Axis Rotation of a Major F1 Fold during a Retrograde F2 Deformation in Southwestern Nigeria
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Date
2017-06-30
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Publisher
Centrepoint Journal (Science Edition), University of Ilorin
Abstract
North east of Ilorin, South-western Nigeria, a NE-SW trending (F1) fold which
was later refolded, was identified in part of the Basement Complex of the
country. The area falls within the West African domain of the Pan African
(600±150Ma) remobilized metamorphic terrain. Polyphase deformations, which
involved at least three tectono-thermal episodes, were also identified. The area
falls within the N-S trending Schist Belts where a large-scale sheeted S1
penetrative metamorphic imprint was successively deformed into an overturned
antiformal (F1)fold and, later, to a broad synformal (F2) fold whose axial traces
are generally NE-SW and E-W, respectively. The first deformation episode was
essentially metamorphic while the subsequent ones were tectonic. Petrological
associations indicate that the rocks are mainly gneisses, psamites and psamitic to
semi-pelitic schists, cut by Pan African granitoids. These rocks were deformed
under high-temperature, low-pressure amphibolite facies conditions, but
downgraded from high to medium grade metamorphic interface during a later
tectonic event, under a high geothermal gradient within the upper crust. An
occurrence of an overturned fold in the northern end of the axial trace of F1 is
attributed to irregular strain distribution that culminated in the open F2 antiform
during the third deformation episode. Of interest, within the context of the
structural geology of Nigeria, is the E-W trend of the axis of the F2 fold, which is
apparently discordant to the observed general N-S structural trends in the
Basement Complex of Nigeria. The origin of this latter structure is attributed to
localized tangential deformation, culminating in fold axis rotation in the direction
of the X axis, and subsequently overturning and tightening of the original F1 fold
during the late- to post- tectonic phases of the Pan African tectonism.