The nonmarine upper Miocene Mint Canyon Formation crops out in a broad southwest-plunging syncline within the Soledad basin, about 30 mi (48 km) north of Los Angeles, California, between the San Gabriel and San Andreas faults. The formation is comprised of fluvial and lacustrine deposits.

Clast counts and paleocurrent directions indicate that the fluvial parts of the Mint Canyon Formation were deposited in a broad westward-draining trough. The distribution of local basement-rock source areas indicates that the alluvial wash crossed the San Andreas fault in the general vicinity of Soledad Pass, near Palmdale. Clasts in the central part of the trough are predominantly of volcanic origin, and most are foreign to the area and have no known local source. They must have been derived from east of the San Andreas fault. Among the wide variety of volcanic-clast types within the Mint Canyon Formation

End_Page 1687------------------------------

is an unusual rapakivi-textured quartz-latite porphyry. A tertiary volcanic field, located about 169 mi (240 km) southeast of the Mint Canyon Formation in the northern Chocolate Mountains east of the San Andreas fault, contains the same variety of volcanic rock types as those that occur as clasts in the Mint Canyon Formation, including the unusual rapakivi-textured porphyry. Chemical analysis and isotope ratios of volcanic clasts from the Mint Canyon Formation and rocks from the volcanic field show them to be strikingly similar.

These data indicate that the Mint Canyon Formation is offset from the volcanic source by about 169 mi (240 km) of right slip along the San Andreas fault.

End_of_Article - Last_Page 1688------------