Expanding Spread Profiles Research Articles

Extreme Mesozoic Crustal Thinning in the Eastern Iberia Margin: The Example of the Columbrets Basin (Valencia Trough)

AbstractEastern Iberia preserves a complex succession of Mesozoic rifts partly or completely inverted during the Late Cretaceous and Cenozoic in relation with Africa‐Eurasia convergence. Notably, the Valencia Trough, classically viewed as part of the Cenozoic West Mediterranean basins, preserves in its southwestern part a thick Mesozoic succession (locally ≈10 km thick) over a highly thinned continental basement (locally only ≈3.5 km thick). This subbasin, referred to as the Columbrets Basin, represents a Late Jurassic‐Early Cretaceous hyperextended rift basin weakly overprinted by subsequent events. Its initial configuration is well preserved allowing us to unravel its 3‐D architecture and tectonostratigraphic evolution in the frame of the Mesozoic evolution of eastern Iberia. The Columbrets Basin benefits from an extensive data set combining high‐resolution seismic reflection profiles, drill holes, seismic refraction data, and expanding spread profiles. The interactions between halokinesis, involving the Upper Triassic salt, and extensional deformation controlled the architecture of the Mesozoic basin. The thick uppermost Triassic to Cretaceous succession displays a large‐scale “syncline” shape, progressively stretched and dismembered toward the basin borders. We propose that the SE border of the basin is characterized by a large extensional detachment fault acting at crustal scale and interacting locally with the Upper Triassic décollement. This extensional structure accommodates the exhumation of the continental basement and part of the crustal thinning. Eventually, our results highlight the complex interaction between extreme crustal thinning and occurrence of a prerift salt level for the deformation style and tectonostratigraphic evolution of hyperextended rift basins.

Using a local Monte Carlo strategy to assess 1-D velocity models from wide-angle seismic travel-time data and application to the Rockall Trough

We present a statistically based search strategy to explore velocity–depth model space derived from the inversion of seismic refraction and wide-angle data. The method is based on the Metropolis algorithm and computes the likelihood of any given model of fitting the observed data. By iteratively perturbing the model, the model space is sampled and the resulting probability density function provides a quantified measure of the velocity resolution as a function of depth. Unlike manual analysis, where a single layer is perturbed to test its sensitivity ignoring the effect on the deeper layers, this method computes the fit for the whole model at each iteration and only selects the models that achieve a specified global fit. We show results of the algorithm from the 1-D inversion of Expanding Spread Profiles from the central part of the Rockall Trough to the west of Britain. As expected, the method highlights the areas of the model that are both well and poorly constrained and shows the degradation of resolution with increasing depth such that for high quality data the accumulated velocity errors at basement depths is +/− 5%. This error increases as the data quality decreases and the estimated pick error increases.

Crustal structure of the NE Rockall Trough from wide‐angle seismic data modeling

Two wide‐angle seismic lines located in the northern Rockall Trough were acquired in May 2000. One line (line E) crosses the trough from the continental shelf off Lewis to normal oceanic crust west of Lousy Bank in NW‐SE direction. The other line (line D) intersects with line E, crosses the Wyville‐Thomson Ridge in a SW‐NE direction and ends in the Faeroe‐Shetland Basin. Sonobuoy data and expanding spread profiles acquired in the same area have been remodeled. Analysis of the seismic data using travel times and amplitudes reveals an up to 5 km thick sedimentary basin including an up to 1.5 km thick basaltic layer which is present in most of the trough. Further conclusions of this study are that the Rockall Trough is underlain by highly stretched continental crust of ∼13 km thickness. The crust thickens to ∼24 km beneath Lousy Bank, which is interpreted to be of continental nature. Beneath the Hebrides continental shelf a three‐layer continental crust of 26 km is modeled. An up to 12 km thick high‐velocity layer is observed underneath the ocean‐continent boundary and is interpreted as magmatic underplating resulting from excess volcanism during rifting. No evidence for an underplate layer could be distinguished beneath the trough area. Modeling of the structure of the Wyville‐Thomson Ridge revealed no existing igneous core of the ridge confirming existing theories, that it is a compressional structure.

Shear wave structure from joint analysis of seismic and seafloor compliance data

SUMMARY Determining shear wave structure is the key to identifying the amount and location of fluid within the crust. Seismic and seafloor compliance methods provide independent estimates of shear wave structure, and a joint analysis of the two data sets should provide better constraints on the properties of the uppermost oceanic crust. We consider an example from 9 ◦ 33 � N on the East Pacific Rise. Seismic data from an on-axis expanding spread profile have been reanalysed to determine a shear wave structure for layer 2B; pS arrivals require a high shear wave velocity within this layer (Poisson ratio in the range 0.22‐0.25). Compliance data from the same location are seemingly inconsistent with this result, requiring that layer 2B is a region of low shear wave velocity (Poisson ratio in the range 0.33‐0.44). The quantitative differences between the two results can be explained by anelasticity and anisotropy; conversely, a knowledge of this discrepancy can be used to constrain the attenuation structure.

Recent progress of deep seismic experiments and studies of crustal structure in northern South China Sea*

The South China Sea (SCS) is one of the largest marginal seas in the western Pacific. Its northern part has the features of a passive continental margin. The studies of deep crustal structure in this area are very important for understanding the tectonic nature, evolution history, basin formation of the northern margin, and the origin of the SCS. In the past decades, the deep seismic experiments of crustal studies in the northern SCS have gone through three stages, namely the sonobucy, two-ship Expanding Spread Profile (ESP), and Ocean Bottom Hydrophone/Seismometer (OBH/OBS). Along the continental slope, the sonobuoy experiments provided useful information about the velocity structure of the upper crust, while the ESP data recorded for the first time the seismic signals from deep crustal structure and Moho interface. And the OBH/OBS profiles revealed the crustal structure in much greater detail. This paper first gives a brief historical review of these deep seismic experiments and studies, then a summary of the latest progress and important research results. The remaining problems and suggestions for further research work are presented as conclusive remarks.

Crustal structure and transform margin development south of Svalbard based on ocean bottom seismometer data

The Barents Sea is located in the northwestern corner of the Eurasian continent, where the crustal terrain was assembled in the Caledonian orogeny during Late Ordovician and Silurian times. The western Barents Sea margin developed primarily as a transform margin during the early Tertiary. In the northwestern part south of Svalbard, multichannel reflection seismic lines have poor resolution below the Permian sequence, and the early post-orogenic development is not well known here. In 1998, an ocean bottom seismometer (OBS) survey was collected southwest to southeast of the Svalbard archipelago. One profile was shot across the continental transform margin south of Svalbard, which is presented here. P-wave modeling of the OBS profile indicates a Caledonian suture in the continental basement south of Svalbard, also proposed previously based on a deep seismic reflection line coincident with the OBS profile. The suture zone is associated with a small crustal root and westward dipping mantle reflectivity, and it marks a boundary between two different crystalline basement terrains. The western terrain has low (6.2–6.45 km s −1) P-wave velocities, while the eastern has higher (6.3–6.9 km s −1) velocities. Gravity modeling agrees with this, as an increased density is needed in the eastern block. The S-wave data predict a quartz-rich lithology compatible with felsic gneiss to granite within and west of the suture zone, and an intermediate lithological composition to the east. A geological model assuming westward dipping Caledonian subduction and collision can explain the missing lower crust in the western block by subduction erosion of the lower crust, as well as the observed structuring. Due to the transform margin setting, the tectonic thinning of the continental block during opening of the Norwegian-Greenland Sea is restricted to the outer 35 km of the continental block, and the continent–ocean boundary (COB) can be located to within 5 km in our data. Distinct from the outer high commonly observed on transform margins, the upper part of the continental crust at the margin is dominated by two large, rotated down-faulted blocks with throws of 2–3 km on each fault, apparently formed during the transform margin development. Analysis of the gravity field shows that these faults probably merge to one single fault to the south of our profile, and that the downfaulting dominates the whole margin segment from Spitsbergen to Bjørnøya. South of Bjørnøya, the faulting leaves the continental margin to terminate as a graben 75 km south of the island. Adjacent to the continental margin, there is no clear oceanic layer 2 seismic signature. However, the top basement velocity of 6.55 km s −1 is significantly lower than the high (7 km s −1) velocity reported earlier from expanding spread profiles (ESPs), and we interpret the velocity structure of the oceanic crust to be a result of a development induced by the 7–8-km-thick sedimentary overburden.

Structure of the Mediterranean Ridge accretionary complex from seismic velocity information

Seismic velocities obtained from ocean-bottom hydrophone, expanding spread profile and multi-channel seismic data were used to compile a velocity model for the Mediterranean Ridge along a 220-km-long transect extending from the Sirte Abyssal Plain to the Cleft region near the Hellenic Trough. A 200–300-m-thin layer of Plio–Quaternary sediments with velocities of 1800–2200 m s −1 covers the whole Ridge. The Messinian evaporites (4000–4500 m s −1) occur in the southwest as a tectonically thickened layer and in a basin just northeast of the crest of the Ridge. In the intervening region however, the evaporites appear absent and the seismic velocities are generally lower. Arched reflectors, imaged in the depth-migrated section, suggest that the sediments beneath the Ridge crest belong to a Pre-Messinian accretionary wedge. Beneath the Messinian evaporites a northeastward-thinning layer of probable Tertiary sediments shows laterally increasing velocities from 3300 m s −1 to 4600 m s −1. Assuming that the layer thinning is caused by compaction due to increased overburden alone, we have calculated a porosity reduction from 15% to 4% and an associated fluid expulsion of 10 km 3 km −1 along the trench axis. This corresponds to c. 60% of the initial fluid volume of an undeformed sediment column from the abyssal plain. The almost impermeable evaporitic cap over these sediments leads to high fluid pressures at the base of the evaporites, likely to make this horizon the basal décollement of the modern accretionary system. A 2.5-km-thick unit of probable Mesozoic carbonates with velocities of 4500–4600 m s −1 is inferred at c. 8 km depth. The top of the oceanic crust occurs at a depth of about 10 km. The results from this study have widespread implications for the understanding of the regional geological history.

A Ligurian (Western Mediterranean Sea) geophysical transect revisited

Data from a new deep‐penetration seismic reflection line (LISA cruise 1995), onland seismic recording (LISA) and marine ESPs (expanding spread profiles, CROC II cruise 1981) are integrated to study the crustal structure of the north Ligurian basin. The correlation of these different seismic data, located along an Antibes–Ile Rousse transect, provides accurate information on the nature of the crust. On the Provençal margin, the multichannel seismic (MCS) profile LISA01 shows a major step in the basement. This structure is associated with a crustal change, corresponding to the ocean boundary. On the Corsica margin, we do not observe such a structure and the ocean boundary is constrained by magnetic anomalies. These anomalies are identical to those observed on the Provençal margin above the main basement structure. On the MCS line, the Messinian salt and the bottom water multiple obscure the crustal image below 7 s two‐way traveltime (TWTT). Nevertheless, the ESPs provide information about the crust below 7 s TWTT. These ESPs were reprocessed and analysed by matching traveltime and amplitude variations in the X–T and τ–p domains. The main result is the identification of a transitional zone on the continental margin (Provençal and Corsica margin) characterized by a layer with velocities of 7.2–7.3 km s−1 in the lower crust. Recordings from land stations on the Corsica margin define the geometry of this layer. In the centre of the basin, the oceanic crust is about 6 km thick. From the Corsica margin to the centre of the basin, the Moho depth decreases from 18 to 13 km. The velocity model deduced from offshore data is consistent with the ESP velocity model and MCS line LISA01. The combined analysis of these data allows us to propose a new boundary of the oceanic crust in the north Ligurian basin.

The abundance of ultramafic rocks in Atlantic Ocean crust

Combined P- and S-wave velocity profiles have been widely used to assess the composition of the oceanic crust, but no quantitative estimates of the abundances of mafic and ultramafic rocks which are compatible with the seismic structure of the crust have been made. In this study, a Voigt–Reuss–Hill (VRH) average model based on the average elastic properties of partially serpentinized peridotites and oceanic diabases and gabbros is used to make quantitative estimates of lithological composition from the seismic structure of the crust. The VRH model is applied to several expanding spread profiles from the north Atlantic, in which P- and S-wave velocities are independently constrained. A purely gabbroic composition is sufficient to explain the seismic structure in most cases, but the lower oceanic crust could also be composed of interlayered gabbros and ultramafic rocks, and the average ultramafic content of the crust could be as high 13 per cent. In that case, the magmatic crust would be about 1 km thinner than the seismic crust. A small volume of serpentine (< 5 per cent) could be distributed through the crust without being detected by seismic methods. Even a 1 per cent volume fraction of serpentine would be a significant source of water delivered to the mantle by subduction of the crust. The seismic properties of the upper mantle are consistent with the properties of dunite, as opposed to peridotite. The low-velocity zone in the crust at NAT ESP 5 could be composed of amphibole-rich gabbros, but the VRH model indicates that it is probably composed of a mixture of gabbro with partially serpentinized peridotite, having a total serpentine content of 10–20 per cent. In either case, the low-velocity zone probably represents a zone of hydrous alteration.

Recovering Shear-wave Anisotropy of the Lower Crust: The Influence of Systematic Errors on Travel-time Inversion

Studies of seismic anisotropy in situ can help to discriminate between different rock types for the lower crust. In this context we investigate the sensitivity of an iterative linearized 3-D travel-time inversion scheme for transversely isotropic media with respect to two types of systematic errors: wrong velocities and interface topography of the hanging wall of the lower crust. The computations simulate realistic field conditions such as found for the Variscan crust at the Urach geothermal anomaly. The study focusses on the possible information content of split S M S arrivals observed along two orthogonal expanding spread profiles. It ensues that an imperfect knowledge of the layer geometry is of minor importance compared to errors in the velocities of the hanging wall. In particular, upper crust anisotropy has to be considered carefully. Generally, the anisotropy of transversely polarized shear waves (SH waves) was recovered with higher accuracy than the anisotropy of vertically polarized shear waves (SV waves).

Expanding spread profiles across the Trans-Hudson Orogen

Four vibroseis expanding spread profiles (ESP) were acquired through the LITHOPROBE Trans-Hudson Orogen Transect program. These ESP's, with a maximum offset of 20 km, were centred on areas where prominent crustal reflectivity was detected by the regional survey. The small source stepout distance (100 m) generated high-fold (>30) data. The Common Midpoint (CMP) stacked sections of ESP and regional reflection data of the 1991 survey exhibit similar crustal images. The regional profiles provide better resolution of shallow and dipping features, whereas the continuity and strength of reflections below 6 s two-way traveltimes (TWT) are better defined on the ESP sections. Modelling results reveal that velocity estimation becomes more robust and accurate when crustal seismic surveys utilize longer offsets than commonly implemented by standard practice. These larger source-receiver separations, however, must be generally limited to offset/depth ratios not exceeding 1.5 when conventional velocity analysis techniques, based on the hyperbolic moveout assumptions, are implemented. The advantages provided by the long-offset data acquisition include increased S/N ratio and a greater number of traces with sufficiently large moveouts which improve velocity resolution, especially below mid-crustal depths. To achieve similar advantages in a regional crustal reflection survey would require the adoption of longer spread lengths than those presently implemented in standard data acquisition procedures.

A new assessment of the abundance of serpentinite in the oceanic crust

The ‘gabbro' model of the composition of the oceanic crust has prevailed for more than two decades, but serpentinites are common along much of the Mid‐Atlantic Ridge, and elsewhere, leading to renewed debate over the abundance of serpentinites in crust created at slow‐spreading ridges. Based on data from both oceanic and ophiolite samples, it has also been suggested that serpentinites are not distinguishable from meta‐gabbros and diabases by their seismic properties. We have made new statistical analyses of the relationship between Vs and Vp in oceanic gabbro and diabase samples and serpentinized peridotites, and compared them to in situ s‐wave and p‐wave velocities derived from Expanding Spread Profiles (ESPs) acquired in the Atlantic. Our findings demonstrate: (1) that there is some overlap of the properties of gabbroic samples taken from ophiolites with the properties of serpentinites, but (2) over the range of typical‐layer 3 seismic velocities, the seismic properties of oceanic gabbros and diabases are distinctly different from the properties of serpentinites, and (3) that in situ seismic velocities are in excellent agreement with the properties of the gabbros, while the shear velocities in serpentinites are significantly lower. Partially‐serpentinized peridotite is certainly present, but not abundant in ‘normal' oceanic crust.

Deep penetration seismic soundings across the northern margin of the South China Sea

Twenty reversed, two‐ship expanding spread profiles (ESPs) with maximum source‐receiver offsets of ∼100 km were collected in three transects across the rifted northern margin of the South China Sea. Source‐receiver offset versus two‐way travel time (X‐T) data were mapped into the intercept time versus ray parameter (τ‐p) domain, and velocity‐depth solutions were obtained by a combination of τ‐sum inversion in the τ‐p domain and ray tracing in both the τ‐p and X‐T domains. Arrivals from the Moho were detected on 17 of the ESPs. The depths to Moho determined for individual ESP interpretations have reproducibilities of ±0.1 km to ±3 km; in most cases the Moho depth has been determined to within ±1.5 km. Moho depths determined in this investigation represent a significant improvement over previous estimates of Moho along the margin from gravity data. Variations in present‐day crustal thickness (measured from top of prerift basement to Moho) are one measure of the amount and nature of the crustal thinning associated with the rifting of continental crust preceding the formation of the adjacent South China Sea Basin. The ESP interpretations reveal that across the eastern portion of the south China margin, the crust appears to thin more or less continuously toward the continent‐ocean boundary. In the west, ESP interpretations also show a general trend of seaward crustal thinning but, in addition, indicate at least two instances of focused, localized crustal thinning. Crustal velocities and the relative proportion of upper crust (VP&lt; 6.4 km/s) and lower crust (VP&gt; 6.4 km/s) are used to identify areas of the south China margin with similar and contrasting crustal structures. Variations in these properties are believed to result primarily from contrasting, prerift crustal structure across the margin. However, magmatic underplating during rifting, depth dependent extension, and Pleistocene igneous intrusions may also have contributed to the variations in present crustal structure. Reliable information about variations in crustal thickness and velocity structure across and along the south China margin is an important prerequisite to understanding better the nature of the spatially variable rifting processes which dominated the formation of this margin.

Gravity anomalies and crustal structure at the Mesozoic Blake Spur Fracture Zone

A number of recent studies have used gravity data from mid‐ocean ridges to infer along‐axis crustal thickness variations, and hence variations in melt supply. A problem with these studies is that it is difficult to distinguish the effects of crustal thickness variations from those of crustal and mantle density variations. We analyze gravity data from the 12–13 km offset Blake Spur Fracture Zone in 140 Ma crust in the western North Atlantic. The advantage of studying this area is that mantle density variations are likely to be much smaller than those at the ridge axis, and crustal thickness variations are well constrained by expanding spread profiles (ESPs). Thus we are able to separately quantify the contributions made to the gravity anomaly by crustal thickness and crustal density variations. After removal of the gravity effects of the seabed and the oceanic basement topography, as defined by a grid of multichannel seismic reflection profiles, the gravity data show a linear 5‐mGal high along the fracture zone. After removal of the effects of crustal thickness variations defined by the ESPs, and of a basal high‐velocity layer beneath the fracture zone interpreted to be partially serpentinized upper mantle, the data show a 5‐mGal gravity low at the fracture zone. An alternative interpretation of the basal layer, as a thickened Moho transition zone, also leads to a residual gravity low. These results, together with other recent results from fracture zones on the Mid‐Atlantic Ridge, suggest that crustal thickness variations at fracture zones may be consistently understimated by studies based on gravity data. The discrepancy may be explained by variations in crustal density due to enhanced fracturing in the vicinity of fracture zones, and to disproportionate thinning of oceanic layer 3 near segment boundaries.

Crustal structure and deep reflector properties: Wide angle shear and compressional wave studies of the midcrustal surrency bright spot beneath southeastern Georgia

Three‐component (P, SH, and SV) expanding spread profiles (ESP), common‐midpoint profiles, and sparse three‐dimensional P wave data were collected over an unusually strong midcrustal reflector, the Surrency Bright Spot (SBS), in southeastern Georgia. Shear wave reflections from the SBS at 10.9 s (16 km depth), and possibly from the lower crust at 18.3 s (29 km depth), were recorded but required substantial source effort (stacking) and were too weak for reliable reflectivity measurements. Reflections on the ESPs delineate a 1.5‐km‐thick Atlantic Coastal Plain section whose seismic properties (Vp=2.53 km/s, Vs=1.51 km/s, Vp/Vs= 1.67) are consistent with quartz‐rich sandstones and siltstones, sitting atop a 15‐km‐thick upper crust (Vp=6.38 km/s, Vs=3.25 km/s, Vp/Vs=1.96), which in turn overlies a 15 km‐thick lower crust of slower material (Vp=6.02 km/s, Vs≈3.26 km/s, Vp/Vs≈1.84). The velocity inversion may result from underthrusting of upper crustal rocks during suturing of Florida to North America. Amplitude‐versus‐offset analyses, combined with an earlier reflection polarity test and waveform modeling, indicate that the SBS originates from a thin (∼80–120 m), high‐impedance layer, most likely a mafic dike or tectonically emplaced ultramafic body.

Poisson's ratio of a seaward-dipping reflector series, Hatton Bank

Seismic reflection profiles across the Hatton Bank continental margin in the eastern North Atlantic have imaged thick sequences of seaward-dipping reflectors. Converted shear waves propagating through one of these sequences were recorded on a two-ship expanding spread profile just landward of the ocean-continent transition. Synthetic seismogram modelling of these shear waves indicates that Poisson's ratio in the sequence is 0.27 ± 0.01, similar to that of normal oceanic Layer 2.

Distribution of magma beneath the East Pacific Rise between the Clipperton Transform and the 9°17′N Deval from forward modeling of common depth point data

We have reprocessed seven cross‐axis common depth point (CDP) profiles between the Clipperton transform and the 9°17′N Deval (deviation in axial linearity) on the East Pacific Rise (EPR) to understand the relationship between axial magma chamber (AMC) width and seafloor morphology. Forward modeling of cross‐axis CDP profiles suggests a segmented AMC in which significant variations in width occur across minor rise axis discontinuities (e.g., Devals). The modeled rise segment widths bounded by the 9°53′N‐9°35′N Devals, the 9°35′N‐9°17′N Devals, and south of the 9°17′N Deval were &lt; 0.7 km, 1.0–1.2 km, and 4.15 km, respectively. Transition in AMC width across these discontinuities is unclear due to the sparseness of cross‐axis line spacing; however, a simple association of Devals with decreased magma supply is doubtful: the minimum (250 m) and maximum (4150 m) AMC widths are found near the 9°35′N and 9°17′N Devals, respectively. The reprocessing of CDP profiles included repicking stacking velocities to ensure a proper stack of the AMC reflector and its associated diffractions, imaging postcritical reflections from the base of layer 2A, finite difference time migration, ray theoretical depth migration, and travel time modeling of AMC diffraction patterns. Constraints on AMC width were derived from forward modeling techniques based on analytic raytracing. Velocity models were constructed from SeaBeam bathymetry and modified expanding spread profile (ESP) velocity‐depth functions. ESP velocity models were altered to compensate for off‐axis thickening of layer 2A as imaged in the CDP reflection data. Continuous two‐dimensional velocity models constructed from modified ESP velocity‐depth functions and SeaBeam bathymetry should account for ray bending at the seafloor/basalt interface and any lateral velocity gradients induced by a thickening layer 2A. Stacked data were time migrated using a finite difference algorithm and extrapolated to depth using ray theoretical depth migration. Reflector positions were input into our forward modeling scheme to produce a zero‐offset travel time response of our migrated solution. The travel time response was then overlain on the stacked section to ensure an adequate match, especially to diffractions generated at the AMC edge. Forward modeling of AMC diffraction patterns reveals that original AMC width estimates were too large. The under‐migration of the AMC reflector resulted from the conversion of stacking to interval velocities without accounting for topographic effects on individual CMP gathers, thus resulting in improperly collapsed diffraction hyperbolae. Ship wandering relative to the AMC edge can account for variations in AMC reflector amplitude and dropout on the along‐axis CDP line. The continuity of the AMC appears unbroken across several ridge axis discontinuities between the Clipperton transform and the 9°17′N Deval which suggests an AMC whose along‐axis dimension exceeds 75 km. Reflectivity modeling of CMP gathers suggests that the available data are consistent with a magma chamber comprising only a thin layer of melt overlying a zone of partially solidified crystal mush. Such a thin layer of melt might inhibit along‐axis mixing of magmas and thereby account for variations in magma composition along the rise crest. This melt lens model for the AMC would also produce strong diffraction patterns as imaged in the CDP data.

A multichannel seismic investigation of upper crustal structure at 9°N on the East Pacific Rise: Implications for crustal accretion

Reprocessed multichannel seismic profiles from the 9°N segment of the East Pacific Rise reveal prominent shallow subbasement events. These events are identified as wide‐angle reflections from the base of seismic layer 2A, based upon modeling of expanding spread profile data and velocity functions. The layer 2A reflections typically increase from 0.15 s after the seafloor reflection at the rise axis to 0.3–0.45 s within 1–2 km of the axis, corresponding to an increase in layer thickness of 200–600 m. No further systematic increase in layer thickness is observed, although lateral variability of the order of a few hundred meters in thickness is observed at greater offsets from the rise axis. However, the intermittent character of the imaged layer 2A reflection is attributed to focusing and defocusing of energy by the seafloor bathymetry rather than necessarily to intrinsic lateral variability at the base of the layer. The base of layer 2A is interpreted as corresponding to the transition between the extrusive section, pillow basalts and sheet flows, and a sheeted dike complex. The rapid thickening of the layer near the rise axis is attributed to successive lava flows burying the initially shallow top of the sheeted dike complex as the layer passes through the neovolcanic zone. Lateral variability of layer 2A can significantly affect the imaging of the underlying axial magma chambers as average velocities within layer 2A are approximately half that of layer 2B. For an along‐axis profile, apparent along‐axis variability in the depth of the axial magma chamber is traced to variability in the thickness of layer 2A caused by wandering of the profile relative to axis. Within the resolution of the data, the time delay of the magma chamber reflection relative to the base of layer 2A is constant.

The ocean-continent boundary in the Gulf of Lion from analysis of expanding spread profiles and gravity modelling

SUMMARY Two-ship multichannel seismic profiles, deep penetration (ECORS) and conventional seismic lines (LIGO surveys) are used to study the crustal structure of the Gulf of Lion (Western Mediterranean). 11 full ESPs (Expanded Spread Profiles) with total shot-receiver ranges up to 60 km were shot in 1981 perpendicular to the margin of the Gulf of Lion and in 1988 a deep MCS seismic profile (ECORS-CROP program) was performed parallel to the ESPs. These ESPs were analysed by matching traveltime and amplitude variations in both the x-t and τ-p domains. The resulting P-wave velocity/depth model has the following features, (a) beneath the continental slope of the Provencal margin a rapid rise of the Moho from 20 to 14 km and the existence of an anomalous 7.2-7.4 km s-1 velocity layer, (b) from the base of the slope to the extensive salt-domes domain a 5-6 km thin crust which does not appear typically oceanic in nature, (c) quite typical oceanic crust up to the Sardinian margin. Gravity modelling is consistent with the seismic results. The OCB (ocean-continent boundary) could be placed north of that postulated by previous authors, where the data indicate a remarkably narrow transition between continental and ‘oceanic’crust, or south where a typical oceanic crust, which correlates well with the domain of the salt domes and of large magnetic anomalies, has been determined. A very prominent reflector is clearly seen, at the base of the continental slope, on the seismic reflection profiles and corresponds to the top of an 7.2-7.4 km s-1 velocity layer. The high-velocity layer is 2-3 km thick where the crust is thinnest and has a limited lateral extent seawards. This anomalous crustal structure could be the result of extremely thinned and possibly broken up, continental crust underplated and intruded by partial melt, or could represent serpentinized peridotite material. Important questions about the evolution of the Gulf of Lion cannot be addressed using these new results alone without addition of other constraints. Nevertheless a two-stage mechanism of drifting and rifting of this part of the Western Mediterranean Sea is proposed.

Shallow structure of oceanic crust in the western North Atlantic from seismic waveform inversion and modeling

Seismic reflection data from Mesozoic oceanic crust in the western North Atlantic, in the vicinity of the Blake Spur Fracture Zone, have imaged a number of short, subhorizontal events in the upper 1 km of the crystalline crust. Wide‐angle expanding spread profile (ESP) data from the same region indicate the presence of first‐order velocity discontinuities in this depth interval. Using ESP data reprocessed to enhance precritical reflection events, we investigate the relationship between the precritical and postcritical reflections with three different approaches: amplitude analysis, forward modeling, and waveform inversion. The precritical reflections have high apparent reflection coefficients (∼ 0.1) and may be divided into two types: those which can be traced out to critical ranges (5–6 km), representing velocity steps, and those which fade out at ranges of 3–4 km, representing shorter‐wavelength velocity variations. Synthetic seismogram modeling indicates that the velocity steps occur within a 50 m depth interval, while the shorter‐wavelength features may correspond to high‐ or low‐velocity zones with a thickness of 20 m (10% of the dominant seismic wavelength) or less. We investigate the velocity structure in detail by full waveform inversion of a selected portion of ESP data, transformed into the intercept time‐slowness domain. The inversion method minimizes the sample‐by‐sample misfit between the data and reflectivity synthetics in a least squares sense. A conjugate gradient algorithm was used, starting from a velocity structure derived from forward modeling of wide‐angle ESP data. The inversion tightly constrains short‐wavelength components of the velocity structure, and indicates that intracrustal reflections result both from major velocity steps and from zones of alternating high and low velocity. Synthetic seismograms generated from sonic log data from Deep Sea Drilling Project hole 418A show reflections of similar amplitude and character to those seen in the ESP data. These reflections probably originate from rather subtle features of the velocity‐depth profile, related primarily to changes in porosity in the pillow lava sequence. A deeper reflector, 800 ms below oceanic basement in the Blake Spur Fracture Zone, may correspond to the top of a zone of serpentinized upper mantle.