The South Tibetan Detachment is one of the major faults in the

Himalaya The Himalayas, or Himalaya (; ; ), is a mountain range in Asia, separating the plains of the Indian subcontinent from the Tibetan Plateau. The range has some of the planet's highest peaks, including the very highest, Mount Everest. Over 100 ...

Mountains.

Background

Understanding the formation of the Himalayan mountains has been a goal of structural geologists for a long time. Many of the problems and disagreements that geologists have with each other concerning the

Himalayan orogeny The geology of the Himalayas is a record of the most dramatic and visible creations of the immense mountain range formed by plate tectonic forces and sculpted by weathering and erosion. The Himalayas, which stretch over 2400 km between th ...

involve the relationship between the observed geometry, or structures, with the various rock units (different types of rock). There are three primary rock units present in the Himalaya: the Lesser Himalayan Sequence (LHS; mainly low-grade Proterozoic

metasediment In geology, metasedimentary rock is a type of metamorphic rock. Such a rock was first formed through the deposition and solidification of sediment Sediment is a naturally occurring material that is broken down by processes of weathering and e ...

s, ''yellow in map''), the Greater Himalayan Crystalline complex (GHC; largely high-grade

paragneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures an ...

es and

migmatite Migmatite is a composite rock found in medium and high-grade metamorphic environments, commonly within Precambrian cratonic blocks. It consists of two or more constituents often layered repetitively: one layer is an older metamorphic rock th ...

, ''orange in map''), and the Tethyan Himalayan Sequence (THS; dominantly low-grade late Proterozoic to Eocene shelf sediments; ''blue in map'') More information about these rock units can be found by reading about the

geology of Nepal The geology of Nepal is dominated by the Himalaya, the highest, youngest and a very highly active mountain range. Himalaya is a type locality for the study of on-going continent-continent collision tectonics. The Himalayan arc extends about fro ...

. Because the Himalaya form on the leading edge of a collisional

tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents ...

event, it stands to reason that many structural features will form in this area. The three main rock units: the LHS, GHC, and THS all have a hotly debated relationship with each other involving two main fault lines. One is the

Main Central Thrust The Main Central Thrust is a major geological fault where the Indian Plate has pushed under the Eurasian Plate along the Himalaya. The fault slopes down to the north and is exposed on the surface in a NW-SE direction (strike). It is a thrust fault ...

(MCT), and the other is the South Tibetan Detachment (STD). Tectonics geologists agree that the GHC, between the LHS and the THS, is between these two faults and is somehow being moved towards the surface and being exposed today. The South Tibetan Detachment is often interpreted as a low angle normal fault and can also represent extension. However, there are new interpretations about old observations that support the idea that the STD can also be a north-directed thrust fault.

Geometry and structure

The main problem that the South Tibetan detachment is expected to help resolve is the method of emplacement of the GHC between the LHS and THS within the Himalaya. There are three models that help explain this emplacement: wedge extrusion, channel flow, and tectonic wedging. The wedge extrusion and channel flow models work under the assumption that the STD is a low angle normal fault, while the tectonic wedge model works as if the STD is a thrust fault.

Importance

The Asia-India collision is one of the most interesting tectonic events of the last 100 million years, and the Himalaya play a critical role as a laboratory to many people studying tectonic processes. The Himalaya offers many opportunities to study all factors that go into mountain building such as continental collisions, erosion, and even climatic changes. In order to understand the structural complexity of the Himalaya orogen, understanding the South Tibet detachment is critical to figuring out the exact time and processes involved in the emplacement of rock units there.

IUGS geological heritage site

In respect of it being 'the place where the highest regional scale low-angle normal fault of the Earth can be directly observed', the International Union of Geological Sciences (IUGS) included 'The South Tibetan Detachment System in the Rongbuk Valley' in its assemblage of 100 'geological heritage sites' around the world in a listing published in October 2022. The organisation defines an IUGS Geological Heritage Site as 'a key place with geological elements and/or processes of international scientific relevance, used as a reference, and/or with a substantial contribution to the development of geological sciences through history.'

References

{{Reflist Geology of the Himalaya Geology of Asia Geology of Nepal First 100 IUGS Geological Heritage Sites