FAULT EVOLUTION IN THE NORTHWEST LITTLE SAN BERNARDINO MOUNTAINS, SOUTHERN CALIFORNIA: A REFLECTION OF TECTONIC LINKAGE BETWEEN THE SAN ANDREAS FAULT AND THE EASTERN CALIFORNIA SHEAR ZONE

• Main Author: Hislop, Ann
• Format: Others
• Published: UKnowledge 2019
• Subjects: Little San Bernardino Mountains Fault Set; San Andreas Fault; Eastern California Shear Zone; Dillon Shear Zone Fault Set; tectonic linkage; Geology; Tectonics and Structure
• Online Access: https://uknowledge.uky.edu/ees_etds/63; https://uknowledge.uky.edu/cgi/viewcontent.cgi?article=1071&context=ees_etds

The Little San Bernardino Mountains (LSBM) Fault Set are N-S dextral faults, east of the restraining bend of the San Andreas Fault (SAF) in southern California, that may form a tectonic linkage between the SAF and the Eastern California Shear Zone. The NW LSBM are a complexly deformed structural domain characterized by the young N-S dextral faults and older NW-oriented Dillon Shear Zone faults. Before the 1992 Joshua Tree (Mw 6.1) and Landers (Mw 7.3) earthquakes, the rugged NW LSBM was the subject of few geologic studies. This bedrock mapping study has further delineated the geometry, distribution, and relative chronology of brittle structures. A 2015 NCALM award of 51 km2 of lidar imagery on Eureka Peak Fault was used to correct fault locations. Bedrock mapping in the epicentral areas of the 1992 Joshua Tree earthquake on Eureka Peak Fault and Landers aftershocks (Mw 5.7, 5.8) focused on the brittle structures of the evolving fault systems and potential connections with historic seismicity. The N-S dextral fault offsets from west to east are; Long Canyon (470 m), Wide Canyon (~150- 340 m), Eureka Peak (~ 225 m), California Riding Trail (850-965 m) and Deerhorn (105 m) faults with a cumulative offset of approximately 2 km. Dolomitic marble, clinopyroxene-hornblende skarn, garnet-epidote skarn and gabbro-diorite intruded by monzogranite are key lithologies used in determining offsets. Joshua Tree Fault, defined by seismicity by Kaven and Pollard (2013) is supported by additional mapped fault data. A “new” fault (Black Rock Canyon) links Wide Canyon and northern Eureka Peak faults. The distribution of aftershock seismicity plotted by depth and latitude along the N-S faults, a prominent broad seismicity trend and bedrock mapping are all consistent with interpreting the N-S faults as an incipient set of faults developing upward from a deeper through-going crustal shear zone. The seismicity since the onset of the Joshua Tree- Landers earthquake sequence on April 23, 1992, forms two distinct trends. Temporally these two trends occurred in sequence; first a N-propagating trend April 23- mid-June along Joshua Tree Fault from the Joshua Tree earthquake epicenter to north of the Pinto Mountain Fault, and secondly a prominent SE trend of Landers aftershocks (including Mw 5.7, 5.8) June 28 onwards, from the Landers earthquake epicenter, along Eureka Peak Fault to the SAF. AFT and (U-Th)/He thermochronology indicate an abrupt boundary on Long Canyon Fault between rapid uplift within ~ 12 km of the SAF and slower uplift more than 12 km north. This boundary is projected along the Dillon Shear Zone structural grain to the 1992 Joshua Tree earthquake epicenter on southern Eureka Peak Fault, dividing the N-striking faults into northern and southern domains. The 14.7 km hypocentral depth of the Joshua Tree earthquake coincides roughly with the depth of the NE dipping SAF intersection with Eureka Peak Fault, forming a hypothesized flower structure which is consistent with rapid uplift of the LSBM escarpment near the SAF. The LSBM Fault Set may be initiated by the upward migration of a through-going mid-crustal break and eastern migration of the current SAF trace bypassing the Big Bend slip impediment. Eureka Peak Fault with a slip rate of 10-20 mm/yr, is the proposed structure tectonically linking the SAF and the Eastern California Shear Zone.