Effect of decollement rheology and deformation rate on the structural development of fold thrust belts in sand box models and their implications for the Naga fold thrust belt (NE India)

This study is intended to explain the deformation style of the Naga fold thrust belt (NFTB, NE India) with the aid of sand box modelling performed at a basal temperature of 50 C and deformed at varying strain rates from 3*10−6 s−1 to 4*10−3 s−1. The models are made up (from bottom to top) of a 0.25 cm thick layer of temperature-sensitive PDMS (polydimethylsiloxane), overlain by 1.75 cm of alternating black and yellow sand. The basal PDMS layer simulates a shale decollement. Decollements in the NFTB are generally developed in the Barail Shale of Oligocene age at 50 C (the depth of the Barail Shale is about 2 km and the prevailing geothermal gradient is 25 C/km). The sand layers simulate the brittlely behaving sandstones which prevail in the NFTB. All of the models were subjected to 35% compression, as the NFTB experienced similar shortening. The varying deformation velocities were chosen to model differing decollement rheologies.