KINEMATICS OF PERIDOTITE EMPLACEMENT DURING NORTH-ATLANTIC CONTINENTAL RIFTING, GALICIA, NORTHWESTERN SPAIN

The western part of the Galicia passive continental margin (western edge of Spain) is bordered by serpentinized peridotites over a N-S distance of about 125 km. These peridotites were emplaced at the end of continental rifting and/or at the very beginning of oceanic accretion. They have been observed in situ and sampled in five sites with the Nautile submersible during the Galinaute cruise (1986). Some crustal continental rocks were also collected at two sites on the tilted blocks adjacent to the peridotite ridge. Most of the escarpments covered by the Nautile on the peridotite ridge have yielded only ultramafic rocks, except at Dive 10 where the peridotites are overlain by a sheared chlonte-bearing schist, and at Dive 14 where the ultramafic rocks are covered by basalts. All the peridotites are plagioclase-bearing harzburgites and Iherzolites, locally cross-cut by rare plagioclase-rich veins and dioritic intrusives. Some pyroxenites and gabbros are associated with the peridotites at Dive 14. The plagioclase-bearing peridotites are similar in composition to those drilled during ODP Leg 103. Primary structures have not been found, except in the northern part of the studied zone where a few samples exhibit magmatic textures. Elsewhere, the peridotites show evidence of a strong mylonitization event which has overprinted a former primary high-temperature fabric. At these localities, boudinage structures and ultramylonitic bands, referred to as shear bands, occur and the rocks have a disrupted mylonitic texture. A few dioritic dykes have also suffered this mylonitization. Shearing occurred in a rotational regime, at high and decreasing temperature (1000-850 °C) and under a high deviatoric stress (> 102 MPa) i.e. under lithospheric conditions. The structural pattern of the peridotite ridge, deduced from measurements on oriented samples, is complex. The trend of the mylonitic foliation appears nearly constant from south to north (N125-N105), but its dip increases (20-45°) northward. From south to north, the stretching lineation pitch varies from subvertical to subhorizontal. Stretching has occurred along a shear zone acting as a normal fault in a northeasterly direction in the southern part of the studied area. To the north, it occurred in a northwesterly direction with a strike-slip component. Stretching in the chlorite-bearing schist and in the gneissose rocks of the continental basement seems to be unrelated to the peridotite deformation. This study confirms the uplift of the Galicia peridotites during the continental rifting of the North Atlantic Ocean in Cretaceous times. During their ascent, the peridotites underwent some melting and high-temperature-low-stress ductile deformation, followed by intense mylonitization at decreasing temperature and increasing stress that has overprinted most of the primary textures. The geometry of the mylonitic structures is related both to the ascent of the peridotite dome and to the extensive stress-strain field prevailing in the continental lithosphere during the rifting episode. It is consistent with that expected in the upper part of a dome locally offset by transverse faults in its northern part due to the vicinity of a triple junction point. The kinematics of emplacement agree well with an E-W opening of the North Atlantic Ocean. © 1990.