Deep structure of the ocean-continent transition in the southern Iberia Abyssal Plain from seismic refraction profiles:: The IAM-9 transect at 40°20′N

We present a crust and mantle velocity structure for the West Iberia passive continental margin derived from a 320-km-long wide-angle seismic profile acquired in the southern Iberia Abyssal Plain. We observe a 170-km-wide ocean-continent transition zone which includes a pair of overlapping peridotite ridges and is bounded by oceanic crust and landward by fault-bounded blocks of continental crust. The profile lies similar to 40 km south of the transect sampled by Ocean Drilling Program (ODP) Legs 149 and 173. The transition zone structure can be divided into an upper layer, 2-4 km thick with velocities of between 4.5 and 7.0 km s(-1) and generally a high-velocity-gradient (1 s(-1)), and a lower layer up to 4 km thick with a velocity of similar to 7.6 km s(-1) and a low-velocity-gradient. A weak Moho reflection in this zone was seen only on wide-angle profiles at an offset of similar to 30 km. The upper layer has a distinctly lower velocity than thinned continental crust adjacent to the continental slope. Conversely, the lower layer has too high a velocity to be magmatically intruded or underplated lower continental crust. On the coincident seismic reflection profile, fault-bounded crustal blocks, identified in unequivocal extended continental crust, are not observed in the transition zone. The upper layer has velocity bounds and gradient similar to oceanic layer 2 observed west of the peridotite ridges, but no oceanic layer 3 velocity structure is present. While magnetic anomalies have been identified within the transition zone, they have not been modeled successfully as seafloor spreading magnetic anomalies, nor do they generally form long linear margin-parallel features. Finally, ODP boreholes, similar to 40 km north of our profile and within the interpreted transition zone, have recovered up to 140-m-thick sections of serpentinite and serpentinized peridotites with little evidence of mafic igneous material. We conclude that the transition zone cannot be dominantly composed of either extended continental crust or oceanic crust. Although current melting models predict a considerably thicker crust of decompression melt products, we interpret this region as exposed upper mantle peridotite with little or no synrift extrusive material and limited amounts of synrift material intruded within the serpentinized peridotite.