On the enigmatic mid-Proterozoic: Single-lid versus plate tectonics

The mid-Proterozoic (ca. 1850-850 Ma) is a peculiar period of Earth history in many respects: ophiolites and passive margins of this age are rare, whereas anorthosite and A-type granite suites are abundant; metamorphic rocks typically record high thermobaric (temperature/pressure) ratios, whereas ultrahigh pressure (UHP) rocks are rare; and the abundance of economic mineral deposits features rare porphyry Cu-Au and abundant Ni-Cu and Fe-oxide Cu-Ag (IOCG) deposit types. These collective observations have been used to propose that a stagnant-lid, or single-lid, tectonic regime operated at this time, between periods of plate tectonics in the Paleoproterozoic and Neoproterozoic. In our reappraisal of the mid -Proterozoic geological record, we not only assess the viability of the single-lid hypothesis for each line of evidence, but also that of the plate tectonic alternative. We find that evidence for the single-lid hypothesis is equivocal in all cases, whereas for plate tectonics the evidence is equivocal or supporting. We therefore find no reason to abandon a plate tectonic model for the mid-Proterozoic time period. Instead, we propose that the peculiarities of this enigmatic interval can be reconciled through the combination of two processes working in tandem: secular mantle cooling and the exceptionally long tenure and incomplete breakup of Earth's first supercontinent, where both of these phenomena had a dramatic effect on lithospheric behaviour and its resulting imprint in the geological record. (c) 2022 British Geological Survey (c) UKRI 2022. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Automated summary

The mid-Proterozoic period is characterized by rare ophiolites and passive margins, abundant anorthosite and A-type granite suites, high thermobaric ratios in metamorphic rocks, and specific types of economic mineral deposits. There have been suggestions of a stagnant-lid tectonic regime during this time, but a reevaluation of the geological record supports the plate tectonic alternative. The unique features of this period can be explained by the combination of secular mantle cooling and the long-lasting and incomplete breakup of Earth's first supercontinent.