Tectonic Geomorphology and Possible Paleoseismic Trench Sites along the Pleito Thrust Fault near Grapevine, CA

Yule, Doug¹, Matthew Birney¹, Shelby Dawson² (1) California State University Northridge, Northridge, CA (2) California State University Northridge, Northridge, Albania

The Pleito thrust is a south-dipping reverse fault located at the southern margin of the San Joaquin Basin, California. It forms part of the fault and fold belt system which generated the 1952 (Mw 7.5) Kern County earthquake. The most recent surface rupture was found to have occurred 345-1465 years ago (Hall,1984) with an recurrence interval estimated by Keller et al. (1998) of 300-500 years.

This study has identified three possible trench sites having a probability to yield evidence for multiple ruptures, therefore better constraining the timing of recent deformation and determining the slip rate across the fault. The Salt Creek site has a variable scarp height ranging from 2.5 m at the mouth of a gully where sedimentation is highest to a scarp of 7 m located in the depositional ‘shadow' between two alluvial fans. At the Tecuya Creek site, incised gullies have eroded the scarp to a height of 2-3 m while the maximum scarp of 6 m occurs away from active alluviation. The third site, ‘Fan on Fan', has a maximum fault scarp of 7 m (with well developed soil horizon) but decreases to ~1.5 m at the mouth of a gully, with active alluviation and poor soil development. Variations in scarp height reflect the rate of sedimentation across the scarp; higher rates of sedimentation allow for partial burying of the scarp. This therefore argues that the 7 m scarp represents multiple events and trenching across the partially buried smaller scarp will reveal a multiple tectonic event.