Journal of South American Earth Sciences 15 (2003) 749–764 www.elsevier.com/locate/jsames
Paleozoic deformation in the Sierras de Cordoba and Sierra de Las Minas, eastern Sierras Pampeanas, Argentina Carol Simpsona,*, Richard D. Lawb, L. Peter Grometc, Roberto Mirod, C.J. Northrupe a Department of Earth Sciences, Boston University, Boston, MA 02215, USA Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA c Department of Geological Sciences, Brown University, Providence, RI 02912, USA d Direccion Nacional del Servico Geologico, Co´rdoba, Argentina e Department of Geology, Boise State University, Boise, ID 83725, USA
b
Received 1 May 2001; accepted 1 January 2002
Abstract Basement orthogneisses, paragneisses, and migmatites in the Sierras de Co´rdoba of the eastern Sierras Pampeanas of central Argentina represent remnants of a Cambrian arc and accretionary prism that initially formed above a subduction zone along the early Cambrian margin of Gondwana. These basement rocks contain many high strain zones that record major events in the tectonic evolution of the western margin of Gondwana during the latest Proterozoic – middle Paleozoic. Initial orthogonal shortening and chevron folding of the accretionary prism rocks occurred prior to a high temperature/low pressure, relatively static metamorphism and migmatization event during 509– 525 Ma, simultaneous with the Pampean orogeny. Localized deformation along a narrow zone of dextral transpression occurred late in the Pampean cycle. After peak metamorphism, the gneisses and migmatites deformed on east-dipping, greenschist-grade, mylonite-, ultramylonite-, and pseudotachylyte-bearing zones. Kinematic data from a selection of these shear zones include field data, microstructural shear sense indicators, and quartz C-axis fabric asymmetry. Almost all show west-directed, dip-slip reverse fault movement, but most do not achieve major crustal shortening. One major ultramylonite zone in western Sierras de Cordoba may represent a major tectonic boundary with the adjacent Sierra de San Luis rocks. This high strain zone is intruded by the Devonian-aged Achala batholith. Other age constraints include a pseudotachylyte vein that has been dated by 40Ar/39Ar method as mid-Silurian and Ordovician I-type plutonic rocks in the Sierra de Las Minas to the west that are deformed into greenschist-grade protomylonite and pseudotachylyte-bearing zones. Our data are consistent with late Ordovician to mid-Devonian orthogonal deformation throughout the Sierras de Co´rdoba, related to the emplacement of the Precordillera and Chilenia terranes to the west. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Basement gneisses; Shear zones; Metamorphism; Pseudotachylyte-bearing zones
1. Introduction The presence of Grenville-aged basement (Abbruzzi et al., 1993; McDonough et al., 1993; Ramos et al., 1998; Vujovich and Kay, 1998) and cover rocks of Laurentian affinity (Astini et al., 1995; Astini, 1998) in the Precordillera and Pie de Palo ranges of the western Sierras Pampeanas, central Argentina (Fig. 1), provide compelling evidence of middle to late Paleozoic juxtaposition of Laurentian crust against western South America. Important insights and constraints on the tectonic * Corresponding author. E-mail address:
[email protected]. (C. Simpson).
evolution of the Paleozoic margin of Gondwana can be obtained from high-grade metamorphic and plutonic rocks exposed in the eastern Sierras Pampeanas (Fig. 1), which separate the Precordillera terrane from cratonic South America (the Rio de La Plata and Amazon cratons). During the Paleozoic, this region saw the final assembly of Gondwanaland, the development of the Pampean and Famatinian arcs, the accretion of the Precordillera to South America, and the clockwise translation of North America relative to South America en route to a late Paleozoic Pangeaic configuration (Dalla Salda et al., 1992a,b, 1993; Dalziel et al., 1994; Astini et al., 1995; Thomas and Astini, 1996; Dalziel, 1997; Astini, 1998; Dickerson and Keller, 1998; Rapela et al., 1998b).
0895-9811/03/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 8 9 5 - 9 8 1 1 ( 0 2 ) 0 0 1 3 0 - X
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Fig. 1. Location map of the Sierras Pampeanas and Precordillera tectonic provinces of central Argentina. Modified from Achilli et al. (1997). SC ¼ Sierras de Co´rdoba; SCh ¼ Sierras de Chepes; SF ¼ Sierra de Famatina; SL ¼ Sierra de San Luis; SLM ¼ Sierra de Las Minas; SN ¼ Sierra del Norte; and PdP ¼ Sierra de Pie de Palo. Dashed lines indicate approximate boundaries between northern, western, and eastern Sierras Pampeanas.
Elongate north– south basement uplifts of the eastern Sierras Pampeanas are separated from adjacent ranges by deep, asymmetrical basins that likely began in the Carboniferous –Permian as a series of isolated, pull-apart basin depocenters (Fernandez-Seveso and Tankard, 1995) within a right lateral, strike – slip fault system that was later reactivated (Ramos, 1988; Simpson et al., 2001). Basement rocks to the east of the Sierras de Cordoba are buried beneath the Parana´ basin. Since the Tertiary, the eastern Sierras Pampeanas have been uplifted on mainly eastdipping, high-angle reverse faults (Jordan and Allmendinger, 1986). Exposed in the uplifts are parts of a Vendian– Middle Cambrian (Pampean) calc-alkaline arc (Rapela et al., 1998a,b,c; Dalla Salda et al., 1998) that developed above an east-dipping subduction zone beneath the Rio de la Plata craton along the margin of Gondwana. I-type plutonic rocks of the Pampean arc are exposed in Sierra del Norte, northern Sierras de Co´rdoba (Fig. 2), and flanked to the southwest by paragneisses, orthogneisses, psammites and metapelites, lenses of amphibolites and associated calc-silicate rocks, and S-type granites and migmatites (Caminos, 1988; Lucero-Michaut et al., 1995). The metasedimentary sequence has been interpreted as part of an accretionary prism that formed adjacent to the Pampean arc (Dalla Salda et al., 1992a,b, 1993, 1998; Dalziel et al., 1994; Dalziel, 1997; Lyons et al., 1997; Northrup et al., 1998a; Rapela et al., 1998a). A general decrease in peak metamorphic pressures, from 9 kb in the south to 3 kb in the north (Cerredo, 1996; Otamendi and Rabbia, 1996), is consistent with a gentle northward plunge of the metasedimentary basement and cover rocks in the Sierras de Co´rdoba (Northrup et al. 1998a).
The substrate to the accretionary prism rocks is not observed in the eastern Sierras Pampeanas. Durand (1996) and Rapela et al. (1998b) illustrate a ca. 530 Ma reconstruction that puts a Pampean terrane or microcontinent outboard of the Pampean arc. Tectonic models put forward by Kraemer et al. (1995), Rapela et al. (1998b), and Sims et al. (1998) suggest that high temperature metamorphism and migmatization of the sedimentary section formed during a continental collision involving cratonic South America and an accreted Pampean terrane. Rapela et al. (1998b) suggest the Pampean terrane is either a rifted piece of the Rio de la Plata or Arequipa –Antofalla craton or a piece of rifted Grenvillian (Laurentian) basement tectonically distinct from the Precordillera terrane to the west, which has a Laurentian affinity. Herein, we report structural and kinematic observations of oriented samples from some of the principal high strain zones in the Sierras de Co´rdoba and adjacent ranges of the eastern Sierras Pampeanas. Evidence for the accretion of a Pampean terrane or microcontinent with the Pampean arc, as well as for the accretion of the Precordillera terrane in the Ordovician (Famatinian) orogeny, should be preserved in these high strain zones. We present field, microstructural, and crystallographic fabric analyses of the deformed metasedimentary and migmatitic rocks that provide a consistent picture of predominantly orthogonal shortening along this sector of Gondwana during the Paleozoic.
2. Early pervasive deformation A pressure solution cleavage (Fig. 3(a)) is the earliest observed structural fabric. It is locally preserved in Tuclame´ Formation psammites in northern Sierras de Co´rdoba, phyllites in central Sierra de San Luis (Fig. 2), and psammites that crop out along the western margin of Sierras de Co´rdoba. Buckle folds in bedding-parallel quartz veins that cross-cut the pressure solution seams in Tuclame´ Formation psammites show pressure-solved limbs, which indicate significant shortening by solution mass transfer. Tight chevron folds are common in metasedimentary rocks throughout the central Sierras de Cordoba, especially north of the Achala batholith (Fig. 2); they have axial surfaces that strike approximately north– south with a steep easterly dip and moderate north or south-plunging fold axes (e.g. Guamanes area, Fig. 4). The early pressure solution cleavage and chevron folds formed a pervasive structural fabric that was subsequently metamorphosed at high temperature and relatively low pressure to form biotite – sillimanite ^ cordierite schists and gneisses. Peak metamorphic mineral assemblages in the highest grade metapelitic rocks generally contain garnet þ biotite þ sillimanite ^ K-feldspar ^ cordierite,
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Fig. 2. Simplified geological map of the eastern Sierras Pampeanas showing locations of main shear zones discussed in the Sierras de Co´rdoba. Modified from Lucero-Michaut et al. (1995).
with peak metamorphic temperatures of 650 – 850 8C (Cerredo, 1996; Otamendi and Rabbia, 1996). Paragneisses, with the exception of psammites, were migmatized during this event and intruded by peraluminous, cordierite-bearing, S-type granite magmas. The S-type granites are predominantly undeformed but locally contain weak foliations defined by aligned quartz ribbons and mica grains subparallel to compositional banding in the surrounding gneisses. Previously folded metasedimentary rocks are preserved as inclusions in the granite migmatites (Fig. 3(b)), and chevron fold hinges in amphibolite gneisses are mimetically overgrown by prograde metamorphic mineral assemblages (Fig. 3(c)). These observations indicate that the regionally significant, pervasive shortening of the accretionary prism rocks in the eastern Sierras Pampeanas ceased before peak metamorphic conditions were attained.
3. Analyses of high strain zones 3.1. Guamanes zone Several of the mylonite zones in the Sierras de Co´rdoba formed at temperatures of 500 8C and higher (Simpson et al., 2001). One of the largest, the Guamanes zone (Martino, 1993), is exposed southeast of Villa de Soto (Figs. 2 and 4) and extends from the La Puerta region through the village of Candelaria to the south (Fig. 5). Outside the zone, upright, chevron-folded psammites and garnet-biotite gneisses, as well as more massive cordierite-bearing anatexites, generally have compositional banding with NW strike, moderate to steep NE dips, and little or no visible lineation (Figs. 4 and 5). Within the Guamanes zone, reoriented and tightened chevron fold hinges in psammites and bi-silli-gneisses plunge at moderate angles to the S or NNE (Fig. 4).
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Fig. 3. Cambrian-metamorphosed metasedimentary rocks of the Sierras de Co´rdoba. (a) Bedding (S0) and refracted pressure solution cleavage (S1) preserved in the cordierite-bearing psammite outcrop of the Tuclame Formation. (b) Inclusions of previously folded psammitic schists in the outcrop of cordierite-bearing Stype granite of the San Carlos massif, near La Higuera. (c) Thin section of unsheared Las Palmas gneiss at Los Tu´neles shows hornblende (dark) and plagioclase (white) mimetically overgrown onto earlier chevron folds in amphibolite boudin. (d) Thin section of unsheared garnet–biotite gneiss from La Calera. Foliation defined by biotite and quartz–plagioclase layers. (e) Thin section of foliated metapelite gneiss from unsheared lens in Guacha Corral zone. Foliation defined by sillimanite grains (sil) and quartz–feldspar bands (qf) in biotite–feldspar matrix. (f) Outcrop of undeformed Cruz de Cana granite intruded into Cambrian high-grade gneisses near Passo del Carmen (locality on Fig. 5). Photomicrographs (c), (d), and (e) in plane light. Scale bars: (a and f) 5 cm, (b) 10 cm, (c) 100 mm, (d and e) 50 mm.
Layering in marble and calc-silicate lenses in the zone is locally boudinaged and refolded into sheath folds (Martino, 1993). Mineral elongation lineations in the reorientation zone are rare, though where present, they generally plunge to the NE (Fig. 4). Small pegmatite bodies mapped by Roberto Caminos (unpublished map) have a right lateral enechelon distribution (Fig. 5), consistent with the right lateral sense of foliation deflection into the steep zone. In thin sections, paragneisses of the Guamanes zone contain fully recrystallized amphiboles that define foliation, polygonal recrystallization of feldspar grains, and Regime III recrystallized quartz (Hirth and Tullis, 1992) with straight, polygonal grain boundaries and little or no internal strain. Sillimanite grains help define the foliation and lineation when present. Superimposed on the high-grade fabrics in the Guamanes zone are many NNW-striking, steeply ENE-dipping shear
zones, often only a few or tens of meters thick and of middle to lower greenschist-grade, which contain strong down-dip stretching lineations (Simpson et al., 2001). In the La Puerta region (Fig. 5), chevron-folded paragneisses and anatexites are deformed into meter-scale, greenschist-grade mylonite zones that often nucleate on the margins of tourmalinebearing pegmatite veins. Mylonitic foliations in this region strike north– south and have steep dips to the east with very strong down-dip mineral lineations. Deflected foliations, rotated feldspar porphyroclast systems (Fig. 6(a), Passchier and Simpson, 1986), and asymmetrical mica ‘fish’ in the greenschist-grade mylonites indicate an east-over-west shear sense. Oriented samples of quartz – feldspar–white mica mylonites from Candelaria (Fig. 5) have quartz C-axis fabrics (Fig. 7, sample 96-35A) and oblique grain shape orientations from Regime III ribbon quartz (Hirth and Tullis, 1992) that also indicate east-over-west movement.
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Fig. 4. Map of Sierras de Co´rdoba (geological units as in Fig. 2) with structural data from principal zones discussed in text. Lower hemisphere equal area stereonets plotted using SterenoetPPC v. 6.0 courtesy of Richard W. Allmendinger; great circles are cylindrical best fit (only shown where statistically valid).
As in many other greenschist-grade shear zones in Sierras de Co´rdoba, late pseudotachylyte veins cross-cut the mylonite foliations (e.g. Simpson et al., 2001). Some veins in the La Puerta area are more than 20 cm thick. 3.2. La Calera zone A thin sliver of granulite-grade rocks crops out in a major Tertiary reverse fault system in the La Calera region on the easternmost flanks of the Sierras de Co´rdoba (Figs. 2 and 8). The only other outcrops of granulite-grade gneisses in the region occur in the southern Sierras de Co´rdoba, south of the Achala batholith (Fig. 2, Otamendi et al., 1999). In the La Calera region, unsheared orthogneisses locally contain hypersthene (750 8C and 6.0 –6.5 kb; Gordillo and Lencinas, 1979; Gordillo, 1984), but the majority of the outcrops are garnet – biotite gneisses (Fig. 3(d)). Foliations in orthoand paragneisses, tonalite sills, and amphibolite and marble lenses strike NE or NW with steep easterly dips and little or no lineation. When present, lineations plunge steeply down dip (Fig. 4). The amphibolite-grade rocks are transected by a 200 m thick, north– south-striking high strain zone west of La Calera (Fig. 8). Unsheared garnet – biotite – sillimanite gneisses, marbles, and calc-silicate rocks are locally deformed into mylonites and ultramylonites. The mylonite foliations generally have easterly dips greater than 808 with a strong, down-dip mineral elongation lineation. The mylonites contain dynamically recrystallized amphibole,
Regime III (Hirth and Tullis, 1992) recrystallized quartz grains, and sigma-type K-feldspar porphyroclasts (Fig. 6(b)). Ultramylonites contain delta-type K-feldspar porphyroclasts and lenticular, relict garnet grains with asymmetrically distributed sillimanite needles in their pressure shadow regions. All field and thin-section shear sense indicators are consistent with a top-to-west, reverse fault movement. Narrow pseudotachylyte veins are contained within and restricted to the mylonitic foliation in outcrops on the banks of the Rio Primero (Fig. 8). A biotite-fusion region of one of these veins (Fig. 6(c)) was used for 40Ar/39Ar spot fusion analysis (Fig. 8 inset; Northrup et al., 1998b). 3.3. Rio Guacha corral zone In the Rio Guacha Corral valley, south of the Achala batholith (Fig. 2), sillimanite-grade paragneisses are transected by a localized zone of shearing (Martino et al., 1995). Lenticular quartz lenses and gneissic fabrics in the metapelites (Fig. 3(e)) are preserved between top-to-west shear bands that crop out adjacent to a Tertiary fault. Stretching lineations in the quartz-rich lithons plunge gently northeast. Quartz C-axis measurements obtained from oriented samples of the 10 cm thick quartz lenses between shear bands show little or no asymmetry (Fig. 7, sample 629). Quartz microstructures in the lenses indicate high temperature and Regime III quartz recrystallization (Hirth and Tullis, 1992), as do microstructures in the adjacent unsheared gneisses.
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Fig. 5. Sketch geological map of region southeast of Villa de Soto, northern Sierras de Co´rdoba. Modified from Caminos (1988). All units except granitoids were metamorphosed to peak conditions in the middle Cambrian. Granite of Passo del Carmen dated at 474 ^ 5 Ma (U –Pb zircon age; Gromet and Simpson, 1999). Achala granite dated at 368 ^ 2 Ma (Dorais et al., 1997). Heavy dashed lines mark the approximate boundaries of the Guamanes (Martino, 1993) shear zone, in which regional WNW-striking foliation is deflected to a more northerly orientation.
The shear bands generally strike NW and dip gently to moderately to the east (Fig. 4) with down-dip stretching lineations defined by new sillimanite needles. The shearbanded zone is locally crossed by NNW-striking sinistral strike – slip faults that contain slickenlines defined by chlorite. These faults are part of a Tertiary fault reactivation event in the Rio Guacha Corral valley region (Simpson et al., 2001) that is likely responsible for local reorientation of shear bands and small ultramylonite zones into unusual southwesterly dips. The shear bands contain a mineral elongation lineation that plunges 308 toward 2508. Quartz Caxes from quartz ribbons in these shear bands (Fig. 7, sample 632) show a top-down-to-west asymmetry, in good agreement with the field observations.
3.4. Ambul zone In the region near Ambul, on the northern margin of the Achala batholith (Fig. 2), moderately dipping mylonite zones and broad areas of retrogressed ultramylonite occur in S-type granite gneisses immediately east of a presumed Tertiary brittle fault. Relict garnet and feldspar porphyroclasts occur throughout the high strain zones. Mylonitic foliations are gently folded around a shallowly NNEplunging axis (Fig. 4). The sense of shear from mica fish, shear bands, folded quartz stringers, and oblique grain shape preferred orientation in quartz is consistently top to the WSW. Quartz ribbon grains in the mylonites (Fig. 7, samples 619, 621, 622) and cm scale quartz veins (Fig. 7,
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Fig. 6. Photomicrographs of mylonitic rocks from Sierras de Co´rdoba. (a) Sigma-type asymmetrical feldspar porphyroclast in biotite-rich mylonite, La Puerta, Guamanes zone, shows top to left (west) displacement. (b) Sigma-type plagioclase porphyroclast in biotite-rich ultramylonite from La Calera zone. Core of grain mantled by rotation-recrystallized new grains; dynamically recrystallized tails show top to left (west) displacement. (c) Pseudotachylyte vein from La Calera zone used for 40Ar/39Ar analysis. Fusion-melted biotite (bottom, gray) with iron oxide-rich border (middle, black) injected into sheared granitic gneiss (top, light). Dark particles in biotite fusion region are iron oxide; light inclusions throughout vein are unmelted feldspar clasts. (d) Regime I grain boundary migration recrystallization in sheared quartz vein, Ambul region. (e) Quartz grains in unsheared gneiss of Los Tu´neles road section. Grains show little or no internal strain; straight, smooth grain boundaries indicate high temperature grain boundary mobility. (f) Quartz subgrains and rotation-recrystallized new grains in mylonitic quartz vein, sample 585, La Higuera zone (see Fig. 10). Note the strong grain shape preferred orientation in addition to the lattice-preferred orientation. Scale bars: (a –c) 50 mm, (d and f) 30 mm, (e) 100 mm. Plane light for (a and c); nicols crossed in (b, d, e, and f).
sample 619V) show grain boundary migration (Fig. 6(d)) and rotation recrystallization textures typical of Regime Ilower Regime II quartz recrystallization (Hirth and Tullis, 1992). The strongly asymmetrical quartz C-axis patterns from these greenschist-grade mylonites indicate top-to-west movement. A few thin, right lateral shear zones with NS strike, moderate east dip, and a moderately NE-plunging lineation have been found in the Ambul area, but they tend to be insignificant in both size and displacement. The retrogressed ultramylonites near Ambul often do not contain an obvious foliation or lineation, even in thin sections (Whitmeyer and Simpson, 2003). Relict rounded garnet and feldspar porphyroclasts are common, and occasional porphyroclasts of muscovite-bearing granite can be found, but asymmetrical porphyroclast systems or recrystallized tails are seldom observed. Occasionally,
foliations are discerned in the ultramylonites, which generally dip moderately east, subparallel to adjacent gneissic foliations. Lineations, when present, plunge moderately to the northeast. On the south side of the Achala batholith, the Ambul zone continues as a major belt of mylonites and ultramylonites known as the Tres Arboles zone, east of Merlo (Fig. 2, Whitmeyer and Simpson, 2003). Moderately eastdipping zones of ultramylonite, tens to hundreds of meters thick, transect pegmatite-rich, garnet – sillimanite –cordierite anatexite. Outside these high strain zones, gneissic foliations dip moderately to steeply east with rare down-dip mineral elongation lineations (Fig. 4). The ultramylonites and mylonites of the hanging wall contain rotated feldspar porphyroclasts, relict stable garnet clasts, and new sillimanite grains (Whitmeyer and Simpson, 2003). Foliation is
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Fig. 7. Lower hemisphere stereographic projections of quartz C-axes, contoured in multiples of uniform distribution, as indicated. n ¼ number of data points. Ambul, Candelaria (Guamanes), and Guacha Corral shear zones located on Figs. 2 and 4.
difficult to detect, and mineral elongation lineations are extremely rare in the more massive ultramylonites, which may reach 16 km thick east of Merlo (Whitmeyer and Simpson, 2003). Rotated feldspar asymmetry, when present, indicates top-to-west movement. 3.5. Los Tu´neles zone A major reverse fault zone with significant westward displacement crops out along the western Sierras de Co´rdoba (Fig. 2). This zone is well exposed in the Los Tu´neles road section (Fig. 2) and may connect with the high strain zones of Ambul and those between Merlo and Guacha Corral (Whitmeyer and Simpson, 2003). In the Los Tu´neles area, sillimanite and K-feldspar-bearing Las Palmas gneisses (Gordillo 1984) in the hanging wall are thrust over chloritic phyllites (Fig. 9), which show a pressure solution cleavage and contain numerous kink bands and small-scale chevron folds. The hanging wall gneisses contain boudins of pegmatites and amphibolite. Relict chevron folds throughout the gneiss were
formed prior to peak metamorphic conditions (Fig. 3(c)). Gneissic foliations generally dip moderately to steeply northeast (Fig. 9) and contain down-dip mineral lineations defined by biotite and sillimanite grains (Fig. 4). The Las Palmas gneiss is retrogressed and deformed into a 2 km thick zone of west-directed shear bands (Fig. 9, Whitmeyer and Simpson, 2003). The shear bands dip more gently northeast than does the gneissic foliation and are typically 5 –10 cm apart, decreasing in spacing and becoming more structurally chlorite-rich down the section toward the thrust plane. Below this are kinked phyllites. Sheared rocks immediately above the fault contact with the underlying phyllites contain cm-scale pseudotachylyte veins that are younger than the shear bands (Fig. 9, Simpson et al., 2001). At the top of the section, in the weakly sheared Las Palmas gneiss, amphibole grains on the sheared margin of an amphibolite boudin have a strong grain shape and latticepreferred orientation and are fully recovered from internal strain. Boudinaged sillimanite grains help define
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movement sense, consistent with the outcrop observations of shear bands. 3.6. La Higuera zone In the footwall of a major Tertiary thrust fault near La Higuera (Fig. 2), cordierite-bearing granite is cut by many thin (cm-scale or smaller) mylonite, cataclasite, and ultracataclasite zones that preferentially localize along narrow, quartz – feldspar–tourmaline veins. These small zones have steep northeast or southwest dips, and almost all contain steeply plunging lineations (Fig. 4). Rotated porphyroclasts, shear bands, and oblique grain-shaped fabrics in the mylonites indicate an overall top-to-west shear sense. Many of the quartz-rich veins contain welldeveloped subgrains (Fig. 6(f)) and recrystallized new grains typical of Regime II recrystallization (Hirth and Tullis, 1992). Quartz C-axes from ribbon grains in the mylonitic veins (Fig. 10, samples 582, 585, 588, 589) show strongly asymmetrical preferred orientation patterns consistent with an overall top-to-west movement. 3.7. Sierra de Las Minas
Fig. 8. (a) Simplified geological map of region near La Calera, eastern Sierras de Co´rdoba, modified from Gordillo and Lencinas (1979). (b) Plot of 36Ar/40Ar against 39Ar/40Ar for biotite-fusion sector of pseudotachylyte vein from starred locality (see Fig. 6(c)). Analyses obtained by C.J. Northrup and M.A. Krol using the CLAIR Argon facility at MIT.
the lineation in more aluminous gneisses nearby. High temperature, Regime III quartz recrystallization (Hirth and Tullis, 1992) in this sample is indicated by the mm-scale grain size, sutured grain boundaries, and absence of internal strain (Fig. 6(e)). Discontinuous ribbon quartz grains in quartz –mica layers from felsic gneiss preserved in lenses between shear bands show symmetrical quartz C-axes that indicate predominantly coaxial deformation (Fig. 10, sample 605). Several quartz vein samples from sheared gneisses near the lowermost thrust plane contain quartz grains with patchy undulose extinction and deformation bands that indicate a lower temperature overprint on the original high temperature fabric. Only one sample was found to contain asymmetrical quartz C-axis fabrics (Fig. 10, sample 598), which indicate a top-to-west (thrust)
Ordovician I-type calc-alkaline plutonic rocks (Pankhurst et al., 1996; Pieters and Skirrow, 1997) form the bulk of the Sierra de Las Minas (Fig. 2). On the eastern flank of the range, near the village of Ulapes, the calc-alkaline pluton is deformed into narrow zones of middle greenschistgrade protomylonites and Type I S/C mylonites (Lister and Snoke, 1984) that contain occasional pseudotachylyte veins. Foliations vary in strike from north to northwest with a gentle to moderate west dip and down-dip lineation defined by quartz ribbons, recrystallized feldspar streaks, and elongate biotite clots. Asymmetrical fabrics in the protomylonites are not well developed in either hand specimen or thin section, but shear bands and quartz C-axes from the eastern side of the pluton indicate both west-side-down (Fig. 11, sample 660) and west-side-up movement (Fig. 11, samples 660A, 662, 663). On the western side of the range, S/C mylonites have S-planes that strike NNW and dip steeply east, as well as C-planes that dip more gently to the east, with a clear top-to-west deflection and strong down-dip mineral elongation. Quartz C-axes from this area also indicate a top-to-west, reverse fault asymmetry (Fig. 11, sample 666).
4. Age constraints on deformation Protolith ages for the metasedimentary rocks of the Sierras de Co´rdoba are poorly constrained. These moderate to high-grade metasedimentary rocks have been correlated along-strike to the north with the structurally shallower, weakly to nonmetamorphosed, Vendian-aged (Durand, 1996), turbiditic, volcanic, and calcareous rocks of
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Fig. 9. (a) Sketch map and (b) schematic cross-section of the Los Tu´neles high strain zone. Younger, brittle faults have reoriented, brecciated, and retrogressed much of the gneiss immediately above the fault contact with chlorite-rich phyllites.
the Puncoviscana Formation (Willner and Miller, 1986; Toselli, 1990; Willner, 1990; Rapela et al., 1998b). Numerous inclusions of folded psammites and biotite schists in the San Carlos S-type granite massif (Fig. 3(b)) illustrate that much of the early shortening of the accretionary prism rocks occurred prior to peak metamorphism and migmatization. Cordierite-bearing S-type granites give a 522 ^ 8 Ma U – Pb SHRIMP monazite age for the migmatization in the Sierras de Co´rdoba (Rapela et al., 1998c). U – Pb ages of 515 – 520 Ma for metamorphic monazites were obtained from central Sierras de Co´rdoba paragneisses (Gromet and Simpson, 1999). These monazite ages are consistent with the U –Pb zircon and monazite data of Rapela and Pankhurst (1996), Lyons et al. (1997), Sims et al. (1998) and Rapela et al. (1998b) for peraluminous bodies in other parts of the eastern Sierras Pampeanas. A sample of foliated calc-silicate from the steep zone of the Guamanes belt, near Candelaria (Fig. 5), yielded a 509 ^ 2 Ma U – Pb metamorphic titanite age for neocrystallized grains and a 490 ^ 2 Ma U –Pb apatite cooling age (Fantini et al., 1998). Muscovite from the Sierras de Co´rdoba S-type granite gave an integrated apparent 40Ar/39Ar cooling age of 502 ^ 5 Ma (Krol and Simpson, 1999). These data are consistent with the relatively rapid cooling of S-type granites and their host gneisses at 650 8C or higher and peak sillimanite-grade
conditions at ca. 510 –520 Ma to approximately 350 8C at ca. 500 Ma. This apparently brief but intense Cambrian (Pampean) thermal event assists us in distinguishing shear zones in the area coeval with the high temperature event from younger, lower grade shear zones. The undeformed, 474 ^ 5 Ma (U – Pb zircon age; Gromet and Simpson, 1999) Cruz de Can˜a granite pluton sharply cross-cuts the reoriented paragneiss foliation on the western edge of the Guamanes shear zone (Figs. 3(f) and 5). Its immediate wall rocks give 515 ^ 2 Ma prograde metamorphic monazite ages (Gromet and Simpson, 1999). These results indicate that the main reorientation of fabrics into the Guamanes steep zone occurred before the middle Ordovician. Retrogressed Ambul ultramylonite and the thick Tres Arboles ultramylonite zone are both sharply cross-cut by undeformed granite of the 368 ^ 2 Ma Achala batholith (U – Pb zircon; Dorais et al., 1997, Fig. 2). Ordovician I-type calc-alkaline plutonic rocks (Pankhurst et al., 1996; Pieters and Skirrow, 1997) in the Sierra de Las Minas exhibit protomylonitic fabrics associated with pseudotachylyte veins. Almost all the mylonite zones in the Sierras de Co´rdoba are cross-cut by pseudotachylyte veins that are rarely found in unsheared rocks. A biotite fusion region of one of these veins from La Calera (Fig. 6(c)) was dated at 428 ^ 12 Ma (MSWD ¼ 0.71) by 40Ar/39Ar spot fusion analysis (Fig. 8, Northrup et al., 1998b). Available data do
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Fig. 10. Lower hemisphere stereographic projections of quartz C-axes, contoured in multiples of uniform distribution, as indicated. n ¼ number of data points. La Higuera and Los Tu´neles shear zones located on Figs. 2, 4, and 9.
not permit any tighter constraints on the ages of most of the post-Pampean mylonite and ultramylonite zones.
5. Discussion Early pervasive shortening of the presumed Vendianaged sedimentary rocks in the Sierras de Co´rdoba region was accomplished by pressure solution and folding at relatively shallow depths into upright chevron and tight folds, now preserved in psammite lenses in migmatites. Evidence for major thrusts that may have shortened the premetamorphic rocks has not yet been identified in the field, though repetitions of marble and amphibolite zones on a regional scale throughout Sierras de Co´rdoba may point to significant early duplication. Preservation of chevron folds in metasedimentary inclusions in S-type granites of middle Cambrian (Pampean) age and mimetic recrystallization of chevron fold hinges preserved in massive amphibolites indicate that much of the shortening of the sedimentary
section was accomplished prior to peak metamorphism at 520 Ma. Mainly orthogonal compression is supported by the presence of many upright chevron folds and the lack of asymmetry in the internal fabrics of the gneisses. Symmetrical C-axis fabrics, such as those in the high temperature lenses of Guacha Corral (Fig. 7, sample 629) and Los Tu´neles (Fig. 10), are consistent with a nonrotational strain path and may be typical of the post-folding, preshearing peak metamorphic event found throughout the region. The Pampean metamorphic event, though widespread, did not produce significant new fabrics in the S-type granites, and the 502 ^ 5 Ma (Krol and Simpson, 1999) muscovite cooling ages for the S-type granite migmatites are consistent with the relatively rapid cooling of the region. However, a local reorientation of the high-grade metamorphic fabrics into north-striking steep zones, such as the Guamanes zone, occurred at around 515 Ma, a late stage in the Pampean orogenic cycle (Gromet and Simpson, 1999). The right lateral deflection of the fabrics and the down-dip
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Fig. 11. Lower hemisphere stereographic projections of quartz C-axes from shear zones in Ordovician I-type granite near Ulapes, eastern Sierras de Las Minas (samples 660, 660A, 662, 663) and San Isidro, western Sierras de Las Minas (sample 666), contoured in multiples of uniform distribution, as indicated. n ¼ number of data points.
lineations in the Guamanes zone suggest a shift from predominantly orthogonal compression to oblique-slip transpression (e.g. Tikoff and Teyssier, 1994) in the region at that time. Throughout the region, postmigmatite mylonite and ultramylonite zones show clear east-over-west, dip-slip movement, which is difficult to reconcile with any significant strike – slip component of terrane emplacement immediately to the west at the time of formation. Foliations in mylonites and ultramylonites affect Ordovician I-type plutons in the Sierras de Las Minas, and at least two zones, Ambul and Guacha Corral, are cut by the Devonian Achala granite. Therefore, the main mylonite zones most likely formed during the Ordovician Famatinian orogeny (Simpson et al., 1998) in response to renewed orthogonal contraction. Lower greenschist-grade protomylonites and pseudotachylytes represent deformation in or close to the brittleductile transition zone at temperatures below 500 8C and at about 12 km depth (Sibson, 1977, 1986). Pseudotachylyte veins in the Ordovician calc-alkaline plutonic rocks of the Sierras de Las Minas, as well as in most mylonite zones in the Sierras de Co´rdoba, and the pseudotachylyte formation age of 428 ^ 12 Ma from the La Calera region suggest that the high-grade basement rocks of the eastern Sierras de Co´rdoba had begun to reach upper crustal levels by at least the late Silurian. In general, structural blocks juxtaposed westward across the dip-slip, reverse shear zones in the eastern Sierras
Pampeanas display similar metamorphic conditions, thus indicating little postmetamorphic differential movement. The lack of major crustal offset is inconsistent with previously mapped lengths of several hundred km for the smaller high strain zones (e.g. Kraemer et al., 1995), unless they are reactivated early normal or strike – slip faults (Simpson et al., 2001). The presence of granulite facies rocks near La Calera could indicate significant crustal shortening across that mylonite zone at the time of its movement, but the narrowness of the high strain zone and the presence of Cretaceous and Tertiary faults in the area (Fig. 6) leaves open the possibility that the uplift of the granulite facies rocks occurred during a younger fault reactivation event (e.g. Simpson et al., 2001). The more regionally significant Los Tu´neles– Ambul-Tres Arboles reverse fault zone, which places sillimanite-grade gneiss westward on phyllites and psammites, may represent a major tectonic boundary with Ordovician-aged metamorphic rocks of the Sierra de San Luis to the west (Whitmeyer and Simpson, 2003). Although we have yet to find any convincing kinematic evidence for significant early strike –slip displacement in most of the long and narrow mylonite zones, Whitmeyer and Simpson (2003) have suggested that the very thick ultramylonites of the Tres Arboles zone may be a result of early strike – slip displacement along that zone. The presence of right lateral strike –slip zones in Ambul further supports this possibility. Several of the major zones, including the Los Tu´neles zone,
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were reactivated during younger brittle faulting events in the region, which may have affected their currently exposed thickness and apparent offset (Simpson et al., 2001). Permian– Cretaceous and Tertiary brittle faults are most likely responsible for the reorientation of crustal blocks and rotation of foliations from their more usual steep easterly dips into areas with westerly dips. Published work on the Ross and Delamerian orogenies of Antarctica and Australia (Bradshaw et al., 1985; Wright and Dallmeyer, 1991; Flo¨ttmann et al., 1993; Goodge et al., 1993; Sandiford et al., 1995; Goodge and Dallmeyer, 1996) indicates that a major early to middle Cambrian calcalkaline arc extended along a significant part of the margin of Gondwana. We agree with other authors (e.g. Dalla Salda et al., 1992a,b, 1993; Dalziel et al., 1994; Lyons et al., 1997; Northrup et al., 1998a) that the eastern Pampeanas arc and accretionary complex is an extension of that complex. The age, structural style, and petrologic characteristics of the eastern Pampeanas arc are remarkably similar to those in the Ross and Delamerian orogens (Acen˜olaza and Miller, 1982; Northrup et al., 1998a). For example (1) Gondwana was the upper plate in both locations, (2) early phases of arc-related tectonothermal activity in the Ross and Delamerian system occurred between 540 and 500 Ma (Bradshaw et al., 1985; Wright and Dallmeyer, 1991; Flo¨ttmann et al., 1993; Goodge et al., 1993; Goodge and Dallmeyer, 1996), (3) plutonism and metamorphism in the eastern Pampeanas arc occurred between ca. 530 – 510 Ma (Rapela and Pankhurst, 1996; Lyons et al., 1997; Gromet and Simpson, 1999), and (4) upper amphibolite to lower granulite facies high temperature/low pressure metamorphism is characteristic of both the Ross and Delamerian (e.g. Sandiford et al., 1995) and the eastern Pampeanas arc systems (e.g. Cerredo, 1996; Otamendi and Rabbia, 1996; Northrup et al., 1998a). Crystallization ages of I-type plutons in the easternmost Sierras Pampeanas indicate that arc magmatism stopped or decreased significantly during latest Cambrian (Rapela and Pankhurst, 1996; Lyons et al., 1997). The termination or reduction of Pampean magmatic activity could have been caused by a sudden change in relative plate movements across the margin, such as the subduction of a mid-ocean spreading center (Northrup et al., 1998a; Gromet and Simpson, 2000). Ridge subduction would not only shut off arc magmatism, but also provide a heat source for the high temperature/low pressure metamorphism and peraluminous magmatism that affected the chevron-folded metasedimentary rocks of the accretionary prism through juxtaposition of the asthenosphere against the accretionary prism base. The duration of the high temperature/low pressure metamorphic event in the Sierras de Co´rdoba was rather short; cooling from the peak temperatures at 520 Ma to the muscoviteblocking temperature for the 40Ar/39Ar system occurred in less than 20 my. The local zones of Pampean-aged, high temperature, right lateral deflection of foliations in the Sierras de Co´rdoba may reflect a change in plate boundary
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configuration from mainly orthogonal subduction to dextral oblique slip transpression. Subduction appears to have been reestablished along the same margin during the development of a new arc outboard (now west) of the abandoned eastern Pampeanas arc during the early stage of the Ordovician-aged Famatinian (equivalent to Taconic) orogen (Dalla Salda et al., 1992a; Pankhurst et al., 1996). Subsequent to the development of this arc (remnants of which are found in the I-type granitic plutons of the San Luis and Las Minas ranges), the Precordillera terrane was emplaced to the west. According to Dalla Salda et al. (1992a) and Dalziel et al. (1994), this collisional event occurred during the closing stages of the Famatinian orogeny. Pieters and Skirrow (1997) and Sims et al. (1998) report Ordovician 40Ar/39Ar muscovite ages for three shear zones in the Sierras de San Luis and Chepes and suggest that other, small-scale shear zones in that region may be as young as Devonian. The greenschist facies protomylonite zones that affect Ordovician granitic rocks near Ulapes in the Las Minas range and leucogranites in the Sierra de San Luis (Sims et al., 1997, 1998) may be related to this late Famatinian event. A major collisional event that involved emplacement of the Precordillera and other terrranes in the late Famatinian could also explain the thick, pre-Achala batholith, Ambul-Tres Arboles ultramylonite zone between the Cambrian-aged metamorphic rocks of the Sierras de Co´rdoba and the Ordovician-aged metamorphic rocks of the eastern Sierras de San Luis (Whitmeyer and Simpson, 2003). Ramos (1984) and Ramos et al. (1986, 1998) suggest that a late Devonian – Early Carboniferous Achalan orogenic event involved the accretion of a Chilenia terrane to the northwest of the Precordillera. The reported 40Ar/39Ar formation age of 428 ^ 12 Ma (Northrup et al., 1998b) from the La Calera pseudotachylyte and the presence of late pseudotachylyte veins in the majority of greenschist facies deformation zones suggest that the collision of the Chilenia block may have been initiated in the Silurian. In addition, reactivation of older faults during the Silurian-mid-Devonian may have been felt as far east as the Sierras de Co´rdoba (Simpson et al., 2001). Although there is little direct evidence for a major late Devonian orogenic or suturing event in the eastern Sierras Pampeanas, the several late Devonian, mainly posttectonic granites (Sims et al., 1997) throughout this region may have formed in response to the termination of Chilenia’s emplacement.
6. Conclusions Structural data from the metamorphic rocks in the eastern Sierras Pampeanas imply mainly orthogonal contraction of a Cambrian accretionary complex on the margin of Gondwana prior to the high temperature/low pressure metamorphism that resulted in migmatization and emplacement of S-type granites. Evidence of high temperature
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deformation that was syn- to postmigmatite formation is very slight and may be limited to the Guamanes dextral transpression shear zone. This early part of the tectonic history of the eastern Sierras Pampeanas is similar, and may be related, to that of the Ross and Delamerian orogenies, which occurred elsewhere along the Gondwana margin during the same time interval. Continued shortening of the metasedimentary rocks after peak metamorphism had waned was accomplished on discrete, east-dipping, greenschist-grade mylonite zones that consistently display an east-over-west, dip-slip movement sense. Evidence for right lateral strike – slip deformation at this time is only known from small zones in the region of Ambul. The predominantly dip-slip mylonites and associated ultramylonites are likely the result of accretion of the Precordillera and other terranes to the west during the Famatinian orogeny. Late stage pseudotachylyte veins, one of which has been dated as mid-Silurian, are found in almost all the high strain zones, including the Ordovician plutonic rocks of Sierra de Las Minas. The pseudotachylytes indicate that the metasedimentary rocks of the Sierras de Co´rdoba and adjacent areas reached the brittle – ductile transition level in the upper crust by mid-Silurian. Reactivation of the high strain zones to form pseudotachylyte veins most likely occurred during the emplacement of the Chilenia terrane in the Silurianearly Devonian.
Acknowledgements This work was supported by NSF grants EAR 93-04326 to R.D. Law, EAR-9628158 and EAR-9903166 to C. Simpson, and EAR-9628285 to L.P. Gromet. We are very grateful to Roberto Martino of Co´rdoba University for introducing CS and RDL to the high strain zones in the area. We thank Kip Hodges for the use of the CLAIR Argon facility at MIT. Mike Krol’s technical assistance with the argon analyses is gratefully acknowledged. The Servicio Geolo´gico Argentino (SEGEMAR) provided much-appreciated logistical support. Detailed, anonymous reviews helped us improve portions of the text. Finally, we acknowledge the contributions and geologic insights of the late Dr Roberto Caminos of SEGEMAR, whose maps clarified the basement geology of the Sierras de Co´rdoba.
References Abbruzzi, J., Kay, S.M., Bickford, M.E., 1993. Implications for the nature of precordilleran basement from precambrian xenoliths in miocene volcanic rocks San Juan Province, Argentina. XII Congreso Geolo´gico Argentina Actas III, 331–339. Acen˜olaza, F.G., Miller, H., 1982. Early Paleozoic orogeny in southern South America. Precambrian Research 17, 133–146. Achilli, F., Anselmi, G., Ardolino, A., Blasco, G., Gzaminos, R., Cobos, J.,
et al., 1997. Mapa geolo´gico de la Repu´blica Argentina, Servicio Geolo´gico Minero Argentino, Buenos Aires. Astini, R.A., 1998. Stratigraphical evidence supporting the rifting, drifting, and collision of the Laurentian Precordillera terrane of western Argentina. Geological Society, London, Special Publications vol. 142, 11–34. Astini, R.A., Benedetto, J.L., Vaccari, N.E., 1995. The early Paleozoic evolution of the Argentine Precordillera as a Laurentian rifted, drifted, and collided terrane: a geodynamic model. Geological Society of America Bulletin 107, 253– 273. Bradshaw, J.D., Weaver, S.D., Laird, M.G., 1985. Suspect terranes and Cambrian tectonics in northern Victoria Land. CicumPacific Council for Energy and Mineral Resources, Earth Science Series 1, Houston, 467 –480. Caminos, R., 1988. Mapa geolo´gico estructural del area situada entre Villa de Soto y La Candelaria, Co´rdoba. Servicio Geologico Argentino (SEGEMAR), Buenos Aires, unpublished report. Cerredo, M.E., 1996. Metamorphic evolution of high grade metapelites of Sierra de Comoechingones, Cordoba, Argentina. XIII Congreso Geolo´gico Argentino y III Congreso de Exploracio´n de Hidrocarburos, Actas V., p. 531. Dalla Salda, L.H., Cingolani, C.A., Varela, R., 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., Dalziel, I.W.D., Cingolani, C.A., Varela, R., 1992b. Did the Taconic Appalachians continue into South America? Geology 20, 1059–1062. Dalla Salda, L.H., Cingolani, C.A., Varela, R., 1993. A pre-Carboniferous tectonic model for the evolution of southern South America. XII International Congress on Carboniferous and Permian Geology and Stratigraphy, Actas I, 371–384. Dalla Salda, L.H., Lopez de Luchi, M.G., Cingolani, C.A., Varela, R., 1998. Laurentia–Gondwana collision: the origin of the Famatinian– Appalachian orogenic belt (a review). Geological Society, London, Special Publication vol. 142, 219–234. Dalziel, I.W.D., 1997. Neoproterozoic –Paleozoic geography and tectonics; review, hypothesis, environmental speculation. Geological Society of America Bulletin 109, 16–42. Dalziel, I.W.D., Dalla Salda, L.H., Gahagan, L.M., 1994. Paleozoic Laurentia–Gondwana interaction and the origin of the Appalachian– Andean mountain system. Geological Society of America Bulletin 106, 243 –252. Dickerson, P.W., Keller, M., 1998. The Argentine Precordillera: its odyssey from the Laurentian Ouachita margin towards the Sierras Pampeanas of Gondwana. Geological Society of London, Special Publication vol. 142, 159–179. Dorais, M.J., Lira, R., Chen, Y., Tingey, D., 1997. Origin of biotite– apatite-rich enclaves Achala Batholith, Argentina. Contributions to Mineralogy and Petrology 130, 31–46. Durand, F.R., 1996. La transicio´n Preca´mbrico-Ca´mbrico en el sur de Sudame´rica. Universidad Nacional de Tucuma´n, Serie Correlacio´n Geolo´gica vol. 12, 195–205. Fantini, R., Gromet, L.P., Simpson, C., Northrup, C.J., 1998. Timing of high-temperature metamorphism in the Sierras Pampeanas of Co´rdoba, Argentina: implications for Laurentia–Gondwana interactions. X Congreso LatinoAmericano de Geologı´a, Actas II, 388 –392. Fernandez-Seveso, F., Tankard, A.J., 1995. Tectonics and stratigraphy of the late Paleozoic Paganzo basin of western Argentina and its regional implications. American Association of Petroleum Geologists, Memoir vol. 62, 285–301. Flo¨ttmann, T., Gibson, G.M., Kleinschmidt, G., 1993. Structural continuity of the Ross and Delamerian orogens of Antarctica and Australia along the margin of the paleo-Pacific. Geology 21, 319–322. Goodge, J.W., Dallmeyer, R.D., 1996. Contrasting thermal evolution within the Ross Orogen, Antarctica: evidence from mineral 40Ar/39Ar ages. Journal of Geology 104, 435–458.
C. Simpson et al. / Journal of South American Earth Sciences 15 (2003) 749–764 Goodge, J.W., Walker, N.W., Hansen, V.L., 1993. Neoproterozoic– Cambrian basement-involved orogenesis within the Antarctic margin of Gondwana. Geology 21, 37 –40. Gordillo, C.E., 1984. Migmatitas cordierı´ticas de la Sierras de Co´rdoba: condiciones fı´sicas de la migmatizacio´n. Boletı´n de la Academia Nacional de Ciencias, Co´rdoba 68, 1–40. Gordillo, C.E., Lencinas, A.N., 1979. Sierras Pampeanas de Cordoba y San Luis. II Simposio de Geologia Regional Argentina, Academia Nacional de Ciencias I, 577–650. Gromet, L.P., Simpson, C., 1999. Age of the Paso del Carmen pluton and implications for the duration of the Pampean Orogeny Sierras de Co´rdoba, Argentina. XIV Congreso Geolo´gico Argentino, Salta, Actas I, 149–151. Gromet, L.P., Simpson, C., 2000. Cambrian orogeny in the Sierras Pampeanas, Argentina: ridge subduction or continental collision? Geological Society of America Abstracts with Programs 32, A-505. Hirth, G., Tullis, J.A., 1992. Dislocation creep regimes in quartz aggregates. Journal of Structural Geology 14, 145– 159. Jordan, T.E., Allmendinger, R.W., 1986. The Sierras Pampeanas of Argentina: a modern analogue of rocky mountain foreland deformation. American Journal of Science 286, 737– 764. Kraemer, P.E., Escayola, M.P., Martino, R.D., 1995. Hipo´tesis sobre la evolucio´n tecto´nica neoproterozoica de las Sierras Pampeanas de Co´rdoba (308400 – 328400 ). Revista de la Asociacio´n Geolo´gica Argentina 50, 47–59. Krol, M.A., Simpson, C., 1999. 40Ar/39Ar cooling ages from micas in the eastern Sierras Pampeanas accretionary prism rocks. Geological Society of America Abstracts with Programs 31, 114– 115. Lister, G.S., Snoke, A.W., 1984. S–C mylonites. Journal of Structural Geology 6, 617–638. Lucero-Michaut, N.H., Gamkosian, A., Jarsun, B., Zamora, Y.E., Sigismondi, M., Caminos, R., Miro, R., 1995. Mapa Geolo´gico de la Province de Co´rdoba, Repu´blica Argentina, 1:500,000. Ministerio de Economı´a y Obras y Servicios Pu´blicos. SEGEMAR, Buenos Aires. Lyons, P., Skirrow, R.G., Stuart-Smith, P.G., 1997. Report on geology and metallogeny of the Sierras Septentrionales de Co´rdoba: 1:250,000 map sheet, Province of Co´rdoba. Geoscientific Mapping of the Sierras Pampeanas. Argentine-Australian Cooperative Project, Australian Geological Survey Organization, unpublished report, 131 pp. Martino, R.D., 1993. La faja de deformacio´n Guamanes; petrografı´a, estructura interna y significado tecto´nico, Sierra Grande de Co´rdoba, Argentina. Revista de la Asociacio´n Geolo´gica Argentina 48, 21–32. Martino, R.D., Kraemer, P., Escayola, M., Giambastiani, M., Arnosio, M., 1995. Transecta de las Sierras Pampeanas de Co´rdoba a los 328 S. Revista de la Asociacio´n Geolo´gica Argentina 50, 60– 77. McDonough, M., Ramos, V.A., Isachsen, C.E., Bowring, S.A., Vujovich, G.I., 1993. Nuevas edades de circones del basamento de la Sierra de Pie de Palo, Sierras Pampeanas Occidentales de San Juan: sus implicancies para los modelos del supercontinente proterozoico de Rodinia. XII Congreso Geolo´gico Argentina, Actas III, 343 –357. Northrup, C.J., Simpson, C., Gromet, L.P., 1998a. Early Paleozoic history of the eastern Pampeanas, Argentina: development of a Cambrian arc and accretionary prism along the margin of Gondwana. X Congreso Latinoamericano de Geologı´a y VI Nacional de Geologı´a Econo´mica, Actas II, 400–403. Northrup, C.J., Simpson, C., Hodges, K.V., 1998b. Pseudotachylyte in fault zones of the Sierras de Cordoba, Argentina: petrogenesis and 40Ar/39Ar geochronology. Geological Sciences of America Abstracts with Programs 30. Otamendi, J.E., Rabbia, O.M., 1996. Petrology of high-grade gneisses from Macizo Rio Santa Rosa: evidence of decompression in the Eastern Sierras Pampeanas. XIII Congreso Geolo´gico Argentino y III Congreso de Exploracio´n de Hidrocarburos, Actas V, 527. Otamendi, J.E., Patin˜o Douce, A.E., Demichelis, A.H., 1999. Amphibolite to granulite transition in aluminous greywackes from the Sierra de
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Comechingones, Co´rdoba, Argentina. Journal of Metamorphic Petrology 17, 415–434. Pankhurst, R.J., Rapela, C.W., Saavedra, J., Baldo, E., Dahlquist, J., Pascua, I., 1996. Sierras de Los Llanos, Malanzan and Chepes: Ordovician Ib and S-type granitic magmatism in the Famatinian orogen. XIII Congreso Geolo´gico Argentino y III Congreso de Exploracio´n de Hidrocarburos, Actas V, 415. Passchier, C.W., Simpson, C., 1986. Porphyroclast systems as kinematic indicators. Journal of Structural Geology 8, 831–843. Pieters, P., Skirrow, R., 1997. Informe Geolo´gico y Metaloge´nico de las Sierras de Chepes, Ulapes, y Las Minas, Provincia de La Rioja. Proyecto de Cooperacio´n Argentino-Australiano, unpublished report, 123 pp. Ramos, V.A., 1984. Chilenia; Un terreno alo´ctono en la evolucı´on paleozoica de los Andes Centrales. Ninth Congreso Geolo´gico Argentino, Actas II, 84 –106. Ramos, V.A., 1988. The tectonics of the central Andes, 308 –338S latitude. Processes in Continental Lithospheric Deformation. Special Paper 218, pp. 31–54. Ramos, V.A., Dallmeyer, R.D., Vujovich, G., 1998. Time constraints on the early Paleozoic docking of the Precordillera, central Argentina. Geological Society of London, Special Publication vol. 142, 143–158. Ramos, V.A., Jordon, T.E., Allmendinger, R.W., Mpodozis, C., Kay, S.M., Corte´s, J.M., Palma, M., 1986. Paleozoic terranes of the central Argentine-Chilean Andes. Tectonics 5, 855 –880. Rapela, C.W., Pankhurst, R.J., 1996. The Cambrian plutonism of the Sierras de Co´rdoba: Pre-Famatinian subduction? and crustal melting. XIII Congreso Geolo´gico Argentino y III Congreso de Exploracio´n de Hidrocarburos, Actas V, 491. Rapela, C.W., Pankhurst, R.J., Casquet, C., Baldo, E., Saavedra, J., Galindo, C., 1998a. Las colisiones continentales Pampeana y Famatiniana. X Congreso Latinoamericano de Geologı´a y VI Nacional de Geologı´a Econo´mica, Actas II, 404. Rapela, C.W., Pankhurst, R.J., Casquet, C., Baldo, E., Saavedra, J., Galindo, C., Fanning, C.M., 1998b. The Pampean orogeny of the southern proto-Andes: Cambrian continental collision in the Sierras de Co´rdoba. Geological Society, London, Special Publication vol. 142, 181– 218. Rapela, C.W., Pankhurst, R.J., Casquet, C., Baldo, E., Saavedra, J., Galindo, C., 1998c. Early evolution of the Proto-Andean margin of South America. Geology 26, 707 –710. Sandiford, M., Fraser, G., Arnold, J., Foden, J., Farrow, T., 1995. Some causes and consequences of high-temperature, low-pressure metamorphism in the eastern Mt Lofty Ranges, South Australia. Australian Journal of Earth Sciences 42, 233–240. Sibson, R.H., 1977. Fault rocks and fault mechanisms. Journal of the Geological Society of London 133, 191 –213. Sibson, R.H., 1986. Earthquakes and rock deformation in crustal fault zones. Annual Review of Earth and Planetary Sciences 14, 149– 175. Simpson, C., Law, R.D.W., Northrup, C.J., Martino, R.D., 1998. Crustal shortening of the Cambrian arc and post-metamorphic shear zones in the Sierras Pampeanas. X Congreso Latinoamericano de Geologı´a y VI Nacional de Geologı´a Econo´mica, Actas II, 394–399. Simpson, C., Whitmeyer, S., De Paor, D.G., Gromet, L.P., Miro, R., Krol, M.A., Short, H., 2001. Sequential ductile through brittle reactivation of major fault zones along the accretionary margin of Gondwana in Central Argentina. Geological Society, London, Special Publication, 186. Sims, J., Stuart-Smith, P., Lyons, P., Skirrow, R., 1997. Informe geolo´gico y metaloge´nico de Las Sierras de San Luis y Comechingones, Provincias de San Luis y Co´rdoba. Argentine–Australian Cooperative Project, Australian Geological Survey Organization, unpublished report, 148 pp. Sims, J., Ireland, T.R., Camacho, A., Lyons, P., Pieters, P.E., Skirrow, R., Stuart-Smith, P., Miro, R., 1998. U – Pb, Th – Pb and Ar – Ar geochronology from the southern Sierras Pampeanas, Argentina: implications for the Paleozoic tectonic evolution of the western
764
C. Simpson et al. / Journal of South American Earth Sciences 15 (2003) 749–764
Gondwana margin. Geological Society, London, Special Publication vol. 142, 259 –281. Thomas, W.A., Astini, R.A., 1996. The Argentine Precordillera: a traveler from the Ouachita embayment of North American Laurentia. Science 273, 752–757. Tikoff, B., Teyssier, C., 1994. Strain modeling of displacement field partitioning in transpressional orogens. Journal of Structural Geology 16, 1575–1588. Toselli, A.J., 1990. Metamorfismo del Ciclo Pampeano. Acenolaza, F.G., Miller, H., and Toselli, A.J. (Eds.), Universidad Nacional de Tucuma´n, Serie Correlacio´n Geolo´gica, 4, pp. 181– 198. Vujovich, G.I., Mahlburg Kay, S., 1998. A Laurentian? Grenville-age oceanic arc/back-arc terrane in the Sierra de Pie de Palo, Western Sierras Pampeanas, Argentina. Geological Society of London, Special Publication vol. 142, 159–179.
Whitmeyer, S.J., Simpson, C., 2003. High strain-rate deformation fabrics characterize a kilometers-thick Paleozoic fault zone in the eastern Sierras Pampeanas, central Argentina. Journal of Structural Geology in press. Willner, A.P., 1990. Divisio´n tectonometamo´rfica del basamento del noroeste Argentino. Acenolaza, F.G., Miller, H., Toselli, A.J. (Eds.), Universidad Nacional de Tucuma´n, Serie Correlacio´n Geolo´gica, 4, pp. 113 –159. Willner, A.P., Miller, H., 1986. Structural division and evolution of the lower Paleozoic basement in the NW Argentine Andes. Zentralblat fu¨r Geologie und Pala¨ontologie B, 1245–1255. Wright, T.O., Dallmeyer, R.D., 1991. The age of cleavage development in the Ross orogen, northern Victoria land, Antarctica: evidence from 40 Ar/39Ar whole-rock slate ages. Journal of Structural Geology 13, 677 –690.
