Raising the Colorado Plateau – tales from xenoliths and petrological modeling

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• Richard Palin, University of Oxford
• Tuesday 08 November 2022, 12:00-13:00
• Department of Earth Sciences, Tilley Lecture Theatre.

If you have a question about this talk, please contact Oscar Branson.

High-elevation, low-relief continental plateaus are major topographic features and profoundly influence atmospheric circulation, sediment transport and storage, and biodiversity. Although orogenic surface-uplift mechanisms for modern continental plateaus near to plate margins, such as Tibet, are well-characterized, they cannot account for examples in intracontinental settings, such as the Colorado Plateau, southwestern USA . While strongly debated, most competing hypotheses that account for Cenozoic uplift of the Colorado Plateau acknowledge the importance of northwest-directed subduction of the Farallon oceanic plate beneath North America since c. 100 Ma. Here, I will describe the results of two in-depth investigations into mechanisms that may have caused uplift of the plateau; one being primarily tectonic, and the other being primarily petrological in nature. First, I show via study of lawsonite-bearing eclogite xenoliths collected from diatremes in the center of the plateau that the shallow-dipping Farallon slab may have penetrated through the proto-plateau SCLM at relatively shallow depth (_{120 km), shearing away} 80 km of its mantle root. This removal likely caused asthenospheric upwelling and isostatic rebound of the plateau region during the Late Cretaceous to the Eocene. Secondly, I report on the results of recent petrological modelling that has investigated whether broad-scale hydration-induced metasomatism of continental lower crust can reduce its density and increase its buoyancy sufficiently to drive isostatic uplift. With reference to devolatilization of the subducting Farallon slab, we have applied open-system reactive transport fluid-rock interaction models to determine the spatial and temporal scales over which mineralogic transformations would have taken place following the infiltration of aqueous fluids into the proto-plateau lower crust. These data show that cumulative surface uplift of over 1,000 m can be achieved by metasomatism of an initial 40-km-thick continental crust, even at relatively modest fluid infiltration rates compared to modern-day subduction zones worldwide. This petrological effect should also be considered alongside purely tectonic arguments when accounting for anorogenic uplift in intracontinental regions, without the need to invoke contributions from extraneous mechanisms that lack a priori evidence, such as mantle plume impingement.

This talk is part of the Department of Earth Sciences Seminars (downtown) series.

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