True Polar Wander: a Key Indicator for Plate Configuration and Mantle Convection during the Late Neoproterozoic

True Polar Wander (TPW) refers to the movement of the Earth compared to its rotational axis and, on geological time-scale, mainly results from changes of the mass density distribution produced by mantle dynamics. Based on paleomagnetic data, large and fast TPW episodes have been proposed from 615 to 590 Ma and 575 to 565 Ma and may correspond to abrupt changes in the mantle dynamics of the Ediacaran (635-541 Ma). To address this question, we explored the effect of the reactivation of a peri-Pacific like girdle of subduction to produce such TPW, coupling paleogeographic reconstructions and simple mantle dynamic simulations.

First, we performed a reconstruction of the main continents between 615 and 520 Ma and defined the position of the main subduction zones including a major girdle of subduction surrounding the continents. We then implemented this paleogeography into our mantle dynamic model and calculated the induced TPW. Our results show that the reactivation of the girdle of subduction along with a reduced effect of the return flow rising from the lower mantle can produce two TPW episodes consistent with the paleomagnetic observations. This mechanism is possible if the sinking velocities associated with the girdle of subduction drastically decrease between 635 and 730 Ma.

This dramatic change of regime is consistent with the global detrital zircon age distribution of the Neoproterozoic which supports a low subduction activity during the Cryogenian (720-635 Ma) and a boost during the Ediacaran. This large-scale subduction fluctuation may explain the protracted rifting history of the supercontinent Rodinia over 200 Ma due to drastic variations of subduction-related stress on the lithosphere. Finally, the weakening in subduction magmatism during the Cryogenian may have strongly decreased the CO2 outgassing into the atmosphere and played a major role in driving the Neoproterozoic global-scale glaciations.