LATE MIOCENE ONSET OF PLATE-BOUNDARY STRAIN IN THE SALTON TROUGH REGION: STRATIGRAPHIC EVIDENCE FROM SPLIT MOUNTAIN GORGE, SOUTHERN CALIFORNIA

Two groups of models have been proposed for the age of the Pacific-North America plate boundary in the Salton Trough and Gulf of California (ST-GC): (1) plate-boundary strain became localized along the ST-GC at ~5-6 Ma and has accumulated ~300 km of dextral offset; or (2) plate motion in the ST-GC initiated ~12-17 Ma and accumulated ~300-450 km of offset. Group-1 models equal or exceed the plate rate (~50 mm/yr), and Group-2 models underestimate it by ~30-60%. Lack of consensus on this question poses a major obstacle to understanding the kinematic evolution of the Pacific-North America plate boundary. Based on recent stratigraphic, paleomagnetic, and micropaleontologic analysis of late Miocene sedimentary rocks at Split Mt. Gorge, we test the applicability of these models and propose an intermediate solution. The Elephant Trees Conglomerate (Me) rests nonconformably on Cretaceous tonalite and is ~8.1 +/- 0.4 Ma at its base in a local paleovalley. Me coarsens up from fluvial sandstone to boulder-rich debris-flow and landslide deposits, onlaps tonalite, and is capped by a large rock-avalanche megabreccia. These deposits filled a complex basin bounded by extension- or transtension-related uplifts that generated steep unstable local topography. The lower megabreccia is abruptly overlain by the marine Fish Creek Gypsum and turbidites of the Latrania Fm, which record rapid marine incursion at 6.27 Ma (top of the C3An.2n subchron). Basal marine deposits of remarkably similar age (~6.3-6.5 Ma) are preserved in a narrow belt for ~600 km along the ST-GC, indicating that the plate boundary was in its present position by or before 6.3 Ma. Thus we infer that deposition of Me in a structurally active basin between ~8 and 6.3 Ma records early off-fault strain associated with initiation of the paleo-San Andreas Fault in the ST-GC corridor. Alternatively, (1) the paleo-San Andreas Fault could have been active prior to 8 Ma without causing any deformation in this area, or (2) these deposits could record intraplate deformation prior to localization of the plate boundary in the Salton Trough. The lead author favors a model in which the Pacific-North America plate boundary was established in the Salton Trough and Gulf of California at about 7-8 Ma, but the alternatives are also plausible.