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

strike slip faulting thrusting and related basins in the cenozoic evolution of the southern branch of the bolivian orocline abstract the evolution of the cenozoic deformation of the cordillera oriental and of the contact zone with the altiplano in southern bolivia is well documented in the tupiza nazareno and estarca basins the tectonic evolution started at about 29 ma the period between 29 and about 22 21 ma is marked by development of a pull apart basin related to n s trending left lateral strike slip faulting during this period initial deposition consisted of andesitic lavas and detrital sediments breccias matrix supported red conglomerates sands and silts of the catati and tupiza formation deposited in alluvial fan and flood plain environments from 21 to 20 ma the tectonic setting evolved to n s oriented thrusts which had a dextral component this event caused the tupiza basin to evolve into a full ramp basin and triggered the development of two piggy back basins nazareno and estarca both basins record detrital deposition nazareno formation in an alluvial fan environment at 10 9 ma deformation in this area stopped allowing peneplanation during which time the san juan de oro erosional surface was formed and the fluvial conglomerates of the oploca formation were deposited this segment of the andes was then tectonically transported to the east while uplift due to thrusting continued in the eastern andean front and in the subandean zone

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

cyclical behavior of thrust wedges insights from high basal friction sandbox experiments scaled sandbox experiments with high basal friction simulating the growth of accretionary wedges display cycles alternating between frontal imbricate thrusting and underthrusting of long undeformed sheets by contrast low basal friction experiments with otherwise similar and constant initial conditions produce a classic frontal imbricate fan through repeated failure along frontal thrusts the cyclical behavior observed in high basal friction experiments is expressed by three quantities 1 the average spacing between frontal thrusts 2 the advance and retreat of the deformation front and 3 the frontal slope alpha of the actively deforming wedge as a long sheet is underthrust the front is steepened through slumping until the maximum critical angle is reached then frontal thrusting resumes and the accretion of imbricate slices builds the wedge forward thereby lowering the taper to the minimum critical angle at shallow tapers a long unit is underthrust and subsequently uplifts shortens and steepens the overlying wedge through backthrust deformation thus completing the cycle underthrusting of long units offers a simple mechanism for underplating overlying units it also provides a possible explanation for temporally and spatially varying wedge geometries in nature when basal frictions attain 80 90 of the internal friction

thrust tectonics in the andean retro foreland basin of northern peru permian inheritances and petroleum implications abstract in northern peru the huallaga moyabamba maranon subandean foreland basin system results from the interaction between thin and thick skinned tectonics geophysical data and the construction of two balanced cross sections show that this structural configuration has been controlled by permian inheritances a fossilized west verging middle permian fold and thrust belt which developed during the gondwanide orogeny has been partly reactivated by the andean compression and controlled thick skinned tectonics propagation this west verging thrust system is still active and causes the crustal and damaging seismicity of the moyabamba region late permian salt which has sealed the middle permian fold and thrust belt controlled thin skinned tectonics propagation and the development of the must large overthrust of the peruvian subandean zone the fossilized and partly reactivated middle permian fold and thrust belt constitutes a new petroleum play for the exploration in the northern peruvian subandean basins sub thrust traps of the moyabamba and huallaga wedge top basins are particularly attractive but stay unexplored

the mechanics of frontal imbrication a first order analysis the detachment and imbrication of thrust slices at the front of a thrust wedge is one of the principle modes by which such wedges grow collapse of the frontal ramp under longitudinal compressional stress cannot explain the regular formation of new slices of finite length unless there are regularly spaced heterogeneities in the footwall layer or the underlying basement surface advance of the thrust wedge over the frontal ramp however increases both the vertical load on the ramp and the traction on the upper flat this will in general produce a peak deviatoric stress in the footwall layer below the leading edge of the thrust wedge failure will occur at this point when the thrust wedge has advanced a distance l such that the deviatoric stress in the footwall layer exceeds its strength l is a function of a rock density b ramp angle c the resistances to motion on the basal detachment the ramp and the upper flat and d the strength and thickness of the footwall layer these mechanical parameters can therefore control the formation of new thrust slices of regular length in the absence of footwall heterogeneities

kinematics of eastern salt range and south potwar basin pakistan a new scenario the salt range of northern pakistan represents an emergent thrust front along which the potwar plateau has been translated southward based on the analysis of subsurface data we have constructed a balanced cross section exhibiting a frontal fault bend fold and a set of detachment folds below the potwar plateau from the restored section we are able to propose a forward kinematic modelling using the thrustpack software the main characteristics of the model are the alternation of periods during which deformation was concentrated on the frontal thrust between 10 and 5 ma and since 1 9 ma and an intervening episode during which deformation was distributed along the whole section between 5 and 1 9 ma the latter phase is marked by development of out of sequence structures the episodes of frontal thrust activation are characterised by high denudation rates on the contrary out of sequence folding occurred during periods of important sedimentary accumulation the consequences for petroleum exploration are also discussed however timing of the petroleum system is strongly dependent on hypotheses made on basal heat flow

gold metallogeny and the copper gold association of the australian proterozoic australian proterozoic gold producing deposits emplaced mainly at 1 55 2 00 ga are divided into the following categories 1 iron oxide dominated brecciahosted cu u au replacement deposits spatially associated with felsic intrusions 273t au 2 stratabound au cu bearing iron formations 152 4t au 3 unconformity style u cu pgm au deposits 53t au 4 iron oxide dominated au cu mineralisation hosted within elements of ductile deformation 146 7t au 5 broken hill and volcanic hosted massive sulphides 150t au 6 iron sulphide dominated veins and replacement zones spatially related to felsic intrusions 150 7t au and 7 iron sulphide dominated veins and replacement zones spatially related to elements of regional deformation 159 9t au categories 1 to 4 are mainly confined to proterozoic rocks constituting an association in which au and cu are commonly present together with variable amounts of u bi co w se te and ree most examples in categories 1 4 fall into either of two groups cu aumagnetite hematite types formed at relatively high temperature 300 450 c and cu u au hematite types formed at 150 300 c we postulate that these ores formed from a common high salinity 15 35 wt nacl equiv low total sulphur a s 10 3 to 10 2 high fo2 fluid type in which metal transport was dominated by chloride complexing the most effective method of metal deposition was fluid mixing achieving a synchronous decrease in fo2 and temperature this unusual oxidised fluid association was favoured in high heat flow extensional settings containing oxidised and or oxidised evaporitic sedimentary sequences the intrusion of oxidised fractionated granites which are commonly temporally associated with metal emplacement acted in some places to heat and focus basinal fluids and in others was the ultimate source of metals

revised deformation history of the central andes inferences from cenozoic foredeep and intermontane basins of the eastern cordillera bolivia 1 investigation of cenozoic deformation sediment accumulation and provenance in the eastern cordillera of bolivia at 17 21 s indicates major shortening 60 140 km and foredeep development followed by limited internal shortening and intermontane basin development contrasting histories of shortening deposition sediment dispersal and detrital composition distinguish a formerly extensive paleogene foredeep exposed along an eastern belt of synclines from a zone of principally neogene intermontane basins in the central eastern cordillera new 40ar 39ar ages of 25 17 ma for interbedded tuffs reveal latest oligocene early miocene accumulation in intermontane basins at rates 80 m myr several times lower than andean foredeeps although poorly dated foredeep evolution probably coincided with middle eocene oligocene deformation and denudation in adjoining regions to the west a mid cenozoic transition from foredeep to intermontane conditions may be attributable to emplacement of a basement involved tectonic wedge beneath the eastern cordillera in this interpretation wedge emplacement drove flexural foredeep subsidence from roughly 40 to 25 ma whereas subsequent accumulation occurred in localized internally drained basins in elevated intermontane areas regardless of the regional subsurface structural geometry 40ar 39ar ages define a 25 21 ma onset of intermontane sedimentation that signifies the termination of major upper crustal shortening over large parts of the eastern cordillera and possibly the age of initial shortening in the interandean and subandean zones to the east in contrast to several popular models the revised tectonic history presented here suggests significant pre neogene shortening in the eastern cordillera and underscores the large uncertainties in estimates of long term shortening rates in the andes

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

double evaporitic decollements influence of pinch out overlapping in experimental thrust wedges abstract this article focuses on how deformation and displacements are transferred between two decollements located at different stratigraphic levels by means of analogue modeling using brittle viscous sand silicone systems we present results from ten analogue models in which we varied key parameters such as the amount of horizontal offset or overlap between the pinch outs of the upper and lower decollements the total amount of shortening and the planform geometry of the upper decollement results indicate that i structures root onto the basal and upper decollement defining an inner and an outer domain and ii the offset overlap of the decollements controls the geometry of the transition zone located between the two decollements the propagation of deformation into the foreland both in space and time and the deformation style and kinematics in the different domains of the model when the pinch out of the upper decollement is at an angle with the backstop oblique structures form and the geometry and propagation mode of the structures change progressively along strike we compare our experimental results with other silicone sand analogue models and with the natural examples of the southern pyrenees where upper triassic and eocene oligocene syn tectonic evaporites acted as basal and upper decollements respectively

the 3 d structural model of an out of sequence earthquake in china implication for the reactivation of positive inversion structures along the northern tianshan fold and thrust belt

spatial and temporal variability in denudation across the bolivian andes from multiple geochronometers abstract we quantify spatial and temporal variations in denudation rates across the central andean fold thrust belt in bolivia with particular focus on the holocene measured and predicted 10 be cosmogenic radionuclide crn concentrations in river sediments are used to 1 calculate catchment averaged denudation rates from 17 basins across two transects at different latitudes and 2 evaluate the sensitivity of holocene climate change on the denudation history recorded by the crn data estimated denudation rates vary by two orders of magnitude from 0 04 to 1 93 mm yr 1 with mean values of 0 40 0 29 mm yr 1 in northern bolivia and 0 51 0 50 mm yr 1 in the south results demonstrate no statistically significant correlation between denudation rates and morphological parameters such as relief slope or drainage basin size in addition the crn derived denudation rates do not reflect present day latitudinal variations in precipitation comparison to 130 previously published denudation rates calculated over long thermochronology derived 10 6 yrs medium crn derived 10 2 10 4 yrs and short timescales sediment flux derived 10 1 yrs indicate temporal variations in denudation rates that increase between 0 and 200 over the last 5 ka crn modeling results suggest that the crn derived denudation rates may not be fully adjusted to wetter climate conditions recorded in the central andes since the mid holocene we conclude that large spatial variability in crn denudation may be due to local variations in tectonics e g faulting while large temporal variability in denudation may be due to temporal variations in climate

effects of mass waste events on thrust wedges analogue experiments and application to the makran accretionary wedge 1 olistostromes cover large portions of active thrust wedges like makran gulf of cadiz and offshore borneo olistostrome emplacement by submarine mass flow represents an instantaneous and massive mass redistribution that may influence thrust wedge mechanics different scenarios are experimentally tested they show that the postolistostrome wedge development depends on the thickness and extent of the added load these results are discussed after the example of the iranian makran wedge situated between the arabian and eurasian plates mass redistribution caused by a late miocene mass flow may explain a change in deformation style from intense folding and thrusting to gentle folding and eventually a jump of thrust imbrication toward the frontal offshore part of this active accretionary wedge

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

tectonics and climate of the southern central andes the history of the southern central andes including the world s second largest plateau and adjacent intermontane basins and ranges of the eastern cordillera and the northern sierras pampeanas of argentina and bolivia impressively documents the effects of tectonics and topography on atmospheric circulation patterns the development of orographic barriers and their influence on erosion and landscape evolution at various timescales protracted aridity in the orogen interior has facilitated the creation and maintenance of the puna altiplano plateau contraction and range uplift filling of basins and possibly wholesale uplift of the plateau increased gravitational stresses in the orogen interior which caused the eastward migration of deformation into the foreland and successive aridification the uplift of the andean orogen has also had a far reaching influence on atmospheric and moisture transport patterns in south america this is documented by the onset of humid climate conditions on the eastern side of the andes in late miocene time which was coupled with the establishment of dramatic precipitation gradients perpendicular to the orogen and changes in tectonic processes in the andean orogenic wedge

sedimentation and preservation of the miocene atacama gravels in the pedernales chanaral area northern chile climatic or tectonic control in recent years longitudinal changes on the thin thick skinned tectonic styles of the central andes has been intensively discussed while other studies have considered the role of mass transfers on the unloading the orogen and on the stress regime along the plate interface arising from changes on the volume of sediment arriving into the peru chile trench the search for paleo climate records is therefore crucial for our understanding of the history of the central andes in this paper we focus on the atacama gravels an extensive blanket of miocene continental deposits filling a neogene paleo valley system along the southern atacama desert in northern chile an east west transect between pedernales and chanaral 26o30 s enabled us to carry out a sedimentological and tectonic study of the atacama gravels based on logging and field observations along the rio salado canyon new 39ar 40ar ages obtained on intercalated and overlying ignimbrites date the beginning of the atacama gravels sedimentation at around the oligocene miocene boundary and cessation of sedimentation in the late miocene thirteen lithofacies included within five facies associations a1 to a5 were identified depositional environments vary from proximal alluvial fan a1 a2 in the precordillera through ephemeral fluvial a3 a4 to distal playa lake a5 in the coastal cordillera no evidences of syn sedimentary deformation have been found showing that the change from sediment removal to sediment preservation cannot be explained by tectonic causes and climate change appears to be the dominant controlling factor of sediment preservation a progressive change from semi arid towards hyper arid climatic conditions during the miocene led to a reduction on the transport capacity of the fluvial system and sediment preservation along the paleovalley system formed during the oligocene

deformation evolution of eastern sichuan xuefeng fold thrust belt in south china insights from analogue modelling abstract the eastern sichuan xuefeng fold thrust belt cxftb located in south china has received wide attention due to its distinctive deformation styles and close relationships with natural gas preservation but its deformation evolution still remains controversial in order to study further this issue we designed three sets of analogue models based on the results of the models we suggest that 1 the deformation in the cxftb may simultaneously initiate along two zones nearby the dayong and qiyueshan faults at 190 ma and then progressively propagate into the interiors of the western hunan hubei and eastern sichuan domains at 140 150 ma and finally reach the front of the huayingshan fault at 120 ma 2 the difference in decollement depth is the main factor determining the patterns of folds in different domains of the cxftb and 3 the eastern sichuan domain may have a basement significantly different from those of the western sichuan and western hunan hubei domains

andean age structure of eastern cordillera province of la paz bolivia a moho root beneath the bolivian andes 40 km deep is consistent with 230 km of overlap of neogene age on a single trenchward dipping transcrustal thrust fault with 10 of finite ramp cutoff main andean thrust mat only 10 of the andean crustal volume is ascribable to magmatic addition the mat is located within south american crust of full thickness it intersects the basement top 450 km inland from the neogene crustal margin it is not a collision suture as shown by persistent pre neogene facies continuity thrusting is not accompanied by terrane accretion the present bilaterally symmetrical thrust belt responds to elastic line loading and to coulomb rheology in the hanging wall of the mat a deep high stress wedge base builds a steep critical slope in the footwall the foredeep response is fast subcritical growth by progradation and blind thrusting on a low stress decollement interaction is maintained by out of sequence renewal of movement on the mat

control on thrust tectonics in the himalayan foothills a view from a numerical model abstract the evolution of an intracontinental accretionary wedge is controlled by a the displacement of several thrust sheets along faults and an underlying decollement b the vertical motion of the rigid basement beneath the accretionary wedge and c superficial processes erosion sedimentation a numerical model is proposed based on the coulomb wedge theory a 2 5 d forward kinematic system progressive tilting of the underthrust basement and an erosion sedimentation model in order to characterise the sequence of fault activation in a thrust system and the syn orogenic sedimentation in such a system the siwalik belt the frontal thrust system of the himalayas represents a structure on which the model can be applied it presents mainly simple structures made of one fault system branched on an underlying decollement conglomerate deposits of the plio pleistocene upper siwalik are overthrust by miocene formations lower siwalik at the front of the wedge and along inner faults structural maps show that thrust sheets are laterally relayed and simultaneously emplaced the comparison between the deformation pattern in the himalayan foothills and the modelling results supports the concept that the himalayan thrust wedge has a steady state type evolution controlled by horizontal convergence and superficial mass transport and characterised by an irregular spatial and temporal distribution of shortening within the whole wedge during this evolution piggy back basins or strongly displaced internal sheets hide some thrust sheets the exhumation path of these sheets is then discontinuous though the shortening of the wedge is regular

coulomb theory applied to accretionary and nonaccretionary wedges possible causes for tectonic erosion and or frontal accretion based on observations from both modem convergent margins and sandbox modeling we examine the possible conditions favoring frontal accretion and or frontal and basal tectonic erosion mean characteristic parameters t t and are used to discuss the mechanical stability of 28 transects across the frontal part of convergent margins where the coulomb theory is applicable natural observations reveal that typical accretionary wedges are characterized by low tapers with smooth surface slope and subducting plate low convergence rates and thick trench sediment while nonaccretionary wedges display large tapers with irregular surface slopes and rough subducting plate high convergence rates and almost no trench fill sandbox experiments were performed to illustrate the effects of seamounts ridges in the subduction zone on the deformation of an accretionary wedge these experiments show that a wedge of sand is first trapped and pushed in front of the seamount which acts as a moving bulldozer this is followed by a tunnelling effect of the subducting seamount through the frontal wedge material which results in considerable sand reworking at an advanced subduction stage the d6collement jumps back from a high level in the wedge to its former basal position we conclude that a high trench sedimentation rate relative to the convergence rate leads to frontal accretion in contrast several conditions may favor tectonic erosion of the upper plate first oceanic features such as grabens seamounts or ridges may trap upper plate material during their subduction process second destabilization of the upper plate material by internal fluid overpressuring causing hydrofracturing is probably another important mechanism

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

stresses in an overthrust sheet and propagation of thrusting an airy stress function solution an airy stress function solution is derived for the stress field in an elastic wedge shaped overthrust sheet subject to horizontal forces at the rear and at the front from this stress solution the state of stress on the basal thrust lower boundary of the sheet is obtained both normal and shear stresses on the basal thrust are affected by the rear pushing and front buttressing forces and by the slopes of the upper and lower boundaries of the wedge shaped block for 1 nonlinear components of external horizontal forces exerted on the vertical edges of the block and or 2 nonzero slopes of the upper and lower boundaries the normal and shear stresses vary systematically with position along the basal thrust the ratio of shear and normal stresses along this plane the frictional function of the thrust can be compared with the friction coefficient to determine where slippage is likely to occur since the frictional function is a function of position the strength is exceeded only on part of the thrust near the rear for most values of the parameters the assumption of simultaneous displacement of the whole thrust sheet on the basal plane is therefore not realistic even simple static models such as the present one indicate that the motion on thrust faults must be envisaged in terms of progressive consecutive dislocation type slippage

subandean thrust and fold belt of northwestern argentina geometry and timing of the andean evolution the subandean ranges of northwestern argentina are an active thin skinned fold and thrust belt the main detachment level in silurian shales dips 2 3w and all the major east verging faults detach from it important intermediate detachment levels in the devonian shales generate lift off structures and the decoupling of the lower and upper structural levels the subandean thrust belt has a minimum shortening of about 60 km 36 at about 2240 latitude the deformation started at about 8 5 9 ma with the uplift of the el pescado range and the formation of an important back thrust at the cinco picachos range fault generation gets young to the east the pintascayo range uplift started at 7 6 ma while the baja orn range uplift began at about 6 9 ma these two ranges continued growing simultaneously until at least 4 7 ma at san antonio range fault movement began at approximately 4 4 ma and the aguarage uplift started at about 2 7 ma an important stage dominated by out of sequence movements spans from about 4 5 ma to present for both proposed models of shortening the quaternary rates of shortening between 8 and 11 mm yr coincide well with global positioning system results from the area the hydrocarbon generation and migration is contemporaneous with the deformation enhancing the possibilities of hydrocarbon entrapment in the area

tectonostratigraphic evolution of the morichito piggyback basin eastern venezuelan basin abstract the morichito piggyback basin mpb is a sw ne oriented depocenter in the eastern venezuelan foreland basin evfb this piggyback basin formed by overlying the pirital thrust during the middle to late miocene as a result of oblique collision between the caribbean and south american plates the mpb covers an area encompassing approximately 1000 km 2 between the serrania del interior range and the pirital high which is a hanging wall uplift along the pirital thrust that acts as a confining barrier on the southern boundary of the mpb previous studies have tried to address the tectonostratigraphic significance of the mpb but new biostratigraphic information and recently acquired 3d seismic data have allowed us to expand the understanding of this basin the mpb occupies a relatively small area of the evfb however the mpb contains a valuable stratigraphic record that can be used to unveil the timing of the main deformational events that took place in the evfb this work presents the tectonostratigraphic evolution of the mpb by defining four tectonostratigraphic sequences t1 t4 each sequence was defined on the basis of integration of well logs biostratigraphy and seismic geomorphological interpretations t1 24 16 ma late oligocene to middle miocene which was deposited in shallow marine environments extends to the south of the pirital high beyond the boundaries of the mpb t1 is equivalent to the early foredeep stage of the evfb having been formed when structural deformation and uplifting were already occurring to the north on the proto serrania del interior range 24 16 ma and the pirital thrust was active 22 ma t2 16 11 ma middle to late miocene is composed of alluvial fan deposits derived from the proto serrania del interior range the geometry and internal configuration of t2 indicate that during this time the basin was transitioning from an open foreland basin to a confined piggyback basin during deposition of t2 the pirital fault was active as an out of sequence thrusting event 16 11 ma t3 late miocene and t4 early pliocene to recent composed of shallow marine and fluvial deposits were deposited in an already restricted piggyback basin the pirital high was already in place during deposition of t3 11 9 3 ma t3 and t4 represent the final phases of mpb infilling when tectonic activity and subsidence were at their lowest rates mpb sedimentary infilling dates the activity of thrusting events in the proto serrania del interior 24 16 ma timing of maximum deformation associated with the pirital out of sequence thrusting event 16 11 ma timing of final emplacement of the pirital high 11 9 3 ma and the beginning of tectonic quiescence

experimental modelling of orogenic wedges a review abstract experimental modelling applied to the study of orogenic wedge dynamics has been a subject of fruitful research for more than 30 years although the technique dates back as far as the early xix th century on one hand several first order parameters controlling the structural evolution of mountain belts have been intensively investigated using the classic tectonic sandbox models the main parameters are the properties of the basal decollement the deforming material the backstop and fluxes kinematics and surface processes on the other hand the morphological evolution of a mountain relief subjected to changing tectonic or climatic forcing has been addressed using another kind of approach called geomorphic models nowadays the literature is extremely rich particularly for the sandbox technique so that it becomes difficult to have an exhaustive view of the effects of the above parameters on mountain evolution in this article we propose a detailed review of the main results obtained using both tectonic and geomorphic approaches our goal is to provide an almost complete state of the art in the experimental study of relief dynamics to guide present and future researchers in their understanding of mountain belt evolution

stochastic modelling of coalbed methane resources a case study in southeast qinshui basin china this paper presents a stochastic analysis of coalbed methane cbm resources for a coal seam in southeast qinshui basin china log and laboratory data are used to predict the coal thickness coal ash content and coal gas content using reservoir modelling the distributions of the karst collapse column kcc coal seam thickness coal quality and coal gas content are generated the convergent interpolation and sequential gaussian simulation methods are used to model the surface and structure of the coal seam the structural models are determined by the surface structure and coal seam thickness based on the structural models the coal and kcc are converted to two facies and their distributions are obtained using object modelling the coal density distributions are simulated based on each facies model using the sequential gaussian simulation finally different realisations are used to study cbm resources the results show that the heterogeneities in coal seam thickness and coal quality lead to significant uncertainty in estimating cbm resources a model with lower heterogeneity gives a greater cbm resource the distributions of kcc coal seam thickness coal quality and gas content are the main sources for uncertainty in cbm resource estimation the density variogram and top structure contribute less to the uncertainty in cbm resources

structural styles and decoupling in stratigraphic sequences with double decollements during thin skinned contractional tectonics insights from numerical modelling abstract six series of particle based numerical experiments were performed to simulate thin skinned contractional tectonics in stratigraphic sequences with double decollements during horizontal shortening the models were assigned with varying rock competence depth and thickness of the upper decollement which resulted in significantly different styles of deformation and decoupling characteristics above and below the upper decollement the models composed of the least competent material produced distributed sinusoidal detachment folds with many shallow structures profoundly decoupled from the deep seated folds the models composed of a more competent material are dominated by faulted box folds with minor disharmonic folds developed in their limbs in contrast the results of models composed of the most competent material are characterised by localised piggyback thrusts fault bend folds and pop up structures with tensile fractures developed in fold hinges depth of the upper decollements also plays an important role in controlling structural decoupling i e the shallower the upper decollements the higher the degree of decoupling becomes thicker upper decollements can provide sufficient mobile materials to fill fold cores and contribute to the formation of secondary disharmonic folds helping enhance structural decoupling our modelling results are comparable to the structural features exhibited in the dezful embayment of the zagros fold and thrust belt with the miocene gachsaran formation acting as the shallow upper decollement and the fars with the triassic dashtak formation as its intermediate decollement this study demonstrates that rock competence depth and thickness of the upper decollements can jointly affect the structural styles and decoupling our modelling results are instructive for structural interpretation of deep zones in fold and thrust belts that exhibit distinct structural decoupling features

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

latitudinal variation of denudation in the evolution of the bolivian andes latitudinal gradients in topography relief climate and deformation have been used to suggest that climate driven erosion has exerted a first order control on the development of the central andes we synthesize the spatial and temporal variations in denudation across the eastern bolivian andes 14 22 s from new and existing estimates to test whether physical evidence exists to support the hypothesis that erosion influences thrust belt evolution basin morphometry channel network indices climate and longitudinal river profiles indicate a northward increase in relative relief fluvial incision and denudation short term denudation rate averages from landslide mapping and sediment flux data range from 1 to 9 mm yr in the north compared to 0 3 to 0 4 mm yr in the south long term denudation rate estimates from thermochronology cosmogenic radionuclides foreland basin sediment volumes stra tigraphy paleoerosion surface degradation and balanced cross sections range from 0 04 to 1 6 mm yr with rates up to more than twice as fast in the north when comparing estimates from the same method the shorter term denudation rates exhibit the greatest variance our denudation synthesis shows that an along strike disparity in denudation greater in the north has existed throughout the holocene and perhaps existed since as early as the late miocene our denudation synthesis also suggests that the disparity and denudation rates have increased to the present correlations between the thrust belt geology geometry geomorphology climate and kinematics of the orogenic wedge provides a case study in observing the regional scale interactions between uplift climate and erosion we conclude that the denudation history uplift history and tectonic geomorphic correlations suggest that models of the evolution of the bolivian andes should incorporate a latitudinal erosion gradient for the last 10 kyrs to perhaps 10 myrs

interaction between deformation and sedimentation in a multidecollement thrust zone analogue modelling and application to the sub andean thrust belt of bolivia fold and thrust belts are influenced by the presence of decollement levels as well as the amount of shortening and synkinematic sedimentation these parameters are studied at the scale of a thrust belt using a field approach combined with analogue and numerical modelling in this study we use analogue modelling to test the evolution of a single structure during sedimentation in a domain containing three prekinematic decollement levels the development of this structure in the analogue experiments shows that the expression of the deformation is strongly dependent on sedimentation rate i the structure propagates forward in an overall asymmetric shape if the sedimentation rate is slower than the uplift velocity or ii the structure grows vertically and its vergence changes at the surface if the sedimentation rate is similar to the uplift velocity or iii the structure grows vertically with a double vergence at the surface and at depth if the sedimentation rate is higher than the uplift velocity the results of the experiments are compared with structures in the subandean thrust belt to aid the interpretation of poor seismic data

experiments on faulting in a two layer cover sequence overlying a reactivated basement fault with oblique slip abstract two series of experiments were undertaken to investigate the development of faults in a cover sequence overlying a reactivated basement fault with oblique slip the experiments represent physical models properly scaled to account for gravitational forces brittle behaviour of the upper cover and ductile behaviour of evaporites in the lower cover models were made with a coulomb layer sand overlying a newtonian ductile layer silicone putty in each series the thickness of the silicone layer was varied from each experiment to the next in all normal wrench experiments a domain of faults appeared in the brittle layer vertical offsets were mostly of normal sense but some reverse faults occurred in experiments with thick silicone layers the thicker the silicone the wider the fault domain the deformation zone was in all cases limited to the faulted domain in the reverse wrench experiments discrete reverse riedel faults appeared without a silicone layer as the silicone layer became thicker the importance of the reverse faults diminished with the thickest layer of silicone 3 cm no faults appeared in the overburden but a monoclinal flexure accommodated the dip slip component in the presence of silicone the deformation zone was not limited to a faulted domain and the strike slip component was accommodated by a bulk shearing of the overburden in the uplifted compartment these experiments demonstrate that in basins deformed in oblique slip mode any deformation that is localized along discrete faults at the basement level becomes distributed over a much wider zone of overburden when this overlies an interval of evaporites

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

fault related fold styles and progressions in fold thrust belts insights from sandbox modeling fault related folds of variable structural styles and assemblages commonly coexist in orogenic belts with competent incompetent interlayered sequences despite their commonality the kinematic evolution of these structural styles and assemblages are often loosely constrained because multiple solutions exist in their structural progression during tectonic restoration we use a sandbox modeling instrument with a particle image velocimetry monitor to test four designed sandbox models with multilayer competent incompetent materials test results reveal that decollement folds initiate along selected incompetent layers with decreasing velocity difference and constant vorticity difference between the hanging wall and footwall of the initial fault tips the decollement folds are progressively converted to fault propagation folds and fault bend folds through development of fault ramps breaking across competent layers and are followed by propagation into fault flats within an upper incompetent layer thick skinned thrust is produced by initiating a decollement fault within the metamorphic basement progressive thrusting and uplifting of the thick skinned thrust trigger initiation of the uppermost incompetent decollement with formation of a decollement fold and subsequent converting to fault propagation and fault bend folds which combine together to form imbricate thrust breakouts at the base of the early formed fault ramps along the lowest incompetent layers which may correspond to basement cover contacts domes the upmost decollement and imbricate thrusts to form passive roof duplexes and constitute the thin skinned thrust belt structural styles and assemblages in each of tectonic stages are similar to that in the representative orogenic belts in the south china southern appalachians and alpine orogenic belts

paleozoic structural controls on shortening transfer in the subandean foreland thrust system ene and southern ucayali basins peru 1 the neogene evolution of the ene and southern ucayali basins of the subandes has been controlled by two stacked thrust wedges that differ in terms of tectonic styles the lower thrust wedge is formed by deep seated decollements within the basement related to thick skinned foreland structures inherited from an early carboniferous thrust system seismic reflection data show that this paleozoic compressional system has been eroded and unconformably covered by late carboniferous clastic sediments it generated an irregular paleozoic sedimentary architecture controlling the neogene thrust propagation the upper thin skinned thrust wedge developed within this paleozoic sedimentary series and constitutes the subandean zone cross section balancing shows an along strike homogenous horizontal shortening of 56 km 30 across the ene southern ucayali thrust system this amount of shortening was vertically partitioned onto the two stacked thrust wedges the n s thickness variations of the paleozoic sedimentary prism controlled the eastward propagation of the upper thrust wedge the southern thickening of the paleozoic series generated major decollements and the shortening excess is of 7 km 16 in comparison to the north consequently the northern lack of shortening onto the upper thrust wedge was transferred to the early carboniferous compressional structures of the lower thrust wedge we suggest that this vertical partitioning of the shortening was accommodated by a regional oblique ramp the tambo transfer zone this geometrical analysis of the ene southern ucayali thrust system provides new perspectives for future hydrocarbon exploration in this region

structural evolution of a fold and thrust belt generated by multiple decollements analogue models and natural examples from the northern apennines italy this study analyses the role played by the mechanical proprieties of the rocks involved in a thrust system focusing on the problem of multiple decollements and of related compressional structures integration of geophysical seismic reflection profiles and geological data and scaled sandbox models are used to study the deformation style of two areas of the northern apennines italy the po plain and the umbria marche apennines both areas are characterised by a complex stratigraphy consisting of decollements located at different depths that influence the geometry and kinematics of the thrust system the main characteristic of the models presented here is the presence of two decollement horizons situated at the base and in an intermediate level of the models during deformation of the models these two decollement horizons generate two sets of structures with different geometrical characteristics and significance through the joint analysis of geophysical and geological data and model results wavelengths of the compressional structures analysed show that the structural style of the two analysed areas is almost similar moreover model results prove that the final configuration of the thrust system follows the general rules of evolution of a wedge deformed above a weak decollement and is largely governed by the larger deep seated structures

influence of syntectonic sedimentation and decollement rheology on the geometry and evolution of orogenic wedges analog modeling of the kuqa fold and thrust belt nw china

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

major temporal variations in shortening rate absorbed along a large active fold of the southeastern tianshan piedmont china the investigation of deformation rates on a mountain piedmont can provide key information for improving our understanding of the overall dynamics of a mountain range here we estimate the shortening rate absorbed by a quaternary emergent detachment fold on the southeastern piedmont of the tianshan china our work is primarily based on new 10be cosmogenic exposure dating of deformed alluvial surfaces the method we have developed combines depth profiling with sampling of surface cobbles thereby allowing exposure time erosion rate and inheritance to be simultaneously constrained the exposure ages of the uppermost uplifted alluvial surfaces are around 140 17 ka140 17 ka 130 9 ka130 9 ka and 47 9 ka47 9 ka from west to east a terrace lying below the 140 ka surface is dated at 65 5 ka65 5 ka the ages of the uplifted and folded alluvial surfaces were then combined with estimates of shortening obtained using two distinct methods 1 the excess area method where sedimentation rates extracted from magnetostratigraphic studies are used to determine the amount of sedimentation after the abandonment of the river and 2 a folding model derived from sandbox experiments the late pleistocene shortening rates are shown to be between 0 4 0 1 mm yr0 4 0 1 mm yr and 0 8 0 5 mm yr0 8 0 5 mm yr on the western part of the fold and 2 1 0 4 mm yr2 1 0 4 mm yr along its central part the central part of the frontal yakeng anticline therefore accommodates up to 25 of the total shortening currently absorbed across the whole eastern tianshan range 8 mm yr however this situation seems to have prevailed for only the last 150 ka as the shortening rate absorbed by this nascent fold was previously ten times slower while the initiation of folding of the yakeng anticline can be traced back to 5 5 ma ago the basinward migration of the active deformation front onto the yakeng fold is a relatively recent phenomenon and appears to be diachronous from west to east probably in relation to the tectonic activity of the folds in the hinterland

quantifying frictional variations and erosion in the mexican fold thrust belt abstract the impact of frictional crustal heterogeneities and surface processes on the evolution of the ancient thin skinned mexican fold and thrust belt mftb is investigated and quantified using numerical finite element simulations these simulations explore the evolution total shortening and structural styles of deforming orogenic wedges the models include spatial lateral variations in the internal friction and the topographic erodibility k using the power law incision rule the mftb exhibits paleogeographical elements manifested as lateral facies changes across two platforms units and two sedimentary basins that show different degrees of deformation the observed kinematics of the mftb requires based on the parameters considered in this study a rheologically heterogeneous crust with at least laterally varying internal friction yet only limited syntectonic and post tectonic erosion are necessary to explain the geometries and internal total shortening constrained by field observations and interpretations application of the analytical and homogeneous critical coulomb wedge ccw theory to the mftb closely predicts the external wedge geometry but fails to predict the observed shortening of the individual litho tectonic units and structural styles

evaluating the role of syn thrusting sedimentation and interaction with frictional detachment in the structural evolution of the sw tarim basin nw china insights from analogue modeling abstract the structural evolution of the fold and thrust belt in sw tarim basin nw china is poorly understood as the region is covered by thick pliocene quaternary conglomerate which significantly decreases the quality of seismic images we here improve the understanding of the thrust belt evolution with an investigation using analogue modeling on the deformation mechanism based on a case of real geological section in which we focus on the impact of syn thrusting sedimentation on the structural style and evolution the results show that syn thrusting sedimentation significantly increases the distance and rate of foreland propagation resulting in a much broader fold and thrust belt the modeling results also suggest that the fold and thrust belt exhibits different structural patterns under different syn thrusting sedimentary conditions in the case of thick syn thrusting sediments a frictional detachment layer of mudstone acts as decollement and the fold and thrust belt develops a duplex pattern in the hinterland in the case of thin syn thrusting deposits the belt develops a pop up structure with backthrusts in the foreland region which cut through the detachment layer the striking similarity between the model with thick syn thrusting sediments and the real geologic section implies that the overlying thick pliocene quaternary sediments play a major role in shaping the structure pattern in the fold and thrust belt of sw tarim basin

influence of synkinematic sedimentation in a thrust system with two decollement levels analogue modelling abstract compressive systems in foreland domains are characterised by fold and thrust belts linked to the presence of one or several ductile layers in depth acting as a decollement level the main parameters controlling the structural evolution are the presence of a decollement level the amount and rate of shortening and the amount of synkinematic sedimentation the effect of these parameters has only been studied on a thrust belt scale furthermore only the effect of synkinematic sedimentation on a simple system with one decollement level has been studied at the scale of a single structure the aim of this study was to use analogue modelling to test the effect of shortening rate velocity and the localization of sedimentation on a single system characterised by the presence of two prekinematic decollement levels the main results showed variations in the structural vergence folding geometry symmetric or asymmetric the evolution of the deformation horizontal propagation versus vertical uplift and the decoupling of the lower and upper brittle structures in relation with the main parameters shortening rate and mass transfer the results of the experiments were then compared to natural examples from the sub andean thrust belt

effect of the brittle ductile transition on the topography of compressive mountain belts on earth and venus the coulomb critical taper model has been very successful in explaining the large scale topography of a number of terrestrial accretionary wedges however this model is limited to cases of purely brittle frictional deformation in this paper we extend the range of applicability of the critical taper model by explicitly including the effects of temperature dependent ductile deformation the new model includes temperature dependent power law flow an assumed velocity field and linear thermal gradients in the atmosphere and within the crust flexural isostasy is also incorporated so that the decollement geometry is computed as a response to the applied load of the wedge material we assume that ductile deformation within the decollement zone is controlled primarily by diffusion flow whereas ductile deformation within the wedge itself is controlled by dislocation creep the topographic profiles predicted by the model are very similar to those of a number of fold and thrust belts on both earth and venus a typical wedge profile includes three distinctive topographic regions a narrow taper toe where both the wedge and the decollement zone deform in a brittle frictional manner a region of relatively steep slope where the wedge base deforms ductilely and the decollement zone is still frictional and a flat plateau region where both the wedge base and the decollement zone are deforming by ductile flow we have applied the model to two fold and thrust belts on venus maxwell montes and uorsar rupes and to the andes on earth and we find good agreement between observed and predicted topography using reasonable parameter values the model accounts for the observed positive correlation between relief and elevation of venusian fold and thrust belts on the basis of different thermal environments at different elevations it is also able to explain the first order differences between terrestrial and venusian fold and thrust belts fundamentally this difference is due to a combination of the lower temperatures and the presence of water on earth

origin of convex accretionary wedges evidence from barbados the surface slopes of submarine accretionary wedges generally decrease away from the toe or defor mation front toward the arc this paper presents evidence that this characteristic convex cross sectional shape is a direct mechanical response to the lithification of sediments during accretion the recently accreted sediments near the toe which are typically unconsolidated and therefore very weak must deform until a critical taper is attained at which they are stable in the presence of the horizontal tectonic compression this critical wedge taper and thus the bathymetric slope of the accretionary prism is relatively high for the porous weak sediments near the toe in contrast the compacted sediments farther back in the wedge are stronger so they require a relatively modest critical taper and therefore a lower bathymetric slope to be stable under the same tectonic compression the result is a convex cross sectional wedge shape we investigate several possible causes of the increase in sediment strength within the wedge and conclude that the primary cause is an increase in cohesive strength with decreasing porosity a straightforward generalization of the exact critical taper model is combined with empirical cohesion porosity velocity relations to infer the distribution of porosity and p wave velocity within the barbados accretionary wedge we estimate that the porosity decreases from its near toe value of 70 to approximately 30 at a depth of 3 km below the seafloor and a distance of 60 km arcward from the front of the wedge

thrust wedges with decollement levels and syntectonic erosion a view from analog models abstract analog sandbox models have been set up to study the impact of syntectonic erosion on thrust wedges with one and two decollement levels different accretion mechanisms are activated depending on interactions between surface processes and wedge mechanics frontal accretion backthrusting underthrusting and underplating due to decollement induced duplex formation at depth these mechanisms may function simultaneously being located at different parts across the wedge for all the experiments a high friction is imposed at the base of models and the volume of eroded material remains equal to the volume of newly accreted material maintaining a constant surface slope during the shortening erosion limits the forward propagation of thrust wedges and favors the underthrusting of basal layers allowing duplex formation erosion promotes development of major backthrusts in the thrust wedges without or with one decollement but no backthrusts was formed in the wedges with two decollements slow erosion allows lower extent of basal underthrusting in comparison with regular rate erosion variations in the erosion taper lead to changes in duplex geometry and exhumation rate in thrust wedges with one or two decollements the 6 erosion taper promotes formation of antiformal stack at the rear part of thrust wedge high rate of basal underthrusting and high extent of erosional removal the cover layers are nearly completely eroded above the antiformal stack and form the synformal klippe in frontal part of thrust wedges the 8 erosion taper allows development of individual ramp anticlines and active forward thrusting of cover layers above the decollement and low rate of basal underplating below it with consequent low extent of erosional removal the results of our experiments support the observations on structural evolution and erosion in the alberta foothills of the canadian rockies

a synthetic stratigraphic model of foreland basin development the processes of erosion and deposition must be included in foreland basin models to predict correctly basin geometry and stratigraphy we present a synthetic stratigraphic model of the development of nonmarine foreland basins that predicts progressive geometry topography and facies patterns in the model steady crustal shortening occurs according to a wedgethickening model erosion and deposition follow a diffusive process and the lithosphere is compensated elastically erosion and deposition are controlled by the transport coefficients r of the diffusion equation for a range of thrust velocities and lithospheric rigidities transport coefficients are of order 102 103 m yr in the mountain belt these values are much higher than those derived from the study of scarp degradation in the sedimentary basin transport coefficients of order 104 m2 yr are appropriate and are compatible with previous studies of fluvial and deltaic deposition rapid thrusting results in a narrow underfilled basin while slow thrusting results in a wide overfilled basin in addition by varying the erosional and depositional transport coefficients while holding other parameters constant we generate both overfilled and underfilled basins these results suggest that changes in the rate of thrust loading the climate or the source rock lithology can create stratigraphic signatures that have been interpreted to record viscoelastic relaxation of the lithosphere clearly to understand either the long term behavior of the lithosphere or to interpret orogenic history from preserved foreland strata the manner in which a basin was filled must be considered we apply the model to the evolution of the modern sub andean foreland and find that an erosional transport coefficient of 3000 m2 yr and a depositional transport coefficient of 20 000 m2 yr successfully predict the observed basin geometry

on the number and spacing of faults orogens and rift zones have a finite number of regional faults the accretionary prisms analysed here have a number of thrusts 50 whereas extensional areas have a number of normal faults ranging between six and 44 the average spacing of thrusts is between 5 and 25 km spacing of normal faults is more restricted into two peaks at 25 29 km and 4 6 km in which the latter is the most common the number and spacing of faults appear to be mainly controlled by the depth of the decollement plane which seems to be more variable in compressive settings with respect to rift zones basementinvolved orogens present fewer and more spaced thrusts by contrast a greater number of thrusts with shorter spacing characterize thin skinned thrust belts the shallower the decollement is the stronger it appears to control the palaeogeography in the sense of rheological lateral variations in the sedimentary cover

geophysical survey of a mud volcano seaward of the barbados ridge accretionary complex a detailed geophysical investigation that included multibeam bathymetry 3 5 khz channel and single channel seismic profiles 20 heat flow measurements and four piston cores was made of a mud volcano located about 8 km seaward of the toe of the barbados ridge accretionary complex in the northern guiana basin the material erupted from the volcano covers an oval area about 3 9 km with its long axis oriented e w roughly perpendicular to the deformation front a flat topped mound on the western side of the feature is the currently active center acoustic stratigraphy suggests that mud volcanism began at the site about 200 000 ma a piston core taken on the flank of the active mound sampled deposits erupted 20 000 30 000 ma that contain late miocene nannofossils heat flow on the active mound is 4 times that in the surrounding basin one dimensional modeling of this anomaly supports a low level continuing flux of mud and water at rates of 2 5 cm yr the mud volcanism appears to have arisen from the combined action of overpressured water that comes from beneath the accretionary prism and existing overpressures in the oceanic section large east west trending basement escarpments underlie the two known mud volcanoes seaward of the barbados ridge accretionary complex and they probably play an important role in developing vertical pathways for fluid to escape and in determining the location where mud volcanism starts

thrust mechanics in highly overpressured accretionary wedges rapid tectonic loading of wet sediment in accretionary wedges is likely to cause the fluid pressure to rise until it is sufficient to cause dilatant fracturing dewatering of sediment that has been underthrust and accreted beneath the wedge can produce a large steady supply of such highly overpressured fluid dilatant fracturing will create escape routes so the fluid pressure is likely to be buffered at the value required for the transition between shear and oblique tensile dilatant fracture which is slightly in excess of the load pressure if the maximum compression is nearly horizontal this in turn buffers the strength of the wedge at the cohesive strength which is not pressure dependent and will not vary greatly throughout the wedge near the wedge front the strength is likely to be that of the cohesion on existing thrust faults in the wedge the shear resistance on the base of the wedge will also be fairly constant and related to the cohesive strength of the weak sediment layer that acts as the basal detachment these assumptions allow the application of a simple plastic continuum model which successfully predicts the observed gently convex taper of accretionary wedges they also allow the use of a mechanical model for frontal imbrication which predicts the spacing of imbricate thrusts at the front of the wedge from the strength and thickness of the imbricated layer the ramp angle and the factional resistances on the bounding faults and decollement surfaces reconstruction of thrust geometries from seismic data across the makran and other accretionary wedges allows the determination of the geometrical parameters and the thrust imbrication model allows the mechanical parameters to be calculated in terms of an assumed cohesive strength for the incoming sediment layer the calculated values are reasonable and successful forward modelling of the profile of the makran shows that the postulate of a buffered value of fluid pressure in excess of the load pressure is consistent with observation

orogeny and stratigraphy numerical models of the paleozoic in the eastern interior of north america a computer model is used to estimate patterns of lithospheric deformation caused by obduction and removal of loads within orogenic belts regions of lithospheric downwarp become sedimentary depocenters and regions of lithospheric upwarp suffer erosion by following the deposition and erosion of sedimentary packages through a number of model time steps it is possible to calculate a synthetic stratigraphy for a given study area this model is applied to the post early ordovician stratigraphy of the eastern interior region of north america the loading that drives lithospheric deformation is associated with the development of the appalachian and ouachita orogens and the independent development of the michigan and illinois intracratonic basins model results are shown to be consistent with presently observed stratigraphy lithology and cumulative erosion and are then used to estimate the configuration of given sediment packages at discrete intervals following their deposition these estimates are presented in the form of isopach maps stratigraphic cross sections and time stratigraphic diagrams as a function of time over the last 470 ma by tracking the burial and erosion history of sedimentary units we are able to establish a cause and effect relationship between particular tectonic events and the extent of preservation of sedimentary units in particular we suggest that the pennsylvanian permian alleghanian orogeny produced major sediment cover over much of the study area

sedimentary basins formed and carried piggyback on active thrust sheets small sedimentary basins that formed on the apennine southern margin of the po basin complex northern italy have been studied using borehole and seismic reflection information and their evolution can be dated using the largely marine biostratigraphy of the miocene to quaternary sediments these studies show that the basins formed and were filled while being carried on moving thrust sheets and they are therefore named piggyback basins each of these basins corresponds to the active phase of a different thrust front ramp on the pyrenean northern margin of the ebro basin complex northern spain similar piggyback basins formed between eocene and mid miocene time they have since been uplifted and eroded and may therefore be studied in outcrop in northern spain much of the basin fill sediment was deposited in nonmarine environments

fold thrust structures where have all the buckles gone special publication title folding and fracturing of rocks 50 years of research since the seminal text book of j g ramsay we dedicate the paper to the memory of martin casey 1948 2008 who did much through good humored argument to ensure that buckling ideas were not lost to what he called the ramping club the thrust belt community the fold thrust research group has been funded by interoil oilsearch and santos we thank paul griffiths and anonymous referee for comments together with hermann lebit for scientific editing the views expressed here of course remain those of the authors

seismic stratigraphy of a neogene piggyback basin argentina depositional sequences composed of neogene nonmarine strata are imaged on seismic reflection data from the iglesia piggyback basin argentina the strata accumulated in an internally drained basin located between the frontal cordillera the high andes and the precordillera thrust belt two reflection geometries onlap and truncation are observed along four unconformities within the neogene section onlap of reflections onto unconformities defines five distinct intervals of basin growth local truncation of reflections beneath unconformities defines four intervening intervals of nondeposition or erosion along strike variation in the amount of truncation suggests that erosion was limited to zones through which major streams entered the basin postdepositional thrusting o the east side of the basin produced field exposures of the five sequences and allowed examination of the nature of the sequence boundaries the iglesia depositional sequences are interpreted to result from neogene crustal shortening within the central andes the iglesia basin has been passively transported above a horizontal decollement that links mid crustal deformation beneath the frontal cordillera with surface thrusting in the precordillera deposition resulting generally from simultaneous uplift of the frontal cordillera and the precordillera can be explained by two scenarios thrust controlled accumulation where space for deposition was created only during faulting or drainage controlled accumulation where deposition occurred only when thrusting cut off through going drainages linking the iglesia basin with the foreland

isolating active orogenic wedge deformation in the southern subandes of bolivia a new gps derived surface velocity field for the central andean backarc permits an assessment of orogenic wedge deformation across the southern subandes of bolivia where recent studies suggest that great earthquakes mw 8 are possible we find that the backarc is not isolated from the main plate boundary seismic cycle rather signals from subduction zone earthquakes contaminate the velocity field at distances greater than 800 km from the chile trench two new wedge crossing velocity profiles corrected for seasonal and earthquake affects reveal distinct regions that reflect 1 locking of the main plate boundary across the high andes 2 the location of and loading rate at the back of orogenic wedge and 3 an east flank velocity gradient indicative of decollement locking beneath the subandes modeling of the subandean portions of the profiles indicates along strike variations in the decollement locked width wl and wedge loading rate the northern wedge decollement has a wl of 100 km while accumulating slip at a rate of 14 mm yr whereas the southern wedge has a wl of 61 km and a slip rate of 7 mm yr when compared to quaternary estimates of geologic shortening and evidence for holocene internal wedge deformation the new gps derived wedge loading rates may indicate that the southern wedge is experiencing a phase of thickening via reactivation of preexisting internal structures in contrast we suspect that the northern wedge is undergoing an accretion or widening phase primarily via slip on relatively young thrust front faults

syn kinematic strata influence the structural evolution of emergent fold thrust belts field research in the salt range was originally supported by historical 1980s research grants from the uk s natural environment research council and the royal society recent research on thrust systems is funded through the fold thrust research group supported by interoil oilsearch and santos from hammerstein j a di cuia r cottam m a zamora g butler r w h eds fold and thrust belts structural style evolution and exploration geological society london special publications 490

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

kinematic evolution of thrusts wedge and erratic line length balancing insights from deformed sandbox models kinematic evolution of fold thrust structures has been investigated by analogue models that include syntectonic sedimentation different decollement dips and basement thicknesses produced different wedge geometries and propagating characteristics a model with one decollement level was characterized by a closely spaced thrust system during early stages of shortening as compared to the late stages the frequency of fault nucleation was rapid during the early stages of deformation conversely the frequency of fault nucleation was low and thrust spacing was significantly wider in a model with two decollement levels individual faults became locked at steep dips and deformation stepped forward as a new fault nucleated in sequence in front of the older locked structure once the thrust system was established up to 27 overall shortening an overlying bed was introduced to simulate syntectonic deformation model sand wedge did not grow self similarly but rather its length and height increased episodically with deformation restoration of deformed models show that layer parallel shortening accommodated for approximately half of the total model shortening across the multilayers calculated error in apparent layer shortening from the restored layers revealed a direct relation with depth of the layers in the models the experimental results are comparable to a natural example from the northern apennines fold and thrust belts

distribution of active faulting along orogenic wedges minimum work models and natural analogue abstract numerical 2 d models based on the principle of minimum work were used to examine the space time distribution of active faulting during the evolution of orogenic wedges a series of models focused on thin skinned thrusting illustrates the effects of arid conditions no erosion unsteady state conditions accretionary influx greater than erosional efflux and steady state conditions accretionary influx balances erosional efflux on the distribution of fault activity for arid settings a general forward accretion sequence prevails although a significant amount of internal deformation is registered the resulting fault pattern is a rather uniform spread along the profile under fixed erosional efficiency settings the frontal advance of the wedge front is inhibited reaching a steady state after a given forward propagation then the applied shortening is consumed by surface ruptures over a narrow frontal zone under a temporal increase in erosional efficiency i e transient non steady state mass balance conditions a narrowing of the synthetic wedge results a rather diffuse fault activity distribution is observed during the deformation front retreat once steady balanced conditions are reached a single long lived deformation front prevails fault activity distribution produced during the deformation front retreat of the latter scenario compares well with the structural evolution and hinterlandward deformation migration identified in southern bolivian subandes ssa from late miocene to present this analogy supports the notion that the ssa is not in steady state but is rather responding to an erosional efficiency increase since late miocene the results shed light on the impact of different mass balance conditions on the vastly different kinematics found in mountain ranges suggesting that those affected by growing erosion under a transient unbalanced mass flux condition tend to distribute deformation along both frontal and internal faults while others under balanced conditions would display focused deformation on a limited number of steady structures

geometry and kinematic evolution of passive roof duplexes deduced from cross section balancing example from the foreland thrust system of the southern bolivian subandean zone the subandean zone of bolivia is a foreland fold and thrust belt which forms the eastern edge of the central andes mountains between 19 s and 22 s latitude the construction of five balanced cross sections shows that the n s trending subandean zone is characterized by the existence of passive roof duplexes these complex structures can be distinguished by the lithotectonic unit within which duplexing occurs the five balanced cross sections permit the geometric and kinematic analyses of these passive roof duplexes the sequential restorations of certain cross sections reveal a possible development of a piggy back sequence of three passive roof duplexes apparently these passive roof duplexes propagated toward the foreland from deeper and deeper lithotectonic units while a passive roof duplex was developing the sole thrust stuck and the major horizontal displacements were then transferred either to out of sequence thrusts or to a new sole thrust in a deeper detachment horizon therefore each passive roof duplex would correspond to the orogenic front of the andean range at one very particular time in the history of the subandean zone of southern bolivia from south to north the quantitative analysis by cross section balancing shows a transfer of displacement from the hinterland structures to the passive roof duplex that forms the present orogenic front available data do not permit us to explain completely this phenomenom

neogene shortening contribution to crustal thickening in the back arc of the central andes to illustrate the neogene shortening distribution in the back arc units of the central andes and to estimate the contribution of the shortening to crustal thickening two balanced crustal cross sections have been constructed across the northern and southern branches of the bolivian orocline total neogene shortening which varies from 191 to 231 km is accommodated by a crustal duplex below the cordillera oriental but is insufficient to produce the 70 km of crustal thickness evidenced by geophysical data below the altiplano the best explanation for this anomalous thickening seems to be crustal underplating by material tectonically eroded from the continental margin this process probably caused the altiplano uplift the subduction of oceanic lithosphere coupled with this underplating and a brief episode of gravity spreading of the altiplano constituted the driving forces that produced neogene shortening and development of the central andes

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