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

zircon u pb geochronology of the nyainqentanglha group from the lhasa terrane new constraints on the triassic orogeny of the south tibet abstract the studied amphibolite facies metamorphic rocks typical of the nyainqentanglha group occur as a tectonic block in the central lhasa terrane south tibet and consist mainly of gneiss amphibolite quartz schist and marble the amphibolite has a similar mineral paragenesis of plagioclase garnet biotite amphibole quartz muscovite and were metamorphosed under p t conditions of 0 63 0 66 gpa and 588 622 c zircons from three dated samples include two types first type of zircons has an inherited magmatic core and a metamorphic rim whereas the other type of zircons is of metamorphic origin la icp ms in situ u pb analysis indicates that the inherited magmatic cores of zircons yield widely variable 206 pb 238 u ages ranging from 1857 ma to 743 ma recording the magmatic thermal events of the material source of the metamorphic rock protoliths in contrast the metamorphic zircons yield concordant 206 pb 238 u ages of 225 213 ma the results indicate that the nyainqentanglha group has been subjected to triassic metamorphism instead of the precambrian metamorphic basement of the lhasa terrane as considered traditionally these rocks together with the coeval sumdo eclogite and pana blueschist form a permian to triassic metamorphic belt with an e w extension of up to 300 km therefore this work provides new constraints on the late paleozoic to early mesozoic orogeny in the central lhasa terrane south tibet

brunhes matuyama polarity transition in three deep sea sediment cores the brunhes matuyama polarity transition was studied in three deep sea sediment cores obtained from the mid northern and equatorial pacific ocean rates of sedimentation vary from 0 7 to 1 1 cm k a 1 the cores were sampled across the transition in 4 mm slices and each level was further subdivided into three specimens inclinations above and below the transition interval are in close agreement with that of an axial dipole field estimates of transition duration based on directional change range from 4900 to 8500 years the transition can be described by a longitudinally confined portion in each of the v g p paths but is complicated by broad loops in all three records while fine scale sampling reveals considerable detail the notable increase in within level dispersion observed in the transition and the apparent differences in v g p paths in nearby cores from the same region indicate that factors other than the geomagnetic field may contribute to these transition records

successive palaeomagnetic reversal records from kauai geomagnetic field reversal is a rapid phenomenon1 2 perhaps taking only a few thousand years knowledge of how the field behaves during these brief and active periods may eventually help to constrain theories of the geodynamo and thus inform us about conditions in the earth s core unfortunately reliable and detailed palaeomagnetic records of transitional field behaviour are few almost entirely from northern hemisphere sites and heavily biased toward reversals of the reverse normal r n sense we report here new data from basalts on the island of kauai hawaii which have recorded successive r n and n r field reversals the sequences of changing field directions for the two reversals are very similar as would be expected if a standing nondipole component controlled the transitional field the data are also consistent with zonal flooding models in which the reversal process depends on the sign of the field other reversal records however cannot be explained by these same models unless the standing field or flooding process changes between some reversals

temporal variations in the cooling and denudation history of the hunza plutonic complex karakoram batholith revealed by 40ar 39ar thermochronology the 40ar 39ar thermochronology of the late cretaceous hunza plutonic complex reveals an episodic cooling and denudation history for this regional scale pluton the 40ar 39ar analyses of biotites from a vertical relief section of 3200 m reveal a pulse of rapid cooling at 20 ma in the interval of 110 27 ma age elevation distributions suggest denudation rates of the order of 0 02 0 003 mm yr at 20 ma denudation rates increased significantly to 2 7 0 7 mm yr then returned to much slower rates until 12 ma a second pulse of rapid cooling beginning at 12 ma is revealed by inverse numerical modeling of multidiffusion domain alkali feldspars from a vertical section of 1700 m decreasing in elevation these samples record the onset of rapid cooling at 12 9 and 7 ma respectively all of the alkali feldspars record a period of nearly isothermal conditions prior to the onset of rapid cooling when rates increased to 30 c m y assuming a geothermal gradient of 30 c km these cooling rates translate into denudation rates of 1 0 mm yr apatite fission track analysis indicates denudation rates of 0 7 0 1 mm yr over the interval of 6 6 ma to 2 4 ma in agreement with the alkali feldspar data these data suggest denudation of 2 9 0 4 km since the plio pleistocene together the alkali feldspar and apatite data indicate that a minimum of 10 km of overburden has been removed since the mid late miocene an electronic supplement of this material may be obtained on a diskette or anonymous ftp from kosmos agu org login to agu s ftp account using anonymous as the username and guest as the password go to the right directory by typing cd apend type ls to see what files are available type get and the name of the file to get it finally type exit to leave the system paper 95tc02424 temporal variations in the cooling and denudation history of the hunza plutonic complex karakoram batholith revealed by 40ar 39ar thermochronology m a krol p k zeitler g poupeau and a pecher diskette may be ordered from american geophysical union 2000 florida avenue n w washington dc 20009 15 00 payment must accompany order

lower pliocene polarity transitions from the suva marl fiji we present a paleomagnetic record of the lower cochiti polarity transition and new results from the lower and upper nunivak transitions obtained from the lower pliocene suva marl fiji 18 s 178 e this formation has yielded a well defined magnetostratigraphy that combined with 40ar 39ar ages from interbedded tuff beds provides excellent chronostratigraphic control clement et al 1997 we previously presented the upper cochiti and the upper and lower nunivak polarity transition records from this unit clement et al 1995 1996 new results were obtained from oriented block samples spanning these reversals each of these reversals was sampled at duplicate sections separated by 4 to 8 km the agreement between the records obtained from different sections suggests that the paleomagnetic records are not artifacts of local sedimentary disturbances or random degrees of incompletely removed overprints instead the similarities in these and other records of these same reversals from different types of paleomagnetic recorders suggest that they provide information about the behavior of the geomagnetic field as it reverses

the upper and lower thvera sedimentary geomagnetic reversal records from southern sicily detailed paleomagnetic records of the upper and lower thvera polarity transitions have been determined from pliocenc marine marls in southern sicily the dominant magnetic mineral is fine grained magnetite the two transitions have vgp paths following a great circle passing south america and the west coast of north america there are strong indications that the vgp paths result from smoothing of the non antipodal stable directions before and after the transitions the upper thvera transitional record is preceded by two excursions and followed by a third one the first excursion is a sedimentary artefact caused by post depositional migration of magnetic minerals and the third one by is caused by weathering of the sediment the upper thvera transition from sicily is compared with the record from calabria about 250 km away the two records show similarities as well as differences both transitions have identical vgp paths but in calabria the transition is recorded lower in the sediment the first and third excursions have different characters and the second one is not present apparently the registration of transitions of the earth s magnetic field in sediments is strongly influenced by smoothing and diagenetic processes after deposition

discovery of a crystalline basement and early ordovician marine transgression in the karakorum mountain range pakistan a granite pluton intruding low grade quartzites and migmatites lies north of the karakorum axial batholith of cretaceous to miocene age the pluton is covered by transgressive litharenite and slate that contain a fairly rich assemblage of acritarchs of arenigian age early ordovician belonging to the peri gondwana biogeographic province this is the first time that crystalline basement has been found in karakorum and acritarchs in central asia these discoveries add new evidence that the karakorum microplate together with the helmand block of central afghanistan and the lhasa block of tibet belongs to a cimmerian domain at the northern fringe of gondwana

stress field evolution in the northwest himalayan syntaxis northern pakistan 1 we have conducted a systematic inversion of striated fault planes throughout northern pakistan in order to better depict the temporal and spatial variations in stress patterns two domains are evidenced at a regional scale separated by the active raikhot fault the western boundary of the nanga parbat spur west of this fault a wrench type stress field with 1 axis oriented around n s predominates in the karakorum and in kohistan it predates pliocene quaternary exhumation of nanga parbat and corresponds to the miocene or earlier regional stress field related to indian asian convergence east of the raikhot fault compression parallel to the belt accounts for initiation of the nanga parbat anticlinorium after 5 ma it is followed by predominant post 2 ma extension both parallel to the belt and nne ssw oriented thus in the n w himalayan syntaxis multidirectional extension is juxtaposed on short timescales to shortening either parallel or perpendicular to the belt such juxtaposition could be characteristic of strain and stress partitioning during oblique convergence

collision tectonics in the nw himalayas west himalayan tectonics involve the collision of microplates between the indian and asian plates the kohistan complex consists largely of tightly folded basic volcanics and sediments generated as late jurassic to late cretaceous island arcs these were intruded by post folding mid cretaceous eocene plutonics produced from continued subduction of the indian plate after closure of a suture between kohistan and the karakorum the himalayan structures show major thrust sheets and the kohistan arc is essentially a crustal pop up with southward upright and northward verging structures developed above a thick ductile decoupling zone the indus suture which can be traced for 100 km beneath kohistan on large reentrants this pop up formed by a two stage process closure of the northern suture followed by closure of the southern indus suture granitic rocks of the kohis tan ladakh batholith dated at 100 40 ma post date most of the structures related to the northern suture but were deformed and carried southwards on shear structures related to the indus suture postcollisional deformation carried this kohistan complex on deep decoupling zones over the indian plate on a series of imbricated gneiss sheets the thrusts climbing up section in the movement direction so that in the far s some override their own molasse debris folds above these deep decoupling zones deformed their overlying thrust sheets into large antiforms i e the nanga parbat and hazara syntaxes the nanga parbat syntaxis probably formed due to a shear couple near a branch line where one of the main himalayan thrusts joined the indus suture beneath kohistan crustal delamination to produce the imbricated gneiss sheets could not account for all the displacement of india into asia suggested by palaeomagnetic data there must also have been lateral displacement as demonstrated by the large oblique slip shear zone in the hunza valley n of kohistan

middle late jurassic tectonostratigraphic evolution of central asia implications for the collision of the karakoram lhasa block with asia mesozoic basins in different regions of central asia provide important records for investigating relationships between intraplate deformation in central asia and tectonic processes at asian boundaries the present study gives a review of the stratigraphic and structural evolution of basins in different regions of central asia during the middle late jurassic it is shown that basins and mountain belts in northwest china experienced compressional deformation and were wholly or partially uplifted during the late middle late jurassic compared to extensively distributed middle jurassic coal bearing strata in northwest china upper jurassic strata characterized by red mudstones and conglomerates have a much smaller distribution in the mean time the tibet pamir plateau also underwent a folding and uplift event and upper jurassic sedimentary rocks are generally missing in the pamir and western tibet the intense compressional deformation and uplift event of the late middle late jurassic from the tibet pamir plateau to northwest china requires a new tectonic model as proposed here we suggest that the karakoram and lhasa blocks were a single giant block which was accreted to asia in the late middle jurassic earliest cretaceous and cross cut by the karakoram fault in the cenozoic during the callovian the western part of the karakoram lhasa block initially collided with the southern asian margin collision and continued convergence during the late middle late jurassic caused sinistral strike slip faulting along the central badakhshan fault and south tian shan suture accommodating crustal shortening in areas to the southeast of the faults the pamir western tibet tarim block qilian qaidam block and bei shan meanwhile the northeastward transpressional motion of the tarim block produced strong compressional stresses to areas north of the tarim block the kyrgyz tian shan central tian shan junggar basin and turfan basin with the northward movement of the karakoram lhasa block the eastern part of the karakoram lhasa block began to collide with the southern asian margin during the latest jurassic earliest cretaceous resulting in strong crustal deformation and thickening in east asia and central asia

a database for the matuyama brunhes magnetic reversal abstract we present a paleomagnetic database designated mbd97 comprising sedimentary and lava data for the matuyama brunhes reversal transition field using criteria regarding suitability of the rock samples measurement methods and temporal resolution of the data we consider 62 different studies of the matuyama brunhes reversal of these only 11 satisfy our selection criteria four lava sites one loess site three shallow marine sedimentary sites and three deep ocean sedimentary sites we discuss our reasons for accepting or disguarding each candidate data set we investigate the distribution of transitional virtual geomagnetic poles vgps they fall along american and asian australian longitudes the spatial and temporal complexity of the data indicate that the transitional field was almost certainly nonaxisymmetric and nondipolar the directional transition of the matuyama brunhes reversal took 2300 5000 years during which time the surface intensity fell to 10 20 of its pre and post transitional value we do not find any systematic bias due to site distribution or inclination shallowing although the errors in the data do vary over the duration of the reversal mbd97 is a small database but it has a fairly good geographic distribution and is suitable for studies of the matuyama brunhes transition nonetheless it is clear that further analysis of transitions will benefit from a continued program of data collection and analysis the data in mbd97 are listed

geographical distribution of transitional vgps evidence for non zonal equatorial symmetry during the matuyama brunhes geomagnetic reversal abstract paleomagnetic records of the matuyama brunhes polarity reversal obtained recently from the northern equatorial and southern latitudes of the atlantic sector display a distinct symmetry with respect to the equator the virtual geomagnetic pole vgp paths from the mid latitude sites in the northern and southern hemispheres dsdp site 609 and core v16 58 are nearly coincident tracking northward through the americas the vgp path from the equatorial site odp site 664 tracks northward through eastern asia nearly antipodal to the other two paths modeling results indicate that superimposing an h 1 3 geometry onto a reversing axial dipole explains the sense of equatorial symmetry exhibited by these three records these results are consistent with the bulk of the data available from other records of this polarity transition suggesting that the transitional vgps during this reversal tend to fall along two distinct longitudinal bands the grouping of transitional vgps suggests that there is a strong geographical control over the reversal process an apparent correlation between the longitudinal bands of transitional vgps and the location of the major flux concentrations in the radial component of the historical geomagnetic field at the core mantle boundary 1 suggests that the processes which cause these flux concentrations may have influenced the geometry of the matuyama brunhes transitional field

cenozoic evolution of the eastern pamir implications for strain accommodation mechanisms at the western end of the himalayan tibetan orogen detailed field mapping geochronologic and thermochronologic analyses and petrologic investigations conducted along the southern segment of the late cenozoic kongur shan extensional system provide new information on the cenozoic tectonic evolution of the eastern pamir at the western end of the himalayan tibetan orogen field relations and cooling age patterns in the hanging wall and footwall of the active faults show a southward decrease in the magnitude of east west extension along the southern kongur shan extensional system from 20 km or less along the muztaghata massif in the north to l3 km along the tashkorgan fault in the south these results in conjunction with previously published work on the northern segment of the kongur shan extensional system show a general southward decrease in east west extension along the entire length of the extensional system consistent with models of extension primarily driven by oroclinal bending or radial thrusting of the pamir petrologic data 40 ar 39 ar cooling ages and monazite th pb ages from schists and gneisses in the footwall of the southern kongur shan normal fault along the muztaghata massif record two tectonic events that immediately preceded late miocene initiation of east west extension 1 high grade schists and gneisses experienced upper amphibolite facies metamorphic conditions 9 10 kbar 700 750 c dated as late oligocene to middle miocene by in situ ion microprobe analyses of monazite inclusions in garnet and 2 high grade schists and gneisses were subsequently rapidly exhumed to shallow crustal levels in the late miocene with 40 ar 39 ar biotite cooling ages of 7 5 9 ma rapid exhumation was accommodated in part by the east west striking south dipping shenti normal fault field relations and regional geologic correlations indicate that this exhumation event was related to the formation of the central pamir gneiss domes and the antiformal muztaghata massif is the eastward continuation of the sares dome of the central pamir these observations suggest that the antiformal gneiss domes of the central pamir have not been offset across the karakorum right slip fault from the qiangtang anticlinorium in tibet instead we propose that the development of the central pamir gneiss domes may have been related to oligocene miocene northward underthrusting and thickening of crust beneath the pamir

middle crustal ductile deformation patterns in southern tibet insights from vorticity studies in mabja dome kinematic kinematic vorticity wm and deformation temperature analyses were performed to test the hypothesis that mid crustal rocks exposed in mabja dome southern tibet were penetratively deformed within a southward flowing mid crustal channel during the late eocene early oligocene to early miocene outcrop and thin section kinematic indicators show a downward transition from mixed top n and top s shear in chloritoid and garnet zone rocks through dominantly top s shear in garnet and kyanite zone rocks to solely top s shear in staurolite zone and deeper rocks along mineral elongation lineation parallel transects wm in schists and orthogneisses decreases with structural depth from w0 80 w40 pure shear to w0 55 w63 pure shear deformation temperature increases from w450 ci n

the zonal harmonic model of polarity transitions a test using successive reversals a recently developed zonal model for the last geomagnetic field reversal which describes time and latitude dependent transitional behavior of intensity and inclination in terms of dominance of low order field harmonics was tested using a latitudinal and chronological succession of transition records the primary data were derived from a high resolution study of five matuyama to brunhes deep sea cores collected along a 40 meridional band in the north central pacific the transitions analyzed were the on sets and terminations of the olduvai and jaramillo events and the matuyama brunhes boundary supplementary data published by previous workers included a jaramillo onset record from the southern indian ocean and a transition recorded in the triassic chugwater formation of north america in a general sense the measured inclination and intensity records indicate the jaramillo transitions and the last reversal to be remarkably similar to each other and to differ from the olduvais crosscorrelation of the measured data sets with synthetic zonal harmonic records for the core latitudes indicates that the zonal harmonic model does predict the behavior of matuyama brunhes reversal in the northern hemisphere for the older reversals however the present model requires substantial modification this is particularly so in the case of the jaramillo onset although two inclination and intensity records of this transition from northern and southern hemisphere sediments strongly resemble each other the current model produces a synthetic record for one hemisphere that is incompatible with that hemisphere s measured data further to model zonally the olduvai reversals for which a wide latitudinal northern hemisphere sampling was available a substantially different ratio of low order multipole components from that of the standard model is required and the modeled solution was unsatisfactory at very low latitude a modified zonal harmonic approach which introduces a small sectoral component may offer a solution to the incompatibilities uncovered by the present study

the matuyama brunhes transition recorded from lake tecopa sediments california abstract the paleomagnetic record of the matuyama brunhes reversal in the sediment of ancient lake tecopa provided the first evidence that the geomagnetic field during transition is not dipolar we resampled the section to increase the time resolution of the record our new data show that the characteristic direction could not be completely isolated by af demagnetization the transitional directions obtained from thermal demagnetization are thus quite different from those of the previous study but the primary conclusion that the field was non dipolar remains the vgp paths lying close to the site longitude is consistent with an axisymmetric transition field

thermobarometric modelling of zircon and monazite growth in melt bearing systems examples using model metapelitic and metapsammitic granulites u pb age data collected from zircon and monazite are used to draw fundamental inferences about tectonic processes in the earth despite the emphasis placed on zircon and monazite ages the understanding of how to relate the timing of growth of zircon and monazite to an evolving rock system remains in its infancy in addition few studies have presented large datasets of geochronological data from zircon and monazite occurring in the same metamorphic rock sample such information is crucial for understanding the growth of zircon relative to monazite in a systematic and predictive manner as per this study the data that exist support the generally held conception that zircon ages tend to be older than monazite ages within the same rock here experimental data for zircon and monazite saturation in melt bearing rocks are integrated with phase diagram calculations the calculations constrain the dissolution and growth behaviour of zircon and monazite with respect to evolving pressure temperature and silicate mineral assemblages in high grade melt bearing metasedimentary rocks several key results emerge from this modelling first that in aluminous metapelitic rocks i e garnet cordierite sillimanite assemblages zircon ages are older than monazite ages in the same rock second that the growth rate of accessory minerals is nonlinear and much higher at and near saturation than at lower temperatures and third that the difference in zircon and monazite ages from the same rock may be ascribed to differences in the temperature s at which zircon and monazite grow rather than differences in closure temperature systematics using our methodology the cooling rate of granulites from the reynolds range central australia have been constrained at 4 c myr 1 this study serves as a first pass template on which further research in applying the technique to a field study can be based

a southern hemisphere record of the matuyama brunhes polarity reversal the authors present a record of the matuyama brunhes 0 73 ma polarity transition from a southern hemisphere deep sea sediment core v16 58 46 degree s 30 degree e the transition is recorded across at least 30 sm of section and is defined by a nearly 180 degree change in directions which occurs during a low in the relative intensities an increase in the sup 10 be sup 9 be ratios associated with the intensity low suggests that the relative intensities may document a decrease in the strength of the geomagnetic field the virtual geomagnetic pole vgp path is neither clearly near or far sided but tracks approximately 120 degree west of the site longitude considered with other matuyama brunhes transition records this southern hemisphere record supports previous interpretations that the matuyama brunhes transitional field was not dipolar instead the v16 58 transition together with other records of this reversal recently obtained from this longitudinal sector suggests that nonzonal terms which are symmetric about the equator were important during this transitional field

tectono metamorphic evolution of the karakorum metamorphic complex dassu askole area ne pakistan exhumation of mid crustal ht mp gneisses in a convergent context the south karakorum margin east of the himalayan syntaxis consist of an e w elongated zone of young 10 3 ma high grade metamorphic rocks m2 and related migmatitic domes this late tectono metamorphic event post dates the palaeogene 55 37 ma phase of thickening of the belt featured by nw se structures and associated m1 amphibolite facies metamorphism 0 7 gpa 700 c this m2 metamorphism is characterised by low pressure high temperature conditions coeval with migmatite formation in response to a thermal increase of c 150 c compared to m1 culminating at a temperature of c 770 c and a pressure of 0 5 0 6 gpa rapid exhumation of migmatitic domes at a rate of 5 mm yr 1 was accommodated by vertical extrusion in the core of e w crustal scale folds these crustal scale folds formed in response to n s syn collisional shortening and were enhanced by thermal weakening of the migmatised continental crust m2 metamorphism is spatially and temporarily associated with granitoids showing a mantle affinity firmly suggesting that this could be the advective heat source for the granite and syenite generation and the subsequent migmatisation of the mid crustal level such relationships between a mantle related magmatism and a high temperature metamorphism in a convergent shortening context are suggestive of the breakoff of the subducted indian slab since 20 ma

oligocene miocene middle crustal flow in southern tibet geochronology of mabja dome new u pb zircon monazite 40 ar 39 ar and apatite fission track ages provide constraints on the timing of formation and exhumation of the mabja dome southern tibet shed light on how this gneiss dome formed and provide important clues on the tectonic evolution of middle crustal rocks in southern tibet zircons from a deformed leucocratic dyke swarm yield a u pb age of 23 1 0 8 ma providing the first age constraint on the timing of middle crustal ductile horizontal extension in the north himalayan gneiss domes zircons and monazite from a post tectonic two mica granite yield ages of 14 2 0 2 ma and 14 5 0 1 respectively indicating that vertical thinning and subhorizontal stretching had ceased by the middle miocene mica 40 ar 39 ar ages from schists and orthogneisses increase structurally down section from 12 85 0 13 ma to 17 0 0 19 ma and then decrease at the deepest structural levels to 13 29 0 09 ma micas from the leucocratic dyke swarm and post tectonic two mica granites yield similar 40 ar 39 ar cooling ages of 13 48 0 13 to 12 84 0 08 ma the low temperature steps of potassium feldspar 40 ar 39 ar spectra yield ages of c 11 0 12 5 ma and apatite fission track analyses indicate the dome uniformly cooled below c 115 c at 9 5 0 6 ma based on these data calculated average cooling rates across the dome range from c 40 60 c million years in schist and orthogneiss and following emplacement of the leucocratic dyke swarm to c 350 c million years following emplacement of the two mica granites the mylonitic foliation peak metamorphic isograds and mica 40 ar 39 ar chrontours are domed whereas the low temperature step potassium feldspar 40 ar 39 ar and apatite fission track chrontours are not suggesting that doming occurred between 13 0 and 12 5 ma and at temperatures between 370 and 200 c our new ages along with field structural and metamorphic data indicate that the domal geometry observed at mabja developed by middle miocene southward directed thrust faulting upward and southward along a north dipping ramp above cold tethyan sediments the structural metamorphic and geochronologic histories documented at mabja dome are similar to those of kangmar dome suggesting a common mode of occurrence of these events throughout southern tibet vertical thinning and horizontal stretching metamorphism generation of migmatites and emplacement of leucogranites in the domes of southern tibet are synchronous with similar events in the greater himalayan sequence that underlie the high himalaya these relations are consistent with previously proposed models for a ductile middle crustal channel bounded above by the south tibetan detachment system and below by the main central thrust in the high himalaya that extended northward beneath southern tibet

high resolution record of the upper olduvai transition from po valley italy sediments support for dipolar transition geometry abstract a detailed record of the upper olduvai polarity transition composed of 100 transitional directions has been obtained from the crostolo section in northern italy a careful examination of the rock magnetic properties of the sediments using standard paleomagnetic techniques thermomagnetic scanning electron microscopy microprobe x ray diffraction and mossbauer analysis shows that greigite is the main magnetic carrier of the remanence the correlation of a shift in the magnetic record to a small scale sedimentary feature observed in the section indicates that the magnetization was acquired at deposition or very shortly after the virtual geomagnetic pole vgp path is largely confined along a great circle over north and south america 90 west of the site and consists of three stages first the vgp moves to southern mid latitudes then after a period of standstill it comes back to almost true north and finally the south pole is reached in a third step although different from a record from the southern indian ocean this path is virtually identical to those obtained for the same transition from north pacific deep sea cores and partly coincides with a north atlantic record which suggests that a dipolar component may be present in the transitional field during the upper olduvai reversal a review of the recently obtained records of various transitions shows that in more than two thirds of the cases the vgp paths are similarly confined along a meridian over the americas or antipodal to them irrespective of the sampling site and of the sense of the transition although not deterministic this tendency suggests that a similar dipolar component might be present in the transitional fields of other reversals of different ages

dome formation and extension in the tethyan himalaya leo pargil northwest india metamorphic dome complexes occur within the internal structures of the northern himalaya and southern tibet their origin deformation and fault displacement patterns are poorly constrained we report new fi eld mapping structural data and cooling ages from the western fl ank of the leo pargil dome in the northwestern himalaya in an attempt to characterize its post middle miocene structural development the western fl ank of the dome is characterized by shallow west dipping pervasive foliation and wnw ese mineral lineation shear sense indicators demonstrate that it is affected by east west normal faulting that facilitated exhumation of highgrade metamorphic rocks in a contractional setting sustained top to northwest normal faulting during exhumation is observed in a progressive transition from ductile to brittle deformation garnet and kyanite indicate that the leo pargil dome was exhumed from the mid crust 40 ar 39 ar mica and apatite fi ssion track aft ages constrain cooling and exhumation pathways from 350 to 60 c and suggest that the dome cooled in three stages since the middle miocene 40 ar 39 ar white mica ages of 16 14 ma suggest a fi rst phase of rapid cooling and provide minimum estimates for the onset of dome exhumation aft ages between 10 and 8 ma suggest that ductile fault displacement had ceased by then and aft track length data from high elevation samples indicate that the rate of cooling had decreased signifi cantly we interpret this to indicate decreased fault displacement along the leo pargil shear zone and possibly a transition to the kaurik chango normal fault system between 10 and 6 ma aft ages from lower elevations indicate accelerated cooling since the pliocene that cannot be related to pure fault displacement and therefore may refl ect more pronounced regionally distributed and erosion driven exhumation

preliminary results of a study of four successive sedimentary geomagnetic reversal records from the mediterranean upper thvera lower and upper sidufjall and lower nunivak abstract the results of a study of four successive early pliocene geomagnetic reversal records are presented the polarity transitions have been recorded in the trubi formation of calabria s italy which is composed of alternating carbonate poor and carbonate rich marly layers with moderately high 5 cm ka 1 accumulation rates the multicomponent nrm contains a well defined primary component in which two dominant magnetic phases can be recognized a magnetite and a low temperature lt i e removed at moderate temperatures phase most probably the lt phase is pyrrhotite the combined presence of pyrrhotite and magnetite in the sediment restricts the validity of a xarm x test for paleointensity studies the magnetite magnetizations reveal a smoother record relative to the records of the lt phase magnetizations between the two phases no significant delay in the moment of remanence acquisition is observed the durations of the major directional changes for the reversals are estimated to be in the range 0 3 3 0 ka the vgp path of the upper thvera record follows a great circle 90 w of the site meridian indicating a clearly non zonal transitional field whereas the vgp paths of the other reversal records are close to the site meridian indicating a mainly zonal transitional field the lower sidufjall and lower nunivak records are nearly identical and have a zonal harmonic content similar to records reported for the matuyama brunhes polarity transition

non axisymmetric behaviour of olduvai and jaramillo polarity transitions recorded in north central pacific deep sea sediments the causes of the reversals of the earth s magnetic field can be investigated by studying the palaeomagnetic record preserved in rocks a proper documentation of reversal phenomena should yield valuable information about the generation of the field deep sea sediments provide a globally and temporally distributed set of polarity transition records our studies2 4 of the olduvai 1 88 1 72 myr 1 and jaramillo 0 94 0 88 myr 1 subchrons using deep sea sediments showed that the time averaged behaviour of the geomagnetic field during these reversals proved to be reliably recorded independent of the latitude and longitude of the sample site here we test some phenomenological transition models and present directional data virtual geomagnetic poles vgps and related directions in rotated declination inclination d i space5 using two records of the olduvai reverse normal reverse r n r and two records of the jaramillo r n r polarity transitions these data are better explained by the flooding rather than the standing field model but they also indicate a degree of asymmetry of the transitional field

evidence for pre cretaceous history and partial neogene 19 9 ma reequilibration in the karakorum nw himalayan syntaxis from 40ar 39ar amphibole dating amphiboles and a biotite from amphibolites and amphibolitic gneisses of the karakorum metamorphic complex kmc nw himalaya were dated using the 40ar 39ar method isotope correlations were compared to electron microprobe analyses both aliv and alvi contents of amphibole increase from the south near the shyok suture zone to the dassu askole area in the north with an increase in metamorphic grade from anchizone to upper amphibolite facies in the lower metamorphic zone discordant age spectra were obtained they are related to epidote amphibolite overgrowth of amphibole rims on older precambrian to mesozoic magmatic cores in the shyok suture zone ar ar ages of 38 62 ma are contemporaneous with the emplacement of the ladakh batholith granodiorites while middle cretaceous magmatic ar ar ages w120 ma are locally preserved in tschermakite relics in the southern karakorum basement a minimum ar ar age of 651 ma was preserved while an age of 208 5g2 ma was preserved in panjal trap like diorites at the margin of the domes zone more recent metamorphism resulted in mineral growth at 20 5 ma in the domes zone amphibole ar ar ages of 17 ma askole and bimodal age spectra of 15 17 and 9 13 ma dassu extend towards the east the zone of neogene amphibolite to granulite facies metamorphism known in the nanga parbat syntaxis

a slab breakoff model for the neogene thermal evolution of south karakorum and south tibet on the south karakorum margin neogene high temperature medium pressure ht mp gneisses define an east west trending thermal anomaly these rocks have been heated from 600 to 750 c during a slight pressure drop from 0 7 to 0 5 gpa their retrogressive path cross cuts the relaxed geotherm of tectonically thickened crust such a p t evolution occurs only if an advective source of heat is involved involvement of an advective heat source is also implied by the occurrence of neogene granitoids and lamprophyres within the ht mp gneiss area these rocks are strongly enriched in large ion lithophile elements relative to primitive mantle and show negative high field strength element anomalies we interpret these geochemical characteristics to be the result of melting of metasomatized asian lithospheric mantle the nd and sr isotopic compositions of the south karakorum neogene magmatic rocks nd 12 to 7 and 87sr 86sr 0 705 0 725 further suggest they could have originated from mixing between asian variously metasomatized mantle and precambrian crust by contrast the origin of the youngest magmatic rocks 10 myr here exemplified by the hemasil syenite and associated lamprophyres requires involvement of a depleted mantle the combined hf nd signature of these rocks hf 10 4 11 5 and nd 3 4 4 3 suggests that the source of the hemasil syenite could have been depleted mantle contaminated by oceanic sediments likely during the earlier subduction of the tethyan ocean neogene magmatic rocks with the same geochemical characteristics and evolution as those of south karakorum have previously been described in south tibet based on their location and the geochemical evolution of their source region we here propose that the neogene magmatic and metamorphic evolution of the south asian margin was controlled by slab breakoff of the subducting indian continental margin starting at about 25 ma this model is supported by available geophysical data from south karakorum and south tibet

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

vertical stretching and crustal thickening at nanga parbat pakistan himalaya a model for distributed continental deformation during mountain building 1 the localization of strain in the continental crust during compressional tectonics is examined using the active structures at the nanga parbat massif an exhumed tract of indian continental crust in the pakistan himalaya this large scale 40 km wavelength structure is considered to involve the whole crust thrusting at the modern surface places gneisses of the indian continental crust onto holocene deposits at the raikhot transect the thrust zone carries a relatively narrow 2 km wide shear zone within which minor structures are asymmetric and the deformation apparently noncoaxial however modeling of foliation and augen preferred orientation ellipticity suggests that the bulk deformation is a combination of relatively small simple shear strains 1 with larger stretching strains heterogeneous stretching within the shear zone was accommodated by localized shearing on metabasic layers so that strain is partitioned outside this shear zone on the transect there is penetrative deformation throughout the nanga parbat massif this broadly distributed deformation shows no asymmetry or evidence of rotation rather this deformation is better described as near pure shear subvertical stretching augen ellipticities suggest subvertical stretches of greater than 200 consideration of plausible changes in crustal thickness during the amplification of the nanga parbat structure suggests the magnitude of vertical stretch decays with depth presumably these strains in the deep crust are more distributed but weaker than in the exposed middle crustal sections assuming conservation of horizontal shortening displacement with depth these studies suggest that penetrative vertical stretching through dominantly pure shear deformation is an effective mechanism for thickening the continental crust and that models which assume that simple shear zones penetrate the whole crust need not be of ubiquitous applicability

anatomy age and evolution of a collisional mountain belt the baltoro granite batholith and karakoram metamorphic complex pakistani karakoram abstract geological mapping and u th pb geochronology from the karakoram metamorphic complex and baltoro granite batholith has resulted in more detailed timing constraints on tectonic evolution of asian crust during and following collision and accretion of the kohistan arc and the indian plate to the southern margin of asia crustal thickening along the karakoram resulted in polyphase deformation metamorphism and melting u pb zircon and monazite ages show that kyanite and sillimanite grade metamorphism in the baltoro region peaked during oligocene early miocene c 28 22 ma times at structurally deeper levels sillimanite grade migmatites of the dassu dome have pliocene c 5 4 3 5 ma ages new u pb ages from crustal melt biotite muscovite garnet leucogranites from the trango towers cathedral peak and paiyu peak across the baltoro batholith range from 19 8 0 5 ma to perhaps as young as 14 ma using data from both the hunza and baltoro regions we suggest that regional high grade metamorphism was diachronous in space and time and could have lasted continuously for at least 37 ma c 50 13 ma or as much as 50 ma c 63 13 ma along the southern margin of the asian plate in the karakoram crustal melting along the baltoro granite batholith spanned at least 13 ma 26 4 13 ma supplementary material analytical methods and description of u th pb data of samples are available at http www geolsoc org uk sup18382

an enhanced image of the pamir hindu kush seismic zone from relocated earthquake hypocentres summary we determine the shape of the seismic zone in the pamir hindu kush region de ned by 30 42 0 n 68 78 0 e by obtaining improved hypocentral locations with 90 per cent con dence limits of less than 30 km the depth error bar for most of the earthquakes of about 6000 shallow and intermediate depth earthquakes available s and depth phase arrival times are also used together with the p wave arrival times in the joint hypocentre determination technique to obtain the best possible hypocentral locations the study region is divided into three depth ranges 0 60 60 160 and 160 km the 0 60 km depth zone is then subdivided laterally into 19 blocks with the deeper regions divided into two blocks each the improved delineation of the seismic zone obtained by using the relocated hypocentres implies that the intermediate depth seismicity in the pamir hindu kush region is most simply explained by a single s shaped seismic zone 700 km long and no more than 30 km wide and with most activity concentrated at 100 300 km depth the main features observed are 1 the eastward steepening of the north dipping hindu kush seismic zone through to its overturning at its eastern end beneath the pamirs where it dips to the southeast 2 the curvature and forking of the subducting slab at depths greater than 200 km within the eastern part of the hindu kush seismic zone 3 the very abrupt cut oi in intermediate depth seismicity at 90 110 km depth with no extension to shallower depths under the pamirs and with a persistent gap between the intermediate and shallow seismicity in the northern pamirs and 4 the unusual horizontal t axes for intermediate depth earthquakes of the pamir seismic zone which align with its curvature this study shows that the seismic zone under the hindu kush has stress axes which follow the classical pattern for subducting slabs controlled by gravity whereas the pamir region has horizontal t axes that follow the trend of the contorted seismic zone this suggests that the pamir seismic zone is a slab deformed due to ow in the upper mantle

do virtual geomagnetic poles follow preferred paths during geomagnetic reversals virtual geomagnetic poles vgps recorded in sediments during reversals of the earth s magnetic field show an apparent preference for two antipodal sectors of longitude1 3 not only in records of the same reversal from different sites but also in records of different reversals if preferred bands really have persisted from one reversal to the next this would imply that the mantle exerts a significant control over the reversal process4 here we analyse the available database of reversal records from the past 12 myr using a statistical test specifically designed to test the hypothesis of two preferred antipodal longitudinal bands our analysis shows that the records taken as a group of independent observations do show an overall preference for two antipodal longitudinal bands however the site longitudes are also strongly grouped5 and a comparison of the transitional vgp longitudes with site longitudes shows an unlikely grouping under the hypothesis of a genuine geographical preference for transitional vgps we conclude that it is premature to accept the hypothesis of mantle control over the core during geomagnetic reversals

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

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

u pb age of the baltoro granite northwest himalaya and implications for monazite u pb systematics the baltoro granite is a major late to post tectonic plutonic phase of the karakoram batholith of the northwest himalaya in northern pakistan u pb zircon analyses indicate both emplacement at 21 0 0 5 ma and significant precambrian zircon inheritance dates on monazite are 17 19 ma and are interpreted to have remained near their closure temperature of about 700 c for several million years after emplacement resulting in pb loss by diffusion we suggest that the granite was emplaced into rocks which were at high temperature and that they remained so until late miocene northeast tilting rapid uplift and or tectonic denudation and cooling

shoshonitic and ultrapotassic post collisional dykes from northern karakorum sinkiang china abstract high k calc alkaline shoshonitic and ultrapotassic post collisional dykes of neogene age have been found in the remote and little known region of northern karakorum located around the shaksgam valley north of the k2 gasherbrum range china the dykes derive from more or less comparable basic magma s and display rather unusual petrographic and geochemical characters with respect to the other k rich rocks the geochemical data are consistent with derivation of the basic magma s from small degrees of partial melting of garnet lherzolites previously enriched in incompatible elements of crustal origin possibly during the subduction of the indian plate beneath karakorum the spectrum of compositions reflects fractional crystallization governed by an early removal of clinopyroxene phlogopite plagioclase garnet amphibole followed by the precipitation of abundant alkali feldspar amphibole apatite quartz additionally assimilation of crustal rocks during magma ascent contributed to the unusual compositional characteristics and is suggested by the abundance of corroded quartz plagioclase xenocrysts and by the occasional presence of granitic xenoliths an apparent connection exists between magmatism and tectonism in the complex karakorum fault zone kfz it is suggested that during the neogene the strike slip kfz and some adjacent post metamorphic faults transiently behaved as extensional fault systems down to deep levels triggering ascent and emplacement of the k rich magma the subsequent re activation of a compressive and transcurrent regime determined the rapid and recent uplift of the more primitive lamprophyres occurring in the plutonites and metamorphites of the upper baltoro glacier and k2 gasherbrum range relative to the more fractionated and contaminated lamprophyres injecting the shallower rocks of the shaksgam valley area

kongur shan normal fault type example of mountain building assisted by extension karakoram fault eastern pamir the northernmost segment of the karakoram fault is an active normal dextral wrench fault that bounds the muji tashgurgan pliocene quaternary basin eastern pamir the kongurshan 7719 m and mustaghata 7546 m mountains are interpreted as twin crustal ramp anticlines formed en echelon on the eastern side of the karakoram fault by subduction of the tarim lithosphere under the pamir west dipping s c mylonitic gneisses that have downdip stretching lineations form the western flank of kongur mountain kinematic criteria consistently indicate a top to the west sense of shear the mylonitic foliation is cut by a steeper active normal fault which is marked by prominent triangular facets on west facing spurs the ductile normal shearing deformation occurred during the interval 5 1 ma therefore like the himalayas the pamir mountain range shows the coeval development of frontal thrusting and rear normal faulting the kongur fault thus appears to be an excellent example of synorogenic extension where normal faulting contributes to relief

do hf isotopes in magmatic zircons represent those of their host rocks abstract lu hf isotopic system in zircon is a powerful and widely used geochemical tracer in studying petrogenesis of magmatic rocks and crustal evolution assuming that zircon hf isotopes can represent initial hf isotopes of their parental whole rock however this assumption may not always be valid disequilibrium partial melting of continental crust would preferentially melt out non zircon minerals with high time integrated lu hf ratios and generate partial melts with hf isotope compositions that are more radiogenic than those of its magma source dissolution experiments with hotplate bomb and sintering procedures of zircon bearing samples demonstrate this disequilibrium effect where partial dissolution yielded variable and more radiogenic hf isotope compositions than fully dissolved samples a case study from the neoproterozoic jiuling batholith in southern china shows that about half of the investigated samples show decoupled hf isotopes between zircons and the bulk rocks this decoupling could reflect complex and prolonged magmatic processes such as crustal assimilation magma mixing and disequilibrium melting which are consistent with the wide temperature spectrum from 630 c to 900 c by ti in zircon thermometer we suggest that magmatic zircons may only record the hf isotopic composition of their surrounding melt during crystallization and it is uncertain whether their hf isotopic compositions can represent the primary hf isotopic compositions of the bulk magmas in this regard using zircon hf isotopic compositions to trace crustal evolution may be biased since most of these could be originally from disequilibrium partial melts

tectonometamorphic evolution of the central karakorum baltistan northern pakistan abstract new structural and metamorphic data from the chogo lungma glacier area and basha valley enlighten the polyphased tectonometamorphic evolution of the metamorphic complex of karakorum a pre to early d1 event is defined by a metamorphic cleavage in the core of some zoned garnet and staurolite porphyroblasts the main tectonometamorphic event d1 is characterized by n110 e south vergent isoclinal folds associated with a metamorphic axial plane cleavage formed in a predominantly flattening regime of deformation it corresponds to a grt bt mu ky peak metamorphism m1 620 730 c and 7 5 11 kbar the d2 tectonometamorphic event corresponds to the development of several conical domes elongated striking n110 e to n140 e the s2 cleavage is seldom individualized as it usually corresponds to a reworking of s1 in some areas late evolution of d2 doming deformation is marked by a crenulation petrologically d2 is characterized by shearing and boudinage of m1 minerals and by development of sillimanite it corresponds to a decompressional evolution 7 5 down to 4 kbar with only slight changes in temperature a late event corresponds to large undulations and open folds which affect the d2 crenulation we propose here a model of vertical plus dextral extrusion of the middle crust to explain the dome structures in karakorum the steep pattern of the metamorphic p t path implies rapid exhumation rates and suggests that the different phases could be part of a continuum of deformation preliminary results by 40 ar 39 ar method give young cooling ages

records of geomagnetic reversals from volcanic islands of french polynesia 1 paleomagnetic study of a polarity transition in a lava sequence from the island of huahine we report the behavior of the earth s magnetic field for a transition recorded in lavas at the volcanic island of huahine in french polynesia south central pacific ocean detailed intermediate fields are observed that record an apparent n n excursion the available k ar ages indicate that most volcanic rocks of the southern part of huahine erupted at a time close to the kaena subchron gaps in the paleomagnetic sampling or a hiatus in the volcanic activity during the kaena event may have led to the partial record of a complete polarity transition missing the fully reversed state nevertheless numerous flows have recorded intermediate fields that somewhat constrain reversal models the low intensities of magnetization associated with the intermediate directions suggest that the transitional field has mainly low intensities the beginning of the transition shows inclination steepening consistent with a zonal model of geomagnetic field reversal based on an axial quadrupole large deviations from a north south planar distribution of the intermediate directions show that non axisymmetrical nondipolar fields are important in the middle of the transition

rates of fluvial bedrock incision within an actively uplifting orogen central karakoram mountains northern pakistan abstract terrestrial cosmogenic nuclide tcn 10 be surface exposure ages for strath terraces along the braldu river in the central karakoram mountains range from 0 8 to 11 ka this indicates that strath terrace formation began to occur rapidly upon deglaciation of the braldu valley at 11 ka fluvial incision rates for the braldu river based on the tcn ages for strath terraces range from 2 to 29 mm a the fluvial incision rates for the central gorged section of the braldu river are an order of magnitude greater than those for the upper and lower reaches this difference is reflected in the modern stream gradient and valley morphology the higher incision rates in the gorged central reach of the braldu river likely reflect differential uplift above the main karakoram thrust that has resulted in the presence of a knickpoint and more rapid fluvial incision the postglacial fluvial incision rate 2 3 mm a for the upper and lower reaches are of the same order of magnitude as the exhumation rates estimated from previously published thermochronological data for the baltoro granite in the upper catchment region and for the adjacent himalayan regions

geochemical constraints on the bimodal origin of high himalayan leucogranites major and trace element and rb sr isotope systematics of the manaslu leucogranite central nepal have been examined to constrain the role of mineral fractionation and fluids in peraluminous granite petrogenesis biotite and tourmaline are for the most part mutually exclusive with a predominance of two mica leucogranites over tourmaline leucogranites the 87sr86sr initial isotopic ratios sri indicate that leucogranitic melts were derived from two different sources the two mica leucogranites having a metagreywacke origin with sri 0 752 nd 15 such a bimodal nature of the source zone does not directly influence the magmatic evolution except that probably the higher initial boron content in the metapelitic rocks may increase the na2ok2o ratio in contrast the amount of water present during melting principally controls in part anatectic processes and element behaviour water saturated conditions probably occured during melting of metagreywackeous rocks and favoured crystallization of two mica leucogranites whereas water absent conditions prevailed during melting of metapelitic layers and favoured biotite plagioclase and monazite fractionation in the source zone and tourmaline crystallization in the leucogranite

how the geomagnetic field vector reverses polarity a highly detailed record of both the direction and intensity of the earth s magnetic field as it reverses has been obtained from a miocene volcanic sequence the transitional field is low in intensity and is typically non axisymmetric geomagnetic impulses corresponding to astonishingly high rates of change of the field sometimes occur suggesting that liquid velocity within the earth s core increases during geomagnetic reversals

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

timing and origin of migmatitic gneisses in south karakoram insights from u pb hf and o isotopic record of zircons abstract the timing and origin of partial melting in collision belts is crucial to understand the thermotectonic evolution and the relationship between ht metamorphism and magmatism in over thickened crust in the present study we used the in situ isotopic hf o and u pb record of zircons to investigate the timing and origin of migmatitic gneisses exposed in the core of the dassu dome in south karakoram the new u pb zircon dating identified the proterozoic inherited cores 1 8 1 9 ga and 2 3 2 5 ga surrounded by a neogene overgrowth with ages ranging from 6 to 20 ma these ages imply that the partial melting in the karakoram metamorphic complex lasted from 20 ma to 6 ma and can be correlated with the miocene magmatism in the adjacent baltoro region oxygen isotopic data from proterozoic inherited cores 1 8 1 9 ga and neogene overgrowths are indistinguishable and generally vary from 8 to 9 5 these values are slightly higher than the most igneous zircons 6 5 8 valley et al 2005 indicating an igneous precursor with heavy initial o composition that later might have equilibrated with low temperature environment or some involvement of supracrustal material is likely however a few low u th relatively old inherited cores 2 3 2 5 ga showed mantle like 18o 5 3 0 6 valley et al 2005 values of 18o 5 5 2 7 the present day weighted mean ehf 0 of the proterozoic inherited cores ranges from 50 1 0 to 44 3 1 2 in contrast the neogene rims are 15 20 e units higher than the inherited core with present day ehf 0 30 6 0 9 this implies that the hf composition of the neogene overgrowth is not controlled exclusively by the dissolution of the inherited cores and that contamination by external melts is likely we suggest a contribution from the neogene less evolved magmatism in the baltoro region ehf 0 4 to 10 the elevated oxygen composition is not consistent with the contribution from pristine mantle derived magmas the observed homogeneous and uniform hf o isotopic composition of the proterozoic inherited cores suggest their derivation from mildly evolved infracrustal sources with minor input from supracrustal material the older inherited zircons 2 3 2 5 ga were precipitated from juvenile mantle derived magmas

age cooling history and origin of post collisional leucogranites in the karakoram batholith a multi system isotope study u pb dating on zircon and monazite from different varieties of leucogranites in the karakoram batholith reveals two distinct pulses of plutonism at 25 5 0 3 0 8 and 21 4 0 3 0 6 ma in both granites these minerals contain inherited components documenting the presence of precambrian material in the magma source a crustal origin of the granites and long crustal residence times of the source rocks are confirmed by highly negative epsilon i nd strongly positive epsilon i sr and t dm nd in excess to 1 ga a biotite gneiss and a granitic dike from the karakoram metamorphic complex south of the batholith yield very similar epsilon i nd epsilon i sr t dm nd in a pb evolution diagram initial isotopic compositions for both the granites and the rocks from the metamorphic complex plot far above the reference field for morb documenting very close affinity to the eurasian tibetan continental crust the striking similarity of isotope signatures between leuco

olduvai onset polarity transition two detailed paleomagnetic records from north central pacific sediments abstract paleomagnetic analyses of two azimuthally oriented deep sea sediment cores from the north central pacific ocean which contain a record of the onset of the olduvai subchron polarity transition show 1 intermediate directions of magnetization with transitional inclinations reaching values up to 70 2 up to three sequential 180 declination changes two of which imply rapid oscillations of the geomagnetic field before the main reversal and 3 a pronounced drop in intensity of magnetization within the transitional interval the data further indicate that the intensity decay and recovery cycle took longer than the directional changes of the reversal whereas the directional changes lasted about 5000 years the duration of the recorded intensity cycles suggest that the complete onset of the olduvai reversal may have lasted as much as 28 000 years anhysteretic remanent magnetization studies and thermomagnetic analyses indicate that uniform magnetic mineralogy characterizes the transition zones of the cores this further suggests that the j 10 arm 70 ratio may represent a relative paleointensity estimate and may thus indicate a decrease of the intensity of the geomagnetic field during the onset of the olduvai reversal virtual geomagnetic pole paths of the two records are near sided directions in projected d i space indicate that the olduvai onset transitional field geometry possessed a contribution from non axisymmetric terms in addition to the dominating zonal harmonics the analyses also suggest that the transitional field during the onset of the olduvai reversal was dominated by harmonics different from those of the matuyama brunhes boundary jaramillo transitions and upper olduvai reversal

transitional paleointensities from kauai hawaii and geomagnetic reversal models thellier paleointensity results from an r n transition zone in kauai hawaii show that field intensity dropped from 0 431 oe to 0 101 oe while the field remained within 30 of the reversed axial dipole direction a recovery in intensity and the main directional change followed this presumably short period of low field strength as the reversal neared completion the field had an intensity of 0 217 oe while still 40 from the final direction the relationship of paleointensity to field direction during the early part of the reversal thus differs from that toward the end a feature that only some reversal models are consistent with for example a model in which a standing nondipole component persists through the dipole reversal predicts only symmetric intensity patterns in contrast zonal flooding models generate suitably complex field behavior if multiple flooding schemes operate during a single reversal or if the flooding process is itself asymmetric

metamorphic evidence for an inverted crustal section with constraints on the main karakorum thrust baltistan northern pakistan abstract the shyok suture zone separates rocks in the asian plate from rocks in the kohistan ladakh island arc in baltistan this suture has been reactivated by the late break back main karakorum thrust mkt the p t histories of metamorphic rocks both north and south of the mkt have been determined in an effort to place constraints on the tectonic history of this zone the terranes north and south of the mkt have different unrelated metamorphic histories rocks from the kohistan ladakh island arc south of the mkt have undergone a static low p 2 4 kbar c 500 c thermal metamorphism the p t paths and metamorphic textures of these rocks are consistent with metamorphism due to emplacement of plutonic rocks into the island arc this metamorphism pre dates folding and deformation of these rocks rocks in the karakorum metamorphic complex north of the mkt have experienced a complex deformational and metamorphic history prograde metamorphic isograds have been deformed by subsequent south verging folding and by gneiss dome emplacement however decompression metamorphic reactions occurred during nappe emplacement higher pressure rocks are associated with higher level nappes creating an inverted pressure metamorphic sequence 8 9 kbar rocks over 5 6 kbar rocks there is little variation in temperature with structural level 550 625 c these two different terranes have been juxtaposed after metamorphism by the late south directed mkt

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

complex behavior of the geomagnetic field during three successive polarity reversals 11 12 m y b p detailed paleomagnetic records of both the directional and relative field intensity changes occurring during three sequential polarity reversals have been obtained from marine clays of serravallian tortonian boundary age in the island of zackinthos greece the three reversals are characterized by a complicated geometrical and temporal structure periods of rapid changes of the geomagnetic field vector alternate with periods of near stationarity different episodes of collapse and recovery of the field intensity and the presence of rebounds have been documented overall the records obtained here are reminiscent of volcanic records and appear much more complex and quite different from those previously obtained from upper tortonian marine clays in crete in the same geographical area the bearing of these results on some of the published reversal models is discussed and it is also suggested that the greater complexity of the zackinthos records might be related to the fact that they are exactly coeval with the last reversal spurt documented by reversal frequency analyses

tectonic controls on the uplift of the nanga parbat massif pakistan himalayas cenozoic mountain belts like the himalayas contain regions that experienced rapid uplift in comparison with the areas around them1 whether this uplift occurred by buoyant rebound of previously thickened crust2 or during the crustal thickening process itself is uncertain3 4 here we show however that the 7 mm yr 1 recent uplift5 of the nanga parbat massif pakistan himalayas was accomplished during nw directed thrusting located along its western margin this 3 km wide liachar thrust zone operated from amphibolite facies to the present topographic surface and accommodates substantial crustal shortening the adjacent indus river valley itself cut by seismogenic faulting completely transects the massif showing that erosion keeps pace with geologically rapid thrust uplift6

gauss matuyama polarity transition the gauss matuyama g m polarity transition has been identified in a 9 cm rotary drilled core from scaries valley california the lithology recovery and preservation of the sediments allow a detailed investigation of the transition the transition is estimated to have lasted approximately 2000 years about 2000 years earlier a short reversal also occurred that lasted 2000 years the relative field intensity decreased by at least 70 both during this short reversal and the g m transition the virtual geomagnetic poles path for the g m transition is restricted to a meridional band 20 wide in the atlantic ocean and closely matches the v g p path for the matuyama brunhes transition recorded at lake tecopa california on the other hand the pole path for the g m transition recorded in russia about 180 in longitude from searles valley is nearly antipodal at low latitudes to the learies valley pole path these data indicate that the transition field associated with the reversal was dominated by non dipole components

age and origin of post collision baltoro granites south karakoram north pakistan insights from in situ u pb hf and oxygen isotopic record of zircons abstract origin of post collision plutonism is critical to understand the tectonothermal evolution of the over thickened continental crust in collision zones this has proven difficult to reconcile with the conventional whole rock geochemical and field based studies alone we report in situ study of zircon u pb hf and o isotopes from five samples of the baltoro plutonic unit bpu in south karakoram the plutonic unit is the western part of the southern asian margin of the india asia convergent zone baltoro granites and a biotite rich enclave yielded similar and overlapping u pb ages ranging from 26 to 15 ma hafnium isotopic composition ehf 0 is very heterogeneous ranging from 17 1 to 4 4 while the oxygen isotopic composition of the granites is homogeneous with mean 18 o ranging from 7 2 to 9 4 based on u pb geochronology and hf o isotopic composition the involvement of two main sources is suggested 1 cretaceous calc alkaline karakoram crust and 2 karakoram gneisses moreover possible involvement of metasomatized asian lithospheric mantle is supported by elevated oxygen composition of granites and identical hf composition of biotite rich enclave to the mantle derived baltoro lamprophyre however direct contribution from juvenile pristine mantle is unlikely as no juvenile mantle type hf and oxygen values were obtained this also precludes the involvement of southward juvenile arc related component of kohistan ladakh batholith our new u pb and hf data are comparable to the mesozoic karakoram batholith miocene two mica leucogranites in the pangong range and magmatism from the lhasa terrane in south tibet suggesting a genetic link between the karakoram and the rocks to the east this magmatic event is best explained by lower crust partial melting promoted by both thermal equilibration following crustal thickening and heat advection by ultrapotassic magmas associated with the breakoff of the indian continental margin

gneiss domes and orogeny many gneiss domes record positive feedback between decompression and partial melting of orogenic middle crust exhumed orogens are riddled with gneiss domes cored by migmatites that underwent dehydration melting during decompression the decreasing buoyancy associated with increasing melt fraction drives further decompression at near isothermal conditions as the partially molten crust rises diapirically this combination of processes may explain the generation and retention of large volumes of crustally derived melt recorded in many deep seated migmatite terranes and inferred for active orogens in exhumed orogens the signature of the rapid ascent of partially molten crust is a gneiss dome cored by migmatite granite the large volume of material involved in the vertical transfer of partially molten crust indicates that the formation of gneiss domes is an efficient mechanism for heat advection during orogenesis

a simple statistical model for geomagnetic reversals the diversity of paleomagnetic records of geomagnetic reversals now available indicate that the field configuration during transitions cannot be adequtely described by simple zonal or standing field models a new model described here is based on statistical properties inferred from the present field and is capable of simulating field transitions like those observed some insight is obtained into what one can hope to learn from paleomagnetic records in particular it is crucial that the effects of smoothing in the remanence acquisition process be separated from true geomagnetic field behavior this might enable us to determine the time constants associated with the dominant field configuration during a reversal

high 18o granitic plutons from the frontenac axis grenville province of ontario canada abstract 18 o 16 o ratios have been determined for whole rock as well as coexisting minerals from nine syn to post tectonic monzonite and quartz monzonite plutons in the westport gananoque area covering approximately 2500 km2 the plutons fall in two distinct isotopic groups 1 i low 18o group 8 0 11 2 rideau lake westport and wolfe lake plutons and 2 ii high 18o group 12 7 16 2 lyndhurst perth road battersea gananoque south lake and crow lake plutons the 18 o 16 o ratios of granitic rocks from the latter group are among the highest ever reported in the literature the isotopic compositions of samples within each pluton bear no relationship to the sample distance from the intrusive contacts nor are they affected by the types of country rocks marbles vs pelitic gneisses into which they are emplaced oxygen isotope fractionations among coexisting quartz feldspar biotite and magnetite all show normal plutonic values thus all lines of evidence suggest that the monzonite and quartz monzonite plutons did not acquire their characteristic 18 o 16 o ratios at the present level of exposure either by isotopic exchange with the surrounding metasedimentary country rocks or by any post crystallization low temperature alteration processes the distinct 18o 16o ratios observed in the high and low 18o groups probably reflect the difference in the isotopic compositions of the source materials the most plausible source for the high 18o group plutons is the grenville supergroup metasedimentary rocks and that for the low 18o group plutons the grenville basement gneisses mixed with a small amount of grenville supergroup metasedimentary rocks

palaeomagnetic constraints on the geometry of the geomagnetic field during reversals palaeomagnetic records of the path of the pole during reversals of the earth s magnetic field provide a test of the hypothesis that dipolar or low order axisymmetric components of the field dominate during reversals multiple records of reversals during the past 12 myr show no simple or consistent geographical pattern although a more robust analysis of the transitional field awaits a greater number of well distributed sampling sites the present data are not inconsistent with the simplest models in which a field reminiscent of the non dipole component of the present day field becomes dominant

the source of granitic gneisses and migmatites in the antarctic peninsula a combined u pb shrimp and laser ablation hf isotope study of complex zircons zircons gneisses and migmatites collected from the antarctic peninsula have different core rim hafnium isotope ratio relationships depending on whether evidence for zircon dissolution is present or absent two samples contain inherited zircon that is partially dissolved in these samples the 176hf 177hf rations of the inherited zircon and new magmatic zircon rims are on average indistinguishable and consistent with in situ melting in such cases the hafnium isotopic composition of the melt was probably strongly influenced by the dissolved zircon component at the source variation in 176hf 177hf within the magmatic zircon rims from grain to grain suggests that hf isotopes were only partially homogenized during melt migration alternatively zircon growth may have taken place within small volumes of partial melt other samples do not preserve textural evidence for zircon dissolution during melt generation in these samples the 176hf 177hf values of the inherited zircon and new magmatic zircon rims are different the zircon rims apparently suggest a source of less evolved hafnium than that contained within the inherited zircon whether this relates to a separate juvenile source or alternatively is derived from minerals other than zircon at the source cannot be resolved inherited zircon irrespective of age has been strongly influenced by the reworking of a juvenile late mesoproterozoic source suggesting that such crust underlies the antarctic peninsula our results therefore suggest that hf isotope analyses provide great potential for future studies investigating the source and processes involved in the generation of crustal melts

zircon u pb dating and in situ hf isotopic analysis of permian peraluminous granite in the lhasa terrane southern tibet implications for permian collisional orogeny and paleogeography article i nfo the lhasa terrane has long been interpreted as a simple tectonic block rifted from gondwana during the late paleozoic and then drifted northward before finally amalgamating with the qiangtang terrane during the early cretaceous in this paper we document permian peraluminous granites near pikang in the southern margin of the central lhasa terrane close to the recently documented songdo eclogite of permian age zircon shrimp and la icpms u pb dating for a pikang granite sample gives an identical crystallization age of about 263 ma and a wide age range of inherited zircons 283 2141 ma in situ hf isotopic analyses for 20 zircons of 263 ma yielded ehf t values of 4 5 to 1 9 the pikang granites have high a cnk values 1 08 and high normative corundum 1 3 2 0 indicative of peraluminous s type granite they are characterized by moderately negative eu anomalies eu eu 0 48 0 61 and strongly negative ba nb sr p and ti anomalies the granites have high end t values 6 4 to 6 0 and low initial 87 sr 86 sr ratios 0 7082 0 7096 relative to melts derived from mature continental crust these rocks are interpreted to have been generated by mixing between mantle melts and their induced melting of mature crustal materials we interpret the pikang peraluminous granite magmatism the regional angular unconformity between the middle and the upper permian and the eclogite of the same metamorphic age 262 ma from the same geotectonic location to represent different products of a common event in time and space we tentatively term this common event as syncollisional orogeny i e the permian gangdese orogeny we further hypothesize that the orogeny may be genetically associated with the collision between the lhasa terrane and the northern margin of australia following the closure of the paleo tethyan ocean south of the lhasa terrane

granites of western karakorum and northern kohistan pakistan a composite mid cretaceous to upper cenozoic magmatism abstract the petrography the chemical mineralogical typology and the ages of six plutonic units four from the karakorum axial batholith darkot pass ghamu bar batura and hunza units and two of northern kohistan gindai and nomal plutons are defined and compared based on field data and analyses of up to 78 samples major elements ree rb s sbnd sr and k ar isotopes the karakorum axial batholith is a composite body three major intrusive stages occurred 1 around mid cretaceous times ca 110 95 ma with the emplacement of subalkaline i e monzonitic darkot pass and calc alkaline hunza ghamu bar units 2 during palaeogene maybe up to 43 ma batura subalkaline unit a strong tectonommetamorphic event recorded in the gneissification of the cretaceous intrusives occurred between these two stages it may be of palaeocene age p t estimates of the highest metamorphic grade rocks of the hunza unit have yielded values of 580 640 c and 5 0 5 kbar and 3 during upper miocene ca 9 ma baltoro subalkaline unit debon et al 1986c in addition a conspicuous network of aplo pegmatitic dykes emplaced into the hunza area possibly from the eocene up to the upper cenozoic with a maximum during the middle miocene ca 15 ma most of these major magmatic stages are met again among the acidic intrusives and dykes of northern kohistan the first one as blastomylonitic tholeiitic plutons nomal ca 102 ma petterson and windley 1985 the second one as subalkaline plutons gindai ca 59 ma and a third one as leucocratic dykes of oligocene age ca 30 ma petterson and windley 1985 these data may be related to the geodynamic evolution of the nw part of the india eurasia suture zone thus allowing better constraints on the major steps of this evolution the partly synchronous closures by n dipping subduction of the two tethys branches which are assumed to have encircled the kohistan arc in upper mesozoic times may have generated both the karakorum and the kohistan intrusives of cretaceous and palaeogene ages the northern branch very likely closed before the southern one at the time of the second intrusive stage palaeogene a kohistan and karakorum had already collided were welded and had suffered the same major tectonometamorphic event b subduction of the southern tethys floor beneath the welded kohistan karakorum was still active c however collision between india and kohistan karakorum may have already begun particularly at the level of the nanga parbat promontory finally it is emphasized that the intrusive processes continued in the karakorum long after the collision e g baltoro granite

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

structural and thermal evolution of the karakoram crust prior to the eocene c 50 ma collision of the indian and asian plates the southern margin of asia along the karakoram plate was an andean type margin dominated by tonalitic granodioritic magmatism of jurassic lower cretaceous age hushe gneiss muztagh tower gneiss and k2 ortho gneiss and associated low pressure andalusite staurolite and garnet grade metamorphism m1 following india asia collision crustal shortening thickening and regional barrovian metamorphism m2 occurred between 50 37 ma thermobarometry of kyanite grade metapelites indicate burial to depths of around 30 35 km simultaneous solution of the garnet biotite geothermometer with the garnet muscovite biotite plagioclase and garnet ai 2 sio 5 quartz plagioclase geobarometers indicates peak m2 p t conditions of 696 20 c at 8 6 0 7kbar 860 mpa temperatures may have exceeded 700 c in sillimanite grade metapelites to produce in situ partial melting and leucogranitic melt pods peak m2 metamorphism occurred prior to 37 0 8 ma the crystallization age of the mango gusar two mica granite pluton which cross cuts syn metamorphic deformation fabrics post m2 thermal relaxation followed from 37 25 ma after which localized high heat concentrations at the base of the thickened crust caused widespread crustal melting and intrusion of the baltoro granite batholith at 25 21 ma a high temperature low pressure thermal aureole m3 along the northern contact is synchronous with the 21 0 5 ma zircon age of the baltoro granite andalusite hornfels along the northern contact of the batholith mitre thermal aureole indicates maximum pressures of 3 75 kbar 375 mpa a 75 c increase of temperature in kyanite sillimanite grade gneisses approaching the southern granite contact of the baltoro granite is interpreted as the thermal upwarping of pre 37 ma barrovian metamorphic m2 isograds around the 21 ma contact aureole m3 isotherms

a detailed record of the lower jaramillo polarity transition from a southern hemisphere deep sea sediment core a detailed record of the lower jaramillo reversed to normal polarity transition was obtained from a southern hemisphere deep sea sediment core latitude 35 91 e longitude 59 97 e the record consists of over 850 samples taken across 140 cm of section the transition itself is recorded across approximately 70 cm and is represented by more than 475 specimens from about 160 levels giving intermediate directions the transition is identified by a nearly 180 shift from directions in good agreement with a reversed axial dipole field to those closely aligned with a normal axial dipole field for the core site latitude the inclinations shallow gradually early in the reversal and pass through very steep negative values 80 late in the transition the declinations show little appreciable variation until the inclinations have moved through the near vertical and then slowly approach values in agreement with a normal polarity field an intensity low accompanies the directional change during which the intensity drops to less than 15 of the maximum values observed in this sample interval the intensity fluctuation spans a wider interval than the directional change decreasing prior to any systematic change in the directions and then increasing to pre transition levels by the same depth at which the directions have stabilized the vgp path constructed for this reversal is longitudinally constrained to a certain extent between 140 and 230 for intermediate vgp latitudes and is roughly centered 120 from the site longitude this path is therefore a far sided vgp path in hoffman s 1977 terminology assuming a constant sedimentation rate 67m ma through the jaramillo subchron the duration of the transition is estimated to be 11 200 years to 4 500 years depending on the criteria for the directional change whereas the associated intensity variation occurred over 15 000 to 20 000 years considered together with records of the most recent reversal matuyama brunhes in light of current transitional field models this record strongly suggests that the lower jaramillo transitional field was dominated by different harmonics than the matuyam brunhes transitional field

equatorial and mid latitude records of the last geomagnetic reversal from the atlantic ocean abstract three records of the matuyama brunhes reversal have been obtained from odp deep sea cores distributed along the west african margin in the atlantic ocean these studies in addition to the record from dsdp site 609b provide a latitudinal transect extending from the equator to 50 n simulations of natural smoothing by post depositional processes show that characteristic features of a transitional field geometry dominated by axisymmetry should be preserved in the records especially at the equator the results do not seem to favour the hypothesis that axisymmetrical terms would dominate during this transition

high resolution sedimentary record of a geomagnetic reversal a detailed study of a geomagnetic reversal shows that during the transition the geomagnetic field undergoes both short and long term fluctuations the first are thought to be linked to the reversal process the second are present with the same time constants both before and during the reversal this suggests that the secular variation at least in its longer period terms is not affected by the reversal of the main field this feature has important consequences for the validity of the frozen flux approximation

the origin and pre cenozoic evolution of the tibetan plateau different hypotheses have been proposed for the origin and pre cenozoic evolution of the tibetan plateau as a result of several collision events between a series of gondwana derived terranes e g qiangtang lhasa and india and asian continent since the early paleozoic this paper reviews and reevaluates these hypotheses in light of new data from tibet including 1 the distribution of major tectonic boundaries and suture zones 2 basement rocks and their sedimentary covers 3 magmatic suites and 4 detrital zircon constraints from paleozoic metasedimentary rocks the western qiangtang amdo and tethyan himalaya terranes have the indian gondwana origin whereas the lhasa terrane shows an australian gondwana affinity the cambrian magmatic record in the lhasa terrane resulted from the subduction of the proto tethyan ocean lithosphere beneath the australian gondwana the newly identified late devonian granitoids in the southern margin of the lhasa terrane may represent an extensional magmatic event associated with its rifting which ultimately resulted in the opening of the songdo tethyan ocean the lhasa northern australia collision at similar to 263 ma was likely responsible for the initiation of a southward dipping subduction of the bangong nujiang tethyan oceanic lithosphere the yarlung zangbo tethyan ocean opened as a back arc basin in the late triassic leading to the separation of the lhasa terrane from northern australia the subsequent northward subduction of the yarlung zangbo tethyan ocean lithosphere beneath the lhasa terrane may have been triggered by the qiangtang lhasa collision in the earliest cretaceous the mafic dike swarms ca 284 ma in the western qiangtang originated from the panjal plume activity that resulted in continental rifting and its separation from the northern indian continent the subsequent collision of the western qiangtang with the eastern qiangtang in the middle triassic was followed by slab breakoff that led to the exhumation of the qiangtang metamorphic rocks this collision may have caused the northward subduction initiation of the bangong nujiang ocean lithosphere beneath the western qiangtang collision related coeval igneous rocks occurring on both sides of the suture zone and the within plate basalt affinity of associated mafic lithologies suggest slab breakoff induced magmatism in a continent continent collision zone this zone may be the site of net continental crust growth as exemplified by the tibetan plateau c 2012 international association for gondwana research published by elsevier b v all rights reserved

the himalayan leucogranites constraints on the nature of their crustal source region and geodynamic setting article i nfo late oligocene miocene leucogranites within southern tibet form part of an extensive intrusive igneous province within the himalayan orogen the main rock types are tourmaline leucogranites tg and two mica leucogranites 2 mg they have high sio2 70 56 75 32 wt al2o3 13 55 15 67 wt and 87 sr 86 sr i 0 724001 0 797297 and low mgo 0 02 0 46 wt and 143 nd 144 nd i 0 511693 0 511906 chondri te normalized rare earth element ree patterns display strong negative eu anomalies whole rock major and trace element and sr nd isotope data for the leucogranites suggest that their source region was a two component mixture between a fluid derived from the lesser himalayan lh crustal sequence and the bulk crust of the higher himalayan hh sequence trace element and sr nd isotope modeling indicate that the proportion of fluid derived from the lh sequence varied from 2 to 19 and the resulting metasomatised source experienced 7 16 melting the amount of fluid derived from the lh sequence increases from north to south northward underthrusting of the indian continent resulted in infiltration of the lh derived fluid into the overlying hh sequence subsequent decompression melting of this metasomatised crust mostly during the miocene 25 9 ma generated the leucogranites this may be linked to steepening of the subducted slab of indian lithosphere beneath the orogenic belt

viscous collision in channel explains double domes in metamorphic core complexes in hot orogens gneiss domes are a response to upper crustal stretching and lower crustal flow two dimensional thermal mechanical modeling shows that localization of extension in the upper crust triggers in the deep crust oppositely verging horizontal flows that converge beneath the extended region upon viscous collision both flowing regions rotate upward to form two upright domes of foliation double domes separated by a steep median high strain zone in such systems horizontal shortening in the infrastructure develops in an overall extensional setting dome material follows a complex depth dependent strain history from shearing in the deep crustal channel to contraction upon viscous collision in the median high strain zone to extension upon advection into the shallow crust this depth dependent strain history is likely a general feature of dome evolution and is arguably well preserved in double domes such as the montagne noire france and naxos greece gneiss domes

parameter sensitivity of primitive equation ocean general circulation models abstract experiments with a low resolution primitive equation ocean general circulation model with idealized basin geometry and surface forcing have been carried out in order to identify the processes controlling the climatically important aspects of the circulation emphasis was placed on the sensitivity of the model solutions to the magnitude of the vertical diffusivity scaling arguments suggest and the numerical experiments confirm that the solutions are most sensitive to the magnitudes of the wind stress curl and the vertical diffusivity for small vertical diffusivity wind forcing dominates the solution the vertical scale of the thermocline is set by the strength of the ekman pumping and there is a multiple gyre circulation in the upper layers for large vertical diffusivity diabatic surface forcing dominates the solution vertical diffusion controls the vertical scale of the thermocline and there is a single large anticyclonic gyre in the upper layers both the meridionally and zonally inte