Shallow crustal fault rocks from the Black Mountain detachments, Death Valley, CA

The Black Mountain detachments denuded crystalline footwalls and extended sedimentary hanging walls from late Pliocene to Recent time. Fault rocks include gouges that crosscut breccias, and are in turn cut by compositionally and texturally distinct shear bands. Breccias have cataclastic textures, noteworthy for abundant transgranular fracture and power-law particle size distributions (D) of 2.77–2.79. Gouges have granular textures, noteworthy for grains with abraded boundaries surrounded by a clay-rich matrix and D = 2.86–3.31. Matrix minerals include phyllosilicates, clay minerals, and oxide aggregates that serve as crude strain indicators. Geochemical data indicate that there was abundant water within the fault zone, but that the water was not plumbed from deeper crustal sources. There are systematic geochemical variations between fault-rock samples, but the inferred mass changes were minor, <10–30%. It is proposed that the fault rocks developed during exhumation since the late Pliocene from ≥3 km to near-surface conditions. Exhumation coincided with the development of granular textures and strain localization. The protracted history of the fault rocks involved multiple deformation mechanisms and authigenic mineral assemblages that hypothetically influenced the frictional properties of the detachment shear zones.