strain heating as a mechanism for partial melting and ultrahigh temperature metamorphism in convergent orogens implications of temperature dependent thermal diffusivity and rheology 1 we explore the importance of strain heating in ductile shear zones for production of leucogranites and high temperature metamorphism in collisional orogens with temperature dependent thermal diffusivity d and rock rheology new measurements on metamorphic rocks from the bohemian massif show that d is 1 3 mm2 s at 25 c and decreases exponentially to as low as 0 4 mm2 s at 600 c this temperature dependence causes model lithospheric geotherms to be straighter compared to geotherms calculated with a constant d using power law parameters for rheology we show that deformation of quartzite at strain rate of 3 10 13 s 1 produces 100 w m3 at 550 c and 7 w m3 at 800 c deformation of stronger clinopyroxenite at this strain rate produces 8 w m3 even at 1050 c when strain heating is introduced by a 3 km thick ductile shear zone with quartz rheology at the depth of 35 km in a lithosphere that has 70 km thick crust and 60 km thick lithospheric mantle the schist solidus is reached by 8 myr in the vicinity of the shear zone deforming at the strain rate of 3 10 13 s 1 for clinopyroxenite rheology ultrahigh temperature uht metamorphic conditions are reached in 40 myr after initiation of strain heating two dimensional models that replicate the exhumation of the greater himalaya crystalline rocks above the main central thrust produce extensive partial melting and an inverted metamorphic field gradient occurrences of leucogranites within crustal shear zone systems may be evidence of the coupling between deformation and heat production in the crust

glaucony and sequence stratigraphy a conceptual framework of distribution in siliciclastic sequences abstract detailed analysis of recent literature on glaucony and selected case studies eocene isle of wight miocene northern apennines shows that the presence of glaucony alone is not diagnostic of a specific systems tract of a depositional sequence a reliable sequence stratigraphic interpretation of glaucony bearing units requires additional information on glaucony including 1 spatial distribution 2 maturity distinction between nascent slightly evolved evolved and highly evolved glaucony and 3 genetic attributes differentiation of autochtonous from allochthonous and intrasequential from extrasequential glaucony autochthonous glaucony is common at various stratigraphic levels in the transgressive systems tract tst and the lower highstand systems tract hst showing an upward increase tst and then decrease hst in abundance and maturity the condensed section can be distinguished from the overlying and underlying deposits by the higher concentration and maturity of glaucony allochthonous intrasequential parautochthonous glaucony can be present in the entire tst hst and lowstand systems tract lst generally showing lower concentration and maturity than its autochthonous counterpart allochthonous extrasequential detrital glaucony is present mainly in the lst its concentration and composition depending on the characteristics of the source horizon the associa ion between autochthonous and allochthonous intrasequential and extrasequential glaucony commonly exists in the lst and in the lower tst

the thermal structure of collisional orogens as a response to accretion erosion and radiogenic heating thermal models of collisional orogens generally predict temperature structures that are much cooler than those recovered by thermobarometric studies here we demonstrate that high temperature low pressure metamorphism and the development of inverted geotherms within collisional belts may be the result of accretion and erosion acting on crust enriched with heat producing elements a new two dimensional finite difference model described here incorporates the subduction of lithosphere with heat producing material in the upper crust accretion of crustal material from the subducting plate to the upper plate and surface erosion of the upper plate these processes result in the development of a wedge of heat producing material within the upper plate the rate of heat production within the wedge and maximum depth of the wedge are the most important parameters controlling the magnitude of upper plate temperatures our model yields inverted upper plate geotherms when heat production rates exceed 0 75 w m3 and the heat producing wedge extends to a depth greater than 35 km temperatures in excess of 500 c at depths of 20 30 km are computed when heat production rates are greater than 1 75 w m3 and the wedge extends to a depth 50 km other processes such as shear heating fluid flow or mantle delamination need not be invoked to explain geologic evidence of high temperatures or inverted thermal gradients in collisional systems

clay microporosity in reservoir sandstones an application of quantitative electron microscopy in petrophysical evaluation clay mineral microporosity in sandstones is measured using computer assisted image analysis of back scattered electron micrographs of petrographic sections diagenetic kaolinite has a variety of textures with microporosity values ranging from 15 to 61 diagenetic chlorite has a generally uniform grain coating texture and microporosity of about 50 fibrous illitic clays are difficult to characterize by the same method an average value of 63 microporosity was recorded but analysis of stereo pair micrographs from scanning electron microscopy analyses reveals that illite commonly has microporosity of approximately 90 clay microporosity data are used to calculate effective pore volumes and volumes of clay bound water for clay minerals in sandstones converting from wei ht percent clay to volume percent clay is important microporosity data are valuable input to vshale evaluation where water saturation is associated with clay mineral type texture and volume

diagenesis and formation water chemistry of triassic reservoir sandstones from southern tunisia the fluvial triassic reservoir subarkoses and arkoses 2409 5 2519 45 m of the el borma oilfield southern tunisia were subjected to cementation by haematite anatase infiltrated clays kaolinite and k feldspar at shallow burial depths from meteoric waters subsequently basinal brines controlled the diagenetic evolution of the sandstones and resulted initially in the precipitation of quartz overgrowths magnesian siderite minor ferroan magnesite and anhydrite the enrichment of siderite in 12c isotope 13cpdb 14 5 to 9 results from derivation of carbon from the thermal decarboxylation of organic matter during further burial the precipitation of dickite and pervasive transformation of kaolinite into dickite occurred followed by the formation of microcrystalline k feldspar and quartz chlorite and illite prior to the emplacement of oil present day formation waters are na ca cl brines evolved by the evaporation of seawater and water mineral interaction and are in equilibrium with the deep burial 3 1 km minerals these waters are suggested to be derived from the underlying silurian and devonian dolomitic mudstones

role of horizontal thermal conduction and finite time thrust emplacement in simulation of pressure temperature time paths abstract comparison of metamorphic thermal histories produced by 1 one vs two dimensional conductive models and 2 instantaneous vs finite emplacement time thrusting indicates that the simple 1d or instantaneous models applied by many petrologists generally produce pressure temperature pt paths with topologies similiar to those generated by 2d time transitive models however maximum temperatures attained by footwall rocks are significantly lower in 1d than in 2d conductive models leading us to recommend that 2d thermal effects be incorporated into models whenever possible pt paths produced by instantaneous and finite duration thrust emplacement differ significantly during the synthrusting period but the peak metamorphic conditions t max and pressure at t max eventually attained by footwall rocks during erosional unroofing are the same within minimum analytical uncertainties 50 k 100 mpa on geothermobarometric data thus if erosion begins only after the end of the thrusting event instantaneous thrusting is a reasonable simplifying assumption that introduces relatively little inaccuracy into pt models devised to examine peak metamorphic conditions

physics controlled thickness of shear zones caused by viscous heating implications for crustal shear localization we evaluate the parameters that control the thickness of ductile shear zones that are generated by viscous heating we employ two dimensional thermomechanical numerical models to simulate shear zone development under compression results show that the shear zone thickness is essentially independent on the numerical resolution and the initial size of a weak inclusion that triggers shear localization a simple scaling law is derived which predicts the thickness with six physical parameters far field stress and strain rate thermal conductivity both constant and temperature dependent initial temperature activation energy and stress exponent the scaling law is confirmed by numerical simulations for a wide range of parameters for crustal deformation conditions typical temperature increase ranges between 50 c and 200 c and the predicted thickness is between 1 km and 7 km these thicknesses agree with natural crustal and lithospheric shear zones suggesting that shear heating is a process controlling crustal shear zone formation

rheological stratification of the lithosphere abstract the rheology of the lithosphere is estimated on the basis of pore pressure dependent frictional failure in the brittle regime and power law steady state creep in the ductile regime different petrological models of the continental and oceanic lithosphere combined with different geotherms and thicknesses are used to generate models covering a variety of geodynamic situations results show that the depth dependence of lithospheric rheology varies with the tectonic province and that in many instances the continental lithosphere has one at the bottom of the crust or two the previous one plus another at mid crustal levels soft ductile layers sandwiched between brittle layers the depth distribution of geophysical parameters seismicity seismic wave velocity and electrical conductivity matches the theological predictions well the lithospheric rheological profiles are used to analyze observed variations in the structural style of deformed continental margin and oceanic terranes in the southeastern canadian cordillera the large scale characteristics of the system show a satisfactory agreement with the inferred rheological structure sub horizontal decollements should be the rule rather than the exception where the lithospheric rheology is strongly stratified

on the meaning of peak temperature profiles in inverted metamorphic sequences inverted metamorphic sequences ims are common features of main thrust systems on earth they exhibit an upwards continuous increase in peak temperature conditions and thereby constitute evidence of the close relationship between the thermal field evolution and tectonic processes heat advection and shear heating are known to allow the formation of such metamorphic signatures heat diffusion also plays an important role in temperature distribution on both sides of the thrust other advection processes such as erosion or accretion may also cause a local peak temperature inversion each one of these processes therefore affects the thermal field around the thrust however despite the crucial importance of all these processes for the interpretation of the inverted peak temperature signatures their respective influences have never been quantified and compared all together to address this issue we propose an innovative coupled approach i we use two dimensional numerical models that simulate various thrust systems allowing for a wide diversity of setups to illustrate this study we focus on intracontinental thrust systems for which all processes listed are likely to play a key role in the thermal evolution we perform a parametric study including kinematic settings i e convergence erosion and accretion thermal properties mechanical strength and heat sources ii dimensionless numbers based on parameters are used to quantify the relative contributions of each process to the thermal budget evolution hence the three thermal processes i e heat diffusion heat advection and shear heating are compared with each other via three dimensionless combinations of the peclet and brinkman numbers rdif radv and rpro respectively erosion and accretion are compared separately based on a fourth dimensionless number rea iii we analytically examine the inverted peak temperature recorded along profiles that are perpendicular to the thrust zone defined in our numerical experiments each peak temperature profile presenting an inversion can then be characterized by a function of approximation involving six meaningful parameters the location ff and width ff of the maximum peak temperature inversion the characteristic peak temperature tcte and gradient glb beneath the inversion zone and the inversion related contrasts in the peak temperature t and gradient g this coupled approach linking numerical modelling and analytical treatment allows to quantitatively interpret ims in terms of the processes involved the application of our method to intracontinental thrust systems demonstrates that shear heating and erosion support significant inversions but that the relative contributions of each process have meaningful consequences our results reveal that competition between shear heating and heat diffusion on the one hand and between erosion and accretion on the other hand have a high impact in particular the variability in the rock s mechanical strength strongly influences the features of peak temperature inversions consequently none of these processes can be ignored our results highlight the major importance of the rheology of rocks in the thermal evolution of shear zones finally our methodology is not only restricted to the analysis of numerical data but also constitutes a way of broad interest to analyse peak temperature signatures around any shear zone

diagenetic history of the surma group sandstones miocene in the surma basin bangladesh abstract this study examines the various diagenetic controls of the miocene surma group sandstones encountered in petroleum exploration wells from the surma basin which is situated in the northeastern part of the bengal basin bangladesh the principal diagenetic minerals cements in the surma group sandstones are fe carbonates with fe calcite dominating quartz overgrowths and authigenic clays predominantly chlorite illite smectite and minor kaolin the isotopic composition of the carbonate cement revealed a narrow range of 18o values 10 3 to 12 4 and a wide range of 13c value 1 4 to 23 1 the 13c vpdb and 18o vpdb values of the carbonate cements reveal that carbon was most likely derived from the thermal maturation of organic matter during burial as well as from the dissolution of isolated carbonate clasts and precipitated from mixed marine meteoric pore waters the relationship between the intergranular volume igv versus cement volume indicates that compaction played a more significant role than cementation in destroying the primary porosity however cementation also played a major role in drastically reducing porosity and permeability in sandstones with poikilotopic pore filling blocky cements formed in early to intermediate and deep burial areas in addition to fe carbonate cements various clay minerals including illite smectite and chlorite occur as pore filling and pore lining authigenic phases significant secondary porosity has been generated at depths from 2500 m to 4728 m the best reservoir rocks found at depths of 2500 3300 m are well sorted relatively coarse grained more loosely packed and better rounded sandstones having good porosities 20 30 and high permeabilities 12 6000 md these good quality reservoir rocks are however not uniformly distributed and can be considered to be compartmentalized as a result of interbedding with sandstone layers of low to moderate porosities low permeabilities owing to poor sorting and extensive compaction and cementation

diagenesis and reservoir quality evolution of incised valley sandstones evidence from the abu madi gas reservoirs upper miocene the nile delta basin egypt abstract elucidation of the distribution of diagenetic alterations in incised valley sandstones within a sequence stratigraphic framework allows a better understanding of their reservoir quality evolution during burial gas reservoirs of the upper miocene abu madi formation present day depth 3350 m temperature 116 the nile delta basin egypt consist of lowstand systems tract lst fluvial and transgressive systems tract tst estuarine sandstones deposited in an incised valley these sandstones have a wide range of porosity 2 to 29 and permeability 0 01 to 6071 md which reflect both depositional facies and diagenetic controls diagenetic events that influenced the reservoir quality evolution include mainly the formation of extensive grain rimming fe mg chlorite mechanical compaction of ductile grains pressure dissolution of quartz and quartz cementation the chlorite rims distributions of which are not clearly related to sequence stratigraphy have preserved reservoir quality by inhibiting quartz cementation but have increased microporosity values in the reservoirs calcite cementation 18o 13 5 to 6 0 13c 14 3 to 1 0 and 87sr 86sr 0 707124 to 0 708181 has not influenced the overall reservoir quality evolution of the sandstones because of its localized occurrence other diagenetic events that have little influence on reservoir quality include the more frequent formation of kaolin in the lst fluvial sandstones and formation of pyrite in the tst estuarine sandstones this study demonstrates the possibility of constraining and evaluating the impact of diagenetic alterations on reservoir quality evolution in incised valley deposits and their sequence stratigraphic distribution and thus has an important impact on hydrocarbon exploration in such settings

shear heating not a cause of inverted metamorphism an archetypal example of inverted metamorphism purportedly resulting from shear heating is found in the pelona schist of southern california united states recent studies demonstrate that the pelona schist was subducted and accreted at the onset of laramide fl at subduction under thermal and kinematic conditions not considered in earlier numerical models to test the shear heating hypothesis under these conditions we constructed a thermokinematic model of fl subduction initiation involving continuous accretion of the schist a neighborhood algorithm inversion demonstrates that available metamorphic and thermochronologic constraints in the sierra pelona mountains are satisfi ed only if accretion rates were 0 2 3 6 km m y and shear heating was minimal shear stress 0 19 mpa minimal shear heating is also consistent with an inversion of models constrained by thermochronology of the east fork of the san gabriel river exposure of the schist shear heating inhibits the formation of modeled inverted gradients during accretion and should not be considered an important factor in their generation

a dimensional analysis to quantify the thermal budget around lithospheric scale shear zones the thermal evolution around shear zones is controlled by three major thermal processes diffusion advection and shear heating we present a dimensional analysis to quantify to first order the relative contributions of these three processes to the thermal evolution around lithospheric scale shear zones we consider 11 parameters that control the kinematics the three dimensional 3 d geometry the initial thermal structure and the average strength of the shear zone three dimensionless parameters are presented to quantify the relative contributions of the three thermal processes we validate the dimensional analysis with 2 d thermo kinematic numerical models the applicability of the dimensional analysis to any kind of shear zone i e thrust normal slip and strike slip shear zones makes it a useful tool that is complementary to previous numerical and analytical studies finally thrust type shear zones are used to illustrate how the three thermal processes control the thermal evolution

the problem of glaucony from the shannon sandstone campanian wyoming terra nova 23 100 107 2011 abstract the shannon sandstone one of the most controversial units in the field of stratigraphy was first interpreted as a shelf sand ridge deposit but subsequent studies have furnished different interpretations from lowstand shoreface to incised valley models despite a general consensus among previous workers about the significance of glaucony for shannon interpretation mineralogical characterization of the green grains has never been undertaken identification of glaucony has been restricted to visual inspection accurate characterization of highly glauconitic deposits from the shannon sandstone powder river basin of wyoming section w3 reveals that sedimentological interpretation of the greensands relies mostly upon mistaken mineral identity glaucony appears as a minor component of the rock and proves to be poorly mature and significantly less abundant 0 6 than previously inferred up to 85 characterization of the glauconitic minerals however provides data that contribute to shannon interpretation

high resolution sequence stratigraphy sedimentology and reservoir quality evaluation of the yaojia formation in the longxi area of the western slope songliao basin china abstract delta front sand bodies with large remaining hydrocarbon reserves are widespread in the upper cretaceous yaojia formation in the longxi area of the western slope songliao basin china high resolution sequence stratigraphy and sedimentology are performed based on core observations well logs and seismic profile interpretations an evaluation of the reservoir quality of the yaojia formation is critical for further petroleum exploration and development the yaojia formation is interpreted as a third order sequence comprising a transgressive systems tract tst and a regressive systems tract rst which spans 4 5 myr during the late cretaceous within this third order sequence nine fourth order sequences fs9 fs1 are recognized the average duration of a fourth order sequence is approximately 0 5 myr the tst fs9 fs5 mostly comprises subaqueous distributary channel fills mouth bars and distal bars which pass upward into shallow lake facies of the tst top fs5 the rst fs4 fs1 mainly contains subaqueous distributary channel and interdistributary bay deposits based on thin sections x ray diffraction xrd scanning electron microscope sem and high pressure mercury intrusion hpmi analyses a petrographic study is conducted to explore the impact of the sedimentary cyclicity and facies changes on reservoir quality the yaojia sandstones are mainly composed of lithic arkoses and feldspathic litharenites the sandstone cements mostly include calcite illite chlorite and secondary quartz occurring as grain coating or filling pores the yaojia sandstones have average core plug porosity of 18 55 and permeability of 100 77 10 3 m2 which results from abundant intergranular pores and dissolved pores with good connectivity due to the relatively coarser sediments and abundant dissolved pores in the feldspars the fs4 fs1 sandstones have better reservoir quality than the fs9 fs5 sandstones developing relatively higher porosity and permeability especially the fs1 and fs2 sandstones the source reservoir cap rock assemblages were formed with the adjoining semi deep lake mudstones that were developed in the nenjiang and qingshankou formations this study reveals the deposition and distribution of the delta front sand bodies of the yaojia formation within a sequence stratigraphic framework as well as the factors controlling the yaojia sandstones reservoir quality the research is of great significance for the further exploration of the yaojia formation in the longxi area as well as in other similar lacustrine contexts

characterization of various stages of calcitization in porites sp corals from uplifted reefs case studies from new caledonia vanuatu and futuna south west pacific abstract this study focuses on the processes of calcitization under meteoric conditions based on observations and analyses of a series of subaerial fossil corals of the genus porites collected from emerged holocene and pleistocene reefs in the pacific ocean we can establish two types of low magnesian calcite lmc precipitates according to their textural characteristics after their transformation from the initial aragonitic skeleton to the calcitized corals in the first variety lmc1 the calcite contains remnants of the original exoskeleton structure in situ insoluble organic tissues pieces of aragonite needles while in the second variety lmc2 filling the primary inter skeletal macro pores such relicts are not present such textural segregation is faithfully reflected in the elemental data on some parts of calcitized corals lmc1 is characterized by mg2 sr2 and mn2 compositions inherited from the aragonite precursor indicating a formation in a semi closed intra skeleton diagenetic environment in disequilibrium with the meteoric bulk aquifer water lmc2 has a chemistry indicative of equilibrium with ambient bulk meteoric water these chemical characteristics can be likened to a two fold mechanism a fine scale process neomorphism typified by the concomitant dissolution of intra fabric aragonite and re precipitation of the lmc1 resulting from the biogenic skeleton and marine waters followed by the cementation of the inter fabric macro voids by an allochthonous subaerial and meteoric lmc2 in the other parts of calcitized corals lmc1 and lmc2 have similar trace element contents in mg2 and sr2 the chemical data are consistent with formation by partial skeletal dissolution followed eventually by cementation of primary voids lmc2 and secondary voids lmc1 by calcite in equilibrium with meteoric bulk aquifer water these two mechanisms of the calcitization of skeletal aragonite mainly differ in the space and time length scales of the intervening transport step and are commonly found juxtaposed in the same thin section taking into account such a heterogeneity is a real challenge for the development of predictive models assessing the effects of diagenetic products on paleoclimatic reconstructions

thermal effects of thrust faulting abstract calculations based on simple models of overthrust sheets in crystalline basement rocks show that significant thermal effects may result from their movements if rates are sufficiently high e g plate tectonic rates the thrust sheets sufficiently thick 5 10 and 15 km are modelled here the distances moved sufficiently large and for reasonable values of the coefficient of friction along the thrust plane overthrusting can cause metamorphic mineral zonations and heat flow anomalies observable in the field regions where large scale overthrusting has occurred should be characterized by a decrease with depth of grade of metamorphic mineral assemblages and anomalously low heat flow the theoretical effects are presented as a series of maximum temperature vs depth and heat flow vs time plots

diagenetic evolution and its impact on reservoir quality of tight sandstones a case study of the triassic chang 6 member ordos basin northwest china abstract the triassic chang 6 member sandstones are important oil reservoirs in the ordos basin of north western china and are actually considered to be typical tight sandstone reservoirs diagenetic alterations play a prominent role in considerably impacting the reservoir quality in this study an integrated approach of thin sections scanning electron microscopy sem x ray diffraction xrd cathodoluminescence cl fluid inclusions laser raman spectrographic analysis and stable isotope analysis was applied to examine the diagenetic history recorded in the chang 6 tight sandstones and investigate the influences of diagenesis on reservoir quality the sandstones consist of fine grained moderated to well sorted arkoses q48 5f40 9r10 6 and lithic arkoses q54 3f29 1r16 6 with an average porosity and permeability of 9 10 and 0 39 md respectively the sandstones have undergone complex diagenetic processes dominant eogenetic features include mechanical compaction leaching of feldspar in meteoric water chlorite coats originating from the transformation of precursor clays and precipitating directly from interstitial water calcites filling intergranular pores as poikilotopic cements and quartz and albite overgrowths on surfaces of grains subsequent mesogenetic events include ankerite cements filling porosity and replacing framework grains extensive dissolution of feldspars and precipitation of kaolinites due to emplacement of organic acids and secondary quartz overgrowths in the late period of mesogenesis early calcite is characterized by 13c 13cpdb ranging from 3 45 to 0 30 mean 2 00 and 18o 18opdb varying from 16 70 to 5 80 mean 10 50 while late ankerite is rich in 12c 13cpdb ranging from 10 71 to 0 80 mean 3 58 and 16o 18opdb varying from 24 30 to 15 80 mean 19 93 moreover calcite hosts single liquid fluid inclusions two phase fluid inclusions are contained in ankerite cements with mean homogenization temperature th of 96 0 c and salinity estimates from 11 47 to 18 96 eq wt nacl these data indicate that the calcite originated from meteoric water and the ankerite was associated with organic matter and recrystallization of the early calcite cements furthermore the metallic ions such as fe2 mg2 and ca2 required for ankerite precipitation were produced through alterations of volcanic fragments feldspars and biotites in comparison compaction has more important role than cementation in the loss of original porosity although cementation exerts enormous influence on reducing porosity quantitatively the compactional porosity loss reaches 54 9 while the cementation porosity loss accounts for up to 42 8 carbonate cements and the other authigenic products quartz and albite overgrowth and kaolinite mostly fill intergranular pores thus reduce porosity chlorite coats have effects in inhibiting compaction and restraining the development of quartz overgrowth thereby protect the intergranular porosity to some extent early secondary porosity was enhanced by leaching and dissolution of feldspar in the eogenetic period however these processes did not increase the net porosity in the mesogenetic stage

recent tectonics associated with the subduction collision of the d entrecasteaux zone in the central new hebrides abstract recent data from the new hebrides vanuatu subduction zone have enabled a morphological description and tectonic interpretation to be made of the complex plate contact between the d entrecasteaux zone and the west torres massif on the one hand and the old santo malekula block on the other between latitudes 14 30 and 17 s no trench occurs owing to the presence of the old rigid santo malekula block at the edge of the overriding plate the outer limit of plate contact describes a tortuous line westward of the projected line of the adjacent trenches this may be explained by local collision of the north d entrecasteaux ridge and the south d entrecasteaux chain with the west flank of santo this collision is compensated for by lateral plastic flow of the inner wall downward into the small basins which separate the d entrecasteaux zone ridges a process which may be explained by plastic plane strain theory the westward jump of the plate contact zone here may result from pieces of the north d entrecasteaux ridge being cut off and incorporated in the frontal arc a review of the geological evidence and the shallow seismicity of the central new hebrides shows that radiant horizontal stresses resulting from this collision have generated a specific compressive stress regime which influences the whole width of the arc the resultant deformation shows a close resemblance to a theoretical strain field produced by the indentation of a long narrow plastic body cf the arc by a rigid flat die cf the d entrecasteaux zone thus the central new hebrides block is both elastically bent under vertical stresses and plastically deformed and pushed eastwards by horizontal stresses recent uplift of the eastern chain can be explained by this model in contrast at the rear of both the northern and southern sections of the arc the tectonics are mainly controlled by a tensional stress regime the strong plate interaction due to the subduction of the d entrecasteaux zone is thought to be responsible for the compressive stress regime across the central new hebrides the same cause can be invoked to account for the tensional stress regime observed in north and south new hebrides rear arc regions

infiltrated clays in fluvial jurassic sandstones of reconcavo basin northeastern brazil abstract fluvial sandstones and conglomerates of the sergi formation a jurassic unit of reconcavo basin northeastern brazil contain significant amounts of interstitial detrital clays which are interpreted as the product of early mechanical infiltration developed in an arid semi arid environment within such an environment the lowered water table allowed muddy waters of episodic runoff to infiltrate through the coarse alluvium concentrating the clays in the upper phreatic zone near the sources of influent seepage petrographic textures considered to be typical of mechanically infiltrated mi clays are herein presented using examples from the sergi formation these textures include 1 ridges and bridges 2 geopetal fabrics 3 loose aggregates 4 anisopachous coatings of tangentially accreted lamellae cutans 5 massive aggregates and 6 shrinkage patterns developed during diagenesis in addition their detrital aspect as revealed by sem observations and the presence of impurities in the interior of the clay aggregates were also used as evidence for the interpreted origin of the clays in most hydrocarbon reservoirs in the sergi formation the clays described in this paper are among the main controls on porosity and permeability distribution commonly zones with maximum clay concentration are the main internal permeability barriers within the sergi formation reservoirs

viscous heating allows thrusting to overcome crustal scale buckling numerical investigation with application to the himalayan syntaxes article i nfo the eastern and western himalayan syntaxes are crustal scale antiforms whose metamorphic evolution is coeval in the india asia collisional history finite element modelling of lithospheric shortening and the resultingcrustal scale folds havepreviously been used todebate the structuralconditions for the development of such structures however numerical models could not account for the metamorphic evolution associated with folding in continuation of previous work we present two dimensional finite element modelling of lithospheric shortening inwhich thermal effects are implemented the new models are consistent with earlier interpretations that crustal folding is the plausible mechanism that shaped the two himalayan syntaxes they further lend support to the concept that lithospheric buckling is a basic response to large scale continental shorteningandan efficientmountainbuilding process introducingviscous shearheating shows however that the buckling mode shifts to a thrusting mode within few shortening depending on the initial thermal rheological structure of the deforming lithosphere the change to the thrusting mode prohibits further fold amplification and lateral fold propagation this mayexplainwhy in continental lithosphere crustal scale folds are isolated whereas regular periodic crustal fold trains are rare focusing deformation on through limbs thrust zones accompanies the establishment of inverted metamorphic gradients these results offer new working hypotheseson howlargethrusts like thehimalayamain centralthrustnucleate results further show that the thermal structure of the lithosphere strongly controls three fundamental deformation and metamorphic modes 1 a cold lithosphere mainly deforms by thermally activated thrusting and exhibits large areas with significant tectonic overpressures twice lithostatic 2 a warm lithosphere is essentially buckled and significant tectonic overpressure builds up in the upper crust and 3 a hot lithosphere tends to thicken homogenously and mainly records lithostatic pressures below the upper crust

factors controlling reservoir properties and hydrocarbon accumulation of the eocene lacustrine beach bar sandstones in the dongying depression bohai bay basin china abstract lacustrine beach bar sandstone reservoirs are economical important exploration targets in the dongying depression bohai bay basin the factors that controls reservoir quality and hydrocarbon accumulation in lacustrine beach bar sandstones were systematically investigated through an integrated petrographic petrophysical and diagenesis analysis the reservoir properties of the beach bar sandstones are mainly affected by compaction carbonate cementation and dissolution significant negative relationships exist between both the porosity and permeability and the volume of carbonate cement carbonate cement is mainly distributed along the sandstone mudstone contacts sandstones with thicknesses less than 1 m are generally tightly cemented to form zones with low petrophysical properties the material source of the carbonate cement in the tightly cemented zone was highly likely from the interbedded mudstone content of carbonate cements in the center of the thick sandstones is low whereas compaction is the dominant diagenetic process with some dissolution there is a positive correlation between the petrophysical properties of the beach bar sandstones and the content of kaolinite the petrophysical properties have an inverse correlation with the contents of illite and illite smectite mixed layer clays reservoir petrophysical property largely controls hydrocarbon accumulation in the beach bar sandstones compared with non oil bearing sandstones oil bearing sandstones have higher porosity and a larger pore throat size sandstones with a porosity of less than 8 5 and or low permeability of less than 1 02 10 3 m2 are mostly incapable of accumulating oil

stratigraphic control on laterally persistent cementation book cliffs utah field data from the upper cretaceous blackhawk formation in the book clitfs utah colorado demonstrate the presence of laterally extensive carbonate cements beneath major marine flooding surfaces the occurrence of firmgrounds compaction around concretions and high minus cement porosities indicate that cementation occurred during early diagenesis it is proposed that the early cementation associated with these marine flooding surfaces occurred as a result of reduced sediment accumulation rates or non depositional hiatuses linked to abrupt deepening during marine flooding these reduced sediment accumulation rates extended sediment residence time within early diagenetic zones leading to enhanced early cementation in shallow marine sandstones and mudstones

meteoric water incursion into marine turbditic sandstones evidence from the andrew formation paleocene uk central graben north sea abstract petrographic and stable carbon and oxygen isotope study revealed extensive meteoric water incursion during diagenesis of deep water marine paleocene turbidite sandstones in the north sea andrew formation uk central graben meteoric water diagenesis resulted in the dissolution and kaolinitization of framework silicates and cementation by calcite and methanogenic siderite and fe dolomite ankerite incursion is envisaged to have occurred over large distances along well connected permeable shelf slope and turbidite sand bodies during major fall in sea level which resulted in exposure of the continental shelf this work urges for consideration of the impact of undersaturated meteoric waters on reservoir quality modifications during hydrocarbon exploration in marine turbidite sandstones

major role of shear heating in intracontinental inverted metamorphism inference from a thermo kinematic parametric study abstract inverted metamorphism corresponds to the stacking of high temperature metamorphic units structurally on top of lower temperature units and is commonly observed along main thrusts in major orogens yet in spite of many existing models the origin and preservation of the metamorphic inversion in intracontinental collision belts are still debated in this study we use a crustal scale 2d thermo kinematic model in order to investigate the key parameters controlling the inversion of the geothermal gradient at crustal scale our results confirm that the kinematic framework strongly impacts the thermal evolution around the thrust erosion velocity and thermal conductivity of rocks are two parameters that control the spatial location of the thermal perturbation and the intensity of inversion respectively however even in extreme kinematic configurations i e convergence velocities 3 cm yr 1 and relatively high thrust dip angles 30 the thermal inversion is fleeting and thrust temperatures cannot reach the high temperature peak values characteristic of natural occurrences 600 c if shear heating is not taken into account conversion of mechanical energy into heat represents a main contribution to the thermal budget along main crustal shear zones it leads to high temperature conditions in the thrust zone and our results attest that it is the only process that allows the preservation through time of an intense thermal inversion our quantification shows that shear heating is much more efficient than other processes such as accretion and surface denudation and is compatible with the observations of inverted metamorphism in the himalayan or variscan belts for example this comparison with natural occurrences suggests that the formation and preservation of intracontinental inverted metamorphism require shear zone viscosity values of the order of 10 20 10 21 pa s for convergence velocities between 1 and 3 cm yr 1

comparison of outcrop and subsurface sandstone permeability distribution lower cretaceous fall river formation south dakota and wyoming quantification of permeability structure in outcrop reservoir analogs documents the distribution of flow units and barriers important for reservoir management and simulation differences in burial history and diagenesis of the outcrop analog and the subsurface reservoir must be quantified however before outcrop permeability data can be used to model subsurface strata appropriately a case study of the lower cretaceous fall river formation shows that permeability differences between facies are accentuated by diagenesis and permeability variation increases with the extent of burial diagenesis the fall river formation which is exposed in outcrop around the black hills uplift in wyoming and south dakota usa produces oil from fluvial and estuarine valley fill sandstones in the adjacent powder river basin outcrop sandstones were buried to 2 km before being uplifted at the end of the cretaceous whereas reservoir sandstones remained at depths of 2 4 km throughout the tertiary the permeability of outcropping fall river sandstones has a log normal distribution the main controls on permeability in outcrop sandstones are 1 ductile grain content 2 grain size both properties being controlled by the energy of the depositional environment and 3 hematite cement an uplift related diagenetic feature not present in the subsurface hematite was preferentially precipitated along zones of permeability contrast including sequence boundaries complicating the determination of changes in primary depositional permeability at important flow unit boundaries in contrast the permeability of fall river reservoir sandstones at a depth of 4 km has a bimodal distribution and is controlled by ductile grain content grain size and quartz cement volume quartz cementation and compaction due to ductile grain deformation were more extensive in the estuarine facies resulting in a greater permeability reduction during burial diagenesis than was observed in fluvial sandstones abundant stylolites developed in sandstones with clay partings or mud clasts features that are common in estuarine sandstones the stylolites acted as an internal source of silica sandstones near the stylolites typically have low permeability that probably resulted from above average volumes of quartz cement diagenesis increased the coefficient of variation of permeability in all facies in the subsurface this diagenetic overprint should be considered if outcrop permeability data are used to model a subsurface reservoir having a very different burial history

impact of structural setting on diagenesis of fluvial and tidal sandstones the bahi formation upper cretaceous nw sirt basin north central libya abstract this petrographic mineral chemical fluid inclusion and stable isotopic study shows that the distribution of diagenetic modifications and their influence on reservoir quality and heterogeneity in tidal and fluvial sandstones of the upper cretaceous bahi sandstones in the rift sirt basin nw libya varies systematically along a series of closely spaced dominantly normal faults between the basin margin and more basinward located areas shallow eogenetic modifications resulting from the percolation of meteoric waters include infiltration of grain coating clays kaolinitization of detrital silicates and cementation by dolomite and k feldspar overgrowths mesogenetic alterations 70 c 2 km include feldspar albitization illitization of infiltrated clay and kaolinite conversion of kaolinite into dickite and cementation by quartz overgrowths t h 112 c 134 c barite t h 145 c 158 c and fe carbonates the restriction of barite and fe carbonate cements to the basinward located sandstones suggests formation by hydrothermal fluids along the faults extensive feldspar dissolution and formation of moldic pores in sandstones from the basin margin were probably caused by deep percolation of meteoric waters results from this study regarding the structural control on the spatial distribution of diagenetic alterations have implications for constraining the flux of pore fluids and by extension reservoir quality in analogous epicratonic rift basins

impact of diagenesis on reservoir quality evolution of the late cenomanian abu roash g member in the sitra field north western desert egypt abstract reservoir quality evolution of the abu roash g member was studied by petrographical analyses from wells in the sitra oilfield in the north western desert of egypt the sandstones of the abu roash g member are predominately subarkose to quartz arenite and exhibit a wide range of porosity 1 2 23 8 and permeability 0 008 126 md diagenetic features that influenced the reservoir quality evolution include mechanical compaction pressure solution cementation and dissolution of framework grains and cements compaction and cementation are generally the main factors that were responsible for the reduction of porosity and permeability in the upper abu roash g sandstones dissolution of detrital feldspars grains was the main drive for porosity enhancement of the upper abu roash g sandstones additionally the corroded contacts between successive carbonate phases and quartz grains followed by dissolution of the carbonate cements left behind secondary porosity on the other hand the main porosity preservation in the upper abu roash g sandstones is due to scattered patches of carbonate cement which prevented compactional collapse of the sandstone framework and to grain coating chlorites that inhibited the precipitation of quartz overgrowths during burial the abu roash g member reservoir was subjected to a high diagenetic overprint resulting in marked reservoir heterogeneity

porosity preserving chlorite cements in shallow marine volcaniclastic sandstones evidence from cretaceous sandstones of the sawan gas field pakistan sandstones that have high porosity and permeability at great burial depth and high temperatures are of economic importance because a significant amount of hydrocarbons have been discovered in such reservoirs the sawan gas field with an expected ultimate recovery of more than 1 tcf lies in the middle indus basin the reservoir rocks cretaceous volcaniclastic sandstones of the lower goru formation show very high porosities at a reservoir temperature of 175c and depths of 3000 to 3500 m 9842 to 11 483 ft the sandstones are mostly feldspathic litharenites strongly altered volcanic rock fragments are the most important lithic component the clay fraction consists of fe rich chlorite chamosite and illite diagenetic features such as compaction quartz overgrowths carbonate cements and feldspar dissolution are observed the most distinguishing feature is a double layer of authigenic chlorite lining the pores of the sandstones chlorite additionally occurs as a pore filling cement and as chloritized detrital components all having similar chemical composition the pore lining cement clearly developed in two stages an earlier poorly crystallized and a later better crystallized growth missing rims at grain contacts show that precipitation occurred after an initial stage of compaction but early relative to other diagenetic phases both chlorite rims grew by direct precipitation from pore waters using products derived from volcanic rock fragments in areas with no thin or discontinuous chlorite rims quartz cementation is common well developed chlorite rims inhibited quartz cementation preserved porosities of up to 20 and good permeabilities porosity preserving chlorite cementation in sawan is restricted to sediments of a shallow marine environment

architecture of fluvial dominated valley fill deposits in the cretaceous fall river formation the fall river formation is a 45 m thick layer of fluvial dominated valley fills and shore zone strata deposited on the stable cratonic margin of the cretaceous western interior seaway fall river deposits in red canyon in the south west corner of south dakota usa expose a cross section of a 3 5 km wide valley fill sandstone and laterally adjacent marine deposits the marine deposits comprise three 10 m thick upward shoaling sequences each composed of multiple metres thick upward coarsening successions the lower two of these sequences are laterally cut by the valley fill sandstone and are capped by metres thick muddy palaeosols the upper sequence spans the top of the valley fill sandstone and is overlain by the skull creek shale the 30 m thick valley sandstone is partitioned into four distinct fills by major erosion surfaces and each of these fills contain many metres thick channel form bodies deposits in the lower parts of these fills are sheet like top truncated channel bodies whereas deposits in the upper parts of fills are upward concave laterally amalgamated channel bodies more completely preserved heterolithic channel bodies or wave deposited sheets each valley fill basal erosion surface records an episode of valley incision and relative sea level fall and the gradual progression from fluvial to more estuarine deposits upwards within each fill records relative sea level rise all fills are dominantly channel deposits and are capped by marine flooding surfaces the dominance of channel deposits the gradual change to more estuarine facies in the upper parts of fills and the location of flooding surfaces at valley fill tops all suggest that sediment supply initially kept pace with relative sea level rise and valleys filled during late marine lowstand and transgression not during subsequent highstands recently proposed facies models have focused on variations in the relative strength of tide wave and river currents as controls on valley fill deposits however relative rates of sediment supply and basin accommodation change and the shift in this ratio along the depositional profile during multiple scale cycles in relative sea level are equally important controls on the style of valley fill deposits

composition porosity and reservoir potential of the middle jurassic kashafrud formation northeast iran abstract in the kopet dagh basin of iran deep sea sandstones and shales of the middle jurassic kashafrud formation are disconformably overlain by hydrocarbon bearing carbonates of upper jurassic and cretaceous age to explore the reservoir potential of the sandstones we studied their burial history using more than 500 thin sections supplemented by heavy mineral analysis microprobe analysis porosity and permeability determination and vitrinite reflectance the sandstones are arkosic and lithic arenites rich in sedimentary and volcanic rock fragments quartz overgrowths and pore filling carbonate cements calcite dolomite siderite and ankerite occluded most of the porosity during early to deep burial assisted by early compaction that improved packing and fractured quartz grains iron oxides are prominent as alteration products of framework grains probably reflecting source area weathering prior to deposition and locally as pore fills minor cements include pore filling clays pyrite authigenic albite and k feldspar and barite existing porosity is secondary resulting largely from dissolution of feldspars micas and rock fragments with some fracture porosity porosity and permeability of six samples averages 3 2 and 0 0023 md respectively and 150 thin section point counts averaged 2 7 porosity reflectance of vitrinite in eight sandstone samples yielded values of 0 64 0 83 in the early mature to mature stage of hydrocarbon generation within the oil window kashafrud formation petrographic trends were compared with trends from first cycle basins elsewhere in the world inferred burial conditions accord with the maturation data suggesting only a moderate thermal regime during burial some fractures iron oxide cements and dissolution may reflect cenozoic tectonism and uplift that created the kopet dagh mountains the low porosity and permeability levels of kashafrud formation sandstones suggest only a modest reservoir potential for such tight sandstones fractures may enhance the reservoir potential

kaolinized potassium leached zones at the contacts of the garn formation haltenbanken mid norwegian continental shelf abstract the middle jurassic garn formation consists mainly of subarkosic arenite in which potassium feldspar is several times as abundant as plagioclase in cores from eight wells the top of the formation upper 6 12 m appears to have undergone relatively intense alteration of potassium feldspar to kaolinite with the resulting loss of bulk postassium content this could be explained as a result of subaerial weathering of the top of the sandstone body except for the fact that in two of the wells the same pattern of alteration is observed at the base of the formation an alternative explanation is that alteration resulted from the introduction of acid released by the maturation of organic matter in adjacent shales in all instances the altered zones display a gradual increase in kaolinite content and a decrease in the ratio of bulk rock potassium aluminium over a distance of several metres approaching the formation boundary this pattern indicates that the active species responsible for the alteration entered vertically from the formation contacts rather than being introduced by lateral flow along the sandstone bed the cores studied are tentatively interpreted as recording different stages in the following depth related sequence of diagenetic events 1 early kaolinization throughout the formation by meteoric water flushing 2 formation of additional kaolinite near the formation contacts by the influx of acid from surrounding shales at approximately 2 7 km depth below the sea floor and 3 extensive alteration of kaolinite to illite at approximately 3 7 km depth illitization has been impeded in the leached zones near the formation contacts due to a local deficiency in reactive potassium here kaolinite survives unaltered whereas kaolinite throughout the rest of the formation has been extensively replaced by illite this implies that the illitization of sandstone can occur by short range diffusion in a chemically closed system solely in response to increasing thermal exposure at greater depth

pore fluid evolution distribution and water rock interactions of carbonate cements in red bed sandstone reservoirs in the dongying depression china the compositions distribution and its interaction with rocks of the evolving pore fluids controls the distribution of carbonate cements and reservoir storage spaces the reservoir quality of the red bed sandstone reservoirs in the dongying depression was investigated by an integrated and systematic analysis including carbonate cement petrology mineralogy carbon and oxygen isotope ratios and fluid inclusions the investigation was also facilitated by probing the mineral origins precipitation mechanisms pore fluid evolution and distribution and water rock interaction of carbonate cements and their influences on reservoir quality diagenetic evolving fluids in the interbedded mudstones are the main source for the precipitation of calcite cements that completely fill the intergranular volume cfiv calcite with heavier oxygen and carbon isotopes the ferro carbonate cements in the reservoir sandstone are enriched in lighter carbon and oxygen isotopes in addition to the cations released by the conversion of clay minerals in reservoirs products of organic acid decarboxylation and the associated feldspar dissolution process provide important sources for such carbonate cementation the carbon isotopes of co2 and the oxygen isotopic composition of fluids equilibrated with the cfiv calcite ferro calcite dolomite and ankerite cements indicate that the pore in the red bed reservoirs experienced high salinity fluids which evolved from the early formed interbedded mudstones through organic acid input and to organic acid decarboxylation pore fluids from nearby mudstones migrated from the edge to the centre of sandbodies causing strong calcite cementation along the sandbody boundaries and forming tight cementation zones pore fluids associated with organic co2 and acids and organic acid decarboxylation are mainly distributed in the internal portion of sandbodies causing feldspar dissolution and precipitation of ferro carbonate cements the distribution of pore fluids caused the zonal distribution of carbonate cements in sandbodies during different periods this may be advantageous to preserve the porosity of reservoirs as exemplified by the distribution of high quality reservoirs in the red bed sandbodies

first insights into mid holocene environmental change in central vanuatu inferred from a terrestrial record from emaotfer swamp efate island abstract here we present the first terrestrial record of mid holocene environmental changes in vanuatu this preliminary multi proxy analysis of core tfer 06 from emaotfer swamp efate island indicates changes in environmental conditions are mainly related to variations in climate over the last 6500 cal yr bp drier periods are broadly correlated with an increase in sustained el nino events recorded in the pacific on a decadal timescale the earliest change is the disappearance of mangroves adjacent to the site around 3200 cal yr bp this could well be due to both local tectonic uplift with subsequent hydrostatic adjustment and the onset of a drier period from c 3250 2500 cal yr bp the prevailing drier conditions can be linked to more persistent el nino conditions local volcanic events had limited ecological impact on the area freshwater diatoms indicate a hydroseral succession species living on submerged plants being common in muds from c 3250 1500 cal yr bp but rare in fibrous peat deposited later palaeoecological indicators of human impact have not been identified throughout this work

geochemistry and diagenesis of stratabound calcite cement layers within the rannoch formation of the brent group murchison field north viking graben northern north sea comment

shear zone and nappe formation by thermal softening related stress and temperature evolution and application to the alps we present the results of two dimensional thermo mechanical numerical simulations of the formation of kilometre scale shear zones and tectonic nappes which are generated by thermal softening during horizontal shortening this model is thermo mechanically coupled and conserves energy by converting mechanical work into heat the results show that in homogeneous material the temperature increase in shear zones caused by viscous heating can be 100 c the characteristic width of the thermally perturbed part of the deforming zone is three times larger than the width of the corresponding shear zone because of diffusive heating of the wall rock therefore temperature variations initiated by shear heating may be difficult to identify based on petrological inferences of field data during lithospheric shortening with a bulk rate of 2 5 10 15 s 1 a nappe forms in the crust at a depth of 45 km peak temperatures in the nappe reach 600 c 100 c increase due to shear heating significant shear heating occurs only locally and is not active continuously in time along the entire crustal shear zone peak values of the maximal principal stress locally reach 2 5 gpa in the nappe peak values of temperature and maximal principal stress increase locally to 770 c 300 c increase due to shear heating and 3 8 gpa respectively if the applied bulk shortening rate is 5 10 15 s 1 and if higher effective viscosities for the crust are applied in the model lithosphere high differential stresses 500 mpa and significant stress deviations from the lithostatic pressure 50 occur only locally and are transient the highest average effective viscosity of the model lithosphere is 2 1022 pa s which is in agreement with the estimates for the continental lithosphere a characteristic feature of the presented dynamic nappe model is a significant stress decrease and cooling within a hundred thousand to 2 million years these rates of decompression and cooling are discussed in the context of microstructural and metamorphic observations the results also show that differential stresses in mature shear zones can be several hundred mpa smaller than at the onset of shear zone formation which suggests that estimates based on microstructural analysis of rocks in ductile shear zones provide minimum values the presented model could explain the formation of coherent alpine nappes with inferred high pressure 1 5 2 5 gpa and ultrahigh pressure 3 gpa conditions in 40 60 km depth at the base of an orogenic wedge

rates of uplift investigated through 230th dating in the gulf of corinth greece abstract the sr u and th isotope compositions of fourteen coral samples from raised terraces on the south side of the gulf of corinth greece have been analysed by high precision mass spectrometry 234u 230th ages of pure coral aragonite together with 87sr 86sr determinations provide the basis for an investigation of tectonic uplift rates in the w aegean over the last 350 ka the corinth basin is a continental fault bounded depression formed by crustal extension northeast of subduction along the hellenic trench and in the vicinity of the aegean volcanic arc the sr isotope composition of present day seawater in the gulf of corinth is indistinguishable from that in the north atlantic but this was not the case during earlier high stands 120 190 and 240 ka ago when 87sr 86sr ratios as estimated from the corals were apparently significantly lower than the ratios of their open ocean counterparts although the cause of the earlier discrepancy between the two reservoirs is unknown it is likely to result from one of two processes either circulation between the gulf of corinth and the open ocean may have been restricted at certain earlier highstands or the lower sr isotope compositions could be the result of diagenetic effects the latter hypothesis is favoured by large variations in the initial 234u 238u ratio of the corals by their unusually high th contents and by the observation that the underlying massive miocene carbonate is severely altered the ages of the corals as determined by mass spectrometry ranges between 70 2 and 385 5 ka for samples from terraces at elevations of 7 to 65 m above present sea level a mean uplift rate for this area of 0 3 mm yr over the last 350 ka has been estimated by combining the coral ages with terrace heights allowing for changes in sea level during this period the uplift here is probably caused by footwall uplift adjacent to large normal faults and put constraints on likely fault slip rates extension rates and the timescales of earthquake recurrence

soulevement et tectonique de l ile d efate vanuatu arc insulaire des nouvelles hebrides au cours du quaternaire recent datations de terrasses soulevees par la methode abstract the uplift of vanuatu formerly new hebrides is related to the subduction of the australian plate beneath the pacific plate the tropical latitude of these volcanic islands favours important quaternary growth of coral reefs during the periodic uplifts several terraces emerged forming typical features of the landscape when eustatic sea level rise is compensated by the uplift relative sea level is therefore constant and favours development of terraces efate situated near the centre of vanuatu archipelago is related to this series of uplifts and reef crest facies lie discordantly on a volcanic substratum thirty five new u th datings of corals in growth position confirm high eustatic sea levels at about 82 000 103 000 and 125 000 yrs b p and perhaps at 140 000 yrs b p the amplitude of the vertical movements is estimated quantitatively from a geomorphological study of the terraces complemented by the dates and the positions of the corals a calculation of the average uplift rates permits the distinction of four major areas on the western part of efate the numerous faults in these areas also favour minor local uplifts the northwestern area is affected by a differential uplift which at about 180 000 yrs b p formed a broad terrace sloping northward from 80 to 50 m the younger about 103 000 and 125 000 yrs b p and the older 300 000 yrs b p terraces are narrower generally the uplift was not uniform during any particular stage of sea level the average uplift rate here ranges from 0 2 to 0 6 mm yr the western area is the best known and shows four prominent major pleistocene terraces their ages range from 103 000 to 300 000 yrs b p but the latter age is uncertain due to the method used and to the high rate of calcification of the samples most of the faults are radially oriented and perpendicular to the terraces forming samoa point the average uplift rate in the western area is 0 9 1 mm yr more than in the north the southwestern area displays two remarkable series of terraces ranging from 50 to 110 m in altitude and dating from 86 000 to 130 000 yrs b p the average uplift rate is 0 8 mm yr the southern area close to port vila is very low in altitude as low as 50 m and bears three terraces dated at 40 000 85 000 and 125 000 yrs b p the uplift rates were 1 mm yr from 40 000 yrs b p to the present but before this time were 0 3 mm yr altitudes as a function of age display a remarkable clustering the levels given by a frequency diagram confirm the stages of sea level previously defined by other authors these results also coincide with the curve of changes in sea level the different average uplift rates between high stages in sea level are shown and appear to be variable they change from one area to the other comparatively slowly between 103 000 and 10 000 yrs b p and generally faster from 10 000 yrs b p to the present a possible effect of a recent local tectonic uplift these periodic uplifts are less important here than in the northern islands of santo and malekula which are affected by the subduction of d entrecasteaux ridge

thermal evolution of the scandian hinterland naver nappe northern scotland the northern highlands terrane of scotland hosts several thrust nappes that were deformed and metamorphosed during the silurian scandian orogeny quantitative petrological analysis of metamorphic assemblages indicates that the hinterland positioned naver nappe experienced decompression heating from 8 9 kbar and 600 c to 6 7 kbar and 700 c monazite xenotime thermometry and geochronology delineate a detailed temperature time history for the naver nappe monazite often exhibits compositional zoning which is used to establish multiple temperature time points in several samples these data indicate that the naver nappe experienced relatively fast heating c 50 c myr 1 and relatively slow cooling 15 20 c myr 1 with peak temperatures occurring at c 425 ma this temperature time evolution is compatible with the early emsian 407 403 ma deposition of unmetamorphosed conglomerates that rest on high grade metamorphic rocks in the naver nappe but requires an acceleration in the cooling rate to 40 50 c myr 1 at 420 410 ma geochronological constraints from this study and previous work suggest that deformation and metamorphism in the hinterland of the scandian orogen in northern mainland scotland are younger than the c 430 ma deformation in the foreland positioned moine thrust zone we postulate that heat from pervasive granitic intrusions in the naver nappe weakened the crust allowing deformation to retreat to the hinterland of the orogen supplementary material a description of our analytical methods all u pb trace element data additional figures explaining our petrological analysis and other relevant data are available at https doi org 10 6084 m9 figshare c 4458041

carbonate cementation in a sequence stratigraphic framework upper cretaceous sandstones book cliffs utah colorado abstract three macroscopic diagenetic features can be recognized in the sandstones of the upper cretaceous desert member of the blackhawk formation and castlegate sandstone of the mesaverde group exposed in the book cliffs utah each of which have distinctive form geometry and stratigraphic distribution diagenetic alterations are 1 leached zones whitecaps up to 10 m thick beneath coal beds 2 large up to 8 m concretionary carbonate cemented bodies in amalgamated shoreface and thin fluvial sandstones and 3 thin up to 2 m laterally extensive carbonate cemented horizons beneath major marine flooding surfaces each feature has distinct petrographic and geochemical signatures and formed through discrete diagenetic processes large isolated carbonate cemented bodies are composed of ferroan dolomite most of which precipitated during early diagenesis field and petrographic data coupled with stable isotope data early cements 13c 2 5 to 3 4o oo vpdb 18o 7 8 to 12 0o oo vpdb 87sr 86sr 0 7078 later cements 13c 3 1 to 5 7o oo vpdb 18o 12 0 to 15 1o oo vpdb 87sr 86sr 0 7093 suggest precipitation from meteoric fluids input into sediments during times of relative sea level fall the source of carbonate for the dolomite cement was dissolution of detrital dolomite from beneath coals by organic acids and subsequent mobilization by meteoric fluids carbonate precipitation in laterally extensive cement horizons appears to have started as a result of hiatus in sediment accumulation during marine flooding events relative sea level rise cement precipitation in these horizons continued through sediment burial as a result of organic matter oxidation reactions in overlying organic rich mudstones the results of this study show a link between sedimentation related to changes in relative sea level and diagenesis leading to the potential for the development of process based predictive models of early diagenesis in depositional successions

albitization of detrital plagioclase in triassic reservoir sandstones from the snorre field norwegian north sea abstract detrital plagioclase an12 an28 in triassic reservoir sandstones of the lunde formation in the snorre field offshore norway has been partially to completely albitized during burial diagenesis depths of about 2 500 3 000 m 75 100 c present temperature calcite caco3 99 mole and kaolinite were formed as by products of plagioclase albitization sodium rich detrital plagioclase an2 an11 are usually unalbitized detrital k feldspar is fresh or rarely partly albitized differences in albitization behavior of detrital plagioclase and k feldspar are shown to be related to the chemistry of formation waters reactivity of kaolinite and temperature the diagenetically formed albite pseudomorphs are very pure ab 9 well ordered low albite dark luminescent vacuolized mostly untwinned and comprised of numerous euhedral albite crystals some of the microporosity within the albite pseudomorphs is caused by volume reduction during albitization of the detrital plagioclase the amount of albite formed in the pseudomorphs is related to the molar proportion of albite component in the detrital unstable ca rich plagioclase the microporosity formed during albitization is proportional to the anorthite component which is believed to have dissolved and resulted primarily in the formation of calcite and kaolinite the albitization of detrital plagioclase in the studied sandstones can thus be explained without assuming the consumption of significant amounts of na from pore solutions

distribution of diagenetic alterations in fluvial deltaic and shallow marine sandstones within a sequence stratigraphic framework evidence from the mullaghmore formation carboniferous nw ireland abstract the distribution of diagenetic alterations in the fluvial deltaic and shallow marine arkosic to subarkosic sandstones average q72f26l02 of the mullaghmore formation carboniferous nw ireland can be predicted within a sequence stratigraphic framework eogenetic calcite 18opdb 13 3 to 6 5 13cpdb 3 0 to 3 4 and 87sr 86sr 0 706721 to 0 709227 and ferron dolomite feco3 8 12 mol 18opdb 14 2 to 7 8 13cpdb 1 4 to 1 0 and 87sr 86sr 0 709051 to 0 709167 occur in bioclast rich transgressive lag deposits at parasequence boundaries and transgressive surfaces and in wave influenced deltaic highstand systems tract hst deposits mesogenetic illite chlorite baroque dolomite feco3 16 mol 18opdb 14 2 to 12 7 13cpdb 3 8 to 1 0 quartz and calcite 18opdb 15 7 to 12 5 13cpdb 5 8 to 3 7 and 87sr 86sr 0 709016 to 0 709122 were formed mainly in the bioclast poor deposits which were not pervasively cemented by carbonates during near surface eodiagenesis these deposits include fluvial incised valley sandstones of lowstand systems tract lst and fluvial dominated deltaic sandstones of transgressive systems tract tst and hst illite is the dominant diagenetic clay mineral in the fluvial incised valley sandstones of lst possibly because of simultaneous albitization of k feldspars conversely chlorite dominates in the fluvial dominated deltaic sandstones of tst and hst because of the presence of suitable precursor clays the integration of diagenesis into sequence stratigraphic framework of clastic sequences should improve the ability to predict the spatial and temporal distribution of diagenetic alterations and related reservoir quality modifications of sandstone deposits

dolomite diagenesis and porosity preservation in lithic reservoirs carmopolis member sergipe alagoas basin northeastern brazil the lithic sandstones and conglomerates of the carmopolis member of the muribeca formation aptian were deposited bp fan deltas alluvial fans and braid deltas that prograded from low grade metamorphic terrains into the sergipe alagoas rift basin during

the interpretation of inverted metamorphic isograds using simple physical calculations several zones of major thrust faulting exhibit a juxtaposition of rocks of higher metamorphic grade over rocks of lower grade this configuration may indicate but does not require that temperature gradients were temporarily inverted near the fault we examine the physical conditions under which such a temperature inversion could occur the overthrusting of hotter rock on colder rock can cause a temporarily inverted gradient both above and below the fault only if the time taken to underthrust rock from the land surface to a given depth z on the fault is less than times the characteristic time for diffusion of heat through the block above the fault where is thermal diffusivity an inverted gradient cannot form without heat generation in the fault zone unless v z sin 100 where v is the rate of underthrusting in millimeters per year z is in kilometers and is the dip of the fault this simple criterion is sufficient to demonstrate that several examples of inverted metamorphic gradients cannot be explained simply by the thrusting of hot on cold rock without heat sources in the fault zone dissipative heating accompanying deformation can cause an inverted temperature gradient within and beneath the thrust zone but whereas the overthrusting of hot upon cold rocks cools the fault zone dissipation heats it thus the overthrusting of hot on cold rock and dissipative heating affect the temperature gradient and the maximum temperatures differently we show that the magnitudes of the inverted temperature gradient and of the maximum temperature above the inverted gradient yield independent estimates of the rate of dissipative heating discrepancy between these estimates implies that some additional process must have occurred such as the post thrusting disruption of the isograds if there is such a discrepancy the maximum temperature probably provides the more reliable estimate of the rate of heating at the fault we illustrate this analysis by applying it to reported inverted metamorphic zonation in the pelona schist the st anthony complex the mt everest region of the main central thrust and the olympos thrust petrological inferences of maximum temperatures depths of metamorphism and magnitudes of apparent inverted gradients imply that shear stresses of about 100 mpa accompanied thrust faulting in some of these regions and that some zones of inverted metamorphism have been tectonically thinned after the metamorphism occured

impact of sequence stratigraphy depositional facies and diagenesis on reservoir quality a case study on the pennsylvanian taiyuan sandstones northeastern ordos basin china abstract the pennsylvanian taiyuan formation of the shenmu field in the northeastern ordos basin of china contains tight sandstone reservoirs that have recently been commercially produced an evaluation of the reservoir quality of the taiyuan sandstones will play a critical role in the development of the field an investigation was conducted to explore possible effects of the sequence stratigraphy depositional facies and diagenetic alterations on the reservoir quality of the taiyuan sandstones based on core descriptions the examination of thin sections and core plug and scanning electron microscope analyses the taiyuan formation consists of lowstand system tracts lst transgressive system tracts tst and highstand system tracts hst the lst sandstones are principally medium to coarse grained and are interpreted to have been deposited in the dominant distributary channels of a delta plain the early tst fine to coarse grained sandstones were largely deposited in the underwater distributary channel and mouth bar of a delta front and the middle tst fine to medium grained sandstones were deposited in the shoreface to foreshore environment the hst silt to medium grained sandstones were mostly deposited in the mouth bar of a delta front and shoreface to foreshore the taiyuan sandstones which are composed of sublitharenites quartzarenites and litharenites suffered from extensive compaction that resulted in a large loss of intergranular porosity before the sandstones were further cemented severely the sandstone cements are mostly composed of illite ankerite calcite and quartz the taiyuan sandstones have a particularly low average core plug porosity of 7 41 and horizontal permeability of 0 916 md which results from that most of the pores of the sandstones are micropores and secondary pores with poor connectivity the lst dominant distributary channel early tst underwater distributary channel and proximal mouth bar and hst shoreface to foreshore sandstones with relatively coarser grain sizes mostly medium to coarse grained have relatively higher porosities and permeabilities than the sandstones with relatively finer grain size which is ascribed to a widespread early dissolution by meteor water leaching and lower content of cements within the coarser grained sandstones however the lst dominant distributary channel sandstones had undergone a more effectively early dissolution and less carbonate cementation than the tst underwater distributary channel and proximal mouth bar and hst shoreface and foreshore sandstones had which lead to the relatively best reservoir quality of the lst dominant distributary channel sandstones among the taiyuan sandstones the study revealed that the reservoir quality of the tight taiyuan sandstones was controlled by sequence stratigraphy framework depositional facies grain size and diagenetic alterations together the sequence stratigraphy framework played a key role in controlling the reservoir quality of the taiyuan sandstones by governing the depositional facies and corresponding diagenetic products while the grain size affected the reservoir quality of the sandstones by controlling the content of cement within the sandstones

pre requisites processes and prediction of chlorite grain coatings in petroleum reservoirs a review of subsurface examples abstract deeply buried reservoirs containing chlorite coated quartz sand grains commonly have higher than expected porosity and permeability although prediction of such positive anomalies still remains elusive a total of 54 published examples based on information and data from 62 scientific papers was collated quantification of some of the most common parameters including depositional environment age and latitude of sand deposition effect on reservoir quality and chemical composition of chlorite is presented the dataset indicates that chlorite coats are found in sandstones deposited in a range of depositional environments but most commonly occur in delta related environments 44 with fluvial environments the second most common 19 age relationships indicate that there is an overall exploration bias in published examples with chlorite coats becoming increasingly common through time the latitude at the time of deposition of sands with chlorite coats is wide 60 n 60 s and indicates that temperate and tropical climates are important for the generation of this clay mineral chlorite can have a variable effect on reservoir quality but is typically positive iron rich chlorites occur overwhelmingly in coastal environments while mixed iron and magnesium rich chlorites are principally found in marine and terrestrial sandstones analysis of these factors suggests that hinterland geology basinal soil development and geochemical weathering and proximity to river systems are essential to the formation of chlorite precursor phases these characteristics have been combined to define situations where chlorite coats were more likely or more unlikely to form these parameters will provide insights into the formation of chlorite and to further refine predictive models for the presence or absence of chlorite coated sandstones

the sequence stratigraphy of fluvial depositional systems the role of floodplain sediment storage abstract the currently used model for alluvial sequence stratigraphy has been criticised miall 1991 and an alternative model is offered here in this model alluviation mainly takes place during transgressive and not highstand phases when accommodation space allows sediment storage the high capacity for some river systems to store floodplain sediments is a critical consideration the effects of climatic and intrinsic geomorphic changes during fluvial deposition are likely to greatly complicate simple base level controlled sequence models and recognising higher order fourth and fifth order cycles will be particularly difficult

review of diagenetic facies in tight sandstones diagenesis diagenetic minerals and prediction via well logs abstract the tight sandstones are characterized by low porosity low permeability complex pore structure and strong heterogeneity due to the extensive diagenetic modifications they experienced understanding of the impact of diagenetic alterations on reservoir quality is crucial to the hydrocarbon exploration and production in tight sandstones diagenetic facies which is the comprehensive description of the diagenesis and diagenetic minerals determines the formation and distribution of sweet spot by correlating the diagenetic facies to well log responses the subsurface distribution of porosity and permeability can be predicted however the prediction of diagenetic facies and reservoir quality via well logs in tight sandstones remains a challenging task this paper critically reviews the impact of diagenesis and diagenetic minerals on reservoir quality in tight sandstones and establishes a model for prediction of diagenetic facies via well logs as assessed from peer reviewed papers in the literature as well as from the authors personal experiences this review begins with reviewing the impacts of compaction cementation dissolution and various types of diagenetic minerals on reservoir quality evolution the definition and classification schemes of diagenetic facies are then discussed and the reservoir quality as well as diagenetic evolution sequence of various diagenetic facies is summarized the same diagenetic facies commonly display similar compositional and textural attributes matrix and cement as well as porosity systems the well log responses gr ac den cnl and rt of various diagenetic facies are summarized by the calibration of log values with cores and related thin sections by translating the diagenetic facies to conventional well logs a predictable model which can be used for subsurface reservoir quality prediction is established then the theory of ecs logs is reviewed and the application of ecs logs in diagenetic facies evaluation is discussed at last the quantitative characterization for various type and degree of diagenesis is reviewed and the subsurface diagenetic facies is predicted by quantitative calculation of the compactional porosity loss cementational porosity loss and dissolution porosity content via well logs correlating the diagenetic facies to well logs provides a powerful tool to predict the distribution of high quality reservoirs in tight sandstones this review will provide insights into the reservoir quality evaluation and sweet spot prediction via well logs in tight sandstones

kinematics and shear heat pattern of ductile simple shear zones with slip boundary condition extrusion by poiseuille flow and simple shear of hot lower crust has been deciphered from large hot orogens and partial slip boundary condition has been encountered in analogue models shear heat and velocity profiles are deduced from a simplified form of navier stokes equation for simple shear together with extrusive poiseuille flow and slip boundary condition for newtonian viscous rheology a higher velocity at the upper boundary of the shear zone promotes higher slip velocity at the lower boundary the other parameters that affect the slip are viscosity and thickness of the shear zone and the resultant pressure gradient that drives extrusion in the partial slip case depending on flow parameters resultant pressure gradient density and viscosity and thickness of the shear zone the velocity profiles can curve and indicate opposite shear senses the corresponding shear heat profiles can indicate temperature maximum inside shear zones near either boundaries of the shear zone or equidistant from them

crustal channel flows 2 numerical models with implications for metamorphism in the himalayan tibetan orogen 1 results from a thermal mechanical model ht1 that includes midcrustal channel flow are compatible with many features of the himalayan tibetan system radioactive self heating and rheological weakening of thickened model orogenic crust lead to the formation of a hot low viscosity midcrustal channel and a broad plateau channel material corresponding to the greater himalayan sequence ghs flows outward from beneath the plateau in response to topographically induced differential pressure at the plateau flank it is exhumed by focused surface denudation and juxtaposed with cooler newly accreted material corresponding to the lesser himalayan sequence lhs the model channel is bounded by coeval thrust and normal sense ductile shear zones interpreted to represent the main central thrust mct zone and south tibetan detachment system respectively inverted metamorphism associated with the model mct zone results from distributed ductile shear along the mct and extrusion of the hot channel a variety of model p t t path styles resembling those observed in the ghs and lhs are produced for points traveling through contrasting tectonic regimes that coexist in different parts of the model predicted times of peak metamorphism cooling and erosion of metamorphic facies are generally compatible with observations although model ghs cooling ages are too young the times of m1 and m2 metamorphic events observed in the ghs correspond to model times of maximum burial and maximum heating respectively the results highlight the need to integrate tectonics and metamorphism in continental collision models and demonstrate the importance of lateral transport of both heat and material in large hot orogens

mountain building in the nepal himalaya thermal and kinematic model we model crustal deformation and the resulting thermal structure across the nepal himalaya assuming that since 20 ma shortening across the range has been primarily taken up by slip along a single thrust fault the main himalayan thrust mht fault and that the growth of the himalayan wedge has resulted mainly from underplating and to the development of a duplex at midcrustal depth we show that this process can account for the inverse thermal metamorphic gradient documented throughout the lesser himalaya lh the discontinuity of peak metamorphic temperatures across the mct as well as the distribution of age of exhumation across the range this study suggests that the metamorphic evolution of the range over about the last 20 million years is compatible with the kinematics of recent crustal deformation deduced from morphotectonic and geodetic studies

spatial variability of palaeosols across cretaceous interfluves in the dunvegan formation ne british columbia canada palaeohydrological palaeogeomorphological and stratigraphic implications a basic sedimentological and palaeopedological framework is now in place for non marine sequence models the variability of interfluve palaeosols has not however been systematically documented nor have the stratigraphic implications of that variability been incorporated into sequence models interfluve palaeosol variability in the deltaic dunvegan formation ne british columbia canada is investigated for which a detailed allostratigraphic and palaeogeographic framework has been developed based on abundant well 2300 logs and outcrop 60 sections control regionally extensive valley fills and interfluves have been mapped in coastal plain deposits over an area of about 50 000 km 2 this palaeogeographic framework permits interfluve surfaces exposed in outcrop to be located in terms of distance from the margins of coeval valleys the micromorphological geochemical and mineralogical characteristics of five representative sequence bounding palaeosols located from 250 m to more than 15 km from coeval valley margins are described these interfluve palaeosols are similar to modern alfisols and all record i aggradation on an alluvial coastal plain ii a subsequent static and or degradational phase related to valley incision nondeposition and soil thickening and iii a final aggradational phase related to valley filling and renewed sedimentation across the coastal plain within this overall template however variations in palaeosol thickness redoximorphic features illuvial clay content and geochemistry suggest developmental control by hydrological characteristics that were influenced by both the nature of the underlying alluvial sediments and distance from the valley margin the presence of mature interfluve palaeosols with complex developmental histories suggests that landscape dissection may have been related to terrace development associated with valley incision palaeosols closer to valley margins are thicker contain more illuvial clay and display characteristics suggesting better drained conditions relative to those palaeosols that developed further from valley margins subsequent deposition on interfluves also reflects distance from valleys with those sites close to valleys accumulating cumulic soils with evidence of brackish groundwater whereas far from valleys 10 15 km groundwater was fresh and clastic supply minimal encouraging peat formation variations in drainage and palaeotopography during landscape dissection resulted in different palaeosol development styles on interfluve surfaces that can be shown on the basis of physical correlation to have the same geomorphic age these observations support the concept of the soil forming interval as a basis for pedostratigraphic correlation in ancient terrestrial deposits palaeosol variability on interfluves is to be expected and recognition and documentation of this variability is an important prerequisite to palaeogeomorphological palaeoclimatic and sequence stratigraphic interpretations

porosity reduction and major controls on diagenesis of cretaceous paleocene volcaniclastic and arkosic sandstone middle park basin colorado abstract thin section and x ray diffraction analyses of 31 outcrop samples show that the early diagenesis of heterogeneous volcaniclastic and arkosic sandstone and conglomerate of the middle park formation upper cretaceous paleocene was strongly influenced by detrital mineralogy rocks with abundant andesitic rock fragments experienced the most compaction and are characterized by an early generation of authigenic chloritic mixedlayer clays and relatively minor illite smectite and chlorite whereas rocks with moderate to minor percentages of volcanic rock fragments experienced less compaction than volcanic rich rocks and are characterized by earlygeneration authigenic illite and lesser illite smectite chlorite and chloritic mixed layer clay compaction and cementation occluded most macropo osity in those samples richest in volcanic detritus thereby sealing the rocks and preventing the development of later stage cements or dissolution porosity unlike that of clay minerals the distribution of authigenic heulandite was not influenced by variations in detrital mineralogy this suggests significant calcium ion transport during early diagenesis and shows that intervals rich in unstable constituents can influence the diagenesis of adjacent intervals without large percentages of plagioclase or volcanic rock fragments late diagenesis was strongly influenced by structural deformation that fractured folded and subaerially exposed the rocks fracturing may have caused local drops in pore fluid pressure causing laumontite to replace heulandite as the stable calcic zeolite penetration of the fractured rocks by oxidizing slightly acidic dilute meteoric water altered the pore fluid chemistry and caused precipitation of minor percentages of illite smectite kaolinite and hematite macroporosity is less than 5 in all but two samples the middle park formation illustrates several factors that can adversely affect porosity during diagenesis of volcaniclastic sandstone 1 lack of early leaching by fresh meteoric water 2 a prolonged episode of compaction before the first authigenic minerals precipitated 3 cementation by first generation clays and heulandite occluding most pores that had survived compaction 4 minor burial dissolution 5 minor late stage cements that further reduced porosity and occluded multigrain fractures and 6 absence of dissolution of framework grains or cement during uplift and subaerial exposure

paleoshoreline record of relative holocene sea levels on pacific islands abstract understanding the history of relative holocene sea levels on pacific islands is important for constraining fundamental geodynamic theories interpreting the environments of early human occupation sites and forecasting future environmental conditions on the islands an observational paleoshoreline record is provided by emergent paleoshoreline indicators formed at higher relative sea levels hence standing at higher elevations than modern counterparts emergent paleoshoreline notches in limestone seacliffs record paleo high tide levels and emergent paleoreef flats record paleo low tide levels whereas emergent paleobeachrock locally records paleo intertidal levels both paleonotches and paleoreefs occur along the coasts of high standing islands exposing volcanic bedrock and uplifted reef complexes but low lying coralline atolls lack sufficient relief to preserve paleonotches controls on relative holocene sea level include global eustatic and regional hydro isostatic changes in ambient sea level relative to island landmasses and shifts in the elevations of islands relative to sea level caused by thermal subsidence of the oceanic lithosphere or thermally rejuvenated loci of hotspot volcanism by flexure of the lithosphere under the load of growing volcanic edifices hawaii samoa society islands by arching of the lithosphere over trench forebulges loyalty islands niue bellona rennell and by tectonism within forearc belts between active volcanic chains and trenches mariana islands tonga vanuatu the dominant pattern of relative sea level change where not overprinted by local tectonism or lithospheric flexure was a uniform early holocene rise in eustatic sea level followed by a regionally variable late holocene hydro isostatic drawdown in sea level the resultant was a mid holocene highstand in relative sea level that affected the development of shoreline morphology throughout the tropical pacific ocean the earliest human migrations into intra oceanic island groups of both the northwestern and southwestern pacific ocean followed close on the heels of the mid holocene sea level highstand and took advantage of newly attractive coastal environments engendered by sea level drawdown the effects of the mid holocene highstand were modified to varying degrees in different island groups by geodynamic uplift or subsidence

temperatures heat flux and frictional stress near major thrust faults we examine the two dimensional advective and conductive transport of heat in a region of thrust faulting both simple theoretical considerations and numerical experiments show that the steady state temperatures near the fault are reduced by a divisor s 1 b z vsin below what they would be with the same heat sources but in the absence of advection in this expression z is the depth to the fault v is the slip rate is the component of the dip of the fault in the direction of underthrusting is thermal diffusivity and b is a dimensionless factor that is essentially equal to one for most forms of heating initial changes in temperatures near the fault are given by t z t t z 0 12vsin t z 0 z these simple formulae are successful because of the neglible influence of lateral conduction of heat at least for slip rates of a few mm yr or more two time constants govern the transition from the initial change in temperature to steady state t1 u v where u is the distance along the fault in the direction of underthrusting from the surface to the depth in question and t2 z 2 2 where z is the depth to the fault when elapsed times exceed the sum of these two time constants temperatures differ from their steady state values by only about 10 percent the numerical experiments indicate that the simple formulae are sufficiently accurate that sophisticiated numerical modeling of temperatures in specific regions is unwarranted putain an application of these simple formulae to measurements of conductive heat flow at island arcs implies that shear stresses at island arcs approach 100 mpa and are greater than 30 mpa calculations of temperatures appropriate for the himalaya suggest that shear stresses of 100 mpa on the main central thrust probably are required to account for the tertiary granites of the region if melting took place after slip began on the thrust similarly the cut off in seismicity at a depth of about 15 km in the himalaya if due to temperatures exceeding 350 to 450 c implies a deviatoric stress close to 100 mpa

analysis of partially emerged corals and reef terraces in the central vanuatu arc comparison of contemporary coseismic and nonseismic with quaternary vertical movements in the central vanuatu arc living and recently deceased reef corals act as natural tide gauges which have allowed us to map vertical tectonic deformation patterns as corals grow the density of the aragonite coral skeletons varies on an annual cycle producing annual growth bands similar to tree rings using coral growth bands we can determine the year coral surfaces died due to emergence we interpret four major coral emergence events as coseismic uplifts that occurred near the epicenters and times of large shallow earthquakes on january 5 1946 ms 7 3 august 11 1965 ms 7 5 october 27 1971 ms 7 1 and december 29 1973 ms 7 5 the 1965 and 1973 events caused maximum uplifts of 120 and 60 cm respectively in the frontal arc also related to these events are uplifts of 10 cm and 6 cm in the back arc on pentecost and maewo islands which lie east of the volcanic chain and the primary forearc zones of uplift and subsidence similar secondary zones of uplift occurred with the great 1960 chile and 1964 alaska earthquakes the amplitude of these secondary uplifts is significantly larger than that predicted by models having a single fault in an elastic half space however the amount of secondary uplift is comparable to that predicted if the fault occurs in a plate of constant thickness overlying a viscoelastic half space at various places in 1957 1969 1970 1977 and 1978 1981 there was about 5 10 cm of emergence not associated with major earthquakes which may indicate nonseismic tectonic uplift however oceanographically lowered sea levels as in el ninos may have determined the times when corals died and recorded these events nevertheless the accumulation of emergence its persistence the limited geographic extent of each event and occurrence in areas of rapid holocene uplift suggest that the causes of the uplifts are tectonic these events suggest that in some areas a third or more of the total accumulated uplift in central vanuatu takes place as aseismic motion however in some areas we find only coseismic emergence in central vanuatu contemporary coseismic vertical deformation holocene uplift and topography have remarkably similar patterns this suggests that the mechanisms and processes causing vertical deformation have varied little over the last 106years apparently the topography structure and seismotectonics are controlled by the subduction of the d entrecasteaux ridge a major bathymetric feature underthrusting this part of the arc the influence of this ridge may have been especially extensive because it migrates very slowly along the arc trend and thus it interacts for a long time with a single portion of the arc system our previous studies of reef terraces indicated the existence of at least four seismotectonic arc segments or blocks along the santo malekula interval of the arc and our present results further support this conclusion each block has uplifted at different times by different amounts at different rates and tilted in a different direction boundaries between the north santo and the south santo segment and between the north malekula and the south malekula segment correlate with the north and south flanks of the d entrecasteaux ridge as does the absence of a physiographic trench west of santo

identification of sedimentary diagenetic facies and reservoir porosity and permeability prediction an example from the eocene beach bar sandstone in the dongying depression china abstract accurate prediction of reservoir porosity and permeability is essential for prospecting hydrocarbon reserves and petroleum production capacity we propose an innovative reservoir porosity and permeability prediction method through identifying sedimentary diagenetic facies determining the porosity permeability trends using core measurement data extrapolating the spatial distribution of the sedimentary diagenetic facies using log data through the bayes discriminant analysis and predicting the reservoir porosity and permeability the essence of the method was illustrated and its effectiveness was demonstrated using the eocene beach bar sandstones in the dongying depression bohai bay basin eastern china the eocene beach bar sandstones are classified into fine sandstone siltstone and argillaceous siltstone based on grain sizes sorting and matrix contents the major diagenetic processes that influence the porosity and permeability of the beach bar sandstones are compaction carbonate cementation and feldspar dissolution seven sedimentary diagenetic facies were identified in the beach bar sandstones based on the lithological types and their corresponding diagenesis and influence extent on reservoir properties the variation ranges of porosity and permeability log k of these sedimentary diagenetic facies are typically less than 6 and 1 2 respectively both the porosity and permeability have well defined functional relationships with depth the sedimentary diagenetic facies can be identified effectively from logging data through the use of the bayes discriminant analysis and corresponding cross plots the porosity and permeability log k of the well evaluated in the study were predicted with errors in the range of 3 and 0 6 respectively based on the distribution and the fitting equation of the trend lines for the seven sedimentary diagenetic facies the predicted porosities and permeability of sedimentary diagenetic facies match the measured porosities and permeability well

on the long lasting sequences of coral reef terraces from se sulawesi indonesia distribution formation and global significance many islands of the eastern indonesian archipelago exhibit late cenozoic sequences of coral reef terraces in se sulawesi on the tukang besi and buton archipelagos we identified 23 islands bearing such sequences remote sensing imagery and field mapping combined to u th and 14c dating enable to establish a chronologic framework of the reef terrace sequences from wangi wangi buton as well as on the neighbouring smaller islands of ular siumpu and kadatua we identified the terraces from the last interglacial maximum mis 5e at elevations lower than 20 m except on w kadatua where it is raised at 34 5 m such elevations yield low to moderate upper pleistocene uplift rates 0 3 mm yr 1 on se buton island a sequence culminates at 650 m and includes at least 40 undated strandlines next to this exceptional sequence on the sampolawa peninsula 18 strandlines culminate at 430 m dated samples at the base of this sequence 40 m yield mean middle pleistocene uplift rates of 0 14 0 09 mm yr 1 extrapolation of these uplift rates compared to the geological setting suggests that the sequences of the sampolawa peninsula provide a record of sea level high stands for the last 3 8 0 6 ma the sequences on se buton island therefore constitute the best preserved long lasting geomorphic record of plio quaternary sea level stands worldwide

diagenesis and reservoir quality evolution of the eocene sandstones in the northern dongying sag bohai bay basin east china abstract the eocene sandstones in the northern dongying sag bohai bay basin china are reservoirs for large accumulations of hydrocarbons the sandstones are mainly lithic arkoses and feldspathic litharenites texturally and compositionally immature these sandstones have a wide range of porosity 0 4 37 and permeability 0 004 6969 md and show an overall decrease in reservoir quality from 1500 m to 5000 m below sea level the reduction in reservoir quality is a product of several digenetic processes these include compaction precipitation of dolomite and calcite in eodiagenetic stage compaction feldspar dissolution precipitation of quartz cements and clays kaolin and illite and precipitation of ferrocalcite and ankerite in mesodiagenetic stage mineral distribution pattern and isotopic composition suggest carbonate cements in sandstones originate from sources outside the sandstones carbonate cementation together with compaction reduced the sandstones porosity and permeability significantly in a sandstone bed marginal sandstones with distance to sandstone mudstone interface less than one meter always have lower porosity than central sandstones as burial depth exceeds 4000 m marginal sandstones have very low porosities 2 m can be potential effective reservoirs feldspar dissolution and precipitation of clays and quartz cements have little impact on absolute porosity mineral distribution pattern and quantitative data show that leached feldspars are the internal source of authigenic quartz and clays in sandstones and the volume difference between feldspar secondary porosity and related authigenic cements is generally less than 0 25 however although there is little or no net import of matter to the sandstones the pore architecture changes dramatically primary macropores are lost as clays and quartz precipitate while the proportion of microporosity increases occurring mainly between clay crystals the overall result is that permeability is significantly degraded

bayesian inference of plastosphere viscosities near the kunlun fault northern tibet 1 flow of the mid crust in the tibetan plateau may strongly influence the patterns of deformation and topography within this area this flow requires the lower crust to have low viscosity and so quantifying this viscosity may be used to test the idea of channel flow an application of bayesian methods to geologic and geodetic data from north central tibet yields lower crustal viscosities assumed newtonian of 1 1019 pa s and 2 1021 pa s 95 bounds the lower bound is larger than the value of 1016 1018 pa s required by models of flow of the lower crust of tibet unless the entire lower crust participates in channel flow

the impact of grain coating chlorite on the effective porosity of sandstones abstract chlorite grain coatings in sandstones can inhibit quartz overgrowths and preserve porosity in hot reservoirs but coatings may also introduce high irreducible water saturation and therefore reduce effective porosity in order to derive a quantitative model to predict microporosity and effective porosity five sets of sandstone samples with well developed chlorite grain coatings were studied scanning electron microscopy and x ray diffraction results on each set indicate various types of chlorite morphology and a broad range of chlorite abundance though the ratio of the average size to the average thickness of chlorite platelets is constant in all samples microporosity defined in this study as the product of total porosity and irreducible water saturation is correlated to chlorite by volume content and chemical composition micro 7 08 vchl 0 39 vchl 2 26 0 927 for iron rich chlorite and micro 7 08 vchl 1 88 vchl 7 83 0 927 for magnesium rich chlorite where micro is microporosity in porosity units p u and vchl is volume percentage of chlorite in the bulk sample when the total porosity for a sample is known its effective porosity is the difference between the total porosity and the microporosity

sequence stratigraphic distribution of diagenetic alterations in coal bearing paralic sandstones evidence from the rio bonito formation early permian southern brazil linking siliciclastic diagenesis to sequence stratigraphy allows a better understanding of the parameters controlling the spatial and temporal distribution of diagenetic alterations and hence of reservoir quality a study of the coal bearing alluvial deltaic estuarine and shallow marine sandstones of the rio bonito formation early permian parana basin southern brazil reveals that the distribution of diagenetic alterations and of related reservoir quality evolution can be constrained within a sequence stratigraphic framework calcite dolomite siderite kaolinite and pyrite cementation is consistently linked to sequence and parasequence boundaries transgressive and maximum flooding surfaces and is systematically distributed within lowstand transgressive and highstand systems tracts diagenesis of coal layers at parasequence boundaries has promoted the formation of stratabound calcite detectable in resistivity wire line logs concretionary pyrite and kaolinite and of silicate grain dissolution in sandstones located above and below these boundaries particularly in the transgressive systems tract meteoric water diagenesis caused grain dissolution and the formation of kaolinite in sandstones below sequence boundaries and in lowstand systems tract sandstones carbonate bioclasts and low sedimentation rates in lag deposits at parasequence boundaries transgressive and maximum flooding surfaces favoured the formation of grain rimming siderite the results of this study are relevant to the exploration of coal bed methane and other coal bearing reservoirs where it is crucial to unravel and predict the distribution and quality of reservoirs and compartments

quaternary uplift of the torres islands northern new hebrides frontal arc comparison with santo and malekula islands central new hebrides frontal arc coral reef terraces on the torres islands have recorded the quaternary uplift of part of the northern new indian plate underthrusts the arc from the west 14c and 230thp34u ages for torres fossil corals indicate that for approximately the past 100 000 years the islands uplifted at a constant rate ranging geographically from 0 7 to 0 9 yr the occurrence of similar uplift rates and eastward tilting directions and the absence of major crosscutting structures or rupture zone boundaries suggest that the islands may be part of a single tectonic block or arc segment the frontal arc morphology in the torres area is typical of frontal arcs farther south the central new hebrides frontal arc has no physiographic trench at the plate boundary west of santo and malekula islands and the axes of the maximum uplift rate are at the seaward edge of the plate the d entrecasteaux ridge dr a massive e w trending bathymetric feature on the indian plate is underthrusting santo and malekula the eastern parts of santo and malekula are physiographically equivalent to the torres islands and seem to have been on a former axis of maximum uplift rate however in response to subduction of the dr the uplift axes of santo and malekula may have shifted westward in a few hundred thousand years the new uplift axes caused the western parts of these islands to achieve their present topographic dominance over the earlier uplift axes holocene uplift rates on southern santo and northern malekula are about twice the average rates of the previous 100 000 years the accelerated holocene uplift could be explained if a particularly prominent part of the dr has recently underthrust santo and malekula d hebrides frontal arc these isles lie about midway between the volcanic chain and trench axis where the

shear heating in continental strike slip shear zones model and field examples summary a two layer crust and upper mantle finite diverence steady state thermomechanical model of a long lived several million years lithospheric strike slip fault is presented and its predictions compared with field observations from various major fault zones in order to estimate the maximum amount of shear heating all mechanical energy is assumed to be dissipated in heat in ductile as well as in brittle layers deformation follows a friction law in the brittle layer s and a power flow law in the ductile one s variations of several independent parameters and their influence on the thermomechanical state of the fault zone and on shear heating are systematically explored shear heating is found to be more important in fault zones avecting an initially cold lithosphere and increases with slip rate friction coeycient and stivness of materials in extreme cases slip rate of 10 cm yr 1 stiv lithosphere shear heating could lead to temperature increases close to 590 c at the moho and 475 c at 20 km depth for more common cases shear heating leads to smaller temperature increases but can still explain high grade metamorphic conditions encountered in strike slip shear zones however modelled temperature conditions often fall short of those observed this could be due to heat transport by mechanisms more eycient than conduction common syntectonic emplacement of granitic melts in ductile strike slip shear zones can be explained by lower crust partial melting induced by shear heating in the upper mantle besides slip rate the possibility of such melting depends mostly on the upper mantle rheology and on the fertility of the lower crust for hard upper mantle and highly fertile lower crust partial melting could occur at rates of 1 cm yr 1 while in most cases it would result from the breakdown of micas for slip rates over 3 cm yr 1 as a result of shear heating partial melting of the upper mantle could occur in the presence of small amounts of fluids rise of magmas and or hot fluids in the shear zone will further enhance the temperature increase in shallower parts of the fault zone in nature shear heating would inevitably cause strain localization in the deeper parts of strike slip faults as is often observed in the field for crustal shear zones

shear heating in extensional detachments implications for the thermal history of the devonian basins of w norway article i nfo the supra detachment basins in western norway were formed during lower to middle devonian in the hangingwall of the extensional system of detachments known as the nordfjord sogn detachment zone nsdz the basins experienced elevated peak temperatures up to 345 c adjacent to the main detachment fault during the terminal stages of the extension in this study we show that the heat generated by the de formation of the rocks within the shear zone of the detachment also known as shear heating played a role in the thermal evolution of the entire region during the extension and could have induced the elevated peak temperatures of the supra detachment basins we numerically analyse the influence of the rheology and the deformation style within the shear zone the rate of exhumation of the footwall and the thickness of the sedimentary accumulation on the top of the system the model reproduces the elevated temperatures recorded in the supra detachment basins where locally 100 c and 25 of the total heat budget can be attrib uted to shear heating we show that this additional heat source may increase the peak temperatures of the hangingwall as far as 5 km away from the detachment