The role of pre-rift tectonic quiescence on rifted margins configuration

During the breakup of the Pangea Supercontinent, rifting localized in portions of the continental lithosphere that comprised orogenic structural inheritances. This heterogeneous orogenic lithosphere is a result of mountain-building processes followed by post-orogenic tectonic quiescence. In the case of the Atlantic Ocean opening in its North, Central, and South segments, the time span between Gondwana-Pangea amalgamation and the onset of rifting is largely different, ranging from tens of Myrs to hundreds of Myrs. In this contribution, we discuss the effects of different tectonic quiescence periods of time on the pre-rift continental lithosphere and consequent variable conjugate rifted margin configurations. Here we present 2D thermo-mechanical numerical models that simulate a sequence of extension, contraction, quiescence, and final extension (i.e. accordion-like models). Through this process, our models self-consistently create the orogenic inheritance that undergoes quiescence and final rifting. We explored wide orogenic structures (i.e. without erosion) and narrow ones (i.e. with erosion). In the case of wide orogens, our models showed that tectonic quiescence periods between 30-60 Myrs developed symmetric conjugate rifted margins, where the lithospheric mantle broke up before the continental crust, which, in turn, hyperextended. Nearly 50% of the previously subducted continental crust remained in the fossil subduction zone after rifting. In the case of wide orogens with 100-300 Myrs of tectonic quiescence, the conjugate rifted margins are strongly asymmetric with one ultra-wide side. Nearly 80% of the previously subducted crust was educted during extension. Still in the wide orogens, but now with less than 30 Myrs of quiescence, the resulting rifted margins are asymmetric, not developing ultra-wide sides and having up to 90% of the previously subducted crust educted. Finally, the narrow orogens were not significantly influenced by tectonic quiescence periods in the construction of the final rifted margins, which resulted all asymmetric and rather narrow. In this case, the longer the quiescence, the more continental crust was preserved in the fossil subduction zone. These simulations show that the final rifted conjugates are strongly affected by an interplay between structural and thermal inheritances in the orogenic lithosphere. Wide orogens are hot due to high concentrations of heat-producing elements and grow laterally by orogenic spreading during longer periods of quiescence. Contrastingly, narrow orogens are cold and lack crustal material for wide rifted conjugates.

Funded by Petrobras Project 2022/00157-6.