t max of asphaltenes a parameter for oil maturity assessment abstract a new maturity parameter determined on both oil and bitumen samples the asphaltene tmax is proposed and discussed this parameter could be very useful to address the maturity of the source rock the asphaltene tmax is measured by programmed rock eval pyrolysis using a modified temperature program some phases of the experimental procedure such as the asphaltene preparation and the rock eval measurement substratum choice are crucial in order to achieve reliable data laboratory simulations were carried out in order to assess the possible effects of both primary and secondary migration on asphaltene tmax in the expelled oil the original value of the asphaltene tmax in the bitumen is not substantially modified and it is very close to that measured on kerogen examples of the determination of asphaltene tmax on many samples collected from different areas and with different organic matter composition are given results show that tmax values from oil asphaltenes are reasonable indicators of source rock maturity

the 1989 macquarie ridge earthquake seismic moment estimation from long period free oscillations we have analyzed the long period 1000 s part of the earth s free oscillation spectrum of the 1989 macquarie ridge earthquake this event stands out as the largest in the last decade and as the first great earthquake to have been recorded digitally on the new generation of very broadband seismic sensors around the globe its spectrum was modeled for five spectral peaks 0s4 0s6 0s9 1s8 and 5s3 taking into account the effects of lateral heterogeneity rotation and ellipticity which cause coupling within the spectral peaks self coupling and between different multiplets multiplet coupling the effects of self coupling were calculated following the formulation of woodhouse and girnius 1982 multiplet coupling only weakly affected the spectral peaks that were used a large discrepancy in phase and amplitude between the calculated spectrum and the data was observed which suggests that the seismic moment was 50 larger than that inferred from surface wave analysis satake and kanamori 1990 an error analysis was performed in order to detect inaccuracies that might have resulted in this discrepancy centroid time source finiteness correction and directivity effects are likely to cause a significant bias both in the moment and phase determination we saw no direct evidence that a precursory event could have been responsible for the additional energy release as was suggested by ihmle et al 1993

teleseismic detection of a slow precursor to the great 1989 macquarie ridge earthquake low frequency spectra for the 1989 macquarie ridge earthquake magnitude 8 2 show an amplitude increase and a phase delay decrease below 6 millihertz that require a short term slow precursor this earthquake can be modeled as a compound event in which a fast rupturing ordinary earthquake was initiated by an episode of slow smooth deformation that began more than 100 seconds before the main shock the moment released in the slow precursor was large about 3 x 10 20 newton meters equivalent to an event of magnitude 7 6 the data are consistent with the precursor being generated in a region of the oceanic upper mantle below the main rupture

the pyrope coesite rocks and their country rocks at parigi dora maira massif western alps detailed petrography mineral chemistry and pt path both the coarse and fine grained varieties of the partly coesite bearing pyrope quartzites their interlayered jadeite kyanite rocks and the biotite phengite gneiss country rock common to all of them were subjected to detailed petrographic and textural studies in order to determine the sequence of crystallisation of their mineral constituents which were also studied analytically by microprobe prior to pyrope and coesite growth the mg rich metapelites were talc kyanite chlorite rutile ellenbergerite schists which upon continued prograde metamorphism developed first pyrope megacrysts in silica deficient local environments at the expense of chlorite talc kyanite and subsequently the smaller pyrope crystals with coesite inclusions from reacting talc kyanite based on geobarometrically useful mineral inclusions as well as on experimentally determined phase relations a prograde pt path simplified for water activity 1 is constructed which passes through the approximate pt conditions 16 kbar and 560 c 29 kbar and 720 c and finally up to 37 kbar at about 800 c where the mg rich metapelite was a pyrope coesite rock with phengite kyanite and talc still present during the retrograde path pyrope was altered metasomatically either into phlogopite kyanite quartz or at a later stage to chlorite muscovite quartz both assemblages yield pt constraints the latter about 7 9 kbar 500 600 c the country rock gneisses have also endured high pressures of at least 15 kbar but they provide mostly constraints on the lowest portion of the uplift conditions within the greenschist facies about 5 kbar 450 c microprobe data are presented for the following minerals pyrope ellenbergerite dumortierite unusually mgti rich jadeite vermiculite formed after na phlogopite paragonite and for several generations of phengite chlorite talc phlogopite dravite and glaucophane in the high pressure rocks as well as for biotite chlorite phengites epidote garnet albite and k feldspar in the country rock gneisses an outstanding open problem identified in this study is the preservation of minerals as inclusions within kyanite and pyrope beyond their pt stability limits

coesite and pure pyrope in high grade blueschists of the western alps a first record and some consequences a pyrope quartzite originally described by vialon 1966 from the dora maira massif was resampled and reinvestigated garnet up to 25 cm in size phengite kyanite talc and rutile are in textural equilibrium in an undeformed matrix of polygonal quartz the garnet is a pyrope almandine solid solution with 90 to 98 mol mg end member it contains inclusions of coesite which has partially inverted to quartz resulting in a typical radial cracking of the host garnet around the inclusions several lines of evidence show that coesite crystallised under nearly static pressure conditions and that the whole matrix has once been coesite

h2o transport and release in subduction zones experimental constraints on basaltic and andesitic systems phase relationships in natural andesitic and synthetic basaltic systems were experimentally investigated from 2 2 to 7 7 gpa and 550 c to 950 c in the presence of an aqueous fluid in order to determine the stability of hydrous phases in natural subducted crustal material and to constrain reactions resulting in the release of water from subduction zones to the mantle wedge water reservoirs in subducted oceanic crust at depths exceeding the amphibole stability field 70 80 km are lawsonite 11 wt h2o mg chloritoid 8 wt talc 5 wt and zoisite clinozoisite 2 wt in basaltic rocks and lawsonite zoisite clinozoisite phengite 4 wt and staurolite 2 wt in andesitic compositions the thermal stability of lawsonite at 6 0 gpa extends to 800 c and 870 c in basaltic and andesitic compositions respectively at pressures above amphibole out 2 3 2 5 gpa lawsonite reacts through continuous reactions with steep positive dp dt slopes to zoisite clinozoisite until 3 0 3 2 gpa and at higher pressures to more than 7 7 gpa to assemblages containing garnet clinopyroxene and garnet clinopyroxene kyanite in basaltic and andesitic compositions respectively on the contrary the breakdown of zoisite clinozoisite is mainly pressure sensitive phengite represents the hydrous phase with the largest stability field encountered in this study in andesite phengite is stable to more than 7 7 gpa and more than 920 c talc and staurolite contribute in minor amounts to the water balance in basaltic and andesitic rock compositions a model for water release from the subducted slab is developed combining thermal models for subduction zones with the experimentally determined phase relationships up to 1 wt and 2 wt h2o in basaltic and andesitic rocks respectively can be stored to depths beyond 200 km in cold subduction zones mainly by lawsonite and phengite dehydration rates are high until amphibole out and relatively low at greater depths the amphibole out reactions are found to release a significant amount of water in a depth interval of several kilometers however they do not represent a discrete pulse of fluid and do not completely dehydrate the descending slab fluid release at depths greater than 200 km through phengite and progressive lawsonite breakdown would hydrate the overlying mantle causing the generation of amphibole or phlogopite peridotite at higher geothermal gradients epidote zoisite contributes to fluid flux to the mantle wedge at 100 120 km depth the extensive stability field of phengite may greatly enhance the role of sediments and the small amount of potassium in mafic compositions for the fluid budget in subduction zones at increasing depth

a gabbro to eclogite transition on flemsoy sunnmore western norway the migmatitic gneisses on the nordoyane islands western norway enclose abundant inclusions of coronitic gabbros and different types of country rock eclogites gabbros on flemsoy show a prograde step wise transition to eclogite via a transitional stage in which garnet and orthopyroxene form coronas between igneous olivine and sodic plagioclase relics after igneous plagioclase cumulates eclogitization proceeds by a complex set of continuous and discontinuous reactions opx and spinel form as transient phases but are consumed in various omphacite forming reactions during eclogite recrystallization relict igneous clinopyroxene shows a continuous increase in jd content while garnet becomes less calcic and more homogeneous from the corona stage to recrystallized eclogite the reactions are strongly diffusion controlled and have been enhanced locally by oxidation and the presence of a fluid phase and deformation while low strain areas of the gabbros remained coronitic whole rock chemistry mineral parageneses and calculated p t conditions are similar for the gabbroeclogites and two types of country rock eclogites some of the layered country rock eclogites appear to have been derived from the coronite bodies p estimates are in the order 15 20 kbar while kgarcpxd values suggest maximum t values of 750 60 c for the eclogite metamorphism in the area

oxygen and carbon isotope evidence for seawater hydrothermal alteration of the macquarie island ophiolite abstract rocks of the miocene macquarie island ophiolite south of new zealand have oxygen and carbon isotopic compositions comparable to those of seafloor rocks basalt glass and weathered basalts have 18o values at 5 8 6 0 and 7 9 9 5 respectively similar to drilled seafloor rocks including samples from the leg 29 dsdp holes near macquarie island compared to the basalt glass the greenschist to amphibolite facies metaintrusives are depleted in18o 18o 3 2 5 9 similar to dredged seafloor samples whereas the metabasalts are enriched 18o 7 1 9 7 although the gabbros are only slightly altered in thin section they have exchanged oxygen with a hydrothermal fluid to a depth of at least 4 5 km there is an approximate balance between18o depletion and enrichment in the exposed ophiolite section the carbon isotopic composition of calcite in the weathered basalts 13c 1 0 2 0 is similar to those of drilled basalts but the metamorphosed rocks have low 13c values 14 6 to 0 9 these data are compatible with two seawater circulation regimes in the upper regime basalts were weathered by cold seawater in a circulation system with high water rock ratios 1 0 based on calcite compositions weathering temperatures were less than 20 c and the carbon was derived from a predominantly inorganic marine source as previously suggested for the samail ophiolite it is postulated that the lower hydrothermal regime consisted of two coupled parts at the deeper levels seawater circulating at low water rock ratios 0 2 0 3 and high temperatures 300 600 c gave rise to18o depleted gabbro and sheeted dikes via open system exchange reactions during reaction the seawater underwent a shift in oxygen isotopic composition 18o 1 0 5 0 and subsequently caused18o enrichment of the overlying metabasalts in the shallower part of the hydrothermal regime the metabasalts were altered at relatively high water rock ratios 1 0 10 0 and temperatures in the range 200 300 c the relatively low water rock ratios in the hydrothermal regime are supported by the low 13c values of calcite interpreted as evidence of juvenile carbon in contrast to the inorganic marine carbon found in the weathered basalts

pressure temperature and time indicators in mafic schist their application to reconstructing the polymetamorphic history of vermont mineral chemistry and overgrowth relationships in mafic schist from vermont are shown to be sensitive indicators of pressure temperature and relative time and to be good chronicles of the paleozoic history of this polymetamorphic terrane within the common assemblage amphibole chlorite epidote plagioclase quartz ti phase carbonate k mica fe 3 oxide electron microprobe analyses show that increasing metamorphic grade as defined by intercalated pelitic schist is recorded by an increase in the edenite glaucophane and tschermakite contents of amphibole in the anorthite content of plagioclase and in the substitution of al vi f 2 ti al iv for fe 2 mg mn si in biotite chlorite and muscovite with increasing pressure the glaucophane component of amphibole increases for medium pressure metamorphism the albite oligoclase gap is in the garnet zone where amphibole has between 1 2 and 1 8 formula proportion al iv and al vi fe 3 ti cr this gap is below the garnet isograd in low pressure mafic schist where al iv and al vi fe 3 ti cr in the amphibole are both less than 0 6 mineral growth periods observed are characterized by metamorphic grade and facies series and are assigned to two ordovician taconic and two devonian acadian events silurian devonian mafic schist in northeastern vermont records two periods of low pressure devonian metamorphism these events are expressed by medium pressure and low pressure metamorphism in the cambrian and ordovician rocks to the west and south in the pre silurian rocks high pressure and medium pressure metamorphism is assigned to the ordovician the observed high pressure metamorphism is confined to a 110 by 40 km area along the green mountain anticlinorium axis in north central vermont

paleoenvironmental reconstruction of the coal and oil shale bearing interval in the lower cretaceous muling formation laoheishan basin northeast china abstract the laoheishan basin in northeast china has been filled with lower cretaceous coal and oil shale bearing sediments the basin fill includes from base to top alluvial conglomerate lower member of muling formation fan delta sediments interbedded with coal and oil prone mudstone denominated as oil shale layers in the upper member of muling formation and volcaniclastic rocks dongshan formation in the present study the maturity of organic matter oil shale quality and paleoenvironment of the coal and oil shale accumulation are investigated based on macro and micropetrographic data proximate and ultimate analyses bulk geochemical parameters biomarkers analysis and stable isotope geochemistry in the laoheishan basin both coal and oil shale layers are derived from land plant organic matter this contrast the oil shale of this basin from oil shale coal intercalations in fault related basins in which oil shale has been found to be of algal origin the coal is sub bituminous in rank hydrogen rich and oil prone the oil shale is of low medium grade and the relatively low oil yield may relate to the abundant resinite and sporinite because of their lower generation potential compared with alginite accumulation of the high ash coal commenced in low lying mire drowned during frequent floodings subsequently high ash low sulfur coal was deposited in a stable low lying mire under oxic conditions and limited bacterial activity afterwards the mire was drowned and formed a freshwater dysoxic to oxic shallow lake in which oil shale layers accumulated finally the depositional environment returned to low lying mire but probably with a brackish influence as indicated by elevated sulfur contents in the uppermost samples petrography and biomarker based proxies indicate that gymnosperms dominated the paleovegetation of the mire accompanied by variable amounts of herbaceous plants such as ferns

low frequency source characteristics of the great 1960 chilean earthquake the low frequency nature of the great 1960 chilean earthquake the earth s largest recorded event is characterized and a source model composed of three events with a combined seismic moment of 5 5 1023 n m and a duration of 1500 s is presented the results show conclusively that large scale slip which is only observed at low frequencies preceded the main shock complex amplitude measurements of normal mode data in the band 1 0 5 0 mhz recorded at eight vertical component stations comprise the data set in this band the earthquake source is assumed to be described by a line source propagating at a constant rupture velocity the data set is the basis for a sequence of least squares inversions to determine the seismic moment rupture velocity spatial slip distribution and the temporal slip function the data are best explained by a source model that begins 1150 s before the main shock with an event of moment 1 9 1023 n m and rise time of 300 s a main shock with moment of 3 2 1023 n m and for which most of the low frequency energy was released beginning about 50 s after the short period origin time and an event 350 s later with moment of 0 4 1023 n m the uncertainty in the moment estimate of the great 1960 chilean earthquake is about 50 and is due primarily to the probable range of fault plane dip which is not well constrained at this time

rifted margin source rock deposition a carbon isotope and biomarker study of a west african lower cretaceous lacustrine section lower cretaceous passive margin sections penetrated in wells peripheral to the deep basinal development of pre salt sediments of the liawenda ponta vermelha troughs in the lower congo coastal basin revealed sustained source potential kerogen type kinetic ea a and carbon isotopic data plus rock extract biomarker signatures confirmed large systematic organofacies variations within these rift phase sediments an inverse correlation between 25 28 30 trisnorhopane and gammacerane abundances suggested three major depositional regimes reflecting salinity and water depth control 13c depleted basal sediments showing strong gammacerane 4 methyl sterane signatures were segregated by a major isotopic excursion this event represented the transition between saline playa lake to deeper water stratified conditions supporting a high level of anoxia for these intermediate sediments sterane and tricyclic diterpane abundances plus sterane hopane ratios had marine connotations and could be interpreted as the result of intermittent marine incursions fore shadowing irreversible oceanic ingression a resurgence of gammacerane abundance in the uppermost sediments typified littoral shallow marine depositional conditions recognition of a possible non ubiquity in lacustrine depositional conditions during the opening of a rifted margin provides further insight into the complexity and attendant controls on hydrocarbon generation in such habitats

talc magnesite and ti clinohumite bearing ultrahigh pressure meta mafic and ultramafic complex in the dabie mountains china

paragonite eclogites from dabie shan china re equilibration during exhumation paragonite in textural equilibrium with garnet omphacite and kyanite is found in two eclogites in the ultrahigh pressure metamorphic terrane in dabie shan china equilibrium reactions between paragonite omphacite and kyanite indicate a pressure of about 19 kbar at c 700 c however one of the paragonite eclogites also contains clear quartz pseudomorphs after coesite as inclusions in garnet suggesting minimum pressures of 27 kbar at the same temperature the disparate pressure estimates from the same rock suggest that the matrix minerals in the ultrahigh pressure eclogites have recrystallized at lower pressures and do not represent the peak ultrahigh pressure assemblages this hypothesis is tested by calibrating a garnet zoisite clinozoisite kyanite quartz coesite geobarometer and applying it to the appropriate eclogite facies rocks from ultrahigh and high pressure terranes these four minerals coexist from 10 to 60 kbar and in this wide pressure range the grossular content of garnet reflects the equilibrium pressure on the basis of the reaction zoisite clinozoisite grossular kyanite quartz coesite h o the results of the geobarometer agree well with independent pressure estimates from eclogites from other orogenic belts for the paragonite eclogites in dabie shan the geobarometer indicates pressures in the quartz stability field confirming that the former coesite bearing paragonite eclogite has re equilibrated at lower pressures on the other hand garnets from other coesite bearing but paragonite free kyanite zoisite eclogites show a very wide variation in grossular content corresponding to a pressure variation from coesite into the quartz field this wide variation partly due to a rimward decrease in grossular component in garnet is caused by partial equilibration of the mineral assemblage during the exhumation

fluid melt rock interaction in mafic eclogites and coesite bearing metasediments constraints on volatile recycling during subduction the question as to whether the metasomatizing slab agent needed to form island arc magmas is an aqueous fluid or a hydrous melt requires a knowledge of fluid melt rock interaction during high pressure low temperature metamorphism in the western alps typical anhydrous mafic eclogites monviso massif and coesite bearing metasediments dora maira massif have experienced a prograde p t path characteristic of a mature cold subduction zone microstructural petrologic geochemical and fluid inclusion studies show that fluid flow was limited during eclogite and high grade blueschist facies metamorphism most of the fluids were driven off the rocks at pressures lower than 1 and 1 6 gpa respectively with increasing pressure and temperature the remaining fluid phase was released during crystal plastic flow processes or devolatilisation reactions but retain the host rocks to form veins partial melts or dense hydrous silicates tectonic erosion at modern convergent margins delivers large amounts of terrigenous sediments kmash system in subduction zones in contrast to the basalt and peridotite systems the kmash system is characterized by a wide range of pressure sensitive reactions which allow the reconstruction of dehydration hydration depths in the subducted slab independently of the thermal structure of subduction zones terrigenous sediments will begin to melt at pressures 2 gpa the melt phase can absorb tremendeous quantities of water and represent major sinks for devolatilisation fluids dense hydrous silicates will form in any subducted sediments at all depths these silicates can store significant amount of water in their structure in the subducted oceanic crust and upper mantle heterogeneous distribution of fluids inherited from hydrothermal alteration at mid ocean ridges will cause contemporaneous dehydration and hydration reactions on a local scale rather than large scale fluid infiltration in the overlying mantle wedge the situation where a cold competent and fluid rich subducted slab is bounded above by a hot relatively weak and anhydrous material is likely to act against buoyant rise of devolatilisation fluids by forcing the fluids to flow downwards at sub arc depths 50 10 to 80150 km and for any geothermal gradient continuous internal buffering of volatile activities will retain the fluid phase in the slab this is in contrast with shallow sub forearc depths 20 to 50 10 km where tectonic vein arrays localized thrust faults serpentine diapirs mud volcanoes and boninite magmas attest to the release of large volume of fluids into the overriding mantle wedge

compositions and sources of fluids in sediments of the peru subduction zone sediments of the middle to lower slope regions of the peru continental margin drilled during ocean drilling program odp leg 112 contain interstitial waters with chloride concentrations as low as 454 mm cl equivalent to 20 dilution of seawater with fresh water strontium isotope data suggest that two separate fluid regimes are present a northerly one in which fluids contain more radiogenic sr than seawater 87sr 86sr up to 0 70996 possibly originating from continental crust and a southerly regime where fluids contain sr less radiogenic than seawater 87sr 86sr down to 0 70722 and which are also enriched in ca2 and depleted in mg2 and 18o that appear to have interacted with oceanic basement o c and he isotopic data support these interpretations

high pressure metamorphism in mafic schist from northern vermont the only confirmed occurrence of glaucophane and omphacite in the appalachian mountains of new england is in mafic schist from tillotson peak 44 48 72 33 north central vermont mineral assemblages observed all with epidote sphene magnetite phengitic muscovite carbonate calcite or dolomite apatite sulfide are 1 actinolite glaucophane chlorite garnet quartz 2 actinolite glaucophane chlorite albite garnet 3 actinolite glaucophane omphacite garnet 4 glaucophane chlorite garnet quartz 5 glaucophane omphacite quartz 6 albite glaucophane chlorite quartz and 7 albite paragonite chlorite quartz electron microprobe investigations show that plagioclase chlorite sphene white mica carbonate and magnetite grains are relatively homogeneous however amphibole pyroxene epidote and garnet grains are zoned indicating incomplete equilibration and changing physical conditions during metamorphism glaucophane in assemblage 6 has the general formula rim composition na 1 9 m4 ca 0 1 mg 1 8fe 1 2 2 al 1 0 vi fe 0 3 3 si 7 9 al 0 1 iv o 22 oh 2 glaucophane in assemblages 1 to 5 is somewhat poorer in the glaucophane endmember and richer in actinolite ca rich amphibole grains are zoned extensively with barroisite cores and actinolite rims omphacite grains in assemblages 3 and 5 have the general formula rim composition na 0 5 ca 0 5 mg 0 4 fe 0 1 2 al 0 4 vi fe 0 1 3 si 2o 6 in spite of the significant zoning the partitioning of elements among the minerals at their grain margins suggests that all seven assemblages above may coexist at the same physical conditions of metamorphism in mafic rocks with a relatively small range in bulk composition correlation of mineral compositions and assemblages with mineral stability data suggests high pressure facies series metamorphism at about 9 2 kb and 450 100 c with p h2o p total it is proposed that the metamorphism occurred during the ordovician

the high pressure stability of hydrous phases in orogenic belts an experimental approach on eclogite forming processes abstract metamorphic evolution of orogenic belts is characterized by reactions involving hydrous phases stability relations of hydrous phases depend on h 2 o activity availability during metamorphism however hydrous phases do occur in synthetic and natural systems also at water undersaturated conditions h 2 o undersaturated experiments in the cao al 2 o 3 sio 2 h 2 o model system were performed in order to predict the role of hydrous phases in h 2 o undersaturated compositions at constant pressure and temperature these experiments yielded different assemblages of hydrous anhydrous phases depending on the experimental bulk h 2 o content in natural eclogites of mafic to intermediate bulk composition common hydrous phases include amphibole lawsonite zoisite mg chloritoid paragonite and talc complex continuous reactions delimit the stability fields of the different assemblages involving these hydrous phases such continuous reactions can be used as powerful petrogenetic tools to derive p t conditions of natural eclogites derivation of h 2 o undersaturated phase relationships for natural and synthetic systems show that hydrous phases also occur in h 2 o undersaturated bulk compositions at eclogite facies conditions comparison of experimentally determined phase relationships in mafic rocks with pressure temperature evolution of orogenic terrains demonstrates that at subsolidus conditions a trend toward h 2 o saturation is necessarily maintained during prograde p t paths in orogenic belts as far as dehydration reactions occur any prograde p t path necessarily leads to a maximized occurrence of hydrous phases and thus a maximized h 2 o preservation is achieved during metamorphism

hydrogeochemistry of the northern barbados accretionary complex transect ocean drilling project leg 110 detailed studies of the major element geochemistry oxygen and hydrogen isotope ratios of pore fluids and the 87sr 86sr ratio of dissolved strontium have made it possible to unravel physical and chemical processes that affect the pore fluid chemistry in a transect of drill holes across the northern barbados accretionary complex these processes include 1 alteration of volcanic ash buried in the pleistocene pliocene sediment column 2 alteration of underlying basalts of layer 2 of the oceanic crust 3 movement of fluids from deep in the accretionary complex along fault zones particularly the decollement and minor permeable layers these fluids from deeper in the complex are characterized by low chloride concentrations and increased 18o h2o values presumably as a result of dehydration of smectite interlayers and 4 mixing processes involving the migrating fluids cause incongruities in the geochemical anomalies of these fluids

continuous measurements of crustal deformation for the 1992 landers earthquake sequence abstract we describe and attempt to interpret continuous measurements of strains and tilts made at pinon flat observatory pfo before during and after the landers and joshua tree earthquake sequences these data show substantial transient deformation following the landers mainshock with a total amplitude of several percent of the co seismic deformation and a decay time of at least several days comparing data from the many types of instruments at pfo allows us to infer possible sources for this deformation the immediate postseismic transient was nearly the same size on three long base strainmeters suggesting either broadscale deformation or local motion near one part of the observatory the latter can largely be ruled out by the similarity of many other measurements in the area covered by these strainmeters and the observations by others of significant postseismic displacements nearer the source possible mechanisms for broad scale deformation include postseismic fault slip time dependent creep in near surface rocks and elastic or thermal responses to water table changes the first two agree best with the observations from pfo but if postseismic fault slip is the source it must have been distributed differently than the co seismic slip and may have included faults other than those that ruptured seismically if one of the other mechanisms is the main source the pfo data imply that the postseismic slip must have been very much smaller than the seismic slip perhaps 2 or less no significant preseismic deformation was observed at a level of 2 10 3 of the co seismic deformation for the days to minutes before the earthquake

the upper thermal stability of chlorite quartz an experimental study in the system mgo al2o3 sio2 h2o experiments up to water pressures of 21 kbar have been undertaken to bracket the reactions chlorite quartz talc kyanite h2o chlorite quartz talc cordierite h2o and talc kyanite quartz cordierite h2o by reversed runs in the system mgo al2o3 sio2 h2o mash these reaction curves intersect at an invariant point ip1 at ph2o 6 4 0 2 kbar and a temperature of 624 4 c the curve of the chlorite quartz breakdown to talc kyanite h2o at water pressures above 6 4 kbar shows a negative dp dt with the slope decreasing with rising pressure whereas the slope of the breakdown curve to talc cordierite h2o at water pressures is clearly positive the composition of the chlorite solid solution reacting with quartz has been estimated to be approximately mg4 85al1 15 al1 15si2 85o10 oh 8 over the entire pressure range investigated the composition of the talc solid solution forming by the breakdown of chlorite quartz appears to be mg2 94al0 06 al0 06si3 94o10 oh 2 at ph2o 2kbar with increasing pressure the al content of talc decreases reaching a value of about 0 06 atoms per formula unit at p h2o 21 kbar as a consequence of the new experimental data the existing phase topologies of the mash system and k2o mash system have been revised for example the invariant point ip1 and the univariant reaction curve kyanite talc h2o chlorite cordierite are stable for this reason the development of medium to high temperature metamorphic rocks compositionally approximating the mash system must be reconsidered the whiteschists from sar e sang afghanistan are treated as an example the application of the present experimental data to metamorphic rocks of more normal composition requires the examination of the influence of further components this leads to the conclusion that the introduction of fe2 into magnesian chlorite extends its stability field in the presence of quartz by 10 15 c in comparison with pure mg chlorite

guidelines for evaluating petroleum source rock using programmed pyrolysis rock eval pyrolysis is used to rapidly evaluate the petroleum generative potential and thermal maturity of rocks accurate conclusions require programs every 30 60 ft 9 18 m understanding of interpretive pitfalls and supporting data such as visual kerogen vitrinite reflectance and elemental analyses the generative potential of coals is commonly overestimated by pyrolysis and is best determined by elemental analysis and organic petrography most coals show high s2 s3 5 and low hi values 300 mg hc g toc migrated oil and mud additives which alter rock eval data can sometimes be removed by special processing for immature rocks bimodal s2 peaks and pi values over 0 2 indicate contamination pyrolysis downgrades organic poor clay rich rocks which show lower hi and higher tmax values than isolated kerogen because of adsorption of pyrolyzate on the clays tmax values for small s2 peaks 0 2 mg hc g toc are unreliable tmax is affected by maturation organic matter type contamination and the mineral matrix s3 is sensitive to inorganic and adsorbed carbon dioxide and to instrumentation problems acidification of carbonate rich samples and proper maintenance improves s3 measurement constant sample weights 100 mg are recommended below a threshold weight tmax increases by up to 10 c and other parameters decrease organic rich samples which overload the detector can be diluted with carbonate detector linearity is determined by pyrolyzing various weights of an organic rich rock

fluid composition and evolution in coesite bearing rocks dora maira massif western alps implications for element recycling during subduction fluid inclusions and f cl concentration of hydrous minerals were analysed in the coesite pyrope quartzite the interlayered jadeite quartzite and their country rock gneiss from the dora maira massif using a combination of microthermometry raman spectrometry synchrotron x ray microfiuorescence and electron microprobe analysis three populations of fluid inclusions were recognized texturally and can be related to distinct metamorphic stages a low salinity aqueous fluid occurs in the retrogressed country gneiss and as late secondary inclusions in jadeite quartzite and chloritized pyrope an earlier secondary population is found in matrix quartz of the jadeite and pyro pe quartzites this population can be related to the early decompression and so to incipient breakdown of garnet into phlogopite bearing assemblages the inclusion fluid is highly saline up to 84 wt equivalent nacl and contains na ca fe cu and zn as major cations in pyrope quartzite additional k was found in these brines which locally coexist with co2 rich inclusions the oldest fluid inclusions are preserved in kyanite grains included in fresh pyrope and in pyrope itself in pyrope all inclusions have decrepitated and contain magnesite an mg phosphate sheet silicate s a chloride and an opaque phase with no fluid preser ved in contrast the kyanite inclusions in pyrope preserve primary h2o co2 low salinity fluid inclusions probably owing to the low compressibility of the kyanite inclusions and host garnet in spite of in situ re equilibration these inclusions can be interpreted as relics of the dehydration fluid that attended pyrope growth these correlations between textural and chemical fluid inclusion data and metamorphic stages are consistent with the fluid composition calculated from the halogen content of different generations of phlogopite and biotite the preservation of different fluid compositions both in time and space is evidence for local control and possibly origin of the fluids in agreement with isotopic data these results in particular the absence of co2 in the jadeite quartzite are best interpreted in terms of a fluid melt system evolution with increasing metamorphism partitioning of h2o na ca fe and heavy metals into melt jadeite quartzite and mg na k f co2 and p into a residual aqueous fluid can account for depletion in na ca and fe of the pyrope quartzite during the retrograde path a h 2 o rose as melt crystallized generating the two populations of hypersaline and water rich fluids that were highly reactive to pyrope the process of fluid melt interaction envisioned here coupled with models of melt extraction in subduction zones provides an attractive opportunity for the instantaneous 1 ma and selective transport of elements between a downgoing slab and the overlying mantle wedge

continuous monitoring of seismic energy release associated with the 1994 northridge earthquake and the 1992 landers earthquake we have developed a method to detect long period precursors for large earthquakes observed in southern california if they occur the method allows us to continuously monitor seismic energy radiation over a wide frequency band to investigate slow deformation in the crust e g slow earthquakes especially before large earthquakes we used the long period records 1 sample sec from terrascope a broadband seismic network in southern california the method consists of dividing the record into a series of overlapping 30 min long windows computing the spectra over a frequency band of 0 00055 to 0 1 hz and plotting them in the form of a time frequency diagram called spectrogram this procedure is repeated daily over a day long record we have analyzed the 17 january 1994 northridge earthquake m w 6 7 and the 28 june 1992 landers earthquake m w 7 3 no slow precursor with spectral amplitude measured over a duration of 30 min larger than that of a magnitude 3 7 was detected prior to either event in other words there was no precursor whose moment was larger than 0 003 of the mainshock

expulsion of oil from petroleum source rocks inferences from pyrolysis of samples of unconventional grain size oil expulsion efficiencies from different lithologies have been investigated and compared employing pyrolyses of samples of unconventional grain size from 300 mg hc g corg show that liquid range hydrocarbons are expelled from coal when the generated oil exceeds the coal sorption capacity this suggests that expulsion of oil from liptinite poor or in general h poor humic coals is primarily limited by the release of generated liquid hydrocarbons from kerogen because such coals generally do not generate sufficient amounts of liquid hydrocarbons to overcome the coal sorption capacity

the 1987 superstition hills earthquake sequence strains and tilts at pinon flat observatory abstract we describe observations at pinon flat observatory of preseismic co seismic and postseismic strain and tilt changes associated with the superstition hills earthquake sequence of 24 november 1987 the data come from two long base strainmeters one long base tiltmeter and a borehole dilatometer co seismic offsets were seen for the two largest earthquakes these offsets roughly agree with those from a dislocation in a half space though with disagreements at the 30 per cent level suggesting some error in one or more of the co seismic observations for the main shock the moment deduced from the offsets is 5 to 6 10 25 dyne cm less than that found from the seismic data no precursive strain changes were seen for the 8 hr before the main shock any strain changes were less than the instrument noise level 1 per cent of the co seismic strain during the last 1000 sec before this earthquake any precursory strains must be less than 0 5 per cent of the co seismic amount postseismic strain during the 2 hr after the main shock was about 10 per cent of the co seismic strain

source parameters of the october 1 1987 whittier narrows earthquake from crustal deformation data offsets in the regional strain field generated by the october 1 1987 whittier narrows earthquake were recorded with large amplitudes on two deep borehole dilational strainmeters at distances of 46 7 and 65 5 km from the hypocenter and marginally on instruments at greater distances in the parkfield area and at pinon flat where laser extensometers also recorded small offsets these data are insufficient to solve for the location and physical parameters of the earthquake but by also using the measured elevation changes in the epicentral area we are able to invert for source models consistent with all available observations of crustal deformation the source models obtained indicate that slip extends to a depth of about 30 km well below the recorded aftershock zone the requirement for deeper and presumably aseismic slip derives from the large negative dilatation experienced by the nearest strainmeter pubs but high frequency data from the same site exclude any significant slow component of moment release by ignoring pubs we obtain a moment of about 0 7 1018 n m on a fault that has a strike and dip of about n60 w and 40 down to the north respectively and extends to a depth of about 16 km the most likely reason for the anomalous offset at pubs appears to be sympathetic slip triggered on a nearby fault by the main shock precursive strain during the month prior to the earthquake is not apparent at the nanostrain level in the data from the closest instrument but the event was accompanied by a change in strain rate at the two nearer sites

episodic aseismic earthquake precursors shallow earthquakes are generally believed to be brittle fractures in a stressed medium with rupture velocity at a speed close to that for shear waves we know however that the earth allows failure over a wide range of timescales creep events occur on the san andreas fault system and for some earthquakes the high speed rupture is accompanied by a slower process1 5 here we report on episodic slow events which occurred before the japan sea earthquake of magnitude 7 7 on 26 may 1983 a sacks evertson borehole strain meter6 90 km from the earthquake recorded about 100 aseismic strain events in a five month period before and immediately following the earthquake none has been detected after the large aftershocks the observed signals are consistent with a precursory redistribution of stress through aseismic slip on a deep extension of the main shock fault plane such episodic aseismic events may provide a mechanism for relatively rapid stress concentration before earthquakes

simple kinetic models of petroleum formation part i oil and gas generation from kerogen modern basin analysis uses kinetic models to predict the extent of petroleum generation within potential source rocks the global kinetic model presented here assigns kinetic parameters based on gross depositional environment and stratigraphic age this is useful in areas of low geochemical knowledge including exploration frontiers five kerogen kinetic organofacies each characterized by a specific organic matter input and early diagenetic overprint can broadly be related to sedimentary facies age associations even using seismic sequence stratigraphy alone a aquatic marine siliceous or carbonate evaporite any age b aquatic marine siliciclastic any age c aquatic non marine lacustrine phanerozoic d e terrigenous non marine ever wet coastal mesozoic and younger and f terrigenous non marine coastal late palaeozoic and younger routine pyrolysis data allow partitioning of organic carbon in the immature source rock into four initial components oil oil generative gas generative and inert separate kinetic parameters for the oil and gas generative fractions allow computation of the evolving concentration and composition of the products it was assumed that the activation energy distributions for each fraction were normally distributed non linear regression of large combined laboratory and field data sets varying by up to 12 orders of magnitude in heating rate established optimum values for the required ten 5 2 kinetic parameter sets a emean e differences in kinetic parameters can be reconciled with known chemical properties although our knowledge base is still inadequate to construct kinetic models from first principles mean activation energies governing oil generation increase systematically in the order a f causing a corresponding increase in generation temperature at a reference heating rate 2 c ma 1 the oil generation window 10 90 oil generative kerogen degraded increases from ca 95 135 c to 145 175 c organofacies c has the tightest energy distribution and narrowest oil generation window the gas generation window ranges from ca 105 155 c to 175 220 c thermal stress results from a combination of temperature and time an order of magnitude increase decrease in heating rate elevates depresses reaction temperatures by ca 15 c heating rates in subsiding sedimentary basins can vary by two orders of magnitude dramatic differences in generation temperature can result if the heating rate and organofacies effects compound thus global oil and gas generation temperature thresholds constitute blunt instruments with which to screen the petroleum potential of sedimentary basins because unexpelled oil provides a potential feedstock for oil to gas cracking in the source rock quantitative models of petroleum generation become really useful only when coupled with models of oil gas cracking and expulsion for example the frequently observed association between gas provinces and coals cannot be explained solely by their generation behaviour

zoned high pressure assemblages in pillow lavas of the zermatt saas ophiolite zone switzerland pillow lavas of the zermatt saas ophiolite zone of the western swiss alps in places consist of zoned high pressure parageneses these assemblages which developed subsequent to the metamorphic peak are of lawsonite eclogite in the cores of pillows and dolomite bearing blueschist or glaucophane eclogite in the rims the matrix between the pillows comprises dolomite glaucophane rich material an analysis of reactions in the system na2o cao feo mgo al2o3 sio2 h2o co2 ncfmas h2o co2 indicates that one possible model is that these assemblages formed cofacially due to systematic intra pillow variations in the chemical potentials of h2o and co2 pillow rims equilibrated at higher values of co2 and lower values of h2o than pillow cores textural evidence suggests that infiltration has controlled the distribution of co2 generated by the breakdown of dolomite bearing assemblages during the return of the rocks to the surface variation in h2o content was caused by ocean floor hydrothermal alteration in these rocks therefore blueschist assemblages have been stabilised by higher values of co2 and eclogites by higher values of h2o if fluids were present as suggested by various lines of evidence then blueschist pillow rims equilibrated at higher co2 co2 h2o than eclogitic pillow cores

organofacies variation control on hydrocarbon generation a lower congo coastal basin angola case history abstract a group of lower congo coastal basin angola carbonate platform petroleums all determined to be of lower cretaceous pre salt bucomazi fm neocomian aptian rift phase provenance showed considerable internal diversity recognised in terms of biomarker abundances and waxiness this variation was particularly evident in their carbon isotopic compositions which ranged from 26 4 to 29 7 ppt although not exclusive to all oils many contained an abundance of an unusual triterpane tentatively characterised as 25 30 bisnorhopane using a combination of kerogen type i ii iii and kinetic data ea and a plus kerogen pyrolysate isotopic signatures a conspicuous variation in organofacies was observed over the vertical and lateral development of this lacustrine source unit in a series of control wells three oil prone organofacies were recognised two of which could be individually characterised as type i 13c depleted and type ii 13 c enriched assemblages a third assemblage of type ii iisuperior character and intermediate isotopic signature not dependent upon mixing of the above end members was similarly recognised significantly the 25 30 bisnorhopane marker observed in many of the oils was found to be exclusively associated with the type ii 13c enriched organofacies on this basis rationalisation of crrde oil character and genera was possible in terms of phased co generation from these limiting organofacies with subsequent co mingling of the resultant oils although dissimilar end member oils could be isotopically matched to source facies representative of the more labile type ii lower ea to the less labile type i higher ea candidates most oils showed a mixed character reflecting their aggregate compostion definition of organofacies make up and variation is crucial to the fine tuning of contemporary modelling practises in their application to the study of hydrocarbon productive habitats

warm humid paleoclimate control of salinized lacustrine organic rich shale deposition in the oligocene hetaoyuan formation of the biyang depression east china abstract lacustrine organic rich shales are widely developed within the oligocene biyang depression in the nanxiang basin in east china and across southeast asia these shales have been proven as hydrocarbon source rocks of crude oil as well as sources and reservoirs of shale oil understanding the deposition of these shales is essential to reconstruct the paleoenvironment and paleoclimate during the oligocene in east china as well as target the potential shale oil resources based on x ray diffraction organic and inorganic petrologic organic and inorganic geochemical and stable isotopic analyses paleoenvironment paleoclimate and hydrocarbon potential of the lacustrine organic rich shales in the oligocene hetaoyuan formation were investigated the mineral composition of hetaoyuan formation organic rich shales is dominated by clay minerals followed by quartz plagioclase calcite dolomite pyrite and gypsum six lithotypes are identified including siltstone silty shale clay shale calcareous shale dolomite shale and massive mudstone the organic matter om of the organic rich shales is dominated by type i kerogen lamalginite maturity parameters tmax biomarker ratios and calculated vitrinite reflectance suggest that the om enters the oil window maturity high toc and generative potential together with high contents of brittle minerals including quartz plagioclase and carbonates reveal the oligocene hetaoyuan formation hold good to very good potential of shale oil exploitation six organic rich shale layers have been identified in the hetaoyuan formation shale layer 1 to 6 from top to bottom among them shale layers 5 and 3 hold the best shale oil potential and therefore be investigated in the present paper shale layer 5 was deposited in a semi deep lacustrine environment under the moderately warm humid paleoclimate further dividing into lower middle and upper units relatively low toc contents av 2 6 wt in the lower unit are probably caused by high sedimentation rate high bioproductivity under favorable preservation conditions led to relatively high toc contents av 3 6 wt in the middle unit less reducing conditions together with moderate low bioproductivity resulted in relatively low toc contents av 2 6 wt in the upper unit warm humid paleoclimate prevailed during deposition of shale layer 3 deepened water depth and formed a deep lacustrine environment the stratified water column led to stable anoxic condition and photic zone euxinia in the saline deep lacustrine environment which enabled the accumulation of high amounts of om av 4 9 wt toc under excellent preservation av hi 710 mg hc g toc the results of core scanning analysis indicate that formation of the laminated shales probably controlled by seasonal fluctuations of paleoclimate

processing of crust in the root of the caledonian continental collision zone the role of eclogitization segments of the root to the caledonian collision zone are exposed along the west coast of norway allowing the study of processes and petrophysical conditions at the deepest crustal levels p t conditions prevailing in these crustal root zones correspond to eclogite facies eclogitization is associated with marked changes in petrophysical properties notably a density 10 15 and vp increase which may give a mantle signature to an eclogitized crust the observed ductility enhancement may be caused by transformation plasticity reduction in grain size formation of new mineral assemblages and presence of fluid in the western gneiss region and the bergen arcs eclogites form along shear zones interpreted as fluid pathways with sharp boundaries to their metastable protolith this makes eclogite shear zones potential deep crustal reflectors dry crust subducted into roots of continental collision zones will undergo eclogitization only if hydrous fluids are available timing of metamorphic reactions does not depend on crossing of equilibrium boundaries in the p t space but rather on the introduction of fluids into the system eclogites are not only passive recorders of p t t paths illustrating very deep subduction of crust 100 km rapid changes in petrophysical properties make fluid induced eclogitization a dynamic process that influences geodynamics stresses set up by the volume changes or by tectonic forces are released by ductile deformation along shear zones and by seismic faulting as evidenced by syn eclogitic pseudotachylytes in the metastable protolith deformation including seismicity enhances fluid infiltration and further eclogitization dense and rheologically weak eclogites may delaminate and sink into the mantle in the absence of fluids crustal doubling may occur without metamorphic re equilibration the evolution of a collision zone depends on the fluid regime which should be considered in geodynamic modelling in addition to p and t

geodetic predictions of a strike slip fault model implications for intermediate and short term earthquake prediction mechanical models of faulting during an earthquake cycle now exist that are based on laboratory derived constitutive laws for rock friction such models allow calculation of geodetic changes as a function of depth distance from the fault and time and are useful for gaining insight on the magnitude and timing of geodetic changes that might occur prior to an earthquake we have used the tse and rice 1986 model of a crustal strike slip fault to study the sensitivity of surface deformation to changes in the values of the friction constitutive parameters and the amount of dynamic overshoot during an earthquake models were examined with values of the characteristic decay distance dc in the constitutive law ranging from 5 to 80 mm with near surface values of the steady state velocity dependence a b of 0 0015 and 0 003 and with either zero or 30 dynamic overshoot the time variations of the predicted displacement velocity strain and strain rate distributions are strongly affected by changes in dc and a b and only slightly affected by the amount of dynamic overshoot the larger values of dc and the less negative value of a b i e large ratios of dc to b a produce larger and earlier precursory signals the most useful signals for predicting the model earthquakes are strain changes near the fault because for nearly all model parameters studied the predicted strain changes are considerably larger than the detectability limits of existing borehole strain meters in the presence of earth noise detectable changes occur a few minutes to a month prior to the earthquake depending on parameter values premonitory increases in velocity also occur a pattern of larger velocities at distances a few kilometers away from the fault trace is a diagnostic indication of accelerating slip at depth that would be useful for predictive purposes unfortunately these velocity changes are detectable with creep meters and two color laser electronic distance measuring devices only for the largest ratios of dc to b a in agreement with recent measurements for several earthquakes the predicted amount of preseismic moment release is 0 5 of the earthquake moment for most simulations and the postseismic moment release is a significant fraction of the earthquake moment this two dimensional model suggests that earthquakes of magnitude 6 or greater on strike slip faults that cut the earth s surface could be predicted using existing instrumentation unless the ratio of dc to b a for natural faults is equal to or smaller than the smallest values investigated by tse and rice 1986 study of three dimensional models is needed since premonitory signals will be smaller if along strike variation is considered

near field high resolution strain measurements prior to the october 18 1989 loma prieta ms 7 1 earthquake high resolution strain recordings were made in deep boreholes throughout california prior to during and following the october 18 1989 ms 7 1 loma prieta earthquake the nearest dilational strainmeters sensitivity 1010 and 3 component tensor strainmeters sensitivity 10 9 were 37 km to 42 km respectively from the main shock high quality data including details of strain offsets were recorded on both instruments through the earthquake these data have been searched for indications of short intermediate and long term strain redistribution and or fault slip that might have indicated imminent rupture short and intermediate term changes in both tensor strain and dilational strain several nanostrain if any during the minutes to months before the earthquake are at least 1000 times smaller than that generated by the earthquake itself if short term preseismic slip did occur at the nucleation point of the earthquake during the previous week and if the type of slip is similar to that during the earthquake its moment could not be more than 1024 dyne cm stated another way slip equivalent to that expected for a m 5 3 earthquake could have occurred in the hypocentral region without the strainmeters detecting it at these distances and azimuthal positions long term changes appear to have occurred in mid 1988 and mid 1989 these changes were both followed by ml 5 earthquakes in the hypocentral region on june 27 1988 and august 8 1989 respectively and since they correspond approximately to changes in geodetic strain rate over the epicenter may indicate precursory strain redistribution in the epicentral region minor postseismic strain recovery 14 occurred in the month following the main shock

continuous borehole strain in the san andreas fault zone before during and after the 28 june 1992 mw 7 3 landers california earthquake abstract high precision strain was observed with a borehole dilational strainmeter in the devil9s punchbowl during the 11 58 ut 28 june 1992 m w 7 3 landers earthquake and the large big bear aftershock m w 6 3 the strainmeter is installed at a depth of 176 m in the fault zone approximately midway between the surface traces of the san andreas and punchbowl faults and is about 100 km from the 85 km long landers rupture we have questioned whether unusual amplified strains indicating precursive slip or high fault compliance occurred on the faults ruptured by the landers earthquake or in the san andreas fault zone before and during the earthquake whether static offsets for both the landers and big bear earthquakes agree with expectation from geodetic and seismologic models of the ruptures and with observations from a nearby two color geodimeter network and whether postseismic behavior indicated continued slip on the landers rupture or local triggered slip on the san andreas we show that the strain observed during the earthquake at this instrument shows no apparent amplification effects there are no indications of precursive strain in these strain data due to either local slip on the san andreas or precursive slip on the eventual landers rupture the observations are generally consistent with models of the earthquake in which fault geometry and slip have the same form as that determined by either inversion of the seismic data or inversion of geodetically determined ground displacements produced by the earthquake finally there are some indications of minor postseismic behavior particularly during the month following the earthquake

kinematic thermal and petrological model of the central alps lepontine metamorphism in the upper crust and eclogitisation of the lower crust abstract seismic and seismological studies as well as gravimetric models indicate that a slab of european lithospheric mantle and lower crust is currently underthrust below the apulian crust we assume a simple kinematic model in which the lower and upper subducted european crusts are decoupled along a decollement the lower crust goes into subduction without deformation the upper crust deforms by pure shear with a horizontal compressional axis the total erosional flux is adjusted to balance upper crust input so that the belt keeps the same geometry different distributions of erosion being used the computed temperature field is steady state if the kinematic model applies during a minimum time of 40 myr for a convergence rate of 8 mm yr equilibrium mineral assemblages and densities are determined from the computed p t conditions for a granodioritic chemical composition of the upper crust and an andesitic composition of the lower crust assuming local isostasy the density model fits the average topographic profile across the central alps the p t t paths obtained for the part of the upper crust initially at depths 10 to 16 km are compatible with the medium pressure oligocene metamorphism in the lepontine dome the peak calculated temperature for the deepest non subducted crustal rocks is 600 c for a pressure of 0 8 gpa near the lower limit of high pressure amphibolites we thus propose that the lepontine metamorphism corresponds to the steady state thermal regime however either faster erosion rates in the internal part of the belt or tectonic denudation are required for exhumation of the deeper portion of the belt the computed temperature field implies eclogitisation of the lower crust at a depth of 55 to 60 km we conclude that the moho limiting the deepest part of the root may correspond to the eclogitisation phase change lower crust eclogites have a density comparable to or higher than that of the mantle depending on their chemical composition 3 37 for andesitic eclogites 3 56 for gabbroic eclogites thus andesitic eclogites may stay in gravitational equilibrium in the mantle below the root whereas gabbroic eclogites are gravitationally unstable and should sink

depositional environment of the lower cretaceous muling formation of the laoheishan basin ne china implications from geochemical and petrological analyses abstract the laoheishan basin located in northeastern china is filled with non marine coal and oil shale oil prone mudstone sediments of the lower cretaceous muling formation based on petrological mineralogical and geochemical methods the depositional environment during organic matter om accumulation is investigated for the first time the coal is subbituminous in rank petrological and bulk geochemical data indicate sapropelic coal deposited in low lying mire under freshwater conditions high pristane phytane ratios provide evidence for a land plant origin of om and oxic conditions during deposition a neutral to slightly alkaline ph is reflected by the presence of carbonate high liptinite contents and terpenoid biomarker composition indicate high amounts of resinous om during oil shale deposition a raised water table was favorable for om preservation under oxic to dysoxic conditions the oil shales in the lower unit of the succession not associated with coal are characterized by high hydrogen index and resinite contents interpreted to reflect deposition at elevated water depths in contrast to the findings of previous studies on oil shales in china indicating its origin from algal om a predominance of terrigenous om rich in waxy and resinous lipids is implied from biomarker and maceral composition the lower cretaceous oil shale and coal in the laoheishan basin contain high amounts of land plant derived terpenoid biomarkers derived from coniferales e g pinaceae cupressaceae taxodiaceae araucariaceae podocarpaceae and ferns the results are in agreement with palynological data the data highlight the potential of resinous and waxy om to form excellent hydrocarbon source rocks

low temperature eclogites and eclogitic schists in mn rich metabasites in ward creek california mn and fe effects on the transition between blueschist and eclogite

petrological constraints on the eclogite and blueschistfacies metamorphism and p t t paths in the western alps phase relationships in the model mafic system and geothermobarometry allow discrimination of four main groups of high p rocks in the nappes of the western alps very high p eclogite facies including kyanite eclogites and coesite pyrope assemblages eclogite facies paragonite zoisite eclogites high t blueschist facies glaucophane garnet lawsonite assemblages and low t blueschist facies glaucophane lawsonite pumpellyite assemblages the blueschist facies eclogite facies transition is promoted chiefly by increasing t low bulk xmg and relatively low h2o the variety of assemblages and the heterogeneous approach to equilibrium observed in the alpine rocks are not only constrained by the intersection of the reaction surfaces in p t x space but also by the effect and timing of the processes which control kinetics i e pervasive deformation and fluid infiltration the faster rate of dehydration reactions relative to hydration reactions along with the fact that different bulk compositions crossed the reaction curves at different temperatures and times all may have induced h2o gradients and contributed to the heterogeneous distribution of deformation through a process of reaction enhanced ductility also mass transfer may have been an effective process in determining the type of high p assemblage in particular rock volumes as regards the p t t paths only the post climax histories are recorded well in the alpine nappes post eclogitic exhumation paths at decreasing temperatures characterize structurally higher nappes which were first subducted during the early alpine cretaceous event in contrast more or less isothermal decompression paths characterize structurally deeper nappes formed by westward propagation of the underthrust surfaces during the early alpine event and the subsequent meso alpine palaeogene collision between the european and african plates in the western alps prevalent eclogite facies conditions were attained during the metamorphic climax of the early alpine subduction while blueschist facies recrystallization characterizes the early alpine exhumation of the eclogitized units and the subsequent intracontinental underthrusts linked to the meso alpine continental collision

diamond inclusions in garnets from metamorphic rocks a new environment for diamond formation diamonds commonly occur in kimberlites lamproites and alluvial sediments derived from these rocks more recently diamonds or their graphite pseudomorphs have been discovered in ultramafic massifs1 and picrites2 here we report the occurrence of diamonds in situ in crustal rocks highly retrograded high pressure metamorphic garnet pyroxene and pyroxene carbonate garnet rocks biotite gneisses and schists from the kokchetav massif northern kazakhstan ussr the diamonds are cubo octahedral averaging 12 m in size and occur in zircons and with euhedral graphite as inclusions in unzoned garnets we believe that the zircon and garnet matrices protected these diamonds from retrogressive transformation to graphite mica rutile titanite clinopyroxene kyanite and zircon also occur as inclusions in garnet often intergrown with the diamonds equilibration relations of inclusions and host garnets indicate that both diamonds and graphite crystallized from a fluid phase under static conditions at pressures of 40 kbar and temperatures 900 1 000 c

the transition between blueschists and lawsonite bearing eclogites based on observations from corsican metabasalts abstract syntectonic eclogites associated with blueschist parageneses have recrystallized in metabasalts from eastern corsica under very low temperatures 420 c the evolution of these eclogites is recorded by the order of development of metamorphic minerals as demonstrated by helicitic inclusions of lawsonite and actinolite in fe rich garnets and the occurrence of fibrous na pyroxenes in the pressure shadows and cracks of boudinaged garnets within a foliated matrix composed of glaucophane ca fe garnets and lawsonite a schreinemakers analysis has been completed on the lawsonite ferroglaucophane actinolite almandine grossular ferro omphacite chlorite system in a t p h 2 o space the resulting three dimensional relationships are applied to the studied samples they are consistent with a prograde increase in temperature at low h 2 o producing eclogites from blueschists and with a subsequent retrogressive decrease in pressure

a mechanism for syn collisional rock exhumation and associated normal faulting results from physical modelling abstract properly scaled physical modelling of the subduction of continental lithosphere is performed with a three layer lithospheric model the model includes a strong upper crust a weak ductile lower crust and a strong mantle part the lithosphere is underlain by a low viscosity asthenosphere subduction is produced by a piston push force and the pull force from the mantle lithospheric layer which is slightly denser than the asthenosphere the results of the modelling are tested on the example of the himalayas in the experiments all lithospheric layers subduct into the mantle to a depth corresponding to 200 300 km in nature until the upper crustal layer fails in front of the subduction zone forming a first major thrust fault the mct in our interpretation of the himalayas underthrusting along this fault increases relief which upon reaching a height corresponding to several kilometres in nature is removed mechanically by a blade the blade is moved horizontally back and forth throughout the experiment this erosional unloading mechanism causes the previously subducted segment of the buoyant upper crust to slide upward over the weak lower crust at a rate comparable to the subduction rate the rise of the crustal slice accelerates displacement along the crustal thrust while producing a normal sense motion and exhumation of the material from great depth up to 40 50 km depending on the model parameters along the upper surface of the slice the rapid uplift of the crust then ceases and the orogenesis occurs further due to continuing underthrusting along the first major thrust at a certain stage the crust fails again producing a new major crustal thrust the mbt in the himalayas

temporal variation of crustal deformation during the days preceding a thrust type great earthquake the 1944 tonankai earthquake of magnitude 8 1 japan the temporal variation in precursory ground tilt prior to the 1944 tonankai japan earthquake which is a great thrust type earthquake along the nankai trough is discussed using the analysis of data from repeated surveys along short distance leveling routes sato 1970 pointed out that an anomalous tilt occurred one day before the earthquake at kakegawa near the northern end of the focal region of the earthquake from the analysis of additional leveling data sato s result is re examined and the temporal change in the ground tilt is deduced for the period of about ten days beginning six days before the earthquake a remarkable precursory tilt started two or three days before the earthquake the direction of the precursory tilt was up towards the south uplift on the southern nankai trough side but the coseismic tilt was up towards the southeast perpendicular to the strike of the main thrust fault of the tonankai earthquake the postseismic tilt was probably opposite of the coseismic tilt the preseismic tilt is attributed to precursory slip on part of the main fault if similar precursory deformation occurs before a future earthquake expected to occur in the adjacent tokai region the deformation may help predict the time of the tokai earthquake

uplift of tibet from eclogites to granulites implications for the andean plateau and the variscan belt abstract using a simplified petrogenetic grid and a one dimensional thermal model to compute the evolution of the density of the crust with time the contribution to the uplift of tibet of metamorphic transitions in the lower crust is shown to be large the computations are made for a lower crust of either mafic or intermediate composition a mafic lower crust would have a mantle like density incompatible with seismological results this conclusion implies that any pre thickening gabbroic lower crust has been removed during the orogenic process a lower crust of intermediate composition would be affected by about 3 km of post thickening uplift over a few tens of millions of years this uplift is essentially the result of the eclogite to granulite transformation the resulting altitude matches the present altitude and does not require any delamination of the mantle lithosphere such a process would also explain why the neogene uplift of tibet to an average elevation of 5 km appears to have occurred without crustal shortening the same model may explain the uplift of the altiplano puna plateau which also occurred without surface shortening since about 10 ma a comparison is made with the now collapsed variscan high mountain belt for which there is evidence of a progressive increase of the temperature in the crust over tens of million years