The geothermal magmatic system at the northern Rahat volcanic field, Saudi Arabia, revealed from 3D magnetotelluric inversion

Abstract

Harrat Rahat (HR) volcanic field is one of the most prominent volcanic areas in western Saudi Arabia, as it is a promising site for geothermal energy. HR has been intensively studied from a geologic point of view. Detailed inspections distinguished 12 stages of volcanic eruption with characteristic volcanics and age intervals. The area is seismologically active and suffered a swarm of earthquake events in 1999. Magnetotelluric measurements were conducted across the northern HR for deep electrical imaging to provide a comprehensive image of the geothermal source and its impact on volcanic eruptions and seismicity. Acquisition sites (65 in total) provided a square grid with ∼5 km intervals. The recording at each site continued for 20 h on average and covered a broad band of frequencies from 1 mHz to 300 Hz. Phase tensor representations and the calculated skew angles indicated three-dimensional subsurface structures. 3D inversion was then carried out using a start model of half space with 100Ωm. The inverted model shows a main magma chamber with very low resistivities in its interior (3 Ωm). Protrusions branch upwards, or first laterally and then upwards, from the main magma reservoir. They represent magma pathways to the surface and account for various eruption locations and stages. The magma chamber is overlain by a continuous zone of higher resistivity (>500 Ωm) that depicts the cooled-down lava sheets. The tabulated and spatially mapped historical eruptions are projected on the results of the 3D MT inversion in the form of slices cut at various depths to obtain clues about the details observed in the model. The epicenters of the 1999 swarm showed no direct spatial matching with features of the inverted model. Similarly, the foci positions are concentrated in the subsurface away from the geothermal source. This confirms the model, as the epicenters usually exist at zones of faults and fractures in the crust, even if the source of stresses that induce seismicity originates away from them. The rocks near the thermal source are ductile and cannot be a source of an elastic rupture. We conclude that the thermal magma reservoir is shallow (10–25 Km), of a large spatial extent (∼1000 km2), impacts surface features and induces earthquake activities.