--> ABSTRACT: A Generalized Inverse Method for Testing Depth-Dependent Extension Models at Highly Extended Continental Margins, by Crosby, Alistair; White, Nicky; Edwards, Glyn; Thompson, Mark; Corfield, Richard; Mackay, Laura; #90135 (2011)

Datapages, Inc.Print this page

A Generalized Inverse Method for Testing Depth-Dependent Extension Models at Highly Extended Continental Margins

Crosby, Alistair 1; White, Nicky 2; Edwards, Glyn 1; Thompson, Mark 1; Corfield, Richard 1; Mackay, Laura 1
(1)BP Exploration, London, United Kingdom. (2) Bullard Laboratories, University of Cambridge, Cambridge, United Kingdom.

A general understanding of rifted margins, which form by thinning of the continental lithosphere, exists. Nevertheless, the exact form of thinning is unclear. This debate has been stimulated by acquisition of dense seismic wide-angle and deep reflection surveys from Atlantic Ocean margins. A central issue concerns the way in which thinning changes with depth, which has important implications for heat flow and structural styles during rifting. We have tackled this issue by developing a generalized inverse model. This model attempts to fit tectonic subsidence, crustal thinning and melting observations by varying strain rate as a function of time and space. It is important to ensure that reconstructions of paleo-water depth and topography are as accurate as possible. Depth-dependent thinning is permitted but we do not prescribe its existence or form except to ensure that mass is conserved. We have applied the algorithm to six margins, including two of the most contentious: Newfoundland-Iberia and Brazil-Angola. In all cases apart from Newfoundland-Iberia, calculated strain rate histories predict thinning estimates which broadly match estimates inferred from normal faulting and strain rates are largely uniform with depth. In contrast, the Newfoundland-Iberian margins formed by a pattern of strongly depth-dependent strain rate. To account for the paucity of syn-rift decompression melting of the underlying asthenosphere, lithospheric mantle close to the ocean-continent transition must have thinned more slowly than the overlying crust. This form of depth-dependency is not common. For example, the Brazil-Angola conjugate margins could have formed by uniform lithospheric thinning provided thick layers of salt were deposited in a pre-existing 400-600 m topographic depression. Depth-dependent thinning is not required to account for the existence of thick, unfaulted sequences of pre-salt sedimentary rocks. More generally, strain rate inversion provides a powerful way to test the validity and tectonic implications of stratigraphic interpretations during the early stages of basin access. The results can be used to calculate basement heat flow during and after rifting. Our workflow therefore reduces uncertainty in both source rock maturity and in the timing of extension in frontier exploration basins.

 

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.