THE GIBRALTAR SUBDUCTION: A DECADE OF NEW GEOPHYSICAL DATA AND THE IMPLICATIONS FOR REGIONAL KINEMATICS AND HAZARD ASSESSMENT

The Gibraltar arc in the Southern Iberia region is one of the most complex portions of the modern day Africa - Eurasia plate boundary. Geophysical data acquired in the past decade (seismic profiling, tomography, hypocenters and other seismological data) reveal a narrow subduction zone here, dipping steeply to the east. Recently acquired seismic refraction data offshore SW Portugal indicate the presence of oceanic crust in the western Gulf of Cadiz. Travel-time tomography of the upper mantle images a steep, east dipping high-velocity body, which is continuous with Atlantic oceanic lithosphere. Recent studies of seismic anisotropy of the mantle from SKS splitting show arc-parallel "fast directions", consistent with toroidal flow around this narrow subducting slab as it retreats to the W. The overall WSW transport direction is borne out by the W, NW and SW vergence of thrust anticlines in the accretionary wedge and by its general shape and symmetry axis. High-resolution bathymetric mapping performed in the Gulf of Cadiz reveal an asymmetric embayment at the deformation front where a 2 km high basement ridge (Coral Patch Ridge) has recently collided. Analog modeling successfully reproduces the current seafloor morphology and confirms the WSW transport direction of the Gibraltar backstop (in response to subduction roll-back). While subduction has slowed significantly (v<1 cm/yr) since 5Ma, deformation of the most recent sedimentary strata suggests it has not altogether ceased. Continued activity of the subduction provides a possible explanation for the source of the 1755 Great Lisbon earthquake (M8.7) and tsunami. This event remains the object of scientific debate and bears strongly on the overall hazard assessment for the area. Recent GPS studies show SW motion of stations in N Morocco (in the SW Rif) at velocities of 3-5 mm/yr in a Nubia fixed reference frame and clearly document the presence of an independent block, a "Rif-Betic-Alboran" microplate, situated between Iberia and Africa. The current GPS displacement field near the straits of Gibraltar may reflect the interseismic phase between major mega-thrust earthquakes (the possible cause of the 1755 earthquake) and thus indicate locking of the shallow portion of the E-dipping subduction interface beneath the central and eastern Gulf of Cadiz.