Geometric, kinematic, and temporal patterns of Quaternary surface rupture on the Eastern Pinto Mountain fault zone near Twentynine Palms, southern California

Christopher M. Menges, Jonathan C. Matti, & Stephanie L. Dudash

Submitted August 15, 2016, SCEC Contribution #7020, 2016 SCEC Annual Meeting Poster #108

Detailed geologic mapping along the eastern Pinto Mountain fault zone (PMfz) in the Twentynine Palms area reveals previously unrecognized time-space variations in internal geometry, kinematics, and Quaternary surface-rupture timing; these variations appear to reflect fault-zone position relative to intersections with transversely-oriented faults at both ends of the mapped zone. The PMfz is an approximately E-W-trending, 80-km-long left lateral fault that forms the structural boundary between domains of transverse NW-trending right-lateral faults in the central Mojave Desert to the north and subparallel E-W-trending left-lateral faults in the eastern Transverse Ranges to the south. Existing data suggest that most mid- to late Quaternary activity is concentrated on sections of the PMfz west of the Twentynine Palms area. However, our mapping along a 10-km section near Twentynine Palms indicates that the overall zone of faulting increases in complexity and width to the east in conjunction with deflection of the average fault zone orientation from an E- to a more SE-direction. Eastward across this 10-km section, the zone of surface ruptures increases in width from < 150 m at the W-end to a >1.5 km near the intersection with the Mesquite Lake transverse fault on the eastern edge. This width increase occurs via an series of E-directed branches and (or) right steps in the composite fault zone that produce a new set of ESE-oriented strands which diverge from the more E- to NE-continuations of the strands entering the bifurcations from the west. Geometrically, the composite fault zone consists of multiple internal zones of clustered faulting, <5 m- to >200 m in width and increasing from one to 4-5 in number eastward across the map area, which are separated by relatively undeformed intervening blocks. Each fault-rich domain comprises a complex array of subparallel to branching discrete multiple component zones of shears and fractures, 2- 8+ in number and centimeters to meters in width, that collectively suggest wide zones of distributed surface rupture. The ages of most recent ruptures--constrained by fault-zone stratigraphy and scarps—vary across components of the composite fault zone. Our youngest age estimates include: (1) late Pleistocene rupture ages on the two main northern zones; (2) multiple (minimum 2-3) Holocene-age ruptures, commonly as young as late Holocene, on the two southern zones. Many internal fault strands in the PMfz display geomorphic and structural features (vertical scarps, fault-bounded ridges and popup structures, oblique slickenlines, net displacement estimates, and folded or tilted strata) indicative of secondary transpression, commonly localized at SE-trending right-shifted constraining bends, branches, or step-overs on internal fault zones; this deformation increases eastward in intensity and coverage as the zones approach transverse-fault intersections.

Key Words
Quaternary faulting, surface ruptures, Pinto Mountain fault

Menges, C. M., Matti, J. C., & Dudash, S. L. (2016, 08). Geometric, kinematic, and temporal patterns of Quaternary surface rupture on the Eastern Pinto Mountain fault zone near Twentynine Palms, southern California. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology