294 INDIAN SUBCONTINENT bending at the eastern end, the mountain ranges join the north-trending Indo-Myanmar Arc represented by the Naga Hills and the Arakan Yoma (Figure 9) The Himalayas are divided axially into the following five units, each showing distinctive litho-tectonic character and evolutionary history: Though not a thrust contact, the ITSZ is an important tectonic contact welding the Indian continental block with the Tibetan block Immediately north of the ITSZ is a belt of 40 Ma to 100 Ma old granitoids, known as the Trans-Himalayan batholith granites i The Sub-Himalayas 10 to 50 km wide belt of Late Tertiary molasse sediments age constituting the Siwalik Group The belt also includes the older Murree formations and their equivalent, the Dharamshalas ii The Lesser Himalayas 60 to 80 km wide belt predominantly comprising Proterozoic lowgrade metamorphosed rocks overlain by thrust sheets of granites and metamorphic rocks iii The Higher (or Great) Himalayas 10 to 15 km thick belt of dominantly Precambrian metamorphites and young granites of Cenozoic age This is also the zone of highest uplift iv Trans-(or Tethyan) Himalayas, a belt of dominantly shelf (usually fossil-bearing) sediments of Late Proterozoic to Cretaceous age, bounded by the Indus-Tsangpo Suture Zone (ITSZ), a relatively narrow belt of ophiolites and associated sediments The tectonic architecture of the Himalayas is built on three prominent intracrustal thrusts From north to south these thrusts are: i The Main Central Thrust (MCT) which separates the crystalline rocks of the Higher Himalayas from the low-grade metamorphites of the Lesser Himalayas ii The Main Boundary Thrust (MBT) which regionally separates the Lesser Himalayas from the SubHimalayas iii The Himalayan (Main) Frontal Thrust (HFT or MFT) which demarcates the tectonic and physiographic boundary between the Siwaliks and the Indo-Gangetic Alluvial Plain A unique feature of the Himalayas is its crustal thickness, which rises from about 35 km in the IGAP to a thickness of between 65 and 80 km over Figure Geological map of the Himalayas showing important tectonic elements MBT Main Boundary Thrust; MCT Main Central Thrust (Adapted with permission from Mahadevan, 1994; ß Geological Society of India, Bangalore.)