Reflective Lower Crust Research Articles

Seismicity pattern and style of seismic faulting at the eastern borderfault of the southern Rhine Graben

Seismicity recorded on high-gain, digital networks in the southern Rhine Graben reveals that the area around the eastern borderfault system shows the highest level of seismic activity. Across this fault system remarkable differences of maximum focal depths exist: 12–13 km are characteristic within the graben, and 18–20 km beneath the western foothills of the southern Black Forest, located approximately between the eastern borderfault of the graben and the river Rhine (inner borderfault). Farther to the east, in the southern Black Forest proper, maximum depths around 20–22 km are found. At particular sites a few earthquakes with depths of up to 26 km, close to the Moho, were detected. The deep cut-off of seismic activity in the southern Black Forest and its western foothills is persistent for about 100 km distance between the Freiburg focal area in the north and the Swiss Jura in the south. Seismic reflection profiling indicates that the deep foci lie well within a reflective lower crust (southern Black Forst, Swiss Jura) or just above a highly reflective top of the lower crust (western foothills of the southern Black Forest, Dinkelberg block). Within the graben proper, seismic activity is restricted to the upper crust and ceases about 5 km above the top of the lower crust.Low seismic activity is found in areas of low P-wave velocities as in the upper crust of the central Black Forest, north of the Freiburg focal area, and at midcrustal levels in the southern Black Forest (at depths of about ±10 km).Well constrained focal mechanisms are presented for 47 events. Most of them occurred adjacent to the eastern borderfault. Nearly all mechanisms show an oblique component of slip. Strike-slip and normal-faulting mechanisms prevail. Only a few events are of thrust type. Dominance of a certain faulting style with depth was not found. Focal mechanisms of events which occur within 10 km of the borderfault show variations of faulting style, indicating local deviations from the average regional stress tensor and/or from the frictional parameters in the proximity of the particular rupture planes. Although the nodal planes correlate with the strike directions of surface trends of major faults in general, it is difficult to recognize their link to individual faults.

Ivrea seismic array: a study of continental crust and upper mantle

SUMMARY A seismic-array study of the continental crust and upper mantle in the Ivrea-Yerbano and Strona-Ceneri zones (northwestern Italy) is presented. A short-period network is used to define crustal P- and S-wave velocity models from earthquakes. The analysis of the seismic-refraction profile LOND of the CROP-ECORS project provided independent information and control on the array-data interpretation. Apparent-velocity measurements from both local and regional earthquakes, and time-term analysis are used to estimate the velocity in the lower crust and in the upper mantle. The geometry of the upper-lower crust and Moho boundaries is determined from the station delay times. We have obtained a three-layer crustal seismic model. The P-wave velocity in the upper crust, lower crust and upper mantle is 6.1±0.2 km s−1, 6.5±0.3 km s−1 and 7.8±0.3 km s−1 respectively. Pronounced low-velocity zones in the upper and lower crust are not observed. A clear change in the velocity structure between the upper and lower crust is documented, constraining the petrological interpretation of the Ivrea-type reflective lower continental crust derived from small-scale petrophysical data. Moreover, we found a VP/VS ratio of 1.69±0.04 for the upper crust and 1.82±0.08 for the lower crust and upper mantle. This is consistent with the structural and petrophysical differences between a compositionally uniform and seismically transparent upper crust and a layered and reflective lower crust. The thickness of the lower crust ranges from about 8 km in front of the Ivrea body (ARVO, Arvonio station) in the northern part of the array to a maximum of about 15 km in the southern part of the array. The lower crust reaches a minimum depth of 5 km below the PROV (Provola) station.

Reflective oceanic crust formed at a fast-spreading center in the Pacific

We present new seismic images of Cretaceous crust formed at a fast-spreading center in the Pacific. The high crustal reflectivity observed in these data contradicts the conventional wisdom that accretionary structures formed at fast-spreading centers are not seismically detectable. Subhorizontal reflections can be traced at 600–800 ms two-way time below the top of basement for tens of kilometres, suggesting the presence of a widespread seismic boundary, possibly a structural discontinuity related to the maximum depth of hydrothermal circulation at the spreading center or the base of the sheeted dikes. Lower-crustal reflections, dipping dominantly toward the paleo–spreading center, may represent mafic-ultramafic banding similar to that observed in the lower crust of reconstructed ophiolite sections.

Deep structure of the southern Kerguelen Plateau (southern Indian Ocean) from ocean bottom seismometer wide‐angle seismic data

Wide‐angle seismic data collected during the Kerguelen ocean bottom seismometer experiment provide the first images of the deep structure of the southern Kerguelen Plateau and support a new interpretation of the origin of the plateau. Velocity models based on travel time inversions and reflectivity synthetic seismograms show a 22‐km‐thick crust composed of ∼1.6 km of sedimentary cover, ∼5.3 km of upper crust, ∼11.0 km of lower crust, and a 4‐ to 6‐km‐thick reflective zone immediately above Moho. Velocities in the upper crust (from 3.8–4.5 km/s at top to 6.0–6.5 km/s at bottom) are consistent with the basaltic nature of this layer, the top of which was sampled during the Ocean Drilling Program. Velocities in the lower crust increase continuously from 6.60 km/s at the top to 6.90 km/s at 19.5 km depth. The reflective zone at the base of the crust identified by wide‐angle reflections is observed only along the NNW‐SSE direction. It consists of alternating high‐ and low‐velocity layers with an average velocity of 6.70 km/s in the NNW‐SSE direction and ≥6.90 km/s in the perpendicular direction. Strong azimuthal anisotropy is also observed in the upper mantle with velocities of 8.60 and 8.00 km/s, in the NNW‐SSE and E‐W directions, respectively. The absence of high velocities at the base of the crust that characterizes many large‐volume mafic provinces, the reflective lower crust, and anisotropy in upper mantle suggest that the southern Kerguelen Plateau represents a stretched continental fragment overlain by basaltic flows isolated from the Antarctic margin during the early opening of the Indian Ocean.

Deep penetrating MCS imaging of the rift-to-drift transition, offshore Douala and North Gabon basins, West Africa

We describe a regional grid of West African PROBE Study (WAPS) deep penetrating multi-channel seismic reflection (MCS) and potential field data crossing the transition from rifted continental crust (RCC) to normal oceanic crust (OC) in the offshore Douala and North Gabon Basins. In profiles oriented sub-parallel to oceanic fracture zones RCC is shown to terminate seaward into a very different form of crust interpreted to be composed of highly attenuated blocks of RCC, wedges of seaward-dipping reflectors representing intermixed mafic volcanics and sediments, mafic lower crust, and possibly exhumed mantle rocks, all of which form a complex faulted terrain. This heterogenous type of crust is referred to here as ‘proto-oceanic crust’ (POC). POC grades seaward into normal to thick OC. In the Douala Basin, sedimentary units deposited in an Early Cretaceous continental rift now form marginal basin highs overlain by < 1 km younger of sediment beneath the shelf. RCC here has undergone little attenuation, as more highly attenuated RCC is believed to occur below the conjugate Sergipe-Alagoas Basin in eastern Brazil. In the North Gabon Basin, RCC has been markedly thinned and resulting rift structures are deeply buried by post-rift deposits. Reflective lower crust within RCC forms discontinuous bands, where normal dip-slip faults sole into it and transform related faults crossthrough it. Interpreted Moho reflections atthe base of RCC form a discontinuous horizon that is sometimes offset at crustal deformation zones; elsewhere it transects the reflective grain of crustal deformation zones. Interpreted WAPS MCS data show that the margin is composed of rift-units separated at ca 40 km intervals by NE trending fracture zones and transfer fault zones of variable length, causing the margin to step progressively southwestward by alternating between normal and transformfaulted segments. Correlation of rift structures between these West African basins and the conjugate Sergipe-Alagoas Basin show that the two margins formed a connected rift-branch that underwent NE-SW oriented extension, ca 35° oblique to the rift axis. This orientation favoured occupation of NE-SW trending preexisting shear fabrics situated in the Late Proterozoic Pan-African/Braziliano mobile belt. At the RCC-POC boundary, wedges of seaward-dipping reflectors abut against RCC and overlie non-reflective lower crust and reflection Moho. They correlate with pronounced negative magnetic anomalies (−200 to −600 nT), probably caused by the juxtaposition of weakly magnetised RCC and more magnetic, mafic volcanic rocks of the POC. Steeply-dipping faults are interpreted to occur at transform-faulted segments, where they separate blocks of RCC from POC, or in some places OC. These NE trending transfer fault zones propagate into oceanic fracture zones (FZs) that compartmentalise and isolate both continental and oceanic rift-units. For example, the Ascension FZ is shown to intersect the African continent south of Bata, Equatorial Guinea, where it forms a 200 km long transform-faulted boundary between RCC and POC. The thickness of POC from the top of the seaward-dipping reflectors to reflection Moho ranges between 2–5 s two-way travel time (twtt). This POC probably formed by volcanic outpouring of partial melt at the time of continental rupture, and then during initial seafloor spreading. Phases of magmatic and amagmatic extension formed within rift-unit spreading cells, owing to variable widths of POC (20–200 km wide) trending parallel to flow lines before evolving seaward into normal OC. Aptial salt extends over RCC and POC, and in some places, salt has been mobilised farther seaward (<10 km) as horizontal sills emanating from salt diapirs. The northernmost known occurrence of this Aptial salt lies 100 km south of Douala, Cameroon. There is no evidence in the MCS data that salt was deposited behind structural highs rimming the margin of the salt basin. Instead, salt overlies sub-horizontal reflectors (0.2–1 s twtt thick) believed to represent sedimentary rocks that drape POC and OC of presumably Aptial age. This interpretation implies that salt in the part of West Africa was deposited in nearshore to deeper basin margin environments without restriction from central basin waters.

Crustal evolution of the Middle Urals based on seismic reflection and refraction data

Integration of seismic refraction and deep reflection data from the Middle Urals of central Russia provides important new constraints on the structure of the Uralian crust. Re-analysis of the GRANIT refraction profile and comparison with coincident reflection data from the ESRU profile shows a high-velocity (7.6–7.8 km/s) root zone from c. 45 to 55 km with low reflectivity beneath the Urals. We interpret this interval as crustal material, consistent with previous Russian interpretations of this velocity anomaly. Above this crustal root, one of the principle features imaged on the ESRU profile is a thick zone (3–4 s TWT) of relatively strong reflectivity which may characterize the lower crust of the East European platform, but is considerably shallower to the east at 8–12 s TWT (25–37 km) than in the west at 10–14 s TWT (31–43 km). Reprocessing of 20 s records from the R-17 Profile in the West Siberian basin, ∼315 km northeast of the ESRU profile, reveals a similar pronounced lower-crustal reflectivity between 11–14 s TWT (34–43 km), in the hinterland of the Middle Urals. This lower-crustal reflection fabric may represent a feature developed during collisional orogenesis, or a younger property imparted through post-orogenic extension. Future deep reflection profiling will be critical to address the continuity, and accordingly, the tectonic significance of this lower-crustal reflectivity in the Urals.

LITHOPROBE reflection studies of Archean and Proterozoic crust in Canada

LITHOPROBE, Canada's national collaborative earth science research project established to develop a comprehensive understanding of the evolution of the North American continent, is a multidisciplinary program spearheaded by seismic reflection studies. Five recently recorded seismic lines, discussed in this paper, are located in Precambrian regions: the Mesoproterozoic Grenville Province, the Paleoproterozoic Trans-Hudson orogen (THO), the Paleoproterozoic/Archean basement of Alberta, and the Archean Superior Province. Data acquired across the Grenville orogen in eastern Quebec show strong reflectivity throughout the crust; upper crustal reflections can be correlated with exposed structural elements, including extensional shear zones and packages of deformed high-pressure rocks (eclogites). In a marine survey across the Grenville orogen off southeastern Labrador, seismic images show variably dipping reflections and a structural high associated with a major gravity anomaly. Data acquired across central Alberta show crustal-scale thrust stacking and imbrication of the Archean Hearne craton. To the east across the Trans-Hudson orogen, images of similar collisional features are observed. Geochronologic constraints indicate contemporaneity of tectonic activity between the two regions at 1.8 Ga, suggesting that collisional tectonic activity was coeval over a broad crustal region, ca. 1000 km across strike. In the Superior Province, seismic data across a collision zone involving the northern Abitibi greenstone belt and the arc-related Opatica plutonic belt show spectacular crustal reflectivity and dipping reflections that extend 8 s (∼30 km) into the mantle. The latter feature is interpreted as representing a relict 2.69-Ga-old suture associated with subduction, providing the first direct evidence that plate tectonics was active in the late Archean. These five examples, supported by other LITHOPROBE results, refute a number of generalizations about crustal reflectivity that have been made in the past and illustrate how reflection studies, combined with other geoscience studies, can lead to a better understanding of Precambrian tectonics. Reflectivity persists throughout the crust; there is no general separation into a poorly reflective upper crust and a reflective lower crust. Crustal reflectivity in Archean and Proterozoic regions is as pervasive as that in areas of more recent tectonism. The Precambrian reflection Moho is generally well defined but shows a range of characteristics. Relative ages of reflectors can be discerned and tectonic significance can be attached to characteristic features of the crustal reflectivity.

The ESRU project ‐ constraints on the geometry of the Main Uralian Fault and lower crustal reflectivity in the Middle Urals

The ESRU project ‐ constraints on the geometry of the Main Uralian Fault and lower crustal reflectivity in the Middle Urals

Relative-amplitude preserving processing for crustal seismic reflection data: an example from western Canada

Most crustal-scale seismic reflection profiles are processed using methods that preserve reflection geometries but not true-relative amplitudes, and therefore do not provide direct constraints on the magnitude of impedance contrasts at major crustal boundaries, the effects of attenuation in the crust or the maximum time of signal penetration. Here we describe a new relative-amplitude preserving (RAP) processing scheme tailored for high-fold, land-based acquisition systems that can be used to provide some of these constraints. Our processing philosophy employs a statistical treatment of amplitudes, and is based on the following assumptions: (1) temporal decay of recorded amplitudes is proportional to the product of zero-offset geometrical spreading in a layered earth and attenuation based on a constant- Q model; and, (2) spatial variations in recorded amplitudes are surface-consistent. We apply this methodology to a profile from western Canada, where well logs from oil and gas exploration are available to calibrate observed seismic reflections. Using a spectral-ratio method tailored for noisy data, we obtain an estimate of ∼ 450 for apparent Q in the sub-Phanerozoic crust. Signal penetration is frequency-dependent, and is estimated to decrease from 20 to 13 s over the frequency range of 10 to 40 Hz. Application of our processing methodology leads to a stack section in which lower-crustal reflectivity is much more prominent than reflectivity in the middle crust, compared with conventional processing of the same data which yields a stack section characterized by conspicuous mid-crustal reflections. The P-wave reflection coefficient for the Moho is estimated to be about 0.1.

Reflectivity characteristics of the crust from modelling refraction/wide-angle and normal-incidence reflection data on BABEL Line 1

Marine normal-incidence reflection and P- and S-wave refraction/wide-angle reflection data from the BABEL project, line 1, in the Bothnian Sea have been used for investigating the nature of the reflectivity in the area. The data are available along the same line, and therefore the same piece of crust is probed by two seismic methods. We use this favourable circumstance for model calculations concerning both data sets by analysing the waveforms of reflections from normal-incidence traveltimes at approximately 4, 12 and 16 s TWT. Strong sub-horizontal reflections are observed on the normal-incidence reflection section at about 4 s TWT. These reflections have been modelled using the reflectivity method (1-D modelling) utilising the water bottom reflection as an estimate of the source wavelet and show a positive polarity. Thickness and P-wave velocity consistent with these reflections are 50 m and 7 km/s, respectively, with a host rock velocity of 6 km/s. From the polarity, velocity and thickness, these reflections are attributed to dolerite sills in the upper crust. Strong lower-crustal reflections are observed on the wide-angle and normal-incidence reflection sections. However, the corresponding S-wave reflections from the lower crust are considerably weaker. The implied high positive impedance contrast for the P-wave lower-crustal reflection and the weak S-wave reflectivity suggests the presence of a highly mafic sheet at about 40 km depth. The wide-angle P- and S-wave sections show strong reflections at the Moho. Coincident normal incidence data show only weak Moho reflectivity indicating a gradient in velocity at the crust/mantle boundary. Dipping events onlap the strong sub-horizontal lower crustal and the P-wave Moho reflections on the wide-angle data. These are explained by dipping structures above the lower crustal reflector and below the Moho, with the latter also consisting of a gradient in velocity.

Causes of high crustal conductivity beneath the Iapetus suture zone in Great Britain

SUMMARY Two magnetic variation anomalies in southern Scotland and northern England have been linked to the position of the Iapetus suture. Previous magnetotelluric measurements along a 140 km profile that crosses both anomalies have been supplemented by new high-quality broad-band observations. Groom-Bailey decomposition of the impedance and hypothetical event analysis of the magnetic variation data agree in defining different strike directions for the two halves of the MT profile: N50°E for the northern part over the Southern Uplands, and N90°E for the southern part over the Northumberland Trough. TE and TM mode apparent resistivity and phase data at representative frequencies have been inverted for the two sub-profiles separately using the smooth inverse of Smith & Booker (1990). The uppermost layer has a low resistivity, is variable in thickness, and correlates well with the known position and thickness of Carboniferous sedimentary rocks. The second layer has a very high resistivity (thousands of Ω m), and reaches the surface where the Lower Palaeozoic metamorphic rocks of the Southern Uplands crop out. A relatively rapid transition to low resistivities occurs at depths of between 8 and 16 km. The conducting ‘layer’ appears to be quasi-continuous, but where the profile crosses the anomalies identified by magnetic variation measurements, the conductance increases, and the upper surface is shallower. The spatial coverage of the magnetic variation data has enabled us to extrapolate conductive features away from the line of section and project the electrical image onto the NEC vertical-incidence seismic reflection profile in the North Sea. There is excellent agreement between a number of features in the acoustical and electrical images. The shallowing of the low-resistivity layer to form a narrow wedge-like feature corresponds to the offshore position of the Stublick fault, while, to the north of the fault, the top of the layer coincides with a south-dipping reflector thought to be a thrust. However, the zones of high conductance and high lower-crustal reflectivity do not in general correlate. The good conductor beneath (he Northumberland Trough spans two zones which were differentiated on ihe NEC profile in terms of their reflectivity. The shape of the conductor's upper surface in the vicinity of the Stublick fault agrees well with the model of Chadwick & Holliday (1991), who proposed that the Iapetus suture was a whole-crustal shear with a gently dipping central ramp. The coincidence between the (present-day) low-resistivity layer and a surface of weakness that was active 300–400 Myr ago is much more readily explained in terms of mineralogy (the presence of graphite) than the presence of fluids. There is also strong support for interpreting the northern conductor in the same way. Its upper surface is relatively flat and occupies the position predicted as the horizontal detachment surface, over which wedges of the Southern Uplands rocks were thrust. Its northeastward extension is sampled by a group of Carboniferous vents from which xenolith suites of crust and upper mantle origin have been obtained. The middle to upper crust is believed to be represented by quartzo-feldspathic gneisses that contain abundant graphite.

Lower-crustal deformation during terrane dispersion along strike-slip faults

After accretion, terranes caught between obliquely converging plates are often displaced several hundreds of kilometers. How this motion is accommodated at depth is not well known. Spurred by our Trans-Alaskan Crustal Transect (TACT) seismic studies of the Yukon-Tanana terrane, Alaska, I propose that the lower crust responds ductilely to accommodate differential movement between upper crust and upper mantle during margin-parallel translation. The locations of these ductile regions are dependent on temperature, composition and strain rate. For geotherms based on moderate surface heat flow (60–90 mWm −2) and reasonable geological strain rates (100% in 3 Ma), typical lower-crustal rocks can deform ductilely. The resultant subhorizontal layered fabric should be detectable with seismic techniques. Two terranes that have experienced hundreds of kilometers of margin-parallel translation, the Yukon-Tanana terrane sampled by our seismic refraction/wide-angle reflection profiles and the Salinian block (California) sampled by both seismic refraction and reflection, have reflective lower crust. Both of these terranes have moderate to high surface heat flows, have relatively thin crust, and are interpreted seismically to be underlain by a quartzo-feldspathic lower crust; conditions favorable for ductile deformation in the lower crust. Interpretations of lower-crustal layering for these two terranes is based on an increase in lower-crustal reflectivity. This layering is attributed to tectonic banding consistent with ductile deformation.

Seismic profiling of Archean crust: Crustal structure in the Morton block, Minnesota River Valley subprovince

A 58 km seismic reflection profile recorded by the University of Wyoming over the early Archean gneisses (3.6–3.8 Ga) of the Minnesota River Valley subprovince (Superior Province) shows more complicated structure and rock variation with depth than the limited surface geology suggests. Wide-angle data show a high average velocity of 6.8 ± 0.1 km/s and a crustal thickness of 49 ± 1 km from a zero-offset time of 14.2 ± 0.2 s. Upper-crustal (0–10 km) reflections are consistent with the gentle dips from surface folding, while middle-crustal (10–24 km) reflections are interpreted as a 40 km long SE dipping thrust or fold structure, 6 km thick. Lower-crustal (24–48 km) reflections are short and low amplitude, consistent with small reflection coefficients and short reflectors. Modeling suggests that this reflectivity is consistent with small (< 2 km horizontal length) lenticular, boudin-like bodies or heterogeneities within the lower crust, and/or the lower crust may be complexly deformed. At 14 s (49 km) the Moho occurs as a locally layered, 10 km long structure at the depth determined for Moho from the wide-angle data. This vertical-incidence Moho reflection demonstrates the occurrence of lateral changes in Moho structure. The seismic profile suggests a model that incorporates a thrust sheet in the middle crust, emplaced by horizontal tectonics in the Archean, and restite bodies in the deep crust, formed from the production of the presently exposed gneisses.

High-resolution images of the lower crust: deep seismic reflections from 15 to 180 Hz

We report the results of an unconventional, broad-band deep-seismic reflection survey undertaken on land in southwest England. Previous marine seismic lines in this area have shown strong layered lower-crustal reflectivity. The broad-band survey shows near-normal-incidence P-wave reflections from the lower continental crust and from the Moho with frequencies that range from 15 to 180 Hz. High-frequency lower-crustal S-wave reflections were also observed. These high frequencies were obtained by placing both sources and receivers in boreholes up to 2 km deep, drilled within a large homogeneous granite pluton. Recording down-hole avoids loss of high frequency energy during transmission through heterogeneous and highly attenuative near-surface layers. The high frequencies were generated using explosive sources, and were recorded on sub-surface accelerometers and hydrophones, and on a conventional array of surface geophones. Seismic attenuation within the granite is low; the quality factor, Q, lies above 2000. These new, high-frequency observations show that the lower crust does not preferentially reflect particular frequencies. The data are most readily explained as resulting from the combination of a frequency-independent Q in the upper crust with a white reflectivity response in the lower crust. The data are not consistent with crustal models which are highly tuned to reflect preferentially the frequencies employed in conventional deep seismic reflection surveys. The data require relatively sharp boundaries within the lower crust; significant variation in physical properties must occur over vertical distances of not more than about 10 m. Sharp boundaries are most easily explained if the reflections result from lithological contrasts.

Structure and seismic response of extended continental crust: Stochastic analysis of the Strona-Ceneri and Ivrea zones, Italy

A two-dimensional stochastic seismic model of the crustal cross section exposed in the Strona-Ceneri and Ivrea zones, northern Italy, was derived from geologic maps and petrophysical data with techniques used in microfabric analysis. The model has a layered self-affine structure, which agrees with the intrinsic scale independence of geologic phenomena. Synthetic seismograms from this model replicate many pertinent features observed in deep seismic reflection data and allow insight into crustal structure: (1) reflectivity depends on the lithological populations present and their distribution within a crustal unit; (2) transparent zones within the reflective lower crust can arise from metamorphic reactions; (3) the transition from upper to lower crust may be a change in small-scale seismic fabric rather than a first-order seismic discontinuity.

Lower crustal reflectivity from waveform inversion

SUMMARY Deep seismic reflection profiles show that the lower part of the continental crust often contains many strong subhorizontal reflections. Based on forward modelling, these reflections are interpreted as a sequence of alternating high- and low-velocity layers. A waveform inversion provides an optimum model by minimizing the misfit between observed data and synthetic seismograms. However, it is very difficult to invert for the whole data set, partly because of the large computation time needed in modelling and partly because the high amplitude reflected energy in the upper crust dominates the misfit function. To overcome these difficulties we propose a three-step strategy for inverting the deeper part of seismic reflection data over a narrow aperture in which the velocity structure can be assumed to be 1-D. First, both source wavelet and recorded wavefield are propagated in the plane-wave domain downwards to a particular depth for a given 1-D medium. Because the source-receiver distances are short for deep reflections it is assumed that these reflections are at normal incidence. It is also assumed that the near-surface velocity is known or has been estimated using other inversion schemes. The downward propagation algorithm takes account of nearsurface effects, such as multiple reflections and attenuation. Second, starting from a known background-velocity model, a non-linear iterative waveform inversion is applied to the propagated data set. The method is based on minimizing the difference, sample by sample, between observed and calculated wavefields in a least-squares sense. The forward modelling, which consists of calculating the data from the model, uses a reflectivity method to compute the synthetic seismogram in the plane-wave domain. The model perturbation is calculated with the help of the conjugate gradients method. The non-linear inversion gives the short wavelengths of impedance contrast, which is a function of density and velocity, in the lower crust. Third, an interpretational step is tried in order to recover the medium wavelengths of velocity. The algorithm is tested on synthetic data and then applied to a part of the WAM data set, south-west of England. A good fit to the real data with the optimum model predicts that the velocity contrast of the layers in the lower crust varies from 0.2 km s-' to 0.5 km s-' and the thickness from at least 0.075 km to 1 km.

Trans-Hudson orogen and Williston basin in Montana and North Dakota: New COCORP deep-profiling results

COCORP (Consortium for Continental Reflection Profiling) deep reflection profiling across the Williston basin and underlying Trans-Hudson orogen shows the following. (1)There is no evidence for a precursor rift basin beneath the axis of the Williston basin. With the exception of small-scale structures (e.g., Nesson and Cedar Creek anticlines), the basement surface beneath the Williston basin is smooth and has a dish-shaped geometry. (2) Crustal penetrating reflections within the eastern Trans-Hudson orogen dip eastward, in apparent contradiction to the conventional that the Superior province underthrusts the eastern side of the orogen. (3) Basement reflections beneath the center of the orogen define a broad crustal-scale antiform, the axis of which coincides approximately with the depocenter of the overlying Williston basin. Midcrustal reflections defining the crest and western flank of this antiform also coincide generally with the location and shape of the source of the North American Central Plains conductivity anomaly. (4) Crustal penetrating reflections beneath the western side of the orogen dip westward, supporting the current view that subduction polarity was to the west beneath the Wyoming province. (5) The eastern edge of the Wyoming province is marked by east-dipping upper-crustal reflections and subhorizontal lower-crustal reflections that are truncated by the west-dipping reflectivity of the Trans-Hudson orogen.

PSV-wave propagation in a medium with local heterogeneities: a hybrid formulation and its application

SUMMARY Hybrid methods, which combine so-called ‘numerical’ and ‘analytical’ methods, are a successful tool for computing synthetic seismograms for media with localized heterogeneous regions. Each of the two methods is applied in that part of the medium in which it can be employed efficiently. This paper deals with a combination of a finite-difference algorithm, applied in the neighbourhood of the heterogeneity, and a frequency–wavenumber filter for the PSV-wave equation applied outside. This method is an alternative to the use of Green's theorem as a wavefield-continuation method (e.g. van den Berg 1984). In the hybrid method presented here, absorption can be incorporated additionally. For different models with first-and second-order discontinuities the hybrid method was tested successfully against the reflectivity method. The hybrid method was applied to study the effects of a heterogeneous lower crust on the wavefield. This application has been inspired by recent deep seismic reflection profiling, which often reveals a highly reflecting lower crust. The scattering models have been divided into three groups: horizontal, vertical and isotropic scatterers. The comparison of the reflection responses of these models shows that thin horizontal or small isotropic scatterers yield high reflected energy, coherent over a few kilometres, in the time window of lower crustal reflections. Vertical structures, characterized by a small correlation length in horizontal direction, have less energy in this time window, but they can generate a complex Moho reflection with a strong Moho coda. It is also shown that the acoustic approximation is mostly not sufficient to describe the wave propagation in such complex media.

High electrical conductivity in a model lower crust with unconnected, conductive, seismically reflective layers

In this paper we derive the electrical conductivity for a model lower crust containing unconnected, highly conductive lamellae within a highly resistive matrix. Lateral overlap, with small vertical separation, of lamellae of the dimensions imaged by seismic reflection profiling (a few hundred metres thick and a few kilometres across) could increase lower-crustal conductivity from the low values predicted by laboratory measurements on dry rocks to the high values observed in field experiments. The model does not depend on the cause of high conductivity within the lamellae. However, lamellation of the lower crust may provide a way of lithologically trapping saline water in permeable, conductive lamellae within an impermeable, non-conductive matrix, and so resolve the apparent contradiction between the low crustal permeabilities required for maintenance of high pore pressure over geological time periods and the high degree of pore interconnection required for the high observed conductivity. The permeable lamellae and impermeable matrix would be of very different lithologies, as implied by the high amplitudes of the lower-crustal reflections. For a typical example the model gives resistivities that compare favourably with the modified Archie's Law. The model can also give anisotropic resistivity effects, which are quantitatively compatible with results from field experiments.

Mapping of quaternary deposits and bedrock topography using high-resolution seismic reflection techniques

Marine normal-incidence reflection and P- and S-wave refraction/wide-angle reflection data from the BABEL project, line 1, in the Bothnian Sea have been used for investigating the nature of the reflectivity in the area. The data are available along the same line, and therefore the same piece of crust is probed by two seismic methods. We use this favourable circumstance for model calculations concerning both data sets by analysing the waveforms of reflections from normal-incidence traveltimes at approximately 4, 12 and 16 s TWT. Strong sub-horizontal reflections are observed on the normal-incidence reflection section at about 4 s TWT. These reflections have been modelled using the reflectivity method (1-D modelling) utilising the water bottom reflection as an estimate of the source wavelet and show a positive polarity. Thickness and P-wave velocity consistent with these reflections are 50 m and 7 km/s, respectively, with a host rock velocity of 6 km/s. From the polarity, velocity and thickness, these reflections are attributed to dolerite sills in the upper crust. Strong lower-crustal reflections are observed on the wide-angle and normal-incidence reflection sections. However, the corresponding S-wave reflections from the lower crust are considerably weaker. The implied high positive impedance contrast for the P-wave lower-crustal reflection and the weak S-wave reflectivity suggests the presence of a highly mafic sheet at about 40 km depth. The wide-angle P- and S-wave sections show strong reflections at the Moho. Coincident normal incidence data show only weak Moho reflectivity indicating a gradient in velocity at the crust/mantle boundary. Dipping events onlap the strong sub-horizontal lower crustal and the P-wave Moho reflections on the wide-angle data. These are explained by dipping structures above the lower crustal reflector and below the Moho, with the latter also consisting of a gradient in velocity.