Paleo movement of continents since 300 Ma, mantle dynamics and large wander of the rotational pole
Résumé
Apparent polar wander (APW) is known to be mainly linked to internal mass distribution changes and in particular to changes in subduction and large-scale upwellings in the mantle. We investigate plate motions during the last 410 million years in a reference frame where Africa is fixed. Indeed, Africa has remained a central plate from which most continents diverged since the break-up of Pangea. The exact amount of subduction is unknown prior to 120 Ma. We propose an approach, based on one hand on the study of the past subduction volcanism to locate ancient subduction activity, and on the other hand microplate motion history in the Tethyan area derived from geology and paleomagnetism. The peri-Pacific subductions seem to be a quasi-permanent feature of the Earth's history at least since the Paleozoic, with however localized interruptions. The “Tethyan” subductions have a complex history with successive collisions of continental blocs (Hercynian, Indo-Sinian, Alpine and Himalayan) and episodical rebirth of E-W subduction trending zones. Assuming that subducted slabs sink vertically into the mantle and taking into account large-scale upwellings derived from present-day tomography and intra-plate volcanism in the past, we compute the time variation of mantle density heterogeneities since 280 Ma. Due to conservation of the angular momentum of the Earth, the temporal evolution of the rotational axis is computed in a mantle reference frame where the Africa plate is fixed, and compared to the apparent polar wander (APW) observed by paleomagnetism since 280 Ma. We find that a major trend of both paleomagnetic and computed APW are successive oscillatory clockwise or counter-clockwise motions, with tracks separated by abrupt cusps (around 230 Ma, 190 Ma and 140-110 Ma). We find that cusps result from earlier major geodynamic events: the 230 Ma cusp is related to the end of active subduction due to the closure of the Rheic Ocean basin after the Hercynian continental collision (340-300 Ma) and to renewed subduction zone West of Laurentia, whereas the 190 Ma cusp results from the Indo-Sinian collision (270-230 Ma) and the subsequent end of the Neo-Tethys ocean subduction.