The importance of west verging structures at the western flank of the Andes, parallel to the subduction zone, appears currently minimized. This hampers our understanding of the Andes‐Altiplano, one of the most significant mountain belts on Earth. We analyze a key tectonic section of the Andes at latitude 33.5°S, where the belt is in an early stage of its evolution, with the aim of resolving the primary architecture of the orogen. We focus on the active fault propagation–fold system in the Andean cover behind the San Ramón Fault, which is critical for the seismic hazard in the city of Santiago and crucial to decipher the structure of the West Andean Thrust (WAT). The San Ramón Fault is a thrust ramp at the front of a basal detachment with average slip rate of ∼0.4 mm/yr. Young scarps at various scales imply plausible seismic events up to Mw 7.4. The WAT steps down eastward from the San Ramón Fault, crossing 12 km of Andean cover to root beneath the Frontal Cordillera basement anticline, a range ∼5 km high and >700 km long. We propose a first‐order tectonic model of the Andes involving an embryonic intracontinental subduction consistent with geological and geophysical observations. The stage of primary westward vergence with dominance of the WAT at 33.5°S is evolving into a doubly vergent configuration. A growth model for the WAT‐Altiplano similar to the Himalaya‐Tibet is deduced.Wesuggest that the intracontinental subduction at theWAT is amechanical substitute of a collision zone, rendering the Andean orogeny paradigm obsolete.Our work has been supported by the binational French‐Chilean ECOS‐Conicyt program (project C98U02), the French Agence Nationale pour la Recherche, Project Sub Chile (ANR‐05‐ CATT‐014), and the Chilean ICM project “Millennium Science Nucleus of Seismotectonics and Seismic Hazard,

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A strong (Mw = 8.1) subduction earthquake occurred on July 30, 1995 in Antofagasta (northern Chile). This is one of the largest events during this century in the region. It ruptured the southernmost portion of a seismic gap between 18°S and 25°S. In 1992 we had used GPS to survey a network with about 50 benchmarks covering a region nearly 500 km long (N‐S) and 200 km wide (E‐W). Part of these marks were re‐surveyed with GPS after the 1995 earthquake. Comparison with 1992 positions indicate relative horizontal displacement towards the trench reaching 0.7 m. The inland subsided several decimeters. The Mejillones Peninsula was uplifted by more than 15 cm. Teleseismic body‐wave modelling of VBB records gives a subduction focal mechanism and source time function with three distinct episodes of moment release and southward directivity. Modelling the displacement field using a dislocation with uniform slip in elastic half‐space suggests a rupture zone with 19°–24° eastward dip extending to a depth no greater than 50 km with N‐S length of 180 km and an average slip of about 5 m. The component of right‐lateral slip inferred both from the teleseismic and geodetic data does not require slip partitioning at the plate boundary. That the well‐constrained northern end of the 1995 rupture zone lies under the southern part of the Mejillones Peninsula increases the probability for a next rupture in the gap north of it.

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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...

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This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards nor with the North American Stratigraphic Code. Any use of trade names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Geological Survey.