evidence and causes of the two stage development of the baikal rift abstract the data on the geological and deep seated structures of the baikal rift zone are briefly summarized based on the analysis of the composition of the sedimentary infill in rift basins two stages of the zone development have been recognized 1 1 slow rifting 2 2 fast rifting as a consequence of the study of the deep seated structure and construction of the thermal model of the lithosphere it was assumed that the first stage is associated with a slow uprise of the surface of the asthenospheric upwelling and the second stage connected with sideways flows of the substance involved in this upwelling after the crustal base was reached

geometry and rate of faulting in the north baikal rift siberia we present a detailed morphotectonic analysis of late quaternary faulting in the north baikal rift nbr a region characterized by ranges and basins distributed over more than 800 km along strike in eastern siberia remote sensing techniques spot meteor scenes and aerial photographs are used to map the active fault network which displays a general en echelon distribution from the northern lake baikal to the easternmost basin with 30 km spaced overstepping segments of 10 80 km in length most faults have a dominant dip slip component over their cenozoic history the inherited crustal fabric strongly influences the overall geometry of the rifted basins we use 54 14c ages of postglacial terraces near the foot scarps of the muya basin to date offsets measured inside alluvial fans the last main postglacial event in this area appears to be the early holocene optimum dated at 10 2 ka following the onset of deglaciation at 13 ka using these time constraints a detailed leveling across two terraces offset by the taksimo fault west muya basin shows consistent minimum vertical slip rates of 1 6 0 6 mm yr 1 using 30 other active scarps analyzed in the field we find a lower bound for horizontal velocity of 3 2 0 5 mm yr 1 across the nbr a rate close to the one found in the southern rift from global positioning system measurements we then compare directions of slip vectors from holocene field data and slip directions from earthquake fault plane solutions although local discrepancies appear the mean directions of lesser horizontal stress 3 inverted from theses values are n130 e and n155 e respectively which are comparable within uncertainties and favor a rifting obliquity of 30 40 extrapolating our holocene rates we estimate basin ages younger than those generally believed less than 7 ma and propose a spatial and temporal evolution of rifted basins consistent with experimental models of oblique rifting total amounts of extension and vertical throw 7 and 12 km respectively across major faults appear rather constant from the central to the northern rift these results favor a progressive development of asymmetric grabens in a rift zone that widens with times and they indicate a strong rheological control on deformation which seems enhanced by other contributions than the far field effects of the indo eurasian collision

evidence for small scale mantle convection in the upper mantle beneath the baikal rift zone 1 inversion of teleseismic p wave travel time residuals collected along a 1280 km long profile traversing the baikal rift zone brz reveals the existence of an upwarped lithosphere asthenosphere interface which causes a travel time delay of about 1 s at the rift axis central high an area with early arrivals relative to the stable siberian platform of up to 0 5 s is observed on each side of the rift about 200 km from the rift axis flank lows while the location of the central high is approximately fixed in the vicinity of the rift axis those of the flank lows vary as much as 200 km with the azimuth of the arriving rays we use three techniques to invert the travel time residuals for velocity anomalies beneath the profile two of the techniques assume an isotropic velocity structure and one of them considers a transversely isotropic velocity model with a vertical axis of symmetry we use independent geophysical observations such as gravity active source seismic exploration and crustal thickness measurements to compare the applicability of the models other types of geophysical measurements suggest that the model involving transverse isotropy is a plausible one which suggests that the central high and flank lows are caused by the combined effects of an upwarped asthenosphere with a 2 5 lateral velocity reduction and a velocity increase due to transverse isotropy with a vertical axis of symmetry we consider the anisotropy to be the result of the vertical component of a lithosphere asthenosphere small scale mantle convection system that is associated with the rifting

early cretaceous denudation related to convergent tectonics in the baikal region se siberia we present 24 new apatite fission track aft ages and 18 track length measurements from the baikal region se siberia most samples have aft ages between 140 and 100 ma with relatively high mean track lengths c 13 2 m the relationship between aft ages elevation and mean track lengths indicate that the samples record rapid cooling during the early cretaceous 140 120 ma as also shown by thermal history inversion of track length distributions cooling took place during a late jurassic early cretaceous orogenic phase related to closure of the mongol okhotsk ocean and reflected in the exhumation of metamorphic core complexes followed by thrusting and reverse faulting basin inversion and large vertical motions the variation in aft ages throughout the study area can be partly explained by differences in geothermal structure but differential denudation also played a role minimum amounts of early cretaceous denudation are estimated at 2 3 km

no moho uplift below the baikal rift zone evidence from a seismic refraction profile across southern lake baikal 1 the late cenozoic baikal rift zone brz in southern siberia is composed of several individual topographic depressions and half grabens with the deep lake baikal at its center we have modeled the seismic velocity structure of the crust and uppermost mantle along a 360 km long profile of the baikal explosion seismic transects best project across the rift zone in the southern part of lake baikal the seismic velocity structure along the profile is determined by tomographic inversion of first arrival times and 2 d ray tracing of first arrivals and reflections the velocity model shows a gently deepening moho from the siberian platform 41 km depth into the sayan baikal fold belt 46 km depth we can exclude the presence of any moho uplift around the 10 km deep sedimentary graben structure of southern lake baikal the lower crust includes a distinct 50 80 km wide high velocity anomaly 7 4 7 6 0 2 km s slightly offset to the northeast from the rift axis we interpret this feature as resulting from mafic intrusions their presence may explain the flat moho by compensation of lower crustal thinning by intrusion of mafic melts the pn wave velocities 8 15 8 2 km s are normal for the area and do not show any sign of decompression melting in the sub moho mantle such that possible lithosphere thinning has not reached the base of the crust on the basis of the results of the best project we suggest that the brz is formed by passive rifting in the rheologically weak suture between the siberian platform and the amurian plate

three dimensional laboratory modelling of rifting application to the baikal rift russia abstract continental rifting is treated as a mechanical instability developing under horizontal tectonic tension the instability results in strain localization and the formation of a neck which is interpreted as a rift zone at the scale of the whole lithospheric plate this process occurs in plane stress conditions and can therefore be modelled to a first approximation by a one layer plate whose properties represent integral over thickness properties of the real lithosphere we have designed a scaled experimental single layer lithosphere model having elasto plastic rheology and lying upon a liquid substratum to study its behaviour under axial horizontal tension in a homogeneous plate the instability develops along a linear zone oriented at an angle of 60 to the tension axis this orientation is preserved even when the divergent displacement of the plate boundaries is not plain parallel but rotational in the latter case the strain localization zone is rapidly propagating when the plate length to width ratio is less than 2 5 the necking develops along two branches conjugated at an angle of about 120 which is frequently observed in actual rift systems if the model contains a local weak zone hot spot or fault zone the rift junction is located at this zone in the lithospheric models comprising strong cratonic and weak segments strain localization depends on the configuration of the boundary between different lithospheres the necking starts to form within the weak segment in the vicinity of the cratonic promontories and propagates in opposite directions again at an angle of ca 60 to the tension axis in the models containing both a strong lithosphere and local weak zones the rift configuration depends on their shape and relative positions with necking always going through the weak zones in a set of models we have reproduced the geometry of the boundary between the siberian craton and the thermally much younger 100 ma sayan baikal lithosphere in the baikal rift area in these models we were able to obtain the well known three branch configuration of the baikal rift system only by introducing a weak zone in the area of lake baikal such a zone simulates the paleozoic suture existing in this area as in nature two wide outer branches eastern and western are oblique to the regional tension axis whereas the central one is narrow and orthogonal to the tension direction in nature and in the model rifting starts in the central branch corresponding to lake baikal the modelling also predicts the formation of a fourth oblique ns trending branch to the south of baikal although poorly expressed in the field this branch has some seismotectonic and magmatic manifestations the orientations of all four branches with respect to each other and with respect to the regional tension direction are remarkably similar in nature and in the model

first study of fossil diatom flora from middle miocene lower pliocene lacustrine sediments in barguzinvalley baikal rift zone abstract a subsequent formation of oligotrophic tectonic paleo lakes bodon 1 and bodon 2 is reconstructed through study of fossil diatom flora in sediments of the upper stratum of the tankhoy formation in the south of barguzin valley the first lake is characterized by the development of middle late miocene planktonic species aulacoseira sp 1 and 2 diatom zones 1 and 2 then of the species pseudoaulacoseira moisseeviae diatom zone 3 the second lake is identified due to early pliocene species a islandica a anadensis and eunotia polyglyphoides diatom zone 4 the diatom flora of zone 3 of paleo lake bodon 1 is correlated to similar zone in paleo lakes in vitim plateau it was assumed that these lakes were formed simultaneously with the paleo lake mukhal on the vitim plateau ca 13 12 million years ago and the paleo lake yuzhnoe ca 10 9 million years ago and later

mantle structure and rifting processes in the baikal mongolia region geophysical data and evidence from xenoliths in volcanic rocks abstract the origin of the baikal rift zone brz has been debated between the advocates of passive and active rifting since the 1970s a re assessment of the relevant geological and geophysical data from russian and international literature questions the concept of broad asthenospheric upwelling beneath the rift zone that has been the cornerstone of many active rifting models results of a large number of early and recent studies favour the role of far field forces in the opening and development of the brz this study emphasises the data obtained through studies of peridotite and pyroxenite xenoliths brought to the surface by alkali basaltic magmas in southern siberia and central mongolia these xenoliths are direct samples of the upper mantle in the vicinity of the brz of particular importance are suites of garnet bearing xenoliths that have been used to construct p t composition lithospheric cross sections in the region for the depth range of 35 80 km xenolith studies have shown fundamental differences in the composition and thermal regime between the lithospheric mantle beneath the ancient siberian platform sampled by kimberlites and beneath younger mobile belts south of the platform the uppermost mantle in southern siberia and central mongolia is much hotter at similar levels than the mantle in the siberian craton and also has significantly higher contents of basaltic major elements ca al na and iron higher fe si and fe mg the combination of the moderately high geothermal gradient and the fertile compositions in the off cratonic mantle appears to be a determining factor controlling differences in sub moho seismic velocities relative to the siberian craton chemical and isotopic compositions of the off cratonic xenoliths indicate small scale and regional mantle heterogeneities attributed to various partial melting and enrichment events consistent with long term evolution in the lithospheric mantle age estimates of mantle events based on os sr nd isotopic data can be correlated with major regional stages of crustal formation and may indicate long term crust mantle coupling the ratios of 143 144nd in many lree depleted xenoliths are higher than those in morb or oib source regions and are not consistent with a recent origin from asthenospheric mantle mantle xenoliths nearest to the rift basins 30 50 km south of southern lake baikal show no unequivocal evidence for strong heating unusual stress and deformation solid state flow magmatic activity or partial melting that could be indicative of an asthenospheric intrusion right below the moho comparisons between xenoliths from older and younger volcanic rocks east of lake baikal together with observations on phase transformations and mineral zoning in individual xenoliths have indicated recent heating in portions of the lithospheric mantle that may be related to localised magmatic activity or small scale ascent of deep mantle material overall the petrographic p t chemical and isotopic constraints from mantle xenoliths appear to be consistent with recent geophysical studies which found no evidence for a large scale asthenospheric upwarp beneath the rift and lend support to passive rifting mechanism for the brz

observational and laboratory insights into salt finger convection improving observational capabilities in the ocean are revealing a substantial body of evidence that the double diffusive instability known as salt fingers is widely present and plays a significant role in oceanic mixing this evidence includes unique fine and microstructure features in particular regions and distinct patterns in the temperature and salinity variations over large areas laboratory experiments have also advanced our understanding of salt fingers the insights gleaned from both laboratory and ocean are reviewed and related to large scale ocean structure and climate modeling

rayleigh wave tomography of china and adjacent regions 1 this paper presents a tomographic study on the s wave velocity structure of china and adjacent regions group velocity dispersions of fundamental rayleigh waves along more than 4000 paths were determined with frequency time analysis the study region was divided into a 1 1 grid and velocities in between grid nodes were calculated by bilinear interpolation the occam s inversion scheme was adopted to invert for group velocity distributions this method is robust and allows us to use a fine grid in model parameterization and thus helps to restore a more realistic velocity pattern checkerboard tests were carried out and the lateral resolution was estimated to be 4 6 in china and its eastern continental shelves the resulting group velocity maps from 10 to 184 s showed good correlation with known geological and tectonic features the pure path dispersion curves at each node were inverted for shear wave velocity structures the three dimensional velocity model indicates thick lithospheres in the yangtze and tarim platforms and hot upper mantles in baikal and western mongolia coastal area and continental shelves of eastern china and indochina and south china sea regions the tibetan plateau has a very thick crust with a low velocity zone in its middle beneath the crust a north dipping high velocity zone mimicking a subducting plate reaches to 200 km in depth and reaches to the kunlun mountains northward in northern tibet a low velocity zone immediately below the moho extends eastward then turns southward along the eastern edge of the plateau until it connects to the vast low velocity area in indochina and the south china sea

characterizing the present day activity of the tunka and sayan faults within their relay zone western baikal rift system russia the sayan and tunka faults are located at the boundary between the northernmost mountain belt of central asia the sayan baikal ranges and the siberian platform these prominent crustal structures were involved in the opening of the southern baikal rift system since the beginning of the cenozoic and define large scale sharp morphotectonic features despite low instrumental seismic activity late pleistocene holocene morphotectonic features along the two faults indicate that the faults are active and have the capacity to produce strong earthquakes a careful mapping of the most recent trace of activity within the southeastern parts of the two faults where they merge within a relay zone demonstrates that they correspond now to left lateral reverse faults suggesting a recent inversion of their vertical component we also show that the two faults are now structurally connected via a young surface rupture and that no obvious post last glacial maximum ruptures are observed along the central part of the sayan fault beyond its junction zone with the tunka fault this suggests that the left lateral strike slip deformation is transferred from the eastern sayan fault to the tunka fault a detailed morphotectonic study along the southeastern sayan fault allows estimating a left lateral slip rate between 1 3 min and 3 9 mm year max finally a critical review of russian paleoseismic data combined with our paleoseismological investigations allows us to propose that the mean recurrence time along the two faults is on the order of 4 kyr and that they may have either ruptured together or during seismic clusters

crustal structure of central lake baikal insights into intracontinental rifting this is the published version copyright 1976 american geophysical union all rights reserved

geodynamics of rifting and implications for hydrocarbon habitat abstract rifting activity is probably mainly governed by frictional forces exerted on the base of the lithosphere by the convecting sub lithospheric upper mantle combined with deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system and by far filed stresses controlling the interaction of lithospheric plates moving relative to each other mantle plumes rising from the deep mantle do not appear to be a major driving force of rifting therefore it is questioned whether a distinction between active and passive rifts is still justified lithospheric extension triggers by decompression partial melting of the lower lithosphere and upper asthenosphere magmatic processes contribute materially to thinning of the subcrustal lithosphere and probably also of the lower crust this could explain the frequently observed discrepancy between upper crustal extension by faulting and lower crustal attenuation as defined by refraction and deep reflection seismic data the duration of the rifting stage of palaeorifts and passive margins is highly variable depending on plate interaction crustal separation can be achieved after as little as 7 ma and as much as 280 ma of rifting activity the availability of crustal discontinuities does not appear to play a significant role in the time required for crustal separation to be achieved the structural style of rifts is controlled by the physical state of the lithosphere the mode and amount of its extension and the lithological composition of pre and syn rift sediments subsidence of rift basins is controlled by the balance between isostatic adjustment of the crust to lithospheric stretching and uplift induced by mechanical and thermal attenuation of the subcrustal lithosphere post rift basin subsidence is mainly governed by the decay of rift induced thermal anomalies densification of the lower crust and intra plate stresses provide overprinting effects in view of this it is unlikely that quantitative analyses of post rift basin subsidence give a direct measure of lithospheric extension tectonically active rifts palaeorifts and passive margin basins contain major hydrocarbon provinces their hydrocarbon charge can rely exclusively on pre rift syn rift or post rift sedimentary sequences or a combination thereof maturation of source rocks can be achieved during the syn and or post rift stage of basin evolution during rifting conductive and convective heat transfer accounts for elevated geothermal gradients these play an important role in the maturation of pre and syn rift source rocks thermal gradients decrease asymptotically during the post rift stage

rupture zones in the aegean region rupture zones of strong shallow earthquakes can be investigated by field observations of surface fault traces sf accurate location of spatial clusters of aftershocks or other small earthquakes cl reliable fault plane solutions fp and information on the spatial distribution of sites with high i viii macroseismic intensities ma the validity of these four techniques was successfully tested by comparing the results of their application to several cases of strong shallow earthquakes these techniques were then applied to spatially define the rupture zones of 150 strong m 6 0 shallow earthquakes in the aegean and surrounding area the type of faulting was determined for all these zones by the use of reliable fault plane solutions which are available for this area the orientation of most of these rupture fault zones is in good agreement with the presently acting stress field with some exceptions which are attributed to the rupture of pre existing faults the tectonic implications of these zones are discussed 1999 elsevier science b v all rights reserved

correlation of lg amplitude ratios from peaceful nuclear explosions to crustal structure in northern eurasia because the lg phase is strongly affected by the variations of crustal structure in some areas understanding these factors is important for nuclear test monitoring this study examines the effects of regional tectonic structure on lg propagation by using the peaceful nuclear explosion pne profiles in russia the logarithms of lg sn and lg pcoda pne amplitude ratios within the 0 5 to 3 hz frequency band are measured and correlated with the regional crustal structures both ratios are found to decrease within the areas with thick low velocity sedimentary cover and across tectonic boundaries with abrupt variations of crustal thickness for the offset derivatives of both the logarithmic ratios a linear relationship to the slopes of the moho and the vertical travel times within the sedimentary cover is determined a strong negative slope is observed along both crustal thinning and crustal thickening segments and a positive slope follows the negative slope across a crustal thickening segment therefore crustal thinning appears to affect the lg propagation stronger than crustal thickening which could be due to focusing lg energy within the crustal wave guide the log amplitude regression provided in this article could be useful for predicting lg behavior in areas where amplitude measurements are not available

control on off rift magmatism a case study of the baikal rift zone abstract the baikal rift zone brz is an active intracontinental rift zone with almost no rift related magmatism along the rift axis most magmatism is observed outside the rift center and e g the major vitim volcanic field is displaced more than 200 km from the rift center the reasons for this regional distribution of the magmatism are enigmatic in particular regarding the off rift magmatism we present results of numerical modeling of rift structures similar to brz that develop in the transition between craton and orogenic belt geophysical evidence suggests that pre existing weak zones control the location of the brz the models therefore include a pre set fault weak zone within the transition zone between the craton and the orogenic belt as an initial condition in the model if the pre existing fault is close to the craton partial melting in the mantle due to sub crustal extension is offset from the surface rift graben by over 200 km a horizontal shear zone in the lower crust transfers the extension from the shallow crust to the lithospheric mantle far from the rift graben a pre set fault close to the orogenic belt zone is required to explain magmatism close to the rift center in the southernmost brz we conclude that the location of the pre existing fault zone relative to a change in lithosphere thickness may shift the magmatism away from the surface expression of the rift graben and may explain the location of the off rift magmatism e g the basaltic vitim plateau the predicted geometry of the sedimentary graben along the rift strike is consistent with seismic observations in the northern and central brz

lithospheric cross sections of the european cenozoic rift system abstract the lithospheric structure of the european cenozoic rift system ecris is presented in transects through the southern rhine graben and the rhenish massif hessen depression emphasizing the geophysical structure of the lithosphere based on seismic refraction reflection investigations teleseismic tomography electromagnetic depth sounding models and gravity aeromagnetic earthquake uplift subsidence and heat flow data the rift is clearly expressed in the rhine graben but is not evident at the surface in the area of the rhenish massif where its existence is indicated by seismicity it is characterized by abnormal crustal and upper mantle structures which vary considerably in horizontal direction for example under the rhine graben the crust is thinned to 25 km but at 40 km depth anomalously high velocities are observed in contrast beneath the rhenish massif the crust is thickened to 35 37 km and under its eastern part a high velocity thin upper mantle slice is seen at 30 km depth within the lower crust which towards the hessen depression is gradually replaced by normal variscan mantle with the moho near 30 km depth under the western part of the rhenish massif p and s wave velocities are reduced below 50 km depth which is not seen east of the rhine river under the rhine graben the existence of a low velocity upper mantle above 100 km cannot be generalized but is restricted to confined regions

finite element modelling of crustal deformation in the baikal rift zone new insights into the active passive rifting debate abstract the origin of the forces responsible for crustal extension in the baikal rift zone siberia is the object of a debate between passive rifting models where crustal extension is primarily caused by horizontal forces related to the kinematics of asia india eurasia collision and active rifting models where crustal extension is primarily caused by a diapiric mantle upwelling in this work we used a two dimensional visco elastic finite element model in order to determine whether horizontal forces alone can account for the present day deformation in the baikal rift zone we tested a number of kinematic boundary conditions and compared predictions of various models against the observed stress and strain field deduced from seismotectonic data earthquake focal mechanisms and microtectonic analyses by adjusting the kinematic boundary conditions and using a three plate model with a differential displacement between the mongolian and amurian plates we found a best fit model that correctly accounts for the observed strain and stress field over the entire baikal rift zone the fact that our model does not take into account vertical forces but still explains most of the observed deformation suggests that the present day opening of the baikal rift is essentially controlled by horizontal forces related to the regional kinematics these forces could have their origin in the india eurasia collision zone further south this result does not imply that the asthenosphere played no role in the rifting process in particular before the fast rifting stage of the baikal rift evolution 3 4 ma but might suggest a recent plio quaternary triggering effect of the india asia collision on the deformation in the baikal rift

mantle transition zone discontinuities beneath the baikal rift and adjacent areas like most other major continental rifts the baikal rift zone brz in siberia ispresumably underlain by a hot and partially molten mantle which has a reduced seismicvelocity relative to surrounding areas recent seismic tomography studies however gaveconflicting results about the depth extent and even the existence of the low velocityanomaly beneath the brz suggesting that additional constraints are needed here wepresent results from stacking of about 1700 radial p to s receiver functions from asingle long running seismic station tly located at the sw tip of lake baikal a clearuplift of the 410 km discontinuity d410 with a magnitude of about 47 km relative tothe south margin of the siberian platform is observed beneath the rift currently availableseismic results suggest that the uplift is unlikely to be caused by addition of water tomantle transition zone mtz silicates but is the result of a 550 c reduction in temperaturein the vicinity of the d410 in addition the 660 km discontinuity d660 shows adownward trend toward the rift from the south suggesting that the entire mtz might havea low temperature beneath the rift the thickening of the mtz suggests a high velocityanomaly of about 2 in the mtz and rules out the possibility that the rifting is caused bya mantle plume originated in or beneath the mantle transition zone

collaborative effects of acidithiobacillus ferrooxidans and ferrous ions on the oxidation of chalcopyrite abstract in recent decades the bioleaching of chalcopyrite has been successfully developed and employed in copper hydrometallurgy understanding the decomposition mechanism of chalcopyrite is also of great significance for environmental remediation because the microbial oxidation of metal sulfides in mining waste and outcrop rocks commonly causes serious environmental contamination this study investigates the influence of acidithiobacillus ferrooxidans a ferrooxidans and added fe2 ions on the oxidation of chalcopyrite the results show that a ferrooxidans and added fe2 ions can collaboratively promote the recovery of cu from chalcopyrite in the bioleaching system with added fe2 ions a ferrooxidans prefer to oxidize soluble fe2 ions rather than decompose chalcopyrite to acquire energy which inhibits the release of cu at the first stage but enhances the growth of a ferrooxidans after reacting for 18 days however the produced fe3 ions greatly promote decomposition and release more cu than the bioleaching system free of fe2 which remained in the rest experiments both the oxidation of chalcopyrite and the release of cu in the bioleaching system are greater than what occurs in the chemical leaching system chalcocite covellite bornite and elemental sulfur were identified as intermediate products and a sulfur transforming route of s2 s22 sn2 s0 so32 so42 can be recognized by x ray photoelectron spectroscopy as the principle end product jarosite covered the chalcopyrite grains and consequently inhibited further oxidation it is noteworthy that the released cu2 ions barely suppressed the growth of a ferrooxidans because they tended to be enriched only in extracellular polymeric substance eps while fe3 ions could be found on both the cell surfaces and the eps which implies a potential mechanism for the survival of cells in a high cu2 solution collectively an integrated model of chalcopyrite oxidation collaborated by both chemical and microbial oxidation has been proposed to elucidate the bioleaching mechanisms and to give a perspective on its hydrometallurgical and environmental applications

dynamics of intracontinental extension in the north baikal rift from two dimensional numerical deformation modeling we model the recent deformation along a lithospheric scale cross section perpendicular to the northern part of the baikal rift zone brz with two dimensional finite element models using realistic lithospheric structure and rheological properties and imposing extension as a far field boundary condition we find a model that matches reasonably well the topography observed deformation pattern gravity anomalies and age of formation of the northern brz our results suggest that 1 extensional strain can occur away from the main rift basin in the sayan baikal range and create depending on the rheological properties of the lithosphere a large off rift basin analogous to the barguzin basin or a series of basins and ranges as observed in the southern part of the sayan baikal range 2 anelasticity plays a major role by participating in the uplift of rift shoulders and hanging wall deformation and allowing the subsidence and tilting of the hanging wall along high angle planar normal faults 3 the lower crust accommodates differential strain between the brittle upper crust and upper mantle by horizontal shear and lateral flow towards the regions of crustal and mantle thinning 4 far field extensional stress and a lithospheric discontinuity inherited from paleozoic tectonic events are sufficient to initiate rifting and basin subsidence and 5 there is no or very little dynamic contribution of a hypothetic asthenospheric plume under the brz at least in the recent phase of its evolution fast rifting stage 3 5 myr

geochemical and mineralogical aspects of sulfide mine tailings abstract tailings generated during processing of sulfide ores represent a substantial risk to water resources the oxidation of sulfide minerals within tailings deposits can generate low quality water containing elevated concentrations of so 4 fe and associated metal loid s acid generated during the oxidation of pyrite fes 2 pyrrhotite fe 1 x s and other sulfide minerals is neutralized to varying degrees by the dissolution of carbonate oxy hydroxide and silicate minerals the extent of acid neutralization and therefore pore water ph is a principal control on the mobility of sulfide oxidation products within tailings deposits metals including fe iii cu zn and ni often occur at high concentrations and exhibit greater mobility at low ph characteristic of acid mine drainage amd in contrast hydr oxyanion forming elements including as sb se and mo commonly exhibit greater mobility at circumneutral ph associated with neutral mine drainage nmd these differences in mobility largely result from the ph dependence of mineral precipitation dissolution and sorption desorption reactions cemented layers of secondary oxy hydroxide and hydroxy sulfate minerals referred to as hardpans may promote attenuation of sulfide mineral oxidation products within and below the oxidation zone hardpans may also limit oxygen ingress and pore water migration within sulfide tailings deposits reduction oxidation redox processes are another important control on metal loid mobility within sulfide tailings deposits reductive dissolution or transformation of secondary oxy hydroxide phases can enhance fe mn and as mobility within sulfide tailings production of h 2 s via microbial sulfate reduction may promote attenuation of sulfide oxidation products including fe zn ni and tl via metal sulfide precipitation understanding the dynamics of these interrelated geochemical and mineralogical processes is critical for anticipating and managing water quality associated with sulfide mine tailings

heat flow variations in continental rifts abstract in the terrestrial thermal field continental rifts stand out as rather narrow regional thermal anomalies in which augmented heat flow is unevenly distributed heat flow variations are discussed on the example of the baikal east african cordilleran and rhine lybian rift systems rift related disturbance of the asthenosphere lithosphere system is accompanied by a general increase in the surficial heat flow regional thermal anomalies are mainly caused by conductive heat transfer from the asthenosphere local thermal anomalies are caused by magmatic mass transfer into the crust and by fluid convection along porous fracture systems

the mechanics of continental extension and sedimentary basin formation a simple shear pure shear flexural cantilever model abstract deep seismic reflection data and earthquake seismology show the fundamental importance of major crustal faults during extension of continental lithosphere controlling the development of rifted sedimentary basins major basement faults are generally planar and extend down to 10 15 km depth corresponding to the base of the seismogenic layer beneath this depth deformation gives way to distributed ductile shear in the lower crust and lithospheric mantle a mathematical model is described of the geometric thermal and flexural isostatic response of the lithosphere to extension by planar faulting of the upper crust and plastic distributed deformation in the lower crust and mantle during faulting upper crustal footwall and hanging wall blocks behave as two mutually self supporting flexural cantilevers their response to isostatic forces induced by extension generates footwall uplift and hanging wall collapse for extension on multiple faults interference of footwall uplift and hanging wall collapse gives rise to the familiar block rotations of rift tectonics the flexural cantilever model is able to predict crustal structure and sedimentary basin geometry for faults of arbitrary spacing horizontal displacement and polarity the effects of fault spacing fault polarity and the amount of extension as well as erosion of footwall uplift on crustal structure and basin geometry during both syn rift and post rift stages of basin evolution are explored models of passive rifting involving lithosphere extension in response to the build up of far field tectonic forces and decompression induced partial melting of the upper asthenosphere and lower lithosphere appear to conform to the main geological observations the role of active rifting driven by locally derived deviatoric tensional forces generated by mantle plumes remains uncertain

fractals in studies of faulting and seismicity in the baikal rift zone abstract fractal analyses have been carried out for spatial patterns of faulting including over 3000 faults and seismicity of more than 60 000 earthquakes in the baikal rift zone brz it is concluded that faulting and seismicity are in balance in the whole brz and out of balance in its flanks by analysing fractal dimensions of seismicity in time it has been revealed that the out of balance state of the processes is predetermined by the fact that development of the spatial patterns of epicentral fields has not been completed yet in the brz flanks

active faults seismicity and recent fracturing in the lithosphere of the baikal rift system abstract a map of major active faults has been constructed for the baikal rift system brs recent active faults are identified using seismological data the brs seismicity of the past 40 years is statistically analyzed areas of a stable concentration of epicenters are revealed on this basis a zone of recent fracturing of the lithosphere is identified and its relation to active and developing faults of the brs is analyzed the zone of the lithosphere fracturing is a major tectonic structure which controls both the recent seismic process and the reactivation of ancient faults it is demonstrated that the available seismological data can provide a basis for a detailed classification of faults by degree of their tectonic activity regularities in the distribution of strong earthquakes along the zone of the recent fracturing of the lithosphere are established as well as regularities in the distribution of strong and weak seismic events relative to transform and other faults the degree of the fault reactivation is determined by their spatial closeness to the axial zone of the recent rupturing of the lithosphere

lower crustal intrusions beneath the southern baikal rift zone evidence from full waveform modelling of wide angle seismic data abstract the cenozoic baikal rift zone brz is situated in south central siberia in the suture between the precambrian siberian platform and the amurian plate this more than 2000 km long rift zone is composed of several individual basement depressions and half grabens with the deep lake baikal at its centre the best baikal explosion seismic transect project acquired a 360 km long deep seismic refraction wide angle reflection profile in 2002 across southern lake baikal the data from this project is used for identification of large scale crustal structures and modelling of the seismic velocities of the crust and uppermost mantle previous interpretation and velocity modelling of p wave arrivals in the best data has revealed a multi layered crust with smooth variation in moho depth between the siberian platform 41 km and the sayan baikal fold belt 46 km the lower crust exhibits normal seismic velocities around the rift structure except for beneath the rift axis where a distinct 50 80 km wide high velocity anomaly 7 4 7 6 0 2 km s is observed reverberant or ringing reflections with strong amplitude and low frequency originate from this zone whereas the lower crust is non reflective outside the rift zone synthetic full waveform reflectivity modelling of the high velocity anomaly suggests the presence of a layered sequence with a typical layer thickness of 300 500 m coinciding with the velocity anomaly the p wave velocity of the individual layers is modelled to range between 7 4 km s and 7 9 km s we interpret this feature as resulting from mafic to ultra mafic intrusions in the form of sills petrological interpretation of the velocity values suggests that the intrusions are sorted by fractional crystallization into plagioclase rich low velocity layers and pyroxene and olivine rich high velocity layers the mafic intrusions were probably intruded into the ductile lower crust during the main rift phase in the late pliocene as such the intrusive material has thickened the lower crust during rifting which may explain the lack of moho uplift across southern brz

inferring the nature of microstructure signals further study is done of holloway and gargett s 1987 suggestion that it is possible to distinguish salt fingering microstructure signals from those induced by turbulence by using the kurtosis the relative flatness of the scalar probability distribution support for their result comes from an examination of towed measurements of conductivity gradient fluctuations from turbulent and salt finger patches in the sargasso sea however when data are examined from presumably salt finger interfaces in the thermohaline staircase east of barbados the caribbean sheets and layers transects c salt experimental region data segments are found having both the suppressed kurtosis values expected of salt fingering and the high values associated with turbulence it is concluded that either turbulence or some other chaotic process is competing with salt fingering in the c salt data or that the kurtosis discriminant is not as useful as holloway and gargett suggest

the geometry of listric normal faults and deformation in their hangingwalls abstract in cross sections containing listric extensional faults area balancing techniques for depth to decollement are usually based on either bed length conservation or displacement conservation listric fault geometry may be constructed from a hangingwall roll over profile using the chevron construction this construction based on conservation of heave necessitates a reduction in fault displacement with decreasing fault dip a modification of this construction utilizing conservation of fault displacement predicts a listric fault that detaches at a shallower depth a new construction based on slip lines uses fault perpendicular displacement segments to generate listric fault shape fault propagation strain may be responsible for anomalous hangingwall geometries and these can be predicted by forward modelling using either a modified chevron construction or a slip line construction

holocene paleoseismicity of the tunka fault baikal rift russia the tunka fault is a major normal oblique transform fault within the nne trending baikal rift that displays geomorphic evidence of recurrent quaternary movement a flight of six fanhead terraces of the kyngarga river is displaced by several parallel faults and has scarps up to 32 m high at arshan the main fault zone is exposed in an abandoned gravel quarry about 1 km east of the kyngarga river thirteen radiocarbon dates from the quarry a shallow trench in a graben and natural streamcuts constrain the timing of the latest two or three holocene paleoearthquakes the latest earthquake is bracketed by the 1024 1315 calendar years cal year b p age of an unfaulted terrace and by the 1947 2179 cal year b p age of a soil buried by scarp colluvium in the graben trench the penultimate earthquake is also relatively well constrained assuming that the displacement event slightly younger than 7091 7867 cal year b p in the upper quarry exposure is the same as the fissuring event slightly older than 6733 7385 cal year b p in the lower quarry exposure evidence for earlier event s between 9 2 12 7 ka depends on ambiguous stratigraphic evidence in the lower quarry exposure on the basis of only the two well dated earthquakes the recurrence interval at arshan may range from 2 9 to 6 8 ka for earthquake displacements of at least 1 3 m or a slip rate of 0 19 0 44 mm yr intermediate term 100 ka and long term circa 500 ka slip rates computed from terrace age estimates and fault scarp heights are 0 08 0 16 mm yr and 0 07 0 11 mm yr respectively rates that are considerably lower than the late holocene rate and the approximately 0 5 mm yr that might be inferred from the tectonic geomorphology of the tunka range front

magnitude of completeness b value and spatial correlation dimension of earthquakes in the south baikal basin baikal rift system abstract magnitude of completeness b value and correlation dimension of spatial epicenter distribution were estimated for the south baikal basin using earthquake catalogue for the period of digital observations all estimated parameters were mapped with a one for all approach that allowed revealing their spatial variability the value of the completeness magnitude which was evaluated on the frequency magnitude distribution was found to range from mc 1 0 to 1 6 over the basin depending on network configuration change with the average value of 1 1 0 1 then b value in the gutenberg richter relation was estimated by maximum likelihood method based on the catalogue with m 1 1 earthquakes the lower b value 0 81 0 03 was revealed for the southern part of the basin which is under transtension stress regime in contrast to pure extension characteristic for the rest of the territory the higher b value 1 01 0 01 was obtained for the area to the north east of the selenga delta where two of the largest baikal earthquakes occurred in the last 150 years and which is characterized by more heterogeneous crust structure correlation dimension in the southern part of the basin was found to be closed to 1 that is interpreted as a tendency of epicenters to be distributed along a line whereas the central part is characterized by a higher value that corresponds to a tendency of epicenters to be distributed over a plane rather than along lineaments a comparison of the southern and central basins which make up the tectonically single south baikal basin shows that seismicity of the central basin is characterized by higher b values and correlation dimension that may imply different stress level and or faulting type

upwarp of anomalous asthenosphere beneath the rio grande rift continental rifts are possible analogues of mid ocean ridges although major plate tectonic features are less clearly observed1 current thermal models of mid ocean ridges2 4 consist of solid lithospheric plates overlying the hotter less viscous asthenosphere with plate thickness increasing away from the ridge axis the lithospheric lower boundary lies at or near the melting point isotherm so that at greater depths higher temperatures account for lower viscosity lower seismic velocities and possibly partial melting upwarp of this boundary at the ridge axis concentrates heat there thus lowering densities by expansion and raising the sea floor to the level of thermal isostatic equilibrium at slow spreading ridges a major central graben forms owing to the mechanics of magma injection into the crust5 topography heat flow gravity and seismic studies support these models on the continents a low velocity channel has been observed although it is poorly developed beneath ancient cratons6 9 plate tectonic models have been applied to continental basins and margins10 12 but further similarities to the oceanic models remain elusive topographic uplift is often ascribed to airy type isostatic compensation caused by crustal thickening rather than thermal compensation in the asthenosphere here we discuss the rio grande rift in southwestern united states teleseismic p wave residuals show that regional uplift is explained by asthenosphere uplift rather than crustal thickening

baikal rift zone structure and geodynamics abstract the cenozoic baikal rift zone is superimposed on the caledonian baikal fold belt representing the suture between the precambrian siberian craton and several microcontinents the baikal rift zone consists of a system of disconnected fault bounded basins and extensional and wrench faults that straddles a major arch having a topographic relief of 2 3 km rifting activity commenced during the oligocene and is still active as evident from high seismicity of the baikal rift zone across the baikal rift zone upper crustal extension is considerably smaller than would be expected from its crustal thickness which decreases by 8 11 km across the baikal basins as compared to the adjacent unextended areas the discrepancy between upper and lower crustal attenuation could be attributed to intrusion of a major diapir indeed geophysical data indicate that the sayan baikal dome coincides with such a diapir the top of which is located at the crust mantle boundary this intrusion is held responsible for oligocene and quaternary doming of the rift zone development of the intra continental baikal rift system is thought to be related to asthenospheric diapirism active rifting however intra plate stresses in conjunction with the himalayan collision may have played some role

the inference of salt fingering from towed microstructure observations discrimination between salt fingering and turbulence induced microstructure has been based upon horizontal wavenumber spectra of temperature or of conductivity information on probability distributions is discarded in such spectral analyses we suggest that a discriminant may be found in probability distributions namely that salt fingering is associated with marked suppression of kurtosis this is demonstrated in records of towed microstructure from the eastern subtropical north pacific

rifting attractor structures in the baikal rift system location and effects abstract the current geodynamics and tectonophysics of the baikal rift system brs as recorded in lithospheric stress and strain are discussed in the context of self organization of nonlinear dissipative dynamic systems and nonlinear media the regional strain field inferred from instrumental seismic moment and fault radius data for almost 70 000 m lh 2 0 events of 1968 through 1994 shows a complex pattern with zones of high strain anisotropy in the central part and both flanks of the rift system the south baikal hovsgol and muya rift basins respectively the three zones of local strain anisotropy highs coincide with domains of predominantly vertical stress where earthquakes of different magnitudes are mostly of normal slip geometry pulse like reversals of principal stresses in the high strain domains appear to be nonlinear responses of the system to subcrustal processes in this respect the brs lithosphere is interpreted in terms of the self organization theory as a geological dissipative system correspondingly the domains of high strain anisotropy and stress change called rifting attractor structures ras are the driving forces of its evolution the location and nonlinear dynamics of the rifting attractors have controlled lithospheric stress and strain of the rift system over the period of observations and the same scenario may have been valid also in the mesozoic cenozoic rifting history the suggested model of a positive feedback fire like evolution of nonlinear dynamical systems with rifting attractors opens a new perspective on the current geodynamics and tectonophysics of the baikal rift system

tectonic evolution and regional setting of the cuu long basin vietnam abstract the cuu long basin is situated off the southeastern coast of vietnam this study utilizes all available seismic data in the cuu long basin vinh chau basin and coastal mekong delta to constrain the tectonic history of the area the linkage between the mae ping fault zone and deformation in the cuu long basin is also examined in detail two major deformational events are observed a transtensional rifting event from the mid eocene to mid early oligocene 40 31 ma followed by transpression from mid early oligocene to mid late oligocene 31 25 ma the data strongly suggest that all the very straight major tributaries of the mekong river delta follow northwest trending faults that link into to the southwestern end of the cuu long basin fault system and that the basin opened between where the mekong delta faults enter and a point of zero extension at the ne termination of the basin the mekong delta fault zone exhibited left lateral displacement during extension in the cuu long basin and right lateral displacement during younger transpression the orientation age and sense of displacement of the mekong delta fault zone are consistent with slip on the mae ping fault zone in thailand and cambodia and hence are considered to be part of the same regional fault system based on a chainat duplex analogue and other data the mekong delta and mae ping fault zones appear to be linked by a nnw striking step over zone referred to here as the mekong ton le sap step over zone this zone follows the eastern boundary of the paleocene to early eocene kampot fold belt based on structural and stratigraphic similarities it appears the adjacent vinh chau basin shares a common tectonic history with the cuu long basin but the linkage to the mekong delta fault zone is less clear

sulfide oxidation and distribution of metals near abandoned copper mines in coastal environments prince william sound alaska usa abstract the oxidation of sulfide rich rocks mostly leftover debris from cu mining in the early 20th century is contributing to metal contamination of local coastal environments in prince william sound alaska analyses of sulfide water sediment precipitate and biological samples from the beatson ellamar and threeman mine sites show that acidic surface waters generated from sulfide weathering are pathways for redistribution of environmentally important elements into and beyond the intertidal zone at each site volcanogenic massive sulfide deposits composed of pyrrhotite and or pyrite chalcopyrite sphalerite with subordinate galena arsenopyrite and cobaltite represent potent sources of cu zn pb as co cd and hg the resistance to oxidation among the major sulfides increases in the order pyrrhotite sphalerite chalcopyrite pyrite thus pyrrhotite rich rocks are typically more oxidized than those dominated by pyrite the pervasive alteration of pyrrhotite begins with rim replacement by marcasite followed by replacement of the core by sulfur fe sulfate and fe al sulfate the oxi dation of chalcopyrite and pyrite involves an encroachment by colloform fe oxyhydroxides at grain margins and along crosscutting cracks that gradually consumes the entire grain the complete oxidation of sulfide rich samples results in a porous aggregate of goethite lepidocrocite and amorphous fe oxyhydroxide enclosing hydrothermal and sedimentary silicates an inverse correlation between ph and metal concentrations is evident in water data from all three sites among all waters sampled pore waters from ellamar beach gravels have the lowest ph 3 and highest concentrations of base metals to 25 000 g l which result from oxidation of abundant sulfide rich debris in the sediment high levels of dissolved hg to 4100 ng l in the pore waters probably result from oxidation of sphalerite rich rocks the low ph and high concentrations of dissolved fe al and so4 are conducive to precipitation of interstitial jarosite in the intertidal gravels although pore waters from the intertidal zone at the threeman mine site have circumneutral ph values small amounts of dissolved fe2 in the pore waters are oxidized during mixing with seawater resulting in precipitation of fe oxyhydroxide flocs along the beach seawater interface at the beatson site surface waters funneled through the underground mine workings and discharged across the waste dumps have near neutral ph 6 7 7 3 and a relatively small base metal load however these streams probably play a role in the physical transport of metalliferous particulates into intertidal and offshore areas during storm events somewhat more acidic fluids to ph 5 3 occur in stagnant seeps and small streams emerging from the beatson waste dumps amorphous fe precipitates in stagnant waters at beatson have high cu 5 2 wt and zn 2 3 wt concentrations that probably reflect adsorption onto the extremely high surface area of colloidal particles conversely crystalline precipitates composed of ferrihydrite and schwertmannite that formed in the active flow of small streams have lower metal contents which are attributed to their smaller surface area and therefore fewer reactive sorption sites seeps containing precipitates with high metal contents may contribute contaminants to the marine environment during storm induced periods of high runoff preliminary chemical data for mussels mytilus edulis collected from beatson ellamar and threeman indicate that bioaccumulation of base metals is occurring in the marine environment at all three sites

observations of salt fingers in the central waters of the eastern north pacific this paper presents direct measurements of the temperature signature of salt fingers observed by a horizontally towed temperature sensor at a station in the middle of the subtropical gyre in the eastern north pacific the observed spectral shape compares favourably with predictions from the spectral model of schmitt 1979a in which a wide range of finger wave numbers grow the buoyancy flux due to temperature across the fingering interface is estimated by three methods a direct determination from the observed temperature gradient variance and predictions from two laboratory flux laws the order of magnitude agreement among these estimates is an encouraging start to the problem of parameterizing the heat salt and buoyancy fluxes associated with salt fingering in the central waters of the subtropical gyres a very real problem in view of the widespread occurrence of salt fingering signatures in the towed temperature records

thickness of the lithosphere beneath the baikal rift zone and adjacent regions abstract methods of separation of gravity anomalies related to mantle density inhomogeneity are developed based on the inversion of these anomalies with regard to some constraints obtained from seismic data the map of lithospheric thickness beneath the baikal rift zone and adjacent regions was made the lithospheric thickness beneath the baikal rift zone is estimated to be 40 50 km i e the lithosphere is thinned here to a crustal thickness beneath the siberian platform the lithospheric thickness increases to 200 km and underneath the trans baikal region of moderate cenozoic tectonic activity it ranges from 75 to 160 175 km therefore a wide asthenospheric upwelling was revealed beneath the rift zone the data available on the configuration of the baikal depression and that of the crust and the lithosphere as a whole made it possible to estimate the magnitudes of extension at different lithospheric levels the significant increase in extension with depth implies that rifting in the baikal zone was caused by asthenospheric diapirism

geodynamics of the baikal rift zone and plate tectonics of asia the baikal rift zone stretches from the southern termination of lake baikal as far as 118 e and includes the baikal basin and the barguzin upper angara and muysk grabens that form its easterly continuation coincident with the rift zone is a narrow seismic belt which extends further east along the stanovoy range to the okhotsk sea this belt can be considered as the boundary between the eurasian and amurian lithospheric plates earthquake focal mechanism solutions that show a consistent orientation of extensional axes across the strike of the rift zone indicate the behaviour of the eurasian and amurian plates as rigid bodies from 83 focal mechanism solutions the pole of rotation of the eurasian and amurian plates was computed to lie at the point 56 95n 117 45e the angular rate estimated from the amount of baikal opening is about 1 10 7 deg year the position of the pole at the eastern termination of the rift zone accounts for the dying out of the rift zone and for the appearance east of the pole of compressive structures in the stanovoy range to the southwest the baikal rift zone is replaced by the mosaic of microplates in central asia that occupy the space between the eurasian and indian lithospheric plates prominent are the mongolian dzungarian alashan tarim tibetan and pamir microplates earthquakes in central asia are dispersed although higher concentrations of earthquakes associated with microplate boundaries are prominent the tectonics of central asia are considered to be due to the interaction of rigid microplates assuming that plastic deformations occur within the relatively wide zones along the microplate boundaries by considering the strike slip faults to define the trajectories of microplate motions the parameters of relative movements of the microplates have been computed the microplate mosaic of central asia is moving northeasterly as a result of the collision and continued convergence of india and eurasia the mosaic acts as a wedge splitting off the amurian plate from eurasia causing it to rotate counterclockwise so opening the baikal rift

tectonic setting of western pacific marginal basins abstract global kinematics as well as magnetic anomalies of marginal basins with continental geology and paleomagnetic data as additional constraints are used to present a set of reconstructions of the western pacific marginal basins between 56 ma and the present at key periods 56 43 32 20 12 and 3 ma our model accounts for the rapid motion of exotic terranes along the whole of the western pacific convergent zone marginal basins appear to open in a great variety of tectonic settings the two extreme examples being the mariana trough where trench suction may be the predominant driving force and the south china sea where intracontinental deformation appears to be the major mechanism the study of marginal basins is a 3 d problem which must take into account the whole tectonic context subduction related tectonics in cross section and upper plate deformation in map view and not only a 2 d problem the classical trench arc back arc problem

faulting within the eastern baikal rift as characterized by earthquake studies earthquake relocations focal mechanism determinations waveform modeling and stress analyses using focal mechanism information are used to examine the faulting processes within the western baikal rift 95 to 108 5 results of the study show that high angle dip 70 faulting is most likely to occur in the more youthful parts of the rift and is most likely to be associated with m 4 5 events like the eastern rift nearly all earthquakes occurred on faults dipping 30 suggesting that in no part of the rift is significant deformation occurring on low angle structures focal mechanism information does not indicate there is a sharp boundary to the western rift as observed in the eastern rift less normal faulting rakes 60 to 120 occurs in the western rift than the eastern rift waveform modeling of the two largest earthquakes 1950 mondy and 1959 srednebaikal in the western rift gives focal depths of 10 to 15 km the 1959 event appears to have occurred along the eastern boundary fault on lake baikal just to the north of proval bay a feature created a m 7 1 earthquake in 1862 the 1950 event may have occurred along the southern boundary fault of the tunka depression stress orientation information indicates a near horizontal nw se to wnw ese striking 3 in the eastern part of the western rift

geometry of the hanging wall above a system of listric normal faults a numerical solution the process of hanging wall deformation above a system of listric faults is controlled by the local stress field rock properties and fault geometry although fault geometry provides a boundary condition the strained hanging wall can be adequately described by a process of simple shear during hanging wall collapse a numerical solution of hanging wall deformation above a set of listric normal faults clearly demonstrates the importance of inclined simple shear the true amount of horizontal extension and sense of inclined shear is estimated based on both rollover and fault geometry this forward modeling technique treats listric faults as continuously curved surfaces and may incorporate any number of faults even or unevenly spaced with variable fault geometry and near surface dip basin width is quite sensitive to sense of shear within the hanging wall and depth of detachment as well as the apparent horizontal extension calculated solely upon the basis of bed length balance the model also indicates that antithetic shear is likely to occur near steeply dipping normal faults whereas geologically reasonable basin geometries are generated under synthetic shear for low angle normal faults ramp and flat listric fault geometries can be modeled with inclined simple shear to generate basins care must be taken when assessing the amount of horizontal extension in extended terranes because the percentage of horizontal extension is quite sensitive to fault geometry and angle of internal shear arrays of listric normal faults may develop extensional geometries that resemble domino style extension if an apparent planar fault geometry observed near the surface assumes a listric shape downward and distributed deformation occurs within the hanging wall the resulting estimation of extension based on the angular relationship between faults and beds is likely to be wrong the model successfully describes the overall geometry of half graben type basins and indicates an important link between fault geometry and sense of shear within the hanging wall of listric normal faults near vertical and synthetic shear are determined near low angle normal faults whereas antithetic shear is likely to be observed near steeply dipping normal faults

faults and tectonic stresses of the baikal rift zone abstract the cenozoic baikal rift system consists of a central extensional segment which links a southwestern and a northeastern sinistral shear segment statistical studies on fault length amount and sense of slip depth of crustal penetration and spacing has led to the establishment of empirical relationships which appear to apply also for non extensional regimes results are graphically displayed in figs 4 7 tectonic stress regimes were determined by means of fault and joint analyses and on the basis of seismological data results of both approaches are compatible and explain the evolution of the baikal rift zone in terms of tensional reactivation of pre existing crustal discontinuities

accommodating large scale intracontinental extension and compression in a single stress field a key example from the baikal rift system the baikal rift system in southern siberia is one of the main intracontinental extensional features on earth the rift system represents the northwestern boundary of the amuria plate and in that respect can be considered as an evolving plate boundary the baikal rift system has been widely studied both in terms of geology and geophysics and many models have been proposed for its formation and evolution however the age of the initiation of deformation and the mechanism driving this deformation are still largely debated while major extension has occurred since the late miocene pliocene the onset of extension seems older than the india asia collision implying that several driving mechanisms may have acted together or in relay through time in this work we review the available data and models for deformation in an area encompassing the baikal rift system the sayan ranges to the west and the transbaikal to the east using a synthesis of this data and our own field and mapping observations we show that the baikal rift system along with transpressional deformation in the sayan ranges and transtension in the transbaikal area can be explained through major left lateral strike slip systems the deformation is strongly controlled by inherited crustal and lithospheric structures and is distributed over a wide area within the western amuria plate that consequently cannot be considered as a rigid block such distributed deformation is likely to have a strong effect on the structure of the future continental margin if extension evolves towards the formation of oceanic crust

low seismic velocity layers in the earth s crust beneath eastern siberia russia and central mongolia receiver function data and their possible geological implication analysis of teleseismic receiver functions at digital stations along the bratsk irkutsk ulanbaatar undurshil profile suggests that low velocity layers in the earth s crust exist not only beneath the baikal rift zone where such a layer was found earlier by deep seismic sounding dss but also beneath early paleozoic sayan baikal paleozoic mongolian early mesozoic mongolia okhotsk fold areas and beneath the siberian platform the reliability of detection of the low velocity layers by receiver function analysis has been checked by numerical modeling the results of this modeling demonstrate that receiver functions can reveal the low velocity layers in the crust if the initial model starting approximation is close to real velocity distribution and if the model medium is divided into thin layers averaged dss velocity model without low velocity layers was used as starting approximation for the inversion of observed receiver functions the low velocity layers are interpreted to reflect inhomogeneities of the earth s crust formed during its evolution most of these layers are presumed to correspond to thick mylonite zones related to large pre cenozoic thrusts the mylonites possess a great seismic anisotropy caused by the mineral orientation formed by the ductile flow in large thrust zones they can result in low velocity layers only for seismic waves whose rays are oriented perpendicular to the mylonite foliation i e in the direction of the minimum velocity the velocities along the foliation direction can be rather high therefore the low angle mylonite zones can be distinguished by the receiver function method which uses the waves from the teleseismic events with nearly vertically oriented rays the suggestion that the low velocity layers mark low angle thrusts is in agreement with gravity and geological data the amount of overthrusting is estimated to be as large as several hundred kilometers multichannel seismic profiling can be used to verify the existence and the deep geometry of the presumed thrusts d 2002 elsevier science b v all rights reserved

development of asymmetric basins along continental transform faults abstract many basins associated with wrench or transform faults are asymmetrical both longitudinally and laterally the sense of basin asymmetry changes along the strike of transform faults geophysical data from the dead sea rift and other transforms indicate that wrench induced asymmetric basins are bounded on only one side by a transform fault this implies that during the evolution of such basins maximum compression was parallel to the transform the direction of extension can change during the geologic evolution of a basin a possible explanation of this observation takes the strength of the fault into into consideration in the case of a weak fault in a strong crust the horizontal principal stresses would probably rotate to orientations parallel and perpendicular to the fault thus minimizing the shear stress on the main fault basins formed by regional extension such as in the east african rift system sometimes show similar features recent studies have shown that these basins often undergo oblique extension this suggests that the asymmetry in these basins may result from simultaneous strike slip motion and fault normal extension similar to the way asymmetry is produced in large transform basins periods of oblique extension can alternate with periods of orthogonal extension the asymmetry formed during periods of oblique extension is often preserved in the stratigraphic record

cenozoic rifting and volcanism in eastern china a mantle dynamic link to the indo asian collision abstract the indo asian continental collision is known to have had a great impact on crustal deformation in south central asia but its effects on the sublithospheric mantle remain uncertain studies of seismic anisotropy and volcanism have suggested that the collision may have driven significant lateral mantle flow under the asian continent similar to the observed lateral extrusion of asian crustal blocks here we present supporting evidence from p wave travel time seismic tomography and numerical modeling the tomography shows continuous low velocity asthenospheric mantle structures extending from the tibetan plateau to eastern china consistent with the notion of a collision driven lateral mantle extrusion numerical simulations suggest that at the presence of a low viscosity asthenosphere continued mass injection under the indo asian collision zone over the past 50 my could have driven significant lateral extrusion of the asthenospheric mantle leading to diffuse asthenospheric upwelling rifting and widespread cenozoic volcanism in eastern china