Thrust systems - an introduction

Summary

This presentation explores thrust systems in geological formations, using a classic cross-section from the Canadian Cordillera. It explains how thrust faults follow staircases through layered rock sequences and discusses the concept of thin-skinned tectonics, where only the sedimentary cover is deformed, detached from the underlying basement. The video highlights fault-bend folds and introduces terms like floor thrusts and back thrusts. Examples from New Zealand, the Himalayas, and Oman are used to illustrate these concepts. The video concludes with a modern cross-section of Alberta’s foothills, emphasizing the complexity of thrust systems and their significant role in mountain formation.

Takeaways

• 🧱 Thrust systems are commonly found on the outer sides of collision belts and mounting structures.
• 🗺️ The example cross-section comes from the outer margins of the Canadian Cordillera, illustrating thin-skinned tectonics.
• 📝 Thin-skinned tectonics involve only the outer sedimentary layers of the Earth's crust, detached from the underlying basement.
• 🛤️ Thrusts follow staircases through layered rock sequences, climbing from a lower floor thrust to upper levels.
• ⏩ Thrusts can move in different directions, but in this example, they predominantly move from west to east, into Canada's interior.
• 🧩 Thrusts repeat stratigraphy by pushing the hanging wall up and over the footwall, often leading to the repetition of rock layers.
• 🔗 Thrusts tend to follow segments that are both parallel to stratigraphy and cut across it, forming staircase patterns.
• 📉 Fault bend folds are created when thrust faults bend, deforming the hanging wall and creating folds.
• 🏔️ In thrust systems, fore thrusts move away from the mountain belt towards the foreland, while back thrusts move towards the mountains.
• 🔺 Thrust systems can create complex triangle zones, where fore thrusts and back thrusts merge, forming wedge-shaped rock structures.

Q & A

What is the key characteristic of thin-skinned tectonics as described in the script?

Thin-skinned tectonics involves only the outer sedimentary layers of the Earth's crust being involved in deformation, detached from the underlying basement.

What is a thrust fault, and how does it impact stratigraphy?

A thrust fault is a type of contractional fault where the hanging wall moves up relative to the footwall. This movement often causes stratigraphy to repeat due to displacement.

What does the term 'floor thrust' refer to in thrust systems?

A floor thrust is a lower thrust that serves as a base for other thrusts to climb from, forming a staircase-like geometry through rock layers.

Why are thrust faults rarely planar as depicted in simple diagrams?

Thrust faults rarely follow a simple planar path. Instead, they form staircase patterns, alternating between segments parallel to the stratigraphy and steeper segments that cut across the layering.

What is a fault-bend fold, and how does it form?

A fault-bend fold forms when rocks in the hanging wall deform as they climb over bends in the fault, creating folds in response to the fault's geometry.

What is the significance of the cross-section through the Canadian Cordillera mentioned in the script?

The cross-section is a classic example of thin-skinned tectonics, showing the structure of thrust systems in the foothills of Alberta. It illustrates the movement of thrusts from west to east, away from the mountain belt.

How do thrust systems create triangle zones, and where is one identified in the script?

A triangle zone forms when forward thrusts and back thrusts converge, creating a wedge-shaped area of rock. One such zone is identified near Turner Valley in the cross-section through Alberta.

What is Anderson's theory of faulting, and how does it relate to thrust systems?

Anderson's theory of faulting predicts the behavior of faults under different stress conditions. It explains why some thrusts move backward (back thrusts) as well as forward (fore thrusts) in a thrust system.

How have cross-sections of the Canadian Cordillera changed since the 1960s?

Since the 1960s, more detailed seismic and geological work has refined cross-sections of the Canadian Cordillera. Modern sections show complex stacks of thrust slices, splaying thrusts, and a more detailed view of thrust systems.

What are fore thrusts and back thrusts in a thrust system?

Fore thrusts move rocks away from the mountain belt, toward the foreland, while back thrusts move rocks back toward the mountain belt.