Competition between fault advection and fluvial aggradation determines channel geometry along strike-slip faults

Abstract:

Deflected channels are a canonical indicator of Holocene fault slip. Measurements of offset channels provide a robust estimate of long term fault slip and a record of recent major earthquakes. Channels are, however, dynamic features of the landscape that evolve in response to tectonic forcing. Ephemeral channels crossing the San Andreas Fault in the Carrizo Plain illustrate how, with increasing offset, channels aggrade, and eventually avulse, limiting the total offset of an active channel. We hypothesize that ongoing fault slip and the resulting reduction in slope incurs a diffusive pulse of upstream aggradation with a fixed avulsion node at the intersection of the fault and the channel. Here, avulsion occurs when deposition overtops the local channel or valley threshold height. Building upon semi-analytical numerical models of transport-limited channel profile evolution, we define a nondimensional avulsion number that determines channel stability. We measure the avulsion number with high-resolution altimetry for a set of active and abandoned channels crossing the San Andreas Fault in the Carrizo Plain. The predicted and observed stability agree well such that a critical avulsion number separates active and abandoned channels. We can, therefore, explain the coexistence of abandoned channels that feature little cumulative offset and drainage systems reoriented along the San Andreas for kilometer stretches. Offset channels appear to faithfully record the coupled fluvial and tectonic system, governed by sediment transport capacity and fault slip rate. Thus, careful characterization of either the tectonic or fluvial systems constrains the other. Our analysis further indicates that subtle changes in any of these governing variables toggle drainage systems between cross-fault to fault guided basins.