present day stress field changes along the baikal rift and tectonic implications intraplate extension in a frame of a global compressional stress field seems linked to local lithospheric perturbations lithospheric thinning or thickening able to modify the resulting state of stress zoback 1992 the baikal rift zone brz siberia is located north of the india asia collision zone and exhibits no direct communication with any oceanic domain it can thus be fully considered as an area of continental extension dominated by the global compressional intraplate stress field resulting from plate driving forces in order to address the problem of its dynamics and kinematics and their links with the india asia collision a comprehensive stress tensor analysis is presented based on 319 focal mechanisms of earthquakes located along the whole baikal rift the stress field is varying at different scales of observation when looking at central asia several thousands kilometers the maximum horizontal stress shmax directions remain rather constant with a fan shape geometry when the tectonic regime goes from compressional himalayas to extensional baikal when observing the baikal rift about 1000 km long clear variations of the stress regime are observed from an extensional regime in the central part of the rift to wrench ones in its northern and southern ends finally at the scale of 100 km systematic shmax reorientations occur close to major rift faults we thus infer that the interaction between collisional processes and inherited structures may have a strong influence on rift dynamics we then use computed stress tensors to predict slip vectors on major rift faults deformation patterns show two distinct parts of the rift the south baikal rift sbr is characterized by a constant trending around n100 e slip vector meanwhile the north baikal rift nbr exhibits a complex block rotation behavior involving at least three crustal blocks we propose to interpret these surficial structures and motions as the result of an interaction between the regional compression coming from the india asia collision and the geometry of the hardly deformable siberian platform this particular setting can explain most of the surficial deformation patterns which suggest a large scale cracking of the lithosphere in the baikal region other possible sources of stress could also be considered like deep mantellic upwelling or trench suction linked to the pacific subduction

aligned buoyant highs across trench deformation clustered volcanoes and deep earthquakes are not aligned with plate tectonic theory abstract bathymetry shows the regional interaction of aseismic buoyant highs in northern pacific subduction zones seamounts ridges and fractures on the seaward side of the trench are associated with events that do not support the accepted plate tectonics paradigm including an altered slab dip angle benioff zone and the clustered volcanoes and earthquakes within the convergent margin most of the examples in this study show a reduction in the number of total earthquakes but an increase in the deeper earthquakes an abnormal amount of across trench deformation and a larger amount of volcanism on the active arc than if no bouyant highs existed in the subduction zone the connections between the seaward highs and the landward clustered highs are the transverse faults which widen by turbidite scour as they age forearc canyons are the modern day bathymetdc expression of these faults all of the parameters introduced disagree with the plate tectonic hypothesis making an alternate explanation for the genesis necessary that explanation falls into the realm of the surge tectonic hypothesis which can explain by fluid mechanics and eastward flow each of the introduced parameters

development of forearcs of intraoceanic subduction zones the uplifted costa rican forearc landward of the middle america trench and the mariana forearc drilled on ipod leg 60 both lack the thick clastic sequences complex deformation and abundant evidence of accretion which characterize more widely known forearcs that border continents both regions contain significant in situ accumulations of pelagic and hemipelagic sediments in place of thick trench and trench slope basin sequences composed of terrigenous turbidites the nicoya peninsula of costa rica contains no significant melange terranes deformation of the mafic igneous basement and its thin cover of pelagic hemipelagic and first cycle volcanogenic material is mild overall with discrete zones of intense deformation disrupting otherwise well preserved stratigraphic sections intraoceanic subduction zones lacking longitudinal trench feed are sites of little or no accretion of sediments and recently suggested experimental and theoretical models of subduction zone processes involving flow melanges are inappropriate for intraoceanic forearcs intraoceanic forearcs generally lack high grade exotic components such as blueschist and eclogite tectonically incorporated as blocks in lower grade matrix although uplift and erosion of the forearc basement may provide detritus of amphibolite and ultramafic rock to the trench and trench slope

global positioning system measurements of strain accumulation across the imperial valley california 1986 1989 global positioning system gps data collected in southern california from 1986 to 1989 indicate considerable strain accumulation across the imperial valley displacements are computed at 29 stations in and near the valley from 1986 to 1988 and at 11 sites from 1988 to 1989 the earlier measurements indicate 5 9 1 0 cm yr right lateral differential velocity across the valley although the data are heavily influenced by the 1987 superstition hills earthquake sequence the 1988 1989 gps displacements are best modeled by 5 0 0 9 cm yr of valley crossing deformation but rates calculated from 15 years of terrestrial geodetic measurements 3 7 0 1 cm yr fit the data nearly as weh1 the relatively high gps rates are suspect because some measurements especially the east trending displacements have large errors there is evidence from gps and very long baseline interferometry observations that the present strain rate along the southern san andreas fault is smaller than the long term geologic estimate suggesting a lower earthquake potential than is currently assumed correspondingly a higher earthquake potential is suggested for the san jacinto fault

geomorphic controls on groundwater arsenic distribution in the mekong river delta cambodia natural arsenic contamination of groundwater is a severe public health crisis affecting nearly 60 million people in south asia and southeast asia alone contamination is spatially heterogeneous and results from the coupled microbial decomposition of organic matter and reductive dissolution of arsenic bearing iron minerals here we demonstrate that elevated arsenic concentrations arise in regions of recent organic matter deposition and thus are controlled by fluvial geomorphic processes arsenic contamination is best expressed within recent geomorphic features such as docked islands scroll bars and avulsions within these features the deposition of rapidly buried reactive organic matter facilitates microbial iron reduction and arsenic release ultimately the organic matter supply is exhausted and the conditions necessary for soluble arsenic to persist diminish

kinematic model of active deformation in central asia the velocity field of present day deformation in central asia is modelled using a set of four rotating blocks siberia tarim tibet india on a spherical earth a best fit is inverted on the basis of estimated shortening rates across the main thrust zones himalayas tien shan and measured slip rates along the principal strike slip faults altyn tagh and karakorum separating those blocks the fit to the data implies that nearly all the present convergence between india and asia can be accounted for by slip partitioning on these four zones with as much as 50 absorbed by northeastwards extrusion of tibet this suggests that localised deformation governs the present mechanical behaviour of the central asian lithosphere

tectonic and geomorphic interpretation of the ogasawara plateau region by multi beam based 3d methods abstract three dimensional views in this case of a subduction complex including the ogasawara plateau significantly enhance the geomorphic interpretation small scale features such as fracturing are apparent in some views and hidden in others so there is merit to producing more than one view this method shows a marked improvement over the use of two dimensional map views coloring or models because the user has less to carry and computer generated products take less time to construct

mode of cenozoic east west extension in tibet suggesting a common origin of rifts in asia during the indo asian collision cenozoic rifts in tibet were traditionally interpreted as a result of topographic collapse of the tibetan plateau reaching the maximum elevation that can be supported by its mechanical strength recent studies have emphasized possible similarities between rifting in tibet and extension in the basin and range of the western united states however when examined in detail one finds that spacing of long 100 km rifts in tibet 100 300 km is significantly greater than that in the basin and range 20 40 km from south to north rift spacing decreases systematically from 191 67 km in the himalaya south of the indus yalu suture to 146 34 km in south tibet between the indus yalu and bangong nujiang sutures and farther north to 101 31 km in central tibet between the bangong nujiang and jinsha sutures instability analysis suggests that the mantle lithosphere must have been involved in east west tibetan extension specifically the widely spaced rifts in the himalaya and tibet may have been related to the presence of a relatively light crust density 2 90 g cm 3 and a strong mantle lithosphere 40 km thick and a factor of 5 stronger than the upper crust the observed systematic decrease in rift spacing can be explained by the known decrease in the crustal thickness in tibet from 70 80 km in the himalaya in the south to 50 55 km in central tibet in the north a regional comparison of rifts in east asia suggests that both the involvement of the mantle lithosphere and the age of rift initiation are similar for tibetan rifts the baikal rift and the shanxi graben this implies that topographic collapse or a convective event in the mantle cannot be the sole cause for the development of the tibetan rifts a regional boundary condition applied throughout east asia must be required

estimating ages of late quaternary stream terraces from analysis of weathering rinds and soils rates of growth of rock weathering rinds and rates of changes of soil morphologic and chemical properties that are calibrated at sites of known ages produce powerful tools for estimating ages of geomorphic surfaces particularly when materials datable by conventional methods are not widespread sequences of latest quaternary stream terraces from the south island of new zealand are ideally suited for application of these calibrated age techniques because climate and source materials for terrace deposits are internally consistent within two regions west land and marlborough modal and mean thicknesses of weathering rinds formed in surface cobbles of torlesse graywacke provide age estimates for latest pleistocene and holocene stream terraces in marlborough with uncertainties estimated at 5 to 40 modal rind thickness of subsurface cobbles provides age estimates for latest quaternary westland sites with uncertainties of 10 to more than 50 few dated soils are available to constrain rates of soil development in westland and marlborough furthermore soils were sampled from only one pedon on each surface morphologic data however provide least squares age estimates for westland sites with uncertainties typically greater than 100 and the calibration data can further bracket likely soil ages to less than 50 uncertainty the regression data from marlborough are less reliable but profile index ratios help bracket ages to at least 50 distributions of oxalate extractable fe and ai and total major element concentrations allow age estimates for westland and marlborough soils with uncertainties estimated at 15 to 50 despite these uncertainties the three techniques combine to provide age estimates with the high resolution needed to examine details of fault slip rates or stream downcutting the calibration data and weathering rates used herein are applicable only to the new zealand region studied although similar calibrated age techniques should provide high resolution dating in many other regions such techniques are particularly useful when multiple approaches are available to provide independent cross checks

the velocity field along the san andreas fault in central and southern california the velocity field within a 100 km broad zone centered on the san andreas fault between the mexican border and san francisco bay has been inferred from repeated surveys of trilateration networks in the 1973 1989 interval the velocity field has the appearance of a shear flow that remains parallel to the local strike of the fault even through such major deflections as the big bend of the san andreas fault in the transverse ranges of southern california across strike profiles of the fault parallel component of velocity exhibit the expected sigmoidal shape whereas across strike profiles of the fault normal component of velocity are flat and featureless no significant convergence upon the fault is observed even along the big bend sector of the fault simple dislocation models can explain most of the features of the observed velocity field but those explanations are not unique about 35 mm yr of relative plate motion is accounted for within the span of the trilateration networks geologic studies indicate that the secular slip rate on the san andreas fault is about 35 mm yr the agreement between these two estimates implies that most of the strain accumulation is elastic and will be recovered in subsequent earthquakes the relative motion observed across the san andreas fault 35 mm yr plus that observed across the eastern california shear zone 8 mm yr accounts for most 43 mm yr of the observed north america pacific relative plate motion 47 mm yr

processes of seamount subduction in the mariana and izu bonin trenches abstract morphology of oceanic plate seamounts near the mariana and izu bonin trenches in the western pacific indicates that the response of these seamounts to subduction related tectonic processes is a function of two factors seamount size and the degree of fracturing of the surrounding sea floor as the oceanic plate bends prior to subduction seamounts smaller than about 40 km in diameter are fractured on the same scale as or to a greater degree than the surrounding sea floor larger seamounts 40 100 km in diameter show less evidence of fracturing prior to subduction since there is no evidence of accretion of large amounts of plate material onto the mariana fore arc it is probable that plate seamounts are subducted at least partially intact the subduction of plate seamounts beneath the mariana arc may be responsible for some areas of uplift on the outer mariana fore arc broad areas of uplift on the mariana and bonin fore arcs are associated in some cases with apparent subduction of ridges of large greater than 100 km diameter seamounts the large seamounts at the western end of the michelson ridge appear to be breaking up into a series of blocks separated by en echelon fractures that are oriented oblique to the trench axis and lie in line with the convergence direction of the pacific plate such large seamounts may interrupt normal subduction processes this suggestion is consistent with the occurrence of gaps in benioff zone seismicity where chains of such large seamounts abut the trenches it appears likely that these large seamounts are also eventually subducted although they may first undergo significant fracturing and reorientation

strain accumulation in southern california 1973 1984 repeated surveys of selected lines from five trilateration networks along the san andreas fault in southern california have been used to deduce the 1973 1984 strain accumulation records at five localities the secular rate of engineering shear strain accumulation is about 0 3 rad yr with the plane of maximum shear parallel to the local strike of the san andreas fault the secular rate of accumulation of areal dilatation is negligible the data were examined to detect evidence for fluctuations in the rate of strain accumulation for this examination 19 lines were removed from the data set four because they exhibited an obvious coseismic offset and 15 others because they contained at least one very anomalous measurement that could reasonably be attributed to a survey blunder the incidence of such blunders appears to be one in every 75 measurements the remaining data consist of 104 lines with an average of 10 measurements each although the strain accumulation plots for the five networks may exhibit marginally significant temporal fluctuations we are not convinced that those fluctuations are greater than could be attributed to survey error in particular we are unable to demonstrate that the 1973 1979 southern california strain anomaly reported by savage and others is real given the uncertainty in the random and systematic errors in measurement the strain measurements in southern california are marginally consistent with linear in time strain accumulation the strain accumulation plots for the salton network clearly established that unlike the deformation reported after the 1940 imperial valley earthquake no acceleration in the shear strain rate has yet been observed following the 1979 imperial valley earthquake

computer simulations of california tectonics confirm very low strength of major faults recent improvements in the technique of modeling fault networks with thin plate finite elements yield full convergence of the solutions without any compromise in the representation of the frictional rheology of the upper crust or the transition to dislocation creep in the lower crust we apply these techniques to model the california region incorporating all faults with estimated slip rate over 1 mm yr as well as variations in elevation heat flow and crustal thickness velocity boundary conditions on the model sides are based on the nuvel 1 plate model and an approximation of deformation in the great basin the frictional and dislocation creep rheologic constants of the crust are calibrated to reproduce the observed variations in the maximum depth of seismicity which occurs at model temperatures of 350 410 c this leaves two free parameters the time averaged coefficient of friction on faults and the apparent activation energy for creep in the lower crust these parameters are systematically varied in three sets of 10 81 and 64 experiments respectively the predictions of each model are tested against three published data sets a set of 79 geologic limits on average slip rate on faults a set of 221 principal stress directions and a set of 841 secular rates of geodetic baselines both trilateration and vlbi very long baseline interferometry the patterns of model scores indicate that the time averaged friction coefficient of major faults is only 0 17 0 25 or only 20 30 of the value 0 85 that is assumed for the friction in the intervening blocks the laser trilateration data taken alone would indicate a preference for a high stress model but this model is one with very little fault slip and seismicity which can be rejected on other grounds in the final three parameter set of 64 models we test whether the anomalous weakness of faults is uniform or proportional to net slip with slip dependent weakening there is a general reduction in prediction errors which supports the hypothesis the geologic data and geodetic data agree in implying that fault weakness that is friction of 0 17 is general with an additional 30 slip dependent weakening of the san andreas to friction of 0 12 or 14 of normal the stress data however are fit best if all weakness is slip dependent effectively only the san andreas is weak thus the hypothesis that all weakening is slip dependent should not be rejected the best models predict all available data with rms root mean square mismatch of as little as 3 mm yr 6 of the relative plate velocity so their predictions may be useful for seismic hazard estimation at least along faults where no data are available this study extends the finding of very low friction from subduction zones where it was previously documented to a dominantly strike slip system in which underthrusting of wet sediment is not widespread return of previously subducted water from the mantle and its chemical or physical binding in fine grained gouge are possible explanations for the very low friction

very long baseline interferometry and active rotations of crustal blocks in the western transverse ranges california changes in baseline vectors between very long baseline interferometry vlbi receiving stations in the western transverse ranges imply that east west blocks of crust in this region rotate clockwise about vertical axes with respect to the pacific and north american plates minimum apparent rotations given by the ratios between components of velocity perpendicular to baseline vectors and the lengths of the baselines imply minimum current rotation rates of a few degrees per million years the relevant vlbi receivers lie on different crustal blocks that are separated by major active faults both geologic and other geodetic observations imply north south convergence between such blocks at several millimeters per year corrections to perpendicular components of velocity for such relative movements between blocks yield likely clockwise rotation rates of 6 m y 2 m y which are indistinguishable from the average rate inferred from paleomagnetic declinations of rocks in the western transverse ranges with ages less than 15 m y thus rotation seems to have occurred continuously and apparently with only small variations in rate during a period when the tectonics of southern california changed dramatically this apparent independence of the rotation rate on the changing surface kinematics is consistent 1 with such rotation being a manifestation of continuous deformation at depth in the lower crust and upper mantle 2 with weak faults separating upper crustal blocks and 3 with the important resistance to continental deformation lying in the upper mantle and or lower crust

on steady rate coupling between an elastic upper crust and a viscous interior although it is commonly accepted that the upper seismogenic layer of the earth s crust deforms elastically except in localized zones faults where deformation is discontinuous the sub seismogenic layer is not as well understand possibilities for the rheology include viscous or viscoelastic behavior and localization of shear to relatively thin zones that can sometimes be 1 km in width if the sub seismogenic layer is viscous or viscoelastic then during interseismic periods a steady rate of deformation will be achieved after transients associated with viscoelastic relaxation have decayed away for sufficiently long earthquake cycles i find that in this situation the motion in the elastic layer is decoupled from that deep within the viscous layer and they may be very different from each other this implies that one can not use contemporary geodetic data to infer steady motion in a viscous sub seismogenic layer although it may be used to study viscoelastic relaxation

erosion and physical transport via overland flow of arsenic and lead bound to silt sized particles understanding of the transport mechanisms of contaminated soils and sediment is essential for the sustainable management of contaminated lands in new england and elsewhere vast areas of agricultural lands are contaminated by the historical application of lead arsenate pesticides left undisturbed the physical and chemical mobility of as and pb in these soils is limited due to their strong affinity for adsorption onto solid phases however soil disturbance promotes erosion and overland flow during intense rainstorms here we investigate the event scale transport of disturbed as and pb contaminated soils through measurement of concentrations of as and pb in suspended sediment and changes in pb isotopic ratios in overland flow investigation of several rain events shows that where land disturbance has occurred physical transport of silt sized particles and aggregates is the primary transport vector of as and pb derived from pesticide contaminated soil although both as and pb are associated with similarly sized particles we find that solid phase as is more effectively mobilized and transported than pb our results demonstrate that anthropogenic land disturbance of historical lands contaminated with lead arsenate pesticides may redistribute through physical transport significant amounts of as and lesser amounts of pb to riparian and stream sediments where they are potentially more bioavailable

paleostress reconstructions and geodynamics of the baikal region central asia part 2 cenozoic rifting abstract investigations on the kinematics of rift opening and the associated stress field present a renewed interest since it has recently been shown that the control of the origin and evolution of sedimentary basins depends to a large extent on the interplay between lithospheric strength and applied stresses it appears that changes of stress field with time are an important factor that either controls or results from the rifting process the object of this paper is to study the changes of fault kinematics and paleostress field with time in the baikal rift system during the cenozoic reduced paleostress tensors were determined by inversion from fault slip data measured in the central part of the rift and its southwestern termination between 1991 and 1995 results show that the stress field varies as well in time as in space two major paleostress stages are determined corresponding broadly to the classical stages of rift evolution late oligocene early pliocene and late pliocene quaternary the first paleostress stage is related to the rift initiation and the second to the major stage of rift development similarities between the recent paleostress field and the present day stress field inverted from focal mechanisms indicate that the second paleostress stage is still active therefore we propose to use proto rift for the late oligocene early pliocene stage and active rift for the late pliocene quaternary stage of rift development during the proto rift stage the stress field was characterized by a compressional to strike slip regime a progressive change from transpression to transtension is suspected for the central part of the rift baikal and barguzin basins during this period in the western termination of the rift sayan massif tunka depression a strongly compressional stress field with oblique thrusting kinematics is well constrained in the late miocene early pliocene interval the active rift stage was initiated by a marked change in fault kinematics and stress regime in the late pliocene in the central part of the rift the stress regime changed into pure extension while in the southwestern extremity it changed into pure strike slip fault kinematics suggests that rifting was initiated by an extrusion mechanism due to the interaction of far field compressional stress on a mechanically heterogeneous crust with the southwards pointing wedge of the siberian craton acting as a passive indentor the cenozoic time space evolution of the stress field is believed to reflect the increasing influence of locally generated buoyancy extensional stresses associated with density anomalies of the lithosphere on intraplate stresses generated by the india eurasia convergence and the west pacific subduction

the use of stream flow routing for direct channel precipitation with isotopically based hydrograph separations the role of new water in stormflow generation abstract understanding the pathways by which event water contributes to stream stormflow provides insight into stormflow generation mechanisms we analyze the impact of storm size on the relative contribution of event water to stormflow by using natural variations in the oxygen isotopic composition of precipitation and stream water to separate multiple stormflow hydrographs from a single fourth order 1212 ha catchment we extend previous isotope based hydrograph separations by independently accounting for the contribution of event water via direct channel precipitation to the stream hydrograph the direct channel precipitation contribution is determined using a numerical model of stream flow routing though the catchment taking precipitation and digital elevation data as input variables for the range of storm sizes sampled having recurrence intervals ranging from less than a week to 4 months essentially all the event water in stream stormflow can be attributed to direct channel precipitation event water not directly falling on the stream channel indirectly contributes to stormflow by increasing the subsurface discharge of pre event water to the stream neither the hydrograph separation data field observations during the precipitation events nor experimental observations of flow in a large scale natural soil column extracted from the watershed are consistent with macropore flow or groundwater ridging as the primary mechanism responsible for increasing subsurface discharge results from a series of artificial rain experiments using the unsaturated natural soil column are consistent with a preferential kinematic flow model and indicate that the discharge of pre event water to the stream during a storm event may be controlled by kinematic flow processes within the watershed soils

focal mechanisms of large earthquakes in the south island of new zealand implications for the accommodation of pacific australia plate motion b the plate motion model nuvel 1 predicts oblique convergence between the pacific and australian plates in the south island of new zealand we used p and sh body waveform analysis to constrain the focal mechanisms of the 15 largest earthquakes ms 5 8 that have occurred in this region since 1964 in order to see how the plate motion is accommodated at the southern end of the alpine fault convergence is achieved by oblique slip movement along a concentrated zone of deformation in the southern offshore region one event may be related to thrusting of the australian plate beneath the pacific plate and another strike slip event probably demonstrates movement on an active strike slip fault system parallel to but offset from the southern limit of the alpine fault this geometry provides a possible mechanism for the rapid uplift of the fiordland region deformation in the northern south island is more distributed in the south west marlborough region partitioning occurs between strike slip faulting in the se and reverse faulting farther nw in the buller region we suggest that the partitioning developed as a consequence of an increasing component of shortening that was accommodated by slip on reactivated pre existing normal faults in the buller region shortening in the buller region may have deflected the ne end of the alpine fault towards the nw forming the prominent bend the marlborough fault system with its youngest and most active faults to the se probably developed in an attempt to maintain a through going strike slip structure as each of the strike slip faults was transported towards the north west partitioning of the opposite polarity with reverse faulting se of the strike slip faulting occurs in north east marlborough the boundary between the two different styles of partitioning in ne and sw marlborough appears to coincide with a change in the nature of the downgoing slab and a change in strike of faults of the marlborough fault system a normal faulting earthquake on the northern edge of the chatham rise probably results from a complex interaction of the buoyant continental crust in that region with the subduction zone and the overlying marlborough fault system

the motion of crustal blocks driven by flow of the lower lithosphere and implications for slip rates of continental strike slip faults geodetic measurements in actively deforming areas of the continents reveal the pattern of deformation in the lithosphere if the dominant forces acting on crustal blocks are tractions at their bases then the long term motion of each block will be given by the average velocity of the underlying lithosphere slip rates between blocks estimated in this way from recent geodetic measurements across fault zones in the south island of new zealand and southern california are in good agreement with slip rates estimated geologically

new finite element techniques for modeling deformation histories of continents with stratified temperature dependent rheology previous techniques for modeling anelastic deformation of the lithosphere have included plane strain models restricted to two dimensional problems and quasi three dimensional plane stress or thin plate models that did not accurately include the effects of the shallow frictional layer or of kinematic detachment of crust from mantle this paper presents techniques to remedy these deficiencies of thin plate models an iteration strategy in which the rheology is linearized using artificial prestress and a particular effective viscosity tensor causes the calculation of horizontal velocities to converge monotonically even with a frictional layer at the top of the lithosphere a technique using two planar grids allows deformation and displacement to be different at the crust and mantle levels at far less cost than that of a three dimensional grid a finite element technique is developed for computing the changes in thickness of these layers caused by pure shear simple shear and pressure gradients a technique based on relaxation of perturbation eigenfunctions solves the heat equation in the lithosphere during deformation accuracy of component numerical methods is good for simple test problems but in realistic nonlinear problems utilizing all components only the precision can be determined because of the lack of analytic solutions precision of the combined program is tested with a realistic sample problem and presented as a function of the number of iterations in each velocity solution convergence factor 0 73 to 0 88 size of time step in the predictor corrector time integration convergence as t0 8 and number of degrees of freedom in the finite element grid convergence as n 0 5 to 0 8 for most variables overall cost of a simulation varies with the fractional precision as 3 3 a new consequence of kinematic detachment a moving wave of crustal thickness is found unfortunately the form of the wave depends on the finite element size this means that element size must be chosen to approximate the smoothing by flexural rigidity effects that were neglected because of cost

strain distribution across the australian pacific plate boundary in the central south island new zealand from 1992 gps and earlier terrestrial observations as part of a geodetic experiment aimed at understanding the deformation associated with the australian pacific plate boundary in the south island of new zealand in 1992 we reoccupied using global positioning system gps techniques a first order triangulation and trilateration network established in 1978 between christchurch on the east coast and hokitika on the west the network crosses the south island a few tens of kilometers southwest of the region where the plate boundary changes from a single throughgoing oblique slip fault the alpine fault to a series of subparallel strike slip faults the marlborough faults the gps data have been analyzed as daily network solutions with about 12 stations in each solution rms repeatabilities for stations with multiple occupations are 4 mm 8 mm and 15 mm in the north east and vertical components respectively the observed strain across the network is consistent with the entire nuvel ia pacific australia plate velocity being accommodated on land across this part of the south island shear strain rates derived from the gps and terrestrial data show that the highest strain rate 0 4 rad yr occurs in the region of the southern alps and alpine fault this rate is about two thirds of the rate predicted from the nuvel ia plate velocity model assuming that all the plate boundary deformation occurs across this region implying that about two thirds of the plate motion is accommodated in the vicinity of the alpine fault significant shear strain rates greater than 0 2 rad yr are also observed farther east particularly in the region of the porters pass amberley fault zone demonstrating that this zone accommodates a significant part of the plate boundary deformation using dislocation modeling we show that the variation in fault parallel shear strain rates across the alpine fault is reasonably consistent with a nonoptimized dislocation model involving a 50 dipping fault that is presently locked to a depth of 12 km one explanation for this observation is that the upper 12 km of the crust in this area is storing most of the right lateral shear component of the relative plate motion as elastic energy that will be released in a future major earthquake the variation in the other component of shear strain which may be interpreted as fault normal relative contraction is not explained by a simple dislocation model implying that the normal component of plate motion is taken up in other ways

geodetic measurement of horizontal deformation in the northern san francisco bay region california analysis of geodolite observations in the coast ranges of northern california indicates that relative plate motion is spread over a wide zone little or no deformation occurs to the southwest of the san andreas fault itself to the northeast of the san andreas fault a region transected by the parallel rodgers creek and maacama faults 25 6 mm yr of right lateral slip is distributed over about 60 km there is a peak shear strain of 0 6 0 1 rad yr near the san andreas fault elsewhere in the deforming zone shear strain rates are 0 3 0 04 rad yr at the northeastern boundary of the study area the strain rate is less than 0 2 rad yr except for the high strain rate near the san andreas fault there is no evidence of any localization of deformation to the surface traces of the faults nor is there any evidence of creep or aseismic slip on any of the faults in the area the data indicate that the relative motion between the pacific and north american plate occurs primarily to the northeast of the san andreas fault the observations are inconsistent with models that include slip at depth on only the san andreas fault the data can be fit equally well by models with slip at depth on all three of the major faults or with motion distributed across the entire zone from the san andreas fault to the napa valley fault preliminary analysis of observations to the farallon islands about 40 km southwest of the san andreas fault suggests that little or no additional relative motion is accommodated on that side of the san andreas fault in particular it is unlikely that the 20 mm yr difference between the plate motion rate about 55 mm yr and the san andreas fault system rate about 35 mm yr can be explained by deformation between the san andreas fault and the farallon islands

normal faulting in the upper continental crust observations from regions of active extension abstract observations of present day normal faulting in regions of active continental extension may be helpful when interpreting the geological record in older extensional basins the most obvious manifestation of active extension is normal faulting in earthquakes earthquake foci are mostly confined to the upper seismogenic continental crust whose thickness imposes a scale on the observed deformation large earthquakes move faults with lengths similar or large compared with the thickness of the seismogenic layer large seismogenic normal faults on the continents appear to be restricted to a dip range of around 30 60 dips significantly gentler than 20 have not been observed in fault plane solutions of large earthquakes large seismographic normal faults are approximately planar in cross section and cut through the base of the upper seismogenic layer rotating about a horizontal axis as they move a reasonable estimate of the regional extension can often be made from the dip of the large normal faults and the tilt of the blocks they bound the domino model such faults are rarely continuous for more than 15 20 km but commonly change strike or step in an en echelon fashion this segmentation may occur on a scale controlled by the thickness of the seismogenic layer and is an important influence on sedimentation and drainage these observations are common to large seismogenic normal faults from a variety of tectonic settings and suggest that the kinematics of normal faulting may not be strongly influenced by the forces responsible for the extension which can vary widely in nature and magnitude small earthquakes which move faults that are small compared with the seismogenic layer thickness show no simple pattern and often can be interpreted as internal deformation of blocks bounded by large faults in some places the seismogenic basement faults do not reach the surface but are decoupled from the sedimentary cover by layers of weak lithology faults in the sedimentary cover may be strongly curved in cross section requiring their hangingwalls to deform internally estimating extension from such faults is not straightforward and requires a knowledge of the hangingwall deformation estimates made in this way are often very non unique and prone to large errors we postulate that the thickness of the seismogenic upper crust controls both the maximum length of large normal fault segments along strike and the maximum size of blocks that can rotate coherently about a horizontal axis

evaluating the role of preexisting weaknesses and topographic distributions in the indo asian collision by use of a thin shell numerical model thin viscous sheet models have proved to be very useful in exploring the interaction between plate boundary and gravitational forces during continental collision however simplifications of these models e g absence of faults planar geometry of the collision zone limit their use in making specific predictions regarding tectonic evolution such as the role of eastward extrusion along major strike slip faults during the indo asian collision this deficiency is overcome by a thin shell finite element model with faults that can assess the effects of preexisting fault configurations and topographic distributions on velocity fields numerical simulations of a palinspastically restored asia at ca 50 ma suggest that preexisting lithospheric weaknesses favor north south shortening during initial collision whereas preexisting high topography in southern asia promotes eastward extrusion these results underscore the first order importance of initial topography and the distribution of preexisting faults in the outcome of geodynamic modeling

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

multi beam surveys of the michelson ridge guyots subduction or obduction abstract this is the first graphic presentation of the michelson ridge in its entirety from a total coverage multi beam survey the ridge splits the izu and bonin trenches is comprised of four guyots and is anomalous to the strike of all other ridges heretofore discovered in the pacific ocean the ridge is obducting offscraping onto the philippine plate instead of subducting

behavior of the brittle crust in wide plate boundary zones in wide and active continental plate boundary zones ductile flow in the deeper and strong parts of the lithosphere may control crustal deformation this is likely if average resistive shear stresses on faults in the brittle crust are much less than 108 pa and the underlying bulk effective viscosity is much greater than 1021 pa s in this case a simple model of distributed deformation referred to as the floating block model may be useful this treats the crust as an array of rotating and translating rigid blocks which are floating on an underlying continuous flow with a constant rheology the model is analyzed in detail in this paper because it has the potential to link detailed observations of crustal deformation with the large scale flow crustal blocks are defined by at least two sets of faults the kinematics of crustal deformation can be described in terms of the motions of these blocks both the relative motion on block boundaries faults and block tilting about a horizontal axis can be described in terms of the underlying flow and block rotation about a vertical axis however rotations about a vertical axis which are an important component of the crustal deformation will depend not only on the underlying flow but also on the shape orientation and arrangement of the crustal blocks the average rotation rate about a vertical axis over finite rotations will be significantly different from that predicted at any instant also the rotation history is considerably complicated if as is likely the underlying flow field or block shape has changed with time these aspects of crustal deformation are discussed with reference to real zones of active deformation in new zealand greece and western north america

geomorphic mesozoic and cenozoic evolution in the oka jombolok region east sayan ranges siberia the east sayan ranges are a key area to understand the interactions between the transpressive deformation linked to the far field effects of the india asia collision and the extension linked to the opening of the baikal rift system the active deformation that affects this range is very recent around 5 ma but occurs in a very complex morphotectonic setting and the understanding of the tertiary deformation relies entirely on a detailed knowledge of the pre deformation situation using apatite fission track thermochronology cosmogenic 10be and morphological study on tertiary lava flows we demonstrate that prior to the oligocene the morphology of the east sayan area was characterized by a wide constantly rejuvenated erosion surface apatite fission track thermal modelling indicates that this surface started to form at least in late jurassic early cretaceous 140 120 ma the long term exhumation rates several tens of million years derived from apatite fission track data 17 5 m ma and the short term erosion rates over a few hundred thousand years derived from cosmogenic 10be data 12 20 m ma are coherent implying a near constant mean erosion rate since late jurassic this constant slow erosion prevented the formation of a lateritic kaolinic weathering crust on the planation surface by oligocene early miocene times a long wavelength uplift that remains to be explained induced incision that created shallow valleys later filled by basaltic lava flows finally the present short wavelength topography initiated during the pliocene

noncohesive critical coulomb wedges an exact solution active fold and thrust belts or submarine accretionary complexes can be modeled as critically tapered wedges of material on the verge of coulomb failure everywhere overlying a basal decollement where frictional sliding is occurring ignoring cohesion the four strength parameters needed to describe a critical coulomb wedge are its internal and basal coefficients of friction and b and its internal and basal hubbert rubey fluid pressure ratios and b an exact relation between surface slope and basal dip of a noncohesive critical wedge with uniform properties is derived the state of stress within such a wedge has the same orientation everywhere and is constant if is and vice versa a coefficient of internal friction 1 1 is consistent with the known surface slope basal dip and pore fluid pressures in the active fold and thrust belt of western taiwan assuming that byerlee s law b 0 85 is valid on the base the wide variety of tectonic styles observed to occur along convergent margins including subduction erosion active accretion subduction without accretion and even extension and normal faulting may be controlled by relatively small spatial or temporal variations in either b or b

differences in trace metal concentrations among fluvial morphologic units and implications for sampling this study examines the segregation of trace metals within and between fluvial morphologic units in sand sized and finer bed sediments in a cobble bed stream the types of fluvial morphologic units sampled are low gradient riffles high gradient riffles glides eddy drop zones lateral scour pools attached bars and detached bars three to nine samples were collected from ten of each type of morphologic unit all 12 metals show significantly different concentrations between some morphologic units in sediments smaller than 2 mm eddy drop zones and attached bars consistently have the highest metal concentrations while low gradient riffles high gradient riffles and glides typically have the lowest concentrations metals showing the greatest between unit variability are al cr fe cu and ti followed by co mn and pb while mg mn v and zn display relatively few differences between units lateral and longitudinal variations of metals within units are not significant and there was no consistent predictable variation in metal concentrations with distance downstream results indicate that metal studies in other gravel and cobble bed streams should include a reconnaissance survey to determine variations between morphologic units stratify sampling by morphologic unit and analyze spatial autocorrelation to determine sample spacing

a review of the source behaviour and distribution of arsenic in natural waters the range of as concentrations found in natural waters is large ranging from less than 0 5 g l 1 to more than 5000 g l 1 typical concentrations in freshwater are less than 10 g l 1 and frequently less than 1 g l 1 rarely much higher concentrations are found particularly in groundwater in such areas more than 10 of wells may be affected defined as those exceeding 50 g l 1 and in the worst cases this figure may exceed 90 well known high as groundwater areas have been found in argentina chile mexico china and hungary and more recently in west bengal india bangladesh and vietnam the scale of the problem in terms of population exposed to high as concentrations is greatest in the bengal basin with more than 40 million people drinking water containing excessive as these large scale natural as groundwater problem areas tend to be found in two types of environment firstly inland or closed basins in arid or semi arid areas and secondly strongly reducing aquifers often derived from alluvium both environments tend to contain geologically young sediments and to be in flat low lying areas where groundwater flow is sluggish historically these are poorly flushed aquifers and any as released from the sediments following burial has been able to accumulate in the groundwater arsenic rich groundwaters are also found in geothermal areas and on a more localised scale in areas of mining activity and where oxidation of sulphide minerals has occurred the as content of the aquifer materials in major problem aquifers does not appear to be exceptionally high being normally in the range 1 20 mg kg 1 there appear to be two distinct triggers that can lead to the release of as on a large scale the first is the development of high ph 8 5 conditions in semi arid or arid environments usually as a result of the combined effects of mineral weathering and high evaporation rates this ph change leads either to the desorption of adsorbed as especially as v species and a range of other anion forming elements v b f mo se and u from mineral oxides especially fe oxides or it prevents them from being adsorbed the second trigger is the development of strongly reducing conditions at near neutral ph values leading to the desorption of as from mineral oxides and to the reductive dissolution of fe and mn oxides also leading to as release iron ii and as iii are relatively abundant in these groundwaters and so4 concentrations are small typically 1 mg l 1 or less large concentrations of phosphate bicarbonate silicate and possibly organic matter can enhance the desorption of as because of competition for adsorption sites a characteristic feature of high groundwater as areas is the large degree of spatial variability in as concentrations in the groundwaters this means that it may be difficult or impossible to predict reliably the likely concentration of as in a particular well from the results of neighbouring wells and means that there is little alternative but to analyse each well arsenic affected aquifers are restricted to certain environments and appear to be the exception rather than the rule in most aquifers the majority of wells are likely to be unaffected even when for example they contain high concentrations of dissolved fe

progressive growth of san clemente island california by blind thrust faulting implications for fault slip partitioning in the california continental borderland we find that the genesis of san clemente island and its surrounding submarine platform is consistent with progressive slip on two southeast striking southwest dipping blind thrust fault segments since their inception 2 to 5 ma 3 km of compression normal to the n150 e fault strike has been accommodated with 1700 m of domal uplift of the san clemente anticlinorium the existence of an extensive suite of pleistocene marine terraces provides evidence that slip and uplift are continuing today based on direct terrace fossil age determinations and correlations of terrace heights with global sea level curves we estimate that san clemente island is currently uplifting at between 0 2 and 0 5 mm yr 1 this translates into 0 6 1 5 mm yr 1 of thrusting on the causative blind thrusts beneath the island unlike the situation at nearby palos verdes where a simple twist in a regional strike slip fault accommodated both fault parallel and fault normal motions the shallow dips of the thrusts suggest that if regional strike slip motion on the san clemente fault exists it must be partitioned onto through going surfaces distinct from the thrusts current gps data are sparse and equivocal but they indicate that 1 4 mm yr 1 of compression and 4 7 mm yr 1 of strike slip are absorbed in the california continental borderland with the palos verdes fault taking some 3 mm yr 1 from the strike slip budget 1 4 mm yr 1 of motion could be present on a through going san clemente fault when translated into an annual moment release rate using kostrov s formula gps strains predict that between 2 5 and 4 9 times 1017 n m yr 1 of earthquake potential is available offshore from san diego to the santa barbara channel distribution of this moment budget among various earthquake magnitudes is arguable but we predict that m 6 quakes in the borderland could recur between 30 and 80 years and m 7 quakes might be found every 310 to 580 years

neogene rotations and quasicontinuous deformation of the pacific northwest continental margin paleomagnetically determined rotations about vertical axes of 15 to 12 ma flows of the miocene columbia river basalt group of oregon and washington decrease smoothly with distance from the plate margin consistent with a simple physical model for continental deformation that assumes the lithosphere behaves as a thin layer of fluid the average rate of northward translation of the continental margin since 15 ma calculated from the rotations using this model is about 15 mm year which suggests that much of the tangential motion between the juan de fuca and north american plates since middle miocene time has been taken up by deformation of north america the fluid like character of the large scale deformation implies that the brittle upper crust follows the motions of the deeper parts of the lithosphere

equivalent strike slip earthquake cycles in half space and lithosphere asthenosphere earth models by virtue of the images used in the dislocation solution the deformation at the free surface produced throughout the earthquake cycle by slippage on a long strike slip fault in an earth model consisting of an elastic plate lithosphere overlying a viscoelastic half space asthenosphere can be duplicated by prescribed slip on a vertical fault embedded in an elastic half space for the case in which each earthquake ruptures the entire lithosphere thickness h the half space equivalent slip rate is as follows depth interval 0 h slip identical to that in lithosphere asthenosphere model i e abrupt coseismic slip and no subsequent slip depth interval 2n 1 h to 2n 1 h n 1 2 slip rate uniform in space and dependent upon time as fn t exp t a where fn is a n 1 degree polynomial in t a is twice the asthenosphere relaxation time and t is measured from the instant after the preceding earthquake the slip rate averaged over the seismic cycle in each depth interval equals the secular rate of relative plate motion for reasonable values of a slip rates below 5h do not vary much from that mean value and can be treated as constant thus the surface deformation due to the earthquake cycle in the lithosphere asthenosphere model can be calculated very simply from the half space model with time dependent slip in the two depth intervals h 3h and 3h 5h and uniform slip at a rate equal to the secular relative plate velocity below depth 5h inversion of 1973 1988 geodetic measurements of deformation across the segment of the san andreas fault in the transverse ranges north of los angeles for the half space equivalent slip distribution suggests no significant slip on the fault above 30 km and a uniform slip rate of 36 mm yr below 30 km one equivalent lithosphere asthenosphere model would have a 30 km thick lithosphere and an asthenosphere relaxation time greater than 33 years but other models are possible

earthquake strain rates and instantaneous relative motions within central and eastern asia summary we use the spatial distribution of moment tensors of earthquakes in this century to estimate the velocity field in asia within a eurasian reference frame in a least squares inversion strain rates on the surface of the earth are matched with continuous spline functions in order to recover the velocity gradient tensor associated with the seismic moment release in asia earthquakes account for 40 60 per cent of the expected motion of india relative to eurasia with the missing component of strain rate equivalent to about 20 mm yr 1 of n s shortening between siberia and india in this solution south china rotates counterclockwise and moves eastwards relative to siberia using rigid plate constraints we next investigate the characteristics of the complete horizontal strain field in asia that accommodates plate motions our strain rate solutions are analogous to the response of a newtonian thin viscous sheet in which the rate of work done by the straining medium in accommodating the velocity boundary conditions is a minimum in these solutions the euler pole for india relative to eurasia is constrained nuvel 1a demets et al 1994 but in the process of fitting the vlbi velocity at shanghai china ward 1994 heki et al 1995 the euler pole for south china is determined in the inversion a solution that both fits the velocity at shanghai china and yields a strain rate field consistent with the earthquake mechanisms is one where the south china block has a motion relative to siberia described by the pole at 51 n 131 e 0 3 deg myr 1 comparison of the complete strain field that accommodates plate motion with the seismic strains indicates that earthquake moment release rates in this century within mongolia are about a factor of 4 larger than the long term rate within gansu ningxia the earthquake moment rates have been about a factor of 2 higher than the long term rate the strike slip faulting within mongolia gansu ningxia western sichuan and yunnan is possibly a direct result of velocity boundary conditions imposed on the south china block by forces unrelated to continental collision of india and eurasia such as forces associated with subduction along the margins of south eastern asia verification of this requires a better understanding of the role of pre existing zones of weakness within the asian continental lithosphere

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

ogasawara plateau multi beam sonar bathymetry and possible tectonic implications the ogasawara plateau bathymetry in the northwest pacific ocean is presented by u s naval oceanographic office multi beam sonar data the plateau is significant in that it separates the izu and bonin trenches and may be obducted onto the overriding plate and or jamming the subduction zone the high resolution bathymetry suggests morphologic evidence of faulting within and possible uplifting on the approximately 20 000 km 2 features

space geodetic measurement of crustal deformation in central and southern california 1984 1992 we estimate the velocity field in central and southern california using global positioning system gps observations from 1986 to 1992 and very long baseline interferometry vlbi observations from 1984 to 1991 our core network includes 12 gps sites spaced approximately 50 km apart mostly in the western transverse ranges and the coastal borderlands the precision and accuracy of the relative horizontal velocities estimated for these core stations are adequately described by a 95 confidence ellipse with a semiminor axis of approximately 2 mm yr oriented roughly north south and a semimajor axis of approximately 3 mm yr oriented east west

late quaternary strike slip on the eastern part of the awatere fault south island new zealand new zealand straddles the obliquely convergent boundary between the pacific and australian plates in central south island plate motion is accommodated by oblique collision of continental crust along the alpine fault in the southern alps and in north island by subduction of oceanic crust beneath the continental hikurangi margin between these two zones oblique convergence is accommodated across the 150 km wide marlborough fault system which is transitional between the two different styles of margins dextral slip in the marlborough fault system is partitioned among four principal faults of which the awatere fault is one our data on the eastern part of the awatere fault provide insight into styles of surface faulting and active deformation in continental transpression zones we 1 document the segmentation and kinematics of oblique slip on the fault including a discrepency between long term and short term accumulation of vertical motion 2 describe an along strike gradient in the degree of slip partitioning of oblique plate motion 3 measure late quaternary strike slip rates of 6 8 mm yr at several sites along the fault rates that decrease eastward to

the relationship between strain rates crustal thickening palaeomagnetism finite strain and fault movements within a deforming zone the deformation of continental lithosphere generally does not occur on a single fault but is distributed over a zone up to a few hundred kilometers wide if the deformation rate is constant throughout the zone and if the velocity is continuous everywhere and does not change with time the deformation gradient tensor can be obtained analytically this tensor directly determines the amount of crustal thickening and is related in a complicated manner to the finite strain tensor the declination of remnant magnetization measured within the zone can also be rotated by the deformation because palaeomagnetic measurements are generally made on rigid blocks which have not been deformed there is no simple relationship between such rotations and the finite strain expressions for all these quantities are obtained in terms of the velocity gradients which are taken to be constant the history of western north america is used to illustrate the relationship between the present deformation by strike slip and thrust motions the rotations of the remnant magnetization vectors and the shape of accreted terranes at the surface distributed deformation is taken up by motion on faults which separate the zone into blocks the displacement rate on the faults their strike and the finite rotations of the blocks are all controlled by the velocity gradients analytic expressions relating these quantities are obtained a deforming zone containing normal faults which crosses central greece takes up both strike slip and extension and illustrates the relationship between the structures within and the motion across the zone a similar but more complicated zone in northeastern iran involves both strike slip and shortening

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

finite strain calculations of continental deformation 1 method and general results for convergent zones a finite element method is used to calculate stresses and strain rates in a thin viscous sheet representing the continental lithosphere that is bordered in one part by an indenting boundary we calculate solutions for velocity fields crustal thickness distributions strain rates rotation time integrated vorticity and finite strain ellipsoids to show the dependence of these quantities on the controlling parameters the stress strain exponent n and the strength of the sheet relative to the gravitational forces which may be expressed as a dimensionless number the argand number ar for n 3 and ar between 1 and 10 a plateau of thickened crust is formed in front of the indenter the strain histories of individual elements within the viscous sheet may be quite complex and the finite strain at the end of deformation may bear no obvious relation to the principal stress orientations during the deformation the strain rate fields are interpreted in terms of the style of faulting that would be expected in the brittle upper crust if it were coupled to the ductile but stronger upper mantle in general the length scale of deformation increases with ar and time since the start of the collision and decreases with n for ar 3 the increase in length scale is accompanied by a change in the style of deformation from predominantly crustal thickening with thrust faulting in the brittle layer to predominantly transcurrent deformation strike slip faulting in the brittle layer

the intraplate character of supercontinent tectonics abstract for several decades geoscientists have recognised intraplate tectonic activity far from plate margins both from modern and ancient examples this apparent disconnect with the drivers of plate tectonics does not necessarily imply unconnected processes but rather an uncertainty in understanding exactly how these systems operate are the driving forces derived locally or do they propagate from plate margins how do these forces interact with a complex tectonic inheritance to generate the observed tectonism furthermore what novel approaches have been applied to understand these processes here we review the general literature and the contents of this special issue to develop some partial answers to these questions key observations include the critical importance of local lithospheric heterogeneities as a control on the mode of orogenesis and also the role of locally derived forces from mantle upwelling or from depositing thick piles of magmatic or sedimentary rocks these processes happen within the overarching tectonic setting provided by far field plate margins which show intimate links in both time and observed processes with many intraplate regions these insights would not be possible without a growing arsenal of geological geophysical and numerical methods that can be applied to intraplate regions and this is reflected in the varied approaches within the issue we envisage that this special issue will provide a stimulus for further progress in understanding intraplate tectonics

the continental collision zone south island new zealand comparison of geodynamical models and observations the south island zone of oblique continent continent convergence occurs along a 400 km long section of the modern australia pacific plate boundary zone across which about 50 km of shortening has been accommodated since about 10 ma the orogen comprises a central mountain range southern alps flanked on both sides by what are interpreted to be foreland basins two essential features that characterize the orogen are 1 the degree of denudation that accompanied deformation and 2 a fundamental structural asymmetry the architectural asymmetry of the orogen can be explained by plane strain finite element models of continental convergence incorporating mantle subduction comparison of model and orogen polarity implies that pacific plate mantle subducts the models predict two crustal scale dipping shear zones that form above the point where the pacific mantle subducts the localized one more distant from the incoming plate retro step up shear zone corresponds to the alpine fault whereas its conjugate pro step up shear zone corresponds to the distributed strain and thrusting along the eastern margin of the mountain belt parameters that modify the model boundary conditions top surface degree of denudation basal zone subduction load crust mantle velocity discontinuity subduction of lower crust mantle retreat and distributed decrease in mantle velocity and the internal strength of the crust two layer crust with moderate coupling temperature distribution strain weakening are varied in a series of numerical model calculations that establish the combination of material properties and boundary conditions that lead to different cross sectional architectures of the modeled collision zone in turn these are compared with observations about the south island orogen the calculations show how the style and extent of deformation across the whole orogen depend on the rheological properties of the crustal layer and on the balance between its internal strength and the combined effects of the boundary and gravitational stresses north to south along strike differences in the width and two dimensional architecture of the orogen simulated in the experiments by varying the model parameters can be explained by a combination of southward increases in preconvergent crustal thickness geothermal gradient convergence and potentially subduction retreat with the added possibility of a southward decrease in the component of lower crustal subduction

mechanics of graben formation in crustal rocks a finite element analysis small 1 10 km wide grabens are ubiquitous tectonic structures that have been observed on the surfaces of all the terrestrial planets and satellites in spite of their frequent occurrence there is no fundamental understanding of how they form other than that they are characteristic of extensional terranes this paper examines the mechanics of the initial stages of graben formation and demonstrates that the configuration of a pair of antithetically dipping normal faults a graben is the energetically most favorable fault configuration in elastic brittle rocks subjected to pure extension a corollary to this hypothesis is that the occurrence of synthetic normal faults is indicative of a stress field other than purely extensional or of nonhomogeneous characteristics of the rock the analysis proceeds by computing the stress field in the vicinity of a single initial normal fault using a two dimensional finite element model it is shown that slip on this fault substantially lowers the proximity to failure of the surrounding rocks except for two small regions one near the surface of the downthrown block and the other near the fault tip the region on the downthrown block is located a horizontal distance from the initial fault slightly less than the fault s depth the proximity to failure actually increases here and a secondary fault develops an antithetic secondary fault would link this region with the other failure zone near the fault tip to demonstrate the preference for antithetic secondary faults more rigorously we show that this fault configuration results in a smaller total strain energy than the synthetic configuration in addition a two layer model is used to show that the graben s width is only weakly dependent upon the presence of mechanically distinct layers in the crust both weak over strong and strong over weak configurations are investigated the depth of the initial fault and the thickness of the top layer are varied and the width of the resulting graben is evaluated by computing the distance between the initial fault and the region of highest proximity to failure near the surface this analysis indicates that the depth of the initial fault is the major factor controlling graben width

present day kinematics of asia derived from geologic fault rates active fault geometry available quaternary rates on major faults and the far field plate motion are used to determine crustal kinematics in the collision zone between india and asia using a finite element formalism to construct a spherical shell model of the asian continent with embedded faults we seek a velocity solution approaching the motion of rigid blocks by minimizing the elastic strain energy in the fault bounded blocks by doing so we test the assumption that long term deformation within continents is mostly localized into major faults in the solution fault motion accounts for more than 80 of the deformation allowing us to describe our velocity model in terms of quasi rigid block rotations on the sphere south china is rotating clockwise about a pole located southwest of borneo implying an e ese velocity vector of 11 mm yr for a point at shanghai in agreement with the velocity vector determined by very long baseline interferometry heki et al 1995 the eastward movement of south china is accommodated by oblique extension along the red river fault at a rate of 10 5 mm yr in the south and by the combination of left lateral strike slip motion on the qinling dabie shan fault and the counterclockwise rotation of the ordos and adjacent blocks in the north the tarim rotates clockwise with respect to dzungaria about a pole located at 44 n 92 e consistent with increasing crustal shortening toward the west throughout the tien shan assuming incompressibility a crustal volume budget over a domain encompassing the tertiary mountain ranges in asia shows that over the last 10 000 years 73 4 of the north south shortening between india and asia has been absorbed by thickening of the lithosphere and 27 has been accommodated by lateral extrusion of continental blocks the present day predominance of thickening in asia results from the relatively slow eastward motion of south china controlled by strike slip faulting in the qinling shan and oblique extension in northeastern china

style and rate of holocene slip palos verdes fault southern california recent studies of the palos verdes fault in the los angeles outer harbor characterize the fault s holocene activity the studies included areal and local geophysical surveys as well as drilling sampling and dating of the subsurface materials the studies identify the fault s location in the shallow subsurface document evidence of holocene strike slip quantify dip slip versus sediment age and quantify the net slip where an early holocene paleochannel crosses the fault the slip rate for the palos verdes fault is constrained to between 2 7 and 3 mm yr for the past 7 8 to 8 ka because the new evidence shows that the palos verdes fault has one of the highest slip rates in the los angeles basin the studies provide significant new insight into the regional tectonics and the seismic risk of the los angeles region using this rate of slip and segmentation models for the fault the coastal los angeles region can expect an mw 7 to 7 2 event about every 400 to 900 years from this potential seismic source

nonlinear strain buildup and the earthquake cycle on the san andreas fault two contrasting models of the earthquake deformation cycle on strike slip faults predict significant temporal declines in shear strain rate near the fault accompanied by a progressive broadening of the zone of deformation adjacent to it in the thin lithosphere model transient deformation results from flow in the asthenosphere due to stress relaxation following faulting through most or all of the lithosphere for an earth model with a thick elastic lithosphere plate thickness depth of seismic slip transient motions are due to postearthquake aseisrnic slip below the coseismic fault plane data from the san andreas fault indicate a long term temporal decrease in strain rate that persists for at least 30 years and may extend through the entire earthquake cycle observations support a cycle long rate decrease and a temporal spreading of the deformation profile only if movement cycles on the northern and southern locked sections of the fault are basically similar if so the usually lower strain rates and broader deformation zone currently observed on the southern san andreas represent a later evolutionary stage of the northern locked section where a great earthquake is a more recent occurrence although the data allow some extreme models to be discarded no sufficiently strong constraints exist to decide between the thin and thick lithosphere models regardless of the appropriate model the geodetic observations themselves indicate that strain buildup is sufficiently nonlinear to cause significant departures from recurrence estimates based on linear strain accumulation and the time predictable model

dynamic causes for the opening of the baikal rift zone a numerical modelling approach abstract previous dynamic models of the baikal rift zone brz are mostly two dimensional on vertical plane in this study a numerical model of neotectonics in the region on map view was constructed using the adapted plates program the present work is an attempt to test different mechanisms for opening baikal rift by comparing the modelled and observed stress and strain rate fields the following rifting scenarios were tested 1 pure northwest southeast extension 2 pure northeast southwest compression 3 oblique rift opening and 4 combined northwest southeast extension and northeast southwest compression the models are calibrated using geologically and gps derived strain rates and stress tensor determinations from fault slip data and earthquake focal mechanisms the most successful model requires a combination of ne sw compression and orthogonal extension the model results indicate that the present extensional regime in brz can be explained by combining the india plate indentation northward into eurasia east west convergence between the north america and eurasia plates and southeastward extrusion of the amur plate in northeastern asia predicted fault slip rates for the best fit model are consistent with the observed holocene fault slip rates in the lake baikal region the generally accepted rotation of the amur and mongolia microplates are used as independent constraints for the choice of the best fit model these data correlate well with the predicted direction of rotation in our best model

temporal variations in latest quaternary slip across the australian pacific plate boundary northeastern south island new zealand rates of latest quaternary slip from stream terraces and moraines displaced by faults of the alpine shear system in ne south island new zealand vary in space and time detailed histories of fault slip are obtained from combining displacement data with estimates of the age of the surface from weathering characteristics precision in surface ages is 5 20 using rock weathering rinds and 15 50 using soil properties the oldest surfaces examined are 15 20 ka and have right lateral fault offsets up to 400 600 m the main faults of the in the ne south island the wairau awatere clarence hope kekerengu and porters pass faults have average latest quaternary right lateral slip rates of 3 10 5 10 7 10 25 40 5 7 and 3 4 mm yr respectively every fault has undergone a substantial decrease in lateral slip rate in the last 3 5 kyr summed across the plate boundary the average latest quaternary slip rates are comparable to long term rates of horizontal slip across the australian pacific plate boundary around 40 mm yr parallel to the boundary and rates of geodetic strain and seismic moment release over the last 50 100 years approximately the same however sums of lateral fault slip rates over the interval from 15 to 5 ka exceed the plate motions whereas late holocene lateral fault slip rates are less than half the long term average the best explanation of these variations is slip across the plate boundary is episodic varying over perhaps 5 kyr intervals this implies that 15 20 kyr is the time interval necessary to average out shorter 5 kyr episodic variations in plate boundary motions

erosion as a driving mechanism of intracontinental mountain growth in nature mountains can grow and remain as localized tectonic features over long periods of time 10 m y by contrast according to current knowledge of lithospheric rheology and neglecting surface processes any intracontinental range with a width that exceeds that which can be supported by the strength of the lithosphere should collapse within a few tens of millions of years for example assuming a quartz dominated crustal rheology the relief of a range initially 3 km high and 300 400 km wide is reduced by half in about 15 m y as a result of lateral spreading of its crustal root we suggest that surface processes might actually prevent such a subsurface collapse removal of material from topographic heights and deposition in the foreland oppose spreading of the crustal root and could eventually drive a net influx of material toward the orogeny we performed a set of numerical experiments in order to validate this hypothesis a section of a lithosphere with a brittle elasto ductile rheology initially loaded by a mountain range is submitted to horizontal shortening and to surface processes if erosion is intense material is removed more rapidly than it can be supplied by crustal thickening below the range and the topography is rapidly smoothed for example a feature 3 km high and 300 400 km wide is halved in height in about 15 m y for an erosion coefficient k 10 3 m 2 yr the erosion rate is of the order of a few 0 1 mm yr this regime might be called erosional collapse if erosion is not active enough the crustal root spreads out laterally and subsurface collapse occurs in the third intermediate regime removal of the material by erosion is dynamically compensated by isostatic rebound and inward flow in the lower crust so that the range can grow in this mountain growth regime the range evolves toward a characteristic graded shape that primarily depends on the erosion law the erosion rate may be high e g 0 5 0 9 mm yr close to the rate of tectonic uplift e g 0 7 1 1 mm yr and few times higher than the rate of topographic uplift 0 15 0 2 mm yr these experiments show that surface processes can favor localized crustal shortening and participate in the development of an intracontinental mountain surface processes must therefore be taken into account in the interpretation and modeling of long term deformation of continental lithosphere conversely the mechanical response of the lithosphere must be accounted for when large scale topographic features are interpreted and modeled in terms of geomorphologic processes

crustal thickening versus lateral expulsion in the indian asian continental collision since the beginning of the continental collision between india and asia there has been about 2500 km of convergence and the northward movement of india has been accommodated by major internal deformation of the asian lithosphere the crustal thickening in and around the tibetan plateau is clearly a direct consequence of this collision but there is considerable debate as to whether a large fraction of the indentation has been accommodated by eastward motion of the lithospheric blocks of southeastern asia and southern china numerical experiments described here test this hypothesis for a range of indentation geometries and rheological models of the lithosphere we employ a thin viscous sheet model of the lithosphere with a depth averaged nonlinear viscous rheology described by a stress strain rate exponent n and including gravitational buoyancy forces scaled by the dimensionless argand number ar the eastern boundary for the collision region is described as a lithostatic boundary the precollision normal stress is determined by static balance and that constant stress is applied throughout the collision the experiments show that during collision the eastern boundary is smoothly displaced to the east at a rate about 1 4 of the indentation rate with only minor variation due to geometry or rheology crustal thickening rates in the region of the plateau are reduced by between 10 and 25 of the corresponding rates determined for similar experiments with a rigid eastern boundary to generalize from the results of these experiments the total north south shortening strain produced by the collision is partitioned between crustal thickening and eastward displacement in the ratio of at least 3 1 and more probably 4 1

the field of crustal velocity in asia calculated from quaternary rates of slip on faults rates of quaternary faulting when treated as measures of strain rates within regions several times larger than the depths of brittle faulting yield a velocity field for asia that matches the average relative velocity between the indian and eurasian plates over the past 2 myr nuvel 1a and the velocity of shanghai with respect to eurasia measured with very long baseline interferometry vlbi by using rates of slip on discrete surfaces faults to define average strain rates we explicitly assume that the large scale deformation can be treated as that of a continuum depending on the jcales over which strain rates are averaged 87 to 93 per cent of the assumed strain ites are fit within assigned 1 sigma uncertainties and 97 to 99 per cent are fit within 2 sigma the internal consistency of these data and the agreement with independent rates nuvel 1a and vlbi test the validity of treating large scale deformation of continents as that of a continuum in the calculated velocity field a large fraction 85 per cent of india s convergence with eurasia is absorbed by crustal or lithospheric thickening among consistently misfit assumed strain rates are those dominated by high rates of strike slip faulting strain associated with 20 30 mm a 1 of slip on the altyn tagh karakax and karakorum faults is inconsistent with the distributions of strain in surrounding regions we conclude that these rates all of which are based on correlations of ages of offset landforms with changes in global climate and not on radiometric dating have been systematically overestimated lateral transport out of india s northward path is relatively minor south china is calculated to move east southeast at less than 10 mm a 1

crustal deformation in the baikal rift from gps measurements three years and four campaigns of global positioning system gps measurements 1994 1997 in the baikal rift zone largest active continental rift system in eurasia show crustal extension at a rate of 4 5 1 2 mm yr in a wnw ese direction a comparison with moment release of large historical earthquakes suggests that elastic strain is currently accumulating in the baikal rift zone along active faults that currently have the potential for a m 7 5 earthquake the gps derived extension rate in the baikal rift zone is at least two times greater than the prediction of most deformation models of asia this result could reflect the dynamic contribution of the pacific eurasia subduction to intracontinental deformation in asia in addition to the effect of the india eurasia collision

continental margin tectonics forearc processes recent studies of convergent plate margins and the structural development of forearc terranes are summarized in a critical review of u s research from the period 1987 1990 topics addressed include the geometry of accretionary prisms coulomb wedge taper and vertical motion in response to tectonic processes offscraping vs underplating or subduction the response to oblique convergence fluids in forearc settings the thermal framework and the effects of fluid advection and serpentinite seamounts also included is a comprehensive bibliography for the period

faulting associated with large earthquakes and the average rate of deformation in central and eastern asia we determine approximate average rates of deformation in portions of eastern and central asia for the last 80 years these rates are based on estimates of source dimensions average displacements seismic moments and orientations of faults of all large earthquakes m 7 5 and as many other major earthquakes m 7 0 as possible in asia since 1900 a major part of this study is the compilation of these data for separate parts of asia we combine seismic moment tensors to estimate the average rotational strain or deformation tensors for these regions for the last 80 years we estimate that shortening across the tien shan has occurred at 11 mm yr between 5 and 19 mm yr left lateral shear on easterly trending planes in mongolia in northwestern tibet gansu and ningxia and on the xianshuihe fault in western sichuan has been very rapid more than 10 mm yr in each region and apparently as rapid as 40 mm yr in northwestern tibet gansu and ningxia slower rates of a few to 10 mm yr appear to characterize east west extension and conjugate north south shortening of tibet the earthquake history of the himalaya is consistent with rates of underthrusting of more than 10 mm yr but is probably less than the convergence rate of india with eurasia of about 50 mm yr the overall strain field is consistent with a large part of india s penetration into eurasia being absorbed by the extrusion of material out of india s way as a result southeast china moved at about 21 mm yr east southeast with respect to eurasia in the last 80 years with an uncertainty of about a factor of 2 in this estimated rate

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

crustal deformation of the marlborough fault zone in the south island of new zealand geodetic constraints over the interval 1982 1994 crustal deformation across the marlborough fault zone in the south island of new zealand has been investigated by resurveying with the global positioning system gps a triangulation and trilateration network across part of the zone the principal strain rates which mostly have errors between 5 and 10 vary systematically across the region the principal axis of horizontal contraction gradually swings from se nw southeast of the hope fault to almost east west in the west the component of velocity parallel to the strike of the faults can account for 95 of the relative motion between the australian and pacific plates this component exhibits little variation west of the alpine wairau fault and shows an almost linear variation from the wairau fault to the east coast the horizontal strain rate northwest of the wairau fault is a small east west uniaxial contraction 0 1 ppm yr 1 if it is assumed that the crustal blocks bounded by the major faults are primarily driven by basal shear tractions with stresses on the faults being relatively insignificant then the slip rates expected on the faults may be derived from the observed velocity variations across the fault zone these predicted slip rates are in good agreement with geologically observed slip rates the larger slip rates on the hope fault are related to the observation that the strain field extends southeast of the fault over a distance large compared with the spacing between the faults the agreement between predicted and geological slip rates lends support to the idea that the observed pattern of deformation reflects that in the lower lithosphere

hope fault jordan thrust and uplift of the seaward kaikoura range new zealand in the northern south island of new zealand displacement at the pacific indian plate boundary is accommodated by the east north east striking right lateral strike slip marlborough fault system the southernmost marlborough fault is the hope fault the late pleistocene holocene horizontal slip rate on this fault is 20 25 mm yr about half of the rate of pacific australian plate motion near the eastern end of the hope fault most displacement is transferred to the north northeast striking jordan thrust but the average dip slip rate at the surface trace of this thrust is less than 4 mm yr we propose that most slip takes place on a blind thrust expressed at the surface by the fault propagation folding of the seaward kaikoura range and that the rate of uplift of this range is as high as that of the southern alps 6 to 10 mm yr the major restraining bend of the alpine fault has the same average slip rate as the wairau fault 4 6 mm yr even though the alpine fault is an east dipping reverse separation fault at the restraining bend this low slip rate results in uplift of the spenser mountains east of the bend at a rate lower than that of the southern alps and seaward kaikoura range

strain localization along the san andreas fault consequences for loading mechanisms the orientation of the near fault strain accumulation associated with the relative motion between the pacific and north american plates is better predicted by the orientation of the local faulting than by the orientation of the regional plate motion this observation follows from consideration of shear strain results from geodetic studies ranging from the imperial valley to shelter cove along the san andreas fault system this set of results includes calculations from regions where the faulting and plate motion are considerably oblique to each other both the localization of the strain associated with pacific north american motion and the orientation of the associated strain can be simply caused by failure of the lithosphere as opposed to localization of plate driving forces two types of models for the rheology and geometry of lithospheric failure have been prevalent in the geodetic literature 1 viscoelastic models which are dominated by horizontal layering of the crust and 2 elastic half space models with geometry which is dominantly vertical horizontal models can accommodate the observed strain results only if postseismic relaxation times are long compared to the repeat times of large earthquakes vertical models produce strains which are parallel to the local structure at all times if the horizontal detachments of the viscoelastic models exist they must be at depths 2 3 times the seismogenic thickness geologic studies also indicate that major transcurrent features extend as ductile shear zones to depths much larger than would be expected in horizontal models these considerations lead us to favor models with vertical geometries the vertical structure of these models can be reconciled with laboratory measurements of rock strength through the strain softening processes associated with flow of composite materials such as quartzo feldspathic rocks independent of the horizontal rheological layering in the starting low strain material progressive deformation will lead to preferential weakening in the plane of shear and to the development of a shear zone which will always be weaker than the surrounding material thus shear zones such as the san andreas system develop vertical tabular geometries which extend far below the brittle plastic transition and which are mechanically persistent

morphotectonic analysis and 10 be dating of the kyngarga river terraces southwestern flank of the baikal rift system south siberia the formation of the baikal rift system basins is controlled by active faults separating each basin from the adjacent horsts the kinematics of these faults is mainly explored through investigation of complex sequences of the fault intersecting river terraces that record both tectonic and climatic events this study focuses on the northern margin of the major tunka basin that develops south west of lake baikal the development of the basin is controlled by the segmented tunka fault we performed a detailed mapping of the kyngarga river terraces the best preserved terraces staircase in baikal rift system at their intersection with the tunka fault in order to decipher the chronology of seismic events and the slip rates along that segment of the fault key terraces were dated using in situ produced cosmogenic 10be we demonstrate that the formation of the terrace staircase occurred entirely during mis1 mis2 the obtained data allowed us to estimate the rate of incision at different stages of the terrace staircase formation and the relationship between the vertical and horizontal slip rates along this sub latitudinal segment of the tunka fault making respectively 0 8 and 1 12 mm yr 1 over the past 12 5 ka analysis of the paleoseismology and paleoclimate data together with terrace dating provided the possibility to estimate the influence of tectonic and climatic factors on the terrace formation our proposed model of the kyngarga river terrace development shows that the incisions into terraces t3 and t6 were induced by the abrupt climatic warming episodes gi 1 and gi 2 respectively whereas terraces t5 t4 and t2 were abandoned due to the vertical tectonic displacement along the tunka fault caused by coseismic ruptures

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

asymmetric upwarp of the asthenosphere beneath the baikal rift zone siberia in the summer of 1991 we installed 27 seismic stations about lake baikal siberia aimed at obtaining accurately timed digital seismic data to investigate the deep structure and geodynamics of the baikal rift zone and adjacent regions sixty six teleseismic events with high signal to noise ratio were recorded travel time and q analysis of teleseisms characterize an upwarp of the lithosphere asthenosphere boundary under baikal theoretical arrival times were calculated by using the international association of seismology and physics of the earth s interior 1991 earth model and travel time residuals were found by subtracting computed arrival times from observed ones a three dimensional downward projection inversion method is used to invert the p wave velocity structure with constraints from deep seismic sounding data our results suggest that 1 the lithosphere asthenosphere transition upwarps beneath the rift zone 2 the upwarp has an asymmetric shape 3 the velocity contrast is 4 9 in the asthenosphere 4 the density contrast is 0 6 and 5 the p wave attenuation contrast t is 0 1 s

the palos verdes terraces california bathtub rings from a buried reverse fault uplift of the palos verdes peninsula has long been associated with a northwest trending southwest dipping reverse fault unfortunately the palos verdes hills fault has no obvious surface displacement and little background seismicity to substantiate its dimension orientation or earthquake potential in this paper we investigate the tectonic style and slip rate of the palos verdes hills fault and the uplift history of the palos verdes hills by analyzing the geometry of 13 marine terraces that encircle the peninsula in a bathtub ring configuration elevations of 211 terrace remnants constrain a fault model with 3 0 to 3 7 mm yr 1 of oblique dextral reverse slip on a fault dipping 67 at 6 to 12 km depth beneath the peninsula if the rate was constant through time fault inception would have occurred 2 4 3 0 ma we propose that the largest credible earthquakes on the fault have magnitude 6 and could revisit every 2000 years

the 1999 mw 6 0 earthquake sequence in the southern baikal rift asia and its seismotectonic implications summary the 1999 february 25 earthquake mw 6 0 is the largest event to have occurred in the baikal rift system since the 1959 m 6 8 earthquake this earthquake sequence took place in the southern basin of lake baikal a region characterized by a sharp transition from strike slip faulting on the main sayan fault to normal faulting along the border faults of the lake thanks to the development of the regional network and a temporal station installed after the main shock we performed a relocation of the aftershocks to describe the rupture area and to determine fault plane solutions we show that the main shock was caused by normal faulting on an intrabasin fault striking northeast and steeply dipping to the northwest aftershock hypocentres did not cluster at the same depth but were spread from 5 to 25 km depth stress inversion carried out using focal mechanisms of the sequence reveals a radial extensive stress regime with a minimum principal stress axis 3 n155 e slightly oblique to the regional stress field n139 e which depicts a stronger intermediate 2 axis this behaviour suggests transient stress perturbation induced by the main shock in the neighbouring zones of the rupture strain release deduced from gps data and historical seismicity shows that within the southern tip of the baikal rift basin a significant stress release has occurred for at least two centuries probably through active normal faulting shared among several faults while the main sayan strike slip fault further west was undergoing interseismic strain accumulation