A record of plate tectonic motion and block rotation at 3.25 Ga

Abstract:

Paleomagnetism is a major source of information about Earth’s geodynamic and paleogeographic histories. Paleomagnetic results from Archean rocks (>2.5 Ga) remain scarce, often with temporal gaps too large (hundreds of Myr) to permit detailed tectonic reconstructions over time. We report a new paleomagnetic pole for the ~3250 Ma Kunagunarrina Formation (East Pilbara Craton, Western Australia), which is supported by a positive fold test, tentatively positive reversal test, and micromagnetic imaging. Together with previously reported paleopoles from the ~3180 Ma Honeyeater Basalt and ~3350 Ma Euro Basalt, our new pole refines late Paleoarchean surface motions of the East Pilbara. This analysis shows that the East Pilbara drifted at least 3-4 cm/yr between 3350 and 3250 Ma, likely accompanied by substantial counterclockwise rotation. It subsequently remained at ~40° latitude between 3250 and 3180 Ma while undergoing a ~1.0°/Myr counterclockwise rotation. Latitudinal translations and vertical axis rotations of these rates and magnitudes are difficult to reconcile with stagnant- or sluggish-lid geodynamic settings and therefore argue strongly for the operation of uniformitarian or episodic plate tectonic processes by 3.25 Ga. If our reversal test is in fact positive, our new pole also documents the oldest geomagnetic reversal described to-date.