Forearc Deformation in the Central Andes: The Northern Terminus of the Atacama Fault Zone

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Allmendinger0087431The Atacama fault zone can be traced for nearly 1000 km in the forearc of the Central Andes. The northern terminus of the fault zone occurs just south of Iquique, Chile, in a series of complex blocks bounded by strike-slip, thrust, and normal faults. Digital topography produced from satellite radar imagery shows that this area is one of the most tectonically active parts of the entire fault system. The area overlies the general region where the Iquique ridge on the Nazca plate is being subducted beneath South America, and correlates well with a seismic gap in the Benioff zone that many seismologists believe will be the site of the next major earthquake on this plate boundary. We are dating these young faults using Ar/Ar and Uranium series isotopic methods, and are also studying their kinematics using measurements of slip directions on the well-exposed faults. These field data are being combined and analyzed in a map view block restoration program developed at the Universite de Rennes (France), which will allow us to quantify the strain and rotation in the region. Then, we can evaluate block rotation versus oroclinal bending hypotheses spurred by paleomagnetic data, as well as investigate the origin of proposed arc parallel compression. Finally, our fieldwork and digital elevation model provides a well-documented "before" picture to compare to the "after" picture to be obtained following the next great earthquake along this plate boundary, site of the largest earthquake on record. Thus, eventually we will be able to document how the forearc of a major ocean continent convergence system deforms during Benioff zone seismicity.