It is accepted that mildly extended sedimentary basins form by largely uniform thinning of continental lithosphere. No such consensus exists for the formation of highly extended conjugate rifted continental margins. Instead, a wide range of models which invoke differing degrees of depth-dependent thinning have been proposed. Much of this debate has focussed on the well-studied Newfoundland-Iberia conjugate margins. We have tackled the problem of depth dependency at this pair of margins in three steps. First, we have reconstructed water-loaded subsidence histories by making simple assumptions about changes in water depth through time. Secondly, we have used these reconstructed subsidence histories to determine the spatial and temporal variation of lithospheric strain rate. An inversion algorithm minimizes the misfit between observed and predicted subsidence histories and crustal thicknesses by varying strain rate as a smooth function of distance across the margin, depth through the lithosphere, and geologic time. Depth-dependent thinning is permitted but, crucially, our algorithm does not prescribe its existence or form. Given the absence of significant volumes of syn-rift magmatism, we have also applied a minimal melting constraint. Inverse modeling has yielded excellent fits to both reconstructed subsidence and crustal observations, which suggest that rifting occurred from similar to 150-135 Ma and at rates of up to 0.3 Ma(-1). Strain rate distributions are depth-dependent, suggesting that lithospheric mantle thins over a wider region than the crust. Beneath highly extended parts of the margin, crustal strain rates greatly exceed lithospheric mantle strain rates. Thirdly, we have tested our strain rate histories by comparing the total horizontal extension with the amount of extension inferred from normal faulting patterns. Both values agree within error. We freely acknowledge that there are important uncertainties in reconstructing the subsidence histories of deep-water margins. Nevertheless, stratigraphic records remain the only, albeit imperfect means of determining how crust and lithospheric mantle thin through time and space. (C) 2008 Elsevier B.V. All rights reserved.
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