173 Preliminary Report

Leg 173 Scientific Report
ABSTRACT

During Leg 173, five sites (Sites 1065, 1067, 1068, 1069, and 1070) were drilled and cored along an east-west transect begun during Leg 149 across the ocean-continent transition (OCT) of the West Iberia continental margin. The main objective of Leg 173 was to investigate the mechanisms of thinning and breakup of the continental lithosphere and the early stages of oceanic crust formation. Subsidiary objectives were to better characterize the OCT, constrain the tectonometamorphic evolution of the continental and oceanic basement, determine the extent of synrift magmatism, examine the nature of the oldest oceanic crust, and investigate the early sedimentary history of the rifted margin. Basement rocks were cored under the sedimentary cover at the tops of structural highs. From east to west, Sites 1065, 1067, 1068, and 1069 lie within the OCT, and the westernmost Site 1070 is located over presumed early oceanic crust. Continental crust appears to underlie Sites 1065 and 1069; the latter site is probably an isolated continental fault block flanked to east and west by synrift melt products and/or exhumed mantle. The basement at Site 1067 consists of a 92-m-thick sequence of mafic rocks (amphibolite, tonalite gneiss, and meta-anorthosite), with enriched to normal mid-ocean ridge basalt tholeiitic affinities that probably represent differentiated products of synrift mantle melting. The meta-anorthosite and the tonalite gneiss are intrusive in the amphibolites. A ductile shear zone developed in the amphibolite at a level that is now at the top of the basement. Site 1068, located on the western flank of the same structural high, encountered breccias with clasts similar to the basement lithologies at Site 1067, and then, after drilling through a fault zone, serpentinized peridotite. The superposition of mafic rocks over ultramafics indicates that the strong seismic reflector that cross cuts the top of the basement on this structural high may be the crust/mantle boundary rather than a major synrift tectonic contact. Site 1070 was located in a region characterized by an oceanic crustal structure that is abnormally thin compared to typical oceanic crust and by seafloor-spreading magnetic anomalies. Basement at this site consists of serpentinized peridotite intruded by gabbroic veins, one of which is pegmatitic. The gabbroic veins may represent the early stages of seafloor spreading mantle melting, but no evidence of upper oceanic crust (lavas or sheeted dikes) was found.