CHANGES IN BEDROCK CHANNEL MORPHOLOGY DRIVEN BY DISPLACEMENT RATE ACCELERATION DURING NORMAL FAULT LINKAGE

Fluvial channel morphology can be influenced by displacement rate acceleration during fault segment interaction and linkage. The Volcanic Tableland, a Late Pleistocene ash-flow tuff in east central California, is dominated by a population of north-south trending normal faults and a fluvial network. Although the fluvial network is presently inactive, previous workers suggest its development has been, in part, coeval with fault activity. We map the faults and fluvial channels associated with four sites, each at a different stage of fault linkage. We utilize high resolution (vertical accuracy of 10 cm) DGPS data to study the effects of fault segment interaction and linkage on bedrock channel morphology. Because the channels are inactive, we estimate downstream changes in channel width and depth using HEC-RAS, a one-dimensional open channel flow model developed by the U.S. Army Corps of Engineers. Trends in channel slope and width to depth ratio suggest that displacement rate acceleration drives incision, and this process appears to begin well before the fault geometry would suggest imminent linkage. This analysis indicates that channel slope and width to depth ratio may adjust independently to changes in fault displacement rate. In some areas, anomalously low width to depth ratios are not associated with profile convexities, whereas in other areas the channel slope and width to depth ratio both appear to respond to fault movement. Our data can be interpreted in one of two ways: either channels respond to increased displacement rates by first decreasing their width to depth ratio and channel slope only changes once a threshold width to depth ratio is achieved or changes in width to depth ratio remain in the channel even after channel convexities have been eroded out of the profile.