Análisis de series temporales en estaciones permanentes GPS

García Cañada, Laura

2016-A
Descargar PDF

Resumen

El extremo occidental de la Cordillera Bética está situado al noroeste del Arco de Gibraltar, un orógeno arqueado formado por la interacción de las placas Euroasiática y Africana y el Dominio de Alborán entre ambas. Constituye una región fundamental para el conocimiento de la evolución alpina del Mediterráneo occidental. Este trabajo de investigación comprende el análisis de la estructura cortical del extremo occidental de la Cordillera Bética y su antepaís así como la actividad actual del frente montañoso en esta región. Además, aporta nuevos datos para la discusión de los modelos de evolución del Arco de Gibraltar propuestos hasta el momento. La integración de datos geológicos, geofísicos y geodésicos ha sido fundamental para conseguir este objetivo, dada la complejidad de la estructura de la zona y la escasez de afloramientos de calidad.

El análisis de anomalías magnéticas a lo largo de varios perfiles que cortan el principal dipolo magnético de la parte suroccidental de la Península Ibérica ha permitido definir la geometría del Complejo Alcalino de Monchique. Este cuerpo aflora al Suroeste de Portugal y se extiende en profundidad hacia la Cordillera Bética. En la parte meridional del cauce del rio Guadiana, se produce un salto brusco del cuerpo anómalo lo que revela la existencia de una falla cortical de orientación N-S, la falla del Guadiana. Datos gravimétricos y geológicos apoyan estos resultados.

Los estudios magnetotelúricos de largo periodo son útiles para determinar la conductividad de la corteza profunda y el manto a través de la cual es posible inferir la estructura de la litosfera y astenosfera. El agua del mar representa el principal conductor en la corteza superficial y su influencia puede enmascarar las estructuras geológicas tridimensionales. En la zona de Gibraltar se ha realizado un estudio a través de modelización forward 3D para definir el efecto en el phase tensor y los tipper vectors de la batimetría y líneas de costa irregulares así como del Estrecho de Gibraltar.

Los datos magnetovariacionales de largo periodo adquiridos en el extremo occidental de la Cordillera Bética, que consideran la influencia del mar en el área, destacan la presencia de un gran cuerpo conductor (0.05 ?·m), Villafranca, en la parte Sur del basamento de la cuenca del Guadalquivir. Las anomalías magnéticas también reflejan la presencia de esta estructura y nos ayudan a definir su naturaleza ígnea, intermedia o básica, con alto contenido de mineralización de sulfuros. El origen de este cuerpo se discute según el contexto geológico regional.

A pesar de que tradicionalmente se ha considerado inactivo el frente occidental de la Cordillera Bética, nuevos datos de GPS reflejan firmemente el movimiento hacia el Oeste con respecto al antepaís. Los valores de desplazamiento máximos se alcanzan en el área del Estrecho de Gibraltar. El carácter rectilíneo del frente montañoso noroccidental junto con el engrosamiento del relleno de la cuenca del Guadalquivir hacia el Sureste confirman la actividad tectónica del extremo occidental de la Cordillera. La deformación en la zona se acomoda principalmente por pliegues que evidencian un acortamiento general de orientación ONO-ESE a O-E al menos desde el Plioceno. Los patrones de desplazamiento concuerdan con la rotación horaria actual de las unidades tectónicas en el extremo occidental de la Cordillera Bética. Todos estos datos soportan la subducción activa junto con slab-rollback como principal modelo de evolución tectónica del Arco de Gibraltar.

Abstract

The westernmost Betic Cordillera is located along the northwestern Gibraltar Arc, which constitutes a curved orogen formed by the interaction of the Eurasian and African plates and the Alborán continental domain in between. This is a key region for understanding the alpine evolution of the western Mediterranean. This Ph.D. Thesis examines the crustal structure in the westernmost Betics and its foreland, also determining the present-day activity of the mountain front in this area. In addition, this research provides new data that contribute to discussion of the main models of the evolution of the Gibraltar Arc proposed to date. Integration of geological, geophysical and geodetic data in the region proved crucial for these purposes, given the complex regional structure and the lack of quality outcrops.

The analysis of magnetic anomalies along several profiles crossing the main magnetic dipole in the southwesternmost Iberian Peninsula helps constrain the geometry of the Monchique Alkaline Complex. This body crops out in southwestern Portugal and extends in depth eastward to the Betic Cordillera. A sharp step in the anomalous body coincides precisely with the southern channel of the Guadiana River, revealing the presence of a deep N-S crustal fault unknown to date, the Guadiana Fault. Gravity and geological observations support these results.

Long period magnetotelluric data are useful to elucidate the conductivity of deep crust and upper mantle, whereby it is possible to infer the lithospheric and asthenosphere structure. Sea water constitutes the main conductive body at shallow crustal levels and its influence can mask 3D geological structures. 3D forward modelling studies have been developed to constrain the bathymetry and irregular coastline effect on phase tensor and tipper vectors in the Gibraltar area and appraise the influence of the narrow Gibraltar Strait.

Long period magnetovariational observations in the westernmost Betic Cordillera, considering the sea influence in the area, highlight the presence of a major conductive body (0.05 ?·m) within the basement of the southern Guadalquivir foreland basin: the Villafranca body. Magnetic anomaly data also evidence this major structure and help constrain its intermediate or basic igneous nature, with high sulphide mineralization. Its origin is discussed in the framework of the regional geological setting.

Although previous research efforts held the western Betic Cordillera frontal area to be inactive, new GPS data suggest a very consistent westward motion with respect to the foreland, reaching maximum displacement values in the Gibraltar Strait area. The rectilinear character of the northwestern mountain front, together with the southeastward increasing thickness of the Guadalquivir basin infill, support the activity of the westernmost Betics. Deformation is mainly accommodated by active folds showing a roughly WNW-ESE to W-E shortening, at least since the Pliocene. The displacement pattern is in agreement with the present-day clockwise rotation of the tectonic units in the westernmost Betic Cordillera. All these data support active lithospheric subduction and slab-rollback as the main model of tectonic evolution of the Gibraltar Arc.

Índice

PART I

Chapter 1. Introduction3

Chapter 2. Aims and Ph.D. Thesis structure7

Chapter 3. Regional settings and background11

3.1 Geological setting12

3.1.1 Iberian Massif12

3.1.2 Betic Cordillera15

3.2.3 Neogene – Quaternary Basins19

3.2 Previous geophysical data20

3.2.1 Gravity data21

3.2.2 Magnetic data22

3.2.3 Seismic Data23

3.2.4 Magnetotelluric data24

3.3 Previous GPS25

3.4 Models of recent tectonic evolution of the Gibraltar Arc27

Chapter 4. Methodology33

4.1 Geological methods33

4.2 Geophysical methods35

4.2.1 Gravity35

4.2.2 Magnetic39

4.2.3 Magnetotelluric41

4.3 Geodetic studies49

PART II

Chapter 5. Magnetic evidence of a crustal fault affecting a linear laccolith: the Guadiana Fault and the Monchique Alkaline Complex (SW Iberian Peninsula)53

5.1 Introduction56

5.2 Geological setting58

5.3 Methodology60

5.4 Magnetic anomaly dipole in SW Iberia62

5.5 Magnetic anomaly models64

5.6 Gravity, geomorphological and fracture evidences of the Guadiana River structure67

5.7 Discussion68

5.8 Conclusions72

Chapter 6. Influence of a narrow strait connecting a large ocean and a small sea on magnetotelluric data: Gibraltar Strait75

6.1 Introduction78

6.2 Methodology80

6.3 The influence of the Gibraltar Strait on phase tensors and induction arrows81

6.4 Discussion85

6.5 Conclusions87

Chapter 7. Long period magnetovariational evidence of a large deep conductive body within the basement of the Guadalquivir foreland basin and tectonic implications (Betic Cordillera, S Spain)89

7.1 Introduction92

7.2 Geological Setting94

7.3 Previous geophysical studies96

7.4 Methodology, data acquisition and processing98

7.4.1 Long Period Magnetovariational observations98

7.4.2 Magnetic observations99

7.5 Induction arrows from Long Period Magnetovariational observations99

7.6 Conductivity models100

7.7 Magnetic anomalies104

7.8 Discussion105

7.9 Conclusions108

Chapter 8. Shallow frontal deformation related to active continental subduction: structure and recent stresses in the westernmost Betic Cordillera111

8.1 Introduction113

8.2 Geological setting115

8.3 Morphology of the westernmost Betic Cordillera mountain front116

8.4 Structure of the mountain front118

8.4.1 Seismic reflection profiles118

8.4.2 Audiomagnetotelluric data119

8.5 Brittle deformation and stresses120

8.6 Discussion122

8.7 Conclusions124

Chapter 9. Active rollback in the Gibraltar Arc: evidences from CGPS data in the Western Betic Cordillera125

9.1 Introduction128

9.2 Geological Setting130

9.3 Methodology133

9.4 Displacements from GPS data in the western Betic Cordillera and its foreland135

9.5 Recent, active and other relevant structures137

9.6 Shortening in the western Betic mountain front and clockwise rotation142

9.7 Discussion142

9.8 Conclusions145

PART III

Chapter 10. General Discussions149

Chapter 11. Conclusions – Conclusiones159

REFERENCES167

Conclusiones

This chapter summarizes the findings compiled in the previous chapters for the purpose to underline the main contribution of this Ph.D. Thesis to the knowledge of the crustal structure and recent tectonic evolution of the westernmost Betic Cordillera and its foreland. Furthermore, the geodynamic model of the Gibraltar Arc supported by this research is highlighted. Integration of geological and geophysical methods is essential to determine the shallow and deep crustal structure mainly in areas with scarce outcrops and complex geological setting as the westernmost Betic Cordillera.

The simultaneous modelling of aeromagnetic and field magnetic data allow accurately constrain the features of the bodies responsible of the magnetic anomalies. The understanding of the magnetic anomalies in the southwestern Iberian Massif made possible to constrain the extension in depth of the Monchique Alkaline Complex with lenticular shape, 50 km wide and its eastward prolongation of more than 150 km. The intrusive body should have a weak remnant magnetization parallel to the induced one. Moreover, combining these data with gravity measurements as well as geomorphological and structural geology observations has been useful to identify the Guadiana fault, interpreted as major N-S oriented blind crustal fault of probable late Variscan age.

Long Period Magnetotelluric data constitute the most suitable tool to determine the conductivity of the deep crustal structure. In that way, MT response functions onshore are strongly influenced by the bathymetry and the coast line geometry in areas surrounded by sea water depending on the distance and the period. In addition, a narrow strait connecting two seas concentrates the electric currents provoking large tipper vectors at sites close to the strait onshore but with negligible effect on phase tensor. Hence, it is essential to take into account the detailed bathymetry and coast line geometry in any MT research performed in areas nearby the Gibraltar Strait.

Long period magnetovariational observations in the westernmost Betic Cordillera supported by magnetic anomaly studies have elucidated the presence of a highly conductive (0.05 ?·m) and magnetic anomalous body, the Villafranca Body. This up to date unknown structure should have sulphure nature and is located at 10 to 30 km depth, probably in an igneous host rock. Villafranca determines also the major magnetic anomaly located in the Variscan basement of the Guadalquivir foreland basin. It may represent a structure related to the Iberian Pyrite Belt, or else a body emplaced along the South Iberian Margin during the Mesozoic Tethys evolution.

The compilation of geological, geophysical and available GPS data supplies consistent arguments to support the present day tectonic activity of the westernmost frontal area of the Betic Cordillera. This fact produces the progressively southeastward increase of the Guadalquivir Basin infill reaching up to Holocene deposits. Recent and active folds accommodate the deformation along the westernmost mountain front and are responsible of its rectilinear character. The presence of soft rocks and the thin-skinned tectonics would have concentrated the deformation along folds at shallow levels over a basal detachment, thus explaining the scarce seismicity in this region. A WNW-ESE to E-W tectonic shortening occurs in the western Betics since the Pliocene, evidenced by a roughly constant rate (2.2-3.25 mm/yr) for Líjar-Trebujena cross-section.

New continuous GPS data evidence the maximum relative westward displacements of western Betic Cordillera with respect to the foreland reaching up to 4.27 mm/yr in the Gibraltar Strait. The rate of displacement decreases along a narrow band towards the northwest mountain front and eastward, to the internal zones of the arcuate orogen. This behaviour supports the variscan foreland as a barrier to the westwards motion of the Betic Cordillera. Such displacement values are compatible with the presence of a region undergoing contraction in the frontal part of the Cordillera, while moderate extension takes place eastwards. In addition, a progressive clockwise rotation during oblique convergence is in agreement with regional GPS results (2.06°/Ma) faster than long-term geological rotations deduced from previous paleomagnetic data (0.916º/Ma). This fact is compatible with progressive rotation of fold axes pattern.

The new geological, geophysical and geodetical data in the Westernmost Betic Cordillera presented in this Ph.D. Thesis support the active subduction and slab rollback as the more confidence model for the recent evolution of the Gibraltar Arc.