Deformed lineation in ductile shear zones: a case study from the Proterozoic fold belt of Singhbhum, eastern India

Abstract

Regional mylonite zones resulting from intense rotational deformation display a strong mylonitic foliation which represents coplanar S- and C- surfaces. The commonly observed stretching lineation indicates the direction of movement. In most models of heterogeneous simple shear the C-surfaces are regarded as invariant, and remain planar during progressive deformation. However, it is well known that perturbations in the flow regime caused by slight variation in material properties may give rise to deflections in the shear parallel foliation which may amplify during progressive deformation, resulting in recognizable folds. Such folds, with strong asymmetry controlled by the sense of shear, are common in the mylonites of ductile shear zones and the stretching lineation is bent by them. The patterns of such deformed lineation are controlled by the initial geometric configuration and the intensity of deformation. The patterns have been theoretically deduced by applying simple shear deformation to gentle initial flexural perturbations. It has been demonstrated that at the initial stages of progressively increasing simple shear the deformed lineation is of Type 1 (l₁ ^ f₂ acute angle opening in the same sense on the two limbs) and at a high value of simple shear the pattern changes to Type 3 (l₁ ^ f₂ acute angle opening in the opposite sense on the two limbs). A systematic study on the patterns of deformed lineation in hand specimens and outcrop-sized folds in a mylonitic banded magnetite quartzite within the Singhbhum Shear Zone in the Proterozoic fold belt of North Singhbhum, eastern India, shows that the commonest type is one in which the l₁ ^ f₂ angle (measured in the same sense) increases continuously from one limb to the other, usually changing from acute angle on one limb to nearly 90° on the other; angles greater than 90° are seen on some folds. The observed patterns are comparable with those obtained in the theoretical model and these suggest an angular shear magnitude of nearly 70° within the shear zone.

Acknowledgements: Dr Tapas Bhattacharya wrote the computer program for the calculations and helped in many ways with the computer work. Mr Nilanjan Dasgupta and Ms Nandini Chattopadhyay helped with the plotting of some data.