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Complex Active Faulting Along the Liquiñe-Ofqui

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Complex Active Faulting Along the Liquiñe-Ofqui UNIVERSIDAD DE CHILE FACULTAD DE CIENCIA FÍSICAS Y MATEMÁTICAS DEPARTAMENTO DE GEOLOGÍA COMPLEX ACTIVE FAULTING ALONG THE LIQUIÑE-OFQUI FAULT SYSTEM: THE LINK BETWEEN CRUSTAL EARTHQUAKES AND POSTGLACIAL GEOMORPHOLOGICAL IMPRINTING IN THE PATAGONIAN ANDES TESIS PARA OPTAR AL GRADO DE DOCTOR EN CIENCIAS MENCIÓN GEOLOGÍA ANGELO ALEXANDER VILLALOBOS CLARAMUNT PROFESOR GUÍA: DR. GABRIEL EASTON VARGAS MIEMBROS DE LA COMISIÓN: DR. SERGIO RUIZ TAPIA DR. RODRIGO FERNÁNDEZ VASQUEZ DR. GREGORY DE PASCALE SANTIAGO DE CHILE 2021 RESUMEN DE LA TESIS PARA OPTAR AL GRADO DE: Doctor en Ciencias, Mención Geología POR: Angelo Alexander Villalobos Claramunt FECHA: 31/03/2021 PROF. GUÍA: Dr. Gabriel Easton Vargas FALLAMIENTO ACTIVO COMPLEJO EN EL SISTEMA DE FALLAS LIQUIÑE- OFQUI: VINCULO ENTRE TERREMOTOS CORTICALES Y LA IMPRONTA GEOMORFOLÓGICA POSGLACIAL EN LOS ANDES PATAGÓNICOS Esta tesis se enfoca en la comprensión del comportamiento tectónico activo del Sistema de Fallas Liquiñe-Ofqui (SFLO) tanto en la escala temporal de corto plazo como en la escala de largo plazo, durante el Cuaternario tardío, entre ~45° and ~46° S. El SFLO es un sistema de fallas intraarco de más de 1000 kilómetros de extensión, que recorre la Cordillera de Los Andes desde el centro-sur de Chile hasta el Itsmo de Ofqui frente al punto triple de la subducción de las placas tectónicas de Nazca y Antártica bajo la placa Sudamericana. Para abordar la naturaleza activa del SFLO durante tiempos post-glaciales se realizaron observaciones y toma de datos geofísicos marinos (perfiles de reflexión sísmica y batimetría), junto al levantamiento geomorfológico en el continente a lo largo de una transecta (~45,5° S) que considera tres dominios morfotectónicos en este sector de los Andes: fiordos, cordillera principal y pampas en la vertiente oriental de la cordillera. El último evento sísmico importante en el área corresponde a la Secuencia Sísmica de Aysén de 2007, la cual comenzó el 10 de enero de 2007 con un pequeño sismo (ML 3) a una profundidad entre 8 – 10 km, extendiéndose hasta febrero del siguiente año. Los sismos más importantes correspondieron a Mw 6.1, asociado a la Falla Quitralco, y el evento principal y más destructivo dentro de esta secuencia, de Mw 6.2, generado a lo largo de la Falla Punta Cola con ruptura del lecho marino, que gatilló inmensos deslizamientos de roca de gran volumen, de los cuales cuatro impactaron el lecho del Fiordo de Aysén, provocando destructivas olas de tsunami. Además de estas estructuras, en el área se encuentran otras fallas con potencial sismogénico como la Falla Río Cuervo. El estudio de registros similares a aquellos provocados durante el evento de 2007, permite proponer siete paleoterremotos previos de características similares, basado en el análisis paleosismológico fundamentado en el estudio de facies sísmicas y estimaciones de las tasas de sedimentación en el lecho marino del Fiordo de Aysén. Con el fin de estimar la edad máxima de rasgos geomorfológicos y paleosismológicos subaéreos y submarinos asociables a SFLO, se presenta por primera vez una transecta de registros geológicos del último retroceso glacial en la región de estudio (45,5° S) en los Andes Patagónicos. Se estudiaron varios sectores utilizando análisis morfoestratigrafico con geocronología de radiocarbono en depósitos paraglaciales, enfocados en determinar la evolución ambiental durante las glaciaciones cuaternarias y la posición de la capa de hielo durante este período. Los resultados se interpretan como un retroceso de casquete glacial que en esta región ocurrió sincrónicamente en ambos flancos de la cordillera principal de los Andes después de 12 ka. Se propone una estrecha relación entre tectónica y clima en la configuración geomorfológica pleistocena-holocena en los Andes Patagónicos, de modo tal que el retiro del casquete glacial luego de la Última Glaciación, habría generado un rebote glacio-isostático el cual provocó una reestructuración de la carga de esfuerzo estático a niveles de la corteza superior, favoreciendo la morfogénesis y los procesos sismogénicos en el SFLO, en particular. Sin embargo, esto sería un proceso secundario sobreimpuesto al proceso mayor según el cual la corteza inferior transfiere esfuerzo tectónico a la corteza superior, generando la transcurrencia dextral que caracteriza dominantemente a este sistema de fallas, tal como ocurrió en el caso de la secuencia sísmica del año 2007 en el Fiordo de Aysén. i PHD THESIS ABSTRACT: PhD in Geological Sciences BY: Angelo Alexander Villalobos Claramunt DATE: 31/03/2021 ADVISOR: Dr. Gabriel Easton Vargas COMPLEX ACTIVE FAULTING ALONG THE LIQUIÑE-OFQUI FAULT SYSTEM: THE LINK BETWEEN CRUSTAL EARTHQUAKES AND POSTGLACIAL GEOMORPHOLOGICAL IMPRINTING IN THE PATAGONIAN ANDES This thesis focuses on understanding the active tectonic behavior of the Liquiñe-Ofqui Fault System (LOFS) over both short-term and long-term scales between ~45° and ~46° S during postglacial times (Late Quaternary) in Chile. Field observations, acquisition of marine geophysical data (seismic reflection profiles and bathymetry) and a geomorphological survey were carried out along a transect (~45.5° S) that comprises three morphotectonic domains: the fjordland, the main Andes mountain range, and the pampas located east of the Andes. From a seismotectonic perspective the study region is situated immediately north of the Chilean triple junction, where the Nazca and Antarctic Plates subduct beneath the South American Plate. LOFS is an intra-arc fault system of more than 1,000 kilometers in length, which runs through the Cordillera de Los Andes from the Ofqui Isthmus towards the north along south-central Chile. The most recent seismic event in the area corresponds to the 2007 Aysén Seismic Sequence, which began on January 10, 2007, with a small earthquake (ML 3) at a depth between 8 - 10 km, extending until February of the following year. Major earthquakes corresponded to Mw 6.1, associated with Quitralco Fault, and the event considered the main and most destructive within this sequence, Mw 6.2, generated along with the Punta Cola Fault with a rupture of the seabed, which also triggered massive landslides of significant volume, of which four entered into the Aysén Fjord, causing destructive tsunami/displacement waves. Other faults with seismogenic potential are known, such as the Río Cuervo Fault, in addition to these structures. The study of records similar to those caused during the 2007 event allows proposing seven previous paleo-earthquakes with similar characteristics, based on the paleoseismological analysis supported by seismic facies features and sedimentation rates estimates. In order to temporarily constrain the maximum age of subaerial and submarine geomorphological and paleoseismological features associated to the fault system, we realized a geomorphological transect (45.5° S) along this area of the Patagonian Andes. Several sectors were studied using a morphostratigraphic approach, and radiocarbon-based geochronology on glacial drifts focused to determine the environmental evolution during Quaternary glaciations and the position of the ice sheet during the Last Glacial Maximum, to constrain the activity of the LOFS. The results show that icesheet retreat in this region occurred synchronously on both flanks of the main mountain range after 12 ka BP. A close relationship between tectonics and climate is proposed in the Pleistocene- Holocene geomorphological configuration in the Patagonian Andes. After the Last Ice Age, the retreat of the icesheet would have generated a glacio-isostatic rebound, which caused a restructuring of the static stress load at levels of the upper crust, favoring morphogenesis and seismogenic processes in the LOFS, in particular. However, this would be a secondary process superimposed on the broader process according to which the lower crust transfers tectonic stress to the upper crust, generating the dextral passage that dominates this fault system, as occurred in the case of the seismic sequence of the year 2007 in the Aysén Fjord. ii Agradecimientos Primero que todo, entrego mis formales agradecimientos a ANID (ex CONICYT) y su programa de Becas de Doctorado Nacional por el financiamiento de mis estudios de posgrado. También quisiera expresar mi más sincero agradecimiento y gratitud hacia todas las personas que me han apoyado a lo largo de mis años de estudio en el Programa de Doctorado en Ciencias mención Geología. Agradezco a mis profesores de la comisión, en especial a mi profesor guía, Dr. Gabriel Easton Vargas, por su confianza y continuo apoyo desde el primer día que llegué a la Universidad de Chile a finales del año 2013 a presentarle mis intensiones de continuar mis estudios en tectónica activa. Agradezco a los profesores Sergio Ruiz, Gregory De Pascale y Rodrigo Fernández por sus correcciones y observaciones que me entregaron diferentes perspectivas que han hecho este trabajo mucho más completo y satisfactorio a nivel personal. A todos mis colegas, coautores y a todos los miembros del Laboratorio de Tectónica Activa, en especial a José “Pepe” González por las incalculables y nutritivas horas de discusión (y verborrea) científica-filosófica que han sido una base fundamental para la construcción de nuevas ideas e hipótesis, y a Jean-Baptiste Ammirati por sus enseñanzas en los aspectos geofísicos y sismológicos. Incluyo además a los estudiantes de posgrado del periodo 2014 -2018 que sumado al intercambio de conocimiento interdiciplinario entregado durante esos años, su agradable compañía hizo
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