What’s a normal fault doing in a collisional orogen?

January 2025 earthquake in southern Tibet

On the 7 January 2025 the people of southern Tibet were hit by a magnitude 7.1 earthquake. Over 100 people died and many more were impacted with reports that thousands of homes were severely damaged.

The USGS confirmed the epicentre of the earthquake was around 10km deep and situated on a north-south trending fault. Solutions for the fault movement confirmed extensional movement had occurred.

Yes, you read that right.

An extensional fault in Tibet, sitting above the greatest collisional mountain range on Earth, In fact, in Tibet there are hundreds of extensional faults, all of which run in a generally north-south orientation, just like the one that failed in Dingri.

What is a normal fault that moved as a result of extension doing in the heart of one of the Earths great collisional orogens?

GoogleEarth images of southern Tibet, including the 'view' from Mt Everest (Chomolungma) with USGS shakemap overlay, earthquake moment tensor, along with images of earthquake damage from BBC.

Tibet

Tibet is a gigantic plateau over 4,300 m average elevation comparable in size to Western Australia or Germany. Some regions are dominated by rugged mountain ranges, snow capped peaks that project thousands of metres above valley floors and run for hundreds of kilometres. Other regions are covered in forest, or wide open grasslands. Some regions, like Dingri, lie in a rain shadow north of the Himalaya and are arid, receiving little more than 400 mm rain a year. Winter is cold in Tibet and the survivors of the Dingri earthquake will be doing it tough right now.

I’ve not been to Dingri region, but I once had the fortune of flying over it, en route to Lhasa, Tibet, from Kathmandu, Nepal. The view outside the left-hand side of the plane out towards Mt Everest and across the high Himalaya is etched in my memory.

For a kid from Adelaide, where our local hills reach just 700 m above sea level, seeing these high ranges and snow capped peaks was an eye-opening thrill that helped propel me down the path of a degree in geoscience that landed me a PhD scholarship a few years later to study the geology of eastern Tibet. But that is another story...

Major fault systems and suture zones of Tibet (Stryon et al., 2010) with location of Dingri earthquake, January 2025 on one of the major north-south trending normal fault systems. Notice how prevalent strike-slip faulting is in eastern Tibet - this is partly in response to the process of topographic ooze, the flow of lower crustal material eastwards from beneath central Tibet and rotation of the upper crust about the eastern Himalayan syntaxis.

Tibetan fault zones

A normal fault is when the rock on one side of the fault moves downward relative to rock on the other side as a result of extensional forces. Normal faults often form valleys or rift zones. Normal faults are different from thrust (or reverse) faults and faults that move with a horizontal component of slip - strike slip - because they move when the rock mass is extended.

India has been colliding with Asia over the past 50 million years or so, in the process the continent is sliding beneath southern Asia (Tibet) forming a series of enormous thrust faults such as the well-known Main Central Thrust (MCT). Many of these are re-activated former suture zones that define the series of accretionary terranes that make up Tibet and much of central Asia, continental ribbons many of which rifted off the north-eastern margin of Gondwana during the Paleozoic and Mesozoic.

 Idealised cross section of the Himalaya-Tibetan Plateau from the pre-print of Knight et al. 2025 and based on many previous studies. Final paper by Knight et al. is in Tectonics and is a very nice paper.

Topographic ooze

The India-Asia collisional process has thickened the crust of Tibet to such an extent however, that it has become gravitationally unstable. The 70 to 80 km thick crust is so thick that the force of India moving northwards is not enough to sustain the excess height caused by the relative buoyancy of crustal rocks, so the orogen is undergoing a process of 'collapse'.

Clark and Royden (2000) identified another aspect of Tibetan tectonics - the process of 'topographic ooze' in a fantastic paper published in Geology.

They showed that the strong cratonic zones that lie beneath a series of basins that border the Tibetan Plateau are barriers to crustal flow forcing the ductile lower crust to 'ooze' out through weaker regions of crust. This crustal flow out east and west from central Tibet over the past 10 to 20 million years.

Clark and Royden (2000) - Their Figure 5. Contour plot of smoothed elevations of Tibetan Plateau and surrounding regions. Contour interval is 1000 m. Areas shaded in dark gray represent regions of cold, strong, continental material; light gray area represents intermediate strength; and white areas represent weak crustal regions. Thus, lower crust escapes from beneath thickened, elevated central plateau through regions where crust is weak.

Living with a dynamic Earth

It is this process of gravitational collapse of the mountain belt (orogen) coupled with the ooze of crustal material out of the thickened zone of central Tibet that drives the formation of the normal fault systems across the plateau. And it is this process that helps to form the many normal fault systems across southern Tibet, one of which ruptured in January this year.

Tibet is a highly active tectonic system. Earthquakes and mountain building and mountain collapse go together, so this is certainly not going to be the last major earthquake in the region. For this reason, tectonically active regions across the world need strict building codes and emergency systems in place to help people coexist with this dynamic Earth.

References

Clark, M.K. and Royden, L.H., 2000. Topographic ooze: Building the eastern margin of Tibet by lower crustal flow. Geology, 28(8), pp.703-706.

Knight, B.S., Capitanio, F.A., Weinberg, R.F. and Dal Zilio, L., 2025. Slowing convergence controls on orogeny: A three‐stage evolution of the Cenozoic India‐Asia collision. Tectonics, 44(1), p.e2024TC008509.

Styron, R., Taylor, M., and Okoronkwo, K., 2010, HimaTibetMap-1.0: new ‘web-2.0’ online database of active structures from the Indo-Asian collision, Eos, vol.91 no. 20.