mechanical interactions between rigid particles in a deforming ductile matrix analogue experiments in simple shear flow abstract the mechanical interaction between two or more particles is investigated in newtonian simple shear flow the experimental models allow observation of both the particle rotation and the deformation pattern around the particles the finite and instantaneous strain patterns around rigid particles are strongly heterogeneous and asymmetric with high finite strain zones aligned to the direction of maximum finite stretch these correspond to local flow with low vorticity number heterogeneous strain patterns around rigid particles spread over a distance of 1 2 times the particle length and this distance increases with increasing bulk strain where rigid particles are more concentrated these patterns coalesce and the overall pattern is then strongly controlled by the particles and cannot be simply related to the external boundary conditions in rocks the characteristic asymmetric strain pattern around rigid isolated particles is a reliable shear criterion which becomes unreliable with high concentrations of rigid particles particle rotation is significantly disturbed when the distance between adjacent particles of equal size is shorter than their length that is only in very concentrated suspensions of rigid particles in a composite shape fabric the development of the sub fabric corresponding to the smaller minerals will be more disturbed

quantitative finite strain and kinematic flow analyses along the zagros transpression zone iran abstract the ghouri area in southwest iran exposes a cross section through the zagros orogenic belt the area provides an opportunity to investigate quantitative finite strain rs kinematic vorticity number wk proportions of pure shear and simple shear components sense of shear indicators steeply plunging lineations and other moderate to steeply plunging stretching lineations in a transpressional zone based on a classical strain analysis of deformed microfossils with oblate strain ellipsoid shape the zagros orogenic belt is classified as a pure shear dominated zone of transpression but asymmetry of shear sense indicators suggests that a significant component of simple shear was involved along the deformation zone boundaries the long axes of the microfossils and stretched pebbles of a deformed conglomerate were used to indicate the stretching direction in this zone the stretching lineations have a steep to moderate plunge but a constant strain magnitude characteristics of dextral inclined transpressional kinematics in the zagros continental collision zone were quantified and indicate an estimated k value

clinoforms and clinoform systems review and dynamic classification scheme for shorelines subaqueous deltas shelf edges and continental margins abstract clinoforms are inclined and normally basinward dipping horizons developed over a range of spatial and temporal scales in both siliciclastic and carbonatic systems the study of clinoform successions underpins sequence stratigraphy and all efforts to reconstruct the relative partitioning of reservoir seal and source rocks along shoreline to basin floor profiles here we review clinoform research and propose a more systematic description and classification of clinoforms this is a crucial step to improve predictions of facies and lithology distribution within shoreline to continental shelf and abyssal plain successions together with the genesis drivers and dynamics of their constituent sedimentary units four basic clinoform types are here distinguished in delta shorelines lacustrine and marine environments on the basis of their overall spatial and temporal scale morphology outbuilding dynamic and geodynamic and depositional setting 1 2 delta scale clinoforms which in turns are sub divided into shoreline and delta scale subaqueous clinoforms 3 shelf edge clinoforms and 4 continental margin clinoforms delta scale clinoform sets are tens of metres high and typically represent 1 103 kyr with progradation rates ranging from 1 000 100 000 m kyr for shorelines and subaerial deltas to 100 20 000 m kyr for subaqueous deltas shelf edge clinoform sets are hundreds of metres high and are nucleated and accreted in 0 1 20 myr usual progradation rates of 1 100 m kyr by successive cross shelf transits of delta scale clinoforms continental margin clinoform sets are thousands of metres high hallmark key geodynamic crustal boundaries e g continent ocean transition and slowly prograde basinwards in ca 5 100 myr with typical rates of 0 1 10 m kyr as a consequence of the very different progradation rates and of the difficulty of large scale clinothems to backstep during transgressions shorelines are the most dynamic clinoforms with regards to position continental margins the least and shelf edges are intermediate shortly after a transgression therefore the four clinoform types may prograde synchronously along shoreline to abyssal plain transects forming compound clinoform systems during the subsequent regressive cycle however due to the dissimilarity in progradation rates different clinoform types will normally merge progressively with each other giving rise to hybrid clinoforms e g shelf edge deltas and fewer depositional breaks in slope are distinguished along a single shoreline to abyssal plain transect overall all clinoform systems are the result of the dynamic evolution of compound and hybrid clinoforms along a temporal and spatial continuum regulated by the cyclical backstepping of the smaller scale system within natural progradational retrogradational cycles of larger scale clinothem outbuilding all clinothem types may show either an accretionary active or draping passive style depending on the proximity to the sediment source draping clinothems are nearly always composed of condensed fine grained sediments while actively accreting clinothems can be composed of predominantly coarse grained i e reservoir forming or predominantly fine grained i e non reservoir lithotypes a novel hierarchical classification scheme for both recent and ancient clinoforms is here proposed consisting of 12 classes the four basic clinoform types delta scale shoreline delta scale subaqueous shelf edge and continental margin are sub divided into eight accretionary active and draping passive sub types 8 division each accretionary sub type is then sub divided into a sandstone prone and mudstone prone variant 12 division which can be at least tentatively predicted on the basis of the clinoform morphology even in the absence of direct stratigraphic logs

early triassic seawater sulfate drawdown abstract the marine sulfur cycle is intimately linked to global carbon fluxes atmospheric composition and climate yet relatively little is known about how it responded to the end permian biocrisis the largest mass extinction of the phanerozoic here we analyze carbonate associated sulfate cas from three permo triassic sections in south china in order to document the behavior of the c s cycle and its relationship to marine environmental changes during the mass extinction and its aftermath we find that 34 s cas varied from 9 to 44 at rates up to 100 myr 1 during the griesbachian smithian substages of the early triassic we model the marine sulfur cycle to demonstrate that such rapid variation required drawdown of seawater sulfate concentrations to 4 mm and a reduction in its residence time to 200 kyr this shorter residence time resulted in positive covariation with 13 c carb due to strong coupling of the organic carbon and pyrite burial fluxes carbon and sulfur isotopic shifts were associated with contemporaneous changes in climate marine productivity and microbial sulfate reduction rates with negative shifts in 13 c carb and 34 s cas linked to warming decreased productivity and reduced sulfate reduction sustained cooling during the spathian re invigorated oceanic overturning circulation reduced marine anoxia and limited pyrite burial as seawater sulfate built to higher concentrations during the spathian the coupling of the marine c and s cycles came to an end and a general amelioration of marine environmental conditions set the stage for a recovery of invertebrate faunas variation in seawater sulfate during the early triassic was probably controlled by climate change possibly linked to major eruptive phases of the siberian traps

testing the utility of the porphyroclast hyperbolic distribution method of kinematic vorticity analysis kinematic vorticity wk is a dimensionless measure of rotation relative to finite stretching and is essential for complete understanding of flow in ductile shear zones the porphyroclast hyperbolic distribution phd method is a widely used technique for estimating wk based on the acute angle between the flow eigenvectors determined from the orientations of back rotated s porphyroclasts the utility of the phd was tested with tertiary ultramylonites from the santa catalina mountains arizona proterozoic ultramylonites from the virgin mountains nevada and a paleozoic ultramylonite from the hylas zone in the virginia piedmont bootstrapping statistics and a computational sieving process were used to analyze the phd data set average standard deviations of the bootstrapped data sets yield a 1s standard error of 9 for a wk value measured as a simple shear sieving results imply that back rotated porphyroclasts may not orient parallel to the extensional eigenvector the phd method is useful for discerning between deformations that are pure shear dominated general shear or simple shear dominated but is not accurate enough to report precise wk values when performed on multiple sections the phd method can identify zones of monoclinic versus triclinic shear 2007 elsevier ltd all rights reserved

implications of strain and vorticity of flow analyses to interpret the kinematics of an oblique convergence event zagros mountains iran abstract finite strain and vorticity of flow analyses were carried out within the deh vazir deformed conglomerates in the sanandaj sirjan metamorphic belt zagros mountains iran this belt is in a sequence of tectono metamorphic complexes made of low to high grade metamorphic rocks affected by a polyphase deformation history using rf technique for two dimensional strain analysis showed that the finite strain of xz plane is rs 3 7 0 70 the calculation of the harmonic mean from the axial ratios of the extracted pebbles and plotting these data in the flinn diagram resulted in k 0 7 0 38 i e oblate strain ellipsoid kinematic vorticity analysis of deformed pebbles showed that wk parameter is 0 59 0 10 i e pure shear dominated non coaxial flow the results quantitatively support the recognition of a partitioned dextral inclined transpressive regime with bulk triclinic symmetry along the sanandaj sirjan metamorphic belt resulted from the oblique collision between the african arabian continent and the iranian microcontinents

detailed 3 d depositional architecture of late jurassic carbonate anhydrite cycles brightling mine weald basin uk abstract quantifying the geometries of evaporite deposits at a 13 c and 18 o isotope analysis the stratigraphic interval contains five facies in stratigraphic order these include supratidal porphyritic nodular evaporite shallow subtidal peloidal packstone with evaporite and two overlying rhythmic sequences of intertidal microbial laminite subtidal shale and subtidal laminar marl capped by nodular anhydrite the interpreted environment of deposition is a supratidal sabkha subject to periodic flooding in which intertidal tidal flat facies and subtidal shallow marine facies laterally passed into the evaporative sabkha the cycles are interpreted as meter scale shoaling upward sequences likely controlled by localized high frequency changes in relative sea level and or sabkha hydrology spatial patterns in the geometries of key stratigraphic surfaces reveal a subtle depression towards the central western region of the mine seam the variation in stratal geometries is interpreted as paleotopography and is a function of individual or composite processes related to dissolution eolian processes and coastal erosion these observations indicate a similar mode of deposition to the modern day sabkha of the persian gulf we conclude that the dynamic process of evaporite deposition led to subtle stratigraphic heterogeneities and changes in bed thicknesses but largely continuous lateral bedding at an interwell scale

glacial versus interglacial sedimentation rates and turbidite frequency in the bahamas the southern tongue of the ocean is a 1300 m deep flat floored basin in the bahamas that receives large amounts of sediment from the carbonate platforms surrounding it on three sides we have examined five 8 13 m long piston cores and determined bulk sedimentation rates turbidite frequency and turbidite accumulation rates for the past two glacial and interglacial periods the mean of bulk sedimentation rates is four to six times higher in interglacial periods average accumulation rates of recognizable turbidites are higher by a factor of 21 to 45 and interglacial turbidite frequency is higher by a factor of 6 to 14 sediment composition indicates that increased interglacial rates are due to higher accumulation of platform derived material additional data from other bahamian basins as well as published material from the caribbean strongly suggest that highstand shedding is a general trend in pure carbonate depositional systems carbonate platforms without a siliciclastic component export more material during highstands of sea level when the platform tops are flooded and produce sediment the response of carbonate platforms to quaternary sea level cycles is opposed to that of siliciclastic ocean margins where sediment is stored on the inner shelf during highstands and passed on to continental rises and abyssal plains during lowstands of sea level

kinematic vorticity flow analysis and 40ar 39ar geochronology related to inclined extrusion of the hp lt metamorphic rocks along the zagros accretionary prism iran abstract the zagros accretionary prism in southwestern iran is exposed along the nw se trending of the zagros thrust system and inclined zagros transpression zone this accretionary prism consists of two units the upper sedimentary melange unit on top and the high pressure metamorphic melange unit at the bottom both units show characteristics of a tectonic wedge the upper unit consists of type i ii and iii melanges which display s c and c type shear band cleavages quartz ribbons and rectangular or fish head boudins the lower unit fabrics display and type porphyroclasts and quartz ribbon mylonites these fabrics formed from a combination of 60 5 simple shear and 39 5 pure shear both components were involved in a lateral exhumation of the high pressure low temperature metamorphic rocks in an inclined transpression wedge shaped geometry the estimated kinematic vorticity number wk was calculated from quartz c axis patterns rotation of porphyroclasts and orientation of finite strain with respect to shear zone boundaries using the mean estimated wm value of 0 84 the inclination angle for the thrust wedge on top of the ne subducting neo tethyan oceanic lithosphere is 18 the 40ar 39ar plateau ages of early generations of biotite from the lower metamorphic melange are 119 95 0 88 ma and 112 58 0 66 ma this late aptian age is related to early thrusting and formation of hp lt metamorphic rocks the dating of two amphibole samples from the amphibolite yields a weighted mean age of 91 23 0 89 ma this turonian cenomanian age suggests a later metamorphic event associated with subduction and obduction of the neyriz ophiolite and later lateral extrusion of hp lt metamorphic rocks along the inclined zagros accretionary prism

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

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

sea level changes an integrated approach sea level changes an integrated approach in october 1985 sepm sponsored a four day conference entitled sea level changes an integrated approach the purpose of the conference was to provide a forum for an interdisciplinary exchange of ideas on sea level changes and to provide an opportunity for integrating various types of evidence in approaching unresolved issues the conference was successful in bringing together scientists from industry academia and government representing all of the major geosciences disciplines presentations of many new papers plus significant releases of data that were previously held proprietary provided fertile ground for discussion this much cited volume represents the best of the material presented at the conference includes the early vail chart

criteria for the recognition of salt pan evaporites layered evaporites can accumulate in 1 ephemeral saline pans 2 shallow perennial lagoons or lakes and 3 deep perennial basins criteria for recognizing evaporites deposited in these settings have yet to be explicitly formulated the characteristics of the ephemeral saline pan setting have been determined by examining eight holocene halite dominated pans salt pans and their deposits marine and non marine from the u s mexico egypt and bolivia these salt pans are typified by alternating periods of flooding resulting in a temporary brackish lake evaporative concentration when the lake becomes saline and desiccation which produces a dry pan fed only by groundwater the resulting deposits consist of alternating layers millimetres to decimetres of halite and mud the layers of halite are characterized by 1 vertical and horizontal cavities rounded crystal edges and horizontal truncation surfaces due to dissolution during flooding 2 vertical chevrons and cornets grown syntaxially on the bottom during the saline lake stage 3 halite cements overgrowths and euhedral cavity linings and disruption of layering into metre scale polygons produced during the desiccation stage the muddy interbeds are characterized by displacive growth of halite during the desiccation stage immediately below the surface of the pan the halite layers are matured by repeated episodes of dissolution and diagenetic crystal growth this results in porous crusts with patches of chevron and cornet crystals truncated by dissolution clear diagenetic halite cement and internal sediment these layers of mature halite closely resemble the patchy cloudy and clear textures of ancient halite deposits holocene salt pans are known to cover thousands of square kilometres and cap halite deposits hundreds of metres thick so they are realistic models for ancient evaporites in scale e g permian salado formation of new mexico texas which preserves many primary salt pan features

evaporitic subtidal stromatolites produced by in situ precipitation textures facies associations and temporal significance abstract the transition between carbonate platforms or isolated carbonate buildups and overlying evaporites commonly is marked by assemblages of stromatolites and interlaminated carbonates and evaporites stromatolites display lamination textures that vary from peloidal and discontinuous on a scale of a millimeter to a few centimeters to isopachous and continuously laminated on a scale of a centimeter to a few meters the isopachous lamination texture may be composed of either 1 micritic or radial fibrous calcite or 2 dolomite isopachous stromatolitic laminae are remarkably uniform varying little in thickness over a given lateral distance in contrast to stromatolites formed of peloidal laminae which show marked variation in thickness over an equivalent lateral distance these isopachous textures are uncommon on most open marine carbonate platforms and apparently developed in transitional carbonate to evaporite settings because of increasing temperature salinity and anoxia related to water stratification which would have created ecologic restriction and an opportunity for stromatolite growth stromatolites with isopachous lamination are here interpreted to have formed as a result of in situ precipitation of sea floor encrusting calcite and possibly dolomite whereas the stromatolites composed of peloidal discontinuous lamination are inferred to have formed by trapping and binding of loose carbonate sediment in microbial mats while the presence of microbes in almost all near surface environments nullifies use of the term abiotic to describe most precipitated minerals we interpret growth of the isopachous stromatolites to have been dominated by chemogenic precipitation in the absence of microbial mats and the growth of peloidal stromatolites to have been controlled by sedimentation in the presence of microbial mats these transitional stromatolite facies are best developed atop proterozoic and paleozoic carbonate platforms that underlie major evaporite successions however inspection of jurassic and younger evaporite basins such as the messinian of the mediterranean region shows that stromatolites with thin isopachous lamination and radial fibrous textures though present are rare instead these facies may have been replaced by stromatolites with peloidal clastic textures and by low diversity diatomaceous and coccolith mudstones accumulation of the mudstones would have imposed two important effects 1 production of coccoliths would have helped extract calcium carbonate from seawater thus lowering the growth potential for precipitation of sea floor encrusting stromatolites 2 settling of both coccoliths and diatoms would have created a sediment flux to the sea floor which would have served to impede growth of precipitated stromatolites because of smothering of growing crystals

comment on numerical models of flow patterns around a rigid inclusion in a viscous matrix undergoing simple shear implications of model parameters and boundary conditions by n mandal s k samanta and c chakraborty journal of structural geology 27 2005 1599 1609

a mohr circle method for 3d strain measurement using the geometry of no finite longitudinal strain and the rxz strain ratio abstract this paper proposes a new method for 3d finite strain analysis this method utilizes a mohr circle construction combined with stereographic projection of the geometry of no finite longitudinal strain and with the strain ratio on the xz plane of the finite strain ellipsoid the method is described using numerical examples and then it is tested by applying it to the deformed deh vazir conglomerate in the southwestern part of the sanandaj sirjan hp lt metamorphic belt within the zagros orogenic belt in iran the results of this method compare well with previous finite strain measurements using strain ratios on three principal planes of finite strain calculation of finite strain from strain ratios on the xy and yz principal planes is advantageous when preparation of 3 perpendicular sections is difficult or impossible

biotic and abiotic processes in the formation and diagenesis of permian dolomitic stromatolites zechstein group ne england abstract the crinkly bed of the upper permian zechstein group succession roker formation hauptdolomit z2c in northeast england is a distinctive 1 4 meter thick unit that has been interpreted as a stromatolite of microbial origin or alternatively as the result of purely abiotic precipitation close examination of this stratigraphically significant unit shows that it is the result of both physical and microbial processes and was deposited in hypersaline shallow water immediately following lowstand evaporite deposition within the zechstein basin the internal structure of the crinkly bed consists of fine millimeter scale laminae of alternating clotted peloidal or aphanitic micritic layers and granular laminae composed of silt size peloids with rare hollow spheroids the lamination is extremely regular in thickness and laterally persistent for up to 4 meters a microbial community composed of coccoid and filamentous microorganisms inferred from fossil evidence colonized the original sediment surface it is suggested that this biofilm was responsible for the precipitation of both the micrite laminae and most of the peloids of the granular laminae the peloids are consequently interpreted as autochthonous this interpretation of a microbial origin is consistent with the microstructure and geochemical signatures of the micritic dolomite which constitutes both the continuous micritic laminae and the peloids themselves primary dolomite precipitation is hypothesized to have taken place from marine to slightly hypersaline waters mediated by bacterial metabolism similar to the process that operates in modern intertidal microbial mats the crinkly bed shows macro structures that vary along a continuum from symmetrical to asymmetrical centimeter scale ripple like structures some showing interference patterns through to domical and conical structures more typical of stromatolites the more ripple like structures have a lamination of microbial origin identical to that within the domal conical stromatolites there is no internal cross lamination their origin is equivocal they could be the result of physical current or wave induced deformation of a surficial biofilm or their form could be inherited from true mechanically deposited ripples not observed that provided a template for biofilm growth changes in the nature and strength of the wave and or current regime and possibly in the community structure of the microbial mat produced this array of microbially induced macrostructures miss the latter plus the absence of desiccation features indicative of intertidal to supratidal deposition suggest subtidal sedimentation at a depth of several meters intergranular gypsum cement precipitated during early diagenesis and ferroan sparry dolomite precipitated during burial tertiary uplift and the consequent exposure to meteoric waters resulted in the precipitation of calcite cements that replaced gypsum and pre existing dolomite probably via the biological mediation of heterotrophic bacteria the interpretation of this distinctive stratigraphic unit illustrates the difficulty in distinguishing between biotic and abiotic processes in the sedimentary record

structural style of buried ediacaran basins in saudi arabia on seismic reflection data abstract several isolated ediacaran basins subcrop phanerozoic strata in saudi arabia the structural styles of these subcropping basins are reviewed here based on interpretation of 2d and 3d seismic reflection data and comparison with arabian shield outcrops the basins are distinguished by layered seismic reflectors that are interpreted as sedimentary and volcanic strata due to limited well penetrations these packages cannot be directly dated but a relative chronology is established anchored by constraints from the arabian shield and regional studies most of the basins are bound by extensional faults and the basin fills show growth architectures and angular unconformities the bounding faults trend broadly north south and are interpreted to be late to post orogenic reactivations of oceanic terrane margins based on their structural characteristics in comparison with sub basins exposed on the shield the basin fills are correlated with the ediacaran jibalah group an equivalent of the omani nafun group these sub basins experienced inversion and exhumation prior to ongoing volcanism and unconformable deposition of late ediacaran to earliest cambrian kurayshah group sediments the kurayshah group is disconformably overlain by the widespread tayma group siliciclastics this disconformity correlates to the angudan unconformity in oman the kurayshah group includes a late ediacaran stratovolcano over 40 km in diameter in the eastern basins evaporitic structural styles indicate the presence of hormuz ara evaporites with local igneous intrusions that correlate at least in part to the kurayshah group the tayma group infills significant accommodation space variations and was affected by ongoing but declining differential subsidence and reactivation of the underlying basin bounding faults

tectonics of the zagros orogenic belt of iran new data and interpretations abstract the zagros orogenic belt of iran is the result of the opening and closure of the neo tethys oceanic realm and consists from northeast to southwest of three parallel tectonic subdivisions 1 1 the urumieh dokhtar magmatic assemblage 2 2 the sanandaj sirjan zone and 3 3 the zagros simply folded belt recent detailed and reconnaissance mapping and stratigraphie structural analyses in various parts of the belt indicate that the tectonics of the sanandaj sirjan zone is dominated by either large scale composite duplex structures or low and high angle ne dipping imbricate systems these thrust systems have transported numerous horses and sheets nappes of metamorphosed and non metamorphosed phanerozoic rocks for tens or possibly hundreds of kilometers stratigraphic evidence and synorogenic conglomerates indicate that thrusting was initiated in the late cretaceous and has continued episodically since that time shear sense indicators in various parts of the zone reveal a ne sw direction of transport thrust systems generated in each episode are superimposed upon the previous structures cutting across and displacing them and producing complicated stacks of thrust sheets southwestward transport and stacking of the thrust sheets has resulted in an increase of about 10 15 km in the thickness of the continental crust in the southwestern part of this zone the so called z main zagros thrust which has been traditionally considered as the boundary between the sanandaj sirjan zone and the zagros simply folded belt is by no means a single high angle reverse fault nor is it a narrow zone of crushed rocks but a series of low angle thrusts locally cut by normal faults these thrusts are indistinguishable from those mapped in the sanandaj sirjan zone and are geometrically part of the sanandaj sirjan zone structures there is no single thrust among them that can be singled out and referred to as the main one the boundary of the sanandaj sirjan zone with the urumieh dokhtar magmatic assemblage to the northeast is characterized by a series of structural depressions formed by compression rock sequences including ophiolite complexes exposed along this boundary are strongly dismembered sheared and mylonitized shearing planes and foliations are either subvertical or dipping at high angles to the northeast this zone is suggested to be the suture between the afro arabian and the iranian plates overall the intensity of deformation increases northeastward from the persian gulf mesopotamian basin where strata are undeformed and horizontal through the zagros simply folded belt which is characterized by en echelon doubly plunging flexural slip folds gentle to open in the southwest and close to locally sw overturned in the northeast the intensity of deformation continues to increase further northeastward through the sanandaj sirjan zone where thrust faulting is dominant this pattern suggests that the deformational front has migrated through time from the northeastern part of the sanandaj sirjan zone i e the suture zone to its present position in the central portion of the persian gulf mesopotamian foreland basin

transpressional deformations within the sanandaj sirjan metamorphic belt zagros mountains iran abstract polyphase deformation structures of the sanandaj sirjan metamorphic belt in the neyriz area zagros mountains iran are consistent with dextral transpressional deformation which is related to the oblique collision between the african arabian continent and the iranian microcontinent the collision started in the late cretaceous and is continuing to the present day quantitative kinematic analyses quartz textures suggest localized shear zones deformed with a significant pure shear component spatial orientation of lineation and semi quantitative kinematic indicators indicate shear zone parallel stretching and shear zone perpendicular shortening the occurrence of a horizontal stretching component parallel to the deformation zone boundary allows a kinematic model of combined transpression and lateral extrusion for this part of the zagros orogeny

strains and vorticity beneath a tabular batholith in the zambesi belt northeast zimbabwe structures on scales of cm to dm in formerly universally orientated quartz veins are used to analyse deformation in part of the pan african e w zambezi orogen by integrating their orientations over length scales of six orders of magnitude 2 cm to 20 km sixty two strain ellipsoids on locality scales 20 200 m and six on district scales 2 20 km demonstrate that each of the constituent lithologies together with the metasedimentary cover sequence as a whole strained as homogeneous materials after the quartz veins developed related to orogenic structures these homogeneous strains demonstrate that major folds in the cover initiated as buckles on e w axes before the quartz veins were inserted but tightened passively as the metamorphic grade rose from greenschist to upper amphibolite facies by contrast post vein f2 flow folds originally on ne axes amplified actively at the same time but without noticeable shortening such f2 folds are interpreted to have been driven by the gravitational stabilization of the relatively dense proterozoic cover and its less dense archaean basement the instability due to this primary density inversion was aided significantly by a secondary density inversion induced by the orogenic thermal gradient while the metamorphic gradient was below 30 c km the basement rose in conformable upright ridges on the same ne trends preserved in the archaean rocks in the kalahari craton to the south the early e w trending folds and later ne trending folds of the zambezi orogeny interfered to generate class ic patterns of elongate domes and basins throughout the cover sequence when the regional thermal gradient rose above 30 c km the basement rose diapirically into or through the cover as domes of migma migmatite or magma on centres 5 10 km apart melts of the deep basement eventually drained up the stems of earlier migma diapirs and presumably fed a tabular batholith in the epizone that has since been removed by erosion diapirs of basement or remobilized basement displaced the earlier class ic fold interference pattern in the cover in all directions this led to uniaxial radial flattening and vorticity down to scales of 100 m in the cover within about 2 km of the basement contact meanwhile away from the immediate influence of the diapirs the cover sequence continued to undergo vorticity about a nnw shortening axis on district scales but only spun on locality scales the mean post vein n s orogenic shortening of the cover sequence is found to be 60 on the locality scale linear strain fields for data from appropriate districts are interpreted in terms of plane strains with about 50 60 of the total shortening being due to uniaxial nnw volume loss of sio2 and water on both locality and district scales all the post vein shortening measured in the orogen can be attributed to lateral displacement of the cover by diapirs of a sialic basement which may not have shortened on the regional scale at any stage pre vein shortening of the cover by thrusts and ductile flow has not been quantified but n s shortening due to early buckles is calculated to have been about 17 the upward loss of sio2 in solution and either solid or molten basement from levels still exposed to others since eroded appears to be a far more characteristic aspect of the eastern part of the zambezi orogen than lateral shortening on the regional scale

strain ellipsoids from incompetent dykes application to volume loss during mylonitization in the singo gneiss zone central sweden we describe how to constrain bulk strain ellipsoids by distinguishing the attitudes of mullioned from planar contacts in incompetent dykes our method is demonstrated by using metabasaltic dykes to

reply to the comment by marques et al on numerical models of flow patterns around a rigid inclusion in a viscous matrix undergoing simple shear implications of model parameters and boundary conditions by n mandal s k samanta and c chakraborty

structural styles in the zagros simple folded zone iran arabia eurasia convergence is achieved in the nw zagros by a combination of shortening on nw se trending folds and thrusts mainly in the simple folded zone and by right lateral strike slip on the nw se trending main recent fault a balanced and restored cross section across this part of the range indicates c 49 km of shortening this probably occurred since c 5 ma providing an estimate of the long term shortening rate across the simple folded zone of c 10 mm a 1 the geometries of exposed structures suggest both basement thrusts and thin skinned decollement levels with major folds possibly nucleated above basement faults fold geometries indicate several decollement horizons shale units are candidates as well as evaporites in the neogene mesozoic lower palaeozoic and upper proterozoic successions the se part of the simple folded zone deforms by north south shortening on broadly east west trending folds and thrusts the link between these regions occurs via a set of fault blocks c 400 km wide in total each bounded by north south right lateral faults incremental changes in the strike of some of the folds occur across these right lateral faults with more east west orientations to the east

dolomite calcitization and cement zonation related to uplift of the raisby formation zechstein carbonate northeast england abstract the raisby formation is the basal zechstein carbonate of northeast england it has undergone a complex diagenetic history during burial including multiphase dolomitization and replacement by sulphates upon uplift meteoric derived fluids have dissolved this replacive sulphate and precipitated calcite cements in the resultant porosity study of these cements by cathodoluminescence and trace element geochemistry shows that they are zoned and different cathodoluminescence zones correlate well with iron and manganese geochemical changes demonstrating a gradual increase in oxygenation of the precipitating fluids through time major diagenetic processes during cement growth include precipitation of anhydrite fluorite and barite calcitization of dolomite and internal sedimentation relating these processes to cement zonation allows this complex diagenesis to be explained in terms of uplift under the influence of meteoric derived groundwaters of constantly changing geochemical and electrochemical characteristics

coeval versus reciprocal mixed carbonate siliciclastic deposition middle devonian baltic basin eastern europe implications from the regional tectonic development the middle devonian narva succession in the baltic basin represents a significant turnaround in the history of the basin the detailed study of core and outcrop sections and the three dimensional correlations across the baltic basin reveal a carbonate dominated mixed retrogressive succession overlain by a siliciclastic dominated progradational succession the palaeogeographic reconstructions show how the shallow tide influenced basin expanded from south west to north east and later during the transgression also to the north south and east the transgressive portion of the basin fill is dominated by carbonate rich sabkha and supratidal to intertidal deposits on the basin margins and subtidal carbonates in the basin centre siliciclastic material was derived by tidal currents and storm waves from the south west through a tidal inlet and flood tidal delta complex this initial transgressive phase is characterized by the lack of subsidence or even episodic uplifts in the northern north western part of the basin margin shown by convergence of timelines and the thin 30 m transgressive succession in contrast on the southern margin the facies associations stack vertically into a 70 to 80 m thick succession indicating significantly higher subsidence rates the upper part of the transgressive phase indicates subsidence across the whole basin the upper progradational portion of the basin fill is dominated by coarse siliciclastic tide influenced deltaic deposits that rapidly prograded from north west to south east this detailed study on the narva succession shows that siliciclastic and carbonate deposition was coeval and that mixing occurred at different temporal and spatial scales the mixing was controlled by grain size volume and location of siliciclastic input rather than relative sea level changes as suggested in widely used reciprocal mixing models it is suggested that the forebulge of the scandinavian caledonian fold and thrust belt migrated to the north western margin of the baltic basin during the earliest eifelian as indicated by the lack of subsidence and probable uplift in the northern north western margin during the early transgressive phase the forebulge migration ceased although the forebulge had already started to subside during the later stages of the transgressive phase the deltaic progradation is interpreted to be associated with the orogenic collapse and uplift in the scandinavian caledonides that caused the erosion of the foreland basin fill and the coarse sediment transport into the baltic basin

upper permian zechstein supergroup carbonate evaporite platform palaeomorphology in the uk southern north sea abstract seismic interpretation geological mapping and depth conversion of the zechstein supergroup z2 cycle using high quality well calibrated three dimensional 3 d seismic has revealed the complex palaeomorphology of a deeply buried ancient carbonate shelf margin on the southwestern margin of the southern permian basin the new mapping shows that the margin comprises a series of corrugated embayments and promontories which extend up to 5 km 3 mi into the basin and locally have a palaeobathymetric relief of 200 m 650 ft well calibration across the margin demonstrates a strongly bipartite lateral thickness distribution between a high velocity anhydrite dominated shelf and a lower velocity halite dominated basin strong lateral and vertical velocity variations in the upper permian zechstein supergroup are known to have major impacts upon seismic imaging and depth conversion in the spb the resulting uncertainty remains one of the major challenges when interpreting and assessing the prospectivity of the underlying upper permian rotliegend group leman sandstone formation reservoirs in the uk southern north sea an understanding of the zechstein shelf edge s 3 d physiography and its velocity variation has implications for the delineation of traps containing the prospective reservoirs that lie below recognition of the complexity and effects of this shelf margin contributed to the recent juliet discovery whose position outside of the presently defined rotliegend group play fairway suggests that prospectivity is more extensive along the basin margin than previously thought as such the work provides a means by which to identify and delineate new structures and extend the life of this mature yet prolific gas province

some new aspects of kinematic vorticity analysis in naturally deformed quartzites abstract two vorticity methods have been proposed to estimate kinematic vorticity number in naturally deformed quartzites these combine the degree of quartz c axis fabric asymmetry either with the strain ratio in the xz plane of finite strain r xz or with the angle between the oblique grain shape fabric and the main foliation here it is suggested that the incorporation of r xz data with angle values can also serve as an additional method of vorticity analysis the use of a new nomogram that provides a rapid means of evaluating the consistency of vorticity estimates obtained by the three methods is also discussed a comparison of vorticity estimates from low grade test samples using all methods yields that three methods may record different parts of deformation history this hypothesis was then used to discuss the spatial and temporal variation of kinematic vorticity in a crustal scale shear zone this natural example showed that the new method is able to record late stage changes in the flow regime which are slightly or not detectable by previous methods also it is noteworthy that vertical vorticity profiles based on results of rigid porphyroclast analysis tend to average profiles obtained by the estimates of quartz based methods

the marl slate a model for the precipitation of calcite dolomite and sulphides in a newly formed anoxic sea detailed studies of a new complete marl slate core in south yorkshire have provided information on isotopic 13c 18o 34s and geochemical variations trace elements and c s ratio which enable the formulation of a model for carbonate and sulphide precipitation in the late permian zechstein sea calcite and dolomite are intimately associated the fine lamination organic character and absence of benthos in the sediments are indicative of anoxic conditions lithologically the core can be divided into a lower predominantly sapropelic marl slate 2 m and an upper transition zone 0 65 m of alternating sapropel and calcite rich and dolomite rich carbonates c s ratios are 2 22 for the marl slate and 1 72 for the transition zone respectively both characteristic of anoxic environments 18o in the carbonates shows a large and systematic variation closely mirrored by variations in calcite dolomite ratio the results suggest a fractionation factor equivalent to a depletion of 3 8 for 18o and 1 5 for 13c in calcite the 34s values of pyrite are isotopically light mean value 32 7 suggesting a fractionation factor for the marl slate of almost 44 typical of anoxic basins the results are related to stratification in the early zechstein sea calcite was precipitated in oxic upper layers above the halocline below the oxic anoxic boundary framboidal pyrite was precipitated resulting in lower sulphate concentration and elevated mg ca ratio due to calcite precipitation as a result of this dolomite formation occurred below the oxic anoxic interface within the anoxic water column and in bottom sediments variations in calcite dolomite ratios and isotopic variations are thus explained by fluctuations in the relative level of the oxic anoxic boundary in the zechstein sea

strain partitioning in ductile shear zones an example from a lower pennine nappe of switzerland abstract the rocks of the maggia nappe a lower pennine nappe of the swiss alps were deformed in at least three phases during alpine orogeny hercynian granites in the core of the nappe developed ductile shear zones during the early phases of deformation but between shear zones they remained almost undeformed both heterogeneous simple shear and heterogeneous volume change played an important role in the formation of the ductile shear zones partitioning of different strain components indicates a volume loss of nearly 80 90 towards the centres of some of the shear zones such a large amount of volume loss is also corroborated by strain analysis of deformed xenoliths in other shear zones of the same region a comparative study of theoretical strain paths and the natural results indicates constant shear strain rate but changing dilation rate during deformation

environmental and sequence stratigraphic implications of anhydrite textures a case from the lower triassic of the central persian gulf abstract the lower triassic kangan formation in the persian gulf south pars gas field and its adjacent areas are composed of carbonate evaporite sequences these sediments were deposited in a shallow marine homoclinal ramp study of the anhydrite bearing intervals shows various structures and textures the anhydrite structures are mainly bedded massive chicken wire and nodular type and the main textures are felted sparse crystal needle shape lath shape equant and fibrous pervasive and poikilotopic cement together with replacement and porphyroblastic gypsum are accounted as the most common diagenetic features in anhydrite evaluation of anhydrite occurrences and features support both primary and secondary formations the nodular to chicken wire anhydrite formed under synsedimentary sabkha conditions whereas anhydrite cements occurred during the late stages of diagenesis shallow burial stage massive to bedded anhydrite could have been formed under subaqueous conditions or originated by coalescing and continued growth of anhydrite nodules in the sabkha zone anhydrite fabrics impose a significant control on the reservoir quality of the kangan carbonates at the south pars gas field thick massive and bedded anhydrite could have been formed as an intraformational seals and anhydrite cements occluded pore spaces and reduced the poroperm values the sequence stratigraphic analysis revealed two depositional sequences in the studied intervals which are composed of tst and hst investigation of anhydrite throughout depositional sequences indicates a change in the content and style of anhydrite texture anhydrite content volume decreases upward through transgressive system tract sea level rise whereas it enhances during highstand system tract sea level fall pervasive and poikilotopic anhydrite cements together with replacement by anhydrite are prevalent features during transgressive and early highstand system tract at the late hst with a progradational stacking pattern anhydrite value increases and felted radial equant crystalline and mosaic texture are the most common anhydrite fabrics sequence boundaries that indicate maximum sea level fall and exposure of successions are marked by the broad anhydrite deposits with massive to bedded and chicken wire structures and various textures that located in late hst package there is an unambiguous relationship between the microfacies associations the evaporite textures and the sea level fluctuations this relationship could lead to a predictable pattern that can be of use as a general guide for the sequence stratigraphic interpretations in the area

heterogeneous shear zone evolution the role of shear strain hardening softening mathematical models are proposed to describe heterogeneous shear zones characterized by variation thickness with time these models take into account several combinations of deformation type regime simultaneous simple shear pure shear and volume change and degree of strain localization increasing or decreasing shear zone thickness results respectively from strain hardening where deformation progressively diffuses into the host rock and strain softening where deformation gradually localizes within a narrow zone shear zone models are made of a series of layers each characterized by homogenous strain a quantitative evaluation of displacement thickness relationship and finite strain development may be obtained from each model temporal evolution depends on the following parameters i rate of shear strain hardening softening ii initial shear strain increment imposed to the system and iii thickness of homogenously deformed units within a layered shear zone based on data coming from natural shear zones displacement rates may be obtained independently of shear zone thickness

facies patterns and stratal geometries clues to the nature of the platform margin during the paleocene northeast sirte basin libya abstract the sirte basin of libya is a mesozoic tertiary rift basin comprising a series of horsts and grabens which were formed as a result of collapse of a n s trending arch in late cretaceous time the paleocene section is made up of two shallowing upward cycles where each cycle is under and overlain by deeper pelagic facies a gradual shallowing trend from basin to shallow platform is seen in both bio and lithofacies these are interpreted as representing prograding ramps with no evidence of a significant break in slope between the basin and the adjacent platform the fact that most facies are dominated by micrites suggests that deposition was in a low energy setting possibly on a leeward margin these ramps were initially foram dominated platforms setting the style for later tertiary ramps large forams filling the role of inner mid ramp carbonate producers after the demise of the rudist coral assemblage at the end of the cretaceous based on local and regional information a number of episodes of relative sea level change can be recognized the two shallowing upward ramp cycles are interpreted as representing two highstand system tracts the precipitation of evaporites in the lower cycle appears to be associated with the earlier highstand system tract culminating during a sea level lowstand a drop in sea level during the late paleocene is also invoked to explain the interpreted influx of meteoric waters following the later highstand this event resulted in the creation of good mouldic and vuggy porosities thus making these porous inner mid ramp facies potential targets for hydrocarbon exploration on a basin wide scale this event is also very important because all the paleocene reservoirs at least in the eastern sirte basin seem to be related to this possible subaerial exposure event

the development of δ clasts in non linear viscous materials a numerical approach abstract winged porphyroclasts are often used as kinematic or strain indicators in shear zones in particular it has been suggested that the geometry of clasts in mylonites can give information about the rheological behaviour of the matrix around the clast passchier et al 1993 we tested this hypothesis with a numerical finite element simulation of the flow field of a viscous matrix around a relatively rigid clast power law viscous flow properties of the matrix with stress exponents of 1 linear viscous 3 and 5 were tested as well as three different sets of approximately simple shear boundary conditions which are relevant to experimental and or natural situations the simulations show that the flow field is primarily determined by the boundary conditions the stress exponent is of secondary importance in general the different boundary conditions produce two types of flow fields about the rigid clast an eye shaped flow field and a bow tie shaped flow field the former is produced by boundary conditions that approximate simple shear applied at an infinite distance from the rigid clast and the latter is produced by boundary conditions which are equivalent to a shear box or ring shear apparatus although both the shear box and ring shear boundary conditions produce bow tie flow patterns the wing geometries which result from the flow patterns for these two cases can be significantly different the geometry of the wings that develop on a clast is a direct function of the flow field around the clast for linear and non linear rheologies only the ring shear boundary conditions produce wings that show stair stepping i e wings that are parallel but not in the same plane in natural shear zones clasts both with and without stair stepping are observed in comparing geologic observations with experimental or numerical modelling it is essential to understand which boundary conditions are appropriate

strain in ductile shear zones in the caledonides of northern sweden a three dimensional puzzle small scale ductile shear zones are well developed in amphibolites of caledonian nappes of the swedish lapland strain in the shear zones is recorded by the shapes of originally sub equant aggregates of plagioclase feldspar classical strain analysis on two dimensional xz profiles across the shear zones leads to results that indicate large volume losses associated with the shear strain large volume losses are however incompatible with geochemical analysis which shows little chemical variation across the shear zones by including data from xy and yz sections information on the three dimensional state of strain can be obtained these data indicate that deformation is not confined to a two dimensional plane particle extension within the shear zone parallel to the shear plane and perpendicular to the shear direction has also occurred the state of strain and strain path in the shear zone can be modeled in a general way by combining stretches in x y and z with simple shearing and volume loss end member models correspond to plane strain with volume loss kx ky 1 kz 1 transpression kx 1 ky 1 kz and pure shear kx 1 kz ky 1 these models produce identical strain ratios and orientations on the xz section but differ in stretch in y and in area of the strain ellipse on the xz section extension in y or x allows for reconciliation of the geometrical and geochemical data for part of an individual shear zone but poses a problem of strain compatibility at the edges and terminations of the shear zones the shear zones show a complex anastomosing pattern in three dimensions and their connectivity provides a way in which strain compatibility is maintained

brine mixing in the middle devonian of western canada and its possible significance to regional dolomitization abstract the desiccation that most large evaporite basins experienced in their history created large elevation heads these heads must have induced flows of ground or formation waters into the evaporite basins the consequences of such groundwater flows are believed to be 1 provision of the necessary hydrodynamic drive to allow dolomitization of carbonates beneath and adjacent to the evaporite basin and 2 the formation of anomalously distributed evaporites in the basin adjacent to sites of groundwater inflow sites where the groundwaters mixed with marine derived brines such brine mixing may also have been responsible for depleting the marine derived brines of their contained sulfate so explaining later development of sulfate impoverished potash deposits anomalous developments of anhydrite surrounding older carbonate buildups in the distal parts of the middle devonian elk point basin of western canada may represent such products of brine mixing and provide evidence of dolomitization of underlying lower palaeozoic and devonian carbonates coevally with the filling of the evaporite basin regional dolomitization in the williston basin is stratigraphically confined beneath the middle prairie evaporite suggesting a genetic link between the dolomitization and evaporite deposition dolomitization by refluxing brines generated in the evaporite basin appears unlikely because of the presence of an aquitard at the base of the evaporite succession developments of laminar and pisolitic carbonates adjacent to the carbonate buildups are believed to indicate the former presence of subaerial springs that carried calcium rich groundwaters into the evaporite basin anhydrite envelopes around the buildups are interpreted to be reaction products that formed when calcium bearing spring waters mixed with sulfate rich but calcium depleted marine brines the calcium in the spring waters is believed to have entered the groundwaters as a by product of the dolomitization of carbonates underlying the evaporite basin precipitation of gypsum now anhydrite as a consequence of brine mixing must have depleted the sulfate content of the marine derived brines it is possible that the sulfate depleted potash deposits in the prairie evaporite originate by this mechanism on the other hand the stratigraphic position of these deposits lends little support for this hypothesis

simple shear of deformable square objects abstract finite element models of square objects in a contrasting matrix in simple shear show that the objects deform to a variety of shapes for a range of viscosity contrasts we catalogue the changing shapes and orientations of objects in progressive simple shear at moderate simple shear 1 5 the shapes are virtually indistinguishable from those in equivalent pure shear models with the same bulk strain r s 4 examined in a previous study in theory differences would be expected especially for very stiff objects or at very large strain in all our simple shear models relatively competent square objects become asymmetric barrel shapes with concave shortened edges similar to some types of boudin incompetent objects develop shapes surprisingly similar to mica fish described in mylonites

tectono metamorphic evolution of the neyriz metamorphic complex quri kor e sefid area sanandaj sirjan zone sw iran abstract the neyriz region includes outcrops of metamorphic rocks that are thrust over the neotethyan ophiolites these rocks are affected by a major deformational event the result of which includes a shearing polyphase foliation present in gneissic core domes overprinted by a crenulation cleavage these fundamental structures developed contemporaneously with a medium pressure metamorphism which is characterized by the syn kinematic crystallization of kyanite and the beginning of anatexis followed by the development of retrometamorphic mineral parageneses the major deformation phase in the area occurred during the early cimmerian orogeny in the late triassic following the orogeny the gneiss domes started to rise into the upper levels of the crust from the geodynamic point of view after the mid permian the studied area was situated at southern passive margin of the iranian plate the central iranian microcontinent at that time was separated by the neotethys ocean from the gondwanian supercontinent after the late triassic the region became an active margin associated with an accretionary prism the margin was finally involved in an orogenic wedge after the closure of the neotethyan oceanic basin in the late mesozoic closure of the basin resulted in a major thrusting of the metamorphic rocks of the southern iranian margin over the neotethyan ophiolites

the epicontinental subsiding margin of the triassic in northern israel north arabian plate abstract the triassic sedimentary pattern of the north arabian margin represents a transition from continental to deep marine we here present a transect part of this suite from southern to northern israel cuttings from the ramalla s and devora n boreholes were studied by petrographic and cathodoluminescence microscopy biotic and sedimentological components were grouped into microfacies upon which a carbonate evaporitic depositional facies model was based mid to inner ramp conditions persisted along the transect essentially following a s n depositional strike with outer ramp to basinal facies to the west five low order lo cycles were identified and integrated into a sequence stratigraphic model with eustatic tectonic and differential subsidence components lo cycle 1 is late anisian pelsonian cycle 2 is late anisian to longobardian cycle 3 is early carnian cycle 4 is late carnian and cycle 5 is at the carnian norian transition facies range from supratidal via lagoonal facies in late hst or lst to calcarenitic shoals and short term deepening to offshore at maximum flooding cycles 1 and 2 are non evaporitic with a clastic influx event in cycle 2 cycles 3 and 4 contain sabkha or salina evaporites in the lagoonal facies that end in the lower part of cycle 5 these facies group into a higher order cyclicity with discrete events that can be correlated between the boreholes depositional and burial history were correlated to alpine and arabian plate stratigraphy over 20 myr from the anisian pelsonian to the norian siliciclastic carbonate and evaporite dominated environments indicate climates ranging from relatively humid to extremely arid rapid subsidence in the late anisian is attributed to neotethyan rifting of northeastern gondwana from latest anisian to early carnian reduced subsidence took place in the face of eustatic sea level rise renewed subsidence in the carnian is attributed to further neotethyan rifting subsidence terminated in the norian with uplift to the north attributed to pre volcanic doming

depositional bias and environmental change important factors in sequence stratigraphy abstract differences among depositional systems here called depositional bias strongly influence sequence patterns siliciclastics and shallow water carbonates for instance shed most of their sediment during opposite phases of a sea level cycle lowstand shedding and highstand shedding respectively furthermore the two systems generate their own system specific relief on the sea floor disperse their sediment load along different avenues and differ in the way they are deactivated reefs and carbonate platforms can be drowned whereas siliciclastic deposition can be shut off and renewed at any depth as a consequence of these differences pronounced unconformities drowning unconformities develop where carbonate platforms are terminated and buried by siliciclastics the siliciclastic to carbonate transition tends to be more gradual drowned platforms and drowning unconformities appeared world wide in great abundance in the miocene cretaceous valanginian turonian jurassic toarcian and devonian frasnian famennian examples of drowning unconformities interpreted as sequence boundaries include those of the early cretaceous platforms off new jersey and off morocco the mid cretaceous unconformity in the gulf of mexico and miocene unconformities on top of reefs in the far east the eustatic cycles postulated from sequence stratigraphy are a very unlikely cause for the mass drownings of reefs and platforms either the rates of rise are an order of magnitude lower than the growth potential of platforms or cycle amplitudes are too small or the cycles are too short and subsidence too slow to remove platforms permanently from the photic zone the critical element in the mass drowning seems to be environmental stress that reduces the growth potential of carbonate systems thus drowning unconformities demonstrate the importance of environmental change as a control on sequence development other examples of dominantly environment controlled sequences include slope deposits shaped by shifting currents e g florida and blake plateau and basin fills that were controlled by changes in sediment input related to tectonic alteration of the drainage in the hinterland e g gulf of mexico environmental change must be considered as a third independent factor that competes with eustatic and tectonically driven regional changes of sea level for control of sequences the definition of sequences and sequence boundaries should be broad enough to include the possibility of non sea level controls

dolomitization associated with transgressive surfaces a mid cretaceous example abstract increased seawater circulation within a carbonate platform generated through transgression and flooding of the platform is proposed here as a cause of the common selective dolomitization of calcitic silts in the upper part of limestone sequences of the barremian aptian urgonian platform of se france which underwent selective porosity creation as a result of karstification during an aptian relative sea level fall parts of this porosity are infilled by microcrystalline dolomite and some of the host limestone is partially to completely dolomitized the microcrystalline dolomite is considered to have formed by dolomitization of calcitic silt internal sediment within the karstic cavities as a result of the pumping of seawater through the upper layers of the platform during subsequent transgression the calcitic silt provided abundant nucleation sites for the dolomitization seawater dolomitization following subaerial exposure of limestone is likely to be a common phenomenon if there is sufficient meteoric diagenesis and porosity production and a prolonged period of submarine erosion winnowing and condensed sequence formation enhancing the amount of circulation of seawater as seafloor sediment is lithified

genesis of limestone megabreccias and their significance in carbonate sequence stratigraphic models a review a re appraisal of the factors controlling the deposition of carbonate megabreccias and their sequence stratigraphic significance is presented based on an examination of the mechanisms for instigating carbonate seafloor slope instability and a review of the depositional settings of these distinctive rock deposits limestone megabreccias have classically been interpreted as formed by the catastrophic collapse of high angle metastable oversteepened carbonate platform margins however a review of megabreccia occurrence shows that these deposits formed on a broad range of carbonate slope angles and that metastable oversteepened slopes are not necessary for their genesis a re evaluation constructed from first principles of mechanisms promoting submarine gravitational instability indicates that pore water overpressure of confined aquifer horizons beneath the seafloor rather than slope oversteepening is the critical control on megabreccia deposition catastrophic release of overpressure may initiate gravitational instability of previously kinematically stable slopes the relative importance of endogenic processes intrinsically linked to the depositional system and exogenic processes operating independently in generating megabreccias is assessed two particularly important mechanisms in generating megabreccias are endogenic processes causing overpressure at discrete hydrologically confined horizons beneath the seafloor during relative sea level falls and increases in stress as pore fluid drains from the sediment when the platform top becomes subaerially exposed during relative lowstands of sea level the increased likelihood of megabreccia genesis during relative sea level falls is supported by an empirical trend identified from a review of the stratigraphic distribution of documented megabreccias exogenic causes of gravitational instability with inherently random periodicity especially seismicity may account for the small number of documented megabreccias that do not conform to this underlying trend the proposed endogenic pattern favouring megabreccia deposition during relative sea level falls is important in a sequence stratigraphic context megabreccia deposits may build volumetrically significant toe of slope wedges and aprons during relative lowstands of sea level partially compensating for the decrease loss in the supply of fresh granular sediment as the platform top carbonate factory contracts lowstand wedges and aprons in carbonate systems therefore generally differ in composition from equivalent highstand deposits in being composed not of calciturbidite deposits but of megabreccias cannibalized from the upper slope

slip partitioning and inclined dextral transpression along the zagros thrust system iran abstract flexural duplex structures and shear zones are well developed in the heneshk area in southwestern iran the zagros thrust system in this area consists of eight sheets of nw striking ne dipping dextral strike slip duplexes that are linked with imbricate fans and oblique slip thrusts the zagros thrust system is a portion of the internal zone of the zagros orogenic belt this internal zone is characterized by penetrative plastic deformation and metamorphism based on the kinematic vorticity number w k 0 73 0 02 calculated from quartz porphyroclasts of the goshti shear zones along the zagros thrust system the estimated angle between the maximum instantaneous strain axis isa and the transpressional zone boundary is 33 the estimated angle of the plate convergence or the flow apophysis of the displacement field is 25 the presence of dominantly dextral shear sense indicators in the area is consistent with dextral inclined transpressional convergence the mean estimated finite deformation w m value indicates relative contributions of 47 pure shear and 53 simple shear for the deformation which involved both strike slip and oblique slip displacements in this inclined transpression about 40 strike slip partitioning was required to accommodate the finite strain and re orientation of instantaneous strain

boudinage and shearband boudins a meso to micro scale tool in structural analysis abstract a classification of boudinage and boudins is made presenting a simplified parametric description of boudin block geometry end members the most important shearband boudins parameters ratios microstructural domains and internal evolutions are described and interpreted shearband boudins are related to a large number of geological meso and microstructures e g pinch and swell drawn or torn boudins mica fish porphyroclasts foliation boudinage tectonic lozenges and c s c several factors can control the asymmetric boudinage process such as the obliquity of a competent layer the orientation of an initial fracture boudin axial ratio the viscosity ratio between matrix and layer flow type and shear strain this review highlights the correlation of shearband boudins with other asymmetrically deformed structures and shows their geological relevance finally this study presents a phenomenological framework for shearband boudins

burial evolution of evaporites with implications for sublacustrine fan reservoir quality a case study from the eocene es4x interval dongying depression bohai bay basin china abstract eocene sublacustrine fan sandstones that developed in a rift basin are important tight gas reservoirs in the dongying depression bohai bay basin northeastern china two units of evaporites developed at the top and bottom of the lower unit of the es4 interval es4x consist predominantly of anhydrite with subordinate gypsum evaporite and related diagenetic processes greatly influenced reservoir quality 13 c values for micritic dolomite cements in es4x are depleted 7 45 to 2 57 due to microbial sulfate reduction msr under shallow burial conditions and this interpretation is supported by large 34 s fractionation between anhydrite and framboidal pyrite precipitation temperatures for micritic dolomite are calculated as 57 5 72 8 c anhydritization of gypsum probably occurred at 100 5 145 2 c during progressive burial as evidenced by homogenization temperatures of aqueous inclusions within anhydrite cements this process resulted in dehydration fractures within anhydrite cements that increased reservoir permeability by connecting isolated pores thermochemical sulfate reduction tsr probably resulted in dissolution of gypsum and anhydrite cements under relatively deep burial conditions ankerite cements are replaced by anhydrite cements and are enclosed by solid bitumen in es4x ankerite cements likely were derived from tsr as reflected in negative 13 c values 7 12 to 3 70 and high calculated temperatures 121 3 185 1 c dissolution by products e g saddle dolomite ankerite nodular pyrite related to tsr precipitated in adjacent pores a lack of significant 34 s fractionation between parent sulfate and nodular pyrite indicates that tsr occurred in a relatively closed system therefore dissolution of gypsum and anhydrite related to tsr contributed little to reservoir quality middle fan lithofacies with better sorting porosity and permeability than inner and outer fan lithofacies constitute high quality reservoirs in es4x

strain and possible volume loss in a high grade ductile shear zone small scale ductile shear zones in high grade metagabbros from the caledonides of northern sweden provide a good opportunity for examining the development of fabric and strain in rocks undergoing progressive non coaxial deformation the rocks contain strain markers in the form of aggregates of plagioclase feldspar a selected shear zone is analyzed assuming monoclinic symmetry which requires for the zone as a whole variable simple shear variable volume change or variable simple shear and volume change plus a homogeneous deformation both size of the feldspar aggregates and aspect ratio of the aggregates plotted against orientation indicate significant volume decrease in excess of 50 and perhaps as much as 80 towards the centers of the zones the data are broadly consistent with a deformation path involving constant shear strain across the zone followed by variable volume change or by a path of progressive loss of volume with increasing shear strain followed by a homogeneous strain they are not consistent with a single path involving rates of shear strain and volume loss that are in constant proportions there is little difference in chemical composition between the shear zone and the surrounding rock an analysis of the concentrations of elements in the shear zone and wall rock suggests that volume changes were slight 15 the discrepancy between the values of volume change estimated from strain analysis and geochemical analysis may be resolved either by geochemical equilibration being attained after deformation or by a departure from the geometric constraints of the monoclinic band model for shear zones modification of marker particle size and shape during dynamic recrystallization may also partly account for the apparent volume loss