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Displacement Histoiy of the Atacama Fault System 25°00'S-27°00'S, Northern Chile

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Displacement Histoiy of the Atacama Fault System 25°00'S-27°00'S, Northern Chile Displacement histoiy of the Atacama fault system 25°00'S-27°00'S, northern Chile M. BROWN Department of Geology, University of Maryland at College Park, Maryland 20742 F. DIAZ* Servicio Nacional de Geología y Minería, Casilla 10465, Santiago, Chile J. GROCOTT School of Geological Sciences, Kingston University, Kingston-upon-Thames KT1 2EE, United Kingdom ABSTRACT ican plate, beneath which subduction of oce- strike of the arc (Mpodozis and Ramos, 1990; anic lithosphere has taken place since early Scheuber and Reutter, 1992; this paper, In the Cordillera de la Costa of the central Paleozoic time (Mpodozis and Ramos, 1990). Fig. 2). Deformation of the magmatic arc Andes in northern Chile, Mesozoic arc com- Strike-slip fault systems in continental litho- rocks is concentrated in the Atacama fault plexes are cut by a trench-parallel strike-slip sphere at convergent plate margins may be system, and we show that this deformation is fault system: the Atacama fault system. Brittle caused by oblique convergence between the contemporary with the development of the faulting in the Atacama fault system is super- overriding and underriding plates (Fitch, arc. In the El Salado segment, the arc base- posed on steeply dipping foliations in ductile 1972; Dewey, 1980; Uyeda, 1982; Wood- ment comprises metasedimentary rocks of shear belts. Between 25°S and 27°S, the west- cock, 1986). Several authors have interpreted Devonian to Carboniferous age and Permian ern part of the fault system was active in Early the AFS to be a sinistral, trench-linked strike- to Triassic intrusive rocks (Bell, 1984, 1987; Cretaceous time as an upper amphibolite fa- slip system formed in response to oblique Bahlburg and others, 1988; Brook and others, des, down-to-the-east, dip-slip ductile shear subduction of the Aluk (Phoenix) plate during 1986; Brown, 1988,1991a, 1991b), exposed in zone. In the eastern part of the fault system, the Cretaceous Period (Naranjo and others, a broad tract along the coast to the west of the ductile deformation is of similar Early Creta- 1984; Woodcock, 1986; Thiele and Pincheira, AFS (Fig. 2). ceous age but occurred under lower-grade met- 1987; Reutter and Scheuber, 1988; Scheuber Formation of the Jurassic to Early Creta- amorphic conditions at the greenschist/lower and Andriessen, 1990; Scheuber and Reutter, ceous magmatic arc was accompanied by the amphibolite facies transition. The mylonites in 1992). In contrast, for most of Cenozoic time, development of a back-arc basin system the eastern part of the fault system were plate convergence has been at a high angle to (Mpodozis and Ramos, 1990), and back-arc formed by sinistral strike-slip displacement. the continental margin (Pilger, 1983; Pardo- basin sequences are preserved as a belt of The dip-slip and sinistral strike-slip dis- Casas and Molnar, 1987), and the AFS has sedimentary and volcanic rocks exposed 70 placements are contemporary with the devel- probably not been active as a major strike- km to 110 km east of the magmatic arc rocks opment of a magmatic arc, and they imply that slip fault system in the Cenozoic (Hervé, (Servicio Nacional de Geología y Minería, the tectonic environment in this part of the arc 1987a; Armijo and Thiele, 1990; Dewey and 1982; Reutter and Scheuber, 1992). Contrac- was transtensional. The ductile deformation Lamb, 1992a). tion of the back-arc basin system occurred, was partitioned spatially into a dip-slip com- The AFS extends for at least 1,000 km be- initially, in the mid-Cretaceous during a de- ponent associated with the emplacement of tween La Serena and Iquique within the Cor- formation phase observed widely in northern magmas and a sinistral strike-slip component. dillera de la Costa of the central Andes Chile (Mpodozis and Ramos, 1990). Brittle fault zones in the EI Salado segment (Fig. 1). The discontinuous and overlapping Coincident with this contractional defor- of the Atacama fault system define large-scale faults that define the AFS strike subparaM to mation, the active magmatic arc began to mi- sidewall ripout structures. Subborizontal slick- the continental margin, although they change grate eastward, from its Late Jurassic to enlines, ripout asymmetry, and S-C-type orientation systematically to define three ar- Early Cretaceous position near the present fabrics in fault gouge indicate that brittle de- cuate segments (Naranjo, 1987; Thiele and coast of northern Chile, toward its current formation involved sinistral strike-slip dis- Pincheira, 1987; this paper, Fig. 1). We focus location in the high Andes (Reutter and placements. The transition from ductile to brit- on the central part of the fault system, re- Scheuber, 1988; Brown, 1991b). Migration of tle sinistral strike-slip displacements may have ferred to by Naranjo (1987) as the El Salado the volcanic arc is marked by mid-Creta- occurred due to cooling, in mid-Cretaceous segment (Fig. 1), and we report our use of ceous intrusive, volcanic and sedimentary time, when the magmatic arc was abandoned. ductile and brittle kinematic indicators to de- rocks, which are exposed immediately to the termine the displacement histoiy of this seg- east of the Jurassic to Early Cretaceous mag- INTRODUCTION ment of the AFS. matic arc rocks, and which cut, or overlie, rocks of the Jurassic to Early Cretaceous The Atacama fault system (AFS) is located GEOLOGICAL SETTING OF THE AFS back-arc basin system (Olson, 1989; Scheu- in the continental margin of the South Amer- BETWEEN 25°S AND 27°S ber and Reutter, 1992). A second contrac- tional deformation occurred in late Oiigocene to early Miocene time when mid-Cretaceous *Present address: Empresa Minera de Mantos The Atacama fault system cuts intrusive Blancos S.A., Avda. Pedro de Valdivia 295, Prov- and volcanic rocks of a Jurassic to Early Cre- and older rocks of the back-arc basin were idencia, Santiago, Chile. taceous magmatic arc and is parallel to the thrust eastward (Olson, 1989; Mpodozis and Geological Society of America Bulletin, v. 105, p. 1165-1174, 8 figs., September 1993. 1165 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/9/1165/3381846/i0016-7606-105-9-1165.pdf by guest on 24 September 2021 BROWN AND OTHERS Figure 1. Major faults of the Atacama fault described from the AFS near Taltal (Ara- system (AFS) in northern Chile. The fault sys- basz, 1971; Naranjo, 1987) and north of Pa- tem offshore north of Taltal is the Coastal poso (Hervé, 1987a). Armijo and Thiele Scarp fault system (CSFS). Displacements on (1990) concluded that, near Antofagasta, nor- this fault system may be linked with displace- mal-slip displacements of Quaternary age on ments on the Atacama fault system (see text for the coastal scarp fault system (Fig. 1) are further discussion). Compilation from linked to strike-slip displacements on the 1:1,000,000 map of Chile (Servicio Nacional de AFS. Dewey and Lamb (1992a, 1992b) have Geología y Minería. 1982); Thiele and studied active displacements in the Andean Pincheira, 1987; Naranjo, 1987; Armyo and plate boundary zone using fault-plane solu- Thiele, 1990. tions. They show that plate convergence is partitioned between Benioff zone slip, short- ening in the offshore fore arc, and shortening in the foreland fold and thrust belt in Argen- tina. According to Dewey and Lamb (1992a), the AFS is not important as an active strike- slip fault system, and the Coast Range of Ramos, 1990). The thrusted sequences are northern Chile is currently undergoing uplift, overlain unconformably by Atacama gravels accommodated by normal slip on the Coastal of mid- to late Miocene age. Scarp Fault System and dextral oblique slip on the AFS. PREVIOUS WORK ON DISPLACEMENT HISTORY DUCTILE DEFORMATION IN THE AFS A study of the AFS was made by Arabasz The AFS contains three major faults near (1971), following work by St. Amand and El Salado (Fig. 3, western, central, and east- Allen (1960), who initially reported evidence ern faults). Brittle deformation is superposed for strike slip. Arabasz (1971) recognized on strongly ductilely deformed rocks along Cretaceous strike-slip displacements and also each of these faults, as implied by the map- later dip-slip displacements on the AFS near ping of Mercado (1978). We examined these Taltal. More recent work has stressed that ductilely deformed rocks in detail, and we both ductilely and brittlely deformed rocks identified kinematic indicators along the are present in the fault system. Ductile, sin- western and eastern faults of the AFS be- istral strike-slip displacements have been de- tween Quebrada del Saladito and Rosario scribed from the AFS near Antofagasta (Figs. 2 and 3). (Hervé, 1987b; Uribe, 1987; Scheuber and Andriessen, 1990; González, 1990; Scheuber Ductile Deformation along the Western Fault and Hammerschmidt, 1991; González and Figueroa, 1991) and near Vallenar (Thiele and The western fault, south of El Salado, jux- Pincheira, 1987). The ductilely deformed taposes Jurassic diorite/tonalite to the west rocks have an Early Cretaceous age (Gonzá- and Cretaceous tonalite/granodiorite of the lez, 1990; Scheuber and Hammerschmidt, Las Tazas pluton to the east for much of its 1991). Other structural and geophysical work length (Figs. 2 and 3). In Quebrada de Gua- in the Antofagasta area (Reutter and others, manga, 8 km west of Manto Verde (Fig. 2), 1991; Scheuber and Reutter, 1992) focused the Jurassic and Cretaceous rocks contain a on arc-normal extension of Late Jurassic and steep, north-striking foliation and a steeply Early Cretaceous age and led to the con- plunging stretching lineation (Fig. 4a) for 150 clusion that the arc was characterized by m on each side of the brittle fault that marks sinistral strike-slip displacements and arc- the contact. The stretching lineations are de- normal extension associated with magma fined by recrystallized aggregates of horn- emplacement.
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