Drainage Analysis and Fluvial Terrace Reconstruction: Assessing Blind Thrust Hazards, Montecitos Anticline, Mendoza, Argentina

Blind thrust faults pose a problem for detailed tectonic and hazard assessment because their lack of surface rupture and sometimes subtle geomorphic expression makes traditional fault recognition and analysis difficult. It is critical to understand and prepare for earthquakes on these faults. The Andean Precordillera between 31° and 33° has experienced several devastating earthquakes in the last century, and geodetic studies indicate that this area has a long-term shortening rate of ~5mm/yr. One of the growing anticlines in this region that partially accommodates this shortening is the Montecito anticline, a fault propagation fold above a blind thrust fault. The Montecito anticline is located just north of Mendoza in the Andean Piedmont and is approximately 7 km long and 2 km wide. Drainage diversions, changes in river sinuosity and deformation of Holocene alluvial fan sediments all indicate that the anticline has been active during the Quaternary. We have used a combination of geomorphic techniques, including drainage analysis and fluvial terrace reconstruction, dating and GPS analysis to understand the geometry and development of the Montecito Anticline. By analyzing differential river profiles of the two transverse drainages we have been able to quantify localized uplift and examine changes in stream gradient across the structure. Stream diversion studies show the average regional flow direction, and the amount and direction of stream deviation from this average in the vicinity of the anticline. 11 river terraces formed along these drainages have been examined using kinematic GPS and ArcGIS for evidence of warping and rotation. Cosmogenic radionuclide dating of 5 of these terraces will constrain uplift rates along the axis. The combination of these techniques has yielded considerable information about the uplift and deformation history of this blind thrust fault. By analyzing these types of faults and continuing to quantify the seismic risk that they pose to nearby population centers we hope to contribute to a greater understanding of earthquake hazards in Mendoza area, as well as to refine geomorphic techniques by which blind thrust faults can be recognized and assessed in other locations.