electromagnetic images of modern and ancient subduction zones abstract one question frequently posed since the advent of plate tectonic theory is when did it begin correlations of zones of enhanced electrical conductivity with orogenic belts dated to half the age of the earth support the suggestion that plate tectonic processes have been in operation since at least the early proterozoic the causes for such zones are examined and whereas anomalies associated with modern and recent subduction collision systems can be readily explained an explanation for palaeozoic and proterozoic ones is more difficult correlation of geophysical characteristics associated with the trans hudson north america and svecofennian fen noscandia orogenic zones and consideration of the paleopole positions of those zones and the superior and slave provinces during the period 1 900 1 825 ga lead to the suggestion that a pan scandamerican orogenic zone was in existence at that time

two pulses of mineralization and genesis of the zhaxikang sb pb zn ag deposit in southern tibet constraints from fe zn isotopes abstract zhaxikang is one large sb pb zn ag deposit located in the north himalaya of southern tibet to date the genesis of this deposit still remains controversial here we present new pyrite fe and sphalerite zn isotopic data for the first three stages of mineralization fe zn isotopic data for mn fe carbonate that formed during the first two stages of mineralization and zn isotopic data for the slate wall rocks of the jurassic ridang formation to discuss the genesis of the zhaxikang deposit the overall 56 fe and 66 zn values range from 0 80 to 0 43 and from 0 03 to 0 38 respectively the 56 fe values of mn fe carbonates are lighter than those of associated pyrite in six mineral pairs indicating that the iron carbonates are preferentially enriched in light fe isotopes relative to pyrite the sphalerite has lighter 66 zn values than associated mn fe carbonates in three mineral pairs the 56 fe values of pyrite that formed during the first three stages of mineralization gradually increase from stage 1 0 33 to 0 09 through stage 2 0 30 to 0 19 to stage 3 0 16 0 43 in comparison the sphalerite that formed during these stages has 66 zn values that gradually decrease from stage 1 0 16 0 35 through stage 2 0 09 0 23 to stage 3 0 03 to 0 22 these data in conjunction with the observations of hand specimens and thin sections suggest that the deposit was overprinted by a second pulse of mineralization this overprint would account for these fe zn isotopic variations as well as the kinetic rayleigh fractionation that occurred during mineralization the temporally increasing 56 fe and decreasing 66 zn values recorded in the deposit are also coincident with an increase in alteration again supporting the existence of two pulses of mineralization the 56 fe values of the first pulse of ore forming fluid were calculated using theoretical equations yielding values of 0 54 to 0 34 that overlap with those of submarine hydrothermal solutions 1 to 0 however the 56 fe values of the stage 3 pyrite are heavier than those of typical submarine hydrothermal solutions which suggests that the second pulse of mineralization was probably derived from a magmatic hydrothermal fluid in addition the second pulse of ore forming fluid has brought some fe and taken away parts of zn which results the lighter 66 zn values of sphalerite and heavier 56 fe values of pyrite from the second pulse of mineralization overall the zhaxikang deposit records two pulses of mineralization and the overprint by the second pulse of mineralization causes the lighter 66 zn values and heavier 56 fe values of modified samples

electrically conductive crust in southern tibet from indepth magnetotelluric surveying the crust north of the himalaya is generally electrically conductive below depths of 10 to 20 km this conductive zone approaches the surface beneath the kangmar dome dipping north and extends beneath the zangbo suture a profile crossing the northern yadong gulu rift shows that the high conductivity region extends outside the rift and its top within the rift coincides with a bright spot horizon imaged on the indepth cmp common midpoint profiles the high conductivity of the middle crust is atypical of stable continental regions and suggests that there is a regionally interconnected fluid phase in the crust of the region

magnetic field modelling and interpretation of the himalayan tibetan plateau and adjoining north indian plains abstract a prominent magnetic low is seen over the himalayan tibetan plateau and the northern indian shield region in satellite anomaly data here we model the sources for the observed magnetic low in the region the induced magnetic field is modelled based on a crustal magnetisation model computed in a geographical information system with horizontal layers containing susceptibility derived from the geology and a tectonic map of the region and global seismic crustal thickness model the modelling results indicate that the upper crust of the himalayan tibetan plateau is magnetised while much of the middle crust and the lower crust appears non magnetic thermal modelling corroborates the predictions placing the curie isotherm in the southern tibetan plateau no deeper than 10 km the results suggest that the sources of the magnetic low observed in the region are likely to be a thin magnetised crust because of the elevated curie isotherm in this region the indian shield region is believed to extend up to the foothills of the himalayas in the north but most of the shield is obscured by a thick sediment cover of the gangetic plains to the north the magnetic field modelling suggests a weakly magnetic crust in the region indicating either the indian shield may not extend up to the base of the himalayas or the lower crust in the region has been modified

erosional modification and gully formation at meteor crater arizona insights into crater degradation processes on mars hydrogeological modification of meteor crater produced a spectacular set of gullies throughout the interior wall in response to rainwater precipitation snow melting and possible groundwater discharge the crater wall has an exceptionally well developed centripetal drainage pattern consisting of individual alcoves channels and fans some of the gullies originate from the rim crest and others from the middle crater wall where a lithologic transition occurs broad gullies occur along the crater corner radial faults deeply incised alcoves are well developed on the soft coconino sandstone exposed on the middle crater wall beneath overlying dolomite in general the gully locations are along crater wall radial fractures and faults which are favorable locales of erosion due to preferential rock breakup from faulting and groundwater flow discharge these structural discontinuities are also the locales where the surface runoff from rain precipitation and snow melting can preferentially flow causing erosion and crater degradation channels are well developed on the talus deposits and alluvial fans on the periphery of the crater floor caves exposed on the lower crater level point to percolation of surface runoff and selective discharge through fractures on the crater wall in addition lake sediments on the crater floor provide significant evidence of a past pluvial climate when the water table was higher and groundwater may have seeped from springs on the crater wall although these hydrological processes continue at meteor crater today conditions at the crater are much more arid than they were soon after impact reflecting a climatic shift this climate shift and the hydrological modifications observed at meteor crater provide insights for landscape sculpturing on mars during various parts of its history

crustal structure of the india asia collision zone southern tibet from indepth mt investigations abstract project indepth was initiated in 1992 to develop a better understanding of the deep structure and mechanics of the himalaya tibet region and magnetotelluric surveying was added during indepth phase ii in 1995 broadband and long period data were acquired in 1995 along a n s transect crossing the india asia collision zone the yarlung zangbo suture at 90 e longitude however these data failed to penetrate to the deep crustal and mantle depths due to low solar activity and the presence of the thick conductive crust deep crustal and upper mantle information is key to test the model of subcretion to southern tibet by the stiff indian mantle lid in order to try to obtain such information and to test along strike variability of the yarlung zangbo suture zone a magnetotelluric experiment was performed in 2001 roughly along longitude 92 e containing an ultra long period component mt data were acquired at 28 locations along the new profile with long period data at 15 of these and ultra long period data at five of these 15 initial comparisons of the models generated from the 1996 and 2001 datasets indicate little variation in electrical conductivity structure along strike

shear velocity structure radial anisotropy and dynamics of the tibetan crust geophysical and geological data suggest that tibetan middle crust is a partially molten mechanically weak layer but it is debated whether this low viscosity layer is present beneath the entire plateau what its properties are how it deforms and what role it has played in the plateau s evolution broad band seismic surface waves yield resolution in the entire depth range of the tibetan crust and can be used to constrain its shear wave velocity structure indicative of crustal composition temperature and partial melting and radial anisotropy indicative of the patterns of deformation we measured love and rayleigh wave phase velocity curves in broad period ranges up to 7 200 s for a few tens of pairs and groups of stations across tibet combining in each case hundreds of interstation measurements made with cross correlation and waveform inversion methods shear velocity profiles were then determined by extensive series of non linear inversions of the data designed to constrain the depth dependent ranges of isotropic average shear speeds and radial anisotropy shear wave speeds within the tibetan middle crust are anomalously low and also show strong lateral variations across the plateau the lowest mid crustal shear speeds are found in the north and west of the plateau 3 1 3 2 km s 1 within a pronounced low velocity zone in southeastern tibet crustal shear wave speeds increase gradually towards southeast whereas in the north the change across the kunlun fault is relatively sharp the lateral variations of shear speeds within the crust are indicative of those in temperature a mid crustal temperature of 800 c reported previously can account for the low shear velocities across lhasa in the north the temperature is higher and exceeds the solidus resulting in partial melting that we estimate at 3 6 per cent strong radial anisotropy is required by the data in western central tibet 5 per cent but not in northeastern tibet the amplitude of radial anisotropy in the crust does not correlate with isotropic average shear speed and by inference with crustal rock viscosity or with surface elevation instead radial anisotropy is related to the deformation pattern and is the strongest in regions experiencing extension crustal flattening as noted previously the growth of tibet by the addition of indian crustal rocks into its crust from the south is reflected in the higher crustal seismic velocities and thus lower temperatures in the southern compared to northern parts of the plateau more recently added rocks having had less time to undergo radioactive heating within the thickened tibetan crust gravity driven flattening the basic cause of extension and normal faulting in the southern western and central tibet is evidenced by pervasive radial anisotropy in the middle crust beneath the regions undergoing extension the overall eastward flow of the crust is directed by the boundaries and motions of the lithospheric blocks that surround tibet

thermoluminescence dating of the kamil impact crater egypt thermoluminescence tl dating has been used to determine the age of the meteorite impact crater at gebel kamil egyptian sahara previous studies suggested that the 45 m diameter structure was produced by a fall in recent times less than 5000 years ago of an iron meteorite impactor into quartz arenites and siltstones belonging to the lower cretaceous gilf kebir formation the impact caused the complete fragmentation of the impactor and the formation of a variety of impactites e g partially vitrified dark and light materials present as ejecta within the crater and in the surrounding area after a series of tests to evaluate the tl properties of different materials including shocked intra crater target rocks and different types of ejecta we selected a suite of light colored ejecta that showed evidence of strong thermal shock effects e g partial vitrification and the presence of high temperature and pressure silica phases the abundance of quartz in the target rocks including the vitrified impactites allowed tl dating to be undertaken the variability of radioactivity of the intracrateric target rocks and the lack of direct in situ dosimetric evaluations prevented precise dating it was however possible to constrain the impact in the 2000 bc 500 ad range if as we believe the radioactivity measured in the fallback deposits is a reliable estimate of the mean radioactivity of the site the narrower range 1600 400 bc at the 2 confidence level can be realistically proposed

imaging and observing the electrical moho abstract defining the depth to the base of the crust is of profound importance for understanding and developing theories of the secular variation of our planet including crustal formation and evolution since its discovery 100 year ago the seismic moho is almost ubiquitously used as the crust mantle boundary reference laboratory studies on crustal and mantle rocks suggest that there should be a concomitant step lie change in electrical conductivity an electric moho emoho at the crust mantle boundary we examine resolution properties of electrical and electromagnetic methods for imaging and observing an emoho for three generic models one to represent the archean one for the proterozoic and the third for the phanerozoic we show that in all three cases given the existence in most localities of a conducting lower crust compared to the upper crust and upper lithospheric mantle the problem is difficult and at the edge and often beyond confident resolution for some highly unusual localities however the lower crust is more resistive than the underlying upper mantle and in those cases it is possible to resolve an emoho but very high quality data are required examples of emoho observations around the globe are discussed focussing on the results from a site on the southwestern part of the slave craton northwestern canada

the canyon diablo impact event projectile motion through the atmosphere meteor crater is one of the first impact structures systematically studied on earth its location in arid northern arizona has been ideal for the preservation of the structure and the surviving meteoric material the recovery of a large amount of meteoritic material in and around the crater has allowed a rough reconstruction of the impact event an iron object 50 m in diameter impacted the earths surface after breaking up in the atmosphere the details of the disruption however are still debated the final crater morphology deep bowl shaped crater rules out the formation of the crater by an open or dispersed swarm of fragments in which the ratio of swarm radius to initial projectile radius cd is larger than 3 the final crater results from the sum of the craters formed by individual fragments on the other hand the lack of significant impact melt in the crater has been used to suggest that the impactor was slowed down to 12 km s by the atmosphere implying significant fragmentation and fragments separation up to 4 initial radii this paper focuses on the problem of entry and motion through the atmosphere for a possible canyon diablo impactor as a first but necessary step for constraining the initial conditions of the impact event which created meteor crater after evaluating typical models used to investigate meteoroid disruption such as the pancake and separated fragment models we have carried out a series of hydrodynamic simulations using the 3d code sova to model the impactor flight through the atmosphere both as a continuum object and a disrupted swarm our results indicate that the most probable pre atmospheric mass of the meteor crater projectile was in the range of 4x10 8 to 1 2x10 9 kg equivalent to a sphere 4666 m in diameter during the entry process the projectile lost probably 30 to 70 of its mass mainly because of mechanical ablation and gross fragmentation even in the case of a tight swarm of particles cd 3 small fragments can separate from the crater forming swarm and land on the plains tens of km away from the crater as individual meteorites starting from an impactor pre atmospheric velocity of 18 km s which represents an average value for earth crossing asteroids we find that after disruption the most probable impact velocity at the earths surface for a tight swarm is around 15 km s or higher a highly dispersed swarm would result in a much stronger deceleration of the fragments but would produce a final crater much shallower than observed at meteor crater

structure of the crust in the vicinity of the banggong nujiang suture in central tibet from indepth magnetotelluric data 1 magnetotelluric data from a 150 km long profile crossing the banggong nujiang suture bns central tibet acquired as part of the international deep profiling of tibet and the himalaya indepth project have been examined for crustal and upper mantle structure strike and dimensionality analyses demonstrate that regional scale electrical structures are two dimensional and oriented approximately parallel to surface geological strike as seen elsewhere in tibet the double thickness crust is generally characterized by resistive upper crust hundreds to thousands of ohm meters overlying conductive middle and lower crust tens to hundreds of ohm meters but in detail there are lateral variations at all levels regionally a northward transition from thick 45 km to thin 15 km resistive upper crust coincides with 1 the surface trace of the bns 2 a prominent strand of the karakorum jiali fault system 3 northward decrease in upper mantle seismic velocities and increase in attenuation and 4 pronounced northward onset of seismic polarization anisotropy the latter two seismological features have been taken to mark the northern limit of indian mantle lithosphere thrust beneath southern tibet on the basis of our electrical model we speculate that 1 the resistive upper crustal root beneath the neogene lunpola and duba basins was produced by crustal shortening localized along the northern edge of the lhasa terrane 2 the low midcrustal resistivity beneath the bns reflects enhanced neogene melting and or metamorphic dewatering of relatively fertile subduction zone complex rocks 3 observed steep upper crustal low resistivity anomalies are produced by hydrothermal fluids within active faults localized within and adjacent to the bns and 4 these strike slip and extensional fault arrays are surface manifestations of lithosphere penetrating shear localized along the northern edge of the underthrust indian plate

receiver function imaging of crustal suture steep subduction and mantle wedge in the eastern india tibet continental collision zone abstract to understand the along strike variation of crustal deformation and tectonic processes in the india tibet continental collision zone we deployed a linear array of broadband seismic stations along 92 e to image lithospheric structure our receiver function cross section reveals a prominent negative converter dipping 20 north from 10 55 km depth below sea level almost through the whole crust beneath the southern lhasa terrane we interpret it to be a manifestation of the yarlung zangbo suture zone yzs separating the continental crust of the indian and eurasian plates this implies the hypothesized channel flows of indian middle crust extruding southwards from tibet are limited at this longitude to the southernmost portion of the lhasa terrane a positive converter consistent with previous suggestions of eclogite formation is seen about 10 15 km above the moho and continuing 50 km north of the 20 dipping yzs converter we image this positive converter continuously from 60 km south of the surface trace of the yzs to the vicinity of the jiali fault supporting the interpretation of sub horizontal underplating of tibetan crust by indian crust to 31 n at 85 e on the hi climb transect however we also show a negative mantle converter sub parallel to the crustal yzs converter from the northern limit of the underplating indian lower crust to at least 140 km depth that we interpret as the base of tibetan lithosphere overlying an asthenospheric mantle wedge based on the lithospheric structure observed in this and other studies we infer that indian mantle lithosphere currently detaches from indian lower crust at the mantle suture that is nearly 50 km south of the surface trace of the yzs at 92 e south of the mantle suture suggested by indepth transect beneath the surface trace of the yzs at 90 e and far south of the mantle suture suggested to be at the 31 n northern limit of underthrusting indian lower crust suggested by hi climb transect at 85 e a change from underplating in the west to steep subduction in the east can reconcile all these observations

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

tl and esr of quartz from the astrobleme of aorounga sahara of chad abstract the present work was intended to evaluate the time that elapsed since the meteorite fall that produced the giant astrobleme of aorounga sahara of chad for this purpose the tl and esr dating techniques using the additive dose method were applied to quartz grains extracted from an impactite and from a sandstone shocked and baked during the impact the esr al centre was measured and resulted in an age of about 800 ka the red tl and the blue tl showed unusual tl features the additive dose response curves were marked by an initial saturated part followed by a second rise at around 0 5 kgy the peaks showed erratic temperature shifts with dose fading was observed for high temperature peaks by comparison with previous work using samples baked by lava flows more than 1 ma ago and presenting some of those features it was assumed that the minimum age of the astrobleme was of the same order of magnitude this is in agreement with other observations a preliminary explanation for those ageing features is proposed it is suspected that radiation induced traps contribute to the tl of the studied quartz grains most probably the malign behaviour of the quartz grains is also connected with shock effects

a paleomagnetic estimate of the age and thermal history of the kentland indiana cryptoexplosion structure the kentland indiana cryptoexplosion structure consists of a centrally uplifted zone of steeply dipping fault blocks surrounded by a ring depression and a low amplitude structural high with a radius of 12 km the age of the deformation is constrained stratigraphically to the interval between mississippian and quaternary paleomagnetic samples were collected from seven carbonate formations and one siltstone unit of these only the middle ordovician quimby s mill limestone contains a stable magnetization with consistent directions stepwise thermal and alternating field demagnetization shows the presence of a northwesterly moderately steep characteristic remanence with a median destructive field of 10 to 30 mt and maximum unblocking temperatures of 300 c the pole position calculated from the mean direction falls near late cretaceous paleomagnetic poles dispersion increases when a bedding correction is applied indicating post deformational magnetization because the magnetization fails the fold test it

seismic reflection imaging of crustal deformation within the eastern yarlung zangbo suture zone abstract various models have been proposed to explain formational mechanisms for the himalayan orogeny but uncertainties remain concerning uplift processes and deeper crustal structure of the orogen imaging of the orogen s deep structure can help resolve these uncertainties this paper describes one migrated deep seismic reflection profile across the eastern yarlung zangbo suture yzs at 92 e which provides a high resolution image of crustal structure that reveals 1 yarlhashampo dome is divided into upper middle and lower tectonic units by two detachments from rim mantle to the core of dome 2 one blind north dipping reverse fault that offsets the south tibet detachment std developed along the northern flank of the yarlhashampo dome 3 a shift of the main himalayan thrust mht dip angle from 18 into relatively gentle dips 6 in the central part of the profile 18 20 s t w t meantime further inferring that 4 north south compression event happened after ca 10 ma in north himalayas 5 multi stage duplexing as the primary mechanism for crustal thickening in the eastern yzs based on interpreted structures

a magnetotelluric survey of ophiolites in the neyriz area of southwestern iran a wide band magnetotelluric study of the ophiolitic zone of the zagros orogenic belt was conducted in the neyriz area of southwestern iran the purpose of the study was to image subsurface structures electrically and relocate the main zagros thrust fault in the region the thrust fault has a complex structure with obscure behavior and is believed to be located within a zone of ongoing continental plate convergence the fault zone with a nw se geological trend is parallel to the zagros orogenic belt and separates the neyriz ophiolite assemblage from the adjacent sanandaj sirjan metamorphic zone magnetotelluric data were collected along a sw ne profile across the geologic strike the study included 18 stations and modeling was performed using a 2 d inversion scheme analysis of both modes of magnetotelluric data te and tm clarifies the signatures of large resistivity variation in the study area due to the presence of a high contrast in resistivity between the ophiolites and neighboring rocks we are able to discern two sharp boundaries as faulting planes and borders of the ophiolite radiolarite zone in the north eastern and southwestern parts of the 2 d resistivity models respectively

the canyon diablo impact event 2 projectile fate and target melting upon impact abstract despite its centennial exploration history there are still unresolved questions about meteor crater the first recognized impact crater on earth this theoretical study addresses some of these questions by comparing model results with field and laboratory studies of meteor crater our results indicate that meteor crater was formed by a high velocity impact of a fragmented projectile ruling out a highly dispersed swarm as well as a very low impact velocity projectile fragmentation caused many fragments to fall separately from the main body of the impactor making up the bulk of the canyon diablo meteorites most of these fragments were engulfed in the expansion plume as they approached the surface without suffering high shock compression and were redistributed randomly around the crater thus the distribution of canyon diablo meteorites is not representative of projectile trajectory as is usual for impactor fragments in smaller strewn fields at least 50 of the main impactor was ejected from the crater during crater excavation and was dispersed mostly downrange of the crater as molten particles spheroids and highly shocked solid fragments shrapnel when compared with the known distribution model results suggest an impactor from the sw overall every model case produced much higher amounts of pure projectile material than observed the projectile target mixing was not considered in the models however this process could be the main sink of projectile melt as all analyzed melt particles have high concentrations of projectile material the fate of the solid projectile fragments is still not completely resolved model results suggest that the depth of melting in the target can reach the coconino sandstone formation however most of the ejected melt originates from 30 40 m depth and thus is limited to moenkopi and upper kaibab material some melt remains in the target based on the estimated volume of the breccia lens at meteor crater our models suggest at most a 2 content of melt in the breccia finally a high water table at the time of impact could have aided strong dispersion of target and projectile melt

scaling craters in carbonates electron paramagnetic resonance analysis of shock damage carbonate samples from the 8 9 mt nuclear near surface explosion crater oak and a terrestrial impact crater meteor crater were analyzed for shock damage using electron paramagnetic resonance epr samples from below the oak apparent crater floor were obtained from six boreholes as well as ejecta recovered from the crater floor the degree of shock damage in the carbonate material was assessed by comparing the sample spectra to spectra of solenhofen and kaibab limestone which had been shocked to known pressures analysis of the oak crater borehole samples has identified a thin zone of allocthonous highly shocked 10 13 gpa carbonate material underneath the apparent crater floor this 5 to 15 m thick zone occurs at a maximum depth of 125 m below current seafloor at the borehole sited at the initial position of the oak explosive and decreases in depth towards the apparent crater edge because this zone of allocthonous shocked rock delineates deformed rock below and a breccia of mobilized sand and collapse debris above it appears to outline the transient crater the transient crater volume inferred in this way is found to be 3 2 0 2 10 6 m 3 which is in good agreement with a volume of 5 3 10 6 m 3 inferred from gravity scaling of laboratory experiments schmidt et al 1986 a layer of highly shocked material is also found near the surface outside the crater the latter material could represent a fallout ejecta layer the ejecta boulders recovered from the present crater floor experienced a range of shock pressures from 0 to 15 gpa with the more heavily shocked samples all occurring between radii of 360 and 600 m moreover the fossil content lithology and sr isotopic composition all demonstrate that the initial position of the bulk of the heavily shocked rock ejecta sampled was originally near surface rock at initial depths in the 32 to 45 m depth below sea level range the epr technique is also sensitive to prehistoric shock damage this is demonstrated by our study of shocked kaibab limestone from the 49 000 year old meteor barringer crater arizona we found shock damage present in the member of the kaibab formation exposed in the crater walls corresponding to peak shock stress in the 0 3 to 0 6 gpa range carbonate ejecta recovered from within the crater experienced shock pressures of up to 0 6 gpa assuming shock damage levels of 0 3 to 0 6 gpa for the lightly shocked carbonate on the walls of the meteor crater combined with the shock pressure versus distance model of moss 1988 and lamb et al 1991 meteor crater impact energies of 2 4 to 8 9 mt are obtained this approximately agrees with energies of 3 3 to 7 1 mt calculated from the crater scaling of schmidt and housen 1987

incision and differential bedrock uplift along the indus river near nanga parbat pakistan himalaya from 10be and 26al exposure age dating of bedrock straths abstract well preserved river cut bedrock surfaces straths abandoned by the progressive incision of the indus river in northern pakistan were dated using in situ cosmogenic 10 be and 26 al exposure age dating measurements on young and modern straths show no significant ingrowth of 10 be and 26 al implying that strath exposure ages closely approximate their abandonment age strath exposure ages range up to 65 ka and increase with height above the river the straths can be divided into two groups low straths dated at about 7 ka which yield very rapid bedrock incision rates 9 12 m ka and high straths 7 ka in age which yield somewhat slower incision rates 1 6 m ka incision rates for both groups increase by 1 3 m ka downstream toward an active reverse fault the raikot fault that forms the western border of the nanga parbat haramosh massif nphm the downstream increases in incision rates for the low straths mimic downstream increases in the modern gradient suggesting that bedrock incision rates have been proportional to stream power over the last 7 ka gradient increases are probably the result of the creation of knickpoints by bedrock uplift along the raikot fault the longitudinal profile and steep gorge of the indus river in the study area indicate that bedrock incision appears to keep pace with differential vertical motions between the nphm and surrounding regions thus the 1 3 m ka difference in incision rates likely reflects the differential bedrock uplift rate between the nphm and the skardu basin the differential incision rate is consistent with differential exhumation rates obtained from apatite fission track ages assuming geotherms of 80 90 c km in the nphm and 35 c km in the skardu basin this relation suggests that rapid bedrock incision of at least 1 3 m ka has persisted within the nphm for the last 0 5 ma the acceleration of incision rates throughout the study area since 15 ka is likely to be a climatic signal that records an increase in the discharge and or sediment load of the indus river in the study area related to deglaciation of the surrounding terrane

luminescence dating of the wabar meteorite craters saudi arabia 1 luminescence dating has been used to find the age of meteorite impact craters at wabar al hadida in saudi arabia the luminescence characteristics of the shocked material were determined using a variety of luminescence dating techniques applied to impactite formed by the meteorite and to the underlying sand the age is found to be 290 38 years a comparison is made with two possible historically recorded ages an impact as young as this has implications for the assessment of hazards from the impact on earth of small meteorites

observations of frontal instabilities on an upwelling filament abstract frontal instabilities were observed along a density front on the cyclonic boundary of an upwelling filament that formed north of pt arguello california in october 1983 observations of the instabilities were conducted using satellite sea surface temperature images and in situ sampling the instabilities formed on the southern cyclonic boundary of the filament at a wavelength of about 20 km and consisted of two lobes one warm and one cool each with a width of about 4 km the time scale for formation of the instabilities is about 1 day near surface distributions of temperature salinity and density within the cool lobes of the instabilities are consistent with local upwelling at the rate of about 30 m d 1 a simple model based on conservation of potential vorticity is presented which accounts for the observed upwelling based on isopycnal displacements and the distribution of salinity the signature of the instabilities appears to be confined to the upper 50 m of the water column

imaging the continental upper mantle using electromagnetic methods abstract the internal structure of the continental lithosphere holds the key to its creation and development and this internal structure can be determined using appropriate seismic and electromagnetic methods these two are complementary in that the seismic parameters usually represent bulk properties of the rock whereas electrical conductivity is primarily a function of the connectivity of a minor constituent of the rock matrix such as the presence of a conducting mineral phase e g carbon in graphite form or of a fluid phase e g partial melt or volatiles in particular conductivity is especially sensitive to the top of the asthenosphere generally considered to be a region of interconnected partial melt knowledge of the geometry of the lithosphere asthenosphere boundary is important as this boundary partially controls the geodynamic processes that create modify and destroy the lithosphere accordingly collocated seismic and electromagnetic experiments result in superior knowledge than would be obtained from using each on its own this paper describes the state of knowledge of the continental upper mantle obtained primarily from the natural source magnetotelluric technique and outlines how hypotheses and models regarding the development of cratonic lithosphere can be tested using deep probing electromagnetic surveying the resolution properties of the method show the difficulties that can be encountered if there is conducting material in the crust examples of data and interpretations from various regions around the globe are discussed to demonstrate the correlation of electromagnetic and seismic observations of the lithosphere asthenosphere boundary also the observations from laboratory measurements on candidate mineralogies representative of the mantle such as olivine are presented

gravity anomalies flexure and deformation of the converging indian lithosphere in nepal and sikkim darjeeling himalayas researchers ubiquitously noted that the common processes of partitioning oblique convergence in response to drag from the trench hanging plate simultaneously produce radial slips along strike translation and extension parallel to the deformation front here we focus on the area between nepal and sikkim darjeeling himalayas and carry out gravity and finite element stress modeling of the strike orthogonal converging indian lithosphere we delineate the geometries of different layers and their interfaces through gravity modeling the optimum model parameters along with rheological parameters of different layers are used for finite element modeling finite element modeling is done with boundary conditions of keeping the upper surface free and rigidly fixing the section of the northern boundary below the main himalayan thrust we impart on its frontal section an amount of 6 1012 n m force equivalent to resistive force of the himalayan tibet system and analyze the maximum and minimum compressive stress fields evolved in the lithosphere we testify our observations with earthquake database and other geophysical and geological studies we note that an increasing flexing of the indian lithosphere beyond the main boundary thrust becomes maxima between the main central thrust and south tibetan detachment in both the areas however more steepening of the moho boundary is identified in the sikkim darjeeling himalaya this abrupt change in lithospheric geometry beneath the greater himalaya is likely correlated with the sharp elevation changes in the topography although the highest seismicity concentration is dominant in this zone the lesser and the tethys himalayas in sikkim darjeeling area also record relatively fair seismic activity more compressive stress field in different layers right within the sharp bending zone supports this observation we thus propose that the sharp bending zone beneath the greater himalaya is suffering maximum deformation and the deformation is continued in the mantle too we also identify both right lateral shear and radial vergence slip which are presumably associated with the general dynamics and kinematics of the himalaya

the onset of india asia continental collision early steep subduction required by the timing of uhp metamorphism in the western himalaya ultrahigh pressure uhp rocks in the nw himalaya are some of the youngest on earth and allow testing of critical questions of uhp formation and exhumation and the timing of the india asia collision initial collision of india with asia is widely cited as being at 55f1 ma based on a paleomagnetically determined slowdown of india s plate velocity and as being at ca 51 ma based on the termination of marine carbonate deposition even relatively small changes in this collision age force large changes in tectonic reconstructions because of the rapid india asia convergence rate of 134 mm a at the time of collision new u pb shrimp dating of zircon shows that indian rocks of the tso morari complex reached uhp depths at 53 3f0 7 ma given the high rate of indian subduction this dating implies that indian continental crust arrived at the asian trench no later than 57f1 ma providing a metamorphic age for comparison with previous paleomagnetic and stratigraphic estimates india s collision with asia may be compared to modern processes in the timor region in which initiation of collision precedes both the slowing of the convergence rate and the termination of marine carbonate deposition the indian uhp rocks must have traveled rapidly along a short hence steep path into the mantle early continental subduction was at a steep angle probably vertical comparable to modern continental subduction in the hindu kush despite evidence for modern day low angle subduction of india beneath tibet oceanic slab break off likely coincided with exhumation of uhp terranes in the western himalaya and led to the initiation of low angle subduction and leucogranite generation d 2005 elsevier b v all rights reserved

petrographic x ray diffraction and electron spin resonance analysis of deformed calcite meteor crater arizona calcite crystals within the kaibab limestone in meteor crater arizona are examined to understand how calcite is deformed during a meteorite impact the kaibab limestone is a silty finegrained and fossiliferous dolomudstone and the calcite crystals occur as replaced evaporite nodules with impact induced twinning twinning in the calcite is variable with deformational regimes based on abundances of crystals with twins and twin densities within crystals the twins are similar to those that are seen in low tectonically deformed regimes low levels of shock are inferred from minor peak broadening of the x ray diffraction patterns xrd of the calcite crystals in addition electron spin resonance esr spectroscopy data also indicates low shock levels 3 0 gpa quantitative shock pressures and correlation between the xrd and esr results are poor due to the inferred low shock levels in conjunction with the analytical error associated in calculation of the shock pressures

crustal rheology of the himalaya and southern tibet inferred from magnetotelluric data the collision between the indian and asian continents which began about 70 million years ago involved a convergence of at least 1 400 km and raised the tibetan plateau since crustal material is too buoyant to be transported downwards into the earth s interior the mechanisms at work in plateau formation are important in understanding the evolution of the continents yet still a matter of debate geophysical observations in the himalaya and tibet using ultra low frequency radio waves now reveal that a partially molten layer is present along the entire length of the himalaya this weak layer implies that widespread crustal flow is occurring and explains many features of plateau evolution the cenozoic collision between the indian and asian continents formed the tibetan plateau beginning about 70 million years ago since this time at least 1 400 km of convergence has been accommodated1 by a combination of underthrusting of indian2 and asian lithosphere crustal shortening3 horizontal extrusion4 and lithospheric delamination5 rocks exposed in the himalaya show evidence of crustal melting1 6 and are thought to have been exhumed by rapid erosion and climatically forced crustal flow7 8 magnetotelluric data can be used to image subsurface electrical resistivity a parameter sensitive to the presence of interconnected fluids in the host rock matrix even at low volume fractions here we present magnetotelluric data from the tibetan himalayan orogen from 77 e to 92 e which show that low resistivity interpreted as a partially molten layer is present along at least 1 000 km of the southern margin of the tibetan plateau the inferred low viscosity of this layer is consistent with the development of climatically forced crustal flow in southern tibet

detection of widespread fluids in the tibetan crust by magnetotelluric studies magnetotelluric exploration has shown that the middle and lower crust is anomalously conductive across most of the north to south width of the tibetan plateau the integrated conductivity conductance of the tibetan crust ranges from 3000 to greater than 20 000 siemens in contrast stable continental regions typically exhibit conductances from 20 to 1000 siemens averaging 100 siemens such pervasively high conductance suggests that partial melt and or aqueous fluids are widespread within the tibetan crust in southern tibet the high conductivity layer is at a depth of 15 to 20 kilometers and is probably due to partial melt and aqueous fluids in the crust in northern tibet the conductive layer is at 30 to 40 kilometers and is due to partial melting zones of fluid may represent weaker areas that could accommodate deformation and lower crustal flow

deep seismic reflection evidence for continental underthrusting beneath southern tibet the himalaya and adjacent tibetan plateau constituting earth s largest region of elevated topography and anomalously thick crust formed as a consequence of cenozoic collision between india and asia itself considered the archetypal continent continent collision1 3 here we report the first results from an attempt to image the structure of the crust beneath this region using deep seismic reflection profiling our 100 km long profile acquired in the tethyan himalaya shows a mid crustal reflection that prob ably marks the active thrust fault along which the indian plate is underthrusting southern tibet upper crustal reflections with geo metries suggestive of large scale structural imbrication of the upper crust and moho reflections from the base of the double normal thickness crust underlying the region these results lend substantial support to the view that crustal thickening beneath southernmost tibet was accomplished by wholesale underthrusting of indian continental crust beneath the structurally imbricated upper crust comprising the tethyan himalaya

thermoluminescence measurements on shock metamorphosed sandstone and dolomite from meteor crater arizona 2 thermoluminescence age of meteor crater thermoluminescence tl dating measurements were performed on shock metamorphosed rocks from meteor crater arizona il ages were obtained for eight specimens four coconino sandstone and four kaibab dolomite which experienced sufficient shock heating during the impact to reset their preimpact tl postimpact radiation doses were determined for each shocked fragment from thermoluminescence measurements on quartz mineral separates effective radiation dose rates were calculated from concentrations of uranium thorium potassium and rubidium in the rocks and their burial environment and the distributions of these elements as determined by fission track and microprobe analyses cosmic radiation dose rates were estimated by altitude scaling of sea level data and were corroborated by tl dosimeter measurements at the crater the mean tl ages for the two rock types were reasonably concordant 50 400 2900 and 46 000 3100 years for sandstones and dolomites respectively the overall mean age 49 000 3000 years is significantly older than the 25 000 year age estimate based on the inferred correlation of the oldest pluvial episode at the crater with radiometrically dated 18o maxima

thermoluminescence measurements on shock metamorphosed sandstone and dolomite from meteor crater arizona 1 shock dependence of thermoluminescence properties the thermoluminescence tl properties of shocked coconino sandstone and kaibab dolomite from meteor crater arizona have been studied equivalent dose ed a measure of stored tl decreases with increasing shock decreasing tl sensitivity but becomes shock independent at high shock grades the minimum shock grade at which ed becomes shock independent the shock threshold for resetting preimpact tl to a negligible level falls within petrographic shock class 2 as defined by kieffer and corresponds to an equilibrated state at about 10 gpa and 700 c shocked coconino fragments suitable for evaluation of the formation age of the crater using tl are those shocked in excess of the resetting threshold the average postimpact temperature of the crater floor about 190 c based on the il of samples heated during burial is consistent with impact energy estimates for the crater

comment on deep resistivity cross section of the intraplate atlas mountains nw africa new evidence of anomalous mantle and related quaternary volcanism citation jones a g d kiyan j fullea j ledo p queralt a marcuello a siniscalchi and g romano 2012 commenton deep resistivity cross section of the intraplate atlas mountains nw africa new evidence of anomalous mantle and relatedquaternary volcanism tectonics 31 tc5011 doi 10 1029 2011tc003051

magnetotelluric constraints on the tectonic setting of grenville aged orogenesis in central australia abstract magnetotelluric mt data were collected over a 480 km long profile in central australia extending from the mesoarchaean palaeoproterozoic gawler craton into the mesoproterozoic neoproterozoic musgrave province the two regions have contrasting geological histories and many reconstruction models of proterozoic australia propose that they were juxtaposed during the mesoproterozoic despite the significant differences in geological background between the gawler craton and the musgrave province the mt data did not image an electrical structure or any change in electrical character between them mt models suggest that southern central australia is electrically separated into two parts the first comprising the northernmost gawler craton and the musgrave province has a 5 km thick layer of moderate resistivity overlying highly resistive crust the second comprising the central northern gawler craton displays moderate resistivity to the base of the crust the lithospheric mantle underlying both electrical regions is uniformly highly resistive our results suggest that the musgrave province and gawler craton are more closely related than is currently recognized we propose that the gawler craton forms basement to the musgrave province and that the grenvillian musgrave orogeny was an intracratonic reworking event that overprinted the older basement

cosmogenic chlorine 36 production rates in terrestrial rocks abstract chlorine 36 is produced in rocks exposed to cosmic rays at the earth surface through thermal neutron activation of 35cl spallation of 39k and 40ca and slow negative moun capture by 40ca we have measured the 36cl content of 14c dated glacial boulders from the white mountains in eastern california and in a 14c dated basalt flow from utah effective time intergrated production parameters were calculated by simultaneous solution of the 36cl production equations the production rates due to spallation are 4160 310 and 3050 210 atoms 36cl yr 1 mol 139k and 40ca respectively the thermal neutron capture rate was calculated to be 3 07 0 24 105 neutrons kg of rock 1 yr 1 the reported values are normalized to sea level and high geomagnetic latitudes production of 36cl at different altitudes and latitudes can be estimated by appropriate scaling of the sea level rates chlorine 36 dating was performed on carbonate ejecta from meteor crater arizona and late pleistocene morainal boulders from the sierra nevada california calculated 36cl ages are in good agreement with previously reported ages obtained using independent methods

ophiolite emplacement in ne oman constraints from magnetotelluric sounding summary magnetotelluric mt data have been acquired across the samail ophiolite oman mountains along a 115 km transect in january 2005 twenty five mt stations were deployed approximately every 4 km along a profile perpendicular to the coastline near muscat the profile extends from the huw l meeh subwindow in the northwestern part of the profile and crosses the saih hatat window a tertiary domal culmination that folds a major ne facing recumbent fold nappe and ends along the southern boundary between the dasir and ibra ophiolite blocks the survey aimed to investigate the tectonic evolution of the emplacement of the samail ophiolite by delineating major faults and geological boundaries on a crustal scale the data shows complex behaviour with the influence of static shift effects and anomalously high phases exceeding 90 phase tensor analysis and 3 d forward modelling hint that the upper plate lower plate up lp shear zone has a 3 d effect on the data in the northern part of the profile rotated mt impedances were inverted using a 2 d code major resistivity interfaces coincide with the location of the up lp shear zone near the surface and dip towards the arabian margin suggesting a southwest orientated underthrusting prior to the ophiolite emplacement the para autochthonous and less deformed upper plate is a resistor whereas the strongly deformed lower plate is more conductive

degradation of selected terrestrial and martian impact craters degraded craters on mars record the cumulative effects of the complex interplay between erosion transport and deposition by competing intermittent processes fortunately impact craters are instantaneous landforms common to both the earth and mars that are characterized by similar topography and local lithofacies deconvolving the signatures of individual degradation processes around young craters on earth e g drainage scale and density changing wall slope deflation features allows calibrating degradation signatures around craters on mars and places first order limits on controlling processes and perhaps past climates such comparisons are valid despite differences in vegetation cover and time scales over which activity occurs on the two planets analysis of degradation at meteor crater lonar crater and talemzane crater reveals a terrestrial sequence of advancing degradation by fluvial eolian and mass wasting processes at meteor crater fluvial processes currently dominate primary erosion however combined deflation from the ejecta and both alluvium and colluvium make eolian processes most important in overall ejecta degradation lonar and talemzane reveal that advancing fluvial degradation is characterized by larger drainages and decreasing wall slopes at talemzane interior drainages breach the rim crest and pirate headward regions of exterior drainage basins when comparable degradation signatures resolvable at viking resolutions are used in selected areas on mars important differences from the terrestrial sequence are revealed all morphologies of craters in southern ismenius lacus sil are typically incised by small valleys with low drainage densities display rare alluvial deposits possess walls sloped at the angle of repose and are sometimes partially covered by airfall deposit remnants analogy with the terrestrial craters indicates signatures associated with advancing fluvial degradation should be readily detected on mars together with other differences between degraded craters on the two planets the paucity of fluvial signatures in sil implies that mass wasting and eolian activity predominated degradation of martian craters preserved since the hesperian crater statistics for sil indicate most craters were degraded during geologically brief intervals in the noachian and mid hesperian therefore fluvial signatures preserved in sil imply any runoff since the hesperian was short lived locally debouched and unrelated to rainfall

partial melt or aqueous fluid in the mid crust of southern tibet constraints from indepth magnetotelluric data summary the indepth project has applied modern geophysical techniques to the study of the crustal structure and tectonic evolution of the tibetan plateau in the lhasa block seismic reflection surveys in 1994 detected a number of bright spots at 15 20 km depths that indicate zones of crustal fluids aqueous fluids or partial melt coincident magnetotelluric mt data collected in 1995 detected a major zone of high electrical conductivity at the same depth as the brightspots using constrained inversion the mt data require a minimum crustal conductance of 6000 s this abnormally high electrical conductance can be best explained by a layered model with fluids partial melt aqueous fluids or a combination of partial melt and aqueous fluids the non uniqueness of the mt method means that a wide range of melt fraction thickness combinations for the above models could all explain the 6000 s conductance to distinguish between these three models other geophysical and geological data are required reflection seismic data suggest that a high fluid content 15 per cent is present at the top of the layer the amplitude versus offset data suggest that the top of this layer may be aqueous fluids rather than partial melt passive seismic data imaged a 20 km thick layer of lower fluid content that is probably partial melt petrological studies suggest that concentrations of aqueous fluids above 0 1 per cent at mid crustal depth cannot be sustained taken together these data show that the high conductivity in southern tibet is most probably the result of a relatively thin layer of aqueous fluids 100 200 m overlying a thicker zone of partial melt 10 km

the subsurface character of meteor crater arizona as determined by ground probing radar ground probing radar has provided new two dimensional stratigraphic and structural information on meteor crater arizona four 50 mhz transects inside the crater and one 100 mhz transect on a northeast radial across the exterior ejecta blanket were carried out the frequencies used were in relation to the desired depth of investigation greater than 50 m at 50 mhz and 30 m for 100 mhz the results obtained inside the crater show that the water table can be clearly detected at depths of 65 m as can the contact between the crater wall and the infilling allochthonous breccia lens in addition we were able to detect 1 point reflectors from relatively coherent blocks 1 5m within the breccia lens 2 a zone 7 m thick relatively free of point reflectors from the immediate fall back unit 3 a series of subhorizontal reflectors a few meters apart due to bedding in the post impact crater sediments and 4 five separate subhorizontal reflectors from what we interpret to be a series of postimpact paleo water table levels in the breccia lens the ejecta blanket transect indicates a 4 10 m thick ejecta layer and the underlying moenkopi formation sandstone as well as the moenkopi kaibab contact and bedding within the kaibab formation in addition reverse faults with displacements up to 4 m were detected in the bedrock as well as changes from subhorizontal to 30 in the dip of the bedrock as the crater rim is approached from the northeast beneath the ejecta blanket the total uplift observed at the crater rim is around 55 m the radar data indicate uplift effects out to a radial distance of 400 m from the rim farther than that determined from drilling 300 m the results demonstrate the utility of continuous ground probing radar cross sections in studying the near subsurface 50 m character at meteor crater this relatively rapid low cost technique can be extended to other structures but confident interpretation requires supplemental ground truth constraints

age and effects of the odessa meteorite impact western texas usa the odessa meteorite craters texas united states include a main crater 160 m diameter 30 m deep plus four smaller meteorite craters the main crater was sampled by coring to 22 m depth to better understand its origin and history dating by optically stimulated luminescence indicates that it was produced immediately prior to ca 63 5 4 5 ka sediment filling the crater includes impact breccias produced at the time of impact wind dominated silts with minor amounts of pond sediments deposited ca 63 5 ka probably just after the impact and ca 53 2 ka wind dominated silt ca 38 1 7 ka and playa muds with a wind blown silt component younger than 36 ka the environment was arid or semiarid at the time of impact based on characteristics of soils on the surrounding landscape the impact caused severe damage within 2 km and produced 1000 km hr winds and thermal pulse animals within a 1 1 5 km diameter area were probably killed this is only the second well dated pleistocene hypervelocity impact crater in north america

crustal structure beneath the eastern margin of the tibetan plateau and its tectonic implications 1 two crustal cross sections through the eastern margin of the tibetan plateau are jointly determined from deep seismic sounding the e w trending line aa passes through the western sichuan plateau including the songpan garze terrane and the longmenshan fault belt and ends in the sichuan basin a part of the yangtze craton line bb has a trend of nne and crosses the songpan garze terrane two dimensional crustal structures along the profiles were jointly determined by the additional use of existing deep seismic sounding data our seismic velocity models indicate that the western sichuan plateau and the sichuan basin have crustal thicknesses of 62 and 43 km average crustal p wave velocities of 6 27 and 6 45 km s and lower crustal vp 6 5 km s thicknesses of 27 and 15 km respectively density models constructed from the seismic velocity models are consistent with observed bouguer gravity anomalies we infer that collision between the tibetan plateau and the yangtze craton has caused thickening of the lower crust and uplift of the western sichuan plateau we detect a low velocity layer in the upper crust of the western sichuan plateau but observe no equivalence in the sichuan basin west dipping thrusts may detach into this low velocity layer the seismic phase pmp in the western sichuan plateau has low amplitude suggesting high attenuation in the lower crust qp of 100 300 we suggest that the high attenuation is a consequence of lower crustal flow caused by the large lower crustal thickness beneath the western sichuan plateau

shock metamorphism of quartz in nature and experiment ii significance in geoscience the occurrence of shock metamorphosed quartz is the most common petrographic criterion for the identification of terrestrial impact structures and lithologies its utility is due to its almost ubiquitous occurrence in terrestrial rocks its overall stability and the fact that a variety of shock metamorphic effects occurring over a range of shock pressures have been well documented these shock effects have been generally duplicated in shock recovery experiments and thus serve as shock pressure barometers after reviewing the general character of shock effects in quartz the differences between experimental and natural shock events and their potential effects on the shock metamorphism of quartz are explored the short pulse lengths in experiments may account for the difficulty in synthesizing the high pressure polymorphs coesite and stishovite compared to natural occurrences in addition post shock thermal effects are possible in natural events which can affect shock altered physical properties such as refractive index and cause annealing of shock damage and recrystallization the orientations of planar microstructures however are unaffected by post impact thermal events except if quartz is recrystallized and provide the best natural shock barometer in terms of utility and occurrence the nature of planar microstructures particularly planar deformation features pdfs is discussed in some detail and a scheme of variations in orientations with shock pressure is provided the effect of post impact events on pdfs is generally limited to annealing of the original glass lamellae to produce decorated pdfs resulting from the exsolution of dissolved water during recrystallization basal 0001 pdfs differ from other pdf orientations in that they are multiple mechanical brazil twins which are difficult to detect if not partially annealed and decorated the occurrence and significance of shock metamorphosed quartz and its other phases namely coesite stishovite diaplectic glass and lechatelierite are discussed for terrestrial impact structures in both crystalline non porous and sedimentary porous targets the bulk of past studies have dealt with crystalline targets where variations in recorded shock pressure in quartz have been used to constrain aspects of the cratering process and to estimate crater dimensions at eroded structures in sedimentary targets the effect of pore space results in an inhomogeneous distribution in recorded shock pressure and temperature which requires a different classification scheme for the variation of recorded shock compared to that in crystalline targets this is discussed along with examples of variations in the relative abundances of planar microstructures and their orientations which are attributed to textural variations in sedimentary target rocks examples of the shock metamorphism of quartz in distal ejecta such as at the k t boundary and from nuclear explosions are illustrated and are equivalent to that of known impact structures except with respect to characteristics that are due to long term post shock thermal effects finally the differences between the deformation and phase transformation of quartz by shock and by endogenic tectonic and volcanic processes are discussed we confirm previous conclusions that they are completely dissimilar in character due to the vastly different physical conditions and time scales typical for shock events compared to tectonic and volcanic events well characterized and documented shock effects in quartz are unequivocal indicators of impact in the natural environment

early cretaceous granitoids and their implications for the collapse of the dabie orogen eastern china shrimp zircon u pb dating and geochemistry abstract two stages of early cretaceous post orogenic granitoids are recognized in the dabie orogen eastern china which recorded processes of extensional collapse of the orogen the early stage granitoids 132 ma are foliated hornblende quartz monzonites and porphyritic monzogranites they are of high k calc alkaline series and metaluminous to weakly peraluminous with high k 2 o and low mgo contents mg values 32 0 46 0 they contain high sr low y and heavy rare earth elements hree and have high sr y and la yb n ratios without clear negative eu sr and ti anomalies the early stage deformed granitoids have adakitic geochemical compositions and are equilibrated with residues rich in garnet and poor in anorthite rich plagioclase and thus indicate the existence of an over thickened 50 km crustal root beneath the orogen at 132 ma the later stage granitoids 128 ma are undeformed fine grained monzogranites fine grained k feldspar granites and coarse grained k feldspar granite porphyry they belong to a peraluminous and high k calc alkaline to shoshonite series and display a flat hree pattern and have strong negative eu sr and ti anomalies with low sr y and la yb n ratios the late stage granitoids are equilibrated with residues rich in anorthite rich plagioclase hornblende ilmenite titanite and poor in garnet indicating that the crust of the dabie orogen became thinner

the terrestrial impact cratering record approximately 130 terrestrial hypervelocity impact craters are currently known due to variations in preservation and in geologic knowledge this sample is biased towards young x 20 km craters on the cratons of australia europe including the former u s s r and north america the rate of discovery of new craters is 3 5 craters per year although modified by erosion terrestrial impact craters exhibit the range of morphologies observed for craters on other terrestrial planetary bodies such as the moon terrestrial craters provide essential ground truth data on the geologic effects of impact and the subsurface structure of impact craters which can be used to constrain interpretations of lunar samples and models of crater formation due to erosion and its effects on form terrestrial craters are recognized primarily by the occurrence of shock metamorphic effects these include shatter cones microscopic planar deformation features solid state and fusion glasses high pressure polymorphs and whole rock melting and vaporization shock recovery experiments indicate that these features occur over shock pressures of 5 gpa to x 100 gpa terrestrial craters have a set of geophysical characteristics which are largely the result of the passage of a shock wave and impact induced fracturing they include gravity and magnetic lows and reductions in seismic velocity the gravity anomalies are seldom greater than 30 mgal due to the limiting effects of lithostatic pressure on fracturing at large complex craters the gravity signature may include a central relative gravity high due to uplift and short wavelength central magnetic anomalies due to a variety of processes much current work is focused on the effects of impact on earth evolution previous work on shock metamorphism and the contamination of impact melt rocks by meteoritic siderophile elements provides a basis for the interpretation of the physical and chemical evidence from cretaceous tertiary boundary sites as resulting from a major impact suggestions that other biological boundaries in the stratigraphie record are due to periodic impacts are not supported by time series analysis of the terrestrial cratering record by analogy with the lunar record and modelling of the effects of very large impacts it has been proposed that biological and atmospheric evolution of the earth could not stabilize before the end of the late heavy bombardment 3 8 ga ago the present terrestrial cratering rate is 5 4 2 7 10 15 km 2a 1 for a diameter 20 km this represents a local threat on historic time scales on a global scale a major impact sufficient to cripple human civilization severely will occur on time scales of 106 a

gradation of the roter kamm impact crater namibia interpretation of ground penetrating radar data from the 3 7 ma old roter kamm impact crater together with results of petrographic and sedimentologic analysis of sediment samples indicates that 40 m or more of slope sensitive degradation of the rim was accompanied by reduction of wall slopes well below the angle of repose and nearly complete erosion of ejecta from around the crater only one patch of in situ ejecta deposit was identified degradation of both the interior and exterior of the crater was dominated by fluvial alluvial activity likely during the first 1 0 2 7 ma history of the crater eolian modification has dominated gradation since the last half of the pleistocene results from roter kamm can help in evaluating evolution of degraded martian craters that may have experienced fluvial alluvial gradation followed by eolian modification on mars extended fluvial alluvial activity should produce a low sloped incised rim thereby eroding most of the continuous ejecta well before removal of easily recognizable rim relief subsequent eolian deposition can bury all but the raised rim of craters thereby creating a focal point for efforts geared towards constraint of gradational history

pre atmospheric depths and thermal histories of canyon diablo spheroids despite having melted during formation seven of eight canyon diablo spheroids weighing from 0 6 to 13 mg retain cosmic ray produced 38ar 38arcos in concentrations 10 10 cm 3 stp g ranging from 0 35 to 68 the presence of 38arcos is consistent with pre atmospheric depths of less than 2 3 meters and most likely rules out an origin for the spheroids deep within the projectile which had a radius of 15 m low levels of 21necos indicate gas loss from these spheroids relative to most canyon diablo meteorites the spheroids contain lower concentrations of cosmogenic noble gases the difference partly reflects diffusion losses from the spheroids especially for 3he and 21ne but also suggests deeper locations on average for the precursor material consistent with independent results from 59ni

aluminum 26 10be and 36cl depth profiles in the canyon diablo iron meteorite the authors have measured activities of the long lived cosmogenic radionuclides sup 26 al sup 10 be and sup 36 cl in 12 fragments of the iron meteorite canyon diablo and have constructed production rate versus depth profiles of those radionuclides profiles determined using differential particle fluxes calculated with the lahet code system are in good agreement with sup 26 al sup 10 be and sup 36 cl experimental data but the agreement for sup 36 cl was obtained only after neutron induced cross sections were modified profiles calculated with lunar particle fluxes are much lower than experimental canyon diablo profiles the cosmic ray exposure ages of most samples are near 540 m y 34 refs 4 figs 2 tabs

electrical evidence of continental accretion steeply dipping crustal scale conductivity contrast 1 many geophysical images of ancient orogenic belts utilise seismic reflection data but increasingly magnetotelluric mt data have been shown to provide insights into orogenic architecture not evident in seismic surveys particularly through their ability to image steep structures a 140km long mt survey carried out in palaeoproterozoic crust in central australia has imaged a crustal scale conductivity contrast across a steeply dipping boundary the surface location of this contrast coincides with the redbank thrust zone previously shown from seismic reflection and gravity data to be a moderately dipping thick skinned fault that produces one of the largest continental gravity gradients known on earth the steeply dipping conductivity interface is interpreted to be a strike slip boundary separating the north australian craton from a younger accreted terrane this result is therefore a first order geophysical image of a proterozoic accretion system in australia the steep boundary is not evident in the seismic data and highlights the utility of mt in defining fundamental crustal boundaries

identifying the lithospheric structure of a precambrian orogen using magnetotellurics the capricorn orogen western australia abstract a 300 km long magnetotelluric mt survey was carried out across the capricorn orogen in western australia the capricorn orogen includes reworked crust of the archaean yilgarn and pilbara cratons and the allochthonous latest archaean to palaeoproterozoic glenburgh terrane in the southern gascoyne complex and the palaeoproterozoic northern gascoyne complex the survey aimed to increase our understanding of the juxtaposition of these crustal elements by obtaining information about their electrical structure with depth phase tensor analysis of the data showed that 2d mt inversion could only be carried out for stations in the centre of the profile crossing the gascoyne complex the resulting model showed that there is no electrical distinction between the glenburgh terrane and the northern gascoyne complex suggesting that the glenburgh terrane may form basement to the whole of the gascoyne complex 3d forward modelling was carried out for the entire survey line incorporating the basement blocks as well as the surrounding ocean and sedimentary basins station data are most closely reproduced by a model in which the margin between the glenburgh terrane and the yilgarn craton the errabiddy shear zone dips south with crust of the glenburgh terrane wedged beneath the northern yilgarn craton forward modelling results also suggest that the main boundary between the gascoyne complex and the pilbara craton to the north is a steeply dipping structure beneath the edmund and collier basins that may correlate with the talga fault

deep geoelectric structure of the sikkim himalayas ne india using magnetotelluric studies broadband 0 001 1000 s magnetotelluric soundings were carried out at 18 locations with a station interval of 5 8 km across the sikkim himalaya northern india along a 120 km long traverse from siliguri in the south to yumthang in the north magnetotelluric transfer functions were computed after robust processing of single site and remote reference sites the two dimensional 2d model derived from the joint inversion of te and tm mode data shows distinct electrical signatures of the main frontal thrust mft the main boundary thrust mbt and the main central thrust mct the mft and mbt zones are expressed by a conductive feature of about 10 40 m indicating the presence of siwalik molasse sediments of gangetic foreland basin an anomalously high conductive 2 5 m in the crust in the depth range of 3 15 km is observed to the north of mbt this may indicate the presence of siwalik molasse sediments together with lesser himalayan sediments with trapped fluids in the fault zone these sediments may act as a lubricant accommodating underthrusting of continental crust the nature of the low resistivity associated with the main himalayan thrust in the higher himalayas north of the mct might indicate presence of metamorphic fluids released due to under thrusting of the indian plate

air blast produced by the meteor crater impact event and a reconstruction of the affected environment using scaling relationships determined from nuclear explosions the radial extent of the air blast produced by the meteor crater impact event is estimated the wind velocity at a distance of 5 crater radii 3 km from the point of impact should have exceeded 2000 km h hurricane force winds would have existed as far away as 20 to 40 km depending on the exact explosive energy of the impact event to determine how this event may have affected the environment surrounding the crater the topography vegetation and animal life that existed at the time of the impact are reconstructed for example if the coniferous woodlands were 100 m lower than they are presently and they had moved farther out onto the plains then the air blast would have flattened trees within a 16 to 22 km radius of the point of impact and damaged them over an area of 4100 to 8500 km2 the distance over which the damage occurred may have been up to 2 larger in some directions around the crater because of additional effects produced by the ballistic shock wave unfortunately since the trajectory of the projectile is not well known the direction of the ballistic shock wave effects cannot yet be determined

erosion of ejecta at meteor crater arizona new methods for estimating erosion at meteor crater arizona indicate that continuous ejecta deposits beyond 1 4 1 2 crater radii from the rim 0 25r 0 5r have been lowered less than 1 m on the average this conclusion is based on the results of two approaches coarsening ofunweathered ejectainto surface lag deposits and calculation of the sediment budget within a drainage basin on the ejecta preserved ejecta morphologies beneath thin alluvium revealed by ground penetrating radar provide qualitative support for the derived estimates although slightly greater erosion 2 3 m of less resistant ejecta locally has occurred such deposits were limited in extent particularly beyond 0 25r 0 5r from the present rim subtle but preserved primary ejecta features e g distal ejecta lobes and blocks further support our estimate of minimal erosion of ejecta since the crater formed 50 000 years ago unconsolidated deposits formed during other sudden extreme events e g landslides exhibit similarly low erosion over the same time frame the common factor is the presence of large fragments or large fragments in a matrix of finer debris at meteor crater fluvial and eolian processes remove surrounding fines leaving behind a surface lag of coarse grained ejecta fragments that armor surfaces and slow vertical lowering

high gradients of lead isotopic composition in north east pacific upwelling filaments substantial differences in stable lead isotopic composition in coastal waters of the north east pacific have been inferred from a single vertical profile taken near western canada1 here we present measurements of lead isotopes in two upwelling filaments off central california 39 n 124 w which confirm that such marked differences exist the distinct filamentary structures appear in association with surface jets which transport cold upwelled water hundreds of kilometres offshore2 surface water ratios of stable lead isotopes 204pb 206pb 207pb 208pb within the core of the first filament for example 206pb 207pb 1 17 contrasted with those of surrounding coastal waters 206pb 207pb 1 19 and those of industrial lead aerosols from the united states 206pb 207pb 1 22 contrasts in the isotopic composition indicated that the lead within the core was primarily derived from aeolian inputs of asian industrial lead 206pb 207pb 1 16 to remote waters which were then upwelled near the north american coastline high gradients of lead isotopic composition in coastal waters were confirmed by surface and sub surface measurements surrounding a second upwelling filament these gradients illustrate the complexity of trace element cycles in coastal waters and the offshore fluxes within the ephemeral filaments demonstrate significant influences of coastal dynamics on composition within the ocean gyre

electromagnetic images of the trans hudson orogen the north american central plains anomaly revealed magnetotelluric studies of the trans hudson orogen over the last two decades prompted by the discovery of a significant conductivity anomaly beneath the north american central plains nacp from over 300 sites yield an extensive database for interrogation and enable three dimensional information to be obtained about the geometry of the orogen from southern north dakota to northern saskatchewan the nacp anomaly is remarkable in its continuity along strike testimony to along strike similarity of orogenic processes where bedrock is exposed the anomaly can be associated with sulphides that were metamorphosed during subduction and compression and penetratively emplaced deep within the crust of the internides of the orogen to the boundary of the hearne margin a new result from this compilation is the discovery of an anomaly within the upper mantle beginning at depths of 80 100 km this lithospheric mantle conductor has electrical properties similar to those for the central slave craton mantle conductor

paleogeographic maps of the continents bordering the indian ocean since the early jurassic paleomagnetic data from the african plate from the early jurassic onward are critically re evaluated and updated resulting in a set of 30 paleopoles from which an apparent polar wander path is derived using 20 ma intervals this path is then used in conjunction with kinematic models of the indian and atlantic oceans to place other continents bordering the indian ocean eurasia india islands and plateaus madagascar seychelles mascarene and blocks involved in the collision of india with eurasia south tibet indochina in an absolute paleogeographic frame fixed with respect to africa paleogeographic maps are presented for the early jurassic pre breakup the jurassic cretaceous boundary anomaly ml5 the early cretaceous m2 the late cretaceous a 34 the cretaceous tertiary boundary a 29 the early eocene a 24 the middle eocene a 21 and the oligocene miocene boundary a 6 observed and predicted paleomagnetic directions for landmasses other than africa i e for western europe china korea india and australia which are not used in deriving the maps are found to be in very satisfactory agreement the quality of reconstructions as measured by rms discrepancies between observed and predicted paleolatitudes is of the order of 5 the main features of the paleogeographic maps are discussed with the aim of relating oceanic and on land geodynamic events the constraints provided by tibetan data on the india asia collision and the 20 ma reorganization of geology and plate boundaries in a band extending from the south china sea through tibet to the red sea system are discussed in more detail we emphasize the episodic nature of the breakup of gondwanaland and formation of the indian ocean and subsequent interaction with eurasia in which six major discrete events are identified separated by 30 ma phases of rather uniform plate motion

vertical concentration profiles of lead in the central pacific at 15 n and 20 s concentrations of lead were measured in a surface transect and at two vertical profile stations 15 n and 20 s in the central pacific these measurements complement similar measurements made earlier in the north pacific at 33 n and in the northwest atlantic at 34 n as well as recent measurements of eolian lead input fluxes near each of these locations the new transect of surface water concentrations of lead corroborates previous measurements which decrease from 13 ng kg at 30 n to 4 ng kg at 17 s in the central pacific this transect gradient is shown to overlie a similar geographic gradient of subsurface maximum concentrations of lead in the three pacific vertical profile stations decreasing from 14 ng kg at 33 n to 11 ng kg at 14 n to 2 5 ng kg at 20 s lead concentrations at each of those locations exhibit maxima at 400 m decreasing concentrations to 2500 m and approximately concentrations of 0 8 1 1 ng kg below that depth the subsurface maximum at the northwest atlantic profile station 36 ng kg at 34 n is also congruent with surface water lead concentrations which decrease from 806 ng kg to 32 ng kg in an offshore transect from rhode island to 34 n 66 w and the shape of the atlantic profile is congruent with those in the pacific there is a positive correlation between the magnitudes of eolian lead input fluxes and the magnitudes of the upper water maxima in lead concentration profiles at corresponding locations as follows south pacific easterlies 3 ng cm 2 yr vs 2 5 ng kg north pacific easterlies 6 ng cm 2 yr vs 11 ng kg north pacific westerlies 50 ng cm 2 yr vs 14 ng kg and north atlantic westerlies 170 ng cm 2 yr vs 36 ng kg this relationship enables one to view the anthropogenic perturbations of the marine lead cycle on a global scale since the industrial origin of eolian and seawater lead has been established by correlations between geographic patterns of industrial lead emissions to the atmosphere and isotopic ratios of industrial leads and by geographic patterns of pb silicate dust ratios and lead isotopic ratios in ocean surface waters these new data coupled with earlier biogeochemical data indicate that surface water concentrations of lead in the north pacific and north atlantic are now conservatively estimated to be 8 to 20 fold greater and those in the south pacific are 2 fold greater than natural concentrations because of industrial emissions of lead to the atmosphere

electromagnetic images of a strike slip fault the tintina fault northern canadian 1 wideband magnetotelluric mt data were acquired along three profiles crossing the strike slip tintina fault in northwestern canada the mt responses obtained exhibit remarkable similarity from all three profiles implying similar two dimensional 2 d electromagnetic behavior of the fault zone over a strike length of at least 350 km analyses of the mt responses for dimensionality corroborate the validity of assuming regional 2 d structures in interpretation several high conductivity anomalies at different depth scales are present in the models obtained and we suggest that both the shallow structures and the deep crustal scale anomalies are caused by electronic conduction mechanisms in interconnected mineralized zones intriguingly the middle and lower crust beneath the surface expression of the tintina fault is highly resistive in contrast to some other large scale strike slip faults this implies that fault zone processes that result in interconnected conducting phases are not generic in nature but are controlled by local conditions

mineralogy composition and alteration of mars pathfinder rocks and soils evidence from multispectral elemental and magnetic data on terrestrial analogue snc meteorite and pathfinder samples major element multispectral and magnetic properties data were obtained at ares vallis during the mars pathfinder mission to understand the compositional mineralogical and process implications of these data we obtained major element mineralogical and magnetic data for well crystalline and nanophase ferric minerals terrestrial analogue samples with known geologic context and snc meteorites analogue samples include unaltered palagonitic and sulfatetic tephra from mauna kea volcano hydrolytic and acid sulfate alteration steam vent material from kilauea volcano hydrolytic alteration and impactites from meteor crater relithification salient results for mars pathfinder include 1 band depths bd530b and bd600 and the reflectivity ratio r800 r750 are consistent with the dominant ferric mineral being nanophase ferric oxide associated with an unknown amount of h2o and occurring in composite particles along with subordinate amounts of other ferric minerals hematite and hematite plus nanophase goethite are most consistent with the data but maghemite akaganeite schwertmannite and nanophase lepidocrocite are also possible interpretations ferric oxides that are consistently not favored by the data as sole alteration products are jarosites and well crystalline goethite and lepidocrocite 2 the strength of the ferric adsorption edge r750 r445 implies the fe3 fe2 values for pathfinder rock and soil are within the ranges 0 7 3 and 3 20 respectively 3 ferrous silicates are indicated for subsets of pathfinder rocks and soils one subset has a band minimum near 930 nm that can attributed to low ca pyroxene alternatively the band could be a second manifestation of certain ferric oxides including nanophase goethite maghemite akaganeite and schwertmannite another subset has a negative spectral slope from 800 to 1005 nm which could result from the high energy wing of a high ca pyroxene and or olivine band a mixture of bright and dark materials and for rocks thin coatings of bright dust on dark rocks 4 chemical data on pathfinder rocks and soils are consistent with two component mixtures between an andesitic rock with low mgo and so3 concentrations soil free rock and a global basaltic soil with high mgo and so3 concentrations rock free soil pathfinder rock free soil can be modeled as a chemical mixture of snc meteorites and the pathfinder soil free rock 5 pathfinder soil cannot be obtained by chemical alteration of pathfinder rocks by any of the hydrolytic and acid sulfate alteration processes we studied presumably global mixing has obscured and possibly erased the elemental signatures of chemical alteration 6 the strongly magnetic phase in palagonitic and sulfatetic tephra is titanomagnetite and possibly its oxidation product titanomaghemite fe ti spinels the saturation magnetization of the tephra samples 0 5 2 0 am2 kg is at or below the low end of the range inferred for martian dust 4 2 am2 kg implying that lithogenic fe ti spinels are a possible candidate for the martian strongly magnetic phase 7 the predominantly palagonitic spectral signature and magnetic nature of martian soil and dust are consistent with glassy precursors with imbedded fe ti spinel particles comparison with lunar glass production rates suggests that production of sufficient quantities of glassy materials on mars by volcanic and impact processes is sufficient to account for these observations

transverse tectonics structures in the garhwal himalaya corridor inferred from 3d inversion of magnetotelluric profile data in the garhwal himalayan corridor ghc we recorded the magnetotelluric mt data at 40 sites in three phases out of these 40 sites useful tipper or vertical magnetic field transfer function vtf data was available only at 19 sites the resistivity model obtained from 3d inversion of the mt data is used to investigate the existence of transverse tectonic structures in the garhwal himalaya through a synthetic inversion experiment on scattered data over a profile like our ghc profile we have demonstrated that the mt data can be used to qualitatively infer about the off profile resistivity structures within about 20 km from the profile we carried out several 3d inversion experiments using different subsets of full impedance tensor and vtf responses individually and jointly to arrive at the final resistivity model the 2d profile section of our 3d resistivity model explains the thrust tectonic and flat ramp flat geometry of the main himalayan thrust mht furthermore the inverted model delineated delhi haridwar ridge dhr as a highly resistive 1000 m feature beneath the low resistive 50 m sediments of the indo gangetic plain igp the dhr continues up to the inner lesser himalayan region and it is bounded by two conductive 10 m fluid saturated fractured zones situated off profile and these run nearly parallel to the dhr from the electrical image of the dhr and of the associated conducting feature which have the geoelectric strike of n13 e we inferred that these features are transverse to the main himalayan arc

crustal structure beneath namche barwa eastern himalayan syntaxis new insights from three dimensional magnetotelluric imaging the eastern terminations of the himalayan orogeny named namche barwa are considered a vital natural laboratory in the tibetan plateau for geodynamics due to its distinctive geological and geomorphological characteristics magnetotelluric mt data measured at 83 sites around the namche barwa are imaged by three dimensional 3d inversion to better reveal the crustal structure of the eastern himalaya the results show a complex and heterogeneous electrical structure beneath the namche barwa the electrical conductors distributed in the middle and lower crust around the namche barwa provide additional evidence for the crustal flow model if they are considered as some parts of the flow in a relatively large scale region the near surface resistivity model beneath the inner part of namche barwa conforms with the locations of hot spring and fluid inclusions the brittle ductile transition and the 300 c 400 c isotherm from previous hydrothermal studies relatively resistive upper crust 800 m is underlain by a more conductive middle to lower crust 80 m the electrical characteristics of the thermal structure at shallow depth indicate an accumulation of hydrous melting a localized conductive steep dipping zone for decompression melting consistent with the tectonic aneurysm model for explaining the exhumation mechanism of metamorphic rocks at namche barwa the results also imply that both surface processes and local tectonic responses play a vital role in the evolution of namche barwa an alternative hypothesis that the primary sustained heat source accounts for the local thermal rheological structure beneath namche barwa is also discussed

a reevaluation of cosmogenic 36cl production rates in terrestrial rocks we have measured 36cl in a suite of 33 rock samples having well constrained exposure histories and ages the 36cl production parameters were estimated by minimizing the squared deviations between the 36cl and independent ages yielding the following production parameters spallation and muon production from ca 2940 200 atoms 36cl mole ca 1 yr 1 spallation from k 6020 400 atoms 36cl mole k 1 yr 1 and neutron production in air 586 40 fast neutrons g air 1 yr 1 the new production constants for spallation on ca and thermal neutron activation are in good agreement with previous results but that for spallation on k is about 50 larger the past decade has seen a rapid growth in the application of terrestrial cosmogenic nuclides produced by cosmic ray particles within rocks at the earth surface nuclear cross sections particularly for reactions initiated by neutrons and muons are not known with sufficient accuracy so production rates for these nuclides must be empirically calibrated using independently dated geological samples direct production estimates for 36cl were first obtained by zreda et al 1991 who measured 36c1 in mineral separates from granitic boulders on moraine crests at chiatovitch creek in the eastern white mountains of california and nevada this data set was the best available at the time but since then several factors have prompted a reevaluation of the 36cl production rates one is that zreda et al 1994 have shown that there can be significant scatter of the cosmogenic ages of individual glacial boulders toward ages younger than that of the moraine due to soil erosion a second factor is that a much wider range of samples is now available for this purpose finally recent theoretical advances in calculating production by thermal neutron absorption liu et al 1994 now permit more realistic calibration of this reaction

in situ10be 26al exposure ages at meteor crater arizona a new method of dating the surface exposure of rocks from in situ production of 10be and 26al has been applied to determine the age of meteor crater arizona a lower bound on the crater age of 49 200 1 700 years has been obtained by this method

age and geomorphic history of meteor crater arizona from cosmogenic 36cl and 14c in rock varnish abstract using cosmogenic 36 cl buildup and rock varnish radiocarbon we have measured the exposure age of rock surfaces at meteor crater arizona our 36 cl measurements on four dolomite boulders ejected from the crater by the impact yield a mean age of 49 7 0 85 ka which is in excellent agreement with an average age of 49 3 ka obtained from thermoluminescence studies on shock metamorphosed dolomite and quartz these ages are supported by undetectably low 14 c in the oldest rock varnish sample

millennial scale varnish microlamination dating of late pleistocene geomorphic features in the drylands of western usa abstract varnish microlamination vml dating is a climate based correlative age determination technique used to correlate and date various geomorphic features in deserts in this study we establish a generalized late pleistocene 18 74 ka millennial scale microlamination sequence in fine grained fast accumulating rock varnish for the drylands of western usa radiometrically calibrate the sequence and correlate it with the 18 o record in the gisp2 greenland ice core we then use this climate correlated varnish microstratigraphy to estimate surface exposure ages for radiometrically dated late pleistocene geomorphic features in the study region the vml dating of debris flow deposits on the sehoo recessional shorelines of lake lahontan at the jessup embayment of central nevada yields a minimum limiting age of 14 95 15 95 ka in good agreement with a calibrated 14 c age of 15 22 0 12 ka for the timing of the lake recession the vml dating of a giant ejecta block on the rim of meteor crater in northern arizona yields a minimum limiting age of 49 15 ka closely matching a thermoluminescence tl age of 49 3 ka and slightly younger than a recently updated cosmogenic 36 cl age of 56 0 2 4 ka for the meteor impact event the vml dating of distal q2c fan surfaces on hanaupah canyon alluvial fan in death valley california yields a minimum limiting age of 73 55 ka in accord with cosmogenic 36 cl depth profile ages of 66 22 14 ka and 72 24 20 ka for the same fan deposits the close agreement between the vml age estimates and the independently derived radiometric ages for these geomorphic features attests to the validity and reliability of millennial scale vml dating to further assess its potential in desert geomorphological research we use the vml method to study alluvial fan responses to millennial scale climatic changes the vml dating of a small tributary fan in death valley reveals two episodes of fan aggradation one ceasing at 73 55 86 75 ka during the dry period of the last interglacial mis 5a and the other finishing at 66 15 ka during the wet period of the last glacial mis 4 the vml and 36 cl dating of the distal q2c fan surfaces on hanaupah canyon fan reveal two episodes of large scale fan aggradation ended at 72 24 20 ka and 73 55 ka during the wet period of mis 4 fanhead incision and associated within channel or fantoe aggradation are found to take place during the relatively dry period of the glacial to interglacial climatic transition 12 24 ka and the holocene interglacial dry period 0 12 ka these data indicate that on the millennial to sub milankovitch timescale 10 3 10 4 years fan aggradation is a discrete sedimentational process under various climatic conditions because fan aggradation is ultimately controlled by the intensity and frequency of precipitation events which in turn are modulated by major climatic oscillations such as heinrich events dansgaard oeschger do events and glacial interglacial shifts these major climatic changes could be the pacemaker of regionally contemporaneous large area fan segmentation