Observations from preliminary 1:24,000 scale bedrock mapping in the western Joshua Tree region: Potential connections between historic seismicity and evolving fault systems

Ann Hislop, Robert E. Powell, Sean P. Bemis, David P. Moecher, & Luke C. Sabala

Submitted August 26, 2016, SCEC Contribution #6963, 2016 SCEC Annual Meeting Poster #102

Bedrock mapping in the epicentral areas of the 1992 Joshua Tree earthquake (Mw 6.1) and Landers aftershocks (Mw 5.7, 5.8) south of Pinto Mountain Fault in southern California has further delineated the geometry, distribution, and relative chronology of brittle structures in the northern Little San Bernardino Mts. Of particular interest are (1) the interactions of the Dillon Shear Zone set of NW-trending faults with three younger N-striking dextral faults named from west to east; Long Canyon, Wide Canyon and Eureka Peak; and (2) the kinematic roles of the Dillon Shear Zone and the N-S faults in relation to the San Andreas Fault. Analysis of petrofabrics and microstructures at hand sample and thin section scale in oriented samples, and comparison to map scale structures for elucidation of kinematics and the collection and analysis of remotely sensed/aerial data was used to determine orientation of outcrop to map scale fractures and joints, and their potential relationship to the N-striking faults ("proto-faults").

Detailed bedrock mapping indicates dextral offsets of 500m on Long Canyon Fault, ~1300m on Eureka Peak Fault SW branch and 180 m on the SE branch all using calc-silicate and marble beds. A new fault (Black Rock Canyon) links Wide Canyon and Eureka Peak Faults and has ~30 m of dextral offset. Two left restraining steps are identified; one between Burnt Mountain and Wide Canyon Faults occupied by Warren Peak and the other at the orientation change between northern and southern Eureka Peak Fault occupied by Eureka Peak. Fracture study sites represent damage zones characterized by increasing concentration of vertical crack sets with mm to centimeter spacing without offsets and unconsolidated breccias.

In a traverse along the Long Canyon Fault, AFT and (U-Th)/He thermochronologic data (Sabala, 2010) indicate an abrupt boundary between rapid uplift within ~ 12 km of the San Andreas Fault (SAF) and slower uplift to the north. When the boundary on this traverse is extended southeastward along the Dillon Shear Zone structural grain it intersects the epicenter of the 1992 Joshua Tree earthquake on the southern Eureka Peak Fault and divides the three N-striking faults into north and south segments. A flower structure is hypothesized along the line described above that links to the SAF at depth and could explain the rapid uplift or pop-up section of the Little San Bernardino Mountain Escarpment closer to the SAF.

The 14 km epicenteral depth of the Joshua Tree earthquake on Eureka Peak Fault coincides roughly with the 17 km projected depth of the NE dipping SAF, supporting the hypothesis that these N-striking faults are a reflection of tectonic linkage between the SAF and the Eastern California Shear Zone. The N-striking faults may be upper crustal brittle fault initiation caused by the upward migration of a through-going break in the middle crust and an eastern migration from the current central California segment of the SAF, partially bypassing the Big Bend slip impediment.

Key Words
N-striking faults, SAF, tectonic linkage, ECSZ

Hislop, A., Powell, R. E., Bemis, S. P., Moecher, D. P., & Sabala, L. C. (2016, 08). Observations from preliminary 1:24,000 scale bedrock mapping in the western Joshua Tree region: Potential connections between historic seismicity and evolving fault systems. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology