Paleomagnetic detection of relative plate motions and a core dynamo by 3.5 Ga - Alec Brenner

Paleomagnetic detection of relative plate motions and a core dynamo by 3.5 Ga
Alec Brenner
Harvard University

The presence or absence of plate tectonics on the early Earth, particularly during the Archean, is hotly debated. Crucially, the few existing paleogeographic constraints on early Archean tectonic motions have not yet demonstrated an example of relative motion between lithospheric blocks beyond 2.5 Ga. This "differential motion" is a defining hallmark of plate motions, documenting internal mobility of the lithosphere.
In this talk, I present the oldest example of differential motion yet discovered at 3.48 Ga, based on 3.48-3.45 Ga paleomagnetic data from North Pole Dome, East Pilbara Craton, Western Australia. These results are supported by a uniquely robust set of age constraints, including inverse baked contact tests on ≥3.44 Ga dikes, a fold test across ≥3.3 Ga deformation, and the presence of a stratabound reversal – the oldest ever documented - at 3.46 Ga. Importantly, these results permit direct comparison with existing data from rocks of the same age range in the Barberton Greenstone Belt, Kaapvaal Craton, South Africa. This comparison reveals that while the Barberton occupied equatorial paleolatitudes throughout the studied interval, the East Pilbara initially moved rapidly from mid-latitudes to polar latitudes at ~3.48-3.47 Ga, and then remained at high latitudes until 3.45 Ga. The Pilbara and Barberton therefore (1) were at very different paleolatitudes during this time, and (2) experienced differential motion around 3.48-3.47 Ga.
The rate of this differential motion, 47(-36/+70) cm/yr, was faster and halted more rapidly than most modern plate motions, suggesting the operation of non-active-lid geodynamics (e.g., sluggish-lid or episodically-mobile geodynamic behavior, or possibly modified plate tectonics). Additionally, the oldest known geomagnetic reversal at ~3.46 Ga shows that the geodynamo reversed infrequently and was stable, axially-aligned, dipolar, and thus generated within Earth’s core. These results provide geodynamic context to the earliest preserved habitats for life, and highlight the ability of paleomagnetism-based paleogeography to advance our understanding of geodynamics in deep time from the surface to the core.