Late neogene and quaternary landscape evolution of the northern California Coast Ranges: Evidence for Mendocino triple junction tectonics

A landscape records the surface response to tectonics at time scales intermediate between short time-scale information derived from seismic imaging and global positioning systems and the long-term geologic record. We link late Neogene and Quaternary deposits and landforms in the northern California Coast Ranges to the tectonics of the Mendocino triple junction. In the northern California Coast Ranges, the Mendocino crustal conveyor geodynamic model describes crustal thickening, thinning, and dynamic topography that produce a double-humped pattern of uplift that migrates northward with the Mendocino triple junction. The tectonics are manifest in the drainage system and elevation pattern of the Coast Ranges. At long wavelengths, the elevation pattern closely matches the predicted double-peaked shape of Mendocino crustal conveyor topography, and the high points of uplift control the location of drainage divides. Presently, the divide between the Russian and Eel Rivers and the divide between the Eel and Van Duzen Rivers approximately correspond to the peaks of uplift predicted by the Mendocino crustal conveyor model. As the triple junction migrates northward, the double-humped pattern of uplift and subsidence migrates, and the Coast Ranges emerge. Smaller drainages develop and evolve by stream capture and flow reversal, and the two main divides migrate in concert with the triple junction. In contrast to the systematic development of the small streams, the largest trunk streams can maintain grade through regions of high uplift, and coastal river mouths remain stationary despite the uplift moving north. Before ca. 2 Ma, the majority of the Coast Range drainage flowed to a southern coastal outlet near the present mouth of the Russian River. At 2 Ma, facilitated by headwater stream capture at key locations, the drainage direction reversed, and the majority of Coast Range rivers now drain into the north-flowing Eel River. The major drainage reorganization at 2 Ma highlights the potential for complexity in geomorphic response to tectonics.