Quaternary slip history of the Morongo Valley Fault, Morongo Valley, CA

The Morongo Valley Fault (MVF) is a northeast trending 18 km-long fault that is a splay from the overall sinistral Pinto Mountain Fault (PMF) system, located along Hwy 62 between Coachella Valley to the south and Yucca Valley to the north. Project results seek to constrain the role of the MVF with respect to the Coachella Valley SAF and Eastern California Shear Zone systems, and the hazard that rupture of this system poses to southern California residents. Mapping of the MVF reveals a series of en-echelon right step overs that resemble a sinistral extensional duplex. Uplifted terrace remnants of Quaternary alluvium occur to the SE of the fault, which dips northwest and is interpreted to be a left-oblique, normal fault system. A trench excavated across the MVF exposes strong evidence for one event at the bottom of the trench overlain by a colluvial wedge, and weak evidence for a second event at the top of the wedge. Calibrated radiocarbon ages from detrital charcoal constrain the first and possible second event to between 2,200-12,850 yrs BP. Three IRSL samples from the trench give much older ages (~20,000-116,000 yrs BP). The IRSL ages are not considered viable ages and probably carry an inheritance signal related to incomplete bleaching of feldspar grains. The trench site’s record of one, and possibly two, events since latest Pleistocene time suggests that the MVF ruptures infrequently relative to the Coachella Valley SAF system (average recurrence interval = ~220 yrs), and eastern PMF (at least four Holocene events). It is possible that the events on the MVF correlate with 1-2 events on eastern PMF. Alternatively, it is equally possible that the MVF breaks independently. The Eastern California Shear Zone (ECSZ) and 1992 Landers earthquake sequence may form the boundary between the eastern PMF, and MVF/western PMF. The paleoseismic record of the western PMF is unknown, but its geomorphic expression is similar to the MVF, and leads to speculation that the two faults possibly share a similar history. It seems likely, therefore that the MVF/western PMF act independently in the region. One explanation is that the SAF, PMF-MVF, and ECSZ form a triangle-shaped structural block at the southern Big Bend region of the SAF that acts as some form of crustal-scale ‘ball-bearing’ or pivot that accommodates slip transfer and gradients at the intersection of the SAF system and ECSZ. By itself, the MVF probably poses little seismic hazard to the region, but its location in the center of this complex region could mean that it breaks during or associated with rare events that involve multiple fault systems.