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Data Analysis of a Magnetotelluric Transect Across the SW Amazon Craton

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Data Analysis of a Magnetotelluric Transect Across the SW Amazon Craton Data analysis of a magnetotelluric transect across the SW Amazon craton Clarisse Monteiro Fernandes (ON-COGE)*; Leonardo Guimarães Miquelutti (ON-COGE); Ved Prakash Maurya (ON- COGE); Emanuele Francesco La Terra (ON-COGE); Sergio Luiz Fontes (ON-COGE). Copyright 2017, SBGf - Sociedade Brasileira de Geofísica and Tapajós-Parima tectonic provinces, south-west This paper was prepared for presentation during the 15th International Congress of sector of the Amazonian Craton. This region comprises the Brazilian Geophysical Society held in Rio de Janeiro, Brazil, 31 July to 3 August, 2017. the Alta Floresta/Juruena-Teles Pires Golden Province, related to an ancient arc-accretion setting (Scandolara Contents of this paper were reviewed by the Technical Committee of the 15th International Congress of the Brazilian Geophysical Society and do not necessarily et al., 2017, Tassinari & Macambira, 2004). The specific represent any position of the SBGf, its officers or members. Electronic reproduction aim of this study is the assessment of the geoelectric or storage of any part of this paper for commercial purposes without the written consent of the Brazilian Geophysical Society is prohibited. strikes, distortions, predominant structures and ____________________________________________________________________ dimensionalities of the lithosphere. Several EM attributes are evaluated including WAL invariants Abstract derived from the magnetotelluric transfer functions, as well as the tipper transfer function relating the vertical Broadband and long-period magnetotelluric data were magnetic field variations with the horizontal magnetic collected along a 200 km long NNE-SSW transect at fields variations, and the phase tensor. the southwest border of the Amazon Craton, in a volcanic to plutonic region related to the Alta Floresta/Juruena-Teles Pires Golden Province. The Tectonic Setting data were processed using a robust regression scheme over the EM cross-spectra for combined single and The Amazon Craton, one of the main tectonic units in remote reference stations, producing estimated the South American Platform, represents a stable transference functions of good to intermediate quality. lithospheric plate composed of Archean to Those were submitted to geometric analysis to assess Mesoproterozoic amalgamated provinces. The dimensionality and geoelectric preferred strikes by published proposed geotectonic models for this region means of WAL invariants and phase tensor can be grouped into two main competing approaches: decomposition. Additionally phase tensor the crustal reworking based (Amaral, 1974; Almeida, pseudosections were estimated, and compared with 1978; Issler, 1977; Hasui et al., 1984; e Costa & Hasui, traditional apparent resistivity and phase in TE and TM 1997; apud Tassinari & Macambira, 2004) and the modes plus tipper magnitude pseudosections. The accretionary based (Cordani et al., 1979; Cordani & obtained results suggest 3-D type regional structures Brito Neves, 1982; Tassinari & Macambira, 1999; moderately to highly distorted by 3-D local, smaller Tassinari et al., 2000; Santos et al., 2000; apud conductive bodies. A highly resistive block is shown in Tassinari & Macambira, 2004) tectonic models. the central to the northern transect sector, for periods from 10-4 to 100 seconds, including a resistant root at The crustal reworking based considers just structural 104 seconds in the central sector. Two main conductive analysis performed over aerogeophysical data, and anomalies are showed for periods from 10-4 to 104: at foresees an Archean to Paleoproterozoic Craton the southern transect sector, which may be related to composed by a group of twelve granite-greenstone the sedimentary and felsic to intermediate volcanic units paleo-plates, with granulite-composed collisional or at the craton south border; and at the middle transect shear belts which were constantly reactivated. The sector, which may be related to intermediate to basaltic accretionary based model (Fig.1), otherwise proposed volcanic units that overlay the contact between the on geochronological ground and complementary craton and the Precambrian sedimentary cover. petrological and restricted geologic mapping evidence, remains the most accepted among the geoscientists and represents the state-of-art of published, peer- reviewed models in terms of tectonics. Introduction The magnetotelluric method is largely employed on regional to global, deep-probing geophysical studies and, in the last decades, on mineral industry exploration studies. Past successful experiences have been reported in magnetotelluric soundings over very resistive lithospheric regions containing large scale conductive anomalies as, for example, cratonic regions embodying metalliferous deposits (Heinson et al., 2006; Maier et al., 2007; Spratt et al., 2014). A question that arises is how to deal with distortion of magnetotelluric responses in such an extreme contrasting environment between usual high resistive bedrock and conductive ore bodies. This work presents a first attempt to investigate the geoelectrical structural information depicted from a magnetotelluric transect across the Rondônia-Juruena Figure 1: Accretionary model geologic map. Fifteenth International Congress of the Brazilian Geophysical Society DATA ANALYSIS OF A MAGNETOTELLURIC TRANSECT This model describes a succession of juvenile melts 1 2 −1 퐼푚{푍푖푗} 휌푎,푖푗 = |푍푖푗| and 휙푖푗 = 푡푎푛 ( ) episodes producing magmatic arcs which amalgamate μ0ω 푅푒{푍푖푗} the older, Archean crustal fragments, followed by a terminal stage of ensialic reworking, anorogenic As a tensor, 푍̿ contains information about magmatism and isotopic rejuvenation at the extreme dimensionality and electromagnetic geometry. The southwest. Different types of geochronological methods simple, regional undistorted regional geoelectric of low- and high-resolution, produces slightly diverging structure should be easily obtained by observing the sub-models, culminating in differences related to provinces boundaries and rock ages. components of the 푍̿ tensor. In a unidimensional Earth the diagonal components of the impedance tensor are Besides that, new regional geologic maps and data zero, whilst the off-diagonal components are equal in continue being published, mainly by the Brazilian magnitude, but have opposite signs; in a bi-dimensional Geologic Service (CPRM), allowing refinements of the Earth, the diagonal components are equal in main models along with more radical reformulation magnitude, but have opposite sign, whilst the off- (Scandolara et al., 2017), but that have not yet diagonal components differ; in a tri-dimensional Earth, culminated in a refereed and broadly accepted all components differ. geotectonic general model for the entire Craton. When real data sense local anisotropic structures, this simple decomposition fails, and the distortion must be Method assessed first. The impedance tensor 푍̿ can be decomposed into regional and local sets of information, The magnetotelluric method uses the natural being local anisotropies regarded as galvanic distortion electromagnetic field variations recorded at the surface over the regional geoelectric structure, affecting only in order to probe the electrical conductivity distribution the measured electric field 퐸̅푚, so the regional electric at depth associated with the Earth's subsurface field 퐸̅푟 is affected by a distortion matrix 퐷̿ as 퐸̅푚 = structure. The natural electromagnetic signal source for 퐷̿ 퐸̅푟. The regional transfer function expression is given the method are the MHD waves and pulsations 푟 푟 푟 by 퐸̅ = 푍̿ 퐵̅/μ0, with 푍̿ being the regional impedance produced by the solar plasma interaction with the tensor, and the measured transfer function is 퐸̅푚 = terrestrial magnetosphere and ionosphere, for the low 푍̿푚 퐵̅/μ , and 푍̿푚 being the measured impedance frequency or long period signal (0.0001 Hz or 10000 s), 0 tensor. Comparing theses equations, we obtain: and the called “spherics” produced in the global atmospheric electrical circuit, for the high frequency or ̿ ̿푟 ̅ ̿ ̅푟 ̅푚 short period signal (10000Hz or 0.0001 s). The resulting 퐷 푍 퐵/μ0 = 퐷 퐸 = 퐸 electromagnetic field is delimited by an atmospheric waveguide that extends from the top of ionosphere to So the impedance tensor, considering distortion is the Earth’s surface. simply given by: 푚 푟 푍̿ = 퐷̿ 푍̿ The magnetotelluric physical mathematics lays on two valid simplifications when considering a distance source A great number of techniques are employed to and a period signal range restricted from 10000 to overcome the problems that arise for the critical 0.0001 seconds: 1) the electromagnetic waves distortion sceneries with varying geometries propagates in a vertical axis as plane waves from the combinations for regional and local information. A type atmospheric waveguide into the solid Earth, inducing of approach summarized in Weaver et al. (2006) is secondary telluric currents in the subsurface and an recovering regional geometry considering the associated magnetotelluric field; 2) a quasi-static decomposition of tensors themselves derived from the approximation allows describing the propagating traditional impedance tensor (푍̿): its equivalent process in the atmospheric waveguide and in the solid magnetotelluric tensor (푀̿), a variation of 푍̿ computing 퐵̅ Earth as diffusive by neglecting the Maxwell's instead of 퐵̅/μ0 (=퐻̅), given by 퐸̅ = 푀̿ 퐵̅; and the phase displacement current correction. tensor (훷̿), a tensor that contain the phase information in 푍̿. Together
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