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

390 PATA Days 2024 R E S U LT S A N D D I S C U S S I O N S The main fault of the CBF juxtaposes late Cenozoic rocks (Abanico Formation) over Quaternary alluvial fan deposits (Figs. 2 and 3; e.g., Estay, 2019). This fault is exposed with an average dip of 40°E, producing a vertical offset of ~3.2 to ~30 m. The alignment of fault outcrops, inflection points of streams and rivers, along with alluvial and fluvial deposits affected by the fault, allow recognition that its most recent activity has a trace length between ~5 and 15 km (Fig. 3). We identified >50 knickpoints along the thalwegs of the area's streams. The heights of these knickpoints range from ~50 cm to ~2 m, mainly of single steps and some representing groups of very close knickpoints. These geomorphological markers are distributed east of the CBF's main fault, aligning predominantly in ~N-S orientation. Clusters of knickpoints were found at ~ 600 m, ~1.5 km, and >2 km east of the CBF's main fault (Fig. 4; e.g., Quezada, in progress). In some cases, knickpoints are located upstream, characteristic of tectonically active regions with recent uplift, where fluvial systems try to reestablish their equilibrium profiles. Additionally, these knickpoints overlap the lineaments proposed for the area (Estay, 2019). Field characterization rules out differential erosion of adjacent lithologies or human intervention origin for the knickpoints, suggesting a tectonic origin. Optically Stimulated Luminescence (OSL) dating from samples taken from both fault-affected and unaffected alluvial fan deposits (Fig. 5) yielded ages ranging from ~2.5 to ~9.0 ka, supporting its Holocene activity, along with ~5 m of fault slip in the last ~8.7 ka, with a vertical rate of ~0.4 mm/a and a fault slip rate of ~0.6 mm/a. Additionally, the Quaternary age (~0.9 Ma, U-Pb) of detrital zircons in terraced alluvial deposits indicates that CBF activity may have begun before ~0.9 Ma, resulting in an accumulated uplift of the hanging block of ~130 m during this period. Based on the relationship between the Aconcagua River and Estero San Francisco incision, the distribution of fluvial terraces to the east, variation in sinuosity and stream power, and fault trace locations (Fig. 3), we suggest that tectonic uplift due to the CBF activity controls river incision in the hanging block from at least ~0.9 Ma to the present. Based on the known length of the main fault of the CBF system, slip, and earthquake scaling relationships, we suggest that this fault has been capable of generating earthquakes of Mw ~7.5. Therefore, the CBF system is active (since at least ~0.9 Ma to the present). In addition, it shares geometric, structural, and seismic rupture chronology similarities with the San Ramón Fault System, the latter located approximately ~50 km to the south along the eastern edge of the Santiago Basin. The potential activation of the CBF represents a seismic risk to the Fig. 2: Natural exposure of the CBF’s main fault, showing different dimensions of fault gouge and its relationship with to the damage zone and tilted alluvial fan sediments (Extract from Estay et al., 2023).