1 Using Basin Analysis and Geochemistry to Reconstruct the San Andreas Fault System: A Symposium in Honor of John Crowell, Tor Nilsen, Tom Dibblee, and Perry Ehlig

Thursday, 27 May 2010
8:40 AM - 12:00 PM
Platinum 2

RECONSTRUCTING THE DILIGENCIA AND VASQUEZ BASINS ACROSS THE SAN ANDREAS FAULT (SOUTHERN CALIFORNIA)

Paper number: 1-3
Program: Symposia Sessions
Time: 9:40 AM – 10:10 AM

Authors

Abstract

Recent work by a UCLA team confirms that a NW-SE-trending normal fault (the Diligencia fault) bounds the northeast side of the Diligencia basin (present orientations): 1. Stratal thicknesses increase toward the northeast. 2. Composition, grain size and paleocurrents of basal Diligencia conglomerate are consistent with derivation from proximal granite, which outcrops to the northeast of the basin. 3. Basal Diligencia conglomerate lies in angular unconformity on Eocene Maniobra Formation along the north side of the basin, without faults. 4. To the NW, the Diligencia fault aligns with a monocline expressed by the Maniobra nonconformity on Mesozoic granite. 5. Geophysical surveys across the Quaternary alluvium along the northeast side of the basin indicate a fault between exposed granite and covered sedimentary or volcanic strata.

When the Diligencia basin and Diligencia fault are unrotated 90° counterclockwise and the Soledad basin is unrotated 45° counterclockwise (as indicated by paleomagnetic data), and 240 km of dextral slip across the San Andreas and closely related faults are removed, the following elements align (NW to SE): 1. Mesozoic granitoids, 2. monoclinal flexure of Maniobra - San Francisquito nonconformity on granitoids, 3. Maniobra-Diligencia and San Francisquito - Vasquez unconformity, 4. half grabens of Diligencia and Charlie Canyon sub-basins (although with opposite polarities), 5. Orocopia detachment - Clemens Well - San Francisquito faults, and 6. Orocopia-Pelona Schist antiforms.

The Diligencia basin probably did not connect directly with any of the Soledad sub-basins (Charlie Canyon, Texas Canyon and Vasquez Rocks), but it formed in a structural setting similar to that of Charlie Canyon sub-basin. This reconstruction refines, but does not refute the reconstructions of Bohannon (1975) and Crowell (1975).