THE CONTROL OF OROGENIC INHERITANCE ON CENOZOIC BASIN AND RANGE EXTENSION

Inheritance control on extension is often taken as the result of reactivation of weak structures and inheritance of hot geotherms. We explored inheritance in the field and via numerical modeling. In the field, the Funeral and Black Mountains bordering the Death Valley region expose shear zones recording an overall retrograde path with relicts of high-grade fabrics and evidence for synkinematic, decompression-driven melt of Late Cretaceous-through-Paleogene, orogenic origin. The highest-grade textures record melt-fluid-rock reactions that produced zircons of Late Cretaceous through Paleogene age. Corresponding quartz c-axes patterns were formed via grain-boundary migration, and overprint by retrograde high-strain zones that underwent grain boundary sliding. These heterogeneous (migmatitic) fabrics then nucleated later Cenozoic extension expressed by lower-temperature mylonitic-to-cataclastic shear zones. These geologic relationships stem from a “rheological inheritance” control wherein the effect of orogenic metamorphic and deformational fabrics affects the mechanics of extension. We use thermomechanical modeling to explore the rheological inheritance; we consider midcrustal compositional heterogeneities and do not assume weakening effects to promote localization. In summary, the widespread superposition of deformational and metamorphic fabrics in shear zones found in extensional domains lead to strain instabilities that nucleate localization, facilitating wide extension as typified by the Basin and Range province.