Time-Space Patterns of Potentially Linked Latest Quaternary Surface Ruptures on the Eastern Pinto Mountain and Southern Mesquite Lake Faults: Implications for Paired Paleoearthquakes on Two Intersecting Conjugate Faults near Twentynine Palms, southern CA

Christopher M. Menges, Stephanie L. Dudash, & Shannon A. Mahan

Published September 11, 2022, SCEC Contribution #12510, 2022 SCEC Annual Meeting Poster #110

Detailed surficial geologic mapping and 21 fault-related luminescence (post-IRSL and OSL) dates provide constraints on latest Quaternary surface ruptures along the eastern Pinto Mountain Fault (PMF) and southern Mesquite Lake Fault (MLF) in the southern Mojave-Desert area of the Eastern California Shear Zone (ECSZ), southern California. The E-trending sinistral PMF and the NW-trending dextral MLF form two conjugate faults that intersect southeast of Twentynine Palms. Focused field- and image-based mapping indicate that the PMF has a composite structure increasing in complexity to the E and SE, including several bifurcating zones of clustered faults, <5 m- to >200 m in width. Each fault-rich domain comprises multiple subparallel to branching zones of individual shears and fractures, 2—8+ in number and centimeters to meters in width; this pattern suggests surface faulting is distributed across wide complex zones with many shear surfaces. Many individual shears and fractures have stratigraphic and geomorphic evidence for one or more specific surface and near-surface rupture events. The most recent event (MRE) on the PMF is evident at three sites on two branches along the south margin of the composite fault zone. This event is stratigraphically bracketed by a faulted late Holocene unit (~1.4 ka) and an overlying undisturbed late Holocene unit (~1.3 ka). Ten km to the SE, depositional features on three late Holocene alluvial fans having ages between ~1.4 and ~3.1 ka are sinistrally offset 3-4 m by a single MRE rupture. A probable correlative MRE lies 15 km to the W along the PMF near Copper Mountain, where faulted and unfaulted deposits have ages of 1.4-1.7 and 0. 9 ka ages, respectively. The NW-striking dextral MLF splays into several zones of surface rupture near the fault’s intersection with the PMF. Specific constraints for the MRE along the MLF include (a) sheared eolian-dune sand having a late Holocene age (~1.3 ka), (b) two late Holocene fans (~2.0 ka and ~2.3 ka) dextrally displaced 3-4 m by a single event, and (c) burial by an unfaulted alluvial fan with a ~1.0 ka age. Our observations indicate that complex MRE ruptures on adjoining intersecting sections of the PMF and MLF have very similar displacements (3-4 m events, ~30-40 km rupture lengths) and latest-Holocene (~1.3 ka) ages, suggesting tightly clustered to coeval ruptures on the fault pair comparable to multiple-fault ruptures observed during several recent historical earthquakes in the ECSZ.

Key Words
Surface Ruptures, Late Holocene activity, Conjugate-fault surface ruptures, Clustered or coeval paleoearthquakes

Citation
Menges, C. M., Dudash, S. L., & Mahan, S. A. (2022, 09). Time-Space Patterns of Potentially Linked Latest Quaternary Surface Ruptures on the Eastern Pinto Mountain and Southern Mesquite Lake Faults: Implications for Paired Paleoearthquakes on Two Intersecting Conjugate Faults near Twentynine Palms, southern CA. Poster Presentation at 2022 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology