Tectonic deformation of soft-sediment convolute folds

The structures of the Precambrian Ghatsila–Galudih fold belt of eastern India have been studied in great detail but in the absence of suitable strain markers there is no estimate of strain for these rocks. However, tectonically deformed soft-sediment convolute folds are plentiful in these rocks. These spectacular structures morphologically resemble sheath folds but have been produced by tectonic deformation of lobes of soft sediment contortions. An estimate of the magnitude of bulk tectonic strain (√λ3=0.31) across the axial plane cleavage, along with the assumption of no volume change during tectonic deformation, gives us crucial information about the pretectonic shape of the convolutes that can be utilised to determine strain along the X and Y axes (√λ1=2.6, √λ2=1.24). The shapes of inclusion trails in garnet porphyroblasts within the mica schists indicate that the axis of rotation is parallel to the Y-axis of bulk strain. The bulk deformation involves a non-coaxial general flattening, with λ1>λ2>1>λ3, with the vorticity vector approximately parallel to the λ2 axis.