the bagdad reflection sequence as tabular mafic intrusions evidence from seismic modeling of mapped exposures two dimensional seismic modeling of exposed rocks implicates subhorizontal tabular mafic intrusions as the likely cause of the unusually prominent reflections that constitute the bagdad reflection sequence brs these basement reflections from depths of 3 15 km are observed on cocorp and other data to extend for some 100 km across the basin and range colorado plateau transition zone in west central arizona the nearby buck mountains fortuitously expose an upended upper crustal block which is used in this study as the basis for a 12 x 6 km seismic model ray tracing through this model produces synthetic reflections from the numerous proterozoic diabase intrusions occurring in the buck mountains these synthetic reflections approximate the scale geometry and character of reflections observed on the cocorp data indicating that the intrusions in the buck mountains furnish a viable geologic analogy for the brs several lines of evidence argue that the major competing hypothesis that tertiary detachment surfaces cause the brs is less satisfactory although we cannot rule out the possibility that the brs is related to hypothetical tertiary intrusions of similar distribution and structure geologic evidence more directly implies that these reflectors are proterozoic in age if so the bagdad reflections serve as strain markers and can be used to constrain subsequent deformation in the arizona transition zone their relatively continuous nature thus argues against widespread disruption of the upper crust in the transition zone during tertiary extension moreover identification of these reflectors as subhorizontal mafic intrusions may have profound consequences for interpretation of similar reflections observed elsewhere by bolstering the contention that mafic intrusions are an important cause of basement reflections in the continental crust

interactive gravity inversion we have developed a new approach for estimating the location and geometry of several density anomalies that give rise to a complex interfering gravity field the user interactively defines the assumed outline of the true gravity sources in terms of points and line segments and the method estimates sources closest to the specified outline to achieve a match between the predicted and observed gravity fields each gravity source is assumed to be a homogeneous body with a known density contrast different density contrasts may be assigned to each source tests with synthetic data show that the method can be of use in estimating 1 multiple laterally adjacent and closely situated gravity sources 2 single gravity sources consisting of several homogeneous compartments with different density contrasts and 3 two gravity sources with different density contrasts of the same sign one totally enclosed by the other the method is also applied to three different sets of field data where the gravity sources belong to the same categories established in the tests with synthetic data the method produces solutions consistent with the known geologic attributes of the gravity sources illustrating its potential practicality

evidence and implications of an extensive early mesozoic rift basin and basalt diabase sequence beneath the southeast coastal plain regional seismic reflection profiles across the georgia and south carolina coastal plain provide a detailed view into the internal structure and stratigraphy of the buried triassic jurassic south georgia basin reflection data indicate that the basin is a complex composite of smaller individual basins which vary drastically along strike these basins are relatively wide along north south profile maximum 100 km but appear to be more restricted along strike in cross section the basins are asymmetric being typically bounded on their south sides by an interpreted north dipping master normal falult the thickness of interpreted basin fill below the cretaceous tertiary overlap also varies drastically locally reaching 6 km analysis and correlation of the reflection profiles when integrated with drill hole and offshore reflection data suggest the presence of an areally extensive basalt flow and or diabase sill sequence of probable early middle jurassic age covering a large part of the triassic jurassic south georgia basin the sequence is traceable on seismic reflection profiles across much of the south georgia basin and may extend as far as from western georgia to offshore south carolina the implied areal extent of this sequence would be more than 100 000 km 2 ranking it among the great basalt flows and or sills of the world on seismic reflection profiles over the south georgia basin the basalt diabase sequence is also important in marking a post rift unconformity which separates tilted and apparently faulted lower mesozoic rift related strata below from a thinner more widespread and generally unfaulted sequence above this configuration is interpreted to mean that basalt extrusion and or hypabyssal intrusion occurred after the main episode of late triassic early jurassic normal faulting and associated localized subsidence syn rift phase and prior to a later jurassic episode of more widespread uniform subsidence post rift phase accompanied by relatively minor normal faulting this two stage evolutionary sequence is analogous to that of many rift basins throughout the world and suggests a hypothesis that the south georgia basin was an area of incipient crustal spreading associated with the early formation of the central atlantic ocean

compositional variation and the origin of deep crustal reflections abstract identifying the sources of crustal reflections is essential for deriving more geological information from deep crustal reflection profiles theoretical and model studies help place constraints on the role of compositional variation in producing deep crustal reflections analysis of laboratory derived velocities and densities of rock types typical for the continental crust indicates that reflection coefficients are generally small but significantly 17 of the possible reflection coefficients have magnitudes between 0 1 and 0 2 comparison between reflection coefficients derived from well logs and reflections observed in associated seismic profiles indicates that constructive interference associated with geological layering is at least as important as the magnitude of the reflection coefficients for producing detectable reflections constructive interference can increase reflection amplitude by two to three times but is limited to a relatively narrow range of layer thickness for a typical 10 40 hz seismic wavelet and typical crustal velocities of about 6 km s constructive interference occurs for layer thickness ranging between about 35 and 80 m layers thinner than 35 m interfere destructively if reflections result from compositional variation seismic models of hypothetical and observed geologic relations provide analogs for interpreting complex reflection patterns observed in deep crustal reflection profiles such models show reflection patterns similar to those observed in the reflection profiles the models indicate that the reflections could originate in the complexly deformed and intruded terranes that are common in the crystalline crust and it may not be necessary to appeal to unobserved phenomena such as special lamellae or fluid filled fractures to explain the reflections

a study of two mid crustal bright spots from southeast georgia usa summary two high amplitude reflections or bright spots occur on cocorp consortium for continental reflection profiling line georgia 16 at a record time of 5 9s corresponding to nearly 16 km depth they are significant because bright spots often mark great contrasts in subsurface physical properties the first is the surrency bright spot sbs an exceptionally bright antiformal reflection and the second is the reedy creek reflection rcr which lies 5 km to the south and resembles the south half of a diffraction the rcr is also imaged on a crossline georgia 18 as a relatively symmetric antiformal reflection these reflections coincide with the brunswick magnetic anomaly thought to mark the late palaeozoic alleghanian suture the sbs and the rcr are reflections from bodies that lie within the plane of georgia 16 migration and 2 d seismic modelling show the aniformal sbs is from a synformal reflector with a buried focus while the rcr is from a relatively small body the inferred geometry of the sbs reflector with its flat portion corresponding to the deepest part of the reflector argues that it does not represent fluids these results combined with recent evidence that the sbs is a reflection from a high velocity thin layer suggest that the sbs and the rcr might be reflections from ultramafic slices emplaced within the alleghanian suture zone during collision or from mafic sheets intruded into the suture zone during subsequent rifting the brightness of the sbs is attributed to a large impedance contrast constructive interference good data quality and a relatively large reflector size with respect to the fresnel zone and neighbouring reflectors

crustal extension and magmatic processes cocorp profiles from death valley and the rio grande rift new crustal scale information on the interaction between normal faulting and magmatic activity is provided by recent cocorp deep seismic reflection profiles in death valley california and by reprocessing of the cocorp data from the socorro area of the rio grande rift new mexico the most striking feature on the seismic sections from these areas is a prominent subhorizontal series of reflectors at mid crustal depth previous studies have suggested that thin tabular magma bodies lie within the mid crustal reflective zones in addition because they are traced without apparent offset beneath faults both mapped at the surface and interpreted from the cocorp data these mid crustal horizons are here inferred to be detachments or zones of tectonic decoupling upper crustal cenozoic faults do not appear to penetrate deeper than 15 km in death valley and 13 km in the rio grande rift in death valley these faults are relatively planar with moderate dips 20 to 35 and appear to bound large basement rocks one such zone of normal faults can be traced from the magma body inferred at 15 km depth beneath central death valley to the surface location of a 690 000 yr old basaltic cinder cone listric and low to moderate angle normal faults are evident on the reprocessed new mexico data and constitute the structural component of upper crustal extension in particular a listric master fault traceable to depths as great as 13 km is inferred to underlie the albuquerque basin unlike the death valley data no faults are observed to merge with the socorro magma body per se rather subhorizontal to moderately west dipping packages of reflections are imaged between the base of the faulted upper crust 13 km depth and the mid crustal magma body about 20 km depth the middle crust marks a major rheological boundary between the faulted upper crust and a ductile lower crust extending by penetrative flow and intrusion events seen in the middle and lower crust are generally subhorizontal and prominent layering is observed a band of reflections attributed to the crust mantle boundary is evident on most seismic sections the upper mantle appears seismically transparent on some of the profiles the events attributed to the base of the crust are the deepest in a series of strong and continuous reflections at least one of which is a layer of magma this association supports the suggestion that magmatic intrusions are a probable cause for the high reflectivity observed in the deep crust of many extensional terranes

grain boundary graphite in rocks and implications for high electrical conductivity in the lower crust the origin of zones of high electrical conductivity in the lower continental crust is a long standing mystery possible explanations include the presence of brines1 2 partial melt3 serpentine4 and graphite5 when discussing the occurrence of graphite in the crust many petrologists have considered phase relations as they would have existed at the peak of metamorphism or during igneous emplacement6 10 here we show that a fine film of graphite is present on the grain boundaries in three rocks from the laramie anorthosite complex in two of these rocks graphite was stable during igneous crystallization but in the other it was not we maintain that in all of the rocks the grain boundary graphite precipitated from a co2 rich fluid during cooling the chemical processes that produced the grain boundary graphite in these rocks are likely to operate in many lower crustal rocks we therefore contend that because the films we observe are capable of producing the high conductivity that is seen in the lower crust grain boundary graphite should be considered as a possible cause for some conductivity anomalies

reflection polarity of the midcrustal surrency bright spot beneath southeastern georgia testing the fluid hypothesis a small reflection seismic experiment with an explosive source was conducted in southeastern georgia to determine the polarity of an unusually strong midcrustal reflector the surrency bright spot sbs which was found at a depth of approximately 16 km during earlier cocorp profiling in the region in addition to being very bright the sbs is notable for being unusually flat and horizontal for about half of its 4 km length as these characteristics are similar to those of fluid caused reflections at shallow depths it has been suggested that the sbs may be caused by in situ midcrustal fluids if caused by fluid enclosed in fracture porosity in solid rock the reflection would be expected to exhibit a negative polarity from the top of the porous zone the vibroseis data however gave ambiguous results with regard to polarity due to the limited bandwidth and inherent uncertainties about the phase of the source signal the new experiment consisted of four dynamite shots each recorded at three receiver stations by seismic group recorders sgr borrowed from amoco production company comparison of the dynamite records with geophone polarity tests indicate that the sbs is characterized by a positive reflection coefficient at its top this result itself does not negate the fluid hypothesis a fluid fluid interface could cause the positive reflection as well as the flat spot nature of the reflector waveform modeling shows that the sbs is caused by two or more thinly spaced reflectors a relatively high impedance layer about 120 m thick provided a good match to the observed dynamite data but requires a lower boundary having a slightly smaller reflection coefficient than the upper boundary 0 7 versus 1 0 a 150 m thick porous zone model also provided a relatively good fit to the observed dynamite data but in this model the polarity test requires that the initial sbs reflection of positive polarity be caused by a fluid fluid interface within the porous zone and that the top of the porous zone be relatively non reflective though these observations do not preclude the fluid hypothesis they certainly make the high impedance model the simpler of the two alternatives presented here the additional constraints imposed by the modeling also suggest that the sbs is more complex than these simple two interface models in either of these cases wavelet tuning contributes in part to the unusually large amplitude of the sbs reflection

direct hydrocarbon detection using comparative p wave and s wave seismic sections two field experiments in the sacramento basin and one in the green river basin demonstrate that comparative p wave and s wave cdp seismic sections can be used to detect gas directly in sandstone reservoirs the lines in the sacramento basin were shot over producing gas fields known to correlate with amplitude anomalies on p wave sections reflections of comparable strength are present on the p wave and s wave sections at lithologic boundaries but gas saturated zones correlating with p wave bright spots show no equivalent s wave amplitude anomalies the responses are consistent with laboratory observations that p wave velocity is more sensitive to the introduction of gas into liquid saturated pore space than s wave velocity the line in the green river basin was shot over a relatively deep gas field producing from an overpressured reservoir not associated with a conventional p wave bright spot the p wave reflection strength of the reservoir is about 50 percent greater than the s wave reflection strength

development of the hatton rockall basin north east atlantic ocean abstract magnetic anomaly and bouguer gravity anomaly maps have been prepared over the hatton rockall basin in the north east atlantic a qualitative interpretation of these maps is presented two cross sectional models have been constructed along lines perpendicular to the axis of the basin the north eastern part of the hatton rockall basin is characterised by circular intrusions and has evolved differently from the south western part which is characterised by linear intrusions that reach the base of the sedimentary sequence it is proposed that the two parts of the basin evolved initially along similar lines and that the south western part of the basin evolved further with a period of extension at the same time as extension between endymion spur and hatton bank

layers and bright spots seismic amplitudes are affected by both geometric and lithologic features of reflecting layers observation of bright spots is a function of not only pore fluid saturations but also of layer thickness and surrounding rock type the numerical model developed uses a finite sum of reflected and mode converted waves to evaluate the application of wave analysis in determining pore fluid types amplitude analysis alone is not sufficient to define realistically pore fluid type but when amplitude analysis is combined with phase and mode converted shear wave behavior it is possible to differentiate between pore fluid types various configurations which lead to bright spots including coal and limestone layers are evaluated an algorithm is developed based on the numerical examples which can differentiate gas zones from other bright spot data thickness of the plane layer is an important factor in defining the overall reflectivity due to interference of the reflected wave components gas saturation of a lay

generalized compact gravity inversion extending the compact gravity inversion technique by incorporating a priori information about the maximum compactness of the anomalous sources along several axes provides versatility thus the method may also incorporate information about limits in the axes lengths or greater concentration of mass along one or more directions the judicious combination of different constraints on the anomalous mass distribution allows the introduction of several kinds of a priori information about the arbitrary shape of the sources this method is particularly applicable to constant linear density sources such as mineralizations along faults and intruded sills dikes and laccoliths in a sedimentary basin the correct source density must be known with a maximum uncertainty of 40 percent otherwise the inversion produces thicker bodies for densities smaller than the true value and vice versa because of the limitations of the inverse gravity problem the proposed technique requires an empirical technique to analyze the sensitivity of solutions to uncertainties in the a priori information the proposed technique is based on a finite number of acceptable solutions presumably representative of the ambiguity region by using standard statistical techniques each parameter is assigned a coefficient measuring its uncertainty the known hematite and magnetite ore body shape in the vicinity of iron mountain mo was reproduced quite well using this inversion technique

reconstruction of geologic bodies in depth associated with a sedimentary basin using gravity and magnetic data summary we present a comprehensive review of the most comm on gravity and magnetic interpretation methods to in depth retrieval of the geometry of two types of geologic bodies associated with a sedimentary basin 1 sedimentary basement relief and 2 salt bodies in reconstructing a basement topography we identify three groups of methods the automatic the spectral and the nonspectral methods the reconstruction of salt bodies geometries from gravity data usually uses the interactive forward modeling but recently gravity inversion methods have been developed to interpret this kind of geological environment

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

geophysical data reveal the crustal structure of the alaska range orogen within the aftershock zone of the mw 7 9 denali fault earthquake geophysical information including deep crustal seismic reflection magnetotelluric mt gravity and magnetic data cross the aftershock zone of the 3 november 2002 mw 7 9 denali fault earthquake these data and aftershock seis micity jointly interpreted reveal the crustal structure of the right lateral slip denali fault and the eastern alaska range orogen as well as the relationship between this structure and seismicity north of the denali fault strong seismic reflections from within the alaska range orogen show features that dip as steeply as 25 north and extend downward to depths between 20 and 25 km these reflections reveal crustal structures probably ductile shear zones that most likely formed during the late cretaceous but these structures appear to be inactive having produced little seis micity during the past 20 years furthermore seismic reflections mainly dip north whereas alignments in aftershock hypocenters dip south the denali fault is nonre flective but modeling of mt gravity and magnetic data suggests that the denali fault dips steeply to vertically however in an alternative structural model the denali fault is defined by one of the reflection bands that dips to the north and flattens into the middle crust of the alaska range orogen modeling of mt data indicates a rock body having low electrical resistivity 10 xm that lies mainly at depths greater than 10 km directly beneath aftershocks of the denali fault earthquake the maxi mum depth of aftershocks along the denali fault is 10 km this shallow depth may arise from a higher than normal geothermal gradient alternatively the low electrical resistivity of deep rocks along the denali fault may be associated with fluids that have weakened the lower crust and helped determine the depth extent of the after shock zone

comparison of p and s wave seismic data a new method for detecting gas reservoirs compressional waves are sensitive to the type of pore fluid within rocks but shear waves are only slightly affected by changes in fluid type this suggests that a comparison of compressional and shear wave seismic data recorded over a prospect may allow an interpreter to discriminate between gas related anomalies and those related to lithology this case study documents that where a compressional wave bright spot or other direct hydrocarbon indicator is present such a comparison can be used to verify the presence of gas in practice the technique can only be used for a qualitative evaluation however future improvement of shear wave data quality may enable the use of more quantitative methods as well

seismic reflectors conductivity water and stress in the continental crust the uppermost 10 15 km of the earth s continental crust differs in several geophysical properties from the lower crust the upper crust is electrically resistive seismically transparent contains nearly all intracontinental earthquake hypocentres and responds to stress elastically with brittle fracture the lower crust is electrically conductive contains many seismic reflectors is aseismic and shows ductile response to stress i show here that the characteristics of both regions can be explained if the entire crust contains saline water in separated cavities in the compressively stressed rocks of the upper crust but in the lower crust forming an interconnected film on the crystal surfaces the model may apply principally to technically active parts of the continental crust beneath shields the lower crust may be dry

a focused and constrained 2d inversion of potential field geophysical data through delaunay triangulation a case study for iron bearing targeting at the shavaz deposit in iran abstract this work illustrates the performance of a focused algorithm for 2d inversion of potential field geophysics gravity and magnetic in regions with rugged topography a non structured model discretization mesh is performed through delaunay triangular cells to accurately represent the rugged observation surface and recover complex shaped sources several synthetic single and multi source targets are simulated to evaluate the applicability of the focused algorithm for simultaneously preserving the edges correct depth estimation and body distribution of the inverted models the algorithm is written in such a way that it is able to improve the modeling result per iteration by imposing physical properties constraining magnetic susceptibility or density contrast depth weighting and focused term in the central system of linear equation whereby a preconditioned conjugate gradient scheme is used iteratively to solve efficiently the l2 norm cost function comprising of a data misfit norm and a model stabilizer potential field data pertaining to a real case study for iron bearing targeting are also inverted along two profiles to investigate the geometry of the sought targets at the shavaz deposit in iran iron oxide ore occurrence was approved by exploratory drilling indicating a shallow hematite target with minor compounds of magnetite of note is that the significant point of this study lies in accurate recovering of causative sources responsible for potential field anomaly when a severe undulating topography occurs

seismic identification of basement reflectors the bagdad reflection sequence in the basin and range province colorado plateau transition zone arizona although seismic reflection profiling has contributed greatly to our knowledge of the crust and upper mantle it is often difficult to unambiguously identify the geologic origin of reflections this is particularly true of intrabasement reflectors whose geometry and origin we need to know in order to understand tectonic processes operating in the crust we present synthetic seismogram modeling of recently collected p and s wave seismic reflection data which suggests that the intrabasement bagdad reflectors in west central arizona are intrusive mafic sheets the identification is based on the determination of the phase positive polarity and amplitudes of the reflected arrivals if the bagdad reflectors are cenozoic age mafic sheets they may be the product of rapid intrusion into an area of weak extension where the principal stresses are approximately equal

crustal bright spots and anisotropy from multi component p and s wave measurements in southern germany abstract controlled shear wave experiments have focussed on deep crustal targets that were earlier recognized by dekorp p wave surveys in southern germany black forest upper palatinate a prominent p wave reflection bright spot at 8 km depth at the ktb deep drilling site is not seen on a comparable s wave reflection section in contrast to deeper reflections this behaviour can be explained either by purely compositional effects layer with anomalously high quartz content or by fluid accumulation reservoir in a fracture zone a 20 km deep 10 km long prominent p wave reflection in the southwestern black forest is only partially accompanied by corresponding s wave reflections the influence of the physical state of the rock e g stress related cracking in addition to possible compositional variations is suggested to explain the contrasting p and s wave reflection strength seismic anisotropy of 10 magnitude revealed by s wave splitting in vertical seismic profiling data at the ktb to more than 4 km depth is attributed to sub vertically dipping and nw striking foliation

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

densities and porosities in the oceanic crust and their variations with depth and age we have used drilling and downhole logging results from deep sea drilling project holes 395a and 418a measured properties of core samples from the seafloor and from ophiolites and seismic velocity structure models of the oceanic crust to constrain empirical statistical relationships between in situ densities porosities and seismic velocities and to estimate densities and porosities in the oceanic crust with the following principal results our review of drilling results and studies of ophiolites suggests that the velocity structure of the oceanic crust is more closely related to changes in porosity and alteration or metamorphic grade than to its igneous structure the transition from layer 2a 3 7 0 4 km s 1 to 2b 5 2 0 4 km s 1 occurs within the extrusive pile the top of layer 2c 6 2 0 2 km s 1 is associated with the top of the sheeted dike complex and with the first appearance of greenschist facies alteration products ophiolite studies have shown that the top of layer 3 occurs within the sheeted dikes where the metamorphic grade of alteration products increases from greenschist to amphibolite facies a comparison of reprocessed and corrected downhole logs from holes 395a and 418a indicates that in situ densities and fractional porosities differ by 0 2 mg m 3 and 0 07 respectively these differences are consistent with prevailing views of the progressive alteration of the upper oceanic crust statistical relationships between in situ density porosity and slowness are highly linear and indicate that the density slowness relation estimated from laboratory samples pb 3 81 0 02 6 0 0 1 s applies to in situ properties as well we obtained a similar empirical relationship for porosities from the hole 418a log data f 0 35 0 03 2 37 0 15 s r2 0 84 s e 0 015 applying these empirical models to seismic velocity structure models yields the following crustal densities and porosities layer 2 2 62 2 69 mg m 3 0 10 0 12 and layer 3 2 92 2 97 mg m 3 the estimated average density of the oceanic crust is 2 86 0 03 mg m 3 densities and porosities estimated from a seismic crustal structure model for old atlantic crust agree very well with the logging data from hole 418a suggesting that in situ densities and porosities can be estimated from seismic velocities high density and porosity gradients are characteristic of the upper oceanic crust increasing densities reflect both a progressive decrease of porosity and increasing grain density with depth based on the seismic properties of layer 2a we estimate that the layer thins at an approximate rate 20 to 45 m m y 1 while the velocity increases by 0 040 km s 1 m y 1 the corresponding rates of change of density and porosity are 0 016 0 009 mg m 3 m y 1 and 0 005 0 003 m y 1 respectively

total variation regularization for depth to basement estimate part 1 mathematical details and applications we have developed an inversion approach that estimates the basement relief of a fault bounded sedimentary basin the sedimentary pack is approximated by a grid of 3d or 2d vertical prisms juxtaposed in the horizontal directions of a right handed coordinate system the prisms thicknesses represent the depths to the basement and are the parameters to be estimated from the gravity data to obtain depth to basement estimates we introduce the total variation tv regularization as a stabilizing function this approach lets us estimate a nonsmooth basement relief because it does not penalize sharp features of the solution we have deduced a compact matrix form of the gradient vector and the hessian matrix of the approximation to the tv function that allows a regularized gauss newton minimization approach because the hessian matrix of the approximation to the tv function is ill conditioned we have modified this hessian matrix to improve its condition and to accelerate the convergence of the gauss newton algorithm tests conducted with synthetic data show that the inversion method can delineate discontinuous basements presenting large slips or sequences of small slip step faults tests on field data from the almada basin brazil and from the san jacinto graben california u s a confirm the potential of the method in detecting and locating in depth normal faults in the basement relief of a sedimentary basin

ductility contrast induced by silicification in pelitic schist of the ryoke metamorphic belt japan abstract contrasting ductility is recognized in the rocks of cretaceous ryoke metamorphic belt in iwakuni area southwest japan pelitic schist is ubiquitous in the region and differences in mineral assemblages mark increase in metamorphic grade the area has been graded as chlorite biotite zone in the north progressing into biotite and muscovite cordierite zones in the south pelitic schist near the boundary between the biotite and muscovite cordierite zones has undergone partial silicification to form whitish silicified schist layers which contain two types of quartz veins those parallel to foliation in the host rock are called schistosity concordant veins and those inclined to host rock foliation schistosity discordant veins in this study we examined the quartz structure in the silicified schist and in both types of veins to understand the ductility contrast induced by the silicification process crystallographic orientations of quartz in the veins and silicified schist rocks were studied using the scanning electron microscopy sem based electron back scatter diffraction ebsd technique quartz c axis orientations in the silicified schist are nearly random demonstrating an absence of post silicification ductile deformation quartz grains in the schistosity concordant veins have preferred c axis orientations perpendicular to the schistosity indicating ductile shortening in contrast schistosity discordant veins display distinct quartz c axis fabric than that found in the schistosity concordant veins this is because the two types of host rocks exhibit a difference in ductility during deformation the presence of deformed quartz veins in the undeformed silicified schist indicates transformation of the ductile pelitic schist into the brittle silicified schist at mid crustal levels where these rocks originate hence forming contrasting rock layers schistosity concordant veins in the biotite rich pelitic schist deformed with its host rock in a ductile manner while the schistosity discordant veins in the neighboring silicified schist were left intact silicification of the pelitic schist may have been caused by the silica rich geofluids produced by subsurface processes geofluids responsible for the occurrence of such mechanically contrasting layers mark an increase in seismic reflectivity at mid crustal depths and may be potential reflectors of seismic waves giving rise to the so called bright spots

basalts water or shear zones in the lower continental crust abstract bright sub horizontal laterally extensive multi cyclic layered reflections have now been recorded in the lower continental crust from many areas in contrast this reflection signature is rare in the continental upper crystalline crust and upper mantle and in the oceanic lithosphere with the present state of knowledge it is sensible to seek a single quantitative explanation for all such layering qualitative explanations for lower crustal layering abound and include igneous metamorphic structural and fluid related models measurements of reflection coefficients from the lower crust place strong quantitative constraints on possible models structurally induced anisotropy metamorphic layering or free fluids cannot realistically produce sufficiently bright reflections igneous layering or the juxtaposition of pre existing heterogeneities by shear zones can produce strong reflections results from crustal xenoliths helium isotope ratios palaeo geothermal gradients and chemistry of flood basalts together with numerical modelling of lithospheric extension and asthenospheric melting suggest that underplating and intrusion of the lower crust with mantle derived mafic igneous rocks is common the apparent spatial relationship between lower crustal layering and continental extension and the strength and continuity of the reflections suggest that underplating related to extension and or elevated mantle temperatures is a good explanation for lower crustal layering

the s reflector west of galicia the seismic signature of a detachment fault the so called s reflector is a group of bright continuous reflections underlying the landward tilted fault blocks of the west galicia rifted margin s has been interpreted as the brittle ductile transition the lop of an intrusion a detachment fault and the crust mantle boundary to constrain the internal structure of the reflector we have carried out detailed analyses of these reflections we compare the waveforms of the seafloor reflection and its first multiple both to determine the amplitude of the seafloor reflection and to show hat the seafloor is in effect a spike in the reflectivity series so that the seafloor reflection can be used as the far field wavelet including both source and receiver ghosts we compare he waveform of the seafloor and 5 and show that within the resolution of our data s is a reflection from a step increase in acoustic impedance this result is confirmed through complex trace analysis and in particular the determination of the apparent polarity of s and the examination of the instantaneous frequency function s is consistently positive polarity and shows no significant frequency anomaly simple modelling shows that s is very unlikely to be a reflection from a thin layer we thus conclude that s is probably a single steplike interface from the varying frequency content of the data we determine a value for the effective q between s and the seafloor and use this to assess the amplitude loss due to attenuation and scattering we use a comparison between the seafloor and the s reflection to constrain the amplitude of s and estimate a reflection coefficient for s of at least 0 2 in places decreasing landwards by analogy with structures developed in the highly extended regions of the western united states we consider that the most likely interpretation of s is as a sharp west dipping detachment fault separating a granitic upper plate from a higher velocity lower plate locally probably serpentinized mantle

imaging of deep fluids in archaean crust despite the importance of aqueous fluids in tectonic metamorphic and magmatic activity in the earth s crust few techniques are available for directly observing their distribution at depth even in stable continental crust such observations as have been made electromagnetic deep sounding which by inference1 2 images saline fluids and super deep drilling3 suggest that fluids are present in varying amounts throughout the crust in direct conflict4 with the normal petrological inference of a very dry deep crust here we report the results of an unusually high resolution electromagnetic sounding method to image the deep electrical conductivity structure below a 30 km profile in ontario which crosses archaean lower crustal material of high metamorphic grade exposed by a proterozoic thrust fault the ivanhoe lake cataclastic zone the electrical resistivity image obtained shows embedded in very resistive crystalline crust two nearly continuous sub horizontal zones of enhanced electrical conductivity at depths of 2 3 and 5 6 km below this the commonly observed2 5 7 mid crustal rise in conductivity is seen at depths of 15 20 km the image does not correlate well with the structure inferred from surface geology suggesting that the transport and distribution of fluids was not strongly controlled by lithological boundaries or the thrust fault itself

seismic constraints on the nature of lower crustal reflectors beneath the extending southern transition zone of the colorado plateau arizona events maintain a constant amplitude with offset which is most consistent with a mafic layering model one exception is a high amplitude 10 db above background event near the base of lower crustal reflectivity which abruptly decreases in amplitude at increasing offsets the event has a pronounced s wave response which along with its negative avo trend is a possible indication of the presence of fluids in the lower crust the arizona transition zone is an active but weakly extended province which causes us to discard models of lower crustal layering resulting from shearing because of the high degree of strain required to create such layers instead we favor horizontal basaltic intrusions as the primary origin of high impedance reflectors based on 1 the fact that most xenoliths in eruptive basalts of the transition zone are of mafic igneous composition 2 indications that a pulse of magmatic activity crossed the transition zone in the late tertiary period and 3 the high regional heat flow observed in the transition zone the apparent presence of fluids near the base of the reflective zone may indicate a partially molten intrusion we present a mechanism by which magma can be trapped and be induced to intrude horizontally at rheologic contrasts in extending crust

juxtaposed mesozoic diabase dike sets from the carolinas a preliminary assessment aeromagnetic anomaly maps of the north carolina piedmont provide confirming evidence of juxtaposed mesozoic diabase dike sets dikes in north carolina may be divided into four domains on the basis of dike orientations most dikes are olivine normative but there are apparent systematic differences in chemical compositions that are reflected in different orientations and ages two orientations dominate northwest and north south the northwest trending set dominates in the south carolina piedmont and the north south set extends through much of the central virginia piedmont and under the coastal plain in south carolina the north south dikes are part of a set that converges near charleston and georgetown south carolina and diverges northward into virginia preliminary information suggests that the north south dikes are younger than the more common northwest trending dikes current models proposed for stress regimes associated with the breakup of pangea can accommodate the northwest set but not the north south convergent set this convergent set may be evidence for crustal deformation above a mantle plume centered near charleston and georgetown in middle jurassic time it is possible that presumed mafic plutons under the coastal plain near charleston regional seismicity patterns and orientations of the diabase dikes are all related to an intersection of zones of crustal weakness inherited from mid mesozoic time the western margin of northwest trending dikes may be related to tectonic features inherited from pre mesozoic time

new cocorp profiling in the southeastern united states part i late paleozoic suture and mesozoic rift basin new cocorp profiling in the southeastern united states has revealed a broad zone of dipping reflections that extends downward through the crust beneath the coastal plain in western georgia the zone is over 50 km wide and most of the reflections dip moderately steeply toward the south regional considerations suggest that this feature marks the late paleozoic suture between north america and africa where crossed by the cocorp survey the suture occurs beneath the north flank of the triassic early jurassic south georgia basin the main depocenter of the south georgia basin occurs about 90 km to the south and is formed by a large half graben containing more than 5 km of rift basin fill farther south the paleozoic suwannee basin sequence beneath northern florida is poorly imaged on the cocorp profiles however weak reflections suggest that these strata including basal felsic volcanics may have an aggregate thickness of about 6 km in north central florida at the northwest end of the cocorp traverse a prominent horizon imaged in the upper crust beneath the inner piedmont probably marks the southern appalachian detachment the detachment appears to be cut off by the towaliga fault implying that the towaliga fault is in part a down to the north normal fault intermittent moho reflections occur at 11 12 s two way time along the length of the cocorp survey indicating that the crust in the region has a roughly uniform thickness of about 33 36 km assuming an average crustal velocity of 6 km s

new evidence for the existence and internal structure of a thin layer of magma at mid crustal depths near socorro new mexico the characteristics of a strong mid crustal reflector in the rio grande rift southwest of socorro new mexico were studied using high quality digital seismograms of microearthquakes the data are from a tight cluster of hypocenters located almost directly beneath the recording station 2 km cross correlation of the direct and reflected microearthquake s phases yields coefficients which are positive and large in absolute value average 0 79 because these two phases leave the focal region 180 out of phase for this swarm reflection must occur at a discontinuity where a 180 phase shift occurs waveforms for the p wave reflections are not identical from event to event within the swarm the character of the p reflection indicates a magma body with rapid lateral changes in internal structure the spectral characteristics of the reflected p phase are suggestive of constructive interference the near normal incidence geometry of both the reflected and direct p phase implies an essentially common path through the upper crust and near surface phanerozoic rocks for these two phases the earthquakes examined in this study are quite small ml 0 hence the initial portion of the direct p phase may be used as the response function for the path through the upper crust during modeling of the reflected p phase synthetic seismograms which include attenuation effects and interbed multiples were calculated and compared to the observed waveforms the results of modeling the stacked spectra of the pzp phase indicate a thin 70 m layer of nonrigid low velocity material underlain by a second thin 60 m layer of slightly higher velocity material this second layer may be an earlier partially solidified intrusion or a crystalline mush at the base of a single magma chamber the total thickness of the magma body is estimated to be 150 m

interactive 2d magnetic inversion a tool for aiding forward modeling and testing geologic hypotheses we present a method for inverting magnetic data with interfering anomalies produced by multiple complex 2d magnetic sources having arbitrary shapes and known magnetization vectors our method is stable and can recover a complex 2d magnetization distribution leading to a reliable delineation of sectionally homogeneous sources with complex shapes our method although similar to interactive forward modeling is unique in that it automatically fits the observations and only requires that the interpreter know the outlines of the sources expressed by simple geometric elements such as points and line segments each geometric element operatesasaskeletaloutlineofaparticularhomogeneoussection of the magnetic source to be reconstructed also the interpreter can define the geometric elements interactively without worrying about data fitting because data are fit automatically the examples with synthetic data illustrate the good performance of the method in mapping the complex geometryofmagneticsources thesolutionsensitivitytouncertaintiesintheaprioriinformationshowsthattoproducegoodresults the uncertainty on the magnetization intensity of each homogeneous extent of the source should be smaller than 40 awrongmagnetizationvectordirectioncanbedetected easily because it often leads to poor data fitting and to estimated sources with abrupt borders the method is also applied to two sets of real data from the northwest ore body at iron mountain mine missouri and the hatton rockall basininthenortheastatlanticocean theestimatedmagnetization distribution in all tests demonstrates a good correlation of estimated magnetic sources with corresponding known geologicfeatures

3d gravity inversion through an adaptive learning procedure we have developed a gravity inversion method to estimate a 3d density contrast distribution producing strongly interfering gravity anomalies the interpretation model consists of a grid of 3d vertical juxtaposed prisms in the horizontal and vertical directions iteratively our approach estimates the 3d density contrast distribution that fits the observed anomaly within the measurement errors and favors compact gravity sources closest to prespecified geometric elements such as axes and points this method retrieves the geometry of multiple gravity sources whose density contrasts positive and negative values are prescribed by the interpreter through the geometric element at the first iteration we set an initial interpretation model and specify the first guess geometric elements and their target density contrasts each geometric element operates as the first guess skeletal outline of the entire homogeneous gravity source or any of its homogeneous parts to be reconstructed from the second iteration on our method automatically redefines a new set of geometric elements the associated target density contrasts and a new interpretation model whose number of prisms increases with the iteration the iteration stops when the geometries of the estimated sources are invariant along successive iterations tests on synthetic data from geometrically complex bodies and on field data collected over a mafic ultramafic body and a volcanogenic sedimentary sequence located in the tocantins province brazil confirmed the potential of our method in producing a sharp image of multiple and adjacent bodies

cocorp new perspectives on the deep crust summary relict sutures from colliding continents regions characterized by a young mono layering and faulting throughout the crust mid crustal magma traps and seismic bright spots which suggest deep crustal fluids are among recent cocorp findings in addition new studies of signal penetration noise mitigation recording geometry and coherency filtering have yielded better understanding of and substantial improvements in data quality amplitude anomalies or bright spots in the basin and range may be due to magma at mid crustal levels in one case a normal fault appears to link the deep magma with young surface volcanics another bright spot 15 km deep in southeastern georgia has a flat geometry that suggests a gas liquid interface perhaps within fluids underthrust along an appalachian suture the mono continues to appear relatively undisturbed in many regions of past deformation suggesting that its formation post dates these major tectonic episodes the diversity of reflection patterns from the u s cordillera casts further doubt on the generality of the common model of a reflective layered lower crust underlying a transparent upper crust