vertical averages of rheology of the continental lithosphere relation to thin sheet parameters abstract we calculate vertically averaged rheologies for continental lithosphere subject to biaxial states of stress corresponding to extensional compressional and strike slip regimes we assume that brittle deformation occurs by byerlee s law and that ductile deformation occurs by steady state creep mechanisms under these conditions the vertically averaged rheology may be approximated by a single power law over the range of strain rates from 10 13 to 10 17 s 1 this is true not only when either brittle or ductile behavior dominates the strength of the lithospere but also when the contributions of brittle and ductile behavior are comparable the value of the power law exponent n and the strength of the lithosphere depend on the depth and stress difference at the brittle ductile transition the temperature at the moho and the geothermal gradient in the range of vertically integrated stress between 1012 and 1013 n m 1n reaches a minimum of between 5 and 10 under other circumstancesn increases rapidly as brittle behavior dominates over ductile behavior the strength of the lithosphere depends most strongly on the thermal state of the upper mantle but also is influenced by the stress supported in brittle portions of the lithosphere

some thermal and mechanical consequences of rapid uplift an example from the southern alps new zealand abstract the thermal evolution of continental crust during active collision is modeled through numerical solutions of the two dimensional heat conduction equation for a rapidly moving medium the boundary conditions used in the modeling are derived from geological and geophysical observations from the active collision zone in the south island of new zealand the problem domain over which the solutions are obtained consists of a 40 km horizontal by 25 km vertical spatial plane with a vertical discontinuity at 10 km from the western boundary to the east of this discontinuity vertical uplift rates of up to 10 mm a occur over a timespan of up to 4 ma temperature distributions are calculated at 10 ka intervals over the 4 ma duration a two dimensional high temperature region is established upon initiation of uplift of the eastern block due to the advective component carrying heat upwards more rapidly than it can be dissipated laterally from the problem domain temperatures within the upper 5 km are greater than 400 c after 2 25 ma with geothermal gradients of up to 200 c km attained within the upper 3 km at times greater than 2 5 ma the vertical temperature distribution changes little while the anomalously high temperatures spread laterally into the stationary crust using rheological equations to describe the brittle behaviour of a water saturated upper crust and the ductile behaviour of a quartz dominated lower crust together with the thermal distribution of the conduction models the mechanical evolution of a collision zone is investigated in addition to high crustal temperatures and associated high heat flow rapid uplift produces a weakening of the crust by raising of the depth of transition from brittle to ductile behaviour within the zone of most rapid uplift the brittle ductile transition rises from 13 km to less than 5 km after 1 5 ma of uplift further uplift reduces the brittle layer to 3 km thickness and causes lateral spreading of the low strength zone the reductions in crustal strength caused by the thermal weakening produce a high strain zone within the region of maximum uplift which is incapable of sustaining large differential stresses this causes horizontal and vertical stress transfer and results in shallow seismicity increases in the adjacent crust as well as in intermediate depth seismicity within the high strength upper mantle because the thermal and mechanical anomalies discussed are a function of rapid uplift all regions of active continental collision may be expected to exhibit similar behaviour some mechanical and thermal characteristics of the himalayan collision zone are briefly examined in light of the numerical modeling

source parameters of regional small to moderate earthquakes in the yunnan sichuan region of china abstract we apply a grid search waveform inversion technique to determine moment tensors for 73 small to moderate earthquakes that occurred in the yunnan sichuan region of china in 2003 and 2004 and were recorded by a passcal deployment together with 6 small to moderate events recorded by the yunnan province digital seismic network ydsn that occurred in 2000 to 2001 the advantage of studying these earthquakes down to an m w 3 03 is that the large number of solutions present a picture of the interaction of the four tectonic blocks in the region we find distinct patterns in the orientation of the maximum compressive stress axes derived from the moment tensor solutions they are compatible with the clockwise rotation of the eastern himalayan syntaxis northeastward compressive motion and the south southeast movement of the east tibetan plateau crustal material and the counterclockwise rotation at the east of the red river fault we also find a subtle variation in source depth for crustal sources across the region

comparison of strong ground motion from the wenchuan china earthquake of 12 may 2008 with the next generation attenuation nga ground motion models a total of 72 free field accelerograms recorded at rupture distances of less than 200 km during the wenchuan earthquake were used to compare the wenchuan earthquake ground motions with those predicted by the recent next generation attenuation nga ground motion models developed using global data from shallow crustal earthquakes but few have recordings from earthquakes with magnitudes as large as 7 9 overall the wenchuan earthquake produced smaller than expected long period t 1 s ground motions but higher than expected short period t 0 5 s ground motions compared with the predicted motions by the nga models this general trend is observed at all distances but it is stronger at distances greater than 50 km the scaling with v s 30 from the wenchuan earthquake is not consistent with the scaling from the nga models the wenchuan data show a much stronger v s 30 dependence at short spectral periods and a weaker v s 30 dependence at long spectral periods the short period hanging wall effects on rock sites are consistent with the nga hanging wall scaling but the short period ground motions on soil sites on the footwall are greater than those predicted by the nga models

lateral extrusion in the eastern alps part 1 boundary conditions and experiments scaled for gravity lateral extrusion encompasses extensional collapse gravitational spreading away from a topographic high in an orogenic belt and tectonic escape plane strain horizontal motion of wedges driven by forces applied to their boundaries in the eastern alps it resulted from 1 an overall northerly compression apulia against eurasia 2 a strong foreland bohemian massif 3 lack of constraint along a lateral boundary carpathian region and 4 a previously thickened gravitationally unstable thermally weakened crust eastern alpine orogenic belt six indentation experiments reproduce lateral extrusion at lithospheric scale the models have two to four lithospheric layers with a mohr coulomb rheology for the upper and a viscous rheology for the lower crust the lithosphere rests upon a low viscosity asthenosphere a broad indenter a narrow deformable area and a weakly constrained eastern margin fullfill as closely as possible conditions in the eastern alps indentation produces both thickening in front of the indenter and escape of triangular wedges lateral variations in crustal thickness become attenuated by gravitational spreading the overall fault pattern includes domains of reverse strike slip oblique normal and pure normal faults strike slip faults in conjugate sets develop serially the narrow width of the deformable area and the strength of the foreland determine the angles between the sets gravitational spreading produces a rhombohedral pattern of oblique and pure normal faults along the unconstrained margin opposite the unconstrained margin the indenter front shows thrusts and folds intersecting with the conjugate strike slip sets a triangular indenter favors spreading high velocity of indentation favors escape high confinement limits lateral motion inhibits spreading and favors thickening lateral extrusion in the eastern alps is best modeled by 1 a weak lateral confinement 2 a broad and straight indenter 3 a narrow width of the deformable area and 4 a rigid foreland crustal thickening lateral escape and gravitational spreading all contribute to the overall deformation

synchronous and velocity partitioned thrusting and thrust polarity reversal in experimentally produced doubly vergent thrust wedges implications for natural orogens a sandbox analog modeling research program was used to study the detailed evolution of doubly vergent thrust wedges high resolution multilayers of thin alternating sand and mica layers were used in contractional coulomb wedge experiments to simulate deformation of anisotropic brittle upper crustal strata in doubly vergent orogens experiments incorporating syntectonic sedimentation in foreland basins and in piggyback basins were also carried out our laboratory models evolved in two main stages 1 initial high velocity thrusting in the retrowedge and high frequency together with low displacement folding and thrusting in the prowedge and 2 low frequency high displacement synchronous thrusting in the prowedge and low velocity thrusting in the retrowedge transition from stage i to stage ii occurred when the growing wedges reached the critical height at which they behaved as a backstop for further prowedge accretion addition of syntectonic sediments increased the persistence of stage i and triggered out of sequence thrusting in the axial zone of the experimental orogens thrust motion was stick slip retrovergent thrusting occurred along a long lived ramp whose lower tip was located at the subduction slot provergent kink bands nucleated at the subduction slot in stage i in contrast during the second stage of wedge evolution kink bands nucleated in a piggyback fashion in the foreland far from the subduction slot and then evolved into high displacement faults that remained active up until the end of the experiments at progressively decreasing rates of thrusting the axial zones of the model wedges were characterized by fast uplift rates during stage i due to backward translation of the belt along the retrovergent long lived ramp and due to the localization of deformation close to the subduction slot outward migration of the deformation front in the prowedge region during stage ii caused the progressive decrease in the rate of the wedge uplift until it eventually stopped analytical models to quantify the coulomb behavior in the wedges validated the reversal of thrust polarity with increasing shortening triggered by the buildup of the topographic load during deformation this thrust polarity reversal highlights problems with the classic concept of back thrusting as a reflection of a significant change in the deformation regime our results compare well with the kinematic evolution of natural accretionary prisms and of thrust and fold belts such as the lesser antilles arc system the mediterranean ridge and the pyrenees

computerized x ray tomography analysis of sandbox models examples of thin skinned thrust systems computerized x ray tomography applied to analog sandbox experiments performed in a normal gravity field makes possible the analysis of the kinematic evolution as well as the three dimensional geometry of models that simulate tectonic deformations most of the plastic or viscous analog materials generally used in a normal gravity field for such models have x ray attenuations compatible with medical scanner images models using soft cohesionless granular materials were studied faults in granular materials corresponding to dilatant zones with lower by 30 50 x ray attenuation are thus clearly visible in the tomography images kinematic analysis was done on imbricate thrust propagation in materials with an internal friction angle of 30 thrust propagation is controlled mainly by a triangular pop up structure resulting from the nucleation of two conjugate thrusts at the basal decollement the pop up structure is created immediately in front of the previous thrust three dimensional geometry analysis was tested on lateral ramps parallel to the transport direction lateral ramps were induced by changing the frictional boundary conditions at the bottom of the model though the lateral ramps strike almost parallel to the transport direction their dip angle is comparable to the dip angle of frontal ramps

comparison of strong ground motion recordings of the lushan china earthquake of 20 april 2013 with the next generation attenuation nga west2 ground motion models abstract a total of 39 free field accelerograms in the 2013 m w 6 7 lushan earthquake with the closest site to rupture distances r rup being less than 200 km are compared with the median ground motions predicted by the recent next generation attenuation nga west2 ground motion prediction equations gmpes overall for most lushan station recordings over the whole range of r rup the spectral acceleration sa of the peak ground acceleration pga and of t 0 06 s are within one standard deviation of the predictions from the five nga west2 gmpes most intraevent residuals also fall between plus and minus one standard deviation from the predictions moreover the standard deviation of intraevent residuals are smaller for sa of t 0 06 s than for pga which suggests that the application of nga west2 in predicting high frequency ground motion due to the lushan earthquake is acceptable the spatial contours of intraevent residuals for pga predicted by the five nga west2 models are almost the same all showing four regions of negative intraevent residuals three of these regions are mainly distributed with the source to site azimuth approximating to 90 or 90 and the other one approximating to 180 which are consistent with the extrusion and rupture directions of the lushan earthquake fault in general the five nga west2 gmpes generate relatively large intraevent residuals at the long period t 1 0 s for r rup exceeding 100 km although the intraevent residuals are not as large the short period hanging wall effects are more consistent with the nga west2 hanging wall scaling than those of long period and the short period ground motions on the footwall are more consistent with the nga west2 models than those on the hanging wall

analogue models of orogenic wedges controlled by erosion abstract indentation in the upper brittle crust where one plate is stiffer than the other produces vertical extrusion of a doubly vergent orogenic wedge sandbox models of this process show that erosion with or without deposition of the eroded material onto one or both margins significantly changes the internal patterns of orogenic shear and compaction within the orogens erosion decreases the vertical stress and changes the criticality of the orogenic wedge whereas redeposition increases the vertical stress on its margins effective indenters of accreted sand which develop in models without erosion if the rigid indenter face dip is 75 e g 45 or 15 are strongly affected by erosion rapid erosion favors thrusting over compaction and decreases both the size and the relevance of the effective indenters redeposition of eroded material on the margins also expands the lifetime of the active shear as the additional load delays initiation of underlying new shears

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

test of goodness of the nga ground motion equations to predict the strong motions of the 2012 ahar varzaghan dual earthquakes in northwestern iran in the first part of this study a set of 87 ground motion records with closest distance to the rupture plane r rup less than 200 km and averaged shear wave velocity over the top 30 m of the subsurface v s 30 between 175 and 1400 m s recorded during the 2012 ahar varzaghan dual earthquakes m w1 6 4 m w2 6 3 were taken into account to examine the predictive capabilities of the next generation attenuation nga ground motion prediction equations gmpes via a set of comparative analyses and tests the first applied method to assess the performance of the nga gmpes is based on the intraevent residual analysis the primary database i e 87 records was also used to develop an event specific gmpe in the case of the ahar varzaghan dual earthquakes by means of regression analyses the derived event specific gmpe has been compared with the nga gmpes for two different site conditions that is v s 30 375 m s rock site and v s 30 375 m s soil site the residual analysis results indicate that the nga gmpes perform better in predicting data recorded at rock sites compared to soil sites for soil sites and at large periods t 2 0 s the observed spectral accelerations are overpredicted by the nga gmpes furthermore as the second part of this study to select the most adequate gmpes 14 strong motion records from the 1997 ardebil earthquake m w 6 1 were added to the primary database the implementation of the likelihood lh and log likelihood methods as modern lh based ranking assessment techniques as well as the nash sutcliffe index reveal that the nga gmpes show good compatibility at short medium periods t 1 0 s on the basis of the data recorded during the 2012 ahar varzaghan dual and 1997 ardebil earthquakes i e 101 records however in the long period range the dispersion in the data does not allow the authors to draw a comprehensible conclusion

high frequency ground motion and source characteristics of the 2008 wenchuan and 2013 lushan china earthquakes the 2008 mw7 9 wenchuan and the 2013 mw6 6 lushan earthquakes which both occurred on the longmen shan thrust belt show some interesting similarities and differences whereas the wenchuan earthquake entailed a rupture zone that extended about 300 km northeastward with fault slip extending to the surface the lushan earthquake was the result of a buried and much more compact zone of rupture the high frequency ground motions however for these two earthquakes as measured by the peak ground acceleration were evidently influenced by neither the extent of rupture nor the presence or absence of surface rupture the source parameters for these two earthquakes tend to confirm the idea that high frequency ground motion is controlled by stress changes in the rupture zone that give rise to the radiated ground acceleration the apparent stresses for the wenchuan and lushan earthquakes are about 0 5 and 0 75 mpa respectively and the stress drops in the same order are about 2 5 and 3 5 mpa the ratios of average stress drop to apparent stress are in the range 4 5 5 for both events consistent with expectations based on the brune j geophys res 75 26 4997 5009 1970 source model

sedimentary basins and crustal thickening abstract we consider the development of sedimentary basins in a tectonic context dominated by horizontal shortening and vertical thickening of the crust well known examples are foreland basins others are ramp basins and buckle basins we have reproduced various styles of compressional basins in experiments properly scaled for gravity a multilayered model lithosphere with brittle and ductile layers floats on a model asthenosphere a computer driven piston provides shortening and thickening synchronous with erosion and sedimentation after a first stage of lithospheric buckling thrust faults appear mainly at inflection points slip on an isolated reverse fault is accompanied by flexure footwall flexure results in a foreland basin and becomes accentuated by sedimentation hangingwall flexure is less marked but may become accentuated by erosion motion on a fault leads to hangingwall collapse at the surface either footwall sedimentation or hangingwall erosion tends to prolong the active life of a reverse fault slip on any pair of closely spaced reverse faults of opposite vergence results in a ramp basin simultaneous slip produces a symmetric ramp basin whereas alternating slip results in a butterfly shaped basin with superposed foredeeps some well developed ramp basins become pushed down until bounding faults meet at the surface and the basin disappears from view at this stage the basin depth is equivalent to 15 km or more slip on any pair of widely spaced reverse faults of opposite vergence results in a pronounced central anticline between two distinct foredeeps in central asia and in western europe cenozoic crustal thickening is due to continental collision for central asia western china kyrgyzstan uzbekistan tajikistan we have compiled a regional structure contour map on the base of the tertiary as well as 4 regional sections foreland basins and ramp basins are numerous and associated with cenozoic thrusts large basins tarim junggar fergana tajik occur around and between mountain ranges but smaller basins issyk kul naryn occur within them in western europe the alps and pyrenees are surrounded by foreland basins ramp basins or intermediate styles in the andes and its foreland neogene thrusts and compressional basins are due to subduction of oceanic lithosphere in colombia they account for much of the cordillera oriental in nw argentina for the altiplano in west central argentina for the sierras pampeanas compressional basins are also common in other areas of older crustal thickening

three dimensional geometry and kinematics of experimental piggyback thrusting the three dimensional geometry and kinematics of piggyback stacks of imbricate thrust sheets are illustrated and discussed using a single model shortened in a squeeze box strike parallel geometric elements simulated include lateral ramps eyed sheath folds splays and thrust thrust interference fine details of these structures were exposed by eroding a shortened wedge of sand using a newly developed vacuum eroding technique a kinematic analysis of the model shows a stepwise increase in imbricate thrust spacing and or a decrease in rate of nucleation of imbricate thrusts in the direction of thrust transport despite the steady forward advance of a rear wall the piggyback wedge accreted episodically recording different strain domains in longitudinal cross sections strain partitioning in single layers by bed length balancing showed an increase in layer shortening with volume loss and a corresponding decrease in imbricate thrusting and ramp folding with depth

modelling the geometry of coulomb thrust wedges internal geometry and detachment of coulomb thrust wedges is examined in a combined gravity and pressure force field under various boundary conditions and coulomb parameters frontally buttressed sand wedges accreted with narrow tapers above soles with negligible friction in these models serial imbricates stepped up in conjugate sets which operated synchronously or serially with increases in sole friction model coulomb wedges accreted in piggyback style with enhanced taper but narrower spacing of imbricates in this mode flat topped box fold anticlines nucleated at the frontal tip of a sole thrust prior to the fore limb failing as a ramp the initial ramp thrusts subsequently rotated and steepened to acquire concave upwards listric geometry when cohesion was a significant component of strength and the sand grains interlocked thoroughly ramp climb during piggyback thrusting occurred through the generation of a stack of backthrusts the taper of cohesionless sand wedges which lack any length scale was consistent with the critical cohesionless coulomb wedge taper in contrast the taper of experimental cohesive sand wedges which are scale dependent was consistent with the theoretical cohesive coulomb narrow taper only when the cohesive length scale was lr sr rar 10 6 this was constrained by comparing geometry of cohesive sand wedges shortened at different body force per unit mass

the contribution of postcritical moho reflections sms to ground motions of the 2008 mw 7 9 wenchuan earthquakethe contribution of postcritical moho reflections sms to ground motions

the obliquely convergent plate boundary in the south island of new zealand implications for ancient collision zones abstract the alpine fault of new zealand forms the western boundary of a zone of distributed deformation formed by the oblique convergence of continental crust belonging to the pacific and australian plates structural and geodetic data from the alpine fault show that a large proportion of the total plate displacement is accommodated by rapid oblique slip on the fault the remaining displacement is distributed over a 200 km wide zone to the east the collision may be modelled as a two sided deforming orogen with the partitioning of deformation being controlled by erosional differences between the narrow high strain inboard and broad low strain outboard sides in ancient collision zones little evidence may remain of the nature and amount of displacement on the inboard side partitioning of deformation among pre existing structures complicates interpretation of the outboard zone radiometric ages may post date collision by several tens of millions of years and indicate slow isostatic uplift and unroofing collision ages if preserved may be recognized by high uplift rates calculated from muscovite biotite pairs

seasonal variations in the south atlantic meridional overturning circulation from observations and numerical models monthly climatologies of temperature and salinity from observations and numerical models are used to estimate the atlantic meridional overturning circulation amoc at 34 s observational estimates suggest that the geostrophic transport plays an equal role to the ekman transport in the amoc seasonal variations at this latitude whereas in the models the ekman transport controls the amoc seasonality the seasonality of the geostrophic transport from observations is largely controlled by the seasonal density variations at the western boundary but in the models the eastern boundary dominates the observed density seasonality at the western boundary is linked to the intensity of the malvinas current which is poorly reproduced in the models the results indicate that the weak seasonal cycle in the model geostrophic transport can primarily be attributed to excessively strong baroclinicity below the surface mixed layer whereas the observations show a strong vertical coherence in the velocity down to 1200 m

evaluation of nga west2 and chinese ground motion prediction equations for developing seismic hazard maps of mainland chinaevaluation of nga west2 and chinese gmpes for developing seismic hazard maps of mainland china

intraplate lithosphere deformation and the strength of the lithosphere summary the intraplate deformation of continental lithosphere in response to applied stress has been investigated using a mathematical model which incorporates the elastic ductile and brittle response of lithosphere material ductile deformation is assumed to be controlled in the crust by dislocation creep in quartz and in the mantle by dislocation creep and plasticity in olivine brittle failure is predicted using modified griffith theory a fundamental feature of the model is the redistribution of stress within the lithosphere following stress release by both ductile and brittle deformation this redistribution produces high levels of stress in the middle or lower crust immediately above the elastic ductile or brittle ductile transition lithosphere deformation is shown to be critically dependent on the temperature structure of which surface heat flow is a convenient indicator for higher geothermal gradients the release of stress in the lower lithosphere by ductile deformation is more rapid and complete and results in large stress levels in the upper lithosphere for sufficiently large applied stresses or steep geothermal gradients the stress levels in the upper and middle crust will cause complete fracture of the upper lithosphere whole lithosphere failure wlf then results by continued brittle and ductile deformation causing geologically significant strains the critical value of applied stress required to give wlf has been calculated for both tensional and compressional deformation as a function of surface heat flow the predicted lithosphere bulk strength is then compared with expected levels of intraplate stress arising from plate boundary forces and isostatically compensated loads which are thought to give net stress levels in the continental lithosphere in the range 0 25 to 0 25 kb using these expected maximum stress levels the model predicts significant extensional deformation in regions of moderate heat flow with q c 60mwm e g central europe as well as for areas of high heat flow like the basinand range province significant compressional deformation is predicted for areas of high heat flow with q c 75 mwm 2 but only for restricted conditions of stress combination these results are in good agreement with

mechanical model for the tectonics of doubly vergent compressional orogens a mechanical model of crustal shortening and deformation driven by the relative convergence of rigid underlying mantle plates explains many features of convergent orogens results based on numerical models and supported by sandbox models show that a coulomb crustal layer subject to basal velocity boundary conditions corresponding to asymmetric detachment and subduction of the underlying mantle passes through three stages of orogenic growth 1 block uplift bounded by step up shear zones 2 development of a low taper wedge over the underthrusting mantle plate and 3 development of a low taperwedge overlying the overthrusting mantle plate and verging in the opposite direction when modified by isostasy basal viscous flow surface erosion and denudation and sedimentation the resultant model orogens exhibit a variety of styles with characteristics in common with small rapidly denuded orogens large orogens with plateaus and extensional characteristics and active subduction margins with doubly vergent accretionary wedges and deformed fore arc basins

lithospheric scale structures from the perspective of analogue continental collision abstract analogue models were employed to investigate continental collision addressing the roles of 1 a suture zone separating different crustal blocks 2 mid crustal weak layers and 3 mantle strengths these models confirmed that low amplitude lithospheric and crustal buckling is the primary response to shortening with a wavelength mainly controlled by the strong upper crust and or lithospheric mantle with increasing shortening bivergent thrusts formed across the brittle layers equivalent to upper crust at inflection points of the buckles while ductile layers lower crust and mantle continued being folded the resulting geometries displayed pop ups and pop downs above the boundaries sutures between blocks further shortening led to wider thrust bounded deformation zones belts in front of the stronger blocks identified as effective indenters during shortening upward extrusion into belts allowed exhumation of deep material weak layers enhanced strain localisation these modelling results have similarity in large scale structures of modern orogens and are relevant to the understanding of natural collisional processes

effect of overburden thickness on thrust belt geometry and development laboratory sand models simulating development of thrust belts indicate that 1 distance between the backstop and the first formed thrust is a linear function of initial sand layer thickness 2 spacing between the first two thrusts is most likely a nonlinear function of initial sand layer thickness 3 initial dip of successive ramps decreases progressively toward the foreland because of variations in principal stress directions with depth and or changes of mechanical properties of sand with increasing overburden 4 faults to the hinterland steepen and become less active as hinterland thrust sheets undergo penetrative shear during sand wedge growth and 5 new ramp initiation occurs to the hinterland of the basal detachment tip line these observations can be interpreted both in context of area balance considerations and soil mechanics theory if initial sand layer thicknesses in our models represent syntectonic overburden in natural thrust belts i e distance between the ground surface and the detachment then our results suggest that locations of some thrust belt salients are controlled by along strike variations in stratigraphic thicknesses and that initial thrust spacing decreases progressively to the foreland if the thrust belt involves a foreland tapering basin furthermore our sand model observations suggest that the concept of a fixed rigid backstop in the hinterland of thrust belts may be misleading as a consequence of thickening the hinterland portion of the thrust belt becomes stabilized and effectively becomes the backstop itself

episodic accretion and strain partitioning in a model sand wedge we model led thrust wedge accretion and deformation partitioning in a passively layered sand detached and shortened above a smooth rigid decollement the thrust wedge accretes in piggyback style during forward advance of a rear wall the passively layered sand accommodates the shortening penetratively by kinking ramp folding and by imbrication in single layers the style of compression changes with time and or stratigraphic position within the sand prism initial penetrative layer shortening above a slipped zone of decollement is succeeded by formation of monoclinal or conjugate shear bands at temporary terminations of decollement the shear bands subsequently narrow down and lock as thrust ramps the model sand wedge accrets episodically rather than steadily episodicity is controlled by the stick slip mode of decollement propagation and by volume loss and compaction of the wedge material in response to convergence this results in formation of a wedge with a convex upwards cross sectional topography

measurements of friction coefficients and cohesion for faulting and fault reactivation in laboratory models using sand and sand mixtures abstract simple laboratory tests have been made on dry quartz sand and on mixtures of sand and clay and sand and cement tests were made on both low and high density specimens of each mixture as determined by physical handling techniques the tests included measurements of density estimates of the coefficient of internal friction based on fault orientation and estimates of the friction coefficients and cohesion values for fault initiation and reactivation in a shear frame apparatus which was also used to test artificial fault zones made from pure clay glass microspheres and wheat flour the results suggest that density is determined more by handling technique than by composition and that friction coefficients and perhaps cohesion both increase with increasing density the shear testing revealed a consistent drop in cohesion for fault reactivation in all densely packed mixtures tested although small this drop may be significant for some types of laboratory models

mechanical energy budget of a fold and thrust belt recent studies1 2 have shown that an active fold and thrust belt is mechanically analogous to the wedge of soil or snow that forms in front of a moving bulldozer such a wedge attains a critical taper in which the regional state of stress is everywhere on the verge of brittle failure here this critical taper model of the regional stress field is combined with a simple kinematic description of the steady state deformation of an active eroding fold and thrust belt to examine the mechanical energy balance of brittle frictional mountain building part of the work performed by the compressive stress at the rear of a fold and thrust belt is dissipated by frictional heating on the decollement fault part is dissipated by internal frictional processes that accompany the brittle deformation within the critically tapered wedge and part is expended against gravitational body forces the total power expenditure in the steady state fold and thrust belt of western taiwan is about five gigawatts of which 45 is being dissipated against basal friction 39 is being expended against gravity and only 16 is being dissipated internally

the strength of intraplate lithosphere abstract the deformation of intraplate lithosphere in response to laterally applied stress has been investigated using a mathematical model of the elastic brittle and viscous response of the lithosphere fundamental to the model is the conservation of the horizontal force associated with the applied stress and the redistribution of stress within the lithosphere after stress release by ductile or brittle failure ductile deformation is assumed to be controlled by dislocation creep in quartz in the crust and by dislocation and dorn law creep in olivine in the mantle brittle failure is predicted using griffith theory the redistribution of stress after brittle and ductile deformation results in large levels of stress in the middle and lower crust immediately above the elastic ductile or brittle ductile transition lithosphere deformation is controlled critically by the lithosphere thermal structure of which surface heat flow q is taken as a convenient indicator for hotter lithosphere stress release by ductile deformation in the middle and lower lithosphere is more extensive and rapidly leads to larger levels of stress in the upper lithosphere for sufficiently large levels of applied stress or sufficiently hot lithosphere complete failure whole lithosphere failure of the lithosphere occurs by ductile and brittle deformation resulting in geologically significant strains critical levels of applied stress required to produce whole lithosphere failure have been computed as a function of the lithosphere surface heat flow the predicted strength of the lithosphere has been compared with expected levels of intraplate stress arising from plate boundary forces and isostatically compensated loads the model predicts significant extensional intraplate deformation in regions of moderate heat flow with q 60 m w m 2 for compressional deformation however a hotter lithosphere is required with q c 75 m w m 2 the model predictions are in general agreement with the comparatively widespread occurrence of extensional intraplate deformation and the restricted occurrence of compressional deformation the model has also been used to investigate the relationship between the depth of the brittle ductile transition and lithosphere temperature structure the brittle ductile transition depth is shown to become shallower with increasing heat flow in good agreement with seismic evidence

two sided orogen collision and erosion from the sandbox to the southern alps new zealand in continental collision zones the indentor is of approximately the same vertical dimension as the material being deformed thereby allowing material to move over the indentor and out of the deforming orogen this simple constraint on the height of the indentor differs from that imposed by the bulldozer model of wedge deformation and results in the formation of an orogen with two opposite dipping topographic slopes these two wedges are mechanically coupled and the mechanics are strongly influenced by the erosion conditions imposed on the wedge where the growing mountains perturb prevailing moisture laden winds the erosion pattern is highly asymmetric erosional work being concentrated at the toe of the steep wedge facing the indentor the inboard wedge concentration of erosional energy results in concentration of mechanical energy in the same region and produces the distinctive geological and geophysical patterns associated with continental collision these include rapid uplift and exposure of deep crustal assemblages adjacent to the indentor accompanied by high crustal heat flow and anomalously low seismicity the outcrop pattern along the inboard wedge evolves from that of a crustal cross section currently exposed in the southern alps to one dominated by repetitive nappe structures consisting predominantly of lower crustal units as in the western alps of europe the other wedge the outboard wedge undergoes relatively little erosion is broad with a more gentle surface slope and maintains an outcrop pattern consisting of upper crustal material

erosional control on the structural evolution of a transpressional thrust complex on the alpine fault new zealand abstract the waikukupa thrust is a 4 km long oblique thrust segment of the new zealand alpine fault that has developed over the last 65 000 years emplacing mylonite and cataclasite over fluvioglacial gravel during the last 20 000 years or so a 2 km long out of sequence imbricate the hare mare thrust has formed on the east side of the waikukupa river valley the average rate of slip on the two thrusts is estimated at 22 30 mm year analysis of the fault structure in terms of critical wedge theory is consistent with imbrication resulting from a reduction in wedge taper below the critical value due to rapid river erosion the internal structure of the waikukupa thrust sheet consists of reverse faults and angular folds forming duplex like systems associated with minor strike slip faults within the limits of uncertainty the structures are consistent with strain accumulating during internal transpressional shear of the thrust sheet while the taper of the latter was subcritical prior to the transfer of fault displacement to the hare mare imbricate the thrust complex forms part of a serrated range front fault system consisting of oblique thrust sections linked by strike slip faults we introduce the term serial partitioning for this type of partitioned transpressional fault system its development is linked to the erosional processes at the range front an alternative system consisting of parallel thrust and strike slip faults we here term parallel partitioning while angle of obliquity is an important parameter in determining which partitioning model develops we suggest that erosion rate is also an influential factor

fault structural control on earthquake strong ground motions the 2008 wenchuan earthquake as an example continental thrust faulting earthquakes pose severe threats to megacities across the world recent events show the possible control of fault structures on strong ground motions the seismogenic structure of the 2008 wenchuan earthquake is associated with high angle listric reverse fault zones its peak ground accelerations pgas show a prominent feature of fault zone amplification the values within the 30 to 40 km wide fault zone block are significantly larger than those on both the hanging wall and the footwall the pga values attenuate asymmetrically they decay much more rapidly in the footwall than in the hanging wall the hanging wall effects can be seen on both the vertical and horizontal components of the pgas with the former significantly more prominent than the latter all these characteristics can be adequately interpreted by upward extrusion of the high angle listric reverse fault zone block through comparison with a low angle planar thrust fault associated with the 1999 chi chi earthquake we conclude that different fault structures might have controlled different patterns of strong ground motion which should be taken into account in seismic design and construction

indenter growth in analogue models of alpine type deformation a series of analogue experiments were carried out to simulate continental convergence as seen in a profile through the central alps a rigid indenter representing the adriatic plate was driven laterally into a sand pack representing the brittle upper crust of europe detached and thickening above its subducting ductile lower crust the rigid indenter advanced at the same steady rate in each experiment but the dip of its front face was steepened in 15 increments from 15 to 90 where the rigid indenter face dipped at 45 or less a sand wedge rose and was bound by a series of forekinks that nucleated at the toe of the indenter where the face of the rigid indenter dipped 60 or more the wedge was defined by a single forekink and one or more backkinks that nucleated from a point advancing in front of the indenter toe we interpret these results as indicating that slices of the sand pack and rising wedge are transferred across kink bands to build an effective indenter with a frontal dip closer to that dictated by the changing shear strength of the sand pile which thickens vertically as it shortens laterally one of our models with a rigid indenter dipping 75 simulates most of the major structures shown in recent syntheses of surface geology and deep seismic data in the central alps without the isostatic lithospheric depression this model accounts for the late collisional stage oligocene to present complex strain and metamorphic histories in the core of the orogenic wedge the rapid rise and extrusion of small pips of alpine eclogites and the current passivity of the insubric line it also emphasizes that lateral extension along gently dipping thrusts orogen normal horizontal escape is confined to the extruded portion of the rising wedge