Multiple Deformation and Interference Structures in the Archaean Iron Ore Group Rocks Around Barda-Ulugutu Area, Singhbhum District, Bihar

Systematic structural analysis of the Archaean Iron Ore Group rocks around Barda and Ulugutu throws new light on the tectonic pattern and structural evolution of the area. The polyphase tectonics (D₁-D₃) is evident by the style and over-printing relationship between three types of s-planes (S_O-S₂), folds (F₁-F₃), and related lineations. Folds (F₁) showing complex geometries, related lineations showing variable plunges and slaty cleavages (S₁) have developed synchronously during the earliest tectonic phase (D₁). The concomitant evolution of a new set of structures and deformation of preexisting D₁ structures took place during the second phase (D₂). The numerous smallscale culminations - depressions of F₂ folds are attributed to the variable orientations of s-surfaces prior to F₂ folding. Critical field evidences suggest the evolution of both F₁ and F₂ folds through similar deformational plans including buckling and post-buckle flattening. The concentric warps (F₃) are envisaged to have developed under flexural conditions during D₃ phase in local domains.

En-echelon arrangement of elliptical interference structures are ascribed to the nonorthogonal axial surfaces of F₁ and F₂ folds, and axial migrations during F₂ folding. The co-existence of type-1 and type-2 patterns is due to F₂-flexure of alternating tight and open folds (F₁) within the same layer. That the bedding (s_O) has been successively subjected to layer parallel compression is implied by buckling nature of repeated folding. Therefore, finite strain-ellipse of the whole area corresponds to constrictional field (1>λ₁>λ₂)·