MANTLE TRANSITION ZONE DISCONTINUITIES BENEATH HOGGAR SWELL, ALGERIA, NORTHWEST AFRICA

The origin of magma beneath the Hoggar swell is still poorly understood. Many previous and recent geophysical investigations give conflicting results about the source of the volcanic material and suggest that additional constraints are needed. As such, here we present results from a single long-running seismic station using stacking of radial P-to-S receiver functions. We note a clear depression of the 410 km discontinuity (d410) from the NW to the SE of the study area. The main changes of the depth of the d410 are towards a major basement faults axis, and consistent with an increase in mantle temperature of about 300˚C in the vicinity of the d410. In addition, the 660 km discontinuity (d660) shows an uplift trend towards the faults axis from the NE to the SW. The uplift in this area may suggest a thermal anomaly with a lateral temperature contrast at the bottom of the MTZ. The average MTZ thickness calculated is 255±14 km, which is similar although slightly thicker than the IASP91 Earth model value of 250 km. We suggest that this slight discrepancy may mean that the entire MTZ has a higher temperature than normal. The thickening of the MTZ suggests a low-velocity anomaly in the MTZ, and rules out the possibility that thinning of the MTZ is caused by a mantle plume originated in or beneath the mantle transition zone.