Geology Outreach: Darma Valley, Uttarakhand

A couple of weeks ago, in partnership with Deep Dive India,  I had taken a group of nature lovers from Bengaluru to the Himalaya for a geology outreach week. We traveled across a section of the Lesser Himalaya up to the town of Dharchula, and then headed north along the Kali valley and then the Dhauliganga valley to the area around the Panchachuli Glacier from where the river Dhauliganga (Darma river) emerges. The picture on the left shows our group at an outcrop of high grade metamorphic rocks. Picture credit: Asha Kini.

The participants were a mix of IT professionals, Chartered Accountants and Business Management executives. And they were an enthusiastic bunch. This was my first Himalaya outreach attempt and I was a bit nervous. But these people made my job much easier with their curiosity and active participation.

The map below shows our route in red.

Source: Geology, Structural and Exhumation History of the Higher Himalayan Crystallines in Kumaon Himalaya, India- R.C. Patel et. al. 2011

During our journey towards Dharchula and ahead, we drove across and learned about 'Klippen'. Beginning about 23 million years ago and continuing until around 15 million year ago, large faults (thrust faults) moved sheets of the high grade metamorphic Greater Himalaya and the oldest rocks of the Lesser Himalaya southwards, and placed them above lower grade metamorphic rocks of the Lesser Himalaya. Subsequently, erosion removed portions of these thrust sheets, leaving behind outliers or islands (Klippen) of these high grade rocks surrounded by the lower grade Lesser Himalayan rocks. We traveled across the Almora, Askot and Chiplakot klippen on our way to the start of our trekking point, which was north of Sobla. The map above shows the Chiplakot klippen surrounded by Lesser Himalayan rocks. 

Dharchula is situated on the low grade metamorphic rocks of the Lesser Himalaya Sequence. A little north of this town, we crossed into the Chiplakot Crystalline Belt (klippen), which is a  high grade metamorphic belt correlated with the  Munsiyari Formation (see map).  The Munsiyari Formation is considered the oldest unit of the Lesser Himalaya Sequence, made up of rocks metamorphosed to a higher grade. It contains the oldest rocks in the Himalaya, a very characteristic augen gneiss (named after the eye shaped clusters of quartz and feldspar), dated to 1.9 billion years.The Chiplakot Crystalline Belt and the Munsiyari Formation rocks both formed by extensive magmatism that was taking place along the Indian northern continental margin in the Paleoproterozoic (~1.9-1.8 billion years ago). These magmatic events were triggered by converging continental blocks, their eventual collision and suturing leading to the formation of a supercontinent known as 'Colombia'.

Just north of Sobla, we encountered the Greater Himalaya. The Main Central Thrust, known locally as the Vaikrita Thrust (VT), places these rocks on top of the Lesser Himalaya Sequence. We remained in this rock group for the rest of the trip. The Greater Himalaya in this area are made up of garnet to sillimanite grade gneiss, mica garnet schists, and migmatites, intruded by leucogranite sills and dikes. These leucogranites formed by the partial melting of buried Indian crust between 24 million and 16 million years ago. The picture, taken near Baaling village,  shows a leucogranite intruding gneiss. Arrows point to fragments of host rock entrapped in the intrusive magma.

Near Dugtu, we caught glimpses of the Tethyan Sedimentary Sequence high up on the ridges to the east and north of the village. And we found boulders of conglomerates and sandstones dislodged from these Tethyan rocks in small streams joining the Dhauliganga river. We also did a memorable walk along the banks of the Dhauliganga river right up to the point it emerges from an ice cave at the snout of the Panchachuli Glacier.

All along our route we stopped for geology observations at selected locations where lithologic breaks, rock folding, and fault zones could be seen. I gave the group small puzzles to solve, wherein they had to use their powers of observation and reasoning to come up with answers on the type of rocks, the sources of pebbles in streams, and differences between river and glacial deposits. In the evenings, informal discussions continued over piping hot delicious meals of roti, subzi, dal, and rajma.

I won't write in detail about the geology of this region, since I have covered it in an earlier post that I wrote when I visited this region two years ago. Please read that post titled 'Chasing the South Tibetan Detachment'.

I will make one addition to the geology covered in that post. Just north of Baaling village there is a sudden change in lithology. High grade gneiss, migmatites and leucogranites, formed at temperatures between 750-800 deg C, are overlain by lower grade metamorphic rocks (400-500 deg C) made up of slates, phyllites and greenschists (minerals like biotite, chlorite and actinolite). These lower grade rocks are locally named Budhi Schist. I could not see the contact between the two lithologic groups since the hillsides along the trail was covered with rubble and forest patches. The change seems to occur a few hundred meters north of Baaling.

I had earlier put this down to a continuous change in pressure temperature conditions within the Greater Himalaya Sequence. But walking across the lithologic transition one can notice the steep change in pressure temperature conditions as evidenced by the different mineral assemblages of the rocks, the absence of significant leucogranite in the lower grade rocks, the presence of dilation and en echelon fractures (evidence of stretching and tensile forces) in this zone, and the strong contrast in folding style between the two rock groups. Folding in the high grade rocks (upper pic) is manifest as ductile flow of dark and light colored mineral domains into wavy,  sigmoidal patterns, rootless isoclinal folds (light or dark colored mineral domains contorted into isolated folds) and ptygmatic folding of quartz-feldspar rich layers (the more competent quartz feldspar layers gets contorted into tight chaotic folds,while the softer surrounding layers flow around it) . In contrast, the strata in lower grade rocks show tight isoclinal and recumbent folding (outlined in  yellow) which can be traced over tens of meters. This indicates that the two rock groups were deformed at different depths under different rheologic conditions.

These abrupt changes in lithology and presence of extensional stress indicators strongly suggest that this transition is bracketed by a northerly dipping ductile shear zone (deeper crustal equivalent of a fault zone along which rocks are deformed and displaced) which separates lower grade hanging wall rocks (block above fault plane) formed in shallower levels of the crust from deeper crustal level and higher grade footwall rocks (block below fault plane). Lower grade hanging wall rocks juxtaposed against higher grade footwall rocks implies normal faulting.

Ideally, shear zones need to be recognized on structural criteria, i.e. the appearance of oriented structures in the rock fabric that indicate the sense of movement. Not having the required structural geology skills, I couldn't document accurately the shear sense (direction of displacement), but previous work carried out on this shear zone shows fabrics indicating a phase of top to the north-northeast normal shear, which means that the hanging wall rocks have been displaced downwards in a northerly direction. 

In the Central Himalaya two strands of the South Tibetan Detachment ( a network of extentional or normal faulting) have been recognized. The shear zone at Baaling likely represents the structurally lower strand of this fault system. The upper strand of this fault zone is present north of Dugtu village and brings into contact unmetamorphosed sediments of the Tethyan Sequence in the hanging wall with lower grade metamorphic rocks (Budhi Schist) in the footwall. 

I'll post below a few pictures of the landscapes around Naagling and Dantu villages. People of the Bhotiya tribes live in this region. We were at about 10,000 to 11,000 feet ASL. These villages  are abandoned for the winter as inhabitants move to lower altitude towns like Dharchula to spend the cold season. People start migrating back in the month of May. When we arrived, only a few families had made their way back. As a result, most villages had an empty feel around them.

1) High grade metamorphic massifs of the Greater Himalaya seen from Naagling.

2) Early morning sunshine hits Dantu Village.

3) Beautiful earthy homes and icy ranges in the background seen at Dantu.

4) Panchachuli Peaks seen from Dantu.

5) Village Goe basking in the sunshine.

6) Golden hues in the countryside around Philum village.

7) The Lassar Yankti valley (tributary of Dhauliganga) seen from Baun village looking north.

8) The picture postcard Baun village.

 9) Realm of the shepherds. Lush meadows with the Greater Himalaya looming all around. Near Baun.

10) Explaining the origin of the Himalaya to the Geo group. Picture credit: Samir Kher.

11) And.. that's me standing at the snout of the Panchachuli Glacier. You can see the river Dhauliganga emerging out of an ice cave. Picture credit: Prakash.

Overall, it was a great learning experience for me. And from the feedback I got, all the participants enjoyed it thoroughly too.

I will be doing this again!