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

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.