Reactive thermodynamics of crustal eclogitization and foundering

Abstract

Regional metamorphism and densification (eclogitization) of the lower crust can affect the lithospheric dynamics of mountain belts, but the coupled effects of reaction rate, temperature, and composition on metamorphism are poorly understood. We present a reactive thermodynamic model of the granulite–eclogite transition to investigate the long-term buoyancy and gravitational stability of the lower crust. First, we characterize the conditions for which orogenic crust attains negative buoyancy by determining its reactive mineral assemblage and density under prescribed pressure–temperature–time paths. Using existing metamorphic rate data, we calibrate a Damkoḧler number (a relative reaction rate) to parameterize the catalytic effect of aqueous fluids. The depth necessary for negative buoyancy is sensitive to temperature and Da, ranging from ∼45 to

for a basaltic-andesite composition (54 wt.% SiO₂). Second, using a Rayleigh–Taylor instability analysis, we suggest that, while cold eclogitic crusts <

would be strong enough to resist foundering within 50 Myr, warm crusts >

could obtain large thicknesses of ∼10 to

and would founder within

. We hypothesize that such foundering events are a natural consequence of convergent tectonics, where the aqueous fluids and high pressures required for metamorphism are known to exist. The Pampean flat slab in the Central Andes provides geophysical evidence linking slab fluids to eclogitization and densification of the thickened continental crust. Lithospheric foundering coupled to convergent tectonics through eclogitization could explain many observations of orogenic hinterland deformation and magmatism.