MYLONITE MÉLANGE IN AN EXTENSIONAL SHEAR-ZONE: A DESCRIPTIVE HISTORY AND STRAIN ANALYSIS

The upper-most section of the footwall of the Badwater Turtleback, an extensional shear-zone situated in the Black Mountains of Death Valley, California, features a mylonite mélange in the much wider extensional shear-zone. This mélange hosts a variety of lithologies affected by both brittle and ductile deformation probably during late-stage, localized, high strain. Our observations come from both micro- and meso-scale work, with the latter being supplemented by detailed mapping made possible with photogrammetry.

We identify this zone as a mélange because of the heterogeneity of both material and structures present at the mesoscopic scale. The mélange is primarily a matrix of fine grained calcite marble with grain diameters of 10 micrometers or less. The mélange is strongly foliated, and contains fragments of mylonitic gneiss, pegmatite, dolomite marble and rare intrusive, mafic material. The longest dimension of fragments is typically sub-parallel with foliation. Fractures in the fragments, particularly in the pegmatite, are sub-perpendicular to the surrounding foliation and terminate at the mylonite-fragment contact. These relations suggest contemporaneity between brittle and ductile deformation, likely occurring as the shear-zone passed through the brittle-ductile transition. The high-strain environment is inferred from the very fine-grained matrix of calcite. Our interpretation of strain localizing within the mylonite is advanced by the observation of a sharp contact structurally below the mélange zone with coarse-grained, crystalline, calcite marble in contrast to the highly deformed mélange. The coarse-grained marbles show little-to-no foliation or preferred orientation and have grain diameters that range from 100 to 400 micrometers. In thin section, the coarse-grained marble features extensive mechanical-twinning less than 1 micrometer thick, supporting a low-strain, low-temperature environment interpretation.