PERVASIVE TENSION FRACTURING OF THE NORTHERN CHILEAN COASTAL CORDILLERA: NEW EVIDENCE FOR FOREARC EXTENSION

Strong coupling between the South American and Nazca plates results in elastic loading of the northern Chilean forearc and produces great thrust earthquakes every 100 to 150 years. Despite this compression, the dominant neotectonic signal demonstrated by normal faults and tension fractures is of arc-normal extension. We have mapped nearly 37,000 open cracks in the Salar Grande region (~21ºS) of the northern Chilean Coastal Cordillera on 1 m resolution IKONOS satellite imagery covering an area of 500 km². The fractures, which are best preserved in a ubiquitous gypcrete surface layer, have both non-tectonic and tectonic origins. Fluctuations in both moisture content and temperature cause bulk shrinking and swelling of the gypcrete crust, resulting in centimeter-scale polygonal fracturing of the surface. Locally, down slope movements of the surface layer have produced cracks trending parallel to the strike of the hill. However, the strong preferred orientation perpendicular to the direction of plate convergence and the regional distribution of the fractures suggest that the majority owe their formation to east-west extension associated with plate margin-scale tectonic processes. Similar fractures were formed during the 1995 M_W=8.0 earthquake near the city of Antofagasta, south of Salar Grande. We suggest that the tension fractures near Salar Grande form primarily during coseismic deformation and may be enhanced by interseismic processes such as subduction erosion. Such fractures may form in other forearcs similar to northern Chile but remain unexposed due to vegetative cover or significant fluvial erosion. The longstanding hyper-arid climate of northern Chile allows for the spectacular preservation of tension cracks, thereby providing evidence of margin-normal extension unaccounted for in other forearcs.