Draw a cross-section from north to south and discuss the tectonic framework of the Himalaya.

Himalayan Tectonic Framework: A North-South Cross-Section

The following describes a generalized north-south cross-section of the Himalayas, coupled with a discussion of the tectonic framework. The cross-section is divided into distinct geological zones, each reflecting a specific stage in the Himalayan orogeny.

(Note: Since I cannot directly draw, I am providing a link to a representative image. A hand-drawn sketch in the exam would be expected.)

Geological Zones (North to South)

1. Eurasian Plate (North of the Indus-Tsangpo Suture Zone)

This represents the relatively stable continental crust of Eurasia. It is characterized by Precambrian basement rocks and Paleozoic-Mesozoic sedimentary sequences. The zone exhibits limited deformation compared to the regions closer to the collision zone.

2. Indus-Tsangpo Suture Zone (ITZS)

This zone marks the primary boundary between the Indian and Eurasian plates. It is a complex region characterized by:

• Ophiolites: Fragments of oceanic crust (e.g., serpentinites, basalts) representing remnants of the Tethys Ocean that were subducted and obducted during the collision.
• Melange: A chaotic mixture of rock fragments, including blocks of oceanic crust, sediments, and continental rocks.
• Granite intrusions: Associated with magmatism related to subduction processes.

3. Higher Himalayas (Greater Himalayas)

This is the highest and most structurally deformed zone, composed of:

• Proterozoic Crystalline Basement: Ancient metamorphic rocks forming the core of the range.
• Tethyan Sedimentary Sequence: Thick sequences of Paleozoic and Mesozoic marine sediments that were thrust over the Indian plate.
• Major Thrust Faults: Including the Main Central Thrust (MCT), Main Boundary Thrust (MBT), and Main Frontal Thrust (MFT). These faults accommodate much of the shortening associated with the collision.

4. Lesser Himalayas (Middle Himalayas)

Lying south of the Higher Himalayas, this zone is characterized by:

• Lower Paleozoic and Mesozoic Sedimentary Rocks: More intensely folded and faulted than those in the Higher Himalayas.
• Regional Metamorphism: Due to the stresses associated with the collision.
• Significant Erosion: Leading to rugged topography.

5. Sub-Himalayan Zone (Siwalik Range)

This is the southernmost zone, composed of:

• Thick Sequences of Cenozoic Molasses: Sediments derived from the erosion of the Himalayas, deposited in foreland basins.
• Gentle Folds and Thrusts: Indicating decreasing deformation towards the south.

6. Indo-Gangetic Plain (Foreland Basin)

A vast alluvial plain formed by the deposition of sediments eroded from the Himalayas. It is a foreland basin, subsiding due to the weight of the mountains.

Tectonic Processes

The formation of the Himalayas is primarily driven by the collision of the Indian and Eurasian plates. The Indian plate, once part of Gondwana, began moving northward after the breakup of the supercontinent. This northward movement led to the closure of the Tethys Ocean and the eventual collision with Eurasia approximately 50-55 million years ago.

Key tectonic processes include:

• Subduction: Initially, the oceanic crust of the Tethys Ocean was subducted beneath the Eurasian plate.
• Obduction: Fragments of the oceanic crust (ophiolites) were obducted onto the Eurasian plate.
• Thrust Faulting: As the Indian plate continued to collide with Eurasia, the crust was shortened and thickened, resulting in the development of major thrust faults.
• Folding: Sedimentary layers were intensely folded due to compressional stresses.
• Isostasy: The thickening of the crust led to isostatic uplift, creating the high elevations of the Himalayas.

The collision is ongoing, with the Indian plate still moving northward at a rate of approximately 4-5 cm per year. This continued convergence results in frequent earthquakes and ongoing uplift.