Electromagnetic constraints for subduction zones beneath the northwest Borborema province: Evidence for Neoproterozoic island arc–continent collision in northeast Brazil Antonio L. Padilha1, Ícaro Vitorello1, Marcelo B. Pádua1, and Maurício S. Bologna2 1
Instituto Nacional de Pesquisas Espaciais—INPE, C.P. 515, 12201-970 São José dos Campos, Brazil Departamento de Geofísica, Universidade de São Paulo—IAG-USP, Rua do Matão 1226, 05508-090 São Paulo, Brazil
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ABSTRACT The Borborema province in northeast Brazil occupies a crucial position in the complex Neoproterozoic West Gondwana reconstruction puzzle. However, correlation attempts between northeast Brazil and West Africa have been hampered because key links in the internal structure of the Borborema province have yet to be identified. To aid such an correlation, a magnetotelluric study was undertaken along two subparallel profiles to image the deep electrical structure in the northwestern part of the province. Despite the occurrence of recurrent tectonothermal episodes that affected the region in the past, two-dimensional models show that a large-scale signature of the assembled terrane during the Neoproterozoic accretion and collision is plausibly preserved in the area. Two resistive features dipping from the upper crust into the upper mantle in downward convergence (opposite directions) are defined beneath one of the profiles that are interpreted to be related to remnants of former subduction slabs, since the observed high-resistivity zone is consistent with a dehydrated oceanic lithosphere depleted of sediments. On the basis of geological and geochemical information, a model of collision of an intraoceanic magmatic arc coalesced into an earlier passive margin is proposed for the Neoproterozoic tectonic evolution of the province, involving processes of reversal of subduction polarity and oceanic slab breakoff. evidence of a long-lasting oceanic subduction in the Borborema province (Santos et al., 2008). In addition, identification of evidence of previous oceans separating the province from the surrounding cratons in the past is crucial to ascertain whether the province grew during the Neoproterozoic by amalgamation of disparate terranes. Deep-penetrating geophysics is the appropriate tool to investigate the subsurface structure and
GEOLOGICAL BACKGROUND The Borborema province is a complex orogenic system in northeasternmost Brazil (Fig. 1). It consists of gneissic and migmatitic
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42°W 41°W 40°W 39°W 38°W Figure 1. Generalized geological map of north0° Atlanti c Ocea west Borborema province BP N n in northeast Brazil (from 3°S 3°S 20°S A′ Bizzi et al., 2001), with M locations of magnetotel60°W 40°W C For luric stations along protale 30 z a B′ files A and B (numbered black dots). Geological 4°S 4°S domains: PB—Parnaíba 25 Basin; MC—Médio CoCC PB 15 reaú; CC—Ceará Central; 20 T Q SP JG—Jaguaribe; RP—Rio 10 Piranhas. Main faults 5°S 15 and lineaments: TB— 5°S R Transbrasiliano; SO—SoO TT bral-Pedro II; SP—Sena5 10 RP dor Pompeu; OR—Orós; PO—Portalegre. AR—ArJG 6°S chean; PP—Paleoprotero- 6°S 5 zoic; NP—Neoproterozoic B (mainly granitoids); PZ— AR PP NP PZ A Paleozoic. TQ—Neoprokm terozoic Tamboril–Santa 0 100 Quitéria complex; TT—Ar- 7°S 7°S 42°W 41°W 40°W 39°W 38°W chean Tróia-Tauá massif. Inset map shows location of study area in Brazil; BP indicates Borborema province.
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INTRODUCTION The history of assembly of the supercontinent Gondwana during the Neoproterozoic has been a matter of strong debate, with an increased focus on correlation between the geology of West and northwest Africa, and east and northeast Brazil. The Borborema province, in northeastern Brazil, occupies a central place in western Gondwana because it would be adjacent to coeval Pan-African fold belts and cratonic terrane of western Africa in pre-drift reconstructions. Moreover, geologic evidence indicates that some major shear zones and lithostructural and tectonic domains of the province have their counterparts in the African continent (Santos et al., 2008). However, the correlations are not without caveats because, unlike Africa where ocean closure and craton convergence features are well defined for the Neoproterozoic (e.g., Castaing et al., 1994), it has been a challenge to recognize such tectonic links in northeast Brazil. As an example, the Transbrasiliano lineament is proposed to be a first-order continental-scale structure through South America, and taken as the continuation of the Kandi lineament in Africa, but its tectonic significance is still not clear, being interpreted either as a Neoproterozoic cryptic suture or as only a large collision-related shear zone (e.g., De Araujo et al., 2012). Consequently, understanding the complex geology of the Borborema province has been considered pivotal for global supercontinent reconstruction models (De Wit et al., 2008). The key missing link in this West Africa– northeast Brazil correlation is, therefore, the
geometry, and thus unravel the geologic history of such patched terranes; however, a recent regional seismic refraction experiment only revealed the general crustal thickness variations, and the internal structures beneath the province are not well defined (Soares et al., 2010). Magnetotellurics (MT) is a passive electromagnetic geophysical method that has been extensively used over ancient and modern subduction and/or collision zones in different parts of the world to derive information on electrical properties of deep crust and upper mantle (see reviews by Jones, 1993; Unsworth, 2010). Our MT soundings across the northwest portion of the Borborema province focused on the pursuit of fossil remnants of ancient tectonic processes. The survey provides models of the crustal–upper mantle electrical conductivity beneath this part of the province and allows mapping, for the first time, of dipping resistive features corresponding to the cryptic remains of Neoproterozoic collision and subduction activities.
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structure. The penetration depth of the electromagnetic signals is period dependent, with longer periods penetrating deeper within the Earth. Broadband and long-period MT data were recorded at 52 sites along two southeast-northwest, subparallel profiles in the northwest Borborema province (Fig. 1), one of them (profile A) coincident with a deep seismic refraction transect (Soares et al., 2010). Time-series data were processed with robust techniques (Egbert, 1997) and dimensionality and geoelectric strike directions were determined using tensor decomposition (McNeice and Jones, 2001) and induction arrows analyses (a full description of MT data collection, processing, inversion, and model validation is available in the GSA Data Repository1). A dominant northeast strike direction was found, parallel to the major geological features and boundaries, and data at a significant number of stations exhibit two-dimensional (2-D)
behavior. However, at the western end of the profiles induction arrows point roughly perpendicular to the coastline at long periods, indicative of coastal effects. Three-dimensional modeling was performed to define the period interval not influenced by the sea at each site. Based on the welldefined strike direction and the overall 2-D nature of the induction arrows at periods insensitive to the coast effect, a 2-D analysis is justified. The 2-D REBOCC (reduced basis Occam’s) inversion code of Siripunvaraporn and Egbert (2000) was used to derive smooth resistivity models of the subsurface conductivity (Fig. 2). The models allow good fits between observations and model predictions (root mean square misfit of 1.79 for profile A, and 1.63 for profile B).
basement complexes, mostly formed during Paleoproterozoic orogenic events that preserved minor Archean blocks, partially covered by Mesoproterozoic to Neoproterozoic metasedimentary and metavolcanic rocks. Its present configuration is associated with the Neoproterozoic–early Phanerozoic Brasiliano–Pan-African orogeny that was responsible for low- to highgrade metamorphism, emplacement of many granitic plutons, and development of continental-scale transcurrent shear zones, mainly striking east-west and northeast-southwest (Brito Neves et al., 2000). The deep crustal and uppermost mantle structure beneath the Borborema province is relatively unknown and so dynamic processes involved in its formation and evolution are still poorly understood. In particular, the geodynamic evolution of the Borborema terrane during the Neoproterozoic is disputed, with two opposing views for the tectonic setting in which magmatic and deformational events took place. In one of the models, the province is regarded as a system of allochthonous orogens amalgamated during accretionary orogenic events in the final stages of a complete and wide tectonic cycle (Brasiliano) spanning most of the Neoproterozoic and Early Phanerozoic times (Brito Neves et al., 2000). The other model proposes that the province occupied a central position in a large Paleoproterozoic continent, so that the Brasiliano orogeny in the region would be related only to far-field stresses reworking preexisting Archean–Paleoproterozoic crust in an intracontinental setting (Neves, 2003). A northeast-southwest–trending complex of granitoid plutons and migmatites (Tamboril– Santa Quitéria complex; TQ in Fig. 1) has been interpreted as remnants of a Neoproterozoic intraoceanic and continental arc (Fetter et al., 2003; Amaral et al., 2011), which would give support to a regime of oceanic lithosphere subduction and continental collision. However, the presence of relics of eclogitic metamorphism on both sides of the Tamboril–Santa Quitéria complex has led to divergent interpretations regarding the polarity of the proposed subduction zone, i.e., eastward (Fetter et al., 2003) or westward (Castro, 2004). MT is a suitable technique to discriminate between the two competing models because they would yield very different conductivity structures at lower-crustal and upper-mantle depths. In the accretionary model, subduction zones would be expected to generate dipping conductive and resistive features throughout the lithosphere. In contrast, in an intracontinental setting, mainly subhorizontal conductive anomalies related to flat-lying fabrics and thin-skinned tectonics would dominate the upper and middle crust.
Figure 2. Two-dimensional resistivity models across profiles A and B in northwest Borborema province derived by inverting magnetotelluric (MT) data and other geophysical constraints. The model for profile A was obtained by joint inversion of TE (transverse electric) and TM (transverse magnetic) data, while profile B used TM and Tzy data. Upper panels of profiles show regional Bouguer gravity anomaly, and lower panels show geoelectric structure derived from two-dimensional MT inversion, with indication of surficial limits of geological domains and main faults. SO—Sobral-Pedro II; SP—Senador Pompeu; OR—Orós; PO—Portalegre; TB—Transbrasiliano; JG—Jaguaribe domain. Inverted open triangles show locations of MT sites. In profile A, there is no signal penetration in the blanked area and dashed line is seismic refraction interpretation of crustal thickness (Soares et al., 2010). Short horizontal black lines represent estimated maximum depth of investigation beneath each site (bars plotted at 100 km level have much deeper depth of penetration).
MT STUDY The MT method utilizes natural electromagnetic fields to derive subsurface conductivity
1 GSA Data Repository item 2014021, magnetotelluric data collection, processing, inversion, and model validation, is available online at www.geosociety.org/pubs/ft2014.htm, or on request from editing@geosociety .org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.
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RESULTS AND DISCUSSION Both models show conductivity variations at different lithospheric depths, not favoring the
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hypothesis of a predominantly crustal intracontinental tectonic setting. More likely, the diversified expressions of conductivity signatures are related to a broad range of very deep tectonic and volcanic activity in the province, including periods of extension in the Paleoproterozoic, subduction and compressional deformation in the Neoproterozoic, and widespread emplacement of postcollisional granitoids from the Neoproterozoic to the Cambrian–Ordovician. The most conspicuous feature is the huge high-conductivity anomaly beneath the Jaguaribe domain in profile A. The deep core of the feature is not resolved by our data set, but it certainly extends down to the upper mantle. High conductivity in a tectonically inactive region is most likely explained by the presence of interconnected graphite, sulfides, or iron oxides. There is no information about the presence of sulfides or iron oxides at the surface of this region of the Borborema province, whereas the presence of carbonate veins in a granitoid suggests that they may be related to CO2-bearing mantle-derived fluids (Santos et al., 2013). Carbon can be introduced into the upper mantle through tectonic processes, such as subduction or lithospheric extension. In the first case, carbon can be transported on the top of subducted slabs, whereas in the latter case enriched deeper mantle material can be transferred to shallower mantle depths, refertilizing the depleted upper mantle with incompatible elements (Selway, 2013). At the surface of the Jaguaribe domain, the Paleoproterozoic extensional event is represented by the volcanic sequences of the Orós fold belt. Subsequent compressional events in the Neoproterozoic are indicated by the curvature of this belt and also by the conductivity enhancement probably resulting from the introduction of subduction-related carbon. Similarly, the predominantly subhorizontal conductive anomalies at upper to mid-crustal depths of the Médio Coreaú domain can be tentatively associated with Paleoproterozoic extension-related magmatism. In this case, the event is represented by the Saquinho volcanic sequence outcrops. The region of the proposed Neoproterozoic collision, involving parts of the Ceará Central domain and the eastern Parnaíba Basin, is marked by a long, low Bouguer anomaly that can be explained by crustal thickening, in agreement with seismic estimates of Moho depths (Soares et al., 2010; Assumpção et al., 2013). In addition, this region is characterized by isolated small conductors at crustal depths showing reasonable correlations between the two MT profiles (Fig. 2). These conductors are spatially coincident with some of the synorogenic and postorogenic granitoids and are approximately coincident with short-wavelength gravity highs, which could be explained by the presence of high-density and high-conductivity intrusions in the crust. The increased conductivity in this
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part of the Borborema province is therefore interpreted as originating from interconnected graphite-rich materials intruded in crevices of synorogenic and postorogenic fissured igneous and metamorphic rocks related to the Neoproterozoic subduction process. Conductors have been observed in a number of other ancient subduction zones (see Jones, 1993; Turkoglu et al., 2009, and references therein), but the main difference here is that the conductors are not showing any preferred dipping direction. However, the model for profile B shows the presence of two moderate- to highresistivity zones, dipping from the upper crust into the upper mantle in opposite but convergent directions. One of these resistive structures dips eastward from the western side of the proposed Transbrasiliano lineament, whereas the other dips westward from the Ceará Central domain (southeast of the Tamboril–Santa Quitéria complex). The resistors merge into each other in a single resistive block at depths of ~50 km. Given the relatively short distance between the two MT profiles and the inferred subduction setting, it would be expected that similar resistive-conductive features would be found beneath both profiles. However, along profile A the only resistive feature that can be correlated to profile B (Fig. 2) is the highly resistive upper mantle beneath the Tamboril–Santa Quitéria complex. As MT data usually resolve conductors more clearly than resistors, the signature of collision-related resistive structure could be obliterated by postcollisional events introducing conductive components into the crustal rocks. In that region, the main difference between the two profiles is the intensity of magmatism. Profile A is characterized by pervasive pulses of postcollisional magmatism spanning an interval of ~150 m.y. after the collisional stage, whereas along profile B the magmatism does not appear to have been as voluminous because outcrops are rare (see Fig. 1). We argue that signatures of resistive Neoproterozoic structures formed during the collisional process were virtually obscured along profile A by the intense postcollisional magmatism. Accordingly, we therefore suggest that the two high-resistivity zones in profile B represent remnants of former subduction zones beneath the northwest Borborema during the Neoproterozoic. MT results from different ancient and modern subduction and/or collision zones have also shown that subducted plates are often imaged as highly resistive slabs (Jones, 1993; Unsworth, 2010). The resistive slab corresponds to a dehydrated oceanic lithosphere depleted of sediments (Wannamaker et al., 1989), an interpretation justified by electromagnetic measurements on the seafloor (Key, 2012). In addition, the shallower style of the proposed oceanic subduction observed in our model, probably constrained by a thinner lithosphere and a hotter mantle, agrees with the suggestion of the
dominant style of subduction during the Archean and Proterozoic (Abbott et al., 1994). TECTONIC IMPLICATIONS AND CONCLUSIONS Because subsurface geoelectrical structures can reflect different snapshots in time, to reconstruct regional development and evolution from the MT model it is necessary to apply an integrated approach combining independent geological and geochemical information. In this case, to explain the observed geoelectrical section the integrated interpretation favors a long-lived and possibly recurrent tectonic activity that culminated in the Neoproterozoic with the collision of an earlier passive margin and an intraoceanic active volcanic arc formed by the decompression melting and degassing of a cold and dense oceanic crust descending back into the mantle. This process sometimes results in the reversal of subduction polarity and slab breakoff (McKenzie, 1969), such as currently observed in Taiwan (Teng, 1996). Evidence supporting this proposition includes the following. (1) Precollision magmatism (age of 795 Ma) is recorded along the eastern border of the Tamboril–Santa Quitéria complex and has been interpreted as a former magmatic arc system (De Araujo et al., 2012). Metamafic rocks occurring in an eclogitic zone in the western side of the same complex show geochemical affinities with island-arc basalts (Amaral et al., 2011). Both may represent records of an active precollisional intraoceanic volcanic arc. (2) Analysis of isotopic signatures, geochemical composition, and the overlapping between the onset of the collisional stage and highpressure metamorphism (650–640 Ma; Amaral et al., 2011) with the magmatic activity in the Tamboril–Santa Quitéria complex has been used to propose a collisional setting for the complex, with an early arc component reworked during the collisional event (De Araujo et al., 2012). (3) Results of this MT study, i.e., the detection of opposed resistive slabs converging at mantle depths beneath the northwest Borborema province, are interpreted as the existence of dual subduction zones with opposite polarity probably caused by a subduction polarity reversal following the collision of an oceanic arc with a passive margin. We propose that the sliding down of the westward-dipping slab beneath the volcanic island arc was interrupted as the backarc crustal gap between the continent and the arc was closed by new eastward-dipping subduction. (4) At the end of the collisional stage (ca. 620–580 Ma), a magmatic pulse in the Tamboril–Santa Quitéria complex, with coeval crustalderived granitoids and mantle-derived magmas, is interpreted to have been triggered by a breakoff of the west-dipping oceanic slab, which contributed to mantle upwelling and emplacement
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of magmas with mantle-derived input (Costa et al., 2010). (5) Postcollisional granitoids are recorded for different time intervals between 590 and 480 Ma (Castro, 2004). The island arc–continent collision model proposed here is similar to the evolution of the Brasília belt in central Brazil (Pimentel and Fuck, 1992), where island-arc formation started ca. 900 Ma and proceeded as arc-type magmatism until the Brasiliano continental collision ca. 650 Ma. Our results support the presence of an ancient seaway in northeast Brazil that was closed by the assembly of Gondwana in the Neoproterozoic. The proposed closure of an ocean resulting in constructive continental margin lithosphere supports assertions for an accretionary origin for the Borborema province, involving a collage of allochthonous terranes (Brito Neves et al., 2000). ACKNOWLEDGMENTS This study was funded by the Brazilian National Research and Development Council (CNPq) through the Instituto Nacional de Ciência e Tecnologia (INCTET) project 573713/2008-1. We also thank CNPq for fellowships to Padilha (grant 303813/2009-1) and Vitorello (grant 302347/2008-9), and the dedicated field and lab crew at Instituto Nacional de Pesquisas Espaciais. Alan Jones, Martyn Unsworth, and one anonymous reviewer made insightful comments that helped to improve the manuscript. REFERENCES CITED Abbott, D., Drury, R., and Smith, W.H.F., 1994, Flat to steep transition in subduction style: Geology, v. 22, p. 937–940, doi:10.1130/00917613(1994)0222.3.CO;2. Amaral, W.S., Santos, T.J.S., and Wernick, E., 2011, Occurrence and geochemistry of metamafic rocks from the Forquilha Eclogite Zone, central Ceará (NE Brazil): Geodynamic implications: Geological Journal, v. 46, p. 137–155, doi:10.1002/gj.1224. Assumpção, M., Feng, M., Tassara, A., and Julià, J., 2013, Models of crustal thickness for South America from seismic refraction, receiver functions and surface wave tomography: Tectonophysics, doi:10.1016/j.tecto.2012.11.014. Bizzi, L.A., Schobbenhaus, C., Gonçalves, J.H., Baars, F.J., Delgado, I.M., Abram, M.B., Leão Neto, R., Matos, G.M.M., and Santos, J.O.S., 2001, Geologia, tectônica e recursos minerais do Brasil: Companhia de Pesquisa de Recursos Minerais, Brasília, Sistema de Informações Geográficas–SIG e mapas na escala 1:2,500,000. Brito Neves, B.B., Santos, E.J., and Van Schmus, W.R., 2000, Tectonic history of the Borborema Province, northeastern Brazil, in Cordani, U.G., et al., eds., Tectonic evolution of South America: Rio de Janeiro, Brazil, Sociedade Brasileira de Geologia, p. 151–182.
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Castaing, C., Feybesse, J.L., Thiéblemont, D., Triboulet, C., and Chèvremont, P., 1994, Palaeogeographical reconstructions of the Pan-African/Brasiliano orogen: Closure of an oceanic domain or intracontinental convergence between major blocks?: Precambrian Research, v. 69, p. 327–344, doi:10.1016/0301-9268(94) 90095-7. Castro, N.A., 2004, Evolução geológica proterozóica da região entre Madalena e Taperuaba, domínio tectônico Ceará Central (província Borborema) [Ph.D. thesis]: São Paulo, Brazil, Universidade de São Paulo, 221 p. Costa, F.G., De Araújo, C.E.G., Vasconcelos, A.M., Palheta, E.S.M., and Justo, A.P., 2010, O complexo Tamboril-Santa Quitéria: Evidências de slab breakoff durante colisão continental neoproterozóica, norte da província Borborema: Belém, Brazil, 45th Congresso Brasileiro de Geologia, abs. PAP165. De Araujo, C.E.G., Cordani, U.G., Basei, M.A.S., Castro, N.A., Sato, K., and Sproesser, W.M., 2012, U-Pb detrital zircon provenance of metasedimentary rocks from the Ceará Central and Médio Coreaú Domains, Borborema Province, NE-Brazil: Tectonic implications for a longlived Neoproterozoic active continental margin: Precambrian Research, v. 206–207, p. 36– 51, doi:10.1016/j.precamres.2012.02.021. De Wit, M.J., Stankiewicz, J., and Reeves, C., 2008, Restoring Pan-African-Brasiliano connections: More Gondwana control, less Trans-Atlantic corruption, in Pankhurst, R.J., et al., eds., West Gondwana: Pre-Cenozoic correlations across the South Atlantic region: Geological Society of London Special Publication 294, p. 399– 412, doi:10.1144/SP294.20. Egbert, G.D., 1997, Robust multiple-station magnetotelluric data processing: Geophysical Journal International, v. 130, p. 475–496, doi:10.1111/j .1365-246X.1997.tb05663.x. Fetter, A.H., Santos, T.J.S., Van Schmus, W.R., Hackspacher, P.C., Brito Neves, B.B., Arthaud, M.H., Nogueira Neto, J.A., and Wernick, E., 2003, Evidence for Neoproterozoic continental arc magmatism in the Santa Quitéria Batholith of Ceará State, NW Borborema Province, NE Brazil: Implications for the assembly of west Gondwana: Gondwana Research, v. 6, p. 265– 273, doi:10.1016/S1342-937X(05)70975-8. Jones, A.G., 1993, Electromagnetic images of modern and ancient subduction zones: Tectonophysics, v. 219, p. 29–45, doi:10.1016/0040 -1951(93)90285-R. Key, K., 2012, Marine electromagnetic studies of seafloor resources and tectonics: Surveys in Geophysics, v. 33, p. 135–167, doi:10.1007 /s10712-011-9139-x. McKenzie, D.P., 1969, Speculations on the consequences and causes of plate motions: Royal Astronomical Society Geophysical Journal, v. 18, p. 1–32, doi:10.1111/j.1365-246X.1969. tb00259.x. McNeice, G., and Jones, A.G., 2001, Multisite, multifrequency tensor decomposition of magnetotelluric data: Geophysics, v. 66, p. 158–173, doi:10.1190/1.1444891.
Neves, S.P., 2003, Proterozoic history of the Borborema province (NE Brazil): Correlations with neighboring cratons and Pan-African belts and implications for the evolution of western Gondwana: Tectonics, v. 22, 1031, doi:10.1029/2001TC001352. Pimentel, M.M., and Fuck, R.A., 1992, Neoproterozoic accretion in central Brazil: Geology, v. 20, p. 375–379, doi:10.1130/0091-7613(1992)020 2.3.CO;2. Santos, R.V., Oliveira, C.G.D., Parente, C.V., Garcia, M.D.G.M., and Dantas, E.L., 2013, Hydrothermal alteration related to a deep mantle source controlled by a Cambrian intracontinental strike-slip fault: Evidence for the Meruoca felsic intrusion associated with the Transbraziliano Lineament, northeastern Brazil: Journal of South American Earth Sciences, v. 43, p. 33–41, doi:10.1016/j.jsames.2012.12.005. Santos, T.J.S., Fetter, A.H., and Neto, J.A.N., 2008, Comparisons between the northwestern Borborema Province, NE Brazil, and the southwestern Pharusian Dahomey Belt, SW Central Africa, in Pankhurst, R.J., et al., eds., West Gondwana: Pre-Cenozoic correlations across the South Atlantic region: Geological Society of London Special Publication 294, p. 101–120, doi:10.1144/SP294.6. Selway, K., 2013, On the causes of electrical conductivity anomalies in tectonically stable lithosphere: Surveys in Geophysics, doi:10.1007 /s10712-013-9235-1. Siripunvaraporn, W., and Egbert, G., 2000, An efficient data-subspace inversion method for 2-D magnetotelluric data: Geophysics, v. 65, p. 791–803, doi:10.1190/1.1444778. Soares, J.E.P., Lima, M.V., Fuck, R.A., and Berrocal, J., 2010, Características sísmicas da litosfera da Província Borborema: Resultados parciais do experimento de refração sísmica profunda, in Proceedings, 4th Simpósio Brasileiro de Geofísica, Brasília: Rio de Janeiro, Brazil, Sociedade Brasileira de Geofísica, CD-ROM. Teng, L.S., 1996, Extensional collapse of the northern Taiwan mountain belt: Geology, v. 24, p. 949–952, doi:10.1130/0091-7613(1996)024 2.3.CO;2. Turkoglu, E., Unsworth, M., and Pana, D., 2009, Deep electrical structure of northern Alberta (Canada): Implications for diamond exploration: Canadian Journal of Earth Sciences, v. 46, p. 139–154, doi:10.1139/E09-009. Unsworth, M., 2010, Magnetotelluric studies of active continent-continent collisions: Surveys in Geophysics, v. 31, p. 137–161, doi:10.1007/s10712 -009-9086-y. Wannamaker, P.E., Booker, J.R., Jones, A.G., Chave, A.D., Filloux, J.H., Waff, H.S., and Law, L.K., 1989, Resistivity cross-section through the Juan de Fuca subduction system and its tectonic implications: Journal of Geophysical Research, v. 94, p. 14,127–14,144, doi:10.1029 /JB094iB10p14127. Manuscript received 17 May 2013 Revised manuscript received 16 October 2013 Manuscript accepted 17 October 2013 Printed in USA
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