Late Quaternary slip rate of the southern Elsinore fault, Southern California: Dating offset alluvial fans via 230Th/U on pedogenic carbonate

Abstract

To understand how strain is accommodated along the Pacific‐North American plate boundary and to assess the associated seismic hazard, estimates of long‐term (103–105 year) slip rates for strands of the San Andreas fault system are needed. The mean slip rate over the past ∼40 kyr for the southernmost Elsinore fault, one of the principal strands of the San Andreas fault system in Southern California, was determined using offset alluvial fans dated by U‐series on pedogenic carbonate. Two displaced fans, correlated with their source drainages using distinctive gravel‐clast assemblages, define offsets of 60 ± 20 and 83 ± 31 m. Milligram‐size samples of dense carbonate clast‐coatings from fan soils provide 230Th/U‐based ages of 39.1 ± 5.3 and 46.4 ± 5.3 kyr (all errors are 2σ), respectively, for the offset fans. Our results indicate that the Elsinore fault near its southern terminus slipped at a mean rate of 1.6 ± 0.4 mm/yr over the past ∼40 kyr. However, slip associated with the most recent earthquake on the southern Elsinore fault increases northward from the study area, and if this pattern has repeated through time, the peak slip rate on the southern Elsinore fault may significantly exceed our measured rate.