STRATIGRAPHIC AND GEOCHRONOLOGIC CONSTRAINTS ON TIME-SPACE PATTERNS OF LATEST QUATERNARY SURFACE RUPTURES ON THE EASTERN PINTO MOUNTAIN AND SOUTHERN MESQUITE LAKE FAULT ZONES NEAR TWENTYNINE PALMS, SOUTHERN CALIFORNIA

Geologic mapping and 18 luminescence ages tightly constrain time-space patterns of latest Quaternary surface ruptures along the E-trending sinistral Pinto Mountain Fault (PMF) and NW-trending dextral Mesquite Lake Fault (MLF) near their intersection in the Twentynine Palms area, southern CA. Our interpretations are based primarily on post-IR IRSL ages because of issues related to application of quartz OSL to deposits in the Mojave Desert region. Structural complexity of the PMF increases E-ward throughout its mapped extent, leading to complex surface-rupture patterns. Multiple late Pleistocene deposits having ages between ~15.01 and ~49.59 ka are highly deformed and laterally displaced ≥ 20 m on all strands of the PMF. Two sites reveal identical movement histories involving at least three mappable faults: (1) A basal alluvial unit shows evidence for at least 3 latest Pleistocene surface ruptures since ~12.31 ka. (2) Overlying upper and middle Holocene alluvial units bracket multiple surface ruptures between ~1.41 and ~5.14 ka, with the latest inferred as the most recent event (MRE). (3) The faulted alluvial succession is capped by an unfaulted unit (~1.27 ka). SE of these localities, a single PMF strand displaces three late Holocene alluvial-fan units (~1.40 to ~3.13 ka); these have bar-and-swale surface features that are offset sinistrally by 3-4 m. We attribute this offset to a single surface rupture correlative with the MRE at the two sites to the W. The dextral MLF traverses mixed eolian and alluvial deposits of Mesquite Lake playa and then projects southeastward to its eventual intersection with the PMF—a geometry we currently are investigating. Late Holocene eolian-dune sand (~1.27 ka) is sheared along the faulted margin of a sag depression on the playa. To the SE, several nested late Holocene alluvial-fan deposits (~1.97 ka and ~2.28 ka) are dextrally offset 3-4 m by what we interpret as a single rupture. Similarity of ages for faulted deposits at the two different locations suggests that the ruptures likely are related and define a MRE for the MLF between 1.27 and 2.28 ka. Significantly, the timing and slip amount for the inferred MLF MRE are comparable to what we infer for the PMF—suggesting temporally clustered to possibly coseismic MREs on both faults similar to paired ruptures of the 2019 Ridgecrest earthquakes.