The correct option is 2 only
Explanation
The Indo-Ganga-Brahmaputra Plain is a vast aggradational plain formed by the deposition of alluvium in a foredeep (depression) that developed between the rising Himalayas in the north and the stable Peninsular Plateau in the south.
- Statement 1 is Incorrect: Originally, the region where the plains now exist was a geo-synclinal depression (or foredeep). This depression was formed due to the collision of the Indian Plate with the Eurasian Plate and the subsequent upliftment of the Himalayas.
- Statement 2 is Correct: The formation of this depression is generally associated with the later stages of the Himalayan orogeny. It attained its maximum development during the third phase of the Himalayan mountain formation (associated with the rise of the Siwaliks and the loading of the lithosphere), not the first phase. The first phase corresponds to the upliftment of the Greater Himalayas (Eocene-Oligocene period).
- Statement 3 is Incorrect: The alluvium deposited in this depression is extremely deep. While estimates vary, the average depth of alluvial deposits ranges from 1,000 to 2,000 meters. In some areas, geological surveys suggest depths up to 6,000 meters. A depth of less than 50 meters is factually incorrect for the major portion of these plains.
Key Takeaway:
The Northern Plains are the youngest physiographic feature of India, formed by the filling of a deep foredeep with sediments brought down by the Himalayan and Peninsular rivers during the Quaternary period.