Chapter 14 Cenozoic Tectonics and Porphyry Copper Systems of the Chilean Andes

Home , Domeyko Fault, Farellones Formation, Flat slab subduction, Salar de Punta Negra

CONSTANTINO MPODOZIS† AND PAULA CORNEJO Antofagasta Minerals, Apoquindo 4001, Piso 18, Santiago, Chile

Abstract Subduction under South America has been active for the past 550 m.y. but large porphyry copper deposits were essentially emplaced during the Paleocene (60−50 Ma) in southern Peru, and mid-Eocene-early Oligocene (43−32 Ma) and late Miocene-Pliocene (10−6 Ma) in north and central Chile. Although the tectonic setting of the Paleocene porphyry deposits is still poorly understood, those of the northern Chile Eocene- Oligocene belt were emplaced along the margin-parallel Domeyko fault system, where active compressional and/ortranspressional deformation and block rotations took place during the formation of the Bolivian orocline. Eocene-early Oligocene oroclinal bending was a consequence of differential tectonic shortening focused along a mechanically weak zone of the Central Andean crust inherited from the Paleozoic. Deformation occurred during an episode of accelerated westward absolute motion of the South American plate, which coincided with very high rates of oceanic crust production in the eastern Pacific. The slow South American-Farallon convergence rates recorded for the Eocene-Oligocene suggest, however, that strong interplate coupling existed during that time. This permitted the transfer of horizontal stresses and large-scale deformation of the Andean margin, creating a favorable scenario for the generation and emplacement of porphyry copper magmas along the Domeyko fault system. The younger, Miocene-Pliocene porphyry copper deposits of central Chile-Argentina were emplaced in a different setting, after the initiation of compressional deformation within a volcano-tectonic depression (Aban- ico basin) that evolved during another, late Oligocene to early Miocene, period of increased East Pacific oceanic crust production. Nevertheless, in contrast to the Eocene-Oligocene situation in northern Chile, the relatively stationary position of the South American plate compared to the mantle reference frame and weak interplate coupling that permitted rapid subduction, increased volcanism, and overriding plate extension. Tec- tonic inversion of the basin and compressional deformation along with crustal thickening and mountain building began at around 20 m.y. ago as interplate coupling increased when the westward motion of South America accelerated and the Nazca-South America convergence velocity decreased in the mid-Miocene. Compression was accompanied, as during the Eocene-Oligocene in northern Chile, by slab shallowing and increased fore- arc subduction erosion. In both cases, the largely structurally controlled, syn- to post-tectonic porphyry copper deposits are associated with long-lived magmatic systems that were active for more than 10 m.y. In northern Chile, the deposits occur as parts of discrete intrusive clusters that comprise a suite of precursor plutons emplaced during multiple events since the Cretaceous. Porphyry copper mineralization is linked to multistage, amphibole-bearing intrusions of intermediate composition derived from hydrous, oxidized magmas with adakitic geochemical signatures. These intrusions appeared when crustal thickness increased to a critical threshold in the course of deformation. Production of magmas with high metal-carrying capacity was fostered as fluids were liberated when amphibole became unstable and was destroyed as the crust thickened. At the same time, source regions within the mantle were contaminated by hydrated fragments of fore-arc continental crust, as the result of enhanced subduction erosion during peaks of compressional deformation.

Introduction Basin and Range extension in the Eocene, far inland from the THE STUDY of the tectonic setting of porphyry copper deposits Pacific margin of North America (Kloppenburgh et al., 2010). is fundamental to understanding their genesis (e.g., Sillitoe, Noncollisional porphyry copper deposit examples in sub- 1998; Kay and Mpodozis, 2001; Cooke et al., 2005; Sillitoe duction-related arc settings include those from the Chagai and Perelló, 2005; Richards, 2009, 2011a; Tosdal et al., 2009). belt in Pakistan, the Laramide porphyry copper province of Some Cenozoic porphyry copper deposits are known to have the western United States and northern Mexico (Lang and Ti- formed during or shortly after continent-continent, conti- tley, 1998; Valencia-Moreno et al., 2007; Perelló et al., 2008) nent-island arc, or island arc-island arc collisions in the Hi- and the Central Andes province, which host some of the malayas-Tibet, the Kerman arc in Iran, and Papua New largest known porphyry copper deposits in the world (Camus, Guinea (Solomon, 1990; Zenqiang et al., 2003; Shaifei et al., 2003; Cooke et al., 2005; Sillitoe and Perelló, 2005). 2009). A Paleozoic example of this type of deposit may be The Andes has long been considered as the type example of Oyu Tolgoi in Mongolia (Perelló et al., 2001). In contrast, a noncollisional orogenic system (e.g., Jordan et al., 1983), other large porphyry deposits such as Bingham Canyon in the where subduction of Pacific oceanic crust beneath South western United States formed during the earliest stages of America has been active for the past 570 m.y. (Cawood, 2005). Nevertheless, the largest porphyry copper deposits are † Corresponding author: e-mail, [email protected] the result of anomalous magmatic systems that developed

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during short periods at specific locations within the Andean Diverse, yet basically subeconomic, porphyry copper deposits orogen. These include the Paleocene to early Eocene (66−52 formed during these events in northern Chile and along the Ma) and middle Eocene to early Oligocene (43−32 Ma) belts Frontal Cordillera in west-central Argentina (Sillitoe, 1977; in southern Peru and northern Chile, and the late Miocene to Sillitoe and Perelló, 2005; Cornejo et al., 2006; Munizaga et early Pliocene (10−5 Ma) porphyry systems in central Chile al., 2008). and contiguous Argentina (Perelló et al., 2003a; Sillitoe and Perelló, 2005). In this contribution, with emphasis on the Jurassic to Early Eocene Tectonics and Chilean belts, we will try to demonstrate how major Cenozoic Metallogeny of the Central Andes tectonic events along the central Andean convergent margin, After the Triassic rifting event, subduction was reestab- prompted by large-scale reorganizations of the global tectonic lished in northern Chile and southern Peru during the Early system, were the main triggers for the formation of large por- Jurassic when a new magmatic arc developed west of the phyry copper deposits. extinct late Paleozoic arc front. Since then, subduction has proceeded uninterrupted to date. Initial Jurassic to Early Pre-Andean History: Cretaceous arc magmatism occurred under extensional con- From Rodinia Dispersal to Pangea Breakup ditions that permitted the formation of a series of intercon- The western margin of South America underwent mag- nected back-arc basins to the east of the main arc, which were matic and tectonic activity at least since the late Neoprotero- progressively filled with marine and continental sedimentary zoic breakup of Rodinia (800−700 Ma), when the separation strata (Mpodozis and Ramos, 1989, 2008). Transpressional of Laurentia from Gondwana produced the opening of the deformation along the arc axis created the intra-arc Atacama proto-Pacific (Iapetus) ocean (Dalziel, 1997). East-directed fault system in northern Chile (Scheuber and González, subduction of newly formed ancestral Pacific crust below 1999) and was accompanied in the Early Cretaceous by the western Gondwana began at ~570 Ma and was fully active emplacement, in northern Chile, of some porphyry copper along the proto-Andean margin by 485 to 465 Ma (Pankhurst deposits at ~140 to 130 Ma (e.g., Antucoya-Buey Muerto, et al., 1998; Cawood, 2005; Chew et al., 2007). Plate conver- 141−139 Ma; Puntillas-Galenosa, 135−132 Ma; Perelló et al., gence in the Central Andes region during the Ordovician to 2003b; Maksaev et al., 2006, 2010). Fast convergence rates Devonian included the progressive collision and accretion of during the global mid Cretaceous superplume event (Larson, a group of tectonostratigraphic terranes of Laurentian and/or 1991) produced an upsurge in volcanism along the Andean Gondwanan affinities (e.g., Ramos et al., 1986; Astini et al., margin, accompanied by intra-arc extension and transtension 1995) against the western South American margin. Terrane which fostered iron oxide-copper-gold (IOCG)−type mineral- amalgamation contributed to the formation of the accre- ization between 120 and 100 Ma in northern Chile and south- tionary Terra Australis orogen, which extended for more than ern Peru (Marschik and Fontboté, 2001; Sillitoe, 2003; Silli- 18,000 km along the Pacific margin of Gondwana from Aus- toe and Perelló, 2005; Chen et al., 2010). Small, low-grade, tralia to South America (Cawood, 2005). The accretionary gold-rich porphyry copper deposits such as Andacollo (104 stage was followed, in the Central Andes, by the buildup of a Ma), Domeyko-Dos Amigos (108−104 Ma), and Pajonales (97 late Carboniferous to Early Permian (320? -280 Ma) supra- Ma) were emplaced under extensional conditions during the subduction magmatic arc on top of the newly accreted terranes, same general period in north-central Chile (Sillitoe and as well as the development of an outboard fore-arc subduc- Perelló 2005; Maksaev et al., 2010). tion complex that extended for more than 1,000 km along the The extensional and transtensional conditions that domi- Chilean segment of the Gondwana margin south of 27° S nated early Andean subduction ended in the early Late Cre- (Mpodozis and Kay, 1992; Hervé, 1988; Willner et al., 2005; taceous, when the back-arc basins were tectonically inverted Chew et al., 2007). (Mpodozis and Ramos, 1989; Tomlinson et al., 2001a). The Magmatism continued from the Permian to the Middle shift to a more contractional, subduction-related regime oc- Triassic (280−240 Ma), when great volumes of intrusive and curred together with the accelerated westward drift of South mostly felsic volcanic rocks, including the Choiyoi large ig- America, in response to the final opening of the Atlantic neous province in Chile and Argentina and the Mitu Group in (Russo and Silver, 1996; Somoza and Zaffarana, 2008). Sub- southern Peru (Kay et al., 1989, Sempere et al., 2002), were sequently, the coastal magmatic arc was abandoned (Fig. 1b) emplaced along the western South American margin. Although and the magmatic front jumped to the east in the Late Creta- geochronologic and geochemical data are still incomplete, ceous, where it remained relatively stationary until the early several competing hypotheses, such as normal or oblique sub- Eocene. Abrupt shifts in the magmatic front such as this has duction, postcollision extension-driven crustal melting, slab been accompanied throughout the Andean history, by tran- breakoff or slab shallowing, have been proposed to explain sient geochemical changes during and after arc migration the prevailing tectonic regime along different segments of the (e.g., Cornejo and Matthews, 2001; Haschke et al., 2006: Fig. Andean margin at that time (Mpodozis and Kay, 1992; Kleiman 1d). Stern (1991, 2011), Kay and Mpodozis (2002), and Kay et and Japas, 2009; Ramos and Folguera, 2009, and references al. (2005) suggested that changes of this type reflect mantle therein). From the Middle Triassic to earliest Jurassic (240− contamination from fore-arc crust removed during enhanced 190 Ma), rifting associated with the incipient stages of Pangea subduction erosion processes associated with major contrac- dispersal (e.g., Veevers, 1989), accompanied by a decreasing tional events along the Andean margin. volume of bimodal magmatism, seems to have occurred In northern Chile, the Cretaceous-Tertiary boundary was along the western margin of South America (e.g., Ramos and marked by another short pulse of contractional deformation, Kay, 1991; Franzese and Spalletti, 2001; Rosas et al., 2007). the K-T event of Cornejo et al. (2003). Paleocene to early

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15º 0 Modern CVZ Arc WC EC SA 20 w E La Paz PERU 40

A Arica ltip la Late Cretaceous- no 60 20º Eocene arc 80 SA CHILE 100 (Ma) Salar de Atacama AGE Mid - Cretaceous arc EC 120 Antofagasta FA Puna 25º 140

A Jurassic - Early Cretaceous N 160 WC arcs Distal tuffs/reset ages

SP ARGENTI 180 Longitude ( ºW ) (a) (b) 70º 65º 200 71 70 69 68 67 66 65

w E Post Incaic intrusions 10 Incaic Event Sm/Yb 20 26 Ma 30 Porphyry Cu Late Cretaceous Eoc-E Olig. Peruvian Event KT deposits Event Incaic intrusions 40 (Ma) Volcanic and

AGE intrusive rocks 50 Ignimbrites ANTOFAGASTA TRANSEC T (20 - 23ºS) 60 Longitude ( ºW ) (c) (d) 72 71 70 69 68 67 66 65 64 90 80 70 60 50 40 30 20 10 0 AGE (Ma)

FIG. 1. (a). Main morphotectonic units of the central Andes, between 15° and 30° S. FA = modern fore-arc zone, including the Coastal Range and, farther to the east, the Precordillera (or Cordillera de Domeyko) shown in Figure 2; WC = Western Cordillera which, north of 27° S, is essentially formed by the active magmatic arc of the Central Andean volcanic zone (CVZ); EC = Eastern Cordillera; SP = Sierras Pampeanas; SA = sub-Andean fold-and-thrust belt. (b). Relationship between age and longitude (distance to the trench) for <200 Ma volcanic and intrusive rocks of the Central Andes (19°−28° S; Haschke et al., 2002, CVZ = modern Central volcanic zone of the Andes). (c). Longitude vs. age for Cenozoic intrusive and volcanic rocks at 20° to 23° S (Iquique to Antofagasta transect; Trumbull et al., 2006). The ~32 to 26 Ma gap may be related to a transient episode of flat subduction produced as a consequence of the Incaic tectonic episode. The east to west migration of the magmatic front during the Miocene is considered to record late-stage slab steepening (Kay et al., 1999; Kay and Coira, 2009). (d). Geochemical changes since the Late Cretaceous near 26° S (Copiapó-El Salvador region; data from Cornejo and Mathews, 2001). Note the short-lived peaks in the Sm/Yb ratio during major tectonic events, superimposed over a more subdued long-term trend to increased values. Peak values may reflect contamination of the mantle magma source regions as a result of massive removal of fore-arc continental crust during periods of enhanced subduction erosion (Kay and Mpodozis, 2002; Kay et al., 2011).

Eocene magmatism included the emplacement of a porphyry built on a relatively thin crust over a steep subduction zone. copper belt between southern Peru (Cerro Verde, 61 Ma; This is consistent with the very low rates of and oblique con- Quellaveco, 54 Ma; Cuajone, 52 Ma) and Cerro Colorado (52 vergence between the South American and Farallon plates Ma), Spence (57 Ma), and Relincho (61 Ma) in northern Chile (Pardo-Casas and Molnar, 1987), and also with within-plate (Sillitoe and Perelló, 2005). Structural and geochemical data geochemical signatures of Paleocene to early Eocene rocks in gathered by the authors suggest that these deposits in north- the Inca de Oro-El Salvador region (Cornejo and Matthews, ern Chile formed in a neutral stress to mildly extensional arc 2001).

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Middle Eocene to Early Oligocene Tectonics a consequence of the formation of the sharp bend of the west- of the Central Andes ern South American margin, known as the Arica elbow or Bo- livian orocline (Fig. 3). Paleomagnetic studies have been es- The Domeyko fault system sential in obtaining a more constrained view of the One of the most relevant tectonic and metallogenic deformational history of this segment of the Central Andes episodes in the Central Andes correlates with the middle and support the tectonic model for orocline formation first Eocene to early Oligocene (45−33 Ma) Incaic tectonic event proposed by Isacks (1988). (Noble et al., 1979; Maksaev and Zentilli, 1988; Mpodozis and Figure 3a is a simplified regional map showing the distrib- Perelló, 2003; Sillitoe and Perelló, 2005); during this period, ution of paleomagnetic (declination) vectors measured for the bending of the continental margin generated the Bolivian Central Andes. Importantly, independent of age, Mesozoic orocline and the Domeyko fault system along the Pre- and Paleogene rocks have been rotated up to 50°. In contrast, cordillera of northern Chile. The Domeyko fault system (Fig. rotations measured in Miocene and younger rocks (<18−11 2) is a >1,000-km-long, 40- to 60-km-wide, orogen-parallel Ma in northern Chile; <20 Ma in southern Peru) are negligi- zone of deformation composed of a complex array of strike- ble, suggesting that most of the rotations were acquired dur- slip, normal, and reverse faults, together with thin- and thick- ing a single Paleogene episode of deformation (Roperch et al., skinned folds and thrusts, which extends along the Cordillera 2006, 2011; Arriagada et al., 2008, and references therein). de Domeyko (also known as Precordillera) in northern Chile Rotations are counterclockwise in southern Peru (Domain B; between 20° and 27° S (e.g., Reutter et al., 1991, 1996; Fig. 3a), clockwise in northern Chile south of Antofagasta Cornejo et al., 1997). Some authors (e.g., Amilibia and (domain D), and almost nonexistent in the intermediate re- Skarmeta, 2003; Amilibia et al., 2008) proposed that most of gion (domain C) between Antofagasta and Arica (Taylor et al., these faults and folds initiated during Late Cretaceous as a 2005; Arriagada et al., 2008). The magnitude of the counter- consequence of the inversion of normal faults inherited from clockwise rotations decreases significantly at the Abancay De- the Mesozoic back-arc extension. However, others (e.g., Tom- flection in central Peru (domain A; Fig. 3a), the latter break linson et al., 2001a; Mpodozis et al., 2005) interpreted that being interpreted as a zone of intense Eocene-early Oligocene the Andean back-arc basins were first inverted during the left-lateral shear along the boundary between the Arequipa early Late Cretaceous to form a proto-Cordillera de and Paracas basement terranes (Ramos, 2009; Roperch et al., Domeyko, while a second main tectonic pulse along the 2011; Fig. 4b). In northern Chile, clockwise rotations de- Domeyko fault system, coincident with the Incaic event, pro- crease progressively south of Antofagasta and essentially dis- duced its final uplift (Reutter et al., 1991, 1996; Scheuber and appear near Vallenar (28°30' S) upon entering the essentially Reutter 1992; Tomlinson et al., 1993; Maksaev and Zentilli, nonrotated domain E (Fig. 3a), which extends southward to 1999). Parts of the Domeyko fault system were subsequently the latitude of Santiago (33° S). reactivated during the Oligocene and the Quaternary (Tom- In his landmark paper, Isacks (1988) proposed that the ob- linson and Blanco, 1997a, b; Audin et al., 2003; Soto et al., served paleomagnetic rotations and the formation of the sea- 2005). ward concave Bolivian orocline are related to along-strike The kinematics of the middle Eocene to early Oligocene variations in the amount of late Cenozoic shortening pro- deformation along the Domeyko fault system is a matter of duced during contractional deformation focused along a controversy; evidence for both left- and right-lateral displace- mechanically and thermally weakened zone located in the ments, including reversal in the sense of shear, has been re- overriding South American plate. Recent structural studies ported along different parts of the faulted domain (Reutter et indicate, however, that the bulk of the shortening (~60%), at al., 1996; Dilles et al., 1997; Tomlinson and Blanco 1997a, b; and near the Arica bend (13°−19° S; Fig. 3), is pre-Neogene Hoffman-Rothe et al., 2004; Niemeyer and Urrutia, 2009). in age and occurred between 40 and 20 Ma (Lamb, 2001; Fission-track age data show that the Cordillera de Domeyko Müller et al., 2002; Kley et al., 2005; McQuarrie, 2006). In- was exhumed between 40 and 30 m.y. ago (Maksaev and caic shortening concentrated within the Eastern Cordillera of Zentilli, 1999; Nalpas et al., 2005) in association with surface Bolivia where >12 km of terrigenous sedimentary strata accu- tectonic uplift and profound erosion, the products of which mulated in a Paleozoic marine basin above highly attenuated accumulated in syntectonic basins east and west of the area of continental crust (Figs. 1, 3). The amount of horizontal short- deformation (Mpodozis et al., 2005; Hong et al., 2007; Wot- ening reaches a maximum near the axis of the orocline, where zlaw et al., 2011). the Paleozoic sedimentary sequence is thickest, and decreases symmetrically along-strike (Oncken et al., 2006; Gotberg et Origin of the Domeyko fault system al., 2010, and references therein, Fig. 4a) as the Paleozoic At first glance, the Domeyko fault system could be consid- sedimentary rocks of the Eastern Cordillera become thinner ered as a trench-linked fault system (Woodcock, 1986) that and give way to metamorphic and crystalline rocks in both nucleated in the thermally weakened crust of the middle Peru (Cordillera de Marañón) and northwest Argentina (Sier- Eocene to early Oligocene magmatic arc of northern Chile ras Pampeanas, see Fig. 3). during a period of suggested fast Eocene oblique conver- Arriagada et al. (2008) attempted to remove the combined gence between the Farallon and South America plates (Pardo- effects of accumulated horizontal shortening and block rota- Casas and Molnar, 1987; Somoza, 1998). However, when re- tions. Figure 3b shows their preferred solution for the restored cent paleomagnetic and structural studies are taken into shape of the continental margin (Peru-Chile trench) at 45 account it becomes apparent that tectonic activation of the Ma, before the formation of the Bolivian orocline. As shown Domeyko fault system during the Incaic episode is essentially in Figure 4b, the ca. 30° E azimuth of the Farallon-South

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La Planada 70º30' YP 67º30'

N Copaquire Rosario (Collahuasi) (36-35)

21º QC Ujina (36-35) 21º Quebrada Blanca (37-35)

Salar de Ascotán

El Abra (38-37) Conchi BOLIVIA CA Radomiro Tomic (36-34) Chuquicamata (36-32) Alejandro Hales (39-36) Toki (39) Miranda (38-37) 22º30' Opache (38-37) 22º30' CALAMA Esperanza- Telégrafo (42-40) Caracoles (42-41) Gaby (42)

CE Centinela (45-44) Salar de Polo Sur (42-41) Atacama ANTOFAGASTA

Chimborazo (41) 24º Zaldívar (38-37) 24º Escondida (38-37) LE Escondida Este (36-35) Salar de Punta Negra

Sierra Juncal (40) ARGENTINA 25º30' 25º30' Exploradora (35) SE Middle Eocene to early Oligo- Sierra del Jardín (42) cene porphyry Cu + Mo+ Au deposits Salar de El Salvador (42-41) Pedernales Eocene plutons (48-38 Ma) PS Paleozoic basement Potrerillos (36) Reverse fault

Normal or strike-slip fault Salar de Maricunga Salars 27º 27º

COPIAPO 0 100km

70º30' 69º 67º30'

FIG. 2. Sketch map of the Cordillera de Domeyko (or Precordillera) and the Domeyko fault system, showing main faults traces, exposures of Paleozoic basement, clusters of Eocene plutonic rocks (YP = Yabricoya-La Planada intrusive cluster; QBC = Quebrada Blanca-Collahuasi; CA = Chuquicamata-El Abra; CE = Centinela, LE = La Escondida; SE = Sierra Ex- ploradora-Juncal; PS = Potrerillos- El Salvador; RF = Río Figueroa) and the locations and ages (in parentheses, Ma) of Eocene-early Oligocene porphyry copper deposits. More detailed maps of CA, LE, and CE clusters are shown in Figures 5, 7, and 8. Based on the 1:1,000,000 geologic map of Chile (SERNAGEOMIN, 2002).

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ILIAN (b) Z D A IEL R H B S 70º 66º 62º 74º

Restoredtrench position at 45 Ma of (45-0 Ma) 78º 250 km Total displacement vectors Total Peru - Chile Trench 14º 18º 22º 26º 10º

IA (a) D L AZILIE H BR S Sierras ARGENTINA Pampeanas Jujuy Santa Cruz A

BOLIVI La Escondida Collahuasi El Morro La Paz Las Bambas 70º 66º 62º Arica Copiapó D E C Antofagasta Chuquicamata PERU

74º ABANCAY DEFLECTION ABANCAY Marañón . 3. (a). Paleomagnetic declination data for the Central Andes in northern Chile and southern Peru that indicate the Central Andean rotation pattern. Data from . 3. (a). Paleomagnetic declination data for the Central Andes in northern Chile and southern Peru that indicate Andean Lima B Cordillera de IG F

Trench et al. (2005), and references cited therein. Distribution of lower Paleozoic sediments of the Bolivian Arriagada et al. (2006, 2008), Roperch 2011), Taylor basin and crystalline metamorphic rocks modified from Ramos Dalla Salda (2011; Fig. 1). Also shown are the main Eocene- Oligocene porphyry copper deposits of the Central Andean belt (Perelló et al., 2003a). Rotation domains designated A to E are discussed in text. (b). Position plate boundary (Peru-Chile trench) at 45 Ma and total (Paleogene to present) displacement vectors for material points on the South American margin according th e two-dimensional restoration model of Arriagada et al. (2008). 250 km 78º A

Peru - Chile Lower Paleozoic sedimentary rocks Lower Paleozoic (Neoproterozoic?) metamorphic and intrusive rocks Middle-Eocene to early Oligocene porphyry copper belt Paleomagnetic declination vector Paleomagnetic domain 10º 14º 18º 22 º D

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0º 10º 15º 20º 25º 30º 35º (a) (b) ng ry one aloda PARACAS r z un Latitude (ºS) TE a bo R She ne R Brazilian ANE terrra Shield

400 Fo ld A a R n rthogonal 45 M TER EQ d O U RAN IP T A h 4 convergence a E r u T s rench 0 Ma Trench t

300 e lt ? Mechanically weak crust (Paleozoic 200 sedimentary rocks 3b Orthogonal

Tectonic shortening (km) shortening Tectonic 3a convergence 100 Initial (non rotated) 21ºS 2 Domeyko fault Domeyko system fault system Distance from orocline axis 1 Oblique 1500 1000 500 0 500 1000 1500 km convergence N S

FIG. 4. (a). Different horizontal tectonic shortening estimates for the Central Andes, showing how values decrease symmetrically north and south of the orocline axis. 1 = Neogene deformation in the Subandean belt (Oncken et al., 2006); 2 = total shortening (Peloegene + Neogene, Oncken et al., 2006); 3a = shortening needed to accommodate crustal area assum- ing initial crustal thickness of 40 km; 3b = same, but considering 35 km as initial thickness (Gotberg et al., 2010); 4 = shortening needed to balance paleomagnetically determined rotations (Arriagada et al., 2008). (b). Tectonic sketch showing how oblique convergence along the southern limb of the orocline may drive strike-slip displacements along the Domeyko fault system at the beginning of the Incaic event. The general scheme comes from figures taken from Isacks (1988) and Lamb (2001). Reverse faults prevailed north of 21° S as a consequence of the original N-NW trend of the continental margin. Note left-lateral shear along the Abancay Deflection along the boundary between the Arequipa and Paracas terranes.

America Eocene to Oligocene convergence vector calculated belt at approximately 30° S and the possibly segmented na- by Somoza (1998, Fig. 4b) was nearly orthogonal to the (re- ture of the belt along its strike length between southern Peru stored) NW-SE−trending section of the margin along the and central Chile (Mpodozis and Perelló, 2003; see below). center and northern limb of the Bolivian orocline. Neverthe- less plate convergence was, at the same time, highly oblique Middle Eocene to Early Oligocene Porphyry Copper along the southern limb of the orocline fostering margin- Province of Northern Chile parallel shear, and (theoretically dextral) strike-slip faulting in northern Chile where the Domeyko fault system was formed Overview (Fig. 4b). Bending of the margin seems to have been accom- Middle Eocene to early Oligocene porphyry copper de- panied at the southern limb of the Bolivian orocline by whole- posits of the Central Andes were emplaced contemporane- sale NE-directed crustal flow (Fig. 3b), which is also required ously with the Incaic tectonic event, when the entire Andean to explain the excess orogenic volume and crustal thickness margin was being reshaped during the formation of the Arica below the Altiplano-Puna reported by Kley and Monaldi bend. Mineralized centers occur in Peru near the eastern end (1998) and Hindle et al. (2005). of the Abancay Deflection (Andahuaylas-Yauri cluster; Perelló Mass transfer, likely associated with lower crustal flow to- et al., 2003a; Fig. 3a), although the vast majority are located ward the center of the orocline (Hindle et al., 2005), increases in Chile along the Domeyko fault system (Sillitoe and Perelló, the possibility of strike-slip faulting along the Domeyko fault 2005; Figs. 2, 3a). El Morro, the southernmost porphyry system. Continued displacement was likely initially blocked copper-gold deposit of economic importance (Perelló et al., near the orocline axis where deformation was dominated by 1996), is located where the rotated and thickened southern margin-normal contraction (Figs. 3b, 4b). As suggested by limb of the Bolivian orocline (domain D; Fig. 3a) terminates McQuarrie (2002) and Boutelier and Oncken (2010), mass and merges with the nonrotated central Chile domain E (see transfer toward the core of the orocline implies crustal thin- below). Most porphyry copper deposits were emplaced along ning and stretching in central Chile, which may have inhib- the Domeyko fault system at long-lived zones of focused mag- ited mountain building south of 28° S. Crustal thinning and a matism, which in some cases were active well before the more extensional tectonic regime prevailed in that region Eocene. They occur as parts of discrete intrusive clusters (south of 28° S) during the Eocene to early Miocene (e.g., separated by large barren areas where only the Paleozoic Jordan et al., 2001; Charrier et al., 2002). These contrasts basement and back-arc basin sedimentary cover is exposed help to explain the termination of the Incaic porphyry copper (Fig. 2).

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The structural control of the porphyry systems is strikingly of the Mesozoic back-arc basin (Ladino et al., 1997; Tomlin- diverse (cf. Sillitoe and Perelló, 2005). At Chuquicamata, por- son et al., 2001a). It is bounded to the west by high-angle, phyry copper-related intrusions were syntectonically emplaced west-vergent reverse faults that place Neoproterozoic to Pa- along an extensional jog in an active dextral strike-slip fault leozoic basement units on top of Jurassic sedimentary strata system (Lindsay et al., 1995); at Potrerillos, intrusions related (Fig. 5a). The east-vergent Arca reverse fault that bounds the to porphyry copper deposits ascended along subvertical, left- eastern side of the Sierra de Moreno (Fig. 5a) has been in- lateral strike-slip faults and migrated laterally near the surface terpreted as a reactivated normal fault, which formed near along an active low-angle thrust (Tomlinson et al., 1993; the eastern edge of the Mesozoic back-arc basin (Tomlinson Niemeyer and Munizaga, 2008). In contrast, porphyry stocks et al., 2001a; Fig. 5a). Syn- to post-tectonic red beds (Tolar related to mineralization at Escondida or El Abra-Fortuna and Tambillos Formations; Fig. 5a) were shed to the east and were emplaced along premineralization reverse and strike-slip west of the uplifting block during the Late Cretaceous. After faults formed during earlier stages of the Incaic event (Dilles deformation, volcanic rocks, which unconformably cover the et al., 2011; Hervé et al., 2012). Other mineralized intrusions basement units and Mesozoic sedimentary strata at Sierra de were cut and displaced by late-stage faults (Esperanza-Telé- Moreno, developed in two separate episodes during the latest grafo, Chuquicamata; Dilles et al., 1997; Tomlinson et al., Cretaceous (Cerro Empexa and Quebrada Mala Formations) 1997a, b). and early to middle Eocene (Icanche Formation; Fig. 5a; Not all porphyry copper deposits are, however, obviously Tomlinson et al., 2001a, 2010). related to major Domeyko fault splays (Sillitoe and Perelló, The eastern, Sierra del Medio basement block (Fig. 5a) was 2005), although the deposits of the Collahuasi-Quebrada uplifted between 43 and 38 Ma during the Incaic event (Tom- Blanca cluster, which crop out to the east of the main fault linson et al., 1997a) along a new set of west- and east-vergent, system (Fig. 2), seem to have been emplaced over a basement high-angle reverse faults. As in other regions of northern discontinuity marked by regional isotopic changes in Neo- Chile, volcanism in the Chuquicamata-El Abra region sharply gene magmatic rocks (Mamani et al., 2010). At El Salvador diminished at this time; however, a syntectonic sedimentary and Polo Sur, 42 to 41 Ma porphyry copper-related intrusions sequence (Sichal Formation; Fig. 5a) that accumulated in a are encapsulated in preexisting, up to 10-m.y. older, caldera- narrow basin between Sierra de Moreno and Sierra del related rhyolite domes, indicating that the middle Eocene to Medio contains a few intercalations of tuffs and volcanic brec- early Oligocene magmas effectively reused the same conduits cias that yield K-Ar and Ar/Ar ages between 43 and 36 Ma (Cornejo et al., 1997). (Tomlinson et al., 2001a). The Incaic deformation also includes These differences in the structural controls of porphyry a strike-slip component that produced segmented N-NE− to copper deposits can be attributed to the changing tectonic NE-trending, dextral strike-slip faults (e.g., the Mesabi fault conditions along different segments of the Domeyko fault in Fig. 5a; Tomlinson et al., 1997a). These faults became system during the >10-m.y. Incaic event. Camus (2003) rec- more important in the southern part of the area near ognized three temporal groupings of mineralized intrusions Chuquicamata (Fig. 5a), in accordance with increasing plate along the Domeyko fault system at 43 to 42, 39 to 37, and 36 convergence obliquity to the south along the Andean margin to 33 Ma. during the Eocene (Fig. 4b). In the Chuquicamata, El Abra, and Quebrada Blanca-Col- As shown in Figure 5, the region between Chuquicamata lahuasi districts, where the three generations of intrusions and El Abra is one of the anomalous zones along the Cor - occur, only the two youngest were fertile in copper (Campbell dillera de Domeyko (Fig. 2), where magmatism was recurrent et al., 2006; Maksaev et al., 2009; Dilles et al., 2011), whereas since the Late Cretaceous. Despite lacking evidence for large in the Escondida district all three events are related to volumes of middle Eocene to early Oligocene volcanic prod- mineralization (Hervé et al., 2012). In contrast, most of the ucts, intrusive magmatism of this age is well recorded. A large copper-bearing intrusions recognized in the Centinela district (>500 km2), composite intrusive complex (Fortuna-El Abra) seem to have been emplaced during the early event. In order was emplaced syntectonically between 45 and 38 Ma near the to illustrate the changing nature of the diverse porphyry clus- southern termination of the Sierra de Moreno (Tomlinson et ters and explore the links between mineralized centers and al., 2001a, 2010; Dillles et al., 2011). Figure 5b shows the re- the evolution of the Domeyko fault system, the salient geo- stored shape of the Fortuna-El Abra batholith before being logic features of three porphyry copper districts, Chuquica- severely dismembered by left-lateral displacements along the mata-El Abra, Escondida, and Centinela (Figs. 2, 5), are ex- West fault (Tomlinson et al., 2007a; Dilles et al., 2011). Field amined below. relationships indicate that the batholith was intruded along the trace of the Quetena reverse fault, an inverted normal Chuquicamata-El Abra fault inherited from Mesozoic back-arc extension. A flattening The Chuquicamata-El Abra intrusive cluster (Fig. 5a) is sit- foliation in metaclastic rocks in the contact aureole is consis- uated along the northern Domeyko fault system, near the tent with emplacement during regional Incaic E-W shorten- boundary between paleomagnetic domains C and D (Fig. 3a). ing (Tomlinson and Blanco, 1997a). The geology of the region records superimposed tectonic and The Fortuna-El Abra batholith, described by Dilles et al. magmatic events beginning in the Mesozoic. In this region, (1997) as a porphyry copper batholith, is a long-lived, com- the Precordillera (Cordillera de Domeyko) is formed by two posite magmatic system that contains intrusive phases em- fault-bounded, N-S−trending basement ranges. The western placed during different stages of the Incaic event; it is similar range, Sierra de Moreno, is a thick-skinned block uplifted in to the Andahuaylas-Yauri batholith of southern Peru, de- the early Late Cretaceous (~85−84 Ma), during the inversion scribed by Perelló et al. (2003a). The batholith comprises an

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Pajonal Diorite Lower to Middle 69º Eocene volcanic (a) (b) rocks (Icanche fm) + Upper Cretaceous ++ sedimentary rocks Cretaceous reverse fault + Los Picos + ++ +++ (Tolar fm) Complex ++ + + + + + Cretaceous reverse fault + ++ + + + + + ++ El Abra (45-42 Ma) + with Eocene reactivation + + + + + + +++ + + + + + ++++++ Small copper ++ + + ++++++ + Eocene reverse fault ++ Arca fault prospects + + + + + +++++ Conchi Viejo + + related to EW shortening + + + + +++ + +++ San Lorenzo + + + ++ ++++ 22º00' porphyry + + + 0 10 20km ++ ++ ++ + dikes + + Clara Granodiorite + + + + + + + ++++ +++ + 21º30' + + + ++ + DEL MEDIO + + Llareta Granodiorite ORENO Antena + + M + + + Granodiorite ++ (39.5-39 Ma) + Early Abra-Antena DE + + ++ Ganodiorite SIERRA + + Fiesta Granodiorite Upper Cretaceous volcanic rocks (Quebrada Mala fm.) SIERRA WEST FAULT (38-37.5 Ma) Alejandro Hales Quetena Toki Genoveva Porphyry Cu deposits Miranda under post mineral cover (Toki Cluster) Quetena Future 0 5 10km fault Opache West Fault

Chuquicamata Porphyry Complex (36-33 Ma)

Co. Jaspe Eocene (45-37 Ma) monzodiorites to granodiorites (including El Abra Fortuna El Abra batholith) Upper Eocene-Oligocene, syntectonic, sedimentary rocks (Sichal 22º and Calama formations) Lower to Middle Eocene volcanic rocks (Icanche formation) ++ Paleocene intrusive rocks Upper Cretaceous volcanosedimentary sequences (Cerro Empexa and Quebrada Mala formation) R. Tomic Cretaceous intrusive rocks Mesabi Fault Upper Cretaceous, syntectonic, red beds (Tambillos and Tolar formations) Jurassic to Lower Cretaceous, back-arc, sedimentary sequences + + + + Chuquicamata +++ + Triassic intrusive rocks (ca. 230 Ma; Elena granodiorite) ++ Alejandro Hales + Toki Triassic volcanic and sedimentary rocks Quetena Fault + CALAMA Sierra del Medio basement block (Upper Paleozoic) 69º 22º30' Sierra de Moreno basement block (Neoproterozoic to Paleozoic)

FIG. 5. (a). Simplified geologic map of the El Abra-Chuquicamata region, indicating age of faults (based on Tomlinson et al., 2001). (b). Restored map of the El Abra-Fortuna batholith after removal of 35 km of Oligocene-early Miocene left-lateral motion on the West fault, showing main intrusive phases and mineralized centers (Dilles et al., 2011). Names of intrusive units east of the future trace of the West fault follow the nomenclature that has been employed for intrusive phases near El Abra.

older group of intrusions, namely the Los Picos complex and al., 2006; Boric et al., 2009; Marquardt et al., 2009; Barra, Pajonal diorite, emplaced between ~45 and 42 Ma (Tomlin- 2011; Fig. 5b). son et al., 2001a; Campbell et al., 2006), which include pre- The youngest intrusive event in the Chuquicamata cluster dominantly mafic pyroxene- and biotite-bearing quartz mon- was related to the emplacement, beyond the southern limits of zodiorites, monzodiorites, and quartz monzonites that are the Fortuna-El Abra batholith, of the Chuquicamata (36−32 copper barren (Fig. 5b). The second intrusive group includes Ma) and Radomiro Tomic (36−34 Ma) porphyry copper de- the Fortuna-El Abra granodiorite complex and is largely com- posits (Lindsay et al., 1995; Reutter et al., 1996; Ballard et al., posed of two hornblende-bearing granodioritic phases (An- 2001; Ossandón et al., 2001; Campbell et al., 2006; Barra, 2011; tena, 39.5−39.0 Ma and Fiesta, 38−37.5 Ma) as well as por- Fig. 5). Sibson (1987), who relied on maps of Perry (1952), in- phyritic components (San Lorenzo porphyries; Fig. 5b). The terpreted the Chuquicamata porphyry copper deposit to have Fortuna-El Abra Complex intrusions, derived from hydrous, been emplaced syntectonically, following an extensional oxidized, and sulfur-rich magmas (Dilles et al., 2011), are as- stepover linking two separate but overlapping, parallel, dextral sociated with large porphyry copper deposits with U-Pb ages strike-slip faults. Alternatively, Lindsay et al. (1995) favored a between 39 and 36 Ma (El Abra, 38−37 Ma; the Toki cluster model in which the required space for magma emplacement including Toki, 39 Ma and Opache, 38−37 Ma; Alejandro coincided with a releasing bend (cf. Cunningham and Mann, Hales, 39−36 Ma; Conchi, 36 Ma; Perelló, 2003; Campbell et 2007) along a single, continuously linked dextral fault.

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After porphyry emplacement at the Chuquicamata and The blocks at Quimal, Los Morros, and Mariposas are lim- Radomiro Tomic, to the south of their present location, the ited to the west and north by left-lateral strike-slip faults deposits were displaced as a consequence of late Oligocene to (Mpodozis et al., 1993a, b). Along the El Bordo Escarpment, early Miocene (31.0−16.3 Ma) left-lateral movements along the eastern margin of the Imilac and Mariposas blocks are the throughgoing regional West fault or West Fissure. Dis- thrust over the sedimentary fill of the Salar de Atacama basin placement along the West fault, which includes, at Chuquica- (Fig. 6a), which includes, among other units, a 2,500-m-thick mata, according to McInnes et al. (1999), a 600 ± 100 m west sequence of Eocene to early Oligocene continental con- sideup vertical displacement component (contested by Tom- glomerates and poorly consolidated gravels. Internal pro- linson et al., 2001b), was able to offset the Fortuna-El Abra gressive unconformities and Ar/Ar ages between 44 and 43 batholith 35 to 37 km in a left-lateral sense (Dilles et al., 1997; Ma from a tuffaceous horizon just above the base of this se- Tomlinson and Blanco, 1997b; Tomlinson at al., 2001a; Fig. quence (Loma Amarilla Formation) indicate that these 5). South of Chuquicamata, at least part of the strike-slip dis- strata-accumulated syntectonically during the regional Incaic placement component was transferred to a set of normal deformation (Hammerschmidt et al., 1992; Mpodozis et al., faults that bound the Cenozoic Calama extensional and/or 2005). transtensional basin (Blanco, 2008; Blanco and Tomlinson, The tectonics of this segment of the Cordillera de Domeyko 2009). Seismic data reveal that a buried normal fault with 1- (Fig. 6a) can be interpreted as a result of the displacement of to 1.5-km down-to-the-east displacement limits the north- a 250- × 50-km basement sliver that was transported northwestern margin of the basin, where up to 2,500 m of silici- ward during the Incaic deformation. According to Mpodozis clastic continental strata accumulated between the Oligocene et al. (1993a, b), the continuous northward shift of the dis- and Miocene (Jordan et al., 2004; Blanco, 2008). placed block was impeded by a buttress located to the north The West fault forms, at present, the sharp western limit of of the moving sliver as the displacement was transferred to the Chuquicamata orebody. Nevertheless, structural inter- the east by tectonic escape (cf. Mann, 1997) toward the pretations by Sibson (1987) and Lindsay et al. (1995) consid- deeply subsiding Salar de Atacama basin. In this model, the ered that the internal architecture of second-order faults and Salar de Punta Negra depression (Fig. 6c) would have formed veins indicates that the deposit is essentially intact. According as an extensional basin at the trailing edge of the displaced to Lindsay et al. (1995), the Oligocene to early Miocene West block. Displacement transfer seems to have occurred by fault propagated northward, along the western edge of the clockwise rotation of small detached blocks, which in turn porphyry complex (previously emplaced at 36−33 Ma), with- generated the local triangular-shaped extensional basins be- out displacing the mineralized intrusive units within the for- tween the rotating blocks as well as contractional deformation mer dextral releasing bend and/or stepover. in their northeastern corners where basement was thrust over the Salar de Atacama basin fill (Fig. 6c). Mpodozis et al. Escondida (1993a, b) located this buttress at Sierra de Limón Verde, Despite also being along the Domeyko fault system, the which is a N-plunging basement half dome that attains one of tectonic history of the Escondida region (Fig. 2) is markedly the highest elevations (3,500 m.a.s.l.) in the Cordillera de different, highlighting the significant changes in tectonic Domeyko (Fig. 6a). However, if along-strike changes in local styles along the >1,000-km strike of the fault system. Defor- stresses resulting from the formation of the Bolivian orocline mation in this area seems to have been dominated by tectonic are considered, the buttressing effect may have been pro- escape linked to passive rotation and transport of brittle vided by the nonrotated paleomagnetic domain C, located upper crustal blocks over hot and ductile lower crust in a way north of Calama (Fig. 3), where initial Eocene deformation similar to the so-called orogenic float or clutch tectonics mod- was taken up by pure east-west shortening (Tomlinson et al., els discussed by Oldow et al. (1990), Lamb (1994), and Tikoff 2001a; see Figs. 3, 4). et al. (2002; see below). At this latitude, the Cordillera de Figure 7 is a more detailed map showing the geologic set- Domeyko appears as a discontinuous mountain range formed ting and distribution of the barren and mineralized intrusions by a group of discrete basement blocks bounded to the west that form part of the Escondida cluster (labeled “LE,” Fig. 2). by a 150-km-long shear lens (Escondida shear lens) devel- The area encompasses the widest part of the regional Escon- oped between the regional Sierra de Varas and Escondida dida shear lens, which is separated to the east from the Sierra faults (Figs. 6a, 7). To the east, the Domeyko range abuts the Imilac and Sierra San Carlos basement blocks by the Escon- Salar de Atacama depression, which is a deep subsiding basin dida fault (the Panadero-Portezuelo fault is an alternative filled by >9 km of Cretaceous to Tertiary continental sedi- name used by Hervé et al., 2012). These two blocks were then mentary strata (Pananont et al., 2004; Mpodozis et al., 2005). separated by the intervening triangular Salar de Hamburgo The basin was built on top of a large positive gravimetric depression (Fig. 7). Drilling shows that the Salar de Ham- anomaly (Central Andean Gravity High; Götze and Krausse, burgo fill includes >1,200 m of red beds, lahars, and pyro- 2002; Fig. 6b), which indicates the occurrence, at depth, of clastic rocks with U-Pb zircon ages of 38 Ma (San Carlos dense crustal rocks that may help to explain its long-lived sub- Strata, Fig. 7; Marinovic et al., 1995; Urzúa, 2009; Hervé et sidence basin history, recorded at least since the Cretaceous. al., 2012); therefore, these units are equivalent to the upper The isolated basement blocks that form the core of the portion of the syntectonic Loma Amarilla Formation in the range are separated by small, triangular basins, with interior Salar de Atacama. The Hamburgo fault is a NE-trending, drainage (Fig. 6a). The southern rhomboid-shaped blocks high-angle reverse fault that places the late Paleozoic base- (e.g., San Carlos and Imilac; Fig. 6a) are bounded along their ment of the San Carlos block over the sedimentary sequences northwestern margins by high-angle, SE-dipping reverse faults. of the Salar de Hamburgo (Fig. 7).

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69º (a) (b) 70º 68º (c) ón V e im r d ?

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Non-rotated e e i

S M

Domain C l

e N BUTTRESS ZONE N

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t ESPERANZA- 23º TELEGRAFO Fortuna-El Abra Q batholith A A' Thrusts

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Los Toros faul am

la F p Transition zone betwen r

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E n CALAMA α

GABY o

Ce d

r Sierra Limón Verde

Centinela Salar de Elvira B Salar de l E Atacama Paleozoic LM basement Salar de los blocks Extensional Morros Salar (rotational) de basins M Atacama ANTOFAGASTA l

u in

as t l Gravity 24º u Salar a High p B s Verónica f a 24º r s Argentina

Cordon de Lila r Va Dee

I Rotated a V

ESCONDIDA Domain D e e d Free face

d ra

Salar de r e

Hamburgo i ul S ra a

r f 0 50 km Salar e

i Escondida fault SC de S s Punta Negra Boundary a c fault n a Salar de ESL Bl Punta Negra La Escondida shear lens (ESL ) Outcrops of Lower Cretaceous s a to Upper Oligocene continental c Cordillera de Domeyko clockwise rotated n clastic strata of the Salar de a eida r basement blocks (Q: Quimal, LM: Los Morros, r lm Atacama basin a A

B M: Mariposas, I: Imilac, SC: San Carlos) N Cordón de Lila-Sierra de Mesozoic to Paleocene strata of Almeida stable domain Sierra de 25º the Centinela District 0 20 40 km Strike-Slip faults Reverse faults Normal faults

FIG. 6. (a). Main structural elements of the Cordillera de Domeyko (between Escondida and Sierra Limón Verde and Salar de Punta Negra (22° 30°−25° S; see location in Fig. 2). Note the large shear lens (Escondida shear lens) flanked by the Escondida and Sierra de Varas strike-slip faults along the western edge of the range and the discontinuous basement blocks (labeled with letters) forming the core of the range. (b). Tectonic sketch of the Cordillera de Domeyko between 21° and 25° S, indicating major Eocene-Oligocene Incaic structures (Tomlinson and Blanco, 1997a). Note contrast between clockwise- rotated blocks in rotated domain D (Fig. 3) and deformation associated with reverse faults in nonrotated domain C. (c). Model of lateral transfer of displacement of a tectonic sliver bounded by a buttress and a free face moving northward along a left-lateral strike-slip fault system. Displacement is impeded, as shown, by a buttress at the leading edge of the block, and transferred toward the right by means of clockwise block rotations. Note extensional basins created between the rotating blocks. A-A' and r-r = position of points and lines before and after rotation (α = rotation angle). Adapted from Beck et al. (1993).

A protracted, >40-m.y. Cretaceous to Tertiary history of Incaic magmatism in the Escondida cluster began in the magmatism is recorded in the Escondida region. The oldest middle Eocene (~44 Ma), when left-lateral displacements intrusive events produced Late Cretaceous (81−71 Ma; Fig. along the Escondida fault and differential rotation between 7) tholeiitic to alkaline pyroxene gabbros and diorites as well the San Carlos and Imilac blocks created the triangular Salar as hornblende-pyroxene monzodiorites and diorites, in addi- de Hamburgo depression (Fig. 7). The oldest intrusions are a tion to early Paleocene (66−64 Ma) pyroxene diorites. These group of 44 to 41 Ma pyroxene-biotite monzodiorites and rocks intruded the sedimentary strata of the Mesozoic back- pyroxene-hornblende granodiorites, emplaced within the Es- arc basin in the Escondida shear lens to the south and west of condida shear lens to the north of Escondida (Marinovic et al., Escondida (Fig. 7); Paleocene to early Eocene volcanic rocks 1995; Richards et al., 2001; Urzúa, 2009). Together, they likely (59−53 Ma) are also present in the area (Marinovic et al., represent the roof of an underlying, partially eroded pluton 1995; Richards et al., 2001; Urzúa, 2009). All of this focused nearly 20 km in diameter (Fig. 7). Most of these rocks, like the and recurrent magmatic activity took place east of the Andean Los Picos complex and Pajonal diorite of the Chuquicamata- arc front, which during the Late Cretaceous to early Pale- El Abra region, are barren, although geochronologic data and ocene (85−50 Ma) was located farther west, in the Central relationships between intrusive phases in the vicinity of the depression of the Antofagasta region (Boric et al., 1990). Chimborazo porphyry copper deposit indicate, according to

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69º

t l 24º u a f

s ila a

a Im e V d

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d ie S Chimborazo (41)

Escondida Norte (38-37) Zaldivar (38-37) Baker (38-37)

Pampa Escondida (36-34.5) Pinta Verde Escondida Este Escondida (36-34.5) (38-37) Salar de ult Hamburgo Imilac fault o fa g ur b am H

Sierra

San Carlos

0 5 10 km

La Escondida fault

Salar de Punta Negra 24º30'

Lower Paleocene (66-64 Ma) px diorites Upper Eocene (42?-36 Ma) sedimentary-volcanic sequence (San Carlos strata) Upper Cretaceous (74-70 Ma) rhyolitic ignimbrites Upper Eocene (38-35 Ma) dacitic to granodioritic Upper Cretaceous (81-72 Ma) gabbro-diorites and hb-px mineralized porphyry intrusions (Escondida cluster) diorites Upper Eocene (39-38 Ma) hb dioritic porphyry intrusions Upper Triassic-Lower Cretaceous sedimentary sequences Eocene (44-41 Ma) px-bt monzodiorites and px-hb Triassic (240-220 Ma) intrusive rocks granodiorites Upper Paleozoic (300-270 Ma) basement Upper Paleocene to Lower Eocene (59-53 Ma) volcanic sequences

FIG. 7. Simplified geologic map of the area around Escondida, highlighting major regional faults and the different intrusive phases that form part of the Escondida intrusive cluster. Compiled and adapted from Marinovic et al. (1995), Richards et al. (2001), Urzúa (2009), Hervé et al. (2012), and field data from the authors (px = pyroxene, hb = hornblende, bt = biotite).

Hervé et al. (2012 ), that an early phase of copper mineral- The final event of Incaic magmatism in the Escondida clus- ization probably occurred at ~41 Ma. ter was related to the emplacement of the Escondida Este The second episode of Eocene-Oligocene magmatism and Pampa Escondida deposits, immediately to the east of began with the emplacement of a closely spaced group of the Escondida fault (Fig. 7), between 36.0 and 34.5 Ma small intrusions distributed across the Escondida fault (Fig. (Hervé et al., 2012). The mineralized porphyries of the Es- 7). These more evolved, amphibole-bearing dioritic stocks, condida cluster, with the exception, perhaps, of Chimborazo, with U-Pb zircon ages of 39 to 38 Ma (Richards et al., 2001; postdate the earlier phase of sinistral faulting and block rota- Urzúa, 2009), intrude both the late Paleocene to early tions along this segment of the Cordillera de Domeyko. De- Oligocene volcanic rocks of the Escondida shear lens and the formation seems to have begun at ~42 Ma (age of the base of late Paleozoic basement units of the Imilac block (Fig. 7). the Loma Amarilla Formation; see above), although the lack Their distribution suggests that they could be apophyses of a of offset on any of the porphyry intrusions across the local larger pluton at depth that intruded along the Escondida fault strands (Panadero-Portezuelo fault) of the larger Escon- fault. The slightly younger group of porphyry copper stocks dida fault indicates that major along-strike fault activity had include a series of multiphase, NE- to N-NE−trending, dike- ceased by 38 Ma, as shown by the across-fault 38−37 Ma por- like intrusions that were emplaced at or near the Escondida phyry dikes (Hervé et al., 2012). fault at 38 to 37 Ma; these include the deposits at Zaldívar, Escondida Norte, Escondida, and Pinta Verde, and, farther Centinela away, at Baker (Richards et al., 2001; Urzúa, 2009; Hervé et Porphyry copper mineralization in the Centinela district (la- al., 2012; Fig. 7). beled “CE,” Fig. 2) occurs within a 25-km-wide, fault-bounded

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belt of Late Cretaceous to early Eocene volcanic rocks, lo- These rocks, emplaced within Jurassic marine limestones of cated between the Paleozoic basement exposures of the the northern Chile back-arc basin (Fig. 8), are almost 100 km Cordillera de Domeyko to the east and an early Cretaceous east of the Early Cretaceous magmatic front, which, as noted volcanic sequence in the Coastal Range to the west (Fig. 8). above, was situated at that time in the Coastal Range (Boric The Centinela district hosts one of the more recently discov- et al., 1990). Younger volcanic and intrusive events occurred ered porphyry copper clusters in northern Chile. Although in the Late Cretaceous, when a volcanic sequence (Quebrada the occurrence of exotic copper mineralization at El Tesoro Mala Formation) and a group of coeval 70 to 66 Ma pyroxene was known for a long time, the full potential of the district diorites to rhyolite porphyries and flow domes, dated (U-Pb only began to be assessed in the mid-1990s (Perelló et al., zircon) between 70 and 66 Ma, were generated after the An- 2010, and references therein). dean arc front migrated eastward into the Centinela region The district records again, a lengthy history (almost 80 m.y.) (see Figs. 1b, 8). Volcanism continued after the Cretaceous- of magmatic activity, from the Early Cretaceous to the Eocene. Tertiary boundary deformation event, with eruptions from The oldest plutonic rocks emplaced within the confines of the stratovolcanoes and small collapse calderas that were active Centinela cluster comprise a group of Early Cretaceous between the early Paleocene (64 Ma) and the early Eocene olivine-pyroxene gabbros and hornblende-bearing quartz dior- (53 Ma). During this interval, a diverse group of epizonal in- ites, with U-Pb zircon and K-Ar ages between 124 and 100 trusions composed mainly of pyroxene-biotite quartz diorite Ma (Mpodozis et al., 1993b; Marinovic and García, 1999). to monzodiorite (60 Ma) and hornblende-biotite granodiorite

69º00 Sierra Limón Verde

Orión (44-41)

as Mirador (41-39) Llano (41)

Esperanza (42-40) Llano fault ierra Telégrafo (42-40) S Caracoles (42-41) Agua Dulce Esperanza fault 23º Centinela (45-44) Coronado fault Penacho Blanco (42)

Pilar (43) t Polo Sur (42-41) Las Lomas duplex ros faul

To Sherezade (44-43)

Los Las Lomas fault

Sierra del Buitre fault

0 5 10 km

Centinela fault

Undifferentiated Cretaceous granitoids Upper Eocene (44-40 Ma) syntectonic, sedimentary and volcanic rocks Upper Cretaceous (78-66 Ma) sedimentary and volcanic sequences (Quebrada Mala fomation) Eocene (44-40 Ma) px-hb monzodioritic to hb-bt granodioritic stocks and mineralized dacitic porphyry Lower Cretaceous (?) volcanic rocks intrusions (Esperanza-Telégrafo and Centinela-Polo Sur) Paleocene (60-56 Ma) px-bt monzodiorites and Lower Cretaceous (124-100 Ma) ol-px gabbros to rhyolitic porphyry intrusions diorites and hb granodioritic porphyry intrusions Paleocene to Lower Eocene (64-53 Ma) volcanic rocks Jurassic to Lower Cretaceous back-arc sedimentary (Cinchado formation ) and volcanic rocks Lower Paleocene (65-64 Ma) diorites and dacitic porphyry Upper Triassic (210-200 Ma) volcanic and sedimentary intrusions rocks Upper Cretaceous (78-68 Ma) px diorites and (minor) Upper Paleozoic (290-270 Ma) basement rhyolitic porphyry intrusions

FIG. 8. Regional geologic map of the Centinela cluster area. Note the 35-km-long NNE trend of 42−40 Ma porphyry copper deposits emplaced during earlier stages of the Incaic event. Multiple superimposed intrusive pulses and volcanic episodes between 120 and 40 Ma show a remarkable recurrence of magmatic events for >80 m.y. All ages are based on recently acquired U-Pb zircon data. (p = prospects, ol = olivine, bt = biotite, px = pyroxene).

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(58−57 Ma) plus andesitic to dioritic porphyritic intrusions, Perelló et al., 2004, 2010; Bisso et al., 2009; Münchmeyer and were emplaced into the Mesozoic units and Paleogene vol- Valenzuela, 2009; Swaneck et al., 2009); they are all associ- canic edifices. ated with tilted porphyry dike swarms. These deposits are Incaic magmatism and mineralization in the Centinela dis- emplaced into moderately to steeply dipping strata, which are trict occurred between 45 and 39 Ma (Mpodozis et al., 2009a; disrupted by major postmineral faults that exhibit reverse, Perelló et al., 2010) and began, as revealed by numerous new normal, and strike-slip displacement components (Figs. 8, 9). U-Pb zircon ages in the intrusive rocks and Re-Os ages in Although the widespread gravel cover makes it difficult to molybdenite, ~12 to 10 m.y. after the termination of volcan- satisfactorily resolve the structural relationships within the ism in the early Eocene. whole district, the regional structure around the Esperanza This event coincides with the mostly copper-barren early and Telégrafo deposits (Fig. 8) comprises a long-wavelength, phase of Incaic intrusions at Chuquicamata-El Abra (45−42 asymmetric, basement-cored anticline, bounded to the west Ma) and Escondida (44−41 Ma). The oldest Incaic intrusive by a moderately E-dipping yet unexposed thrust fault that rocks include a small group of 45 Ma pyroxene-biotite and was discovered during exploration drilling (Telégrafo fault; quartz diorites yet, in contrast to Chuquicamata-El Abra and Perelló et al., 2004, 2010; Bisso et al., 2009; Münchmeyer Escondida, at Centinela, numerous mineralized porphyry and Valenzuela, 2009; Fig. 9). The hinge zone of the anticline centers were emplaced between 44 and 39 Ma. They form, is, in turn, sliced by two subvertical faults (Coronado and together with some barren stocks, a 40-km-long, N- to NE- Llano faults; Figs. 8, 9) linked to the N-S−trending regional trending belt, which includes at least 10 discrete intrusive fault zone. Figure 9 includes a west-east structural section complexes (Fig. 8). A syntectonic sequence of conglomerates across the Esperanza deposit, where mineralization is associ- and volcaniclastic sandstones, which accumulated at the same ated with a group of easterly inclined porphyry dikes em- time as porphyry copper emplacement, comprises interbed- placed within the ~40° to 50° W-dipping Triassic to Upper ded layers of dacitic block-and-ash deposits and tuffs with U- Cretaceous strata that form the frontal limb of the anticline. Pb zircon ages between 42 and 39 Ma. This panel, containing in part mineralized and altered host The oldest porphyry systems (45−43 Ma) occur along the rocks to the porphyry deposits, is upthrown to the west, southwest end of the belt, and the age decreases systematically along the Telégrafo fault, over barren, unaltered mid-Eocene to the northeast until reaching 39 Ma at the northeast edge of (42−39 Ma) sedimentary and volcanic rocks that accumu- the porphyry trend (Fig. 8). The geometry of the porphyry lated when porphyry intrusions were being emplaced at depth. complexes is controlled by their position relative to the main The tilted, frontal-limb panel of the anticline is, in turn, structural feature of the district, a 3- to 5-km-wide, N-S−trend- bounded to the east by the subvertical Esperanza fault (Fig. ing fault zone that cuts obliquely across the porphyry belt. This 9) that places Jurassic limestones over Late Upper Creta- zone of intense deformation constitute to the northern termi- ceous strata. The rectilinear fault trace and the mismatch of nation of the Sierra de Varas fault, which stretches for >250 the lithology and age of the Late Cretaceous volcanic rocks km along the western border of the Cordillera de Domeyko across the Esperanza fault show it includes an important (Mpodozis et al., 1993b; Soto et al., 2005; Figs. 2, 6a), and was component of strike-slip movement, although the precise active both during and after porphyry emplacement (Fig. 8). age of deformation and genetic links between both faults re- Porphyry deposits located west and east of this zone of con- main to be determined. The Llano and Coronado faults are, centrated deformation are largely undeformed. Copper miner- however, as shown in Figure 9, younger faults that are su- alization in mineralized porphyry systems, located west of the perimposed over the Telégrafo-Esperanza system, which ex- fault zone, are related to subvertical, hornblende-biotite dacites hibits both left-lateral and large, down-to-the-east compo- dike swarms intruded into Paleocene volcanic/subvolcanic units nents of displacement, part of which has a late Miocene or (e.g., Centinela) or early Eocene rhyolitic dome complexes younger (<10 Ma) age. (Polo Sur, Perelló et al., 2010). The oldest deposits where em- Structural relationships at Caracoles, in spite of being only placed at 45 to 44 Ma (Centinela) and 44 to 43 Ma (Sher- 10 km to the south, are entirely different and difficult to erezade) to be followed by the intrusion by several barren py- match with those observed at Esperanza-Telégrafo. In the roxene-hornblende dioritic stocks and lacoliths dated at 43 Ma, southern part of the district (Fig. 8), the fault zone is dis- although a porphyry copper system with the same age has been placed to the west, out of strike with the fault zone to the also recognized at Pilar. A new pulse of copper-bearing intru- north, and includes a strike-slip duplex (Las Lomas duplex) sions occurred, finally, between 42 to 41 Ma, at Polo Sur, while bounded by two N-S−trending subvertical faults (Las Lomas dacitic porphyries with a similar age (42 Ma) but apparently and Centinela faults; Fig. 8). Kinematic indicators show evi- barren have been documented at Penacho Blanco (Fig. 8). dence for an episode of left-lateral displacement, even if the Mirador, the youngest porphyry deposit recognized so far in the internal geometry of the duplex is compatible with an earlier district (41−39 Ma; Mora et al., 2009) and located east of the (Late Cretaceous?) event of dextral shear (Marinovic and fault zone (Fig. 8), is also structurally undisturbed. The copper García, 1999; Mpodozis et al., 2009a). The Caracoles deposit mineralization, hosted within Jurassic marine limestones and (42−40 Ma, Swaneck et al., 2009) is hosted in early Paleocene evaporites (Mora et al., 2009) is associated with a group of (64−60 Ma) volcaniclastic rocks beneath the gravel cover. multiphase, W- to NW-trending intrusions that, as in the older Copper mineralization is linked to a steeply SE-dipping Centinela and Polo Sur deposits, appear to be subvertical. swarm of thin dacite porphyry dikes that follow the internal By contrast, deposits emplaced along the fault zone (Fig. 8) structural trends of the Las Lomas duplex; this is consistent have intermediate ages of 42 to 40 Ma (Caracoles, 42−41 Ma; with emplacement of the porphyry along a zone of (earlier or Telégrafo, 42−40 Ma; Esperanza, 42−40 Ma; Llano, 41 Ma; synmineral?) strike-slip deformation.

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49000 49200 49400 49600 UTM (E ) m.a.s.l.

3200 W Esperanza E fault

Coronado 2800 Llano fault fault Mi-Pl Ks Pal Jur Mi-Pl Ol Ep 2400

Ks Ks 2000 Trv

1600 Eo Ks

1200 0 0.5 1 km Ep Esperanza copper porphyry intrusions (41-40 Ma) Telégrafo fault 800 Pal Lower Paleocene dacitic domes (64-63 Ma)

Ks Upper Cretaceous volcanic rocks (Quebrada Mala Mi-Pl Miocene to Pliocene gravels formation, 70-66 Ma) Ol Oligocene sedimentary rocks Jurassic marine sedimentary sequences Jur Eo Eocene sedimentary and volcanic strata Trv Upper Triassic volcanic rocks (210-200 Ma) Left-lateral displacement on faults 0.5% CuT

FIG. 9. Schematic structural section across the Esperanza porphyry Cu deposit, Centinela district. The Esperanza orebody is part of a W-dipping sliver of Jurassic and Late Cretaceous strata intruded by Eocene porphyry dikes (41−40 Ma), thrust to the west (Telégrafo fault) on top of Eocene (42−37 Ma) sedimentary and pyroclastic sequences. The Esperanza and Coronado faults have a complex and younger displacement history, including strike-slip components. The down-to-the-east displacement shown along the Coronado and Llano faults corresponds only to the youngest (late Miocene-Pliocene?) episode of deformation. Location of section shown in Figure 8.

Long-lived intrusive clusters and porphyry deposits: Even if these models explain porphyry spacing, they do not What is the relationship? elucidate why clustered magmatism began long before the As discussed above, Eocene to Oligocene porphyry copper Eocene. As more data are gathered, an even more striking deposits in northern Chile are located in districts of focused spatial relationship is emerging in several districts (Escon- and long-lived magmatic activity distributed along the Cor- dida, Centinela, Chuquicamata, Quebrada Blanca-Collahuasi, dillera de Domeyko (Fig. 2). In some districts, magmatism Sierra Exploradora; Figs. 5, 7, 8), between extended, Creta- was active for 50 m.y. (Chuquicamata-El Abra, Escondida) or ceous and younger magmatic activity, porphyry copper de- even 80 m.y. (Centinela). Magmatism began in a back-arc set- posits, and Triassic intrusions which, near Escondida and Col- ting while the arc front was located much farther west in the lahuasi, show evidence of weakly developed porphyry-style Coastal Range or Central depression (Boric et al., 1990); to copper mineralization (Cornejo et al., 2006; Munizaga et al., date, the cause of these zones of long-term magmatism is un- 2008). In addition, indistinguishable geochemical signatures certain. Yañez and Maksaev (1994) suggested that porphyry reported in a pilot study by Wilson et al. (2011), which com- spacing may be related to Rayleigh-Taylor diapirism along the pared Triassic granodiorites to mineralized Eocene porphyry mid Eocene-early Oligocene arc. Behn et al. (2001) noted deposits at Alejandro Hales (Fig. 5), may suggest tapping of a that a correlation exists between the location of porphyry cop- common source region at depth. The problem remains open. per deposits and regional E-W−trending negative magnetic Neogene Tectonic Province of Central Chile anomalies that extend from the coast to the modern Andean and Argentina arc, and which may represent crustal structures favorable for magma ascent as the arc migrated eastward since the Jurassic. Late Eocene (?) to early Miocene extension and Richards (2003) speculated that porphyry copper deposits the Abanico intra-arc basin were emplaced at the intersections of the Domeyko fault sys- The tectonic evolution of the Andean orogen in central tem with NW-trending transcordilleran crustal-scale struc- Chile (31°−34° S) and contiguous Argentina differs from that tures, yet with the exception of Potrerillos (Fig. 2), none of of northern Chile in that the main deformation events are these has been documented on the western (Chilean) side of younger, the tectonic style is different, and the principal age the Andes. To address these observations, Tomlinson and of porphyry copper mineralization is much younger (late Cornejo (2012) proposed a hybrid model to explain porphyry Miocene to early Pliocene). At present, a zone of flat subduc- spacing, which combines Rayleigh-Taylor diapirism with tion (Central Chile or Pampean flat-slab region; Cahill and crustal-scale structural control. Isacks, 1992; Kay and Mpodozis, 2002; Ramos et al., 2002;

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Fig. 10) that was formed by progressive slab shallowing, begin- assigned to the Abanico, Coya-Machalí, and Cura-Mallín For- ning in the early Miocene (Kay et al., 1987), extends from 27° mations (e.g., Charrier et al., 1996, 2002; Jordan et al., 2001; to 33° S. As in northern Chile, the Coastal Range is formed by Kay et al., 2005; Farías et al., 2008). These sequences accu- the volcanic and intrusive remnants of a Jurassic to Cretaceous mulated in extensional volcano-tectonic depressions or intra- arc system. From 32° S southward, a thick sequence of coeval arc basins that are referred to as the Abanico basin; Ar/Ar marine and terrestrial sedimentary rocks exposed along the ages at the latitude of Santiago (33° S) range from latest eastern slope of the Cordillera Principal corresponds to sedi- Eocene to early Miocene (35−21 Ma; Muñoz et al., 2006). ments that accumulated within the Mesozoic Neuquén back- Volcanic rocks are calc-alkaline to tholeiitic in composition, arc basin (Mpodozis and Ramos, 2008; Figs. 10−12). and the overall geochemical signature suggests that volcanic The most notable geologic feature, however, is a several activity occurred over a relatively thin crust (<35 km; Kay et kilometer-thick volcano-sedimentary sequence that forms al., 2005; Muñoz et al., 2006). most of the western part of the Cordillera Principal between Equivalent units extend for more than 1,500 km to the 32° and 37° S (Figs. 10b, 11, 12), which has been traditionally south along the crest of the Andean range into the northern

PERU 73º 71º 69º 80º (a) 75º (b) Aconcagua 33º Santiago 4º BOLIVIAAltiplano-P 20º Maipo Orocline 32º

40 Ma rench una Antofagasta T Figure 11 35º

hile Peru-Chile trench Peru-Chile CVZ

erú- C

Chile

tre Region 3.4 cm/yr Flat-slab Concepción Central Valley

30º 37º Perú-Chile trench Perú-Chile

e Juan Fernández RidgeSantiago 30 Ma

Coastal Range

39º ARGENTINA

t Concepción ARGENTINA

fqui faul SVZ 20 M Somuncurá 40º Plateau a Liquiñe - O Bariloche 41º

Oligocene to Miocene intrusive rocks Puerto Montt

Middle and upper Miocene volcanic rocks (with major sedimentary participation in the south) Oligocene to middle Miocene volcanic sequences and interbedded 43º sedimentary strata Late Oligocene to middle Miocene continental strata Late Oligocene to middle Miocene 0 100 km Somuncurá plateau volcanic rocks Isla Magdalena Eocene volcanic and intrusive rocks

FIG. 10. (a). Map showing position of the Chilean-Pampean flat-slab region and the distribution of Quaternary volcanoes (CVZ = Central Andean volcanic zone, SVZ = Southern Andes volcanic zone). Light-colored area shows region with elevations >3 km. (b). Simplified geologic map showing the distribution of Oligocene-Miocene volcanic and sedimentary strata in south-central Argentina and Chile from 32° to 45° S. Based on the compilation by Jordan et al. (2001) and the 1:1,000,000 geologic map of Chile (SERNAGEOMIN, 2002). Arrows north and south of the Maipo orocline (or Maipo mega kink; Arriagada, et al., 2009) indicate the average values of paleomagnetically determined block rotations (Arriagada et al., 2009).

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CHILE ARGENTINA 71º 69º CORDILLERAFRONT Los Andes AL Mendoza Valparaíso 33º

RIO BLANCO -

LOS BRONCES

P COA

Tupungato

Santiago

S T S

R I N C I P I C N I R A A A

L San José R

A N G E G N

34º

Rancagua A L C O D I L L E R A R E L L I D O C L EL TENIENTE Maipo MAIP O ORO SYM CL MET INE R Y AXIS

35º Curicó M

0 25 50 km

Early Cretaceous sedimentary strata Upper Cenozoic foreland basin strata (Neuquén basin) Quaternary stratovolcanoes Miocene intrusive rocks (Principal Triassic to Jurassic sedimentary strata Cordillera) (Neuquén basin) Miocene volcanic rocks (Farellones Quaternary volcanic rocks Jurassic to Cretaceous volcano-sedimentary formation) sequences (Coastal Codillera) Cretaceous intrusive rocks Plio-Pleistocene tuffs Late Paleozoic to Jurassic batholiths (Coastal Cordillera) Eocene? - Early Miocene volcano- Fore arc Miocene sedimentary strata sedimentary sequences (Coya - Machalí/ Fontal Cordillera Paleozoic basement Abanico formations)

FIG. 11. Geologic map of the area around the Maipo orocline from 32°30' to 35° 30' S (location in Fig. 10). Note change in the structural trend of the Coastal Range and Principal Cordillera across the Maipo orocline (Farías et al., 2008; Arriagada et al., 2009), from N-S, to the north, to N-NE, to the south. A = Aconcagua fold-and-thrust belt, M = Malargüe fold-and- thrust belt (adapted from Farías et al., 2008). See text for more details.

Patagonian Andes (Fig. 10b). Even though the erosion level asthenospheric upwelling during a transient Oligocene to increases and the volume of preserved volcanic products early Miocene event of very rapid subduction. Deep mantle decreases southward, these sequences define a progressively upwelling has also been suggested by Kay et al. (2007a) to ex- southward-widening belt that extends from the Coastal plain the origin of the 33 to 17 Ma basaltic flows of the large Range to the eastern slope of the Andes at the latitude of back-arc Somuncurá volcanic plateau in Patagonia (Fig. 10b). Puerto Montt (41° S, Fig. 10b). The nature of interbedded Oligocene to Miocene submarine basaltic pillow lavas with sedimentary rocks changes along strike, from continental to MORB geochemical features in the Aysén region indicate ex- lacustrine facies in the northern part of the belt to marine treme crustal thinning along the arc farther south, between facies toward the south; geochemical signatures also display 43° and 46° S (Silva et al., 2003). evidence for crustal thinning and extension increasing to the south. Muñoz et al. (2000) indicated that 37 to 20 Ma mafic Miocene compressional failure of the Abanico basin lavas exposed at the latitude of Puerto Montt (41° S; Fig. 10b) The late Eocene to early Miocene extensional period ter- and erupted in an extensional setting possess island-arc minated at ~20 to 18 Ma, followed by compressional defor- geochemical affinities and were likely produced by deep mation leading to the emergence of the modern Andes in

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70º Quaternary stratovolcanoes Chile Argentina Quaternary volcanic rocks Upper Cenozoic foreland sedimentary sequences Miocene intrusive rocks Los Azules (10-8) Volcanic rocks of the Abanico basin (Eocene? to Miocene) include the Abanico, Rincones de Araya (9) Coya-Machali and Farellones formations Jurassic to Cretaceous sedimentary sequences of the Neuquén basin Piuquenes (11) Coastal Range block (Paleozoic to early Cretaceous) El Altar (12-10) Frontal Cordillera Paleozoic basement El Pachón (9-8)

Los Pelambres (14-10) El Yunque (15) Mercedario ( 13)

32º Cerro Mercedario Cerro Bayo del Cobre (12-10)

Amos- Andrés (9-8)

LR Vizcachitas (11-10) Morro Colorado Pimentón (11-9) West Wall (11-9) Aconcagua San Felipe

Novicio (15-13) Pocuro fault Pocuro

Rio Blanco-Los Bronces Valparaiso 33º (7.7-4.7) A

Los Machos (15-14) Los Piches (14-12.5) Los Sulfatos (7-6)

Tupungato Santiago

San José

34º Rancagua El Teniente (6.5-4.6)

0 50 km Maipo M

71º 70º

FIG. 12. Tectonic sketch of the northern end of the Abanico intra-arc basin (31°−34° S), showing the location and age (Ma, in parentheses) of Miocene to early Pliocene porphyry copper deposits of central Chile and contiguous Argentina and the composite fold-and-thrust belt developed along the eastern margin of the basin (LR = La Ramada, A = Aconcagua, M = Malargüe fold-and-thrust belts).

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central and southern Chile and contiguous Argentina (e.g., the colder and rigid western Coastal Range and eastern Giambiagi et al., 2003). Between 31° and 34° S, much of this Frontal Cordillera blocks (Fig. 13). Under these conditions, deformation was focused along the eastern border of the the late Eocene to early Miocene volcanic sequences were Abanico basin, which finally collapsed and was inverted by com- tectonically transported to the east and juxtaposed over the pressional deformation in response to the collision between sedimentary sequences of the Neuquén back-arc basin to

W E Rapid Convergence Intra-arc Weak intraplate Abanico basin Mesozoic coupling sedimentary wedge

Coastal Range Block Frontal Cordillera block

Moho

Lithosphere S te e p sl ab Asthenosphere 35-21 Ma

Slow Convergence Collapsed Strong intraplate Farellones volcanism & inverted Aconcagua- coupling Abanico basin La Ramada Accelerating Syntectonic intrusions FTB SAM plate Subduction erosion

Zone of partial melting Shallowing sla

12-5 Ma b

Slow Convergence Aconcagua- El Teniente Strong intraplate Los Pelambres La Ramada Frontal Cordillera coupling Los Bronces FTB

Lower crust melting and mixing with mantle-derived magmas Delaminated lower crustal blocks? 10-5 Ma

FIG. 13. Schematic diagram near 33° to 34° S, showing the evolution of the Abanico intra-arc basin during the Oligocene and Miocene (SAM = South America, FTB = fold and thrust belt). Major porphyry copper deposits began to be formed at 10 Ma when the deformation front migrated to the east and the Frontal Cordillera was uplifted as a consequence of shallowing of the subducted Nazca plate. Even though the diagrams combine geologic relationships observed at different latitudes it shows the tectonic position of the Los Pelambres intrusions along the boundary thrusts of the deformed Abanico basin and the Río Blanco-Los Bronces and El Teniente intrusions in the less deformed rocks near the center of the former basin farther to the west. Red arrows below the Abanico basin show hypothetical magma paths. (SAM = South American plate, FTB= fold and thrust belt)

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form the La Ramada, Aconcagua, and Malargüe fold-and- deposits, at Los Pelambres, Río Blanco-Los Bronces, and El thrust belts (Ramos et al., 1996; Figs. 11−13). In central Chile, Teniente (Camus, 2003; Cooke et al., 2005; Sillitoe and Perelló, a sharp decrease in the volume of volcanism ensued, the arc 2005), as well as numerous smaller deposits and prospects front migrated eastward, and the geochemical and isotopic that straddle the Chile-Argentina border region from 31° to signatures of younger, middle to late Miocene volcanic 35° S (Fig. 12). Although the first evidence of mineralization sequences (e.g., Farellones Formation; Figs. 10−11) indicate that postdates the beginning of Miocene deformation is as old progressive crustal thickening as a consequence of increased as 15 to 13 Ma (Novicio, 15–13 Ma; Los Machos, 14−12.5 Ma; horizontal shortening (Ramos et al., 1996; Kay and Mpodozis, Los Piches, 14−12.5 Ma; Maksaev et al., 2009; Toro et al., 2002; Stern et al., 2010). 2009; Fig. 12), most of the deposits north of 33° S (Los Between 33° and 35° S, the amount of Miocene and Azules, Rincones de Araya, El Altar, Los Pelambres, El younger back-arc shortening decreased along strike from Pachón, Vizcachitas, Amos-Andrés, Pimentón, Novicio, plus north to south, as the tectonic style in the deformed back-arc West Wall in the San Felipe cluster) were emplaced between sequences changed from the narrow, thin-skinned Aconcagua 12 and 8 Ma (Sillitoe and Perelló, 2005; Maksaev et al., 2009; fold-and-thrust belt to the wider, mixed thin- and thick- Toro et al., 2009; Maygadán et al., 2011; Perelló et al., 2012). skinned style of the Malargüe fold-and-thrust belt to the At this time, the E-NE−trending fragment of the Juan Fer- south (Ramos et al., 1996, 2004; Giambiagi et al., 2011; Figs. nández Ridge began to subduct below the Los Pelambres re- 11−12). The transition zone, at 34° S, coincides with the sym- gion (Yañez et al., 2001; Kay and Mpodozis, 2002), causing metric axis of the W-NW−trending Maipo orocline (Farías et the Andean deformation front to move eastward; uplift of the al., 2008; Arriagada et al., 2009), and is revealed by the Frontal Cordillera (Fig. 11) commenced and was accommo- change in orientation of both the Chile trench and the main dated by regional thick-skinned faults (Ramos et al., 1996, structural trends of the Principal Cordillera, from N-S to N- 2004; Pérez, 2001; Giambiagi et al., 2003; Fig. 12). By con- NE (Fig. 11). The Maipo orocline, a more subtle feature than trast, south of 33° S, the youngest and largest deposits, in- the Bolivian orocline (see above), is also shown in paleomag- cluding Río Blanco-Los Bronces and El Teniente (Maksaev et netic studies, as the magnitude of paleomagnetic block rota- al., 2004; Deckart et al., 2005; Maksaev et al., 2009; Fig. 12), tions determined in all rocks older than 10 Ma changes from were emplaced from 7 to 4 Ma during a period (7−5 Ma) of 4°clockwise north of the orocline to 32° clockwise to the transient shallowing of the subducting slab below the south (Arriagada et al., 2009; see Fig. 10b). These changes co- Neuquén basin, when volcanism with arc-like geochemical incide with a rapid fall of the absolute elevation of the Andean signatures expanded up to 500 km east of the present day range, as well as an overall decrease of crustal thickness, from Perú-Chile trench (Ramos and Folguera, 2005; Kay et al., 50 km at 32° S to <40 km at 36° S (Introcaso et al., 1992; 2006). Gilbert et al., 2006; Anderson et al., 2007). From a structural point of view, porphyry copper deposits These along-strike differences seem to reflect the effect of the of central Chile and contiguous Argentina include a group of more contractional conditions prevailing during the Neogene stocks that postkinematically intruded regional thrusts along north of latitude 33° S, as shallowing of the subducting Nazca the boundary between the former Abanico basin and the thin- slab progressed. This, in turn, lead to the establishment of the skinned La Ramada fold-and-thrust belt (Los Pelambres, modern Chilean or Pampean flat-slab region between 28° Amos-Andrés, Pimentón; Figs. 12, 14a), along the northern and 33° S (e.g., Kay and Mpodozis, 2002). Slab shallowing is part of the belt. Another, southern group (e.g., Vizcachitas, generally attributed to the buoyancy effect introduced by the West Wall, Río Blanco-Los Bronces, and El Teniente; Figs. subduction of the E-NE−trending Juan Fernández Ridge 11−12) comprises intrusive complexes emplaced farther west during the late Miocene (Yañez et al., 2001; Ramos et al., of the thrust belt in gently folded sequences of the Abanico 2002, and references therein), although recent analogue and basin, with no obvious relationship to regionally significant numerical experiments (e.g., Martinod et al., 2005) show that tectonic structures. A distinctive feature of the porphyry sys- moderate-sized, buoyant ridges that impinge on a trench are tems at Los Pelambres, Río Blanco-Los Bronces, and El Te- not able to alone induce formation of flat-slab segments of the niente is the occurrence of barren and/or mineralized mag- dimensions observed in central Chile and contiguous Ar- matic hydrothermal breccia complexes (Skewes and Stern, gentina. Other authors (Manea et al., 2012) suggest that a 1994). combination of trenchward motion of thick cratonic lithosphere accompanied by trench retreat may better explain the Los Pelambres formation of the Chilean-Pampean flat slab during the The late Miocene Los Pelambres porphyry copper-molyb- Miocene. denum deposit and its smaller gold-bearing satellite, the Frontera deposit (Fig. 12), are located in a narrow belt of in- Late Miocene to Early Pliocene Porphyry Copper tense deformation that involves Oligocene to early Miocene Deposits of Central Chile and Contiguous Argentina (33−18 Ma) volcanic rocks of the Abanico basin; these volcanic rocks, previously described as the Los Pelambres For- Overview mation, form part of the northern termination of the La Ra- The major porphyry copper deposits in central Chile and mada fold-and-thrust belt at 31°42' S (Mpodozis et al., 2009b; contiguous Argentina are preferentially located at or near the Perelló et al., 2012 ; Fig. 14a). The deposit is formed by mul- transition zone between the Chilean (or Pampean) flat-slab tiple magmatic-hydrothermal centers with ages of 12.3 to 10.8 zone and the steeper subduction zone beneath southern Ma, hosted by a precursor quartz diorite pluton (Los Pelam- Chile. This region contains three of the world’s largest copper bres stock, 13.6−13.0 Ma) and adjacent andesitic (Abanico

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Dacitic porphyry intrusions (5 Ma)

Magmatic-hydrothermal breccias Qz dioritic porphyry intrusions ( San Francisco Batholith (14?-8 Ma) Miocene volcanic rocks (Farellones Formation) La Copa rhyolite-breccia complex (4 Ma)

e r Y

o c a

ero

La Paloma s E Donoso 70º18' San Enrique-Monolito Infiernillo

Brecha Sur o

BATHOLITH n a

a S ío SAN FRANCISCO R El Plomo San Manuel Los Piches Ortiga 0 2 4km (b) 70º24' RIO BLANCO-LOS BRONCES 33º12' 33º6'

ta Cruz Intrusive rocks associated with copper mineralization at Los Pelambres and El Pachón (14-8 Ma) Post tectonic monzodiorites and hb-bearing grandiorites and dacites (16-15 Ma) Lower Miocene (21-18 Ma) volcanic sequences Pretectonic px diorites and granodiorites (23-21 Ma)

an Syntectonic (?) gabbros to px monzogranites (18 Ma) ra de S rdille Co Chalinga Intrusive Complex and related intrusives (23-15 Ma) 0 10 km Argentina

El Yunque

í at r z Cru Santa Río Mondaca fault Mondaca

70º20' Los Pelambres fault Pelambres Los Chile Pachón

Frontera Los Pelambres El

70º30'

ío

ocuro fault ocuro P . 14. Comparison between the geologic setting of (a) the Los Pelambres-El Pachón and (b) Río Blanco-Los Bronces districts. Despite the difference in scale, both . 14. Comparison between the geologic setting of (a) Los Pelambres-El Pachón and (b) Río Blanco-Los Bronces districts. Despite IG Cuncumén F porphyry complexes are associated with N-NW−trending belts of intrusions and magmatic-hydrothermal breccias emplaced during the waning stages of long-lived ear- porphyry complexes are associated with N-NW−trending belts of intrusions and magmatic-hydrothermal breccias emplaced during the et al. (2010). lier magmatic centers (Chalinga intrusive complex, San Francisco batholith). Map of the Río Blanco-Los Bronces region is from Irarrázaval Triassic rift strata Strongly deformed, Oligocene to Lower-Miocene (33-18 Ma) volcanic unit ("Los Pelambres Formation") Upper Cretaceous to Eocene intrusive rocks Cretaceous to Paleocene volcanic and sedimentary units Jurassic-Early Cretaceous, back-arc sedimentary sequences Oligocene to Lower Miocene (25-21 Ma) volcanic sequences Upper Paleozoic basement (a) 31º40' 31º50' 32º00' CHALINGA INTRUSIVE COMPLEX LOS PELAMBRES

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Formation) country rocks. The precursor stock and its con- along this trend, beginning at 7.7 Ma, during the waning tained porphyry copper mineralization were postkinematically stages of the San Francisco batholith. Intrusive activity termi- emplaced along the high-angle, Los Pelambres reverse fault, nated with the emplacement of late-mineral dacitic porphyry which constitutes the eastern limit of the above-mentioned intrusions at ~5 Ma, and the postmineral La Copa rhyolite zone of concentrated deformation (Mpodozis et al., 2009b; breccia complex at 4.7 Ma (Skewes et al., 2003; Deckart et al., Perelló et al., 2009, 2012). At a more regional scale, however, 2005, 2012; Irarrázaval et al., 2010; Toro et al., 2012; Fig. the Los Pelambres stock appears to be a satellite intrusive 14b). body of a much larger (>250 km2) and long-lived, multistage pluton located a short distance to the west (Chalinga intrusive El Teniente complex; Fig. 14), which was active for at least 8 m.y. between El Teniente, 100 km south of Río Blanco-Los Bronces, is 23 and 15 Ma. The complex includes a pretectonic (with ref- located near the axis of the Maipo orocline, at the center of erence to regional deformation) suite of gabbros, pyroxene the deformed Coya-Machalí (Abanico) basin and 30 km west diorites, and granodiorites, with U-Pb zircon ages between 23 of the most internal thrust sheets of the Aconcagua fold- and 21 Ma; a group of 18 Ma syntectonic olivine gabbro-dior- and-thrust belt (Fig. 15). The deposit is hosted by gently ites and granodiorites; and a younger and larger group of folded volcanic rocks of the El Teniente volcanic complex post-tectonic, 16 to 15 Ma hornblende-bearing granodiorites. (informal unit equivalent to the Farellones Formation), Rocks of this latter group form the eastern margin of the which unconformably overlies Oligocene to early Miocene Chalinga intrusive complex, where they cut the traces of re- volcanic rocks of the Coya-Machalí Formation (Kay et al., gional thrust faults (Fig. 14a). 2005; Stern et al., 2010). Mineralization at El Teniente is as- Several small granodiorite to quartz diorite stocks and sociated with a magmatic-hydrothermal center that records hornblende-bearing dacite porphyry intrusions, dated at 15 to at least 6 m.y. of continuous activity between ~9 and 3 Ma. 13 Ma (Fig. 14a), form a NW-SE−trending string that extends As at Los Pelambres and Río Blanco-Los Bronces, magma- from the eastern Chalinga intrusive complex across the inter- tism at El Teniente includes a large premineral intrusive national frontier to Cerro Mercedario, ~70 km to the south- complex containing older mafic facies (andesite intrusive east in Argentina showing a southeastward propagation of sills and olivine-pyroxene gabbros), described as the Te- intrusive magmatism from the Chalinga complex during the niente Mafic Complex, and a younger core of hydrous, horn- middle to late Miocene (Figs. 12, 14a). Some of these intru- blende-bearing intrusions (Sewell Tonalite; Cannell et al., sions are associated with large porphyry-related hydrothermal 2005; Stern et al., 2010; Vry et al., 2010; Fig. 15). Ages for alteration zones, such as El Yunque (Fig. 14a). Porphyry mag- this group of intrusions are still not well constrained (Te- matism and copper mineralization at Los Pelambres evolved niente Mafic Complex: 8.4 Ma K-Ar whole-rock; Sewell along this trend between ~14 and 10 Ma, whereas much more Tonalite: 7.05 Ma total gas, Ar/Ar; Maksaev et al., 2004, limited data suggest that El Pachón was active between 9.2 Stern et al., 2010, and references therein). and 8.4 Ma, and the Cerro Mercedario porphyry copper de- The copper mineralization at El Teniente is associated with posit at ~13 Ma (Sillitoe, 1977; Bertens et al., 2006; Perelló et a >2-km-long, N-NW−trending body of dacite porphyry and al., 2012). a series of small magmatic-hydrothermal breccias (Vry et al., 2010 ), which in contrast to Los Pelambres or Río Blanco-Los Río Blanco-Los Bronces Bronces, were emplaced within the earlier intrusions (Te- The world’s largest copper district at Río Blanco-Los Bronces niente Mafic Complex and Sewell Tonalite; Fig. 15). Detailed (Serrano et al., 1996; Skewes et al., 2003; Frikken et al., 2005; U-Pb zircon, Ar/Ar, and Re-Os geochronology (Maksaev et Irarrázaval et al., 2010; Toro et al., 2012), is located, farther al., 2004) has allowed recognition of five pulses of felsic in- south, near the center of the former Abanico basin (Figs. trusions linked to mineralization emplaced between 6.5 and 11−12). Although younger than Los Pelambres, it also formed 4.6 Ma. Subsequently, the late-mineral 4.81 Ma Braden brec- during the final stages of evolution of a long-lived, >10-m.y, cia pipe and a family of narrow, E-NE−oriented olivine-horn- magmatic system, which includes a large premineral intrusive blende lamprophyre dikes; ages from 3.9 to 2.9 Ma (Stern et complex (San Francisco batholith; Fig, 14b); this is remarkably al., 2011) mark the last magmatic pulses in the district before similar to the Chalinga intrusive complex of the Los Pelambres the magmatic front migrated ~50 km eastward to the Chile- area and was emplaced within gently folded, early Miocene Argentine frontier to form the northernmost active volcanoes (18−15 Ma) volcanic rocks (Farellones Formation; Fig. 14b). of the modern Andean Southern volcanic zone (Fig. 12). Deckart et al. (2005, 2012) recognized that the San Fran- cisco batholith includes three main intrusive phases, emplaced Origin of Miocene-Pliocene porphyry copper alignments between12 and 8 Ma, although an older U-Pb zircon age (14.7 One of the most striking features of porphyry copper de- Ma) for pyroxene monzodiorites (Jerez, 2007; Deckart 2012) posits in central Chile is their relationship to N-NW−trending indicates that magma emplacement began during the middle alignments of multiple magmatic-hydrothermal centers, em- Miocene. The main mineralization at Río Blanco-Los Bronces placed during the final stages of long-lived magmatic systems occurred along an 8-km-long, N-NW−trending corridor, that include breccia complexes whose formation has been which commences in the San Francisco batholith and extends considered to be triggered by decompression during rapid ex- from Río Blanco in the northwest to Los Sulfatos in the south- humation and tectonic uplift (Warnaars et al., 1985; Skewes east (Fig. 14b). Multiple magmatic-hydrothermal breccia and Stern, 1994). complexes associated with porphyry copper-bearing, amphi- When the overall geometry of the porphyry alignments of bole-rich quartz diorite porphyry intrusions were emplaced the three main centers described in this paper are considered,

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Teniente Dacite Porphyry N Mine 6232 5.28±0.10 Ma (b) Level Teniente 5 (a) Laguna La Negra (2.284m) 1600 Laguna 0 400 km La Huifa Mafic Complex 8.9±1.4 Ma?

Dacitic porphyries 6.09±0.18 Ma Braden 800 6228 Pipe Supergene zone

Braden Pipe 4.81±0.10 Ma

00 Marginal Breccia 0 2 km Porphyry A 6224 5.67±0.19 Ma

Sewell Tonalite (7.05± 0.14 Ma) Braden Pipe (4.81±0.10 Ma) Sewell Tonalite Mafic Complex (8.9±14.4 Ma) Hydrothermal breccias Latite Dike 7.05±0.14 Ma Teniente Volcanic Complex Unconsolidated deposits 800 4.82±0.09 Ma 400 1200 Mafic Complex Sewell Tonalite Porphyry "A" Dacite Porphyry Braden Pipe Biotite breccia Igneous breccia Anhydrite breccia Tourmaline breccia Lamprophyre dykes

FIG. 15. Intrusive complexes in the area of El Teniente porphyry copper deposit (Stern et al., 2010). Note the N-NW trend of mineralized porphyry intrusions and magmatic-hydrothermal breccias, hosted within the precursor Teniente mafic complex and Sewell Tonalite. Late-stage lamprophyre dikes are perpendicular to the trend of the copper-bearing porphyry deposits and breccias.

their orientation is similar and almost perpendicular to the di- Discussion rection of the Miocene and Pliocene plate convergence (So- moza and Ghidella, 2005). In contrast, the late lamprophyre Linking geochemistry and tectonics: dikes at El Teniente (Fig. 15) are nearly parallel to the the suggested adakite connection Miocene plate convergence vector (see Fig. 16). Recent stud- Both the middle Eocene to early Oligocene and late Miocene ies on the relationships between volcanism and tectonics in to early Pliocene porphyry copper-bearing intrusions include the modern Southern volcanic zone of the Andes (Sepúlveda intermediate rocks (SiO2 >56%) with abundant hydrous min- et al., 2005; Cembrano and Lara, 2009) have shown that eralogy, dominated by hornblende-bearing granodiorites and primitive mantle-derived basalts ascend along NE-trending dacites; these intrusions show geochemical and isotopic sig- max fractures and faults, parallel to σH , which is regionally nature (SiO2 >56%, Sr >400 ppm, high Sr/Y ratios, low close to the orientation of plate convergence. Nevertheless, HREE contents, high La /Yb ratios, 87Sr/86Sr <0.704) similar during the recent eruptions of southern Andes volcanoes like to those described for typical adakitic rocks (cf. Defant and Puyehue-Cordón Caulle (40°30' lat S, 1960, 2011) evolved Drumond, 1990; Castillo, 2012). Concave middle REE pat- rhyolites erupted along NW-trending basement faults that terns in Chilean porphyry coppers indicate hornblende frac- are, in theory, severely disoriented in relationship to re- tionation, whereas the lack of negative europium anomalies gional stresses to allow magma ascent. To overcome this denotes a high oxidation state of the magmas (see Kay et al., paradox, Sepúlveda et al. (2005) and Lara et al. (2006) have 2005). suggested that coseismic or postseismic stress relaxation A relationship between adakites and mineralized porphyry during large subduction-zone earthquakes can produce systems was proposed by Thiéblemont et al. (1997) and Kay transient episodes of extension that allow ascent of evolved and Mpodozis (2001), while some authors (e.g., Sun et al., magmas that otherwise would remain trapped in crustal 2011) have suggested that the adakitic signatures of copper- reservoirs. Similarly, decompression during seismic events rich magmas are indicative of direct melting of subducted (see Sibson, 1987, 1994) appears to be a plausible mecha- oceanic crust. In accord with these views Oyarzún et al. nism to explain repeated cycles of breccia formation and (2001) proposed that the middle Eocene to early Oligocene porphyry intrusion along now-concealed N-NW− to NE-ori- porphyry copper belt of northern Chile may have been ented faults that may have tapped the roofs of overpres- formed when fast, oblique convergence led to flat subduction sured, deeper seated magma chambers during the evolution and direct melting of the downgoing plate, whereas Reich et of the central Chile porphyry coppers systems. al. (2003) proposed that the porphyry copper intrusions at

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10 East Pacific (a) 9 West Pacific South Pacific 8 7 Average 6 preserved 5 half-spreading rate 4 (cm/yr) 3 2

Age (Ma) 1 0 140 130 120 110 10090 80 70 60 50 40 30 20 10 0

(b) 10 4 Convergence E W velocity velocity 2 5 (cm/yr) 2 1 3

0 S N velocity (d) 1 Velocity (cm/yr) Velocity -40º Angle between convergence vector -20º and present-day Age (Ma) South American 0 margin 80 70 60 50 40 30 20 10 0 0(E-W)

4 +20º Age (Ma) 60 5040 30 2010 0 Ma

FIG. 16. Temporal changes of critical plate parameters that can be linked to adjustments in the tectonic regime along the Andean margin. (a). Ocean crust production (average half-spreading rates since 140 Ma) for different regions of the Pacific basin (Conrad and Lithgow-Bertelloni, 2007). (b). Absolute velocity of the South American plate since 80 Ma, treated as an angular velocity vector decomposed into its ~ E to W and S to N components (Silver et al., 1998). (c). Cenozoic convergence rates between the Farallon-Nazca and South American plates. Data from: 1 = Sdrolias and Muller (2006), 2 = Pardo-Casas and Molnar (1987), 3 = Somoza (2008), 4 = Somoza and Ghidella (2005). (d). Direction of the Farallon-Nazca plate motion, shown as the deviation angle from the E-W path (0°, north = positive, south = negative). Adapted from Somoza and Ghidella (2005). Shaded areas in all graphics indicate the time of porphyry copper emplacement during the Eocene to Oligocene and Miocene to Pliocene.

Los Pelambres formed by melting of the subducting east- they formed during and/or after the most important tectonic northeast arm of the Juan Fernández Ridge under flat-slab episodes that reshaped the whole Andean margin leading to conditions. However, as noted by Kay and Kay (2002) and mountain building and crustal thickening. Castillo (2012), thermal models for subducting plates show Kay et al. (1999) and Kay and Mpodozis (2001) argued that that sufficiently high pressure-temperature conditions for part of the water content of mineralizing Andean magmas can slab fusion can be reached only in exceptional circumstances, be derived from the exsolution of fluids during the transfor- including subduction of very young and hot oceanic crust, as mation of hydrous lower crustal amphibolite to dry garnet- prevailed during emplacement of the 12 Ma Cerro Pampa bearing eclogite during crustal thickening. When the crust of adakites near the Chile Triple Junction in Patagonia (Kay and the arc thickens to a critical value of ~45 km, amphibole and Kay, 2002). plagioclase break down, water is liberated, and eclogite forms. Richards and Kerrich (2007) and Richards (2011b) strongly These relationships explain why the largest porphyry copper argued against slab melts being a necessary ingredient in deposits preferentially form during contractional events, such porphyry copper-gold mineralization, pointing out that the as the Incaic episode of southern Peru and northern Chile, critical factors for adakite genesis include elevated water and and the late Miocene to early Pliocene event in central Chile sulfur contents as well as high oxidation state of the magmas, and contiguous Argentina. which together result in hornblende fractionation and sup- Another process that can also contribute to generation of pression of plagioclase crystallization. Richards (2011b) con- water-rich adakitic magmas during periods of deformation is sidered that such hydrous, oxidized conditions are typical in subduction erosion (von Heune and Scholl, 1991; Stern, 1991, normal arc settings. Nevertheless, this view is inconsistent 2011; Kay et al., 2005). Subduction of oceanic crust, pelagic with the fact that giant porphyry copper deposits are not and terrigenous sediments, and crust tectonically eroded widespread throughout the Andean history; as we have shown from the edge of the continent into the mantle-source region

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of Andean magmas may provide large amounts of water (e.g., American plate (Silver et al., 1998) sharply increased (Fig. Stern et al., 2010). Transient geochemical changes, such as 4b). However, as shown in Figure 4c these effects were not those depicted in Figure 1d, showing adakitic signatures balanced, as should have occurred, by an associated increase (Haschke et al., 2002; Kay et al., 2005) are consistent with the in the Farallon-South America convergence rate. According loss of slivers of fore-arc crust by subduction erosion (Fig. 13) to Somoza (1998) and Somoza and Ghidella (2005), the Far- during, or immediately following, major Mesozoic to Ceno- allon-South America convergence velocity was rather low zoic deformation events. Intermittent and massive loss of (6−7 cm/yr) or, as alternatively suggested by Pardo-Casas and fore-arc crust is a possibility that has been recognized in re- Molnar (1987) and Sdrolias and Muller (2006), was rapidly cent numerical models of subduction erosion reported by decreasing (Fig. 16c). Keppie et al. (2009) and may explain the abrupt eastward This apparent paradox can be resolved if a high degree of shifts of the magmatic front that punctuated the tectonic his- coupling existed at the plate interface at that time. If this was tory of the Central Andes. Without ignoring other models for the case for northern Chile and southern Peru during the adakite formation (e.g., Castillo, 2012), high Sr/Y hydrous Eocene, the mechanically weak margin of the Central Andes, magmas linked to the middle Eocene to early Oligocene and pushed from the east and west, may have started to bend and late Miocene to early Pliocene porphyry copper deposits of contract to form the Bolivian orocline and the Domeyko fault northern and central Chile, respectively, are most likely at- system. High interplate coupling may also explain the forma- tributed to a combination of melting of mantle derived mag- tion of a flat-slab zone and the subsequent Oligocene magmas, including asthenosphere contaminated with pieces of matic lull recorded in southern Peru and northern Chile (e.g., fore-arc crust that entered the mantle through subduction Sandeman et al., 1995; James and Sacks, 1999; Kay et al., 1999; erosion, that mixed with fluids derived from dehydration of Perelló et al., 2003a; Hasckhe et al., 2006; Kay and Coira, the base of thickened amphibole-rich lower crust during 2009). Slab flattening intensifies interplate frictional forces, these two main periods of Andean deformation (e.g., Kay et which increases the possibility of subduction erosion; this, in al., 2007b). turn, would promote eastward migration of the shortening and magmatic fronts toward the Andean foreland (e.g., Espurt et Plate tectonics and Andean tectonic regimes al., 2008; Keppie et al., 2009; Martinod et al., 2010). Conver- As discussed above, giant Andean porphyry copper de- gence rates increased during the Oligocene and Miocene, posits were emplaced during regional deformation events leading to steepening of the Incaic flat slab; accumulation of that reshaped the entire Andean margin. Regional tectonic related mafic magmas below the already thickened crust led and tectonomagmatic events are, ultimately, the result of to the production of crustal melts and the eruption of large changes in plate interaction parameters that could influence volumes of ignimbrites during the inception of volcanism the state of stress in the overriding plate. Among these along the modern Central Andean volcanic zone during the changes on sea-floor spreading and plate convergence rates, Miocene (Kay and Coira, 2009). hot-spot activity, absolute plate velocity, and shifts in the position of the plate contact (trench roll-back), together with Tectonic regime: Central Chile variations in slab age, width, and/or dip has been considered Although the tectonic evolution of central and south-cen- to strongly influence the dynamics of convergent margins tral Chile appears to be different from that of northern Chile, worldwide (e.g., Uyeda and Kanamori, 1979; Heuret and it is in fact complementary and preconditioned by the forma- Lallemand, 2005; Schellart and Rawlinson, 2010, and refer- tion of the Bolivian orocline. As noted above, crustal flux to- ences therein). In recent years, variations in absolute plate ward the north along the southern limb of the orocline during velocities (Russo and Silver, 1996; Silver et al., 1998) as well the middle Eocene to early Oligocene Incaic contraction may as variations on the degree of mechanical interplate cou- have caused stretching and thinning of the upper crust in cen- pling (Yañez and Cembrano, 2004; Luo and Liu 2009a, b; tral Chile, thereby facilitating the opening of the Abanico Iaffaldano et al., 2012) have been proposed as some of the basin (McQuarrie, 2002; Arriagada et al., 2008; Boutelier and fundamental controls on Andean orogeny. Lamb and Davis Oncken, 2010). The along-strike differences in tectonic style (2003) even suggested that differences in plate coupling are also agree with the numerical simulations by Schellart (2008), possibly linked to along-strike differences in climate that which show that higher compression occurs near the center of modulated the supply of sediments to the trench along the wide subduction zones where the trench remains stationary Andean margin which, upon subduction, lubricated the or advances toward the continent. Rapid trench retreat (roll- plate interface and, as a result, determined the degree of back) along the lateral slab edges may explain extension in the mechanical coupling. overriding plate and be compatible with the opening of the Abanico basin in central Chile. Tectonic regime: Northern Chile The late Eocene to Oligocene opening of the Abanico basin Figure 16 shows time-dependent variations for some plate occurred during a period of steady westward displacement of tectonic parameters that can be compared with the geologic the South America plate (Silver et al., 1998), a period that also record of the Andean margin. A correlation can be made be- coincided with another transient episode of very rapid gener- tween the Incaic compressional event and an episode of very ation of oceanic crust in the eastern Pacific, which com- rapid oceanic crust production in the eastern Pacific, which menced immediately after the formation of the Nazca plate peaked in the middle Eocene at ~40 Ma (Fig. 16a; Conrad (Conrad and Lithgow-Bertelloni, 2007; Fig. 16a). In contrast and Lithgow-Bertelloni, 2007). At the same time the east- to the situation during the Incaic event, the convergence rate west component of the absolute velocity vector of the South between the Nazca and South American plates reached, at

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this time, a record high (15 cm/yr; Somoza, 1998; Sdrolias and crustal thickening and eastward migration of the magmatic Muller, 2006; Fig. 16b). Such a disparity may be explained if front. At the same time, the subduction angle shallowed, weak plate coupling permitted rapid subduction and, as a leading to the formation of a flat-slab region between 27° and consequence, the generation of large volumes of magma and 33° S as the Juan Fernández Ridge was being subducted be- extension in the overriding plate during the Oligocene to neath the western edge of South America. These changes early Miocene in central Chile and contiguous Argentina. again created favorable conditions for the formation of fertile The beginning of contraction in central Chile and contigu- hydrous magmas. ous Argentina at ~20 Ma coincides (as shown in Fig. 16d) The relationships described above demonstrate that the with an acceleration of the absolute motion of South America concentration of huge porphyry copper deposits in the (see discussion in Kay and Copeland, 2006), drastic decrease Chilean Andes resulted directly from the tectonic evolution in oceanic crust production in the eastern Pacific, and a drop of the margin and indicate that a tectonic trigger is essential in plate convergence rates between the Nazca and South for the formation of giant porphyry coppers systems. American plates (Somoza, 1998; Conrad and Lithgow-Bertel- loni, 2007; Fig. 16). Inversion of the Abanico basin resulted, Acknowledgments north of 35° S in continued deformation during the Miocene, This contribution is the result of long years of work with causing an increase in crustal thickness to >50 km (Ramos et many colleagues at the Chilean Geological Survey, Antofagasta al., 2004) and enhanced subduction erosion. Contamination Minerals, and various universities both in Chile and abroad. of the asthenosphere through subduction of fore-arc crust We are especially grateful to Sue Kay, Andy Tomlinson, Terry created favorable conditions to produce water-rich mafic Jordan, Cesar Arriagada, Moyra Gardeweg, Rick Allmendinger, melts with high sulfur and metal contents; these melts had Victor Ramos, Pierrick Roperch, Francisco Camus, Stephen the capacity to ascend and evolve within an upper crustal Matthews, Nicolás Blanco, Francisco Hervé, Reynaldo Char- magma chamber to generate large porphyry copper deposits. rier, Carlos Münchmeyer, Ricardo Muhr, José Cembrano, Carlos Arévalo, and many others who, for lack of memory, we Concluding Remarks are here unable to mention. We thank Jeff Hedenquist, Dick There are few studies that consider the relationships be- Sillitoe, José Perelló, and Francisco Camus for pushing us tween the regional-scale tectonic evolution of the Andes and through this endeavor, and Antofagasta Minerals for provid- the formation of giant Cenozoic porphyry copper deposits. ing time and support for the writing. Francisco Morales However, it is apparent that these deposits formed during helped with the preparation of the figures. Victor Ramos, Sue critical moments in the tectonic evolution of the Andean mar- Kay, José Perello, Jeff Hedenquist, and Dick Sillitoe carefully gin. 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