Directions for future research

Several possible research avenues can be followed to increase our understanding of the structural geology of the Andes of central Chile and its relation with the transport of magmas and hydrothermal fluids. Some of them are:

- The results of this work suggest that the Piuquencillo fault is a major arc- oblique fault system, which bounds two different segments of the

Abanico Basin with contrasting tectonic and stratigraphic histories. More detailed work along this fault system is needed to better understand the stratigraphic facies changes associated with it, in particular the transition between the Abanico and Farellones formation in the northern block to the Coya-Machali Formation and the Teniente Volcanic Complex in the south. Several Miocene plutonic bodies are emplaced along this fault; additional studies could be done to look for evidence of syn-tectonic

plutonic emplacement and to study the exploration potential of their associated hydrothermal systems.

- To the south of the Piuquencillo fault, most of the Tertiary rocks can be grouped in the Miocene Coya-Machali Formation and the Teniente Volcanic Complex. However, previous work has shown that at specific localities there are outcrops of late Eocene rocks coeval with the Abanico Formation. The contact relation between these older rocks and the Coya- Machali Formation sensu stricto remains unclear. Is there a temporal hiatus? What is the correct stratigraphic assignation of the older rocks? They could be considered as part of the Abanico Formation, as a lower member of the Coya-Machali Formation, or as an entirely different unit. Detailed stratigraphic mapping and radiometric dating is needed to solve this problem.

- The arc-oblique fault systems recognized by this study are commonly associated with syn-tectonic hydrothermal veins (e.g., Fig. 3.11). Some of them contain base and precious metal mineralisation and are exploited by small-scale operations. As most of the scientific studies in the area have focused on the giant porphyry Cu-Mo systems, these veins have received little attention. When were they formed? Under which conditions? Are they coeval with the porphyry Cu-Mo deposits? Are some of them related with “blind” porphyry Cu systems, or do they have no further exploration potential? All these questions remain unanswered.

- 40Ar/39Ar ages on volcanic rocks presented by this and previous studies suggest the possibility that in the Rio Blanco-Los Bronces and Maipo segments, Farellones Formation rocks were affected by the thin-skinned deformation at the eastern margin of the Abanico Basin. However, the reliability of the 40Ar/39Ar ages is questionable. Detailed structural and stratigraphic mapping and U-Pb zircon geochronology will be required to confirm or discard the presence of blocks of Farellones Formation rocks bounded by segments of the eastern basin-margin faults.

Conclusions

Chapter 8

Conclusions

Structural mapping and the study of regional geophysical datasets show that the Western Main Cordillera of central Chile is dominated by NE- and NW-striking fault systems, oblique to the continental margin and to the axes of the Meso-Cenozoic magmatic arcs (Fig. 4.1). Based on their correlation with older faults observed in the Coastal Cordillera to the west (Fig. 6.10) and in the Andean basement in Argentina to the east, it is proposed that these arc-oblique faults correspond to long-lived, reactivated basement structures. Some of them bound regional-scale gravimetric domains (Fig. 4.3B), suggesting that they define the deep architecture of the crust beneath the Andes of central Chile.

The orogen-oblique, NE- and NW-striking faults are associated with major changes in facies, thicknesses and age of the Tertiary volcanic deposits of the Western Main Cordillera (Figs. 3.11, 4.2), with stratigraphic correlations and syn-tectonic deposits indicating they were active as normal faults during the late Eocene-Early Miocene (Fig. 4.2, 6.3). During that period they bounded individual sub-basins within the intra-arc Abanico Basin. Fault plane kinematics and the geometry of fault-related folds, in turn, demonstrates that most arc-oblique faults were reactivated as strike- slip faults with varying degrees of reverse component during basin inversion in the middle Miocene – early Pliocene (Figs. 3.5A, 3.7, 6.4).

Orogen-oblique faults accommodated most of the internal deformation of the

Abanico Basin during tectonic inversion, in combination with low-angle detachments and ramp-flat thrusts developed at different stratigraphic levels (Fig. 3.9). Intrusive contacts, dykes, hydrothermal breccias and veins show strong NE and NW preferred orientations (Figs. 3.2, 3.4B, 3.5B, 3.7, 7.4), indicating that hydrothermal fluids and magmas were channelled during tectonic inversion by the pre-existing NW- and NE- striking fault systems. Intersections of pairs of conjugate, arc-oblique strike-slip faults controlled the localization of the Rio Blanco-Los Bronces and El Teniente porphyry Cu-Mo deposits, and in general those intersections seem to have been highly favourable for the emplacement of Mio-Pliocene magmas and hydrothermal systems (Fig. 7.4).

The kinematic and dynamic analysis of fault-slip data shows that structural reactivation during tectonic inversion was concentrated around major plutons and porphyry Cu-Mo deposits, where there is widespread evidence of syn-tectonic precipitation of hydrothermal minerals on fault planes (Fig. 6.4). In these areas the fault-slip dataset is consistent with reactivation during E- to ENE-directed

compression (Fig. 6.9). This suggests feedback between magmatic and hydrothermal activity, fluid pressure, and the reactivation under compression of the structural architecture inherited from the extensional period. In the margins of the inverted Abanico Basin, compression was accommodated by reverse faulting (sub-vertical σ3), while in the central part of the basin, where the rock column is considerably thicker and the topography higher, a strike-slip regime (sub-vertical σ2) was

predominant during Miocene-early Pliocene tectonic inversion (Figs. 6.7, 6.8). The age of tectonic inversion was confirmed with three Late Miocene 40Ar/39Ar ages obtained on syn-tectonic hydrothermal minerals (Fig. 6.5, Table 6.1).

Based on a combination of U-Pb geochronology, (U-Th)/He thermochronology, whole rock geochemistry and structural data, it is proposed that the Abanico Basin in central Chile can be divided into two main segments (Fig. 5.8). The Rio Blanco-Los Bronces deposit is located in the northern segment, while El Teniente is in the southern one. The boundary between the two segments is controlled by the NW- striking Piuquencillo fault and probably by conjugate, NE-striking faults (Fig. 5.8). It is proposed that, while in the northern segment all the Tertiary volcanic and

sedimentary rocks can be grouped in the Abanico and Farellones formations, this is not possible in the southern segment. The Abanico Formation may still be present, but it is overlain by the Coya-Machali Formation, which is equivalent in age to the Farellones Formation but it is strongly deformed and has a much larger volcano- sedimentary component. This unit is in turn covered by the Teniente Volcanic Complex, which is several m.y. younger than the youngest volcanic rocks of the northern segment (Table 5.1).

The Tertiary geologic evolution of both segments is characterized by the opening of an extensional basin and its subsequent inversion. However, they differ in their

Conclusions

exhumation history and geochemical variations over time. An early deformation event beginning at ~22 Ma affected only the northern segment, and is associated with the formation of progressive unconformities between the syn-extensional Abanico Formation and the syn-compression Farellones Formation (Fig. 3.12). This was the time of initiation of plutonic activity in this segment and coincided with the onset of crustal thickening as reflected in the geochemistry of the Farellones Formation (Fig. 5.6). A second crustal thickening and exhumation event began at ~12 Ma, reflected by a sharp increase of La/Yb ratios in the already thickened crust of the northern segment (Fig. 5.7). In the southern segment this event is characterized by a moderate increase of La/Yb ratios, the initiation of plutonic activity and the transition between the syn-extensional Coya-Machali Formation and the syn-compression Teniente Volcanic Complex (Fig. 5.7, 7.3). Finally, a third exhumation and crustal thickening event affected both segments at ~6 Ma. This was the main exhumation event

affecting the Tertiary rocks of central Chile (Fig. 5.10), and is coeval with the formation of large hydrothermal breccia complexes associated with the porphyry Cu- Mo deposits. Geochemically, this event is reflected by a sharp increase in La/Yb ratios at the southern segment (Fig. 5.6, 5.7).

Seismic data show that arc-oblique fault systems control the distribution of upper- crustal hypocenters at the Western Main Cordillera of central Chile (Fig. 4.3A), showing that these faults are still active under the current stress regime. The

distribution of thermal springs and active volcanoes in central Chile, in turn, shows a strong correlation between the arc-oblique fault systems identified by this study and the location of active hydrothermal cells and volcanic complexes (Figs. 4.1C, 7.6, 7.7).

Eighteen mineral exploration targets were defined at the intersections of major arc- oblique, conjugate fault systems (Fig. 7.5). This targets are particularly relevant for the exploration of totally or partially “blind” porphyry systems, which might have not been eroded enough to be exposed at the present-day surface.

A first-order role of arc-oblique fault systems in the geologic evolution of this Andean segment has never been suggested before, but there is compelling geological

and geophysical evidence to support this hypothesis. The presence of pre-existing, regional-scale arc-oblique fault systems such as the ones described in this study, might be the key to understand the structural controls on the emplacement of mineral deposits in other metallogenic belts, both in the Andes and in orogenic systems elsewhere. They can also lead to a better understanding of the structural controls on volcanism, geothermal activity and crustal seismicity.

References

References

Acocella, V., Gioncada, A., Omarini, R., Riller, U., Mazzuoli, R., and Vezzoli, L., 2011, Tectonomagmatic characteristics of the back-arc portion of the Calama- Olacapato-El Toro Fault Zone, Central Andes: Tectonics, v. 30, TC3005, doi:10.1029/2010TC002854.

Aguirre, L., 1960, Geologia de los Andes de Chile Central, Provincia de Aconcagua, 9, Instituto de Investigaciones Geologicas, 70 p.

Aguirre, L., Calderon, S., Vergara, M., Oliveros, V., Morata, D., and Belmar, M., 2009, Edades isotópicas de rocas de los valles Volcán y Tinguiririca, Chile central: XII Congreso Geologico Chileno, Santiago, 2009, Proceedings, S8.

Aguirre, L., Feraud, G., Vergara, M., Carrasco, J., and Morata, D., 2000, 40Ar/39Ar ages of basic flows from the Valle Nevado stratified sequence (Farellones

Formation), Andes of central Chile: IX Congreso Geologico Chileno, Puerto Varas, 2000, Proceedings, p. 583-585.

Allmendinger, R. W., Cardozo, N., and Fisher, D. M., 2012, Structural Geology algorithms: vectors and tensors: New York, Cambridge University Press, 302 p. Almandoz, G., Zulliguer, G., and Marquez-Zavalia, F., 2005, Altar: mineralizacion de alta sulfuracion vinculada a un sistema de porfido cuprifero, San Juan, Argentina: Congreso Geologico Argentino, La Plata, p. 369-376.

Alvarado, P., Barrientos, S., Saez, M., Astroza, M., and Beck, S., 2009, Source study and tectonic implications of the historic 1958 Las Melosas crustal earthquake, Chile, compared to earthquake damage: Physics of the Earth and Planetary Interiors, v. 175, p. 26-36.

Amilibia, A., Sabat, F., McClay, K. R., Munoz, J. A., Roca, E., and Chong, G., 2008, The role of inherited tectono-sedimentary architecture in the development of the central Andean mountain belt: Insights from the Cordillera de Domeyko: Journal of Structural Geology, v. 30, p. 1520-1539.

Arabasz, 1971, Geological and geophysical studies of the Atacama Fault Zone in Northern Chile: PhD thesis, Pasadena, Californian Institute of Technology. Araneda, M., Avendaño, M., and Merlo, C., 2000, Modelo gravimetrico de la Cuenca de Santiago, etapa II final: IX Congreso Geologico Chileno, Puerto Varas, 2000, Proceedings, p. 404-408.

Armijo, R., Rauld, R., Thiele, R., Vargas, G., Campos, J., Lacassin, R., and Kausel, E., 2010, The West Andean Thrust, the San Ramon Fault, and the seismic hazard for Santiago, Chile: Tectonics, v. 29, TC2007, doi:10.1029/2008TC002427.

Aubouin, J., Borrello, A. V., Cecioni, G., Charrier, R., Chotin, P., Frutos, J., Thiele, R., and Vicente, J. C., 1973, Paleogeographic and structural sketch of the Southern Andes: Revue De Geographie Physique Et De Geologie Dynamique, v. 15, p. 207- 216.

Baeza, O., 1999, Analisis de litofacies, evolucion depositacional y analisis estructural de la Formacion Abanico en el area comprendida entre los rios Yeso y Volcan, Region Metropolitana: Honours thesis, Santiago, Universidad de Chile, 119 p. Baker, J., Peate, D., Waight, T., and Meyzen, C., 2004, Pb isotopic analysis of standards and samples using a Pb-207-Pb-204 double spike and thallium to correct for mass bias with a double-focusing MC-ICP-MS: Chemical Geology, v. 211, p. 275-303.

Baker, M., Cooke, D., and Hollings, P., 2011, LA-ICP-MS U-Pb zircon ages and Nd isotope data for the Teniente Plutonic Complex: implications for the evolution of the El Teniente Porphyry Cu-Mo Deposit: SGA Biennial Meeting, Antofagasta, 2011, Proceedings, p. 853-855.

Baros, M. C., 1995, El Teniente, los hombres del mineral: Santiago, Grafica Andes, 500 p.

Barrientos, S., Vera, E., Alvarado, P., and Monfret, T., 2004, Crustal seismicity in central Chile: Journal of South American Earth Sciences, v. 16, p. 759-768.

References

Benavente, O., Aguilera, F., Gutierrez, F., Tassi, F., Reich, M., and Vaselli, O., 2012, Los sistemas hidrotermales de Chile Central (33-36°S): XIII Congreso Geologico Chileno, Antofagasta, 2012, Proceedings, p. 559-561.

Bevins, R. E., Robinson, D., Aguirre, L., and Vergara, M., 2003, Episodic burial metamorphism in the Andes - A viable model?: Geology, v. 31, p. 705-708. Black, L. P., and Gulson, B. L., 1978, The age of the Mud Tank Carbonatite, Strangways Range, Northern Territory: BMR Journal of Australian Geology and Geophysics, v. 3, p. 227-232.

Black, L. P., Kamo, S. L., Allen, C. M., Aleinikoff, J. N., Davis, D. W., Korsch, R. J., and Foudoulis, C., 2003, TEMORA 1: a new zircon standard for Phanerozoic U- Pb geochronology: Chemical Geology, v. 200, p. 155-170.

Boric, R., 1997, Nuevos antecedentes sobre el modelo geologico del yacimiento de cobre El Soldado, Chile Central: VIII Congreso Geologico Chileno, Antofagasta, 1997, Proceedings, p. 862-866.

Bott, M. H. P., 1959, The mechanics of oblique slip faulting: Geological Magazine, v. 96, p. 109-117.

Caceres, J., Comte, D., Charrier, R., Diaz, D., Pardo, M., Vera, E., Garcia, R., Parra, J. C., Piquer, J., and Yanez, G., 2006, En busqueda de un modelo conceptual para la genesis de porfidos gigantes en la Cordillera Principal de Chile Central, Proyecto Anillo: resultados preliminares en transectas geofisicas de Teno, Tinguiririca y Cachapoal: XI Congreso Geologico Chileno, Antofagasta, 2006, Proceedings, p. 167-170.

Camus, F., 2003, Geologia de los sistemas porfiricos en los Andes de Chile: Santiago, Servicio Nacional de Geologia y Mineria, 267 p.

Cannell, J., Cooke, D. R., Walshe, J. L., and Stein, H., 2005, Geology,

mineralization, alteration and structural evolution of the El Teniente porphyry Cu- Mo deposit: Economic Geology, v. 100, p. 979-1003.

Cannell, J., Cooke, D. R., Walshe, J. L., and Stein, H., 2007, Geology,

mineralization, alteration and structural evolution of El Teniente porphyry Cu-Mo deposit - A reply: Economic Geology, v. 102, p. 1171-1180.

Castelli, J. C., and Lara, L., 1999, Geología de exploración básica generativa entre Andina y Río Colorado, 1:50000, Unpublished report for Codelco.

Cembrano, J., and Lara, L., 2009, The link between volcanism and tectonics in the southern volcanic zone of the Chilean Andes: A review: Tectonophysics, v. 471, p. 96-113.

Cembrano, J., Herve, F., and Lavenu, A., 1996, The Liquine Ofqui fault zone: A long-lived intra-arc fault system in southern Chile: Tectonophysics, v. 259, p. 55-66. Cembrano, J., Lavenu, A., Yanez, G., Riquelme, R., Garcia, M., Gonzalez, G., and Herail, G., 2007, Neotectonics, in Moreno, T., and Gibbons, W., eds., The Geology of Chile: London, The Geological Society, p. 231-261.

Charrier, R., and Munizaga, F., 1979, Edades K-Ar de vulcanitas cenozoicas del sector cordillerano del rio Cachapoal (34°15' de latitud Sur): Revista Geologica de Chile, v. 7, p. 41-51.

Charrier, R., and Vicente, J. C., 1972, Liminary and geosyncline Andes: major orogenic phases and synchronical evolutions of the central and Magellan sectors of the Argentine Chilean Andes: Solid Earth Problems Conference, Upper Mantle Project, Buenos Aires, 1972, p. 451-470.

Charrier, R., Wyss, A. R., Norell, M. A., Flynn, J. J., Novacek, M. J., McKenna, M. C., Swisher, C. C., Frassinetti, D., and Salinas, P., 1990, Hallazgo de mamíferos fósiles del Terciario Inferior en el sector de Termas del Flaco, Cordillera Principal, Chile Central: implicaciones paleontológicas, estratigráficas y tectónicas: II

Simposio sobre el Terciario de Chile, 1990, p. 73-84.

Charrier, R., Wyss, A. R., Flynn, J. J., Swisher, C. C., Spichiger, S., and Zapatta, F., 1994, Nuevos antecedentes estratigráficos y estructurales para las Formaciones

References

Coya-Machalí y Abanico, entre 33º50’ y 35ºS, Cordillera Principal Chilena: VII Congreso Geologico Chileno, 1994, p. 1316-1319.

Charrier, R., Wyss, A. R., Flynn, J. J., Swisher, C. C., Norell, M. A., Zapatta, F., McKenna, M. C., and Novacek, M. J., 1996, New evidence for late Mesozoic early Cenozoic evolution of the Chilean Andes in the Upper Tinguiririca Valley (35 degrees S), Central Chile: Journal of South American Earth Sciences, v. 9, p. 393- 422.

Charrier, R., Baeza, O., Elgueta, S., Flynn, J. J., Gans, P., Kay, S. M., Munoz, N., Wyss, A. R., and Zurita, E., 2002, Evidence for Cenozoic extensional basin

development and tectonic inversion south of the flat-slab segment, southern Central Andes, Chile (33°-36° SL): Journal of South American Earth Sciences, v. 15, p. 117- 139.

Charrier, R., Bustamante, M., Comte, D., Elgueta, S., Flynn, J. J., Iturra, N., Munoz, N., Pardo, M., Thiele, R., and Wyss, A. R., 2005, The Abanico extensional basin: Regional extension, chronology of tectonic inversion and relation to shallow seismic activity and Andean uplift: Neues Jahrbuch Fur Geologie Und Palaontologie-

Abhandlungen, v. 236, p. 43-77.

Charrier, R., Pinto, L., and Rodriguez, M. P., 2007, Andean Tectonostratigraphy, in Moreno, T., and Gibbons, W., eds., The Geology of Chile: London, The Geological Society, p. 21-114.

Chernicoff, C. J., Richards, J. P., and Zappettini, E. O., 2002, Crustal lineament control on magmatism and mineralization in northwestern Argentina: geological, geophysical, and remote sensing evidence: Ore Geology Reviews, v. 21, p. 127-155. Corti, G., Van Wijk, J., Bonini, M., Sokoutis, D., Cloetingh, S., Innocenti, F., and Manetti, P., 2003, Transition from continental break-up to punctiform seafloor spreading: How fast, symmetric and magmatic: Geophysical Research Letters, v. 30, 1604, doi:10.1029/2003GL017374.

Darwin, C., 1846, Geological observations on South America, in The geology of the voyage of the Beagle: London, Smith Elder and Co.

Davidson, J., 1971, Contribución al estudio geológico de los Andes Meridionales Centrales: Geología del área de las Nacientes del Teno, Provincia de Curicó: Honours thesis, Universidad de Chile, 145 p.

Deckart, K., Clark, A. H., Aguilar, C., Vargas, R., Bertens, A., Mortensen, J. K., and Fanning, M., 2005, Magmatic and hydrothermal chronology of the giant Rio Blanco porphyry copper deposit, central Chile: Implications of an integrated U-Pb and Ar- 40/Ar-39 database: Economic Geology, v. 100, p. 905-934.

Deckart, K., Godoy, E., Bertens, A., Jerez, D., and Saeed, A., 2010, Barren Miocene granitoids in the Central Andean metallogenic belt, Chile: Geochemistry and Nd-Hf and U-Pb isotope systematics: Andean Geology, v. 37, p. 1-31.

Deckart, K., Clark, A. H., Cuadra, P., and Fanning, M., 2013, Refinement of the time-space evolution of the giant Mio-Pliocene Rio Blanco-Los Bronces porphyry