UKPSC Prelims 2025 General Studies Paper-I · Question 101 of 149

Correct — B, Lesser Himalayan zone.

The Himalaya is sliced lengthwise by major dislocation surfaces that appear in a fixed order as you travel south to north: the Himalayan Frontal Thrust, then the Main Boundary Thrust, then the Main Central Thrust, then the South Tibetan Detachment.

Each longitudinal belt is bracketed by two of them. The Siwalik belt lies above the Frontal Thrust and below the MBT. Cross the MBT going north and you step into the Lesser Himalayan zone.

The MCT closes that belt off in the north, carrying the Greater Himalayan crystallines southward over it. The pair named in the stem therefore brackets exactly one belt.

Carry away the ladder itself: a Himalayan zone is defined by the structure below it and the structure above it, read in south-to-north order.

• (a)
  Shiwalik zone — The Siwalik belt lies one step further south. Its northern limit is indeed the Main Boundary Thrust, but its southern limit is the Himalayan Frontal Thrust, so the MCT is not what closes it off.

  This option is the right answer to "which zone lies between the Himalayan Frontal Thrust and the Main Boundary Thrust" — the belt of young sandstone and conglomerate fringing the plains.

• (c)
  Central crystalline zone — These high-grade gneisses and migmatites make up the Greater Himalaya, and the Main Central Thrust is the surface they ride southward on. That places them above the MCT, on its northern side.

  This option is the right answer to "which zone lies between the Main Central Thrust and the South Tibetan Detachment" — the belt carrying peaks such as Nanda Devi.

• (d)
  Tethys Himalayan zone — The Tethyan belt is the fossil-bearing sedimentary succession lying still further north, resting above the South Tibetan Detachment. In Uttarakhand it is exposed in the Niti and Mana valley areas of Chamoli district, beyond the high peaks.

  This option is the right answer to "which zone lies north of the South Tibetan Detachment, also called the Trans-Himadri Fault".

The Himalaya can be divided along its length by four major dislocation surfaces rather than by height alone. From the plains northward these are the Himalayan Frontal Thrust, the Main Boundary Thrust, the Main Central Thrust and the South Tibetan Detachment.

The first three are north-dipping thrusts along which older rock has been pushed south over younger rock, a product of continued India-Eurasia convergence. The South Tibetan Detachment is a normal-sense structure sitting on top of the crystalline pile.

The belts they define — Siwalik, Lesser Himalaya, Greater Himalaya or Central crystallines, and Tethys Himalaya — differ in rock type, metamorphic grade and elevation.

• The Main Boundary Thrust separates Siwalik rocks to its south from Lesser Himalayan rocks to its north.
• The Main Central Thrust carries the Greater Himalayan crystalline rocks southward over the Lesser Himalayan sequence.
• The Lesser Himalaya is built largely of low- to medium-grade metasedimentary rocks such as slate, phyllite, quartzite, limestone and dolomite.
• The Himalayan Frontal Thrust marks the southern edge of the Siwalik belt against the Indo-Gangetic plain.
• The South Tibetan Detachment, also called the Trans-Himadri Fault, lies above the Greater Himalayan crystallines and below the Tethyan sediments.
• The Mussoorie and Nainital ranges of Uttarakhand lie within the Lesser Himalayan zone.
• Duns such as the Dehradun valley are longitudinal valleys lying between the Siwalik range and the Lesser Himalaya.

• Reading the two named structures in the wrong order: the MCT lies north of the MBT, so the belt wanted is the one above the MBT, not the one below it.
• Letting the word "Central" in "Central crystalline zone" suggest that the belt sits south of the Main Central Thrust; the crystallines ride over the thrust and lie north of it.
• Taking the Tethyan belt's southern limit to be the MCT; the structure below the Tethyan sediments is the South Tibetan Detachment.
• Switching between the physiographic names (Outer, Lesser, Greater, Tethys Himalaya) and the tectonic ones without keeping the south-to-north order of the structures fixed.

• practice — not a real PYQ

  The Main Boundary Thrust forms the boundary between which of the following pairs?

  • (a)Indo-Gangetic plain and Shiwalik zone
  • (b)Shiwalik zone and Lesser Himalayan zone
  • (c)Lesser Himalayan zone and Central crystalline zone
  • (d)Central crystalline zone and Tethys Himalayan zone
  Answerb — The MBT is the surface that places Lesser Himalayan rocks over Siwalik rocks, so it is the boundary between those two belts.

  (a) describes the Himalayan Frontal Thrust, which brings the Siwalik over the alluvium of the plains. (c) describes the Main Central Thrust. (d) describes the South Tibetan Detachment.

• practice — not a real PYQ

  The Central crystalline zone of the Himalaya is bounded on its southern side by which one of the following?

  • (a)Himalayan Frontal Thrust
  • (b)Main Boundary Thrust
  • (c)Main Central Thrust
  • (d)South Tibetan Detachment
  Answerc — The crystalline belt rides southward on the Main Central Thrust, so the MCT is its floor and its southern limit at the surface.

  (d) is its northern limit, lying above the crystallines. (b) is two belts to the south, and (a) is the outermost structure, against the plains.

• practice — not a real PYQ

  The hill stations of Mussoorie and Nainital lie in which of the following zones of the Himalaya?

  • (a)Shiwalik zone
  • (b)Lesser Himalayan zone
  • (c)Central crystalline zone
  • (d)Tethys Himalayan zone
  Answerb — Both sit on the Lesser Himalayan ranges, the belt between the Main Boundary Thrust and the Main Central Thrust.

  (a) is the low outer range fringing the plains. (c) is the high crystalline belt of peaks such as Nanda Devi, and (d) lies beyond it in the Tethyan sedimentary tract.

• practice — not a real PYQ

  Travelling northwards from the Indo-Gangetic plain into the Himalaya, in which order are the following structures crossed?

  • (a)Main Boundary Thrust — Himalayan Frontal Thrust — Main Central Thrust
  • (b)Himalayan Frontal Thrust — Main Boundary Thrust — Main Central Thrust
  • (c)Main Central Thrust — Main Boundary Thrust — Himalayan Frontal Thrust
  • (d)Main Boundary Thrust — Main Central Thrust — Himalayan Frontal Thrust
  Answerb — Moving north from the plains you meet the Himalayan Frontal Thrust first, then the Main Boundary Thrust, then the Main Central Thrust.

  The other options disturb that order: (a) and (d) place the MBT ahead of the frontal thrust, and (c) reverses the traverse into a north-to-south one.