Abstract

Models of fault zones development are very important in order to predict the geometry and distribution of fractures at all scales, providing useful information for both industry and scientific research. Currently, most models are based on analyses of microfractures generated in rock-deformation laboratories [1-3] or on field measurement analysis of outcrop- and regional-scale faults [4, 5]. However, models including the nature and progressive development of structures from micro- to regional-scale are relatively scarce [6], mainly because of the lack of well-exposed fault zones exhibiting fractures at different scales. Here, we present and analyze structures related to the propagation and evolution of the strike-slip Bolfín fault of the Atacama Fault System [7, 8] observed within a broad range of scales covering 9 orders of magnitude. Also, we present a model for the evolution and development of fault zones. 2. Bolfín Fault It is one of the main faults or “first-order faults” within the Caleta Coloso Duplex, which is part of the Atacama Fault System located in the Coastal Cordillera ca. 20 km south of Antofagasta [4]. The Bolfín fault splays off from the Caleta Coloso fault with an average trend of 162º, and a length of at least 45 km. The fault cuts mainly meta-diorites and exhibits a cataclastic core of about 100- 200m wide characterized by sub-vertical bands of cohesive fault-rocks and fractured host rock (fig. 1.B and C). , very similar to those previously reported for the core of the Caleta Coloso fault [4, 9-11]. Cataclasites display centimetric shear foliation (C) with an orientation of 175º/50º (RHR) and consists of microcrystalline epidote and angular fragments of plagioclase (> 0,1mm). A microstructural analysis of two thin sections (horizontal and vertical, both perpendicular to the fault-strike) reveals two main families of epidote-filled fractures: I: 145º/53º and II: 110º/68º, forming acute angles of 24º and 57º (Counterclockwise), with respect to C surfaces. According to these angular relationships, families I and II can be interpreted as been in R and R’ position with respect to C. 3. Subsidiary Faults at the Tip Point of Bolfín Fault According to its internal structure, these faults can be divided in two types: (1) Simple Core Faults and (2) Multiple-Strands Core Faults. Simple Core faults commonly show a green cataclastic core of about 0.5-2 m wide. They have two sets of different planar structures, one defined by compositional and textural bands, and another defined by discrete preferentially-oriented discontinuities (fractures). The compositional and textural bands are parallel to the core boundaries andconsist of layer of different colours and different matrix/fragments ratios that can be classified as different fault rocks. The discrete fractures show a preferred orientation between 20º and 75º with respect to the core boundaries. These fractures cut both the cataclastic core and the surrounding host rock (damage zone), but the majority are in the core. Thus, the Bolfin fault can be considered a larger and more developed simple-core fault. Multiple-core faults consist of several sub-parallels centimeter-to-meter-long and milimiter-to-entimeter wide cataclastic-core faults separated by domains of lessfractured protolith. Most of the deformation in the multiple-strands-core faults is accommodated by sub-parallel faults which are separated by distances one order ofmagnitude smaller than its lengths, here named main faults. These main faults are connected by several smaller oblique subsidiary fractures, named secondary fractures. The secondary fractures show both evidence of extensional and shear displacement, with the largest extension amounts focused on the central part. They show characteristic sigmoid shapes, or sub-asymptotic, which becomes parallel to the main faults as they approach them. The geometrical and spatial distribution of these two types of fractures gives a metric-size multi-duplex shape to the internal structure of the multiple-core faults, very similar to the kilometric-scale distribution of the faults within the Caleta Coloso Duplex. Two examples of this geometry are the Parulo and the Palmera faults, both located at the tip-point of Bolfín fault. These faults show mostly sinistral horizontal separation, even though some main faults show dextral displacements. 4. Conclusion The detailed observation of fault zone structure at different scales, strongly suggest that simple-core faults initiated from multiple-core faults. Main faults developed during initial stages; secondary faults then formed in the regions between main faults, connecting them. Thus, as the total strain increased in the multiple-core faults, secondary fractures increasingly developed, until they became banded zones of fault-rocks, crush breccias or cataclasites (depending on the amount of strain). In this way, the total area affected by main faults during their initial developing stages, turned into cataclastic zones as the strain increase.