NEW CONSTRAINTS ON CRUSTAL STRUCTURE IN EASTERN AFAR FROM THE ANALYSIS OF RECEIVER FUNCTIONS AND SURFACE WAVE DISPERSION IN DJIBOUTI

Abstract

Crustal structure beneath the GEOSCOPE station ATD in Djibouti has been investigated using H-κ stacking of receiver functions and a joint inversion of receiver functions and surface wave group velocities. We obtain consistent results from the two methods. The crust is characterized by a Moho depth of 23 ± 1.5 km, a Poisson's ratio of 0.31 ± 0.02, and a mean Vpof c. 6.2 km s-1but c. 6.9-7.0 km s-1below a 2-5 km-thick low-velocity layer at the surface. Some previous studies of crustal structure for Djibouti placed the Moho at 8 to 10 km depth, and we attribute this difference to how the Moho is defined (an increase of Vpto 7.4 km s-1in this study vs. 6.9 km s-1in previous studies). The crustal structure we obtained for ATD is similar to crustal structure in many other parts of central and eastern Afar. The high Poisson's ratio and Vp, throughout most of the crust indicate a mafic composition and are not consistent with models invoking crustal formation by stretching of pre-existing Precambrian crust. Instead, we suggest that the crust in Afar consists predominantly of new igneous rock emplaced during the late syn-rift stage where extension is accommodated within magmatic segments by dyking. Sill formation and underplating probably accompany the dyking to produce the new and largely mafic crust. © The Geological Society of London 2006.