Overlying the only part of the South American continental crust that is in direct contact with the subducting Nazca Plate, the Coastal Cordillera of northern Chile and southern Peru should provide the most complete geological record of the coupling between the two plates. This record of coupling is exquisitely preserved in the hyperarid Atacama Desert. This preservation is both one of the major advantages and major challenges of working in this region: On the positive side, exposure of geomorphic surfaces is complete and unencumbered by erosion or vegetation, and brittle saline soils preserve subtle deformation features that would quickly be obliterated in more humid environments. On the negative side, ancient geomorphic features are just as fresh as Recent one and the lack of organic material precludes radiocarbon dating, a traditional tool of paleoseismology. During the last several years, we have concentrated on documenting three fundamental characteristics of late Cenozoic forearc deformation: (1) NS shortening on reverse faults striking at a high angle to, and dextral-reverse faults striking oblique to, the continental margin; (2) N-striking normal faults of the forearc and their reactivation, locally, as reverse faults; and (3) extensive suites of tension cracks. Reverse faults striking at a high angle to the margin are present between 19 and 21.5°S, straddling the topographic symmetry plane that marks the axis of the Bolivian orocline. Limited dating of tuffs and surfaces shows that these structures have been active for at least the last 6 Ma. At least 5 of these structures -- Atajaña, Pisagua, Iquique north and south, and Barranco Alto -- cut the Pleistocene marine terraces of the coastal platform, producing 20 to 50 m of vertical offset. A forearc crustal earthquake just south of Pisagua in March 2007 demonstrates that margin parallel shortening continues to the present and that permanent deformation occurs during the interseismic part of the plate boundary seismic cycle. NS-striking normal faults are well known in the forearc thanks to spectacular exposure on the Mejillones Peninsula, where normal faults dip 45-70° E. Simple kinematic analysis suggests that these listric faults flatten at a few kilometers depth, well above the subduction zone, and thus do not directly facilitate subduction erosion. High-angle normal faults in the Atacama fault zone near Antofagasta and in the Salar Grande (Paposo, Salar del Carmen, Hombre Muerto) area have locally experienced minor reactivation as steep reverse faults, indicating that the fault zones are very weak and may experience movement during both interseismic and coseismic phases. Open tension cracks are widespread and well preserved due to the gypcrete crust that characterizes the Coastal Cordillera between 500 and 1200 m, but also penetrate well into bedrock. We have mapped more than 75,000 cracks, the majority due to tectonic mechanisms, on 1 m and 2.5 m resolution satellite imagery. Tectonic cracks can be subdivided into to groups: those related to bending on the crest of fault bend/propagation folds associated with the EW reverse faults, and those produced coseismically. We suggest, based on orientation and distribution, that this latter group may actually be used to define long-term segment boundaries of rupture zones of interplate earthquakes.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 8002 Continental neotectonics (8107);
- 8011 Kinematics of crustal and mantle deformation;
- 8104 Continental margins: convergent;
- 8123 Dynamics: seismotectonics