True polar wander and supercontinents

I present a general model for true polar wander (TPW), in the context of supercontinents and simple modes of mantle convection. Old, mantle-stationary supercontinents shield their underlying mesosphere from the cooling effects of subduction, and an axis of mantle upwelling is established that is complementary to the downwelling girdle of subduction zones encircling the old supercontinent. The upwelling axis is driven to the equator by TPW, and the old supercontinent fragments at the equator. The prolate axis of upwelling persists as the continental fragments disperse; it is rotationally unstable and can lead to TPW of a different flavor, involving extremely rapid (Vm/year) rotations or changes in paleolatitude for the continental fragments as they reassemble into a new supercontinent. Only after several hundred million years, when the new supercontinent has aged sufficiently, will the downwelling zone over which it amalgamated be transformed into a new upwelling zone, through the mesospheric shielding process described above. The cycle is then repeated. The model explains broad features of the paleomagnetic database for the interval 1200–200 Ma. Rodinia assembled around Laurentia as that continent experienced occasionally rapid, oscillatory shifts in paleolatitude about a persistent axis on the paleoequator, an axis that may have been inherited from the predecessor supercontinent Nuna. By 800 Ma, long-lived Rodinia stabilized its equatorial position and disaggregated immediately thereafter. Gondwanaland assembled as its constituent fragments documented rapid, oscillatory shifts of apparent polar wander, here interpreted as TPW. The Gondwanaland–Pangea centroid migrated to the equator immediately prior to Jurassic–Cretaceous breakup. Lack of substantial TPW since 200 Ma may indicate the stabilizing effects of specific plate boundary conditions (i.e., persistent convection patterns in the Tethys–Indian Ocean region), possibly superimposed on a secular geodynamic shift governed by increased lower-mantle viscosity associated with long-term planetary cooling. TPW is a significant geodynamic process that, in terms of continental motions, may even dominate plate tectonics for certain intervals of Earth history. The effects of such rapid TPW may be found among regional tectonics and sea-level changes, and possibly global climate change and biological evolution. D 2002 Elsevier Science B.V. All rights reserved.