T7 Tectonometamorphic Evolution of the Mediterranean from the End of the Variscan Orogeny to Present

Sunday, 10 October 2021
8:00 AM - 12:00 PM
A106

QUANTIFYING CRUSTAL THICKNESS AND PALEOELEVATION IN THE VARISCAN OROGEN: GEOCHEMICAL EVIDENCE FOR TWO SHORT-LIVED CRUSTAL THICKENING AND THINNING CYCLES

Paper number: T7-12
Program: Technical Sessions
Time: 10:35 AM – 10:50 AM

Authors

Abstract

The Variscan orogeny is widely recognized as a major mountain building event associated with the collision of Laurentia and Gondwana during the formation of Pangea. Yet the topographic and crustal evolution of the Variscan orogen are poorly constrained due to the deep level of exposure. Competing models suggest either subdued topography underlain by relatively thin crust (≤50 km) (e.g., Franke, 2014) or a Tibet-like, high elevation orogenic plateau supported by thickened crust (≥60 km) (e.g., Dorr and Zulauf, 2010). To test these models, we quantified crustal thickness and paleo-elevation using trace element (Sr/Y; La/Yb) geochemical proxies with careful filtering of data from syn-collisional plutons. The proxies suggest significant changes in crustal thickness with space and time across Variscan massifs. Two or more cycles of crustal thickening/thinning are indicated. In the eastern sector of the orogen (the Bohemian and Vosges massifs and Alpine/Carpathian inliers), the data suggest thickening to ~55 km by 355 Ma and thinning at 340-330 Ma. To the west in the Massif Central, crustal thicknesses may have reached ~70 km at 335-320 Ma. A second phase of thickening (to ~50 km) is indicated in the eastern massifs and Iberia at ca. 315 Ma. Crustal thinning to ~35 km is indicated in all domains at 315-290 Ma. The timing of crustal thickening and thinning indicated by geochemical proxies are consistent with existing geologic and petrologic data. If the proxies are correct, Variscan paleoelevations (~2-4 km) were likely lower than Tibet, excepting the Massif Central (~5 km). Short lived (5-15 m.y.) crustal thickness peaks contrast with the 40-50 m.y. lifespan of ≥60 km thick crust in the Tibetan plateau and the Devonian Acadian altiplano in the Appalachian Mountains. We speculate that high temperatures may have favored gravitationally driven crustal flow and thus, thinner crust. The ~340 Ma onset of the Late Paleozoic Ice Age coincides with crustal thinning and post-dates thickening, suggesting that the development of high topography may not have been the main driver of CO2 drawdown. If long-term climate change in the Late Paleozoic was driven by tectonics, our data suggest that enhanced chemical weathering of rapidly exhuming basement rocks during orogenic collapse may have been a major driver of global climate change.

Geological Society of America Abstracts with Programs. Vol. 53, No. 6

doi: 10.1130/abs/2021AM-369542

© Copyright 2021 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.