Rheological implications of heterogeneous deformation at multiple scales in the Late Cretaceous Sierra Nevada, California

Abstract

Late Cretaceous deformation in the east-central Sierra Nevada arc of California was heterogeneous at multiple scales. We quantify this heterogeneous deformation at centimeter, meter, and kilometer scales in the vicinity of the Gem Lake shear zone and infer variations in effective viscosity from our data. At the centimeter scale, variations in strain of different clast types in conglomerate suggest that effective viscosities varied by less than an order of magnitude. Lithology controlled the magnitude and nature of deformation recorded by clasts. At the meter scale, cleavage refraction between stratigraphic layers records variations in finite strain. Comparison of our observations with cleavage refraction models suggests a maximum effective viscosity contrast of ∼10 between layers. Bulk composition controls variations in deformation of the different layers. At the kilometer scale, variation in finite-strain magnitude and orientation in similar rock types both within and outside the shear zone demonstrates that deformation inside the zone was relatively intense. Comparing these results to numerical models of heterogeneous regional deformation, we estimate that the angle of oblique convergence inside the zone was ∼15 ± 10°, while outside it was greater than 60°. These kilometer-scale results imply regional deformation was moderately strike-slip partitioned during the Late Cretaceous and suggest regional effective viscosity varied by a factor between 6 and 17. At each scale of observation, the apparent range of effective viscosity varies by an order of magnitude or less. Consequently, we infer that relatively modest strength variations produced the structures observed at hand sample to tectonic scales.