Collaborative Research: Forearc Extension in a Strongly Coupled Subduction System, Northern Chile

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Work on the extensional structures of the forearc of northern Chile is providing new insight into the nature of coupling between the subducted Nazca oceanic plate and the overriding South American continental plate. The nature of this coupling is not only of scientific interest but of tremendous practical importance as this plate boundary is responsible for some of the greatest earthquakes on Earth. Every 100 to 150 years a magnitude 8+ earthquake occurs in various segments. The area in which the research is being carried out last had a major earthquake 130 years ago. Although the plate boundary appears to be strongly locked between great earthquakes and is strongly elastically compressed during that time, the geology of the Coastal Cordillera, which overlies the zone of interplate coupling, displays only features associated with horizontal extension parallel to the plate convergence direction. This extension is manifest both as north-striking normal faults and as a spectacular suite of tension fractures which, due to the uniquely dry environment can be mapped quite completely. The extensional structures may be related to elastic rebound or visco-elastic relaxation during or following great earthquakes, to shaking during earthquakes, to subduction erosion at the plate boundary, or perhaps to an overly steep continental slope going down to the oceanic trench at 8 km depth just 75 km offshore. This project is assessing the relative contributions of these factors by combining: (1) field work with geochronology to construct kinematic models of normal faults to yield magnitude and age of extension as well as depth to detachment; (2) study of the origin, distribution, and magnitude of extension represented by the tension fractures; (3) differential interferometric side aperture radar to document vertical motions in the forearc, including, potentially, incipient coastal escarpment collapse and retreat; and (4) finite element modeling to evaluate the role of the topography of the Coastal Escarpment and the continental slope on the regional stress field.