Abstract
At least three successive thermo-tectonic episodes, the last of which involved a largescale heterogeneous simple shear deformation, can be structurally distinguished in the Okene area of the basement complex of Nigeria. Evidence for the first episode is only observed in a suite of banded gneisses (Early Gneiss). The earliest metamorphic fabric in the banded gneisses predates the fabric in a swarm of minor metamorphosed intermediate-basic dykes. The second episode is distinguished by the earliest fabric in a suite of metasediments and meta-intrusives (Main Phase Gneiss) which is co-eval with the fabric of the intermediate-basic dykes. The Early Gneiss occurs as competent relics interleaved between the foliation planes of the Main Phase Gneiss, both suites constituting a large-scale sheeted rock body — the Gneiss Complex. Sigmoidal, gently plunging asymmetric folds of the Gneiss Complex, and intrusives of Pan-African age (Late Intrusive Suite) which cross-cut the Gneiss Complex, distinguish the third episode. Because a sub-horizontal stretching lineation superimposed on the Gneiss Complex is sub-parallel to the axes of the folds, and the Late Intrusive Suite is tectonised only in the steep limb and tight hinge regions of the folds, the last episode is inferred to have involved a lateral heterogeneous simple shear of an originally flat-lying Gneiss Complex.
Published Version
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