Some Recent Results for Geologically Instantaneous Plate Motions

Geologically "instantaneous" plate motions, i.e., those averaged over the past few Myr, provide a fundamental constraint on the terrestrial reference frame. In this talk I review some recent results for geologically instantaneous plate motion, which indicate that the motion of some plates differ significantly from those of the NNR-NUVEL1A set of angular velocities, which are currently incorporated into the international terrestrial reference frame. The set of relative angular velocities for plate motions in the Indian Ocean have been revised in many ways since the construction of the NUVEL-1 and NUVEL-1A angular velocities. The Indo-Australian composite plate is now interpreted as three component plates (Indian, Capricorn, and Australian) with multiple diffuse plate boundaries rather than merely two component plates [Royer and Gordon 1997]. The African composite plate, which was treated as a single plate in NUVEL-1 is now treated as two component plates (Nubian and Somalian) with an intervening diffuse plate boundary [Chu and Gordon 1999; Horner-Johnson et al., this meeting]. The motion of India relative to Eurasia has also been significantly revised (slower and more clockwise) because of new plate motion data in the Indian Ocean, the revised geometry of African and Indian Ocean plates, and a re-weighting of data from the Atlantic [Argus and Gordon 1999]. These results change not only the relative plate angular velocities but also the motion of all plates relative to the no-net-rotation reference frame. Modifications to the no-net-rotation reference frame are tiny compared with the differences between the no-net-rotation frame and the hotspot frame, however. Gripp and Gordon [in press] recently constructed a new set of data from the young traces of hotspot tracks and combined it with NUVEL-1A to obtain a set of angular velocities of the plates relative to the hotspots, which they call HS3-NUVEL1A. The angular velocity of any plate relative to the NNR-NUVEL1A no-net-rotation reference frame differs significantly from the corresponding angular velocity relative to the hotspots specified in HS3-NUVEL1A. Thus, the lithosphere has a net rotation relative to the hotspots of 0.44 +/- 0.11° per Myr (95% confidence level) about a pole at 56S, 70E. This net rotation will be compared with that predicted from models for plate driving forces.