Structure of the accretionary complex of Barbados, I: Chalky Mount

Abstract

The island of Barbados is underlain by a basal tectonic complex that may be representative of rocks and structures of the structural high of the accretionary prism of the Lesser Antilles arc. At Chalky Mount, the complex consists of six fault-bounded packets of east-northeast strike and steep dips. The packets are composed of deformed but generally coherent Cenozoic sedimentary rocks, terrigenous turbidite in five of the packets and hemipelagic rocks in the sixth. There is no evident stratigraphic affiliation among beds in any of the six packets. Rocks in fault packets underwent sequential deformations, most of which can be timed as early, contemporaneous, or late with respect to cessation of interpacket motions. Early structures in packet 1 consist of a main fold train with easterly striking axial planes and related thrusts, broken formation, and homoaxially refolded folds. Packet 1 also contains later open folds with northeast strikes. Packets 2–4 are relatively thin slices within a fault complex, the Chalky Mount fault zone, in which late left-oblique slip occurred on two of the four faults of the zone. Packet 5 contains three phases of deformation: an early near-recumbent major fold, marginal belts of fault-related second folds, and open post-fault third folds. Packet 6 includes early folds that are tightened and reoriented toward its northern fault boundary where it is locally a thrust. Early structures in each packet are the most pervasive and record the major stratal shortening. Shortening in each packet is approximately normal to packet boundaries, and early axes lie in the plane of the boundaries. Packet boundaries are interpreted as surfaces of accretion and early structures as deformations generated during transfer of layer successions from an ocean floor to the front of the accretionary prism. Contemporaneous and post-fault structures are local and are due to displacements during reactivation of faults, mainly inter-packet, within the accretionary prism. Early structures at Chalky Mount developed in the brittle state, apparently with displacements taken up by sliding between grains and along layer boundaries. All structures record distinctly nonmetamorphic environments.