piggyback basin development above a thin skinned thrust belt with two detachment levels as a function of interactions between tectonic and superficial mass transfer the case of the subandean zone bolivia abstract the subandean fold and thrust belt of bolivia is characterised by two major detachment levels and large piggyback basins sand box and numerical models have been used to study sedimentation and erosion control on thrust belt evolution and to study the retroactive effects of tectonics on piggyback development in thin skinned thrust belts with two detachment levels analogue models show that surface processes play a dominant role in controlling wedge evolutions erosion promotes fault reactivation and tectonic delamination passive roof duplex while sedimentation promotes forward shifting of the frontal thrust and consequently piggyback basin development numerical models were used to understand the development of the subandean fold and thrust belt of bolivia numerical experiments show that the simultaneity of basement tilting and high sedimentation rates promotes the formation of a stable tectonic wedge outer and inner faults are alternately active during the beginning of deformation a kinematic evolution that favours the development of piggyback basins between the step by step history of the thrust belt predicts that each change in tectonic location is recorded with large unconformities in basins but these unconformities are not well preserved from progressive erosion in the final geometry

brittle frictional mountain building 2 thermal structure and heat budget this paper describes a simple thermal model of an actively deforming critically tapered fold and thrust belt the model determines the steady state temperature distribution and heat flow as well as the pressure temperature time histories of rocks that outcrop at the surface the main parameters controlling the thermal structure are the accretion and erosion rates the undisturbed geothermal gradient at the toe and the amount of frictional heating both shear heating on the decollement fault and internal strain heating within the deforming brittle wedge are incorporated in a mechanically consistent manner and they dominate the effect of radiogenic heating except in fold and thrust belts with significantly overpressured pore fluids the mean stresses temperatures and surface heat flow all increase with an increase in the basal and internal coefficients of friction and this dependence is used to constrain the level of friction on the decollement fault beneath the steady state fold and thrust belt in taiwan rocks outcropping in the core of the central mountain range of taiwan experience maximum theoretical temperatures in excess of 400 c and maximum mean pressures in excess of 500 mpa if the coefficient of basal friction is b 0 5 qualitatively these conditions are in good agreement with the observed high greenschist facies metamorphism the theoretical surface heat flow which increases from 95 mw m2 at the front of the fold and thrust belt to 240 mw m2 at the rear is in excellent agreement with the results of a recent geothermal survey of taiwan and theoretical cooling histories are in good agreement with fission track and other geochronologic studies taken together these results provide strong evidence that sliding on the basal decollement fault beneath taiwan is governed by a coefficient of friction in the range of typical laboratory measurements b 0 5 0 2 approximately 35 of the total surface heat flux of 3 gw is heat conducted into the base of the wedge from the top of the basal decollement fault and somewhat more than 30 is heat advected into the toe by accretion the remaining heat is generated internally about 25 by internal strain heating and about 10 by radiogenic heating either an increase in the coefficient of basal friction b or a reduction in the pore fluid pressure ratio b leads to an increase in the surface heat flow because of the increased frictional heating within the wedge and on the basal decollement fault the overall balance of energy in a steady state fold and thrust belt is described by the equation e w g q where is the rate at which both mechanical and heat energy are added from external sources w g is the rate at which work is performed against gravitational body forces in a reference frame attached to the overriding plate and q is the rate at which waste heat flows out of both the upper and lower boundaries the total power input into the taiwan fold and thrust belt is approximately 4 2 gw the mechanical work being done on the base and front of the fold and thrust belt accounts for 3 of these 4 2 gw in addition 0 9 gw of heat are being advected from the subducting plate into the toe by accretion the remaining 0 3 gw are being supplied by in situ radiogenic heating the outgoing energy is dominated by the 3 gw of heat conducted out the top in the surface heat flow however another 0 8 gw are conducted down beneath the rear portion of the basal decollement fault to heat the underlying subducting slab only 0 4 of the incoming 4 2 gw do useful mechanical work against gravity within the wedge the efficiency of brittle frictional mountain building in taiwan is therefore 10

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

rates of shortening propagation underthrusting and flexural wave migration in continental orogenic systems the rate of horizontal shortening in an orogenic wedge is the rate at which the length of undeformed crust decreases as it is incorporated into the orogen this rate equals the rate of convergence of the foreland lithosphere toward the central surface of the orogenic belt and the rate of subduction of foreland lithosphere beneath the central surface the rate of propagation of an orogenic wedge is the rate at which it elongates in the direction of horizontal shortening this rate is controlled by the rates of mass accretion to the orogenic wedge and erosion the orogenic belt drives a flexural isostatic wave through the foreland lithosphere at a velocity equal to the rate of propagation plus the rate of subduction or convergence or shortening in orogenic belts where the total amount of shortening cannot be reliably estimated from balanced regional cross sections it may be possible to determine total shortening from the distance of flexural wave migration in the foreland basin and the width of the orogenic wedge in addition orogenic wedges may accelerate solely in response to a reduction in taper

crustal and upper mantle responses to lithospheric segmentation in the northern apennines lithospheric tear faults are expected to develop in response to along strike variations in the rates of slab rollback however the exact geometry of such structures and their crustal and upper mantle expressions are still debated we present an analysis of seismic structural and morphological features that possibly represent the expression of lithospheric segmentation in the northern apennines geophysical observations show evidence for the existence of a discontinuity in the lithospheric structure beneath the northern apennines characterized by a change in the spatial distribution of intermediate depth seismicity along strike variations in the pattern of crustal seismicity and a bend in the moho topography the near surface expression of this discontinuity is associated with an abrupt change in the morphology and exhumation history of the northern apennines in the proximity of the livorno sillaro lineament we interpret these features as evidence for incipient tearing of the lithospheric slab beneath the northern apennines marking the boundary between domains that underwent contrasting styles of lithospheric deformation which are either associated with different rates of slab rollback or a transition from underplating to retreat we suggest that similar types of structures may play a crucial role in the evolution of convergent plate boundaries allowing segmentation of orogenic belts and facilitating the development of orogenic curvatures ultimately further tearing along such structures could potentially lead to the occurrence of tear related magmatism and the formation of slab windows

controls on the strength of coupling among climate erosion and deformation in two sided frictional orogenic wedges at steady state 1 many important insights regarding the coupling among climate erosion and tectonics have come from numerical simulations using coupled tectonic and surface process models however analyses to date have left the strength of the coupling between climate and tectonics uncertain and many questions unanswered we present an approximate analytical solution for two sided orogenic wedges obeying a frictional rheology and in a condition of flux steady state that makes explicit the nature and sensitivity of the coupling between climate and deformation we make the simplifying assumption that the wedge grows in a self similar fashion consistent with airy isostasy such that topographic taper is invariant with orogen width tectonic influx rate and climate we illustrate first how and why the form of the erosion rule matters to orogen evolution and then derive a physically based orogen scale erosion rule we show that steady state orogen width crest elevation and crustal thickness are controlled by the ratio of accretionary flux to erosional efficiency to a power dictated by the erosion process remarkably we show that for most combinations of parameters in the erosion law rock uplift rate is more strongly controlled by erosional efficiency than it is by the accretionary flux further assuming frontal accretion with no underplating the spatial distribution of erosional efficiency dictates the relative rock uplift rates on the pro wedge and retro wedge and the time averaged trajectories of rocks through the orogen the restriction to invariant frictional properties is conservative in these respects systems subject to positive feedback between erosion and rheology will exhibit even stronger coupling among climate erosion and deformation than shown here

mechanics of fold and thrust belts and accretionary wedges cohesive coulomb theory a critically tapered fold and thrust belt or submarine accretionary wedge is one that is on the verge of coulomb failure everywhere including its base where frictional sliding along a decollement is assumed to be occurring cohesion within a wedge can add significantly to the overall strength near the toe the effect of this is to decrease the near toe taper leading to a critical topographic profile that is concave upward if the decollement is planar we obtain an approximate self consistent solution for the state of stress within a thin skinned cohesive critical coulomb wedge and determine the relationship between the wedge taper and its strength and basal friction the theory is then applied to the presently deforming fold and thrust belt of western taiwan fitting of theoretical critical wedge shapes to topographic profiles and measurements of the step up angles of thrust faults from the basal decollement are used to constrain the taiwan wedge strength parameters an attractive assertion fully consistent with all the observations is that the mechanics of fold and thrust belts and accretionary wedges is governed by normal frictional and fracture strengths of rocks measured in the laboratory in particular if byerlee s law b 0 85 is adopted as the coefficient of sliding friction on the base we find a coefficient of internal friction 0 9 1 0 in the wedge and a wedge cohesion so 5 20 mpa other solutions having strengths and ambient stresses up to 4 times lower than this can also however satisfy the data

a review of brittle compressional wedge models abstract fold and thrust belts and accretionary wedges form by compression of sedimentary sequences and basement rocks into forward and backward thrusts folds nappes and duplexes for over a century models have been used to investigate the essential characteristics of such brittle wedges here i review model studies of brittle thrust wedges in orthogonal compression focussing on critical taper theory analogue and numerical techniques for modelling brittle behaviour and the most commonly investigated variations in wedge model studies those in basal dip basal strength internal strength and surface processes many model results can be placed in the context of critical taper theory which provides analytical solutions for wedge taper angle and slip line orientations for a homogeneous material on the verge of failure throughout dynamic forward models have confirmed critical taper predictions of decreasing surface dip for increasing basal dip wider wedges with steeper forward thrusts and shallower backward thrusts for decreasing basal strength narrower wedges and enhanced exhumation for surface erosion and decreasing surface dip for increasing internal strength but analogue and numerical models have been able to take these results a step further by investigating the evolution of non critical thrust wedges and the effects of non homogeneous materials with decollement layers and strain weakening shear zones these results have highlighted the strong impact of heterogeneous materials on the internal structures of thrust wedges and raise the question whether the complexity of structures that are observed in many natural fold and thrust belts requires that lithological layering inherited faults or a wide range of rheologies need to be included in forward models of brittle thrust wedges

epicenter distribution and magnitude of earthquakes in fold thrust belts insights from sandbox models scaled analogue models are used to illustratethe eect of basal friction and erosion on fault activity andhence on epicentre distribution and magnitude of earth quakes in the sedimentary cover of active fold thrust belts model results suggest that in fold thrust belts shortenedabove low friction ductile decollements rock salt or over pressured mudstone low to moderate magnitude earth quakes mw 5 3 5 6 distributed over a wide area occuralong several long lived thrust faults conversely in areasshortened above high friction decollements large magnitudeearthquakes mw 6 6 6 8 distributed over a narrowzone are likely to occur along few short lived thrust ramps calculated magnitude of earthquakes from models and theirdistribution are in agreement with recorded earthquake pat tern from the zagros mountain belt which is partially short ened above a ductile decollement of hormuz salt formation model results also showed that erosion reactivates older in active thrusts and promotes formation of out of sequencethrusts

how fast is the denudation of the taiwan mountain belt perspectives from in situ cosmogenic 10be quantifying denudation rates in a wide range of climatic and tectonic settings at various time and space scales is a critical step in calibrating and validating landscape evolution models focusing on taiwan we quantified centennial rates of denudation at the scale of the whole orogen using in situ 10be concentrations measured in stream sediments collected at the outlets of major rivers to assess denudation rates that are statistically significant we applied both the mean square weighted deviation approach and the bootstrap technique for the central segment of the belt where the collision is considered to be near mature the orogen scale pattern of denudation shows a two fold pattern 1 higher denudation values on the order of 4 5 mm yr characterize the eastern side of the belt i e retro wedge with a slight increase towards the south and 2 lower denudation values on the order of 1 3 mm yr on the western side of the belt pro wedge with a minimum value centered on the main recess of the deformation front to the north and to the south of the central segment the denudation rates converge towards lower values on the order of 2 3 mm yr at the scale of the mountain belt drainage basin metrics such as relief hypsometric index and slope values seem to explain the observed variance in the data population conversely to the first order average precipitation pattern suggesting a strong tectonic control on the regional pattern of denudation rates applied to the whole orogen such field based approach thus provides important input data to validate and calibrate the parameters to be supplied to landscape evolution models

formation and evolution of strike slip faults rifts and basins during the india asia collision an experimental approach the processes which have governed the formation and evolution of large tertiary strike slip faults during the penetration of india into eastern asia are investigated by means of plane strain indentation experiments on layered plasticine models the steady state deformation of plasticine obeys a power creep flow law e c t n the stress exponent n is between 6 and 9 at 25 c uniaxial plane strain tests on cubic specimens show that the growth of faults in layered plasticine results from strain softening a process observed for strain rates ranging from 3 5 10 5 to 3 6 10 3 s 1 fault or shear zones form after only 7 10 bulk strain subsequent deformation is controlled by the geometry of the fault pattern rather than the physical properties of the plasticine a series of nine plane strain indentation experiments shows the influence of boundary conditions as well as that of the internal structure of the plasticine model on the faulting sequence the ubiquity of strain softening in experimental deformation of a variety of rocks as well as the widespread occurrence of shear zones in nature suggest that long term deformation of the continental lithosphere may also be primarily influenced by the geometry of large faults which rapidly develop with increasing strain the deformation and faulting sequence observed in the plasticine indentation experiments may thus be compared to collision induced strikeslip faulting in asia particularly to total offsets and rates of movements on the faults the experiments simulate the evolution of the western ends of the strike slip faults which have probably been analogous to trench fault fault triple junctions the experiments also illustrate mechanisms for the formation of extensional basins such as the south china sea north china basin and andaman sea near active continental margins the basins which appear to absorb terminal offsets along major strike slip faults near such margins may result from mismatch between the sharply angular shape of the deformed continental edge and the more regularly curved trench along which the smoothly flexed oceanic lithosphere subducts the existence of distinct phases of strike slip extrusion corroborates the idea that the discontinuities in time which typify intracontinental tectonics and orogenic cycles may often result from strain localization and the ensuing discontinuous non steady state deformation of the continental lithosphere

the impact of analogue material properties on the geometry kinematics and dynamics of convergent sand wedges abstract simulation of geodynamic processes in sandbox experiments requires analogue materials with a deformation behaviour that reproduces the deformation mechanisms of typical crustal rocks we present data on the frictional strength of different sand types employing static and dynamic shear tests the sand types analysed are characterised by an elastic frictional plastic mechanical behaviour with a transient strain hardening and strain softening phase prior to transition to stable sliding this is in conflict with the standard assumption of an ideal cohesionless coulomb material with constant frictional properties the influence of the identified transient material properties on the kinematics growth mechanisms and internal deformation patterns of convergent sand wedges results in characteristic wedge segments which vary depending on material compaction between wedges with well defined segments i e frontal deformation zone frontal imbrication zone and internal accumulation zone with straight slopes and wedges with a continuous convex topographic profile for most materials only the frontal part of the wedge is critical during experimental runs taper and strength of the wedge segments can be shown to be controlled by the frictional properties of active faults wedge segmentation is controlled by a bulk strength increase toward the rear of the wedge due to fault rotation in mechanically less favourable orientations and plastic material hardening the limit between the frontal critical parts of a wedge and internal stable parts is largely controlled by a critical state of stress upon which either renewed failure or fault inactivation occurs on this basis we suggest that critical taper analysis of wedges must be restricted to specific kinematic segments comparison of the experimental results with the nankai accretionary wedge suggests that our interpretation also applies to natural convergent wedges moreover we provide constraints for the selection of adequate granular analogue materials to simulate typical crustal rocks in natural convergent wedges

model experimentation of the effect of gravity on tectonic processes summary it seems likely that many of the tectonic phenomena in the earth s crust are generated by processes within the mantle our view of such a large scale tectonic phenomenon as the evolution of orogenic belts consequently ought to reflect on the theorizing of the behaviour of the mantle the author finds it therefore appropriate to present in this symposium some results from experimental model studies related to the origin of geosynclines and orogens some models simulating convection currents in the mantle are also discussed

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

channel changes in badlands stripping of the vegetation and soil from a 13 hectare site in virginia underlain by coastal plain sediments created a rapidly evolving badland topography two types of channels developed 1 sand bed alluvial channels were graded to transport the bed material load supplied from slope erosion with available runoff but they also generally eroded their beds slowly and 2 steeper bedrock floored channels incised rapidly in bedrock channels the erosion rate was proportional to the 4 9ths power of drainage area and the 2 3rds power of gradient these exponents are consistent with a model in which the erosion rate is proportional to the bed shear during high flows due to rapid mass wasting and reduced runoff the alluvial channels became as much as 50 steeper during the winter than the summer with an attendant yearly cycle of winter aggradation and summer entrenchment the gradients their seasonal variability and their downstream hydraulic geometry were consistent with the predictions of total load transport formulas for sand beds and high loads the hydraulic geometry of alluvial channels in the virginia badlands were similar to that on the morrison formation in the western united states

orogeny and orography the effects of erosion on the structure of mountain belts a numerical model of the coupled processes of tectonic deformation and surface erosion in convergent orogens is developed to investigate the nature of the interaction between these processes crustal deformation is calculated by a two dimensional finite element model of deformation in response to subduction and accretion of continental crust erosion operates on the uplifted surface of this model through fluvial incision which is taken to be proportional to stream power the relative importance of the tectonic and erosion processes is given by a dimensionless erosion number relating convergence velocity rock erodibility and precipitation rate this number determines the time required for a system to reach steady state and the final topographic shape and size of a mountain belt fundamental characteristics of the model orogens include asymmetric topography with shallower slopes facing the subducting plate and an asymmetric pattern of exhumation with the deepest levels of exhumation opposite to subduction these characteristics are modified when the regional climate exhibits a dominant wind direction and orographically enhanced precipitation on one side of the mountain belt the two possible cases are dominant wind in the direction of motion of the subducting plate and dominant wind direction in the opposite direction of the subducting plate velocity models of the former case predict a broad zone of exhumation with maximum exhumation in the orogen interior models of the latter case predict a focused zone of exhumation at the margin of the orogen and at high erosion number a reversal in the topographic asymmetry natural examples of these two cases are presented the southern alps of new zealand exhibits the climate and exhumation asymmetry characteristic of wind in the direction opposite to motion of the subducting plate the asymmetry of topography suggests that erosion is not efficient enough to have reversed the topographic asymmetry the contrasting example of dominant wind in the direction of subduction motion is provided by the olympic mountains of washington state in this case exhumation of deep levels of the cascadia accretionary wedge shows a broad domal pattern consistent with the observed orographic precipitation

predicting orogenic wedge styles as a function of analogue erosion law and material softening 1 the evolution of a compressive frictional wedge on a weak frictional and planar decollement subjected to frontal accretion is predicted with a two step method called sequential limit analysis the first step consists in finding with the kinematic approach of limit analysis the length of the active decollement and the dips of the emerging ramp and of the conjugate shear plane composing the emerging thrust fold the second step leads to a modification of the geometry first because of the thrust fold development due to compression and second because of erosion erosion consists in removing periodically any material above a fictitious line at a selected slope as done in analogue experiments this application of sequential limit analysis generalizes the critical coulomb wedge theory since it follows the internal deformation development with constant frictional properties the deformation is mostly diffuse a succession of thrust folds being activated so that the topographic slope reaches exactly the theoretical critical value frictional weakening on the ramps results in a deformation style composed of thrust sheets and horses applying an erosion slope at the critical topographic value leads to exhumation in the frontal central or rear region of the wedge depending on the erosion period and the weakening erosion at slopes slightly above or below the critical value results in exhumation toward the foreland or the hinterland respectively regardless of the erosion period exhumation is associated with duplexes imbricate fans antiformal stacks and major backthrusting comparisons with sandbox experiments confirm that the thickness dips vergence and exhumation of thrust sheets can be reproduced with friction and erosion parameters within realistic ranges of values

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

on steady states in mountain belts the dynamic system of tectonics and erosion contains important feedback mechanisms such that orogenic systems tend toward a steady state this concept is often invoked but the nature of the steady state is commonly not specified we identify four types of steady state that characterize the orogenic system and illustrate these cases by using numerical model results and natural examples these types are 1 flux steady state 2 topographic steady state 3 thermal steady state and 4 exhumational steady state they refer to the erosional flux the topography the subsurface temperature field and the spatial pattern of cooling ages respectively models suggest that the topography will reach a steady mean form at the scale of an orogenic belt but perfect topographic steady state is unlikely to be achieved at shorter length scales thermal steady state is a precondition for exhumational steady state and in the case of temperature dependent deformation topographic steady state exhumational steady state is characterized by reset age zones spatially nested according to closure temperature as illustrated in natural systems from new zealand the cascadia accretionary margin and taiwan

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

structural and tectono stratigraphic review of the sicilian orogen and new insights from analogue modeling abstract the apennines sicilian maghrebian fold and thrust belt originated from the subduction of the alpine tethys and the later collision of drifted continental blocks against the african and apulian paleomargins from north to south the sicilian fold and thrust belt sftb is divided in four main tectono stratigraphic domains 1 the calabro peloritani terrane drifted from the european margin 2 the remnants of the alpine tethys accretionary wedge atw related to the subduction of the tethys 3 the folded and thrusted platform panormide and deep water imerese sicanian series of the offscrapped african margin and 4 the african foreland hyblean unfortunately scarce quality seismic lines and outcrops of key tectono stratigraphic units make the structure and dynamic evolution of the central eastern part of the sftb controversial first this study outlines through a review of the tectono stratigraphic evolution of the central eastern sector of the sicilian orogen the major remaining issues concerning 1 the occurrence of inferred alpine tethys units far from the region where the remnants of the atw outcrop nebrodi mountains both in a forearc position above the peloritani block to the north and in an active foreland context along the present day southern front of the belt and 2 the diverging tectonic styles from stacked large scale tectonic nappes to foreland imbricated thrust systems rooted into a main basal decollement secondly new constraints are given using analogue modeling to test mechanically the hypothesized structural and tectono stratigraphic evolution of the sftb the experiment simulates the orogenic evolution of the sftb at crustal scale from the oligocene tethys subduction to the middle miocene late pliocene continental collision between the european and african paleomargins the tectono stratigraphic synthesis is used to model the first order mechanical stratigraphy of the sedimentary units involved in the sicilian belt as well as the imprint of the african margin structural inheritance the experiments succeed in reproducing the general structure and tectono stratigraphic evolution of the sftb in particular the models support field observations hypothesing a gravity driven origin of the inferred alpine tethys units intercalated within the forearc and foreland syntectonic sedimentation moreover the model testifies of the main tectonic steps that led to the sftb building first a low tapered accretionary wedge was accreted above the alpine tethys oceanic crust from the oligocene to the early miocene the following underthrusting of the stretched african continental margin and its frontal panormide platform shortened and thickened the accretionary wedge this phase provided favorable conditions for significant pulses of reworked alpine tethys units that intercalated within the forearc and foredeep successions during the middle miocene the decollement of the african meso cenozoic cover panormide platform and imerese sicanian deep water basin enhanced a deep seated deformation phase along with duplexing of the panormide platform beneath the alpine tethys wedge leading to its emersion since the late messinian activation of basement faults led to a generalized emersion of the orogenic units through large wavelength fold culminations accompanied by syn tectonic deposition at their southern limbs concurrently the prism front was partly indented to the southeast by the thick and locally already emerged hyblean platform

estimating basal friction in accretionary wedges from the geometry and spacing of frontal faults elastic theory applied to the deformation in accretionary wedges is used to calculate the condition for slip along an active frontal fault and the basal decollement the equations for calculating the stresses can be solved for the coefficient of basal friction in the situation of the formation of a new frontal thrust fault this allows us to calculate the efficient coefficient of basal friction which includes the weakening effect of pore fluid pressure from geometric parameters and material properties only the geometric parameters like fault dip and layer thickness can be derived from high resolution seismic cross sections application of our analysis to the makran and the nankai accretionary wedge allows us to estimate the upper limit of the effective coefficient of basal friction wbw0 16 and wbw0 2 in these two areas respectively fl 2001 elsevier science b v all rights reserved

spatial and temporal distribution of deformation at the front of the andean orogenic wedge in southern bolivia new observations from an active orogenic wedge help link the seismotectonic behavior of individual faults to wedge deformation rates and patterns over multiple timescales we provide the first detailed constraints on the distribution and timing of deformation at the front of the andean orogenic wedge in southern bolivia where a recent study suggests that great mw 8 earthquakes could rupture the master fault underlying the wedge we use stratigraphic relationships across fault related folds and elastic dislocation modeling of seismic reflection horizons to obtain probabilistic estimates of wedge front fault ages and slip rates our analyses reveal that at least half of the previously determined gps based wedge loading and quaternary whole wedge shortening rates are absorbed across a 20 40 km wide wedge front zone consisting of 1 4 en echelon and partially to fully overlapping faults and folds associated with blind thrust faults the difference between our slip rates and the geodetic geologic observations combined with evidence for activity across internal wedge structures supports the notion that nonsteady state mass balance conditions coupled with elevated erosional efficiency result in distributed wedge deformation the orogenic wedge in southern bolivia behaves in a similar fashion to the taiwanese and himalayan ranges slip accumulates at downdip locations along the master fault and is released incrementally by earthquakes that rupture the wedge front fault zone the faults and folds comprising this zone pose a major source of seismic hazard accumulating slip is also released in the wedge interior and older internal wedge faults must be considered in any future assessment of regional earthquake risk

episodic growth of fold thrust belts insights from finite element modelling abstract the sequential development of a fold thrust belt was investigated using 2d finite element modelling fem the new model results show that a thrust system is typically composed of three distinct regions the thrust wedge pre wedge and undeformed region the thrust wedge involves growth that repeats episodically and cyclically a cycle of wedge building starts as frontal accretion occurs which is accompanied by a rapid increase in wedge width reducing the taper angle below critical in response to this the wedge interior tracked here by the 50 m displacement position rapidly propagates forwards into a region of incipient folding the taper angle progressively increases until it obtains a constant apparent critical value 10 during this period the wedge experiences significant shortening after a new thrust initiates at the failure front leading to a decrease in wedge width successive widening of the wedge and subsequent shortening and thrusting maintain a reasonably constant taper angle the fold thrust belt evolves cyclically through a combination of rapid advancement of the wedge and subsequent gradual slow wedge growth the new model results also highlights that there is clear although minor deformation 0 10 m horizontal displacement in front of the thrust wedge

a sedimentation device to produce uniform sand packs abstract in physical modelling using sand the construction of a sand pack is one of the main experimental difficulties existing solutions are generally based either on delicate manipulations pouring and scraping or on motorized sedimentation devices and the qualities of packs thus produced are known to affect experimental outcomes however uniformity planarity reproducibility and efficiency can be achieved with a simple sedimentation device without motors a rectangular reservoir pierced with holes rests on a support plate pierced with wider holes sand flows when the reservoir is displaced so that its holes match those of the support plate sand jets are diffused by planar horizontal sieves below which the sand sets into the experimental box tests on a 250 m median grain size sand show that the density is at its maximum value reproducible and uniform the spatial variations are only of 0 4 of the mean density thickness of the sand layers shows spatial variations around 2 of the mean thickness and 6 near the side walls very fine grains 90 m median grain size produce less uniform and less planar packs because of their greater sensitivity to air currents caused by the sedimentation according to direct shear tests the sand pack has a well defined static friction coefficient decreasing to a lower dynamic value after about 3 mm of slip with dilatancy in contrast poured sand packs which are initially less dense develop only the dynamic friction coefficient no peak shear stress during slip without dilatancy hole diameters hole spacing and the number and openings of the sieves are the parameters controlling the qualities of the sand pack for a given grain size distribution

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

climatic forcing of erosion landscape and tectonics in the bhutan himalayas a fundamental objective in studies of climate erosion tectonics coupling is to document convincing correlation between observable indicators of these processes on the scale of a mountain range the eastern himalayas are a unique range to quantify the contribution of tectonics and climate to long term erosion rates because uniform and steady tectonics have persisted for several million years while monsoonal precipitation patterns have varied in space and time specifically the rise of the shillong plateau the only orographic barrier in the himalayan foreland has reduced the mean annual precipitation downwind in the eastern bhutan himalaya at the miocene pliocene transition apatite fission track aft analyses of 45 bedrock samples from an e w transect along bhutan indicate faster long term erosion rates outside of the rain shadow in the west 1 0 1 8 mm yr than inside of it in the east 0 55 0 85 mm yr furthermore an aft vertical profile in the latter segment reveals a deceleration in erosion rates sometime after 5 9 ma in this drier segment of bhutan there are remnants of a relict landscape formed under a wetter climate that has not yet equilibrated to the present climatic conditions uplift and preservation of the paleolandscape are a result of a climate induced decrease in erosion rates rather than of an increase in rock uplift rate this study documents not only a compelling spatial correlation between long term erosion and precipitation rates but also a climatically driven erosion rate change on the scale of the eastern himalayas a change that in turn likely influences that region s recent tectonic evolution

minimum work mountain building minimum work methods may provide a useful way to examine mountain building processes here we present forward models of crustal deformation in which faults form along paths that minimize the total frictional and gravitational work associated with slip our numerical model produces thrust wedges analogous to the wedges produced by viscous models but with explicit fault paths rather than diffuse internal deformation the model reproduces the general geometry of real compressional belts highlighting the basic competition between friction and topography gravitational work which may drive the evolution of orogenic zones in particular we replicate observed geologic features such as dendritic faulting patterns thin skinned thrust systems and back thrusting while some modeled features differ from observations the results suggest that for many applications minimum work may be a good approximation for macroscale crustal deformation

river transport effects on compressional belts first results from an integrated analogue numerical model 1 erosion deposition and tectonic processes interact to define the style of deformation in compressional belts this paper introduces a method of integrating a numerical model for surface transport with analogue sandbox models of compressional belts to further improve our understanding of this interplay the analogue model provides the numerical model with the rising topography whereas the numerical model calculates the river network and the resulting amount of erosion and sediment transport compared to previous analogue models with uniform two dimensional erosion this method provides a tool to incorporate process based rates of erosion and deposition with spatial distributions that depend on the river network developed on the evolving topography rivers running parallel to the evolving mountain belt induce along strike changes in erosion and deposition that in turn result in asymmetric tectonic deformation the additional load of sediments controls the propagation of thrust as well as the direction of vergence of the main thrust high erosion rates do not always lead to fewer thrusts within the compressional belts but generally steepen and lengthen the lifetime of the affected faults

finite strain calculations of continental deformation 2 comparison with the india asia collision zone numerical experiments on a thin viscous sheet model for deformation of continental lithosphere subjected to an indenting boundary condition yield distributions of crustal thickness of stress and strain rate and of latitudinal displacements that may be compared with observations in the india asia collision zone a simple indenting boundary condition applied to initially laterally homogeneous sheets obeying a power law rheology produces results that are in broad agreement with the observations provided that the power law exponent is three or greater and the sheet can support vertically integrated stress differences of 2 1013 5 1012 n m 1 in regions in front of the indenter under these conditions the calculated deformation shows accommodation of convergence primarily by crustal thickening to produce a plateau in front of the indenter palaeomagnetic data from india and tibet and the observed distribution of topography suggest that much of the post eocene convergence of india with asia has been taken up by deformation within asia that involved crustal thickening the principal difference between calculation and observation is the absence from the calculated strain rate fields of east west extension of the plateau in front of the indenting boundary the calculations show that once such a plateau is formed the buoyancy force associated with the crustal thickness contrast inhibits further thickening and the plateau strains at less than half the rate of its immediate surroundings seismically determined regional strain rates exhibit a similar distribution with the tibetan plateau straining at about one quarter the rate of the tien shan and ningxia gansu regions calculated principal compressive stress orientations and regional strain rates agree with the seismically determined quantities in the mongolia baikal tien shan tibet and ningxia gansu regions of asia to within the uncertainty of the latter the vertically integrated stresses that are calculated for the viscous sheet are comparable with those that can be supported by a theologically stratified continental lithosphere obeying laboratory determined flow laws we suggest that the thin viscous sheet model described in this paper and its companion gives a simple and physically plausible description of the observed deformation in central asia in this description the predominant mechanism of accommodation of continental convergence is diffuse crustal thickening with shear on vertical planes playing a subsidiary role once large crustal thickness contrasts have been established

erosional control of the kinematics and geometry of fold and thrust belts imaged in a physical and numerical sandbox 1 we investigate the effect of systematically varying erosion intensity k on the geometry and kinematics of fold and thrust belts modeled using equivalent conditions in physical and numerical experiments similar material properties and boundary conditions were used to compare numerical experiments to those modeled in the sandbox using an erosion rule that removes mass from the scaled sand wedges according to rates expected from bedrock fluvial incision the geometry of both modeling approaches is quantitatively compared with that expected from analytical theory the fold and thrust belt growth rate is inversely proportional to k and is well predicted by theory except for the high erosion case in which both the numerical and the experimental sandbox grows supercritically a direct relationship exists between the erosion intensity and the number of fore shear bands and shear strain magnitude the number of fore shear bands decreases and their strain increases with erosion intensity the results indicate that realistic and mechanistic erosion rules can be applied to physical experiments to model mass removal and this approach opens up the possibility of calibrating strain history to erosion intensities for predictions in natural settings quantitative comparisons between the physical and numerical sandbox experiments indicate that deformation style and geometry of the thrust and fold belts are similar when identical rheologies and boundary conditions are used this benchmarking suggests that numerical experiments realistically model conditions observed in the simple cohesionless rheologies commonly employed in sandbox experiments this suggests that numerical simulation may reliably model experimental deformation in geological situations that are difficult to represent in scaled experiments because of the different scales and rheologies of natural orogenic wedges

erosion and exhumation in accretionary orogens experimental and geological approaches 1 this study examines the role of erosion on exhumation and deformation history of accretionary orogens through experimental and geological methods fault propagation kinematics and exhumation rates in an eroded thrust wedge are analyzed through scaled sandbox simulations by testing different parameters friction rheologic layering and sediment input output ratio a model coulomb wedge is submitted to erosion under flux steady state conditions that is the volume of eroded material remains equal to the volume of newly accreted material maintaining a constant surface slope during shortening results show that the way of exhumation depends on the internal dynamics of thrust wedges and conversely on how this dynamics is modified by erosion in the eroded thrust wedges the diversity of exhumation patterns is controlled by the mode of fault propagation depending on the basal friction high or low the vertical component of exhumation is generally higher for the wedges with high basal friction than for low friction wedges the uplift of material occurs along a cluster of subvertical thrusts in the middle part of the eroded thrust wedge with low basal friction the material is exhumed along a series of inclined 20 50 thrusts in the rear of the high friction wedge the zone of maximum exhumation is generally localized in the middle part of the thrust wedge and migrates toward the backstop with continued shortening the vertical exhumation rate increases with time and the material accreted later is rapidly transferred to the main exhumation zone compared to the material accreted during the early stages the exhumation occurs at rates 2 times faster in the wedges with half the thickness than in full thickness wedges the total quantity of eroded material at the end of convergence constitutes 36 50 of initial model area for the thrust wedges with low basal friction and 20 40 for the high basal friction wedges the extent of basal underplating increases with total shortening the area of basal underplated material constitutes up to 30 and 40 of the eroded thrust wedge area for the models with low and high basal friction respectively presence of decollements in the accreted series allows underplating of thrust units developing an anticlinal stack whose growth and location is favored by erosion our results are compared to different present day active and ancient convergent orogens himalaya taiwan southern alps of new zealand and cascadia orogen allowing the different specific exhumation processes to be characterized

shear test results for cohesion and friction coefficients for different granular materials scaling implications for their usage in analogue modelling abstract laboratory tests have been carried out on dry granular materials such as quartz sand glass microspheres and sugar with different grain size rounding and sphericity the measurements have been made with a simple shear test machine for different values of normal stress 50 900 pa shear stress has been plotted against normal stress in order to determine the cohesion and coefficient of internal friction for the investigated materials resulting values of cohesion and coefficient of internal friction are mainly dependent on rounding and sphericity while grain size has a less significant influence further the behaviour of the materials for very small normal stresses 0 400 pa is more complex than previously assumed the fracture envelopes for all materials investigated are convex outward for this small range and converge towards a straight failure envelope with increasing normal stress finally in extensional faulting experiments there is no significant change in fault dip with increasing depth therefore the non linear behaviour for small normal stresses is best described as a dependence of the cohesion on the normal stress and not as a dependence of the coefficient of internal friction on the normal stress values for cohesion increase from 0 pa 15 pa at zero normal stress to 137 247 pa 15 pa for normal stresses greater than 250 400 pa the results show that well rounded spherical material is better suited to model brittle behaviour of rocks in crustal and lithospheric scale analogue models than less well rounded material since it has a smaller cohesion and a coefficient of internal friction which is closer to values of natural rocks

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

initiation of a thrust fault revealed by analog experiments abstract to reveal in detail the process of initiation of a thrust fault we conducted analog experiments with dry quartz sand using a high resolution digital image correlation technique to identify minor shear strain patterns for every 27 m of shortening with an absolute displacement accuracy of 0 5 m the experimental results identified a number of weak shear bands and minor uplift prior to the initiation of a thrust in cross section view the observations suggest that the process is closely linked to the activity of an adjacent existing thrust and can be divided into three stages stage 1 is characterized by a series of abrupt and short lived weak shear bands at the location where the thrust will subsequently be generated the area that will eventually be the hanging wall starts to uplift before the fault forms the shear strain along the existing thrust decreases linearly during this stage stage 2 is defined by the generation of the new thrust and active displacements along it identified by the shear strain along the thrust the location of the new thrust may be constrained by its back thrust generally produced at the foot of the surface slope the activity of the existing thrust falls to zero once the new thrust is generated although these two events are not synchronous stage 3 of the thrust is characterized by a constant displacement that corresponds to the shortening applied to the model similar minor shear bands have been reported in the toe area of the nankai accretionary prism sw japan by comparing several transects across this subduction margin we can classify the lateral variations in the structural geometry into the same stages of deformation identified in our experiments our findings may also be applied to the evaluation of fracture distributions in thrust belts during unconventional hydrocarbon exploration and production

conceptual models for short eccentricity scale climate control on peat formation in a lower palaeocene fluvial system north eastern montana usa fluvial systems in which peat formation occurs are typified by autogenic processes such as river meandering crevasse splaying and channel avulsion nevertheless autogenic processes cannot satisfactorily explain the repetitive nature and lateral continuity of many coal seams compacted peats the fluvial lower palaeocene tullock member of the fort union formation western interior williston basin montana usa contains lignite rank coal seams that are traceable over distances of several kilometres this sequence is used to test the hypothesis that peat formation in the fluvial system was controlled by orbitally forced climate change interacting with autogenic processes major successions are documented with an average thickness of 6 8 m consisting of ca 6 m thick intervals of channel and overbank deposits overlain by ca 1 m thick coal seam units these major coal seams locally split and merge time stratigraphic correlation using a cretaceous palaeogene boundary event horizon several distinctive volcanic ash fall layers and the c29r c29n magnetic polarity reversal shows consistent lateral recurrence of seven successive major successions along a 10 km wide fence panel perpendicular to east south east palaeo flow the stratigraphic pattern complemented by stratigraphic age control and cyclostratigraphic tests suggests that the major peat forming phases resulting in major coal seams were driven by 100 kyr eccentricity related climate cycles two distinct conceptual models were developed both based on the hypothesis that the major peat forming phases ended when enhanced seasonal contrast at times of minimum precession during increasing eccentricity intensified mire degradation and flooding in model 1 orbitally forced climate change controls the timing of peat compaction leading to enhancement of autogenic channel avulsions in model 2 orbitally forced climate change controls upstream sediment supply and clastic influx determining the persistence of peat forming conditions at the scale of the major successions model 2 is supported because interfingering channel sandstones do not interrupt lateral continuity of major coal seams

analogue modeling of large transport thrust faults in evaporites floored basins example of the chazuta thrust in the huallaga basin peru abstract the huallaga basin is a deformed foreland basin located in north peru the basin comprises several syntectonic depocenters the most significant is the biabo syncline located at the back of the chazuta thrust a long flat floored thrust detaching on an evaporitic decollement which has accommodated more than 40 km of horizontal displacement the hangingwall of the chazuta thrust has remained remarkably intact with little or no internal deformation and has incorporated a large volume of evaporites at its base in order to unravel the formation and evolution of this thrust we conducted a series of physical experiments that tested the role of various parameters the goal is to investigate a system in which most of the deformation is accommodated in the frontal part of the chain chazuta thrust whereas deformation of the thrust sheet itself remains minor results from our experimental investigations suggest that the three key parameters that have allowed for such a long lived large slip frontal thrust to operate are 1 the wedge shaped syn kinematic sedimentation 2 the presence of the biabo syncline which acted as a bulldozer pushing the evaporites forward forcing their distal inflation and 3 the erosion at the front that favored farther advance of the frontal thrust dragging passively large volumes of evaporites along with it

influence of attenuated lithosphere and sediment loading on flexure of the deep water magallanes retroarc foreland basin southern andes flexural subsidence in foreland basins is controlled by applied loads such as topography water sediment and subcrustal forces and the mechanical properties of the lithosphere we investigate the controls on subsidence observed within the upper cretaceous magallanes retroarc foreland basin of southern south america to evaluate the impact of lateral variations in flexural rigidity due to late jurassic extension conventional elastic models cannot explain the observed basin deflection and thick accumulation of deep water cenomanian turonian basin strata however models in which the lithosphere has been previously thinned and deflects under topographic and sedimentary loads successfully reproduce regional subsidence patterns results satisfy paleobathymetric observations in the magallanes basin and suggest that lithospheric thinning is necessary to produce both long wavelength and deep subsidence during late cretaceous basin evolution results indicate that elastic thickness decreases westward from 45 25 km in the distal foreland to 37 15 km beneath the foredeep these findings are consistent with a westward reduction in crustal thickness associated with the jurassic extensional history of the patagonian lithosphere our results also show that sediment loading exerts an important control on regional deflection patterns and promotes a wider region of subsidence and reduced forebulge uplift we propose that lateral variations in mechanical properties and large sediment loads restrict depocenter migration and may cause the foredeep to remain fixed for prolonged periods of time these findings confirm that loading of thinned lithosphere imposes different mechanical controls on the flexural profile and have potential implications for other retroarc foreland basins characterized by earlier extensional histories

brittle frictional mountain building 1 deformation and mechanical energy budget an active fold and thrust belt is analogous to a wedge of soil or snow that forms in front of a moving bulldozer such wedges exhibit a critical taper and a regional state of stress that is everywhere on the verge of coulomb failure the width of such a critically tapered fold and thrust belt does not depend on its brittle strength or frictional properties but rather on the accretionary influx rate of fresh material at its toe and on the rate of erosion a steady state fold and thrust belt is one in which the accretionary influx is balanced by the erosive efflux rocks are accreted at the toe and then horizontally shortened as they are transported toward the rear those that enter lower in the accreted section are more deeply buried before being uplifted by erosion mass balance and isotropy constrain the kinematics of this large scale deformation enabling us to infer the trajectories residence times and stress strain histories of rocks incorporated into eroding fold and thrust belts a typical rock resides in the steady state taiwan wedge for 2 3 m y before it is uplifted and eroded during its motion through the wedge it experiences strain rates in the range 10 13 to 10 14 s 1 the mechanical energy budget of brittle frictional mountain building is described by the equation where is the rate of work performed on the base and front of the fold and thrust belt by the subducting plate is the rate at which energy is dissipated against friction on the decollement fault is the rate at which energy is dissipated by internal frictional processes within the deforming brittle wedge and is the rate of work performed against gravitational body forces in a reference frame attached to the overriding plate the total mechanical power being supplied by the subducting eurasian plate to the active fold and thrust belt in taiwan is slightly over 3 gw approximately 60 of this work of steady state mountain building is being dissipated against friction on the decollement fault and about another 25 is being dissipated against internal friction this leaves only 15 or roughly half a gigawatt available to do useful work against gravity in general fold and thrust belts with moderate pore fluid pressures are dominated by work done against friction on the decollement fault however those with nearly lithostatic pore fluid pressures may be dominated by work done against gravity internal frictional dissipation is always less than basal frictional dissipation as it is in taiwan an alternative and equivalent description of the mechanical energy balance of a steady state fold and thrust belt is provided by the equation in this version the quantity on the left is the rate at which work is performed on the back of the wedge by the overriding plate and is the rate of work performed against gravity in a reference frame attached to the subducting plate the latter quantity is always positive for any critically tapered fold and thrust belt whose decollement fault dips toward its rear in contradiction to the central premise of the gravity spreading theory of fold and thrust tectonics

geologic constraints on bedrock river incision using the stream power law denudation rate in unextended terranes is limited by the rate of bedrock channel incision often modeled as work rate on the channel bed by water and sediment or stream power the latter can be generalized as ka m s n where k represents the channel bed s resistance to lowenng whose variation with lithology is unknown a is drainage area a surrogate for discharge s is local slope and m and n are exponents whose values are debated we address these uncertainties by simulating the lowering of ancient river profiles using the finite difference method we vary m n and k to match the evolved profile as closely as possible to the corresponding modern river profile over a time period constrained by the age of the mapped paleoprofiles we find at least two end member incision laws ka 0 3 0 5 s for australian rivers with stable base levels and k f a 0 1 0 2 s n for rivers in kauai subject to abrupt base level change the long term lowering rate on the latter expression is a function of the frequency and magnitude of knickpoint erosion characterized by k f incision patterns from japan and california could follow either expression if they follow the first expression with m 0 4 k varies from 10 7 10 6 m 0 2 yr for granite and metamorphic rocks to 10 5 10 4 m 0 2 yr for volcaniclastic rocks and 10 4 10 2 m 0 2 yr for mudstones this potentially large variation in k with lithology could drive strong variability in the rate of long term landscape change including denudation rate and sediment yield

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

steady state erosion of critical coulomb wedges with applications to taiwan and the himalaya 1 orogenic structure appears to be partially controlled by the addition to and removal of material from the mountain belt by tectonic accretion and geomorphic erosion respectively we developed a coupled erosion deformation model for orogenic wedges that are in erosional steady state and deform at their coulomb failure limit erosional steady state is reached when all material introduced into the wedge is removed by erosion that is limited by the rate at which rivers erode through bedrock we found that the ultimate form of a wedge is controlled by the wedge mechanical properties sole out depth of the basal decollement erosional exponents basin geometry and the ratio of the added material flux to the erosional constant as this latter ratio is increased wedge width and surface slopes increase we applied these models to the taiwan and himalayan orogenic wedges and found that despite a higher flux of material entering the former the inferred ratio was larger for the latter calculated values for the erodibility of each wedge showed at least an order of magnitude lower value for the himalaya relative to taiwan these values are consistent with the lower precipitation regime in the himalaya relative to taiwan and the exposure of crystalline rocks within the himalayan orogenic wedge independently determined rock erodibility estimates are consistent with the accretionary wedge sediments and metasediments and the crystalline and high grade metamorphic rocks exposed within taiwan and the himalaya respectively therefore differences in rock type and climate apparently lead to key differences in the erosion and hence orogenic structure of these two mountain belts

growth and erosion of fold and thrust belts with an application to the aconcagua fold and thrust belt argentina 1 the development of topography within and erosional removal of material from an orogen exerts a primary control on its structure we develop a model that describes the temporal development of a frontally accreting critically growing coulomb wedge whose topography is largely limited by bedrock fluvial incision we present general results for arbitrary initial critical wedge geometries and investigate the temporal development of a critical wedge with no initial topography increasing rock erodibility and or precipitation decreasing mass flux accreting to the wedge front increasing wedge sole out depth decreasing wedge and basal decollement overpressure and increasing basal decollement friction lead to narrow wedges large power law exponent values cause the wedge geometry to quickly reach a condition in which all material accreted to the front of the wedge is removed by erosion we apply our model to the aconcagua fold and thrust belt in the central andes of argentina where wedge development over time is well constrained we solve for the erosional coefficient k that is required to recreate the field constrained wedge growth history and these values are within the range of independently determined values in analogous rock types using qualitative observations of rock erodibilities within the wedge we speculate that power law exponents of 1 3 m 0 4 and 2 3 n 1 characterize the erosional growth of the aconcagua fold and thrust belt this general model may be used to understand the development of mountain belts where orogenic wedges grow as they deform at their coulomb failure limit

deformation exhumation and topography of experimental doubly vergent orogenic wedges subjected to asymmetric erosion we explore the effect of asymmetric erosion and dip angle of the indenter on the internal kinematics deformation and exhumation of an experimental doubly vergent orogenic wedge during transient and steady state flux conditions we calculate displacement fields and derive strain and exhumation for several erosion and non erosion cases using particle image velocimetry piv analysis we apply calculated displacement fields to a simple heat equation to explore virtual evolution of thermal structure erosion exerts a primary control in the internal kinematics of the experimental wedge in non erosion cases early formed fore shears rotate toward the indenter such that the shear bands become steeper this effect is more pronounced in prowedge erosion cases where shear bands become very steep and even overturned conversely retrowedge erosion results in back rotation of early formed fore shears and the shear bands evolved to gently dipping structures wedge erosion in combination with the condition of flux and topographic steady state increases density of fore shears and controls size width and topographic divide migration steeply dipping indenters produce overall gentle particle trajectories and gently dipping indenters favor steep trajectories particle velocity with respect to isotherms increases where erosion is concentrated and near active fore shears