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

hierarchy of sea level cycles in the medial silurian siliciclastic succession of pennsylvania the medial silurian siliciclastic succession in central pennsylvania records two hierarchically superimposed cycles of sea level fluctuations the larger of these cycles controlled lithofacies architecture at the magnitude of formations and members five alternations of transgression and regression with an average cycle period of about 2 5 m y produced wedges of coarser shallow water facies pointing basinward northwest and wedges of deeper water basin center mudrock facies extending marginward southeast superimposed on these are smaller scale shallowing upward cycles typically 1 3 m thick that occur in three different contemporaneous facies basin margin tidal flats mid shelf offshore bars and below wave base in the storm influenced transition to the basin center average recurrence interval for such smaller cycles was about 100 ka their period was regular and was caused by sea level fluctuations of about 1 3 m

changes in quartz solubility and porosity due to effective stress an experimental investigation of pressure solution experimental compaction of quartz sand was conducted in semistatic flow through systems at 150c to examine the chemical and physical processes associated with pressure solution pore fluid chemistry and porosity were monitored through time to investigate the role of effective stress on silica solubility and compaction rate the concentration of silica in the pore fluid varied directly with effective stress at constant pore fluid pressure but not in a linear fashion increases in silica were not transient and could be partially reversed by removal of the effective stress porosity decreased steadily under constant nonzero effective stress at 150c but remained essentially constant under identical loading conditions at room temperature initial compaction rates increased linearly with effective stress deformed samples show indented contacts with fractures and dissolution features healed microfractures and a decrease in average grain size multiple mechanisms pressure solution subcritical crack growth crack healing appear to operate at grain contacts during compaction the authors interpret long term time dependent compaction accompanied by stress induced changes in fluid chemistry as evidence for pressure solution

origin of a thrust related fold geometric vs kinematic tests abstract geometric tests to determine the origin of fault related folds are common but as is typical in structural geology more than one fold origin may yield the final natural geometry thus the results of geometric tests are usually non unique in contrast kinematic tests of origin which employ both geometry and data about deformation commonly yield more constrained if not unique results unfortunately the necessary data collection requires much more work than for a geometric test in this study the thrust related barclay anticline is analyzed both geometrically and kinematically to determine which test is more effective geometric tests using angular relationships indicate three possible origins fault bend fault arrest and break thrust for the kinematic test predicted deformations for interlayer and flexural slip flexural flow simple shear and bending strains are compared to micro and mesostructural distributions solution strain and finite strain from the anticline strain measurements indicate that microscale deformation is uniformly distributed through the structure and is lithification dominated the microscale deformation does not match kinematic predictions and did not accommodate fold formation fold growth was achieved primarily through layer parallel slip restricted mostly to the forelimb and absent in the hinge which eliminates fault bend and fault arrest origins that require material transport from forelimb to backlimb the barclay anticline is therefore interpreted to be a break thrust structure interestingly a suite of contraction faults in the forelimb and hinge indicates material transport from forelimb to backlimb such transport has been discounted for break thrust folds the most important result of this study is that a kinematic test was a necessary step for distinguishing fold origin geometric testing alone was insufficient given that the barclay anticline has geometric characteristics typical of many thrust related folds kinematic testing appears necessary to determine their structural origin

two contrasting accretion v collision orogenies insights from early paleozoic polyphase metamorphism in the altun qilian north qaidam orogenic system nw china

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

synfolding remagnetization and deformation results from palaeozoic sedimentary rocks in west virginia summary palaeomagnetic analysis was conducted at 27 sites from two stratigraphically adjacent palaeozoic units along two transects meldey and rig of the patterson creek anticline in the appalachian orogen of west virginia to better understand the relationship between deformation and remagnetization while no primary directions were observed we isolated identical well defined secondary late palaeozoic magnetizations in carbonates from the late silurian tonoloway formation and the early devonian helderberg group standard application of conventional palaeomagnetic fold tests suggests that the magnetizations are synfolding in the helderberg group and pre folding to early synfolding in the tonoloway formation a fold test on all data combined indicates a synfolding magnetization best grouping at 60 65 per cent unfolding but the resulting distributions are not circular an alternative interpretation based on optimal differential unfolding at the site mean level yields one group with a nearly pre folding or early synfolding magnetization 70 per cent unfolding that is both circular and concordant with north america s apparent polar wander path the second group has a variable synfolding magnetization that is elliptical at the optimum unfolding level and falls off the path differences in either the age of remagnetization or viscous partial thermoremanent magnetization contamination cannot account for this result the sites falling into the pre folding group are biased towards the tonoloway formation the medley transect the northwest limb away from small folds and lower dip angle relative to the sites in the synfolding group the two groups defined by the fold test results also have a relationship with both total strain and strain partitioning the rocks containing the early synfolding remanence exhibit less total strain and in particular less pressure solution strain relative to grain boundary sliding and calcite twinning it is likely that the remagnetization in the entire area was coincident and occurred prior to folding even though some sites reveal synfolding directions the variation in the behaviour of the remagnetization relative to folding is unclear at present but it indicates that pressure solution and stylolite formation are important processes in remagnetization in any event the relationship between deformation and magnetization is not simple apparently identical magnetizations from adjacent sites may have very different histories and or responses to deformation while the exact relationship between remagnetization and deformation remains elusive this study demonstrates that significant variation in behaviour can exist at a very local scale

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

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

microfractures and their role in deformation of a quartz arenite from the central appalachian foreland abstract microfractures are pervasive in the massanutten sandstone a quartz arenite exposed in a doubly plunging synclinorium in the central appalachian foreland they are present as fluid inclusion planes fips and microveins parallel the fips fips occur in three orthogonal sets horizontal vertical and normal to the fold axis and vertical and parallel to the fold axis the geometry distribution and age of fips relative to other microstructures shows that microfractures developed during folding microfracturing followed layer parallel shortening by dislocation flow and was coeval with the peak of pressure solution that marked the final stages of folding microfracture densities are extremely high 160 mm 1 in many samples rapid closure of microfractures suppressed brecciation and prevented microfractures from propagating unstably to form joints microfracture opening accounts for significant finite strain with extensional strains of up to 11 1 and associated volume increases of over 16 0 the general lack of shortening microstructures indicates that a net volume increase is likely and that the massanutten sandstone may have served as a sink for dissolved phases from adjacent units undergoing volume loss deformation

structural evolution of the pulaski thrust system southwestern virginia the pulaski fault system is a complex series of alleghanian thrusts which has juxtaposed rocks of the pulaski thrust sheet over rocks as young as early to middle mississippian maccrady formation of the saltville thrust sheet the salem branch a later major branch of the pulaski fault system within the pulaski thrust sheet displaced the complexly deformed plate containing a broken formation a lithotectonic assemblage of complexly folded and faulted elbrook and rome carbonates and shales with associated breccias over rocks as young as early mississippian lower price formation of the salem synclinorium the classic ubiquitous max meadows breccias generally are confined to the broken formation which forms the basal part of the complexly deformed plate and which is interpreted as an exhumed lower level decollement zone formed during an earlier alleghanian stage and transported during a later alleghanian stage of deformation to a much higher structural level minimum displacement of the complexly deformed plate is on the order of 100 110 km on the basis of a new palinspastic reconstruction of the pulaski thrust sheet on the basis of this reconstruction and of two balanced and restored cross sections for the western portion of the valley and ridge minimum shortening for the pulaski thrust sheet is 80 whereas that for the western part of the valley and ridge province is only 10 20 the pulaski fault system is interpreted to have originated below the appalachian wide basal decollement zone near the contact between the cambrian rome and elbrook formations

acadian dextral transpression and synorogenic sedimentary successions in the appalachians the successive seboomook littleton northern appalachians and catskill pocono central appalachians clastic wedges suggest oblique convergence and southwestward migration of acadian orogeny beginning in early devonian and continuing into early mississippian wrench fault movement in maritime canada coincided with deposition of all but the earliest components of the catskill pocono clastic wedge and continued into the pennsylvanian contrasts between a thin lower to middle devonian shallow shelf facies in the alabama appalachian fold thrust belt and a time equivalent thick shallowing upward sedimentary to volcanic succession in the adjacent talladega slate belt are interpreted to reflect a wrench fault basin a wrench fault setting for devonian rocks in alabama integrated with manifestations of oblique convergence during the acadian orogeny in the central and northern appalachians can be accommodated in dextral transpression along the entire length of the acadian appalachian orogen

displacement transfer at thrust terminations the saltville thrust and sinking creek anticline virginia u s a abstract the saltville thrust terminates in the core of the sinking creek anticline changes in the amplitude of the anticline may preserve profile shortening as the thrust displacement decreases towards the fault termination consequently thrust displacement would be transferred into the fold we combine newly published maps and new strain and mesostructural data from the tuscarora sandstone to argue that the sinking creek anticline did perform this kinematic role we propose that the anticline developed as a fault propagation fold that experienced both decollement and anticlinal breakthrough by the saltville thrust thrust displacement was accommodated by the development of the modified fault propagation fold and by transfer to a roof flat from the decollement breakthrough new strain and mesostructural data indicate no fixed pin lines in either the hinge or backlimb throughout folding consequently layer slip occurred through the hinge during folding and the forelimb deformed internally during breakthrough towards the plungeout of the sinking creek anticline the displacement on the saltville thrust is transferred to a floor thrust we apply this interpretation to the termination of the st clair thrust which has a similar surface geometry where the thrust terminates into an anticline a testable implication of this proposed interpretation for the st clair thrust is that the appalachian plateau must absorb over 20 km of thrust displacement as horizontal shortening

the problem of strain marker centers and the fry method abstract the fry method and its variants will underestimate finite strain when applied to strain markers deformed heterogeneously at marker scale despite homogeneous behavior at a population scale this difference results from the post deformation centers not coinciding with the original pre deformation centers the amount of error may rapidly approach 50 for small strains in ideal cases but is likely to be in the range of 20 30

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

spatial arrangement of decollements as a control on the development of thrust faults abstract we used two dimensional finite element models to explore different configurations of weak layers in undeformed sedimentary sequences to investigate the occurrence of three characteristic types of thrust configurations ramp flat imbricate and duplex in our models we embedded two low friction weak layers with a finite spatial extent in a sequence of stronger rock these two weak layers were initially horizontal were separated vertically by 1 km and were arranged in three different relative positions to each other when the models were deformed and these weak layers developed into decollements they interacted to produce one of the three types of thrust faults as a function of their initial configurations when the tips of weak layers were separated by a large gap 10 km only the lower level decollement became active producing imbricate thrusts when the two weak layers overlapped for a large distance 10 km they simultaneously became active decollements producing duplexes in the overlapped zone when the gap or overlap was small

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

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

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

implications of thermal events on thrust emplacement sequence in the appalachian fold and thrust belt some new vitrinite reflectance data interpretation of existing geothermometry data combined with new vitrinite reflectance data within the framework of a detailed composite tectonic setting elucidates the evolution of structural sequencing of thrust sheets during the alleghanian event in the valley and ridge province in virginia that the pulaski thrust sheet preceded the saltville thrust sheet in the emplacement sequence and that both reached thermal maxima prior to or during respective emplacement may be inferred from vitrinite and other geothermometry data in contrast the narrows and st clair thrust sheets probably each attained their thermal maximum after emplacement new vitrinite reflectance data are consistent with cai and other temperature sensitive information heretofore ascertained in the valley and ridge province and support previously established maximum temperatures of ca 200 c for strata of the saltville thrust sheet as young as mississippian r max values from mississippian coals in the price formation of the sal

thrust related strain gradients and thrusting mechanisms in a chevron folded sequence southeastern australia an e directed imbricate fan thrust system within a chevron folded monotonous quartz rich turbidite succession shows marked gradients in strain from the upper to lower parts of individual thrust sheets upper parts of thrust sheets show low to moderate strains xz strains 30 1 tight inclined chevron folds ila 30 with predominantly w dipping axial surfaces intense phyllitic cleavage with a strong down dip mineral stretching lineation and long curved tapering quartz pressure shadows on pyrite such differences in fold geometry the intensity of the cleavage and the geometry of quartz fibres in pressure shadows require thrust sheet emplacement to be synchronous with the internal deformation of sheets thrust sheets show a change from an approximately constant volume plane strain coaxial deformation in their upper parts to non coaxial high strain deformation with incremental i between those expected for pure shear and simple shear i e 0 i 5 45 in their basal parts observed geometries and intensities reflect partitioning of strain and deformation mechanisms within individual thrust sheet volumes both during the initiation and propagation of subsurface detachment faults and with their emplacement to shallower crustal levels strain softening related to chevron folding and more intense cleavage development within a 500 m basal zone has controlled the emplacement of these turbidite dominated thrust sheets transport and internal deformation of the thrust sheets did not involve gravitationally induced spreading but required either a push from the rear or some form of underthrusting

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

intergranular pressure solution in four quartzose sandstones abstract cathodoluminescence petrography and scanning electron microscopy reveal that several geologic variables significantly influenced intergranular pressure solution and quartz cementation in the bromide ordovician oklahoma upper minnelusa permian wyoming nugget jurassic utah and tuscarora silurian pennsylvania sandstones a linear relationship exists between mean grain size and intergranular pressure solution with finer grained samples having experienced more intergranular pressure solution than coarser grained ones samples that display pervasive illite grain coatings have also experienced more intergranular pressure solution than those with no or poorly developed grain coatings sorting and limited framework grain compositional variability have exerted little influence o intergranular pressure solution silica budgets indicate that in the bromide upper minnelusa and nugget sandstones more quartz has been dissolved by intergranular pressure solution than is present as cement these sandstones have exported silica during burial diagenesis in the tuscarora the amount of quartz dissolved by intergranular pressure solution is about equal to the amount of quartz cement present implying approximate mass balance however petrographic evidence suggests that most of the quartz cement in at least three of these sandstones predates most intergranular pressure solution implying that they acted as silica importers during shallow diagenesis and silica exporters during deeper diagenesis intergranular pressure solution has played varying roles in determining the ultimate porosity of these four sandstones in the nugget intergranular pressure solution was the single most important diagenetic process that influenced porosity evolution in the bromide intergranular pressure solution was slightly more important than mechanical compaction and considerably more important than cementation in the tuscarora intergranular pressure solution was slightly more important than mechanical compaction and of about equal importance to quartz cementation in the upper minnelusa intergranular pressure solution was less important than both mechanical compaction and gypsum anhydrite cementation

strain partitioning of deformation mechanisms in limestones examining the relationship of strain and anisotropy of magnetic susceptibility ams abstract in order to investigate the relationship between rock strain and anisotropy of magnetic susceptibility ams strain partitioning and ams analysis was conducted at 35 sites from two stratigraphically adjacent paleozoic limestone units in the patterson creek and wills mountain anticlines in the central appalachian orogen of west virginia in addition anisotropy of anhysteretic remanent magnetization aarm was conducted on selected samples to examine the role of preferentially oriented magnetite on the ams fabric strain is partitioned into bed normal shortening due to compaction solution strain 35 0 shortening bed parallel shortening due to tectonic solution strain 13 3 shortening calcite twinning strain 5 8 shortening and grain boundary sliding 26 7 shortening the ams fabrics in the rocks were found to be a result of a complex interaction between rock lithology deformation mechanisms and strain magnitude although all the rocks have experienced the same deformation conditions six different ams fabrics are exhibited each of the different ams fabrics is a composite fabric resulting from the overprinting of three components 1 an inherent primary depositional ams fabric that is attributed to preferentially oriented phyllosilicates in the rock matrix 2 a diagenetic and or compaction ams fabric formed during burial that is due to preferentially oriented phyllosilicates in solution structures and in the rock matrix and 3 a tectonic ams fabric that was imparted on the rocks by layer parallel shortening deformation prior to folding and is also attributed to preferentially oriented phyllosilicates in solution structures and in the rock matrix as well as twinning of ferroan calcite

determination of pressure solution shortening in sandstones abstract a new method for the determination of pressure solution shortening in sandstones uses the geometry of grain to grain interpenetrations and grains truncated against solution surfaces these features are used to construct plots from which the magnitude and direction of the pressure solution shortening can be determined using simulated pressure solution deformation of artificial and natural grain populations the new method is shown to correctly assess a variety of coaxial and non coaxial shortenings although primarily intended to determine shortening the method can also quantify extension related to growth of beards or overgrowths during pressure solution application of the method to naturally deformed quartz arenite samples shows that pressure solution shortening of up to 26 occurred during compaction and 22 during layer parallel shortening

quantifying the geometry displacements and subresolution deformation in thrust ramp anticlines with growth and erosion from models to seismic reflection profile a b s t r a c t we quantify the evolution of thrust ramp anticlines exhibiting growth and or erosion using the area depthstrain ads method we illustrate validate and explore the limitations of the ads method for a series of kinematic models and experimental sand analog models we then apply the method to a seismically imaged thrust ramp anticline from the deep water niger delta the ads method allows determination of the boundary displacements displacement rates and layer parallel strains using the excess areas areas of marker beds above their regional level of multiple horizons the kinematic and experimental models demonstrate that excess areas for pre deformation units lie on a straight line on an area depth graph the inverse slope of which is the boundary displacement and the intercept is the elevation of the basal detachment for growth strata the line connecting an area depth value to the value representing the basal detachment gives the displacement after deposition of that horizon for partially eroded markers the calculated displacements are less than the actual displacements however originally planar unconformities yield the displacements that accrued after the formation of the unconformities the experimental sand models show substantial layer parallel shortening in the anticlines displacements on the main thrust ramp that are only half the displacement on the basal detachment and small tectonic dilations the seismic example is an area balanced ramp anticline with clearly defined pre deformation and growth sequences fault separation at the center of the ramp in the pre deformation interval is approximately equal to the total boundary displacement but decreases both upward and downward the layer parallel strains have the opposite pattern with the lowest values at the center of the ramp and increasing values both upward and downward

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

strain partitioning and factorization in a quartz arenite abstract most rocks deform by multiple grain scale deformation mechanisms in order to assess completely the contributions of each mechanism the strain associated with that mechanism must be known and compared to the finite strain this study describes the partitioning of strain between three mechanisms pressure solution dislocation creep and microfracturing in a quartz arenite deformed at low temperatures pressure solution which occurred primarily during compaction dominates the finite strain with an average of 24 shortening normal to bedding dislocation creep which occurred during layer parallel shortening accounts for about 2 shortening parallel to bedding dip microfractures which occur in three orthogonal sets resulted in 2 4 extension normal to bedding parallel to bedding strike and parallel to bedding dip the validity of the mechanism strains was tested using strain factorization models were constructed using the mechanism strains in an order determined by their relative ages because porous well sorted quartz arenites such as the one studied are likely to undergo some mechanical compaction in the early stages of diagenesis variable amounts of mechanical compaction were included in factorization models models using the measured deformation mechanism strains with an initial 5 10 mechanical compaction yield finite strains in close agreement with the measured finite strains this suggests that the mechanisms identified the strain associated with each mechanism and the deformation sequence are plausible

predicting pore pressure in active fold thrust systems an empirical model for the deepwater sabah foldbelt abstract measurements related to mudrock shale and siltstone porosity such as acoustic velocity density or electrical resistivity have traditionally been used to predict pore pressures in extensional stress settings the underlying assumption is that burial and vertical effective stress ves which is the overburden minus the pore pressure controls the compaction of these rocks through porosity loss the dataset presented here compares ves and acoustic velocity of similar composition mudrocks in both an extensional and a compressional stress setting in the extensional stress environment the mudrocks follow a typical compaction trend with a porosity loss and increase in acoustic velocity that can be related to ves in an active fold thrust belt the compressive stresses further reduce the porosity and increase the acoustic velocity of the mudrocks first a layer parallel shortening compacts sediments beyond what is observed for the ves this additional compaction is further enhanced near thrust faults and in anticlinal forelimbs presumably due to additional shear stress in these areas the mudrocks located in folds that are buried by additional sedimentation do not compact again until the tectonic compaction is overridden by enough new burial after that the mudrocks follow the observed extensional setting compaction trend in the fold thrust belt the observed reduction in porosity by stresses other than burial leads to an under prediction of pore pressure using traditional methods to account for this we present a correction that can be applied to the acoustic velocity or porosity using two parameters a proximity to thrust faults and anticlinal forelimbs and b the amount of burial after fold formation with these corrections the extensional velocity ves compaction trend can be used to accurately predict pore pressure within the active fold thrust belt the correction is calibrated with well data and is empirical none the less it is a first step toward understanding the magnitudes of the compressive lateral stresses involved in generating the observed porosity loss within the fold thrust belt

assessing brittle volume gain and pressure solution volume loss processes in quartz arenite abstract quartz arenite of the tuscarora sandstone has been deformed by dislocation flow pressure solution and microfracturing of which the last two are most important pressure solution involved shortening with no extension most samples have approximately 20 shortening normal to bedding interpreted to be compaction and a few have up to 20 layer parallel shortening of tectonic origin because there is no extension these shortenings resulted in an equivalent amount of bulk volume loss multiple sets of microfractures in the form of fluid inclusion planes and microveins account for up to 10 extension and bulk volume increases up to 17 when both microfracture volume gains and pressure solution volume losses are considered together all samples show a net bulk volume loss ranging from 14 to 35 changes in material volume are different than bulk volume because of the presence of porosity at the time of pressure solution whereas all samples show bulk volume losses most samples show net material volume gains of up to 16 with only those having experienced both compactional and tectonic pressure solution showing material volume losses pressure solution during compaction can account for most but not all cement compactional pressure solution surfaces do not extend into pore filling cement suggesting that much of the cement was externally derived after compaction therefore the flux of material may have been considerably greater than the material volume changes would indicate

evolution of solution structures in a deformed quartz arenite geometric changes related to permeability changes abstract quartz arenite of the silurian keefer sandstone at a site in west virginia u s a records an early pervasive solution related deformation by grain interpenetration volume losses measured by the pss method show shortening directions from both diagenetic compaction and early tectonic layer parallel shortening later solution related tectonic deformation during folding formed transgranular stylolites along pre existing faults or in rock volumes with abundant microfractures thus the geometry of solution related deformation changed progressively from pervasive at grain scale to focused along transgranular surfaces we propose that this geometric change resulted from intense porosity and permeability reduction during diagenetic cementation and early layer parallel shortening which required fluids to exploit tectonically created pathways along faults or networks of microfractures during later deformation

origin of fracture porosity example from altamont field utah the occurrence of natural fracture systems in subsurface rock can be predicted if careful evaluation is made of the geologic processes that affect sedimentary strata during their cycle of burial diagenesis uplift and erosional unloading variations in the state of stress within rock arise for example from changes in temperature pore pressure weight of overburden or tectonic loading hence geologic processes acting on a sedimentary unit should be analyzed for their several contributions to the state of stress and this information used to compute a stress history from this stress history predictions may be made as to when in the burial cycle to expect fracture joint formation what type of fractures extension or shear may occur and which geologic factors are most favorable to development of fractures a stress history is computed for strata of the naturally fractured altamont oil field in utah s uinta basin calculations suggest that fractures formed in extension that the well cemented rocks are those most likely to be fractured that fractures began to develop only after strata were buried to great depth and that the fracture system continued to develop as strata were uplifted and denuded of overburden geologic evidence on fracture genesis and development is in accord with the stress history prediction stress history can be useful in evaluating a sedimentary basin for naturally fractured reservoir exploration plays

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

structural chronology of the alleghanian orogeny in southeastern west virginia the trend change between central and southern appalachian structures is sharply defined in southeastern west virginia there n30 35 e trends coincident with major central appalachian folds such as the browns mountain anticline end abruptly at n60 e trending southern appalachian structures along the st clair fault analysis of local and regional folds cleavage bedding perpendicular stylolite seams and bedding and fault slickenlines reveals that layer parallel shortening directed n10 30 w occurred in nonfolded greenbrier group mississippian carbonates well into the present appalachian plateau area this structural event is early and is associated with the evolution of southern appalachian folds and faults south of the st clair fault central appalachian folds and mesoscopic structures were superimposed on this early layer parallel shortening fabric this structural chronology indicates that southern appalachian folds and faults predated the development of central appalachian structures in the region

aftershocks are well aligned with the background stress field contradicting the hypothesis of highly heterogeneous crustal stress 1 it has been proposed that the crustal stress field contains small length scale heterogeneity of much larger amplitude than the uniform background stress this model predicts that earthquake focal mechanisms should reflect the loading stress rather than the uniform background stress so if the heterogeneous stress hypothesis is correct focal mechanisms before and after a large earthquake should align with the tectonic loading and the earthquake induced static stress perturbation respectively however i show that the off fault triggered aftershocks of the 1992 m7 3 landers california earthquake align with the same stress field as the pre landers mechanisms the aftershocks occurred on faults that were well oriented for failure in the pre landers stress field and then loaded by the landers induced static stress change aftershocks in regions experiencing a 0 05 to 5 mpa coseismic differential stress change align with the modeled landers induced static stress change implying that they were triggered by the stress perturbation contrary to the heterogeneous stress hypothesis these triggered aftershocks are also well aligned with the pre landers stress field obtained from inverting the pre landers focal mechanisms therefore the inverted pre landers stress must represent the persistent background stress field earthquake focal mechanisms provide an unbiased sample of the spatially coherent background stress field which is large relative to any small scale stress heterogeneity the counterexample provided by the landers earthquake is strong evidence that the heterogeneous stress model is not widely applicable

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

mesostructural development in detached folds an example from west virginia an alleghanian duplex of three rootless anticlines in the central appalachians containing oriskany sandstone provides an example of mesostructural development in detached folds these mesostructures which include stylolites veins vein arrays joints and faults developed before during and after duplex formation non alleghanian mesostructures are the most ubiquitous joint sets began forming during the late pennsylvanian in response to burial stresses and fluid overpressure subsequent pre alleghanian n s compression reactivated the joint trends to form stylolites and vein arrays again in the presence of large fluid pressures during alleghanian deformation mesostructures developed only locally in the folds in response to layer rotation interlayer slip and hydrofracturing during thrusting during alleghanian and post alleghanian uplift and erosion thermal elastic relaxation and arc length changes reactivated a joint trend to form new joints the resulting varied distribution of mesostructures

a model for sandstone compaction by grain interpenetration abstract the compaction of an idealized sandstone is described quantitatively in terms of its stress temperature history and the microstructure of its monomineralic load bearing grain cement framework the model relationship is developed from simple energy and volume balance considerations although chemistry mineral pore fluid and transport diffusion fluid flow play major roles in most diagenetic processes they do not appear explicitly in this mechanical model for grain interpenetration in sandstones for a model sandstone at compaction equilibrium the only quantity determined uniquely by the ambient stress and temperature is its burial constant equal to the ratio of its grain cement contact diameter to the separation of its grain centers the burial constant determines what the maximum porosity can be whether primary or secondary sandstones with identical burial constants may have very different grain interpenetrations and minus cement porosities depending on the amount of supporting cement and the time of its emplacement supporting load bearing cement increases the effective intergrain contact area thereby increasing a sandstone s resistance to compaction supporting cement usually preserves more pore space than it occupies this is in direr contrast to passive non lo d bearing cement which may destroy even more effective pore space than its solids occupy the removal of passive cement increases porosity but the removal of supporting cement initiates further compaction and causes porosity to decrease with sufficient supporting cement primary porosity can retain economic values to great depths model predictions are uniformly consistent with available observational data on quartzose sandstones agreement among data from geographically diverse sandstone reservoirs ranging in age from jurassic to pleistocene suggests that compaction equilibrium is reached in times that are short on the geological time scale and that age is not a factor in the compaction of sandstones