughout the himalaya suggests that the paleoshoreline was striking near n s approximately perpendicular to the regional orientation of the northern indian margin progressive eastward thickening of the sandstone layers and eastward younging of the quartzite series suggest diachronous sedimentation such stratal patterns and the strike of the late triassic paleoshoreline were attributed to the neo tethys transgression from the zanskar range to southern tibet

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

multicyclic history of the northern india continental margin northwestern himalaya 1 the geologic evolution of northern india is best recorded in the stratigraphic succession of the zanskar range northwestern himalaya which represents the most complete transect through this ancient continental margin sedimentary history began in the late proterozoic and recorded a late pan african orogenic event around the cambrian ordovician boundary when the gondwana supercontinent was eventually assembled the following long period of epicontinental deposition in shallow seas linked to palaeo tethys lasted until the early permian when a neo tethyan rift began to open between paleo india and the cimmerian microcontinents neo tethyan history can be subdivided into two sedimentary megasequences both recording a major tectonic and magmatic event in the lower part the first one began with breakup in the late permian and lasted until the end of the jurassic the second one started in the early cretaceous with the final detachment of india from gondwana and the opening of the indian ocean and ended with the india eurasia collision in the early eocene the two megasequences can be in turn subdivided into six transgressive regressive supersequences bounded by tectonically enhanced unconformities basal sandstone units of early permian late permian norian callovian early cretaceous and paleocene age are invariably associated with oolitic ironstones or reworked glauco phosphorites and mark the transgre sive part of each supersequence next condensed nodular carbonates or shales with pelagic fauna are typically overlain by thick shallowing upward marly units capped by regressive platformal carbonates the six tectono eustatic supercycles reflect successive rifting episodes which punctuated the progressive separation of india from the rest of gondwana and document the combination of plate microplate reorganizations and eustatic climatic and oceanographic changes in the tethyan domain after the onset of collision between india and asia close to the paleocene eocene boundary obduction of the remnants of the neo tethys ocean floor onto the indian margin began and the latter underwent multiphase deformation with fold thrust shortening followed by heating and extension after the main metamorphic event ophiolitic nappes were re thrusted and finally emplaced with their sedimentary sole on top of the passive margin succession

isotopic and structural constraints on the location of the main central thrust in the annapurna range central nepal himalaya five isotope enhanced geologic transects in the southern annapurna range of central nepal elucidate structural geometries near the main central thrust whole rock nd isotopes and u pb ages of detrital zircons unambiguously distinguish greater himalayan hanging wall and lesser himalayan footwall metasedimentary rocks nd 0 values for lower lesser himalayan rocks typically range from 20 to 26 whereas greater himalayan rocks usually have nd 0 values of 19 to 12 lower lesser himalayan rocks yield detrital zircons with an age peak at ca 1880 ma and no detrital zircons younger than ca 1550 ma in contrast greater himalayan rocks yield detrital zircons with a prominent broad peak of ages at ca 1050 ma and no detrital zircons younger than ca 600 ma the protolith boundary between greater and lesser himalayan rocks is up to 1 km farther south than usually mapped on the basis of lithology field and microstructural observations indicate the presence of a top to the south ductile shear zone superimposed on this boundary confirming this shear zone as the main central thrust no evidence exists for large scale structural mixing of greater and lesser himalayan rocks along the main central thrust in the annapurna range

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

stratigraphy and sedimentary history of the nepal tethys himalaya passive margin abstract the sedimentary history of the nepal tethys himalaya began with deposition of thick carbonates in the cambro ordovician followed by a mixed siliciclastic carbonate epicontinental succession recording two major deepening events in the early silurian and late devonian fossiliferous carbonate ramp deposits in the tournaisian were disconformably followed by white quartzose sandstones and black mudrocks with locally intercalated diamictites derived from sedimentary rocks and deposited in asymmetric tectonic basins rift stage break up in the mid early permian locally associated with effusion of tholeiitic lava flows was followed by a transgressive sandy to shaly locally coal bearing or bioclastic unit capped by condensed pelagic carbonates in the middle to late permian juvenile ocean stage subsidence of the cooling stretched crust led close to bathyal water depths in the olenekian but then slowed down in the middle triassic to increase again sharply in the late triassic owing to renewed extensional tectonic activity and sediment loading during up and out building of the indian continental terrace deposition of tropical platform carbonates finally became widespread in the middle liassic mature passive margin stage the initial fragmentation of gondwana in the middle jurassic led to rejuvenation of the indian craton and deposition of quartzo feldspathic hybrid arenites capped by condensed oolitic ironstones deposited at warm subtropical latitudes in the late bathonian middle callovian next a discontinuous pelagic grey marly limestone unit was followed by the ammonoid rich offshore spiti shale in the late jurassic the final disintegration of gondwana began in the berriasian when quartzose siliciclastics derived again from the rejuvenated indian craton and partly from recycling of older clastic successions were followed by thick deltaic to shelf volcaniclastics documenting eruption of alkali basalts in the valanginian followed in the hauterivian to albian by more felsic differentiates such as the trachyandesites exposed in the lesser himalaya 120 km to the south a widespread drowning episode fostered by waning volcaniclastic supply during a global eustatic rise is documented by a major glauconitic horizon deposited at middle southern latitudes in the late albian overlain by scaglia like pelagic limestones in the latest albian the final part of sedimentary history during the rapid northward flight of india and its collision with eurasia is not documented anywhere in nepal due to later erosion of upper cretaceous to lower tertiary strata

the himalayan passive margin from precambrian to cretaceous times abstract the himalaya passive margin should be separated into the indian himalaya and the pakistan himalaya divided by the nanga parbat haramosh basement uplift in the northwestern syntaxes the indian passive margin shows a complex history involving splitting of microcontinents off the northern gondwana margin from early paleozoic times until the jurassic in contrast the pakistan himalaya formed part of the stable indian shelf until separation from africa started in the jurassic each margin has a distinct stratigraphic history until the mid mesozoic after which they show similar histories until the tertiary the himalaya mountains in india consist of three distinct tectonic units juxtaposed during the neogene by southward thrusting of the northern indian precambrian to mesozoic passive margin the lesser himalaya on the south forms a late precambrian passive margin usually directly overlain by late cretaceous to tertiary clastics its stratigraphy is closest to that of the indian shield in contrast the high himalaya has a thin ordovician to carboniferous shelf sequence deposited after early ordovician deformation and granite intrusion above this thick permian and mesozoic shelf sequences mark the separation of continental blocks off the northern indian margin and the opening of the neotethys ocean the north himalaya formed the slope and basin of this ocean and consists of reactivated paleozoic gneiss domes overlain by thick mesozoic sediments which pass in the northeastern himalaya into an enormously thick sedimentary sequence resting on oceanic crust paleomagnetic and structural evidence indicates at least 500 km of southward thrusting along the main central thrust between the lesser and high himalaya this thrusting together with a further 250 km of erosion at the front of the nappes has removed the entire inner shelf of the himalaya passive margin and can explain the startling contrast between the high and lesser himalaya stratigraphic sequences but there are still discrepancies particularly in the eastern himalaya where precambrian basement is juxtaposed with the north himalaya mesozoic slope these discrepancies can be partly resolved by strike slip movements roughly parallel to the himalaya which have removed parts of the northern indian passive margin such faults of the requisite orientation displacement and age occur in southeast asia where their cumulative displacements can add up to several thousand kilometres similar faults of undoubted pre tertiary age occur in the pakistan himalaya here the passive margin shows a much simpler history of jurassic to miocene subsidence

black smokers massive sulphides and vent biota at the mid atlantic ridge the discovery of black smokers massive sulphides and vent biota in the rift valley of the mid atlantic ridge demonstrates that this assemblage of hydrothermal phenomena is not limited to intermediate to fast spreading oceanic ridges hydrothermal exchange processes may thus be important at the ridges which extend through the atlantic ocean and western indian ocean comprising more than half of the 55 000 km global length of seafloor spreading centres

himalayan metamorphic sequence as an orogenic channel insight from bhutan the bhutan himalayas differ from the rest of the himalayas in two major ways i low grade sedimentary rocks lie above the greater himalayan sequence as klippen i e erosional remains of the south tibetan detachment and ii an out of sequence thrust the kakhtang thrust lies structurally above the klippen and it doubles the exposed thickness of the greater himalayan sequence our field observations and geochronological data constrain the main kinematic events in the bhutan himalayas crystallisation ages of leucogranite dykes deformed by the main central thrust and the south tibetan detachment indicate that these two structures operated together between 16 and 22 ma the out of sequence kakhtang thrust was active at 10 14 ma and was concurrent with reactivation of the south tibetan detachment restoration of the bhutan himalayas prior to the out of sequence thrusting shows that the greater himalayan sequence was the core of a long low viscosity crustal channel extending under the tibetan plateau we propose that the gravity driven southward extrusion of the channel material from underneath the tibetan plateau caused the inverted metamorphic sequence in the lesser himalayan sequence and in the greater himalayan sequence this process also led to occurrences of present day surface rocks that were derived from variable distances down dip but from similar crustal depths such an exhumation pattern can explain the similar peak pressures for the greater himalayan sequence along the length of the himalayas

implications of shortening in the himalayan fold thrust belt for uplift of the tibetan plateau 1 recent research in the himalayan fold thrust belt provides two new sets of observations that are crucial to understanding the evolution of the himalayan tibetan orogenic system first u pb zircon ages and sm nd isotopic studies demonstrate that 1 greater himalayan medium to high grade metasedimentary rocks are much younger than true indian cratonic basement and 2 these rocks were tectonically mobilized and consolidated with the northern margin of gondwana during early paleozoic orogenic activity these observations require that greater himalayan rocks be treated as supracrustal material in restorations of the himalayan fold thrust belt rather than as indian cratonic basement in turn this implies the existence of greater himalayan lower crust that is not exposed anywhere in the fold thrust belt second a regional compilation of shortening estimates along the himalayan arc from pakistan to sikkim reveals that 1 total minimum shortening in the fold thrust belt is up to 670 km 2 total shortening is greatest in western nepal and northern india near the apex of the himalayan salient and 3 the amount of himalayan shortening is equal to the present width of the tibetan plateau measured in an arc normal direction north of the indus yalu suture zone this information suggests that a slab of greater indian lower crust composed of both indian cratonic lower crust and greater himalayan lower crust with a north south length of 700 km may have been inserted beneath the tibetan crust during the cenozoic orogeny we present a modified version of the crustal underthrusting model for himalayan tibetan orogenesis that integrates surface geological data recent results of mantle tomographic studies and broadband seismic studies of the crust and upper mantle beneath the tibetan plateau previous studies have shown that incremental mesozoic and early cenozoic shortening had probably thickened tibetan crust to 45 55 km before the onset of the main cenozoic orogenic event thus the insertion of a slab of greater indian lower crust with maximum thickness of 20 km tapering northward could explain the cenozoic uplift of the tibetan plateau the need for tibetan crust to stretch laterally as the greater indian lower crust was inserted may explain the widespread east west extension in the southern half of the plateau our reconstruction of the himalayan fold thrust belt suggests that indian cratonic lower crust of presumed mafic composition and high strength should extend approximately halfway across the tibetan plateau to the banggong suture from there northward we predict that the tibetan plateau is underlain by more felsic and therefore weaker lower crust of greater himalayan affinity two slab break off events are predicted by the model the first involved neotethyan oceanic lithosphere that foundered 45 35 ma and the second consisted of greater indian lithosphere most likely composed of greater himalayan material that delaminated and foundered 20 10 ma asthenospheric upwelling associated with the break off events may explain patterns of cenozoic volcanism on the tibetan plateau although the model predicts a northward migrating topographic front due solely to insertion of greater indian lower crust the actual uplift history of the plateau was complicated by early middle tertiary shortening of tibetan crust

lithostratigraphic correlations in the western himalaya an isotopic approach we present the results of a whole rock nd isotopic study of two contrasting regions of the western himalaya using the neodymium model age approach on the scale of a single orogen high grade metasedimentary rocks from zanskar yield model ages t dm that are similar to those of the high himalayan crystalline series t dm 1 2 2 0 ga nd 6 to 16 and distinct from values from the lesser himalaya t dm 2 3 3 4 ga nd 18 to 27 hence these two lithological sequences can be recognized for 2000 km along the strike of the orogen data for the basement of the nanga parbat massif at the western extremity of the himalaya t dm 2 3 2 8 ga nd 18 to 30 suggest that these rocks are not equivalent to the high himalaya as previously supposed but have affinities with the lesser himalaya a thin metasedimentary cover sequence on the margins of the nanga parbat massif is isotopically indistinguishable from the high himalaya t dm 1 6 1 8 ga nd 10 to 14 the prior misidentification of the provenance of the massif stems from its high metamorphic grade characteristic of the high himalaya but in this case related to the unique neogene history of the nanga parbat massif which has exhumed a higher grade equivalent of the lesser himalaya that is not seen elsewhere

exhumation history of a section of the sikkim himalayas india records in the metamorphic mineral equilibria and compositional zoning of garnet the exhumation history of pelitic migmatite samples from the high himalayan crystalline complex hhc near the south tibetian detachment system in the sikkim darjeeling section has been determined on the basis of thermo barometric analyses retrograde breakdown reactions and compositional zoning of garnet the peak metamorphic condition is estimated to be 10 4 kbar 800 c from thermo barometric and phase equilibrium constraints the observed retrograde breakdown of garnet to spinel and cordierite requires near isothermal and hence extremely rapid 15 mm yr exhumation up to the depth of 15 km numerical modeling suggests that the initial rapid exhumation must have been followed by a much slower process 2 mm yr up to at least 5 km depth to lead to the development of the observed compositional zoning of garnet the dramatic change of the exhumation velocity vz might reflect a process of tectonic thinning followed by erosion and or horizontal flow at shallow depth assuming that the vz did not change significantly below 5 km depth these results suggest that the hhc samples studied in this work exhumed from a depth of 34 km within 8 ma

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

thrust tectonics crustal shortening and the structure of the far eastern nepal himalaya balanced and restored structural sections across the far eastern nepal himalaya have been constructed in order to determine the structure and evolution of the himalayan orogenic wedge and the amount of tectonic shortening the region has undergone since the initiation of thrusting along the main central thrust mct the far eastern nepal himalaya is comprised of three distinct thrust bound tectonic packages the higher himalayan crystalline thrust sheet the lesser himalayan metasediment thrust package and the sub himalayan imbricate fan the higher himalayan crystallines consisting of kyanite and sillimanite bearing gneisses intruded by the miocene jannu leucogranites have been thrust over the lesser himalayan metasediments along the mct for a distance of 140 km to 175 km the lesser himalayan metasediments are a 12 km thick unit consisting primarily of phyllites metaquartzites and mylonitic augen gneisses in which garnet biotite and chlorite metamorphic zones are exposed in progressively deeper structural levels the lesser himalayan metasediment thrust package is underlain by a decollement the main detachment fault mdf which lies at a calculated depth of between 6 and 10 km underneath the mahabharat lekh and at a calculated depth of 20 to 25 km north of the tamar khola dome the tamar khola dome overlies a footwall ramp along the mdf where the mdf cuts upsection through the lesser himalayan metasediments the lesser himalayan thrust package probably has an internal structure aproximating a hinterland dipping duplex with the mct and the mdf corresponding to the roof and floor thrusts respectively both the tamar khola thrust an out of sequence breach thrust and the main boundary thrust mbt are splay thrusts off of the mdf the sub himalaya consisting of nonmetamorphosed sedimentary rocks displays an emergent imbricate fan geometry and is underlain by the southern continuation of the mdf which lies at a depth of 5 5 km to 6 km beneath the siwalik hills folding and thrusting within the lesser himalayan thrust package and the sub himalayan imbricate fan have accomodated 45 to 70 km of tectonic shortening total north south shortening across the higher lesser and sub himalaya of far eastern nepal south of the tibetan plateau has been of the order of 185 km to 245 km and has occurred at an average rate of 7 4 mm to 15 3 mm per year since the initiation of the mct between 16 and 25 ma

duplexes as slip transfer zones application to the himalayan fold thrust belt abstract duplex fault zones and imbricate fans originally termed schuppen schuppenstruktur or imbricate structure were first described in the late 1800s from the alps nw highlands scotland and montana rockies lewis thrust sheet as originally defined duplexes contained shingled imbricate slices bounded above and below by through going thrusts these duplex fault zones commonly occur along the base of major regional thrust sheets or within the body of such sheets accommodating regional bed parallel shortening and transferring slip from a lower stratigraphic horizon to a higher one thereby providing a mechanical advantage to the emplacement of large thrust sheets however faulting is not the only means of layer parallel shortening lps and displacement transfer within duplexes lps cleavage and detachment folds are also common and can play the same role as imbricate faults based on some selected examples we recognize three types of duplexes 1 fault duplex 2 cleavage duplex and 3 fold duplex the style of duplex that forms is likely a function of mechanical stratigraphy and deformation conditions the relationships among the three basic duplex styles can be plotted on a ternary diagram each duplex type constituting an apex given the complexities of most sedimentary sequences hybrid duplexes with a mix of fault fold and cleavage shortening can also form hybrid duplexes plot along the side of the triangle as fault fold fold cleavage and fault cleavage duplexes while those combining all three mechanisms plot in the middle if the partitioning of shortening by the different mechanisms can be quantified the ternary diagram provides a means of representing the contributions of the three mechanisms in shortening and slip transfer along much of the himalayan fold thrust belt a significant portion of the lesser himalayan shortening is taken up by the lesser himalayan duplex a first order fault duplex with subsidiary cleavage and fold development the geometry of and shortening within the duplex varies significantly along the length of the himalaya

age of initiation of collision between india and asia a review of stratigraphic data abstract the collision of india with asia is perhaps the most profound tectonic event to have occurred in past 100 ma it is responsible for the uplift of the himalayas and tibetan plateau and has been argued to have been responsible for geological geochemical and climatological consequences of global extent yet the age of initiation of this collision remains poorly constrained the literature is replete with estimates that range from the late cretaceous 65 ma to latest eocene

pyrite and phosphate in anoxia and an origin of life hypothesis the metal sulfide pyrite fes2 plays a key role in the elaborate origin of life hypothesis presented by wachtershauser g wachtershauser proc natl acad sci usa 87 1990 200 204 it is envisaged that the in situ formed positively charged pyrite was electrostatically concentrating the de novo synthesized prebiotic anionic organic molecules in statu nascendi here we show that anionic phosphate and phosphorylated organic molecules 5 adenosine mono phosphate and phosphoglyceric acid interact with pyrite regardless of whether the overall surface charge is positive or negative we demonstrate that aqueous fe2 metal ion h2s and s2o2 3 influence the interaction of phosphate with pyrite phosphate itself controls the interaction of adenosine ade and 5 amp with pyrite 5 amp interacts via its phosphate functional group with pyrite surfaces the reported experiments illustrate the intermediary role of phosphate between pyrite mineral surfaces and aqueous organic molecules we suggest here that on a prebiotic earth phosphate could have been concentrated on metal sulfide minerals which in turn were selectively concentrating organic molecules favoring phosphorylated compounds

iron and sulfur in the pre biologic ocean tentative geochemical cycles for the pre biologic earth are developed by comparing the relative fluxes of oxygen dissolved iron and sulfide to the atmosphere and ocean the flux of iron is found to exceed both the oxygen and the sulfide fluxes because of the insolubility of iron oxides and sulfides the implication is that dissolved iron was fairly abundant and that oxygen and sulfide were rare in the atmosphere and ocean sulfate produced by the oxidation of volcanogenic sulfur gases was the most abundant sulfur species in the ocean but its concentration was low by modern standards because of the absence of the river borne flux of dissolved sulfate produced by oxidative weathering of the continents these findings are consistent with the geologic record of the isotopic composition of sedimentary sulfates and sulfides except in restricted environments the sulfur metabolism of the earliest organisms probably involved oxidized sulfur species not sulfide

pyrite oxidation during sample storage determines phosphorus fractionation in carbonate poor anoxic sediments abstract we investigated the phosphorus p and iron fe fractionation in four cores with anoxic sediments deposited during the mid cretaceous oceanic anoxic event 2 94 ma and the paleocene eocene thermal maximum 55 ma that were exposed to oxygen after core recovery surprisingly p associated with iron oxyhydroxides fe bound p was a major p phase in these laminated sediments deposited under euxinic conditions a significant fraction of total fe was present as poorly crystalline ferric fe this fraction increased with increasing storage time of the investigated cores in carbonate poor samples fe bound p accounted for up to 99 of total p and its abundance correlated with pyrite contents in samples with higher caco3 contents 5 wt in the investigated samples p was mostly present in authigenic ca p minerals irrespective of pyrite contents we conclude that the p fractionation in anoxic carbonate poor sediments is strongly affected by pyrite oxidation that occurs when these sediments are exposed to oxygen pyrite oxidation produces sulfuric acid and iron oxyhydroxides the abundance of poorly crystalline fe oxyhydroxides provides further evidence that these were indeed formed through recent post recovery oxidation rather than in situ tens of millions of years ago the acid dissolves apatite and the released phosphate is subsequently bound in the freshly formed iron oxyhydroxides pyrite oxidation thus leads to a conversion of authigenic ca p to fe bound p in more calcareous samples caco3 can act as an effective buffer against acidic dissolution of ca p minerals the results indicate that shielding of sediments from atmospheric oxygen is vital to preserve the in situ p fractionation and to enable a valid reconstruction of marine phosphorus cycling based on sediment records

high p t polymetamorphism dehydration melting and generation of migmatites and granites in the higher himalayan crystalline complex sikkim india the higher himalayan crystalline complex hhc in sikkim india consists of pelitic migmatites interlayered with calc silicate rocks and minor metabasites microstructural relationships between the mineral phases and deformational fabric elements and zoning characteristics of garnet indicate a prolonged and complex polymetamorphic history for the hhc the pelitic rocks in the upper part of the hhc contain the assemblage plagioclase quartz garnet k feldspar biotite sillimanite and are devoid of muscovite most of the mineral phases grew syn to post tectonically mineral growth coincided with development of a pervasive fabric s 2 during prograde metamorphism m 2 in the early stages of the collisional event some garnet grains display texturally distinct cores and rims which are separated by calcic plagioclase this texture suggests an earlier metamorphic episode m l m may represent pre himalayan metamorphism and decompression of the hhc later the collisional event led to renewed burial of the hhc and m 2 reactions m 2 is reflected by dehydration melting of muscovite and biotite to form granitic melts which either crystallized in situ to form leucosomes or migrated from their source regions to form larger granitic bodies geothermobarometric estimates for peak m 2 conditions indicate p 10 12 kbar t 800 850 c a subsequent metamorphic event m 3 occurred because of 5 kbar of decompression m 3 is recorded by the breakdown of porphyroblastic garnet in all hhc lithologies higher temperature and pressure estimates come from progressively higher structural levels of the complex the thermal gradient of 5 5 c km is anomalous and may be a consequence of thermal buffering during melting however the pressure gradient of 0 25 kbar km resembles a normal lithostatic gradient which suggests that the hhc in sikkim represents an inverted barrovian sequence this inverted zonation of the hhc is probably the result of large scale structural inversion and or tectonic juxtaposition because of ductile shearing

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

tectonostratigraphic subdivisions of the himalaya a view from the west 1 indian plate rocks in the central himalaya have traditionally been divided into orogen parallel fault bound tectonostratigraphic zones a straightforward westward extrapolation of these zones has proved problematic in part because of a lack of consensus on the existence or significance of major faults within the metamorphic zone of the indian plate in pakistan where more than 10 locations for the main central thrust mct have been proposed we address this ambiguity by systematically tracing established central himalayan tectonostratigraphy around the western himalayan syntaxis and across pakistan this exercise reveals the following stratigraphic and structural relationships 1 there is a westward decrease in neogene shortening across the himalayan fold and thrust belt such that there is no age equivalent thrust in pakistan with displacement and metamorphic juxtaposition equivalent to the central himalayan mct 2 shortening across the fold and thrust belt in western pakistan is concentrated in the unmetamorphosed foreland as opposed to the metamorphic zone in the central himalaya 3 lesser himalayan higher himalayan and tethyan rocks are in stratigraphic order within the metamorphic zone of pakistan which appears to be the metamorphic equivalent of kashmir tethyan stratigraphy 4 the combination of early paleozoic and late paleozoic tectonism in pakistan has locally eliminated upper proterozoic higher himalayan rock and lower to middle paleozoic tethyan rock from the metamorphic zone of pakistan 5 late cretaceous and or early paleocene proto himalayan deformation in the pakistan foreland telescoped and eroded stratigraphy prior to the main phase of himalayan orogeny 6 tectonostratigraphic zones are offset in eastern pakistan by the transverse jhelum balakot fault 7 there is no evidence within the indian plate of pakistan for a large scale normal fault system comparable to the south tibetan detachment system 8 stratigraphy as well as the age and tectonic setting of deformation and metamorphism must be taken into account when drawing tectonostratigraphic zones

do along strike tectonic variations in the nepal himalaya reflect different stages in the accretion cycle insights from numerical modeling abstract whereas the large scale morphology and dynamics of orogenic wedges are well explained by critical taper theory many questions remain unanswered regarding the details of how deformation is accommodated internally here we investigate the dynamics of a collisional orogenic wedge bounded by an over thickened continental plateau using two dimensional thermo mechanical numerical models these models applied to the himalayan orogen and compared with reference cross sections lead us to propose a new hypothesis to explain along strike variations in tectonic style topography and exhumation patterns observed along the himalayan range by a combination of two mechanisms first numerical models produce a cycle of crustal ramp formation and advection toward the rear of the wedge the asynchronous evolution of this cycle along different segments of the range may account for the well documented lateral variations in the geometry of the main himalayan thrust mht and for the existence of a well defined topographic transition in some segments of the range second the models suggest that the formation of duplexes leading to the isolation of klippen along the range front may be controlled by rheological contrasts between the tibetan plateau and or the greater himalayan sequence and the colliding indian plate