Abstract: Uniformitarianism and Global Tectonics

Eli A. Silver

The foundation of plate-tectonics theory is a quantitative description of relative movements between spherical caps or plates on a globe. As long as the ancient earth experienced such movements and was nearly spherical in shape, plate tectonics applies in principle to past events. However, many of the basic observations on which we rely for quantitative control of plate motions, such as marine magnetic anomalies, are destroyed with age because of subduction, and earthquakes are not available to define ancient plate boundaries. A further restriction is the geometric requirement that at least one out of three poles of rotation which describe the movement between plates must migrate with time relative to the plates whose motion it describes, meaning that even where present-d y motions are well determined we cannot assume constant motion in earlier times. Moreover, there are periods of up to 50-m.y. duration in which magnetic-reversal frequency is very low, leaving crust of that age without magnetic age stripes. Past reconstructions using paleomagnetic pole determinations have been moderately successful, and represent the major tool for quantitative study of Precambrian plate motions. In addition to the rigorously quantitative aspects, plate tectonics provides a very useful qualitative description of tectonic events. In such a framework, many ancient orogenic belts are the result of processes of subduction and of collision between island arcs (ARC), active (AND) and passive (ATL) continental margins, and seamount chains (SMT). Subduction polarity in ancient c nvergence zones may be determined using geochemical (e.g., K₂O), metamorphic (paired High P-High T belts), structural and stratigraphic indicators. In addition to numerous subduction zones, the modern world shows a few good examples of ATL-AND (Himalayas), ARC-ATL (Taiwan, Timor), and ARC-ARC (Molucca Sea) collisions, and some ARC-SMT and AND-SMT collisions. ATL-AND collisions result in intracontinental orogenic belts and suture zones, and may partly or completely mask earlier ARC or SMT collisions. AND-AND collisions should have rather similar effects but no modern examples are known. Because metamorphism, erosion, weathering, burial, and later faulting also obscure ancient collisions, an important job for today's uniformitarian is to gain sufficient understanding of modern c llisions to recognize the subtle clues in the scrambled orogenic legacy of the past.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma