CCP Receiver-Function Imaging of the Moho beneath Volcanic Fields in Western Saudi Arabia

Abstract:

We are searching for structural complexity in the crust and upper mantle beneath the Neogene volcanic fields (‘harrats’) of western Saudi Arabia. We determined P-wave seismic receiver functions for 50 broadband seismographic stations located within or adjacent to three volcanic fields: Harrats Lunayyir, Rahat, and Khaybar. There are 18 seismographic stations within Lunayyir, 11 in Khaybar, and 15 in Rahat with average interstation spacing of 10 km, 30km, and 50 km. For each station we calculated 300 to 600 receiver functions with an iterative time-domain deconvolution; noisy receiver functions (outliers) were rejected by cross correlating each receiver function with a station stack; we only accepted those with a cross correlation coefficient ≥ 0.6. We used these receiver functions to create a common-conversion point (CCP) image of the crust and upper mantle. The Moho and lithosphere-asthenosphere boundary (LAB) are clearly imaged, particularly beneath Lunayyir, and have average depths of about 38 km and 60 km. We do not find any evidence for structural disruption of the Moho within our ~70 km x 70 km image of the Moho beneath Lunayyir. We image a clear crust-mantle boundary beneath Rahat and Khaybar also at ~38 km, 2–3 km deeper than anticipated from prior receiver function results outside of the harrats. Mid-crustal low velocity zones seen locally beneath all three harrats, most commonly at 10–15 km or 15–20 km in depth, may more likely represent silicic Precambrian basement than accumulations of magma. Estimates of up to ~0.5 km³ of magma erupted during each eruptive episode are consistent with the lack of a disrupted Moho. However, the total erupted volume of magma, e.g. > 1000 km³ at Rahat, together with associated intrusions from the mantle, is consistent with crustal thickening of ~2 km beneath the harrats.