Margin segmentation prior to continental break-up: A seismic–stratigraphic record of multiphased rifting in the North Atlantic (Southwest Iberia)

A dense grid of multichannel (2D) seismic profiles, tied to borehole, dredge and outcrop data are used to analyze the multiphased rifting, structural architecture and rift-locus migration across the southwest Iberian margin. In the study area, three distinct sectors show different structural evolutions from the Late Triassic to the Late Jurassic–earliest Cretaceous. The three structural sectors are distinguished by: 1) the presence of incipient tilt-blocks on the inner proximal margin, which denotes limited syn-rift subsidence; 2) developed tilt-blocks on the outer proximal margin; 3) evidence of significant fault-related subsidence on the outer proximal margin during the Middle Jurassic, followed by an Oxfordian–Tithonian/Berriasian(?) rift phase leading to seafloor spreading; and 4) marked crustal stretching on the distal margin, where highly-rotated tilt-blocks overlain by thick Late Triassic to Late Jurassic units are observed. This work demonstrates that significant subsidence occurred in Southwest Iberia several millions of years prior to the latest Jurassic–earliest Cretaceous extensional episode leading to continental breakup. The magnitude of early-subsidence episodes approaches that of the last subsidence pulse preceding continental breakup. Across the southwest Iberian margin the observed structural sectors differ from each other in terms of the age of rift climax of syn-rift strata. We interpret the multiple extensional pulses recorded in Southwest Iberia as resulting not only from continental rifting between Iberia and Newfoundland, but also between Nova Scotia and Morocco. Thus, it is considered that pre-breakup units in the deep-offshore basins of Iberia comprise multiple rift-related sequences whose distribution and relative thickness depends on local subsidence rates, on the diachronous northward-migration of rifting, and on the relative crustal stretching experienced by individual sub-basins.

Research highlights
► SW Iberia reveals significant rift subsidence from the Triassic to the Late Jurassic.
► Multiphased rifting shows strong middle Jurassic extension and subsidence.
► Continental extension during rifting generates discrete growth stratal geometries.
► Relative locus of subsidence migrates westwards across the margin.