Imaging subsurface northern Rahat Volcanic Field, Madinah city, Saudi Arabia, using Magnetotelluric study
Introduction
Rahat Volcanic Field (RVF) is one of the Cenozoic lava fields in the Kingdom of Saudi Arabia. It occupies a 50 km wide plateau extending from north to south for almost 300 km between the Hejaz coastal range (Fig. 1) on the west and the high plains of Najd on the east (Berthier et al., 1981; Durozoy, 1970). It has an approximate volume of 2000 km3 and an estimated average thickness of about 150–400 m (Blank and Sadek, 1983). The most recent eruption of 1256 CE, which lasted for 52 days, extruded 0.5 km3 of alkali-olivine basalt forming a 2.25 km long fissure and produced 6 scoria cones and a 23 km-long lava flow that came to within 8 km of Madinah city (Camp et al., 1987; Camp and Roobol, 1989). Three major hazards were documented and located within the study area: the historic eruption of 1256 CE, fissure eruption or Five Fingers eruption, and the earthquake swarm in 1999 (El Difrawy et al., 2013).
In 1256 CE an effusive basaltic eruption sent lava flows ~20 km from the source vent towards the city of Al-Madinah (Camp et al., 1987); the volcanic field has the potential to produce another volcanic eruption close to the city. Furthermore, an earthquake swarm in 1999 was interpreted as a possible magmatic intrusion (Mokhtar et al., 2013), suggesting that local volcanism has potential for activity on human timescales. A borehole seismic network, installed in 2012 as a part of VORiSA project, recorded microearthquakes in the swarm area (at only ~20–45 km depth) indicating possible on-going movement/activity at that location.
King Abdulaziz University (KAU), Saudi Arabia, and the University of Auckland (UoA), New Zealand, jointly collaborated to evaluate the geohazards in the RVF through Volcanic Risks in Saudi Arabia (VORiSA) project. This project implemented geophysical and geological surveys over 3 years. Gravity, MT, and seismic surveys were the most important tasks in the VORiSA project. Aboud et al. (2015) published the results of gravity data combined with the available aeromagnetic data. Abdelwahed et al. (2016) published the seismic results. In this research, we will discuss the analysis of MT data taking into consideration previously published works.
MT is a passive exploration technique that utilises a broad spectrum of naturally occurring geomagnetic variations as a power source for electromagnetic induction in the Earth (Simpson and Bahr 2005). By measuring electric and magnetic fields in orthogonal directions at the surface, the technique maps the electrical resistivity distribution of the subsurface. Penetration depth depends on the electromagnetic sounding period and the conductivity structure of the target site; depths range from a few tens of metres to several tens of kilometres. This makes MT one of the most useful tools for mapping lateral and vertical geo-electrical variations of the sub-surface.
A total of 65 MT stations were deployed in the northern part of RVF to map subsurface geo-electrical resistivity variations in the area. This study was conducted with the aim of identifying minor and major geological structural systems and possible intrusive bodies that could be related to local volcanism and possible magmatic activities in the area.
Section snippets
Geologic setting
The northern terrains of RVF, referred to as Harrat Al-Madinah, are largely constructed of monogenetic, Strombolian-style basaltic volcanoes and basaltic lava flows, which extruded onto the Precambrian basement rocks of the Arabian Shield through NNW-trending en echelon-vent zones (Moufti et al., 2010). The basalts range from olivine transitional basalt and alkali olivine basalt to hawaiite, whereas the associated evolved rocks of mugearite, benmoreite, and trachyte occur mainly as domes and
Acquisition
A total of 65 MT soundings were deployed over an area of about 1050 km2 using 4 sets of the Phoenix 5-channelMTU-5A. Sites were chosen to ensure adequate coverage of the main basaltic and trachytic volcanic centres, and the location of the 1999 seismic swarm. 56 MT sites were placed on a grid-orientedNW with about 5 km spacing between stations (Fig. 2). The remaining 9 sites were placed around the 1256 CE historical eruption and the “Five Fingers” fissure flow to intensify coverage around these
Results and discussions
The results from the 3D inversion of the MT data are presented in Fig. 6, Fig. 7. These clearly illustrate the thick high resistive zone and the irregular-shaped low resistivity zones seen at depth below ~12 km. It can be realized that the 3D view (Fig. 6A) shows low resistivity body (nearly green) as seen through the resistive zone (blue). This low resistive body has two peaks, northern and southern peak. The northern peak is shown in side views (Figs. 6A, 6B, and 6C) with resistivity values
Acknowledgements
The authors thank King Abdulaziz University (KAU) for providing the necessary support to conduct the ground magnetotelluric survey as a part of the collaborative project ‘Volcanic Risk in Saudi Arabia’ (VORiSA) between KAU and the University of Auckland, New Zealand. The authors are also grateful to all the staff members of the Geohazards Research Centre (GRC) at KAU and the Institute of Earth Science and Engineering (IESE) at UoA for the invaluable support during the labour-intensiveMT field
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