Tectonism rather than "snowball Earth" glaciation is responsible for the Great Unconformity

Abstract

The Great Unconformity (GUn)-a widely recognized discontinuity and associated gap in the rock record between Precambrian and Cambrian rocks-represents a globally important interval of continental exposure and erosion that is notable also for the first appearance of all major animal phyla on Earth. However, its origin remains the subject of vigorous debate. Here, we present 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.

Keywords: Cambrian explosion; Great Unconformity; basement exhumation; multiple thermochronology; supercontinent cycle.