2D Magnetotelluric interpretation of the crust electrical resistivity across the Pampean terrane–Río de la Plata suture, in central Argentina

Tectonophysics 459 (2008) 54ndash65

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2D Magnetotelluric interpretation of the crust electrical resistivity across thePampean terranendashRiacuteo de la Plata suture in central Argentina

Alicia Favetto a Cristina Pomposiello a Moacutenica G Loacutepez de Luchi a John Booker b

a CONICET INGEIS Ciudad Universitaria C1428EHA Buenos Aires Argentinab University of Washington Department of Earth and Space Sciences Seattle WA 98195 USA

Corresponding author Fax +54 11 4783 3024E-mail addresses favettoingeisubaar (A Favetto)

(C Pomposiello) deluchiingeisubaar (MG Loacutepez de Lbookeresswashingtonedu (J Booker)

0040-1951$ ndash see front matter copy 2008 Elsevier BV Aldoi101016jtecto200711071

A B S T R A C T

A R T I C L E I N F O

Article history

Magnetotelluric data were o

Received 1 March 2006Received in revised form 29 November 2006Accepted 11 November 2007Available online 4 April 2008

KeywordsSutureSierras de CoacuterdobaSierras PampeanasMagnetotelluricsElectrical resistivity

btained along a 450 km almost westndasheast profile at approximately 315deg S whichextends from La Rioja to Santa Feacute provinces in central Argentina The profile crosses two main crustaldomains that were juxtaposed during the EarlyndashMiddle Cambrian Pampean Orogeny the Pampean terraneto the west and the Riacuteo de la Plata craton to the east The electrical resistivity structure of the crustal domainstogether with their boundary is presented Through dimensionality analysis of the data it was demonstratedthat regional-scale electrical structures are mainly two-dimensional with a strike direction oriented parallelto the surface geological strike The resistivity model shows a subvertical limit approximately along theeastern border of the Sierra Chica de Coacuterdoba To the east the shallower structure is the ChacondashParanensebasin extending to a depth of 6 kmwith resistivities between 1 and 30 Ohm-m whereas below the basin theca 21ndash23 Ga Riacuteo de la Plata craton shows resistivities in a range of 300ndash10000 Ohm-m The Pampeanterrane presents a 6 km layer with a resistivity higher than 10000 Ohm-m whereas below this layer theresistivity values range from 50 to 200 Ohm-m Based on both the geological information and themagnetotelluric results the sharp lateral discontinuity observed in the resistivity model to the east ofthe Sierras de Coacuterdoba is conjectured to represent the boundary between the Riacuteo de la Plata craton and thePampean terrane which may correspond to the Early Cambrian suture

copy 2008 Elsevier BV All rights reserved

1 Introduction

The location of crustal blocks boundaries and their geometry atdepth is in some cases controversial because boundary definition isoften based on relatively limited surface and borehole geologicinformation and seismic reflection data Terranes are generallyconsidered as exotic crustal blocks separated from a continentalnucleus by suture zones In general these zones result from theaccretion of terranes after subduction of an intervening ocean aprocess that usually also produces a series of ophiolites and volcanism(Keary and Vine 1990 Meissner 1996) Reliable reconstructions ofcollisional and accretionary processes are almost impossible becausemany of the characteristic features of collision are no longer preservedin the deeply eroded orogens (Key et al 1989)

The magnetotelluric (MT) method involves the measurement oforthogonal components of natural electric and magnetic fields whichcontain information about the electrical resistivity distribution at crustaland upper mantle depths This method is well suited for studyingregional structures and may help in defining crustal variations and

cpomposiingeisubaaruchi)

l rights reserved

terrane boundaries based on lateral changes of resistivity In particularthis methodology has been successful in identifying suture zones ieacross the Ossa Morena Zone and South Portuguese Zone suture(Monteiro Santos et al 1999) the Palaeoproterozoic Trans-Hudsonorogen of North America (Jones et al 2005) and in the vicinity of theBanggongndashNujiang in central Tibet (Solon et al 2005)

In southern South America geological and paleomagnetic dataindicate a remarkably sharp crustal boundary along the westernmargin of West Gondwana between the Riacuteo de la Plata craton (RIC)and the basement of the Sierras Pampeanas (Fig 1) The basementunderlying Late Palaeozoic and younger sediments in central andsouthern Coacuterdoba is juvenile Palaeoproterozoic crust of the Riacuteo de laPlata craton (Rapela et al 2005) whereas the basement of the SierrasPampeanas shows a common crustal history with the ArequipandashAntofalla massif (Steenken et al 2004 and references therein)

The first hypothesis about a boundary between the basement of theSierrasPampeanasand theRICwasbasedon thebelts of ophiolites of theSierras de Coacuterdoba the easternmost mountain range of the SierrasPampeanas The eastern belt of ophiolites of the Sierraswas identified asa lherzolite ophiolite with an inferred back-arc origin (Kraemer et al1995) which led to the hypothesis of the Coacuterdoba terrane as a part of theRIC Thewestern belt of ophiolites of the Sierraswas considered to showmid-oceanic ridge affinities (Ramos et al 2000) and to represent thesuture resulting from the accretion of the Pampia terrane to the RIC On

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 Kroumlner and Cordani 2003) Dashed lines indicate inferred positions of major cratonic fragments belowPhanerozoic cover Cratons abbreviations SFmdashSatildeo Francisco RAmdashRio Apa PRmdashParana RPmdashRio de la Plata PAmdashPampiaPampean AAmdashArequipandashAntofalla LAmdashLuiz Alves CGmdashCentral Goias

55A Favetto et al Tectonophysics 459 (2008) 54ndash65

the contrary an Early CambrianndashMiddle Cambrian collision between thePampean Terrane (PAT) and the RIC with the corresponding suturerunning along the east of the Sierras deCoacuterdoba (Fig 2)wasproposed byRapela et al (1998)

Fig 2 Geological sketch of the studied area showing the locations of the MT sites LocationSierra de Coacuterdoba and San Luis is based on the Geological Map of Coacuterdoba (1500000) and GGa correspond to TDM (Depleted mantle model age) after Rapela et al (1998) and Steenken etthe studied crustal units

Uncertainties in the definition of the limits of the accreted terranethe structural mechanisms of the collisional event and the unknownwidth of the former oceanic basins together with younger amalgama-tion of terranes along the western border of the PAT and subsequent

of the proposed sutures containing ultramafic rocks after Ramos (1999) Geology of theeological Map of San Luis (1500000) (Servicio Geoloacutegico Minero Argentino) Values inal (2004) They are included in order to show some of the isotopic differences between

56 A Favetto et al Tectonophysics 459 (2008) 54ndash65

tectonic events have led to an imprecise location of the Early Cambriansuture

Loacutepez de Luchi et al (2005) analyzed data from 19 MT sites along a200 km westndasheast profile around 315deg S that extends from the top ofthe Sierra Grande de Coacuterdoba across the plains up to the west of theSanta Feacute province Along the profile the crustal structure indicates thepresence of lateral resistivity variations As the sector with the highresistivity contrast between two major blocks coincides with ageological feasible terrane boundary between the RIC and the PATthese authors proposed that the suture may be located close to theeastern border of the Sierra Chica de Coacuterdoba

The aim of this paper is to improve the previous interpretation inthe vicinity of the inferred suture zone namely the PampeanndashRiacuteo de laPlata suture (PARIS) between the RIC and PAT Ten new MT sites wereadded to the original profile in Loacutepez de Luchi et al (2005) extending itup to a total lengthof around450 km to avoid border effects close of thearea of interest Therefore twenty nine MT sites were included in thisstudywhich is focused on analyzing the eastern border of the Sierra deCoacuterdoba in order to detect lateral variations in electrical resistivitiesthat may correspond to a major geological terrane boundary

2 Geological background

Different paleogeographic reconstructions that mainly rely onpaleomagnetic data have been proposed for the chronology andsequence of accretions that led to Gondwana assembly (eg Brito-Neves et al 1999 Trompette 2000 Alkmim et al 2001 SanchezBettucci and Rapalini 2002 Veevers 2004 Rapalini 2005 andreferences therein) The South American and African continentalblocks (Fig 1) assembled into West Gondwana (Amazonia WestAfrica Riacuteo de la Plata Satildeo FranciscoCongo and Borborema Brito-Neves and Cordani 1999 Campos-Neto 2000 Alkmim et al 2001Cordani et al 2001 2003) or alternatively into a smaller WesternGondwana (West AfricandashAmazonia Riacuteo Apas Sanchez Bettucci andRapalini 2002) and Central Gondwana (West NilendashCongondashSatildeoFranciscondashRiacuteo de la PlatandashArabia Brito-Neves et al 1999 SanchezBettucci and Rapalini 2002) during several stages of ocean closuresand collisions between 900 and ~550 Ma (Cordani et al 2001 2003)which resulted in the Pan-AfricanndashBrasiliano fold belts

Collision of Amazonia (proto-western Gondwana) with Riacuteo de laPlatandashSatildeo Francisco (central Gondwana) as one of the latest events inWestern Gondwana amalgamation has been recently proposed byAlkmim et al (2001) The closure of a large ocean separating thesecratons from Amazonia and West Africa that is supported by availablepaleomagnetic data probably occurred by the end of the Ediacaran orbeginning of Cambrian (Rapalini 2005) Paleogeographic reconstruc-tions indicate that the Pampean terranendashRiacuteo de la Plata craton suture(PARIS) would result from the closure of a large oceanic domain theGoiaacutesndashPharusian Pampean Ocean (Rapalini 2005 and referencestherein) Drifting apart of Laurentia and Amazonia (Fig 1) may haveled to the closure of this ocean being the collision between the RIC andthe PAT the final event (Cordani et al 2001 2003 Rapalini 2005)Paleomagnetic data from the Rio de la Plata craton suggest that thisblock was already assembled to most major Gondwana blocks by theend of the Proterozoic (Sanchez Bettucci and Rapalini 2002 andreferences therein) Although the PAT lacks pre-Late Cambrianpaleomagnetic data recently obtained Late Cambrian paleomagneticdata suggest consistent paleo-latitudes with the rest of Gondwana(Rapalini 2005 Rapela et al 2005) As the geological evidencesupports a link between Amazonia and the Pampean terrane(Schwartz and Gromet 2004 Steenken et al 2004) it is reasonableto infer that the RIC and the PAT were juxtaposed between the end ofthe Proterozoic and the beginning of the Cambrian

The Riacuteo de la Plata craton (Fig 1) is made up by a Palaeoproterozoicnucleus 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 2003Saalmann et al 2005 and references therein) Towards the west theRIC is covered by the very thick Phanerozoic sedimentary deposits ofthe ChacondashParanense basin

The juvenile Palaeoproterozoic ages and isotopic signature in theArgentina outcrops of the RIC corresponds to localities in Tandilia andto boreholes in the western sector of the ChacondashParanense basin(Pankhurst et al 2003 Rapela et al 2005)

The Sierras Pampeanas a morphotectonic unit that constitutesgenerally NndashS striking mountain ranges in central and northwesternArgentina are characterized by Late-PrecambrianndashEarly Paleozoicmetamorphic and igneous rocks The Sierras Pampeanas were dividedinto two main parts the Eastern and the Western Sierras Pampeanas(Caminos 1979) The Eastern Sierras Pampeanas are composed largely ofOrdovician Cambrian or older metasedimentary rocks of variablemetamorphic grade intrudedbydifferent suites of CambriantoDevoniangranitoids (Rapela et al 1998 Sims et al 1998 Rapela 2000 amongothers) Regional metamorphism is attributed to the Cambrian Pampeanand to the Ordovician Famatinian orogenies (Pankhurst et al 1998Rapela et al 1998 Thomas and Astini 2003 and references therein)

The basement blocks of Sierras Pampeanas are located on thedeformed and faulted foreland of the Pampean flat-slab segment ofthe Nazca plate (Ramos et al 2002) Older crustal discontinuities inthe foreland ie sutures and shear zones played a strong role in theinception and geometry of the main faults that differentially upliftedthe blocks of Sierras Pampeanas (Ramos 1994) Uplift of the basementblocks of the Sierras Pampeanaswas the result of Andean compressionduring late Cenozoic times (Ramos et al 2002 and references therein)

The Sierras de Coacuterdoba the easternmost block of the EasternSierras Pampeanas records a dominantly Cambrian high temperaturelow- to medium-pressure metamorphism and the intrusion of I- andS-type granitic plutons that results from the Pampean Orogeny(Rapela et al 1998)

The concept of the Pampean Orogeny a continental scale collisionthat affected a large sector of the proto-Andean margin of Gondwanafrom at least 17deg to 33deg S involves the accretion to the western marginof Gondwana ie to the RIC of the PAT a Neoproterozoic rift-relatedcontinental fragment during an east-facing subduction event The PAT(Pampia and Coacuterdoba terranes see below) as defined by Rapela et al(1998) encompasses a crustal block that extends from the easternborder of the Sierras de Coacuterdoba up to the boundary with the CuyaniaPrecordillera terrane (~68deg W)

Ramos (1999) considered that a latest Precambrian collisionbetween the RIC and a so-called Pampia Terrane followed the closureof an ocean basin located to the west of a magmatic arc developed inthe western part of the RIC The suture between the RIC and thePampia Terrane would be represented by the western ultramafic beltof ophiolites of the Sierras de Coacuterdoba (Kraemer et al 1995) whereasthe eastern ultramafic belt would be the result of the closure of a back-arc basin Therefore the so-called Coacuterdoba terrane that underlay mostof the Sierras de Coacuterdoba should be part of the RIC (Fig 1)

Most models that encompass the idea of a suture between thebasement of the Sierras Pampeanas and the RIC consider that themetasedimentary rocks of the Sierras de Coacuterdoba are part of an originalpassive margin sequence which developed into an accretionary prismduring a period of eastward-facing subduction along the SouthAmerican margin (Kraemer et al 1995) Rapela et al (1998) proposedthat the metasedimentary rocks of the Sierras de Coacuterdoba are syn-subduction deposits on or marginal to the PAT that were part of thePuncoviscana basin andwere deformed andmetamorphosed during thePampean Orogeny Therefore in his hypothesis most of the Sierras areunderlain by the PATand the suture zone should be located somewhereto the east of the Sierras de Coacuterdoba

The Pampeanmobile belt in the Sierras de Coacuterdoba and in generalmost meta-sediments of the Eastern Sierras Pampeanas do not recordzircon patterns with conspicuous RIC provenance (Schwartz and


Gromet 2004 Rapela et al 2005 Steenken et al 2006) On thecontrary they share a within errors uniform TDM (time of extraction ofthe depleted mantle) of 16ndash17 Ga (Rapela et al 1998 Pankhurst etal 1998 Steenken et al 2004 and references therein) that does not fitthe data for the RIC (Pankhurst et al 2003 Saalmann et al 2005) butsuggests a common crustal history for the PAT and the ArequipandashAntofalla massif (Steenken et al (2004 and references therein) Nosystematic TDM differences or contrasting metamorphic grade(Escayola and Kraemer 2003) were seen at both sides of the westernophiolite belt of the Sierras de Coacuterdoba (Rapela et al 1998 Steenkenet al 2004) Additionally the PARIS also appears to represent a broadzone of rheological discontinuity along which the eastern front of theAndean was developed (Booker et al 2004)

Fig 3 Induction vectors for 10s 107s and 1280s periods (Parkinsons convention) for the 29current concentrations

This evidence together with our previous results (Loacutepez de Luchiet al 2005) led us to consider the feasibility of the model of the PATwhere only one suture zone should be present

3 Magnetotelluric interpretation

31 Data analysis

MT long period data were acquired between 2001 and 2004 at 29sites along a 450 km westndasheast profile at about 315deg S crossingthrough Alta Gracia city It extended from the east of La Rioja province(~66deg W) across the Sierra Grande Sierra Chica de Coacuterdoba and theplains up to the east of the Santa Feacute province as seen in Fig 2

MT sites In a two-dimensional situation they are normal to strike and point towards

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-mand degrees The measured data are shown on the left column and the response of the model corresponding to Fig 7A and B with nmrs=15 is presented on the right column



These data were collected with a new generation of low powerlong period GPS-controlled MT systems (NIMS) They were processedusing robust statistical time series analysis and remote referencemultisite method (Egbert 1997)

The regional strike was determined to be very close to NndashS at mostof the sites using impedance tensor decomposition for electric andmagnetic distortion (Chave and Smith 1994) this fact is in agreementwith the approximately northndashsouth tectonic fabric of the Sierras deCoacuterdoba In this analysis we found the error level at which a 2Dmodelis an acceptable representation of the structure and we used this errorlevel in subsequent inversions that assume a regional 2D Earth Thesites located close to the eastern border of the Sierra Chica (745 750and 755) had a significant vertical magnetic field distortion at highfrequencies (more than 100 Hz) and could be neglected at lowfrequencies The three last sites at the western end of the profileshowed a poor determination of strike direction with larger errorsthan the other sites

32 Induction vectors

Induction vectors are a widely accepted tool for studying bidimen-sionality Parkinson vectors pointing towards current concentrationregions were calculated They should be directed orthogonal to strikedirection in 2D structures and their magnitudes are larger in resistiveterrains (Parkinson1962) Their real parts for three different periods arepresented in Fig 3 Short period induction vectors are more influencedby the shallow structure At 10s they have a small magnitude all alongthe profile except in those sites near 750 At this zone (Fig 2) which iscentered in the proposed suture zone they are especially large For 107sand 1280s vectors are mostly controlled by regional structures and arealmost orthogonal to the strike direction Their magnitude is moderateexcept for the easternmost sites where vectors are very small For 107svectors their direction in the area of the suture zone is coincident to theone at 10s but they are smaller in magnitude

Vectors at the three westernmost sites are pointing to the SE theirdeviations from EndashW direction were consistent with the single-sitedeterminations of the strike This is in agreementwith the fact that theregional strike changes its direction slightly towards the SE at thosesites The fault that borders the Sierra to the west may be related to asubvertical conductor considering that the vectors close to the faultare pointing to it

Fig 5 Electrical resistivity model for the eastndashwest magnetotelluric transect across Argentinasuture The inverted triangles denote the location of the MT sites

Vectors for 1280s show a more uniform EndashW aligned pattern overmost of the profile pointing to the east and mainly controlled by thevery conductive ChacondashParanense basin Vectors at the three western-most sites keep on pointing to the east which could be related with adeeper conductive layer

33 2D inversion

For2Dstructures data canbeseparated into two independentmodeswith electric current flowing parallel (TE) and perpendicular (TM) to thestrike direction (NndashS) Pseudosections of apparent resistivity (ρ) andphase (ϕ) for both polarizations and real and imaginary parts of thetransfer function between vertical and horizontalmagnetic field (Tzy) inmeasurement coordinates for the frequency range of 10minus4ndash10minus1 Hz areshown in Fig 4

MT impedance data were inverted using the NLCG algorithm ofRodi and Mackie (2001) Usually apparent resistivities and phases inboth polarizations TE (ρxyϕxy) and TM (ρyxϕyx) and Tzy can beinverted using this algorithm It minimizes model roughness subjectto fitting the data to a prescribed misfit Version 610 allows us toinclude discontinuities between arbitrary blocks in the model

We started inverting Tzy to explore if the obtainedmodel shows thelocation of the suture zone considering that Tzy might be particularlysensitive to vertical discontinuities It is assumed thatmeasured verticalmagneticfield ismainly dominated by currents induced in the Earth andonly thehorizontalmagnetic EWfield component is involved Thereforethis interpretation is less liable to static shift and certain 3D effects

To invert Tzy data the starting model included two conductiveblocks imbedded into a half-space of 1000 Ohm-m whose resistivitieswere fixed during the inversion The first block with a resistivity of03 Ohm-m and a thickness of 4000 m was placed at the top of themodel far from the profile (west of the coast of Chile ~72deg W)representing the Pacific Ocean trench at themost western end and theother with a resistivity of 3 Ohm-m simulating the conductivemantle was located deeper than 700 km Different values of thestarting half-space resistivity (from10 to 10000 Ohm-m)were used tocheck their influences on the final model and it resulted in a finalmodel practically not dependent on that initial value This is mainlydue to the presence of the fixed conductive blocks The inversionprocess started using a large value of the regularization parameter(τ=1000) and 003 as its error floor This produced a very smooth

at about 315deg S derived from inversions of only Tzy data The dashed line is the inferred

Fig 6 NRMS as a function of model roughness obtained setting different regularizationparameter values (τ) Misfit improves and model roughness increases as τ decreasesτ=10 was the value selected to obtain the MT models


model that was used as starting model for subsequent inversionMisfit improved and model roughness increased as τ decreased anderror floor was set to 002 When the inversion normalized root meansquare (NRMS) was about 12 and τ was 10 the model was achievedThis model provides clear evidence of boundaries with resistivity

Fig 7 A) Top Topographic profile Below First 50 km geoelectric cross-section derived fromdenote the location of the MT sites Only the alternate sites are numbered here for the sake

contrasts as seen in Fig 5 The vertical discontinuity is observed aroundsite 750 and it is indicated byan imaginary line projected surfacewardsalong this interface points close to the vicinity of Alta Gracia city

Multiple inversions were performed to establish the set ofparameters to be used in the inversion process in order to obtain amodel consistent with the geology and a similar misfit for the wholedata set Both modes TE and TM and Tzy were jointly inverted Dataerror floors of 30 in TM (ρyx) an error large enough to avoid using it inthe inversion in TE (ρxy) (to prevent static shift effects and its sus-ceptibility to distortion by off-profile structure) and 29deg in the phases(ϕyx and ϕxy) were set These values are consistent with the error floorsused in the decomposition and special features of individual sitestrike and 002 in Tzy to avoid overfitting it (Fig 5) We beganinverting TMand Tzy data in order to avoidfitting TE data tighter thanthe other data for this particular dataset The first inversion startedusing the same initial model used for Tzy inversion (with the half-space of 1000 Ohm-m and two conductive blocks) the same largestarting value of the regularization parameter (τ=1000) and largerstarting errors floors for Tzy and ρyx Again this produced a verysmooth model that was used as a starting model for subsequentinversion misfit improves and model roughness increases as τdecreases Then TE mode was included in the inversion which wasfinished for τ=10 with a final NMRS of 15 to satisfy the 2D condi-tion the variation of NMRS with roughness is shown in Fig 6 Thebidimensional behavior is further reinforced by the fact that themodels obtained by the separate inversion of the different data sets

inversions of the MT data The dashed line is the inferred suture The inverted trianglesof clarity B) Deep geoelectric cross-section for the first 200 km


(Tzy TE and TM not shown) are very similar to the final model Awide resistivity variation range and a main lateral discontinuity areclearly apparent at around 160 km from the site at the westernend (Fig 7A) It can be observed that the geometry of the mainstructures and resistivity values are coincident with the modelrepresented previously by inverting only Tzy (Fig 5) Both models

Fig 8 Normalized residual From top to bottom TE and

show two low resistive structures given by the ChacondashParanensebasin (1 Ohm-mbρb30 Ohm-m) to the east and a block more than6 km deep (50 Ohm-mbρb200 Ohm-m) to the west Resistivityimage to depths up to 200 km is shown in Fig 7B The presence of aconductor at a depth of 150ndash200 km in the central part of the studiedprofile produces a gradual decrease in resistivity above this anomaly

TM Impedance phase and real and imaginary Tzy

Fig 9 First 50 km geoelectric cross-section derived from inversions of theMT data using a vertical discontinuity between sites 760 and 765 The inverted triangles denote the locationof the MT sites


The responses predicted by the last model (Fig 7A and B) are alsoshown as pseudosections in Fig 4 to the right of the correspondingmeasured data A comparison between observed data and modelresponses indicates that this model provides a very good fit for phases(ϕyx andϕxy) and Tzy (real and imaginary parts) a reasonablefit for TMapparent resistivity (ρyx) and aworsefit for TE (ρxy) (in agreementwiththe large errors adopted in the inversion) These observations areconsistent with the 12 value obtained for NRMS in the inversionFurthermore thenormalized residuals are randomly distributed and ingeneral they are in a range of minus3 to 3 These normalized residuals forphase (ϕyx andϕxy) and Tzy (real and imaginary parts) are presented inFig 8

Since the inversion code permits us to include discontinuitieswith no roughness penalty in the inversion process this valuableinversion option was used to clarify the interpretation of differentparts of the model Therefore we included a vertical discontinuityin the inversion process to simulate a possible sharp contrast due tothe suture This test is useful to evaluate the degree of connectionbetween the shallow resistive structure to the west of this verticaldiscontinuity and the deeper resistive structure to the east of itBoth resistive structures seem to differ across the vertical discon-tinuity as it was expected from the geological data that suggestthat there is a boundary between the RIC and the PAT (Fig 9) The

Fig 10 Horizontal conductance (conductivity integrated horizontally fromwest to east for enormalized by the length in km for each side of the profile 150 km westward and 250 km

resistivity is very high to the west below the Sierra whereas verylow values within the ChacondashParanense basin are found to the eastup to around 6 km deep Comparing average resistivity at the samedepths within the 6ndash50 km range the eastern area is much moreresistive than the western one This becomes clear when oneexamines the horizontal conductance (conductivity integratedhorizontally from west to east for each depth) The horizontalconductance is expressed normalized by the length (in km) for eachside of the profile 150 km westward and 250 km eastward In thefirst 10 km the conductance is very high to the east because of thebasin and on the contrary it is very low to the west because of theldquoSierrasrdquo Deeper than 10 km resistivity tends to constant averagevalues which differ between 1 and 2 orders of magnitude at bothsides These results provide conclusive evidence about structuraldifferences in resistivity values for both sides of the vertical limitbetween RIC and PAT as seen in Fig 10

4 Discussion

Our preliminary results in Loacutepez de Luchi et al (2005) led us toconsider that the limit between the PAT and the RIC might be locatedto the east of the Sierra Chica de Coacuterdoba (Fig 2) This fact is alsosupported by the lack of systematic TDM differences at both sides of

ach depth) for both sides of the vertical limit between RIC and PAT This magnitude waseastward

Fig 11 Maximum penetration depth reachable for different periods It was calculated using the skin depth approximation and the conductance for each depth (conductivityintegrated vertically from the top of the surface to depth) for the model shown in Fig 7A


the inferred limit between the Coacuterdoba and the Pampia terranes andthe comparable metamorphism ages as mentioned in the Geologicalbackground section

The location of a major lithospheric boundary along the easternborder of the Sierra Chica de Coacuterdoba is further supported by theeasternmost recorded seismic activity (Alvarado et al 2005) relatedwith the Andean compression acting on a sector where the flat-slabsteepens (Cahill and Isacks 1992) Isotopic results (Rapela et al 1998)do not fit the idea of the eastern and the western Coacuterdoba belt ofophiolites as limits for terranes (Kraemer et al 1995) ie thehypothesis of the Coacuterdoba terrane as a part of the Riacuteo de la Platacraton since not only no RIC provenance was identified in the Sierrasde Coacuterdoba meta-sediments (Schwartz and Gromet 2004 Steenkenet al 2004 Rapela et al 2005) but the provenance patterns arecomparable in the entire Sierras de Coacuterdoba and in the adjacent Sierrade San Luis (Steenken et al 2004 2006)

The geoelectric cross-section presents a sharp lateral discontinuityseparating structures with a high resistivity contrast and roughly atwo layer resistivity structure These results have been obtained in allthe models that were presented ie inverting both MT polarizationsand Tzy and only Tzy (Figs 7 and 5) Tzy inversion offers an alternativemodel with especially sensitive to vertical discontinuities at depth andnot influenced by static shift and other effects Both models showwestndasheast changes in the electrical structure within the analyzed first50 km of the continental crust Similar changes at both sides of amajordiscontinuity located at the longitude of Alta Gracia were described byLoacutepez de Luchi et al (2005)

Induction vectors for the 10s and 107s periods exhibit a sharpchange of direction next to Alta Gracia and for 1280s period thevectors are mainly eastward aligned Interaction between the suturezone the fault that borders the eastern sector of the Sierra Chica theexistence of the deep crustal conductive layer and the ChacondashParanense basin may produce a complex pattern To clarify themaximum penetration depth reachable for different periods wascalculated using the skin depth approximation and the conductancefor each depth (conductivity integrated vertically from the top of thesurface to depth) for the model shown in Fig 7A Fig 11 shows theestimated depth skin plotted for each period along the profile This isan approximated simplification due to the fact that the reflections atinterfaces should be taken into account in 2D structures In the easternarea where the basin is highly conductive it is necessary to processmore than 100s periods in order to pass through the first 10 kmwhereas in the western sector a maximum depth of up to 30ndash40 kmcould be reached at the same period due to the extremely highresistive values at the first 5ndash10 km from the surface It may be notedthat the induction arrows shown at different periods along the profile(Fig 3) could be influenced by structures placed at quite differentdepths between eastern and western parts of the assumed location ofthe suture complicating their interpretation

Around the 755 site there is a subvertical limit between thehighly resistive vertical zone of the RIC and the more conductivezone below Pampa de Achala that runs approximately along the

eastern border of the Sierra Chica de Coacuterdoba which supports thePampean terrane hypothesis (Rapela et al 1998) and additionallyseems to be correlated with the steepening of the subducting oceanicNazca plate This electric boundary persists even at the uppermost6 km of the crust where it would control the border of the ChacondashParanense basin against the highly resistive basement of the Sierrasde Coacuterdoba

In the eastern sector between sites 760 and 845 the firstkilometers with low resistivity (1ndash100 Ohm-m) correspond to thesedimentary sequences of the ChacondashParanense basin as it wasdescribed by Favetto et al (2004) Below this basin the highly resistive(10000 Ohm-m) zones that involve up to 150 km of the lithosphere(Fig 7B) are interpreted as the RIC The highly resistive zones areseparated by a less resistive zone which reaches 30 Ohm-m at a depthof 150ndash200 km between sites 790 and 834 This electrical structure issimilar to what was observed in a previous MT model across theSierras 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 deCoacuterdoba are the highest resistivity zone around 10000 Ohm-m inagreement with the presence of metamorphic and igneous rocks thatcharacterizes the Sierras Pampeanas in combinationwith the fact thatthe Sierras Pampeanas are at present under compressionwhich wouldimply the difficulty of fluid circulation (Booker et al 2004)

The conductor at 30ndash70 km deep in the PAT also appears in modelswhere the vertical discontinuity is set The deep crustal conductivelayer may indicate the existence of graphite in fossilized former shearzones developed as the result of the PATndashRIC collision or alternativelyit may suggest the existence of a partial melted zone During acollisional orogeny major thrusts form a linked fault system rooted inthe original subduction zone (eg Willett et al 1993 Beaumont andQuinlan 1994) Multiple activations of these major thrusts would leadto brittle overprinting of earlier ductile shear zones along discretenarrow faults Deep reaching crustal fracture zones are suggested bythe regional intrusions of mantle derived basaltic magmas through thecrust in the Cretaceous and the Cenozoic in the Sierras de CoacuterdobaThe present electrical structure of the PAT is influenced by theirlocation on the flat-slab segment of the subducting oceanic Nazcaplate Therefore at present the real evidence for the former suture isprovided by the different electrical structure of the two blocks

5 Conclusions

The MT results allowed us to draw interesting conclusions on thegeoelectrical structures down to a depth of about 200 km The RIC is aresistive structure (ρasymp10000 Ohm-m) that extends down to 150 kmin depth The PAT has a heterogeneous structure with a thin highresistivity layer down to 6 km and a conductive layer down to 70 km

The boundary between these two crustal units is located along theeastern border of the Sierra Chica de Coacuterdoba To the east of thisboundary the RIC a highly resistive crustal segment grades into asector of relatively enhanced resistivity below the ChacondashParanense


basin To the west the PAT appears as a less resistive crustal segmentbelow themost resistive 6 km top layer that corresponds to the Sierrasde Coacuterdoba

In summary the sharp lateral discontinuities observed in theresistivity model in combination with the geological informationsuggest that the transition between the RIC and the PAT at present islocated at the east of the Sierras de Coacuterdoba In consequence themeaning of the ophiolite belts as representing the location of crustalscale sutures remains unclear

Acknowledgements

This research was supported by National Science Foundation GrantsEAR99-09390 and EAR0310113 and by Agencia Nacional de PromocioacutenCientiacutefica y Tecnoloacutegica PICT 99 07-06313 The MT systems wereprovided by the EMSOC Instrument Facility supported by NSF GrantsEAR02-36538 andEAR96-16421 DiscussionswithRMartino A Rapaliniand A Steenken were especially helpful for the understanding of thetectonic evolution of the Gondwana margin Aurora Burd helped inprocessing data Gabriel Giordanengo of INGEIS provided valuabletechnical assistance during the field campaigns

References

Alkmim FF Marshak S Fonseca MA 2001 Assembling West Gondwana in theNeoproterozoic clues from the Satildeo Francisco craton region Brazil Geology 29 (4)319ndash322

Alvarado P Beck S Zandt G Araujo M Triep E 2005 Crustal deformation in thesouthndashcentral Andes back-arc terranes as viewed from regional broad-band seismicwaveform modeling Geophys J Int 163 (2) 580ndash598

Basei MAS Siga Jr O Masquelin H Harara OM Reis Neto JM Preciozzi F 2000The Dom Feliciano Belt and the Riacuteo de la Plata Craton tectonic evolution andcorrelation with similar provinces of southwestern Africa In Cordani UG MilaniEJ Thomaz Filho A Campos DA (Eds) Tectonic Evolution of South America 31st

International Geological Congress Riacuteo de Janeiro Brazil pp 311ndash334Beaumont C Quinlan G 1994 A geodynamic framework for interpreting crustal-scale

seismic-reflectivity patterns in compressional orogens Geophys J Int 116 (3)754ndash783

Booker J Favetto A Pomposiello MC 2004 Low electrical resistivity associated withplunging of the Nazca flat slab beneath Argentina Nature 429 399ndash403

Brito-Neves BB Cordani UG 1999 Tectonic evolution of South America during theLate Proterozoic Precambrian Res 53 23ndash40

Brito-Neves BB Campos-Neto MC Fuck RA 1999 From Rodinia to WesternGondwana an approach to the BrasilianondashPan African cycle and orogenic collageEpisodes 22 155ndash166

Cahill T Isacks B 1992 Seismicity and shape of the subducting Nazca plate J GeophysRes 97 17503ndash17529

Caminos R 1979 Sierras Pampeanas de Tucumaacuten La Rioja y San Juan In Turner JCM(Ed) 2nd Simposio de Geologiacutea Regional Argentina Academia Nacional de CienciasCoacuterdoba pp 41ndash80

Campos-Neto MC 2000 Orogenic systems from southwestern Gondwana In CordaniUG Milani EJ Thomaz-Filho A Campos DA (Eds) Tectonic Evolution of SouthAmerica 31st International Geological Congress Riacuteo de Janeiro pp 335ndash365

Cingolani CA Dalla Salda LH 2000 Buenos Aires cratonic region In Cordani UGMilani EJ Thomas Filho A Campos DA (Eds) 31st International GeologicalCongress Rio de Janeiro pp 139ndash146

Chave AD Smith JT 1994 On electric and magnetic galvanic distortion tensordecompositions J Geophys Res 99 4669ndash4682

Cordani UG Brito-Neves BB DAgrella-Filho MS 2001 From Rodinia to Gondwanaa review of the available evidence from South America Gondwana Res 4 600ndash602

Cordani UG DAgrella-Filho MS Brito-Neves BB Trindade RIF 2003 Tearing upRodinia the Neoproterozoic palaeogeography of South American cratonic frag-ments Terra Nova 15 (5) 350ndash359

Dalla Salda LH 1999 Cratoacuten del Riacuteo de la Plata 1 Basamento graniacutetico-metamoacuterficode Tandilia y Martin Gracia An Inst Geol Recur Miner (SEGEMAR) 29 97ndash100

Dalla Salda LH Bossi J Cingolani CA 1988 The Riacuteo de la Plata cratonic region ofsouthwestern Gondwanaland Episodes 11 263ndash269

Egbert GD 1997 Robust multiple station magnetotelluric data processing Geophys JInt 130 (2) 475ndash496

Escayola MP Kraemer PE 2003 Significado geotectoacutenico de las suturas de las SierrasPampeanas orientales en la faja Orogeacutenica Coacuterdoba posible correlacioacuten con losoroacutegenos brasilianos Rev Bras Geocienc 33 69ndash76

Favetto A Pomposiello MC Benedit T Booker J 2004 Magnetotelluric model of theChacoparanense sedimentary basin at 315degS Argentina Proceedings of the 17thWorkshop on Electromagnetic induction in the Earth IAGA WG 12 Available atwwwemIndia 2004org

Hartmann LA Santos JOS Cingolani CA McNaughton NJ 2003 Two Palaeopro-terozoic orogenies in the evolution of the Tandilia Belt Buenos Aires as evidencedby zircon UndashPb SHRIMP geochronology Int Geol Rev 44 528ndash543

Jones AG Ledo J Ferguson IJ 2005 Electromagnetic images of the Trans-Hudsonorogen the North American Central Plains anomaly revealed Can J Earth Sci 42457ndash478

Keary P Vine FJ 1990 Global Tectonics Blackwell Scientific Publishing London 320pp

Key RM Charsley TJ Hackman BDWilkinson AF Rundle CC1989 Superimposedupper Proterozoic collision-controlled orogenies in the Mozambique orogenic beltof Kenya Precambrian Res 44 197ndash212

Kraemer PE Escayola MP Martino RD 1995 Hipoacutetesis sobre la evolucioacuten tectoacutenicaneoproterozoica de las Sierras Pampeanas de Coacuterdoba (30deg40primendash32deg40prime) Rev AsocGeol Argent 50 47ndash59

Kroumlner A Cordani UG 2003 African southern Indian and South American cratonswere not part of the Rodinia supercontinent evidence from field relationships andgeochronology Tectonophysics 375 325ndash352

Loacutepez de Luchi MG Favetto A Pomposiello MC Booker J 2005 Magnetotelluricevidence for the suture between the Riacuteo de la Plata and the Pampean cratons at 31deg40prime Coacuterdoba province Argentina 6th International Symposiumon AndeanGeodynamics (ISAG 2005 Barcelona) Extended abstracts pp 446ndash449

Meissner R 1996 Faults and folds fact and fiction Tectonophysics 264 (1) 279ndash293Monteiro Santos FA Pous J Almeida EP Queralt P Marcuello A Matias H Mendes

Victor LA 1999 Magnetotelluric survey of the electrical conductivity of the crustacross the Ossa Morena Zone and South Portuguese Zone suture Tectonophysics313 (4) 449ndash462

Pankhurst RJ Rapela CW Saavedra J Baldo E Dahlquist J Pascua I Fanning CM1998 The Famatinian magmatic arc in the central Sierras Pampeanas an Early toMid-Ordovician continental arc on the Gondwana In Pankhurst RJ Rapela CW(Eds) The Proto-Andean Margin of Gondwana vol 142 Geological Society ofLondon pp 343ndash367 Special Publication

Pankhurst RJ Ramos A Linares E 2003 Antiquity and evolution of the Riacuteo de la Platacraton in Tandilia southern Buenos Aires province Argentina J South Am EarthSci 16 5ndash13

Parkinson WD 1962 The influence of continents and oceans on geomagneticvariations Geophys J R Astron Soc 6 441ndash449

Ramos VA 1994 Terranes of southern Gondwanaland and their control in the Andeanstructure (30ndash33degS lat) In Reutter KJ Scheuber E Wigger PJ (Eds) Tectonics ofthe Southern Central Andes Structure and Evolution of an Active ContinentalMargin Springer Berliacuten pp 249ndash261

Ramos V1999 Rasgos Estructurales del territorio argentino Geologiacutea Argentina Institutode Geologiacutea y Recursos Minerales Anales vol 29 pp 715ndash784 Buenos Aires

Ramos VA Escayola M Mutti D 2000 Proterozoicndashearly Paleozoic ophiolites in theAndean basement of southern South America In Dilek Y Moores EM Elthon DNicolas A (Eds) Ophiolites and Oceanic Crust New Insights from Field Studies andOcean Drilling Program vol 349 Geological Society of America pp 331ndash349Special Paper

Ramos VA Cristallini EO Peacuterez DJ 2002 The Pampean flat-slab of the CentralAndes J South Am Earth Sci 15 (1) 59ndash78

Rapalini AE 2005 The accretionary history of southern South America form thelatest Proterozoic to the Late Paleozoic some paleomagnetic constraints InVaughan APM Leat PT Pankhurst RJ (Eds) Terrane Processes at the Margins ofGondwana vol 246 Geological Society of London pp 305ndash328 Special Publications

Rapela CW 2000 The Sierras Pampeanas of Argentina Paleozoic building of theSouthern Proto-Andes In Cordani UG Milani EJ Thomaz-Filho A Campos DA(Eds) Tectonic Evolution of South America 31st International Geological CongressRiacuteo de Janeiro Brazil pp 381ndash387

Rapela CW Pankhurst RJ Casquet C Baldo E Saavedra J Galindo C Fanning CM1998 The Pampean Orogeny of the southern proto-Andes Cambrian continentalcollision in the Sierras de Coacuterdoba In Pankhurst RJ Rapela CW (Eds) The Proto-Andean Margin of Gondwana vol 142 Geological Society of London pp 181ndash217Special Publication

Rapela CW Fanning M Pankhurst RJ 2005 The Rio de la Plata Craton the search forits full extent In Pankhurst RJ Veiga G (Eds) Gondwana 12 Mendoza 2005 AbsAcademia Nacional de Ciencias p 304

Rodi W Mackie R 2001 Non-linear conjugated gradient algorithm for 2-Dmagnetotelluric inversion Geophysics 66 174ndash178

Saalmann K Hartmann LA Remus MVD Koester E Conceiccedilatildeo RV 2005 SmndashNdisotope geochemistry of metamorphic volcano-sedimentary successions in the SatildeoGabriel Block southernmost Brazil evidence for the existence of juvenileNeoproterozoic oceanic crust to the east of the Rio de la Plata craton PrecambrianRes 136 159ndash175

Sanchez Bettucci L Rapalini AE 2002 Paleomagnetism of the Sierra de Las Aacutenimascomplex southern Uruguay its implications in the assembly of western GondwanaPrecambrian Res 118 243ndash265

Schwartz JJ Gromet LP 2004 Provenance of Late Proterozoicndashearly Cambrian basinSierras de Coacuterdoba Argentina Precambrian Res 129 1ndash21

Sims JP Ireland TR Camacho A Lyons P Pieters PE Skirrow RG Stuart-Smith PGMiroacute R 1998 UndashPb ThndashPb and ArndashAr geochronology from the southern SierrasPampeanas Argentina implications for the Paleozoic evolution of the westernGondwana margin In Pankhurst RJ Rapela CW (Eds) The Proto-Andean Marginof Gondwana vol 142 Geological Society of London pp 259ndash281 SpecialPublication

Solon KD Jones AG Nelson KD Unsworth MJ Kidd WF Wei W Tan H Jin SDeng M Booker JR Li S Bedrosian P 2005 Structure of the crust in the vicinityof the BanggongndashNujiang suture in central Tibet from INDEPTH magnetotelluricdata J Geophys Res 110 B10102ndashB10122

Steenken A Loacutepez de Luchi MG Siegesmund S Wemmer K Pawlig S 2004 Crustalprovenance and cooling of the basement complexes of the Sierra de San Luis an


insight into the tectonic history of the proto-Andean margin of GondwanaGondwana Res 7 1171ndash1195

Steenken A Siegesmund S Loacutepez de Luchi MG Wemmer K Frei R 2006 NewConstraints on the Famatinian Geodynamic Evolution of the proto-Andean Marginof Gondwana (Sierra de San Luis Argentina) J Geol Soc Lond 163 965ndash982

Thomas W Astini R 2003 Ordovician accretion of the Argentine Precordillera terraneto Gondwana a review J South Am Earth Sci 16 (1) 67ndash79

Trompette R 2000 Gondwana evolution its assembly at around 600 Ma ComptesRendus de lAcadeacutemie des Sciences Series IIA- Earth Planet Sci 330 (5) 305ndash315

Veevers JJ 2004 Gondwanaland from 650ndash500 Ma assembly through 320 Ma mergein Pangea to 185ndash100 Ma breakup supercontinental tectonics via stratigraphy andradiometric dating Earth-Sci Rev 68 (1ndash2) 1ndash132

Willett S Beaumont C Fullsack P 1993 Mechanical model for the tectonics of doublyvergent compressional orogens Geology 21 371ndash374

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 Kroumlner and Cordani 2003) Dashed lines indicate inferred positions of major cratonic fragments belowPhanerozoic cover Cratons abbreviations SFmdashSatildeo Francisco RAmdashRio Apa PRmdashParana RPmdashRio de la Plata PAmdashPampiaPampean AAmdashArequipandashAntofalla LAmdashLuiz Alves CGmdashCentral Goias


the contrary an Early CambrianndashMiddle Cambrian collision between thePampean Terrane (PAT) and the RIC with the corresponding suturerunning along the east of the Sierras deCoacuterdoba (Fig 2)wasproposed byRapela et al (1998)

Fig 2 Geological sketch of the studied area showing the locations of the MT sites LocationSierra de Coacuterdoba and San Luis is based on the Geological Map of Coacuterdoba (1500000) and GGa correspond to TDM (Depleted mantle model age) after Rapela et al (1998) and Steenken etthe studied crustal units

Uncertainties in the definition of the limits of the accreted terranethe structural mechanisms of the collisional event and the unknownwidth of the former oceanic basins together with younger amalgama-tion of terranes along the western border of the PAT and subsequent

of the proposed sutures containing ultramafic rocks after Ramos (1999) Geology of theeological Map of San Luis (1500000) (Servicio Geoloacutegico Minero Argentino) Values inal (2004) They are included in order to show some of the isotopic differences between























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Documents

2D Magnetotelluric interpretation of the crust electrical resistivity across the Pampean terrane–Río de la Plata suture, in central Argentina