Rifting in the presence of accreted terranes – a numerical modelling study

Many rifted margins have formed in areas that have previously experienced subduction and orogenesis, completing the Wilson cycle of closing and opening oceans. Often the subduction phase is accompanied by the accretion of bathymetric highs, such as oceanic plateaus, continental fragments, seamounts and microcontinents. Such accretionary orogeneses result in a more complex structural, rheological and thermal inheritance than continent-continent collision without terranes. Here we use 2D thermo-mechanical numerical models to investigate how accretionary, rather than collisional orogens, affect a subsequent phase of continental rifting. Our models build an orogen through subduction, terrane accretion and collision before the onset of rifting. We examine the structure of the resulting rifted margins and the degree in which inherited compressional structures are utilized.

For rifting of collisional systems without terrane accretion, we find that there is a competition between structural and thermal inheritance that has a first order control on rifted margin architectures. For smaller, colder collisional systems, localized reactivation of the old subduction interface promotes the formation of narrow margins. Conversely, in larger, hotter collisional orogens, wide margins develop through distributed extension, initiating away from the inherited suture in the hot, weak regions of the pre-rift orogen. This dynamic persists even in the presence of accreted terranes, where the orogens preserve multiple suture-zones that dissect the lithosphere. In smaller orogens, the optimally oriented, steepest and as a result shortest, and hence weakest suture experiences the highest degree of inversion, localizing the rifting.. In larger, hotter accretionary orogens, deformation is not primarily focused on inherited shear zones but is instead concentrated in the thickest, hottest part of the orogen. We interpret this as thermal inheritance dominating over the influence of structural inheritance. Depending on the pre-rift lithosphere configuration, accreted terranes can be preserved in one or both rifted margins. Our results show that the size of the accretionary orogen prior to extension has the strongest influence on the style of the resulting rifted margins and that the presence of multiple sutures between the accreted terranes plays a smaller role in localizing extension.

Our experiments demonstrate that a wide range of features such as continental fragments, allochthons or hyper-extended segments that can form in the presence of inherited compressional structures and emphasize the importance of the deformation history in the evolution of continental rifting. These results can be further used to understand how various stages of the Wilson cycle affect each other.