post rift tectonic history of the songliao basin ne china cooling events and post rift unconformities driven by orogenic pulses from plate boundaries

applying the coulomb failure function with an optimally oriented plane to the 2008 mw 7 9 wenchuan earthquake triggering the coulomb failure function cff quantitatively describes static stress changes in secondary faults near the source fault of an earthquake cff can be employed to monitor how static stress transfers and then shed some light on the probability of successive events occurring around a source fault in this paper we focus on the cff and particularly on optimally oriented planes we present a unified model to determine an optimally oriented plane and its corresponding coulomb stress then apply the model to the 2003 mw 6 6 bam iran earthquake and the 2008 mw 7 9 wenchuan china earthquake thereby checking its effectiveness our results show that spatial correlation between positive coulomb stress changes and aftershocks are for the 2003 bam earthquake 47 06 when elastic coulomb stress changes are resolved on uniform planes and 87 53 when these are resolved on optimally oriented planes at depth for the 2008 wenchuan earthquake the correlations are 45 68 and 58 20 respectively it is recommended that account be taken of optimally oriented planes when drawing a coulomb stress map for analyzing earthquake triggering effects

tethyan and indian subduction viewed from the himalayan high to ultrahigh pressure metamorphic rocks abstract the himalayan range is one of the best documented continent collisional belts and provides a natural laboratory for studying subduction processes high pressure and ultrahigh pressure rocks with origins in a variety of protoliths occur in various settings accretionary wedge oceanic subduction zone subducted continental margin and continental collisional zone ages and locations of these high pressure and ultrahigh pressure rocks along the himalayan belt allow us to evaluate the evolution of this major convergent zone 1 cretaceous 80 100 ma blueschists and possibly amphibolites in the indus tsangpo suture zone represent an accretionary wedge developed during the northward subduction of the tethys ocean beneath the asian margin their exhumation occurred during the subduction of the tethys prior to the collision between the indian and asian continents 2 eclogitic rocks with unknown age are reported at one location in the indus tsangpo suture zone east of the nanga parbat syntaxis they may represent subducted tethyan oceanic lithosphere 3 ultrahigh pressure rocks on both sides of the western syntaxis kaghan and tso morari massifs formed during the early stage of subduction exhumation of the indian northern margin at the time of the paleocene eocene boundary 4 granulitized eclogites in the lesser himalaya sequence in southern tibet formed during the paleogene underthrusting of the indian margin beneath southern tibet and were exhumed in the miocene these metamorphic rocks provide important constraints on the geometry and evolution of the india asia convergent zone during the closure of the tethys ocean the timing of the ultrahigh pressure metamorphism in the tso morari massif indicates that the initial contact between the indian and asian continents likely occurred in the western syntaxis at 57 1 ma west of the western syntaxis the higher himalayan crystallines were thinned rocks equivalent to the lesser himalayan sequence are present north of the main central thrust moreover the pressure metamorphism in the kaghan massif in the western part of the syntaxis took place later 7 m y after the metamorphism in the eastern part suggesting that the geometry of the initial contact between the indian and asian continents was not linear the northern edge of the indian continent in the western part was 300 to 350 km farther south than the area east of the nanga parbat syntaxis such en baionnette geometry is probably produced by north trending transform faults that initially formed during the late paleozoic to cretaceous gondwana rifting farther east in the southern tibet the collision occurred before 50 6 0 2 ma finally high pressure to ultrahigh pressure rocks in the western himalaya formed and exhumed in steep subduction compared to what is now shown in tomographic images and seismologic data

hp uhp exhumation during slow continental subduction self consistent thermodynamically and thermomechanically coupled model with application to the western alps abstract the exhumation of ultra high pressure uhp crustal material often occurs at the rear of sedimentary accretion wedges e g alps himalaya norway however the mechanisms of deep 100 km burial in slow 10 mm yr 1 and slow at the later stage

the modulation of groundwater exploitation on crustal stress in the north china plain and its implications on seismicity abstract in the north china plain henceforth ncp continuous over exploitation of groundwater resources forms widespread groundwater depression cones the removal of massive groundwater unloads the local lithosphere and therefore significantly perturbs regional crustal deformation and stress evolution patterns unfortunately its ramifications to seismicity are yet poorly understood in this work we adopt a full three dimensional 3d viscoelastic finite element model to estimate the crustal deformation and stress changes triggered by groundwater unloading and analyze its effect on major active faults by calculating the induced coulomb stress changes meanwhile variations on regional seismicity in association with the successive groundwater removal over the past 60 years in ncp is examined under scrutiny the stunning results emphasize apparent crustal uplift occurs in most of the groundwater depression zones reaching up 4 0 cm and vertical normal stress increases by up to 70 kpa at the depth of 10 km concentrating in the deepest groundwater depression cone overall coulomb stress significantly increases by above 10 kpa on major active faulting zones compared with the tectonic stress loading rate of 0 5 kpa yr the stress perturbation by groundwater removal is definitely non negligible which may eventually introduce an extra incentive to expedite the failure of active faults and even trigger earthquake events in ncp

exact timing of granulite metamorphism in the namche barwa eastern himalayan syntaxis new constrains from sims u pb zircon age zircon grains separated from 2 granulites from the eastern himalaya were investigated by raman spectroscopy cathodoluminescence imaging and secondary ion mass spectrometry these grains have a thin homogeneous rim and an oscillatory inner zone domain with or without a relict inherited core garnet kyanite and rutile inclusions were identified within only the rim domain of zircon grains indicating that the rim had formed during peak granulite facies metamorphism u pb zircon data record three distinct age populations 1 805 ma for the inherited core ca 500 ma oscillatory inner zone as well as 24 25 ma and ca 18 ma for the metamorphic rim these new precision ages suggest that the peak metamorphic age for the hp granulite is at ca 24 25 ma and subsequent amphibolite facies retrograde metamorphism occurred at ca 18 ma

faults of the central part of the lewis and clark line and fragmentation of the late cretaceous foreland basin in west central montana the lewis and clark line is a prominent zone of strike slip dip slip and oblique slip faults that extends from near wallace idaho to east of helena montana faults of this zone have been intermittently active from middle proterozoic to holocene time and because of numerous tectonic overprints controversy continues about displacement directions and times of displacement along specific faults geologic mapping shows evidence that many principal faults of the lewis and clark line such as the st marys helena valley bald butte ninemile and osburn faults had right separation or slip that ranged between 28 and 11 km and this displacement probably occurred during late cretaceous time other faults such as the elevation mountain placer creek and ranch creek faults have late cretaceous right separations that range between 8 and 3 2 km and the mount sentinel fault zone has between 6 5 and 3 km of right separation of probable late cretaceous age subsidiary structures of the lewis and clark line postdate paleozoic and lower cretaceous rocks and predate late cretaceous stocks at some places subsidiary faults and folds that have age constraints have slip directions compatible with right slip along adjacent principal faults sedimentation patterns of lower and upper cretaceous rocks indicate that faults of the lewis and clark line fragmented the foredeep region of the foreland basin into separate northern and southern basins in late cretaceous time the lower and upper cretaceous blackleaf formation albian and lower cenomanian was deposited in a continuous foredeep basin that extended across the lewis and clark line from north of the canadian border to southwestern montana a distance of about 450 km north of the lewis and clark line middle and upper cenomanian rocks are absent and a thin sequence of uppermost cenomanian to campanian rocks was deposited in a marine environment that changed to a strand line and continental environment in early campanian time south of the lewis and clark line middle and upper cenomanian deposits are also absent but a thick sequence of turonian to campanian rocks was deposited in brackish water and strand line environments and during later campanian time in a continental environment in the region between the st marys helena valley and bald butte faults a barrier may have formed that served as a local sediment source between foredeep regions in the northern and southern foreland basin during the period 91 to 75 ma south of the bald butte fault an extensional tectonic regime contributed to a higher sediment accumulation rate in the foredeep region along the north border of the southern basin 30 cm 1 000 yr as compared to lower sediment accumulation rates 6 9 and 7 8 cm 1 000 yr in the foredeep region of the south part of the northern basin

hp uhp metamorphism as an indicator of slab dip variations in the alpine arc hp uhp and lt metamorphic units that commonly occur in the inner parts of mountain belts result from the subduction of continental and oceanic material most often exhumed prior to continental collision the prograde pressure temperature history of hp uhp rocks strongly depends on the convergence rate and on the subduction zone geometry the maximum pressure recorded provides a proxy for the depth of shearing off and stacking of hp metamorphic nappes a 2 d thermal model of continental subduction at lithospheric scale is used to compute the length and pressure peak of detached hp metamorphic units as a function of the slab dip angle and the convergence rate model results are applied to the metamorphic nappe pile of the inner alps a mean convergence rate of 1 cm year during the subduction of the brianconnais terrane is indicated by the paleogeographic reconstructions between 46 and 38 ma on this basis the available petrological data and lengths of metamorphic units are used to compute the variations of the slab dip angle the slab dip angle is shown to increase from the northeast to the southwest along the alpine arc with estimated values of 20 for suretta 30 45 for monte rosa and gran paradiso and 60 for dora maira from eocene to oligocene times the increase in slab dip angle is controlled by changes of buoyancy due to the spatial configuration of the valaisan trough and the incoming of crustal material within the subduction zone

seismic moments local magnitudes and coda duration magnitudes for earthquakes in central california onscale seismograms recorded at stations in the u s geological survey s usgs central california seismic network calnet have been used to estimate the seismic moment m and local magnitude ml for earthquakes of 1 ml 4 located on the san juan bautista and parkfield sections of the san andreas fault the coyote lake section of the calaveras fault the sargent fault and near livermore these data together with m and ml estimates for 4 ml 6 earthquakes in these areas cannot be fit with a single linear log m versus ml relation rather the data are consistent with log m 1 5 ml 16 for 3 ml 6 with log m 1 2 ml 17 for 1 1 2 ml 3 1 2 and with a slope of to 1 fro 1 2 ml 1 1 2 whereas usgs coda duration magnitude md is consistent with ml for 1 1 2 ml 3 md is larger than ml at ml 1 1 2 and smaller than ml at ml 3 log m can be estimated to a precision of 0 2 for 1 md 3 1 2 earthquakes in central california by applying log m 1 2 md 17 to the md that have been routinely published by the usgs

fault plane solutions for the 1984 morgan hill california earthquake sequence evidence for the state of stress on the calaveras fault fault plane solutions for 946 aftershocks of the april 24 1984 morgan hill california m6 2 earthquake reveal a pattern of complex faulting within a 10 km wide zone surrounding the calaveras fault the fault plane solutions fall into three groups strike slip mechanisms located along the calaveras fault with north northwest striking dextral slip planes nearly parallel to the fault strike slip mechanisms located northeast of the calaveras fault with north striking dextral slip planes and reverse mechanisms located southwest of the calaveras fault with northeast or southwest dipping slip planes the average azimuth of p axes for aftershocks located on the calaveras fault is n10 1 e in contrast the average azimuths of p axes for aftershocks northeast and southwest of the calaveras fault are n49 7 e and n37 3 e respectively by assuming that the earthquakes occur on preexisting cracks in response to a uniformly oriented regional stress field we are able to infer from the observed average slip directions an orientation for 1 the axis of maximum horizontal compression 63 80 from the local strike of the calaveras fault such a high angle orientation of 1 is incompatible with the classical andersonian model of strike slip faulting but is consistent with an alternate model in which a weak calaveras fault is driven by plate motion slightly convergent with respect to the san andreas fault system in central california if this inferred high angle uniform stress field is combined with the changes in the static stress field calculated with elastic dislocation theory for the 1984 morgan hill main shock the resulting stress field predicts most of the observed spatial pattern of the aftershocks and their fault plane solutions the orientations of the inferred aftershock slip planes are generally consstent with the fracture orientations predicted by the coulomb failure criterion areas with predicted increases in the coulomb failure function of only a few tenths of a megapascal correspond to areas of intense aftershock activity the invariance of focal mechanisms in the vicinity of the south end of the main shock rupture before and after the main shock suggests that the magnitude of compressive stress in this area is at least 10 mpa in addition to explaining a preponderance of the seismic observations of the aftershock sequence the presence of a predominantly fault normal compressive regional stress field accounts for the contemporary development of folds reverse faults and topographic relief observed throughout the central california coast ranges

reply to comment by p j o brien on the onset of india asia continental collision early steep subduction required by the timing of uhp metamorphism in the western himalaya by mary l leech s singh a k jain simon l klemperer and r m manickavasagam earth planetary science letters 234 2005 83 97

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

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

how does the mid crust accommodate deformation in large hot collisional orogens a review of recent research in the himalayan orogen abstract the presence of hot weak crust is a central component of recent hypotheses that seek to explain the evolution of continent continent collisions and in particular may play an important role in accommodating the 3000 km of convergence within the himalaya tibetan collision over the last 55 myr models that implicate flow of semi viscous midcrustal rocks south toward the front of the himalayan orogen channel flow are able to account for many geologic observations in the himalaya while alternative models of collision particularly thrust wedge taper demonstrate that much of the observed geology could have formed in the absence of a low viscosity mid crustal layer several recent studies synthesized here have prompted a shift from initial assumptions that channel flow and thrust wedge taper processes are by definition mutually exclusive these new studies reveal the presence of several tectonometamorphic discontinuities in the midcrust that appear to reflect a continuum of deformation in which both channel and wedge type processes operate in spatially and temporally distinct domains within the orogen and further that the system may migrate back and forth between these types of behavior this continuum of deformation styles within the collisional system is of crucial importance for explaining the evolution of the himalayan orogen and hence for understanding the evolution of earth s many continent continent collision zones

two stages of granulite facies metamorphism in the eastern himalayan syntaxis south tibet petrology zircon geochronology and implications for the subduction of neo tethys and the indian continent beneath asia the eastern himalayan syntaxis in southeastern tibet consists of the lhasa terrane high himalayan rocks and indus tsangpo suture zone the lhasa terrane constitutes the hangingwall of a subduction zone whereas the high himalayan rocks represent the subducted indian continent our petrological and geochronological data reveal that the lhasa terrane has undergone two stages of medium p metamorphism an early granulite facies event at c 90 ma and a late amphibolite facies event at 36 33 ma however the high himalayan rocks experienced only a single high p granulite facies metamorphic event at 37 32 ma it is inferred that the late cretaceous c 90 ma medium p metamorphism of the southern lhasa terrane resulted from a northward subduction of the neo tethyan ocean and that the oligocene 37 32 ma high p 1 8 1 4 gpa rocks of the high himalayan and coeval medium p 0 8 1 1 gpa rocks of the lhasa terrane represent paired metamorphic belts that resulted from the northward subduction of the indian continent beneath asia our results provide robust constraints on the mesozoic and cenozoic tectonic evolution of south tibet

initial geometry of western himalaya and ultrahigh pressure metamorphic evolution ultrahigh pressure metamorphic rocks on both sides of the western himalayan syntaxis show different p t t paths the kaghan unit was metamorphosed under the uhp conditions significantly later 46 ma than the tso morari unit 53 ma implying that the tso morari was subducted earlier 57 ma than the kaghan unit 52 ma the age difference likely reflects the initial shape of greater india with the kaghan unit located greater than 300 km south of the tso morari before the collision of two continents we calculate the dip of the subducting plate using two independent methods the results show gentle dipping subduction east of the western syntaxis and steep subduction west of the syntaxis since the time of india eurasia collision to the present time we propose that the steep subduction in the western part is likely related to the proto chaman and karakorum faults along which the indian plate moved northward in the eastern part the overlying eurasian plate extruded to east which allowed gentle dipping subduction of the indian continent although the main period of eastward extrusion of the eurasian continent occurred between 30 and 15 ma our results suggest that this was likely taking place since the early india asia collision using those geometrical constraints a 3d image of the slab is reconstructed in the western part showing the sharp bending of the western syntaxis along the proto chaman fault this bending resulted in the warping of the slab surface to form a conical fold with a north dipping axis located near the western syntaxis 2007 published by elsevier ltd

review of published rockburst events and their contributing factors abstract through a review of selected published case study data from around the world in situ conditions that contribute to rockburst events have been identified this review was carried out to assist research associated with developing numerical modelling techniques to reproduce rockbursting within a large scale mine models for predictive purposes contributing factors identified through the case study review include unfavourable stress states excavation geometry size and orientation in relation to the principal stresses and rate and direction of advance factors also incorporated unfavourable rock mass characteristics that included mineralogy contrasts in geomechanical properties and geological intensifiers dykes faults etc

a possible seismic gap and high earthquake hazard in the north china basin in this study we use combined historical records and results of early paleo earthquake studies to show that a 160 km seismic gap has existed along the northeast striking right slip tangshan hejian cixian fault china over more than 8400 yr the seismic gap is centered in tianjin a city in the north china basin with a population of 11 million and located 100 km from beijing which has a population of 22 million current data indicate that the recurrence interval of major earthquakes along the tangshan hejian cixian fault is 6700 10 800 yr this implies that a large earthquake with an estimated magnitude of m 7 5 is either overdue or will occur within the next 2000 3000 yr along the inferred seismic gap if it is ruptured by a single event alternatively the seismic gap may be explained by aseismic creeping strain transfer between adjacent faults or much longer recurrence times than the current knowledge indicates

three dimensional rheological structure of the lithosphere in the ordos block and its adjacent area summary the 3 d rheological structure of the lithosphere in the ordos block and its adjacent area 32 43 n 102 116 e is obtained using iterative and averaging methods based on the seismic velocity structure thermal structure and composition of the lithosphere the thermal structure is obtained from the newly published surface heat flow data sets the brittle fracture mechanism is taken into account when the rheological structure is calculated the strain rate obtained from gps observation in the area under study is used to calculate the strength and the viscosity of the lithosphere the results show that there are low viscosity layers at the bottom of the upper crust the lower crust and the bottom of the lithosphere both under the ordos block and its adjacent regions and the viscosity varies horizontally the viscosity in grabens surrounding the ordos block is one to two orders of magnitude lower than that in the ordos block the results also show that the rheological structure coincides with the major geological structure below the major fault systems the viscosity is relatively low the earthquakes with ms 4 0 occurred along the large active faults most of the hypocentres of the earthquakes are in the upper and middle crust with high viscosity the maximum depth of the earthquake is above the viscosity contour line of 1022 pa s numerical studies show that the active geological structures must be taken into account when the 3 d rheological structure is used for geodynamics study

reconstruction of the deformed collision zone between india and asia by backward motion of lithospheric blocks 1 on the basis of a synthesis of tectonic data available on the india asia collision we present a first attempt to reconstruct the evolution of the collision zone assuming that the deformation of the lithosphere is localized along narrow shear zones and that the interiors of mantle blocks in between remain relatively undeformed we define block contours from the fault pattern and move back the blocks along their boundary faults along convergent or extensional boundaries the crust is assumed to shorten or stretch coherently step by step we go backward in time to finally reach the collision onset for each time step we find a solution compatible with the data set available and the position of the adjacent blocks for each block the search for compatibility at the scale of the entire collision zone allows for solving the kinematics of regions with fewer data and suggests plausible scenarios for regions where data is lacking for each step we calculate large scale displacement maps and determine euler poles for each block for the most recent time step the map proposed is compared to gps motions the deformation budget implies that extrusion absorbed 30 of the convergence between india and siberia during the entire collision span but varied with time accounting for as little as 3 or as much as 60 of this convergence at different epochs

mining induced microseismic event location errors accuracy and precision of two location systems the accuracy and precision of microseismic event locations were measured analyzed and compared for two types of location systems anolog and digital in the first system relative times of first arrival were estimated from analog signals using automated hardware circuitry station positions were estimated from mine map coordinates and event locations were determined using the bld blake leighton and duvall direct solution method in the second system arrival times were manually measured during interactive displays of digital waveforms station coordinates were surveyed and the sw gbm salamon and wiebols godson bridges and mckavanagh direct basis function was used to solve for locations both systems assume constant isotropic seismic velocity of slightly different signals data sets calibration blast signals with known source site and origin time and microseismic event signals were recorded by each location system employing the same array of high frequency 5 khz accelerometers with 150 m maximum dimension the calibration blast tests indicated a location precision of 2 m and accuracy of 10 m for the analog system location precision and accuracy for the digital system measured 1 m and 8 m respectively numerical experiments were used to assess the contributions of errors in velocity arrival times and station positions on the location accuracy and precision for each system measured and estimated errors appropriate to each system for microseismic events were simulated in computing source locations for comparison with exact synthetic event locations discrepancy vectors between exact locations and locations calculated with known data errors averaged 7 7 and 1 4 m for the analog and digital systems respectively these averages are probably more representative of the location precision of microseismic events since the calibration blast tests produce impulsive seismic arrivals resulting in smaller arrival time pick errors in the analog system for both systems location accuracy is limited by inadequate modeling of the velocity structure consequently when isotropic velocity models are used in the travel time inversions the increased effort expended with the digital location system does not for the particular systems studied result in increased accuracy

exhumation history of eastern ladakh revealed by 40ar 39ar and fission track ages the indus river tso morari transect nw himalaya fission track and 40 ar 39 ar ages place time constraints on the exhumation of the north himalayan nappe stack the indus suture zone and molasse and the transhimalayan batholith in eastern ladakh nw india results from this and previous studies on a north south transect passing near tso morari lake suggest that the sw directed north himalayan nappe stack comprising the mata tetraogal and tso morari nappes was emplaced and metamorphosed by c 50 45 ma and exhumed to moderately shallow depths c 10 km by c 45 40 ma from the mid eocene to the present exhumation continued at a steady and slow rate except for the root zone of the tso morari nappe which cooled faster than the rest of the nappe stack rapid cooling occurred at c 20 ma and is linked to brittle deformation along the normal ribil zildat fault concomitant with extrusion of the crystalline nappe in the south data from the indus molasse suggest that sediments were still being deposited during the miocene

geothermal production and reduced seismicity correlation and proposed mechanism abstract at brady hot springs a geothermal field in nevada heated fluids have been extracted cooled and re injected to produce electrical power since 1992 analysis of daily pumping records and catalogs of microseismicity between 2010 and 2015 indicates a statistically significant correlation between days when the daily volume of production was at or above its long term average rate and days when no seismic event was detected conversely shutdowns in pumping for plant maintenance correlate with increased microseismicity we hypothesize that the effective stress in the subsurface has adapted to the long term normal operations deep extraction at the site under this hypothesis extraction of fluids inhibits fault slip by increasing the effective stress on faults in contrast brief pumping cessations represent times when effective stress is decreased below its long term average increasing the likelihood of microseismicity

two stage eastward diachronous model of india eurasia collision constraints from the intraplate tectonic records in northeast indian ocean abstract the onset of timing and the model of the india eurasia collision are strongly debated based on continental works without systematic marine tectonic constraints two aseismic ridges the laccadives maldives chagos ridge lmcr formed after the deccan volcanism in western india and the ninetyeast ridge ner formed after the emplacement of the rajmahal traps in eastern india are prominent features in the northeast indian ocean the onset of a major and steady increase in magma production rates along the aseismic ridge associated with the significant slowdown of the northward moving indian plate is a likely indicator of the onset of the india eurasia collision using gravity derived crustal thickness we calculated the magma production rates along the lmcr and the ner respectively the steady increasing magma production rate along the lmcr and a southwestward jump of the central indian ridge constrained the soft india island arc and hard india eurasia collisions between western india and eurasia to 50 52 ma and 41 ma respectively the steady increasing magma production rate along the ner and the formation of the mammerickx microplate constrained the soft collision between eastern india and eurasia to 47 49 ma and the extinction of the wharton ridge constrained the hard collision between eastern india and eurasia to 38 ma both the soft and hard collisions between western india and eurasia are 3 myr earlier than the soft and hard collisions between eastern india and eurasia respectively with systematic marine tectonic constraints a two stage eastward diachronous model of the india eurasia collision was proposed

crustal structure and extensional deformation of thinned lithosphere in northern china abstract we herein present an interpretation of a 320 km long wide angle seismic profile between anxin and kuancheng which was obtained in 2002 the profile runs from the north china plain ncp where the lithosphere is just 70 km thick to the yanshan mountain folded belt ymfb where the lithosphere is180 km thick our model shows a crustal thickness that varies from 31 km under the ncp to 36 km under the ymfb the observed thinning of the crust in the ncp is about 14 which compares with an average extension of 24 41 at basin scale and 25 at lithosphere scale this finding suggests that the extensional deformation of the lithosphere in the north china block depends on depth the thin high velocity crust mantle transition zone has most likely originated after a delamination of the bottom of the crust and a concomitant intrusion of materials from the mantle the lower velocity of the lower crust may be attributed to the destruction of the lithosphere which permitted the lateral flow of melting materials above the moho from the ncp to the ymfb the differences found between the crust and the lithospheric mantle help to dispel any of the remaining uncertainty in the extensional factors and they may be attributed to detachment of middle crust and an intrusion of magma that originated in either lithosphere or asthenosphere we infer that the detachment of the middle crust lower crustal flow and magma intrusion probably lead to the underestimation of the crustal scale extensional factor and may represent the crustal response to the thinning of the lithosphere

palaeomagnetic estimates of crustal shortening in the himalayan thrusts and zangbo suture this article reports new palaeomagnetic results from precisely dated late palaeocene limestones from the former northern margin of the indian subcontinent now in tibet between the major himalayan thrusts and the india tibet suture zone convergence in the suture and intracontinental thrusting can be estimated separately and are respectively found to be of the order of 550 km 650 km and 400 km 400 km comparison with kinematic models derived from magnetic anomalies constrains the age of the collision at about 50 myr

observations of heterogeneous stope convergence behavior and implications for induced seismicity analysis of stope convergence measurements in a burst prone lead zinc silver mine is being used by the u s bureau of mines to study rock burst processes three convergence gages were installed in a 3 m wide stope of the galena mine with an inter gage distance of approximately 8 m the two month period immediately after installation during which the subsequent cut of the pillar was made is the subject of the analysis described in this paper microseismicity rates and locations local magnitude 5 and above were also recorded during this period coseismic convergence steps within the 10 minute sample interval are observed at various times on all three gages the convergences of the three gages do not track one another indicating the blocky nature of deformation in this stope at a 10 meter or smaller size scale the behavior of the three gages is different with respect to coseismic and interseismic convergence during the study period for one of the gages 82 of total closure occurs coseismically the other two gages show much smaller percentages of coseismic convergence 30 and 35 and proportionately higher percentages of interseismic steady creep this heterogeneous convergence behavior may be indicative of the relative stability of the faults in the vicinity of each gage an examination of stability models shows that the mine geometry related spatial variation in normal stress acting on an existing set of faults may explain the observed behavior

an early india asia contact paleomagnetic constraints from ninetyeast ridge odp leg 121 new paleomagnetic results from sedimentary rock and basement of the ninetyeast ridge ocean drilling program leg 121 sites 756 758 detail the northward movement of the indian plate for the past 80 m y analysis of the combined paleolatitude age profile indicates a distinct reduction in india9s northward movement rate at 55 ma interpreted as completion of suturing of greater india and asia india9s northward motion slowed from 18 19 5 cm yr to 4 5 cm yr for the location of site 758 comparison of this profile with paleomagnetic data from the wider himalayan region indicates that initial contact between northwestern greater india and southern asia was already established by cretaceous tertiary time this supports a possible causal link between the india asia convergence and the deccan traps extrusion

mining induced seismicity in faulted geologic structures an analysis of seismicity induced slip potential relationships between the locations of mining induced seismic events local fault structure and mine geometry were examined in a deep hard rock mine in northern idaho stopes experiencing rock bursts and other large seismic events were found to fall into two structural regimes the silver vein and the n48 w trend a steeply dipping plane of seismic activity that is subparallel to major local steeply dipping faults which bound blocky structures the n48 w trend also intersects a shaft that was seriously damaged when fault gouge was expelled into the opening during a 3 month period of high seismic energy release models of stress interaction are used to support the hypothesis that mining induced deformation was mobilized along a 1 5 km length of the n48 w trend specifically numerical models are used to simulate rupture of seismic events and estimate induced changes in the quasi static stress field a coulomb failure criterion is used with these results to estimate the spatial variation in potential for slip on planes parallel to local faulting increases in the potential for slip on fault planes subparallel to the n48 w trend are consistent with activation of deformation along its 1 5 km length for events with constant seismic moment stress drop is shown to be far more important than source dimension in elevating slip potential along the observed plane of seismic activity

land subsidence in tianjin china land subsidence has been affecting tianjin for the past 50 years it leads to comprehensive detrimental effects on society the economy and natural environment overpumping of groundwater is the main cause in 2008 the maximum cumulative subsidence reached 3 22 m and the total affected area nearly 8 000 km2 the subsidence reached its most critical state in the early 1980s when it occurred at a rate as high as 110 mm year at the same time groundwater extraction had also reached a maximum of 1 200 million m3 by importing the luan river to tianjin and restricting exploitation of groundwater hydraulic heads gradually recovered after 1986 in all aquifers and this has continued to the present in the second aquifer the subsidence rate in urban areas dropped to 10 15 mm year the area of groundwater extraction expanded to the suburban area with economic growth in the 1990s and it was shifted to the third and fourth aquifers at present with a subsidence rate of 30 40 mm year four new suburban subsidence centers have been formed several measures were adopted to mitigate and prevent land subsidence disasters these included restricting groundwater exploitation groundwater injection prohibiting use in the specific zone a pricing policy for water resources advocating water saving technology and strict enforcement of groundwater laws although the subsidence area is still increasing slowly the subsidence rate is being controlled

the response of creeping parts of the san andreas fault to earthquakes on nearby faults two examples rates of shallow slip on creeping sections of the san andreas fault have been perturbed on a number of occasions by earthquakes occurring on nearby faults one example of such perturbations occurred during the 26 january 1986 magnitude 5 3 tres pinos earthquake located about 10 km southeast of hollister california seven creepmeters on the san andreas fault showed creep steps either during or soon after the shock both left lateral ll and right lateral rl steps were observed a rectangular dislocation in an elastic half space was used to model the coseismic fault offset at the hypocenter for a model based on the preliminary focal mechanism the predicted changes in static shear stress on the plane of the san andreas fault agreed in sense ll or rl with the observed slip directions at all seven meters for a model based on a refined focal mechanism six of the seven meters showed the correct sense of motion two possible explanations for such coseismic and postseismic steps are 1 that slip was triggered by the earthquake shaking or 2 that slip occurred in response to the changes in static stress fields accompanying the earthquake in the tres pinos example the observed steps may have been of both the triggered and responsive kinds a second example is provided by the 2 may 1983 magnitude 6 7 coalinga earthquake which profoundly altered slip rates at five creepmeters on the san andreas fault for a period of months to years the xmm1 meter 9 km northwest of parkfield california recorded ll creep for more than a year after the event to simulate the temporal behavior of the xmm1 meter and to view the stress perturbation provided by the coalinga earthquake in the context of steady state deformation on the san andreas fault a simple time evolving dislocation model was constructed the model was driven by a single long vertical dislocation below 15 km in depth that was forced to slip at 35 mm yr in a rl sense a dislocation element placed in the seismogenic layer under xmm1 was given a finite breaking strength of sufficient magnitude to produce a parkfield like earthquake every 22 years when stress changes equivalent to a coalinga earthquake were superposed on the model running in a steady state mode the effect was to make a segment under xmm1 that could slip in a linear viscous fashion creep ll and to delay the onset of the next parkfield like earthquake by a year or more if static stress changes imposed by earthquakes off the san andreas can indeed advance or delay earthquakes on the san andreas by months or years then such changes must be considered in intermediate term prediction efforts

assessing elastic coulomb stress transfer models using seismicity rates in southern california and southwestern japan 1 we investigated whether observed changes in seismicity rate following the 1992 landers and 1995 kobe earthquakes correspond with predictions from coulomb stress transfer models to quantify the relationship between coulomb stress change and seismicity rate change we compared spatial changes in observed seismicity rates to the modeled changes in coulomb failure stress cfs for the two subject earthquakes we begin with the simplest and most common cfs model which assumes that regional stress orientations are uniform and the stress change is resolved on optimally oriented strike slip planes we then extended the cfs models to include the more geophysically realistic assumptions of spatially varying regional stress orientations and optimally oriented planes in three dimensions for all the tested cfs models we found that virtually everywhere seismicity rate changes could be resolved the rate increased regardless of whether the cfs theoretically promoted or inhibited failure and that we were not able to reliably resolve seismicity rate decreases an improvement in the correlation between the sign of the cfs and where rate increases occur when estimating the cfs on optimally oriented planes in three dimensions rather than two dimensions is a consequence of using optimally oriented planes and the ability to primarily observe rate increases the region of positive cfs increases by allowing more degrees of freedom so the correlation with positive rate change appears to increase the areas of highest seismicity rate following the main shock did not correlate with the sign or the magnitude of the cfs rather the areas most active after the main shock were also the most active before the main shock

continuous catchment scale monitoring of geomorphic processes with a 2 d seismological array 1 distributed activity of geomorphic processes with different spatiotemporal scales is hard to monitor in detail with conventional methods but might be detected with seismometers from july to september 2010 we deployed 14 seismometers to evaluate the ability of a two dimensional array with small interstation distances 11 km to continuously monitor geomorphic processes in a mountain catchment 370 km2 in taiwan spectral analysis of seismic records highlights different sources with high frequency content 1 hz consistent with hillslope and river processes using a common detection algorithm and a location technique based on the timing of seismic amplitude we have located 314 near surface events most of which 69 occurred during daily convective storms event activity was positively correlated with the precipitation intensity but this relation was not uniform in the catchment high resolution satellite images and air photos did not show geomorphic change during the study which did not have any episodes of extreme precipitation a majority of events 61 were collocated with preexisting geomorphic features landslide scars gullies within the uncertainty on location 9 of interstation distance the combination of event location and timing suggests a geomorphic source of recorded signals and most events had the seismic characteristics of rockfall debris avalanches or slides reactivation of prior erosion sites by such processes is difficult to detect with imagery but can possibly be resolved by seismic monitoring when proven this approach will allow a spatially comprehensive survey of geomorphic activity at the catchment scale with temporal detail sufficient to evaluate the exact meteorological conditions under which process events occur

the lithosphere of north china craton from surface wave tomography abstract fundamental mode rayleigh wave group velocity dispersion data were used in a tomographic study to reconstruct the crust and upper mantle shear velocity structure in east asia this article expounds the features of lithosphere structure in different sub regions of north china and discusses their implications for the tectonic evolution and present tectonics of north china craton ncc the study revealed that strong earthquakes took place in the low velocity zones and along the boundaries between high and low velocities in the middle and lower crust the active nw trending zhangjiakou bohai seismic belt corresponds to a prominent low velocity zone in the crust and upper mantle lid in addition to the well known structural difference between western and eastern ncc we are able to resolve clearly the lithospheric features of sub regions in eastern ncc the southern north china basin and north yellow sea are characterized by a high velocity crust and upper mantle lid down to 70 80 km depth while the northern north china basin has a low velocity crust and the velocity below moho to about 130 km is rather uniform and low there is also a general trend of velocity decrease in the upper mantle from west to east the velocity images bear the imprints of subduction and collision that took place in the periphery of ncc since mesozoic and shed some light on the mechanism of mesozoic lithospheric thinning of eastern ncc

eclogitic metasediments from the tso morari area ladakh himalaya evidence for continental subduction during india asia convergence metasediments in the tso morari area ladakh himalaya provide new insights into the higher himalayan metamorphism in the northwestern part of the himalayan belt whole rock analyses and petrologic observations show that the metasediments correspond to fe rich metapelites mg rich metapelites intermediate metapelites and metagreywackes of the indian continental margin jadeite chloritoid paragonite garnet in the fe rich metapelites indicate pressures of 20 2 kbar at temperatures of 550 50 c according to major element partitioning thermobarometry stability fields of minerals and thermocalc p t estimates these results are consistent with p t estimates on other metasediments and with the occurrence of eclogites subsequent retrogression at the eclogite blueschist facies transition from 18 to 13 kbar and 540 50 c was followed by an increase in temperature to 630 30 c at amphibolite facies conditions the metamorphic evolution is related to subduction of the indian continental margin beneath the southern asian margin at the onset of the indian eurasian collision

palaeolatitude and age of the indo asia collision palaeomagnetic constraints summary ongoing controversies on the timing and kinematics of the indo asia collision can be solved by palaeomagnetically determined palaeolatitudes of terranes bounding the indo asia suture zone we show here based on new palaeomagnetic data from the linzizong volcanic rocks 54 47 ma near the city of lhasa that the latitude of the southern margin of asia was 22 8 4 2 n when these rocks were deposited this result combined with revised palaeomagnetic results from the northernmost sedimentary units of greater india and with apparent polar wander paths of india and eurasia palaeomagnetically constrain the collision to have occurred at 46 8 ma 95 per cent confidence interval these palaeomagnetic results are consistent with tomographic anomalies at 15 25 n that are interpreted to locate the tethyan oceanic slab that detached following collision and with independent 56 46 ma collision age estimates inferred from the timing of slowing down of india high pressure metamorphism the end of marine sedimentation and the first occurrence of suture zone and arc detritus on the greater indian margin when compared with apparent polar wander paths of india and eurasia the 46 ma onset of collision at 22 8 4 2 n implies 2900 600 km subsequent latitudinal convergence between india and asia divided into 1100 500 km within asia and 1800 700 km within india

exhumation of the ultrahigh pressure tso morari unit in eastern ladakh nw himalaya a case study exhumation processes of the ultra high pressure uhp tso morari dome nw himalaya are investigated using structural petrological and geochronological data the uhp tso morari unit is bounded by the low grade metamorphic indus suture zone to the ne and mata unit to the sw three deformation phases d1 d2 and d3 are observed only d3 is common to the uhp unit and the surrounding units in the uhp unit the first deformation phase d1 produced upright folds under eclogitic conditions 20 kbar 580 60 c d1 is overprinted by d2 structures related to a nw se trending open anticline this phase is characterized by blueschist mineral associations and corresponds to the quasi isothermal decompression from a depth of 90 km eclogitic conditions up to 30 40 km the final exhumation phase of the tso morari unit is dominated by tectonic denudation and erosion d3 associated with a slight temperature increase radiochronological analyses indicate that the uhp exhumation process began during the eocene exhumation was fast during d1 d2 and slowed down through d3 in oligocene time the change in the deformation style from d1 d2 to d3 in the tso morari unit coincides with changes in the exhumation rates and in the metamorphic conditions these changes may reflect the transition from an exhumation along the subduction plane in a serpentinized wedge to the vertical uplift of the tso morari unit across the upper crust

seismic shaking in the north china basin expected from ruptures of a possible seismic gap a 160 km long seismic gap which has not been ruptured over 8000 year was identified recently in north china in this study we use a dynamic source model and a newly available high resolution 3 d velocity structure to simulate long period ground motion up to 0 5 hz from possibly worst case rupture scenarios of the seismic gap we find that the characteristics of the earthquake source and the local geologic structure play a critical role in controlling the amplitude and distribution of the simulated strong ground shaking rupture directivity and slip asperities can result in large amplitude i e 1 m s ground shaking near the fault whereas long duration shaking may occur within sedimentary basins in particular a deep and closed quaternary basin between beijing and tianjin can lead to ground shaking of several tens of cm s for more than 1 minute these results may provide a sound basis for seismic mitigation in one of the most populated regions in the world

glaucophane and barroisite eclogites from the upper kaghan nappe implications for the metamorphic history of the nw himalaya

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

tethyan subducted slabs under india abstract tomographic imaging of the mantle under tibet india and the adjacent indian ocean reveals several zones of relatively high p wave velocities at various depths under the hindu kush region in northeastern afghanistan and southern tajikistan a regional northward dipping slab is seen in the entire upper 600 km of the mantle and is apparently still attached to the lithosphere of the indian plate under northern pakistan this same slab shows a roll over structure with the deeper portion overturned and dipping southward as can also be seen in the distribution of earthquake hypocenters farther east southeast e g in the vicinity of nepal a well resolved anomaly below 450 km depth is connected to the slab under the hindu kush but seems to be separated from the lithosphere above 350 km these upper mantle anomalies are interpreted as the remnants of delaminated sub continental lithosphere that went down when greater india continued to converge northward with asia after 45 ma the deeper high velocity anomalies under the indian sub continent appear clearly separated from the shallower ones as well as from each other and are inferred to represent remnants of oceanic lithospheric slabs that have sunk into the lower mantle and were subsequently overridden by the indian plate they occur at depths between 1000 and 2300 km and occasionally descend down to the core mantle boundary the anomalies form three parallel wnw ese striking zones we interpret the two southern zones as remnants of oceanic lithosphere that was subducted when the neo tethys ocean closed between india and tibet in the cretaceous and earliest tertiary the northern deep mantle zone under northern afghanistan the himalayas and the lhasa block in southern tibet may represent the last subducted remnant of the paleo tethys ocean which is thought to have closed before the hauterivian stage of the early cretaceous the middle zone continues southeastward as a rather straight high velocity zone towards sumatra where it becomes convex southward and parallel to the subduction zone under the sunda arc comparison of this straight middle zone near india with the shallower upper 600 1000 km northern zone which displays a cusp like shape near the yunnan sw china syntaxis of the eastern himalayas supports the notion that the shallow northern zone represents later subduction than the deeper middle zone the suggestion of a counterclockwise rotation 20 of the indian plate during tertiary indentation of asia is supported by these features the present day latitudes of 5 35 n of the deep slabs under india and adjacent areas correspond to the approximate paleolatitudes of the cretaceous subduction zones the slab remnants in the middle mantle occur therefore near the ancient locations where they started their downward journey which implies that lateral movements in the deeper mantle were not large

comparison of ground motion from tremors and explosions in deep gold mines seismic body waves from tamped chemical explosions two with yields of 50 and one of 150 kg were compared with corresponding data from three mining induced tremors with a view to testing methods of discriminating between the two types of events detonated at depths of about 2 km all three explosions generated p waves for which the low frequency spectral asymptotes agree well with corresponding results scaled down from nuclear shots at the nevada test site for the two smaller explosions recorded underground at hypocentral distances ranging from 234 to 871 m the p wave corner frequencies defined by the intersections of the low and high frequency asymptotes are also in fine agreement with scaled down results from the nevada test site for the 150 kg explosion which was recorded only at the surface it appears that 2 39 km of upward propagation caused the corner frequency to be at least a factor of 4 lower than anticipated due to attenuation all three explosions generated s waves that appear to be a consequence of deviatoric stress release in the immediate environs of the explosions the three tremors analyzed here generated p and s waves whose spectra and source parameters agree well with standard earthquake source models with regard to discrimination based on p wave spectra we conclude that for events of fixed low frequency spectral asymptotes the explosions typically have higher corner frequencies than tremors or earthquakes although counterexamples certainly exist interestingly the 150 kg explosion was identified as such on the basis of p and s wave polarities that are incompatible with the normally expected double couple source model instead these initial motions are consistent with an explosion in conjunction with normal faulting the body wave spectra of this explosion and those of a nearby tremor however were indistinguishable

implications of shortening in the himalayan fold thrust belt for uplift of the tibetan plateau 1 recent research in the himalayan fold thrust belt provides two new sets of observations that are crucial to understanding the evolution of the himalayan tibetan orogenic system first u pb zircon ages and sm nd isotopic studies demonstrate that 1 greater himalayan medium to high grade metasedimentary rocks are much younger than true indian cratonic basement and 2 these rocks were tectonically mobilized and consolidated with the northern margin of gondwana during early paleozoic orogenic activity these observations require that greater himalayan rocks be treated as supracrustal material in restorations of the himalayan fold thrust belt rather than as indian cratonic basement in turn this implies the existence of greater himalayan lower crust that is not exposed anywhere in the fold thrust belt second a regional compilation of shortening estimates along the himalayan arc from pakistan to sikkim reveals that 1 total minimum shortening in the fold thrust belt is up to 670 km 2 total shortening is greatest in western nepal and northern india near the apex of the himalayan salient and 3 the amount of himalayan shortening is equal to the present width of the tibetan plateau measured in an arc normal direction north of the indus yalu suture zone this information suggests that a slab of greater indian lower crust composed of both indian cratonic lower crust and greater himalayan lower crust with a north south length of 700 km may have been inserted beneath the tibetan crust during the cenozoic orogeny we present a modified version of the crustal underthrusting model for himalayan tibetan orogenesis that integrates surface geological data recent results of mantle tomographic studies and broadband seismic studies of the crust and upper mantle beneath the tibetan plateau previous studies have shown that incremental mesozoic and early cenozoic shortening had probably thickened tibetan crust to 45 55 km before the onset of the main cenozoic orogenic event thus the insertion of a slab of greater indian lower crust with maximum thickness of 20 km tapering northward could explain the cenozoic uplift of the tibetan plateau the need for tibetan crust to stretch laterally as the greater indian lower crust was inserted may explain the widespread east west extension in the southern half of the plateau our reconstruction of the himalayan fold thrust belt suggests that indian cratonic lower crust of presumed mafic composition and high strength should extend approximately halfway across the tibetan plateau to the banggong suture from there northward we predict that the tibetan plateau is underlain by more felsic and therefore weaker lower crust of greater himalayan affinity two slab break off events are predicted by the model the first involved neotethyan oceanic lithosphere that foundered 45 35 ma and the second consisted of greater indian lithosphere most likely composed of greater himalayan material that delaminated and foundered 20 10 ma asthenospheric upwelling associated with the break off events may explain patterns of cenozoic volcanism on the tibetan plateau although the model predicts a northward migrating topographic front due solely to insertion of greater indian lower crust the actual uplift history of the plateau was complicated by early middle tertiary shortening of tibetan crust

tomographic imaging of the lowermost mantle with differential times of refracted and diffracted core phases pkp pdiff the mapping of variations in p wave speed in the deep mantle is restricted by the uneven sampling of p waves in particular beneath the southern hemisphere to enhance data coverage we augmented the 1 6 million summary rays of p pp and pwp that we used in previous studies with differential travel times of diffracted and refracted core phases for the core refracted differential travel time residuals pkpab pkpdf and pkpab pkpbc we used 1383 cross correlated digital waveforms as well as 27 000 routinely processed bulletin data we used the waveform data to define quality criteria for the selection and reduction of the bulletin pkp data for pkpdf pdiff we only considered 543 records derived from waveform cross correlation no pcp data were used in this study we used optical ray theory to calculate the ray paths associated with the p pp pwp and pkp data which are measured at 1 hz however to account for the large fresnel zones of the low frequency 50 mhz pkpdf pdiff data we estimated the three dimensional shape of the frechet sensitivity kernels from kernels calculated by normal mode summation the use of these kernels allows us to properly distribute the sensitivity for a given seismic phase over a large mantle volume while allowing the high frequency data to constrain small scale structure the differential times are relatively insensitive to source mislocation and to structure in the shallow mantle beneath source and receivers and they have previously been interpreted exclusively in terms of lateral structure directly above the core mantle boundary cmb however images thus obtained can be contaminated by effects of small scale structure elsewhere in the mantle here we do not make a priori assumptions about the mantle source of anomalous time differentials from test inversions we conclude that both upper and lower mantle structures that are poorly resolved by p data can be mapped into the core along pkp paths but that the effect of outer core structures if any on the mantle model is small compared to the inversion of the p pp and pwp alone the inclusion of the pkpab pkpdf and pkpab pkpbc and pkpdf pdiff data improves the resolution of structure beneath 2200 km depth in particular the joint inversion puts better constraints on the long wavelength variations in the very deep mantle and yields an increase in the amplitude of velocity perturbations near the cmb that is in agreement with but still smaller than inferences from shear wave studies resolution tests indicate that in some regions the enhanced definition of structure is significant but in most regions the improvements are subtle and structure remains poorly resolved in large regions of the mantle

mechanics of seismic instabilities induced by the recovery of hydrocarbons we review earthquake distributions associated with hydrocarbon fields in the context of pore pressure diffusion models poroelastic stress transfer and isostasy theory these three mechanisms trigger or induce seismic instabilities at both local scale d 5 km and at regional scale d 20 km the modeled changes in stress are small 1 mpa whatever the tectonic setting each mechanism corresponds to different production processes 1 local hydraulic fracturing due to fluid injection induces seismic slip on cracks ml 3 within the injected reservoir through decreasing the effective stress 2 pure fluid withdrawal causes pore pressure to decrease within the reservoir it triggers adjustments of the geological structure to perturbations related to the reservoir response to depletion poroelastic mechanisms transfer this stress change from the reservoir to the surrounding levels whereml 5 seismic instabilities occur either above or below the reservoir 3 massive hydrocarbon recovery induces crustal readjustments due to the removal of load from the upper crust it can induce larger earthquakes ml 6 at greater distance from the hydrocarbon fields than the two other mechanisms

constraints on the tectonic evolution of the northwestern himalaya from geochronologic and petrologic studies of babusar pass pakistan petrologic geochronologic and structural studies of indian plate rocks in the footwall of the main mantle thrust indicate that peak metamorphism occurred in thrust imbricates well before the final assembly of the kohistan island arc with the indian plate as the northern margin of the indian plate was thickened by this early thrusting the metamorphic rocks evolved along increasing pressure temperature paths and in some regions reached temperatures sufficient for partial melting it is difficult to constrain the timing of this early deformation and metamorphism but 40 ar 39 ar hornblende ages and u pb ages of crosscutting leucogranite dikes suggest that in most areas peak metamorphism was earlier than 40 ma and in some areas earlier than 50 ma after peak metamorphism in the interval from 40 to 25 ma rocks of the indian plate continued to be thrust southward creating sharp metamorphic discontinuities at thrust boundaries from 25 to 20 ma rocks of the indian plate underwent rapid cooling

def3d a fem simulation tool for computing deformation near active faults and volcanic centers abstracts most analytical solutions of deformation near active and volcanic centers are limited to simple geometries and homogeneous media to solve more complex problems we have developed a fem software package called def3d for modeling deformation caused by earthquakes or magma chambers in laterally inhomogeneous media features presently available include modeling of split node technique equivalent body forces and surface tractions def3d presently supports the isotropic elastic and viscoelastic materials behaving as a maxwell body the program has been verified in various test cases by strict comparison with analytical solutions displacements following the 2008 wenchuan earthquake and magma induced surface deformation near changbaishan volcano are used to test the reliability of the software package

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

seismicity and tectonic stress in the coeur d alene mining district abstract the coeur d9alene mining district lies within a minor zone of seismicity in northern idaho along the lewis and clark zone a 400 km long system of west northwest trending faults rockbursts seismic events that cause damage in mines have become increasingly common in the coeur d9alene district during recent years our epicenter map of the district shows only 52 seismic events since 1982 however magnitude frequency analysis suggests that regional seismograph coverage is complete only for events above magnitude 2 8 and that 90 per cent of the seismic events above magnitude 1 5 are not located by the regional seismograph network a fault plane solution for the 1 august 1988 magnitude 4 1 tectonic earthquake that occurred 15 km northeast of mullan idaho shows together with previous in situ stress measurements that the coeur d9alene mining district lies within a regional northeast southwest extensional regime with the maximum compressive stress directed northwest southeast this orientation is deleterious for mining as it favors right lateral slip on the steeply dipping west northwest trending faults and bedding planes common in the district low level seismicity in the immediate vicinity of the lucky friday mine is driven in the short term by mining rate as is evident from the decreased levels of seismic activity on weekends when mining does not take place in the longer term cycles of tectonic stress build up and release may be important large rockbursts at the mine seem to be followed by several months of seismic quiescence

dating the indian continental subduction and collisional thickening in the northwest himalaya multichronology of the tso morari eclogites multichronometric studies of the low temperature eclogitic tso morari unit ladakh india place timing constraints on the early evolution of the northwest himalayan belt several isotopic systems have been used to date the eclogitization and the exhumation of the tso morari unit lu hf sm nd rb sr and ar ar a ca 55 ma age for the eclogitization has been obtained by lu hf on garnet omphacite and whole rock from mafic eclogite and by sm nd on garnet glaucophane and whole rock from high pressure metapelites these results agree with a previously reported u pb age on allanite and together these ages constrain the subduction of the indian continental margin at the paleocene eocene boundary during exhumation the tso morari rocks underwent thermal relaxation at about 9 3 kbar characterized by partial recrystallization under amphibolite facies conditions ca 47 ma as dated by sm nd on garnet calcic amphibole and whole rock from metabasalt rb sr on phengite apatite and whole rock and ar ar on medium si phengite from metapelites ar ar analyses of biotite and low si muscovite from metapelites which recrystallized at 5 kbar toward the end of the exhumation show that the tso morari unit was at upper crustal levels ca 30 ma these results indicate variable exhumation rates for the tso morari unit beginning with rapid exhumation while the indian margin subduction was still active and later proceeding at a slower pace during the crustal thickening associated with the himalayan collision

numerical modelling of anomalous microseismicity influenced by lithological heterogeneity in longwall top coal caving mining abstract mining induced microseismicity has been extensively used to evaluate the potential for rock bursts and coal and gas outbursts in underground coal mines in a research project completed a few years ago it was observed that characteristics of microseismicity around a working longwall panel were fairly consistent over the monitoring period until a lithological heterogeneity zone with a relatively high coal strength was reached the current research presented in this paper aims at achieving a better understanding of the effect of lithological heterogeneity on microseismic activity in longwall coal mining the heterogeneous zone inferred from the tomography measurements was first digitalised and implemented into a 3d geomechanical model a microseismicity modelling approach which combines deterministic stress and failure analysis together with a stochastic fracture slip evaluation was used to simulate the evolution of microseismicity induced by the progressive face advance passing through the heterogeneous zone the heterogeneity was taken into account by varying the material strength and the fracture attributes of the elements within the high strength zone results have shown that both the high rock strength of coal lithotype and low power law scaling exponent of fractures within this zone contribute to the reduction in fitted b values from frequency magnitude distribution of microseismicity and the increase in fitted gaussian distribution parameters to the logarithmic event energy these deviations are believed to result from the combined effects of increased stress drops and slipped fracture sizes when the heterogeneous zone is approached

velocity structure of the pamir hindu kush region possible evidence of subducted crust the arrival times of compressional p and shear s waves from approximately 580 microearthquakes recorded by a temporary array in the pamir hindu kush region in central asia are used to deduce one and three dimensional velocity structures of this region the results for one dimensional structures imply that the moho is at 70 5 depth also there is a velocity reversal near 160 km depth which is inferred to be the beginning of the low velocity zone this reversal continues to depths of approximately 230 km below 230 km velocities are somewhat higher than those of normal mantle at similar depths 9 3 km s versus 8 4 km s for p waves the outstanding feature of the results for three dimensional velocity structures is a broad 40 km centrally located region with 8 to 10 lower velocities than those in the surrounding regions this low velocity region envelopes the seismic zone at depths between 70 and 150 km the region may actually extend beyond these depths but the results for shallower and deeper structure lack sufficient resolution to decide several tests using both hypothetical and real data were performed to estimate the reliability of the three dimensional solutions the results of these tests suggest that the inferred velocities are reasonably accurate representations of the average velocities in the blocks although one must be cautious of the effects of averaging in interpreting the solution the low velocity region is inferred to be a manifestation of substantial quantities of subducted continental crust therefore while subduction has occurred in the pamir hindu kush the results of the three dimensional inversions suggest that continental rather than only oceanic lithosphere has been subducted to depths of at least 150 km appendices are available with the entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j81 013 1 00 payment must accompany order

subduction ultrahigh pressure metamorphism and regurgitation of buoyant crustal slices implications for arcs and continental growth abstract intracontinental collisional belts that retain mineralogic relics of phanerozoic ultrahigh pressure uhp metamorphism are increasingly being recognized adjacent regions generally lack evidence of coeval calcalkaline volcanism plutonism following consumption of intervening oceanic lithosphere each orogen marks the site of a deeply subducted microcontinental promontory fragments of which later returned to midcrustal levels mafic and ultramafic rocks are volumetrically minor pressures in uhp complexes approached or exceeded 2 8 gpa at 600 900 c subduction zones involve low t prograde gradients and constitute the only plate tectonic realm where such conditions exist although internal portions of descending plates may have even lower geothermal gradients crustal upper margins are typified by p t trajectories of 5 10 c km attending underflow clinoamphibole rich metabasaltic rocks dehydrate at pressures 2 2 2 5 gpa this accounts for the kinetically favorable solid state production of mafic eclogites and partial fusion of morbs to form hydrous andesitic melts devolatilization of partly serpentinized mantle lithosphere also introduces h 2 o to the magmagenic zone as large masses of continental crust enter deeper portions of subduction zones mineralogic transformations slow or cease because micas the major oh bearing phases in such rocks retain volatile contents to pressures 4 gpa persistence of hydroxyl bearing layer silicates may explain the metastable preservation of low density assemblages in continental crust subjected to uhp conditions and the dearth of calcalkaline magmatism in most collision belts exhumed uhp terranes are intensely deformed and disaggregated all consist dominantly of quartzofeldspathic compositions mafic eclogites garnet peridotites constitute only a few volume percent eventual decoupling of the relatively low density subducted crust promotes buoyancy propelled transport to shallow depths along the subduction channel with the hanging wall lithosphere acting as a stress guide extensional collapse and erosional unroofing of rising masses result in the continued ascent of deeply subducted but buoyant crustal material although equidimensional massifs must return surfaceward large volume to surface areas render the contained uhp phase assemblages virtually unquenchable surviving uhp terranes thus consist of relatively thin slabs of continental crust allochthons that retain uhp relics apparently lost heat by conduction across both upper normal fault and lower reverse fault contacts based on present exposures estimated thicknesses of uhp sheets are qinling dabie sulu belt 5 km kokchetav complex 1 3 km dora maira massif 1 2 km western gneiss region 1 km slices evidently rose to midcrustal levels at exhumation rates approaching 10 mm per year back reaction attending decompression in all cases was nearly complete where uhp mineral inclusions persisted retrogression apparently was limited by the relative impermeability of the rocks to catalytic aqueous fluids and by declining temperatures the coarse grain size of host phases also disfavored complete retrogression

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