effects of topography on the curvature of fold and thrust belts during shortening of a 2 layer model of continental lithosphere abstract we have used analogue experiments to investigate the effects of surface topography on the curvature of fold and thrust belts under conditions of 1 initial relief but no erosion and 2 no initial relief but differential erosion sedimentation and transport in experiments where a 2 layer model lithosphere shortened and thickened in front of an advancing straight piston the geometry of the developing thrust wedge was very sensitive to variations in surface topography in models with an initially flat horizontal surface and in the absence of erosion and sedimentation thrusts were straight propagated forwards and nucleated at buckle folds far in front of an advancing piston around an initial topographic high plateau or cone thrusts tended to be arcuate forming salients towards the foreland initial plateaux and cones tended to behave rigidly while arcuate thrust slices formed around them to accommodate differential slip transfer zones developed on both sides of initial highs fault blocks rotated about vertical axes and thrusts moved in oblique slip within transfer zones in models with initially horizontal surfaces which were subject to differential erosion sedimentation and transport thrusts initially were straight but then progressively rotated around non eroded thickened and stronger areas these worked as indenters in front of which new thrusts nucleated at curved buckle folds these thrusts were also curved their apices being in front of the thickened non eroded areas in nature arcuate structural patterns are to be found around the altiplano of the central andes and around the tromen volcanic ridge in the neuquen basin of northern patagonia we infer that these areas behaved in quasi rigid fashion protected as they were by their high elevations and that differential erosion at the scale of the entire andes may have contributed to oroclinal bending

duplex style and triangle zone formation insights from physical modeling duplexes are a common feature in thrust belts at many scales their geometries vary significantly from antiformal stacks with significant forethrusting in the cover e g southern pyrennes spain to triangle zones e g foreland canadian rockies to low displacement individually spaced ramp anticlines e g sub andean thrust belt bolivia we present a series of physical experiments demonstrating that the strength of the decollements relative to that of the intervening and overlying rock layers plays a significant role in controlling the duplex style the models comprise brittle layers made of dry quartz sand and decollements made of two types of viscous silicone polymers the strength of the decollements in the models is a function of the shortening rate applied to the model the relative strength of the decollements and surrounding rocks affects the development of active or passive roof duplexes triangle zones it also affects the amount of translation of individual thrust blocks and the spacing of thrust ramps which in turn determine if a duplex evolves into an antiformal stack or into individually spaced ramp anticlines model results indicate that specific associations of structural features form systematically under similar rheological and boundary conditions the presence of relatively strong decollements promotes local underthrusting of the cover individual ramp anticlines internal deformation of thrust sheets low early layer parallel shortening and sequential towards the foreland propagation of structures weak decollements promote forethrusting of the cover antiformal stacks coeval growth of structures and low internal strain with the exception of significant early layer parallel shortening no underthrusting at a regional scale occurred in any model

field scale acoustic investigation of a damaged anisotropic shale during a gallery excavation in the opalinus clay formation at the mont terri underground rock laboratory the gallery ga08 was excavated in august 2008 to join the end face of the pre existing gallery ga04 the aim of the present work was to perform in situ acoustic experiments to monitor the evolution of the excavation damaged zone edz induced during the gallery construction the end face of ga04 was instrumented with two arrays of acoustic transducers allowing for the active and passive seismic monitoring i e acoustic survey and micro seismicity from the acoustic survey data which required a high energy acoustic source to emit high frequency signals 21 25 31 and 38 khz the rock mass was observed to be anisotropic and heterogeneous at the scale of the experiment p wave velocities were determined to be in average between 3300 m s along a structural bedding plane and 2700 m s at 70 incidence relative to that assuming a transversely isotropic shale formation the p wave velocity dependence versus was modeled using thomsen s weak transverse isotropy model thomsen 1986 36 thomsen s p wave anisotropy parameter was found to be 0 15 and the fifth thomsen s parameter controlling the deviation of the wave front from an ellipsoidal geometry was found to be 0 16 the s wave velocity was estimated along a single direction of aligned receivers and turned out to be around 1560 m s at 30 we also show that the rock mass acts as a frequency filter for acoustic waves related to the rock mass heterogeneities i e the inter bedding structure which induces wave scattering and refraction from the micro seismicity data we identified a large number of micro seismic events mses detected on the acoustic arrays during and following the excavation most of the mses were induced on the excavated face but we also located some mses inside the rock mass itself we show that these events are located close to a major fault which seems to be reactivated by the excavation process

the resolving power of cross borehole tomography the data collected in a cross borehole tomography problem is inherently incomplete since the boreholes do not completely surround the media under study the ability of the experiment to resolve structure is therefore limited by poor experimental geometry we use the method of backus and gilbert to investigate the ability of this technique to resolve variations in acoustic velocity and attenuation between the two boreholes both integrals that appear in the backus gilbert formulation can be performed analytically thus expediting the calculations the resolving kernels have significant horizontal width demonstrating that the horizontal position of structures can be only poorly resolved

seismic travel time and attenuation tomography to characterize the excavation damaged zone and the surrounding rock mass of a newly excavated ramp and chamber abstract seismic travel time and attenuation tomography were applied to characterize the excavation damaged zone and the adjacent rock mass in the gfz underground lab within the research and education mine reiche zeche of the technical university bergakademie freiberg germany the lab is situated in gneiss rocks at 150 m depth and comprises three galleries which enclose an area of approximately 50 m 100 m along these galleries two seismic surveys were performed before and after the excavation of a new ramp and chamber for both measurements travel time and attenuation tomographies for p waves were performed with the ray based inversion algorithm simulps14 the seismic velocities were calculated from first arrival travel times whereas a logarithmic spectral ratio approach was used to calculate the corresponding quality factors q for attenuation tomography a comparison of the tomograms reveals a decrease of average p wave velocity values from 5 64 km s to 5 54 km s and of average q values from 29 8 to 26 5 in the whole area after the excavation of the new cavities the maximum changes are located at already weakened zones either at the conjunction of two galleries or of a major fracture zone with a gallery the attenuation tomography shows a higher sensitivity to rock mass changes than the travel time tomography however the calculation of the q values demands a higher signal quality than the determination of the seismic travel times

detachment folding fold amplification and diapirism in thrust wedge experiments 1 the relations between detachment folding fold amplification and salt diapirism in contractional settings have been investigated by means of scaled analogue models the viscosity of the silicone layer simulating salt in nature and the shortening rates were combined in order to reproduce weak type 1 models and strong type 2 models decollements deformation patterns in the roof sequence exhibited two contrasting styles 1 outward propagation of detachment folding along the decollement ofp mode and 2 passive roof duplex prd mode in type 2 models detachment folding propagated away from the most external thrust in the floor sequence while in type 1 models long lived detachment folds almost invariably localized amplified above a floor thrust tip as a result of strain localization a silicone wall intruded occasionally into the crestal graben of detachment folds in type 1 and ofp models best fitting of transition models data points indicates nonlinear relations with regression curves close to the equilateral hyperbola equation for both ofp prd and amplified detachment folds box folds transitions a quantitative comparison of model results with nature has been attempted by plotting salt based fold and thrust belts data points on the scaled transition curves obtained from the modeling such a comparison relates shear stress products and ratios to the conditions favoring the amplification of detachment folds and the potential emplacement of ductile diapirs in their core by reducing the roof sequence strength pore fluid pressure b is inferred to shift the equilibrium of fold and thrust belts toward the field of ofp and diapirism

sandbox modelling of thrust wedges with fluid assisted detachments abstract we have used granular materials to model the development of thrust wedges where migrating pore fluids assisted in the formation of detachments the governing equations yield practical scales for linear dimensions stresses and time using compressed air as a pore fluid models a few centimetres thick were deformed in about half an hour model materials were sands of three different grain sizes and a loess they had suitable values of density permeability cohesion and internal friction fluid flow obeyed darcy s law at yield the materials satisfied a coulomb criterion for effective stresses models with various sequences of layers were submitted to horizontal shortening in a rectangular box compressed air entered through a sieve at the base the fluid pressure was uniform over the basal boundary in models made from a single material the style of deformation depended on the fluid pressure for no fluid flow the thrust wedge was short and high the surface slope attained large angles 30 and internal structures were mainly forethrusts for fluid pressures approaching lithostatic values thrust wedges were longer and lower and surface slopes attained smaller angles in models containing basal layers of small permeability detachments formed beneath them and the structural style was dominated by interacting forethrusts and backthrusts in multilayered models thin skinned detachments formed beneath less permeable layers in the sequence to understand how fluid flow controlled the first stages of detachment we calculated ideal vertical profiles of fluid pressure vertical normal stress effective stress and horizontal shear stress for multilayered models in the undeformed state the profiles are segmented because material properties vary from layer to layer sharp drops in shear strength occur at positions where detachments were observed in the sandbox models we infer that detachments resulted from large fluid pressures beneath relatively impermeable layers

lateral extrusion of lower crust from under high topography in the isostatic limit where there is isostasy the rocks of the lower continental crust are subject to an effective lateral pressure gradient equal to the gradient of topographic load whether compensation is in crustal roots or in the mantle the result is a poiseuille flow planar channel flow in the weak lower crust which removes crust from under mountains and smooths and levels the topography assuming cubic power law creep the flux of crust is proportional to the third power of the topographic gradient to the usual arrhenius term to the tenth power of the absolute temperature of the lower crust and to the negative fifth power of the geothermal gradient the result is that any initial condition with an isolated high tends eventually toward a state where the topography is pancake shaped with a flat central plateau and steeper flanks spreading outward when adjacent pancakes merge most of the relief is eliminated flow may either roughen or smooth the moho depending on whether there are lateral density contrasts in the mantle lithosphere or not although it is difficult to find analytic solutions for this evolution it is easily simulated numerically using various published flow laws for plausible lower crustal rocks lateral extrusion is shown to be insignificant under the oceans marginally significant under shields and platforms important under elevated plains and dominant beneath high plateaus and or hot delaminated regions in particular the basin and range province of the western united states cannot maintain short wavelength 100 km moho relief of more than 1 km for times greater than 10 20 m y at most if the tibetan plateau of china has been delaminated as recently suggested then it may flatten even faster reducing 100 km wavelength topographic features to 200 m relief in no more than 0 03 0 13 m y and leaving only features with wavelengths over 400 km at the present therefore the lower crust of tibet may resemble a hydraulic reservoir as previously suggested because the flatness of the moho is self maintained in these regions information on past tectonics is continually lost and the present moho shape cannot be used to balance cross sections for times in the past the flatness of the moho today is not a constraint on the mechanisms of extension or shortening in these regions nor is it evidence for underplating by intrusions it also follows that very intense and inhomogeneous dilational strains may have occurred locally as in the metamorphic core complexes of the basin and range province

potential of surface to tunnel seismic tomography to detect vertical faults application to the tournemire underground research laboratory france the tournemire underground research laboratory aveyron france developed by the french institute for radiological protection and nuclear safety is composed of a tunnel excavated in an argillaceous geological layer belonging to a limestone argillite limestone sub horizontal sedimentary sequence sub vertical fault zones are intercepted by drifts and boreholes excavated from the tunnel in the argillaceous layer at a depth of about 250 m these fault zones are characterized by velocities lower than those of the embedding rocks in this study we assess the capacity of first arrival traveltime tomography to detect and characterize such fault zones in a surface to depth transmission configuration with a limited angle aperture the source at the surface is a sledgehammer and turns out to be very repetitive the source trigger is transmitted to the acquisition bay via a vhf transmitter receivers are located at depth in underground works first arrivals are clear and traveltime picking is straightforward before inverting real data we conduct synthetic tests with the real acquisition geometry to assess limits and validity of first arrival traveltime tomography to detect a lower velocity fault zone using different initial configurations and inversion constraints during the inversion we show that introducing a priori information on velocities in the shallow subsurface limestones and argillaceous layers and constraints on maximum velocities greatly helps improving results and subsequent interpretation velocities obtained from the real data display a main lower velocity zone that extends from the galleries in the argillaceous layer right up to the surface in limestones the location and extension of this lower velocity zone are consistent with fault and fracture zones mapped from geological observations

flexural subsidence by 29 ma on the ne edge of tibet from the magnetostratigraphy of linxia basin china abstract this study provides a detailed magnetostratigraphic record of subsidence in the linxia basin documenting a 27 myr long sedimentary record from the northeastern edge of the tibetan plateau deposition in the linxia basin began at 29 ma and continued nearly uninterruptedly until 1 7 ma increasing rates of subsidence between 29 and 6 ma in the linxia basin suggest deposition in the foredeep portion of a flexural basin and constrain the timing of shortening in the northeastern margin of the plateau to late oligocene late miocene time by late miocene early pliocene time a decrease in subsidence rates in the linxia basin associated with thrust faulting and a 10 clockwise rotation in the basin indicates that the deformation front of the tibetan plateau had propagated into the currently deforming region northeast of the plateau

modelling hanging wall accommodation above rigid thrust ramps abstract experimental models are used to study the role of material rheology in hanging wall accommodation above rigid flat ramp flat thrust footwalls the deformation in the hanging wall was accomplished by forwards sliding along a rigid basal staircase trajectory with a variable ramp angle ranging from 15 to 60 we model different ramp angles to examine hanging wall accommodation styles above thrust ramps of overthrust faults ranging from 15 to 30 as well as above pre existing normal faults ranging from 45 to 60 for the hanging walls we used stratified frictional sand and viscous silicone putty materials in this paper we study three types of models type 1 models represent purely frictional hanging walls where accommodation above thrust ramps was by layer parallel thickening and by generating a series of back thrusts type 2 and 3 models represent stratified frictional viscous hanging walls in these models accommodation was by a complex association of reverse and normal faults mainly controlled by the rheological anisotropy as well as by the ramp inclination angle in type 2 models the silicone covered only the lower flat while in type 3 models it also covered the rigid ramp for 30 in type 2 models and 45 in type 3 models the viscous layer inhibited the development of back thrusts in the frictional hanging wall instead the silicone thickened to develop a ductile ramp for values higher than 30 in type 2 models and 45 in type 3 models back thrusts develop in response to the bulk compression the experiments simulate many structures observed above natural thrust ramps with 30 and pre existing normal faults with 45 the models emphasise the importance of a basal ductile layer which allows the hanging wall to step up over the rigid ramp by building up its own ductile ramp the models also emphasise that foreland directed normal faulting can develop at a thrust front in the case that the vertical stress due to gravity exceeds the horizontal stress due to end loading within a thrust wedge

role of decollement material with different rheological properties in the structure of the aljibe thrust imbricate flysch trough gibraltar arc an analogue modelling approach abstract the miocene aljibe thrust imbricate flysch trough in the external zone of the gibraltar arc shows large changes in structural style towards the foreland rheological variation of the decollement rocks triassic evaporitic rocks and mesozoic limestones appears to be the main factor controlling the contrasts in structural style that characterise adjacent wedge domains in order to test this hypothesis we carried out analogue experiments involving three dimensional variations of decollement material a plate of viscous material was placed in the central part of the models at the base of the sand multilayers different geometries of the viscous plate were used experimental results show a strong link between the rheological properties of the decollement material and the structure of the overlying thrust wedge these results compare well with the internal geometry and vergence distribution in the aljibe thrust imbricate indeed the tectonic style of the wedge can be explained as a consequence of variations of rock type along the basal decollement

indentation of a continent with a built in thickness change experiment and nature orogens oblique to the direction of plate convergence are currently attributed to obliquity between the margins of one or both of the sutured continents to their direction convergence we use a single analogue experiment and natural examples to illustrate a potential additional factor variations in strength of the indented continent at a high angle to the convergence direction the wavelengths of structures in laterally shortened lithosphere depend on the strength of the most competent layers lateral variations in crustal thickness must therefore lead to structures oblique to any applied lateral compression an analogue experiment was performed to explore this phenomenon a two layer indented continent was modelled by a brittle upper crust of sand above a lower crust of high viscosity polymer floating on a single layer of low viscosity syrup representing the mantle the well known strike slip structures allowing lateral escape to distant weak boundaries were hindered by lateral boundaries in front of the indenter this allowed us to focus on the effects of a thickness change built into the indented continent along a zone parallel to the direction in which a vertical rigid wall advancing at a steady rate represented the indenter vertical escape led to an orogenic belt oblique to the advancing wall this obliquity influences subsequent lateral escape model scaling and interpretations are based on extended thin sheet approximation etsa and standard theories of faulting four sectors of the alpine himalayan orogen iran tunisia the eastern alps and the himalaya are oblique to the continental convergence direction and we point to thickness changes at high angles to the suture that may account for this geometry as crustal thicknesses north of oblique sectors of the himalayas are not yet known we speculate on them we infer from the main difference between our experiment and all our examples chosen from nature that vertical orogenic escape was oblique to our model suture but can be parallel to natural sutures

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

geological and hydraulic characterisation of the excavation disturbed zone in the opalinus clay of the mont terri rock laboratory abstract the fracture network in the excavation disturbed zone edz of an argillaceous formation the opalinus clay in the mont terri rock laboratory in northwest switzerland is characterised using structural and hydrogeological techniques sampling of unloading fractures is done by analysing resin injected overcores taken from the edz resulting in a structural data set of fracture orientations frequencies and extent of the edz these parameters are derived in tunnel sections which were excavated by blasting pneumatic hammering and road header techniques pneumatic and hydrogeological tests are subsequently carried out to derive hydraulic parameters such as permeability and transmissivity distributions of the fracture network synthesis of the structural and hydrogeological data led to a conceptual model of the edz in the mont terri rock laboratory which is defined by a 2 m plastically deformed zone around the tunnel this plastic zone is further divided into an inner 1 m zone consisting of an air filled fracture network and an outer zone with mostly saturated and isolated fractures transmissivities in the inner zone are orders of magnitude higher than those of the undisturbed opalinus clay finally a methodology is proposed for the structural and hydrogeological characterisation of the edz this methodology is not site specific and is thus applicable at any argillaceous underground facility

experimental and field study of the effects of lithological contrasts on thrust related deformation 1 the influence of mechanical contrasts related to stratigraphy on thrust related deformation are investigated by sandbox modeling using combinations of granular materials possessing different mechanical properties and by structural analysis of a natural example from the spanish pyrenees the models used in this study consist of loose sand and glass microbeads in varied stratigraphic thicknesses and sequences and the natural example consists of a weakening upward sequence of sandstones and carbonates in both cases layers of different composition had a mechanical significance and accommodated shortening differently model sand and natural sandstone exhibited most shortening by thrust imbrication irrespective of stratigraphic position whereas glass microbeads and carbonates exhibited wider deformation zones and enhanced folding accompanied by displacement reduction along thrust surfaces cutting them transitions of structural style were guided by the stratigraphy rather than by a simple unidirectional propagation from a step up layer parallel shortening was accommodated by diverse mechanisms but did not vary significantly in layers which had different properties the magnitude with which mechanical contrasts were expressed in the models was influenced by the strength at the decollement zone the effect of mechanical contrast was more profound in models shortened above a low friction decollement

convergence rate dependent growth of experimental viscous orogenic wedges the effects of convergence rate variations on the evolution of mature orogenic wedges are investigated using temperature dependent variations in the viscosity of paraffin as an analogue for the natural strength distribution in the crust experiments were performed in a thermomechanical shortening apparatus for strain rates ranging between 10 5 and 5 10 5 s 1 experimental wedges accreted at fast convergence rates show steep surface tapers and narrow deformed areas whereas at low strain rates they show lower taper topographies and wider deformed areas during self similar growth a decrease in convergence rate during deformation corresponds to a change from localised to delocalised deformation in concomitance with the outward migration of the deformation front the model results provide insights into deformation partitioning geometry and accretionary mechanisms within orogenic wedges

seismic investigation of the excavation damaged zone in opalinus clay a mini sonic borehole probe was used to investigate the excavation damaged zone edz around an underground opening in the opalinus clay in the mont terri rock laboratory in switzerland the edz could be determined by using seismic parameters as interval velocities and amplitudes of first arrivals derived from measurements in eight radial drilled boreholes the extent of the edz is about 1 m but varies between 0 6 and 2 2 m

experiments on shortening of a 4 layer model of the continental lithosphere davy ph and cobbold p r 1991 experiments on shortening of a 4layer model of the continental lithosphere in p r cobbold editor experimental and numerical modelling of continental deformation tecronophysics 188 l 25 we present an experimental method for modelhng lithospheric deformation the method is based on a simplified profile of lithosphere rheology where only four layers are considered brittle crust ductile crust brittle mantle and ductile mantle the strengths of the four layers calculated from laboratory determined flow laws depend strongly on geothenns and slightly on strain rates and deformation regimes the rheological profiles of the lithosphere have been recreated in experimental models built with analog materials sand silicone putty and golden syrup which flow under their own weight during lithospheric shortening the style of deformation depends on the relative strengths of the four layers especially those in the mantle of special importance is the existence or non existence of a brittle mantle the initial stages of compression often produce flexures or buckles without local isostatic compensation buckling exerts a strong control on the localisation of thrust faults in later stages the thrusts tend to form at the inflexion points of buckles the style of crustal thickening depends on the relative thicknesses and resistances of the lithospheric layers in 2 layer lithospheric models without resistant mantle shortening was concentrated upon a small number of thrusts widely spaced in 3 layer models shortening was concentrated into a large number of reverse faults more closely spaced and more steeply dipping than before in clayer models a single thrust formed in the brittle mantle motion on this thrust dragged the crustal layers down forming an asymmetric crustal root with steep imbricate slices

landward vergence and oblique structural trends in the oregon margin accretionary prism implications and effect on fluid flow abstract the central oregon margin spans a regional transition in accretionary structures from seaward verging in the south to landward verging in the north new multichannel seismic mcs data image both landward and seaward vergent provinces along the central and northern oregon margin landward vergence is characterized by a deep decollement with deformation distributed across a broad lower continental slope in a coherent structural style in the landward vergent area virtually all 4 km of incoming trench sediments overthrust the preceding thrust sheet forming fault bend folds and a distinctive ridge trough morphology this style of landward vergence is not explained by existing models in contrast seaward vergence correlates with a shallower decollement approximately 1 4 km above the oceanic crust and a more intense style of deformation within a narrower slope initial thickening of the trench sediments occurs across a well developed protothrust zone the frontal thrust forms a ramp anticline that is cut by a prominent backthrust previously observed seafloor vent sites in both regions correlate with thrusts that exhibit high amplitude reversed polarity reflections suggestive of enhanced porosity along the faults potential fluid sources and migration paths are strongly influenced by changes in the level of the decollement and vergence along the margin abrupt changes in structural style occur both along strike and updip and are bounded by two sets of oblique slip faults three nw striking left lateral faults are imaged in both mcs and seabeam data plunging anticlines developed along the nw striking faults are venting fluids and were previously interpreted as mud volcanoes the deformation front is locally disrupted where these faults intersect the prism but they appear to have limited influence on the structural evolution of the prism in contrast the ne striking right lateral faults are confined to the deforming sediments of the upper plate these faults interact with the thrusts within the prism forming a rhomboidal pattern of three dimensional deformation

shortening of analogue models with contractive substrata insights into the origin of purely landward vergent thrusting wedge along the cascadia subduction zone and the deformation evolution of himalayan tibetan orogen abstract origin of certain uncommon fold and thrust belts such as the purely landward vergent rearward vergent thrusting wedge along the cascadia subduction zone and the wide flat toped himalayan tibetan orogen were proposed to be closely related to their ductile substrata results of a series of sandbox models with contractive substrata of various contracting velocities and different contracting directions designed to study the influences of horizontal contraction of ductile substrata on deformation evolution and structural style of fold and thrust belts show that purely rearward vergent thrusting wedges can be reproduced under the condition of syn directionally horizontal contraction of the ductile substrata without other requirements of forward dipping backstops landward dipping detachments and strong wedges relative to weak bases therefore syn directionally horizontal contraction of the ductile substrata can be considered an important cause for the formation of purely landward vergent thrusting wedge along the cascadia subduction zone our modeling results together with the results of some other previous physical and numerical models reinforce the opinion that the deformation evolution of the himalayan tibetan orogen were likely governed by its weak lower crust and suggest that the north and south edges of the himalayan tibetan orogen may have undergone almost synchronous cenozoic deformation evolution owing to the existence of weak lower crust it is also proposed that 1 development of thrusts in the himalayan tibetan orogen may not necessarily occur in a frontward sequence 2 much later uplift deduced from cenozoic sedimentary records in the basins along the northeastern margin of the himalayan tibetan orogen does not necessarily imply much later initiation of uplift and deformation at the northeastern margin of the orogen

high resolution quantitative seismic imaging of a strike slip fault with small vertical offset in clay rocks from underground galleries experimental platform of tournemire france imaging tectonic faults with small vertical offsets in argilittes clay rock using geophysical methods is challenging in the context of deep radioactive waste disposals the presence of such faults has to be assessed since they can modify the rock confining properties in the tournemire experimental platform tep france fault zones with small vertical offsets and complex shape have been identified from underground works however 3d high resolution surface seismic methods have shown limitations in this context that led us to consider the detection and characterization of the faults directly from underground works we investigate here the potential of seismic full waveform inversion fwi applied in a transmission configuration to image the clay rock medium in a horizontal plane between galleries and compare it with first arrival traveltime tomography fatt our objective is to characterize seismic velocities of a block of argilittes crossed by a subvertical fault zone with a small vertical offset the specific measurement configuration allows us to neglect the influence of the galleries on the wave propagation and to simplify the problem by considering a 2d isotropic horizontal imaging domain our fwi scheme relies on a robust adaptation of early arrival waveform tomography the results obtained with fatt and fwi are in accordance and both correlate with the geological observations from the gallery walls and boreholes we show that even though various simplifications are done in the inversion scheme and only a part of the data is used fwi allows to get higher resolution images than fatt and is especially less sensitive to the incomplete illumination as it uses also diffracted energy the results provided in this study highlight the complexity of the fault zone showing a complex interaction of the main fault system with a secondary system composed of decimetric fractures associated with the presence of water

anisotropy of electrical conductivity of the excavation damaged zone in the mont terri underground rock laboratory summary electrical resistivity measurements were performed to characterize the anisotropy of electrical resistivity of the excavation damaged zone edz at the end face of a gallery in the opalinus clay of the mont terri underground rock laboratory url the data were acquired with a combination of square arrays in 18 zones on the gallery s face and in two series of four boreholes perpendicular to the face each data set is independently inverted using simulated annealing to recover the resistivity tensor both the stability and the non uniqueness of the inverse problem are discussed with synthetic examples the inversion of the data shows that the face is split in two domains separated by a tectonic fracture with different resistivity values but with a common orientation the direction of the maximum resistivity is found perpendicular to the bedding plane and the direction of minimum resistivity is contained in the face s plane these results show that the geo electrical structure of the edz is controlled by a combination of effects due to tectonics stratigraphy and recent fracturing produced by the excavation of the gallery

radiocarbon dating of flood events katherine gorge northern territory australia radiocarbon analyses of organic materials provide accurate dating of flood events in the katherine gorge northern territory australia selected flood events in the period 1955 1980 can be very precisely dated essentially to the calendar year through the use of curves showing the time variation of anomalously high 14 c activity in the troposphere that was generated by nuclear testing conventional radiocarbon analysis of alluvial charcoal and organic litter is used to establish dates for the largest katherine river floods of the past several centuries the improved geochronologic techniques demonstrated here will help improve paleoflood hydrologic interpretations

cenozoic deformation history of the qaidam basin nw china results from cross section restoration and implications for qinghai tibet plateau tectonics abstract the deformation and uplifting history of the qinghai tibet plateau is a controversial issue and so far there has been insufficient evidence for interpretation investigations of the cenozoic deformation in the northeastern edge of this plateau may provide some relevant information in this paper the cenozoic deformation history of the qaidam basin has been studied based on results from the balanced cross section restoration of six geological sections they indicate that 1 the qaidam basin has been undergoing continuous shortening since the beginning of cenozoic with two relatively fast shortening phases the first during 42 8 40 5 ma and the second during 2 8 ma present 2 the shortening rate of the qaidam basin reached its maximum value since 2 8 ma and approximately 32 of the average total cenozoic shortening took place in this period and 3 the shortening of all sections reached approximately 50 of their respective total shortening by miocene the cenozoic deformation history of the qaidam basin shows good synchroneity with uplifting history of the qinghai tibet plateau it is therefore proposed that the deformation and uplifting in the south and north edges of the qinghai tibet plateau was almost synchronous with the altyn tagh fault formed at the beginning of india asia collision accordingly it is postulated that since the india asia collision at the beginning of cenozoic the indian plate moved northward continuously with two relatively fast phases the first during 42 8 40 5 ma and the second during 2 8 ma present approximately 50 of the total cenozoic northward displacement of the indian plate of the total amount of crustal shortening and uplifting of the qinghai tibet plateau and of the total cenozoic strike slip displacement of the altyn tagh fault were probably reached by the early miocene moreover another 30 40 might have taken place during 2 8 ma present the periods during 42 8 40 5 ma and during 2 8 ma present may also be the two fast phases for crustal shortening and uplifting of the qinghai tibet plateau and strike slip displacement of the altyn tagh fault

uplift of tibetan plateau a history of the elevation and crustal thickness of the tibetan plateau since the continental collision at 45 ma has been computed using a kinematic model based on plate tectonic reconstructions and conservation of crustal volume in our reconstructions the indochina block was attached to the southern margin of eurasia before being extruded to the southeast as evidenced by the opening of the south china sea this opening affected the mass input into tibet so that tibet did not uplift to an elevation higher than 1 km until early miocene when spreading of the south china sea ceased the model then gives a steady rise of tibet from middle miocene to its present elevation

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

crustal channel flows 1 numerical models with applications to the tectonics of the himalayan tibetan orogen 1 plane strain thermal mechanical numerical models are used to examine the development of midcrustal channel flows in large hot orogens in the models radioactive self heating reduces the viscosity of tectonically thickened crust and increases its susceptibility to large scale horizontal flow channels can be exhumed and exposed by denudation focused on the high relief transition between plateau and foreland we interpret the himalaya to have evolved in this manner channel flows are poorly developed if the channel has a ductile rheology based on wet quartz flow laws and well developed if there is an additional reduction in viscosity to 1019 pa s this reduction occurs from 700 c to 750 c in the models and is attributed to a small percentage of in situ partial melt melt weakening model ht1 provides an internally consistent explanation for the tectonic evolution of many features of the himalayan tibetan orogenic system erosional exhumation exposes the migmatitic channel equivalent to the greater himalayan sequence ghs between coeval normal and thrust sense ductile shear zones corresponding to the south tibetan detachment and the main central thrust systems outward flow of unstable upper crust rotates these shears to low dip angles in the model both the ghs and the lesser himalayan sequence are derived from indian crust with the latter from much farther south similar models exhibit a range of tectonic styles including the formation of domes resembling north himalayan gneiss domes model results are relatively insensitive to channel heterogeneities and to variations in the behavior of the mantle lithosphere beneath the model plateau

northeastward growth of the tibet plateau deduced from balanced reconstruction of two depositional areas the qaidam and hexi corridor basins china we address the problem of late cenozoic uplift erosion and growth of northeastern tibet by reconstructing from isopach maps and drill holes the volumes and masses of sediments deposited in the qaidam and hexi corridor basins since 35 myr ago the mass budget is based on simple geometrical assumptions such as regional similarity of the thickness ratios between strata of different ages in the qaidam where our record extends back to the oligocene the budget shows a huge rise of the accumulation rates after the beginning of the pliocene 5 3 ma the early pliocene seems to be the period of maximum deposition with accumulation rates in excess of 1 mm yr 1 2 7 kg m 2 yr 1 of compacted rocks throughout the basin there also seems to be a southeastward shift of the largest depocenters between the upper pliocene 3 4 1 6 ma and the quaternary in the hexi corridor sedimentation is confined to small foreland flexural depressions associated with the frontal thrusts of the qilian shan and occurs at an average rate one order of magnitude smaller than in the qaidam basin the accumulation rate is maximum in the quaternary the sedimentation history appears to support a plateau building mechanism resulting from the combination of two geologicallly common processes crustal scale thrusting and sedimentary basin infilling the time needed to completely fill the qaidam basin and make its catchment a plateau closely resembling that of the highest part of tibet qangtang is of the order of 9 myr the mechanism now at work north of the kunlun which involves rapid infilling of broad flat areas separated by relatively narrow mountain ranges has thus probably been important in producing the high smooth topography that characterizes much of central tibet

topography as a major factor in the development of arcuate thrust belts insights from sandbox experiments abstract we have used sandbox experiments to investigate and to illustrate the effects of topography upon the development of arcuate thrust belts in experiments where a sand pack shortened and thickened in front of an advancing rectilinear piston the geometry of the developing thrust wedge was highly sensitive to variations in surface topography in the absence of erosion and sedimentation the surface slope tended to become uniform as predicted by the theory of critical taper under these conditions the wedge propagated by sequential accretion of new thrust slices in contrast where erosion or sedimentation caused the topographic profile to become irregular thrusts developed out of sequence for example erosion throughout a hinterland caused underlying thrusts to remain active and inhibited the development of new thrusts in the foreland where initial topography was irregular in plan view accreting thrusts tended to be arcuate they were convex towards the foreland around an initially high area concave towards the foreland around an initially low area initial plateaux tended to behave rigidly while arcuate thrust slices accreted to them thrust motions were radial with respect to each plateau within transfer zones to each side fault blocks rotated about vertical axes and thrust motions were oblique slip at late stages of deformation the surface slope of the thrust wedge tended towards a uniform value initial mountains of conical shape representing volcanoes also escaped deformation except at depth where they detached arcuate thrust slices accreted to front and back where a developing thrust wedge was subject to local incision accreting thrust slices dipped towards surrounding areas of high topography forming vs across valleys arcuate structural patterns are to be found around the three highest plateaux on earth tibet pamirs and altiplano and around the tromen volcanic ridge in the neuquen basin of northern patagonia we infer that these areas behaved in quasi rigid fashion protected as they were by their high topography

receiver coupling effects in seismic waveform inversions seismic waveform inversion offers opportunities for detailed characterization of the subsurface however its full potential can only be exploited when any systematic source and receiver effects are either carefully avoided or appropriately accounted for during the inversions repeated crosshole measurements in the mont terri switzerland underground laboratory have revealed that receiver coupling may significantly affect the seismic waveforms more seriously coupling conditions may vary during the course of a monitoring experiment to address this problem we have developed a novel scheme that estimates medium properties frequency dependent source functions and frequency dependent receiver coupling factors we demonstrate the efficacy of the new scheme via a synthetic 2d crosshole experiment in which realistic receiver coupling factors are incorporated because determination of medium parameters and estimation of source functions and receiver coupling factors are largely separated the method can be

experimental insights on the geometry and kinematics of fold and thrust belts above weak viscous evaporitic decollement abstract this paper discusses the differences in mechanical properties and kinematics between fold and thrust belts detaching on evaporitic decollements and those detaching on stronger detachments physical experiments are described that model shortening of a thick brittle cover overlying a weak viscous decollement to gain a better understanding of these differences we tested the influence of 1 the decollement layer thickness and 2 the presence of a deformable backstop on the hinterland side and of a decollement pinch out on the foreland side because of the very low shear strength of the viscous decollement folds and thrusts did not propagate according to a piggy back sequence but by centripetal back and forth propagation additional shortening was accommodated by growth of all existing structures fold symmetry and thrust vergence varied between experiments models confined between two rigid vertical endwalls always deformed by symmetric folding and thrusting in models having a deformable backstop and a foreland decollement pinch out forethrusting initially dominated folds were asymmetric and fault blocks rotated in models having a thick decollement layer folds kept growing asymmetrically diapirism also depended on the initial decollement geometry diapirs formed only in models having a deformable backstop and were restricted to the hanging wall of a fault related fold finally the implications of such kinematics on the thermal history are discussed as well as the likelihood of hydrocarbon maturation and preservation

influence of syntectonic sedimentation on thrust wedges in analogue models scaled analogue sandbox models have been used to simulate the growth of coulomb thrust wedges that include syntectonic sedimentation during thrusting increases in the syncontractional sedimentation rate dramatically change the shape of the coulomb thrust wedge decreasing the number of thrust slices the internal shortening and the critical taper of the wedge thrust trajectories change from ramp flat to sigmoidal with increased syncontractional sedimentation the analogue models show many similarities to thrust fault geometries in accretionary prisms and foreland fold and thrust belts in which thrust slices prograded into basins with active sedimentation

tertiary deformation history of southeastern and southwestern tibet during the indo asian collision geologic mapping and geochronological analysis in southwest kailas area and southeast zedong area tibet reveal two major episodes of tertiary crustal shortening along the classic indus tsangpo suture in the yalu river valley the older event occurred between ca 30 and 24 ma during movement along the north dipping gangdese thrust the development of this thrust caused extensive denudation of the gangdese batholith in its hanging wall and underthrusting of the xigaze forearc strata in its footwall examination of timing of major tectonic events in central asia suggests that the initiation of the gangdese thrust was approximately coeval with the late oligocene initiation and development of north south shortening in the eastern kunlun shan of northern tibet the nan shan at the northeastern end of the altyn tagh fault the western kunlun shan at the southwestern end of the altyn tagh fault and finally the tian shan north of the tarim basin such regionally synchronous initiation of crustal shortening in and around the plateau may have been related to changes in convergence rate and direction between the eurasian plate and the indian and pacific plates the younger thrusting event along the yalu river valley occurred between 19 and 10 ma along the south dipping great counter thrust system equivalent to the locally named renbu zedong thrust in southeastern tibet the backthrust system in south central tibet and the south kailas thrust in southwest tibet the coeval development of the great counter thrust and the north himalayan granite gneiss dome belt is consistent with their development being related to thermal weakening of the north himalayan and south tibetan crust due perhaps to thermal relaxation of an already thickened crust created by the early phase of collision between india and asia or frictional heating along major thrusts such as the main central thrust beneath the himalaya

anomalies of noble gases and self potential associated with fractures and fluid dynamics in a horizontal borehole mont terri underground rock laboratory abstract before the excavation of the new gallery ga08 in the mont terri underground rock laboratory switzerland which joined the existing gallery ga04 in 2008 the end face of gallery ga04 was instrumented in 2007 to characterize the evolution of the rock mass with geochemical and geophysical methods the noble gas content of a 12 m long horizontal borehole evidenced that desaturation processes occurred in the first 2 m where pre existing fractures accommodated the stress change during excavation of gallery ga04 four years before these first 2 m are associated with the so called excavation damaged zone edz as an inflow of pore water was observed in this borehole few weeks after its drilling continuous self potential sp measurements were performed to characterize its dynamics after the drilling of new sub horizontal boreholes in the end face strong localized variations of sp occurred the comparison with the geological features of the rock mass suggests that these dynamic anomalies have to be associated with fluid circulation in pre existing tectonic fractures that were primarily reactivated by the excavation of the gallery ga04 and subsequently by the drilling operations

tectonic history of the altyn tagh fault system in northern tibet inferred from cenozoic sedimentation the active left slip altyn tagh fault defines the northern edge of the tibetan plateau to determine its deformation history we conducted integrated research on cenozoic stratigraphic sections in the southern part of the tarim basin fission track ages of detrital apatites existing biostratigraphic data and magnetostratigraphic analysis were used to establish chronostratigraphy whereas composition of sandstone and coarse clastic sedimentary rocks was used to determine the unroofing history of the source region much of the detrital grains in our measured sections can be correlated with uplifted sides of major thrusts or transpressional faults implying a temporal link between sedimentation and deformation the results of our studies together with existing stratigraphic data from the qaidam basin and the hexi corridor suggest that crustal thickening in northern tibet began prior to 46 ma for the western kunlun shan thrust belt at ca 49 ma for the qimen tagh and north qaidam thrust systems bounding the north and south margins of the qaidam basin and prior to ca 33 ma for the nan shan thrust belt these ages suggest that deformation front reached northern tibet only 10 5 m y after the initial collision of india with asia at 65 55 ma because the aforementioned thrust systems are either termination structures or branching faults of the altyn tagh left slip system the altyn tagh fault must have been active since ca 49 ma the altyn tagh range between the tarim basin and the altyn tagh fault has been a long lived topographic high since at least the early oligocene or possibly late eocene this range has shed sediments into both the tarim and qaidam basins while being offset by the altyn tagh fault its continuous motion has made the range act as a sliding door which eventually closed the outlets of westward flowing drainages in the qaidam basin this process has caused large amounts of oligocene miocene sediments to be trapped in the qaidam basin the estimated total slip of 470 70 km and the initiation age of 49 ma yield an average slip rate along the altyn tagh fault of 9 2 mm yr remarkably similar to the rates determined by gps global positioning system surveys this result implies that geologic deformation rates are steady state over millions of years during continental collision

injection of indian crust into tibetan lower crust a two dimensional finite element model study we propose a model in which the stronger indian crust injects into the weaker lower crust of tibet if the viscosity of the tibetan lower crust is below a critical value the continuous advancing of the indian crust beneath tibet would produce negligible variation in stresses on the bottom of the stronger brittle tibetan upper crust and would result in a spatially uniform uplift and a virtually undisturbed tibetan plateau surface this process is analogous to that of a piston rising in a hydraulic jack as additional fluid is forced into the chamber we find that if the viscosity of the tibetan lower crust is assumed to be constant throughout the computed critical viscosity is 6 1018 pa s for the present day lower crustal thickness of 58 km in addition to uniform viscosity two hypothetical depth dependent viscosities are also tested which give rise to essentially the same distribution of stress along the bottom of brittle upper crust our model also interprets the 4 km elevation contrast between the tibetan plateau and the tarim block a 1 160 viscosity contrast between the tibetan and tarim lower crust is required in order to maintain the current thickening rate of the tibetan crust 1 2 mm yr while keeping the thickness of the tarim crust unchanged

appraisal of waveform repeatability for crosshole and hole to tunnel seismic monitoring of radioactive waste repositories countries worldwide are seeking solutions for the permanent removal of high level radioactive waste from the environment surrounding the waste with multiple engineered barriers and emplacement in deep geological repositories is widely accepted as a safe means of isolating it from the biosphere for the necessary 105 106 years as a precautionary measure society demands that repositories be monitored for 100 300 years after they are backfilled and sealed effective monitoring that does not compromise the engineered and natural barriers is challenging to address this issue we investigate the viability of crosshole and hole to tunnel seismic methods for remotely monitoring high level radioactive waste repositories measurements are made at two underground rock laboratories in switzerland one within granitic rock and one within clay rich sediments numerical simulations demonstrate that temporal changes of the monitored features i e bentonite plug excavation damage zone sand filled microtunnel sh

the mont terri rock laboratory a new international research project in a mesozoic shale formation in switzerland abstract in many countries argillaceous formations are being considered as potential host rocks for repositories of radioactive waste therefore in 1995 several organisations decided to start an international research project in the reconnaissance gallery of the mont terri motorway tunnel in north western switzerland in a mesozoic shale formation the opalinus clay aalenian this project is under the patronage of the swiss national hydrological and geological survey the following organisations are also partners in the project andra and ipsn france bgr germany enresa spain nagra switzerland jnc and obayashi japan sckcen belgium the aims of the project are to analyse the hydrogeological geochemical and rock mechanical properties of an argillaceous formation the changes of these properties induced by the excavation of galleries and to evaluate and improve appropriate investigation techniques in january 1996 eight niches were excavated for the project and 15 experiments were started in boreholes of no more than 30 m in length in the winter of 1997 1998 a new research gallery was excavated to host further experiments different excavation drilling and investigation techniques have been tested and improved significant results of the different experiments are already available some of those dealing with hydrochemistry porewater pressure measurements and the evolution of the excavation disturbed zone edz are reported in this paper

shortening of analogue models of the continental lithosphere new hypothesis for the formation of the tibetan plateau initial stages of compression produce periodic buckling of models analogous to a four layer continental lithosphere with further shortening amplification of the buckles occurs by thrust faulting at inflection points in both brittle layers of the upper crust and upper mantle the rescaled results suggest that a continental lithosphere under compression will buckle with a first order wavelength of about 200 km and a second order wavelength of 20 30 km continued shortening results in the amplification of undulations troughs sink and become compressional basins while deformation keeps going on major thrusts within the upper mantle thus giving rise to an irregular topography of the moho fifty percent shortening results in a 60 km thick continental crust we argue that this model history may explain the elevation and deep crustal structure of the tibetan plateau

seismic tomography of the excavation damaged zone of the gallery 04 in the mont terri rock laboratory summary an endoscopic antenna is used to perform a seismic cross hole tomography in the excavation damaged zone edz of the new g04 gallery of the mont terri underground rock laboratory switzerland excavated in opalinus clay more than 800 seismic traces were recorded between two vertical boreholes by combining 22 source and 48 receiver locations a vertical area of 1 2 3 4 m under the floor of the gallery is investigated with a high resolution tomography data with a very good quality allow to determine the traveltimes and the amplitudes of a 40 khz source wavelet propagating between the two boreholes the analysis of the traveltimes shows that the wave velocity is homogeneous but anisotropic with a minimum value of 2490 45 m s 1 in the direction normal to the bedding and a maximum of 3330 90 m s 1 parallel to the bedding the amplitude of the first arrivals strongly varies depending on the source receiver locations and suggesting an heterogeneous distribution of the attenuation coefficient of the seismic waves a bayesian inversion provides likely models of attenuation that are compared with geological observations the areas where fractures or cracks exist in the opalinus clay appear as highly absorbing the seismic waves

evolutionary model of the himalaya tibet system geopoem based on new modelling geological and geophysical data a two dimensional thermo mechanical laboratory modelling of continental subduction was performed the subducting continental lithosphere includes a strong brittle upper crust a weak ductile lower crust and a strong upper mantle the lithosphere is underlain by a low viscosity asthenosphere subduction is produced by a piston push force and the pull force from the mantle lithospheric layer which is denser than the asthenosphere the lithospheric layers are composed of material whose strength is sensitive to and inversely proportional to temperature throughout the experiment the model surface was maintained under relatively low temperature and the model base at higher temperature the subduction rate satisfied the peclet criterion modelling confirms that the continental crust can be deeply subducted and shows that slab break off delamination and tectonic underplating are fundamental events with drastic consequences on the subsequent evolution of the convergent system combining these results with previous purely mechanical modelling we elaborate a new evolutionary model for the himalaya tibet convergent system the principal successive stages are 1 subduction of the indian continental lithosphere to 200 250 km depth following subduction of the tethys oceanic lithosphere 2 failure and rapid buoyancy driven uplift of the subducted continental crust from ca 100 km depth to some depth that varies along the mountain belt 20 30 km on average 3 break off of the indian subducted lithospheric mantle with the attached oceanic lithosphere 4 subduction underplating of the indian lithosphere under asia over a few to several hundred kilometers 5 delamination roll back and break off of the indian lithospheric mantle 6 failure of the indian crust in front of the mountain belt formation of the main central thrust and underthrusting of the next portion of indian lithosphere beneath tibet for a few hundred kilometers at the beginning of stage 6 the crustal slice corresponding to the crystalline himalayas undergoes erosion activated uplift and exhumation fl 2000 elsevier science b v all rights reserved

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

seismic monitoring of radioactive waste repositories abstractsafe isolation of high level radioactive waste hlw and spent nuclear fuel sf from the biosphere is an important concern for humanity many countries consider monitoring the evolving conditions of a hlw sf repository over a long period noninvasively in such a way as to not compromise the sealing of the repository remote active seismic methods are possible options we performed extensive seismic measurements at the mont terri rock laboratory located within a clay formation that is a potential host rock in the swiss hlw sf program a scaled down hlw sf repository was simulated as a 13 m long microtunnel of 1 m diameter the dry sand filled microtunnel was progressively water saturated and slightly overpressured the monitoring system consisted of a pressure seismic source and a hydrophone cable located in two boreholes drilled perpendicular to the microtunnel and eight geophones equally distributed around the periphery of the microtunnel nonlinear anisotropic tomography of crosshole traveltim

mechanical role of backstops in the growth of forearcs we use numerical and laboratory modeling to analyze the mechanical role of backstops in the overriding plate at subduction zones a backstop is defined as a region within a forearc that has significantly greater shear strength than the sediment lying farther trenchward it can be thought of as the bulldozer behind an accretionary wedge we calculate the stress and displacement fields within forearcs for various backstop models using the finite element method and we simulate deformation over a backstop using a small scale laboratory model in this way we model the effects of the mechanical properties and geometry of a backstop on forearc structures we find that the growth of an outer arc high the development of an inner deformation belt with landward vergence and the seemingly paradoxical presence of an undeformed forearc basin within an otherwise highly deformed forearc can be explained by assuming the existence of a geometrically simple backstop with geologically reasonable contrasts in mechanical properties compared to the sediments just trenchward of it the structures produced in our numerical models are quite insensitive to the rheology boundary conditions and exact mesh geometry employed the general types of structures observed in the laboratory models depend only weakly upon the strength and the geometry of the backstop these results suggest that a detailed picture of an underlying backstop cannot be determined from surface information alone backstops in which the contact with the accretionary wedge dips arcward rather than trenchward however should produce only slightly different forearc structures with less development of both the outer arc high and the landward vergent inner deformation belt although natural forearcs are far more complex than our simple models they exhibit many of the same features indicating that relatively simple backstop mechanics may be a very important factor in the overall growth of forearcs

northeastward growth and uplift of the tibetan plateau magnetostratigraphic insights from the guide basin 1 most of the evidence for the uplift of the tibetan plateau comes from its southern and central parts although the northern rim has been less studied it may greatly contribute to the understanding of the mechanism and timing of the uplift recent studies on the northeastern part of the plateau suggest that the uplift can largely be explained by cenozoic thrusting and folding linked to the movement of the altyn tagh and kunlun faults and that diffuse shortening might be a realistic way to explain the thickening of the continental interior nevertheless the age of sediments that record the tectonic processes is poorly known the terrigenous formations in the guide gonghe basin ne tibet show several phases of deformation and include thick deposits of conglomerates interbedded with sandstones and siltstones that are thought to be the result of an increased sediment flux dating of these gravel deposits with magnetostratigraphic and faunal studies has permitted the precise timing of the basin infilling and its tectonic evolution to be established our results indicate that the onset of conglomerate accumulation occurs in the pliocene an observation that is in agreement with those of the linxia basin just east of the plateau if the high sedimentation rates 22 cm kyr in the guide basin are mostly due to tectonism then significant uplift occurred as late as pliocene

active thrusting and folding in the qilian shan and decoupling between upper crust and mantle in northeastern tibet fieldwork south of the city of gaotai gansu province china shows that active shortening of surface sediments in the foothills of the yumu shan a large fore mountain of the qilian shan at the northeastern edge of tibet involves both overthrusting and flexural slip folding north of this mountain we found and mapped a prominent north facing thrust scarp that offsets a holocene fan sloping gently 3 4 to the north part of this scarp appears to be related to the m 7 5 180 a d earthquake that may have led to the demise of the han dynasty city of luo tuo chen in the hexi corridor a set of 10 100 150 m long profiles measured across this scarp 3 2 m high on the average can be made to fit the diffusion degraded morphology of a surface break related to the 180 a d event using a value of about 3 3 m 2 10 3 yr for the mass diffusivity of fanglomerates in this part of gansu province smaller mountain facing scarps on a terrace capped foothill result from bedding slip concurrent with active folding of underlying steeply northdipping plioquaternary sandstone and conglomerate beds holocene uplift rates along the yumu shan which is only one of the qilian shan ranges are estimated to be between 0 4 and 1 9 mm yr which implies that much of the mountain formed in the quaternary the periclinal structure of the plioquaternary envelope under which the paleozoic core of the yumu shan plunges towards the west suggests that the whole 3200 m high mountain is a basement ramp anticline mountains striking parallel to the yumu shan with similar structure and comparable or greater sizes north and south of the hexi corridor probably also correspond to recent crustal ramp anticlines this implies that the wide mountainous upper crustal wedge making the northeastern edge of the tibet qinghai plateau is detached from the underlying lower crust and upper mantle

modeling of thrust fronts above ductile and frictional detachments application to structures in the salt range and potwar plateau pakistan series of scaled sandbox models are used to simulate the development of thin skinned simultaneous shortening above adjacent ductile and frictional substrates these models simulate the evolution of the potwar plateau and salt range in pakistan where pale

large scale crustal deformation of the tibetan plateau the topography velocity and strain fields calculated from a three dimensional newtonian viscous model for large scale crustal deformation are generally in good agreement with results from geological geodetic and earthquake studies in and around the tibetan plateau provided that the model rheology incorporates a weak zone within the deep crust beneath the plateau equivalent to a viscosity of 1012 pa s within a 250 m thick channel or 1018 pa s within a 15 km thick channel model studies and observations show a plateau at 5 km elevation with steep topographic gradients across the southern and northern plateau margins and more gentle gradients across the southeastern and northeastern margins rapid shortening strain is concentrated along the lower portions of the northern and southern plateau margins at rates 20 mm yr model results show north south shortening 10 mm yr in reasonable agreement with gps data 5 8 mm yr of north south shortening across the northern two thirds of the plateau and east west stretching 10 15 mm yr across the eastern half of the high plateau in reasonable agreement with seismic geologic and gps data upper crustal material moves eastward from the plateau proper into a lobe of elevated topography that extends to the south and east clockwise rotation of material around the east himalayan syntaxis at rates up to 10 mm yr occurs partly as a result of dextral shear between indian and asian mantle at depth and partly as a result of gravitational spreading from the high plateau to the south and east there is little difference in model surface deformation for assumptions of moderately weak or extremely weak lower crust except in southern and northern tibet where margin perpendicular extension occurs only for the case of an extremely weak lower crust our results suggest that the tibetan plateau is likely to have gone through a two stage development the first stage produced a long narrow high orogen whose height may have been comparable to the modern plateau the second stage produced a plateau that grew progressively to the north and east east west stretching eastward plateau growth and dextral rotation around the east himalayan syntaxis probably did not begin until well into the second stage of plateau growth perhaps becoming significant after 20 m y of convergence

a mechanism for syn collisional rock exhumation and associated normal faulting results from physical modelling abstract properly scaled physical modelling of the subduction of continental lithosphere is performed with a three layer lithospheric model the model includes a strong upper crust a weak ductile lower crust and a strong mantle part the lithosphere is underlain by a low viscosity asthenosphere subduction is produced by a piston push force and the pull force from the mantle lithospheric layer which is slightly denser than the asthenosphere the results of the modelling are tested on the example of the himalayas in the experiments all lithospheric layers subduct into the mantle to a depth corresponding to 200 300 km in nature until the upper crustal layer fails in front of the subduction zone forming a first major thrust fault the mct in our interpretation of the himalayas underthrusting along this fault increases relief which upon reaching a height corresponding to several kilometres in nature is removed mechanically by a blade the blade is moved horizontally back and forth throughout the experiment this erosional unloading mechanism causes the previously subducted segment of the buoyant upper crust to slide upward over the weak lower crust at a rate comparable to the subduction rate the rise of the crustal slice accelerates displacement along the crustal thrust while producing a normal sense motion and exhumation of the material from great depth up to 40 50 km depending on the model parameters along the upper surface of the slice the rapid uplift of the crust then ceases and the orogenesis occurs further due to continuing underthrusting along the first major thrust at a certain stage the crust fails again producing a new major crustal thrust the mbt in the himalayas

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

the influence of backstop dip and convergence velocity in the growth of viscous doubly vergent orogenic wedges insights from thermomechanical laboratory experiments abstract we investigate the effects of convergence velocity and backstop geometry on the evolution of experimental viscous doubly vergent orogenic wedges experiments are performed in a thermomechanical shortening box using the newtonian properties of 52 54 en type paraffin as analogue to the natural strength distribution in the continental crust a velocity discontinuity is imposed at the subduction slot where the base plate is pulled below a rigid backstop the mobile base plate dips 5 two different backstop dips 60 and 30 are used in combination with constant convergence velocities of 2 and 10 cm h 1 results point out that backstop geometry exerts a first order control on the evolution of doubly vergent experimental wedges steeply dipping backstops cause the stationary uplift of the wedge axial region above the subduction slot whereas shallow dipping backstops allow its retroward translation in concomitance with a more pronounced deformation in the retrowedge convergence velocity controls the uplift in the wedge axial region and the amount of the outward migration of the deformation front in both the prowedge and retrowedge regions for fast convergence velocity and shallow dipping backstop deformation in the retrowedge is favoured an increase in the backstop dip and or a decrease in the convergence velocity favour deformation in the prowedge both prowedge and retrowedge tend to grow in self similar conditions with an aspect ratio depending upon the backstop dip and convergence velocity these results may have important consequences for the evolution and deformation style of ductile dominated orogenic wedges suggesting that location of the axial region and deformation progression in natural orogens can be strongly controlled by boundary and kinematic conditions

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

landward vergence in accretionary prisms the role of the backstop and thermal history the accretionary histories recorded along the convergent margins in southwest alaska and southwest japan are punctuated by two anomalous events near trench magmatism closely associated in time and space with landward structural vergence in both areas the near trench magmatism is believed to be related to the subduction of an active spreading center it is postulated that these anomalous thermal events caused the backstop i e mechanical boundary within the prism to be reoriented to a seaward instead of a landward dip this seaward dipping backstop resulted in the development of landward verging structures for the only time in the almost 100 m y history of these two margins a modern analog may be the washington convergent margin these examples illustrate the potential importance of thermal history and the dip of the backstop in controlling the style of accretion in submarine wedges

thrust geometry controlled by erosion and sedimentation a view from analogue models analogue sandbox models have been set up to study thrust systems formed above two decollements and affected by erosion and sedimentation the deformation was imaged by x ray scanner three erosion sedimentation conditions were tested for all the experiments a duplex develops between the two decollement levels for the experiment without erosion and without sedimentation ramp anticlines form in the upper series for the model with syntectonic sedimentation and without erosion the shortening in the upper series is first characterized by fault related folds and then by a forward propagation of the decollement that induces the development of a piggy back basin for the experiment with erosion and sedimentation a passive roof duplex is formed relationships between structural style and syntectonic superficial conditions in classical thin skinned thrust belts are similar to those found in the analogue models the ramp anticlines of the jura developed with low erosion and low sedimentation and the large sub andean thrust belt is controlled by syntectonic sedimentation whereas the passive roof duplex of pakistan is synchronized with intense erosion this study outlines the crucial effect of the superficial mass transport phenomena in the tectonic style of thrust belts

material transfer in accretionary wedges from analysis of a systematic series of analog experiments the structural evolution of accretionary wedges was modeled in a series of analog experiments with systematically varying initial boundary conditions the two principle parameters investigated were basal friction and thickness of sediment input relative to subducted sediment output material transfer in terms of degree and location of accretion and erosion is quantified as a function of these two parameters low basal friction produces long thin wedges by continuous frontal accretion of imbricate thrusts simultaneously substantial basal erosion may occur with little influence on the frontal dynamics the formation of an outer arc high with an accompanying forearc basin depends upon the relative amount of basal erosion the opening of the subduction window spacing of thrust faults is proportional to the layer thickness confirming the principle of self similar wedge growth from critical wedge theory high basal friction typically generates steep wedges marked by an erosional mass wasting frontal slope in the cases i o this is due to underthrusting and or subduction of the entire sedimentary section the case i o also features the late development of normal faulting and an extensional forearc basin for both low and high basal friction the material balance affects the wedge slope erosive conditions i o for low basal friction the primary modes of material transfer are frontal accretion and basal erosion for high basal friction the primary modes of material transfer are underthrusting underplating and frontal erosion applying the experimental results to active convergent margins parallels may be drawn thus the barbados ridge complex is interpreted to be a good example of a low basal friction wedge with an accretionary material balance while the japan and mariana margins provide examples of an erosive material balance

simultaneous development of noncylindrical folds frontal ramps and transfer faults in a compressional regime experimental investigations of himalayan examples experiments were performed with modeling clay models to understand the simultaneous development of folds frontal ramps and transfer faults oblique ramps in layered sequences the models were deformed by layer parallel contraction at a late stage of deformation the models were subjected to a maximum contraction parallel to the layering and perpendicular to the early axis of maximum contraction to study the effect of the superposed deformation the experimental results reveal that a transfer fault joining two frontal thrust ramps acts as a shear zone and controls the orientation of fold hinge lines in the adjacent region hence two orientations of fold hinge lines may be observed 1 oblique to the axis of maximum compression in the vicinity of the transfer fault and 2 normal to the axis and parallel to the frontal ramps away from the transfer fault during the longitudinal fold propagation the two hinge lines may coalesce to form a fold curvature opposite to the tectonic transport direction the curvature varies along the transfer fault depending on the shear zone geometry additional fold hinge line curvatures may result during the superposed deformation both in the early and the superposed fold hinge lines all these curvatures produce a systematic pattern which can be analyzed with the help of the experimental results the fold hinge line orientation may also be used to establish an oblique ramp in field as demonstrated by examples from the lesser himalayas india

non coulomb wedges wrong way thrusting and natural hazards in cascadia landward vergence in accretionary wedges is an uncommon phenomenon not readily explained by classical mohr coulomb critical wedge theory predominantly landward vergent thrust faults are observed along the cascadia convergent margin from 45 n to 48 n we present depth migrated multichannel seismic images of the internal structure of the accretionary wedge offshore washington collected during the orwell project in 1996 these reveal a high p wave velocity 4 km s basal layer that thickens landward and serves as a decollement for a series of overlying landward vergent thrust faults analog modeling using a ductile basal layer consisting of silicone putty produces an array of trenchward propagating landward vergent thrusts and offers a plausible mechanical model for the evolution of these structures the rheological properties of a basal calcareous mudstone layer offshore cascadia are discussed in relation to the mechanics of landward vergence and to rapid loading due to the prograding nitinat and astoria deep sea fans a viscoelastic layer beneath the accretionary wedge is considered to be capable of rupturing during great interplate earthquakes and thus represents an increased risk for both the maximum size of such an event and for the generation of tsunamis

monitoring the excavation damaged zone by three dimensional reconstruction of electrical resistivity a damaged zone is formed during the excavation of underground galleries altering the rock properties from a perspective of nuclear waste storage in deep geological sites there is a clear interest to monitor the rock properties in such zones we constructed electrical resistivity tomograms as a function of time to monitor the damaged area in gallery 04 of the mont terri underground rock laboratory switzerland measurements were performed using electrode rings surrounding the gallery the experience showed a heterogeneous distribution of damages around the gallery and their fast formation after the excavation two main areas were concerned by damage formation located in regions where the bedding was tangential to the excavated gallery such regions represented an extension of about 2 m along the gallery walls and reached a depth of 1 5 m main damages were created during the next months following the excavation process slight variations were still observed 3 yr after the excavation that may be related to the gallery environmental condition fluctuation the method applied here demonstrates the interest to monitor the whole region surrounding excavated galleries dedicated to host nuclear wastes