STRATIGRAPHIC EVIDENCE FOR AGES OF DIFFERENT EXTENSIONAL STYLES IN THE SALTON TROUGH, SOUTHERN CALIFORNIA

The Salton Trough has been demonstrated to be a supra-detachment basin, at least in part, but the timing of the detachment phase and the degree of similarity to other supra-detachment basins in the SW U.S. have been subjects of disagreement. In the west-central Salton Trough (WCST), where the Neogene basin-fill succession is best exposed, we recognize both low-angle (<45º) and high-angle (>45º) normal faults that pre-date Quaternary strike-slip faulting. The age of detachment faulting is constrained by stratigraphic relationships in two localities (southern end of Santa Rosa Mountains, and eastern flank of Whale Peak), where upper-plate fanglomerates are in low-angle fault contact with lower-plate Cretaceous plutonic rocks. These fanglomerates and associated arkoses interfinger with Pliocene to early Pleistocene fluvial deposits of the ancestral Colorado River. No unequivocally Miocene strata have been observed in contact with low-angle faults.

High-angle normal faults observed in three localities -- Painted Gorge (PG), No Return Canyon (NRC), and Barrett Canyon (BC) -- are clearly of Miocene age. Syn-tectonic strata comprise early Miocene mafic volcanics (PG), late Miocene fanglomerates (NRC), and latest Miocene to earliest Pliocene marine sandstones (BC). Post-tectonic overlap is demonstrated by late Miocene to early Pliocene marine sandstones (PG), fanglomerates (NRC), and marine pholad borings (PG).

We infer three phases of extension in the Salton Trough. (1) High-angle normal faulting began during the early Miocene, although a topographic rift basin did not develop until the late Miocene. This phase was characterized by relatively slow extension and sedimentation, for a total fill <1 km thick. (2) Low-angle detachment faulting occurred during rapid Pliocene to early Pleistocene transtensive opening of the Salton Trough, accompanied by high sediment input from both the Colorado River and basin flanks, for a total accumulation up to 5 km thick. (3) In the WCST, strike-slip faulting and transpressive uplift replaced low-angle extension post-1.5 Ma. During the Quaternary, Salton Trough extension was dominated by basin-centered "leaky transform" tectonics, although the timing of its inception is not well constrained.