The Cape Fold Belt and Syntaxis and the rotated Falkland Islands: dextral transpressional tectonics along the southwest margin of Gondwana

Abstract

Two enigmas concerning the Cape Fold Belt (CFB), part of the Gondwanide Orogen that formerly stretched across southern Gondwana, are (1) its apparent development far-removed (≥1 500 km) from the convergent margin of Gondwana; and (2) the origin of the Cape Syntaxis, a 40–80° bend in the strike of the belt that occurs near Cape Town. An additional puzzle is how the Falkland Islands, which are believed to have originally been situated off the southeastern coast of Africa and may have formed the eastern continuation of the CFB, came to be rotated 180° during, or prior to, the break-up of Gondwana. In an attempt to address these enigmas, I review recent developments in the study of the CFB and of the South American, Falkland Islands and Antarctic portions of the Gondwanide Orogen, provide reinterpretations of some data and suggest a new tectonic model. Structural data (orientations relative to an east-west-trending CFB) are consistent with interpretation of the South America and Antarctic portions of the Gondwanide Orogen as northwest-trending dextral transpressional belts. The east-west-trending CFB, including the Falkland Islands, forms an inboard- or left-step within this intracontinental dextral shear zone. Dextral margin-parallel translation of the crustal block outboard of the orogen (extending from the Gondwanide Belt south to the margin of Gondwana) was accommodated by strike-slip deformation in South America and Antarctica, and by convergence and the development of a foreland-verging fold and thrust belt, across the CFB. The Cape Syntaxis, and the partially obscured Port Elizabeth Antitaxis, are oroclinal bends of the CFB that developed in response to continued dextral shear along the Gondwanide Belt. Clockwise rotation of the Falkland Islands occurred in two stages: (1) ~ 90° during oroclinal bending (the islands were incorporated in the short east limb of the Port Elizabeth Antitaxis); and (2) > 60° during solid body rotation about the Euler Pole to the Agulhas-Falkland Fracture Zone, a major dextral transform fault that propagated through the hinge region of the Port Elizabeth Antitaxis during break-up of Gondwana.