a theoretical and numerical study of density currents in nonconstant shear flows the previous idealized two fluid model of a density current in constant shear is extended to the case where the inflow shear is confined to the low levels the analytical solution is determined by the conservation of mass momentum vorticity and energy it is found that a low level shear acts in a similar manner to a uniform vertical shear in controlling the depth of a steady state density current when the shear enhances the low level flow against the density current propagation the current is deeper than half of the domain depth time dependent numerical experiments are conducted for a variety of parameter settings including various depths and strengths of the shear layer the numerical results agree closely with the theoretical analyses numerical experiments are also performed for a case where the initial depth of the density current is set to be comparable to the low level shear which is much shallower than that given by the steady state solution the circulation at the density current head remains shallow and is nonsteady in this case whereas the time averaged flow still exhibits a deep jump updraft pattern that is close to the theoretical solution suggesting the applicability of the theoretical results to even more transient flows the simulated flow features are discussed in terms of balanced and unbalanced dynamics and in the context of forcing and uplifting at the frontal zone in long lived convective systems here the term balance refers to a flow configuration that satisfies the steady state solution of the idealized theoretical model

the sea breeze front analytical model abstract analytical solutions to the nonlinear equations of motion are used to describe the sea breeze front it is found that the front can develop when the atmosphere stratification in the heated layer is neutral or unstable the temperature drop due to front passage is proportional to the square of the difference between the front speed and the synoptic wind this square of the difference on the other hand is proportional to 2 s m ga where s is the mean potential temperature drop across the front and a is the front radius fronts which have the same speed of propagation and size have a larger temperature drop when the synoptic wind blows in the opposite direction to the direction of propagation onshore winds associated with strong convergence are obtained in the lower part of the front while the return current associated with strong divergence is observed in the upper part the vertical velocity reaches values of some meters per second in the front region the front propagation is studied

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

unroofing of active metamorphic core complexes in the d entrecasteaux islands papua new guinea metamorphic core complexes in the d9entrecasteaux islands are forming as a result of active extension at the western end of the woodlark basin spreading center high grade metamorphic rocks exist in the cores of gneissic domes including eclogites formed at minimum depths of 45 75 km i e 13 21 kbar high temperatures were maintained during uplift of basement rocks from 13 21 kbar t 730 900 c through 7 11 kbar t 570 730 c to 5 6 kbar t 730 c subsequent rapid cooling 100 c m y of the basement occurred between 1 and 2 ma on the basis of 40 ar 39 ar geochronology the core zone rocks were transected by kilometre scale amphibolite facies ductile shear zones active at depths of 25 35 km extensive intrusion of granodiorite plutons occurred and the age of granodiorite intrusions constrains late stage shear zone activity to 2 ma and younger unroofing was accomplished by faults and shear zones bounding the gneiss domes juxtaposing them against ultramafic rocks and sedimentary deposits surface uplift continues today and has led to topographic elevations of up to 2 5 km

deformation metamorphism and imbrication of the indian plate south of the main mantle thrust north pakistan south of the main mantle thrust in north pakistan rocks of the northern edge of the indian plate were deformed and metamorphosed during the main southward thrusting phase of the himalayan orogeny in the hazara region between the indus and kaghan valleys metamorphic grade increases northwards from chlorite zone to sillimanite zone rocks in a typically barrovian sequence metamorphism was largely synchronous with early phases of the deformation the metamorphic rocks were subsequently imbricated by late north dipping thrusts each with higher grade rocks in the hanging wall than in the footwall such that the metamorphic profile shows an overall tectonic inversion the rocks of the hazara region form one of a number of internally imbricated metamorphic blocks stacked after the metamorphic peak on top of each other during the late thrusting this imbrication and stacking represents an early period of post himalayan uplift

some thermal and mechanical consequences of rapid uplift an example from the southern alps new zealand abstract the thermal evolution of continental crust during active collision is modeled through numerical solutions of the two dimensional heat conduction equation for a rapidly moving medium the boundary conditions used in the modeling are derived from geological and geophysical observations from the active collision zone in the south island of new zealand the problem domain over which the solutions are obtained consists of a 40 km horizontal by 25 km vertical spatial plane with a vertical discontinuity at 10 km from the western boundary to the east of this discontinuity vertical uplift rates of up to 10 mm a occur over a timespan of up to 4 ma temperature distributions are calculated at 10 ka intervals over the 4 ma duration a two dimensional high temperature region is established upon initiation of uplift of the eastern block due to the advective component carrying heat upwards more rapidly than it can be dissipated laterally from the problem domain temperatures within the upper 5 km are greater than 400 c after 2 25 ma with geothermal gradients of up to 200 c km attained within the upper 3 km at times greater than 2 5 ma the vertical temperature distribution changes little while the anomalously high temperatures spread laterally into the stationary crust using rheological equations to describe the brittle behaviour of a water saturated upper crust and the ductile behaviour of a quartz dominated lower crust together with the thermal distribution of the conduction models the mechanical evolution of a collision zone is investigated in addition to high crustal temperatures and associated high heat flow rapid uplift produces a weakening of the crust by raising of the depth of transition from brittle to ductile behaviour within the zone of most rapid uplift the brittle ductile transition rises from 13 km to less than 5 km after 1 5 ma of uplift further uplift reduces the brittle layer to 3 km thickness and causes lateral spreading of the low strength zone the reductions in crustal strength caused by the thermal weakening produce a high strain zone within the region of maximum uplift which is incapable of sustaining large differential stresses this causes horizontal and vertical stress transfer and results in shallow seismicity increases in the adjacent crust as well as in intermediate depth seismicity within the high strength upper mantle because the thermal and mechanical anomalies discussed are a function of rapid uplift all regions of active continental collision may be expected to exhibit similar behaviour some mechanical and thermal characteristics of the himalayan collision zone are briefly examined in light of the numerical modeling

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

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

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

frontogenesis in a fluid with horizontal density gradients the adjustment under gravity of a fluid containing a horizontal density gradient is described the fluid is initially at rest and the resulting motion is calculated as the flow accelerates driven by the baroclinic density field two forms of the initial density distribution are considered in the first the initial horizontal gradient is constant a purely horizontal motion develops as the isopycnals rotate towards the horizontal the vertical density gradient increases continually with time but the horizontal density gradient remains unchanged the horizontal velocity has a uniform vertical shear and the gradient richardson number is constant in space and decreases monotonically with time to the second density distribution consists of a piecewise constant gradient with a jump in the gradient along a vertical isopycnal the density is continuous in this case frontogenesis is predicted to occur on the isopycnal between the two constant density gradient regions and the timescale for the formation of a front is determined laboratory experiments are reported which confirm the results of these calculations in addition lock exchange experiments have been carried out in which the horizontal mean gradient is represented by a series of step density differences separated by vertical gates

heat focussing granite genesis and inverted metamorphic gradients in continental collision zones abstract the thermal conductivity of sedimentary rocks is usually much lower than that of crystalline rocks gneisses and granites continental collision zones are characterized by large scale thrusts which superpose crystalline basement and sedimentary layers this leads to heat refraction effects of significant magnitude sedimentary layers act as a barrier to the transport of heat we show that the peculiar geometry of a gently sloping sedimentary layer thrust into more conductive rocks is very efficient in diverting the vertical flux of heat temperatures can vary by more than 50 along the horizontal and reach a marked maximum at the top of crystalline layers these results are in general agreement with the main features of orogenic belts as exemplified by the himalayas they explain why the young leucogranites of the main central thrust mct are confined to a narrow belt at the top of the basement slab they also account for the inverted metamorphic gradient observed there in a crustal accretionary wedge folding and thrusting modify the structure of the upper crust by thickening the sedimentary cover and by introducing sedimentary rocks at depth this leads to high temperatures at shallow depths in specific places les zones de collision continentale se caracterisent par de grands chevauchementsafaible pendage qui empilent les unes sur les autres des couches de socle cristallin et des couches sedimentaires cette structure particuliere engendre d importants effets de refraction thermique en effet leur conductibilitethermiqueetant normalement bien plus faible que celle des roches cristallines les formations sedimentaires font office de barrieres pour le transfert thermique le flux de chaleur se trouve fortement devie dans un plan horizontal la temperature peut varier de plus de 50 les maximaetant situesal interface superieur des dalles cristallines en ces endroits precis les isothermes profonds remontent tres significativement ces resultats rendent bien compte de plusieurs faits essentiels observes dans l himalaya zoneographie inverse du metamorphisme localisation des leucogranites le long d une bandeetroite situee au sommet de la dalle du tibet

anatomy of a continental subduction zone the main mantle thrust in northern pakistan the himalayas form an ideal natural laboratory to study the deformation processes of continental crust during collision orogeny new information is presented concerning the structural evolution of the main mantle thrust zone in the himalayas of n pakistan in the region around nanga parbat the hanging wall lies at relatively high levels within the kohistan arc terrane which has been emplaced onto indian continental rocks this thrust probably originated as a breakback structure in the hanging wall to the pre collisional oceanic subduction zone the present hanging wall contains a shear zone of about 1 km width developed under amphibolite facies conditions simple shear dominant strains have developed new fabrics parallel to the main thrust zone however these structures are redeformed by discrete extensional and compressional shears within about 100 m of the thrust contact again developed under amphibolite facies conditions the footwall consists of an old basement complex the nanga parbat gneisses overlain by a distinct suite of metasediments now at amphibolite facies this cover assemblage of psammites pelites and marbles with local metabasites consistently lies directly against rocks derived from the kohistan arc along the mmt the absence of structures suggestive of consistent rheological contrasts within the cover assemblages suggests that the vast majority of the deformation features they contain were developed only once they experienced substantial tectonic overburdens prior to this the indian cover rocks will have been passively subducted beneath the kohistan arc until into amphibolite facies we discuss these inferences in terms of evolving shear zone width with time and comment on the implications for predicting the character of mid deep crustal shear zones particularly from seismic reflection profiles

a note on the structure of the head of an intrusive gravity current the head of an intrusive flow advancing along the interface between two fluids is studied experimentally when the two layers are of equal depth and the density of the intrusion is the mean of the two densities the dependence of the flow on the interface thickness and the depth of the intrusion is determined when the interface is very thin the flow is similar to the nominally inviscid gravity currents observed by britter simpson 1978

an investigation of convection behind the cape canaveral sea breeze front abstract a three dimensional numerical simulation of land water circulations near cape canaveral florida is performed using the advanced regional prediction system the role of kelvin helmholtz instability khi in determining the time and location of convection behind the sea breeze front is examined the model configuration attempts to improve upon limitations of previous work e g resolution surface characteristics initial state it provides a detailed and realistic simulation of the desired features the simulation exhibits a single precipitating storm that forms behind the sea breeze front this postfrontal storm develops when an outflow boundary intersects a deep layer of upward motion above the marine air the region of ascent initially is the remnant of a cell that formed along the sea breeze front but before the cell decays a portion of its upward motion is intensified and displaced the modification of the ascent is a product of khi that is occurring on top of the sea breeze interface i

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

dynamics of a thunderstorm outflow abstract the kinematic and thermodynamic structures of a thunderstorm outflow are examined by means of dual doppler radar analysis mesonet lower and sounding data the data were collected in the denver colorado area during june 1984 the dual doppler analysis shows that the cold outflow is ducted beneath the pbl inversion along the gust front there is a narrow quasi two dimensional updraft kelvin helmholtz instability khi developed along the top of the gust front head near the surface front and propagated backwards dissipating in the wake of the head region an isothermal layer aloft appears to have limited billow amplification to the quasi neutral layer below the gust front s leading edge had numerous inflections which are believed to result from barotrophic instabilities small vortices develop at some of the inflection points detailed analysis of one such circulation shows evidence of the formation of two enhanced updrafts separated by an occlusion downdraft these observations are the firs

numerical simulation of thunderstorm outflow dynamics part i outflow sensitivity experiments and turbulence dynamics abstract in this first paper of a two part series a two dimensional numerical model is developed and used to investigate the dynamics of thunderstorm outflows by focusing only on the outflow and using essentially inviscid equations and high spatial resolution we are able to explicitly represent important physical processes such as turbulent mixing to simplify interpretation of the results the model atmosphere used in all experiments is calm and dry adiabatic this approach allows us to establish basic characteristics of modeled outflows in simple physical settings and provides a foundation for future studies using more realistic environments all simulated outflows are initialized by prescribing a controlled horizontal flux of cold air into the model domain through a lateral boundary in a series of sensitivity tests we examine three parameters of the cold air source region 1 the vertical temperature deficit profile 2 the magnitude of the temperature deficit and 3 the cold air depth by hol

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

duration of strike slip movements in large shear zones the red river belt china abstract sixty four size fractions of zircon monazite xenotime and titanite were dated by the u pb method to temporally and spatially constrain left lateral movements in the red river shear zone between the depths of 15 and 20 km the shearing led to the formation of the high grade metamorphic gneisses that form the bulk of the 4200 m high diancang shan and 3100 m high ailao shan mountain ranges the omnipresence of melt layers and pockets in these gneisses shows that temperatures associated with tectonometamorphism lie at and above the solidus of continental crust and deformation patterns are very homogeneous over the entire length of the belt north of the gneisses subvolcanic bodies intrude unmetamorphosed cover rocks in particular eocene red beds along the projected trend of the red river shear zone for a leucogranitic layer in the diancang shan gneisses an age of 24 2 0 2 ma 2 is obtained for monazite and 22 4 0 2 ma is obtained for xenotime whereas zircon formed through two growth stages at 24 23 ma these ages record solidification of the anatectic melts over a period of at least 1 8 0 4 ma during which left lateral shear was continuously active in the ailao shan gneisses zircon and titanite ages are 26 3 0 3 and 26 1 0 2 ma for two monzonitic bodies and 24 1 0 2 and 22 4 0 2 ma for two pegmatitic layers that occur within the monzonitic intrusions these results show that zircon monazite titanite and probably also xenotime are closed systems for u and pb under melting conditions 650 c therefore the ages date mineral formation in the melt rather than cooling below certain closure temperatures a significantly older age of 35 0 0 1 ma is obtained for a monzonitic subvolcanic intrusion north of the diancang shan gneisses because these intrusions follow the trend of the shear zone they most likely represent the surface expression of early activity along that zone indicating that strike slip movement had already occurred in latest eocene time the total set of u pb ages shows that magmatic and metamorphic activity occurred from 35 to 20 ma strengthening the idea that the red river shear zone absorbed most of the intraplate movements required to compensate the opening of the south china sea between 32 and 16 ma the existence of monzonitic syenitic intrusions along the red river zone implies that melting affected source rocks other than the diancang shan and ailao shan gneiss protoliths potential sources to produce such rocks lie in the deep crust possibly including melts from the mantle consequently the red river shear zone must be a deep rooted structure along which melts migrated to the surface moreover the contemporaneity of shear motion and magma generation suggests that melting occurred in direct relation to strike slip activity

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

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

plutonism and the origin of metamorphic core complexes the formation of metamorphic core complexes may be triggered by plutonic activity during episodes of continental extension pulses of ductile deformation have taken place during short lived thermal events initiated by the heat input from intruded plutons sills or dikes such intrusions may be the underlying cause for differential uplift of the footwall during tectonic denudation of metamorphic core complexes fast cooling inferred from 40 ar 39 ar apparent ages may have taken place after periods of magma arrival and need not be the result of rapid erosional or tectonic denudation heterogeneity of 40 ar 39 ar apparent ages can be explained by rapid cooling of deforming mylonites formed at shallow crustal levels in the thermal aureoles of intruded sills or sill swarms under such circumstances apparently isothermal decompression paths may actually link pressure temperature points set during periods of transient mineral growth during and after periods of igneous activity

role of lithospheric strength heterogeneities in the tectonics of tibet and neighbouring regions unlike rigid oceanic plates continents show diffuse deformation for example the tibetan plateau that has accommodated much of the convergence of india with asia we interpret the tertiary deformation of asia as the three dimensional strain of a non linear viscous continental lithosphere lateral strength contrasts modify this deformation stress concentration around the relatively strong tarim basin gives rise to local crustal thickening in the tien shan to the north and to strike slip in the altyn tagh along its southern margin

mazeno pass pluton and jutial pluton pakistan himalaya age and implications for entrapment mechanisms of two granites in the himalaya zircon and monazite u th pb ion microprobe analysis were performed on the mazeno pass pluton and the jutial pluton two leucogranite bodies within the nanga parbat haramosh massif nphm pakistan himalaya zircon rim ages and monazite ages indicate the mazeno pass pluton in southwest nphm intruded at 1 40 0 05 ma the jutial pluton to the north similarly yields concordant zircon and monazite ages suggesting crystallization at 9 45 0 06 ma the jutial pluton was subsequently intruded by leucogranite dikes at 5 3 ma as revealed by monazite ages concordancy of u pb and th pb accessory mineral ages demonstrates the robustness of the technique on young rocks both plutons some of the youngest in the himalaya have a general association with nearby shear zones that we interpret to have played an integral role in granite evolution and emplacement setting deformation enhanced ascent together with new field observations these results provide an insight on the spatial and temporal relationship between plutonism and deformation relating to the development of the massif

zircon response to diamond pressure metamorphism in the kokchetav massif ussr ultrahigh pressure metamorphism of the diamond bearing gneisses in the kokchetav massif ussr has been dated at 530 7 ma 2 sgr by shrimp ion microprobe analysis of metamorphic zircons zircon xenocrysts as old as 2000 ma have retained original isotopic compositions through the extreme metamorphic conditions and indicate that the protolith that was buried to diamond forming pressures included early proterozoic components highly variable reset ting of u pb isotopic systems in zircon xenocrysts at the time of metamorphism suggests that diffusive lead loss had commenced but was incomplete a disequilibrium observation consistent with relatively transient exposure of the rocks to extreme crustal depths

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

the gangotri granite garhwal himalaya laccolithic emplacement in an extending collisional belt the gangotri miocene leucogranite is composed of several laccoliths 6 7 km long 1 5 2 km thick which can be divided into two sets of lenses the southern lenses intruded in the lower part of the high himalaya sedimentary cover and the northern lenses intruded in an older porphyritic biotite bearing granite in both cases the magmatic fabric is commonly weak with no dominant stretching direction although a rough e w trend is present in the northern lenses the magnetic fabric is characterized by the fact that drastic changes in the direction in the magnitude of the magnetic parameters or in the fabric type planar or linear may take place over very short distances measurements of preferred orientations in thin sections indicate that the strain regime was largely dominated by a coaxial component for both sets of lenses in agreement with the large dispersion observed in both the field and magnetic lineations the comparison of the granite and host rock structures shows that the leucogranite emplacement dates the onset of the extensional tectonism in the high himalaya range and is not related to a southward directed thrust event associated with the main central thrust this is exemplified by the vertical attitude of the feeder dikes that intrude the metasedimentary rocks beneath die southern lenses the presence of these dikes indicates in turn that magma ascent occurred by fracture propagation the spatial disposition of the southern lenses could have resulted either from the disruption of a single laccolithic intrusion by crustal scale boudinage due to a northern gravity backslide of the top of the tibetan slab or from the intrusion of independent laccoliths the use of the elastic bending theory of pollard and johnson 1973 shows that in both hypotheses the current laccolith sizes are compatible with a laccolithic mode of magma emplacement however neither the density contrast between the magma and its enclosing rocks nor the lithological boundary between the tibetan slab and the overlying tibetan metasedimentary series controlled the level of magma emplacement rather flat lying collapse structures which intersected the upward propagating magma dikes are the most likely causes of magma arrest such a mechanism was favored by the schist rich lithology of the metasedimentary host rocks in addition field relationships indicate that the melt supply through the dike system was a continuous rather than pulsed process existing numerical treatments on the rates of magma transport through fractures show that in such a case the laccoliths could have been built in less than 100 years this short time of emplacement the small size of the laccoliths and their peripheral disposition relative to the badrinath granite suggest that the gangotri lenses may represent the initial stage of pluton accretion in the high himalaya which ultimately gave rise to a much larger massif such as the manaslu granite

brittle and ductile deformation in a zone of rapid uplift central southern alps new zealand schists exposed in the central southern alps new zealand 1 5 km east of the alpine fault zone have been rapidly uplifted during the late cenozoic a regionally consistent sequence of mesoscopic structures discordant to the schistosity is recognized extending backward in time from recent brittle displacements of deglaciated surfaces through structures exhibiting brittle ductile transitional behaviour to ductile deformational features kinematic analysis of these structures gives consistent principal subhorizontal shortening directions similar to the present day principal horizontal shortening direction indicating their relation to late cenozoic uplift of the southern alps analysis of deformed veins suggests a minimum shortening strain of about 50 perpendicular to the foliation the veins crosscut upright mesoscopic to macroscopic folds which commonly develop high strain zones on their limbs within such zones a strong stretching lineation plunges gently sw approximately perpendicular to the stretching lineation in the mylonites along the alpine fault the folds and high strain zones do not appear to be related to late cenozoic uplift but may have originated during an earlier phase of dominantly strike slip motion fluid inclusion studies give depth temperature estimates of around 3 km and 285 c for the development of the brittle structures and 6 8 km at 310 350 c for the brittle ductile transition the deformed veins show evidence of extensive fluid infiltration during the later phases data from fluid inclusions and metamorphic assemblages give an estimate of 15 20 km and 400 450 c for their deformation these data combined with uplift rates determined by other studies allow the construction of a depth temperature time path which indicates nearly isothermal decompression associated with rapid uplift a shallow brittle ductile transition 6 8 km is consistent with numerical modeling of thermal and mechanical behavior of the crust during rapid uplift associated with continental collision and with the presence of high heat flow in the area because of the small temperature change over a large amount of decompression pressure may be an important factor in controlling the depth of the brittle ductile transition the marked temperature drop above the brittle ductile transition is thought to be enhanced by the influence of convection as an effective cooling mechanism

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

in situ particle size spectra and density of particle aggregates in a dredging plume abstract an in situ laser particle sizer the lisst 100 was used to describe the spatial variation of beam attenuation coefficient in situ particle size spectra and aggregate densities in a dredging plume in the sound oresund between denmark and sweden the results proved that the above mentioned parameters varied significantly within the investigated length of the plume approximately 2 km it is shown how the small single primary particles aggregate and change the in situ particle size spectra into a slightly better sorted and coarser size distribution and at the same time how the mean density of particles aggregates decreases significantly this shift in the state in which the particles exist in the water effectively changes the optical response of the mass of particles suspended in the water it is shown that adequate correlations between mass concentrations and beam attenuation coefficients can only be obtained if parameters describing the in situ quality and state of the mass of particles e g standard deviation of the size spectra and volume concentration is included in the regression from the spatial variation in mean density and in situ particle size it was possible to calculate the spatial difference in settling velocity it was found that the difference in settling velocity was only about a factor of 1 7 because increasing in situ particle size was counter balanced by decreasing mean density furthermore the time scale of flocculation within the plume was found to be in the order of 50 min

tibet uplift and erosion abstract the 5 km high tibetan plateau is an outstanding topographic feature on the earth today its horziontal extent elevation and location cause significant effects on modern atmospheric circulation and climate so the history of uplift of the surface of tibet is linked to cenozoic climate changes at local regional and perhaps global scales geological and geophysical studies of the plateau are contributing data on the present and past deformation of the tibetan lithosphere that has formed the plateau primarily during the cenozoic the principal of isostasy then can be used to estimate the elevation history of the surface for a given deformation history different parts of tibet probably had different uplift histories but presently available data are not sufficient for distinguishing many contrasts in one scenario cretaceous and early cenozoic north south distributed shortening of tibetan crust and mantle lithosphere probably caused significant uplift of the surface relative to sea level to perhaps half of the present elevation by the early miocene thinning of the high density mantle portion of the lithosphere during the miocene may then have allowed the thick tibetan crust to rise close to its present elevation perhaps higher before 8 ma since then slow east west extension of tibet probably reduced the crustal thickness slightly and may have caused the elevation of the plateau to decrease during the late cenozoic erosion of tibet unlike narrow mountain belts has been unable to match the uplift of a broad plateau orographic precipitation and efficient river networks concentrate erosion on the edges while the interior is protected from significant erosion despite its lofty elevation the southern edge of the plateau the himalaya has suffered a minimum of 25 km of denudation since the miocene while central tibet shows little or no sign of major erosion since that time the gangdese arc in southern tibet was rapidly eroded during the mid miocene when 4 km of rock apparently were removed from the surface as shown by mineral cooling ages this pulse of erosion was probably caused by a combination of local thrust system movement and changes in base level and precipitation due to relative elevation changes between the gangdese and the himalaya to the south the modern long wavelength flatness of tibet is unlikely to have been caused by erosion and indicates viscous flow at some level of the lithosphere has been acting to level the plateau surface

exhumation overrated at nanga parbat northern pakistan abstract new 40 ar 39 ar laserprobe and thermobarometric data from the nanga parbat haramosh massif nphm in northern pakistan provide important constraints on thermal models of the evolving geotherm of the massif simple thermal models indicate that the unusually young cooling ages from this area do not necessarily imply extreme exhumation rates of about 7 mm y because the advection of heat resulting from rapid exhumation leads to a steepened near surface geotherm the most important control on the present day thermal structure of the nphm appears to be the thermal structure resulting from obduction of the kohistan arc some 50 m y ago transient geotherms calculated for the past 10 ma can only be reconciled with published geochronological and thermobarometric studies if exhumation rates lay in the range of 3 to 4 mm y still considerably greater than exhumation rates determined from the main himalayan orogen

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

melt enhanced deformation a major tectonic process convergent tectonics between continental crustal blocks result in deep burial and anatexis of supracrustal rocks anatectic and or mantle derived melts combine to form a melt weakened zone in the thickened lower crust the accumulation of melt eventually leads to crustal failure along melt lubricated shear zones rapid 1 mm yr vertical component movements of large crustal blocks result we refer to these crustal displacements as tectonic surges the melt lubricated shears are characterized by the close association of sheared country rocks with foliated or massive igneous sills and plutons rocks that formed at different crustal levels are juxtaposed across these shear zones one result of surges with large lateral component of movement is metamorphic inversion with high pressure and high temperature metamorphic rocks structurally over lower pressure and temperature assemblages large and rapid vertical surges may displace crust containing abundant melt and may result in high t p including granulite facies metamorphism and preservation of metamorphic textures caused by rapid decompression

kinetics of pb release during the zircon evaporation technique abstract the release of silica and pb from zircon by the thermal evaporation technique is shown to relate to the conversion of zircon to a porous baddeleyite structure the zircon baddeleyite reaction occurs at a sharp reaction front which progressively migrates towards the centre from the grain margin significant reaction in non metamict zircon commences in the mass spectrometer at temperatures of the order of 1500 c the progressive migration of zircon baddeleyite transformation towards grain interiors explains why the evaporation technique with its inherent advantage of not requiring chemical separation of pb and u progressively samples pb in zones towards the centre of the grain and gives 207 pb 206 pb ages as accurate as conventional u pb dating methods the rates of pb and silica loss are primarily determined by the rate of advance of this de silicification reaction front towards the interior of the grain most estimates of pb diffusion coefficients in zircon imply that diffusion distances in the unreacted zircon are negligible 10 2 m at the temperatures and duration of the pb evaporation experiments isothermal experiments demonstrate the rate of zircon baddeleyite transformation with time and temperature and provide an estimate of 130 10 1 kcal mol 1 for the activation energy of the process significantly higher than estimates of pb diffusion activation energy

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

time relationships between magmatism tectonics and metamorphism in three plutonic belts in southern tibet new k ar data from north to south the three plutonic belts in southern tibet are the transhimalaya or kangdese the north himalaya or lhagoi kangri and the high himalaya 25 new mineral and whole rock k ar dates added to previous data enable a precise analysis to be made of the time relationships between different magmatic tectonic and metamorphic events that occurred on both sides of the indus tsangpo suture zone these include 1 the emplacement of a large part of the eastern transhimalaya magmatism occurred during eocene time suggesting a close relationship between this emplacement and the late stages of conver gence between india and eurasia 2 south of the suture the ages obtained fall mainly between 11 and 15 ma the greatest age allows us to fix a lower limit on the timing of metamorphic recrystallization of the paleozoic north himalaya granites an intermediate age may correspond to a minimum age of emplacement of the high himalaya and part of the north himalaya cenozoic granitoids finally the youngest age may characterize the northernmost cenozoic granitoids of the north himalaya belt the consistency of these tenuous age variations with field and structural data suggests that they may be significant 3 our determinations give ages often significantly younger than those usually proposed for the same events as suggested by some authors such a discrepancy might reflect relatively late india eurasia collision in this eastern part of the suture

observations of atmospheric density currents using a tethered balloon borne turbulence probe system a tethered kite balloon with multiple instrumented probes clamped to the tethering cable is used to investigate atmospheric density current fronts passing through three sites at varying distances from the coast the system allows high frequency measurements of the three components of wind temperature and humidity to be made simultaneously at up to ten levels as the front passes through this enables a two dimensional visualization of the current and its circulation to be constructed as a result the structure of some of the wide variety of atmospheric density currents is revealed by direct observation the internal circulations and the effects of varying environments on the currents and their movement are described together with turbulence statistics one current was associated with what may have been a coastal trapped disturbance and in one case two opposing sea breezes appear to have collided inland two currents penetrating into surface inversions were dissociated from the surface one of these was travelling downwind and formed a strong near surface jet associated wave and bore structures were found to be travelling in layers above the currents in two cases in one case environmental air was found to intrude into the head region and convect internally while in another air from the head region extruded into the environmental flow the measurement array enabled the different terms of the dynamic equations to be evaluated and their relative contribution assessed a remarkable correlation is found between thermal and dynamic variables a study of the internal flux balance of daytime density currents shows that the internal flow plays a significant part in maintaining the capping inversion of the current eddies were observed at interfacial regions in which richardson numbers were significantly greater than 0 25 confirming some modelling studies frontal slopes in most cases are in accord with theoretical values but in one downwind case the slope is considerably less than predicted the inland currents are found to be deeper than coastal currents which may explain the increase in speed observed later in the day water vapour frontal structures are generally less marked than thermal fronts

a numerical study on sea land breezes as a gravity current kelvin helmholtz billows and inland penetration of the sea breeze front abstract a two dimensional nonhydrostatic compressible and dry numerical model was developed by using high spatial resolution and an appropriate turbulence model fine structure and dynamics of the sea breeze head were investigated kelvin helmholtz kh instability was found to occur in the foremost part of the head consequently kh billows were generated there and grew in amplitude while travelling backward relative to the advancing front along the zero velocity boundary the kh billows entrained the upper air into the sea breeze the resultant turbulent mixing and wave perturbations induced a top friction force acting on the sea breeze head structure of the kh billow and the wavelike motion induced by it near the ground were also investigated and compared with the results obtained in laboratory density currents linear theories and observations the kh instability khi did not occur at all stages of the sea breeze and this resulted in a diurnal variation of the fine structure and dynamics of

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

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

the tectonic history of kohistan and its implications for himalayan structure the tectonic history of kohistan northern pakistan involves two collisional events cleavage and folding developed at 90 100 ma along the northern suture between the kohistan island arc and the asian plate at the same time there was major folding and shearing of the lower part of the kohistan arc approximately 100 km south of the suture this deformation was followed by ocean subduction south of the kohistan arc generating the kohistan calc alkaline batholith with subsequent ocean closure during the eocene and obduction of the kohistan arc together with the adjacent part of the asian plate over the indian continental crust the construction of balanced cross sections through the imbricated upper part of the indian continental crust in the footwall to this southern suture indicates a minimum displacement of 470 km requiring the western himalayan hinterland to be underlain by a large wedge of indian middle to lower crust there is some shortening of the overriding kohistan and asian plates by thrusts and shear zones but it is insufficient to satisfy the palaeomagnetic data there must be major crustal shortening involving thrusts in the hindu kush and pamirs north of kohistan the post eocene thrust direction which for most of pakistan is towards 160 is almost perpendicular to that immediately to the east in the himalayan belt generating complex refolded thrust patterns in the hazara syntaxis and large scale folding and rapid uplift with associated brittle faulting and seismic activity adjacent to the nanga parbat syntaxis these different thrust trends indicate that major thrust movement as well as the folds and deformation fabrics cannot always be related to plate movement vectors but are modified by or develop from complex rotations during place collision or from the gravitational spreading of a thickened crust a regional approach is required to recognize and correctly attribute the various components in thrust displacements

cryptic crustal events elucidated through zone imaging and ion microprobe studies of zircon southern appalachian blue ridge north carolina georgia compositional zoning reveals multistage growth histories and resorption events in zircon from a high grade terrane in the eastern blue ridge of north carolina and georgia these zoning patterns were used to guide high resolution ion microprobe dating that places important constraints on the evolution of the southern appalachian crust zircons from granulite facies metapelite have unzoned rims that yield concordant u pb ages of 495 14 ma we interpret this as the time of rim growth which occurred during peak metamorphism early in the protracted orogenic history of the region detrital cores characterized by truncated euhedral zoning are of grenville age 1 04 1 26 ga zircons from the whiteside and rabun plutons have well defined rounded inherited cores and euhedral oscillatory zoned magmatic rims rims of rabun zircons record magmatic crystallization at 374 4 ma whereas whiteside rims yield a 466 10 ma crystallization age cores from both plutons include 1 1 1 3 ga and 2 6 2 7 ga ages these data indicate that there was no single voluminous episode of plutonism in this area that similar material underpinned the region at least from 370 to 470 ma and that previously unrecognized archean basement or archean basement derived sedimentary rock was present in the southern appalachians results of this study verify the value of combining zoning and ion microprobe studies using conventional u pb methods or ion microprobe dating without knowledge of zoning would have made interpreting the events recorded in these zircons and the ages that they yield difficult or impossible

diamond inclusions in garnets from metamorphic rocks a new environment for diamond formation diamonds commonly occur in kimberlites lamproites and alluvial sediments derived from these rocks more recently diamonds or their graphite pseudomorphs have been discovered in ultramafic massifs1 and picrites2 here we report the occurrence of diamonds in situ in crustal rocks highly retrograded high pressure metamorphic garnet pyroxene and pyroxene carbonate garnet rocks biotite gneisses and schists from the kokchetav massif northern kazakhstan ussr the diamonds are cubo octahedral averaging 12 m in size and occur in zircons and with euhedral graphite as inclusions in unzoned garnets we believe that the zircon and garnet matrices protected these diamonds from retrogressive transformation to graphite mica rutile titanite clinopyroxene kyanite and zircon also occur as inclusions in garnet often intergrown with the diamonds equilibration relations of inclusions and host garnets indicate that both diamonds and graphite crystallized from a fluid phase under static conditions at pressures of 40 kbar and temperatures 900 1 000 c

lead diffusion in apatite and zircon using ion implantation and rutherford backscattering techniques abstract ion implantation was used to introduce pb into the minerals zircon and apatite diffusion profiles were obtained using rutherford backscattering and the results were fitted with a model to determine the diffusion coefficients this approach is both simple and useful in studying diffusion over a temperature range of geologic interest without inordinate annealing times results for apatite over the temperature range 600 900 c are in good agreement with earlier results obtained for higher temperatures watson et al 1985 and are described by the following arrhenius law d 1 27 10 4 cm 2 s 1 exp 54 6 1 7 kcl mol 1 rt this suggests that radiation damage induced by ion implantation has little effect on pb diffusion in this case due to rapid annealing of induced damage at low temperatures the activation energy stated above is somewhat smaller than that previously determined 70 kcal mol 1 reflecting the larger range in 1 t sampled by the present results closure temperatures calculated with these diffusion parameters are in good agreement with those inferred from geochronologic data diffusion coefficients for pb in zircon determined in this study are greater than those obtained from a single experimental measurement and estimates of pb diffusion based on geochronologic data and u pb zoning in natural zircons the differing results for zircon and apatite obtained in this study appear to be related to each mineral s ability to anneal the radiation damage induced by ion implantation

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

non normal growth of kelvin helmholtz eddies in a sea breeze the generalized stability of a sea breeze front is analyzed using a two dimensional model the objective is to understand the mechanisms leading to the shedding of eddies behind the sea breeze front as seen in observations laboratory experiments and numerical models regions with ri 1 4 the energy source for optimal growth is the vertical shear of the mean horizontal wind the vertical shear in the upper part of the front and the horizontal shear in the lower part the growth begins with vertical advection by the perturbation velocity of the mean flow momentum located in the upper part of the front perturbations eventually propagate away from the localized shear area and a feedback mechanism is needed for this growth to be sustained this feedback occurs through temperature anomalies in the upper part of the front inducing pressure gradient anomalies in the lower part these gradients lead to a growing vertical wind component and this vertical wind component then enters the upper part of the front which reinforces the extraction of energy thereby closing the feedback loop and leading to both normal mode instability and in the stable regime large non normal growth we find that both the instability and the non normal growth are vulnerable to parameter changes that weaken this feedback loop