PLATE MOTIONS, CRUSTAL AND LITHOSPHERIC MOBILITY, AND PALEOMAGNETISM - PROSPECTIVE VIEWPOINT

The shift in viewpoint from a static to a mobilistic solid Earth, as b rought about by paleomagnetism, opened many new directions of research that continue to be exciting today and hold prospects for an exciting future. Here five topics of current research on Earth's mobilistic su rface are reviewed and their future directions discussed. First, divid ing Earth's solid surface into two domains, (1) nearly rigid plate int eriors and (2) plate boundaries, some of them very wide, leads to a re view and discussion of the kinematics of wide plate boundaries. Many m ore adjustable parameters will be needed to describe the complex kinem atics of plate boundaries, which cover similar to 15% of Earth surface , than the few dozen parameters needed to describe the kinematics of p late interiors, which covet similar to 85% of Earth's surface. Second, the question of the degree of rigidity of plate interiors is discusse d. The integral of the velocity gradient across plates interiors is at least a few hundredths or tenths of millimeters per year and in sever al well-determined cases less than 2 or 3 mm/yr. Future investigations will no doubt be aimed at narrowing these limits and to expanding the area of Earth's surface over which limits are known. Third, space geo detic data, mainly from very long baseline interferometry, satellite l aser ranging, and the Global Positioning System, have demonstrated a r emarkable similarity of velocities of stable plate interiors averaged over a few to a dozen years with plate velocities averaged over a few million years. Already, however, significant differences of a few mill imeters per year are emerging, and the tectonic and dynamic significan ce of these differences need to be evaluated. Fourth, despite more tha n two decades of investigation, the question of how fast hotspots move relative to one another is still a contentious issue. One group of re searchers maintains that maximum speeds are 3 mm/yr, whereas another m aintains that speeds are 10-20 mm/yr or more. The differences cannot b e reconciled until the uncertainties in plate reconstructions relative to hotspots are systematically and properly incorporated into analyse s of hotspot motion. Fifth, paleomagnetists have estimated true polar wander over the past 200 Myr by equating it with the apparent polar wa nder of hotspots. However, the possible motion among hotspots and the neglect of the uncertainties in reconstructions relative to the hotspo ts have left these analyses unconvincing. Further progress requires th e incorporation of these uncertainties. A possible exception is the ap parent polar wander of the hotspots over the past similar to 10-20 Myr or less, which requires only small adjustments for plate motion and h as presumably smaller uncertainties due to plate reconstructions. The cause of this apparently significant, geologically recent shift of the pole is poorly understood. Compared with the rate and direction of th e secular shift of the pole observed this century from the Internation al Latitude Service and for 1976 to 1994 from space geodetic data, the paleomagnetically observed shift is unlikely to be due to the removal of northern hemisphere midlatitude ice sheets, which is the commonly accepted explanation for the shift observed over the past century. I s peculate instead that the apparently rapid uplift of the Tibetan Plate au sometime during the past similar to 10-20 Myr is at least partly re sponsible for the geologically recent shift of the pole. Finally, I co nclude that the theory of plate tectonics has provided a framework tha t leads naturally to further quantification of the kinematics and defo rmation of Earth's solid surface chiefly because of the key assumption of the rigidity of plate interiors, which permits specific prediction s to be made. This revolution in quantification still has far to So an d holds exciting prospects for future tectonic studies.