TECTONIC EVOLUTION OF THE MORONGO VALLEY BLOCK IN SOUTHERN CALIFORNIA—IMPLICATIONS FOR LATE QUATERNARY INTERACTION OF THE WESTERN PINTO MOUNTAIN FAULT ZONE WITH THE SAN ANDREAS FAULT ZONE

The triangular Morongo Valley block (MVb) is bounded by the Big Morongo and Morongo Valley strands (BMs and MVs respectively) of the western sinistral Pinto Mountain fault zone (PMfz) and the Mission Creek strand (MiCs) of the dextral San Andreas fault zone (SAfz) at a 20-km-wide sinistral step within the San Gorgonio structural knot. We propose a four-stage model for the tectonic evolution of the MVb, that is consistent with Kendrick et al. (2015, 2022) and Matti et al. (2023). Our model implicitly links the MVb to interaction between the SAfz and western PMfz, following 1.5 myr of alternating dextral and sinistral faulting and punctual strand switching and interspersed with transpressional faulting (Kendrick et al., 2015, 2022; Matti et al., 2023).

Stages 1–2: By 1.5 Ma the MVs and the MiCs were the active strands of the western PMfz and SAfz respectively. The MVs was propagating southwestward towards the MiCs, gradually bending it NW and ultimately displacing it ~15 km. Stage 3: at ~375 ka the Mill Creek strand (MCs) replaced the MiCs as the active strand of the SAfz. Dextral shear along the MCs then deflected the MVs ~20° to its present orientation, displacing it ~12 km and resulting in the transtensional opening of Morongo Valley. Stage 4: At ~125 ka, BMs activates and propagates towards MCs, which bends the western part ~20° SE. BMs offsets MCs and adjacent stream drainages 1–1.25 km in and merged with the MVS. IRSL dating by Kendrick et al. (2015) implies a slip rate of 10 to 12.5 mm/yr on the BMs for the past 125 ka. Gabriel (2017) interprets ¹⁰Be cosmogenic dates of granitic boulders to suggest the eastern BMs was active by 89 ka with slip rate slowing to ~3 mm/yr.