The recent evolution of ideas on the Mediterranean region has been triggered by very active data acquisition over the last 15 years. Seismic tomography provides an unique view of mantle heterogeneities, space geodesy leads to precise determinations of the present strain and velocity fields, the combination of structural geology, radiometric dating and metamorphic petrology allows the description of P-T-t-D paths of exhumed metamorphic rocks, and exploration geophysics, onshore and offshore, gives a detailed view of the crustal geometry. Extension started in the Gulf of Lion and propagated eastwards and southwestwards to form the Liguro-Proven~ai basin, Tyrrhenian Sea and the Alboran Sea. It also started, at much the same time, in the Panonnian basin as well as in the Aegean back-arc region. Thus a seminal event occurred some 30 Ma ago that produced a sharp change from overall compression to back-arc extension. Although gravitational forces due to the collapse of a thick crust have affected most basins, it is now almost certain that this event ultimately originated in the mantle, either by slab detachement, slab rollback or both processes acting in sequence.
During Leg 110, the northern Barbados Ridge was penetrated at six sites ranging from 6 km seaward to 17 km arcward of the deformation front. Reference Site 672 in the abyssal plain provides a unique record of paleoenvironments prevailing at the latitude of the Caribbean in the western Atlantic. At this site Senonian to lower Pleistocene pelagic and hemipelagic sedimentation was punctuated by (1) locally derived middle Eocene and Oligocene calcareous turbidites, (2) South American derived middle and upper Eocene and Oligocene terrigenous turbidites, and (3) arc-derived Neogene ash falls. The Neogene section is currently accreting at the deformation front while low-angle thrusts are propagating seaward from the decollement. Balanced cross sections, with corrections for porosity loss, suggest 40% shortening of the Neogene accreted section within 7 km of the deformation front. The underthrust series show little or no deformation, but slightly higher porosities than expected suggest overpressuring. A decrease in the percentage of smectite interlayers in the underthrust sequence suggests diagenetic processes possibly involving neoformation of illite and the release of free water. Possibly underplated, stratally disrupted sediments occur at the sites located furthest uplsope. Anomalies in temperatures and pore-water geochemistry specify fluid migration pathways including the decollement zone, active thrusts, and Eocene sand layers in the underthrust sequence. Anomalies of probable thermogenic methane in the decollement zone suggest long-distance lateral migration in this conduit. The negative chloride anomalies characteristic of the migration conduits might be generated by smectite dehydration at the fluid source areas. The decollement and thrust faults probably have a cyclic history of variable fluid flow and related variable fluid pressure and permeability, superimposed on a lower background flow rate. This apparent episodic flow is consistent with the model of deformational pumping associated with incremental displacement along faults. However, the ddcollement could undergo faulting and simultaneous fluid influx, in a steady state, because it can be a surface of minimum head and minimum effective stress, given reasonable pressure gradients.
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