balanced section in thrust belts part 1 construction balanced geological cross sections are an important aid to understanding thrust belt structures and to estimating their hydrocarbon bearing potential two approaches to section balancing have been taken in the past construction of retrodeformable sections from raw data and modification of previous interpretations after an evaluation procedure these approaches may be unified by use of a langrangian grid projection of datum locations dips and stratigraphic thicknesses onto the section plane is performed directly by calculation or graphically using plunge lines spline interpolation and parallel fold modeling are the simplest ways of filling gaps in the sectional data but isogon interpolation yields rheologically more realistic results faults are interpolated using cutoff geometries as constraints fault tips are located from distance displacement plots whereas the depth to detachment is obtained by a modified chamberlin construction interpolations made on the section are directed back to the geologic map via plunge lines although these techniques of section construction do not guarantee a balance especially when complexities such as growth structures diapirs or strike slip faults are present they eliminate many potential errors

modelling growth stratal architectures associated with double edge fault propagation folding abstract fault related folds in fold and thrust belts and accretionary prisms are commonly simulated by three end member classes of geometric models decollement folding fault propagation folding and fault bend folding a given model produces a fold shape that depends on the amount of shortening the fault shape and eventually the fault propagation history the great variety of geometric solutions implies that the final fold shape is adequate to distinguish among different folding mechanisms only in a limited suite of cases additional information to better constrain fold kinematics is provided by the analysis of growth strata patterns which passively register the progressive evolution of deformation in the substratum availability of growth strata pattern templates associated with different geometric and kinematic solutions provides a useful tool for inferring the dominant kinematic pathways of natural thrust related structures in this paper we present growth strata patterns associated with double edge fault propagation folding the influence of the sedimentation rate the fault shape and the fault s p ratio i e slip versus ramp propagation rate on growth strata architectures are investigated geometrical modelling and application to natural examples indicate that double edge fault propagation anticlines are characterised by growth triangles on both limbs their position and geometry relate to the ramp shape and propagation history and on the sedimentation rate

fault reactivation and structural inversion backarc and intraplate compressive deformations example of the eastern sunda shelf indonesia the tectonic evolution of the eastern sunda platform in indonesia is revealed by the study of seismic profiles and comprises three general tectonic periods during the cenozoic extension with graben infill from the paleogene to the early miocene a tectonically quiet period with only broad subsidence and a middle miocene recent period of folding inversion and thrusting these structures reveal changes in the intraplate stress field which was controlled by the changes in the boundary conditions of the plates collisions on the northern and eastern sides and interplate coupling southward along the java subduction zone inversions often occurred without reactivation of pre existing normal fault planes at the upper levels of these faults the factors controlling these deformations are discussed with special interest placed on the conditions for the potential reactivation of cohesionless normal faults inversion and reactivations were mostly controlled by the orientation of the pre existing fault planes in relation with the regional stress field and by the tectonic stress regime in a strike slip regime effective principal stresses 1 and 3 horizontal with 1 2 3 the optimum conditions for reactivation in isotropic rocks are pre existing faults that strike close to the potential two vertical conjugate failure planes the same mechanical properties as for pre existing fault prevail in such conditions strike slip movements and reverse strike slip movements and associated en echelon folds were generally observed in a compressive regime 1 and 2 horizontal with 1 2 3 typical steeply dipping normal faults are difficult to reactivate by slip movements we suggest that reverse slip movements occurred in the deeper levels of faults and were accommodated by flexure above the pre existing fault planes and the inversion of grabens the strongly inverted grabens are those whose orientation is perpendicular to the regional compression nevertheless with strong regional shortening large uplifts and thrust faults were observed with reactivation in the form of reverse or reverse strike slip movements along pre existing steeply dipping normal fault planes

conical drag folds as kinematic indicators for strike slip fault motion abstract conical drag folds are developed in the vicinity of faults where rock sequences with significantly different mechanical properties are subjected to oblique shear the drag folds formed by passive rotation of a competent bed surrounded by flowing incompetent media mesoscale conical drag folds can serve as kinematic indicators the conical fold axis coincides with the rotation axis of slip and the angle between the small circle of poles to bedding and the conical axis is equal to the angle between the pole to tilted bedding and the rotation axis of slip the sense of asymmetry of the drag fold reflects the sense of shear the relationship between conical folding and sense of shear established for the mesoscale can be applied to larger fault zones for example the geometry of conical drag folds along with other fault related structures reveal sinistral strike slip motion in the mount arif area of the negev

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

kinematics of fault related folding derived from a sandbox experiment we analyze the kinematics of fault tip folding at the front of a fold and thrust wedge using a sandbox experiment the analog model consists of sand layers intercalated with low friction glass bead layers deposited in a glass sided experimental device and with a total thickness h 4 8 cm a computerized mobile backstop induces progressive horizontal shortening of the sand layers and therefore thrust fault propagation active deformation at the tip of the forward propagating basal decollement is monitored along the cross section with a high resolution ccd camera and the displacement field between pairs of images is measured from the optical flow technique in the early stage when cumulative shortening is less than about h 10 slip along the decollement tapers gradually to zero and the displacement gradient is absorbed by distributed deformation of the overlying medium in this stage of detachment tip folding horizontal displacements decrease linearly with distance toward the foreland vertical displacements reflect a nearly symmetrical mode of folding with displacements varying linearly between relatively well defined axial surfaces when the cumulative slip on the decollement exceeds about h 10 deformation tends to localize on a few discrete shear bands at the front of the system until shortening exceeds h 8 and deformation gets fully localized on a single emergent frontal ramp the fault geometry subsequently evolves to a sigmoid shape and the hanging wall deforms by simple shear as it overthrusts the flat ramp system as long as strain localization is not fully established the sand layers experience a combination of thickening and horizontal shortening which induces gradual limb rotation the observed kinematics can be reduced to simple analytical expressions that can be used to restore fault tip folds relate finite deformation to incremental folding and derive shortening rates from deformed geomorphic markers or growth strata

bed by bed fold growth by kink band migration sant llorenc de morunys eastern pyrenees ahstract rowth strata deposited over and against the flank of the sant llorenc de morunys fold during its final stages of deformation have been mapped at high resolution as the basis for unraveling the kinematics of fold growth we use restoration techniques based on normal balancing assumptions to decipher the detailed kinematic history of folding the progressive restorations as well as balanced forward modeling show that the last few hundred meters of fold growth were dominated by kink band migration of a sort that is typical of much faultrelated folding the kink band migration has produced complex anticlinal hinge zone geometry including segmented fold hinges linked by disconformities and unconformities which has direct and detailed explanation in terms of fluctuations in deposition rate relative to curved hinge kink band migration rate large fluctuations in the convolution of non steady sedimentation and deformation are demonstrated although the absolute fluctuations in deformation and sedimentation are unknown at a length scale of 100 m kink band migration with little or no deposition is interspersed with sedimentation with little or no deformation at the length scale of 500 m deposition ranges from 200 to 50 of uplift 0 1997 elsevier science ltd all rights reserved

strains developed in the hangingwalls of thrusts due to their slip propagation rate a dislocation model reply abstract fault planes propagate radially from a point source and this failure propagation process is very similar to the movement of a dislocation through a crystal lattice an elastic strain represented by an extra half plane is necessarily imparted to the lattice at an edge dislocation and this is equivalent to the ductile bead which accompanies the propagating tip of a thrust fault this ductile bead migrates with the fault tip and imparts a characteristic internal strain to the thrust sheet a two dimensional model is presented to illustrate the inter relationship between fault plane slip fault tip propagation and internal strain in multilayered sequences internal strain is usually represented by asymmetric folds verging in the thrust transport direction a simple technique the displacement distance plot utilizes the fact that displacement dies out towards the fault tip this plot can be used to quantify the relative stretch measured parallel to the fault movement which is dependent on the slip propagation rate and it may be used to define exactly the position of the fault tip on a cross section examples of fold thrust structures from devon england and pembrokeshire wales are used to illustrate the technique

a unified kinematic model for the evolution of detachment folds abstract detachment folds represent a major structural element in a number of fold belts they are common in the jura mountains the zagros fold belt the central appalachian fold belt the wyoming fold belt the brooks range the parry islands fold belt and parts of the subandean belt these structures form in stratigraphic packages with high competency contrasts among units the competent upper units exhibit parallel fold geometries whereas the weak lower unit displays disharmonic folding and significant penetrative deformation two distinct geometric types disharmonic detachment folds and lift off folds have been recognized however these structures commonly represent different stages in the progressive evolution of detachment folds the structures first form by symmetric or asymmetric folding with the fold wavelength controlled by the thickness of the dominant units volumetric constraints require sinking of units in the synclines and movement of the ductile unit from the synclines to the anticlines continuing deformation results in increasing fold amplitudes and tighter geometries resulting from both limb segment rotation and hinge migration initially limb rotation occurs primarily by flexural slip folding but in the late stages of deformation the rotation may involve significant internal deformation of units between locked hinges the folds eventually assume tight isoclinal geometries resembling lift off folds variations in the geometry of detachment fold geometry such as fold asymmetry significant faulting and fold associated with multiple detachments are related to variations in the mechanical stratigraphy and pre existing structure

three dimensional retro deformation of the lechtal nappe northern calcareous alps we determine the amount of shortening depth of detachment and the style of deformation of the lechtal nappe the largest nappe within the northern calcareous alps german austrian border by three dimensional retro deformation analysis of ca 40 km2 area a three dimensional model was constructed by splining lines from eight n s cross sections spaced e w at ca 4 km intervals to define fault geometrics and seven stratigraphic layers within the permo triassic to cretaceous strata the modelled volume has four main thrusts which link to a common detachment at 2 5 km depth three dimensional fault displacements and heaves were determined using allan diagrams the algorithms fault parallel flow and flexual slip unfolding were used to restore northwards movement on the thrusts and folding of beds over thrust planes respectively minimum shortening estimates vary from east to west from 25 to 42 typical error 6 but additional shortening in the west is due only to folding

progressive evolution of a fault related fold pair from growth strata geometries sant llorenc de morunys se pyrenees abstract new structural stratigraphical mapping constrains the three dimensional kinematics and mechanisms of eocene oligocene growth folding at sant llorenc de morunys ne ebro basin spain a 1 km wide sub vertical panel of syntectonic alluvial gravels passes southwards via a highly asymmetrical growth fold pair to shallowlydipping strata the axial surface of the anticline comprises either continuous or en echelon segments while that of the syncline is concave and usually continuous while converging upwards the axial surfaces do not define growth triangles principal and subsidiary growth unconformities and thickness changes occur across both axial surfaces and the common limb dips within the common limb decrease up stratigraphy and up dip mesostructures indicate that internal deformation was ongoing during folding at all stratigraphical levels and concentration of cleavage in the syncline indicates that this hinge was essentially fixed sequential restoration of three profiles shows that folds amplified principally by limb rotation but incorporated minor passive hinge migration particle movement vectors generated by section restoration are arcuate about a hinterland pinpoint a new trishear model of fault propagation folding involving non rigid limb rotation reproduces the rounded hinge forms thickening geometries and limb dip variations observed simple kink band migration models fixed axis and constant thickness theories do not replicate these features

numerical modeling of trishear fault propagation folding in contrast to kink band migration modeling methods trishear numerical models produce fault propagation folds with smooth profiles and rounded hinges modeled fold hinges tighten and converge downward within a triangular zone of distributed deformation which is focused on the fault tip such features have been reported from field studies and are also seen in analogue models of compressional deformation however apart from its initial application to laramide folds little quantitative work has been undertaken on trishear fault propagation folding in other settings in addition no study has been undertaken into the growth strata which might be associated with such structures this paper uses an equivalent velocity description of the geometric model of trishear together with models of erosion and sedimentation to investigate trishear fault propagation folding of both pregrowth and growth strata the trishear model is generalized to include a variety of fault propagation to slip ratios and fault propagation from a flat decollement the models show continuous rotation of the forelimb with the characteristic development of cumulative wedges within growth strata when total slip on a structure is high the model predicts overturned pregrowth and growth strata during the initial stages of deformation beds in the forelimb thicken but later thin when they become steep or overturned the effect of variations in fault propagation to slip ratios on two dimensional finite strain in the models is assessed by the use of initially circular strain markers high fault propagation to slip p s ratios lead to narrow zones of high finite strain while lower p s ratios lead to more ductile deformation and broader zones of high strain in all cases hanging wall anticlines and footwall synclines originate as early ductile folds which are later cut by the propagating fault modeled structures are compared with natural examples

an elusive blind thrust fault beneath metropolitan los angeles seismic reflection profiles petroleum wells and relocated earthquakes reveal the presence of an active blind thrust fault beneath metropolitan los angeles a segment of this fault likely caused the 1987 whittier narrows magnitude 6 0 earthquake mapped sizes of other fault segments suggest that the system is capable of much larger magnitude 6 5 to 7 and more destructive earthquakes

investigating fault shadows in the niger delta the stratigraphic evolution of the niger delta is dominated by a continuous stepwise progradation occurring during a major tertiary regressive phase that continues to the present the successive phases of the delta growth form transient depobelts that are bounded by mega structural growth faults

thrust surface geometry implications for thrust belt evolution and section balancing techniques abstract the application of modern concepts of thrust tectonics to orogenic belts has yielded many detailed restorable sections thrusts generally restore with a staircase geometry onto the undeformed stratigraphic template since the work of rich 1934 it has been generally assumed that thrusts have propagated with a staircase trajectory and that hangingwall structures result from the movement of the thrust sheet up footwall ramps however thrusts often cut through previously folded strata with a smooth trajectory smooth trajectory thrusts can be identified from footwall geometry and strain state once a smooth trajectory thrust has been identified it is insufficient to produce a balanced deformed section and its restored counterpart sequential balanced sections must be constructed reversing the deformation history the restoration of thrusts to a staircase trajectory has no mechanical significance as the thrust is merely acting as a marker if restored back beyond the time at which it propagated thrust ramps are not necessarily actual steps in the thrust surface they are often just zones where the thrust cuts through bedding in a smoothly curving trajectory ductile deformation in thrust belts can reduce bed length at depth thus removing the need for structures which are necessary to preserve constant bed lengths ductile deformation in the internal zones of orogenic belts renders the construction of balanced sections in these zones highly problematical

different modes of the late cretaceous early tertiary inversion in the north german and polish basins several selected seismic lines are used to show and compare the modes of late cretaceous early tertiary inversion within the north german and polish basins these seismic data illustrate an important difference in the allocation of major zones of basement thick skinned deformation and maximum uplift within both basins the most important inversion related uplift of the polish basin was localised in its axial part the mid polish trough whereas the basement in the axial part of the north german basin remained virtually flat the latter was uplifted along the sw and to a smaller degree the ne margins of the north german basin presently defined by the elbe fault system and the grimmen high respectively the different location of the basement inversion and uplift within the north german and polish basins is interpreted to reflect the position of major zones of crustal weakness represented by the wnw ese trending elbe fault system and by the nw se striking teisseyre tornquist zone the latter underlying the mid polish trough therefore the inversion of the polish and north german basins demonstrates the significance of an inherited basement structure regardless of its relationship to the position of the basin axis the inversion of the mid polish trough was connected with the reactivation of normal basement fault zones responsible for its permo mesozoic subsidence these faults zones inverted as reverse faults facilitated the uplift of the mid polish trough in the order of 1 3 km in contrast inversion of the north german basin rarely re used structures active during its subsidence basement inversion and uplift in the range of 3 4 km was focused at the elbe fault system which has remained quiescent in the triassic and jurassic but reproduced the direction of an earlier variscan structural grain in contrast n s oriented mesozoic grabens and troughs in the central part of the north german basin avoided significant inversion as they were oriented parallel to the direction of the inferred late cretaceous early tertiary compression the comparison of the north german and polish basins shows that inversion structures can follow an earlier subsidence pattern only under a favourable orientation of the stress field a thick zechstein salt layer in the central parts of the north german basin and the mid polish trough caused mechanical decoupling between the sub salt basement and the supra salt sedimentary cover resultant thin skinned inversion was manifested by the formation of various structures developed entirely in the supra salt mesozoic cenozoic succession the zechstein salt provided a mechanical buffer accommodating compressional stress and responding to the inversion through salt mobilisation and redistribution only in parts of the ngb and mpt characterised by either thin or missing zechstein evaporites thick skinned inversion directly controlled inversion related deformations of the sedimentary cover inversion of the permo mesozoic fill within the mid polish trough was achieved by a regional elevation above uplifted basement blocks conversely in the north german basin horizontal stress must have been transferred into the salt cover across the basin from its sw margin towards the basins centre this must be the case since compressional deformations are concentrated mostly above the salt and no significant inversion related basement faults are seismically detected apart from the basin margins this strain decoupling in the interior of the north german basin was enhanced by the presence of the elbe fault system which allowed strain localization in the basin floor due to its orientation perpendicular to the inferred late cretaceous early tertiary far field compression

coexistence of fault propagation and fault bend folding in curve shaped foreland fold and thrust belts examples from the northern apennines italy terra nova 24 396 406 2012 abstract fault bend and fault propagation folds have unique profiles in foreland fold and thrust belts that are caused by different fault related folding models and controlled by the mechanical characteristic of the multilayer and or by reactivation of normal faults in a positive inversion tectonics context in this work nne ssw and nw se trending anticlines related to the neogene curve shaped olevano antrodoco sibillini thrust northern apennines italy are investigated to reconstruct fault related folding mechanisms geological and structural analysis allows us to interpret the nne ssw trending anticlines as fault bend reactivation folds and the nw se trending anticlines as fault propagation shortcut anticlines the coexistence of fault bend and fault propagation folding processes involving the same multi layered succession in different arms of a curve shaped thrust is explained in an inversion tectonics context this along strike variation of different folding mechanisms might be recognised in similar curve shaped orogenic thrust belts controlled by structural inheritance

a kinematic model for the origin of footwall synclines abstract the origin of footwall synclines at the base of thrust ramps can be related to the kinematic evolution of fault propagation folds the conventional fault propagation fold model does not predict the development of footwall synclines because it assumes a high propagation to slip ratio for development of the lower flat which does not allow for folding before the thrust cuts up section an alternative model where the propagation to slip ratio remains finite throughout the development of the fault propagation fold produces a detachment fold at the thrust tip during propagation of the thrust along the lower flat if the thrust cuts up through the ductile core of the detachment fold during ramp formation it will leave behind the forelimb of the detachment fold as part of a footwall syncline

cover deformation above steep basement normal faults insights from 2d discrete element modeling abstract a discrete element model is used to investigate progressive cover deformation above a steep 70 basement normal fault the cover materials are homogenous with frictional material behavior in the model shown here both normal and reverse faults in the cover accommodate displacement on the underlying basement fault the earliest faults are curved reverse faults which propagate upwards from the basement fault tip into the proto hanging wall these are replaced progressively towards the footwall by subvertical to steep normal faults and finally by a normal fault which dips at an angle predicted by mohr coulomb theory thus most early secondary structures are located in the hanging wall of the final through going fault this structural evolution produces an asymmetric triangular zone of deformation above the basement fault tip which superficially resembles that associated with trishear however its progressive development is quite different results also emphasize that the occurrence of reverse faults in extensional settings is not diagnostic of inversion

seismic expression and kinematics of a fault related fold termination rosario structure maracaibo basin venezuela abstract fold terminations are primary features of deformed belts and are critical elements in understanding the 3 d kinematics of fault related folds typically such terminations form due to loss of displacement on the genetically related thrust fault and or to along strike change in fault attitude forming a lateral or oblique ramp to unambiguously determine the mechanism responsible for a given fault related fold termination footwall cutoffs and along strike displacement variation must be constrained reflection seismic data can provide the detail necessary to uniquely describe these features the rosario structure in the maracaibo basin venezuela is an example of a natural fault related fold that possesses a southern termination whose fold fault geometry and along strike displacement variation are constrained by industry reflection seismic and well data we interpret that the fold plunge near the southern termination is due to an along strike decrease in displacement the fault geometry associated with the southern termination changes from a flat ramp flat at the crest of the structure where displacement is greatest to simply a ramp near the lateral fault tip without forming a lateral or oblique ramp we combine these observations regarding displacement and fault geometry with the assumption that along strike spatial variations reflect temporal variations during development of a fault related fold to propose a kinematic model for the rosario structure specifically we suggest that the structure first initiated as an isolated fault ramp within the stiff carbonate and clastic units with increased shortening the fault grew to link with upper and lower detachments in the weaker shale units to create a hybridized fault bend fold our model suggests a possible explanation for the evolution of the rosario structure and also provides an alternative to existing pseudo three dimensional models for fault related fold growth that relaxes the rigidity in the assumption of along strike self similarity

influence of detachment layer thickness on style of thin skinned shortening abstract there are several parameters which may determine whether detachment folds or thrust faults develop in thin skinned compressional systems a fundamental but little publicised parameter is apparent from consideration of detachment fold growth as a detachment fold amplifies its core must be filled by redistribution of ductile material if the mobile material were insufficient to supply the volume required by the fold core through redistribution fold growth would be inhibited and eventually faulting would occur the volume of a detachment fold core is compared here with the volume of available underlying mobile material for various amounts of shortening and sizes of fold the occurrence of detachment folds versus thrusts in several compressional belts is successfully predicted by this relationship

a balanced cross section of the 1994 northridge earthquake southern california the northridge earthquake of 17 january 19941 was the latest in a series of very damaging thrust fault generated earthquakes to strike california following the san fernando2 1971 coalinga3 1983 and whittier narrows4 5 1987 events like the last two of these the northridge event occurred along a fault that did not reach the surface and which had not been detected by traditional seismic hazard methods6 7 balanced cross sections8 9 which flatten and remove the crustal deformation can be used to identify and quantify the seismic hazard posed by thrust faults here we present a balanced cross section through the northridge portion of the transverse ranges fold and thrust belt10 which shows that the earthquake occurred on what we call the pico thrust a cross section of this sort constructed before the earthquake would have revealed the fault although it would not have predicted the earthquake cross sectional modelling of the pico thrust yields an average slip rate of 1 4 1 7 mm yr 1 and a recurrence interval of northridge sized mw 6 7 earthquakes every 1 500 1 800 years we show that the pico thrust is the back thrust to the 170 km elysian park thrust3 4 which underlies some of the most densely urbanized portions of the los angeles basin

effect of initial fault geometry on the development of fixed hinge fault propagation folds abstract this paper describes how a model of fixed hinge basement involved fault propagation folds may be adapted to apply to thin skinned thrust faults to generate footwall synclines fixed hinge fault propagation folding assumes that the fold axial surfaces diverge upwards fold hinges are fixed in the rock the fault propagated through the forelimb thickness changes occur in the forelimb and the forelimb progressively rotates with increasing displacement on the underlying fault the original model for fixed hinge fault propagation folds was developed for the case of a planar fault in basement with a tip line that was at the interface between basement and the overlying sedimentary cover rocks the two geometries applicable to thin skinned thrusts are for the cases where a fixed hinge fault propagation fold develops above an initial bedding parallel detachment and an initial fault ramp of constant dip which flattens down dip into a bedding parallel detachment

fault related folds above the source fault of the 2004 mid niigata prefecture earthquake in a fold and thrust belt caused by basin inversion along the eastern margin of the japan sea 1 the 2004 mid niigata prefecture earthquake occurred in a fold and thrust belt that has been growing since late pliocene time in a miocene rift basin along the eastern margin of the japan sea we constructed the trajectory of the subsurface faults responsible for the growth of the folds from the geologic structure and stratigraphy in the source region assuming that the folds have been growing as fault related folds because of inclined antithetic shear with a dip of 85 in the hanging wall above a single reverse fault the fault trajectory constructed from the fold geometries nearly coincides with the geometries of the source fault of the main shock of the 2004 earthquake revealed by aftershocks which supports that the rupture was along a geologic fault that has ruptured repeatedly during the last a few million years a three dimensional fault model based on 12 fault trajectories constructed along parallel sections revealed that the main shock occurred on a convex bend in the fault surface and that the southern termination of the aftershock distribution nearly coincides with a concave bend in the fault the close relation between the source fault and the geologic structure shows that it is possible to construct source fault geometry assuming inclined shear as deformation mechanism of a hanging wall and to infer the rupture areas from geologic data

predicting the orientation of joints from fold shape results of pseudo three dimensional modeling and curvature analysis we treat layers of sedimentary rock as elastic plates and predict the orientations of joints by assuming that they open parallel to the maximum instantaneous stretch of a layer because the direction of maximum instantaneous stretch is parallel to the maximum curvature of a surface we hypothesize that joints will trend parallel to the minimum curvature of an elastically deformed layer after constructing pseudo three dimensional trishear models of laramide style uplifts that grow self similarly we calculated the direction of minimum curvature axes during the evolution of the fold our analysis of minimum curvature axes in evolving folds suggests several important characteristics for fold related joint sets 1 joints that are neither parallel nor perpendicular to the fold axis may be induced by local fold related strains 2 at any time during folding joint orientations may vary according to the structural position on a fold 3 at any location on a fold joint orientation may depend on when a joint forms during the evolution of the fold and 4 joint patterns in trishear folds may vary with stratigraphic position natural folds that evolve along simple geometric pathways may develop fold related joint sets the orientation dominance abutting relationships spacing and continuity of which will vary systematically throughout the structure this variation in joint system architecture may reflect the history of fold growth

geometries of syntectonic sediments associated with single layer detachment folds abstract three kinematic models have been proposed to account for the geometry and kinematics of detachment folds involving a homogeneous competent layer detached over a ductile unit model 1 variable limb dip constant limb length model 2 constant limb dip variable limb length and model 3 variable limb dip variable limb length because the same fold shape can be generated by any of the above mechanisms fold kinematics are best determined by the geometries of syntectonic sediments in single layer detachment folds growth strata patterns are controlled by axial surface activity limb rotation limb lengthening fold uplift rates sedimentation rates and deformation mechanisms of the syntectonic sediments asymmetric kink folds have been modelled and compared with natural examples from the southern spanish pyrenees under conditions of high sedimentation rates model 1 folds produce characteristic fanning growth stratal wedges that initially onlap and then progressively overlap the detachment anticline whereas at low sedimentation rates anticlines with fanning growth wedges on both limbs are formed model 2 folds under conditions of high syntectonic sedimentation rates form anticlines with growth strata largely parallel to the pre growth units and thinned over the fold crest in contrast at low sedimentation rates model 2 folds show offlapping growth strata onto both limbs model 3 growth folds under conditions of high syntectonic sedimentation rates form anticlines with intermediate features from model 1 and model 2 at low sedimentation rates anticlines are formed with offlapping growth structures

critical state finite element models of contractional fault related folding part 2 mechanical analysis abstract finite element modeling of contractional fault related folding with critical state mechanics concepts reveals fundamentally different behaviors at two levels depending on 1 mechanical stratigraphy and 2 initial fault geometry deviatoric plastic strain rate and strain patterns show that strong layers like normally consolidated sandstone or overconsolidated shale develop strain localization in form of narrow plastic shear bands soft layers like normally consolidated shale fail to develop localizations and respond by diffuse distributed plastic strain inter layered overburdens display both deformation modes models with listric thrust fault seeds develop basement cored anticlines in the hangingwall that are characterized by locally tensile stress regimes within overall compressive regional stress fields the anticline crests of these models show evidence for strain softening by contrast anticlines emerging in detached fault related fold models are subject to locally compressive stress regimes consistent with the applied far field shortening these anticline crests strain harden during deformation stresses resulting from the finite element models deviate from stress estimates based on integrated densities the discrepancy is small for basement involved models but significant for detached style models suggesting that stress predictions based on integrated densities may be inaccurate for contractional systems first order predictions of plastic deformation suggest that footwall synclines in basement involved fold styles may host reservoirs with relatively poor quality owing to horizontal and vertical compaction anticlines found in detached deformation styles are bounded by compactive domains whereas the crestal regions are less affected by volumetric plastic deformation thus producing relatively better reservoir quality

the oak ridge fault system and the 1994 northridge earthquake the 17 january 1994 northridge earthquake moment magnitude m w 6 7 in the san fernando valley in southern california illuminated a hitherto unrecognized blind reverse fault with a moderate southward dip 1 this fault lies beneath the active north dipping santa susana fault system and uplifted both the footwall and the hanging wall of the santa susana fault during the earthquake1 here we argue that footwall uplift on the santa susana fault before the earthquake could have been used to identify the northridge blind thrust as an active fault moreover we propose that the northridge earthquake occurred on a continuation of the oak ridge fault system which reaches the surface in the ventura basin to the west slip rates on the western part of this fault system 2 3 are nearly three times larger than on the northridge blind thrust 4 increasing the probability of a n or t bridge sized earthquake in the heavily populated ventura basin

beef veins and thrust detachments in early cretaceous source rocks foothills of magallanes austral basin southern chile and argentina structural evidence for fluid overpressure during hydrocarbon maturation abstract we describe 1 bedding parallel veins of fibrous calcite beef and 2 thrust detachments which we believe provide good evidence for fluid overpressure in source rocks for petroleum our examples are from the surface or subsurface of the magallanes austral basin which lies at the southern tip of south america there the best source rocks for petroleum are of early cretaceous age in the central parts of the basin these source rocks have become overmature but at the eastern edge onshore and offshore they are today either immature or in the oil window in tierra del fuego the foothills of the andes consist mainly of sedimentary rocks which have undergone thin skinned thrusting in the vicuna area chile early cretaceous source rocks have reached the surface above thrust detachments which are visible on seismic data and well data at the surface we have found calcite beef containing hydrocarbons solid and or fluid in the rio jackson and vicuna formations which have reached the wet gas window in the rio gallegos area argentina the source rocks have not reached the surface but seismic and well data provide good evidence for thin skinned thrusting above flat lying detachments in early cretaceous source rock where it is in the early oil window in contrast there is little or no deformation where the source rock is still immature thus the deformation front coincides with the maturity front next to the central parts of the basin where the source rocks have reached the surface within the andes proper they have undergone low grade metamorphism within these source rocks we have found beef veins but of quartz not calcite to the east within the foreland basin seismic and well data provide evidence for a few compressional structures including thin skinned detachments in the deeply buried source rock finally in the northern part of the basin santa cruz province argentina where it is shallower the source rocks have reached the surface in the foothills above a series of back thrusts at lago san martin the source rocks have reached the oil window and they again contain calcite beef in conclusion where we have examined early cretaceous source rocks at the surface they contain either calcite beef if they have reached the late oil window or wet gas window or quartz beef if they are overmature independent evidence for overpressure in the form of source rock detachments comes from subsurface data especially at the southern end of the basin where the source rocks are not overmature and deformation is relatively intense thus we argue that hydrocarbon generation has led to overpressure as a result of chemical compaction and load transfer or volume changes or both

reverse drag revisited why footwall deformation may be the key to inferring listric fault geometry abstract although reverse drag the down warping of hanging wall strata toward a normal fault is widely accepted as an indicator of listric fault geometry previous studies have shown that similar folding may form in response to slip on faults of finite vertical extent with listric or planar geometry in this study we therefore seek more general criteria for inferring subsurface fault geometry from observations of near surface deformation by directly comparing patterns of displacement stress and strain around planar and listric faults as predicted by elastic boundary element models in agreement with previous work we find that models with finite planar planar detached and listric detached faults all develop hanging wall reverse drag folds all of these model geometries also predict a region of tension and elevated maximum coulomb stress in the hanging wall suggesting that the distribution and orientation of near surface joints and secondary faults may also be of limited utility in predicting subsurface fault geometry the most notable difference between the three models however is the magnitude of footwall uplift footwall uplift decreases slightly with introduction of a detachment and more significantly with the addition of a listric fault shape a parametric investigation of faults with constant slip ranging from nearly planar to strongly listric over depths from 1 to 15 km reveals that footwall fold width is sensitive to fault geometry while hanging wall fold width largely reflects fault depth application of a graphical approach based on these results as well as more complete inverse modeling illustrates how patterns of combined hanging wall and footwall deformation may be used to constrain subsurface fault geometry

displacement distance methods in the analysis of fold thrust structures and linked fault systems a new method for analysing the internal strain in thrust sheets is described the displacement distance diagram can be used to quantify the relative shortening or relative stretch between the hangingwall and footwall beds of a thrust and is a useful way of analysing commonly occurring fold thrust structures the stretch is a function of the relative rates of thrust propagation and thrust slip the cumulative displacement distance diagram described here can be used to calculate the absolute shortening or extension when this is accommodated above a single decollement in a linked fault system this method offers a useful way of analysing duplex structures and large scale foreland thrust zones as well as extensional fault systems and can be used as an alternative to or in conjunction with balanced cross sections both methods can be used to quantify the variation in shortening above a single detachment zone this variation may be mechanically significant reflecting lithological control on thrust or normal fault growth

fault bend folding as an end member solution of double edge fault propagation folding fault bend folding is the most commonly used kinematic mechanism to interpret the architecture and evolution of thrust related anticlines in thrust wedges however its basic requirement of an instantaneous propagation of the entire fault before hangingwall deformation limits its kinematic effectiveness to overcome this limitation we used the interdependence between fold shape and fault slip vs propagation rate s p ratio implemented in double edge faultpropagation folding we show that very small s p values produce fault propagation anticlines that when transported

energy balance for large thrust sheets and fault bend folds abstract thrust sheet movement over ramps requires energy because of the frictional resistance and deformation within the fault zone fault bend folding at the base and top of the ramp and changes in the gravitational potential energy because of uplift to model the energy usage a kinematic model of a foreland thrust sheet is constructed assuming 1 the ramp is planar and the flats are parallel to bedding 2 the fault bend folds are concentric 3 thickness is preserved for beds that enter the folds parallel to the basal thrust fault and 4 cross sectional area is preserved for rocks deformed by folding equations for the work done within the fault zone and during uplift and fault bend folding are derived by combining the kinematic model with stresses that increase in proportion to depth the relative amounts of energy consumed by friction along the fault uplift and fault bend folding are estimated to be 2 7 1 0 25 for a ramp angle of 30 the energy balance for the movement of large thrust sheets thus depends principally upon friction in the fault zone and changes in the gravitational energy

why blind thrust faults do not propagate to the earth s surface numerical modeling of coseismic deformation associated with thrust related anticlines high fault tip stress concentrations are associated with coseismic slip on blind thrust faults and suggest that these structures should readily propagate to the earth s surface seismic profiles of blind thrust related earthquakes reveal diffuse zones of aftershocks surrounding the fault tip which are attributed to inelastic deformation such as flexural slip or extensional fracturing the complex interaction between blind thrust faults and secondary structures may control the evolution of blind thrust systems the influence of bedding plane slip on fault propagation is simulated with numerical models using the boundary element method we use two parameters to estimate the tendency for thrust fault propagation 1 the mode ii stress intensity factor and 2 the maximum coulomb stress near the fault tip calculations from both analyses suggest that shallow thrust faults may exhibit an increased tendency to propagate as a result of interaction with the earth s surface and slip along bedding planes above the fault tip and a decreased tendency to propagate due to slip along bedding planes at or below the fault tip our results demonstrate that the magnitude and style of inelastic deformation in active fault systems control fault propagation

orogens and slabs vs their direction of subduction abstract subduction zones appear primarily controlled by the polarity of their direction i e w directed or e to nne directed probably due to the westward drift of the lithosphere relative to the asthenosphere the decollement planes behave differently in the two end members in the w directed subduction zone the decollement of the plate to the east is warped and subducted whereas in the e to nne directed it is ramping upward at the surface there are w directed subduction zones that work also in absence of active convergence like the carpathians or the apennines w directed subduction zones have shorter life 30 40 ma than e or ne directed subduction zones even longer than 100 ma the different decollements in the two end members of subduction should control different ptt paths and therefore generate variable metamorphic assemblages in the associated accretionary wedges and orogens these asymmetries also determine different topographic and structural evolutions that are marked by low topography and a fast eastward migrating structural wave along w directed subduction zones whereas the topography and the structure are rapidly growing upward and expanding laterally along the opposite subduction zones the magmatic pair calc alkaline and alkaline tholeiitic volcanic products of the island arc and the back arc basin characterise the w directed subduction zones magmatic rocks associated with e or ne directed subduction zones have higher abundances of incompatible elements and mainly consist of calc alkaline shoshonitic suites with large volumes of batholithic intrusions and porphyry copper ore deposits the subduction zones surrounding the adriatic plate in the central mediterranean confirm the differences among subduction zones as primarily controlled by the geographic polarity of the main direction of the slab the western margin of the adriatic plate contemporaneously overridden and underthrust europe toward the west to generate respectively the alps and the apennines while the eastern margin subducted under the dinarides hellenides these belts confirm the characters of the end members of subduction zones as a function of their geographic polarity similarly to the pacific subduction zones

structural and depositional evidence for repeated ice marginal oscillation along the eastern margin of the late devensian irish sea ice stream abstract late devensian glacigenic sediments and landforms along the north west coast of wales document the advance and subsequent retreat of the eastern margin of an irish sea ice stream that met coalesced and ultimately uncoupled from ice radiating outwards from the adjacent welsh ice cap centred over snowdonia across the boundary between the two former ice masses is a set of sediment landform assemblages that reflect rapidly changing erosional and depositional conditions during ice interaction from the inner part of the ice stream the assemblages range outwards from a subglacial depositional assemblage characterised by drumlin swarms through a subglacial erosional assemblage marked by prominent bedrock scours and large subglacial rock channels through an ice marginal assemblage identified by closely spaced glaciotectonised push moraines and intervening marginal sandur troughs into a freely expanding proglacial sandur and lacustrine delta assemblage the ice marginal assemblage provides evidence for numerous oscillatory episodes during retreat and at least 20 ice marginal limits can be identified at least 11 of these display multiple criteria for identifying readvance and in the ideal case is characterised by a moraine form built by localised tectonic stacking of diamict to the rear fronted by a clastic wedge of ice front alluvial fan gravel and intercalated flow till the distribution of sediment landform assemblages suggests a highly dynamic convergent ice stream flow pattern with high ice velocity a sharply delineated lateral shear margin pervasive ice marginal glaciotectonic deformation and a tightly focused ice marginal sediment delivery system all signature characteristics of contemporary ice streams

mechanics of triangle zones and passive roof duplexes implications of finite element models abstract finite element models of passive roof duplexes simulate the evolution and distribution of fault slip and deformation as functions of boundary conditions friction on the roof fault cover strength and amount of overburden a foreland buttress is essential for backthrust displacement in the models this foreland buttress is analogous to a pin line between the cover and footwall forelandward of the triangle zone in a balanced cross section and could be caused by a stratigraphic pinchout of the upper detachment or a strong cover in the foreland with boundary conditions that prevent the cover from slipping toward the hinterland relative to the wedge incipient forethrust and backthrust ramps zones of high equivalent plastic strain cut through the cover in the models there is more backthrust displacement with a lower friction coefficient on the upper detachment and with a stronger cover because of pressure dependence both fault strength and material strength increase with increasing overburden therefore the effect of overburden on backthrust displacement is determined by the nature of the pressure dependencies for fault friction and material strength in models with multiple imbricate faults the distribution of equivalent plastic strain adjacent to each fault is similar to that in single fault models with differences resulting from bending strains in the wedge and cover the strain distribution in the cover of the models is similar to that above natural passive roof duplexes characterized by layer parallel shortening above the tip of each horse in the duplex

deepwater north west borneo hydrocarbon accumulation in an active fold and thrust belt in the deepwater acreage of north west borneo an active offshore fold and thrust belt hosts a number of proven hydrocarbon accumulations and promises to deliver considerable additional hydrocarbon volumes as a result of ongoing exploration campaigns typical trapping geometries observed in this neogene large scale linked fold and thrust belt are hanging wall anticlines foreland folds and ridges and sub thrust footwall cut offs commercial drilling targets lie within the deformed miocene and pliocene sediment pile and these have been charged with oil and gas from a petroleum system that is still active at the present day a major challenge here is to avoid drilling traps that have expelled their hydrocarbons during periods of active deformation and uplift in recent geological time studies of tectonic deformation phases are beginning to illuminate some key controls on trap formation and hydrocarbon retention history this paper focuses on providing an overview of the north west borneo deepwater fold and thrust belt and introduces some key issues and challenges for future hydrocarbon exploration in the region

structural evolution in response to fold and thrust belt tectonics in northern assam a key to hydrocarbon exploration in the jaipur anticline area abstract the upper assam basin is an intermountain basin surrounded on two sides by eastern himalayan fold belt and the third side by assam arakan fold belt the basin has the contradictory characteristics of being a historic oil producing province in one of the most tectonically active areas of the world assam is a seismically active area and the location of major earthquakes yet contains one of the world s largest oil and gas fields an integrated study based on surface and subsurface data and relying on the sequence stratigraphic approach was undertaken to provide a re appraisal of the upper miocene to recent succession and its relationship to the region s structural history detailed data in the jaipur anticline region of the basin is viewed in regional context while the seismic database area includes only the northeastern part of the basin we suggest that the interpretations are applicable to the entire upper assam basin for interpreting the complex relationship between structural evolution and the generation and remigration of hydrocarbons in tectonically active regions a regional geo history analysis is a required component of the petroleum system description necessary for a successful exploration model

sedimentary basin inversion and intra plate shortening abstract sedimentary basin inversion the shortening of formerly extensional basins is accommodated mainly by compressional reactivation of extant faults and fractures across a wide range of scales as such inversion is a large scale manifestation of byerlee friction the dynamic criterion for fault reactivation governing the effective shear strength of the shallow crust basin inversion generates distinctive deformational architecture and it is implicated strongly in sedimentary basin exhumation as a principal source of horizontal stress inversion drives significant sedimentary porosity reduction and resultant fluid flow upper crustal deformation is critically dependent on fluid overpressure i e pore fluid pressures greater than would be calculated from a hydrostatic gradient and perhaps more than in any other tectonic setting overpressures are potentially large during sedimentary basin inversion this review therefore includes discussion of the role of fluid overpressure in inversion and the evidence for it collectively inversion has profound implications good and bad for the prospectivity of many petroliferous sedimentary basins thus recognizing the evidence for basin inversion quantifying its magnitude and understanding the mechanisms that accommodate inversion and other phenomena affected by it have become essential components of the basin analyst s remit

the structure of mountain fronts commonly the part of a mountain front which is visible at the surface consists of foreland dipping thrust belt rocks elevated above their regional structural position and overlain more or less conformably by molasse several explanations for their geometry are possible 1 major detachments exist within or beneath the molasse resulting in transport of the foreland basin examples of this geometry come from the swiss molasse plain the southern pyrenees and the mackenzie mountains of canada 2 displacement is lost on major backthrusts beneath the frontal monocline examples cited here are the rockies of alberta the sulaiman ranges of pakistan the mackenzies and the andes in peru 3 thrust sheets travelled over an old land surface and syntectonic molasse contemporaneously offlaps the topographic high of the thrust front this phenomenon occurs along the alpine thrust front in haute provence 4 the frontal fold represents deformation above a large scale thrust tip no unequivocal example of tip line strain at this scale has been recorded although this type of deformation may occur in the brooks range of alaska in many areas mountain fronts show a combination of these idealized geometries

estimation of fault propagation distance from fold shape implications for earthquake hazard assessment a numerical grid search using the trishear kinematic model can be used to extract both slip and the distance that a fault tip line has propagated during growth of a fault propagation fold the propagation distance defines the initial position of the tip line at the onset of slip in the santa fe springs anticline of the los angeles basin we show that the tip line of the underlying puente hills thrust fault initiated at the same position as the 1987 magnitude 6 0 whittier narrows earthquake

models of strain variation in nappes and thrust sheets a review abstract strain within nappes and thrust sheets can be modelled using simple boundary conditions an original rectangular prism of material resting on a thrust plane is considered to be subject to various pure and simple shear components operating parallel to the thrust deformation gradient tensors are determined for the models and solutions found for the principal strains a two dimensional model considers heterogeneous simple shear combined with a stretch in the direction of shear simple shear zones arise as a special case where 1 recumbent fold nappes characteristic of gravitational collapse can be modelled with 1 and thrust sheets with layer parallel shortening and folding with differential transport of nappes gives rise to additional wrench type shears solutions for the principal strains in this type of model are found and used to interpret side wall ramps and steep zones parallel to nappe movements by including a stretch in the transport direction these differential transport models produce prolate strains 1 and oblate strains finally a brief consideration of bending strains produced in a sheet passing over a ramp indicates reversals in the shear direction during the strain history these predict possible crenulation of early thrust fabrics at ramps

fault related folding in sandbox analogue models of thrust wedges abstract the nucleation and growth of thrust fault related folds has been simulated using high resolution sandbox coulomb wedge type analogue models thrust related anticlines grew progressively from initial layerparallel shortening to decollement folding thrust tip folding and eventually thrust ramp folding thrust ramps nucleated in the central sector of the overturned forelimb of decollement folds as a consequence of limb thinning thrust tip folding occurred during outward migration of the ramp tips the fault propagation rate was greater at the lower ramp tip which quickly joined the basal thrust flat while folding continued beyond the upper tip the development of the analogue model thrust fault related folds by progressive changes in kinematics from decollement folding to thrust tip folding and finally thrust ramp folding compares well with analyses of naturally occurring thrust fault related folds

geometric and numerical model of progressive limb rotation in detachment folds a simple area balanced geometric model of progressive limb rotation in detachment folds allows the quantification of displacement uplift and rotation and the assessment of their effect upon growth strata geometries translation of this geometric model into a velocity model of deformation allows the modeling of associated growth strata resulting from background sedimentation and local erosion transport and deposition we show that with constant displacement and sedimentation rate this model predicts a progressive unconformity indicating the onlap and eventual overlap of the fold limb caused by a decreasing uplift rate this decrease is a consequence of the kinematics of progressive limb rotation increases in displacement rate with sedimentation rate kept constant can lead to offlap and angular unconformities although subsequent onlap does not necessarily reflect a return to a lower displacement rate modeled growth strata geometries exhibit many of the features reported from the literature

large scale thin skinned thrusting in the southern alps kinematic models the tectonics of the southern alps are dominated by thrusting on extensive ramp flat systems and by folding some of which is ramp folding basement slivers are involved that frontally are not thicker than 5 km the most unequivocal section is that of the grigna mountains east of lake como for which kinematic modeling based on material balance considerations results in a shortening of 40 km it is separated from the grona section in the west and the valtorta section in the east by probably pre existing transverse zones that cut through the important decollement horizons and define blocks of different depths to basement local tectonics across these transverse zones change abruptly while over all shortening remains constant in particular both east and west of the grigna section basement ramp folds were wedged into the very incompetent raibl beds provoking back thrusting of the thick hauptdolomit and higher sediments with the substitution of the original cover of the frontal limb of the basement fold this new kinematic interpretation is compatible with the information facilitates the construction of balanced sections and is required for lateral constancy of shortening

multibeam bathymetric survey and high resolution seismic investigations on the barbados ridge complex eastern caribbean a key to the knowledge and interpretation of an accretionary wedge abstract an important problem of active margin processes is the way accretionary wedges form in response to oceanic crust subduction new data have been obtained along the barbados ridge in front of the lesser antilles tertiary island arc they include high resolution multichannel seismic data and accurate sea beam bathymetric maps both have been used to propose a 3 dimensional interpretation of tectonic and sedimentary processes related to subduction such as oceanic off scraping ductile folding relative uplift and subsidence fore arc basin evolution and mud diapirism they all improved our knowledge of the evolution of accretionary prisms and more specifically of the barbados ridge development in plio quaternary time previous tectonic events are deduced from recently published geological data from barbados and the lesser antilles islands

fault related folding a review of kinematic models and their application abstract folding that is directly related to fault activity is an important deformation feature that occurs all over the world in mountain belts accretionary wedges fold and thrust belts and intra plate settings in either strike slip compressional or extensional regimes due to their widespread occurrence knowledge about the development of these structures is important to a broad spectrum of geoscience sub disciplines such as structural geology seismology geomorphology petroleum geology and quaternary geology fault related folding has been analyzed intensively over the last 140 years for the sake of this review we divide the folds according to the way the faults and the folds form that is into detachment fault bend and fault propagation folds all fault related folds are caused by changes in fault parameters the simplest method to produce folds is to transport material along faults that have stepped flat ramp flat geometries fault bend fold alternatively the slip can decrease along the length of the fault and depending on whether the fault remains within a detachment layer or steps up through mechanical stratigraphy either a detachment fold or a fault propagation fold is formed respectively detachment folding was first investigated in the early 20th century whereas the full significance of fault propagation folds was recognized quite late in the 1980s seminal work on fault related folding was carried out in the 1930s but quantitative kinematic models have only been available in the last 30 years these models are extremely valuable because they allow a comprehensive understanding of the evolution of fault related folds and lead to more accurate predictions of the sub surface structure from the mid 1990s onwards numerical simulations have been used to identify how fault parameters such as dip and fault bend angle propagation to slip ratio and shape of the trishear zone influence the geometry of the related folding this is directly applicable to the analysis of the shape of anticlines produced however this does not mean that fold geometry is uniquely related to fault geometry on the contrary different kinematic approaches can lead to a similar fold shape

kinematic modelling of extensional fault propagation folding abstract many studies have shown that in extensional basins discrete faulting at depth is commonly linked to more distributed deformation in particular folding at higher levels such extensional fault propagation folds are particularly common where there is a distinct mechanical contrast between faulted basement and sedimentary cover outcrop and analogue modelling studies indicate that such folds form as upward widening zones of distributed deformation monoclines above discrete faults at depth with increasing displacement strain the folds are cut by faults as they propagate upwards into the cover to date however there has been little investigation into the kinematics of linked basement faulting and extensional fault propagation folding here we present a two dimensional kinematic model of linked basement faulting and fault propagation folding which is based upon trishear the model allows investigation of the influence of shear zone geometry and the rate of fault propagation upon the style of folds and the strains associated with them the evolution of linked basement faulting and folding predicted by the model is compared in detail to that observed in an analogue model the kinematic model reproduces well many of the features seen both in the analogue model and reported from outcrop and seismic studies

experimental models of extensional forced folds we have used single layer and multilayer clay models to study the development of forced folds above normal faults our modeling results show that the deformation patterns associated with extensional forced folding depend on the dip of the underlying normal fault and the presence of layer parallel detachments in single layer clay models extensional forced folds are upward widening monoclines anticlinal axial surfaces dip in the same direction as underlying master normal faults and synclinal axial surfaces dip in the opposite direction of master normal faults most secondary faults are upward steepening normal faults if master normal faults are steeply dipping however many secondary normal faults become high angle reverse faults at shallow depths the propagation and linkage of secondary faults into through going normal faults terminates the development of extensional forced folds more folding occurs prior to fault linkage if the master normal fault is steeply dipping rather than gently dipping most dipping beds and secondary faults are preserved in the hanging walls of the through oing normal faults in multilayer clay models with layer parallel detachments extensional forced folds are also upward widening monoclines slip on the lowest detachment laterally transfers extension induced by normal faulting and forced folding from the master normal fault to the detachment edge slip on overlying detachments accommodates minor thickness changes associated with upward widening of the fold secondary faults include low angle normal faults near the anticlinal axial surface minor thrust faults near the synclinal axial surface and high angle normal faults above the detachment edge the model predicted deformation patterns are similar to those of extensional forced folds from the gulf of suez and offshore norway this similarity suggests that our modeling results apply to extensional forced folds and can provide guidelines for interpreting field well and seismic data

decollement depth versus accretionary prism dimension in the apennines and the barbados 1 along representative cross sections of the apennines and the northern barbados accretionary prisms we measured the area the decollement depth the angle a of the upper envelope and the angle b of the dip of the regional monocline the continental sections of the apennines accretionary prism have a deeper decollement than the oceanic sections of the northern barbados 6 10 km depth and 1 km depth respectively because the sediment pile is thinner on the incoming barbados plate and its denser oceanic structure is more easily subducted considering the frontal 50 km the apennines have an average cross sectional area of 500 km 2 and the northern barbados ridge of 100 km 2 the total area is a function of the depth of the decollement plane therefore at a given amount of subduction the deeper the decollement depth is the bigger the area of the wedge will result assuming negligible compaction and erosion as a consequence the larger area volume and higher elevation of the apennines with respect to the barbados is determined by the apenninic deeper decollement despite these differences the geometry of both decollements is in some cases comparable in particular close to the boundary between the crystalline crust and the sediment pile where the main density and strength contrasts are concentrated variations in depth of the decollement occur moving along strike in both accretionary prisms the geometry of the prisms is further controlled by the different values of a and b their sum and the distance of the accretionary prism relative to the subduction hinge index terms 8099 structural geology general or miscellaneous 8150 tectonophysics evolution of the earth plate boundary general 3040 8122 tectonophysics dynamics gravity and tectonics keywords decollement depth accretionary prism apennines barbados citation bigi s f lenci c doglioni j c moore e carminati and d scrocca decollement depth versus

alabama gulf coast groundwaters 4he and 14c as groundwater dating tools analyses of 4 he and 1 4 c in groundwaters from a miocene quartz aquifer on the alabama gulf coast show the usefulness of 4 he for dating these holocene groundwaters in this aquifer system of low alkalinities and low phs radiocarbon ages can be used without model correction the groundwaters studied ranged from 42 3 to 95 5 modern carbon and yielded uncorrected ages of 375 6790 yr radiogenic 4 he ages ranged from 50 7500 yr

distribution of fault slip in outcrop scale fault related folds appalachian mountains abstract current kinematic models of ramp related folds predict a direct relationship between ramp angle and fold shape and imply specific sequences of deformation analyses of outcrop scale structures in the valley and ridge province of the appalachians reveal configurations that depart from model predictions the models fail to account for the presence of footwall synclines and are inconsistent with measured displacement distributions on some natural faults observations support the interpretation that faults can grow by propagation both up and down dip from a nucleation point fault propagation in either direction may result in the formation of folds primarily on the side of the fault that is displaced in the direction of fault propagation