regional variations in deformation mechanisms and structural styles in the central appalachian orogenic belt orogenic belts such as the central appalachian fold and thrust belt consist of distinct tectonic provinces characterized by different structural styles as a result of deformation under different physical conditions a quantitative study of deformation mechanisms was undertaken for quartzites and sandstones of the central appalachian orogenic belt in order to understand the controls of different physical parameters on the relative importance of these mechanisms and on the geometry and mechanics of formation of structures in the different provinces in the blue ridge province the dominant deformation mechanisms are pressure solution dislocation creep and extension fracturing with some accompanying grain boundary sliding and recrystallization the proportion of dislocation creep to pressure solution strain increases both with temperature and deviatoric stress an axial plane slaty cleavage which contains the maximum and intermediate principal directions of the strain ellipsoid maintains a uniform orientation in both the blue ridge and the great valley provinces folds in the blue ridge province have a similar geometry formed by progressive flattening of parallel folds due to internal deformation ductile deformation zones on the overturned limbs of folds acted as precursors for the propagation of thrust faults in the late stages of folding in the valley and ridge and massanutten synclinorium important deformation mechanisms in the sandstone units include pressure solution plastic deformation cataclasis brecciation extension fracturing and slip along minor faults and bedding planes major folds are strongly disharmonic and show variations between concentric and angular geometries they are associated with thrust ramps that propagated through progressively developing folds in the late stages of folding brittle deformation zones characterized by high intensities of extension fractures and cataclastic bands formed on the steep forelimbs these features are interpreted to be the result of layer parallel extension and the propagation of splay faults through these limbs cleavage developed during layer parallel shortening prior to the onset of folding and was rotated to its present fanned geometry during folding on the basis of the results of this study the central appalachian orogenic belt can be subdivided into two deformational provinces dominated by different deformation mechanisms and characterized by different geometries and mechanics of formation of the structures for orthoquartzites the tectonite front which separates the tectonites of the hinterland from the non tectonites of the foreland is located along the blue ridge front for carbonates and argillaceous sandstones the tectonite front is located along the north mountain fault

paleostress magnitudes determination by using fault slip and fluid inclusions planes data paleostress magnitudes can be estimated by using fault slip data rupture and friction laws angelier 1989 for dry conditions however their estimation is difficult if fluids are present during deformation the lithostatic load and the fluid pressure being generally unknown this paper shows that the quantitative estimation of the lithostatic load and the fluid pressure during a tectonic event can be derived from paleofluid analysis in fluid inclusion planes fip as fip are healed mode i cracks oriented in a consistant manner relative to regional or local structures stress and fluid features may be obtained for a given deformation event this approach has been applied to a fault system affecting an hercynian granite of the nw french massif central a nw se compression has been defined from a population of 51 faults characterized by orientations around n60 e to n110 e for dextral strike slips and n135 e to n175 e for sinistral movements the stress ratio has been determined with fault slip data around 0 52 0 08 and by using angelier s method the ratio 3 1 was estimated around 0 27 0 03 and the friction coefficient was estimated around 0 58 0 1 fip orientation were measured in a sample collected at 1 meter from the fault the dominant trend is nw se vertical or dipping toward the sw inclusion fluids from fip are characterized by homogenization temperatures in the 260 380 c range with a mode around 300 c and melting temperatures from 0 c to 1 5 c 0 5 equivalent nacl with a mode around 1 0 c 1 7 equivalent nacl these data yield assuming a geothermal gradient in the 60 80 c km range and hydrostatic conditions a fluid pressure estimate of about 50 10 mpa and a v around 132 10 mpa thus a major stress ratio 1 3 can be estimated in the 70 105 mpa range

relations between fluid inclusion trails and regional stress field a tool for fluid chronology an example of an intragranitic uranium ore deposit northwest massif central france abstract episyenites within the uranium ore bearing le bernardan granite northwest french massif central were formed through the dissolution of quartz by hydrothermal solutions these solutions may now be found fossilized as fluid inclusions f i the f i form rectilinear trails along microcracks which trapped the percolating fluids simultaneous study of the orientations of the trails and the chemistry of the inclusions shows that 1 1 the trails may be grouped according to direction each related to successive regional stress fields 2 2 the fluids of each group are compositionally distinct marking different stages of the hydrothermal event this example shows that the geometric relations between secondary f i trails and regional stress fields may be used to establish the chronology of both during a region s geological evolution

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

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

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

climate change record in subsurface temperatures a global perspective analyses of underground temperature measurements from 358 boreholes in eastern north america central europe southern africa and australia indicate that in the 20th century the average surface temperature of earth has increased by about 0 5 degreesc and that the 20th century has been the warmest of the past five centuries the subsurface temperatures also indicate that earth s mean surface temperature has increased by about 1 0 degreesc over the past five centuries the geothermal data offer an independent confirmation of the unusual character of 20th century climate that has emerged from recent multiproxy studies

are fluid inclusion planes useful in structural geology abstract quantification of deformation or stresses in structural geology is often difficult due to the uncertainity of the paleodepth or fluid pressure the best records of fluid percolation are paleofluids trapped as fluid inclusions in healed microcracks of the rock forming minerals fluid inclusion planes fip are mode i cracks which form in sets with a predominant orientation perpendicular to the least principle stress 3 however the repeated microfracturing and healing of the rock forming minerals yield complex superimposed patterns of healed microcracks such patterns are often difficult to interpret due to the lack of suitable chronological criteria these problems have been recently documented and solved by coupling deformation studies detailed examination at all scales of the relationships between trapped fluids and their host structures and studies of fluid inclusions this paper summarizes recent advancements in fip analysis and suggests further ways for research

the role of microcracking and grain boundary dilation during retrograde reactions department of geology university of southampton southampton oceanography centre southampton so14 3zh uk email ajb1 soc soton ac ukveined metamorphic rocks are common and give evidence of fracture controlled fluid flow however the processesinvolved in fluid infiltration in domains between fractures is rarely well established reaction products concentrated atgrain boundaries give an indication that diffusion of elements along grain boundaries may be important and linearfluid inclusion trails provide evidence of grain scale microcracking despite these common observations the relativeimportance of grain boundary processes and microcracking at different metamorphic grades is not well characterised etheridge et al 1983 developed a metamorphic porosity model comprising grain boundary bubbles and tubules ofwhite white 1981 but also incorporating dilatant grain boundaries and intra grain tensile cracks variations in thelocal granular stress state will initiate transient micro cracking and grain boundary dilatancy promoting enhancedporosity permeability and grain scale fluid infiltration this in turn causes retrogression the extent to which it devel ops being influenced by the degree of connectivity of the dilatant cracks and grain boundaries and whether for particu lar p t x conditions the infiltrating fluid is at equilibrium or not with the host being infiltrated with reference to examples of greenschist facies retrogression from troms norway and south harris scotland this study establishes that the processes of microcracking and grain boundary dilatancy are highly heterogeneous atthin section scale in examples where reaction products are not related to mutual contact of specific phases it is shownthat the development of reaction products varies according to grain boundary orientation and density of microcracks an example from norway shows a quartz rich garnet mica schist where a centimetre scale brittle fracture has in duced extensive microcracking and fluid infiltration in the adjacent matrix fluid inclusions defining healed microcracks perpendicular to schistosity have h

the role of fluids in low temperature fault related deformation of quartz arenite abstract the microstructural and fluid histories in quartz arenite deformed in a regional scale fault zone were investigated to determine the role of fluids in the selection of grain scale deformation mechanisms using cathodoluminescence fluid inclusion microthermometry and stable isotope geochemistry of quartz cements three chemically distinct aqueous fluids were identified that affected the rock at different times during deformation the earliest deformation microstructures are distributed cataclasites which were cemented by reddish brown luminescing quartz precipitated from an aqueous cacl 2 brine fluid i once cemented brittle deformation continued but localized along discrete shear surfaces in narrow zones 20 1000 m wide and on discrete microfractures microveins associated with this phase were cemented with bluish green luminescing quartz precipitated from an aqueous cacl 2 brine fluid ii water channelized along dissolution surfaces and in pervasive microfracture networks that formed at this time promoted hydrolytic weakening and crystal plastic deformation especially near fluid pathways temperatures during deformation associated with fluids i and ii were 225 235 c as determined from fluid inclusion data in the later stages of deformation an influx of iron bearing aqueous fluid fluid iii greatly accelerated the rate of quartz dissolution and filled fractures stylolites and vugs with goethite the progression of deformation mechanisms indicated by the microstructures and their relationship to the different fluids show that the selection of grain scale deformation processes is affected not only by the presence of fluids but also by their chemistry

near surface in situ stress 3 correlation with microcrack fabric within the new hampshire granites we studied the correlation between near surface in situ stress and the preferred orientation of microcracks at two quarries in the milford granite and one quarry in the conway granite new hampshire the orientation of in situ stress was determined by overcoring doorstoppers and from the strike of induced borehole fractures produced by a pressurized packer the preferred orientation of vertical microcracks was determined using ultrasonic measurements to determine p wave velocity vp on core and in situ and from thin section analyses in situ vp anisotropy was determined from interborehole travel time data at all three sites directions of maximum compressive stress 1 and induced borehole fractures are aligned with the preferred orientation of open microcracks determined from core vp and thin section data an analysis of the microcracking sequence within each pluton suggests 1 that the quarry grain resulted from cooling induced thermal stresses and 2 a method of distinguishing the paleostress axes at the time of cooling from contemporary stress

deformation in the hinge region of a chevron fold valley and ridge province central pennsylvania the hinge region of an asymmetrical chevron fold in sandstone taken from the tuscarora formation of central pennsylvania u s a was studied in detail in an attempt to account for the strain that produced the fold shape the fold hinge consists of a medium grained quartz arenite and was deformed predominantly by brittle fracturing and minor amounts of pressure solution and intracrystalline strain these fractures include 1 faults either minor offsets or major limb thrusts 2 solitary well healed quartz veins and 3 fibrous quartz veins which are the result of repeated fracturing and healing of grains the fractures formed during folding as they are observed to cross cut the authigenic cement deformation lamellae and in a few cases pressure solution occurred contemporaneousl y with folding the fibrous veins appear to have formed as a result of stretching of one limb the cross cut all other structures based upon the spatial relationships between the deformation features we believe that a neutral surface was present during folding separating zones of compression and extension along the inner and outer arcs respectively using the strain data from the major faults the fold can be restored back to an interlimb angle of 157 however the extension required for such an angle along the outer arc is much more than was actually measured this disparity between observed and required deformation suggests that the rest of the folding strain may be attributed to minor faulting isolated severe pressure solution and to slight grain movements we were not able to recognize the latter we propose that a single episode of deformation produced the chevron fold causing the brittle deformation after the sandstone had been lithified this brittle deformation was accomplished by faulting together with the translation of individual sandstone blocks which do not contain significant internal deformation

mode i microfracturing and fluid flow in damage zones the key to distinguishing faults from slides abstract we have examined the distribution of microfractures in arenites and the evolution of vein forming fluids in the matrix of carbonate breccias within the damage zones of large detached blocks in order to characterize their modes of emplacement previous studies of microfractures in the damage zone associated with tectonic faulting have shown a clear pattern of increasing density as the fault is approached previous studies of carbonate breccia within damage zones of tectonic faults typically document evidence of multiple fluid events representing repeated rupture healing processes however in this study we find no change in the microfracture density with distance from the 45 km displaced gravity driven slide block at heart mountain wyoming in a previous study of the same massive slide block there was no evidence of multiple fluid infiltration events related to emplacement we interpret these observations as indicating the absence of rupture cycling that would be expected in the development of a process zone instead being consistent with catastrophic emplacement of gravity driven slide blocks we use this distinct pattern of microfracture density and fluid infiltration to demonstrate that several large 1 km2 detached blocks in the basin and range previously thought to be allochthons related to hyperextension detachment faults are actually slide blocks whose detachment surfaces represent no crustal extension

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

fluid inclusions tectonic indicators abstract during the first half of the 20th century fluid inclusions have been studied in ore geology in order to determine the chemistry and physical parameters density of the mineralising fluids apart from a few pioneering works in the middle of the century fluid inclusions were not used as structural markers before the late 1980s during the same time experiments as well as observations of natural samples have shown that the volume shape and density of fluid inclusions may change when submitted to pressure temperature conditions different from their trapping isochores in view of these experiments and based on natural examples from the higher himalaya metamorphic rocks it is evident how fluid inclusions can be used as thermobarometric structural and tectonic markers points of interest for further studies on fluid inclusions in nature and experiments are also stressed

microfracturing and regional stress field a study of the preferred orientations of fluid inclusion planes in a granite from the massif central france abstract a study of the deformation of a granitic massif indicates a relationship between palaeostress fields and the geometry of microfractures as defined by fluid inclusion trails that is healed microfractures a statistical study of the distribution of the trails leads to the following conclusions trails exhibit several distinct preferred orientations on the scale of a grain which may be observed in many samples the orientations of the trails are similar to those of micro and mesoscale fractures in the granite and the dominant trail direction is parallel to the main direction of regional shortening thus fluid inclusion trails are mode i cracks which can be used as excellent markers of palaeostress fields

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

petrofabric test of viscous folding theory abstract compression and extension axes are deduced from quartz deformation lamellae in a quartzite and a graywacke folded into an asymetrical syncline deformation lamellae fabrics in the two sandstones are distinctly different in the graywacke regardless of bedding orientation or position on the fold compression axes are normal or nearly normal to the axial planar rough cleavage extension axes generally lie in the cleavage plane parallel to dip in most quartzite samples compression axes are parallel or subparallel to bedding at high angles to the fold axis and extension axes are normal to bedding two samples from the very base of the formation indicate compression parallel to the fold axis with extension parallel to bedding at high angles to the fold axis one of these two shows both patterns the lamellae fabric geometry in these two samples suggests the presence of a neutral surface in the quartzite the lamellae derived compression and extension axes are in good agreement with the buckling behavior of a viscous layer quartzite embedded in a less viscous medium graywacke and shale below and shale and carbonate above

strain cleavage and microstructure variations in sandstone implications for stiff layer behaviour in chevron folding sandstone layers within tightly chevron folded quartz rich turbidites of the lachlan fold belt southeastern australia show a fanning spaced cleavage overprinted by a non fanning grain alignment cleavage low internal strains recorded by quartz grain separations xz 1 4 1 8 and low fold flattening strains xz 1 8 angular relations between spaced cleavage and bedding in folds with varying tightness suggest that sandstone layers initially act as stress supporting members with spaced cleavage development due to pressure solution taking place at limb dips up to 15 during fold development sandstone layers undergo rigid body rotation involving interlayer slip without marked internal deformation except at fold hinges after fold lock up bedding spaced cleavage angles change due to inner arc collapse by pressure solution and subsequent flattening with formation of a grain alignment cleavage and mica beard growth internal deformation of sandstone is characterized by microstructures related to solution redeposition processes and growth of mica chevron folds form by homogeneous shortening and pressure solution due to layer parallel shortening tangential longitudinal strain due to buckling fold tightening by inner arc collapse due to pressure solution and fold flattening at metamorphic peak with growth of mica geometric modelling of internal strains and bedding cleavage angles requires a significant component of interlayer slip during chevron fold development under greenschist facies conditions

correlation between deformation lamellae microfractures macrofractures and in situ stress measurements white river uplift colorado dynamic analyses of quartz deformation lamellae and microfractures in lower paleozoic quartzites from the white river uplift record a northwest direction of compression which can be related to joint and fault orientations and in situ stress determinations crystallographic orientations of deformation lamellae resemble those formed in the subbasal i relatively nonselective domain microfractures in quartz grains form orthogonal sets of generally north northeast and west northwest trending fracutres which are approximately normal to bedding the strongest set of microfractures in most samples lies in the 2 plane deduced from deformation lamellae analyses suggesting that the microfractures are extensional features maxima of microfractures correspond to maxima of joints measured at adjacent locations the strikes of prominent west northwest sets of microfractures and joints are subparellel to the dominant northwest strike of faults on the uplift in situ stress measurements conducted in the piceance basin and rangely colorado areas indicate principal stress orientations similar to those determined from quartz deformation lamellae microfractures and joints which are pre uplift to early uplift late laramide fabric elements

diagenesis of quartz in the upper proterozoic kaimur sandstones son valley central india abstract the upper proterozoic kaimur sandstones in central india are quartz sublithic and lithic arenites cemented by quartz illite and hematite diagenetic quartz occurs in five modes syntaxial overgrowths fracture healings aggregates of small euhedral crystals quartz resulting from the alteration of detrital silicates and from the recrystallization of quartz intergranular pressure dissolution is suggested as the main source of silica with smaller contribution from other sources such as silica dissolved in meteoric waters stylolitization clay mineral diagenesis and the alteration of detrital silicates studies on the fluid inclusions and oxygen isotopes of diagenetic quartz suggest that meteoric water modified by diagenetic reactions has mediated the quartz cementation

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

crack propagation sequence and pore fluid conditions during fault bend folding in the appalachian valley and ridge central pennsylvania bedding parallel bpv strike sv and cross fold cfv veins represent a sequence of polyphase fracturing during the development of a fault bend fold in lower paleozoic carbonate beds in the appalachian valley and ridge province pennsylvania brecciation and layer parallel shearing played important roles in the development of the earliest vein set bpv which propagated prior to folding fluid inclusions in calcite bpv are highly saline brines 23 4 wt naci trapped at conditions close to lithostatic p 180 mpa t 267 c while passing through the lower kink plane of a fault bend fold strike joints propagated in dolomitic beds located on the extensional side of neutral surfaces stylolitization of strike joint surfaces accompanied slip of the hanging wall up a ramp renewed extension upon passing through a second kink plane led to propagation of antitaxial sv along the stylolitized joints by the crack seal process slightly less saline fluids 22 4 wt naci were trapped in sv at fluid pressures 144 mpa and temperatures 217 c with the concurrent formation of lateral ramps the carbonates moved to the upper flat and were subjected to strike parallel extension as manifested by the propagation of antitaxial cfv due to further mixing of fresh waters the salinity of fluids forming cfv decreased 20 5 wt nacl with trapping conditions at p 116 mpa and t 179 c the fluid evolution path from bpv to cfv through sv shows a modest decrease in salinity with a sharp decrease in possible trapping pressures

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

tattekere conglomerate quartzite association origin and stratigraphic position of a disputed archean formation in the supracrustals of karnataka india abstract the origin and stratigraphic interpretation of the conglomerate quartzite rocks occurring in and around tattekere and kumkumna hosuru villages in the holenarsipur supracrustal belt is an important aspect of the precambrian geology of karnataka india due to the controversy regarding its origin and stratigraphic status the geology of the holenarsipur supracrustal belt has been interpreted by previous workers in different ways in the present work attempts have been made to resolve the controversy based on recent observations and analytical results the petrographic characteristics of conglomerates and quartzites the occurrences of primary sedimentary structures and their similarity to proven sedimentary quartzites in the northern sector along with the chemical composition of the quartzites point to a sedimentary origin for this association the heavy mineral and chemical composition of the quartzites suggest a major source rock contribution from a granitic terrain with subordinate contributions from metamorphic and or metasedimentary rocks al 2 o 3 and cr enrichment in quartzites resulted from deposition of al 2 o 3 rich chromite grains derived from isolated exposures of ultramafic rocks possible k metasomatism has also been inferred from the chemical composition of the quartzites from field distributions petrographic characteristics chemical compositional data and inferred source rock contribution it may be inferred that the conglomerate quartzite association occurring in and around tattekere and kumkumna hosuru villages in the holenarsipur supracrustal belt represents a single horizon with a near basal position in the stratigraphic sequence this suggests a basal near basal unconformity in the stratigraphic succession of holenarsipur supracrustal belt

paleostress directions from the preferred orientation of closed microfractures fluid inclusion planes in sandstone east texas basin u s a abstract closed extensional microfractures are useful indicators of paleostress directions in shallowly buried quartz cemented quartz arenites of the lower cretaceous travis peak formation a tabular sandstone and shale unit approximately 670 m thick that was deposited in the gradually subsiding northern gulf of mexico basin the closed microfractures are syndiagenetic fluid inclusion rich planes that have an ene strike that is subparallel to contemporary quartz filled extension veins and regional normal faults indicating that closed microfractures contain the maximum paleostress direction these microstructures are potentially useful for mapping paleostress trajectories in passive margin and platform settings particularly where samples are obtained from core previous experimental studies of microfracture closure suggest that microfractures evolved into fluid inclusion planes by dendritic quartz precipitation in east texas microfracture closure may have occurred at temperatures as low as 85 c and depths as shallow as 1500 m

microcracks in rocks a review abstract in the past decade the number of studies about microcracks in rocks has rapidly increased this review of recent work concentrates on microcracks in rock as separate entities emphasizing microcrack morphogenesis kinematics dynamics population statistics and observational techniques cracks are produced when the local stress exceeds the local strength the local stress may be augmented by twin lamellae interactions kink bands and deformation lamellae stress concentrations at grain boundary contacts and around intracrystalline cavities local strength may be reduced along cleavage planes along grain boundaries and along any internal surface as a result of corrosion by chemically active fluids dislocations appear not to be a significant factor for crack nucleation below about 500 c in silicates spatial and temporal changes in temperature can also induce microcracking as a result of differential thermal expansion between grains with different thermoelastic moduli and thermal conductivities the amount of quartz in the rock has a significant effect on thermally induced microcracks because of its large and variable thermal expansivity the application of hydrostatic pressure between 100 and 200 mpa effectively closes most cracks but the closure may not be uniform if crack wall asperities exist hydrostatic pressure appears to stabilize cracks and make crack growth more difficult the number and average size of mechanically induced microcracks is greater in rock deformed at higher pressures the application of a deviatoric stress field on the boundaries of a rock mass results on a microscopic scale in a very complex stress system which greatly affects nucleation and propagation paths the relative amount of intragranular and intergranular cracking appears to depend upon mineralogy rock type and stress state the vast majority of stress induced microcracks in rocks appear to be extensional statistically they are predominantly oriented within 30 of the macroscopic maximum stress direction crack densities increase as macroscopic deviatoric stress increases above a threshold level crack size distributions may be either lognormal or exponential fracture in rock under compressive boundary loads is a result of the coalescence of many microcracks not the growth of a single crack some crack configurations are more favorable for coalescence than others as deviatoric stress increases and rock failure is approached the microcrack population changes spatially from random to locally intense zones of cracking away from the fault the crack density dies off rapidly to the background level a few grains away under lesser deviatoric stresses slow subcritical microcrack growth can occur as a result of stress aided corrosion at the crack tip the rate governing mechanism may be either the chemical reaction rate or the rate at which water can get to the crack tip important details still remain to be worked out