Origin of terraced hillslopes on active folds in the southern San Joaquin Valley, California

Abstract

The northern-facing forelimbs of the active San Emigdio and Wheeler Ridge anticlines in the southern San Joaquin Valley (SSJV) are marked by numerous, sharply defined terracettes. Terracette treads and risers are 3–5 and 7–11 m wide, respectively. The terracettes are discontinuous along their length, with the longest being ∼300 m, although most terminate more abruptly forming lobate mounds. The terracettes form on 10–25° slopes underlain by loosely consolidated coarse conglomerate beds of the Pleistocene Tulare Formation that dip subparallel to terracette risers. Soil developed on the slopes is comprised of a 1- to 2-m-thick calcic Bk horizon, overlain by a 0.5-m-thick argillic claypan Bt horizon, followed by 1- to 3-m-thick AB and A horizons that have been extensively bioturbated. A number of hypotheses have been proposed for the genesis of similar terracettes. The two most viable tectonic models for our study area are based on growth of active fault-bend folds and include (i) sediment onlap with seismic folding and (ii) flexural slip faulting. We produced a very high-resolution digital elevation model (DEM) (1.67-m spatial resolution and ±5-cm vertical accuracy) using NASA's RASCAL (RAster SCanning Airborne Lidar) instrument and completed numerical modeling that we interpret to disprove both of the active fold growth models. The discovery and mapping of similar terracettes on NW- to NE-facing hillslopes on the Kettleman Hills anticline and Comanche Point in the northern Tehachapi Mountains suggest that a northern-facing aspect is an important factor contributing to terracette development. The terracettes are also present only on hillslopes that overlie blind or emergent thrust faults. We propose that a soil creep model based on flow-dominant mass wasting is the most likely cause of terracette development. Creep is confined to the thick A and AB cumulic soil horizons that delaminate above the rigid Bt claypan and occurs predominantly on northern-facing slopes due to increased soil moisture retention. The barrier to creep caused by the decrease in slope at the base of fold limbs leads to the development of terracettes that become more sharply defined in a downslope direction. We interpret the overthickened A and AB soil horizons to form preferentially on northern-facing slopes due to increased bioturbation and deposition of eolian silt. Strong ground motions produced by earthquakes may facilitate soil creep, and thereby contribute to the overall development of the terracettes.