Crustal‐scale structural architecture in central Chile based on seismicity and surface geology: Implications for Andean mountain building

Abstract

[1] We document a crustal-scale structural model for the central Chile Andes based on seismicity and surface geology, which consists in a major east verging ramp-detachment structure connecting the subduction zone with the cordillera. The ramp rises from the subducting slab at ∼60 km depth to 15-20 km below the western edge of the cordillera, extending eastward as a 10 km depth flat detachment. This structure plays a fundamental role in the Andean orogenesis because most of the shortening has been accommodated by structures rooted in it and allows the distribution of crustal thickening in a "simple shear deformation mode." Indeed, despite shortening distribution being very asymmetric (∼16 km versus ∼70 km in the western and eastern side, respectively), the western side is higher and thicker than what is expected. Yield strength envelopes show strong rheological control on this structure. V p and V p /V s variations in the upper mantle and in the deepest limit of the seismogenic interplate contact mark the intersection of the ramp with the slab, which coincides with the blueschist-eclogite transition. Therefore, subduction processes would control the depth where the major east verging structure may merge with the slab. Such a ramp-flat structure is observed in other parts of the Chilean margin; hence, it seems to be a first-order feature in the Andean subduction zone. This structure delimitates upward the rocks, transmitting part of the plate convergence stress from the plate interface, and controls mountain-building tectonics, thus playing a key role in the Andean orogeny.

Improving seismotectonics and seismic hazard assessment along the San Ramón Fault at the eastern border of Santiago city, Chile

“…Contrarily, it is difficult to associate the observed local shallow seismicity under the optic of an east vergent-dominant lithospheric scale fault responsible for the structure of the west Andes Cordillera (Farías et al 2010). Therefore, to set earthquake rupture scenarios, we preferred the continental-scale West Andean Thrust (Armijo et al 2010), in which the San Ramón fault participates playing a major role in building the mountain front at the western border of the main Andes Cordillera of Central Chile, like a tectonic framework consistent with the seismological observations shown and discussed in this work.…”

“…The main Andes Cordillera in this region is mostly constituted by Mesozoic to Cenozoic volcanic and sedimentary rocks together with Cenozoic intrusives (Thiele 1980;Charrier et al 2002Charrier et al , 2005Farías et al 2010;Armijo et al 2010). The San Ramón Fault is an N-S fault system located at the eastern border of the city of Santiago at the foot of the mountain front associated with the continental-scale West Andean Thrust (Armijo et al 2010), where the San Ramón hill range reaches 3,249 m a.s.l.…”

“…Almost all the cities in the Chilean country have experienced a megaor large-thrust earthquake in the last century. In addition to that, seismic hazard is also associated with large inland intermediate-depth earthquakes like the M s 8.3 Chillán earthquake in 1939 (Campos and Kausel 1990), less recorded shallow crustal events which have occurred mainly along the main Andes Cordillera (M w 6.3 Las Melosas earthquake in 1958, Alvarado et al 2009;Sepulveda et al 2008;Legrand et al 2007; M w 6.3 Aroma earthquake in 2001, Farías et al 2010; M w 6.5 Curicó earthquake in 2004) and large magnitude aftershocks like those following the 2010 Maule earthquake which occurred near Pichilemu in March 11th, 2010 (Farías et al 2011;Ryder et al 2012). Reliable assessment and mitigation of seismic hazards along active tectonic zones like the Chilean subduction margin are therefore challenging problems with clear economic and societal implications.…”

Mapping a Part of Neuquén Basin in Argentina by Global-phase H/V Spectral Ratio

“…For the conversion to depth with equation (4), we use constant P-and S-wave velocity model for the sediments below the TN-and TE-array using Farías et al (2010) as a guide. We show the GloPHVestimated depths in Fig.…”

El terremoto de 1647 de Chile central como un evento intraplaca: ¿otra amenaza para Chile metropolitano?

“…Otras fallas corticales, que también podrían presentar una amenaza para las ciudades localizadas en la depresión intermedia de Chile central, han sido reconocidas en levantamientos geológicos recientes (Charrier et al, 2005;. Entre ellas destaca una posible extensión hacia el norte de la falla El Fierro (Comte et al, 2008;Pardo et al, 2008;Farías et al, 2010b). Una expresión de este tipo de sismicidad cortical andina se observó durante el terremoto de Las Melosas en 1958, cuyo epicentro estuvo solo a 60 km al sureste de Santiago (Barrientos, 2007).…”