Ocean Drilling Program, United States

The ocean/continent transition (OCT) of the Iberia Abyssal Plain nonvolcanic rifted margin has been intensively drilled during Leg 149 of the Ocean Drilling Program. Site 897 is located over a serpentinized peridotite ridge interpreted as the southern expression of the peridotite ridge drilled at the OCT of the Galicia Margin during ODP Leg 103. In the Iberia Abyssal Plain, the ridge is associated with a lower Miocene synsedimentary reverse fault which merges with its eastern flank. Using the recently developed Pasisar system, a near-bottom single-channel seismic profile was acquired over this ridge during the Fluigal cruise (July 1993). Site 897 biostratigraphic gaps are correlated with seismic unconformities identified on this high-resolution seismic profile. The mean vertical velocity within the sedimentary column at Site 897 is 1820 m/s. The major reverse fault, which develops within the synsedimentary lower Miocene front, merges with the eastern flank of the serpentinized peridotite ridge at a significant angle of about 60 degrees . Shear has been observed on Site 897 samples collected close to the serpentinized body, both within the serpentinite breccias and the overlying claystones. Thus, the lubricant effect of serpentinite facilitates the concentration of shear along the flanks of the serpentinized peridotite ridge. We suggest that the synsedimentary reverse fault extends along the eastern flank of the ridge and across the whole brittle crust. As reverse faulting is preferentially located where serpentinized peridotite ridges exist, reverse faults located at the base of passive continental margins could generally help to locate the ocean/continent transition if the margin has undergone compression.