From microcontinents to extensional allochthons: witnesses of how continents rift and break apart?

The conjunction of high-quality seismic surveys, deep sea drilling, and progress in numerical modelling has changed the way of thinking about how continents rift and oceans form. In particular the discovery of exhumed continental mantle and hyper-extended crust in deep-water rifted margins has led to a paradigm shift in research into the evolution of rifted margins. Although rifted margins now appear to be more complex and their architecture more diverse than previously thought, their study worldwide shows that there are in fact a limited number of structures observed in seismic images that characterize their architecture. These ‘building stones’ include crustal blocks of various sizes, often referred to as microcontinents, continental ribbons, H-blocks, extensional allochthons and outer highs. The aim of this paper is to define the characteristics of these continental blocks and to describe their relationship and position within the rifted margins, and to understand the underlying processes that govern their formation. We propose, using the example of the North Atlantic, that these crustal blocks are the result of specific rift processes that correspond to the sequential evolution from stretching, to thinning and exhumation of the continental lithosphere. We show that the relationship between the various rift structures provides fundamental insights into the controlling processes that thin and finally rupture continental lithosphere.