Abstract
The Liquine-Ofqui fault zone (LOFZ) in the Patagonian Andes is an active major transpressional intra-arc fault system along which Quaternary faulting and volcanism develop. Subaerial and submarine geomorphologic and structural characterization of latest Pleistocene-Holocene faults and monogenetic volcanoes allows us to assess geological cartography of active faults and the kinematic model for recent tectonics during postglacial times, since 12,000 cal. years BP. This allows increasing the basic geological knowledge necessary for determining the seismic hazard associated with cortical structures in the Aysen region in southern Chile. Fault cartography and field observations suggest dominant dextral-reverse strike slip along north-south and locally NNW-striking faults, dextral-normal strike slip along NE to NNE- striking faults, and sinistral strike slip along east-west faults. This kinematics is consistent with regional SW-NE shortening in the context of a major transpressional fault zone. Holocene and even historic monogenetic and sub-aquatic volcanism occurred in this tectonic setting in a close spatial relationship and probably favored by the activity and local architecture of faults. Submarine fault scarps and deformed sediments observed at the bottom of the Aysen Fjord were associated with the destructive April 2007 Mw6.2 earthquake located along the LOFZ. Our observations show that this earthquake occurred along dextral 15-20 km long N-S structure named Punta Cola Fault (PCF). This fault system is located some kilometres to the east of the main N-S Rio Cuervo Fault (RCF). Most of the epicentres of the seismic swarm during 2007 were located along or in between both structures. The study area is a transference zone between N-S regional branches of the LOFZ. The cartography of fault segments proposed here together with geophysical and geologic data suggest that large earthquakes Mw6.2-6.5 can be typically expected along most of the active faults. Besides, seismic hazard assessment should also consider the possibility of earthquake magnitude in the order of 7.1 along the main fault systems like the RCF.
Highlights
Earthquake size and occurrence in active fault systems result from regional and local stress field, fault kinematics and fault architecture, which is closely associated to structurally-controlled permeability and fluids (Scholz, 1982; Wells and Coppersmith,1994; Caine et al, 1996; Kim et al, 2004)
Along the Liquiñe-Ofqui fault zone in the Patagonian Andes, close to the subduction of the Nazca and Antarctic plates beneath the South American plate, active faulting and volcanism seem closely associated with tectonically driven fluid circulation through exhumed basement rocks along the magmatic arc, which result in seismic activity and large earthquakes (Cembrano et al, 2002; Lange et al, 2008; Cembrano and Lara, 2009; Legrand et al, 2011)
Through detailed surface mapping of postglacial geomorphologic features affected by faults, together with field observations of mesoscale outcrops of faults affecting rocks and sediments, we propose a cartography of active fault segments and a kinematic model for recent tectonics, providing new data for seismic hazard assessment in one of the most important urban and commercial areas in the Aysén region of southern Chile
Summary
Earthquake size and occurrence in active fault systems result from regional and local stress field, fault kinematics and fault architecture, which is closely associated to structurally-controlled permeability and fluids (Scholz, 1982; Wells and Coppersmith,1994; Caine et al, 1996; Kim et al, 2004). Along the Liquiñe-Ofqui fault zone in the Patagonian Andes, close to the subduction of the Nazca and Antarctic plates beneath the South American plate, active faulting and volcanism seem closely associated with tectonically driven fluid circulation through exhumed basement rocks along the magmatic arc, which result in seismic activity and large earthquakes (Cembrano et al, 2002; Lange et al, 2008; Cembrano and Lara, 2009; Legrand et al, 2011). We assess the cartography and kinematics of Late PleistoceneHolocene fault systems and spatially associated monogenetic volcanic centres in the Aysén Fjord area of southern Chile, with the aim to improve critic knowledge for seismic hazard assessment in the Patagonian Andes. The right lateral strike-slip, oblique slip, reverse slip ductile to brittle deformation characterizing a transpressional regime along this system has been active during the late Cenozoic (Cembrano et al, 1996, 2002; Lavenu and Cembrano, 1999; Arancibia et al, 1999; Thomson, 2002; Rosenau et al, 2006; Adriasola et al, 2006), associated to Vargas et al / Andean Geology 40 (1): 141-171, 2013
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