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2D Magnetotelluric interpretation of the crust electrical resistivity across the Pampean terrane–Río de la Plata suture, in central Argentina
Tectonophysics 459 (2008) 54–65
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Tectonophysics j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t e c t o
2D Magnetotelluric interpretation of the crust electrical resistivity across the Pampean terrane–Río de la Plata suture, in central Argentina Alicia Favetto a, Cristina Pomposiello a,⁎, Mónica G. López de Luchi a, John Booker b a b
CONICET, INGEIS, Ciudad Universitaria, C1428EHA Buenos Aires, Argentina University of Washington, Department of Earth and Space Sciences, Seattle, WA 98195 USA
A R T I C L E
I N F O
Article history: Received 1 March 2006 Received in revised form 29 November 2006 Accepted 11 November 2007 Available online 4 April 2008 Keywords: Suture Sierras de Córdoba Sierras Pampeanas Magnetotellurics Electrical resistivity
A B S T R A C T Magnetotelluric data were obtained along a 450 km, almost west–east profile at approximately 31.5° S, which extends from La Rioja to Santa Fé provinces in central Argentina. The profile crosses two main crustal domains that were juxtaposed during the Early–Middle Cambrian Pampean Orogeny: the Pampean terrane to the west and the Río de la Plata craton to the east. The electrical resistivity structure of the crustal domains together with their boundary is presented. Through dimensionality analysis of the data, it was demonstrated that regional-scale electrical structures are mainly two-dimensional with a strike direction oriented parallel to the surface geological strike. The resistivity model shows a subvertical limit approximately along the eastern border of the Sierra Chica de Córdoba. To the east, the shallower structure is the Chaco–Paranense basin extending to a depth of 6 km with resistivities between 1 and 30 Ohm-m, whereas below the basin the ca 2.1–2.3 Ga Río de la Plata craton shows resistivities in a range of 300–10,000 Ohm-m. The Pampean terrane presents a 6 km layer with a resistivity higher than 10,000 Ohm-m whereas below this layer the resistivity values range from 50 to 200 Ohm-m. Based on both the geological information and the magnetotelluric results, the sharp lateral discontinuity observed in the resistivity model to the east of the Sierras de Córdoba is conjectured to represent the boundary between the Río de la Plata craton and the Pampean terrane which may correspond to the Early Cambrian suture. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The location of crustal blocks boundaries and their geometry at depth is in some cases controversial because boundary definition is often based on relatively limited surface and borehole geologic information and seismic reflection data. Terranes are generally considered as exotic crustal blocks separated from a continental nucleus by suture zones. In general, these zones result from the accretion of terranes after subduction of an intervening ocean, a process that usually also produces a series of ophiolites and volcanism (Keary and Vine, 1990; Meissner, 1996). Reliable reconstructions of collisional and accretionary processes are almost impossible because many of the characteristic features of collision are no longer preserved in the deeply eroded orogens (Key et al., 1989). The magnetotelluric (MT) method involves the measurement of orthogonal components of natural electric and magnetic fields, which contain information about the electrical resistivity distribution at crustal and upper mantle depths. This method is well suited for studying regional structures and may help in defining crustal variations and
⁎ Corresponding author. Fax: +54 11 4783 3024. E-mail addresses: [email protected] (A. Favetto), [email protected] (C. Pomposiello), [email protected] (M.G. López de Luchi), [email protected] (J. Booker). 0040-1951/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2007.11.071
terrane boundaries based on lateral changes of resistivity. In particular, this methodology has been successful in identifying suture zones, i.e. across the Ossa Morena Zone and South Portuguese Zone suture (Monteiro Santos et al., 1999), the Palaeoproterozoic Trans-Hudson orogen of North America (Jones et al., 2005) and in the vicinity of the Banggong–Nujiang in central Tibet (Solon et al., 2005). In southern South America, geological and paleomagnetic data indicate a remarkably sharp crustal boundary along the western margin of West Gondwana between the Río de la Plata craton (RIC) and the basement of the Sierras Pampeanas (Fig. 1). The basement underlying Late Palaeozoic and younger sediments in central and southern Córdoba is juvenile Palaeoproterozoic crust of the Río de la Plata craton (Rapela et al., 2005) whereas the basement of the Sierras Pampeanas shows a common crustal history with the Arequipa– Antofalla massif (Steenken et al., 2004 and references therein). The first hypothesis about a boundary between the basement of the Sierras Pampeanas and the RIC was based on the belts of ophiolites of the Sierras de Córdoba, the easternmost mountain range of the Sierras Pampeanas. The eastern belt of ophiolites of the Sierras was identified as a lherzolite ophiolite with an inferred back-arc origin (Kraemer et al., 1995) which led to the hypothesis of the Córdoba terrane as a part of the RIC. The western belt of ophiolites of the Sierras was considered to show mid-oceanic ridge affinities (Ramos et al., 2000) and to represent the suture resulting from the accretion of the Pampia terrane to the RIC. On
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Fig. 1. a) Map of Gondwana at the end of the Neoproterozoic showing the locations of cratons, orogenic belts, and inferred plate limits (modified from Steenken et al., 2004); b) Precambrian tectonic framework of Central South America (modified from Kröner and Cordani, 2003). Dashed lines indicate inferred positions of major cratonic fragments below Phanerozoic cover: Cratons abbreviations: SF—São Francisco; RA—Rio Apa; PR—Parana; RP—Rio de la Plata; PA—Pampia/Pampean; AA—Arequipa–Antofalla; LA—Luiz Alves; CG— Central Goias.
the contrary, an Early Cambrian–Middle Cambrian collision between the Pampean Terrane (PAT) and the RIC with the corresponding suture running along the east of the Sierras de Córdoba (Fig. 2) was proposed by Rapela et al. (1998).
Uncertainties in the definition of the limits of the accreted terrane, the structural mechanisms of the collisional event, and the unknown width of the former oceanic basins together with younger amalgamation of terranes along the western border of the PAT and subsequent
Fig. 2. Geological sketch of the studied area showing the locations of the MT sites. Location of the proposed sutures containing ultramafic rocks after Ramos (1999). Geology of the Sierra de Córdoba and San Luis is based on the Geological Map of Córdoba (1:500,000) and Geological Map of San Luis (1:500,000) (Servicio Geológico Minero Argentino). Values in Ga correspond to TDM (Depleted mantle model age) after Rapela et al. (1998) and Steenken et al. (2004). They are included in order to show some of the isotopic differences between the studied crustal units.
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tectonic events have led to an imprecise location of the Early Cambrian suture. López de Luchi et al. (2005) analyzed data from 19 MT sites along a 200 km west–east profile around 31.5° S that extends from the top of the Sierra Grande de Córdoba across the plains up to the west of the Santa Fé province. Along the profile, the crustal structure indicates the presence of lateral resistivity variations. As the sector with the high resistivity contrast between two major blocks coincides with a geological feasible terrane boundary between the RIC and the PAT, these authors proposed that the suture may be located close to the eastern border of the Sierra Chica de Córdoba. The aim of this paper is to improve the previous interpretation in the vicinity of the inferred suture zone, namely the Pampean–Río de la Plata suture (PARIS) between the RIC and PAT. Ten new MT sites were added to the original profile in López de Luchi et al. (2005) extending it up to a total length of around 450 km to avoid border effects close of the area of interest. Therefore, twenty nine MT sites were included in this study, which is focused on analyzing the eastern border of the Sierra de Córdoba in order to detect lateral variations in electrical resistivities that may correspond to a major geological terrane boundary. 2. Geological background Different paleogeographic reconstructions that mainly rely on paleomagnetic data have been proposed for the chronology and sequence of accretions that led to Gondwana assembly (e.g. BritoNeves et al., 1999; Trompette, 2000; Alkmim et al., 2001; Sanchez Bettucci and Rapalini, 2002; Veevers, 2004; Rapalini, 2005 and references therein). The South American and African continental blocks (Fig. 1) assembled into West Gondwana (Amazonia, West Africa, Río de la Plata, São Francisco/Congo and Borborema, BritoNeves and Cordani, 1999; Campos-Neto, 2000; Alkmim et al., 2001; Cordani et al., 2001, 2003) or alternatively into a smaller Western Gondwana (West Africa–Amazonia, Río Apas, Sanchez Bettucci and Rapalini, 2002) and Central Gondwana (West Nile–Congo–São Francisco–Río de la Plata–Arabia, Brito-Neves et al., 1999; Sanchez Bettucci and Rapalini, 2002) during several stages of ocean closures and collisions between 900 and ~550 Ma (Cordani et al., 2001, 2003) which resulted in the Pan-African–Brasiliano fold belts. Collision of Amazonia (proto-western Gondwana) with Río de la Plata–São Francisco (central Gondwana) as one of the latest events in Western Gondwana amalgamation has been recently proposed by Alkmim et al. (2001). The closure of a large ocean separating these cratons from Amazonia and West Africa that is supported by available paleomagnetic data probably occurred by the end of the Ediacaran or beginning of Cambrian (Rapalini, 2005). Paleogeographic reconstructions indicate that the Pampean terrane–Río de la Plata craton suture (PARIS) would result from the closure of a large oceanic domain, the Goiás–Pharusian Pampean Ocean (Rapalini, 2005 and references therein). Drifting apart of Laurentia and Amazonia (Fig. 1) may have led to the closure of this ocean being the collision between the RIC and the PAT, the final event (Cordani et al., 2001, 2003; Rapalini, 2005). Paleomagnetic data from the Rio de la Plata craton suggest that this block was already assembled to most major Gondwana blocks by the end of the Proterozoic (Sanchez Bettucci and Rapalini, 2002 and references therein). Although the PAT lacks pre-Late Cambrian paleomagnetic data, recently obtained Late Cambrian paleomagnetic data suggest consistent paleo-latitudes with the rest of Gondwana (Rapalini 2005; Rapela et al., 2005). As the geological evidence supports a link between Amazonia and the Pampean terrane (Schwartz and Gromet, 2004; Steenken et al., 2004), it is reasonable to infer that the RIC and the PAT were juxtaposed between the end of the Proterozoic and the beginning of the Cambrian. The Río de la Plata craton (Fig. 1) is made up by a Palaeoproterozoic nucleus with scarce influence of Neoproterozoic orogenic events (Dalla Salda et al., 1988; Dalla Salda, 1999; Basei et al., 2000; Cingolani
and Dalla Salda, 2000; Hartmann et al., 2003; Pankhurst et al., 2003; Saalmann et al., 2005 and references therein). Towards the west, the RIC is covered by the very thick Phanerozoic sedimentary deposits of the Chaco–Paranense basin. The juvenile Palaeoproterozoic ages and isotopic signature in the Argentina outcrops of the RIC corresponds to localities in Tandilia and to boreholes in the western sector of the Chaco–Paranense basin (Pankhurst et al., 2003; Rapela et al., 2005). The Sierras Pampeanas, a morphotectonic unit that constitutes generally N–S striking mountain ranges in central and northwestern Argentina, are characterized by Late-Precambrian–Early Paleozoic metamorphic and igneous rocks. The Sierras Pampeanas were divided into two main parts: the Eastern and the Western Sierras Pampeanas (Caminos, 1979). The Eastern Sierras Pampeanas are composed largely of Ordovician, Cambrian or older metasedimentary rocks of variable metamorphic grade, intruded by different suites of Cambrian to Devonian granitoids (Rapela et al., 1998; Sims et al., 1998; Rapela, 2000 among others). Regional metamorphism is attributed to the Cambrian Pampean and to the Ordovician Famatinian orogenies (Pankhurst et al., 1998; Rapela et al., 1998; Thomas and Astini, 2003 and references therein). The basement blocks of Sierras Pampeanas are located on the deformed and faulted foreland of the Pampean flat-slab segment of the Nazca plate (Ramos et al., 2002). Older crustal discontinuities in the foreland, i.e. sutures and shear zones, played a strong role in the inception and geometry of the main faults that differentially uplifted the blocks of Sierras Pampeanas (Ramos, 1994). Uplift of the basement blocks of the Sierras Pampeanas was the result of Andean compression during late Cenozoic times (Ramos et al., 2002 and references therein). The Sierras de Córdoba, the easternmost block of the Eastern Sierras Pampeanas, records a dominantly Cambrian high temperature, low- to medium-pressure metamorphism and the intrusion of I- and S-type granitic plutons that results from the Pampean Orogeny (Rapela et al., 1998). The concept of the Pampean Orogeny, a continental scale collision that affected a large sector of the proto-Andean margin of Gondwana from at least 17° to 33° S, involves the accretion to the western margin of Gondwana i.e. to the RIC of the PAT, a Neoproterozoic rift-related continental fragment during an east-facing subduction event. The PAT (Pampia and Córdoba terranes, see below) as defined by Rapela et al. (1998) encompasses a crustal block that extends from the eastern border of the Sierras de Córdoba up to the boundary with the Cuyania/ Precordillera terrane (~ 68° W). Ramos (1999) considered that a latest Precambrian collision between the RIC and a so-called Pampia Terrane followed the closure of an ocean basin located to the west of a magmatic arc developed in the western part of the RIC. The suture between the RIC and the Pampia Terrane would be represented by the western ultramafic belt of ophiolites of the Sierras de Córdoba (Kraemer et al., 1995) whereas the eastern ultramafic belt would be the result of the closure of a backarc basin. Therefore the so-called Córdoba terrane that underlay most of the Sierras de Córdoba should be part of the RIC (Fig. 1). Most models that encompass the idea of a suture between the basement of the Sierras Pampeanas and the RIC consider that the metasedimentary rocks of the Sierras de Córdoba are part of an original passive margin sequence, which developed into an accretionary prism during a period of eastward-facing subduction along the South American margin (Kraemer et al., 1995). Rapela et al. (1998) proposed that the metasedimentary rocks of the Sierras de Córdoba are synsubduction deposits on or marginal to the PAT that were part of the Puncoviscana basin and were deformed and metamorphosed during the Pampean Orogeny. Therefore in his hypothesis most of the Sierras are underlain by the PAT and the suture zone should be located somewhere to the east of the Sierras de Córdoba. The Pampean mobile belt in the Sierras de Córdoba and, in general, most meta-sediments of the Eastern Sierras Pampeanas do not record zircon patterns with conspicuous RIC provenance (Schwartz and
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Gromet, 2004; Rapela et al., 2005; Steenken et al., 2006). On the contrary, they share a within errors uniform TDM (time of extraction of the depleted mantle) of 1.6–1.7 Ga, (Rapela et al., 1998; Pankhurst et al., 1998; Steenken et al., 2004 and references therein) that does not fit the data for the RIC (Pankhurst et al., 2003; Saalmann et al., 2005) but suggests a common crustal history for the PAT and the Arequipa– Antofalla massif (Steenken et al. (2004 and references therein). No systematic TDM differences or contrasting metamorphic grade (Escayola and Kraemer, 2003) were seen at both sides of the western ophiolite belt of the Sierras de Córdoba (Rapela et al., 1998; Steenken et al., 2004). Additionally, the PARIS also appears to represent a broad zone of rheological discontinuity along which the eastern front of the Andean was developed (Booker et al., 2004).
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This evidence, together with our previous results (López de Luchi et al., 2005), led us to consider the feasibility of the model of the PAT where only one suture zone should be present. 3. Magnetotelluric interpretation 3.1. Data analysis MT long period data were acquired between 2001 and 2004 at 29 sites along a 450 km west–east profile at about 31.5° S crossing through Alta Gracia city. It extended from the east of La Rioja province (~ 66° W), across the Sierra Grande, Sierra Chica de Córdoba, and the plains up to the east of the Santa Fé province as seen in Fig. 2.
Fig. 3. Induction vectors for 10s, 107s and 1280s periods (Parkinson's convention) for the 29 MT sites. In a two-dimensional situation, they are normal to strike and point towards current concentrations.
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Fig. 4. Pseudosections for both polarizations TE (xy) and TM (yx), from top to bottom: rho xy, phi xy, rho yx, phi yx, real Tzy, imaginary Tzy for sites 705 to 845 expressed in Ohm-m and degrees. The measured data are shown on the left column and the response of the model corresponding to Fig. 7A and B, with nmrs = 1.5, is presented on the right column.
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These data were collected with a new generation of low power, long period, GPS-controlled MT systems (NIMS). They were processed using robust statistical time series analysis and remote reference multisite method (Egbert, 1997). The regional strike was determined to be very close to N–S at most of the sites using impedance tensor decomposition for electric and magnetic distortion (Chave and Smith, 1994); this fact is in agreement with the approximately north–south tectonic fabric of the Sierras de Córdoba. In this analysis, we found the error level at which a 2D model is an acceptable representation of the structure and we used this error level in subsequent inversions that assume a regional 2D Earth. The sites located close to the eastern border of the Sierra Chica (745, 750 and 755) had a significant vertical magnetic field distortion at high frequencies (more than 100 Hz) and could be neglected at low frequencies. The three last sites at the western end of the profile showed a poor determination of strike direction with larger errors than the other sites. 3.2. Induction vectors Induction vectors are a widely accepted tool for studying bidimensionality. Parkinson vectors pointing towards current concentration regions were calculated. They should be directed orthogonal to strike direction in 2D structures and their magnitudes are larger in resistive terrains (Parkinson, 1962). Their real parts for three different periods are presented in Fig. 3. Short period induction vectors are more influenced by the shallow structure. At 10s they have a small magnitude all along the profile except in those sites near 750. At this zone (Fig. 2), which is centered in the proposed suture zone, they are especially large. For 107s and 1280s, vectors are mostly controlled by regional structures and are almost orthogonal to the strike direction. Their magnitude is moderate except for the easternmost sites where vectors are very small. For 107s vectors, their direction in the area of the suture zone is coincident to the one at 10s, but they are smaller in magnitude. Vectors at the three westernmost sites are pointing to the SE; their deviations from E–W direction were consistent with the single-site determinations of the strike. This is in agreement with the fact that the regional strike changes its direction slightly towards the SE at those sites. The fault that borders the Sierra to the west may be related to a subvertical conductor, considering that the vectors close to the fault are pointing to it.
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Vectors for 1280s show a more uniform E–W aligned pattern over most of the profile, pointing to the east and mainly controlled by the very conductive Chaco–Paranense basin. Vectors at the three westernmost sites keep on pointing to the east which could be related with a deeper conductive layer. 3.3. 2D inversion For 2D structures, data can be separated into two independent modes with electric current flowing parallel (TE) and perpendicular (TM) to the strike direction (N–S). Pseudosections of apparent resistivity (ρ) and phase (ϕ) for both polarizations and real and imaginary parts of the transfer function between vertical and horizontal magnetic field (Tzy) in measurement coordinates, for the frequency range of 10− 4–10− 1 Hz are shown in Fig. 4. MT impedance data were inverted using the NLCG algorithm of Rodi and Mackie (2001). Usually, apparent resistivities and phases in both polarizations, TE (ρxy,ϕxy) and TM (ρyx,ϕyx), and Tzy can be inverted using this algorithm. It minimizes model roughness subject to fitting the data to a prescribed misfit. Version 6.10 allows us to include discontinuities between arbitrary blocks in the model. We started inverting Tzy to explore if the obtained model shows the location of the suture zone considering that Tzy might be particularly sensitive to vertical discontinuities. It is assumed that measured vertical magnetic field is mainly dominated by currents induced in the Earth and only the horizontal magnetic EW field component is involved. Therefore, this interpretation is less liable to static shift and certain 3D effects. To invert Tzy data, the starting model included two conductive blocks imbedded into a half-space of 1000 Ohm-m, whose resistivities were fixed during the inversion. The first block, with a resistivity of 0.3 Ohm-m and a thickness of 4000 m, was placed at the top of the model, far from the profile (west of the coast of Chile ~72° W), representing the Pacific Ocean trench at the most western end and the other, with a resistivity of 3 Ohm-m, simulating the conductive mantle, was located deeper than 700 km. Different values of the starting half-space resistivity (from 10 to 10,000 Ohm-m) were used to check their influences on the final model; and it resulted in a final model practically not dependent on that initial value. This is mainly due to the presence of the fixed conductive blocks. The inversion process started using a large value of the regularization parameter (τ = 1000) and 0.03 as its error floor. This produced a very smooth
Fig. 5. Electrical resistivity model for the east–west magnetotelluric transect across Argentina at about 31.5° S derived from inversions of only Tzy data. The dashed line is the inferred suture. The inverted triangles denote the location of the MT sites.
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Fig. 6. NRMS as a function of model roughness obtained setting different regularization parameter values (τ). Misfit improves and model roughness increases as τ decreases, τ = 10 was the value selected to obtain the MT models.
model that was used as starting model for subsequent inversion. Misfit improved and model roughness increased as τ decreased and error floor was set to 0.02. When the inversion normalized root mean square (NRMS) was about 1.2 and τ was 10, the model was achieved. This model provides clear evidence of boundaries with resistivity
contrasts as seen in Fig. 5. The vertical discontinuity is observed around site 750 and it is indicated by an imaginary line projected surfacewards along this interface points close to the vicinity of Alta Gracia city. Multiple inversions were performed to establish the set of parameters to be used in the inversion process in order to obtain a model consistent with the geology and a similar misfit for the whole data set. Both modes TE and TM, and Tzy were jointly inverted. Data error floors of 30% in TM (ρyx), an error large enough to avoid using it in the inversion in TE (ρxy) (to prevent static shift effects and its susceptibility to distortion by off-profile structure) and 2.9° in the phases (ϕyx and ϕxy) were set. These values are consistent with the error floors used in the decomposition and special features of individual site strike and 0.02 in Tzy to avoid overfitting it (Fig. 5). We began inverting TM and Tzy data in order to avoid fitting TE data tighter than the other data for this particular dataset. The first inversion started using the same initial model used for Tzy inversion (with the halfspace of 1000 Ohm-m and two conductive blocks), the same large starting value of the regularization parameter (τ = 1000) and larger starting errors floors for Tzy and ρyx. Again, this produced a very smooth model that was used as a starting model for subsequent inversion; misfit improves and model roughness increases as τ decreases. Then, TE mode was included in the inversion which was finished for τ = 10 with a final NMRS of 1.5 to satisfy the 2D condition; the variation of NMRS with roughness is shown in Fig. 6. The bidimensional behavior is further reinforced by the fact that the models obtained by the separate inversion of the different data sets
Fig. 7. A) Top: Topographic profile. Below: First 50 km geoelectric cross-section derived from inversions of the MT data. The dashed line is the inferred suture. The inverted triangles denote the location of the MT sites. Only the alternate sites are numbered here for the sake of clarity. B) Deep geoelectric cross-section for the first 200 km.
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(Tzy, TE and TM, not shown) are very similar to the final model. A wide resistivity variation range and a main lateral discontinuity are clearly apparent at around 160 km from the site at the western end (Fig. 7A). It can be observed that the geometry of the main structures and resistivity values are coincident with the model represented previously by inverting only Tzy (Fig. 5). Both models
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show two low resistive structures given by the Chaco–Paranense basin (1 Ohm-m b ρ b 30 Ohm-m) to the east and a block more than 6 km deep (50 Ohm-m b ρ b 200 Ohm-m) to the west. Resistivity image to depths up to 200 km is shown in Fig. 7B. The presence of a conductor at a depth of 150–200 km in the central part of the studied profile produces a gradual decrease in resistivity above this anomaly.
Fig. 8. Normalized residual. From top to bottom: TE and TM Impedance phase and real and imaginary Tzy.
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Fig. 9. First 50 km geoelectric cross-section derived from inversions of the MT data using a vertical discontinuity between sites 760 and 765. The inverted triangles denote the location of the MT sites.
The responses predicted by the last model (Fig. 7A and B) are also shown as pseudosections in Fig. 4 to the right of the corresponding measured data. A comparison between observed data and model responses indicates that this model provides a very good fit for phases (ϕyx and ϕxy) and Tzy (real and imaginary parts), a reasonable fit for TM apparent resistivity (ρyx) and a worse fit for TE (ρxy) (in agreement with the large errors adopted in the inversion). These observations are consistent with the 1.2 value obtained for NRMS in the inversion. Furthermore, the normalized residuals are randomly distributed and in general they are in a range of −3 to 3. These normalized residuals for phase (ϕyx and ϕxy) and Tzy (real and imaginary parts) are presented in Fig. 8. Since the inversion code permits us to include discontinuities with no roughness penalty in the inversion process, this valuable inversion option was used to clarify the interpretation of different parts of the model. Therefore, we included a vertical discontinuity in the inversion process to simulate a possible sharp contrast due to the suture. This test is useful to evaluate the degree of connection between the shallow resistive structure to the west of this vertical discontinuity and the deeper resistive structure to the east of it. Both resistive structures seem to differ across the vertical discontinuity as it was expected from the geological data that suggest that there is a boundary between the RIC and the PAT (Fig. 9). The
resistivity is very high to the west below the Sierra whereas very low values within the Chaco–Paranense basin are found to the east up to around 6 km deep. Comparing average resistivity at the same depths within the 6–50 km range, the eastern area is much more resistive than the western one. This becomes clear when one examines the horizontal conductance (conductivity integrated horizontally from west to east for each depth). The horizontal conductance is expressed normalized by the length (in km) for each side of the profile, 150 km westward and 250 km eastward. In the first 10 km, the conductance is very high to the east because of the basin and, on the contrary, it is very low to the west because of the “Sierras”. Deeper than 10 km, resistivity tends to constant average values which differ between 1 and 2 orders of magnitude at both sides. These results provide conclusive evidence about structural differences in resistivity values for both sides of the vertical limit between RIC and PAT, as seen in Fig. 10. 4. Discussion Our preliminary results in López de Luchi et al. (2005) led us to consider that the limit between the PAT and the RIC might be located to the east of the Sierra Chica de Córdoba (Fig. 2). This fact is also supported by the lack of systematic TDM differences at both sides of
Fig. 10. Horizontal conductance (conductivity integrated horizontally from west to east for each depth) for both sides of the vertical limit between RIC and PAT. This magnitude was normalized by the length in km for each side of the profile, 150 km westward and 250 km eastward.
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Fig. 11. Maximum penetration depth reachable for different periods. It was calculated using the skin depth approximation and the conductance for each depth (conductivity integrated vertically from the top of the surface to depth) for the model shown in Fig. 7A.
the inferred limit between the Córdoba and the Pampia terranes and the comparable metamorphism ages as mentioned in the Geological background section. The location of a major lithospheric boundary along the eastern border of the Sierra Chica de Córdoba is further supported by the easternmost recorded seismic activity (Alvarado et al., 2005) related with the Andean compression acting on a sector where the flat-slab steepens (Cahill and Isacks, 1992). Isotopic results (Rapela et al., 1998) do not fit the idea of the eastern and the western Córdoba belt of ophiolites as limits for terranes (Kraemer et al., 1995), i.e. the hypothesis of the Córdoba terrane as a part of the Río de la Plata craton since not only no RIC provenance was identified in the Sierras de Córdoba meta-sediments (Schwartz and Gromet, 2004; Steenken et al., 2004; Rapela et al., 2005) but the provenance patterns are comparable in the entire Sierras de Córdoba and in the adjacent Sierra de San Luis (Steenken et al., 2004, 2006). The geoelectric cross-section presents a sharp lateral discontinuity separating structures with a high resistivity contrast and roughly a two layer resistivity structure. These results have been obtained in all the models that were presented, i.e. inverting both MT polarizations and Tzy and only Tzy, (Figs. 7 and 5). Tzy inversion offers an alternative model with especially sensitive to vertical discontinuities at depth and not influenced by static shift and other effects. Both models show west–east changes in the electrical structure within the analyzed first 50 km of the continental crust. Similar changes at both sides of a major discontinuity located at the longitude of Alta Gracia were described by López de Luchi et al. (2005). Induction vectors for the 10s and 107s periods exhibit a sharp change of direction next to Alta Gracia and for 1280s period the vectors are mainly eastward aligned. Interaction between the suture zone, the fault that borders the eastern sector of the Sierra Chica, the existence of the deep crustal conductive layer, and the Chaco– Paranense basin may produce a complex pattern. To clarify, the maximum penetration depth reachable for different periods was calculated using the skin depth approximation and the conductance for each depth (conductivity integrated vertically from the top of the surface to depth) for the model shown in Fig. 7A. Fig. 11 shows the estimated depth skin plotted for each period along the profile. This is an approximated simplification due to the fact that the reflections at interfaces should be taken into account in 2D structures. In the eastern area, where the basin is highly conductive, it is necessary to process more than 100s periods in order to pass through the first 10 km, whereas in the western sector, a maximum depth of up to 30–40 km could be reached at the same period, due to the extremely high resistive values at the first 5–10 km from the surface. It may be noted that the induction arrows shown at different periods along the profile (Fig. 3) could be influenced by structures placed at quite different depths between eastern and western parts of the assumed location of the suture, complicating their interpretation. Around the 755 site there is a subvertical limit between the highly resistive vertical zone of the RIC and the more conductive zone below Pampa de Achala that runs approximately along the
eastern border of the Sierra Chica de Córdoba which supports the Pampean terrane hypothesis (Rapela et al., 1998) and additionally seems to be correlated with the steepening of the subducting oceanic Nazca plate. This electric boundary persists even at the uppermost 6 km of the crust where it would control the border of the Chaco– Paranense basin against the highly resistive basement of the Sierras de Córdoba. In the eastern sector, between sites 760 and 845, the first kilometers with low resistivity (1–100 Ohm-m) correspond to the sedimentary sequences of the Chaco–Paranense basin as it was described by Favetto et al. (2004). Below this basin, the highly resistive (10,000 Ohm-m) zones that involve up to 150 km of the lithosphere (Fig. 7B) are interpreted as the RIC. The highly resistive zones are separated by a less resistive zone which reaches 30 Ohm-m at a depth of 150–200 km between sites 790 and 834. This electrical structure is similar to what was observed in a previous MT model across the Sierras Pampeanas (Booker et al., 2004). In the western sector of the profile, between sites 715 and 750 (approximately 100 km width), the first 6 km below the Sierras de Córdoba are the highest resistivity zone, around 10,000 Ohm-m, in agreement with the presence of metamorphic and igneous rocks that characterizes the Sierras Pampeanas in combination with the fact that the Sierras Pampeanas are at present under compression which would imply the difficulty of fluid circulation (Booker et al., 2004). The conductor at 30–70 km deep in the PAT also appears in models where the vertical discontinuity is set. The deep crustal conductive layer may indicate the existence of graphite in fossilized former shear zones developed as the result of the PAT–RIC collision or alternatively it may suggest the existence of a partial melted zone. During a collisional orogeny, major thrusts form a linked fault system rooted in the original subduction zone (e.g. Willett et al., 1993; Beaumont and Quinlan, 1994). Multiple activations of these major thrusts would lead to brittle overprinting of earlier ductile shear zones along discrete narrow faults. Deep reaching crustal fracture zones are suggested by the regional intrusions of mantle derived basaltic magmas through the crust in the Cretaceous and the Cenozoic in the Sierras de Córdoba. The present electrical structure of the PAT is influenced by their location on the flat-slab segment of the subducting oceanic Nazca plate. Therefore at present the real evidence for the former suture is provided by the different electrical structure of the two blocks. 5. Conclusions The MT results allowed us to draw interesting conclusions on the geoelectrical structures down to a depth of about 200 km. The RIC is a resistive structure (ρ ≈ 10,000 Ohm-m) that extends down to 150 km in depth. The PAT has a heterogeneous structure with a thin high resistivity layer down to 6 km and a conductive layer down to 70 km. The boundary between these two crustal units is located along the eastern border of the Sierra Chica de Córdoba. To the east of this boundary the RIC, a highly resistive crustal segment grades into a sector of relatively enhanced resistivity below the Chaco–Paranense
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basin. To the west, the PAT appears as a less resistive crustal segment below the most resistive 6 km top layer that corresponds to the Sierras de Córdoba. In summary, the sharp lateral discontinuities observed in the resistivity model in combination with the geological information suggest that the transition between the RIC and the PAT at present is located at the east of the Sierras de Córdoba. In consequence the meaning of the ophiolite belts as representing the location of crustal scale sutures remains unclear. Acknowledgements This research was supported by National Science Foundation Grants EAR99-09390 and EAR0310113 and by Agencia Nacional de Promoción Científica y Tecnológica PICT 99: 07-06313. The MT systems were provided by the EMSOC Instrument Facility supported by NSF Grants EAR02-36538 and EAR96-16421. Discussions with R. Martino, A. Rapalini, and A. Steenken were especially helpful for the understanding of the tectonic evolution of the Gondwana margin. 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Tectonophysics j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t e c t o
2D Magnetotelluric interpretation of the crust electrical resistivity across the Pampean terrane–Río de la Plata suture, in central Argentina Alicia Favetto a, Cristina Pomposiello a,⁎, Mónica G. López de Luchi a, John Booker b a b
CONICET, INGEIS, Ciudad Universitaria, C1428EHA Buenos Aires, Argentina University of Washington, Department of Earth and Space Sciences, Seattle, WA 98195 USA
A R T I C L E
I N F O
Article history: Received 1 March 2006 Received in revised form 29 November 2006 Accepted 11 November 2007 Available online 4 April 2008 Keywords: Suture Sierras de Córdoba Sierras Pampeanas Magnetotellurics Electrical resistivity
A B S T R A C T Magnetotelluric data were obtained along a 450 km, almost west–east profile at approximately 31.5° S, which extends from La Rioja to Santa Fé provinces in central Argentina. The profile crosses two main crustal domains that were juxtaposed during the Early–Middle Cambrian Pampean Orogeny: the Pampean terrane to the west and the Río de la Plata craton to the east. The electrical resistivity structure of the crustal domains together with their boundary is presented. Through dimensionality analysis of the data, it was demonstrated that regional-scale electrical structures are mainly two-dimensional with a strike direction oriented parallel to the surface geological strike. The resistivity model shows a subvertical limit approximately along the eastern border of the Sierra Chica de Córdoba. To the east, the shallower structure is the Chaco–Paranense basin extending to a depth of 6 km with resistivities between 1 and 30 Ohm-m, whereas below the basin the ca 2.1–2.3 Ga Río de la Plata craton shows resistivities in a range of 300–10,000 Ohm-m. The Pampean terrane presents a 6 km layer with a resistivity higher than 10,000 Ohm-m whereas below this layer the resistivity values range from 50 to 200 Ohm-m. Based on both the geological information and the magnetotelluric results, the sharp lateral discontinuity observed in the resistivity model to the east of the Sierras de Córdoba is conjectured to represent the boundary between the Río de la Plata craton and the Pampean terrane which may correspond to the Early Cambrian suture. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The location of crustal blocks boundaries and their geometry at depth is in some cases controversial because boundary definition is often based on relatively limited surface and borehole geologic information and seismic reflection data. Terranes are generally considered as exotic crustal blocks separated from a continental nucleus by suture zones. In general, these zones result from the accretion of terranes after subduction of an intervening ocean, a process that usually also produces a series of ophiolites and volcanism (Keary and Vine, 1990; Meissner, 1996). Reliable reconstructions of collisional and accretionary processes are almost impossible because many of the characteristic features of collision are no longer preserved in the deeply eroded orogens (Key et al., 1989). The magnetotelluric (MT) method involves the measurement of orthogonal components of natural electric and magnetic fields, which contain information about the electrical resistivity distribution at crustal and upper mantle depths. This method is well suited for studying regional structures and may help in defining crustal variations and
⁎ Corresponding author. Fax: +54 11 4783 3024. E-mail addresses: [email protected] (A. Favetto), [email protected] (C. Pomposiello), [email protected] (M.G. López de Luchi), [email protected] (J. Booker). 0040-1951/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2007.11.071
terrane boundaries based on lateral changes of resistivity. In particular, this methodology has been successful in identifying suture zones, i.e. across the Ossa Morena Zone and South Portuguese Zone suture (Monteiro Santos et al., 1999), the Palaeoproterozoic Trans-Hudson orogen of North America (Jones et al., 2005) and in the vicinity of the Banggong–Nujiang in central Tibet (Solon et al., 2005). In southern South America, geological and paleomagnetic data indicate a remarkably sharp crustal boundary along the western margin of West Gondwana between the Río de la Plata craton (RIC) and the basement of the Sierras Pampeanas (Fig. 1). The basement underlying Late Palaeozoic and younger sediments in central and southern Córdoba is juvenile Palaeoproterozoic crust of the Río de la Plata craton (Rapela et al., 2005) whereas the basement of the Sierras Pampeanas shows a common crustal history with the Arequipa– Antofalla massif (Steenken et al., 2004 and references therein). The first hypothesis about a boundary between the basement of the Sierras Pampeanas and the RIC was based on the belts of ophiolites of the Sierras de Córdoba, the easternmost mountain range of the Sierras Pampeanas. The eastern belt of ophiolites of the Sierras was identified as a lherzolite ophiolite with an inferred back-arc origin (Kraemer et al., 1995) which led to the hypothesis of the Córdoba terrane as a part of the RIC. The western belt of ophiolites of the Sierras was considered to show mid-oceanic ridge affinities (Ramos et al., 2000) and to represent the suture resulting from the accretion of the Pampia terrane to the RIC. On
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Fig. 1. a) Map of Gondwana at the end of the Neoproterozoic showing the locations of cratons, orogenic belts, and inferred plate limits (modified from Steenken et al., 2004); b) Precambrian tectonic framework of Central South America (modified from Kröner and Cordani, 2003). Dashed lines indicate inferred positions of major cratonic fragments below Phanerozoic cover: Cratons abbreviations: SF—São Francisco; RA—Rio Apa; PR—Parana; RP—Rio de la Plata; PA—Pampia/Pampean; AA—Arequipa–Antofalla; LA—Luiz Alves; CG— Central Goias.
the contrary, an Early Cambrian–Middle Cambrian collision between the Pampean Terrane (PAT) and the RIC with the corresponding suture running along the east of the Sierras de Córdoba (Fig. 2) was proposed by Rapela et al. (1998).
Uncertainties in the definition of the limits of the accreted terrane, the structural mechanisms of the collisional event, and the unknown width of the former oceanic basins together with younger amalgamation of terranes along the western border of the PAT and subsequent
Fig. 2. Geological sketch of the studied area showing the locations of the MT sites. Location of the proposed sutures containing ultramafic rocks after Ramos (1999). Geology of the Sierra de Córdoba and San Luis is based on the Geological Map of Córdoba (1:500,000) and Geological Map of San Luis (1:500,000) (Servicio Geológico Minero Argentino). Values in Ga correspond to TDM (Depleted mantle model age) after Rapela et al. (1998) and Steenken et al. (2004). They are included in order to show some of the isotopic differences between the studied crustal units.
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tectonic events have led to an imprecise location of the Early Cambrian suture. López de Luchi et al. (2005) analyzed data from 19 MT sites along a 200 km west–east profile around 31.5° S that extends from the top of the Sierra Grande de Córdoba across the plains up to the west of the Santa Fé province. Along the profile, the crustal structure indicates the presence of lateral resistivity variations. As the sector with the high resistivity contrast between two major blocks coincides with a geological feasible terrane boundary between the RIC and the PAT, these authors proposed that the suture may be located close to the eastern border of the Sierra Chica de Córdoba. The aim of this paper is to improve the previous interpretation in the vicinity of the inferred suture zone, namely the Pampean–Río de la Plata suture (PARIS) between the RIC and PAT. Ten new MT sites were added to the original profile in López de Luchi et al. (2005) extending it up to a total length of around 450 km to avoid border effects close of the area of interest. Therefore, twenty nine MT sites were included in this study, which is focused on analyzing the eastern border of the Sierra de Córdoba in order to detect lateral variations in electrical resistivities that may correspond to a major geological terrane boundary. 2. Geological background Different paleogeographic reconstructions that mainly rely on paleomagnetic data have been proposed for the chronology and sequence of accretions that led to Gondwana assembly (e.g. BritoNeves et al., 1999; Trompette, 2000; Alkmim et al., 2001; Sanchez Bettucci and Rapalini, 2002; Veevers, 2004; Rapalini, 2005 and references therein). The South American and African continental blocks (Fig. 1) assembled into West Gondwana (Amazonia, West Africa, Río de la Plata, São Francisco/Congo and Borborema, BritoNeves and Cordani, 1999; Campos-Neto, 2000; Alkmim et al., 2001; Cordani et al., 2001, 2003) or alternatively into a smaller Western Gondwana (West Africa–Amazonia, Río Apas, Sanchez Bettucci and Rapalini, 2002) and Central Gondwana (West Nile–Congo–São Francisco–Río de la Plata–Arabia, Brito-Neves et al., 1999; Sanchez Bettucci and Rapalini, 2002) during several stages of ocean closures and collisions between 900 and ~550 Ma (Cordani et al., 2001, 2003) which resulted in the Pan-African–Brasiliano fold belts. Collision of Amazonia (proto-western Gondwana) with Río de la Plata–São Francisco (central Gondwana) as one of the latest events in Western Gondwana amalgamation has been recently proposed by Alkmim et al. (2001). The closure of a large ocean separating these cratons from Amazonia and West Africa that is supported by available paleomagnetic data probably occurred by the end of the Ediacaran or beginning of Cambrian (Rapalini, 2005). Paleogeographic reconstructions indicate that the Pampean terrane–Río de la Plata craton suture (PARIS) would result from the closure of a large oceanic domain, the Goiás–Pharusian Pampean Ocean (Rapalini, 2005 and references therein). Drifting apart of Laurentia and Amazonia (Fig. 1) may have led to the closure of this ocean being the collision between the RIC and the PAT, the final event (Cordani et al., 2001, 2003; Rapalini, 2005). Paleomagnetic data from the Rio de la Plata craton suggest that this block was already assembled to most major Gondwana blocks by the end of the Proterozoic (Sanchez Bettucci and Rapalini, 2002 and references therein). Although the PAT lacks pre-Late Cambrian paleomagnetic data, recently obtained Late Cambrian paleomagnetic data suggest consistent paleo-latitudes with the rest of Gondwana (Rapalini 2005; Rapela et al., 2005). As the geological evidence supports a link between Amazonia and the Pampean terrane (Schwartz and Gromet, 2004; Steenken et al., 2004), it is reasonable to infer that the RIC and the PAT were juxtaposed between the end of the Proterozoic and the beginning of the Cambrian. The Río de la Plata craton (Fig. 1) is made up by a Palaeoproterozoic nucleus with scarce influence of Neoproterozoic orogenic events (Dalla Salda et al., 1988; Dalla Salda, 1999; Basei et al., 2000; Cingolani
and Dalla Salda, 2000; Hartmann et al., 2003; Pankhurst et al., 2003; Saalmann et al., 2005 and references therein). Towards the west, the RIC is covered by the very thick Phanerozoic sedimentary deposits of the Chaco–Paranense basin. The juvenile Palaeoproterozoic ages and isotopic signature in the Argentina outcrops of the RIC corresponds to localities in Tandilia and to boreholes in the western sector of the Chaco–Paranense basin (Pankhurst et al., 2003; Rapela et al., 2005). The Sierras Pampeanas, a morphotectonic unit that constitutes generally N–S striking mountain ranges in central and northwestern Argentina, are characterized by Late-Precambrian–Early Paleozoic metamorphic and igneous rocks. The Sierras Pampeanas were divided into two main parts: the Eastern and the Western Sierras Pampeanas (Caminos, 1979). The Eastern Sierras Pampeanas are composed largely of Ordovician, Cambrian or older metasedimentary rocks of variable metamorphic grade, intruded by different suites of Cambrian to Devonian granitoids (Rapela et al., 1998; Sims et al., 1998; Rapela, 2000 among others). Regional metamorphism is attributed to the Cambrian Pampean and to the Ordovician Famatinian orogenies (Pankhurst et al., 1998; Rapela et al., 1998; Thomas and Astini, 2003 and references therein). The basement blocks of Sierras Pampeanas are located on the deformed and faulted foreland of the Pampean flat-slab segment of the Nazca plate (Ramos et al., 2002). Older crustal discontinuities in the foreland, i.e. sutures and shear zones, played a strong role in the inception and geometry of the main faults that differentially uplifted the blocks of Sierras Pampeanas (Ramos, 1994). Uplift of the basement blocks of the Sierras Pampeanas was the result of Andean compression during late Cenozoic times (Ramos et al., 2002 and references therein). The Sierras de Córdoba, the easternmost block of the Eastern Sierras Pampeanas, records a dominantly Cambrian high temperature, low- to medium-pressure metamorphism and the intrusion of I- and S-type granitic plutons that results from the Pampean Orogeny (Rapela et al., 1998). The concept of the Pampean Orogeny, a continental scale collision that affected a large sector of the proto-Andean margin of Gondwana from at least 17° to 33° S, involves the accretion to the western margin of Gondwana i.e. to the RIC of the PAT, a Neoproterozoic rift-related continental fragment during an east-facing subduction event. The PAT (Pampia and Córdoba terranes, see below) as defined by Rapela et al. (1998) encompasses a crustal block that extends from the eastern border of the Sierras de Córdoba up to the boundary with the Cuyania/ Precordillera terrane (~ 68° W). Ramos (1999) considered that a latest Precambrian collision between the RIC and a so-called Pampia Terrane followed the closure of an ocean basin located to the west of a magmatic arc developed in the western part of the RIC. The suture between the RIC and the Pampia Terrane would be represented by the western ultramafic belt of ophiolites of the Sierras de Córdoba (Kraemer et al., 1995) whereas the eastern ultramafic belt would be the result of the closure of a backarc basin. Therefore the so-called Córdoba terrane that underlay most of the Sierras de Córdoba should be part of the RIC (Fig. 1). Most models that encompass the idea of a suture between the basement of the Sierras Pampeanas and the RIC consider that the metasedimentary rocks of the Sierras de Córdoba are part of an original passive margin sequence, which developed into an accretionary prism during a period of eastward-facing subduction along the South American margin (Kraemer et al., 1995). Rapela et al. (1998) proposed that the metasedimentary rocks of the Sierras de Córdoba are synsubduction deposits on or marginal to the PAT that were part of the Puncoviscana basin and were deformed and metamorphosed during the Pampean Orogeny. Therefore in his hypothesis most of the Sierras are underlain by the PAT and the suture zone should be located somewhere to the east of the Sierras de Córdoba. The Pampean mobile belt in the Sierras de Córdoba and, in general, most meta-sediments of the Eastern Sierras Pampeanas do not record zircon patterns with conspicuous RIC provenance (Schwartz and
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Gromet, 2004; Rapela et al., 2005; Steenken et al., 2006). On the contrary, they share a within errors uniform TDM (time of extraction of the depleted mantle) of 1.6–1.7 Ga, (Rapela et al., 1998; Pankhurst et al., 1998; Steenken et al., 2004 and references therein) that does not fit the data for the RIC (Pankhurst et al., 2003; Saalmann et al., 2005) but suggests a common crustal history for the PAT and the Arequipa– Antofalla massif (Steenken et al. (2004 and references therein). No systematic TDM differences or contrasting metamorphic grade (Escayola and Kraemer, 2003) were seen at both sides of the western ophiolite belt of the Sierras de Córdoba (Rapela et al., 1998; Steenken et al., 2004). Additionally, the PARIS also appears to represent a broad zone of rheological discontinuity along which the eastern front of the Andean was developed (Booker et al., 2004).
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This evidence, together with our previous results (López de Luchi et al., 2005), led us to consider the feasibility of the model of the PAT where only one suture zone should be present. 3. Magnetotelluric interpretation 3.1. Data analysis MT long period data were acquired between 2001 and 2004 at 29 sites along a 450 km west–east profile at about 31.5° S crossing through Alta Gracia city. It extended from the east of La Rioja province (~ 66° W), across the Sierra Grande, Sierra Chica de Córdoba, and the plains up to the east of the Santa Fé province as seen in Fig. 2.
Fig. 3. Induction vectors for 10s, 107s and 1280s periods (Parkinson's convention) for the 29 MT sites. In a two-dimensional situation, they are normal to strike and point towards current concentrations.
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Fig. 4. Pseudosections for both polarizations TE (xy) and TM (yx), from top to bottom: rho xy, phi xy, rho yx, phi yx, real Tzy, imaginary Tzy for sites 705 to 845 expressed in Ohm-m and degrees. The measured data are shown on the left column and the response of the model corresponding to Fig. 7A and B, with nmrs = 1.5, is presented on the right column.
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These data were collected with a new generation of low power, long period, GPS-controlled MT systems (NIMS). They were processed using robust statistical time series analysis and remote reference multisite method (Egbert, 1997). The regional strike was determined to be very close to N–S at most of the sites using impedance tensor decomposition for electric and magnetic distortion (Chave and Smith, 1994); this fact is in agreement with the approximately north–south tectonic fabric of the Sierras de Córdoba. In this analysis, we found the error level at which a 2D model is an acceptable representation of the structure and we used this error level in subsequent inversions that assume a regional 2D Earth. The sites located close to the eastern border of the Sierra Chica (745, 750 and 755) had a significant vertical magnetic field distortion at high frequencies (more than 100 Hz) and could be neglected at low frequencies. The three last sites at the western end of the profile showed a poor determination of strike direction with larger errors than the other sites. 3.2. Induction vectors Induction vectors are a widely accepted tool for studying bidimensionality. Parkinson vectors pointing towards current concentration regions were calculated. They should be directed orthogonal to strike direction in 2D structures and their magnitudes are larger in resistive terrains (Parkinson, 1962). Their real parts for three different periods are presented in Fig. 3. Short period induction vectors are more influenced by the shallow structure. At 10s they have a small magnitude all along the profile except in those sites near 750. At this zone (Fig. 2), which is centered in the proposed suture zone, they are especially large. For 107s and 1280s, vectors are mostly controlled by regional structures and are almost orthogonal to the strike direction. Their magnitude is moderate except for the easternmost sites where vectors are very small. For 107s vectors, their direction in the area of the suture zone is coincident to the one at 10s, but they are smaller in magnitude. Vectors at the three westernmost sites are pointing to the SE; their deviations from E–W direction were consistent with the single-site determinations of the strike. This is in agreement with the fact that the regional strike changes its direction slightly towards the SE at those sites. The fault that borders the Sierra to the west may be related to a subvertical conductor, considering that the vectors close to the fault are pointing to it.
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Vectors for 1280s show a more uniform E–W aligned pattern over most of the profile, pointing to the east and mainly controlled by the very conductive Chaco–Paranense basin. Vectors at the three westernmost sites keep on pointing to the east which could be related with a deeper conductive layer. 3.3. 2D inversion For 2D structures, data can be separated into two independent modes with electric current flowing parallel (TE) and perpendicular (TM) to the strike direction (N–S). Pseudosections of apparent resistivity (ρ) and phase (ϕ) for both polarizations and real and imaginary parts of the transfer function between vertical and horizontal magnetic field (Tzy) in measurement coordinates, for the frequency range of 10− 4–10− 1 Hz are shown in Fig. 4. MT impedance data were inverted using the NLCG algorithm of Rodi and Mackie (2001). Usually, apparent resistivities and phases in both polarizations, TE (ρxy,ϕxy) and TM (ρyx,ϕyx), and Tzy can be inverted using this algorithm. It minimizes model roughness subject to fitting the data to a prescribed misfit. Version 6.10 allows us to include discontinuities between arbitrary blocks in the model. We started inverting Tzy to explore if the obtained model shows the location of the suture zone considering that Tzy might be particularly sensitive to vertical discontinuities. It is assumed that measured vertical magnetic field is mainly dominated by currents induced in the Earth and only the horizontal magnetic EW field component is involved. Therefore, this interpretation is less liable to static shift and certain 3D effects. To invert Tzy data, the starting model included two conductive blocks imbedded into a half-space of 1000 Ohm-m, whose resistivities were fixed during the inversion. The first block, with a resistivity of 0.3 Ohm-m and a thickness of 4000 m, was placed at the top of the model, far from the profile (west of the coast of Chile ~72° W), representing the Pacific Ocean trench at the most western end and the other, with a resistivity of 3 Ohm-m, simulating the conductive mantle, was located deeper than 700 km. Different values of the starting half-space resistivity (from 10 to 10,000 Ohm-m) were used to check their influences on the final model; and it resulted in a final model practically not dependent on that initial value. This is mainly due to the presence of the fixed conductive blocks. The inversion process started using a large value of the regularization parameter (τ = 1000) and 0.03 as its error floor. This produced a very smooth
Fig. 5. Electrical resistivity model for the east–west magnetotelluric transect across Argentina at about 31.5° S derived from inversions of only Tzy data. The dashed line is the inferred suture. The inverted triangles denote the location of the MT sites.
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Fig. 6. NRMS as a function of model roughness obtained setting different regularization parameter values (τ). Misfit improves and model roughness increases as τ decreases, τ = 10 was the value selected to obtain the MT models.
model that was used as starting model for subsequent inversion. Misfit improved and model roughness increased as τ decreased and error floor was set to 0.02. When the inversion normalized root mean square (NRMS) was about 1.2 and τ was 10, the model was achieved. This model provides clear evidence of boundaries with resistivity
contrasts as seen in Fig. 5. The vertical discontinuity is observed around site 750 and it is indicated by an imaginary line projected surfacewards along this interface points close to the vicinity of Alta Gracia city. Multiple inversions were performed to establish the set of parameters to be used in the inversion process in order to obtain a model consistent with the geology and a similar misfit for the whole data set. Both modes TE and TM, and Tzy were jointly inverted. Data error floors of 30% in TM (ρyx), an error large enough to avoid using it in the inversion in TE (ρxy) (to prevent static shift effects and its susceptibility to distortion by off-profile structure) and 2.9° in the phases (ϕyx and ϕxy) were set. These values are consistent with the error floors used in the decomposition and special features of individual site strike and 0.02 in Tzy to avoid overfitting it (Fig. 5). We began inverting TM and Tzy data in order to avoid fitting TE data tighter than the other data for this particular dataset. The first inversion started using the same initial model used for Tzy inversion (with the halfspace of 1000 Ohm-m and two conductive blocks), the same large starting value of the regularization parameter (τ = 1000) and larger starting errors floors for Tzy and ρyx. Again, this produced a very smooth model that was used as a starting model for subsequent inversion; misfit improves and model roughness increases as τ decreases. Then, TE mode was included in the inversion which was finished for τ = 10 with a final NMRS of 1.5 to satisfy the 2D condition; the variation of NMRS with roughness is shown in Fig. 6. The bidimensional behavior is further reinforced by the fact that the models obtained by the separate inversion of the different data sets
Fig. 7. A) Top: Topographic profile. Below: First 50 km geoelectric cross-section derived from inversions of the MT data. The dashed line is the inferred suture. The inverted triangles denote the location of the MT sites. Only the alternate sites are numbered here for the sake of clarity. B) Deep geoelectric cross-section for the first 200 km.
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(Tzy, TE and TM, not shown) are very similar to the final model. A wide resistivity variation range and a main lateral discontinuity are clearly apparent at around 160 km from the site at the western end (Fig. 7A). It can be observed that the geometry of the main structures and resistivity values are coincident with the model represented previously by inverting only Tzy (Fig. 5). Both models
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show two low resistive structures given by the Chaco–Paranense basin (1 Ohm-m b ρ b 30 Ohm-m) to the east and a block more than 6 km deep (50 Ohm-m b ρ b 200 Ohm-m) to the west. Resistivity image to depths up to 200 km is shown in Fig. 7B. The presence of a conductor at a depth of 150–200 km in the central part of the studied profile produces a gradual decrease in resistivity above this anomaly.
Fig. 8. Normalized residual. From top to bottom: TE and TM Impedance phase and real and imaginary Tzy.
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Fig. 9. First 50 km geoelectric cross-section derived from inversions of the MT data using a vertical discontinuity between sites 760 and 765. The inverted triangles denote the location of the MT sites.
The responses predicted by the last model (Fig. 7A and B) are also shown as pseudosections in Fig. 4 to the right of the corresponding measured data. A comparison between observed data and model responses indicates that this model provides a very good fit for phases (ϕyx and ϕxy) and Tzy (real and imaginary parts), a reasonable fit for TM apparent resistivity (ρyx) and a worse fit for TE (ρxy) (in agreement with the large errors adopted in the inversion). These observations are consistent with the 1.2 value obtained for NRMS in the inversion. Furthermore, the normalized residuals are randomly distributed and in general they are in a range of −3 to 3. These normalized residuals for phase (ϕyx and ϕxy) and Tzy (real and imaginary parts) are presented in Fig. 8. Since the inversion code permits us to include discontinuities with no roughness penalty in the inversion process, this valuable inversion option was used to clarify the interpretation of different parts of the model. Therefore, we included a vertical discontinuity in the inversion process to simulate a possible sharp contrast due to the suture. This test is useful to evaluate the degree of connection between the shallow resistive structure to the west of this vertical discontinuity and the deeper resistive structure to the east of it. Both resistive structures seem to differ across the vertical discontinuity as it was expected from the geological data that suggest that there is a boundary between the RIC and the PAT (Fig. 9). The
resistivity is very high to the west below the Sierra whereas very low values within the Chaco–Paranense basin are found to the east up to around 6 km deep. Comparing average resistivity at the same depths within the 6–50 km range, the eastern area is much more resistive than the western one. This becomes clear when one examines the horizontal conductance (conductivity integrated horizontally from west to east for each depth). The horizontal conductance is expressed normalized by the length (in km) for each side of the profile, 150 km westward and 250 km eastward. In the first 10 km, the conductance is very high to the east because of the basin and, on the contrary, it is very low to the west because of the “Sierras”. Deeper than 10 km, resistivity tends to constant average values which differ between 1 and 2 orders of magnitude at both sides. These results provide conclusive evidence about structural differences in resistivity values for both sides of the vertical limit between RIC and PAT, as seen in Fig. 10. 4. Discussion Our preliminary results in López de Luchi et al. (2005) led us to consider that the limit between the PAT and the RIC might be located to the east of the Sierra Chica de Córdoba (Fig. 2). This fact is also supported by the lack of systematic TDM differences at both sides of
Fig. 10. Horizontal conductance (conductivity integrated horizontally from west to east for each depth) for both sides of the vertical limit between RIC and PAT. This magnitude was normalized by the length in km for each side of the profile, 150 km westward and 250 km eastward.
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Fig. 11. Maximum penetration depth reachable for different periods. It was calculated using the skin depth approximation and the conductance for each depth (conductivity integrated vertically from the top of the surface to depth) for the model shown in Fig. 7A.
the inferred limit between the Córdoba and the Pampia terranes and the comparable metamorphism ages as mentioned in the Geological background section. The location of a major lithospheric boundary along the eastern border of the Sierra Chica de Córdoba is further supported by the easternmost recorded seismic activity (Alvarado et al., 2005) related with the Andean compression acting on a sector where the flat-slab steepens (Cahill and Isacks, 1992). Isotopic results (Rapela et al., 1998) do not fit the idea of the eastern and the western Córdoba belt of ophiolites as limits for terranes (Kraemer et al., 1995), i.e. the hypothesis of the Córdoba terrane as a part of the Río de la Plata craton since not only no RIC provenance was identified in the Sierras de Córdoba meta-sediments (Schwartz and Gromet, 2004; Steenken et al., 2004; Rapela et al., 2005) but the provenance patterns are comparable in the entire Sierras de Córdoba and in the adjacent Sierra de San Luis (Steenken et al., 2004, 2006). The geoelectric cross-section presents a sharp lateral discontinuity separating structures with a high resistivity contrast and roughly a two layer resistivity structure. These results have been obtained in all the models that were presented, i.e. inverting both MT polarizations and Tzy and only Tzy, (Figs. 7 and 5). Tzy inversion offers an alternative model with especially sensitive to vertical discontinuities at depth and not influenced by static shift and other effects. Both models show west–east changes in the electrical structure within the analyzed first 50 km of the continental crust. Similar changes at both sides of a major discontinuity located at the longitude of Alta Gracia were described by López de Luchi et al. (2005). Induction vectors for the 10s and 107s periods exhibit a sharp change of direction next to Alta Gracia and for 1280s period the vectors are mainly eastward aligned. Interaction between the suture zone, the fault that borders the eastern sector of the Sierra Chica, the existence of the deep crustal conductive layer, and the Chaco– Paranense basin may produce a complex pattern. To clarify, the maximum penetration depth reachable for different periods was calculated using the skin depth approximation and the conductance for each depth (conductivity integrated vertically from the top of the surface to depth) for the model shown in Fig. 7A. Fig. 11 shows the estimated depth skin plotted for each period along the profile. This is an approximated simplification due to the fact that the reflections at interfaces should be taken into account in 2D structures. In the eastern area, where the basin is highly conductive, it is necessary to process more than 100s periods in order to pass through the first 10 km, whereas in the western sector, a maximum depth of up to 30–40 km could be reached at the same period, due to the extremely high resistive values at the first 5–10 km from the surface. It may be noted that the induction arrows shown at different periods along the profile (Fig. 3) could be influenced by structures placed at quite different depths between eastern and western parts of the assumed location of the suture, complicating their interpretation. Around the 755 site there is a subvertical limit between the highly resistive vertical zone of the RIC and the more conductive zone below Pampa de Achala that runs approximately along the
eastern border of the Sierra Chica de Córdoba which supports the Pampean terrane hypothesis (Rapela et al., 1998) and additionally seems to be correlated with the steepening of the subducting oceanic Nazca plate. This electric boundary persists even at the uppermost 6 km of the crust where it would control the border of the Chaco– Paranense basin against the highly resistive basement of the Sierras de Córdoba. In the eastern sector, between sites 760 and 845, the first kilometers with low resistivity (1–100 Ohm-m) correspond to the sedimentary sequences of the Chaco–Paranense basin as it was described by Favetto et al. (2004). Below this basin, the highly resistive (10,000 Ohm-m) zones that involve up to 150 km of the lithosphere (Fig. 7B) are interpreted as the RIC. The highly resistive zones are separated by a less resistive zone which reaches 30 Ohm-m at a depth of 150–200 km between sites 790 and 834. This electrical structure is similar to what was observed in a previous MT model across the Sierras Pampeanas (Booker et al., 2004). In the western sector of the profile, between sites 715 and 750 (approximately 100 km width), the first 6 km below the Sierras de Córdoba are the highest resistivity zone, around 10,000 Ohm-m, in agreement with the presence of metamorphic and igneous rocks that characterizes the Sierras Pampeanas in combination with the fact that the Sierras Pampeanas are at present under compression which would imply the difficulty of fluid circulation (Booker et al., 2004). The conductor at 30–70 km deep in the PAT also appears in models where the vertical discontinuity is set. The deep crustal conductive layer may indicate the existence of graphite in fossilized former shear zones developed as the result of the PAT–RIC collision or alternatively it may suggest the existence of a partial melted zone. During a collisional orogeny, major thrusts form a linked fault system rooted in the original subduction zone (e.g. Willett et al., 1993; Beaumont and Quinlan, 1994). Multiple activations of these major thrusts would lead to brittle overprinting of earlier ductile shear zones along discrete narrow faults. Deep reaching crustal fracture zones are suggested by the regional intrusions of mantle derived basaltic magmas through the crust in the Cretaceous and the Cenozoic in the Sierras de Córdoba. The present electrical structure of the PAT is influenced by their location on the flat-slab segment of the subducting oceanic Nazca plate. Therefore at present the real evidence for the former suture is provided by the different electrical structure of the two blocks. 5. Conclusions The MT results allowed us to draw interesting conclusions on the geoelectrical structures down to a depth of about 200 km. The RIC is a resistive structure (ρ ≈ 10,000 Ohm-m) that extends down to 150 km in depth. The PAT has a heterogeneous structure with a thin high resistivity layer down to 6 km and a conductive layer down to 70 km. The boundary between these two crustal units is located along the eastern border of the Sierra Chica de Córdoba. To the east of this boundary the RIC, a highly resistive crustal segment grades into a sector of relatively enhanced resistivity below the Chaco–Paranense
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basin. To the west, the PAT appears as a less resistive crustal segment below the most resistive 6 km top layer that corresponds to the Sierras de Córdoba. In summary, the sharp lateral discontinuities observed in the resistivity model in combination with the geological information suggest that the transition between the RIC and the PAT at present is located at the east of the Sierras de Córdoba. In consequence the meaning of the ophiolite belts as representing the location of crustal scale sutures remains unclear. Acknowledgements This research was supported by National Science Foundation Grants EAR99-09390 and EAR0310113 and by Agencia Nacional de Promoción Científica y Tecnológica PICT 99: 07-06313. The MT systems were provided by the EMSOC Instrument Facility supported by NSF Grants EAR02-36538 and EAR96-16421. Discussions with R. Martino, A. Rapalini, and A. Steenken were especially helpful for the understanding of the tectonic evolution of the Gondwana margin. 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