Study links slow mantle motion and glaciers to the Great Unconformity

Left · 2A Nature study by geoscientists Nicolas Flament and Santosh Dhungana of the University of Wollongong, Australia, proposes that slow convection in Earth's mantle helped create the Great Unconformity, a gap in the geological record. 1 2 Left · 2At sites such as the Grand Canyon, a flat layer of sedimentary rock rests directly on much older crystalline basement. Flament says the gap can span anywhere from 200 million years to more than a billion years. 1 2 Left · 2The unconformity has been found across most of North America, parts of northern China, and beneath younger rocks in northern Europe. The sediments above it appear around the time of the Cambrian explosion, the large diversification of animal life. 1 2

Left · 2Earlier explanations included erosion by ice sheets during the Snowball Earth period (roughly 720 to 635 million years ago), rifting, and a global fall in sea level. Flament notes that global glaciation or sea-level change should have left the unconformity everywhere, and it is not found everywhere. Mantle-driven uplift and subsidence (dynamic topography) had been proposed but not tested quantitatively. 1 2 Left · 2The team ran six simulations of mantle convection going back 1 billion years, driven by reconstructions of tectonic plate motion, and used the Australian supercomputer Gadi. The mantle is about 2,900 kilometres thick, and its hot rising and cold sinking rock can raise or lower the surface by hundreds of metres. 1 2 Left · 2In the models, Laurentia (the core of North America), Baltica (northern Europe) and North China were lifted for hundreds of millions of years and then sank during the Cambrian. Ars Technica reports that five of the six models showed this for Laurentia and four for northern Europe and North China. 2 1

Left · 2According to the authors, uplift alone was too gentle to strip away enough rock, but it coincided with Snowball Earth glaciation, and ice sheets erode even gentle slopes more efficiently than rivers. When the continents later sank, seas flooded in and buried the eroded surface under new sediment. 1 2 Left · 2The models also account for why the unconformity is hard to see elsewhere: Australia and South America went through two uplift-and-subsidence cycles from about 900 million years ago, which the researchers say is not conducive to forming and preserving it. Ars Technica adds that Africa's erosion was shaped by surrounding mountain belts rather than the mantle. 2 1 Left · 1Flament acknowledges uncertainties: the size of mantle-driven surface motion is still debated, continents' starting positions (their longitude) are uncertain, and ancient ocean floor had to be invented in the models. He also said it would be misleading to say the mantle set the pace of the Cambrian explosion, though he called it one of the forces working together. 1