classification of huminite iccp system 1994 abstract in the new classification iccp system 1994 the maceral group huminite has been revised from the previous classification iccp 1971 int handbook coal petr suppl to 2nd ed to accommodate the nomenclature to changes in the other maceral groups especially the changes in the vitrinite classification iccp 1998 the new vitrinite classification iccp system 1994 fuel 77 349 358 the vitrinite and huminite systems have been correlated so that down to the level of sub maceral groups the two systems can be used in parallel at the level of macerals and for finer classifications the analyst now has according to the nature of the coal and the purpose of the analysis a choice of using either of the two classification systems for huminite and vitrinite this is in accordance with the new iso coal classification that covers low rank coals as well and allows for the simultaneous use of the huminite and vitrinite nomenclature for low rank coals

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

hot orogens tectonic switching and creation of continental crust comment and reply reply this reply touches on several issues that reflect brown s and my different emphasis on the tectonic setting of granulites essentially brown suggests that the tectonic switching model should produce counterclockwise pressure temperature time p t t paths then argues that most documented

geophysical geological transect and tectonic evolution of the swiss italian alps a complete alpine cross section integrates numerous seismic reflection and refraction profiles across and along strike with published and new field data the deepest parts of the profile are constrained by geophysical data only while structural features at intermediate levels are largely depicted according to the results of three dimensional models making use of seismic and field geological data the geometry of the highest structural levels is constrained by classical along strike projections of field data parallel to the pronounced easterly axial dip of all tectonic units because the transect is placed close to the western erosional margin of the austroalpine nappes of the eastern alps it contains all the major tectonic units of the alps a model for the tectonic evolution along the transect is proposed in the form of scaled and area balanced profile sketches shortening within the austroalpine nappes is testimony of a separate cretaceous age orogenic event west directed thrusting in these units is related to westward propagation of a thrust wedge resulting from continental collision along the meliata hallstatt ocean further to the east considerable amounts of oceanic and continental crustal material were subducted during tertiary orogeny which involved some 500 km of n s convergence between europe and apulia consequently only a very small percentage of this crustal material is preserved within the nappes depicted in the transect postcollisional shortening is characterized by the simultaneous activity of gently dipping north directed detachments and steeply inclined south directed detachments both detachments nucleating at the interface between lower and upper crust large scale wedging of the adriatic or apulian lower crust into a gap opening between the subduced european lower crust and the pile of thin upper crustal flakes alpine nappes indicates a relatively strong lower crust and detachment between upper and lower crust

heat producing elements and the thermal and baric patterns of metamorphic belts the character of sedimentary basins before they are deformed and metamorphosed may strongly influence the thermal and baric patterns of metamorphic belts crustal thickening of anoxic sedimentary basins and subsequent thermal reequilibration may produce large areas of high grade metamorphic rocks and granites because the concentrations of the heat producing elements are high in such basins in new england there is a spatial association among granites and high grade metasedimentary rocks rich in u and th that now form the central maine terrane the high content of heat producing elements in these rocks is attributed to fixing of u and th in highly reduced sediments that were deposited in an anoxic basin that formed in the silurian when the basin was thickened during the devonian acadian orogeny the thermal energy generated by the u and th rich sediments produced the observed broad zone of high grade rocks and anatectic granites this hypothesis was tested with thermal calculations that reproduce most of the first order thermal and baric patterns of the acadian appalachians if pretectonic lateral variations in heat production are assumed

granite ascent and emplacement during contractional deformation in convergent orogens based on a case study in the central maine belt of west central maine u s a it is proposed that crustal scale shear zone systems provide an eaective focussing mechanism for transfer of granite melt through the crust in convergent orogens during contractional deformation flow of melt in crustal materials at depths below the brittle plastic transition is coupled with plastic deformation of these materials the flow is driven by pressure gradients generated by buoyancy forces and tectonic stresses within the oblique reverse central maine belt shear zone system stromatic migmatite and concordant to weakly discordant irregular granite sheets occur in zones of higher strain which suggests percolative flow of melt to form the migmatite leuco somes and viscous flow of melt channelized in sheet like bodies possibly along fractures cyclic fluctuations of melt pressure may cause instantaneous changes in the eaective permeability of the flow network if self propa gating melt filled tensile and or dilatant shear fractures are produced due to melt enhanced embrittlement in homogeneous migmatite and schlieric granite occur in zones of lower strain which suggests migration of partially molten material through these zones en masse by granular flow and channelized flow of melt carrying entrained residue founded on the central maine belt case study we develop a model of melt extraction and ascent using the driving forces stress conditions and crustal rheologies in convergent especially transpressive orogens ascent of melt becomes inhibited with decreasing depth as the solidus is approached for intermedi ate a h2o muscovite dehydration melting the water saturated solidus occurs between 400 and 200 mpa near the brittle plastic transition during high t low p metamorphism where the balance of forces favors sub horizontal fracture propagation emplacement of melt may be accommodated by ductile flow and or stoping of wall rock and inflation may be accommodated by lifting of the roof at shallower crustal levels and or sink ing of the pluton floor the resultant plutons have sub horizontal tabular geometries with floors that slope down to the ascent conduits although these plutons may have locally discordant relations with country rock structures when viewed at the crustal scale granite ascent and emplacement in convergent orogens are syn tec tonic processes 1998 elsevier science ltd all rights reserved

age constraints on the paleoproterozoic tectonometamorphic history of the committee bay region western churchill province canada evidence from zircon and in situ monazite shrimp geochronology in situ u pb sensitive high resolution ion microprobe shrimp analyses of monazite from upper amphibolite facies paragneiss of the committee bay supracrustal belt central rae domain canada reveal three age populations ca 2350 1850 and 1780 ma the ca 1850 ma age also corresponds to growth of low th u zircon as indicated by u pb shrimp analyses of zircon separates from melanosome and leucosome the contextual advantage of the in situ monazite analysis allows evaluation of the geochronological data in terms of the regional structural and metamorphic evolution the region is dominated by a northeast striking s2 s1 fabric axial planar to tight northwest vergent f2 folds early garnet is enveloped by this biotite sillimanite cordierite s2 fabric garneti hosts ca 1850 ma monazite inclusions with ca 2350 ma cores placing a maximum age on garneti growth and s2 development d2 metamorphic conditions progressed through 3 5 kbar 1 kbar 100 mpa and 600 c to near peak conditions of 5 kbar and 675 c a minimum age for s2 is provided by unstrained ca 1820 ma monzogranite that locally and regionally truncates s2 dominantly ca 1780 ma matrix monazite is interpreted to date post s2 garnetii and cordierite which record 5 kbar and 675 c these data indicate that the committee bay region experienced penetrative d2 tectonometamorphism at ca 1850 1820 ma with a subsequent static overprint the absence of a ca 1 85 ga plutonic suite in the region suggests that low pressure metamorphism was a response to thick skinned crustal thickening initiated at ca 1870 ma the new data highlight the importance of paleoproterozoic reworking of the central rae domain in the hinterland of the trans hudson orogen

the western desert versus nile delta a comparative molecular biomarker study abstract the western desert and the nile delta are prolific petroleum producing basins the correlation and matching between their petroleum systems have been briefly studied overall geochemical and biological marker analyses of twelve crude oil and condensate samples taken from seven oil gasfields located in the nile delta and the western desert basins of egypt were effective in identifying and geographically defining two major oil families related by age and source rock depositional environment these samples were classified respectively as late cretaceous tertiary and jurassic early cretaceous the source rocks giving rise to the late cretaceous tertiary oils are associated with abundant terrestrial organic matter the source rocks giving rise to jurassic early cretaceous oil are presumably generated from a mixed terrestrial marine source meanwhile the western desert oil samples show prominent c 30 steranes elevated c 27 monoaromatic steroids and a distinct homohopane distribution suggesting this oil type is derived from a more marine influenced source rock than terrigenous sourced oil in the nile delta the most crucial geochemical fingerprinting biomarkers that have been used to differentiate between nile delta and the western desert oil are oleananes highly branched isoprenoids hbi sterane isomerization 24 norcholestanes aromatic steroid hydrocarbons and c 30 24 n propylcholestanes

metamorphic response to collision in the central himalayan orogen abstract closure of the neotethys ocean and high angle continent continent collision between india and asia after about 55 ma resulted in low angle subduction of the indian plate below the tibetan plateau and by 30 ma established an arcuate 2300 km long shallow north dipping metamorphic fold thrust belt in the foreland this himalayan metamorphic front quickly established an upper plate lower plate paired metamorphic architecture centred on the median high himal thrust that largely controlled subsequent evolution the upper plate is a thick slab of high t moderate p high grade migmatized metamorphic rocks whereas the lower plate is an inverted series of moderate t high p schists in two crustal wedges the broad main central thrust zone and at lowest structural levels the footwall below the basal main central thrust spatial and temporal patterns of metamorphic response in the evolving himalayan metamorphic front has been characterized in a large scale integrated structural metamorphic study based on 8 profiles across eastern nepal metamorphic response at all structural levels is established using a large dataset n 160 of internally consistent quantitative pt determinations petrology of metapelite samples semi quantitative p t paths metamorphic mapping and metamorphic field gradients these results are integrated with previously published metamorphic studies structural profiles and metamorphic chronology from these datasets the architecture and evolution of the himalayan metamorphic front is constrained by rock kinematics metamorphic field gradients showing discontinuities and diachronous metamorphism with contrasting p t evolutions at different structural levels each of the three panels constituting the himalayan metamorphic front upper plate main central thrust zone and footwall experienced distinctly different tectono metamorphic histories crustal processes operating during metamorphism and exhumation differ between the upper and lower plates the upper plate experienced long lived metamorphism starting from at least 28 38 ma and tracking low p t clockwise p t paths that culminated at 19 27 ma in peak high grade condition with 27 31 c km thermal regimes protracted high grade conditions produced significant partial melt which facilitated gravity driven southward extrusion involving internal ductile flow processes southward extrusion of the upper plate was accommodated by coeval reverse movement on the high himal thrust and top down to the north normal reactivation of the south tibet detachment system between 22 10 ma transport of this thick slab to the south resulted in further prograde burial of the lower plate below culminating in peak metamorphism at the highest pressures attained in the lower plate rocks the lower plate consists of at least two deeply buried 7 5 9 4 kb crustal wedges both of which experienced steep p t hairpin clockwise p t paths with isothermal decompression the main central thrust zone experienced 20 26 c km metamorphism at 12 22 ma and was exhumed along the basal main central thrust after 11 ma whereas loading by the main central thrust zone gave rise to further prograde burial of the footwall which experienced 16 21 c km metamorphism at 4 10 ma and was exhumed along the main boundary thrust at 3 9 ma in contrast to the upper plate both crustal wedges were exhumed by a process of out wedging which involved upthrusting along basal and internal thrusts with coeval extensional slip in the hanging walls this three stage foreland propagating lateral exhumation history resulted in telescoping of the himalayan metamorphic front in concert with peak metamorphic events at different structural levels during main phase orogenesis and is not the result of superimposed retrograde reactivation of the belt

devonian extension in northwestern newfoundland 40ar 39ar and u pb data from the ming s bight area baie verte peninsula abstract the ming s bight group of northwestern newfoundland an outlier of humber zone continental margin rocks is entirely surrounded by ophiolitic rocks of the dunnage zone structures in the ming s bight group and adjacent units record three main phases of deformation the earliest structures relate to silurian sinistral transpression previously documented in the region two later phases of extensional deformation produced a series of dextral oblique normal shear zones and faults that now separate the ming s bight group in the footwall from ophiolitic and granitoid rocks in the hangingwall 40ar 39ar and u pb data constrain the times of oblique normal shear and cooling metagabbro in the point rousse ophiolite complex which lies in the hangingwall preserves disturbed ordovician hornblende 40ar 39ar ages whereas adjacent shear zones record devonian ages hornblendes in pacquet harbour group amphibolites within extensional shear zones mainly record 40ar 39ar ages of 390 380 ma synkinematic titanite

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

the petrogenesis and emplacement of the new hampshire plutonic suite a longstanding problem associated with the acadian orogeny in new england is whether or not the orogeny involved a redistrubution of mass and energy within the midcrust without the addition of magmas and or anomalous heatfrom the mantle and or lower crust several closed system models suggest the new hampshire plutonic suite nhps resulted from essentially in situ partial melting of the metasedimentary rocks of the central maine terrane cmt at midcrustal levels crustal thickening models shear heating along the decollement and the presence of u enriched metasedimentary rocks have been suggested as heat sources for anatexis while applicable to numerous nhps plutons there are several difficulties with these models as explanations for the nhps as a whole 1 the syntectonic bethlehem kinsman and spaulding members are too cao and na 2 o rich to have been derived from the exposed metapelites deeper seated metagraywackes may account for some of the nhps source rocks but not for the metaluminous spaulding tonalite and other more mafic nhps rocks 2 some nhps 1 8 o nd and sr isotopic values are outside the range of cmt and bronson hill terrane metasedimentary rocks requiring a more primitive mantle like component 3 the temperatures of the kinsman and the spaulding magmas were higher than those attained during regional metamorphism associated with crustal thickening greater than 850 c 4 magmatic epidote and hornblende geobarometry for the winnipesaukee pluton of the spaulding tonalite suggest intratelluric crystallization initially at pressures 6 kbars deeper than the inferred decollement between the allochthonous cmt terrane metasedimentary rocks and underlying basement these data suggest that while the nhps contains a significant component from the cmt metasedimentary rocks the metaluminous spaulding rocks mafic enclaves in the kinsman and mafic rocks in northeastern vermont western maine and southeastern new hampshire provide evidence of heat and mass beyond that associated with closed midcrustal system processes

source rock characteristics and hydrocarbon generation modelling of upper cretaceous mukalla formation in the jiza qamar basin eastern yemen abstract the upper cretaceous mukalla coals and other organic rich sediments which are widely exposed in the jiza qamar basin and believed to be a major source rocks were analysed using organic geochemistry and petrology the total organic carbon toc contents of the mukalla source rocks range from 0 72 to 79 90 with an average toc value of 21 50 the coals and coaly shale sediments are relatively higher in organic richness consistent with source rocks generative potential the samples analysed have vitrinite reflectance in the range of 0 84 1 10 ro and pyrolysis t max in the range of 432 454 c indicate that the mukalla source rocks contain mature to late mature organic matter good oil generating potential is anticipated from the coals and coaly shale sediments with high hydrogen indices 250 449 mg hc g toc this is supported by their significant amounts of oil liptinite macerals are present in these coals and coaly shale sediments and py gc s 2 pyrograms with n alkane alkene doublets extending beyond nc 30 the shales are dominated by type iii kerogen hi one dimensional basin modelling was performed to analysis the hydrocarbon generation and expulsion history of the mukalla source rocks in the jiza qamar basin based on the reconstruction of the burial thermal maturity histories in order to improve our understanding of the of hydrocarbon generation potential of the mukalla source rocks calibration of the model with measured vitrinite reflectance ro and borehole temperature data indicates that the present day heat flow in the jiza qamar basin varies from 45 0 mw m 2 to 70 0 mw m 2 and the paleo heat flow increased from 80 ma to 25 ma reached a peak heat flow values of approximately 70 0 mw m 2 at 25 ma and then decreased exponentially from 25 ma to present day the peak paleo heat flow is explained by the gulf of aden and red sea tertiary rifting during oligocene middle miocene which has a considerable influence on the thermal maturity of the mukalla source rocks the source rocks of the mukalla formation are presently in a stage of oil and condensate generation with maturity from 0 50 to 1 10 ro oil generation 0 5 ro in the mukalla source rocks began from about 61 ma to 54 ma and the peak hydrocarbon generation 1 0 ro occurred approximately from 25 ma to 20 ma the modelled hydrocarbon expulsion evolution suggested that the timing of hydrocarbon expulsion from the mukalla source rocks began from 15 ma to present day

petrographic composition and depositional environments of cretaceous coals and coal measures in the middle benue trough of nigeria abstract cretaceous coals and coal measure sequences in the area around the obi lafia part of the middle benue trough of nigeria were studied petrographically the turonian santonian awgu formation and the maastrichtian lafia formation constitute the sedimentary successions in both outcrop and borehole profiles of this area maceral analyses of 32 coal samples from this area allow subdivision of the coal beds into three different coal facies the vitrinite fusinite coal facies the trimaceritic coal facies and the shaly coal facies in the vitrinite fusinite coal facies vitrinite constitutes on average 67 3 liptinite 4 2 inertinite 17 1 and mineral matter 11 4 the trimaceritic coal facies has 42 3 vitrinite 22 4 liptinite 23 8 inertinite and 11 5 mineral matter in the shaly coal facies vitrinite constitutes 29 9 liptinite inertinite and mineral matter are 13 8 22 0 and 34 3 respectively correlation of the distribution of microfossils in the interseam sediments with the tissue preservation gelification groundwater and vegetation indices of the coal facies indicates that the vitrinite fusinite coal facies was deposited in wet forest swamp subenvironments along and within lagoons the trimaceritic facies was deposited in limno telmatic clastic marsh subenvironments in lower delta plains the shaly coal facies shows characteristics of various subenvironments but generally tends to be more marine than the other two facies

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

pressure temperature time paths from two dimensional thermal models prograde retrograde and inverted metamorphism two dimensional thermal models of time transitive crustal thickening and subsequent unroofing in large scale overthrust terrains generate pressure temperature time ptt paths that generally resemble those produced by earlier one dimensional instantaneous models but that differ in detail these differences in ptt path morphology are most pronounced proximal to major fault zones where postthrusting geotherms are characterized by large temperature inversions in one dimensional models but generally lack inversions in two dimensional models tests using the two dimensional fault model developed here indicate that 1 burial rate proportional to dp dt not thrust fault geometry dip angle controls the topology of synthrusting pt paths but plays only a minor role in determining the maximum temperature that rocks attain during the later unroofing stage in their thermal histories 2 normal ranges of thrusting and erosion rates and fault parameters result in pt paths with the usual sense clockwise on conventional pt diagrams counterclockwise in our diagrams 3 the amount of heating and duration of heating following the end of thrusting are a function of the rate of unroofing dp dt during this period 4 fast unroofing rates lead to the attainment of lower maximum temperatures after greater amounts of unroofing 5 the initial thermal state of the lithosphere prior to thrusting has a profound effect on pt path morphologies and on the peak metamorphic conditions attained by samples 6 for excess heat distributed across the entire lithosphere e g due to increased background thermal gradients a plot of peak temperatures experienced by metamorphic rocks versus structural depth tmax plot closely represents the initial geotherm 7 excess heat in the crust only e g increased radioactive heating in a layer yields a different result with the tmax plot corresponding to the initial geotherm only near the top of the hanging wall tmax plots for both 6 and 7 show no temperature inversion which exceeds the nominal uncertainties 50 k for geothermometric data 8 shear heating can lead to significant temperature inversions at the fault zone if the frictional coefficient is 0 6 or greater and 9 simultaneous thrusting and erosion produce pt loops significantly narrower than those resulting from sequential thrusting and erosion suggesting that any formulation which fails to account for some degree of simultaneity between thrusting and erosion represents a far endmember model forward models like those presented here provide important guidelines for understanding the sensitivity of metamorphic ptt paths to various thermal mechanical and geometric factors related to tectonism but they are generally inappropriate for reconstructing metamorphic thermal histories from actual petrologic and geochronologic data analytical inversion techniques that use a postthrusting thermal regime consistent with two dimensional forward models and that integrate values of dp dt dt dt and radioactive heating rates extracted from suites of metamorphic rocks provide the best hope for furthering our understanding of the thermal evolution of metamorphic terrains

time dependent crustal temperature modeling central alps abstract the conventional modeling of crustal temperatures is based on the steady state approach which is reliable in tectonically stable regions only the present work focuses on the assessment of the influence of the transient effects on the thermal field at regions of ongoing continental collision in the first part of this study the thermal effects of most evident processes operating in collision zones thickening and flaking of the crust frictional heating on the surfaces of flakes bulk replacement of mantle lithosphere by hotter rocks and also uplift and erosion are estimated on the basis of 1 d time dependent synthetic models the results show that surface heat flow caused by the most of investigated transient processes may differ from the steady state one by quantities of the order of magnitude larger than the uncertainties of the heat flow measurements these differences may exist over tens of millions of years in the second part three 2 d time dependent temperature sections gradually including the above assessed effects have been calculated for the central alps the results show that time dependent models give better agreement of the calculated and measured heat flow patterns than previously developed for the same region steady state model cermak et al 1990 best agreement is achieved when only the transient effects of thickening and flaking and increased heat inflow from the mantle are taken into account a poorer coherence of the other versions with the measured heat flow is probably attributable to uncertain knowledge and consequently great simplification of spacing and time dependence of erosion and or frictional heating

interaction among upper crustal lower crustal and mantle materials in the port mouton pluton meguma lithotectonic zone southwest nova scotia the port mouton pluton is unique among the late devonian peraluminous granitoid bodies in the meguma lithotectonic zone of southwestern nova scotia in its lithological heterogeneity extensive physical and chemical interac tion with the country rocks clear evidence for mingling and mixing with mafic magmas and highly abundant pegmatites new u pb age determinations on monazite establish an intrusion age of 373 1 ma similar to the ages of other meguma lithotectonic zone granitoid plutons and mafic intrusions field relations petrology and geochemistry define three stages of intrusion of the port mouton pluton i early stage discontinuously exposed around the outer margin of the pluton dominated by coarse grained tonalite granodiorite and with rb sr 0 40 and gdn lun 0 55 eu eu 2 and iii late stage consisting of abundant minor sheets throughout the pluton dominated by fine grained tonalite granodiorite and leucogranite that are similar to rocks of the early and middle stages the port mouton pluton shows a wider range of 87 sr 86 sri 0 7036 0 7154 and a wider range and generally higher endi 3 72 to 2 12 than other granitoid rocks in the meguma lithotectonic zone potentially reflecting a com plex partially equilibrated interaction among mantle lower crust and upper crust field petrological and chemical evi dence for the involvement of mantle derived magmas and melting of upper crust permit modelling of the port mouton pluton granitoid compositions by three simultaneous mixing equations these mixing model results suggest that the early stage granitoid rocks can form from simple three component mixing relationships when the bulk distribution coefficients between residuum and melt for sr and nd range from 1 05 to 1 18 or two component mixing combined with fraction ation of material like the known felsic lower crust the middle stage granitoid rocks only yield solutions involving two component mixing and fractionation of material unlike the known felsic lower crust we conclude that the late devonian mafic magmas played a major role in the formation of granitoid magmas in the meguma lithotectonic zone by supplying heat and material to cause partial fusion of the avalon lower crust

coal as a source rock for oil a review the geological debate about whether and to what extent humic coals have sourced oil is likely to continue for some time despite some important advances in our knowledge of the processes involved it is clear that not only liptinites but also perhydrous vitrinites have the potential to generate hydrocarbon liquids in the course of natural coalification some liptinites especially alginite cutinite and suberinite contain a higher proportion of aliphatic moieties in their structure than other liptinites such as sporinite and resinite and are therefore more oil prone it is of potential value to be able to predict the several environments of deposition in which coals with high liptinite contents or containing perhydrous vitrinites may have been formed review of the distribution of oil prone coals in time and space reveals that most are jurassic tertiary with key examples from australia new zealand and indonesia methods based both on experimental simulations and the examination of naturally matured samples have been used to determine the order of generation of hydrocarbons from different macerals results are not entirely consistent among the different approaches and there is much overlap in the ranges of degradation but it seems probable that in the natural environment vitrinites begin to generate early followed by labile liptinites such as suberinite then cutinite sporinite and finally alginite petroleum potential may be determined by experimental simulation of natural coalification or inferred through various micro techniques especially fluorescence and infrared ir spectroscopy or bulk techniques such as elemental analysis and 13c nmr spectroscopy the latter three techniques enable a measure of the polymethylene component of the coal which now appears to be one of the best available approaches for determining petroleum potential no method of experimental simulation of petroleum generation from coals is without criticism and comparative results are highly variable however hydrous pyrolysis confined pyrolysis and forms of open system hydrous pyrolysis approach acceptable simulations whether and to what degree generated liquid hydrocarbons are expelled has long been the central problem in oil from coal studies the structure of vitrinite was believed until recently to contain an interconnected microporous network in which generated oil would be contained until an expulsion threshold was attained recent studies show the pores are not interconnected combined with a dynamic model of pore generation it now seems that expulsion of hydrocarbons is best explained by activated diffusion of molecules to maceral boundaries and ultimately by cleats and fractures to coal seam boundaries the main reason for poor expulsion is the adsorption of oil on the organic macromolecule which may be overcome 1 if coals are thin and interbedded with clastic sediments or 2 if the coals are very hydrogen rich and generate large quantities of oil the existence of oil in vitrinite is attested to by solvent extractions fluorescence properties and by microscopic observations of oil and bitumen experimental simulation of expulsion of oil from coals has only recently been attempted the relative timing of release of generated co2 and ch4 could have considerable importance in promoting the expulsion of liquid hydrocarbons but the mechanism is unclear as it is universally agreed that dispersed organic matter dom in some shales readily generates and expels petroleum it is curious that few consistent geochemical differences have been found between coal macerals and dom in interbedded shales unambiguous evidence of expulsion from coals is limited and in particular only a few commercial oil discoveries can be confidently correlated to coals these include upper cretaceous fruitland formation coals in the usa from which oil is produced new zealand tertiary coals and middle jurassic coals from the danish north sea it is likely that coals have at least contributed to significant oil discoveries in the gippsland basin australia in the turpan basin china and in the kutei and ardjuna basins in indonesia but this remains unproven early reports that early jurassic coals in mid norway were a major source of the reservoired oils have been shown to be inaccurate none of the proposed rules of thumb for generation or expulsion of petroleum from coals seem particularly robust decisions on whether a particular coal is likely to have been an active source for oil should consider all available geological and geochemical information the assumptions made in computational models should be well understood as it is likely with new understandings of processes involved that some of these assumptions will be difficult to sustain

organic geochemical characteristics and depositional environments of the jurassic coals in the eastern taurus of southern turkey abstract in this study organic matter content type and maturity as well as some petrographic and physical characteristics of the jurassic coals exposed in the eastern taurus were investigated and their depositional environments were interpreted the total organic carbon toc contents of coals in the feke akkaya kozan gedikli and kozan kizilinc areas are 24 54 66 78 and 49 15 respectively the feke akkaya and kozan kizilinc coals have low hydrogen index hi values while the kozan gedikli coals show moderate hi values all coal samples display very low oxygen index oi values the kozan gedikli coals contain type ii organic matter om the feke akkaya coals contain a mixture of type ii and type iii om and the kozan kizilinc coals are composed of type iii om sterane distribution was calculated as c27 c29 c28 from the m z 217 mass chromatogram for all coal samples tmax values for the feke akkaya kozan gedikli and kozan kizilinc coals are 439 412 and 427 c vitrinite reflectance values ro for the feke akkaya and kozan kizilinc coal samples were measured as 0 65 and 0 51 and these values reveal that the feke akkaya and kozan kizilinc coals are at subbituminous a or high volatile c bituminous coal stage on the basis of biomarker maturity parameters these coals have a low maturity the pristane phytane pr ph ratios for the feke akkaya kozan gedikli and kozan kizilinc coals are 1 53 1 13 and 1 25 respectively in addition all coals show a homohopane distribution which is dominated by low carbon numbers and c35 homohopane index is very low for all coal samples all these features may indicate that these coals were deposited in a suboxic environment the high sterane hopane ratios with high concentrations of steranes low pr ph ratios and c25 c26 tricyclic ratios 1 may indicate that these coals formed in a swamp environment were temporarily influenced by marine conditions

thermal and kinematic modeling of bedrock and detrital cooling ages in the central himalaya 1 we introduce a new method that convolves detrital mineral cooling ages with digital elevation models to test numerical models of erosion in collisional orogens along a trans himalayan transect in central nepal we develop a kinematic and thermal model to predict variations in bedrock cooling ages in modern himalayan topography the model assumes a thermal steady state and utilizes a simple ramp and flat style decollement representing the main himalayan thrust the model also assumes a topographic steady state such that overthrusting is balanced by erosion to maintain a constant topographic profile erosion rates display strong spatial variations as a function of the angle between the slope of the topographic surface and the trajectories of rock particles approaching the surface to predict the detrital cooling age signal we combine the distribution of bedrock cooling ages within a catchment with the rate of erosion and distribution of muscovite predicted cooling age distributions are compared with detrital 40ar 39ar muscovite data to assess varying tectonic and erosion scenarios such cooling age distributions are very sensitive to how much of the total plate convergence is expressed as erosion of the overthrusting plate the best fit model assigns 4 6 km myr 1 of overthrusting equivalent to as much as 1 5 2 km myr 1 of vertical erosion to the asian plate although a trade off exists between ramp geometry along the decollement and the best fit rate only a narrow range of ramp dips decollement depths and erosionally compensated overthrusting rates are compatible with the observed detrital ages

geochemical characterisation of early cretaceous lacustrine sediments of bima formation yola sub basin northern benue trough ne nigeria organic matter input preservation paleoenvironment and palaeoclimatic conditions the bima formation is the oldest lithogenetic unit occupying the base of the cretaceous successions in the yola sub basin of the northern benue trough it is differentiated into lower b1 middle b2 and upper b3 bima members integrated organic and inorganic geochemical studies were applied on the lacustrine sediments of the bima formation with the aim of reconstructing the palaeodepositional environment and organic matter input in response to climate and tectonism the analysed sediments were deposited in a freshwater lacustrine environment with a low salinity stratified water column and suboxic to relatively anoxic conditions as indicated by the bulk geochemical parameters the biomarkers provide evidence for a contribution of aquatic algae and microorganisms with a significant amount of terrigenous organic matter input the preservation of the organic matter is possibly attributed to the stratified water column with low salinity and suboxic to relatively anoxic conditions based on the geochemistry of major and trace elements the sediments were deposited during semi arid climatic conditions within passive continental margin setting this agrees with the tectonic events in the west and central african rift system wcars during the early cretaceous period

organic facies and maturation of jurassic cretaceous rocks and possible oil source rock correlation based on pyrolysis of asphaltenes scotian basin canada abstract this paper establishes source rock characterization and oil oil and oil source rock correlations of selected organic rich shale source rocks and selected light oils and condensates from jurassic and cretaceous formations of the scotian basin multiple source rocks of kerogen types iia oil prone iia iib oil and condensate prone iib condensate and gas prone iii gas prone and iv nonsource are identified in various stratigraphic horizons vitrinite reflectance and t inmax data indicate variable maturation histories in different parts of the basin which are related to differences in lithology related heat conductivity and the presence of older sediments closer to areas of rifting multiple linear regression and cluster analyses of aromatic hydrocarbon ratios reveal two oil families some kerogen type iib rock extracts match many of the light oil and condensates in terms of the aromatic hydrocarbon distributions a second family of oils could not be correlated with any of the analyzed source rock extracts pyrolysis gas chromatography of asphaltenes from the cohasset a 52 and sable island 3h 58 wells are significantly different in n alkane aromatics and phenol content the asphaltene pyrolyzates of samples from venture b 52 and s desbarres o 76 have a high aromatic content low content of n alkanes and high phenol values s desbarres which is typical of a terrestial source they correlate well with one another and also with sable island light oil the n triumph g 43 whole rock and extract asphaltene pyrolyzates are dominated by normal hydrocarbons with aromatic phenolic and thiophenic compounds in low concentrations suggesting an algal source algal type organic matter may have made a contribution to the cohasset petroleum as its asphaltene pyrolyzate is also dominated by normal hydrocarbons

organic geochemical characteristics and depositional setting of paleogene oil shale mudstone and sandstone from onshore penyu basin chenor pahang malaysia abstract the eastern chenor pahang area of the penyu basin contains an untapped potential in part due to poor knowledge of its organic facies characteristics the sedimentary succession of eastern chenor pahang area is dominated by fine to coarse grained sandstone occurring in association with oil shale and mudstone in a predominantly fining upward sequence detailed geochemical and petrographic characterization was carried out on oil shale and mudstone samples to evaluate the origin and type of organic matter om as well as the paleoenvironmental conditions and organic richness of the paleogene sedimentary succession the facies are only enriched in total organic carbon toc potential hydrocarbon yield s2 and extractible organic matter eom the types of organic matter are important index reflecting the potential of the source rock s based on elemental ratios the analyzed samples are shown to contain types i and ii kerogens this finding is supported by the high hydrogen index hi of 300 mg hc g toc and the presence of abundant liptinitic materials biomarker signatures n alkanes n c25 high c29 c30 17 h hopane ratios 0 8 3 3 and high c29 steranes 49 78 low total sulfur and v v ni ratio suggest a predominantly terrestrial source input with some contribution from aquatic microorganisms as indicated by the common occurrence of amorphous organic matter combining this results with n alkane patterns kerogen types and organic petrography predominance of c29 sterane probably indicates a contribution of aquatic organisms such as green and brown algae consistent with botrycoccus braunii race a and pediastrum contribution and some influence of higher plants om integration of biomarker studies with bulk geochemical and petrographic analysis suggests that the analyzed samples were deposited under suboxic conditions in a fresh water lacustrine environment this finding is corroborated by the ratios of pristane phytane v ni and sr br high bioproductivity and a stratified fresh water column with suboxic bottom water condition enhanced om preservation in the analyzed mudstone and shale facies the high om content in these samples maybe related to the fine grained matrix of the mud rocks and the paleoclimate in the area

petrographical and geochemical characterization of sub bituminous coals from mines in the cesar rancheria basin colombia abstract tertiary coals from the la jagua and calenturitas coal mines in the cesar sub basin colombia were investigated for the first time using organic petrological and organic geochemical methods to assess the thermal maturity and palaeoenvironmental conditions during peat accumulation thirteen seams were sampled from the los cuervos formation at two mines evaluation based on organic petrological analyses and rock eval pyrolysis reveals that the coals are thermally immature and of sub bituminous b to sub bituminous a rank with a predominance of type iii mixed with type ii kerogen 22s 22s 22r homohopane ts ts tm mpi mpr and 1 mp 9 mp ratios are very low this low thermal maturity corresponds to high volatile matter contents petrographically the la jagua and calenturitas coals are dominated by vitrinite with low to high amounts of inertinite and low to moderate amounts of liptinite the detailed petrographic data indicate a predominantly herbaceous plant input and oxidative conditions during deposition mostly with strong tissue destruction further conclusions are deduced from petrographic ratios such as tissue preservation gelification groundwaterac and vegetation indices ash and sulfur contents iso and n alkane distribution and 17 h homohopane ratio in summary the data support a formation in tropical ombrogenic rather wet peats with high bacterial activity

large scale deformation in the india asia collision constrained by earthquakes and topography analysis of earthquake focal mechanisms allows division of the india asia collision into kinematic domains that strongly correlate with topography these kinematic domains indicate strain partitioning dominated by oblique slip deformation the kunlun and south tibetan fault systems mark discontinuities in the strain field and bound the high flat topography of the plateau which deforms by transtension the northern and southern margins of tibet deform by transpression or contraction and are topographically steep correlations between seismicity and topography are due to mohr coulomb wedge mechanics at the northern and southern plateau margins which produce naturally steep surface slopes whereas the flat interior and eastern margin of the tibetan plateau is underlain by viscous crust which supports subdued topography further muted by cenozoic basin fill these data indicate that the long wavelength topography of the india asia collision is controlled by seismically caused surface displacements which are linked to deep crustal deformation mechanics

the interdependence of deformational and thermal processes in mountain belts crustal temperatures within collisional orogens are anomalously high compared with temperatures at comparable depths in stable continents which is evidence of thermal processes that are fundamental to orogenesis these temperatures can be explained by the redistribution of crust enriched in heat producing elements through the accretion of crust from the down going plate to the upper plate and surface erosion with the use of geologically reasonable rates the model results predict high temperatures over 600 c and inverted upper plate geotherms about 100 c over 20 kilometers at shallow depths 20 to 40 kilometers by 25 to 35 million years after collision this study emphasizes the interdependence of deformational surficial and thermal processes

tectonic overpressure in weak crustal scale shear zones and implications for the exhumation of high pressure rocks 1 a two dimensional numerical simulation of lithospheric shortening shows the formation of a stable crustal scale shear zone due to viscous heating the shear zone thickness is controlled by thermomechanical coupling that is resolved numerically inside the shear zone away from the shear zone lithospheric deformation is dominated by pure shear and tectonic overpressure i e pressure larger than the lithostatic pressure is proportional to the deviatoric stress inside the shear zone deformation is dominated by simple shear and the deviatoric stress decreases due to thermal weakening of the viscosity to maintain a constant horizontal total stress across the weak shear zone i e horizontal force balance the pressure in the shear zone increases to compensate the decrease of the deviatoric stress tectonic overpressure in the weak shear zone can be significantly larger than the deviatoric stress at the same location implications for the geodynamic history of tectonic nappes including high pressure ultrahigh pressure rocks are discussed

source rock characteristics depositional setting and hydrocarbon generation potential of cretaceous coals and organic rich mudstones from gombe formation gongola sub basin northern benue trough ne nigeria abstract the ongoing exploration activities at the maiganga coal mine uncover some missing gaps in the stratigraphy of the late cretaceous gombe formation therefore there is a need to further investigate this sequence including the organic facies variations and distributions of the coals mudstones and sandy shaly sediments in the gombe sandstone formation gongola sub basin northern benue trough of northeastern nigeria geochemical and petrographic methods were used to assess the organic matter source input paleodepositional conditions thermal maturity and hydrocarbon generation potential of the analysed maiganga and yaya ngari samples field observations showed four coal seams interbedded with mudstones and shales deposited in coarsening upward sequence of a deltaic environment evaluation based on the source rock analyses sra reveals that maiganga sedimentary facies is dominated by terrestrial source input of type iii kerogen this is supported by the biomarker study suggesting land plant origin for the organic matter with minor lacustrine and marine influences in maiganga that grades into predominantly mixed organic matter input of a transitional terrestrial to marine environment in the yaya ngari area this shows a progressive movement towards marine source input paleodepositional conditions of organic matters were sub oxic to oxic in maiganga while the mixed organic matters in yaya ngari were deposited in sub oxic conditions based on organic petrology and bitumen extraction data the maiganga coal was distinctly observed to be immature of pre oil generation window while the yaya ngari is in early oil generation window these analysed sediments are mainly gas prone based on the dominance of type iii kerogen although the presence of type ii iii kerogen supported by relatively higher hi may indicate gas oil potential for the maiganga coal on the other hand type iv kerogen dominated the yaya ngari samples this suggests that these shales are gas prone or do not possess any hydrocarbon generation potential as indicated by the very low hi values

comments on the paper grain size dependent potential for self generation of magnetic anomalies on mars via thermoremanent magnetic acquisition and magnetic interaction of hematite and magnetite by kletetschka et al 2005

unusual biological marker geochemistry of oils and possible source rocks offshore beaufort mackenzie delta canada abstract tertiary oils and source rocks from the offshore beaufort mackenzie delta have been examined for their biological marker content the triterpane and sterane distributions suggest a predominant land plant contribution to the oils the oils range in maturity from extremely immature to moderately mature 18 h oleanne is a major triterpane of the oils together with probable bisnorlupanes and an excellent correlation is observed in the concentration of these components in individual oils an excellent correlation is also observed between the concentration of bisnorlupanes and oil maturity based on sterna and triterpane maturation parameters suggesting that the decrease in relative concentration of these components with increasing maturity is a result of progressive dilution by hydrocarbons generated by the cracking of kerogen only one formation in cores taken from the delta has been found to contain bisnorlupanes and were present they are the dominant hydrocarbons of the branched cyclic fraction

on the thermomechanical evolution of compressional orogens summary we present results from a newly developed fully coupled thermomechanical model of the continental crust in which crustal shortening at a convergent plate boundary is driven by a basal velocity discontinuity which represents delamination and subduction in the underlying mantle this new dynamical model incorporates complex rheologies elasticity thermally activated creep and brittle frictional behaviour allows for extremely large deformation and is coupled along its top boundary to a complex erosion sedimentation model the model is based on the dynamic lagrangian remeshing dlr method which uses lagrangian spatial discretization of the crust and therefore allows for an accurate tracking of rock particles as they travel through the deforming orogen this information is in turn used to produce synthetic ptt paths we have used this model to predict the distribution of apparent ages for a wide range of isotopic systems at the surface of an actively deforming orogen we have also predicted other geophysical and geological observables such as the metamorphic grade of exposed rocks regarded here as a first order approximation for total denudation topography surface heat flux and the thickness of sediment deposited in the adjacent foreland basins results clearly demonstrate that the highest exhumation rates and thus the youngest isotopic ages are found in regions of maximum topography near the centre of the orogen but that the most deeply exhumed rocks are found on the side of the orogen in the vicinity of the retro shear zone a crustal scale fault which accommodates most of the crustal shortening within the orogen it is also in this region that the isotopic systems characterized by the greatest closure temperatures display the youngest ages and the highest grade metamorphic rocks are found these conclusions are derived from the assumption of uniform erosion across the orogen in cases where rainfall and thus erosion is orographically controlled the tectonic style of the orogen is different from the uniform erosion case as is the distribution of isotopic ages and metamorphic grades across the orogen

organic geochemistry of the lower suban coal seam south sumatra basin indonesia palaeoecological and thermal metamorphism implications abstract hydrocarbons extracted from the tertiary age coals from the lower suban seam south sumatra basin indonesia have been investigated using gas chromatography gc and combined gas chromatography mass spectrometry gc ms low rank vitrinite huminite reflectance 0 41 0 45 coals from the tambang air laya mine represent different maceral assemblages of an ideal succession of ombrogenous palaeo peat development in a vertical section high rank coals vitrinite reflectance 1 42 5 18 from the suban mine have been thermally metamorphosed by an andesitic intrusion variations in the distributions of n alkanes isoprenoids and saturated and aromatic biomarkers in the low rank coals reflect variations in local source input and palaeomire conditions terpenoid biomarkers such as cadinane and eudesmane type sesquiterpenoids and oleanane and ursane type triterpenoids indicate the predominance of angiosperm plants in the palaeomire particularly dipterocarpaceae the distribution of hopanoids is affected by the organic facies of the coal and their maturity and correlates with the palaeomire evolution as derived from petrological studies close to the igneous intrusion rapid thermal stress has destroyed most of the biomarkers but variations in n alkane distributions attributable to palaeomire conditions remain reversals in the trends for molecular parameters based on aliphatic hydrocarbons n alkane distribution and pristane phytane ratio and aromatic hydrocarbons methyl phenanthrenes with coal rank are observed

zoned poikiloblastic garnets p t paths and syn metamorphic uplift through 30 km of structural depth wopmay orogen canada

timing and heat sources for the barrovian metamorphism scotland abstract new shrimp u pb zircon ages of 472 2 5 8 ma and 471 2 5 9 ma are presented for the age of peak metamorphism of barrovian migmatites 40 ar 39 ar ages for white mica from the barrovian metamorphic series are presented and are recalculated using recently proposed revisions to the 40 k decay constants to allow more precise and accurate comparison with u pb ages the 40 ar 39 ar ages are found to vary systematically with increasing metamorphic grade between c 465 ma for the biotite zone and c 461 ma for the sillimanite zone there is no evidence for any significant metamorphic heating during the first 15 myr of the grampian orogeny before c 473 ma or the final 4 myr after c 465 ma the barrovian metamorphism occurred over a period of 8 myr within the 27 myr grampian orogeny the barrovian metamorphism records punctuated heating was temporally and spatially associated with large scale bimodal magmatism and developed within crust that was not overthickened the temporally distinct nature of the barrovian metamorphic episode within the grampian orogeny and its heating pattern and tectonic context are not consistent with significant heat contribution from thermal equilibration of overthickened crust rather the barrovian metamorphism records a transient phase of crustal thermal disequilibrium during the grampian orogeny temporal and spatial association with grampian bimodal magmatism is consistent with production of the barrovian metamorphic series within the middle crust as the result of advection of heat from the lower crust and or mantle the barrovian metamorphic series the classic example of orogenic regional metamorphism did not form in response to crustal thickening and thermal relaxation but appears to record large scale contact metamorphism

lateral variations in the crustal structure of the indo eurasian collision zone we thank michael ritzwoller and two anonymous reviewers for constructive comments that have helped improve the manuscript the majority of the seismic data used in this study were downloaded from iris dmc data for the ngri stations in india were provided by s s rai and zahid ra provided the pmd pakistan data kajal borah provided the ambient noise cross correlations for the uttaranchal network figures were prepared using generic mapping tools gmt software wessel and smith 1998 we thank robert herrmann for making the computer programs in seismology freely available

tectonic mode switches and the nature of orogenesis abstract the birth and death of many mountain belts occurs in lithosphere that over rides major subduction zones here the tectonic mode shortening versus extension can abruptly switch even during continuous and otherwise smooth convergence if the hinge line of the foundering slab rapidly retreats i e rolls back the foundering slab creates a gravitational potential well into which the orogen collapses this motion coupled with stress guides can pull the orogen apart a slowing of roll back or of hinge retreat means that the subduction flexure may subsequently begin to be pushed back or be pushed over by the advancing orogen the consequence of such changes in relative motion is that orogenic belts are affected by abrupt tectonic mode switches the change from push to pull leads to a sudden change from horizontal extension to horizontal shortening potentially throughout the entire mass of the orogenic lithosphere that over rides the subducting slab the sequencing of these tectonic mode switches affects the thermal evolution of the orogen and thus fundamentally determines the nature of orogenesis this insight led to us to our quite different views as to how orogens work it is evident that orogens affected by abrupt push pull mode switches are characterized by high pressure metamorphism whereas orogens affected by abrupt pull push mode switches are characterized by high temperature metamorphism magmatism and anatexis

some remarks on high temperature low pressure metamorphism in convergent orogens many high temperature low pressure high t low p metamorphic terranes show evidence for peak mineral growth during crustal thickening strain increments at pressures near the maximum attained during the heating cooling cycle such terranes are not readily explained as the conductive response to crustal thickening since the resulting moho temperatures would greatly exceed the crustal liquidus and because heating due to conductive equilibration on length scales appropriate to lithospheric scale strains must greatly outlast the deformation consequently high t low p metamorphism may be generated during crustal thickening only when significant heat is advected within the crust as for example may occur during the segregation of granitic melts we show that without the addition of asthenospheric melts and at strain rates appropriate to continental deformation the conditions required for significant lower crustal melting during deformation are only likely to be attained if heat flow into the lower crust during crustal thickening is increased substantially for example by removing the mantle part of the lithosphere a simple parameterization of lithospheric deformation involving the vertical strain on the scale of the crust c and the lithosphere 1 respectively allows the potential energy of the evolving orogen to be readily evaluated using this parameterization we show that an important isostatic consequence of the deformation geometries capable of generating such high t low p metamorphism during crustal thickening with c1 is an imposed upper limit to crustal thicknesses which is much lower than for homogeneous deformations fc f1 for the same initial lithospheric configuration

benue trough and benue chain it is suggested that the benue trough may have been initiated by the extension of the equatorial oceanic fracture zones onto the continent during the early stages of the separation of africa and south america a transcurrent movement along deep seated faults of the basement is responsible for the structure of the trough after a period of extension characterized by the sedimentary infilling of the trough and igneous activity the trough is turned into a chain through a short phase of compression a discrete pattern of en echelon folds and the strain pattern deduced from the analysis of large scale and microfracturing correspond to a n s to n160 e direction of compression coupled to a sinistral transcurrent movement along a ne sw direction the difference in age of the folding phase in the northern and southern parts of the chain indicates a diachronous nature for this tectonic event in relation to the wrench regime the tertiary evolution of the trough marked by a tensional regime resulting in an e w extension is to be related to the cameroon volcanic line

mechanical model for the tectonics of doubly vergent compressional orogens a mechanical model of crustal shortening and deformation driven by the relative convergence of rigid underlying mantle plates explains many features of convergent orogens results based on numerical models and supported by sandbox models show that a coulomb crustal layer subject to basal velocity boundary conditions corresponding to asymmetric detachment and subduction of the underlying mantle passes through three stages of orogenic growth 1 block uplift bounded by step up shear zones 2 development of a low taper wedge over the underthrusting mantle plate and 3 development of a low taperwedge overlying the overthrusting mantle plate and verging in the opposite direction when modified by isostasy basal viscous flow surface erosion and denudation and sedimentation the resultant model orogens exhibit a variety of styles with characteristics in common with small rapidly denuded orogens large orogens with plateaus and extensional characteristics and active subduction margins with doubly vergent accretionary wedges and deformed fore arc basins

organic geochemical characteristics and oil generating potential of the upper jurassic safer shale sediments in the marib shabowah basin western yemen the organic rich safer shales exposed in the north central part of onshore marib shabowah basin are evaluated and their depositional environments are interpreted total organic carbon contents toc of the shales range from 1 02 16 8 wt and yield hydrogen index hi values ranging from 130 to 820 mg hc g toc consistent with mainly type ii with minor contributions from type i and mixed types ii iii kerogens the safer shale samples have vitrinite reflectance values in the range of 0 5 1 0 ro indicating early mature to peak mature stage for oil generation tmax values range from 429 438 c which are in reasonably good agreement with vitrinite reflectance data kerogen microscopy shows that the safer shales are characterized by high amounts of organic matter consisting predominantly of yellow fluorescing amorphous organic matter and alginite of marine origin this is supported by their high content of hydrogen rich type ii and i oil prone kerogen the biomarker distributions of the upper jurassic safer extracts are characterized by dominant low to medium molecular weight compounds n c14 to n c20 low pr ph ratio 1 0 high phytane n c18 ratios 0 82 2 68 and predominant regular sterane c27 all biomarker parameters clearly indicate that the organic matter was derived from marine algal inputs and deposited under anoxic reducing conditions hypersaline conditions also prevailed during deposition of these sediments as indicated by the presence of gammacerane

the alpine geodynamic evolution of penninic nappes in the eastern central alps the eastern central alps consist of several pennine nappes with different tectonometamorphic histories the tectonically uppermost units oceanic avers bundnerschiefer continental suretta and tambo nappes oceanic vals bundnerschiefer show cretaceous early tertiary w directed thrusting with associated blueschist facies metamorphism related to subduction of the pennine units beneath the austroalpine continental crust this event caused eclogite facies metamorphism in the underlying continental adula nappe the gross effect was crustal thickening the tectonically lower continental simano nappe is devoid of any imprint from this event in the course of continent continent collision high t metamorphism and n directed movements occurred both affected the whole nappe pile more or less continuously from amphibolite to greenschist facies conditions crustal thinning commenced during the regional temperature peak a final phase is related to differential uplift under retrograde p t conditions further thinning of the crust was accommodated by e to ne directed extensional deformation

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

vapour absent melting from 10 to 20 kbar of crustal rocks that contain multiple hydrous phases implications for anatexis in the deep to very deep continental crust and active continental margins

the role of mantle delamination in widespread late cretaceous extension and magmatism in the cordilleran orogen western united states extension during plate convergence and mountain building is widely recognized yet the causes of synconvergent extension remain controversial here we propose that delamination of lithospheric mantle aided by decoupling of the crust from the mantle via a reduction in the viscosity of the lower crust through heating incursion of fl uids and partial melting explains many enigmatic yet prevalent aspects of the metamorphic magmatic and kinematic history of the sevier laramide orogen of the western united states during the late cretaceous extension heating anatexis magmatism and perhaps rock uplift were widespread during a restricted time interval in the late cretaceous 75 67 ma along the axis of maximum crustal thickening within the mojave sector of the sevier orogen and to a lesser extent within the interior of the idaho utah wyoming sector to the north similar processes may have been active in the peninsular range sierran western mojave and salinian segments of the mesozoic cordilleran arc these processes are viewed as predictable consequences of the thermal rheological and dynamic state of the overlying crust following delamination of mantle lithosphere beneath isostatically compensated mountain belts the proposed delamination would have occurred immediately prior to eastward propagation of lowangle subduction of the farallon plate during the inception of the laramide orogeny following delamination extension and anatexis of the north american crust were aided locally by egress of slab derived fl uids from the low angle farallon slab we suggest that lithospheric delamination may have aided in the shallowing of the slab to achieve lowangle subduction geometry delamination has been proposed to be common in areas of thickened continental lithosphere in the terminal phase or late in orogenesis the late cretaceous delamination event proposed here for the sevier laramide orogen occurred during protracted plate convergence and was synchronous with and followed by continued shortening in the external part of the orogen

striated conical structures and related fractures in bituminous coal of the southern canadian rocky mountains abstract striated structures many of which are conical in form are common mesoscopic features in bituminous and semi anthracite coals of the southern canadian rocky mountains the structures are planar conical to pyramidal in outline and are characterized by striae which radiate from a common apex and bifurcate or horsetail to form secondary structures on the master surface they are up to 20 cm in length 4 cm in width and have apical angles varying from 10 to 50 the striated structures occur at numerous localities in both highly sheared and relatively unsheared coal seams the conical striated structures closely resemble some shatter cones developed in fine grained rocks whereas the planar striated structures are somewhat similar to chevron structured hackle marked joints the pyramidal structures are apparently unique to coal all three types of structures are considered to be the result of brittle shear fracture based on their occurrence orientation and morphology the apparently restricted occurrence of the structures to coal is likely a function of the homogeneity and low tensile and compressive strength of bituminous and semi anthracite coals possibly augmented by high inter and or intra particle gas pressure resulting from devolatization of the coal during progressive coalification

bimodal behavior of extended continental lithosphere modeling insight and application to thermal history of migmatitic core complexes abstract extensional migmatitic core complexes micc expose partially molten crustal rocks exhumed during post thickening extension low heat flow in active collisional mountains suggests that there is i either input from extra crustal heat sources or ii under assessment of the pre extensional collisional heat content to form high grade metamorphic core complexes we evaluate these two hypotheses by varying thermal gradients and crustal thicknesses of two dimensional thermo mechanical numerical models simulating the extension of a stratified lithosphere for moho temperatures 35 and 50 km extension leads to a rifting mode dictated by the upwelling of the asthenosphere in that case no micc is formed for moho temperatures 700 c and or crustal thicknesses 60 km extension develops micc while maintaining a flat moho similar to wide extensional systems such as the aegean and the basin and range however when crustal strain softening is switched on it produces for both mentioned conditions miccs with similar crustal geometries testifying that partial melting in miccs can result from both extra crustal syn extensive heat asthenospheric advection and collisional heat typically miccs induced by asthenospheric heat show the sequence lower crustal migmatization doming and crustal ultra high temperature uht conditions and mantle melting in a short temporal range from 5 to 10 model ma after asthenospheric upwelling the convective migmatitic dome is shifted horizontally with respect to the asthenospheric culmination and dome migmatites experience isobaric heating to peak conditions in contrast collisional heat induced miccs exhibit pre extensional migmatization and subsequent doming at 3 4 model ma in this case the asthenosphere generating lower crustal uht conditions and mantle melts upwells lately and is not symptomatic for micc formed in short extensional pulses

himalayan metamorphic sequence as an orogenic channel insight from bhutan the bhutan himalayas differ from the rest of the himalayas in two major ways i low grade sedimentary rocks lie above the greater himalayan sequence as klippen i e erosional remains of the south tibetan detachment and ii an out of sequence thrust the kakhtang thrust lies structurally above the klippen and it doubles the exposed thickness of the greater himalayan sequence our field observations and geochronological data constrain the main kinematic events in the bhutan himalayas crystallisation ages of leucogranite dykes deformed by the main central thrust and the south tibetan detachment indicate that these two structures operated together between 16 and 22 ma the out of sequence kakhtang thrust was active at 10 14 ma and was concurrent with reactivation of the south tibetan detachment restoration of the bhutan himalayas prior to the out of sequence thrusting shows that the greater himalayan sequence was the core of a long low viscosity crustal channel extending under the tibetan plateau we propose that the gravity driven southward extrusion of the channel material from underneath the tibetan plateau caused the inverted metamorphic sequence in the lesser himalayan sequence and in the greater himalayan sequence this process also led to occurrences of present day surface rocks that were derived from variable distances down dip but from similar crustal depths such an exhumation pattern can explain the similar peak pressures for the greater himalayan sequence along the length of the himalayas

styles of crustal deformation in compressional orogens caused by subduction of the underlying lithosphere abstract crustal scale deformation is calculated for models in which the driving mechanism corresponds to the asymmetric detachment and underthrusting of the underlying mantle lithosphere the plane strain finite element model results provide indications of the styles of deformation to be expected in small compressional orogens in particular these styles occur where shortening of the mantle lithosphere is achieved by the nearly rigid convergence between lithospheric mantles and the subduction of one mantle beneath the other the crust is modelled using coulomb plastic frictional and thermally activated power law viscous rheologies and the effects of compositional layering and variable geothermal gradients are included results are presented for a range of models in which the strength of the coupling between the model crust and its basal boundary surface denudation partial and total subduction of the crust and compositional layering are examined the results show the development of inclined step up shear zones which are a consequence of conjugate thrusting in regions of coulomb controlled rheology these zones link to sub horizontal shear zones which occur where composition and temperature render the crust viscously weak the model strain fields are interpreted in terms of deformation on discrete planes and the seismic reflectivity fabric that may be associated with this discrete deformation finally we ask whether similar seismic reflectivity fabric can be recognized in observations from small compressional orogens

linking metamorphic textures to u pb monazite in situ geochronology to determine the age and nature of aluminosilicate forming reactions in the northern monashee mountains british columbia abstract the monashee mountains of the canadian cordillera are thought to expose a classic barrovian facies series of isograds the timing of aluminosilicate growth in the region was determined for four pelitic schist samples by combining textural relationships with monazite compositional zoning and monazite u pb geochronology conducted directly on thin sections by the laser ablation method three distinct phases of kyanite growth are recorded in the kyanite zone at c 153 ma between 122 and 94 ma and between 76 and 58 ma for each phase monazite and garnet grew synchronously with kyanite probably by a reaction involving the breakdown of staurolite in contrast sillimanite growth by muscovite dehydration melting occurred at or before c 104 ma in the sillimanite zone and retrograde sillimanite grew in schists previously metamorphosed at the kyanite grade during the first two phases by the influx of hot acidic fluids during top to the east shearing at ca 71 ma these results indicate that rocks metamorphosed at different places and different times in the orogen were juxtaposed prior to being overprinted at the sillimanite grade in the late cretaceous early paleocene during the influx of hot fluids in a structurally coherent body deforming by easterly directed shearing this study also provides new insight into monazite petrogenesis and suggests that at least in some circumstances monazite formation is linked to the staurolite out reaction that produces kyanite

neoproterozoic high pressure low temperature metamorphic rocks in the avalon terrane southern new brunswick canada high p low t metamorphic rocks of the hammondvale metamorphic suite hms are exposed in an area of 10 km2 on the nw margin of the caledonian avalon terrane in southern new brunswick canada the hms is in faulted contact on the se with c 560 550 ma volcanic and sedimentary rocks and co magmatic plutonic units of the caledonian terrane the hms consists of albite and garnet porphyroblastic mica schist with minor marble calc silicate rocks and quartzite pressure and temperature estimates from metamorphic assemblages in the mica schist and calc silicate rocks using twq indicate that peak pressure conditions were 12 4 kbar at 430 c peak temperature conditions were 580 c at 9 0 kbar 40ar 39ar muscovite ages from three samples range up to 618 615 ma a minimum age for high p low t metamorphism in this unit these ages indicate that the hms is related to the c 625 600 ma subduction generated volcanic and plutonic units exposed to the se in the caledonian terrane the ages are also similar to those obtained from detrital muscovite in a neoproterozoic cambrian sedimentary sequence in the caledonian terrane suggesting that the hms was exposed by latest neoproterozoic time and supplied detritus to the sedimentary units the hms is interpreted to represent a fragment of an accretionary complex similar to the sanbagawa belt in japan it confirms the presence of a major cryptic suture between the avalon terrane sensu stricto and the now adjacent brookville terrane

the organic geochemistry of the eocene oligocene black shales from the lunpola basin central tibet abstract this paper reports on the depositional paleoenvironment and the potential hydrocarbons of the eocene oligocene black shales from the dingqinghu and niubao formations in the lunpola basin central tibet nineteen samples from two outcrop profiles were analysed the contents of the total organic carbon toc and sulphur were measured other analyses included rock eval pyrolysis solvent extraction and gas chromatography mass spectrometer gc ms the results indicated that the shales from the dingqinghu and niubao formations are thermally immature the pyrolysis data show that the shales contain type i organic matter and that lacustrine algal are the main organic matter sources the low pristane to phytane ratios and the high gammacerane indices indicate that the shales were deposited in a reducing stratified and hypersaline palaeo lake which is consistent with the climate information provided by the development history of palaeo lakes from the eocene to the oligocene epochs

hot orogens tectonic switching and creation of continental crust comment and reply comment in a stimulating paper collins 2002 1 p 535 asserted that many granulite terrains were too hot to have formed during continental collision in spite of evidence to the contrary e g harley 1989 2 o brien and rotzler 2003 3 based on this assertion collins 2002 1 p

fractionation and incipient self granulitization during deep crust emplacement of lower ordovician valle fertil batholith at the gondwana active margin of south america abstract large granite batholiths were emplaced at the gondwana active margin during lower ordovician in south america these have contributed to crustal growth by net addition of silicic rocks to the continental crust new u pb shrimp zircon age determinations together with thermobarometric and geochemical data yield that batholith magma intrusion is the responsible of heating and self granulitization of early gabbro pulses partially molten granulitic gabbros which appear as either early intruded into the metasedimentary host or as large inclusions within the batholith forming qtz diorites contain opx bearing trondhjemitic leucosomes surrounding hbl opx pl mafic mesosomes forming typical agmatitic structures hornblende plagioclase equilibria applied to mineral pairs of granoblastic aggregates in textural equilibrium of metagabbro mesosomes yield temperatures in the range 850 910 c for core to core pairs and in the range 1000 1075 c for rim to rim pairs at pressures of about 0 7 gpa shrimp zircon age revealed that the whole batholith was emplaced over a narrow time interval of 20 ma from 465 to 485 ma with most ages clustered at about 470 ma the age of metagabbros is 473 7 ma for older zircons and 454 4 for younger zircons these ages are almost coincident within error with the age of host migmatites 477 5 ma and those of batholith intrusion of 476 9 ma and 475 3 ma for qtz diorites and 475 5 ma for granites zircon overgrowths of these intrusive rocks yield sages clustered around 450 ma revealing a protracted thermal history more complex than previously believed the geochemical study reveals that qtz diorites tonalites and granodiorites form a continuous trend produced by magmatic fractionation from a parental dioritic magma a weak adakitic tendency with sr y 15 in several samples implies the presence of grt in the source or magma chamber at a minimum pressure of 1 0 gpa higher than the depth of emplacement at 0 7 gpa the high temperature of magma emplacement which induced the incipient self granulitization of early magmatic pulses together with the cotectic like fractionation linking coeval qtz diorites tonalites and granodiorites is compatible with fractionation at the lower crust of a deep generated infracrustal sublithospheric intermediate magma

styles of post subduction collisional orogeny influence of convergence velocity crustal rheology and radiogenic heat production abstract a number of models of continental collision already exist but the role of foregoing oceanic plate subduction or variable coupling between plates still remains to be explored in addition heat generation by radioactive decay may be quite variable for example our geochemical data from low to high grade metamorphic rocks of the lesser and higher himalayan sequences reveal measured heat production values on the order 4 5 w m 3 which is considerably higher than normal inferred upper crustal values 3 therefore we performed 2d numerical experiments of oceanic continental subduction followed by continental collision for varying continental lower crustal strength convergence velocity and radiogenic heating rate oceanic subduction produces thinning and upwarping of the overriding lithospheric wedge such as is observed in andean type collisional settings cascadia and central andes after collision two main orogenic styles develop depending on the values of key parameters either a slow convergence rates weak shear heating and consequently strong rheological coupling between plates and or a weak lower crust generate a two sided thick and narrow symmetrical collisional zone or b high convergence rates strong shear heating and therefore weak rheological coupling between plates and a strong lower crust produce a one sided thin and broad asymmetrical collisional zone in this case a vertical lithospheric body divides two upper crustal wedges that develop distinct deformation patterns in both cases enhanced radiogenic heating favors the formation of midcrustal partially melted channels that propagate toward the surface and the pro foreland partial melting triggers lateral extrusion of metamorphic rocks from different depths 15 23 kbar 700 c on top of coherent lp lt units the geometry of metamorphic isograds inverted shapes of p t paths at different grades and exhumation rates are similar to those observed in parts of the himalayan chain indicating that the models capture the essence of real systems the model results demonstrate that internal heat production rate is a crucial parameter in determining the timing of slab breakoff polarity reversal and exhumation of the inverted metamorphic sequence with respect to the timing of partial melting late stages of some numerical experiments showed interesting geodynamical evolution of the collisional zone with the appearance of polarity reversal roll over of both plates and roll back of the subducting slab irrespective of these details hundreds of km of continental lithosphere are recycled into the mantle in all models supporting the idea that continental crustal consumption is indeed a common and considerable process during collision this affects inferences about extents of convergence and or shortening during orogenesis moreover crustal recycling into the deep mantle during collisional orogeny needs to be considered in models of crustal growth and evolution

controls on low pressure anatexis low pressure anatexis whereby rocks melt in place after passing through the andalusite stability field develops under more restricted conditions than does low pressure metamorphism our thermal modelling and review of published work indicate that the following mechanisms operating alone may induce anatexis in typical pelitic rocks without inducing wholesale melting in the lower crust i magmatic advection by pervasive flow ii crustal scale detachment faulting and iii the presence of a high heat producing layer of these only magmatic advection by pervasive flow and crustal scale detachment faulting have been shown quantitatively to provide sufficient heat to cause widespread melting combinations of the above mechanisms with pluton scale magmatic advection shear heating removal of the lithospheric mantle or with each other provide additional means of developing suitable high temperatures at shallow crustal levels to generate low pressure anatexis

the phillips pluton maine usa evidence of heterogeneous crustal sources and implications for granite ascent and emplacement mechanisms in convergent orogens the phillips pluton age of 403 8 1 3 ma was assembled at a crustal level below the contemporary brittle plastic transition during regional dextral reverse transpressive deformation the pluton is composed dominantly of medium to coarse grained leucogranite sensu lato s l but within its bounds includes decametric massive outcrop of fine to medium grained granodiorite s l in places the leucogranite contains centimetric enclaves apparently of the granodiorite granodiorite is host to more biotite than muscovite and more calcic oscillatory zoned plagioclase compared to the leucogranite pegmatitic granite and composite pegmatite aplite occur as metric sheets within the pluton and as larger bodies outside the pluton to the sw magmatic fabrics defined by biotite schlieren occur locally in the leucogranite the attitude of these fabrics and layering within the leucogranite are concordant with the ne striking steeply dipping country rock foliation k2o contents rb sr ratios rb sr and ba covariations and chondrite normalized rare earth element ree patterns of leucogranite are consistent with high to moderate a h2o muscovite dehydration equilibrium eutectic melting of a predominantly pelite source similar to metasedimentary rocks of the surrounding central maine belt cmb the ree patterns and rb sr ratios of granodiorite also suggest derivation from a metasedimentary source but more likely by moderate to low a h2o muscovite biotite dehydration equilibrium eutectic to non eutectic minimum melting of a protolith dominated by greywacke in which garnet and plagioclase were residual phases both granite s l types have heterogeneous initial nd isotope compositions samples of granodiorite define a range in end 404 ma of 1 8 to 0 1 0 3 2 uncertainty and samples of leucogranite define a range in end 404 ma of 8 0 to 5 3 0 3 2 uncertainty this bimodal distribution suggests that melts were derived from a minimum of two sources the data are consistent with these sources being cmb metasedimentary rocks end 404 ma 4 for the granodiorite the range of nd isotope compositions within each granite type most likely reflects isotopic heterogeneity inherited from the source these data imply that the integrity of individual melt batches was maintained during ascent and that extensive mixing of melt batches during emplacement at this level in the pluton did not occur although centimetric enclaves have intermediate nd isotope compositions consistent with small scale interactions between magmas we infer that the phillips pluton represents the root of a larger pluton and that what remains of this larger pluton is the feeder constructed from multiple melt batches arrested during waning flow of granite magma through a crustal scale shear zone system

oleanane parameter verification by quantitative study of the biomarker occurrence in sediments of the niger delta abstract the variation of the absolute concentration of oleananes in three selevted sedimentary columns of the tertiary niger delta is paralleled by the corresponding oleanane parameter op profile thereby confirming that the latter can be utilized for estimating the relative contribution of terrigenous organic matter it is shown that op features a dynamic range especially at the oil generative window ogw and can precisely delimit the top of the ogw which for the niger delta is shown to coincide approximately with the 80 c isotherm and ro 0 55 this study provides the first direct evidence of the inadequacy of vitrinite reflectance as a maturity parameter for type iii kerogen furthermore based on the new findings an alternative model for oleanene oleanane interconversions in the geosphere is proposed

microfacies and petroleum properties of coals as revealed by a study of north sea jurassic coals abstract the influence of the composition and microstructural aspects of coal microfacies on petroleum properties were investigated in light of petrographic and geochemical studies the first study concerns a collection of north sea jurassic coals of various maceral compositions the second study is a pyrolysis gas chromatography study on pure microlithotypes from a similar french jurassic coal the results show that hydrocarbon potential is logically increased by a high content in liptinite but more often in hydrogen rich vitrinite in the same way the hydrocarbon potential is reduced by a high inertinite content mainly due to pre sedimentary oxidation of the original biomass but also due to early diagenetic processes that lead especially to the formation of micrinite or to post sedimentary alterations these trends indicate that total hydrocarbon potential is increased by an autochthonous origin of the organic matter that was deposited in a relatively closed paleoenvironment and thus protected from detrital and organo detrital inputs finally the oil potential of humic coals as determined by open pyrolysis gas chromatographic studies as related to the weight of rock is similar to classical source rocks the structural aspects of the coal microfacies which probably play a role in the release of oil from coal seams in natural conditions are also discussed

tectonic assembly of inverted metamorphic sequences inverted metamorphic sequences are characterized by peak metamorphic temperatures that increase structurally upward with isograds that are typically parallel to associated thrust faults a fully coupled thermal mechanical model for convergent orogens shows how an inverted sequence can be tectonically assembled in a crustal scale ductile shear zone at moderate to high rates of synorogenic erosion model inverted sequences form by tectonic juxtaposition of points with widely differing initial positions that reach peak temperatures at different times and in different places within the model orogen and thus do not represent metamorphic or thermal gradients inverted crustal isotherms are not required to produce model inverted isograds model results agree well with metamorphic pressure temperature data from the main central thrust zone of the central nepal himalayas

modelling tectonic styles and ultra high pressure uhp rock exhumation during the transition from oceanic subduction to continental collision the tectonic styles of the transition from oceanic subduction to continental collision and their implications for exhumation of uhp material are tested in a series of numerical thermal mechanical models that address subduction collision dynamics at constant convergence rate reference models examine the sensitivity to continental mantle lithosphere density pro continental margin temperature and rheology and retro continental margin rheology three tectonic styles are observed 1 continental subduction without significant deformation of the over riding continent favoured by relatively dense mantle lithosphere and a strong retro continental margin 2 deformation and subduction of the leading edge of the retro continent favoured by low density mantle lithosphere and strong retro continental crust and 3 backthrusting of the retro continental margin and subsequent double subduction favoured by low density mantle lithosphere and weak retro continental crust continental u hp material is formed in all three tectonic styles although it is only efficiently exhumed in the first case three mechanisms contribute to u hp exhumation their importance depending on the evolving geometry of the subduction zone the strength of the continental margin crust and the density of the subducting mantle lithosphere weaker hotter continental margins decouple in the subduction channel and are subsequently exhumed by buoyancy and or driven circulation of material and or forced expulsion in response to insertion of strong continent into the subduction channel plunger effect stronger colder continental material is subducted without detachment and is then transported laterally underplating the retro continental mantle lithosphere neither surface erosion nor slab break off are required for uhp exhumation

petrographic and geochemical characterization of the upper cretaceous coal and mudstones of gombe formation gongola sub basin northern benue trough nigeria implication for organic matter preservation paleodepositional environment and tectonic settings abstract under exploration of the upper cretaceous petroleum systems in the northern benue trough nigeria has been attributed largely to lack of subsurface samples and non integration of organic and inorganic geochemical methods recent access to surface sampling from maiganga coalfield revealed a need for detailed evaluation of the sedimentary facies types for better understanding of the hydrocarbon potential of the area detailed maceral point count py gc chnso as well as major oxides and trace element icp ms analysis were used to characterize the gombe formation coals and the mudstones for determination of the organic matter origin preservation paleodepositional environment tectonic setting and paleoclimatic condition during sedimentation evaluation based on the maceral composition tpi gi values plot of trace elements v versus ni and sedimentary succession suggest deposition of predominantly terrigenous organic matter 45 to 73 huminite maceral in an upper deltaic setting for the maiganga facies the maceral analysis associated with py gc studies indicates that the sedimentary facies of gombe formation have potential to generate liquid hydrocarbon this can be estimated from an average of 21 50 liptinite content and the ternary plot of the py gc pyrolysate m p xylene n octene and phenol suggesting presence of type ii and type ii iii kerogen within the basin assessment of the paleodepositional environment based on the trace elements composition indicates a moderate stratified water column and salinity within a relatively sub oxic to oxic conditions during sedimentation the major oxide composition as well as the binary plots of sio2 versus al2o3 k2o na2o and log sio2 versus k2o na2o for the shales and mudstones revealed a predominance of semi arid to slightly humid warm paleoclimatic conditions and a passive continental margin setting this is in accordance with the cretaceous tectonic events of the west and central africa which affected the petroleum system in the entire benue trough

an arbitrary lagrangian eulerian formulation for creeping flows and its application in tectonic models summary this paper presents and analyses numerical techniques developed to investigate viscoplastic stokes flows within a model of lithospheric deformation in particular the techniques are related to a subduction model of compressional orogens the driving mechanism in the model corresponds to the near rigid convergence and subduction of one mantle lithosphere beneath another in plane strain and this boundary condition forces flow in an overlying viscoplastic model crust the numerical techniques use the arbitrary eulerian lagrangian formulation in which flows with free surfaces and large deformation are computed on an evolving eulerian finite element grid that conforms to the material domain a regridding algorithm allows the associated lagrangian motion and fields to be followed and in addition coupled back to the eulerian calculation of the flow mass flux boundary conditions are used so that the effects of erosion and deposition by surface processes and mass loss by subduction can be included in the model calculation the evolving model crustal layer is flexurally compensated using a general elastic beam formulation the applicability of the numerical techniques to problems ranging from accretionary wedges to crustal and lithospheric scale deformation is discussed simple flows a linear viscous subduction model a whirl flow and a quasi convection model are used to show that the mass conservation regridding and surface tracking errors are small the broader applicability of the modelling techniques is reviewed

organic geochemical characteristics of crude oils from the masila basin eastern yemen the masila basin is an important hydrocarbon province in yemen but the origin of hydrocarbons and their generation history are not fully understood in this regard 10 crude oils from different petroleum reservoir sections in the masila basin were characterized by a variety of biomarker and non biomarker parameters using gc gc ms and stable carbon isotope techniques oils from the masila basin display pristane phytane pr ph ratios ranging from 1 7 to 2 0 low sulfur content high c35 homohopane index relatively high c27 sterane concentrations and relatively high tricyclic terpanes suggesting a marine clay source rock that was deposited in mildly anoxic to suboxic conditions with dominantly algal organic matter c29 20s 20s 20r steranes and bb bb aa sterane ratios indicate that the masila oils have reached peak oil window maturity another related feature of these oils is the absence of 18a h oleanane which suggests a source age older than cretaceous the carbon isotope compositions are similar to those of the potential source rocks which range from 25 4 to 28 3 indicating a marine environment the new data presented in this paper suggest that the masila oils constitute one oil family and that the oil originated from the upper jurassic madbi source rock in the basin

990 and 1100 ma grenvillian tectonothermal events in the northern oaxacan complex southern mexico roots of an orogen inliers of f1 0 1 3 ga rocks occur throughout mexico and form the basement of the oaxaquia microcontinent in the northern part of the largest inlier in southern mexico rocks of the oaxacan complex consist of the following structural sequence of units from bottom to top which protolith ages are 1 huitzo unit a 1012f12 ma anorthosite mangerite charnockite granite amcg suite 2 el catro n unit z1350 ma orthogneiss migmatized at 1106f6 ma and 3 el marquez unit z1140 ma para and orthogneisses these rocks were affected by two major tectonothermal events that are dated using u pb isotopic analyses of zircon a the 1106f6 ma olmecan event produced a migmatitic or metamorphic differentiation banding folded by isoclinal folds and b the 1004 978f3 ma zapotecan event produced at least two sets of structures z1 recumbent isoclinal class 1c 3 folds with gently nw plunging fold axes that are parallel to mineral and stretched quartz lineations under granulite facies metamorphism and z2 tight upright subhorizontal wnw to nne trending folds accompanied by development of brown hornblende at upper amphibolite facies metamorphic conditions cooling through 500 jc at 977f12 ma is documented by 40 ar 39 ar analyses of hornblende fold mechanisms operating in the northern oaxacan complex under zapotecan granulite facies metamorphism include flexural and tangential longitudinal strain accompanied by intense flattening and stretching parallel to the fold axes subsequent phanerozoic deformation includes thrusting and upright folding under lower grade metamorphic conditions the zapotecan event is widespread throughout oaxaquia and took crustal rocks to a depth of f25 30 km by orogenic crustal thickening and is here designated as zapotecan orogeny modern analogues for zapotecan granulite facies metamorphism and deformation occur in middle to lower crustal portion of subduction and collisional orogens contemporaneous tectonothermal events took place throughout oaxaquia and in various parts of the genvillian orogen in laurentia and amazonia d 2003 elsevier science b v all rights reserved