Holocene activity and seismogenic capability of intraplate thrusts: Insights from the Pampean Ranges, Argentina

Abstract

Trench excavations across the El Molino fault in the southeastern Pampean Ranges of central-western Argentina have revealed a deformation zone composed of opposite-verging thrusts that deform a succession of Holocene sediments. The west-verging thrusts place Precambrian basement over Holocene proximal scarp-derived deposits, whereas the east-verging thrusts form an east-directed fault-propagation fold that deforms colluvium, fluvial and aeolian deposits. Ages for exposed fault-related deposits range from 7.1 ± 0.4 to ~0.3 ka. Evidence of surface deformation suggests multiple rupture events with related scarp-derived deposits and a minimum of three surface ruptures younger than 7.1 ± 0.4 ka, the last rupture event being younger than 1 ka. Shortening rates of ~0.7 ± 0.2 mm/a are near one order of magnitude higher than those estimated for the faults bounding neighboring crustal blocks and are considered high for this intraplate setting. These ground-rupturing crustal earthquakes are estimated to be of magnitude Mw ≥ 7.0, a significant discrepancy with the magnitudes Mw < 6.5 recorded in the seismic catalog of this region at present with low to moderate seismicity. Results highlight the relevance of identifying primary surface ruptures as well as the seismogenic potential of thrust faults in seemingly stable continental interiors.