Why are thrust faults older than normal faults in the Himalayan-Tibetan plateau?

Question

In the Himalayan-Tibetan plateau, both normal and thrust faults are observed. Geological age dating suggests thrust faults are older (before 18-20 Ma) than the normal faults (after 18-20 Ma). Explain this observation.

Why is this the case? I'm unable to explain this using whatever I have learned so far. I am a beginner in the field of Earth Sciences, and I would appreciate a detailed answer (or any helpful pointers) so I can better understand these concepts!

Thank you!

P.S. I'm also new to this website, so pardon me if my question isn't in line with any of the guidelines. Let me know how I can improve it.

Answer 1

In line with what strawberry-sunshine has answered, and to put it into more layman terms, after a compressive episode there's a distensive one.

Imagine you have a volume of sand, and you are compressing it horizontally in one direction, it is confined in the other horizontal direction and the only force acting vertically is gravity.

During compression, the sand will reorder itself in response to the applied stress: this reorganization are the equivalent of the thrust faults. You keep applying force until movement is negligible. Then you stop applying it althogether. The weight of the sand will force a second reorganization to adapt to the new conditions. This is the equivalent of a distensive period, and the displacements inside the sand mass are the equivalent of the normal faults developed in the second stage of the orogen.

Although reality is more complicated that that, I hope this example helps you understanding why normal faults can be younger than thrust faults.

Best regards

Answer 2

Among all continents, during the Pangea separation, India traveled the maximum distance. At the beginning the convergence velocity of India was very high ($100-125$ mm/yr, from $70$ to $55$Ma). The velocity slowed down to $70-75$mm/yr after the collision ($55-20$Ma). In this time ($55-20$Ma) most thrust faults in Himalaya developed due to extreme compression. The convergence velocity reduced to about $40-50$mm/yr after $20$ Ma. Since $20$ Ma, the mantle-convection below the Indian Plate, that dragged India (from ~$70-20$ Ma) to form Himalaya, could not drag the Indian plate further due to (i) the collision and (ii) the fact that the Tibetan asthenosphere acted as a buttress. The mantle rocks below the Indian plate, being elastic-viscous in terms of rheology, started relaxing (viscous relaxation), which significantly reduced the compressive force. The relaxing nature of the Mantle below the Indian plate also transmitted to the crust, expression of which is the formation of normal faults after $20$ Ma.

The ages could be $\pm 5$ Ma.