The influence of accretionary orogenesis on subsequent rift dynamics

Abstract

The Wilson Cycle of closing and opening of oceans is often schematically portrayed with "empty"oceanic basins. However, bathymetric and geophysical observations outline anomalous topographic features on the ocean floor, such as microcontinents and oceanic plateaus, that can be accreted or subducted when oceans close in subduction. If later rifting and extension localizes in the area of former oceanic closure, this implies that the rifted margins formed in regions are characterized not only by continent-continent collision but also by the presence of accreted continental terranes. An excellent example of such a system can be found in the North Atlantic, where the late Paleozoic to Mesozoic opening of the Atlantic Ocean followed the early Paleozoic Caledonian orogeny, which formed during the collision of Baltica and Laurentia continents but also incorporated allochthonous continental terranes. The full evolution from subduction to accretion-collision and how those processes bear on continental rifting has not been studied systematically. Potential factors that can influence the evolution and structural style of a rift in such a tectonic setting include the thermo-tectonic age of the orogen, the number and type (size, rheology) of accreted terranes, and the nature of terrane boundaries, as well as the velocity of rifting. Here, we use 2D finite-element thermo-mechanical models to investigate how accreted microcontinents and the size of the orogen affect the style of continental rifting. Our models demonstrate that there is a competition between thermal and structural inheritance that has a first-order effect on the style of rifting. In large, warm orogens thermal inheritance dominates over structural inheritance, leading to the formation of new major extensional shear zones, whereas in small, cold orogens structural inheritance dominates over thermal inheritance, allowing for efficient deformation localization along pre-existing sutures. In comparison, the presence of accreted terranes within the orogen only has secondary effects. In small, cold orogens, when multiple sutures are present, the oldest, shortest, and most optimally oriented suture is reactivated extensively, with the others experiencing only limited activity. In contrast, in large, warm orogens, the suture closest to the centre of the orogen is inverted the most. Additionally, the presence of accreted terranes within the pre-rift lithosphere leads to the formation of continental fragments in the rifted margin architecture.