Submarine earthquake rupture, active faulting and volcanism along the major Liquiñe-Ofqui Fault Zone and implications for seismic hazard assessment in the Patagonian Andes

Abstract

ABSTRACT. The Liquiñe-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 Aysén 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 Aysén 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 Río 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. RESUMEN.Ruptura sísmica submarina, tectónica y volcanismo activo a lo largo de la Falla Liquiñe-Ofqui e implicancias para el peligro sísmico en los Andes patagónicos. La Zona de Falla Liquiñe-Ofqui es un sistema de intraarco activo y paralelo al contacto entre las placas tectónicas de Nazca y Sudamérica, a lo largo del cual el fallamiento y el volcanismo cuaternarios están estrechamente asociados. A partir de observaciones geomorfológicas y estructurales subaéreas, la prospección subacuática de fallas con evidencia de actividad pleistocena tardía-holocena y la caracterización de volcanes monogenéticos, se propone una cartografía y un modelo cinemático para la tectónica reciente, con énfasis en los últimos 12.000 años, asociada a fallas activas en el área del Fiordo Aysén. Esto permite incrementar sustancialmente el conocimiento geológico básico necesario para la determinación del peligro sísmico asocia...

Holocene tephrochronology of the lower Río Cisnes valley, southern Chile

“…6; Table 1; Naranjo and Stern, 2004;De Porras et al, 2012. AMS radiocarbon ages were determined on organic matter and converted to calendar years before present (cal yrs BP) by applying the ShCal13 curve (Hogg et al, 2013) to the CALIB 7.0.4 program (Stuiver et al, 1998(Stuiver et al, , 2013. FIG.…”

(Un)Coupled thrust belt‐foreland deformation in the northern Patagonian Andes: New insights from the Esquel‐Gastre sector (41°30′–43°S)

“…North to the Chilean Triple Junction, the North Patagonian Batholith includes Middle Jurassic to Miocene rocks of calc‐alkaline composition [ Castro et al ., ; Pankhurst et al ., ] intruding the Upper Paleozoic basement. In this sector the Liquiñe‐Ofqui Fault Zone is a major transpressional intra‐arc fault system (Figure b) that accommodates the oblique convergence between the Nazca and South America plates and along which most of the Quaternary volcanism and faulting develop [ Hervé , ; Cembrano et al ., ; Rosenau et al ., ; Vargas Easton et al ., ]. This is an ~ 1000 km long strike‐slip fault characterized by dextral motion during the Cenozoic, when it was active within the arc roots in a thermally weakened crust [ Cembrano et al ., ; Lavenu and Cembrano , ; Thomson et al ., ], and by previous left‐lateral motion during the Mesozoic, when it favored the emplacement of the Patagonian Cordilleran Batholith [ Cembrano et al ., ; Castro et al ., ].…”

Tefrocronología en curso superior del valle del río Cisne (44°S), Chile Austral.

“…Melimoyu and Hudson are two of the largest volcanic edifices in the SSVZ (Völker et al, 2012). Also the many small monogenetic cones in the region have produced basaltic scoria deposits as well as lava flows (López-Escobar et al, 1995a;Gutiérrez et al, 2005;Watt et al, 2013;Vargas et al, 2013), but the potential regional extent of distribution of tephra from these generally small volume mafic eruptions is uncertain The previously published interpretations of the source volcanoes of tephra in the SSVZ were made in part on the basis of tephra major element chemistry compared with published whole rock chemistry of samples of lavas from the volcanoes of the SSVZ (Naranjo and Stern, 2004;Weller et al, 2014). In a similar fashion, the possible sources of seven of the tephra in the Lago Shaman core were made on the basis of bulk tephra trace-element chemistry compared to published whole-rock trace-element analysis of lava samples from the SSVZ volcanoes to the west of the core site (de Porras et al, 2012).…”