Ocean Drilling Program, United States

The drilling of peridotite during ODP Leg 149 confirms that the peridotite ridge, previously explored on the Galicia Margin, is a major feature continuous over more than 250 km in the ocean/continent transition of the west Iberian Margin. At Site 897, the ultramafic rocks are serpentinized spinel and plagioclase-bearing lherzolite and websterite, with minor harzburgite and dunite. The serpentinized peridotites underwent four main events during their high-temperature evolution: (1) high temperature (1000 degrees -900 degrees C) shear deformation; (2) limited partial melting and (3) subsolidus reequilibration in the plagioclase stability field; and (4) poorly developed mylonitic shear deformation at a lower temperature (700 degrees C), under high deviatoric stress and at low pressure (i.e., under lithospheric conditions). As for the Galicia peridotites, this evolution is compatible with mantle dome uplift beneath a continental rift zone, and it confirms that simple shear is a major deformation mechanism involved in extension of the continental lithosphere. A complex deformation under subsurface conditions during the serpentinization of the rocks documents the late emplacement of the tectonically denuded dome at the rift axis. Hydrothermalism had an important role during this late evolution, which suggests that the mantle can be stretched over a large width after continental breakup, thus creating a wide (150 km) ocean/continent transition along the passive margin.