Chapter 18: Constraints on the kinematics and timing of late Miocene-Recent extension between the Panamint and Black Mountains, southeastern California

Detailed mapping of extensional structures and synextensional volcanic rocks exposed in the eastern Panamint Mountains, southeastern California, place new constraints on the rates and geometry of late Miocene extension in the Death Valley area. At the eastern edge of the central Panamint Mountains, the Burro Trail and Amargosa "thrust" faults of Hunt and Mabey (1966) form subparallel roof and sole faults, respectively, of a kinematically related system of currently low-angle normal faults and subvertical strike-slip faults named here the Eastern Panamint fault system. The roof and sole faults initiated as 40 to 60°W-dipping normal faults in late Miocene time and were subsequently rotated to shallow dips by later, structurally lower, normal faults. Roughly 150 to 160 percent cumulative extension can be demonstrated for Eastern Panamint structures exposed in Trail Canyon. Late Miocene tuffs and andesites exposed in the eastern Panamint Mountains are correlative with the Sheephead Andesite and Rhodes Tuff in the southern Black Mountains. Reconstruction of these sequences indicates 25 to 55 km of post-9 Ma extension between the eastern Panamint and southern Black Mountains along an azimuth of N55° ± 3 °W. This extension was dominantly accommodated by movement on the Amargosa fault system and late Neogene faults exposed along the western front of the Black Mountains. These data constrain the extension rate between the Black and Panamint Mountains to be between 6.4 and 2.7 mm/yr over the past 9 m.y. Existing data on the direction and age of initiation and cessation of extension for faults in the Death Valley area show two periods of extension. The earlier period, from 15 to 10 Ma, appears to have widely varying extension directions; the later period, from 10 to 0 Ma, is characterized by consistently northwestern extension directions.