Evolution of Interference structures and Successive Sheath Folding during a Single Phase of Ductile Shearing: A Case Study from Basement Rocks in the Northwestern Indian shield
Deepak C. Srivastava
Department of Earth Sciences, I. I. T., Roorkee, Roorkee-24766 (Uttaranchal), India
dpkesfes@iitr.ernet.in
Several generations of interference structures and successive sheath-folds are demonstrated to have evolved during a single phase of progressive ductile shearing in the gneissic-migmatitic terrain of the northwestern Indian shield. Exceptionally well-preserved overprinting relationships enable the classification of folds, occurring within the sheared basement into two main groups, F1 and F2, and further classification of these fold groups into two or more sets of folds.
Successively developed folds have assumed isoclinal and sheath geometries due to rotation of limbs and axial planes towards the direction of maximum extension and accentuation in the hinge line curvature during the course of progressive shearing. Both type-2 and type-3 interference patterns, formed during the ductile shearing, are tightened so much that the axial planes of successive folds parallel each other, except at the hinge zone of relatively later folds. Structural analysis reveals that the large-scale structure is a type-2 interference pattern formed due to overprinting of F2 folds on F1 folds during the progressive ductile shearing.
Plane non-cylindrical geometry of F1 folds, occurring within the shear zones that bear insignificant imprints of F2 folding, represent first phase of sheath folds. Analysis of deformed F1 lineations substantiates that the F1 hinge lines were curvilinear before the initiation of F2 folding on horizontal-NNE trending hinge lines. The angular variation between F2 hinge lines and F1 lineations are the main cause for the diversity in geometrical pattern of curvilinear F1 lineations patterns; namely, V-, sigmoidal- or arc-shaped. The deformation during F2 folding was accomplished by a combination of simple shear and pure shear, and the X-, Y- and Z-axes of the finite strain ellipsoid were vertical, horizontal-NNE and horizontal-WNW, respectively. It is due to rotation of the limbs and axial planes towards XY plane and hinge lines towards X-axis during the ductile shearing that F2 folds also assumed isoclinal sheath geometry.