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

calcite twin widths and intensities as metamorphic indicators in natural low temperature deformation of limestone abstract twins in naturally deformed coarse grained limestones of the northern subalpine chain france show a direct relationship of twin width and intensity number of twins per mm to the peak metamorphism under diagenetic and anchimetamorphic conditions the transition from dominant thin twinning to dominant thick twinning occurs between a mean vitrinite reflectance ro of 1 60 and 2 60 or temperatures of between 150 and 200 c below the transition temperature increasing strain is apparently accommodated by adding new twins and above the transition temperature increasing strain is accommodated by twin widening similar transitions from thin to thick twinning with increasing metamorphism are also present in the helvetic alps the prealps and the central appalachian valley and ridge province these data for naturally deformed limestones support the interpretation from experimental studies of a temperature effect on twin width and intensity and indicate that the twinning in the northern subalpine chain occurred at or near peak metamorphism

laurentide cordilleran and montane glaciation in the western peace river grande prairie region alberta and british columbia canada abstract geological investigations over the last decade in the western peace river grande prairie district of northwestern alberta and northeastern british columbia have provided important chronostratigraphic information regarding laurentide cordilleran and montane quaternary glacial events numerous 14 c age determinations as well as stratigraphic and sedimentological analyses of numerous exposures indicate that this area was first glaciated by cordilleran ice sometime before the late wisconsinan during the late wisconsinan ca 22 000 14 000 bp laurentide glacier ice covered much of the area between approximately 15 000 and 10 000 bp montane ice affected only the western part of the peace region there is no evidence for synchroneity of the ice advances and therefore no evidence for ice coalescence in this region the resultant ice free corridor was most likely a laterally fluctuating zone which existed during a several thousand year period in the late pleistocene climatic conditions along the corridor during this period were probably unfavorable for human migration but adequate for some plants and small animals numerous isolated areas along the western foothills of the northern rocky mountains west of the laurentide glaciation limit were never glaciated during the late wisconsinan

deformation conditions in the northern subalpine chain france estimated from deformation modes in coarse grained limestone abstract in low temperature experiments the deformation mode and ductility in a single rock type are most directly controlled by the confining pressure and strain magnitude a graphical technique is developed from published experimental microstructural data that allows for the determination of deformation mode for coarsegrained limestone on the basis of microstructural point count data the deformation mode and strain are used to constrain the confining pressure during thin section scale deformation by comparison with experimental deformation results the technique is applied to coarse grained samples of cretaceous and tertiary limestone from the northern subalpine chain france and reveals that the majority of samples deformed mostly by twinning in the uniform flow mode at minimum depths estimated from deformation mode and strain of between 0 and 6 0 km and maximum depths interpreted from peak metamorphic temperatures and geothermal gradient of about 2 7 9 7 km in the remaining samples brittle failure probably occurred at relatively shallow depths

lateral and vertical changes of deformation style in the osen roa thrust sheet oslo region abstract changes in deformation style and amounts of shortening in the osen roa thrust sheet of the oslo region occur vertically and laterally approaching the thrust front in the south deformation in the cambromiddle ordovician sequence passes laterally from closely spaced imbricates in the north 50 60 shortening through triangle pop up and imbricate zones toward the south 20 37 shortening to widely spaced zones of deformation up to 20 shortening approaching the thrust front changes in deformation style are attributed to changing boundary conditions across the klekken thrust declining end of orogenic forces and an increase in thickness of competent units in the ordovician rocks to the south vertical changes in deformation style are attributed to the increasing percentage of competent units upward in the cambro silurian sedimentary rocks in the north the accompanying decrease in shortening upwards requires a structurally necessary upper detachment horizon to separate folded late middle ordovician silurian sediments from imbricated early cambro middle ordovician sediments below while southward in the oslo area the upper detachment needs to be placed between silurian and cambro ordovician units finally in eiker with less than 20 shortening the whole cambrosilurian sequence appears to have deformed as a single unit in the northern oslo region the upper detachment probably has a backthrust sense of motion above an imbricate stack passive roof duplex further south the upper detachment is probably directed toward the foreland

cover deformation above a blind duplex an example from west virginia u s a abstract pretectonic cover above blind thrust systems responds to underlying thrusting by combinations of forethrusting backthrusting and coupling the role of these responses was examined in the hanging rock cacapon mountain anticlinorium of the central appalachians this cover fold consists of middle ordovician and younger sedimentary rocks above a blind horse of cambro ordovician carbonates the cover has abundant folding and subsidiary faulting cover cleavage in carbonate and fine grained clastic rocks formed before regional folding in response to layer parallel shortening lps this 15 lps also produced grain suturing and fluid inclusion trails in quartzarenites the lps formed during cover forethrusting accommodating older blind horses in the hinterland with the formation of the underlying horse cover coupling occurred with bending folding additional secondary buckling and cleavage rotation by flexural flow with perhaps minor flattening this cover coupling was insufficient to accommodate horse formation no evidence exists for backthrusting but forelandwards abundant subsurface evidence exists for cover forethrusting with imbricate thrusting this thrusting accommodates horse formation and differs in style from the lps of the older forethrusting event so for this blind system the cover responded with a combination of coupling and forethrusting

seismic data geometry evolution and shortening in the active sulaiman fold and thrust belt of pakistan southwest of the himalayas surface and subsurface data from the sulaiman fold and thrust belt are integrated to analyze the deep structure tectonic shortening and kinematics of the sulaiman fold and thrust belt at the western margin of the indian subcontinent seismic reflection data show that nearly all the 10 km thick sequence of dominantly platform 7 km and molasse strata is detached at the deformation front these strata thicken tectonically to about 20 km in the hinterland without significant thrust faults in the foreland a balanced structural cross section suggests that structural uplift in the sulaiman fold and thrust belt is a result of a thin skinned passive roof duplex style of deformation the duplex sequence consists of jurassic and older strata it is separated from the roof equence by a passive roof thrust in thick cretaceous shales that has a backthrust sense of displacement over foreland propagating duplexes the roof sequence with incipient out of sequence thrusting is intact for about 150 km northwards where it emerges along a passive backthrust sequential restoration of the balanced section reveals a series of structural and geometrical features including 1 development of low amplitude broad concentric detachment folds sui and loti at the tip of the decollement 2 increase in amplitude of a detachment fold to a critical level for development of ramp and duplex structures and 3 out of sequence thrusting to create required critical taper for an outward translation of the foreland fold and thrust belt a balanced structural cross section 349 km long from the sulaiman fold and thrust belt restores to an original length of 727 km suggesting a maximum of 378 km of shortening since 21 ma in the cover strata of the indian subcontinent calculation of displacement rates over the sulaiman fold and thrust belt 18 mm yr added to the resolved rate of the chaman fault vector for the component parallel to the plate convergence direction 15 mm yr are close to the current india asia plate convergence rate 37 mm yr

a comparison of the strain of crinoid columnals with that of their enclosing silty and shaly matrix on the appalachian plateau new york abstract a pole figure goniometer was used to measure the preferred orientation of the basal planes of chlorite in 87 samples from the upper devonian clastic rocks of the appalachian plateau new york interpretation of the preferred orientation according to the theory of march yields an estimate of how much an element of the sediment has changed shape since deposition this deformation can be interpreted according to three distinct models for the strain history all begin with compaction followed by one of three types of tectonic strain 1 a plane strain that conserves bedding plane area by compensating for horizontal shortening with horizontal elongation at right angles 2 one that uniaxially shortens a horizontal line normal to the fold axes without compensating elongation and 3 a plane strain again which conserves the area in the vertical plane containing a shortened horizontal direction by stretching it vertically for lack of suitable names we will simply call the three total strains type b u and v from the initials of the distinguishing features of their tectonic increment the horizontal strain components estimated from preferred orientation are compared at a number of localities with those implicit in the principal horizontal diameters of the elliptical outlines of deformed crinoid columnals found on bedding planes expressed for constant bedding plane area or type b total strain the strain in the direction of greatest elongation measured by preferred orientation was found to range from no recorded strain to 0 19 with a median of 0 04 as a rule however the more intensive strains were recorded in shales with a fine grained phyllosilicate content exceeding 85 by volume in which the mean was 0 07 deformed crinoid columnals in the same set of samples have elongations parallel to their long axes that range from 0 04 to 0 10 with a median and average of 0 07 the discrepancy between the two sets of strain measures at least in the coarser and more permeable rock types may either be real and caused by easier flushing of dissolved ions or may be caused by imperfect strain recording in coarse matrix materials

the role of strain in area constant detachment folding abstract the classical constant area relationship for balancing cross sections is reformulated to accommodate layer parallel strain leading to a general equation for the layer parallel strain in a fold formed above a stratigraphically fixed detachment horizon the strain required for area balance can be calculated from a knowledge of the fold shape and the depth to detachment the strains predicted from a simple kinematic model are realistic in the order of 10 30 layer parallel shortening the strain may be penetrative or occur as small scale disharmonic folds and faults the tip top anticline a fold at the leading edge of the wyoming thrust belt is used to illustrate the applicability of this reformulation the computed layer parallel strains in the well controlled upper portion of the structure are less than 5 because most of the shortening is accommodated by second order folds and faults

the structural geometry and evolution of foreland thrust systems northern virginia seismic reflection data reveal that the structural geometry of the central appalachians of northern virginia consists of three distinct thrust systems each thrust system is characterized by a unique internal geometry the blue ridge thrust sheet is a composite thrust sheet composed primarily of imbricated precambrian crystalline rocks it over rode cambrian ordovician carbonates and formed a sheared basement cored fault bend fold thrusts within the sheet may be taconic and earliest alleghanian whereas final thrusting and emplacement of the sheet were probably slightly younger but still early alleghanian the north mountain thrust sheet is characterized by imbricated cambrian ordovician carbonates that are deformed into large amplitude mode ii fault bend folds and fault propagation folds rocks of this sheet were transported more than 60 km across a similar section of carbonates the leading edge of the north mountain thrust sheet was deformed into a fold with a mode ii fault bend fold geometry and was juxtaposed against middle paleozoic rocks the middle paleozoic rocks occur in a ramp across which displacement along the north mountain thrust was transferred to a higher detachment more than 60 km of cover rocks displaced during the emplacement of the north mountain thrust sheet either were transported across this ramp and thrust over a similar section in the western valley and ridge province or were backthrust above the sheet the timing for imbrication and emplacement of the north mountain thrust sheet is probably main phase alleghanian the lower carbonate duplex extends from beneath the blue ridge and north mountain thrust sheets westward across the western valley and ridge province the lower carbonate duplex is characterized by imbricated cambrian ordovician carbonates that form low amplitude fault bend folds this thrust system also probably formed during main phase alleghanian deformation

variation in quartz arenite deformation mechanisms between a roof sequence and duplexes abstract microstructural abundances and histories in quartz arenite of the lower silurian tuscarora sandstone were used to determine the nature and role of microscale deformation in a cover sequence and underlying thrust system and to assess the degree to which the cover sequence accommodated emplacement of the thrust system sandstone samples are located across the transition from the central to southern appalachian foreland thrust system where the thrusts change from blind to emergent southwards and where southern deformation of early alleghanian age has been previously shown to be overprinted by central deformation of alleghanian age microstructures observed with transmitted light and cathodoluminescence microscopy indicate that grain scale deformation occurred by dislocation flow pressure solution and microfracturing with the last being generally the most important the sequence of deformation mechanisms is the same for the cover sequence and the thrust system pressure solution during sedimentary compaction dislocation flow during layer parallel shortening and localized microfracturing with limited pressure solution near major thrust ramps and in steep fold limbs a greater abundance of dislocation flow microstructures in the cover sequence from layer parallel shortening indicates grain scale accommodation in the tuscarora sandstone of some shortening associated with emplacement of the thrust system the transition zone between the central and southern appalachians contains the greatest occurrence of every microstructure which is consistent with the area having been affected by diachronous central and southern alleghanian deformations

application of the rfφ technique to ellipitical markers deformed by pressure solution abstract the applicability of the r f technique of finite strain analysis to elliptical markers whose shape is modified by pressure solution is investigated variably oriented elliptical markers shortened by removing material from one or both sides in open and closed systems are compared to markers shortened homogeneously by the same amount for the models examined it was found that the r f technique correctly predicts the principal directions the initial marker axial ratio and the axial ratio of a circle deformed in a like manner interpretation of strain ratios in terms of percent shortening requires knowing how much volume was lost equations relating the strain ratio to percent shortening are given for both constant volume and volume loss deformation

three dimensional structural interrelationships within cambrian ordovician lithotectonic unit of central appalachians a block diagram of the cambrian ordovician lithotectonic unit illustrates three dimensional structural relationships within that sequence along the length of the central appalachian valley and ridge and high plateau provinces the diagram shows that the valley and ridge province is divisible into areas within which shortening is relatively constant in the cambrian ordovician lithotectonic unit these areas are bounded by zones across which significant differences in shortening occur these transition zones contain major cross strike structural discontinuities in surface structure in some instances these discontinuities extend across the valley and ridge province and into the high plateau province increases in fold amplitude and number occur in the cover of the plateau across strike from the more intensely deformed areas of the valley and ridge where shortening within the cambrian ordovician unit is significantly greater than elsewhere within that province structurally controlled gas accumulations are more prevalent in these areas of the plateau

relationship of spaced cleavage to folds and thrusts in the idaho utah wyoming thrust belt the idaho utah wyoming thrust belt of the rocky mountain cordillera preserves structures that developed at very shallow levels of the crust t 200 c the geometry of major structures is well documented and synorogenic conglomerates record the timing of movement on major thrusts spaced cleavage and pencil fracturing are developed within micrites of the jurassic twin creek formation the cleavage developed dominantly by pressure solution early in the deformation history of each thrust sheet aided by elevated temperatures and pressures in areas where increased overburden resulted from emplacement of earlier overriding sheets cleavage within any sheet is well developed in the area covered by the pre erosional extent of the previous sheet forelandward from this area cleavage intensity decreases drastically thermal modelling of the sheets agrees well with temperature estimates from illite crystallinity studies on material from the cleavage planes cleavage is axial planar to small folds and formed concurrently with small scale buckle folds and contraction faults during the early history of movement on each thrust however cleavage strongly fans about large scale folds formed later during movement suggesting passive rotation of cleavage with bedding during the later stages of thrust history the rotated cleavage may also be folded and overprinted by a second weaker cleavage during a second ramping or during movement on a later lower thrust cross folding may also develop as a result of lateral ramping thus complex patterns of interfering folds multiple cleavages and associated fiber filled fractures may be produced during motion on a single thrust fault and so magnify the problem of correlating cleavages and deciphering multiple deformations

laurentide and montane glaciation along the rocky mountain foothills of northeastern british columbia mapping the surficial geology of the trutch map area nts 94g provides new data on the timing of continental and montane glaciations along the foothills of northeastern british columbia striated surfaces on mountain crests were dated to the late wisconsinan substage by cosmogenic dating the striations were produced by eastward flowing ice emanating from the region of the continental divide this ice was thick enough to cross the main ranges and overtop the rocky mountain foothill summits at 2000 m above sea level asl it is argued here that such a flow unhindered by topography could only have been produced by the cordilleran ice sheet and not by local cirque glaciation during this time the cordilleran ice sheet dispersed limestone and schist erratics of western provenance onto the plains beyond the mountain front conversely the laurentide ice sheet did not reach its western limit in the foothills until after cordilleran ice retreated from the area during its maximum the laurentide ice penetra

structural interpretation of the eastern sulaiman foldbelt and foredeep pakistan the sulaiman foldbelt is an active tectonic feature on the northwestern margin of the indian plate seismic reflection data have been combined with surface geological mapping and drill hole data to interpret the structural style and tectonic shortening of the eastern sulaiman foldbelt and its adjacent foredeep the data show that the basement is more than 8 km deep near the deformation front and that it deepens towards the west the foredeep adjacent to the eastern sulaiman foldbelt is a broad synclinal structure with a steep western limb the more gentle eastern limb has monoclinal dips from near zero to 2 5 several salt cored anticlines are observed on the eastern part of the monocline but the salt structures are lacking in the western foredeep and in the frontal part of the sulaiman foldbelt the basal decollement under the sulaiman lobe lies either in ductile eocambrian salt or within a deep zone of other material that undergoes ductile behavior near the deformation front ramps from the basal detachment become flats in lower triassic and lower cretaceous shales a steep and highly elevated zone immediately to the west of the frontal folds is interpreted to be underlain by a passive roof duplex the culmination wall of this oblique duplex is separated from the overlying roof sequence by a passive roof thrust in lower cretaceous shales the roof thrust has a backthrust sense of movement relative to the forelandward propagating duplex and may extend obliquely 100 km into the interior of the foldbelt fault propagation and fault bend folds are present in the frontal portion of the eastern sulaiman foldbelt palinspastic restoration of a balanced cross section shows 108 km of shortening in the eastern sulaiman foldbelt east of the kingri strike slip fault the presence of more than 11 km of pre neogene strata on top of basement suggests that the mesozoic rifted margin of the indian subcontinent may lie beneath the sulaiman foldbelt this interpretation is also favored by the presence of ophiolites and flysch deposits overthrusted on late cretaceous shelf strata

the role of salt in fold and thrust belts davis d m and engelder t 1985 the role of salt in fold and thrust belts in n l carter and s uycda editors collision tectonics deformation of continental lithosphere tectonophysics 119 67 88 the style of deformation in thin skinned fold and thrust belts is critically dependent upon the resistance to sliding along the detachment between the mass of deforming sediments and the underlying rocks evaporites can provide an extremely weak horizon within which a basal detachment can form and along which only a relatively small shear traction can be supported fold and thrust belts that form atop a salt layer such as the appalachian plateau the franklin mountains in northwestern canada and the jura of the alps among others share several readily observable characteristics as predicted by a simple mechanical model for fold and thrust belts a detachment in salt permits a thrust belt to have an extremely narrow cross sectional taper in addition predicted orientations of the principal stress axes over a salt dtcollement are consistent with the commonly observed lack of a consistently dominant vergence direction of structures within the thrust belt other common attributes of salt basal thin skinned deformation include the presence of several widely but regularly spaced folds and abrupt changes in deformational style at the edge of the salt basin

strain fabric associated with the meade thrust sheet implications for cross section balancing abstract emplacement of the meade thrust sheet southeastern idaho u s a was accompanied by the development of a strain fabric and significant footwall deformation characteristics of the strain fabric are based on strain estimates using pressure solution cleavage and deformed fossils within the jurassic twin creek formation the strain fabric formed during early layer parallel shortening at the thrust front and accommodates from 10 to 35 shortening within the incipient thrust sheet footwall strata were later folded above a detachment at the base of the twin creek formation and the early strain fabric was passively rotated the structural development of this area consisting of 1 strain fabric development 2 folding of the footwall and 3 the truncation of footwall structures by imbricates of the meade thrust are placed in a relative time frame to aid in section reconstruction the passive reorientation of the strain fabric and the truncation and incorporation of footwall structures into the thrust sheet are used as kinematic and geometric constraints for cross section balancing we present a sequential retrodeformation of the meade thrust sheet in which strain is removed from the cross section at the appropriate geometric stages in the deformation these sections illustrate some methods by which strain data can be incorporated into cross sections and also indicate the difficulty involved in balancing cross sections of internally deformed thrust sheets interpretation of these sections suggests that a significant amount of footwall deformation accompanied the emplacement of the meade thrust sheet and that footwall deformation is strongly controlled by the local stratigraphic package the sections also illustrate that the earliest deformation within a thrust sheet may be unrelated to motion on the underlying incipient thrust and that the trace of a thrust fault which cuts previously deformed strata will not restore to a typical ramp and flat geometry

flow pattern evolution of the laurentide and cordilleran ice sheets across west central alberta canada implications for ice sheet growth retreat and dynamics during the last glacial cycle this paper presents a reconstruction of the geometry dynamics and flow pattern of the laurentide and cordilleran ice sheets along the albertan portion of the eastern canadian rocky mountains during the last glaciation sediment landform associations relating to the evolution of these ice sheets document their initial convergence and mutual deflection across west central alberta the continued advance of laurentide ice locally displaced cordilleran ice westward into the front ranges and deflected trunk ice emerging from the athabasca river valley south eastwards along the foothills these convergent flow patterns remained major features in the regional geometry of both ice sheets throughout the last glacial maximum the onset of deglaciation was characterized by the northward retreat of the laurentide ice sheet which progressively unblocked valleys along the mountain front the configuration of buttress removal initiated the lateglacial expansion and sequential drawdown of the eastern cordilleran ice sheet along fast flowing outlet glaciers subsequent regional retreat of laurentide and cordilleran ice was associated with the development of large proglacial lakes which amplified deglaciation by destabilizing the margins of both ice sheets triggering extensive readvances due to periodic changes in water level associated with the opening and closing of outlets by ice margin fluctuations

joints microfractures and the formation of solution cleavage in limestone the geometry of solution cleavage surfaces in limestones indicates a close connection between cleavage formation and joint and microfracture surfaces these relations suggest that networks of microfractures and joints act both as conduits for transport of dissolved phases and as surfaces on which the cleavage nucleates material transport is by darcyian type bulk flow and is thus not a rate limiting process this contrasts with metamorphic terranes where the transport mechanism diffusion has been identified as a rate limiting step cleavage surfaces of weak to moderate intensities or spacing may form parallel to nonprincipal planes of finite strain

coalescence of late wisconsinan cordilleran and laurentide ice sheets east of the rocky mountain foothills in the dawson creek region northeast british columbia canada abstract geomorphic stratigraphic and geochronological evidence from northeast british columbia canada indicates that during the late wisconsinan approximately equivalent to marine oxygen isotope stage mis 2 a major lobe of western sourced ice coalesced with the northeastern sourced laurentide ice sheet lis high resolution digital elevation models reveal a continuous 75 km long field of streamlined landforms that indicate the ice flow direction of a major northeast flowing lobe of the cordilleran ice sheet cis or a montane glacier 200 km wide was deflected to a north northwest trajectory as it coalesced with the retreating lis the streamlined landforms are composed of till containing clasts of eastern provenance that imply that the lis reached its maximum extent before the western sourced ice flow crossed the area since the lis only reached this region in the late wisconsinan the cis montane ice responsible for the streamlined landforms must have occupied the area after the lis withdrew stratigraphy from the murray and pine river valleys supports a late wisconsinan age for the surface landforms and records two glacial events separated by a non glacial interval that was dated to be of middle wisconsinan mis 3 age

structure and tectonics of central and southern appalachian valley and ridge and plateau provinces west virginian and virginia the valley and ridge and eastern allegheny plateau from pennsylvania into tennessee is subdivided into distinct allochthonous sheets emplaced over an essentially featureless precambrian basement surface sheet development is related to variations in stratigraphic thickness and lithology as well as regional basement configuration three sheets control central appalachian structural style the massanutten blue ridge sheet is easternmost and is bounded on the west by the little north mountain pulaski fault system continuing westward into the plateau the martinsburg sheet contains anticlinoria and synclinoria uncut by major thrust faults both massanutten blue ridge and martinsburg sheets are seated in upper ordovician martinsburg reedsville shales the underlying waynesboro sheet is rooted in lower cambrian waynesboro rome shales and its imbrication has controlled the locations of major anticlinoria and synclinoria in the upper sheet within the southern appalachians greater imbrication of the lower sheet and increased thrust fault displacement at longitudinal ramps have destroyed the lateral continuity along strike of the central appalachian waynesboro and overlying martinsburg sheets there imbrication to the surface has formed the saltville sheet continuous into alabama as well as the st clair narrows copper creek cove mountain and st paul honaker sheets southwest of the trend change the holston stone mountain sheet adjacent to the blue ridge is rooted primarily in lower and middle cambrian strata and is underlain by the pulaski sheet in the westernmost sector of this region the pine mountain and richlands sheets are seated in devonian shales the pulaski and massanutten blue ridge sheets rooted primarily in upper ordovician shales form a continuous structural unit and include blue ridge and piedmont rocks

strain partitioning in the helvetic thrust belt of eastern switzerland from the leading edge to the internal zone abstract the strain and strain partitioning in the helvetic thrust belt are analysed in order to determine their variation across the belt from the external thin skinned portion to the internal thick skinned portion and in order to begin the detailed mechanical analysis of the entire fold and thrust belt coarse grained limestones of jurassic and cretaceous age were chosen for analysis because they can be traced from the external to the internal zones the strain within a bed is partitioned into three components transgranular pressure solution microfaults veins twinning granular and intragranular exclusive of twinning crystal plasticity associated with recrystallization transgranular strain was very important in the early history of the helvetic nappes giving way to twinning at a later stage under tectonic burial twinning strain was early in the infrahelvetic complex giving way to intracrystalline deformation at the later stage of burial the transition from transgranular to intragranular dominated strain occurred at a vitrinite reflectance value of 3 5 significantly below the greenschist grade of metamorphism in a large scale fold at the leading edge of the helvetic nappes transgranular strains were associated with the buckling stage of the fold and twinning occurred during fold tightening in more internal parts of the helvetic nappes twinning strains are also late with regard to transgranular strains but here their pattern corresponds to the regional finite strain pattern in the internal zone the infrahelvetic complex twinning seems to have been an early event and records an initial layer parallel shortening followed by regional internal deformation with intragranular strains

cosmogenic 36cl dating of the foothills erratics train alberta canada the foothills erratics train of alberta was carried into place during coalescence between montane glaciers and the laurentide ice sheet its age has been controversial late wisconsinan and pre wisconsinan ages have been assigned to it cosmogenic 36 cl exposure dating was carried out on erratics over a 130 km segment of the erratics train seven of eight age determinations support emplacement during the late wisconsinan glaciation these ages eliminate the possibility of an ice free corridor during the last glacial maximum they are also consistent with the hypothesis that the laurentide ice sheet was the most extensive of all pleistocene continental ice sheets in western canada

the use of finite strain data in constructing a retrodeformable cross section of the meade thrust sheet southeastern idaho u s a abstract a method for drawing a balanced cross section is presented which uses the constraint of both the interpretation of the geometry and the analysis of the internal deformation within the meade thrust sheet southeastern idaho u s a earliest deformation associated with the meade thrust sheet is layer parallel shortening which is indicated by a spaced solution cleavage and deformed fossils in the jurassic twin creek formation this early layer parallel shortening fabric is rotated by folding in front of the main meade thrust sheet these folds are then truncated by the meade thrust locally finite strain markers indicate fault parallel shear

abrupt bolling warming and ice saddle collapse contributions to the meltwater pulse 1a rapid sea level rise elucidating the source s of meltwater pulse 1a the largest rapid sea level rise caused by ice melt 14 18 m in less than 340 years 14 600 years ago is important for understanding mechanisms of rapid ice melt and the links with abrupt climate change here we quantify how much and by what mechanisms the north american ice sheet could have contributed to meltwater pulse 1a by driving an ice sheet model with two transient climate simulations of the last 21 000 years ice sheet perturbed physics ensembles were run to account for model uncertainties constraining ice extent and volume with reconstructions of 21 000 years ago to present we determine that the north american ice sheet produced 3 4 m global mean sea level rise in 340 years due to the abrupt bolling warming but this response is amplified to 5 6 m when it triggers the ice sheet saddle collapse

reconstructing the confluence zone between laurentide and cordilleran ice sheets along the rocky mountain foothills south west alberta landform mapping near the athabasca valley was fundamental to determining whether the late wisconsinan laurentide and cordilleran ice sheets lis cis coalesced in this paper we examine the detailed landform record using new lidar digital elevation and hillshade models of the area of confluence and eventual desuturing of these ice sheets this work reveals an earlier more extensive cordilleran advance before coalescence with the lis when the ice sheets coalesced the flow pattern was dominated by ice flow along the mountain front with cordilleran ice flowing out of major trunk valleys but laurentide ice flowing into the foothills up smaller valleys this flow pattern implies that when the ice sheets coalesced the cis was already waning or at least was not at its maximum deglaciation was interrupted by several re advances probably related to the destabilizing effect of proglacial lakes along the ice margin

late wisconsinan cordilleran and laurentide glaciation of the peace river valley east of the rocky mountains british columbia in the past researchers have disagreed over the maximum extent of the cordilleran ice sheet in the peace river valley during the late wisconsinan some workers argued that cordilleran ice reached beyond the rocky mountains and briefly coalesced with the laurentide ice sheet on the westernmost interior plains in contrast others asserted that cordilleran ice did not reach beyond the eastern front of the rocky mountains stratigraphic interpretation of three sections within a middle wisconsinan paleovalley and re examination of a previously published regional stratigraphic framework show that western sourced ice likely the cordilleran ice sheet extended east of the mountain front during the late wisconsinan prior to the incursion of the laurentide ice sheet into the area this conclusion has implications for cordilleran ice sheet reconstruction and modelling and provides insight into the interactions between the cordilleran and laurentide ice sheets during the last glaciation

electromagnetic resistivity anisotropy in till from the kiskatinaw streamlined landform field northeastern british columbia canada abstract applications of the azimuthal electromagnetic em method that establish the internal structure of landforms consisting of sediment are sparse in the kiskatinaw streamlined landform field of northeastern british columbia ice flow parallel till ridges formed beneath the cordilleran ice sheet during the late wisconsinan em anisotropy data from the kiskatinaw field are systematically related to ridge elongation and orientation and in part to clast fabrics however in contrast to previous studies we find that resistivity measurements do not necessarily record the orientation of ice flow and electrical anisotropy in till may not be a simple indicator of ice flow direction for highly elongate ridges l w 3 the long axes of resistivity ellipses are at high angles to the long axes of the landforms tills in these elongate ridges are matrix rich 85 and display strong unimodal ice flow parallel clast fabrics for more equant ridges l w

fractal strain distribution and its implications for cross section balancing abstract rock units having different physical properties show contrasts in structural styles massive competent rocks have relatively simple geometry and control the structural framework whereas thin bedded less competent units are characterized by complex deformation at smaller scales these contrasting structural styles can be quantified by a fractal dimension d a fold profile with a d close to 1 has a simple geometry a value of 2 d 1 indicates a fractal profile a fractal profile has a fractal strain distribution such that the measured shortening increases with resolution for rock units having different values of d the shortening strains measured with the same resolution cannot be compared because a significant amount of deformation takes place at smaller scales for these units with a larger d proper balancing requires high resolution analysis for these units to resolve strain at smaller scales in the central appalachians west virginia the cacapon mountain anticlinorium has a large ramp anticline in cambrian ordovician rocks which has a net shortening of 10 510 km as measured by section restoration in contrast restoration of the overlying complexly folded and faulted upper ordovician through devonian units indicated a 3 660 km shortening thin section analysis revealed a 15 20 strain equivalent to another 3 km shortening and the remaining difference in shortening between the cover and the underlying blind thrust sheet has been previously attributed to forethrusting fractal analysis indicates that the cambrian ordovician thrust sheet has a d 1 001 and the cover fold profile has a d 1 072 which indicates a fractal geometry if thin section scale through outcrop scale to map scale strains are included in the cross section balancing the cover sequence has a comparable shortening to that in the underlying thrust sheet

evidence of cordilleran late wisconsinan glaciers in the ice free corridor abstract glaciers flowing down the athabasca river valley during the last glacial period were fed both by local montane glaciers and by cordilleran ice moving easterly across the continental divide from british columbia resulting in a major eastward surge into the ice free corridor the timing of this event has been disputed for many years and has important implications for the study of paleoenvironmental conditions and the north south migration of biota in the corridor chronologic and lithostratigraphic data from the jasper region suggest that this event took place in the late wisconsinan subtill radiocarbon dates on wood in the athabasca river valley indicate that non glacial alluvial sediments began accumulating there sometime before 48 ka a finite date of 29 100 560 bp gsc 3792 on wood under till near jasper provides a maximum age for the onset of the last glaciation in the region although ice from west of the rocky mountain trench probably did not cross the continental divide until after 21 5 ka the occurrence of supraglacial metamorphic erratics as well as the distribution and nature of mapped glacial deposits and the trend of subglacial paleoflow indicators in the region east of the rockies suggests that late wisconsinan glaciers flowing from the mountains coalesced with laurentide glaciers and were deflected southeasterly along the mountain front an even more extensive glacial event may have effected the region in pre late wisconsinan times the model of late wisconsinan glaciation presented here is in agreement with all known radiocarbon data from the area and is especially attractive when supratill dates on non woody lacustrine organics are viewed in light of possible contamination by old carbon

strain variation in thrust sheets across the sevier fold and thrust belt idaho utah wyoming implications for section restoration and wedge taper evolution abstract although strain in fold and thrust belts is typically inhomogeneous on the largest scale strain within thrust sheets shows regular patterns the strain which is a quantitative measure of the deformation of any thrust sheet is independent of the translation of the sheet the internal deformation can be represented by a deformation profile the change in shape during thrusting of a fault perpendicular line which can then be used to unstrain the sheet during restoration in cross section balancing deformation profiles in well studied thrust sheets show regular geometric patterns and compatibility constraints can be used to minimize the data needed to define the profiles in other sheets from the geometry of deformation profiles and direct field observations the most important component of internal deformation of thrust sheets in many fold and thrust belts is early layer parallel shortening lps this can be removed in a straightforward manner during restoration and significantly affects the geometry i e taper of the restored section original basin wedge taper may have important implications for the mechanical evolution of fold and thrust belts thus changes in original basin taper resulting from strain removal may affect significantly the way in which we interpret the evolution of a fold and thrust belt in an area

blind thrust systems blind faults are documented from 20 thrust systems throughout the world they occur as single faults sets of faults or entire thrust systems the overlying cover responds to the formation of the blind system by backthrusting coupling forethrusting and deposition the relative importance of the first three responses for pretectonic cover is represented in a ternary backthrusting coupling forethrusting diagram pretectonic cover of blind systems deforms by either a combination of coupling and forethrusting or a combination of backthrusting and coupling

kinematic model for the curvature of the northern subalpine chain france abstract analyses of cross sectional shortening mapped faults and calcite twin strain are used in conjunction with geometric kinematic modeling to interpret the mechanism of curvature formation in the northern subalpine chain of france there is a 45 change in strike around the chain that is convex towards the foreland northwest five models for fold thrust belt curvature are applied to the northern subalpine chain they can reproduce the geometry and the kinematic history of a curved fold thrust belt and can be used to predict the magnitude and orientations of strains and the three dimensional geometry of the curvature the models are pure bending radial thrusting curve parallel simple shear uniform displacement uniform shortening and transport parallel simple shear two principal criteria for distinguishing among the models geometrically are the horizontal tangential strain extension or contraction parallel to strike and cross sectional shortening the transport parallel simple shear model predicts no tangential extension in the central domain and greater radial shortening and significant tangential extension in the northeastern and southern domains this is the pattern observed in the northern subalpine chain therefore the curvature of the northern subalpine chain is attributed to a transport parallel simple shear mechanism

broad top thrust sheet an extensive blind thrust in the central appalachians interpretation of seismic data from the central appalachians valley and ridge province reveals that a large blind thrust the broad top thrust is much more extensive than previously realized this thrust sheet extends across the 15 mi 24 km width of the nittany anticlinorium a dominate structural element that lies along the western margin of the valley and ridge province adjacent to the plateau province in the appalachian foreland it extends for a distance along strike of at least 100 mi 160 km and duplicates a 7800 ft 2380 m thick sequence of cambrian and ordovician limestone and dolomite referred to as the cambrian ordovician lithotectonic unit thinning of the duplicated cambrian ordovician sequence along the length of the nittany anticlinorium by as much as 2500 ft 760 m observed on seismic lines is attributed to removal of less brittle intervals from the base of the upper sheet and top of the lower sheet the missing section from this inter sheet detachment zone may have been transported toward the foreland and incorporated into the leading parts of the thrust sheet the tectonic thinning solves space problems associated with duplication of the lithotectonic unit beneath what have previously been considered abnormally deep synclines of the anticlinorium the abnormal depth of these synclines occurs where tectonic thinning is located on the seismic lines structural traps in a triangle zone along the allegheny structural front have been attributed to the intense shortening associated with emplacement of the broad top thrust sheet the presence of the thrust sheet along the length of the structural front in west virginia and virginia enhances the probability of successful exploration for triangle zone structures along the entire length of the structural front structures produced by variable tectonic thinning in the inter sheet detachment zone may also produce additional traps within the anticlinorium in the form of closure or associated fracture systems

precision and accuracy of the calcite strain gage technique the twinned calcite strain gage technique allows the determination of strain magnitude and direction in mildly deformed sedimentary rocks in that the measurements required for the calculation are rather time consuming it is important to know the minimum number required to obtain a precision that is comparable with the accuracy of the result this paper documents a series of tests on three different limestones having different textures and porosities in order to determine the precision and accuracy of the technique as a function of the number of measurements and the number of perpendicular thin sections the tests show that for most purposes the best measurement strategy will be to make 25 measurements on each of two perpendicular thin sections the best determined principal strain axis is the one having a magnitude that is opposite in sign from the other two over the range of zero to 11 shortening the magnitude of the odd signed strain axis will than have a precision and accuracy of about 11 in magnitude and a precision and accuracy of about 6 to 8 in orientation differences in texture and porosity do not appear to have significant effects upon precision and accuracy

quaternary stratigraphy and glacial history of the peace river valley northeast british columbiathis article is one of a selection of papers published in this special issue on the theme geology of northeastern british columbia and northwestern alberta diamonds shallow gas gravel and glaciers two cordilleran and three laurentide glacial advances are recorded in quaternary sediments and landforms in the peace river valley northeast british columbia the advances are inferred from fluvial gravels glaciolacustrine sedi ments and tills within nested paleovalleys excavated during three interglaciations and from the distribution of granitoid clasts derived from the canadian shield till of the last late wisconsinan laurentide glaciation occurs at the surface ex cept where it is overlain by postglacial sediments the advance that deposited this till was the most extensive in the study area and the only advance definitively recognized in western alberta south of the study area late wisconsinan cordil leran till has not been found in the study area but cordilleran and laurentide ice may have coalesced briefly during the last glaciation support for this supposition is provided by the inferred deflection of laurentide flutings to the southeast by cordilleran ice the earliest laurentide advance may have been the least extensive of the three laurentide events recog nized in the study area erratics attributed to this advance occur only east of the halfway river beatton river drainage divide

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

a computer program for the calcite strain gauge technique abstract the calcite strain gauge technique has been used successfully to determine strains in slightly to moderately deformed limestones the technique involves the use of a universal stage to collect calcite twin orientation thickness and frequency data the data are reduced by a least squares method to give the orientations and magnitudes of the principal strains the purpose of this paper is to describe an easy to use computer program for the technique and an accompanying manual on collecting data for the strain gauge analysis the program calculates principal strain magnitudes and orientations total strain due to twinning j20 5 standard errors for the strains average twin width twin intensity and percent negative expected values in addition the program can plot stereonets with compression axes tension axes poles to e twin planes crystallographic c axes g axes average twin widths per twin set twin intensity per twin set and positive and negative expected values the stereonets may also be rotated all calculated orientation data may be saved in a computer file for later input into a stereonet contouring program the user may choose automatically to remove 20 of the twin sets with the largest deviation or manually to remove selected twin sets from the data being analyzed in order to clean the data twin sets that give positive or negative expected values may also be analyzed separately

late paleozoic deformation of the cratonic carbonate cover of eastern north america paleozoic carbonates that cover the eastern half of the north american craton preserved a bedding parallel shortening fabric up to 800 km away from the appalachian ouachita orogenic front as recorded by mechanical twins in calcite a strain fabric is present in 29 of 31 samples from three traverse within the flat lying cratonic cover sequence jasper arkansas to minneapolis bluefield west virginia to madison wisconsin and albany new york to hamilton ontario twinning is found only in paleozoic carbonates and is absent in cretaceous limestones in the two southernmost traverses the orientation of the principal shortening axis e1 is perpendicular to the thrust front and parallel to the thrusting direction of the southern appalachians and the ouachitas the magnitude of calcite shortening strain 6 and the calculated differential stress 90 mpa decrease exponentially away from the orogenic front until twinning is absent the northernmost traverse shows greater complexity probably because of its location in the bend of the appalachian trend the authors conclude that the regional strain fabric preserved in the cratonic cover sequence of eastern north american was produced by late paleozoic orogenic stresses of the appalachian ouachita orogeny and that compressive stresses were transmitted over distances greater than 1200 km awaymore from the active plate margin less

the pattern and style of deglaciation at the late wisconsinan laurentide and cordilleran ice sheet limits in northeastern british columbia this paper reports on the landform assemblages at the northern confluence of the late wisconsinan laurentide and cordilleran ice sheets with montane and piedmont glaciers in the northern rockies and southern mackenzie mountains recent observations in northeastern british columbia refine our knowledge of the pattern and style of ice sheet retreat glacial lake formation and meltwater drainage at the onset of deglaciation confluent laurentide and cordilleran terminal ice margins lay between 59 n 124 30 w and 60 n 125 15 w from this terminal limit ice sheets retreated into north central british columbia and yukon territory with remnant cordilleran ice and montane glaciers confined to mountain valleys and the liard plateau distinctive end moraines are not associated with the retreat of cordilleran ice in these areas laurentide ice retreated northeastward from uplands and the plateaus then separated into lobes occupying the fort nelson and petitot river valleys ice retreat landforms include recessional end moraines sometimes overridden and drumlinized hill hole pairs crevasse fill deposits de geer like ribbed till ridges hummocky moraines kames meltwater features and glacial lake deposits that fall within the elevation range of glacial lake liard and glacial lake fort nelson ca 840 380 m meltwater and sediment transport into glacial lakes fort nelson liard nahanni and mackenzie was sustained by remnant ice in the liard river and fort nelson river drainage basins until the end of glaciation optical dating of sand from stabilized parabolic dunes on the liard plateau indicates that proglacial conditions lake formation and drainage began before 13 0 0 5 ka calendar years the petitot fort nelson and liard rivers all occupy spillways incised into glacial deposits and bedrock by meltwater overflow from glacial lakes peace and hay

roof sequence response to emplacement of the wills mountain duplex the roles of forethrusting and scales of deformation abstract this paper focuses on the behavior of a roof sequence in the appalachian plateau of west virginia u s a and emplacement of the wills mountain duplex with 17 5 km of displacement unlike the plateau along strike in pennsylvania and new york where forethrusting was previously documented this roof sequence lacks an underlying salt dominated roof decollement kinematic analyses reveal that the roof sequence in the west virginian plateau accommodated about two thirds of the 17 5 km of shortening by the adjacent wills mountain duplex as a forethrusting kinematic response the remaining shortening imbalance of about 5 km between the duplexes and younger roof sequence rocks is accommodated by additional forethrusting further into the foreland and local compensation this kinematic response matches that along strike in the central appalachians despite the loss of the salt decollement we interpret that an ordovician shale dominated formation was sufficiently weak to substitute for the salt horizon thus a weak mechanical unit rather than specifically a salt decollement is a necessary prerequisite for forethrusting a contributing factor to forethrusting may be the subvertical front of the wills mountain duplex which inhibited other responses by the roof sequence mesoscale and smaller processes including grain to grain pressure solution twinning and cleavage formation account for over 75 of the shortening in the roof sequence and if ignored would result in an erroneous interpretation of backthrusting or local compensation this result suggests that failure to consider all deformation scales could lead to incorrect kinematic conclusions in other tectonic systems

deformation of helderberg limestones above the blind thrust system of the central appalachians helderberg limestones have been subjected to late burial solution and layer parallel shortening by cleavage formation with twinning they also record deformation during passive macrofolding due to emplacement of the underlying blind thrust system including passive cleavage rotation during flexural flow perhaps by grain boundary sliding cleavage intensification in gentle fold limbs and cleavage dilation in steep limbs by cleavage normal untwinned calcite fibers cleavage dilation was the second dilation event as bedding solution structures were previously opened with bedding normal fiber growth during layer parallel shortening thus pre existing solution structures were commonly exploited as fluid conduits and precipitation sites

migration of deformation fronts during progressive deformation evidence from detailed structural studies in the pennsylvania anthracite region u s a abstract detailed study of selected outcrops in the pennsylvania anthracite region has resulted in identification of five at least partly continuous stages of alleghanian progressive deformation these structural stages are in sequence a layer parallel shortening and top to the foreland layer parallel shear b a second episode of layer parallel shortening c flexural slip and flexural flow folding d fold modification and late thrust faulting and e late veining and fracturing at each outcrop an average shortening direction was determined for each observed structural stage the mean regional shortening direction for each structural stage was calculated and maps showing the spatial distribution of the shortening directions for each stage were constructed the mean regional shortening directions for the five structural stages successively rotated 25 30 clockwise during the evolution of the fold and thrust belt shortening directions for structural stages b and c over the region show a systematic clockwise change in orientation from hinterland to foreland indicating that these structural stages of progressive deformation were time transgressive on a regional scale

postglacial viability and colonization in north america s ice free corridor during the last glacial maximum continental ice sheets isolated beringia northeast siberia and northwest north america from unglaciated north america by around 15 to 14 thousand calibrated radiocarbon years before present cal kyr bp glacial retreat opened an approximately 1 500 km long corridor between the ice sheets it remains unclear when plants and animals colonized this corridor and it became biologically viable for human migration we obtained radiocarbon dates pollen macrofossils and metagenomic dna from lake sediment cores in a bottleneck portion of the corridor we find evidence of steppe vegetation bison and mammoth by approximately 12 6 cal kyr bp followed by open forest with evidence of moose and elk at about 11 5 cal kyr bp and boreal forest approximately 10 cal kyr bp our findings reveal that the first americans whether clovis or earlier groups in unglaciated north america before 12 6 cal kyr bp are unlikely to have travelled by this route into the americas however later groups may have used this north south passageway

mari bugti pop up zone in the central sulaiman fold belt pakistan abstract the sulaiman fold and thrust belt is an active tectonic feature of the himalayan mountain system in pakistan seismic reflection profiles borehole surface geology data and bouguer gravity modeling suggested a passive roof duplex geometry over a transitional crust related to the former passive margin of the indian subcontinent in the frontal part of the sulaiman fold belt a passive roof sequence of cretaceous and younger rocks is structurally uplifted at the surface the roof sequence displays a coherent stratigraphy over the underlying duplex sequence of jurassic and older strata the folds in the roof sequence reflect blind faults in the duplex sequence the duplex style of deformation persists throughout the central sulaiman fold belt however unlike the frontal sulaiman fold belt stratigraphy at the surface in the central sulaiman is disrupted by e w and ne trending faults with apparent map lengths of tens of kilometers these foreland and hinterland verging high angle faults juxtapose cretaceous rocks in the cores of tight symmetrical anticlines against eocene ghazij shale and kirthar limestone according to seismic reflection data they have only minor vertical offsets of 1 2 km and are mostly restricted to the roof sequence as a result cretaceous rocks bounded between reverse faults are exposed at the surface in the cores of tight anticlines as pop up structures this implies that 1 the exposed faults in the central sulaiman fold belt are not primary structures with major shortening and 2 recognition of these faults in the roof sequence may reflect an early stage of development of overstep back thrusts from the upper decollement passive roof thrust

mechanisms of thrust propagation some examples and implications for the analysis of overthrust terranes abstract evidence is presented for the existence of three fundamental mechanisms by which thrust sheets move the mechanisms are 1 failure of a stiff layer to form ramp flat geometry imbricated thrust sheets 2 detachment of a layer by folding decollement thrust sheets 3 differential layer parallel shortening lps thrust sheets the mechanisms are independent but may operate interactively to form families of hybrid thrust sheets since lps thrust sheets have not been previously described or documented data from the new york plateau deformed almost exclusively by differential lps is presented to demonstrate the physical characteristics of this thrust mechanism information from the plateau indicates that finite strain behavior closely reflects the geometric boundary conditions and is independent of temperature and depth of burial finite strain data are used to construct a set of iso strain maps in both the deformed and undeformed states the iso strain maps are in turn used to determine the displacement field for the thrust sheet the displacement field allows visualization of the effects of both lagrangian and eulerian transformations on an initially orthogonal grid utilizing sections from a number of overthrust belts it is shown that the three mechanisms occur universally although in various proportions lps is regionally developed throughout the central and northern appalachians the presence of the lps thrust sheets probably accounts for the failure of structural cross sections to bed length balance in these blind thrust terranes finally a series of examples drawn from the field as well as the literature are used to illustrate both the characteristics of hybrid thrust sheet families as well as to indicate how knowledge of their behavior can be utilized in developing strategies for the construction and balancing of structural cross sections

deformational behavior of shale interlayers in evaporite detachment horizons jura overthrust switzerland shale deformation was studied in a series of naturally strained evaporites and shales deformed at low temperatures 70 high strain rates 10 14s 1 and low pressures overburden 2 km as well as in rock samples and sintered specimens experimentally deformed in coaxial and simple shear configurations at temperatures of 20 350 c strain rates of 10 4 10 6s 1 and confining pressures of 0 1 400 mpa within naturally strained shale sulfate multilayers three deformation regimes can be recognized that are separated by the inversion of relative competence of shale and anhydrite and the onset of gypsification respectively in deeper domains strain is focused on slickensides which are often coated with sulfate that acts as a lubricant in shallower domains and especially in domains with significant access to syntectonic water strain becomes pervasive and very low strength ductile shale gypsum tectonites form experiments reveal that confining pressure and water content have a significant influence on style of deformation pervasive or focused on discrete surfaces whereas temperature and strain rate have virtually no influence on either strength or deformation style except at temperatures 200 c that cause dehydration and embrittlement

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

duplex structures and imbricate thrust systems geometry structural position and hydrocarbon potential duplexes and imbricate thrust systems form some of the most complex hydrocarbon traps in overthrust belts the geometry of a duplex is controlled by the ramp angle thgr and height hr the final spacing between adjacent thrusts a prime and the relative displacements on them d1 d2 for constant thgr and hr three different classes are recognized 1 independent ramp anticlines and hinterland sloping duplexes 2 true duplexes and 3 overlapping ramp anticlines imbricate thrust systems consist of several thrust faults each of which loses displacement upsection and eventually dies out by progressively transferring its displacement to a fold at its tip or by distributing it among several splays hybrid duplexes are ormed from systems of fault propagation folds that are subsequently carried over ramps so they have a more complex geometry and a greater structural relief these different types of thrust systems occur as outcrops and prospect scale structures and they define the regional structural styles of most fold and thrust belts second and third order duplexes and imbricate thrust systems occur in specific structural positions such as footwalls hanging walls and frontal zones of major thrust ramps and in cores of major anticlines several types of duplexes and imbricate thrust systems form important hydrocarbon traps examples include the system of independent anticlines of the turner valley and highwood oil and gas fields the hinterland sloping duplex consisting of the chestnut ridge sandy ridge system of the ben hur oil field the partly overlapping anticlines of the waterton and savanna creek gas fields and the completely overlapping anticlines of the rose hill oil field

a classification of thrust fronts variations in structural style at thrust fronts are examined to determine whether they represent different stages of typical thrust sheet development or unique events as orogenic stresses declined thrust fronts are classified as buried or emergent in which either the sole thrust died out in the subsurface or emerged to the synorogenic surface buried thrust fronts are subdivided into two types type 1 fronts end in broad zones of low strain type 2 fronts end in zones of high strain emergent thrust fronts may display significant horizontal displacement strongly emergent or limited displacements of about 1 km weakly emergent tip stick thrust fronts exhibit high strains and out of sequence thrusting different types of thrust fronts are linked by transitional tip li e anticlines and strike slip zones most thrust front types represent different stages of typical thrust sheet development but those in type 1 display atypical development unique to thrust fronts

passive roof duplex geometry in the frontal structures of the kirthar and sulaiman mountain belts pakistan abstract exploration for hydrocarbons over the past few years has greatly improved our understanding of the geometry of frontal mountain belt structures in this study we introduce and discuss the concept of the passive roof duplex using as the main example the kirthar and sulaiman ranges in the baluchistan province of pakistan structures similar to those described here have been recognized previously in other mountain belts and they appear to exist as a common feature in many more frontal regions of mountain belts our example of a passive roof duplex which we describe from pakistan is compared briefly with similar structures reported by others the passive roof duplex is here defined as a duplex whose roof thrust has backthrust sense passive roof thrust and whose roof sequence those rocks lying above the roof thrust remains relatively stationary during foreland directed piggy back style propagation of horses within the duplex