three dimensional velocity imaging of the kachchh seismic zone gujarat india abstract to understand the causative mechanism of the continued occurrence of earthquakes in kachchh gujarat for the last six years we estimated high resolution three dimensional vp vs and vp vs structures in the aftershock zones of the 2001 mw7 7 bhuj and 2006 mw5 6 gedi earthquakes we used 13 862 p and 13 736 s wave high quality arrival times collected from the seismograms of 2303 aftershocks recorded at 5 18 three component seismograph stations during 2001 06 seismic images revealed a marked spatial variation in the velocities from 20 to 14 in vp from 12 to 13 in vs and from 12 to 12 increase in vp vs in the 0 34 km depth range beneath the bhuj aftershock zone relatively more increase in vp than vs resulting in an increase in vp vs in the crust beneath the seismically active causative fault north wagad fault nwf zone of 2001 bhuj mainshock suggests a rigid mafic crust beneath the region they also delineate an increase of 8 in vp and 14 in vs and a decrease of 4 in vp vs in the almost vertical rupture zone of the 2006 gedi earthquake extending up to 12 km depth this high velocity body associated with the gedi mainshock is inferred to be a gabbroic intrusive the banni region and the wagad uplift are found to be associated with high velocity intrusive bodies inferred to be mafic extending from 5 to 35 km depth which might have intruded during the rifting in early jurassic 160 ma aftershock activity is mainly confined to the zones characterized by high vp high vs and low vp vs ratio which might be representing the strong competent and brittle parts of the fault zone intrusive bodies that could accumulate large strain energy for generating aftershocks for more than six years it is inferred that the crustal stress concentrations associated with the intrusive bodies are contributing significant perturbation to the crustal stress regime to generate the intraplate earthquakes in the kachchh rift zone a few patches of slow vp and vs and high vp vs between 10 to 30 km depth have also been detected on the causative 45 south dipping north wagad fault nwf for the 2001 mainshock which may be attributed to the fluid filled fractured rock matrix interestingly the 2001 bhuj mainshock hypocenter is found to be associated with such a low velocity patch thus it is inferred that the occurrence of 2001 bhuj mainshock might have facilitated by the presence of fluids at hypocentral depth

variations in the provenance of sediment from ice sheets surrounding baffin bay during mis 2 and 3 and export to the labrador shelf sea site hu2008029 0008 davis strait based on quantitative x ray diffraction mineralogical analysis of cores hu2008029 0006 0008 857 m water depth located south of the davis strait sill we seek answers to two questions i the temporal correlation of nw labrador shelf sea heinrich h and baffin bay detrital carbonate bbdc events and ii the provenance of the sediments a sediment unmixing model sedunmix is used to determine provenance variations although 12 radiocarbon dates were obtained there were many age reversals and we favour a depth age model for hu2008029 0008pc based on correlations to well dated neighbouring cores north and south of davis strait we find no correlation between h and bbdc events except for h 0 which appears to be a broadly synchronous event along the ne canadian margin and is correlative with an episode of major moraine building along this flank of the laurentide ice sheet unmixing of the sediment composition based on six potential glacial source regions indicates that the bulk of the sediment in 0008pc is derived from glacial erosion of west greenland but is punctuated by intervals when dolomite rich sediments derived from outcrops north of baffin bay dominate the composition estimated dates on these bbdc events are 13 25 and 30 38 cal ka bp copyright 2013 john wiley sons ltd

characteristics and origin of the zhulazhaga gold deposit in inner mongolia china abstract the zhulazhaga gold deposit in the northern margin of the north china craton ncc is a large tonnage hydrothermal deposit with a reserve of about 50 t at about 4 g t au the deposit is characterized by an epigenetic style of mineralization in metasedimentary rocks dominated by slates low grade metamorphic clastic rocks and minor volcanic rocks gold mineralization is dominantly controlled by structures manifested by stratabound orebodies located immediately to the east and west of a major n s trending fault and associated extensional fractures and ne trending fault zones the mineralization is also preferentially located in the fourth sub member of first member of the agulugou formation there are many auriferous quartz veins parallel to the bedding of the host rocks which contain the orebodies and high grade ore preferentially occurs near to these quartz veins the structures provided pathways for transportation of gold bearing fluids within the formation that appears to form a particularly favorable host rock and depositional setting hydrogen and oxygen isotope data indicate that ore forming fluids were dominated by magmatic water mixed with minor meteoric water sulfur and lead isotope data indicate that some sulfur and lead came from the magmatic fluid the metallogenic age 280 ma of this deposit and the crystallization age 280 290 ma of the granite porphyry in the district indicate a link between the hercynian magmatism and gold mineralization the hercynian tectono magmatism and associated hydrothermal activity remobilized gold and drove ore forming fluids to the favorable depositional environment the northern margin of the ncc is prospective for gold mineralization hosted in metasedimentary rocks which may be one of the most important deposit types for future gold exploration in northern china

a low seismic wavespeed anomaly beneath northwestern india a seismic signature of the deccan plume abstract seismic tomography of the indian region using p wave arrival times reveals an approximately cylindrical region of lowered seismic velocities in the upper mantle to the north of the gulf of cambay and the present exposure of the deccan flood basalts this anomaly with low velocities interrupts the characteristically high seismic velocities in the lithosphere beneath peninsular india and extends from shallow depth down to a more extensive low velocity zone beneath 200 km the centre of the low velocity anomaly lies just below the earliest alkaline magmatism which has been directly associated with the deccan plume in the northern portion of the cambay graben these magmatic events precede the main deccan flood eruptions by at least 3 myr the lowered seismic wavespeeds link into the ancient narmada lineament and to some extent to the south but here unfortunately the tomographic image runs out of resolution synthetic tests indicate that such a zone of lowered wavespeeds can be recovered with some loss of amplitude towards the south the association of the current seismic anomaly with the deccan traps is intriguing since the cambay graben remains a place of raised heat flow one explanation would be that the seismic anomaly represents the signature of the initial conduit through which plume material forced its way through the lithosphere beneath the cambay rift the zone of lowered seismic velocities beneath the indian lithosphere might then represent the remains of the initial plume head which would need to have moved with india over the 65 myr since the deccan eruptions

source parameters and tectonic implications of aftershocks of the mw 7 6 bhuj earthquake of 26 january 2001 we present and discuss the spatial distribution of more than 1000 aftershocks of the largest continental intraplate earthquake to occur during the modern seismological period the data were recorded on a network of eight portable digital seismographs deployed for 3 weeks starting 17 days after the mainshock we have calculated high quality single event locations based on a 1d velocity model determined for the region for earthquakes with magnitudes between 2 and 5 aftershock locations reveal activity concentrated on a nearly east striking south dipping plane trapezoidal in outline the active zone tapers from about 45 km along strike at the shallow end which is about 5 km deep to no more than 25 km long at a depth of 35 km the total rupture area was about 1300 km 2 we estimate the static stress drop of the mainshock at 16 2 mpa aftershocks extend nearly through the entire crust with concentrations in the lower crust at about 26 km and in the upper crust at about 10 km the fault that ruptured was not mapped at the surface and not known to have been active prior to the 2001 earthquake the aftershock data are consistent with the bhuj earthquake resulting from reactivation in contraction of a fault formed under extension within a failed rift

seismic imaging of the aftershock zone of the 2001 mw 7 7 bhuj earthquake india 1 detailed seismic tomography reveals that the 2001 mw 7 7 bhuj earthquake was associated with a 10 14 increase in vp and vs and a 10 decrease in vp vs in the 10 to 35 km depth range covering a 2750 km2 area beneath the aftershock zone this anomaly could be attributed to the existence of a lithological heterogeneity or a pluton of mafic composition that might have intruded during the rifting in early jurassic 160 ma the banni basin and the wagad uplift are found to be associated with high velocity intrusive bodies extending from 5 to 35 km depth a few patches of low vp and vs and high vp vs between 10 to 30 km depth have also been detected on the causative 45 south dipping north wagad fault nwf for the 2001 mainshock which may be attributed to a fluid filled fractured rock matrix

spatiotemporal variation of fractal properties in the source zone of the 2001 mw 7 7 bhuj earthquake the kachchh seismic zone ksz in gujarat india has been experiencing earthquakes since the occurrence of the 2001 m w 7 7 bhuj mainshock which led to a homogeneous catalog of 2808 earthquakes of m w 3 0 between 2001 and 2011 this enabled us to investigate the spatiotemporal variation of fractal dimensions d b values and seismic velocities in the ksz through 3d mapping of these three independent parameters the estimated fractal dimensions for the gedi fault gf events range from 0 7 to 1 0 whereas those of north wagad fault nwf events vary from 1 2 to 2 4 the b values for gf events also show smaller values in comparison to the nwf events the d values associated with events occurring in the main rupture zone mrz of the 2001 mainshock vary from 1 6 to 2 1 whereas those associated with events occurring in the surrounding unrifted zones show a wider range of variation from 1 2 to 2 4 we also observe that events associated with the nwf satisfy the aki s relationship d 2 b aki 1981 whereas events associated with gf do not follow this relationship both b and d values tend to increase with an increase in the earthquake density which could be connected with the process of disintegration of crustal rocks in the mrz where large material heterogeneities in terms of low and high seismic velocity patches are being mapped through local earthquake velocity tomography through examining the temporal changes in b and d values we notice several episodes of abnormally low b and d values associated with the occurrences of bursts of seismic activity in 2001 2005 2006 2008 and 2010 this decrease in b and d values is attributed to a reduction in rock friction in the causative fault zone at 10 35 km depths probably due to the presence of metamorphic fluids or volatile co2 thereby enhancing the probability of occurrence of stronger earthquakes

effects of ice coverage and ice rafted material on sedimentation in the fram strait as little is known about pelagic sedimentation processes in arctic environments1 the interpretation of biological and chemical processes as well as the reconstruction of ancient conditions including those in the glacial north atlantic is difficult here we provide sediment trap results which show that the position of the sea ice boundary significantly influences the particle flux the seasonal variability of the particle flux differed markedly in the various sediment trap sites in fram strait depending on the behaviour of the sea ice under complete ice cover sedimentation is very low whereas maximum sedimentation is found at the ice margin the highest particle flux observed showing a large lithogenic component was observed at the ice edge where the water was warmer 2 c we find that high biogenic opal fluxes are characteristic of the summer ice margin indicating that the sedimentary record of opal fluxes may allow the position of ice margins in the past to be reconstructed

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

iterative de convolution of the local waveforms characterization of the seismic sources in kachchh india abstract the deviatoric and double couple dc constrained moment tensor inversions of multiple point sources 10 20 s for regional or local earthquakes developed by zahradnik et al 2005 has been applied on the data of nine significant bhuj aftershocks of mw4 4 mw5 6 recorded at three component 5 15 accelerograph and 5 11 seismograph stations epicentral distances

three dimensional seismic velocity imaging of the kachchh rift zone gujarat india implications toward the crustal mafic pluton induced intraplate seismicity abstract three dimensional tomographic images of seismic velocities have been constructed for the 2001 mw7 7 bhuj earthquake region using data from a dense 58 station seismograph network during 2001 16 two distinct prominent high velocity anomalies 10 14 increase in vp and vs and 10 11 decrease in vp vs have been mapped at 2 40 km depths below the banni and wagad uplift regions which could be attributed to mafic plutons we also image several high velocity zones intrusive mafic rocks and some low velocity patches meteoric water metamorphic fluids volatile co2 within the main rupture zone of the 2001 bhuj earthquake where maximum confinement of seismicity is noticed we propose that stresses induced by mafic plutons in response to the n s compression of the indian plate might have resulted in a stress regime near to critical for thesub parallel rift associated fault system within the main rupture zone thereby generating small to large earthquakes through small stress perturbations caused by crustal fluid flows and stress transfer

results of pilot re os dating of sulfides from the sukhoi log and olympiada orogenic gold deposits russia abstract the pilot study with re os dating of sulfides from sukhoi log and olympiada gold deposits revealed early paleozoic ages of the auriferous sulfides from the two largest orogenic gold systems in the neoproterozoic orogens of the baikalides framing the siberian craton the age dating results indicate that gold mineralization is therefore epigenetic the formation of the dated orogenic gold deposits is synchronous with some regional metamorphic events in the baikalides at least in case of the sukhoi log deposit the metamorphic events occurred in the rear parts of the early paleozoic magmatic arcs where coeval subduction related magmatism produced porphyry copper molybdenum mineralization

three dimensional ground motion modelling in the kachchh rift zone gujarat india abstract the strong motion dataset of the 6 april 2006 bhuj aftershock of mw5 5 from 16 three component accelerograph stations are used here to perform a detailed ground motion modelling in the kachchh rift basin first we estimate the 1 d velocity structure model down to the bedrock at 16 accelerograph sites through the genetic algorithm inversion of strong motion waveforms then a 3 d velocity structure model of the basin is constructed using the modelled 1 d velocity structure and constraints from earlier seismological vs30 estimation by ps logging and masw and seismic exploration studies in the region we modelled low frequency 0 1 1 hz ground motions excited by the 6 april 2006 event using the finite difference method fdm assuming a point source to access the robustness of the constructed velocity structure model at most of the stations the observed and simulated velocity waveforms are found to be in good agreement in terms of both amplitude and ground motion duration we also computed synthetic ground velocities at numerous locations within the basin for both the 2001 bhuj mainshock finite fault source and the 2006 aftershock point source cases using the 3 d velocity structure model for both the cases we notice significant amplifications of amplitudes and elongation of shaking at stations lying within the basin while smaller amplifications are noticed at stations away from the basin our fdm modelling for the 2001 bhuj mainshock at lower frequency

an appraisal of the 2001 bhuj earthquake mw 7 7 india source zone fractal dimension and b value mapping of the aftershock sequence we examined seismic characteristics b value and fractal dimension of the aftershock sequence of the january 26 2001 bhuj earthquake mw 7 7 that occurred in the kutch failed rift basin western margin of the stable continental region scr of india a total of about 2 000 events m 2 0 were recorded within two and a half months immediately after the main shock some 795 events were precisely relocated by simultaneous inversion these relocated events are used for mapping the frequency magnitude relation b value and fractal correlation dimension dc to understand the seismic characteristics of the aftershocks and the source zone of the main shock the surface maps of the b value and dc reveal two distinct tectonic arms or zones of the v shaped aftershock area western zone and eastern zone the b value is relatively higher 1 6 in the western zone compared to a lower value 1 4 in the eastern zone the dc map also shows a higher value 1 2 1 35 in the western zone compared to a lower dc 0 80 1 15 in the eastern zone this implies a positive correlation between dc and b value two cross sections e w and n s are examined the e w sections show similar characteristics higher b value and higher dc in the western zone and lower in the eastern zone with depth the n s sections across the fault zones however show unique features it imaged both the b and dc characteristics convincingly to identify two known faults the kutch mainland fault and the south wagad fault swf one stepping over the other with a seismogenic source zone at depth 20 35 km the source zone at depth is imaged with a relatively lower b and higher dc at the fault end of the swf showing a negative correlation these observations corroborated with the seismic tomography as well as with the proposed geological tectonic model shed a new light to our understanding on seismogenesis of the largest scr earthquake in india in the recent years

seismic imaging of the 2001 bhuj mw7 7 earthquake source zone b value fractal dimension and seismic velocity tomography studies abstract we use precisely located aftershocks of the 2001 mw7 7 bhuj earthquake 2001 2009 to explore the structure of the kachchh seismic zone by mapping the 3 d distributions of b value fractal dimension d and seismic velocities from frequency magnitude analysis we find that the catalog is complete above mw 3 0 thus we analyze 2159 aftershocks with mw 3 0 to estimate the 3 d distribution of b value and fractal dimensions using maximum likelihood and spatial correlation dimension approaches respectively our results show an area of high b d and vp vs ratio values at 15 35 km depth in the main rupture zone mrz while relatively low b and d values characterize the surrounding rigid regions and gedi fault gf zone we propose that higher material heterogeneities in the vicinity of the mrz and or circulation of deep aqueous fluid volatile co2 is the main cause of the increased b d and vp vs ratio values at 15 35 km depth seismic velocity images also show some low velocity zones continuing in to the deep lower crust supporting the existence of circulation of deep aqueous fluid volatile co2 in the region probably released from the eclogitasation of olivine rich lower crustal rocks the presence of number of high and low velocity patches further reveals the heterogeneous and fractured nature of the mrz interestingly we observe that aki 1981 s relation d 2b is not valid for the spatial b d correlation of the events in the gf d2 1 2b zone however the events in the mrz d2 1 7b show a fair agreement with the d 2b relationship while the earthquakes associated with the remaining parts of the aftershock zone d2 1 95b show a strong correlation with the aki 1981 s relationship thus we infer that the remaining parts of the aftershock zone are probably behaving like locked un ruptured zones where larger stresses accumulate we also propose that deep fluid involvement may play a key role in generating seismic activity in the mrz therefore a similar episode of deep fluid inflow could play a crucial role in causing the next strong earthquakes in the surrounding blocks of nwf

source to sink statistical identification of regional variations in the mineralogy of surface sediments in the western nordic seas 58 n 75 n 10 w 40 w the non clay and clay mineralogies of the 2 mm sediment fraction from 157 seafloor samples in the western nordic seas is calculated by quantitative x ray diffraction analysis our sample coverage includes the iceland shelf the ice impacted east greenland margin and the greenland iceland and irminger seas the bedrock geology of source areas includes precambrian basement rocks devonian red beds and younger marine and fluvial sediments and large outcrops of early tertiary to recent flood basalts and other volcanic facies sediment erosion and transport in the area are associated with ice sea ice and icebergs and glacial meltwater and fluvial sediment plumes we use principal component analysis fuzzy k means clustering and discriminant function analysis to reduce the original 31 minerals 157 sites matrix to more manageable 5 regional coherent clusters of sites which are well discriminated by the weight of six minerals namely quartz pyroxene illites magnetite and maghemite amorphous silica e g glass and muscovite the close spatial relationship between bedrock geology and discrete cluster membership stresses the importance of this variable and indicates that far distant sediment transport by sea ice icebergs or other mechanisms although occurring does not radically change the mineral composition of present day sea floor sediments our improved knowledge of the regional mineral assemblages allows for tracking distinct eastern greenland and icelandic sources

magnetotelluric studies in the epicenter zone of 2001 bhuj earthquake abstract some earthquakes to mw5 level are occurring along two faults viz kachchh mainland fault and south wagad fault near the epicenter of mw7 7 bhuj earthquake of 2001 these faults are not the causative fault of the 2001 earthquake and suspect for future large earthquake in absence of detail geophysical surveys the sub surface orientation of these faults was not known and mt profiles have proved useful in delineating the subsurface trends two mt profiles having lengths of 24 km and 15 km with an inter station spacing of 1 3 km are taken about 10 km east and west of the epicenter of mw7 7 2001 bhuj earthquake respectively the profiles are in nearly n s direction to detect the faults which trend e w in the area from 2 d inversion of mt data of 23 stations it is found that there are two distinctive conductive features matching with the south wagad fault and kachchh mainland fault south of it both being about 10 km apart the south wagad fault is seen extending to banni low lying area 30 km west of the present trace

geochronology isotopic chemistry and gold mineralization of the black slate hosted haoyaoerhudong gold deposit northern north china craton abstract the haoyaoerhudong gold deposit is the largest gold deposit in the north margin of the north china craton gold province and contains over 7 moz of gold at an average grade of 0 62 g t the deposit is hosted in the carbonaceous and pyritic slate phyllite and schist and is controlled by a tight syncline and shear zones the high grade orebodies contains abundant pyrite veins and pyrite quartz veins three stages of pyrite have been identified including the diagenetic disseminated pyrite pyrite veins caused by peak matamorphism and pyrite quartz veins forming during post peak metamorphism native gold has been observed in pyrite veins and pyrite quartz veins the 40ar 39ar plateau age of a biotite separate from a tails of boudinaged pyrite quartz veins is 260 1 2 9 ma combined with previously published 40ar 39ar mica age data and low closure temperature of mica these results suggest that pyrite veins and pyrite quartz veins were formed during the peak to post peak metamorphism during 285 260 ma respectively the lower limit of the formation age of sedimentary disseminated pyrites has constrained to 1670 1560 ma by the intruded mafic ultramafic dikes disseminated pyrite separates have 34s values ranging from 39 40 to 17 85 206pb 204pb of 19 144 21 892 207pb 204pb of 15 681 15 864 and 208pb 204pb of 37 502 38 925 suggesting they were formed from seawater sulfate and has experienced strong sulfur isotopic fractionation in contrast hydrothermal pyrites from pyrite veins and pyrite quartz veins have 34s values ranging from 6 8 to 16 47 206pb 204pb of 18 566 18 922 207pb 204pb of 15 645 15 684 and 208pb 204pb of 38 924 38 983 which may reflect dissolution reprecipitation of disseminated sulfides from the pre existing organic rich sediments the mineral paragenetic geometric and cross cutting relationships of pyrite veins and pyrite quartz veins at haoyaoerhudong suggest that gold was most likely introduced into pyrite accompanying sedimentation of the organic rich shales and then became enriched during diagenesis subsequently the hydrothermal fluids following metamorphism and shear zone activity make dissolution of the gold in the diagenetic pyrite and precipitated in the intersection of shear zone and tight syncline

re os dating of low level highly radiogenic llhr sulfides the harnas gold deposit southwest sweden records continental scale tectonic events to obtain geologically meaningful and precise results on low level sulfides that are highly radiogenic blank corrections that account for the extreme difference in the os isotope composition of sample and blank are necessary and the resulting data is best plotted in 187 re 187 os space much like the approach used for molybdenites we here introduce the acronym llhr as a means of characterizing low level and highly radiogenic sulfides use of the traditional 187 re 188 os vs 187 os 188 os plot for llhr sulfides leads to large analytical uncertainties and highly correlated errors as 188 os is poorly determined and can generally be accounted for by the blank we now recognize that llhr sulfides such as those encountered at the harnas au deposit are a common occurrence and characteristic of many crustally affiliated ore systems the harnas au deposit in southwest sweden is located in the mjosa vanern mineral belt that transects from northwest to southeast the prominent geologic fabric of the sveconorwegian province including the mylonite zone the deposit is hosted in a strongly sheared and locally brecciated sulfide bearing quartz vein that crosscuts the foliation of orthogneisses belonging to the 1600 maa mal granitoid suite a 187 re 187 os isochron of 973 34 ma 2 for samples of pyrite and chalcopyrite from outcrops of the harnas alma and silvergruvan veins was obtained the harnas and subsidiary alma veins are au bearing whereas the silvergruvan 2 5 km to the southeast is not of the au bearing type four replicate analyses of an additional pyrite sample from the harnas vein yield a model age of 616 40 ma 95 c l in all samples re and 187 os concentrations are extremely low 0 35 to 7 1 and 0 005 to 0 074 ppb respectively and total common os 0 002 0 028 ppb is less than half the calculated radiogenic 187 os two samples of galena from the harnas vein yielded blank level re and os neither the 973 nor 616 ma time of mineralization is related to development of the mylonite zone at 920 ma or intrusion of the anorogenic bohus blomskog granites at 920 to 900 ma previously proposed sources for generating mjosa vanern mineral belt ore fluids an age of 973 ma for harnas alma silvergruvan suggests that sulfide deposition was associated with the waning stages 0 97 0 90 ga of the sveconorwegian grenvillian orogeny 1 15 0 90 ga postdating metamorphism and deformation that produced a regionally overthickened crust in the northern part of the sveconorwegian province the vein systems for these and numerous other small deposits in the mjosa vanern mineral belt could have resulted from regional exhumation in the late sveconorwegian with associated extension and the development of fracture systems at cooler middle to upper crustal levels in the southern part of the sveconorwegian province dominated by granulitic facies rocks no mineralization is present perhaps due only to increased exhumation and erosion that has exposed deeper crustal levels the late sveconorwegian age 973 ma records the main period of sulfide deposition for the harnas alma silvergruvan vein systems and indicates the robustness of the chronometer within tectonically complex terranes the late neoproterozoic age 616 ma records a later time of pyrite deposition probably associated with shearing and reopening of the harnas vein system because the au mineralization at harnas has been described as associated with shearing and occurring in fracture fillings in early pyrite we suggest that part or all of the au mineralization could have been introduced at 616 ma during reactivation of the vein system the 616 ma age for pyrite suggests a previously unrecognized and much younger mineralizing episode in the harnas region most likely reflecting continental scale extension and the late neoproterozoic opening of the iapetus ocean

three dimensional tomography of the 1992 southern california earthquake sequence constraints on dynamic earthquake rupture tomographic inversion of p wave arrival times from aftershocks of 1992 southern california earthquakes is used to produce three dimensional images of subsurface velocity the preliminary 1992 data set augmented by the 1986 m 5 9 north palm springs sequence consists of 6458 high quality events recorded by the permanent regional network providing 76306 raypaths for inversion the target area consisted of a 104 x 104 x 32 km 3 volume divided into 52 x 52 x 10 rectilinear blocks significant velocity perturbations appear to correlate with rupture properties of recent major earthquakes preliminary results indicate that a low velocity anomaly separates the dynamic rupture of the m 6 5 big bear event from the m 7 4 landers main shock a similar low velocity region separates the m 6 1 joshua tree sequence from the landers rupture high velocity anomalies occur at or near nucleation sites of all four recent main shocks north palm springs joshua tree landersbig bear a high velocity anomaly is present along the san andreas fault between 5 and 12 km depth through san gorgonio pass this high velocity area may define an asperity where stress is concentrated and where future large earthquakes may begin

low coda qc in the epicentral region of the 2001 bhuj earthquake of mw 7 7 on 26 january 2001 03 46 55 ut a devastating intraplate earthquake of m w 7 7 occurred in a region about 5 km nw of bhachau gujarat 23 42 n 70 23 e the epicentral distribution of aftershocks defines a marked concentration along an e w trending and southerly dipping 45 zone covering an area of 60 40 km2 the presence of high seismicity including two earthquakes of magnitudes exceeding 7 7 in the 200 years is presumed to have caused a higher level of shallow crustal heterogeneity in the kutch area a site lying in the seismic zone v zone of the highest seismicity for potentially m8 earthquakes on the seismic zoning map of india attenuation property of the medium around the epicentral area of the bhuj earthquake covering a circular area of 61 500 km2 with a radius of 140 km is studied by estimating the coda q c from 200 local earthquakes of magnitudes varying from 3 0 4 6 the estimated q 0 values at locations in the aftershock zone high seismicity are found to be low in comparison to areas at a distance from it this can be attributed to the fact that seismic waves are highly scattered for paths through the seismically active and fractured zone but they are well behaved outside the aftershock zone distribution of q 0 values suggests that the local variation in q 0 values is probably controlled by local geology the estimated q 0 values at different stations suggest a low value of q 102 0 80 f 0 98 0 02 indicating an attenuative crust beneath the entire region the frequency dependent relation indicates a relatively low q c at lower frequencies 1 3 hz that can be attributed to the loss of energy due to scattering attenuation associated with heterogeneities and or intrinsic attenuation due to fluid movement in the fault zone and fluid filled cracks the large q c at higher frequencies may be related to the propagation of backscattered body waves through deeper parts of the lithosphere where less heterogeneity is expected based on the attenuation curve estimated for q 0 102 the ground acceleration at 240 km distance is 13 of 1 g i e 0 13 g agreeing well with the ground acceleration recorded by an accelerograph at ahmedabad 0 11 g hence it is inferred that the q 0 value obtained from this study seems to be apt for prediction of ground motion for the region

sinking mafic body in a reactivated lower crust a mechanism for stress concentration at the new madrid seismic zone we propose a geodynamic model for stress concentration in the new madrid seismic zone nmsz the model postulates that a high density mafic body situated in the deep crust directly beneath the most seismically active part of the nmsz began sinking several thousands of years ago when the lower crust was suddenly weakened based on the fact that deformation rates in the nmsz have accelerated over the past 9 k y we envision the source of this perturbation to be related to the last north american deglaciation excess mass of the mafic body exerts a downward pull on the elastic upper crust leading to a cycle of primary thrust faulting with secondary strike slip faulting after which continued sinking of the mafic body reloads the upper crust and renews the process this model is consistent with the youth of activity the generation of a sequence of earthquakes and the velocity evolution during interseismic periods which depend upon the density contrast of the mafic body with respect to the surrounding crust its volume and the viscosity of the lower crust

seismogenesis of the lower crustal intraplate earthquakes occurring in kachchh gujarat india abstract large intraplate continental earthquakes like the 1811 12 new madrid m w 8 0 and the 2001 bhuj mw7 7 were highly destructive because they occurred in strong crust but the mechanisms underlying their seismogenesis are not understood here we show using local earthquake velocity tomography and joint inversion of receiver functions and surface wave group velocity dispersion that the crust and uppermost mantle beneath the 2001 bhuj earthquake region of western india is far more complex than hitherto known through previous studies a new image of the crust and underlying mantle lithosphere indicates the presence of a 18 km thick high velocity vp 7 15 8 11 km s differentiated crustal and mantle magmatic layer above a hot and thin lithosphere only 70 km in the epicentral region of 2001 bhuj earthquake this magmatic layer begins at the depth of 24 km and continues down to 42 km depth below this region brittle ductile transition reaches as deep as the moho 34 km due to the possible presence of olivine rich mafic magma our 1 d velocity structure envisages an initial phase of plume activity deccan plume at 65 m y ago resulting in basaltic magma in the eclogitic layers at sub lithospheric levels wherein they were subjected to crystallization under ultra high pressure conditions our study also delineates an updoming of moho 4 7 km as well as asthenosphere 6 10 km below the kachchh rift zone relative to surrounding areas suggesting the presence of a confined body of partial melts below the lithosphere asthenosphere boundary restructuring of this warm and thin lithosphere may have been caused due to rifting at 184 and 88 m y ago and tholeiitic and alkalic volcanism related to the deccan traps k t boundary event at 65 m y ago recent study of isotopic ratios proposed that the alkalic basalts found in kachchh are generated from a co 2 rich lherzolite partial melts in the asthenosphere that ascended along deep lithospheric rift faults into the lithosphere it appears that such kind of crust mantle structure deepening of brittle ductile transition and a high input of volatiles containing co 2 emanating from mantle control the seismogenesis of lower crustal earthquakes in the kachchh continental rift zone

results from local earthquake velocity tomography implications toward the source process involved in generating the 2001 bhuj earthquake in the lower crust beneath kachchh india to comprehend the source processes of the 26 january 2001 bhuj earthquake sequence of m w 7 7 and its influence on the seismic hazard of the kachchh we estimate various seismological parameters using reliable and accurate aftershock data the estimated parameters led to several important findings including the delineation of an east west trending south dipping 45 fault north wagad fault nwf which touches the surface about 25 km north of kachchh mainland fault kmf the aftershock zone is confined to a 60 km long and 40 km wide region lying between the kmf to the south and nwf to the north extending from 10 to 45 km depth focal mechanism solutions of the mainshock and 25 significant aftershocks of m w 3 0 obtained from waveform inversion of broadband data and local earthquake moment tensor inversion suggest that the region between the kmf and nwf is mainly characterized by reverse faults with east west trend and southerly dip matching with the geological faults in the region the tomographic inversion technique is used to invert 5516 p wave travel times and 4061 s p travel time differences from 600 aftershocks recorded at 8 18 stations tomographic results suggest a regional high velocity body characterized by high v p 7 0 8 5 km sec high v s 4 0 4 8 km sec and low 0 24 0 26 with a head extending 60 km in north south and 40 km in east west at 10 40 km depths this high velocity anomaly is inferred to be a mafic pluton rift pillow which might have intruded during the rifting time 135 ma this crustal mafic pluton must be contributing significantly in accumulating large crustal stresses resulting in the generation of large earthquakes in this intraplate area another important result of our study is the detection of a low velocity zone low v p 6 5 7 0 km sec low v s 3 6 4 0 km sec large 0 26 0 265 within the mafic body at the hypocentral depth of the mainshock 18 25 km which is inferred to be a fluid filled trapped aqueous fluid resulting from metamorphism fractured rock mass the analysis of depth distribution of b values suggests a high b value zone between 15 and 25 km depths which further supports this contention hence the presence of fluids at the hypocenter might have facilitated the occurrence of the 2001 bhuj earthquake within the inferred mafic body in the lower crust

transient electromagnetic investigations in a tectonic domain of the kachchh intraplate region western india a morphotectonic study of the kachchh mainland fault

deep faulting and structural reactivation beneath the southern illinois basin abstract the investigation of deep fault structure and seismogenesis within stable continental interiors has been hindered by the paucity of detailed subsurface information and by low levels of seismicity outstanding seismotectonic questions for these areas include whether pre existing structures govern the release of seismic energy as earthquakes can reactivation of such structures be recognized and to what extent have precambrian basement structures exerted long lived controls on the development of overlying phanerozoic features the southern portion of the illinois basin provides a premier area in which to study the relation between contemporary seismicity and pre existing structures due to the frequency of seismic events the concentration of available geophysical data and the wealth of borehole information we have integrated the study of this information in order to create a 2 5 dimensional picture of the earth for local seismogenic depths 0 15 km for a study area of moderate 20th century earthquake activity the area is located along the western flanks of two of the major structures within the illinois basin the wabash valley fault system wvfs and the la salle anticlinal belt lsa the results of reprocessing seismic reflection profiles combined with earthquake hypocenter parameters suggest three distinct seismotectonic environments in the upper crust first we have delineated a fault pattern that appears to correspond to the steep nodal plane of a strike slip mechanism event 1974 04 03 mb 4 7 the fault pattern is interpreted to be a deeply buried rift zone or zone of intense normal faulting underpinning a major paleozoic depocenter of the illinois basin fairfield basin second a similar event 1987 06 10 mb 5 2 and its well located aftershocks define a narrow zone of deformation that occurs along and parallel to the frontal thrust of the lsa third the hypocenter of the largest event in the study area 1968 11 09 mb 5 5 may be spatially associated with a prominent zone of dipping middle crustal reflections just west of the wvfs which have been interpreted as a deeply buried blind thrust the proposed correlation of pre existing structures with earthquakes having consistently oriented structural parameters supports the reactivation of old deformation zones by contemporary stresses as previously proposed by earlier workers however the degree to which deformation has propagated upward from precambrian basement into the paleozoic rocks varied significantly even over a small study area the societal value of associating an earthquake with a specific pre existing deformation zone in the seismogenic crust is to improve the assessment of seismic hazard or to assess the integrity of a stratigraphic formation being considered as a target for natural gas storage or carbon sequestration

rb sr geochronology and geochemistry of the xiaotongchang basalt hosted copper deposit in the jinping area sw china abstract the xiaotongchang basalt hosted copper deposit in jinping southern yunnan province sw china is located in the southern part of the emeishan large igneous province here we use rb sr re os and s pb c o isotope data to constrain the age and genesis of the deposit petrological investigations reveal two main stages of mineralization the early stage is characterized by disseminated or massive sulfides in an assemblage of pyrite chalcopyrite quartz albite calcite rb sr isotopic dating of early stage chalcopyrite yields an age of 256 9 4 ma mswd 5 4 which is slightly younger than the eruption age of the jinping basalts the 34 s cdt values of the sulfides range from 5 8 to 7 7 suggesting that the metals of the early stage copper mineralization originated from the emeishan basalts and that the ore forming fluids were contaminated by crustal rocks these units may indicate that the early stage orebodies represents syngenetic hydrothermal mineralization related to the eruption of emeishan flood basalts the late stage mineralization is characterized by a pyrite chalcopyrite bornite chalcocite quartz calcite assemblage that is developed in veins or breccia the re os isotopic dating of late stage chalcopyrite samples reveals that they formed at 240 231 ma the pb isotopic compositions 206 pb 204 pb t 17 521 19 227 207 pb 204 pb t 15 480 15 760 and 208 pb 204 pb t 37 778 38 731 t 235 ma show that the late stage copper orebodies have a heterogeneous source and originated mainly from the crust the late stage sulfides have higher 34 s cdt values 7 9 to 10 7 than the early stage sulfides the narrow range of sulfur isotopic compositions of the sulfides associated with both stages and the pb isotopic compositions of late stage chalcopyrite indicate that some of the metals associated with late stage mineralization originated from the jinping basalts the c o isotopic compositions of calcite samples show that the magmatic hydrothermal fluids in the late stage flowed through and were contaminated by the maokou limestone therefore we suggest that the late stage orebodies represent magmatic hydrothermal mineralization associated with the late triassic rhyolite porphyry and that the tectono magmatic hydrothermal fluid remobilized copper from the basalts the copper deposits formed at favourable interfaces within the basalt stratigraphy or in favorable structural locations

upper mantle seismic anisotropy in the intra continental kachchh rift zone gujarat india abstract shear wave splitting study of 411 sks skks phases covering backazimuth range of 13 to 305 recorded by 12 broadband stations in the kachchh rift has led to estimates of fast axis orientations and splitting times for 118 good measurements the average vector mean of fast axis orientation 86 14 corresponds to the e w axis of the kachchh rift and the delay time 1 6 s is attributed to the 184 km thick upper mantle layer with 4 anisotropy the anisotropic character observed for the kachchh rift kr is comparable to other continental rifts and these are related to the high temperature lattice preferred orientation fabric of olivine inherited from the mantle flows the source of the rift axis parallel anisotropy is traced to the rift parallel flows within the 76 6 km thick lithosphere additionally the rift parallel pockets of partial melts also induce anisotropy within the asthenosphere both these are inherited from the plume lithosphere interaction during the deccan reunion plume episode 65 ma

magnetotellurics study to identify subsurface resistivity structure in the eastern part of kachchh little rann area of gujarat india a magnetotelluric mt survey has been conducted in the eastern part of the kachchh region of gujarat state crossing little rann starting from adesar village in the north to enjar village in the south where limited geophysical data is available the profile has been acquired in the n s direction having a total length of 52 km with the interstation spacing of 3 4 km from two dimensional 2d inversion of the mt data four conductive zones are identified the analysis reveals that the depth of the basement varies from 1 2 km in the south to 1 8 km in the north the first conductive zone is found at a distance of 3 km in the south of adesar and is identified at the location of the kanmer fault kf the eastern extension of south wagad fault swf and might be the kf the second conductive zone is found at a distance of 42 km in the south of adesar and 13 km north of enjar village and is located at the contact zone of kachchh and saurashtra peninsula at the location of the north kathiawar fault nkf it is inferred as the nkf the third conductive zone at a distance of 27 km in the south of adesar is found between the first and second conductor this is interpreted as a step fault formed during the rifting process of kachchh the lower crust below 20 km is found conductive 120 m and might indicate the presence of fluid filled mafic ultramafic rocks in the lower crust

three dimensional p and s wave velocity structure of redoubt volcano alaska the three dimensional p and s wave structure of redoubt volcano alaska and the underlying crust to depths of 7 8 km is determined from 6219 p wave and 4008 s wave first arrival times recorded by a 30 station seismograph network deployed on and around the volcano first arrival times are calculated using a finite difference technique which allows for flexible parameterization of the slowness model and easy inclusion of topography and source receiver geometry the three dimensional p wave velocity structure and hypocenters are determined simultaneously while the three dimensional s wave velocity model is determined using the relocated seismicity and an initial s wave velocity model derived from the p wave velocity model assuming an average vp vs ratio of 1 78 convergence is steady with approximately 73 and 52 reduction in p and s wave arrival time rms respectively after 10 iterations the most prominent feature observed in the three dimensional velocity models derived for both p and s waves is a relative low velocity near vertical pipelike structure approximately 1 km in diameter that extends from 1 to 6 km beneath sea level this feature aligns axially with the bulk of seismicity and is interpreted as a highly fractured and altered zone encompassing a magma conduit the velocity structure beneath the north flank of the volcano between depths of 1 and 6 km is characterized by large lateral velocity variations high velocities within this region are interpreted as remnant dikes and sills and low velocities as regions along which magma migrates no large low velocity body suggestive of a magma chamber is resolved in the upper 7 8 km of the crust

crustal and lithospheric thinning beneath the seismogenic kachchh rift zone gujarat india its implications toward the generation of the 2001 bhuj earthquake sequence a high resolution passive seismic experiment in the kachchh rift zone of the western india has produced an excellent dataset of several thousands teleseismic events from this network 500 good teleseismic events recorded at 14 mobile broadband sites are used to estimate receiver functions for the 30 310 back azimuth ranges which show a positive phase at 4 5 6 1 s delay time and a strong negative phase at 8 0 11 0 s these phases have been modeled by a velocity increase at moho i e 34 43 km and a velocity decrease at 62 92 km depth the estimation of crustal and lithospheric thicknesses using the inversion of stacked radial receiver functions led to the delineation of a marked thinning of 3 7 km in crustal thickness and 6 14 km in lithospheric thickness beneath the central rift zone relative to the surrounding un rifted parts of the kachchh rift zone on an average the kachchh region is characterized by a thin lithosphere of 75 9 5 9 km the marked velocity decrease associated with the lithosphere asthenoshere boundary lab observed over an area of 120 km 80 km and the isotropic study of xenoliths from kachchh provides evidence for local asthenospheric updoming with pockets of partial melts of co2 rich lherzolite beneath the kachchh seismic zone that might have caused by rifting episode at 88 ma and the associated deccan thermal plume interaction at 65 ma episodes thus the coincidence of the area of the major aftershock activity and the moho as well as asthenospheric upwarping beneath the central kachchh rift zone suggests that these pockets of co2 rich lherzolite partial melts could perhaps provide a high input of volatiles containing co2 into the lower crust which might contribute significantly in the seismo genesis of continued aftershock activity in the region it is also inferred that large stresses in the denser and stronger lower crust at 14 34 km depths induced by ongoing banni upliftment crustal intrusive marked lateral variation in crustal thickness and related sub crustal thermal anomaly play a key role in nucleating the lower crustal earthquakes beneath the kachchh seismic zone

variability of surface water characteristics and heinrich like events in the pleistocene midlatitude north atlantic ocean biomarker and xrd records from iodp site u1313 mis 16 9 1 a reconstruction of milankovitch to millennial scale variability of sea surface temperature sst and sea surface productivity in the pleistocene midlatitude north atlantic ocean marine isotope stage mis 16 9 and its relationship to ice sheet instability was carried out on sediments from integrated ocean drilling program iodp site u1313 this reconstruction is based on alkenone and n alkane concentrations u37k index total organic carbon toc and carbonate contents x ray diffraction data magnetic susceptibility and accumulation rates increased input of ice rafted debris occurred during mis 16 12 and 10 characterized by high concentrations of dolomite quartz and feldspars and elevated accumulation rates of terrigenous matter minimum input values of terrigenous matter on the other hand were determined for mis 13 and 11 peak values of dolomite coinciding with quartz plagioclase and kalifeldspar peaks and maxima in long chain n alkanes indicative for land plants are interpreted as heinrich like events related to sudden instability of the laurentide ice sheet during early and late deglacial phases of the glacials the coincidence of increased toc values with elevated absolute concentrations of alkenones suggests increased glacial productivity probably due to a more southern position of the polar front alkenone based sst reached absolute maxima of about 19 c during mis 11 3 and absolute minima of 10 c during mis 12 and 10 within mis 11 prominent cooling events mis 11 22 and 11 24 occurred the absolute sst minima recorded directly before and after the glacial maxima mis 10 2 and 12 2 are related to heinrich like event meltwater pulses as supported by the coincidence of sst minima and maxima in c37 4 alkenones and dolomite these sudden meltwater pulses especially during terminations iv and v probably caused a collapse of phytoplankton productivity as indicated by the distinct drop in alkenone concentrations ice sheet disintegration and subsequent surges and outbursts of icebergs and meltwater discharge may have been triggered by increased insolation in the northern high latitudes

multi stage enrichment processes for large gold bearing ore deposits abstract a review of previous studies of the world s large hydrothermal gold deposits indicates that the largest deposits tend to show complicated parageneses where multiple gold enrichment events and processes have been involved in the deposit generation these observations suggest that multistage processes may even be a requirement for the formation of large deposits in some deposits e g witwatersrand boddington cadia sukhoi log or carlin the different enrichment processes occur millions of years apart in others such as many large porphyry deposits the different stages are much closer in time in many deposits particularly sedimentary hosted deposits early diffuse enrichment occurs within a particular province that is then upgraded by more focused processes e g sukhoi log kalgoorlie the presence of this early diffuse enrichment could explain the tendency for gold deposits to cluster into camps this model has important implications as the presence or absence of multiple gold events could be used to discriminate at the exploration and feasibility stages between small deposits with single stage ore genesis and more complicated deposits with multistage enrichment and the potential for larger gold endowment

earthquake depth distributions in central asia and their relations with lithosphere thickness shortening and extension this paper examines the relationship between seismogenic thickness lithosphere structure and rheology in central and northeastern asia we accurately determine earthquake depth distributions which reveal important rheological variations in the lower crust these variations exert a fundamental control on the active tectonics and the morphological evolution of the continents we consider 323 earthquakes across the tibetan plateau the tien shan and their forelands as well as the baikal rift ne siberia and the laptev sea and present the source parameters of 94 of these here for the first time these parameters have been determined through body wave inversion the identification of depth phases or the modelling of regional waveforms lower crustal earthquakes are found to be restricted to the forelands in areas undergoing shortening and to locations where rifting coincides with abrupt changes in lithosphere thickness such as the ne baikal rift and w laptev sea the lower crust in these areas is seismogenic at temperatures that may be as high as 600 c suggesting that it is anhydrous and is likely to have great long term strength lower crustal earthquakes are therefore a useful proxy indicating strong lithosphere in places that are too small in areal extent for this to be confirmed independently by estimating effective elastic thickness from gravity topography relations the variation in crustal rheology indicated by the distribution of lower crustal earthquakes has many implications ranging from the support of mountain belts and the formation of steep mountain fronts to the localization and orientation of rifting in combination these processes can also be responsible for the separation of the front of the thin skinned mountain belts from their hinterlands when continents separate

multistage sedimentary and metamorphic origin of pyrite and gold in the giant sukhoi log deposit lena gold province russia gold mineralization at sukhoi log in eastern siberia is hosted in a deformed neoproterozoic organic bearing and pyritic black shale and siltstone sequence that is folded into a tight overturned anticline the deposit contains about 30 million ounces of gold at an average grade of 2 0 g t au and is one of the largest known undeveloped gold resources the high grade gold zone forms a gently dipping tabular body in the core of the anticline the best gold grades occur in narrow bedding parallel pyrite quartz veinlets that have been folded during the main deformation event lower grade gold is associated with disseminated pyrite developed in and around the high grade core of the deposit detailed paragenetic studies of the mineralization and host rocks have defined six stages of pyrite development in the carbonaceous sediments the two earliest forms of pyrite termed py1 and py2 are commonly developed in stratiform layers of micron sized crystals framboids and fine euhedra which are interpreted as synsedimentary to early diagenetic in origin coarser grained bedding parallel aggregates of inclusion rich pyrite termed py3 contain inclusions of arsenopyrite native gold and gold tellurides and are interpreted to form during late diagenesis and earliest deformation coarse euhedral pyrite py4 overgrows the earlier pyrite py1 py2 and py3 and the slaty cleavage developed in the host rocks indicating a syndeformation timing late stage inclusion free pyrite py5 overgrows and replaces earlier sulfides and is considered to be syn to late deformation laser ablation inductively coupled plasma mass spectrometry la icpms analyses of the various pyrite types indicate that the synsedimentary py1 contains the highest levels of invisible gold varying from 0 4 to 12 1 ppm with a mean of 3 22 ppm au and 1 900 ppm as py1 is also enriched in a suite of trace elements mo sb ni co se te ag cu pb zn mn ba cr u v which are similar to those concentrated by organic processes in euxinic sedimentary environments later generations of pyrite from py2 to py5 including pyrite in bedding parallel pyrite quartz veinlets contain progressively lower contents of invisible gold and most other trace elements however this metamorphic and postmetamorphic pyrite contains microinclusions of free gold arsenopyrite pyrrhotite sphalerite and chalcopyrite the paragenetic textural and chemical relationships at sukhoi log suggest that gold was clearly initially introduced prior to cleavage development accompanying sedimentation of the organic rich shales and fixed during diagenesis within the structure of diagenetic arsenian pyrite subsequently accompanying deformation gold was liberated from recrystallized diagenetic pyrite to become concentrated as free gold and gold tellurides within metamorphic pyrite and folded bedding parallel pyrite quartz veinlets two key processes are considered vital to the formation of the sukhoi log deposit original synsedimentary and early diagenetic concentration of gold dissolved within arsenian pyrite in organic rich black shales and metamorphic processes that liberated gold from the early forms of arsenian pyrite to be concentrated as free gold and gold tellurides within late diagenetic and metamorphic pyrite and associated pyrite quartz veinlets in the core of an overturned anticline these ore forming processes are unlikely to be unique to sukhoi log other black shale and turbidite hosted deposits that occur in rifted continental margin environments which have undergone collision and basin inversion may form by similar processes

stress in the indo australian plate cloetingh s and wortel r 1986 stress in the ind austr ian plate in b johnson and a w bally editors intraplate deformation characteristics processes and causes tectonophysics 132 49 67 we modelled the state of stress in the indo australian plate in order to investigate quantitatively variations observed in tectonic style the numerical procedure incorporates the dependence of slab pull and ridge push on the age of the oceanic lithosphere estimates are presented for the average net resistive forces at the himalayan collision zone the suction force acting on the overriding indo australian plate segment at the tonga kermadec trench and the drag at the base of the lithosphere our modelhng shows a concen ation of compressive stresses of the order of 3 5 lcbar in the ninetyeast ridge area the effects of the compressive resistance associated with himalayan collision and subduction of young lithosphere off the northern part of the sunda arc are focused in this region the stress field as calculated gives a consistent explanation for the observed concentration of seismic activity stein and okal 1978 and significant deformation in the oceanic crust weissel et al 1980 mcadoo and sandwell 1985 in the area the calculated stress field in the area adjacent to the southeast and central india ridges is characterized by tension parallel to the spreading axis this explains the concentration of near ridge normal faulting seismicity with t axes subparaile1 to the spreading ridge in the indian ocean as recently observed by bergman et al 1984 and wiens and stein 1984 the regional stress field along the strike of the sunda arc varies from compression seaward of and parallel to the sumatra trench segment to tension perpendicular to the java flores segment this explains the selective occurrence of well developed grabens seaward off the java flores segment of the trench observed by hilde 1983 our modelling shows that the observed rotation of the stress field denham et al 1979 in the australian continent is mainly the consequence of its geographic position relative to the surrounding trench segments and the variations of the forces acting on the down going slab in each of these the state of compression in west and central australia is induced by the action of resistive forces at the himalayan and banda arc collision zones the joint occurrence of high levels of compression in the plate s interior and normal faulting seismicity in the near ridge areas is a transient feature unique to the present dynamic situation of the indo australian plate

coulomb static stress variations in the kachchh gujarat india implications for the occurrences of two recent earthquakes mw 5 6 in the 2001 bhuj earthquake region summary double difference algorithm was used to relocate aftershock sequences of two mw 5 6 kachchh earthquakes india which have occurred on 2006 march 7 and april 6 along almost vertical strike slip gedi fault gf and south dipping reverse north wagad fault nwf respectively the relocated focal depths delineate a marked variation of 4 and 7 km in the brittle ductile transition depths beneath gf and nwf respectively we modelled the coulomb failure stress change cfs produced by the 2001 mainshock and 2006 gedi events on an optimally oriented plane a strong correlation with occurrences of aftershocks and regions of increased cfs is obtained on both the faults the predicted cfs on the gf increased by 0 14 mpa at 3 km depth where the 2006 march 7 mw 5 6 event occurred the variation in brittle ductile transition depths may be related to the perturbation in geothermal gradient resulted from the presence of intrusives or varying saturation state of fluid filled fractured rock matrix beneath both the fault zones

estimation of sedimentary thickness in kachchh basin gujarat using sp converted phase an inexpensive method using natural earthquake data is utilized for determining the sedimentary thickness in kachchh the institute of seismological research isr is operating a network of broadband seismographs and strong motion accelerographs in gujarat we used data from 13 broadband seismographs and two strong motion accelerographs in the study the stations are within 5 to 80 km from the epicenters in this study the s to p converted phase sp is used this phase is generated due to large impedance contrast between sediments and basement this phase is clear in the vertical component the difference in the travel times of s and sp phases and velocities of p and s waves is used for determining the sedimentary layer thickness the thickness of sediments beneath each of these 15 stations was determined covering an area of 23 500 sq km

the sources of mineralizing fluids of orogenic gold deposits of the baikal patom and muya areas siberia constraints from the c and n stable isotope compositions of fluid inclusions abstract the compositions of fluid inclusions in quartz and sphalerite from five gold deposits of the baikal patom sukhoi log verninsk and dogaldyn and baikal muya uryakh and irokinda foldbelts in the northern margin of the central asian orogenic belt along its boundary with the siberian craton indicate that the gold mineralization was formed predominantly from heterogeneous co2 h2o orogenic fluid all deposits are structurally controlled and composed of veinlet disseminated and vein to stockwork types of ores that are hosted by rocks that underwent regional metamorphism to the greenschist facies in baikal patom and up to the granulite facies in baikal muya the gold quartz veins of all the baikal patom deposits and the uryakh were formed by fluids of similar composition with salinity of 1 4 9 5 wt nacl equiv and co2 content of 1 4 8 6 mol kg of solution under comparable physical conditions at temperatures of 128 385 c and pressures of 570 3290 bar fluids of the granulate hosted irokinda deposit are distinguished for the highest temperatures and pressures and involve relics of medium to high temperature 179 453 c brines with salinity of 4 6 46 3 wt nacl equiv the temperature and pressure of the mineralizing fluid increase roughly from north to south which correlates with the increase in the metamorphic grade of the host rocks the increase is accompanied by the simultaneous systematic shift in the c isotope composition of co2 released from the quartz by the chrush leach method from 1 9 to 5 5 13cco2 in the baikal patom and uryakh from 0 6 to 0 7 13cco2 at irokinda the c and n isotope compositions of co2 suggest that irokinda fluids were partly derived from decarbonated marine limestone whereas the orogenic gold bearing fluids of the other deposits are interpreted to have been composed mostly of fluid from a crustal magmatic granitic source

long and short wavelengths of indian ocean geoid and gravity lows mid to upper mantle sources rapid drift and seismicity of kachchh and shillong plateau india abstract spectral analysis of the indian ocean geoid low provides depth to the large wavelength sources as 1300 700 and 340 km that are supported from the spectral analysis and the modeling of the corresponding large wavelength regional gravity anomaly with negative density contrasts at these levels the three levels coincide with the sharp changes in the gradient of the seismic velocities related to the olivine spinel transformation of successively increasing fe mg ratio as depth increases known as transition zones the first two segments are supported from continuous wavelet transform analysis of the large wavelength component of the corresponding gravity field the low density rocks in this section appear to be related to the subducted indian tethyan lithosphere that roll back and drifted southwards after subduction as inferred from tomography experiments the relatively short wavelength sources of the spectrum of the geoid data at depths of 162 and 85 km suggest sources along the lithosphere asthenosphere boundary lab under the indian continent and surrounding oceans respectively a low viscosity zone has been envisaged below 660 km discontinuity that may correspond to the low density rocks in this section which is popularly referred to as graveyards of the subducted rocks under geoid lows the subducted slab is hydrated due to dehydration from metamorphism that causes upwelling in the mid to upper mantle which is likely to set in baby plumes presence of fluid may transform part of olivine to serpentine further reducing the bulk density of rocks in this section they would make this region buoyant that appears to be responsible for the central indian ocean deformation zone with large scale folding faulting seismicity and high heat flow the same also appear to be responsible for the rapid drift of the indian plate short wavelength component of the indian ocean geoid low reflects most of the surface shallow tectonics of the region similar to the gravity anomaly providing an additional data set for this purpose kachchh and the shillong plateau inspite of being in the intra plate region are highly seismogenic that compare almost to the activity along the plate boundaries tectonics and residual geoid anomalies of the indian continent and adjoining regions suggest that the geoid highs of kachchh and the shillong plateau along the nw and the ne corners of india are connected to the geoid highs of the plate boundaries through several lineaments and faults that also show geoid highs and trends of seismic activity leading to plate boundaries geoid highs indicating high density mafic rocks suggest their connections extensions to plate boundaries indicating them to be plausibly part of diffused plate boundaries besides both these regions are affected by i prominent lithospheric flexure of the indian plate ii large scale mafic intrusions and iii interaction of proterozoic mobile belts with effects of present day himalayan orogeny that make them more vulnerable for seismic activity

a possible origin of intraplate earthquakes in the kachchh rift zone india since the 2001 mw7 7 bhuj earthquake abstract we herein use the joint inversion of p receiver functions and fundamental mode group velocity dispersion data 9 70 s of rayleigh and love waves to estimate crustal and lithospheric thicknesses at twenty three component mobile broadband stations in kachchh gujarat modeled moho depths range from 35 to 43 km while lithospheric thicknesses vary from 64 to 106 km the main result of our modelling is the delineation of a marked crustal 2 4 km as well as lithospheric 10 20 km thinning and a 2 6 drop in vs across the lithosphere asthenosphere boundary lab within the samkhiali graben associated with gravity high underlying the central kachchh rift zone krz where 95 of the continued aftershock activity took place since 2001 such a large drop in vs could be attributed to the presence of carbonatite melts in the upper mantle our modeling reveals a 4 km crustal thinning below the central krz and a 4 km crustal thickening below the surrounding riftless regions this kind of crustal structure is inferred to induce large flexural deviatoric stresses 50 100 mpa in the upper crust thereby these stresses in the presence of regional plate tectonic stresses can bring the samkhiali graben below the central krz near to the critical stress level while stress transfer meteoric water in the upper crust and metamorphic fluid as well volatile co2 flows in the lower crust provide the required triggering effect to the critically stressed graben structure down to 35 km depth for generating continued aftershock activity in the kachchh rift zone since 2001 we also propose that the deeper circulation of volatile co2 through the inferred conduit related to the 65 ma deccan plume activity extending from lower crust down to asthenosphere plays a key role in the generation of uninterrupted occurrence of earthquakes in the kachchh rift zone gujarat india

quantitative mineralogy of the yukon river system changes with reach and season and determining sediment provenance the mineralogy of yukon river basin sediment has been studied by quantitative x ray diffraction bed beach bar and suspended sediments were analyzed using the rockjock computer program the bed sediments were collected from the main stem and from selected tributaries during a single trip down river from whitehorse to the yukon river delta during the summer of 2001 beach and bar sediments were collected from the confluence region of the tanana and yukon rivers during the summer of 2003 suspended sediments were collected at three stations on the yukon river and from a single station on the tanana river at various times during the summers of 2001 through 2003 with the most complete set of samples collected during the summer of 2002 changes in mineralogy of yukon river bed sediments are related to sediment dilution or concentration effects from tributary sediment and to chemical weathering during transport carbonate minerals compose about 2 wt of the bed sediments near whitehorse but increase to 14 wt with the entry of the white river tributary above dawson thereafter the proportion of carbonate minerals decreases downstream to values of about 1 to 7 wt near the mouth of the yukon river quartz and feldspar contents of bed sediments vary greatly with the introduction of pelly river and white river sediments but thereafter either increase irregularly quartz from 20 to about 50 wt or remain relatively constant feldspar at about 35 wt with distance downstream clay mineral content increases irregularly downstream from about 15 to about 30 wt the chief clay mineral is chlorite followed by illite smectite there is little to no kaolinite the total organic carbon content of the bed sediments remains relatively constant with distance for the main stem generally 1 to 2 wt with one exception but fluctuates for the tributaries 1 to 6 wt the mineralogies of the suspended sediments and sediment flow data were used to calculate the amount of mineral dissolution during transport between eagle and pilot station a distance of over 2000 km we estimate that approximately 3 wt of the quartz 15 wt of the feldspar 1 wt of the alkali and 25 wt of the plagioclase and 26 wt of the carbonates 31 wt of the calcite and 15 wt of the dolomite carried by the river dissolve in this reach the mineralogies of the suspended sediments change with the season for example during the summer of 2002 the quartz content varied by 20 wt with a minimum in mid summer the calcite content varied by a similar amount and had a maximum corresponding to the quartz minimum these modes are related to the relative amount of sediment flowing from the white river system which is relatively poor in quartz but rich in carbonate minerals suspended total clay minerals varied by as much as 25 wt with maxima in mid july and suspended feldspar varied up to 10 wt suspended sediment data from the summers of 2001 and 2003 support the 2002 trends a calculation technique was developed to determine the proportion of various sediment sources in a mixed sediment by unmixing its quantitative mineralogy results from this method indicate that at least three sediment sources can be identified quantitatively with good accuracy with this technique sediment mineralogies can be used to calculate the relative flux of sediment from different tributaries thereby identifying sediment provenance

sedimentary and crustal structure beneath kachchh and saurashtra regions gujarat india abstract strong and distinct ps conversions from moho and top sediments characterize the radial receiver functions rf estimated for stations in kachchh basin while these for stations in saurashtra are characterized by a strong ps conversion from moho and a weak or absence of ps conversion from top sediments the jumping inversion of stacked radial rfs suggest that 1 3 km thick top low velocity sediments v p 1 8 2 5 km s 0 28 0 34 and a 39 48 km thick crust 0 25 thicker than average precambrian crust characterize the kachchh basin area while the crust beneath saurashtra is characterized by a 0 1 km thick moderate velocity sediments v p 2 0 3 56 km s 0 25 0 28 and a north dipping shallow moho 35 42 km 0 248 the estimated average crustal values from the arrival times of pppms and ppsms pspms phases suggest a relatively stronger crust in kachchh 0 22 0 27 in comparison to the crust in saurashtra 0 18 0 19 in the epicentral zone of 2001 bhuj earthquake the estimated moho thickness reveals a moho upwarping 3 5 km change in the e w direction and a 6 9 km change in the n s direction beneath the epicentral zone of 2001 bhuj earthquake which might have induced large intraplate stresses to cause the m w 7 7 earthquake at 23 km depth

influence of deccan volcanism synrift magmatism on the crust mantle structure and its implications for the seismogenesis of earthquakes occurring in the kachchh rift zone

occurrences of large magnitude earthquakes in the kachchh region gujarat western india tectonic implications abstract moderate to large damaging earthquakes in the peninsular part of the indian plate do not support the long standing belief of the seismic stability of this region the historical record shows that about 15 damaging earthquakes with magnitudes from 5 5 to 8 0 occurred in the indian peninsula most of these events were associated with the old rift systems our analysis of the 2001 bhuj earthquake and its 12 year aftershock sequence indicates a seismic zone bound by two linear trends nnw and nne that intersect an e w trending graben the bouguer gravity values near the epicentre of the bhuj earthquake are relatively low 2 mgal the gravity anomaly maps the distribution of earthquake epicentres and the crustal strain rate patterns indicate that the 2001 bhuj earthquake occurred along a fault within strain hardened mid crustal rocks the collision resistance between the indian plate and the eurasian plate along the himalayas and anticlockwise rotation of the indian plate provide the far field stresses that concentrate within a fault bounded block close to the western margin of the indian plate and is periodically released during earthquakes such as the 2001 m w 7 7 bhuj earthquake we propose that the moderate to large magnitude earthquakes in the deeper crust in this area occur along faults associated with old rift systems that are reactivated in a strain hardened environment

weak mantle in nw india probed by geodetic measurements following the 2001 bhuj earthquake abstract far reaching transient surface deformation following the 2001 mw 7 6 bhuj intraplate earthquake in nw india reveals visco elastic flow in the mantle with only modest contributions from crustal relaxation processes the relatively rapid decay of gps measured deformation rates indicates increasing effective viscosities of the mantle from 3 1018 pa s in the first 6 months to 2 1019 pa s during the 6 year observation period consistent with a time and stress dependent rheology such as power law flow by dislocation creep the observed data do not require relaxation of the lower crust over these time scales and indicate a lower bound of 1020 pa s on its effective viscosity the unusually low viscous strength of the mantle below the earthquake epicentral region may be the long lasting result of thermal weakening by the late cretaceous deccan plume and may be responsible for the unusually active intraplate seismicity in the region

stress perturbation associated with the amazonas and other ancient continental rifts the state of stress in the vicinity of old continental rifts is examined to investigate the possibility that crustal structure associated with ancient rifts specifically a dense rift pillow in the lower crust may modify substantially the regional stress field both shallow 2 0 2 6 km depth breakout data and deep 20 45 km depth crustal earthquake focal mechanisms indicate a n to nne maximum horizontal compression in the vicinity of the paleozoic amazonas rift in central brazil this compressive stress direction is nearly perpendicular to the rift structure and represents a 75 rotation relative to a regional e w compressive stress direction in the south american plate elastic two dimensional finite element models of the density structure associated with the amazonas rift as inferred from independent gravity modeling indicate that elastic support of this dense feature would generate horizontal rift normal compressional stresses between 60 and 120 mpa with values of 80 100 mpa probably most representative of the overall structure the observed 75 stress rotation constrains the ratio of the regional horizontal stress difference to the rift normal compressive stress to be between 0 25 and 1 0 suggesting that this rift normal stress may be from 1 to 4 times larger than the regional horizontal stress difference a general expression for the modification of the normalized local horizontal shear stress relative to the regional horizontal shear stress shows that the same ratio of the rift normal compression relative to the regional horizontal stress difference which controls the amount of stress rotation also determines whether the superposed stress increases or decreases the local maximum horizontal shear stress the potential for fault reactivation of ancient continental rifts in general is analyzed considering both the local stress rotation and modification of horizontal shear stress for both thrust and strike slip stress regimes in the amazonas rift case because the observed stress rotation only weakly constrains the ratio of the regional horizontal stress difference to the rift normal compression to be between 0 25 and 1 0 our analysis is inconclusive because the resultant normalized horizontal shear stress may be reduced for ratios 0 5 or enhanced for ratios 0 5 additional information is needed on all three stress magnitudes to predict how a change in horizontal shear stress directly influences the likelihood of faulting in the thrust faulting stress regime in the vicinity of the amazonas rift a rift normal stress associated with the seismically active new madrid ancient rift may be sufficient to rotate the horizontal stress field consistent with strike slip faults parallel to the axis of the rift although this results in a 20 40 reduction in the local horizontal shear stress within the seismic zone sparse stress data in the vicinity of the seismically quiescent midcontinent rift of the central united states suggest a stress state similar to that of new madrid with the local horizontal shear stress potentially reduced by as much as 60 thus the markedly different levels of seismic activity associated with these two subparallel ancient rifts is probably due to other factors than stress perturbations due to dense rift pillows the modeling and analysis here demonstrate that rift normal compressive stresses are a significant source of stress acting on the lithosphere and that in some cases may be a contributing factor to the association of intraplate seismicity with old zones of continental extension

3d crustal resistivity structure beneath the wagad aftershock zone of the 2001 bhuj earthquake kutch india heterogeneous resistivity structure controlled by widespread fluid infiltration and clues to aftershocks pattern abstract the epicentral region of the 26 january 2001 bhuj earthquake 7 7 m w in the kutch region of india has been experiencing a vigorous and unique intraplate aftershock activity which shows a v shaped pattern and n ne migration along the western margin of the wagad uplift located to the northeast of the mainshock this study presents 3d resistivity structure of the wagad aftershock zone imaged using magnetotelluric mt data to understand the crustal geoelectric structure and subsurface geometry of faults lineaments which could provide critical inputs to evaluate and strengthen the earthquake generation mechanism and aftershocks pattern the obtained 3d conductivity structure showed a thick 3 km and homogeneous conductive 1 30 m layer of tertiary mesozoic sedimentary cover in the area a highly heterogeneous upper and middle crust characterized by a mix of conductive 300 m structures is imaged below the sedimentary cover enhanced conductivity zones showed spatial correlation with the surface trace of various faults lineaments and are interpreted as fluid enriched zones in the crust which play a critical role in earthquake triggering significantly higher fluid enrichment noticed in the crust below two minor faults khadir transverse fault ktf and manfara fault mf which are hitherto considered to have an insignificant role in the seismogenesis of the area the aftershock events seem to occur mostly on the resistive parts of the crust and its pattern shows good alignment with the anomalous conductive zones associated to the above two faults the results provide a strong clue to infer that the ktf and mf have a vital role in the observed aftershock activity and control the v shaped aftershocks pattern

tomography of the source area of the 1995 kobe earthquake evidence for fluids at the hypocenter seismic tomography revealed a low seismic velocity 5 and high poisson s ratio 6 anomaly covering about 300 square kilometers at the hypocenter of the 17 january 1995 magnitude 7 2 kobe earthquake in japan this anomaly may be due to an overpressurized fluid filled fractured rock matrix that contributed to the initiation of the kobe earthquake

seismicity analysis of the kachchh aftershock zone and tectonic implication for 26 jan 2001 bhuj earthquake abstract we have carried out a detailed analysis of seismicity in the vicinity of the 26 jan 2001 bhuj earthquake mw 7 7 from the depth sections of 24 parallel profiles and from the b value cross section we claim the existence of a hidden fault which conjugates to the major rupture fault i e north wagad fault of the 2001 bhuj earthquake the proximity of the intersection of these faults to the focus suggests a close association bhuj main shock generation a circular pattern in the profiles also provides evidence for the existence of an intrusive consistent with earlier findings from gravity magnetic modeling and tomography studies the location of the fault intersection within the intrusive support a model where both play a significant role in the earthquake generation the intersection of the conjugate faults acts as a stress concentrator while their presence within a big pluton possibly will facilitate the stress amplification this mechanism might explain the occurrence of two mw 7 7 earthquakes in a relatively short time span of 182 years in the kachchh rift the b value cross section displays high and low b value patches along the two intersecting faults this suggests a model of a faulted block that consists of two kinds of segment the locked and the unlocked locked segments do not easily participate in creeping and therefore generate strong magnitude aftershocks m 3 while unlocked segments easily creep and result in only weak aftershocks m

regional tectonic framework structure and evolution of the western marginal basins of india abstract the kutch saurashtra cambay and narmada basins are pericontinental rift basins in the western margin of the indian craton these basins were formed by rifting along precambrian tectonic trends interplay of three major precambrian tectonic trends of western india dharwar nnw sse aravalli delhi ne sw and satpura ene wsw controlled the tectonic style of the basins the geological history of the basins indicates that these basins were formed by sequential reactivation of primordial faults the kutch basin opened up first in the early jurassic rifting was initiated in late triassic along the delhi trend followed by the cambay basin in the early cretaceous along the dharwar trend and the narmada basin in late cretaceous time along the satpura trend the evolution of the basins took place in four stages these stages are synchronous with the important events in the evolution of the indian sub continent its breakup from gondwanaland in the late triassic early jurassic its northward drifting during the jurassic cretaceous and collision with the asian continent in the early tertiary the most important tectonic events occurred in late cretaceous time the present style of the continental margins of india evolved during early tertiary time the saurashtra arch the extension of the aravalli range across the western continental shelf subsided along the eastern margin fault of the cambay basin during the early cretaceous it formed an extensive depositional platform continuous with the kutch shelf for the accumulation of thick deltaic sediments a part of the saurashtra arch was uplifted as a horst during the main tectonic phase in the late cretaceous the present high thermal regime of the cambay bombay high region is suggestive of a renewed rifting phase

a new insight into crustal heterogeneity beneath the 2001 bhuj earthquake region of northwest india and its implications for rupture initiations the seismic characteristics of the 2001 bhuj earthquake mw 7 6 has been examined from the proxy indicators relative size distribution 3d b value mapping and seismic tomography using a new data set to understand the role of crustal heterogeneities in rupture initiations of the 2001 bhuj earthquake of the gujarat india one of the disastrous indian earthquakes of the new millennium the aftershocks sequence recorded by 22 seismograph stations of gujarat seismic network gsnet during the period from 2006 to 2009 encompassing approximately 80 km 70 km rupture area had revealed clustering of aftershocks at depth of 5 35 km which is seismogenic layer responsible for the occurrence of continued aftershocks activity in the study region the 3d b value mapping estimated from a total of 3850 precisely located aftershocks with magnitude of completeness mc 2 7 shows that a high b value region is sandwiched within the main shock hypocenter at the depth of 20 25 km and low b value region above and below of the 2001 bhuj main shock hypocenter estimates of 3 d seismic velocity vp vs and poisson s ratio structure beneath the region demonstrated a very close correspondence with the b value mapping that supports the similar physicochemical processes of retaining fluids within the fractured rock matrix beneath the 2001 bhuj mainshock hypocenter the overall b value is estimated close to 1 0 which reveals that seismogenesis is related to crustal heterogeneity which in turn also supported by low vs and high structures the high b value and high anomaly at the depth of 20 25 km indicate the presence of highly fractured heterogeneous rock matrix with fluid intrusions into it at deeper depth beneath the main shock hypocenter region low b value and high vp in the region is observed towards the north east and north west of the main shock that might be an indication of the existence of relatively competent rock masses with negligible volume of cracks that may have contained over pressurized fluids without molten rocks

crustal scale weak zone along a collisional suture revealed by spatial variations in velocity structures and seismicity abstract high quality first arrival data collected with a high density temporary seismic array and regional seismic network were used to construct a p wave velocity model and identify the precise location of earthquakes in the active collision zone of southeastern taiwan a crustal scale weak zone defined by high seismicity is characterized by a steeply east dipping lower velocity anomaly and represents a boundary between an uplifted upper mantle and the luzon volcanic arc the main features of the weak zone with its associated low velocity zone and seismicity pattern vary significantly along the collisional boundary of southeastern taiwan along the weak zone unlithified sediments or highly fractured materials are compacted as the philippine sea plate moves toward the eurasian plate in areas of more advanced collision soft material is more compacted and can therefore store significantly greater amounts of strain energy this energy is released through episodic earthquakes

the 2001 bhuj earthquake tomographic evidence for fluids at the hypocenter and its implications for rupture nucleation 1 the january 26 2001 bhuj earthquake mw 7 6 is one of the most catastrophic indian earthquakes we have investigated the 3 d seismic velocity and poisson s ratio structures of the bhuj source area to understand the probable cause of triggering the earthquake we used 1948 p and 1865 s wave high quality arrival times from 331 aftershocks recorded at a temporary seismic network significant variations up to 5 in velocity and 10 in poisson s ratio are revealed in the aftershock area the mainshock is located in a distinctive zone characterized by high vp low vs and high poisson s ratio in the depth range of 20 to 30 km and extending 15 to 30 km laterally this feature is very similar to that of the 1995 kobe earthquake zhao et al 1996 the anomaly may be due to a fluid filled fractured rock matrix which might have contributed to the initiation of the bhuj earthquake

molybdenite re os zircon u pb dating and hf isotopic analysis of the shuangqing fe pb zn cu skarn deposit east kunlun mountains qinghai province china the shuangqing fe pb zn cu deposit is located in the xiangride county of qinghai province china and is a typical example of skarn deposits in the east kunlun mountains skarnization and mineralization took place along the contact zone between carboniferous carbonates and the concealed triassic plagiogranite la icp ms u pb dating of zircons from the plagiogranite has yielded ages of 227 2 1 0 and 226 54 0 97 ma which are interpreted as the emplacement age of the plagiogranite molybdenites separated from ore bearing quartz veins yielded a re os isochron age of 226 5 5 1 ma these age data confirm that both intrusion and related skarn mineralization initiated at 227 ma re contents of molybdenite zircon ehf t and 176 hf 177 hf values fall into the ranges 3 31 to 6 58 g g 8 6 to 0 0 and 0 282403 to 0 28263850 respectively the timing of the shuangqing fe pb zn cu mineralization coincided with a major change in the stress field in east kunlun from transpression to extension related to the partial melting of thickening crustal materials in a post collisional tectonic setting

evidence for massive discharges of icebergs into the north atlantic ocean during the last glacial period sediments in the north atlantic ocean contain a series of layers that are rich in ice rafted debris and unusually poor in foraminifera1 here we present evidence that the most recent six of these heinrich layers deposited between 14 000 and 70 000 years ago record marked decreases in sea surface temperature and salinity decreases in the flux of planktonic foraminifera to the sediments and short lived massive discharges of icebergs originating in eastern canada the path of the icebergs clearly marked by the presence of ice rafted detrital carbonate can be traced for more than 3 000 km a remarkable distance attesting to extreme cooling of surface waters and enormous amounts of drifting ice the cause of these extreme events is puzzling they may reflect repeated rapid advances of the laurentide ice sheet perhaps associated with reductions in air temperatures yet temperature records from greenland ice cores appear to exhibit only a weak corresponding signal moreover the 5 10 000 yr intervals between the events are inconsistent with milankovitch orbital periodicities raising the question of what the ultimate cause of the postulated cooling may have been

crustal structure of the gujarat region india new constraints from the analysis of teleseismic receiver functions abstract in this study receiver function analysis is carried out at 32 broadband stations spread all over the gujarat region located in the western part of india to image the sedimentary structure and investigate the crustal composition for the entire region the powerful genetic algorithm technique is applied to the receiver functions to derive s velocity structure beneath each site a detail image in terms of basement depths and moho thickness for the entire gujarat region is obtained for the first time gujarat comprises of three distinct regions kachchh saurashtra and mainland in kachchh region depth of the basement varies from around 1 5 km in the eastern part to 6 km in the western part and around 2 3 km in the northern part to 4 5 km in the southern part in the saurashtra region there is not much variation in the depth of the basement and is between 3 km and 4 km in gujarat mainland part the basement depth is 5 8 km in the cambay basin and western edge of narmada basin in other parts of the mainland it is 3 4 km the depth of moho beneath each site is obtained using stacking algorithm approach the moho is at shallower depth 26 30 km in the western part of kachchh region in the eastern part and epicentral zone of the 2001 bhuj earthquake large variation in the moho depths is noticed 36 46 km in the saurashtra region the crust is more thick in the northern part it varies from 36 38 km in the southern part to 42 44 km in the northern part in the mainland region the crust is more thick 40 44 km in the northern and southern part and is shallow in cambay and narmada basins 32 36 km the large variations of poisson s ratio across gujarat region may be interpreted as heterogeneity in crustal composition high values of 0 30 at many sites in kachchh and few sites in saurashtra and mainland regions may be related to the existence of high velocity lower crust with a mafic ultramafic composition and locally to the presence of partial melt the existing tectono sedimentary models proposed by various researchers were also examined

relocations and local earthquake tomography implications toward the mafic pluton induced crustal seismicity in kachchh gujarat india for last 18 years abstract the study presents new probable crustal model based on precise double difference relocations of 2420 aftershocks of the 2001 bhuj earthquake mw7 7 during 2001 16 and combining other geophysical and geological studies in the region which will lead to a better understanding of the cause of uninterrupted aftershock activities in the region relocations of aftershocks delineate two main seismic clusters in the central aftershock zone caz i e eastern a1 and western a2 the alignments of relocated hypocenters of the aftershocks delineate three main fault segments viz the south dipping north as well as south wagad faults and north dipping kachchh mainland fault relocations also detect some zones of devoid of aftershocks with a typical dimension of 3 9 km within the caz which could be related to locales of future earthquakes seismic velocity tomograms depict that both seismic clusters a1 and a2 are associated with high velocity anomalies inferred to be mafic plutons below the wagad and banni regions respectively the e w depth section of relocated hypocenters delineates an 18 km variation of the seismogenic base which could be attributed to the increase in crustal maficness due to the intrusion of mafic plutons below the caz stress inversions of selected focal mechanisms depict a rotation of 44 in the 1 orientation within the caz which could be attributed to large local stresses induced by mafic plutons the continued seismicity at kachchh is proposed to be caused by the presence of large stresses associated with mafic plutons owing to the n s plate compression associated with northward movement of the indian plate wherein crustal fluid flows aqueous metamorphic fluids or volatile co2 might provide the triggering to generate continued aftershock activities at kachchh india

re os geochronological evidence for multiple paleoproterozoic gold events at the scale of the west african craton the archean proterozoic craton of west africa hosts numerous gold deposits which are spatially and temporally related to the eburnean orogeny that took place between 2250 and 1980 ma and included multiple deformation events the majority of these gold deposits are located along shear zones the structural history is relatively well established for most gold deposits but absolute timing of the mineralization is commonly lacking five deposits hosted in the baoule mossi domain located in the southwestern part of the west african craton were studied to better constrain timing of gold events nassara and kiaka in burkina faso and wassa damang and obuasi in southern ghana gold mineralization was structurally characterized at each deposit and dated by the re os method on pyrite arsenopyrite and pyrrhotite grains that were coeval with the gold deposition combined structural and re os geochronological constraints allow two groups of gold deposits to be distinguished early orogenic gold formed during the eoeburnean orogeny i e between 2190 and 2125 ma as represented by kiaka1 at 2157 24 ma and wassa1 at 2164 14 ma these early gold occurrences are expressed as early disseminated gold enrichment 1 3 g t au at kiaka and gold hosted in folded and boudinaged quartz veins that are transposed along the primary s foliation at wassa these re os ages on the early sulfides at wassa and kiaka are significant because they definitively provide the first direct age constraints on gold only eoeburnean mineralization in the birimian of west africa late orogenic gold hosted by brittle structures and with higher gold grades up to 60g t at kiaka formed during late eburnean deformation d d events between 2120 and 2000 ma as represented by the re os dating of these ores at wassa at 2055 18 ma and obuasi at 2045 40 ma similar style ores dated at nassara at 2131 99 ma and damang at 2080 135 ma but both with large uncertainties and an undated younger event at kiaka are also suggested to be part of this later mineralizing episode the high uncertainties on the nassara and damang ages may be directly linked to the low rhenium and osmium contents of the studied samples these results highlight the polyphase character of the widespread gold mineralization in the west african craton although the late stages of the eburnean orogeny constitute a prolific period for the formation of high grade gold mineralization identification of less well studied early stage gold deposits which can also contain large quantities of gold is critical for mineral exploration in the west african craton

passive source seismic imaging of the crust and upper mantle beneath the 2001 mw 7 7 bhuj earthquake region gujarat india abstract variations in crustal and lithospheric thicknesses below the kachchh rift kr zone have been investigated employing the joint inversion of p receiver functions rf and rayleigh wave and love wave group velocity dispersion 5 70 s on the broadband waveforms of teleseismic events estimated crustal and lithospheric thicknesses at 15 broadband sites led to the delineation of an updoming of 4 7 km in the moho and 6 12 km in the asthenosphere beneath the central kr relative to the surrounding unrifted regions the moho depths vary from 35 to 42 km and lithospheric thicknesses range from 62 to 78 km the observed marked shear velocity reduction 2 6 at the lithosphere asthenosphere boundary lab over an area of 120 km 80 km suggests the presence of patches of partial melt in the asthenosphere below the kr evidencing the possible presence of imprints of the deccan reunion mantle plume the coincidence of the area of updoming of the moho and asthenosphere and the confined aftershock activity below the central kr implies that these patches could provide a high input of volatile co 2 into the lower crust which might play a key role in generating the continued aftershock activity in the region since the occurrence of the 2001 m w 7 7 bhuj mainshock

the tectonic evolution of western india and its impact on hydrocarbon occurrences an overview abstract the largest hydrocarbon accumulations of india were localized in western india by a unique convergence of tectonic events mesozoic rifting along the structural trends of proterozoic mobile belts formed the passive margin basins of the indian coast during the campanian rifting of madagascar from india north south striking normal faults propagated into the area of the cambay graben late maastrichtian doming over the incipient deccan reunion hotspot imparted extensional stresses to the northwestern indian coast and formed the fault block that became the bombay high after eruption of the deccan flood basalts in early paleocene extension continued in the weakened crust this resulted in enhanced subsidence of the cambay graben and surat danahu depression rifting of the seychelles microcontinent from india and reactivation of normal faults on the emergent bombay high the cambay graben and the surat danahu depression filled with organic rich source shales as they underwent thermal subsidence shallow water eocene to miocene carbonates developed on the slowly subsiding bombay platform and sea level oscillations fostered secondary porosity development late tertiary maturation of the surat danahu depression shales generated hydrocarbons that migrated into the carbonate reservoirs on the bombay high konkan kerala and the older basins of the east coast of india were unaffected by the post campanian tectonic events and lack the favorable play elements that characterize the giant fields

zooming in on heinrich layers theories explaining the origin of the abrupt massive discharges of ice rafted detritus ird into the glacial north atlantic the heinrich layers hls generally point to the laurentide ice sheet as the sole source of these events until it was found that the irds also originated from icelandic and european ice sheets bond and lotti 1995 snoeckx et al 1999 grousset et al 2000 this apparent contradiction must be reconciled as it raises fundamental questions about the mechanism s of hl origin we have analyzed two 12 cm thick hls in an ultrahigh resolution mode 1 2 century intervals in a mid atlantic ridge piston core the 18o record n pachyderma left coiling reveals strong excursions induced by the melting of the icebergs these excursions are associated with a strong decrease in the amount of planktic foraminafersand with a 3 c cooling of the surface waters counts of coarse detrital grains reveal that ird are deposited according to a typical sequence 1 volcanic glass 2 quartz and feldspars 3 detrital carbonate that implies a chronology in the melting of the differentpan atlantic ice sheets sr and nd isotopic composition confirm that in both heinrich layers h1 and h2 precursor ird came from first europe iceland followed then by laurentide derived ird an internal cyclicity can be identified during h1 and h2 about four to six major abrupt discharges occurred roughly on a century timescale the 13c and 15n records reveal that dominant inputs of continent derived organic matter are associated with ird within the hls hiding the plankton productivity signal