STRETCHING AND SUBSIDENCE - RIFTING OF CONJUGATE MARGINS IN THE NORTH-ATLANTIC REGION

Subsidence on rifted conjugate continental margins around the North At lantic is analyzed to derive the amount and areal distribution of stre tching in the crust and in the lower lithosphere during continental ri fting. Study areas are the Grand Banks and Orphan Basin regions of the eastern Canadian continental margin and the Goban Spur and Galicia Ba nk regions off western Europe. In all areas, maps of synrift and postr ift sediment thickness and bathymetry were used to derive maps of post - and synrift subsidence. A two-layer lithospheric stretching model wi th independent amounts of stretching in the crust and in the lower lit hosphere was assumed to be applicable, with the rifting history approx imated by several instantaneous episodes of extension. This model was used to derive estimates of stretching at all points on a 0.05-degrees geographical grid, where subsidence values were available within the study regions. The models are constrained with seismic measurements of crustal thickness. The results imply that pure shear stretching predo minates at a lithospheric scale, while simple shear is more localized laterally and confined to the crust. In places there is significant de coupling between crustal and mantle stretching. Near the continent-oce an boundary, final continental breakup may be localized on one side of the rift between conjugate margin pairs, rather than symmetrically lo cated. Total extension of the margins is compatible with that estimate d from normal fault geometries and indicates that the continent-ocean boundary has been extended up to 350 km seaward of its original positi on, which should be considered in plate kinematic reconstructions.