Constrictional strain in a non-coaxial shear zone: implications for fold and rock fabric development, central mojave metamorphic core complex, california

Abstract

Finite strain and fold analyses of footwall mylonites in the central Mojave metamorphic core complex (CMMCC) reveal two distinct deformation paths that generated non-coaxial constrictional strain. The first path, recorded in the Waterman Hills, accomplishes constrictional strain through the formation of L-tectonites at the grain scale. The second path, recorded in the Mitchel Range, involves a combination of plane strain at the grain scale and Y-axis shortening through syn-mylonitic folding. The ductile shear zone in the Waterman Hills is approximately 500 m thick and is developed entirely within a Miocene syn-kinematic granodiorite. Strain magnitude decreases structurally downward. The mylonites contain abundant NE-directed non-coaxial microstructures yet are L-tectonites. R f/ø analysis of quartz-ribbon grain shapes indicate prolate strains ( K=6) and bulk chemical data suggest that mylonitization was isochemical or involved volume loss. Therefore, ductile shearing in the Waterman Hills involved true constrictional strain at the grain scale. Immediately to the southeast in the Mitchel Range, the lateral continuation of the shear zone is more than 1000 m thick and primarily involves lithologically heterogeneous pre-Tertiary basement. The mylonitic fabric comprises a well-developed foliation and NE-trending stretching lineation. R f/ø strain analysis was applied to grain shapes of cataclasized and strongly altered garnet porphyroclasts in a peraluminous granite. In contrast to the Waterman Hills, finite strains in these L- S-tectonites approximate plane strain ( K = 1.1). However, the mylonitic fabric is strongly folded about axes uniformly oriented subparallel to the stretching lineation. The coaxial folds range from open to isoclinal. We interpret the folds to have nucleated with axes parallel to the stretching lineation during mylonitization and to have accomplished Y-axis shortening in the shear zone. Macroscopic folds of the brittle detachment spatially correspond to open folds in the ductile shear zone, suggesting that Y-axis shortening continued after the fault zone had reached the brittle regime. Corrugations and folds oriented subparallel to the transport direction are characteristic features of fault zones in Cordilleran metamorphic core complexes. Therefore, constrictional strain may be common in core complexes. Strain compatibility with upper plate rocks in core complexes may be maintained by Y-axis shortening along conjugate strike slip faults. A nearly uniaxial stress field ( σ 3 < σ 2 ≈ σ 1) is consistent with most of the structures formed in Basin and Range extension and could generate the observed constrictional strain.