District-scale fault systems

High-angle fault systems recognized in the district can be grouped in three main sets, according to their orientation: NW-, NE- and N-striking. The first two sets are

predominant in most of the study area, while the N-striking faults are restricted to the western and eastern inverted margins of the Abanico Basin (Fig. 3.2). Apart from these three sets of high-angle fault systems, locally there are low-angle detachments and ramp-flat thrust faults rooted in stratigraphic boundaries between contrasting packages of Tertiary volcanic rocks. All the individual faults mentioned in this section are labeled in Figure 3.2.

NW-striking fault systems: they occur in the central part of the district (Fig. 3.2). The general strike is N35°W, dipping 60-70°W. The strike of individual fault planes varies between N20°W and N60°W. Fault activity generated major drag folds, which indicate east-vergent reverse movements (Fig. 3.5A). Drag folds affected the

Abanico Formation and the lower and middle members of the Farellones Formation (Fig. 3.5A), showing that their last reverse-reactivation increment was post-early Miocene. Steps in syn-tectonic hydrothermal mineral fibers coating fault planes indicate that the reverse movement was coupled with a sinistral strike-slip

component. However, stratigraphic correlations indicating normal movements (Fig. 3.5A), together with the high dip angle of the fault planes, provide evidence that these structures correspond to reactivated normal faults.

It is possible to distinguish three main parallel NW-striking fault systems in both the northern and the southern parts of the district. The correlation along strike between these two groups of faults is not evident as their trace is obscured by syn-tectonic

magmatic and hydrothermal activity and by displacements of up to 400 meters along cross-cutting NE-striking faults (Fig. 3.2).

In the northern part of the district, three main faults can be distinguished, from west to east: the Cerro Pelado, Las Amarillas and Matancilla faults (Fig. 3.2). These faults are spatially associated with the emplacement of dikes and elongated stocks of both the northernmost Rio Blanco – San Francisco batholith and the Rio Colorado plutonic complex (Figs. 3.2, 3.5). The contacts between the plutons and their volcanic host rocks are rectilinear and located on strike of major faults (Figs. 3.2, 3.5) but they are of intrusive nature, without evidence of significant post-magmatic fault reactivations. The Matancilla fault is also related to major drag folds that affect strata of the Abanico Formation and the lower member of the Farellones Formation, indicative of reverse movement (Fig. 3.5A). However, stratigraphic correlations across the fault indicate an earlier normal movement (Fig. 3.5A). In the southern area, from west to east the Ortiga, Rio Blanco – Los Bronces and Barriga fault systems are recognized (Fig. 3.2). Rhyolitic domes were emplaced along the Ortiga fault in the south-western margin of the Rio Blanco – San Francisco batholith. The Rio Blanco – Los Bronces fault system (Castelli and Lara, 1999; Silva and Toro, 2009) extends both to the NNW and to the SSE of the Rio Blanco – Los Bronces cluster (Figs. 3.2, 3.4B). Steps in syn-tectonic tourmaline and actinolite fibers systematically indicate sinistral strike-slip movements, occasionally with a reduced reverse component. Subvolcanic porphyries and hydrothermal breccias of the cluster were emplaced along the central part of the fault system (Fig. 3.4B). Individual branches of the Rio Blanco – Los Bronces fault system controlled the emplacement of syn-tectonic hydrothermal biotite veins in late Miocene times (Silva and Toro, 2009) and have been recognized by mine geologists as the most prominent fault system in the Rio Blanco sector of the cluster (Skarmeta et al., unpub. report for CODELCO, 2004). The Barriga fault system forms the north-eastern margin of the Rio Blanco – San Francisco batholith (Fig. 3.2, 3.4B), and shows evidence of sinistral strike-slip movement with a smaller reverse component, according to steps in syn-tectonic actinolite fibers.

Chapter 3 – Structural Evolution of the Rio Blanco-Los Bronces District

NE-striking fault systems: these fault systems have a general strike of N40°E, although the strikes of individual fault planes vary from N30° to N70°E. They generally dip at high angles to the NW, varying from 60° to 90°. These faults show a dextral strike-slip movement, with minor reverse dip-slip components (Fig. 3.7). They commonly displace NW- and N-trending faults and fold axis, such as the Matancilla and Alto del Juncal faults and their related fold trends (Figs. 3.2, 3.4A). The observed offsets are of up to 400 meters. As with the NW fault systems, the stratigraphic correlations across these faults generally indicate normal movements, showing they also correspond to reactivated normal faults. In the core of these faults, there are abundant zones rich in fault gouge and tectonic breccia, while in the

damage zone there are abundant E-W vertical extensional structures filled mainly by quartz and calcite, which commonly occur in the transition to undamaged rock. The main faults are commonly surrounded by several individual fault planes displaying syn-tectonic hydrothermal minerals fibers of quartz, chlorite, epidote, calcite or tourmaline (Fig. 3.7D, E).

At least three individual major fault systems with NE strikes can be recognized across the Rio Blanco – Los Bronces district. Magmatic bodies and syn-tectonic hydrothermal veins have been emplaced along all of them (Fig. 3.7B, C, D, E). From north to south, they are the Saladillo, Flores and El Salto fault systems (Fig. 3.2). The Escondida fault, identified in the Los Bronces sector of the cluster by previous workers (Warnaars et al., 1985), is considered here as part of the El Salto fault system. Several individual faults belonging to this system have been recognized by mine geologists in the Rio Blanco and Sur Sur areas as well (Skarmeta et al., 2004). The average strike of fault planes within the El Salto fault system is N40°E, while the average dip is 70-80°NW. Across the district, several plutonic bodies and subvolcanic porphyries occur at the intersection of this fault system with the NW- and N-striking faults. Quartz and tourmaline veins with sericitic haloes,

hydrothermal breccia dikes and dacitic and rhyolitic dikes were emplaced along different branches of the El Salto fault system (Fig. 3.7B, C). Some of these branches are the only faults that record evidence of post-mineral reactivation in the Rio Blanco – Los Bronces cluster, as they cross-cut the tourmaline-cemented mineralized

hydrothermal breccias with displacements in the order of meters to tens of meters (Fig. 3.4B).

Figure 3.7. NE-striking fault systems in the Rio Blanco-Los Bronces district. A. Pyroclastic layers of the lower member of Farellones Formation displaced by strike-slip dextral movement along a N45°E fault, part of the Flores fault system at 380687mE, 6348584mN. The offset is approximately seven meters. B. Tourmaline-quartz-pyrite-chalcopyrite vein with sericitic halo and clasts of granodiorite porphyry emplaced along a N60°E-striking, subvertical fault. View NE at 383262mE, 6339743mN. C. Dacite porphyry emplaced along a N35°E structure, part of the Flores fault system. All the contacts of the porphyritic intrusion are of intrusive nature. View NE from 379198mE, 6342426mN. D. Main fault of the El Salto fault system at 386364mE, 6339053mN. The orientation of the fault plane is N40°E/80°NW and it displays syn-tectonic quartz and chlorite fibers indicating dextral strike-slip movement with a reduced normal component. Black arrow indicates sense of movement of the missing block. E. Fault plane belonging to the Saladillo fault system at 381485mE, 6357206mN. Orientation of the fault plane is N55°E/80°N. Steps in syn-tectonic epidote fibers indicate a dextral strike-slip movement with

Chapter 3 – Structural Evolution of the Rio Blanco-Los Bronces District

N-striking fault systems: in the eastern part of the study area, there is a system of east-vergent reverse faults striking N-S and dipping 55°-70°W, with some local, west-vergent back-thrusts (Figs. 3.2, 3.4A). The main faults of this system are the El Fierro and Alto del Juncal faults (Figs. 3.2, 3.4A). Miocene intrusive bodies have been emplaced along the trace of both of these faults (Figs. 3.2, 3.4A), without evidences of post-magmatic faulting. Between these two structures, the lava flows of Abanico Formation have been tightly folded in a series of east-vergent, overturned folds (Fig. 3.8A).

The El Fierro fault juxtaposes the Tertiary Abanico Formation hanging wall rocks to the west against the Mesozoic San Jose or Lo Valdes Formation footwall

sedimentary rocks to the east (Fig. 3.8B). Bedding in the hanging wall is parallel to the fault. This structure was originally defined by Davidson (1971) in southern central Chile as a high angle reverse fault which juxtaposes Tertiary volcanic rocks (Piquer et al., 2010) on top of older Mesozoic sedimentary rocks. This structure has been correlated with equivalent faults exposed to the north of Davidson’s (1971) original locality, where they have been given different names (Las Leñas and El Diablo faults, Charrier et al., 2002; Fock et al., 2006; Farias et al., 2010). Here we retain the original nomenclature, as the structure observed in our study area fits with the original definition given by Davidson (1971).

The faults associated with the western margin of the inverted Abanico Basin are almost entirely located outside the study area, with the exception of some outcrops in the north-west (Fig. 3.2) where the Pocuro fault can be observed. This structure strikes N-S and dips 60-80°E, and juxtaposes different andesitic packages of the Abanico Formation. The fault is associated with a trend of parallel folds in the Tertiary volcanic rocks and, outside of the study area, has controlled the

emplacement of several intrusive bodies of small dimensions and variable textures and compositions (Castelli and Lara, 1999). The contact with the Late Cretaceous rocks of the Lo Valle Formation is located further west, associated with the Infiernillo fault (Fig. 1.2B; Fuentes et al., 2002; Farias et al., 2010).

Figure 3.8. Deformation style at the inverted eastern margin of the Abanico Basin. A. East- vergent anticline-syncline pair affecting Abanico Formation lava flows. View N from 399397mE, 6349379mN. B. Immediately to the east of (A), the Tertiary volcanic rocks are in fault contact with Mesozoic limestones of the Lo Valdes Formation along the El Fierro fault. View N from 400682mE, 6348304mN.

Detachments and ramp-flat thrusts: local low-angle thrusts detachments are developed in stratigraphic contacts within the Tertiary volcanic sequence. The most continuous of these faults occurs in the western part of the district (Fig. 3.2), where

Chapter 3 – Structural Evolution of the Rio Blanco-Los Bronces District the middle andesitic member of Farellones Formation is detached from the lower pyroclastic member by a west-vergent thrust, which propagates upwards forming a ramp which repeats the middle member (Fig. 3.9).

Figure 3.9. Ramp-flat fault in the Farellones Formation (solid black line). Andesitic lavas of the middle member are detached from the lower pyroclastic member, and are repeated by a west- vergent ramp. Dashed line represents the contact between the two members. Aerial view S from 366600mE, 6346600mN.