Mantle Convection, Not Just Ice, May Have Carved Earth's Great Unconformity

In the walls of the Grand Canyon, a flat line separates ancient crystalline rock from far younger sedimentary layers, and the missing time between them is vast. That surface is one exposure of the Great Unconformity, an erosional boundary found across the globe that dates to the interval before the Cambrian explosion of life. A new study in Nature argues that the cause was not only ice at the surface but slow motion deep below it: mantle convection lifting and lowering whole continents.

The explanations already on the table include glacial erosion during Snowball Earth, rifting and plume activity, a fall in sea level, and mantle-driven dynamic topography. The authors note that the dynamics behind these proposals had not been tested. They tested them by running mantle convection models with the code CitcomS, simulating a billion years of flow from 1,000 million years ago to the present. The models use about 13 million nodes in a three-dimensional spherical mantle. They were driven by plate tectonic reconstructions, with plate motion and the thermal structure of oceanic lithosphere and subducting slabs imposed in one-million-year steps. The team ran six model cases, three of them drawn from earlier work.

The results point to long-wavelength dynamic topography: continental-scale uplift and subsidence. For Laurentia, Baltica and North China, the models predict prolonged uplift in the Neoproterozoic followed by long-term subsidence in the Phanerozoic. Uplift through the Cryogenian would have exposed the land to glaciers, which the authors say erode at rates an order of magnitude higher than rivers and can do so at lower elevations. Dynamic topography on its own is thought to have about a tenth the amplitude of tectonic topography, and tilting alone would not create slopes steep enough for the fluvial erosion required. Glaciers, in this account, did the cutting while the mantle set the stage.

The number of cycles mattered. A single episode of Precambrian uplift followed by Phanerozoic subsidence is the ideal recipe for both forming and preserving the surface, and it matches thermochronology studies. Laurentia showed that pattern in every model except one, M3, in which it sat above a mantle downwelling at 640 million years ago and did not uplift notably. M3 also matched present-day mantle structure poorly, which the authors take as support for a no-net-rotation reference frame over the paleomagnetic one. South America and Australia, by contrast, mostly showed two uplift-subsidence cycles, which would have eroded older unconformities and formed new ones, and they preserve less sediment.

The models also track the sediment record. Predicted uplift followed by rapid Cambrian subsidence generally matched increased sediment flux across the Precambrian–Cambrian boundary for every continent except Africa. There, old orogens and a central position in Gondwana, with mountain belts in Antarctica and South America blocking drainage to the sea, produced inland sedimentation that complicates the match. Baltica, in all models, was uplifted as it moved from the southern to the northern hemisphere between 540 and 520 million years ago, then subsided quickly.

Geography explains where the surface can be seen. Laurentia is large and had limited reworking at its margins, with no subduction directly along them from about 930 to 510 million years ago. The Grand Canyon exposure is tied to Laramide uplift from Farallon flat-slab subduction and later Colorado River incision. Baltica, repeatedly deformed along its eastern margin, mostly hides the unconformity at depth. North China's exposure is unexpected given its small area and four orogenic episodes between roughly 500 and 200 million years ago, and the authors suggest Early Cretaceous lithospheric delamination. The rarity of such unconformities may reflect the more than 900-million-year span needed to make one, combined with rare widespread glaciation and poorer preservation further back in time.

The authors say the uplift and subsidence changed landscapes and sediment flux, and in turn seawater chemistry, which may have catalysed the Cambrian explosion. That link remains a direction for future work. So does the model's main weakness: it depends heavily on tectonic reconstructions, whose uncertainty grows with age. The authors point to alternative reconstructions with continuously closing plate polygons, or different ways of predicting past mantle flow, as the next tests.