Tuesday, June 20, 2017

Quiz: Spot The Granite Intrusion

I came across this glacially transported boulder in the Duktu village valley near the Panchachuli Glacier in the Kumaon Himalaya.

It is a block of high grade gneiss intruded by a granite. Without scrolling beyond the first photograph, try to work out the contact between the gneiss and the granite.



Answer:

The boulder is encrusted by moss. There is some mineral staining too. And sunlight falling on the rock gives it a speckled appearance.. All this reduces the contrast in color between the gneiss and the granite.

But there is a vital clue in the orientation of structures. Both the gneiss and the granite have a planar fabric imprinted on them.

The fabric of the gneiss is due to the orientation of platy minerals like micas stacked in layers, alternating with layers richer in quartz and feldspars. Assume this is the original disposition of the rock as well. The gneiss layering you see is due to the trace of horizontal planes of separation of different mineral layers. I have outlined some of this planar fabric in brown lines.

The granite has a planar fabric too, but this is due to near vertical fractures. The rock has been broken in to thin slabs  by fractures (red lines) which may have formed during the cooling of the magma. These fractures don't pass into the surrounding host gneiss. Two arms of the granite have penetrated between the gneiss layers forming mini sills.

You can see the contact (black line) between the gneiss and the granite roughly where my wallet is. Here, the horizontal planar fabric of the gneiss abruptly juxtaposes against the vertical planar fabric of the granite.



Thursday, June 15, 2017

Field Photo: Glacial Erratic

Inspired by this xkcd comic:


I saw quite a few of these glacial erratics in the Dhauliganga river valley around the villages of Duktu and Dantu. Here is my friend sitting on one of them.


This boulder is a high grade gneiss. It is an erratic because the surrounding bedrock is all low grade phyllite and slate. The source of the high grade gneiss boulder is the snow capped range you see in the background. These are the Panchachuli peaks and the Panchachuli glacier has eroded, transported and deposited gneiss rocks all the way down the valley onto a different bedrock.

The photo below shows another erratic from this valley. If you look closely it is a mixed rock made up of high grade gneiss intruded by light colored granite. A big patch of dark grey banded gneiss is visible in the lower right corner of the boulder. The cliffs in the background and the substrate on which the boulder rests is low grade phyllite.


And a long view of village Duktu with glacial erratics strewn all over the hill slope (blue arrows).


I have been promising a post on the glacial deposits of the Dhauliganga river valley. That post will come soon. Meanwhile, here is a view of some of the moraines I saw near village Duktu.  Photo taken from near the snout of the glacier facing downstream.


The linear ridge in the center of the photo made up of rust, brown and light colored boulders is a medial moraine. It was formed when two glacial streams carrying debris along their edges joined. As these glaciers receded the debris along their edges (lateral moraines) coalesced and formed a ridge in the center of the valley. You can see the milky white colored Dhauliganga river flowing to the right of the ridge. The blue arrows to the right of the picture high up along the mountain slopes point to older lateral moraines deposited when the Panchachuli glacier was thicker and extended further down in the valley...

more on these deposits later..

Tuesday, June 13, 2017

Books: Origins Of Complexity ; China Water History

These just arrived.

Extract:

I hope to persuade you that energy is central to evolution, that we can only understand the properties of life if we bring energy into the equation..... I want to show you that the origin of life was driven by energy flux, that proton gradients were central to the emergence of cells, and that their use constrained the structure of both bacteria and archaea. I want to demonstrate that these constraints dominated the later evolution of cells, keeping the bacteria and archaea forever simple in morphology, despite their biochemical virtuosity. I want to prove that a rare event, an endosymbiosis in which one bacterium got inside an archaeon, broke those constraints, enabling the evolution of vastly more complex cells. .....Finally, I want to convince you that thinking in these energetic terms allows us to predict aspects of our own biology, notably a deep evolutionary trade-off between fertility and fitness in youth, on the one hand, and ageing and disease on the other.

The last book I read on the evolution of complexity was Mark Ridley's The Cooperative Gene which described the many evolutionary inventions that suppress genomic conflict and make multicellular bodies workable. Nike Lane writes at a more fundamental level of the energy currency of the cell. Feeling very excited about this book. I am sure to learn a lot.

Extract:

But the ubiquitous and ambivalent relationship that the Chinese people have had with water has made it a powerful and versatile metaphor for philosophical thought and artistic expression, and its political connotations can be subverted and manipulated in subtle ways for the purposes and protest and dissent. These meanings of water are more than metaphorical. Because the lives of everyday folk has always depended on water, the river and canals mediate their relationship to the state. Water -too much of it,or too little - has incited the people to rise up and overthrow their governments and emperors. Burgeoning economic growth now places unprecedented pressure on the integrity and sometimes the very existence of China's waterways and lakes. Not only can China's leaders ill afford to ignore this potential brake on economic growth, but the environmental problems are leading to more political pluralism in a nominally one party state.

Sweeping... from the Qin Dynasty (200 B.C.) to the present..

Thursday, June 1, 2017

The Serpents Of Nagling- Granite Intrusions Into Greater Himalayan Sequence Metamorphics

Over chai, elders told us about large serpents invading their village. A curse, they said. Only the correct prayers and purification rituals saved them, forcing the serpents to retreat deep into the forest. Some serpents remain trapped in the rock faces near the village, which was renamed Nagling (Nag means cobra..or more generically serpent).

The picture below are the entombed serpents of Nagling (trekkers for scale).


Geologists recognize them to be granite dykes (intrusions cutting across host rock layering) and sills (intrusions parallel to host rock layering) intruding the high grade metamorphic rocks of the Greater Himalayan Sequence (GHS).

The GHS is a block of the Indian crust bounded between the Main Central Thrust (MCT) at the base and the South Tibetan Detachment System (STDS) at the top. It represents mid crustal material which was metamorphosed and then was extruded and exhumed during Himalayan orogeny between 25 million years ago to about 16 million years ago. These dates vary somewhat along the strike of the Himalaya. Thrusting along the MCT took place earlier in the western Himalaya. Eastern regions like the Sikkim Himalaya record younger dates for the movement of the MCT.

The grade of metamorphic varies within the GHS. The figure below is a schematic section of the Greater Himalayan Sequence. It is from a study on the nature of the MCT by Michael Searle and colleagues from the Nepal Himalaya and is a very useful guide to think about the internal structure of the GHS.


 Source: Searle et. al. 2008

From the base of the MCT the grade of metamorphism increases towards higher structural levels. This is recognized as an "inverted metamorphic gradient", since minerals that are formed at higher and higher temperatures and pressures are occurring at structurally higher and by implication apparently shallower levels of the crust. The inverted gradient is recognized by the successive appearance of  biotite, garnet, sillimanite and finally kyanite. The sillimanite-kyanite zone transitions into the zone of partial melting and granite intrusives. This is the zone where the crust experienced conditions that lead to the formation of in situ melts and their mobilization and intrusion into surrounding rock. Above this zone the grade of metamorphism reduces towards the STDS. In the figure, the granite intrusion zone is directly overlain by the STDS and the Tethyan sequence. However, there is variation in this theme across the Himalaya. In the Kumaon region where I was, the "melt zone" is overlain by a sequence of lower metamorphic grade phyllite rocks.

What caused this melting and production of granitic magma? Many geologist point to the STDS. They suggest that this zone of extentional faulting stretched and thinned the crust, resulting in " decompression-related anatexis". This means that when extentional faulting along the STDS and exhumation reduced the overburden on deeply buried hot rocks, the release in pressure resulted in the lowering of rock melting point. This led to a partial melting of the crust (anatexis). Other geologists disagree with this explanation. They point out that since decompression has a minor effect on melting the likely source rock compositions you would require unreasonably large amounts of denudation along the STDS.  Rather, they suggest that crustal thickening by the continued convergence of India with Asia elevated temperatures in the middle levels of the crust to a range where partial melting began. These melts then moved along weak planes and intruded the surrounding GHS above the sillimanite and kyanite grade gneisses. The main pulses of this magma generation took place between 24 million years and 19 million years ago.

Geologists estimate the temperatures of this melt zone to be around 650 deg C to 750 deg C, corresponding to a  burial depth of about 20-25 km. Yes, the GHS represents crust that has traveled from that depth to the Himalayan heights it now commands by a combination of thrust faulting and erosional unroofing i.e. the stripping away of shallower levels of the crust!

During one of my previous treks in the Kumaon region I had walked across the GHS from the base of the MCT to the sillimanite zone in the Goriganga valley from the town of Munsiari to village Paton. This time, one valley to the east,  we began our trek at village Nagling in the zone of  partially melting. All around us were rock faces intruded by sill complexes and dykes. The picture below shows multiple sills of granite cross cut by dykes.


High up from Nagling village towards Nagling Glacier I saw this granite dyke complex (outlined by red dotted lines ) cutting across metamorphic banding (black lines).


And in the stream near Nagling Glacier I came across this rounded stream boulder showing granite cross-cutting banded migmatitic gneiss.


We traveled north and  reached Duktu. Earlier, somewhere near the village of Baaling, we had crossed the zone of partial melting and were in the uppermost levels of the GHS made up of phyllite grade metamorphic rocks. The phyllites are not intruded by granite.

However, granite was present at Duktu too, but only in the Dhauliganga river bed. This river emerges from the Panchachuli Glacier. The Panchachuli ranges which fall lower in the GHS are made up of high grade gneiss intruded by granite.

As a result, the Dhauliganga river bed near Duktu village is choked with boulders of granite and migmatite rocks.


This is a very distinctive  biotite-tourmaline granite. The picture below shows blocks of granite with tabular black tourmaline.


Here is a picture of me looking intently at a block of GHS made up of a granite intruding in to a gneiss.


And another close up of light colored granite intruding dark grey banded gneiss and encircling and enclosing rafts of the metamorphic host rock (red arrows).


And finally, from the sheer rock faces near Nagling Glacier, one of my favorite examples of the granite intrusions. A near vertical dyke (red broken outline) cut and displaced by a fault (yellow broken lines). Metamorphic banding shown in black lines.


... Pleistocene-Holocene glacial deposits of the Panchachuli Glacier area.. coming up next!

Tuesday, May 30, 2017

On The Moth's Inordinate Love For Salt

I am reading The Forest Unseen- A Year's Watch In Nature by David George Haskell.

During a vigil in the forest, a moth landed on the author's finger and refused to let go.. why? A passage from the book...

Only males have such an exaggerated antennae. They comb the air for scent released by females and fly upwind, guided to a mate by their enormous feathery noses. But finding a mate is not enough. The male must provide a nuptial offering to his mate. My finger provides him with an essential ingredient for this gift.

Diamonds may be the crystal of choice for wooing humans, but moths seek a different, altogether more practical mineral, salt. When the moth mates he will pass to his partner a package containing a ball of sperm and a packet of food. This food is generously seasoned with sodium, a precious gift that looks forward to the needs of the next generation. The female moth passes the salt to the eggs and thus to the caterpillars. Foliage is deficient in sodium, so the leaf-munching caterpillars need their parents salty bequest. The moth's arduous attachment to my finger prepares him for mating and will help his offspring survive. The salt in my sweat will make up for the deficiencies in caterpillar diets.

Full of nuggets like this... recommended.

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. This is the famous Darma Valley trek in the Dhauliganga river valley of the Kumaon Himalaya. For years, the trek began at village Sobla, about an hours drive north of Dharchula. From Sobla, it is a two to three day hike to Duktu, which is the base village to approach the Panchachuli Glacier. However, when we went there in the first week of May 2017, a serviceable road had reached village Nagling. This cut two days of our walk. We began our trek at Nagling. It was a days walk to Duktu. There I got a chance to look at the geology and especially search for an important fault zone I had been wanting to see. I begin this post with some geology background and then my observations during our walks around Duktu.

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 northern extent of the Indian plate would have been made up of Archean granite and granite-gneiss terrains representing the earliest stable crust and greenstone belts (metamorphosed and deformed volcano-sedimentary rocks). On this Archean-lowermost Proterozoic foundation were deposited successions of sedimentary packages, some intruded by igneous rocks. These range in age from the Mesoproterozoic to the Eocene.  The Archean and lowermost Proterozoic rocks are not exposed anywhere in the Himalaya. The Lesser Himalaya Sequence and the Greater Himalaya Sequence are slices of crust containing the Mesoproterozoic to Phanerozoic successions which have been metamorphosed to varying degrees during Himalayan orogeny. The Tethyan Sedimentary Sequence represents late Neoproterozoic to Eocene successions which largely escaped metamorphism during Himalayan orogeny.

In the early Cenozoic, when the Indian plate impinged into Asia this Neoproterozoic to Eocene 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 Duktu (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 right 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.

Thursday, April 13, 2017

Oceanic Crustal Thickness Since The Breakup Of Pangea

Of interest:

Decrease in oceanic crustal thickness since the breakup of Pangaea - Harm J. A. Van Avendonk, Joshua K. Davis, Jennifer L. Harding and Lawrence A. Lawver

Earth’s mantle has cooled by 6–11 °C every 100 million years since the Archaean, 2.5 billion years ago. In more recent times, the surface heat loss that led to this temperature drop may have been enhanced by plate-tectonic processes, such as continental breakup, the continuous creation of oceanic lithosphere at mid-ocean ridges and subduction at deep-sea trenches. Here we use a compilation of marine seismic refraction data from ocean basins globally to analyse changes in the thickness of oceanic crust over time. We find that oceanic crust formed in the mid-Jurassic, about 170 million years ago, is 1.7 km thicker on average than crust produced along the present-day mid-ocean ridge system. If a higher mantle temperature is the cause of thicker Jurassic ocean crust, the upper mantle may have cooled by 15–20 °C per 100 million years over this time period. The difference between this and the long-term mantle cooling rate indeed suggests that modern plate tectonics coincide with greater mantle heat loss. We also find that the increase of ocean crustal thickness with plate age is stronger in the Indian and Atlantic oceans compared with the Pacific Ocean. This observation supports the idea that upper mantle temperature in the Jurassic was higher in the wake of the fragmented supercontinent Pangaea due to the effect of continental insulation.

Continental insulation refers to the idea that an unbroken continental crust such as that provided by a supercontinent may act as a blanket resulting in a slow build up of heat over tens to hundreds of millions of years in the underlying mantle. Eventual continental breakup will lead to enhanced magmatism and thicker ocean crust along these previously insulated regions.

The Pangaean paleogeography of the Triassic (252 million to 201 million years ago) is depicted in the map below. The distribution of continents is lopsided covering the sites of the future Atlantic and Indian Oceans.


 Source: Paleobiology Navigator