DILIGENCIA BASIN (SOUTHERN CALIFORNIA) REVISITED: SEDIMENTATION IN HALF GRABEN BOUNDED ON THE NORTHEAST BY NORMAL FAULT

Previous workers have concluded that sedimentation and basaltic magmatism that produced the Lower Miocene Diligencia Formation occurred within an extensional setting. Some have proposed east-west-trending normal faults for either the northern or southern boundaries of the half graben, although evidence for either interpretation is lacking. These interpretations have been influenced by the fact that present exposures of the Diligencia Formation are found in an east-west-trending syncline resulting from younger north-south compression related to transpression along the San Andreas fault system.

Recent work by a UCLA team confirms the alternative interpretation that a NW-SE-trending normal fault (herein named the Diligencia fault) bounds the northeast side of the basin (present orientations): 1. Stratal thicknesses increase toward the northeast. 2. Basal Diligencia conglomerate (previously interpreted by some as Quaternary) has grain-size and paleocurrent trends 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; the lack of Maniobra detritus in basal Diligencia precludes a fault. 4. 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.

These new data and model for the Diligencia basin resolve long-standing controversy concerning extension direction (present orientation) during the Early Miocene in this tectonic block.