Rheological inheritance: lessons from the Death Valley region, US Basin and Range Province

Abstract

AbstractRheological inheritance occurs when older metamorphic and deformational fabrics impact the mechanics of younger tectonic provinces, such as occurs in extensional provinces developed on sites of previous orogenesis. The Funeral and Black Mountains from the Death Valley region of the US Basin and Range provide the opportunity to study such rheological inheritance. The Funeral Mountains expose shear zones containing high-grade metamorphic fabrics and evidence for synkinematic, decompression-driven melt of Late Cretaceous, orogenic origin. Quartz <c>- and [a]-axes patterns from the shear zones correlate with high-temperature slip systems. The quartz microstructures were formed via grain-boundary migration, and these are overprinted by high-strain layers of mixed-phase aggregates that underwent grain boundary sliding. Reaction textures from the Funeral Mountains illustrate that much of the fabric development post-dates melting, but locally involved melt–rock reactions. In contrast with the Funeral Mountains, the basement complex in the Black Mountains preserves few peak-metamorphic textures, largely owing to the overprinting by Cenozoic magmatism and deformation. However, local relicts of high-grade deformational fabrics yielding Late Cretaceous-through-Eocene magmatic zircon ages are overprinted by greenschist grade fabrics. Using outcrop and microstructural (including electron backscatter diffraction) observations, and thermodynamic modelling, we detail how segregation of melt products during orogenic partial melting resulted in chemically isolated compositional domains, favouring localization via the formation of fine-grained retrograde fabrics. We propose a conceptual model that builds on our results wherein the heterogeneous distribution of peak, orogenic metamorphic phases and melt products governs lower crustal strength and fabric evolution during extension. The Wilson Cycle may be sensitive to rheological inheritance as the width of continental margins formed during rifting will be sensitive to the fabrics and compositions formed during collision.