experimental study of fluid transport processes in the matrix system of the european organic rich shales i scandinavian alum shale abstract this contribution presents results from a laboratory study investigating the fluid gas water transport properties in the matrix system of the scandinavian alum shale the maturity of the organic matter of the shale samples ranged between 0 5 and 2 4 vitrinite reflectance vr r gas he ar ch 4 and water flow properties were determined at effective stresses ranging between 5 and 30 mpa and a temperature of 45 c the effects of different controlling factors parameters on the fluid conductivity including permeating fluid moisture content anisotropy heterogeneity effective stress pore pressure and load cycling were analyzed and discussed pore volume measurements by helium expansion were conducted under controlled in situ effective stress conditions on a limited number of plugs drilled parallel and perpendicular to bedding for alum shale the intrinsic permeability coefficients measured parallel and perpendicular to bedding 6 10 22 8 10 18 m 2 were within the range previously reported for other shales and mudstones permeability coefficients were strongly dependent on permeating fluid moisture content anisotropy effective stress and other sample to sample variations the intrinsic absolute permeabilities measured with helium were consistently higher up to five times than those measured with argon and methane permeability coefficients he ch 4 measured on a dry sample were up to six times higher than those measured on an as received sample depending on effective stress the effect of moisture on measured permeability coefficients became more significant as effective stress increased permeability coefficients he ch 4 measured parallel to bedding were up to more than one order of magnitude higher than those measured perpendicular to bedding parallel to bedding all samples showed a nonlinear reduction in permeability with increasing effective stress 5 30 mpa the stress dependence of permeability could be well described by an exponential relationship

thick skinned deformation observed on deep seismic reflection profiles in western argentina one of the first deep seismic reflection profiles in south america has resulted from reprocessing nearly 90 km of 96 channel vibroseis data that were originally collected by ypf the argentine national oil company across the sierra de la huerta in the sierras pampeanas of western argentina reprocessed sections show the following features in the upper 20 km of crust 1 prominent layering within carboniferous to quaternary strata and 2 a complex suite of faint reflections which are interpreted to define both extensional and compressional faults the mesozoic strata are bounded by one or more west dipping normal faults one or two east dipping thrust faults shallow with depth a geometry similar to the rocky mountain foreland of the western u s a one thrust has a clear surface trace although lacking evidence of surface scarps or clear fault plane reflections panels of constant dip domain seen in the seismic data justify the application of fault bend folding techniques to suggest that a second deeper fault becomes nearly flat at 15 20 km cumulative cenozoic offset on the thrusts beneath the sierra de la huerta is about 13 km a well located earthquake hypocenter lies within the footwall block of a major thrust and is presumed to represent susidiary thrust motion along a secondary high angle fault gravity models suggest that the thrust ramps may nucleate at an anomalous dense mass in the middle or lower crust interpretation of lower crustal structure is hampered by the lack of clear reflections presumably due to small scale and low amplitude impedance contrast variations of lithologic units at those depths consideration of recorrelation limitations and signal penetration constraints provide likely rationale for the scarcity of clear reflections in the basement rocks of the region but low reflectance of mafic metamorphic rocks remains a possible explanation

stable isotope geochemistry of crude oils and of possible source rocks from new zealand 2 sulfur abstract the sulfur isotopic composition of new zealand low s crude oils and components asphaltenes absorptive filtration fractions and source rock components extracted hydrocarbons kerogen pyrite sulfates were determined oils from the taranaki basin have 34s values from 3 to 14 o the lower values possibly represent plant assimilation of fresh water sulphate higher values particularly those from the west coast basin around 20 suggest closed ocean basin conditions or fast sedimentation rate whereas negative 34s values from east coast basin suggest open ocean or low sedimentation rate conditions it is suggested than an oil source rock relation should show as a similarity in 34s values for total oil and source rock extract kerogen and pyrite shale samples analysed plot outside the corresponding oil range and cannot be considered as possible source rocks however coal samples from the maui 4 well and nelson outcrops show strong isotopic correlations with the taranaki oils

fennoscandian borderzone thermal and tectonic history of a tuffaceous sandstone and granite from fission track analysis bornholm denmark abstract fission track ages for zircon and apatite together with track length distributions for apatite have been obtained from a tuffaceous sandstone of wenlock age from bornholm denmark the zircon yields an age close to the depositional age of c 420 ma which is similar to the age of bentonites from gotland sweden the apatite yields a much younger age which is interpreted as a cooling uplift age on the basis of the track length distribution it is concluded that due to burial since deposition the sediment has been heated into the 130 190 c interval prior to 261 ma the complex apatite fission track age and length distribution obtained for a granite suggest reheating or prolonged heating in the 70 130 c interval probably associated with individual block movements in the area bornholm is located at the rim of the fennoscandian borderzone and thus fission track analysis of the thermal and tectonic history is important in understanding the evolution of this structure which is part of a major structural discontinuity stretching from oslo to the black sea

atmospheric weathering of scandinavian alum shales and the fractionation of c n and s isotopes abstract subaerial exposure and oxidation of organic carbon c org rich rocks is believed to be a key mechanism for the recycling of buried c and s back to earth s surface importantly processes coupled to microbial c org oxidation are expected to shift new biomass 13 c org composition towards more negative values relative to source however there is scarcity of information directly relating rock chemistry to oxidative weathering and shifting 13 c org at the rock atmosphere interface this is particularly pertinent to the sulfidic c org rich alum shale units of the baltoscandian basin believed to constitute a strong source of metal contaminants to the natural environment following subaerial exposure and weathering consistent with independent support we show that atmospheric oxidation of the sulfidic c org rich alum shale sequence of the cambrian devonian baltoscandian basin induces intense acid rock drainage at the expense of progressive oxidation of fe sulfides sulfide oxidation takes priority over microbial organic matter decomposition enabling quantitative massive erosion of c org without producing a 13 c shift between acid rock drainage precipitates and shale moreover 13 c enrichment in inorganic carbon of precipitates does not support microbial c org oxidation as the predominant mechanism of rock weathering upon exposure instead a 34 s 34 s shale 34 s precipitates 0 accompanied by elevated s levels and the ubiquitous deposition of acid rock drainage sulfate minerals in deposited efflorescent precipitates relative to shales provide strong evidence for quantitative mass oxidation of shale sulfide minerals as the source of acidity for chemical weathering slight 15 n depletion in the new surface precipitates relative to shale coincides with dramatic loss of n from shales collectively the results point to pyrite oxidation as a major driver of alum black shale weathering at the rock atmosphere interface indicating that quantitative mass release of c org n s and key metals to the environment is a response to intense sulfide oxidation consequently large scale acidic weathering of the sulfide rich alum shale units is suggested to influence the fate and redistribution of the isotopes of c n and s from shale to the immediate environment

influence of paleoenvironmental factors on preservation of organic matter in middle cretaceous greenhorn formation pueblo colorado sedimentological and geochemical studies of the upper cretaceous hartland shale and overlying bridge creek limestone members of the greenhorn formation reveal close associations between abundance of current induced sedimentary structures extent of bioturbation and type of preserved organic matter abundant hydrogen rich organic matter 4 5 wt and low sulfur to organic carbon ratios are characteristic of the laminated facies which lack current induced sedimentary structures sparse hydrogen poor organic matter 0 1 0 5 wt and relatively high sulfur to organic carbon ratios are characteristic of the bioturbated facies which contain numerous sedimentary structures indicative of currents the concentration of oxygen in the benthic environment and the degree of bio urbation of the sediment apparently were determined principally by the rate that oxygen was supplied advectively by currents rather than by the rate of oxygen consumption by decomposition thus paleoclimatic and paleo oceanographic factors that influenced mixing and current strength in the water column profoundly affected the amount and type of organic matter preserved in these units it is proposed that episodes of high river discharge lead to density stratification of the water column and in turn to quiescent oxygen depleted bottom water and high preservation of organic matter thus accounting for the intermittent widespread deposition of organic rich strata during maximum transgression and depth of the greenhorn sea the lateral association of voluminous deltaic deposits with dee water black shales in other elongate or restricted marine basins may have causes similar to those envisioned for the hartland shale and bridge creek limestone members

plate tectonics and the evolution of the british isles this paper is an essay on the rationalism and difficulties of the geological corollaries of plate tectonics with illustrations particularly from the geological history of the british isles there is special emphasis on the mechanics of continental lithosphere in extension when basins are formed and in convergence whereby continental lithosphere thinned in extension is shortened and restacked to form orogenic belts an upper lithosphere stress concentrating elastic lid suffers extremely complex inhomogeneous deformations above a simpler viscoelastic lower lithosphere whose deformation drives strain and displacement in the elastic lid the very nature of plate tectonics as relative motion among a multi plate mosaic of plates provides a rational semi quantitative framework for large scale geological complexity in space and time the caledonian and hercynian history of the british isles may be explained semi quantitatively in terms of plate boundary evolution but it is not possible to define unequivocal solutions the rationalism of plate tectonics is exhibited on a smaller more direct field scale by many of the complexities of ophiolite complexes such as ballantrae scotland that can be explained as the effects of the evolution of ridge ridge transforms and by diachronous deformation and sedimentation in subduction accretion prisms such as the southern uplands deformational and sedimentary patterns in nw europe from triassic times to the present day resulted from a complex interaction of extensional events related to diachronous atlantic opening and compressional events related to sequential alpine shortening phases those widespread events are typical of intracontinental plate boundary systems and create great difficulties for the geologist wishing to decipher relative plate motion from geological sequences because huge areas much larger than the british isles must be analysed and synthesized before much sense can be extracted further difficulties arise from the palinspastic and preservation problems of convergent boundaries orogenic shortening telescopes and partially subducts facies belts most orogenic belts were assembled as a collage of displaced and rotated terrain elements that were subsequently flattened elongated and straightened to an apparent but deceiving simplicity therefore while plate tectonics provides a rational geological framework against which for example british basin evolution can be tested and explained we cannot yet use the geological effects of plate boundary evolution to reconstruct relative plate motion quantitatively

carbon and hydrogen stable isotope variations in kerogen during laboratory simulated thermal maturation carbon and hydrogen stable isotope measurements on sapropelic and humic kerogen isolated from laboratory heated sediments reveal several isotopic trends an initial depletion of 13c and d 2h or deuterium in kerogen isolated from the heated sapropelic sediment appears to be the result of isotopic heterogeneity interaction with soluble organic matter and or volatilization of a heat labile fraction of the protokerogen during the main isotopic trend the sapropelic kerogen becomes enriched in 13c and d by about 5 and 50 ppt parts per thousand respectively theoretical calculations based on a rayleigh fractionation process suggest that this trend is largely controlled by the generation of isotopically light methane directly from the ker gen this argument also applies to the main hydrogen isotopic trend for the humic kerogen which becomes enriched in d by about 90 ppt the main 13c trend for the humic kerogen however results in a maximum enrichment in 13c of only about 1 6 ppt and appears to be largely controlled by factors other than methane a final depletion trend in 13c and d occurs for the sapropelic kerogen at very high temperatures this may be due to a reduced isotopic fractionation at higher temperatures or to the indirect generation of methane from kerogen through a liquid intermediate which is subsequently coked to pyrobitumen

on the reactivation of extensional fault systems in terranes that have undergone substantial extension three sets of faults dominate a shallow to steep dipping commonly rotational normal faults b a really extensive shallow dipping normal detachment faults and c steep dipping transfer faults that strike at high angles to the normal faults these fault systems may extend through a large fraction of the crust reactivation of these fault systems will depend primarily on the relative strengths of the faults shear zones and their host rock and their orientation in the prevailing stress field it is concluded that reactivation is generally mechanically favoured but that it will probably only take place when the fault shear zones are in near ideal orientations consideration of the tectonic setting of extended terranes and of the limited number of well described examples suggests that reverse thrust reactivation of the normal and detachment faults and wrench reactivation of transfer faults are the most likely styles examples of these styles are described from the bass strait basins of southeastern australia because extended terranes commonly underlie sedimentary basins for example on passive continental margins reactivation of extensional faults may be a key control on the tectonic evolution of such basins i e basin inversion

organic petrology of selected oil shale samples from the lower carboniferous albert formation new brunswick canada incident light microscopy was used to describe maturation and composition of organic material in oil shale samples from the lower carboniferous albert formation of new brunswick the maturation level was determined in normal white light by measuring vitrinite reflectance and in fluorescent light by measuring fluorescence spectra of alginite b results indicate low to intermediate maturation for all of the samples composition was determined by maceral analysis alginite b is the major organic component in all samples having significant oil potential oil yields obtained from the fischer assay process and oil and gas potentials from rock eval analyses correlate to the amounts of alginite b and bituminite determined in the samples in some of the samples characterized by similar high concentrations of alginite b decrease in fischer assay yields and oil and gas potentials is related to an increase in maturation as expressed by increase in the fluorescence parameter lgrmax and red green quotient of alginite b incident light microscopy particularly with fluorescent light offers a valuable tool for the identification of the organic matter in oil shales and for the evaluation of their oil and gas potentials end of article last page 494

footwall uplift associated with late jurassic normal faulting in the northern north sea tilted fault blocks in the north sea show varying degrees of erosion such footwall erosion is predicted by the domino model of extension in which rigid fault blocks rotate as extension proceeds profiles of three north sea fields snorre brent and heather are analysed to estimate the amount of uplift that accompanied late jurassic normal faulting and these estimates are compared with the footwall uplift predicted by the domino model agreement between observed and predicted uplift is excellent varying from 1 km at snorre to c 100 m at heather for a given amount of extension large fault blocks tend to be bounded by large faults and to show a greater degree of erosion the brent province of the northern north sea is an area of prolific hydrocarbon discoveries almost all of the major oil fields occur in tilted fault blocks which were formed by late jurassic extension and subsequently buried by cretaceous tertiary thermal subsidence badley et al 1988 marsden et al 1990 the crests of the tilted blocks are typically eroded or degraded with varying amounts of middle lower jurassic and upper triassic section removed this footwall erosion is commonly ascribed in the literature to pre rifting doming ziegler 1982 heterogeneous stretching coward 1986 local transpression beach 1985 or even regional compression frost 1987 the purpose of this paper is to show that the uplift and degradation is a direct consequence of the late jurassic normal faulting and fault block rotation the degree of uplift is related in a simple way to the size across strike of

thrust tectonics thin skinned or thick skinned and the continuation of thrusts to deep in the crust abstract the paper analyses the geometry of thin skinned thrust zones where the thrusts shallow out at depth and of thicker skinned fault zones where much of the crust is involved and where the thrusts are frequently observed to become steeper downwards in the interiors of many orogenic belts the steep dip of faults is not original but due to the folding above lower decoupling zones the energy involved in the internal deformation of hanging wall rocks may prohibit many faults becoming more shallow upwards such shallowing upwards faults may occur in more ductile rocks to maintain compatibility between zones which have experienced different deformation intensities but displacements on the faults are unlikely to be large another mechanism for producing faults which steepen downwards is proposed for the major thrusts which form the southern margin to the himalayas these carry large thicknesses 30 to 100 km of crustal and upper mantle rocks to the south causing flexuring and isostatic depression of the indian plate the steeply dipping thrusts are not footwall ramps these may be some distance behind the steepened zone this thrust induced isostatic bending of the crust has important implications when considering regional seismic interpretations as well as thrust mechanics and kinematics

processes of water rock interaction in the turonian aquifer of oum er rabia basin morocco possible water rock interaction processes in the moroccan basin of oum er rabia were discussed by a geochemical study of groundwater from the turonian limestone aquifer the most important water resource in the region different types of water according to the classification of piper were defined waters have shown an evolution from dominant cho3 ca mg type through mixed to so4 cl ca mg type the use of geochemical diagrams and chemical speciation modeling method has shown that water rock interaction is mainly controlled by carbonate and anhydrite dissolution ion exchange and reverse ion exchange processes water rock equilibrium conditions are favorable for the precipitation of calcite dolomite kaolinite and magnesian smectite

a geophysical study of the earth s crust in central virginia implications for appalachian crustal structure a regional seismic reflection line i 64 across the virginia piedmont has provided a stacked section suitable for an integrated interpretation of geophysical data in the region a highly reflective upper crust an allochthonous blue ridge province underlying thrust sheets including the blue ridge master decollement and a basal decollement at a depth of about 9 km 3 s are confirmed on the seismic data immediately east of the blue ridge province appalachian structures plunge to as much as 12 km 4 s depth the evington group hardware terrane and chopawamsic metavolcanic rocks carolina terrane crop out in the piedmont province and numerous eastward dipping reflections originate from these rocks in the subsurface these eastward dipping reflectors overlie a gently west dipping 10 15 highly reflective zone that varies in depth from 1 5 s 4 5 km beneath the goochland terrane to 4 s 12 km beneath the rocks of the evington group some of the overlying eastward dipping reflections apparently root in this zone the zone may include decollement surfaces along which the overlying rocks were transported relatively few reflections originate from within autochthonous grenville basement at the western end of the profile the goochland granulite terrane is interpreted to be a westward thrust nappe structure that has overridden a portion of the chopawamsic metavolcanic rocks a broad zone of east dipping 20 45 reflections bounds the goochland terrane on the east these reflections may originate from deformation zones and continue to moho depths they appear to be correlative with similar events seen on other appalachian lines the pervasiveness of the zone of east dipping events on other seismic reflection lines and the continuity of the adjacent piedmont gravity high suggest continuity of crustal features along the length of the appalachians a major conclusion of this study is that crustal thinning is responsible for the main components of the gravity field in virginia that is the appalachian gravity gradient and the piedmont gravity high the crust thins from about 52 km beneath the appalachian mountains to about 35 km beneath richmond virginia and then rethickens by up to 10 km beneath the zone of east dipping reflections mylonites east of richmond the i 64 seismic data also contain a sequence of reflections at about 9 12 s indicative of lower crustal layering the base of this zone of reflections coincides almost exactly with the mohorovicic discontinuity interpreted from earlier refraction work the layering extends about 70 km west from richmond virginia and is interpreted as a lower crustal transition zone that is believed to persist across most of virginia

active normal faulting and crustal extension a world wide review of fault plane solutions and focal depths for large normal faulting earthquakes on the continents shows that the overwhelming majority of such earthquakes nucleate in the depth range 6 15 km on faults dipping between 30 and 60 in the few cases where levelling or seismic data are good enough these normal faults are shown to be approximately planar from the surface to their nucleation depth at the base of the brittle crust there is evidence that in some cases as a result of the enormous transitory increase in strain rate during large earthquakes rupture continues into the normally ductile lower crust on surfaces with substantially gentler dips these low angle surfaces may be analogous to some of the detachments seen in metamorphic core complexes of the western usa but the nature of the motion on them depends on strain rate as well as on rheological contrasts between the detachments and the blocks on either side such contrasts however are unlikely to introduce substantial curvature to an originally planar shear zone if the observed spread in active normal fault dips is caused by rotation of the faults and the blocks they bound during extension then a maximum value of 1 7 can be accommodated by seismic activity on a single generation of normal faults with continued extension either a new generation of steeper faults cutting the rotated first faults is likely to form or the deformation will continue aseismically on faults dipping at less than 30 large earthquakes have not been observed to nucleate on very low angle 20 normal faults within the continental crust anywhere in the world such faults can of course move aseismically but are unlikely to do so on a large scale within the upper crust in areas where steep normal faults are seismically active thus extensional models that require concentrated simple shear on large sub horizontal faults within the brittle upper crust will also require a spatial separation between aseismic very low angle faulting and seismic highangle faulting since seismogenic high angle faults dominate the topography of extending regions upper crustal very low angle faulting if it occurs on a large scale presumably does so in areas that are relatively fiat

upb dating of calcite concretions from cambrian black shales and the phanerozoic time scale the u pb dating of carbonate offers considerable potential for accurate and direct dating of sedimentary rocks here we show that carbonate concretions from the upper cambrian black shales in southern scandinavia have remarkably high u to pb ratios 238u204pb up to 288 000 permitting precise geochronology a fine crystalline early diagenetic concretion yields an age of 509 8 5 1 ma providing an accurate age for the late cambrian whereas a coarse cone in cone concretion has yielded an age of 478 2 4 9 ma defining the final stage of sediment compaction and indicating a protracted dewatering and diagenetic history these results demonstrate the power of this method for directly calibrating the geological time scale and constraining diagenetic histories

normal faulting in the upper continental crust observations from regions of active extension abstract observations of present day normal faulting in regions of active continental extension may be helpful when interpreting the geological record in older extensional basins the most obvious manifestation of active extension is normal faulting in earthquakes earthquake foci are mostly confined to the upper seismogenic continental crust whose thickness imposes a scale on the observed deformation large earthquakes move faults with lengths similar or large compared with the thickness of the seismogenic layer large seismogenic normal faults on the continents appear to be restricted to a dip range of around 30 60 dips significantly gentler than 20 have not been observed in fault plane solutions of large earthquakes large seismographic normal faults are approximately planar in cross section and cut through the base of the upper seismogenic layer rotating about a horizontal axis as they move a reasonable estimate of the regional extension can often be made from the dip of the large normal faults and the tilt of the blocks they bound the domino model such faults are rarely continuous for more than 15 20 km but commonly change strike or step in an en echelon fashion this segmentation may occur on a scale controlled by the thickness of the seismogenic layer and is an important influence on sedimentation and drainage these observations are common to large seismogenic normal faults from a variety of tectonic settings and suggest that the kinematics of normal faulting may not be strongly influenced by the forces responsible for the extension which can vary widely in nature and magnitude small earthquakes which move faults that are small compared with the seismogenic layer thickness show no simple pattern and often can be interpreted as internal deformation of blocks bounded by large faults in some places the seismogenic basement faults do not reach the surface but are decoupled from the sedimentary cover by layers of weak lithology faults in the sedimentary cover may be strongly curved in cross section requiring their hangingwalls to deform internally estimating extension from such faults is not straightforward and requires a knowledge of the hangingwall deformation estimates made in this way are often very non unique and prone to large errors we postulate that the thickness of the seismogenic upper crust controls both the maximum length of large normal fault segments along strike and the maximum size of blocks that can rotate coherently about a horizontal axis

thin skinned thrusting in the northern new england orogen central queensland australia regional cross sections of the northern new england orogen and eastern bowen basin in eastern queensland indicate that the climactic permian triassic deformation involved thin skinned tectonics with the development of a mountain front represented by a syncline formed by crustal delamination with wedging of a thrust sheet from the east the thrust sheet is exposed in the folded zone of the bowen basin and is characterized by upright nnw trending folds in late permian sediments the folds have a chevron to flattened chevron style and formed mainly by flexural slip at about 60 shortening with detachment from an underlying basement of paleozoic rocks farther east the basement is overlain by pervasively cleaved rocks of the gogango overfolded zone in which structures show a consistent westerly vergence these rocks overlie an inferred passive roof duplex in the underlying basement together the folded zone of the bowen basin and the gogango overfolded zone constitute a foreland fold thrust zone with a shortening of 50 90 km this implies that the new england orogen has been thrust west along a major east dipping detachment

a lower crustal bright spot under the bay of plenty north island new zealand abstract marine seismic reflection measurements across the central bay of plenty northern north island new zealand detected a strong seismic reflection in the lower crust at a depth of about 10 km 5 s twt this reflection is observed on the stacked seismic section for about 8 5 km across and below a young actively rifting basin that lies along the offshore extension of the active taupo volcanic zone the reflection is unusual in that its maximum extent on any shot record is about 1250 m about half the fresnel zone diameter suggesting a series of point reflectors or reflectors of limited extent wave equation modelling indicates that the reflection event probably represents a positive increase in acoustic impedance and thus does not represent a magma body or partial melt as has been suggested previously the preferred model suggests a thin layer such as a mafic sill with the large reflectivity and apparent wavelength caused by wavelet tuning similar to that deduced for the surrency bright spot in the usa a mafic sill would be consistent with the geological setting although other explanations such as a highly altered shear zone cannot be excluded from the restricted extent of the reflections on the shot gathers and their spatial relationship we infer that the sill is fragmented

a deep seismic profile of 800 km length recorded in southern queensland australia summary in 1984 the australian bureau of mineral resources and the geological survey of queensland recorded a regional seismic reflection profile of over 800 km length from the eastern part of the eromanga basin to the beenleigh block east of the clarence moreton basin a relatively transparent upper crustal basement with an underlying more reflective lower crust is characteristic of much of the region prominent westerly dipping reflectors occur well below the sediments of the eastern margin of the clarence moreton basin and the adjacent beenleigh block and provide some of the most interesting features of the entire survey a wide angle reflection refraction survey of 192 km length and an expanding reflection spread of 25 km length were recorded across the nebine ridge the only clear deep reflectors are interpreted as p to sv or sv to p converted reflections from a mid crustal boundary at a depth of about 17 km the combined nebine ridge data provide well constrained p and s wave velocity models of the upper crust and suggest a crustal structure quite different from that beneath the adjacent mesozoic basins

geophysical evidence for thick skinned crustal deformation in central australia until now there has been no clear evidence from deep seismic reflection profiles of crustal faults that displace the moho new data from central australia show such a structure in a region characterized by large thrust structures large gravity anomalies and abrupt changes in teleseismic travel times here the principal thrust the redbank deformed zone cuts through the crust and the moho is significantly displaced across it the geological and geophysical evidence leads to a model of thick skinned crustal deformation which occurred in an intra cratonic environment in late devonian time

main results of the ecors pyrenees profile abstract the 250 km long deep seismic reflection survey across the pyrenees from one foreland ebro basin to the other aquitaine basin was designed to study the structure of the deformed mesozoic boundary between the iberian and european plates the results are 1 1 the entire crust shows well defined reflectors with a general fan shape geometry 2 2 a deep layered domain is evident above the moho discontinuity 3 3 near the surface in the external domains reflectors accurately define the geometry of major thrusts and structures affecting the mesozoic and cenozoic cover of the pyrenees 4 4 the middle crust shows two major sets of seismic events interpreted as hercynian structures using these results a balanced structural section of the pyrenean belt is proposed the mode of thickening of the crust during the alpine orogeny is briefly discussed only the iberian crust was significantly thickened by a southward stacking of slabs

seismic characteristics and identification of negative flower structures positive flower structures and positive structural inversion negative and positive flower structures and positive inverted structures imply specific modes of formation and their distinctive characteristics make them important criteria for the identification of certain structural styles a negative flower structure from the andaman sea consists of a shallow synform bounded by upward spreading strands of a wrench fault that have mostly normal separations paralleling monoclines and oblique en echelon normal faults flank the divergent wrench fault a positive flower structure from the ardmore basin oklahoma consists of a shallow antiform displaced by the upward diverging strands of a wrench fault that have mostly reverse separations en echelon folds are present on either side of this convergent wrench fault flower structures may resemble extensional or contractional fault blocks on individual seismic profiles flower structures are identified by their unique internal fault and fold architecture by their association with a straight and throughgoing zone of deformation and by their lack of a consistent sense of separation or regional upthrown side positive structural inversion at the rambutan oil field south sumatra basin has formed a shallow anticlinorium and has partly uplifted the underlying graben deeper fault segments bounding the graben have retained their normal fault profiles but at shallow levels some of these faults have reverse separations similar changes of separation with depth may be produced by strike slip but sets of faults with listric profiles and nearby blocklike trend patterns identify the inversion as a dip slip event where correlation of reflections is uncertain or deep control is unobtainable positive inverted structures may also resemble contrational fault blocks detached normal fault assemblages and certain negative inverted structures

the growth of geological structures by repeated earthquakes 2 field examples of continental dip slip faults a strong test of our understanding of the earthquake cycle is the ability to reproduce extant fault bounded geological structures such as basins and ranges which are built by repeated cycles of deformation along strike slip faults the coseismic and interseismic deformation can be nearly equal in magnitude and opposite in sign resulting in little permanent deformation except for the fault offset for dip slip faults portions of the crust are lifted and dropped and so buoyancy forces are exerted the seismic and interseismic deformations do not balance and structures grow and become subject to erosion and deposition we consider three examples for which the structure and fault geometry are well known the white wolf reverse fault in california site of the 1952 kern county m 7 3 earthquake the lost river normal fault in idaho site of the 1983 borah peak m 7 0 earthquake and the cricket mountain normal fault in utah site of quaternary slip events basin stratigraphy and seismic reflection records are used to profile the structure and coseismic deformation measured by leveling surveys is used to estimate the fault geometry to reproduce these structures we add the deformation associated with the earthquake cycle the coseismic slip and postseismic relaxation to the flexure caused by the observed sediment load treating the crust as a thin elastic plate overlying a fluid substrate the cumulative deformation is principally dependent on the elastic plate thickness modestly sensitive to the sediment substrate density difference and insensitive to the fluid viscosity for the 4 to 8 ma structures we deduce a longterm flexural rigidity of 2 15 1019 nm this is equivalent to an elastic plate thickness of 2 4 km for a young s modulus of 2 5 1010 nm 2 this value is found where independent estimates of the elastic thickness from the coherence between surface topography and gravity yield values of about 4 km but where coseismic fault slip extends to a depth of 10 15 km thus much of the seismogenic crust must weaken substantially during the life of active faults causing the fault bounded basins to narrow over time

modelling of bed contours and cross sections adjacent to planar normal faults the geometry of beds in the volume surrounding a single blind planar fault depends on i the axial ratio of the fault surface ellipse ii the ratio between the maximum displacement and the maximum dimension of the fault surface iii the displacement distribution on the fault surface from the point of maximum displacement to zero displacement at the tip line loop fault surface displacement profile and iv the rate at which the displacement decreases to zero along a normal to the fault surface reverse drag displacement profile these relationships which are either known or can be estimated are incorporated in a computer model which generates structural contours on horizons intersected by a planar fault structural contour patterns also vary with the pre faulting dip and strike of beds the dip and strike of the fault and the level at which a bed intersects the fault surface when the boundary conditions are relaxed to include synsedimentary faults the ratio of hangingwall to footwall displacement varies systematically with fault dip a synthetic structural contour map is matched with that of an actual fault model isopachs based on simple depositional models are generated for synsedimentary units cross sections are generated for synsedimentary faults by sequential fault growth and sedimentary deposition and show features similar to actual cross sections the soft domino model is applicable primarily to dip slip tectonic faults with maximum displacements of up to about 5 km listric faults are not considered

origin source mixing and thermal maturity of natural gases in the panyu lower uplift and the baiyun depression pearl river mouth basin northern south china sea multiple natural gas fields have been discovered in the baiyun depression and the adjoining panyu lower uplift in the pearl river mouth basin northern south china sea the natural gases are associated with condensate and are characterized by relatively heavy carbon isotopes with methane and ethane 13c values ranging from 44 2 to 33 6 and 30 0 to 25 4 respectively nearly all methane and ethane are derived from oil prone type ii kerogen in the wenchang formation source rock whereas the heavy hydrocarbon gases propane butanes and pentanes are derived from both the wenchang and enping type iii kerogen formations based on an integrated comparison of carbon isotopic compositions of the natural gases typical type i ii and type iii kerogen derived gases and the enping and wenchang kerogens the gases from the eastern parts of the baiyun depression and the panyu lower uplift mainly originate from secondary oil cracking and primary kerogen cracking respectively the gases from the northern slope of the baiyun depression are a mixture of oil cracking and kerogen cracking gases both oil cracking and kerogen cracking gases were mainly generated from the wenchang formation source rock in the maturity range of 1 5 2 5 vitrinite reflectance with a corresponding present day depth range of 5400 6500 m 17 700 21 300 ft the apparent contribution of the wenchang formation to the discovered gas accumulations demonstrates that it is the most important source rock in the area instead of the enping formation the search for more gas derived from oil cracking will be the next natural gas exploration direction in the baiyun depression

structural stratigraphic models for extensional basins of half graben type abstract the equations describing subsidence in isostatically compensated basins formed by lithospheric stretching can be modified to define a notional depth to decollement which relates subsidence to the stretching factor in much the same way as the depth to a physical sole fault relates subsidence to extension in uncompensated thin skinned basins for a constant density basin fill this notional depth to decollement corresponds to the level of no vertical motion within the lithosphere in general the physical sole to a system of detached normal faults will not lie at the notional depth to decollement and must move vertically during extension this vertical movement must be implicitly corrected for before area bed length balancing is used to determine the depth to a physical sole fault the notional depth to decollement has been calculated numerically for an extensional basin loaded by sediments which follow a north sea shaly sand compaction law and increases with from 5 to 9 km a set of model half grabens has been constructed by combining subsidence curves generated by this shaly sand model with geometric relationships inherent in the rotated planar domino mechanism of upper crustal faulting for comparison a similar set of half grabens was constructed for an uncompensated basin with a sole fault at 7 5 km depth in both cases large fault to fault spacings give rise to large basement relief and consequent footwall uplift and erosion small fault spacings give rise to simple listric growth faults intermediate values generate more complex uplift subsidence histories which are capable of giving rise to such commonly observed features as rounded eroded fault block shoulders local regression followed by onlap or exponentially increasing subsidence at the crest of the fault block

the geometrical evolution of normal fault systems abstract the purpose of this paper is to examine the kinematic behaviour of normal fault systems and see what general conditions govern their geometrical evolution we pay particular attention to seismological and surface data from regions of present day active normal faulting as the instantaneous three dimensional geometry at the time of fault movement is better known in active regions than in areas where the faults are now static most normal faults are concave upward or listric this shape can be produced by geometric constraints either because the faults reactivate curved thrusts or because they must be curved to accommodate rotations another effect which will produce curved faults is the variation of rheology with depth brittle failure at shallow depths produces less fault rotation than does distributed creep in the lower part of the crust an important geometric feature of normal faulting is the uplift of the footwall the amount of such uplift is related not only to the elastic properties of the lithosphere but also to the throw and dip of the fault a striking feature of active normal faults is that they occur in groups in which all the faults dip in the same direction this behaviour arises because the faults cannot intersect if they do one must cease to be active the rotation which such fault systems produce reduces the dip of the faults until a new steeply dipping fault is formed once a new fault cuts pre existing faults the earlier faults become locked and a new set of faults must propagate rapidly across the whole region involved many of these geometric constraints also apply to thrust faulting

scaling kinematics and evolution of a polymodal fault system hail creek mine ne australia we analyse a system of normal faults that cut sandstones siltstones mudstone coal and tuff at hail creek coal mine in the bowen basin ne australia our detailed mapping utilized the dense borehole network and strip mining operations the fault surfaces have complex geometries yet the components of the individual faults show similar orientation variability to the whole fault system the faults and their components dip to the se nw nnw and sse with an orthorhombic symmetry that we refer to as polymodal there are multiple displacement peaks with complementary changes on adjacent faults this observation suggests kinematic coherence between neighbouring faults twin displacement peaks on some faults suggest that segment linkage occurred on a scale of hundreds of m these polymodal faults follow the same displacement length scaling laws as other normal faults fault dip is affected by lithology with steeper dips in more competent sandstone beds an odd axis construction using whole fault planes suggests that they formed in a triaxial strain state three different principal strains with vertical shortening and horizontal extension along principal directions of 148 and 058 odd axis constructions using individual fault components as opposed to whole faults give similar principal strain orientations and maximum strain ratios the variable component orientations and the consistency of fault kinematics on different scales suggest that the faults evolved by the propagation or linkage of smaller components with variable orientations within the same bulk strain state

the growth of geological structures by repeated earthquakes 1 conceptual framework in many places earthquakes with similar characteristics have been shown to recur if this is common then relatively small deformations associated with individual earthquake cycles should accumulate over time to create geological structures following this paradigm we show that existing models developed to describe leveling line changes associated with the seismic cycle can be adapted to explain geological features associated with a fault in these models an elastic layer containing the fault overlies a viscous half space with a different density fault motion associated with an earthquake results in immediate deformation followed by a long period of readjustment as stresses relax in the viscous layer and isostatic equilibrium is restored deformation is also caused as a result of the loading and unloading due to sediment deposition and erosion in this paper the parameters that control the growth of dip slip structures are identified we find that the flexural rigidity of the crust or the apparent elastic thickness provides the main control of the width of a structure the loading due to erosion and deposition of sediment determines the ratio of uplift to subsidence between the two sides of the fault the flexure due to sediment load is much more important in this respect than whether the fault is normal or reverse in character we find that in general real structures are associated with apparent elastic thicknesses of 4 km or less and thus with very low flexural rigidities

balanced cross section construction from seismic sections in areas of extensional tectonics abstract the application of the technique of balanced section construction initially developed for areas of compressional folding and faulting is reviewed with reference to extensional tectonics a number of examples are discussed where these techniques have been successfully applied in the north sea the interpretation of geoseismic sections is considered to be greatly assisted by careful application of geometrical balance and a consideration of strain even in areas of low crustal extensions the nature of seismic sections however places limitations on the validity of balancing which must be borne in mind with such interpretations and wherever possible the balancing of a geoseismic section should be confirmed by complete depth conversion the rapid testing of the integrity of the geoseismic section by attempting to balance the section at the interpretational phase can eliminate many problems as well as allowing the fullest use to be made of the geophysical information

carbon isotope composition of lower palaeozoic kerogen effects of maturation abstract the carbon isotope composition of type i kerogen from a marine organic rich black shale deposit of late cambrian age the alum shale formation of southern scandinavia varies from 28 to 30 pdb the thermal maturity is rated by hi oi index t max pi and vitrinite reflectance values and published conodont alteration data the results range from immature conditions with regard to oil generation in central sweden to overmature and submetamorphic conditions in southern sweden denmark and norway in the immature areas the shale is found to be extremely hydrogen rich and oil prone hydrocarbon potential up to 85 kg per ton rock the observed 13 variations are not correlated with maturity it is therefore concluded that thermal maturation processes have no influence on the isotopic composition of a purely marine kerogen the observed range in lower palaeozoic 13 kerogen values 27 to 31 is in accordance with published results from other sedimentary basins

seismic imaging of late palaeozoic early mesozoic extensional and contractional structures in the bowen and surat basins eastern australia abstract the bowen and surat basins in eastern australia form part of a sedimentary basin system that was initiated in the latest carboniferous or earliest permian the basin system developed in a back arc plate tectonic setting and has had a complex evolutionary history with early phases of normal and oblique extension being followed by later phases of contraction and transpression the geometries of the phases are best observed in selected industry seismic data and in long regional bmr deep seismic reflection profiles recorded across the basins in 1984 and 1989 the denison trough along the western margin of the bowen basin was initiated as a series of extensional half grabens bounded by north to north northwest striking faults the polarity of the half grabens are relatively consistent along strike with the bounding faults being mainly on the western side of the haif grabens farther east the taroom trough is the main depositional centre in and around seismic line bmr 84 14 the taroom trough also displays a half graben geometry but only a very limited amount of horizontal extension 3 3 km maximum can be accommodated on the steep bounding fault at its eastern margin this trough developed by oblique extension with the bounding fault having a significant strike slip component reactivation of the bounding faults in the denison trough in the late permian to middle triassic led to tectonic inversion and the development of contractional structures at the same time to the east of the denison trough in the central bowen basin major shortening was accommodated by thin skinned thrusting on a series of listric thrust faults that dip to the east these thrusts root in a major east dipping detachment that appears to flatten in the middle crust in and around seismic line bmr 84 14 the zone of deformation is extremely localised confined to near the basin bounding faults and the structures are interpreted to result from transpression resulting from reactivation of the strike slip faults thus seismic data from the bowen and surat basins provide excellent images of early normal and oblique extensional structures and also of the later contractional structures in the denison trough and central bowen basin and the transpressional structures associated with the major bounding faults in the east

stable isotope and organo geochemical studies on phanerozoic sediments of the williston basin north america abstract carbon isotope studies have been carried out on marine phanerozoic sediments of the williston basin north america special attention was given to the recognition of systematic variations with age 13c measurements on the carbonate fraction show a 3 0 depletion from the cambrian to the mississippian and a 1 2 0 enrichment to the jurassic both the sign of these displacements and a good correlation with the organic carbon content suggest that real variations in the rate of photosynthesis may have been the driving force for these changes with age because of the large isotopic heterogeneities in the organic biomass systematic fluctuations with age in the 13c values of the total organic carbon record are hard to prove nevertheless a significant 3 0 depletion was observed from the devonian to the end of the carboniferous this depletion seems to occur on a worldwide scale it is therefore proposed that a rise in the rate of photosynthesis as a consequence of the colonization of the continents by higher plants is the global cause of this displacement in the isotope record of the total organic carbon the bitumen fraction does not show any systematic isotope variations with age comparison of coexisting 13corg 13cbit pairs demonstrates that during the ordovician mississippian time period no significant 1 0 differences in the isotope values of these two organic carbon sources can be observed in contrast the bitumen fraction is depleted in 13c as compared to the total organic carbon in the cambrian as well as in the pennsylvanian cretaceous time interval several parameters i e the depth of burial of these sediments the 1 2 pristane n c17 phytane n c18 ratio and the n alkane distribution pattern support the theory that the rank of the organic matter may be responsible for the observed isotopic pattern

review of recent results from continental deep seismic profiling in australia abstract the australian geological survey organisation regularly collects 450 500 km of onshore deep seismic reflection data and up to 4500 km offshore each year in australia these recordings are made in a wide range of tectonic provinces including in the last few years late palaeozoic mesozoic intracontinental and palaeozoic mesozoic cenozoic continental margin extensional basins moderately deformed palaeozoic transtensional basins and compressional fold belts and archaean greenstone terranes several of these provinces are major petroleum exploration provinces whereas others contain significant mineral deposits the primary purpose of the deep seismic profiling program is to resolve the tectonic history of the australian continent and thereby to encourage exploration for hydrocarbons and mineral resources in australia on the northwest australian continental margin major basin systems including the bonaparte basin formed as a result of complex interactions since the carboniferous involving episodes of extension followed by strike slip movements and inversion which reactivated both the primary extensional and ancient basement structures off southeastern australia basins such as the gippsland basin formed as part of a linked transtensional system related to movement on a common mid crustal detachment complex on continental australia the bowen basin in the northeast was deformed by thrust faults that root in a major e dipping detachment that flattens in the middle crust the cobar basin in the southeast is a case where the seismic data support a detachment model in which the upper plate displacement vector can be calculated by plate reconstructions linking the geometry of the detachment surface with that of the basin the greenstone terranes within the eastern goldfields region of western australia show crustal scale fault systems that are planar and steep dipping more in keeping with those interpreted in data from other precambrian provinces rather than those of the palaeozoic provinces

thermal alteration of cretaceous black shale by diabase intrusions in the eastern atlantic ii effects on bitumen and kerogen abstract the thermal effects of three one major and two minor miocene diabase intrusions on cretaceous black shales from dsdp site 41 368 have been analyzed a concentration gradient was observed especially for the hydrocarbons decreasing towards the major intrusion and between the three sills the thermally altered samples in the proximity of and between the sills contained elemental sulfur and an excess of thermally derived pristane over phytane whereas the unaltered sediments contained no elemental sulfur and more phytane than pristane a maximum yield of the extractable hydrocarbons was observed at a depth of 7 m below the major sill two classes of molecular markers were present in this bitumen suite the first was sesqui di and triterpenoids and steranes which could be correlated with both terrigenous and autochthonous sources they were geologically mature and showed no significant changes due to the thermal stress the second class was found in the altered samples which contained only polynuclear aromatic hydrocarbons with low alkyl substitution and sulfur and oxygen heterocyclic aromatic compounds these compounds were derived from pyrolytic reactions during the thermal event kerogen was isolated from all of these samples but only traces of humic substances were present the h c n c 13 c 34 s and d all exhibit the expected effects of thermal stress the kerogen becomes more aromatized and richer in 13c 34s and d in the proximity of and between the sills maturation trends were also measured by the vitrinite reflectance and electron spin resonance where the thermal stress could be correlated with an elevated country rock temperature and an increased degree of aromaticity the effects of in situ thermal stress on the organic rich shales resulted in the generation and expulsion of petroliferous material from the vicinity of the sills

faunal evidence for oceanic separations in the palaeozoic of britain the faunal criteria for the recognition of the relative separation of old continents are 1 the contrast between shallow not deep water faunas off opposite shorelines 2 the recognition of deeper water facies surrounding the edges of continents and 3 the recognition of the disposition of ancient climatic belts these criteria are defined and used to assess the relative position of the various parts of what is now britain in the palaeozoic the iapetus ocean is well characterized by faunal differences from the cambrian until the lower ordovician but from caradoc times onwards these differences dwindle until by the latest silurian only the distribution of the ostracods separates the two sides of the ocean on faunal grounds in the early ordovician s british faunas are comparable with those from bohemia france and elsewhere to the s indicating connection with gondwanaland and these faunas differ from those in the baltic area a distribution which we attribute to a true ocean tornquist9s sea between n and s europe tornquist9s sea probably closed in the upper ordovician and from ashgill times onwards the s british and baltic faunas remained similar during the middle and late silurian there is evidence of increasing faunal distinction between britain the baltic area and north america on the one hand and southern europe and gondwanaland on the other indicating an opening rheic ocean which appears to have persisted until late carboniferous times

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

effects of thermal maturation on stable organic carbon isotopes as determined by hydrous pyrolysis of woodford shale acquiring crude oils that have been expelled from the same rock unit at different levels of thermal maturation is currently not feasible in the natural system this prevents direct correlation of compositional changes between the organic matter retained in a source rock and its expelled crude oil at different levels of thermal maturation alleviation of this deficiency in studying the natural system requires the use of laboratory experiments natural generation of petroleum from amorphous type ii kerogen in the woodford shale may be simulated by hydrous pyrolysis which involves heating crushed rock in contact with water at subcritical temperatures 374 c four distinct stages of petroleum generation are observed from this type of pyrolysis 1 pre oil generation 2 incipient oil generation 3 primary oil generation and 4 post oil generation the effects of thermal maturation on the 13c values of kerogen bitumen and expelled oil like pyrolysate from the woodford shale have been studied through these four stages of petroleum generation similar to the natural system the kerogens isolated from the pyrolyzed rock showed no significant change in 13c this suggests that the 13c value of kerogens may be useful in kerogen typing and oil to source rock correlations 13c values of bitumens extracted from the pyrolyzed rock showed an initial decrease during the incipient oil generation stage followed by depletion during the primary and post oil generation stages this reversal is not favorable for geochemical correlation or maturity evaluation saturated and polar components of the bitumen show the greatest 13c variations with increasing thermal maturation the difference between the 13c of these two components gives a unidirectional trend that serves as a general indicator of thermal maturation and is referred to as the bitumen isotope index bii 13c values of the expelled pyrolysates show a unidirectional increase with increasing thermal maturation the constancy and similarity of 13c values of the aromatic components in the expelled pyrolysates and bitumens with increasing thermal maturation encourages their use in oil to oil and oil to source rock correlations isotopic type curves for expelled pyrolysates indicate that they may be useful in oil to oil correlations but have a limited use in oil to source rock correlations

determination of kerogen maturity by the pyrolysis carbon isotope method abstract the premise of the proposed maturation procedure is that immediately after post depositional microbial alteration of organic matter there is a degradable but not renewable pool of non extractable methane precursor carbon that becomes smaller as methane is produced with increasing time and temperature in addition the methane progressively becomes isotopically heavier because of mass balance and bonding considerations a maturity index based on the isotopic difference between the total accumulated methane produced by pyrolysis and the kerogen carbon and or the mole ratio of methane to kerogen carbon is presented published data by chung and sackett geochim cosmochim acta43 1979 1988 1979 for a suite of 19 coals and 12 shales of various ranks and maturities support this procedure

the hsi bright reflector further evidence for extensive magmatism in the precambrian of western canada abstract seismic reflection lines acquired in 1995 as part of lithoprobe canada s national geoscientific research program revealed unusually bright reflectors of extensive lateral scale in the archaean medicine hat block of southern alberta the herein named head smashed in hsi reflector is imaged over an area of at least 6000 km 2 in the upper half of the crust the reflector appears as a mostly horizontal to subhorizontal feature although continuous it has a complex and laterally variable internal structure comparison with a reflector of known impedance contrast in the overlying western canadian sedimentary basin indicates a high impedance structure to cause the reflections dense one dimensional modelling is consistent with the reflector being a multilayered structure with layer thicknesses in the range of tens to hundreds of metres and lateral scales of discrete reflecting elements between tens of metres and hundreds of metres polarity constraints overall geometry and reflection character strongly suggest that mafic ultramafic horizontal sheets are the cause of hsi reflectivity the hsi is the third major sequence of strong crustal reflectivity imaged by lithoprobe deep seismic data in the precambrian crust of western canada all three features are very similar in overall geometry reflector strength and character and have been interpreted as horizontal sheetlike intrusions small scale variations of the reflectivity signatures suggest a pronounced rheological heterogeneity in the upper crustal rocks within the host tectonic domains