No abstract

The Lesser Antilles island arc is composed of two Cenozoic volcanic arcs, the older (early Eocene to mid-Oligocene) and the Recent (early Miocene to Present) arcs, superimposed on a Meszoic proto-arc belonging to a larger geodynamic feature, the Mesozoic Caribbean arc (M.C.A.), initiated in the Early Cretaceous. The Lesser Antilles arc experienced two major volcanic interruptions of 8-to-10 m.y. duration each. The first interruption accompanied the Paleocene opening of the Grenada Basin, with oceanic crust forming in the southern part of the basin. The splitting of the central part of the M.C.A. (eastern Caribbean) isolated a remnant arc, the Aves Swell, from an active arc, the Lesser Antilles. The second volcanic interruption occurred in late Oligocene, as a consequence of a first-order tectonic event: the colli sion of a buoyant Atlantic aseismic ridge. Thereafter, the subduction geometry of the renewed volcanic line (Recent arc) was modified in the northern half by a westward jump from the outer arc to the inner arc. In the Neogene, the subduc tion of non-buoyant ridges had second-order, but nevertheless significant, effects on the tectonics and volcanism of the arc.The development pattern of the construction of the arc ridge by volcanic processes is described. Emphasis is placed on the role of subducted sediments in the generation of orogenic magmas and, conversely, on the role of subducted ridges in the local interruption of arc volcanism. The morphology of the underthrust oceanic crust controls the mag matic activity of the island arc, and particularly the development, in space and time, of "arc compartments."

No abstract

Abstract--Detailed analysis of ODP Leg 110 cores provides evidence for a complex structural evolution of the frontal thrust system in the northern Barbados Accretionary Prism. Initial east-directed in-sequence thrusts are biostratigraphically documented, and are overprinted towards the west by out-of-sequence overthrusts, largescale folds and penetrative deformation of the accreted rocks. This structural progression is interpreted to reflect the need for secondary shape adjustments of the accretionary prism in order to maintain a critical taper. At present the basal d6collement is located within Lower Miocene strata. Accreted Eocene sediments suggest either fossil frontal off-scraping at a lower stratigraphic level or duplex formation and underplating.The present localization of the basal detachment is provoked by a zone of initial high porosity in the Lower Miocene radiolarian-rich mudstones. Shearing is accompanied by partial porosity breakdown in a 30--40 m wide zone of intense scaly fabrics and stratal disruption that constitutes the Atlantic-Caribbean plate boundary at present. Large displacements are taken up by a relatively narrow zone of contact strain. Initially the preferred orientation of clay minerals (incipient scaly fabrics) may be a flattening fabric, but with progressive rotational flow the scaly planes may act as discrete microshears and provide small-scale flow partitioning. Alternatively, scaly fabrics may be interpreted as brittle microshears. Mud-filled veins are present in the accreted rocks as well as at the oceanic reference site east of the deformation front. Carbonate-filled veins are restricted to the accreted rocks. This indicates persistence of high fluid pressures, but a fundamental change in the type of fluid-rock interaction during frontal accretion tectonics.

No abstract