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Stratigraphic correlation shows that the main district-scale high-angle fault systems had early normal movements, and controlled the location of depocenters of Tertiary volcanic and sedimentary rocks. In particular, the three main NE fault systems (Saladillo, Flores and El Salto faults, Fig. 3.2) dip to the NW, and their normal movements seem to have produced progressively deeper sub-basins towards the NW, which were only partly filled by volcanic and sedimentary rocks of the Abanico Formation, producing a NW-dipping paleo-topography. This paleosurface was covered by the volcanic products of the Farellones Formation, explaining why the Abanico-Farellones unconformity is found at ~3100 meters a.s.l. (above sea level) close to the Rio Blanco – Los Bronces cluster, while in the northern part of the district is located at ~2600 meters a.s.l. A paleo-topography controlled by normal faults can also explain why the Abanico-Farellones transition is found at

considerably lower elevations in the hanging wall of the Matancilla fault compared with the foot wall (Fig. 3.5).

Kinematic indicators demonstrate that during tectonic inversion, after the deposition of the Farellones Formation, the NE-striking structures were reactivated as dextral strike-slip faults, whereas NW-NNW faults typically show a combination of sinistral and reverse movements (Figs. 3.2, 3.4, 3.5, 3.7). This suggests that the bulk of the measured fault planes were reactivated under E-W contraction, and that compressive stresses were accommodated in the central part of the inverted Abanico Basin by a combination of strike-slip movement along arc-oblique, high-angle fault systems and

local ramp-flat reverse faults (Fig. 3.9). Structural style is different in the easternmost part of the inverted basin (to the east of the Alto del Juncal fault) and in the Mesozoic rocks of the Eastern Main Cordillera, where compressive deformation was

accommodated by widespread thrusting and folding (Fig. 3.8, Aconcagua fold and thrust belt), not by strike-slip faults.

The Abanico and Farellones formations and the Miocene plutons are all cut by fault planes with kinematic indicators consistent with E-W contraction, suggesting that deformation associated with tectonic inversion was active at least throughout the Miocene.

3.7.1 The emplacement of subvolcanic porphyries and hydrothermal breccias

The Rio Blanco-Los Bronces cluster is defined by a NW-NNW-trend of subvolcanic porphyries and hydrothermal breccias, which were emplaced along the Rio Blanco- Los Bronces fault (Fig. 3.4B). As noted by Mpodozis and Cornejo (2012), the orientation of this fault is, in theory, not favorable for dilation under an E-W compressive regime. They suggested that a possible answer to this paradox is the sudden release of magmas and hydrothermal fluids triggered by major earthquakes, allowing their ascent through non-ideally oriented faults, decompressing a magma chamber which under normal conditions would remain sealed.

The geometry of the deposit can also be explained by the interaction of two conjugate fault systems, the NE-striking dextral El Salto fault and the NW-NNW- striking sinistral Rio Blanco-Los Bronces fault (Fig. 3.4B). Incremental movement of the El Salto fault system can produce dilation in the NW-NNW-striking structures, creating space for the emplacement of porphyries and hydrothermal breccias even under E-W compression, as shown in Figure 3.16. A similar mechanism was proposed by Hodgson (1989) for the emplacement of gold-rich veins in one of two sets of intersecting conjugate faults. This could be considered an analog to the fluid- pump behavior of faults suggested by Sibson (1986). The original model was proposed for the epithermal environment under hydrostatic fluid pressures, but the same geometric arrangement could promote even more catastrophic fluid suction and

Chapter 3 – Structural Evolution of the Rio Blanco-Los Bronces District hydrothermal brecciation in the deeper porphyry environment, which under stable conditions (inter-seismic periods) is under quasi-lithostatic pressures.

3.7.2 The role of structures oblique to the magmatic axis

This study shows that NE- and NW-striking faults controlled the

compartmentalization of the Abanico Basin during the late Eocene-Oligocene, absorbed most of the internal deformation of the basin during inversion in the Mio- Pliocene and also controlled the emplacement of magma and hydrothermal veins. This has not been suggested previously for the Rio Blanco-Los Bronces district, but Rivera and Cembrano (2000) and Rivera and Falcon (2000) presented similar conclusions from regional mapping around the El Teniente deposit, located about 100 km to the south in the same metallogenic belt (Figs. 1.2B, 3.1).

The origin of these arc-oblique structural systems may be found in the basement of the Main Cordillera. The main faults identified in published geological maps at the Coastal Cordillera of central Chile, where Paleozoic and Mesozoic rocks are exposed without the Tertiary cover, also have a NW- and NE orientation, with a complete absence of major faults parallel to the continental margin (Fig. 1.2B, 3.1;

SERNAGEOMIN, 2002). These orogen-oblique faults controlled the location of Mesozoic plutons and also seem to be responsible for the interruption of the late Paleozoic-Triassic belt, which gets interrupted to the north of Santiago by a major NW-striking structural system (Fig. 1.2B, 3.1). This regional-scale structural architecture of the Coastal Cordillera is remarkably similar to the structural pattern observed at the Rio Blanco-Los Bronces district (Fig. 3.2).

During the Triassic, subduction at the Gondwanan margin is thought to have been interrupted, or at least greatly diminished, signaling a pause in the continental drift of

Figure 3.16. Diagram illustrating how space (represented by dark gray rectangles) can be created along NW- NNW-striking faults under E-W compression, through incremental slip along an active, NE-striking, dextral fault system.

Gondwana (Charrier et al., 2007). This pause favored the accumulation of heat in the upper mantle, melting of the lower crust, the generation of large volumes of silicic magma, and the onset of extensional tectonics in the upper crust (Charrier et al., 2007). Palaeo-geography was dominated by fault-bounded NNW to NW-trending extensional basins and NE-striking transfer faults (Mpodozis and Ramos, 1989; Niemeyer et al., 2004; Charrier et al., 2007; Ramos, 1996; Giambiagi et al., 2003a). According to Ramos (1994), the orientation of the Triassic rifts reflects the existence of even older, NW-trending weakness zones in the crust associated with suture zones bounding allochthonous terranes accreted during the Proterozoic and the Paleozoic to the western margin of Gondwana. Other authors (e.g., Salfity, 1985; Jacques, 2003) have shown that these NW and NE structures are major features of the entire South American continent, acting as weakness zones which are repeatedly reactivated, playing a major role in the tectono-stratigraphic evolution of the continent. These arc-oblique fault systems are interpreted to have played an important role controlling the emplacement of plutonic bodies and the location of volcanic centers and mineral deposits in different segments of the Andes of Chile and Argentina, sometimes at their intersections with arc-parallel strike-slip faults (Richards et al., 2001; Chernicoff et al., 2002; Cembrano and Lara, 2009; Acocella et al., 2011).

Considering all of the evidence outlined above, we suggest that both the NW- and NE-trending fault systems observed in the Rio Blanco-Los Bronces district

correspond to pre-Andean structures which characterize the structural architecture of the Paleozoic-Triassic basement. These structures have been reactivated several times throughout the Mesozoic and Cenozoic, with different kinematics depending on the prevailing tectonic regime. The middle to late Tertiary movements described in this paper corresponds to the last part of this reactivation history. The fact that they are deep, penetrative basement structures explains their effectiveness in channeling and focusing magmatism through the crust.