Imaging subsurface northern Rahat Volcanic Field, Madinah city, Saudi Arabia, using Magnetotelluric study

Highlights

• Layer of intermediary resistivity is thicker to the south-west and thinner to the eastern edge.

• This layer is underlain by resistive (>1000 ohm-metres) granitic basement.

• Two conductive “channels” trending NW-SE and NE-SW at depths of about 18 km were mapped.

• The NE-SW and the NW-SE trending channels run in the middle of the study area and connects the mapped intrusive.

• The conductive intrusive and “channels” may be attribute partial melts stored in the pre-existing structures within the lower crust.

Abstract

Rahat volcanic field is one of the basaltic fields in Saudi Arabia that has three major geohazard events; the historical eruption (1256 CE), the fissure eruption (~4500–1500 BP), and seismic swarm (1999). These events were studied and evaluated using geophysical and geochemical studies as well as volcanological studies. Geophysical studies include gravity, seismic, and magnetotelluric surveys. In the current research, the magnetotelluric data will be analyzed using 3D inversion technique in order to image the subsurface resistivity setting of the study area.

Results from 3D inversion of the MT data revealed four major resistivity structures. The first is a layer of intermediate resistivity (40–250 Ω-meters) which is thicker (~800 m) to the south-west and thinner to the eastern edge, mostly covered by exposed Precambrian basement. This layer is underlain by resistive (>1000 Ω-metres) granitic basement. Intruding into the resistive basement are two near-vertical conductive (<20 Ω-metres) structures. One is located immediately west of the historic eruption (1256 CE) centre in the north, at a depth of about 15 km. The other intrusive is on the southern end of the survey area, also at a depth of about 15 km. Two conductive “channels” trending NW-SE and NE-SW were observed at depths of about 18 km. The NE-SW aligned “channel” runs through the northern intrusive while the NW-SE trending “channel” runs in the middle of the study area and connects both intrusive. The conductive intrusive and “channels” may be attribute partial melts stored in the pre-existing structures within the lower crust. Although the youngest known trachytic eruptions from the study area are several hundred thousand years old, recent and ongoing seismicity strongly suggest there may be magmatic activities in the lower and probably upper crust.

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.