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

200 PATA Days 2024 The Puente Hills are composed almost exclusively of the Puente Formation, a Miocene marine sequence of sandstones and shales (Morton & Miller, 2006). The southern flank of the Puente Hills has a well-ordered fluvial drainage network consisting of primary trunk streams and hundreds of tributary streams. In the western half of the Puente Hills, periodic stream capture has complicated the drainage history, but in the eastern half the drainage basins are all distinct and separable. This paper focuses on the eastern half of the Puente Hills. South of the Puente Hills lie the Santa Ana Mountains, a plutonic-cored range that dominates over the Puente Hills. The Santa Ana Mountains are a 6 mm/yr indenter driven into the eastern Los Angeles Basin by the Elsinore fault (Gath, 2022). Over the past million years, the indenter has thrust the entire LA Basin sedimentary sequence over the nose of the pluton. The Santa Ana Mountains indenter is presently only 300 m from physically colliding with the south-vergent Puente Hills, separated only by the antecedent Santa Ana River. This collision plays a critical role in the uplift rate of the Puente Hills. M E T H O D S 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, andobserved that the larger streamswere 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 (Rockwell et al., 1988; Gath, 1997).