Regional significance of crustal and sub-crustal rheological heterogeneities beneath the Harrat Lunayyir and their continuity into the neighboring harrats, Western Saudi Arabia – Perspectives of the Afar plume activity

Abstract

The crustal and sub-crustal rheological parameters computed from the Receiver Function Analysis (REF) results on 24 seismic stations underlying the Harrat Lunayyir, Northern Rahat and Khaybar are investigated down to a depth of 150 km to gain an insight on the plume head and regional effects of the Afar Plume. The harrats are part of the Large Igneous Province with its spine along the Makkah-Madinah-Nafud (MMN) volcanic line. Rheological parameters are analyzed by individual plots to infer the variation of lithological composition introduced underneath the harrats by magmatic intrusions and metamorphism at crustal depths. The spatial mapping of the parameters indicates a pervasive and interconnected ‘Zone of Anomalous Rheological Parameters’ (ZARP) in the upper and lower crust with major features: (i) a clear continuity between the upper and lower crust, (ii) gap area between harrats Rahat and Lunayyir is a ZARP high, and (iii) the correlation between ZARP maps and detailed structural maps indicate a causal pathway of magmatic intrusion in crust connecting the harrats laterally. We investigate their vertical continuity by tracking the rheological parameters to lithospheric depths that unravels two anomalous zones at 30–50 km (the immediate sub-crust) and 80–150 km (sub-lithosphere) depths respectively. We speculate that the former acts as the zone of de-compressed crustal melting linked to crustal thinning that possibly acts as magma feeder zone for the harrats along MMN Line, and represent the plume head. The deeper zone at sub-lithospheric depth represents the extensive and persistent zone of horizontal mantle flow from the Afar region.