Differential Late Paleozoic Active Margin Evolution in South-Central Chile (37°S-40°S) -The Lanalhue Fault Zone

Originally published as: Glodny, J., Echtler, H., Collao, S., Ardiles, M., Burón, P., Figueroa, O. (2008): Differential Late Paleozoic active margin evolution in South-Central Chile (37°S-40°S) -The Lanalhue Fault Zone. - Journal of South American Earth Sciences, 26, 4, 397-411 DOI: 10.1016/j.jsames.2008.06.001. Accepted Manuscript Differential Late Paleozoic active margin evolution in South-Central Chile (37°S-40°S) -The Lanalhue Fault Zone Johannes Glodny, Helmut Echtler, Santiago Collao, Mary Ardiles, Pablo Burón, Oscar Figueroa PII: S0895-9811(08)00053-9 DOI: 10.1016/j.jsames.2008.06.001 Reference: SAMES 759 To appear in: Journal of South American Earth Sciences Received Date: 17 January 2008 Accepted Date: 5 June 2008 Please cite this article as: Glodny, J., Echtler, H., Collao, S., Ardiles, M., Burón, P., Figueroa, O., Differential Late Paleozoic active margin evolution in South-Central Chile (37°S-40°S) -The Lanalhue Fault Zone, Journal of South American Earth Sciences (2008), doi: 10.1016/j.jsames.2008.06.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. 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ACCEPTED MANUSCRIPT 1 Differential Late Paleozoic active margin evolution in South-Central Chile (37°S-40°S) - 2 The Lanalhue Fault Zone 3 4 5 Johannes Glodny1*, Helmut Echtler1, Santiago Collao2, 6 Mary Ardiles2, Pablo Burón2, Oscar Figueroa2 7 8 9 (1) GeoForschungsZentrum Potsdam, Telegrafenberg C2, D-14473 Potsdam, Germany 10 (2) Departamento Ciencias de la Tierra, Universidad de Concepción, Casilla 2-D, 11 Concepción, Chile 12 13 14 15 *: Corresponding author: Johannes Glodny 16 Phone: +49 331 2881375 17 Fax: +49 331 2881370; 18 e-mail: [email protected] 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 1 ACCEPTED MANUSCRIPT 57 Abstract 58 The N-S oriented Coastal Cordillera of South Central Chile shows marked lithological 59 contrasts along strike at ~38°S. Here, the sinistral NW-SE-striking Lanalhue Fault Zone 60 (nomen novum) juxtaposes Permo-Carboniferous magmatic arc granitoids and associated, 61 frontally accreted metasediments (Eastern Series) in the northeast with a Late Carboniferous 62 to Triassic basal-accretionary forearc wedge complex (Western Series) in the southwest. The 63 fault is interpreted as an initially ductile deformation zone with divergent character, located in 64 the eastern flank of the basally growing, upwarping, and exhuming Western Series. It was 65 later transformed and reactivated as a semiductile to brittle sinistral transform fault. Rb-Sr 66 data and fluid inclusion studies of late-stage fault-related mineralizations revealed Early 67 Permian ages between 280 and 270 Ma for fault activity, with subsequent minor erosion. 68 Regionally, crystallization of arc intrusives and related metamorphism occurred between 69 ~306 and ~286 Ma, preceded by early increments of convergence-related deformation. Basal 70 Western Series accretion started at >290 Ma and lasted to ~250 Ma. North of the Lanalhue 71 fault, Late Paleozoic magmatic arc granitoids are nearly 100 km closer to the present day 72 Andean trench than further south. We hypothesize that this marked difference in paleo- 73 forearc width is due to an Early Permian period of subduction erosion north of 38°S, 74 contrasting with ongoing accretion further south, which kinematically triggered the evolution 75 of the Lanalhue Fault Zone. Permo-Triassic margin segmentation was due to differential 76 forearc accretion and denudation characteristics, and is now expressed in contrasting 77 lithologies and metamorphic signatures in todays Andean forearc region north and south of 78 the Lanalhue Fault Zone. 79 80 81 Keywords: forearc deformation, accretion, tectonic erosion, isotopic dating, Permian, Chile 82 83 84 85 86 87 2 ACCEPTED MANUSCRIPT 88 1. Introduction 89 The Coastal Cordillera of South Central Chile is dominated by Upper Carboniferous to 90 Triassic crystalline basement. Petrologic work has documented marked contrasts in the 91 composition and metamorphic evolution of this basement across a NW-SE striking, 92 lineament-like fault zone within the Cordillera Nahuelbuta (Fig. 1). Northeast of this fault 93 zone, i.e., north of ~38°S, Upper Carboniferous to Early Permian granitoids are associated 94 with metasediments, termed ‘Eastern Series’, which show distinct, very low-grade, medium 95 pressure to HT-LP contact-metamorphic overprints (Aguirre et al., 1972; Hervé, 1977; Kato, 96 1985; Willner et al., 2000; Lucassen et al., 2004). Southwest of the lineament, the Coastal 97 Cordillera is made up of a Permotriassic forearc accretionary wedge known as ‘Western 98 Series’. Its intercalated metasediments and metabasites show a distinct high P / low T 99 signature, with a regionally homogeneous metamorphic grade of transitional greenschist- to 100 blueschist facies (Aguirre et al., 1972; Hervé, 1988; Willner et al., 2001; Willner, 2005; 101 Glodny et al., 2005). 102 On a larger scale, the contact between Western and Eastern Series has been recognized as 103 a major crustal discontinuity due to the associated lithological contrasts (Ernst, 1975; Kato, 104 1976, 1985; Hervé, 1977). Western and Eastern Series have been interpreted as near- 105 contemporaneous and genetically associated, constituting a paired metamorphic belt (sensu 106 Miyashiro, 1961; Aguirre et al., 1972; Ernst, 1975). The contact between the Western and 107 Eastern Series can be traced or inferred for more than 1000 km south of ~34°S. The nature 108 of this contact is regionally variable, from transitional (on a km scale) to tectonic (Davidson et 109 al., 1987; Richter et al., 2007) The NW-SE striking segment at ~38°S is unique in that it is the 110 only segment where this contact significantly deviates from a trench-parallel, NNE-SSW 111 orientation (Fig. 1). The first-order fault zone at ~38°S is thus a key tectonic feature for a 112 better understanding of the regional forearc basement architecture. 113 The exact age and tectonic nature of fault activity has so far been unknown (cf. Martin et al., 114 1997, 1999), leaving open questions about the geodynamic role of this fault zone. Beside 115 reference to it as the regional ‘Eastern Series-Western Series contact’, a variety of names 3 ACCEPTED MANUSCRIPT 116 exist in the literature. While the term ‘Coast Range Suture’ is unspecific in comprising the 117 entire contact zone all along the South Chilean coast, the term ‘Linea de Purén’ is misleading 118 as the town of Purén is not located on the fault trace. From a tentative correlation of the fault 119 zone with the similarly NW-SE striking dextral Jurassic Gastre Fault System (cf. Rapela & 120 Pankhurst, 1992) in Central Patagonia, Argentina, it was termed ‘Gastre Fault Zone’ or 121 ‘Gastre-Purén Fault Zone’. However, in the present work we show that this correlation is 122 incorrect. The lake ‘Lago Lanalhue’, in the western slopes of the Cordillera Nahuelbuta (Fig. 123 1) is located on the fault trace and shows a NW-SE-elongated shape. We therefore propose 124 the new term ‘Lanalhue Fault Zone (LFZ)’ as an appropriate name for the here discussed 125 fault zone. 126 In this work, we present the first direct geochronological data for Lanalhue fault activity, 127 combined with macro- and microstructural observations, to clarify evolution and geodynamic 128 significance of the fault zone. New geochronological data on the regional basement units 129 indicate that the activity of the LFZ initiated in Early Permian times, immediately after 130 consolidation of the involved basement segments. Fluid inclusion data on fault-related quartz 131 mineralizations help to constrain the post-metamorphic exhumation history. We present a 132 new model on the assembly and subsequent segmentation of the Late Carboniferous to 133 Triassic active continental margin in South Central Chile. 134 135 136 2. Geological setting 137 Along the Pacific coast of Central Chile, Late Paleozoic to Triassic processes assembled and 138 shaped the southwestern margin of Gondwana (Hervé et al., 1987; Mpodozis & Ramos, 139 1989, Glodny et al., 2006). Whereas in northern Chile Late Paleozoic to Triassic structures 140 are largely overprinted by the Andean orogeny in Jurassic to Recent times, the South Central 141 Chilean margin shows abundant pre-Andean basement, preserved in a remarkably stable, 142 stationary active margin setting. Below, we characterize the pre-Andean geologic units of a 4 ACCEPTED MANUSCRIPT 143 composite NE-SW profile across the LFZ, i.e, from the granitoids of the Cordillera 144 Nahuelbuta into the Western Series paleoaccretionary forearc complex (Fig. 2). 145 146 The granitoids of the Cordillera Nahuelbuta (37°-38°S), as part of the Southern Coastal 147 batholith of South-Central Chile, are calc-alkaline rocks, in particular granodiorites, tonalites, 148 and diorites, with minor granites, intruded at upper crustal levels into metasedimentary 149 sequences. Age determinations for these granitoids cluster tightly around 295-305 Ma (Hervé 150 et al., 1988; Lucassen et al., 2004). The Nahuelbuta granitoids are remnants of a Late 151 Carboniferous to Permian magmatic arc. While towards the north equivalents of the 152 Nahuelbuta granitoids and their host rocks are found in the Coastal Cordillera, south of 153 38°30’S equivalents are located in the western flank of the Andean Main Cordillera, in the 154 Lake District of South-Central Chile at ~40°S, east of Valdivia (Fig.

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