Tectonic evolution

3.6.1 Late Eocene to early Miocene: extension and intra-arc basins

This period was characterized by the opening and infill of the Abanico intra-arc volcano-tectonic basin. The main basin-margin normal faults (Pocuro-Infiernillo and Alto del Juncal faults) were N-striking, and the area within them was completely covered by the products of the Abanico Formation (Figs. 3.2, 3.10). However, strong changes in thickness and volcano-sedimentary facies in this unit indicate the

presence of various sub-basins and depocenters. These internal sub-basins were bounded by NW- and NE-striking normal faults (e.g., Figs. 3.5A, 3.11). The tectonic regime during this period was characterized by broadly E-W extension, and the ascent of magma to the surface was favored by a thinned continental crust (Kurtz et al., 1995) and several deep-tapping, high-angle normal faults. From 34 to 22 Ma as much as 5 km of volcanic rocks were deposited in the basin (Figs. 3.6, 3.10), with no coeval plutonic bodies recognized (Table 3.2).

3.6.2 Early Miocene to early Pliocene: Tectonic inversion, plutonism and porphyries

During this period the high angle, ~N35°W and ~N40°E, intra-basin extensional faults of the Abanico Basin were reactivated in strike-slip sinistral and dextral mode, respectively. Some reverse dip-slip movement occurred, locally associated with intense folding of the nearby Abanico and Farellones formations (Fig. 3.5A). The mainly N-striking basin-margin faults were reactivated as reverse faults.

Table 3.2. Summary of the tectonic, stratigraphic and magmatic evolution of the Rio Blanco-Los Bronces district

Age Tectonic regime

Stratigraphic units Plutonic units Porphyries and breccias Active structures

Pliocene- Quaternary (4-0 Ma) Post- compression relaxation

Moraines, colluvial and alluvial unconsolidated deposits

NE faults are reactivated as post-mineral normal

faults

Late Miocene-early Pliocene (7-4 Ma)

Compression Rhyolitic pyroclastic deposits within the diatreme of the La Copa subvolcanic complex

Dacitic, rhyolitic and andesitic porphyries, tourmaline and rock-flour breccias, dacitic and rhyolitic diatremes; porphyry- style mineralization

NW faults control the emplacement of early syn- tectonic veins and breccias, but late veins were emplaced along dextral NE faults; uplift is controlled by reverse faults located in the eastern part of the district and in Argentina

Middle to late Miocene (15-8 Ma)

Compression Farellones Formation, upper member

Monzonite, monzodiorite, granodiorite, diorite, tonalite

Andesitic dikes, early tourmaline and actinolite- magnetite breccias

Strike-slip dextral NE faults, reverse-sinistral NW faults, reverse N-S faults

Upper early Miocene-middle Miocene (17-16 Ma)

Compression Farellones Formation, middle member

Syenogranite Andesitic dikes Strike-slip dextral NE faults, reverse-sinistral NW faults, reverse N-S faults

Early Miocene (23- 17 Ma)

Compression Farellones Formation, lower member

Granodiorite, diorite, gabbro

Andesitic dikes, dacitic and rhyolitic porphyries

N-S faults are reactivated as reverse faults; NW faults reactivated as composite sinistral-reverse faults; NE faults reactivated as dextral strike-slip faults

Late Eocene-late Oligocene (34-25

Chapter 3 – Structural Evolution of the Rio Blanco-Los Bronces District This selective reactivation of pre-existing normal faults with different orientations generated the present-day structural architecture, whereby sub-basins are bounded by high-angle faults, each with its own thickness of local volcano-sedimentary facies, intensity of folding and exhumation level (Fig. 3.10). None of the pre-existing fault systems was ideally oriented for reactivation under E-W compression (e.g., Sibson, 2000). After the early Miocene supra-lithostatic fluid pressures were achieved in the study area, as evidenced by the presence of abundant dilational, sub-horizontal sills of Miocene age (Fig. 3.13B). This, together with the abundance of syn-tectonic hydrothermal minerals, demonstrates that conditions were appropriate for the reactivation of non-ideally oriented faults (Sibson, 1985, 2000). Tectonic inversion was coeval with the deposition of the Farellones Formation, which differs markedly from the Abanico Formation in that it covers a considerably smaller surface area and it reaches a maximum thickness of only 1.5 km (Figs. 3.6, 3.10). The basal layers of Farellones Formation cover the Abanico Formation in progressive unconformity. The 22.7 Ma age obtained in the basal pyroclastic flow (Table 3.1) marks the inception of the compressive regime in this segment of the Tertiary Andean magmatic arc (see Table 3.2). This is in good agreement with field evidence showing that NW-striking, subvertical dikes dated at ~22 Ma were emplaced syn-tectonically under an E-W compression direction (Fig. 3.13A).

Initiation of plutonic activity was coeval with Farellones Formation volcanism and tectonic inversion of the Abanico Basin. The earliest plutons in the district

correspond to granodioritic bodies currently exposed at the Colorado (21.76 ± 0.53 Ma; U-Pb in zircon, this study, Table 3.1, sample AN12JP011) and San Francisco river valleys (20.1 ± 2.0 Ma; K-Ar in hornblende, Warnaars et al., 1985). Magmatism culminated with the emplacement of structurally-controlled rhyolitic porphyries and diatremes (~4.69 Ma, U-Pb zircon ages; Deckart et al., 2013; this study, Table 3.1, sample AN13JP013). The units dated between 20.1 and 8.16 Ma, are equigranular plutonic rocks, while those with ages between 7.12 and 4.69 Ma are subvolcanic rocks directly associated with hydrothermal activity and mineralization (Table 3.2; Deckart et al., 2005, 2013). The host rocks of these subvolcanic complexes include the older equigranular plutons. This implies that in a 1 m.y. period between 8.16 and 7.12 Ma, this area was subject to high exhumation rates, unroofing of the older plutonic rocks and exposure to the subvolcanic environment, with porphyry dikes

and diatremes being fed by a deeper, unexposed magma chamber. Given the

characteristics and erosion level of the Rio Blanco-Los Bronces cluster (e.g., Proffett, 2009; Sillitoe, 2010), this magma chamber is inferred to have been located at 5-7 km below the present-day surface. This depth coincides well with the uppermost of the three detachment levels interpreted in our cross-sections, and also with a notable area of low Vp/Vs ratios in seismic tomography (Fig. 3.15), which we speculate correlates with the very young (<4 Ma) crystalline rocks of the deep magma chamber that solidified after volatile exsolution and formation of the deposits of the Rio Blanco- Los Bronces cluster. Isotopic data from anhydrite in the Rio Blanco and Sur Sur sectors (Frikken, 2004; Frikken et al., 2005), suggest an interaction of the

mineralizing fluids with Mesozoic evaporites, indicating their source must be located below the base of the Tertiary volcanic rocks.

Figure 3.15. Upper panel: geological cross-section through the porphyry deposits of the Rio Blanco-Los Bronces cluster (legend in Fig. 3.10). Lower panel: bedding planes and intrusive contacts, faults, Vp/Vs tomography and distribution of hypocentres in the same cross-section. RB-LB = Rio Blanco-Los Bronces cluster.

Chapter 3 – Structural Evolution of the Rio Blanco-Los Bronces District Intrusive contacts, porphyry dikes, hydrothermal breccias and veins, all show strong NW and NE preferred orientations (Fig. 3.2, 3.4, 3.5, 3.7B, C), indicating that reactivated faults channeled the ascent and emplacement of magma and

hydrothermal fluids during the compressive stage, acting as feeders both to the batholiths and the mineralized systems. In the Rio Blanco – Los Bronces cluster in particular, the main breccia bodies are aligned and elongated parallel to the N35°W Rio Blanco-Los Bronces fault system (Fig. 3.4B), while most of the late veins (quartz – tourmaline – sulfide veins with sericitic haloes and carbonate – sulfide – sulfosalts veins) are emplaced along the ~N40°E faults (Fig. 3.7B).