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_Journal of Structural Geology 33 (2011) 343e361_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com locate jsg An Alpine-style Ordovician collision complex in the Sierra de Pie de Palo, Argentina: Record of subduction of Cuyania beneath the Famatina arc C.R. van Staal a,*, G.I. Vujovich b,c, K.L. Currie d, M. Naipauer b,c a Geological Survey of Canada, 625 Robson Street, Vancouver, British Columbia, V6B 5J3 Canada b CONICET, Buenos Aires, Argentina c Laboratorio de Tectonica Andina-FCEyN, Universidad de Buenos Aires, Pabellon II, Ciudad Universitaria, 1428-Buenos Aires, Argentina d Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, K1A 0E8 Canada Article info Article history: Received 12 April 2010 Received in revised form 23 September 2010 Accepted 25 October 2010 Available online 20 November 2010 Keywords: Cuyania Collision complex Fold nappes Thrusts Subduction channel Abstract The Caucete Group and structurally overlying Pie de Palo Complex in western Argentina are characterised by two generations of west-verging folds and thrust-related shear zones, which formed under amphibolite facies conditions. The Caucete Group is separated from the Pie de Palo Complex by the Pirquitas thrust. These structures are interpreted to have formed as a result of progressive deformation generated during Middle Ordovician, underthrusting of the Laurentian-derived Cuyania microcontinent beneath the active Famatina margin. Geometrical relationships are most simply explained if the Pie de Palo Complex was basement to the Caucete Group prior to Ordovician orogenesis. We propose that this basement-cover relationship was established during Cambrian rifting of the Cuyania microcontinent from Laurentia. The Pirquitas fault may have been initiated during this extension prior to its long-lived remobilization as a thrust. We cannot rule out the possibility that the Pie de Palo Complex was exotic with respect to the Caucete Group, but for this to be possible we have to introduce an extra generation of structures, for which no evidence is preserved. The deformation was characterised by early strain localization followed by more homogeneously distributed non-coaxial flow during F2. Thermal softening probably dominated over fabric softening during this stage. Crown Copyright ? 2010 Published by Elsevier Ltd. All rights reserved. 1. Introduction The proto-Andean margin of west Gondwana accreted terranes from the late Neoproterozoic to the Late Paleozoic (Ramos et al., 1986, 2010; Ramos, 2004, 2008; Escayola et al., 2007). The docking of the composite Cuyania (Precordillera terrane sensu stricto plus Pie de Palo terranes, Ramos et al., 1998) or greater Precordillera terrane (Finney, 2007) with the Early Paleozoic west-facing, proto-Andean Famatina arc (Pankhurst et al., 1998) has been extensively discussed in terms of provenance, time of accretion, and regional extent. Fossil evidence (Benedetto et al., 1999), subsidence history, isolation of the carbonate platform from clastic input during the Early to Middle Ordovician (Astini, 1998; Keller et al., 1998) and paleomagnetic evidence (Rapalini and Astini, 1998; Rapalini, 2005) suggest that at least the Precordillera terrane had a Laurentian provenance during the Early Cambrian, to which it remained attached after the break-up of Rodinia, and moved as an independent microcontinent across Iapetus (Ramos et al., 1986; Astini et al., 1995; Thomas and Astini, 1996). The time of initial accretion (Ordovician; Astini, 1998; Casquet et al., 2001; Vujovich et al., 2004) or Siluro-Devonian (Rapela et al., 1998; Keller et al., 1998) and whether the colliding Famatinian arc was separated from the proto-Andean margin by a significant marginal basin or seaway (Bahlburg and Herv?, 1997; Rapela et al., 1998; Rapalini, 2005) remain contentious. We present results of studies carried out in the western Sierra de Pie de Palo, in west-central Argentina (Fig. 1), which is located close to the boundary between the Cuyania composite terrane and remnants of the proto-Andean margin. The tectonostratigraphy, style and age of deformation and metamorphism of these rocks, and the relationship between foliation development, folding and thrusting bear on the provenance and tectonic setting of these rocks and how deeply buried rocks flow and accommodate ductile deformation in an A-subduction collision zone. Our data suggest that an A-subduction collision zone comprises a regime characterised by early strain localization that transforms to more homogeneously distributed flow in response to evolving thermal structure, and support earlier suggestions that during folding thin ductile thrust sheets formed as a secondary feature by shearing-out of the common, lower limb of overturned folds (e.g. Heim, 1919; Ramsay, 1992). 0191-8141 $ e see front matter Crown Copyright ? 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016 j.jsg.2010.10.011 344 C.R. van Staal et al. Journal of Structural Geology 33 (2011) 343e361 Fig. 1. Map of the Cuyania composite terrane (Precordillera ? Pie de Palo terranes) after Ramos et al. (1998, 2010) and map of the Sierra de Pie de Palo after Ramos and Vujovich (2000) and Naipauer et al. (2010a). Structure, and support earlier suggestions that during folding thin ductile thrust sheets formed as a secondary feature by shearing-out of the common, lower limb of overturned folds (e.g. Heim, 1919; Ramsay, 1992)._ 2. Geological setting The intensely deformed metamorphic rocks of the Pie de Palo terrane (Fig. 1) lie immediately west of the Valle F?rtil lineament (e.g. Chernicoff et al., 2009), generally inferred to mark the suture between the Famatina arc and the exotic Cuyania composite terrane to the west (Ramos et al.,1998; Ramos, 2004). The Pie de Palo terrane, the leading edge of Cuyania, is separated from the Precordillera terrane, which represents Cuyania’s trailing part, by a wide (w50 km) valley (Valle del Tulum) filled with sand and salt flats (Fig. 1). Hence, the relationships between these two Cuyanian terranes are contentious. Early workers inferred that at least part of the rocks in the Sierra de Pie de Palo represented exhumed Mesoproterozoic basement which was assembled into a composite ’Cuyania’ terrane during the Grenville orogeny (Ramos et al., 1998; Vujovich and Kay, 1998; Chernicoff et al., 2009). In contrast, Mulcahy et al. (2007) interpreted new radiometric age and isotopic data (Galindo et al., 2004; Mulcahy et al., 2007) as suggesting that the Pie de Palo terrane represents a fragment of imbricated Mesoproterozoic basement to the Famatina forearc. The key to establishing the affinities of the Pie de Palo terrane rocks lies in the metasedimentary rocks of the Caucete Group, which occur along the western fringes of the Sierra de Pie de Palo, and its structural relationships with the structurally overlying Mesoproterozoic rocks of the Pie de Palo Complex (Vujovich et al., 2004). The rocks of the Caucete Group were traditionally correlated with the Cambrian-lower Ordovician platform rocks of the Precordillera terrane (Schiller, 1912) (Figs. 1 and 2). This correlation, supported by tectonostratigraphic studies (van Staal et al., 2002), carbonate isotope data (Galindo et al., 2004) and detrital zircon studies (Naipauer et al., 2010a), implies a direct tectonic linkage between the Precordillera and at least part of the Pie de Palo terranes. Our investigations (Fig. 2) focussed on the Caucete Group, the structurally overlying 1e1.2 Ga ultrama?c to intermediate rocks of the Pie de Palo Complex (Vujovich et al., 2004), and the shallow to moderately, east-dipping Pirquitas thrust (Ramos and Vujovich, 2000; Chernicoff et al., 2009) that separates the two units (Figs. 2 and 3). The preserved tectonostratigraphy of the metasedimentary rocks combined with nearly continuous outcrop and significant relief allows elucidation of the complex structure and determination of the relationships between folding and thrusting. 3. Caucete Group 3.1. Tectono-stratigraphy Rocks of the Caucete Group (Borello, 1969; Vujovich, 2003) were intensely deformed by ductile shear (Fig. 4A) and at least two generations of tight to isoclinal folds (Fig. 4E), which transposed the compositional layering of the rocks into an orientation parallel C.R. van Staal et al. Journal of Structural Geology 33 (2011) 343e361 345 Fig. 2. Geological map of the central-western Sierra de Pie de Palo. Geology was mapped on 1:25,000 air photos and than transferred to a satellite-based topographic map. Where there is little or no outcrop (sand-?lled dry creeks and ?ats) colours are faded or left white. UM ? ultrama?c rocks. to a complex, generally shallowly to moderately east-dipping composite fabric. Strain appears high throughout the sequence as documented below. Despite intense deformation, the rocks preserve an easily recognisable tripartite tectonostratigraphy, comprising varicoloured psammite (El Quemado and La Paz formations), poorly sorted arkosic and calcareous red beds interlayered with dolomite (El Desecho Formation), and marble (Angacos Formation). The integrity of the lithological units is generally preserved on map-scale, except in the hinges of large folds, where units are typically complexly structurally interleaved (Fig. 4C) and zones of very high attenuation (Fig. 2). Tectonostratigraphic integrity despite such intense folding is most easily explained if the folds are asymmetrical on all scales such that the overall shallowly dipping enveloping surface to the folded rocks makes a small angle with the compositional layering. All three major units display compositional layering on cm to m-scale (Fig. 4E), which we interpret as bedding. It is difficult to explain layers of such contrasting compositions as dolomite and sandstone in any other way. Some of the layering in the psammites could conceivably be the result of extreme transposition of completely recrystallised felsic sills or dikes, but no evidence for such intrusions is observed where strain is relatively low. 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