Variability of rock uplift rates in space and time along the Mendocino Coast, CA, as determined from channel profile analysis and catchment denudation rates

Abstract:

Off fault seismic activity associated with the principally strike-slip fault zones of California can be quite hazardous, but difficult to assess. Using cosmogenically derived denudation rates and channel profile analysis, we mapped the spatial and temporal distribution of rock uplift rates along the Mendocino Coast in northern California. Specifically, we used measured 10Be concentrations in river sands to determine catchment-averaged denudation rates, and then used these rates with basin-averaged channel steepness values to infer the erosive properties of the underlying bedrock. We then used the spatial distribution of channel steepness with these erosive properties to map the spatial distribution of local channel incision rate onto this landscape. In so doing, we identified generally lower elevation areas of this landscape that showed high channel steepness (and hence denudation rate) that were distinct from a higher-elevation area that showed systematically lower channel steepness and denudation rate values. These two areas are separated by distinct channel knickpoints, but not lithologic contacts. Assuming that changes in rock uplift rates are manifest in channel steepness values as a headward propagating kinematic wave that separates high and low channel steepness values, we bravely use the distance that this transition has moved upstream to estimate the time at which rock uplift rates increased. This analysis suggests that rock uplift rates along the coast changed from 0.3 to 0.75 mm/yr between 350 and 450 ka. Finally, the preservation and back-tilting of a Pliocene marine sediment package within 5-15 km of the San Andreas Fault supports the patterns suggested by the channel profile analyses, and further suggests that relatively rapid uplift may indicate a zone of elevated off-fault seismic hazard.