Effects of tectonic quiescence between orogeny and rifting.

Tectonic quiescence is a period of nearly null relative plate motion. During the Atlantic Ocean opening, rifting cut across continental segments of supercontinent Pangea that had been through different periods of quiescence, which varied from tens to hundreds of Myr. Using numerical models, we explored the effects of quiescence between orogeny and rifting on the pre-rift lithosphere and subsequent conjugate rifted margin configuration. Our model results showed that quiescence of ∼30–60 Myr caused wide orogenic wedges (>300 km) to be heated beneath their centers, which led to the breakup of the lithospheric mantle before the breakup of the continental crust during rifting, resulting in hyperextended conjugate rifted margins. In these scenarios, continental lithosphere breakup away from the suture contributed to the preservation of previously subducted lower crust along the subduction zone. Long periods of tectonic quiescence (∼100 Myr) experienced by a wide orogen permitted efficient orogenic collapse and lateral spreading, with pronounced lithospheric thermal weakening. Consequently, post-quiescence rifting reactivated the suture and effectively drove plate eduction, exhuming most of the subducted continental lower crust and mantle lithosphere. Ultimately, ultra-wide domains developed into asymmetric conjugate margins. Contrastingly, in narrow orogenic wedges (<200 km), different quiescence durations had minor effects on the margin architecture. In these cases, a longer quiescence time contributed to a larger preservation of subducted crust in the fossil subduction zone after rifting. Our models agree with conjugate rifted margins in the North and South Atlantic, which probably developed after the collapse of wide orogens and subsequent periods of tectonic quiescence.