ABSTRACT STRUCTURAL CONTROLS ON EXTENSIONAL-BASIN DEVELOPMENT TRIASSIC ISCHIGUALASTO FORMATION, NW ARGENTINA By Kristin Guthrie This thesis reports the results of a structural and stratigraphic field study of the Ischigualasto Formation in the Ischigualasto Basin, NW Argentina. Previous stratigraphic investigations have documented an along-strike increase in Ischigualasto Formation thickness of ~275 m over a distance of 7 km. While this thickness change has been attributed to syndepositional faulting, the exact location and characteristics of controlling faults have never been documented. In order to determine the influence of fault-related accommodation development on thickness variations in the Ischigualasto Formation a detailed structural mapping project was conducted in the eastern part of the basin. Field mapping identified two groups of intrabasinal normal faults with near perpendicular strike azimuths. These fault orientations, and their calculated extension directions, are best explained by the development of basin-margin perpendicular release faults coupled with normal-sense reactivation of preexisting basement structures. Calculation of cumulative observed displacements along intrabasinal normal faults in the study area, however, revealed a negligible contribution to accommodation. The presence of interpreted release faults in the study area, however, suggests that observed changes in Ischigualasto Formation thickness were controlled by a northwest increase in basin- bounding fault displacement during the time of deposition. STRUCTURAL CONTROLS ON EXTENSIONAL-BASIN DEPOSITION, UPPER TRIASSIC ISCHIGUALASTO FORMATION, NORTHWESTERN ARGENTINA A Thesis Submitted to the Faculty of Miami University In partial fulfillment of The requirements for the degree of Master of Science Department of Geology By Kristin Guthrie Miami University Oxford, OH 2005 Advisor ______________________________________ (Dr. Brian Currie) Reader _______________________________________ (Dr. Yildirim Dilek) TABLE OF CONTENTS INTRODUCTION 1 GEOLOGIC SETTING 2 STRATIGRAPHY 3 STRATIGRAPHIC VARIATIONS 9 METHODS 10 DEFORMATIONAL FEATURES NORMAL FAULTS 13 FOLDS 13 REVERSE FAULTS 14 STRUCTURAL SYNTHESIS NORMAL FAULT ORIENTATION AND DISPLACEMENT 22 CONTROLS ON INTRABASIN-FAULT ORIENTATION 26 REACTIVATION OF BASEMENT STRUCTURES 28 CHANGES IN REGIONAL-EXTENSION DIRECTION 28 OBLIQUE EXTENSION 29 ALONG-STRIKE VARIATIONS IN BASIN-BOUNDING 29 FAULT DISPLACEMENT STRUCTURAL CONTROLS ON ISCHIGUALASTO BASIN THICKNESS 32 VARIATION CONCLUSIONS 34 ii LIST OF TABLES 1. RELATIVE ABUNDANCES OF ISCHIGUALASTO FORMATION 9 ARCHITECTURAL ELEMENTS 2. NORMAL FAULT ORIENTATIONS, DISPLACEMENTS, FAULT LENGTHS 25 3. CUMULATIVE DISPLACEMENT 32 iii LIST OF FIGURES 1. MAP OF THE ISCHIGUALASTO BASIN 5 2. LOCATION OF THE ISCHIGUALASTO BASIN IN REFERENCE TO 6 TERRANE BOUNDARIES 3. DISTRIBUTION OF TRIASSIC BASINS 7 4. CHRONOSTRATIGRAPHIC CHART 8 5. STRATIGRAPHIC SECTIONS AND PALEOCURRENT ANALYSIS 11 6. STRUCTURAL MAP OF THE ISCHIGUALASTO BASIN 12 7. PHOTOGRAPH OF NORMAL FAULT 15 8. PHOTOGRAPH OF FAULT ZONE 16 9. PHOTOGRAPH OF SLICKENLINES 17 10. MODELS OF FAULT RELATED FOLDING 18 11. PHOTOGRAPH OF FOLD AREA 19 12. ENLARGEMENT OF AREA OF REVERSE FAULTING 20 13. PHOTOGRAPH OF REVERSE FAULTING 21 14. LOWER HEMISPHERE STEREOGRAPHIC PROJECTIONS 23 15. CHART OF VARIATION IN FAULT AZIMUTH AND FREQUENCY 24 16 . LANDSAT IMAGE OF MAPPING AREA WITH BASEMENT TRENDS 27 17. OBLIQUE FAULT PATTERN 30 18. SCHEMATIC DRAWING OF HALF-GRABEN STRUCTURE AND 31 RELEASING FAULT iv INTRODUCTION The Ischigualasto basin of northwestern Argentina contains ~4 km of continental strata that was deposited in an extensional half-graben during Mesozoic time (Milana and Alcober, 1994). The Upper Triassic Ischigualasto Formation consists of > 700 m of fluvial/alluvial conglomerates, sandstones, and mudstones. Recent stratigraphic investigations of the Ischigualasto Formation have determined that it displays significant lateral changes in thickness. For example, in the southeastern part of the basin, the Ischigualasto Formation increases from in thickness ~400 m to > 700 m over a distance of ~8 km (Currie et al., 2001). Although this thickness change has been attributed to syndepositional faulting (Currie et al., in review), the location, geometry, and displacement of these proposed faults have never been documented. The stratal thickness variations observed in the Ischigualasto Formation are common manifestations of syndepositional faulting in extensional basins (Childs et al., 2003; Gupta et al., 1998; Schlische, 1991; Withjack et al., 2002; Morley et al., 1999 among others). Recent research into the controls on rift-basin development has shown that both basin-bounding and intrabasinal faults strongly control basin deposystems and resulting stratigraphic architecture (Schlische, 1991; Contreras et al., 1997; Gawthorpe et al, 1993; Gibson et al 1989; Gupta et al, 1998; Lambiase et al., 1995; Leeder et al., 1987; Morley et al., 1999; Schlische et al., 2003 among others). By combining stratigraphic observations with detailed structural mapping, it is possible to extrapolate the evolution of fault-controlled basin accommodation development and discern the kinematic history of extensional deformation in a region. In turn, these factors can be of further use in determining the driving forces behind observed variations in basin deposystems, stratigraphic and facies architecture, and paleopedological variations within continental-rift strata. The purpose of this investigation is to document the structural controls on stratigraphic- thickness variations observed in the Ischigualasto Formation through detailed field mapping. The study area is situated in the Agua de la Peña drainage, within the boundaries of the Ischigualasto Provincial Park, San Juan. In this area, the Triassic strata are completely exposed, allowing observed faults and folds to be mapped in detail. Stratigraphic data used in the investigation were derived from measured sections of the Ischigualasto Formation presented in Currie et al. (in review). 1 Observed structures and stratal-thickness variations are evaluated within the context of current models of fault-related accommodation development in rift basins (Walsh and Waterson, 1988; Schlische et al., 1993, 1995; Janecke et al., 1998; Gupta et al., 2000). The results of kinematic analyses are evaluated in terms of possible controlling mechanisms of fault-orientation including: 1) normal-sense reactivation of preexisting basement structures (Lezzar et al, 2002); 2) oblique extension (Clifton and 2001); 3) temporal changes in regional-extension direction (Morley et al., 1990); and 4) along-strike variations in basin-bounding fault displacement (Destro et al., 2003). Documented structural controls on changes in formation thickness provide a better understanding of how intrabasinal deformation influenced the stratigraphic architecture of the Ischigualasto Formation. GEOLOGIC SETTING The Ischigualasto basin is located near the border of San Juan and La Rioja provinces in northwestern Argentina (Fig. 1). The basin is situated along the boundary between the Precordilleran and Sierras Pampenas terranes which were accreted to the southwestern margin of Godwanaland during Paleozoic time (Fig. 2) (Ramos et al., 1988). The northwest-trending boundaries of these terranes were reactivated as crustal-scale normal faults during regional Mesozoic extension (Ramos and Kay, 1991; Lopez-Gamundi et al., 1994). This extension resulted in the development of a series of subparallel, en-echelon, north/northwest-trending basins (Uliana and Biddle et al., 1992) (Fig. 3). Rift-related deposition in the Ischigualasto Basin began during the Early Triassic, as normal displacement on the paleo-Valle Fértil fault led to the development of a structural half- graben (Milana and Alcober, 1994). Deposition in the basin continued throughout the Triassic and resulted in accumulation of up to 3.5 km of nonmarine and volcanic strata (Georgieff et al., 1992; Alcober, 1996). Quaternary shortening in the Andean foreland produced reverse- displacement reactivation of Mesozoic normal faults and structural inversion of Ischigualasto Basin strata (Zapata and Allmendinger, 1996). The rocks of the Ischigualasto basin are exposed in the hanging wall of the Valle Fertíl and Alto faults (Fig. 1), both of which are interpreted to be reverse-reactivated zones of Triassic normal faulting (Milana and Alcober, 1994; Zapata and Allmendinger, 1996). The paleo-Valle Fértil fault is interpreted as the main, basin-bounding normal fault separating Proterozoic-Paleozoic crystalline and sedimentary rocks of the footwall 2 from the Mesozoic sedimentary rocks of the hanging wall. The paleo-Alto Fault is interpreted as a E-NE dipping normal fault (Milana and Alcober, 1994) that merged with Valle Fertíl fault to the south (Lopez et al., 1996). STRATIGRAPHY Triassic rocks of the Ischigualasto basin reach a maximum thickness of ~3.5 km and consist of the Lower Triassic Tarjados and Talampaya formations, the Middle Triassic (Ladinian) Chañares, Ischichuca, and Los Rastros formations, and the Upper Triassic (Carnian- Norian) Ischigualasto and Los Colorados formations (Fig. 4) (Romer and Jensen, 1966; Alcober, 1996). All the stratigraphic units in the basin were deposited in continental environments (Milana and Alcober, 1996). While the focus of this project is the Ischigualasto Formation, mapping included documenting the contact between the underlying Los Rastros Formation and the