A Brittle-Ductile Transition Preserved in the Sierra Crest Shear Zone, Sierra Nevada, CA: a Natural Laboratory for Examining Rheologically-Controlled Brittle and Ductile Deformation Partitioning

Abstract:

The Sierra Crest shear zone (SCSZ), an ~300 km long Late Cretaceous dextral transpressive ductile-brittle shear system in the eastern central Sierra Nevada, CA partitioned tectonic boundary conditions during a fundamental rheological transition in the upper crust from ductile to brittle deformation due to the exhumation and cooling of the arc. The SCSZ represents a well-exposed and data-rich ‘natural laboratory’ to study the mechanisms driving evolving strain partitioning and rheology.

The SCSZ transitioned from a broad swath of partitioned ductile shear zones, comprised of anastomosing simple shear dominated zones separated by pure shear dominated domains, to a complex partitioned brittle fault system, expressed as brittle slip, veining, brecciation, and pseudotachylyte formation along discrete structures, as arc magmatism shut down, the arc cooled, and exhumation rates increased. Previous studies have documented evolving deformation partitioning in the ductile system indicated by variable fabric development ranging from preserved bedding to mylonites, the spread of lineation orientations, and variable kinematics. Multi-generational brittle fabrics that are variably ductilely deformed and the orientation of 1^st and 2^nd order brittle structures, both concordant and discordant with ductile shears, indicate that partitioning also evolved during the complex rheological transition.

Structural, strain, P-T-t, geochronologic, and field data provide controls on parameters (e.g. lithology, fluids, strain, preexisting structure, timing, P-T conditions) needed to model the development of the SCSZ in anisotropic crust undergoing a transition in rheology and tectonic boundary conditions. As rheological heterogeneity will lead to deformation partitioning throughout intervening scales, it is unrealistic to apply single scale models to this investigation. Thus, we propose to compare the above observations to predictions made by a model (MOPLA; Jiang, 2014) of progressive deformation that incorporates rheology-controlled partitioning.