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

Abstract

Mud diapirism in sediments on the ocean floor ahead of the Barbados accretionary wedge, previously believed to be a consequence of fluid overpressure and fluid migration caused by the advance of the accretionary wedge, is shown to have been initiated at about 3.4Ma by a change in the motion between the North and South American plates across fracture-zones. Neither the deposition of the Orinoco Fan nor the advance of the Barbados accretionary wedge played a part in triggering this Pliocene mud diapirism, although the former 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 23km ahead of it, in linear diapir fields that are aligned WNW–ESE, along the northern sides of north-facing basement scarps flanking oceanic fracture zones. The basement troughs on the northern sides of the basement ridges flanking the oceanic fracture zones contain increased thicknesses of low-permeability pelagic–hemipelagic sediments, which are prone to overpressuring and undercompaction, and are therefore a likely diapiric source. There is evidence of Plio-Pleistocene diapirism in one of the fracture-zone troughs, nearly 350km east of the accretionary wedge. During most of the Tertiary, compression and transpression across the fracture zones tectonically thickened the already thicker accumulation of sediment in the fracture-zone troughs, increasing its diapiric potential, which was increased further by the deposition of overlying Orinoco submarine fan sediments from 12Ma. Pliocene diapirism was triggered by a change in the relative plate motion between North and South America from compression to dextral strike–slip movement, between 3 and 5Ma. While fluid expelled from beneath the accretionary wedge, travelling through faults opened up in the lower part of the sediment fill of the fracture-zone troughs, cannot be excluded as the mechanism driving the Pliocene diapirism, the very long distance required for transmission of fluid favours alternative explanations. One of these alternatives is that mud was mobilised by the water generated by the smectite–illite transformation in sediment heated by fluid flow in the oceanic crust beneath the fracture zones. This flow may have resulted from a reorganisation 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.