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Deep Seismic Structure of the SW Iberian Margin: Crustal Affinity of the Geological Domains and Geodynamic Implications

Valenti Sallares, Sara Martínez-Loriente, Eulàlia Gràcia, Cesar R. Ranero, Audrey Gailler, Marc-Andre Gutscher, Jordi Diaz, Juanjo Dañobeitia, and Nevio Zitellini

We investigate the deep structure of the SW Iberian margin along two regional wide-angle seismic profiles. Profile P1 crosses the central Gulf of Cadiz from N to S starting in the Portuguese continental margin. It is 270 km-long and include 16 Ocean Bottom Seismometers (OBS) and 7 landstations. P2 goes from the Tagus to the Seine abyssal plains crossing the Gorringe Bank, is 320 km-long and includes 30 OBS. The data were modeled by joint refraction and reflection travel-time inversion to obtain the seismic velocity field and the main reflectors geometry.

P1 shows the ~30 km-thick Variscan crust in the north, a 60 km-wide transition zone, and a ~7 km-thick and ~150 kmwide oceanic crustal section. The sharp transition between continental and oceanic crust is best explained by an initial rifting setting as a transform margin during the Early Jurassic that followed the Central Atlantic continental break-up. The narrow oceanic basin would have formed during a seafloor spreading episode between Iberia and Africa that lasted up to the initiation of North Atlantic oceanic spreading. This oceanic plate segment could be the last remnant of an oceanic corridor that once connected the Alpine-Tethys with the Atlantic ocean, so it is, in turn, one of the oldest oceanic crustal fragments currently preserved on Earth. The presence of oceanic crust is consistent with the postulated existence of a narrow, westward retreating slab beneath the Gibraltar Arc.

In P2, the basement structure is similar in the Tagus and Horseshoe plains. It shows a top layer with a velocity gradient twice stronger than oceanic Layer 2 that change to an underlying layer with a five-fold weaker gradient, and there is no evidence of a sharp crust-mantle boundary, suggesting that sediment overlays directly exhumed mantle. The estimated serpentinization degree decreases from ~80 % near the top of Gorringe Bank to less than 5% at ~20 km deep. These domains could have been originally part of a single exhumed mantle band, of nature and origin similar to the Iberia Abyssal Plain, generated at the beginning of the North Atlantic opening (Early Cretaceous). During the Miocene, the NW-SE trending Eurasia-Africa convergence resulted in thrusting of the SE segment of the exhumed mantle band over the NW one, forming the Gorringe Bank. The Seine abyssal plain shows a 6-7 km-thick oceanic crust, probably coeval to the southern P1, but generated during the Central Atlantic opening.

AAPG Search and Discovery Article #90161©2013 AAPG European Regional Conference, Barcelona, Spain, 8-10 April 2013