india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

apatite fission track and u th he ages from the higher himalayan crystallines kaghan valley pakistan implications for an eocene plateau and oligocene to pliocene exhumation abstract apatite fission track and apatite and zircon u th he ages were obtained from high and ultra high pressure rocks from the kaghan valley pakistan four samples from the high altitude northern parts of the valley yielded apatite fission track ages between 24 5 3 7 and 15 6 2 1 ma and apatite u th he ages between 21 0 0 6 and 5 3 0 2 ma these data record cooling of the formerly deeply subducted high grade metamorphic rocks induced by denudation and exhumation consistent with extension and back sliding along the reactivated normal acting main mantle thrust overlap at around 10 ma between fission track and u th he ages is recognised at one location besal showing that fast cooling occurred due to brittle reactivation of a former thrust fault widespread miocene cooling is also evident in adjacent areas to the west deosai plateau tso morari most likely related to uplift and unroofing linked to continued underplating of the indian lower crust beneath ladakh and kohistan in the late eocene to oligocene in the southernmost part of the study area near naran two significantly younger late miocene to pliocene apatite fission track ages of 7 6 2 1 to 4 0 0 5 ma suggest a spatial and temporal separation of exhumation processes these younger ages are best explained by enhanced late miocene uplift and erosion driven by thrusting along the main boundary thrust

kar and 40ar39ar study of metamorphic rocks associated with the oman ophiolite tectonic implications abstract k ar analyses on extracted minerals are reported for a variety of metamorphic rocks associated with the sumail ophiolite amphibolites lying at the sole of the ophiolite yield ages of 95 100 ma which are viewed as reflecting times of crystallization high pressure metamorphics of the saih hatat reveal complex results white micas range in age from 80 to 131 ma whereas blue amphiboles indicate ages that are systematically lower than those of coexisting white micas investigation of a few white micas by the 40 ar 39 ar step heating method yields rather intricate age spectra featuring low apparent ages in the first and the last stages of gas release and high apparent ages in between two explanations can be equally envisaged for these convex upward age spectra the first is the mixing of two generations of micas corresponding to two main metamorphisms the first one m1 is a low to medium temperature high pressure event which conceivably occurred between 130 and 114 m y ago the second m2 overprints m1 and has produced rocks typical of the greenschist facies it took place 80 m y ago and also affected the sole of the ophiolites the second is the presence of excess argon in mica mixtures with complex degassing properties thus the two metamorphic phases identified by microscopic inspection are not significantly different in age they occurred in the 70 80 ma interval moreover k ar dates on amphibole from gabbroic dikes intersecting the peridotites suggest that they are genetically linked to the mafic part of the ophiolites assuming that metamorphism is a tracer of tectonic events we view the infraophiolitic amphibolites as the result of an intraoceanic thrusting which took place near a spreading center nevertheless the uncertainty as to the age of the blueschist metamorphism precludes the possibility of indicating a timetable based on metamorphic ages for the motion shift of africa relative to eurasia during the late cretaceous a tentative model of the obduction of the sumail ophiolite is proposed assuming an age of about 130 ma for m1 we stress differences with respect to earlier reconstructions which advocated an 80 m y old blueschist metamorphism

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

paleomagnetic results from oman ophiolites related to their emplacement abstract the paleomagnetic results from the cretaceous oman ophiolites suggest large rotations of the ophiolitic nappe during the intraoceanic thrusting and continental obduction which preceded its last emplacement onto the arabian margin and they confirm that the ophiolites were formed and thrust at equatorial latitudes furthermore they may indicate the occurrence of short reversals during the cretaceous long normal polarity period our paleomagnetic study was mainly conducted on metalliferous sediments interbedded within the volcanics of the ophiolites ninety nine cores from 31 samples from fifteen outcrops distributed along the nappe were analysed alternating field and stepwise demagnetisation revealed complex multicomponent magnetisations carried by hematite and magnetite among the various components a stable component was commonly found in each hand specimen from the same outcrop with varying unblocking temperature or field spectra because of varying composition from one sample to another after tilt correction the inclinations of these stable components were all found to be low and the declinations grouped around nw se and nne ssw axes each group containing roughly antipodal directions the occurrence of antipodal directions is inconsistent with the ages of the sediments situated in the middle of the cretaceous long normal polarity period but is consistent with previously published findings of dual polarity in gabbros and dikes the two nw se and nne ssw paleomagnetic groups may correspond to two successive episodes of magnetisation acquired during diagenesis or during high or low temperature hydrothermal activity the large rotations of the nappe that these magnetisations suggest would be consistent with the transport directions recorded within the metamorphic soles in further studies the magnetic mineralogy and the paleomagnetic results will have to be related to hydrothermal activity and the particular magnetic properties of our manganese rich sediments will need to be better understood

the transition from tethys to the himalaya as recorded in northwest pakistan early cenozoic sedimentary rocks exposed in the kohat plateau of northwestern pakistan record tectonic closure of the tethys sea and development of a restricted marine basin that formed during himalayan collision between india asia and a series of microplates in the paleocene initial subsidence of the basin was caused by the downward deflection of the indian plate in response to loading of the asian plate the western margin of the early eocene basin was dominated by deposition of shale sandstone and conglomerate derived from microplates located to the north and west of the indian continental margin the eastern margin of the basin was a carbonate shelf and sabkha flat salt deposition occurred subaqueously in the central parts of the basin in the late early eocene redbeds derived from the northwest were deposited by a fluvial and or deltaic system this influx of clastic sediments marks the earliest record of terrestrial foreland basin deposition in northwest pakistan during the middle eocene the basin was reflooded and a carbonate shelf developed relative sea level rise may reflect subsidence of the indian plate in response to continued crustal loading in the himalayan suture zone uplift and erosion occurred between the late eocene and miocene possibly related to a peripheral bulge south of the himalayan suture zone the main phase of himalayan foreland basin fluvial deposition began in the miocene renewed uplift related to final collision of india and afghanistan during the pliocene is recorded by a thick sequence of conglomerate in the western kohat plateau correlation with eocene sedimentary rocks from southern pakistan to northern india delineates the depositional systems that developed as the tethys sea closed during himalayan convergence

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 geometry and timing of orogenic extension an example from the western italian alps contacts between rocks recording large differences in metamorphic grade are indicative of major tectonic displacements low p upon high p contacts are commonly interpreted as extensional i e material points on either side of the contact moved apart relative to the palaeo horizontal but dating of deformation and metamorphism is essential in testing such models in the western alps the piemonte ophiolite consists of eclogites t 550 600 c and p 18 20 kbar structurally beneath greenschist facies rocks t 400 c and p 9 kbar mapping shows that the latter form a kilometre wide shear zone the gressoney shear zone gsz dominated by top se movement related to crustal extension rb sr data from micas within different gsz fabrics which dynamically recrystallized below their blocking temperature are interpreted as deformation ages ages from different samples within the same fabric are reproducible and are consistent with the relative chronology derived from mapping they show that the gsz had an extensional deformation history over a period of c 9 myr between c 45 36 ma this overlaps in time with the eclogite facies metamorphism the gsz operated over the entire period during which the footwall evolved from eclogite to greenschist facies and was therefore responsible for eclogite exhumation the discrete contact zone between eclogite and greenschist facies rocks is the last active part of the gsz and truncates greenschist facies folds in the footwall these final movements were therefore not a major component of eclogite exhumation pressure estimates associated with old and young fabrics within the gsz are comparable indicating that during extensional deformation there was no significant unroofing of the hangingwall since there are no known extensional structures younger than 36 ma at higher levels in this part of the alps exhumation since the final juxtaposition of the two units at 36 ma seems to have been dominated by erosion key words deformation age eclogite exhumation rb sr dating tectonic

cooling and uplift histories of the crystalline thrust stack of the indian plate internal zones west of nanga parbat pakistan himalaya abstract amphibole and mica k ar ar ar and rb sr geochronology for the crystalline internal zones of the indian plate define both an extensive pre himalayan thermal history and a post himalayan metamorphism cooling history south of the main mantle thrust near besham hornblende ar ar ages from basement gneisses record an ca 1850 ma mid proterozoic thermal event hornblende muscovite and biotite cooling ages from cover sequences metamorphosed during the himalayan orogeny are 35 4 30 to 24 and 29 to 22 ma respectively the mica ages together with those derived from zircon and apatite fission track data zeitler 1985 demonstrate a rate of cooling of about 30 c ma during the late oligocene to early miocene that was greater than that either before or since this rapid cooling was initiated during the post metamorphic evolution of the indian plate south verging crustal scale thrust stack during which cover sequences metamorphosed during the himalayan orogeny were imbricated with basement rocks thermally unaffected during that event most of the cooling which happened during the stripping of some 10 2 km of overburden reflects exhumation due to a combination of erosion recorded in the miocene molasse sediments of the foreland basin and major crustal extension within the mmt zone both erosion and extension were the direct consequence of the evolution of the thrust stack

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

continental subduction and a mechanism for exhumation of high pressure metamorphic rocks new modelling and field data from oman abstract the physical model presented reveals two principal regimes of continental subduction a highly compressional hc regime and a low compressional lc regime characterised by high and low pressure between the overriding and subducting plates respectively the pressure is inversely proportional to the pull force which depends on the difference between average density of the subducting lithosphere and density of the mantle the subducting continental crust reaches a maximum depth which is proportional to the strength of the crust inversely proportional to the interplate pressure and is ca 200 km on average the crust then fails forming a major crustal thrust the location of failure depends on the regime of subduction on the interplate pressure in the hc regime failure occurs near the front of the subduction collision zone whereas the crust fails at greater depth under the base of the overriding plate in the lc regime the failure is followed by buoyancy driven uplift of the subducted crustal slice while the lithospheric mantle keeps subducting the uplift causes a normal sense displacement formation of a normal fault along the upper surface of the crustal slice for the lc regime failure of the crust near the base of the overriding plate a rapid spontaneous crustal uplift intrusion into the interplate zone brings the deeply subducted crust to shallow depths under the pressure of this crust the frontal part of the overriding plate undergoes local extension and then fails forming a tectonic window the rising material the high pressure rocks is exhumed within this window in the hc regime uplift of the subducted crust after its failure in front of the overriding plate is possible only with erosion of the relief which provides an unloading effect allowing the subducted crustal slice to rise up the exhumation depth is generally smaller in this regime but the volume of the exhumed material is larger the hc subduction regime has been shown earlier to match the himalayan situation the lc regime fits the situation in the oman mountains considered in this paper

strain deformation temperatures and vorticity of flow at the top of the greater himalayan slab everest massif tibet this paper presents quantitative data on strain deformation temperatures and vorticity of flow at the top of the greater himalayan slab the data were collected from the tibetan side of the everest massif where two low angle normal faults bound the upper surface of the greater himalayan slab the earlier and structurally lower lhotse detachment and the later and structurally higher qomolangma detachment greenschist to sillimanite grade quartz rich metasedimentary rocks exposed in the rongbuk to north col region of the everest massif are characterized by cross girdle quartz c axis fabrics indicating approximate plane strain conditions fabric opening angles progressively increase with depth beneath the overlying lhotse detachment and indicate progressively rising deformation temperatures of 525 625 50 c at depths of 300 600 m beneath the detachment deformation temperatures of c 450 c are indicated by fabric opening angles in epidote amphibolite facies mylonites located closer to the overlying detachment a top down to the north normal shear sense is indicated by the asymmetry of microstructures and c axis fabrics but the degree of asymmetry is low at distances greater than 400 m beneath the detachment and sillimanite grains are drawn into adjacent conjugate shear bands but still appear pristine indicating that deformation occurred at close to peak metamorphic temperatures these quenched fabrics and microstructures indicate rapid exhumation in agreement with previous isotopic dating studies mean kinematic vorticity numbers w m were independently calculated by three analytical methods calculated w m values range between 0 67 and 0 98 and indicate that although a simple shear component is generally dominant particularly in greenschist facies mylonites located between the lhotse and overlying qomolangma detachments there is also a major component of pure shear in samples located at 400 600 m beneath the lhotse detachment pure and simple shear make equal contributions at w k 0 71 our integrated strain and vorticity data indicate a shortening of 10 30 perpendicular to the upper surface of the greater himalayan slab and confirm that the upper surface of the slab is a stretching fault with estimated down dip stretches of 10 40 assuming plane strain deformation measured parallel to the flow plane transport direction

ground temperature changes in eastern canada borehole temperature evidence compared with proxy data borehole temperature logs have been inverted to infer ground temperature histories gth in eastern and central canada regional ground temperature histories were obtained by simultaneous inversion of several temperature profiles from the same areas simultaneous inversion of 21 temperature logs sampled across all of eastern and central canada yielded an average solution for this region all but three of the studied sites show signs of warming in the last 150 years this period of warming which started after 1800 ad was found throughout this part of canada the warming followed a cooler period corresponding to the little ice age the inferred ground temperature histories exhibit long term trends similar to those obtained from treering growth indices in nearby regions and stable isotope data in the southern hemisphere the modern warming appears correlated with the atmospheric concentration of co2 as measured in ice cores

the himalayan main central thrust pile and its quartz rich tectonites in central nepal as a consequence of the india asia collision a deeply rooted crustal slice has been moved southwards over the northern boundary of the indian plate at the present level of observation the main central thrust pile a rock pile of several kilometers thickness is affected by shear deformation which imprinted a flat foliation and a constant n s trending lineation these tectonites were deformed under a wide range of metamorphic conditions grading from lower greenschists up to the sillimanite field the m c t shear zone is studied from the view point of the quartz rich rocks over 100 samples helped to define a microstructural zonation roughly parallel to the thermal one 35 samples particularly rich in quartz have been used for the preferred orientation study of their c axes and a few of them for their a axes strong lattice orientations when undisturbed by mainly secondary recrystallization attest to large plastic strains in quartz the c axes are mostly distributed along unequally populated crossed girdles which may evolve into an inclined single girdle the a axes concentrate perpendicular to the more populated girdle concerning quartz the following is suggested 1 1 the deformation has a dominant shear component responsible for the dominant stretching lineation it could be simple shear in case of inclined single girdles 2 2 one set of a axes line up parallel to the shear direction slip on various planes in the a direction may be an important deformation mechanism 3 3 asymmetry in the lattice preferred orientations could be used to predict the sense of shear for the himalayan thrusting in more than 80 of cases

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

exhumation of high pressure rocks a review of concepts and processes the exhumation of high pressure metamorphic rocks requires either the removal of the overburden that caused the high pressures or the transport of the metamorphic rocks through the overburden exhumation cannot be achieved simply by thrusting or strike slip faulting it may be caused by erosion of shortened and thickened crust but this is unlikely to be the only mechanism for exhuming rocks from depths greater than about 20 km one or more of the following additional mechanisms may be involved 1 corner flow of low viscosity material trapped between the upper and lower plates in a subduction zone can cause upward flow of deeply buried rock and may explain some occurrences of high pressure tectonic blocks in melange this process does not however appear to be adequate to explain the exhumation of regional high pressure terrains 2 buoyancy forces acting directly on metamorphic rock bodies may cause them to rise relative to more dense surroundings this is likely to be the most important mechanism of exhumation of crustal rocks subducted into the mantle but cannot explain the emplacement of coherent tracts of high density metamorphic rock into shallow crustal levels some high pressure blocks emplaced at shallow levels in accretionary terrains may have been entrained in diapiric intrusions of low density mud or serpentinite 3 extension driven by the forces associated with contrasts in surface elevation may explain the exhumation and structural setting of many high pressure terrains extension may occur in the upper part of an accretionary wedge thickened by underplating or it may affect the whole lithosphere in a region of intracontinental convergence if surface elevation has been increased by the removal of a lithospheric root in the second case extension may be accompanied by magmatism and an evolution towards higher temperature during decompression of the metamorphic terrain

genesis of the batinah melange above the semail ophiolite oman abstract the batinah melange which overlies the late cretaceous semail ophiolite in the northern oman mountains comprises mostly sedimentary rocks of deep water facies alkalic lavas and intrusives all of continental margin affinities together with smaller volumes of semail ophiolitic and metamorphic rocks four intergradational textural types of melange can be recognized sheet melange has large 1 km intact sheets either with little intervening matrix or set in other melange types and with an organised sheet orientation fabric slab melange is finer textured 100 m and more disrupted block melange has smaller m blocks with some matrix and a weak to random block fabric clast melange is matrix supported rudite with a weak depositional clast fabric structural relationships particularly the absence of tectonic fabrics the decreasing strength of fragment fabrics with increasing fragmentation and the abundance of brittle fragmentation suggest that these melange types formed by either gravity driven sedimentary processes or superficial sliding or thrusting of individual rock slabs in the slab melange long sequences can be pieced together passing up from upper triassic mafic sub marine extrusives and sediments into radiolarian cherts hemipelagic and redeposited limestones and terminating in non calcareous radiolarities with mn deposits of early cretaceous age mafic sills are numerous these sequences can be matched with sub ophiolite rocks now exposed in fault corridors through the semail these sequences become progressively disrupted upwards in the corridors and can be traced continuously into overlying melange which then thins away from the corridors we argue that during late cretaceous emplacement over the arabian margin active fault corridors split the semail slab and acted as conduits up which sub ophiolite rocks were supplied to the ophiolite surface there the rocks were redisributed by superficial processes

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

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred

the upper batinah complex oman allochthonous sediment sheets above the semail ophiolite the upper batinah complex consists of allochthonous sedimentary sheets that structurally overlie both the semail ophiolite and its cover of melange lower batinah complex sequences represent a former sedimentary transition across the outer part of a passive mesozoic continental margin from proximal to distal units include redeposited quartzitic sandstones and shallow water limestones large carbonate buildups floored by triassic mafic lavas and pelagic radiolarites or hemi pelagic calcilutites also underlain by triassic mafic lavas sheets of these upper batinah complex sequences were emplaced onto the upper surface of the semail ophiolite in late cretaceous time during or soon after the advance of the semail nappe onto the arabian margin emplacement was along subhorizontal fault zones with sheets of distal facies followed by successively more proximal sheets structural style suggests a low overburden during deformation with ductile flow of argillaceous material but more brittle behavior of othe

timing of himalayan ultrahigh pressure metamorphism sinking rate and subduction angle of the indian continental crust beneath asia coesite relics were discovered as inclusions in clinopyroxene in eclogite and as inclusions in zircon in felsic and pelitic gneisses from higher himalayan crystalline rocks in the upper kaghan valley north west himalaya the metamorphic peak conditions of the coesite bearing eclogites are estimated to be 27 32 kbar and 700 770 c using garnet pyroxene phengite geobarometry and garnet pyroxene geothermometry respectively cathodoluminescence cl and backscattered electron bse imaging distinguished three different domains in zircon inner detrital core widely spaced euhedral oscillatory zones and thin broadly zoned outermost rims each zircon domain contains a characteristic suite of micrometre sized mineral inclusions which were identified by in situ laser raman microspectroscopy core and mantle domains contain quartz apatite plagioclase muscovite and rutile in contrast the rim domains contain coesite and minor muscovite quartz inclusions were identified in all coesite bearing zircon grains but not coexisting with coesite in the same growth domain rim domain 206 pb 238 u zircon ages reveal that the quartz bearing mantle domains and the coesite bearing rim were formed at c 50 ma and 46 2 0 7 ma respectively these facts demonstrate that the continental materials were buried to 100 km within 7 9 myr after initiation of the india asia collision palaeomagnetic data from the indian oceanic floor supports an initial india asia contact at 55 53 ma combination of the sinking rate of 1 1 1 4 cm year 1 with indian plate velocity of 4 5 cm year 1 suggests that the indian continent subducted to about 100 km depth at an average subduction angle of 14 19

miocene dextral shearing between himalaya and tibet the main central thrust zone is the major structural feature of the himalayas it was active during miocene time and accommodated at least 100 km and possibly as much as 300 km of convergence between india and tibet we present evidence that late in the tectonic history another large shear zone located north of the high himalayas the north himalayan shear zone underwent a phase of dextral strike slip motion the most plausible explanation for this phase of motion is that it reflects the onset of extension in tibet it is usually thought that extension began about 2 5 ma the phase of dextral shear we report occurred between 25 and 15 ma if our explanation for this movement is correct it places the onset of extension at least 12 m y earlier than previously thought

the hawasina basin a fragment of a starved passive continental margin thrust over the arabian platform during obduction of the sumail nappe abstract a complete revision of the lithostratigraphy of the hawasina nappes in the eastern and central oman mountains as compared to that of the previous stratigraphy leads to a new palaeogeographical reconstruction of the basin five very distinct zones have been determined representing five principal structural units that succeed one another from the external to the internal zones as follows 1 1 the hamrat duru basin 2 2 the baid platform 3 3 the al avidh trough 4 4 the misfah platform and 5 5 the umar basin the history of the hawasina basin began in the late permian with the formation on the northeastern edge of gondwana of a vast intracontinental basin the hamrat duru basin however this basin became a passive continental margin only during the middle late triassic formation of the neo tethys by crustal spreading and accretion began at this time in the internal part of the basin and in continuity with the umar basin the misfah platform and the al aridh trough which formed at the same time within the hawasina basin from this period onwards the main geomorphological features of the basin and of the margin were fixed and persisted up to the late cretaceous the phase of extension by which the basin was subsequently affected during the late tithonian berriasian was mainly manifested by generalized subsidence these main stages in the formation and evolution of the hawasina basin are marked by extensive development of in general alkaline but in places transitional and locally tholeiitic morb magmatism the sediments of the hamrat duru basin are dominated by turbidites composed of material mainly derived from the arabian platform whereas the al aridh trough and umar basin are characterized by pelagic sediments however the al aridh and umar grabens also contain layers of breccia of variable extent containing clasts and blocks of late permian and late triassic shallow water carbonates derived from the adjacent baid and misfah platforms sedimentation in the hawasina basin thus began locally in the late permian and by the triassic it was taking place in the remaining parts of the basin continuing up to the late turonian early senonian when the basin closed and overthrusting during obduction of the sumail ophiolite occurred sedimentation ended the overthrusting took place in three stages 1 1 a southerly directed compressive pre early obduction phase during which the hawasina basin was detached from its substratum to form an obduction prism 2 2 an obduction phase sensu stricto which by gravitational mechanisms induced major displacement of the principal hawasina units and their piling up on the arabian platform with the outermost units at the base and the innermost at the top and 3 3 a late obduction phase which caused local north northeastward backfolding possibly associated with the development of a fracture cleavage which in the basal structural levels may pass into true schistosity

thermal effects of metamorphic fluids in subduction zones in subduction zones large volumes of fluid are released from the subducting slab during prograde metamorphism of hydrated oceanic crust once released metamorphic fluids tend to migrate upward owing to buoyancy fluid migration can affect the thermal structure of a subduction zone by 1 advecting heat and 2 causing retrograde hydration carbonation reactions in the hanging wall under most p t conditions the volumetric heat capacities of h 2 o and rocks are similar therefore the thermal effects of fluid migration and uplift are comparable calculations suggest that advective heat transport by fluid flow causes relatively minor thermal effects pervasive upward fluid flow results in thermal effects analogous to regional uplift rates on the order of 0 2 mm yr fluid flow channelized along the subduction shear zone translates the local thermal structure of the shear zone updip at 1 mm yr a more important thermal effect results from exothermic retrograde reactions in the hanging wall that release 30 kj mol of h 2 o consumed retrograde reactions significantly retard cooling of the hanging wall just as prograde reactions retard warming of the subducting slab

transtensional deformation in the central himalaya and its role in accommodating growth of the himalayan orogen 1 field mapping structural analysis and geochronologic data from northwestern nepal reveal major normal right slip motion along a previously unrecognized west northwest striking system of shear zones that we term the gurla mandhata humla fault system gmh the gmh obliquely cuts across the greater himalayan crystalline sequence and into the lesser himalayan imbricate thrust belt via two right step over structures the average slip direction on the gmh parallels the strike of the himalayan orogen motion along this fault system has resulted in an apparent left separation of the south tibet detachment main central thrust zone and lesser himalayan imbricate thrust belt along a north striking segment of the fault system we estimate a minimum of 21 km of net slip on the southern branch of the gmh by restoring the trace of the main central thrust zone parallel to the average slip direction on the fault taking into account slip estimates from the northern branch of the gmh yields a minimum net slip estimate of 24 4 to 32 4 km for the gmh the 232th 208pb ion microprobe monazite ages from leucogranite bodies indicate that motion on the gmh occurred after 15 ma its initiation immediately followed crustal thickening between the main central thrust zone and indus yalu suture zone motion on the gmh is contemporaneous with arc normal contraction in the southernmost himalayan orogen these observations can be explained by a model that involves foreland propagating structural systems facilitating arc normal contraction in the foreland and arc parallel extension in the hinterland that work together to maintain the arcuate shape of the himalayan orogen

gps measurements from the ladakh himalaya india preliminary tests of plate like or continuous deformation in tibet observations of relative motion in a geodetic network in ladakh india and across southern tibet indicate slow shear on the karakorum fault rapid east west extension across the whole of southern tibet and constant arc normal convergence between india and southern tibet along the himalayan arc measurements of ten campaign style and six permanent sites with global positioning system gps precise geodesy provide these bounds on the style and rates of the large scale deformation in the tibet himalaya region divergence between sites at leh ladakh india and shiquanhe western tibet as well as slow relative motion among sites within the ladakh network limit right lateral slip parallel to the karakorum fault to only 3 4 5 mm yr this low rate concurs with a recent estimate of 3 4 mm yr for late holocene time but disagrees with the much higher rate of 30 35 mm yr that has been used to argue for plate like behavior of the tibetan plateau convergence between ladakh and the indian subcontinent at 18 8 3 mm yr at 224 17 1 differs little from estimates of convergence across the central segment of the himalaya finally lengthening of the baseline between leh ladakh and lhasa in southeastern tibet at 17 8 1 mm yr or between leh and bayi farther to the southeast at 18 3 mm yr is consistent with an extrapolation of rates of east west extension of the tibetan plateau based both on shorter gps baselines e g lhasa simikot and on diverging slip vectors of earthquakes in the himalaya we interpret these results to indicate that tibet behaves more like a fl uid than like a plate

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

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

ductile extrusion of the higher himalayan crystalline in bhutan evidence from quartz microfabrics quartz textures measured from deformed quartz tectonites within the lesser himalaya and higher himalaya crystalline of bhutan show similar patterns orientation and distribution of the quartz crystallographic axes were used to confirm the regional shear sense the asymmetry of c axis and a axis patterns consistently indicates top to the south shearing the obliquity of the texture and the inferred finite strain plane strain to moderately constrictional suggest the strain regime had a combination of rotational and irrotational strain path in most of the samples from the bhutan himalaya the inferred deformation mechanisms suggest moderate to high temperature conditions of deformation that produced the observed crystallographic preferred orientation much higher temperature of deformation is indicated in the quartz veins from a leucogranite the observed ductile deformation is pervasively developed in the rocks throughout the investigated area the intensity of deformation increases only slightly in the vicinity of the main central thrust simultaneous southward shearing within a large part of the higher himalaya crystalline near and above the main central thrust and normal faulting across the south tibetan detachment is explained by the tectonically induced extrusion of a ductily deforming wedge the process of extrusive flow suggested here can be approximated quantitatively by channel flow models that have been used to describe subduction zone processes channel flow accounts for some observed phenomena in the himalayan orogen such as inverted metamorphic sequences near the main central thrust not related to an inversion of isotherms and the syntectonic emplacement of leucogranites into the extruding wedge locally leading to an inversion of isotherms due to heat advection

uranium lead isotopic ages of the samail ophiolite oman with applications to tethyan ocean ridge tectonics plagiogranites are a minor but widespread component of the samail ophiolite plutonic member they crystallized from the most fractionated melts generated by magmatic crystallization and differentiation of a steady state magma chamber beneath the tethyan spreading ocean ridge and their ages are thought to mark the time of ocean crust formation isotopic u pb ages of zircons from 13 plagiogranites collected along a 270 km segment of the samail ophiolite subparallel to the regional trend of the sheeted dike complex the former spreading ridge axis direction define a narrow time interval of 93 5 97 9 m y with a pronounced clustering about 95 m y the zircon ages of the plagiogranites agree remarkably well with the early cenomanian to early turonian biostratigraphic ages of sediments that are intercalated within the ophiolite pillow lavas and that lie just above them tippit et al 1981 the agreement of radiometric and biostratigraphic ages provides strong support for the conclusion that the plagiogranite u pb ages closely date the time span of ocean crust formation no step changes in age patterns are observed along the ridge axis sheeted dike direction suggesting that there are no major internal offsets of the ophiolite by transform or other faults along most of the traverse one possible exception occurs at the southeastern end of the sampled interval ibra area where a 3 m y discontinuity might be caused by an unmapped fault assuming that the regional trend of the sheeted dikes n10 25 w marks the direction of the former spreading ridge axis the present array of sample localities spans a distance of 130 to 195 km normal to that axis i e in the spreading direction the data as a whole do not define a clear cut age trend normal to the spreading axis but by eliminating samples that may be aberrant due to faulting the data array suggests a pattern of increasing ages from east to west this would indicate that the samail ophiolite is derived from the western southwestern side of the tethyan spreading system a conclusion reached independently from geological evidence also the gentle age gradient in the spreading direction i e between 130 and 195 km of ocean crust were formed within an age span of no more than 2 m y suggests a very fast spreading rate this conclusion also is supported by independent geologic evidence

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

metamorphic pressure temperature conditions of indian plate rocks south of the main mantle thrust lower swat pakistan the lower swat rock sequence in northern pakistan is composed of greenschist and amphibolite facies indian plate rocks that crop out in a dome directly south of the main mantle thrust zone and kohistan arc terrane the metamorphism is eocene to oligocene in age and records the collision of the indian plate with the kohistan arc this paper presents mineral assemblage mineral composition and garnet zoning data on the lower swat rock sequence in order to estimate the pressure temperature conditions of metamorphism and to infer a possible pressure temperature time path the stratigraphic sequence consists of precambrian to cambrian manglaur formation unconformably overlain by late paleozoic to early mesozoic alpurai group the alpurai group is further subdivided into the marghazar kashala saidu and nikanai ghar formations the manglaur formation in the core of the dome is at kyanite grade metamorphic grade decreases in the overlying alpurai group where a garnet isograd is mapped in calcareous schist of the kashala formation mineral composition geothermometers and geobarometers indicate final metamorphic equilibrium conditions of about 600 700 c and 9 11 kbar in the manglaur and marghazar formations these conditions are associated with a phase of static recrystallization which postdates the main phases of deformation correlation of the structural tectonic history with the pressure temperature estimates and with the garnet zoning analysis suggests that in the eocene the lower swat rock sequence was subducted to a depth of 35 to 45 km beneath the mmt suture melange subduction was followed immediately by exhumation the initiation of exhumation is believed to have been triggered by a change from subduction to strike slip motion in the mmt thereby allowing the relatively buoyant lower swat crust to rise the entire metamorphic deformational cycle lasted from 7 to 16 m y ending in the late eocene about 38 ma when the rocks cooled through the argon blocking temperature in hornblende 550 c

the low temperature brittle ductile transition in a quartzite and the occurrence of cataclastic flow in nature the paper describes the mechanical and microstructural characteristics associated with the brittle to cataclastic flow transition in an orthoquarzite oughtibridge ganister and compares its microstructural development with features of cataclastic deformation of rocks in nature

effects of subsurface heterogeneity on the inference of climate change from borehole temperature data model studies and field examples from canada all existing approaches to the analysis and interpretation of borehole temperatures in terms of a ground surface temperature history are based on the one dimensional theory of heat conduction deviations from this idealization are manifest as noise in the interpretation we present numerical experiments that explore the effects of three dimensional subsurface heterogeneity on ground surface temperature histories inferred from borehole temperature profiles inversions of steady state temperature profiles containing such three dimensional noise reveal that in an inversion formulation incorporating a priori information spurious temperature histories can emerge when the a priori constraints on subsurface temperatures and thermophysical properties are too tight i e when arbitrarily small variations in subsurface temperatures and thermophysical properties are interpreted to have significance for the derived climate history relaxation of the a priori constraints enables an effective muting of the spurious histories similar experiments conducted on synthetic transient signals confirm the steady state results but also reveal that relaxation of the a priori constraints will also lead to a loss of signal if the level of relaxation is excessive our experiments suggest that relaxation of constraints on thermal conductivity is more efficient than relaxation of borehole temperatures in suppressing artifacts arising from the three dimensional effects we identify a range of constraints in which reasonable noise suppression and signal recovery are both achieved and then reprocess data from 22 canadian boreholes for which surface temperature histories had earlier been derived by ourselves and others our reprocessed results reveal a remarkably simple surface temperature history common to much of eastern canada this history comprises a recent warming interval commencing in the nineteenth century in which the surface temperature increased by some 1 4 c approximately 0 5 1 0 c of this increase was a recovery from a preceding cooler interval in which the temperature was below the long term mean the remainder of the increase represents warming that exceeds the long term mean

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

thermal controls on early tertiary short lived rapid regional metamorphism in the nw himalaya pakistan abstract during tertiary collision in the nw himalaya the leading edge of the indian plate was subducted beneath the kohistan island arc along the main mantle thrust mmt metamorphism within indian plate cover sediments was synchronous with ductile shearing and took place along a path of increasing pressure during subduction beneath the island arc initial collision cannot have pre dated 65 ma and probably shortly pre dated 50 ma radiometric data constrain the metamorphic peak as shortly post dating 50 ma as firstly initially subducted units are now probably located beneath tibet secondly the subduction thrust separating the kohistan arc terrane from the indian plate was probably cooled by continued underthrusting and thirdly the heat producing indian plate cover sediments were delaminated from the basement during collision metamorphism was more rapid than can be predicted by purely conductive models of thermal relaxation although dissipative shear heating along the mmt doubtless contributed to early stages of heating of the footwall rocks the temperatures attained in the footwall are too high to support the shear stresses required to generate them solely through shear heating a model is derived to account for both the rapid regional metamorphism and the equally rapid post metamorphic cooling dissipative shear heating along the mmt generated an early inverted thermal profile in the upper units of the indian plate as the hanging wall mafic rocks have a low thermal conductivity they would have acted as a thermal reflector and the heat would have been conducted away only slowly as footwall temperatures increased through the brittle ductile transition the role of dissipative shear heating decreased and continued heating became a function of internal heat generation within the footwall rocks together with hanging wall thermal reflectivity the metamorphic inversion was reinforced by imbrication of the metamorphic stack as it accreted onto the mmt footwall during early stages of uplift and exhumation dissipative shear heating within thrust systems provides a potentially important mechanism by which areas of large regional extent can undergo regional metamorphism over short timescales in the absence of magmatic heating

deformation of peridotite at high pressure in a new molten salt cell comparison of traditional and homologous temperature treatments abstract we have conducted a series of constant strain rate experiments on a synthetic peridotite composed of isostatically hot pressed olivine and orthopyroxene over a pressure range of 700 2200 mpa and a temperature range of 1250 1700 k by using a sample assembly in which specimens are surrounded by molten salt the data are of much higher quality than previously obtained in solid pressure medium apparatus we find that both the temperature and pressure dependence of creep in our experiments can be combined quantitatively as the homologous temperature dependence of creep this quantitative relationship between creep and melting greatly simplifies the thermodynamic treatment of creep and provides a simple model for the extrapolation of laboratory creep data for peridotite to conditions throughout the earth s upper mantle

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

unusual transition in quartzite dislocation creep regimes and crystal slip systems in the aureole of the eureka valley joshua flat beer creek pluton california a case for anhydrous conditions created by decarbonation reactions abstract microstructures and quartz c axis fabrics were analyzed in five quartzite samples collected across the eastern aureole of the eureka valley joshua flat beer creek composite pluton temperatures of deformation are estimated to be 740 50 c based on a modified c axis opening angle thermometer of kruhl j metamorph geol 16 1998 142 in quartzite layers located closest 140 m to the pluton wall rock contact flattened detrital grains are plastically deformed and partially recrystallized the dominant recrystallization process is subgrain rotation dislocation creep regime 2 of hirth and tullis j struct geol 14 1992 145 although grain boundary migration dislocation creep regime 3 is also evident complete recrystallization occurs in quartzite layers located at a distance of 240 m from the contact and coincides with recrystallization taking place dominantly through grain boundary migration regime 3 within the quartzites strain is calculated to be lowest in the layers closest to the pluton margin based on the aspect ratios of flattened detrital grains the c axis fabrics indicate that a slip operated within the quartzites closest to the pluton wall rock contact and that with distance from the contact the operative slip systems gradually switch to prism c slip the spatial inversion in microstructures and slip systems apparent high temperature deformation and recrystallization further from the pluton contact and apparent low temperature deformation and recrystallization closer to the pluton contact coincides with a change in minor phase mineral content of quartzite samples and also in composition of the surrounding rock units marble and calc silicate assemblages dominate close to the pluton wall rock contact whereas mixed quartzite and pelite assemblages are dominant further from the contact we suggest that a thick marble unit located between the pluton and the quartzite layers acted as a barrier to fluids emanating from the pluton decarbonation reactions in marble layers interbedded with the inner aureole quartzites and calc silicate assemblages in the inner aureole quartzites may have produced high x co 2 water absent fluids during deformation the presence of high x co 2 fluid is inferred from the prograde assemblage of quartz calcite and not wollastonite diopside k feldspar in the inner aureole quartzites we suggest that it was these dry conditions that suppressed prism c slip and regime 3 recrystallization in the inner aureole and resulted in a slip and regime 2 recrystallization which would normally be associated with lower deformation temperatures in contrast the prograde assemblage in the pelite dominated outer part of the aureole is biotite k feldspar these wet pelitic assemblages indicate fluids dominated by water in the outer part of the aureole and promoted prism c slip and regime 3 recrystallization because other variables could also have caused the spatial inversion of c axis fabrics and recrystallization mechanisms we briefly review those variables known to cause a transition in slip systems and dislocation creep regimes in quartz our conclusions are based on a small number of samples and therefore the unusual development of crystal fabrics and microstructures in the aureole to the ejb pluton suggests that further study is needed on the effect of fluid composition on crystal slip system activity and recrystallization mechanisms in naturally deformed rocks

how perfect is the himalayan arc the himalayan plate boundary because it is entirely subaerial is both the most dramatic and the most accessible to direct observation of all active convergent boundaries on earth the shape of this boundary can be described as a small circle of radius 1696 55 km centered at long 91 6 1 6 e and lat 42 4 2 1 n for the extent of the arc between long 77 2 and 92 1 e the pole of this small circle is consistent whether seismicity topography or stress state is used to define the position of the tectonic boundary the defined small circle also coincides with a peak in microseismicity the maximum horizontal strain rate and a peak in the vertical velocity field this quantitative definition of a stable curved tectonic boundary is a prerequisite to modeling the dynamics of curvature in convergent arcs and applying appropriate boundary conditions to other regional models

thermal diffusivity of igneous rocks at elevated pressure and temperature thermal diffusivity measurements of seven igneous rocks were made to temperatures of 400 sup 0 c and pressures of 200 mpa the measuring method was based on the concept of cylindrical symmetry and periodic heat pulses the seven rocks measured were westerly rhode island granite climax stock nevada quartz monzonite pomona washington basalt atikokan ontario canada granite creighton ontario canada gabbro east bull lake ontario canada gabbro and stripa sweden granite the diffusivity of all the rocks showed a positive linear dependence on inverse temperature and excluding the east bull lake gabbro showed a linear dependence on quartz content quartz content varied from 0 to 31 by volume diffusivity in all cases rose or remained steady with increasing confining pressure the pressure effect was strongest at lowest pressures and vanished by levels between 10 and 100 mpa depending on rock type the pressure effect measured as a percentage change in diffusivity is stronger in the four rocks of granite composition than in the three of basaltic composition our results agree well with existing thermal diffusivity measurements at atmospheric pressure

differential stress strain rate and temperatures of mylonitization in the ruby mountains nevada implications for the rate and duration of uplift kno ledge of the m agnitude of the differential stress during the formation of mylonitic rocks provides constramts on mechanical and thermal models for the exhumation of the metamorphosed footwalls of major low angle detachment faults we have analyzed the differential flow stress during the mylonitization of quartzose rocks in the ruby mountains nevada using grain size piezometers and kinetic laws for grain g owth quartzites from mylonitic shear zones in lamoille canyon and secret creek gorge have grain sizes of 91 151 m and 42 64 m respectively the peak temperature during mylonitization was 630 i 50 c and analysis of grain growth kinetics indicates that mylonitization continued during cooling to temperatures s450 c quartz grain size piezometers suggest that the mylonitization occurred under differential stresses crl cr3 of 38 64 mpa or maximum shear stresses of 19 32 mpa extrapolation of quartzite flow laws indicates that the mylonitization occurred at strain rates between 10 10 and 10 13 1 arguments presented m the paper suggest that the likely range of strain rates is 10 11 to 10 12 1 these strain rates are compatible with displacement rates of the order of 23 mm yr l along a 1 5 km thick simple shear zone such a shear zone dipping 15 would produce an uplift rate of 5 8 km m y 1 and a horizontal extensioo rate of 22 km m y l this uplift rate indicates that midcrustal mylonitic rocks could have been lifted up along a 1 5 km thick simple shear zone dipping 15 in 2 6 m y

orogen parallel and orogen perpendicular extension in the central nepalese himalayas north dipping gravitationally driven extensional structures that unroof the high grade core of the himalayas have been recognized in several parts of the orogen recent work in west central nepal demonstrates that the extensional history at the top of the greater himalayan metamorphic sequence in this region is laterally variable in the upper marsyandi valley there is evidence for three generations of early to middle miocene west directed orogen parallel extensional structures the study area consists of two tectonostratigraphic sequences the lower sequence consists of a 12 km thick section of high grade metasedimentary rocks of the greater himalayan sequence the higher sequence consists of a 4 km thick section of medium grade marble of the tibetan sedimentary sequence it overlies the greater himalayan sequence in the marsyandi valley along a northwest dipping ductile shear zone the chame detachment mylonitic fabrics in footwall rocks show evidence of west directed displacement at sillimanite grade through greenschist grade conditions a cataclastic zone indicates progressively more brittle deformation at the eastern end of the outcrop trace of the chame detachment in the dudh khola valley 15 km west of the marsyandi valley the tibetan sedimentary sequence is juxtaposed with the greater himalayan sequence by an early miocene north dipping ductile shear zone the dudh khola detachment that has well developed north trending stretching lineations formed during sillimanite grade metamorphism north directed orogen perpendicular extension in the dudh khola region is interpreted as coeval with west directed orogen parallel extension in the marsyandi region along strike variations in the orientation of extensional strain at the top of the greater himalayan sequence may be explained by lateral crustal thickness gradients

u pb zircon ages from the indian plate in northwest pakistan and their significance to himalayan and pre himalayan geologic history u pb zircon isotopic ages of seven rock samples are combined with field data to provide constraints on deposition intrusion and metamorphism in the northwestern part of the indian plate pakistani hinterland south of the indus suture zone in pakistan the results suggest deformation intrusion and regional metamorphism at ca 2174 ma an event that may correlate with granulite facies metamorphism in the nanga parbat area this was followed by erosion and deposition prior to a second proterozoic deformation at ca 1850 ma which was associated with widespread intrusion and possibly with low grade regional metamorphism metasedimentary and intrusive rocks of this age are present within the lesser himalaya of nepal but are apparently absent in the high himalayan crystalline slab north of the mct this intrusive deformational event was followed by erosion and late proterozoic deposition with development in the cambrian of an epicontinental shallow marine shelf intrusion in late cambrian middle ordovician resulted only in minor uplift erosion and development of a disconformity marine shelf conditions were reestablished in the middle paleozoic prior to a widespread intrusive event in the carboniferous permian and normal faulting erosion and syndeformational deposition and volcanism in the late permian marine shelf conditions were reestablished in the triassic prior to the himalayan orogeny zircons with a concordant age of 89 ma coupled with field and published isotopic age data suggest that himalayan deformation and metamorphism in the pakistani hinterland began between 90 and 75 ma due to subduction of the indian plate beneath indus ophiolitic melange and reached peak amphibolite facies conditions between 70 and 48 ma this metamorphism precedes eocene 54 50 ma collision of india with the kohistan arc complex field data suggest that the presently exposed pakistani hinterland from afghanistan to babusar was never significantly overthrust or buried by the kohistan arc rather than initiating the metamorphism the collision of kohistan with india resulted in uplift exhumation and cooling of the metamorphic pile

focal depths and fault plane solutions of earthquakes and active tectonics of the himalaya synthetic seismograms were compared with long period body waves for nine earthquakes with epicenters in the himalayan arc to determine depths of foci and to improve fault plane solutions focal depths are shallow 10 20 km inferred slip vectors are locally perpendicular to the mountain range they plunge very gently about 10 deg in the eastern sections of the range and more steeply about 25 deg in western sections assuming india to be a rigid plate the radially oriented slip vectors imply that southern tibet extends at about half the rate of underthrusting in the himalaya and therefore probably at about 5 10 mm yr the shallow depths and gentle dips of the fault planes at least for the events in the eastern half of the range are consistent with coherent underthrusting of the indian plate beneath at least the lesser himalaya the steeper dips of fault planes in the western part of the arc might reflect deformation of the overriding thrust plate or simply a steepening of the main underthrusting zone beneath the greater himalaya

fault plane solutions of earthquakes and active tectonics of the tibetan plateau and its margins summary fault plane solutions of earthquakes within and on the margins of the tibetan plateau show diverse styles of faulting and deformation with thrust faulting and crustal shortening normal to the margins of the plateau and with normal and strike slip faulting resulting in roughly east west crustal extension within the plateau the direction of overthrusting of the himalaya onto the indian shield is radially outward varying from southwest in the western himalaya to south southeast in the east assuming that the indian shield behaves rigidly this requires a west northwest divergence of western tibet from southeastern tibet at a rate of 18 9 mm yr 1 comparable with the rate of convergence at the himalaya fault plane solutions of earthquakes in the southern portion of the tibet plateau consistently show large components of normal faulting on roughly north striking planes and corroborate such extension within the high plateau where elevations exceed 5000 m normal and strike slip faulting occur so that an overall east southeast west northwest extension of the region at about 10 mm yr 1 is partitioned into roughly equal parts of crustal thinning and north northeast south southwest crustal shortening about 5 mm yr 1 in general strike slip faulting characterizes solutions for earthquakes within eastern tibet where mean elevations drop below 4500 5000 m but the orientations of the strike slip faults vary across the region in central tibet left lateral slip occurs on planes trending roughly northeast but for earthquakes farther east the orientations of that plane become progressively east west and then southeast this variation in orientation implies a rotation of material along curved left lateral shear zones thus the eastward extrusion of tibet appears to be facilitated not only by rapid left lateral shear but also by large clockwise rotations of the material in eastern tibet the rate of eastward extrusion of material in eastern tibet relative to the tarim basin to its north is roughly 30 40 mm yr 1 fault plane solutions of earthquakes in the northern and eastern margins of tibet show large components of thrust faulting with the p axes oriented radially outward from the plateau and approximately perpendicular to the regional topographic contours of the plateau the orientation of this crustal shortening is northeast southwest on the northeastern margin east west on the eastern margin and northwest southeast in the longmenshan on the southeastern margin thus at least some of the extrusion of eastern tibet out of india s northward path into asia is absorbed by crustal shortening on the margins of the plateau the variation from normal faulting in the high tibetan plateau where elevations exceed 5000 m to dominantly strike slip faulting farther east where elevations are lower and then to thrust faulting on the margins of the plateau where elevations drop below 3000 m surely results at least in part from a decrease in the value of the vertical stress the magnitude of the east west compressive stress need not vary across the plateau

criteria for identifying structures related to true crustal extension in orogens faults and shear zones in orogens may be related to crustal shortening or extension or both true extensional structures should ultimately intersect the earth s surface when traced out in a direction opposite to that of hangingwall transport the present dip of a fault in an orogen and its relation to local layering are unreliable guides the observation of low pressure metamorphic rocks structurally above high pressure metamorphic rocks is not sufficient to diagnose crustal extension a valid metamorphic criterion is the difference in pressure time histories between footwall and hangingwall if the footwall was decompressing faster than the hangingwall during shear zone movement then the shear zone was truly extending the crust the pressure time history can be constrained via the temperature time and pressure temperature histories ideally crustal extension inferred from structural data should be confirmed using diagnostic ptt information

the tectonic expression slab pull at continental convergent boundaries examination of five thrust belt systems developed at continental subduction boundaries suggests that they comprise two distinct groups that display pronounced and systematic differences in structural style topographic elevation denudation metamorphism postcollisional convergence and foredeep basin geometry and facies the distinctive geological features developed within each thrust belt group appear to be causally linked to the relative rates of subduction and convergence via the magnitude of horizontal compressional stress transmitted across the subduction boundary at subduction boundaries where the rate of overall plate convergence is less than the rate of subduction termed here retreating subduction boundaries the transmission of horizontal compressive stress across the plate boundary is small and regional deformation of the overriding plate is by horizontal extension the tectonic expression of these retreating subduction boundaries includes topographically low mountains little erosion or denudation low grade to no metamorphism little to no involvement of crystalline basement in shortening little to no postcollisional convergence anomalously deep foredeep basins and a protracted history of flysch deposition within the adjacent foredeep basin analysis of deflection and gravity data across three retreating subduction boundaries apennine carpathian and hellenic systems shows that subduction is driven by gravitational forces acting on dense subducted slabs at depths between about 40 and 80 km carpathians 50 and 150 km apennines and 50 and 250 km hellenides the total mass anomalies represented by the slabs are approximately 3 1012 6 1012 and 12 1012 n m respectively the slabs are partially supported by flexural stresses transmitted through the subducted lithosphere to the foreland and partially supported by dynamic viscous stresses in the asthenosphere at subduction boundaries where the rate of overall plate convergence is greater than the rate of subduction termed here advancing subduction boundaries the transmission of horizontal compressive stress across the plate boundary is large and regional deformation of the overriding plate is by horizontal shortening the tectonic expression of these advancing subduction boundaries includes topographically high mountains antithetic thrust belts large amounts of erosion and denudation exposure of high grade metamorphic rocks at the surface extensive deformation of crystalline basement to midcrustal depths protracted postcollisional convergence tens of millions of years and a protracted history of molasse deposition within the adjacent foredeep basins analysis of gravity and deflection data across two advancing subduction boundaries developed within the continental lithosphere western to eastern and southern alps and himalayas shows that the thrust sheets have been translated for great distances over the foreland lithosphere relative to the point at which the subduction forces are applied thus obscuring any flexural and gravity signals from the subducted slab however it appears that far field stresses presumably related to global plate motions drive most of the convergent motion across these subduction boundaries the concept that orogenic belts formed above retreating subduction boundaries have recognizable tectonic signatures that differ from those of orogenic belts formed above advancing subduction boundaries suggests that it may be possible to interpret the plate boundary settings in which ancient orogenic belts evolved appendix b is available with entire article on microfiche order from the american geophysical union 2000 floridaavenue n w washington d c 20009 document t92 004 2 50 payment must accompany order

strain partitioning along the himalayan arc and the nanga parbat antiform shortening along the himalayan arc of continental convergence is approximately in the radial direction if the underthrusting foot wall block india is not deformed the hanging wall block tibet needs to stretch along the arc as suggested by radial grabens in southern tibet in contrast the nanga parbat haramosh massif and the western himalayan syntaxis are part of a 250 km long antiform that strikes in the radial direction northeast and verges northwest the nanga parbat antiform is the structural and topographic expression of arc parallel shortening that compensates for arc parallel extension in southern tibet this shortening is predicted to be as high as 12 mm yr

ground temperature history in central europe from borehole temperature data summary ground temperature histories gth are inferred from temperature measurements in several boreholes of south eastern germany and western bohemia czech republic the gths that can be extracted from these boreholes ranging in depth between 150 and 700m cover the past 250yr both data sets were inverted separately and yield consistent gths they were also inverted jointly to yield a regional gth of the past 250yr for this part of europe the results indicate two main episodes in the mean ground temperature with 1 a cooling period from 1750 1800 to 1930 1950 followed by 2 short colder and warmer periods until now the same trends are found in the meteorological records at four nearby weather stations bayreuth jena munchen and praha but the meteorological record in berlin is clearly distinct the gth for this part of europe is also markedly different from one obtained in central france these differences are consistent with the spatial variability of climatic trends

himalayan structure and metamorphism south of the main mantle thrust lower swat pakistan abstract during the eocene oligocene the indian plate collided with the kohistan arc along the main mantle thrust mmt zone the structure of the lower swat rock sequence on the indian plate directly south of the mmt is a dome with a basement of granitic gneiss and quartz rich schist unconformably overlain by amphibolitic and calcareous schist the earliest superposed small scale folds f1 f2 represent a progressive f1 f2 deformation that is associated with a single set of wsw vergent large scale folds termed f2 these folds are inferred to have developed during oblique wsw directed overthrusting of the mmt suture complex onto the lower swat rock sequence metamorphism began during f1 f2 as indicated by an s1 foliation that developed during biotite grade metamorphism s1 is preserved as a relict texture in porphyroblasts that grew during a subsequent interkinematic phase during garnet and higher grade metamorphism the dominant regional foliation s2 developed following the interkinematic phase s2 is associated with transposition of s1 and rotation or dismemberment of porphyroblasts annealing recrystallization followed s2 and continued during f3 thereby destroying or masking possible pre existing stretching fabrics superposed f3 folds are upright and open with n s axial trends they may correlate with early doming of the lower swat rock sequence and with strike slip displacement in the northern part of the mmt zone north of the lower swat area f3 was followed by retrograde metamorphism and development of e w trending s vergent f4 folds f4 may be associated with a final phase of southward directed thrusting and inactivity in the mmt zone correlation of published 40ar 39ar ages with the metamorphic fabrics suggests that f1 f2 and f3 occurred in the eocene and that f4 developed in the oligocene f4 is the earliest indication of southward verging structures on this part of the indian plate

metamorphism and crustal stacking in the north indian plate north pakistan abstract the northern part of the indian plate in north pakistan is composed of a number of large scale crustal nappes each of which are stratigraphically distinct and which were stacked late in the main phase of southeasterly directed thrusting associated with the himalayan event the major nappes recognised in the swat to kaghan area of north pakistan are the besham swat hazara banna lower kaghan and upper kaghan nappes metamorphism was synchronous with early stages of deformation within each nappe the metamorphic grade increases upwards an overall inversion that represents post metamorphic imbrication within individual nappes synchronous with the main phase of nappe stacking rather than a hot iron type inversion as described under the mct in nepal and india as a result of this within nappe imbrication each thrust slice within any particular nappe contains rocks of a higher metamorphic grade than those in the slice below with sharp metamorphic breaks across the imbricating thrusts as well as across the major shears that bound the crustal scale nappes uplift along these imbricating thrusts initiated cooling of the stack k ar mica and hornblende cooling ages imply that much of this uplift was completed by 30 ma or within 20 ma of the collision

the contact between the higher himalaya crystallines and the tibetan sedimentary series miocene large scale dextral shearing space and time evolution of the synmetamorphic structures across the metamorphic pile have been studied in several areas along the himalayan belt east and central nepal garhwal zanskar from one area to the other the evolution is very similar 1 at the base of the pile in the main central thrust mct shear zone the stretching lineation penetrative and regularly oriented n0 e to n30 e indicates the mct transport direction very constant all along the belt from the eohimalayan main metamorphic development up to the late metamorphic movements 2 at the top of the pile at the contact between the crystalline unit and its sedimentary cover gravity driven structures are confirmed north vergent folds ductile normal faulting however there are numerous local indications of late miocene syn to late emplacement of the leucogranitic plutons dextral shearing 3 in between across the medium part of the pile the stretching lineation shows a conspicuous progressive regional clockwise rotation clearly indicated by the strain trajectories mapped in nepal and garhwal points 1 and 2 show that the crystalline sedimentary boundary despite its apparent structural and metamorphic continuity is not only a normal fault but also an important dextral shear zone which has acted since the upper miocene as the main southern limit of the eastward extruding tibetan block

structural evidence for back sliding of the kohistan arc in the collisional system of northwest pakistan in the naran region of the pakistan himalayas a regionally distributed second generation of folds results from northward directed shear deformation these folds are collapse structures associated with back sliding of the hanging wall namely the kohistan paleo island arc they are explained by a geometrical model that combines coeval vertical and horizontal shortening accordingly they are synconvergence collapse features that indicate at least 5 km of vertical shortening of the imbricate thrust slices derived from the indian continent in the northwest himalayas

kinematic reworking and exhumation within the convergent alpine orogen abstract kinematic data from the internal zones of the western alps indicate both top to se and top to nw shearing during synkinematic greenschist facies recrystallisation rb sr data from white micas from different kinematic domains record a range of ages that does not represent closure through a single thermal event but reflects the variable timing of synkinematic mica recrystallisation at temperatures between 300 and 450 c the data indicate an initial phase of accretion and foreland directed thrusting at ca 60 ma followed by almost complete reworking of thrust related deformation by se directed shearing this deformation is localised within oceanic units of the combin zone and the base of the overlying austroalpine basement and forms a regional scale shear zone that can be traced for almost 50 km perpendicular to strike the timing of deformation in this shear zone spans 9 ma from 45 to 36 ma the se directed shear leads to local structures that cut upwards in the transport direction with respect to tectonic stratigraphy and such structures have been interpreted in the past as backthrusts in response to ongoing alpine convergence however on a regional scale the top to se deformation is related to crustal extension not shortening and is coincident with exhumation of eclogites in its footwall during this extension phase deformation within the shear zone migrated both spatially and temporally giving rise to domains of older shear zone fabrics intercalated with zones of localised reworking top nw kinematics preserved within the combin zone show a range of ages the oldest 48 ma may reflect the final stages of emplacement of austroalpine units above piemonte oceanic rocks prior to the onset of extension however much of the top to nw deformation took place over the period of extension and may reflect either continuing or episodic convergence or tectonic thinning of the shear zone 40ar 39ar data from the region are complicated due to the widespread occurrence of excess 40ar in eclogite facies micas and partial ar loss during alpine heating reliable ages from both eclogite and greenschist facies micas indicate cooling ages in different tectonic units of between 32 and 40 ma these ages are slightly younger than rb sr deformation ages and suggest that cooling below ca 350 c occurred after juxtaposition of the units by se directed extensional deformation our data indicate a complex kinematic history involving both crustal shortening and extension within the internal zones of the alpine orogen to constrain the palaeogeographic and geodynamic evolution of the alps requires that these data be integrated with data from the more external zones of the orogen complexity such as that described is unlikely to be restricted to the western alps and spatially and temporally variable kinematic data are probably the norm in convergent orogens recognising such features is fundamental to the correct tectonic interpretation of both modern and ancient orogens

simulation of the metamorphic and deformational history of the metamorphic sole of the oman ophiolite emplacement of ophiolites onto less dense continental crust is an enigma the rates of emplacement the mechanisms of emplacement and the stress and temperature histories of the rocks are all incompletely understood the oman ophiolite is well exposed and has been studied extensively by field geologists an integrated thermal and kinematic model of the temperature stress rock type and displacement fields during the early stages of the emplacement of the oman ophiolite is developed here to explain field relations and provide insight into the development of the metamorphic rocks at the base of the ophiolite the thermal evolution was calculated by a finite difference algorithm for heat conduction considering heats of metamorphic reactions deformational heating and heat advection by rock the stress and displacement fields were calculated by an analytical model using a velocity boundary condition power law constitutive relations and a brittle frictional sliding relationship in simulations involving young hot subducted lithosphere a wider portion of the subduction zone deforms by power law creep and deformation occurs at lower stresses and slower strain rates minimizing the deformational heating compared to colder older subducted lithosphere dehydration occurs at an accelerated rate in younger hotter lithosphere removing heat from the subducted plate at an earlier stage and causing the zone of brittle deformation to propagate downward more rapidly power law creep of subducted basaltic rocks is limited to temperatures greater than 500 c and occurs at differential stresses of the order of 100 mpa the simulations predict that metamorphic field gradients and the spatial distribution of rock types in the metamorphic sole of tethyan ophiolites might be used to infer the emplacement direction of the ophiolite

foliation intersection axes preserved in garnet porphyroblasts from the swat area nw himalaya a record of successive crustal shortening directions between the indian plate and kohistan ladakh island arc abstract we have investigated the geometry and tectonic significance of porphyroblast inclusion trails in 61 oriented samples from the swat region of north pakistan nw himalaya the studied samples represent the indian plate cover sequence deformed and metamorphosed to medium and high grade during the himalayan orogeny two complementary techniques were applied to measure the orientation of foliation intersection inflection axes fia in porphyroblasts in the studied samples and reveal the existence of three principal fia sets in the study area measuring approximately 60 30 km each fia set is characterized by a distinctive regionally consistent trend and relative timing from old to young ese wnw fia set1 e w fia set 2 and broadly nne ssw fia set 3 this sequence is inferred to record an anticlockwise rotation of the direction of tectonic transport or bulk crustal shortening perpendicular to fia through time either progressively or in a stepwise fashion consistent with previous kinematic and geochronological data regarding collisions between the indian plate kohistan ladakh island arc klia and eurasia between ca 55 and 40 ma fia set 1 and 2 correlate well with early formed ese wnw and e w trending thrust related structures in the study area respectively whereas the trend of fia 3 matches that of relatively young regional scale folds and shear zones with a conspicuous n s trend hence the fia succession has successfully resolved the relative timing of formation between the e w versus n s trending structures in the region which was a longstanding question in the tectonics of the nw himalaya in addition a comparison of orientation data for symmetrical millipede inclusion trails versus asymmetrical curved trails indicates that porphyroblasts experienced very limited differential rotations in the region despite the complex polyphase deformation history recorded perhaps the most fascinating aspect of our findings is the direct relationship that is suggested between fia trends and successive vectors of relative indian plate motion since collision with the klia

differences in recent ground surface warming in eastern and western canada evidence from borehole temperatures because of the perturbation caused by a recent climatic warming most borehole temperature depth profiles in canada show reduced or negative gradients at shallow depths however the onset time and magnitude of the warming are different in eastern and western british columbia and southern yukon canada we determined the average subsurface temperature perturbations associated with the recent warming from borehole data for 34 and 51 respectively well distributed sites in these two regions of canada if the ground surface temperature is assumed to have increased linearly since and onset time the results indicate that the ground surface has warmed by 1 5 k since the mid 19th century in eastern canada but only by 0 8 k since the late 19th century in british columbia and southern yukon

ground temperature histories in eastern and central canada from geothermal measurements evidence of climatic change abstract inverse and direct methods have been used to analyze a large number of borehole temperature logs in order to infer past climatic changes results indicate a warming of 1 2 c in eastern and central canada during the past 150 years a period of cooling between 500 and 200 years before present corresponding to the time of the little ice age has also been identified in the same areas a regional ground temperature history is estimated for eastern and central canada from the simultaneous inversion of several temperature logs the inferred temperature changes appear correlated with the concentration of atmospheric carbon dioxide as reported from a greenland ice core and agree with existing meteorological and dendrochronological records for the area

miocene north vergent extensional displacements along the main mantle thrust nw himalaya pakistan structural data from n pakistan show that early tertiary s vergent structures within the upper part of the indian plate and the himalayan suture zone were overprinted by regionally developed n vergent extensional structures such extension has been suggested but not previously described from the nw himalaya cooling histories suggest extension was miocene in age extension was most likely the result of isostatic rebound resultant from crustal or lithospheric thickening and or delamination

estimations of the past groundwater recharge rate from deep borehole temperature data abstract in order to estimate the past groundwater recharge rates from deep borehole temperature data observed temperature depth profiles are compared with the profiles calculated from heat conduction convection equation under the condition of linear increase in surface temperature due to global warming and urbanization in tokyo metropolitan area the results show the increase in groundwater recharge rate from the 1890s to 1940s and it decreased from the 1940s to 1990s these estimations agree well with the precipitation record in tokyo during the last 100 years therefore the deep borehole temperature data can be used for estimating not only surface temperature history but also the past groundwater recharge rate in tokyo area

age of initiation of the india asia collision in the east central himalaya abstract we document the stratigraphy and provenance of the lower tertiary terrigenous sections in the zhepure shan region of the tethyan himalaya southern tibet using petrographic and geochemical whole rock and single grain techniques the cretaceous early tertiary shelf deposits of shallow marine carbonates and siliciclastics of the former indian passive margin near the western end of the zhepure shan are conformably overlain by lower tertiary clastic rocks sandstones in the jidula formation paleocene mostly contain monocrystalline quartz grains of cratonic origin in contrast significant amounts of immature framework grains with a distinct ophiolitic and volcanic arc influence are present in the youxia early eocene and shenkeza post early eocene formations major trace and rare earth element concentrations in both sandstones and shales complement the petrographic data and indicate that the source of the jidula formation consisted primarily of quartzose basement rocks probably of indian con

post metamorphic cooling history of the indian plate crystalline thrust stack pakistan himalaya a post metamorphic himalayan cooling history for the crystalline internal zones of the northern indian plate is defined by amphibole and mica k ar and ar ar geochronology hornblende muscovite and biotite cooling ages from cover sequences metamorphosed during the himalayan orogeny are 35 40 30 to 23 and 29 to 23 ma respectively the mica ages together with those derived from zircon and apatite fission track data demonstrate a cooling rate of about 30 c ma during the late oligocene to early miocene this rapid cooling was initiated during the post metamorphic development of the indian plate south verging crustal scale thrust stack most of the cooling occurred during the stripping of some 10 2 km of overburden through a combination of erosion recorded in the miocene molasse sediments of the foreland basin and major crustal extension within upper levels of the indian plate and the main mantle thrust zone both erosion and extension were the direct consequence of the evolution of the thrust stack

calibrating the bulk rheology of active obliquely convergent thrust belts and forearc wedges from surface profiles and velocity distributions the geometrical profiles and surface velocity distributions in active forearc wedges depend on the bulk rheology and the surface tractions analytical and numerical solutions of the force balance and comer flow circulation equations indicate that the mechanical parameters should in principle all be directly determinable from the observational variables the analysis suggests the following preliminary conclusions 1 the surface profiles of forearc wedges are con sistent with either a bulk plastic rheology or a viscous rheol ogy with a transition to plastic or coulomb behavior in the upper few kilometers 2 comer flow circulation in the absence of accretion is diagnostic of wedges with a bulk viscous rheology the surface strike normal velocity and the surface profile both depend on the bulk viscosity and the viscous coupling at the base allowing these mechanical vari ables to be determined current estimates of this velocity in some wedges suggest the bulk viscosity is not less than 3x10 9 pa s 3 obliquely converging nonaccreting plastic and coulomb wedges show no distributed strike parallel shear if the obliquity is above a critical angle the wedge is separated from the upper plate by a strike slip fault defining a forearc sliver the critical angle of obliquity the velocity of the forearc sliver and the wedge geometry allow the boundary tractions to be determined in obliquely convergent viscous wedges the strike parallel velocity is distributed throughout the wedge decaying exponentially from the rear to the front its value at the rear and the length scale for the decay allow the bulk viscosity and the coupling at the rear to be determined slip vectors in some forearc wedges and thrust belts suggest complete partitioning of the strike parallel component of motion characteristic of a bulk viscous rheology others show partitioning above a limiting angle of obliquity suggesting bulk plastic behavior there are no documented cases of active margins where there is an absence of partition ing for obliquity 45 o which appears to role out coulomb behavior as a general description for the bulk rheology of forearc wedges

creation and preservation of subduction related inverted metamorphic gradients peak metamorphic temperatures and recrystallization increase structurally upward in the lower plate of several paleosubduction zones in the westernmost u s cordillera inverted metamorphic gradients are preserved in the pelona orocopia schist southern california the catalina schist california borderland the central metamorphic belt klamath province the south fork mountain schist northern california coast ranges and possibly the shuksan suite northern washington with the exception of the south fork mountain schist peak metamorphic temperatures increase structurally upward from 400 c to 650 c over 1 2 km indicating metamorphic gradients in excess of 100 c km 1 estimates of metamorphic gradients in some terranes such as the catalina schist depend critically on the effects of postmetamorphic faulting the tectonic settings and high metamorphic pressures 800 mpa suggest that the inverted metamorphic gradients formed in paleosubduction zones four out of five inverted metamorphic gradients lie structurally beneath ultramafic hanging walls whereas the pelona orocopia schist formed beneath granitic crystalline rocks the high temperatures recorded by the metamorphic rocks strongly suggest that inverted metamorphic gradients form during the early stages of subduction and thus provide insight into the early thermal history of subduction zones two separate two dimensional numerical models were constructed in order to constrain the thermal conditions under which inverted metamorphic gradients are created and preserved rapid subduction 10 cm yr 1 beneath young oceanic lithosphere 10 ma results in rapid heat conduction downward from the hanging wall and the creation of inverted thermal gradients in excess of 100 c km 1 in the top portion of the downgoing slab in order to be preserved in the geologic record the inverted metamorphic gradient must be accreted to the base of the hanging wall accretion of the upper portion of the subducting slab may occur because of declining temperatures that cause downward migration of the viscous slip zone and or greenschist facies devolatilization reactions that weaken the descending slab alternatively inverted metamorphic gradients may represent the continuous accretion of material at rates of 1 10 mm yr 1 under conditions of declining temperature continuously accreted inverted metamorphic gradients could form in oceanic lithosphere as old as 30 ma continued subduction of 1000 km of oceanic lithosphere effectively refrigerates the early accreted inverted metamorphic gradient synsubduction uplift and or moderately fast postsubduction uplift rates of 1 mm yr 1 are required to preserve the inverted metamorphic gradient the models predict that 1 the high grade metamorphic rocks in inverted metamorphic gradients form early in the subduction process and therefore effectively date the initiation of subduction and 2 inverted metamorphic gradients mark sites of major plate convergence in excess of 1000 km

deformation thermometry based on quartz c axis fabrics and recrystallization microstructures a review abstract for quartz rich tectonites two types of deformation thermometer are currently commonly employed 1 the quartz c axis fabric opening angle thermometer that provides an estimate of deformation temperatures when fabrics were locked in during dislocation creep and dynamic recrystallization 2 the quartz recrystallization thermometer that indicates a range of likely deformation temperatures based on observed microstructures and inferred mechanisms of dynamic recrystallization a critically important caveat in applying both thermometers is the assumption that deformation temperature is the primary controlling factor in recrystallization mechanisms and fabric development however fabric opening angles and recrystallization mechanisms are also sensitive to other variables such as strain rate and water weakening in this paper the development of these thermometers is reviewed and their potential sensitivities to competing factors such as temperature strain rate water weakening and in the case of opening angles 3d strain type are discussed examples of the application of these potential thermometers to naturally deformed quartz rich rocks are given and case studies of correlations between deformation temperatures estimated by these thermometers and temperatures of synkinematic metamorphism determined by petrology based thermobarometers are highlighted in the review attention is focused on problems associated with applying these thermometers to natural deformation and examples of such problems are discussed

role of oblique convergence in the active deformation of the himalayas and southern tibet plateau noting similarities with subduction along curved oceanic trenches and using a simple block model we show that radial vergence evident in earthquake slip vectors along the himalayan deformation front east west extension on north trending normal faults in the himalayas and southern tibet and right lateral strike slip on the karakorum jiali fault zone can all result from basal shear caused by the indian plate sliding obliquely beneath tibet along a gently dipping arcuate plate boundary within the framework of this mechanism the normal faults in the himalayas and southern tibet are not proxies for the uplift history of tibet the distribution and style of the faults in the himalayas and southern tibet suggest that the basal drag from the underthrusting indian lithosphere extends northward beneath most of southern tibet

last 250 years climate reconstruction inferred from geothermal measurements in the czech republic abstract the subsurface of the earth has a certain capability to remember what has happened on its surface tens to hundreds of years ago ground surface temperature gst history reflecting past climate conditions can be evaluated by analysing the excursions left on the present day temperature depth distribution measured by precise temperature loggings in the boreholes temperature logs from fifteen selected holes were inverted into gst histories to assess the corresponding climatic changes these temperature records generally represent typical conditions existing on the territory of the czech republic in combination with the gst reconstructions from our previous works these data were used to assess the regional pattern of the last 250 year climatic change the gsts were obtained by applying the generalized least squares inversion method jackson d d 1972 interpretation of inaccurate insufficient and inconsistent data geophys j r astron soc 28 97 109 whereby the additional information on interdependence of both measured data and climate changes can be incorporated into the model in the form of covariance matrices three main episodes can be distinguished in the last 250 years a warm period between 1790 and 1830 a slightly colder period in 1940 1960 and a general warming with the average rates of 0 01 0 03 k year since then the geographical distribution of the recent warming rate may indicate an impact of human activities the verification of the gst assessment was accomplished by the independent past climate reconstructions gained by the use of historical sources and instrumental records

evolutionary model of the himalaya tibet system geopoem based on new modelling geological and geophysical data a two dimensional thermo mechanical laboratory modelling of continental subduction was performed the subducting continental lithosphere includes a strong brittle upper crust a weak ductile lower crust and a strong upper mantle the lithosphere is underlain by a low viscosity asthenosphere subduction is produced by a piston push force and the pull force from the mantle lithospheric layer which is denser than the asthenosphere the lithospheric layers are composed of material whose strength is sensitive to and inversely proportional to temperature throughout the experiment the model surface was maintained under relatively low temperature and the model base at higher temperature the subduction rate satisfied the peclet criterion modelling confirms that the continental crust can be deeply subducted and shows that slab break off delamination and tectonic underplating are fundamental events with drastic consequences on the subsequent evolution of the convergent system combining these results with previous purely mechanical modelling we elaborate a new evolutionary model for the himalaya tibet convergent system the principal successive stages are 1 subduction of the indian continental lithosphere to 200 250 km depth following subduction of the tethys oceanic lithosphere 2 failure and rapid buoyancy driven uplift of the subducted continental crust from ca 100 km depth to some depth that varies along the mountain belt 20 30 km on average 3 break off of the indian subducted lithospheric mantle with the attached oceanic lithosphere 4 subduction underplating of the indian lithosphere under asia over a few to several hundred kilometers 5 delamination roll back and break off of the indian lithospheric mantle 6 failure of the indian crust in front of the mountain belt formation of the main central thrust and underthrusting of the next portion of indian lithosphere beneath tibet for a few hundred kilometers at the beginning of stage 6 the crustal slice corresponding to the crystalline himalayas undergoes erosion activated uplift and exhumation fl 2000 elsevier science b v all rights reserved

quantitative kinematic flow analysis from the main central thrust zone nw himalaya india implications for a decelerating strain path and the extrusion of orogenic wedges quantitative kinematic indicators from the main central thrust zone mctz in the nw himalaya have been used to characterize the type of flow during deformation different generations of tension gashes have been rotated by variable angles with respect to the mylonitic foliation forming associated fringe folds these record the late stage brittle ductile flow and reveal that a strong pure shear component of deformation occurred throughout the mctz to characterize earlier deformation increments fabrics from highly deformed quartz ribbons were analyzed well developed shape and lattice preferred orientation patterns show a systematic change of the glide systems and suggest inverted palaeotemperatures within the mctz investigations of the c axes patterns reveal a strong asymmetry at the top of the mctz whereas the samples from the base of the mctz show almost perfectly symmetrical type i crossed girdles deformation within the mctz probably started close to simple shear flow at higher temperatures which progressively became a more general shear during cooling and ended in a pure shear dominated flow during the final stages of brittle ductile deformation i e a decelerating strain path using the higher himalaya crystalline as an example a kinematic model for the extrusion of crustal wedges above major thrust zones is suggested

paleogeographic and paleotectonic evolution of the himalayan range as reflected by detrital modes of tertiary sandstones and modern sands indus transect india and pakistan detrital modes of sandstones derived from the himalayan suture belt record the history of the mountain range since initial collision between india and asia which began in latest paleocene time tertiary clastic wedges deposited in fore arc foreland and remnant ocean basins and exposed along the indus transect from northernmost india to the arabian sea represent the best opportunity to study sedimentary responses to successive tectonic events during continental collision quartzose continental block and feldspatholithic magmatic arc sandstones were deposited respectively on the passive indian tethys himalayan succession and active asian indus group continental margins during late cretaceous paleocene time closure of the neotethys was marked by sudden arrival of volcanic and ophiolitic detritus on the passive continental margin of the indian plate during deposition of sediments dated at foraminiferal zones p6 pakistan to p8 india starting in early eocene time deposition of chulung la formation and murree supergroup volcanic and ophiolitic to metasedimentary detritus was accumulated in rapidly subsiding piggy back and foreland basins homogeneous petrographic composition within the eocene lower miocene murree supergroup with only slight progressive increase of detrital feldspars suggests erosion of largely supracrustal rocks involved in thrusting in the north in middle miocene time marked enrichment in medium to high grade metamorphic detritus in foreland sandstones siwalik group reflects rapid uplift of a warm wedge of indian crust which was carried southward along the main central thrust this major paleogeographic change was recorded also by quartzolithic remnant ocean turbidites which were fed great distances along transverse fracture zones and later accreted in the coastal makran subduction complex panjgur association and makran group recycled orogen detritus derived from the elevated himalayan chain is still accumulating today in the indus fan enrichment in feldspars with respect to ancient sandstones reflects deep erosion levels into mid crustal rocks along the core of the growing orogen

a case of obduction related high pressure low temperature metamorphism in upper crustal nappes arabian continental margin oman p t paths and kinematic interpretation in the southeastern oman mountains the saih hatat tectonic window reveals arabian continental material beneath the oceanic sensu lato nappes i e the hawasina sedimentary units and the huge sumail ophiolite the northeastern internal half of the window suffered an intense deformation associated with a high pressure low temperature hp lt metamorphism approximately dated at 80 75 ma according to the available stratigraphie data after the deformation due to the absence of collision the obduction inherited structural and metamorphic pattern was virtually unmodified a metamorphic map is established based on mineral assemblages in metapelites and metabasites low grade assemblages occur in the highest units muscat nappes quryat unit characteristic minerals are femg carpholite lawsonite associated with kaolinite or pyrophyllite and locally sudoite in the metapelites while lawsonite and blue amphiboles are found in the metabasites high grade rocks outcrop in the deepest and easternmost units as sifah antiform and are typified by an epidote glaucophane bearing eclogite assemblage intermediate units hulw show assemblages of intermediate grade chloritoid schists and glaucophane bearing metabasites higher grade assemblages such as talc chloritoid were not found inverted gradients are observed locally in the muscat nappes pyrophyllitebearing units above kaolinite quartz bearing units and these are ascribed to late metamorphic thrust displacements which are demonstrated by microstructures a complex p t evolution is fairly well documented by the mineral assemblages prograde evolution in the subophiolitic continental rocks began with a rather hot gradient of about 30 35 c km with chloritoid growth in the higher units thereafter pressure increased with the temperature remaining constant or even decreasing chloritoid replaced by kaolinite oxides and carpholite growth in synkinematic conditions the peak p t conditions are close to 8 kbar 270 c for the muscat nappes and 11 kbar 400 c for the as sifah unit the corresponding surface related gradient is low 10 15 c km during the retrograde evolution pressure and temperature generally decreased concomitantly but in some of the upper units temperature remained constant or increased slightly for some time at which point sudoite growth from carpholite took place the metamorphic pattern and mineral evolution are interpreted in the framework of the late cretaceous obduction the already cold oceanic lithosphere was thrust upon a relatively hot thinned passive margin with coeval basaltic volcanism pressure progressively increased in the passive margin due to the wedge shape of the advancing ophiolite the maximum thickness of which was about 25 km in the as sifah area in this area the overburden was also attributable to the accumulation of continental material up to 5 km thick by means of a complex set of synmetamorphic often conjugated folds and thrusts the basal peridotites of the obducted slab were probably not hotter than 300 c during its thrusting upon the continental margin the temperature of the tectonic pile below the ophiolite remained low because it was thrust above more external and still colder continental areas late thrusting in the same southerly direction controlled the retrograde evolution together with erosional and gravity driven tectonic unloading

age of initiation of collision between india and asia a review of stratigraphic data abstract the collision of india with asia is perhaps the most profound tectonic event to have occurred in past 100 ma it is responsible for the uplift of the himalayas and tibetan plateau and has been argued to have been responsible for geological geochemical and climatological consequences of global extent yet the age of initiation of this collision remains poorly constrained the literature is replete with estimates that range from the late cretaceous 65 ma to latest eocene

ground warming patterns in the northern hemisphere during the last five centuries abstract changes in the earth s surface energy balance recorded underground were used to reconstruct the temperature of the ground surface for the last 500 years in the northern hemisphere we reconstructed ground surface temperature histories gsths from temperature versus depth profiles measured at 558 sites distributed between 30 and 60 n in the northern hemisphere we show that the ground has warmed about 0 5 k in the last 100 years spatial analysis reveals that spatial variability is important and that the weighted average northern hemisphere gsth shows some consistency with multiproxy and meteorological records reconstructions for the last two centuries

exhumation of early tertiary coesite bearing eclogites from the pakistan himalaya early tertiary coesite bearing eclogites crop out in the internal zones of the indian plate in the kaghan valley of north pakistan field and geochronological data suggest that uhp metamorphism at c 46 ma accompanied subduction of the leading edge of continental india beneath kohistan a new u pb rutile age of 44 1 1 1 ma can be interpreted as either a growth or a cooling age given the uncertainty of the closure temperature of rutile for pb but clearly documents a very short lived metamorphic peak with cooling occurring between 44 and 40 ma eclogitic assemblages are preserved in permo triassic mafic rocks in both cover and basement sequences with the best preserved eclogites hosted in the anhydrous basement the eclogite facies rocks lie on the hanging wall of a major south directed ductile thrust zone which placed basement rocks that contain coesite eclogites southward onto lower pressure cover metasediments shear criteria show that those sequences that contain eclogite facies rocks are flanked above by a zone of pervasive north directed extensional shearing mineral assemblages contained within both the thrust and extensional shear zones are consistent with them both having operated synchronously during the amphibolite to greenschist facies transition amphibole ar ar cooling history data suggest that this was at c 40 42 ma as in the alps this implies exhumation from the metamorphic peak to greenschist facies conditions within a few million years this short time scale has important implications as it is likely that both the thrust and extensional shear zones represent the late stage of a deformation continuum that commenced at uhp and brought the uhp rocks back to mid crustal levels this late eocene extensional shearing and associated exhumation of hp or uhp rocks has not previously been recorded in the pakistan himalaya the thrusts and shears were later folded by major east trending folds themselves cut by miocene aged north vergent extensional faults that currently separate the indian plate from the structurally overlying kohistan island arc

fault systems and paleo stress tensors in the indus suture zone nw pakistan abstract analysis of fault striations measured in the kohistan part of the indus suture zone nw himalaya pakistan has been carried out to document dynamic evolution during the brittle stage of the collision of india and asia processing of the data with a direct inversion method identified four stress fields which were chronologically ordered from field evidence as sse nnw compression e w compression radial extension and ssw nne compression the last corresponds to the present day stress field defined from seismic activity the earlier stress fields are related to times during the miocene when convergence related stresses were disturbed by the formation of the nearby nanga parbat and indus syntaxes

dynamic recrystallization of quartz correlation between natural and experimental conditions quartz veins in the eastern tonale mylonite zone italian alps were deformed in strike slip shear due to the synkinematic emplacement of the adamello pluton a temperature gradient between 280 c and 700 c was effected across this fault zone the resulting dynamic recrystallization microstructures are characteristic of bulging recrystallization subgrain rotation recrystallization and grain boundary migration recrystallization the transitions in recrystallization mechanisms are marked by discrete changes of grain size dependence on temperature differential stresses are calculated from the recrystallized grain size data using paleopiezometric relationships deformation temperatures are obtained from metamorphic reactions in the deformed host rock flow stresses and deformation temperatures are used to determine the strain rate of the tonale mylonites through integration with several published flow laws yielding an average rate of approximately 10 14s 1 to 10 12s 1 the deformation conditions of the natural fault rocks are compared and correlated with three experimental dislocation creep regimes of quartz of hirth tullis linking the microstructures of the naturally and experimentally deformed quartz rocks a recrystallization mechanism map is presented this map permits the derivation of temperature and strain rate for mylonitic fault rocks once the recrystallization mechanism is known