A large variation in elevation and gravity anomaly
prevails from the Red Sea coast to the interior of the Arabian Shield (AS)
across the Asir Igneous Province (AIP); The Asir Mountain
(AM) is developed on AIP. Here the elevation varies from 45 - 2700 m,
corresponding changes in F.A. are from –30 to + 220 mgal and B.A.
from +22 to –175 mgal. Regression relationships between
elevation and gravity anomalies demonstrate significant changes in trend at
about 400 m threshold of elevation across the pediment west of AM, at about 45
km inland of the shoreline, flanking the Hizaz-Asir Escarpment (HAE). Gravity
anomaly variation along a traverse taken across HAE and AIP is interpreted here
in terms of anomalous masses in crust as well as due to deeper crustal
configuration. 2D gravity interpretation is, in part, constrained by surface
geology, available geologic cross-sections for crust, interpretations from the
IRIS Deep-Seismic Refraction Line, and to a lesser extent by the available
gross results from shear-wave splitting and receiver function analysis. The
gravity model provides probable solutions for the first time on geometric
configuration and geophysical identification: a) for the seaward margin of the
mid-Tertiary Mafic Crust (TMC) below sediment cover of the Asir pediment that
coincides with the 400 m threshold elevation. This signifies an anomalous
uplift at the rifting phase. Moho below TMC extends from 10 - 22 km
depth across HAE and west margin of AIP, b). Thinned continental
crust below the Asir margin whose upper layer coincides with a seismic
reflector is at about 22 km depth, c). Rift-margin characteristic detachment
fault associated with basaltic flows on top surface of TMC at its inner margin,
d). Two geologically mapped low-angle normal faults dipping to the east
developed between the basic rocks intruding the AIP and e). felsic pluton
farther east within AS. Large scale igneous activity followed by intense
deformation affecting AIP clearly owes their origin to the rifting architecture
of the AS at the Red Sea extensional margin.
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