contact metamorphism and depth of emplacement of the manaslu granite central nepal implications for himalayan orogenesis abstract the manaslu massif central nepal provides a well exposed example of a deeply eroded pluton its contact aureole can be followed from the base to the top along medium to low grade mesozonal to anchizonal tethyan metasediments contact metamorphic mineral assemblages and thermobarometric estimations suggest that the granite was emplaced at 18 21 km for the base and 9 13 km for the roof calculated temperatures within the aureole at 550 40 c are compatible with the intrusion temperature of a leucogranitic magma microstructural evidence shows that the temperature remained high 500 c at the base of the massif during and after granite emplacement whereas towards the top of the granite deformation proceeded rapidly at lower temperature heating of the abundant calcareous rocks in the contact aureole released a local co2 rich fluid whereas a h2o and boron rich fluid seems to have pervaded the whole zone this fluid is probably exsolved from the granitic melt during its crystallization the depth of emplacement of the massif has an important implication for the reconstruction of the himalayan geodynamic evolution implying a burial of the tethyan metasediments by a major refolding of the sedimentary cover or more probably by extensive development of the north himalayan nappes towards the south for more than 70 km before granite emplacement i e before the miocene the young age of the granite and its depth of emplacement suggest a rapid tectonic denudation in the order of 1 mm a 1 probably by normal faulting north of the massif during its cooling

thermo tectonic history of the tethyan himalayas deduced from the palaeomagnetic record of metacarbonates from shiar khola central nepal abstract palaeomagnetic measurements were carried out on low grade metamorphic carbonates of mesozoic age from the shiar area 85 1 e 28 6 n of the tethyan himalaya th in north central nepal two characteristic remanence components carried by pyrrhotite chrm 1 and magnetite chrm 2 could be identified by their unblocking temperature spectra of 270 340 and 430 580 c respectively fold tests are not significant due to the uniform bedding of all sites however according to results from other areas of the th the pyrrhotite component has been probably acquired as a secondary p trm during exhumation and cooling thus the age of remanence acquisition can be related to the last cooling event 25 17 ma in the surrounding areas the inclination of the magnetite component matches the value expected from the indian apwp this may the primary origin of the chrm 2 pyrrhotite site mean directions show a small circle distribution with a best fit parallel to the n s direction backtilting to the expected inclination i exp by intersection of the remanence small circle with the small circle of constant i exp yields a clockwise block rotation of 30 35 with respect to the indian plate characteristics of the pyrrhotite component small circle distribution of site means secondary origin p trm with unblocking temperatures below about 300 c allow the interpretation of the chronologic order of the thermo tectonic history i an earlier main folding phase at elevated temperatures ii a later event of cooling through about 300 c coinciding with the acquisition of chrm 1 iii clockwise block rotations with respect to the indian plate and iv long wave folding as the youngest tectonic event

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

gypsum as a precursor to pyrrhotite in metamorphic rocks during low grade regional metamorphism pyrrhotite can form from gypsum by the reaction caso4 2h2o fe sol 2 2corg fes ca sol 2 2h2o 2co2 this reaction takes place in the anchizone below 350 c and might be initiated by the thermal dehydration of gypsum 200 c and aided by the generation of gaseous hydrocarbons evidence for the reaction is the occurrence in dolomitic layers in the ballachulish slate east larroch quarry argyll of lath and swallow tail shaped quartz dolomite pyrrhotite pseudomorphs after gypsum quartz pyrrhotite bodies in slate represent replaced and deformed mainly flattened crystals concretions and possible veinlets of gypsum pyrite porphyroblast growth after the peak of metamorphism and under relatively high fs2 conditions failed to destroy some early pyrrhotite because it is encapsulated in quartz pyrite silicate reactions and hydrothermal exhalations have been suggested previously to account for pyrrhotite enriched horizons in regionally metamorphosed rocks replacement of gypsum by pyrrhotite is an additional explanation for pyrrhotite enriched horizons especially in dolomitic and graphitic lithologies

coupling of late orogenic tectonics and secondary pyrrhotite remanences towards a separation of different rotation processes and quantification of rotational underthrusting in the western himalaya northern india abstract the total amount of crustal shortening within the indian plate includes straight forward and rotational shortening the complex tectonic history of the western himalaya impedes a direct observation of rotational shortening between india and its northern margin a combination of palaeomagnetic results and tectonic structures along a 300 km nw se section from nw zanskar 33 9 n 76 5 e to malari 30 0 n 79 6 e in northern india enable a separation of rotations on different scales correction of the observed rotations and quantification of uniform rotational shortening new palaeomagnetic results from metacarbonates of the tethyan himalaya in sarchu losar and spiti and earlier studies from nw zanskar and malari reveal a characteristic remanent magnetisation carried by pyrrhotite unblocking spectrum 250 330 c negative fold tests 99 level indicated remanence acquisition after the main folding of the himalaya the formation of pyrrhotite during thermal events t 200 c suggests a remanence acquisition related to exhumation and cooling temperatures of about 200 300 c indicate a thermochemical remanence acquisition with an eohimalayan age dated in two areas by 40k 39ar techniques on illites the resulting rotation angles in respect to stable india show an anomalous counterclockwise rotation in nw zanskar and a trend of decreasing clockwise rotations to the se these variations may be attributed to meso scale 100 km tectonic features and oroclinal bending for the first time block rotations with respect to stable india are separated into local meso scale and regional significant ones after elimination of local and meso scale effects using structural analysis of late orogenic features modelled rotations due to oroclinal bending were subtracted regionally significant rotations of 0 20 clockwise are interpreted to be caused only by uniform rotational underthrusting

palaeomagnetic investigation in the pamirs and its tectonic implications abstract this paper presents a palaeomagnetic investigation in the pamirs of tadjikistan remanences are mainly carried by magnetite and are of secondary synfolding origin application of a new method the small circle reconstruction allows to extract palaeomagnetic rotation and tectonic tilting since the time of remanence acquisition assumed to be 20 ma in some regions consistent rotations are observed i clockwise in the muzkol area of the central pamirs ii following the trend of oroclinal bending in the northern pamirs and iii counterclockwise in the northern folded sequence of the southern pamirs however variations within different regions are obvious and scattering even occurs within smaller areas most likely local block rotations are caused by n s shortening palaeomagnetic and tectonic results reveal that tilt axes are rotated around vertical axes in the course of progressive shortening folding surprisingly the scatter of the tilt axes is reduced hereby by comparing the distributions of the present and former tilt axes the amount of shortening is estimated for the southern pamirs shortened to 40 and the central pamirs shortened to 60

palaeomagnetism of cretaceous to tertiary sediments from southern tibet evidence for the extent of the northern margin of india prior to the collision with eurasia abstract palaeomagnetic studies have been carried out on mid cretaceous to latest palaeocene sediments from the tethyan himalaya on the northern margin of the indian plate near the localities of gamba 28 3 n 88 5 e and duela 28 0 n 89 2 e in southern tibet a total of 61 sites more than 700 cores have been sampled mainly from limestones zongshan formation jidula formation member ii zongpu formation sandstones jidula formation member iii and graywackes gamba group member ii with ages well defined from detailed lithological and biostratigraphic analyses carriers of characteristic remanences are magnetite in limestones and graywackes and hematite in sandstones reliable palaeodirections were obtained from 35 sites 356 specimens positive fold tests antiparallel normal and reverse polarities a northward trend of the palaeolatitudes with decreasing age and absence of any metamorphic overprint indicate a primary origin for the characteristic remanences the results have been compared with expected palaeodirections according to available data for the indian subcontinent based on its present day geometry the late maastrichtian sites fourteen sites 156 specimens and the middle late palaeocene sites fourteen sites 113 specimens yield palaeolatitudes of 5 7 s and 4 0 n for the sampling localities in southern tibet respectively these palaeolatitudes are much further north than expected and imply that crustal shortening between the tethyan himalaya and the indian shield since the time of india eurasia collision has occurred on the order of 1500 km at the longitude of gamba duela comparison of our observed palaeolatitudes with palaeolatitudes for the lhasa block during cretaceous and palaeocene times indicate that initial contact of the northern indian margin with the lhasa block was already established at 65 60 ma with suturing completed at about 55 50 ma

paleomagnetic evidence for clockwise rotation and tilting in the eastern tethyan himalaya se tibet implications for the miocene tectonic evolution of the ne himalaya abstract crustal movement around and away from the namche barwa syntaxis is indicated in the asian velocity field inferred from gps data and quaternary fault slip rates nevertheless there is a limited field based control on the rotational history of the north eastern himalayan arc exploring the poly phase nature of deformation within the cretaceous diorite dykes and their host rock triassic flysch in the eastern tethyan himalaya 90 92 e combined with new remote sensing data and existing thermo geochronological data allow us to unravel the kinematic relationship between paleomagnetic remanence vectors and the deformation phases decay at 325 c in high temperature susceptibility curves and in the thermal demagnetization of the sirm indicate that the characteristic remanent magnetization in the cretaceous diorite dykes is carried by pyrrhotite the pyrrhotite component unblocks at 280 350 c revealing normal and reverse polarities it is of post folding origin with a mean remanence direction of 019 28 and 001 20 in the eastern qonggyai valley and western nagarze part of the sampling area respectively the 22 ma k ar age of the last metamorphic event support that the remanence is post eohimalayan folding and likely of thermoremanent or thermo chemical origin comparison of the declination with respect to the early miocene reference direction yields a trend from no apparent rotation in the west to 20 clockwise rotation in the east with respect to the stable indian plate this result can be kinematically related to the middle to late miocene strain partitioning between far field southeast extrusion of se tibet and near field strike slip faulting and e w extension furthermore the observed pattern of tilting around horizontal axis may reflect concealed north himalayan doming

tectonic versus mineralogical contribution to the magnetic fabrics of epimetamorphic slaty rocks an example from the ardennes massif france belgium abstract the discrimination of the respective contribution of strain and mineralogy of the bulk anisotropy of low field magnetic susceptibility ams of rocks is addressed by a study of the ardennes variscan thrust belt unmetamorphosed to low medium grade rocks in the study area define a succession of magnetic parageneses hematite maghemite hematite maghemite magnetite magnetite pyrrhotite a direct control of the magnetic mineralogy on the magnitude of ams axes is shown in addition in all cases the interpretation of the directions of ams axes as kinematic indicators also applies to these rocks we show a progressive change in shearing direction from the south of the rocroi massif where the nappe transport is n directed in the north the vergence changes to northwestward

magnetic properties of the high himalayan leucogranites structural implications the magnetic properties of the high himalayan leucogranites have been investigated on 527 specimens in three plutons everest makalu 6 sites and manaslu 40 sites in nepal and gangotri 43 sites in india susceptibility varies between 2 and 100 10 6 si with an anisotropy ratio up to 1 16 high field and low temperature magnetic measurements together with comparison with weight percent iron demonstrate that anisotropy of magnetic susceptibility is carried by paramagnetic biotite and tourmaline the latter produces an inverse fabric i e with the minimum axis parallel to mineral lineation the magnetic fabric demonstrates complex patterns of stretching lineations during magmatic emplacement and its usefulness in semi quantitatively estimating petrofabric intensity is demonstrated for the biotite bearing facies natural remanent magnetization was measurable at only two sites in everest makalu where there are well defined reverse directions carried by titanomagnetite and pyrrhotite comparison of these preliminary results with predicted directions for stable india suggests northward tilting of about 10 and a small clockwise rotation of this massif

andradite skarn garnet records of exceptionally low δ 18 o values within an early cretaceous hydrothermal system sierra nevada ca skarn garnets in the mineral king roof pendant of the south central sierra nevada within sequoia national park california usa reveal variable fluid chemistry with a significant component of meteoric water during metasomatism in the early cretaceous sierra nevada batholith we focus on andradite garnet associated with pb zn mineralization in the white chief mine laser fluorination oxygen isotope analyses of 18o of garnet 18o grt from sites along the skarn show a large range of values 8 8 to 4 6 vsmow ion microprobe sims analyses elucidate that individual andradite crystals are strongly zoned in 18o grt up to 7 of variation total rare earth element concentrations ree across individual garnets show progressive depletion of skarn forming fluids in these elements during garnet growth these findings support a skarn model of earliest red high 18o grandite garnet consistent with a magmatic dominated equilibrium fluid 18o h2o as high as 8 later green andradite crystallized in equilibrium with a low 18o fluid indicating a significant influx of meteoric fluid 18o h2o 6 to 5 following a hiatus in garnet growth associated with late stage pb zn mineralization latest orange overprint rims have higher 18o values 18o h2o 0 2 and depleted total rees suggesting influx of high 18o trace element depleted fluid derived from regional metamorphism of the carbonate host remarkably low 18o grt values in the white chief canyon skarn require a significant proportion of meteoric fluid available during 400 c andradite forming metasomatism fluid flow was channelized at the pluton wallrock contact evidenced by the narrow extent of skarn

magnetic monitoring of pyrrhotite alteration during thermal demagnetization pyirhotite progressively oxidizes via magnetite to hematite during thermal demagnetization of porous artificial samples containing well defined natural pyrrhotite rockmagnetic and x ray diffraction determinations show that the intermediate alteration stages consist of pyrrhotite magnetite pyrite intergrowths alteration is quite differently expressed in two ctaiically virtually identical pyrrhotites

the case of the 1981 eruption of mount etna an example of very fast moving lava flows 1 mount etna despite being an extremely active volcano which during the last 400 years has produced many lava flow flank eruptions has rarely threatened or damaged populated areas the reconstruction of the temporal evolution of potentially hazardous flank eruptions represents a useful contribution to reducing the impact of future eruptions by and analyzing actions to be taken for protecting sensitive areas in this work we quantitatively reconstructed the evolution of the 1981 lava flow field of mt etna which threatened the town of randazzo this reconstruction was used to evaluate the cumulated volume the time averaged discharge rate trend and to estimate its maximum value the analysis was conducted by comparing pre and post eruption topographic surfaces extracted by processing historical photogrammetric data sets and by utilizing the eruption chronology to establish the lava flow front positions at different times an unusually high discharge rate for etna of 640 m3 s was obtained which corresponds well with the very fast advance rate observed for the main lava flow a comparison with other volcanoes presenting high discharge rate was proposed for finding a clue to unveil the 1981 etna eruptive mechanism a model was presented to explain the high discharge rate which includes an additional contribution to the lava discharge caused by the interception of a shallow magma reservoir by a dike rising from depth and the subsequent emptying of the reservoir

magnetic polarity stratigraphy of siwalik group sediments of karnali river section in western nepal the remanent magnetization of siltstones and sandstones sampled at 476 levels sites throughout a 3560 m thick molasse sequence belonging to the siwalik group 0 2015 m lower 2015 3560 m middle has been studied by stepwise thermal demagnetization this section is exposed along the karnali river in nepal the natural remanent magnetization nrm usually consists of two components a viscous or thermoviscous component of recent field origin and an ancient characteristic component chrm the former component is of normal polarity and resides either in goethite unblocking temperature 150 c resistant to afd up to 150 mt or in maghemite unblocking temperature 150 400 c goethite contributes up to 90 per cent of the total intensity in the finer variegated muddy samples belonging to the lower half of the section maghemite content is significant in the grey mud free lithologies from the upper half of the section the main component unblocked in the high temperature range commonly 610 680 c and believed to reside in haematite presumably of mostly detrital origin represents a characteristic remanence chrm the tilt corrected chrm directions at individual sites show antipodal clusters ratio of normal to reverse polarity sites 0 62 and yield mean inclinations recording significant inclination shallowing a feature well recorded in the siwaliks this chrm is interpreted to represent a largely primary detrital remanence the chrm data from 430 sites yield the karnali river magnetic polarity sequence whose correlation with the geomagnetic polarity timescale cande kent 1995 suggests a depositional age of 16 ma younger than chron c5cn 1n to 5 2 ma around the top of chron c3r for the 3560 m section sampled hence the karnali river exposes the oldest part of the siwalik group in nepal estimates of the sediment accumulation rate sar average to 32 9 cm kyr 1 for the 10 8 myr time span of deposition

the himalayan arc large scale continental subduction oroclinal bending and back arc spreading abstract palaeomagnetic results from the himalayan arc and southern tibet compared with simulated apparent polar wander paths for the indian plate show a consistent pattern of rotations of the himalayan arc relative to the indian shield varying gradually from 45 clockwise in the northwestern himalaya to slightly counterclockwise in the lhasa region this pattern is consistent with continental underthrusting of greater india beneath the tibetan plateau since the early miocene over at least 650 km at the longitude of western nepal and oroclinal bending since the latest miocene available palaeomagnetic observations support the steady state model for the formation of the himalayan arc with refinements as follows 1 collision between greater india s northern boundary and southern asia occurred at equatorial latitudes with progressive suturing from palaeocene in the northwestern himalaya until early eocene in the eastern himalaya 2 continuing convergence and indentation of greater india into southern asia over about 2000 km up to the early miocene resulted in southeastward extrusion of indochina and 3 neogene counterclockwise rotational underthrusting of greater india along the main central thrust with pliocene quaternary oroclinal bending of the himalayan arc

hydrothermal fluid controlled remagnetization of sedimentary rocks in korea tectonic importance of pervasive tertiary remagnetization abstract in order to better understand the tectonic environment of the korean peninsula since cretaceous a paleomagnetic study was carried out on the jinan basin located in southwestern korea from the demagnetization of 597 samples we found that remanence is carried by pyrrhotite and or magnetite black siltstone and hematite red siltstone which is a common magnetic mineral assemblage of the rocks remagnetized in early tertiary in korea all the rock units recorded identical ancient geomagnetic field directions in particular paleodirections were better clustered in geographic coordinates indicating a secondary origin the presence of abundant silicates as inclusions within fe oxides and fe sulfides confirms the authigenic origin of the magnetic carriers precipitated by fluid mediated chemical processes compilation of the korean cretaceous and tertiary poles indicates that the jinan basin was remagnetized in the early tertiary age in addition comparison of the korean cretaceous and tertiary poles with those from adjacent major terrains including china and siberia reveals that a dominant feature of tectonic motions in korea was a vertical axis rotation sense of vertical axis rotation was time dependent with clear distinction between clockwise rotation in cretaceous and counter clockwise rotation in early tertiary such differences in the mode of vertical axis rotation might be caused by the activities of basin bounding strike slip faults in korea possibly driven by the changes of subduction regime of the kula pacific plate in early tertiary

revised and synthetic apparent polar wander paths of the african eurasian north american and indian plates and true polar wander since 200 ma we have reviewed paleomagnetic data available for the eurasian african north american and indian plates over the last 200 ma selection criteria are those generally accepted with an emphasis on evidence for lack of remagnetization accurate dating and proper structural analysis this results in 23 35 51 and 2 poles for eurasia africa north america and india respectively we believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander apw paths than averaging of larger data sets involving less stringent selection criteria we propose and describe revised apw paths but more importantly we next use relative motion models to transfer all data in a common reference frame we find good agreement between transferred data when they are averaged separately for each plate in independent 20 ma windows this is a check on consistency of paleomagnetic data kinematic models and of the geocentric dipole hypothesis transferred data from all plates are averaged in 20 ma windows to generate synthetic apw paths for all plates studied these synthetic paths are in agreement with the original revised apw paths that use only data from a single plate moreover both geographic and time resolution are improved spatial confidence intervals are of the order of 5 the synthetic paths display interesting features such as a previously ill recognized apw loop for eurasia paleomagnetic and hotspot apw are next compared and a determination of true polar wander tpw is derived we find significant tpw amounting to over 20 in the last 200 ma tpw appears to be episodic with a standstill between 180 and 110 ma there is general agreement with a previous study of livermore et al 1984 the tpw standstill appears to correlate with a time of decreasing reversal frequency ending with the cretaceous long normal superchron other periods of fast tpw would seem to correspond to increasing reversal frequency however it is suggested that the major tpw hairpin at 50 ma might correspond to the collision of india with eurasia tentative correlations with core or mantle indicators can be understood as a result of couplings between the core mantle and lithosphere we believe primarily related to episodic evolution of the d layer

temperature and age constraints on the metamorphism of the tethyan himalaya in central nepal a multidisciplinary approach abstract metasediments of devonian to triassic age of the tethyan himalaya th from several areas in central and western nepal between western dolpo and marsyandi valley were sampled for thermo metamorphic studies illite and chlorite crystallinity vitrinite reflectance calcite dolomite and chlorite chloritoid geothermometers k ar dating on illite rich fractions and zircon fission track thermochronology this paper reports for the first time the occurrence of chloritoid in the th out of the range of granite contact aureoles it also presents the first zircon fission track dating performed on th metasediments the peak temperatures of metamorphism have been around 250 300 c 320 350 c 330 370 c and 400 450 c in the western dolpo hidden valley manang and marpha areas respectively in the manang and hidden valley areas illite k ar data are interpreted as ages of recrystallized k white micas newly formed during metamorphism at around 25 30 ma in the marpha area illite k ar and zircon fission track ages 12 15 ma are consistent representing cooling ages after metamorphic overprinting of higher grade than in the other areas the joint investigation of the organic maturation and zircon ft chronology yields insight on the thermal calibration of zircon reset the rmax of 5 7 to 8 0 indicates a temperature range of ca 315 325 c which is in the partial annealing zone of the zircon ft thermochronometer our results might be explained by the presence of a thick thrust sheet once existing above the study area

magnetic transition at 30 34 kelvin in pyrrhotite insight into a widespread occurrence of this mineral in rocks abstract a characteristic magnetic transition at 30 34 k is shown to provide a powerful tool for the identification of pyrrhotite with concentration down to 10 ppm through the same low temperature techniques as applied to magnetite and hematite extended down to liquid helium temperature a review of rock magnetic and petrological data on pyrrhotite suggests that this mineral should be considered as a major carrier of paleomatnetic signals unblocking temperature up to 350 c and extreme resistance against af may be encountered in fine grained pyrrhotite

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

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

a time framework based on magnetostratigraphy for the siwalik sediments of the khaur area northern pakistan the magnetostratigraphy of eleven new sections in the khaur area of northern pakistan is presented all specimens have been subjected to thermal demagnetization the sections taken in adjacent ravines incising the siwalik formations nearly penpendicular to strike form a temporal framework in which to interpret the biological and lithological information a composite section is constructed from several long sections and is correlated to the magnetic polarity time scale chron 6 to chron 15 providing absolute age information for the biostratigraphic zonations of barry et al 1982 detailed sections along the strike of major stratigraphic marker beds document an isochronous horizon and form the basis for paleoenvironmental reconstructions the preservation of a polarity transition in fluvial sediments suggests continuity of sedimentation on a time scale of 103 104 years