Breakup of a supercontinent between 625 Ma and 555 Ma: new evidence and implications for continental histories

Abstract

A method developed recently for constructing tectonic subsidence curves in early Paleozoic miogeoclines has produced new evidence for the breakup of a late Proterozoic supercontinent. Tectonic subsidence analyses in miogeoclines of eastern and western North America, northwestern Argentina, the Middle East and northwestern Australia limit the timing of the continental breakup to between 625 and 555 Ma. These results refine the implications of a much broader range of radiometric ages of rift-related igneous rocks and biostratigraphic ages of the transition from active extension to passive subsidence in miogeoclines. The recognition of the timing and extent of rifting has led to testable hypotheses for latest Proterozoic and early Paleozoic continental histories. Breakup and onset of drift along an extensive system of continental fractures within a relatively short period of time would generate a large amount of young ocean floor, thereby reducing the volume of the global ocean basin and causing a sea level rise. Maximum reduction of ocean basin volume would postdate the time of breakup, probably by about 70 m.y., placing the transgressive peak at a time not older then about 510–520 Ma. That age agrees well with the time of maximum flooding on the continents close to the end of the Cambrian. Restriction of the breakup to between 625 and 555 Ma reduces the time gap between an essentially intact late Proterozoic supercontinent and the oldest reliable paleomagnetic reconstruction of the dispersed continents at about 560 Ma. A continental reconstruction produced by rotating Laurentia and Baltica into Gondwana a minimum distance from the 560 Ma position is consistent with limited geologic data. However, that reconstruction places Laurentia and Baltica in low latitudes which is difficult to reconcile with the absence of evaporites in syn-rift complexes in both continents.