Geological and structural controls on La Garrotxa monogenetic Volcanic Field (NE Iberia) and structural controls on La Garrotxa monogenetic Volcanic Field (NE Iberia)

Abstract

Monogenetic volcanism represents the most common volcano type on Earth. The distribution of it depends in each case on their regional and local tectonic controls. The great variety of eruptive styles, edifice morphologies, and deposits shown by monogenetic volcanoes is the result of a complex combination of internal (magma composition, gas content, magma rheology, magma volume, etc.) and external (regional and local stress fields. stratigraphic and rheological contrasts of substrate rock, hydrogeology, etc.) parameters during magma transport from the source region to the surface.

The present PhD Thesis focuses on the geological and structural controls of monogenetic volcanism. It pays particular attention to the uppermost part of the lithosphere and its role to determine the distribution of eruptive vents and eruptive styles. We have selected La Garrotxa Volcanic Field (GVFj as a case study. This Quaternary volcanic field is located in the Northeast of the Iberian Peninsula and includes more than SO well-preserved volcanoes. It covers an area of 600 km2, between the cities of Olot and Girona and belongs to the Catalan Volcanic Zone, one of the alkaline volcanic provinces of the European Rift System. The GVF is still poorly known and has become an ideal place for the application of the multidisciplinary studies, including geophysical methods, geological fieldwork and geomorphological and structural analyses, which constitute this PhD Thesis.

The first work was carried out at the Northern sector of the GVF. This study was mainly based on the application of gravimetry and self-potential techniques, in order to identify the main tectonic structures of the volcanic area at depth. A second work was performed in order to obtain a much better detail of the shallower structures and to relate the subsurface geology to the feeding system of these monogenetic volcanoes. In this case, we applied the electrical resistivity tomography method, comparing the new data with the self-potential results. A third work was carried out at the Southern sector of the GVF, at La Crosa de Sant Dalmai volcano (10 km SW of Girona), one of the biggest maar-diatreme edifices of the Iberian Peninsula. Here, several geophysical techniques were applied, including gravimetry, magnetometry, self-potential and electrical resistivity tomography. A model of the uppermost part of the diatreme was obtained, determining the internal structure and its origin. The results obtained with the application of these geophysical methods in this volcanic field suggest that electrical resistivity tomography is a useful tool for the study the internal structures of different types of monogenetic landforms. In this way we present a short work that illustrates different examples of internal structures of monogenetic volcanic cones from GVF. Furthermore, combining the results from these geophysical studies with the geological informations obtained by fieldwork, we performed the first volcanic stratigraphy map of the GVF. Finally, we also led a detailed volcano-structural analysis of the whole volcanic field, including geostatistical distribution of faults, fissures and vents, morpho structural lineaments identified by remote sensing, a morphometrical analysis of the volcanic cones and craters, location of regional seismic events recorded in the area (since 1978), and mantle derived gases in springs and water wells, as a guide to identify active faults and open fractures and to define the structural controls of this volcanism.

The results obtained from all these studies have permitted understanding how magma was transported into the lithosphere and erupted at the surface, and represent an essential tool for a correct volcanic hazard assessment of the GVF. Furthermore, the methodologies described in this PhD Thesis establish general guidelines to study active monogenetic volcanic fields and we hope that it will contribute to improve their understanding.

La present Tesi Doctoral es centra en els controls geològics i estructurals del vulcanisme monogenètic. S'ha posat especial atenció a la part més superficial de la litosfera i en el paper que juga aquesta zona en determinar la distribució de punts emissors i estils eruptius. S'ha seleccionat com a cas estudi el Camp Volcànic de la Garrotxa (GVF) (NE d'lberia). Aquest és encara poc conegut i ha esdevingut un lloc ideal per l'aplicació d'estudis multidlsciplinaris com els treballs que constitueixen aquesta Tesi Doctoral, integrant mètodes geofísics. treballs de camp, ¡ anàlisis geomorfològics i estructurals.

Els primers dos treballs s'han portat a terme al sector Nord del GVF. Aquests estudis s'han basat en l'aplicació de diverses tècniques geofísiques, identificant així les estructures tectòniques principals I obtenint una informació detallada del basament. El tercer treball també s'ha realitzat a partir d'un estudi multigeofísic. En aquest cas al sector Sud del GVF, concretament al volcà de la Crosa de Sant Dalmai. Els resultats han permès la realització d'un model de la part superior de la diatrema, determinant la seva estructura interna i origen. Els resultats obtingut suggereixen que la tomografia elèctrica resulta una eina eficaç per l'estudi dels diferents tipus d'edificis monogenètics.

D'aquesta manera es presenta un treball breu amb diferents exemples de l'estructura dels cons volcànics de la zona. Combinant aquests resultats amb informació geològica de camp, s'ha realitzat la primera cartografia vulcanoestratigràfica del GVF. Finalment, s'ha fet un anàlisi volcanoestructural de tot el camp volcànic, el qual Inclou la distribució geoestadística de falles, fissures i punts emissors, identificació morfo-estructural de lineacions, anàlisi morfomètric dels cons i cràters, localització de la sismicitat registrada de la zona i gasos derivats del mantell en surgències i pous, com a guia per identificar falles actives o fractures obertes, amb l'objectiu de definir els controls estructurals d'aquest vulcanisme.

Tot els resultats obtinguts han permès entendre el transport del magma des de la litosfera fins la superfície, i representen un eina essencial per una correcta avaluació de la perillositat volcànica al GVF. Tanmateix, les metodologies descrites estableixen una pauta general per l'estudi dels camps monogenètics actius.