Precambrian crustal structure in Africa and Arabia : Evidence lacking for secular variation

We review the thickness and shear wave velocity structure of Precambrian crust in Africa and Arabia, where over 90% of the landmass is comprised of Archean and Proterozoic terranes, and examine the data for evidence of secular variation. The data come from many published 1D shear wave velocity profiles obtained by jointly inverting receiver functions and surface wave dispersion measurements, 35 new 1D shear wave velocity profiles for locations in eastern Africa, and a new map of crustal thickness for Africa and Arabia derived from modeling satellite gravity data. We find for both Archean and Proterozoic terranes a similar range of crustal thicknesses (~ 33–45 km), similar mean crustal shear wave velocities (~ 3.6–3.7 km/s), and similar amounts of heterogeneity in lower crustal structure, as reflected in the thickness of lowermost crust with shear wave velocities ≥ 4.0 km/s. There is little evidence for secular variation in crustal structure, indicating that there may have been few changes over much of Earth's history in the processes that form the continental crust. Post-formation tectonic events also may have modified many of the terranes to such an extent that secular trends arising from crustal genesis may be difficult to recognize.