development of cleavage in limestones of a fold thrust belt in eastern new york abstract tectonic cleavage has developed in lower devonian limestones of the hudson valley fold thrust belt in new york state morphology and distribution of cleavage in these limestones is controlled by the amount of clay quartz matrix present and by strain only limestones with greater than 10 clay quartz matrix developed widespread cleavage x ray diffraction analyses indicate that the clay matrix was altered during deformation the width of the 001 illite peak in cleaved lime wackestone is less than that in uncleaved lime wackestone from the same stratigraphic level a minimum of 10 clay is necessary to provide interconnectivity between sites of dissolution and the free fluid system preliminary analyses of bulk chemistry indicate that calcite is removed from the local rock system during cleavage development supporting proposals that circulation of fluid through the rock plays a major role in the development of cleavage textural evidence suggests that both pressure solution and free face dissolution contribute to the removal of ions at grain boundaries cross cutting relations indicate that cleavage was initiated early during the development of the fold thrust belt but continued to develop during the late stages of folding

how long do structures take to form in transpression zones a cautionary tale from california it is generally assumed that individual sets of coplanar and colinear deformation structures form together during events that are of relatively short duration 1 5 m y the record of deformation in a sequence of late cretaceous to holocene sedimentary rocks from the northern salinian block of california spans at least 30 m y and illustrates that this assumption is sometimes incorrect at different localities geometrically and kinematically identical contractional structures either predate or postdate local unconformities of varying age within the succession so that it is possible to define at least four chronologically distinct but otherwise indistinguishable deformation episodes in the absence of the unconformities the punctuated nature of the deformation would not be apparent therefore suggesting that subparallel structures may form during successive distinct deformations spread out over long time periods in the northern salinian block the inferred contractional strain field is approximately normal to the adjacent san andreas fault and appears to have been consistently oriented in this direction during deformation events recorded over the past 30 45 m y the strain pattern is most easily explained by efficient partitioning of transpressional strains into fault normal shortening and right lateral faulting during episodic regional deformations we propose that reactivation of preexisting structural anisotropies controls the observed partitioning of deformation in many transpression zones

cleavage fold relationships and their implications for transected folds an example from southwest virginia u s a abstract a non coaxial deformation involving pre folding initiation of cleavage perpendicular to bedding is proposed to explain non axial planar cleavage associated with mesoscopic folds in part of the appalachian foreland thrust belt of southwest virginia folds are gently plunging asymmetric upright to slightly inclined sinusoidal forms with non axial fanning cleavage they show extreme local variations in type and degree of transection and the consistency of transection direction these relations are further complicated by hinge migration cleavage fan angles bedding cleavage angles and transection values appear influenced by fold tightness and in part by fold flattening strain fold flattening increments are considered simultaneous with folding axial surface traces and not cleavage traces coincide with the principal extension direction in fold profiles geometric modelling of cleavage fanning and bedding cleavage angle variations for various theoretical folding modes suggest that folding in limestone and sandstone layers was by tangential longitudinal strain significant shape modification and change in bedding cleavage relations occurred after limb dips of 40 and 50 were attained in limestone and sandstone respectively mud rock class 1c folds with convergent cleavage fans show features transitional between buckling and flexural flow initiation of cleavage fabrics during layer parallel shortening prior to significant folding may be important for cleavage evolution in some deformed rocks

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

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

interrelations of mesoscopic structures and strain across a small regional fold virginia appalachians abstract small regional folds such as the clover hollow anticline of the narrows thrust sheet in southwest virginia u s a are considered to be large buckle folds expressing lateral shortening above a subsurface decollement cleavage mesoscopic and regional folds and contraction faults have developed in these rocks under anchimetamorphic conditions in a single protracted deformation during thrust sheet emplacement the contraction faults dominate the structure at all scales three fault associations isolated contraction faults contraction faults in series and complex fault zones with intense folding determine the pattern and intensity of local structures regional displacement transfer of strain along and across faults has produced local variations in structural style duplex like systems of second order faults terminate laterally into zones of intense folding and third order faulting fold tightness cleavage intensity strain magnitude and total longitudinal strain e t are maximum in these regions contraction faults in this thrust sheet have propagated along zones of high strain rate associated with mesoscopic folding and intense cleavage regional hinge migration and greater structural complexity along the southeast limb of the clover hollow anticline are considered to be due to emplacement of the adjacent thrust sheet

strain patterns and shortening in a folded thrust sheet an example from the southern appalachians abstract total strain patterns estimated across the pulaski thrust sheet of the southwest virginia appalachians show an approximately homogeneous plane strain deformation associated with folding and distortion above a subsurface decollement estimated strains are low 1 2 x z

cleavage development within a foreland fold and thrust belt southern pyrenees spain abstract in the southern pyrenees lithologically distinct cleavage fronts are each parallel to bedding and dip 20 towards the foreland pressure solution was the dominant mechanism of cleavage development the mudstone cleavage front is coincident with the 195 c paleoisotherm and is associated with a pressure solution strain of 5 a mechanical twin strain of 4 and a deviatoric stress magnitude of 65 mpa illite crystallinity measurements define a geothermal gradient of 15 c km 1 and indicate that the paleoisotherms are bedding parallel deviatoric stress magnitudes from calcite twins were regionally constant at 65 mpa and principal stress axes were perpendicular to cleavage temperature was the primary control on deformation micromechanisms and the position and orientation of the cleavage front within the foreland thrust wedge deformation below the cleavage front occurs predominantly by pressure solution which in conjunction with mechanical twinning and microfracturing produces a quasi plastic rheology stress magnitudes determined from mechanical twinning of carbonate grains and long term 10 6 10 76 y strain rates determined for regional folds and faults suggest an apparent macroscopic viscosity of 9 8 10 18 to 7 2 10 19 pa s for the lower thrust wedge above the cleavage front temperature pressure solution strain total strain and mesoscale deformation diminish the region of the thrust wedge above the 100 c paleoisotherm is characterized by large brittle faults with cataclastic fault zones and negligible grain scale deformation indicating an elastico frictional rheology

the evolution of mesoscopic imbricate thrust faults an example from the vermont foreland u s a abstract an outcrop of a thin 30 cm bed of micrite surrounded by thicker sequences of well cleaved calcareous shale in the ordovician flysch of western vermont records a complex history of imbricate faulting and associated folding the calcareous shale has been shortened by pressure solution cross cutting relations among the floor ramp and roof faults indicate that faulting progressed from the hinterland east to the foreland west all the ramp faults developed from arrays of w climbing enechelon extension fractures all other faults were controlled by the weaker bedding planes that surround the micrite and are found at various levels in the shale each fault is marked by layers of highly twinned sparry calcite and black carbonaceous shale selvedge simple shear along the floor thrust has rotated the s1 cleavage toward the foreland and produced along with volume change a pressure solution cleavage in the fault zone all cleavages have progressively developed from the hinterland to the foreland during folding and faulting of the micrite layer parallel shortening measured in both rock types is between 11 and 16

mobile hinge kinking in layered rocks and models abstract an unresolved question in structural geology is whether kink folds grow by the gradual rotation of the fold limbs about a fixed hinge by the nucleation of kink bands that grow by fold hinge migration or by some combination of these mechanisms the importance of mobile hinge kinking has previously been recognized in experimentally deformed slate and phyllite as well as decks of paper cards but its importance in the folding of multilayers consisting of alternating competent and incompetent layers has been uncertain this study shows that mobile hinge kinking can be an important folding mechanism in both natural and experimental multilayers the natural folds are in pelagic limestones from the umbria marche apennines of italy the geometrical properties of these folds as well as the small scale structures in the rocks indicate that these folds formed by the nucleation and expansion of kink bands with mobile hinges not by a fixed hinge folding mechanism the experimental folds were produced by the deformation of multilayered models consisting of layers of lead separated by layers of wax impregnated cloth the progressive growth of these folds by the nucleation and expansion of kink bands was directly observable a rough estimate of the rate at which fold hinges migrated through the apennine limestones was obtained indirectly using the displacement history recorded by calcite fibers on a fault which cuts a kink fold and was active during the formation of the fold it appears that the rate at which fold hinges migrated was approximately 100 times greater than the displacement rate along a fault that was moving by pressure solution slip

deformation within foreland thrust sheets by populations of minor faults the dominant deformation mechanism within foreland thrust sheets is the displacement of blocks of weakly deformed rock on discrete mineral coated minor faults in highly deformed layers at the bases of three southern appalachian foreland sheets such minor faults are members of families or populations of consistently oriented structures the deformation style is similar in all three sheets rock nearest each thrust was shortened in the direction of sheet transport and thickened by a family of faults oriented at low angles to the thrusts rock further above each thrust experienced only the first of these two deformation stages statistical analyses of fault populations from these three sheets demonstrate that their deformation is quantitatively as well as qualitatively similar deformation due to movement on minor faults is statistically homogeneous at the scale of an outcrop in one of these sheets strain due to minor faults can be measured in that sheet a strain profile across this sheet is similar to strain profiles across continuously deformed sheets from internal portions of mountain belts suggesting that their responses to emplacement are similar

calculation of shortening due to outcrop scale deformation and its relation to regional deformation patterns abstract the objectives of this study are to assess the regional contribution of outcrop scale structures in well exposed rocks of a tectonic province and evaluate how failure to measure this scale of structure affects the assessment of regional deformation the contribution of outcrop scale structures to regional deformation has not been quantified previously because of the need for long continuous exposures to regionally characterize the structures this study used 18 6 km of semi continuous roadcut and streambed exposures in well exposed devonian clastic rocks to construct 1 200 scale profiles to address the objectives key results are 1 outcrop scale structures contribute about 1 6 to 1 4 of total regional shortening 2 submapscale shortening components from microscale and outcrop scale structures equal mapscale shortening that would be interpreted from a regional cross section 3 analysis of submapscale structures in the target blind thrust belt is needed to identify that the roof sequence deformation preceded and continued through duplex formation 4 the kinematic response of the roof sequence would be incorrectly identified as backthrusting if the shortening contribution of outcrop scale structures is ignored 5 although measured outcrop scale shortening does not match that predicted by the fractal analysis of other workers the fractal approach to predicting deformation as a function of scale may still be valid

extensional layer parallel shear and normal faulting abstract an extensional fault system in bare mountain nevada u s a contains abundant evidence of layer parallel shear deformation contemporaneous with faulting layer parallel shear is manifest by deformation of pre faulting fabrics and cleavage at low angles to bedding that indicate shear in the down dip direction perpendicular to fault bedding intersections layer parallel shear along discrete bedding planes locally offsets normal faults and shear distributed within layers reorients block bounding normal faults in simple rigid block models of extension accommodated by normal faults above a low angle detachment or decollement extension causes faults to rotate to progressively shallower dips while originally horizontal beds rotate to steeper dips these rotations reorient faults out of originally optimum conditions for slip into orientations of a lower slip tendency whereas bedding rotates to steeper dips with progressively higher slip tendency the timing or amount of rotation before the initiation of layer parallel shear depends on the frictional resistance to sliding or resistance to shearing within layering in fault blocks offset or deflection of block bounding normal faults may cause faults to lock as extension increases alternatively bedding and faults may become simultaneously active progressively lowering dips of faults and bedding until neither is well oriented for slip at which point new faults are required to accommodate additional extension at bare mountain early extension within the fault system was accomplished by fault slip and associated block rotation continued extension took place by slip along bedding within fault blocks

kink detachment fold in the southwest montana fold and thrust belt the hossfeldt anticline in the southwest montana thrust belt is characterized by a kink geometry and probably overlies a thrust detachment at depth the mesofabric distribution in the limbs documents that the eastern overturned limb has undergone most of the rotation and internal deformation during folding leaving the gently dipping western limb virtually undeformed the anticline exhibits unique mesofabrics in its hinge region that require a pinned anticlinal hinge during its evolution the half wavelength of the hossfeldt anticline eustis syncline pair coincides with that predicted from buckling theory if one considers the massive carbonates of the paleozoic section as a competent beam although the geometry and mesofabric distribution of the hossfeldt anticline satisfy the geometric requirements of either a fault propagation fold or a detachment kink fold the buckling wavelength strongly suggests that its origin was as a kink buckle fold above a flat detachment rather than as a fault propagation fold above a thrust ramp

regional structural synthesis wyoming salient of western overthrust belt surface geologic mapping regional and high density reflection seismic data and information from approximately 370 wells have been combined to describe geometrically that area of the western overthrust belt between the snake river plain and the uinta uplift particular care has been taken to verify two dimensional interpretations of multiple thrust sheets by linear restoration by establishing equivalence of pre thrust lengths of affected beds one can gain confidence in interpreted structural geometries as well as generate data in regard to internal shortening and deformation intensity because of irresoluable geometric problems within sections 47 cross sections were developed at roughly 6 mi 10 km spacing to help verify changes in displacement or placement of key st uctural elements displacements of 0 to 32 mi 0 to 51 km have been demonstrated jurassic to paleocene sevier thrusts were active across a previously deformed cratonic shelf terrane and interacted with active structural elements such as the uinta uplift gros ventre range and moxa arch thrusts are progressively younger to the east except for the darby prospect pair in which the darby system is younger

lower crustal thinning in the rhinegraben implications for recent rifting seismic near vertical and undershooting experiments carried out in the southern upper rhinegraben area between 1984 and 1991 show pronounced lateral variations of deep crustal properties significant differences in the apparent thickness of the reflective lower crust and the transparent upper crust appear to be related to different structural settings a 12 to 14 km thick reflective lower crust beneath a 15 km thick transparent upper crust of the eastern graben shoulder black forest probably originated during permo carboniferous reequilibration of thickened variscan orogenic crust thinning of this lower crust by about 5 km beneath the graben and rifted domains transitional between graben and shoulder the dinkelberg block is interpreted to be related to cenozoic extensional faulting a discrepancy between moderate extension of upper crust and lower crustal geometry indicates mechanical decoupling at depth during extension congruent modification of other physical properties is suggested by strong single reflective elements in the topmost parts of the thinned lower crust in the transitional dinkelberg block such an anomalously strong reflector occurs at a depth of 20 km below a pronounced local maximum of earthquake activity it is interpreted to be the presently active zone of decoupling which in time shifted from the rift axis to the eastern transition area

correction to repeating waveform initiated by a 180 190 ka geomagnetic excursion in western north america implications for field behavior during polarity transitions and subsequent secular variation by robert m negrini et al

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

including strain data in balanced cross sections abstract almost all published balanced cross sections include cleaved and or strained rocks and may therefore violate the requirement that bed lengths and formation areas are preserved between undeformed and deformed states knowledge of distortions presence and type of cleavage and overprinting relations in poly deformed segments are necessary to define area changes related to volume loss strike elongation and poly deformation work in the southern and central appalachians has shown variation in structural complexity and changing deformation styles both along and across the thrust belt this necessitates separate treatment of individual sheets during balancing regions of little or no interstratal slip pinning points should be chosen for each thrust sheet and each fold strain in the sheet in external sheets where deformation intensity increases towards thrusts pin points should be back limb inflection points in intermediate and internal sheets where cleavage is approximately coeval with folding pin points should be at fold hinges where cleavage and strain are heterogeneously developed throughout a stratigraphic sequence a line length balance using massive relatively uncleaved beds e g knox group southern appalachians can be used as a basis for restoring the section unstraining of deformed layers given strain data and cleavage trajectories for individual thrust sheets provides line lengths and formation areas which can be fitted to the key bed of the undeformed section cleavages which partially or fully postdate folding indicate continued shortening in the thrust wedge and modify earlier pinning points in upper sheets these cleavages and poly deformed segments are difficult to restore as strain data are generally scarce in the metamorphosed internal thrust sheets

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

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

scales of structural heterogeneity within neotectonic normal fault zones in the aegean region abstract structural heterogeneities within neotectonic normal fault zones in the aegean region are important controls on the geomorphic expression of their uplifted footwall blocks contrasting fault rocks and slip plane phenomena corrugations comb fractures slip parallel fractures and pluck holes are responsible for mesoscopic inhomogeneities within fault scarps along strike disruptions in the continuity of fault scarps step over zones step up zones step down zones footwall cross faults and hangingwall cross faults are products of perturbations in slip plane geometry or intersections with transverse faults large scale variations in range front morphology reflect contrasting patterns of deformation within fault zones some major fault zones and many minor fault zones are characterized by a migration of the active slip plane towards the hangingwall resulting in a distributed network of high angle faults and a range front which is stepped in profile the presence of a localized mass of uneroded bedrock hangingwall salients protruding from the hangingwall of some major fault zones results in intense deformation along a principal slip plane and a range front which is ramp like in profile as a consequence of structural heterogeneities within neotectonic normal fault zones fault scarps and range fronts in the aegean region are subject to a temporally and spatially variable pattern of degradation

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

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

displacement and strain patterns of extensional orogens rock fabrics that result from displacement in extensional orogens provide a means of identifying geometric models responsible for extension of continental crust strain compatibility arguments indicate that a finite extension can be accommodated by displacements across 1 planar nonrotating faults or ductile shear zones 2 shear zones which rotate above a horizontal detachment the domino model or 3 shear zones which rotate as a result of a horizontally oriented pure shear stretching component the plastic model listric normal shear zone geometries may develop as the result of a depth dependent change in the pure shear component in the plastic model or the area loss component in the planar shear zone model within the geometric framework of these various models the effects of superposed simple shear pure shear extension and area change on the rock fabric are investigated these displacement components which may be superposed sequentially or simultaneously determine the state of finite strain associated with a given magnitude of tectonic extension the relationships between displacement and strain are expressed as graphs of foliation dip or strain ratio versus shear strain and as graphs of foliation dip or strain ratio versus tectonic extension the shear strain graphs illustrate the effects of the different displacement components on the rock fabric in a single shear zone whereas the tectonic extension graphs better illustrate these effects on the regional scale the shear strain and tectonic extension graphs can be used by field geologists 1 to determine tectonic extension across a region from measurements of strain ratio or orientation and fluctuation of foliation 2 to distinguish between possible extension models from field data and 3 to evaluate the influence of shear zone attitude and thickness upon the amount of displacement applications of these methods are illustrated for field areas in metamorphic core complexes of arizona and in the basin and range province of the north america cordillera

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

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

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

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