nature and origin of seismic reflection fabric ruby east humboldt metamorphic core complex nevada seismic reflection profiling across exposed upper and middle crustal rocks of the ruby east humboldt metamorphic core complex delineates important characteristics of the crustal structure developed during tertiary extensional deformation the goals of this study were to trace a tertiary extensional shear zone from mylonitic surface outcrops into a seismic section and to characterize the deeper crustal fabric associated with the polyphase deformational history of the complex reflections in the shallow surface correlate with a plastic to brittle shear zone that formed during the tectonic unroofing of the middle crustal rocks constructive interference from strong planar layering in the mylonitic shear zone is considered chiefly responsible for generating the reflections east and southwest dipping reflectors in the seismic section appear to correlate with exposed opposing dipping mylonitic foliation domains the opposing dips of the mylonitic layering may reflect warping of the normal sense shear zone during tectonic exhumation coupled with an overall anastomosing character deeper in the section a heterogeneous reflection character correlates with increases in velocity shown from wide angle measurements in this region and is interpreted as penetrative fabric originating from extensional flow facilitated by broad scale pure shear and localized simple shear lower crustal rocks apparently achieved granulite facies metamorphism during extension a maximum of 6 8 km of mafic material could have been added to the crust during cenozoic extension the moho reflection has such a high amplitude that it may be caused by partially molten rocks interleaved with peridotite reflections from the moho show no significant upwarping of the base of the crust beneath the core complex this suggests that the lower crustal configuration is at least as young as the period of extensional activity that lasted from approximately 40 to 20 ma which was responsible for the exhumation of the metamorphic core complex

supratidal evaporitic dolomite at ojo de liebre lagoon mineralogical and isotopic arguments for primary crystallization abstract this pair deals with a new site of dolomite formation in supratidal evaporite flats at the southernmost edge of the ojo de liebre evaporitic complex baja california mexico the sedimentary context is mostly siliciclastic except for calcareous outcrops around the flats both chemical and isotopic data on the interstitial solutions demonstrate a mixing between mg2 rich marine derived brines and calcium bicarbonate bearing continental waters which are subjected to concentration by capillary evaporation the lack of any in situ solid caco3 in the sedimentary matrix argues for direct crystallization of dolomite from aqueous solutions this is documented by the oxygen isotope compositions of the dolomite which are consistent with deposition under equilibrium conditions of ambient temperature and 18o of water of the interstitial solutions furthermore the low 13c values of the dolomites and the absence of living plants or organisms in the basin indicate that a fraction of the total dissolved inorganic carbon reservoir is biogenic and is mostly provided by continental groundwaters therefore crystallization of dolomite occurs within the sands infiltrated by solutions whose state of saturation relative to dolomite is mainly governed by the chemistry of continental groundwaters mixing with the inflowing marine brines

isotope geochemistry of fluid inclusions in permian halite with implications for the isotopic history of ocean water and the origin of saline formation waters abstract d and 18o values have been determined for fluid inclusions in 45 samples of permian halite the inclusions are enriched in 18o relative to the meteoric water line but are depleted in d relative to ocean water inclusions with the more positive values coincide with the isotopic composition expected for evaporating sea water which follows a hooked trajectory on a d 18o diagram inclusions with more negative values may represent more highly evaporated sea water but probably reflect synsedimentary or diagenetic mixing of meteoric water with evaporite brines the isotope systematics in these inclusions are sufficiently similar to those of a modern evaporite pan to indicate that permian sea water was isotopically similar to modern sea water connate evaporite brines can have negative values because of the probable hooked isotope trajectory of evaporating sea water and or synsedimentary mixing of evaporite brines with meteoric waters subsurface formation waters composed of mixtures of remnant primary evaporite brines and later meteoric waters may be more common than previous isotopic evidence has suggested

seismic signatures of complex geological structures in the cue weld range area murchison domain yilgarn craton western australia abstract the murchison domain forms the northwest part of the youanmi terrane a tectonic unit within the neoarchean yilgarn craton in western australia in the cue weld range area the murchison domain has experienced a complex magmatic and deformation history that resulted in a transposed array of greenstone belts that host significant iron gold and base metal deposits in this study we interpret the upper 2 s about 6 km of a deep crustal seismic profile 10ga yu1 near the town of cue and correlate rock units and structures in outcrop with corresponding reflections we performed 3d constant velocity ray tracing and calculate the corresponding travel times for the reflections for time domain pre stack and post stack seismic data this allows us to link shallow reflections with mafic volcanic rocks of the glen group and basaltic rocks of the polelle group in outcrop based on our interpretation and published geological maps and data we propose a model in which the local stratigraphy represents a refolded thrust system to test our hypothesis we applied 2d acoustic finite difference forward modeling the corresponding synthetic data were processed in the same way as the acquired data comparisons between the acquired and the synthetic data show that the model is consistent with observations we propose a new model for the subsurface of the cue weld range area and argue that some of the lithologies in the area are repeated structurally at different levels our approach highlights the benefit of imaging and modeling of deep seismic transects to resolve local structural complexity in archean granite greenstone terrains

scattering of seismic waves simulated by finite difference modelling in random media application to the gravberg 1 well sweden we generate surface and vsp synthetic seismograms using finite difference modelling of the elastic wave equation in self similar media the elastic model is determined by analyses of the sonic log from the deep gravberg 1 borehole in central sweden the upper 1200 m is highly fractured and the velocities are best described by a log normal distribution rather than a gaussian distribution analyses of this interval after removing the deterministic trend and assuming a self similar gaussian distribution of the random component give a standard deviation of 370 m s a correlation distance of 45 m and a hurst number of 0 18 these values and others from deeper levels are used to generate the 2 d elastic model and the synthetic seismograms are compared with real data synthetic surface seismic data show a poor qualitative match when compared to real data synthetic vsp data match the real vsp better qualitatively in general the synthetic data show considerably more scattering effects than the real data possible explanations for this discrepancy include 1 intrinsic attenuation has been ignored 2 a gaussian distribution of the random component was assumed or 3 the heterogeneities have a preferred orientation the poor match implies that the method for extracting the 2 d or 3 d velocity variations in the uppermost crust from 1 d sonic log data need to be studied further

crustal and upper mantle velocity structure of the salton trough southeast california this paper presents data and modelling results from a crustal and upper mantle wide angle seismic transect across the salton trough region in southeast california the salton trough is a unique part of the basin and range province where mid ocean ridge transform spreading in the gulf of california has evolved northward into the continent in 1992 the u s geological survey usgs conducted the final leg of the pacific to arizona crustal experiment pace two perpendicular models of the crust and upper mantle were fit to wide angle reflection and refraction travel times seismic amplitudes and bouguer gravity anomalies the first profile crossed the salton trough from the southwest to the northeast and the second was a strike line that paralleled the salton sea along its western edge we found thin crust 21 22 km thick beneath the axis of the salton trough imperial valley and locally thicker crust 27 km beneath the chocolate mountains to the northeast we modelled a slight thinning of the crust further to the northeast beneath the colorado river 24 km and subsequent thickening beneath the metamorphic core complex belt northeast of the colorado river there is a deep apparently young basin 5 6 km unmetamorphosed sediments beneath the imperial valley and a shallower 2 3 km basin beneath the colorado river a regional 6 9 km s layer between 15 km depth and the moho underlies the salton trough as well as the chocolate mountains where it pinches out at the moho this lower crustal layer is spatially associated with a low velocity 7 6 7 7 km s upper mantle we found that our crustal model is locally compatible with the previously suggested notion that the crust of the salton trough has formed almost entirely from magmatism in the lower crust and sedimentation in the upper crust however we observe an apparently magmatically emplaced lower crust to the northeast outside of the salton trough and propose that this layer in part predates salton trough rifting it may also in part result from migration of magmatic spreading centers associated with the southern san andreas fault system these spreading centers may have existed east of their current loca tions in the past and may have influenced the lower crust and upper mantle to the east of the current salton trough

hydrocarbon filling and leakage history of a deep geopressured sandstone fulmar formation united kingdom north sea diagenetic minerals in a water filled borehole from the jurassic fulmar formation united kingdom central north sea record two phases of hydrocarbon filling and emptying initial oil charge was during the late cretaceous at shallow burial depths of about 1 5 km 0 9 mi as we consider that hydrocarbon has preserved porosity during burial this has implications for the understanding of the porosity evolution of the fulmar formation which in other locations is an important hydrocarbon reservoir the early oil charge as recorded by illite k ar ages progressively filled the structure from 84 to 59 ma and possibly precipitated bitumen because of biodegradation the first oil predated many of the burial diagenetic reactions within the sandstone after leak off at ca 60 ma diagenetic reactions continued in an open geochemical system with possible import of co2 products of these reactions include ankerite and quartz overgrowth cements hydrocarbon staining postdates these phases and provides evidence of a hydrocarbon charge probably gas condensate the second hydrocarbon charge also leaked off and the sandstone is now water bearing previous work on the fulmar formation has incorrectly placed all the diagenetic reactions as predating the first arrival of hydrocarbon the present day pore fluids are high salinity high 18o fluids derived from the underlying permian zechstein evaporates these fluids entered the reservoir during a phase of overpressure release that caused fracturing of the framework quartz grains possibly coincident with the second phase of hydrocarbon leak off at 2 5 ma mark wilkinson studied sandstone hosted calcite concretions for his ph d and has subsequently tried to take a more holistic view of the diagenesis of sedimentary basins his career has been funded by a variety of oil companies although he is currently involved in geological disposal of some of the co2 that he was partly responsible for producing stuart haszeldine is a professor at edinburgh university he received his ph d on clastic and coal sedimentology in 1981 from the university of strathclyde after an attempted reeducation in the oil industry he has collaborated with many companies to research on hydrocarbon reservoir quality radioactive waste disposal and coal resources he currently heads the united kingdom s largest group investigating carbon dioxide sequestration tony fallick was educated at the university of glasgow scotland in physics b sc and chemistry ph d he did research at mcmaster university ontario canada and cambridge england before moving in 1980 to the scottish universities research and reactor center where he is currently the director

the use of a high resolution 3d chirp sub bottom profiler for the reconstruction of the shallow water archaeological site of the grace dieu 1439 river hamble uk the remains of henry v s flagship the grace dieu currently lie buried within the inter tidal sediments of the river hamble s england previous archaeological investigations have been hindered by difficult excavation conditions resulting in a poor understanding of the dimensions shape and degradation state of the hull s deeper structure this study therefore aimed to image characterize and reconstruct the buried remains of this vessel using a high resolution 3d acoustic sub bottom chirp system with rtk gps positioning capability the accurate navigation and high resolution data that were acquired enabled the construction of a full 3d image of the site that not only identified the remains of the wooden hull but also features buried within it in addition the degradation state of these buried wooden remains were investigated by calculating reflection coefficients while a hypothetical larger reconstruction of the grace dieu s hull was achieved through the use of the shipshape ship design software package the results of this project demonstrate that i acoustic data can be used to successfully image buried wooden shipwrecks ii artefacts are buried within the hull of the grace dieu iii there is variation in the degradation state of the buried timbers as calculated from the acoustic data with the shell of the vessel being moderately well preserved and iv the grace dieu was a very large ship for its time possibly over 60 m long and 16 m wide the outcomes of this research not only have considerable implications for the management and monitoring of submerged and buried archaeological sites but also for planning intrusive surveys should they be required

crustal structure of eastern nevada from cocorp deep seismic reflection data the western nevada segments of the cocorp 40 n deep seismic reflection survey of the north american cordillera reveal the geometry of structures of cenozoic and possibly earlier ages to travel times of 10 s corresponding to depths of 30 km the most striking feature of the data is a band of prominent reflections typically at traveltimes of 9 5 to 10 5 s that are present discontinuously across the entire data set few reflections are observed from beneath the base of this reflective zone which is interpreted as the crust mantle transition this reflection mono is inferred to be continuous across the survey area varying gradually in depth but without resolvable offsets it appears to have taken its present form or position during basin range crustal extension the middle to lower crust in much of the survey area is characterized by discontinuous reflections that are typically subhorizontal and locally dip gently west these reflections may represent intrusions or shear zones related to basin range or pre basin range extension but some are likely to be inherited from earlier compressional deformation reflections from the upper crust are interpreted as images of basin fill strata basin range normal faults and mesozoic and paleozoic thrusts related to back arc thrusting and accretion of oceanic and arc related rocks

wide angle seismic reflections from two dimensional random target zones we have analyzed the shot gather seismic response of a buried zone of isotropic random velocity fluctuations scattering analysis predicts that events at offsets greater than the depth of the target zone will appear more continuous and coherent than events at zero and small offsets due to an effective dip filter inherent to the source target receiver geometry the scattering theory is confirmed with finite difference synthetic seismograms calculated for an idealized crustal model the closely spaced synthetic seismograms show bright events having the appearance of refraction arrivals which suggest a horizontally layered structure in the target zone these effects appear to be independent of the magnitude of the velocity fluctuations provided that the magnitude of the fluctuations is small 3 0 of the mean velocity we have made a second finite difference calculation of a crustal scale model to reinterpret previously published wide angle data from the black forest germany sandmeier and wenzel 1986 the response of the lower crust seen in the conventionally recorded field data can be explained by a lower crustal model of isotropic small amplitude random velocity fluctuations we present this in contrast to a laterally homogeneous model of a finely layered lower crust of alternating high and low velocities proposed by sandmeier and wenzel 1986 the synthetic studies and the scattering analysis demonstrate that the geometry of the wide angle experiment produces signals which appear to be from layered target zones even where no layering exists they also underscore the importance of conducting seismic surveys which make use of closely spaced receivers and which integrate vertical incidence and wide angle recording

small scale heterogeneity and large scale velocity structure of the continental crust since conrad 1925 first identified a midcrustal refraction event in central europe seismologists have developed layered models of the continental crust with velocity steps at various levels modern analysis of crustal refraction data including automated inversion relies on ray theoretical techniques or wave theory for plane layered media which generally parameterize the earth as a series of grossly horizontal layers the bias toward layered models is in part based on physical and geologic intuition but it is to a large measure a result of available interpretation methods for example layer based reflectivity and ray tracing methods in contrast reflection seismology has produced a picture of a highly heterogeneous crust with wavelength scale variations in velocity existing at different levels of the crust in different tectonic regimes finite difference modeling shows that zones of small amplitude random velocity fluctuations not only produce short discontinuous reflection segments as observed on deep seismic reflection data but also produce strong wide angle reflections some of the events identified as crustal wide angle reflections in field data may in fact be the result of scattering from wavelength scale heterogeneities and need not result from reflection from a first or second order velocity discontinuity significant changes in the large scale velocity structure associated with zones of random velocity fluctuations produce relatively small changes in the dynamic properties of the backscattered wave field this observation is in disagreement with the common credo that the seismic refraction method is primarily sensitive to changes in the large scale velocity structure as a consequence the ray theoretical interpretation of such wide angle reflections as first order discontinuities leads to crustal velocity structures that are nonunique and may be erroneous in detail while correctly predicting average crustal velocity and thickness we are not suggesting that crustal velocity does not change with depth on the large scale rather we are proposing that crustal seismic events observed at intermediate and large offsets may be the result of scattering from wavelength scale velocity fabric rather than specular reflection or refraction from first order discontinuities

the mohorovicic discontinuity beneath the continental crust an overview of seismic constraints abstract the seismic signature of the moho from which geologic and tectonic evolution hypotheses are derived is to a large degree a result of the seismic methodology which has been used to obtain the image seismic data of different types passive source earthquake broad band recordings and controlled source seismic refraction densely recorded wide angle deep seismic reflection and normal incidence reflection using vibroseistm explosives or airguns have contributed to the description of the moho as a relatively complex transition zone of critical importance for the quality and resolution of the seismic image are the acquisition parameters used in the imaging experiments a variety of signatures have been obtained for the moho at different scales generally dependent upon bandwidth of the seismic source this variety prevents the development of a single universally applicable interpretation in this way source frequency content and source and sensor spacing determine the vertical and lateral resolution of the images respectively in most cases the different seismic probes provide complementary data that gives a fuller picture of the physical structure of the moho and its relationship to a petrologic crust mantle transition in regional seismic studies carried out using passive source recordings the moho is a relatively well defined structure with marked lateral continuity the characteristics of this boundary change depending on the geology and tectonic evolution of the targeted area refraction and wide angle studies suggest the moho to be often a relatively sharp velocity contrast whereas the moho in coincident high quality seismic reflection images is often seen as the abrupt downward decrease in seismic reflectivity the origin of the moho and its relation to the crust mantle boundary is probably better constrained by careful analysis of its internal details which can be complex and geographically varied unlike the oceanic moho which is formed in a relatively simple well understood process the continental moho can be subject to an extensive variety of tectonic processes making overarching conclusions about the continental moho difficult speaking very broadly 1 in orogenic belts still undergoing compression and active continental volcanic arcs the moho evolves with the mountain belt 2 in collapsed phanerozoic orogenic belts the moho under the collapse structure was formed during the collapse often by a combination of processes 3 in regions having experienced widespread basaltic volcanism the moho can result from underplated basalt and basaltic residuum in precambrian terranes the moho may be as ancient as the formation of the crust in others precambrian tectonic and magmatic processes have reset it we note that seismic reflection data in phanerosoic orogens as well as from precambrian cratonic terranes often show thrust type structures extending as deep as the moho and suggest that even where crust and mantle xenoliths provide similar age of formation dates the crust may be semi allochothonous

quartz veins record vertical flow at a graben edge fulmar oil field central north sea the fulmar oil field lies at the faulted western margin of the central graben in the central north sea chalcedony and quartz and later dolomite occur as diagenetic cements in veins within the upper jurassic fulmar formation quartz in one particular vein contains primary aqueous fluid inclusions with homogenization temperatures ranging from 85 degrees to 140 degrees c with a mean of 117 degrees c the measured d18o of the authigenic quartz ranges from 25 1 to 27 1o oo smow the d18o of the water from which most of the quartz precipitated is calculated to have been around 8o oo which is significantly more positive than the measured present day formation water d18o of 4o oo quartz and chalcedony cements in the vein are postdated texturally by dolomite that has a d18o of 23 5o oo smow high salinities are recorded in the fluid inclusions in the quartz the majority ranging from 15 to 19 wt nacl equivalent also the quartz contains inclusions of anhydrite high salinity aqueous inclusions combined with anhydrite inclusions in the quartz and the 18o enriched calculated water composition suggest that the quartz precipitated from an evaporite influenced fluid the same fluid contributed mg for dolomite precipitation more deeply buried sections including the permian zechstein evaporites underlying the fulmar field are the most obvious source of an 18o enriched saline fluid mineralization of the vein resulted from cross formational flow of warm fluids from depth through the zechstein and into the jurassic succession over vertical distances of 600 2000 m fluids most probably migrated up faults and fractures during episodes of salt movement or overpressure release

reservoir quality and rock properties modeling triassic and jurassic sandstones greater shearwater area uk central north sea abstract the complex burial and diagenetic histories of the jurassic fulmar and triassic skagerrak sandstones in the uk central north sea present significant challenges with regard to reservoir quality and rock property prediction commercial reservoir quality is retained despite deep burial and associated high temperatures and pressures shallow marine fulmar sands are normally compacted mean igv 26 3 yet have porosities of 21 33 porosity was preserved through inhibition of quartz cementation by clay and microquartz coatings and enhanced by dissolution of framework grains 5 skagerrak fluvial sands are more compacted mean igv 23 2 exhibit minor feldspar dissolution we modeled reservoir quality evolution using the forward diagenetic model touchstone which simulates porosity loss due to compaction and quartz cementation quantitative petrographic analyses and burial history data were used to calibrate touchstone model parameters the results were applied to deeper prospects for pre drill prediction of porosity and permeability in parallel petrophysical data were used to characterize the elastic properties of the sandstones to provide a basis for quantitative seismic forward modeling experimental data and core calibrated petrophysical results reflecting variable in situ fluids and saturations were used to build an elastic properties model the model is robust and was used to generate fluid filled sandstone properties incorporating touchstone results for prospect specific seismic attribute modeling well results from exploration wells are in good agreement with pre drill touchstone and elastic properties model predictions

saline groundwaters in the canadian shield a first overview abstract the chemical and isotopic compositions of saline groundwaters and brines in the canadian shield are described the occurrence of such waters is very widespread not related to any specific rock type or mineralization and generally can be expected at depths below 1 km the most saline brines have salinities exceeding 200 g l 1 dissolved solids and their chemistries tend to suggest that in the past saline waters penetrated the canadian shield the original chemistry of these solutions was subsequently modified through strong rock water interactions and it appears that their final chemical compositions are independent of the original chemistries stable isotope analyses indicate that these waters are not simple evaporated seawater sedimentary basin brines hydrothermal solutions or residual ore forming fluids although any of these could be a precursor of the brines secondary isotope exchange reactions and the hydration of primary silicates accompanied by the formation of clay minerals could have modified not only the primary isotopic composition but may have also profoundly influenced the chemistries of these brines to be effective these processes would have had to occur in closed systems

a seismic refraction survey of the imperial valley region california the u s geological survey conducted an extensive seismic refraction survey in the imperial valley region of california in 1979 the imperial valley is located in the salton trough an active rift between the pacific and north american plates forty shots fired at seven shot points were recorded by 100 portable seismic instruments at typical spacing of 0 5 1 km more than 1300 recording locations were occupied and more than 3000 usable seismograms were obtained we analyzed five profiles using a standard ray tracing program constructed a contour map of reduced travel times from our most widely recorded shot point and modeled an existing gravity profile across the salton trough results are itemized 1 all models have in common a sedimentary layer vp 1 8 5 0 km s a transition zone vp 5 0 5 65 km s a basement vp 5 65 km s in the imperial valley 5 9 km s on the bordering mesas and subbasement vp 7 2 km s 2 the sedimentary layer ranges in thickness along the axis of the salton trough from 3 7 km salton sea to 4 8 km u s mexican border on the bordering mesas it is quite variable in thickness 3 the transition zone is about 1 km thick in most places in the imperial valley there are no marked velocity discontinuities in this zone between the sedimentary layer and basement on the bordering mesas however there is a discontinuity at the top of this zone 4 there are apparently two types of basement on the bordering mesas basement is crystalline igneous and metamorphic rocks in the imperial valley basement is mostly lower greenshist facies sedimentary rocks based primarily on the smooth transition in character from sediment to basement arrivals the low value of basement velocity and the fact that deep 4 km wells in the valley penetrate only the upper part of the known cenozoic stratigraphic column for the salton trough 5 the subbasement or intermediate crustal layer ranges in depth along the axis of the salton trough from 16 km salton sea to 10 km u s mexican border gravity modeling requires that this layer deepen and or pinch out beneath the bordering mesas and mountain ranges based on its high velocity and the presence of intrusive basaltic rocks in the sedimentary section in the imperial valley the subbasement is thought to be a mafic intrusive complex similar to oceanic middle crust 6 several structures are seen that affect basement transition zone and deeper parts of the sedimentary layer they include a scarp along the imperial fault as much as 1 km down to the northeast and a scarp passing roughly along the topographic boundary between the imperial valley and the bordering mesa to the west as much as 3 km down to the east we interpret the latter scarp to be the suture or rift boundary between the older crystalline basement on the mesa and the younger metasedimentary basement in the imperial valley 7 on a contour map of reduced travel time from our most widely recorded shot point subtle patches of early arrivals among otherwise late arrivals in the central imperial valley correlate well with known geothermal resource areas having reservoir temperatures of more than 150 c apparently the salton trough is a location where new crust is being generated as the rift opens mafic intrusive rocks fill it from below as sedimentary rocks fill it from above rifting and intrusion produce high heat flow that metamorphoses the sedimentary rocks to shallow depth metasedimentary basement in the imperial valley and thus consolidates the new crust

stochastic characterization and seismic response of upper and middle crustal rocks based on the lewisian gneiss complex scotland summary we have developed statistical models of upper and middle crustal seismic velocity heterogeneity based on geologic maps and petrophysical data from the lewisian gneiss complex scotland the level of heterogeneity we have measured is relevant to seismic exploration of the crystalline crust using conventional reflection and refraction techniques we digitized two 1 10560 geologic maps of laxfordian archean age lewisian rocks on a 26 8m grid both maps are believed to be representative of the upper and middle crust in north western scotland and both are believed to provide cross sectional views of parts of the crust the digital maps were characterized by the statistics of their lithologic populations and by their 2 d spatial autocorrelation functions different lithologies were assigned seismic velocities appropriate for the mid crust using petrophysical data three lithologies are dominant silicic gneisses vp 6 2 km s 1 mafic amphibolites vp 6 75 kms 1 and intermediate composition schists vp 6 5kms 1 both maps have self affine spatial fabrics the first map covers the core of a syncline its autocorrelation function defines a medium with a fractal dimension of 2 78 a horizontal characteristic length of about 244m and a vertical correlation of about 133m aspect ratio is 2 1 it has an essentially trimodal velocity lithology population consisting of 37 per cent silicicgneiss 43 per cent mafic amphibolites and 20 per cent schists this map is representative of 30 40 per cent of laxfordian rocks the second map is a plan view which can be rotated 90 about an axis perpendicular to strike to give a cross section this map is characterized by a fractal dimension of 2 55 with a horizontal correlation length of about 111m and a vertical correlation of about 38m aspect ratio 3 1 it has a nearly bimodal population consisting of 77 per cent silicic gneisses and 22 per cent mafic amphibotites it is representative of 60 70 per cent of laxfordian rocks lastly we examine the seismic response of an upper to middle crust based on our statistical models using acoustic and elastic 2 d finite difference synthetic seismograms short offset shot records demonstrate that a lewisian upper crust produces scattered waves which significantly disrupt signals reflecting from deeper levels measurements of transmission scattering q and coda decay rates confirm that seismic scattering in lewisian type crust is strong the migrated cmp response of a lewisian crustal model shows the characteristic salt and pepper pattern often observed in the upper crust and described incorrectly as transparent we suggest that translucent is a more appropriate descriptor

the effects of oil emplacement on diagenetic processes examples from the fulmar reservoir sandstones central north sea geologic note 1 sedimentary petrography and petroleum geochemistry studies have been done on fulmar formation upper jurassic cores from fulmar field central north sea the extent of quartz overgrowths and albitization of k feldspars is lower in oil saturated zones compared with water saturated zones however microthermometric studies of aqueous fluid inclusions in quartz overgrowths show similar temperature ranges for both oil and water saturated zones 85 125 degrees c with maximum recorded temperatures close to the current reservoir temperature about 130 degrees c this suggests that quartz overgrowth development in the oil saturated zones continued after oil emplacement but was retarded compared with development in the water saturated zones the quartz cement therefore mus have been essentially locally derived by pressure solution albitization of k feldspars is significantly less about 50 in the oil saturated zones compared with albitization in the water saturated zones because albitization of k feldspars depends on a supply of sodium and removal of potassium this process is more likely to be terminated by oil emplacement the observed difference in the albitization has been used to suggest that the fulmar reservoir probably was filled during the neogene this age estimate agrees broadly with timing determined independently by modeling of oil generation in adjacent source basins the fulmar sandstones contain little or no kaolinite and show no evidence of substantial leaching of feldspars or carbonates this is probably because the fulmar sandstones being partly turbiditic offshore sand deposits were not exposed to meteoric water their close proximity to the marginally mature to mature kimmeridge clay formation seems to have little effect in terms of developing secondary porosity suggesting that the role of organic acids or co 2 generated in these source rocks is of minor importance

decimeter resolution 3d seismic volume in shallow water a case study in small object detection the 3d chirp subbottom profiler provides high resolution imaging of coastal and inshore seabed and subseabed structure by combining the known highly repeatable source waveformofchirpprofilerswiththecoherentprocessingand interpretation afforded by true 3d seismic volumes comprising 60 hydrophone groups arranged around a maltese cross of four chirp transducers 3d chirp permits acquisition of a true 3d volume with a horizontal resolution of 12 5 cm providing an excellent base for shallow water engineering archaeological military and geologic studies here we presentresultsfromsurveyinganatidalbasinonthesouthern coastofenglandtomapbedrockprotrusionsandthesizeand distributionofburiedobjects thestudyareaof150250 m provided a series of unique challenges including a large number of discrete objects ranging from tens of centimeters toseveralmetersinsize buriedinathinveneer0 5to1 5 m of unconsolidated silt overlaying a flat bedrock surface that showed high acoustic contrast and short wavelength roughness by comparing comprehensive postsurvey dredging of the entire site with a prestack time migrated 3d volume it is possibletoconfirma100 detectionrateforalldiscreteburied objects larger than 0 300 30 m in an illuminated area although one acoustic anomaly could not be accounted for in thedredgingresults

pore fluid evolution in the kimmeridge clay formation of the uk outer moray firth implications for sandstone diagenesis abstract carbonate concretions in upper jurassic kimmeridge clay formation from three overpressured wells provide a detailed record of pore fluid evolution in the outer moray firth northern central graben the concretions contain multiple generations of septarian cements with morphologies ranging from simple cracks to complex fractures discrepancy exists between diagenetic studies which indicate these concretions formed during the initial 1 1 5 km of burial and palaeotemperature predictions based on a thermal history calibrated from the vitrinite reflectance kinetic model which indicate formation at much greater depths of between 1 5 and 2 5 km modelling undertaken for this study indicate that the concretions formed during the initial stages of burial under high heat flows fitting the early diagenetic model for their formation these conclusions have important implications for understanding the cementation of adjacent sandstone reservoirs where cement sequences have similar mineralogy and isotopic compositions with precipitation in the same temperature range from mudrock derived pore fluids early cementation of these sandstones is implied by analogy with the mudrock concretions

nature and origin of upper crustal seismic velocity fluctuations and associated scaling properties combined stochastic analyses of ktb velocity and lithology logs the main borehole of the german continental deep drilling program ktb extends over 9000 m into a crystalline upper crust consisting primarily of interlayered gneiss and metabasite we present a joint analysis of the velocity and lithology logs in an effort to extract the lithology component of the velocity log covariance analysis of lithology log approximated as a binary series indicates that it may originate from the superposition of two brownian stochastic processes fractal dimension 1 5 with characteristic scales of 2800 m and 150 m respectively covariance analysis of the velocity fluctuations provides evidence for the superposition of four stochastic process with distinct characteristic scales the largest two scales are identical to those derived from the lithology confirming that these scales of velocity heterogeneity are caused by lithology variations the third characteristic scale 20 m also a brownian process is probably related to fracturing based on correlation with the resistivity log the superposition of these three brownian processes closely mimics the commonly observed 1 k decay fractal dimension 2 0 of the velocity power spectrum the smallest scale process characteristic scale 1 7 m requires a low fractal dimension 1 0 and accounts for 60 of the total rms velocity variation a comparison of successive logs from 6900 7140 m depth indicates that such variations are not repeatable and thus probably do not represent true velocity variations in the crust the results of this study resolve disparity between the differing published estimates of seismic heterogeneity based on the ktb sonic logs and bridge the gap between estimates of crustal heterogeneity from geologic maps and borehole logs

marine electrical resistivity tomography for shipwreck detection in very shallow water a case study from agropoli salerno southern italy abstract a marine electric resistivity survey was carried out over a submerged beach along the agropoli shore salerno italy to detect buried objects of archaeological interest below the sandy seabed we found a shipwreck a military vessel that probably sunk during the salerno landing operations of the allied forces in the second world war resistivity data provide information on the vertical and horizontal extension of the shipwreck which is characterized by very low calculated resistivity values about 2 5 ohm m such values differ significantly from the sand and the bedrock values 5 40 ohm m although the presence of the shipwreck is clearly visible from geoelectric data the joint application of electric magnetic and multibeam bathymetric techniques reduces the ambiguities inherent in each method as shown in the electrical resistivity tomography ert and confirmed by the digital elevation model obtained from the processing of bathymetric data the shipwreck extends more than 30 m in ne sw direction and it is about 13 m wide the global extension of the relic is consistent with the magnetic data that are characterized by a magnetic anomaly with an amplitude of about 1800 nt and similar dimension as inferred from the estimation of source boundaries obtained from the computation of the analytic signal the results of our survey encourage the use of marine geoelectrical methods for the detection of buried archaeological targets particularly in locations where the use of seismic prospecting is not effective e g very shallow water with sandy sea bottoms the integration of different geophysical methods allows to better define the extension depth and thickness of buried objects suggesting that such an approach is the most effective for underwater archaeological investigations

seismic imaging in hardrock environments the role of heterogeneity abstract we investigate the effect of petrophysical scale parameters and structural dips on wave propagation and imaging in heterogeneous media seismic wave propagation effects within the heterogeneous media are studied for different velocity models with scale lengths determined via stochastic analysis of petrophysical logs from the matagami mine quebec canada the elastic modeling study reveals that provided certain conditions of the velocity fluctuations are met strong local distortions of amplitude and arrival times of propagating waves are observed as the degree of scale length anisotropy in the p wave velocity increases the location of these local amplitude anomalies is related to the dips characterizing the fabric of the host rocks this result is different from the elliptical shape of direct waves often defined by effective anisotropic parameters used for layered media although estimates of anisotropic parameters suggest weak anisotropy in the investigated models these effective anisotropic parameters often used in vti tti do not sufficiently describe the effects of scale length anisotropy in heterogeneous media that show such local amplitude travel time and phase distortions in the wavefields numerical investigations on the implications for reverse time migration rtm routines corroborate that mean p wave velocity of the host rocks produces reliable imaging results based on the rtm results we postulate the following weak anisotropy in hardrock environments is a sufficient assumption for processing seismic data and seismic scattering effects due to velocity heterogeneity with a dip component is not sufficient to cause mislocation errors of target structures as observed in the discrepancy between the location of the strong seismic reflections associated to the matagami sulfide orebody and its true location future work will investigate other factors that may provide plausible explanations for these mislocation problems with the objective of providing a mitigation strategy for incorporation into the seismic data processing sequence when imaging in hardrock settings

seismic structure of gneissic granitic upper crust geological and petrophysical evidence from the strona ceneri zone northern italy and implications for crustal seismic exploration summary we present a stochastic seismic model of gneissic granitic upper crust by estimating the amplitude probability distribution and the autocorrelation function of the inferred seismic velocity field of the strona ceneri zone northern italy unlike previous investigations of the adjacent ivrea zone and the lewisian gneiss complex scotland the lithologies mapped in the strona ceneri zone cannot be directly correlated with laboratory measured seismic velocities the amplitude probability distribution of the extensive petrophysical database and the relatively uniform mineralogical composition of the lithologies indicate that the velocity distribution in the strona ceneri zone can be approximated as being continuous and gaussian based on this assumption and using the spatial autocorrelation function measured from a digitized geological cross section we have modelled the fine scale structure of the strona ceneri zone as a self affine medium with a fractal dimension of 2 50 0 15 and horizontal and vertical characteristic scales of 2 6 0 4 km and 1 4 0 3 km respectively on near offset shot gather seismic data strona ceneri type upper crustal heterogeneity produces a reflected field with the appearance of incoherent scattered noise characteristic of non reflective transparent upper crust found in extended phanerozoic provinces this type of heterogeneity has a considerable effect on seismic imaging of underlying reflective lower crust individual reflection segments are disrupted and shifted in time by up to several tenths of a second and the boundaries of the reflective zone become distorted and blurred at far offsets seismic scattering from heterogeneity of this type reproduces the commonly observed rapid decay and scattered distribution of pg amplitudes comparable amplitude decay patterns result from layered deterministic upper crustal velocity structures with velocity inversions

the displacement of connate water from aquifers a mathematical model is developed for the mixing of meteoric and original formation waters in homogeneous aquifers the theoretical analysis indicates that geologic formations that have had a few to a few tens of pore volumes of meteoric water pass through them may be at steady state with respect to further displacement of their contained fluids depending on constraints in the recharge area the steady state may vary from near meteoric water throughout the whole formation to a spatial variation in concentration ranging from meteoric water in the recharge areas to highly concentrated water at the discharge ends of the system the former condition is attained when the recharge area incorporates the total depth and breadth of the formation where this condition is not satisfied the concentrations that evolve are controlled by the original concentration of the formation water the composition of recharging meteoric water the dispersion parameters and the size of the recharge area in comparison to the total breadth of the formation as measured along its strike for heterogeneous formations the concentration patterns that evolve seem to depend further on contrasts in hydraulic conductivity this is demonstrated in the milk river sandstone which is characterized by several extensive zones of dilute water within narrow high conductivity pathways and marked increases in concentration in adjacent lower permeability rock in parts of the aquifer where higher permeability pathways appear to be absent the isochlores form a broad linear band with a marked increase in concentration in the flow direction

pore water evolution in oilfield sandstones constraints from oxygen isotope microanalyses of quartz cement abstract oxygen isotope microanalyses of authigenic quartz in combination with temperatures of quartz precipitation constrained by fluid inclusion microthermometry and burial history modelling are employed to trace the origin and evolution of pore waters in three distinct reservoirs of the brae formation in the miller and kingfisher fields north sea oxygen isotope ratios of quartz cements were measured in situ in nine sandstone thin sections with a cameca ims 4f ion microprobe in conjunction with quartz cement paragenesis in the reservoirs constrained from textural and cathodoluminescence cl microscopy studies pore water evolution was reconstructed from the time of deposition of the sandstones in the upper jurassic until the present cl photomicrographs of quartz overgrowths in the brae formation sandstones show three cement zones a b and c which can be related to different oxygen isotope compositions 1 the earliest and thinnest zone a homogeneous cl pattern with probable 18 o values between 23 and 26 direct measurements were not possible precipitated in the sandstones at temperatures 18 o values between 15 and 18 precipitated in the sandstones most likely between 70 and 90 c 3 the third zone c homogeneous cl pattern and directly measured 18 o values between 16 and 22 precipitated in the sandstones most likely at temperatures 90 c calculated oxygen isotope compositions of pore waters show that zone a quartz cements and enclosing concretionary calcite precipitated from a meteoric type fluid 7 during shallow burial 18 o 7 to a more 18 o enriched fluid 18 o 4 as burial continued to 3 0 km data from zone c quartz cements are consistent with further fluid evolution from 18 o 4 to basinal type fluids with 18 o similar to the present day formation water oxygen isotope composition 0 6 at 4 0 km burial a similar pore water evolution can be derived for all three reservoirs studied indicating that hydrogeologic evolution was similar across sandstones of the whole brae formation the quartz cement zones observed in the brae formation sandstones and the pore water history derived for the area studied is analogous to published petrographic and pore water evolution data from the nearby brent group reservoirs and from reservoirs located in the haltenbanken area on the atlantic margin offshore norway considering quartz cement is a major porosity occluding phase in many reservoir sandstones and because pore waters both dissolve quartz and carry the dissolved silica to cementation sites the data presented are valuable for improving the understanding and prediction of reservoir quality development in sandstones globally

reequilibration of fluid inclusions in low temperature calcium carbonate cement calcium carbonate cements precipitated in low temperature near surface vadose environments contain fluid inclusions of variable vapor to liquid ratios that yield variable homogenization temperatures cements precipitated in low temperature phreatic environments contain one phase all liquid fluid inclusions neomorphism of unstable calcium carbonate phases may cause reequilibration of fluid inclusions stable calcium carbonate cements of low temperature origin which have been deeply buried contain fluid inclusions of variable homogenization temperature and variable salt composition most inclusion fluids are not representative of the fluids present during cement growth and are more indicative of burial pore fluids therefore low temperature fluid inclusions probably reequilibrate with burial fluids during progressive burial reequilibration is likely caused by high internal pressures in inclusions which result in hydrofracturing the resulting fluid inclusions population could contain a nearly complete record of burial fluids in which a particular rock has been bathed

evidence for resetting of fluid inclusion temperatures from quartz cements in oilfields abstract quartz is a major pore occluding cement in north sea oilfield reservoir sandstones fluid inclusions yield homogenization temperatures t h which have been thought to represent growth temperatures for the host quartz however early diagenetic inclusions from the detrital grain overgrowth boundary yield excessively high palaeotemperatures which do not match the growth temperatures calculated from the depth of cementation the temperatures of these inclusions increase with present day depth of burial and approach present day reservoir temperatures this suggests that the inclusions have begun to reset towards present day pressure temperature conditions the t h of the inclusions hence represent burial temperatures rather than growth temperatures for the quartz experimental studies indicate that the amount of resetting undergone by an inclusion varies depending on its size shape and fluid composition a relationship between inclusion t h and these variables has been observed in inclusions from deeply buried quartz cements in the north sea

chemical diagenesis of a multicomponent carbonate system 2 stable isotopes abstract this study examines the oxygen and carbon isotopic distribution in the brachiopods crinoids rugose corals and matrix cement from the mississippian burlington limestone iowa and missouri and the silurian read bay formation arctic canada the isotopic data can be reconciled with textural and trace element observations only if the 18o content of ancient oceans differed from that of the present oceans this postulated secular variation in the 18o content of seawater requires a separate evaluation of the mississippian and silurian carbonate components examination of the 18o content of the least altered low mg calcite fossil components of the two formations suggests that the 18o of mississippian seawater was on average about 1 5 lighter and that of silurian seawater was about 5 5 lighter than the 18o of present day ocean water the average diagenetic shift in 18o for the read bay formation which stabilized in a partly closed diagenetic phreatic meteoric system is about 1 for the original low mg calcite and about 2 for the original high mg calcite and aragonite components conversely for the burlington limestone which stabilized in an open diagenetic phreatic meteoric system the average diagenetic shift in 18o is about 3 for the original low mg calcite and about 4 f r the original high mg calcite components the 13c values of the originally low mg calcite or high mg calcite with less than 7 mole mgco3 brachiopods and rugose corals the originally high mg calcite crinoids and the originally aragonite lime mud of the burlington limestone and read bay formation are bimodally distributed this bimodal 13c distribution is independent of geologic age inferred original mineralogy of the component degree of diagenetic alteration and the type of diagenetic meteoric system with components from both the read bay formation and burlington limestone contributing to both 13c groups the mode of the light 13c group is 0 25 and for the heavy 13c group it is 3 25 pdb both 13c groups also show a decrease in 13c of about 1 with increasing diagenetic alteration stabilization

geochemistry and petrogenesis of the marginal zone of the mount lowe intrusion central san gabriel mountains california the marginal zone of the mount lowe intrusion mli is composed of a mineralogically and geochemically distinctive suite of quartz poor alkali enriched monzodiorites and quartz monzodiorites low initial 87sr 86sr rb and rb sr coupled with high ab or k rb ba and sr suggest that feldspathic continental crust did not contribute significantly to the magma the inferred original magma composition is most consistent with experimental data for about 20 batch modal melting of eclogite or quartz eclogite of altered oceanic tholeiite composition the high absolute ba and sr abundances require in addition an enriched component perhaps derived from dehydration processes within a subducted slab at the level of exposure the range of observed compositions resulted from fractionation of hornblende plagioclase sphene apatite and magnetite fractionation resulted in formation of a peraluminous mn enriched residual liquid which crystallized garnet the grandite enriched composition of the garnet is inferred to be due to high pressure ca 6 7 kb high f text o text 2 crystallization relative to typical plutonic garnets the composition of the marginal zone of mli has many chemical similarities to early mesozoic plutonism in the cordilleran continental margin magmatic arc of western north america

mechanical compaction of deeply buried sandstones of the north sea abstract sandstones experience mechanical compaction when the overburden load exceeds the compressive strength petrographic evidence is rarely sufficient to determine the timing of mechanical compaction it is often assumed from indirect evidence such as regional porosity depth trends that mechanical compaction is a process that occurs exclusively during shallow or intermediate burial mechanical compaction without grain fracturing may occur at depth following pervasive framework grain dissolution and or if anomalously high porosity has been preserved due to the presence of small amounts of cement we describe examples from the fulmar sandstone formation of the central graben north sea that experienced late stage mechanical compaction following sponge spicule dissolution and microcrystalline quartz cementation deep burial mechanical compaction involving grain crushing may occur if the rate of grain contact quartz dissolution and or quartz overgrowth development cannot compete with the rate of stress increase at grain contacts some rotliegendes sandstones of the southern north sea that have been buried to 4 5 km offer a good example where the suppression of chemical compaction due to the presence of grain coating clays resulted in pervasive grain fracturing mineral veins are frequently associated with sandstones that have experienced pervasive mechanical compaction during deep burial these may reflect the sudden development of overpressure resulting from the transfer of load to the fluid during collapse of the sandstone framework

the seismic reflection character of the continental mohorovicic discontinuity the continental mohorovicic discontinuity is most often interpreted as a step function velocity boundary however on deep crustal seismic reflection profiles reflections at depths where refraction data places the moho have laminated character and are laterally discontinuous these observations on continental reflection data point to a model of the moho that has thin layered rather than block structure previous workers who have synthetically generated seismic responses from various crust mantle boundary models have shown that thin layered models of alternating high and low velocity generate responses that best emulate observed moho arrivals geologic interpretations of the thin layering include relatively undeformed metasediments cumulate layering tectonic banding and lenses of partial melt to obtain more direct geologic evidence a synthetic seismogram is generated from geologic cross sections of the ivrea verbano zone an exposed section through the lower crust and upper mantle the synthetic shows laminations similar to those observed on reflection data without more direct evidence no single geologic interpretation of the moho is reasonable indeed the moho may be a laterally variable boundary its composition and structure dependent upon the geologic history of the overlying crust

tomographic image of the southern california mantle p wave teleseismic travel time delays recorded by the southern california array are inverted by the method of backprojection tomography to obtain images of variations in the p wave velocity structure to a depth of 750 km two major upper mantle features arc resolved one beneath the transverse ranges region and another beneath the salton trough region the transverse ranges feature appears as a curtainlike east trending high velocity anomaly this feature is 60 km thick extends most deeply on its eastern end to 250 km and attains a maximum velocity 3 greater than average southern california mantle of the same depth the salton trough feature which is not as well resolved as the transverse ranges feature is composed of low velocities in the upper 70 100 km these p wave velocities arc depressed 3 4 compared to average southern california mantle tests of the inversion indicate the major aspects of the imaged structure are authentic

continental rupture and the creation of new crust in the salton trough rift southern california and northern mexico results from the salton seismic imaging project a refraction and wide angle reflection seismic profile along the axis of the salton trough california and mexico was analyzed to constrain crustal and upper mantle seismic velocity structure during active continental rifting from the northern salton sea to the southern imperial valley the crust is 17 18 km thick and approximately one dimensional the transition at depth from colorado river sediment to underlying crystalline rock is gradual and is not a depositional surface the crystalline rock from 3 to 8 km depth is interpreted as sediment metamorphosed by high heat flow deeper felsic crystalline rock could be stretched preexisting crust or higher grade metamorphosed sediment the lower crust below 12 km depth is interpreted to be gabbro emplaced by rift related magmatic intrusion by underplating low upper mantle velocity indicates high temperature and partial melting under the coachella valley sediment thins to the north and the underlying crystalline rock is interpreted as granitic basement mafic rock does not exist at 12 18 km depth as it does to the south and a weak reflection suggests moho at 28 km depth structure in adjacent mexico has slower midcrustal velocity and rocks with mantle velocity must be much deeper than in the imperial valley slower velocity and thicker crust in the coachella and mexicali valleys define the rift zone between them to be 100 km wide in the direction of plate motion north american lithosphere in the central salton trough has been rifted apart and is being replaced by new crust created by magmatism sedimentation and metamorphism

structural evidence for northeastward movement on the chocolate mountains thrust southeasternmost california the late cretaceous chocolate mountains thrust of southeastern california and southwestern arizona places a block of proterozoic and mesozoic continental crust over the late mesozoic continental margin oceanic sedimentary and volcanic rocks of the regionally distinctive orocopia schist the chocolate mountains thrust is interpreted as a thrust burial subduction fault rather than a low angle normal exhumation unroofing uplift fault an important parameter required to understand the tectonic significance of the chocolate mountains and related thrusts is their sense of movement the chocolate mountains thrust zone contains sparse to locally abundant mesoscopic asymmetric folds fabric relations supported by regional geologic evidence indicate that these folds are an integral part of and coeval with the thrust zone on a lower hemisphere equal area plot representing the orientation and sense of asymmetry of 80 thrust zone folds from 36 localities spread over an area 60 by 10 km z folds plot northwest of and s folds plot southeast of a northeast southwest striking vertical plane of overall monoclinic symmetry the only sense of movement consistent with the collective asymmetry of the thrust zone folds is top to the northeast asymmetric microstructures studied at several localities also indicate top to the northeast movement paleomagnetic data suggest that the original sense of thrusting prior to neogene vertical axis tectonic rotation related to the san andreas fault system was northward the essential point is that movement of the upper plate of the chocolate mountains thrust evidently was continentward continentward thrusting suggests a tectonic scenario in which an insular or peninsular microcontinental fragment collided with mainland southern california the suture predicted by this model is elusive but the chocolate mountains thrust and underlying orocopia schist themselves may represent the suture at the present level of exposure alternative tectonic models involving subduction of the orocopia schist eastward beneath continental southern california circumvent the suture problem but are presently not supported by any direct structural evidence

alteration and remelting of nascent oceanic crust during continental rupture evidence from zircon geochemistry of rhyolites and xenoliths from the salton trough california abstract rhyolite lavas and xenoliths from the salton sea geothermal field southern california provide insights into crustal compositions and processes during continental rupture and incipient formation of oceanic crust salton buttes rhyolite lavas contain xenoliths that include granophyres fine grained altered rhyolites felsite and amphibole bearing basalts zircon is present in lavas and xenoliths surprisingly even in the basaltic xenoliths where it occurs in plagioclase rich regions interpreted as pockets of crystallized partial melt zircons in the xenoliths are exclusively late pleistocene holocene in age and lack evidence for inheritance u th isochron ages are 20 5 1 2 1 2 ka granophyres 18 3 3 5 3 6 ka felsite 30 1 12 4 14 1 ka and 9 2 6 6 7 0 ka basalts all errors 1 the dominant zircon population in the rhyolite lavas yielded u th ages between 18 and 10 ka with few pre quaternary xenocrysts present 18ozircon values are lower than typical crustal basement values thus ruling out rhyolite genesis by melting of continental crust moreover 18ozircon values are 0 5 1 0 lower than compositions achievable by zircon crystallization from residual melt in equilibrium with unaltered mid ocean ridge basalt suggesting that basaltic crust and silicic plutons in the subsurface of the salton sea geothermal field isotopically exchanged with meteoric waters this is evidence for deep reaching hydrothermal circulation and indicates rhyolite genesis by episodic remelting of altered basalts instead of fractional crystallization of unaltered basaltic magma

experimental stretching of fluid inclusions in calcite implications for diagenetic studies homogenization temperatures of fluid inclusions trapped in calcite crystals may change with increased heating beyond the homogenization temperature fluid inclusions trapped in calcite during basin subsidence will tend to yield homogenization temperatures approaching maximum burial temperatures diagenetic studies that depend on these data will be biased coarse crystalline calcite containing saline fluid inclusions was used to examine the influence of increased temperatures and pressures on measured homogenization temperatures stepped heating experiments were conducted at 1 atm over a temperature range of 90 to 245 c along with hydrothermal experiments at confining pressures of 173 and 691 bar and temperatures of 160 250 and 350 c in all cases significant increases in the homogenization temperatures of the inclusion population were recorded presumably stresses generated during heating of the fluid inclusions exceeded the combined strength of the calcite crystal and the external pore pressure and thus caused fluid inclusion stretching size is a secondary factor that controls fluid inclusion reequilibration

analysis of lateral coherency in wide angle seismic images of heterogeneous targets the character of deep reflections recorded in wide angle seismic experiments often suggests fine scale layering in the structure of the reflecting target the lateral continuity of wide angle reflections is enhanced however because energy arriving at a long off set receiver is confined to a narrow range of apparent slowness the distribution of energy with slowness was studied by levander and gibson this issue who show that the restricted range amounts to a dip filter their energy slowness distribution is used here to relate lateral correlation in a reflected wave field to the correlation properties of a randomly heterogeneous target tests with finite difference synthetic data from levander and gibson confirm a simple convolutional relation between the statistics of the wave field and those of a target with small magnitude velocity variations field data recorded in the basin and range province nevada show a progressive increase in reflection continuity with increasing source receiver offset as expected interpretation of reflector heterogeneity for this data set however is complicated by noise contamination and scattering above the target

intraformational fluid flow in the thuringian syncline germany evidence from stable isotope data in vein mineralization of upper permian and mesozoic sediments abstract vein mineralization observed in boreholes quarries and outcrops of upper permian and mesozoic sediments in the thuringian syncline germany was investigated for the first time by petrography electron microprobe analysis and stable isotope analysis the objective was a characterization of paleo fluid systems within this basin veins are mostly restricted to nw se trending fault systems and comprise carbonates calcite dolomite minor sulfates gypsum anhydrite celestite barite and rarely sulfides they are variably distributed within the late paleozoic to mesozoic succession calcite vein mineralization occurs in almost all triassic strata though dominating in the lower muschelkalk subgroup mu gypsum anhydrite veins occur in the middle keuper subgroup km middle muschelkalk subgroup mm middle and upper buntsandstein subgroup sm so as well as in the upper permian zechstein sulfate and carbonate isotope analyses indicate that most vein mineralization dominantly crystallized from intraformational fluids within the upper permian and triassic sediments crystallization from descending meteoric waters was observed especially for calcite veins in the lower muschelkalk but also for gypsum anhydrite veins in the upper buntsandstein mineralization from ascending carbonate ph buffered warm waters was restricted to individual parts of the southern deep reaching main border fault zone of the thuringian syncline mixing and fault crossing fluid movements are subordinate within the thuringian syncline we conclude that no large scale cross formational fluid flow occurred in thuringian syncline over the last millions of years

estimation of crustal stochastic parameters from seismic exploration data stochastic models for the crystalline crust produce synthetic seismograms that compare well with recorded data for a variety of crustal ages and tectonic environments in this paper we explore the parameter space describing such stochastic models as a basis for formulating the inverse problem that is we wish to estimate the parameters which define a stochastic model from the recorded backscattered wave field we base the estimation on approximate relations between the primary reflectivity series which is the ideal wave field response of a medium and various seismic gathers a two dimensional lateral correlation method is used to investigate the sensitivity of synthetic wave fields to horizontal characteristic length a derived empirical relationship relates the scale length to the half width of the correlation coefficient a horizontal wavenumber time domain spectral analysis successfully estimates the horizontal characteristic length ax and the fractal dimension d of the stochastic medium from which the wave field was backscattered we employ a monte carlo based optimization scheme to fit the observed spectral values to a one dimensional von karman function and simultaneously estimate values for ax and the hurst number the method yields consistent results when tested on synthetic data application of this technique to data sorted into different domains shows that zero offset sections yield the best estimate we also present results of our estimates using this method on a program for array seismic studies of the continental lithosphere passcal dynamite source shot gather and a vibroseis source consortium for continental reflection profiling cocorp data set recorded in the basin and range province

cocorp arizona transect strong crustal reflections and offset moho beneath the transition zone the consortium for continental reflection profiling cocorp transect across the southwest margin of the colorado plateau in arizona reveals prominent subhorizontal zones of reflections here termed the bagdad reflection sequence brs which dominate the upper and middle crust in the transition zone tz between the colorado plateau and the basin and range province a cenozoic origin for the brs is considered most likely it may represent fragmented or sheared zones related to detachment faulting or a series of intrusions possibly emplaced along preexisting zones of weakness moho reflections observed beneath the tz and the basin and range province contrast with the nonreflective moho beneath the colorado plateau suggesting that extension and its associated igneous processes probably played a key role in the development of reflectors at the moho the moho and a prominent midcrustal reflector are offset by about 3 km in a normal sense in at least one place this is the best example of offset moho yet found by cocorp and contrasts sharply with cocorp observations of a smooth moho elsewhere this offset is probably a late tectonic expression of crustal extension and thinning across the tz

some attributes of wavefields scattered from ivrea type lower crust abstract the ivrea zone in northern italy is a well studied sliver of extended lower continental crust which was brought to the surface in the course of alpine lithospheric shortening based on two 1 25 000 geological maps from the central ivrea zone and petrophysical parameters of the corresponding lithologies we have constructed geologically constrained stochastic seismic models of ivrea type lower crust a stochastic rather than a deterministic approach is applied due to the limited availability of detailed geological maps and the high degree of small scale structural and petrophysical complexity of the ivrea zone the primary characteristic of the resulting model is its layered self affine or fractal structure and its bimodal velocity distribution while we are not suggesting that ivrea type structure is universal for extended lower continental crust synthetic seismograms may help to constrain some pertinent quantitative aspects observed in deep seismic reflection data 1 1 ivrea type lower crust can explain the igh observed reflection coefficients and bright reflectivity in the lower crust 2 2 observed lower crustal q factors can be largely explained by scattering losses and intrinsic q factors of reflective lower crust are likely to be larger than 1000 which is incompatible with the presence of free volatiles 3 3 in agreement with recent broad band studies of the lower crust ivrea type lower crust does not create any tuning effects and 4 4 for ivrea type heterogeneity the lateral correlation of the reflected signal is largely independent of the lateral characteristic scale of the scattering structures which suggests that multiple scattering may be important in the reflective lower crust

oxygen isotopic indications of the mechanisms of silica transport and quartz cementation in deeply buried sandstones the oxygen isotopic compositions and inferred precipitation temperatures of quartz cements in a suite of deeply buried sandstones from the north sea gulf coast and western canada basins indicate that in almost every case the quartz precipitated from 18 o rich basinal waters 18 o rich waters are found in strictly limited volumes in sedimentary basins silica transport and quartz cementation could not therefore have occurred as a result of large scale advection of basinal or meteoric waters because this would require very large fluid fluxes quartz cementation was driven by processes operating on a local scale the isotopic data do not uniquely indicate the mechanisms of silica transport but are consistent with both diffusion and local recycling of fluid

a deterministic and stochastic velocity model for the salton trough basin and range transition zone and constraints on magmatism during rifting as a high resolution addition to the 1992 pacific to arizona crustal experiment pace a 45 km long deep crustal seismic reflection profile was acquired across the chocolate mountains in southeastern california to illuminate crustal structure in the transition between the salton trough and the basin and range province the complex seismic data are analyzed for both large scale deterministic and fine scale stochastic crustal features a low fold near offset common midpoint cmp stacked section shows the northeastward lateral extent of a high velocity lower crustal body which is centered beneath the salton trough off end shots record a high amplitude diffraction from the point where the high velocity lower crust pinches out at the moho above the high velocity lower crust moderate amplitude reflections occur at midcrustal levels these reflections display the coherency and frequency characteristics of reflections backscattered from a heterogeneous velocity field which we model as horizontal intrusions with a von karman fractal distribution the effects of upper crustal scattering are included by combining the mapped surface geology and laboratory measurements of exposed rocks within the chocolate mountains to reproduce the upper crustal velocity heterogeneity in our crustal velocity model viscoelastic finite difference simulations indicate that the volume of mafic material within the reflective zone necessary to produce the observed backscatter is about 5 the presence of wavelength scale heterogeneity within the near surface upper and middle crust also produces a 0 5 s thick zone of discontinuous reflections from a crust mantle interface which is actually a first order discontinuity

lower crustal reflectivity modeled by rheological controls on mafic intrusions orogenic collapse lithospheric extension and associated basaltic magmatism have affected many phanerozoic regions this has led to the hypothesis that the high lower crustal reflectivity of these areas originates from horizontal mafic dikes embedded in a more felsic ma trix tectonic extension however favors the intrusion of unreflective vertical dikes stress distribution in an extending lithosphere varies over several orders of magnitude which explains the simultaneous intrusion of vertical and horizontal dikes vertical dikes penetrate rheologically strong zones and transform into horizontal dikes in weak zones where stress conditions are quasi isostatic and a subhorizontal strain fabric is likely to exist the seismic response of a corresponding crustal model faithfully reproduces banded reflectivity patterns typical of extended provinces because the loci of mechanical weakness vary with temperature composition and fluid content other common reflectivity patterns can be explained similarly this combination of two prominent end member models on the origin of lower crustal reflections i e ductile flow and mafic intrusions is compatible with pertinent features associated with postorogenic lithospheric extension such as high heat flow bimodal volcanism anatectic granites granulite facies metamorphism and uniform crustal thickness

chemical constraints and origins of four groups of gulf coast reservoir fluids discussion the authors take to task stoessell and moore s 1983 review of some previous ideas on the origin of gulf coast formation fluids taking specific exception to their interpretation of data from the edwards formation six points are emphasized edwards brines are significantly enriched in bromide relative to chloride over seawater evaporated to halite saturation edwards brines have significantly higher k c1 and ca mg c1 ratios than seawater evaporated to halite saturation edwards brines cannot achieve molar ca mg ratios of 16 by dolomization progressive stabilization of metastable dolomite by updip fluid cannot raise the magnesium content of edwards water in the absence of dolomite dissolution the volume of brine contained in the cretaceous section exceeds any reasonable volume of hypothetical connate mesozoic brine and isotopic differences between edwards brines and jurassic seawater evaporated to halite saturation are extreme and therefore preclude recognition of any primary jurassic component in the current brines

stochastic modeling of the reflective lower crust petrophysical and geological evidence from the ivera zone northern italy crustal scale seismic wave equation modeling demonstrates that wavelength scale velocity fluctuations can dominate the seismic response over a wide range of offsets and that a wide range of velocity distributions qualitatively can explain deep seismic data this range can be narrowed by studying the detailed petrophysical and structural properties of middle and lower crustal exposures the ivrea zone in the southern alps is a well studied sliver of extended lower continental crust brought into its present upright position as a result of late alpine lithospheric shortening the high degree of structural and petrophysical complexity and in particular the limited exposure of the ivrea zone favor a statistical rather than deterministic approach to develop a two dimensional crustal scale seismic model for this purpose we digitized two 1 25 000 geological maps from the central ivrea zone on a 60 m grid we find that the ivrea zone is characterized by a bimodal velocity distribution and a band limited self affine structure having a fractal dimension of 2 7 a characteristic horizontal scale of 500 to 1000 m and an aspect ratio around 4 0 assuming statistical stationarity a realization of this stochastic model was used to develop synthetic seismograms typical of ivrea type lower continental crust our results indicate that 1 ivrea type lower crust can explain qualitatively commonly observed lower crustal reflectivity 2 relatively minor changes in bulk composition and or metamorphic grade may lead to dramatic changes in reflectivity which cannot be explained by current generic models on the origin of lower crustal reflections and 3 travel time interpretation of the synthetic wide angle data may lead to erroneous large scale velocity structures

wave equation datuming for improving deep crustal seismic images abstract reflection quality in deep crustal seismic sections is often poor due to surface waves irregularities in the surface layer and variations in topography in an effort to remove the effects of the variable near surface layer and topography we have used wave equation datuming to downward continue shot gathers to the base of the surface layer and then redatumed upward to a flat surface using the basement velocity as the replacement velocity we discuss briefly the theory behind the method present a simple synthetic example of a datumed shot gather and conclude with an example of a datumed shot gather from the risc crustal seismic experiment in southeastern california

deep seismic image enhancement with the common reflection surface crs stack method evidence from the aravalli delhi fold belt of northwestern india summary imaging of deep crustal features from narrow angle deep seismic reflection data especially from fold belt region has been a challenging task the common reflection surface crs stack is an alternative seismic imaging technique for multicoverage reflection data it is an automatic stacking process which does not require explicit knowledge of stacking velocity this crs stack is especially useful when the data quality is poor and foldage is low in this paper we demonstrate an application of the crs stack to a deep seismic reflection data set acquired across the aravalli delhi fold belt of the northwestern india which provides a seismic stack section with much improved signal to noise ratio comparing the conventional common midpoint cmp stack with the crs stack we find that the moho and other crustal reflections have been resolved clearly and the continuity of the reflectors has also been enhanced with the crs stack method the major findings from our crs processing include clear image of the moho discontinuity below the marwar basin and sandmata complex and prominent upper and mid crustal reflections our study for the first time images an extension of crustal scale jahazpur thrust below the sandmata complex which becomes listric at the moho some of the crustal features derived in this study were not identified in the earlier investigations using the cmp stack our study clearly demonstrates that the crs stacking method is more appropriate for imaging the crustal and subcrustal structures of the thrust fold belt region than the conventional cmp method where limited velocity information is available crustal thickness across the proterozoic orogenic aravalli delhi fold belt varies between 38 and 50km global correlation of the seismic results suggests no relation between crustal thickness and age of the crustal block but it depends on the thermorheological and tectonic history of the region palaeosignatures of the proterozoic subduction and collision processes are still preserved in major portion of the crust in this study area as well as in several parts of the world

the effects of crustal heterogeneity on ray based teleseismic imaging summary we evaluate the efficacy of crustal scale receiver function imaging in the presence of crustal scattering first we show that the resolution of an image is not appreciably affected by geologically realistic crustal scattering rather we show that the image contrast is reduced the image of actual targets get buried in the noise created by arrivals arising from crustal scattering then we construct three classes of models in which we simulate a flat moho a moho with topography and a subduction zone for all the models we simulate the moho as a finite thickness velocity transition zone and the crust with a three layer heterogeneous zone that contains binary von karman statistics with varying degrees of rms density and velocity contrasts we use finite difference simulations to generate a suite of varying slowness seismograms which we then image with pre stack 2 d kirchhoff migration and ccp stacking we find that for a small degree of rms density velocity contrasts both imaging methodologies faithfully image the models but for rms density velocity variations greater than approximately 5 per cent per cent we observed distortion and or artefacts to the images for migration the distortion takes the form of amplitude variations for horizontal model features whereas for the ccp imaging the main affect is the enhancement of any horizontal feature in the model

evaluation of infiltration from losing disconnected rivers using a geophysical characterisation of the riverbed and a simplified infiltration model summary despite their significance to alluvial aquifer water balances there are few field derived estimates of infiltration from losing disconnected rivers infiltration was estimated over a 2 km section of billabong creek murray darling basin australia using a combination of field measurements and modelling techniques a method was developed whereby in river and riverbank electrical resistivity surveys were used to map the spatial coverage and thickness of the riverbed clay layer thought to be the key control on infiltration in this environment inverted resistivity measurements were used to generate a surrogate for a spatial map of clay layer thickness with constraint provided by coring the riverbed at selected high and low resistivity locations to independently determine the thickness of the clay the survey showed that the clay layer was continuous across the study reach but varied in thickness 1 m to 4 m a simple infiltration model was developed for infiltration through a clay layer and shown to be accurate under steady state conditions when compared to estimates obtained using a variable saturation numerical model for idealised riverbed cross sections infiltration rates across the study reach were estimated to range between 1700 and 7800 m 3 km 1 year 1 with an average of 3400 m 3 km 1 year 1 a sensitivity analysis showed that infiltration rates were most sensitive to clay hydraulic conductivity not clay layer thickness however it is anticipated that when applied at a larger scale 10 100 km infiltration rates will be more sensitive to the presence or absence of a clay layer in the riverbed the proposed methodology can provide independent estimates of infiltration in losing disconnected rivers at a scale suitable for the calibration of regional groundwater models

the crust as a heterogeneous optical medium or crocodiles in the mist abstract based on petrophysical data geologic maps and a well log we present statistical descriptions of likely upper middle and lower crustal rocks to characterize the fine scale heterogeneity observed in crustal exposures and inferred from deep crustal seismic data the statistical models developed for granitic and metamorphic upper crust and for an extended metamorphic lower crust are used to construct whole crustal models of seismic velocity heterogenity we present finite difference synthetic cmp data from several models which compare favorably with field data the statistical models also permit classification of the seismic reflection experiment and the crustal heterogeneity according to scattering regime the optical or scattering properties of importance for classification are the velocity fluctuation intensity the horizontal and vertical correlation lengths of the medium the correlation function of the medium and the velocity population function for the crustal properties we measured the bandwidth of a typical deep crustal experiment overlaps from the weak to the strong scattering regime with implications for crustal seismic data processing and imaging notably deep crustal signals are likely to have experienced multiple scattering making common seismic imaging techniques of questionable value moreover the details of the unmigrated cmp stacked section bears little resemblance to the underlying medium

evaporites marine or non marine division of evaporites into those types that retain information about primary mineralogy textures and structures primary evaporites and those types so extensively altered that few or no primary features remain secondary evaporites only primary evaporites can give unequivocal data bout their marine or non marine origin to help evaluate such primary deposits they have been further divided using the following attributes 1 whether the depositional setting was marine or non marine 2 whether the source water was a seawater b meteoric c hydrothermal d diagenetic e volcanogenic or f mixed specific criteria for making distinctions are 1 kinds of fossils 2 sedimentology of the associated non saline facies 3 kinds of primary saline minerals 4 association of such saline minerals both in qualitative and quantitative terms 5 trace element isotope and fluid inclusion geochemistry of such saline minerals modified journal abstract

the geological hubble a reappraisal for shallow water traditional two dimensional 2d seismic acquisition techniques image the subsurface using a grid of orthogonal lines dips are recorded only in the along track direction limiting migration to a 2d along track approximation of the inherently 3d wavefields this produces profiles that are often complicated by out of plane reflections and with resolution for structural interpretation constrained by the line spacing rather than the source wavelength the acquisition of true 3d seismic reflection data in contrast provides dip information for the reflected wavefields in both along and across track directions this allows a full treatment of the 3d wavefields during migration affording accurate 3d structural reconstruction significantly improved resolution theoretically 1 2 source wavelength and increasing signal to noise ratio snr through more effective noise cancellation

the origin and evolution of saline formation water lower cretaceous carbonates south central texas u s a abstract land l s and prezbindowski d r 1981 the origin and evolution of saline formation water lower cretaceous carbonates south central texas u s a in w back and r letolle guest editors symposium on geochemistry of groundwater 26th international geological congress j hydrol 54 51ndash 74 systematic chemical variation exists in formation water collected from a dip section through lower cretaceous rocks of south central texas chemical variation can be explained by an interactive water rock diagenetic model the cyclic lower cretaceous shelf carbonates of the edwards group dip into the gulf of mexico coast geosyncline and can be considered to a first approximation as part of a complex aquifer contained by paleozoic basement beneath and by relatively impermeable upper cretaceous clay and chalk above the hydrodynamic character of this carbonate system is strongly controlled by major fault systems major fault systems serve as pathways for vertical movement of basinal brines into the lower cretaceous section formation water movement in this sytem has strong upfault and updip components the parent na ca cl brine originates deep in the gulf of mexico basin at temperatures between 200 and 250 c by the reaction halite detrital plagioclase quartz water albite brine other dissolved components originate by reaction of the fluid with the sedimentary phases k feldspar calcite dolomite anhydrite celestite barite and fluorite significant quantities of pb zn and fe have been mobilized as well as the brine moves updip out of the overpressured deep gulf of mexico basin and encounters limestones of the stuart city reef trend the buried platform margin small amounts of galena precipitate in late fractures continuing to rise upfault and updip the brine becomes progressively diluted on encountering significant quantities of dolomite in the backreef facies the ca rich brine causes dedolomitization although thermochemical consideration suggests that small amounts of several authigenic phases should precipitate most have yet to be found minor amounts of several kinds of calcite spar are present however as the brine evolves by dilution and by cooling no systematic changes in any cation cl ratio occur except for regular updip gain in mg as a result of progressive dedolomitization the formation water highly diluted by meteoric water eventually discharges along faults as hot mineral water

chemical constraints and origins of four groups of gulf coast reservoir fluids pitzer s method for computing activity coefficients has been applied to reported fluid compositions in four groups of gulf coast reservoirs the mississippi and arkansas smackover formation the texas edwards group the texas frio formation and the louisiana miocene sequence the activity quotients in fluids from carbonate reservoirs imply a reaction relation between calcite and disordered dolomite in which ordering increases at higher temperatures this relationship indicates that an increasing mg ca molality ratio with decreasing temperature does not necessarily imply dolomite dissolution by an updip moving fluid instead it may reflect an approach to equilibrium between calcite and metastable dolomite activity quotients in fluids from clastic reservoirs imply a rea tion relation between chlorite illite potassium feldspar and quartz at temperatures above 100 c 212 f present fluid compositions in the smackover and the edwards appear to be related to the updip movement of louann brines from the gulf coast basin accompanied by dolomitization in particular bromide plots for the edwards fluids indicate simple dilution of louann brines with interstitial fluids similar plots for fluids from the smackover show the complicating effects of illite formation sylvite dissolution and halite recrystallization albitization may have occurred during fluid movement through clastics prior to entering the smackover and edwards present fluid compositions in the frio and miocene are apparently not related to louann brines the compositional data reflect the effects of halite dissolution the subsequent albitization of plagioclase and potassium feldspar and the dissolution of carbonates during secondary porosity formation

seismic imaging of extended crust with emphasis on the western united states understanding of the crust has improved dramatically following the application of seismic reflection and refraction techniques to studies of the deep crust this is particularly true in areas where the last tectonic event was extensional such as the basin and range province of the western united states and much of western europe in these regions a characteristic reflective pattern has emerged whereby the lower crust is highly reflective and the upper crust and upper mantle are either poorly reflective or strikingly nonreflective in the metamorphic core complex belt in the western united states where extension can be as much as an order of magnitude greater than in the more classic continental rift zones the lower crustal reflectivity thickens and rises yielding a picture of a crust that is reflective throughout synthetic seismic studies have documented that the reflectivity in these regions can be modeled by numerous laminae tens of meters thick and hundreds of meters across characterized by inter layered high and low velocities two geologic factors are interpreted as contributing to this layered character ductile strain responding to stress in the thermally weakened middle and lower crust and intrusive layering corresponding to injection of subhorizontal sheets of mantle derived magmas these two processes yield a variety of geologic structures including transposed compositional layering mylonitic ductile shear zones and intrusive mafic sheets all of which occur at the proper scales to cause the prominent reflectivity observed if metamorphic core complexes are representative of extended continental crust world wide then these results suggest that magmatism and ductile flow have also contributed to the evolution of the middle and lower crust in many other areas around the world

cenozoic volcanism and two stage extension in the salton trough southern california and northern baja california pronounced differences in age and geochemistry between cenozoic volcanic rocks within the salton trough southern california and northern baja california imply their formation within two distinct tectonic settings rather than the previously proposed single phase of continental rifting the emplacement of oligocene miocene alkaline lavas overlapped with crustal extension and basin formation in the region during this time post 4 ma subalkaline magmatism related to the gulf of california east pacific rise divergent plate margin exploited crust already ruptured during miocene extension new rare earth element and sr nd isotopic data and k ar 40ar 39ar and u pb ages for basalt andesite and granophyric granite xenoliths contained within the salton buttes rhyolite domes part of the post 4 ma oceanic spreading regime indicate that they are the components of a differentiated magmatic system these data also refute the presence of crystalline basement beneath this part of the salton trough we suggest that the rupture of crystalline basement beneath the axis of the salton trough was more effective during oligocene miocene time

seismic structure of the continental crust based on rock velocity measurements from the kapuskasing uplift the michipicoten greenstone belt mgb wawa gneiss terrane wgt and kapuskasing structural zone ksz in ontario canada are regarded as a 25 km thick partial cross section through the crust of the superior province we measured compressional wave velocities vp at confining pressures up to 600 mpa for representative rock samples from these terranes vp for mgb is variable 6 1 7 km s and depends on composition and anisotropy compressional wave velocities for low anisotropy quartzofeldspathic gneisses and intrusive rocks from the wgt and the ksz show little variation over a wide range of silica content but mafic gneisses and anorthositic rocks show a wide velocity range over a narrower silica range a trend corroborated by data for other high grade rocks a model of the continental crust based on the laboratory data shows three megalayers with differing seismic characteristics the shallowest levels mgb are seismically heterogeneous with high average velocities 6 6 km s at 100 mpa because of the dominance of metabasalts the balance of the upper crust wgt is seismically homogeneous as it is dominated by a variety of isotropic quartzofeldspathic rocks with average velocities of 6 35 km s at 100 mpa and 6 6 km s at 600 mpa the uppermost lower crust ksz is layered seismically heterogeneous and transversely isotropic the average velocity at 600 mpa is 6 84 km s the boundary between wgt and ksz is marked by a velocity increase of between 0 2 and 0 3 km s an increase similar to that commonly associated with midcrustal discontinuities i e conrad in shield areas here this boundary corresponds to the amphibolite granulite facies transition and to a change from a tonalitic middle crust to a lithologically heterogeneous uppermost lower crust refraction results show an anomalous high velocity zone in the upper crust under the ksz that on the basis of laboratory velocity data can be correlated with ksz lithologies these data indicate that ksz rocks can be traced to the west to depths as great as 20 km levels deeper than 20 km not exposed at the surface in the ksz are characterized by velocities greater than 7 0 km s and may be dominated by mafic and anorthositic lithologies or by more garnet rich rocks the velocity model of the superior province crust based on geological reconstruction and laboratory velocity data is in general agreement with the velocity structure to depths of about 25 km determined for the crust below the mgb from refraction experiments the velocity structure of this model is also generally similar to the velocity structure of shield areas

quartz cementation and related sedimentary architecture of the triassic solling formation reinhardswald basin germany abstract the olenekian solling formation of the reinhardswald basin was studied with the aim to examine the dependency of the degree of silica diagenesis on the sedimentary architecture the solling formation is composed of stacked sandstone clay siltstone cycles deposited by braided i e subarkoses and meandering i e subarkosic wackes river systems the sand dominated braided rivers of the lower solling formation are characterized by channels and downstream accretion whereas mixed load meandering rivers of the upper solling formation are represented by lateral accretion and laminated sands diagenetic processes which modified the sandstone siltstone successions include compaction feldspar alteration and cementation sandy clay poor successions are highly quartz cemented i e silica importers whereas clay and mica rich successions are more compacted i e silica balanced or silica exporters three quartz cement generations with a total volume of up to 18 indicate that silica from several different sources cemented the sandy fluvial architectural elements of the lower solling formation cementation includes silica supply from internal and external to the sandstone layers which are silica from grain margin dissolution from feldspar alteration silica released from clay siltstones and silica precipitated from basinal brines this cementation history is supported by cathodoluminescence observations and fluid inclusion studies and a consistent burial history was successfully modeled by simulation of the thermal subsidence in contrast to the lower solling formation the fluvial architectural elements of the mixed load systems of the upper solling formation contain only one quartz cementation phase with a total volume up to 8 the reinhardswald basin shows a dependency of the fluvial architecture and the diagenetic features for that we define the term diagenetic architecture

evidence against natural deformation of fluid inclusions in diagenetic quartz abstract this paper describes evidence for and against burial induced deformation of fluid inclusions in diagenetic quartz fluid inclusions in diagenetic minerals are prone to non elastic deformation stretching or leakage if subjected to progressive burial in a sedimentary basin if they deform they cannot record mineral growth conditions correctly a correlation between homogenization temperature and depth may be interpreted as indicating wholesale leakage or stretching experimental work and theoretical studies suggest that heating 200 c higher than the homogenization temperature is required to produce significant deformation but it may not be possible to extend the results to natural conditions the experiments further predict a correlation between homogenization temperature and inclusion size in a sample of inclusions that have undergone natural deformation this is not observed in natural examples but the absence of a correlation is not proof that leakage has not occurred several features of inclusion data in quartz cements do however argue against stretching or leakage homogenization temperatures commonly show narrow distributions and are compatible with independent data such as illite potassium argon ages and petrography different generations of inclusions in a single sample set also retain homogenization temperature distributions compatible with their sequential origin taken as a whole the body of evidence argues against widespread stretching or leakage of fluid inclusions in diagenetic quartz

late cretaceous juxtaposition of metamorphic terranes in the southeastern san gabriel mountains california new structural petrological and geochronological data from the southeastern san gabriel mountains define the amalgamation history of suspect terranes in this region four metamorphic terranes constitute the area the cucamonga san antonio san gabriel and baldy terranes and were juxtaposed in two discrete episodes during the late cretaceous the cucamonga san antonio and san gabriel terranes were juxtaposed along subparallel east west trending left lateral mylonitic fault zones during the first episode the cucamonga terrane consists of lower cretaceous granulite facies gneissic rocks these rocks were intruded in the late cretaceous by granitic rocks as the cucamonga terrane was faulted ductilely against the southern margin of the san antonio terrane mylonitized units were subjected concurrently to retrograde amphibolite facies metamorphism metasedimentary pendants in the san antonio terrane were metamorphosed under upper amphibolite facies conditions as plutonic rocks were emplaced syntectonically along its northern boundary the san antonio terrane was juxtaposed concurrently against the san gabriel terrane a terrane consisting of precambrian gneiss and precambrian and mesozoic intrusive rocks the boundary is represented by middle fork complex a ductile zone intermingling rocks of the two terranes after palinspastic restoration of local cenozoic fault displacements the left lateral mylonitic fault zones separating the terranes appear as parts of a major sinistral transform system along the southern margin of the san gabriel mountains displacement for each of the mylonitic fault zones was approximately several tens of kilometers and was part of a broad left lateral shear system this system is modeled as a lateral ramp or tear fault to a west directed synplutonic ductile thrust system underlying the san gabriel terrane crosscutting syn and post tectonic intrusives indicate that most of the displacement occurred between 88 and 78 m y ago no lithologic isotopic or structural data support previous assertions that these terranes were accreted to north america during the tertiary the late cretaceous synplutonic juxtaposition of the cucamonga san antonio and san gabriel terranes was followed by underthrusting of oceanic rocks of the baldy terrane that is pelona schist along the vincent thrust precise timing of this episode is as yet uncertain

a stochastic view of lower crustal fabric based on evidence from the ivrea zone despite its complicated history the ivrea zone is considered to be a representative surface exposure of extended continental crust we have digitized two standard 1 25 000 geological maps from this area and evaluated their structural statistics because of the subvertical orientation of the ivrea zone these maps can be considered as small scale cross sections through the lower continental crust the autocorrelation functions of the digitized lithologies measured from these maps show a clear self similar or fractal rather than a gaussian or deterministic trend we found that an anisotropic von karman correlation function with an aspect ratio around 4 and a hurst number of 0 3 corresponding to a fractal dimension of 2 7 matches the observed data our results represent an explicit confirmation of previous indirect evidence for the fractal nature of lithospheric heterogeneities and provide the means to construct realistic crustal scale seismic models of ivrea type lower continental crust

interpreting fracture development from diagenetic mineralogy and thermoelastic contraction modeling four sets of thin section scale mode i open mode cemented microfractures are present in sandstone from the eocene misoa formation maracaibo basin venezuela the first set of microfractures is intragranular f1 formed early during compaction and are filled with quartz cement precipitated at temperatures equal to or higher than 100 8c the second set of microfractures f2 is cemented by bituminite pyrite formed at temperatures between 60 and 100 8c and are associated with kerogen maturation and hydrocarbon migration from underlying overpressured source rocks the third set of microfractures f3 is fully cemented by either quartz cement or calcite cement the former has fluid inclusion homogenization temperatures between 149 and 175 8c these temperatures are mostly higher than maximum burial temperatures 160 8c suggesting that upward flow caused by a pressure gradient transported silica vertically which crystallized into the fractures upward decompression may have also caused a pco2 drop which at constant temperature allowed simultaneous carbonate precipitation into the third microfracture set the fourth set of thin section scale microfractures f4 is open or partially cemented by siderite hematite and other iron oxides the presence of hematite and iron oxides in microfractures is evidence for oxidizing conditions that may be associated with the uplift of the misoa formation in order to time and place constraints on the depth of formation of the fourth set of microfractures we have coupled published quartz cementation kinetic algorithms with uniaxial strain equations and determined if in fact they could be associated with the uplift of the formation our results suggest that thermoelastic contraction caused by the formation s uplift erosion and consequent cooling is a feasible mechanism for the origin of the last fracture set hence we infer that meteoric water invasion into the fractures at the end of the uplift cause the precipitation of oxides and the transformation of siderite to hematite d 2005 elsevier b v all rights reserved

a kinematic model for the late cenozoic development of southern california crust and upper mantle a model is presented for the young and ongoing kinematic deformation of southern california crust and upper mantle that accounts for both the known late cenozoic geologic record and upper mantle structure in this model the high velocity upper mantle transverse ranges anomaly is created through the convergence and sinking of the entire thickness of subcrustal lithosphere using typical scaling relations we estimate this anomaly to be 500 c colder and 1 more dense than average southern california mantle of the same depth we infer that subcrustal lithosphere from both sides of the convergence zone participates nearly equally in the construction of the anomaly the low velocity upper mantle anomaly beneath the salton trough region is attributed to high temperatures and 1 4 partial melt related to adiabatic decompression during mantle upwelling our model has crustal motions in the central transverse ranges region differing from mantle motions while the subcrustal lithosphere south of the transverse ranges moves toward and sinks beneath the transverse ranges the corresponding crust largely avoids convergence by adopting a more westerly direction of motion the differing velocity fields necessitates of a decoupling zone immediately south of the transverse ranges near the moho we interpret the upper mantle anomalies as results of small scale convection beneath southern california to which we attribute much of the recent tectonic activity in the region

absolute reflection coefficients from deep seismic reflections abstract many deep seismic reflection profiles show bright sub horizontal layered reflections in the lower continental crust accurate estimates are obtained for normal incidence reflection coefficients for the brightest of these reflections by using a marine seismic acquisition system that has been calibrated using deep water sea bottom multiples minimum bounds are placed upon these reflection coefficients by assuming minimum losses due to scattering anelastic attenuation and geometric spreading reflection coefficients of around 0 1 are obtained from the layered lower crust and around 0 15 from the moho it is argued that constructive interference between successive reflections from a layered sequence of reflectors is often unimportant consequently these coefficients can be used to place useful limits on the absolute acoustic impedance contrasts present at depth