EVIDENCE FOR INACTIVITY SINCE ~100,000 YRS BP ON THE NORTHERN ROUTE OF THE SAN ANDREAS FAULT, SOUTHERN CALIFORNIA

From San Bernardino to Indio, CA, the San Andreas fault (SAF) follows two distinct routes. A northern route (NR) follows the Mill Creek and Mission Creek strands and a southern route (SR) follows the San Bernardino, Banning, and Garnet Hill strands, and San Gorgonio Pass (SGP) fault zone. We think that reconfiguration of the SAF zone in the SGP region at ~100 ka shifted motion from the NR to the SR. Surprisingly, the complex SR has experienced low slip rates and relatively infrequent paleoearthquakes, particularly in SGP. SR complexity and low slip rates in part led Fosdick & Blisniuk (F&B) (2018) to propose that the NR has generated 2.5 km of dextral slip since ~100 ka, yielding a slip rate of 20-30 mm/yr. Their offset and its timing are based on correlating a small strath terrace (Qt2) with an expansive fill terrace (Qf2) 5-7 km away that has a published OSL age of ~100 ka, then using those deposits to define a paleodrainage that has been offset along the NR since that 100 ka—thereby showing that the NR remains a major active plate boundary. We question this proposal using four arguments that support little, if any, slip on the NR since ~100 ka: (1) The Qt2/Qf2 correlation is implausible because of (a) differences in soil development, elevation above modern channel, and genetic terrace styles (strath vs fill), (b) the lag on Qt2 consists of material cannibalized from older alluvial deposits into which the strath was carved, and (c) a profile connecting the Qt2-Qf2 terraces defines a gentle gradient much different from steeper profiles on Pleistocene fan remnants in the area. (2) The NR does not deform Holocene units for most of its route (Matti et al., this session). In contrast there is abundant evidence for Holocene deformation along the SR over the same distance. (3) Stream deflections of ~50 m in >>60 ka fill proposed on the NR near Mission Creek (F&B, 2018) equate to slip rates of <<1 mm/yr. (4) Dextral motion of ~6 mm/yr on the SR through SGP (Heermance & Yule, 2017) matches estimates from geodetic models for SAF motion here. Thus, there is no reason to require significant motion on the NR. Though the NR once carried ~20 mm/yr of SAF motion, it shut down following the latest SAF reorganization at ~100 ka. This relatively recent event created the geometrically complex SR and its traits characteristic of a poorly organized, youthful fault system.