Quaternary activity and seismogenic potential of the Sierra Chica Fault System, Pampean Ranges of Argentina

The Sierra Chica Fault System (SCFS) is a west-vergent thrust bounding the easternmost basement uplift in the Pampean Ranges of Argentina. Neogene-Recent activity along this structure is responsible for ongoing mountain-building in the broken foreland of the Andes, some 800 km away from the Pacific-South American subduction zone. We characterize Quaternary activity along the SCFS and provide the first numerical age constraints at sites where crystalline rocks of the Sierra Chica range overthrust Quaternary sediments as recent as ~50 ka. Detailed trench logging of the deformation zone defines average slip rates along the SCFS to 0.01–0.36 mm/a over the past ~90 ka. Historical earthquakes of magnitudes ≤7.4 occur along Pampean Range faults without any primary surface ruptures. Thus, evidence for surface ruptures suggests that late Pleistocene slip along the SFCF was likely associated with prehistoric earthquakes with magnitudes >7, although the application of global scaling laws to this tectonic setting tends to underpredict seismogenic potential along the SCFS. Our study aims to highlight the challenges of evaluating the seismic hazard of a fault system that has been active during the late Quaternary, but lack diagnostic geomorphic expression of past surface ruptures. This observation underscores that caution is needed when applying global scaling laws to intracontinental deformation in the broken forelands of subduction zones.