A reinterpretation of the large-scale structure of Precambrian rocks in the Adola Goldfield (Ethiopia) based on two generations of interference pattern

Abstract

Four generations of folds (F2-F5) have been recognized and mapped in Precambrian rocks of the Adola Goldfield (Ethiopia). The F2 phase folded pre-existing planes (D1) that were responsible for the large-scale thrusting of deep crustal and supracrustal rocks and oceanic crust. Pinching and thrusting of deep crustal rocks and oceanic crust was followed by an F2, recumbent folding phase. The style of F2 folds is now isoclinal with subvertical axes. F3 is tight and shows folds with horizontal axes and a general N-S reoriented trend. F4 produced tight to open folds, coaxial with F2, with fold attitudes depending upon existing S surfaces. On a large scale, F4 imprints the final structural pattern. F5 produced very open folds with subvertical axes and an E-W axial plane, and can only be seen on a large scale. The distinction between fold phases and related structures is based on regional and meso-scale overprinting relationships. The overprinting of F3/F2 gives a type-2 Ramsay interference pattern and F4/F2 + F3 produced some deviations from the general pattern. Major wrench faults, striking NE-SW, account for the last tectonic event (D6). Structural interpretation provides compelling evidence that some kind of obduction-accretion process has been active in the late Precambrian of southern Ethiopia.