Preliminary Geochronology data for Cajon Pass Terraces- implications to the regional chronosequence

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

Published August 14, 2019, SCEC Contribution #9649, 2019 SCEC Annual Meeting Poster #141

In Southern California, the two largest fault systems (San Andreas and San Jacinto) converge at the Cajon Pass area. Here a sequence of terraces is displaced by both fault systems and therefore record their evolution and interactions through time. As such, their ages are crucial for estimating slip rates 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. Due to their relevance, we sampled at several locations for TCN and OSL with the purpose to obtain numerical ages and consequently improve slip-rates. This work was conducted in two phases, an initial one aiming to investigate Qoa-e surface exposure ages at several locations across the Cajon Pass and a second one aiming to quantify Qoa-d and Qoa-c surface ages at specific locations where the terraces are displaced, which will allow to estimate directly more accurate slip-rates. 10Be TCN depth profiles age results from the initial phase indicate much younger ages than the one inferred based on soil chronology (500±200 ka), with ages roughly ranging from 70 to 100 ka. This can suggest that Qoa-e was not formed during a major single sedimentation phase, but that might have had several periods of sedimentation, being the last event measured with the TCN depth profile. In addition, because ages vary, it might indicate that sedimentation could have been diachronous though Cajon Pass. If so, then a more detailed and careful analysis need to be conducted to improve understanding about the surface processes at the Cajon Pass area, namely to understand the tectonic-climate relationship that ultimately controls incision, sedimentation and erosion. Samples for Qoa-d and Qoa-c have been collected for TCN (10Be and 36Cl) as well as also for OSL and samples for both geochronology methods are now being processed.

Key Words
Cajon Pass, Pleistocene, slip-rate, Geochronology

Citation
Figueiredo, P. M., Weldon, R. J., Owen, L. A., & Onderdonk, N. W. (2019, 08). Preliminary Geochronology data for Cajon Pass Terraces- implications to the regional chronosequence. Poster Presentation at 2019 SCEC Annual Meeting.

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