crustal reworking at nanga parbat pakistan metamorphic consequences of thermal mechanical coupling facilitated by erosion within the syntaxial bends of the india asia collision the himalaya terminate abruptly in a pair of metamorphic massifs nanga parbat in the west and namche barwa in the east are actively deforming antiformal domes which expose quaternary metamorphic rocks and granites the massifs are transected by major himalayan rivers indus and tsangpo and are loci of deep and rapid exhumation on the basis of velocity and attenuation tomography and microseismic magnetotelluric geochronological petrological structural and geomorphic data we have collected at nanga parbat we propose a model in which this intense metamorphic and structural reworking of crustal lithosphere is a consequence of strain focusing caused by significant erosion within deep gorges cut by the indus and tsangpo as these rivers turn sharply toward the foreland and exit their host syntaxes the localization of this phenomenon at the terminations of the himalayan arc owes its origin to both regional and local feedbacks between erosion and tectonics

boiling fluids in a region of rapid uplift nanga parbat massif pakistan abstract the nanga parbat massif of northern pakistan is currently undergoing rapid uplift 5 10 mm a 1 resulting in near surface elevated temperatures numerous quartz veins cut geologically young structures 100 c km 1 and the isotherms describe a giant pillar of heat above about 4 km fluid temperature is greater than conductive rock temperature in permeable zones which carry convecting boiling meteoric fluid

brittle tectonic evolution along the western margin of south africa more than 500 myr of continued reactivation abstract the brittle structural history of western south africa has been investigated by remote sensing and field studies to build a conceptual scheme for its 500 ma long evolution paleostress tensors were computed from a significant fault slip dataset and a relative geochronological succession of brittle deformation events was established this was aided by separating in time faulting events through the usage of cretaceous weathering horizons silicified fluvial deposits paleosols and 77 54 ma olivine melilitite plugs as time markers the oldest features recognized formed during four compressional episodes assigned to the neoproterozoic pan african evolution this history is expressed by sub vertical conjugate fracture sets and fits well the inferences derived from remote sensing the greatest compressive direction rotated from nw se to nne ssw and finally to almost e w a subsequent ene wsw oriented extensional episode is associated with the local effects of the opening of the atlantic ocean and was followed by a second ca e w extensional episode linked to the well acknowledged mid cretaceous 115 90 ma event of margin uplift a late santonian 85 83 ma nw se compressive paleostress deformed the late cretaceous sequences and was in turn followed firstly by a renewed episode of ne sw extension and later by ca nne ssw late maastrichtian 69 65 ma shortening the latter is broadly coeval with the emplacement of the gamoep magmatic suite a phase of wnw ese cenozoic extension is assigned to the extensional phase recorded in the okawango delta interpreted as reflecting propagation of the east african rift system into southern africa no stress tensor was computed for the present day wegener anomaly stress field oriented nw se however in situ stress measurements were used to perform slip tendency analysis which indicates that under the currently existing stress conditions wnw ese and nnw sse striking faults are critically stressed and are the most likely reactivated in agreement with the present seismicity

on the inference of denudation rates from cooling ages of minerals abstract the inference of denudation rates from the temperature time histories of rocks is not straightforward because the geothermal gradient evolves with time as denudation advects heat towards the land surface we show that the curvature in the temperature time path recorded by an exhumed rock is strongly dependent upon denudation rate and upon the temperature of the rock before denudation began but only weakly dependent upon the initial geothermal gradient the original depth of burial of the rock or the duration of the denudation for the case in which denudation rate is constant a systematic search of parameter space yields robust estimates of denudation rate and of the initial temperature if there is prior knowledge of the initial geothermal gradient the original depth of burial of the rock or the duration of the denudation this may be used further to limit the acceptable ranges of the other two parameters application of this procedure to two geological examples suggests that denudation rates in the nanga parbat massif are lower and less variable than previously suggested and that the inference drawn from accelerated cooling rates of accelerating denudation in the andes is incorrect

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

paleostress field reconstruction in the oslo region abstract the geodynamic history of a region is archived in its geologic record which in turn may reflect deformation patterns that causally can be related to certain configurations of paleostresses in the oslo region the exposed geological record ranges from precambrian high grade metamorphic rocks through cambro silurian sedimentary rocks to permo carboniferous sedimentary and magmatic rocks the latter being related to the development of the oslo rift system we investigate the kinematics of outcrop scale faults to derive the diversity of paleostress states responsible for the observed strain for this purpose we combine different graphical and numerical approaches to separate heterogeneous fault slip data sets and estimate the associated reduced stress tensors a reduced stress tensor consists of the directions of the three principal stress axes with 1 2 3 and the ratio of principal stress differences r 2 3 1 3 the present study gives evidence for three major regional paleostress fields that affected the oslo region a compressional stress field with a nw se directed maximum compression 1 has been identified as a caledonian imprint the most prominent regional stress field however is tensional characterised by a wnw ese directed minimum compression 3 and related to the permo carboniferous rifting later the area was affected by a wrench regime with a roughly n s directed 1 and a maximum age of permian whereas the absolute timing of this stress field is unclear due to the lack of exposed rocks younger than permian for a large number of estimated stress states none of the principal axes are sub vertical these oblique paleostresses cannot be integrated into a common regional stress field but rather correspond to local effects of intense magmatic activity during the permo carboniferous phase of rifting the present study tends to show that the oslo region remained unaffected by major tectonic activity for much of the mesozoic and cenozoic which is when the central european basin system to the south experienced several phases of intense deformation

neotectonics of the nanga parbat syntaxis pakistan and crustal stacking in the northwest himalayas the northwest termination of the himalayan arc is marked by two large antiformal structures termed syntaxes and a nw se zone of intermediate depth earthquakes which have yielded both strike slip and thrust sense fault plane solutions this contribution presents new structural data from the nanga parbat area the northern syntaxis one of the fastest rising portions of the orogen 7 mm yr 1 our studies show uplift related to syntaxis growth to be accommodated by structures located along its western margin fault kinematics in the southern part of the margin indicate nw directed thrusting along a shear zone and high level cataclastic faults termed the liachar thrust zone this carries amphibolite facies basement rocks out onto quaternary sediments further north along the western margin active faults are dominantly dextral strike slip oriented north south e g the shahbatot fault overprinting relationships suggest that the strike slip fault zone has migrated southwards into the nw directed thrust zone these relationships are consistent with the northwest termination of the arcuate himalayan thrust belt at a lateral tip generating folding and radial thrust directions faulting patterns at nanga parbat suggest that this tip zone has migrated southwards active faulting is now concentrated in the northwest continuation of the hazara southern syntaxis along a seismogenic strike slip and thrust zone these deep level tips lie on the crustal stacking thrusts which cut through the higher level sse directed thrusts of pakistan

documentation of neogene regional metamorphism in the himalayas of pakistan using u pb in monazite abstract in the himalayas investigators have constrained the timing of metamorphism associated with the eocene orogeny through the use of cooling ages on a number of minerals the majority of such studies give minimum metamorphic ages of 30 40 ma 1 2 3 4 5 6 largely confirming theoretical studies predicting peak metamorphic conditions occurring tens of millions of years after large scale collision e g 7 in northwestern pakistan however the nanga parbat haramosh massif nphm exhibits much more recent activity with rapid and accelerating cooling over the past 10 ma 1 2 the nphm also exhibits unexpectedly recent igneous activity three leucogranite dikes give minimum zircon u pb ages of 7 5 and 2 3 ma 8 the problem we address in this paper is whether the thermal structure suggested by recent leucogranite production at depth is discernible in the shallower metamorphic rocks of the nphm to date metamorphism we have used the u th pb system in monazite a common accessory mineral in high grade metamorphic rocks which can be a viable metamorphic geochronometer dating the formation of the mineral under moderate metamorphic conditions 9 10 monazite separated from schists and gneisses of the nphm gives u pb ages of 4 11 ma we interpret these young ages as evidence for neogene metamorphism affecting the nphm an event heretofore unsuspected in the greater himalaya and only intimated in the nphm this recent metamorphism roughly coincided with leucogranite production at depth 8 and with the onset of high denudation rates 1 2 as such our conclusions offer important constraints on further modeling of similarly anomalous behavior within tectonically active regions suggesting that any reasonable model should strive to explain the relationship between recent rapid uplift and coincident recent metamorphism

exhumation and incision history of the lahul himalaya northern india based on u th he thermochronometry and terrestrial cosmogenic nuclide methods abstract low temperature apatite u th he ahe thermochronology on vertical transects of leucogranite stocks and 10 be terrestrial cosmogenic nuclide tcn surface exposure dating on strath terraces in the lahul himalaya provide a first approximation of long term 10 4 10 6 years exhumation rates for the high himalayan crystalline series hhcs for northern india the ahe ages show that exhumation of the hhcs in lahul from shallow crustal levels to the surface was 1 2 mm a and occurred during the past 2 5 ma bedrock exhumation in lahul fits into a regional pattern in the hhcs of low temperature thermochronometers yielding plio pleistocene ages surface exposure ages of strath terraces along the chandra river range from 3 5 to 0 2 ka two sites along the chandra river show a correlation between tcn age and height above the river level yielding maximum incision rates of 12 and 5 5 mm a comparison of our ahe and surface exposure ages from lahul with thermochronometry data from the fastest uplifting region at the western end of the himalaya the nanga parbat syntaxis illustrates that there are contrasting regions in the high himalaya where longer term 10 5 10 7 years erosion and exhumation of bedrock substantially differ even though holocene rates of fluvial incision are comparable these data imply that the orogen s indenting corners are regions where focused denudation has been stable since the mid pliocene however away from these localized areas where there is a potent coupling of tectonic and surface processes that produce rapid uplift and denudation plio pleistocene erosion and exhumation can be characterized by disequilibrium where longer term rates are relatively slower and shorter term fluvial erosion is highly variable over time and distance the surface exposure age data reflect differential incision along the length of the chandra river over millennial time frames illustrate the variances that are possible in himalayan river incision and highlight the complexity of himalayan environments

structural evolution and asymmetric uplift of the nanga parbat syntaxis pakistan himalya the nanga parbat syntaxis in the nw himalaya is a still growing crustal scale north trending antiformal structure in the core of which indian plate gneisses have been uplifted from beneath the overthrust rocks of the kohistan island arc isotopic and fission track geochronology show that uplift rates within the syntaxis have increased to present day rates of 6 mm yr uplift has been accommodated by a combination of initial northwest verging thrusting on the western margin of the syntaxis followed by crustal scale folding within the syntaxis and latterly by dextral reverse faulting on the western margin this thrusting folding and faulting is the effect of deformation at the north western lateral tips of the main himalayan thrusts where they interfere with the south southeast verging thrusts of the northwest himalaya

analysis of fault slip inversions do they constrain stress or strain rate fault slip data commonly are used to infer the orientations and relative magnitudes of either the principal stresses or the principal strain rates which are not necessarily parallel or equal at the local scale of an individual fault the shear plane and slip direction define the orientations of the local principal strain rate axes but not in general the local principal stress axes at a large scale the orientations of p and t axes maxima for sets of fault slip data do not provide accurate inversion solutions for either strain rate or stress the quantitative inversion of such fault slip data however provides direct constraints on the orientations and relative magnitudes of the global principal strain rates to interpret the inversion solution as constraining the global principal stresses requires that 1 the fault slip pattern must have a characteristic symmetry no lower than orthorhombic 2 the material must be mechanically isotropic and 3 there must be a linear constitutive relationship between the global stress and the global strain rate isotropic linear elastic constitutive equations are appropriate to describe the local deformation surrounding an individual slip discontinuity fault slip inversions however constrain the characteristics of a large scale cataclastic flow which is described by constitutive equations that are probably but to an unknown degree anisotropic and nonlinear such material behavior would not strictly satisfy the requirements for the stress interpretation thus at the present state of knowledge fault slip inversion solutions are most reliably interpreted as constraining the principal strain rates

a stress inversion procedure for polyphase calcite twin and fault slip data sets in rocks that are polydeformed an approach which separates faults prior to stress inversion is more appropriate the traditional stress inversion approach involving the concept of the best fit stress tensor e g a tensor which minimises the misfit between calculated and measured fault striae data often risks computing artificial stress tensors that are some form of average of mixed sets of real stress tensors a new approach is proposed in which fault data are pre processed to group the faults on the basis of their response to all possible orientations and magnitudes of applied stress a computer method is described which utilises cluster analysis based on the right dihedra method to divide dynamically mixed fault populations to monophase subsets this division is based on the ranked similarity coefficients of each fault pair from the raw data set the data clusters form dynamically homogeneous subsets which are used for the composite right dihedra solution this solution is re computed for the reduced stress tensor defined by the orientation of principal stress axes and the ratio of their magnitudes

geochronologic constraints on syntaxial development in the nanga parbat region pakistan 40ar 39ar data hornblende biotite muscovite and k feldspar and u pb data zircons were obtained from the nanga parbat haramosh massif nphm nw pakistan along three transects in the southern regions of the nphm we have based our interpretations on our new data as well as geochronologic dates from previous studies in the northern regions of the massif geochronologic data show that the nphm has experienced exceptionally high denudation and cooling rates over the past 10 m y u pb ages determined through sensitive high resolution ion microprobe shrimp depth profiling experiments on metamorphic zircons and conventional u pb monazite dates suggest that the timing of metamorphism varied across the massif in addition we have documented that the massif has experienced postmetamorphic differential cooling both along and across strike thermochronologic data on currently exposed surface rocks suggest that cooling occurred more recently and at greater rates in the south central regions of the massif representing deeper crustal levels than along the margins and northern regions of the massif within the tato region cooling following peak metamorphic temperatures of 600 700 c was as high as 140 c m y following partial melting of pelitic units biotites from this area record plateau ages of 0 9 0 1 ma along the astor and indus gorges cooling was less rapid approximately 70 80 c m y following peak metamorphism as indicated by u pb monazite ages of 6 8 ma and 40ar 39ar muscovite cooling ages of 2 2 3 4 ma cooling over the last 3 m y occurred at rates of 100 140 c m y the overall cooling age pattern within the massif is interpreted syntaxial growth through the development of north plunging antiforms prior to 3 ma followed by reverse faulting along east dipping fault zones along the raikot river transect the biotite cooling age pattern is consistent with the folding of isotherms during folding of the foliation surfaces the age pattern was disrupted at 1 ma due to faulting along the raikot and tato faults an electronic supplement of tables a1 a2 and a3 may be obtained on a diskette or anonymous ftp from kosmos agu org login to agu s ftp account using anonymous as the username and guest as the password go to the right directory by typing cd apend type ls to see what files are available type guest and the name of the file to get it finally type exit to leave the system paper 95tc00032 geochronologic constraints on syntaxial development in the nanga parbat region pakistan david m winslow peter k zeitler c page chamberlain and ian s williams diskette may be ordered from american geophysical union 2000 florida avenue n w washington dc 20009 15 00 payment must accompany order

seismic characterization of an active metamorphic massif nanga parbat pakistan himalaya earthquakes recorded by a dense seismic array at nanga parbat pakistan provide new insight into synorogenic metamorphism and mass flow during mountain building micro seismicity beneath the massif drops off sharply with depth and defines a shallow transition between brittle failure and ductile flow the base of seismicity bows upward mapping a thermal boundary with 3 km of structural relief over a lateral distance of 12 km anom alously low seismic velocities are observed at the core of the massif and extend to depth through the crust the main locus of seismicity and low velocities correlates with a region of high topography rapid exhumation high geothermal gradients young metamorphic and igneous ages and crustal fluid flow we suggest a genetic link between these phenom ena in which hot rocks rapidly advected from depth are pervasively modified at relatively shallow levels in the crust

the svecofennian orogen a tectonic model abstract recent geological studies have provided new concepts concerning the timing of the final break up of the archaean craton age data on detrital zircons from sedimentary rocks in sweden and finland show marked similarities and indicate pervasive homogenization of accumulated material geochemical and isotopic studies in sweden and finland suggest the existence of two volcanic belts separated by an area dominated by sedimentary rocks however the ages of metamorphism and plutonism differ in particular the late post 1 85 ga granitoid magmatism was more voluminous in sweden than in finland the svecofennian crust in finland is anomalously thick up to 65 km and is characterized by a high velocity lower crust of variable thickness the evolution of the svecofennian orogen is modelled by opening of the pre svecofennian ocean at 1 95 ga and progressive accretion of two arc complexes to the archaean craton between 1 91 1 87 ga initial accretion at 1 91 ga led to thrusting of the slices of an island arc complex and proterozoic marine strata onto the thickening craton margin subduction reversal with simultaneous anti clockwise rotation of the converging plate or convergence of another plate resulted in subduction below the newly accreted arc complex the tectonic style at the craton margin changed from thrusting to lateral displacement in the east in finland in the west in sweden the subduction zone retreated along a transform fault a large sedimentary basin closed in the east 1 89 ga ago when another arc complex collided against the accreted one collision resulted in lithospheric thickening subsequent delamination of the thickened lithosphere and compensation by a hot mantle underplate and large scale granitoid magmatism as a result of mixing between mantle magma and remelted arc complex material the thick dense layer that formed in the lower crust as the result of underplating kept the thickened crust in isostatic balance thus the exhumation rate in this area was similar to the rate in areas of thinner crust subduction and sediment accumulation continued in the west until at least 1 87 ga when large areas in the east were already stabilized an intracratonic transpressional zone developed as the result of continued convergence subsequent extensional collapse along the transpressional zone caused melting of svecofennian sedimentary rocks migmatization and generation of the 1 84 1 82 ga anatectic granites of southern finland and central sweden the voluminous post 1 85 ga granitoid magmatism in sweden is tentatively associated with an east west extension that is not related to the extensional collapse

nature s diffusion experiment the cooling rate cooling age correlation correlation between the age of orogeny and the cooling rate of metamorphic rocks through 500 180 c as determined through various radioisotopic dating methods suggests that ancient orogens cooled at rates as much as two orders of magnitude slower than those of active orogens i propose that the correlation has arisen because ancient orogens have undergone long postorogenic periods as long as hundreds of millions of years during which the rocks remained effectively isothermal laboratory experiments suggest that argon diffusion in biotite may be sufficiently rapid at temperatures of 220 c to produce the correlation by slow loss of argon during extended isothermal periods one interesting realization is that nature has provided a long term argon diffusion experiment the results of which have been inverted in this paper to give an independent estimate of the diffusivity of argon in biotite under natural conditions this estimate compares favorably with a projection of the results of laboratory hydrothermal experiments down 10 orders of magnitude the analysis confirms the theory that there is a critical thermal window below accepted closure temperatures in which long residence will result in significant partial resetting of thermochronometers

direct evidence for a steep geotherm under conditions of rapid denudation western himalaya pakistan recent fluid inclusion and 40 ar 39 ar cooling age data show that currently exposed basement rocks in the raikhot glacier valley of the nanga parbat haramosh massif pakistan himalaya were at temperatures of 350 50 c at depths of 6 2 km hydrostatic pressure correction these data imply the presence of a steep thermal gradient in the upper crust at 1 ma 29 100 c km and denudation rates over the past 1 0 m y of 3 6 mm yr providing independent corroboration of previous estimates of rapid denudation at nanga parbat 4 5 mm yr over 3 3 m y our data provide direct documentation of near surface compaction of isotherms under conditions of rapid denudation a result that has long been supported by thermal modeling

geochemistry of a dry steam geothermal zone formed during rapid uplift of nanga parbat northern pakistan abstract natural dry steam zones vapour only are relatively rare most geothermal systems contain both liquid and vapour and typically follow a boiling point depth bpd relationship the nanga parbat uplift driven conductive thermal anomaly results in a geothermal system which follows a bpd relationship at shallow levels but below about 3 km fluid inclusions show that the hydrothermal fluid is dry steam with fluid densities from 0 36 to as low as 0 07 g cm 3 this dry steam zone may persist down to the brittle ductile transition the dry steam has salinities up to 5 wt dissolved salts and up to 22 mole dissolved co 2 the dry steam originated as meteoric water high on the slopes of nanga parbat with 18 o as low as 16 oxygen isotopic exchange with the host rock was facilitated by high temperatures 340 to 450 c and low fluid densities so that the fluid meteoric isotopic signature was completely obliterated hence quartz veins formed by the migrating dry steam have 18 o between 9 and 15 a range which is indistinguishable from quartz in the host rocks quartz vein precipitation from dry steam requires 3 to 5 orders of magnitude greater volume of fluid than typical hydrothermal fluids the dry steam zone at nanga parbat has formed due to near isothermal depressurization of very hot fluid during rapid tectonic uplift at rates 3 6 mm year

mafic sheets from indian plate gneisses in the nanga parbat syntaxis their significance in dating crustal growth and metamorphic and deformation events

the eastern himalayan syntaxis major tectonic domains ophiolitic melanges and geologic evolution abstract geologic mapping in the eastern himalayan syntaxis confirmed the three regional tectonic elements outlined by previous geologic workers our studies however show that the indus yarlung tsangpo suture iys is a continuous melange zone that forms an inverted u in map view around the namche barwa antiform the namche barwa and nyainqentanglha crystalline complexes lie below and above the iys suture respectively and both were parts of the northern indian plate basement rocks with petrologically and geochronologically correlative protoliths both units were deformed metamorphosed and intruded at the end of the proterozoic the zhibai formation the lower part of the namche barwa group extends along the northwest slope of the himalaya and is mainly composed of highly deformed aluminous felsic gneiss containing sporadic boudins of high pressure granulite in the namche barwa antiform the upper part of the namche barwa group includes a calcareous rock assemblage characteristized by marble and diopside bearing calcsilicate rocks interlayed with felsic gneiss petrochemical studies show that the iys contains lenses of oceanic crustal rocks originated from fore arc trench island arc and back arc basin environments which implies they were derived from a ssz type supra subduction zone ophiolite our field mapping identifies the jiali parlung tsangpo remnant suture jps that lies north of the namche barwa antiform as a possible branch of the neo tethyan oceanic realm subduction of the mesozoic neo tethyan oceanic plate resulted in both mesozoic and cenozoic granite intrusions in the northwest trending gangdise magmatic belt along the southern edge of asia uplift and exhumation have been the most recent dominant tectonic processes in the late cenozoic for the high himalayan crystalline rocks namche barwa group in the core of the namche barwa antiform

a stress inversion procedure for polyphase fault slip data sets palaeostress estimation from striated fault data is frequently frustrated by the fact that natural fault data are heterogeneous in the sense that they cannot satisfactorily be explained by reactivation brought about by a single stress tensor in many instances there is clear evidence to show that striation data can record the effects of multiple stress events any attempt to find the tensor which best fits or explains such dynamically mixed data sets risks determining a spurious stress state which is some form of compromise between the real stress states corresponding to different tectonic phases to avoid this problem a strategy is proposed here which involves an initial separation of the raw data into coherent sub sets prior to formal stress inversion this separation is performed by assigning attributes to each fault which describe the fault s compatibility with trial stress tensors using these attributes faults can be grouped into dynamically homogeneous families using the statistical techniques of cluster analysis

isotopic constraints on the cooling history of the nanga parbat haramosh massif and kohistan arc western himalaya nineteen 40ar 39ar and 22 rb sr mineral ages are presented for metamorphic rocks and granites collected from the northern part of the nanga parbat haramosh massif nphm and adjacent parts of the kohistan arc western himalaya the majority of these ages are interpreted as exhumation related cooling ages and they therefore place important constraints on the exhumation history of the region the 40ar 39ar and rb sr mica cooling ages for areas of the nphm to the north of the indus river lie in the range 2 8 ma and are similar to previously published ages from the indus gorge itself cooling histories for these areas show little systematic variation from east to west suggesting that the basement may have been exhumed as a relatively rigid block after the main stages of folding mica ages increase dramatically from the nphm westward into the kohistan arc where ages lie in the range 13 30 ma the contrast in both cooling ages and the age differential between mineral pairs between the two terranes indicates rapid cooling of the nphm in the last 10 m y compared with the kohistan arc the relationship between rates of cooling and exhumation is poorly constrained in regions of rapid uplift where the characteristics of the evolving geotherm are not well understood however the results are consistent with an anomalously rapid exhumation history of the nphm during the last 10 m y compared with the adjacent kohistan arc the interpretation of some of the ages documented in this study is problematical two biotite samples from contrasting lithologies within the nphm have yielded higher 87sr 86sr ratios than the corresponding whole rocks but the cause of this isotopic disequilibrium remains unclear other biotite samples from the region have consistently yielded younger rb sr ages than 40ar 39ar ages which may be a reflection of either preferential leaching of 87sr during fluid infiltration on a regional scale or the presence of excess argon alternatively the relative ages indicate slightly lower closure temperature for strontium diffusion in biotite than for argon diffusion in biotite where samples have undergone rapid cooling

first evidence for high grade himalayan age synconvergent extension recognised within the western syntaxis nanga parbat pakistan we present evidence for horizons of medium to high grade 400 600 c deformation accompanying normal sense displacement in high strain himalayan gneisses and schists from the southeastern nanga parbat massif the western syntaxis in the pakistan himalaya in their present orientation the broadly n s trending steeply dipping gneisses show microstructural and outcrop scale evidence for dextral and sinistral shear in discrete layers we interpret these as horizons of thrust and normal motion within the footwall of the original main mantle thrust once neogene antiformal folding is removed this is the first report of significant normal motion in the main mantle thrust footwall in the nanga parbat syntaxis and may indicate that synconvergent extension in the himalaya extended to the western syntaxis this episode of extension could correspond to a period of similar normal motion in the central himalaya or represent a separate event earlier in the orogeny

improved confidence in u th he thermochronology using the laser microprobe an example from a pleistocene leucogranite nanga parbat pakistan 1 the newly developed laser microprobe u th he thermochronometer permits for the first time the ability to generate precise u th he cooling ages for even very young 1 ma samples with a spatial resolution on the order of tens of micrometers this makes it possible to test the reproducibility of independent u th he age determinations within individual crystals further increasing the reliability of the method as an example we apply it here to a pleistocene granite from nanga parbat pakistan where previous constraints on the thermal history are consistent with rapid exhumation and cooling twenty one u th he dates determined on two monazite crystals from a single granite sample yield a mean of 748 000 years with a 95 confidence level of 19 000 years there is no discernible variation in the distribution of u th he ages in the cores of these crystals and therefore no evidence for the development of substantial diffusive loss 4he zoning over 80 of the interior of the monazite crystals during postcrystallization cooling of the granite modeling of these data suggests that cooling at a mean rate of 300 k ma would be necessary to produce the observed ages and the lack of a 4he gradient which is consistent with preexisting constraints for nanga parbat increased precision in thermochronology permits more tightly constrained exhumation models which should aid geologic interpretation

metamorphism and melting of the lithosphere due to rapid denudation nanga parbat massif himalaya the nanga parbat haramosh massif of the western himalaya has undergone a complex metamorphic and denudational history over the past 1 8 ga metamorphism in the tato region is characterized by a barrovian type metamorphism followed by partial melting and recrystallization along shear zones during nearly isothermal decompression while the massif was still at temperatures 550 c mineralogical and textural evidence for this path includes garnet zoning patterns late stage migmatization abundant cordierite both in schists and leucogranite dikes replacement of high pressure assemblages by low pressure assemblages and abundant low density inclusions thermobarometry on the nonmigmatized gneisses reveals conditions of 540 740 c at 7 1 13 1 kbar thermobarometry in the migmat ized rocks reveals final equilibration at 608 675 c at 3 9 6 8 kbar early fluid inclusions occur in quartz inclusions within garnet porphyroblasts that grew during decompression the last fluid inclusions to be trapped occur in microfrac

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

cooling history of the nw himalaya pakistan fission track and 40ar 39ar cooling ages indicate that the late tertiary cooling history of the himalayan ranges of northern pakistan is largely a function of uplift and erosion interpretation of cooling ages which range from under 0 5 ma to over 80 ma suggests that during the late tertiary long term uplift rates at least doubled from under 0 2 mm yr to in some cases well over 0 5 mm yr uplift rates show strong and systematic regional variations as well which reflect the greater uplift of eastern and northern regions the association of very rapid uplift and erosion with the nanga parbat haramosh massif can be explained by a locally vigorous collision of india with eurasia near a promontory of indian crust the resultant rapid uplift of the nanga parbat haramosh massif reactivated the main mantle thrust melange zone with a reversed sense of motion discontinuities in the coolingage distribution along the main mantle thrust in the southern swat hazara region may be the result of the thermal effects of overthrusting

the metamorphic signature of contemporaneous extension and shortening in the central himalayan orogen data from the nyalam transect southern tibet abstract geological relationships and geochronological data suggest that in miocene time the metamorphic core of the central himalayan orogen was a wedge shaped body bounded below by the n dipping main central thrust system and above the n dipping south tibetan detachment system we infer that synchronous movement on these fault systems expelled the metamorphic core southward toward the indian foreland thereby moderating the extreme topographic gradient at the southern margin of the tibetan plateau reaction textures thermobarometric data and thermodynamic modelling of pelitic schists and gneisses from the nyalam transect in southern tibet 28 n 86 e imply that gravitational collapse of the orogen produced a complex thermal structure in the metamorphic core amphibolite facies metamorphism and anatexis at temperatures of 950 k and depths of at least 30 km accompanied the early stages of displacement on the main central thrust system our findings suggest that the late metamorphic history of these rocks was characterized by high t decompression associated with roughly 15 km of unroofing by movement on the south tibetan detachment system in the middle of the metamorphic core roughly 7 8 km below the basal detachment of the south tibetan system the decompression was essentially isothermal near the base of the metamorphic core roughly 4 6 km above the main central thrust the decompression was accompanied by about 150 k of cooling we attribute the disparity between the p t paths of these two structural levels to cooling of the lower part of the metamorphic core as a consequence of continued and probably accelerated underthrusting of cooler rocks in the footwall of the main central thrust at the same time as movement on the south tibetan detachment system

paleostress reconstruction from kinematic indicators in the oslo graben southern norway new constraints on the mode of rifting abstract the oslo graben southern norway is a n s trending carboniferous permian rift system characterized by major mafic to silicic magmatism n s trending faults reactivation of preexisting precambrian faults and formation of half grabens magmatism is expressed by the presence of lavas dyke injections cauldron formation and the intrusion of batholiths paleostress analyses mainly based on slickensides have been performed in the area both within and outside the rift structure combined with the current tectono magmatic model of the oslo graben area the analyses show the following stress evolution from the caledonian orogeny with its compressional tectonics in the silurian to the late stages of the carboniferous permian rifting nw se compression occurred in silurian times due to the continental collision of the caledonian orogeny after a long period of a missing geological record the area was affected by n s compression during the late carboniferous a shallow sedimentary basin developed indicative for the pre and proto rift phases the transition from the proto rift phase to the initial rift phase is marked by a transition from a transpressional regime into a transtensional regime during the early permian the stress regime changed from pure extension to radial extension we suggest that the oslo graben can not simply be explained in terms of or passive or active rifting but that a combination of both evolving through time is more suitable to explain the observations we propose a model in which the oslo graben is initially triggered by far field stresses in the latest carboniferous and earliest permian which infers opening due to passive rifting these far field stresses are suggested to be linked to the hercynian orogeny active to the south in central europe in carboniferous times and the reorganisation of the pangea supercontinent in early permian times the stress regime caused radial extension indicative for uplift and a change to an active mode of rifting simultaneously with the radial extension large volumes of magma are emplaced at near surface levels despite all alternative models an anomalous thermal gradient is necessary to create these large volumes of magmatic material we propose therefore that shortly after the onset of rifting the dominant rifting mode becomes active plume related although far field stresses might still be present

active hydrothermal systems during the recent uplift of nanga parbat pakistan himalaya during the last 10 m y the nanga parbat haramosh massif in the northwestern himalaya has been intruded by granitic magmas has undergone high grade metamorphism and anatexis and has been rapidly uplifted and denuded as part of an ongoing project to understand the relationship between tectonism and petrologic processes we have undertaken an isotopic study of the massif to determine the importance of hydrothermal activity during this recent metamorphism our studies show that both meteoric and magmatic hydrothermal systems have been active over the last 10 m y we suggest that the rapid uplift of the massif created a dual hydrothermal system consisting of a near surface flow system dominated by meteoric water and a flow regime at deeper levels dominated by magmatic metamorphic volatiles meteoric fluids derived from glaciers near the summit of nanga parbat were driven deep into the massif along the transpressional faults causing 18 o and d depletions in the gneisses and marked oxygen isotopic disequilibriuui between mineral pairs from the fault zones the discharge of these meteoric fluids occurs in active hot springs that are found along the steep faults that border the massif at deeper levels within the massif infiltration of low 18 o magmatic fluids caused 18 o depletions in the gneisses within the migmatite zone these low 18 o fluids were derived from the young 4 ma relatively low 18 o granites 8 that are found within the core of the massif geochronological evidence in the form of fission track and 40 39 ar cooling ages and u pb ages on accessory minerals from the granites and gneisses provide a constraint on the timing of fluid flow in the surface outcrops we examined fluid infiltration in the migmatite zone rocks located along the tato traverse was coeval with metamorphism granite emplacement and rapid denudation in the interval 0 8 3 3 ma finally we infer from the presence of active hot springs that significant flow systems continue to be active at depth within the central portion of the nanga parbat haramosh massif

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

tectonothermal evolution of the high himalayan crystalline sequence langtang valley northern nepal the high himalayan crystalline sequence in north central nepal is a 15 km thick pile of metasediments that is bound by the main central thrust to the south and a normal fault to the north the langtang section through the metasediments shows an apparent inversion of metamorphic isograds with high p kyanite grade rocks exposed beneath low p sillimanite grade rocks textural evidence confirms that the observed inversion is a result of a polyphase metamorphic history and phase equilibria studies indicate that thermal decoupling has occurred within a mechanically coherent section of crust rocks now exposed at the base of the high himalayan thrust sheet underwent barrovian regional metamorphism m1 prior to 34 ma in the early stages of the himalayan orogeny recording metamorphic conditions of t 710 30 c p 9 1 kbar after the activation of the main central thrust which emplaced these metapelites southwards onto the lower grade lesser himalayan formations the upper part of the thrust sheet was overprinted by a second heating event m2 resulting in sillimanite grade metamorphism and anatexis of metapelites at t 760 30 c p 5 8 0 4 kbar between 17 and 20 ma crustally derived leucogranite magmas have been emplaced into low grade tethyan sediments on the hangingwall of the normal fault that bounds the northern limit of the metapelitic sequence the cause of the selective heating of the upper section of the metasediments during m2 cannot be reconciled with either post thrusting thermal relaxation or advection models the cause of m2 remains problematical but it is suggested that heat focusing has occurred at the top of the high himalayan crystalline sequence as a result of movement on the normal fault blanketing metapelites of high heat productivity with low grade sediments of low thermal conductivity this model implies that the normal fault was active before m2 consistent with decompression textures that formed during or shortly after sillimanite grade metamorphism

permeability function for oil sands tailings undergoing volume change during drying the coefficient of permeability function is an important unsaturated soil property required when modeling seepage and contaminant transport phenomena inaccuracies in the estimation of the permeability function can lead to significant errors in numerical modeling results changes in void ratio and degree of saturation are factors that influence the permeability function presently available methodologies for estimating the unsaturated permeability function make the assumption that there is no volume change as soil suction is changed as a result volume changes are interpreted as changes in degree of saturation the commonly used estimation techniques for the permeability function are reasonable for soils such as sands that experience little volume change as soil suction is changed on the other hand inaccurate results are generated when soils undergo volume change as is the case with oil sands tailings revisions to previous methodologies are proposed to render the estimation of the permeability functio

timing quantification and tectonic modelling of pliocene quaternary movements in the nw himalaya evidence from fission track dating abstract variable exhumation rates deduced from the pliocene quaternary ft zircon apatite ages from the himalayan metamorphic belt hmb of the nw himalaya along the sutlej valley in himachal pradesh have been modelled in the tectonic framework of fast exhumed lesser himalayan windows which caused lateral extensional sliding of the metamorphic nappe cover along the well known main central thrust mct and differential movements along thrust zones as well in the northern belt of the higher himalayan crystallines hhc two distinct clusters of the ft apatite ages have been deciphered apatite ages having a weighted mean of 4 9 0 2 ma 1 in basal parts on the hanging wall of the mct and 1 49 0 07 ma 1 in the hanging wall of a newly recognized ne dipping chaura thrust further north fast exhumation of the chaura thrust hanging wall has been inferred at a rate of 4 82 0 55 mm yr from the zircon apatite cogenetic pairs during 1 54 ma and 0 97 ma and 2 01 0 35 mm yr since 1 49 ma in comparison its foot wall has been exhumed at a much slower rate of 0 61 0 10 mm yr since 4 9 ma the overlying vaikrita thrust zone rocks reveal an exhumation rate of 1 98 0 34 mm yr from 2 70 0 40 ma to 1 31 0 22 ma and 2 29 0 66 mm yr since 1 31 0 22 ma using these data a vertical displacement of ca 2 08 0 68 km has been calculated along the chaura thrust between 4 9 and 1 50 ma on an average rate of 0 6 mm yr it is of the order of 1 18 km from 2 70 ma to 1 54 ma along the vaikrita thrust and 0 78 mm yr from 1 31 ma to 0 97 ma and has behaved as an extensional normal fault during these periods tectonic modelling of the exhumation rates in the nw himalaya reveals fastest uplifting himalayan domes and windows like the nanga parbat in pakistan suru and chisoti domes in zanskar and kishwar kulu rampur window axis in se kashmir and himachal pradesh during pliocene quaternary these windows appear to have caused lateral extensional sliding of the himalayan metamorphic nappes in the lower parts the middle parts of the hhc belt have witnessed both overthrusting and extensional faulting due to complex and variable exhumation patterns within the hanging and foot walls of the mct and vaikrita thrust along the sutlej valley thus causing movement of upthrust crustal wedge between the extensional ones thus ft zircon apatite ages provide evidence for the presence of a number of crustal wedges having distinct tectonothermal history within the hhc

petrogenetic and tectonic significance of young leucogranites from the northwestern himalaya pakistan in the himalaya of northwestern pakistan small discordant bodies of leucogranite intrude metamorphosed basement of the indian crust as the first step in assessing the tectonic significance of these granites we have used the shrimp ion microprobe to obtain u pb dates on zircon zircons of igneous appearance are scarce in the leucogranites and with one exception are associated with xenocrystic components which occur as discrete grains and as cores rimmed by young zircon zircons of igneous appearance are high in u 5000 to 40 000 ppm low in th u 0 01 to 0 05 and frequently colored a distinct blue xenocrystic components give a range of proterozoic ages consistent with ages previously reported for indian crust in sharp contrast zircons of igneous appearance yield clusters of concordant tertiary ages two leucogranite dikes from southern localities yield rather old emplacement ages of about 35 ma swat and about 50 ma naran together with previously determined 40ar 39ar and fission track cooling ages these ages document immediate and rapid metamorphism and denudation following eocene collision three leucogranite dikes from northern localities within the nanga parbat haramosh massif are very young having emplacement ages of about 2 3 ma 5 ma and 7 ma the formation and emplacement of these leucogranites during the rapid late tertiary denudation of the nanga parbat region suggests to us that decompression melting may be a viable mechanism for leucogranite genesis

new constraints on 1 7 gyr of brittle tectonic evolution in southwestern finland derived from a structural study at the site of a potential nuclear waste repository olkiluoto island abstract the brittle deformation history of southwestern finland was reconstructed through paleostress inversion of fault slip data from outcrops and drill cores at the site of a planned nuclear waste repository seven distinct paleostress states have been resolved spanning 1 7 ga of geological history 1 nw se to nnw sse transpression acted soon after 1 75 ga when conditions leading to brittle deformation were first attained 2 n s to ne sw transpression caused partial reactivation of 1 3 nw se extension followed during the gothian event and at the time of rapakivi magmatism 4 renewed ne sw transtension occurred between 1 60 and 1 30 ga forming the nw se elongated satakunta graben 5 a significant and so far unrecognised phase of ne sw compression has been found to postdate both rapakivi granites and 1 27 ga olivine diabase sills 6 e w transpression followed during the early stages of the mesoproterozoic sveconorwegian orogeny and predated 7 almost coaxial e w extension of the late sveconorwegian orogenic collapse our results prove that the shield s crust fragmentation i e generation of new fractures was short lived in comparison to its jostling phase i e reactivation of old fractures the shield thus reached structural maturity with respect to its saturation with brittle structures already in the late mesoproterozoic we predict that that future stress changes will most probably be accommodated by reactivation of the existing and inherited brittle structures rather than by formation of new deformation zones

thermochronologic constraints on the tectonic evolution of active metamorphic core complexes d entrecasteaux islands papua new guinea metamorphic core complexes in the d entrecasteaux islands papua new guinea formed as the result of active extension at the western end of the propagating woodlark basin spreading center domes of high grade metamorphic rocks i e amphibolites eclogites and migmatites intruded by large granodiorite bodies comprise the lower plate of the d entrecasteaux metamorphic core complexes the domes are transected by kilometer scale shear zones a thermochronologic study of the d entrecasteaux islands utilizing k ar 40ar 39ar and fission track techniques has documented the unroofing history of these active metamorphic core complexes gneisses in the cores of the domes cooled rapidly 100 c m y as indicated by hornblende and biotite 40ar 39ar apparent ages of 2 7 to 3 0 ma and 1 6 to 1 7 ma respectively and apatite fission track ages of 0 4 to 0 9 ma 40ar 39ar apparent ages on white mica biotite and potassium feldspar and fission track ages on apatites from shear zone gneisses indicate extremely rapid cooling in some cases 500 c m y and suggest shear zones were active from 4 0 to 3 5 ma and 1 9 to 1 4 ma in general 40ar 39ar mineral ages for retrogressed core zone gneisses shear zone gneisses and granodiorites are 2 0 to 3 0 ma amphiboles 1 5 to 1 7 ma muscovites and 1 4 to 1 8 ma biotites and 1 0 to 2 0 ma k feldspars apatite fission track ages from core zone gneisses shear zone gneisses and granodiorites range from 0 4 to 1 0 ma thermochronologic results indicate that emplacement of granodiorites closely coincided with retrogression of the metamorphic basement and movement on the outer shear zones bounding the gneiss domes the granodiorite bodies associated with the d entrecasteaux islands domes represent syn kinematically emplaced granitoids intruded into an area of active continental extension

the namche barwa syntaxis evidence for exhumation related to compressional crustal folding abstract geological observation in the eastern end of the himalayas shows that the asia india suture is folded and continues southward into india and burma where the continuation of the transhimalayan plutonic complex can be identified metamorphic rocks derived from india occur structurally below the suture in the core of a regional antiform isotopic and fission track dating are consistent with the geological interpretation in addition they establish cooling exhumation of rocks from c 30 km depth within the last 4 ma rapid exhumation of deep crustal levels in mountain systems is generally related to extension subsequent to thermal softening of a thickened continental lithosphere however in the eastern himalayas we argue that exhumation is caused by c 10 mm yr 1 erosion coeval with crustal scale folding the general history of this syntaxis resembles the evolution of the western himalayas syntaxis in pakistan

tectonic analysis of fault slip data sets using data that include the direction and the sense of motion on individual fault surfaces determined by slickenside lineations it is possible to reconstruct reduced stress tensors that correspond to the orientation of stress axes and to the ratio 2 3 1 3 between principal stress values 1 2 3 compression being positive no assumption is made concerning the orientation of fault planes relative to stress axes so that reactived faults are taken into account as well as newly created ones qualitative and quantitative methods for analysis of fault slip data were developed during the last 10 years the practical limitation of the methods and the necessity for critical field observations are emphasized these methods can be applied to focal mechanisms of earthquakes a more complex analysis of heterogeneous data sets involving an iterative separation of different stress systems is also discussed this analysis enables one to distinguish successive faulting events careful qualitative study in the field is in all cases essential

synchronous anatexis metamorphism and rapid denudation at nanga parbat pakistan himalaya the nanga parbat haramosh massif of the western himalaya is a north trending half window of indian crust that provides spectacular exposures of precambrian basement gneisses that have been overprinted by himalayan metamorphism we report here petrologic data and u pb dates on zircon and monazite which document that nanga parbat gneisses underwent a pliocene pleistocene episode of high grade metamorphism and anatexis during an interval in which the nanga parbat massif was undergoing rapid denudation at mean rates of 5 mm yr we speculate that by initiating decompression melting this denudation may be at least partly responsible for the anatexis and high grade metamorphism

u th pb systematics of morphologically characterized zircon and allanite a high resolution isotopic study of the alpine rensen pluton northern italy abstract the rensen pluton is a typical member of the tertiary calc alkaline plutonic suite structurally connected to the periadriatic lineament it intrudes the austroalpine polymetamorphic basement of the eastern alps southwest of the tauern window northern italy zircons and allanites from tonalitic quartz dioritic and monzogranitic lithologies were studied systematically on the basis of refined grain by grain microanalytical techniques high resolution isotope analyses show that morphology and u pb systematics of zircon are highly correlated with magma evolution zircons from the main tonalitic part of the intrusion display a gradual increase of u content from early stage s type morphology to late stage g1 type morphology most of the zircon crystals include premagmatic seeds derived from old crustal sources unevolved s type zircons contain inherited components of late proterozoic early paleozoic apparent age whereas late stage g1 type zircons show inheritance of early proterozoic apparent age both types of inherited components are present in zircons from a fine grained tonalitic domain located at the center of the pluton which shows restitic structures the crustal affinity of this rock is corroborated by its relatively low e nd value 6 0 as compared to values obtained for the main tonalite 3 9 and the quartz diorite 3 5 the accuracy of cainozoic zircon ages is affected by initial radioactive disequilibrium in the u pb decay chains particularly uncertain is the fractionation behavior of protactinium which could be enriched in zircon due to the high stability of the pa 5 oxidation state because the th pb system is devoid of long lived intermediate radioactive nuclides the best approximation to the emplacement age of the rensen pluton is given by allanite th pb ages of 31 70 0 25ma 95 c l for the main tonalite and by a slightly younger age of 31 09 0 32ma for quartz diorite

constraints on the tectonic evolution of the northwestern himalaya from geochronologic and petrologic studies of babusar pass pakistan petrologic geochronologic and structural studies of indian plate rocks in the footwall of the main mantle thrust indicate that peak metamorphism occurred in thrust imbricates well before the final assembly of the kohistan island arc with the indian plate as the northern margin of the indian plate was thickened by this early thrusting the metamorphic rocks evolved along increasing pressure temperature paths and in some regions reached temperatures sufficient for partial melting it is difficult to constrain the timing of this early deformation and metamorphism but 40 ar 39 ar hornblende ages and u pb ages of crosscutting leucogranite dikes suggest that in most areas peak metamorphism was earlier than 40 ma and in some areas earlier than 50 ma after peak metamorphism in the interval from 40 to 25 ma rocks of the indian plate continued to be thrust southward creating sharp metamorphic discontinuities at thrust boundaries from 25 to 20 ma rocks of the indian plate underwent rapid cooling

thermochronology of the modern indus river bedload new insight into the controls on the marine stratigraphic record 1 the indus river is the only major drainage in the western himalaya and delivers a long geological record of continental erosion to the arabian sea which may be deciphered and used to reconstruct orogenic growth if the modern bedload can be related to the mountains in this study we collected thermochronologic data from river sediment collected near the modern delta u pb ages of zircons spanning 3 gyr show that only 5 of the eroding crust has been generated since india asia collision the greater himalaya are the major source of zircons with additional contributions from the karakoram and lesser himalaya the 39ar 40ar dating of muscovites gives ages that cluster between 10 and 25 ma differing from those recorded in the bengal fan biotite ages are generally younger ranging 0 15 ma modern average exhumation rates are estimated at 0 6 km m y or less and have slowed progressively since the early miocene 20 ma although fission track ft dating of apatites may indicate a recent moderate acceleration in rates since the pliocene 1 0 km m y driven by climate change the 39ar 40ar and ft techniques emphasize the dominance of high topography in controlling the erosional flux to the ocean localized regions of tectonically driven very rapid exhumation e g nanga parbat s karakoram metamorphic domes do not dominate the erosional record

tectonic exhumation of the nanga parbat massif northern pakistan abstract structural analysis in the nanga parbat region of northern pakistan indicates that extensional deformation has been in part responsible for the exhumation of gneissic rocks of the indian plate basement complex that dominates the nanga parbat massif this massif has been mapped as a syntaxis at the boundary between the indian and asian plates in the northwest himalaya and has received recent attention for evidence of rapid exhumation within the last 10 ma field work along the main mantle thrust mmt and within the indian plate rocks in the area between babusar pass and toshe gali southwest of the peak of nanga parbat provided us with evidence from shear fabrics and a shallowly wsw plunging stretching lineation for a dominant phase of extensional deformation during this deformation rock of the kohistan sequence and the indian plate cover sequence moved to the wsw relative to rocks of the indian plate basement including the nanga parbat gneiss a post metamorphic ductile shear fabric pervades the mmt contact the indian plate cover sequence below the mmt but decreases in intensity in the indian plate basement and the nanga parbat gneiss using 39 ar 40 ar geochronology of hornblendes we have constrained the age of pre deformational amphibolite facies metamorphism to be 40 56 ma published cooling ages from the babusar pass region further document that the extensional deformation was completed by 20 ma we interpret this ductile extensional shear as the mechanism responsible for significant tectonic exhumation of the gneissic and granitic rocks of the nanga parbat massif

the tectonic evolution of the kohistan karakoram collision belt along the karakoram highway transect north pakistan the kohistan arc terrane comprises an intra oceanic island arc of cretaceous age separating the indian plate to the south from the karakoram asian plate to the north within the indus suture zone of north pakistan the intra oceanic arc volcanics chalt dras group were built on a foundation of dominantly mid ocean ridge basalt morb related amphibolites of the kamila group the subarc magma chamber is represented by multiple intrusions of a huge gabbro norite complex chilas complex which includes some ultramafic assemblages of residual mantle harzburgite and dunite layered cumulates and hornblendites cut by late stage dikes of hornblende plagioclase pegmatites the chilas complex norites intrude the gilgit metasediments of lower amphibolite and greenschist facies in northern kohistan which also form xenolithic roof pendants within the top of the chilas complex along the southern margin of kohistan jijal and sapat complex ultramafics dunites harzburgites and websterites form remnant suprasubduction zone ophiolitic mantle rocks along the hanging wall of the main mantle thrust the cretaceous obduction plane along which kohistan was emplaced onto indian plate rocks garnet granulites of the jijal complex formed at 12 14 kbars represent original magmatic lower crustal rocks subducted to depths of at least 45 km and metamorphosed during high pressure and high temperature subduction of earlier arc related rocks obduction of the sapat ophiolite and kohistan arc occurred between 75 and 55 ma the closure of the shyok suture zone separating kohistan from the karakoram plate must have occurred prior to 75 ma the age of the jutal basic dikes which crosscut the closure related fabrics mainly late north directed backthrusting in the lower hunza valley andean type granitoid gabbrodiorite granodiorite granite emplacement along the kohistan ladakh batholith ended at the time of india asia collision 60 50 myr ago postcollisional crustal thickening along the karakoram led to multiple episodes of metamorphism from latest cretaceous and throughout the tertiary sillimanite grade metamorphism in hunza was actually pre india asia collision and may have resulted from the earlier kohistan collision localized and sporadic crustal melting episodes across northern kohistan indus confluence and parri granite sheets and the southern karakoram hunza dikes and sumayar and mango gusar leucogranites occurred from 51 to 9 ma and culminated in the huge baltoro monzogranite leucogranite intrusion 25 21 myr ago a vast network of leucogranitic and pegmatitite dikes containing gem quality aquamarine muscovite tourmaline garnet biotite quartz are younger than 5 ma and form the final phase of intrusion in the haramosh area and parts of the southern karakoram area

the metamorphism in the central himalaya all along the himalayan chain an axis of crystalline rocks has been preserved made of the higher himalaya crystalline and the crystalline nappes of the lesser himalaya the salient points of the metamorphism as deduced from data collected in central himalaya central nepal and kumaun are 1 the higher himalaya crystalline also called the tibetan slab displays a polymetamorphic history with a first stage of barrovian type overprinted by a lower pressure and or higher temperature type metamorphism the metamorphism is due to quick and quasi adiabatic uplift of the tibetan slab by transport along an mct ramp accompanied by thermal refraction effects in the contact zone between the gneisses and their sedimentary cover the resulting metamorphic pattern is an apparent diachronic inverse zonation with the sillimanite zone above the kyanite zone 2 conversely the famous inverted zonation of the lesser himalaya is basically a primary pattern acquired during a one stage prograde metamorphism its origin must be related to the thrusting along the mct with heat supplied from the overlying hot tibetan slab as shown by synmetamorphic microstructures and the close geometrical relationships between the metamorphic isograds and the thrust 3 thermal equilibrium is reached between units above and below the mct far behind the thrust tip there is good agreement between the maximum temperature attained in the hanging wall and the temperature of the tibetan slab during the second metamorphic stage but closer to the mct front the thermal accordance between both sides of the thrust is due to a retrogressive metamorphic episode in the basal part of the tibetan slab

suppression of earthquakes by large continental ice sheets the interior regions of antarctica and greenland are aseismic no earthquake larger than body wave magnitude 4 5 5 0 is known for either except along coastal zones or continental shelves here i advance an explanation for this lack of seismic activity in terms of pressure effects produced by the continental ice sheets that mantle both continents

timing and duration of himalayan metamorphism within the indian plate northwest himalaya pakistan in indian plate rocks defining the footwall of the indo asian suture in the upper kaghan valley ukv of northern pakistan we resolve four phases in a continuum of himalayan metamorphism and deformation with peak metamorphic pressures of 7 2 0 7 kbar and a minimum estimate for peak temperature of approximately 540 c to attach an absolute age to himalayan metamorphism we used the shrimp ion microprobe to obtain u pb ages on the rims of zircons from granites metamorphosed during the earliest phase of himalayan orogeny four granite samples give a mean u pb age of 47 3 ma zircons from the malakand granite of the swat region 200 km west of the ukv have rims that also give a mean u pb age of 47 3 ma cores of these zircons yield u pb ages of 254 291 ma which approach the previously published rb sr isochron age of 297 4 ma for the peshawar plain igneous province suggesting an expanded areal extent of this carboniferous igneous suite the metamorphic ages of 47 3 ma document that the thermal cu

unsupported radiogenic pb in zircon a cause of anomalously high pb pb u pb and th pb ages ion microprobe u th pb isotopic analyses of zircons from a granodioritic orthogneiss from the napier complex mount sones enderby land antarctica have identified an unambiguous example of unsupported radiogenic pb in a 3 950 ma old crystal at one 40 m spot on the crystal an unusually large heterogeneity in pb content was found the concentration of radiogenic pb ranging from 5 to 50 percent higher than could have been generated in 3 950 ma by radioactive decay of the co existing u and th this relative excess of radiogenic pb is attributed to pb gain rather than to u and th loss because first the pb content varied by more than the u or th contents and secondly changes in the pb u pb th and pb isotopic composition correlated directly with changes in the pb concentration the individual 207pb 206pb apparent ages ranged from 4 000 ma to 4 145 ma all greater than the inferred age of the crystal a correlation between 207pb 206pb and pb u shows that the pb excess has not resulted from recent pb movement the spot apparently gained radiogenic pb about 2 500 ma ago at the same time as the majority of the other zircons in the rock suffered substantial pb loss the pb movement occurred in response to a discrete geologic event reverse discordance is a phenomenon that must be considered when interpreting zircon u pb ages especially 207pb 206pb ages of single crystals or portions of crystals

climatic limits on landscape development in the northwestern himalaya the interaction between tectonism and erosion produces rugged landscapes in actively deforming regions in the northwestern himalaya the form of the landscape was found to be largely independent of exhumation rates but regional trends in mean and modal elevations hypsometry frequency distribution of altitude and slope distributions were correlated with the extent of glaciation these observations imply that in mountain belts that intersect the snowline glacial and periglacial processes place an upper limit on altitude relief and the development of topography irrespective of the rate of tectonic processes operating

geology of the northern part of the nanga parbat massif northern pakistan and its implications for himalayan tectonics the northern outcrop termination of indian continental crust lies at the nanga parbat massif from where this contribution presents new field data the tectonic contact with the structurally overlying kohistan island arc is concordant and ductile is associated with interleaving of nanga parbat and kohistan lithologies and may be correlated with the main mantle thrust found elsewhere in the nw himalayas this ductile shear zone is locally overprinted by cataclastic faults associated with exhumation of the massif but overall the northern outcrop termination of the massif is controlled by erosion through a gently northward plunging antiformal structure this folds both the main mantle thrust and the underlying concordant 9layered unit9 of the nanga parbat basement thus there is no indication that the massif acted as a promontory to the indian continent during collision nor that it is a structure entirely bound by neotectonic faults ductile shear fabrics associated with the 9main mantle thrust9 are cross cut by leucogranite sheets and pegmatites these may represent the stockwork to a significant crustal melt granite body described here in the northern massif this jutial granite shows many geochemical characteristics in common with similar bodies in the high himalayas which are consistent with anatexis of the buried indian continental basement rocks however the granite is enriched in heat producing elements particularly th 22 ppm and shows extremely high 87 sr 86 sr ratios 0 88 the high concentrations of radiogenic sr are also an attribute of a suite of hitherto enigmatic leucogranitic pegmatites that laces the nanga parbat massif suggesting that these may represent a stockwork to a largely buried body of which the jutial granite is a small exposure the enrichment in heat producing elements within the granite may reflect similarly high heat production in the source indian continental crust requiring in turn a fundamental re examination of the thermal evolution of this crust during himalayan metamorphism and exhumation

strain and stress discussion the three sections of the paper by marrett and peacock consider respectively the historical development of structural analysis the conceptual role of strain and stress in the mechanical analysis of structures and the use of these and related terms in the literature of structural geology marrett and peacock 1999 in keeping with the spirit of the 20th anniversary special issue as described in the preface evans and treagus 1999 marrett and peacock have presented both factual information and opinions about these topics some of these opinions are challenged in this discussion which also seeks to correct a few misleading statements and to advocate a research methodology that integrates geometry kinematics and dynamics

episodic fluvial incision of rivers and rock uplift in the himalaya and transhimalaya abstract seventeen strath terraces in northern india were dated using 10 be surface exposure methods ages range from c 7 to c 735 ka and provide fluvial incision rates of 0 02 0 003 to 2 6 1 9 mm a 1 on the northern side of the ladakh range incision rates are c 1 mm a 1 in the northern zanskar range they are 0 06 0 005 mm a 1 new and published incision rates in southernmost lahul range from 0 1 0 02 to 13 2 6 2 mm a 1 rates for ages 35 ka are 0 4 0 2 mm a 1 across the himalaya and transhimalaya holocene fluvial incision rates range from c 0 02 to c 26 0 mm a 1 whereas pleistocene incision rates are 5 mm a 1 many of the holocene incision rates exceed exhumation rates whereas pleistocene incision rates are comparable with exhumation rates this suggests that long term fluvial incision is in dynamic steady state with exhumation the temporal pattern for rates of fluvial incision is probably controlled by episodic incision linked to significant precipitation changes throughout the quaternary suggesting that strath terraces with ages 35 ka can be used for assessing long term rates of rock uplift in contrast rates of fluvial incision based on late glacial and holocene strath terraces reflect changes in monsoon intensity and deglaciation events by determining ages for multiple samples on flights of strath terraces it is possible to document changes in incision rate assess whether post abandonment transient shielding has occurred and help elucidate tectonic v climatic controls on their formation supplementary material tables ds1 4 for previously published data and recalculated ages for strath terraces and figures ds1 and ds2 showing plots of incision rates against time for the himalaya and transhimalaya are available at http www geolsoc org uk sup18454