ABSTRACT: Lateral Movement on the Soledad Fault
PATCHICK, PAUL F., Patchick & Associates, Antioch, CA
The Soledad Fault occurs immediately north of the San Gabriel Mountains, Los Angeles County, California. The fault lies in the Transverse Ranges between Antelope Valley and the San Fernando Valley, site of the 1971 earthquake. This fault separates Oligocene(?) and middle Miocene deposits from Precambrian(?), anorthosite, diorites, amphibolite, and dikes comprising the San Gabriel basement complex.
A remarkable succession of 3000-ft-thick nonmarine Oligocene(?) rocks is exposed in the east-central Lang Quadrangle.
This herein-named Ague Dulce formation, of piedmont alluvial-fan environment, is characterized by the size of its completely unsorted clasts (from several millimeters to >10 ft), and their shapes (varying from very angular to subrounded). The dominant composition is anorthosite (75-95% pure) in a typical arkose matrix.
Examining matrix, fragment size (a statistical analysis of 1000 samples for nine sizes of clasts -- from west to east -- from six localities for 3 mi of outcrops), rounding, shape, sorting, stratification, and thickness factors led to the conclusion that this is predominantly a fanglomerate, with certain parts definitely conglomeratic.
Spatial distribution of anorthosite clasts reveals that, to explain the two cycles of sedimentation (the result of tectonism), faulting had to be the result of a postulated lateral movement, as well as vertical on the Soledad fault. The northern block must have slipped southwestward, relative to the southern block, for a distance of several miles at the very least.