Structural blocks

The study area was subdivided into structural blocks, defined here as geographically restricted areas with a well-defined lithotype and age range and a homogeneous deformation style. The structural blocks identified contain from 1 to 21 structural stations. Thirty one structural blocks were defined; their distribution is shown in Figure 6.7, and a summary of their lithological characteristics and age is provided in Table 6.1.

Figure 6.8 shows the result of the kinematic and dynamic analysis for each of the structural blocks. Four of them (2, 4, 15 and 29) contain less than four measured fault planes, and consequently were not used for the dynamic analysis. Structural blocks 19 and 28 contain four and five fault planes respectively, but all the possible

subgroups of faults were found to be incompatible with any unique stress tensor. As a result of this, it was only possible to carry out dynamic analyses on 25 of the 31 structural blocks shown on Figure 6.7.

Figure 6.7. Location of the 31 structural blocks into which the study area was subdivided, shown over the geological map. Legend as in Fig. 6.2.

Chapter 6 – Structural evolution of the Andean Main Cordillera of Central Chile

Table 6.2. Summary of the lithological units present in each of the 31 structural blocks defined in the study area. Age ranges from Gana and Wall (1997), Kurtz et al. (1997), Rivera and Falcon (1998), Baeza (1999), Aguirre et al. (2000), Charrier et al. (2002), Deckart et al. (2005, 2010), Muñoz et al. (2006), Montecinos et al. (2008) and this study.

Structural

block Geological unit Lithology Age range

1 Abanico Formation Andesitic lava flows, pyroclastic intercalations 34-22 Ma

2 Rio Colorado batholith Granodiorite 22-21 Ma

3 Abanico Formation Andesitic lava flows, pyroclastic intercalations 34-25 Ma

4 Sills in Abanico Formation Granodiorite 12-11 Ma

5 Abanico Formation Andesitic lava flows, pyroclastic intercalations 34-25 Ma 6 Abanico Formation Andesitic lava flows, pyroclastic intercalations 34-25 Ma 7 Farellones Formation Andesitic lava flows and pyroclastic deposits 22-16 Ma 8 Estero Barriga Intrusive Complex Quartz-monzonite 15-14 Ma

9 San Francisco batholith Syenogranite 16-15 Ma

10 Rio Blanco Granodiorite Granodiorite 12-11 Ma

11 Farellones Formation Andesitic lava flows 17-16 Ma 12 Abanico Formation Volcano-sedimentary deposits, pyroclastic intercalations 26-18 Ma 13 Farellones Formation Andesitic lava flows, dacitic and rhyolitic intercalations 22-19 Ma 14 Abanico Formation Andesitic lava flows, pyroclastic intercalations 31 Ma 15 Abanico Formation Andesitic lava flows, pyroclastic and volcano-sedimentary deposits 31-19 Ma

16 Meson Alto pluton Granodiorite 12-11 Ma

17 Rio Damas, Lo Valdes and Colimapu formations Limestones, sandstones and conglomarates Oxfordian-Albian 18 Abanico Formation Andesitic lava flows, volcano-sedimentary intercalations 27-25 Ma

19 San Gabriel pluton Granodiorite 12-11 Ma

21 Abanico Formation Andesitic lava flows 34-25 Ma 22 Abanico Formation Andesitic lava flows, volcano-sedimentary intercalations 34-21 Ma 23 Teniente Volcanic Complex Pyroclastic deposits 13-12 Ma 24 Teniente Volcanic Complex Andesitic lava flows 12-8 Ma 25 Teniente Volcanic Complex Andesitic lava flows 12-8 Ma 26 Coya-Machali Formation Volcano-sedimentary deposits, pyroclastic intercalations 23-13 Ma 27 Teniente Volcanic Complex Andesitic lava flows 12-8 Ma

28 Pangal Intrusive Complex Granodiorite 10-9 Ma

29 Cortaderal Intrusive Complex Granodiorite 12-11 Ma 30 Teniente Volcanic Complex Andesitic lava flows, pyroclastic intercalations 12-8 Ma 31 Coya-Machali Formation Andesitic lava flows, volcano-sedimentary intercalations 16-12 Ma

Chapter 6 – Structural evolution of the Andean Main Cordillera of Central Chile Strong spatial patterns are evident from the analysis of structural blocks. In the

structural blocks located close to the Rio Blanco-Los Bronces porphyry Cu-Mo cluster (structural blocks 7, 9, 10 and 11; Figs. 6.7, 6.8),fault-slip data is consistent with strike-slip regime under E-W compression (E-W trending, sub-horizontal shortening axis and σ1; N-S oriented extension axis and σ3; and sub-vertical intermediate kinematic axis and σ2). No secondary clusters were recognized by the Multiple Inverse Method. Toward the east, structural block 6 shows a similar pattern of E-W compression and N-S extension, but a secondary cluster of sub-vertical σ3 is evident using the Multiple Inverse Method. To the east of block 6, block 5 is

composed of strongly deformed rocks of Abanico Formation, close to the eastern inverted basin-margin faults and the contact with the Mesozoic units (Fig. 6.7). Block 5 was also affected by E-W compression, but the stereographic projection for σ3 shows two main steeply plunging clusters, while the average stretching kinematic axis is sub-vertical (Fig. 6.8). This indicates that in this area E-W shortening was mostly accommodated by reverse faulting. The rocks affected by faulting at blocks 5 and 6 belong to the same unit and age range (Table 6.1), and the same syn-tectonic hydrothermal minerals are found on the fault planes. This suggests that different stress regimes operated at the same time in different parts of the Abanico Basin during tectonic inversion. A similar situation can be observed around the El Teniente porphyry Cu-Mo deposit, at the transition between structural blocks 30 and 31 (Figs. 6.7, 6.8) which are both composed of late Tertiary volcanic rocks. In block 30, located in the central part of the inverted Abanico Basin, faulting is consistent with a strike-slip deformational regime, while at block 31, located immediately to the east and closer to the eastern basin margin, a dominant sub-vertical cluster for σ3

indicates shortening was accommodated by reverse faulting.

Fault-slip data of most structural blocks is consistent with faulting under E- to ENE- directed compression. However there are several exceptions, particularly in the Maipo area. Some structural blocks (e.g., blocks 17, 18, 23 and 24) clearly show a cluster of sub-vertical σ1 and pressure axes, indicating that faulting occurred in response to extensional deformation. Also, a unique characteristic of structural block 17 is the presence of a major cluster of almost N-S trending σ1.

Figure 6.8. Results of the analysis of fault-slip data for the 31 structural blocks. All stereoplots are lower-hemisphere, equal-area projections. For each structural block, the first plot illustrates the fault planes. The second stereoplot shows the P and T axes for each fault plane as blue and red dots respectively, together with the average kinematic axes (1=shortening, 2=intermediate, 3=stretching). The third and fourth stereoplots show the calculated orientations of σ1 and σ3 for

Chapter 6 – Structural evolution of the Andean Main Cordillera of Central Chile

Figure 6.8. (Cont.)