Deep structure of the ocean‐continent transition in the southern Iberia Abyssal Plain from seismic refraction profiles: Ocean Drilling Program (Legs 149 and 173) transect

Abstract

Abstract. We present a wide-angle seismic refraction study of an 80x40 km region of the southern Iberia Abyssal Plain, south of Galicia Bank. An intersecting grid of two E-W and four N-S wide-angle reflection/refraction profiles is used to define variations of the basement velocity blocks immediately south of Galicia Bank. This crust is underlain by a high-velocity layer (7.3-7.9 km/s) of weakly serpentinized (i.e., 0-25%) peridotite, which exists throughout the eastern part of the survey area. Basement within the OCT appears to consist dominantly of a broad region of exposed upper mantle that has been serpentinized heterogeneously both vertically and horizontally. In the southeast sector of our survey where basement topography deepens and becomes subdued, continental fault blocks are absent; instead, basement contains an upper layer of more pervasively serpentinized (i.e., 25-45%) peridotite that is ~2 km thick. This layer is characterized by low velocity at the top of basement (4.2 km/s) that increases rapidly with depth, and it probably corresponds to a seismically unreflective layer, previously identified in reflection profiles to the south of our survey. In the western section of our survey, beneath a series of elevated basement ridges, velocities are reduced within both the upper •asement layer (3.5-6.0 km/s) and lower layer (6.4-7.5 km/s). These changes suggest that both upper and lower layers have become more highly serpentinized (with values of 60-100% in the upper layer and 25-45% in the lower layer) probably during the last stages of rifting and immediately before formation of oceanic crust. A normal or slow spreading oceanic crustal structure is not found within the survey region. Thus it appears that the onset of seafloor spreading occurs in the region west of the peridotite ridge sampled at ODP Site 897 and east of the J magnetic anomaly.

The role of rifting in the development of the continental margins of the southwest subbasin, South China Sea: Insights from an OBS experiment

“…6), which represent the top basement detachment faults exhumed in the lower crust footwall. Our observation of a sharp velocity boundary is compatible with the wide-angle refraction seismic data in the Iberia-Newfoundland rifted margins (Chian et al 1999). Thus, we used the final detachment faults as the boundary between the proximal domain and distal domain of the SWSB margins by employing the definition from the Iberia-Newfoundland margins (Péron-Pinvidic and Manatschal 2009).…”

East Greenland Ridge in the North Atlantic Ocean: An integrated geophysical study of a continental sliver in a boundary transform fault setting

“…Carlson and Miller, 1997;Chian et al, 1999;Whitmarsh et al, 1996;Pickup et al, 1996]. If the upper basement layer is thin (<7 km) [Skelton et al, 2005], serpentinization can be achieved by hydrothermally driven water influx, possibly aided by circulation along deep faults.…”

Non-volcanic rifted margins, continental break-up and the onset of sea-floor spreading: some outstanding questions

“…This is necessarily located in the most distal part of the margin, where hydrotherreal circulation and metamorphism of mantle rocks are possible, owing to the thinning and fracturing of the upper continental crust (Boillot et al 1989b). In this case, the definition as lower crust is based on the seismic character (P-wave velocities ranging between 6.5 and 7.9kin s-l; see Recq et al 1996;Whitmarsh et al 1996c;Chian et al 1999;Dean et al 2000) and not on its geological nature. Actually, 'undercrusting' (crustal thickening by addition of serpenfinite at the bottom of the crust) results in this case from hydrothermal transformation of fresh peridotite into serpentinized peridotite of lower density and seismic velocity.…”