TECTONIC INHERITANCE IN CORDILLERAN METAMORPHIC CORE COMPLEXES

The Cordilleran metamorphic core complexes of the western United States are largely considered to be products of large magnitude extension in the Tertiary and have served prominently in the development of geometric and kinematic models for how continental crust extends. Many models for continental extension, and the evolution of presently low-angle normal faults in metamorphic core complexes, incorporate strain localization due to changes in rock strength with depth resulting from temperature-dependent variations in deformation mechanisms, reaction softening, and/or the presence of partial melt. However, despite the common superposition of extension on sites of prior crustal thickening, the influence of crustal-scale rheological stratification resulting from pre-existing structure and lithologic architecture is under appreciated. Both lithologically-controlled rheological stratification and anisotropies resulting from prior deformation have influenced the resulting geometries of many extensional detachment fault systems, thus reducing the mechanical paradox of low-angle normal faults and lessening the need for explanations such as large fault rotations, stress rotations, and elevated pore fluid pressure for detachment faults. In the western US, detachment faults in the core complexes of the northern and central Basin and Range, which developed within thick passive margin strata, commonly show stratigraphic separations less than structural depth separations, demonstrating the influence of pre-existing tilt; in the southern Basin and Range, Tertiary core complex shear zones and detachment faults, which developed in crystalline rocks, commonly reactivate older anisotropic ductile fabrics. These points will be addressed by reviewing the geology of several metamorphic core complexes from the northern, central, and southern Basin and Range. For example, in the Funeral Mountains metamorphic core complex, the Miocene Boundary Canyon detachment fault reactivates a Late Cretaceous mylonitic extensional shear zone and utilizes the pre-extensional dip of rheologically-layered strata resulting from Jurassic thrust burial.