In October I had the great opportunity to visit Nepal for nearly a month. As part of a trekking group (organised by Dhamma Yatra) I spent close to two weeks hiking up and down the Kali Gandaki valley, which is the deepest gorge of the world as it is set between two 8000+ m high peaks, the Dhaulagiri and the Anapurna I. In this post I want to show you some of the awesome geology one can see in this part of Nepal!
Geology of Nepal
The geology of Nepal has been marked by the India-Eurasia continent-continent collision which started about 65 million years ago and is still ongoing today. Since the start of the collision there has been a crustal shortening of about 2500km (which is basically as much as India is from N-S!) and much of that shortening can be found in the worlds highest mountain range, the Himalaya, which makes up most of Nepal. The shortening is accommodated by deformational features such as folds and thrust faults. Folds occur when previously flat lying layers of rock are bend or curved while thrust faults are faults on which older rocks have been pushed over younger rocks. I have previously written about faults and fold here and here. Anyhow, big thrust faults divide Nepal into several tectonostratigraphic units (read: different rock types of different ages), with some units showing not much deformation (e.g. Sub himalaya) while other units have been greatly deformed and are made up of metamorphic rocks (e.g. Greater Himalaya). Those thrust faults are also the sliding planes on which earthquakes occur commonly, such as the devastating earthquake that occurred on April 25th 2015 and was triggered along the Main Frontal Thrust, which is located in a depth of ~8km at the epicentre (note that it is shown much further south on the map below as that is where it crops out on the surface).
The former kingdom of Mustang lies mainly in the Tethyan Himalaya (blue on the map above), which consists of slightly metamorphosed and deformed sedimentary rocks that formed in the ocean that once was between Asia and India! That is something pretty cool, the rocks that now from high mountains (up to 8000m+, e.g. the Anapurna I or the Dhaulagiri) formed in the ocean in up to several km water depth 🙂 As the rocks have only been slightly deformed one can still see sedimentary features and can find fossils. Overall the Tethyan sequence has a thickness of about 11km (Parsons et al., 2016).
Something that is not shown on the geological map is that erosion of the high mountains has formed younger rock layers that now pile up in the valleys of the Himalaya. In Mustang there is a thick (~500-800m) sequence of rock fall and slides, debris flow deposits, alluvial fans and fluvial deposits and glacial deposits, which in turn are cut through by the river Kali Gandanki.
I think studying these erosional deposits might be very interesting as the history of the mountain building and erosion could be understood in more detail. As far as I am aware there is no publication describing the rocks in detail out there yet. So if you are looking for a cool mapping/PhD project you just need to find a suitable supervisor and some funding! One thing is sure, the landscape will take your breath away and the local people are very courteous and friendly 🙂