pressure temperature evolution in the kathmandu and gosainkund regions central nepal abstract the kathmandu and gosainkund regions in central nepal are occupied by a metamorphic sequence of upper amphibolite to granulite facies metasedimentary rocks on the basis of structure and lithology three units are distinguished the gosainkund crystalline nappe gcn corresponding to the southward continuation of the higher himalayan crystallines hhc of langtang and the kathmandu crystalline nappe kcn representing an out of sequence thrust sheet in the lesser himalaya the boundary between the two nappes corresponds to the main central thrust mct the gcn and the kcn are thrust over the lesser himalaya along the mct and the mahabharat thrust mt respectively structural and petrographical data demonstrate that the metamorphic assemblage in the three units is related to movements on the mct and mt with a well preserved inverted thermal gradient in the mct zone both conventional and thermocalc thermobarometric methods are used to calculate pressure and temperature the upper lesser himalayan sample records syn mct metamorphic conditions of 750 150 mpa and 566 136 c the kcn samples record p t conditions from 900 140 to 720 420 mpa and 700 60 to 484 194 c while the gcn samples equilibrated at upper amphibolite to granulite facies conditions from 890 260 to 583 198 mpa and 754 40 to 588 100 c error estimation quoted at 2 the final equilibration of the upper section of the gcn occurred with a pressure and temperature drop of 170 mpa and 60 c respectively associated with nnw normal ductile shearing the good preservation of the metamorphic conditions in the different units with no field and petrographic evidence of a metamorphic discontinuity across the mct and mt zones suggests that the final exhumation of the nappe piles occurred along the mbt

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

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

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

ductile thrusting in the himalayas shear sense criteria and stretching lineations the most useful criteria used for the determination of the sense of shear can be found in the huge himalayan ductile shear zone known as m c t they allow the determination of the sense of shear in relation with an unequivocal southward directed intracontinental thrust the various criteria analyzed at different scales of observation show the permanence of the shear flow regime throughout the metamorphic evolution however most of the observed shear criteria are due to late nappe emplacement postdating the climax of the metamorphism the most striking feature is the stretching lineation which is roughly normal to the thrust trace all along the belt it parallels everywhere the shear direction and thus indicates the transport direction of the nappes the observed radial pattern of stretch trajectories implies that the local motion does not correspond to the average plate convergence

a late miocene pliocene origin for the central himalayan inverted metamorphism abstract perhaps the best known occurrence of an inverted metamorphic sequence is that found immediately beneath the himalayan main central thrust mct generally thought to have been active during the early miocene however in situ 208 pb 232 th dating of monazite inclusions in garnet indicates that peak metamorphic recrystallization of the mct footwall occurred in this portion of the central himalaya at only ca 6 ma the apparent inverted metamorphism appears to have resulted from activation of a broad shear zone beneath the mct which tectonically telescoped the young metamorphic sequence this explanation may resolve some outstanding problems in himalayan tectonics such why the mct and not the more recently initiated thrusts marks the break in slope of the present day mountain range it also renders unnecessary the need for exceptional physical conditions e g high shear stress to explain the generation of the himalayan leucogranites

an evaluation of the inverted metamorphic gradient at langtang national park central nepal himalaya the crystalline core of the himalayan orogen in the langtang area of nepal located between the annapurna manaslu region and the everest region contains middle to upper amphibolite grade pelitic gneisses and schists these rocks are intimately associated with the main central thrust mct one of the major compressional structures in the northern indian plate which forms a 3 7 km wide zone containing rocks of both footwall and hangingwall affinity an inverted metamorphic gradient is noticeable from upper footwall through hangingwall rocks where metamorphic conditions increase from garnet grade near the mct zone to sillimanite k feldspar grade in the upper hangingwall petrographic data distinguish two metamorphic episodes that have affected the area a high pressure moderate temperature episode m1 and a moderate pressure high temperature episode m2 comparison with appropriate reaction boundaries suggests that conditions for m1 in the hangingwall were approximately 900 1200 mpa and 425 525 c thermobarometric results for 24 samples from the footwall mct zone and hangingwall reflect p t conditions during the m2 phase of 400 1200 mpa and 490 660 c the decrease in estimated palaeopressures from footwall to hangingwall approximate a lithostatic gradient of 27 mpa km 1 with slight fluctuations in the mct zone reflecting structural discontinuities in contrast to the palaeopressures palaeotemperatures are indistinguishable across the entire area sampled although field evidence suggests the presence of the inverted palaeothermal gradient well known in the himalaya quantitative thermobarometry indicates that temperatures of final equilibration were all within error of each other across 17 km of section at langtang change in pressure is responsible for the presence of the sequence of index minerals through the section i interpret these data to reflect diachronous attainment of equilibrium temperature conditions in a lithostatic palaeopressure profile after ductile faulting of the sequence

imprinted textures on apatite a guide to paleoporosity and metamorphic recrystallization fluids play a key role in the transfer of both heat and mass within the crust and in the attainment of chemical equilibrium in metamorphic rocks controlling both reaction rates and deformation mechanisms however our understanding of fluid movement through the deep crust is limited because rocks readily recrystallize and fluids are invariably lost from grain boundaries little is known about how textures evolve during metamorphism since minerals rarely preserve much textural history that can be directly related to grain boundary processes here we report on a study of the surfaces of apatite within gneisses a variety of textural features is preserved on these surfaces including imprinted texture of fossil grain boundaries this texture is linked to deformation that caused a temporary excursion to fine grain size in the surrounding silicates the geometry of paleoporosity is also recorded by microhollows on the apatite surface and gives a unique insight into both fluid behavior within the crust and the textural evolution of rocks

tropical and extratropical responses of the north atlantic atmospheric circulation to a sustained weakening of the moc abstract the tropospheric response to a forced shutdown of the north atlantic ocean s meridional overturning circulation moc is investigated in a coupled ocean atmosphere gcm the third climate configuration of the met office unified model hadcm3 the strength of the boreal winter north atlantic storm track is significantly increased and penetrates much farther into western europe the changes in the storm track are shown to be consistent with the changes in near surface baroclinicity which can be linked to changes in surface temperature gradients near regions of sea ice formation and in the open ocean changes in the sst of the tropical atlantic are linked to a strengthening of the subtropical jet to the north which combined with the enhanced storm track leads to a pronounced split in the jet structure over europe eof analysis and stationary box indices methods are used to analyze changes to the north atlantic oscillation nao there is no consistent signal of a change in the variability of th

effective radium concentration across the main central thrust in the nepal himalayas effective radium concentration ec ra of 622 rock samples from 6 different sites in the nepal himalayas was measured in the laboratory using radon accumulation experiments these sites located from lower dolpo in western nepal to eastern nepal are divided into 9 transects which cut across the main central thrust zone mct zone separating low grade meta morphic lesser himalayan sequence lhs units to the south and higher grade metamorphic greater himalayan sequence ghs units to the north this boundary remains difficult to define and is the subject of numerous debates ec ra values range from 0 03 0 03 to 251 6 4 0 bq kg a1 and appear to be representative of the formation and clearly related to the local lithology for example for the upper trisuli and langtang valleys site in central nepal the most studied place with 350 available ec ra values lhs rocks are characterized by a mean value of 5 3 1 3 bq kg a1 while ghs rocks of formations i and ii show significantly lower values with a mean value of 0 69 0 11 bq kg a1 thus leading to a lhs ghs ec ra ratio of 7 8 2 2 this behavior was systematically confirmed by other transects ratio of 7 9 2 2 in all other sites with a threshold ec ra value separating lhs from ghs of 0 8 bq kg a1 thus bringing forward a novel method to characterize within the mct shear zone which rocks belong to the ghs and lhs units in addition ulleri augen gneiss belonging to lhs rocks occurred in several transects and were characterized by high ec ra values 17 9 4 3 bq kg a1 easy to distinguish from the ghs gneisses characterized by low ec ra values at the bottom of the ghs thus providing a further argument to locate the mct the measurement of ec ra data thus provides a cost effective method which can be compared with neodymium isotopic anomalies or estimates of the peak metamorphic temperature this study therefore shows that the measurements of ec ra provides additional information to discriminate different geological formations and can be particularly useful in areas where geology mapping is not straightforward or still remains controversial

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

spatio temporal microseismicity clustering in the cretan region abstract spatio temporal clustering of microseismicity in the central forearc of the hellenic subduction zone in the area of crete is investigated data for this study were gathered by temporary short period networks which were installed on the islands of crete and gavdos between 1996 and 2004 the similarity of waveforms is quantified systematically to identify clusters of microseismicity waveform similarities are calculated using an adaptive time window containing both the p and s wave onsets the cluster detection is performed by applying a single linkage approach clusters are found in the interplate seismicity as well as in the intraplate seismicity of the continental crust in the region of the transtensional ptolemy structure the majority of the clusters are off the southern coast of crete in a region of elevated intraplate microseismic activity within the aegean plate clusters in the gavdos region are located at depths compatible with the plate interface while cluster activity in the region of the ptolemy trench is distributed along a nearly vertical structure throughout the crust extending down to the plate interface most clusters show swarm like behaviour with seismic activity confined to only a few hours or days without a dominant earthquake and with a power law distribution of the interevent times for the largest cluster precise relocations of the events using travel time differences of p and s waves derived from waveform cross correlations reveal migration of the hypocenters this cluster is located in the region of the ptolemy trench and migration occurs along the strike of the trench at 500 m day relocated hypocenters as well as subtle differences in the waveforms suggest an offset between the hypocenters and thus the activation of distinct patches on the rupture surface the observed microseismicity patterns may be related to fluids being transported along the plate interface and escaping towards the surface in zones of crustal weakness ptolemy structure triggering swarm like cluster activity along its way

kinematic history of the main central thrust zone in the langtang area nepal abstract this paper describes for the first time evidence for a northeastward brittle extensional movement of the main central thrust mct zone overprinting the southward ductile thrust movement the mct zone is the major tectonic boundary between the lesser himalayan metasedimentary sequence lhs and higher himalayan crystalline sequence hhs and contributes to the kinematic and tectonic evidence of continent continent collision in the neogene the langtang area 50 km north of kathmandu is a matter of interest in considering the movement of the kathmandu nappe because the general trend of foliation of the hhs and the underlying mct zone is turned from wnw ese to n s or nne ssw the major tectono lithostratigraphic units of the mct zone in the study area are divided into three the lower and middle units included in the lhs and the upper unit even included in the hhs characteristic mesoscopic quartz lenses are developed in the schists of the lower unit forming asymmetric boudins this asymmetry indicates an extensional top to the ne shear sense however a thrust sense of top to the sw is also preserved in microscopic ductile shear bands and mica fish the extensional shear movement in the lower unit took place after the major thrust movement in the mct zone as a negative inversion the middle unit contains ulleri type augen gneiss syabru bensi augen gneiss which is widely distributed in the mct zone in the other parts of the nepalese himalaya asymmetric clast tails or clast pressure shadows and asymmetric boudins observed in the syabru bensi augen gneiss give a dextral thrust oblique sense of shear this dextral component is regarded as movement of a n s trending lateral ramp during the overthrusting of the kathmandu nappe the upper unit above the mct is commonly made up of kyanite bearing medium pressure type amphibolite facies pelitic gneiss asymmetric deformational features such as mica fish and shear bands in the pelitic gneisses commonly show a thrust top to the w sense of movement after the emplacement of the kathmandu nappe 9 6 ma brittle extensional shear occurred only in the crystalline schists of the lower unit presumably associated with layer parallel slip this extensional movement in the lower unit is probably caused by the change in strike of foliations e g from a general trend of e w in the other areas to n s as seen in the study area this is because n s contraction during continued continent continent collision produced e w extension normal to the contraction axis n s thus the relative east directed movement on normal faults took place only in the n s trending domain of the mct zone another possibility for the extensional movement is squeezing of the upper part of the lhs between the mct above and an out of sequence thrust or the main detachment fault below

tectonic and polymetamorphic history of the lesser himalaya in central nepal abstract the lesser himalaya in central nepal consists of precambrian to early paleozoic low to medium grade metamorphic rocks of the nawakot complex unconformably overlain by the upper carboniferous to lower miocene tansen group it is divided tectonically into a parautochthon two thrust sheets thrust sheets i and ii and a wide shear zone main central thrust zone from south to north by the bari gad kali gandaki fault the phalebas thrust and the lower main central thrust respectively the lesser himalaya is overthrust by the higher himalaya along the upper main central thrust umct the lesser himalaya forms a foreland propagating duplex structure each tectonic unit being a horse bounded by imbricate faults the umct and the main boundary thrust are the roof and floor thrusts respectively the duplex is cut off by an out of sequence fault at least five phases of deformation d 1 d 5 are recognized in the lesser himalaya two of which d 1 and d 2 belong to the pre himalayan pre tertiary orogeny petrographic microprobe and illite crystallinity data show polymetamorphic evolution of the lesser and higher himalayas in central nepal the lesser himalaya suffered a pre himalayan probably early paleozoic anchizonal prograde metamorphism m 0 and a neohimalayan syn to post umct diagenetic to garnet grade prograde inverted metamorphism m 2 the higher himalaya suffered an eohimalayan pre or early umct kyanite grade prograde metamorphism m 1 which was in turn overprinted by neohimalayan syn umct retrograde metamorphism m 2 the isograd inversion from garnet zone in the lesser himalaya to kyanite zone in the higher himalaya is only apparent due to post metamorphic thrusting along the umct both the lesser and higher himalayas have undergone late stage retrogression m 3 during exhumation

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

chronology of tectonic events in the crystalline core of the himalaya langtang national park central nepal the main central thrust mct is an important intracontinental subduction zone that accommodated a significant proportion of shortening between india and asia during tertiary himalayan orogenesis argon 40 argon 39 geochronology indicates at least two distinct periods of thrust movement on the mct in the langtang national park region of central nepal ductile deformation and associated mylonitization characterizing the earlier event are constrained by 40ar 39ar dating of muscovites to have occurred sometime before 5 8 ma a later period of brittle deformation resulting in the juxtaposition of rocks of different lithology within the mct zone occurred at approximately 2 3 ma based on 40ar 39ar dating of neoblastic muscovites from the brittle fault zones the hanging wall of the mct contains amphibolite to upper amphibolite grade gneisses and small leucogranite bodies assigned to the greater himalayan sequence argon 40 argon 39 cooling ages of muscovite and biotite from the gneisses range from 4 6 to 9 7 ma these dates contrast with previously obtained 16 21 ma u pb monazite and zircon ages for metasedimentary rocks from the same structural levels parrish et al 1992 indicating relatively slow cooling over the early to middle miocene interval for much of the mct hanging wall however one 19 3 ma biotite 40ar 39ar cooling age for a sample from the uppermost portion of the hanging wall is only slightly younger than u pb monazite ages for nearby anatexites possibly suggesting rapid cooling of the uppermost greater himalayan sequence by tectonic denudation associated with the structurally higher south tibetan detachment system the general consistency of 40ar 39ar ages throughout the 11 km thick greater himalayan sequence suggests rapid cooling in late miocene time probably due to an increase in erosion rate related to ramping on the structurally lower main boundary thrust an alternative possibility would be massive hydrothermal resetting in late miocene time of the entire sequence

synthrusting metamorphism cooling and erosion of the himalayan kathmandu complex nepal in this paper we tackle some of the outstanding problems of the himalaya in particular the external zone in the kathmandu complex using an integrated approach involving field mapping microstructure thermobarometry and geochronology the result is a new model showing the evolution of one major main central thrust therefore we refute suggestions that the kathmandu complex is a klippe or separate thrust sheet compared to the main central thrust sheet in the high himalaya the kathmandu complex shows differences in deformational and metamorphic features and timing of metamorphism that are consistent with its position some 100 km south of the high himalaya fairly near the leading edge unless there was substantial volume loss between the time of peak metamorphism and the beginning of thrusting then our geobarometry results indicate that the main central thrust wedge was 40 km thick on the northern side of the kathmandu complex and 20 km thick on the southern margin initiation of the main central thrust occurred at 22 ma possibly during the closing stages of peak amphibolite facies metamorphism slip at elevated temperature 500 600 c continued until 14 ma this is a little longer than has previously been proposed in marked contrast to the famous inverted metamorphism on the main central thrust in the high himalaya the metamorphic zonal scheme in the kathmandu complex is right way up with the exception of a thin zone of greenschist facies thrust related dynamically metamorphosed rocks at the base these mylonites postdate the high grade regional amphibolite metamorphism and give an illusion of inverted metamorphism a likely reason for the contrast is that the main central thrust cut up section toward the foreland and therefore at kathmandu carries high levels in the metamorphic structure our model involves reactivation of the main central thrust at 7 8 ma as inferred from published monazite and mica ages but because the kathmandu rocks show no evidence for high temperature reactivation at this time we presume that the late reactivation involved only the internal high himalaya zone while the main central thrust was inactive in the external kathmandu zone we attempt to quantify rates of cooling exhumation and thrusting during time period 22 ma to the present

a structural transect in the lower dolpo insights on the tectonic evolution of western nepal abstract we present the results of a structural transect in lower dolpo cross cutting the upper part of the lesser himalaya lh the higher himalayan crystallines hhc and the lower part of the tibetan sedimentary sequence tss the mct zone affects the upper part of the lh as well as the lower part of the hhc and shows a later brittle reactivation mean vorticity in the mct points to non coaxial deformation these data together with available kinematic data along the belt on the south tibetan detachment system stds and in the core of the hhc point to increasing simple shear toward the tectonic boundaries a top to the sw high temperature shear zone toijem shear zone is recognized in the middle part of the hhc at the boundary between units 1 and 2 it developed during the earlier stages of exhumation of the hhc enhancing the decompression of the hanging wall and the emplacement of leucogranite dykes and sills its development could be explained by a change in the velocity profile during the extrusion of the hhc triggered by first order changes in rock types of the tectonic unit the stds is marked by a wide zone of high strain and by a metamorphic jump from amphibolite facies in the carbonate rocks of the upper part of the hhc to greenschist facies marbles in the lower part of the tss the development of a pervasive foliation towards the bottom of the tss indicates increasing strain related to top down to the ne tectonic transport a low p metamorphic event marked by the growth of post d1 biotite porphyroblasts at the base of the tss is related to the conductive heating from the underlying hhc

plio plistocene in sequence thrust propagation along the main central thrust zone kumaon garhwal himalaya india new thermochronological data abstract the kumaon and garhwal himalayas nw india have similar geologic tectonic topographic and precipitation pattern however low temperature thermochronological age pattern varies significantly along the strike as well as across the strike of major faults here we interpret the new set of thermochronological age data across the strike of major faults namely vaikrita thrust vt munsiari thrust mt berinag thrust bt and the baijnath nappe along pindari valley in the kumaon garhwal himalaya in the present work 18 new apatite fission track aft ages of samples collected along a north south transect have been reported ages in the hanging wall of vt which range from 2 1 0 2 to 4 2 0 7 ma have been found to be becoming younger linearly with distance from north to the vt this trend crosses the vt with significant jump in ages in the south of the vt ages are lying between 0 3 0 1 and 3 9 0 8 ma and show linear variation with distance from the vt to bt no significant jump in ages across the mt is observed and the linear trend of younging southward is continuing till the bt the break and displacement in age pattern with youngest ages close to thrusts possibly indicate progressive late thrust movement it reflects sequential uplift and cooling towards the south from the vt to bt which is consistent with an in sequence style of thrust propagation we compare our new data to the published thermochronological data from its adjacent traverses along the goriganga and dhauliganga valleys 25 km away from the present traverse towards east and west respectively in the kumaon garhwal himalaya in order to understand potential along strike variations in the kinematics our observations indicate a dynamic coupling between tectonic and surface processes in the garhwal kumaon himalaya and support that the geometry of crustal scale faults and their associated kinematics control exhumation pathways of rocks

a structural analysis of the main central thrust zone langtang national park central nepal himalaya the main central thrust mct is one of the most tectonically significant structures in the himalayan orogen detailed geologic mapping and structural analysis of the mct in the langtang national park region of central nepal reveals that this segment of the fault zone experienced multiple episodes of south directed movement under both brittle and ductile conditions during the tertiary period early mid miocene movement resulted in the development of mylonitic fabrics synchronous with amphibolite facies metamorphism the mean orientation of the dominant mylonitic foliation is n28 w 38 ne an associated mineral stretching lineation plunges 40 to n40 e kinematic indicators suggest hanging wall movement to the southwest relative to the footwall along the north dipping fault it is not possible to constrain the magnitude of high temperature displacement on the mct at the longitude of langtang late stage structures in the mct zone at langtang include a series of imbricate brittle thrust faults that separate different lithostratigraphic units and correspond to metamorphic discontinuities we interpret this fault system as a duplex structure muscovite 40 ar 39 ar cooling ages from the mct zone range from 8 9 6 9 ma because the nominal closure temperature of ar diffusion in muscovite approximately 625 k is higher than the apparent temperature conditions under which late brittle deformation occurred we suggest that brittle deformation was a latest miocene pliocene phenomenon another major himalayan fault the main boundary thrust mbt was developing to the south of langtang at approximately the same time we speculate that brittle faulting within the mct zone may have initiated as the mct zone was transported over a ramp in the mbt

structure of the main central thrust zone and extrusion of the high himalayan deep crustal wedge kishtwar zanskar himalaya the main central thrust is a crustal scale ductile shear zone between 1 5 and 3 km wide which places the oligocene miocene metamorphic rocks of the high himalayan zone south or sw over the unmetamorphosed or weakly metamorphosed rocks of the lesser himalaya the high strain zone of the main central thrust is coincident with an inverted metamorphic field gradient from biotite to kyanite grade over a structural thickness of 1500 m kyanite grade rocks metamorphosed at 9 5 10 kbar were exhumed from depths of 33 37 km along the main central thrust hanging wall and emplaced over lesser himalayan rocks never buried deeper than 10 12 km exhumation of the deepest buried kyanite grade rocks occurred along the zone above the main central thrust zone approximately 45 km width 28 km structural thickness of the high himalaya exposes sillimanite grade gneisses migmatites and 19 5 21 5 ma old leucogranites formed at pressures between 4 5 and 7 kbar and depths of 16 25 km a ne dipping normal fault ductile shear zone at the top of the slab zanskar shear zone shows condensed but right way up isograds from sillimanite to chlorite grade over 2 400 metres structural thickness 40 ar 39 ar geochronology shows that most of the high himalayan slab had cooled below 350 c by 16 ma supporting models linking the two bounding faults of the high himalaya both kinematically and temporally there is no evidence of melting along the main central thrust zone and the himalayan leucogranites were generated 10 30 km structurally above the main central thrust frictional heating along the main central thrust could not therefore have played any role in generating the leucogranites thus far there is little evidence for late miocene reactivation along the main central thrust as seen elsewhere along the main central thrust in nepal

origin of the inverted metamorphism of the lower himalayas central nepal abstract the southern slope of the central nepal himalayas is divided geologically by the main central thrust zone into the underlying midland metasediment zone lower himalayas in the south and the overlying himalayan gneiss zone higher himalayas in the north the former zone consists of a thick pile of various kinds of metasediments folded into an anticlinorium and the latter zone consists of high grade crystalline rocks with a north dipping homoclinal structure and is further overlain by the tethys himalayas composed of non or slightly metamorphosed tethyan sediments the main central thrust zone shows a characteristic lithology such as calcareous schist quartzite phyllitic schist and augen gneiss and is intensely mylonitized the mineral composition and paragenesis of the rocks in the present area indicate an inverted metamorphism from the midland metasediment to the main central thrust zones that is the metamorphic grade of the midland metasediment zone increases upward toward the thrust zone from the chlorite to the biotite zones of the greenschist facies and the greenschist amphibolite transitional zone is attained in the thrust zone the himalayan gneiss zone is apparently much higher in metamorphic grade than the thrust zone on the basis of the compositional zoning of garnets and muscovite compositions however it is concluded that the himalayan gneisses suffered a polymetamorphism the younger metamorphism resulted in a retrograde metamorphism of the himalayan gneisses together with the above mentioned inverted metamorphism and an anatectic melting in depth this was caused by shear heating due to the thrust movement along the main central thrust zone during the alpine tectonism the older metamorphism which seems to be precambrian in age was much higher in metamorphic grade than the younger one and formed the pyrope rich cores of garnets in the himalayan gneisses the maximum metamorphic temperature of 550 c estimated from the mineral paragenesis in the thrust zone is compatible with that caused by shear heating due to the thrusting with thrusting velocity of 5 10 cm yr maximum stress of 300 bars and thrusting distance of 250 km bird 1978