Mid‐Tertiary structural evolution of the Old Woman Mountains Area: Implications for crustal extension across southeastern California

Abstract

Investigations in the Old Woman Mountains area provide insights into mid‐Tertiary extensional tectonism in southeastern California. The Old Woman Mountains area is a moderately extended region that lies between two highly extended terranes, the Central Mojave Extensional Complex and the Colorado River Extensional Corridor. Two normal faults and a high‐angle fault that we interpret to be a transfer structure divide the area into four distinct structural blocks. These blocks correspond to the four mountain ranges of the area: the Old Woman, Piute, Little Piute, and Ship mountains. Other major faults involved in tilting are inferred to be buried beneath alluvium in surrounding valleys. Deformation occurred in part before, but primarily after, deposition of the Peach Springs Tuff, an 18.5 Ma regional stratigraphic marker. Transport directions of hanging walls are westward on the west side of the Old Woman Mountains area and east‐southeastward on the east side. We suggest that the Old Woman Mountains area is a less extended portion of a continuous extensional terrane that stretches from the central Mojave Desert across the Colorado River to the transition zone of the Colorado Plateau. Lower strain in the Old Woman Mountains area than in most of the Colorado River Extensional Corridor and Central Mojave Extensional Complex and age relations across the terrane can be explained by a model in which the principal locus of upper crustal breakaway to an east dipping low‐angle simple shear zone shifted at about 18–20 Ma from a position far to the west to just east of the Old Woman Mountains area.