A Case Study from the Atacama Fault System

U N I V E R S I D A D D E C O N C E P C I Ó N DEPARTAMENTO DE CIENCIAS DE LA TIERRA 10° CONGRESO GEOLÓGICO CHILENO 2003

FAULT-FRACTURE NETWORK DEVELOPMENT IN UPPER-CRUSTAL, STRIKE-SLIP SETTINGS: A CASE STUDY FROM THE ATACAMA FAULT SYSTEM

CEMBRANO, J1., GONZÁLEZ, G2., AHUMADA, I3., OLIVARES, V3., HERRERA, V4.

1. Departamento Ciencias Geológicas, Universidad Católica del Norte. Casilla 1280, Antofagasta. E-mail: [email protected] 2. Departamento Ciencias Geológicas, Universidad Católica del Norte. Casilla 1280, Antofagasta. E-mail: [email protected] 3. Departamento Ciencias Geológicas, Universidad Católica del Norte. Casilla 1280, Antofagasta. 4. Programa de Doctorado en Ciencias Mención Geología, Departamento Ciencias Geológicas, Universidad Católica del Norte-MECE Educación Superior, Casilla 1280, Antofagasta.

INTRODUCTION The Atacama Fault System (AFS) is the most important structure of the Central Andes forearc (Arabasz, 1971; Scheuber and González, 1999). The AFS extends for more than 1000 km between Iquique and La Serena. The large-scale geometry of the AFS was formed during the late Jurassic and Early Cretaceous time at the end of a Jurassic magmatic arc that dominated a great part of the present-day Coastal Cordillera area. During the formation of the AFS, large brittle structures -more than 60 km in length- were formed by sinistral strike-slip movement (Hervé, 1987; Scheuber and Andriessen, 1990). Some NS-striking master faults and subsidiary NW striking splay faults are organized into strike-slip duplexes that occur at various scales from regional to local (e.g. Taylor et al., 1998) (Figure 1). We here describe and discuss the nature, geometry and kinematics of the Caleta Coloso Duplex which is exposed in the Coastal Cordillera south of Antofagasta (Scheuber and González, 1999). Our aim is to better understand the processes that govern the master and subsidiary fault development, the nature of the linkage between overstepping fault strands, and the strain partitioning patterns arising from fault interaction in strike-slip settings.

FAULT ARCHITECTURE AND KINEMATICS The Caleta Coloso duplex is formed by two, NNW-striking, subvertical master faults (Caleta Coloso, Cerro Bolfin) joined by a set of NW to WNW-striking imbrícate faults (Figure 1). Both master and subsidiary faults host spatially and temporally related hydrothermal mineral associations indicating a strong link between fluid transport and duplex development.

CALETA COLOSO FAULT It is marked by discontinuous tabular zones of steeply-dipping, north-northwest-striking cataclasite, phyllonite, fault gouge and discrete mesoscopic fault networks. The cataclasite zone is up to 500 m wide, contains randomly oriented mesoscopic faults with well-developed striated surfaces. Meter-wide, strongly foliated fault gouge zones and phyllonites, exhibit subhorizontal striae and sinistral kinematic indicators in locally high strain zones. Discrete faults show calcite

Todas las contribuciones fueron proporcionados directamente por los autores y su contenido es de su exclusiva responsabilidad. growth fibers indicating that sinistral displacement was coeval with hydrotermal fluid transport.

We interpret these structural organization as a central zone made up of high strain phyllonite and gouge, flanked by a wider zone of low strain cataclasite.

Figure 1. Regional scale geometry of the Atacama Fault System (AFS) in the Coastal Cordillera of northern Chile. Inset shows the Caleta Coloso Duplex formed by NW striking faults linking the ~NS-striking Bolfin and Caleta Coloso faults (modified of Brown et al., 1993; Scheuber and González, 1999).

BOLFIN FAULT The Cerro Bolfin fault strikes NNW, has a curved shape, and consists of meter-wide zones of fault gouge and cataclasite with shallowly plunging lineations. Last movement on the fault is normal dip-slip. Multiple sets of subsidiary northwest striking faults splay off the Caleta Coloso fault joining it with the Bolfin fault as a duplex structure.

IMBRICATE FAULTS The northwest imbricate structures dip steeply to the southwest and consist of normal-sinistral meter-wide fault-fracture zones and epidote-chlorite/calcite-siderite veins. The overall branched geometry and kinematics of faults and veins resembles consecutive sets of Riedel shears as seen in analog experiments (e.g. Naylor et al., 1986). Individual veins, up to 25 cm thick, are commonly banded and show isolated inclusions of altered wallrock fragments. The dioritic wallrock shows alteration haloes of albite and sericite.

FAULT-FRACTURE DEVELOPMENT The imbricate faults of the duplex show a displacement gradient with respect to the distance from the master faults. For instance, the horizontal separation of markers is greater along the Jorgillo fault at increasing distance from the Caleta Coloso fault (Figure 2). Furthermore, the Jorgillo fault changes from a cataclastic zone close to the Caleta Coloso fault to a more complex strongly foliated fault gouge/phyllonyte zone away from it. This suggests that the imbricate faults have propagated from the central part of the stepover to the master faults; which is in conflict with previous hypothesis of duplex formation (e.g Woodcock and Fisher, 1986 and Swanson, 1990).

FAULT KINEMATICS AND STRAIN COMPATIBILITY The geometry and kinematics of mesoscopic faults and striae was used to obtain local strain tensors using Marrett and Almendinger’s (1990) method (Figure 2). Pure strike-slip tensors were obtained from the main NNW striking master faults zones, compatible with a NE-trending principal extension axis. The fault populations from within the duplex, in contrast, yield transtensional to purely extensional incremental strain tensors, with a nearly N-S trending streching axis. These results suggest that strain incompatibility arising from continuos strike- slip/normal displacement on the duplex imbricate faults may have been accommodated by lateral extrusion and vertical displacement of rock lenses (e.g. Woodcock and Fisher, 1986).

CONCLUSIONS Structural and kinematic observations in the Caleta Coloso duplex provides important insights into fault-fracture network development in upper crustal strike-slip settings. The strike-slip duplex appears to have formed in several stages: An early, widely distributed cataclastic deformation along NNW-striking, overstepping fault zones of the Atacama Fault System. As deformation and cooling progressed, deformation became more localized in the internal parts of the cataclastic zones through discrete faults. These anisotropies channeled hydrothermal fluids that enhanced fault gouge and phyllonite formation, perhaps favoring locally plastic deformation. Coevally with master fault displacement a set of subsidiary imbricate faults nucleated from the central part of the stepover to the master faults as a response to a critical strain state resulting from master fault interaction. These imbricate faults, as normal dip-slip faults with a small sinistral component, accommodated extensional deformation in the stepover zone by NNW stretching, fluid influx, precipitation and sealing and by a small vertical displacement.

ACKNOWLEDGMENTS This research is being funded by Fondecyt grant 1020436 (JC) and Fundación Andes Project C- 13755(12)(GG). Verónica Herrera’s Ph.D. thesis work is partially funded by MECESUP project UCH0010.

Figure 2. Geologic and structural map of the Caleta Coloso Duplex in the Atacama Fault System, south of Antofagasta. Incremental strain tensor solutions, as calculated from fault-slip data, are shown by locality. Number of faults per site is shown for each diagram. Strike-slip deformation dominates along the master faults; transtensional to extensional strain prevails along the imbricate faults of the duplex.

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