Seismic structure of the volcanic apron north of Gran Canaria

High-resolution reflection seismic profiles through the volcanic apron north of Gran Canaria collected during Meteor Cruise 24 were interpreted in the light of results from Leg 157 (Sites 953 and 954). The shape of the submarine island flanks of Gran Canaria and the two adjacent islands of Fuerteventura to the east and Tenerife to the west were reconstructed by interpretating seismic profiles that penetrated the sediments covering the deeper portions of the volcanic pedestals. The ~4750-m-deep flank of Fuerteventura is the oldest submarine island flank, influencing the subsequent shield-building of Gran Canaria to the east, whose 16- to 15-Ma shield is ponded against Fuerteventura, forming a topographic barrier between the islands. The associated reduction of the current cross section has caused strong bottom currents, indicated by erosional features and contourites. To the north, the flank of Gran Canaria extends 60 km seaward to a depth of ~4500 m. The shield of the Anaga massif on northeast Tenerife onlaps the flank of Gran Canaria to the east. Seismic correlation of the feathered edge of the Anaga shield (~50 km off Tenerife at a depth of 4000 m) to the bio- and magnetostratigraphy at Site 953 results in an age of ~6 Ma. The surrounding sedimentary basin is characterized by chaotic and discontinuous reflection patterns of the slope facies, turning into well-stratified basin facies ~30–40 km off the coast. The westward decrease of reflectivity in the northern apron is interpreted to be caused by the submarine ridge off Galdar at the western limit of the north coast of Gran Canaria, through which mass flows from Gran Canaria entering the sea in the north were diverted to the northeastern part of the apron. The volcanic activity correlates with the sedimentation rates in the apron. The lowest rate corresponds to the volcanic hiatus on Gran Canaria (9–5 Ma) with 3–4 cm/k.y., and the highest rate (up to 12 cm/k.y.) was found during the voluminous Miocene volcanism on the island. A number of large mass-wasting events could be identified, interbedded with the pelagic background sedimentation. The basaltic breccia drilled at Site 954 (lithologic Unit IV) is interpreted to represent the deposits associated with a slope failure at the northern flank of Gran Canaria at 12 Ma. The seismic mapping reveals >60 km3 of debris advanced at least 70 km into the apron. The volume fits well with the dimensions of an amphitheater at the northern flank of Gran Canaria. The Quaternary volcanism on La Isleta at northeast Gran Canaria extends further seaward, where the seismic data show young lava flows. Other submarine volcanism occurred in the channel between Gran Canaria and Fuerteventura

Comparison of seismic reflection data to a synthetic seismogram in a volcanic apron at Site 953

The volcanic apron of Gran Canaria at Site 953 is characterized by numerous, closely spaced reflectors, allowing a highresolution stratigraphic correlation. The calibration of the presite survey seismic data (during the Meteor Cruise 24) with regard to the lithology and stratigraphy found at the drill site was achieved by computing a synthetic seismogram serving as the link between seismic and borehole data. Because logging data were available for only 53% of the hole, velocity and density measurements taken from the recovered cores were used in the missing intervals to obtain a complete synthetic seismogram. Most reflectors in the upper ~900 m of the sequence (lithologic Units I–V) turned out to be thin volcaniclastic layers intercalated to the nonvolcanic background sediments. Their thicknesses are generally <2 m, and the reflections from their tops and bases overlap, forming a single reflection. The limit of the seismic detection of such interbeds is on the order of several decimeters and thus requires special care for the processing of the velocity and density data to avoid destruction of the signal from these thin layers