Peter Buhl

Multi-channel seismic imaging of a crustal magma chamber along the East Pacific Rise

A reflection observed on multi-channel seismic profiles along and across the East Pacific Rise between 8°50′ N and 13°30′ N is interpreted to arise from the top of a crustal magma chamber located 1.2–2.4 km below the sea floor. The magma chamber is quite narrow (<4 – 6 km wide), but can be traced as a nearly continuous feature for tens of kilometres along the rise axis.

Direct mapping of seismic data to the domain of intercept time and ray parameter -A plane-wave decomposition

Marine seismic data recorded as a function of source‐receiver offset and traveltime are mapped directly to the domain of intercept or vertical delay time and horizontal ray parameter. This is a plane‐wave decomposition based on beam forming of wide‐aperture seismic array data to determine automatically the loci of coherent seismic reflection and refraction events. In this computation, semblance, in addition to the required slowness or horizontal ray parameter stack, is found for linear X — T trajectories across subarrays. Subsequently, semblance is used to derive a windowing filter that is applied to the slowness stack to determine the points of stationary phase and eliminate aliasing. The resulting filtered slowness stacks for multiple subarrays can then be linearly transformed and combined according to ray parameter, range, and time. The resulting function of intercept time and horizontal ray parameter offers significant computational and interpretational advantages for the case of horizontal homogeneou...

SEISMIC STRUCTURE OF THE SOUTHERN EAST PACIFIC RISE

Seismic data from the ultrafast-spreading (150 to 162 millimeters per year) southern East Pacific Rise show that the rise axis is underlain by a thin (less than 200 meters thick) extrusive volcanic layer (seismic layer 2A) that thickens rapidly off axis. Also beneath the rise axis is a narrow (less than 1 kilometer wide) melt sill that is in some places less than 1000 meters below the sea floor. The small dimensions of this molten body indicate that magma chamber size does not depend strongly on spreading rate as predicted by many ridge-crest thermal models. However, the shallow depth of this body is consistent with an inverse correlation between magma chamber depth and spreading rate. These observations indicate that the paradigm of ridge crest magma chambers as small, sill-like, midcrustal bodies is applicable to a wide range of intermediate- and fast-spreading ridges.

Cross section of an accretionary wedge; Barbados Ridge Complex

Many major geological terranes are interpreted as accretionary complexes, and there are several speculative models for their structure and mode of formation. The seismic reflection section across the Barbados Ridge complex at lat 16°12′N presented here shows, for the first time, the entire cross-sectional shape of a large accretionary wedge and its forearc basin. Atlantic oceanic crust underlies 122 km of the wedge and then passes beneath the crust of the forearc of the Caribbean plate, where it can be traced 15 km farther; it dips landward at 9°. The forearc basement dips seaward to meet the ocean crust. The maximum thickness of the wedge is about 10 km. A layer of sediments, 1 km thick, is drawn beneath the accretionary wedge on the surface of the oceanic crust, with little disturbance, for a distance of 70 km, and some sediments still appear to adhere to the ocean crust to where it passes beneath the forearc basement. It is not clear whether sediment is subducted deeper, but it appears probable. The principal resistance to landward motion of the accretionary wedge is provided by the weight of up to 6 km of forearc-basin sediments on the seaward-dipping forearc basement. Both the forearc sediments and the basement have been deformed as a consequence of the horizontal compression produced by the subduction of ocean 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 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.

Uniform accretion of oceanic crust south of the Garrett transform at 14°15′S on the East Pacific Rise

Using migrated common depth point reflection profiles, we find the structural differences along the ultrafast spreading (>150 mm/yr) East Pacific Rise south of the Garrett fracture zone are second-order, suggesting a remarkably uniform process of crustal accretion. The rise axis south of the Garrett transform is underlain by a narrow (<1.0 km) melt lens which shows great along-strike continuity. The depth of the axial melt sill is approximately 1200 m beneath the seafloor which is about 400 m shallower than along the slower spreading East Pacific Rise at 9°30′N. This observation strengthens the argument that the depth to the top of the crustal velocity inversion is spreading rate dependent. Melt sill width, however, shows little variation along the East Pacific Rise, suggesting no dependence of magma chamber size on spreading rate. The melt reservoir decreases in width toward/across the 14°27′S ridge axis discontinuity by a modest 250–300 m and appears to be continuous across this feature. Given the small aspect ratio (∼1.0 km by ∼50 m by tens of kilometers) of the axial melt lens, the previously recorded jump in MgO content across the 14°27′S offset is likely the result of a mixing boundary which is sustained through an along-strike impedance in convection. Wide-angle reflections originating at the base of seismic layer 2A, assumed to coincide with the extrusive layer, reveal a twofold to threefold increase (200–250 to 500–600 m) in thickness within 1–2 km of the rise axis. The pattern of extrusive thickening imaged south of the Garrett transform is similar to that observed along the slower spreading (110–120 mm/yr) East Pacific Rise at 9°N. Outside of the neovolcanic zone mean extrusive thickness is relatively invariant along a profile and from profile to profile. This implies a degree of temporal stability of the along-strike magma supply when integrated over the 10 kyr that corresponds to the width of the neovolcanic zone. The inferred uniformity of off-axis mean extrusive thickness is inconsistent with the conjecture that decreases in axial volume toward the 14°27′S discontinuity are caused by long-term reductions in magma supply. Second-order differences in the style of extrusive thickening may be related to structural differences within the low-velocity zone underlying the rise axis and/or changes within the stress field in the overlying carapace which results in the diffuse emplacement of lavas near the rise axis. Images of Mono on cross-axis profiles may be traced to within ∼1.0 km of the melt sill edge; this observation is in agreement with rise crest models which generate the lower crustal section through the advection of material down and outward from the axial melt lens rather than through cumulate deposition at the base of a large magma chamber.

Crustal structure at the Blake Spur Fracture Zone from expanding spread profiles

We present results from WKBJ and reflectivity synthetic seismogram modeling of 10 reversed expanding spread profiles (ESPs), along flow lines parallel to the Blake Spur fracture zone across 140 Ma Atlantic crust. These profiles provide detailed constraints on variations in crustal structure at the fracture zone. Seven profiles on either side of the fracture zone show a normal structure for old oceanic crust. A 2–3 km thick upper layer with a steep velocity gradient is underlain by 4–5 km of crust with velocities of 6.5–7.2 km/s and low gradient and a sharp transition to upper mantle velocities of 8 km/s. Several velocity discontinuities were detected within the upper 2–3 km, but these do not generally coincide with intracrustal reflectors detected by simultaneous normal incidence reflection profiles. This structure shows little regional variation toward the fracture zone. Despite the small magnetic anomaly offset (∼12 km) and the indistinct topographic signature of the fracture zone, three ESPs within a 10–20 km wide ribbon centered on the fracture zone trough show clearly anomalous crustal structure, relative to normal oceanic crust. A 2–4 km thick high gradient upper layer is underlain by a thick prism of material with a velocity of 7.2–7.6 km/s and high Poissons ratio (probably at least 0.29), which is consistent with 15–30% serpentinization of upper mantle peridotites. This interpretation requires the action of off-axis hydrothermal circulation, penetrating the cracked and relatively permeable fracture zone lithosphere to a depth of at least 7 km. The original igneous crust in a narrow region close to the fracture zone is thus inferred to have been much thinner than adjacent “normal” crust, which may imply a sharp reduction in the magma budget at the ends of the adjacent spreading segments.

New seismic images of oceanic crustal structure

Remarkable new images of the internal structure of oceanic crust have been recorded from a two-ship multichannel seismic survey in the western North Atlantic. Results include strong, planar dipping reflectors in the lower crust, evidence of whole-crustal failure generating reflectors from the top of the basement to the Moho, and banded layering in the lower crust and in some places in the upper mantle. Anomalous crust in the Blake Spur Fracture Zone is documented from both normal incidence and wide-angle seismic profiles The internal structure of the crust records the magmatic and tectonic history of the crust as it was generated at, and then moved away from, the spreading center.

Seismic structure of oceanic crust in the western North Atlantic

We examine the seismic structure of Cretaceous-aged ocean crust north of the Blake Spur fracture zone in the western North Atlantic by using a combination of multichannel seismic and wide-angle reflection/refraction data. Although the oceanic crust in this area is characterized by a relatively uniform thickness and seismic velocity structure, it displays large variations in crustal reflectivity on both ridge-normal and ridge-parallel profiles. The upper crust is highly reflective and contains both subhorizontal (or shallow dipping) events and more steeply dipping reflectors. Subhorizontal reflectors are typically present between 1.5 and 2 km depth and are, in some cases, laterally continuous for distances of 15 to 20 km. Generally, these events do not correlate with the depth of the seismic layer 2/layer 3 boundary determined from refraction data, and their origin is still poorly understood. The steeply dipping events are generally confined to the upper 2.5 km of the crust. We interpret these reflectors as the subsurface expression of ridge-parallel extensional faults commonly mapped at mid-ocean ridges. The lower crust in this area exhibits alternating regions of high and low reflectivity. The highly reflective zones are made up of packages of linear or concave-upward dipping reflectors that flatten out in a diffuse zone of high reflectivity near the base of the crust. Some of these dipping reflectors appear to cut through the whole crustal section, although most fade out in the acoustically transparent midcrust. On ridge-normal profiles the majority of these events dip to the east, toward the paleo-spreading center, whereas on ridge-parallel profiles the events typically dip south, toward the Blake Spur fracture zone. The base of the crust is generally not associated with a strong, discrete Moho reflection but is a comparatively indistinct boundary usually associated with a 1- to 2-km-thick band of diffuse high reflectivity. We interpret the lower crustal and whole crustal dipping events as the subsurface expression of major fault systems that have ruptured the entire crustal section down to depths of 8–10 km. With the available data we cannot unambiguously determine the geometrical relationship between the dipping lower crustal reflectors on the ridge-normal and ridge-parallel profiles. We may be imaging a single major detachment surface that dips both toward the ridge axis and away from accommodation zones linking major boundary faults. Alternatively, these events may represent two different classes of fault systems that form during different stages of the emplacement and aging of oceanic lithosphere.

Tertiary sedimentary history and structure of the Valencia trough (western Mediterranean)

Abstract We present here main results of the Common Depth Point (CDP) data acquired during the Valsis 2 Cruise in 1988 in the Valencia trough. The profiles are tied in with industrial well data and this correlation allows the sedimentary and structural history of the region to be deduced. The Valsis Cruise seismic profiles have been supplemented by a very dense grid of industrial seismic lines and these data permit us to establish an accurate depth to basement map. The formation of the initial grabens, coeval with those of the Gulf of Lions, is related to the Early Miocene opening of the northwestern Mediterranean basin and the Barcelona graben is filled by the same sedimentary layers, including evaporites, as that of the Provencal region. Nevertheless, the Valencia-Catalan grabens have been reactivated by young extensional tectonics which could be a consequence of the convergence of Africa relative to Europe. The Valencia trough is segmented by transfer faults which trend NW-SE. These faults, which have a more accentuated structural expression than the Valencia and Catalonia grabens, may act as transform faults separating the individual Balearic Islands. The transfer faults are in strike with volcanic ridges which have been sampled during the DSDP Leg 13. The dense seismic grid allows us to delineate several widespread volcanic features in the Valencia trough which have been active from the Early Miocene to the Pleistocene. However, we note that the volcanic features are mainly Miocene in age whereas the recent volcanism is restricted to a narrow zone (Columbretes Islands). The compressional tectonics which deformed the Balearic Islands does not appear to extend far towards the North. We delineate the compressional front north of Ibiza, but we failed to determine any thrust or fold north of Mallorca, whereas an extensional tectonics is evident.