Incipient axial collapse of the Main Cordillera and strain partitioning gradient between the central and Patagonian Andes, Lago Laja, Chile (Oct 06)
Daniel Melnick , François Charlet et al.
...Lago Laja is a late Quaternary volcanic-dammed lake located near the drainage divide of the south central Andes. Field observations, lake reflection seismic profiles, bathymetry, and remote sensing data reveal an active fault system that runs parallel to the volcanic arc along the axis of the Main Cordillera, the Lago Laja fault system (LLFS). Normal faults of this extensional system cut late Pleistocene volcanics, <7.1 ka still water lacustrine sediments, 6.3 ka pyroclastic deposits, and Holocene alluvial fans. We divide the LLFS in three segments on the basis of fault geometry, width, and slip magnitude. The underwater faults of the central segment in the lake's deepest part have the maximum Holocene vertical slip rate of >2.7 mm/yr. Since 7.1 ka, the LLFS accounts for ~0.7% of arc-normal extension at an average minimum rate of 1.2 mm/yr and strain rate of ~10-14 s-1. Seismites and surface ruptures evidence M>6 paleoearthquakes. The Main Cordillera at ~37°S is a large-scale pop-up structure uplifted by thrusting along its foothills.
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