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Deep structure of Parecis Basin, Brazil from 3D magnetotelluric imaging
Journal Pre-proof Deep structure of Parecis Basin, Brazil from 3D magnetotelluric imaging S.L. Fontes, M.A. Meju, V.P. Maurya, E.F. La Terra, L.G. Miquelutti PII:
S0895-9811(19)30113-0
DOI:
https://doi.org/10.1016/j.jsames.2019.102381
Reference:
SAMES 102381
To appear in:
Journal of South American Earth Sciences
Received Date: 16 March 2019 Revised Date:
6 September 2019
Accepted Date: 4 October 2019
Please cite this article as: Fontes, S.L., Meju, M.A., Maurya, V.P., La Terra, E.F., Miquelutti, L.G., Deep structure of Parecis Basin, Brazil from 3D magnetotelluric imaging, Journal of South American Earth Sciences (2019), doi: https://doi.org/10.1016/j.jsames.2019.102381. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
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Deep structure of Parecis Basin, Brazil from 3D magnetotelluric imaging
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S.L. Fontes1, M. A. Meju2, V. P. Maurya1#, E.F. La Terra1 and L.G. Miquelutti1†
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† Currently at Universidade Federal de Uberlandia, Brazil
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# Currently at National Geophysical Research Institute, India
MCTIC-Observatorio Nacional, Rio de Janeiro, Brazil Petronas Upstream Exploration, Kuala Lumpur, Malaysia
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Corresponding author: Sergio L Fontes ([email protected])
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Abstract
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The resistivity structure beneath the Parecis basin in central Brazil has been studied using
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broadband (0.001 - 5000 Hz) magnetotelluric (MT) data for 455 stations recorded along five
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lines in the Juruena sub-basin and over the Precambrian basement at the northern and
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southern basin borders. We found that the reconstructed resistivity-versus-depth profiles from
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our 3D MT inversion models satisfactorily match the average values of the resistivity well
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logs in the top 2200 m at two exploration well locations chosen for model validation. The low
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resistivity Parecis Formation at shallow depth can be traced across the region while an areally
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restricted conductive sequence at about 2000 m depth can be imaged in the central part of
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Juruena sub-basin and is interpreted as synrift deposits or remnants of a buried
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Neoproterozoic basin. Sedimentary formations deeper than 2200 m could be sensed but not
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individually resolved in our resistivity models. The deeper resistivity structure consists of a
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resistive upper crust, severely thinned over the Neoproterozoic fold belts bounding the basin
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to the north and south where it is underlain by conductive zones at mid-lower crustal depths.
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We interpret the deep crustal steep conductors as pre-existing shear-zones possibly associated
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with the Upper Cretaceous to Tertiary alkaline magmatism in the region.
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1 Introduction
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Parecis Basin is an elongate east to west intracratonic basin (Figure 1) in central Brazil
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adjacent to the subandean depression. It has an area of about 355,000 km2 and contains as
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much as 6 km thick sediments and metasediments proven by deep exploration wells in the
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region [BDEP, 2008]. The basin initiated as a NW-SE rift system [Braga and Siqueira, 1996]
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whose abortion led to thermal subsidence over a wide area, and to the deposition of the
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Parecis sag-type basin. The rifts formed over reactivated older structures and the preserved
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grabens may contain Silurian alluvial fans and Carboniferous glacial deposits. The basement 2
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and deep crustal structure across the basin remain poorly understood and are the subject of a
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long running debate [e.g., Braga and Siqueira, 1996; Barros et al., 2009; Barros and
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Assumpção, 2011; Santos and Flexor, 2012; Assumpção and Sacek, 2013; La Terra et al.,
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2016; Fontes et al., 2016; Loureiro et al., 2017]. According to Siqueira [1989], the Parecis
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Basin is separated into Alto Xingu, Juruena and Rondônia sub-basins by the Serra Formosa
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Arch to the east and the Vilhena Arch to the west (Figure 1). From north to south, the
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structural framework is interpreted to consist of several tectonic domains notably Tapajós
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mobile belt, Brasnorte high, Pimenta Bueno graben, Rio Branco high, Colorado graben and
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the Neoproterozoic North Paraguay fold belt, and the basin is surrounded by crystalline
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igneous basement rocks (Figure 1).
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Figure 1. Location map of the Phanerozoic Parecis Basin in western Brazil showing the MT
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and seismic survey lines used in this study. Geological map and inset modified after Siqueira
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[1989] and Bahia et al. [2007]. Shown are old MT line P0 (green circles), 4 new MT lines 3
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(black traces), 4 seismic lines (blue dots) coinciding with MT profiles (i.e., 295-002 with
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PC02, 295-003 with PC03, 295-007 with PC07, 295-009 and 295-0010 with PC09+10), 2
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ANP wells A and B (2-FI-0001-MT and 2-SM-0001-MT) and CPRM well C (PB-01-RO) as
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large yellow crosses. Light grey lines represent previously interpreted structural framework
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consisting of BH (Brasnorte high), PBG (Pimenta Bueno graben), RBH (Rio Branco high),
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CG (Colorado graben), NFB (Northern Paraguay fold belt), TMB (Tapajós Mobile Belt). S1
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is a single MT station west of Well B. The small red star is the epicenter of 1998/2005
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earthquakes while the large red star is the epicenter of the 1955 6.2mb earthquake [Barros et
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al., 2009; Barros and Assumpção, 2011].
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The trends of these structural domains mirror those of the gravity anomalies. Braga and
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Siqueira [1996] interpreted the Parecis Bouguer gravity anomalies to reveal grabens and
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structural highs but the inferred regional basement variation from gravity can be ambiguous
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with regards to some important local structures unless constrained by other geophysical data
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[e.g., Barros and Assumpção, 2011]. Contrary to the gravity-derived model [Braga and
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Siqueira, 1996], Barros and Assumpção [2011] suggest from receiver function analysis that
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the basement depth tends to increase towards the south from the northern border of Parecis
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basin. Barros and Assumpção [2011] also conclude that the decrease in gravity toward the
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north (as used by Braga and Siqueira [1996]) must be caused by variations of deeper crustal
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structure, such as variation in crustal thickness in the Parecis basin. Using 2D magnetotelluric
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(MT) imaging of a roughly N-S regional line (line P0 in Figure 1), Santos and Flexor [2012]
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inferred the presence of a pile of ~ 8 km thick sedimentary formations at the center of Parecis
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basin. La Terra et al. [2016] modeled the basement relief of the Parecis basin using MT data
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and constraints from potential field anomalies. Their 2D conductivity models imaged
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relatively conductive sedimentary sequences, intercalated with highly resistive horizontal and
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vertical bodies interpreted as possible volcanic intrusions (dikes and sills). They also 4
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observed anomalous low resistivity in the lower crust and ascribed it to earlier tectonics
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before basin formation. Fontes et al. [2016] show evidence for the presence of up to 8 km
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deep Mesozoic-Palaeozoic sediments in the centre of Parecis basin from preliminary 3D MT
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inversion. Recent integrated interpretation of seismic and gravity data by Loureiro et al.
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[2017] suggest the presence of a Mesoproterozoic basin of low Bouguer gravity, directly
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overlying the crystalline basement. They also mapped the Neoproterozoic sedimentary
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packages using seismic horizons interpreted with the aid of 2 ANP wells (2-FI-1-MT and 2-
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SM-1-MT). However, the seismic data are of poor quality and lateral correlation away from
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the well locations may be somewhat conjectural.
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In the present paper, we apply the 3D inversion code of Kelbert et al. [2014] to the full MT
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datasets from four MT survey lines of Fontes et al. [2016] and an additional MT line P0
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previously inverted in 2D by Santos and Flexor [2012]. We aim to better understand the
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resistivity structure of the crust and upper mantle beneath the basin in order to help resolve
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past controversies about the basin. We first calibrate our 3D inversion models at the locations
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of two ANP (Brazilian Agency of Oil, Natural Gas and Biofuels) exploration wells along our
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MT lines, and also validate our 3D models with seismic interpretation in the regions away
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from the well sites. After model calibration, we then attempt to resolve some of the above
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issues identified in past geophysical studies of the deep structure across the basin. The rest of
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our paper is structured as follows. In section 2, we discuss the result of standard tensorial
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analysis of the MT field data. We show that the characteristics of the MT impedance tensor
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allows 1D analysis of part of the data at high frequencies but that 3D character dominates at
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intermediate to low frequencies. In section 3, we discuss the detailed study of conventional
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smoothness-constrained 3D MT inversion for deep resistivity structure with its validity tested
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at stations near two hydrocarbon exploration wells where electrical resistivity and gamma-ray
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logs are available for ground-truthing, as well as along two coincident survey lines where 5
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independent models from integrated 2D seismic and gravity interpretation using constraints
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from both wells are available. In section 4, we discuss the possible relationship of our
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resistivity models to past geophysical models. The main conclusions from our studies are
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presented in section 5.
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2 MT Data Analysis
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The new MT survey consists of broadband recording (0.001 - 5000 Hz) at 382 stations along
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four MT profiles covering mostly what is termed the Juruena sub-basin of the Parecis basin.
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The new profiles are PC02 (central region of the Juruena sub-basin), PC03 (western domain
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of the Juruena sub-basin), PC07 and PC9+10 (both covering the eastern domain of Juruena
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sub-basin). The average station spacing is 3.6 km along PC02 and 1.8 km for the others.
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These data were acquired by the commercial company Schlumberger for ANP. Additional
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MT (0.001 - 100 Hz) data were available from an earlier more extensive regional line P0
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(Figure 1) acquired by MCTIC-Observatório Nacional [Santos and Flexor, 2012], yielding a
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total of 455 stations. Both electric (E) and magnetic (H) fields were acquired at all the MT
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sites. Three of the new MT profiles (PC02, PC03 and PC09+10) coincide with 2D seismic
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lines (295-002, 295-003, 295-009 and 295-0010) from ANP. Two petroleum exploration
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wells 2FI-0001-MT (or Well A) and 2SM-0001-MT (or Well B) are located at the southern
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end of profiles PC02 and PC09+10 respectively (Figure 1). We will use the seismic and well
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data for the geological calibration of the results of MT depth imaging in the next section.
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As is standard in MT practice, the recorded time series for the E and H field components at
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each station were processed in single station and remote reference mode using the robust
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processing scheme of Egbert [1997] based on multivariate statistical methods. The results
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include the full impedances (Z, i.e., all four components Zxx, Zxy, Zyx and Zyy) and vertical
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magnetic transfer functions (T). The Z data for most of the stations show small scatter and
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small error bars, suggesting good data quality in general. The T data for P0 are small and
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relatively noisy (with large error bars) and will thus be given less emphasis than those
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resulting from the newer MT data. We computed various parameters for conventional
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dimensionality analysis such as tipper (Parkinson induction arrows), Bahr skew and the phase
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tensor [Caldwell et al., 2004, Simpson and Bahr, 2005], in order to determine reliable
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dimensionality distribution. We also computed two other parameters derived from the phase
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tensor: beta is the skew of the phase tensor and lambda is its ellipticity, and their threshold
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values for structural classification are: beta < 1 and lambda < 0.1 for 1D structures; beta < 1
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and lambda > 0.1 for 2D structures; and beta is non-zero or >1 for 3D structures.
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The real part of the induction arrows for representative frequencies (0.001-100 Hz) for all the
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MT profiles are shown in Figure 2. In general, the induction arrows are very small at 100 Hz,
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suggesting conductive structures at shallow depth and 1D nature for the given induction
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volume. At low frequencies (<0.1 Hz), large induction arrows point to the southwest on
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profiles PC07 and PC09+10 possibly suggesting the presence of a major NW-SE trending
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localized 3D conductor at the location where the Paleozoic deepest section of the Pimenta
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Bueno graben has been proposed [Fontes et al., 2016]. The induction arrow shows a reversal
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in behavior along PC02 suggesting the presence of a linear NW-SE trending conductor near
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the middle of the profile. There is also a reversal of induction arrows at the northeastern
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segment of profile PC03. The pattern of variations thus suggests the presence of localized
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bands of NW-SE trending conductors, rather than one continuous major lineament across the
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area.
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Figure 2. Induction arrows for all five MT lines for representative frequencies (100 - 0.001
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Hz). The arrows are real parts of the Parkinson induction arrow and point towards a major
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conductor. The larger the size of the arrow, the stronger the conductivity contrast between
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the anomalous conductor and the surrounding media.
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The phase tensor invariants from Caldwell et al. [2004] and phase skew from Bahr [1991] are
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presented in Figure 3. They enable us to infer the structural dimension of the conductivity
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variation along the MT profiles and the approximate geometry of the Parecis basin. The
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combined inspection of phase tensor invariants, i.e., beta (< ±1º) and lambda (< 0.1) suggests
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mostly 1D bodies for frequencies higher than 0.1 Hz and the presence of 3D structures for
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frequencies lower than 0.1 Hz since both beta and lambda exceed their threshold limits
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(Figure 3). The main shallow 3D features are highlighted in the beta section for profile PC02
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with non-zero values for frequencies higher than 1 kHz (Figure 3). Bahr skews smaller than
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0.12 indicates 1D/2D structure for frequencies between 1 kHz - 0.1 Hz, while it becomes
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more than 0.12 for frequencies less than 0.1 Hz depicting 3-D nature for all MT sites.
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Moreover, frequencies greater than 1 kHz for one of the MT profile PC02 also shows shallow
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3D behavior.
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Figure 3. Pseudosections of dimensionality parameters for the five MT lines. Shown are the
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parameters beta, lambda, Bahr skew, and the minimum phase determined from phase tensor.
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The minimum phase pseudo-section (Figure 3) may be used to qualitatively infer the
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conductivity variation of a region [Caldwell et al., 2004] and Hill et al. [2009] showed the
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presence of melt beneath the Mount St Helens of Washington State (USA) using minimum
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phase data. In general, an increase in apparent resistivity is associated with phase decrease
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and vice versa. In Figure 3, the high minimum phases (> 45º) will be related to the
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electrically conductive zones while the lower minimum phases (< 45º) will relate to the
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electrically resistive zones. The high minimum phases observed at frequencies above 1 Hz
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would suggest the presence of conductive sediments in the basin. Interestingly, patches of
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slightly lower phases are also observed within those high minimum phases. This possibly
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suggests the presence of resistive bodies (possibly intrusions or thrust wedges) within the
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conductive sediments. In addition, the occurrence of higher minimum phases for frequencies
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lower than 0.1 Hz in the southern segments of profiles PC03, PC02 and P0 possibly suggests
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the presence of steep deeply-lying conducting features (fault-zones or partial melt?) in the
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southern border of the Juruena sub-basin. The major regionally present zones of low
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minimum phases (~10-20º) represents the resistive basement of the Parecis basin in all
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profiles. The minimum phase variation enables us to identify the basement topography and
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possible locations of deep faults within the basement. Thus, as expected, the basement is
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deeper in the center and shallower at the northern and southern ends of profiles PC02 and P0
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(Figure 3). Given the identified presence of both 1D and 3D features in the various MT
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profiles, a 3D inversion of the field data is considered more appropriate than 2D inversion.
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First, we need to test the appropriate data types and initial resistivity models for the 3D
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inversion.
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3 Three-Dimensional MT Inversion and Ground-truthing of Results
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3.1 Testing prior Models and Data types
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We first performed 3D inversion of the MT profiles using ModEM code [Kelbert et al., 2014]
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focusing on investigating the effects of prior models and data types on the inversion results.
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We did not incorporate the available a priori information from the 2 hydrocarbon exploration
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wells in any of our 3D inversions in order to assess the actual predictive capability of 3D MT
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inversion of real data. We inverted the full impedance and tipper data using different initial
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half-space models (10, 50, 100, 200 and 500 Ωm). The considered error floor both for
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impedance and tipper was 5% of absolute values for all impedance and tipper components.
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First, we tested 3D inversion runs for MT lines PC02 and PC09+10 with different initial
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uniform half space models. Following Meqbel et al. [2014], fine mesh discretization at
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shallow depths in 3D inversion was adopted to allow compensating possible effects of static
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shifts in the MT data. The model domain was discretized into 190×100×45 cells with
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horizontal cell size of 1 km × 1 km for profile PC07. The first cell thickness was considered 10
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about 20 m and the cell thickness increases by a factor of 1.2 down to a depth of about 360
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km. The first cell or layer thickness was chosen to be one-tenth of the electromagnetic skin
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depth for the first frequency (~ 5 kHz). Ten padding cells increasing in size outwards with a
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factor of 1.2, were included in both horizontal directions to reduce inversion artifacts within
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the model domain. We increased the horizontal cell size from 1 to 2 km for profile PC02
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considering its wider station spacing, which is twice that of the other profiles. However, other
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model discretization parameters were kept similar to PC07, except the number of horizontal
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cells for each profile, included for 3D inversion, which depends upon station spacing and
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profile length. The discretized horizontal cell domains for profiles PC02, PC03 and PC09+10
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are 160×145, 140×120 and 190×105 respectively. The regularization parameter, λ, was
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assigned an initial value of ~ 10 and successively reduced by a factor 10, down to a value of
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10-8. Smoothing was chosen 0.2 for x, y and z directions. A large λ value provides smooth
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structures at initial iterations and its subsequent size reduction allows the reconstruction of
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rougher (and perhaps more geologically realistic) structures in later iterations. The results of
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the various inversion runs with different starting half-space models are somewhat similar as
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shown in Figure 4. The normalized RMS misfit values were 1.66, 1.62, 1.57, 1.41 and 1.51
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for the 10, 50, 100, 200 and 500 Ωm initial models, respectively. A half-space model of 100
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Ωm was chosen as the preferred initial model for all subsequent inversion runs, despite
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yielding a normalized RMS higher than other initial models – this choice was based on both
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its comparatively clearer geologic features and better fit to well A log data.
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Figure 4. Testing the effect of prior models for MT line PC02. The inversion models were
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reconstructed from initial half-space models of resistivity: (a) 10 Ωm, (b) 50 Ωm, (c) 100
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Ωm, (d) 200 Ωm and (e) 500 Ωm. The normalized RMS misfit value for each inversion run is
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stated for each cross-section.
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Figure 5. Testing the effect of MT data types on the inversion result for line PC02. (a) Tipper
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T, (b) full impedance Z, and (c) combined Z and T inversions.
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To test the effect of various MT data types, we inverted MT data types such as tipper (T), full
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impedance (Z) and combination of both full impedance and tipper (Z+T) for line PC02
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(Figure 5). This procedure helped us to identify the most robust model features common to
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all the data types. Tipper data inversion (Figure 5a) imaged the major conductive features but
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may not have depth resolution. Note the sheet-like conductor at the central portion of the 13
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profile and the steep conductors at the southern end of the profile. The full impedance
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inversion (Figure 5b) resolved the basin geometry, which is deepest at the center and
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shallows up at both northern and southern basin edges. Deep conductors were found at mid
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crustal depths at the southern end of profiles PC03, PC02 and P0. In the case of Z+T
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inversion (Figure 5c), we found that most of the model features are generally similar to those
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for Z inversion but with some loss in resolution details. We finally selected Z+T joint
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inversion as the approach to use in this study and then applied it to all the other lines. The
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results are summarized in Figure 6; interestingly, we found that steep conductors are not only
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present in the southern basin margin but also at the northern end of lines PC02 and P0. The fit
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between the field data and the computed model responses for all the lines is good (normalized
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RMS of 1.8 to 2.1) and can be seen in the pseudosections presented in Figure 7. Notice that
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the observed and predicted MT apparent resistivity and phase responses are generally well
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matched for both Zxy and Zyx components. Further examples of the fit are shown in Figure 8
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for eight representative stations picked from the five MT lines. These individual line plots
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suggest that a satisfactory fit was achieved.
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Figure 6. Deep electrical structure across Parecis basin from 3D imaging of individual MT
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lines. (a) Upper crustal resistivity structure. (b) Crust and upper mantle resistivity structure.
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Figure 7. Pseudosections of the observed apparent resistivity and phase data for the five MT
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lines and those predicted by the joint Z+T 3D inversion models. (a) XY data. (b) YX data.
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Figure 8. Observed versus predicted MT apparent resistivity and phase responses for eight
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representative MT stations for the 3D inversion models. Individual points with error bars are
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the observed xy (red) and yx (blue) components of the impedance tensor at each station. The
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corresponding predicted responses for xy and yx data components are shown as solid curves.
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Inset map shows the locations of the representative stations for each survey line.
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3.2 Evaluation of MT inversion Result at Deep Well Sites
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To test MT model reliability, first we compared the resistivity logs of wells with the depth
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wise resistivity variation from 3D inversion. We also overlay the seismic lines coinciding
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with the MT profiles to verify the main structures of the basin such as faults, basement and
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sedimentary formations imaged by the 3D inversion model. We utilized two Parecis basin
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wells [Dardenne et al., 2006; Vasconcelos et al., 2014], namely A (2-FI-1-MT) and B (2-SM-
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1-MT) from ANP, in the southern ends of profiles PC02, and PC09+10 respectively (Figure
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1).
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The composite litholog from well A shows Parecis Group ~ 600 m, Guape gabbro intrusives
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~ 670 – 1300 m, Parecis Group ~1300 –1800 m, and phyllitic basement rocks starting at ~
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1800 m. The composite litholog from well B shows Utiariti Formation ~ 10 – 250 m and the
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Salto das Neves Formation ~ 250 – 520 m of Parecis Group of rocks; Diamantino Formation
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~ 520 – 1450 m, Sepotuba Formation ~ 1450 – 2530 m, Raizama Formation ~2530 – 3780 m,
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and Serra Azul Formation ~ 3780 – 3970 m of Alto Paraguai Group; Nobres Formation ~
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3970 - 4280 m, Guia Formation ~ 4280 – 4570 m and Mirassol D’Oeste Formation ~ 4570 –
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4800 m of Araras Group; Puga Formation ~ 4800 – 4920 m and Bauxi Formation ~ 4920 –
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5150 m of Jangada Group; Carbonato Salto Magessi Formation ~ 5150 – 5670 m, and Cuiaba
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Group probed at ~ 5670 m downwards. All these mentioned ranges correspond to well depths
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of formation tops and bases.
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Thus, well A mostly sampled resistive igneous bodies (intrusives or thrust wedges?) while
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well B has sedimentary formations down to 5700 m depth. We extracted the resistivity
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variation with depth from 3D MT resistivity model at the location of the Wells A and B.
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These were then plotted alongside the gamma-ray and resistivity logs of both wells with the
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major formation boundaries from the composite lithology log (Figure 9). We found that the
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resistivity-depth profiles from our 3D MT inversion models satisfactorily matched the
313
average values of the resistivity well logs in the top 2200 m (Figure 9). At the shallowest
314
levels in both wells, there is an electrically conductive formation (C1) of high gamma-ray
315
response which is correctly predicted by the MT models and correlates with Parecis Group in
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the composite lithological log. Thus, there is consistency in our 3D inversion results with the 18
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independently measured well log data at least down to the explored depths at these well
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locations. Well A (southwest of Juruena sub-basin), drilled down to a depth of 2.4 km,
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penetrated metamorphic phyllitic basement rocks from a depth of about 1.8 km (Figure 9).
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These metasedimentary rocks are only moderately radiogenic and electrically resistive
321
compared to the overlying Parecis Group (possibly Utiariti Fm) which is of low resistivity.
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Since the Utiariti Fm of Parecis Group has a distinct electrical signature, it could thus be used
323
as a marker horizon (C1) for interpreting or correlating our MT models with seismic images
324
at shallow levels (<2200 m) in this basin. The gabbroic body sampled at ~ 1000 m depth in
325
Well A could be a thrust wedge or a sill and is detected by MT imaging (Figure 9).
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327 328 329
Figure 9. Result of 3D MT inversion versus actual gamma-ray and resistivity well logs at the
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locations of wells A and B. Interpreted lithologies from the composite logs of both wells are
331
also shown for comparison. C1, C2 and C3 are target conductors in the sedimentary section.
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(see Figure 1 for site location). Stratigraphy from Vasconcelos et al. [2014].
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Well B, drilled down to a depth of 5.8 km, did not penetrate crystalline basement. This well
336
penetrated a thick column of sedimentary formations. The variation of resistivity with depth
337
have very similar trends in the 3D inversion model and well logs. The Utiariti Fm of Parecis
338
Group has characteristic low resistivity and high gamma-ray values and can be confidently
339
interpreted as marker horizon C1 (Figure 9). Another conductive marker (C2) is evident
340
within the Alto Paraguai Group in the well log and the MT model at about 2000 depth. Below
341
about 2200 m depth, whilst the MT resistivity trend correctly mimics the average well log
342
resistivity pattern, the individual formations cannot be discriminated in the MT model. This
343
means that we cannot use the present result to distinguish the Nobres Fm and Guia Fm of
344
Araras Group and Puga Fm and Bauxi Fm reservoirs of Jangada Group and the intervening
345
conductive Mirassol D’Oeste Fm of Araras Group or map the basal conductive Cuiaba Group
346
occurring at the bottom of the drilled section (C3).
347 348
The major technical limitations faced here were associated with grid-size optimization for the
349
selected combinations of data type for such a large-scale practical inverse problem on a small
350
computational platform and the natural loss of MT resolution with depth. For the size of
351
regional problem considered here, the computational grid-size limitation and the reduction in
352
the number of inverted MT stations from 455 to 199, coupled with the diffusive nature of MT
353
signals, affected stratigraphic resolution to a large extent at depths greater than 2 km. Note
354
that we did not incorporate any a priori information from Wells A and B in our MT
355
inversions. It is possible that incorporating resistivity-versus-depth constraints from the well
356
logs could improve the detection and resolution of unit C3.
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Due to the diffusive nature of the MT signals and skin depth control, it is noteworthy to
358
mention that the MT inversions reflect the average subsurface resistivity over an increasing
359
hemisphere dropping resolution laterally and with depth whereas the well log data provides 20
360
localized resistivity values with depth. Nonetheless, we consider the MT results to be
361
satisfactory at these well locations.
362 363
4 Discussion
364
The main anomalous features found in the resistivity cross-sections obtained from 3-D
365
inversion of the individual MT lines (Figures 6 and 10) are: i) intrasedimentary conductive
366
features marked by stratiform conductors C1, C2 and C3, and ii) steep conductive features
367
(C4) in the basement and sub-horizontal conductors (C5) at 10-30 km depth. For the upper 10
368
km of the crust, the question may be asked if it was justifiable to invert individual MT lines
369
in 3D rather than 2D? We consider the expensive 3D inversion to be necessary in this fold-
370
thrust and potentially magmatic region. It is instructive to compare the 2D inversion result of
371
Santos and Flexor [2012] and our new result of 3D inversion of the same datasets since the
372
former model has been used by ANP for promoting petroleum exploration licensing for this
373
basin. This is shown in Figure 10. The resistivity structures are very different in both models
374
especially over the fold belts at the southern and northern basin borders. The 3D structures
375
depicted in our dimensional analysis for intermediate to lower frequencies cause the observed
376
disagreement between both inversions as 2D inversion of 3D data can usually create artifacts.
377
Our 3D models have been shown to be consistent with well log data (Figure 9) and are
378
preferred for geological interpretation here. The previously suggested lateral extents of the
379
basement structural domains [Bahia et al., 2007] are also indicated at the top of the MT
380
sections in Figure 10 for comparison. The 3D inversion result shows consistent lateral
381
changes as those derived earlier from potential field studies for the Brassnorte high, Pimenta
382
Bueno graben, and Rio Branco high [Bahia et al., 2007] and, in addition, refine the lateral
383
and depth boundaries of the tectonic domains. However, in the northern parts of lines P0 and
384
PC02 (Figure 6 and 10), it is clear that the up to 9 km depth to basement implied by Braga
385
and Siqueira [1996] is not tenable, as also found by Barros and Assumpção [2011]. 21
386
387 388
Figure 10. Comparison of 2D and 3D inversion models for line P0 for the top 8 km of the
389
crust. Dotted horizontal white line at 4 km depth is for reference. C1 and C2 are stratiform
390
conductive units in the sedimentary basin. C4 is a steep conductive zone extending deep into
391
the crust.
392 393
Loureiro et al. [2017] propose a new structural interpretation for the Parecis basin based on
394
the integration of 2D seismic and gravity data constrained by data from wells A and B
395
(Figures 11 and 12). They distinguished the economic (~ 6 km) and crystalline (~ 10 km)
396
basement, supported by the gravity models constrained with seismic horizons tied to the
397
available wells. From top to bottom, the marked horizons are base of the Mesozoic group, top
398
of Araras group, siliciclastic sequences, carbonate sequences, economic and metamorphic
399
basement rocks (Figures 11 and 12). The economic basement consists of phyllitic rocks (~
400
2.52 g/cm3) which are less dense than the crystalline basement, made up of mainly igneous
401
rocks (~2.67 g/cm3). Our MT sections along the same seismic-gravity lines are also shown in
402
Figures 11 and 12 for comparison. For basin-wide or lateral correlation purpose, we
403
identified three regional marker units in the sedimentary section labelled C1, C2 and C3. We
404
also identified steep conductive zones in the crystalline basement labelled C4. Our resistivity 22
405
models for the two MT lines crossing the ANP deep exploration wells and the available
406
resistivity well logs are also superimposed on the seismic-gravity models of Loureiro et al.
407
[2017] for comparison in Figures 11 and 12.
408 409
Figure 11. Comparison of MT model and independent integrated seismic-gravity model for
410
line PC02. Top graphic is the resistivity cross-section from unconstrained joint 3D inversion
411
of Z and T data. Bottom graphic superposes the MT Z+T inversion model, the interpreted
412
seismic-gravity model [Loureiro et al., 2017] and geologic formations.
413
23
414
In Figure 11, our feature C1 is in good agreement with Well A data (Fig. 9) and is also
415
conformable with the shallowest boundary interpreted by Loureiro et al. [2017] although the
416
seismic data are of poor quality. No other significant conductor was found in the Well A but
417
our MT model suggests lateral continuity of C1 across the entire line, consistent with the
418
shallowest boundary interpreted by Loureiro et al. [2017]. The localized deepening and the
419
presence of our feature C3 at profile distance 175-255 km agrees with the thick sequence
420
occurring between the top basement and the shallowest boundary of Loureiro et al. [2017].
421
The vertical conductive zone C4 imaged at profile distance 45 – 60 km on this line is also in
422
excellent agreement with a major fault interpreted from seismic data by Loureiro et al. [2017]
423
in the basement. In Figure 12, our marker unit C1 in the sedimentary section is in excellent
424
agreement with Well B data and is also conformable with the shallowest well-based seismic-
425
gravity boundary interpreted by Loureiro et al. [2017]. Our unit C2 is also in agreement with
426
Well B data. Its lateral continuity up-dip from the well location (where it lies at ~2 km depth)
427
is remarkably consistent with the seismic data. We can only infer the presence of the
428
conductive unit C3 down to 4-5 km towards the location of Well B but cannot resolve it in
429
our unconstrained 3D MT models. It is possible that the resolution of our MT models can be
430
improved further by incorporating well-based constraints as in the gravity-seismic work of
431
Loureiro et al. [2017] but we consider our present 3D MT models adequate for gross
432
geological interpretation.
433 434
24
435 436 437
Figure 12. Comparison of MT model and independent seismic-gravity model for line
438
PC09+10. Top graphic is the resistivity cross-section model from unconstrained 3D joint Z
439
and T inversion. Bottom graphic superposes the MT Z+T model, the interpreted seismic-
440
gravity model [Loureiro et al., 2017] and geological formations. C1, C2 and C3 are layered
441
conductors in the sedimentary section as observed in Well B shown in Figure 9.
442 443 25
444
What is the geological significance of the deep conductivity anomalies? The deep crustal and
445
upper mantle resistivity structure across the region (Figure 6) is crosscut by major subvertical
446
crustal conductive features (marked C4 and C5). These conductors are restricted to the basin
447
margins characterized by Neoproterozoic mobile (fold-thrust) belts. They are deepest at the
448
SW end of line PC02 (20-30 km depth), lie at 10-20 km depth on PC03 to the west, and occur
449
at 10-15 km depth on line P0 to the east. It is also interesting that Cretaceous and Tertiary
450
kimberlites are found in the northern and southern margins of the basin [Schobbenhaus,
451
1981]. So, the steep conductors C4 and C5 (Figure 6) are likely reactivated shear-zones in the
452
crystalline crust beneath the basin that probably served for magma transport during the
453
Cretaceous and Tertiary periods. Apparently, this region is presently undergoing compression
454
due to the convergence between the South American plate and Nazca plates [Assumpção and
455
Sacek, 2013].
456 457
5 Conclusion
458
Broadband (0.001 – 5000 Hz) magnetotelluric (MT) data for 455 stations, recorded along
459
four lines in the Juruena sub-basin and one regional line cutting across the Parecis basin into
460
the outcropping Precambrian basement at its northern and southern borders, have been
461
inverted to image the sediments and the deep crustal structure beneath the basin. We
462
performed 3D inversion for each line separately using the full datasets. We found that the
463
reconstructed resistivity-versus-depth profiles from our 3D MT inversion models
464
satisfactorily match the average values of the resistivity well logs in the top 2200 m at two
465
well locations chosen for model validation. The Parecis Formation of low resistivity and
466
shallow depth can be traced across the region in the MT models including the intruded or
467
thrusted gabbroic body which is of high resistivity. We found that the sedimentary formations
468
deeper than 2200 m could not be individually resolved by the coarsely discretized MT depth
26
469
models although the total sedimentary cover thickness may be approximated from the 3D
470
imaging results. An areally restricted conductive sequence (dubbed C3 here) occurs at about
471
2000 - 5000 m depth in the central part of Juruena sub-basin and is interpreted as possible
472
synrift deposits. The most significant features of the deep basement structure at mid-lower
473
crustal depths are the NW-SE trending steep conductive zones restricted to the mobile (fold-
474
thrust) belts at basin margins. They are deepest at the SW end of line PC02 (20-30 km depth),
475
lie at 10-20 km depth on line PC03 to the west, and occur at 10-15 km depth on line P0 to the
476
east. We interpret these conductors as possibly ophiolites broken by thrust-faults or shear-
477
zones in the resistive (crystalline) crust beneath the basin currently known to be undergoing
478
compression and we suggest that they are possibly associated with the Upper Cretaceous and
479
Tertiary alkaline magmatism in the region.
480 481
Acknowledgments
482
The new MT data used in this study were acquired by Schlumberger for the Brazilian Agency
483
of Oil, Natural Gas and Biofuels (ANP). The authors thank ANP for supporting this study
484
(TC ANP-ON 01/2013) and providing the MT and well log data for our investigation. The
485
MT data are available free of charge from ANP database (BDEP) on request to
486
[email protected]. We thank the editor Franck Audemard, Naser Meqbel and an
487
anonymous reviewer for comments and suggestions that improved the manuscript.
488 489
References
490 491
27
492 493
Assumpção, M., and V. Sacek (2013), Intra-plate seismicity and flexural stresses in central Brazil, Geophys. Res. Lett., 40, 487–491, doi:10.1002/grl.50142.
494 495
Bahia, R. B. C., M. A. M. Neto, M.S.C. Barbosa, and A. J. Pedreira (2007), Análise da
496
evolução tectonossedimentar da bacia dos Parecis através de métodos potenciais, Brazilian
497
Journal of Geology, 37, 639–649. In Portuguese.
498 499 500
Barros, L.V. Assumpção, M. Quinteros, R. and Caixeta, D., (2009), The intraplate Porto dos Gaúchos seismic zone in the Amazon craton – Brazil, Tectonophysics, 469, 37-47.
501 502
Barros, L.V. and Assumpção M. (2011), Basement depths in the Parecis Basin (Amazon)
503
with receiver functions from small local earthquakes in the Porto dos Gaúchos seismic
504
zone, Journal of South American Earth Sciences, 32, 142-151.
505 506 507
BDEP (2008), Banco de dados de exploração e produção: CD-003295, Pedido: 4015; Br: 4966, Convênio ANP/CPRM, BDEP – ANP/SDT.
508 509
Braga L.F.S, and Siqueira L.P. (1996), Three-Dimensional Gravity Modelling of the
510
basement topography beneath Parecis Basin, Brazil, constrained by DBNM, spectral
511
estimates of depth to magnetic sources, In: Proceedings of the 5th Latin American
512
Petroleum Congress, Rio de Janeiro, Brazil, 1996, 1–5.
513 514 515
Caldwell, T.G., Bibby, H.M. and Brown, C. (2004), The magnetotelluric phase tensor. Geophysical Journal International, 158: 457–469.
516
28
517
Dardenne, M.A., Alvarenga, C.J., Oliveira, C.G. and Lenharo, S.L.R. (2006), Geologia e
518
Metalogenia do Depósito de Cobre do Graben Colorado na Fossa Tectônica de Rondônia,
519
In: Marini, O.J., Queiroz, E.T., Ramos, B.W. (Eds.), Caracterização dos Depósitos
520
Minerais em Distritos Mineiros da Amazônia, pp. 553-596. In Portuguese.
521 522 523
Egbert, G. D. (1997), Robust multiple-station magnetotelluric data processing, Geophys. Journal. International, 130, 475–496.
524 525
Fontes, S. L., Maurya, V. P. and La Terra, E. F. (2016), Magnetotelluric Evidence of Deep
526
Mesozoic-Paleozoic Sediments beneath Parecis Basin, West Central Brazil, American
527
Geophysical Union, Fall General Assembly, abstract #GP51A-1359.
528 529
Hill, Graham J., T. Grant Caldwell, Wiebke Heise, Darren G. Chertkoff, Hugh M. Bibby,
530
Matt K. Burgess, James P. Cull and Ray A. F. Cas (2009), Distribution of melt beneath
531
Mount St Helens and Mount Adams inferred from magnetotelluric data, Nature
532
Geoscience, 2, 785–789.
533 534 535
Kelbert, A., Meqbel, N., Egbert, G.D. and Tandon, K. (2014), ModEM: a modular system for inversion of electromagnetic geophysical data, Computers & Geosciences, 66, 40–53.
536 537
Loureiro, E.M.L, Menezes, P.T.L, Zalán, P.V., and Heilbron, M. (2017), Tectonic
538
Framework of Parecis Basin: a Seismic-Gravity Integrated Interpretation, 15th
539
International Congress of the Brazilian Geophysical Society, Rio de Janeiro, Brazil, 31
540
July to 3 August 2017.
541
29
542
La Terra, E.M.F, Vital, L., Santos, I., Miquelutti, L.G. and Fontes, S.L. (2016), Mapping of
543
basement relief and volcanic intrusions of Parecis basin in Brazil based on 3D
544
magnetotelluric imaging and potential field data. International Conference and Exhibition,
545
Barcelona, Spain, 3-6 April 2016: pp. 172-172. https://doi.org/10.1190/ice2016-6529755.1
546 547
Meqbel, N.M., Egbert, G.D., Wannamaker, P.E., Kelbert, A., Schultz, A. (2014), Deep
548
electrical structure of the northwestern U.S. derived from 3-D inversion of USarray
549
magnetotelluric data. Earth and Planetary Science Letters 402, 290-304.
550 551
Santos, H.S. & Flexor, J.M. (2012), Geoelectric directionality of a Magnetotelluric (MT)
552
survey in the Parecis Basin, Brazil, Revista Brasileira de Geofísica, 30(1): 81-92, ISSN
553
0102-261X.
554 555
Schobbenhaus, C. (1981), Mapa Geológico do Brasil e da Área Oceânica Adjacente
556
Incluindo Depósitos Minerais: Escala 1:2 500 000. Geologic Map of Brazil and Adjoining
557
Ocean Floor Including Mineral Deposits: Scale 1:2,500,000, Brasília: República
558
Federativa do Brasil, Ministério das Minas e Energia, Departamento Nacional de
559
Produção Mineral.
560 561 562
Simpson, F. and Bahr, K., 2005. Practical Magnetotellurics, Cambridge University Press, 254pp.
563 564
Siqueira, L.P. (1989), Bacia dos Parecis. Boletim Geociências PETROBRAS 3 (12), 3-16.
565 566
30
567
Vasconcelos, C.S., Morales, I.V.F., Trosdorf Jr., I., Santos, S.F., Figueiredo, M.F., 2014.
568
Revisão da estratigrafia na seção perfurada pelo poço 2-SM-1-MT (Salto Magessi), Bacia
569
dos Parecis- Alto Xingu, MT. Boletim Geociências Petrobras, Rio de Janeiro, v. 22, n. 1,
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p. 171-178. In Portuguese.
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31
Ref: SAMES_2019_97 Title: Deep structure of Parecis Basin, Brazil from 3D magnetotelluric imaging Journal: Journal of South American Earth Sciences
Highlights
3-D resistivity structure of Parecis basin using 455 magnetotelluric soundings was imaged at depths down to 12 km; Low resistivity shallow Parecis formation was traced across the basin; Resistive upper crust is thinned over the Neoproterozoic fold belts; Deep crustal steep conductors possibly associated to alkaline magmatism in the region.
S0895-9811(19)30113-0
DOI:
https://doi.org/10.1016/j.jsames.2019.102381
Reference:
SAMES 102381
To appear in:
Journal of South American Earth Sciences
Received Date: 16 March 2019 Revised Date:
6 September 2019
Accepted Date: 4 October 2019
Please cite this article as: Fontes, S.L., Meju, M.A., Maurya, V.P., La Terra, E.F., Miquelutti, L.G., Deep structure of Parecis Basin, Brazil from 3D magnetotelluric imaging, Journal of South American Earth Sciences (2019), doi: https://doi.org/10.1016/j.jsames.2019.102381. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
1
Deep structure of Parecis Basin, Brazil from 3D magnetotelluric imaging
2 3
S.L. Fontes1, M. A. Meju2, V. P. Maurya1#, E.F. La Terra1 and L.G. Miquelutti1†
4 5
1
6
2
7
† Currently at Universidade Federal de Uberlandia, Brazil
8
# Currently at National Geophysical Research Institute, India
MCTIC-Observatorio Nacional, Rio de Janeiro, Brazil Petronas Upstream Exploration, Kuala Lumpur, Malaysia
9 10
Corresponding author: Sergio L Fontes ([email protected])
11 12
1
13
Abstract
14
The resistivity structure beneath the Parecis basin in central Brazil has been studied using
15
broadband (0.001 - 5000 Hz) magnetotelluric (MT) data for 455 stations recorded along five
16
lines in the Juruena sub-basin and over the Precambrian basement at the northern and
17
southern basin borders. We found that the reconstructed resistivity-versus-depth profiles from
18
our 3D MT inversion models satisfactorily match the average values of the resistivity well
19
logs in the top 2200 m at two exploration well locations chosen for model validation. The low
20
resistivity Parecis Formation at shallow depth can be traced across the region while an areally
21
restricted conductive sequence at about 2000 m depth can be imaged in the central part of
22
Juruena sub-basin and is interpreted as synrift deposits or remnants of a buried
23
Neoproterozoic basin. Sedimentary formations deeper than 2200 m could be sensed but not
24
individually resolved in our resistivity models. The deeper resistivity structure consists of a
25
resistive upper crust, severely thinned over the Neoproterozoic fold belts bounding the basin
26
to the north and south where it is underlain by conductive zones at mid-lower crustal depths.
27
We interpret the deep crustal steep conductors as pre-existing shear-zones possibly associated
28
with the Upper Cretaceous to Tertiary alkaline magmatism in the region.
29 30
1 Introduction
31
Parecis Basin is an elongate east to west intracratonic basin (Figure 1) in central Brazil
32
adjacent to the subandean depression. It has an area of about 355,000 km2 and contains as
33
much as 6 km thick sediments and metasediments proven by deep exploration wells in the
34
region [BDEP, 2008]. The basin initiated as a NW-SE rift system [Braga and Siqueira, 1996]
35
whose abortion led to thermal subsidence over a wide area, and to the deposition of the
36
Parecis sag-type basin. The rifts formed over reactivated older structures and the preserved
37
grabens may contain Silurian alluvial fans and Carboniferous glacial deposits. The basement 2
38
and deep crustal structure across the basin remain poorly understood and are the subject of a
39
long running debate [e.g., Braga and Siqueira, 1996; Barros et al., 2009; Barros and
40
Assumpção, 2011; Santos and Flexor, 2012; Assumpção and Sacek, 2013; La Terra et al.,
41
2016; Fontes et al., 2016; Loureiro et al., 2017]. According to Siqueira [1989], the Parecis
42
Basin is separated into Alto Xingu, Juruena and Rondônia sub-basins by the Serra Formosa
43
Arch to the east and the Vilhena Arch to the west (Figure 1). From north to south, the
44
structural framework is interpreted to consist of several tectonic domains notably Tapajós
45
mobile belt, Brasnorte high, Pimenta Bueno graben, Rio Branco high, Colorado graben and
46
the Neoproterozoic North Paraguay fold belt, and the basin is surrounded by crystalline
47
igneous basement rocks (Figure 1).
48
49 50
Figure 1. Location map of the Phanerozoic Parecis Basin in western Brazil showing the MT
51
and seismic survey lines used in this study. Geological map and inset modified after Siqueira
52
[1989] and Bahia et al. [2007]. Shown are old MT line P0 (green circles), 4 new MT lines 3
53
(black traces), 4 seismic lines (blue dots) coinciding with MT profiles (i.e., 295-002 with
54
PC02, 295-003 with PC03, 295-007 with PC07, 295-009 and 295-0010 with PC09+10), 2
55
ANP wells A and B (2-FI-0001-MT and 2-SM-0001-MT) and CPRM well C (PB-01-RO) as
56
large yellow crosses. Light grey lines represent previously interpreted structural framework
57
consisting of BH (Brasnorte high), PBG (Pimenta Bueno graben), RBH (Rio Branco high),
58
CG (Colorado graben), NFB (Northern Paraguay fold belt), TMB (Tapajós Mobile Belt). S1
59
is a single MT station west of Well B. The small red star is the epicenter of 1998/2005
60
earthquakes while the large red star is the epicenter of the 1955 6.2mb earthquake [Barros et
61
al., 2009; Barros and Assumpção, 2011].
62 63
The trends of these structural domains mirror those of the gravity anomalies. Braga and
64
Siqueira [1996] interpreted the Parecis Bouguer gravity anomalies to reveal grabens and
65
structural highs but the inferred regional basement variation from gravity can be ambiguous
66
with regards to some important local structures unless constrained by other geophysical data
67
[e.g., Barros and Assumpção, 2011]. Contrary to the gravity-derived model [Braga and
68
Siqueira, 1996], Barros and Assumpção [2011] suggest from receiver function analysis that
69
the basement depth tends to increase towards the south from the northern border of Parecis
70
basin. Barros and Assumpção [2011] also conclude that the decrease in gravity toward the
71
north (as used by Braga and Siqueira [1996]) must be caused by variations of deeper crustal
72
structure, such as variation in crustal thickness in the Parecis basin. Using 2D magnetotelluric
73
(MT) imaging of a roughly N-S regional line (line P0 in Figure 1), Santos and Flexor [2012]
74
inferred the presence of a pile of ~ 8 km thick sedimentary formations at the center of Parecis
75
basin. La Terra et al. [2016] modeled the basement relief of the Parecis basin using MT data
76
and constraints from potential field anomalies. Their 2D conductivity models imaged
77
relatively conductive sedimentary sequences, intercalated with highly resistive horizontal and
78
vertical bodies interpreted as possible volcanic intrusions (dikes and sills). They also 4
79
observed anomalous low resistivity in the lower crust and ascribed it to earlier tectonics
80
before basin formation. Fontes et al. [2016] show evidence for the presence of up to 8 km
81
deep Mesozoic-Palaeozoic sediments in the centre of Parecis basin from preliminary 3D MT
82
inversion. Recent integrated interpretation of seismic and gravity data by Loureiro et al.
83
[2017] suggest the presence of a Mesoproterozoic basin of low Bouguer gravity, directly
84
overlying the crystalline basement. They also mapped the Neoproterozoic sedimentary
85
packages using seismic horizons interpreted with the aid of 2 ANP wells (2-FI-1-MT and 2-
86
SM-1-MT). However, the seismic data are of poor quality and lateral correlation away from
87
the well locations may be somewhat conjectural.
88 89
In the present paper, we apply the 3D inversion code of Kelbert et al. [2014] to the full MT
90
datasets from four MT survey lines of Fontes et al. [2016] and an additional MT line P0
91
previously inverted in 2D by Santos and Flexor [2012]. We aim to better understand the
92
resistivity structure of the crust and upper mantle beneath the basin in order to help resolve
93
past controversies about the basin. We first calibrate our 3D inversion models at the locations
94
of two ANP (Brazilian Agency of Oil, Natural Gas and Biofuels) exploration wells along our
95
MT lines, and also validate our 3D models with seismic interpretation in the regions away
96
from the well sites. After model calibration, we then attempt to resolve some of the above
97
issues identified in past geophysical studies of the deep structure across the basin. The rest of
98
our paper is structured as follows. In section 2, we discuss the result of standard tensorial
99
analysis of the MT field data. We show that the characteristics of the MT impedance tensor
100
allows 1D analysis of part of the data at high frequencies but that 3D character dominates at
101
intermediate to low frequencies. In section 3, we discuss the detailed study of conventional
102
smoothness-constrained 3D MT inversion for deep resistivity structure with its validity tested
103
at stations near two hydrocarbon exploration wells where electrical resistivity and gamma-ray
104
logs are available for ground-truthing, as well as along two coincident survey lines where 5
105
independent models from integrated 2D seismic and gravity interpretation using constraints
106
from both wells are available. In section 4, we discuss the possible relationship of our
107
resistivity models to past geophysical models. The main conclusions from our studies are
108
presented in section 5.
109 110
2 MT Data Analysis
111
The new MT survey consists of broadband recording (0.001 - 5000 Hz) at 382 stations along
112
four MT profiles covering mostly what is termed the Juruena sub-basin of the Parecis basin.
113
The new profiles are PC02 (central region of the Juruena sub-basin), PC03 (western domain
114
of the Juruena sub-basin), PC07 and PC9+10 (both covering the eastern domain of Juruena
115
sub-basin). The average station spacing is 3.6 km along PC02 and 1.8 km for the others.
116
These data were acquired by the commercial company Schlumberger for ANP. Additional
117
MT (0.001 - 100 Hz) data were available from an earlier more extensive regional line P0
118
(Figure 1) acquired by MCTIC-Observatório Nacional [Santos and Flexor, 2012], yielding a
119
total of 455 stations. Both electric (E) and magnetic (H) fields were acquired at all the MT
120
sites. Three of the new MT profiles (PC02, PC03 and PC09+10) coincide with 2D seismic
121
lines (295-002, 295-003, 295-009 and 295-0010) from ANP. Two petroleum exploration
122
wells 2FI-0001-MT (or Well A) and 2SM-0001-MT (or Well B) are located at the southern
123
end of profiles PC02 and PC09+10 respectively (Figure 1). We will use the seismic and well
124
data for the geological calibration of the results of MT depth imaging in the next section.
125 126
As is standard in MT practice, the recorded time series for the E and H field components at
127
each station were processed in single station and remote reference mode using the robust
128
processing scheme of Egbert [1997] based on multivariate statistical methods. The results
129
include the full impedances (Z, i.e., all four components Zxx, Zxy, Zyx and Zyy) and vertical
6
130
magnetic transfer functions (T). The Z data for most of the stations show small scatter and
131
small error bars, suggesting good data quality in general. The T data for P0 are small and
132
relatively noisy (with large error bars) and will thus be given less emphasis than those
133
resulting from the newer MT data. We computed various parameters for conventional
134
dimensionality analysis such as tipper (Parkinson induction arrows), Bahr skew and the phase
135
tensor [Caldwell et al., 2004, Simpson and Bahr, 2005], in order to determine reliable
136
dimensionality distribution. We also computed two other parameters derived from the phase
137
tensor: beta is the skew of the phase tensor and lambda is its ellipticity, and their threshold
138
values for structural classification are: beta < 1 and lambda < 0.1 for 1D structures; beta < 1
139
and lambda > 0.1 for 2D structures; and beta is non-zero or >1 for 3D structures.
140
The real part of the induction arrows for representative frequencies (0.001-100 Hz) for all the
141
MT profiles are shown in Figure 2. In general, the induction arrows are very small at 100 Hz,
142
suggesting conductive structures at shallow depth and 1D nature for the given induction
143
volume. At low frequencies (<0.1 Hz), large induction arrows point to the southwest on
144
profiles PC07 and PC09+10 possibly suggesting the presence of a major NW-SE trending
145
localized 3D conductor at the location where the Paleozoic deepest section of the Pimenta
146
Bueno graben has been proposed [Fontes et al., 2016]. The induction arrow shows a reversal
147
in behavior along PC02 suggesting the presence of a linear NW-SE trending conductor near
148
the middle of the profile. There is also a reversal of induction arrows at the northeastern
149
segment of profile PC03. The pattern of variations thus suggests the presence of localized
150
bands of NW-SE trending conductors, rather than one continuous major lineament across the
151
area.
152 153
7
154 155
Figure 2. Induction arrows for all five MT lines for representative frequencies (100 - 0.001
156
Hz). The arrows are real parts of the Parkinson induction arrow and point towards a major
157
conductor. The larger the size of the arrow, the stronger the conductivity contrast between
158
the anomalous conductor and the surrounding media.
159 160
The phase tensor invariants from Caldwell et al. [2004] and phase skew from Bahr [1991] are
161
presented in Figure 3. They enable us to infer the structural dimension of the conductivity
162
variation along the MT profiles and the approximate geometry of the Parecis basin. The
163
combined inspection of phase tensor invariants, i.e., beta (< ±1º) and lambda (< 0.1) suggests
164
mostly 1D bodies for frequencies higher than 0.1 Hz and the presence of 3D structures for
165
frequencies lower than 0.1 Hz since both beta and lambda exceed their threshold limits
166
(Figure 3). The main shallow 3D features are highlighted in the beta section for profile PC02
167
with non-zero values for frequencies higher than 1 kHz (Figure 3). Bahr skews smaller than
168
0.12 indicates 1D/2D structure for frequencies between 1 kHz - 0.1 Hz, while it becomes
169
more than 0.12 for frequencies less than 0.1 Hz depicting 3-D nature for all MT sites.
8
170
Moreover, frequencies greater than 1 kHz for one of the MT profile PC02 also shows shallow
171
3D behavior.
172 173 174
Figure 3. Pseudosections of dimensionality parameters for the five MT lines. Shown are the
175
parameters beta, lambda, Bahr skew, and the minimum phase determined from phase tensor.
176 177
The minimum phase pseudo-section (Figure 3) may be used to qualitatively infer the
178
conductivity variation of a region [Caldwell et al., 2004] and Hill et al. [2009] showed the
179
presence of melt beneath the Mount St Helens of Washington State (USA) using minimum
180
phase data. In general, an increase in apparent resistivity is associated with phase decrease
181
and vice versa. In Figure 3, the high minimum phases (> 45º) will be related to the
182
electrically conductive zones while the lower minimum phases (< 45º) will relate to the
183
electrically resistive zones. The high minimum phases observed at frequencies above 1 Hz
184
would suggest the presence of conductive sediments in the basin. Interestingly, patches of
185
slightly lower phases are also observed within those high minimum phases. This possibly
186
suggests the presence of resistive bodies (possibly intrusions or thrust wedges) within the
187
conductive sediments. In addition, the occurrence of higher minimum phases for frequencies
9
188
lower than 0.1 Hz in the southern segments of profiles PC03, PC02 and P0 possibly suggests
189
the presence of steep deeply-lying conducting features (fault-zones or partial melt?) in the
190
southern border of the Juruena sub-basin. The major regionally present zones of low
191
minimum phases (~10-20º) represents the resistive basement of the Parecis basin in all
192
profiles. The minimum phase variation enables us to identify the basement topography and
193
possible locations of deep faults within the basement. Thus, as expected, the basement is
194
deeper in the center and shallower at the northern and southern ends of profiles PC02 and P0
195
(Figure 3). Given the identified presence of both 1D and 3D features in the various MT
196
profiles, a 3D inversion of the field data is considered more appropriate than 2D inversion.
197
First, we need to test the appropriate data types and initial resistivity models for the 3D
198
inversion.
199 200
3 Three-Dimensional MT Inversion and Ground-truthing of Results
201
3.1 Testing prior Models and Data types
202
We first performed 3D inversion of the MT profiles using ModEM code [Kelbert et al., 2014]
203
focusing on investigating the effects of prior models and data types on the inversion results.
204
We did not incorporate the available a priori information from the 2 hydrocarbon exploration
205
wells in any of our 3D inversions in order to assess the actual predictive capability of 3D MT
206
inversion of real data. We inverted the full impedance and tipper data using different initial
207
half-space models (10, 50, 100, 200 and 500 Ωm). The considered error floor both for
208
impedance and tipper was 5% of absolute values for all impedance and tipper components.
209
First, we tested 3D inversion runs for MT lines PC02 and PC09+10 with different initial
210
uniform half space models. Following Meqbel et al. [2014], fine mesh discretization at
211
shallow depths in 3D inversion was adopted to allow compensating possible effects of static
212
shifts in the MT data. The model domain was discretized into 190×100×45 cells with
213
horizontal cell size of 1 km × 1 km for profile PC07. The first cell thickness was considered 10
214
about 20 m and the cell thickness increases by a factor of 1.2 down to a depth of about 360
215
km. The first cell or layer thickness was chosen to be one-tenth of the electromagnetic skin
216
depth for the first frequency (~ 5 kHz). Ten padding cells increasing in size outwards with a
217
factor of 1.2, were included in both horizontal directions to reduce inversion artifacts within
218
the model domain. We increased the horizontal cell size from 1 to 2 km for profile PC02
219
considering its wider station spacing, which is twice that of the other profiles. However, other
220
model discretization parameters were kept similar to PC07, except the number of horizontal
221
cells for each profile, included for 3D inversion, which depends upon station spacing and
222
profile length. The discretized horizontal cell domains for profiles PC02, PC03 and PC09+10
223
are 160×145, 140×120 and 190×105 respectively. The regularization parameter, λ, was
224
assigned an initial value of ~ 10 and successively reduced by a factor 10, down to a value of
225
10-8. Smoothing was chosen 0.2 for x, y and z directions. A large λ value provides smooth
226
structures at initial iterations and its subsequent size reduction allows the reconstruction of
227
rougher (and perhaps more geologically realistic) structures in later iterations. The results of
228
the various inversion runs with different starting half-space models are somewhat similar as
229
shown in Figure 4. The normalized RMS misfit values were 1.66, 1.62, 1.57, 1.41 and 1.51
230
for the 10, 50, 100, 200 and 500 Ωm initial models, respectively. A half-space model of 100
231
Ωm was chosen as the preferred initial model for all subsequent inversion runs, despite
232
yielding a normalized RMS higher than other initial models – this choice was based on both
233
its comparatively clearer geologic features and better fit to well A log data.
234
11
235 236
Figure 4. Testing the effect of prior models for MT line PC02. The inversion models were
237
reconstructed from initial half-space models of resistivity: (a) 10 Ωm, (b) 50 Ωm, (c) 100
238
Ωm, (d) 200 Ωm and (e) 500 Ωm. The normalized RMS misfit value for each inversion run is
239
stated for each cross-section.
240
12
241 242
Figure 5. Testing the effect of MT data types on the inversion result for line PC02. (a) Tipper
243
T, (b) full impedance Z, and (c) combined Z and T inversions.
244 245
To test the effect of various MT data types, we inverted MT data types such as tipper (T), full
246
impedance (Z) and combination of both full impedance and tipper (Z+T) for line PC02
247
(Figure 5). This procedure helped us to identify the most robust model features common to
248
all the data types. Tipper data inversion (Figure 5a) imaged the major conductive features but
249
may not have depth resolution. Note the sheet-like conductor at the central portion of the 13
250
profile and the steep conductors at the southern end of the profile. The full impedance
251
inversion (Figure 5b) resolved the basin geometry, which is deepest at the center and
252
shallows up at both northern and southern basin edges. Deep conductors were found at mid
253
crustal depths at the southern end of profiles PC03, PC02 and P0. In the case of Z+T
254
inversion (Figure 5c), we found that most of the model features are generally similar to those
255
for Z inversion but with some loss in resolution details. We finally selected Z+T joint
256
inversion as the approach to use in this study and then applied it to all the other lines. The
257
results are summarized in Figure 6; interestingly, we found that steep conductors are not only
258
present in the southern basin margin but also at the northern end of lines PC02 and P0. The fit
259
between the field data and the computed model responses for all the lines is good (normalized
260
RMS of 1.8 to 2.1) and can be seen in the pseudosections presented in Figure 7. Notice that
261
the observed and predicted MT apparent resistivity and phase responses are generally well
262
matched for both Zxy and Zyx components. Further examples of the fit are shown in Figure 8
263
for eight representative stations picked from the five MT lines. These individual line plots
264
suggest that a satisfactory fit was achieved.
265
14
266 267
268 269
Figure 6. Deep electrical structure across Parecis basin from 3D imaging of individual MT
270
lines. (a) Upper crustal resistivity structure. (b) Crust and upper mantle resistivity structure.
271
15
272
273 274 275
Figure 7. Pseudosections of the observed apparent resistivity and phase data for the five MT
276
lines and those predicted by the joint Z+T 3D inversion models. (a) XY data. (b) YX data.
277 278 279
16
280
281 282
Figure 8. Observed versus predicted MT apparent resistivity and phase responses for eight
283
representative MT stations for the 3D inversion models. Individual points with error bars are
284
the observed xy (red) and yx (blue) components of the impedance tensor at each station. The
285
corresponding predicted responses for xy and yx data components are shown as solid curves.
286
Inset map shows the locations of the representative stations for each survey line.
287 288
3.2 Evaluation of MT inversion Result at Deep Well Sites
289
To test MT model reliability, first we compared the resistivity logs of wells with the depth
290
wise resistivity variation from 3D inversion. We also overlay the seismic lines coinciding
17
291
with the MT profiles to verify the main structures of the basin such as faults, basement and
292
sedimentary formations imaged by the 3D inversion model. We utilized two Parecis basin
293
wells [Dardenne et al., 2006; Vasconcelos et al., 2014], namely A (2-FI-1-MT) and B (2-SM-
294
1-MT) from ANP, in the southern ends of profiles PC02, and PC09+10 respectively (Figure
295
1).
296
The composite litholog from well A shows Parecis Group ~ 600 m, Guape gabbro intrusives
297
~ 670 – 1300 m, Parecis Group ~1300 –1800 m, and phyllitic basement rocks starting at ~
298
1800 m. The composite litholog from well B shows Utiariti Formation ~ 10 – 250 m and the
299
Salto das Neves Formation ~ 250 – 520 m of Parecis Group of rocks; Diamantino Formation
300
~ 520 – 1450 m, Sepotuba Formation ~ 1450 – 2530 m, Raizama Formation ~2530 – 3780 m,
301
and Serra Azul Formation ~ 3780 – 3970 m of Alto Paraguai Group; Nobres Formation ~
302
3970 - 4280 m, Guia Formation ~ 4280 – 4570 m and Mirassol D’Oeste Formation ~ 4570 –
303
4800 m of Araras Group; Puga Formation ~ 4800 – 4920 m and Bauxi Formation ~ 4920 –
304
5150 m of Jangada Group; Carbonato Salto Magessi Formation ~ 5150 – 5670 m, and Cuiaba
305
Group probed at ~ 5670 m downwards. All these mentioned ranges correspond to well depths
306
of formation tops and bases.
307
Thus, well A mostly sampled resistive igneous bodies (intrusives or thrust wedges?) while
308
well B has sedimentary formations down to 5700 m depth. We extracted the resistivity
309
variation with depth from 3D MT resistivity model at the location of the Wells A and B.
310
These were then plotted alongside the gamma-ray and resistivity logs of both wells with the
311
major formation boundaries from the composite lithology log (Figure 9). We found that the
312
resistivity-depth profiles from our 3D MT inversion models satisfactorily matched the
313
average values of the resistivity well logs in the top 2200 m (Figure 9). At the shallowest
314
levels in both wells, there is an electrically conductive formation (C1) of high gamma-ray
315
response which is correctly predicted by the MT models and correlates with Parecis Group in
316
the composite lithological log. Thus, there is consistency in our 3D inversion results with the 18
317
independently measured well log data at least down to the explored depths at these well
318
locations. Well A (southwest of Juruena sub-basin), drilled down to a depth of 2.4 km,
319
penetrated metamorphic phyllitic basement rocks from a depth of about 1.8 km (Figure 9).
320
These metasedimentary rocks are only moderately radiogenic and electrically resistive
321
compared to the overlying Parecis Group (possibly Utiariti Fm) which is of low resistivity.
322
Since the Utiariti Fm of Parecis Group has a distinct electrical signature, it could thus be used
323
as a marker horizon (C1) for interpreting or correlating our MT models with seismic images
324
at shallow levels (<2200 m) in this basin. The gabbroic body sampled at ~ 1000 m depth in
325
Well A could be a thrust wedge or a sill and is detected by MT imaging (Figure 9).
326
327 328 329
Figure 9. Result of 3D MT inversion versus actual gamma-ray and resistivity well logs at the
330
locations of wells A and B. Interpreted lithologies from the composite logs of both wells are
331
also shown for comparison. C1, C2 and C3 are target conductors in the sedimentary section.
332
(see Figure 1 for site location). Stratigraphy from Vasconcelos et al. [2014].
333 19
334 335
Well B, drilled down to a depth of 5.8 km, did not penetrate crystalline basement. This well
336
penetrated a thick column of sedimentary formations. The variation of resistivity with depth
337
have very similar trends in the 3D inversion model and well logs. The Utiariti Fm of Parecis
338
Group has characteristic low resistivity and high gamma-ray values and can be confidently
339
interpreted as marker horizon C1 (Figure 9). Another conductive marker (C2) is evident
340
within the Alto Paraguai Group in the well log and the MT model at about 2000 depth. Below
341
about 2200 m depth, whilst the MT resistivity trend correctly mimics the average well log
342
resistivity pattern, the individual formations cannot be discriminated in the MT model. This
343
means that we cannot use the present result to distinguish the Nobres Fm and Guia Fm of
344
Araras Group and Puga Fm and Bauxi Fm reservoirs of Jangada Group and the intervening
345
conductive Mirassol D’Oeste Fm of Araras Group or map the basal conductive Cuiaba Group
346
occurring at the bottom of the drilled section (C3).
347 348
The major technical limitations faced here were associated with grid-size optimization for the
349
selected combinations of data type for such a large-scale practical inverse problem on a small
350
computational platform and the natural loss of MT resolution with depth. For the size of
351
regional problem considered here, the computational grid-size limitation and the reduction in
352
the number of inverted MT stations from 455 to 199, coupled with the diffusive nature of MT
353
signals, affected stratigraphic resolution to a large extent at depths greater than 2 km. Note
354
that we did not incorporate any a priori information from Wells A and B in our MT
355
inversions. It is possible that incorporating resistivity-versus-depth constraints from the well
356
logs could improve the detection and resolution of unit C3.
357
Due to the diffusive nature of the MT signals and skin depth control, it is noteworthy to
358
mention that the MT inversions reflect the average subsurface resistivity over an increasing
359
hemisphere dropping resolution laterally and with depth whereas the well log data provides 20
360
localized resistivity values with depth. Nonetheless, we consider the MT results to be
361
satisfactory at these well locations.
362 363
4 Discussion
364
The main anomalous features found in the resistivity cross-sections obtained from 3-D
365
inversion of the individual MT lines (Figures 6 and 10) are: i) intrasedimentary conductive
366
features marked by stratiform conductors C1, C2 and C3, and ii) steep conductive features
367
(C4) in the basement and sub-horizontal conductors (C5) at 10-30 km depth. For the upper 10
368
km of the crust, the question may be asked if it was justifiable to invert individual MT lines
369
in 3D rather than 2D? We consider the expensive 3D inversion to be necessary in this fold-
370
thrust and potentially magmatic region. It is instructive to compare the 2D inversion result of
371
Santos and Flexor [2012] and our new result of 3D inversion of the same datasets since the
372
former model has been used by ANP for promoting petroleum exploration licensing for this
373
basin. This is shown in Figure 10. The resistivity structures are very different in both models
374
especially over the fold belts at the southern and northern basin borders. The 3D structures
375
depicted in our dimensional analysis for intermediate to lower frequencies cause the observed
376
disagreement between both inversions as 2D inversion of 3D data can usually create artifacts.
377
Our 3D models have been shown to be consistent with well log data (Figure 9) and are
378
preferred for geological interpretation here. The previously suggested lateral extents of the
379
basement structural domains [Bahia et al., 2007] are also indicated at the top of the MT
380
sections in Figure 10 for comparison. The 3D inversion result shows consistent lateral
381
changes as those derived earlier from potential field studies for the Brassnorte high, Pimenta
382
Bueno graben, and Rio Branco high [Bahia et al., 2007] and, in addition, refine the lateral
383
and depth boundaries of the tectonic domains. However, in the northern parts of lines P0 and
384
PC02 (Figure 6 and 10), it is clear that the up to 9 km depth to basement implied by Braga
385
and Siqueira [1996] is not tenable, as also found by Barros and Assumpção [2011]. 21
386
387 388
Figure 10. Comparison of 2D and 3D inversion models for line P0 for the top 8 km of the
389
crust. Dotted horizontal white line at 4 km depth is for reference. C1 and C2 are stratiform
390
conductive units in the sedimentary basin. C4 is a steep conductive zone extending deep into
391
the crust.
392 393
Loureiro et al. [2017] propose a new structural interpretation for the Parecis basin based on
394
the integration of 2D seismic and gravity data constrained by data from wells A and B
395
(Figures 11 and 12). They distinguished the economic (~ 6 km) and crystalline (~ 10 km)
396
basement, supported by the gravity models constrained with seismic horizons tied to the
397
available wells. From top to bottom, the marked horizons are base of the Mesozoic group, top
398
of Araras group, siliciclastic sequences, carbonate sequences, economic and metamorphic
399
basement rocks (Figures 11 and 12). The economic basement consists of phyllitic rocks (~
400
2.52 g/cm3) which are less dense than the crystalline basement, made up of mainly igneous
401
rocks (~2.67 g/cm3). Our MT sections along the same seismic-gravity lines are also shown in
402
Figures 11 and 12 for comparison. For basin-wide or lateral correlation purpose, we
403
identified three regional marker units in the sedimentary section labelled C1, C2 and C3. We
404
also identified steep conductive zones in the crystalline basement labelled C4. Our resistivity 22
405
models for the two MT lines crossing the ANP deep exploration wells and the available
406
resistivity well logs are also superimposed on the seismic-gravity models of Loureiro et al.
407
[2017] for comparison in Figures 11 and 12.
408 409
Figure 11. Comparison of MT model and independent integrated seismic-gravity model for
410
line PC02. Top graphic is the resistivity cross-section from unconstrained joint 3D inversion
411
of Z and T data. Bottom graphic superposes the MT Z+T inversion model, the interpreted
412
seismic-gravity model [Loureiro et al., 2017] and geologic formations.
413
23
414
In Figure 11, our feature C1 is in good agreement with Well A data (Fig. 9) and is also
415
conformable with the shallowest boundary interpreted by Loureiro et al. [2017] although the
416
seismic data are of poor quality. No other significant conductor was found in the Well A but
417
our MT model suggests lateral continuity of C1 across the entire line, consistent with the
418
shallowest boundary interpreted by Loureiro et al. [2017]. The localized deepening and the
419
presence of our feature C3 at profile distance 175-255 km agrees with the thick sequence
420
occurring between the top basement and the shallowest boundary of Loureiro et al. [2017].
421
The vertical conductive zone C4 imaged at profile distance 45 – 60 km on this line is also in
422
excellent agreement with a major fault interpreted from seismic data by Loureiro et al. [2017]
423
in the basement. In Figure 12, our marker unit C1 in the sedimentary section is in excellent
424
agreement with Well B data and is also conformable with the shallowest well-based seismic-
425
gravity boundary interpreted by Loureiro et al. [2017]. Our unit C2 is also in agreement with
426
Well B data. Its lateral continuity up-dip from the well location (where it lies at ~2 km depth)
427
is remarkably consistent with the seismic data. We can only infer the presence of the
428
conductive unit C3 down to 4-5 km towards the location of Well B but cannot resolve it in
429
our unconstrained 3D MT models. It is possible that the resolution of our MT models can be
430
improved further by incorporating well-based constraints as in the gravity-seismic work of
431
Loureiro et al. [2017] but we consider our present 3D MT models adequate for gross
432
geological interpretation.
433 434
24
435 436 437
Figure 12. Comparison of MT model and independent seismic-gravity model for line
438
PC09+10. Top graphic is the resistivity cross-section model from unconstrained 3D joint Z
439
and T inversion. Bottom graphic superposes the MT Z+T model, the interpreted seismic-
440
gravity model [Loureiro et al., 2017] and geological formations. C1, C2 and C3 are layered
441
conductors in the sedimentary section as observed in Well B shown in Figure 9.
442 443 25
444
What is the geological significance of the deep conductivity anomalies? The deep crustal and
445
upper mantle resistivity structure across the region (Figure 6) is crosscut by major subvertical
446
crustal conductive features (marked C4 and C5). These conductors are restricted to the basin
447
margins characterized by Neoproterozoic mobile (fold-thrust) belts. They are deepest at the
448
SW end of line PC02 (20-30 km depth), lie at 10-20 km depth on PC03 to the west, and occur
449
at 10-15 km depth on line P0 to the east. It is also interesting that Cretaceous and Tertiary
450
kimberlites are found in the northern and southern margins of the basin [Schobbenhaus,
451
1981]. So, the steep conductors C4 and C5 (Figure 6) are likely reactivated shear-zones in the
452
crystalline crust beneath the basin that probably served for magma transport during the
453
Cretaceous and Tertiary periods. Apparently, this region is presently undergoing compression
454
due to the convergence between the South American plate and Nazca plates [Assumpção and
455
Sacek, 2013].
456 457
5 Conclusion
458
Broadband (0.001 – 5000 Hz) magnetotelluric (MT) data for 455 stations, recorded along
459
four lines in the Juruena sub-basin and one regional line cutting across the Parecis basin into
460
the outcropping Precambrian basement at its northern and southern borders, have been
461
inverted to image the sediments and the deep crustal structure beneath the basin. We
462
performed 3D inversion for each line separately using the full datasets. We found that the
463
reconstructed resistivity-versus-depth profiles from our 3D MT inversion models
464
satisfactorily match the average values of the resistivity well logs in the top 2200 m at two
465
well locations chosen for model validation. The Parecis Formation of low resistivity and
466
shallow depth can be traced across the region in the MT models including the intruded or
467
thrusted gabbroic body which is of high resistivity. We found that the sedimentary formations
468
deeper than 2200 m could not be individually resolved by the coarsely discretized MT depth
26
469
models although the total sedimentary cover thickness may be approximated from the 3D
470
imaging results. An areally restricted conductive sequence (dubbed C3 here) occurs at about
471
2000 - 5000 m depth in the central part of Juruena sub-basin and is interpreted as possible
472
synrift deposits. The most significant features of the deep basement structure at mid-lower
473
crustal depths are the NW-SE trending steep conductive zones restricted to the mobile (fold-
474
thrust) belts at basin margins. They are deepest at the SW end of line PC02 (20-30 km depth),
475
lie at 10-20 km depth on line PC03 to the west, and occur at 10-15 km depth on line P0 to the
476
east. We interpret these conductors as possibly ophiolites broken by thrust-faults or shear-
477
zones in the resistive (crystalline) crust beneath the basin currently known to be undergoing
478
compression and we suggest that they are possibly associated with the Upper Cretaceous and
479
Tertiary alkaline magmatism in the region.
480 481
Acknowledgments
482
The new MT data used in this study were acquired by Schlumberger for the Brazilian Agency
483
of Oil, Natural Gas and Biofuels (ANP). The authors thank ANP for supporting this study
484
(TC ANP-ON 01/2013) and providing the MT and well log data for our investigation. The
485
MT data are available free of charge from ANP database (BDEP) on request to
486
[email protected]. We thank the editor Franck Audemard, Naser Meqbel and an
487
anonymous reviewer for comments and suggestions that improved the manuscript.
488 489
References
490 491
27
492 493
Assumpção, M., and V. Sacek (2013), Intra-plate seismicity and flexural stresses in central Brazil, Geophys. Res. Lett., 40, 487–491, doi:10.1002/grl.50142.
494 495
Bahia, R. B. C., M. A. M. Neto, M.S.C. Barbosa, and A. J. Pedreira (2007), Análise da
496
evolução tectonossedimentar da bacia dos Parecis através de métodos potenciais, Brazilian
497
Journal of Geology, 37, 639–649. In Portuguese.
498 499 500
Barros, L.V. Assumpção, M. Quinteros, R. and Caixeta, D., (2009), The intraplate Porto dos Gaúchos seismic zone in the Amazon craton – Brazil, Tectonophysics, 469, 37-47.
501 502
Barros, L.V. and Assumpção M. (2011), Basement depths in the Parecis Basin (Amazon)
503
with receiver functions from small local earthquakes in the Porto dos Gaúchos seismic
504
zone, Journal of South American Earth Sciences, 32, 142-151.
505 506 507
BDEP (2008), Banco de dados de exploração e produção: CD-003295, Pedido: 4015; Br: 4966, Convênio ANP/CPRM, BDEP – ANP/SDT.
508 509
Braga L.F.S, and Siqueira L.P. (1996), Three-Dimensional Gravity Modelling of the
510
basement topography beneath Parecis Basin, Brazil, constrained by DBNM, spectral
511
estimates of depth to magnetic sources, In: Proceedings of the 5th Latin American
512
Petroleum Congress, Rio de Janeiro, Brazil, 1996, 1–5.
513 514 515
Caldwell, T.G., Bibby, H.M. and Brown, C. (2004), The magnetotelluric phase tensor. Geophysical Journal International, 158: 457–469.
516
28
517
Dardenne, M.A., Alvarenga, C.J., Oliveira, C.G. and Lenharo, S.L.R. (2006), Geologia e
518
Metalogenia do Depósito de Cobre do Graben Colorado na Fossa Tectônica de Rondônia,
519
In: Marini, O.J., Queiroz, E.T., Ramos, B.W. (Eds.), Caracterização dos Depósitos
520
Minerais em Distritos Mineiros da Amazônia, pp. 553-596. In Portuguese.
521 522 523
Egbert, G. D. (1997), Robust multiple-station magnetotelluric data processing, Geophys. Journal. International, 130, 475–496.
524 525
Fontes, S. L., Maurya, V. P. and La Terra, E. F. (2016), Magnetotelluric Evidence of Deep
526
Mesozoic-Paleozoic Sediments beneath Parecis Basin, West Central Brazil, American
527
Geophysical Union, Fall General Assembly, abstract #GP51A-1359.
528 529
Hill, Graham J., T. Grant Caldwell, Wiebke Heise, Darren G. Chertkoff, Hugh M. Bibby,
530
Matt K. Burgess, James P. Cull and Ray A. F. Cas (2009), Distribution of melt beneath
531
Mount St Helens and Mount Adams inferred from magnetotelluric data, Nature
532
Geoscience, 2, 785–789.
533 534 535
Kelbert, A., Meqbel, N., Egbert, G.D. and Tandon, K. (2014), ModEM: a modular system for inversion of electromagnetic geophysical data, Computers & Geosciences, 66, 40–53.
536 537
Loureiro, E.M.L, Menezes, P.T.L, Zalán, P.V., and Heilbron, M. (2017), Tectonic
538
Framework of Parecis Basin: a Seismic-Gravity Integrated Interpretation, 15th
539
International Congress of the Brazilian Geophysical Society, Rio de Janeiro, Brazil, 31
540
July to 3 August 2017.
541
29
542
La Terra, E.M.F, Vital, L., Santos, I., Miquelutti, L.G. and Fontes, S.L. (2016), Mapping of
543
basement relief and volcanic intrusions of Parecis basin in Brazil based on 3D
544
magnetotelluric imaging and potential field data. International Conference and Exhibition,
545
Barcelona, Spain, 3-6 April 2016: pp. 172-172. https://doi.org/10.1190/ice2016-6529755.1
546 547
Meqbel, N.M., Egbert, G.D., Wannamaker, P.E., Kelbert, A., Schultz, A. (2014), Deep
548
electrical structure of the northwestern U.S. derived from 3-D inversion of USarray
549
magnetotelluric data. Earth and Planetary Science Letters 402, 290-304.
550 551
Santos, H.S. & Flexor, J.M. (2012), Geoelectric directionality of a Magnetotelluric (MT)
552
survey in the Parecis Basin, Brazil, Revista Brasileira de Geofísica, 30(1): 81-92, ISSN
553
0102-261X.
554 555
Schobbenhaus, C. (1981), Mapa Geológico do Brasil e da Área Oceânica Adjacente
556
Incluindo Depósitos Minerais: Escala 1:2 500 000. Geologic Map of Brazil and Adjoining
557
Ocean Floor Including Mineral Deposits: Scale 1:2,500,000, Brasília: República
558
Federativa do Brasil, Ministério das Minas e Energia, Departamento Nacional de
559
Produção Mineral.
560 561 562
Simpson, F. and Bahr, K., 2005. Practical Magnetotellurics, Cambridge University Press, 254pp.
563 564
Siqueira, L.P. (1989), Bacia dos Parecis. Boletim Geociências PETROBRAS 3 (12), 3-16.
565 566
30
567
Vasconcelos, C.S., Morales, I.V.F., Trosdorf Jr., I., Santos, S.F., Figueiredo, M.F., 2014.
568
Revisão da estratigrafia na seção perfurada pelo poço 2-SM-1-MT (Salto Magessi), Bacia
569
dos Parecis- Alto Xingu, MT. Boletim Geociências Petrobras, Rio de Janeiro, v. 22, n. 1,
570
p. 171-178. In Portuguese.
571
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Ref: SAMES_2019_97 Title: Deep structure of Parecis Basin, Brazil from 3D magnetotelluric imaging Journal: Journal of South American Earth Sciences
Highlights
3-D resistivity structure of Parecis basin using 455 magnetotelluric soundings was imaged at depths down to 12 km; Low resistivity shallow Parecis formation was traced across the basin; Resistive upper crust is thinned over the Neoproterozoic fold belts; Deep crustal steep conductors possibly associated to alkaline magmatism in the region.