Rotation of the Falklands microplate reassessed after recognition of discrete Jurassic and Cretaceous dyke swarms

Recently acquired aeromagnetic data for the Falkland Islands have shown that previous interpretations of the dolerite dyke swarms are inadequate. In particular, most of the dykes previously described from West Falkland as forming a ‘north–south’ swarm of Jurassic age are associated with a set of NE–SW linear magnetic anomalies that are entirely separate from another set of truly N–S anomalies. Very few dykes had been previously reported from East Falkland, but the aeromagnetic survey demonstrates clearly that dykes of both the NE–SW and the N–S swarms are present. Ar–Ar age dating of East Falkland dykes has confirmed the Jurassic age of the NE–SW dykes but has established an early Cretaceous age for the N–S dyke swarm. The Jurassic dykes are generally considered a part of the regional Karoo–Ferrar magmatism linked to the initial break-up of Gondwana. We consider the Cretaceous dykes to be associated with the later opening of the North Falklands Basin during the early development of the South Atlantic Ocean. The Jurassic and Cretaceous dykes must respectively pre-date and post-date the microplate rotation envisaged in most models for the Falklands break-out from Gondwana. The shapes of the aeromagnetic anomalies associated with dykes from each of the swarms support the hypothesis that the early Jurassic dykes have experienced a pre-Cretaceous, clockwise microplate rotation of about 120°.