Imaging of magma intrusions beneath Harrat Al-Madinah in Saudi Arabia

Abdelwahed, Mohamed F.; El-Masry, Nabil; Moufti, Mohammed Rashad; Lewis-Kenedi, Catherine B.; Zhao, Dapeng; Zahran, Hani; Shawali, Jamal

doi:10.1016/j.jseaes.2016.01.023

Cited by 21 publications

(11 citation statements)

References 54 publications

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“…Therefore, it is still ambiguous if there is causal relation between the observed velocity anomalies and the Cenozoic volcanism. Also, we note high crustal P wave velocity anomalies in Harrat Lunayyir (Hansen et al, ; Koulakov et al, ), but low crustal P velocity features are observed below northern Harrat Rahat (Abdelwahed et al, ), while we detect crustal S velocity reductions under both harrats. Hence, the mechanisms which trigger the observed crustal seismic velocity anomalies within different local regions may be different, requiring further local high‐resolution investigations to fully understand the origin of the surface volcanism in western Saudi Arabia.…”

Section: Discussionsupporting

confidence: 51%

“…Koulakov et al (), concentrating on Harrat Lunayyir, found anomalous areas with lower S velocities, higher P velocities, and higher Vp / Vs ratios in the upper crust. Additionally, Abdelwahed et al () reported crustal low P velocity zones underneath the 1256 CE eruption center and the 1999 seismic swarm area in northern Harrat Rahat. These local investigations support our observations—crustal velocity anomalies that are possibly related to the volcanism.…”

Section: Discussionmentioning

confidence: 99%

“…The crustal thickness ranges between 27 and 45 km, while the crustal S velocity varies between 3.44 and 3.95 km/s with bulk Vp / Vs ratios of 1.61–2.03 in the Arabian Plate (Al‐Damegh et al, ; Hansen et al, ; Tang et al, ). Local tomography studies (e.g., Abdelwahed et al, ; Hansen et al, ; Koulakov et al, ) have investigated crustal seismic velocity anomalies under some harrats in Saudi Arabia. Fast P velocities, slow S velocities, and high Vp / Vs ratio anomalies were found in the upper crust beneath Harrat Lunayyir (Koulakov et al, ), while crustal P wave velocity reductions in the northern Harrat Rahat were imaged by Abdelwahed et al ().…”

Section: Introductionmentioning

confidence: 99%

“…Local tomography studies (e.g., Abdelwahed et al, ; Hansen et al, ; Koulakov et al, ) have investigated crustal seismic velocity anomalies under some harrats in Saudi Arabia. Fast P velocities, slow S velocities, and high Vp / Vs ratio anomalies were found in the upper crust beneath Harrat Lunayyir (Koulakov et al, ), while crustal P wave velocity reductions in the northern Harrat Rahat were imaged by Abdelwahed et al (). Additionally, a thin lithosphere, strong polarization anisotropy, and anomalously low seismic velocities in the sublithospheric mantle have been inferred below western Arabia (Chang & Van der Lee, ; Koulakov et al, ; Park et al, ; Tang et al, ; Tkalčić et al, ; Yao et al, ).…”

Section: Introductionmentioning

confidence: 99%

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Shear Velocity Structure Beneath Saudi Arabia From the Joint Inversion of P and S Wave Receiver Functions, and Rayleigh Wave Group Velocity Dispersion Data

et al. 2019

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We develop a new 3-D shear velocity model for the lithosphere and sublithospheric mantle under Saudi Arabia by jointly inverting P wave receiver functions, S wave receiver functions, and fundamental-mode Rayleigh wave group velocities. P and S receiver functions are calculated from earthquakes recorded between 2012 and 2015 at 156 Saudi National Seismic Network stations operated by the Saudi Geological Survey. Rayleigh wave dispersion data are extracted from independent tomographic studies. Our model reveals significant lateral variations in crustal and upper-mantle S velocity below Saudi Arabia. Particularly, a low-velocity zone, with minimum S velocity of~4.0 km/s in the depth range of 70-190 km, is observed under the Arabian Shield coinciding with Cenozoic surface volcanism. The low-velocity zone is found consistent with the presence of partial melts in the mantle and interpreted as a potential deep magma source for the volcanism in western Arabia. We propose that magmas responsible for the Arabian volcanism may be derived from multiple sources. Both lithospheric thinning and the Afar plume trigger magma production beneath southwestern Arabia, while decompression melting caused by the lithospheric thinning may be the main factor in the central and northern portions. Furthermore, we find evidence for localized crustal low shear velocity anomalies (2-4% reductions) that appear spatially correlated to the volcanism; these may be due to fractures caused by magma ascent and small amounts of partial melt in the crust. This spatial distribution of S velocity reductions may indicate the plumbing system for magma ascent underneath western Arabia.

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Section: Discussionsupporting

confidence: 51%

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Shear Velocity Structure Beneath Saudi Arabia From the Joint Inversion of P and S Wave Receiver Functions, and Rayleigh Wave Group Velocity Dispersion Data

et al. 2019

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“…Geophysical investigations local to HR include body wave and ambient‐noise tomography studies (Abdelwahed et al, ; Civilini et al, ) that image a high‐velocity lower crust. The exception is a region of low velocity in the body wave model in the vicinity of a 1999 earthquake swarm (circle, Figure ).…”

Section: Introductionmentioning

confidence: 99%

Crustal Magmatism and Anisotropy Beneath the Arabian Shield—A Cautionary Tale

et al. 2019

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Volcanism in Saudi Arabia includes a historic eruption close to the holy city of Al Madinah. As part of a volcanic hazard assessment of this area, magnetotelluric (MT) data were collected to investigate the structural setting, the distribution of melt within the crust, and the mantle source of volcanism. Interpretation of a new 3-D resistivity model includes a shallow graben beneath thin lava fields (Harrats), a melt-free upper crust, and decompression melting in the asthenosphere below thin lithosphere. Within the lower crust the model images elongate conductivity anomalies, one of which was attributed in a previous MT study to melt. The regional MT data, combined with perspective from geology and geophysical modeling, suggest the lower crust is anisotropic with no interconnected melt zones. These divergent interpretations have distinct hazard implications and highlight the importance of large survey aperture and anisotropic modeling to MT studies of volcanic regions. Lower-crustal anisotropy extends beyond the Harrat, with the most conductive direction oriented N10°E and a factor of 3-5, determined from 2-D anisotropic inversion, between the most and least conductive directions. The enhanced conductivity is likely due to interconnected grain boundary graphite, while the anisotropy direction reflects either frozen-in fabric from Neoproterozoic stabilization of the Arabian Shield or modern ductile deformation driven by channelized asthenospheric flow coupled through a thin rigid mantle lid. Asthenospheric melt is interpreted to transect the crust primarily through diking, with limited melt storage and short residence times in the crust.Plain Language Summary Volcanism within the lava fields, or Harrats, of Saudi Arabia includes historic eruptions, such as an eruption in 1256 CE near the holy city of Al Madinah. As part of a volcanic hazard assessment of northern Harrat Rahat, geophysical studies produced a three-dimensional model of electrical resistivity of the crust and upper mantle. In contrast to a more local study, the model shows no evidence of magma in the crust. This is consistent with other evidence in the region that suggests magma does not stay in the crust for long times but ascends rapidly from the upper mantle to the surface. This finding provides a snapshot in the lifecycle of this volcanic field and is important to understanding the hazards it poses. Our model also finds deep electrical anisotropy, probably due to a preferred direction or "fabric" in the lower crust. This fabric may be ancient, frozen in millions of years ago when the crust formed. Alternately, it may be caused by ongoing flow in the ductile lower crust in response to deeper mantle flow.

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