diagenetic origin for quartz pebble conglomerates the occurrence of quartz pebble conglomerates qpc in the rock record increases backward through time from the tertiary through the precambrian the positive correlation between qpc abundance and age is valid both for numbers of reported qpc and for qpc as a percentage of all conglomerate and at both the era and the period level qpc are usually interpreted as being due to intense chemical weathering protracted transport or sediment recycling but none of these can account for the age distribution of qpc which is the opposite of the global mass age distribution for sedimentary rocks precambrian and tertiary conglomerates with similar sources and sedimentology have vastly different clast populations nonquartzose clasts being much more abundant in the younger rocks comparison of the petrology of qpc and polymict conglomerates shows that qpc have consistently higher proportions of diagenetic secondary matrix and pressure solved grain contacts we conclude that diagenetic factors play an important role in qpc formation by preferentially destroying less durable clasts

coseismic extension recorded within the damage zone of the vado di ferruccio thrust fault central apennines italy abstract recent high resolution hypocentral localisation along active fault systems in the central apennines illuminates the activation of seismogenic volumes dipping at low angle here we report field and microstructural evidence of seismogenic extensional faults localized within pre existing thrust fault zones the vado di ferruccio thrust fault vftf is a narrow fault zone

membrane seal and tertiary migration pathways in the bodalla south oilfield eromanga basin australia abstract the middle jurassic hutton sandstone oil accumulation at the bodalla south oilfield in the eromanga basin australia is primarily a structural trap with stratigraphic control in common with many others in this basin it does not appear to be full to the structural spill point analysis of mercury injection curves has revealed that the caprock of the hutton sandstone i e the birkhead formation which is predominantly a lithic sandstone is a membrane type seal the calculated maximum size of oil column that will accumulate beneath this caprock is 12 64 m and the maximum oil column encountered to date is 11 25 m there is no waste zone above the hutton sandstone reservoir at bodalla south

the influence of depth on reactivation in normal faulting abstract the study examines the influence of depth on the development of steep normal faults above a gently dipping fault which is reactivated in its deeper part and abandoned in its upper part the isotropic and anisotropic mohr coulomb anderson theory is used to determine the ability of any part of a fault to be reactivated by calculating the difference between the stress deviator necessary to induce reactivation and that required to induce fracturing when linear depth dependent properties are assumed it appears that a new normal fault can branch off a gentle pre existing fault only if the friction coefficient of the latter is much less than that used by byerlee and if the ratio cohesion of fault cohesion of intact rock is non negligible for a 30 dip fault the friction coefficient has to be less than 0 35 friction angle less than 20 neither pore fluid pressure nor constant excess fluid pressure affects the above conditions when the cohesion of the surrounding rocks increases with depth steep normal faults can branch off byerlee faults with dips as gentle as 30 a low shear strength of the lowest part of the preexisting fault also increases the chances of reactivation of the deeper part and abandonment of the upper part

microstructural observations on natural syntectonic fibrous veins implications for the growth process abstract syntectonic antitaxial and ataxial fibrous veins were investigated using sem microprobe cathodoluminescence cl and optical microscopy in antitaxial calcite veins fibres and surrounding selvage grew simultaneously with similar growth rates of crystallographically differently oriented grains new material precipitated at the vein margin in antitaxial and bi mineralic ataxial microstructures bridges of country rock material formed during vein growth in an initial en echelon vein system in our antitaxial and bi mineralic ataxial samples the spacing of solid inclusions does not reflect individual crack seal openings

slip sense inversion on active strike slip faults in southwest japan and its implications for cenozoic tectonic evolution abstract analyses of deflected river channels offset of basement rocks and fault rock structures reveal that slip sense inversion occurred on major active strike slip faults in southwest japan such as the yamasaki and mitoke fault zones and the median tectonic line mtl along the yamasaki and mitoke fault zones small size rivers cutting shallowly mountain slopes and quaternary terraces have been deflected sinistrally whereas large size rivers which deeply incised into the mio pliocene elevated peneplains show no systematically sinistral offset or complicated hairpin shaped deflection when the sinistral offsets accumulated on the small size rivers are restored the large size rivers show residual dextral deflections this dextral offset sense is consistent with that recorded in the pre cenozoic basement rocks s c fabrics of fault gouge and breccia zone developed in the active fault zones show sinistral shear sense compatible with earthquake focal mechanisms whereas those of the foliated cataclasite indicate a dextral shear sense these observations show that the sinistral strike slip shear fabrics were overprinted on dextral ones which formed during a previous deformation phase similar topographic and geologic features are observed along the mtl in the central eastern part of the kii peninsula based on these geomorphological and geological data we infer that the slip sense inversion occurred in the period between the late tertiary and mid quaternary period this strike slip inversion might result from the plate rearrangement consequent to the mid miocene japan sea opening event this multidisciplinary study gives insight into how active strike slip fault might evolves with time

a kinematic model for deformation within brittle shear zones textural analysis of fault gouge indicates that displacement of particles within brittle shear zones occurs by three diaerent modes synthetic riedel shears antithetic riedel shears and shearing particulate flow a kinematic model is developed which incorporates these three slip modes and constrains the combined deformation to be simple shear parallel to the shear zone boundaries penetrative fabric p foliation and shape preferred orientation of survivor grains commonly observed in foliated fault gouge is assumed to be a result of the shearing particulate flow only the model which relates penetrative fabric orientation to the relative shear rates on riedel shears and shearing particulate flow reconciles inclined p foliation and high g 10 shear strain 1999 elsevier science ltd all rights reserved

geometry styles and quantification of inversion structures in the jiyang depression bohai bay basin eastern china abstract inversion structures of jiyang depression evolved during mesozoic and cenozoic are classified into positive and negative inversion geometry styles the geometry styles of the reactivated faults in the study area are attributed to their multi phase history which includes at least four phases of compressive deformation and subsequent subsidence caused by extensional reactivation of faults since mesozoic in the case of negative inversion style the negative inversion ratios cannot be calculated by the same method of calculating the degree of positive inversion thus a new method of quantifying the negative inversion ratio is presented in this paper negative inversion ratio is defined as the ratio of extensional displacement to contractional displacement measured along the fault this method provides a useful complement to conventional estimation of the degree of inversion and adapts to any negative inversion faults based on this method we obtained the negative inversion ratios of some faults in the jiyang depression which indicates a decrease in deformational degree from east to west in the whole region

extensional reactivation of an abandoned thrust a bound on shallowing in the brittle regime abstract shallow dip angles 45 suggested by field observations of continental extensional faults are not predicted by classical isotropic mohr coulomb anderson theory earthquake data indicate that normal faults exist in the upper crust with dip angles commonly as shallow as 30 some structural evidence suggests brittle normal faulting with dip angles as shallow as 10 one explanation of the apparent conflict between theory and structural seismic observations is that intrinsically weak shallow dipping pre existing faults are preferentially reactivated any reduction in frictional strength of the pre existing structure below that of surrounding rock increases the likelihood that such structures will be first breaking when extension is initiated enhanced fluid pore pressure on the pre existing fault reduces the effective strength and can further enhance shallow fault reactivation an analytical treatment clarifies the roles of geometry intact pre existing fault strengths and fluid pore pressures frictional strength ratios of 3 or greater could account for extremely shallow normal faults dips 10 20 without consideration of pore pressures in excess of the least principal stress or of principal stress systems rotated away from the gravity vector moderate reduction in friction 3 4 with respect to wall rock can reduce the dip to 30 and can account for shallow normal slip earthquakes

book reviewthe techniques of modern structural geology vol 1 strain analysis j g ramsay and m i huber academic press london 1983 xiii 307 pp us 55 00 28 00 hardcover

aluminium loss during sandstone diagenesis petrographical observations from a sandstone buried to between 3 2 and 5 7 km in the north sea central graben suggest a significant change in bulk chemistry during burial the sand contained 35 or more feldspar upon deposition but due to extensive dissolution only 5 remains in the deep sandstones this has released much more al than is currently contained as authigenic minerals up to 1460 mol cm 3 of al have been exported this deep secondary porosity is economically important but its recognition by petrographic textures is unreliable as are bulk chemical provenance studies of deeply buried sedimentary and metamorphic strata

porphyroclast systems as kinematic indicators porphyroclasts of relatively strong minerals in mylonites commonly have an internal monoclinic shape symmetry defined by tails of dynamically recrystallized material the geometry of a porphyroclast and its tails called a porhyroclast system can serve as a valuable indicator of the sense of vorticity porphyroclast systems have been divided into and types on the basis of the geometry of the tails types have wedge shaped recrystallized tails whose median lines lie on opposite sides of a reference plane parallel to the tails and containing the symmetry axis for the system types are further subdivided into a a types in which the porphyroclast is isolated in a relatively homogeneous matrix and b types in which the porphyroclast system is associated with a shear band foliation in the matrix types typically have narrow recrystallized tails whose median lines cross the reference plane adjacent to the porphyroclast consequently embayments of matrix material occur adjacent to the porphyroclasts and the tails display characteristic bends a porphyroclast system in a mylonite develops when the relatively weak dynamically recrystallized grain aggregate in the porphyroclast mantle changes its shape due to non coaxial flow in the adjacent matrix this behaviour has been modelled in shear box experiments passive marker lines around rigid cylinders embedded in silicone putty were subjected to simple shear the experiments were modified to simulate a change in recrystallization rate r with respect to rate of deformation by decreasing the diameter of the rigid cylinder during deformation at variable rates the ratio r appears to be one of the most important factors in determining which porphyroclast system will develop at high r values flow of recrystallized material away from the porphyroclast is continuously appended by the production of new grains and wedge shaped a type tails develop at low r values relatively few new grains are added to the tails which become thinned and deflected by drag due to the spinning motion of the porphyroclast in addition most porphyroclast systems at low shear strains are of a type or lack monoclinic symmetry whereas types are only developed at high shear strain values complex porphyroclast systems characterized by two generations of tails are observed in many of the natural and model shear zones studied and may form due to fluctuating r conditions that allow isolated a and type porphyroclast systems to be used as sense of vorticity indicators are the systems should have a monoclinic shape symmetry matrix grain size should be small with respect to porphyroclast size matrix fabric should be homogeneous deformation history should be simple and observations should be made on sections normal to the inferred bulk vorticity vector for the mylonite

multidisciplinary approach to constrain kinematics of fault zones at shallow depths a case study from the cameros demanda thrust north spain thrusting at shallow depths often precludes analysis by means of structural indicators effective in other geological contexts e g mylonites sheath folds shear bands in this paper a combination of techniques including structural analysis magnetic methods as anisotropy of magnetic susceptibility and paleomagnetism and paleothermometry is used to define thrusting conditions deformation and transport directions in the cameros demanda thrust north spain three outcrops were analyzed along this intraplate large scale major structure having 150 km of outcropping length 30 km of maximum horizontal displacement and 5 km of vertical throw results obtained by means of the different techniques are compared with data derived from cross sections and stratigraphic analysis mixed layer illite smectite and vitrinite reflectance indicating deep diagenetic conditions and mature stage of hydrocarbon generation suggests shallow depths during deformation thus confirming that the protolith for most of the fault rocks is the footwall of the main thrust kinematic indicators foliation s c structures and slickenside striations indicate altogether a dominant nnw movement of the hanging wall in the western zone and ne in the eastern zone of the thrust thus implying strain partitioning between different branches of the main thrust the study of ams in fault rocks nearly 400 samples of fault gouge breccia and microbreccia indicates that the strike of magnetic foliation is oblique to the transport direction and that the magnetic lineation parallelizes the projection of the transport direction onto the k max k int plane in sites with strong shear deformation paleomagnetism applied to fault rocks indicates the existence of remagnetizations linked to thrusting in spite of the shallow depth for deformation and a strong deformation or scattering of the magnetic remanence vectors in the fault zone the application of the described techniques and consistency of results indicate that the proposed multidisciplinary approach is useful when dealing with thrusts at shallow crustal levels

an example of inversion in a brittle shear zone abstract a useful tool to document structural inversion inside a shear zone is the identification of overprinted structures which can increase the complexity of the internal fabric of shear zones in the case of thrust faults shear zones can also be problematic due to the occurrence of structures connected to flexural slip along bedding in the recumbent forelimb therefore identification of superimposition among different structures can help to distinguish among these configurations inside a shear zone in central apennines italy favourable conditions along the outcrop of mt tancia s thrust allowed the discovery of two sets of superimposed fabric elements referring to two opposite senses of shear the first is related to thrusting top to e ne whereas the second is likely associated to reactivation in normal sense top to w sw the fabric elements have been mapped and their distribution inside a semi brittle shear zone has been defined high angle extensional shear planes occurring only in the uppermost part of the shear zone closest to the main thrust plane have been interpreted as a new generation of c planes connected to negative reactivation instead of riedel shear planes the overprinting relationship defined inside the shear zone helps in discriminating whether the observed structures developed in the footwall of the main thrust or on the recumbent limb of the hanging wall s anticline the deformation inside the shear zone can be described using a simple shear model at the outcrop scale where reoccurrence of fibrous calcite on shear c planes should control rotation of s planes these are dissolution planes parallel to the xy plane of the local strain ellipsoid at thin section scale instead veins maintaining the same orientation suggest a coaxial deformation due to repeated crystallization events controlled by fluid pressure and differential stress variation

superimposed fabrics due to reversal of shear sense an example from the bergen arc shear zone western norway abstract a profile through the upper margin of a large scale ductile shear zone the bergen arc shear zone western norway has been studied in detail within the shear zone asymmetric structures indicate dextral or top to nw movement which is interpreted as representing late or post caledonian extensional tectonics the hanging wall of the shear zone is characterised by kinematic indicators such as shear bands associated with curved or inclined foliation and asymmetric boudins indicating sinistral or top to se movements these are interpreted as related to thrusting during the caledonian orogeny the studied profile shows the progressive overprinting of the sinistral structures by the later dextral shear this first led to a decrease in the obtuse angle between shear bands and the curved inclined foliation subsequently backfolding of asymmetric boudins in quartz veins occurred and a composite cleavage developed from shear bands and associated curved inclined foliation dextral asymmetric extensional shear bands were developed at a later stage when the folded quartz veins had attained tight to isoclinal geometries

architecture and evolution of an extensionally inverted thrust mt tancia thrust central apennines geological structural geochemical and k ar geochronological constraints abstract fault inversion may lead to significant obliteration of earlier tectonic structures thus preventing the straightforward interpretation of the complete kinematics and deformation history of faults we adopt a multidisciplinary approach to 1 reconstruct the tectonic evolution through space and in time of the extensionally inverted mt tancia thrust central apennines italy 2 understand the deformation mechanisms and the transition and reactivation of compressional structures during negative tectonic inversion and 3 constrain the origin of fluids involved during tectonic processes to this end we combined 1 detailed geological mapping and multiscale structural analysis 2 illite smectite paleothermal indicators 3 c o and clumped isotopes on calcite mineralizations and 4 k ar dating of authigenic and or syn kinematic illite from the mt tancia thrust fault rocks we show that shortening occurred between 9 and 7 ma during possibly multiple events of fluid overpressure and shear rupturing involving fluids entrapped over long term within the host rocks post compressive tectonic inversion occurred at 3 ma under fluid pressure fluctuations during shear events with an input of meteoric derived fluids tectonic inversion is spatially confined within the first few metres below the thrust surface in a volume dominated by the partial overprinting folding transposition and re utilization of the earlier compressional fabric

a stress inversion procedure for polyphase calcite twin and fault slip data sets in rocks that are polydeformed an approach which separates faults prior to stress inversion is more appropriate the traditional stress inversion approach involving the concept of the best fit stress tensor e g a tensor which minimises the misfit between calculated and measured fault striae data often risks computing artificial stress tensors that are some form of average of mixed sets of real stress tensors a new approach is proposed in which fault data are pre processed to group the faults on the basis of their response to all possible orientations and magnitudes of applied stress a computer method is described which utilises cluster analysis based on the right dihedra method to divide dynamically mixed fault populations to monophase subsets this division is based on the ranked similarity coefficients of each fault pair from the raw data set the data clusters form dynamically homogeneous subsets which are used for the composite right dihedra solution this solution is re computed for the reduced stress tensor defined by the orientation of principal stress axes and the ratio of their magnitudes

diagenetic quartzarenite and destruction of secondary porosity an example from the middle jurassic brent sandstone of northwest europe significant amounts of feldspar have been dissolved from middle jurassic sandstone oil reservoirs in the north sea northwest europe during burial diagenesis sandstones of the middle jurassic brent group become increasingly quartzose with increasing burial depth at statfjord field 2500 m depth sandstones are arkose to subarkose at hutton field 3050 m depth they are subarkose to quartzarenite and at lyell field 3500 m depth sandstones are typically quartzarenite in spite of extensive feldspar dissolution the abundance of secondary porosity due to feldspar dissolution is similar for all fields averaging 2 9 of the total rock volume thus far more feldspar has been dissolved than is recorded as secondary porosity the limit on preservation of secondary porosity may be largely the effect of the mechanical strength of the rock an excessive number of large secondary pores lowers the rock strength below the point at which the rock can withstand overburden stress thus causing collapse of some of the secondary pores

the recognition of reactivation during continental deformation reactivation involves the accommodation of geologically separable displacement events intervals 1 ma along pre existing structures the definition of a significant period of quiescence is central to this phenomenological definition and the duration of the interval chosen represents the resolution limit of reactivation criteria found in most ancient settings in neotectonic environments reactivation can be further defined as the accommodation of displacements along structures that formed prior to the onset of the current tectonic regime this mechanistic definition cannot always be applied to ancient settings due to the uncertainties in constraining relative plate motion vectors four sets of criteria may be used to recognize reactivation in the geological record stratigraphic structural geochronological and neotectonic some structural criteria may not be reliable if used in isolation to identify reactivated structures much of the previously published evidence cited to invoke structural inheritance is equivocal as it uses similarities in trend dip or three dimensional shape of structures numerous fault and shear zone processes can cause significant weakening both synchronously with deformation and in the long term and may be invoked to explain reactivation the collage of fault bounded blocks forming most continents therefore carries a long term architecture of inheritance which can explain much of the observed complexity of continental deformation zones

fabric reactivation an example from the hualapai mountains nw arizona usa abstract foliation reactivation involves renewed movement on preexisting fabrics and can lead to difficulty in determining temporal and spatial relationships of fabrics to larger structures as well as correlation of fabrics to tectonic events porphyroblasts that contain internal fabrics allow for interpretation of the structural history of an area including the potential for foliation reactivation in boriana canyon hualapai mountains nw arizona three vertical foliations s 2 s 4 are preserved in the contact aureole of the 1 40 ga boriana granite foliation s 2 strikes northeast and is parallel with regional foliation that formed post or syn 1 68 ga foliation s 3 occurs throughout the canyon but is most prevalent in the contact aureole where close to the pluton it forms the main cleavage andalusite and garnet porphyroblasts overgrew s 3 crenulations foliation s 4 is parallel to the s 2 regional fabric and in some outcrops deformation associated with s 4 has decrenulated the s 3 fabric although s 3 is preserved in garnet and andalusite porphyroblasts in these outcrops s 4 is also axial planar to f 2 folds recognition of fabric reactivation of this aureole allows for discrimination of foliation forming episodes that are separated by 250 m y and a better understanding of the proterozoic geologic history for this part of nw arizona

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

deformation partitioning and foliation reactivation during transpressional orogenesis an example from the central longmen shan china progressive localization of strain within the songpan garze fold belt at the eastern margin of the tibetan plateau has resulted in the formation of the wenchuan maowen shear zone and preservation of excellent overprinting relationships between three major phases of ductile and semi ductile deformation these relationships show that deformation in the eastern songpan garze fold belt evolved from crustal thickening during sw directed compression d1 to localized sinistral transcurrent shear d2 and finally to increasingly localized high temperature d3p and retrograde d3r reverse shear in response to se directed compression both d2 and d3 deformations were co planar and reactivation of the s2 foliation led to the formation of a composite s2 3 fabric however inclusion trails in garnet porphyroblasts preserve evidence of a transition between these strike slip and reverse shear end members indicating that d2 and d3 were part of an episode of progressive transpressional shear metamorphic grade increases from chlorite to kyanite zone within the wenchuan maowen shear zone reaching syn d3p peak conditions of 600 c and 10 kbar the d3p and d3r reverse shears were responsible for tectonic exhumation of the shear zone and resulted in the formation of retrograde d3r high strain zones along the margins of the xuelongbao granite and adjacent to the wenchuan maowen fault

palaeostress estimation using calcite twinning experimental calibration and application to nature abstract by means of high temperature deformation experiments on calcite rocks of different grain sizes we have established the relationships between differential stress and twinning incidence twin density and volume fraction and the way that these relationships depend on grain size both twinning incidence and volume fraction increase with grain size at a constant level of differential stress log twin density is proportional to stress but is independent of grain size at a given grain size volume fraction of twinning increases non linearly with differential stress twinning activity measured by twinning incidence increases linearly with stress the behaviour of each of these parameters is independent of temperature strain and strain rate these relationships can be used as a basis for the estimation of palaeostress values in naturally deformed calcite rocks this is illustrated by a study of twinning activity in limestones close to thrust faults in the cantabrian zone of the variscan orogenic belt in northwestern spain high peak differential stresses were measured ranging from 150 to 250 mpa using the turner dynamic analysis technique however it is clear that shear stresses resolved parallel to the fault planes were very low the attainment of high stress differences when the resistance to faulting was low is attributed either to bending forces induced during the motion of the non planar surfaces relative to each other or to the stress peak being attained prior to the localization of deformation onto the fault surfaces

stress space striated faults deformation twins and their constraints on paleostress abstract stress space is used to illustrate a discussion of the range of stress tensors theoretically compatible with striated faults and with deformation twinning other than friction laws each of the main constraints placed on stress by such data specifies the magnitude of a component of traction being linear in elements of the stress tensor such a constraint by each datum is represented in stress space by a hyperplane comparison at each level of constraint orientation sense of shear assumption of critical ratio or value of shear stress for faulting twinning is made for a single datum followed by consideration of sets of data and stress tensor reduction at no level of constraint are striated faults and deformation twins equivalent fault data cannot constrain absolute values of stress components data for untwinned crystals within twinned untwinned sets provide the fullest limit to values of deviatoric stresses paleostress estimates from twinned untwinned data are generally preferable to those from striated faults

insights into shallow level processes of mountain building from the northern apennines italy recent observations from submarine convergent margins reveal that sediments at shallow levels of burial undergo intricate and irregular fluctuations in physical conditions as they progressively deform and lithify the early stages of mountain building similar on land evidence is comparatively sparse as a result of difficulties of exposure and of early features having been obscured here we describe aspects of three structures from the northern apennines of italy that corroborate these notions of complex shallow level orogenic processes the upper cretaceous castiglioncello melange formed during the subduction phase of apennine orogenesis the major cervarola thrust and the smaller intra formational balduini thrust both of tortonian age represent the ensuing collisional phase all three structures formed at burial depths less than 2 or 3 km and are syn diagenetic although differing in detail all three show features such as clay fabrics and mineralized veins that record complex histories of fluid flow overpressure and deformation the clays show varying types of scaliness some of which had to involve pore dilation and collapse operating in cycles the veins typically evolved from networks of bedding parallel and bedding perpendicular veinlets through dilation breccias and mosaic textures to shear and extensional fibre growth during hydrofracture such observations imply fluctuating physical conditions and dynamic tectonic processes operating on shallow level sediments in a way that is highly heterogeneous with depth areally and through time

secondary cleavages in ductile shear zones abstract secondary cleavages developed at late stages in ductile shear zones show several features that are inconsistent with progressive simple shear in the zone these are the orientation of a single secondary cleavage oblique to the shear zone boundaries conjugate sets with opposite senses of shear and multiple sets with the same sense of shear these features can be explained if the bulk flow is partitioned into slip along discrete failure planes parallel to the primary foliation s coaxial stretching along the foliation and spin

calcite twins from microveins as indicators of deformation history abstract in order to investigate whether the evidence of the deformation history is preserved in the microvein calcite twinning microstructures several families of calcite veins have been studied from weakly deformed mesozoic limestones of the western iberian chain eastern iberian peninsula the maximum shortening directions established with this method correspond to the previously established paleostress evolution of this chain the reconstructed stress regimes evolve from predominantly compressional four compressional episodes with the maximum shortening direction oriented successively nw se n s ne sw and e w to extensional the established orientations of maximum shortening axes are consistent with the directions of paleostrains that have been independently deduced from stylolites and fault striations in the same region moreover the results clearly show that the e twins record some paleostress orientations that are not registered in the studied area by fault population analysis in this sense the e twins record older paleostress orientations than the small faults probably because the faults were reactivated several times and the twins were not modified since their formation the calculated values of twin strains 1 differential stress magnitudes 62 mpa and the existence of type i twins formed below 200 c confirm that this region was slightly deformed under low p t conditions

geometry classification and kinematics of s c and s c fabrics in the mushandike area zimbabwe planar fabrics in small dextral shear zones in the mushandike area of the zimbabwe archaean craton consist of a sigmoidal primary foliation s displaced by discrete shears c or c these planar fabrics can be divided into porphyroclastic megacrystic and banded types depending mainly on the values of the angles between s and c or c surfaces the structures that define s surfaces and the spacing of c or c surfaces the values of the angles between the foliations are proposed as the basis for a general classification of planar fabrics in shear zones the values of the angles depend on microstructures and strain microstructural evidence shows that slip did not occur on s surfaces but only on c or c surfaces a model for the development of the fabrics proposes that s surfaces lie parallel to the xy plane of the local finite strain ellipsoid in domains of simple shear between c or c surfaces c surfaces may form in the orientation of a coulomb failure surface the field evidence suggests that an increase in bulk strain is at least partly accommodated by an increase in local strain in domains between c or c surfaces as the s surfaces rotate with the local finite strain ellipsoid

open system chemical behavior in deep wilcox group mudstones texas gulf coast usa abstract wilcox group mudstones have been mechanically and geochemically transformed over a temperature range of 20 200 c our research controlled for provenance and age by sampling from five wells parallel to the paleodepositional axis all within the houston delta system across the sampled depths mudstone porosity has been reduced from 25 to

kinematics of compressional and extensional ductile shearing deformation in a metamorphic core complex of the northeastern basin and range abstract analysis of shear criteria enables the kinematics of two main ductile shearing events d 1 and d 2 to be established in the raft river grouse creek and albion metamorphic core complex the first event d 1 is a nne thrusting and corresponds to mesozoic shortening a well developed non coaxial ductile deformation d 2 of cenozoic age is marked by the occurrence of opposing eastward in raft river and westward shear criteria in albion grouse creek these characterize an arch structure where the shear strain increases outwards in the axial zone of the complex d 2 seems coaxial cenozoic extension is considered to be related to gravitational instability induced by mesozoic overthickening of the crust involving uplift erosion and abnormal heating brittle extension occurs in the upper part of the uplifted domain it is transformed laterally above undeformed basement towards stretched domains of the middle and lower crust through the ductile shear zones localized at the precambrian paleozoic interface this extension of the middle and lower crust occurred in the vicinity of the root zones of the former mesozoic thrusts which may thus have been reactivated as ductile normal faults during cenozoic extension

book review the techniques of modern structural geology volume 2 folds and fractures john g ramsay and martin i huber academic press london 1987 xi 309 398 pp price stg17 50 us 34 50 paperback

syn diagenetic evolution of shear structures in superficial nappes an example from the northern apennines nw italy abstract in the superficial nappes of the northern apennines meso and microstructures in sheared mudstones and fine grained turbidites consist mainly of networks of striated faults and calcite vein arrays the striated faults correspond to the first generation of structures they form two sets respectively synthetic and antithetic to the general shearing locally a sigmoidal microfabric is associated with the synthetic faults in scaly deformation bands up to a few metres thick late laterally extensive calcite veins are superposed on these structures they result from the opening of multiple releasing oversteps between shear surfaces the crack seal mechanism of deformation played a major role in their formation structures of the first generation result from heterogeneous ductile deformation of initially poorly lithified and water rich sediments the late formation of the calcite veins corresponds to a transitional ductile brittle behaviour in sufficiently lithified sediment evolution of structures probably depended on a strain hardening process and was dependent on the progressive compaction and dewatering of the sediment the gently dipping faults and veins probably formed the main pathway for the interstitial fluids possibly overpressured expelled from the sediment we emphasize that the geometry of the structures described provides important shear sense criteria for the study of superficial nappe tectonics

the influence of pre existing thrust faults on normal fault geometry in nature and in experiments relations between normal faults and pre existing thrust faults are classically described in terms of three basic situations normal faults can cross cut thrust faults they can branch out from thrust faults at depth on a de collement level or they can entirely reactivate thrust planes the mechanical aspects of these types of interaction were studied by analogue modelling in which sand simulates the brittle rocks and silicone putty an interlayered de collement the models underwent compression producing thrust faults with variable dips followed by extension three possible ways of interaction are described here a no interaction occurs in the case of low dip thrust faults 32 1 and normal faults are developed independently displaying a listric geometry b branching at depth on the de collement level occurs when dip of the thrust faults reaches 32 1 in this case the dip of the normal faults whose geometry becomes planar decreases with increasing thrust dip we suggest that this change in dip of normal faults depends upon the rotation of stress tensor axes along the pre existing fault zone where a drop in the friction coefficient is likely to occur c reactivation occurs in brittle material when dip of the pre existing fault exceeds 41 1

structure of the western albany mobile belt southwestern australia evidence for overprinting by neoproterozoic shear zones of the darling mobile belt abstract the albany mobile belt is dominated by ductile fabrics and regional structures formed during progressive dextral transpression at 1190 ma d1 resulting from nw se to nnw sse shortening and represents the oblique convergence between the west australian and east antarctic shields deformation was accompanied by metamorphism up to granulite facies and granitoid intrusion neoproterozoic extension and progressive deformation within a sinistral transcurrent to transtensional shear zone in the leeuwin complex of the southern darling mobile belt may be contemporaneous with transpressional shearing along the donnybrook nannup shear zone on the western yilgarn craton margin during this neoproterozoic event e w mesoproterozoic trends in the western albany mobile belt have been rotated into a n s orientation ductile reactivation and overprinting of mesoproterozoic structures is accompanied by retrogression of granulite facies assemblages to amphibolite facies migmatisation and pegmatite and granitoid intrusion in demonstrating that the n s orientation of the western albany mobile belt results from a younger event a continuation of this mobile belt is expected in northeast india whilst a continuation of the darling mobile belt may exist in the mirnyy area of east antarctica folding and the formation of conjugate brittle ductile shear zones during ne sw shortening and early cambrian conjugate faulting and jointing resulting from ese wnw shortening is common to both the western albany mobile belt and leeuwin complex

fault deformation mechanisms and fault rocks in micritic limestones examples from corinth rift normal faults abstract a multidisciplinary study investigates the influence of different parameters on fault rock architecture development along normal faults affecting non porous carbonates of the corinth rift southern margin here some fault systems cut the same carbonate unit pindus and the gradual and fast uplift since the initiation of the rift led to the exhumation of deep parts of the older faults this exceptional context allows superficial active fault zones and old exhumed fault zones to be compared our approach includes field studies micro structural optical microscope and cathodoluminescence geochemical analyses 13 c 18 o trace elements and fluid inclusions microthermometry of calcite sin kinematic cements our main results in a depth window ranging from 0 m to about 2500 m are i all cements precipitated from meteoric fluids in a close or open circulation system depending on depth ii depth in terms of p t condition determines the development of some structures and their sealing iii lithology marly levels influences the type of structures and its cohesive non cohesive nature iv early distributed rather than final total displacement along the main fault plane is the responsible for the fault zone architecture v petrophysical properties of each fault zone depend on the variable combination of these factors

emplacement of foreland thrust systems emplacement driving mechanisms for thrust systems have been briefly reviewed and compared with distinctive structural styles in thrust belts a b type mode which implies a push from the rear and an elastic brittle behavior of the rock mass appears to be suitable as a general model for the emplacement of thrust sheets in thin skinned fold and thrust belts where stratal shortening of sedimentary covers and imbrication of upper level crustal rocks occur in such an environment that the mechanical behavior of the crustal section involved the mode and timing of deformation and the geometry of thrust systems are all strongly dependent upon the coupling between induced tectonic strain and transient close to lithostatic pore fluid pressure buildup over a critical areal extent the model presented here does not require a weak basal horizon for the detachment of thrust sheets and pore pressure pp does not need to be high at once under the entire length of the thrust wedge rather the transient buildup of pp and hence the spatial and temporal progressive weakening of the rock mass are directly coupled with deformation

coupled mineral fluid evolution of a basin and high kaolinization in the sw england granites in relation to the development of the plymouth basin

calcite twins their geometry appearance and significance as stress strain markers and indicators of tectonic regime a review twin lamellae are a widespread feature in calcites of any type and origin twinning in calcite is an important intracrystalline deformation mechanism with low critical resolved shear stress and virtually no temperature dependence in contrast to most slip systems in rock forming minerals twinning therefore permits intracrystalline deformation at very low temperature and very little confining pressure twinning alone however cannot lead to large strains because only one independent slip system is effectively available twins lead to discontinuous shearing on the grain scale which gives rise to considerable strain incompatibilities at grain boundaries twinning consequently leads to strain hardening detailed observations of the geometry of twin lamellae show that incompatibilities at grain boundaries are eliminated by pressure solution on the grain scale at low temperature and dynamic recovery processes at higher temperatures twins are found in various tectonic environments and record very small strains in rocks deformed under very little cover different twinning strain and twinning stress methods and palaeopiezometers are critically reviewed all methods are limited to strains of less than 15 shortening or extension by twinning twinning nevertheless is an important although not directly measurable contribution to the total strain in highly strained calcite rocks at least up to middle greenschist facies under which conditions it is the most effective contributor to the formation of strong lattice preferred orientations in calcite tectonites the appearance of calcite twin lamellae changes systematically as a function of deformation temperature micro twins and straight narrow 1 5 m but fewer twins are developed above approximately 200 c curved twins twinned twins and completely twinned grains indicate the progressive importance of other slip systems and larger intracrystalline strains are possible at ca 250 c and above ancient straight twin lamellae are modified into irregular geometries by recrystallization and grain boundary migration the appearance of calcite twins is proposed as an approximate but rapid and easy to use geothermometer in low temperature environments

calcite twin morphology a low temperature deformation geothermometer abstract twinning of the e plane is the dominant crystal plastic deformation mechanism in calcite deformed below about 400 c calcite in a twin domain has a different crystallographic orientation from the host calcite grain so called thin twins appear as thin black lines when viewed parallel to the twin plane at 200 320 magnification under a petrographic microscope thick twins viewed in the same way have a microscopically visible width of twinned material between black lines calcite e twin width and morphology has been correlated with temperature of deformation in naturally deformed coarse grained calcite in this paper we present a compilation and analysis of data from limestones of the frontal alps france and switzerland and the appalachian valley and ridge and plateau provinces eastern united states to document this temperature dependence mean calcite twin width correlates directly with temperature of deformation such that thin twins dominate below 170 c and thick twins dominate above 200 c above 250 c dynamic recrystallization is an important deformation mechanism in calcite mean twin intensity twin planes mm correlates negatively with temperature and a cross plot of twin intensity with twin width can yield information about both strain and temperature of deformation these relationships provide a deformation geothermometer for rocks that might otherwise yield little or no paleotemperature data

foreland directed gravitational collapse along curved thrust fronts insights from a minor thrust related shear zone in the umbria marche belt central northern italy gravitational collapse occurs during the mature evolution of orogenic belts but its signature is difficult to discriminate in macroscopic structures from that of pre syn or late post orogenic extension so reliable mesoscopic examples are particularly useful a composite fabric developed along a lateral thrust ramp in the apennines reveals mesoscopic normal faults that truncate the thrust surface overprint the s fabric and merge downwards in a foreland directed splay leaving the thrust footwall undeformed these relationships indicate syn late thrusting extension which we interpret as induced by hanging wall gravitational collapse our study provides critical constraints for reconstructing the kinematic evolution of collapsing thrust fronts