Extensional tectonics in a convergent margin setting: Deformation of the northern Chilean forearc
The Coastal Cordillera of the northern Chilean forearc overlies a portion of the plate boundary where great subduction zone earthquakes originate. Geological structures in this region exhibit deformation that has accumulated over millions of years as a result of the subduction earthquake cycle. These structures primarily demonstrate extension in the direction of convergence between the Nazca and South American plates. The details of the permanent strain field have significant implications for the long-term interaction between the forearc and subduction zone processes. In this thesis, I combine observations of faults and surface cracks, radar interferometric analyses of ground deformation, and numerical modeling of the stress, strain, and displacement fields produced in the forearc by the subduction zone earthquake cycle to provide new insight into this interaction. I present spatially-complete maps of meter-scale surface cracks throughout the forearc. Through analysis of crack strikes, morphological observations, and elastic dislocation modeling of stress fields generated by subduction zone earthquakes, I find that the distribution of cracks is consistent with formation by repeated interplate seismic events. Concentrations of stress along upper plate faults can explain the enhanced opening of cracking. Movement on the upper plate faults themselves also results from stresses related to the subduction earthquake cycle. Models presented in this thesis show consistency between the complicated deformation field expressed by the forearc structures and the varying stress fields associated with the subduction zone. Interseismic deformation encourages failure on normal faults, while strong underthrusting earthquakes are capable of pushing these faults toward either normal or reverse failure. Minor reverse motion superimposed on dominantly normal faults is described for several locations in the forearc. The dependency of the upper plate stress field on the distribution of subduction zone strain accumulation and release and the sense of forearc faulting have implications for fault mechanics. The style of permanent deformation observed in the forearc can be used to constrain the distribution of interseismic strain accumulation and coseismic slip. Additionally, because the upper plate faults respond to low-magnitude stress perturbations induced by the seismic cycle, the absolute level of stress on these structures must be very low.
forearc; subduction zone; earthquake cycle; Atacama Desert; normal faults; tension cracks
dissertation or thesis