accelerated extension of tibet linked to the northward underthrusting of indian crust the tibetan plateau is extending numerical simulations suggest that regional scale extension is caused by gravitational collapse of the plateau whereas rapid extension in the south is caused by underthrusting of the indian slab

extension during continental convergence with application to the tibetan plateau the tibetan plateau is the product of crustal thickening caused by the collision between india and asia and is the largest active example of extensional tectonics in a zone of continental collision throughout most of the tertiary the tectonics of the plateau were dominated by north south shortening a significant proportion of which took place on east west striking thrust faults for the last 5 m y or so the plateau has been thinning by the mechanism of extension on north south trending normal faults previous investigations of the collision have been able to account for the large scale features of the tertiary deformation but have failed to explain the transition in the late tertiary to quaternary strain rate field of the plateau from north south compression to east west extension the transition could in principle be effected by a reduction in the rate of convergence between india and asia or by a uniform reduction in strength of the whole continental lithosphere of asia it could not however be effected by a reduction in strength of the elevated region alone this produces increased compressional strain rates in the weakened zone an alternative explanation for the transition to extension comes from considering the thermal evolution of thickened continental lithosphere the lower part of the lithosphere consists of a thermal boundary layer which when thickened by horizontal shortening is colder and denser than its surroundings convective instability of the thickened thermal boundary layer and its replacement by hot asthenosphere would rapidly raise the surface elevation and gravitational potential energy of the overlying part of the lithosphere the convective instability would happen in a time brief compared with the collision time scale 50 m y in the case of india and asia but would only occur after there had already been substantial thickening of the lithosphere numerical experiments show that for a range of lithospheric parameters the increase in surface height as much as 2 km and of potential energy 5 to 10 1012 n m 1 resulting from convective instability of the lower lithosphere are sufficient for east west extension to replace north south compression as the dominant feature of the stress field of the tibetan plateau

crustal deformation of the eastern tibetan plateau revealed by magnetotelluric imaging the ongoing collision of the indian and asian continents has created the himalaya and tibetan plateau through a range of deformation processes these include crustal thickening detachment of the lower lithosphere from the plate delamination and flow in a weakened lower crust 1 6 debate continues as to which of these processes are most significant 7 in eastern tibet large scale motion of the surface occurs but the nature of deformation at depth remains unresolved a large scale crustal flow channel has been proposed as an explanation for regional uplift in eastern tibet 6 8 9 but existing geophysical data 10 11 do not constrain the pattern of flow magnetotellurics uses naturally occurring electromagnetic waves to image the earth s subsurface here we present magnetotelluric data that image two major zones or channels of high electrical conductivity at a depth of 20 40 km the channels extend horizontally more than 800 km from the tibetan plateau into southwest china the electrical properties of the channels imply an elevated fluid content consistent with a weak crust 12 13 that permits flow on a geological timescale these findings support the hypothesis that crustal flow can occur in orogenic belts and contribute to uplift of plateaux our results reveal the previously unknown complexities of these patterns of crustal flow many previous studies of the indiaasia

crustal electrical structures and deep processes of the eastern lhasa terrane in the south tibetan plateau as revealed by magnetotelluric data abstract a 3d magnetotelluric mt inversion for a 2d broadband mt profile along 92 e in the eastern lhasa terrane was applied to understand the crustal electrical structures and deep processes in the india tibet continental collision zone the middle and lower 20 to 50 km crust is distributed with conductors that are primarily concentrated north of the yarlung zangbo sutures yzs the results imply that the hypothesis of middle and or lower crustal flow between the tethyan himalaya and lhasa terrane are not supported by the mt data within the profile area we suggest that given the possibility of the existence of channel flow in the middle and or lower crust extruding southward from tibet the southernmost portion should be limited in the northern yzs the electric model also indicates that the primarily conductive region in the middle to lower crust can be imaged from the yzs to 30 8 n while the crust of the northern lhasa terrane north of 30 8 n has a higher resistivity from this result it can be inferred that the northern lhasa terrane might have a cold and strong middle to lower crust and that the front of the india crust might be halted in the northern lhasa terrane 30 8 n along 92 e

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

miocene jiali faulting and its implications for tibetan tectonic evolution the karakoram jiali fault zone kjfz comprises a series of right lateral shear zones that southerly bound the eastward extrusion of northern tibet relative to india and stable eurasia here we present new 40 ar 39 ar age data from the puqu and parlung faults two easternmost branches of the jiali fault zone which indicate a main phase of the kjfz shearing from v18 to 12 ma thus the tibetan eastward extrusion bounded by principal strike slip fault zones started and was probably most active around the middle miocene an interval marked also by active east west extension in southern tibet the coincidence of these two tectonic events strongly suggests a common causal mechanism which is best explained as oblique convergence between india and asia under the framework of this mechanism the extension in southern tibet is not a proxy for the plateau uplift the kjfz activity was furthermore coincident with right lateral displacements along the gaoligong and sagaing faults in southeast asia this defines a miocene deformation record for the regional dextral accommodation zone that in response to the continuing india asia collision may have accounted for the initiation and prolonged history of clockwise rotation of the tibetan extrusion around the eastern himalayan syntaxis 2002 elsevier science b v all rights reserved

electrical conductivity of clinopyroxene nacl h2o system at high temperatures and pressures implications for high conductivity anomalies in the deep crust and subduction zone

propagation of the deformation and growth of the tibetan himalayan orogen a review abstract long standing problems in the geological evolution of the tibetan himalayan orogen include where the india asia convergence was accommodated and how the plateau grew to clarify these problems we review the deformations and their role in the plateau s growth our results show that 1630 km of shortening occurred across the tibetan himalayan orogen since 55 ma with more than 1400 km accommodated by large scale thrust belts these thrust belts display an outward expansion from central tibet and couple with the surficial uplift the development of the tibetan plateau involved three significant steps primitive plateau 90 55 ma proto plateau 55 40 ma and neoteric plateau 40 0 ma several processes have collaborated to produce the proto plateau including the pre existing primitive plateau the india asia collision and subductions of greater india and songpan ganzi beneath the lhasa qiangtang terrane since 40 ma the proto plateau which was dominated by a topographic gradient lower crustal flow and continuous india asia convergence experienced three periods of rapid outward growth 40 23 23 10 and 10 0 ma in general the n s trending rifts were caused by the eastward growth of the plateau dominated by thrusting and crust flow in central tibet while they were the results of intense n s shortening in himalaya

indian punch rifts tibet the strikes of late cenozoic normal faults in tibet fan systematically from a northwesterly to northeasterly direction from west to east they exhibit an axis of symmetry that bisects the himalayan arc and trends subparallel to india asia motion the two dimensional stress field inferred from the spatial pattern of normal fault orientations can be explained solely by collisional stresses localized along a southern part of the himalayan arc continued insertion of indian crust into tibet can explain the persistence of extension and may be increasing plateau crustal thickness and elevation coeval with rifting

3 d electrical structure across the yadong gulu rift revealed by magnetotelluric data new insights on the extension of the upper crust and the geometry of the underthrusting indian lithospheric slab in southern tibet abstract the approximately north south trending cenozoic yadong gulu rift ygr in the eastern lhasa block is an ideal location to investigate the extensional kinematic mechanism of the upper crust and the deformation characteristics of the indian lithospheric slab in southern tibet the magnetotelluric mt method has been widely used in probing subsurface structures at lithospheric scale and is sensitive to high electrically conductive body conductor a three dimensional 3 d inversion of mt data was conducted to derive the east west electrical structures across the northern segment of the ygr the result reveals that the conductors in the middle crust are not continuous in the east west direction the deep conductor underneath the ygr is interpreted to result from the tearing of the indian lithospheric slab the upper crust to the east of the ygr is significantly intruded by underlying conductors based on the features of the 3 d inversion result from this study and other geophysical observations the formation of the ygr is most likely caused by tearing of the indian lithospheric slab through the pull of mid lower crustal conductors that have locally weak strength beneath the ygr

geophysical evidence of deep hydrocarbon flow in mottled zone areas dead sea transform zone the origin of unusual magnetic initially sedimentary rocks of the mottled zone mz in israel and jordan remained enigmatic for several decades until integrated characterization of the mz area was achieved by ground magnetic measurements and geologic observations along representative profiles in parallel with reprocessing and reinterpretation of available aeromagnetic and gravity data micromagnetic profiling was combined with gamma radioactivity measurements and representative samples were selected to determine rock physical properties results of field measurements reduction and transformation of geophysical fields anomaly inversion and forward modeling accompanied by geologic analysis suggest that two types of magnetic anomalies and local gravity minima in the mz areas are related to the same event i e deep hydrocarbon flow associated with fossil mud volcanism physicochemical interaction of deep hydrocarbon flow and surrounding sedimentary rocks caused widespread weak magnetization and correspo

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

weakly coupled lithospheric extension in southern tibet a b s t r a c t west east extension is a prominent tectonic feature of southern and central tibet despite ongoing north south n s convergence between india and eurasia knowledge of deep structure beneath the n s trending rifts is key to evaluating models proposed for their origin including gravitational collapse oblique convergence along the arcuate plate boundary and mantle upwelling we model direct s and moho reflected sspmp phases at teleseismic distances to constrain variations in crustal thickness across the major rifts crossed by a 900 km long w e broadband array in the lhasa terrane crustal thicknesses are 70 80 km however moho depth decreases by 10 km within a horizontal distance of 100 km west of the yadong gulu rift ygr and nyainquentanghla mountains nqtl this moho uplift taken with deep extensional focal mechanisms and reduced seismic velocity in the upper mantle suggests that asthenospheric upwelling has significantly contributed to the pattern of extension across the ygr and nqtl the 100 km separation between surface rift and moho uplift is likely enabled by partial decoupling across a ductile middle crust 2015 the authors published by elsevier b v this is an open access article under the cc by nc nd license http creativecommons org licenses by nc nd 4 0

low angle range flank faults in the panamint inyo and slate ranges california implications for recent tectonics of the death valley region four of the mountain ranges near death valley california display exhumed low angle normal faults on their flanks features originally referred to as turtleback structures these fault surfaces are smooth planar to curviplanar and defined by faceted spurs and coincident interfluve crests the lowest parts of these faults are overlain in tectonic contact by poorly indurated fanglomerate and locally by quaternary volcanic rocks steep fault scarps in alluvium are present at the foot of each turtleback type flank although these may be examples of active low angle normal faults it is difficult to establish the dip histories of these faults the problem with constraining the dip histories is one of missing and concealed information because the exposed hanging wall rocks are some of the youngest material cut and displaced by the currently shallow dipping faults another important relationship along these range flanks is the intersection between the low angle faults and steep scarp forming faults that cut alluvium at the range fronts although this relationship remains unconstrained in most of the ranges a shallow dipping fault is cut by one of the steep faults in panamint valley suggesting that in one area at least the low angle faults are not active the low angle faults and not the steep neotectonic faults are probably responsible for most of the opening of the present valleys because each low angle fault intersects the valley floor at the range front none of the low angle faults are perched high in the ranges as would be the case if the steep faults had uplifted them significantly therefore even if there has been a geologically recent transition from low angle normal faulting to steep normal with or without strike slip faulting the modern topography still reflects the former regime strictly speaking the death valley region may not be an example of active low angle normal faults and supradetachment basins

association between magnetic susceptibilities and hydrocarbon deposits in the barinas apure basin venezuela we have measured the magnetic susceptibility of 5425 drill cuttings coming from 20 oil wells distributed in eight fields of the petroliferous barinas apure basin barinas venezuela seeking evidence of magnetic anomalies associated with the presence of hydrocarbon deposits the rock samples are located between the near earth s surface and the basement s top approximately 4000 m in the magnetic susceptibility profiles we observed high magnetic susceptibilities at the top of the sedimentary units corresponding to reservoir and source rocks of oil producing wells whereas in the case of nonproducing wells we observed low magnetic susceptibilities a basic statistical analysis of the magnetic susceptibility measurements shows a significant difference between the mean values across producing and nonproducing wells at the correspondent oilrelated formations with a ratio of 4 1 of producing to nonproducing wells in the producing wells we have found a relation between the magnitudes of the magnetic susceptibility and the age of the formation the larger the magnetic susceptibility the younger the geological stratum the age and depth of the formations where the cuts were obtained exclude the presence of cultural human origin contamination of the samples suggesting the evidence of authigenic origin of the magnetic minerals due to the reductive effect of hydrocarbons in rocks

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

direct stratigraphic dating of india asia collision onset at the selandian middle paleocene 59 1 ma the collision of india with asia had a profound influence on cenozoic topography oceanography climate and faunal turnover however estimates of the time of the initial collision when indian continental crust arrived at the transhimalayan trench remain highly controversial here we use radiolarian and nannofossil biostratigraphy coupled with detrital zircon geochronology to constrain firmly the time when asian derived detritus was first deposited onto india in the classical sangdanlin section of the central himalaya which preserves the best paleocene stratigraphic record of the distal edge of the indian continental rise deep sea turbidites of quartzarenite composition and indian provenance are replaced upsection by turbidites of volcano plutoniclastic composition and asian provenance this sharp transition occurs above abyssal cherts yielding radiolaria of paleogene radiolarian zones rp 4 6 and below abyssal cherts containing radiolaria of zone rp6 and calcareous shales with nannofossils of the paleocene calcareous nannofossil zone cnp 7 constraining the age of collision onset to within the middle paleocene selandian the youngest u pb ages yielded by detrital zircons in the oldest asia derived turbidites indicate a maximum depositional age of 58 1 0 9 ma collision onset is thus mutually constrained by biostratigraphy and detrital zircon chronostratigraphy as 59 1 ma this age is both more accurate and more precise than those previously obtained from the stratigraphic record of the northwestern himalaya and suggests that within the resolution power of current methods the india asia initial collision took place quasi synchronously in the western and central himalaya

crustal and upper mantle structure of northern tibet imaged with magnetotelluric data 1 magnetotelluric mt data were collected in northern tibet along the amdo to golmud highway during the 1995 and 1999 project indepth international deep profiling of tibet and the himalaya surveys broadband and long period mt data were collected and the te mode tm mode and vertical magnetic field data were inverted to yield a minimum structure two dimensional resistivity model the model obtained from inverting all responses simultaneously shows that a pervasive midcrustal conductor extends from the kunlun shan to the bangong nuijiang suture the vertically integrated conductivity conductance of this crustal layer is greatest in the northern qiangtang terrane at latitude 34 n the electrical resistivity of the upper mantle is constrained by the mt data to be in the range of 10 30 m across the songpan ganze and qiangtang terranes this is lower than would be expected if asian lithosphere underthrusts northern tibet as far as the qiangtang terrane the mt responses are more consistent with a model in which asian lithosphere extends as far south as the kunlun shan and the upper mantle beneath the songpan ganze and qiangtang terranes is sufficiently hot to contain a small fraction of interconnected partial melt

p wave tomography and dynamics of the crust and upper mantle beneath western tibet abstract the continental collision between the indian and asian plates plays a key role in the geologic and tectonic evolution of the tibetan plateau in this article we present high resolution tomographic images of the crust and upper mantle derived from a large number of high quality seismic data from the antilope project in western tibet both local and distant earthquakes were used in this study and 35 115 p wave arrival times were manually picked from the original seismograms geological and geochemical results suggested that the subducting indian plate has reached northward to the lhasa terrane whereas our new tomography shows that the indian plate is currently sub horizontal and underthrusting to the jinsha river suture at depths of 100 to 250 km suggesting that the subduction process has evolved over time the asian plate is also imaged clearly from the surface to a depth of 100 km by our tomography and it is located under the tarim basin north of the altyn tagh fault there is no obvious evidence to show that the asian plate has subducted beneath western tibet the indian and asian plates are separated by a prominent low velocity zone under northern tibet we attribute the low velocity zone to mantle upwelling which may account for the warm crust and upper mantle beneath that region and thus explain the different features of magmatism between southern and northern tibet but the upwelling may not penetrate through the whole crust we propose a revised geodynamic model and suggest that the high velocity zones under lhasa terrane may reflect a cold crust which has interrupted the crustal flow under the westernmost tibetan plateau

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

thermal characteristics of the main himalaya thrust and the indian lower crust with implications for crustal rheology and partial melting in the himalaya orogen abstract the main himalaya thrust mht is the current tectonic boundary between the subducting indian lithosphere and the overlying himalayan orogenic prism and the tibetan crust we present thermo kinematic calculations and metamorphic p t t paths of the indian lower crust ilc that constrain the thermal structure of the mht and the southern tibetan crust lhasa block and explain the origin of a thin seismic low velocity zone that was revealed by the recent hi climb experiment from receiver functions of teleseismic waves northward of the himalayas the low velocity zone occurs within the ductile regime of the crust and is thought to extend along the mht into the lhasa block in the lhasa block the low velocity zone occurs directly above the ilc predicted evolution of mineralogy of the ilc along its subduction p t t path shows that its dehydration can potentially induce wet melting within the orogenic prism above the inclined portion of the mht however north of the yarlung tsangpo suture yts below the southern lhasa block where subduction of the ilc is flat the ilc is predicted to be anhydrous eclogite and therefore it cannot supply h 2 o to the overlying crust the seismic low velocity zone above this portion of the ilc is best explained by dehydration melting due to strain heating the mht there appears to be localized by the rheological contrast between the ductile lower lhasa block and the strong eclogitic ilc southward thrusting of the himalaya orogenic prism which contains accreted indian upper crust causes advection of hot middle crustal rocks to shallower levels thereby producing a shallow ductile regime between the himalayas and the yts the shallow ductile regime is evident in the limit of upper crustal earthquake foci to shallow depths in this region

construction and destruction of the north china craton with implications for metallogeny magnetotelluric evidence from the hengshan wutai fuping region within trans north china orogen abstract the hengshan wutai fuping region within the trans north china orogen is an important corridor that preserves the records of precambrian evolutionary history and phanerozoic magmatic and metallogenic events in the north china craton here we present 2 d and 3 d inversion models of magnetotelluric data collected from the hengshan wutai fuping region and surrounding regions our data reveal nearly horizontal high conductivity layer at lower crustal and upper mantle depths of 35 45 km beneath the hengshan and wutai mountains providing direct evidence for crust mantle decoupling and magmatic underplating in the mesozoic yixingzhai gold deposits and boqiang cu mo au deposits two nearly upright conductors cut through the crust and connect the lower crustal high conductivity layer suggesting frozen channels of fluids and magmas a west dipping slab shaped conductor within the high resistivity lithosphere beneath the wutai complex appears to be the deep extension of the exposed longquanguan ductile shear zone these resistivity anomalies can provide new constraints on the subduction collision events during precambrian tectonic evolution of the trans north china orogen as well as on the phanerozoic tectonic magmatic and metallogenic events in the hengshan wutai fuping region the west dipping slab shaped conductor may indicate a westward oceanic subduction between the wutai and fuping complexes with subsequent collision associated with the formation of the trans north china orogen in the paleoproterozoic we interpret the crustal and upper mantle conductors to be part of an interconnected fluid melt transport channel which probably played an important role for the mesozoic tectonic magmatic and metallogenic events as well as the cenozoic basaltic volcanism in the hengshan wutai fuping region

penetration of crustal melt beyond the kunlun fault into northern tibet the transition between the weak lithosphere of the tibetan plateau and the surrounding rigid crustal blocks has a key role in the ongoing collision between india and asia a reanalysis of existing magnetotelluric data suggests that crustal melt penetrates north from the tibetan plateau beyond the kunlun fault and weakens the crust beneath the kunlun shan

crustal lithospheric structure and continental extrusion of tibet crustal shortening and thickening to c 70 85 km in the tibetan plateau occurred both before and mainly after the c 50 ma india asia collision potassic ultrapotassic shoshonitic and adakitic lavas erupted across the qiangtang c 50 29 ma and lhasa blocks c 30 10 ma indicate a hot mantle thick crust and eclogitic root during that period the progressive northward underthrusting of cold indian mantle lithosphere since collision shut off the source in the lhasa block at c 10 ma late miocene pleistocene shoshonitic volcanic rocks in northern tibet require hot mantle we review the major tectonic processes proposed for tibet including rigid block continuum and crustal flow as well as the geological history of the major strike slip faults we examine controversies concerning the cumulative geological offsets and the discrepancies between geological quaternary and geodetic slip rates low present day slip rates measured from global positioning system and insar along the karakoram and altyn tagh faults in addition to slow long term geological rates can only account for limited eastward extrusion of tibet since mid miocene time we conclude that despite being prominent geomorphological features sometimes with wide mylonite zones the faults cut earlier formed metamorphic and igneous rocks and show limited offsets concentrated strain at the surface is dissipated deeper into wide ductile shear zones

40ar 39ar dating of the jiali and gaoligong shear zones implications for crustal deformation around the eastern himalayan syntaxis abstract we conducted a comprehensive 40ar 39ar geochronological study of the jiali and gaoligong shear zones to obtain a better understanding of crustal deformation and tectonic evolution around the eastern himalayan syntaxis ehs the new age data reveal that the main phase of deformation in the jiali and gaoligong shear zones occurred from 22 to 11 ma and from 18 to 13 ma respectively structural data collected during this study indicate that the jiali shear zone underwent a change in shear sense from sinistral to dextral during its movement history based on a comparison with the deformation histories of other major shear zones in the region we argue that the initial sinistral motion recorded by the jiali shear zone was coincident with that of the ailao shan red river shear zone which marked the northern boundary of the southeastward extrusion of the indochina block during the early miocene from the middle miocene 18 ma the jiali shear zone changed to dextral displacement becoming linked with the dextral gaoligong shear zone that developed as a consequence of continued northward indentation of the indian continent into asia since this time the jiali and gaoligong shear zones have been united defining the southwestern boundary of the ehs during clockwise rotation of the eastward extruding tibetan block as revealed by recent gps data the temporal change in regional deformation pattern from southeastward block extrusion to clockwise rotation of crustal fragments may have played an important role in the development of the eastern himalayan drainage system around the ehs

quaternary low angle slip on detachment faults in death valley california detachment faults on the west flank of the black mountains nevada and california dip 29 36 and cut subhorizontal layers of the 0 77 ma bishop ash steeply dipping normal faults confined to the hanging walls of the detachments offset layers of the 0 64 ma lava creek b tephra and the base of 0 12 0 18 ma lake manly gravel these faults sole into and do not cut the low angle detachments therefore the detachments accrued any measurable slip across the kinematically linked hanging wall faults an analysis of the orientations of hundreds of the hanging wall faults shows that extension occurred at modest slip rates 1 mm yr under a steep to vertically oriented maximum principal stress the black mountain detachments are appropriately described as the basal detachments of near critical coulomb wedges we infer that the formation of late pleistocene and holocene range front fault scarps accompanied seismogenic slip on the detachments

crustal density structure from 3d gravity modeling beneath himalaya and lhasa blocks tibet abstract the himalaya and lhasa blocks act as the main belt of convergence and collision between the indian and eurasian plates their crustal structures can be used to understand the dynamic process of continent continent collision herein we present a 3d crustal density model beneath these two tectonic blocks constrained by a review of all available active seismic and passive seismological results on the velocity structure of crust and lower lithosphere from our final crustal density model we infer that the present subduction angle of the indian plate is small but presents some variations along the west east extension of the orogenic belt the dip angle of the moho interface is about 8 9 in the eastern and western part of the orogenic belt and about 16 in the central part integrating crustal p wave velocity distribution from wide angle seismic profiling geothermal data and our crustal density model we infer a crustal composition model which is composed of an upper crust with granite granodiorite and granite gneiss beneath the lhasa block biotite gneiss and phyllite beneath the himalaya a middle crust with granulite facies and possible pelitic gneisses and a lower crust with gabbro norite troctolite and mafic granulite beneath the lhasa block our density structure

lithospheric electrical structure in the central tibetan plateau and its tectonic significance abstract the himalaya tibetan plateau that is situated in the southwest china is an ideal natural field laboratory for continental dynamics in this study through the careful processing and analyzing of the magnetotellurics data the reliable 2 d electrical model was obtained to examine the relationship between the lithospheric electrical structure and dynamics in the central himalaya tibetan plateau six conductors mainly result from the partial melting and aqueous fluids and three resistors are related to different tectonic evolution in the model combined with other observations we summarize that the upwelling hot material partial melting aqueous fluids subduction erosion and crushed zone contribute to the error between the electrical location and the geographic location of the indus yarlung zangbo suture the subducted indian plate is shown by the different angles and different locations of its frontier in different areas the electrical structure beneath the lhasa terrane corresponds well with the geological features of an old locally reworked crust dominantly in the central lhasa terrane with two young juvenile crustal blocks surrounding the central terrane and it also shows the relationship among the dynamics magmatism and mineralization in the gangdise metallogenic belt meanwhile we indicate that the subducted polarity of the bangong nujiang tethyan ocean lithosphere was double sided and the smooth subduction of the longmu tso shuanghu tethyan ocean lithosphere may cut through the moho surface approximately 20 km above all these results can provide helpful constrains to understand the evolution mechanism of the himalaya tibetan plateau

structure of the central altyn tagh fault revealed by magnetotelluric data new insights into the structure of the northern margin of the india asia collision abstract the altyn tagh fault atf is a left lateral strike slip fault that forms the northern margin of the tibetan plateau and plays a significant role in accommodating the convergence between the colliding indian and eurasian plates as a part of the fourth phase of the indepth project magnetotelluric mt data were collected across the central segment of the atf to determine the lithospheric scale structure of the fault system dimensionality analyses demonstrated that the mt data can be interpreted using two dimensional approaches but some localized 3 d effects are seen consequently both 2 d and 3 d inversions were carried out and a joint interpretation was made on the basis of these two types of models inversion models revealed two major conductors beneath the qaidam basin qb and altyn tagh range atr respectively the conductive region beneath the qb was interpreted as a ductile layer in the lower crust to upper mantle that might represent flow beneath the western margin of the qb whereas the large scale south dipping conductor beneath the atr is interpreted as a region with high fluid content formed by metamorphism associated with the oblique underthrusting of the tarim block beneath the northern tibetan plateau these fluids migrate upwards through the fault system and have formed serpentinized zones in the crust combining these interpretations a structural model compatible with diverse geophysical observations is proposed in which we suggest the competing end member rigid block model and continuum model are reconcilable with the continuum model locally dominant for the study region as evidenced by a thickened crust

cenozoic crustal scale duplexing and flat moho in southern tibet evidence from reflection seismology abstract although the lhasa terrane of southern tibet is an integral part of the tibetan plateau how the area has achieved its current high altitude of 5000 m remains poorly understood this is largely due to the fact that little is known about deep 15 km crustal structures and the depth and geometry of the moho below the terrane in this study we address this question by acquiring and analyzing high resolution seismic reflection data gathered from four high energy dynamite shots along a 137 km north south traverse spanning the southern lhasa terrane our new seismic data reveal the presence of north dipping reflectors in the crust a seismically weak reflection layer at the lowermost crust and a sub horizontal and locally discontinuous moho at depth of 69 72 km with a crustal velocity of 6 30 km s the discordant relationship between the crustal dipping reflectors the subhorizontal lower crustal layer and the flat moho suggests that the 12 km basal section of the lhasa terrane crust is a decoupling zone accommodating different styles of lithospheric scale deformation because the north dipping reflectors can be projected and or connected with the reflectors in the cenozoic himalayan thrust system and the southernmost lhasa terrane we propose that the dipping crustal reflectors are parts of a south directed crustal scale thrust duplex created by the cenozoic india asia collision the relatively discontinuous and horizontal geometry at the base of the crust indicates that the moho was reconstituted through ductile deformation in the lowermost crust caused by abnormally high temperature

present day crustal motion within the tibetan plateau inferred from gps measurements 1 using the measurements of 726 gps stations around the tibetan plateau we determine the rigid rotation of the entire plateau in a eurasia fixed reference frame which can be best described by an euler vector of 24 38 0 42 n 102 37 0 42 e 0 7096 0 0206 ma the rigid rotational component accommodates at least 50 of the northeastward thrust from india and dominates the eastward extrusion of the northern plateau after removing the rigid rotation to highlight the interior deformation within the plateau we find that the most remarkable interior deformation of the plateau is a glacier like flow zone which starts at somewhere between the middle and western plateau goes clockwise around the eastern himalayan syntaxis ehs and ends at the southeast corner of the plateau with a fan like front the deformation feature of the southern plateau especially the emergence of the flow zone could be attributed to an eastward escape of highly plastic upper crustal material driven by a lower crust viscous channel flow generated by lateral compression and gravitational buoyancy at the later developmental stage of the plateau the first order feature of crustal deformation of the northeastern plateau can be well explained by a three dimensional elastic half space dislocation model with rates of dislocation segments comparable to the ones from geological observations in the eastern plateau although gps data show no significant convergence between the eastern margin of the plateau and the sichuan basin a small but significant compressional strain rate component of 10 5 2 8 nstrain yr exists in a relatively narrow region around the eastern margin in addition a large part of the eastern plateau northeast of the ehs is not undergoing shortening along the northeastward convergence direction of the ehs but is stretching

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

three dimensional model of the late cenozoic history of the death valley region southeastern california the accumulation of a large database on the timing and kinematics of late cenozoic deformation in the death valley region of southeastern california indicates a complex three dimensional history on the basis of paleogeographic reconstructions we suggest the system was initiated as a localized pull apart between two conjugate strike slip faults the garlock and furnace creek faults and evolved into a system characterized by distributed transtension related to the eastern california shear zone our reconstructions differ from previous models in the incorporation of significant vertical axis rotations of a number of crustal blocks to explain paleomagnetic data from the region the model may resolve 1 a long standing problem of the eastern termination of the garlock fault which is explained here as a complex system of splays that initially terminated in the pull apart between the furnace creek and garlock systems and 2 the complex architecture of the black mountains which is explained here in terms of initial extreme attenuation between the garlock and furnace creek systems with overprinting by a fold and normal fault system that operated simultaneously as a result of distributed transtension this model suggests much of the displacement field is taken up in rotations and translations and the actual crustal thinning in our model is relatively small 50 66 of original thickness

fold fault relationships in low angle detachment systems a close kinematic and timing relationship between low angle extensional faulting and upright to moderately inclined folding with fold axes parallel to the extension direction is established for two well exposed neogene detachment systems in the simplon region of the alps and in the death valley region of california fold amplitude is largest in the oldest mylonitic foliation of the footwall these folds are transected by younger mylonitic foliation s and cataclastic detachment faults which are themselves folded about similar axes in general the amplitude decreases and the wavelength increases for progressively younger fold structures consistent with a history of progressive folding during faulting the interplay between folding and faulting in the footwall reflects a component of shortening perpendicular to fold axial surfaces during extension parallel to fold axes this strain pattern can develop in regions with a transcurrent component or during lateral extrusion and does not necessarily require overall crustal thinning

seismic images of crust and upper mantle beneath tibet evidence for eurasian plate subduction seismic data from central tibet have been combined to image the subsurface structure and understand the evolution of the collision of india and eurasia the 410 and 660 kilometer mantle discontinuities are sharply defined implying a lack of a subducting slab beneath the plateau the discontinuities appear slightly deeper beneath northern tibet implying that the average temperature of the mantle above the transition zone is about 300 c hotter in the north than in the south there is a prominent south dipping converter in the uppermost mantle beneath northern tibet that might represent the top of the eurasian mantle lithosphere underthrusting the northern margin of the plateau

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

tibetan plate overriding the asian plate in central and northern tibet at the southern boundary between india and tibet the indian tectonic plate subducts northwards beneath the tibetan plateau seismic imaging shows that at the northern boundary of tibet the asian plate is also subducting southwards beneath the plateau and the tibetan lithosphere is separate

mapping crustal structure beneath southern tibet seismic evidence for continental crustal underthrusting abstract receiver function imaging along a temporary seismic array antilope 2 reveals detailed information of the underthrusting of the indian crust in southern tibet the moho dips northward from 50 km to 80 km beneath the himalaya terrane and locally reaches 85 km beneath the indus yalung suture it remains at 80 km depth across the lhasa terrane and shallows to 70 km depth under the qiangtang terrane an intra crustal interface at 60 km beneath the lhasa terrane can be clearly followed southward through the main himalaya thrust and connects the main boundary thrust at the surface which represents the border of the indian crust that is underthrusting until south of the bangong nujiang suture a mid crustal low velocity zone is observed at depths of 14 30 km beneath the lhasa and himalaya terranes probably formed by partial melt and or aqueous fluids

geochemical variations in miocene adakitic rocks from the western and eastern lhasa terrane implications for lower crustal flow beneath the southern tibetan plateau abstract it is generally believed that miocene adakitic rocks in the lhasa terrane block of southern tibet were derived from a thickened lower crust and that their compositions will therefore reflect the geochemical characteristics of their lower crustal source s adakitic rocks in the western part of the lhasa terrane possess geochemical and sr nd isotopic characteristics that are clearly different from those in the eastern part of the lhasa terrane the former display a more enriched nd sr isotopic signature together with an older model age relative to the latter suggesting that the compositions of the lower crust under the western and eastern parts of the lhasa terrane also differ in addition the adakitic rocks in the himalayas on the southern side of the indus yalu suture iys have similar geochemical characteristics and sr nd isotopic compositions to the adakitic rocks of the western lhasa terrane but differ from rocks in the eastern lhasa terrane and from igneous rocks derived from crust beneath the himalayas we suggest that the lower and or middle crustal materials beneath the western lhasa terrane probably extend to the southern side of the iys due to channel flow if mid lower crustal flow has occurred below southern tibet the southeastward moving anatectic mid lower crustal material was probably derived from below the western part of the lhasa terrane rather than the eastern moreover the emplacement ages of adakitic rocks from mayum on the southern side of the iys imply that southeastward ductile flow of mid lower crust beneath western lhasa may have occurred as early as 17 ma

seismic velocities in southern tibet lower crust a receiver function approach for eclogite detection summary beneath the tibet plateau the deficit of crustal thickening with respect to what is expected from the plate tectonic constraints is thought to be absorbed either by lateral extrusion or by vertical rock mass transfer to nourish the unsettled debate of the relative importance of these two processes we propose a new approach based on the s to p and the p to s wave conversions enabling the precise determination of the seismic velocities the weighted amplitudes of the direct conversion and of reverberations are stacked at their predicted arrival times for various values of layer thickness and vp vs ratio separately for two sets of p and s receiver functions for each set of receiver functions coherent stack gives the vp vs ratio and thickness for the considered layer the grid search stacking method the values of vp vs ratio and layer thickness are functions of the velocity used for stacking the set of receiver functions but using the p and s receiver functions allows us to solve this indetermination and to find the effective parameters of the layer velocity vs vp vs ratio and thickness we use a bootstrap resampling of the receiver function data sets to estimate the parameters uncertainties for the southern lhasa block the migrated sections of both p and s receiver functions hi climb experiment data show a layer in the lower crust that may be related to the lower indian crust underplated beneath tibet with the grid search stacking method high shear wave velocities vs 4 73 km s 1 and low vp vs ratios 1 69 are detected in this layer such values are typical for high grade eclogites and the low vp vs ratio precludes the confusion with mafic granulites there is no evidence for partial eclogitization near and south of the yarlung tsangpo suture and the about 19 km thick eclogitic layer extends northwards only to about the middle of the lhasa terrane

correlating biodegradation to magnetization in oil bearing sedimentary rocks abstract a relationship between hydrocarbons and their magnetic signatures has previously been alluded to but this is the first study to combine extensive geochemical and magnetic data of hydrocarbon associated samples we report a detailed study that identifies a connection between magnetic mineralogy and oil biodegradation within oil bearing sedimentary units from colombia canada indonesia and the uk geochemical data reveal that all the oil samples are derived from mature type ii kerogens deposited in oxygen poor environments biodegradation is evident to some extent in all samples and leads to a decrease in oil quality through the bacterially mediated conversion of aliphatic hydrocarbons to polar constituents the percentage of oil components and the biodegradation state of the samples were compared to the magnetic susceptibility and magnetic mineralogy a distinct decrease in magnetic susceptibility is correlated to decreasing oil quality and the amount of extractable organic matter present further magnetic characterization revealed that the high quality oils are dominated by pseudo single domain grains of magnetite and the lower quality oils by larger pseudo single domain to multidomain grains of magnetite and hematite hence with decreasing oil quality there is a progressive dominance of multidomain magnetite as well as the appearance of hematite it is concluded that biodegradation is a dual process firstly aliphatic hydrocarbons are removed thereby reducing oil quality and secondly magnetic signatures are both created and destroyed this complex relationship may explain why controversy has plagued previous attempts to resolve the connection between magnetics and hydrocarbon deposits these findings reinforce the importance of bacteria within petroleum systems as well as providing a platform for the use of magnetization as a possible exploration tool to identify subsurface reservoirs and a novel proxy of hydrocarbon migration

implications of crustal property variations for models of tibetan plateau evolution shear coupled teleseismic p waves sampling the interior of the tibetan plateau provide evidence of systematic variations in crustal structure the crust thins by up to 20 km from south to north with a concomitant increase in poisson s ratio from normal values in the south to unusually high values in the north this suggests that the crust of the northern plateau is partially melted due to high temperatures these changes imply spatial and perhaps temporal variations in the way the elevation of the high plateau is created and maintained

mapping hydrocarbon charge points in the wessex basin using seismic geochemistry and mineral magnetics abstract this study reports a multidisciplinary approach to determining hydrocarbon charge points and migration in the wessex basin southern england geochemical analysis of reservoir core material bridport sandstone and inferior oolite using gas chromatography mass spectrometry gc ms suggests that the oil in the wessex basin is from a single source and that small variations in environmentally sensitive biomarkers are likely due to small differences in maturity or depositional conditions during the formation of the oil over millions of years using seismic data basin modelling revealed two potential hydrocarbon migration pathways from the hanging wall of the purbeck fault into the sherwood sandstone reservoir at wytch farm one of these potential pathways is represented by cores termed creech and the other bushey farm to try to distinguish between the two potential pathways cores were studied using mineral magnetic techniques the magnetic signature was characterised using low temperature

the exhumation history of collision related mineralizing systems in tibet insights from thermal studies of the sharang and yaguila deposits central lhasa abstract the large newly discovered sharang porphyry mo deposit and nearby yaguila skarn pb zn ag mo deposit reside in the central lhasa terrane northern gangdese metallogenic belt tibet multiple mineral chronometers zircon u pb sericite 40 ar 39 ar and zircon and apatite u th he reveal that ore forming porphyritic intrusions experienced rapid cooling 100 c ma during a monotonic magmatic hydrothermal evolution the magmatic hydrothermal ore forming event at sharang lasted 6 0 myr 1 8 myr for cooling from 900 to 350 c and 4 0 myr for cooling from 350 to 200 c whereas cooling was more prolonged during ore formation at yaguila 1 8 myr from 900 to 500 c and a maximum of 16 myr from 900 to 350 c all porphyritic intrusions in the ore district experienced exhumation at a rate of 0 07 0 09 mm yr apatite he ages between 37 and 30 ma combined with previous studies this work implies that uplift of the eastern section of the lhasa terrane expanded from central lhasa 37 30 ma to southern lhasa 15 12 ma at an increasing exhumation rate all available geochronologic data reveal that magmatic hydrothermal exhumation activities in the sharang yaguila ore district occurred within four periods of magmatism with related mineralization significant porphyry type mo mineralization was associated with late cretaceous eocene felsic porphyritic intrusions in the central lhasa terrane resulting from neotethyan oceanic subduction and india asia continental collision

field tests of rolling hinges existence mechanical types and implications for extensional tectonics in rolling hinge models a locus of isostatic footwall uplift migrates together with the withdrawing hanging wall of a large displacement tens of kilometers normal fault a fault bend fold that migrates through the footwall results causing progressive deactivation of the updip part of the fault as it is tilted to a lower dip rolling hinge migration should leave a discernible geologic record but compelling tests are difficult to design because predictions of rolling hinge models are nonunique and alternative models commonly can explain individual data sets field tests based on thermochronology paleomagnetism and structural analysis are typically applied to footwall rocks whereas sedimentology geochronology of volcanic rocks and structural analysis are typically applied to upper plate rocks we review existing theoretical models and case studies in 14 detachment terranes some negative results are reasonably compelling but positive tests as yet tend to permit rather than demand a rolling hinge locally both positive and negative conclusions have been reached about the passage of a hinge within the same terrane such differences in interpretation may depend more on the scale of observation or on the specific model considered than on the geological development of the area subvertical simple shear and flexural failure mechanisms with or without significant induced flow of lower crustal material are thought to dominate hinge related footwall deformation in individual sites although rolling hinge models offer an alternative to low initial fault dips hinge related deformation does not demand a steep initial fault dip few areas in which rolling hinge tectonics are permissible had initially steep faults 35 rather where the evidence most strongly favors a rolling hinge detachment faults had initial dips 35 in the future conclusive results are most likely to be obtained if diverse tests are applied to both the upper and lower plates of detachment terranes where other geological sources of ambiguity are minimal at best the rolling hinge mechanism does not universally explain the observation that earthquakes are rare on low angle normal faults nor solve the mechanical problems of frictional slip on such faults

the black mountains turtlebacks rosetta stones of death valley tectonics the black mountains turtlebacks expose mid crustal rock along the western front of the black mountains as such they provide keys to understanding the tertiary structural evolution of death valley and because of the outstanding rock exposure they also provide valuable natural laboratories for observing structural processes there are three turtlebacks the badwater turtleback in the north the copper canyon turtleback and the mormon point turtleback in the south although important differences exist among them each turtleback displays a doubly plunging antiformal core of metamorphic and igneous rock and a brittle fault contact to the northwest that is structurally overlain by miocene pleistocene volcanic and or sedimentary rock the turtleback cores contain mylonitic rocks that record an early period of top southeastward directed shear followed by top northwestward directed shear the earlier formed mylonites are cut by and locally appear concurrent with 55 61 ma pegmatite we interpret these fabrics as related to large scale basement involved thrust faults at the turtlebacks now preserved as areally extensive metamorphosed basement over younger cover contacts the younger and far more pervasive mylonites record late tertiary extensional unroofing of the turtleback footwalls from mid crustal depths available geochronology suggests that they cooled through 300 8c at different times 13 ma at badwater 6 ma at copper canyon 8 ma at mormon point at mormon point and copper canyon turtlebacks these dates record cooling of the metamorphic assemblages from beneath the floor of an 11 ma tertiary plutonic complex collectively these relationships suggest that the turtlebacks record initiation of ductile extension before 14 ma followed by injection of a large plutonic complex along the ductile shear zone ductile deformation continued during extensional uplift until the rocks cooled below temperatures for crystal plastic deformation by 6 8 ma subsequent low angle brittle fault slip led to final exposure of the igneous and metamorphic complex the turtleback shear zones can constrain models for crustal extension from map view as well as cross sectional perspectives in map view the presence of basement involved thrust faults in the turtlebacks suggest the black mountains were a basement

structural geology and kinematic history of rocks formed along low angle normal faults death valley california several late cenozoic low angle normal or detachment faults on the western flank of the black mountains are each characterized by the following in descending structural order 1 a hanging wall of upper tertiary to quaternary sedimentary and volcanic strata that displays little evidence for fault related damage other than widely spaced planar or listric normal faults 2 a sharp planar and locally striated principal slip plane forming the lower boundary of the hanging wall 3 an upper zone zone i of very fine grained fault generated rocks composed dominantly of gouge 4 a lower zone zone ii of coarser grained fault rocks consisting chiefly of foliated breccia and 5 a variably damaged footwall consisting of partly mylonitic precambrian units or tertiary plutonic rocks that has been exhumed from depths of 10 12 km since late miocene time the fault rocks which were mostly derived from the footwalls preserve evidence for cataclastic and particulate flow fault rocks contain authigenic minerals but lack the cyclically deformed mineral filled syntectonic veins that are abundant in some other late cenozoic high angle and strike slip faults meso and microscale fabrics in zones i and ii indicate that finite shear strains increase progressively upward toward the principal slip plane in a conceptual kinematic model of a shear box displacements of the hanging wall produced a shearing flow in the fault rocks below some of the slip on the principal slip plane was also partitioned into localized slip on discrete sliding surfaces in zones i and ii since 770 ka during the latest stages of incremental deformation in the brittle shear zones distributed flow in the fault rocks alternated with slip that was chiefly localized on the principal slip planes in this respect the detachment faults differ from inactive segments of the san andreas system along which displacements were progressively and irreversibly localized onto a single principal slip surface the strain gradients and corresponding changes in grain size in the shear zones resemble those in mylonitic shear zones except in their symmetry strain gradients in the black mountains are notably asymmetric they are present only below the principal slip planes not above

curie point depth estimates from aeromagnetic data from death valley and surrounding regions california aeromagnetic data were analyzed to determine the curie point depth cpd by power density spectral and three dimensional inversion methods within and surrounding death valley in southern california we calculated the cpd for 0 5 regions using 2d power density spectral methods and found that the cpds varied between 8 and 17 km however the 0 5 region may average areas that include shallow and deep cpds and because of this limitation we used the 3d inversion method to determine if this method may provide better resolution of the cpds the final 3d model indicates that the depth to the bottom of the magnetic susceptible bodies varies between 5 and 23 km even though both methods produced roughly similar results the 3d inversion method produced a higher lateral resolution of the cpds the shallowest cpds occur within the central and southern death valley panamint valley coso geothermal field and the tecopa hot springs region deeper 15 km cpds occur over outcropping granitic and precambrian lithologies in the panamint range grapevine mountains black mountains and the argus range the shallowest cpd occurs within the central death valley over a possible seismically imaged magma body and slightly deeper values occur within the panamint valley southern death valley and tecopa hot springs the shallow cpd values suggest that partially molten material may also be found in these latter regions the cpd computed heat flow values for the region suggest that the entire area has high heat flow values 100 mw m 2 on the other hand locally extremely high values 200 mw m 2 occur within the panamint valley the southern and central death valley and tecopa hot springs region these locally high heat flow values may be related to midcrustal magma bodies but additional geophysical experiments are needed to determine if the magma bodies exist

two dimensional interpretation of three dimensional magnetotelluric data an example of limitations and resolution summary interpretation of magnetotelluric mt data for three dimensional 3 d regional conductivity structures remains uncommon and two dimensional 2 d models are often considered an adequate approach in this paper we examine 2 d interpretation of 3 d data by considering the synthetic responses of a 3 d structure chosen specifically to highlight the advantages and limitations of 2 d interpretation 2 d models were obtained from inversion of the synthetic 3 d data set with different conditions noise and distortion applied to the data we demonstrate the importance of understanding galvanic distortion of the data and show how 2 d inversion is improved when the regional data are corrected prior to modelling when the 3 d conductive structure is located below the profile the models obtained suggest that the effects of finite strike are not significant if the structure has a strike extent greater than about one half of a skin depth in this case the use of only tm mode data determined better the horizontal extent of the 3 d anomaly when the profiles are located away from the 3 d conductive structure the use of only tm mode data can imagine phantom conductive structures below the profile in this case the use of both polarizations produced a better determination of the subsurface structures it is important to note that the main structures are identified in all the cases considered here although in some cases the large data misfit would cause scepticism about features of the models

an intermediate depth earthquake beneath tibet source characteristics of the event of september 14 1976 this paper presents the results of a detailed study of an intermediate depth earthquake beneath southern tibet september 14 1976 0643 51 6 ut 29 8l n 89 57 e focal depth 90 10 km mb 5 4 from the differences in arrival times of p pp and sp and from rayleigh wave spectral densities the focus is estimated to be at depth of 90 10 km and therefore in the uppermost mantle from the polarities of the first motions of p waves inversion of the relative amplitudes of p pp and sp and synthesis of their waveforms the source mechanism is constrained to be predominantly normal faulting with the t axis approximately horizontal and trending east west this mechanism is similar to those of shallow earthquakes in tibet the occurrence of an earthquake at depth of 90 km is consistent with rapid deformation at a temperature of about 700 to 800 c in the mantle beneath the tibetan crust the seismic moment of an equivalent double couple point source is estimated to be about 9 1024 dyne cm with an uncertainty factor of 1 3 from the sp amplitudes in the period range near 10 s and 5 1024 dyne cm also with an uncertainty factor of 1 3 from the spectral density of rayleigh waves with periods of 50 s

magnetic and clast fabrics as measurements of grain scale processes within the death valley shallow crustal detachment faults 1 the rock product of shallow crustal faulting includes fine grained breccia and clay rich gouge many gouges and breccias have a fabric produced by distributed deformation the orientation of fabric elements provides constraints on the kinematics of fault slip and is the structural record of intrafault strain not accommodated by planar and penetrative surfaces however it can be difficult to quantify the deformational fabric of fault rocks especially the preferred orientations of fine grained minerals or to uniquely determine the relationship between fabric geometry and finite strain here we present the results of a fabric study of gouge and breccia sampled from low angle normal detachment faults in the black mountains death valley ca we measured a preferred orientation of the long axes of the clasts inherited from the crystalline footwall of the fault and compared the shape preferred orientation to the anisotropy of magnetic susceptibility of the fault rocks the two measurements of fabric exhibit systematic similarities and differences in orientation and anisotropy that are compatible with the large scale kinematics of fault slip the dominant carriers of the magnetic susceptibility are micron and sub micron scale iron oxides and clay minerals therefore even the finest grains in the fault rock were sensitive to the distributed deformation and the micro mechanics of particle interaction must have departed from those assumed by the passive marker kinematic model that best explains the fabric

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

remagnetization and chemical alteration of sedimentary rocks remagnetization is a common phenomenon in rocks and developing a greater understanding of its mechanisms has several benefits acquisition of a secondary magnetization is usually tangible evidence of a diagenetic or thermal event which can be dated using palaeomagnetic techniques this is important because the timing of diagenetic and thermal events is commonly difficult to determine remagnetization can also obscure primary magnetizations and a better understanding of remagnetization could improve our ability to uncover primary magnetizations many chemical remagnetization mechanisms have been proposed including those associated with chemical alteration by a number of different fluids e g orogenic weathering mineralizing hydrocarbons and burial diagenetic processes e g clay diagenesis maturation of organic matter this book contains case studies and review articles that focus on remagnetization chemical remagnetization mechanisms and magnetic changes associated with chemical alteration by hydrocarbons

formation of magnetic minerals at hydrocarbon generation conditions in this paper we report the pyrolysis and formation of magnetic minerals in three source rock samples from the wessex basin in dorset southern england the experimental conditions in the laboratory recreated the catagenesis environment of oil source rocks magnetic analysis of both the heated and the unheated samples at room temperature and at very low temperatures 5 k coupled with transmission electron microscopy imaging and x ray analysis revealed the formation of nanometre sized 10 nm magnetic particles that varied across the rock samples analysed but more importantly across the pyrolysis temperature range magnetic measurements demonstrated the formation of these magnetic minerals peaked at 250 c for all rock samples and then decreased at 300 c before rising again at 320 c the newly formed magnetic minerals are suggested to be primarily pyrrhotite though magnetite and greigite are also thought to be present the sizes of the magnetic minerals formed suggest a propensity to migrate together with oil potentially explaining the magnetic anomalies observed above and within oil fields

a statistical and graphical workflow for the analysis of magnetic susceptibility data from drill cuttings in a hydrocarbon exploration setting abstractwe have analyzed the magnetic susceptibility measurements for more than 5000 samples of drill cuttings from oil gas wells in the venezuelan oil fields to facilitate information extraction and improve data presentation including the determination of threshold values used in hydrocarbon exploration the values for each well typically ranged over several orders of magnitude we have developed a new scale called the magnetization contrast magnitude scale m based on a logarithmic transformation of to better represent the data collectively the m data appear to be influenced by more than one process source resulting in skewed bimodal distributions with a significant degree of overlap between the individual modes each mode or subpopulation of the m data could be modeled with a lognormal distribution we have determined that a method based on normal probability plots could be used to decompose the observed bimodal distributions into their individual components and to obtain initial estim

lithospheric electrical structure of the middle lhasa terrane in the south tibetan plateau abstract the lhasa terrane in southern tibetan plateau is a huge tectono magmatic belt and an important metallogenic belt its formation evolution process and mineralization are affected by the subduction of oceanic plate and subsequent continental collision however the evolution of lhasa terrane has been a subject of much debate for a long time the lithospheric structure records the deep processes of the subduction of oceanic plate and continental collision the magnetotelluric mt method can probe the sub surface electrical conductivity newly dense broadband and long period magnetotelluric data were collected along a south north trending profile that across the lhasa terrane at 88 89 e dimensionality analyses demonstrated that the mt data can be interpreted using two dimensional approaches and the regional strike direction was determined as n110 e based on data analysis results a two dimensional 2 d resistivity model of crust and upper mantle was derived from inversion of the transverse electric mode transverse magnetic mode and vertical magnetic field data inversion model shows a large north dipping resistor that extended from the upper crust to upper mantle beneath the himalaya and the south of lhasa terrane which may represent the subducting indian continental lithosphere the 31 n may be an important boundary in the lhasa terrane the south performs a prominent high conductivity anomaly from the lower crust to upper mantle which indicates the existence of asthenosphere upwelling while the north performs a higher resistivity and may have a reworking ancient basement the formation of the ore deposits in the study area may be related to the upwelling of the mantle material triggered by slab tearing and or breaking off of the indian lithosphere and the mantle material input also contributed the total thickness of the present day tibetan crust the results provide helpful constrains to understand the mechanism of the continent continent collision and the regional exploratory prospect of the deep resources

tracing the indian lithospheric mantle beneath central tibetan plateau using teleseismic tomography abstract we determined a detailed three dimensional p wave velocity structure of the crust and upper mantle down to 400 km depth beneath central tibet by applying teleseismic tomography to 28 146 high quality p wave arrival times the data were collected very carefully from the original seismograms of 1625 teleseismic events recorded by 131 broadband stations of five portable seismic networks deployed in central tibet our results show that the indian lithospheric mantle has subducted beneath central tibet and its frontier has passed through the bangong nujiang suture and extended northward beneath the qiangtang terrane at 34 north latitude a prominent low velocity anomaly is revealed within the tethyan himalayan sequences close to the yarlung zangbo suture which is the boundary between the tethyan himalayan sequences and the lhasa terrane the subduction of the indian lithospheric mantle has caused the east west extension in central tibet the indian slab beneath the region has not sutured with the asian lithospheric mantle in the north the obtained tomographic images can explain many geological and geophysical features such as strong seismic anisotropy in the upper mantle and k rich and ultra potassic lavas in northern tibet

faulting at mormon point death valley california a low angle normal fault cut by high angle faults new geophysical and fault kinematic studies indicate that late cenozoic basin development in the mormon point area of death valley california was accommodated by fault rotations three of six fault segments recognized at mormon point are now inactive and have been rotated to low dips during extension the remaining three segments are now active and moderately to steeply dipping from the geophysical data one active segment appears to offset the low angle faults in the subsurface of death valley

upper mantle seismic velocities and anisotropy in china determined through pn and sn tomography 1 we have obtained velocity images of the uppermost mantle beneath china by performing tomographic inversion of both pn and sn traveltimes from the annual bulletin of chinese earthquakes 99 139 pn arrivals and 43 646 sn arrivals were selected pn anisotropy was also obtained simultaneously with pn velocity average pn and sn velocities are 8 05 and 4 55 km s respectively and maximum velocity perturbations are about 3 4 the pn and sn velocities are low in eastern china and high in western china particularly high velocities are associated with old basins for example tarim junggar turpan hami qaidam and sichuan and stable craton for example ordos low sn velocities are found mainly throughout north china in addition velocities are relatively low beneath the central tibetan plateau and the north south seismic zone along 103 e in tarim and central china where we observe strong anisotropy the fast pn velocity directions are consistent with the directions of maximum principal compressive stress as well as directions of crustal movement determined from global positioning system beneath the india eurasia collision zone the pn anisotropy direction is parallel to the collision arc and nearly perpendicular to both the direction of maximum compression and crustal movement resulting from pure shear deformation both the velocity variations and anisotropy indicate that the tibetan plateau was extruded and the mantle material beneath the plateau has flowed around the east himalaya syntax while the remaining material has diverted northwestward beneath the tarim basin

an overview of the crustal structure of the tibetan plateau after 35 years of deep seismic soundings abstract since the pioneer wide angle seismic profile along the yadong gulu rift acquired in 1974 by the ex institute of geophysics chinese academy of sciences cas several research programs aimed to deep geophysics performed thanks to the participation of chinese national and international institutions have been developed during last 35 years including 23 wide angle seismic profiles with total length of about 6000 km these profiles are unevenly distributed most of them in eastern tibet and few profiles in western tibet in this paper we make a summarized presentation of all these wide angle seismic profiles and provide an overall view of the seismic velocity structure of the crust beneath the broad tibetan plateau which is the product of the continuous convergence and collision of the indian and eurasian plates since about 50 ma ago different patterns of crustal thickness variation related to the tectonic blocks and along suture zones of the region are displayed the crust thickness is confirmed to be about 70 75 km under southern tibet and 60 65 km under northern northeastern and southeastern tibet the leading edge of the subducted lithosphere reaches the northern margin of the plateau and directly contacts with tarim basin westward of the 90 e boundary the indian crust is moving towards the northern edge of the plateau and collides with tarim basin at 80 e while reach the bangong nujiang suture belt at 88 e eastward of the 90 e boundary the northern edge of the crust should be at 50 100 km south of bangong nujiang suture the results supply helpful constrains to understand the mechanism of the continent continent collision and its consequences in the plateau and neighbouring areas

underplating in the himalaya tibet collision zone revealed by the hi climb experiment we studied the formation of the himalayan mountain range and the tibetan plateau by investigating their lithospheric structure using an 800 kilometer long densely spaced seismic array we have constructed an image of the crust and upper mantle beneath the himalayas and the southern tibetan plateau the image reveals in a continuous fashion the main himalayan thrust fault as it extends from a shallow depth under nepal to the mid crust under southern tibet indian crust can be traced to 31 n the crust mantle interface beneath tibet is anisotropic indicating shearing during its formation the dipping mantle fabric suggests that the indian mantle is subducting in a diffuse fashion along several evolving subparallel structures

tertiary structural evolution of the gangdese thrust system southeastern tibet structural and thermochronological investigations of southern tibet xizang suggest that intracontinental thrusting has been the dominant cause for formation of thickened crust in the southernmost tibetan plateau since late oligocene two thrust systems are documented in this study the north dipping gangdese system gts and the younger south dipping renbu zedong system rzt west of lhasa the gangdese thrust juxtaposes the late cretaceous forearc basin deposits of the lhasa block the xigaze group over the tethyan sedimentary rocks of the indian plate whereas east of lhasa the fault juxtaposes the late cretaceous eocene andean type arc the gangdese batholith over tethyan sedimentary rocks near zedong 150 km southeast of lhasa the gangdese thrust is marked by a 200 m thick mylonitic shear zone that consists of deformed granite and metasedimentary rocks a major south dipping backthrust in the hanging wall of the gangdese thrust puts the xigaze group over tertiary conglomerates and the gangdese plutonics north of xigaze and west of lhasa a lower age bound for the gangdese thrust of 18 3 0 5 ma is given by crosscutting relationships the timing of slip on the gangdese thrust is estimated to be 27 23 ma from 40 ar 39 ar thermochronology and a displacement of at least 46 9 km is indicated near zedong the age of the gangdese thrust gt is consistent with an upper age limit of 24 ma for the initiation of movement on the main central thrust in places the younger renbu zedong fault is thrust over the trace of the gt obscuring its exposure the rzt appears to have been active at circa 18 ma but had ceased movement by 8 1 ma the suture between india and asia has been complexly modified by development of the gts rzt and locally strike slip and normal fault systems

upper mantle deformation beneath central southern tibet revealed by shear wave splitting measurements abstract seismic anisotropy in the upper mantle has been inferred from shear wave splitting measurements at the temporary seismic array antilope 2 in the central southern tibet from september 2005 to october 2006 teleseismic shear waves mainly sks and skks are used to determine the splitting parameters fast polarization direction and delay time weak splitting with average delay time of 0 3 s is observed at stations in the southern lhasa terrane and northernmost tethyan himalaya significant seismic polarization anisotropy with delay times from 0 8 to 1 5 s and ne oriented fast direction is obtained for stations located farther north in the northern lhasa terrane the transition in shear wave splitting characteristics occurs at 30 5 n and suggests distinct upper mantle deformation from south to north the negligible anisotropy south of this transition indicates that there is no appreciable large scale mantle seismic polarization anisotropy the substantial anisotropy in the north may be caused by the eastward flow in a lithospheric crush zone which is squeezed and sheared between the advancing indian plate to the south and eurasian plate to the north

new constraints on crustal structure and moho topography in central tibet revealed by sinoprobe deep seismic reflection profiling abstract from october 2009 to may 2010 through the support of the sinoprobe project a series of deep seismic reflection profiles were conducted in order to image deep crustal structure of central tibet these profiles extend from the northern lhasa terrane to the qiangtang terrane by crossing the bangong nujiang suture bns the moho depth varies from 75 1 km beneath the northmost lhasa terrane to 68 9 km beneath southmost qiangtang terrane there is a 6 2 km sharp moho offset across the bns within 25 km to the north of the bns the moho rises smoothly to 62 6 km distinct moho reflector lies at 62 6 67 3 km beneath the qiangtang terrane the boundary of the middle and upper crusts and that between the middle and lower crusts interpreted from indepth velocity model correspond to the strong reflection horizons at about18 8 km and 31 3 km depth beneath south qiangtang terrane respectively a north dipping series of reflection packages in the mid to lower crust may mark subduction of the lhasa terrane beneath the qiantang terrane the central portion of the reflection profile exhibits an antiformal structure at 14 1 km in the upper crust which corresponds with the blueschist bearing metamorphic belt

deep thermal state on the eastern margin of the lhasa gangdese belt and its constraints on tectonic dynamics based on the 3 d electrical model abstract since the mesozoic the lhasa gangdese block has undergone several stages of tectonism magmatism caused by the closure of the new tethys ocean and the collision between the indian and eurasian plates which led to the lithospheric shortening and crustal uplift through the careful processing and analysis of the magnetotelluric data on the eastern margin of the lhasa gangdese block near 92 e a reliable 3d electrical model was obtained to examine the relationship between the lithospheric electrical structure and geodynamics some conductive and resistive layers were obtained which may be formed in some tectonic processes the thermal model of the lithospheric uppermost mantle 75 100 km were calculated using the arrhenius equation and the hashin shtrikman hs boundary condition combined with the terrestrial heat flow point and curie isothermal in the study area the temperature distribution at the 30 100 km depths was calculated the melt fractions of the mid lower crust calculated by the classical archie s law pressure calculated by density and temperature at different depths can be obtained in a meantime the resistor beneath the tethys himalaya terrane represented the subduction of the indian plate with a deep angle the subduction of the indian crust may not exceed the indus yarlung zangbo suture while the indian lithospheric mantle may not reach the luobadui milashan fault additionally our electrical and thermal structure indicated that the indian lithospheric mantle probably detached from the indian crust our study further discussed the underplating of the mantle derived materials and the southern extrusion along the main himalaya trust and offered a subsidiary evidence for the possible south eastward crustal flow in a local area finally the relationship between the electrical structure and the deep metallogenic mechanism was briefly established

interseismic deformation and geologic evolution of the death valley fault zone 1 the death valley fault zone dvfz located in southeastern california is an active fault system with an evolved pull apart basin that has been deforming over the past 6 myr we present a study of the interseismic motion and long term stress accumulation rates to better understand the nature of both past and present day loading conditions of the dvfz using a 3 d semi analytic viscoelastic deformation model combined with geodetic velocities derived from the mobile array of gps for nevada transtension magnet network and the southern california earthquake center scec crustal motion map version 4 cmmv4 gps data we establish parameters for interseismic slip rate and apparent locking depth for four dvfz fault segments our preferred model provides good fit to the data 1 0 mm yr and 1 5 mm yr rms misfit in the fault perpendicular and fault parallel directions respectively and yields apparent locking depths between 9 8 17 1 km and strike slip rates of 3 7 mm yr for the segments we also determine subsidence 0 5 0 8 mm yr and extension 1 0 1 2 mm yr rates in the pull apart basin region with these parameters we construct a dvfz evolution model for the last 6 myr that recreates the motion of the fault blocks involved in the formation of the present day geological structures in death valley finally using coulomb stress accumulation rates derived from our model 0 25 0 49 mpa 100 yr combined with earthquake recurrence interval estimates of 500 to 2600 years we assess present day seismic hazards with calculated moment magnitudes ranging from 6 7 7 7

electrical resistivity structures northeast of the eastern kunlun fault in the northeastern tibet tectonic implications abstract only sparse deep geophysical data have been obtained in the region to the north of the eastern kunlun fault eklf in the tibetan plateau which leads to inconsistent and ambiguous descriptions of the area in various geodynamic models the detailed geophysical issues in the region including the northeast boundary of the plateau and the crust mantle coupling mode northeast of the eklf are under dispute to resolve these disputes we have collected magnetotelluric mt data along five profiles in the region to the north of the eklf since 2009 the interpretation of data from the latest profile labeled l5 which starts from the eastern margin of the qaidam basin crosses the qilian mountain and the hexi corridor and finally reaches the southern alashan block is the focus of this paper these new results are comparable with our previously determined structures the two dimensional 2d resistivity model supports three divisions of the qilian mountain a complex northern qilian a relatively simple sandwiched southern qilian and a transitional central qilian in between them our data indicate a relatively low resistivity layer at the middle lower crust below the eastern margin of the qaidam basin and the southern qilian mountain this result indicates a weak crustal zone wcz existing northeast of the eklf and is interpreted as a large north dipping thrust at this depth the presumed imbricated thrust in the crust in the northeastern plateau resulting from the indenting of the mantle wedge from the south is the primary process in the crustal thickening in the region the mt results indicate that the northeastern boundary of the tibetan plateau is not below the eklf instead we suggest that the northeastern boundary of the outgrowth of the tibetan plateau is along and below the northern rim of the hexi corridor

are the benches at mormon point death valley california usa scarps or strandlines the benches and risers at mormon point death valley usa have long been interpreted as strandlines cut by still stands of pluvial lakes correlative with oxygen isotope stage ois 5e 6 120 000 186 000 yr b p and ois 2 10 000 35 000 yr b p this study presents geologic mapping and geomorphic analyses gilbert s criteria longitudinal profiles which indicate that only the highest bench at mormon point 90 m above mean sea level msl is a lake strandline the other prominent benches on the north descending slope immediately below this strandline are interpreted as fault scarps offsetting a lacustrine abrasion platform the faults offsetting the abrasion platform most likely join downward into and slip sympathetically with the mormon point turtleback fault implying late quaternary slip on this low angle normal fault our geomorphic reinterpretation implies that the ois 5e 6 lake receded rapidly enough not to cut strandlines and was 90 m deep consistent with independent core studies of the salt pan no evidence of ois 2 lake strandlines was found at mormon point which indicates that the maximum elevation of the ois 2 lake surface was 30 m msl thus as measured by pluvial lake depth the ois 2 effective precipitation was significantly less than during ois 5e 6 a finding that is more consistent with other studies in the region the changed geomorphic context indicates that previous surface exposure dates on fault scarps and benches at mormon point are uninterpretable with respect to lake history

magnetic and mineralogical studies to characterize oil reservoirs in venezuela high resolution aeromagnetic surveys over oil fields and magnetic susceptibility ms measurements in soils sediments and drill cuttings have been proposed as complementary or alternative means of exploration and assessment of hydrocarbon reservoirs some micromagnetic anomalies detected by these studies could be the likely result of the existence at shallow depths of authigenic magnetite and or fe sulphides it has been hypothesized that such magnetic minerals are the chemical byproducts of primary fe oxides exposed to a reducing environment induced by the underlying reservoir secondary magnetite could appear as spherical aggregates framboids of submicronic crystals identifiable by scanning electron microscopy sem the formation of these framboids seems linked either to inorganic chemical processes associated with the presence of hydrocarbons or to oil biodegradation in the last few years we have examined the causal relationship between framboids of magnetic minerals and hydrocarbons via rock magnetic and electron paramagnetic resonance epr analyses of drill cuttings the samples used have been taken at different depths within the first 1500 m of producing and nonproducing wells from two oil fields in southwestern venezuela we have complemented ms results with s ratio logs sem x ray energy dispersion edx irm acquisition curves and high temperature susceptibility measurements more recently we have also studied soil samples from an oil prospective area in western venezuela where rock magnetic and epr results were compared with ethane gas analyses and remote sensing data rock magnetic and epr studies have been performed in 14 oil wells from la victoria 3 and guafita 11 fields figure 1 since our research has focused at the near surface expression of hydrocarbon migration the samples analyzed drill cuttings were taken from different depth at intervals of about 15 m between 100 and 1500 m all samples belong to a single group of molasses from strata that lie far above the producing levels

tearing of the indian lithospheric slab beneath southern tibet revealed by sks wave splitting measurements a b s t r a c t shear wave birefringence is a direct diagnostic of seismic anisotropy it is often used to infer the northern limit of the underthrusting indian lithosphere based on the seismic anisotropy contrast between the indian and eurasian plates most studies have been made through several near north south trending passive source seismic experiments in southern tibet to investigate the geometry and the nature of the underthrusting indian lithosphere an east west trending seismic array consisting of 48 seismographs was operated in the central lhasa block from september 2009 to november 2010 splitting of sks waves was measured and verified with different methods along the profile the direction of fast wave polarization is about 60 in average with small fluctuations the delay time generally increases from east to west between 0 2 s and 1 0 s and its variation correlates spatially with north south oriented rifts in southern tibet the sks wave arrives 1 0 2 0 s later at stations in the eastern part of the profile than in the west the source of the anisotropy estimated by non overlapped parts of the fresnel zones at stations with different splitting parameters is concentrated above ca 195 km depth all the first order features suggest that the geometry of the underthrusting indian lithospheric slab in the himalayan tibetan collision zone beneath southern tibet is characterized by systematic lateral variations a slab tearing and or breakoff model of indian lithosphere with different subduction angles is likely a good candidate to explain the observations

magnetic characterization of oil sands at osmington mills and mupe bay wessex basin uk this study combines magnetic experimentation and geochemical analysis on oil sands from osmington mills and mupe bay wessex basin uk to investigate the possibility of a relationship between hydrocarbons and magnetic mineralogy removal of hydrocarbons by chemical extraction was conducted to allow comparison of 1 oil sands and 2 cleaned sands detailed magnetic analysis including low temperature and high temperature experimentation revealed that all but one sample was dominated by siderite identified by the neel transition at 37 38 k as well as containing large grains of multidomain magnetite verwey transition 110 k and hematite morin transition 250 k scanning electron microscopy and energy dispersive x ray analysis confirmed the presence of iron oxides in particular framboids 500 nm 45 mm in diameter probably magnetite hysteresis parameters showed distinct grouping of oil sands compared to their clean counterparts and a negative linear regression in log space was observed r 1 4 0 7 between the percentage of extractable organic matter and magnetic susceptibility these results suggest a relationship exists between magnetic minerals and the alteration of oil due to biodegradation which is not yet fully understood possible mechanisms are suggested to be due to anaerobic bacteria or the transportation of the oil as it migrates through the host rock aeromagnetic surveys have established a general connection between magnetic anomalies and hydrocarbon accumulations on a regional scale but with a large degree of ambiguity e g donovan et al 1979 reynolds et al 1991 gay 1992 leblanc morris 1999 several major remagnetizations worldwide e g mccabe et al 1984 1987 elmore leach 1990 suk et al 1993 garner cioppa 2006 are associated with hydrocarbon provinces and anomalous aeromagnetic signals this is thought to be due to chemical alteration of minerals as fluids such as hydrocarbons migrate through the host rock but no definitive mechanism has been determined recent research by several groups has tried to identify the magnetic minerals themselves for hydrocarbon identification e g liu et al 1998 aldana et al 1999 diaz et al 2000 aldana costanzoalvarez 2003 ivakhnenko potter 2004 costanzo alvarez et al 2006 liu et al 2006 rijal et al 2010 aldana et al 2011 guzman et al 2011 rijal et al 2011 for example costanzo alvarez et al 2000 and liu et al 2006 conducted rock magnetic measurements on producing and nonproducing wells and identified anomalous values of magnetic susceptibility x at certain depth intervals within both oil producing and non producing wells characterized by a low coercivity mineral probably magnetite aldana et al 1999 categorized these anomalies into two types 1 fe rich spherical aggregates of authigenic origin as seen by scanning electron microscopy sem analysis only found in producing wells costanzo alvarez et al 2000 aldana costanzo alvarez 2003 and most likely magnetite and 2 characterized by magnetite and hematite but no association with spherical aggregates present in both non producing and producing wells costanzo alvarez et al 2000 aldana costanzo alvarez 2003 liu et al 2006 also found variances between oil producing and non oil producing layers using basic hysteresis parameters saturation remanence magnetization mrs saturation magnetization ms coercivity bc and remanent coercivity bcr the results showed that within an oil bearing layer its x ms and mrs values were between 24 and 36 times higher than the corresponding layer obtained from a dry well with maghemite siderite and pyrite contributing to the enhanced magnetic signature another study by ivakhnenko potter 2004 investigated the magnetic susceptibility of crude oils refined oil fractions and formation waters from sites worldwide it was found that all the fluids were diamagnetic but had distinct differences in magnetic susceptibility for example the crude oils were more negative than formation waters from the same location the physical and chemical properties of the oils appeared to correlate with the magnetic susceptibility such as gravity metal concentration density and api american petroleum index from elmore r d muxworthy a r aldana m m mena m eds 2012 remagnetization and chemical alteration of sedimentary rocks geological society london special publications 371 http dx doi org 10 1144 sp371 6 the geological society of london 2012 publishing disclaimer www geolsoc org uk pub ethics geological society london special publications published online june 27 2012 as doi 10 1144 sp371 6