Abstract:
The Atlantic margin of Spain and Portugal is widely considered to be the classic non-volcanic passive continental edge. Mesozoic rifting between North America and Europe/Africa occurred in three stages, the last of which, during the Early Cretaceous, corresponded to a northward-propagating breakup of Iberia from its conjugate margin, the Grand Banks of Newfoundland. Relatively thin sediment cover and a lack of syn- and post-rift volcanism make this margin one of the best places on Earth to image clearly the Moho and deep crustal structures created by such rifting. Results from ODP Legs 149 and 173 have shown that crustal structure of the western Iberia margin is characterized by a approximately 10 km wide ocean-continent transition (OCT). Drilling of topographic highs indicates that uppermost crust within the OCT consists mostly of serpentinized peridotite, believed to be exhumed upper mantle. A seismic experiment, R/V Maurice Ewing cruise 97-05, conducted in July-August 1997 by an international consortium led by Rice University, acquired MCS reflection data across and along the OCT. Simultaneous far-offset reflection and refraction data were recorded along many of the lines, with ocean-bottom seismometers (OBSs) from UTIG, ocean-bottom hydrophones (OBHs) from GEOMAR, and land stations from the universities of Madrid and Barcelona. MCS profile 9 runs north-south, parallel with and approximately 200 km west of the coast of Portugal. This profile characterizes the bathymetric contrast between Galicia Bank in the north ( approximately 2 km water depth) and the Iberia Abyssal Plain (IAP) in the south ( approximately 5 km water depth). Profile 9 is 270-km-long, with 10 OBS receivers located at 15-50 km spacing along its length. Along the southern half of profile 9, interpreted sediments are continuous and up to 4500 m (2.7 s twt) thick. Farther north, on southeastern Galicia Bank, sediment accumulation occurs only in isolated basins. Acoustic basement in the IAP is occasionally overlain by interpreted pre/syn-rift sediments. Strong, multiple sub-basement reflections reveal complex extensional faulting beneath the IAP. Some of these faults, occurring at 3 to 4 s sub-seafloor, may be related to a southward extension of "S", a group of bright reflectors which underlies tilted fault blocks beneath western Galicia Bank. The "S" reflector has previously been interpreted either as Moho or as an intra-crustal detachment fault, but has not yet been sampled. Ray-tracing and inversion of the travel times observed on the OBS data reveal a southward-thinning wedge of presumed extended continental crust stretching the entire N-S length of the IAP imaged, from the flank of Galicia Bank to the southern end of profile 9. A clear Moho reflection, with a P-wave velocity of approximately 8.0 km/s, lies at the base of this wedge, reaching depths as shallow as approximately 6 km below the sea floor or just approximately 1700 m below the top of acoustic basement. There is no evidence of substantial serpentinization of the upper mantle along profile 9, as defined by the presence of a lower crustal layer with P-wave velocities of 7.0-7.6 km/s.