Dominance of Tectonics Over “snowball Earth” Glaciation in Driving the Great Unconformity

The Great Unconformity—a widely recognized discontinuity and associated gap in the rock record between Precambrian and Cambrian rocks—is arguably the most iconic but enigmatic gap in Earth’s stratigraphic record. Among puzzling features requiring explanation is evidence for many kilometers of exhumation and a hiatus of hundreds of millions to billions of years in continental interiors. Existing models predict either a glacial or a plate tectonic origin in the Neoproterozoic. Here, we show field relationships, and zircon and monazite U–Pb, biotite and muscovite Rb–Sr, and zircon (U–Th)/He thermochronology data for Precambrian crystalline basement rocks from North China to constrain the exhumation history below the unconformity. Dates from multichronometers and thermal history inversions show that the most substantial cooling of continental basement took place from ~2,100 to 1,600 Ma. Comparison with thermal history data from Laurentia, Baltica and Amazonia suggests that protracted plate tectonics broadly modulated by supercontinent cycles, and not 'snowball Earth' glaciation, is responsible for crustal exhumation below the unconformity. The most pronounced erosion evident in both the thermochronologic record and geochemical indicators of continental weathering is shown to correspond with development of Earth’s first true supercontinent (Columbia), rather than with either the Cambrian explosion or the emergence of modern plate tectonics. This does not detract from our understanding of the impacts that formation of the Great Unconformity may have on the long-term evolution of the Earth-life system because the late Paleoproterozoic was also a decisive time in geologic history. If our interpretation is correct, it demonstrates that that development of the Great Unconformity had an even more profound impact on evolution of the Earth-life system than previously appreciated. Further tests of our interpretations will be provided by thermochronologic studies on basement rocks on other paleocontinents and a more thorough examination of erosion below the Great Unconformity at a global scale.