Pliocene-Quaternary Tectonic Subsidence of Part of the Central Depression Forearc, 20-22°S

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F u n 2 d 6 la serena octubre 2015 ada en 19 ! Pliocene-Quaternary Tectonic Subsidence of part of the Central Depression Forearc, 20-22°S

Teresa Jordan1, Nicolás Cosentino1, and Arturo Jensen2 1Department of Earth & Atmospheric Sciences, Cornell University, Ithaca, NY 14853 USA 2Facultad de Ingeniería y Ciencias Geológicas, Universidad Católica del Norte, Antofagasta, Chile contact email: [email protected]; [email protected]; [email protected]

Abstract. Between 20-22°S the forearc basin of northern between the location of the Loa River, the facies history, Chile consists of the Quillagua-Llamara sub-basin and the and possible tectonic uplift history of the forearc have been Pampa del Tamarugal. A distinguishing modern feature is incompletely understood. This paper focuses on a study that the Loa River flows in a deeply incised canyon through area that spans the Coastal Cordillera and Central this atypically low elevation sector of the forearc basin, and Depression at 20°-22°S (18,000 km2), with emphasis on then cuts west across the Coastal Cordillera. The basin fill the region where the entrenched Loa Canyon connects the facies demonstrates a persistent tendency to be a low forearc to the Pacific Ocean. sector of the forearc during the Late Miocene and the Late Pliocene. A new method to measure paleo-altitudes in the Atacama Desert system was applied to 18 locations with ancient gypsic soil in the Coastal Cordillera and forearc basin. The method determines whether soil gypsum formed under the influence of marine fog, and uses the modern altitude-fog relations to estimate paleo-altitude. For soil ages spanning about 5.5 – 1 Ma at three locations east of the Loa River, the ancient soil indicates that subsidence during the Pliocene and during the Quaternary lowered the forearc elevation by minimum values ranging between 60 m and 230 m.

Key Words. forearc, tectonics, paleoaltitude, Chile, paleosol

1 Introduction

The nonmarine forearc of northern Chile (20°–22°S) consists, from west to east, of a narrow coastal platform, an angle-of-repose coastal cliff (500–1200 m relief), a Coastal Cordillera with altitudes lower than 2000 m, and a 50–75 km wide valley (Central Depression) that gently tilts to the west and extends between altitudes of ~800 and ~2500 m.a.s.l. (Fig. 1). The Central Depression is the modern expression of the Pampa del Tamarugal forearc Figure 1. Topography and locations of paleosol and relict soil basin, which is filled with strata of Oligocene to recent age samples. Faults and folds from Carrizo et al. (2005) and photo (Nester and Jordan, 2012). The basin is bounded to the east interpretation. Inset shows study area and path of Loa River. by the Precordillera, which in turn rises to the volcanic arc (Western Cordillera). The forearc is within the hyperarid Few prior studies have developed quantified uplift Atacama Desert. histories for the non-marine Andean forearc in northern Chile and southern Peru. To fill this gap in knowledge, The Loa River cuts across the Coastal Cordillera at Cosentino et al. (submitted) developed a new absolute ~21.5°S. A final deeply entrenched canyon of the Loa paleoaltimetry proxy based on 87Sr/86Sr of gypsic paleosols connects the forearc sedimentary basin to the Pacific Coast. and relict soils of the Atacama Desert. The paleoaltimeter The sector of the Central Depression within which the Loa is based on the observed altitude dependence of 87Sr/86Sr in flows first north and then west is referred to as the Holocene accumulations of salts in the area, which is a Quillagua-Llamara sub-basin. Prior studies of the relationship controlled by the altitude to which marine- sedimentary fill of the Quillagua-Llamara sub-basin report sourced fogs reach. The Holocene 87Sr/86Sr - altitude sedimentary environments suggestive of a topographic distribution is bi-modal, with high 87Sr/86Sr ratios inside depression since the Late Miocene (Bao et al., 1999; Sáez the height band of time-averaged (104 yrs) influence by et al., 1999; Sáez et al., 2012). Yet the genetic relations marine aerosols, and low values outside of that elevation

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range. This height band, the time-averaged fog zone, spans (Jordan et al., 2010). Imbrication data from three horizons amount of displacement across the larger individual faults an altitude above the top of the contemporaneous fog zone, ~225–1075 m.a.s.l (Cosentino et al., submitted). By in these well sorted gravels indicate a consistent paleo- is 100–350 m. Although not systematically mapped, a and yet the site now occurs in the altitude range of modern analogy, data for the 87Sr/86Sr of Miocene, Pliocene and flow direction toward the NW and NNW. Upper Miocene series of north-trending reverse faults, strike-slip faults, fog. These results indicate a subsidence of the surface Pleistocene gypsum-anhydrite soils quantify the surface and Pliocene sedimentary rocks accumulated in lacustrine and fold axes are common in the center of the Quillagua- relative to sea level (Fig. 1, red stars). A site north of uplift history of the Coastal Cordillera and Central environments (Sáez et al., 1999; Sáez et al., 2012), whose Llamara basin, straddling 70°W. The Soledad Formation is 20°30’S (Fig. 1, blue star) near the Coastal Escarpment Depression. lithologies are strongly dependent upon the paleoclimate offset vertically by 100–200 m. contains a paleosol which formed within the fog zone but and paleohydrology (Fig. 2). The Upper Miocene Hilaricos then rose to a modern altitude above the height of fog. anhydrite formed in a gypsum-generating salar. It is A broader scale young deformation in the Quillagua- Between 20°30’–21°S, four sites in the eastern Coastal overlain by the deposits of a relatively fresh-water lake, the Llamara sub-basin is displayed by the outcrop patterns. Cordillera and Central Depression produced ambiguous 2 Geomorphology Quillagua Formation. After another major change in the The northern, western and southern margins of the exposed results (N. Cosentino, in progress, Cornell University). ~4 Ma Soledad Formation (Quezada et al., 2013) are The geomorphological framework of the Andean forearc water balance the subsequent unit, the Soledad Formation, inclined gently toward the basin center (0.3–2.0°dips; Fig. follows the structural grain. North of 21°10’S, the major reflects a salar environment. An upper member of the 1). This defines a broad structural basin. Because the tectonic features trend ~N340°–N350°; farther south the Soledad is widespread (Fig. 1) and dominated by gypsum. 7 Discussion eastern zone of the Soledad paleo-salar is exposed only grain is ~N05°–N20°. A set of faults that traverses the A lower member is found in the east and consists of sand that was saturated by brines that led to a halite cement that adjacent to faults and folds (e.g., Cerro Soledad; Lomas de Coastal Cordillera and enters the western side of the Four locations yield clear signals of a change in altitude Central Depression departs from that pattern; many of now dominates the rock volume. The northern limit of the Sal), no analysis of long-wavelength deformation of the Hilaricos and Quillagua lacustrine stages is not fully east side is available. since 5 Ma: three show subsidence and one shows uplift these faults are east-trending (Figure 1). Drainages in the (Fig. 1). Sites in the Quillagua-Llamara sub-basin Precordillera and Central Depression dominantly flow known because of limited subsurface data. The available data suggest that the Late Pliocene (Quezada et al., 2013) systematically show subsidence or an indeterminate result. from east to west, down the regional topographic gradient. Two sites that flank the Lomas de Sal indicate decreases in Because the drainage basins are mostly endorheic, the Soledad salar was at least 100 km in N-S length (Nester, 5 Methods for Sr-ratio analysis of ancient 2008; Sáez et al., 1999). altitude exceeding 230 m (west) and 30 m (east). A site Quaternary depositional systems terminate against the soil near the town of Quillagua indicates subsidence in excess western boundary of the Central Depression, resulting in of 60 m. A location in the Coastal Cordillera near 20°20’S north- elongated salars and paleo-lakes. Targets for field sampling were locations with likely (Fig. 1) must have risen in altitude by at least 100 m during ancient gypsic soil at altitudes below 2000 m.a.s.l. for a similar timespan. Sites farther north than 20°S also The Loa River is an anomaly in the Atacama Desert for which there existed local age constraints. Samples indicate uplift during this time span (Cosentino, several reasons, one of which is its circuitous route from collected in 2011 and 2012 were obtained at one or two dissertation in progress, 2015). the Andean highlands to the Pacific Ocean (Fig. 1). The depths (Cosentino 2015, dissertation in progress). During Loa headwaters are within the volcanic arc. After a short the 2013 field campaign, relict soils were sampled through During the time interval in which there was a net west-directed segment, the river flows 150 km south on the vertical profiles of 6-10 samples. Laboratory procedures to subsidence of the Quillagua-Llamara sub-basin there was east side of the Precordillera, then cuts west to follow the extract Sr are detailed in Cosentino et al. (submitted). widespread displacement across individual fault scarps by regional topographic gradient across the Precordillera for 87 86 Sr/ Sr was measured with either thermal ionization mass magnitudes that are similar to the changes documented by nearly 90 km. At Chacance, the Loa again turns abruptly spectrometry (VG Sector 54, Cornell University) or paleo-altimetry. However, the new data record subsidence and flows north for about 90 km in the Central Depression, inductively coupled plasma mass spectrometry (Neptune for three different locations relative to the local-scale past Quillagua. The final change in direction, to flow Multicollector ICP-MS, Rutgers University). NBS-987 Sr structures: one on the down-thrown block west of Lomas westward, occurs where the Quebrada Amargo joins the standards run in association with samples averaged de Sal, one on the tilted eastern block of Lomas de Sal, and Loa. 0.71024 ± 0.00003 (2σ, n = 32) and 0.710282 ± 0.000012 one east of Quillagua village where there are no nearby Several 20-30 km long channels parallel the Loa River (2σ, n = 18), respectively. Corrections were applied to faults. The consistent evidence for subsidence in a zone not between Chacance and Quillagua. As a set, these account for this inter-laboratory standard deviation. strictly associated with individual fault blocks suggests Figure 2. Time correlations between variable paleoclimate and that there is a longer-wavelength (tens of kilometers) secondary ephemeral channels and the Loa Canyon are the only axially oriented erosional features in the Central the accumulation of sedimentary units in the Quillagua-Llamara subsidence in the study area. Subsidence of long- Depression. Whereas the Loa Canyon is incised at least 10 sub-basin. Chronology from Saez et al. (2012), Quezada et al. 6 Results wavelength is also indicated by the tilt of the Soledad (2013) and Jordan et al. (2014). Paleoclimate conditions from m everywhere in this reach, the incision depth of the minor Formation around the northern, western and southern Jordan et al. (2014). margins of the sub-basin. This subsidence tendency occurs channels is near zero in the south and increases toward Relict soils are widespread in the study area, developed on each channel’s point of merger with the Loa in the north. where the forearc basin altitudes are ~300 m lower than At a large spatial scale, the study area appears to have been Pampa del Tamarugal basin-fill strata as old as ~11 Ma along strike to the north and south (Fig. 1). We suggest dominated by erosion since about 3 Ma. Yet localized and as young as ~2.2 Ma [Jordan et al., 2014]. The that true tectonic subsidence since ~4 Ma is responsible for accumulation of evaporite deposits likely continued during distribution of parent material age and of soil burial an important part of the modern forearc topography. 3 Quillagua-Llamara sub-basin Miocene- some parts of the Quaternary (e.g. Sáez et al., 2012) where relations allow a comprehensive examination of paleo- Quaternary sedimentary record hydrological barriers formed. altitude history for only the last ~5 million years. Samples A relationship may exist between the Loa River history from 18 locations in the study area (Fig. 1) resulted in 10 and tectonic subsidence within the Quillagua-Llamara sub- 87 86 The Miocene(?) to Quaternary sedimentary fill of the sites with no detectable change in altitude: the Sr/ Sr basin. The facies history indicates that the Quillagua- Quillagua-Llamara sub-basin is known from exposures in 4 Deformation patterns value of the sampled horizons in the gypsic soil indicates Llamara zone was a relative topographic low within the the walls of the Loa Canyon and its tributaries, from formation at a position relative to the paleo-fog altitude forearc basin system throughout the Late Cenozoic, and structurally uplifted blocks in a zone near 70°W (Fig. 1), Two sets of faults and folds are common in the study area range that is the same as its current position relative to the thus tended to be occupied by rivers or lakes. The basal and in two oil exploration boreholes (Fig. 2). The older (Fig. 1). Allmendinger et al. (2005) and Carrizo et al. modern fog altitude range. For these sites, the sample may conglomerate was most likely deposited by a north- strata are fluvial conglomerates of uncertain age, although (2008) showed that numerous small-offset faults, trending have moved either up or down in altitude but it did not flowing river located where the modern Loa occurs. The they are inferred to interfinger eastward with Late WNW, WSW and NW within the Coastal Cordillera, were cross a Sr-ratio threshold. For 3 sites located south of 21°S, Hilaricos salar facies suggests that the region was Oligocene – Middle Miocene distal alluvial fan facies active during the Latest Miocene and Pliocene. The the Sr-isotope data indicate that the ancient soil formed at endorheic during the Late Miocene. During the latest

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Miocene and Early Pliocene a fresh-water lake formed in the same region because of a wetter climate (Sáez et al., References Cited 2012). The subsequent return to hyperaridity terminated the lake. Another cycle of diminished aridity followed by Allmendinger, R.W.; González, G.; Yu, J.; Hoke, G.D.; Isacks, B.L. renewed hyperaridity created the Soledad Formation salar 2005. Trench-parallel shortening in the northern Chilean forearc: during the Late Pliocene. There is no evidence of drainage tectonic and climatic implications. Geological Society of America by a major river like the Loa from the high Andes to the Bulletin 117(1): 89-104, doi:10.1130/B25505.1. Bao, R.; Sáez, A.; Servant-Vildary, S.; Cabrera, L. 1999. Lake-level Pacific across the study area during either the Hilaricos or and salinity reconstruction from diatom analyses in Quillagua Soledad deposition. In contrast, during Quillagua time a Formation (late Neogene, central Andean forearc, northern Chile). paleo-Loa River fed the basin and the lake may have been Palaeogeography, Palaeoclimatology, Palaeoecology, 153(1-4): 309- an open system. But the location of an expected spill-point 335. for the Quillagua lake has not been documented (Sáez et Carrizo, D.A.; González L., G.; Dunai, T.J. 2008. Constricción Neógena en la Cordillera de la Costa norte de Chile: Neotectónica y al., 1999). We propose that the east-trending set of faults 21 datación de superficies con Ne cosmogénico. Revista Geologica De between 21°10’–21°40’S (Fig. 1) provided low topography Chile, 35(1): 1-38, doi:10.4067/S0716-02082008000100001. and weakened rock which were utilized by overspill from Cosentino, N.; Jordan, T.E. 2012. 87Sr/86Sr en paleosuelos salinos the Quillagua lake system. Although the paleo-Loa likely como paleoaltímetro; resultados preliminares para el norte de Chile did not drain through this basin during Soledad time, (19-22°S), In Congreso Geológico Chileno 13, 669-671, Antofagasta, groundwater leaked from the southern part of the Soledad Chile. Cosentino, N.J.; Jordan, T.E.; Derry, L.A.; Morgan, J.P.. In review. salar system may have further weathered the relict Loa 87Sr/86Sr in recent accumulations of calcium sulfate on landscapes Canyon. of hyperarid settings as an altitude proxy: results for northern Chile (19.5-21.5°S). Geochemistry Geophysics Geosystems. We hypothesize that several hundred meters of tectonic Jordan, T.E.; Kirk-Lawlor, N.E.; Blanco, N.; Rech, J.A.; Cosentino, subsidence since ~4 Ma led again to capture of both local N.J. 2014. Landscape modification in response to repeated onset of and regional drainage systems. Since that time surface hyperarid paleoclimate states since 14 Ma, Atacama Desert, Chile. Geological Society of America Bulletin 126, B30978-B30971. water and ground water flow have been focused through Jordan, T. E.; Nester, P. L.; Blanco, N.; Hoke, G. D.; Dávila, F.; the Quillagua-Llamara area. The north-trending paleo- Tomlinson, A. J. 2010. Uplift of the Altiplano-Puna Plateau: A View channels that parallel the Loa between Chacance and from the West. Tectonics 29 (TC5007), doi:10.1029/2010TC002661. Quillagua may be relicts of initial attempts to re-connect Nester, P. (2008) Basin and Paleoclimate Evolution of the Pampa del the Loa headwaters to the Quillagua-Llamara sub-basin Tamarugal Forearc Valley, Atacama Desert, Northern Chile, Cornell late in Soledad Formation time. Their pattern is consistent University, Ph.D. Dissertation, 253 p. Nester, P. L.; Jordan, T.E. 2012. The Pampa del Tamarugal Forearc with a long-lived northward inclination of the southern Basin in Northern Chile: The Interaction of Tectonics and Climate, In flank of the region of latest Pliocene–Quaternary tectonic Tectonics of Sedimentary Basins: Recent Advances (Busby, C.; Azor, subsidence. A.; editors). Blackwell Publishing Ltd: 369-381. Oxford, England. Quezada, A.; Vásquez, P.; Sepúlveda, F. 2013. Soledad Formation: detailed mapping and radiometric ages, In International Geological Congress on the Southern Hemisphere 2, (GEOSUR 2013). Acknowledgements Bollettino di Geofísica teorica ed applicata 54 (Supplement), 242. Sáez, A.; Cabrera, L.; Garcés, M.; Bogaard, P.; Jensen, A.; Gimeno, The U.S. National Science Foundation supported this D. 2012. The stratigraphic record of changing hyperaridity in the Atacama desert over the last 10 Ma. Earth and Planetary Science research (EAR-0208130, EAR-0609621 and EAR- Letters 355: 32-38. 1049978). The authors benefited from shared field work Sáez, A.; Cabrera, L.; Jensen, A.; Chong, G. 1999. Late Neogene with Nicolás Blanco, Andrew Tomlinson, Fernando lacustrine record and palaeogeography in the Quillagua-Llamara Sepúlveda, Paulina Vasquez, and Andrés Quezada of basin, Central Andean fore-arc (northern Chile). Palaeogeography, SERNAGEOMIN. We thank Antonio Díaz for managing Palaeoclimatology, Palaeoecology 151(1): 5-37. the field work logistics.

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