QUANTIFYING OFFSET ON THE QUATERNARY PAINTED GORGE FAULT, SOUTHERN CALIFORNIA

The Painted Gorge fault is a NW-striking dextral fault located in the Carrizo Badlands northeast of the Coyote Mountains of southern California. Its trace is at least 11 km long and it lies between the NW-striking Elsinore and San Jacinto faults, two active segments of the San Andreas plate boundary. The Painted Gorge fault forms 1 m SW-facing scarps where it cuts across Quaternary alluvial fans, suggesting that it may pose a seismic hazard.

Detailed geologic mapping near the southeast end of the fault trace reveals two hornblende-phyric andesite fields within the lower Miocene Alverson Formation. These fields are located on opposite sides of the fault. The andesites in these fields are mineralogically and petrographically indistinguishable, with similar phenocryst assemblages; yet they differ markedly from other local lithologies within the Alverson Formation, which consist primarily of basalt. The two andesite fields are semicircular with diameters of about 1.3 km. The fault bounds the straight sides of these semicircular fields, which are dextrally offset ~1 km across the fault. Thus, these two andesite fields appear to represent what was once a single circular 1.3 km diameter volcanic field that was later cut and offset ~1 km by the Painted Gorge fault.

To test this hypothesis, 12 samples were collected throughout the two volcanic fields for geochemical analysis. The results indicate that the rocks in the two fields are all calc-alkaline andesites with closely overlapping LIL and HFS trace element abundances, indicating a similar mantle source and magma evolution. Therefore, it is probable that the andesite fields on the two sides of the fault were emplaced during the same eruption and thus represent a piercing point that records ~1 km of dextral slip on the Painted Gorge fault – the same order of magnitude as that of the nearby Elsinore fault (0.6 - 5.2 km).