131-3 Crustal Overturn During Extension Drives Exhumation of Deep Orogenic Crust”.

Session: Going with the Shear - New Insights into Lithospheric Extensional and Strike-Slip Systems

Presenting Author:

Authors:

Abstract:

Metamorphic core complexes can expose deformed rocks sourced from the middle to lower crust which represent the final product of complex pressure-temperature-time-deformation paths. Continental crust can be exhumed from near-Moho depths (>50 km) to near-surface levels (<15-5 km) in <10 million years, and the history recorded by these deeply sourced rocks is a critical facet of both the formation mechanisms of metamorphic core complexes and the vertical and lateral mass transport that occurs within extensional settings.

We present 2D numerical experiments where rheological properties of the lithosphere are systematically varied, and these experiments predict that orogenic crust that was deepest at the start of extension can be exhumed to the shallowest levels, resulting in a partial overturn of the crust. Experiments also predict the magnitude of extension-driven lateral flow of deep crust prior to exhumation: parcels of low viscosity deep crust initially separated by up to 200 km flow towards a central, steep exhumation channel and are then exhumed to shallow levels (in some cases to <5 km), where they experience rapid cooling. If the deep crust is an order of magnitude more viscous than in lower-viscosity models, it is solicited from a smaller range of initial locations (<50 km lateral separation), and crust exhumed to similar shallow levels is derived from a greater range of pre-extension depths. In the case of low-viscosity deep crust, most exhumed rocks experience isothermal decompression or decompression + heating depending on their starting position at the time of extension. Most exhibit a large difference (up to ~0.7 GPa) between the maximum lithostatic pressure and the pressure at the thermal maximum, consistent with observations from nature that high-grade metamorphic rocks exhumed under extension typically record mid-crustal conditions even if they formerly resided in the deep crust. This is particularly the case for quartzofeldspathic rocks, which are easily overprinted; more refractory lithologies may preserve some relics of their former deep crustal location. These models provide insight into the deep source of high-grade rocks exhumed by extension to shallow crustal levels, such as in some metamorphic core complexes.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-11120

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