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Modelling plate kinematics in the Scotia sea

Eagles, Graeme (2000) Modelling plate kinematics in the Scotia sea. PhD thesis, University of Leeds.

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Abstract

A new model of plate kinematics in the Scotia Sea region is presented in which continental crustal blocks and the signatures of seafloor spreading are defined semi-automatically using gravity and total field magnetic anomalies and some of their residuals, transformations and derivatives. This study is the first of the region to integrate gridded magnetic and gravity data in order to make reconstructions, and one of the first anywhere to make full use of gridded magnetic data in an inverse procedure. The context provided by the quantitative reconstructions allows qualitative assessment of visually-derived reconstructions of small movements in the region. The Scotia Sea floor consists of three large oceanic magnetic provinces: the west,central and east Scotia seas, and four smaller sub-basins, all enclosed within the elevated submarine and emergent Scotia Arc. The Scotia Arc consists of Mesozoic continental and Cenozoic island-arc fragments. Only the east Scotia Sea remains active; the west and central parts are the products of extinct spreading centres. West Scotia sea spreading is reasonably well described by tectonic flowlines expressed in satellite free-air gravity anomalies and magnetic reversal isochrons in total field anomalies. These data are combined in an inversion to reconstruct the west Scotia Sea's margins between its inception at thron C8 (- 26.5 Ma) and extinction at chron C3a (- 6 Ma). The results suggest strongly, and for the first time, that the west Scotia Sea formed as a small ocean basin whose passive margins were Tierra del Fuego and the central Scotia Sea, and not as a back-arc basin in the strict sense. During its growth the kinematics of the west Scotia Sea's margins approximated those of the South American and Antarctic plates. The small kinematic differences are suggested to be due to convergence at the `proto-South Sandwich-Discovery' subduction zone, to the east of the central Scotia Sea, and to dextral strike-slip (pre-C6 (N 20 Ma)) and oblique convergence (post-C6) at the North Scotia Ridge, the Mesozoic northern arm of the Scotia Arc. The most widely-accepted interpretations of the central Scotia Sea hold that it is a back-arc basin, but model flowlines about published reconstruction poles in the region show that instead it could have originated by accretion to the South American plate at the ancestral South American-Antarctic Ridge in the Weddell Sea, later to move eastwards as the eastern passive margin to the west Scotia Sea. Magnetic reversal anomalies in the central Scotia Sea are consistent with its accretion in this way during the Cretaceous, probably between chrons M4 and M20 (- 126-149 Ma). All of this material was hitherto thought to have been destroyed completely by subduction at the ancestors of the South Sandwich subduction zone. The central Scotia Sea is thus also re-assigned in the model to have an oceanic, rather than back-arc basin, origin. Hence, the bulk of the Scotia Sea floor formed as a consequence of the predictable movements of major plates following the break-up of Gondwana, with back-arc basins in the strict sense only forming small subbasins until the inception of the east Scotia Sea at or soon after C5c. The development of this much larger, oceanic, back-arc basin occurred following a change in the direction of relative motion at the West Scotia and South American-Antarctic Ridges at C6. This new model of Scotia Sea kinematics, presented as a series of reconstructions of total field and Bouguer anomalies, is the first to be both self-consistent and consistent within the context of known major plate motions. The new interpretation of the central Scotia Sea is at odds with previous reconstructions which place South Georgia in the heart of a reconstructed compact connection between Tierra del Fuego and the Antarctic Peninsula. The altered position of South Georgia, south of Maurice Ewing Bank (Falkland Plateau) helps explain the puzzling provenance of its turbidites and suggests, as previous workers have done, that it may be appropriate to redefine the genesis of at least the eastern part of the Rocas Verdes Basin as an oceanic basin formed by accretion at a propagating rift, rather than (as before) a rare example of a back-arc basin formed behind an east-directed subduction zone. Although the new model is self-consistent, it is not uniquely so for two of the small basins in the Scotia Sea (Protector and Dove Basins) whose age remains poorly defined.It can be speculated that either or both of these basins may have opened deep-water gateways in the Drake Passage region prior to spreading in the west Scotia Sea, possibly in the Middle Eocene. A very tentative correlation between such events and initial cooling prior to the onset of Antarctic glaciation, via the cooling effect of establishing an efficient Antarctic Circumpolar Current, is suggested.

Item Type: Thesis (PhD)
Additional Information: This thesis has accompanying materials which can be accessed through the British Library's EthOS service http://ethos.bl.uk
Academic Units: The University of Leeds > Faculty of Environment (Leeds) > School of Earth and Environment (Leeds)
Depositing User: Ethos Import
Date Deposited: 12 Mar 2010 12:20
Last Modified: 08 Aug 2013 08:44
URI: http://etheses.whiterose.ac.uk/id/eprint/258

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