The absolute plate motions for early Tertiary time are reconsidered. The no-net-torque determination of Solomon and others indicated absolute plate motions at 55 m.y. ago that were considerably different from the present motions. To test whether these motions are truly characteristic of early Tertiary time or merely a consequence of uncertainties, an alternative model for the relative motions is considered. A single major change is made in the relative motions scheme: Antarctica is treated as two plates. The role of Antarctica is particularly important in the relative motion scheme because it is the sole link between the large Pacific Ocean plates and the other, largely continental plates. Although this relative plate motion model for early Tertiary time does not yield absolute motions that are compatible with those of the present, there are considerable differences from Solomon and others' determination. Thus, past relative plate motions must be further constrained to give absolute motions that can confidently be compared to the present set and used in testing hypotheses about driving forces.
- Geological Society of America