Abstract Structural Controls on Extensional

Home , Ischigualasto Formation, Los Colorados Formation, Los Rastros Formation

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

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 overlying Los Colorados Formation. As such, lithologic and stratigraphic descriptions of the three formations are presented below. The Los Rastros Formation consists of up to ~1000 m of mudstone, sandstone, conglomerate, and basalt (Martinez et al., 1994; Lopez et al., 1995; Alcober, 1996). An altered volcanic tuff near the base of the Los Rastros Formation yielded a sanidine single-crystal 40Ar/39Ar age of 233 +/- 0.5 m.y. (P. Renne, pers. Comm.., 2003). In addition, a K/Ar date from a basalt flow correlated with the upper Los Rastros Formation was dated at 229 +/- 5 ma (Odin et al., 1982; Alcober, pers. comm., 2001). These dates correspond well with fossil-plant assemblages from the Los Rastros Formation that indicate deposition during the Ladinian (234- 227 Ma) according to the time scale of Gradstein et al. (1995) (Bonaparte et al., 1966; Alcober, 1996; Martins-Neto et al., 2001). The Ischigualasto Formation consists of ~300-700 m of fluvial-alluvial mudstone, sandstone, and conglomerate. The Ischigualasto Formation consists primarily channel and overbank deposits of fluvial systems sourced in highlands to the west and southwest of Valle Fértil paleofault (Alcober, 1996; Currie et al., in review). The formation contains abundant paleosols as well as altered volcanic ash horizons (Tabor et al., in review). In this study, individual channel sandstones, paleosols, and ash deposits all served as marker beds from which vertical-fault displacements could be measured. The lower contact of the Ischigualasto Formation is placed at the base of the first pebble-cobble conglomerate/conglomeratic sandstone

3 or smectitic mudstone overlying carbonaceous sandstones and mudstones of the Los Rastros Formation. Abundant vertebrate fossils in the lower 2/3 of the Ischigualasto formation indicate a Carnian age of deposition (Rodgers et al., 1993; Alcober, 1996). Altered ash beds in the Ischigualasto Formation have provided additional chronostratigraphic control. Sanidine crystals from a bentonite sampled ~80m above the base of the formation yielded an 40Ar/39Ar date of 227.8 ± 0.3 Ma (Rogers et al., 1993), while plagioclase crystals from a bentonite ~70 m from the top of the formation in Section 1 yielded a date of 223.0 ± 0.7 Ma (Currie et al., in review).Collectively, these data support a Carnian age of deposition based on the time-scale of Gradstein et al. (2004). The Los Colorados Formation conformably overlies the Ischigualasto Formation and contains up to ~700 m of sandstone, mudstone, and evaporites that were deposited by ephemeral fluvial/lacustrine and eolian deposystems (López Gamundi et al., 1989; Milana and Alcobar, 1994). Paleocurrent data indicate sediment transport from the W-NW. Vertebrate fossils in the uppermost part of the Los Colorados Formation indicate a Norian age of deposition (O. Alcober, personal communication, 2004). The contact between the Ischigualasto and the Los Colorados formations is gradational with dominantly gray-colored Ischigualasto rocks vertically transitioning into red-colored Los Colorados lithologies (Alcober, 1996). For the purpose of this study, the contact of the two formations is placed at the top of the upper-most variegated mudstone underlying dominantly red-colored sandstones and mudstones in the transition interval.

4 A

Figure 1. (A) Map of the southern Ischigualasto basin including locations of measured stratigraphic sections. Inset of map shows the location of the study area within Argentina (Modified from Currie et al., (in review). (B) Diagrammatic cross-section of the Ischigualasto basin.

5 Figure 2. Generalized isopach maps of the Triassic Basins surrounding the Ischigualasto basin. Dashed lines represent sutures between Paleozoic accreted terranes (Modified from Ramos and Kay, 1991).

Figure 3. Reconstructed distribution of Triassic Basins in southern South America and Africa. Extensional basins lie exclusively to the west of the suture between Paleozoic terranes and the Gondwanan craton (Modified from Ramos, 1988).

Figure 4. Chronostratigraphic chart of Ischigualasto basin strata. Stratigraphic position of constraining dates are noted with A = radiometric age date of intercalated ash/basalt, V = vertebrate fossils. (From Currie et. al., in review.)

The Ischigualasto Formation exhibits dramatic changes in thickness and lithologic variability from east to west across the study area. Three detailed sections across the basin were previously measured documenting these changes (Fig. 1, Fig. 5) (Currie et al., in review). In section 1, the Ischigualasto Formation is ~ 690 m thick, dominated by fluvial-channel sandstones, particularly in the upper half of the formation. Between section 1 and 2, the formation thins to ~ 415 m, a change in stratigraphic thickness of ~ 275 m (Fig. 5). Further to the east in Section 3, the Ischigualasto Formation is roughly the same thickness (~ 400 m) as in Section 2. In addition to the pronounced change in stratal thickness between sections 1 and 2, the alluvial architecture of the Ischigualasto Formation also greatly varies between the two locations. For example, Section 1 contains a larger percentage of fluvial-channel sandstones than those areas to the southeast (Table 1). Also, thickness-normalized percentage of both distal overbank deposits and paleosols increases in areas where the Ischigualasto Formation is thin.

Table 1. Relative abundances of Ischigualasto Fm. architectural elements. Section 1 Section 2 Section 3 Thickness 691 413 397 % Channel Deposits 30% 24% 21% % Proximal Overbank Deposits 23% 23% 24% % Distal Overbank Deposits 47% 53% 55% % Paleosols 23% 29% 38%

Both the observed formation thickness and alluvial-architecture variations across the study area may be related to syndepositional fault activity including displacement variations along basin-bounding fault systems; displacement along synthetic/antithetic intrabasin faults; and folding associated with terminations of intrabasin fault zones (Walsh and Waterson, 1988; Schlische et al., 1993, 1995; Janecke et al., 1998; Gupta et al., 2000). The structural mapping and kinematic analysis described below were performed in order to determine which of these

9 potential structural influences contributed to the thickness and facies variations observed in the Ischigualasto Formation.

METHODS Structural mapping for this study was conducted in the area between measured Sections 1 and 2 where the thickness variations in the Ischigualasto Formation are most pronounced (Fig. 1). The mapping area is approximately 25 km2, elongated in the direction parallel to the NW-SE trend of the basin (Fig. 6). Structural mapping was performed to document the nature and location of faults, folds, and other structural features within the formation. Field investigations took place during July and August, 2004.Prior to this investigation no base map was available for use. Aerial photographs were the only high-resolution imagery available. In order to allow GPS confirmation of our spatial location in the field, it was necessary to embed a digital UTM grid on air photos through the process of georectification. This was accomplished by assigning a digital UTM coordinate to individual pixels within the aerial photograph. ERDAS imagine software was used to add a digital reference grid to the scanned aerial photograph. Using this software the aerial photograph was rectified to a SPOT image using panchromatic band 8 with accuracy of ± 5m. This georectified image was then imported into ArcView 9.0 and a UTM grid was overlain on the photo. In order to be useful in the field, the photo was then sectioned and enlarged using Adobe Photoshop to a 1:12,000 scale. The overall positioning accuracy of the map is ± 15 m.

Figure 5. (A) Logs of Ischigualasto Formation measured sections in the study area (B) paleocurrent orientations of trough-cross stratified channel sandstones in each section. (Modified from Currie et. al., in review)

Figure 6. Structural map of the study area.

12 DEFORMATIONAL FEATURES In order to evaluate the processes affecting syndepositional accommodation development within the basin, all intrabasinal features within the mapping area were documented. Structural components present in the basin included normal faults, reverse faults, fault-displacement folds and fault-bend folds. Though all of these features are present, the relative affect on accommodation development within the basin varies significantly between them. Below is a summary of the types of features documented by this study.

Normal Faults Normal faults are the most common deformation feature observed in the study area. Normal faults dip between 54°-85° and exist as individual faults with narrow fault zones (Fig. 7), or as multiple, variably-dipping faults defining a zone of deformation up to 15 m wide (Fig. 8). Normal faults have traces that are parallel, oblique, and normal to the overall trend of the basin (~330°). Two major fault sub-sets identified are approximately north-south striking and east-west striking. Cross-cutting relationships between sets indicate that they were most likely coeval. In some cases individual faults range in length from < 50 m to > 3 km. Fault displacement is primarily dip-slip, with observed slickenline rakes > 70° (Figure 9). Normal-sense drag folding is a common feature in both hanging wall and footwall strata. Based on the measured vertical offset of marker beds (channel sandstones, paleosols, bentonite layers) identified in both the hanging wall and footwall, fault throws range between < 1 m to >100 m. Fault throw, however, varies significantly along strike, with maximum vertical displacements commonly observed near the mid-point of individual fault segments.

Folds Folds are less commonly observed than normal faults. Folds are generally low amplitude anticlines and synclines that are closely associated spatially with mapped normal faults. Though folding is normally associated with compressional regimes, many mechanisms exist to develop folds in extension environments (Janecke, et al. 1998). The most common type of folding observed in the mapping region are rollover or extensional fault-bend folds with a component of fault drag near the fault surface (Fig. 10). One example of this type of fold is located in the central part of the study area (Fig. 11). In this region, two low amplitude folds are present in

13 conjunction with the same east-dipping normal fault. A small scale syncline lies directly next to the associated normal fault, most likely the result of fault drag. A broad-open anticline is also associated with this same fault. Though no subsurface data of this area exists it is possible that the anticline represents a fault-bend fold, formed as the associated normal-fault becomes listric at depth.

Thrust-Reverse Faults Intrabasinal thrust and reverse faults are restricted to a small area in the northern part of the study area (Fig. 12). In this location, a series of northwest-striking (~330°), west-dipping reverse and thrust faults (dips 25-50°) deform the upper Ischigualasto Formation and lower Los Colorados Formation. Based on map patterns and observed field relationships, these faults are imbricates (Fig.13). In places, beds between imbricates are steeply dipping to overturned. Maximum vertical displacement (~ 50 m) occurs on the frontal imbricate. Given that these reverse faults have strikes that parallel the ~ 330° trend of the Valle Fertíl fault to the west, they are most likely associated with Pleistocene-Recent Sierras Pampeanas contractional deformation.

Figure 7. Photograph of a north-south oriented normal fault within the Ischigualasto Formation. This fault has approximately 5 m of throw.

Figure 8. Fault zone consisting of two variably-dipping east-west striking normal faults within the Ischigualasto Formation. Deformation zone is ~ 5 m wide.

Figure 9. Typical set of slickenlines with near vertical rake orientations indicating dip-slip fault displacement.

Figure 10. Schematic drawing of fault-related fold mechanisms. The diagram shows fault-bend and drag folds associated with listric normal-fault displacement. (Modified from Janecke, 1998).

Figure 11. Area of folding located in central portion of the basin. Hanging wall syncline produced by fault drag and interpreted fault bend anticline possibly associated with a change in fault dip at depth.

Figure 12. Enlargement of area affected by thrust-reverse faulting.

Figure 13. Interpreted photograph of imbricated thrust faults in the northern part of the study area.

21 STRUCTURAL SYNTHESIS As discussed above, many types of structural features exist in the Ischigualasto basin. The importance of each type of feature must be evaluated in order to determine which, if any, played a controlling factor in the thickness changes observed in the Ischigualasto Formation in the study area. Intrabasinal reverse faulting is only locally present, and interpreted as being associated with the recent phase of contractional deformation and basin inversion. Normal- fault related folding has only been observed in a small portion of the mapping area, and also at a much smaller scale than needed to explain the observed changes in formation thickness. Normal faults are the only pervasive structural feature observed throughout the mapping area. Intrabasinal faulting has the potential to produce dramatic thickness variations in extensional basin strata (Schlische, 1991; Gawthorpe and Leeder et al., 2000; Cowie et al., 2000). However, if displacement on the normal faults observed in the study area produced the observed along- strike thickness variations in the Ischigualasto Formation, the orientation and displacement on observed faults must be in accord with observed stratal thickness changes. In order to evaluate these relationships, fault-orientation and displacement data from the study area are presented below.

NORMAL FAULT ORIENTATION AND DISPLACEMENT

Although there is considerable variation in normal-fault distribution and displacement in the study area, faults can be separated into two general groups based on the overall orientation. These groups consist of an approximately east-west striking group of faults and a north-south striking group (Fig. 6, Fig. 14). Overall fault orientation was determined by averaging the strike of faults over the entire length of a continuous fault segment. Sense of slip on individual faults was determined using the orientation of fault-surface slickenlines and drag folds. Fault throw was determined by the measured stratigraphic offset of individual marker beds such as ash layers, laterally traceable channel sandstones and paleosols. Kinematic analysis was performed using Allmendinger’s FaultKin program. Figure 15a shows a graph of the distribution of mapped fault azimuths. The majority of faults (76%) strike approximately east-west (between 050°-115°) while north-south (310°-049°) striking faults make up only 24% or the observed faults (Table 2). The north-south-striking

Figure 14. Lower hemisphere stereographic projections of (a) east-striking (b) north-striking normal faults. Orientations of greatest (1) and least principle stresses (3) are noted.

Figure 15. (A.) Histogram of frequency of fault azimuths. (B.) Histogram of total displacements at various azimuths.

24 faults, however, comprise almost all of the observed fault displacement within the study area (Figure 15b). East-west striking faults have average displacements of 3 m (Table 2), whereas the average fault displacement on the north-trending faults is ~18 m. Approximately 70 % of the total recorded displacement in the study area, however is recorded on only three of north-striking faults (Table 2). Studies of ancient normal fault systems show a positive correlation between fault length and maximum displacement. This relationship predicts that faults increase in length and cumulative displacement with increased extension (Walsh and Watterson, 1988; Gillespie et al., 1992; Schlische et al., 1996). This premise may be used to evaluate the relative maturity of fault sets examined in this study. North-striking faults in general exhibit much longer traces than the east-striking faults, with four traces over 5 km in length (Table 2). These faults also have much larger displacements. East-west faults have much shorter traces, with an average length of only 750 m, and with comparatively much smaller amounts of displacement. Comparison of the two fault sets indicates that north-striking faults experienced more extension than east-striking faults. North-oriented fault traces display more curvature indicating their growth was at least partly the result of fault linkage (Schlische et al., 1996; Clifton et al., 2000). Fault linkage could also be the cause of changes in dip angles and displacement variation along strike. Though some east-west fault traces also display some curvature, the majority of faults in this group have straighter traces. These characteristics also indicate a greater degree of extension for the north-south oriented faults than for the east-west oriented faults.

Table 2. Normal Fault Orientations, Displacements, Fault Lengths Total Max Min Ave. N (D) (D) (D) (D) Max (L) Min (L) Ave. (L) σ3 North South 19 336 100 1 18 6 km 500m 2 km 093° East West 61 184.5 10 0.5 3 6 km 50 m 750 m 357°

There is also an inverse relationship between overall abundance of faults with a given orientation and mapped fault length (Table 2). The abundance reflects the maturity of these two fault sets in that the shorter, less-developed east-west striking segments were not given the opportunity to link, increasing their relative abundance to the longer north-south, through-going

25 faults. The more mature north-trending group of faults indicates roughly east-west extension (σ3 = 093°) while the less developed, east-striking group, indicates north-south directed extension

(σ3 = 357°). Currently, we do not have any definite proof that the observed faults in the study area were active during Ischigualasto Formation deposition. However, paleocurrent orientations derived from Ischigualasto fluvial-channel sandstones (Fig. 6b) are similar to north-trending group of faults, suggesting a possible fault control on paleoflow direction.

CONTROLS ON INTRABASIN-FAULT ORIENTATION The orientation of intrabasinal faults can be closely related to the orientation of basin- bounding normal faults and pre-existing anisotropies in the depositional basement (Morley et al., 1990; Schlische et al., 1991, Destro et al., 2002). Figure 16 is a LANDSAT image showing the location of both the mapping area and general orientations of the Valle Fertíl and Alto faults. The Ischigualasto basin is bordered to the west by the Valle Fertíl fault, which strikes ~ 330°. The Valle Fertíl fault has been previously interpreted as the main basin-bounding fault for the Ischigualasto basin (Lopez, 1996; Milana & Alcober, 1994). The Alto fault is located to the south and east of the basin and displays a variable orientation. Along most of its length adjacent to the study area, the Alto fault strikes north-northeast. North of the study area, the buried trace of the Alto Fault swings to NW where it is roughly parallel to the Valle Fertíl fault. Potential anisotropies in the area are the product fabrics in the underlying Proterozoic- Paleozoic crystalline basement. Lineaments in the basement of the northern Sierras Valle Fertíl are highlighted in yellow on Figure 16. These traces are parallel to the strike of metamorphic foliation planes, faults cross-cut by Paleozoic intrusive rocks, and Paleozoic pegmatite/diabase dikes that have been documented in the region (SGMA, 1995), and thus represent zones of pre- existing weakness that may have influenced normal-fault geometry during Triassic regional extension. One conspicuous feature of the intrabasinal faults in the Ischigualasto basin is the orientation of the majority of the mapped faults are ~30°-60° from the orientation of the interpreted basin-bounding Valle Fertíl fault. While the observed fault orientations are not expected in rift basins that have undergone purely orthogonal extension (Clifton, 2002), a large range in normal-fault azimuths can be explained if basins 1) have experienced normal-sense

Figure 16. LANDSAT image of the southern part of the Ischigualasto basin and the northern Sierra Valle Fertíl showing the location of the Valle Fertil and Alto faults, as well as interpreted basement fabrics (yellow lines). Mapping area outlined in white. reactivation of preexisting basement structures (Lezzar et al, 2002); 2) have undergone oblique 27 extension (Clifton and Schlische, 2001); 3) have experienced temporal changes in regional- extension direction (Morely, 1999; Russell and Snelson, 1994); or 4) contain along-strike variations in basin-bounding fault displacement (Destro et al., 2003). Each of these potential influences on intrabasin-fault orientation, as they apply to the study area, are discussed below in more detail.

Reactivation of Basement Structures It has been shown that pre-existing fabrics underlying rift basins also play an integral role in controlling the orientation and development of intrabasinal faults. Often, preexisting structures result in a divergence in orientation of intrabasinal faults from the trend of the boundary-fault systems. For example intrabasinal dip-slip dominated faults in continental rifts are often oriented at angle to the overall trend of basin-bounding fault systems, and are interpreted to be the result of reactivation of pre-existing basement structural trends (Morley et al., 1987; Russell and Snelson, 1994; Lezzar et al., 2002). Within the Ischigualasto basin the north-striking fault group are oriented parallel to slightly oblique to the overall trend of the basin-bounding Valle Fertíl fault (330°). The trend of these faults is similar to the NW-NE striking basement structures mapped in northern Sierras Valle Fertíl. Given the preexisting similarities between the two trends, the north-striking faults in the study area may have been controlled by the normal-sense reactivation of pre-existing basement structures.

Changes in regional-extension direction The existence of two distinct fault groups also presents the possibility of change in the direction of regional extension through time. Changes in regional extension direction would have produced distinct fault families with differing orientations. However, the relative timing of development of each fault family could potentially be discerned based on cross-cutting relationships along intersecting traces. Analysis of fault intersections in the study area, however, indicates displacement on both the north- and east-striking fault groups were coeval. Without a separation in timing of the two fault groups it is unlikely that a reorientation in the direction of regional extension resulted in the two observed fault sets.

28 Oblique Extension The fault patterns observed within the Ischigualasto basin cannot be explained by the excepted model of orthogonal rifting due to presence of many faults oriented oblique to the basin-bounding fault system. Figure 17 presents a model of oblique-extension derived from clay modeling experiments of Clifton (2000). Models of oblique extension were used by Clifton (2000) to show how fault orientations vary as a function of the acute angle between the rift trend and the displacement direction, α (Fig. 17). This model predicts that as rift obliquity, α, decreases variation in fault azimuth increases. Comparison between fault orientation patterns and faults mapped in the Ischigualasto basin indicates a possible α value of 30°. The models presented by Clifton, however, indicate an abrupt change in style of faulting between α = 30° and α = 45°. Experimental results show that at α = 45° faults are predominantly dip-slip in nature. At α = 30° faulting begins to incorporate a significant oblique slip component. As all observed kinematic indicators in the study area suggest dip-slip fault displacement, oblique extension is therefore an improbable cause of the observed fault orientations in the study area.

Along-Strike Variations in Basin-Bounding Fault Displacement Release faults form in the hanging-walls of large-scale basin-bounding faults to relieve the stresses associated with differential displacement along the fault trace (Destro, 2003). Along- strike displacement on the boundary fault commonly increases such that the largest amount of accommodation forms at the center of the fault segment (Fig. 18a). Varying amounts of displacement along strike results in bending of the hanging-wall. Stress associated with the bending, manifests as normal faults oriented perpendicular to the orientation of the basin- bounding fault (Fig. 18b) (Destro et al., 2003). In the study area, the mapped east-striking group of faults is roughly perpendicular to the trend of the basin-bounding Valle Fertíl fault. It is possible that these shorter, less developed fault segments represent releasing faults associated with differential normal-sense displacement along the Valle Fertíl fault during the Triassic. This interpretation offers an explanation of the existence of two perpendicularly oriented extension directions in faults that cross-cutting relationships indicate were coeval.

Figure 17. (A) Model of oblique faulting in relation to observed fault patterns in the Ischigualasto basin (B) (Modified from Clifton, 2000).

Figure 18 (A). Schematic diagram of half-graben geometry due to a displacement gradient along a basin-bounding fault (Modified from Schlische 2003). (B) Schematic diagram of intrabasinal release faults generated by along-strike changes in basin-bounding fault displacement showing perpendicular σ3 (modified from Destro et al., 2003).

STRUCTURAL CONTROLS ON ISCHIGUALASTO FORMATION THICKNESS VARIATIONS In the previous section, the different potential causes of intrabasinal normal faulting were discussed. With the information available, faulting has been attributed to normal-sense reactivation of pre-existing basement structures as well as development of release faults associated with an along-strike increase in displacement on the basin-bounding Valle Fertíl fault. While it is uncertain if the mapped fault zones were active during Ischigualasto Formation deposition, by evaluating the overall displacement of faults in the study area, the potential contribution of intrabasinal faults to the stratigraphic-thickness changes observed in the Ischigualasto Formation can be determined. In order to accomplish this task, the total maximum displacements for all faults in the area of observed Ischigualasto Formation thickening were calculated. To account for conjugate faults in overall fault-related accommodation generation between the two measured sections, faults that display a down-to-the-west or -north dip-slip were combined into one group while those that display a down-to-the-east or -south displacement were combined into another. The difference in total displacement between these two sets is the potential along-strike fault-related accommodation. These calculations show that there is only 1 m of net positive displacement produced by intrabasinal faults in the study area. As such, the faults themselves cannot account for the observed ~300 m thickness increase in the Ischigualasto Formation between Section 1 and Section 2.

Table 3. Cumulative Displacement North and West (positive accommodation) 263 m South and East (negative accommodation) 262 m

Cumulative Displacement +1 m

Although the cumulative displacement on observed intrabasinal structural features cannot account for the observed changes in Ischigualasto Formation stratigraphic thickness, the along- strike displacement variations associated with the Valle Fertíl boundary-fault system may be responsible for the observed thickness changes (Fig. 18). The presence of release faults in the map area strongly suggests that this is a possibility. Fault growth models (Walsh & Watterson,

32 1988; Gillespie et al., 1992; Schlische, 1991) show that the along-strike fault displacement gradient affects basin accommodation development both perpendicular and parallel to the trace of the boundary fault. The resulting geometry is that of a trough with an increase in thickness of synrift strata approaching and towards the center of the boundary fault (Anders and Schlische, 1994; Withjack et al., 2002; Schlische, 1991). Release faults, oriented perpendicular to the trend of the basin-bounding fault, produce relatively small amounts of displacement, many with a conjugate geometry, resulting in negligible changes in accommodation . Figure 18 shows a hypothetical cross section through the basin at the time of deposition. This shows that the displacement gradient would be producing an increase in stratal thickness towards the center of the basin-bounding fault. In the case of the Ischigualasto Formation, the observed thickness increase between Section 1 and Section 2 would represent an along-strike increase in displacement on the Valle Fertíl fault to the northwest. Given the ~275 m thickness increase in the Ischigualasto Formation and the ~7 km distance between Sections 2 and Section 1, the along-strike dip of the basal contact of the formation, given the model, would have been < 2.25 °. In addition, since the present-day exposures of the Ischigualasto Formation are ~ 10 km from the basin-bounding fault, the observed formation thickness change of ~275 m is most likely a small percentage of the actual fault displacement increase.

33 CONCLUSIONS The goal of this study was to document and evaluate the structural controls on the development of the stratigraphic architecture of the Ischigualasto Formation. Detailed structural mapping documented the presence of many types of intrabasinal structures. These features were evaluated in terms of their contribution to accommodation development within the basin. Reverse faulting was shown to be only locally present and interpreted to be the result of a recent phase of contractional deformation. Folding was shown to be associated with normal faulting and present in a small portion of the mapping area. The only structure mapped consistently within the study area, with the potential to result in significant increases in stratigraphic thickness, were normal faults. Evaluation of these characteristics showed that intrabasinal normal faults display significant variations in fault azimuth, displacement and length. Structural analysis of intrabasinal faults revealed the presence of two kinematically distinct fault groups with nearly perpendicular directions of extension (σ3) The orientation of intrabasinal faults can be closely related to the orientation of basin-bounding normal faults and pre-existing basement anisotropies. The results of the kinematic analysis were placed in the context of models possible controls of intrabasinal fault orientation including 1) normal sense reactivation of basement structures; 2) oblique extension; 3) temporal changes in regional-extension direction; and 4) along strike variation in basin-bounding fault displacement. Through the use of these models it was determined that the north-striking group most likely represents the reactivation of structures in the underlying crystalline basement rocks. The east-trending family of faults were interpreted as release faults that formed as the result of a displacement gradient along the basin-bounding paleo-Valle Fertíl fault. Though intrabasinal faults were shown to result in negligible amounts of potential accommodation development within the basin, their existence signaled the presence of a displacement gradient along the basin-bounding fault system. This displacement gradient was shown to be the most likely cause of the observed increase in Stratigraphic thickness of the Ischigualasto Formation.

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