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.

Usefulness of Compiled Geophysical Prospecting Surveys in Groundwater Research in the Metropolitan District of Quito in Northern Ecuador

“…The profile features included a prospecting length of 1300 m, a prospecting depth of 1800 m, ER in the 10-8010 Ω m range, and an average RMSE of 14.32. The ER values were similar to that reported for similar volcano-sedimentary rocks with variable degrees of fissuring, fracturing, and saturation [52,56]. 2, updated and reinterpreted from Peñafiel et al [18].…”

“…The prospecting depths were 130 m for the VLF-EM survey and 1500, 1800, and 4000 m for the three low-frequency MTS surveys, of which only the first two were in the IAV sector (Figure 1b, Table 2). The hydrogeological reinterpretation of these two low-frequency MTS surveys [18,52] provided two significant findings: (i) the delineation of first-order thrusts and strike-slip faults controlling the geometry and stacking structure of Holocene to late Pliocene formations; and (ii) the identification of hitherto unknown disconnections (evidenced as high-resistivity fringes) between aquifers (evidenced as lowresistivity spaces) previously defined as hydraulically connected [26,36], resulting in less groundwater storage than previously known. An example is given in Figure 4c.…”

“…Four surveys used electromagnetic techniques, specifically VLF-EM and low-frequency MTS, for the geometry and structure of Holocene to late Pliocene formations resulting from the action of first-order thrusts and strike-slip faults [18,39,45,52]. In general, electromagnetic techniques infer subsurface ER from measurements of natural geomagnetic and geoelectric field variations at the ground surface [73,74].…”

“…One survey used VLF-EM (<10 Hz) in 13 profiles with a prospecting length in the 160-750 m range, a maximum prospecting depth of 130 m, and ER in the 15-280 Ω m range [39]. Three surveys used low-frequency MTS (>10 Hz) [18,45,52] with a total prospecting length of 33.3 km, a maximum prospecting depth of 4000 m, and ER in the ranges of 10-220 Ω m for geological formations catalogued as aquifers and 220-27,100 Ω m for geological formations catalogued as aquitards (Table 2).…”

Diverging Water Ages Inferred From Hydrodynamics, Hydrochemical and Isotopic Tracers in a Tropical Andean Volcano-Sedimentary Confined Aquifer System

“…As stated by Saffer and Tobin (2011), in subduction zones such as Ecuador, the persistent seismic movements and the high pressures, combined with the porosity and hydraulic characteristics of faults, can favor interactions between oceanic flow and other processes inside the continent. According to Alvarado et al (2014), using geomorphological studies, GPS data and seismic methods, and Reyes et al (2020), who worked in the same area using a magnetotelluric technique, this fault is a very old structure that is possibly connected to the oceanic terrains accreted to the continent. Schipper et al (2017) measured the δ 13 C of CO 2 emanating from three volcanoes in the Andes of South Peru and Chile.…”