Seismic structure and crustal nature of the geological provinces off the SW Iberian margin: results of the NEAREST-SEIS wide-angle seismic survey
Abstract
The region offshore the SW Iberian margin hosts the present-day NW-SE plate convergence between the European and African Plates causing continuous seismic activity of moderate magnitude. In autumn 2008 a Spanish-French team carried out a wide-angle seismic survey in the area (NEAREST-SEIS cruise), in the framework of the EU-funded NEAREST project. During the survey two long profiles were acquired using a pool of 36 Ocean Bottom Seismometers (OBS), to provide information about the crustal structure and nature of the different geological provinces, and to reveal the deep geometry of the main fault interfaces. A total of 30 OBS were deployed along profile P1, which is 356 km long and trends NW-SE from the Tagus abyssal plain (TAP), crossing the Gorringe bank (GB), the Horseshoe abyssal plain (HAP) and the Coral Patch Ridge (CPR), up to the thrust-and-fold belt of the Seine abyssal plain (SAP). The inverted model along this profile show four different domains. In the TAP a 3-4 km-thick, low velocity sedimentary layer covers the basement, which shows a velocity >7 km/s just 2-3 km below its top, similarly to that of the basement outcropping in the Gorringe bank. In the HAP the sedimentary cover is thicker, showing an upper unit with low velocity corresponding to the Horseshoe gravitational unit, on top of a higher velocity lower unit, which may represent the highly consolidated Mesozoic sedimentary sequence, with a total thickness of 5 km. The basement shows the same velocity distribution as in TAP and GB, suggesting a common nature and origin. According to its seismic structure, we interpret this basement as highly serpentinized, exhumed mantle. In contrast, the CPR and SAP show evidences for the presence of a well-developed, 6-7 km-thick oceanic crust, underlying the sedimentary sequence. Profile P2 is 256 km long, and trends S-N from the easternmost SAP beyond the NW Moroccan margin, crossing the Gulf of Cadiz imbricated wedge and the Portimao bank ending at the Iberian margin shelf. 15 OBS and 7 land-stations were deployed along this profile. The inverted model shows two main domains: In the southern half, there is a 3-4 km-thick cover of low velocity sediments, which represents the western edge of the wedge that covers the internal Gulf of Cadiz, overlying a 7-8 km-thick oceanic crust. According to recent tectonic reconstructions, this crustal segment should have been emplaced there during the early phase of continental spreading between Iberia and Africa, in the context of Mesozoic Atlantic spreading. The northern part of P2 displays a relatively sharp ocean-continent transition zone concentrated in a ∼50 km-wide band, that ends with the ∼30 km-thick continental crust of the SW Iberian shelf.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.T14D..06S
- Keywords:
-
- 7220 SEISMOLOGY / Oceanic crust;
- 7270 SEISMOLOGY / Tomography;
- 8104 TECTONOPHYSICS / Continental margins: convergent