Spatial distribution of the rheological heterogeneities at crustal depths underneath the Harrat Rahat, western Saudi Arabia and their correspondence to Bouguer anomalies

Abstract

The Harrat Rahat (HR) is the largest harrat in the Large Igneous Province in Saudi Arabia. Evolution of HR is enigmatic; two main debated postulates are the Red Sea rift margin volcanism vis-a-vis the Afar plume activity. Our objective in the present study is to investigate the rheological parameters for the crustal columns beneath the HR; computed from Receiver Function analysis results based on 11 seismic stations in the Northern HR (NHR) and three broadband stations from the Southern HR (SHR). The computed rheological parameters (Vp, Vp/Vs, Lame's First Constant, Shear Modulus, Density, Poisson's Ratio, Young's Modulus, Bulk Modulus and Acoustic Impedance) are utilized to investigate the rheological character individually for the upper and lower crust (UC and LC) underneath the NHR and SHR by adopting the following: (i) mutual correlation between the computed elastic constants in order to make inferences on the probable rock composition prevalent at crustal depths, (ii) the rheological parameters are then compared to those derived for the host terrain, the Arabian Shield Transitional Crust (ASTC) for understanding on the intrinsic changes locally introduced in HR by magmatic intrusions, (iii) spatial mapping for selected rheological parameters unravel a pervasive ‘Zone of Anomalous Rheological Parameters’ (ZARP), the resultant maps provide the variation of rheological properties in UC and LC. The gross features appearing on the ZARP maps demonstrate the rheological heterogeneity within the bounds of NHR and SHR. The anomalous zones are further examined for their physical property correspondence to the Bouguer gravity anomaly map for NHR. The gravity axial low zone in NHR is regarded as probable locale for fractured dyke invaded crust which act as feeder zone; where a raised thermal regime heralded gross compositional changes in LC in consequence to crust-mantle interaction.