metamorphism melting and extension age constraints from the high himalayan slab of southeast zanskar and northwest lahaul abstract we present evidence for two distinct stages of tertiary metamorphism m1 and m2 in the high himalayan slab of southeast zanskar and northwest lahaul as well as evidence for an older pre himalayan metamorphism pre m1 the m1 was a regional barrovian type event related to crustal shortening and thickening of the indian plate margin while m2 was associated with crustal melting and the emplacement of the gumburanjon leucogranite into the zanskar shear zone at the top of the slab u pb dating of metamorphic and magmatic accessory phases constrains the timing of m1 between 30 and 37 ma and the crystallization and emplacement age of the gumburanjon leucogranite at 21 22 ma inherited accessory phases in metamorphic and magmatic samples suggest that the protoliths of the slab are at least lower paleozoic in age and that they experienced a major pre m1 thermal perturbation at ca 450 500 ma whether this was associated with a regional barrovian type metamorphism or whether it was a thermal event rel

a p t t d discontinuity in east central nepal implications for the evolution of the himalayan mid crust abstract rock specimens collected from lower middle and upper portions of the exhumed himalayan mid crust in the upper tama kosi region of east central nepal yield pressure temperature time deformation p t t d paths that demonstrate the presence of a structural metamorphic and geochronologic discontinuity a p t t d path from below the discontinuity defines a loading path with deformation at staurolite grade metamorphic conditions occurring at 10 8 ma p t t d paths from above the discontinuity in contrast record an earlier and protracted metamorphic history that includes decompressional heating monazite ages from specimens collected above the discontinuity constrain prograde garnet growth to be 21 ma with decompression related garnet breakdown initiating at c 19 ma and likely continuing until c 15 ma these contrasting p t t d histories separated by a tectonometamorphic discontinuity are consistent with an evolutionary model for the himalaya in which rocks above the discontinuity were metamorphosed in the deep hinterland of the orogen and deformed during lateral extrusion from beneath tibet while rocks below the discontinuity were deformed and metamorphosed later within the shallower foreland of the orogen these results demonstrate the spatial and temporal relationships and compatibility between lateral midcrustal extrusive flow and wedge taper processes within an evolving orogen

models of magnetic anisotropy variations in sedimentary thrust sheets abstract magnetic anisotropy variation in sedimentary thrust sheets was modelled through the extension of strain models in thrust sheets coward and kim 1981 sanderson 1982 using the empirical relationship between the magnetic anisotropy and strain and respecting the pre deformational anisotropy of sedimentary rocks in thrust sheets deformed by simple shear and lateral lengthening the magnetic foliations remain near the bedding while in thrust sheets deformed by simple shear and lateral shortening the magnetic foliation may deviate strongly from the bedding even creating a girdle pattern in its poles

estimation of deformation temperatures flow stresses and strain rates from an intra continental shear zone the main boundary thrust nw himalaya uttarakhand india abstract this work generates thermal and tectonic data from the mylonitized rocks of the main boundary thrust mbt south and north to this major thrust the siwalik range and the lesser himalaya respectively have been receiving a renewed interest amongst the petroleum geoscientists understanding temperature flow stress and strain rates related to a major thrust is crucial in thermal mechanical modelling of the terrains adjacent to it strongly sheared proterozoic chandpur phyllites and abundant syntectonic quartz veins occur along the mbt zone one of the major foreland verging thrusts in the himalayan orogen near the sahenshahi temple at dehradun uttarakhand state india microstructure analyses show intense ductile top to s shear in terms of s c fabric and xrd analyses indicate presence of clays e g illite and clinochlore these clays not only bear the signatures of the intense fluid activity but also enhance the fault movement by reducing the mechanical strength of the fault zone rocks thermometry based on qualitative quartz grain boundary mobility 300 550 c and quantitative laser raman spectroscopy for carbonaceous materials 340 370 c reveal the maximum estimates of the deformation temperature and metamorphic temperature respectively the higher temperature microstructures can be found in lower temperature domains under the presence of fluids hence the quartz thermometry estimates a broader temperature range than the raman thermometry previous works report that the phyllites present at the base of a thrust sheet experiences deformation at

two dimensional strain determination by the inverse surfor wheel abstract a simple method of two dimensional strain determination from the preferred orientation of deformed lines is presented it is based on the change of probability of intersecting lines on a given traverse as a function of traverse orientation the method can be applied to real lines such as grain boundary outlines or to virtual lines such as the tie lines between centre points of neighbouring grains in this manner estimates for the strain of surfaces i e of the particles enclosed by the surfaces and of bulk strain can be obtained

subduction kinematics and dynamic constraints the kinematics of subduction zones shows a variety of settings that can provide clues for dynamic understandings two reference frames are used here to describe the simple 2d kinematics of subduction zones in the first the upper plate is assumed fixed whereas in the second frame upper and lower plates move relative to the mantle relative to a fixed point in the upper plate u the transient subduction hinge h can converge diverge or be stationary similarly the lower plate l can converge diverge or be stationary the subduction rate vs is given by the velocity of the hinge h minus the velocity of the lower plate l vs vh vl the subduction rate 1 increases when h diverges and 2 decreases when h converges combining the different movements at least 14 kinematic settings can be distinguished along the subduction zones variable settings can coexist even along a single subduction zone as shown for the 5 different cases occurring along the apennines subduction zone apart from few exceptions the subduction hinge converges toward the upper plate more frequently along e or ne directed subduction zone whereas it mainly diverges from the upper plate along w directed subduction zones accompanying backarc extension before collision orogen growth occurs mostly at the expenses of the upper plate shortening along e ne directed subduction zones whereas the accretionary prism of w directed subduction zones increases at the expenses of the shallow layers of the lower plate the convergence shortening ratio is 1 along e or ne directed subduction zones whereas it is 1 along accretionary prisms of w directed subduction zones backarc spreading forms in two settings along the w directed subduction zones it is determined by the hinge divergence relative to the upper plate minus the volume of the accretionary prism or in case of scarce or no accretion minus the volume of the asthenospheric intrusion at the subduction hinge since the volume of the accretionary prism is proportional to the depth of the decollement plane the backarc rifting is inversely proportional to the depth of the decollement on the other hand along e or ne directed subduction zones few backarc basins form e g aegean andaman and can be explained by the velocity gradient within the hangingwall lithosphere separated into two plates when referring to the mantle the kinematics of subduction zones can be computed either in the deep or in the shallow hotspot reference frames the subduction hinge is mostly stationary being the slab anchored to the mantle along w directed subduction zones whereas it moves w or sw ward along e or ne directed subduction zones surprisingly along e or ne directed subduction zones the slab moves out of the mantle i e the slab slips relative to the mantle opposite to the subduction direction kinematically this subduction occurs because the upper plate overrides the lower plate pushing it down into the mantle as an example the hellenic slab moves out relative to the mantle i e sw ward opposite to its subduction direction both in the deep and shallow hotspot reference frames in the shallow hotspot reference frame upper and lower plates move westward relative to the mantle along all subduction zones this kinematic observation casts serious doubts on the slab negative buoyancy as the primary driving mechanism of subduction and plate motions w directed subduction zones rather provide about 2 3 times larger volumes of lithosphere re entering into the mantle and the slab is pushed down this opposite behavior is consistent with the down dip extension seismicity along e ne directed subduction zones and the frequent down dip compression along the w directed subduction zones subduction kinematics shows that plate velocity is not dictated by the rate of subduction along the w directed subduction zones the rate of subduction is rather controlled i by the hinge migration due to the slab interaction with the easterly trending horizontal mantle wind along the global tectonic mainstream ii by the far field plate velocities and iii by the value of negative buoyancy of the slab relative to the country mantle alternatively e ne nne directed subduction zones have rates of sinking chiefly determined i by the far field velocity of plates and ii by the value of negative buoyancy of the slab relative to the country mantle along this type of subduction the subduction hinge generally advances e ne ward toward the upper plate decreasing the subduction rate but it moves w sw ward relative to the mantle all this indicates that subduction zones have different origin as a function of their geographic polarity and the subduction process is more a passive feature rather than being the driving mechanism of plate motions a rotational component combined with mantle density and viscosity anisotropies seems more plausible for generating the global tuning in the asymmetry of subduction zones

regional scale strain variations in banded iron formations of eastern india results from anisotropy of magnetic susceptibility studies anisotropy of magnetic susceptibility ams data are used as a tool to determine strain variations in different parts of the banded iron formations bifs of the bonai synclinorium eastern india ams data of 88 cylindrical cores drilled from 29 samples collected from the limb and hinge parts of mesoscopic scale folds as well as different parts of the entire synclinorium are presented it is found that the samples from limbs of small scale folds and also from limbs of the regional scale synclinorium have higher degrees of anisotropy than the hinges this is inferred to indicate that the limbs accommodated higher strain than the hinges ams orientation data are analysed in conjunction with field data it is concluded that the magnetic fabric developed in the limbs as well as hinges of the bifs of the study area is related to deformation and is not a manifestation of sedimentary fabric

erratic shortening from balanced cross sections of the western himalayan foreland basin causes and implications for basin evolution abstract balanced cross sections from the western himalayan foreland foothill belt reveal a variation in shortening amounts from 22 to 71 3 over a lateral distance of 85 km the large variation may be attributed to the fact that the method of cross section balancing considers the initial layering as horizontal which is an unlikely situation in foreland basins simple trigonometric relationships are used to minimize the error in shortening estimates however the suggested corrections could not be applied to the himalayan foreland basin because of unavailability of the required data on initial basin geometry the study also indicates that shortening rates for the area based on these shortening amounts cannot be used for earthquake prediction

superposed folding of a blind thrust and formation of klippen results of anisotropic magnetic susceptibility studies from the lesser himalaya abstract the rocks of the garhwal lesser himalaya have undergone a weak superimposed deformation hence linear and planar structures are either absent or poorly developed this puts a severe limitation on application of conventional methods of finite strain determination in understanding the deformation pattern however the geometry orientation and distribution of magnetic susceptibility strain ellipses clearly reveal the effects of early and superposed deformations in the area the orientation patterns of the ellipses also help to identify reversal of displacement along an oblique fault ramp during the superposed deformation the hrouda double plot reveals a combination of lateral shortening and simple shear thereby suggesting a small translation along the klippe detachment thrust the study has important implications for understanding the structural evolution of the lesser himalayan klippen because the earlier models in the absence of the relevant data are based on assumptions concerning thrust displacement the present field studies and the ams data favour an alternate model for the structural evolution of the lesser himalayan klippen that lie in the core of the mussoorie syncline the model explains structural features and outcrop patterns as due to a combination of fault bifurcation back thrusts pop up and subsequent superposed deformation the klippen lie over their roots and are described as pop up klippen

thrust tectonics thin skinned or thick skinned and the continuation of thrusts to deep in the crust abstract the paper analyses the geometry of thin skinned thrust zones where the thrusts shallow out at depth and of thicker skinned fault zones where much of the crust is involved and where the thrusts are frequently observed to become steeper downwards in the interiors of many orogenic belts the steep dip of faults is not original but due to the folding above lower decoupling zones the energy involved in the internal deformation of hanging wall rocks may prohibit many faults becoming more shallow upwards such shallowing upwards faults may occur in more ductile rocks to maintain compatibility between zones which have experienced different deformation intensities but displacements on the faults are unlikely to be large another mechanism for producing faults which steepen downwards is proposed for the major thrusts which form the southern margin to the himalayas these carry large thicknesses 30 to 100 km of crustal and upper mantle rocks to the south causing flexuring and isostatic depression of the indian plate the steeply dipping thrusts are not footwall ramps these may be some distance behind the steepened zone this thrust induced isostatic bending of the crust has important implications when considering regional seismic interpretations as well as thrust mechanics and kinematics

the age and rate of displacement along the main central thrust in the western bhutan himalaya abstract in western bhutan the main central thrust mct is broadly folded creating multiple exposures of the fault surface over a 70 km across strike distance this unusual map pattern presents a unique opportunity to map the mct and document both the magnitude and age of displacement in situ th pb sims and la icp ms geochronology of metamorphic monazite from the immediate hanging wall of the mct indicates that prograde monazite growth in greater himalayan gh rocks continued until 20 8 1 1 ma whereas crystallization of in situ melts characterized by high y monazite overgrowths occurred during cooling from ca 15 10 ma prograde monazite growth at 15 ma in lesser himalaya lh rocks in the immediate footwall requires that lh footwall strata began to be buried at this time and the mct had reached its southernmost exposed extent by combining prograde monazite ages in the immediate hanging wall and footwall the duration of mct displacement is bracketed between 20 8 1 1 and 15 0 2 4 ma immediately north of our study area a published estimate of shearing along the outer south tibetan detachment std argues for displacement between 20 and 15 ma coeval with the age range for mct displacement that we document in this study however retrograde monazite grains as young as 10 ma suggest that gh rocks were cooling until 10 ma 5 myr later than motion on the outer std immediately to the north this cooling was either the result of continued displacement on the mct or growth of a duplex that passively folded the mct using a sequential reconstruction we estimate a total displacement of 230 km which is the sum of displacements on the mct and the structurally lower paro thrust over a duration of 5 8 2 6 myr this indicates a horizontal shortening rate of 4 0 3 2 1 3 cm yr which exceeds present rates estimated from geodetic measurements across the himalaya and mct displacement rates c 2 cm yr inferred from petrologic and thermal models in central nepal but is indistinguishable from plate convergence rates calculated for eastern bhutan between 23 and 20 ma 3 3 0 7 cm yr our study highlights that displacement on the mct alone achieved plate velocity rates in western bhutan and that the age and rate of mct displacement varied significantly across the himalaya

significance of ams analysis in evaluating superposed folds in quartzites quartzites tend to be compositionally homogeneous and because of this deformation related fabric elements foliations and lineations are poorly developed in them this makes structural analysis of deformed quartzites challenging the measurement of anisotropy of magnetic susceptibility ams is useful for recognizing structural imprints in rocks that lack mesoscopic fabrics and the present study is carried out with an aim to demonstrate the robustness of ams in analysing such deformation imprints in quartzites ams data of samples from folded quartzites located in an approximately 10 km 2 area around galudih eastern india are presented although on a regional scale superposed deformation and ductile shearing are known from the area the investigated quartzites do not preserve mesoscopic evidence of these large scale features and have developed folds that plunge gently towards the se with a vertical nw se striking axial plane the magnetic foliation recorded from ams analysis is parallel to the axial plane while the orientation of the magnetic lineation varies from se through vertical to nw this is similar to the large scale fold axis variations recorded in various regional domains mapped over an area of about 200 km 2 it is concluded that although the imprint of regional superposed deformation is not obvious on the mesoscopic scale in the quartzites around galudih this imprint can be detected from the magnetic fabric the present study thus highlights the usefulness of ams in analysing superposed folds in quartzites

superimposed folding and thrusting by two phases of mutually orthogonal or oblique shortening in analogue models abstract orogens may suffer more than one phase shortening resulting in superposition of structures of different generations superimposition of orthogonal or oblique shortening is studied using sandbox and centrifuge modelling results of sand models show that in orthogonal superimposition the two resulting structural trends are approximately orthogonal to each other in oblique superimposition structures trend obliquely to each other in the relatively thin areas of the model foreland and mutually orthogonal in areas where the model is thickened during the first phase of shortening i e the hinterland thrusts formed during the first shortening phase may be reactivated during the later shortening phase spacing of the later phase structures is not as wide as expected considering they across the pre existing thickened wedge superposition of structures results in formation of type 1 fold interference pattern bedding is curved outwards both in the dome and basin structures folded layers are dipping and plunging outwards in a dome while they are dipping and plunging inwards in a basin in the areas between two adjacent domes or basins i e where an anticline is superimposed by a syncline or a syncline is superimposed by an anticline bedding is curved inwards and the anticlines plunge inwards and the synclines outwards the latter feature could be helpful to determine the age relationship for type 2 fold interference pattern in tectonic regions where multiple phases of shortening have occurred the orogenic scale dome and basin and arrowhead shaped interference patterns are commonly formed as in the models however in some areas the fold interference pattern might be modified by a later phase of thrusting similar to models results superimposition of two and or even more deformation phases may not be recorded by structures all over the tectonic area

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

two mica and tourmaline leucogranites from the everest makalu region nepal tibet himalayan leucogranite genesis by isobaric heating in the higher himalaya of the region from cho oyu to the arun valley northeast of makalu the miocene leucogranites are not hosted only in the upper high himalayan crystallines hhc a network of dykes also cuts the lower hhc and the lesser himalayan crystallines lhc the plutons and dykes are mainly composed of two mica muscovite biotite tourmaline cordierite andalusite sillimanite leucogranite with tourmaline 2 6 and biotite 1 5 modal and tourmaline muscovite tourmaline biotite sillimanite garnet kyanite andalusite spinel corundum leucogranite with tourmaline 2 2 and biotite 1 5 modal both leucogranite types were produced by partial melting in the andalusite sillimanite facies series under lp ht conditions constrained by the occurrence of peritectic andalusite and cordierite the geochemical features of the leucogranites suggest that tourmaline leucogranite was produced by muscovite dehydration melting in muscovite rich metapelites at p 350 mpa and t 640 c whereas two mica leucogranite was produced by biotite dehydration melting in biotite rich metapelites at p 300 mpa and t 660 710 c melting in fertile muscovite rich metapelites of the top of both the hhc and lhc produced magmas which were emplaced at the same structural level in which they had been generated melting in the biotite rich gneiss of both the hhc and lhc produced hotter magmas which were transported upwards by dyking and eventually coalesced in the plutons of the upper hhc a similar process also produced a network of two mica granite at the top of the lhc in the ama drime nyonno ri range northeast of makalu the prograde character of leucogranite melt producing reactions in the everest makalu area suggests that here the generation of miocene leucogranites took place in a regime of nearly isobaric heating following nearly adiabatic decompression

superposed deformation fabrics in the precambrian metabasic rocks of the iron ore group singhbhum craton eastern india evidences from anisotropy of magnetic susceptibility studies abstract anisotropy of magnetic susceptibility ams data have been utilized to build up the multiple deformation sequences and strain variation from an apparently undeformed metabasic lava sequence of the basal part of the paleo mesoarchean greenstone succession of the iron ore group rocks in the western iron ore basin singhbhum craton eastern india the basal lava crops out bordering the regional horseshoe shaped outcrop the horseshoe synclinorium structure of the overlying bif rocks ams data of 892 cylindrical specimens drilled from 83 samples collected from different structural domains within the synclinorium structure are presented in the analysis ams orientation data have been analyzed in the light of superposed fold type 1 interference fabrics developed within bif as a result of successive deformation episodes d1 and d2 it is found that the lava samples ubiquitously present two consistent magnetic fabrics homogeneous over smaller sub areas which have been successfully correlated with the d1 and d2 deformation fabrics from the bif rocks moreover a regional scale variation in intensity and nature of strain is noticed from the ams data which have been utilized to decipher folding mechanism as well as the changing deformation pattern during successive folding events

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

probing the depths of the india asia collision u th pb monazite chronology of granulites from nw bhutan 1 rocks metamorphosed to high temperatures and or high pressures are rare across the himalayan orogen where peak metamorphic conditions recorded in the exposed metamorphic core the greater himalayan sequence ghs are generally at middle to upper amphibolite facies however mafic garnet clinopyroxene assemblages exposed at the highest structural levels in bhutan eastern himalaya preserve patchy textural evidence for early eclogite facies conditions overprinted by granulite facies conditions monazite hosted within the leucosome of neighboring granulite facies orthopyroxene bearing felsic gneiss yields la mc icp ms u th pb ages of 13 9 0 3 ma monazite associated with sillimanite grade metamorphism in granulite hosting migmatitic gneisses yields u th pb rim ages between 15 4 0 8 ma and 13 4 0 5 ma monazite associated with sillimanite grade metamorphism in gneiss at structurally lower levels yields u pb rim ages of 21 17 ma these data are consistent with miocene exhumation of ghs material from a variety of crustal depths at different times along the himalayan orogen we propose that these granulitized eclogites represent lower crustal material exhumed by tectonic forcing over an incoming indian crustal ramp and that they formed in a different tectonic regime to the ultrahigh pressure eclogites in the western himalaya their formation and exhumation in the miocene therefore do not require diachroneity in the timing of the initial india asia collision

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

microstructures and strain variation evidence of multiple splays in the north almora thrust zone kumaun lesser himalaya uttarakhand india abstract the north almora thrust zone natz marks the boundary of the almora crystalline complex acc against the lesser himalayan sedimentary sequence lhs in the north its southern counterpart the south almora thrust sat is a sharply marked contact between the acc and the lhs in the south published studies argue various contradictory emplacement modes of the north almora thrust recent studies have implied splays of smaller back thrusts in the natz the present study investigates meso and microstructures and strain distribution in the natz and compares it with strain distribution across the sat in the natz field evidence reveals repeated sequence of 10 500 m thick slices of proto to ultra mylonite thrust over the lesser himalayan rautgara quartzite in accordance with the field evidence the strain analysis reveals effects of splays of smaller thrust in the natz the study therefore argues that contrary to popular nomenclature the northern contact of the acc with the lhs is not a single thrust plane but a thrust zone marked by numerous thrust splays

orogen transverse tectonic window in the eastern himalayan fold belt a superposed buckling model abstract the eastern lesser himalayan fold thrust belt is punctuated by a row of orogen transverse domal tectonic windows to evaluate their origin a variety of thrust stack models have been proposed assuming that the crustal shortening occurred dominantly by brittle deformations however the rangit window rw in the darjeeling sikkim himalaya dsh shows unequivocal structural imprints of ductile deformations of multiple episodes based on new structural maps coupled with outcrop scale field observations we recognize at least four major episodes of folding in the litho tectonic units of dsh the last episode has produced regionally orogen transverse upright folds f 4 the interference of which with the third generation f 3 orogen parallel folds has shaped the large scale structural patterns in dsh we propose a new genetic model for the rw invoking the mechanics of superposed buckling in the mechanically stratified litho tectonic systems we substantiate this superposed buckling model with results obtained from analogue experiments the model explains contrasting f 3 f 4 interferences in the lesser himalayan sequence lhs the lower order terrain scale folds have undergone superposed buckling in mode 1 producing large scale domes and basins whereas the rw occurs as a relatively higher order dome nested in the first order tista dome the gondwana and the proterozoic rocks within the rw underwent superposed buckling in modes 3 and 4 leading to type 2 fold interferences as evident from their structural patterns

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

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

pressure temperature time path discontinuity in the main central thrust zone central nepal metapelites collected in central nepal reveal a discontinuity in metamorphic pressuretemperature time p t t paths near the base of the main central thrust zone despite an absence of obvious structural breaks garnets in the structurally lowest rocks grew with increasing t and p loading whereas garnets 1 3 km upsection grew with increasing t but decreasing p exhumation monazite grains in structurally lower rocks yield ionmicroprobe th pb ages of 8 9 ma structurally higher monazite grains range from 10 to 22 ma the p t t paths confirm previous interpretations that footwall metamorphism in part resulted from thrust reactivation ca 8 ma but also reflect thermal relaxation following older 20 ma or older thrust movement the main central thrust zone formed during pulses of movement that resulted in progressive transfer of material from the lower to upper plate

role of inversion tectonics in structural development of the himalaya abstract analysis of surface and subsurface structures variation of shortening amounts obtained by restoration of deformed cross sections and occurrence of younger hangingwall rocks over the older footwall rocks across the vaikrita thrust in the higher himalaya suggests reactivation of early normal faults as thrusts based on this an inversion tectonics model is proposed for structural development of the himalaya the model explains the geometrical shape of the himalaya as primary arcuation and helps in resolving superimposed deformation in the region

thermal model for the zanskar himalaya abstract crustal thickening along the northern margin of the indian plate following the 50 ma collision along the indus suture zone in ladakh caused widespread high temperature medium pressure barrovian facies series metamorphism and anatexis in the zanskar himalaya metamorphic isograds are inverted and structurally telescoped along the main central thrust mct zone at the base of the high himalayan slab along the zanskar valley at the top of the slab isograds are the right way up and are also telescoped along northeast dipping normal faults of the zanskar shear zone zsz which are related to culmination collapse behind the miocene himalayan thrust front between the mct and the zsz a metamorphic anatectic core within sillimanite grade rocks contains abundant leucogranite granite crustal melts of probable himalayan age a thermal model based on a crustal scale cross section across the zanskar himalaya suggests that m1 isograds developed during early himalayan barrovian metamorphism were overprinted during high grade mct related anatexis and folded around a large scale recumbent fold developed in the hanging wall of the mct

reconciling himalayan midcrustal discontinuities the main central thrust system the occurrence of thrust sense tectonometamorphic discontinuities within the exhumed himalayan metamorphic core can be explained as part of the main central thrust system this imbricate thrust structure which significantly thickened the orogenic midcrustal core comprises a series of thrust sense faults that all merge into a single detachment the existence of these various structures and their potential for complex overprinting along the main detachment may help explain the contention surrounding the definition mapping and interpretation of the main central thrust the unique evolution of specific segments of the main central thrust system along the orogen is interpreted to be a reflection of the inherent basement structure and ramp position and structural level of exposure of the mid crust this helps explain the variation in the timing and structural position of tectonometamorphic discontinuities along the length of the mountain belt

implications of channel flow analogue models for extrusion of the higher himalayan shear zone with special reference to the out of sequence thrusting the higher himalayan shear zone hhsz contains a ductile top to n ne shear zone the south tibetan detachment system lower stdsl and an out of sequence thrust oost as well as a top to n ne normal shear at its northern boundary and ubiquitously distributed compressional top to s sw shear throughout the shear zone the oost that was active between 22 ma and the holocene varies in thickness from 50 m to 6 km and in throw from 1 4 to 20 km channel flow analogue models of this structural geology were performed in this work a newtonian viscous polymer pdms was pushed through a horizontal channel leading to an inclined channel with parallel and upward diverging boundaries analogous to the hhsz and allowed to extrude to the free surface a top to n ne shear zone equivalent to the stdsu developed spontaneously this also indirectly connotes an independent flow confined to the southern part of the hhsz gave rise to the stdsl the pdms originally inside the horizontal channel extruded at a faster rate through the upper part of the inclined channel the lower boundary of this faster pdms defined the oost the model oost originated at the corner and reached the vent at positions similar to the natural prototype some time after the channel flow began the genesis of the oost seems to be unrelated to any rheologic contrast or climatic effects profound variations in the flow parameters along the hhsz and the extrusive force probably resulted in variations in the timing location thickness and slip parameters of the oost variation in pressure gradient within the model horizontal channel however could not be matched with the natural prototype channel flow alone presumably did not result in southward propagation of deformation in the himalaya

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

miocene to holocene exhumation of metamorphic crustal wedges in the nw himalaya evidence for tectonic extrusion coupled to fluvial erosion the himalayan crystalline core zone exposed along the sutlej valley india is composed of two high grade metamorphic gneiss sheets that were successively underthrusted and tectonically extruded as a consequence of the foreland directed propagation of crustal deformation in the indian plate margin the high himalayan crystalline sequence hhcs is composed of amphibolite facies to migmatitic paragneisses metamorphosed at temperatures up to 750 c at 30 km depth between eocene and early miocene during early miocene combined thrusting along the main central thrust mct and extension along the sangla detachment induced the rapid exhumation and cooling of the hhcs whereas exhumation was mainly controlled by erosion since middle miocene the lesser himalayan crystalline sequence lhcs is composed of amphibolite facies para and orthogneisses metamorphosed at temperatures up to 700 c during underthrusting down to 30 km depth beneath the mct the lhcs cooled very rapidly since late miocene as a consequence of exhumation controlled by thrusting along the munsiari thrust and extension in the mct hanging wall this renewed phase of tectonic extrusion at the himalayan front is still active as indicated by the present day regional seismicity and by hydrothermal circulation linked to elevated near surface geothermal gradients in the lhcs as recently evidenced in the himalayan syntaxes active exhumation of deep crustal rocks along the sutlej valley is spatially correlated with the high erosional potential of this major trans himalayan river this correlation supports the emerging view of a positive feedback during continental collision between crustal scale tectono thermal reworking and efficient erosion along major river systems

full fit palinspastic reconstruction of the conjugate australian antarctic margins 1 despite decades of study the prerift configuration and early rifting history between australia and antarctica is not well established the plate boundary system during the cretaceous includes the evolving kerguelen broken ridge large igneous province in the west as well as the conjugate passive and transform margin segments of the australian and antarctic continents previous rigid plate reconstruction models have highlighted the difficulty in satisfying all the available observations within a single coherent reconstruction history we investigate a range of scenarios for the early rifting history of these plates by developing a deforming plate model for this conjugate margin pair potential field data are used to define the boundaries of stretched continental crust on a regional scale integrating crustal thickness along tectonic flow lines provides an estimate of the prerift location of the continental plate boundary we then use the prerift plate boundary positions along with additional constraints from geological structures and large igneous provinces within the same australian and antarctic plate system to compute full fit poles of rotation for australia relative to antarctica our preferred model implies that the leeuwin and vincennes fracture zones are conjugate features within gondwana but that the direction of initial opening between australia and antarctica does not follow the orientation of these features rather the geometry of these features is likely related to the earlier rifting of india away from australia antarctica previous full fit reconstructions based on qualitative estimates of continental margin overlaps generally yield a tighter fit than our preferred reconstruction based on palinspastic margin restoration

kinematic evolution of the eastern tethyan himalaya constraints from magnetic fabric and structural properties of the triassic flysch in se tibet anisotropy of magnetic susceptibility ams combined with structural analysis are used in this work with the aim to characterize the tectonic evolution of the triassic flysch within the eastern tethyan himalaya thrust belt in se tibet the attitude of the magnetic foliation and lineation are concordant with the planar and linear structures of tectonic origin defined by the preferred orientation of the iron bearing silicates two different tectonic domains can be defined a the southern domain is controlled by the eohimalayan tectonic foliation s1 recorded in the magnetic foliation which trends east west and dips to the north b the northern domain is dominated by the neohimalayan magnetic foliation with wnw ese strike and dips to the south opposite to the vergence of the main structures a slightly prolate magnetic ellipsoid has been found in between the two domains recording the intersection of s1 and the subtle development of the s2 tectonic foliation hinterland propagation of the deformation lead to the great counter backthrust generation pointed out by the ssw steeply plunging magnetic lineation furthermore different orientations of magnetic foliation may indicate an early miocene c 208 clockwise vertical

a review of temporal constraints for the palaeoproterozoic large positive carbonate carbon isotope excursion the lomagundi jatuli event abstract the palaeoproterozoic lomagundi jatuli event is one of the largest magnitude and earliest known positive carbonate carbon isotope excursions preserving 13 c values between 5 and 16 and even higher it is recorded in sedimentary rocks on all continents bar antarctica and spans stratigraphic thicknesses ranging from several to many tens of metres this unique positive 13 c interval signals fundamental changes in the global carbon cycle and is a key event in earth system evolution following oxygenation of earth s atmosphere here we present a comprehensive review of the age constraints on the lomagundi jatuli event the first such effort in two decades this new chronology compilation focuses on the u pb and re os chronometers and demonstrates that global synchronicity of the lomagundi jatuli event is permissible an interpretation supported by the apparent wide dispersion of lomagundi jatuli event bearing successions in the most recent palaeoproterozoic plate reconstructions assuming the lomagundi jatuli event is synchronous worldwide then the bounds on its duration range from a maximum of 249 9 myr 2306 9 ma to 2057 1 ma to a minimum of 128 9 4 myr 2221 5 ma to 2106 8 ma

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

structural evolution of the kangra recess himachal himalaya a model based on magnetic and petrofabric strains abstract structural style supplemented by magnetic and petrofabric strain studies suggests that the kangra recess is the result of combined frontal and oblique thrust ramps which were initiated as a listric normal fault during the pre himalayan tensional phase and were later reactivated as a listric thrust fault during the himalayan orogeny the displacement out of the tectonic transport plane has resulted in the formation of a pull apart basin parallel to the oblique ramp the shear strain associated with the faulting was accommodated within a narrow zone characterized by the presence of salt the structures in the hanging wall were developed as a result of a combination of simple shear and pure shear the orientations of the folds were controlled by interference patterns between simultaneously developing folds and thrust ramp structures there is no marker horizon across the main boundary thrust mbt which could be used to measure the amount of displacement but a method based on the geometry of the pull apart basin is proposed to estimate the strike slip displacement along the oblique ramp the method indicates a horizontal displacement of 33 5 km along the mbt which corresponds well to other geological data a sequence of stages in the structural evolution of the area is proposed

evidence for a far traveled thrust sheet in the greater himalayan thrust system and an alternative model to building the himalaya the galchhi shear zone underlies the kathmandu klippe in central nepal and has emerged as a key structure for discriminating competing models for the formation of the himalayan orogenic wedge new chronologic data from the galchhi area suggest a new structural and orogenic interpretation zircons from quartzites and an orthogneiss restrict protolith deposition to between 467 7 10 ma and 570 ma with metamorphic zircon growth at 23 29 ma comparable data from the greater himalayan sequence ghs at the intra ghs langtang thrust north of galchhi similarly restrict ghs deposition to between 475 7 3 and 660 ma undeformed pegmatites near galchhi constrain movement of the galchhi shear zone to 22 5 2 3 ma long before slip of the main central thrust in the region 17 ma shear sense indicators in the galchhi area indicate both top to the south and top to the north shears the old age of movement neoproterozoic youngest detrital zircons occurrence of top to the south shear sense indicators and intrusive paleozoic granites all suggest that the galchhi shear zone is an intra ghs top to the south thrust rather than either a thrust involving lesser himalayan rocks or a top to the north shear zone that juxtaposed tethyan and ghs rocks during coeval movement of the main central thrust the ghs in nepal was not emplaced as a single body of rock but consists of at least two ductile thrust sheets present in both the hinterland at langtang and toward the foreland at galchhi ghs thrust sheets sequentially underplated during southward propagation of the thrust belt

20 years of geological mapping of the metamorphic core across central and eastern himalayas abstract the largest crystalline unit representing the mid crust in the himalayan belt is the greater himalayan sequence ghs which stretches all over the 2400 km of length of the belt the ghs recognized since the first geological explorations of the himalayas has been considered for a long time as a coherent tectonic unit exhumed by the contemporaneous shearing along the main central thrust and the south tibetan detachment system in the time span 25 17 ma a multidisciplinary approach integrating geological mapping structural analysis petrology and geochronology allowed to better constrain on its internal architecture characterized by several levels of tectonic metamorphic discontinuities on the regional scale with a top to the s sw sense of shear and active since 40 ma the ghs is consequently divided in three main tectonic units exhumed progressively from the upper part to the lower one by ductile shear zones later involving the lesser himalayan sequence above the main central thrust a cryptic tectono metamorphic discontinuity higher himalayan discontinuity hhd has been recognized and mapped in central eastern himalaya the mapping of the hhd has been allowed by the use of a multidisciplinary approach involving structural analysis geochronology and petrology a new map of western nepal is presented in this framework the popular models of exhumation of the ghs mainly based on the contemporaneous activity of the two bounding shear zones main central thrust and the south tibetan detachment and considering the ghs as a coherent tectonic unit should be reconsidered an in sequence shearing tectonic model from the deeper to the upper structural levels further affected by out of sequence thrusts is more appropriate to explain the deformation metamorphism and exhumation of the mid crust in the himalayan belt geological mapping of the himalayan belt is very far away to be exhaustively completed anyway during the last 20 and particularly during the last few years it has been notably improved due to a new multidisciplinary approach

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

reconstructing the greater indian margin a balanced cross section in central nepal focusing on the lesser himalayan duplex across much of the himalaya rocks in the lesser himalayan duplex are covered by roof thrusts of other paleoproterozoic lesser himalayan rocks or greater himalayan rocks however in central nepal between the main central thrust and the main boundary thrust lesser himalayan rocks are exposed in structurally complex relationships within the lesser himalayan duplex we present two balanced cross sections with different stratigraphy involved in the duplex one with the basal kuncha formation and one without this unit to test stratigraphic assumptions both cross sections have roof thrust sheets consisting of the main central thrust ramgarh munsiari thrust and trishuli thrust folded over the hinterland dipping lesser himalayan duplex dissected by late faults cross section 1 has a shortening estimate from the main central thrust to the main boundary thrust including motion on the main central thrust of 359 km or 78 cross section 2 has a shortening estimate of 349 km or 76 over the same region because the amount and percentage of shortening are not significantly different between the two cross sections the different stratigraphic assumptions did not change the shortening results this similarity suggests that many of the choices made when constructing a cross section may be less important than researchers previously thought

simultaneous development of noncylindrical folds frontal ramps and transfer faults in a compressional regime experimental investigations of himalayan examples experiments were performed with modeling clay models to understand the simultaneous development of folds frontal ramps and transfer faults oblique ramps in layered sequences the models were deformed by layer parallel contraction at a late stage of deformation the models were subjected to a maximum contraction parallel to the layering and perpendicular to the early axis of maximum contraction to study the effect of the superposed deformation the experimental results reveal that a transfer fault joining two frontal thrust ramps acts as a shear zone and controls the orientation of fold hinge lines in the adjacent region hence two orientations of fold hinge lines may be observed 1 oblique to the axis of maximum compression in the vicinity of the transfer fault and 2 normal to the axis and parallel to the frontal ramps away from the transfer fault during the longitudinal fold propagation the two hinge lines may coalesce to form a fold curvature opposite to the tectonic transport direction the curvature varies along the transfer fault depending on the shear zone geometry additional fold hinge line curvatures may result during the superposed deformation both in the early and the superposed fold hinge lines all these curvatures produce a systematic pattern which can be analyzed with the help of the experimental results the fold hinge line orientation may also be used to establish an oblique ramp in field as demonstrated by examples from the lesser himalayas india