Regional deformation of the Sierra de San Luis, Argentina ...

TECTONICS, VOL. 23, TC1005, doi:10.1029/2003TC001542, 2004

Regional deformation of the Sierra de San Luis, Argentina: Implications for the Paleozoic development of western Gondwana Steven J. Whitmeyer and Carol Simpson Department of Earth Sciences, Boston University, Boston, Massachussetts, USA

Received 10 May 2003; revised 8 September 2003; accepted 22 September 2003; published 17 January 2004.

[1] Metamorphic and ductile deformation fabrics within the Sierra de San Luis, central Argentina, provide evidence for the Early to Middle Paleozoic development of the paleo-Pacific margin of Gondwana. Presumed Vendian-aged metasedimentary rocks within both the Sierra de San Luis and the Sierras de Co´rdoba preserve early pressure solution cleavage development. Cambrian peak metamorphism in the Sierras de Co´rdoba and Ordovician peak metamorphism in the Sierra de San Luis indicate that juxtaposition of these two terranes did not occur prior to the Late Ordovician. Peak metamorphism and regional folding of the Sierra de San Luis metasedimentary rocks is equated with early stages of the development and shortening of the Famatinian magmatic arc along the western margin of Gondwana. Extensive, NNE trending zones of upper greenschist- to amphibolite-facies ductile deformation within the Sierra de San Luis and along the western margin of the Sierra de Comechingones record the dextral oblique juxtaposition of the Sierra de San Luis terrane against the southwestern margin of the Sierras de Co´rdoba, coincident with, or closely followed by, the suture of the exotic Precordillera terrane to the western margin of Gondwana by the Early Devonian. Middle to Late Devonian restricted, lower greenschist-grade reactivation of ductile faults may record the accretion of the Chilenia terrane, the final stage of Paleozoic convergent tectonism along this segment of western I NDEX T ERMS: 8010 Structural Geology: Gondwana. Fractures and faults; 8030 Structural Geology: Microstructures; 8005 Structural Geology: Folds and folding; 8102 Tectonophysics: Continental contractional orogenic belts; 8157 Tectonophysics: Plate motions—past (3040); KEYWORDS: Sierras Pampeanas, Gondwana, Laurentia, mylonite, Precordillera, shear zone. Citation: Whitmeyer, S. J., and C. Simpson (2004), Regional deformation of the Sierra de San Luis, Argentina: Implications for the Paleozoic development of western Gondwana, Tectonics, 23, TC1005, doi:10.1029/2003TC001542.

1. Introduction [2] Considerable recent attention has been directed at potential connections between Laurentia and Gondwana in Copyright 2004 by the American Geophysical Union. 0278-7407/04/2003TC001542$12.00

the early Paleozoic. Much of this research has focused on the Precordillera of western Argentina, an exotic terrane most often linked with a Laurentian origin [Dalla Salda et al., 1992a, 1992b; Astini et al., 1995; Thomas and Astini, 1996; Dalziel, 1997], but also interpreted as a migratory terrane of Gondwanan affinity [Acenõlaza and Toselli, 1988, 2000; Acenõlaza et al., 2002; Finney et al., 2003]. Interaction of the Precordillera with Gondwana is part of a complex tectonic sequence that likely initiated with east directed subduction off the west coast of Gondwana (present-day coordinates) in the Early Cambrian, and included multiple collisional episodes throughout the Early to Middle Paleozoic [Acenõlaza and Toselli, 1976; Ramos, 1988; Acenõlaza et al., 1990; Durand and Acenõlaza, 1990; Astini et al., 1995; Kraemer et al., 1995; Durand, 1996; Rapela et al., 1998; Davis et al., 1999]. In order to decipher and constrain the distinct early Paleozoic tectonic events of western Gondwana, we focus on the Eastern Sierras Pampeanas, located east of the Precordillera in central Argentina (Figure 1). Here deformational, metamorphic and intrusive fabrics preserve evidence for the tectonic environment and the relative, and in places absolute, timing of events along the paleo-Pacific margin of Gondwana. [3] The Eastern Sierras Pampeanas has two principal components; the Sierras de Co´rdoba and del Norte in the east and the Sierras de San Luis and Chepes in the west (Figure 1). Well-exposed plutonic and metamorphic rocks and zones of ductile deformation within the Eastern Sierras Pampeanas provide evidence for a succession of Early Paleozoic tectonic events, including the Cambrian Pampean Orogeny [Acenõlaza et al., 1990; Pankhurst and Rapela, 1998; Gromet and Simpson, 1999], the Early to Middle Ordovician Famatinian Orogeny [Pankhurst and Rapela, 1998; Sims et al., 1998], and possibly the Ocloyic and/or Achalian Orogenies (Late Ordovician through Middle Devonian [Sims et al., 1998; Stuart-Smith et al., 1999]). Most previous work has viewed the Sierras de Co´rdoba and the Sierra de San Luis as an interrelated unit throughout the early Paleozoic [Ramos, 1988; Prozzi, 1990; Von Gosen and Prozzi, 1998], and therefore subject to consistent styles of deformation and metamorphism. However, recent work [Simpson et al., 2001; Whitmeyer and Simpson, 2003] suggests that the Sierra de San Luis was not positioned adjacent to the Sierras de Co´rdoba prior to the Late Ordovician, which requires a reevaluation of the Paleozoic tectonic development of western Gondwana. [4] In this paper, we present evidence from the Sierra de San Luis that documents Early Paleozoic regional metamorphism and deformation prior to juxtaposition along the

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WHITMEYER AND SIMPSON: DEFORMATION OF THE SIERRA DE SAN LUIS, ARGENTINA

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Figure 1. Location map of the Paleozoic tectonic provinces of northwestern Argentina. Modified from Achilli et al. [1997]. PC, Precordillera; NSP, Northern Sierras Pampeanas; SF, Sierra de Famatina; WSP, Western Sierras Pampeanas; SCh, Sierras de Chepes; SL, Sierra de San Luis; SC, Sierras de Co´rdoba; SN, Sierra del Norte. southwestern margin of the Sierras de Co´rdoba. We show that subsequent deformational events within the Sierra de San Luis can be correlated with events in the Sierras de Co´rdoba and likely resulted from one or more phases of the collision of the Precordillera and Chilenia terranes. Finally, we relate the deformational history of the Eastern Sierras Pampeanas to existing evidence for the early Paleozoic growth of the western Gondwana margin, and provide a temporal framework for the suture of the Precordillera terrane.

2. Sierra de San Luis Geology [5] The Sierra de San Luis consists of broad, NNE trending bands of greenschist through upper amphibolite facies pelitic and quartzo-feldspathic rocks [Kilmurray and Dalla Salda, 1977; Ortiz Sua´rez et al., 1992; Prozzi and Ortiz Sua´rez, 1994]. Distinctive suites of Ordovician and Devonian granitoids intrude the metasedimentary rocks [Ortiz Sua´rez et al., 1992; Rapela et al., 1992; Stuart-Smith et al., 1999], and Tertiary volcanic rocks crop out through the center of the region [Brogioni, 1987, 1990; Ramos et al., 1991] (Figure 2). Metasedimentary lithologies are divided

into three principal groups: a western complex of predominantly gneissic and migmatitic rocks (the ‘‘Nogoli Metamorphic Complex’’ of Sims et al. [1997] and ‘‘western basement complex’’ of Von Gosen and Prozzi [1998]; see inset in Figure 3), a central region of slate-grade through sillimanite-grade pelitic and quartzo-feldspathic rocks (includes the ‘‘San Luis Formation’’ of Prozzi and Ramos [1988], ‘‘Phyllite Group’’ of Von Gosen and Prozzi [1998], ‘‘Pringles Metamorphic Complex’’ of Sims et al. [1997], and ‘‘Micaschist Group’’ of Von Gosen and Prozzi [1998]; Figure 3), and an eastern region of biotite-grade through migmatitic quartzo-feldspathic schists, quartzites and minor pelites (the ‘‘Conlara Metamorphic Complex’’ of Sims et al. [1997] and ‘‘eastern basement complex’’ of Von Gosen and Prozzi [1998]; Figure 3). Below we discuss the lithologic and metamorphic characteristics of each region in turn. 2.1. Western Complex [6] The western complex is composed of biotite + muscovite and sillimanite + biotite ± garnet paragneisses and orthogneisses, with abundant local quartz and K-feldsparrich migmatite injections. Foliations strike NNE and dip

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Figure 2. Simplified geologic map of the Sierra de San Luis and southern regions of the Sierra de Comechingones. CB, Chacras batholith; GZ, Guzman zone; LE, La Escalarilla pluton; RC, Rio Claro fault zone; RP, Renca pluton; TA, Tres Arboles fault zone; TF, Trapiche fault zone. steeply west, with less prevalent vertical and steeply east dipping foliations (Figure 4a). In the northwest, occasional intrusions of peraluminous granitic migmatite reach hundreds of square meters in map view. Zones of amphibolite boudins are frequent along the eastern and western margins of the complex, as centimeter- to meter-sized stringers and pods within discrete shear zones. Some of the mafic bodies preserve granulite facies metamorphism within some opxand cpx-rich cumulates. Minor greenschist-grade retrogression is confined to postpeak metamorphism shear zones in the northwest. The eastern margin of the complex is abruptly terminated by an extensive, NNE trending, decameter-thick mylonite and ultramylonite zone, here termed the Rio Claro fault zone (RC, Figure 2). 2.2. Central Region [7] Slates and phyllites crop out within the core of a major upright synform in the south central region of the

Sierra de San Luis (Figures 2 and 3). Foliations strike NNE with predominantly subvertical dips, but vary from steeply east dipping to moderately west dipping (Figure 4g). Chlorite slates occupy the synform core and grade into biotite + chlorite phyllites, then biotite + muscovite schists, and finally sillimanite schists on either side. The lightly metamorphosed pelites and quartzites have been interpreted as a turbiditic sequence correlative with the Vendian to Lower Cambrian Puncoviscana Fm. to the north [Prozzi, 1990; Von Gosen and Prozzi, 1996], although fossils have yet to be identified in the San Luis rocks. Present depositional age constraints are limited to rhyolitic and dacitic ash beds within the phyllites that yielded Cambrian ages (529 ± 12 Ma, discordant U/Pb zircon from four small populations [So¨llner et al., 2000]; Figure 5). [8] On the west limb of the synform, foliations strike NE and dip moderately to steeply to the west (Figure 4c). Within the sillimanite-grade zone, mafic and ultramafic pods and

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Figure 3. Geologic map of the Sierra de San Luis showing isograds of regional peak metamorphism. Inset shows the three regions discussed in text. 4 of 16

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Figure 4. Geologic map of the Sierra de San Luis with lower hemisphere stereographic projections of foliations (triangle) and outcrop-scale fold axes (diamonds). (a) Western complex. (b) – (g) Central region. (h) – ( j) Eastern region.

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Figure 5. Geologic map of the Sierra de San Luis including geochronologic data. Reported analyses include U/Pb crystallization ages of zircons from plutonic rocks (age with line connecting to pluton), U/ Pb crystallization ages of zircons within extrusive rocks (‘‘z’’ indicates locations), U/Pb of metamorphic monazites (‘‘m’’ indicates locations) and 40Ar/39Ar cooling ages of muscovite (‘‘a’’ indicates locations). Data from Brogioni [1993], Krol et al. [2000], Sims et al. [1998], So¨llner et al. [2000], Stuart-Smith et al. [1999], Von Gosen et al. [2002], and L. P. Gromet (personal communication, 2002).

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lenses are common. Sillimanite schists are thrust to the west over biotite and chlorite phyllites along a tens to hundreds of meters thick mylonite and ultramylonite zone, here termed the Trapiche fault zone (TF, Figures 2 and 3). East of the central slates and phyllites, quartzo-feldspathic rocks become more prevalent at the expense of pelites. Garnet is conspicuously absent in sillimanite-grade rocks, perhaps due to the gradual eastward change to more quartz-rich rocks. 2.3. Eastern Region [9] Rocks of the eastern region are predominantly biotite + quartz ± muscovite quartzo-feldspathic schists and quartzites (stipled area; Figures 2 and 3), with abundant quartz injections and locally migmatized regions. Foliations strike N to NNE and dip moderately to the east (Figures 4h and 4j). Pelitic schists are uncommon, and decrease in outcrop frequency to the east. Sillimanite + biotite + muscovite schists are abundant in the south along with K-feldspar migmatites, often near S-type granitic intrusions (e.g., the Chacras batholith and related intrusions; CB, Figure 2). Migmatitic biotite + quartz schists and quartzites occur near the eastern margin of the Sierra de San Luis and along the southwestern margin of the Sierras de Co´rdoba, west of the Tres Arboles Fault Zone (TA, Figure 2). In areas where quartzo-feldspathic rocks predominate, we have estimated peak metamorphic grade from quartz and feldspar microstructures [De Paor and Simpson, 1993; Stipp et al., 2002; Rosenberg and Stu¨nitz, 2003] and presence of migmatitic melt. 2.4. Intrusive Rocks [10] Plutonic rocks consist of pretectonic to syntectonic biotite tonalites and granites, and posttectonic, largely discordant, biotite + muscovite ± garnet granites and granodiorites [Llambias et al., 1998]. Biotite granites in the western half of the Sierra de San Luis are notably elongate along a NNE trend, and often contain a well defined, subvertical, NNE striking magmatic fabric. However, in several locations pluton margins are defined by ductile shear zones (e.g., the Rio Claro pluton and northern tip of the La Escalarilla pluton; RC, LE, Figure 2) with a foliation subparallel to the internal fabric, which suggests syntectonic crystallization. Crystallization ages obtained from this suite of pretectonic to syntectonic granitoids are constrained to Early to Middle Ordovician by U/Pb zircon geochronology from several plutons (490 ± 15 Ma to 468 ± 6 Ma [Sims et al., 1998; Stuart-Smith et al., 1999; Von Gosen et al., 2002]; Figure 5). In addition, recent U/Pb zircon ages from the northern lobe of the elongate La Escalarilla granite (507 ± 24 Ma [Von Gosen et al., 2002] and 482 ± 23 Ma (L. P. Gromet, Brown University, personal communication, 2002) suggest that this section of the La Escalarilla pluton is a member of the EarlyMiddle Ordovician suite of syntectonic granitoids. [11] Plutonic rocks within the eastern half of the Sierra de San Luis are predominantly S-type biotite + muscovite ± garnet granitoids with an elliptical, discordant outcrop pattern. In several locations, plutons crosscut or truncate metamorphic isograds, indicating that emplacement was postpeak metamorphism (Figure 3). Crystallization ages from the eastern, peraluminous granitoids range from

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408 ± 25 Ma (U/Pb zircon [Brogioni, 1993]) for the Chacras batholith (CB, Figure 5) to 393 ± 5 Ma (U/Pb zircon [Stuart-Smith et al., 1999]) for the Renca pluton in the southeast (RP, Figure 5). A minimum age constraint on regional folding of 403 ± 6 Ma [Sims et al., 1998] is provided by a southern outcrop of the undeformed La Escalarilla pluton where it clearly intrudes folded metasedimentary country rocks (Figure 6a). This Devonian age contrasts with the Late Cambrian and Ordovician ages obtained from the northern lobe of the intrusion and suggests a complex and extended intrusional history for this pluton.

3. Deformation 3.1. Pressure Solution Cleavage (D1) [12] The earliest deformational fabric (D1) evident within the Sierra de San Luis is a pressure solution cleavage visible in isolated exposures among biotite phyllites in the southern core of the main synform. Associated centimeter-thick quartz veins are tightly buckled (Figure 6b), with fold axes that generally plunge shallowly to the SSE, and indicate approximately 60% shortening from ESE directed compression (present-day coordinates). Xenoliths of similar, chevronfolded, biotite + quartz schists occur within migmatitic rocks. [13] Peak metamorphism followed D1 deformation in both the Sierra de San Luis and the Sierras de Co´rdoba. Within the Sierras de Co´rdoba, Middle Cambrian peak metamorphism during the Pampean Orogeny occurred at temperatures of 650 to 950C at pressures of 7 – 8 kbar [Rapela et al., 1998; Otamendi et al., 1999]. Central regions of the Sierras de Co´rdoba indicate Pampean-related peak metamorphism within gneisses and migmatitic S-type granites, reaching 810 ± 50C and 8.6 ± 0.8 kbar in the east at 522 ± 8 Ma [Rapela et al., 1998], and 685 – 725C at 4.5 – 5.8 kbar in the west [Baldo, 1992; Gordillo, 1984]. Middle to Late Cambrian high-temperature-low-pressure metamorphism has been documented across northern regions of the Sierras de Co´rdoba at 515 –525 Ma (U/Pb monazite [Rapela et al., 1998; Gromet and Simpson, 1999]), and rapid cooling from the Pampean thermal peak is indicated by muscovite cooling ages of 502 ± 5 Ma [Krol and Simpson, 1999]. At present, there is no evidence for Ordovician high temperature metamorphism within the Sierras de Co´rdoba. Temporal constraints for peak metamorphism within the San Luis range include zircons from amphibolites and sillimanite-grade gneisses just east of the La Escalarilla pluton that yield U/Pb crystallization ages of 478 ± 6 Ma and 484 ± 7 Ma [Sims et al., 1998] (Figure 5), and zircon rims and monazites that provide U/Pb metamorphic ages of ca. 460 Ma and 451 ± 10 Ma [Sims et al., 1998]. Metamorphic monazites from the central and eastern regions yielded Ordovician ages of 453 ± 2 Ma and 470 –482 Ma, respectively (L. P. Gromet, Brown University, personal communication, 2002) (Figure 5). 3.2. Regional Folding (D2) [14] The most significant deformational event within the Sierra de San Luis is a pervasive folding (D2) that has

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Figure 7. Cross sections interpreting geology along three approximately W-E transects across the Sierra de San Luis. Locations of A – A0, B– B0, and C – C0 are indicated on Figure 2. No vertical exaggeration, but note the slight change in scale between A –A0 section and B– B0 section.

affected the entire region. The pattern of NNE trending, alternating metamorphic isograds suggests a regionalscale, postpeak metamorphism folding event, where chlorite-grade phyllites occupy synformal cores, and sillimanite schists ± migmatite ± amphibolite occupy antiformal cores (Figure 3). Upright to west vergent, 1 – 2 km amplitude folds plunge shallowly to the NNE and SSW (Figure 4), and pervasive centimeter to meter-scale parasitic folds mimic the regional pattern (Figure 6c). Axial planar foliations and fold limbs are steeply east to steeply west dipping (Figures 4 and 7), and bedding/cleavage intersections within phyllitic rocks (Figure 6d) also document west vergent folding. Peak metamorphic minerals, such as sillimanite and/or biotite are predominantly oriented parallel to the axial planar foliation. However, within hinge regions of parasitic folds, sillimanite, biotite and muscovite are folded, which indicates mineral growth prior to the folding event (Figure 6e). Garnets are

typically euhedral to subhedral, lack an internal fabric, and likely pretectonic or syntectonic. 3.3. Ductile Deformation (D3, D4) [15] Ductile shear zones (D3) deform the folded metasedimentary rocks in several locations. Extensive upper greenschist- to amphibolite-facies mylonite and ultramylonite zones juxtapose sillimanite-grade schists against biotite grade metasediments along the Rio Claro, Trapiche and Tres Arboles fault zones (Figures 2 and 3). In addition, several small (i.e., 5 cm to 2 m in width), discontinuous mylonite zones enclose discrete pods and stringers of amphibolite and affect rocks east of the La Escalarilla pluton (see Figure 2). [16] The Rio Claro fault zone defines the eastern boundary of the western complex and extends for >50 km along a NNE strike (RC, Figure 2). Subvertical shear zone foliations strike NNE, and stretching lineations are predominantly down dip (Figure 8a). Mylonitic fabrics enclose the 490 ±

Figure 6. (a) Outcrop photo of Devonian granite intruding folded metasedimentary rocks. (b) Outcrop photo of pressure solution cleavage in psammites with buckled quartz vein indicating approximately 60% shortening. (c) Outcrop photo showing east-over-west climbing parasitic folds. (d) Outcrop photo showing bedding-cleavage relationship in biotite-grade metasedimentary rocks. (e) Photomicrograph showing sillimanite needles within foliation around fold. (f ) photomicrograph of syntectonic garnet with fibrolitic sillimanite in pressure shadow regions; principal movement direction is east-over-west. Ford Escort for scale in Figure 6a; coin diameter in Figure 6b is 2 cm.; field book width in Figure 6c is 12 cm.; pencil length in Figure 6d is 10 cm.; scale bars for Figures 6e and 6f are 30 mm and 100 mm, respectively. 9 of 16

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Figure 8. Geologic map of the Sierra de San Luis with lower hemisphere stereographic projections of foliations (triangle) and stretching lineations (dots). (a) Rio Claro fault zone. (b) Lower greenschist-grade shear zone along northwest side of La Escalarilla granite. (c) Lower greenschist-grade shear zone along northeast side of La Escalarilla granite. (d) Lower greenschist-grade shear zone along southeast side of La Escalarilla granite. (e) Trapiche fault zone. (f) ‘‘Guzman’’ deformation zone of Sims et al. [1997]. 10 of 16

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15 Ma (U/Pb zircon crystallization age [Von Gosen et al., 2002]) elongate, Rio Claro biotite granite that itself contains a strong NNE striking, synmagmatic foliation. Sigma and delta porphyroclasts preserve evidence for both east side up and east side down oblique movement, which may indicate a large component of pure shear [Simpson and De Paor, 1997]. However, in places along the shear zone there is evidence of an overprinting lower greenschist-facies deformation, which locally obscures any evidence for a uniform simple shear component. [17] The Trapiche fault zone (TF, Figure 2), composed of upper greenschist- to amphibolite-grade mylonites and ultramylonites, extends the length of the Sierra de San Luis along a NNE to NE strike. Syntectonic garnets exhibit fine-grained fibrolite needles within pressure shadows (Figure 6f ), and indicate predominantly east-over-west movement. Stretching lineations within subvertical foliations predominantly plunge steeply to the NE (Figure 8e), which indicates a component of dextral movement. Quartz microstructures include deformation bands and subgrains with equant recrystallized grains that exhibit a strong lattice-preferred orientation. Feldspar porphyroclasts contain deformation twins and some deformation-induced myrmekite [e.g., Simpson and Wintsch, 1989]. Collectively, quartz and feldspar microstructures suggest that deformation occurred at temperatures between 400 and 500C [De Paor and Simpson, 1993; Stipp et al., 2002; Rosenberg and Stu¨nitz, 2003]. [18] The Tres Arboles fault zone extends for 200+ km along the western boundary of the Sierras de Co´rdoba and is considered to be the eastern boundary of San Luis-related metasedimentary rocks [Whitmeyer and Simpson, 2003] (Figures 2 and 7). Here, a 16-km-thick zone of mylonites and ultramylonites thrusts granulite-grade gneisses and migmatitic rocks to the west over biotite gneisses, quartzites and migmatites. Fault zone foliations dip 30– 50E and locally contain down-dip stretching lineations. Sigma and delta porphyroclasts record east over west movement, although dextral movement indicators and subhorizontal lineations can be seen within nearby, and presumed coeval, small-scale shear zones. Despite the lack of preserved transcurrent kinematic indicators within the main fault zone rocks, Whitmeyer and Simpson [2003] presented arguments that the zone overall records significant transpressive movement. Quartz and feldspar microstructures, syntectonic sillimanite needles and garnet-biotite geothermometry suggest deformation at temperatures of 500+C in the hangingwall to 350 – 450C in the footwall [Whitmeyer and Simpson, 2003]. [19] Localized lower greenschist-facies ductile shear zones (D4) occur along the margins of the La Escalarilla pluton and in the ‘‘Guzman zone’’ of Sims et al. [1997] (GZ, Figure 2). Mylonitic foliations on both sides of the northern extension of the La Escalarilla pluton are parallel to the pluton boundaries, with subvertical or steep dips to the east and west (Figures 8b– 8d). Stretching lineations are mostly down dip to the SSE. Sigma porphyroclasts and stretching lineations indicate east over west and sinistral movement. S/C fabrics defined by chlorite and biotite are common, and in places apparently overprint an earlier

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amphibolite-facies deformation. Local ultramylonitic regions of the Tres Arboles fault zone also preserve evidence of fault reactivation, but lack definitive movement indicators.

4. Discussion [20] Most existing models for the development of the convergent western margin of Gondwana view the Sierra de San Luis and the Sierras de Co´rdoba as a single interrelated tectonic unit beginning with Late Precambrian deposition of coeval turbiditic sequences [Ramos, 1988; Prozzi, 1990; Von Gosen and Prozzi, 1998]. Initial D1 pressure solution cleavage and chevron folding within the Sierra de San Luis and similar structural features in the Tuclame´ Fm., northeast of the Tres Arboles fault zone in the Sierras de Co´rdoba, both indicate orthogonal shortening prior to peak metamorphism [Simpson et al., 2003]. These pressure solution fabrics have been interpreted as the earliest evidence for shortening of the accretionary prism during Pampean orogenesis [Simpson et al., 2003]. However, prism-related metasedimentary rocks potentially extended laterally along the Cambrian Gondwana margin well beyond their present exposure within the Eastern Sierras Pampeanas [Rapela et al., 1998]. Therefore depositional and structural similarities in pressure solution fabrics within the Sierra de San Luis and the Sierras de Co´rdoba do not necessarily constrain their relative positions during the Early to Middle Cambrian. [21] At present there is no evidence for Cambrian high grade metamorphism within the San Luis rocks. Nor is there evidence for a significant Ordovician thermal event in the Sierras de Co´rdoba, with the exception of a few small-scale Ordovician plutons in the north. The disparity in relative timing of peak metamorphism between the Sierras de Co´rdoba (Cambrian) and the Sierra de San Luis (Ordovician) indicates that they could not have been juxtaposed in their present-day close position prior to the Middle to Late Ordovician. [22] The regional pattern of metamorphic isograds in the Sierra de San Luis suggests a postpeak metamorphism regional folding event (D2) with shallow SSW plunges, caused by WNW-ESE compression (present-day coordinates). Folding postdates Early to Middle Ordovician peak metamorphism (482 – 460 Ma) and presumably followed intrusion of the elongate Early to Middle Ordovician plutons (490– 468 Ma). The southwest extension of the La Escalarilla granite (403 ± 6 Ma) intrudes the folded country rocks and brackets regional folding within the Sierra de San Luis between Middle Ordovician and Early Devonian. [23] The simple structure of folded metamorphic isograds in the Sierra de San Luis range is cut by the Rio Claro and Trapiche mylonite and ultramylonite zones. These zones record upper greenschist- to amphibolite-grade deformation conditions, roughly equivalent to peak metamorphic conditions in hangingwall country rocks. D3 shear zone foliations are generally subparallel to country rock foliations produced by the D2 regional folding event (compare Figure 8e to Figure 4c), and sigma and delta porphyroclasts indicate predominantly east-over-west movement with a

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dextral component. Unsheared, folded metasedimentary rocks east of the Trapiche fault zone exhibit equivalent metamorphic conditions and display similar orientations and vergence to fabrics within the fault zone, consistent with the initiation of D3 ductile shearing late in the D2 regional folding event. [24] Sims et al. [1997] suggested that the discrete shear zones east of the La Escalarilla pluton may have facilitated upward emplacement of amphibolites into felsic upper crust, consistent with our representation of the amphibolites as the core of a regional-scale antiform (Figure 7). The mafic rocks would have occupied the deepest level of the San Luis crustal section prior to folding, which suggests a prefolding mafic underplating of the San Luis region. An extensive mafic underplate could have resulted from Famatinian magmatism and/or shallow subduction, and may have provided the regional-scale heat source for Ordovician peak metamorphism across the Sierra de San Luis. Significant Ordovician metamorphism is not in evidence in the Sierra de Comechingones, east of the Tres Arboles fault zone, nor in the rest of the Sierras de Co´rdoba, which indicates that the Sierra de San Luis must have been fairly distant from the Co´rdoba terrane during this period. [25] The kilometers thick Tres Arboles mylonite zone likely facilitated the juxtaposition of the Sierra de San Luis against the Sierras de Co´rdoba along the western margin of the Sierra de Comechingones [Whitmeyer and Simpson, 2003]. Maximum age of deformation is constrained by the Early to Middle Ordovician peak metamorphic event in the Sierra de San Luis. The intrusion of the Achala granite (368 ± 2 Ma, U/Pb zircon [Dorais et al., 1997]) truncates the amphibolite-facies fault zone northeast of Merlo (Figure 2) and provides a minimum age constraint for the initial deformation along the fault zone. Other, smaller, upper greenschist- to amphibolite-grade ductile deformation zones across the Sierra de San Luis, such as the Rio Claro and Trapiche fault zones, may also be related to the emplacement of the Sierra de San Luis adjacent to the Sierras de Co´rdoba. [26] Lower greenschist-grade D4 deformation zones along the eastern and western borders of the La Escalarilla pluton (Figure 2) overprint amphibolite-grade dextral mylonites and record east-over-west and sinistral movement, which indicates a change in movement direction. 40 Ar/39Ar cooling ages of foliation-forming muscovites

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from these D4 shear zones indicate that deformation ended between 376 Ma and 353 Ma [Sims et al., 1998; Krol et al., 2000] (Figure 5). The ultramylonite matrix of the Tres Arboles zone lacks kinematic indicators that would enable conclusive determination of late stage movement, and at present, we cannot rule out possible Late Devonian sinistral oblique reactivation along this fault zone, similar to what we document in the western Sierra de San Luis. The change from predominantly dextral transpression to predominantly sinistral transpression suggests that Late Devonian deformation was a distinct event, although perhaps only by a few million years, and may represent the adjustment of terranes along the western margin of Gondwana.

5. Summary of Tectonic Events 5.1. Early to Middle Cambrian [27] Following the final breakup of the Proterozoic supercontinent Rodinia into Laurentia and Gondwana in the Late Proterozoic [Dalziel, 1997; Karlstrom et al., 2001], Laurentia was positioned off the southwest coast of Gondwana (present-day coordinates). During this period, the Precordillera terrane may have undergone the initial stages of rifting from the Ouachita embayment of southeastern Laurentia (present-day Texas [Thomas and Astini, 1996]; Figure 9a). Disagreement continues regarding the source location of the Precordillera, and other interpretations represent it as an extended ‘‘plateau’’ that remained attached to Laurentia following Neoproterozoic breakup [Dalziel, 1997], or as a suspect terrane that originated along the southwestern margin of western Gondwana (south of the Kalahari craton region [Acenõlaza et al., 2002]; Figure 9a). [28] Coincident with the initial stages of the Paleozoic clockwise migration of Laurentia around Gondwana [Dalziel et al., 1994], east directed subduction initiated off the western coast of Gondwana. The earliest evidence for this is seen in the Sierra del Norte (SN, Figure 9b), an early Cambrian I-type plutonic arc comprising the northeast section of the Sierras de Co´rdoba [Rapela et al., 1998]. During the Early to Middle Cambrian, accretionary prism sediments accumulated outboard of the volcanic arc, eventually comprising most of the metasedimentary rocks exposed in the Sierras de Co´rdoba. As in the Sierra de San Luis, these metasediments are commonly considered to

Figure 9. Summary of Early to Middle Paleozoic tectonic events along the paleo-Pacific margin of Gondwana, as interpreted from metamorphic and deformation fabrics within the Sierra de San Luis. (a) Early-Middle Cambrian, Precordillera terrane (PC) outboard of Laurentia [Thomas and Astini, 1996; Dalziel, 1997] or detached from southwestern Gondwana [Acenõlaza and Toselli, 1988, 2000]; Sierra del Norte (SN) magmatic arc develops along the margin of western Gondwana. (b) Middle-Late Cambrian, development of Sierras de Co´rdoba (SC) accretionary prism along the western margin of Gondwana due to east directed subduction; period of peak metamorphism within the Sierras de Co´rdoba. (c) Early Ordovician, development of the Famatinian magmatic arc with southern extension in the Sierras de Chepes (SCh) and the Sierra de San Luis (SL). (d) Middle Ordovician, dextral oblique convergence of the Precordillera and northeast directed shortening of the Famatinian arc along the western margin of Gondwana; period of peak metamorphism and regional folding within the Sierra de San Luis. (e) Late Ordovician-Early Devonian, northward movement of the Sierra de San Luis relative to the Sierras de Co´rdoba; final suture of the Precordillera terrane to western Gondwana. (f ) Middle-Late Devonian, sinistral deformation and plutonism within the Sierra de San Luis and reactivation of the Tres Arboles fault zone which is potentially produced by sinistral oblique convergence of the Chilenia terrane (CH). 12 of 16

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be the depositional equivalents of the Vendian Puncoviscana turbiditic sequence to the north [Toselli, 1990; Durand, 1996]. Emplacement of the I-type plutonic arc and subsequent accretion and shortening of prism-related rocks probably comprises most of what has been termed the Pampean Orogeny [Acenõlaza et al., 1990], although others have inferred the collision of a microcontinent during this period [Rapela et al., 1998].

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continental plate, considerably west of the present-day location of the Sierra de San Luis. In this scenario, metamorphism of the San Luis metasedimentary rocks is related to early Famatinian plutonism, and regional folding would have occurred during postintrusional shortening of the Famatinian arc as east directed subduction continued along western Gondwana. 5.3. Late Ordovician to Early Devonian

5.2. Early to Middle Ordovician [29] The next significant tectonic episode along the Gondwana margin began with the development of a Famatinian arc west of, and distinct from, the Pampean (Sierra del Norte) arc. The Famatinian magmatic arc was initially recognized within the Sierra de Famatina, northwest of the Eastern Sierras Pampeanas [Acenõlaza and Toselli, 1976; Ramos, 1988] (SF, Figure 1) and consists of metaluminous to peraluminous granitic rocks [Rapela et al., 1992; Saavedra et al., 1992]. Southward extension of the arc includes the voluminous granitic rocks of the Sierras de Chepes [Pankhurst et al., 1998; Saavedra et al., 1998] and likely includes the elongate granitoids of the western Sierra de San Luis [Sims et al., 1997; Stuart-Smith et al., 1999] (Figure 9c). Development of the arc in the Sierras de Chepes and San Luis is constrained to the Early Ordovician by U/Pb and Rb/Sr granitoid crystallization ages of 490– 468 Ma [Pankhurst et al.,1998; Sims et al., 1998; Stuart-Smith et al., 1999; Von Gosen et al., 2002]. The Late Cambrian (507 ± 24 Ma) crystallization age, reported by Von Gosen et al. [2002] for the northern finger of the La Escalarilla pluton, is difficult to interpret within the Famatinian context, however the error bars permit emplacement during the early stages of the Famatinian Orogeny. [30] Early models for emplacement of the Famatinian arc depicted an outboard island arc that was accreted to western Gondwana during the Middle Ordovician [Conti et al., 1996; Toselli et al., 1996]. In contrast, Pankhurst et al. [1998] described the Famatinian magmatic belt as a continental arc that intruded along the Gondwana margin, primarily on the basis of initial Sr ratios and Nd model ages. At present, evidence from the San Luis rocks does not conclusively favor either of the proposed tectonic scenarios. Cambrian ash layers within schists of the Sierra de San Luis central synform are located west of a wide expanse of quartzo-feldspathic rocks, which suggests that an extensive marginal platform separated the San Luis and Co´rdoba blocks, prior to Famatinian-related, E-W compression. Early-Middle Ordovician peak metamorphism is conspicuously lacking within the Sierras de Co´rdoba rocks. This indicates that the Sierra de San Luis was subjected to a significant thermal event while still some considerable distance from the Sierras de Co´rdoba, and that these two components of the present-day Eastern Sierras Pampeanas could not have been juxtaposed prior to the Late Ordovician. No evidence of ophiolitic or other significant ultramafic rocks has been found between the Sierra de San Luis and the Sierras de Co´rdoba, from which we infer that this region does not represent a collisional plate boundary suture. Therefore we propose that the Famatinian arc developed along the western margin of the Gondwana

[31] Movement of the Sierra de San Luis toward its current position was facilitated by the Tres Arboles fault zone along the western border of the Sierra de Comichingones [Whitmeyer and Simpson, 2003]. Fabrics within the fault zone indicate amphibolite-facies deformation conditions and predominantly east-over-west and dextral oblique movement, at least initially. These conditions are duplicated within other high-grade mylonite zones in the Sierra de San Luis and suggest their coeval development. This suite of ductile fault zones may represent the final stages of the Famatinian Orogeny. More probably, the major mylonite zones are part of the initial phase of the Ocloyic Orogeny, an east directed, dextral oblique collisional cycle [e.g., Dalziel, 1997; Acenõlaza et al., 2002] that culminated with emplacement of the Precordillera by the early Devonian [e.g., Pankhurst and Rapela, 1998; Sims et al., 1998; Keller, 1998] (Figures 9d and 9e). [32] It is certainly possible that instead of two distinct tectonic events, the Famatinian and Ocloyic orogenies are better depicted as one extended collisional sequence. In such a model, post-Pampean east directed subduction produced the Famatinian magmatic arc along the western margin of Gondwana, probably after development of a wide, marginal, quartzitic platform. Continued NE directed convergence shortened the Famatinian-related plutonic and metasedimentary rocks, and the approach of the Precordillera terrane ultimately moved the Sierra de San Luis close to its present-day position next to the Sierras de Co´rdoba. 5.4. Middle to Late Devonian [33] The final stage of Paleozoic convergent tectonism included possible reactivation of the Tres Arboles fault zone, sinistral, lower greenschist-facies reactivation of fault zones within the Sierra de San Luis, and widespread, mostly discordant, peraluminous plutonism across the Eastern Sierras Pampeanas (the ‘‘Achalian Orogeny’’ of Sims et al. [1997]). Minor, Late Devonian, sinistral displacement documented in the western Sierra de San Luis may be correlative with local reactivation of the Tres Arboles fault zone, although this is poorly constrained at present. If the Late Devonian shear zones throughout the Eastern Sierras Pampeanas are correlative, this suggests that the Sierra de San Luis was located northwest of the Sierras de Co´rdoba prior to final juxtaposition. In this scenario, the Sierra de San Luis, initially located significantly southwest of the Sierras de Co´rdoba, slides past it to the north during FamatinianOcloyic convergence, before moving southeast to its present position adjacent to the Sierra de Comechingones. [34] Sinistral convergence is also associated with the Devonian suture of Chilenia along the western margin of

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the Precordillera (likely already attached to Gondwana [Astini, 1996; Davis et al., 1999]). Although some workers consider Devonian tectonism as yet another component of the suture of the Precordillera [e.g., Pankhurst and Rapela, 1998], we suggest that the distinct change from dextral oblique to sinistral oblique movement must represent a significant reorientation in the plate convergence vectors. Therefore we agree with Astini [1996], Sims et al. [1998], and Davis et al. [1999] and interpret Middle-Late Devonian tectonism as a distinct event that records the sinistral oblique collision of Chilenia to the

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west of the Precordillera (Figure 9f). This event represents the culmination of Early-Middle Paleozoic convergent tectonism along the western margin of Gondwana.

[35] Acknowledgments. This work was funded by NSF grant EAR 9628158 to C. Simpson and L. P. Gromet. The authors wish to thank the Geological Survey of Argentina (SEGEMAR) for logistical support, and Peter Gromet, Roberto Miro, and Ariel Ortiz Suarez for helpful discussion. Stereoplots were constructed with the aid of StereonetPPC v.6.0.2 by Richard W. Allmendinger. This manuscript was substantially improved by reviews from Sarah Roeske and an anonymous reviewer.

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Congreso de Exploracioń de Hidrocarburos, Serv. Geol. Minero Argent., Buenos Aires. Conti, C. M., A. E. Rapalini, B. Coira, and M. Koukharsky (1996), Paleomagnetic evidence of an early Paleozoic rotated terrane in northwest Argentina: A clue for Gondwana-Laurentia interaction?, Geology, 24, 953 – 956. Dalla Salda, L. H., C. A. Cingolani, and R. Varela (1992a), Early Paleozoic orogenic belt of the Andes in southeastern South America: Result of Laurentia-Gondwana collision?, Geology, 20, 617 – 620. Dalla Salda, L. H., I. W. D. Dalziel, C. A. Cingolani, and R. Varela (1992b), Did the Taconic Appalachians continue into South America?, Geology, 20, 1059 – 1062. Dalziel, I. W. D. (1997), Neoproterozoic-Paleozoic geography and tectonics: Review, hypothesis, environmental speculation, Geol. Soc. Am. Bull., 109, 16 – 42. Dalziel, I. W. D., L. H. Dalla Salda, and L. M. Gahagan (1994), Paleozoic Laurentia-Gondwana interaction and the origin of the Appalachian-Andean mountain system, Geol. Soc. Am. Bull., 106, 243 – 252. Davis, J. S., S. M. Roeske, W. C. McClelland, and L. W. Snee (1999), Closing the ocean between the Precordillera terrane and Chillenia: Early Devonian ophiolite emplacement and deformation in the southwest Precordillera, in Laurentia-Gondwana Connections Before Pangea, edited by V. A. Ramos and J. D. Keppie, Spec. Pap. Geol. Soc. Am., 336, 115 – 138. De Paor, D., and C. Simpson (1993), New directions in structural geology, short course notes, 117 pp., U.S. Geol. Surv., Reston, Va. Dorais, M. J., R. Lira, Y. Chen, and D. Tingey (1997), Origin of biotite-apatite-rich enclaves, Achala Batholith, Argentina, Contrib. Mineral. Petrol., 130, 31 – 46. Durand, F. R. (1996), La transicıón Preca´mbrico-Ca´mbrico en el sur de Sudame´rica, in Early Paleozoic Evolution in NW Gondwana, Ser. Correl. Geol., vol. 12, edited by B. Balsia and F. G. Acenõlaza, pp. 195 – 205, Univ. Nac. de Tucumań, Tucumań, Argentina. Durand, F. R., and F. G. Acenõlaza (1990), Caracteres biofaunı´sticos, paleoecolo´gicos y paleogeogra´ficos de la Formacioń Puncoviscana (Preca´mbrico superior-Ca´mbrico inferior) del noroeste Argentino, in El Ciclo Pampeano en el Noroeste Argentino, Ser. Correl. Geol., vol. 4, edited by F. G. Acenõlaza and H. Miller, 71 – 112, Univ. Nac. de Tucumań, Tucumań, Argentina. Finney, S., J. Gleason, G. Gehrels, S. Peralta, and G. Acenõlaza (2003), Early Gondwanan connection for the Argentine Precordillera terrane, Earth Planet. Sci. Lett., 205, 349 – 359. Gordillo, C. E. (1984), Migmatitas cordierı´ticas de la Sierras de Co´rdoba: Condiciones fı´sicas de la mig-

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Andean Margin of Gondwana, edited by R. J. Pankhurst and C. W. Rapela, Geol. Soc. Spec. Publ., 142, 343 – 367. Prozzi, C. R. (1990), Consideraciones acerca del basamento de San Luis, Argentina, paper presented at XI Congreso Geolo´gico Argentino, Serv. Geol. Minero Argent., San Juan, Argentina. Prozzi, C. R., and A. E. Ortiz Sua´rez (1994), Rocas metamo´rficas de bajo grado en las Sierras Pampeanas (Argentina), paper presented at 7 Congreso Geolo´gico Chileno, Serv. Nac. de Geol. y Miner., Santiago, Chile. Prozzi, C. R., and G. Ramos (1988), La Formacioń San Luis, paper presented at Primeras Jornadas de Trabajo de Sierras Pampeanas, Univ. Nac. de San Luis, San Luis, Argentina, 24 – 26 Aug. Ramos, V. A. (1988), Late Proterozoic-Early Paleozoic of South America: A collisional history, Episodes, 11, 168 – 174. Ramos, V., F. Munizaga, and S. Mahlberg Kay (1991), El magmatismo Cenozoico a los 33S de latitud: Geocronologıá y relaciones tectońicas, paper presented at 6 Congreso Geolo´gico Chileno, Serv. Nac. de Geol. y Miner., Santiago, Chile. Rapela, C. W., B. Coira, A. Toselli, and J. Saavedra (1992), The Lower paleozoic magmatism of southwestern Gondwana and the evolution of the Famatinian Orogen, Int. Geol. Rev., 34, 1081 – 1142. Rapela, C. W., R. J. Pankhurst, C. Casquet, E. Baldo, J. Saavedra, C. Galindo, and C. M. Fanning (1998), The Pampean Orogeny of the southern proto-Andes: Cambrian continental collision in the Sierras de Co´rdoba, in The Proto-Andean Margin of Gondwana, edited by R. J. Pankhurst and C. W. Rapela, Geol. Soc. Spec. Publ., 142, 181 – 218. Rosenberg, C. L., and H. Stu¨nitz (2003), Deformation and recrystallization of plagioclase along a temperature gradient: An example from the Berger tonalite, J. Struct. Geol., 25, 389 – 408. Saavedra, J., E. Pellitero, J. Rossi, and A. Toselli (1992), Magmatic evolution of the Cerro Toro granite, a complex Ordovician pluton of Northwestern Argentina, J. S. Am. Earth Sci., 5, 21 – 32. Saavedra, J., A. Toselli, J. Rossi, E. Pellitero, and F. Durand (1998), The Early Palaeozoic magmatic record of the Famatina System: A review, in The Proto-Andean Margin of Gondwana, edited by R. J. Pankhurst and C. W. Rapela, Geol. Soc. Spec. Publ., 142, 283 – 285.

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C. Simpson and S. J. Whitmeyer, Department of Earth Sciences, Boston University, 685 Commonwealth Avenue, Boston, MA 02215, USA. ([email protected])