Magnetotelluric studies in detecting an old suture zone and major crustal scale shear zones (Iberia)
Resumen Abstract Índice Conclusiones
Alves Ribeiro, Joana
2019-A
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This PhD thesis presents the application of the Magnetotelluric (MT) method to image the electrical conductivity distribution in two distinct areas in the Variscan belt in Portugal: Pinhel-Trancoso (Beira Interior; NE Portugal) and Grandola-Serpa (Alentejo; SW Portugal) areas. The aim of this investigation is to detect and characterise the main large-scale Variscan structures (D2 extensional shear zones and D3 folds and strike-slip shear zones), as well as the geometry of the Carboniferous plutons and major late faults.
In the Pinhel-Trancoso area, a new magnetotelluric (MT) survey comprising 17 MT soundings throughout a 30 km long N300 W transect in the Iberian autochthons domain of NW Iberia (Central Iberian Zone) is presented. This study provided the frst insight of the electrical conductivity in depth in this region of the Variscan Orogen. The 2-D inversion model reveals the resistivity structure of the continental crust up to 10 km depth. The model shows a wavy structure separating a conductive upper layer underlain by a resistive layer. This large-scale structure is interpreted as two main tectonic blocks of aVariscanD2extensionalshearzone(i.e.,Pinhelshearzone). Theupperlayerrepresents alowergrademetamorphicdomain (LGD) characterisedbylow-grademetamorphicrocks including graphite-rich rocks. The lower layer, a high-grade domain (HGD), consists of high-grade metamorphic rocks that experienced partial melting and are associated with granites (more resistive) emplaced during crustal thinning. A superimposed crustal shortening results on the wavy structure observable in the 2D model, which is responsible for the development of large-scale D3 folds (e.g., Marofa synform). This structure is later defected by a D4 strike-slip shear zone (e.g., Juzbado-Penalva do Castelo shear zone). Subsequent events of the crust structure are marked by the intrusion of Permo-Carboniferous plutons and the propagation of steeply dipping brittle fault zones.
A 3D inversion was performed over the single profle in the Pinhel-Trancoso area. It is visible some degree of similarity between the 2D and 3D inversion, although, several diferences between the two models are evident mainly on the north part of the profle. These diference are probably due to the site coverage which infuenced the inversion model. In this particular case, the 3D model did not contribute to a better understanding of the subsurface.
The Grandola-Serpa area compromises a total of 68 MT sites over fve profles that were acquired during 1997 and 2002. These profles are approximately 16 to 22 km apart, where the site spacing is ? 5 km. In this region it was performed the frst 3D inversion of the SW Iberia. The 3D model identifes the limits of the Ossa-Morena Zone (OMZ), PulodoLoboZone (PLZ) and South PortugueseZone (SPZ). IntheOMZ isrecognised a wavy structure separating a more resistive upper layer (allochthonous units) underlain by a more conductive layer (autochthonous including graphite-rich rocks). The wavy structure is the result of a superimposed crustal shortening D3. To the west of the Messejana fault, this wavy structure has a deeper signature which may indicate a larger volume of plutonic rocks (belonging to the Evora Massif and the Beja Igneous Complex) intruding the autochthonous. The boundary between the OMZ and PLZ at the east of Messejana fault is represented by the WNW-ESE-trending Ferreira-Ficalho thrust which extends parallel to the Beja-Acebuches Ophiolitic Complex. A high conductive anomaly if found in the vicinity of the Sto. Aleixo da Restaura<ao thrust maybe justify as possible mineralisation caused by fuid migration. The NNW-SSE-trending Santa Susana shear zone located at the westernmost part of the study area, cuts the allochthonous units, the autochthonous and the Beja Igneous Complex. Several hidden pluton are also identifed in the model as well as later faults (e.g., Grandola and Vidigueira-Moura faults) that extend in depth and are responsible for the movement of crustal blocks.
Joana Alves Ribeiro – «Magnetotelluric studies in detecting an old suture zone and major crustal scale shear zones (Iberia)»
Índice
Agradecimientos …………………………………………………………………………………………………… 5
Resumen ……………………………………………………………………………………………………………… 9
Capítulo 1. Introducción. Motivación y objetivos. ……………………………………………………. 17
1.1. Motivación ………………………………………………………………………………………………. 19
1.2. Objetivos …………………………………………………………………………………………………. 21
Capítulo 2. El campo geomagnético. Origen y medida ……………………………………………… 23
2.1. Introducción histórica ……………………………………………………………………………….. 25
2.2. Campo geomagnético de origen interno ……………………………………………………… 28
2.3. Campo geomagnético de origen externo …………………………………………………….. 32
2.4. Elementos del campo geomagnético ………………………………………………………….. 36
2.4.1. Datos geomagnéticos ……………………………………………………………………………. 38
2.4.1.1. Datos marinos …………………………………………………………………………………… 38
2.4.1.2. Datos de observatorios y estaciones seculares ……………………………………… 40
2.4.1.3. Datos de satélites. Índices geomagnéticos. …………………………………………… 43
Capítulo 3. Modelización del campo geomagnético ………………………………………………… 47
3.1. Expresión armónica del campo geomagnético en el espacio ………………………….. 49
3.2. Dependencia temporal del campo geomagnético ………………………………………… 53
3.3. Diferencias respecto la media ……………………………………………………………………. 54
3.5. Modelos de campo geomagnético a escala regional …………………………………….. 64
3.5.1. Análisis armónico en un casquete esférico (SCHA). …………………………………… 65
3.5.2. Revisión de la técnica SCHA: R-SCHA ……………………………………………………….. 70
3.6. Inversión y regularización ………………………………………………………………………….. 75
Capítulo 4. Datos empleados ………………………………………………………………………………… 79
4.1. Datos marinos de cruce …………………………………………………………………………….. 82
4.2. Datos de observatorios y estaciones seculares …………………………………………….. 85
4.2.1. Selección de la intensidad total de los datos de observatorio …………………….. 85
4.2.2. Selección de datos vectoriales de observatorio ………………………………………… 87
4.2.3. Selección de la intensidad total de los datos de estaciones seculares………….. 89
4.2.4. Selección de los datos vectoriales de estaciones seculares ………………………… 90
4.3.Datos de satélite ………………………………………………………………………………………. 93
Capítulo 5. Modelo regional NAGSVM-F ………………………………………………………………… 97
5.1. Modelización de la variación secular de la intensidad total del campo
geomagnético a partir de un nuevo conjunto de datos marinos de cruce: modelo
NAGSVM-F …………………………………………………………………………………………………………. 99
Índice
Agradecimientos …………………………………………………………………………………………………… 5
Resumen ……………………………………………………………………………………………………………… 9
Capítulo 1. Introducción. Motivación y objetivos. ……………………………………………………. 17
1.1. Motivación ………………………………………………………………………………………………. 19
1.2. Objetivos …………………………………………………………………………………………………. 21
Capítulo 2. El campo geomagnético. Origen y medida ……………………………………………… 23
2.1. Introducción histórica ……………………………………………………………………………….. 25
2.2. Campo geomagnético de origen interno ……………………………………………………… 28
2.3. Campo geomagnético de origen externo …………………………………………………….. 32
2.4. Elementos del campo geomagnético ………………………………………………………….. 36
2.4.1.
Datos geomagnéticos ……………………………………………………………………………. 38
2.4.1.1. Datos marinos …………………………………………………………………………………… 38
2.4.1.2. Datos de observatorios y estaciones seculares ……………………………………… 40
2.4.1.3. Datos de satélites. Índices geomagnéticos. …………………………………………… 43
Capítulo 3. Modelización del campo geomagnético ………………………………………………… 47
3.1. Expresión armónica del campo geomagnético en el espacio ………………………….. 49
3.2. Dependencia temporal del campo geomagnético ………………………………………… 53
3.3. Diferencias respecto la media ……………………………………………………………………. 54
3.5. Modelos de campo geomagnético a escala regional …………………………………….. 64
3.5.1. Análisis armónico en un casquete esférico (SCHA). …………………………………… 65
3.5.2. Revisión de la técnica SCHA: R-SCHA ……………………………………………………….. 70
3.6.Inversión y regularización ………………………………………………………………………….. 75
Capítulo 4. Datos empleados ………………………………………………………………………………… 79
4.1. Datos marinos de cruce …………………………………………………………………………….. 82
4.2. Datos de observatorios y estaciones seculares …………………………………………….. 85
4.2.1. Selección de la intensidad total de los datos de observatorio …………………….. 85
4.2.2. Selección de datos vectoriales de observatorio ………………………………………… 87
4.2.3. Selección de la intensidad total de los datos de estaciones seculares………….. 89
4.2.4. Selección de los datos vectoriales de estaciones seculares ………………………… 90
4.3.Datos de satélite ………………………………………………………………………………………. 93
Capítulo 5. Modelo regional NAGSVM-F ………………………………………………………………… 97
5.1. Modelización de la variación secular de la intensidad total del campo
geomagnético a partir de un nuevo conjunto de datos marinos de cruce: modelo
NAGSVM-F …………………………………………………………………………………………………………. 99
This thesis applies the MT method to study two distinct areas in the Variscan Orogeny in Portugal: Pinhel-Trancoso (NE of Portugal) and Grandola-Serpa (SW of Portugal) to characterise the electrical conductivity distribution using 2D and 3D inversions.
The evolution of MT inversions algorithms have been signifcant since its frst steps in the late 60’s (Fig. 2.12), a considerable part has to do also with the development of computation. This has encouraged to use more 3D inversions. The 3D inversion has the advantage that is not necessary perform a dimensionality analysis priorly. However, it is recommended in order to have an idea of the inversion outcome. Tietze & Ritter [2013] points out the advantages and disadvantages of the 3D over the 2D inversions. The 3D inversion has clear advantages on a complex surface situation, while the 2D inversion provides more options and control on the resolution of small-scale structures, the handling of the static shift and detailed bathymetry and a fner mesh discretisation. Thus, Tietze & Ritter [2013] suggested that 2-D and 3-D inversion should be complementary.
Although several investigations performed over a single profle have revealed good results (eg.: Beka et al. [2017], Patro & Egbert [2011], Siripunvaraporn et al. [2005]), they will always depend on the quality of the data and on the complexity of the geological structure. The 3D inversion of the single profle in Pinhel-Trancoso has revealed not to be this case. It must also be kept in mind that Profle A even though it was only used the of-diagonal for the 3D inversion, the data still presented noise. This profle also features some gaps between sites, which has been demonstrated that coverage can infuences the inversion model [Meqbel et al., 2016].
Grandola-Serpa area compromises a total of 68 sites over fve profle. These profles are approximately 16 to 22 km apart, where the site spacing is ? 5 km. It was made the option of a tighter mesh grid in order to obtain a more detailed information in the
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vicinity of the MT sites. It has been demonstrated that if a coarse grid is applied this would have a negative consequence on the data misft [Meqbel et al., 2014].
The impedance tensor diagonal of profle A (Trancoso-Pinhel) and profles B, C and D (Grandola-Serpa area) has a signifcant presence of noise thus, this component was not used in the inversions. This may have compromised the visualization of a more detailed subsurface [Patro & Egbert, 2011].
As a fnal consideration, it must be kept in mind that diferent users can provide different models for the same data set (eg.: Bedrosian & Feucht [2014] vs Meqbel et al. [2014]). This has to due with personal choices of the operator since beginning of the data processing. In the inversion methodology, is necessary to make certain options such as: frequencies to use, full impedance or just the of-diagonal, model grid, inversions parameters, and the inversion algorithm. The main structures will be present, however small diferences will be evidenced.
Joana Alves Ribeiro – «Magnetotelluric studies in detecting an old suture zone and major crustal scale shear zones (Iberia)»