The West Iberia (WIM) and Newfoundland (NFM) continental margins formed over a succession of rift events related to the opening of the North Atlantic between the Late Triassic and the Early Cretaceous. They are characterized by a variable width Ocean- Continent Transition (Zone) where exhumed and serpentinized mantle has been cored. Results from 1-D well backstripping along the Portuguese shelf suggest 40-45% greater extension factors (β) than previous estimates. In addition, the age and duration of both the Late Triassic-earliest Jurassic and the Late Jurassic rifting episodes have been better constrained. It appears, for example, that the Late Jurassic rift propagates northwards along the margin, as inferred for the continental break-up during the Early Cretaceous. Combined backstripping and gravity modelling techniques (POGM), together with new bathymetry and sediment thickness grids, have been used to estimate the effective elastic thickness, T_e, of the lithosphere. Results along closely space profiles in the WIM reveal that T_e decreases from 15-40 km over unthinned Variscan basement to ≤ 15 km over stretched continental and transitional crust. Along strike, a good correlation is found between the modelled mechanical structure and the segmentation of the Variscan basement onshore. Discrepancies between observed and calculated anomalies are discussed in terms of other processes that affected the margin, such as serpentinization and inversion. Thermo-mechanical rheological modelling techniques have also been tested. The compiled software inputs the β's constrained from POGM to determine the thermal structure of the margin, which, in turn, has been used to construct models of rheology and, hence, T_e. The models predict an increase in the T_e of thinned continental lithosphere with age since rifting, between < 20 and > 35 km, and suggest that processes such as ductile shearing and mantle serpentinization might have permanently weakened the lithosphere. Results from POGM along conjugate profiles off WI and NF reveal a greater longterm strength of extended continental and transitional basement in the NFM (T_e's of 10-40 km). This analysis further supports the hypothesis that the low T_e's modelled in the WIM might be related to its complex structural framework. In order to explain the large asymmetries observed in the amount of basement subsidence (500-1300 in) between the WI and NF margins a lithospheric-scale "simple shear" rift model is proposed for the latest stages of rifting and continental break-up.