strain heating as a mechanism for partial melting and ultrahigh temperature metamorphism in convergent orogens implications of temperature dependent thermal diffusivity and rheology 1 we explore the importance of strain heating in ductile shear zones for production of leucogranites and high temperature metamorphism in collisional orogens with temperature dependent thermal diffusivity d and rock rheology new measurements on metamorphic rocks from the bohemian massif show that d is 1 3 mm2 s at 25 c and decreases exponentially to as low as 0 4 mm2 s at 600 c this temperature dependence causes model lithospheric geotherms to be straighter compared to geotherms calculated with a constant d using power law parameters for rheology we show that deformation of quartzite at strain rate of 3 10 13 s 1 produces 100 w m3 at 550 c and 7 w m3 at 800 c deformation of stronger clinopyroxenite at this strain rate produces 8 w m3 even at 1050 c when strain heating is introduced by a 3 km thick ductile shear zone with quartz rheology at the depth of 35 km in a lithosphere that has 70 km thick crust and 60 km thick lithospheric mantle the schist solidus is reached by 8 myr in the vicinity of the shear zone deforming at the strain rate of 3 10 13 s 1 for clinopyroxenite rheology ultrahigh temperature uht metamorphic conditions are reached in 40 myr after initiation of strain heating two dimensional models that replicate the exhumation of the greater himalaya crystalline rocks above the main central thrust produce extensive partial melting and an inverted metamorphic field gradient occurrences of leucogranites within crustal shear zone systems may be evidence of the coupling between deformation and heat production in the crust

etude petro structurale des chevauchements ductiles himalayens sur la transversale de l everest makalu nepal oriental tectonic and microtectonic data in eastern nepal indicate that the major observed thrusting 100 km on the main central thrust mct postdates the barrovian metamorphism of the high himalaya gneisses this result at variance with the famous reverse metamorphism model better explains the abnormal metamorphic superpositions in the himalayas and accounts for the lack of high pressure assemblages under the thick allochtonous high himalaya tibetan slab pressure and temperature estimates by microprobe analysis on plagioclase biotite garnet kyanite sillimanite and cordierite assemblages are presented for samples collected along the mct shear zone and across the gneiss slab in the everest makalu area since there is very little difference in pressure at the front of the slab kathmandu klippe and its root these estimates support the existence of important late metamorphic thrusting the decrease of pressure towards the top of the gneiss pile combined with a small temperature increase explains the k

geologic thermobarometry of retrograded metamorphic rocks an indication of the uplift trajectory of a portion of the northern scandinavian caledonides kyanite grade pelitic schists from the efjord area 68 15 n 16 45 e northern norway show abundant textural evidence of retrograde reequilibration during late orogenic uplift and cooling of this part of the scandinavian caledonides graphical analysis of mineral compositions in seven samples that show a range of retrograde textures indicates that they reached final chemical equilibrium under nonuniform pressure temperature conditions three element partitioning geothermometers and geobarometers yield consistent pt estimates in individual samples which together define a trend in pt space the sample showing the fewest signs of retrogression yields the highest pressure and temperature estimates whereas samples exhibiting abundant reequilibration textures give lower pt estimates we suggest that the trend indicated by these samples reflects a portion of the pt trajectory assumed by the efjord area during uplift thus the judicious use of element partitioning geothermometers and geobarometers on samples showing signs of retrograde metamorphism may yield important new information concerning the late tectonic evolution of metamorphic terrains

metamorphic response to collision in the central himalayan orogen abstract closure of the neotethys ocean and high angle continent continent collision between india and asia after about 55 ma resulted in low angle subduction of the indian plate below the tibetan plateau and by 30 ma established an arcuate 2300 km long shallow north dipping metamorphic fold thrust belt in the foreland this himalayan metamorphic front quickly established an upper plate lower plate paired metamorphic architecture centred on the median high himal thrust that largely controlled subsequent evolution the upper plate is a thick slab of high t moderate p high grade migmatized metamorphic rocks whereas the lower plate is an inverted series of moderate t high p schists in two crustal wedges the broad main central thrust zone and at lowest structural levels the footwall below the basal main central thrust spatial and temporal patterns of metamorphic response in the evolving himalayan metamorphic front has been characterized in a large scale integrated structural metamorphic study based on 8 profiles across eastern nepal metamorphic response at all structural levels is established using a large dataset n 160 of internally consistent quantitative pt determinations petrology of metapelite samples semi quantitative p t paths metamorphic mapping and metamorphic field gradients these results are integrated with previously published metamorphic studies structural profiles and metamorphic chronology from these datasets the architecture and evolution of the himalayan metamorphic front is constrained by rock kinematics metamorphic field gradients showing discontinuities and diachronous metamorphism with contrasting p t evolutions at different structural levels each of the three panels constituting the himalayan metamorphic front upper plate main central thrust zone and footwall experienced distinctly different tectono metamorphic histories crustal processes operating during metamorphism and exhumation differ between the upper and lower plates the upper plate experienced long lived metamorphism starting from at least 28 38 ma and tracking low p t clockwise p t paths that culminated at 19 27 ma in peak high grade condition with 27 31 c km thermal regimes protracted high grade conditions produced significant partial melt which facilitated gravity driven southward extrusion involving internal ductile flow processes southward extrusion of the upper plate was accommodated by coeval reverse movement on the high himal thrust and top down to the north normal reactivation of the south tibet detachment system between 22 10 ma transport of this thick slab to the south resulted in further prograde burial of the lower plate below culminating in peak metamorphism at the highest pressures attained in the lower plate rocks the lower plate consists of at least two deeply buried 7 5 9 4 kb crustal wedges both of which experienced steep p t hairpin clockwise p t paths with isothermal decompression the main central thrust zone experienced 20 26 c km metamorphism at 12 22 ma and was exhumed along the basal main central thrust after 11 ma whereas loading by the main central thrust zone gave rise to further prograde burial of the footwall which experienced 16 21 c km metamorphism at 4 10 ma and was exhumed along the main boundary thrust at 3 9 ma in contrast to the upper plate both crustal wedges were exhumed by a process of out wedging which involved upthrusting along basal and internal thrusts with coeval extensional slip in the hanging walls this three stage foreland propagating lateral exhumation history resulted in telescoping of the himalayan metamorphic front in concert with peak metamorphic events at different structural levels during main phase orogenesis and is not the result of superimposed retrograde reactivation of the belt

correlation between melting deformation and fluid interaction in the continental crust of the high himalayas langtang valley nepal combined metamorphic stable isotope and structural studies from the high himalayan crystalline sequence in the langtang valley of north central nepal reveal a strong positive correlation between distance above the base of the section the main central thrust the amount of melt material and evidence of prolonged fluid and deformation histories thus suggesting that these processes are strongly interdependent kyanite grade rocks at the base of the section are unmelted and have undergone little syn or post metamorphic internal deformation and little prograde or retrograde fluid rock interaction by contrast sillimanite grade rocks higher in the section contain progressively larger volumes of melt have suffered increasingly complex syn and post metamorphic deformation and show increasing evidence for the presence of fluids although the factors that initiated these processes remain problematic it is suggested that fluid distributions within the langtang section have been passively controlled by the movement of melts these melts may have provided a primary control on deformation during both the magmatic stage and subsequently through the exsolution of exsolved aqueous fluids

regulation of algal blooms in antarctic shelf waters by the release of iron from melting sea ice during summer 1995 96 we measured iron in the water column and conducted iron enrichment bottle incubation experiments at a station in the central ross sea 76 30 s 170 40 w first in the presence of melting sea ice and 17 days later in ice free conditions we observed a striking temporal change in mixed layer dissolved iron concentrations at this station from 0 72 2 3 nm with sea ice present to 0 16 0 17 nm in ice free conditions these changes were accompanied by a significant drawdown in macronutrients and an approximate doubling of algal diatom biomass our incubation experiments suggest that conditions were iron replete in the presence of sea ice and iron deficient in the absence of sea ice we surmise that bioavailable iron was released into seawater from the melting sea ice stimulating phytoplankton production and the biological removal of dissolved iron from the mixed layer until iron limited conditions developed these observations suggest that the episodic release of bioavailable iron from melting sea ice is an important factor regulating phytoplankton production particularly ice edge blooms in seasonally ice covered antarctic waters

records of the evolution of the himalayan orogen from in situ th pb ion microprobe dating of monazite eastern nepal and western garhwal abstract in situ th pb monazite ages from rocks collected along two transects the dudh kosi everest eastern nepal and the bhagirathi river garhwal himalaya india perpendicular to the main central thrust mct suggest a striking continuity of tectonic events across the himalaya the youngest age reported in this study 5 9 0 2 ma mswd 0 4 from matrix monazite grains collected beneath the mct in the garhwal region is consistent with several age data from rocks at similar structural levels in central nepal providing support for widespread late miocene mct activity the lateral parallelism of orogenic events is further manifested by the 20 7 0 1 ma age of a high himalayan leucogranite from an injection complex that outcrops along the dudh kosi everest transect resembling ages of these bodies reported elsewhere the youngest monazite grain analyzed along the dudh kosi everest transect is 10 3 0 8 ma the absence of 7 3 ma monazite ages in eastern nepal may reflect a different nappe structure which obscures the reactivated ramp equivalent exposed in the garhwal and central nepal garnets from the mct hanging wall greater himalayan crystallines and footwall lesser himalaya display different major element zoning and the patterns are useful for constraining the location of the thrust system that separates the two lithologies pressure temperature paths for two upper lesser himalayan garnets that contain monazite inclusions indicate the utility of an in situ methodology to constrain the metamorphic evolution of the shear zone along the dudh kosi everest transect upper lesser himalayan monazite grains from three rocks record a clear signal at 14 5 0 1 ma mswd 8 4 and the 23 ma age that characterizes the hanging wall is notably absent monazites collected within a large scale greater himalayan crystallines fold yield the 14 ma age consistent with the structure forming due to mct related compression paleo mesoproterozoic 1407 35 ma matrix monazite grains are found within an augen gneiss unit located beneath the mct whereas cambro ordovician 436 8 548 17 ma inclusions are preserved within garnets of the greater himalayan crystallines the presence of 45 2 2 1 ma grains from lower structural levels of the greater himalayan crystallines indicates the unit realized conditions conducive for monazite growth during the eocene

himalayan leucogranites and migmatites nature timing and duration of anatexis widespread anatexis was a regional response to the evolution of the himalayan tibetan orogen that occurred some 30 ma after collision between asia and india this paper reviews the nature timing duration and conditions of anatexis and leucogranite formation in the greater himalayan sequence ghs and compares them to contemporaneous granites in the karakoram mountains himalayan leucogranites and associated migmatites generally share a number of features along the length of the mountain front such as similar timing and duration of magmatism common source rocks and clockwise p t paths despite commonalities most papers emphasize deviations from this general pattern indicating a fine tuned local response to the dominant evolution there are significant differences in p t xh2o conditions during anatexis and timing in relation to regional decompression further to that some regions underwent a second event recording melting at low pressures zircon and monazite ages of anatectic rocks range between c 25 and 15 ma suggesting prolonged crustal melting typically a single sample may have ages covering most of this 10 ma period suggesting recycling of accessory phases from metamorphic rocks and early formed magmas recent studies linking monazite and zircon ages with their composition have determined the timing of prograde melting and retrograde melt crystallization thus constraining the duration of the anatectic cycle in some areas this cycle becomes younger down section towards the leading front of the himalayas whereas the opposite is true in other areas the relationship between granites and movement on the south tibetan detachment std reveals that fault motion took place at different times and over different durations requiring complex internal strain distribution along the himalayas the nature and fate of magmas in the ghs contrast with those in the karakoram mountains ghs leucogranites have a strong crustal isotopic signature and migration is controlled by low angle foliation leading to diffuse injection complexes concentrated below the std in contrast the steep attitude of the karakoram shear zone focused magma transfer feeding the large karakoram baltoro batholith anatexis in the karakoram involved a cretaceous calcalkaline batholith that provided leucogranites with more juvenile isotopic signatures the impact of melting on the evolution of the himalayas has been widely debated melting has been used to explain subsequent decompression or conversely decompression has been used to explain melting weakening due to melting has also been used to support channel flow models for extrusion of the ghs or alternatively to suggest it triggered a change in its critical taper in view of the variable nature of anatexis and of motion on the std it is likely that anatexis had only a second order effect in modulating strain distribution with little effect on the general history of deformation thus despite all kinds of local differences strain distribution over time was such that it maintained the well defined arc that characterizes this orogen this was likely the result of a self organized forward motion of the arc controlled by the imposed convergence history and energy conservation balancing accumulation of potential energy and dissipation independent of the presence or absence of melt

uncertainties in differential thermodynamic gibbs method p t paths a combined analytical and numerical evaluation of the uncertainties in p t paths is made for three assemblages that propagates errors in the parameters initial pressure initial temperature initial composition change of composition of monitor parameters endmember entropy and endmember volume propagated errors along an isobaric heating path t 77 c for assemblage 1 grt bt pl qtz ms chl h2o using as monitor parameters the mole fractions of almandine spessartine grossular and anorthite and ignoring uncertainties in thermodynamic properties are approximately 320 bars 1 and 8 3 c 1 if rim compositional uncertainties of 5 in major cations are assumed or 2 5 c 1 and 50 bars 1 if electron microprobe analytical uncertainties are assumed for compositions the largest source of uncertainty is from the errors in the monitor parameters and p t path uncertainties can depend critically on which monitor parameters are used if the mole fraction of annite is used as a monitor parameter in place of the anorthite content of plagioclase then propagated uncertainties are worse than 29 c 1 and 5800 bars 1 p t path uncertainties also depend on assemblage 1 precisions in assemblages 2 grt bt pl qtz ms sil h2o and 3 grt bt qtz kfs sil h2o using as monitor parameters the mole fractions of almandine spessartine grossular and anorthite are calculated to be 267 bars and 43 4 c and 372 bars and 8 2 c respectively estimates of the accuracies in p t paths that include potential errors in endmember entropy of 1 j mol k and in endmember volume of 1 cm3 mol are 324 bars and 8 5 c assemblage 1 341 bars and 48 6 c assemblage 2 and 388 bars and 9 7 c assemblage 3 use of different garnet plagioclase and muscovite activity models can change the length of a p t path by as much as 15 but does not typically change directions in p t space significantly models that incorporate changes of fluid composition shorten p t paths in assemblages 1 and 3 but do not change trajectories significantly assemblage 2 is virtually unaffected by fluid phase models for the mineral assemblages considered here and using appropriate monitor parameters propagated errors are small compared to the total path length suggesting that the differential thermodynamic approach is a precise and accurate method for determining amounts of heating or thickening during metamorphism and hence for interpreting orogenic processes

thermal effects of thrust faulting abstract calculations based on simple models of overthrust sheets in crystalline basement rocks show that significant thermal effects may result from their movements if rates are sufficiently high e g plate tectonic rates the thrust sheets sufficiently thick 5 10 and 15 km are modelled here the distances moved sufficiently large and for reasonable values of the coefficient of friction along the thrust plane overthrusting can cause metamorphic mineral zonations and heat flow anomalies observable in the field regions where large scale overthrusting has occurred should be characterized by a decrease with depth of grade of metamorphic mineral assemblages and anomalously low heat flow the theoretical effects are presented as a series of maximum temperature vs depth and heat flow vs time plots

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

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

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

cause and effect among thrust and normal faulting anatectic melting and exhumation in the himalaya the transition from compression to extension in a regime of overall convergence in a narrow mountain belt requires a change with time in at least one of the following horizontal normal stress applied to the belt by its surroundings the distribution of shear traction on the base of the belt the constitutive relation between stress and strain rate of material in the belt itself the surface height of the belt the following processes could have caused one or other of these changes in the himalaya in miocene time and thus could have triggered the onset of extension by normal faulting on planes parallel to thrust faults that were active at the same time the shear stress on the thrust fault at the base of the himalaya could have decreased for instance because of melting on the fault plane alternatively the creation of a new deeper thrust fault and hence a new locus of convergence could have initiated extensional faulting if the distribution of shear stress on the new thrust fault were unable to support the himalaya above it the himalaya could have been weakened by an increase in pore fluid pressure for instance owing to metamorphic dehydration reactions within the wedge the himalaya could have been elevated for instance following the detachment of a tongue of mantle lithosphere from beneath it or by the underthrusting of a lens of material beneath its base the apparent simultaneity of thrust and normal faulting on nearly parallel planes of anatexis and of exhumation in the himalaya in miocene time can be explained if normal faulting was responsible for most of the exhumation and through decompression melting for anatexis existing data suggest that the most probable cause for miocene normal faulting was either uplift of the himalaya or the cessation of slip on the main central thrust at the base of the orogenic wedge and the creation of a new deeper thrust fault and a new base to the deforming himalaya 1993 the geological society

younger hanging wall rocks along the vaikrita thrust of the high himalaya a model based on inversion tectonics abstract in sharp contrast to the common observed characteristic of areas of thrust tectonics where older rocks are thrust over younger along the vaikrita thrust in the high himalaya younger hanging wall rocks i e vaikrita group late mesoproterozoic to early neoproterozoic lie above the older footwall rocks i e munsiari formation paleoproterozoic the phenomenon is explained by an inversion tectonics based model where normal faulting and metamorphism were followed by thrusting in which the thrust displacement was less than the displacement during the earlier normal faulting the present day hanging wall tilt towards north may have been caused by a later thrust initiated as a piggy back sequence accompanied by folding and himalayan metamorphism

interpretation of monazite ages obtained via in situ analysis abstract monazite grains from nepal and vietnam were compositionally analyzed with an electron microprobe and dated th pb with an ion microprobe five sources of uncertainty explain age distributions from single samples that appear inconsistent with a single population 1 pb loss due to diffusion 2 dissolution reprecipitation reactions along a retrograde path 3 analytical uncertainties 4 analyses of overlapping age domains and 5 episodic monazite growth the influence of these factors is sample dependent but can be evaluated 1 using peak metamorphic conditions and x ray element maps to assess potential polymetamorphism or retrogression 2 obtaining other geochronologic data including previous work or dating other minerals in the sample 3 evaluating any method related uncertainty including counting statistics for electron microprobe analyses or calibration reproducibility for ion microprobe analyses and 4 ascertaining the potential growth mechanism of the monazite grain including dissolution of detrital grains or production from rare earth element ree oxide or allanite chemical contents of monazite grains analyzed in this study fail to reflect timing information or mineral growth mechanisms instead of relying on monazite chemical composition major mn fe mg ca and minor y element garnet zoning patterns and peak p t conditions should be used to facilitate age interpretation this thermobarometric data records the sample s thermal history changes in garnet growth rate and mechanisms and accessory mineral breakdown

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

thermobarometric constraints on the thermal history of the main central thrust zone and tibetan slab eastern nepal himalaya the main central thrust mct south of mt everest in eastern nepal is a 3 to 5km thick shear zone separating chlorite bearing schist in the lower plate from sillimanite bearing migmatitic gneiss in the overlying tibetan slab the metamorphic grade increases through the mct zone toward structurally higher levels previous workers have suggested that either post or synmetamorphic thrust movement has caused this inversion of metamorphic isograds in an effort to quantify the increase in grade and to constrain proposed structural relations between metamorphism and slip on the fault four well calibrated thermobarometers were applied to pelitic samples collected along two cross strike transects through the mct zone and tibetan slab results show an increase in apparent temperature up section in the mct zone from 778 k to 990 k and a decrease in temperature to 850 k in the lower tibetan slab which is consistent with synmetamorphic thrust movement a trend in calculated pressures across this section is less well defined but on average decreases up section with a gradient of 28mpa km resembling a lithostatic gradient pressure temperature paths for zoned garnets from samples within the mct zone modelled using the gibbs method show a significant decrease in temperature and a slight decrease in pressure from core to rim which might be expected for upper plate rocks during synmetamorphic thrust movement samples from the uppermost tibetan slab yield higher temperatures and pressures than those from the lower tibetan slab which may be evidence for later resetting of thermobarometers by intrusion of the large amounts of leucogranite at that structural level

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

the interpretation of inverted metamorphic isograds using simple physical calculations several zones of major thrust faulting exhibit a juxtaposition of rocks of higher metamorphic grade over rocks of lower grade this configuration may indicate but does not require that temperature gradients were temporarily inverted near the fault we examine the physical conditions under which such a temperature inversion could occur the overthrusting of hotter rock on colder rock can cause a temporarily inverted gradient both above and below the fault only if the time taken to underthrust rock from the land surface to a given depth z on the fault is less than times the characteristic time for diffusion of heat through the block above the fault where is thermal diffusivity an inverted gradient cannot form without heat generation in the fault zone unless v z sin 100 where v is the rate of underthrusting in millimeters per year z is in kilometers and is the dip of the fault this simple criterion is sufficient to demonstrate that several examples of inverted metamorphic gradients cannot be explained simply by the thrusting of hot on cold rock without heat sources in the fault zone dissipative heating accompanying deformation can cause an inverted temperature gradient within and beneath the thrust zone but whereas the overthrusting of hot upon cold rocks cools the fault zone dissipation heats it thus the overthrusting of hot on cold rock and dissipative heating affect the temperature gradient and the maximum temperatures differently we show that the magnitudes of the inverted temperature gradient and of the maximum temperature above the inverted gradient yield independent estimates of the rate of dissipative heating discrepancy between these estimates implies that some additional process must have occurred such as the post thrusting disruption of the isograds if there is such a discrepancy the maximum temperature probably provides the more reliable estimate of the rate of heating at the fault we illustrate this analysis by applying it to reported inverted metamorphic zonation in the pelona schist the st anthony complex the mt everest region of the main central thrust and the olympos thrust petrological inferences of maximum temperatures depths of metamorphism and magnitudes of apparent inverted gradients imply that shear stresses of about 100 mpa accompanied thrust faulting in some of these regions and that some zones of inverted metamorphism have been tectonically thinned after the metamorphism occured

fluid inclusions tectonic indicators abstract during the first half of the 20th century fluid inclusions have been studied in ore geology in order to determine the chemistry and physical parameters density of the mineralising fluids apart from a few pioneering works in the middle of the century fluid inclusions were not used as structural markers before the late 1980s during the same time experiments as well as observations of natural samples have shown that the volume shape and density of fluid inclusions may change when submitted to pressure temperature conditions different from their trapping isochores in view of these experiments and based on natural examples from the higher himalaya metamorphic rocks it is evident how fluid inclusions can be used as thermobarometric structural and tectonic markers points of interest for further studies on fluid inclusions in nature and experiments are also stressed

the seismology of greece abstract a review and a bibliography are given of the seismological literature for the area of greece in the period from 1950 up to the present time the aim is to incorporate all aspects of seismology starting with instrumental installations passing on to earthquake catalogues statistics and effects and ending up with structural properties and seismotectonics the purpose is to provide a service to every geophysicist who has an interest in this tectonically complex area and particularly to the numerous researchers engaged in geophysical expeditions to greece

linked fluid and tectonic evolution in the high himalaya mountains nepal fluid inclusions were studied in a quartz lens from the structurally highest unit of the himalaya mountains in nepal from a textural geometrical chemical and isotopic point of view six types of fluid inclusions were distinguished one of these types consists of annular inclusions this shape is attributed to a confining pressure increase in a non isotropic stress field two successive stress fields were deduced from the orientation of the inclusion planes relative to the schistosity the bulk composition of the fluid was dominated by co2 84 mol and h2o the composition remained constant during the whole history of the sample indicating that it was buffered by the carbonaceous host rock and or that one single fluid was reworked in situ by decrepitation stable isotope of fluids and minerals indicate 1 that fluids were buffered by surrounding rocks for o and c and 2 that at least two types of water metamorphic and meteoric were involved finally a p t t e path is proposed for the sample taking into account the southward thrusting along the main central thrust the northward tectonic denudation of the himalaya mountains inducing tectonic burying below the annapurna range and lastly rapid uplift

emplacement of himalayan leucogranites by magma injection along giant sill complexes examples from the cho oyu gyachung kang and everest leucogranites nepal himalaya abstract the upper part of the high himalayan slab in north central nepal is comprised of a thick layer parallel sheet of biotite muscovite tourmaline garnet sillimanite cordierite leucogranite up to 3 4 km thick and dipping north at 5 20 these strongly peraluminous magmas were emplaced into high temperature low pressure sillimanite and cordierite bearing gneisses calc silicates and rare amphibolites which were metamorphosed at temperatures of 600 650 c some time during the oligocene early miocene parallel stringers of black xenolithic gneisses within the leucogranites suggest passive magmatic intrusion along fractures parallel to the schistosity in the country rocks in the mountains of cho oyu gyachung kang pumori lingtren and the base of the everest massif these leucogranites form part of a single structural horizon bounded at the top by the lhotse detachment the lower of two n dipping normal faults of the south tibetan detachment std system and below by the khumbu thrust kt an out of sequence fault which was partly responsible for the uplift erosion and exhumation of the leucogranites a model for the emplacement of these leucogranites is proposed where they represent viscous minimum melts produced by melting of a pelitic protolith similar to the underlying sillimanite grade gneisses through muscovite breakdown either during fluid absent melting at

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

zoned poikiloblastic garnets p t paths and syn metamorphic uplift through 30 km of structural depth wopmay orogen canada