Mud diapirism in front of the Barbados accretionary wedge: the influence of fracture zones and North America-South America plate motions

Mud diapirism in sediments on the ocean floor ahead of the Barbados accreti onary wedge, previously believed to be a consequence of fluid overpressure and fluid migration caused by the advance of the accretionary wedge, is sho wn to have been initiated at about 3.4 Ma by a change in the motion between the North and South American plates across fracture-zones. Neither the dep osition of the Orinoco Fan nor the advance of the Barbados accretionary wed ge played a part in triggering this Pliocene mud diapirism. although the fo rmer was important in creating the conditions that favoured diapirism, and the latter has sustained and reactivated diapirs to the present day. Active mud diapirism occurs from the deformation front of the accretionary wedge to at least as far as 23 km ahead of it, in linear diapir fields that are a ligned WNW-ESE, along the northern sides of north-facing basement scarps fl anking oceanic fracture zones. The basement troughs on the northern sides o f the basement ridges flanking the oceanic fracture zones contain increased thicknesses of low-permeability pelagic-hemipelagic sediments, which are p rone to overpressuring and undercompaction, and are therefore a likely diap iric source. There is evidence of Plio-Pleistocene diapirism in one of the fracture-zone troughs, Dearly 350 km east of the accretionary wedge. During most of the Tertiary, compression and transpression across the fracture zo nes tectonically thickened the already thicker accumulation of sediment in the fracture-zone troughs, increasing its diapiric potential, which was inc reased further by the deposition of overlying Orinoco submarine fan sedimen ts from 12 Ma. Pliocene diapirism was triggered by a change in the relative plate motion between North and South America from compression to dextral s trike-slip movement, between 3 and 5 Ma. While fluid expelled from beneath the accretionary wedge, travelling through faults opened up in the lower pa rt of the sediment fill of the fracture-zone troughs, cannot be excluded as the mechanism driving the Pliocene diapirism, the very long distance requi red for transmission of fluid favours alternative explanations. One of thes e alternatives is that mud was mobilised by the water generated by the smec tite-illite transformation in sediment heated by fluid flow in the oceanic crust beneath the fracture zones. This flow may have resulted from a reorga nisation of the fluid-flow regime of the igneous oceanic crust, in response to the change of stress and strike-slip faulting along the fracture zones. (C) 2001 Elsevier Science Ltd. All rights reserved.