DEPTH DEPENDENT STRETCHING AND THE OPENING OF THE NORTH ATLANTIC

Analysis of the structural and stratigraphic architecture of basin-systems along the northwest margin of Europe illustrates how the magnitude, location and mechanism of rifting changed through time. The mid-Cretaceous basins formed in an intracratonic setting between Greenland and Northwest Europe. Estimates of stretching from reconstruction and forward modeling of upper crustal fault-systems are in reasonable balance with estimates of stretching from crustal-thinning measurements, and post-rift basin-subsidence analysis. Uniform-stretching models can reasonably predict the observed structural and stratigraphic basin-architecture.

A fundamental change in deformation mechanism occurred, however, during Early Tertiary continental separation. A critical observation from the outer reaches of the Northwest European Atlantic Margin, and one that has been made in other basin systems around the world, is that stretching that led to breakup at the continental margin, was not uniform – it was depth-dependent with observed stretching of the upper-crust being significantly lower than that observed and predicted for the lower-crust and continental lithospheric mantle.

Most conceptual models that attempt to illustrate the transition from intracontinental rifting to seafloor spreading assume that in the final state, continental crust is directly juxtaposed against oceanic crust. Data emerging from continental margins worldwide do not support this view. New kinematic models for passive-margin development need to account for the basic observation that conjugate passive-margins are always structured as upper-plates – lower-plate margins are never observed. Models also need to explain depth-dependent stretching and how lower crust and continental lithospheric mantle rocks are exhumed at the seabed during the breakup at magma-poor margins.