Geophysical and petrological evidence for the presence of an “anomalous” upper mantle beneath the Sahara basins (Algeria)

Abstract

North of 20°N, the northwest part of the stable African shield is characterized by an anomalous high heat flow zone (80–110 mW m −2). This east-west elongated zone is oblique to major Proterozoic structural units and cannot be related to differences in crustal heat production. Maximum heat flow occurs in the southern part of the Sahara basins (100–120 mW m −2). Only a mantle heat flow contribution of 60–70 mW m −2 can account for this very high surface heat flow. The estimated geotherms suggest that the temperature of upper mantle is anomalously high and partial melting may be present at shallow depths. This hypothesis is supported by the existence of a long-wavelength negative component of the gravity field that corresponds with the high heat flow axis and with the very low lithospheric S-wave velocities (4.2–4.4 km/s) down to 160 km obtained from surface wave studies. In addition, the chemistry of the scarce Cenozoic melilitite lavas (Illizi volcanic district), geographically associated with the thermal anomaly, as well as the mineralogy of their inclusions (phlogopitized garnet/spinel peridotite and alkali clinopyroxenite) show that the upper mantle beneath this region is locally highly metasomatised and partially melted. From these preliminary results, it appears that the northwest African upper mantle has been modified by recent thermal rejuvenation processes (including gas transfer) that are likely to be more extensive in the southern part of the Sahara basins.