Interaction Between Stream Development and Propagating Extensional Faults: The Volcanic Tableland, Owens Valley, California

Surface processes are extremely sensitive to the evolving topography around faults, and the changing mosaics of erosion, incision and deposition provide a valuable record of the influence of the contemporary fault pattern. However, there remains a desperate need for quantitative information about the rates and scales of the landscape's response to tectonic forcing. This study focuses on a series of recently-abandoned fluvial networks around a well-exposed [~7km long] extensional fault array on the Volcanic Tableland, a 0.76Ma outflow sheet of Bishop Tuff in the tectonically-active Owens Valley. Interaction and linkage of the faults in the array have been previously documented by Dawers and Anders, 1995 [JSG 17, 607-14]. Detailed mapping using GPS with centimetre accuracy demonstrates that the evolution of the streams has been intimately linked to that of the faults. The streams show a rapid response to interactions between neighbouring faults, by adjustments of their profile, course and channel geometry and by large scale network reorganisations, which in turn have altered the sediment distribution. Variations in the jointing and welding of the tuff have also influenced the stream patterns and incision styles. In situ-produced cosmogenic ²⁶Al and ¹⁰Be have been used to date samples from bedrock channels in an area of interaction between two overlapping fault segments. This has constrained the timing and length of stream occupation, the relative influence of tectonic and climatic forcing, and rates of both surface and tectonic processes. Long-term average erosion rates of 1.7-9.1mMy^-1 are calculated for streams draining the overlap. Incision in the footwall of the main fault has been dominated by the upstream migration of steep knickpoints developed in response to fault movements, and aided by the geometry of a pre-existing joint pattern. Knickpoints propagated at long-term average rates of 0.9-1.5mmyr^-1, but possibly as fast as 3.55mmyr^-1 during wet glacial periods. Exposure ages suggest that stream evolution on the Volcanic Tableland has been influenced more strongly by local faulting than by the regional glacial cycles. This combined study has important implications for the sensitivity of streams to even subtle tectonic movements, and their ability to provide detailed pictures of fault growth and interaction in an evolving extensional environment.