A paleomagnetic analysis of Cambrian true polar wander

The latest Neoproterozoic through Cambrian is one of the most remarkable in tervals in geologic time. Tectonically, the period from 580 to 490 Ma marks a time of rapid plate reorganization following the final stages of superco ntinental breakup and Gondwana assembly. The apparent speed at which this r eorganization occurred led some to propose a link between tectonic events, biologic changes and climatic changes. One of the more intriguing proposals is that the tectonic changes were triggered by an episode of inertial inte rchange true polar wander (IITPW) which resulted in a rapid (6 degrees/m.y. ) shift of the spin asis relative to the geographic reference frame. IITPW is a special case of true polar wander (TPW) that makes specific demands on the length of apparent polar wander paths (APWPs) recording the motion. Sp ecifically, each path must allow for similar to 90 degrees of synchronous m otion during the interval from 523 to 508 Ma. A review of paleomagnetic dat a for Laurentia, Baltica, Siberia and Gondwana indicates that none of the A PWPs approaches the necessary length, each path is of a different length an d the apparent motions are non-synchronous. Collectively, these observation s negate the premise of a Cambrian IITPW event. Since the IITPW hypothesis was proposed as an alternative to rapid plate motion of Laurentia and Gondw ana during the Neoproterozoic-Cambrian interval, any alternative model must account for this rapid motion. I suggest that a reasonable explanation for 'anomalously' high rates of plate motion for some continents, possibly on the order of 20-40 cm yr(-1), is enhanced plate motion driven by lower-mant le thermal anomalies and possibly true polar wander. In fact, the enhanced plate motions driven by these lower-mantle sources may provide a dynamic fe edback triggering true polar wander. (C) 1999 Elsevier Science B.V. All rig hts reserved.