Proceedings of the 12th International INQUA meeting on paleoseismology, active tectonic and archaeoseismology

204 PATA Days 2024 Fig. 7 illustrateswhere theWhittier fault enters the Santa Ana River canyon in the lower right corner. It is possible that a 3 kmmeander in the river is reflecting a right-lateral fault offset and is not purely an entrenched meander. This scale ofmeander is similar to another that is fully shown on Fig. 7 farther to the west, so the connection to theWhittier fault may be coincidental. But, if not, retrodeforming the meander at 3 mm/yr would imply an ~1 Ma age for the current position of the Santa Ana River and similarly, an ~1 Ma age for the initial emergence of the Puente Hills. At 1 Ma, the northern end of the Santa Ana Mountains would have been approximately 6 km to the south (Gath, 2022), not compressing the Santa Ana Canyon as it is today, though certainly having a progressive and increasing effect on entraining the river into its current path. Fig. 7 shows the terrace ages based on soil profile development of the MIS Stage 5e and 7 fill terraces. Typical correlations of terraces with the eustatic sea level curve assume a constant uplift rate (Grant et al., 1999). However, the OSL ages for the Qt1&2 terraces do not permit a constant uplift rate, necessitating consideration Table 2: Ages of the fill terraces in Santa Ana Canyon (modified from Grant, et al., 2006). e Hills. a change in basin rate the impact of the Hills. Ana indenter distance e Puente Hills. ssively increasing he uplift rate of the increasing as the Fig. 7: Terrace map through the Santa Ana River canyon (Fig. 1) showing age estimates for the strath and fill terraces (Table 2). Whittier fault shown as thin dashed and dotted red lines, possible 3 km Santa Ana River offset shown as red arrows. Photos of the 120 ka fill terrace (left) with the Whittier fault offsetting Qt3 to surface (note lack of vertical surface offset) and highest (600 ka) strath surface (right). Terrace Height above Santa Ana River (m) Soil Age (ka) OSL Age (ka) Discussion Qal 0 [100 m elev.] 0 N/A Modern river Qt0 -25 N/A N/A SAR thalweg Qt1 27 60 31.9±4.8 [LG-6] 29.3±3.0 [LG-8] 23.5±1.9 [LG-9] Cut and filled into toe of Qt2 Qt2 39 80 38.0±3.9 [LG-7] 45.0±5.6 [LG-11] Fan deposits graded to Santa Ana River Qt3 100 120 N/A Incised into by streams offset 400 m Qt4 155 210 N/A Remnants, all N of WF Qt5 195 N/A N/A Strath surface Qt6 285 N/A N/A Strath surface Qt7 345 N/A N/A Strath (Fig. 7) Table 2: Ages of the fill terraces in Santa Ana Canyon (modified from Grant, et al., 2006). Fig. 7 illustrates where the Whittier fault enters the Santa Ana River canyon in the lower right corner. It is possible of a variable uplift rate to accommodate both sets of data. Fig. 8 shows that a variable uplift rate can indeed be applied to the Puente Hills and that doing so honors all the data available. Fig. 8 illustrates the impact of the Santa AnaMountains indenter, and its 6 mm/yr collision with the Puente Hills (Gath, 2022). At 1 Ma, the Santa Ana Mountains were 6 km south of the Santa Ana River, but the river was already becoming constrained into its antecedent position. By 600 ka, the collision had broadened sufficiently to uplift the entire eastern Puente Hills area (Fig. 1) and initiate formation of its three principal drainages and primary internal tributaries at 300-400 ka. During this time the uplift rate doubled from 0.3 to 0.6 mm/yr. In the last 200 ka, as the collision intensified, and particularly as the northernmost tip of the Santa Ana Mountains’ impacted the easternmost corner of the Puente Hills, the uplift rate in the Santa Ana canyon doubled again to over 1.2 mm/ yr, consistent with Bergen et al.’s (2017) finding of a late Pleistocene–Holocene doubling of slip rate on the Puente Hills Thrust farther to the west.