INTRODUCTION AND GEOLOGICAL ENVIRONMENT

The Azores archipelago (see Fig.1) lies in the area of the Triple Junction ([1], [2]) between the North-American, Euroasiatic and African tectonic plates. It is characterized by an irregularly shaped volcanic plateau, and is formed by nine islands, divided into three major groups. Volcanism in this region of extension has created giant linear volcanic ridges, notably those forming the Islands of S. Jorge and Pico. These are some of the largest such volcanic ridges actively forming anywhere on earth and represent an important mode of crustal formation, analogous to that observed at mid-ocean ridges. The Azores area is also unique for having so many active volcanic ridges over a wide range of water depth.

Figure 1: Location of the Azores Islands. Also shown the plate boundaries (heavy lines) and the main fracture zones (data taken from http://www.gsj.go.jp/dMG/free/plates/Intro.html, compiled by NOAA Global Relief Data on CD-ROM (93-MGG-01, original digitizing performed by PLATES project at the University of Texas, edited for plotting with GMT [14] by Kensaku Tamaki, Ocean Research Institute, University of Tokyo).

A joint Italian (CNR) and British (Univ. of Oxford) project was presented during 1998 to study this distribution to test theories for how volcanic morphology varies with pressure, due to effects of variable gas exsolution from the magma, and address the interplay between volcanism and tectonics in the genesis of the ridges. The main practical objective of work at sea was to map the structures with TOBI (the Towed Ocean Bottom Instrument of the SOC, Southampton, UK)([3],[4]), in order to resolve their origin, in particular by resolving relationships between recent volcanism and major faults.

Our other objectives were (a) to locate hydrothermal springs and determine what geological structures favour them in particular their relation to historical submarine eruptions and faults observed in TOBI images, and (b) to contribute to the general understanding of volcanic and submarine land slides hazards in the Azores region.

Figure 2: R/V Urania, CNR, operated by SO.PRO.MAR Spa.

Funding for the TOBI operation and personnel was provided by a grant from the EEC under the EASSS (European Access to Sea Floor Survey System) scheme. Since TOBI had already been deployed on R/V Urania during the 1998 TIVOLI cruise, the Italian research vessel has been considered to be suitable for the installation. A request to CNR for Urania ship-time was submitted and approved, and a cruise of 35 days (AZ99) was scheduled for july 1999. The EEC funding also included travel and subsistance costs for researchers and graduate students involved in post-cruise data processing and training at SOC. In order to allow as wide experience of TOBI surveying as possible, participation of researchers from several European Institutes was planned, including researchers of Portuguese Universities. Participation included all research aspects, from data acquisition to interpretation.

Taking the opportunity of the transit to Gibraltar, as a part of a collaboration agreement with the ENEA (Centro di Ricerche Ambiente Marino of La Spezia, Italy), two days of the cruise time were devoted to water sampling in the Gibraltar Strait within the framework of the EC MAST III Programme CANIGO (Canary Islands Azores Gibraltar Observations). The ocean domain covered by the CANIGO Project includes the sub-tropical North Atlantic gyre and the Mediterranean-Atlantic exchange. The main objective of the oceanographic research was to contribute to the general understanding of water fluxes trough the Strait of Gibraltar, by the analysis of trace metals, metalloids and radionuclides content in the water sampled collected. In addition, the data collected will allow improvements in estimate of exchanges between the Atlantic and the Mediterranean Sea, solving the fine structure of the surficial Atlantic inflow and deep Mediterranean outflow.

Figure 3: Navigation data.

R/V Urania (Fig.2) left Civitavecchia 30-jun-1999 on schedule. Water samples were collected by the ENEA team in the Mediterranean and Atlantic waters around the strait and on 5 july the team was disembarked in Portimao (Portugal). Ship arrived in the Azores I. 8-jul-1999, and started to work with TOBI after a short bathymetric and CTD survey.

During the first leg, we had 10 days of continuous TOBI, chirp Sonar, bathymetry and magnetics data collection, southeast of Pico, S. Jorge and Terceira Islands. The first leg ended 19-july-1999 with a port call in Horta (Faial I.), where a partial changeover of the scientific crew occured. Weather conditions were good for almost all of the first leg.

Figure 4: Location of the stations of the CANIGO sampling.

The second leg started 21-jul-1999 and ended late 28-jul-1999, when the ship headed to Civitavecchia, docking 4-aug-1999

Whilst passing Terceira 28-jul-1999, the ship passed close to the eruption of the submarine volcano Cerreta. We observed vapour jets and floating lava blocks.

Fig.3 shows the navigation data including the transits. Fig.4 shows the locations of water samples collected by the CANIGO team. Fig.5 shows the navigation data of the two legs, along with the areas we originally intended to survey with TOBI.

Figure 5: TOBI Planned areas, actual navigation lines and CTD/XBT stations (black circles).

Hereafter, a description of the equipment and of their usage is given, along with details of the general setting, performances and results (Chapter 4). Chapter 5 presents the Oceanographic setting and the problems encountered in the sound velocity data acquisition and processing. Chapter 6 describes the sidescan sonar and swath bathymetry data processing.