Artigo Acesso aberto Revisado por pares

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

2009; Zoological Society of London; Volume: 15; Issue: 3 Linguagem: Inglês

10.1144/1354-079309-847

ISSN

2041-496X

Autores

P. Stone, G. S. Kimbell, Philip C. Richards,

Tópico(s)

Paleontology and Stratigraphy of Fossils

Resumo

Recently acquired aeromagnetic data for the Falkland Islands has 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 predate 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°.- ---------oooooooooo----------A clastic sedimentary sequence ranging in age from Siluro-Devonian to Permian crops out across the Falkland Islands and is cut by numerous dolerite dykes (Fig. 1).Until recently the dykes were regarded as exclusively of Jurassic age (Mussett & Taylor 1994), but are now known to have been intruded during separate Jurassic and Cretaceous episodes of magmatism (Stone et al. 2008).Palaeomagnetic results from the Jurassic dykes (Taylor & Shaw 1989) have been taken to support the proposition that the Falkland Islands form part of a microplate that during the break-up of Gondwana was rotated through 180 o from an original position adjacent to the east coast of South Africa (Adie 1952).Rotation has been described as either a late Jurassic (Marshall 1994;Storey et al. 1999) or early Cretaceous (Thomson 1998) phenomenon.A comprehensive account of the Falkland Islands' geology is provided by Aldiss & Edwards (1999).Of the sedimentary divisions, the Siluro-Devonian to Carboniferous quartzite, sandstone and mudstone of the West Falkland Group have the largest outcrop, covering most of the western archipelago, including the western of the two main islands, the eponymous West Falkland.In the eastern of the two main islands -East Falklandthe West Falkland Group crops out in the north, whereas the south of East Falkland, and its smaller peripheral islands, is underlain by the tillite, mudstone and sandstone of the Carboniferous to Permian Lafonia Group.At the southern extremity of West Falkland the West Falkland Group rests unconformably on the Proterozoic crystalline basement of the Cape Meredith Complex, which has been dated at approximately 1100 -1000 Ma (Jacobs et al. 1999).Pre-Devonian, mafic dykes cut the Cape Meredith Complex (Thistlewood et al. 1997) with the widespread, polyphase swarms of Mesozoic dolerite dykes cutting the basement and all of the overlying sedimentary succession.Three discrete episodes of heating and coolingwith the cooling phases occurring in the late Permian, early Jurassic and late Cretaceoushave been by described by Thomson et al. (2002) and linked to major changes in the plate tectonic setting of the Falklands.This paper discusses the implications of recently-acquired aeromagnetic and geochronological (Ar-Ar: Stone et al. 2008) data for the rotational model of Falklands tectonics.The aeromagnetic survey was flown as part of a mineral exploration programme by Falklands Gold and Minerals Ltd.The Ar-Ar analyses were performed by R. Esser at the New Mexico Geochronological Research Laboratory.These results establish a more complex history of dyke intrusion than had been hitherto appreciated and here, in particular, we consider the newly-discovered, early Cretaceous dyke swarm.This additional element in the geological history of the Falklands has significance for the currently resurgent hydrocarbons exploration offshore, and also allows a better assessment of the widely accepted (though not universally so) regional model of microplate rotation.

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