Constraining Rates of Neoarchean Plate Motion through Magnetostratigraphy and Integrated High-Precision Geochronology of the Fortescue Group, Pilbara, Western Australia

Abstract:

Plate tectonics influences the global climate and carbon cycle. Continental arrangement affects Earth’s albedo, and rates of plate motion control heat flux from the interior and rate of volcanic degassing. While rates of Phanerozoic plate movements and magnetic field reversals have been well studied, little is known about these phenomena in Earth’s early tectonic history due to a paucity of unaltered Archean rocks retaining primary magnetization. Archean tectonics and an active geodynamo would have played an important role in creating environments suitable for life. A magnetostratigraphic study integrated with high-precision geochronology has the potential to quantify plate velocities and magnetic field reversal rates.

The 2.7 Ga Fortescue Group on the Pilbara craton in Western Australia is one of the oldest and best-preserved basalt successions. The Fortescue records two intervals of potentially rapid cratonic motion, but velocity estimates ranging from 4–50 cm/yr suffer from current ages’ analytical errors on the order of a few Ma. We measured five stratigraphic sections across these intervals, and collected paleomagnetic and geochronology samples at known stratigraphic heights. With our calculated virtual geomagnetic poles and U-Pb CA-ID-TIMS zircon ages with precision and accuracy of <1 Ma, we can more precisely quantify the velocity of the Pilbara craton as it underwent a ~10,000 km displacement during the eruptive history of the group’s lowest stratigraphic unit, the Mount Roe Basalt, and a more modest displacement of ~2000 km during the deposition of the Tumbiana Formation and Maddina Basalt. We also observe and constrain the age of a previously undescribed magnetic field reversal in the Tumbiana Formation, doubling the described magnetic field reversal rate to four reversals over the 60 Ma depositional history of the Fortescue. Our calculated plate velocities will enlighten the debate on the role of tectonics in the cooling of early Earth.