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

Fig. 2: Sorted plot of stream deflections across the Whittier fault, showing approximate incision ages based on a 3 mm/yr Whittier fault slip rate. Age of the MRE at 2 ka is from Gath et al., 2017. Chart modified from Gath, 1997 & Gath et al., 2017. Fig. 3: Delineation of the drainage basin areas (red boundaries) in the eastern Puente Hills (Fig. 1); Whittier fault shown in green. These stream deflections provide another approach to determining the uplift age of the Puente Hills by looking at the evolution of the drainage network as a proxy for time under the assumption that a larger drainage basin takes more time to form than a smaller one. The Puente Hills provide a unique location for this analysis because the drainages are all oblique to theWhittier fault, displaced northwesterly (up gradient) and as such the basins remain distinct. In the eastern Puente Hills, only Soquel and Brea Canyons have been captured, and their former channels remain as wind gaps from which total displacement and basin area can be measured. The areas of 15 drainage basins in the eastern Puente Hills (Fig. 3) were measured (Table 1). The three canyons (Brea, Carbon, and Tonner) offset the most by fault deformation (1400-1700 m, Table 1), exhibit the largest areas as well (15-24 km2), with Brea being the smallest of the three. This is not surprising because Brea and Tonner canyons merge at the Whittier fault where Tonner captures the Brea drainage. Plotting basin area against basin age (calculated from channel displacement) forces a log normal relationship (Fig. 4) with an R2 of 0.92 which indicates a strong relationship between basin area and age. 12th International INQUA Meeting on Paleoseismology, Active Tectonics and Archaeoseismology (PATA), October 6 th -11 th , 2024, Los Andes, Chile METHODS Two separate geomorphic techniques were employed to independently determine the uplift rate of the eastern Puente Hills: 1) drainage basin development rates and 2) elevated fluvial and strath terraces. Drainage Basin Development: Gath (1997) mapped and measured 155 streams that were deformed when crossing the Whittier fault, and observed that the larger streams were deformed greater amounts than the smaller streams, indicating a temporal relationship of progressive growth faulting (Fig. 2). The distance that a stream has been offset can be used as a proxy for the age of the stream by dividing the offset by the rate of slip on the fault. In this paper, I exploit this unique dating method to quantitatively model the Quaternary temporal and physical evolution of the Puente Hills through the progressive deformation of the drainage network. By retrodeforming the offset streams using the 2.5-3.0 mm/yr rate of slip on the fault [dividing the total stream offset by the fault slip rate] one can calculate the age of the stream before it was initially offset by the fault. The three largest rivers in the eastern Puente Hills (Carbon, Tonner and Brea; see Figs. 2, 3 & Table 1) are all deflected ~1700 meters, implying an age of about 600 ka (Marine Isotope Stage 15), which I interpret as the minimum emergence age for the Puente Hills. Another strong deflection cluster is at ~400 meters which correlates with the 150-120 ka glacial incision period between a prominent set of MIS 7 & 5e interglacial terraces along the southern California coast (Grant et al., 1999). These canyons, incised into their flanking terrace during the corresponding glacial, were also used to calculate a 2.5-3.3 slip rate (Rockw ll et al., 1988; Gath, 1997). Fig. 2: Sorted plot of stream deflections across the Whittier fault, showin approxim te incision ages based on a 3 mm/yr Whittier fault slip rate. Age of the MRE at 2 ka is from Gath et al., 2017. Chart modified from Gath, 1997 & Gath et al., 2017. These stream deflections provide another approach to determining the uplift age of the Puente Hills by looking at the evolution of the drainage network as a proxy for time under the assumption that a larger drainage basin takes more time to form than a smaller one. The Puente Hills provide a unique location for this analysis because the drainages are all oblique to the Whittier fault, displaced northwesterly (up gradient) and as such the basins remain distinct. In the eastern Puente Hills, only Soquel and Brea Canyons have been captured, and their former channels remain as wind gaps from which total displacement and basin area can be measured. The areas of 15 drainage basins in the eastern Puente Hills (Fig. 3) were measured (Table 1). The three canyons (Brea, Carbon, and Tonner) offset the most by fault deformation (1400-1700 m, Table 1), exhibit the largest areas as well (15-24 km 2 ), with Brea being the smallest of the three. This is not surprising because Brea and Tonner canyons merge at the Whittier fault where Tonner captures the Brea drainage. Fig. 3: Delineation of the drainage basin areas (red boundaries) in the eastern Puente Hills (Fig. 1); Whittier fault shown in green. Map Basin Area Offset Age ID Name (km 2 ) (km) (ka) A Brea 15.40 1.37 457 B Tonner 24.21 1.68 559 C Olinda Landfill 0.93 0.27 90 D Olinda a 1.37 0.42 140 E Olinda b 1.01 0.31 102 F Carbon 22.57 1.68 559 G Telegraph 10.46 1.17 390 H Travis Ranch 3.03 0.73 242 I Yorba Linda 0.89 0.41 137 X Soquel 10.00 1.21 403 J Blue Mud 2.68 0.37 122 K Lomas de Yorba 0.69 0.35 117 L Box 1.33 0.40 132 M Bee 2.21 0.38 127 N Bryant 0.52 0.26 87 Table 1: Delineation of the drainage basin areas in the eastern Puente Hills (Fig. 3) showing the measured basin area, the right- lateral deflection along the Whittier fault, and the calculated basin age by retrodeforming the stream along the fault at a 3 mm/yr rate. Plotting basin area against basin age (calculated from channel displacement) forces a log normal relationship (Fig. 4) with an R 2 of 0.92 which indicates a strong relationship between basin area and age. T ble 1: D lineati of the drainage basin ar as in th eastern Puente Hills (Fig. 3) showing the measured basin area, the right- lateral deflection along the Whittier fault, and the calculated b sin age by retrodeforming the stream along the fault at a 3 mm/yr rate.