Shear Luck Near Sunderdunga River

As I settled down for lunch by the Sunderdunga riverside during my recent Kumaon trek, I noticed a polished boulder nearby. It had a striking appearance dominated by a large crystal of feldspar set in a much finer grained material. This finer matrix had a pronounced streaky fabric, as if made up of very fine layers. Upon closer examination, these layers or foliation was due to the planar arrangement of minerals like amphiboles, mica, quartz, and feldspar. The larger eye catching feldspar grain in the center of the boulder seems a little flattened along one axis and elongated along the orthogonal, giving it a crude sigmoid shape.

I had chanced upon a rock caught up in a shear zone. These are fault zones where movement of the crust causes intense rock deformation. The type of deformation I observed typically occurs at a deeper level where high temperatures make rocks soft and ductile. Rocks caught up in fault zones at shallower levels undergo brittle deformation. They have a broken appearance, made up of sharp edged fragments set in a crushed finer matrix. The rock is fractured, and these cracks get filled with minerals like calcite and quartz. 

This typical brittle like deformation was absent in this rock. There was no sign of any fracturing and breakage of the rock. Instead, the finer grained minerals seemed to flow around the larger feldspar crystal. Grain size reduction occurs by plastic rearrangement of atomic layers and recrystallization of softer minerals during deformation. The stronger resistant minerals which remain as large crystals are called porphyroclasts. Since rocks are sliding past, there is a rotational component to deformation also. Larger grains often show signs of being rotated, while finer groundmass wraps around.

The end stage of such ductile deformation are rocks known as mylonites. These have a flinty or glassy appearance due to the extreme grain size reduction.

I suspect this particular rock has not quite reached the mylonite stage. Let us call it a protomylonite. It does show a clear contrast between the finer matrix made up of stretched and elongated minerals and a large porphyroclast.  

The asymmetry of the porphyroclast gives geologists an idea of the sense of motion along faults. The annotated photo below shows the relative sense of shear or motion.  

Hundreds of such measurements have been made in the Greater Himalaya. When measured in-situ,  the direction of relative motion is 'top to the south', indicating the general direction of movement of Himalaya thrust faults.  Deformation is not uniformly distributed throughout the Greater Himalaya but appears restricted to narrow zones. These zones of intense shearing containing deformed rocks including mylonites have allowed the recognition of  major thrust fault zones such as the Main Central Thrust which emplaces the Greater Himalaya slab on top of the Lesser Himalaya.

There are minor shear zones too. I think this rock was eroded from one such shear zone in the Sunderdunga valley. 

Coming back to the brittle versus ductile deformation regimes. Almost all the deformation you observe in the Greater Himalaya took place in the ductile regime. Here are a few examples from the Greater Himalaya of ductile deformation seen in schists and gneisess. These are my observations from various treks in the Kumaon. 

A cross section of the Himalaya is presented here to showcase the metamorphic gradients along the Greater Himalaya slab (green). For this reading, you can ignore the rest of the Himalaya orogen shown in the figure. Temperature gradient increases towards the core of the slab with kyanite (k) and sillimanite (sill) as the prime high grade metamorphism indicators. This example is from the Nepal Himalaya, but the arrangement of the different Himalaya divisions is identical in adjacent Kumaon. 

 Source: Mike Searle et.al. Tectonophysics 2017

Notice the localization of mylonites along the Main Central Thrust zone. Metamorphism of rocks above around 600 degree centigrade during the Eocene (~35 million years ago) and in the Miocene (~25-16 million years ago) has resulted in the ubiquity of ductile deformation observed in the Greater Himalaya. In hotter pockets in the core, metamorphic rocks partially melted and the resulting granitic magma was injected along penetrative weak planes, forming dikes, sills, and small plutons.

Channeled between two great fault zones, the Main Central Thrust at the base and the South Tibetan Detachment as roof, this hot mushy crustal material was then tectonically extruded to shallower levels, its ductile fabrics frozen and preserved as the rocks cooled. Subsequent tectonism has superimposed brittle deformation on the Greater Himalaya ductile structures. 

Finally, another beautiful example of a gneiss showing ductile shearing.  Fish shaped white feldspar are set in a biotite mica and quartz matrix which flows around the porphyroclasts. Can you guess the sense of relative motion?

Observing features that you have seen only in a textbook - that is the great joy of going out in the field.

Landscapes: Sunderdunga Valley Kumaon Himalaya

In mid November, I explored the Sunderdunga valley in the Kumaon region of Uttarakhand. It was a good rigorous walk through some extraordinarily beautiful landscapes. This area is better known for the famous Pindari Glacier trek. Kafni Glacier is another option for trekkers. All three routes begin at village Khati. The picture taken of the high ranges from nearby Dhakori shows the three glacial valleys.

And here are some more photos of the route with a brief commentary.

The entrance to Sunderdunga valley with the vigorous Sunderdunga river flowing through.

The first day walk to Jatoli village was through golden and green forests.

Village Jatoli in the mid November sun. We stayed there overnight at the Kumaon Mandal tourist guesthouse. They provide excellent clean accommodation. 

About 4 km walk upstream from Jatoli the next day and the land cover changes abruptly. The forest is gone. There is no marked trail from here on and a walk over a rugged boulder strewn region begins. 

We navigate our way over steeply dipping metamorphic rocks and scree cones. 

Numerous rock falls make for tricky passages. You can spot my companions climbing their way up the steep slope. 

After slogging for about 8 km through this terrain we arrive at Kathaliya, situated at about 10,500 feet ASL. We have climbed about 2500 feet from Jatoli to Kathaliya. A small trekkers shed has been constructed here. We stayed there for the next couple of days. 

Next morning, ahead of Kathaliya camp, we encountered the full glory of Sunderdunga valley. Here, it is a occupied by a wide boulder strewn river bed with several small active channels. The earthy colors of rock and grass were in stunning contrast to the blue sky. A solitary shepherd's hut can been seen in the lower right. 

Another view of the valley.

Boulder bed! The surrounding Greater Himalaya are made up of high grade metamorphic rocks. You can spot quartzo- feldspathic gneiss, amphibolite gneiss, mica schist and gneiss, and mica, garnet, and kyanite bearing schist and gneiss in the river bed. Quite a treat to walk along this metamorphic treasure! 

 Crossing the wider channels on rickety wooden bridges was fun! 

Here we are near Maiktoli Top, a high vantage point. Jagdish Bisht, me, Ratan Singh Danu, Lucky, and Kapil. They made my trip safe, comfortable, and memorable.

Why I go to these places. A clear view of the bands of metamorphic rocks exposed along the spectacular cliff face of the Sunderdunga ridge!

 

Village Khati is such a pretty place.

The high bare peaks in the background speak of a worrying trend. Everyone I talked to told me that this trek would have been impossible a few years ago in mid November. The upper part of the valley and the rocky ridges would have been blanketed in a thick snow pack. This area still had not received a single snowfall when I left on 22nd November. The two or three big snowfalls of the year now occur mostly in January and February. The pastoral and agriculture economy depends on a healthy winter snow cover to rejuvenate the high meadows, and to replenish springs and streams.

My guide tells me that Sunderdunga valley is the tougher route amongst the three treks to the nearby glaciers. I am so glad I walked this valley!

Darwin's House Plants, Water Diviners, Geology Podcast

 A couple of good articles and a geology podcast.

1) “Spontaneous Revolutions” Darwin’s Diagrams of Plant Movement: Darwin's unbounded curiosity for nature led him down many unexpected research pathways. Towards the end of his long career, his restless mind noticed the growth patterns of his house plants. Determined to understand more about their motion and the stimuli, he spent hours tracking tendrils grow and came up with innovative ways to record their movements on paper. Natalie Lawrence has written a lovely essay on this lesser known chapter of Darwin's life and work. 

2) Trust, cost go greater depths to sustain unscientific water divining practice: Large swaths of Indian agriculture is desperately dependent on access to groundwater. Simrin Sirur explores the reliance on water diviners in south India. Diviners use sticks, coppers tongs, coconuts, magnetic compass, and chains with keys as their instruments for sensing groundwater. Despite all this unscientific baggage, many diviners are not all that ignorant. They have a knowledge of the local landscape and groundwater availability. Their prediction relies more on their past experience and a dollop of common sense. 

I must tell you about my experience with a diviner. My neighbor requested that I accompany her to a plot of land outside Pune. She had hired a diviner to help her locate groundwater. We picked him up en route. He was the late Pandit Bhimsen Joshi's son! On reaching my friend's property he got to work with copper tongs. After a few minutes of walking  up and down the site the copper tongs started shaking. He indicated the spot to drill and suggested going down to a depth of 150 feet. On the way back he cheerfully told us that he knew that the adjacent plot owner had struck water at 150 feet. Past experience and common sense go a long way! 

3) Geology Bites Podcast:  Conversations with Geologists: Oliver Strimpel has had quite an unusual career beginning with a doctoral degree in astrophysics. He later became the director of the Computer Museum in Boston and then a patent attorney. But geology beckoned him. He has worked alongside geology researchers trying to date rocks and unravel the timing of movement of the Karkoram fault in Ladakh. Geology Bites grew out of his passion for the subject. You will find a wide range of geology topics discussed on this site. 

I have so far listened to experts talk about radioactive waste disposal, continental crust composition, the inherent bias in the global sedimentary record, and on the evolution of minerals through geologic time. All have been excellent. The talks are about half hour, so they don't tax your patience too much. 

If you have free time coming up this Diwali, I recommend you dive into this collection of geology talks.

Dr. V.V. Peshwa, Geologist Extraordinaire, 1939-2024

My Guruji, Dr. V. V. Peshwa passed away on August 27, 2024. He was my thesis advisor during my Master's education in Pune. His career as a faculty with the Department of Geology, Pune University (now Savitribai Phule Pune University), was full of distinction and dedication to the noble cause of teaching. Field geology, remote sensing, and mineralogy. He had a mastery over these subjects and taught them with extraordinary clarity. His lectures on mineral optics, delivered without the aid of notes, remain some of the most lucid explanations I have heard on any aspects of geology.  

Dr. Peshwa also set up the remote sensing lab at Pune University in the early 1970's,  having received a specialized Master's degree from the Netherlands. Over the years he amassed a vast collection of aerial photographs and satellite imagery of Indian landscapes, teaching with great panache the fine skills of image interpretation. He was a formidable researcher too, with publications in igneous and metamorphic petrology, remote sensing of the Deccan Basalts and Proterozoic sedimentary basins, and on geohazards. 

I will recount two incidents from my association with him. I had to choose a thesis guide at the end of my first year of Master's course at Pune. I asked Dr. Peshwa if he was willing to be my guide. As was his style, he promptly said no! I was unsure how to persuade him, but fortunately my senior, Anand Kale, came up with a brilliant plan. I was told to sit on a chair outside his room and poke my head inside every few minutes until he said yes. I agreed, and like a security guard sat outside his room all morning. Towards early afternoon Dr. Peshwa had given up trying to ignore this motionless sentry outside his door and agreed to my request, but on one condition. I had to go and map an area in Andhra Pradesh in the Cuddapah Basin.  He had some aerial photos of this place and wanted someone to study a fold structure that was spectacularly exposed near Nandyal town. The imagery below is from ISRO Cartosat.

Folded Cuddapah Group and Kurnool Group sediments south of Gani.

Dr. Peshwa accompanied me during my second trip to the field area. It was mid January and one early morning we set off to the low range of hills, about an hour walk from where we were staying. We worked till the afternoon, and by around 3 pm decided to call it a day. We were running out of water and were famished. We thought we should walk to the next village which was just 15 minutes away, have a snack and then turn back to our camp in Gani village. To our surprise every shop in the village was closed. Dejectedly we started walking to Gani. After a while we spotted a man on a bicycle coming in our direction. We recognized him as a shopkeeper from Gani. He stopped and explained that it was the auspicious day of Pongal and everything was closed. He slipped his hand into a bag and gave us two round dry buns to eat and cycled away. We tried to bite into them, but they was rock hard, harder than the Cuddapah quartzites we were trying to break with a hammer. We collapsed with laughter and trudged along, where our host was waiting for us with a hot sumptuous meal! 

For all his exuberance and light heartedness, Dr. Peshwa never compromised on the quality of work he expected from his students. He supervised with an eagle eye my petrographic analysis, read every word of my thesis, and even sent me back to the library because he thought my literature search was not exhaustive enough. He gave me full independence to follow my interest in carbonate sedimentology, but cautioned me to remain within the bounds of data. He did not like grand theorizing or explanations by 'arm waving'. Some might call him conservative, but it made us into careful researchers, and brought a rigor to our work. 

He remained active in geology long after his retirement, accompanying younger faculty and students to the field and acting as their mentor and advisor. I live near his house and used to stop by once in a while for a chai and long conversations about geology. He will be missed greatly. The picture below shows Dr. Peshwa, seated center, on his 80th birthday.

Now, only all those memories remain to serve as inspiration and to help us stay true to what the rocks are telling us.

Map: Paris Olympics Purple

I was hoping that this 1874 geological map of the Paris area was the inspiration behind the startling purple color theme for the recently concluded Paris Olympics. The map was shared on X (formerly Twitter) by the Geological Society of London.

The sedimentary strata are folded into an arc that looks like the purple athletics track! 

Alas, no. The purple color was selected because the organizers wanted a unique identity for the games. And apparently it made for better television viewing.

There is a geology connection to the athletics track though. A big component of the flooring is calcium carbonate usually obtained by grinding down quarried limestone. For these games, in keeping with the theme of sustainability promoted by the organizers, the purple track was made up of discarded mussel shells obtained from an Italian fishing cooperative. 

Oh well. I like my geology connection story better.

Remotely India: Bundelkhand Mafic Dikes and Quartz Veins

Remotely India #14

Do you see anything striking (pun intended) about this geologic map of the Bundelkhand craton?

Source: Sarada Mohanty 2023- Proterozoic Basins of the Bundelkhand Craton

Notice that the green lines are predominantly oriented in a NW-SE direction. The pink lines are predominantly striking NE-SW. These are magmatic and fluid intrusions into the Bundelkhand granitic crust. The green lines represent mafic dikes (Mg and Fe rich basaltic magma), and the pink lines represent quartz veins. 

The Bundelkhand craton is an Archean age block of continental crust. Like other Archean age terrains, it has a long history of magmatism, volcanism, and sedimentation. The oldest rocks, a suite of granitic rocks going by the term 'tonalite–trondhjemite–granodiorite', and associated volcanics and chemical sediments are as old as 3.4 billion years. Through the Archean the crust grew by repeated injections of magma. Voluminous magmatism petered out by around 2. 4 billion years ago with the formation of the Bundelkhand granodiorite batholith, an enormous subsurface body of congealed magma. Granodiorite is a calcium feldspar bearing variant of granite. This younger rock type covers most of the surface area of this terrain.

Geologic activity continued for hundred of millions of years after the emplacement of this batholith with the intrusion of these impressive dike swarms and quartz vein clusters.

Staying true to the objective of this series on Indian geology as seen from satellite imagery, the emphasis here will be on the field features of these intrusive bodies.

Giant Quartz Veins:

Locality- Northeast of Mauranipur, Uttar Pradesh.

The quartz vein stands out as a high long ridge. Steep sided blocks of quartz make up the spine of the ridge. Weathered boulders shed from the quartz vein have formed the surrounding slopes. This distinctive landform is instantly recognizable in the imagery as you explore this region.  

Locality: Southeast of Mohangarh, Madhya Pradesh.

Here you can observe the intrusive relationship between the giant quartz vein and the older Bundelkhand granite (BG) which crops up as low hills made up of a light toned fractured rock. The linear vein can be traced cutting across the host rock.

Locality: Southeast of Mauranipur, Uttar Pradesh.

At this location you can observe an unusual feature. Two quartz veins have split to form a tuning fork shaped geomorphic feature.

These quartz veins intruded the crust around 2.15 to 2 billion years ago. The quartz crystals contain bubbles of gas and minuscule amounts of liquid trapped inside them. They inform us about the temperature and pressure during precipitation of the crystals and about the salinity of the fluid. There are also tiny crystals of other hydrous minerals like chlorite and epidote found inside the quartz. These reveal the source of the fluid. Such studies conducted by Duttanjali Rout and colleagues identify two distinct sources of fluids involved in the formation of these veins. A hot moderate salinity fluid derived from the Bundelkhand granodiorite mixed with meteoric water percolation downwards through fractures. The deeper fluids were sourced from not more than 5 km in the subsurface.

A drop in the temperature and pressure of the rising silica saturated fluid as it encountered the colder meteoric water resulted in decrease of silica solubility and the precipitation of quartz. The giant quartz veins are the product of a vigorous Proterozoic geothermal system that lasted tens of millions of years. The researchers have drawn a comparison with Broadlands-Ohaaki geothermal system in Northland, New Zealand, and the Kakkonda geothermal system in NE Japan. Both are in granitic terrains and could be loose analogs for the processes in operation during the formation of the Bundelkhand quartz veins.  

There are differences in what we can observe in these ancient and modern systems. In the Proterozoic example, the surface expression of the silica rich geothermal system, the hot springs and geysers, have long since eroded away. We can study only the subsurface plumbing system. In the modern settings, the surface processes are apparent and the underground patterns of fluid flow have to be inferred. 

Mafic Dikes:

Locality- Northeast of Lalitput, Uttar Pradesh,

A NNW-SSE trending dike is exposed near Tera village. The surface expression of mafic dikes is very different from the quartz veins. The dikes weather away faster and are exposed as low relief hills with extensive boulder fields derived from the weathering of the dolerite rock. In the satellite imagery, you can see the dark toned nature of the boulders hinting at its mafic composition. Due to the spread of boulders around the dike, the width of the intrusion appears far more that its true width. 

Locality- Mahoba , Uttar Pradesh

An ENE-WSW trending mafic dike is surrounded by Mahoba town. As with the NW-SE trending cluster, these E-W trending intrusions also appear as dark toned low relief boulder strewn hills.

Locality- Mahoba, Uttar Pradesh.

This is a synoptic view of the E-W trending dike, captured by ISRO Cartosat. The white rectangle in the lower left of the image is the bounding area covered by the previous imagery. It is quite an extensive intrusion, and to the eastern end it can be seen cutting across outcrops of the Bundelkhand granite. 

Geochronologic work on these mafic dikes shows that the NW-SE trending dike swarm intruded around 1.9 to 1.8 billion years ago. The E-W trending group of dikes are much younger, dated to about 1.1 billion years ago. 

The geochemistry of these dikes point to an upper mantle source of the magma. The dikes are a variety of thoeliitic basalt, not too much different from the basalts of the Deccan Traps in Maharashtra. Unlike the shallow sourced fluid of the quartz veins, the source magma of the dikes was generated at least 50 km down in the mantle lithosphere.

The crisscrossing lines you see on a geologic map of the Bundelkhand craton are a record of geologic activity that continued long after voluminous granitic magmatism ended. In rare exposures, mafic dikes are seen cutting across quartz veins, indicating that they are the younger of the intrusives. Most of the geochronology data collected so far supports this field observation. Studies of the spatial patterns of the dikes and quartz veins too hint that they represent two independent deformation events. The formation of both these systems required extensive fracturing and faulting  of the crust by extensional forces. Geologists are still working out the reasons for these crustal disturbances. 

In the case of the quartz veins, the fracture systems tapped relatively shallow sources of heat and fluids. In the subsequent reactivation of the crust, much deeper fracture systems cutting across the crust tapped upper mantle sources of heat,  providing conduits for the passage of mafic magma to shallower crustal levels. 

These deep crust penetrating fractures and Proterozoic mafic dike swarms tell another story about the strength of the crust and the advent of plate tectonics, but that is fuel for another post!

I am having fun resurrecting my Remotely India series. Stay in touch for more explorations of Indian geology on this blog.

Field Photo: Unusual Himalaya Metamorphic Rock

My friend Emmanuel Theophilus, who spends a lot of time wandering in the high Kumaon Himalaya, sent me this photo of a feldspar rich gneiss.,

He observed this loose boulder near the small settlement of Bugdiyar in the Goriganga valley, north of Munsiyari town. Bugdiyar is located in the Greater Himalaya. This is a high grade metamorphic rock terrain. As you walk along the many trails that lead to places like Nandadevi Base Camp and Milam Glacier,  you can observe mica and amphibole rich schist with gleaming garnets, quartz and feldspar rich gneiss, migmatite gneiss (partially melted gneiss), and leucogranite (quartz and feldspar rich magma) intruding this high grade ensemble. 

This traverse takes you into the core of the Himalaya orogen, where high temperature and pressure during mountain building that took place 35 to 15 million years ago transformed the sedimentary protolith into metamorphic rocks. 

This particular gneiss rock has an extraordinary texture. I have never before seen such large feldspar (white crystals) in a metamorphic rock. Judging by the pebbles and other rocks strewn by the side, these are inches long feldspar grains. 

I want to introduce two terms used to describe texture in metamorphic rocks; porphyroblastic and porphyroclastic. Both these terms describe rocks with very large crystals surrounded by fine grained minerals. These are rocks with two distinct crystal size classes. 

Porphyroblastic texture forms when one mineral grows more quickly than other minerals during metamorphism. Large crystals of the rapidly growing mineral are set in a finer crystalline matrix. Both the large and small sized minerals have recrystallized, but at different rates.  

In contrast, porphyroclastic texture forms when there is a size reduction of some minerals , leaving one unaffected mineral larger than the rest. This situation occurs most commonly in fault zones where softer minerals may get crushed more easily leaving the resistant mineral as a large porphyroclast. These types of rocks have a broken appearance. The softer minerals become aligned to give the rock a prominent streaky banded texture. The more competent mineral may also develop an elongated shape.

Which of the above is the rock Theo found? My guess is that it is a porphyroblastic gneiss. Take a closer look at the beautiful large grains. They seem to be the result of growth during metamorphism, in the process engulfing small pockets of mica in their interiors. The rock lacks the streakiness and the often broken, bent, and stretched large grains characteristic of a porphyroclastic texture.

However, there is a subtle sign of deformation too. Have a look at this close up. 

The black arrows point to rugby ball shaped feldspar grains. They have a long axis and a short axis and appear to be stretched in one direction. Also notice the grey cracks running along the longer axis of many of these crystals and continuing into the rock. These are paper thin zones where force or stress was localized. The change in shape (strain) in the feldspar grains follows these very narrow zones of deformation. 

All of the above is my reasoned speculation on the origin of this texture. The next step is to meet up with Theo near Bugdiyar and walk along the Goriganga in search of the outcrop.

The Goriganga near Bugdiyar. It is spectacular out there!

Remotely India: Chittagong Tripura Fold Belt

Remotely India #13

Did you know that the easternmost part of the Bengal delta is being compressed into folded hill ranges? These go by the name Chittagong Tripura Fold Belt (CTFB), also referred to by geologists as the Outer Indo Burman (Myanmar) Ranges.

Take a look at the annotated satellite image below. The CTFB appears as a series of north south oriented ridges and valleys, extending from northern Tripura to south of Cox Bazaar in Bangladesh. 

Structurally they are made up of strata folded into anticlines (upwarps) and synclines (downwarps). To the east, they are separated from the inner Indo Burman (Myanmar) Ranges (IBR) by the north south trending Kaladan Fault. The Chittagong Coastal Fault marks the westernmost boundary of this fold belt, although the sedimentary pile below the sea bed of the Bay of Bengal to the west is also deforming. The 'deformation front' of this terrain is therefore further to the west of the Coastal Fault. 

As you might have guessed, these fold belts are a result of the Indian tectonic plate converging with Asia. But the nature of tectonic plate interaction is different from the plate collision that formed the Himalaya. In the case of the Himalaya, the continental crust of the Indian plate has collided with the continental crust of the Asia plate. The lower part of the Indian continental crust has slid under Tibet while thick slices of the Indian upper crust have been thrust up by faults to form the different geologic units of the Himalaya. 

Tracing the mountain arc southwards from its bend around Arunachal Pradesh, a different type of tectonic plate interaction is unfolding. In the Himalaya collision zone the more buoyant continental crust is sliding at a shallow angle underneath Tibet, a process known as underplating. In contrast, the Indian tectonic plate along this eastern convergence zone is made up of denser oceanic crust. As a result, along the zone of contact with Asia, this dense plate is subducting or taking a deep dive at a steeper angle into the mantle. 

Another difference apparent from the surface structure is the presence of both vertical and sideways movement of crustal blocks. This occurs because the Indian plate is pressing into Asia at an angle. Oblique convergence results in thrust faulting wherein rocks are moved up along east sloping fault planes. Collision at an angle also causes blocks to slide past each other along strike slip faults.  

The IBR is an older mountain chain formed by the subduction of the Tethyan oceanic crust underneath the Asia plate and the smaller Myanmar plate. This process, initiated in the Late Cretaceous around 100 million  years ago, eventually led to the formation of a complex fold belt by mid Miocene times (15-20 million years ago). 

This fold belt is made up of deep sea sediments and fragments of the Tethyan oceanic plate. These rocks were subjected to very high pressures during mountain building. Sheared and fractured rock units occur in a melange made up of dismembered blocks of varied rock types juxtaposed by faults. Heat and high pressure acting on rocks rich in aluminum, calcium, iron, titanium, and magnesium has resulted in the formation of deposits of exquisite gemstones such as jade, rubies, sapphires, spinel, and peridote. The IBR is studded with precious stones!

By Miocene (~20 million  years ago) the IBR had emerged above sea level as elevated ranges and had started eroding. Sediments shed from these hills were deposited in delta and shallow marine environments of the Bengal Basin to the west. During continued subduction of the Indian plate, between 2-4 million  years ago, this thin skin of the crust made up of about 5 km of sediment was scraped off, faulted, and crumpled up to form the CTFB. Geologists call these scraped off wedges of sediments that form along subduction zones as 'áccretionary prisms'. 

Further to the south, the Andaman Islands is also an accretionary prism formed along the plate junction between India and Asia.

The deformation of the CFTB diminishes from the east to the west. There are two distinct structural domains of this belt. To the east is a more tightly folded belt known as the Eastern Highly Compressed Fold Thrust Zone. Towards the west, is the more open Western Fold Thrust Zone. The emergent part of this fold belt is bounded to the west by the Chittagong Coastal Fault. However, geophysical studies show that the strata below the Bay of Bengal sea bed is also being warped and can be considered part of a westward growing CTFB.

The annotated satellite image below is a close up of the CTFB and the IBR. The black line is the Kaladan Fault separating the two, but even without my annotation, the two terrains have a distinctly different appearance. The older IBR have been more deeply dissected by streams. They have an etched faceted texture. To the west, the younger ranges of the CTFB have a more uniform even texture. 

Finally, I just wanted to put up a structural cross section of the CTFB. The folded and faulted nature of the sedimentary strata is apparent, as is the difference between the more tightly folded eastern zone compared with the more open western domain. Source: Md. Sakawat Hossian et.al. 2022: Lithosphere.

Scientists study terrains like the Chittagong Tripura Fold Belt to understand the mechanical response of the crust to different types of tectonic plate interactions. There is an economic incentive too. The IBR with its precious stone deposits has long been a target of exploration. In the CTFB natural gas seepage has been observed at many places. Geologists are interested in understanding the subsurface structure to target search for hydrocarbon accumulations.

As always, exploring Indian geology from satellite imagery is fun and a great learning experience for me. Stay tuned for more such stories!

Geological Contacts: Angular Unconformity Kaladgi Basin

 Remotely India Series #12

Through the Proterozoic Eon, beginning around 2 billion years ago,  extensional forces acting on continental crust opened up several sedimentary basins across what is now peninsular India. Crustal blocks subsided along faults and these depressions filled in with sediments deposited in fluvial and shallow marine environments. These basins were long lived, some lasting for more than a billion years. 

Sedimentation was not continuous.  Pulses of sediment deposition were punctuated by long periods of non deposition. Tectonic movements deformed early deposited piles of sediment. They were uplifted and an extensive basin wide erosional surface formed.

There was then a renewed phase of basin development. Sediment of these successor basins were deposited on tilted and folded older strata. Commonly, these younger packages of sediments are relatively undeformed. They are preserved as mesas and plateaus made up of flat lying strata. This discordance in attitude between two sets of strata separated by a widespread erosion surface is known as an angular unconformity.

In this post I will highlight an angular unconformity from the Kaladgi Basin from north Karnataka, south India. I have used high resolution imagery from Indian Space Research Organization's Cartosat.  Imagery is available for browsing and download from ISRO's Bhuvan 2D web maps.

The first image shows the area around Ramdurg village. The multi-stage history of the basin is readily apparent. The light colored strata exposed along narrow ridges are folded, while the rust brown hills are made up of undeformed sediments. The light toned strata are quartzites of the Bagalkot Group. The brown sandstone which rest on the Bagalkot quartzites are the Badami Group. Standard annotations show the varying dip and strike of the folded Bagalkot sediments. The white cross in grey circle denotes horizontal Badami strata. 

Kaladgi Basin history has become clearer based on recent geochronologic work by Shilpa Patil Pillai, Kanchan Pande, and Vivek S.Kale. They infer that basin initiation occurred around 1.4 billion years ago. Sedimentation of the Bagalkot Group terminated by 1.2 billion years ago. Movement along major WNW-ESE and tranverse NNE-SSE to NE-SW trending faults deformed the Bagalkot sediments into a series of folds around 1.1 billion years ago. This was followed by uplift and erosion of these folded sediments. Deformation was accompanied by low grade metamorphism of these rocks.

The basin floor subsided again around 900 million years ago initiating deposition of the Badami Group of sediments. The famous cave temples of Badami have been cut out from the lower part of the Badami sedimentary sequence.

The next imagery is a good example on how to recognize the relative timing of deformation events. Arrows point to fracture sets in the Bagalkot quartzites. These lineaments do not extend into the Badami sediments implying that fracturing occurred during an earlier phase of deformation. 

Let's look at a location that shows the angular discordance between the Bagalkot and Badami sediments. This is near Shirur town, north of Badami.  The lighter toned steeply tilted Bagalkot sediments outcrop as E-W trending narrow ribbons, north of Budanagad village. The brown colored Badami sediments form a more extensive plateau. Since these strata are horizontal, the traces of bedding planes form concentric bands mimicking contour lines. 

The final location is just south of Ramdurg village. The unconformity here is a little harder to decipher, but you can make out the tilt of the light colored Bagalkot quartzites, annotated by the standard notation of strike and dip. The quartzites form triangular facets sloping eastwards. Like the previous example, the concentric bands of brown in the adjacent hill indicates that this is the overlying horizontally disposed Badami sandstone.

Many Proterozoic basins of India contain such unconformity bounded sequences. Some more classic examples come from the Chattisgarh, Cuddapah, and Vindhyan basins. These sequences from different basins were not deposited synchronously. Each basin has it own trajectory of sedimentation, deformation, and erosion. 

Detailed field mapping, supplemented by absolute dating of rocks wherever possible, is elucidating the complex poly-phase history of Indian Proterozoic sedimentary basins in the context of global continental breakup and reassembly. For arm chair geologists and enthusiasts, easily available web mapping technology makes it possible to join in the excitement of teasing out these terrain's many secrets hiding in plain sight.

Links: Earthquake Detectives, Origin Of Life, India Water Act

Reading from the past few weeks- 

1) How earthquake scientists solved the mystery of the last “Big One” in the Pacific Northwest. The American northwest is a tectonically active region. About 150 km west of the Pacific coast is the Cascadia subduction zone. Here, the Juan de Fuca, Explorer, and Gorda tectonic plates slide underneath the continental plate of North America. Large earthquakes have occurred in the past and will occur in the future. 

Reporter Gregor Craige has written a book, On Borrowed Time: North America’s Next Big Quake, in which he explores the region's earthquake potential and the cross disciplinary studies that enable scientists to understand past earthquake history as well as the impact a big future earthquake will have. Canadian Geographic has shared an abstract from his book. The earthquake puzzle was solved by combining information from tree rings, Native American peoples memories of past events, and Japanese record of tsunamis. It is fascinating reading. 

2) To unravel the origin of life, treat findings as pieces of a bigger puzzle. Was life's beginnings in a warm little pond or in a deep sea hydrothermal vent? Did lightning provide the energy, did asteroids provide the organic matter? There are many many scenarios that try to provide an explanation to this vexing question. 

One of the leading researchers of this field, Nick Lane, and his colleague Joana Xavier, have summarized some of the key arguments and problems of the field in this tour de force of science writing. Highly recommended! 

3) Analysis: The Great Indian Water Act Of 2024. In more good news for industries, factories and foreign investors, yet another Indian environmental law has been diluted to facilitate “ease of business”. Shailendra Yashwant begins his analysis of The Water Amendment (Pollution and Prevention) Act, 2024 Bill on this depressing note. Amendments seek to "rationalize criminal provisions". Polluters can now escape jail time and get away by just paying a fine. All this when climate change and water security is one of the big challenges facing India. 

Is It A Lava Tube?

My latest field geology video is about a small cave in the basalt lava near my house in Pune city. The location is Hanuman tekdi, also known as Fergusson College Hill. The cave is along the slope right behind IMDR canteen. 

Is the cave a remnant of a lava tube, or has it formed by some other process? 

Sound on. Permanent Link - Fergusson College Hill Cave.

You can access this cave by walking along the path starting from the main gate of Gokhale Institute of Politics and Economics. Turn left as you approach the hill and after a few steps look up to the right. 

Visit quickly. As you can see from this photo, rubble from the construction works of water tanks at the top of the slope is slowly spreading and might cover up this cave. I hope not. 

More geology videos soon!

The Making Of Iceland

Committees are underappreciated. 

xkcd comics.

Happy New Year everyone! 

Geological Contact: Sirohi Fold Belt And Erinpura Granite

One of  my favorite courses during graduate work in Pune was geological Remote Sensing. We focused a lot on visual interpretation of aerial photographs and satellite imagery. Lab work meant hours of poring over different areas of India and making preliminary geology maps based on observations of the landforms and rock structures visible from space. The rock composition, structure, and weathering patterns controls the surface expression of geological units. These are manifest on satellite imagery and on aerial photographs as differences in tone, texture, structural styles, and relief. 

I came across a nice example of the juxtaposition of two distinct geologic rock units just northeast of the town of Sirohi in Rajasthan. 

These two terrains made up of the Erinpura Granite and the Sirohi Group sediments are easily recognizable. 

The terrain west of the yellow line and isolated ribbons between the purple lines is made up of layered metamorphosed sedimentary rocks of the Sirohi Group. These sediments were deposited on the Erinpura Granite basement around 850 million years ago. Hard rock layers form northeast southwest oriented ridges with intervening valleys underlain by a softer lithology.  The texture appears fine and the ridge and valley terrain has a substantial vegetation cover. The rock layers can be traced as a series of folds. The Sirohi Group sediments were deformed and metamorphosed by 820 million years ago.

The Erinpura Granite represents magmatism that occurred around 900 million years ago.  Occurring mostly east of the yellow line, the granite also forms enclaves (orange outline) in the fold belt.

The granite appears as a light toned surface. It is massive body of rock, lacking the layering seen in the western sedimentary basin. The landform is a low elevation plain as compared to the Sirohi rocks.The rock has a coarse granular texture. Its surface has scant vegetation. Several lineaments (dark colored lines) are seen crisscrossing the granite body. These are fractures and dikes. Shrubs and trees growing along the fractures and dikes give them a darker appearance and make them stand out against the lighter granite body. The fracture sets have also controlled the local drainage. Several streams can be seen flowing along straight courses.

Below are two close up images of the contact zones between the two terrains. 

At this scale you can appreciate why the granite as a coarser texture in the synoptic imagery. The fracture systems has broken the granite in to a blocky surface. 

In this image below the Erinpura Granite occurs as an enclave surrounded by Sirohi metasedimentary rocks. 

And finally, a close up of structure controlled drainage in the Eripura Granite terrain. The fir tree like pattern, known as trellis drainage,  is due to streams flowing along orthogonal fracture sets. 

Geology mapping done. Now to visit Rajasthan for a field check! 

Darwin's Earthworms, Ocean Currents, Geology Heritage Lost

My latest set of readings.

1) Why Darwin Admired the Humble Earthworm. A delightful essay by Philip Ball on Darwin's work on earthworms. Published towards the end of his career, this book apparently sold more copies than the Origin of Species! As Philip Ball wittily observes, that tells us something about the English passion for gardening. Darwin's research on earthworms consisted of detailed observations and cleverly designed experiments, often carried out with the help of family members. 

His powers of observation and analysis remained undimmed - "Darwin reported that 80 percent of leaves he removed from worm burrows had been inserted tip first—a far from random distribution".

2) No, the Gulf Stream isn't going to shut down. The premise of the movie The Day After Tomorrow is that of a catastrophic cold snap engulfing north America and Europe, triggered by the shutting down of the Gulf Stream. This massive ocean current forms in the subtropics in the western side of the Atlantic and transports heat from the lower latitudes to northern Europe, moderating the temperatures in these northern regions. Media reports claim that recent work might be pointing to a collapse of the Gulf Stream, but as Frank Jacobs explains, people are conflating two very different current systems. 

Some studies are suggesting that the Atlantic Meridional Overturning Circulation, a much smaller and restricted circulation system, might be slowing down and might even collapse by 2050. This will result in some cooling in the Greenland and Norwegian seas, but will not affect the larger Gulf Stream. The article has a nice animation of global ocean currents which I found informative.

3) They Have Put Geology in Coffins. For long, geologists have been complaining about the utter indifference shown by successive Indian governments to our natural heritage. Here is one more example from Himachal Pradesh. Along the Kalka-Shimla highway, on the stretch between Parwanoo and Solan lay a treasure. This was a section of sedimentary rocks recording the retreat of the Tethys Sea which began after the collision between India and Asia started creating high topography. Along this stratigraphic section, marine sediments give way to freshwater deposits. The outcrop was a natural outdoor laboratory for students and researchers. Now it is gone. The National Highway Authorities of India has covered it with concrete and stone walls. Science is the big loser again. 

Arundeep Ahluwalia expresses the anguish of geologists who knew and loved this part of the Himalaya- "It forever denies coming generations any chance to study the long stretches of such highways and to the nature lovers in society the excitement of the history and grandeur of the earth".  

Read and weep. 

Field Photos: Boudinage In Sandstone

Boudinage is a structure that is developed when rock layers are being pulled and stretched. The term is derived from the French word for sausage. A rock layer is deformed by necking and is segmented into a string of sausages. The structure is best developed when there is contrast in competence or strength in a rock pile. The stronger layer is broken up in boudins, while the weaker layer 'flows' around and fills the gaps created in the neighboring boudin layer.

I wanted to showcase examples of boudins from igneous, sedimentary, and metamorphic rocks. 

The first example has been recently documented in a paper on silica rich magmatism from Sausrashtra by Anmol Naik and colleagues. The photograph shows a rhyolite with boudinage (black arrow) developed in a lava flow band. This deformation is due to layer parallel stretching during flow of a viscous lava.

 Source: Anmol Naik et.al. Geological Magazine 2023

The second instance is from Darma Valley, Kumaon,  near the village of Philum, not so far from the Panchachuli Glacier. It is a remarkable instance of boudins developing in a sandstone. Black arrows highlight elliptical and oval knobs, fragments of a once continuous sand rich layer. Notice how the surrounding thinly layered material has flown into the gaps between these boudins. The more competent sand layer broke up, while the softer clay rich material got wrinkled and warped but did not break. 

And lastly, here is an example of the more common variety of boudinage encountered in the field. This has developed in a high grade metamorphic rock. The lighter more competent layer made up of quartz and feldspar has been deformed into a series of boudins, while the mica rich grey layers are ductile. This outcrop is also from Darma Valley, Kumaon, near the village of Baaling. 

The three examples showcase deformation which produced similar structures in very different settings. Internal forces generated during flow of a viscous lava can result in folding and boudins. In sedimentary basins, similar forces may  affect a sand rich slurry moving down slope as a gravity flow or on semi consolidated sediment shaken during an earthquake. And forces acting on high grade metamorphic rocks  produce some spectacular boudins at high temperatures during episodes of mountain building.

Documenting rock deformation in the field is fun! 

Septarian Concretion from Khambhat

My friend Bhushan Panse, who is a geology enthusiast and an avid rock and mineral collector, handed this specimen to me over a coffee meeting. He had bought it from a mineral supplier from Khambhat, Gujarat.

I commented that it is a septarian concretion. These hard ellipsoidal or oval shaped lumps form in mud and silt layers by the precipitation of calcite  around a nucleus. Khambhat and many other parts of Gujarat are underlain by Mesozoic and Cenozoic age sedimentary rocks. The process of concretion formation would have taken place at shallow burial depths when these sediments were still porous and water saturated. Mineral deposition in pore spaces often takes place in concentric layers. The calcium carbonate comes from saturated marine pore water or is derived from shells as they start dissolving during shallow burial. Notice the rust to brown color of the concretion. It is likely due to the presence of iron oxide and hydroxides which formed in the pore spaces from the iron contained in clay minerals.

The term Septarian Concretion refers to the radiating cracks or Septaria (derived from Septum). Cracks come in a variety of shapes. There are radiating cracks as seen in this specimen. These cracks are wider near the center and taper outwards. Other concretions may show concentrically oriented cracks, or overlapping sigmoidal shapes. Cracks may intersect, pointing to multiple cracking events. They are filled with either calcite or silica. The crystals filling these cracks are sometimes broken and displaced, and cracks may contain mud and silt. These features indicate a variety of stresses at play in concretions interiors. 

There are many ideas on how these cracks form. They have been interpreted as shrinkage cracks due to desiccation and hardening of mud. Dehydration during chemical transformation of clay minerals is another explanation.  A third hypothesis links the formation of cracks to gas expansion released during putrefaction of organic material. 

Sedimentologist Brian Pratt has offered another novel explanation. He proposed that these cracks result due to shaking of sediment during synsedimentary earthquakes. Shaking during ground motion results in variable stress fields in the interior of the concretion forming a large variety of crack geometries. These concretions may be preserving signals of  seismicity affecting that sedimentary basin!

Here is his compilation of the large variation in septarian concretion cracks from various sedimentary basins across Canada.

 Source: B. Pratt: Septarian concretions: internal cracking caused by synsedimentary earthquakes

A geologist friend who worked with the Geological Survey of India suggested another intriguing explanation. Parts of the region near Khambhat experienced explosive volcanic activity towards the waning phases of Deccan Volcanism. Ash expelled from volcanoes can coat small broken lava fragments forming lumps known as  'áccretionary lapilli'. Aggregations of ash and pyroclastic material if larger than 64 mm are known as volcanic bombs. This concretion fits the size range of a bomb. The dark fragments in the center of the concretion do resemble a fine grained igneous rock. A closer examination under a microscope is needed for a confirmation of its origin.

It is fun to examine hand specimens that friends collect from various part of the world and try to identify the rocks and minerals. But often a clear cut answer is not possible due to the need for additional information from a higher resolution or the chemical makeup. But a guessing game over coffee is always welcome. 

Geodes, nodules, and concretions found in volcanic and sedimentary rocks are mystery objects. You never know what you will see inside when you break open one of these lumps. There may be an array of perfectly faceted purple amethyst crystals and multicolored calcite. Or a trapped fossil. Or a crack network filled with bright and shiny calcite and quartz. These crystal rich interiors give us important information on the composition of fluids which react with rock at many different times during their geologic history. This water rock interaction is of interest to mineralogists and  economic geologists who want to understand the history of fluid flow through sedimentary basins and the conditions that lead to the concentration and deposition of metals. 

Geological investigation at all scales inform us about how the earth works. One can stand and gape at great mountain ranges and wonder about the movement of tectonic plates. But you can also crack open a rather dull colored lump from a shale and marvel at its insides, all telling a story of groundwater flow and chemical reactions, and who knows, past earthquakes as well.