THE SURFACE/SUBSURFACE RELATIONSHIP BETWEEN DRAINAGE AND BURIED FAULTS AS OBSERVED IN THE ANDEAN FORELAND OF CENTRAL-WESTERN ARGENTINA Thesis Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By Peter Andreas Enderlin, B.S. Graduate Program in Geological Sciences The Ohio State University 2010 Master’s Examination Committee: Dr. Lindsay Schoenbohm, Advisor Dr. Ian Howat Dr. Lawrence Krissek Copyright by Peter Andreas Enderlin 2010 ABSTRACT The Andean foreland of central-western Argentina (30°-35°S) is characterized by the interaction of the east-vergent, thick-skinned Sierras Pampeanas and the west-vergent, thin-skinned Precordillera. Blind thrust faults are associated with the transition between these structural provinces, and large earthquakes have resulted from their interplay beneath the cities of Mendoza and San Juan. This study develops and applies a geomorphic approach to reveal buried tectonic features at both the scale of individual structures and the regional-scale. We interpret changes in bank heights and sinuosity from three rivers located between the Cerro Salinas and Montecito anticlines to suggest the existence of a third, buried structure. Inflections in elevation swaths indicate these three structures may be connected by the southern continuation of the Cerro Salinas thrust, which would tie the three to the Sierras Pampeanas structural province. Regional- scale drainage in the Andean foreland shows that neither current- nor paleorivers have flowed across the Alto del Desaguadero area. Inflections in W-E and N-S elevation swaths across this area suggest the influence of tectonic forcing, possibly due to a rising basement structure similar to the Sierra Pie de Palo to the north. ii ACKNOWLEDGEMENTS First and foremost I thank my advisor, Dr. Lindsay Schoenbohm, for her guidance, encouragement, flexibility, and for giving me this opportunity. I would also like to thank my committee members, Dr. Ian Howat and Dr. Lawrence Krissek, as well as Dr. Terry Wilson, for their feedback, contributions and time. Thanks to my collegues in Argentina for their assitance: Dr. Carlos Costa and Emillio Ahumada, of the Universidad Nacional de San Luis and Dr. Jorge Baron of CEDIAC. Also, a big thank you goes to Cesar Distante for his help in the field, his humor, and his friendship. Finally, a very special thanks to my family for all their support, and to Ellyn McFadden for keeping me smiling through it all. Funding provided by NSF Grant #EAR-60014033 and by AAPG Ohio Geological Society Named Grant. iii VITA January 18, 1983…………………………………………………Born – Staten Island, NY 2001……………………………………………………….Morristown-Beard High School September 2004……………………………………………..Poster, GSA Annual Meeting 2005………………………....…B.S. Geology/Environmental Science, Dickinson College Graduated with Departmental Honors 2005-2006……………………Geologist, Science Applications International Corporation 2006-2008……………………Geologist, Langan Engineering & Environmental Services 2008-2009……………………...Graduate Teaching Associate, The Ohio State University May 2009………………………………… AAPG Ohio Geological Society Named Grant December 2009…………...…………………………………….Poster, AGU Fall Meeting 2009-present…………………....Graduate Research Associate, The Ohio State University FIELDS OF STUDY Major Field: Geological Sciences iv TABLE OF CONTENTS Abstract……………………………………………………………………………………ii Acknowledgements………………………………………………………………………iii Vita………………………………………………………………………………………..iv List of Tables…………………………………………………………………………….vii List of Figures…………………………………………………………………………...viii Chapters: 1. Introduction and Background……………………………………………………..1 1.1 Purpose………………………………………………………………………...1 1.2 Tectonic Setting……………………………………………………………….3 1.3 Climate………………………………………………………………………...9 1.4 Rivers………………………………………………………………………...11 1.5 River Response to Uplift……………………………………………………..14 2. The Cerro Salinas Thrust Case Study……………………………………………18 2.1 Methods………………………………………………………………………22 2.2 Results………………………………………………………………………..25 2.3 Discussion……………………………………………………………………32 3. Regional-Scale Drainage Deflection……………………………………………..38 3.1 River Deflections…………………………………………………………….38 3.1.1 Methods…………………………………………………………….38 3.1.2 Results……………………………………………………………...42 3.1.3 Discussion………………………………………………………….50 3.2 Alto del Desaguadero………………………………………………………...52 3.2.1 Methods…………………………………………………………….52 3.2.2 Results……………………………………………………………...54 3.2.3 Discussion………………………………………………………….58 v 4. Conclusions………………………………………………………………………61 References………………………………………………………………………………..64 Appendix A: Drainage anomaly data…………………………………………………….69 vi LIST OF TABLES Table Page 2.1. Bank Heights of Rivers 1, 2, and 3………………………………………………….26 2.2. Sinuosity Indices of Rivers 1, 2, and 3……………………………………………...28 vii LIST OF FIGURES Figure Page 1.1. Global distribution of earthquakes caused by blind or buried thrust faults…………..2 1.2. Regional map of study area…………………………………………………………...4 1.3. Schematic drawing of flat slab and steep subduction regions in South America…….6 1.4. Cerro Salinas study area………………………………………………………………8 1.5. Distribution of annual precipitation of south-western South America……………...10 1.6. Current and paleodrainage of the study area………………………………………...12 1.7. Important drainage ditches and irrigation canals of Mendoza, circa 1761………….14 2.1. Interpretation of seismic profile 31017……………………………………………...20 2.2 Structural interpretation of the Cerro Salinas study area…………………………….21 2.3. Thrust-related tectonic topography curve…………………………………………...24 2.4. Bank height plot of Rivers 1 through 3……………………………………………...27 2.5. Sinuosity plot of Rivers 1 through 3………………………………………………...27 2.6. Elevation swath profiles across the projected Cerro Salinas thrust............................31 2.7. Schematic cross section inferred from seismic profile 31017………………………37 3.1. Example of HydroSHEDS stream-network vectors deviating from the ASTER imagery beyond the 2-km buffer……………………………………………………..40 viii 3.2. Tools used for stream deflection identification……………………………………..42 3.3. Trans-contour method identified anomalies, 5-km tool, 20-m contour interval, HydroSHEDS elevation model………………………………………………………44 3.4. Result comparison of different variables in a limited test area using the HydroSHEDS elevation model………………………………………………………45 3.5. Burrato-method identified anomalies, 5-km tool, 10-m contour interval, HydroSHEDS elevation model………………………………………………………47 3.6. Trans-contour method identified anomalies, 5-km tool, 20-m contour interval, ERSDAC elevation model…………………………………………………………..48 3.7. Burrato-method identified anomalies, 5-km tool, 10-m contour interval, ERSDAC elevation model………………………………………………………………………49 3.8. Alto del Desaguadero elevation swath and elevation inflection point locations……53 3.9. W-E elevation swath profiles across the Alto del Desaguadero…………………….55 3.10. N-S elevation swath profiles across the Alto del Desaguadero……………………57 ix CHAPTER 1: INTRODUCTION AND BACKGROUND 1.1 Purpose Blind thrust faults are faults that have not propagated to intersect the Earth’s surface. The growth of these faults has resulted in numerous moderate- to large-scale earthquakes throughout the world (Fig.1.1). The explicit danger with these earthquakes is that the buried faults are often not recognized until they rupture. Highly populated metropolitan areas, such as Los Angeles (pop. 10 million), that are situated in a tectonically active zone are constantly at risk of earthquakes. The most significant blind thrust earthquake (M 5.9) on record for Los Angeles was the Whittier Narrows earthquake, which occurred on October 1, 1987. This event resulted in 8 deaths, several hundred injuries, and over $360 million in property damages (Bilodeau et al., 2007; USGS website). The Andean foreland of central-western Argentina is among the most seismically active regions in the world (Siame et al., 2006). Underlying blind thrusts pose significant risk to the cities of Mendoza and San Juan as they are capable of generating large earthquakes. One of the largest on record, a magnitude 7.4 earthquake, occurred in 1944 under the city of San Juan, resulting in over 10,000 deaths (Siame et al., 2006). In 1861, a magnitude 7.0 earthquake practically destroyed the city of Mendoza (USGS website). Although 1 earthquakes cannot be prevented, monitoring stress in active faults can be used to indicate when an earthquake may occur. Figure 1.1 Locations of earthquakes caused by blind or buried thrust faults. Some dots represent more than one event. The area of interest is boxed in red. Modified from Lettis et al. (1997). Blind thrust faults are difficult to monitor because they must first be identified. The identification of these faults has traditionally been limited to seismic exploration and/or the recognition of anticlinal hills above the projected fault tip. However, even if there is no topographic expression, blind thrust faults can be identified geomorphically (e.g. Burrato et al., 2003). Deviations in drainage patterns, such as longitudinal and lateral changes in stream gradient (i.e. river diversions, shifts in sinuosity, and gradient changes), as well as overall changes in basin shape, have been associated with tectonics. 2 Areas of low-gradient, such as the Andean foreland, are especially sensitive to vertical deformation. Furthermore, the rivers in this region flow from the Andes Mountains across active blind thrust faults before reaching the foreland. This presents an ideal study location to refine our methods for recognizing