TOPOMED-GASSIS_LEG1 Cruise, RV Sarmiento de Gamboa

Abstract

The TOPOMED is a Collaborative Research Project is based in the characterization of the lithospheric structure of the Gibraltar Arc region, including the Gulf of Cadiz and Alboran Sea, and the syudy of the resulting effects in the relief. Within the TOPOMED national project, GASSIS (Gibraltar Arc System Seismic Investigations Survey) is the first of two cruises that will be carried out in the South Iberian Margin. The TOPOMED-GASSIS cruise has been has been successfully accomplished onboard the R/V Sarmiento de Gamboa, from the 3rd to the 17th October (Leg 1) leaded by E. Gràcia and R. Bartolomé (UTM-CSIC) and from 18th to 29th October (Leg 2) leaded by C.R. Ranero (ICREA/ICM-CSIC). In addition to the seismic imaging of active faults and large structures of the South Iberian Margins, the TOPOMED-GASSIS cruise also included the implementation of new cutting edge marine seismic instrumentation (> 3.7 km streamer). Thus, Leg 1 team departed from Vigo on the 27th September and tests along the Galician and Huelva coasts were carried out until the 2nd October included. The tests included the deployment and recovery operations of a 5.3 km streamer and one seismic array of > 3000 c.i. Two test lines were shot and quality control was carried out by the Leg 1 scientific party. Failures detected (i.e. low winch speed and problems with the winch break) were solved on board before the start of the cruise with the help of a specialized engineer who embarked for a day. The main aim of the cruise is to characterize the crustal and upper mantle structure of the South Iberian Margin. We focused on: a) the investigation of shallow processes (i.e. active faults, fluid venting, mass fluxes) and b) the imaging and characterization of the deep crustal and mantle structure of the area. These objectives could only be achieved by using the acoustic and new MCS instrumentation of the Spanish RV Sarmiento de Gamboa, which has provided unprecedented quality of data in the region. The data will permit to efficiently attenuate the seafloor multiple energy, and to use state of the art processing and imaging techniques to obtain the deep tectonic structure of the margin. To accomplish this goal we have used up to two high volume G-gun arrays and up to 6 km long Sercel multichannel digital streamer (480 channels) towed behind the vessel at a survey speed of 4.5-5 knots. Simultaneously to the seismic survey, we have used the hull mounted swath-bathymetry system to map unsurveyed areas, the hull mounted sub-bottom profiler Parasound to image the shallow sediments 0-100 m below the seafloor, a hull-mounted gravimeter and a shallowtowed magnetometer