Revisiting the Cajon Pass Quaternary Terraces with Geochronology dating­ implications for the long term slip rates of the San Jacinto and San Andreas systems

Paula M. Figueiredo, Ray J. Weldon, & Lewis A. Owen

Published August 15, 2018, SCEC Contribution #8767, 2018 SCEC Annual Meeting Poster #261

The San Andreas (SA) and the San Jacinto (SJ) Faults are the two major active fault systems in Southern California, accommodating 50-70 % of the 52 mm/yr North America–Pacific plate boundary motion. At the Cajon Pass (CP) area, both fault systems converge and became parallel, displacing a terrace sequence younger than 700 ka (post-dating Noble’s Old Alluvium unit). Since the inception of SJ, both systems have been evolving and interacting at different rates and time, and their long-term tectonic interaction is not sufficiently understood, particularly at the CP area. This poses limitations for the understanding of their structural evolution but also for the regional seismic hazard analysis. SA slip-rates decrease from N of CP (Palette Creek, 35±5 mm/yr, for Holocene and ~ 400 ka) to CP area (Holocene, 24.5 mm/yr) and south of it (Plunge Creek, 6.3-18.5 mm/yr). This decrease of about 5-10 mm/yr from Palette Creek to CP, most likely indicates transfer to other structures like Lytle Creek fault (2.5 mm/yr), or the Glen Helen fault, or the main geologic strand of the SJ. South of CP, a significant amount of slip seems to have been transferred to the northern SJ during late Pleistocene, while Holocene slip-rates indicate a similar or slightly higher rate for SJ. The terrace sequence at the CP displaced by SA and SJ is crucial to access slip-rates and tectonic interactions between these fault systems through time. The upper deposits, Qoa-e (500±200 ka), Qoa-d (55±12 ka), and Qoa-c correspond to significant alluvial fill deposits, over 50 m thickness widely present that can be correlated throughout the basin. The chronology of the sequence was established through a soil chronosequence: while the younger sequence (<35 ka) was dated and its soil chronosequence calibrated, the older sequence, namely the Qoa-e and the Qoa-d were never dated with absolute dating techniques. Aiming to refine ages for the older terraces and improve long term slip-rates for SA and SJ and understand their interaction during Pleistocene, we sampled depth profiles for TCN 10Be in the Qoa-e at 3 locations: Summit area (displaced by SA and Cleghorn F.), Texas Hill (displaced by Lytle Creek F.) and Cucamonga (displaced by Cucamonga T.). Samples are presently being measured at the Prime Lab Accelerator Mass Spectrometer and we will present preliminary results for the surface ages of the Qoa-e terrace as well as discuss further locations to sample and constrain the age of other terraces.

Key Words
Cajon Pass; Quaternary; Geochronology; Slip-rates

Figueiredo, P. M., Weldon, R. J., & Owen, L. A. (2018, 08). Revisiting the Cajon Pass Quaternary Terraces with Geochronology dating­ implications for the long term slip rates of the San Jacinto and San Andreas systems. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
San Andreas Fault System (SAFS)