Structure of the Alima and Associated Anticlines in the Foreland Basin of the Southern Atlas Mountains, Tunisia

Structure of the Alima and associated anticlines in the foreland basin of the southern Atlas Mountains, Tunisia P. Riley1, C. Gordon2, J.A. Simo3, B. Tikoff1, and M. Soussi4 1DEPARTMENT OF GEOSCIENCE, UNIVERSITY OF WISCONSIN–MADISON, 1215 W. DAYTON ST., MADISON, WISCONSIN 53706, USA 2DRAGON MINING SWEDEN AB, PAUTRÄSK 100, 921 31 STORUMAN, SWEDEN 3EXXONMOBIL UPSTREAM RESEARCH COMPANY, 3120 BUFFALO SPEEDWAY, HOUSTON, TEXAS 77098, USA 4DÉPARTEMENT DE GÉOLOGIE, FACULTÉ DES SCIENCES DE TUNIS, UNIVERSITÉ DE TUNIS EL MANAR, 2092 TUNIS, TUNISIA ABSTRACT Based on data presented in this study, the E-W–trending Alima anticline in the Metlaoui region of the southern Tunisian Atlas Mountains formed due to far-foreland, brittle deformation. The Alima anticline is one in a series of en echelon folds in the Atlas fold-and-thrust belt of North Africa. Geologic mapping indicates that the Alima anticline has a steep southern limb, a gently dipping northern limb, and per- vasive normal fault sets. Fracture orientations suggest that fracturing occurred early in the fold history as a synfolding process, not as a pre- or postdeformational process. Gravity data show positive Bouguer anomalies near fold crests, not the negative anomalies that would be expected if the anticline were salt cored. Seismic data, collected along lines in basins surrounding the Alima anticline, suggest the presence of several high-angle reverse faults. Based on surface and subsurface studies, we attribute the development of the Alima anticline to far-foreland deformation associated with late Cenozoic contraction. N-S–directed elongation in the Triassic reoriented to NW-SE–directed shortening in the Miocene, causing Triassic normal faults to be reactivated as oblique-slip reverse faults. A comparison of the Alima anticline to other anticlines in the region suggests that several different styles of folding are present, each representing a different time of initiation. LITHOSPHERE; v. 3; no. 1; p. 76–91. doi: 10.1130/L119.1 INTRODUCTION tion. Additionally, folds associated with fault reactivation may exhibit synchronous, local brittle extensional structures and particular joint pat- In the far-foreland of mountain belts, including the Sunda Shelf of terns (Unruh and Twiss, 1998; Cooper et al., 2006). Indonesia, the Alpine Mountains of Europe, the Algerian Atlas Moun- This study examines the E-W–trending Alima anticline to investigate tains, the southern North Sea, and the Rocky Mountains of the western folding mechanisms in the Metlaoui region of the southern Tunisian Atlas United States, fault reactivation is documented as a mechanism for fold Mountains. The Alima anticline is one in a series of en echelon folds in development (e.g., Ziegler, 1983; Letouzey et al., 1990; Wise and Obi, the distal Atlas fold-and-thrust belt of central Tunisia. Geologic map- 1992; Vially et al., 1994; Remmelts, 1995). Preexisting faults have long ping, fracture data, gravity data, and seismic data are utilized to establish been acknowledged as zones of weakness in crustal rocks during subse- whether far-foreland, brittle deformation associated with the Alpine orog- quent tectonic activity, and they are commonly favored for reactivation eny generated folds in the Metlaoui region. In addition, these data are used over new fault generation (e.g., Donath and Cranwell, 1981; Etheridge, to investigate whether salt diapirism or fault reactivation played a larger 1986; Holdsworth et al., 1997). Factors such as the orientation of the fault, role in the generation of the Alima anticline and similar folds in the south- the coeffi cient of sliding friction, and the state of stress are thought to ern Tunisian Atlas Mountains. Geophysical data are then used to establish control the conditions under which a fault is reactivated (Letouzey, 1990). episodes of folding since the Cretaceous. Examples of fault reactivation have been documented in physical experi- ments (e.g., Koopman et al., 1987; Buchanan and McClay, 1991; Sassi et TECTONIC SETTING al., 1993), in seismic data (Ziegler, 1983; Etheridge et al., 1985; Letouzey et al., 1990), and in the fi eld (e.g., Holdsworth et al., 1997). The Atlas Mountains extend 2000 km from the Atlantic margin of In order to study reactivated faults directly, they must breach the sur- Morocco to the Mediterranean coast of northern Tunisia (Fig. 1). The face and retain kinematic indicators from multiple episodes of activity or Tunisian Atlas Mountains are bounded to the north by the Tell-Rif chain, retain changes in deformation products such as cataclasites within brec- to the south by the Saharan Platform, and to the east by the Sahel block cias (White, 1986; Holdsworth et al., 1997). The presence of salt can (Fig. 1; Ben Ferjani et al., 1990; Burollet, 1991; Swezey, 1996; Hlaiem, further complicate the causal relationship between reactivated faults and 1999; Bouaziz et al., 2002). folding. The movement of salt through the subsurface can generate salt- In Tunisia, the Atlas Mountains began as a series of failed rift basins cored anticlines, which can be diffi cult to distinguish from folds formed associated with the breakup of Pangea and opening of the Neotethys from fault reactivation. Thus, it is necessary to gather evidence from from the Permian to the Early Cretaceous (Laville and Petit, 1984; stratigraphy (e.g., Etheridge, 1986; White, 1986), geometric similarity Winterer and Hinz, 1984; Andrieux et al., 1989; Stets, 1992; Grasso (e.g., Etheridge, 1986; Holdsworth et al., 1997), and geophysical data to et al., 1999; Dhahri and Boukadi, 2010). Although extension occurred deduce whether fault reactivation is a viable mechanism for fold forma- throughout this time, two main rifting events are recognized in the 76 For permission to copy, contact [email protected] | |Volume © 2011 3 Geological | Number Society1 | LITHOSPHERE of America Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/3/1/76/3044932/76.pdf by guest on 02 October 2021 Structure of the Alima and associated anticlines, southern Atlas Mountains | RESEARCH northern and central Tunisia from north to south. Metamorphism and Apennines N Pyrenees nappe emplacement occurred in the Tell-Rif Chain in northern Tunisia, Iberian Range and basin inversion is interpreted to have formed the vergent NE-SW– 40°N trending folds in the northern Tunisian Atlas (Boccaletti et al., 1990). In the southern Tunisian Atlas Mountains, this orogeny again reactivated major Mesozoic normal faults, such as the Gafsa fault (Bobier et al., 1991). Southwest of the Gafsa fault, in the far-foreland of the orogeny, Betic chain Mediterranean Sea E-W– to ENE-WSW–trending folds developed (Bouaziz et al., 2002; Atlantic Ocean Khomsi et al., 2009). After a period of late Pliocene extension, contrac- Tell-Rif tion resumed during the early Pleistocene, reactivating strike-slip faults Rif Tunisian Atlas and Atlas folds (Ben Ayed, 1986; Chihi, 1992; Bouaziz et al., 2002). High plateau Minor contractional events continued throughout the Pleistocene, and Moroccan Saharan Atlas Meseta NW-SE regional shortening continues as a result of slow convergence High Atlas between North Africa and Eurasia (Dlala and Hfaiedh, 1993; Rebai, 30°N 1993; Chihi, 1995; Bouaziz et al., 2002). Continental Intracontinental Crystalline Alpine Thrust platforms fold belts 0° massifs chains fronts 10°E STRUCTURE AND STRATIGRAPHY OF THE METLAOUI REGION Figure 1. Map of mountain belts to the north and south of the Western Mediterranean Sea. Boxed area indicates location of Figure 2A. Figure is adapted from Zargouni (1986) and Bouaziz et al. (2002). This study focuses on the southern Tunisian Atlas Mountains, specifi - cally the Metlaoui region between the Gafsa fault and the Fault of Chotts; the Fault of Chotts is interpreted as a blind thrust fault that cores the Chott Range (Hlaiem, 1999) (Fig. 2). The Gafsa fault bounds the area to the Tunisian Atlas Mountains. The fi rst extensional deformation occurred north, and the Fault of Chotts marks the southern boundary. Both faults from the Jurassic to the Early Cretaceous and formed E-W–striking are major right-lateral oblique-slip features interpreted as reactivated normal faults and basins such as the Fault of Chotts and Chotts Basin basement faults (Hlaiem, 1999). The Gafsa fault currently displays right- (Guiraud et al., 1987; Ben Ferjani et al., 1990; Zouari et al., 1999; Soua lateral strike-slip movement, although evidence from seismic data sug- et al., 2009). The second phase of rifting occurred during the Early gests multiple past movement directions and reactivations (e.g., Boukadi Cretaceous and generated NW-SE–striking normal faults and basins, and Bédir, 1996; Hlaiem 1999). The Fault of Chotts is associated with a such as the Gafsa fault and basin (Guiraud et al., 1987; Boccaletti et al., number of E-W–oriented anticlines, and evidence from seismic studies 1990; Soua et al., 2009). The development of the basins (grabens and suggests several reactivation episodes (Hlaiem, 1999). half grabens) resulted in stratigraphic variations in southern and central The Metlaoui region is characterized by broad synclines and tight anti- Tunisia (Bishop, 1988; Bédir, 1995; Hlaiem et al., 1997; Hlaiem, 1999; clines that create large depressions and narrow, continuous E-W–trending Soussi and Ben Ismaïl, 2000). In late Early Cretaceous (late Aptian to ranges, respectively (Fig. 2). The anticlines are south vergent, with south- early Albian), extension was interrupted by a brief period of contrac- ern limbs near vertical and locally overturned, and regional-scale faults are tion, termed the “Austrian Phase” (Ben Ayed, 1986; Guiraud et al., oriented NW-SE and E-W. The en echelon folds that comprise the Met- 1987; Khomsi et al., 2004), but extension resumed in the Late Creta- laoui and Moulares Ranges, including the Alima anticline, are similar in ceous (Bédir, 1995; Bouaziz et al., 2002). deformational style, with arcuate and segmented, right-stepping fold axes. Although the timing is not well established (Zargouni, 1986; Zouari Fold axes are 8–25 km long, but typically ~17 km, and anticline widths et al., 1999; Khomsi et al., 2006), Mesozoic extension switched to con- vary from 5 to 10 km.

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