Investigating the effect of a weak lower crust on Basin and Range extension history.

Abstract:

The formation of the Basin and Range (B&R), a physiographic region that spans much of the western United States, had significant impacts on faunal, floral, and climatic evolution. The western United States has a complex deformational history, with both thin-skinned and thick-skinned deformation being overprinted by extensional and glacial processes. Previous authors have estimated crustal thickness, indicating a thick crustal root under the paleotopography of the B&R (Bahadori et al., 2018). Because the modern Mohorovicic Discontinuity (Moho) is flat below regions of metamorphic core complexes (MCCs), it suggests that the paleo-root was flattened as lower crustal rock was exhumed (Bahadori et al., 2018). This indicates a weak lower crust facilitating the broadening of paleo-roots and paleotopography prior to- and during extension in the western United States (Bahadori et al., 2018). Current research estimates that some regions of the B&R have undergone up to 200% East-West extension (Wernicke et al., 1992). Extension of these magnitudes produces regions of exposed MCCs in the B&R. To understand the relationships between extensional mechanisms and MCC migration, we performed instantaneous and time-dependent 3-D geodynamic models with variable lower crustal viscosity and crust mantle strength partitioning to investigate the lower crustal flow during extensional processes where gravitational collapse is the primary driving force. Models predict upper crust/lower crust differential motion and rates consistent with the observed MCC vergence and timing, producing deformation of the Moho that leads to flattening.