AbstractsEarth & Environmental Science

Constraining the most recent surface rupture on the Garnet Hill strand, San Andreas Fault, Coachella Valley, California

by Jose Cardona

Institution: California State University – Northridge
Year: 2016
Keywords: Geology; Dissertations, Academic  – CSUN  – Geological Sciences.
Posted: 02/05/2017
Record ID: 2084861
Full text PDF: http://hdl.handle.net/10211.3/171784


The slip rate and rupture history of the Garnet Hill strand, San Andreas Fault, remain the least understood of the Holocene-active strands of the San Andreas Fault system in the northern Coachella Valley. Here I report new trenching data that constrain the timing of the most recent surface rupture on the Garnet Hill strand from the East Whitewater Hill Paleoseismic site located near the Hwy 62/I-10 interchange. The Garnet Hill strand is ~30 km-long extending northwest from Edom Hill to the Whitewater River area where it appears to merge with the Banning strand of the SAF. At the eastern end of the San Gorgonio Pass (SGP), the Garnet Hill strand consists of oblique dextral-slip segments with compressive left stepovers. Quaternary alluvium capped by a thick, reddish-orange soil is deformed at one such stepover to the east of Whitewater Canyon. The Garnet Hill strand is interpreted to occur here because a) its surface trace at Whitewater Canyon forms a ~10 m-high scarp in Holocene alluvium that projects east-southeastward to where higher sedimentation rates have buried the surface trace, and b) an emergent fault is required to construct a balanced cross section across the structure. The map trace of the Garnet Hill strand is not well located here due to burial by young sediment but it is mapped in Qfaults at the base of the hillslope as it is inferred to cause uplift of the hills. The trench excavated for this study revealed no definitive evidence of surface faulting. The trench exposed a charcoal-rich colluvial wedge-shaped deposit (Qcf) at its north end, with underlying coarse-grained cobble-gravel and overlying finer-grained fluvial material. The Qcf is light orangish-brown, matrix supported, with sub-rounded to sub-angular clasts and abundant ~1???2 cm-long pieces of detrital charcoal. The color suggests that the Qcf is derived from the steep, orangish-brown weathered gravels from the hillslope just north of the trench. I interpret that the Whitewater River sourced the underlying cobble gravel and smaller drainages north of the site provided the overlying finer fluvial material. The youngest charcoal ages from the base (A.D. 1404???1435) and top (A.D. 1474???1635) of the Qcf provide a maximum age for its formation of ~600 years ago. Given our inference that the fault lies beneath the trench, two possible hypotheses can explain the absence of observed faulting: (1) the colluvial deposit formed after an earthquake with a thrust tip that is undetectable (or blind) in the cobble gravels, or (2) the Whitewater River eroded and/or buried the near-surface expression of the fault. Both explanations provide a minimum age for the timing of the last earthquake at ~600 years ago. These results argue against the possibility that the ~A.D. 1690 event observed at sites on the Coachella section ruptured through the trench location. However, the data permit that the most recent rupture (~A.D.1400) documented to the northwest in San Gorgonio Pass (SGP) did rupture at this site and may be continuous with paleoearthquakes on the… Advisors/Committee Members: Yule, John D (advisor), Heermance, Richard V (committee member).