Free-air gravity over the Hoggar Massif, northwest Africa: Evidence for alteration of the lithosphere

Analysis of the free-air gravity field over the broadly elevated Hoggar region in northwest Africa suggests that the uplift is similar in origin to the midplate hotspot swells observed on the seafloor. The Hoggar Massif is a dome of Precambrian basement, approximately 1 km high, 1000 km wide, and capped by late Tertiary alkali basalt volcanos. Gravity measurements reveal a broad free-air high, with maximum amplitude of approximately 40 mGal, which is coincident with the areal extent of the Hoggar. Using the reasonable assumption that the Hoggar is in local isostatic balance, calculations indicate that the amplitude of the gravity anomaly is best explained if the isostatic root of the elevated area is 60 km below ground surface. This root depth is similar to that observed for both the Hawaiian Swell in the Pacific and the Bermuda Rise in the Atlantic, and is shallower than the expected base of the lithosphere. The gravity data suggest that the lithosphere beneath the Hoggar has been reduced in density, perhaps by reheating as inferred for oceanic swells.