Abstract
The crustal and sub-crustal rheological parameters are investigated down to a depth of 150 km for an area of 650 x 215 km2 of the Large Igneous Province of Arabia whose spine is constituted by the Makkah-Madinah-Nafud (MMN) volcanic line. This is done with an aim to gain an insight on the plume head as well as on the regional crustal effects brought by the Afar Plume. The rheological parameters are: Vp, Vp/Vs, Density (ρ), Acoustic Impedance, Lame’s First constant (λ), Shear modulus (μ), Poisson’s Ratio (σ), Bulk modulus (K) and Young’s modulus (E). These are computed from the Receiver Function Analysis results based on data from 24 seismic stations underlying the Harrat Lunnayir, Northern Rahat and Khaybar (Figure 1). Actual calculations are done by the following constitutive relations: (i) Vp (km/s) = 0.9409 + 2.0974Vs–0.8206 Vs2 + 0.2683Vs3 – 0.0251Vs4, where, VP = P-wave velocity, VS = Shear-wave velocity, (ii). ρ = 1.6612Vp – 0.4721Vp2 +0.0671Vp3 – 0.0043Vp4 + 0.000106Vp5, (iii). λ = ρ (Vp 2 – 2VS 2), (iv). μ, K and E are calculated following equations [μ = Vs2 ρ; K = λ +2/3(μ); E = μ (3λ+2μ)/(λ+ μ)], (v). σ = 0.8835–0.315Vp + 0.0491Vp2 – 0.0024Vp3 and (vi). Acoustic Impedance (AI) = Vp*ρ. The parameters are analyzed by: (a) individual plots to infer the variation of lithological composition and metamorphism at crustal depths, (b) spatial mapping of the parameters to identify their 2D-variation. The former accounts for the changes locally introduced underneath the harrats by magmatic intrusions. Mutual correlations of computed rheological parameters for upper and lower crustal columns underneath HL indicate linear relations in the upper crust but represented mostly as linear pattern in the lower crust. These are inferred as changes in rock composition from felsic to mafic, where the lower crust attains granulite facies of metamorphism. The rheological parameters for HL, when compared to those for the host region, indicate fundamental changes in crustal composition which were locally introduced in HL by the mantle flow produced by Afar plume activity. The Vp/Vs values are higher in the lower and upper crust below HL in comparison to the entire area. In the lower crust, Vp/Vs attains an average value of 1.73 (with λ = μ) represent a change in crustal composition from quartz-diorite to diorite. These results are best described by a pervasive and interconnected ‘Zone of Anomalous Rheological Parameters’ (ZARP) in the upper and lower crust. Their major features are: (i) a clear continuity between the upper and lower crust, (ii) the surface gap between the harrats Rahat and Lunayyir is actually represented by a ZARP high underneath, and (iii) the ZARP maps when correlated to detailed structural maps provide an initial indication on the causal pathway of magmatic intrusion at crustal depths connecting the harrats laterally. Next we look for their vertical continuity by tracking the rheological parameters to lithospheric depths that unravels, for the first time, two anomalous zones at respective depth intervals of 30-50 km (the immediate sub-crust) as well as at greater lithospheric depths or still beyond (80-150 km). We speculate that the former acts as the zone of de-compressed crustal melting linked to crustal thinning that possibly acts as the magma feeder zone for the harrats represented by the MMN Line. By definition, this should thus represent the plume head. The deeper zone at sub-lithospheric depth is likely to represent the more extensive and persistent zone of horizontal mantle flow from the Afar region (Figure 2). Further studies are warranted to substantiate these initial findings.
Biography
Prof. Manoj Mukhopadhyay holds an M.Sc. (Applied Geophysics) (1971) degree from Indian School of Mines (ISM), Dhanbad and Ph.D. (Geophysics) (1976) from ISM & the National Geophysical Research Institute, Hyderabad, India. He worked as a faculty with the Department of Applied Geophysics at ISM from 1976 – 2006, when, he was elevated to the rank of Professor in 1989. He also held the Departmental Chair for seven years (1994-97 and 2004-06). His teaching areas cover: Gravity Exploration, Offshore Geophysics and Geodynamics. He conducted gravity mapping over extensive areas in central and eastern India. He set up the ISM Seismological Observatory Dhanbad in 1998. He was a Visiting Fellow of the Natural Sciences & Engineering Research Council, Govt. of Canada, working at the Earth Physics Branch, Ottawa (formerly; the Dominion Observatory) (1978-80), and a Visiting Fellow of the Royal Norwegian Council of Scientific & Industrial Research, Oslo, working at the Norwegian Institute of Technology, Trondheim (1991-93). He was a Visiting Professor at the University of Kuwait (2006-08), and subsequently, a Professor of Geophysics, Department of Geology & Geophysics, King Saud University, Riyadh, Saudi Arabia (2008-15). Since 2017, he is with the Department of Applied Physics, University of Technology, Lae, Papua New Guinea. His research publications are indexed in: (i) Canadian Geophysical Atlas (1987) published by the Geological Survey of Canada, Ottawa, (ii) Bouguer Gravity Anomaly Map of Bangladesh (1990) published by the United States Geological Survey, Denver, (iii) Seismotectoinc Atlas of India & its Environs (2000) published by the Geological Survey of India, Kolkata, & (iv) World Stress Map, Helmholtz Centre Potsdam – GFZ. His Research Gate ‘Research Interest Score’ (Aug. 2022): 1070 (67 publications), and h-index (Elsevier Science alert): 24.