causes of the uniformly high crystallinity of graphite in large epigenetic deposits in low pressure environments precipitation of graphite is hindered at low to moderate temperatures by the high solubility of carbon in c o h fluids and by kinetic barriers to nucleation those low temperature fluids that do attain saturation tend to precipitate graphite continuously during flow and cooling thereby producing widely dispersed films of low crystallinity graphite in contrast at high temperatures particularly when combined with high pressures the precipitation of graphite is enhanced by decreased solubility of carbon in c o h fluids and by improved nucleation under those conditions the longevity of fluid systems in high temperature high pressure terranes permits efficient long term scavenging of dispersed carbon from the crust the latter may be redistributed in a much more concentrated form as fluids rise cool and decompress and as the carbon is finally precipitated as highly crystalline graphite in fracture systems the combined effects of the thermochemical controls on carbon solubility and the geological controls on fluid generation movement and p t pathways are the reason that large epigenetic graphite deposits form dominantly at high temperatures and pressures those high temperature high pressure conditions in turn account for the uniformly high crystallinity of the fluid deposited graphite

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

electrically conductive crust in southern tibet from indepth magnetotelluric surveying the crust north of the himalaya is generally electrically conductive below depths of 10 to 20 km this conductive zone approaches the surface beneath the kangmar dome dipping north and extends beneath the zangbo suture a profile crossing the northern yadong gulu rift shows that the high conductivity region extends outside the rift and its top within the rift coincides with a bright spot horizon imaged on the indepth cmp common midpoint profiles the high conductivity of the middle crust is atypical of stable continental regions and suggests that there is a regionally interconnected fluid phase in the crust of the region

deformation enhanced fluid distribution in feldspar aggregates and implications for ductile shear zones experiments at 900 c and 1 4 gpa show that the distribution of aqueous fluid in fine grained feldspar aggregates changes from isolated pores under hydrostatic conditions to mostly wetted grain boundaries during deformation the isolated pore distribution is rapidly regained during annealing following deformation the deformation enhanced fluid distribution causes a switch from dislocation creep to diffusion creep accompanied by a significant decrease in strength it also increases the bulk transport rate through the aggregate by more than an order of magnitude a change in fluid distribution is not observed in fine grained quartz aggregates deformed at similar conditions if deformation enhanced fluid distribution occurs in naturally deformed feldspathic rocks it could help to explain the localization of strain and enhancement of bulk transport in ductile shear zones

a magnetotelluric study of the alpine fault new zealand summary magnetotelluric soundings have been made at seven locations on a 4 km profile crossing the alpine fault in the south island of new zealand the distortion techniques of groom bailey 1989 and lilley 1998a b have been used to derive regional apparent resistivity and phase curves that correspond to electromagnetic induction in orientations parallel and perpendicular to the fault 2 d inversion of the regional responses reveals that a narrow 1 km wide conductive zone is associated with the alpine fault this conductor is most probably related to the heating of deep circulating meteoric water in a region in which enhanced temperatures occur at shallow depth due to the tectonic uplift of the southern alps

a comparison of vitrinite reflectance measurements made on whole rock and dispersed organic matter concentrate mounts abstract a comparison of mean random vitrinite reflectance r v r values calculated from measurements on polished whole rock and on concentrates of dispersed organic matter dom mounts of the same samples generally show little difference compared to measurements on dom mounts r v r measurements by the same microscopist on corresponding whole rock samples are up to a few tenths of a percent lower over the range of 0 6 3 2 r v r and have a slightly higher standard deviation whole rock mounts have an advantage over dom mounts in the identification of first cycle vitrinite and differentiation of solid bitumen from vitrinite

preliminary results from a geophysical study across a modern continent continent collisional plate boundary the southern alps new zealand abstract the southern alps of south island new zealand is a young transpressive continental orogen exhibiting high uplift rates and rapid transcurrent movement a joint us nz geophysical study of this orogen was carried out in late 1995 and early 1996 to derive a three dimensional model of the deformation the measurements undertaken include active source and passive seismology magnetotelluric and electrical studies and petrophysics preliminary models for the active source seismic measurements across south island confirm in general terms a thickened crust under the southern alps a high velocity lower crustal layer and a major crustal discontinuity associated with the alpine fault the anisotropy in physical properties of the rocks of the plate boundary zone is clearly demonstrated in the preliminary results of laboratory seismic velocity measurements shear wave splitting and resistivity the mid crust under the southern alps coincides with a major electrical conductivity high which possibly corresponds to fluid in the crust the top lies at about 15 km close to the base of shallow seismicity east of the alpine fault offshore the marine reflection data have consistently imaged a reflective lower crust adjacent to south island these data are showing complex structure particularly off western and southeastern south island the complexity in structure high quality data and consistency in results from several techniques indicates that the south island experiment will contribute significantly to our knowledge of transpressive plate boundaries in particular and the continental lithosphere in general

fluid generation and pathways beneath an active compressional orogen the new zealand southern alps inferred from magnetotelluric data 1 forty one wideband magnetotelluric mt soundings were collected in a 150 km long transect across the southern alps of the central south island of new zealand an active compressional orogen decomposed mt impedance tensors vertical magnetic field relations and reconnaissance soundings at two locations off line imply an approximately two dimensional geometry here with average regional geoelectric strike of n40 e similar to surface geologic trends two independent two dimensional inversion algorithms were applied to the mt data and both imply a concave upward u shaped middle to lower crustal conductive zone beneath the west central portion of the island the average conductivity of this zone in the strike direction appears to be much higher than that required across strike and may represent anisotropy or along strike conductive strands narrower than the transverse magnetic cross strike mode mt data can resolve the deep crustal conductor under the southern alps is interpreted to represent mainly a volume of fluids arising from prograde metamorphism within a thickening crust fluid interconnection and electrical conduction are promoted by shear deformation the conductor rises northwestward toward the trace of the alpine fault but attains a near vertical configuration at a depth of 10 km and reaches close to the surface 5 10 km inland of the fault trace itself the transition to vertical orientation at this depth is interpreted to occur as fluids ascend across the brittle ductile transition in uplifting schist and approach the surface through induced hydrofractures the high grade schist becomes resistive after depletion of fluids and continues to extrude toward the alpine fault shallow extensions of the deep high conductivity are coincident with modern hydrothermal veining and gold mineralization interpreted to be of deep crustal provenance to the southeast high conductivity also reaches the surface coincident with a major back thrust fault zone of the doubly vergent southern alps orogen which also exhibits evidence for expulsion of high temperature fluids the higher conductivity inferred along strike possible anisotropy could reflect more efficient fluid interconnection in this higher strain direction as well as possible contributions by sheared fluid deposited graphite conductivity of the uppermost mantle of the south island is low consistent with advection of cold mantle lithosphere into the underlying asthenosphere as suggested by p wave delay studies

organic matter characteristics of cenomanian turonian source rocks implications for petroleum and gas exploration onshore senegal one hundred and fifteen samples of cenomanian turonian sediments from the senegal margin basin believed to be the main source rocks in the area have been characterised to determine their organic facies and their oil generating potential the samples have been subjected to detailed organic petrographic studies palynofacies maceral analysis vitrinite reflectance measurements combined with rock eval and leco determinations total organic carbon values comprised between 0 3 and 3 5 indicate poor to moderate primary bioproductivity and or unfavourable organic matter om preservation rather high c n ratio values supported by the presence of well preserved ligno cellulosic debris and of collotelinite suggest a notable contribution of terrestrial om this is supported by hydrogen and oxygen index values that testify to type iii om s c ratio values lower than 0 36 together with the presence of abundant reworked organic particles indicate that the studied sediments were deposited in a transitional and mixing zone between a coastal area with well oxygenated waters and a fully marine environment with more or less pronounced anoxia the rather low marine productivity and poor om preservation are here strongly reinforced by a very marked dilution of the om by mineral inputs in addition to these combined factors indicating that the om is mainly of the gas prone type a moderate maturity is shown by tmax and vitrinite reflectance values ranging from 435 to 445 c and 0 4 to 0 9 respectively the reservoir facies consisting of sandstones interbedded with shales are characteristic of prograding fluvio deltaic formations developed during maastrichtian regression faults and rollover structures most probably constitute the main migration pathways and traps for hydrocarbons respectively overall the characteristics of the studied formations highlight the importance of local factors in om accumulation even during the so called global cenomanian turonian anoxic event

numerical simulation of the thermal maturation oil generation and migration in the songliao basin northeastern china abstract the discovery of the giant daqing oil field in the songliao basin led to the realisation of the significant petroleum potential of non marine basins in order to reconstruct the basin evolution and oil formation an integrated organic geochemical basin modelling study along a regional transect across the songliao basin was conducted it provided a regional heat flow evolution model and revealed post orogenic or late syn orogenic maturation in the central depression and pre orogenic maturation in the southeast uplift zone kinetic parameters of petroleum generation for the lacustrine source formations are the basis for the simulation of oil generation and migration in the songliao basin using the principle activation energy peaking at 54 kcal mol and a pre exponential factor of about 4 2 1027 ma 1 the simulation obtained a relatively good match with the measured transformation ratios the qingshankou formation in the west and east central depressions constituted the major source in the basin major oil generation migration and accumulation occurred during the early tertiary in the west central depression the generated oils migrated upwards into the yaojia formation followed by the updip migration into the daqing anticline and towards the local structural high along the west slope in contrast the oil migration in the east central depression was dominated by the downward movement from the lower member of the qingshankou formation followed by the updip migration towards the caoyang anticline the simulated oil accumulations are in good agreement with discovered oil fields implying a potential application of the model for prediction and evaluation of new exploration targets in the basin

origin of the torlesse terrane and coeval rocks south island new zealand the carboniferous to lower cretaceous torlesse terrane and haast schist derivatives constitute the major part of the complexly deformed facies of the eastern province of new zealand strata consist mainly of quartzofeldspathic graywacke and mudstone intercalated with minor but widely distributed conglomerate and volcanics with associated chert and limestone clastic rocks were deposited largely by sediment gravity flow mechanisms in a deep marine environment also present are a few highly fossiliferous shallow marine and terrestrial deposits of limited areal extent which rest unconformably on or in fault contact with torlesse flysch several periods of deformation are recognized and melange is present on both local and regional scales metamorphism ranges from zeolite to greenschist facies the bulk of the rocks fall into five areally extensive and mutually exclusive fossil zones of the following ages permian atomodesma middle triassic early late triassic torlessia late triassic monotis and late jurassic early cretaceous contacts between major fossil zones are mainly tectonic petrographic analysis permits subdivision of torlesse sandstones into five major petrofacies that correspond in age to the five major fossil zones the sandstone petrofacies mainly arkosic together with the composition of conglomerate clasts mainly indurated torlesse rocks indicate that the source terrane was a continental volcano plutonic arc probably part of gondwanaland coupled with autocannibalistic reworking of older uplifted torlesse rocks in contrast to the quartzofeldspathic nature of the torlesse coeval sedimentary rocks of the eastern province are volcanogenic they are thought to represent related forearc basin maitai murihiku terranes and trench complex caples terrane deposits derived from a volcanic island arc brook street terrane three petrofacies are established for maitai murihiku and caples sandstones the petrofacies indicate a common evolving immature to submature volcanic island arc source for these terranes a reconstruction of new zealand s eastern and western provinces is proposed in permian and triassic times the torlesse was deposited in trench slope or borderland basins along a trench transform margin fronting a continental volcano plutonic arc source western province gondwanaland deposition was spasmodic but voluminous and was accompanied by concurrent deformation and accretion resulting in parallel belts of torlesse rock younging outward from the gondwanaland margin at the same time the brook street terrane volcanic arc and associated terranes were forming to the west of the torlesse site separated from gondwanaland by a marginal sea in latest triassic or early jurassic times the torlesse was rafted into the volcanic arc system via transform faulting approximately parallel to the gondwana margin the collision event resulted in tectonic thickening of torlesse and caples rocks at the plate interface and metamorphism to haast schist the source was then dominated by older partly metamorphosed torlesse terrane newly uplifted along the collision front closing of the marginal sea behind the brook street terrane in late jurassic early cretaceous times resulted in juxtapositioning with the western province gondwanaland along the median tectonic line

identification of indigenous vitrinites for improved thermal maturity evaluation abstract the four methods that we use to improve the analysis of vitrinite reflectance of well cuttings samples are discussed the criteria for properly identifying the indigenous vitrinites a five fold classification of dispersed vitrinites in sediments first cycle second cycle low gray high gray and gray vitrinites a recording of the reflectance value vitrinite type data for each vitrinite particle analyzed a computer assisted microscope system that generates reflectance histograms of the identified vitrinite type data for easy interpretation using this approach we have minimized inaccuracies caused by the presence of mud additives cavings reworked vitrinites and solid bitumen

magnetovariational soundings across the south island of new zealand difference induction arrows and the southern alps conductor abstract magnetovariational mv data from 14 sites on a transect of the south island of new zealand are presented the induction arrows clearly show the effects of induction in the surrounding oceans comparison with the responses of an analogue model of the effect of the oceans on induction in the new zealand region indicates that the induction arrows also contain information on anomalous on land conductivity structure the use of difference induction arrows to remove the effect of induction in the oceans from the observed responses is validated using both two and three dimensional numerical models pertinent to the new zealand situation difference induction arrows calculated by subtracting the analogue model results from the observed data show a clear reversal in both real and quadrature arrows across the south island two dimensional numerical modeling of the projections of the difference arrows on to the transect shows that the response is consistent with the existence beneath the southern alps of an anomalously conducting region previously identified by magnetotelluric mt sounding use of the difference arrow technique is seen to be valid over a large period band because of the reduction in the coast effect response range as a result of the island situation

variations of organo mineral microfacies of posidonia shale from the lower saxony basin and the west netherlands basin application to paleoenvironmental reconstruction abstract high resolution organic petrological analysis has been carried out on core samples of the posidonia shale from four wells two located in northwestern germany lower saxony basin and two in the western netherlands west netherlands basin the organo mineral microfacies defined by maceral composition and associated mineral matter show a wide range of variations related to the proportions of individual liptinite macerals of marine origin and of terrestrial macerals macerals of vitrinite inertinite groups and sporinite variations in organo mineral microfacies in which the organic components are sensitive to paleoenvironmental changes are considered as a potential indicator of changes in depositional conditions sedimentation of organic matter is controlled by several factors the most important in the marine environment being primary biomass productivity and influx of transported terrestrial organic matter biological degradation related to bacterial activity and the oxygen content of the bottom water the level of degradation in organic rich sediments is mainly related to oxygen availability which in turn influences the diversity of algae and the quality and quantity of the preserved organic matter these two factors can be directly linked to maceral preservation and the organo mineral microfacies in order to evaluate the relationships between different types of organic matter and paleoenvironmental conditions two indices the oxidation index and the degradation index have been proposed these indices illustrate the transition from highly degraded to well preserved macerals provide information on the types of organic matter and permit recognition of precursors for degraded amorphous organic matter aom use of these indices not only enables determination of the organo mineral microfacies but also sheds light on the aom precursors in the posidonia shale information derived from the indices can also be used to reconstruct the paleoenvironments

detection of widespread fluids in the tibetan crust by magnetotelluric studies magnetotelluric exploration has shown that the middle and lower crust is anomalously conductive across most of the north to south width of the tibetan plateau the integrated conductivity conductance of the tibetan crust ranges from 3000 to greater than 20 000 siemens in contrast stable continental regions typically exhibit conductances from 20 to 1000 siemens averaging 100 siemens such pervasively high conductance suggests that partial melt and or aqueous fluids are widespread within the tibetan crust in southern tibet the high conductivity layer is at a depth of 15 to 20 kilometers and is probably due to partial melt and aqueous fluids in the crust in northern tibet the conductive layer is at 30 to 40 kilometers and is due to partial melting zones of fluid may represent weaker areas that could accommodate deformation and lower crustal flow

electrical resistivities of crustal fluids results of calculations on the electrical resistivities of crustal fluids as a function of temperature pressure and fluid composition indicate that crustal fluids can range in resistivity from approximately 1 m the dominant contribution to decreasing the resistivities of crustal fluids comes from increased concentrations of chloride and bicarbonate salts and increased temperature and density of the fluids calculations indicate that the sharpest decrease in fluid resistivity occurs at shallow depths 10 km and temperatures 300 c can not be readily explained as a product of variations in resistivities of crustal fluids these results indicate that a transition in mineralogy or pore shape geometry is more likely responsible for the observed decrease in electrical resistivity of the middle crust than a major change in fluid resistivity

teleseismic p wave delays and modes of shortening the mantle lithosphere beneath south island new zealand a high speed zone in the mantle directly beneath the southern alps of new zealand is required by the recorded pattern of teleseismic p waves two parallel lines of 80 seismographs spaced at 2 km intervals recorded three earthquakes from the western pacific with epicentral distances of 52 53 and 78 azimuthal bearings were all within 15 degrees of the mean trends of the seismograph lines differences between measured delays and those predicted from the crustal structure reach 0 8 s along one line and 1 0 s along the other with the rays for the earliest arriving signals passing the depth of 120 km beneath the center of the island assuming these early arrivals are due to structure within the mantle shallower than 200 km they imply that the core of the high speed zone lies beneath the thickest crust which has been shortened by 100 km of convergence during the past 6 7 myr although the shape and position of the high speed body cannot be fixed uniquely a roughly symmetric body centered about a depth of 120 km 80 100 km wide with a depth extent of 100 km and with a maximum speed advance of 7 satisfies the observations the pattern of residuals does not fit with those predicted by simple models of intracontinental subduction in which crust and mantle lithosphere are detached and one slab of mantle lithosphere underthrusts the other rather the residuals favor thickening of mantle lithosphere by a more homogenous straining of it as if mantle lithosphere beneath continental crust behaved as a continuum an excess mass in the mantle is also required by the observed gravity anomalies once allowance is made for the seismically determined crustal thickness this high density mantle anomaly provides sufficient force per unit length to maintain the crustal root which is approximately twice as thick as that necessary to support the topography

material and thermal roles of basalt in crustal magmatism case study from eastern nevada thousands of cubic kilometres of calc alkaline intermediate to silicic magma erupted in eastern nevada at 35 ma on the basis of petrologic models crustal contribution to magmatism is slightly greater than basaltic input the volume of crystals generated during differentiation is roughly twice that of erupted magma implying that intrusive complements are enriched in accumulated crystals the amount of basalt needed to drive the system thermally can be derived from the volume of crust that is heated and assimilated and is about twice the volume of basalt needed materially

reversed determination of the reaction phlogopite quartz enstatite potassium feldspar h 2 o in the ranges 750 875 degrees c and 2 12 kbar at low h 2 o activity with concentrated kcl solutions the reaction of phlogopite plus quartz to enstatite potassium feldspar and aqueous fluid in the system kmash kcl was reversed at 2 12 kbar and 750 875 degrees c and at low h 2 o activities by reversal of the h 2 o content of concentrated kcl solutions equilibrated with product and reactant assemblages synthetic 1m phlogopite kmg 3 alsi 3 o 10 oh 2 and enstatite mgsio 3 maintained end member stoichiometry throughout and the potassium feldspar kalsi 3 o 8 was a high sanidine based on unit cell refinements the broad p t xh 2 o and narrow reversal ranges of this investigation were possible because of the low and well defined h 2 o activity yet powerful fluxing action of concentrated kcl solutions solubility experiments on quartz and potassium feldspar in our p t xh 2 o range showed that fluid phase solution of silicate constituents was too small to have affected the h 2 o activity in the experiments the new determinations are more definitive than previous work done at very low pressures with pure h 2 o or in co 2 h 2 o mixtures they establish the standard free energy of the reaction in the experimental range with an uncertainty of about 1 kj and indicate that the synthetic phlogopite has maximal al si disorder under our experimental conditions the standard enthalpy of reaction at 298 k is 106 54 or 2 0 kj 2sigma based on our reversals a value 6 kj less positive than that currently used by many workers in calculations of biotite stability and h 2 o activity in the petrogenesis of high grade metamorphic rocks the lower thermal stability that we find for phlogopite requires revision in estimates of h 2 o activity of granulite facies metamorphism typical values for the natural assemblage orthopyroxene biotite garnet potassium feldspar plagioclase quartz at deep crustal metamorphic conditions 750 850 degrees c 5 10 kbar are alpha h2o 0 4 0 6 compared with values of 0 15 0 30 which would have been estimated with previously available data on phlogopite stability an important consequence of the expanded h 2 o activity range of granulites is that alkali chloride solutions of only moderate concentration xh 2 o 0 5 0 7 which are the values observed in actual fluid inclusions in many kinds of igneous and metamorphic rocks are a feasible alternative to the vapor absent conditions considered necessary by many workers based on previous low estimates of h 2 o participation of concentrated brines in deep crust upper mantle metamorphic processes enables alkali metasomatism and other kinds of chemical transport in an aqueous fluid without large scale melting of the crust

magnetotelluric evidence for crustal suture zones bounding the southern great valley california a geoelectric section inferred from a regional magnetotelluric study across the coast ranges the great valley and the sierra nevada reveals significant variations in electrical resistivity zones of lower resistivity interpreted at depths from 10 km to at least 30 km lie near mapped geologic boundaries between the coast ranges and the great valley and beneath the eastern side of the great valley the former boundary is inferred by others to separate the subduction complex of the coast ranges from the mafic basement of the great valley the lower resistivities are most likely associated with metasediments trapped between the coast ranges ophiolite and the former oceanic crust beneath the great valley the latter boundary is problematic but may be evidence for a deep metasedimentary section trapped between the ophiolites beneath the great valley and the granitic rocks of the sierra nevada the lack of change in the magnetotelluric phase across the great valley indicates that a suture zone marked by lower resistivities is unlikely to be present beneath the valley however this does not preclude the existence of a resistive suture zone

electrical conductivity and paleo proterozoic foredeeps sedimentary facies associated with paleo proterozoic foredeeps are apparently highly conductive the conductive lithology appears to be a euxinic carbonaceous shale deposited on preforedeep continental margin facies the shale is overlaid by axial turbidites and often incorporated into the succeeding fold and thrust belt carbonaceous shales can be made highly conducting if graphitized by exposure to temperatures exceeding 400 c although foredeep facies develop diachronously the most conductive unit is usually found in the core of the fold and thrust belt suggesting that deformation may be essential for the complete graphitization of the carbon the foredeep hypothesis offers an attractive explanation for several highly conductive features detected in the upper and middle crust in laurentia and possibly throughout the world the euxinic facies represent highly specific tectonic marker horizons that are readily delineated by using electromagnetic methods

should fluorescence alteration replace vitrinite reflectance as a major tool for thermal maturity determination in oil exploration abstract the fluorescence alteration multiple macerals famm technique is closely related to vitrinite reflectance until recently fluorescence alteration study of dispersed organic matter dom in sedimentary rocks has been regarded as technically difficult with the introduction of optical microprobes equipped with small continuous wave gas lasers having a very stable output of blue radiation readily focused to 1 2 m on the sample this work has now become routine famm has important advantages over vitrinite reflectance stemming from the possibility of resolving the data into two parameters one related to the level of maturation and the other related to the hydrogen richness of a maceral this enables the resolution of what are often difficult problems for the single parameter vitrinite reflectance technique namely the identification of indigenous vitrinite in dom the recognition of caved and reworked organic matter and the suppression of vitrinite reflectance for these reasons famm is a powerful tool for investigating vitrinite reflectance anomalies especially those associated with unconformities it is also well suited to investigate the tacit assumption in the vitrinite reflectance technique that all telocollinite matures along a single compositional pathway the famm technique is not without problems the range is restricted to equivalent mean random vitrinite reflectance rm o 0 4 1 2 the results are more sensitive to ambient temperature oxidation than vitrinite reflectance and the technique demands a high level of technical expertise the famm technique is calibrated on coals and it is therefore more likely to give high quality results on types ii and iii kerogens rather than type i kerogens which have not yet been intensively investigated although famm is unlikely to replace vitrinite reflectance considerable confidence may be had in a maturation assessment when consistent results are obtained from these two closely related but independent techniques

origin of small scale segmentation and transpressional thrusting along the alpine fault new zealand the alpine fault is the major structural feature of the australian pacific plate boundary in the south island of new zealand geologic evidence suggests that half to three quarters of the plate boundary displacement is accommodated by movement on the fault detailed investigation of the central section of the alpine fault has revealed that it consists of oblique thrust sections striking 020 050 that are linked by subvertical right lateral faults striking between 065 and 090 the segmentation is on a scale of 1 10 km similarly oriented right lateral faults are abundant southeast of the alpine fault and are consistent with stresses induced in an elastic layer by an oblique slip ductile fault zone below propagation of the fault to the surface is predicted to result in an en echelon arrangement of strike slip and thrust segments it is suggested that the spatial distribution of segments is affected by the existence of deeply incised valleys in the hanging wall that disturb the stress field to depths of 1 4 km the segmentation is near surface and does not appear to act as a barrier to the propagation of large earthquakes

upper mantle anisotropy in the new zealand region shear wave splitting parameters of fast polarization direction and delay time t are determined using data from the southern alps passive seismic experiment sapse on the south island of new zealand and in the surrounding region our results clearly show that are subparallel to trends of the alpine and marlborough faults and to the pacific australian plate boundary the t values range from 0 6 2 2 s with an average value of 1 6 s the largest values are from the central south island the main source of the observed shear wave splitting is an anisotropic region between 40 400 km the width of the zone is approximately 200 km we attribute the coincidence of surface structural trends with the measured and the large t values to significant shear deformation in a 200 km thick zone along the plate boundary extending from the surface to deep within the upper mantle

crustal deformation during 1994 1998 due to oblique continental collision in the central southern alps new zealand and implications for seismic potential of the alpine fault the positions of 115 ground marks in a 150 100 km area of oblique continental collision in the central southern alps new zealand have been measured by global positioning system gps two to four times between 1994 and 1998 contemporary velocity and strain rate fields derived from these observations are largely invariant along the northeasterly strike of the mountains and alpine fault across strike more than 60 of the strain occurs within a band from 5 km nw to 20 km se of the alpine fault but significant strain continues at least a further 60 km se to near the edge of the southern alps foothills projections of the fault parallel and fault normal components of velocity onto an alpine faultnormal profile show that about 85 of the nuvel 1a model relative plate motion is observed within the gps network the surface displacements in the high strain rate region are well fit by a model in which stable slip or shearing is occurring at 50 70 of the relative plate rate in a region deeper than about 5 8 km on the down dip extension of the se dipping alpine fault material shallower than this is behaving elastically and thus storing elastic strain in the region of the alpine fault the longer wavelength displacements can be modeled either as distributed deformation beneath the southern alps or by localization of elastic strain around the upper end of a discrete nw dipping fault or shear zone that is slipping stably below about 30 km depth and would outcrop near the se boundary of the mountains if extrapolated to the surface strain determined from a small scale survey network crossing the alpine fault indicates no significant near surface aseismic fault slip on the central alpine fault over the past 25 years our results are consistent with independent geological evidence that the central section of the alpine fault is capable of producing large to great earthquakes

the tectonic evolution of the southern alps new zealand insights from fully thermally coupled dynamical modelling summary we present here the results of numerical models of the southern alps in which the thermal and dynamical developments of the orogen are fully coupled to one another this interlinkage allows the use of a wide range of thermal and physical features of the orogen as constraints on the applicability of our models in particular the thermal aspect of this method enables the prediction of the distribution of thermochronological ages at the surface of the model perturbation of the geothermal structure due to rapid uplift and exhumation causes an intrinsic weakening and concentration of strain along the equivalent of the alpine fault zone in these models this thermal weakening of the crust also produces a zone of high strain antithetic to the alpine fault in the upper crust that is comparable to the main divide fault zone within the southern alps the introduction of orographic rainfall and erosional processes into the model leads to the development of high asymmetric topography comparable to that of the southern alps this topographic profile results from the capture of available precipitation by the windward side of the orogen and the resultant rain shadow on the leeward side due to the time required to accumulate suycient topography for this rain capture evect to become significant the establishment of this high asymmetric topography lags the initiation of surface uplift by several million years comparison of the observed thermal history of the southern alps with that seen in models based on divering hypotheses of the tectonic evolution of the south island shows that the present tectonic regime of the orogen most probably developed in a single rapid reorganization of plate motions at approximately 5 ma with relative stability of the tectonic regime since that time the variation in isotopic ages along the southern alps is consistent with that expected from variation in accumulated uplift and exhumation along the orogen arising from the obliquity of convergence of the australian and pacific plates

magnetotelluric evidence of lithospheric mantle thinning beneath the southern sierra nevada a wideband 0 01 1000 s magnetotelluric survey across the southern sierra nevada has identified zones of enhanced conductivity in the lower crust and upper mantle that underlie the resistive batholithic rocks at depths greater than 10 20 km the eastern zone underlies the highest topography of the range and extends eastward this eastern conductive zone extends to depths in excess of 100 km based on model sensitivities and lies well below the moho depth of 32 38 km from seismological studies therefore the enhanced conductivity in the mantle cannot be attributed to conventional explanations for conductive continental crust and is instead likely due to partial melt such an interpretation is consistent with gravitational seismological and geologic evidence estimates of the partial melt fraction range from 2 to 5 at depths of 40 70 km which are consistent with fractions of melt inclusions observed in xenoliths from the eastern sierra nevada this partial melt accounts for approximately one third of the density decrease required in the mantle for support of the present high elevations the other two thirds of the density decrease could be due to thermal expansion of the upper mantle or due to mineralogical changes the western conductive zone also straddles the moho and extends into the mantle beneath western foothills of the sierra nevada and the great valley sediments to the west however xenoliths from this region indicate high velocity crustal rocks to depths in excess of 60 km and we therefore attribute the enhanced conductivity to graphitic metasediments and or dehydration of metaserpentinite emplaced by downward return flow of the host rocks during intrusion of the sierran plutons

subcrustal earthquakes in the central south island new zealand and the root of the southern alps the pukaki microearthquake network has revealed significant earthquake activity deeper than 15 km in the central south island new zealand extending to a depth of 73 km beneath the crest of the southern alps events deeper than 25 km define a planar seismic zone that has a strike of 41 and dips northwest at 19 gravity models suggest that earthquakes in this zone are occurring in a seismogenic uppermost mantle underlying an aseismic lower crustal root to the southern alps the 17 difference in strike of the dipping seismic zone and the alpine fault provides an explanation for the pattern of uplift and crustal thickening seen in the central south island

diffusion creep in feldspar aggregates experimental evidence a combination of mechanical and microstructural results for experimentally deformed albite aggregates of 2 10 m grain size indicates a regime of grain boundary diffusion creep that depends upon the presence of water temperature and strain rate at 900 c and 10 5 s 1 deformation occurs dominantly by recrystallization accommodated dislocation creep when the aggregates contain 0 2 wt water but by grain boundary diffusion creep when the aggregates contain 0 9 wt water experiments at other temperatures and strain rates indicate that fine grained aggregates with added water may go directly from cataclastic flow to grain boundary diffusion creep with increasing temperature or decreasing strain rate bypassing the field of dislocation creep microstructural evidence for grain boundary diffusion creep in the wet fine grained aggregates includes the development of rectangular grains rather than polygonal grains grain growth a very low average dislocation density open grain boundaries including pores and channels and the presence of overgrowths of a different composition on grains in a two plagioclase an1 and an79 aggregate additional evidence for diffusion creep in the wet fine grained aggregates is indicated by their similar microstructures and strengths when deformed at 600 and 1500 mpa in contrast dry feldspar aggregates deforming by dislocation creep show an increase in strength and a transition to semi brittle behavior at lower confining pressures grain boundary diffusion creep of feldspars may be common in naturally deformed rocks due to the ease of grain size reduction by both cataclasis and dynamic recrystallization

the continental collision zone south island new zealand comparison of geodynamical models and observations the south island zone of oblique continent continent convergence occurs along a 400 km long section of the modern australia pacific plate boundary zone across which about 50 km of shortening has been accommodated since about 10 ma the orogen comprises a central mountain range southern alps flanked on both sides by what are interpreted to be foreland basins two essential features that characterize the orogen are 1 the degree of denudation that accompanied deformation and 2 a fundamental structural asymmetry the architectural asymmetry of the orogen can be explained by plane strain finite element models of continental convergence incorporating mantle subduction comparison of model and orogen polarity implies that pacific plate mantle subducts the models predict two crustal scale dipping shear zones that form above the point where the pacific mantle subducts the localized one more distant from the incoming plate retro step up shear zone corresponds to the alpine fault whereas its conjugate pro step up shear zone corresponds to the distributed strain and thrusting along the eastern margin of the mountain belt parameters that modify the model boundary conditions top surface degree of denudation basal zone subduction load crust mantle velocity discontinuity subduction of lower crust mantle retreat and distributed decrease in mantle velocity and the internal strength of the crust two layer crust with moderate coupling temperature distribution strain weakening are varied in a series of numerical model calculations that establish the combination of material properties and boundary conditions that lead to different cross sectional architectures of the modeled collision zone in turn these are compared with observations about the south island orogen the calculations show how the style and extent of deformation across the whole orogen depend on the rheological properties of the crustal layer and on the balance between its internal strength and the combined effects of the boundary and gravitational stresses north to south along strike differences in the width and two dimensional architecture of the orogen simulated in the experiments by varying the model parameters can be explained by a combination of southward increases in preconvergent crustal thickness geothermal gradient convergence and potentially subduction retreat with the added possibility of a southward decrease in the component of lower crustal subduction

magnetotelluric transect of long valley caldera resistivity cross section structural implications and the limits of a 2 d analysis twenty four magnetotelluric mt soundings have been collected in an east west profile across the center of long valley caldera the average station spacing is approximately 1 km and appears adequate to sample the important features of the upper crustal and deeper resistivity structures additional control on the shallowest resistivity is provided by a continuous profile of time domain electromagnetic soundings coincident with the western portion of the mt line our mt data set reveals numerous resistivity structures which illuminate the evolution and present state of the long valley system many of these have been quantified through two dimensional 2 d finite element modeling emphasizing the transverse magnetic tm mode important structural components include low resistivity layers 0 5 1 5 km deep under the eastern half of the caldera beneath the axial graben of the resurgent dome and under the west caldera moat most of this layering appears to lie in post caldera early rhyolite tuffs and the upp

using reflectance crossplots and rotational polarization for determining first cycle vitrinite for maturation studies abstract vitrinite reflectance on dispersed sedimentary organic matter is traditionally determined from histograms of vitrinite and inertinite reflectance measured on either whole rock samples or kerogen concentrates or from particles usually in whole rock preparations chosen by morphology and relative reflectance to be first cycle vitrinite this paper discusses an alternative method discrimination of first cycle vitrinite using apparent maximum and minimum reflectance data plotted on reflectance crossplots the use of crossplots increases the confidence that the macerals identified as first cycle vitrinite are optically similar the maximum minimum data in this study were collected on a quantitative reflectance microscope outfitted with rotational polarization a technique that eliminates the usual difficulty in collecting such data on very small macerals by stage rotation

statistical analysis of vitrinite reflectance data a new approach abstract the reflectance of vitrinite collotelinite particles is a widely used parameter as a geothermometer for the estimation of the thermal maturity of organic matter enclosed in rocks however several problems have occurred during the last decades which can be traced back to basically three causes human mistakes technical problems and problems associated with the structural and compositional inhomogeneity of organic matter whilst in most cases the first two types of uncertainties can be handled by standardization the third can cause significant problems during interpretation due to its generally inestimable character the suppression of vitrinite reflectance and statistical problems originated from small sample size and outliers belong to this latter type international standards such as the astm and the iso define the vitrinite reflectance parameter as a statistical average of measured data disregarding the fact that the average is an unresisting and unrobust statistical parameter in other words the average is very sensitive to outliers and distribution the aim of this research was to find and test a better more resistant and robust statistical parameter used by traditional parametric and nonparametric statistics which can be applied in practice instead of the average three categories of statistical problems were studied on coal and disperse organic matter dom samples the distribution of measured values the effect of data number and the effect of outliers on statistical parameters the statistical experiments carried out on numerous original and generated sample sets show that the median med and the most frequent value mn a special weighted average are better parameters to estimate the thermal maturity of organic matter especially above 1 reflectance value

a ramm study of thermal maturity of dispersed organic matter in marine source rocks abstract the raman maturity method ramm is a non graphical technique for thermal maturity determination based on the multi linear regression analysis of parameters derived from carbon raman spectra of humic macerals it is calibrated against vitrinite reflectance with a suite of australian mostly permian coals in a previous communication an equation to cover the range 0 4 1 2 equivalent vitrinite reflectance was derived in this report we extend the range with a second equation for the range 1 0 2 5 and validate its use by testing a series of australian permian and german carboniferous coals the special features of the technique are that it is equally applicable to vitrinite and inertinite it corrects internally for the suppression of vitrinite reflectance effect grains as small as 5 m are suitable and it does not require well depth information to be available the technique has been applied to two wells cape range 2 and dampier 1 from the rift margin of the north west shelf of australia both wells intersect large thicknesses of fine grained marine jurassic and cretaceous sediments including the major source rock formation of the carnarvon basin there are no terrestrial sediments for control there is good agreement between ramm and vr results for vr 1 2 for cape range 2 however within the oil window in both wells ramm results indicate the vitrinites in many of the samples have reflectance suppression to such a degree that the vr results would be misleading or useless for maturity modeling it is likely that this problem which is well documented for marine influenced coals to some degree affects organic matter in marine source rocks worldwide

anatomy of the southern cordilleran hingeline utah and nevada from deep electrical resistivity profiling to address outstanding questions in mesozoic cenozoic structure and present day deep physicochemical state in the region of the southern cordilleran hingeline a detailed east west profile of magnetotelluric mt soundings 155 km in length was acquired from these soundings a resistivity interpretation was produced using an inversion algorithm based on a structural parameterization in the upper ten kilometers of the transect the interpretation shows two segments of low resistivity lying beneath allochthonous rocks of the late mesozoic sevier thrust sheet subsequent industry drilling motivated in part by our surveying confirms the existence and position of the eastern subthrust conductor and more spectacularly identifies the presence of yet deeper autochthonous mesozoic rocks the conductors cannot be specified uniquely with present public data because their electrical characteristics appear consistent with paleozoic pyrolized graphitic strata of either late devonian mississippian or middle ordo

provenance of permian triassic and ordovician metagraywacke terranes in new zealand evidence from 40ar 39ar dating of detrital micas single crystal 40 ar 39 ar ages of detrital micas from greenland group ordovician low grade metagraywackes of the west coast south island new zealand indicate a source area having muscovite age patterns of 490 515 ma the detrital muscovite ages are characteristic of granitoid and gneiss terranes of the ross adelaide fold belt of antarctica therefore the source area for the greenland group sedimentary rocks was the immediately adjacent pacific margin of gondwana similar age determinations for rakaia terrane permian triassic torlesse supergroup metagraywackes of the wellington and canterbury regions of new zealand indicate that their source area included rocks having prominent muscovite cooling ages populations of 210 290 ma 80 and 410 460 ma 15 the torlesse detrital muscovite age data thus suggest that the source area of the metagraywackes was not within the lachlan fold belt of southeast australia and analogues in antarctica nor in the adjacent ross adelaide fold belt rather the rakaia terrane of the torlesse metagraywackes is a true suspect terrane derived from a source area within the hunter bowen fold belt of northeast australia and the immediately adjacent older orogens of eastern queensland

hydrocarbon prospects across narmada tapti rift in deccan trap central india inferences from integrated interpretation of magnetotelluric and geochemical prospecting studies abstract as the mesozoic sediments contribute most of the oil and gas reserves of the world we present an integrated interpretation approach using magnetotellurics mt and surface geochemical prospecting studies to demarcate hydrocarbon prospective gondwana mesozoic formations underneath the deccan flood basalts of late cretaceous age across narmada tapti rift between bhusawal and barwah in central india the mt interpretation shows deep 5 km basement structure between southern and central part of the mt profile however it gradually becomes shallower to either ends of the profile with a predominant basement depth reduction in the northern end compared to the southern end the geophysical results suggest thick 2 3 5 km mesozoic sediments in the area characterized by deep basement structure the geochemical analysis of the near surface soil samples indicate higher concentrations of light gaseous hydrocarbons constituents over the area marked with thick sub basalt mesozoic formations analyses of the geochemical data imply that these hydrocarbons are genetically related generated from a thermogenic source and these samples fall in the oil producing zone the temperature depth estimations in the region supports favorable temperature conditions 80 120 c for oil generation at basement depths

thermal modeling of the southern alps new zealand finite element modeling of the thermal regime across the southern alps of new zealand has been carried out along two profiles situated near the franz josef and haast valleys the modeling involves viscous deformation beneath the southern alps including both uplift and erosion and crustal lithospheric thickening as a result of crustal shortening extending to 20 mm y of a 25 km thick crust published uplift rates and crustal thickness variations along the two profiles are used to constrain the modeled advection of crustal material and results are compared with the recent heat flow determinations 190 50 mw m2 in the franz josef valley and 90 25 mw m2 in the haast valley comparisons of the model with published k ar and fission track ages show that the observed heat flow in the franz josef valley is consistent with observed zircon fission track ages of around 1 ma if the present day uplift rate is close to 10 mm y major thermal differences between the franz josef and haast profiles appear to be due to different uplift and erosion rates there is weak evidence that frictional heating close to the alpine fault zone is not significant the modeling provides explanations for the distribution of seismicity beneath the southern alps and predicts a low surface heat flow over the eastern foothills due to the dominant thermal effect of crustal thickening beneath this region predicted temperatures at mid crustal depth beneath the zone of maximum uplift rate are 50 100 c cooler than those indicated in previously published models which implies that thermal weakening of the crust may not be the main factor causing the aseismicity of the central southern alps the results of the modeling demonstrate that the different types of reset age data in the region within 25 km of the alpine fault are critical for constraining models of the deformation and the thermal regime beneath the southern alps

ramm raman maturity method study of samples used in an interlaboratory exercise on a standard test method for determination of vitrinite reflectance on dispersed organic matter in rocks abstract ramm raman maturity method is a thermal maturity tool for humic macerals in coals or as dispersed organic matter dom expressed in equivalent vitrinite reflectance eqvr the eqvr values are based on a recently established calibration according to multi linear regressions of selected parameters from raman spectra of vitrinite and inertinite in a suite of australian coals with vitrinite reflectance vr in the range 0 4 2 5 the study contributes to the application of raman spectroscopy for thermal maturity determinations as applicable to the petroleum exploration industry and researched by many groups worldwide a suite of samples to apply the technique was made available from an interlaboratory round robin exercise on application of astm d7708 a method for determination of vr of dom in sedimentary rocks this exercise involved the distribution of two sets of six unknown rock samples to all participating laboratories collated vr results from some samples of the first set having high maturities and low organic matter om contents showed a disturbingly high degree of scatter although general agreement exists between ramm determinations and the average of mean random vr values determined at the various laboratories some important differences emerged for some samples in the range eqvr 1 2 1 8 on ramm diagrams which involve plots of eqvrs derived from the two regression equations defining the method the pattern of distribution of vitrinite and inertinite changes with thermal maturity for normal coals and with om composition in subhydrous versus perhydrous coals most of the round robin samples contained solid bitumens having reflectances close to those of the associated vitrinite measurements taken from solid bitumen were avoided as much as possible but because the ramm study was done in air medium some contamination of the humic maceral data was inevitable many of the solid bitumens and other potentially complicating components such as tasmanites related alginite can be identified on the ramm diagram and eliminated from ramm results for lower maturity samples eqvr 1 8 the humic dom of marine samples having eqvr 1 0 has spectral properties that differ from those of humic macerals of the same maturity in terrestrially deposited rocks this could possibly explain any discrepancies between ramm eqvr and vr data for the marine rocks the presence of contaminating solid bitumen with reflectance similar to that of vitrinite is the main complicating factor for thermal maturity evaluation using either vr or ramm analyses of gas shales and tight oil reservoir rocks to enable the best results from ramm studies of these types of rocks a preliminary study of the solid bitumen component should be carried out to determine its field on the ramm diagram with the object of eliminating contaminating solid bitumen data from eqvr calculations

vertical and lateral fluid flow related to a large growth fault south eugene island block 330 field offshore louisiana data from sediments in and near a large growth fault adjacent to the giant south eugene island block 330 field offshore louisiana indicate that the fault has acted as a conduit for fluids whose flux has varied in space and time core and cuttings samples from two wells that penetrated the same fault about 300 m apart show markedly different thermal histories and evidence for mass flux sediments within and adjacent to the fault zone in the u s department of energy pennzoil pathfinder well at about 2200 m sstvd subsea true vertical depth showed little paleothermal or geochemical evidence for throughgoing fluid flow the sediments were characterized by low vitrinite reflectances ro averaging 0 3 ro moderate to high d18o and d13c values and little difference in major or trace element composition between deformed and undeformed sediments in contrast faulted sediments from the a6st well which intersects the a fault at 1993 m sstvd show evidence for a paleothermal anomaly 0 55 ro and depleted d18o and d13c values sodium is depleted and calcium is enriched in a mudstone gouge zone at the top of the fault cut in the well this effect diminishes with distance from this gouge zone cuttings from other wells in south eugene island block 330 show slightly elevated vitrinite reflectance in fault intercepts relative to sediments outside the fault zone overall indicators of mass and heat flux indicate the main growth fault zone in south eugene island block 330 has acted as a conduit for ascending fluids although the cumulative fluxes vary along strike this conclusion is corroborated by oil and gas distribution in downthrown sands in blocks 330 and 331 which identify the fault system in northwestern block 330 as a major feeder simple modeling of coupled heat and mass flux indicates the paleothermal anomaly in the fault zone intersected by a6st well was short lived having a duration less than 150 yr the anomaly could have been produced by a 2 106 m3 pulse of fluid ascending the fault at an actual velocity of over 1 km yr darcy flux of 330 m yr from 3 km deeper in the basin simple darcy law computation indicates a transient fault permeability on the order of 110 md during this flow pulsing of fluid up the fault was probably the norm although most flow did not produce such strong thermal anomalies as the one detected in the a6st well analysis of fluid pressures shows that the main fault is a profound lateral permeability barrier having up to 1800 psi of water pressure differential across it the hydrocarbon sealing capacity of the fault depends on the pressure difference across the fault fault permeability is best understood in terms of effective stress under ambient conditions the fault is at high pressure relative to downthrown reservoirs a pulse of high pressure fluid ascending end page 244 the fault lowers effective stress in the fault zone sufficiently to produce a significant transient increase in permeability if the fluid is in an area of the fault adjacent to downthrown relatively low pressure reservoir sands the fluid will discharge into them permeability in and adjacent to the fault then decreases such that fluid cannot reenter the fault zone and escape from the reservoir

melting of albite and dehydration of brucite in h 2 o nacl fluids to 9 kbars and 700 900 c implications for partial melting and water activities during high pressure metamorphism the melting reaction albite solid h2o fluid albite h2o melt has been determined in the presence of h2o nacl fluids at 5 and 9 2 kbar and results compared with those obtained in presence of h2o co2 fluids to a good approximation albite melts congruently at 9 kbar indicating that the melting temperature at constant pressure is principally determined by water activity at 5 kbar the temperature t mole fraction x h2o melting relations in the two systems are almost coincident by contrast h2o nacl mixing at 9 kbar is quite non ideal albite melts 70 c higher in h2o nacl brines than in h2o co2 fluids for x h2o 0 8 and 100 c higher for x h2o 0 5 the melting temperature of albite in h2o nacl fluids of x h2o 0 8 is 100 c higher than in pure water the p t curves for albite melting at constant h2o nacl show a temperature minimum at about 5 kbar water activities in h2o nacl fluids calculated from these results from new experimental data on the dehydration of brucite in presence of h2o nacl fluid at 9 kbar and from previously published experimental data indicate a large decrease with increasing fluid pressure at pressures up to 10 kbar aqueous brines with dissolved chloride salt contents comparable to those of real crustal fluids provide a mechanism for reducing water activities buffering and limiting crustal melting and generating anhydrous mineral assemblages during deep crustal metamorphism in the granulite facies and in subduction related metamorphism low water activity in high pressure temperature metamorphic mineral assemblages is not necessarily a criterion of fluid absence or melting but may be due to the presence of low a h2o brines

correction criteria for the suppression of vitrinite reflectance in hydrogen rich kerogens preliminary guidelines abstract thermal maturity is an important factor when assessing the petroleum potential of a basin or particular source rocks although the vitrinite reflectance vr technique provides excellent results from coal samples it has limitations when source rocks are used one problem is the anomalously low vrs the suppression of vr vr suppression in hydrogen rich kerogens the anomalously low suppressed vrs discussed in this article are mainly caused by the problems that organic petrographers encounter in finding and measuring appropriate vitrinite particles in hydrogen rich kerogens vr suppression is recognized when lower than normal vrs are measured on exinite rich or amorphous rich intervals in a vr well profile vr vs depth it can also be recognized when anomalously low vrs relative to the regional coalification maturation pattern or to regional oil gas occurrences are encountered in an interval although vr suppression in organic rich rocks coals and oil shales is well documented guidelines for its correction are lacking a preliminary model fig 1 is presented to help correct the vr suppression in hydrogen rich kerogens measured suppressed vr in r o along with original when immature hydrogen index hio estimated via rock eval pyrolysis and or organic petrography data of a sample can be used to estimate the maximum true unsuppressed vr although the vr and hio data for an individual sample is sufficient to estimate the maximum true vr we recommend the use of data from several closely spaced samples the model was tested with examples from published data and found to reasonably correct vr suppression in hydrogen rich kerogens we plan to refine the model using data from rock samples suppressed unsuppressed pairs and pyrolysis data

fluid evolution during metamorphism of the otago schist new zealand i evidence from fluid inclusions fluid inclusion salinities from quartz veins in the otago schist new zealand range from 1 0 to 7 3 wt nacl eq in the torlesse terrane and from 0 4 to 3 1 wt nacl eq in the caples terrane homogenization temperatures from these inclusions range from 124 to 350 c with modal values for individual samples ranging from 163 to 229 c but coexisting low salinity inclusions exhibiting metastable ice melting show a narrower range of t h from 86 to 170 c with modes from 116 to 141 c these data have been used in conjunction with chlorite chemistry to suggest trapping conditions of 350 400 c and 4 1 6 0 kbar for inclusions showing metastable melting from lower greenschist facies rocks with the densities of many other inclusions reset at lower pressures during exhumation of the schist the fluid inclusion salinities and br cl ratios from veins from the torlesse terrane are comparable to those of modern sea water and this suggests direct derivation of the vein fluid from the original sedimentary pore fluid some modification of the fluid may have taken place as a result of interaction with halogen bearing minerals and dehydration and hydration reactions the salinity of fluids in the caples terrane is uniformly lower than that of modern sea water and this is interpreted as a result of the dilution of the pore fluid by dehydration of clays and zeolites the contrast between the two terranes may be a result of the original sedimentary provenance as the torlesse terrane consists mainly of quartzofeldspathic sediments whilst the caples terrane consists of andesitic volcanogenic sediments and metabasites which are more prone to hydration during diagenesis and hence may provide more fluid via dehydration at higher grades

stable isotopic evidence for mixing between metamorphic fluids and surface derived waters during recent uplift of the southern alps new zealand recent studies have shown that the southern alps of new zealand have active hydrothermal systems driven by tectonic uplift these studies have concentrated on the rapidly exhumed rocks immediately adjacent to the alpine fault we present new stable isotopic evidence that shows that fluid flow and fluid mixing processes thought to be restricted to rocks near the alpine fault also occurred in the low uplift rate region of the southern alps orogen during the kaikouran orogeny 10 ma to present low 18ocalcite values of post metamorphic veins in the eastern southern alps indicate that meteoric waters have penetrated to hot midcrustal levels 350 300 c 5 km depth and mixed with metamorphic fluids in areas far removed from the alpine fault in addition the isotopic values of calcites precipitated in active faults in the mackenzie basin define a trend of increasing 18o decreasing 13c and decreasing temperature and form an isotopic mixing line between fault and vein calcites crosscutting metamorphic rocks and authigenic calcites precipitated in mackenzie basin sediments the isotope data indicate a second phase of mixing between a modified metamorphic fluid and a surficial diagenetic fluid at shallow depths we suggest that mixed metamorphic meteoric hydrothermal systems have developed in the east side of the southern alps during uplift and that active faults have played a critical role in transporting metamorphic fluids outward from the mountain front and into the adjacent intermontane basins

vitrinite recycling diagnostic criteria and reflectance changes during weathering and reburial abstract the aim of this study was first to review or even identify reliable diagnostic criteria to distinguish recycled and autochthonous vitrinite particles and second to examine and try to explain the impact of weathering and reburial on optical changes reflectance of recycled material the work was based on indigenous and recycled particles of two sample sets from two wells drilled in the senegalese margin basin and the ardeche paleomargin se france respectively in addition to reflectance measurements the studied samples have also been analysed by rock eval pyrolysis for both sample sets hydrogen index hi and oxygen index oi values suggest a type iii organic matter of dominant terrestrial origin two main diagnostic criteria appear particularly useful to identify indigenous and recycled vitrinite particles the first is the clear genetic relationship between telinite and collotelinite the second is the presence of pyrite framboid inclusions in indigenous vitrinite the pyrite vitrinite association illustrates the relationship between sulphate reduction pyritisation and in situ organic matter transformation through gelification process however application of both diagnostic criteria requires some additional information about depositional conditions i e anoxicity and thermal maturity of the studied samples weathering has no or only very limited effect on vitrinite reflectance during their new burial history recycled vitrinites follow a reflectance evolution path close to that of inertinites and thus appear as transitional materials between vitrinite and inertinite macerals consistently this behaviour implies that recycled vitrinite reflectance changes during burial diagenesis do not solely depend on their chemical composition but most probably too on combined chemical and structural changes inherited from their past burial history and from the impact of weathering during their recycling stage

partial melting below the magmatic arc in the central andes deduced from geoelectromagnetic field experiments and laboratory data abstract magnetotelluric and geomagnetic deep soudings in northern chile revealed a pronounced high conductivity zone hcz below the western cordillera which constitutes the present magmatic arc with active volcanism of the south american continental margin conductivities in the range of 1 s m are observed the anomalously high conductivities in a broad depth range from approximately 20 km to at least 60 km are interpreted in terms of partial melting other geophysical observations such as a zone of low seismic velocities lvz at similar depths high heat flow values 100 mw m2 and a pronounced negative anomaly in the residual gravity field are also considered impedance spectroscopic laboratory experiments up to and in the temperature range of partial melting were performed under controlled oxygen fugacities at sub solidus temperatures electrical behavior is described by defect electrons with an activation energy of 1 34 ev and a conductivity of 2 5 ms m at 900 c model calculations using a modified brick layer model mbl were compared with experimental observations a good agreement between calculations and experiments is achieved with an electrical resistivity of the melt phase of 7 s m at 1250 c assuming an activation energy of 1 ev the same mbl model is used to calculate melt proportions beneath the western cordillera between 14 and 27 vol of interconnected melt are necessary to explain the observed hcz the stability of the melt rich crust is explained by a dynamic melting crystallisation behavior during crustal anatexis and by magma filled dikes

long period magnetotelluric measurements near the central california coast a land locked view of the conductivity structure under the pacific ocean summary telluric data from hollister california were combined with magnetic data from the fresno california magnetic observatory to determine the mt response at long periods the shifted eigenstate analysis of latorraca et al 1986 was used to determine the maximum apparent resistivities and corresponding eih phases for periods from 1 to 30 h the apparent resistivity and phase at hollister are greatly influenced by the ocean with the mantle branch beginning at a period of about 12 h in typical continental areas the mantle branch usually begins at periods of 10s or less a maximum likelihood inversion algorithm was used to invert the observed data and to determine the sensitivity of the model parameters the data required a resistivity thickness product of approximately 1 x lo9 q m2 1 x lo6 q m km for the oceanic lower crust and upper mantle the oceanic mantle shows a rather small gradient of conductivity from approximately 200 to 640 km in depth and a rapid conductivity increase below 640km the sensitivity analysis shows the data are most sensitive to the oceanic parameters except for the electric field amplitudes which are also sensitive to the resistivities under the observation site

tectonic architecture of the mountain front foreland basin transition south island new zealand assessed by fission track analysis oblique continental convergence across the alpine fault since the mid miocene has loaded the australia plate with the leading edge of the pacific plate and formed a foreland basin the basin occurs mostly offshore beneath the continental shelf but remnants of the marine basin fill overlie ordovician basement in a narrow coastal strip 10 20 km wide in westland between the tasman sea and the alpine fault the results of fission track analyses of apatite and zircon separates from this basement integrated with the regional structure and stratigraphy indicate that the foreland basin formerly extended across the whole of westland and that its succession increased in thickness eastward toward the alpine fault reaching a maximum of about 4 km continuing covergence across the main thrust alpine fault has caused its footwall to thicken by reverse fauting in a transitional zone 10 20 km wide immediately west of the mountain front southern alps consequently the inner margin of the foreland basin has been inverted and the succession almost completely eroded in the southern part of westland upper crustal shortening in the australia plate amounted to about 2 km associated uplift started about 5 m y ago and involved only one major fault bounded block in northern westland upper crustal shortening amounted to about 12 km two major fault bounded blocks were involved and uplift was underway by 10 ma assuming a preuplift geothermal gradient of 25 c km the long term rate of uplift ranged between 0 8 and 2 0 mm yr in southern westland and between 0 4 and 1 2 mm yr in different parts of northern westland

two case studies of thermal maturity and thermal modelling within the overpressured jurassic rocks of the barrow sub basin north west shelf of australia abstract thermal maturity within the jurassic thick overpressured source rocks of the bambra 2 and barrow deep 1 wells barrow sub basin western australia was investigated and modelled using a temperature time related kinetic model the aim of this study is to investigate whether overpressure has a determinable influence on thermal maturation repeat formation tests rfts drill stem tests dsts and mud weights indicate that the jurassic section in the barrow sub basin is highly overpressured it appears that the overpressured zone is associated with high sonic transit times and low resistivity values in the jurassic fine grained rocks anomalously low measured vitrinite reflectance r o values in the overpressured zone appear to be due to either r o suppression or inaccurate identification of vitrinite anomalously low t max values are found to be caused by contamination from drilling mud additives a good correlation is found between the reliable maturity data including biomarker maturity and predicted maturity values r o between 0 8 and 2 2 and suggests that the anomalously low r o and t max values are probably not related to the overpressuring fluid pressure about 40 80 mpa at 3000 4600 m these results demonstrate that the true maturity level in the jurassic overpressured section probably depends on temperature and time alone we therefore conclude that overpressure is not an important factor in thermal maturity and hydrocarbon generation in this region

low seismic wave speeds and enhanced fluid pressure beneath the southern alps of new zealand a region of low seismic wave speed is detected beneath the central southern alps of new zealand on the basis of traveltime delays for both wide angle reflections and p waves from teleseismic events respective ray paths for these p waves are mutually perpendicular ruling out anisotropy as a cause of the delays the low speed region measures about 25 km by 40 km has a speed reduction of 6 10 and is largely above the downward projection of the alpine fault the most likely cause of the low speed zone is high fluid pressure due to excess water being released by prograde and strain induced metamorphism into the lower crust because enhanced fluid pressure reduces the work required for deformation the existence of the central southern alps low speed zone implies that this part of the australian pacific plate boundary is relatively weak

role of fluids in the metamorphism of the alpine fault zone new zealand models of fluid rock interaction in and adjacent to the alpine fault in the hokitika area south island new zealand were investigated using hydrogen and other stable isotope studies together with field and petrographic observations all analysed samples from the study area have similar whole rock d values dwr 56 to 30 average 45 n 20 irrespective of rock type degree of chloritization location along the fault or across strike distance from the fault in the garnet zone the green chlorite rich fault rocks which probably formed from australian plate precursors record nearly isothermal fluid rock interaction with a schist derived metamorphic fluid at high temperatures near 450 500 c d of water in equilibrium with the green fault rocks dh2o green 18 d of water in equilibrium with the greyschists and greyschist derived mylonites dh2o grey 19 at 500 c dh2o green 17 dh2o grey 14 at 450 c there is no indication of an influx of a meteoric or mantle derived fluid in the alpine fault zone in the study area the alpine fault zone at the surface shows little evidence of late stage retrogression or veining which might be attributed to down temperature fluid flow it is probable that prograde metamorphism in the root zone of the southern alps releases metamorphic fluids that at some region rise vertically rather than following the trace of the alpine fault up to the surface owing to the combined effects of the fault the disturbed isotherms under the southern alps and the brittle ductile transition such fluids could mix with meteoric fluids to deposit quartz rich possibly gold bearing veins in the region c 5 10 km back from the fault trace these results and interpretations are consistent with interpretations of magnetotelluric data obtained in the south island geophysical transects sight programme

southern alps cu au hydrothermal system westland new zealand the southern alps of new zealand is an actively rising mountain belt which displays a thermal anomaly adjacent to the alpine fault the australian pacific plate boundary extensive fluid movement occurs in this uplift zone resulting in metallic vein mineralization gold mineralization is confined to greenschist facies rocks while younger veins in amphibolite facies rocks near the alpine fault are enriched in copper transport and deposition of metals in this complex hydrothermal system is governed by interaction between rising metamorphic fluids and downward percolating meteoric fluid metamorphic fluids have equilibrated with graphitic schist country rock and are relatively reduced infiltration and mixing of meteoric water increases oxygen activity and decreases sulphur activity in the fluid oxidised meteoric water heats up and dissolves cu during downward percolation this cu is deposited as the fluid becomes more reduced hence there is a progressive increase in copper content in the middle portions of the hydrothermal system especially in the more permeable highly fractured rocks near the alpine fault

lithospheric and upper mantle structures results of electromagnetic soundings in europe abstract information on electrical conductivity of the subsurface at a crustal scale is inferred mainly from magnetotelluric mt and magnetometer data the latter include both magnetometer array studies mv and geomagnetic depth soundings gds locally controlled source data e g data from d c and very low frequency vlf resistivity surveys frequency soundings and airborne electromagnetic mappings provide information on near surface structures e g on exposures of deeper conductors the variety of electromagnetic methods provides an efficient zooming ability for structures from a regional scale of hundreds of kilometres to small local details of some metres thus both the mapping of large scale crustal conductivity structures and a detail study of the anomalous structures are possible the electrical conductivity of the crust and upper mantle is described within the plate tectonic framework examples of structures which become electrically conductive at different phases of the process are shown using the results of electromagnetic soundings in various tectonic environments in europe it is shown how the crustal conductors can be interpreted as ancient and modern tectonic markers these include structures from the fennoscandian shield precambrian extensional basins and collisional zones the carpathians scotland and ireland suture zones and the pannonian basin extensional basin and thrust zones as well as conductors detected along the european geotraverse e g the terrain boundaries and the decollement of the variscan thrusting an example which considers the lower crustal conductivity from the fennoscandian shield shows rather large lateral variations in conductance of the lower crust varying from 1 s to several hundreds of siemens estimates of the depth of the asthenospheric conducting layer within the european territory also vary considerably shallowest depths are detected beneath extensional regions the pannonian basin and the rhinegraben 40 80 km whereas depths of 200 km or over or the absence of a conducting layer are detected beneath the alps and old precambrian cratons among the key areas where electromagnetic investigations should play an essential role are the deep structure of the east european platform boundaries of archaean and proterozoic terrains the structure of the teisseyre tornquist zone the nature of the present day continental lower crust and the characteristics of the electrical asthenosphere beneath europe

resistivity structure of the underconsolidated sediments of the cascadia basin summary the cascadia basin is located on the abyssal plain of the juan de fuca plate off the west coast of vancouver island the sediments of the cascadia basin are very thick over 2 km and are underconsolidated because of a high rate of sedimentation seismic results suggest that the sediment acoustic velocities are low and increase slowly and monotonically with depth from the sea floor a vertical electric dipole ved survey was conducted to obtain the resistivity structure of the upper kilometre of sediments to determine the physical properties of a thick sequence of abyssal basin sediments and for comparison with seismic results while models with multiple layers can be found which fit the data the best fitting models share a common feature decreasing resistivity with depth which is suggestive of a gradient in the resistivity the feature is consistent with the results of other geological and geophysical studies in particular the seismic results do not indicate any significant contrast in the physical properties i e layering in the upper 500 m we extend the ved theory to the case where resistivity varies continuously with depth and obtain a gradient model that fits the cascadia basin data marginally better than simple layered models the model resistivity is combined with heat flow measurements for the determination of physical properties versus depth the physical property models the porosity for example are not consistent with the physical properties derived from seismic survey results the mismatch is diagnostic of the underconsolidation and is primarily due to the effects of the high rate of sedimentation on the seismic velocity and on the heat flow which in turn influence the physical properties

a focused look at the alpine fault new zealand seismicity focal mechanisms and stress observations the alpine fault is the pacific australian plate boundary in the south island of new zealand this study analyzes 195 earthquakes recorded during the 6 month duration of the southern alps passive seismic experiment sapse in 1995 1996 and two mr 5 0 earthquakes and aftershocks in 1997 which occurred close to the central part of the alpine fault precise earthquake locations are derived by simultaneous inversion for hypocenter parameters a one dimensional velocity model and station corrections together with focal mechanisms calculated using a first motion and amplitude ratio method these results provide a picture of the seismotectonics in the central south island over a 6 month period moment tensor inversions of three earthquakes provide an independent means of comparison to the focal mechanisms derived using the amplitude first motion method to validate our observations over time we compare the sapse seismicity with the seismicity recorded by the new zealand national seismic network nznsn and a local network at lake pukaki east of the southern alps 6 months versus 8 years our study indicates that the alpine fault releases elastic strain seismically from the surface down to 10 12 km depth between milford sound in the south and the hope fault in the north the seismicity rate of the alpine fault is low but comparable to locked sections of the san andreas fault with large earthquakes expected seismicity decreases north of bruce bay at the alpine fault and within a triangular region along the alpine fault located between the hope and porters pass fault zones we interpret this as the result of deformation distributed on the alpine fault and the hope and porters pass fault zones the base of the seismogenic zone is fairly uniform at 12 km 2km over large parts of the south island the high alps region has a shallower base of the seismogenic zone indicating localized elevated temperatures east of the alpine fault most of the study region deforms under a uniform stress field with a maximum principal horizontal shortening direction of 110o 120 o similar to geodetic observations and plate motions this confirms that the region is not undergoing strain partitioning the earthquake data show that the deformation away from the alpine fault is distributed on mainly nne trending thrust faults and strike slip transfer faults with a maximum seismogenic depth of 12km

magnetotelluric soundings across the south island of new zealand electrical structure associated with the orogen of the southern alps summary magnetotelluric soundings have been made at 31 locations on two transects across the central part of the south island of new zealand the calculated impedance tensor at each site has been decomposed using an assumed tectonic strike of n35 e to yield 2 d regional apparent resistivity and phase responses these have been inverted to produce 2 d models of electrical conductivity structure associated with the southern alps orogen the resulting structure is consistent with proposed models for the orogen and shows a zone of enhanced conductivity at a depth of only 10 km beneath the southern alps this correlates with suggestions of enhanced temperatures at shallow depths and can be interpreted as being due to the presence of wet greenschist facies rocks upthrust from lower crustal depth by the collision of the pacific and australian plates reduced resistivity also occurs at 25 km depth beneath the canterbury plains and can be tentatively associated with the existence of a lower crustal detachment surface the derived resistivity structure close to the alpine fault is highly complex and may be indicative of either 3 d induction in igneous intrusions or anisotropic structure associated with the fault zone

verification of five magnetotelluric systems in the mini emslab experiment given the degree of complexity of modern magnetotelluric mt instrumentation comparison of the total performance for two or more systems is an important verification test this paper compares the processed data from five mt systems which were designed and constructed separately and which employ different electrode types electrode separations magnetometers and methods of signal processing the comparison shows that there is a high degree of agreement among the data from the different systems the study also demonstrates the compatibility and reliability of the mt systems employed as part of emslab juan de fuca electromagnetic sounding of the lithosphere and asthenosphere beneath the juan de fuca plate this project proposed by a consortium of institutions involves not only magnetotellurics studies but also studies of magnetic variation on land and on the sea bottom the project calls for the real time mt systems to occupy stations along segments of a profile in oregon a composite profile will be created from the segments prior to commencing the main mt profiling phase one week was set aside in august 1984 for all groups to record and process mt data sequentially at six sites in diverse geologic terrains this experiment was called mini emslab