Petrology and provenance of the Modelo Formation, Santa Monica Mountains, Southern California; constraints on displacement along the San Gabriel Fault

The Modelo Formation of the central and eastern Santa Monica Mountains is an Upper Miocene siliceous-diatomaceous shale and sandstone submarine-fan sequence, which was deposited in the northern Los Angeles basin. The graded, poorly sorted, and angular sandstones comprise channelized mid-fan facies deposited at middle-to-upper-bathyal depths; they are arkosic (58% feldspar) with variable amounts of matrix. Published paleocurrent data yield a south-southwest flow direction. The direction of the paleocurrents constrains the timing of deposition to be after rotation of the Transverse Ranges (after 15 Ma.) Biostratigraphic data indicate that deposition commenced during the Late Mohnian (after 10.8 Ma). QFL plots indicate that the provenance was basement uplift to transitional arc (e.g. Dickinson, 1985). The meta-igneous lithic fragments and the high feldspar content of the rocks suggest an anorthositic, gneissic, tonalitic, or mylonitic source terrane. Sandstone modes and conglomerate clasts indicate that the basal section was derived locally from the Santa Monica Slate. The upper section represents a volcanic unroofing sequence, as evidenced by decreasing volcanic lithic fragments upsection. Crowell (1982) suggested 60 km of right slip on the San Gabriel fault between 12 and 5 Ma. Palinspastic restoration places the Santa Monica Mountains immediately south of the San Gabriel Mountains, whichmore » wee the likely source for Modelo sandstones. Based on plagioclase composition (95% oligoclase and albite) of Modelo sandstone, the anorthosite body in the western San Gabriel Mountains is an unlikely source. However, a source in the mylonitic, gneissic, and tonalitic bodies of the eastern San Gabriel Mountains could explain the observed petrography. This source for the Modelo is consistent with 60 km of movement on the San Gabriel fault.« less