How Can Asymmetric Detachment Faults Lead To Final Symmetric Distal Ocean Continent Transitions?

Abstract:

Asymmetry or symmetry of magma-poor rifted margins refers commonly to the crustal architecture and the occurrence or absence of large-scale in-sequence extensional detachment faults. While hyper-extended domains at magma-poor rifted margins are often considered to be asymmetric, the downlapping sedimentary sequences observed over Ocean Continent Transitions (OCT) at conjugate pairs of margins suggest a symmetric evolution for these domains prior to lithospheric breakup. In this context, it appears that the current models implying a single detachment fault to explain the presence of exhumed serpentinized mantle domains cannot account for the observations made OCT.

On the base of small-scale structural and stratigraphic observations along the Australia-Antarctica and Iberia-Newfoundland margins we propose that this final large-scale symmetric architecture of the OCT can be related to the development of multiple, out-of-sequence asymmetric detachment faults. Detailed mapping of these faults suggests that they are related to a cyclic de- and re-localization of deformation resulting in an apparent symmetric pattern. These cycles of deformation appear to be strongly influenced by magmatic increase and by alternations between pure shear and simple shear modes. In this context, the presence of a decoupling interface between the upper brittle deformation in the serpentinized exhumed mantle and underlying levels is of major importance. The complex interaction between out-of-sequence detachment systems and the successive rise of the asthenosphere may explain the observed transition from a fault- to a magma-controlled strain accommodation and the transition to more symmetric and localized accretion associated with the formation of a stable spreading center.