Bimodal behavior of extended continental lithosphere: Modeling insight and application to thermal history of migmatitic core complexes

Extensional migmatitic core complexes (MiCC) expose partially molten crustal rocks exhumed during post-thickening extension. Low heat flow in active collisional mountains suggests that there is (i) either input from extra-crustal heat sources or (ii) under-assessment of the pre-extensional collisional-heat content to form high grade metamorphic core complexes. We evaluate these two hypotheses by varying thermal gradients and crustal thicknesses of two-dimensional, thermo-mechanical, numerical models simulating the extension of a stratified lithosphere. For Moho-temperatures <700 degrees C and crustal thicknesses >35 and <= 50 km, extension leads to a rifting mode dictated by the upwelling of the asthenosphere. In that case, no MiCC is formed. For Moho-temperatures >700 degrees C and/or crustal thicknesses >60 km, extension develops MiCC while maintaining a flat Moho, similar to wide extensional systems such as the Aegean and the Basin and Range. However, when crustal strain softening is switched on, it produces for both mentioned conditions MiCCs with similar crustal geometries, testifying that partial melting in MiCCs can result from both extra-crustal, syn-extensive heat (asthenospheric advection) and collisional heat. Typically, MiCCs induced by asthenospheric-heat show the sequence lower crustal migmatization, doming and crustal Ultra-High-Temperature (UHT) conditions and mantle melting in a short temporal range (from 5 to 10 model-Ma) after asthenospheric upwelling; the convective migmatitic dome is shifted horizontally with respect to the asthenospheric culmination and dome migmatites experience isobaric heating to peak conditions. In contrast, collisional-heat induced MiCCs exhibit pre-extensional migmatization and subsequent doming (at 3-4 model-Ma). In this case, the asthenosphere, generating lower crustal UHT conditions and mantle melts, upwells lately and is not symptomatic for MiCC formed in short extensional pulses (<12 model-Ma); convective core migmatites experience isothermal decompression during doming. The Eocene Rhodope domes exemplify asthenosphere-heat induced MiCCs. The Miocene Naxos dome is instead interpreted as a collisional-heat induced MiCC. (C) 2012 Elsevier B.V. All rights reserved.