The splitting of the ancient Indo–Brahma river into the Indus, Ganga, and Brahmaputra drainage systems is believed to have been primarily caused by which geological event?

The correct option is Pleistocene upheaval.

During the Miocene period, a continuous river known as the Indo-Brahma or the Shiwalik river flowed along the entire longitudinal extent of the Himalayas from Assam to the Punjab and finally discharged into the Gulf of Sind near Karachi.

However, during the Pleistocene epoch, significant geological events took place:
1. The upheaval of the Western Himalayas, including the uplift of the Potwar Plateau (Delhi Ridge), acted as a water divide.
2. This tectonic movement dismembered and split the ancient Indo-Brahma river system into three major drainage systems:
  • The Indus system in the west
  • The Ganga and its Himalayan tributaries in the central part
  • The Brahmaputra in Assam and its tributaries in the east

Therefore, the splitting of the Indo-Brahma river into the Indus, Ganga, and Brahmaputra drainage systems was primarily caused by the Pleistocene upheaval.