HR: 13:40h
AN: T53E-01    [Abstracts]
TI: Transition From Rift to Drift at Obliquely Divergent Continental Rifts: the Paired Rio Muni (W Africa) and NE Brazilian Margins
AU: * TURNER, J P
EM: j.p.turner@bham.ac.uk
AF: University of Birmingham, School of Geography, Earth & Environmental Sciences, Birmingham B15 2TT, UK, Birmingham, B15 2TT United Kingdom
AU: Green, P F
EM: mail@geotrack.com.au
AF: Geotrack International, 37 Melville Road, Brunswick West, Victoria, 3055 Australia
AU: WILSON, P G
T53E-01 AF: University of Birmingham, School of Geography, Earth & Environmental Sciences, Birmingham B15 2TT, UK, Birmingham, B15 2TT United Kingdom
AU: WESTBROOK, G K
EM: g.k.westbrook@bham.ac.uk
AF: University of Birmingham, School of Geography, Earth & Environmental Sciences, Birmingham B15 2TT, UK, Birmingham, B15 2TT United Kingdom
AU: LAWRENCE, S
EM: s.lawrence@ecqc.co.uk
AF: Exploration Consultants, Highlands Farm, Greys Road, Henley-on-Thames, RG9 4PR United Kingdom
AB: We develop a synoptic model for the breakup and drift of a major sheared continental margin system: the Rio Muni basin and its NE Brazilian counterpart. It relates the evolution of crustal structure determined from seismic interpretation (including gravity-modelling of the deep-imaging PROBE dataset) to the cooling history of this margin system yielded from thermal history data (mainly apatite fission track analysis - AFTAr - and vitrinite reflectance data). Shear margins initiate as a leaky transform fracture system accommodating the oblique (i.e. non-orthogonal) divergence of opposing rifted continental margins. As such, the transition from continental breakup (i.e. rupture) to continental drift (i.e. ocean opening) at shear margins exhibits significant differences from that of the much better understood normally divergent rifts, where the spreading vector is normal to the strike of the opposing rift margins. For example, unlike at normally divergent rifts, continental breakup and drift are separate episodes in the early evolution of shear margins. In Rio Muni-NE Brazil, they are recorded by separate breakup and drift unconformities spanning a 15-20Ma. interval, the time taken for the ocean ridge to traverse the length of the margin before a continuous arm of oceanic crust separated Rio Muni from its Brazilian counterpart. In the Rio Muni basin, the c.70km-wide Ascension Fracture Zone (AFZ) exhibits oblique-slip faulting and synrift half-graben formation that accommodated oblique extension during the period leading up to and immediately following whole-lithosphere failure and continental breakup 117Ma. Gravity-modelling of PROBE seismic profiles reveals a land-locked precursor oceanic basin that preceded full ocean opening and which subsequently was stranded on the African margin. Its existence supports the idea of multiple rift suture lines accommodating episodic breakup and it emphasizes the significance of separate breakup and drift episodes at shear margins. Oblique extension is recorded also by strike- and oblique-slip fault geometry within the AFZ and buckling of Aptian synrift rocks in response to block rotation and local transpression. Principal cooling episodes affecting both margins identified from the AFTAr occurred in the Mid-Cretaceous 110-90Ma., following a period of high heatflow during which the geothermal gradient was more than twice that of the present day 25§C/km gradient, and 45-35Ma. coinciding with a major change in the pole of rotation at Chron 34 some 84Ma. Additional cooling episodes between 85-75Ma. and 10-2Ma. are restricted to the African side. Given that this margin system underwent breakup some 117Ma., it appears that thermal equilibrium was maintained throughout the initial breakup meaning that no significant uplift and erosion is recorded by the thermal history data during this period. Instead, Mid-Cretaceous cooling is interpreted as a response to the influence of i) the c.105Ma. St Helena Plume and ii) transpressional shortening and inversion that accompanied the breakup to drift period.
DE: 8105 Continental margins: divergent (1212, 8124)
DE: 8106 Continental margins: transform
DE: 8111 Continental tectonics: strike-slip and transform
DE: 9305 Africa
DE: 9360 South America
SC: Tectonophysics [T]
MN: Fall Meeting 2005