Ocean Drilling Program, United States

The Iberia Abyssal Plain segment of the western Iberian Atlantic margin is characterized by highly extended and thinned continental crust bounded westward by a ridge of serpentinized peridotite within the transition zone between continental and oceanic crust. To better understand the evolution of this margin, we have analyzed the margin-normal profile Lusigal 12 (LG12) between Sites 898 and 901. After optimum processing, including prestack depth migration, the seismic sections image strong reflections below the breakup unconformity in this segment of the Iberian nonvolcanic margin. We show that these intracrustal reflections are overlain by probable basement rocks (Vp>5 km/s), whereas synrift or prerift sedimentary units exhibit generally lower velocities (Vp<4.7 km/s). Between Site 901 (identified by drilling as probable continental crust) and Site 900 (amphibolite facies metamorphosed gabbro) to the west, basement is broken up into landward-tilted crustal blocks bound by normal faults. These block-bounding faults appear to detach onto bright intracrustal reflections, which thus mark the lower boundary of the tilted blocks. By analogy with the similar seismic image of the Galicia Bank area to the north, where continental breakup has been controlled by a detachment fault structure (the S reflector), we suggest that continental breakup in the Iberia Abyssal Plain could have been also controlled by detachment faults that were active at different times during rifting. Extension of the upper lithosphere along the detachment system may have exposed a cross-section through the entire lithosphere from the upper crust (in the east, at Site 901), through the lower crust (Site 900), to the uppermost mantle exposed at the western edge of the drilling transect (Site 897, just landward of oceanic crust).