Critical Wedge Theory: a Himalayan example (C8)

Summary

This video explores the critical wedge theory, a geological concept explaining the formation and dynamics of the Himalayan mountain range. It delves into the collision of India with Eurasia, the development of the orogenic wedge, and the role of thrust faults in shaping the range. The script also examines how orographic precipitation influences the wedge's evolution, causing out-of-sequence thrusting and rapid exhumation of high-grade metamorphic rocks.

Takeaways

• 🌏 The Himalayas were formed by the northward collision of India with Eurasia, creating a vast mountain range.
• 🏔️ The critical wedge theory helps explain the formation, size, and dynamics of the Himalayan mountain range, including how the collision unfolded.
• 📚 The structural architecture of the Himalayas consists of four geologic belts, each with different levels of metamorphism, separated by major fault zones.
• 🛑 The South Tibetan detachment is significant as it was initially a thrust fault but later became an extensional normal fault, indicating the dynamic nature of the region's geology.
• 📏 Most of the shortening across the Himalayas is accommodated by the main frontal thrust, which is the youngest and most active thrust.
• 🔄 The critical wedge concept involves the growth of the orogenic wedge through the formation of new thrusts as older ones become too burdened with overburden to continue slipping.
• 🔧 The physics of the critical wedge are governed by the balance of shear stress and normal stress, which dictate where new faults can form and where existing ones may stall.
• 🌧️ Orographic precipitation, particularly intense rainfall and glacial erosion, can reduce the normal stress on faults in the rear of the wedge, potentially causing out-of-sequence thrusting.
• 📈 Fission track thermochronology provides evidence for rapid exhumation of rocks above the main central thrust, supporting the occurrence of out-of-sequence thrusting.
• ⛰️ The high peaks of the Himalayas, composed of the Tethyan sedimentary series, are subject to extension along the South Tibetan detachment, demonstrating the ongoing geological activity.
• 🌡️ The critical state of the main boundary thrust indicates that it is at the threshold of slipping, highlighting the delicate balance of forces within the critical wedge.

Q & A

What is the critical wedge theory and how does it relate to the Himalaya?

The critical wedge theory is a concept in geology that explains the formation and dynamics of mountain ranges, particularly how thrust faults develop within an orogenic wedge. In the context of the Himalaya, the theory helps to understand how the mountain range's shape, size, and fault dynamics are influenced by the collision of the Indian plate with Eurasia, leading to the formation of an orogenic wedge with multiple thrust faults.

How did the Himalaya mountain range form?

The Himalaya mountain range formed as a result of the northward collision of the Indian plate with the Eurasian plate. This collision caused the sediments from the Indian plate to be scraped off and piled up against the rigid backstop of the Tibetan Plateau, leading to the formation of the orogenic wedge and the thrust faults that characterize the mountain range.

What are the four geologic belts of the Himalaya and how do they differ?

The four geologic belts of the Himalaya are the Sub-Himalayan series, the Lesser Himalayan series, the Greater Himalayan series, and the Tethyan Himalayan series. They differ in terms of their geological composition and the degree of metamorphism they have undergone, with the Sub-Himalayan series consisting of lightly buried slates, the Lesser Himalayan series of deeper, higher-grade schists, the Greater Himalayan series of deeply metamorphosed gneisses, and the Tethyan series of lightly metamorphosed sedimentary rocks.

What is the role of the South Tibetan detachment in the formation of the Himalaya?

The South Tibetan detachment is a major fault zone in the Himalaya that has played a significant role in the mountain range's formation. Initially, it functioned as a thrust fault, but as the Himalaya grew higher than Tibet, the fault was reactivated as an extensional normal fault, allowing the Tethyan series sedimentary rocks to slide northward off the mountain range.

How does the critical wedge theory explain the pattern of shortening along faults within the Himalayan mountain range?

According to the critical wedge theory, as the orogenic wedge thickens, the thrust faults in the rear of the wedge accumulate more overburden, increasing the normal stress and eventually causing them to lock up or stall. This makes it favorable for new faults to break out towards the front of the wedge where there is less overburden and lower normal stress, leading to a pattern of shortening focused on the youngest thrusts at the front of the range.

What is the significance of orographic precipitation in the context of the critical wedge theory?

Orographic precipitation, which is intense rainfall caused by the upward movement of air masses against a mountain barrier, is significant in the context of the critical wedge theory because it can lead to rapid erosion and removal of overburden from the rear of the wedge. This can reduce the normal stress on locked thrust faults, potentially reactivating them in an out-of-sequence manner.

What evidence supports the occurrence of out-of-sequence thrusting in the Himalaya?

Evidence for out-of-sequence thrusting in the Himalaya includes the presence of high-grade metamorphic rocks, such as gneisses, in the high Himalaya series, which suggests rapid exhumation driven by thrusting. Additionally, fission track thermochronology data indicates that rocks above the main central thrust have been brought to the surface very quickly, further supporting the idea of out-of-sequence thrusting.

How does the critical wedge theory account for the height and width of the Himalaya mountain range?

The critical wedge theory accounts for the height and width of the Himalaya by explaining how the thrust faults within the orogenic wedge carry heavier loads as the wedge thickens. This leads to an increase in normal stress that causes older thrusts to stall, while new thrusts form at the front of the wedge with less overburden, thus controlling the range's height and width.

What are the prerequisites for the formation of a critical wedge according to the script?

The prerequisites for the formation of a critical wedge include the presence of a material that can be easily faulted and deformed, such as sedimentary rocks; a weak basal ramp or detachment that allows the thrust sheets to slide; and a rigid backstop, like the Tibetan Plateau in the case of the Himalaya, which pushes against the wedge and contributes to its growth.

How does the script describe the process of thrust faults breaking out towards the foreland in the critical wedge?

The script describes the process as one where, as the orogenic wedge thickens due to compression, older thrusts in the rear of the wedge become locked under a thicker overburden. This causes new thrusts to break out towards the foreland or the southern tip of the wedge, where there is less overburden and lower normal stress, allowing for easier faulting and deformation.