Time constraints on exhumation of the East African Orogen from field observations and 40Ar/ 39Ar cooling ages of low-angle mylonites in Eritrea, NE Africa

Abstract

Neoproterozoic high-grade metamorphic rocks of the lower to middle crust are exposed along parts of the East African Orogen (EAO). However, the mechanisms and rates of exhumation of these deep orogenic rocks remain unclear. In eastern Eritrea, the rocks of the EAO comprise two lithotectonic domains: Ghedem and Bizen. The Ghedem domain consists of low-angle mylonites among lower to middle crustal gneisses and schists that were metamorphosed at amphibolite facies conditions before being retrogressed locally. Here we quantitatively constrain time increments in the exhumation history of the low-angle Ghedem mylonites using 40Ar/ 39Ar-cooling ages for metamorphic hornblende and white mica samples extracted from them. The hornblende samples gave an average plateau age of about 579 ± 6 Ma and the white mica samples an average plateau age of about 567 ± 5 Ma. Geothermobarometry had already shown that these rocks experienced progressive syn-deformation metamorphism that peaked when P– T conditions were near 12 kbar and 650 °C at 593 ± 5 Ma. These P– T conditions and the new 40Ar/ 39Ar cooling ages indicate that cooling and exhumation of the low-angle mylonites of the Ghedem domain were accomplished in three increments of different rate as they progressively rose from as deep as 45 km while the orogen was collapsing during the late Neoproterozoic. In the first increment, the rocks cooled ∼11 °C/Ma as they rose at about 1.07 km/Ma to cool through 500 °C at a depth of 30 km at 579 ± 5 Ma. In the second increment, the rocks cooled about ∼10 °C/Ma after their exhumation rate increased to about 1.25 km/Ma so that they cooled through 300 °C at 567 ± 5 Ma. A third increment, poorly constrained by unconformable Permo-Carboniferous sediments, implies minimum cooling rates of 1.4 °C/Ma and exhumation rates of only 0.06 km/Ma. The first two increments of exhumation occurred when the EAO in eastern Eritrea was undergoing gravitational collapse probably much faster and later than in the southern EAO. Such regional differences in gravitational collapse rates map local differences in maximum lithospheric thickness built by the final convergence of East and West Gondwana.