segmentation and differential post rift uplift at the angola margin as recorded by the transform rifted benguela and oblique to orthogonal rifted kwanza basins abstract we analyse tectonic and sedimentary field and subsurface data for the angola onshore margin together with free air gravity anomaly data for the offshore margin this enables us to characterize the mode of syn rift tectonism inherited from the precambrian and its impact on the segmentation of the angola margin we illustrate that segmentation by the progressive transition from the benguela transform rifted margin segment to the oblique rifted south kwanza and orthogonal rifted north kwanza margin segments the spatial variation in the intensity of post rift uplift is demonstrated by the study of a set of geomorphic markers detected in the post rift succession of the coastal benguela and kwanza basins upper cretaceous to cenozoic uplifted palaeodeltas erosional unconformities palaeovalleys quaternary marine terraces and perched gilbert deltas the onshore benguela transform margin has a distinctive mainly progradational stratigraphic architecture with long term sedimentary gaps and high elevation marine terraces resulting from moderate upper cretaceous cenozoic to major quaternary uplifting i e 775 1775 mm ky or m ma by contrast repeated synchronous episodes of minor cenozoic to quaternary uplift occurred along the orthogonal rifted north kwanza segment with its cenozoic aggradational architecture short term sedimentary gaps and low elevation pleistocene terraces margin style likewise governs spatial variations in the volume of offshore sediment dispersed in the associated deep sea fans along the low lying north kwanza margin sedimentation of the broad cenozoic to pleistocene kwanza submarine fan was probably governed by the width of the kwanza interior palaeodrainage basin combined with the wet tropical neogene climate along the high rising benguela margin the small size of the benguela deep sea fan is related to the interplay between moderate continental sediment dispersal from long lived small catchments and a warm very arid neogene climate however the driving forces behind the epeirogenic post rift uplift of the angola coastal bulge remain a matter of speculation

relay zone geometry and displacement transfer between normal faults recorded in coal mine plans abstract overlap lengths separations and throw gradients were measured on 132 relay zones recorded on coal mine plans throws on the relay bounding fault traces are usually 2 m and individual structures are recorded on only one seam throw gradients associated with relay zones are not always higher than on single faults but asymmetry of throw profiles is diagnostic of relay zones bed geometries around larger faults in opencast mines are used to assess the displacement accommodated by shear in the vertical plane normal to the faults and displacement transfer accommodated by shear in the fault parallel plane three dimensional structure is defined for two relay zones each recorded on five seam plans these relay zones are effectively holes through the fault surfaces and overlap occurs between salients or lobes of the parent fault surfaces lobes initially terminated at tip lines but as the faults grew gradually rejoined the main fault surfaces along branch lines this type of relay zone originates by bifurcation of a single fault surface at a locally retarded tip line and is an almost inevitable result of a tip line irregularity

geophysical characterisation of active thermogenic oil seeps in the salt province of the lower congo basin part i detailed study of one oil seeping site abstract we report the geophysical characterisation of natural oil seep sites through a combination of sea surface evidence of oil leakage from spatial imagery with a large collection of seafloor and subsurface geophysical data this paper provides a detailed characterisation of one selected active seep site and identifies possible specific feature of oil seep sites the oil seep is a complex shaped feature on the seafloor consisting of a cluster of heterometric pockmarks inside a main depression area and peripheral metre scale seafloor mounds a strong deformation related to salt tectonics controls the location of the seafloor source by fracturing the overburden the associated thermal anomaly induces a vertical modification position of the base of the gas hydrate stability zone bghsz that is used as a fluid migration route towards the crest of the diapir the combination of local depressions and seafloor amplitude anomalies linked with vertical high amplitude pipes rooted on the bghsz suggests a focused fluid flow towards the seafloor in peripheral areas the seafloor mounds are linked by shallow faults to buried high amplitude patches on sub bottom profiler sections the combination of restricted size seafloor mounds with a progressive deepening of the high amplitude from the seafloor suggests a substantial decrease of the hydrocarbon flow towards peripheral areas the proximity of actively oil supplying seafloor depressions and seafloor mounds shows that the hydrocarbon flow rapidly decreases laterally the thermogenic seep site is affected by two consistent and sub parallel reflections with negative polarity the first is interpreted as the methane related bghsz the second could correspond to the base of a thermogenic bghsz produced by a mixture of heavier gas the seafloor roughness and double bsr appear to be specific features of oil seep sites the geophysical features revealed at a localised study area will be extrapolated towards a larger province for relevance validation

origin of dolomites in the cretaceous maha sarakham evaporites of the khorat plateau northeast thailand abstract the khorat plateau of northeast thailand and laos was an area of widespread deposition of evaporites and siliciclastics maha sarakham formation during the cretaceous and three types of dolomites are associated with this formation 1 limpid dolomite 2 coarse subhedral and euhedral dolomite and 3 saddle dolomite limpid dolomite is present as isolated crystals in recrystallized clear halite beds while the other two types are only present as authigenic clusters and individual large crystals within the widespread basal anhydrite unit bau 1 1 m which defines the lowest member of the formation the 18 o pdb isotopic values and the 87 sr 86 sr ratios of the limpid dolomite are within the range of cretaceous seawater petrography and carbon and oxygen isotopes suggest that limpid dolomite was formed under shallow burial during times of freshening in a mostly marine fed aggrading halite salina dolomite formation was possibly influenced by bacterial metabolism the coarse crystalline and saddle dolomite types float within a matrix of coarse recrystallized anhydrite in the basal anhydrite the 18 o isotopic values of the coarse subhedral and euhedral dolomite types are more negative than the limpid dolomite and the 87 sr 86 sr ratios are more radiogenic than typical marine dolomite of the cretaceous saddle dolomite has even more negative 18 o isotopic values than the coarse euhedral dolomite the 87 sr 86 sr ratios of the saddle dolomite are also radiogenic the isotopic and petrographic data indicate that the dolomites in the basal anhydrite unit are late and authigenic but do not appear to replace a precursor carbonate rather they appear to have precipitated within the deep burial environment along a 1 2 m thick subsurface mixing interface between warm upwelling basinal brines and dense highly saline brines created by dissolving the underside of the overlying impervious halite bed the upwelling basinal waters presumably rose through the underlying siliciclastics of the khorat group and then moved laterally along the base of the maha sarakham salt the result of this inferred hydrologic process apparently is an accumulation of burial dolomite and recrystallized residual anhydrite in a petrographically complex unit that ultimately appear to be a solution residue the basal anhydrite unit

effect of mechanical interaction on the development of strike slip faults with echelon patterns abstract the overlapping geometry of echelon strike slip faults is well known we have quantified this observation by measuring the amount of overlap and separation and plotting them against each other for over 120 examples although there is large scatter the data show a linear trend suggesting that the overlap increases proportionally with separation up to a limiting value we have analyzed this conspicuous relationship in terms of fault interaction by using a numerical model based on displacement discontinuity the results show that fault interaction is in fact an important factor in contributing to the overlapping geometry of echelon strike slip faults

pre rift and synrift exhumation post rift subsidence and exhumation of the onshore namibe margin of angola revealed from apatite fission track analysis abstract a series of cooling events in the development of the namibe margin of angola is defined by apatite fission track analysis data from samples of outcropping cretaceous sandstones and crystalline precambrian basement regional exhumation in the late carboniferous early permian and jurassic preceded early cretaceous rifting further episodes of uplift and erosion affected the margin in the early and late cretaceous before it was buried by up to 2 km of post break up section which was subsequently removed during cenozoic uplift and erosion beginning between 35 and 20 ma this timing is consistent with published analyses of river profiles suggesting that uplift of the margin began at c 30 ma estimates of between 1 5 and 2 km of section removed during cenozoic exhumation are consistent with burial depths estimated from the diagenesis of former evaporite horizons now at outcrop these results add further support to a growing body of evidence showing that the evolution of passive margins is anything but passive the key episodes of exhumation defined in this study are broadly synchronous with events identified in many areas of the west africa margin from namibia to equatorial guinea and are regarded as representing a continent scale response to stresses related to tectonic plate movements

inversion of field data in fault tectonics to obtain the regional stress iii a new rapid direct inversion method by analytical means summary a new method for determining the reduced stress tensor with four degrees of freedom the orientations of the three principal stress axes as well as the ratio of principal stress differences using fault slip data or focal mechanisms of earthquakes is presented from a computational point of view the inversion of fault slip data is made in a direct way by purely analytical means as a result the determination process is extremely fast and adaptable on small microcomputers from a physical point of view the method aims at simultaneously i minimizing the angles between theoretical shear stress and actual slip vector and ii having relative magnitudes of shear stress large enough to induce slip despite rock cohesion and friction examples of application to actual fault slip data sets with good or poor variety of fault slip orientations are shown the double significance of the basic criterion adopted results in a more realistic solution of the inverse problem than the single minimization of the shear stria angle

a refined astronomically calibrated 40ar 39ar age for fish canyon sanidine abstract intercalibration between the astronomical and radio isotopic dating methods provides a means to improving accuracy and reducing uncertainty of an integrated multi chronometer geologic timescale here we report a high precision 40 ar 39 ar age for the fish canyon sanidine fcs neutron fluence monitor by multi collector noble gas mass spectrometry through cross calibration with a1 tephra sanidines a1ts of the direct astronomically tuned faneromeni section crete the astronomically intercalibrated 40 ar 39 ar age of fcs of 28 172 0 028 ma 2 external errors is within the uncertainty of but more precise 0 10 than the previous 40 ar 39 ar age determined by intercalibration with astronomically tuned tephras from the melilla basin morocco using this proposed age for fcs combined with measurements using the a1ts as the neutron fluence monitor a weighted mean bishop tuff 40 ar 39 ar sanidine age of 0 7674 0 0022 ma 2 external errors is indistinguishable from the id tims u pb zircon age 0 7671 0 0019 ma the consistency between the astronomically calibrated 40 ar 39 ar sanidine age and u pb zircon age for this quaternary unit suggests that agreement between these two radio isotopic dating techniques is now achievable at better than 0 3 2 in the youngest part of geologic time

from pull apart to the rifting process the formation of the pannonian basin abstract during the middle and the late miocene intra carpathian basins have developed south and west of the carpathian arc these basins are bounded by normal faults and connected by conjugate shear zones sinistral strike slip fault systems trend ne sw whereas dextral ones trend nw se by means of geometrical analyses of fault system and paleostress tensor determinations it has been possible to consistently reconstruct the succession of tectonic events in the hungarian mountains the two main stages in the structural development of the intra carpathian basins are 1 1 the initiation of pull apart basins along conjugate shear zones in a n s compressional and e w extensional regime and 2 2 the development of these basins in a tectonic regime characterized by a dominant e w trending extension

burial diagenetic sabkha like gypsum and anhydrite nodules abstract anhydrite nodules in the late devonian dismal creek and blue ridge members in the subsurface of alberta canada recovered from depths in excess of 3000 m resemble sabkha nodules however several features are inconsistent with a sabkha origin or are consistent with both a shallow and a deep burial origin these nodules occur in facies that represent deposition on a marine carbonate platform or ramp close to or below fair weather wave base almost all nodules are associated with dark bituminous argillaceous seams and pockets carbonaceous halos anastomosing bituminous veinlets and stylolitic contacts petrographic uv fluorescence x ray diffraction and pyrolysis data indicate that the dark material consists of diagenetic residues from pressure solution and concretionary cementa ion and that the organic matter is mature kerogen solidified crude oil or a mixture of the two sulfur and oxygen isotope ratios of the anhydrite suggest bacterial sulfate reduction before or during nodule growth all features taken together indicate a burial diagenetic origin for the dismal creek and blue ridge nodules it is suggested that gypsum or anhydrite nodules resembling those commonly formed in sabkhas can be formed during burial at depths of at least several tens to several hundreds of meters the main criteria for recognizing a burial origin for sulfate nodules are 1 nodules occur within or juxtaposed to residues of pressure solution seams stylolites and diagenetic argillaceous and carbonaceous pockets between diagenetic carbonate nodules 2 nodules replace host rock and they may crosscut the boundaries between host rock and pressure solution accumulates 3 nodules contain inclusions that had formed during burial e g authigenic pyrite dolomite etc 4 there is an absence of features indicative of breakage during mechanical compaction e g broken pile of brick texture withi the nodules 5 anhydrite crystals in nodule margins have circum nodular tangential alignment 6 compared to sabkha nodules burial diagenetic nodules may occur in facies that are not lagoonal to peritidal and supratidal and that are unlikely to have been subaerially exposed in evaporitic climates

late pleistocene granodiorite source for recycled zircon and phenocrysts in rhyodacite lava at crater lake oregon abstract rhyodacite tephra and three lavas erupted 27 ka interpreted to be early leaks from the climactic magma chamber of mount mazama contain ubiquitous resorbed crystals antecrysts that were recycled from young granodiorite and related plutonic rocks of the same magmatic system the shallow composite pluton is represented by blocks ejected in the 7 7 ka climactic eruption that formed crater lake caldera plagioclase crystals in both rhyodacite and granodiorites commonly have cores with crystallographically oriented fe oxide needles exsolved at subsolidus conditions at least 80 of plagioclase crystals in the rhyodacite are antecrysts derived from plutonic rocks other crystals in the rhyodacite notably zircon also were recycled sims 238 u 230 th dating indicates that zircons in 4 granodiorite blocks crystallized at various times between 20 ka and 300 ka with concentrations of analyses near 50 70 110 and 200 ka that correspond to periods of dacitic volcanism dated by k ar u th ages of zircon from a rhyodacite sample yield similar results no analyzed zircons from the granodiorite or rhyodacite are pre quaternary zircon minimum ages in blocks from different locations around the caldera reflect ages of nearby volcanic vents and may map the distribution of intrusions within a composite pluton survival of zircon in zircon undersaturated hydrous magma and of fe oxide needles in plagioclase suggests that little time elapsed from entrainment of antecrysts to the 27 ka eruption of the rhyodacite the 27 ka rhyodacite is an example of young silicic magma that preserved unstable antecrysts from a known source early during growth of a large high level magma chamber in contrast the voluminous 7 7 ka climactic rhyodacite pumice is virtually lacking in zircon indicating dissolution of any granodioritic debris in the intervening period mineralogical evidence of assimilation may be destroyed in hot vigorously growing silicic magma bodies such as ultimately produced the climactic eruption of mount mazama

diagenetic origin of basal anhydrite in the cretaceous maha sarakham salt khorat plateau ne thailand development of a diagenetic anhydrite bed at the base of the cretaceous maha sarakham saline formation the basal anhydrite member of the khorat plateau in north eastern thailand took place due to leaching and or pressure dissolution of salt at the contact between an underlying active sandstone aquifer system and an overlying massive halite dominated evaporite sequence basal evaporites composed of halite with intercalated anhydrite of the latter sequence are undergoing dissolution as a result of subsurface flushing with anhydrite produced as the insoluble residue the result is a 1ae1 m thick interval of nodular anhydrite displaying unique basin wide continuity observed textures petrographic features and chemical data from the anhydrite and associated authigenic minerals support the origin of the basal anhydrite member as an accumulation residue from the dissolution of the maha sarakham salts petrographically the anhydrite in this unit is made up of crystals that are blocky and recrystallized sheared generally elongated and broken and is bounded at the bottom by organic rich stylolite surfaces authigenic and euhedral dolomite and calcite crystals are associated with the anhydrite traces of pyrite galena and chalcopyrite are present along the stylolite surfaces suggesting supply of fresh water from the underlying sandstone at highly reducing conditions of burial the d 34 s of sulphate in the basal anhydrite averages 15 cdt and falls within the isotopic composition of the anhydrite in the cretaceous maha sarakham formation proper and the cretaceous values of marine evaporites measured d 18 o in dolomite range from 4ae37 to 14ae26 pdb suggesting a re equilibration of dolomite with basinal water depleted in 18 o and possible recrystallization of dolomite under relatively elevated temperatures the d 13 c however varies from 1ae57 to 2ae53 pdb suggesting a contribution of carbon from oxidation of organic matter this basal anhydrite bed similar to basinwide beds found at the bottom of many giant evaporite sequences has always been considered to be depositional here at the base of the maha sarakham formation we demonstrate that the anhydrite is diagenetic in origin and was formed by accumulation of original anhydrite by dissolution of interbedded halite from waters circulating though the underlying aquifer it represents an upsidedown caprock

sequence stratigraphy of carbonate evaporite basins models and application to the upper permian zechstein of northeast england and adjoining north sea the successions of carbonate evaporite basins which are periodically cut off from the world ocean are considered in terms of their sequence stratigraphy two basin types are distinguished those where incomplete drawdown takes place and those showing complete drawdown and wholesale basin desiccation it is argued that following relative sea level falls sequences begin with the evaporites precipitated as marginal lowstand gypsum wedges in the first basin type and as lowstand basin centre halite fills in the second the evaporites pass up into carbonate sediments which are mainly deposited on shallow water platforms around the basin during transgressive and highstand systems tracts the concepts and models developed are applied to the zechstein upper permian strata of northeast england and adjoining north sea seven sequences are proposed with some containing thin parasequences the sequence approach to carbonate evaporite basins which considers both basin margin and basin centre facies gives a more encompassing perspective of sedimentation than the traditional cycle concept with its emphasis on the basin centre

kinematic analysis of faults in a physical model of growth faulting above a viscous salt analogue abstract syn sedimentary faults overlying an extended ductile layer within a sand model provide an analogue for studying some aspects of the geometry and kinematics of basin scale extensional listric faults associated with salt rollers the displacement analysis techniques used to examine displacement variations over fault surfaces within the sand model show that displacement varies in a regular and predictable manner deviations from simple fault displacement patterns are due to transfers of displacement onto adjacent synthetic faults aggregation of the displacements on adjacent synthetic faults produces more regular patterns of displacement distribution and demonstrates the geometric coherence of the aggregated faults rapid upward decreases in displacement on the model faults are due to fault growth during sedimentation elsewhere in the model post sedimentary faulting is associated with very low upward displacement gradients and pre and syn faulting sedimentation are readily distinguished on cross sectional fault traces in three dimensions a non horizontal line separating the pre faulting and syn faulting sequences tracks the lateral extent of a fault through time backstripping of fault displacements by subtracting the fault displacement of a sedimentary horizon from the displacements of underlying horizons gives the displacement distribution on a fault surface at the time of deposition of the backstripped horizon backstripped fault displacement distributions also allow examination of the general fault structure through time backstripping of fault displacements on one of the larger faults within the model and on an associated hanging wall synthetic splay shows that the two structures originated as a relay zone which was breached by lateral propagation of one of the overlapping faults which form the relay zone the backstripping technique allows determination of the geometrical changes during growth of a fault system

stratigraphic control on facies and diagenesis of dolomitized oolitic siliciclastic ramp sequences pinda group albian offshore angola pervasively dolomitized oolitic to siliciclastic ramp sequences are prolific oil reservoirs in the albian pinda group of northern angola during the late cretaceous this reservoir series was segmented by salt tectonic gravitational gliding into halokinetic turtle back and raft structures that provide hydrocarbon traps today the albian ramp sequences contain a large variety of reservoir facies reaching from littoral sandstones with interparticle porosity to various types of dolomitized marine shelf to shoreline carbonates with moldic vuggy and intercrystalline porosity reservoir prediction is substantially improved by an integrated approach combining high resolution sequence stratigraphy with carbonate geochemistry as high resolution stratigraphy documents most of the important sandstone reservoirs formed at the base of stratigraphic sequences during shoreface transgression pisolitic dolostones with moldic porosity are the best carbonate reservoirs they developed as transgressive shoreface sand sheets above the basal sandstone reservoirs regressive facies tracts contain oolitic and pisolitic dolomites with intercrystalline and moldic porosity that originated in progradational barrier and sand belt settings the combined stratigraphic and geochemical approach allowed us to determine the pinda group dolomites as early diagenetic in origin whereas fluid inclusion and oxygen isotope analysis alone suggested a late burial dolomite formation dolomite neomorphism at higher burial temperatures modified most of the original geochemical signatures of early near surface dolomitization cathodoluminescence data and trace element analysis suggest that dolomitization was associated with gradually decreasing pore water salinity accordingly barrier oolites were completely dolomitized and developed intercrystalline porosity during the earliest stages of diagenesis at a later phase of early diagenesis as waters became less saline and undersaturated with respect to calcium carbonate fabric selective dissolution of the remaining calcareous components occurred the decrease in pore water salinity and associated porosity generation are genetically linked with high frequency events of relative sea level lowering this possible relationship implies that high resolution sequence stratigraphy can be a valid reservoir prediction tool even in pervasively dolomitized series

maximum link distance between strike slip faults observations and constraints the maximum fault link distance lateral separation between two interacting strike slip faults is studied in relation to fault length combined length of the two interacting faults data collected for laboratory generated strike slip faults indicate that fault linkage takes place when the lateral separation between two strike slip faults is less than one tenth of the combined length of the two faults strike slip faults developed in clay models require a smaller link distance than those developed in gouge materials about 98 of the data collected for natural strike slip faults and 93 of earthquake rupture data derived from knuepfer 1989 follow the rule these observations support a simple scaling relationship for strike slip faults i e w 0 1 l where wis lateral link distance and l the combined fault length an and sammis 1996a two possible explanations are discussed assuming a fault continues to be dominantly strike slip after various fault coalescences the fault link distance for both compressional coalescence leading to restraining bend and extensional coalescence leading to releasing bend can be constrained within 0 134 l assuming fault coalescence is caused by interaction between breakdown zones at fault tips then the scaling relationship b 0 1 l between the size of a breakdown zone b and the individual fault length l scholz et al 1993 leads to the scaling relationship w max 0 1 l where w max is the maximum link distance the above observed relationship between w max and l may not hold if faults coalesce through preexisting faults if a fault has a mixed mode other than strike slip or if a fault bend is caused by cross faulting rather than fault coalescence limited exposure of natural fault traces may also lead to errors in link distance measurements

relay ramp forms and normal fault linkages canyonlands national park utah fieldwork within a series of mesoscale grabens in southeast utah has revealed a particularly well exposed system of interlinked extensional faults a series of down faulted grabens are developed within a 460 m thick brittle layer of upper paleozoic sandstone and shale which overlies a ductile layer with a high gypsum content all the major grabens consist of two or more overlapping elements which are composed of fault segments these segments may be hard linked fault surfaces are joined or soft linked fault surfaces are isolated but linked by ductile strain of the rock volume between them in map view relay structures are defined as zones connecting the footwalls and hanging walls of overlapping fault segments representing soft linkage of fault segments in the canyonlands grabens the transfer of displacement between soft linked fault segments is characterized by well de fined dipping relay ramps commonly rotated and extended to accommodate the ductile strain between the overlapping fault segments relay ramps develop as ephemeral structures eventually becoming breached by hard linkage of the fault segments breakdown of ramps by breaching is part of the process of fault growth by segment linkage within the canyonlands graben system four orders of segmentation are present this may be a consequence of different scales of mechanical heterogeneity but evidence from the canyonlands grabens and elsewhere points to a fundamental process of discontinuous fault growth by segment linkage

strike slip relay ramps areas of reorientated bedding at contractional oversteps between strike slip faults are here called strike slip relay ramps metre scale examples are described from the jurassic sediments at east quantoxhead somerset u k larger strike slip relay ramps occur in the rio de peixe basin ne brazil along the newport inglewood trend california and in the bovey basin sw england although the geometry and development of strike slip relay ramps are similar to those of relay ramps in normal fault systems there are differences in the structures which accommodate the transfer of displacement between the overstepping faults whereas strike slip relay ramps are typically transpressional with pressure solution often occurring relay ramps in normal fault systems are dominated by extension or transtension care needs to be taken when interpreting areas of reorientated bedding between overstepping faults particularly when displacement directions are unknown for example when using seismic data this is because relay ramps can occur in both strike slip and normal fault zones

alpine tectonics of the pannonian basin the neogene evolution of the pannonian basin is fairly well understood in terms of back arc extension behind the carpathian thrust fold belt this middle miocene holocene basin however is superimposed on an earlier middle eocene early miocene basin traditionally called the paleogene basin of hungary although the paleogene stratigraphy of the basin is well known its structural evolution is not clear however a transtensional origin is assumed analogous to the overlying pannonian basin the existing data set on the paleogene basin can be integrated easily into a compressional basin model other intra carpathian paleogene basins such as the transylvanian and slovenian basins are now considered to be flexural in origin systematic interpretation of reflection seismic data shows that these tertiary basins are superimposed on a cretaceous alpine thrust fold belt the internal geometry of this belt in the basement of the pannonian basin indicates a close relationship to the surrounding mountain belts alps carpathians and dinarides whereas the neogene pannonian basin is in a mature stage of exploration the underlying paleogene flexural basins are much less explored furthermore the cretaceous thrust fold belts should be regarded as frontier areas for future oil and gas exploration efforts

meganodular anhydritization a new mechanism of gypsum to anhydrite conversion palaeogene neogene ebro basin north east spain a number of palaeogene to early neogene gypsum units are located along the southern margins of the ebro basin north east spain these marginal units of eocene to lower miocene age formed and accumulated deposits of ca sulphates gypsum and anhydrite in small shallow saline lakes of low ionic concentration the lakes were fed mainly by ground water from deep regional aquifers whose recharge areas were located in the mountain chains bounding the basin and these aquifers recycled and delivered ca sulphate and na chloride from mesozoic evaporites triassic and lower jurassic in outcrop the marginal sulphate units are largely secondary gypsum after anhydrite and exhibit meganodules from 0 5 to 5 m across and large irregular masses in the sub surface these meganodules and masses are mostly made of anhydrite which replaced the original primary gypsum the isotopic composition 11 1 to 17 4 for 18ovsmow 10 7 to 15 3 for 34svcdt of secondary gypsum in this meganodular facies indicates that the precursor anhydrite derived from in situ replacement of an initial primary gypsum as a result of ascending circulation of deep regional fluid flows through the gypsum units near the basin margins the gypsum was partly altered to anhydrite within burial conditions from shallow to moderate depths from some metres to a few hundred metres at such depths the temperatures and solute contents of these regional flows exceeded those of the ground water today these palaeoflows became anhydritizing solutions and partly altered the subsiding gypsum units before they became totally transformed by deep burial anhydritization the characteristics of the meganodular anhydritization for example size and geometry of the meganodules and irregular masses spatial arrangement relations with the associated lithologies and the depositional cycles presence of an enterolithic vein complex and palaeogeographic distribution are compared with those of the anhydritization generated both in a sabkha setting or under deep burial conditions and a number of fundamental differences are highlighted

origin of satin spar veins in evaporite basins abstract satin spar fibrous gypsum veins occur in rocks overlying evaporites in the amadeus basin australia appalachian basin usa cheshire basin england elk point basin canada palo duro basin usa paradox basin usa and zechstein basin england these antitaxial veins which are characterized by central partings and near vertical fibers fill horizontal and inclined fractures most satin spar veins occur in highly fractured or brecciated clastic strata that overlie or are associated with bedded halite anhydrite and gypsum saline springs are commonly present indicating that halite dissolution is active the similarities in morphology and occurrences of these veins suggest a common genesis satin spar veins result from several simultaneously active processes recharge of low salinity surface water results in dissolution of shallow 200 m to 750 m deep halite development of cavernous porosity and formation of extensional fractures in the rock column overlying salt dissolution zones greater solubility of anhydrite than gypsum at low temperatures and at salinities below halite saturation causes hydration of anhydrite to gypsum which takes place without significant volume change once groundwater is saturated with respect to gypsum any further anhydrite hydration and solution must result in supersaturation and the excess caso4 carried by ground water flowin from dissolution zones precipitates as gypsum in open high permeability fractures

development of pull apart basins a comparative study of well mapped active and ancient pull apart basins suggests a qualitative model for their continuous development pull aparts evolve from incipient to extremely developed basins through a sequence of closely related states new and compiled map data from several areas including the northern caribbean and turkey suggests the following stages in pull apart development 1 in rigid intracontinental strike slip plate boundary zones larger pull aparts nucleate at releasing fault bends along segments of the principal displacement strike slip fault zone which are oblique to the theoretical interplate slip lines 2 initial opening across releasing fault bends produces spindle shaped basins defined and often bisected by oblique slip faults connecting the discontinuous ends of the strike slip faults 3 increased strike slip offset produces basin shapes which we colloquially call lazy s shape for basins between sinistral faults and lazy z shape for basins between dextral faults 4

iron rich encrustation on the footwall of the gant bauxite vertes hills hungary evidence for preservation of organic matter under exceptional conditions abstract investigation of the iron rich encrustation under the gant bauxite in the vertes hills hungary provides good evidence for preservation of organic matter the organic matter probably was trapped during rapid redeposition of bauxitic material by episodic mudflow debris flow onto a karstic surface triggered by synsedimentary tectonism associated with early to middle eocene strike slip motions according to the morphogenetic study of hematite pseudomorphs after pyrite in the crust the cathodoluminescence study of the cement of the underlying dolostone and the crust the high negative values of 13 c and 18 o of the cement and the chemical analysis of trace element composition of the crust the bauxite and the dolostone bedrock show pyrite precipitation in a meteoric phreatic organic rich environment at the contact of the bauxite and the underlying dolostone this environment was probably created as a result of the slow rise of the water table preceding the middle eocene transgression sulphur needed for the formation of the pyrite may have been supplied by the organic matter which may have been concentrated at the bottom of the deposit as tropical vegetation trapped underneath the mudflow or by introduction of marine pore water during transgression

geometry and development of relay ramps in normal fault systems relay ramps occur between normal fault segments that overstep in map view the geometry and evolution of exposure scale relay ramps are described from the somerset coast england and are compared with larger scale ramps from elsewhere relay ramps can be classified into four groups based on the degree of interaction and linkage between the overstepping segments these groups are interpreted as being evolutionary stages in stage 1 the segments do not interact stage 2 involves the reorientation of bedding between two interacting faults to produce a relay ramp in stage 3 connecting fractures start to break the relay ramp stage 4 is when the relay ramp is destroyed to produce a single fault that has an along strike bend these evolutionary stages can develop through ti e but they can also be seen spatially a branch line between normal faults or an along strike bend may represent a stage 4 relay with progressively earlier stages occurring updip or downdip characteristic variability in displacement distance profiles for fault segments and linked faults accompanies the interaction and linkage processes displacement transfer by relay ramps is accompanied by steep displacement gradients along fault segments at oversteps relay ramps often contribute to a minimum in total fault displacement at a linkage point

timing of post depositional events in the burano formation of the secchia valley upper triassic northern apennines clues from gypsum anhydrite transitions and carbonate metasomatism the burano evaporite formation from the secchia river valley is an up to 2200 m thick sequence composed of meter to decameter scale interbeds of gypsum anhydrite and dolostones with minor halite the deposit has been affected by a complex array of post depositional modifications thermal events and large scale evaporite dissolution preventing a satisfactory reconstruction of the environment of deposition the modifications are intense because these rocks were the main decollement horizon during the formation of the northern apennines chain the carbonate rocks are massive and or laminated dolomitic mudstone wackestone oolitic packstones and oolitic peloidal bioclastic grainstones which commonly appear as mega boudins within a sulfate groundmass the dolostones to 3 7 to 3 0 pdb have been affected by mg metasomatic replacement by magnesite to 2 6 to 1 4 pdb induced by hydrothermal circulation total homogenization temperatures of fluid inclusions in hydrothermal magnesite range from 275 to 310 c the anhydrite rocks are characterized by flow structures such as centimeter scale pseudo lamination composed of aligned prismatic crystals with transposed isoclinal folds outlined by dolostones fragments homogenization temperatures of fluid inclusions in authigenic quartz incorporated into sulfate rocks range from 260 to 305 c emilia and from 230 to 315 c tuscany the gypsum rocks are composed of xenotopic irregular cloudy crystals and more rarely by centimeter scale idiotopic crystals showing the same structures as the anhydrite rocks the origin of the gypsum rocks is due to late alteration of anhydrite by migration of sharp hydration fronts the hydration is a two step process and is revealed by the presence in the gypsum rocks of corroded anhydrite micro relics and authigenic quartz crystals which include anhydrite the role of the burano evaporites during the apennines tectogenesis can be depicted as follows a prevalent deposition of gypsum in the upper triassic b gypsum dehydration at burial conditions to form anhydrite cretaceous c syn tectonic flow of anhydrite rocks brecciation of dolostones syn tectonic growth stage of quartz euhedra at deep burial conditions possibly related to the development of the oligocene miocene greenschist facies apuane metamorphic complex d hydrothermal deposition of sparry magnesite and partial mg metasomatic replacement of dolostones by magnesite e sub surface dissolution of halite to form thick matrix supported residual caprock like anhydrite mega breccias f complete gypsification of anhydrite at sub surface conditions and g evaporite dissolution at surface exposure producing dolostone breccias with partial calcitization and removal of most clasts calcare cavernoso

a comment on titaniq under pressure the effect of pressure and temperature on the solubility of ti in quartz by jay b thomas e bruce watson frank s spear philip t shemella saroj k nayak and antonio lanzirotti trace concentrations of ti in quartz are used to indicate the pressure and temperature conditions of crystallization in the titaniq geothermobarometer of thomas et al contrib miner petrol 160 743 759 2010 it utilises the partitioning of ti into quartz as an indicator of the pressures and or temperatures of crystal growth for a given value of tio2 activity in the system if temperatures are inferred to 20 c pressure is constrained to 1 kbar and vice versa there are significant contrasts however between the conclusions from titaniq and those for natural quartz as well as other mineral phases in volcanic rocks application of the titaniq model to quartz from the 27 ka oruanui and 760 ka bishop high silica rhyolites where the values of t p and tio2 activity are constrained by other means fe ti oxide equilibria melt inclusion entrapment pressures in gas saturated melts melt and amphibole compositions yields inconsistent results if realistic values are given to any two of these three parameters then the value of the third is wholly unrealistic the model yields growth temperatures at or below the granite solidus pressures in the lower crust or upper mantle or tio2 activities inconsistent with the mineralogical and chemical compositions of the magmas cl imagery and measurements of ti and other elements in quartz are of great value in showing the growth histories and changes in conditions experienced by crystals but direct linkages to p t conditions during crystal growth cannot be achieved

diagenetic evolution of aptian evaporites in the namibe basin south west angola the widespread and dissected nature of the angolan gypsiferous salt residuals offers a uniquely detailed view of the lateral and vertical relations inherent to secondary evaporite textures which typify exhumed salt masses worldwide such secondary textures are sometimes misinterpreted as primary evaporite textures thin metre scale and patchy dome like gypsum accumulations are well exposed within strongly incised present day river valleys along the eastern margin of the namibe and benguela basins south west angola these sections are time equivalent to the main basinward subsurface evaporites aptian loeme formation which mostly consist of halite the gypsum here called the bambata formation is interpreted to represent the final residual product of fractional dissolution and recrystallization of the halite mass that occurred during late cretaceous margin uplift and continues today this halite underwent multiple episodes of diagenetic alteration between its deposition and its final exhumation leading to the formation of various secondary gypsum fabrics and solution related karst and breccia textures that typify the current evaporite outcrop four different diagenetic gypsum fabrics are defined thinly bedded alabastrine nodular alabastrine displacive selenite rosettes and fibrous satin spar gypsum current arid conditions are responsible for a thin weathered crust developed at the top of the outcropping gypsum but the fabrics in the main core of the current at surface evaporite unit mostly formed during the telogenetic stage of uplift prior to complete subaerial exposure alteration occurred as various dissolving and rehydrating saline minerals encountered shallow aquifers in the active phreatic and vadose zones geomorphological and petrographic analyses mostly based on the cross cutting relations and crystallographic patterns in the outcrop are used to propose a sequence of formation of these different fabrics

comparison of the rift and post rift architecture of conjugated salt and salt free basins offshore brazil and angola namibia south atlantic abstract this study presents a regional comparison between selected 2d seismic transects from large conjugated salt and salt free basins offshore southern brazil campos basin santos basin pelotas basin and southwest africa kwanza basin northern and southern namibe basin walvis basin tectonic stratigraphic interpretation of the main rift and post rift units free air gravity data and flexural isostatic backstripping were used for a comprehensive basin to basin documentation of key mechanisms controlling the present day differences in conjugated and neighbouring south atlantic basins a significant variation in the tectonic sedimentary architecture along strike at each margin and between the conjugated basins across the south atlantic reflects major differences in 1 the structural configuration of each margin segment at transitional phase between rifting and breakup as emphasized in the highly asymmetric settings of the large santos salt basin and the conjugated salt free southern namibe basin 2 the post breakup subsidence and uplift history of the respective margin segment which caused major differences for example between the campos and espirito santo basins and the conjugated northern namibe and kwanza basins 3 variations in the quantity and distribution of post breakup margin sediments which led to major differences in the subsidence history and the related present day basin architecture for example in the initially rather symmetric siliciclastic pelotas and walvis basins and 4 the deposition of aptian evaporites in the large rift and sag basin provinces north of the rio grande rise and walvis ridge highly contrasting the siliciclastic basins along the margin segments south of the ridges the resulting present day architecture of the basins can be generally classified as i moderately symmetric salt free and magma rich in the northern part of the southern segment i highly asymmetric salt bearing and magma poor vs salt free and magma rich in the southern part of the central segment and iii moderately symmetric to asymmetric salt rich and magma poor in the central part of the central segment of the south atlantic

4d analogue modelling of transtensional pull apart basins abstract scaled sandbox models were used to investigate the 4d evolution of pull apart basins formed above underlapping releasing stepovers in both pure strike slip and transtensional basement fault systems serial sectioning and 3d volume reconstruction permitted analysis of the full 3d fault geometries results show that very different pull apart basins are developed in transtension compared to pure strike slip both types of models produced elongate sigmoidal to rhomboidal pull apart systems but the transtensional pull apart basins were significantly wider and uniquely developed a basin margin of en echelon oblique extensional faults dual opposing depocentres formed in the transtensional model whereas a single central depocentre formed in pure strike slip in transtension a distinct narrow graben system formed above the principal displacement zones pdzs cross basin fault systems that linked the offset pdzs formed earlier in the transtensional models sequential model runs to higher pdz displacements allowed the progressive evolution of the fault systems to be evaluated in cross section transtensional pull aparts initiated as asymmetric grabens bounded by planar oblique extensional faults with increasing displacement on the pdzs basin subsidence caused these faults to become concave upwards and lower in dip angle due to fault block collapse towards the interior of the basin in addition strain partitioning caused fault slip to become either predominantly extensional or strike slip the models compare closely with the geometries of natural pull apart basins including the southern dead sea fault system and the vienna basin austria

dense brine refluxing a new genetic interpretation of widespread anhydrite lumps in the oligocene lower miocene asmari formation of the zagros foreland basin ne iraq abstract the asmari formation in the zagros basin of ne iraq contains a thick sequence of oligocene lower miocene carbonates and it forms one of the best known carbonate reservoirs in the world the formation mainly consists of dolostones limestones sandstones and mudstones and there is a general upward increase in the abundance of evaporites generally in the form of anhydrite lumps in the dolostones of the upper part the anhydrite types and textures provide evidence on the salinity depth temperature environment and sea level fluctuations which can be used for paleoenvironmental and climatic interpretations and they also exert a significant control on reservoir quality which has attracted the attention of many petroleum industries previous studies suggested that the anhydrite lumps in the upper asmari formation aquitanian burdigalian were synsedimentary deposits in a hypersaline setting of a sabkha salt flat however this paper shows that 1 the upper asmari formation was deposited in an inner to mid ramp setting based on the sedimentological petrographical and paleontological characteristics 2 the anhydrite lumps are irregular and variable in size and are generally found within or associated with karst systems and 3 fossil assemblages indicate that the synsedimentary salinity was normal or only slightly hypersaline which means that the anhydrite was not directly deposited from the synsedimentary marine water after comprehensive analysis of the anhydrite lumps it is proposed in this paper that the anhydrite lumps were formed within the upper asmari formation aquitanian burdigalian due to downwards seepage of the overlying hypersaline water which is termed as dense brine refluxing a new genetic model it is suggested 1 that the upper asmari formation aquitanian burdigalian was deposited in an inner to mid ramp setting and subjected to eogenetic karstification generating karst systems in local area with relative high landscape due to sea level change 2 that during the middle miocene tectonic compression resulted in the zagros basin isolated and the seawater becoming hypersaline with higher salt concentrations and a high density and 3 the hypersaline water seeped downwards through the underlying porous sediments forming anhydrite lumps and resulting in a decrease in the porosity of the upper asmari formation aquitanian burdigalian this paper provides new perspectives on the origin of anhydrite lumps and the resulting effects on reservoir quality in the zagros foreland basin

solution collapse breccias of the minkinfjellet and wordiekammen formations central spitsbergen svalbard a large gypsum palaeokarst system large volumes of carbonate breccia occur in the late syn rift and early post rift deposits of the billefjorden trough central spitsbergen breccias are developed throughout the moscovian minkinfjellet formation and in basal parts of the kazimovian wordiekammen formation breccias can be divided into two categories i thick cross cutting breccia bodies up to 200 m thick that are associated with breccia pipes and large v structures and ii horizontal stratabound breccia beds interbedded with undeformed carbonate and siliciclastic rocks the thick breccias occur in the central part of the basin whereas the stratabound breccia beds have a much wider areal extent towards the basin margins the breccias were formed by gravitational collapse into cavities formed by dissolution of gypsum and anhydrite beds in the minkinfjellet formation several dissolution fronts have been discovered demonstrating the genetic relationship between dissolution of gypsum and brecciation textures and structures typical of collapse breccias such as inverse grading a sharp flat base breccia pipes collapse dolines and v structures cave roof collapse are also observed the breccias are cemented by calcite cements of pre compaction shallow burial origin primary fluid inclusions in the calcite are dominantly single phase containing fresh water final melting points are ca 0 c suggesting that breccia diagenesis occurred in meteoric waters cathodoluminescence cl zoning of the cements shows a consistent pattern of three cement stages but the abundance of each stage varies stratigraphically and laterally d 18 o values of breccia cements are more negative relative to marine limestones and meteoric cements developed in unbrecciated minkinfjellet limestones there is a clear relationship between d 18 o values and the abundance of the different cement generations detected by cl paragenetically later cements have lower d 18 o values recording increased temperatures during their precipitation carbon isotope values of the cements are primarily rock buffered although a weak trend towards more negative values with increasing burial depth is observed the timing of gypsum dissolution and brecciation was most likely related to major intervals of exposure of the carbonate platform during gzhelian and or asselian sakmarian times these intervals of exposure occurred shortly after deposition of the brecciated units and before deep burial of the sediments

the origin of evaporitive limestones an example from the messinian of sicily italy a good example of an extensive evaporative carbonate deposit which has altered little since formation is found in the messinian upper miocene evaporites of sicily in this study these messinian carbonates the calcare di base are examined through a combination of field observations together with petrologic mineralogic and stable isotope studies these evaporitic carbonates are found in massive nearly featureless beds both calcite and aragonite commonly composed of automorphic breccias and or weakly layered micrite additionally fine scale features are observed within the carbonates such as small algal stromatolites and algally controlled laminae petrologic study of thin sections reveals ubiquitous algal filaments and grumeleuse to pelletal micrites as the primary components this suggests by analogy to modern hypersaline environments that the original calcium carbonate precipitated as algally controlled aragonite in a hypersaline environment displacive and interbedded halite still present within the deposit in the subsurface in some areas has been leached in others leaving residual breccias cubic voids and halite pseudomorphs the carbonate has been partially calcitized during one or more phases of early diagenesis the diagenetic systems which affected these sediments were not wholly open the halite was removed in some areas early in the sediment s history but after partial lithification probably synsedimentary as indicated by breccias containing well preserved halite voids these breccias lie within a nonhalite bearing micritic matrix later alteration has removed more of the halite leaving cubic void spaces porous layers and halite pseudomorphs filled with calcite celestite and strontianite the presence in some areas of considerable amounts of unaltered aragonite and in other regions of secondary strontium minerals indicate that the systems which affected these sediments were not open ones based on the oxygen isotope values obtained from the unaltered aragonites it appears that the large volumes of calcium carbonate in this deposit were derived from a combination of continual influx of seawater together with a probable contribution from continental sources coming into a restricted hypersaline environment at the margins of the basin the isotopic composition of the altered carbonates suggests that meteoric waters participated in the diagenesis while secondary bacterial processes oxidation sulfate reduction and methanogenesis contributed carbonate ions depleted in carbon 12 to the final product

rupture progression along discontinuous oblique fault sets implications for the karadere rupture segment of the 1999 izmit earthquake and future rupture in the sea of marmara abstract large earthquakes in strike slip regimes commonly rupture fault segments that are oblique to each other in both strike and dip this was the case during the 1999 izmit earthquake which mainly ruptured e w striking right lateral faults but also ruptured the n60 e striking karadere fault at the eastern end of the main rupture it will also likely be so for any future large fault rupture in the adjacent sea of marmara our aim here is to characterize the effects of regional stress direction stress triggering due to rupture and mechanical slip interaction on the composite rupture process we examine the failure tendency and slip mechanism on secondary faults that are oblique in strike and dip to a vertical strike slip fault or master fault for a regional stress field well oriented for slip on a vertical right lateral strike slip fault we determine that oblique normal faulting is most favored on dipping faults with two different strikes both of which are oriented clockwise from the strike slip fault the orientation closer in strike to the master fault is predicted to slip with right lateral oblique normal slip the other one with left lateral oblique normal slip the most favored secondary fault orientations depend on the effective coefficient of friction on the faults and the ratio of the vertical stress to the maximum horizontal stress if the regional stress instead causes left lateral slip on the vertical master fault the most favored secondary faults would be oriented counterclockwise from the master fault for secondary faults striking 30 oblique to the master fault right lateral slip on the master fault brings both these secondary fault orientations closer to the coulomb condition for shear failure with oblique right lateral slip for a secondary fault striking 30 counterclockwise the predicted stress change and the component of reverse slip both increase for shallower angle dips of the secondary fault for a secondary fault striking 30 clockwise the predicted stress change decreases but the predicted component of normal slip increases for shallower angle dips of the secondary fault when both the vertical master fault and the dipping secondary fault are allowed to slip mechanical interaction produces sharp gradients or discontinuities in slip across their intersection lines this can effectively constrain rupture to limited portions of larger faults depending on the locations of fault intersections across the fault intersection line predicted rakes can vary by 40 and the sense of lateral slip can reverse application of these results provides a potential explanation for why only a limited portion of the karadere fault ruptured during the izmit earthquake our results also suggest that the geometries of fault intersection within the sea of marmara favor composite rupture of multiple oblique fault segments

evidence for episodic basin dewatering in salt dome cap rocks a detailed record of episodic basin dewatering events is preserved in the anhydrite cap rocks of two gulf coast salt domes metalrich brines were intermittently expelled from geopressured zones deep in the stratigraphic section and were channeled upward along escape structures bounding the salt diapirs overhanging anhydrite cap rock helped to focus some escaping fluid into the zone of dissolution between the top of salt and overlying residual anhydrite cap rock iron lead and zinc sulfide solubilities were exceeded in this zone possibly in response to dissolution and reduction of cap rock sulfates because the metalliferous brines entered the dissolution zone intermittently they were recorded as relatively thin horizontal bands of sulfide sandwiched between thicker accumulations of anhydrite continued dissolution of salt and underplating of residual anhydrite caused the sulfide bands to be displaced upward relative to the base of the cap leading to an inverted stratigraphic record of basin dewatering events paleomagnetic data from the winnfield salt dome suggest that sulfide producing basin dewatering events and anhydrite cap rock accumulation occurred between 157 and 145 ma

calibrating the duration and timing of the messinian salinity crisis in the mediterranean linked tectonoclimatic signals in thrust top basins of sicily the messinian 9salinity crisis9 which affected the mediterranean represents one of the most dramatic examples of base level fluctuation known in the geological record an amplitude of perhaps 2 km within a stage with a duration of less than 2 ma deposits within the caltanissetta basin of central sicily are used to calibrate the duration and timing of these fluctuations two successions of evaporites termed 9first cycle9 and 9second cycle9 are separated by an inter regional unconformity the first cycle is regionally regressive while the second is transgressive chronostratigraphic calibration of these deposits has been provided by a linked magnetostratigraphic structural and sedimentological study the regression was protracted the earliest evaporites in our study accumulated early in chron c3ar pre 6 88 ma and the youngest accumulated late in chron c3an post 6 0 ma during this interval the basinward shift in coastline was 70 km and in vertical section implies a relative fall in sea level at 0 3 0 4 m ka 1 lowstand probably finally occurred at 5 8 5 5 ma transgression marked by accumulation of the 9second cycle9 deposits which all record reversed magnetizations c3r apparently occurred far more rapidly 200 ka prior to the return to 9normal9 marine conditions in the central mediterranean late in chron c3r local rates of tectonic deformation are relatively slow within the thrust belt which underlies the caltanissetta basin therefore it is likely that the timing and rates of the messinian 9salinity crisis9 on sicily are generally applicable to other basins in the region and help to underpin rates of climate change within this part of the neogene

the primary lower gypsum in the mediterranean a new facies interpretation for the first stage of the messinian salinity crisis abstract the detailed facies and physical stratigraphic analysis of the primary lower gypsum in the mediterranean indicates a surprising bed by bed correlation at basin scale spain italy hellenic arc and cyprus arc that is tuned to the orbital calibration for the first stage of the messinian salinity crisis from 5 96 to 5 61 ma ago a total of 16 precessionally controlled gypsum cycles were deposited rapidly in less than 350 ka forming sequences up to 300 m thick the lack of subaerial exposure features and the common facies associations and stacking pattern for sections located thousands of kilometers apart in different geological settings indicates a modest depositional depth not extremely shallow selenite deposition occurred only at the bottom of restricted marginal basins less than 200 m deep while no gypsum could precipitate in the deeper euxinic mediterranean portions where only thin and barren shale dolostone couplets formed the lowermost selenite beds pass laterally to dolomite rich limestones interbedded with barren euxinic shales in poorly oxygenated settings indicating that the gypsum sedimentation was diachronous and did not necessarily mark the onset on the messinian salinity crisis evaporite facies sequences ef1 to 8 within individual gypsum beds show small scale subaqueous sedimentary cycles that mimic regressive transgressive cycles a initial evaporite precipitation at relatively low supersaturation produced the massive selenite facies ef3 in a relatively deep setting b continuous evaporation and drawdown by oscillating brine level formed the banded selenite ef4 at the aridity acme of the precessionally controlled cycle c general progressive brine level rise with strong brine flow led to the formation of large selenite supercones branching laterally supercones in spain and branching selenite ef5 in the rest of the mediterranean and d flooding by undersaturated continental water terminated gypsum precipitation with the deposition of argillaceous sediments ef1 northern apennines and or limestone ef2 sicily and spain during the humid phase in the precession climate cycle the stacking pattern and selenite facies associations suggest an overall shallowing upward trend with a basin wide hydrologic change starting from the 6th cycle 5 84 ma which is marked by the appearance of the branching selenite facies supercones in spain and indicates that the brines became current dominated the sr isotope stratigraphy suggests that in the northern apennines the brines were strongly modified by continental waters 87 sr 86 sr 0 708893 to 0 708998 and received direct pulses of atlantic seawater 87 sr 86 sr 0 70900 to 0 709024 only in the upper part of the section in areas away from the mainland such as sicily the continental input was by far less important

tertiary paleomagnetic results and structural analysis from the transdanubian range hungary rotational disintegration of the alcapa unit abstract we carried out an integrated paleomagnetic and structural study in the transdanubian range western and central hungary as a result the tertiary tectonic history of this area can be characterized by three events of counterclockwise ccw rotation and four or five phases of brittle deformation the change of the orientation of stress axes between phases is mainly apparent and reflects the rotation of the faults predating a particular rotation event the first two rotation events r1 and r2 were probably governed by the rollback mechanism of the subducting european plate we suggest that these rotations were taking place from 18 17 and 16 14 5 ma respectively i e simultaneously with the rotations of the north hungarian paleogene basin and the main part of the western carpathians however the angle of both rotations was less in the transdanubian range due to increasing distance from the subduction front the differential rotation was accommodated by extensional faulting by formation of a graben system on the other hand the youngest rotation event r3 seems to be connected to the renewed rotation of the adriatic plate around 5 ma our combined data set strongly supports earlier conclusions namely that the different subunits of the eastern alpine western carpathian northern pannonian unit alcapa did not form a rigid unit although they moved in similar manner

analog models of restraining stepovers in strike slip fault systems scaled sandbox models have successfully simulated the geometries and progressive evolution of antiformal pop up structures developed in a weak sedimentary cover above restraining stepovers in offset sinistral strike slip fault systems in rigid basement models were run both with and without synkinematic sedimentation which was added incrementally to cover the growing antiformal structures vertical and horizontal sections of the completed models permit the full three dimensional 3 d structure of the pop ups to be analyzed in detail three representative end member experiments are described 30 underlapping restraining stepovers 90 neutral restraining stepovers and 150 overlapping restraining stepovers the experimental pop ups are typically sigmoidal to lozenge shaped antiformal structures having geometries that are dependent on both the stepover angle and stepover width in the underlying basement faults underlapping restraining stepovers typically form elongate lozenge shaped pop ups 90 neutral restraining stepovers produce shorter squat rhomboidal pop ups and overlapping restraining stepovers produce sigmoidal antiformal pop ups trans pop up cross fault systems are characteristic at large displacements on the basement fault system above the offset principal displacement zones the pop ups are commonly small narrow positive flower structures whereas in the stepover region they widen out and become markedly asymmetric this pop up asymmetry switches across the center of the stepover where the pop ups are largely symmetical maximum rotations measured within the central highly uplifted region of the pop ups increase from 7 counterclockwise for the underlapping 30 stepovers to 14 counterclockwise for the neutral 90 stepovers to 16 counterclockwise for the overlapping 150 stepovers in models where no synkinematic sediments were added during deformation the pop up structures are bound by convex flattening upward oblique slip reverse fault systems that link downward to the offsets in the basement fault system in contrast in the begin page 234 experiments where synkinematic sediments were added incrementally during deformation the pop ups are formed by oblique slip reverse faults that steepen upward into the synkinematic strata with the formation of fault propagation growth folds the analog models are compared with natural examples of pop up structures and show strong similarities in structural geometries and stratal architectures these models may provide structural templates for seismic interpretation of complex contractional structures in offset strike slip fault systems

the flexural slip mechanism abstract detailed field studies of turbiditic sequences from south georgia south atlantic north devon england and cardigan bay wales show that flexural slip occurs on discrete movement horizons between rock packets in which the beds have welded contacts stair stepping displacements of sedimentary dykes and of early quartz veins show that the movement horizons generally have a decimetre to metre spacing and are marked by bedding parallel quartz veins these veins are from 1 mm to several cm thick and can be used to identify movement horizons in the absence of displaced markers they consist of several sheets of quartz fibres which each carry a slickenfibre lineation and which together preserve a record of the displacements on an individual surface complex slickenfibre patterns and departures from ideal behaviour in which slip occurs orthogonal to the fold hinge probably result from changes in slip vector on the limbs of growing non cylindrical folds movement horizons show many of the features associated with large scale thrusting such as ramps duplexes and imbricate structures and the shear sense given by fibre steps on these surfaces and by duplexes within stratigraphically restricted packages changes across fold hinges chevron folds are thought to develop mainly by flexural flow in the early stages with flexural slip becoming dominant later as the beds become lithified new slip surfaces are generated as the dip of the fold limbs increases

what controls relay ramps and transfer faults within rift zones insights from analogue models abstract structures within rift zones exhibit two main types of interaction relevant at the rift scale relay ramps and transfer faults at high angle to the rift analogue experiments have been performed to investigate whether these types of interaction may be affected by differential extension along the rift in these models sand brittle crust analogue overlies two adjacent silicone ductile crust analogue layers with different viscosity in order to simulate different percentage extension rates e along rifts the experiments show a distinct behaviour as a function of e for e e 21 3 the interaction occurs by means of transfer faults striking subparallel to the extension direction experimental data are consistent with the geometries and extension rates of rift zones relay ramps characterize narrow rifts and oceanic ridges where the mean percentage of extension is low e e 39 where the rift more likely achieves differential extension e 21

microbial mediated pre salt carbonate deposition during the messinian salinity crisis calcare di base fm southern italy abstract a multi scale analysis of sedimentary carbonate facies and post sedimentary diagenetic features of the calcare di base formation the precursor to evaporites in upper messinian successions of northern calabria and central sicily has revealed their microbial bio mediated origin massive to laminated microbial boundstones represent the most common sedimentary facies forming flat to low relief cm to m scale stromatolitic and thrombolitic bodies the fabric of the micrite varies from peloidal to aphanitic and almost always preserves filamentous bacteria which characterized the original microbial mat the mat was dominated by sulphur oxidizing bacteria belonging to the thiotrichaceae but there is evidence for a more complex community with sulphate and or nitrate reducing bacteria all being responsible for the mediation of the carbonate precipitation microbial boundstones are rich in pseudomorphs of ca sulphate and halite which formed during the deposition of the microbial carbonate layers of primary gypsum are interbedded locally with carbonates suggesting the presence of restricted marine conditions the stable o and c isotopic composition of the carbonates that vary from dolomite to aragonite and calcite suggests a complex interplay between arid to humid climatic changes expressed cyclic interbedding of the carbonate with marl marlstone later diagenetic events mainly consist of phreatic meteoric recrystallization and cementation although considered as diachronous the microbial carbonates can be mapped out over a distance of more than 500 km across southern italy this indicates near constant environmental conditions upon the central mediterranean shelf at the beginning of the salinity crisis deposition of the extensive subaqueous microbial deposit that largely comprises the calcare di base is envisaged to have taken place across a shallow to moderate depth platform with local slopes into deeper water areas where some resedimentation occurred

diversity and origin of quartz cements in continental carbonates example from the lower cretaceous rift deposits of the south atlantic margin abstract silica precipitation in continental carbonates is a common process occurring during sedimentation and diagenesis the lower cretaceous rift deposits of the south atlantic equatorial margin which are intensively explored by petroleum companies provide good examples of such silicifications in carbonates exhibiting a wide diversity of petrographic habit of early to late quartz cements in order to understand the palaeoenvironmental and diagenetic conditions leading to this diversity we integrated detailed petrography of diagenetic sequences and quartz habit with 18oquartz measurements by sims of individual cements observed in samples from the offshore and onshore basins of the west african margin the petrographic description highlights the omnipresence of early fibrous microquartz cements exhibiting either length fast or length slow habit in addition to laminated microquartz and micro or mega quartz forms amongst the isotopic analysis the 18oquartz data show that length slow cements are generally strongly enriched in 18o 18oquartz ranging from 31 to 37 smow whereas length fast forms show less elevated values 25 c and

fibrous gypsum and fibrous anhydrite in veins fibrous calcium sulphate occurs in nature mainly as gypsum satinspar in extensional veins several mechanisms have been put forward to account for these veins and their fillings but a number of features remain unexplained or are not in accord with the interpretation s given this paper describes anhydrite as a fibrous vein precipitate which shows most features typical of satinspar subhorizontal attitude of the vein s vertical fibres a parting separating a lower from an upper seam and incorporated wall rock fragments new observations include the stacked nature of the fibres pressure shadows next to wall rock fragments and parabolic alignment of the latter these features suggest that the veins opened due to vertically oriented tensile stress the vein filling crystals grew centrifugally outwards from the initial plane of rupture which now forms the parting keeping pace with the dilation the mode of incorporation of wall rock fragments in the fibres requires repeated differential opening of the fissure thereby either wall rock fibre interface ruptured at a time and statistically the ruptures alternated so that trace element trends across the vein s are roughly symmetrical to the parting the source of the calcium and sulphate ions may have been adjacent evaporite beds but an external source is indicated where no evaporite beds are present in contrast to previous interpretations hydration of anhydrite to gypsum in the host rocks is not a prerequisite for the formation of satinspar veins the exact causes for the vertical dilation remain somewhat speculative and may have been different in various locations the vertically oriented tensile stress may have resulted from lateral tectonic compression or hydraulic overpressure or a combination of the two

variation in slip on intersecting normal faults implications for paleostress inversion numerical models based on linear elasticity theory predict asymmetric slip distribution with a steep slip gradient near the line of intersection of intersecting normal faults they also predict a discrepancy between the direction of slip on the fault plane and the direction of resolved shear stress both variations in slip magnitude and direction are due to mechanical interaction between the faults with intersecting patterns these interactions cause local perturbations of the shear stress field acting on the plane of the adjacent fault field observations from the chimney rock area of central utah show that slickensides on normal faults cutting the navajo sandstone change rake away from the expected downdip direction as the intersection line with adjacent faults is approached the sense and magnitude of this change in orientation are similar to those computed by using the numerical models the good correspondence between field observations and theoretical results from this paper not only provides insight into the mechanics of intersecting faults but suggests that care is required when using standard inverse methods to compute paleostresses from slickenside data the slickenside orientation near intersection lines will generally not be in the direction of the remote maximum shear stress as resolved on the fault plane a parameter study of this change in orientation provides helpful results for evaluating field data prior to a paleostress analysis

slip distributions on intersecting normal faults abstract slip distributions on normal faults often are asymmetric and display multiple local maxima numerically computed slip distributions from elastic models indicate that such irregular slip distributions can be caused by mechanical interaction between intersecting faults that produce local perturbations of the stress field resolved on the faults a three dimensional boundary element model based on the displacement discontinuity method has been used to analyze the mechanical interaction of faults that form y or t shaped intersections slip distributions are asymmetric with a steeper slip gradient toward the line of intersection multiple slip maxima occur depending on the angle between the faults but generally they are not located along the intersections nor at the fault centers examples of intersecting normal faults taken from oil reservoir seismic surveys and sandbox experiments have been used to confirm the application of the theoretical results despite considerable simplifications in terms of geometry and boundary conditions the computed slip distributions capture the characteristics of the observed slip distributions

comparison of depositional sequences and tectonic styles among the west african deepwater frontiers of western ivory coast southern equatorial guinea and northern namibia the recent success of tertiary plays in the deepwater offshore central west africa has generated exploration interest in underexplored upper to middle cretaceous plays along the margin three areas are examined in this article including the upper slopes of western ivory coast southern equatorial guinea and northern namibia depositional sequences in the three areas have similar development but differ in depositional character and form initial identification of sequences is based on unconformity bound units seen on 3d seismic data there are 6 7 major seismic sequences identified in each area each seismic sequence is correlated to well control for age and approximate lithologic calibration using 2d seismic data sets the central west african margin includes offshore territories from the transform bounded gulf of guinea to the north down to a southern limit just north of walvis ridge figure 1 the areas considered in this study include the offshore areas of western ivory coast southern equatorial guinea and northern namibia the ivorian margin is bound by two transform zones the st paul to the north and the romanche to the south the ivory coast data consist of a 1200 km2 3d seismic survey offshore the westernmost edge of the country the data were acquired and processed in 2000 the area is approximately 150 200 km west of and along trend with the discoveries in a present day shelf to upper slope position the second centrally located study area overlies the ascension transform zone in southernmost equatorial guinea a 3120 km2 3d seismic survey forms the basis of the interpretation in this area a prestack time migration was applied to the data which were acquired and processed in 2001 the data are 20 30 km south of the rio muni basin discoveries and extend to the northern edge of the gabonese salt basin the third most southerly study

structural features of an active strike slip fault on the sliding flank of mt etna italy abstract the strike slip pernicana fault system pfs was activated along the eastern flank of mt etna during an earthquake in september 2002 and one month later during the eruption of the ne rift structural and volcanological data suggest that the pfs was activated as a result of the slide of the ne flank of etna this activation produced surface fracturing on walls and on paved and unpaved roads the segments of the pfs arranged in a right stepping en echelon configuration show a an inverse proportion between length and frequency b fractal behavior over scales of 10 2 10 1 m between their length overstep and overlap c consistent strike with regard to their fault array and d a progressive eastward decrease in the displacement along the smallest faults the consistent geometric and kinematic features of the pfs related to the sector collapse of etna are similar to those of faults in strike slip settings

recognition of interstitial anhydrite dissolution a cause of secondary porosity san andres limestone new mexico and upper minnelusa formation wyoming rectangular and stair step pore reentrants in carbonate mudstones have been recognized previously as indirect evidence for anhydrite dissolution in this study direct evidence for subsurface dissolution of interstitial anhydrite in both dolomite grainstones and quartz sandstones includes 1 cleavage related dissolution fringe on anhydrite crystal surfaces and 2 isolated remnants of optically continuous formerly poikilotopic anhydrite influenced by the prominent cleavages the dissolution fringe on the surfaces of the anhydrite crystals consists of a series of sharp right angled projections and reentrants experimentally etched anhydrite surfaces exhibit features that directly compare to the dissolution fringe whereas experimentally grown anhydrite does not we deduced the following sequence of anhydrite dissolution within dolomite grainstones and quartz sandstones slow incipient dissolution began along the boundaries between anhydrite and adjacent minerals from these intercrystalline boundaries solutions penetrated anhydrite cleavages leading to more rapid preferential dissolution perpendicular to the more prominent cleavage planes the widened cleavage planes together with intercrystalline boundaries acted as conduits for the removal of dissolved ions in the final stage as dissolving anhydrite borders retreated toward pore throats dissolution slowed and was again restricted to intercrystalline boundaries this process was then repeated in adjacent interstices

giant polygons in the realmonte mine agrigento sicily evidence for the desiccation of a messinian halite basin abstract the messinian realmonte salt deposit consists of a 400 600 m thick halite succession that can be divided into four main lith ologic units base to top a cumulates of plates settled out from a stratified water column b plate cumulates in a shallowing upward sequence containing six kainite layers near the top of the unit c cu mulates of skeletal hoppers with chevron overgrowths precipitated from a nonstratified water body and d skeletal halite and anhydrite spectacular vertical fissures cut through the upper part of unit b at its boundary with salt unit c these fissures are spaced at intervals of up to 5 m apart extend down to a depth of 6 m and are filled by red mud the salt beds affected by the fissures are commonly upturned buckled and are truncated and onlapped by the succeeding flat lying halite beds of unit c additionally the upturned layers are cut by vertical dissolution pipes that extend down to as much as a depth of 4 m below the fissured surface to explain the observed superimposed structures we propose two mechanisms both induced by the desiccation of the evaporite basin 1 when the salt surface was exposed the halite layers were buck led broke into polygonal crusts and formed tepee structures the buckling was caused by a net volume increase due to thermal expan sion and what is more important precipitation of new halite induced by evaporative pumping of brines moving up from the groundwater table the buckled salt layers were then affected by meteoric dissolu tion which created vertical pipes 2 the tepee structures and the associated zone of dissolution pipes are further crosscut by a new network of large polygons these new polygonal sets are outlined by deep fissures in which red silt collected blown by the wind or carried by episodes of surface flooding this type of deep contraction crack is known to be produced by volumetric changes induced by annual temperature fluctuations the documentation of contraction polygons and other exposure fea tures represents the first report of desiccation events during the de position of the sicilian salt no geological data are yet available to settle the questions if the salt of sicily is correlatable with the halite present beneath the floor of the mediterranean and if the desiccation of the realmonte salt basin could have been induced by a local tectonic causes uplift of the basin floor by thrust activity b simple evaporitic drawdown c basinwide drop of the mediterranean sea level or d complex interaction among these factors

evolution of the stress fields in the carpatho pannonian area during the neogene abstract observations of microtectonic faults in and around the pannonian basin suggest that the tertiary evolution of the area was controlled by large scale strike slip fault activity statistical analysis of the fault pattern for different localities makes it possible to separate several conjugate sets of strike slip faults and related dilatational and compressive structures these stress fields have a distinct rotation pattern with time inversion of fault data suggests a characteristic rotation of the principal stress axes with time some rigid body rotations are indicated by paleomagnetic data there are basically three contrasting mechanisms to explain this observation 1 1 appropriate rotation of the regional e g european stress field 2 2 rotation of larger tectonic units e g microcontinents under a stable regional stress field and 3 3 coherent rotation of smaller detached continental blocks under a stable regional stress field our model for the neogene tectonic evolution of the pannonian area combines these mechanisms moderate microcontinent rotation may have occurred during the paleogene early miocene in the early middle miocene the microcontinents were dissected by a system of strike slip and normal faults created by a n s compressive e w extensive stress field gravity spreading of the pannonian units and block rotation within bounding strike slip faults are suggested to explain rotated magnetic directions and fault patterns in the late neogene clockwise rotation of the general stress field by at least 40 is thought to be responsible for the reactivations of older strike slip faults and for the creation of new sets this change in the orientation of the stress field is related to the eastward and then southward shift of the carpathian subduction activity

micro analytical perspectives on the bishop tuff and its magma chamber new in situ major and trace element analytical data are presented for crystals sanidine plagioclase biotite orthopyroxene clinopyroxene and matrix glasses from juvenile materials representing the full bishop tuff sequence from the earliest fall unit f1 to the latest ignimbrite package ig2nc these data are combined with published information to investigate the nature and zonation of the pre eruptive bishop magma chamber our data confirm that this magma chamber was a single unitary body that was thermally and compositionally zoned the zonation was largely established prior to the growth of crystals and also prior to mixing in the lower parts of the chamber induced by late stage intrusion of a magma of contrasting composition and slightly higher temperature the bright rim magma sparse mixed swirly and dacitic pumices show enrichments in ba sr and ti that identify these pumices as possible representatives of the bright rim magma a model revised from previously published work for the pre eruptive magma chamber comprises three main parts 1 an upper volumetrically dominant 2 3 relatively unzoned region that was the source of the earlier eastern erupted ignimbrite units and their coeval fall units 2 a volumetrically minor transition zone that shows evidence for minor degrees of mixing and was the dominant source for the latest eastern erupted part of ig1eb sherwin subunit and the earlier part of the northern erupted ignimbrite ig 2na 3 a lower volumetrically subordinate 1 3 region that was affected by mixing with the bright rim invasive magma in the lead up to the eruption and fed later northern erupted units ingress of the bright rim magma introduced orthopyroxene and bright rimmed zircon crystals and induced partial resorption then overgrowth of rims enriched in ti sr and ba on sanidine and quartz and development of zoning in clinopyroxene based on pumice proportions and associated crystal and glass chemistries through the eruptive sequence we infer that the roof and floor of the magma chamber were stepped down to the north such that the transition zone magma formed the floor of the southern part of the melt dominant chamber and the roof of the northern part our data reinforce the previous concept of a single compositionally and thermally zoned bishop magma chamber and additionally support a temporally constrained role for pre eruptive magma mixing and the introduction of melts and minerals with contrasting compositions to the resident bishop magma

alternate interpretation of the messinian salinity crisis controversy resolved a high resolution chronology of the mediterranean messinian salinity crisis is proposed two types of evaporite deposition may be distinguished those in marginal areas vs those in basinal ones their diachroneity is deduced from the stratigraphic relationships linking these evaporites to a major messinian erosional surface a two step model is proposed for the evolution of the salinity crisis through time during the first phase from 5 75 to 5 60 ma the deposition of marginal evaporites took place in response to a modest sea level fall in the second interval from 5 60 to 5 32 ma the mediterranean basin became isolated during this later period the deposition of basinal evaporites and the cutting of the mediterranean canyons took place

origin and diagenetic evolution of gypsum and microbialitic carbonates in the late sag of the namibe basin sw angola abstract ephemeral evaporitic conditions developed within the uppermost part of the transgressive late sag sequence in the namibe basin sw angola leading to the formation of extensive centimetre to metre thick sulphate bearing deposits and correlative microbialitic carbonates rich in pseudomorphs after evaporite crystals the onshore pre salt beds examined in this study are located up to 25 m underneath the major mid aptian evaporitic succession which is typified at the outcrop by gypsiferous bambata formation and in the subsurface by the halite dominated loeme formation carbonate evaporite cycles mostly occur at the top of metre thick regressive parasequences which progressively onlap and overstep landward the former faulted rift topography or fill major pre salt palaeo valleys the sulphate beds are made up of alabastrine gypsum associated with embedded botryoidal nodules dissolution related gypsum breccia and are cross cut by thin satin spar gypsum veins nodular and fine grained fabrics are interpreted as being diagenetic gypsum deposits resulting from the dissolution and recrystallisation of former depositional subaqueous sulphates whereas gypsum veins and breccia result from telogenetic processes the carbonates display a broader diversity of facies characterised by rapid lateral variations along strike thin dolomitic and calcitic bacterial mediated filamentous microbialitic boundstones enclose a broad variety of evaporite pseudomorphs and can pass laterally over a few metres into sulphate beds dissolution related depositional breccias are also common and indicate early dissolution of former evaporite layers embedded within the microbialites sulphate and carbonate units are interpreted as being concomitantly deposited along a tide dominated coastal supra to intertidal sabkha and constitute high frequency hypersaline precursor events prior to the accumulation of the giant saline mid aptian bambata and loeme formations petrographic and geochemical analyses reveal successive dissolution recrystallisation and cementation phases that occurred during burial uplift and exhumation implying a complex diagenetic evolution of both gypsum and carbonates influenced by pore fluids of diverse composition which distinctly varied from meso to telogenetic domains

tectonic analysis of fault slip data sets using data that include the direction and the sense of motion on individual fault surfaces determined by slickenside lineations it is possible to reconstruct reduced stress tensors that correspond to the orientation of stress axes and to the ratio 2 3 1 3 between principal stress values 1 2 3 compression being positive no assumption is made concerning the orientation of fault planes relative to stress axes so that reactived faults are taken into account as well as newly created ones qualitative and quantitative methods for analysis of fault slip data were developed during the last 10 years the practical limitation of the methods and the necessity for critical field observations are emphasized these methods can be applied to focal mechanisms of earthquakes a more complex analysis of heterogeneous data sets involving an iterative separation of different stress systems is also discussed this analysis enables one to distinguish successive faulting events careful qualitative study in the field is in all cases essential

zoned quartz phenocrysts from the rhyolitic bishop tuff cathodoluminescence cl reveals growth zones in quartz phenocrysts from the rhyolitic bishop tuff melt inclusions occur in various zones and record the evolving melt composition during zonal growth the zones form an oscillatory pattern between bright and dark cl quartz there are three recognizable patterns of cl zoning in these crystals 1 weakly zoned cores and bright cl rims 2 weakly zoned cores and dark cl rims and 3 no cl intensity difference from core to rim dark cl quartz generally occurs at crystal edges contains most of the melt inclusions and is interpreted as fast growing zones that occur along recognizable crystal edges edge zones are thicker than the same zone on adjacent faces consistent with relatively fast growth of these zones in each successive zone these edge zones decrease in size toward the rim while the zones along the crystal faces increase some of the melt inclusions have bright cl quartz locally associated with them this is interpreted as the postentrapment crystallization of slow growing quartz in the melt inclusions many crystals display zone discordance from the weakly zoned interiors to the rims most of the discordant surfaces are rational and probably are primary growth features pumice clasts from the southern vents are largely compositionally and texturally distinct from those from the northern vents and this distinction is also evident in the quartz cl the crystals that have bright cl rims are all associated with the late erupted northern part of the bishop tuff melt inclusion compositions and cl zoning patterns suggest a common origin for early and middle erupted quartz and the interior zones of late erupted quartz however the bright cl rim on the late erupted quartz indicates an additional stage of crystallization in late erupted magma melt inclusions in individual early erupted crystals have small variations in ba whereas inclusions in late erupted crystals markedly increase in ba toward the rim which is opposite to the normal zoning of sequentially trapped melts expected during closed system crystallization differentiation the quartz zoning features are consistent with the hypothesis of crystal settling in evolving magma that erupted late from northern vents

superposed deformation and inherited structures in an ancient dilational step over zone post mortem of the rengali province india abstract in the eastern indian shield a dextral strike slip system juxtaposed the archaean singhbhum province against the proterozoic eastern ghats belt at 490 470 ma two wnw ese trending strands of the strike slip system enclose a multiply deformed d1 to d3 intervening domain called the rengali province with d3 representing dextral shearing in a granulite lens within the province an early fabric sgr was deformed by an amphibolite facies d1 d2 deformation continuum in the late archaean time forming cylindrical folds in the surrounding quartzofeldspathic gneisses quartzites and mica schists of the province superimposition of syn d3 shortening on d1 d2 folds generated complex non cylindrical geometries the granulites escaped d3 strain microstructures in the province bounding shear zones confirm that d3 deformation was associated with mylonitization dynamic recrystallization and greenschist facies metamorphism in the quartzites syn d3 folds can be correlated with rotation of d1 d2 structures through the shortening zone of bounding dextral shears since the province bounding shears form a step over zone the structural complexity within the rengali province arises from superposition of syn d3 shortening structures on initially asympathetically oriented inherited cylindrical d1 d2 folds hydrous fluid channeling causing greenschist facies metamorphism and quartz vein emplacement accompanied d3 as the step over zone was dilational in nature

oblique slip and the geometry of normal fault linkage mechanics and a case study from the basin and range in oregon abstract map patterns of normal fault linkages near summer lake oregon show a systematic relationship between echelon step sense oblique slip sense and the position of linking faults where the step sense is the same as the sense of oblique slip e g left step and left oblique slip the faults are linked in the lower part of their relay ramp where the step sense and slip sense are opposite e g left step and right oblique slip the faults are linked in the upper part of the ramp a boundary element code is used to calculate the stress field around echelon normal faults during oblique slip and the model results reveal a relationship similar to the field observations if step sense and oblique slip sense are the same there is a greater potential for deformation ahead of the tip of the front fault and in the lower part of the ramp if step sense and oblique slip sense are opposite there is a greater potential for deformation ahead of the tip of the rear fault and in the upper part of the ramp the field model comparison confirms that oblique slip modifies the mechanical interaction among fault segments and thus influences fault growth and the geometry of fault linkage

structural segmentation inversion and salt tectonics on a passive margin evolution of the inner kwanza basin angola the kwanza basin angola is divided into the inner and outer kwanza salt basins separated by a chain of synrift basement highs on which aptian 112 122 ma salt is thin or absent north to northwest trending basement structures in the inner kwanza basin have repeatedly been reactivated since neocomian 144 127 ma rifting reactivation formed three northwest striking fold and thrust belts near basement uplifts the thrust belts are bounded by northeast striking rift related transfer fault zones that were apparently reactivated during subsequent shortening three episodes of postrift basement involved shortening are documented in the inner kwanza basin 1 albian early cenomanian 112 96 ma 2 senonian 89 65 ma and 3 oligocene holocene 34 0 ma we relate the albian early cenomanian event to ridge push the senonian event to global plate reorganization and oligocene holocene shortening to uplift of the african superswell structural segmentation of the inner kwanza basin controlled the evolution of salt structures adjacent to basement uplifts diapirs were initiated as buckle folds some anticlines were unroofed by erosion and evolved into passive salt walls elsewhere broad salt walls were triggered by either detached extension or basement block uplift these walls grew until they exhausted their supply of salt thereafter dissolution rates exceeded rates of salt inflow so the walls began to subside withdrawal of salt from the walls produced the elongate sedimentary troughs for which the basin is famous trough fill ranges in age from cenomanian to pliocene and this age varies greatly from trough to trough and along strike within troughs

comparison of sequences formed in marine sabkha subaerial and salina subaqueous settings modern and ancient marine evaporites occurring in modern subaqueous salina settings and subaerial sabkha settings are different subaqueous holocene evaporites occur as shoaling upward lacustrine sequences up to 10 m thick they are evaporite dominated and are composed primarily of bottom nucleated crystals that may be deposited as massive laminated or rippled units each coastal lake is dominated by laminated evaporites with subordinate carbonate sediments in plan view they show a well developed bull s eye pattern with a sulfate center and a carbonate rim in contrast subaerial sabkha evaporites occur as part of a laterally prograding shoaling upward peritidal sequence in which the supratidal unit is usually no more than 1 m thick sabkha sequences are matrix dominated not ev porite dominated with the bulk of the sulfate phase occurring as diagenetic nodules enteroliths or diapirlike structures these sulfates were formed during syndepositional diagenesis by replacement and displacement processes the various facies of the sequence tend to accumulate in belts parallel with the shoreline relative to the sea level or the brine level sabkhas tend to form over paleotopographic highs whereas salinas tend to occur in paleotopographic lows some of the characteristics that distinguish holocene subaerial and subaqueous evaporite sequences can be used to do the same for similar ancient facies even when gypsum has been converted to nodular anhydrite the distinction is important for it can be used by explorationists in the oil industry to define the paleotopography of the associated underlying porous and nonporous carbonates

morphology and micro fabrics of weathering features on gyprock exposures in a semiarid environment ebro tertiary basin ne spain abstract gyprock is a common rock in evaporitic formations which have a wide distribution across all continents however outcrops of gyprock are rather limited occurring mainly in low rainfall areas i e southwest of usa italy spain gyprock is one of the most common lithology in the ebro tertiary basin here semiarid conditions propitiate the development of specific weathering processes and landforms with significant implications for understanding the genesis of soils and the geomorphological evolution of these areas gypsum crystallization within the pores and fissures of gyprock close to the surface leads to volume increase and pressures with the consequent weathering of this rock as a result considerable porosity develops producing peculiar morphologies on gyprock outcrops domes and decimetre scale blisters specific microscopic features voids with plain walls horse shoe pores and serrated pores as well as microscopic fabrics grating fabric skeletal fabric fan fabric and botryoidal fabric this paper presents a morphological characterization of the material forming the various types of weathering features from microscopic to mesoscopic scale the macromorphology and micromorphology of those features were used to interpret their genesis

formation of segmented normal faults a 3 d perspective the interpretation of fault kinematics from geometric data is an essential step in developing an understanding of the growth of fault systems constraints on fault geometry are however often restricted to 2 d maps or cross sections in this article we consider the extent to which kinematic interpretations of faulting benefit from a 3 d rather than 2 d geometrical perspective concentrating on relay zones and segmented normal fault arrays we suggest that very different interpretations of their evolution arise from the recognition that the propagation directions of faults and fault segments will rarely be contained within the inspection plane of 2 d data a 3 d perspective favours an interpretation in which the segments of a fault array are kinematically interrelated from their initiation individual segments in such systems may link into a single fault surface out of the plane of inspection or may be unconnected in 3 d we argue that this interpretation which conflicts with the often suggested model of incidental overlap of originally isolated faults should be the preferred model for the generation and growth of segmented normal fault arrays

solution and recrystallisation processes and associated landforms in gypsum outcrops of sicily abstract four small areas of messinian upper miocene age gypsum outcropping in western sicily are described messinian age evaporites are found in sicily over a 1000 km 2 area here gypsum outcrops extensively as a consequence of soil erosion induced by human impact geomorphological maps show how the rocky surfaces are characterized by a wide range of forms there are large medium small and microsized forms which can be identified as belonging to different morphotypes the morphotypes can be classified into two main categories those that originated by solution and those that originated through recrystallisation four areas illustrated by geomorphological maps were specifically chosen to describe a type of medium sized form dome like hills these medium sized forms are covered by a mosaic of smaller forms related to both the previous categories different types of karren and of expansion forms the types of karren can be explained as the results of the solution process under different hydro dynamical behaviour the dome like hills and other related expansion forms are more difficult to understand these expansion forms can be explained by the same process that leads to the development of gypsum tumuli the outcrops of gypsum lacking soil cover and influenced by alternating seasonal water conditions of surplus and deficit are affected by both solution and recrystallisation processes during the wet season the water soaks into the rocky mass filling all the fissures and pores of the outer rocky layer from a few centimetres to some metres below the surface during the dry season there is a capillary upward motion of the water solution near the surface gypsum precipitates from the oversaturated solution increasing the crystal size or forming new crystals in this way during the dry season there is a pressure increase in the outer gypsum layers which is responsible for the development of a gypsum weathering crust and characterised by many different forms such as gypsum tumuli pressure ridges pressure humps and other related small forms the crust may also lead to the development of mega tumuli and dome like hills from the morphostructural point of view the dome like hills do not seem to be controlled by the strike dip or fissuring of the gypsum beds their evolution seems to be linked to the fact that on most of the dome surfaces the weathering crust is evolving through a nearly isotropic field of stresses resulting in volume increase in the outer gypsum layer

evaluation of halite dissolution at a radioactive waste disposal site andrews county texas waste control specialists wcs has been granted permits to dispose of radioactive waste at their surface facility in western andrews county texas u s a the facility is located over permian age halite bearing formations and the possibility of dissolution and its effects on the long term performance of the disposal site must be considered we compare data from the wcs site and vicinity to three conceptual hydrologic models of dissolution processes shallow deep and stratabound geophysical logs yield stratigraphic and lithofacies data from the underlying halite bearing units the permian rustler and salado formations the halite units are continuous and thickness and lithofacies vary by depositional process with no discernible postdepositional dissolution modest structural trends in these units differ from local changes in evaporite thickness therefore deformation is not related to thickness differences local thickness changes in the middle of the salado formation indicate depositional variations core obtained from upper evaporite bearing formations at the wcs site show normal depositional features and no brecciation that would be associated with postdepositional dissolution deeply buried bedded halite behaves as a ductile material and the effective stress is close to zero so that the pressure in pore fluids approaches lithostatic hydraulic gradients are outward from halite toward overlying and underlying rocks eliminating access for low salinity fluids formation fluids at depth are commonly saline and slow moving further limiting the dissolution process the hydrologic systems at the site limit the potential for future dissolution

morphology and geometry of duplexes formed during flexural slip folding abstract bedding parallel slip on the limbs of chevron style folds during flexural slip folding results occasionally in the formation of small scale duplexes the duplexes described here from upper carboniferous turbidites in sw england generally range in thickness from 0 3 to 30 cm and are up to 2 4 m long they have smooth nearly flat roofs and both the internal thrusts and the slickenfibres on them are commonly oblique to the mean transport direction as indicated by slickenfibres on the floor and roof thrusts the morphology of the duplexes suggests that they developed between active floor and roof thrusts and they show a characteristic lack of hangingwall anticlines on the link thrusts their shear sense always corresponds to that required by flexural slip and the total bedding parallel displacement across each is commensurate with that predicted by the flexural slip model all thrust surfaces in the duplexes are marked by quartz fibre veins a feature which distinguishes them from soft sediment and other pre folding duplexes and carry slickenfibres whose mean orientation and complex variations in slip direction on different fibre sheets on the same slip surface are identical to those on flexural slip movement horizons from the same fold limb the duplexes form either as a result of resistance to thrust propagation by local facies or bed thickness changes or develop as transfer structures between the tips of movement horizons propagating along adjacent slip surfaces late stage duplexes develop from imbricated fibre veins and also form on slip surfaces oblique to the axial planes of major folds