4. PRELIMINARY RESULTS OF THE GALINAUTE CRUISE: DIVES OF THE SUBMERSIBLE NAUTILE ON THE WESTERN GALICIA MARGIN, SPAIN 1 Gilbert Boillot 2 , Maria C. Comas 3 , Jacques Girardeau 4 , Jacques Kornprobst 5 , Jean-Paul Loreau 6 , Jacques Malod 7 , Denis Mougenot 2 , and Michel Moullade 8 ABSTRACT In 1986, the French submersible Nautile was used to investigate at depths of 3500 to 5500 m in the area west of Gali- cia (Spain) where the JOIDES Resolution had drilled 12 holes in 1985 during ODP Leg 103. Three different kinds of seafloor were investigated with the Nautile: (1) the Galicia passive margin, where crystalline continental basement and Paleozoic(?)-Mesozoic sediments outcrop; (2) a basaltic seafloor, interpreted as a piece of Cretaceous oceanic crust that is presently incorporated into the northern slope of Galicia Bank as a result of Eocene tectonics; and (3) an ultramafic seafloor, composed of serpentinized peridotite locally cut by dolerite dikes. The ultramafic rocks form a 100-km-long belt between North Atlantic oceanic crust to the west and thinned continental crust of the Galicia margin to the east. New data from the Nautile dives complement and confirm some of the ODP Leg 103 results; they also force reas- sessment of some of the preliminary interpretations proposed by the Leg 103 shipboard scientific party. INTRODUCTION The passive continental margin west of Galicia (Spain) is characterized by a thin and discontinuous sedimentary cover. These conditions are favorable for studying local outcrops on the seafloor of the crystalline basement of the margin and the first sediments deposited on that basement; equivalent rocks are deeply buried under younger sediments on most other margins. Subsequent to Ocean Drilling Program (ODP) Leg 103 drilling on the Galicia margin in 1985 (see the Initial Reports of Leg 103; Boillot, Winterer, et al., 1987), we dove in the same region with the French submersible Nautile during the Galinaute cruise of the Nadir (21 May-2 July 1986). Nautile, with two pilots and one scientist aboard, is capable of operations to a maximum depth of 6000 m. During the Galinaute cruise, we dove to depths ranging from 3500 to 5500 m at 15 different sites (Fig. 1), collecting 104 rock samples from the seafloor over a distance of 30 km (Boillot et al., 1986a). These new data complement the Leg 103 data. The general scientific objectives of the Galinaute cruise were the same as those of ODP Leg 103. The Galicia margin was chosen for drilling and diving investigations in order to recon- struct the initial history of a passive margin and to understand the effects of horizontal stretching of the lithosphere during rifting as well as during the birth of a new ocean (the North At- lantic) between two continents (Newfoundland and Iberia) mov- ing away from each other. The Galinaute cruise, scheduled only one year after Leg 103, aimed at three specific objectives: 1 Boillot, G., Winterer, E. L., et al., 1988. Proc. ODP, Sci. Results, 103: Col- lege Station, TX (Ocean Drilling Program). 2 Laboratoire de Geodynamique Sous-Marine, Villefranche-sur-Mer, France. 3 Instituto Andaluz de Geologia Mediterranea, CSIC, Universidad de Gra- nada, Granada, Spain. 4 Laboratoire de Petrologic Physique, Institut de Physique du Globe et Uni- versite de Paris VII, Paris, France. 5 Centre de Recherches Volcanologiques, Clermont-Ferrand, France. 6 Laboratoire de Geologie, Museum National d'Histoire Naturelle, Paris, France. 7 Departement de Geologie Dynamique, Universite Pierre et Marie Curie, Paris, France. 8 Laboratoire de Geologie et Micropaleontologie Marines, Universite de Nice, Nice, France. 1. To complete our knowledge of the initial history of the margin during the Triassic and Jurassic. Serious difficulties ham- pered coring sedimentary rocks of that age on Leg 103, and we hoped to recover the missing section by sampling normal fault escarpments visited with the Nautile. 2. To sample the ultramafic basement bounding the deep margin. Studies of a sample dredged in 1978 (Boillot et al., 1980) and of cores drilled at Site 637 (see "Site 637" chapter; Shipboard Scientific Party, 1987a) showed that upper mantle serpentinized peridotites crop out on the seafloor in a zone be- tween the continental crust of the Iberian margin and the basal- tic crust of the Atlantic Ocean. The Galinaute scientific party hoped to confirm that discovery by increasing the number of in- situ observations at different localities and thereby developing a better understanding of the transition between the ultramafic seafloor and the adjacent continental and oceanic crusts. 3. To sample the "acoustic basement" overlying the S reflec- tor. Seismic lines recorded on the deep Galicia margin (Fig. 2) show a strong reflector within the acoustic basement, labeled the "S" horizon by de Charpal et al. (1978). Interpretation of this reflector is controversial. It could be a detachment surface at the boundary between the basement and the overlying deformed sedimentary cover, as proposed by the Leg 103 scientific party (see "Site 640" chapter; Shipboard Scientific Party, 1987c). Alter- natively, it could be the boundary between brittle and ductile crust, where listric normal faults root (Montadert et al., 1979), or some other intracrustal geophysical boundary (Boillot et al., 1980). By taking samples on fault escarpments from the "acous- tic basement" overlying the S reflector, we hoped to constrain the interpretations of this enigmatic feature. Most of these three objectives were achieved by the Galinaute cruise. In addition, the Nautile recorded unexpected data con- cerning the Atlantic oceanic crust close to the margin and the structure of the northwestern slope of the Galicia Bank. In this paper we present preliminary results of dive observations and the present state of post-cruise sample studies; further studies of the data are in progress at the time of this writing. MORPHOLOGIC AND STRUCTURAL SETTING OF THE DIVE SITES Taking the scientific objectives into account, we selected the dive sites on the basis of previously collected bathymetric and 37