Rapid True Polar Wander Oscillations Preserved In Continuous Orosirian Strata

Abstract:

Previous paleomagnetic study of the Orosirian (2050-1800 Ma) Great Slave Supergroup, well exposed on islands in Great Slave Lake on the southeastern margin of the Slave craton, revealed evidence of a regularly reversing, or oscillatory, apparent polar wander path. The paleogeographic utility of paleopoles from the Great Slave Supergroup has been brought into question owing to the possibility of vertical-axis rotations associated with the McDonald transcurrent fault array, but structural corrections can be applied. In a high-resolution paleomagnetic re-sampling of portions of the Great Slave Supergroup, concentrating on the Pethei and Christie Bay Groups in particular, we demonstrate systematic and correlative shifts in declination within structurally uniform homoclinal sections. Positive fold tests further substantiate the primary nature of the paleomagnetic oscillations. Smooth, albeit rapid, declination changes preserved within and between our Great Slave Supergroup sections cannot be trivially explained by structural rotation. True polar wander oscillations (expressed as declination variation locally) may be used to correlate Orosirian strata regionally and globally. Geochronological refinement of the Great Slave Supergroup, both directly and indirectly, should help facilitate any such correlations and those with intrusive rocks both locally and globally. Unconformably overlain by the Et-Then Group that is contemporaneous with the McDonald Fault system, the Great Slave basin and intrusive Compton laccoliths pre-date transcurrent faulting. Increased precision achieved using modern U-Pb (zircon) geochronologic methods on the ca. 1865 Ma Compton laccoliths calibrates the minimum age constraint for Great Slave deposition. Additionally, magnetic susceptibility of a ~350 m-thick section of the Pethei Group reveals a hierarchy of cycles. If interpreted as astronomical cycles, possibly expressed through the effects of insolation on biogenic carbonate precipitation relative to continental siliciclastic export, sediment accumulation rates could be constrained for the Pethei carbonates. Such age constraints have implications for maximum rates of true polar wander in Paleoproterozoic time, which is linked through mantle viscosity to the secular cooling of Earth.