60 to 80 km wide belt predominantly comprising Proterozoic low-grade metamorphosed rocks overlain by thrust sheets of granites and metamorphic rocks.. The Higher or Great Himalayas.. Tra
Trang 1bending 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
follow-ing five units, each showfollow-ing distinctive litho-tectonic
character and evolutionary history:
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
low-grade 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
metamor-phites and young granites of Cenozoic age This
is also the zone of highest uplift
iv Trans-(or Tethyan) Himalayas, a belt of
domin-antly 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
Though not a thrust contact, the ITSZ is an import-ant 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 batho-lith granites
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 region-ally separates the Lesser Himalayas from the Sub-Himalayas
iii The Himalayan (Main) Frontal Thrust (HFT or MFT) which demarcates the tectonic and physio-graphic 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 9 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.)
294 INDIAN SUBCONTINENT