Publicação

Magnetic and gravimetric modeling of the Monchique magmatic intrusion in south Portugal

Detalhes bibliográficos
Resumo:	The Monchique alkaline complex (MAC) crops out in southern Portugal with a roughly elliptical shape of about 80 km2 elongated along the ENE-WSW direction. The MAC dates to the Late Cretaceous (69- 72 Ma) and intrudes the Carboniferous Flysh formation of the South Portuguese Zone. At the surface, it comprises two main types of syenites: a central homogeneous nepheline syenite surrounded by a heterogeneous syenite unit, and some less expressive outcrops of mafic rocks (gabbros, hornfels, breccia and basalts). This igneous complex was formed during the Upper Cretaceous West Iberia alkaline magmatic event, which is characterized by alkaline magmatism of sublithospheric origin and was active from approximately 100 Ma to 69 Ma. The Monchique region hosts the most active seismic cluster of mainland Portugal, with low magnitude earthquakes (M < 4) that occur along two lineations with NNE–SSW and WNW–ESE preferred orientations. In this work we study the Monchique region through gravimetric and magnetic methods in order to: 1) better understand how the MAC influences the geomagnetic and gravimetric field in the region; 2) to create new and consistent 2D and 3D models for the intrusion; and 3) to help constrain the origin of the observed seismicity and its possible relationship with the existence of sub cropping magmatic bodies. We process recently acquired data - ground gravity survey (49 points) and drone-borne aeromagnetic survey – and integrate it with existing data. The interpretation of the gravimetric results is complemented by a density analysis of the magmatic and host rocks. We perform 3D magnetic and gravity inversion to model the geometry of gravity and magnetic sources, and 2D magnetic forward modeling along a representative profile. The calculated Bouguer gravity anomaly shows a positive gradient towards the southwest with a negative peak in the center of the Monchique Mountain. However, after applying the terrain correction (complete Bouguer anomaly), this peak vanishes. This is justified by the similar mean density values for the syenite and host rocks, respectively 2560 kg/m3 and 2529 kg/m3. The gravity field shows a significant positive anomaly (both free-air and Bouguer) in the SW part of the study region, close to Ponta de Sagres. This anomaly and its associated gradient, complemented by results from our 3D density inversion model, imply the presence of high density rocks in this region suggesting a decrease in the crustal thickness and consequent shallowing of mantle materials. The new aeromagnetic data allows to map the Monchique magnetic anomaly with unprecedented detail and reveal a 10 km elongated anomaly with 3 km wavelength and maximum 1707 nT amplitude. 3D magnetic susceptibility inversion models show a 15 km long body with maximum depth between 5-10 km, and susceptibility >0.02 SI, in agreement with previous susceptibility analysis of Monchique samples. The highest magnetic anomaly is found at Picota hill (east), but the deepest parts of the intrusion seem to be bellow Fóia hill (west). It is noteworthy that earthquake hypocenters concentrate at depths of 5-20 km, thus mostly below the modeled magmatic intrusion, indicating a fracture zone oriented preferentially ENE-WSW with hydrothermal activity.
Autores principais:	Camargo, Gabriela Cunha
Assunto:	Complexo Alcalino de Monchique Gravimetria Geomagnetismo Oasis Montaj Modelação 3D Teses de mestrado - 2022
Ano:	2022
País:	Portugal
Tipo de documento:	dissertação de mestrado
Tipo de acesso:	acesso aberto
Instituição associada:	Universidade de Lisboa
Idioma:	inglês
Origem:	Repositório da Universidade de Lisboa