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 contractional 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.

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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, shortening 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. blende, feldspar, and quartz. The fabric A mid-Cretaceous age for brittle strike-slip asymmetry between domains of variable fab- displacements in the AFS is implied by K-Ar ric intensity implies that the ductile move- ages obtained for mineralization associated ment sense was east-side down (compare with faulting (Arabasz, 1971; Zentilli, 1974; with Ramsay and Graham, 1970; this paper, Colley and others, 1990), and sinistral, brittle Fig. 5a). The zone of ductile deformation strike-slip displacements were reported on along the western fault has been traced north- major faults in the AfS near Vallenar (Thiele ward to a point 5 km north of Quebrada del and Pincheira, 1987). Normal-slip, brittle dis- Saladito, close to the northern end of the Las placements of late Miocene age have been Tazas pluton (Fig. 2). Ductile deformation is

1166 Geological Society of America Bulletin, September 1993

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 i r 70°00Jt\i Taltal Figure 2. The Atacama Fault System between 25°20'S and 27°00'S in the Cordillera de la Costa of northern Chile. (1) Paleozoic

• 25° 30' metasedimentary basement containing Per- mian-Triassic and Lower Jurassic plutonic complexes. (2) Jurassic volcanic and sedimentary rocks. (3) Mid-Jurassic plutonic complexes. (4) Cretaceous volcanic and sedimen- Cifuncho tary rocks. (5) Cretaceous plutonic complexes (LT = Las Tazas pluton). (6) Middle to late Miocene Atacama gravels. Compiled from Mercado (1977, 1978), Naranjo (1978), Naranjo and Puig (1984), Servicio Nacional de Geología y Minería (1982), and our own maps.

Esmeralda

26°00' absent along the western fault between this point and Taltal. Both ductile and brittle deformation along the western margin of the Las Tazas intrusion decrease south of Que- brada de Guamanga, and the contact between Cretaceous and Jurassic plutonic rocks is undeformed 2 km south of the Quebrada. In deformed rocks derived from the Las Tazas pluton, granitic veins, lenses, and seg- regations lie parallel to the shear surfaces of minor shear zones. This relationship, and the observation that some veins are unfoliated while others have a strong foliation, implies that residual melt was still present during deformation. Recrystallization of diopside, hornblende, and biotite indicates that ductile deformation along the western fault occurred under upper amphibolite facies conditions. The microstructure of the rocks is dominated by strain-free grains with straight or gently curved grain boundaries implying that static recrystallization has followed dynamic recrystallization. North of Quebrada de Guamanga, the zone of ductilely deformed rocks along the western fault increases in width. At Quebrada del Saladito, about 14 km north-northwest of El Salado, the shear zone is 800 m wide and contains mylonitic rocks derived from Jurassic and Cretaceous diorite and tonalite (Naranjo and others, 1984; this paper, Fig. 2). The mylonitic foliation contains a steeply plunging ^ Mina Marito Verde stretching lineation defined by recrystallized aggregates of hornblende (Fig. 4a). Asymmet- A Cuesta La Reina ric foliation boudinage (Hanmer, 1988), a- porphyroclast systems (Passchier and Simp: 5 6 • Cuesta los Pozos son, 1987; this paper, Fig. 5b), and S-C —• Major faults mylonites (Lister and Snoke, 1986) all indicate east-side-down displacement. The my-

1167 Geological Society of America Bulletin, September 1993

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 lonite formed and was subsequently recrystallized statically under amphibolite facies conditions. Field relationships at Quebrada del Sal- adito provide further evidence for the timing of mylonitization relative to Cretaceous magmatism. Granodiorite dikes of Cretaceous age from the Las Tazas intrusion cut the mylonitic foliation. Some of these dikes are al- most concordant and are strongly foliated, whereas others are discordant and undeformed (Fig. 5c). Therefore, emplacement of the dikes was syn- to postkinematic with re- spect to ductile displacements, with an east- side-down movement sense.

Ductile Deformation along the Eastern Fault

The eastern fault of the AFS, 4 km southeast of Manto Verde, marks a boundary between Cretaceous volcanic rocks to the west and Cretaceous tonalite/granodiorite to the east (Figs. 2 and 3). The Cretaceous plutonic rocks adjacent to the brittle fault trace are intensely ductilely deformed in a 1-km-wide belt of steeply east-dipping mylonite. The most intensely deformed rocks have a horizontal to gently plunging biotite mineral lineation, and elongate aggregates of recrystal- Iized quartz define a subhorizontal stretching fabric (Fig. 4b). Fold hinge lines are mostly subhorizontal but bend through 90° or more, which indicates that sheath folds (Cobbold and Quinquis, 1980) are present. The orientation of the planar and linear fabric elements indicates that horizontal surfaces expose the kinematic xz plane, parallel to the stretching lineation and perpendicular to the foliation. Viewed on such surfaces, these folds have an S-asymmetry, which suggests that sinistral strike-slip deformation has produced the mylonite (Fig. 5d). In zones of heterogeneous strain, the fabric asymmetry between zones of variable fabric intensity provides farther evidence for sinistral displacement. This zone of high ductile strain may be traced southward to where the eastern fault juxtaposes Jurassic and Cretaceous intrusive rocks near Porvenir (Fig. 2). The brittle fault is exposed 2 km north of Porvenir, marked by a 10-m-wide, mineralized crush breccia (fault-rock terminology after Sibson, 1977). Figure 3. The Atacama Fault System between 25°20'S and 27°00'S. The fault system is defined East of the fault, Cretaceous tonalite is by an overlapping system of brittle faults shown with thickened lines and is a locus of intense strongly ductilely deformed in a 1-km-wide mineralization. Four sidewall ripout structures are identified by cross-hatched ornament. E = belt of steeply dipping mylonite. Subhorizon- trailing extensional ramp; C = leading contractional ramp. An "ideal" sidewall ripout structure tal stretching lineations, defined by elongate for sinistral slip is shown in the top right for comparison. Fault traces are compiled from maps aggregates of quartz, feldspar, and horn- of Mercado (1977, 1978), Naratyo (1978), Naranjo and Puig (1984), Servicio Nacional de Geo- blende, imply strike-slip displacement, and logía y Minería (1982), and our own maps. The faults between the AFS and the coast have not S-C fabrics imply sinistral sense of shear. been studied. West of the brittle fault, ductile deformation

1168 Geological Society of America Bulletin, September 1993

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a) Stretching c) Slickenlines epidote, albite, and quartz in mylonite de- Lineations rived from diorite and granodiorite indicate that deformation occurred under upper greenshist to lower amphibolite facies conditions, at lower metamorphic grade than in the ductile shear zones in the western part of the AFS.

Age of Ductile Displacements

In the Jurassic to Early Cretaceous magmatic arc, U-Pb zircon (Berg and Baumann, 1985), Rb-Sr biotite, K-Ar mineral (Berg and Breitkreuz, 1983), and 40Ar-39Ar hornblende ages (Brown and others, 1991) from undeformed rocks in individual plutons all yield western ages which are, within error, identical. The ductile shear 18 points Taltal 15 points ages of the arc plutons decrease systemati- zone cally from west to east across the arc and are interpreted to date rapid, postmagmatic cool- b) Stretching d) Slickenlines ing of the plutons (Brown and others, 1991). Lineations Naranjo and others (1984) determined K-Ar hornblende and biotite mineral ages of rocks within, and adjacent to, ductile shear zones along the western branch of the fault system at Quebrada del Saladito. Both undeformed Cretaceous intrusive igneous rocks adjacent to, and within, the shear zone and my- lonitized Jurassic diorite gave ages of ca. 130 Ma. Also along the western branch of the fault system, Brown and others (1991) used hornblende to determine ""Ar-^Ar isotope correlation ages of ca. 132 and ca. 130 Ma from ductilely deformed Jurassic diorite bor- dering the Las Tazas pluton. Hornblende from undeformed tonalite of the Las Tazas eastern 40 39 ductile shear 39 points Manto verde 23 points pluton gave a Ar- Ar isotope correlation zone age of ca. 127 Ma. We interpret the similarity 40 39 Figure 4. Orientation of stretching lineations in intensely ductilely deformed rocks exposed (a) between the Ar- Ar hornblende postmagmatic cooling age from the Las Tazas pluton along the western branch of the AFS between Quebrada de Guamanga and Quebrada del Sal- 40 39 adito and (b) along the eastern branch of the AFS between El Salado and Rosario. (c) Orientation and the Ar- Ar hornblende ages from my- of slickenlines along the eastern branch of the AFS between Taltal and Quebrada Pan de Azucar lonite in the country rock to mean that the and (d) along the central and eastern branches of the AFS at Mina Manto Verde. These locations heat for the ductile deformation of the country rock was provided by the intrusion. This are shown in Figure 2. interpretation is consistent with the field evidence that ductile deformation accompanied in Jurassic diorite is present in a 500-m-wide quartz, feldspar, and hornblende, and S-C emplacement of the intrusion. From the belt of anastomosing, steep ductile shear fabrics indicative of sinistral strike-slip dis- geochronologic evidence for rapid, postmagmatic cooling of the plutons, it follows that zones. Recrystallized aggregates of horn- placement (Figs. 5e and 5f). Exposure is poor 40 39 blende and quartz define shallow- to steeply immediately to the west of the eastern fault at the Ar- Ar hornblende cooling age of the north-plunging stretching lineations. S-C fab- Rosario, so that the width of the mylonite belt mylonite is close to the deformation age of rics imply sinistral displacements with an in Jurassic protolith is unknown. the mylonitic rocks. oblique east-side-down component. The belt The microstructure in mylonite along the Along the eastern branch of the fault sys- of mylonite may be traced farther southward eastern branch of the AFS is characterized by tem at Porvenir, hornblende from mylonitic to Rosario (Fig. 2), where mylonitic rocks de- sutured grain boundaries and strong crystal- Cretaceous diorite yielded a 40Ar-39Ar corre- rived from Cretaceous tonalite are exposed lographic preferred orientations in quartz, lation age of ca. 126 Ma (Brown and others, on the east side of the brittle fault trace, but and variably deformed new grains in quartz 1991). Weakly deformed rocks of this intru- here the belt is only 200 m wide. The mylonite and hornblende. These microstructures are sion, 5 km to the northwest, gave a U-Pb zir- contains horizontal to gently plunging good evidence of dynamic recrystallization. con age of 126.8 ±1.3 Ma (Berg and Bau- stretching lineations, defined by aggregates of Parageneses containing hornblende, biotite, mann, 1985). We interpret these ages to imply

Geological Society of America Bulletin, September 1993 1169

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 Figure 5. a. Low deformation domain flanked by high strain zones from the western part of the AFS south of El Salado. Rotation of the foliation indicates east-side-down movement on the ductile shear zone. The shear zone is viewed looking to the south and the stretching direction is vertical and in the plane of the photograph. b. Porphyroclast system from the western part of the AFS at Quebrada del Saladito. A mineral fabric defined by biotite and hornblende around a feldspar porphyroclast defines an asymmetric system indicating east-side-down movement on the ductile shear zone. The photomicrograph is viewed looking north and the stretching direction is vertical within the plane of the photomicrograph. The scale bar is 1 mm long. c. Mylonitic foliation developed in Jurassic diorite/tonalite cut by unfoliated, Early Cretaceous granodiorite along the western trace of the AFS at Quebrada del Saladito. The rocks are viewed in a vertical exposure, looking south. The geologic hammer in the lower left foreground is 50 cm long. d. Mylonitic foliation with asymmetric fold in early Cretaceous tonalite/granodiorite along the eastern part of the AFS, 8 km south-southeast of Manto Verde mine. The mylonite is viewed on a horizontal surface with north at the top of the photograph. The stretching direction in the mylonite is horizontal. Fold hinge line orientation varies through >90°, but folds consistently have S-asymmetry on horizontal surfaces as shown, implying that the sense of shear is sinistral. e and f. Mylonitic foliation with asymmetric foliation boudins in Jurassic tonalite along the eastern part of the AFS at Porvenir (e) and Rosario (f). The position of the boudins is indicated by an arrow in each photograph. The mylonitic rocks are exposed on a horizontal surface with north to the left. The stretching direction in the mylonite is horizontal, and the sense of asymmetry d indicates that the displacement was sinistral strike-slip.

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tematic mapping at 1:100,000 scale (Mer- The surfaces of faults bounding the slabs con- cado, 1977,1978; Naranjo, 1978; Naranjo and tain subhorizontal slickenfibers (Fig. 4c). Puig, 1984). Between Taltal and Quebrada This part of the El Salado segment con- Pan de Azúcar, the AFS is dominated by two trasts sharply with that between Quebrada major, subparallel faults (Fig. 2) linked by Pan de Azúcar and 27°S in that it does not subsidiary faults to define asymmetric fault- contain broad belts of mylonitic rocks. Fault bounded slabs tapering to the north (Fig. 3, rocks produced by intense ductile deforma- shaded domains). Along the eastern fault, tion are comparatively rare, even in plutonic prominent, steeply dipping fracture surfaces rocks along the eastern side of the fault sys- with slickenlines cut zones of mineralized tem (Fig. 2), which we would expect to con- crush breccia as much as 10 m thick. Slick- tain mylonitic rocks if the displacement his- enfibers and slickensides measured on these tory were similar to that found between fracture surfaces typically plunge 0°-10° Quebrada Pan de Azúcar and 27°S. north or south (Fig. 4c) and show that the shear direction was subhorizontal, consistent Quebrada Pan de Azúcar-27°S with an important component of brittle strike slip in the eastern part of the fault system be- South of Quebrada Pan de Azúcar, where tween Taltal and Quebrada Pan de Azúcar. the brittle faults are superposed on ductilely Smaller, asymmetric fault-bounded slabs ta- deformed rocks, the overlapping pattern of pering to the north are exposed on horizontal major and linking subsidiary faults defines surfaces along the eastern branch of the fault two fault-bounded slabs that taper to the 1 km south of Quebrada Cifuncho (Fig. 6). north (Fig. 3, shaded domains). Gently plung-

Figure 6. Sidewall ripout structures in grano- a displacement on principal fault diorite from the eastern branch of the AFS near Taltal (2 km south of Quebrada Cifuncho—see Fig. 2). The diagonally shaded domains identify two sidewall ripouts.Ripou t 2 overprinted ripout 1, and its trailing ramp has been truncated by fracture A-A\ Subhorizontal slickenfibers are present on the fault surfaces (Fig. 4c). The figure is drawn from a field map. E = trailing extensional ramp; C = leading contractional ramp. b ripout initiation principal fault sticks that ductile, sinistral strike-slip displacement \ in the eastern part of the AFS is of similar age to, although may slightly postdate, east-side- down ductile displacements along the west- leading ' trailing ern branch of the fault system. contractional ramp extensional ramp

BRITTLE DEFORMATION IN THE AFS

Asymmetric fault-bounded slabs in fault c ripout displacement and deformation of principal fault systems have been called sidewall ripouts by Swanson (1988,1989; this paper, Fig. 6), who pointed out their utility as kinematic indicators (Fig. 7). Similar structures have been produced in shear box experiments (Marsh and others, 1990). Asymmetric fault-bounded slabs from the AFS between 25°S and 27°S are described below and we interpret these structures to be sidewall ripouts. Figure 7. Geometry and kinematics of sidewall ripoutformation , (a) Displacement on principal fault. Incipient sidewall ripout shown by broken line, (b) Ripout initiation. Increase in shearing Taltal-Quebrada Pan de Azûcar resistance causes principal fault to stick. Ripout is carved from the sidewall as slip bypasses the inactive segment of the fault, (c) Ripout displacement and deformation of principal fault. The The pattern of major brittle faults between releasing bend so formed may be reactivated (dotted line) if increments of slip subsequently return Taltal and 27°S is known as the result of sys- to the principal fault. Diagrams of sidewall ripout structure are modified after Swanson (1989).

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ing to horizontal slickenlines were measured fagasta (Fig. 1) by González (1990), who oblique convergence toward the southeast along the central fault zone, 2 km northwest linked steep stretching lineations in the coun- would lead to trench-linked sinistral strike of Manto Verde (Fig. 3), where the fault zone try rock west of the Early Cretaceous Cerro slip on the AFS (see also Uyeda, 1982; has placed Cretaceous tonalite/granodiorite Cristales intrusion to diapiric emplacement. Woodcock, 1986). Our work confirms that to the west in contact with Cretaceous vol- In the El Salado segment of the fault system, between 25°S and 27°S also, ductile, sinistral canic rocks (Fig. 2). At Cuesta Los Pozos high-temperature mylonitic rocks in Jurassic strike-slip displacements of Early Cretaceous (Fig. 2), the central fault zone is contained tonalite at the western margin of the Early age have an important role in the displace- within a zone of fracturing at least 1 km wide. Cretaceous Las Tazas intrusion (Fig. 2) are ment history of the AFS. Brittle deformation and mineralization are in- similarly characterized by steeply plunging Therefore, the El Salado segment of the tense and, in contrast to the western and east- stretching fabrics. The displacement sense on AFS, south of Quebrada Pan de Azúcar, is ern faults, earlier ductilely deformed rocks these mylonitic rocks, however, is east-side characterized both by dip-slip and strike-slip have not been recognized at this locality. down, toward the intrusion. Field relation- ductile deformation in two major Early Cre- Along the central and eastern branches of the ships and age determinations clearly indicate taceous ductile shear belts in the western and fault system near Manto Verde, S-C-type that ductile deformation and emplacement of eastern parts of the fault system, respec- fabrics in fault gouge indicate sinistral sense the Las Tazas intrusion are closely related in tively. These belts appear to be of broadly of shear during brittle deformation. time. Therefore, the magmas probably pro- similar age, and our results support the con- vided the heat source for the high-tempera- tention that Early Cretaceous deformation in Sidewall Ripouts in the AFS ture ductile deformation. The east-side- the AFS was transtensional (Reutter and oth- down, dip-slip displacements may then be ers, 1991; Scheuber and Reutter, 1992). In the We interpret the faults that link the eastern related to extension of the arc rather than to El Salado segment, this deformation is parti- and western principal faults of the AFS be- the diapiric emplacement of the intrusion, tioned between shear zones that accommo- tween Taltal and Quebrada Pan de Azúcar as and we suspect that the steeply dipping, east- dated magma emplacement and arc-normal the leading contractional and trailing exten- side-down shear zones along the western extension, and shear zones that accommo- sional ramps of sidewall ripout structures branch of the fault system are linked to a low- dated sinistral strike-slip deformation. We re- (Fig. 3), and the geometry should be com- angle extensional detachment. Preliminary main unable to estimate the amount of ductile pared with Figure 7. We identify also two results of our current work in the AFS show displacements on these shear zones. sidewall ripout structures between Quebrada that a linked system of listric normal faults Pan de Azúcar and 27°S (Fig. 3). These side- cuts Jurassic volcanic rocks northwest of El IMPLICATIONS—BRITTLE REGIME wall ripouts have sinistral asymmetry, and Salado (Fig. 2), and we emphasize that we taken together with subhorizontal slicken- have seen no evidence of thrust-related struc- Age of Brittle Deformation and Cooling of lines (Figs. 4c and 4d) and S-C-type fabrics in tures in Jurassic and Cretaceous rocks of the the Magmatic Arc fault gouge, they show that sinistral strike- magmatic arc in the El Salado segment of the slip displacements in the brittle regime have fault system. This interpretation supports the The age of brittle sinistral strike-slip dis- occurred in the AFS between 25°S and 27°S. arguments of Reutter and others (1991) and placements in the AFS is not well deter- After formation, a sidewall ripout may be Scheuber and Reutter (1992) that extension mined. Thiele and Pincheira (1987) and translated along a fault system, and, as a re- of the arc accompanied emplacement of the Hervé (1987a, 1987b) imply that brittle, sin- sult, the principal fault may be folded Early Cretaceous intrusions. istral strike slip is Early Cretaceous in age, (Fig. 7c). Ripouts between Taltal and Que- Several authors have speculated that a link consistent with oblique subduction to the brada Pan de Azúcar and the ripout to the exists between displacements on the AFS southeast at that time. K/Ar data for the age west of El Salado have, apparently, not been and the development of the Mesozoic back- of fault-related mineralization support this significantly translated to the north, because arc basin to the east (Brown, 1991b; Colley view (Zentilli, 1974; Colley and others, 1990). the bounding principal faults are not folded. and others, 1990). Recognition of Early Cre- An Early Cretaceous age (ca. 126 Ma) for In contrast, the principal fault to the east of taceous dip-slip displacements in the AFS ductile, sinistral strike-slip displacements the sidewall ripout at Manto Verde (Fig. 3) with downthrow to the east is consistent with along the eastern branch of the fault system, has been folded, presumably to accommo- this idea; the details of any kinematic link between Manto Verde and Rosario, gives a date some northward translation of the with extension in the back-arc area and dis- lower limit for the age of the superposed brit- ripout. The fold in the principal fault is a re- placements on the AFS remain obscure. tle, sinistral strike-slip displacements. The leasing bend for sinistral strike slip (Figs. 3 In the eastern part of the El Salado seg- youngest intrusions in the Jurassic to Early and 7c) and Fe-Cu mineralization is particu- ment, ductile deformation is characterized by Cretaceous arc are about 120 m.y. old (Berg larly intense at this locality. sinistral strike-slip displacement. This lower- and Baumann, 1985; Scheuber and Reutter, temperature strike-slip deformation is of sim- 1992), and therefore the change from ductile IMPLICATIONS—DUCTILE REGIME ilar age to, or possibly slightly postdates, the to brittle displacements could have coincided dip-slip movements discussed above. Strike- with the abandonment and cooling of the arc Early Cretaceous ductile displacements in slip displacements on the AFS have been when magmatic activity stepped east in the the western part of the El Salado segment linked by several authors to early and mid- mid-Cretaceous. occurred at upper amphibolite to amphibolite Cretaceous oblique convergence of the Aluk Abandonment of the Jurassic-Early Cre- facies conditions and had a dip-slip, east- and South American plates (Naranjo and oth- taceous arc may have been a consequence of side-down movement sense. Ductile dis- ers, 1984; Thiele and Pincheira, 1987; Reutter more rapid advance of the South American placements with a dip-slip component have and Scheuber, 1988; Scheuber and Andries- plate toward the trench following the initia- also been recognized in the AFS near Anto- sen, 1990). These authors have argued that tion of sea-floor spreading in the South At-

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A Cenozoic age for the brittle sinistral ment are Cretaceous in age, but they strike strike-slip displacements on the AFS is made toward the younger coastal scarp fault off- unlikely by the plate kinematics of this era. shore to the north of Taltal (Figs. 1 and 8). 24° 24" S Strongly oblique convergence of the Aluk This apparent continuity implies that the plate ceased in Late Cretaceous time, after coastal scarp fault has reactivated part of the which the Farallón plate and then the Nazca AFS offshore, north of Taltal. Onshore, late plate subducted below the margin (Zonen- Miocene dip-slip displacements have reacti- shayn and others, 1984). The convergence vated the AFS between Taltal and Quebrada history between the Farallón and South Cifuncho (Arabasz, 1971; Naranjo, 1987), American plates for the end of the Creta- and we imply that these displacements are ceous Period and for Paleocene time is un- linked to those on the coastal scarp fault certain (Pilger, 1983; Pardo-Casas and Mol- offshore. nar, 1987), although Paleocene igneous activity in the Andes to the east of the El Sal- CONCLUSIONS ado segment is consistent with continued subduction (Olson, 1989). From early The western branch of the Atacama fault Eocene time, plate reconstructions are more system in the El Salado segment was active certain and show that the direction of con- under upper amphibolite conditions in the vergence has been only slightly oblique to the Early Cretaceous period as a dip-slip ductile continental margin toward the northwest shear zone with downthrow to the east. My- (Pardo-Casas and Molnar, 1987). If the AFS lonitic deformation along the eastern branch has acted as a trench-linked strike-slip fault of the fault system occurred under lower- during post-Eocene time, therefore, then grade metamorphic conditions at the amphib- dextral rather than sinistral displacement olite facies/greenschist facies transition and is would be expected. of similar age to the ductile deformation along the western branch. Mylonitic rocks along Offset of Mylonite-Bearing Segments of the the eastern branch of the fault system formed AFS by sinistral strike-slip displacement. Dip-slip and sinistral strike-slip ductile dis- Between Taltal and Quebrada Pan de Azú- placements are contemporary with the devel- car, broad zones of intense ductile shear are opment of the Jurassic-Early Cretaceous absent, and in this part of the AFS the fault magmatic arc, and they imply that the tec- rocks are characteristically brittle. Fault tonic environment in the arc was transten- rocks formed by ductile deformation are sional. The deformation was partitioned spa- present north of Taltal and south of Quebrada tially into a dip-slip component, associated Pan de Azúcar. We interpret this distribution with the emplacement of magmas, and a sin- of fault rocks to mean that the Taltal-Que- istral strike-slip component. Figure 8. Map of the Atacama fault system brada Pan de Azúcar part of the AFS has cut Brittle, sinistral strike-slip faults are super- in the vicinity of Taltal showing offset of belts and displaced the mylonite-bearing segments posed on ductilely deformed rocks along the of ductile deformation by brittle faults between of the AFS (Fig. 8). If this interpretation is western and eastern branches of the AFS be- Taltal and Quebrada Pan de Azúcar (QPA). correct, then the amount of brittle, sinistral tween Quebrada Pan de Azúcar and 27°S. The brittle faults between Taltal and Que- displacement is about 70 km: the distance be- The pattern of overlapping brittle faults de- brada Pan de Azúcar are part of the AFS but tween the cut-offs of the mylonite-bearing fines asymmetric, fault-bounded slabs, which are oblique to the general trend of the system segments of the fault system at Quebrada Pan are interpreted as sidewall ripout structures. with a sense that would give riset o transtension de Azúcar and north of Taltal (Figs. 3 and 8). The transition from ductile to brittle sinistral during sinistral strike-slip deformation on the strike slip on the AFS may have occurred due fault system. Linkage with the Coastal Scarp Fault to cooling as the magmatic arc migrated to the east at the end of Early Cretaceous time. The coastal morphology of Chile between To the north of Quebrada Pan de Azúcar, lantic (Brown, 1991b). This change in the Taltal and Iquique is controlled by a normal broad belts of intensely ductilely deformed movement vector of the overriding plate was fault system (Fig. 1), associated with uplift of rocks are absent, although they are present not, apparently, accompanied by change in the Cordillera de la Costa, which has pro- north of Taltal between Paposo and Antofa- the convergence angle of the underriding duced a spectacular coastal scarp. Near An- gasta. This gap in the belts of ductile strain plate, and oblique subduction of the Aluk tofagasta, in Neogene and Quaternary time, along the AFS is present because the mylo- plate persisted until late in the Cretaceous Pe- as much as 1 km of normal dip-slip displace- nitic rocks are cut and displaced about 70 km riod (Zonenshayn and others, 1984). Conse- ments accumulated on this fault system sinistrally by a segment of the fault system quently, brittle, sinistral strike-slip displace- linked to sinistral oblique-slip displacements characterized by brittle faults. This brittle ment could have continued on the AFS on the AFS (Armijo and Thiele, 1990; this segment of the fault system also contains throughout the mid- and Late Cretaceous paper, Fig. 1). In our study area, brittle, sin- sidewall ripout structures and is of probable Period. istral strike-slip faults of the El Salado seg- Early Cretaceous age. The fault system has

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been reactivated during late Cenozoic time to 27°00'S: 6th Congreso Geológico Chileno, Actas, v. 1, Universidad de Chile, Santiago, Comunicaciones, v. 34, p. 631-634. p. 57-66. accommodate uplift of the Cordillera de la Brown, M., Dallmeyer, R. D., Díaz, F., and Grocott, J., 1991, Dis- Olson, S. F., 1989, The stratigraphic and structural setting of the placement histoiy and tectonic significance of the Atacama Potrerillos porphyry copper district, northern Chile: Revista Costa. fault system (El Salado segment) N Chile: 40Ar-39Ar mineral Geológica de Chile, v. 16, p. 3-29. age* constraints: Eos (American Geophysical Union Transac- Pardo-Casas, F., and Molnar, P., 1987, Relative motion of the tions), v. 72/17, p. 263. Nazca (Farallón) and South American plates since Late Cre- ACKNOWLEDGMENTS Cobbold, P. R., and Quinquis, H., 1980, Development of folds in taceous time: Tectonics, v. 6, p. 233-248. shear regimes: Journal of Structural Geology, v. 2, Passchier, C. W., and Simpson, C., 1986, Porphyroclast systems as p. 119-126. kinematic indicators: Journal of Structural Geology, v. 8, Colley, H., Treloar, P. J., and Díaz, F., 1990, Gold-silver mineral- p. 831-843. Fieldwork in Chile was supported by the ization in the El Salvador region, northern Chile, in Keays, Pilger, R. H., 1983, Kinematics of the South American subduction National Geographic Society, the Royal So- R., Ramsey, R., and Groves, D., eds., The geology of gold zone from global plate reconstructions, in Ramón-Cabre, deposits: The perspective in 1988: Economic Geology Mono- S. J., ed., Geodynamics of the eastern Pacific region, Carib- ciety of London, the Servicio Nacional de graph 6, p. 208-217. bean and Scotia arcs: American Geophysical Union Geody- Dewey, J. F., 1980, Episodicity, sequence and style at convergent namics Series, v. 9, p. 113-125. Geología y Minería de Chile (SERNA- plate boundaries, in Strangeway, D. W., ed., The continental Ramsay, J. G., and Graham, R. G., 1970, Strain variations in shear GEOMIN), and the National Environment crust and its mineral deposits: Geological Association of Can- belts: Canadian Journal of Earth Sciences, v. 7, p. 786-813. ada Special Paper 20, p. 553-576. Reutter, K.-J., and Scheuber, E., 1988, Relation between tectonics Research Council (NERC) Research Grant Dewey, J. F., and Lamb, S. H. 1992a, Active tectonics of the and magmatism in the Andes of Northern Chile and adjacent Andes: Tectonophysics, v. 205, p. 79-95. areas between 21° and 25°S: 5th Congreso Geológico Chileno, (GR9/476). We are grateful for this support. Dewey, J. F., and Lamb, S. H., 1992b, Neotectonics of the Andes: Santiago, Tomo III, p. 1345-1363. We would particularly like to thank our Geological Society of America, Abstracts with Programs, Reutter, K.-J., Heinsohn, W. D., Scheuber, E., and Wigger, P., v. 25, p. A63. 1991, Crustal structure of the Coastal Cordillera near Anto- driver, Tuco, for his help during fieldwork. Fitch, T. J., 1972, Plate convergence, transcurrent faults and inter- fagasta, northern Chile: 6th Congreso Geológico Chileno, Ac- nal deformation adjacent to southeast Asia and the western tas, v. 1, p. 862-866. We thank Arthur Sylvester, Eric Nelson, Pacific: Journal of Geophysical Research, v. 77, St. Amand, P., and Allen, C. R., 1960, Strike-slip faulting in North- Terry Pavlis, Mark Swanson, David p. 4432-4460. ern Chile: Geological Society of America Bulletin, v. 71, González, G. A., 1990, Patrones estructurales, modelo de ascenso, p. 1965. Schwartz, Thomas Rockwell, and Robert emplazamiento y deformación del Plutón de Cerro Cristales, Scheuber, E., and Andriessen, P.A.M., 3990, The kinematic and Cordillera de la Costa al sur de Antofagasta, Chile: Memoria geodynamic significance of Atacama fault zone, northern Whitney for reviews, which led to consider- de Título (Inédito), Universidad Católica del Norte, Chile: Journal of Structural Geology, v. 12, p. 243-257. able improvements in the paper. Antofagasta. Scheuber, E., and Hammerschmidt, K., 1991, ^Ar-^Ar and Rb-Sr González, L. G., and Figueroa, A. O., 1991, Análisis tectónico de data from ductile shear zones from the Atacama fault zone la zona de falla Caleta Coloso, sur de Antofagasta, Chile: Una (AFZ), northern Chile: An attempt to determine the age of REFERENCES CITED zona de cizalle de dominio dúctil y rígido: 6th Congreso Geo- deformation: Terra Abstracts, v. 3, p. 364-365. lógico Chileno, Actas, v. 1, p. 693-696. 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A., Hervé, F., Prieto, X., and Munizaga, F., 1984, Ac- MANUSCRIPT RECEIVED BY THE SOCIETY SEPTEMBER 24,1990 in northern Chile: Geochemical evidence from 25°30'- tividad Cretácica de la falla Atacama al este de Chañaral: REVISED MANUSCRIPT RECEIVED JANUARY 21,1993 Milonitizadón y plutonismo: Departamento de Geología, MANUSCRIPT ACCEPTED FEBRUARY 4,1993

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1174 Geological Society of America Bulletin, September 1993

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