Rift strength controls rapid plate accelerations: A global analysis of Pangea fragmentation

Abstract:

Motions of Earth’s plates are thought to be driven by slab pull, basal drag, and ridge push. Here we propose that plate motions during supercontinental fragmentation are decisively controlled by the non-linear decay of a resistive force: rift strength.

We use state-of-the-art global tectonic reconstructions and the new geotectonic analysis tool pyGPlates to analyze the transition from rifting to sea-floor spreading of well-studied post-Pangea rift systems (Central Atlantic, South Atlantic, Iberia/Newfoundland, Australia/Antarctica, North Atlantic, South China Sea, Gulf of California). In all cases, continental extension starts with a slow phase (< 10 mm/yr, full extension velocity) followed by a rapid acceleration over periods of a few My that introduces a fast rift phase (> 10 mm/yr). The transition from slow to fast extension takes place long before crustal break-up. In fact, we find that approximately half of the present day rifted margin area was created during the slow, and the other half during the fast phase. We reproduce the transition from slow to fast rifting using numerical forward models with force boundary conditions, such that rift velocities are not imposed but instead evolve naturally in response to changing strength of the rift. These models show that the two-phase velocity behavior during rifting and the rapid speed-up are intrinsic features of continental rupture that can be robustly inferred for different crust and mantle rheologies.

It has been proposed that abrupt plate accelerations can be caused by plume-lithosphere interaction, subduction initiation, and slab detachment. However, none of these mechanisms explains our result that plate speed-up systematically precedes continental break-up. We therefore propose dynamic rift weakening as a new mechanism for rapid plate motion changes.