The Oligocene-Miocene boundary has an important effect on the structure of the basin. Carbon Isotope data and Pollen analysis indicates of a climatic change around SE Asia which significantly increases humidity of the region. From this, reconstruction of monsoonal records was established and it is inferred that around 24-20 Ma is the time when the monsoon intensified. The intensification of the monsoon also resulted in intensification of erosion around the Himalayas. This erosion then caused a reduction in mass of the Himalaya which made the foreland basin partly invert, rebound, and essentially uplift. This is supported by the discovery of the older marine deposition of the Subathu formation in parts of the thrust slices in sub-Himalayas, at a high elevation than it would normally be found.