Mechanisms for accommodation of Miocene extension: Low‐angle normal faulting, magmatism, and secondary breakaway faulting in the southern Sacramento Mountains, southeastern California

Abstract

The Colorado River extensional corridor (CREC) accommodated up to 100% crustal extension between ∼23 and 12 Ma. The southernmost Sacramento Mountains core complex lies within this region of extreme extension and exposes a footwall of Proterozoic, Mesozoic, and Miocene crystalline rocks as well as Miocene volcanic and sedimentary rocks in the hanging wall to the regionally developed Chemehuevi‐Sacramento detachment fault (CSDF) system. New structural, U‐Pb‐zircon, Ar‐Ar, and fission track geochronologic and paleomagnetic studies detail the episodic character of both magmatic and tectonic extension in this region. Extension in this part of the CREC was initiated with tectonic slip along a detachment fault system at a depth between 10 and 15 km. Magmatic extension at these crustal levels began at ∼20–19 Ma and directly account for 5–18 km of extension (10–20% of total extension) in the southern Sacramento Mountains. Three discrete magmatic episodes record rotation of the least principal stress direction, in the horizontal plane, from 55° to 15° over the following ∼3 Myr. The three intrusions bear brittle and semibrittle fabrics and show no crystal‐plastic fabric development. The final 3–4 Myr of stretching were dominated by amagmatic or tectonic extension along a detachment fault system, with extension directions rotating back toward 75°. The data are consistent with extremely rapid cooling and uplift of Miocene footwall rocks; the ∼19 Ma Sacram suite was emplaced at a mean pressure of ∼3.0 kbars and uplifted rapidly to a level in the crust where brittle deformation was manifested by movement on the detachment fault at ∼16 Ma. By ∼14 Ma the footwall was exposed at the surface, with detritus shed off and deposited in adjacent hanging wall basins.