Megathrust Earthquakes Rattled Earth Over 3 Billion Years Ago

The world’s most powerful earthquakes happen in subduction zones, where one tectonic plate slides beneath the other. When these plates become stuck together, stress builds in the crust of the Earth — like a rubber band being stretched. When enough stress builds up to overcome the friction holding the plates together — like a rubber band snapping — an earthquake occurs. Such an event is also known as megathrust earthquake.

Strange rock deposits found in the Barberton Greenstone Belt preserve evidence of what could be the oldest megathrust earthquakes known so far.

The Barberton Greenstone Belt in southern Africa formed at different times over a critical 800-million-year span from 4.15 to 3.3 billion years ago, when Earth’s tectonic switched from a rigid rock surface to moving plates.

A greenstone belt includes ancient rocks formed or deposited on the seafloor and later tectonically thrusted on younger continental rocks. Generally such a tectonic belt still shows an undisturbed succession from seafloor lava to marine sediments, but in a new study, Simon Lamb of Victoria University in Wellington, and Cornel de Ronde, Institute of Geological and Nuclear Sciences New Zealand, describe an unusually chaotic rock formation from the Barberton Greenstone Belt as the result of sedimentation along an active subduction zone.

According to the two authors, the chaotic rock formation in the Greenstone Belt is very similar to the Great-Marlborough-Conglomerate, a geological formation found in the mountains of New Zealand.

Around 24 million years ago, the Pacific Plate sliding beneath the continental plate hosting Australia and New Zealand triggered a series of megathrust earthquakes. The shaking destabilized vast areas of the continental shelf, parts of which slid into deeper basins and formed a rock deposit consisting of a chaotic succession of well-rounded boulders to angular blocks of great size, comprising shallow-water sediments and deep-sea volcanic rocks. As the two tectonic plates collided, the conglomerate was eventually uplifted and incorporated into the landmass of New Zealand.

Previous research explained the chaotic rock formations found in the over 3.3-billion-old Greenstone Belt as breccia formed during large meteorite impacts, but according to the new study, the extent and frequency of such deposits would suggest an unrealistic high impact rate on early Earth. The new interpretation as large submarine landslide deposits fits better with what we know of Earth at the time.

The full research paper “Large-scale submarine landslides in the Barberton Greenstone Belt, southern Africa—Evidence for subduction and great earthquakes in the Paleoarchean” was published in the journal Geology and can be found online here.