PALEOZOIC PLATE DYNAMICS

Current plate motions can be accounted for by a balance of active forc es, slab pull, ridge push, and, for continental plates, trench suction , with drag beneath the plate as a resistive force. If we assume that the same forces have acted through time, we can reconstruct plate moti ons from the geometry of past plate boundaries. Paleozoic reconstructi ons are made with paleomagnetic, tectonic, climatic, and biogeographic data, as no ocean floor remains. PALEOMAP reconstructions are used to estimate past plate speeds and to test simple dynamical models in ord er to determine which ranges of forces best accounts for the observati ons. Over the last 600 m.y., plate speeds averaged over 40- to 100-m.y . intervals show considerable variation; Gondwana's speed oscillates f rom 20 to 60 km/m.y. over a long timescale (200-400 m.y.) with conside rable noise superposed. Over the Paleozoic Era motions for large conti nental regions average 28 km/m.y.; force balance models based on prese nt-day observations suggest that continental regions without a large a ttached slab would move 30 mm/yr. The opening and closing of the ocean between Laurentia and Gondwana 560-400 Ma is used to test dynamical m odels and the parameter values assumed. In the late Precambrian, Laure ntia rifted away from Gondwana. In the earliest Cambrian it was near 4 0 degrees S; by Late Cambrian and Ordovician it had moved to the equat or. During the Silurian and Devonian, Laurentia reversed direction and later collided with Gondwana at 40 degrees S. In a model of the force s acting on the plates, slab pull, ridge push, and trench suction are assumed to balance plate drag. Only certain ranges of ridge-push and t rench parameters can model both the opening and subsequent closing of the ocean. The dynamic models, with parameter values inferred from pre sent rates, bracket the rates required by the reconstructions.