Are some of the deep crustal conductive features found in SW Iberia caused by graphite?

Santos, Fernando A. Monteiro; Mateus, António; Almeida, Eugénio P.; Pous, J.; Mendes-Victor, L.

doi:10.1016/s0012-821x(02)00721-5

Cited by 45 publications

(27 citation statements)

References 82 publications

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“…If this conductor appeared more elongated, then fluids along a fault zone would be a plausible explanation, but the conductor beneath site 9 and 10 does not have this usual shape. Another possibility is the presence of a basement including graphite‐bearing rocks, as occurs in the SW Iberia [ Monteiro‐Santos et al , 2002].…”

Section: Discussioncontrasting

confidence: 57%

Crustal resistivity structure of the southwestern transect of the Rif Cordillera (Morocco)

Anahnah¹,

Galindo-Zaldı́var²,

Chaloüan³

et al. 2011

Geochem. Geophys. Geosyst.

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[1] A NE-SW magnetotelluric 110 km-long profile including 18 sites was acquired across the western Rif Cordillera along the Eurasian-African plate boundary, allowing to constrain its poorly known deep structure. It extends from the Internal Zones, close to the Alboran coast, crossing the External Zones and up to the Gharb foreland basin. The periods recorded range from 0.001 s to 1000 s. The combination of magnetotelluric data with available geological data provides new insight regarding the relationship Copyright 2011 by the American Geophysical Union 1 of 14 between deep and shallow crustal structures of the Rif Cordillera. Analyses of structural dimensionality suggest a preferential NW-SE direction, and a 2D joint inversion was performed. A 3D inversion extending the 2D model along the strike confirmed the reliability of this approach. The magnetotelluric model shows a heterogeneous upper crust in agreement with the geological structures observed at surface. The Internal Zones correspond to resistive (metamorphic rocks) and conductive (peridotites) bodies, while the External Zones and the foreland basin are characterized by large conductive bodies of variable thickness. A crustal detachment level separating shallow geological units from a probable variscan basement was inferred. At depth, the most relevant feature consists of large resistive bodies with a shallow irregular top, located below the frontal part of the Rif. The outcrops of exotic metapelitic, granitic and gneissic blocks in the frontal part of the Cordillera suggest that these large resistive bodies may correspond to a gneissic or granitic basement surrounded by metapelitic rocks. Late contractive thrust and diapiric processes were responsible for their uplift and shallow emplacement. The Rif constitutes an active southwestward vergent orogenic wedge, oblique to the present-day NW-SE convergent Eurasian-African plate boundary.

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Section: Discussioncontrasting

confidence: 57%

Crustal resistivity structure of the southwestern transect of the Rif Cordillera (Morocco)

Anahnah¹,

Galindo-Zaldı́var²,

Chaloüan³

et al. 2011

Geochem. Geophys. Geosyst.

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“…Dense wide‐angle seismic data, partially superposed to the IBERSEIS profile, show relatively high seismic velocities from 4 s to the Moho depth, thus providing evidence of a prevailing mafic composition (mafic granulites with mafic intrusions) for the middle and lower crust in the SPZ, OMZ and southern CIZ (from the Central Unit to the Alcudia Anticline) [ Palomeras et al , 2009]. The moderate conductive middle to lower crust should then be attributed to a mafic granulite layer, perhaps bearing interconnected graphite films as proposed by Monteiro Santos et al [2002] and Muñoz et al [2008] for OMZ. The earlier data are in accordance with the conductive middle to lower crustal layer (C1) imaged by MT (Figure 3c).…”

Section: Discussion Of the Mt Image In The Frame Of Geological And Gementioning

confidence: 93%

“…Furthermore, the upper section of C1 can be interpreted as corresponding to the lower part of the Serie Negra Group (SN, Figures 3c and Figure 12), presumably recrystallized under higher metamorphic conditions. Note that the Serie Negra Group has proved to be highly conductive because of the presence of interconnected graphite [ Pous et al , 2004; Monteiro Santos et al , 2002]. …”

Section: Discussion Of the Mt Image In The Frame Of Geological And Gementioning

confidence: 99%

“…Magnetotellurics has also proved to be an efficient technique for imaging old structures affecting rocks that bear (interconnected) conductive mineral phases (e.g., graphite, sulphides, iron oxides). In SW Iberia, five magnetotelluric (MT) profiles provided evidence of the orogenic sutures at depth, which are characterized by high conductivities [ Monteiro Santos et al , 1999, 2002; Almeida et al , 2001, 2005; Pous et al , 2004; Muñoz et al , 2005; Vieira da Silva et al , 2007]. A midcrustal conductive layer along the whole OMZ was revealed, suggesting a major middle crust décollement between a granulitic basement and the overlying sequence, mostly composed of metasedimentary and metavolcanic rocks [ Muñoz et al , 2008].…”

Section: Introductionmentioning

confidence: 99%

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Constraints on the crustal structure of the internal Variscan Belt in SW Europe: A magnetotelluric transect along the eastern part of Central Iberian Zone, Iberian Massif

Pous¹,

Poyatos²,

Heise³

et al. 2011

J. Geophys. Res.

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[1] The Iberian Massif is the best exposed segment of the European Variscan Belt. It includes relatively well preserved terranes that were accreted by transpression along time and resulted in a number of geotectonic units that formed part of the Late Paleozoic assembly of the Pangaea Supercontinent. In SW Iberia, these units are the Central Iberian Zone (CIZ), Ossa Morena Zone (OMZ), and the South Portuguese Zone (SPZ). A 210 km long NE-SW magnetotelluric profile was carried out through the CIZ, from the OMZ-CIZ boundary toward the north, reaching the Tagus (Cenozoic) basin. Data dimensionality analysis resulted in a suitable 2-D electrical resistivity structure, allowing a 2-D inversion of the data set. Complementary available geophysical data (deep seismic, gravity and aeromagnetic) and a comparison with a detailed geological cross section led us to constrain the interpretation of the 2-D electrical resistivity structure of the CIZ crust. The results show, for the upper crust, the existence of diverse conductive/resistive bodies that correlate well with known geological features (sedimentary basins, faults, granitic plutons, mineralized systems). A mild but steady conductive band is located along the middle and lower crust that is interpreted as a mafic granulite basement. The upper section of this band connects with several elongated shallow conductors, providing further evidence for the existence, in the Central Iberian Zone, of a complex décollement system where the major faults are rooted. Such a crustal architecture is viewed as the northward continuation of the Variscan large-scale structures previously recognized in the southern sectors (OMZ and SPZ).

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“…Our interpretation takes into consideration new and previous data to show subduction of a continental margin and continental collision, top to south shearing contemporaneous with subduction, postorogenic collapse and exhumation, and to set an upper age to the Variscan eclogite facies. The critical data that serve as ground for our hypothesis are (Figure 12) (1) subduction toward the north (so far undisputed), (2) structural features and reflectors dipping to north, as shown by surface geology and geophysical soundings [ Monteiro Santos et al , 1999; Almeida et al , 2001; Monteiro Santos et al , 2002; Simancas et al , 2003; Pous et al , 2004], (3) subduction of continental margin sediments with embedded mafic rocks (carbonates with blueschists inside), (4) opposite senses of shear, of different age, associated with the structures developed either during early HP prograde tectonometamorphic events (top to south shearing, TM n ) or during later retrogression (top to north shearing, TM n+2 ), respectively (see Figure 2b), (5) present occurrence of HP rocks at the surface, and (6) along‐strike variations in metamorphic record. We propose a simpler way of explaining the exhumation of the HP metamorphic rocks, and of unifying observations along the western and eastern segments of the SW Iberian Variscan suture, and (7) extension in the neighboring South Portuguese Terrane with formation of a wide and deep basin between circa 365 and 310 Ma, filled with sediments eroded from the OMZ, and with volcanics.…”

Section: Geodynamic Evolution Of the Sw Iberian Variscan Orogen: Discmentioning

confidence: 97%

Geodynamic evolution of the SW Variscides: Orogenic collapse shown by new tectonometamorphic and isotopic data from western Ossa‐Morena Zone, SW Iberia

et al. 2008

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International audienceThe pre-Mesozoic geodynamic evolution of SW Iberia has been investigated on the basis of detailed structural analysis, isotope dating, and petrologic study of high-pressure (HP) rocks, revealing the superposition of several tectonometamorphic events: (1) An HP event older than circa 358 Ma is recorded in basic rocks preserved inside marbles, which suggests subduction of a continental margin. The deformation associated with this stage is recorded by a refractory graphite fabric and noncoaxial mesoscopic structures found within the host metasediments. The sense of shear is top to south, revealing thrusting synthetic with subduction (underthrusting) to the north. (2) Recrystallization before circa 358 Ma is due to a regional-scale thermal episode and magmatism. (3) Noncoaxial deformation with top to north sense of shear in northward dipping large-scale shear zones is associated with pervasive hydration and metamorphic retrogression under mostly greenschist facies. This indicates exhumation by normal faulting in a detachment zone confined to the top to north and north dipping shear zones during postorogenic collapse soon after 358 Ma ago (inversion of earlier top to south thrusts). (4) Static recrystallization at circa 318 Ma is due to regional-scale granitic intrusions

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Are some of the deep crustal conductive features found in SW Iberia caused by graphite?

Cited by 45 publications

References 82 publications

Crustal resistivity structure of the southwestern transect of the Rif Cordillera (Morocco)

Crustal resistivity structure of the southwestern transect of the Rif Cordillera (Morocco)

Constraints on the crustal structure of the internal Variscan Belt in SW Europe: A magnetotelluric transect along the eastern part of Central Iberian Zone, Iberian Massif

Geodynamic evolution of the SW Variscides: Orogenic collapse shown by new tectonometamorphic and isotopic data from western Ossa‐Morena Zone, SW Iberia

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