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Detecting a master thrust system by magnetotelluric sounding along the western Andean Piedmont of Quito, Ecuador

Abstract:

The Quito Basin, a narrow NE-trending tectonic depression, contains a perched Quaternary volcaniclastic sequence juxtaposed against the Interandean Depression through the Quito fault system (QFS). Syntectonic infilling, which occurs during a piggy-back phase, consists of alluvial fan sets grading eastward into lacustrine facies. The western piedmont margin overlies the foothills of Quaternary volcanoes. Classic thrust tectonic models predict basinward propagation of low-angle thrusts ahead of master faults. A magnetotelluric survey was used to investigate the western Quito piedmont. Three resistivity sections show similar geoelectrical assemblages. From west to east, a wedge-shaped high-resistivity structural high abuts a low-resistivity domain, representing a fold-thrust belt overriding a foreland basin. This set continues to the east with a new high-resistivity structural high sliced by low-angle imbricated thrusts that culminate in local thrust-top troughs before grading into a deeper low-resistivity trough against the younger QFS anticlines. Thus, the Quito master thrust system results in a prominent basin-facing monocline affecting the foothills of Quaternary volcanoes.