joint sets that enhance production from middle and upper devonian gas shales of the appalachian basin the marine middle and upper devonian section of the appalachian basin includes several black shale units that carry two regional joint sets j1 and j2 sets as observed in outcrop core and borehole images these joints formed close to or at peak burial depth as natural hydraulic fractures induced by abnormal fluid pressures generated during thermal maturation of organic matter when present together earlier j1 joints are crosscut by later j2 joints in outcrops of black shale on the foreland northwest side of the appalachian basin the east northeast trending j1 set is more closely spaced than the northwest striking j2 set however j2 joints are far more pervasive throughout the exposed devonian marine clastic section on both sides of the basin by geological coincidence the j1 set is nearly parallel the maximum compressive normal stress of the contemporary tectonic stress field shmax because the contemporary tectonic stress field favors the propagation of hydraulic fracture completions to the east northeast fracture stimulation from vertical wells intersects and drains j2 joints horizontal drilling and subsequent stimulation benefit from both joint sets by drilling in the north northwest south southeast directions horizontal wells cross and drain j1 joints whenever present then staged hydraulic fracture stimulations if necessary run east northeast i e parallel to the j1 set under the influence of the contemporary tectonic stress field thereby crosscutting and draining j2 joints

early rotation and late folding in the pennsylvania salient u s appalachians evidence from calcite twinning analysis of paleozoic carbonates calcite twinning analysis of paleozoic limestones from 42 sites reveals that the change in regional strike along the frontal edge of the pennsylvania salient is accompanied by an equal magnitude rotation of paleostress directions of up to 60 degrees the rotations recorded at 22 reliable sites show no discernible difference between sites with rocks of cambrian ordovician and silurian devonian age evidence for similarly fanned orientations is not present in foreland sites scatter in the data is attributed to grain scale rotations and compaction overprinting as demonstrated in prior studies and it was reduced by data cleaning methods as well as by the use of contouring and data averaging methods comparisons of paleostress directions within the belt reveal only minor rotations in the southwest region of the salient and the bulk of rotation is accommodated by the northern limb we hypothesize that these rotations resulted from convergence in the thrust wedge against a northerly bounding rigid basement block around pennsylvanian times this created a structural anisotropy in the evolving belt that guided the post rotational formation of folds in early permian times and produced the current pattern of the salient this model of decoupled thrusting rotation and folding explains a variety of previously conflicting observations in the area while offering a kinematically consistent scenario that may also apply to other curved orogenic belts

controls on subsurface methane fluxes and shallow gas formation in baltic sea sediment aarhus bay denmark shallow gas accumulates in coastal marine sediments when the burial rate of reactive organic matter beneath the sulfate zone is sufficiently high and the methanogenic zone is sufficiently deep we investigated the controls on methane production and free methane gas accumulation along a 400 m seismo acoustic transect across a sharp transition from gas free into gas bearing sediment in aarhus bay denmark twelve gravity cores were taken in which the pore water was analyzed for inorganic solutes while rates of organic carbon mineralization were measured experimentally by so42 s 35 radiotracer method the thickness of organic rich holocene mud increased from 5 to 10 m along the transect concomitant with a shallowing of the depth of the sulfate methane transition from 4 m to 2 5 m in spite of drastic differences in the distribution of methane and sulfate in the sediment along the transect there were only small differences in total mineralization and methanogenesis was only equivalent to about 1 of sulfate reduction shallow gas appeared where the mud thickness exceeded 8 9 m rates of methanogenesis increased along the transect as did the upward diffusive flux of methane interestingly the increase in the sedimentation rate and holocene mud thickness had only a modest direct effect on methanogenesis rates in deep sediments this increase in methane flux however triggered a shallowing of the sulfate methane transition which resulted in a large increase in methanogenesis at the top of the methanogenic zone thus our results demonstrate a positive feedback mechanism that causes a strong enhancement of methanogenesis and explains the apparently abrupt appearance of gas when a threshold thickness of organic rich mud is exceeded c 2016 elsevier ltd all rights reserved

cross correlation analysis of long term ambient seismic noise recordings in the caribbean netherlands to monitor the volcanoes on saba and st eustatius

deformation along a salient transverse zone junction an example from the leamington transverse zone utah sevier fold thrust belt usa abstract within fold thrust belts the junctions between salients and recesses may hold critical clues to the overall kinematic history of fold thrust belts the deformation history within these junctions is best preserved in areas where thrust sheets extend from a salient through an adjacent recess we examine one such junction within the sevier fold thrust belt western united states along the leamington transverse zone northern utah the canyon range thrust sheet can be traced continuously from the leamington transverse zone to its adjacent salient to the south the central utah segment deformation within the canyon range thrust sheet took place by faulting and cataclastic flow analyses of these fault networks preserved throughout the canyon range thrust sheet are used to develop a kinematic history of the leamington transverse zone field data is supplemented by analog sandbox experiments this study suggests that in detail deformation within the overlying thrust sheet may not directly reflect the underlying basement structure moreover these junctions may contain several types of accommodating structures that helped to maintain critical taper and that serve as potential targets for natural resource exploration

new model for evolution of fold and thrust belt curvature based on integrated structural and paleomagnetic results from the pennsylvania salient in a number of curved fold and thrust belts worldwide such as the cantabrian arc and the wyoming idaho salient paleomagnetic data indicate vertical axis rotations inconsistent with structural findings this apparent conflict is especially pronounced in the pennsylvania salient where the paleomagnetically defined vertical axis rotations are opposite of the rotation sense indicated by structural studies we resolve this apparent disparity by developing an integrated and kinematically admissible model that has implications for other curved mountain belts the curvature of the pennsylvania salient may be explained by deformation partitioning in an initially laterally tapered basin between the cambrian ordovician carbonates and the overlying siliciclastic rocks earliest deformation occurred in the lowest strata and progressed toward the foreland and up section with time lateral differences in initial layer parallel shortening in the cambrian ordovician carbonates caused differential translation and rotations about a vertical axis in the cover sequence because of initial basin geometry the orogenic wedge developed a lateral taper with further shortening in the wedge and involvement of the cover rocks in the fold and thrust deformation gravitational driving forces became more important and the paleostress trajectories diverged in opposite directions on the two arms of the salient in response to the lateral taper

strain variations and three dimensional strain factorization at the transition from the southern to the central appalachians abstract the transition between the southern and the central appalachian foreland thrust system is a region subjected to at least two non coaxial deformations where typical two dimensional analyses such as cross section balancing or plane strain retro deformation cannot be applied three dimensional factorization can simulate the non coaxial strain history simulation results for compaction and layer parallel shortening lps were compared to finite strains of 16 quartz arenite samples from the tuscarora sandstone successful simulations indicate in the southern appalachians 30 35 total compaction by volume loss including a 4 5 9 e w horizontal shortening during lithification and 5 lps along 150 330 by plane strain or axial symmetric flattening in the central appalachians 30 35 total compaction by volume loss including a 3 7 n s horizontal shortening during lithification and 10 lps along 120 300 by plane strain or axial symmetric flattening in the transition zone 30 compaction by volume loss including a 3 e w horizontal shortening during lithification 5 southern lps along 150 330 by axial symmetric flattening and 5 10 central shortening along 130 or 140 by plane strain or axial symmetric flattening at 10 to the normal central trend from the simulations horizontal shortening in the southern appalachians during lithification in the mississippian was greater than the later alleghanian deformation this shortening probably represents a very early lps developed in unlithified sand that required less deviatoric stress to deform than did the cemented quartz arenites during later tectonic deformation also the oblique and weaker central tectonic lps in the transition zone probably represents behavior at the limits of central appalachian deformation finally the lps simulations indicate that axial symmetric flattening oblate strain is more successful than plane strain as the representative behavior so caution is recommended in assuming plane strain as the dominant strain behavior in orogenic forelands

overprinted strike slip deformation in the southern valley and ridge in pennsylvania abstract two major faults over 32 km long and 6 4 km apart truncate or overprint most previous folds and faults as they trend more northerly than the previous n25 e to n40 e fold trends the faults were imposed as the last event in a region undergoing sequential counter clockwise generation of tectonic structures the western big cove anticline has an early nw verging thrust fault that emplaces resistant rocks on its nw limb a 16 km overprint by the cove fault is manifested as 30 small northeast striking right lateral strike slip faults this suggests major left lateral strike slip separation on the cove fault but steep dip slip separation also occurs from south to north the cove fault passes from se dipping beds within the big cove anticline to the vertical beds of the nw limb then it crosses four extended separated tuscarora blocks along the ridge brings cambro ordovician carbonates against devonian beds and initiates the zone of overprinted right lateral faults finally it deflects the lat 40 n fault zone as it crosses to the next major anticline to the northwest to the east the major path valley fault rotates and overprints the earlier carrick valley thrust the path valley fault and cove fault may be mesozoic in age based upon fault fabrics and overprinting on the east west lat 40 n faults

significance of the deformation history within the hinge zone of the pennsylvania salient appalachian mountains abstracttwo competing models exist for the formation of the pennsylvania salient a widely studied area of pronounced curvature in the appalachian mountain belt the viability of these models can be tested by compiling and analyzing the patterns of structures within the general hinge zone of the pennsylvania salient one end member model suggests a nw directed maximum shortening direction and no rotation through time in the culmination an alternative model requires a two phase development of the culmination involving nnw directed maximum shortening overprinted by wnw directed maximum shortening structural analysis at 22 locations throughout the valley and ridge and southern appalachian plateau provinces of pennsylvania are used to constrain orientations of the maximum shortening direction and establish whether these orientations have rotated during progressive deformation in the pennsylvania salient s hinge outcrops of paleozoic sedimentary rocks contain several orders of folds conjugate faults steep

continental margins orogenic belts and intracratonic structures angular bends promontories and embayments in rifted and passive continental margins are interpreted to be the precursors of curves recesses and salients respectively in orogenic belts convergence at an irregularly shaped continental margin results in along strike diachroneity of closing and thus of orogeny and in a variable trajectory of stress into continental crust both timing and sense of movement of intracratonic structures apparently have complex relationships to the progress of convergence at irregular continental margins

pennsylvania salient of the appalachians a two azimuth transport model based on new compilations of piedmont data new compilations of geometry and tectonic transport in the pennsylvania salient suggest that these features continue uninterrupted from the piedmont through the valley and ridge provinces these data are the basis for a new two azimuth transport model for origin of the curvature edge geometry of an eocambrian craton and a thickened stratigraphy acted as templates for alleghanian regional decollement first as reading prong n35 w motion followed by n68 w blue ridge motion both displacements involved some drag rotation and overprinting in a central zone this model avoids the problem of minimal tangential stretching explains the anthracite basins as trailing edge graben and basin structures against the autochthonous pocono plateau and produces the nittany juniata culmination as an overprinted pileup of duplexes in the central zone the model of detachment followed by changing azimuth of motion of a megadecollement sheet may apply to other salients throughout the world

characterization of the seismically imaged tuscarora fold system and implications for layer parallel shortening in the pennsylvania salient abstract the tuscarora fold system tfs is located in the pennsylvania salient in the foreland of the valley and ridge province the tfs is imaged in high quality 3d seismic data and comprises a system of small scale folds within relatively flat lying lower silurian tuscarora formation strata we characterize the tfs structures and infer layer parallel shortening lps directions and magnitudes associated with deformation during the alleghany orogeny previously reported lps data in our study area are from shallow devonian and carboniferous strata based on outcrop and core analyses above the shallowest of three major detachments recognized in the region seismic data allows us to characterize lps at depth in strata beneath the shallow detachment our lps data orientations and inferred magnitudes are consistent with the shallow data leading us to surmise that lps during alleghanian deformation fanned around the salient and was distributed throughout the stratigraphic section and not isolated to strata above the shallow detachment we propose that a nw se oriented alleghanian maximum principal stress was perturbed by deep structure associated with the non linear margin of laurentia resulting in fanning of shortening directions within the salient

structural geology of stewart peak culmination idaho wyoming thrust belt a structural culmination is the highest point along a structural axis or fold system away from which the folds plunge structural culminations are an important and predictable component of foreland fold thrust belts and are prime targets for oil and gas accumulations the stewart peak culmination is a doubly plunging anticlinorium in the absaroka thrust sheet of western wyoming located in the northern salt river range major thrust faults in the stewart peak culmination comprise a large scale duplex fault zone the roof fault of the duplex is the absaroka thrust the floor fault is the basal or regional decollement and the intervening murphy and firetrail thrusts are trailing edge imbricate faults in the darby plate that have locally uplifted and deformed the absaroka into an asymmetric breached anticlinorium or culmination in addition these major thrust faults subdivide the culmination into fault bound structural horses each with a characteristic style of deformation as a function of its stratigraphic and structural level in the duplex a recent exploratory test of the culmination provided data which suggest that the absaroka thrust fault locally cuts downsection in the transport direction this may be explained by initial folding and breaching of the firetrail anticline followed by main motion on the absaroka thrust which overrode the fold the available data suggest that the stewart peak culmination is the result of 1 polyphase uplift or arching of the absaroka thrust sheet by the murphy and firetrail thrust sheets 2 lateral imbrication and ramping and 3 arching of the precambrian basement complex beneath the northern salt river range in addition a basic kinematic model of deformation applicable to structural culminations includes emplacement of an allochthonous mass over a parautochthonous substratum closely followed by or contemporaneous with deformation and translation of the footwall section to produce a duplex fault zone that folds the overlying allochthon the stewart peak culmination bears many similarities in structural style and origin to polyphase folded fault domes in the canadian rocky mountains specifically the stewart peak area is compared to the savanna creek and moose mountain culminations

history of the sevier orogenic wedge in terms of critical taper models northeast utah and southwest wyoming problems with applying critical taper models to ancient orogenic wedges are overcome in the late cretaceous late paleocene sevier orogenic wedge in utah and wyoming by a symptomatic approach in which wedge behavior is inferred from the time space distribution of thrust faulting erosion and synorogenic sediment accumulation in association with the orogenic wedge in turn the causes of wedge behavior are interpreted in terms of features that are known about the sevier wedge such as the locations of major decollements and the lithologic constituents of major thrust sheets from coniacian through late paleocene time 35 m y basement and cover rocks in the sevier wedge were shortened by 100 km in three major and one minor events an overall eastward progression of thrusting was punctuated by several episodes of out of sequence and hinterland vergent thrusting over the long term wedge taper was controlled by internal wedge strength initial taper and changes in the durability of the upper surface of the wedge and its internal lithostratigraphy critical taper was maintained by the offsetting effects of basement duplexing in the rear of the wedge and forward imbrication at the front of the wedge while the basal decollement was mainly in basement rocks the wedge was highly tapered and thrust spacing was relatively close 15 km beginning ca 75 ma the basal decollement ramped upward and eastward into weak cambrian shale and jurassic salt thrust spacing increased to 25 45 km and wedge taper decreased drastically the highly tapered rear part of the wedge carried strong basement rocks above moderately weak mylonitic and cataclastic decollements whereas the frontal lower taper part of the wedge carried sedimentary rocks above extremely weak salt and shale decollements through time internal strength of the wedge decreased as it incorporated an increasing proportion of sedimentary rocks the thrust belt was able to advance eastward in spite of its decreasing internal strength and decreasing taper because the basal decollement of the wedge also became weaker through time both by strain softening beneath the rear of the wedge and by propagating along extremely weak rocks beneath the front of the wedge provenance data from associated synorogenic conglomerates indicate that the durability of the upper surface of the wedge probably increased over the long term history of the sevier wedge as an increasing proportion of the erosional surface became occupied by resistant precambrian quartzite and basement rocks this would have had the effect of decreasing the rate of erosion through time and maintaining a steeper surface slope which would have helped the wedge to remain at critical to supercritical taper the sevier wedge also exhibits several shorter term 10 km and 3 was subjected to regional erosion marked by major unconformities that ultimately caught up with tectonic thickening and caused the wedge to stall this cyclicity is attributed to the need for wedges to become supercritical before they can propagate forward over large distances and a lag time between wedge thickening and uplift and wedge erosion erosion eventually catches up to the rate of uplift the wedge stalls and the locus of deformation shifts to the rear of the wedge in order to rebuild taper this analysis suggests that orogenic wedges continually deform and find a way to maintain taper sufficient to allow forward propagation as long as a sufficient push from the rear exists the actual taper value may change substantially through time as lithostratigraphy internal strength upper surface durability and presence absence of weak potential stress guides conspire to maintain a critical taper

paleomagnetic considerations of the development of the pennsylvania salient in the central appalachians abstract late paleozoic remagnetizations of central appalachian clastic and carbonate rocks of ordovician to devonian age constrain the structural evolution of the pennsylvania salient new paleomagnetic results from 26 sites located on nine folds combined with previously published results from 60 sites yield a mean direction for the northeastern limb of the salient at 172 1 k 49 and a mean direction for sites southwest of the salient at 173 1 k 70 these two directions are not statistically different from each other and are not significantly rotated from the expected permian paleomagnetic direction for north america individual fold test results show that this secondary magnetization is synchronous with or even older than folding across the region these paleomagnetic results suggest that curvature of fold axes around the salient is an original feature of fold growth and not the result of post orogenic oroclinal bending in contrast analysis of the primary magnetization in the silurian bloomsburg formation shows that directions from the two limbs of the salient are significantly different new results from twenty sites yield a mean direction for the northeastern limb at 4 40 k 31 and for the southwestern limb at 338 8 k 23 combining these results with previously published directions from the bloomsburg formation yields a mean direction for the northeastern limb at 1 34 k 32 and for the southeastern limb at 343 34 k 30 the difference in declination does not appear to be the result of internal deformation of the rocks or acquisition of magnetization during a period of rapid apparent polar wander rather these directions indicate that the northeastern limb has been rotated clockwise 18 9 relative to the southwestern limb the sense and amount of this rotation is consistent with the curvature of the orogen around the salient however within each limb of the salient the primary component declinations do not directly correlate with the orientation of the first order fold axes collectively therefore these paleomagnetic results suggest that the structural evolution of the salient included pre orogenic rotations or rotations that occurred early in the alleghanian orogeny followed by later remagnetization and folding of the central appalachian foreland

formation of regional cross fold joints in the northern appalachian plateau abstract fold axis parallel elongation associated with the development of arcuate fold and thrust belts is proposed as a causal mechanism for syn orogenic cross fold joints such a mechanism can be coupled with other joint propagation models providing a widely applicable resolution to the enigmatic origin of regional systematic joints the fold axis parallel elongation model is compatible with kinematic indicators as recorded by a sequence of cross fold joints and related deformational fabrics in the central and northern appalachian plateau modeling of an arcuate tectonic boundary subjected to normal compressive loading demonstrates that tangential tensile stress can be large enough to initiate cross fold joints on the convex side of the tectonic boundary simulated stress trajectories from boundary element modeling bear a strong resemblance to the stress trajectories inferred from the regional cross fold joint patterns in the central and northern appalachian plateau modeling also displays a cratonward decrease in both the tangential stress and the tangential strain such a decrease is consistent with the deformation styles observed in the central and northern appalachian plateau

the age and timing of folding in the central appalachians from paleomagnetic results comparison of the paleomagnetic pole positions of late paleozoic secondary magnetizations with a time averaged reference apparent polar wander path for north america shows that sedimentary rocks within the central appalachians were remagnetized in the later half of the permian between ca 255 and 275 ma ages of remagnetization do not vary for sites distributed along or across strike of the central appalachian fold and thrust belt or across the appalachian plateau in new york nor do they vary with stratigraphic position of the host rocks however the pattern of folding relative to remagnetization across the valley and ridge and great valley in pennsylvania maryland and west virginia shows a distinct temporal variation individual fold tests yield postfolding magnetizations near the hinterland margin of the fold and thrust belt synfolding magnetizations in the central part of the belt and prefolding magnetizations near the foreland because of the similar age of the late paleozoic magnetization we conclude that rocks across the central appalachians were remagnetized in a relatively short period of time as folding and thrusting propagated more slowly toward the foreland we interpret this pattern to indicate that older folds near the hinterland grew prior to remagnetization folds in the central region grew during remagnetization and folds near the foreland grew after remagnetization rapid remagnetization relative to folding is also evident from the physical characteristics and magnetic properties of secondary hematite in remagnetized synfolding folds in such folds rocks were apparently remagnetized during an increment of fold growth in which there was partial and possibly negligible fold limb rotation collectively these results may constrain the mechanism for basinal fluid mobilization during deformation current models for remagnetization involve the migration of chemically active brines into the foreland during tectonism if realistic our results bear on this model by suggesting that brine migration was not directly related to the successive emplacement of individual thrust sheets as has been suggested for other fold and thrust belts

the distribution of layer parallel shortening fabrics in the appalachian foreland of new york and pennsylvania evidence for two non coaxial phases of the alleghanian orogeny

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

fringe cracks key structures for the interpretation of the progressive alleghanian deformation of the appalachian plateau vertical joints in devonian clastic sedimentary rocks of the finger lakes area of new york state are ornamented with arrays of fringe cracks that reveal the complex deformational history of the appalachian plateau detachment sheet during the alleghanian orogeny three types of fringe cracks were mapped gradual twist hackles abrupt twist hackles and kinks gradual twist hackles are curviplanar en echelon fringe cracks that propagate with an overall vertical direction within the bed hosting the parent crack and are found in all clastic lithologies of the detachment sheet abrupt twist hackles propagate as planar features in thick shale beds above or below the siltstone beds hosting parent joints kinks propagate horizontally as planar surfaces from the tips of parent joints in siltstone beds the breakdown of the parent joint into either gradual or abrupt twist hackles depends on the orientation and magnitude of the remote stress field internal fluid pressure and the elastic properties of the bed the twist angle of gradual twist hackles is larger in coarser clastic beds indicating that stress and internal pressure are more important parameters than elastic properties in controlling breakdown assuming that the vertical stress axis s v equals 78 mpa at 3 km burial depth the difference in twist angle between sandstone and shale beds is used to estimate the maximum horizontal stress difference in the shale beds as s h s h 2 5 mpa when s h s h 12 mpa in sandstone beds the twist angle of the fringe cracks and the abutting relationships of parent joints give an indication of the overall change in stress field orientation within the detachment sheet during alleghanian tectonics these parent joints indicate a regional clockwise stress rotation of alleghanian age concordant with the twist angle of fringe cracks throughout the western part of the study area a counterclockwise twist angle in the eastern portion indicates a local stress attributed to drag where no salt was available to detach the eastern edge of the plateau sheet the clockwise change in stress orientation is consistent with the rotation in stress orientation found in the anthracite belt of the pennsylvania valley and ridge but is opposite to the sense of rotation in the southwestern portion of the detachment sheet western pennsylvania and west virginia the two regional rotation domains are separated by the juniata culmination