Seismic reflection profiles across many continental margins have imaged bottom-simulating reflectors (BSRs) parallel to the seabed; these are often interpreted as the base of a zone in which methane hydrate "ice" is stable. Waveform inversion of seismic reflection data can be used to estimate from seismic data worldwide the velocity structure of a BSR and its thickness. A test of this method at a drill site of the Ocean Drilling Program predicts that sediment pores beneath the BSR contain free methane for approximately 30 meters. The hydrate and underlying gas represent a large global reservoir of methane, which may have economic importance and may influence global climate.

No abstract

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 Poisson's 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.

Abstract.The Southwest Indian Ridge is a slow spreading end-member of the mid-ocean ridge system. The deepest borehole penetrating the lower oceanic crust, Ocean Drilling Program hole 735B, lies on the eastern transverse ridge of the Atlantis II Fracture Zone at 57øE. A wide-angle seismic survey in the vicinity of the borehole reveals a crustal structure that is highly heterogeneous. To the east of Atlantis Bank, on which hole 735B is located, the crust consists of a 2-2.5 krn thick high-velocity-gradient oceanic layer 2 and a 1-2 krn thick low-velocity-gradient layer 3. The transform valley has a 2.5-3 km thick crust with anomalously low velocities interpreted to consist largely of highly serpentinized mantle rocks. The seismically defined crust is thickest beneath the borehole, where layer 2 is thinner and the lower crust is inferred to contain 2-3 krn of partially serpentinized mantle. The seismic velocity models are consistent with gravity data which show weak residual mantle Bouguer anomalies because the regions of thinner crust have lower crustal densities. Stress variations deduced from mass balances between the transform valley floor and the adjacent transverse ridges are much larger than the likely threshold for lithospheric failure and therefore indicate that the relief is supported dynamically. The variation of crustal thickness with spreading rate defined by data from the Southwest Indian Ridge and elsewhere is consistent with models of melt generation in which the upwelling mantle is cooled by conductive heat loss at very slow spreading rates, resulting in reduced melt generation under the spreading axis. Large segment-scale variations in crustal thickness suggest subcrustal along-axis migration of melt toward segment centers.

Abstract. Strong bottom-simulating reflectors (BSR) have been mapped over a regionof approximately 50,000 km 2 on the southeastern U.S. margin and have been associated with possible abundance of natural gas hydrates. In June 1992, coincident single-channel seismic and wide-angle ocean bottom seismic data were acquired in the region, focusing on the Blake Ridge and the Carolina Rise. Wide-angle reflections from the BSRs were clearly observed at offsets up to •6 km. Joint travel time inversion was conducted with wide-angle and vertical-incidence data in order to explore possible regional variation, and the resultant two-dimensional average velocity models imply higher background velocities on the Carolina Rise. Full waveform inversion was then performed to determine the seismic origin of the BSRs. The best fit model shows a similar low velocity (• 1.4 km/s) beneath the BSR at both sites, indicating trapped free gas with low saturation (< 10%). The inversion results also indicate that a thin, high-velocity wedge, with a maximum velocity of •2.3 km/s, is present just above the Blake Ridge BSR. Sediment reflectivities were also calculated, and higher reflectivities are observed on the Carolina Rise. An increase in reflectivity below the BSR seems to correspond to the gas-bearing zone at both sites.Concentration of hydrates were e•timated based on these velocity models. Whereas average hydrate concentration of 3% of the total sediment volume is suggested for the lower half of the hydrate stability zone at the Blake Ridge, only a very low average concentration of hydrate can be expected at the Carolina Rise. The hydrates seem to be concentrated near the base of the hydrate stability zone, and the maximum hydrate concentration is estimated as •20% at the Blake Ridge and •7% at the Carolina Rise, both of which are too high to be explained by in situ biogenic activity only and require some secondary accumulation mechanism. It is suggested that hydrate recycling caused by the stability field migration may have effectively condensed hydrates at both sites. Additional enhancement by upward fluid expulsion may also be viable for the Blake Ridge.