Pervasive cracking of the northern Chilean Coastal Cordillera:: New evidence for forearc extension

Despite convergence across the strongly coupled seismogenic interface between the South American and Nazca plates, the dominant neotectonic signature in the forearc of northern Chile is arc-normal extension. We have used 1 m resolution IKONOS satellite imagery to map nearly 37,000 cracks over an area of 500 km(2) near the Salar Grande (21 S). These features, which are best preserved in a ubiquitous gypcrete surface layer, have both nontectonic and tectonic origins. However, their strong preferred orientation perpendicular to the plate convergence vector suggests that the majority owe their formation to approximate east-west extension associated with plate boundary processes such as interseismic loading, coseismic and postseismic strain, and long-term instability resulting from subduction erosion. Similar structures were formed during or shortly after the 1995 M-w = 8.0 earthquake near the city of Antofagasta, south of Salar Grande, and in conjunction with the 2001 M-w = 8.2-8.4 Arequipa, Peru, event. Cracks such as these may form in other forearcs but remain largely unexposed because of vegetative cover or marked fluvial erosion-factors that are absent in northern Chile as a result of its hyperarid climate.