Global reference frames Distinguishing plate motions relative to the mantle and true polar wander

Описание: Dr. Bernard Steinberger (GFZ German Research Centre for Geosciences, Germany) gave a guest lecture via Skype in Fall 2008 for a graduate class on Advanced Plate Tectonics in SOEST, University of Hawaii at Manoa. The students recorded the lecture and produced video podcasts of the event. This is one of 8 such lectures from the course which was taught by Prof. Paul Wessel.

ABSTRACT:

"In order to understand the relation between plate motions and surface processes on one side, and structure and flow deeper in the mantle on the other, a common reference frame is needed. For the past ~130 Myrs, such a reference frame can be based on hotspot tracks ("hotspot reference frame"). Here I show how this reference frame is modified by considering the motion of hotspots in a convecting mantle, and how with such a "moving hotspot reference frame" hotspot tracks can be fit globally, at least back to ~65 Ma. For times older than ~130 Ma, such a reference frame has to be based on paleomagnetism, as no suitable hotspot tracks are available. However, plate motions in the paleomagnetic frame may represent either motion over the deeper mantle, or a re-orientation of the entire Earth relative to its spin axis (true polar wander). Moreover, they do not determine the East-West component of motion. Here I present an approach to distinguish plate motions relative to the mantle and true polar wander: I essentially propose that coherent rotations of all continents in the paleomagnetic frame around an axis close to the joint "center of mass" of all continents represent true polar wander. The best assumption regarding East-West motion is that the African plate has not moved longitudinally. I will also show that these assumption are both physically reasonable and consistent with observations over the past 130 Myrs, for which a hotspot-based reference frame is available."