Tectonic implications of exposure of lower continental crust beneath the Iberia Abyssal plain, northeast Atlantic Ocean: Geophysical evidence

Abstract

We present new seismic data from a basement high beneath the southern Iberia Abyssal Plain that appears, from the latest Ocean Drilling Program (ODP) results and indirectly from various seismic observations, to consist of lower continental crustal material. This is a unique opportunity to use seismic profiles and other geophysical evidence to investigate the tectonic process which led to the exhumation of these rocks. We infer that the lower crust was exhumed at the seafloor and then uplifted toward the end of rifting. Our results lead to the following new observations: low-angle detachment faults, previously reported along the east-west Lusigal-12 profile across the southern Iberia Abyssal Plain, penetrate both the 3-10 km thick continental crust and, unusually, the uppermost mantle. Processing of new multichannel seismic reflection profiles across the basement high on which ODP Sites 900, 1067, and 1068 are located reveals that the high is bounded by a previously unsuspected steep, landward dipping normal fault on its east flank. Basement cores from the above ODP sites also reveal that the high is not capped by the previously predicted early syn-rift sediment but by rocks from the lower continental crust. These unexpected observations are incorporated into a new tectonic model of the development of this part of the west Iberia margin which is also consistent with other geophysical observations. A novel feature of this model is the proposal that the seaward edge of the continental crust is thinned to 3-6 km by a currently poorly understood process. The model implies three stages of deformation; (1) lithospheric extension, principally by symmetric pure shear, which leads to a - 10 km thick crust in which the lower crust is largely absent over the axial zone and the crust-mantle boundary forms a shear zone, (2) further thinning and then dissection of the most distal continental crust by seaward dipping, low-angle normal faults, (3) inception of a high-angle, landward dipping normal fault that offsets the tectonized crust-mantle boundary and uplifts the lower crust to the crest of the basement high on which ODP Sites 900, 1067, and 1068 were drilled.