Two‐Phase Exhumation of the Santa Rosa Mountains: Low‐ and High‐Angle Normal Faulting During Initiation and Evolution of the Southern San Andreas Fault System

• Published in: Tectonics (Washington, D.C.) Vol. 36; no. 12; pp. 2863 - 2881
• Main Authors: Mason, Cody C., Spotila, James A., Axen, Gary, Dorsey, Rebecca J., Luther, Amy, Stockli, Daniel F.
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-12-2017
• Subjects: Apatite; Basins; Boundaries; Crystals; Earth; Earth crust; Evolution; Exhumation; Fault lines; Fossils; Geologists; Helium; low‐temperature thermochronometry; Miocene; Modelling; Mountains; Plate boundaries; Plate motion; Pleistocene; Rotation; Salton Trough; Santa Rosa Mountains; southern San Andreas Fault system; Strike-slip faults; Uplift; West Salton Detachment Fault; San Andreas Fault; Santa Rosa Mountains; California; United States > US; North America
• ISSN: 0278-7407; 1944-9194
• DOI: 10.1002/2017TC004498

Low‐angle detachment fault systems are important elements of oblique‐divergent plate boundaries, yet the role detachment faulting plays in the development of such boundaries is poorly understood. The West Salton Detachment Fault (WSDF) is a major low‐angle normal fault that formed coeval with localization of the Pacific‐North America plate boundary in the northern Salton Trough, CA. Apatite U‐Th/He thermochronometry (AHe; n = 29 samples) and thermal history modeling of samples from the Santa Rosa Mountains (SRM) reveal that initial exhumation along the WSDF began at circa 8 Ma, exhuming footwall material from depths of >2 to 3 km. An uplifted fossil (Miocene) helium partial retention zone is present in the eastern SRM, while a deeper crustal section has been exhumed along the Pleistocene high‐angle Santa Rosa Fault (SFR) to much higher elevations in the southwest SRM. Detachment‐related vertical exhumation rates in the SRM were ~0.15–0.36 km/Myr, with maximum fault slip rates of ~1.2–3.0 km/Myr. Miocene AHe isochrons across the SRM are consistent with northeast crustal tilting of the SRM block and suggest that the post‐WSDF vertical exhumation rate along the SRF was ~1.3 km/Myr. The timing of extension initiation in the Salton Trough suggests that clockwise rotation of relative plate motions that began at 8 Ma is associated with initiation of the southern San Andreas system. Pleistocene regional tectonic reorganization was contemporaneous with an abrupt transition from low‐ to high‐angle faulting and indicates that local fault geometry may at times exert a fundamental control on rock uplift rates along strike‐slip fault systems.
Plain Language Summary
The southern San Andreas Fault system is an important component of the Pacific‐North America plate tectonic boundary, but the age of initiation of this significant geological feature in southern California is disputed. We investigated the uplift history of the Santa Rosa Mountains, west of the San Andreas Fault in Coachella Valley, California, by measuring helium in crystals called apatite. The technique known as thermochronometry lets us learn the cooling history and thus the uplift history of mountain ranges. Our work shows that the Southern San Andreas system began forming with extension of Earth's crust along a low‐angle normal fault 8 Ma ago. The timing of southern San Andreas formation was probably synchronous with the beginning of widespread strike‐slip faulting in the Baja Mexico area that would later become the Gulf of California. Much later, around 1 Ma ago, a new strike‐slip fault, called the San Jacinto Fault, deactivated and cut the low‐angle fault and uplifted the southwest Santa Rosa Mountains. This event was associated with punctuated, surprisingly high rates of uplift. These results help geologists to reconstruct the history of this plate boundary and to learn how mountains form along oblique‐extensional and strike‐slip fault systems.
Key Points
Exhumation of the Santa Rosa Mountains (SRM) initiated at circa 8 Ma via low‐angle extension along the West Salton Detachment Fault
Correction of Pleistocene to recent NE tilt of the SRM and thermal modeling improve precision of exhumation initiation timing
Local fault geometry, not plate motion obliquity, exerts primary control on rock uplift rates in the SRM since circa 1.2 Ma