age and cooling history of the manaslu granite implications for himalayan tectonics abstract the manaslu granite is one of the high himalayan leucogranites which are believed to be derived by partial melting of quartzofeldspathic gneissess of the high himalayan crystallines hhc and emplaced at the contact between the hhc and the tethyan sedimentary series we have analyzed a suite of muscovite 15 biotite 3 and alkali feldspar 10 by the 40 ar 39 ar method to help constrain the age and cooling history of the granite 40 ar 39 ar ages ranges are 18 4 to 13 3 ma 17 0 to 14 7 and 16 4 to 3 4 for muscovite biotite and k feldspar respectively based on the muscovite analyses and a re interpretation of a previously published u pb monazite age we conclude that the crystallization age of the manaslu granite is everywhere 20 ma the postcrystallization cooling history given by the micas and the feldspars suggests the presently exposed parts of the pluton were emplaced at depths of 8 to 15 km these data place a minimum age of movement on the mct in this area of 20 ma we estimate the time of magma segregation and transport to be no more than 5 ma

trace element modelling of pelite derived granites the presence or absence of a vapour phase during incongruent melt reactions of muscovite and biotite together with the composition of the protolith determines the trace element characteristics of the resulting melt provided that equilibrium melting occurs for those phases that host the tracc elements of interest for granitic melts rb sr and ba provide critical constraints on the conditions that prevailed during melting whereas ree are primarily controlled by accessory phase behaviour mass balance constraints for eutectic granites that are formed by the incongruent melting of muscovite in pelites indicate that melting in the presence of a vapour phase will result in a large melt fraction and deplete the restite in feldspar hence the melt will be characterized by low rb sr and high sr ba ratios in contrast vapour absent melting will result in a smaller melt fraction and an increase in the restitic feldspar consequently high rb sr and low sr ba ratios are predicted vapour absent melting will also enhance the negative eu anomaly in the melt granites that result from the incongruent melting of biotite in the source will be characterized by higher rb concentrations than those that result from the incongruent melting of muscovite the himalayan leucogranites provide an example of unfractionated crustally derived eutectic melts that are enriched in rb but depleted in sr and ba relative to their metasedimentary protoliths these compositions may be generated by the incongruent melting of muscovite as a low melt fraction f 0 1 from a pelitic source under vapour absent conditions

new insights into the origin of two contrasting himalayan granite belts the two parallel belts of miocene granite that extend along much of the himalaya differ in age petrogenesis and emplacement style we suggest that their origin is linked to shear heating on a continuously active decollement that cuts through previously metamorphosed indian supracrustal rocks that were transformed into basement during the initial stages of the indian asian collision numerical simulations assuming a shear stress of 30 mpa indicate that initiation of slip on the himalayan thrust at 24 ma could trigger discontinuous melting reactions leading to formation of the high himalayan granite chain from 24 to 20 ma and the north himalayan belt from 18 to 12 ma this model result agrees well with observation as do model predictions regarding emplacement style and location

heat sources for tertiary metamorphism and anatexis in the annapurna manaslu region central nepal following the collision of india with southern tibet crustal rocks of the leading edge of india 1 underwent regional metamorphism to upper amphibolite grade 2 melted locally to produce anatectic granitoids and 3 were sheared and thrust onto lower grade rock along the main central thrust yielding an inverted metamorphic sequence this sequence is exemplified in the annapurna manaslu region we use simple physical calculations to examine the heat sources involved in the different phases of metamorphism the regional metamorphism apparently is due to the burial of the northern edge of india beneath the accretionary prism along the southern edge of tibet the observed temperatures and pressures of this first phase of metamorphism are consistent with the thermal relaxation during the 10 30 m y before slip on the main central thrust began of thickened continental lithosphere whose original surface heat flux was between 50 and 70 mw m 2 the release of water from the footwall of the main central thrust apparently facilitated melting of the overlying crust in the second phase such melting could have occurred in the first million years or so of thrusting if warm 550 650 c crust in the footwall contained the necessary water if melting did not occur in the earliest stages of slip on the main central thrust dissipative heating with shear stresses of 10 to 100 mpa is required for temperatures near the main central thrust to have remained high enough to generate melting above the fault during the underthrusting of cold material the thickness 6 8 km of the zone of inverted isograds associated with the fault if undisrupted and due solely to thermal diffusion implies that the time required to carry the rocks preserving the inverted metamorphism from the surface to depths of 30 40 km was 4 8 m y the apparent inverted temperature gradients about 10 25 c km in this zone can be understood as the combined result of two processes diffusion of heat from hot rock thrust over cold rock expunged the original temperature gradient near the fault and could have created an inverse gradient of 10 20 c km the peak temperatures in such a zone however would not have exceeded about 350 c without an additional source of heat dissipative heating at shear stresses of about 100 mpa can account for peak temperatures in excess of 600 c during slip on the fault and would have contributed as much as 13 c km to the inverse gradient although inversion of metamorphic isograds could have occurred as a result of deformation within the main central thrust zone the high temperatures during slip on this zone still require dissipative heating unless the duration of slip exceeded 25 m y

low temperature replacement of monazite in the ireteba granite southern nevada geochronological implications the ireteba pluton is a relatively homogeneous 64 ma zircon ion probe age two mica granite that was intruded by two 16 ma miocene plutons at depths ranging from 5 to 13 km deeper levels of the ireteba and miocene plutons were ductilely deformed at 15 16 ma at shallow levels remote from the miocene plutons the ireteba granite appears to have experienced little miocene heating and deformation monazites from different portions of the pluton reflect the different histories experienced by the host rock irregularly shaped patchy zones with high huttonite component thsio4 are widespread in monazite at deep levels adjacent to miocene plutons but less common in shallow level rock monazite grains with extensive replacement generally have irregular embayed surfaces in undeformed rocks distant from the miocene plutons monazites are less modified and more nearly euhedral though fine networks of replacement veins are common and irregular rims are evident in some grains secondary monazite from these samples is poorer in huttonite ion probe th pb dating yields 60 65 ma ages for magmatic and some replacement zones in monazite from the shallow samples and veins yield apparent ages as young as mid tertiary monazites from deep samples yield a few 55 65 ma ages for remnant magmatic zones and abundant miocene ages for replacement zones 14 18 ma these data demonstrate extensive miocene replacement of magmatic monazite especially at deep levels near miocene plutons and they suggest an early replacement episode as well both events were probably related to influxes of fluid the first may have been associated with initial solidification of the ireteba pluton and the second with the miocene plutons and or extensional deformation ambient temperatures at the time of replacement indicate that secondary monazite growth occurred at t as low as 400 c or less

observations and controls on the occurrence of inherited zircon in concord type granitoids new hampshire u pb analyses of zircons separated from two concord type plutons near sunapee and dixville notch new hampshire reveal differences in the pattern and magnitude of zircon inheritance which are related to differences in melt chemistry the sunapee pluton contains only slightly more zr than required to saturate the melt at the peak temperature of 700 30 sup 0 c traces of inherited zircon in this separate are inferred to be present as small largely resorbed grains in contrast the long mountain pluton near dixville notch contains about 240 more zr than required to saturate the melt thus more than half of the zr existed as stable inherited zircon crystals during the partial fusion event consistent with the observation of substantial inheritance in all grain size fractions ion probe intra grain analyses of zircon from the long mountain pluton indicate a complex pattern of inheritance with contributions from at least two proterozoic terranes and caution against simple interpretations of upper and lower intercepts of chords containing an inherited component ion probe analyses of zircons from the sunapee pluton reveal clear evidence of u loss which results in incorrect apparent conventional u pb ages ages of crystallization for the long mountain and sunapee pluton are approx more 350 and 354 5 ma respectively a sm nd measurement for the long mountain pluton yields a depleted mantle model age of 1 5 ga consistent with the observed inheritance pattern in contrast a sm nd model age for the sunapee pluton is improbably old due to minor monazite fractionation less

stratigraphy structure and tectonic evolution of the himalayan fold thrust belt in western nepal regional mapping stratigraphic study and 40ar 39ar geochronology provide the basis for an incremental restoration of the himalayan fold thrust belt in western nepal tectonostratigraphic zonation developed in other regions of the himalaya is applicable with minor modifications in western nepal from south to north the major structural features are 1 the main frontal thrust system comprising the main frontal thrust and two to three thrust sheets of neogene foreland basin deposits 2 the main boundary thrust sheet which consists of proterozoic to early miocene lesser himalayan metasedimentary rocks 3 the ramgarh thrust sheet composed of paleoproterozoic low grade metasedimentary rocks 4 the dadeldhura thrust sheet which consists of medium grade metamorphic rocks cambrian ordovician granite and granitic mylonite and early paleozoic tethyan rocks 5 the lesser himalayan duplex which is a large composite antiformal stack and hinterland dipping duplex and 6 the main central thrust zone a broad ductile shear zone the major structures formed in a general southward progression beginning with the main central thrust in late early miocene time eocene oligocene thrusting in the tibetan himalaya north of the study area is inferred from the detrital unroofing record on the basis of 40ar 39ar cooling ages and provenance data from synorogenic sediments emplacement of the dadeldhura thrust sheet took place in early miocene time the ramgarh thrust sheet was emplaced between 15 and 10 ma the lesser himalayan duplex began to grow by 10 ma simultaneously folding the north limb of the dadeldhura synform the main boundary thrust became active in latest miocene pliocene time transport of its hanging wall rocks over an 8 km high footwall ramp folded the south limb of the dadeldhura synform thrusts in the subhimalayan zone became active in pliocene time the minimum total shortening in this portion of the himalayan fold thrust belt since early miocene time excluding the tibetan zone is 418 493 km the variation depending on the actual amounts of shortening accommodated by the main central and dadeldhura thrusts the rate of shortening ranges between 19 and 22 mm yr for this period of time when previous estimates of shortening in the tibetan himalaya are included the minimum total amount of shortening in the foldthrust belt amounts to 628 667 km this estimate neglects shortening accommodated by small scale structures and internal strain and is therefore likely to fall significantly below the actual amount of total shortening

joint seismic tomography for bulk sound and shear wave speed in the earth s mantle high quality p and s travel times are now available from careful reprocessing of data reported to international agencies a restricted data set has been extracted for which comparable ray coverage is achieved for p and s and used for a joint inversion to produce a three dimensional model for shear and bulk sound velocities represented in terms of 2 2 cells and 18 layers in depth through the mantle about 106 times for each of p and s are combined to produce 312 549 summary rays for each wave type linearizing about the ak135 reference model 583 200 coupled tomographic equations are solved using an iterative partitioned scheme clear high resolution images are obtained for both bulk sound speed and shear wavespeed the bulk and shear moduli have differing sensitivity to temperature and mineral composition and so the images of the two velocity distributions help to constrain the nature of the processes which produce the variations different heterogeneity regimes can be recognised in the upper mantle the transition zone most of the lower mantle and the lowermost mantle in the upper mantle many features can be explained by thermal effects but in some orogenic zones e g western north america the opposite sense of the bulk sound and shear wave speed variation requires compositional effects or volatiles to outweigh any thermal effects in the lower mantle pronounced narrow structures which may represent remnant subduction are most marked in shear the level of large scale variations in bulk sound speed compared to shear diminishes with depth in the lower mantle reaching a minimum near 2000 km below this depth the variability of both wave speeds increases near the core mantle boundary the variations of the two wave speeds show little concordance suggesting the presence of widespread chemical heterogeneity

evolution of the himalaya abstract the compression and attendant deformation of a thick and vast sedimentary prism formed since early riphean times on the northern continental margin of the indian craton gave rise to the himalaya mountains as a result of convergence and collision of the indian and asian plates the oceanic trench sediments tectonically implanted with sea floor material and intimately associated with calc al kaline volcanics in the narrow sindhu tsangpo belt extending from kohistan through dras leh darchen mansarovar to shigatse and beyond represent the subduction island arc complex which developed south of the dynamic southern margin of the asian continent and was welded to the colliding indian plate during the late eocene to oligocene period this complex is fringed to the north by a wide zone of andean type granitic bodies the evolution of the himalayan orogen is closely connected with the development of the present day andaman nicobar indonesia island arc subduction system in the southeast and the makran ranges oman trench in the southwest the evolution of the himalaya was accomplished in four major phases of tectonic upheaval during the late cretaceous to palaeocene karakoram phase late eocene to oligocene malla johar phase middle miocene to pontian sirmurian phase and late pliocene to middle pleistocene siwalik phase while the karakoram phase marks the convergence of continents and the malla johar phase represents the collision and subduction it was during the sirmurian upheaval that the main tectonic features developed and the himalaya acquired its distinctive structural complexion

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

comment on rossby gravity waves in the central equatorial pacific ocean during the norpax hawaii to tahiti shuttle experiment by s m chiswell and r lukas

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

crustal channel flows 1 numerical models with applications to the tectonics of the himalayan tibetan orogen 1 plane strain thermal mechanical numerical models are used to examine the development of midcrustal channel flows in large hot orogens in the models radioactive self heating reduces the viscosity of tectonically thickened crust and increases its susceptibility to large scale horizontal flow channels can be exhumed and exposed by denudation focused on the high relief transition between plateau and foreland we interpret the himalaya to have evolved in this manner channel flows are poorly developed if the channel has a ductile rheology based on wet quartz flow laws and well developed if there is an additional reduction in viscosity to 1019 pa s this reduction occurs from 700 c to 750 c in the models and is attributed to a small percentage of in situ partial melt melt weakening model ht1 provides an internally consistent explanation for the tectonic evolution of many features of the himalayan tibetan orogenic system erosional exhumation exposes the migmatitic channel equivalent to the greater himalayan sequence ghs between coeval normal and thrust sense ductile shear zones corresponding to the south tibetan detachment and the main central thrust systems outward flow of unstable upper crust rotates these shears to low dip angles in the model both the ghs and the lesser himalayan sequence are derived from indian crust with the latter from much farther south similar models exhibit a range of tectonic styles including the formation of domes resembling north himalayan gneiss domes model results are relatively insensitive to channel heterogeneities and to variations in the behavior of the mantle lithosphere beneath the model plateau

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

crustal generation of the himalayan leucogranites abstract detailed studies of the himalayan two mica leucogranites such as the manaslu pluton indicate that they have very uniform mineralogical petrological and structural characteristics one can relate their occurrence to the thickest zones of the underlying tibetan slab in these zones migmatization attains its greatest development and vertical extension the granite is emplaced at first along the main disharmonic plane above the main central thrust mct at the top of the tibetan slab infrastructure ductile deformation of the granite is variable the granite being syn to late kinematic with regard to the functioning of the mct major elements are very homogeneous except for na and k implying that p t conditions of melting were relatively uniform the melted material was of a similar composition over a vast volume and the percentage of melting was small 10 15 trace elements are highly variable some are characteristic for very evolved material ta rb cs u or show the link with the tibetan slab ba sr whilst others are problematic th ree ree and th abundances being much less in the granite than in the tibetan slab imply that they have been extracted during one of the main stages of formation possibly by monazite radiogenic pb sr nd and stable o isotopes are consistent with the origin of the granite from the tibetan slab however the heterogeneous sr isotopic ratios make age dating difficult and imply poor mixing or little fluid interaction during its evolution h isotope data indicate that magmatic compositions of the main body of the manaslu granite have been preserved late or post magmatic alterations are extremely local in the main pluton to the north another belt of two mica granites occurs whose characteristics are very similar to the high himalaya belt they were probably generated in a similar way during this recent intracontinental evolution

the kangmar dome a metamorphic core complex in southern xizang tibet the kangmar metamorphic igneous complex is one of the most accessible examples of an enigmatic group of gneiss domes the north himalayan belt that lies midway between the greater himalaya and the indus tsangpo suture in southern tibet structural analysis suggests that the domal structure formed as a consequence of extensional deformation much like the tertiary metamorphic core complexes in the north american cordillera unlike its north american counterparts the kangmar dome developed in an entirely convergent tectonic setting the documentation of metamorphic core complexes in the himalayan orogen supports the emerging concept that extensional processes may play an important role in the evolution of compressional mountain belts

detection of inherited monazite in the manaslu leucogranite by 208pb232th ion microprobe dating crystallization age and tectonic implications abstract although leucogranites are among the least petrologically variable of all igneous rocks ironically they are among the most difficult to reliably date the high himalayan leucogranites have been the subject of numerous geochronological investigations because of their interrelationship with the most significant tectonic features of that mountain belt for a variety of reasons linked to the minimum melt composition of these leucogranites these dating studies have not been entirely successful we report results of a new ion microprobe dating method based on the decay of 232 th to 208 pb in monazite that has directly revealed the presence of inherited pb in monazite from the manalsu granite casting doubt on its previously accepted age monazite ages obtained from this leucogranite yield two distinct populations a large number of ages with a normal distribution and mean age of 22 4 0 5 ma 2 s e that we interpret to be the crystallization age and a smaller inherited fraction with an age of ca 600 ma because formation of the granite is thought to be related to slip on the main central thrust both this date and a second less precise result from a structurally similar pluton near mt everest indicate that the main central thrust was active in the interval 24 22 ma but do not constrain its initiation together with crosscutting relationships these data require that movement on the north himalayan fault occurred prior to 22 ma at both locations

when did the roof collapse late miocene north south extension in the high himalaya revealed by th pb monazite dating of the khula kangri granite th pb ion microprobe measurements made on 12 monazite grains from the khula kangri granite tibet bhutan frontier are interpreted to indicate that crystallization occurred at 12 5 0 4 ma the leucogranite is cut by the gonto la detachment part of the southern tibet detachment system that has allowed upper level north directed extension of the himalayan orogen significant orogen normal extension in southern tibet appears to have continued 8 10 m y later than previously recognized this is the first reported crystallization age for a leucogranite east of the yadong cross structure an apparent 70 km offset of the high himalaya and southern tibet detachment west of the yadong cross structure reliable ages for high himalaya events major main central thrust slip granite generation and emplacement attainment of critical topography and major detachment extension group between ca 24 and 19 ma we interpret the west to east change across the yadong cross structure to be due to either 1 an abrupt 10 m y younging of principal high himalayan events or 2 a deeper thus younger exposed part of the footwall of the southern tibet detachment near khula kangri the southern tibet detachment is cut by the highly oblique yadong gulu rift a manifestation of tibet plateau east west extension integrated estimates of magnitude and rate of detachment displacement suggest that the observed postcrystallization north directed extension lasted for 1 3 m y after which time the yadong gulu rift formed this interpretation is consistent with initiation of east west extension of tibet at ca 8 ma

slab breakoff a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens we present a model proposing that oceanic lithosphere detaches from continental lithosphere during continental collision slab breakoff allowing an explanation of syn to post collisional magmatism and metamorphism continental collisions are preceded by subduction of dense oceanic lithosphere and followed by attempted subduction of buoyant continental lithosphere this situation of opposing buoyancy forces leads to extensional deformation in the subducted slab a narrow rifting mode of deformation will result if there is strain localization slab breakoff results we have assessed the plausiblity of this process by quantitatively evaluating an upper bound for the strength of the lithosphere and have compared it with the change in buoyancy force during continental sbduction whether breakoff will occur and the depth at which it will occur is a strong function of temperature and hence the subduction velocity for a subduction velocity of 1 cm yr breakoff could occur at depths of between 50 and 120 km while at higher velocities it is still likely to occur but at deeper depths as a result of the rifting during breakoff the asthenosphere upwells into the narrow rift and following breakoff it impinges on the mechanical lithosphere of the overriding plate the resulting conducted thermal perturbation leads to melting of the metasomatised overriding mantle lithosphere producing basaltic magmatism that leads to granitic magmatism in the crust dry asthenospheric mantle will melt only if breakoff occurs at a depth shallower than ca 50 km breakoff removes the force at the downdip side of the continental crust while the enhanced heating leads to a reduction of the strength of the underlying crust both effects facilitate the freeing of buoyant crustal sheets which can then rise towards the surface leading to the rapid exhumation of eclogite facies continental crust the cessation of subduction and replacement of the cold oceanic lithosphere by asthenosphere leads to rapid uplift of the orogen we have tested the variety of predicted expressions of slab breakoff in the geological record of the alps the aegean islands and the dabie shan a comparison of the various belts highlights 1 that the magmatism and metamorphism are found near the suture and the centre of the orogen demonstrating the general steepening of suture during collision 2 that a vague inverse correlation exists between maximum depth of metamorphism and volume of syn orogenic magmatism and 3 that the melts can be emplaced in both compressive and extensional environments we suggest that slab breakoff is an important step in the evolution of many orogenic belts and it allows an explanation for the combined presence in collisional orogens of magmatism with a mantle parentage and ultra high pressure metamorphics

a model for the origin of himalayan anatexis and inverted metamorphism the origin of the paired granite belts and inverted metamorphic sequences of the himalaya has generally been ascribed to development of the main central thrust mct although a variety of models have been proposed that link early miocene anatexis with inverted metamorphism recent dating studies indicate that recrystallization of elements of the mct footwall occurred in the central himalaya as recently as 6 ma the recognition that hanging wall magmatism and footwall metamorphism are not spatially and temporally related renders unnecessary the need for exceptional physical conditions to explain generation of the high himalayan leucogranites and north himalayan granites which differ in age petrogenesis and emplacement style we suggest that their origin is linked to shear heating on a continuously active thrust that cuts through indian supracrustal rocks that had previously experienced low degrees of partial melting numerical simulations assuming a shear stress of 30 mpa indicate that continuous slip on the himalayan decollement beginning at 25 ma could trigger partial melting reactions leading to formation of the high himalayan granite chain between 25 and 20 ma and the north himalayan belt between 17 and 8 ma the ramp flat geometry we apply to model the himalayan thrust system requires that the presently exposed rocks of the hanging wall resided at middle crustal levels above the decollement throughout the early and middle miocene late miocene out of sequence thrusting within the broad shear zone beneath the mct provides a mechanism to bring these rocks to the surface in their present location i e well to the north of the present tectonic front and has the additional benefit of explaining how the inverted metamorphic sequences formed beneath the mct we envision that formation of the mct zone involved successive accretion of tectonic slivers of the lesser himalayan formations to the hanging wall and incorporate these effects into the model the model predicts continued anatexis up to 400 km north of the himalayan range consistent with the timing and geochemistry of leucogranites exhumed on the flank of a south tibetan rift

the zedong window a record of superposed tertiary convergence in southeastern tibet determining the timing magnitude and location of deformation due to the indo asian collision is widely acknowledged as an important step in understanding how the lithosphere responds during continental collision a puzzling result of geological investigations of the lhasa block over the past 2 decades has been the apparent lack of significant tertiary deformation there perhaps the most important structural feature of the lhasa block is the south directed gangdese thrust system which developed along its southern edge the thrust system which separates the andean type batholith of southern asia from rocks of indian affinity is obscured at most locations across southeastern tibet by back thrusts of the younger north directed renbu zedong thrust system the best documented site where both thrusts are exposed occurs near zedong zedong window systematic geochronologic analyses were conducted in this area u pb zircon dating of three samples of a synkinematically to postkinematically deformed hanging wall granitoid the yaja granodiorite cut by the gangdese thrust indicates a crystallization age of 30 4 0 4 ma 2 thus placing an upper bound on the initiation of the thrust u pb zircon dating of granitoid samples structurally higher in the gangdese hanging wall yields emplacement ages of 42 5 1 0 ma and 48 9 0 8 ma 2 similar to other magmatic complexes within the gangdese arc geochemical results are consistent with these plutons forming in the same environment as precollisional intrusions within the gangdese batholith suggesting a significant post collisional input of juvenile heat the 40ar 39ar thermochronology of samples from a vertical section through the hanging wall within the yaja granodiorite coupled with results of a numerical thermal model indicate an average slip rate along the gangdese thrust of 7 mm yr between 30 and 23 ma and a minimum displacement of 50 km farther east in the zedong window thermal effects produced by later north directed thrust sheets of the renbu zedong system appear to have obscured thermal history signatures in the gangdese hanging wall related to earlier south directed thrusting thermochronological results from this region indicate that thermal overprinting related to the north directed thrusting occurred between 25 and 10 ma consistent with previous estimates

crustal channel flows 2 numerical models with implications for metamorphism in the himalayan tibetan orogen 1 results from a thermal mechanical model ht1 that includes midcrustal channel flow are compatible with many features of the himalayan tibetan system radioactive self heating and rheological weakening of thickened model orogenic crust lead to the formation of a hot low viscosity midcrustal channel and a broad plateau channel material corresponding to the greater himalayan sequence ghs flows outward from beneath the plateau in response to topographically induced differential pressure at the plateau flank it is exhumed by focused surface denudation and juxtaposed with cooler newly accreted material corresponding to the lesser himalayan sequence lhs the model channel is bounded by coeval thrust and normal sense ductile shear zones interpreted to represent the main central thrust mct zone and south tibetan detachment system respectively inverted metamorphism associated with the model mct zone results from distributed ductile shear along the mct and extrusion of the hot channel a variety of model p t t path styles resembling those observed in the ghs and lhs are produced for points traveling through contrasting tectonic regimes that coexist in different parts of the model predicted times of peak metamorphism cooling and erosion of metamorphic facies are generally compatible with observations although model ghs cooling ages are too young the times of m1 and m2 metamorphic events observed in the ghs correspond to model times of maximum burial and maximum heating respectively the results highlight the need to integrate tectonics and metamorphism in continental collision models and demonstrate the importance of lateral transport of both heat and material in large hot orogens

age and thermal history of eo and neohimalayan granitoids eastern himalaya abstract four distinct granitoid suites outcrop across a n s transect in the eastern himalaya between the main central thrust mct and the indus tsangpo suture the arunachal and tsona leucogranites outcrop respectively in the mct hanging wall and adjacent the south tibetan detachment both crystallized at about 19 ma and appear to be lateral equivalents of the high himalayan leucogranites in the central tethyan himalaya the dala granitoids form a suite of undeformed plutons emplaced into deformed tethyan himalayan sequence metasediments in the northern tethyan himalaya the yala xiangbo granitoids are part of an igneous complex in the core of the yala xiangbo dome both the dala and yala xiangbo granitoids have eocene magmatic ages 44 1 1 2 ma and 42 5 ma respectively that are notably older than the vast majority of post collisional granitoids along the main himalayan arc pelitic units from the core of the yala xiangbo dome record two episodes of amphibolite grade metamorphism and monazite growth coinciding with well documented episodes of eohimalayan and neohimalayan metamorphism the thermochronological histories of the dala and yala xiangbo units suggest that the eastern tethyan himalaya was tectonically quiescent from ca 40 ma until ca 15 ma miocene exhumation of the dala and yala xiangbo units and all north himalayan domes studied to date appears best explained by a structural control such as concurrent north directed motion on the great counter thrust we hypothesize that an extension of the ninety east ridge may have been responsible for eocene high heat flow beneath the eastern himalaya and explain the localization of magmatism there

evidence for early 44 ma himalayan crustal thickening tethyan himalaya southeastern tibet abstract comprehensive understanding of himalayan orogenesis is limited in part by a poor knowledge of the eohimalayan episode a phase of tectonic activity predating the better understood neohimalayan crustal thickening associated with coaxial deformation along the main central thrust the cambo ordovician to tertiary metasedimentary sequences outcropping within the tethyan himalaya are the structurally highest units of the himalayan fold and thrust belt they and are inferred to have been the first to have accreted to asia a compilation of data from transects along the length of the main himalayan arc shows that the tethyan sequences have experienced at least five deformation events although the timing of these episodes is poorly constrained emplacement of a series of undeformed granitoid bodies following the second of these events which accounts for the majority of crustal thickening in the tethyan himalayan units is constrained by u pb zircon dating to be older than 44 1 1 2 ma thus significant crustal thickening had occurred along the length of the proto himalayan arc by the mid eocene or within 10 to 20 myr of the initiation of himalayan orogenesis

fluid enhanced melting during prograde metamorphism anatexis is a commonly recognized feature of high grade metamorphism but segregated melts are generally ascribed to anatexis during peak metamorphic conditions and little is known about melting along the prograde path a suite of small volume deformed two mica leucogranites has been recognized within the high himalayan crystalline series of the garhwal himalaya these granites are consistently more siliceous than minimum melt granite compositions and are characterized by low rb sr ratios high ba low abundances of hfs elements and positive eu anomalies such trace element characteristics contrast strongly with the geochemistry of the well studied early miocene leucogranites of the high himalaya derived from fluid absent melting sm nd garnet dating of one deformed granite indicates a crystallization age of 39 3 ma c 15 ma before the emplacement of the more voluminous high himalayan leucogranites whilst some entrainment of restitic phases cannot be excluded trace element signatures suggest a low temperature 2 o activity positive eu anomalies and unusually low rb sr ratios are indicative of rapid disequilibrium melting fluid enhanced melting may be a common feature of prograde upper amphibolite facies metamorphism of orogenic belts predating peak metamorphism by at least 15 ma these melts will only crystallize within this period if they segregate from their protoliths subsequent dating of long lived melts would indicate erroneously young ages for the prograde melting events however melts formed in this way may be recognized by their distinctive trace element chemistry the persistence of early formed melts within an orogen provides insights into the prograde heating path and may be critical in controlling the rheology of the middle crust and hence its deformational history

paleocene eocene record of ophiolite obduction and initial india asia collision south central tibet 1 uppermost cretaceous to eocene marine sedimentary sequences occur both to the south and north of the yarlung zangbo suture in south central tibet they consist of indian margin strata of the northern tethyan himalaya and asian margin strata of the gangdese forearc both assemblages are characterized by major changes in depositional environment and sedimentary provenance at 65 ma and an appearance of detrital chromium rich spinel of ophiolite affinity tio2 generally 0 1 wt during the paleocene ophiolitic melange exposed along the suture could have provided a source for detrital spinel the melange occurs in the hanging wall of a north dipping south directed mylonitic shear zone which includes a tectonic sliver of mafic schist amphibole from the schist yields 40ar 39ar ages of 63 ma which we attribute to cooling during slip along the shear zone and southward obduction of the melange melange obduction was coeval with the development of an angular unconformity within the gangdese forearc basin to the north between late maastrichtian time and 62 ma upper paleocene to middle eocene sandstones in the northern tethyan himalaya yield 200 120 ma u pb detrital zircon ages and 190 170 ma 40ar 39ar detrital mica ages these detrital grains were most likely sourced from regions north of the yarlung zangbo suture suggesting that onset of india asia collision in south central tibet is middle eocene or older in age collectively our results support previous suggestions that oceanic rocks were obducted onto the northern margin of india during latest cretaceous earliest tertiary time coeval changes in gangdese forearc sedimentation raise the possibility that this obduction event marks onset of tectonic interaction between india and asia at 65 ma alternatively in concert with the conventional view of eocene collision initiation the obducted oceanic rocks may be of intraoceanic origin while coeval changes in gangdese forearc sedimentation may be a consequence of an increase in the rate of ocean continent convergence following the demise of the intraoceanic subduction zone

elemental abundance anomalies in the late cenomanian extinction interval a search for the source s elemental abundances have been measured by neutron activation methods across the cenomanian turonian late cretaceous extinction interval in samples collected from sixteen sites in the western interior basin of north america and from twelve widely separated locations around the globe including six odp dsdp sites in most western interior basin sites in colombia and in western europe weaker two closely spaced elemental abundance peaks occur in the upper cenomanian 92 m y spanning the ammonite zones ofsciponoceras gracile throughneocardioceras juddii elements with anomalously high concentrations include sc ti v cr mn co ni ir pt and au the lower peak coincides with the disappearance extinction of the foraminiferrotalipora cushmani in north american sectionsr greenhornensis also disappears at or just below this horizon but in europe it disappears considerably earlier thanr cushmani a series of molluscan extinction and speciation or migration events also begins near the stratigraphic level of the lower elemental abundance peak the well documented positive 13c excursion begins just before the extinctions and the elemental anomalies and continues into the lower turonian well above the upper anomaly this carbon isotope excursion has been observed in east european sections where we find little or no evidence of the elemental anomalies suggesting that the two phenomena may not be tightly coupled elemental abundance ratios in the anomalies closely resemble those of mid atlantic ridge basalt or hawaiian lava tholeiitic but not those of c1 chondrite black shale average crustal rocks or lamproite and kimberlite of roughly similar age in southeastern kansas the excess ir and other siderophiles hint at possible large body impact s for the source however we have not located microspherules other than biogenic calcispheres or shocked mineral grains in any of our samples furthermore sc ti v and mn are not enriched in differentiated solar system bodies although the weak geochemical signal from comet impact s could be masked by the strong terrestrial like overprint these anomalies more likely resulted either from intense seafloor spreading activity or merely from increased circulation of deep metal rich water associated with the large late cenomanian through early turonian eustatic rise and deep water opening of the south atlantic the flooding of continental seaways and margins also could have contributed to the anomalies by preventing much continental detritus from diluting the normal background marine geochemical component

isotopic study of the manaslu granite himalaya nepal inferences on the age and source of himalayan leucogranites a detailed isotopic study of the manaslu leucogranite was carried out a u pb age of 25 ma and a whole rock rb sr age isochron of 18 ma were obtained suggesting that the magmatic activity lasted at least 7 ma initial sr isotopic ratios are very high 0 740 to 0 760 and initial nd isotopic ratios are low nd in 13 to 16 and they show the existence of large isotopic variations even at the metre scale these are not the result of perturbations by fluids but rather they reflect the initial isotopic heterogeneity of the source material which has not been obliterated by magmatic processes e g fusion mixing by convection these results also support the crustal origin of this leucogranite the tibetan slab paragneisses whose sr and nd isotopic ratios are very similar to those of the granite at an age of 20 ma are the most probable parental material nd model ages for both the leucogranite and the gneisses are in the range 1 5 2 ga a model of formation of the manaslu granite by coalescence of different batches of magma is in agreement with the present data

tertiary structural evolution of the gangdese thrust system southeastern tibet structural and thermochronological investigations of southern tibet xizang suggest that intracontinental thrusting has been the dominant cause for formation of thickened crust in the southernmost tibetan plateau since late oligocene two thrust systems are documented in this study the north dipping gangdese system gts and the younger south dipping renbu zedong system rzt west of lhasa the gangdese thrust juxtaposes the late cretaceous forearc basin deposits of the lhasa block the xigaze group over the tethyan sedimentary rocks of the indian plate whereas east of lhasa the fault juxtaposes the late cretaceous eocene andean type arc the gangdese batholith over tethyan sedimentary rocks near zedong 150 km southeast of lhasa the gangdese thrust is marked by a 200 m thick mylonitic shear zone that consists of deformed granite and metasedimentary rocks a major south dipping backthrust in the hanging wall of the gangdese thrust puts the xigaze group over tertiary conglomerates and the gangdese plutonics north of xigaze and west of lhasa a lower age bound for the gangdese thrust of 18 3 0 5 ma is given by crosscutting relationships the timing of slip on the gangdese thrust is estimated to be 27 23 ma from 40 ar 39 ar thermochronology and a displacement of at least 46 9 km is indicated near zedong the age of the gangdese thrust gt is consistent with an upper age limit of 24 ma for the initiation of movement on the main central thrust in places the younger renbu zedong fault is thrust over the trace of the gt obscuring its exposure the rzt appears to have been active at circa 18 ma but had ceased movement by 8 1 ma the suture between india and asia has been complexly modified by development of the gts rzt and locally strike slip and normal fault systems

mid eocene high sr y granites in the northern himalayan gneiss domes melting thickened lower continental crust abstract within the himalayan collisional belt granotoids occur along two sub parallel belts the northern himalayan gneiss domes nhgd and the high himalayan crystalline series hhcs in the yardoi area of nhgd two mica granite a new type granite occurs in the core of the yardoi gneiss dome ygd dala and quedang from north to south and extends at least 50 km long these granites have similar mineral composition elemental and radiogenic isotope geochemistry and age of formation shrimp zircon u pb dating indicates that the yardoi and the quedang two mica granites formed at 42 6 1 1 ma and 42 8 0 6 ma respectively similar to the dala pluton these two mica granites have 1 high sio2 68 wt al2o3 15 wt and a cnk 1 0 2 relatively high sr and lree but low y 40 and up to 250 and la yb 30 4 very weak or no eu anomalies and 5 as compared with those in the himalayan leucogranites low initial sr 87sr 86sr i

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

new constraints on the age of the manaslu leucogranite evidence for episodic tectonic denudation in the central himalayas the manaslu leucogranite of central nepal transacts one segment of the south tibetan detachment system a major extensional feature that helped to moderate large topographic gradients in the himalayan orogen in middle tertiary time 40 ar 39 ar ages for hornblendes from the northeastern contact aureole of the pluton indicate that intrusion occurred prior to 22 23 ma providing a minimum age for formation of the detachment system in the manaslu region that is at least 2 m y older than previous estimates from other parts of the orogen 40 ar 39 ar mica ages from the aureole similar to previously published mica ages from the upper part of the leucogranite indicate an episode of rapid cooling at 19 16 ma we attribute this cooling event to tectonic denudation of the pluton by movement on structurally higher exten sional faults and we suggest that gravitational collapse of the orogenic front occurred episodically over the late oligocene early miocene interval in the manaslu area

trace element composition of igneous zircon a thermal and compositional record of the accumulation and evolution of a large silicic batholith spirit mountain nevada hafnium u th and ree content of zircons from the spirit mountain batholith in southern nevada correlate with calculated temperatures from the ti in zircon thermometer to support field and petrologic evidence of rejuvenation of crystal mush and melt extraction events during the 2 million year accumulation of the granitoid batholith marked variation in zircon composition from sample to sample from grain to grain within individual samples and from zone to zone within individual grains documents in detail a history of fluctuating conditions with repeated episodes of replenishment reheating crystal mush rejuvenation fractional crystallization and melt segregation the zircons exhibit compositional and thermal variability indicative of variations in host melt composition due to 1 melt rejuvenation mixing and fractionation 2 coeval growth of other ree rich accessory minerals and possibly 3 fluctuation in fo2

a thermal model for the distribution in space and time of the himalayan granites abstract in southern tibet crustal thickening due to the india asia collision has led to the formation of two granite belts one is located at the southern edge of the accretionary wedge of tethyan sedimentary rocks close to the contact with basement gneisses of the tibetan slab the other is found within the wedge itself close to the kangmar thrust trace available ages suggest that the granites appeared first in the southern belt and then in the kangmar belt this sequence seems to violate the chronology of thrusting another feature of the himalayas is that melting started only about 20 ma after the onset of thickening which is much less than the thermal time constant of thick crust we give a thermal model based on the assumption of conductive heat transfer which explains these features the model relies on the geometry of a sedimentary accretionary wedge bounded by low angle thrust faults and on the existence of a thermal conductivity contrast between old basement and young sedimentary rocks the wedge of sedimentary rocks acts as an insulating cap and its southern edge heats up along the contact with basement rocks on a horizontal cross section there is a temperature maximum along this southern edge which explains why melting starts there the early thermal evolution is sensitive to local conditions and granites first appear in the vicinity of the most radiogenic parts of the basement the distribution of granites in space and time is seemingly random reflecting different melting events in different radiogenic environments in the heterogeneous basement this model predicts a relationship between radioactivity and age which is compatible with available data the results emphasize that there are large horizontal temperature variations across a thickened region and that granite ages are not related simply to the timing of tectonic phases