1. (b) Discuss the basis of various explanations for the formation of Shiwalik. 10Marks (PYQ/2024)

Answer:

Introduction

The Shiwalik Range, also known as the Siwalik Hills or the Sub-Himalayas, forms the outermost foothills of the Himalayas and stretches across parts of northern India, Nepal, and Pakistan. Unlike the high, crystalline structures of the main Himalayan chain, the Shiwaliks are composed largely of unconsolidated to semi-consolidated sediments. Over the decades, geoscientists have proposed several explanations for its formation, reflecting the intersection of tectonic uplift, sedimentation, and erosional processes. These explanations can broadly be grouped into a tectonic accretionary model, a sediment-dominated foreland basin model, and an integrative model that emphasizes the interplay between active faulting and sediment dynamics.

Tectonic Accretionary Model

One major explanation for the formation of the Shiwaliks is rooted in plate tectonics and the concept of an accretionary wedge:

• Plate Collision and Thrusting: According to plate tectonic theory, the ongoing collision between the Indian Plate and the Eurasian Plate is the primary driver for Himalayan orogeny. As the Indian Plate advances northward, it is forced beneath the Eurasian Plate (a process known as underthrusting), resulting in intense compressional forces. The Shiwalik Range is believed to have formed as an accretionary wedge—or thrust belt—where sediments scraped off from the overlying Himalayan units are progressively thrust and stacked along the Main Frontal Thrust (MFT). This process creates a series of imbricate thrusts that build up the Shiwalik Range over time. Researchers have used seismic reflection profiles and structural mapping to support this view of active, ongoing thrusting as the land is folded and faulted.

• Fold and Thrust Belt Dynamics: In this model, the Shiwaliks represent the peripheral expression of deep-seated tectonic shortening. The interplay between fold development and thrust faulting (often documented through field observations and geomorphological analysis) leads to the characteristic gentle undulations and elongated ridges that define the range. The work of geologists using structural restoration techniques has further highlighted the role of differential uplift and fault-propagation folds in shaping the sub-Himalayan landscape.

Sedimentary and Foreland Basin Model

Another explanation emphasizes the role of erosion, sedimentation, and the evolution of a foreland basin:

• Erosional Debris and Depositional Processes: As the Himalayas rise due to tectonic activity, they undergo intense erosion. The eroded material—comprising sand, silt, and clay—is transported by rivers and deposited in the adjacent foreland basin. Over millions of years, these sediments accumulate and gradually lithify to form the sedimentary strata of the Shiwalik Range. Paleoclimatic studies and stratigraphic analyses have shown that the sediments record a history of fluvial deposition, influenced by monsoon variability and climatic shifts. This hypothesis suggests that the Shiwaliks are essentially an archive of Himalayan erosion and are continually built up by sediment input from the main orogen.
• Foreland Basin Dynamics: In this context, the Shiwalik sediments are deposited in a flexural basin created by the loading of the Himalayan thrust belt. The magnitude and pattern of subsidence are influenced by the weight of overlying thrust sheets, ensuring that the foreland basin remains a favorable setting for sediment accumulation. Geologists have used basin modeling and sedimentological studies to understand the interplay between tectonic loading, subsidence, and sediment supply.

Integrative Perspectives

Most contemporary explanations of the Shiwalik formation acknowledge that both tectonic and sedimentary processes have acted in concert:

• Coupled Tectonics and Sedimentation: Recent integrative models posit that the Shiwalik Range has developed through the simultaneous action of thrust faulting and continuous sediment deposition. This view rests on the observation that tectonic uplift and active faulting not only rearrange older sediments but also create accommodation space for new deposits. This dual process is supported by high-resolution geophysical studies and radiometric dating of sedimentary sequences that reveal periodic pulses of thrusting interleaved with significant depositional events.
• Dynamic Feedback Mechanisms: Furthermore, modern theories consider the feedback between erosion (which provides the sediment supply) and tectonic uplift (which creates topographic relief and promotes continued erosion). This dynamic interplay is fundamental to the cyclic nature of mountain building observed in the sub-Himalayan region, where erosion, sedimentation, and tectonic uplift are intricately linked.

Conclusion

The formation of the Shiwalik Range is best explained as the outcome of multiple, interacting processes. The tectonic accretionary model highlights the role of thrust faulting associated with the collision between the Indian and Eurasian Plates, while the sedimentary model underscores the importance of foreland basin dynamics and sediment accumulation from Himalayan erosion. An integrative approach, which recognizes the complex feedbacks between active tectonics and sedimentary processes, currently offers the most comprehensive explanation. This multifaceted understanding not only deepens our insight into the evolutionary history of the Himalayas but also informs our broader comprehension of mountain-building processes in convergent plate boundary settings.