Thursday, May 25, 2017

Chasing The South Tibetan Detachment- Panchachuli Glacier Area Kumaon Himalaya

This is a geology travelogue of my recent trek to the Panchachuli Glacier.

The South Tibetan Detachment System (STDS) is an important fault zone in the Himalaya, bringing in to structural contact the Tethyan Sedimentary Sequence (TSS) with the underlying metamorphic rocks of the Greater Himalayan Sequence (GHS). It is a northerly dipping extensional or normal fault. This means that the Tethyan sediments which make up the hanging wall of the fault have moved down relative to the footwall made up of the Greater Himalayan Sequence. As the name suggests the STDS is most prominently developed in the southerly Tibet plateau like physiographic province of the Himalaya, north of the great Himalayan summits.

What is the history of the TSS and the STDS? As is the case with Peninsular India, the foundation of the northern extent of the Indian plate is made up of Proterozoic rocks which were deformed and metamorphosed during the Mesoproterozoic times (1.8 billion to 1 billion years ago). On this "crystalline basement" were deposited a succession of sedimentary rocks ranging in age from the Neoproterozoic to the Eocene.

In the early Cenozoic, when the Indian plate impinged into Asia this sedimentary "cover" was folded, faulted and scraped off to form an early "Tethyan" mountain range. As collision continued and as lower tiers of the Indian crust subducted under Asia, thrust faults moved slices of deeply buried and metamorphosed crust upwards. These slices are the Greater Himalayan Sequence. They are bounded by the Main Central Thrust at its base and by the STDS at the top. Concurrent with the movement of the Main Central Thrust, fault zones developed at the base of   the Tethyan sedimentary cover, perhaps along the same planes of breakages that had earlier uplifted the Tethyan ranges. This fault zone evolved into the STDS.

There are different hypothesis on how important the STDS is to the evolution of the Himalayan orogen. One school of thought suggests that the extention and thinning of the crust along the detachment zone accelerated the exhumation of the deeply buried GHS and brought these deeper levels of the crust in to structural contact with the Tethyan cover sequence. Alternative scenarios argue that thrust faulting played a more prominent role in the southward propagation and exhumation of the GHS with the STDS playing only a minor role in the exhumation of the footwall GHS.

Whichever scenario is correct, there is no doubt that the South Tibetan Detachment is a major structural boundary separating two distinct lithologic terrains.

The outcrops around me during my trek where all metamorphic rocks of the Greater Himalayan Sequence. I had hypothesized that the South Tibetan Detachment and Tethyan rocks if they indeed were present in the area would be making up the summits of the ranges around Duktu and in the Tidang area.

After days of observation I was proved right about that. I used three types of indicators to infer the  presence of Tethyan sedimentary rocks high up on the summits and to recognize the fault boundary between them and the underlying Greater Himalayan Sequence metamorphic rocks.

1) Structural discordance between the Greater Himalayan Sequence and the Tethyan Sedimentary Sequence. This could be clearly seen near the summits of the ranges north and east of Duktu.

2) Boulders of sedimentary rocks like conglomerate and planar and cross bedded sandstones in the streams draining these ranges.

3) Dilation fractures in both the phyllite grade metamorphic rocks of the Greater Himalayan Sequence and in sandstones of the Tethyan Sedimentary Sequence. This indicated the presence of an extensional stress regime. The South Tibetan Detachment is a zone of normal faulting. The crust has been broken and pulled apart by tensional forces. These stresses were felt over a broad zone and impacted the footwall and hanging wall rocks.

As I am writing up these three criteria I have to admit that my thinking about these lines of evidence was not at all clear when I started the trek. Rather, my ideas and understanding of the local geology evolved haphazardly over the days as I walked the valley and started noticing structural orientations, stream rubble and fracture patterns.

We began our trek at Nagling village. Our destination was the village of Dukti (Lat 30.2486, Long 80.5460). We walked northwards. As Himalayan thrust sheets dip north, we were going structurally higher and higher up the Greater Himalayan Sequence. At, and ahead of Nagling, we were in a zone of partial melting and granite intrusions. High grade gneiss and migmatites were intruded by dykes and sills of granite. I'll be posting about this section separately. This high grade gneiss zone was overlain by a sequence of phyllite grade metamorphic rocks. These phyllites show tight isoclinal and recumbent folding. The internal structure of the Greater Himalayan Sequence is interesting. There is an increase in metamorphic grade from the base to the higher levels and then a decrease towards the very top.

Above the phyllite grade rocks separated by the STDS are the Tethyan sediments. I figured I would have traveled north enough, i.e. structurally high enough along the GHS to cross the phyllite zone and into the overlying Tethyans. I had an expectation that at the very least I would notice them capping some of the ranges I was going to encounter between the villages of Duktu and northward towards Sipu.

Here is an interactive map of the area I trekked, which you can use to follow the text and check on the locations of the samples.

Day1 - The northerly walk takes a left turn as we enter the Panchachuli Glacier valley. The river Dhauliganga is a west to east flowing river in this valley near Duktu village. We entered the village of Duktu in pouring rain. Every mountain range was covered in clouds, and in any case the rain was heavy enough to keep us indoors for the evening.

Day 2- More rain! It happened during my last trek in the Munsiari valley too. We go stuck there for two days due to heavy rain and snow. As it happened, the rain stopped by afternoon and we could go for a short walk to the twin village of Dantu across the Dhauliganga river. The river bed was chocked with boulders of a distinct biotite-tourmaline bearing granite (Picture to the right). Both Duktu and Dantu villages are located in phyllite grade rocks. This conspicuous granite does not intrude these rocks. Its source lies in the Panchachuli ranges, lower in the GHS. The Panchachuli Glacier has gouged it from the Panchachuli ranges and transported the debris to this valley. All the summits were still covered by clouds and I was resigned to wait it out for any further observations of the geology.

Day 3- Perfect weather. It was bright and sunny. But I hardly did any geology this day. We took a spectacular 5 kilometer walk westwards to the Panchachuli glacier.  The terrain was covered by forest, shrubs and grass and higher up by ice. We walked along the lateral moraines of glaciers past. The Panchachuli glacier was much bigger during the Pleistocene ice ages and glacial deposits are piled up high in the valley. I'll be posting on these deposits too. If only I had just glanced to the east of Duktu and looked carefully at the ice snow covered ranges!!

Day 4- Great weather again! We took a northerly course towards the village of Tidang. Our original plan was to walk up further north to the village of Sipu. However,  the ITBP (Indo-Tibetan Border Police) were restricting movements of civilians in that area and we got a nod to go only to Tidang on a day trip. This is fantastic terrain. We passed through pine forests and then into a landscape of open woodlands and scrublands. We were now in the Lassar Yankti valley. The picture below shows a north facing view of the Lassar Yankti valley.

 This river joins the Dhauliganga near the village of Duktu. There were enormous mountain ranges on both sides of the valley. Here below is a view of the mountain ranges on the right bank of the Lassar Yankti near the village of Dakad.  The north dipping rock faces in the foreground are Greater Himalayan Sequence phyllites. I had a feeling that if there were Tethyan sediments here they would be making up the summits of the range in the background.   I was keeping my eyes peeled for anything interesting.

And soon I began noticing that phyllite grade rock fragments scattered along scree slopes showed dilation fractures (Pic to the left). These fractures occur when the crust is being subjected to tensile forces. I now strongly suspected that these upper structural levels of the GHS were close enough to the STDS to have experienced extensional stresses. The picture show a phyllite grade rock with foliation displaced along a fault (black line) and showing dilation fractures (above) and another phyllite with parallel sets of dilational fractures (below). The fractures have been filled or healed with secondary quartz.

We passed the village of Dakad (Lat 30.2756, Long 80.5291). A few hundred meters ahead I had the first of the big "aa-haa" moments of the trek. A large boulder of sandstone showing planar and cross bedding lay just a few meters aside of the trail. It must have been transported there either during a rock fall or by glaciers from high up on the ranges on the left bank of the Lassar Yankti. A few minutes ahead we came across a stream draining these ranges and joining the Lassar Yankti. In that stream near the bridge connecting to village Tidang I saw a conglomerate boulder (Lat 30.2822,  Long 80.5262). Sedimentary rocks of the TSS were definitely present high up in that range. Here is a picture of the cross bedded sandstone (above) and the conglomerate (below).

Looking up towards the ranges, I could not identify a lithologic or structural boundary, but the presence of dilation fractures and sedimentary debris pointed to the presence of the STDS and the TSS high in those ranges.

Day 5- The weather Gods were kind again. We trekked westwards from Duktu along the left bank of the Dhauliganga river towards the terminal moraine of the Panchachuli glacier. The rock walls on the left bank of the river were phyllite grade rocks. Again, I found dilation fractures in them. And in a small stream draining those ranges... another conglomerate (Pic to the right) ! (Lat 30.2471, Long 80.5181). I looked up to the summits carefully. Perhaps my viewing angle was just right or perhaps my mind was now better prepared but... there it was... a clear structural discordance between steep northwesterly-dipping rocks and the overlying more gently northeasterly-dipping rocks. I was looking at the South Tibetan Detachment Fault that had placed Tethyan sediments over the Greater Himalayan Sequence (picture below; join the tips of the arrows to trace the detachment fault).

I then looked through the valley straight towards the ranges to the east of Duktu. Again, that same structural discordance was clearly visible in the snow capped summits. The picture below (photo credit: Swati Pednekar )  shows this eastern range, the detachment fault (join the tips of the arrows to trace the fault) and the lithologic units.

Day 6- A trek to villages of Goe, Philam and Bon. We walked north from Duktu, crossed the Lassar Yankti river a little ahead of Dantu village and entered village Goe (Lat 30.2602, Long 80.5411) and then walked southwards. That morning I had confidently predicted that we would find sedimentary rock with dilation fractures on this trail. These villages are at the base of the ranges shown in the picture above. Although not diagnostic, there was another strong hint that these ranges had sedimentary rocks at the summits. The summit rocks have weathered into a blocky square edged pattern typical of jointed sandstones and quartzites.

And I was right! Sandstones along with low grade phyllite rocks (from the lower levels of the mountain) were being used to build walls and pavements in all the three villages. Picture on the left (above) shows a cross bedded sandstone block making up part of a wall in village Goe. And a cross bedded sandstone slab (left, below) is being used as a pavement stone for a village trail between Goe and Philam. Further south ahead of village Bon, a large stream draining these mountains contained boulders of bedded sandstones. And at a small bridge at the bottom of the valley  (Lat 30.2370, Long 80.5450) I came across a sandstone block (picture below) with slickensides (black arrows) and dilation fractures (red arrows). Slickensides are striations on rock surfaces formed by frictional movement of rocks along a fault. This was a strong indicator that these sandstones were sourced from an extentional fault zone high up near the summit.

We continued walking southwards, into lower levels of the GHS. Soon, we were back in the Nagling area, in the zone of partial melting and granite intrusions.

This ended our trek in the Panchachuli Glacier area. To date, it was the most satisfying trek I had done in the Himalaya. Although the STDS was high up and I could not actually walk across it, I had hypothesized, made observations and validated my expectations of the presence of the detachment faults and Tethyan sedimentary rocks. This would be a good field exercise for students! And I am hoping this post will be used by trekkers wanting to explore and understand the geology of this area.

Day 7- We trekked to the Nagling Glacier which has carved a perfect U shaped valley. Certainly one of the most beautiful sites I have been to.

... more geology posts on glacial deposits and granite intrusions... coming soon.. !

Tuesday, May 16, 2017

Landscapes: Panchachuli Glacier And Lassar Yankti River Valley Kumaon Himalaya

I'm back. It was epic. There was geology. I saw the South Tibetan Detachment fault zone. I saw rock deformation. I saw Pleistocene -Holocene glacial deposits. I saw glaciers... I trekked, I photographed, I lived with the local nomads and farmers.

I need a little time to write more on the geology. Meanwhile, here is a glimpse of the absolutely wonderful landscape I wandered through for the past couple of weeks.

Here is an interactive map of the area I traveled through.

and these lands...

1) The crown jewels of the region- The Panchachuli Range seen from village Dantu. There are five peaks. From this angle, the fifth is hidden behind the peak on the left.

2) Sunrise at the village of Nagling.

3) Himalayan valleys, forested slopes and snowy peaks. En route from Nagling to Duktu. View looking south towards Nagling.

4) Village Baaling with northerly dipping metamorphic rocks of the Greater Himalayan Crystalline Sequence

5) Climbing towards the Panchachuli Glacier. This is a superb 2 hour walk from village Duktu passing through birch and pine forests, scrubland, meadows and finally glacial moraines and ice.

6) On the Glacier! About 13,500 feet ASL.

7) Terminal Moraine and the place of origin of the river Dhauliganga.

 8) The Dhauliganga river with biotite-tourmanline granite boulders sourced from the Panchachuli massifs. This is a Miocene granite intrusive into the Greater Himalayan Crystalline Sequence metamorphics.

9) The Lassar Yankti river valley with village Goe at a distance.

10) View from village Tidang of the surrounding rock massifs. The northerly dipping rock slabs are phyllite to medium grade metamorphic rocks of the Greater Himalayan Crystalline Sequence.

11) Another view of the Lassar Yankti river from village Tidang

12) View from village Philam looking east towards some impressive mountains. These are mostly made up of phyllite grade metamorphic rocks of the Greater Himalayan Crystalline Sequence... but with mystery rocks at the very top! 

13) A little piece of heaven. Nagling Glacier over the Pleistocene ice ages has carved a perfect U shaped valley

 14) Village Duktu. We were close to ten and half thousand feet ASL here. Most of these villages were still uninhabited. People who had migrated to lower altitudes the previous November had locked up by placing wooden shafts and thorny scrub branches against their doors to ward of evil spirits...  and I suspect the occasional Himalayan bear who might fancy hibernating in their home. When we reached here, villagers were just beginning to return with their livestock for their summer stay.

 15)  Relaxing at village Dantu with my friends.

 16) Mystery solved. That's me pointing to the South Tibetan Detachment Zone.

How did I figure that out? What were the geological indicators?.. Coming soon!

Sunday, April 30, 2017

Gone Hiking! Panchchuli Glacier And Beyond- Kumaon Himalaya

I'm leaving today for a trek in the Kumaon Himalaya, Uttarakhand. The destination is Panchchuli Glacier in the Darma Valley. We will also be going over on to the next ridge to the east and hiking toward the village of Tidang and finally Sipu in the Lasser Yankti river valley, gateway to Ralam Glacier.

I've embedded below an interactive map of the area.

The Panchchuli Glacier base camp is around 13,900 feet ASL. From what I've heard from friends and the pictures I have seen, the trek offers some pretty stunning views of the Himalaya. Hopefully I'll come across some interesting geology too. This time I made a decision not to read up on the geology. My recent Himalaya trips have given me some familiarity with the lithology and structure of the region. I am guessing that most of the early part of the trek will be in the hanging wall of the Main Central Thrust. High grade metamorphic rocks of the Greater Himalaya Crystalline Sequence are exposed here. Towards the village of Sipu I am hoping to get a glimpse (even at the distance will do!) of the Southern Tibetan Detachment, a fault zone that separates the Greater Himalaya metamorphic rocks from the overlying Tethyan sedimentary sequence.

Let's see.

I'll be posting on my trip later in the month of May. Depending on connectivity I may be able to send a few field dispatches via Twitter.

Stay tuned.


Friday, April 28, 2017

Himalayan Gravel Flux And Flood Risk

Why should an understanding of sediment transport distance and whether that sediment gets broken down into coarser gravel or finer sand be of any practical use?

Here is a good example from the Himalaya.

Abrasion-set limits on Himalayan gravel flux- Elizabeth H. Dingle, Mikaƫl Attal & Hugh D. Sinclair

Rivers sourced in the Himalayan mountain range carry some of the largest sediment loads on the planet, yet coarse gravel in these rivers vanishes within approximately 10–40 kilometres on entering the Ganga Plain (the part of the North Indian River Plain containing the Ganges River). Understanding the fate of gravel is important for forecasting the response of rivers to large influxes of sediment triggered by earthquakes or storms. Rapid increase in gravel flux and subsequent channel bed aggradation (that is, sediment deposition by a river) following the 1999 Chi-Chi and 2008 Wenchuan earthquakes reduced channel capacity and increased flood inundation. Here we present an analysis of fan geometry, sediment grain size and lithology in the Ganga Basin. We find that the gravel fluxes from rivers draining the central Himalayan mountains, with upstream catchment areas ranging from about 350 to 50,000 square kilometres, are comparable. Our results show that abrasion of gravel during fluvial transport can explain this observation; most of the gravel sourced more than 100 kilometres upstream is converted into sand by the time it reaches the Ganga Plain. These findings indicate that earthquake-induced sediment pulses sourced from the Greater Himalayas, such as that following the 2015 Gorkha earthquake, are unlikely to drive increased gravel aggradation at the mountain front. Instead, we suggest that the sediment influx should result in an elevated sand flux, leading to distinct patterns of aggradation and flood risk in the densely populated, low-relief Ganga Plain.

Behind paywall, but I thought this is a good illustration of how insights into very fundamental earth processes can potentially help save lives.

Wednesday, April 19, 2017

Evolution Of The Konkan-Kanara Coastal Plain

The Konkan coastal plains is a beautiful getaway from west coast city life. Palm fringed beaches, quiet rivers and estuaries, betel nut plantations and forest tracts. Small villages and settlements dot the landscape. To the east, the coastal plains abut against the imposing Western Ghat escarpment.

How did this coastal plain of Maharashtra form? (Kanara refers to the stretch south of Maharashtra in the state of Karnataka).  I came across a paper by Mike Widdowson on the evolution of laterite in Goa. It also has a broader discussion on the conditions that led to the formation of geomorphology of the coastal lowlands extending all along the west coast of India.

Here it is summarized nicely in this figure below:

Source: Evolution of Laterite in Goa: Mike Widdowson  2009

After Deccan Volcanism ended, rifting of the Indian west coast and down faulting of the western side led to the formation of a west facing fault scarp. Erosion of this scarp over the early mid Cenozoic (from about 60 million years ago) has caused it to retreat eastwards. The Western Ghat escarpment is this retreated scarpThe coastal plain formed as an erosional surface that became broader and broader with the progressive eastward retreat of this cliff to the current location. The fault which caused the western side to subside thus lies in the Arabian Sea along the west coast.

In Mid-Late Miocene (~10 million years ago), a phase of humid climate resulted in intense chemical weathering of the basalts and pediment (rock debris) exposed along the coastal plains. This alteration of the basalts formed thick iron rich soils. The reddened and indurated crust of this soil is commonly termed laterite. In the Western coastal lowlands this laterite may be a few meters thick.

Subsequent uplift of the west coast and concomitant down cutting by west flowing rivers formed a dissected landscape composed of laterite capped mesas (table lands) and entrenched meandering streams. These mesas reach altitudes of 150-200 m in the eastern parts of the coastal plain. Nearer the coast they are about 50 -100 m above sea level. 

The western margin of India has seen multiple episodes of extensive laterite formation. The famous table lands of the hill stations of Panchgani and Mahabaleshwar are also made up of laterite. They occur at altitudes of around 1200 m to 1500 m.  However, this upland or high altitude laterite is much older, having formed about 60- 50 million years ago in the early Cenozoic, soon after Deccan volcanism ended. The Konkan and Goa lowland laterites point to another younger phase of laterization. Sheila Mishra and colleagues have identified two more surfaces in the Deccan Traps at 650 m ASL and 850 m ASL that preserve remnants of laterite cover. This suggests a complex polyphase history of denudation and chemical weathering and tectonic stability of the Sahaydri ranges of the Western Ghats.

The sea cliffs that one encounters as you travel along the Konkan and Goa coastline are a result of a late Cenozoic uplift. I remember with fondness a trek I did during my college days from the town of Ratnagiri south to the town of Malvan. There were absolutely majestic sections where we walked on the edge of laterite capped sea cliffs with the Arabian Sea heaving and thundering below us. Little coves and beaches of sparkling white sand lay between the cliffs. Here and there local fisherman had kept their fish catch to dry out in the sun. The pungent smell urged us on!

The satellite imagery below shows a section of the coastal plains from Ratnagiri in the north to Devgarh in the south. White arrows point to the laterite capped table lands dissected by stream networks. Orange arrows point to sea cliffs. Black arrows shows the Western Ghat escarpment.

This is a very interesting paper. Open Access.