Enigmatic Mid‐Proterozoic Orogens: Hot, Thin, and Low. Issue 16 (18th August 2021)

• Record Type: Journal Article
• Authors: Spencer, Christopher J.; Mitchell, Ross N.; Brown, Michael
• Is Part Of: Geophysical research letters. Volume 48:Issue 16(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-08-18
• Subjects: orogeny -- crustal thickness -- ultra‐high temperature metamorphism; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2021GL093312
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Since the Archean, secular change in orogenic style is demonstrated through evolution of metamorphic conditions and geochemical proxies. Linked to orogenic style is the amount of crustal thickening and elevation, whereas orogenic vigor is related to the supercontinent cycle. An array of Proterozoic orogens spanned the assembly of supercontinents Columbia and Rodinia, but the vigor of orogenesis is debated, with proposals for both Mesoproterozoic quiescence and climax. We show mid‐Proterozoic orogenesis occurred globally and was broadly continuous; furthermore, orogens exhibit elevated metamorphic thermobaric ratios with large volumes of high‐temperature felsic magmatic rocks. These features reflect higher mantle heat flux leading to increased mid‐crustal flow and lower elevation. In this context, proposals that geochemical proxies for crustal thickness record orogenic quiescence are inconsistent with the geological record. Alternatively, secular change in crustal thickness is attributed to orogenic style, namely the prevalence of hot, thin, and low orogens in the mid‐Proterozoic. Plain Language Summary: Orogenesis is the process whereby tectonic plates converge and mountain systems are created. In the case of the Andes and Himalayas, orogenesis resulted in significant thickening of the continental crust. Recent attempts to provide geochemical proxies for crustal thickness have allowed geologists to track the thickness of the crust through geologic time. One period of time in particular between 1850 and ∼850 million years ago—the mid‐Proterozoic—was characterized as having relatively thin crust. Some have argued this is evidence for a period of "orogenic quiescence." However, the geologic record is rife with ancient orogenic belts during this time as evidenced by the metamorphic and igneous rock records. In particular, the metamorphic rocks display higher than normal temperature/pressure ratios indicating unusually hot crust. We propose that the thin crust at this time is a product of high temperatures resulting in greater crustal flow and therefore lower mountain ranges. Key Points: The mid‐Proterozoic was characterized by unusually hot orogenesis Elevated orogenic temperatures limited crustal thickening and promoted crustal flow and hinterland extension Secular change in orogenic process explains independent estimates of variation in Proterozoic crustal thickness