earth science

Main Himalayan Thrust

Main Himalayan Thrust is a earth science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Main Himalayan Thrust rather than just read about it. In short: The Main Himalayan Thrust (MHT) is a décollement under the Himalaya Range. This thrust fault follows a northwest-southeast strike, reminiscent of an arc, and gently dips about 10 degrees towards the north, beneath the region.

Key takeaways

Main Himalayan Thrust belongs to earth science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Main Himalayan Thrust to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Main Himalayan Thrust from memory before moving on to harder problems.

Reference excerpt

The Main Himalayan Thrust (MHT) is a décollement under the Himalaya Range. This thrust fault follows a northwest-southeast strike, reminiscent of an arc, and gently dips about 10 degrees towards the north, beneath the region. It is the largest active continental megathrust fault in the world.

Overview The MHT accommodates crustal shortening of India and Eurasia as a result of the ongoing collision between the Indian and Eurasian plates. The MHT absorbs around 20mm/yr of slip, nearly half of the total convergence rate. This slip can be released from small-scale earthquakes and some plastic deformation, but the MHT still accumulates a deficit of moment of 6.6×1019 Nm/yr. The MHT also remains locked with the overlying Eurasian plate from its surface expression to the front of the higher Himalayas, nearly 100 kilometres away. This locking mechanism combined with the rapid accumulation of deficit of moment are concerning, as some professionals estimate that earthquakes up to the size of 8.9 on the Richter scale could be in order for regions such as western Nepal. Earthquakes of this magnitude are estimated to have a return period of over 1000 years in this region. Deformation of the crust is also accommodated along splay structures including the Main Frontal Thrust (MFT), Main Boundary Thrust (MBT), Main Central Thrust (MCT), and possibly the South Tibetan Detachment. The MHT is the root detachment of these splays. Currently, the MFT and MHT accounts for almost the entire rate of convergence (15–21 mm/yr). This fault defines where the Indian subcontinent is underthrust beneath the Himalayan orogenic wedge.

Geometry The MHT is a large north-dipping detachment fault separating the northern boundary of the Indian plate and southern boundary of the Himalayas. It is a gently sloping decollement that is predominantly subhorizontal, but its dip angle varies along strike and regions, and different research propose contradicting results. The MHT generally extends north towards southern Tibet, where it reaches a depth of 40 km (25 mi). In some sections, the MHT features a flat-ramp-flat geometry due to the presence of a midcrustal ramp beneath the High Himalayas before it resumes its subhorizontal dip beneath the Tethys Himalaya in southern Tibet. Along the Bhutan section, Xavier Robert and others proposed that the MHT has a dip of 5–7° with limited variability along strike. However, Isabelle Coutand and others postulated that the MHT was a complex structure using thermochronology that features a flat-ramp-flat configuration with significant variability along strike. They inferred that the MHT at the surface (known as the Main Frontal Thrust (MFT)) is a steep frontal ramp that extends into a flat planar surface at 10–15 km (6.2–9.3 mi) before encountering a 30° ramp at depth. The midcrustal ramp is also found in the Garhwal, Nepal, and Sikkim Himalaya, where its reaches a maximum steepness of 40°. Beneath the Garhwal Himalayas, a flat-ramp-flat geometry is also inferred north of the surface expression of the MHT. The shallower flat, dipping north at 2°, occurs at a 10 km (6.2 mi) below sea level. It connects with a northeast dipping 16° ramp at about 100 km (62 mi) north of the MFT, where the Lesser Himalayas transitions to the High Himalayas. This ramp occurs at 10–20 km (6.2–12.4 mi) depth before it connects to a deeper flat. The deeper flat dips 4° north at 20–25 km (12–16 mi) depth. In Gorkha District of central Nepal, at the site of the 2015 earthquakes, the MHT features a flat-ramp-flat-ramp-flat geometry, where a middle and deeper ramp bound a flat middle segment of the MHT that ruptured. The area where more than a meter of displacement occurred during the earthquake was consistent with the elliptical shape (about 100 by 50 km (62 by 31 mi) across) of this subhorizontal fault segment isolated from other parts of the MHT by these ramps. Beyond the 2015 rupture patch, both ramps combine to form a larger ramp which is prominent as it extends eastwards. To the west of this zone, after middle and deeper ramps combine, they diverge again, to form another flat-ramp-flat-ramp-flat feature. This flat middle segment is larger; extending 400 km (250 mi) westwards. The MHT emerges towards the surface at depth on its gently sloping plane via a splay fault. Its surface expression in the northern Gangetic Plain is represented by the Main Frontal Thrust (MFT). The MFT is the southernmost and youngest thrust structure of the greater Himalayan fold and thrust belt. At Bardibas, southern Nepal, the MHT lies horizontally at a depth of 2 km (1.2 mi); emerging steeply towards the surface as two local splay faults, the Patu and Bardibas thrust. In Jammu and Kashmir of the northwestern Himalayas, the arc geometry of the plate boundary causes the MHT to dip in a northeast direction. The MHT maintains a gently-sloping subhorizontal thrust structure. Beneath the Kashmir Valley, seismic imaging has revealed a flat decollement at 12–16 km (7.5–9.9 mi) depth. The 4° decollement is also present beneath the Sub- and Lesser Himalayas of Jammu and Kashmir. Further northeast towards the Kishtwar Himalaya and Zanskar Range, the MHT forms a 13–17° dipping midcrustal ramp at 10–16 km (6.2–9.9 mi) depth.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Main Himalayan Thrust

Start with the simplest possible case. Write down what Main Himalayan Thrust claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In earth science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Main Himalayan Thrust before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Main Himalayan Thrust ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Main Himalayan Thrust

In research: Main Himalayan Thrust appears in earth science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Main Himalayan Thrust in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Main Himalayan Thrust is common in secondary-school and first-year university syllabi. It links to neighbouring topics Active faults, Geology of Asia, Geology of Bhutan, so understanding it makes those chapters shorter.
In everyday life: Look for Main Himalayan Thrust outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Main Himalayan Thrust in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Main Himalayan Thrust means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Main Himalayan Thrust out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Main Himalayan Thrust in simple terms?

Why does Main Himalayan Thrust matter?

Because it connects several earth science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Main Himalayan Thrust?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Main Himalayan Thrust.