Cenozoic stratigraphic development in the north Chilean forearc: Implications for basin development and uplift history of the Central Andean margin

Tectonophysics 495 (2010) 67–77

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Cenozoic stratigraphic development in the north Chilean forearc: Implications for basin development and uplift history of the Central Andean margin Adrian J. Hartley ⁎, Laura Evenstar Department of Geology and Petroleum Geology, School of Geosciences, University of Aberdeen, Aberdeen AB24 3UE, UK

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Article history: Received 21 September 2008 Accepted 20 May 2009 Available online 24 May 2009 Keywords: Andes Forearc Chile Chronostratigraphy Basin Andean uplift

a b s t r a c t Analysis of the Cenozoic stratigraphic development of the forearc of northern Chile between 18°S and 23°30′ S, allows constraints to be placed on the timing and nature of basin formation and the uplift history of the Central Andes. Chronostratigraphic charts have been constructed from 20 lithostratigraphic sections distributed throughout the forearc. Sections were taken from the Longitudinal Valley, Central Depression, Calama Basin, Salar de Atacama, Precordillera and the western flank of the Western Cordillera. Correlation and timing of events is largely based on the presence of dated volcanic horizons in all the studied sections. Three chronostratigraphic units are defined based upon the presence of regional unconformities. Deposition of the Late Eocene to Early Miocene chronostratigraphic unit (38–19 Ma) commenced across an irregular unconformity surface between ∼ 38 and 30 Ma with alluvial fan and fluvial sediments derived from the east interbedded with rhyolitic ignimbrites. Aggradation after 25 Ma resulted in development of a large broad basin over much of northern Chile that expanded eastwards through onlap onto basement. Deposition terminated around 19 Ma with the development of an angular unconformity over much, but not all of the study area. During deposition of the Early to Late Miocene chronostratigraphic unit (18–10 Ma) emergent volcanic source areas to the east provided catchments for large fluvial systems that drained westwards into endorheic ephemeral lacustrine basins. Fold growth affected sedimentation restricting accommodation space to small intra-thrust basins in the Precordillera and localised disruption and unconformity development in the Longitudinal Valley. The Late Miocene to present day chronostratigraphic unit (10–0 Ma) followed the development of a regional angular unconformity at 10 Ma. Sedimentation was restricted to a series of thrustbounded endorheic basins in both the Central Depression and the Precordillera sourced from the east with volcanic activity limited to the periodic eruption of extensive ignimbrite sheets. Alluvial fan, fluvial and lacustrine sedimentation dominated within the endorheic basins from ∼ 8 to 3 Ma. After development of a regional unconformity at 3 Ma a change to isolated evaporite sub-basins took place in the Central Depression with small lacustrine basins developed along the flank of the Western Cordillera. The scale and grain size recorded in the sedimentary systems indicates that a substantial source area was located in the present day area of the Western Cordillera by 30 Ma and that this has persisted to the present day. This area also shed material eastwards into the Altiplano. The presence of such a topographic feature by 30 Ma suggests that a significant proportion of Andean uplift had occurred prior to the Late Miocene. This important uplift phase should be incorporated into any model of Andean uplift. The evidence from the basin-fill succession suggests that sediments accumulated in a basin developed in front of a broad monocline between 38 and 19 Ma and that a transition to a thrust-bounded foreland style basin took place after the development of the unconformity at 19 Ma. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The formation of the Central Andean mountain range, the ‘type’ convergent continental margin, is considered to be generated by shortening and thickening of the South American continent above the ⁎ Corresponding author. Tel.: +44 1224 273712; fax: +44 1224 272785. E-mail addresses: [email protected] (A.J. Hartley), [email protected] (L. Evenstar). 0040-1951/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2009.05.013

descending Nazca plate and intervening mantle wedge (Isacks, 1988). However, the physical details of how this actually takes place are poorly constrained with contrasting hypotheses suggesting that uplift and deformation is: 1) controlled solely by the absolute motion of the South American plate over the Nazca plate (Silver et al., 1998), 2) related to periods of rapid convergence generated by increased plate coupling (Pardo-Casas and Molnar, 1987; Somoza, 1998), 3) decoupled from changes in convergence rate due to mechanical weakening of the crust by progressive melting (Isacks, 1988), 4) controlled by high shear

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stress at the plate interface resulting from a lack of lubricating sediment in the trench (Lamb and Davis, 2003) and 5) related to mantle delamination (Garzione et al., 2006). To be able to constrain which of these mechanisms either individually or together have controlled uplift and deformation, it is important to have a well constrained temporal and spatial framework for determining the timing of uplift and deformation. Here we use the Cenozoic stratigraphic record in the north Chilean forearc to assess the onset and timing of uplift of the Western Cordillera and place constraints on the mechanisms responsible for Andean uplift. A brief review of the Late Eocene to recent tectonostratigraphic development of the forearc is followed by a detailed stratigraphic analysis. 1.1. The Andean Forearc The Central Andes can be divided into a number of physiographic provinces outlined in Figs. 1 and 2. Within the forearc these include the Coastal Cordillera, the Central Depression or Longitudinal Valley and the Precordillera. The Precordillera is bounded to the east by the Western Cordillera, which forms the active volcanic arc, and which in turn forms the western margin of the Altiplano–Puna plateau. 1.1.1. Coastal Cordillera The Coastal Cordillera is up to 3 km high (average 1 km) and 50 km wide and extends throughout northern Chile up to 18°30′S, north of this latitude the cordillera progressively decreases in elevation and grades into a gently seaward-dipping alluvial plain in northernmost Chile and southern Peru. The cordillera is bounded to the west by the coastal scarp which is up to 1 km high, and is considered to represent a Miocene cliff line (Mortimer and Saric, 1975; Hartley and Jolley, 1995). It is composed of the remnants of a Jurassic arc (granodiorite and andesite) now cut by a series of active N–S to NE–SW trending extensional faults. Uplift and

Fig. 2. Geographic setting of study area in Atacama Desert (digital elevation model based on GTOPO30). Numbers correspond to sections shown in Figs. 3 to 5. Area is outlined on Fig. 1.

Fig. 1. General location map of western South America showing the main physiographic provinces. AFZ — Atacama Fault System, CS — Cordillera de la Sal, SA — Salar de Atacama, CB — Calama Basin. The box shows area of Fig. 2.

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delineation of the cordillera as a separate morphotectonic unit (i.e., distinct from the adjacent Central Depression) is considered to have taken place in the Late Eocene to Early Oligocene and continues to the present day (Hartley et al., 2000). The western flank of the cordillera contains a number of north-trending half-grabens that extend offshore toward the trench (von Huene and Ranero, 2003). Onshore, adjacent to the present day shoreline, half-grabens contain as much as 500 m of lower Miocene to Holocene shallow marine and shelfal deposits. Within the Coastal Cordillera (east of the scarp), Oligocene–Miocene alluvial deposits are preserved as a series of erosional remnants, some of which infill small half-grabens.

1.1.2. Central Depression/Longitudinal Valley The north-south trending topographic low between the Coastal Cordillera and Precordillera is referred to as the Central Depression in the southern part of the study area and the Longitudinal Valley in the north. It has an average elevation of 1000 m in the west and rises to 2000 m in the east. It is 25 to 100 km wide and separated from the Coastal Cordillera by the 900-km-long, north-trending extensional Atacama fault zone (Fig. 1). During the Jurassic and Early Cretaceous, the fault formed a major intra-arc shear zone (Scheuber and Andriessen, 1990) and has had a transtensional and extensional history for much of the Cenozoic (Okoda, 1971). As much as 1100 m of Late Eocene to Holocene strata are preserved within the Longitudinal Valley–Central Depression (Hartley et al., 2000). Sedimentary rocks include fluvial, lacustrine, playa, and nitrate deposits with interbedded volcanic ashes (Fig. 3), alluvial-fan deposits are restricted to the flanks of the Central Depression. The eastern basin margin grades into the coarse grained alluvium of the Precordillera.

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2. Database We have used descriptions of areas between 18°S to 23°30′S within the Central Depression–Longitudinal Valley and Precordillera/Western Cordillera to erect a litho- and chronostratigraphic correlation for the Cenozoic of northern Chile (Figs. 1 to 5). Sedimentary logs (Fig. 3) were constructed from the Salar de Atacama area (Jolley et al., 1990; Flint et al., 1993; Naranjo et al., 1994; Kape, 1996; Mpodozis et al., 2000; Muñoz et al., 2002), the Calama Basin (May et al., 1999, 2005), the Central Depression/Longitudinal Valley (Sáez et al., 1999; Victor et al., 2004; Farías et al., 2005; Pinto et al., 2004; Charrier et al., 2005; García and Hérail, 2005), the Precordillera (Victor et al., 2004; Charrier et al., 2005) and the western flank of the Altiplano (Kött et al., 1995; Gaupp et al., 1999; Charrier et al., 2005). Regional correlations have been made primarily using dated volcanic strata and lithological similarities. The lithostratigraphy is summarised in Fig. 3 and the chronostratigraphy in Figs. 4 and 5. The location of sections is shown in Fig. 2. 3. Cenozoic succession The lithostratigraphy of the Cenozoic succession in northern Chile is complex as illustrated in Fig. 3, and for simplicity is considered here within a chronostratigraphic context. Throughout the Central Depression–Longitudinal Valley, Precordillera and Western Cordillera, the Cenozoic succession can be divided into three broad, generally unconformity-bounded chronostratigraphic units of Late Eocene to Early Miocene, Early to Late Miocene and Late Miocene to recent age (Figs. 3 and 4). 3.1. Late Eocene to Early Miocene (?35 to ∼18 Ma)

1.1.3. Precordillera The Precordillera forms a topographic ‘bench’ between 1900– 2300 m in the west and 3200–3670 m altitude in the east. It is 10 to 70 km wide and consists of a number of reverse-fault and strike-slipfault-bounded blocks of Palaeozoic and Mesozoic strata. Sedimentation in the Precordillera took place across a subdued, low relief pediplain developed during the Eocene to Early Oligocene (Fig. 3). On the basis of apatite fission track data Maksaev and Zentilli (1999) showed that exhumation of rocks underlying the pediplain occurred rapidly during the Eocene with very little subsequent erosion of the Precordillera. Cenozoic sedimentary successions within the Precordillera range in age from Eocene to Late Miocene although in some areas they may extend into the Pliocene. The deposits comprise a series of alluvial systems interbedded with ignimbrites derived from the uplifting Western Cordillera (e.g., Pinto et al., 2004; Victor et al., 2004; Farías et al., 2005). These fluvial systems pass westwards into basin centre deposits of the Central Depression. Sediment thicknesses are up to a maximum of 1500 m in the east grading westwards to a few hundred metres in the Longitudinal Valley–Central Depression.

1.1.4. Western Cordillera The Western Cordillera is 50 to 100 km wide, has an irregular topography with peaks of up to 6350 m and an average altitude of between 3800 and 4500 m. It forms the modern day volcanic arc and comprises the eastern margin of the Precordillera. Precambrian–Early Palaeozoic metamorphic rocks together with Triassic, Jurassic and Cretaceous volcanic, sedimentary and plutonic rocks form the basement to the Western Cordillera. These strata are unconformably overlain by up to 2500 m of andesitic and dacitic lavas, rhyolitic ignimbrites and lacustrine and alluvial sediments of Late Oligocene to Pleistocene age separated into a number of unconformity bound packages. These deposits accumulated in small interarc basins.

3.1.1. Age and lithostratigraphic units The Late Eocene to Early Miocene chronostratigraphic unit (Unit 1 Figs. 4 and 5) is present throughout northern Chile and displays significant variations in thickness and lithology. This chronostratigraphic unit includes the Azapa, Oxaya, Lupica, Chucal, Sichal, Calama and lower parts of the Latagualla and Altos de Pica Formations (Fig. 3). The age of the base of the unit is poorly constrained with few datable units directly above the unconformity surface. Radiometric dating of volcanic ashes between 18°30′S and 21°S within the lower part of the stratigraphy indicate an age of 26 Ma although the presence of N300 m of sediment between the ashes and the unconformity in some areas (e.g., Farías et al., 2005) suggests an Early Oligocene age. On the basis of dated ignimbrites and calculated sedimentation rates García (2002) proposed that the onset of sedimentation commenced after 34 Ma in the Arica region. Further south in the Pica region deposition of Member 1 of the Altos de Pica Formation is inferred to have been initiated between 27–29 Ma (Victor et al., 2004). South of 21°S the onset of Cenozoic sedimentation has been taken at 38 Ma based on dating of volcanic rocks that overlie an angular unconformity (Hammerschmidt et al., 1992). Recent work suggests that sedimentation in the Calama Basin (22–23°S) may have commenced in the Early to mid Eocene between 52 and 47 Ma (Blanco et al., 2003; May et al., 2005). Further east in the Salar de Atacama basin, Kape (1996) used magnetostratigraphy to suggest that deposition of the Artolla and Tambores Members of the Paciencia Group commenced before 30 Ma. An Early Oligocene age is also supported by the presence of a 300 m thick section of mudstones, evaporites and conglomerates present beneath an ash dated at 26.8 ± 1.4 Ma the San Pedro Formation (Mpodozis et al., 2000). The top of this chronostratigraphic unit is marked by an angular unconformity developed sometime between 16 and 19 Ma in most of the sections. It should be noted that in the central part of the study area (the Aroma, Pica and northern Quillagua areas) no unconformity is developed, with continuous sedimentation up to 15 Ma. The upper

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contact of the unit is erosional throughout the Longitudinal Valley and Precordillera in Northern Chile. In Peru this unconformity forms the Pampas Lagunas Pediplain at 19–20 Ma (Quang et al., 2005). In northernmost Chile, the youngest volcanics below the unconformity are ∼19 Ma in Lauca and Caragua (García and Hérail, 2005), 19.5 ± 0.7 Ma in the Pachani region (Charrier et al., 2005), 19.9 ± 1.0 Ma in Umagata (Charrier et al., 2005) and 19.3 ± 0.8 Ma in Quebrada Camiña (Naranjo and Paskoff, 1985). The oldest dated volcanics in the overlying sediments are considerably higher stratigraphically than the unconformity itself and are dated at 16.2 ± 0.7 Ma (Muñoz and Sepúlveda, 1992). It is therefore concluded that the unconformity formed between 19–18 Ma. Within the Calama Basin this time period corresponds to a hiatus between Calama Formation deposition and the initiation of El Loa Group sedimentation just before 19 Ma (May et al., 2005). It also correlates with the termination of sedimentation in the northwestern part of the Salar de Atacama at 20 Ma (Kape, 1996). In the central part of the Salar de Atacama an unconformity has been recognised from seismic data (Muñoz et al., 2002) but the age of this surface is poorly constrained. 3.1.2. Lithology, thickness and depositional setting The conglomerates and interbedded sandstones that overlie the Early Cenozoic unconformity are referred to as the Azapa Formation in the Longitudinal Valley. These sediments are up to 500 m thick and unconformably overlie and infill an older palaeotopography formed over Mesozoic sediments mainly in the eastern part of the Longitudinal Valley and the Precordillera, and are particularly well developed in the Arica area. Further south these conglomerates pass into more volcanic dominated succession in the Suca and Camiña areas before becoming more conglomerate dominated in the Aroma and Pica regions. These conglomerates have temporal and lithological equivalents in the Quillagua to Calama area (Sichal and Calama Formations respectively) and in the Salar de Atacama (Tambores Member) where fine grained evaporites, mudstones and thin sandstones of the Artolla Member are also present (Flint, 1985; Kape, 1996). From Pica northwards the top of this chronostratigraphic unit is dominated by rhyolitic ignimbrites with the regionally extensive Suca Ignimbrite forming a prominent marker horizon (Fig. 3). South of Quillagua volcanic intercalations are restricted to occasional ashes. Sections in the Precordillera (as well as those at Suca and Camiña) are dominated by rhyolitic ignimbrites interbedded with subordinate sandstones and conglomerates that directly overlie preCenozoic basement. They are referred to as the Lupica Formation in the Precordillera/Western Cordillera and the Oxaya (and Latagualla) Formation in the Longitudinal Valley and can reach thicknesses of nearly 900 m (e.g., Pachani, section 14). It is clear that the rhyolites thin rapidly westwards (e.g., García and Hérail, 2005) with the only development in the Longitudinal Valley restricted to the Suca and Camiña areas. The only substantial accumulation of sediments during this chronostratigraphic interval in the Precordillera was in the Chucal region, where up to 500 m of fluvio-lacustrine sandstones, conglomerates, shales and limestones are developed above a local unconformity (Charrier et al., 2005). 3.1.3. Synthesis The Late Eocene to Early Miocene succession consists of wellrounded fluvial conglomerates and sandstones interbedded with occasional overbank and lacustrine mudstones (Pinto et al., 2004;

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García and Hérail, 2005; May et al., 2005). Palaeocurrent and grain size analysis suggests bedload dominated fluvial systems derived from an easterly source area. The rhyolitic ignimbrites are considered have formed as a result of an influx of hot asthenosphere into the mantle wedge and a steepening of the subduction zone at 25 Ma (Wörner et al., 2000). Victor et al. (2004), however, relate the increase in ignimbrites at this time to an increase in shortening rate. The overlying unconformity has been interpreted in the Calama area as being related to local tectonic activity (May et al., 2005) but the large regional extent from Southern Peru down to the Salar de Atacama Basin suggests a more regional control. The timing of the onset of sedimentation varies considerably within the study area. In the Central Depression–Longitudinal Valley it ranged from 38 Ma to about 28 Ma suggesting that deposition took place initially, in a series of isolated basins. By 25 Ma these basins were connected into a single basin which also included the Precordillera/ Western Cordillera where aggradation resulted in eastward onlap onto basement. The Calama and Salar de Atacama Basins remained isolated throughout the Oligocene and Miocene. It should be noted however, that the timing of the onset of deposition is poorly constrained in most areas. 3.2. Early to Late Miocene (∼18 to 10 Ma) 3.2.1. Age and lithostratigraphic units The Early to Late Miocene chronostratigraphic unit (Unit 2 Figs. 4 and 5) can be correlated throughout northern Chile from 18°30′ to the Salar de Atacama at 23°30′ and displays significant thickness and lithological variations. This chronostratigraphic unit includes the El Diablo Formation, Members 2 and 3 of the Latagualla Formation (Pinto et al., 2004), Members 4 and 5 of the Altos de Pica Formation and the lower part of the Hilaricos Formation in the Central Depression and the Jalquinche and Lasana Formations in the Calama Basin (Figs. 3, 4 and 5). Sediments of this age are preserved in the central part of the Salar de Atacama Basin, where an unconformity bound package has been identified between the overlying San Bartolo Group ignimbrites and an underlying unit, however age constraints are poor (Muñoz et al., 2002). On the NW flank of the Salar de Atacama a thin succession of gravels and mudstones has been identified below the San Bartolo Group ignimbrites (Kape, 1996). In the Precordillera the lithostratigraphic units within this time interval include the Joracane, Zapahuira and Chucal Formations. The age of the base of this chronostratigraphic unit is variable ranging between 11.4 ± 0.3 Ma at Pachani to 19.14 ± 0.3 Ma in the Calama Basin. Full sections are only developed in the northern part of the Quillagua area, the Calama Basin and at Chucal. All the other sections contain locally developed angular unconformities. In the Longitudinal Valley these unconformities occur in the Aroma–Pica area at about 14.5 Ma and between Suca and Pica at about 12 Ma. In most areas deposition appears to have ceased at an earlier stage (i.e., post 10.7 ± 0.3 Ma in the Caragua region, post 11.7 ± 0.4 Ma in the Aroma area and pre-8.2 ± 0.5 Ma in the Camin a area). The top of the unit (the El Diablo Formation in the north) forms the current land surface, termed the Tarapacá pediplain in the Longitudinal Valley, or forms a regional unconformity within the Precordillera which is capped by the Lauca ignimbrite, except in the case of the Lauca Basin where it is overlain by younger sediments. North of 21°S with the exception of the Tana Lava and the possible Pliocene section drilled in

Fig. 3. Simplified lithostratigraphy of the north Chilean forearc between18°18′ to 23°30′. For location of sections see Fig. 2. Section details: 0. Salar de Atacama — main basin (Bevacqua, 1991; Muñoz et al., 2002); 1. NW Salar de Atacama, southern section (Mpodozis et al., 2000); 2. NW Salar de Atacama, northern section (Kape, 1996); 3. Calama Basin east (de Silva, 1989; May et al., 2005); 4. Calama Basin west (May et al., 2005); 5. Quillagua–Llamara Basin south (Sáez et al., 1999); 6. Quillagua–Llamara Basin north (Maksaev, 1978; Sáez et al., 1999); 7. Pica (Victor et al., 2004); 8. Pampa Caya (Baker, 1977; Victor et al., 2004); 9. Salar de Copasa (Baker, 1977; Vergara and Thomas, 1984; Victor et al., 2004); 10. Aroma (Farías et al., 2005); 11. Camiña (Pinto et al., 2004; Farías et al., 2005); 12. Suca (Pinto et al., 2004); 13. Umagata Region (Charrier et al., 2005); 14 Pachani (Charrier et al., 2005); 15. Arica (Parraguez, 1998; García, 2002; García and Hérail, 2005; Von Rotz et al., 2005); 16. Belén (Charrier et al., 2005); 17. Lauca Basin (Horn et al., 1992; Kött et al., 1995; Gaupp et al., 1999); 18. Caragua (García and Hérail, 2005); 19. Chucal (Charrier et al., 2005). APF = Altos de Pica Formation, Cop. Fm = Coposa Formation, SBG = San Bartolo Group, PIg = Pelon Ignimbrite, YBIg = Yerbas Buenas Ignimbrite, SIg = Sifon Ignimbrite, AIg = Artolla Ignimbrite.

Fig. 4. Chronostratigraphic diagram for the Longitudinal Valley, Central Depression, Calama Basin and Salar de Atacama. (Numbers in parentheses refer to numbered references in Fig. 3).

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Fig. 5. Chronostratigraphic diagram for the Precordillera/Western Cordillera. (Numbers in parentheses refer to numbered references in Fig. 3).

the oil well Pintados 1 (Victor et al., 2004), no post-10 Ma strata are preserved within the Longitudinal Valley, as such it is not possible to determine the time at which sedimentation ceased in this area. In contrast, further south, in the Quillagua area sedimentation was continuous throughout this time interval. 3.2.2. Lithology, thickness and depositional setting In the Longitudinal Valley in the northern part of the study area between Aroma and Arica, the Early to Late Miocene chronostratigraphic unit comprises sediments of the El Diablo Formation. In the Arica region these sediments comprise sandstones, limestones and siltstones coarsening up to largely fluvial–alluvial gravels with occasional ignimbrites (García and Hérail, 2005). From Suca to Pica the Nama ignimbrite forms a good stratigraphic marker and occurs within easterly-derived fluvial sandstones and conglomerates (e.g., Pinto et al., 2004). The thickest succession within this chronostratigraphic unit is developed between Suca and Camiña (up to 650 m), north of Suca the unit is truncated and full thicknesses are not preserved, south of Camiña a locally developed unconformity interrupts the succession at Aroma and Pica. The sandstone and conglomerate succession developed above the sub-regional unconformity at ∼ 12 Ma between Suca and Pica is similar to that beneath the unconformity (except in the Camiña area where the Tana Lava directly overlies the unconformity). In the Quillagua–Llamara area a continuous section is present in the north which comprises interbedded carbonates, evaporites and sandstones that form the upper part of the Hilaricos Formation. This interval is time equivalent to the carbonates, sandstones and mudstones of the Jalquinche and Lasana Formations in the Calama Basin, and to the thin succession of gravels and mudstones that occur below the San Bartolo Group ignimbrites in the northern part of the Salar de Atacama (Kape, 1996). This interval records a period of predominantly low energy ephemeral lacustrine, salt flat deposition with occasional fluvial and alluvial fan development along the basin

flanks. In the Calama Basin the top of the succession is marked by an angular unconformity which is developed prior to eruption of volcanic ashes that are equivalent in age to the San Bartolo Group ignimbrites (Fig. 4). In the Precordillera and western flank of the Western Cordillera this chronostratigraphic interval comprises a series of discrete unconformity-bounded packages largely composed of rhyolites and ignimbrites with subordinate sediments. In the Chucal region a virtually continuous but poorly exposed succession is developed between 18 and 11 Ma. The succession is dominated by ignimbrites with occasional interbedded sandstones and is referred to as the Quebrada Macusa Formation (Charrier et al., 2005). Elsewhere volcanics are only developed towards the top of this chronostratigraphic unit between 11 and 14 Ma and include the andesites of the Zapahuira Formation between Belén and Caragua which are equivalent to the upper part of the Quebrada Macusa Formation at Chucal. In the Precordillera a thick sedimentary succession is only developed in the Belén area between 18 and 16 Ma where it comprises 600 m of conglomerates referred to as the Joracane Formation (García, 1996). The conglomerates were derived from the east and record deposition in a fluvial environment (Charrier et al., 2005). Over 50 m of coarse clastic material of presumed mid to Late Miocene age has been identified in the Lauca area but has not been described in detail (Gaupp et al., 1999). In the Caragua area a 150 m thick unconformity-bounded package of sandstones that includes the 10.7 Ma Caragua/Tignamar Ignimbrite is locally developed (Figs. 3 and 4). This unit is referred to as the Huaylas Formation (García and Hérail, 2005). 3.2.3. Synthesis The Early to Late Miocene chronostratigraphic unit shows considerable variations in thickness and lithology. In general all the successions indicate derivation from an uplifted volcanic terrain to the east. From Quillagua northwards thick successions of fluvial conglomerates and sandstones in the Longitudinal Valley/Central Depression

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interbedded with occasional overbank deposits indicate substantial run-off in the catchment. Further south the Calama Basin was endorheic with alluvial fan and fluvial systems draining into a central playa (May et al., 2005). Similarly, endorheic basins were present in a small sub-basin in the north of the Salar de Atacama and in the central part of the main Salar de Atacama, although post-depositional erosion may have removed some time-equivalent sediments along the NW flank. In the Precordillera and Western Cordillera during the Early to Late Miocene sediment accumulation was limited to the Chucal and Caragua areas and to a lesser extent the Lauca area. Elsewhere rhyolites, ignimbrites and andesites dominate the succession particularly towards the top of the unit. The local to sub-regional development of unconformities within the Precordillera and Longitudinal Valley can be related directly to growth folds associated with movement on specific thrust faults (Pica and Suca — Pinto et al., 2004; Aroma — Farías et al., 2005; Pica — Victor et al., 2004; Chucal — Charrier et al., 2005; Caragua — García and Hérail, 2005). This fault activity clearly resulted in changes in accommodation space generation within the study area, in particular restricting areas for sediment accumulation in the Precordillera/ Western Cordillera and disrupting sedimentation in the Longitudinal Valley/Central Depression. 3.3. Late Miocene to present day (∼ 10 Ma to present day) 3.3.1. Age and lithostratigraphic units The Late Miocene to present day chronostratigraphic unit (Unit 3, Figs. 4 and 5) is only present from Quillagua southwards in the Central Depression and in the Precordillera/Western Cordillera. North of Quillagua, the Tana Lava represents the only post-10 Ma strata described from the Longitudinal Valley. It should be noted however, that a thick (N400 m) succession of possible Pliocene sediments was drilled in the well Pintados 1 at 20°30′S in the Longitudinal Valley (Victor et al., 2004). This is likely to indicate that sediments were deposited during this time period within endorheic basins in the Longitudinal Valley but are either not exposed or have been removed by erosion. The lithostratigraphic units in the Quillagua area include the upper part of the Hilaricos Formation and the Quillagua and Soledad Formations, in the Calama Basin they include the Opache, Chiquinaputo and Chiuchiu Formations and in the Salar de Atacama all sediment deposited after the San Bartolo Group Ignimbrites which includes the Vilama Formation and a number of unnamed units. In the Precordillera lithostratigraphic units within this time interval include the Lauca, Huaylas, Caya and Coposa Formations, and possible equivalents to Members 4 and 5 of the Altos de Pica Formation (Victor et al., 2004). The age of the base of the unit is taken at 9.8 Ma in the Salar de Atacama and Calama Basins and occurs at the base of the first San Bartolo Group ignimbrite. In the Precordillera, the age of the base is poorly constrained with a 7 Ma ignimbrite present towards the base of the Lauca Formation (Kött et al., 1995) and 7 Ma rhyolitic tuff in the upper part of the Caya Formation (Baker, 1977). A regional unconformity is developed between 4 and 3 Ma in both the Precordillera and Quillagua to Salar de Atacama area. Above the unconformity the age of the top of the unit varies with sedimentation close to the present day occurring in the Quillagua–Llamara–Calama and Chucal areas and the central Salar de Atacama and Lauca Basins. In contrast no post-3 Ma deposits are preserved between the eastern flank of the Calama Basin and the Salar de Atacama and in the Precordillera between Belén and Caragua. 3.3.2. Lithology, thickness and depositional setting In the Quillagua area sedimentation appears to have been continuous from the underlying Hilaricos Formation into the Quillagua Formation up to 3 Ma. Between 10 and 6 Ma sedimentation in the Quillagua area was restricted to the north of the basin and

includes up to 200 m of alluvial fan deposits derived from both the Coastal Cordillera to the west and the Precordillera to the east, the latter including the Arcas fan (Kiefer et al., 1997; Sáez et al., 1999). In the Calama area sedimentation was restricted to the eastern half of the basin between 10 and 6 Ma with deposition of 130 m of fluvial conglomerates and sandstones of the Chiquinaputo Formation (May et al., 2005). After 6 Ma onlap over a basement high resulted in expansion and linkage of the Quillagua and Calama Basins (May et al., 2005). Between 6 and 3 Ma up to 100 m of marginal lacustrine and fluvial sedimentation was deposited across the two basins (Quillagua and Opache Formations). Sedimentation ceased shortly after 3.3 Ma due to uplift associated with gentle folding and resumed shortly after 3 Ma with deposition of up to 100 m of evaporites and marginal lacustrine deposits in small, isolated basins in the Quillagua–Calama area (Chui–Chui and Soledad Formations, May et al., 2005). Sedimentation along the western flank of the Salar de Atacama Basin commenced sometime after 8 Ma with alluvial fan deposits up to 100 m thick derived from the west feeding into a lacustrine basin represented by the Vilama Formation. Uplift shortly after 3 Ma resulted in development of an angular unconformity along the western basin margin and uplift of the Cordillera de la Sal (Fig. 1; Jolley et al., 1990). Sedimentation within the basin centre was continuous from 10 Ma to the present day with accumulation of a thick (1000 m) evaporite and shale succession as recorded from borehole data (Muñoz et al., 2002). In the Precordillera/Western Cordillera deposition between 10 and 3 Ma was restricted to a series of small, isolated basins including the Belen–Pachani (Huaylas Formation) area, the Caya–Coposa (Caya– Coposa Formations) area and the Lauca Basin which extended southwards to include the Chucal area (Lauca Formation). All these units comprise conglomerates derived from a volcanic terrane sourced from the east and are interbedded with volcanics. Thin sandstone and limestone layers are present in the Lauca Basin indicating lacustrine development (Kött et al., 1995; Gaupp et al., 1999; Charrier et al., 2005). A regional unconformity developed across the whole of the area prior to eruption of the Lauca/Huaylas Ignimbrite at ∼ 2.9 Ma. Above the ignimbrite, sedimentation only continued in the Lauca Basin with development of a 35 m thick lacustrine succession. In the Coposa area, ignimbrite deposition prevailed after 3 Ma. 3.3.3. Synthesis The Late Miocene to present day chronostratigraphic unit shows considerable variations in thickness, lithology and preservation. Sediment accumulation and preservation was restricted to endorheic basins such as the Quillagua–Llamara, Calama, Lauca, Caya–Coposa and Salar de Atacama. Within these basins sedimentation likely commenced between 10 and 7 Ma (age constraints are limited) and continued up to 3 Ma when a regional unconformity developed across much of the forearc. In the Quillagua–Llamara and Calama Basins sedimentation was dominated initially by alluvial fan and fluvial deposition before giving way to more lacustrine sedimentation after 6 Ma. Alluvial fan and lacustrine sedimentation also dominated along the flanks of the Salar de Atacama area between 8 and 3 Ma with evaporite deposition prevailing in the central part of the basin from 3 Ma onwards. Elsewhere in the study area after 3 Ma sedimentation was restricted to isolated evaporite-dominated basins in the Central Depression–Longitudinal Valley (Hartley and Chong, 2002) and small lacustrine basins along the edge of the Western Cordillera (Lauca Basin). This chronostratigraphic unit is largely characterised by the development of discrete, fault-bounded endorheic basins within the forearc. The basins appear to have been initially isolated but in the case of the Quillagua–Llamara and Calama Basins by 6 Ma sediment aggradation outpaced subsidence resulting in linkage between the two. The generation of a regional angular unconformity at ∼ 3 Ma was followed by a significant change in basin size and depositional style

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with restriction of sedimentation to small, isolated, fault/fold-bounded sub-basins that filled with evaporites in the Central Depression and lake deposits along the flank of the Western Cordillera. The general lack of preserved strata younger than 10 Ma in the Longitudinal Valley north of Quillagua makes it difficult to assess the controls on sediment distribution within this area. The presence of a thick succession of Pliocene sediments in the Pintados 1 well in the Salar de Pintados (Victor et al., 2004) suggests that sediment was deposited and preserved in the Longitudinal Valley during this time period, but that accumulation was restricted to small endorheic basins. Outside of these basins it is probable that sediment was either not deposited or removed by subsequent erosion. In particular in the north of the study area (from Quebrada Aroma northwards) the post10 Ma development of deep canyons that link the drainage systems of the western Altiplano directly to the Pacific Ocean (Fig. 2) will have resulted in net erosion rather aggradation during this time period. 4. Summary of Late Eocene to recent sedimentation and basin development Deposition of the Late Eocene to Early Miocene chronostratigraphic unit commenced across an irregular unconformity surface between ∼ 38 and 30 Ma in the Longitudinal Valley and Central Depression (possibly earlier in the Calama Basin) and from ∼ 25 Ma onwards in the Precordillera. Alluvial fan and fluvial conglomerates and sandstones dominated in the northern and southern parts of the study area with rhyolitic ignimbrites in the central area and the Precordillera. Aggradation particularly after 25 Ma resulted in basin expansion and onlap of the sedimentary succession eastwards. By 20 Ma alluvial fan, fluvial and lacustrine sedimentation took place in a single linked basin across the whole of the study area from Quillagua northwards with separate accumulations in the Calama and Salar de Atacama basins (although the precise relationship between these basins at this time is unknown, for example the Calama and Quillagua basins may have been linked). Throughout the forearc, with the exception of the Chucal area, no local unconformities are recorded within this chronostratigraphic unit. Deposition terminated in the Late Early Miocene associated with the development of an angular unconformity around 19 Ma over much, but not all of the study area. During development of the Early to Late Miocene chronostratigraphic unit (18–10 Ma) emergent volcanic source areas to the east provided catchments for large fluvial systems that drained westwards into endorheic ephemeral lacustrine basins. Fold growth associated with fault movement strongly affected sedimentation in the Precordillera restricting accommodation space to small intra-thrust basins. In the Longitudinal Valley–Central Depression fold growth resulted in localised disruption and progressive unconformity development rather than basin reorganisation. Extensive volcanic activity was largely restricted to the Precordillera/Western Cordillera with only occasional large ignimbrites and volcanic ashes preserved in the Longitudinal Valley, Calama Basin and Salar de Atacama. The Late Miocene to present day chronostratigraphic unit followed the development of a regional angular unconformity at 10 Ma. Sedimentation was restricted to a series of largely thrust-bounded endorheic basins in both the Central Depression and the Precordillera–Western Cordillera. Volcanic activity was limited to the periodic development of extensive ignimbrite sheets (e.g., San Bartolo Group in the Salar de Atacama area and the Coposa area). Sediments were largely derived from an emergent volcanic source terrain to the east with local sediment input from other sources related to uplift associated with individual basin-bounding fault systems. Alluvial fan, fluvial and lacustrine sedimentation dominated within the endorheic basins from ∼ 8 to 3 Ma. After development of the regional unconformity at 3 Ma a change to isolated evaporite sub-basins took place in the Central Depression with small lacustrine basins developed along the flank of the Western Cordillera.

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5. Discussion Lithostratigraphic and chronostratigraphic analysis of the Cenozoic succession in northern Chile allows some constraints to be placed on a number of problems that are important to understanding the development of the Central Andes. In particular deposition and deformation of the sedimentary and volcanic sequence can give insights into the nature of basin development and the source areas that supplied these basins and, contribute to an understanding of when and how uplift commenced and whether it was continuous or intermittent up to the present day. 5.1. Onset of sedimentation and uplift of the Western Cordillera The coarse grained nature of Late Eocene to Late Oligocene sediment, particularly the Azapa Formation, is commonly interpreted as relating to the initiation of Andean uplift (Dingman and GalliOliver, 1965; Mortimer and Saric, 1975; Naranjo and Paskoff, 1985; Wörner et al., 2002; Victor et al., 2004 and Farías et al., 2005) although some uncertainty exists over the exact timing of the onset of sedimentation (e.g., Wörner et al., 2002; Victor et al., 2004). The discrepancy over timing is likely to be related to the fact that as shown in Figs. 4 and 5, sedimentation commenced at different times in different parts of the forearc. The earliest likely age is 38 Ma in the Quillagua area, however older ages have been reported from the Calama Basin and it is possible that this basin developed at an earlier stage than elsewhere. It appears that sedimentation was occurring across much of the Longitudinal Valley–Central Depression, Calama Basin and Salar de Atacama by 30 Ma, and that by 24 Ma deposition was taking place in the Precordillera and Western Cordillera. The widespread and coarse grained nature of the sediment that was deposited indicates an organised and well established drainage system capable of transporting well rounded pebbles and boulders. The material in the source area included Cenozoic volcanics and Mesozoic plutonic, volcanic and sedimentary rocks as well as in some areas metamorphic basement (Wörner et al., 2002; Pinto et al., 2004). The mixed provenance also supports the idea of a well established drainage network. It is clear therefore that probably before 30 Ma and certainly before 24 Ma a well established integrated drainage network capable of supplying large fluvial systems was in place along the eastern flank of the present day Precordillera/Western Cordillera. This indicates that substantial differential uplift of the western flank of the Altiplano relative to the forearc must have taken place by 30 Ma. The presence of a substantial, emergent source area in the vicinity of the present day Western Cordillera is also supported by studies of sedimentary successions from the Bolivian Altiplano immediately to the east of our study area. In this area a 3000–6500 m thick succession of westerly-derived, Late Eocene to Oligocene fluvial sandstones and mudstones (the Potoco Formation), has been described (Horton et al., 2001). The source for this material is considered to be where the present day Western Cordillera is located (Horton et al., 2001). It appears therefore, that an important phase of uplift affected the area of the present day Western Cordillera by at least 30 Ma if not before, resulting in establishment of a source terrain that shed alluvial sediment into the forearc to the west, into the Altiplano to the east and was also sourced the thick rhyolites and ignimbrites present in the Chilean Precordillera. The significant large-scale uplift that must have taken place in the area of the Western Cordillera in the Late Eocene and Early Oligocene would need to be incorporated into any reconstruction of Central Andean uplift in order to accurately model the development of this area. It is likely that this phase of Late Oligocene to Early Miocene uplift in the area of what is now the Western Cordillera/Precordillera is one of a series of pre-Neogene phases of uplift that affected this part of the Andes (e.g., Coira et al., 1982). Charrier et al. (2007) identified an ‘Incaic’ or ‘proto-Domeyko’ range in northernmost Chile

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which they recognised as the source for the Neogene detritus shed westwards towards what is now the Coastal Cordillera. These observations further indicate that the pre-Neogene uplift history of the Central Andes is poorly defined. Without a more detailed knowledge of the pre-Neogene history of the Central Andes it is difficult to constrain the starting point for reconstructing the uplift history of this area.

5.2. Basin development and deformation style The general form of basin development during the Cenozoic in northern Chile allows some constraints to be placed on the nature of the uplift affecting the source terrain. For example there appears to have been a significant change in tectonic style and basin size following unconformity development at 19 Ma. Prior to 19 Ma sedimentation took place initially in small isolated basins which coalesced through aggradation resulting in onlap onto the Precordillera to the east. After 19 Ma more discrete thrust-bounded basins developed in the Precordillera with thrust activity influencing sedimentation along the east flank of the Longitudinal Valley and Central Depression as shown by development of growth folds (e.g., Pinto et al., 2004; Victor et al., 2004; Farías et al., 2005; Charrier et al., 2005; García and Hérail, 2005). Basin development between 30 and 19 Ma would suggests a broad area of uplift to the east with little or no active faulting within the basin itself similar to the broad monoclinal upwarp favoured by Isacks (1988). After 19 Ma the Longitudinal Valley–Central Depression area appears to have behaved more as a foreland-type basin where progressive westerly directed thrust activity resulted in uplift and segmentation of the forearc in the foreland to the developing proto-Western Cordillera. This style of deformation is similar to that proposed by Muñoz and Sepúlveda (1992) and Muñoz and Charrier (1996) for the development of the western edge of the Western Cordillera, who considered uplift to be generated by large-scale thrusting rather than a broad crustal scale monocline. The evidence from the basin-fill succession suggests that sedimentation was initiated in a basin developed in front of a broad monocline with a transition to a thrust-bounded foreland style basin after 19 Ma.

6. Conclusions A lithostratigraphic and chronostratigraphic analysis of the Late Eocene to present day succession in the north Chilean forearc has been undertaken which allows assessments to be placed on the nature, timing and mechanisms that influenced Andean uplift. Three chronostratigraphic units are defined based upon the presence of regional unconformities. Sedimentation commenced across an irregular unconformity surface between ∼ 38 and 30 Ma with alluvial fan and fluvial sediments derived from the east interbedded with rhyolitic ignimbrites. After 25 Ma aggradation resulted in a large basin that expanded eastwards through onlap onto basement. Deposition terminated around 19 Ma with the development of an angular unconformity. From 18 to 10 Ma emergent volcanic source areas to the east provided catchments for large fluvial systems that drained westwards into endorheic ephemeral lacustrine basins. Fold growth resulted in small intra-thrust basins in the Precordillera and unconformity development in the Longitudinal Valley. Following development of a regional unconformity at 10 Ma, sedimentation was restricted to a series of thrust-bounded endorheic basins in both the Central Depression and the Precordillera. These were sourced from the east with volcanic activity limited to the periodic eruption of extensive ignimbrite sheets. Alluvial fan, fluvial and lacustrine sedimentation dominated within endorheic basins from ∼ 8 to 3 Ma. After development of a regional unconformity at 3 Ma a change to isolated evaporite sub-basins took place in the Central Depression with small lacustrine basins developed along the flank of the Western Cordillera. The scale and grain size recorded in the sedimentary systems indicates that a substantial source area was located in the present day area of the Western Cordillera by 30 Ma and that this has persisted to the present day. This area also shed material eastwards into the Altiplano. The presence of such a topographic feature by 30 Ma suggests that a significant proportion of Andean uplift had occurred prior to the Late Miocene and such a feature needs to be incorporated into any model for uplift of the Central Andes The evidence from the basin-fill succession suggests that sediments accumulated in a basin developed in front of a broad monocline between 38 and 19 Ma and that a transition to a thrust-bounded foreland style basin took place after the development of the regional unconformity at 19 Ma. References

5.3. Unconformity generation Two types of angular unconformity can be recognised: 1) Local unconformities which can be related directly to tip folds associated with a specific fault and are only present in one section or two adjacent sections. This type of unconformity is present throughout much of the post-19 Ma succession. 2) Regional unconformities which are related to individual faults or fault systems, but occur at the same time in different basins across much of the forearc. Three regional unconformities can be recognised at around 19, 10 and 3 Ma. It is possible that uncertainty associated with the precise dating of these unconformities means they may not be regionally synchronous events. The unconformities could be associated with individual faults that were operative over broadly similar time frames, which if coupled with a lack of erosion due to the arid/hyper-arid Cenozoic climate in this area (Hartley et al., 2005), could result in the development of apparently synchronous unconformities. Alternatively if the angular unconformities are synchronous, a causal mechanism that is operative on a large spatial (nearly 6° of latitude) and short temporal scale is required. The only mechanism that could operate at such spatial and temporal scales is a large subduction zone earthquake. For example the area of coseismic uplift associated with the 1960 Central Chilean earthquake stretched for over 7° of latitude, and a similar scale of coseismic uplift (900 km by 200 km) was associated with the 1964 Alaskan earthquake (Plafker, 1972).

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