Bengal Delta, Africa Rifting, India Sand Mining

A few readings for your perusal- 

1) The future of Bengal Delta. With this succinct title Dipen Bhattacharya has written an informative article on the origin and evolution of the Bengal Delta. The Bay of Bengal was created when India broke away from eastern Antarctica about 130 -120 million years ago in the early mid Cretaceous. The basin expanded as India drifted northwards. From Cretaceous to Oligocene times (120-25 million years ago) rivers from Peninsular India were providing most of the sediment being deposited in the Bay. Himalaya derived sediment started overwhelming Peninsular river input from about 25 million years ago. K.S. Krishna and coworkers have very elegantly demonstrated this in their study of sediment pathways in to the Bay of Bengal.

Dr. Bhattacharya has traced the evolution of the delta into more recent times, explaining the role of the Pleistocene ice ages in delta growth. The delta’s future too is at risk with dam building and ground water extraction amplifying the changes due to global warming induced sea level rise. Well worth reading.

2) Eastern Africa Is Splitting Apart, but Not Where We Expected.  Africa is tearing apart along a north south oriented corridor from the Red Sea to Mozambique. Plate motion has formed the famous rift valleys of Ethiopia, Kenya, and Tanzania, as the crust stretches and subsides along faults. Kimberley Cartier explains the geological set up of the region and the stages in which continents break apart with oceanic basins eventually forming along the initial zones of continental rifts.

Why this region of Africa is rifting is not all that easy to explain. If you look at the plate tectonic map of Eastern Africa and the adjacent Indian Ocean and Arabian Sea you will notice that the oceanic Somalia and Indian Ocean tectonic plates are pushing into East Africa. For continents to split and be pulled apart, there have to be extensional forces generated. These are usually provided at the locus of rifting by the mantle doming up, thereby breaking and pushing the lithosphere away, and by one end of the plate subducting underneath another overriding plate. The subducting oceanic crust becomes denser and heavier as it sinks deeper, pulling the rest of the plate with it.

After the breakup of Gondwanaland, the northerly movement of the India plate through the Cretaceous was sustained by the pull force of the northern edge of the India plate sinking under Asia. On the other hand, Eastern Africa is surrounded by plate spreading zones. There is no pull force available for eastern Africa, only the localized extensional stresses due to mantle upwelling.  Is that providing adequate horizontal traction at the base of the Africa Plate for the crust to break apart and stretch? For a deeper understanding into the mantle forces responsible for this, I will recommend J Micheal Kendall and Carolina Lithgow-Bertelloni ‘s article- Why is Africa Rifting?

3)  India’s rivers bear lasting scars from relentless sand mining. Some years ago I heard a podcast on Planet Money about a Jamaican beach that was stolen. An estimated 500 truckloads of sand was hauled away in the middle of the night. Sand is big business all over the world. Indian river beds too are being plundered for their sand to satisfy the demands of the booming construction industry. Sahana Ghosh explores how scientists are surveying Indian rivers using field observations and satellite data. They are trying to track down the amount of sand being extracted and the environmental impact of sand mining.

Iran Oil, Flowering Plants, India Aquifers

Some readings and a podcast from the past few weeks- 

1) The Geological controls on Iran Oil- Geology lovers who like to explore satellite imagery would have surely noticed the landscape of southern Iran. The crust is wrinkled up into unending fold mountains. These have resulted due to the collision of the Arabian plate with the Eurasian plate. And that convergence earlier in history created a depression which filled  with organic rich mud, the source of all that Iranian oil reserves. Stephanie Pappas has written a nice primer on the quirk of geology that explains Iran’s oil bounty.

2) How Flowers Transformed Planet Earth-  “Both in the evolution of life on this planet, and in human culture, flowers have been a critical engine of connection and cooperation”. I had enjoyed David George Haskell’s earlier book, “The Forest Unseen”, in which he observed one square meter of forest floor through different seasons to track changes in fauna and flora and what that teaches us  about ecologic relationships. Viviane Callier talks to him about his new book on flowering plants and their significance in science and society.

3) Indian Hydrogeology- Groundwater is the lifeline of Indian agriculture. That makes understanding aquifers a critical aspect of exploring and utilization of this resource. Recently, Pune based groundwater researcher Dr. Himanshu Kulkarni was awarded the International Water Prize by the University of Oklahoma for his contributions to Indian hydrogeology. They span nearly 4 decades of work in the Deccan basalts, as well as other Indian geologic terrains. His work includes not just the science of aquifers, but also efforts on involving local communities in sharing and managing this resource.

Veena Srinivasan of Well Labs has a long conversation with Dr. Kulkarni about his life’s work. On a personal note, Himanshu was my senior in University. It was really good to hear about this recognition for his important contributions to Indian groundwater science.

Easterly Tilt Of The Deccan Plateau - Update

I first wrote about this topic in 2011 in response to a question by a reader. I thought I would update my post with some new maps and explanations. Why is there such a pronounced pattern of easterly flow of the rivers in the Indian Peninsula.  I keep getting asked this question.  It was time for an update on this interesting topic on geology and landscapes. 

The region south of the Tapi river covering the Deccan basalts and the southern Indian peninsula exhibits an easterly drainage with the rivers flowing into the Bay of Bengal. The map below shows the Indian peninsular region with easterly drainage. The Deccan Plateau is mostly but not entirely covered by the Deccan basalts. South of this region is the Karnataka Plateau with a Precambrian geology. Along the east coast there are Permian-Triassic and Cretaceous basins.

Source: Hetu C. Sheth: Deccan Beyond the Plume Hypothesis

The question posed to me was - What is the relationship between the Deccan volcanics and the easterly tilt of the Indian plateau (i.e. the plateau covering the Deccan volcanics and the southern Indian peninsular region)?

The easier more intuitive answer would have been that the western ghats provide the topography and Deccan volcanism created a lava pile that is thicker to the west and which thins to the east, thus generating an east sloping surface. Rivers follow the slope to the Bay of Bengal. 

There are some geologic age inconsistency in this answer and this also does also not fully explain why the region south of the Deccan Volcanics too has an easterly drainage. Clearly, something more is going on. 

To understand the evolution of the Peninsular drainage patterns let us look back to the time when the Peninsula didn't exist. In early Mesozoic, India was part of Gondwanaland and was joined to Antarctica and Australia to the east, and Africa to the west. The triangular shape of south India with characteristic eastern and western coastlines had not formed yet. 

How can we find out the direction rivers were flowing back then? Geologists look to clues in the sedimentary basins of that age. The composition of sand in sandstone is matched to the most likely source terrain. And current directions can be inferred from studying ripples preserved on the surface of ancient sand. 

The paleo geographic maps below shows Gondwanaland and the location of the Pranhita Godavari basin in the Mesozoic. 

 

Source: Sankar Kumar Nahak and Coworkers 2024.

West North West flowing rivers originating in the highlands of the future Antarctica and in the Eastern Ghats were funneling sediment to the basin. Much of the interior of the region that would become the southern Peninsular India was a peneplain. There wasn't much topography towards the west for an easterly drainage network to develop. 

India broke away from Antarctica beginning about 140 million years ago. A distinct eastern continental margin formed. Several NE- SW oriented basins developed along the edge of the Indian continent. Since by this time an expanding Indian Ocean lay to the east, the orientation of a natural drainage system would have been from the west towards the east. 

We can say with some confidence that by 90 to 80 million years ago, east flowing rivers originating in the interior of the Indian continent were depositing sediment along the eastern Indian margin. See this map of sediment distribution along the Indian east coast. 

 Source: K.S. Krishna and Coworkers 2016.

It shows the thickness of  Mid- Late Cretaceous sediment, ranging in age from about 100 million years ago to 65 million years ago. The sediment lobes coincide with the mouths of the Godavari, Krishna rivers and other southern rivers, indicating that the paleo Godavari and the paleo Krishna system had begun building deltas from that time. Since there were no Western Ghats then, these rivers may have been shorter, with their source somewhere in the Archean and Proterozoic terrain of Peninsular India. 

Further to the south, geologists find a similar story with the ancient Cauvery. The Cauvery basin formed when Sri Lanka detached from the Indian continent. Its delta and marine deposits too contains sediment from the Late Cretaceous. 

The easterly drainage pattern of Peninsular India developed before Deccan Volcanism and the formation of the Western Ghats. 

India broke away from Madagascar about 88 million years ago  and subsequently from the Seychelles about 66-64 million year ago. The latter separation coincided with Deccan Volcanism and the eventual formation of the western Indian continental margin. Block faulting that accompanies continental breakup would have created a north south oriented high area, which would eventually evolve into the present day Western Ghats. The thinning of the lava pile to the east also would have created an easterly slope. Rivers originating in the western highland now would flow across the length of the Peninsula. 

New streams would have incised the fresh volcanic surface as lava buried the older etched landscape. But the regional  pattern of easterly flow persisted.

Some geologists maintain that there has been some fairly recent Cenozoic age (past 15-20 million years) uplift of the Western Ghats which has accentuated relief and produced the youthful looking topography of scarps, waterfalls, and deep canyons. These earth movements would have certainly given new energy to the drainage system, but there is some geologic evidence to suggest that the streams originating in the western ghat region are antecedent to the uplift of the ranges. 

For example, in the Mahabaleshwar area easterly drainage cuts across the axis of a north south oriented gentle anticlinal structure, implying that the drainage predates the uplift and warping of lava flows. Evidence from sedimentation patterns of the eastern river deltas also show that the easterly drainage originated much earlier than the formation of the Western Ghats. 

What then created that initial slope to the east that imprinted the drainage network that continues today? 

One reason is that the eastern margin formed first. Basin formation along the eastern edge of the continent would have created a relief difference between the western interior and the eastern depressions, resulting in stream networks flowing eastwards.  Secondly, the new oceanic crust made of lava that formed when India and Antarctica separated in the Late Jurassic and Early Cretaceous would have cooled by Late Cretaceous times. Becoming colder and denser it has been sinking and dragging the Peninsular region with it. 

Earlier eastern basin formation and a tug from the floor of the Bay of Bengal may have been enough to impress an east flowing drainage. Later, the east sloping lava surface and the rise of the Western Ghats reinforced this distinction between the west and the east perpetuating the direction of river flow initiated since Cretaceous times. 

How Old Is Himalaya Topography?

 Is this true?

The answer is no. To be fair, aside from the click bait, the article itself does not make any such claim. It covers a new study on some igneous rocks from Arunachal Pradesh that formed during an earlier pre-Himalaya stage of the India Asia plate convergence. 

By mid late Cretaceous times, between 100 to 66 million years ago, the dense ocean lithosphere that was the front edge of the Indian plate was sinking underneath the Ladakh terrain, a splinter of Asian continental plate disconnected from the mainland. The subduction of the Indian plate triggered extensive melting in the deep subsurface, building over time a magmatic arc. The schematic below shows the plate tectonic scenario. 

Source: Sankar Chatterjee and coworkers: Gondwana Research.

This arc has been well studied in the Kohistan and Ladakh areas. The new work found that a body of granitic rock known as the Lohit pluton is also an eastern extension of the Kohistan Ladakh arc complex. 

Eventually, the oceanic crust of the Indian plate was consumed and the Indian continental crust collided with the Ladakh terrain, which by this time had sutured with the Asian mainland. Himalayan mountain building begins from this point on, roughly sometime after 50 million years ago. 

How do geologists know when topography began to form in the Himalayan region? Various dating methods tell us when a particular rock or mineral crystallized or cooled below a particular temperature. But how do we date the formation of topography? Sedimentary geology, my field of specialization, has played a big role in giving us insights into the tectonic and topographic evolution of the Himalaya. I'll summarize how the story of the formation of the different Himalaya ranges came to be written by these geologists. 

Due to inherited geologic history and subsequent conditions during continental collision, the entire region between Ladakh and the Himalaya front can be subdivided into six geologic terrains running along the length of the mountain arc. These are, from north to south,  the Gangdese magmatic arc, the Indus-Tsangpo suture zone, the Tethys Himalaya, the Greater Himalaya, the Lesser Himalaya, and the sub Himalaya (Siwalik). Each is made up of distinctive rock associations. 

The earliest topography began to from in the north, at the zone of contact between the two continental plates. Over time, there was a step wise progression of southwards topography formation. As each of the geologic terrains rose up, newly developed stream networks started eroding the rocks and delivered distinctive sediment mixtures to two types of sedimentary basins which had developed adjacent to the mountains. The Indus and Bengal basins at the two extremities of the Himalaya received sediments from the Indus and the Yarlung-Tsangpo/Brahmaputra. A second type of basin, known as a foreland basin formed to the south of the orogen. This moat like depression which runs parallel to the Himalaya, and is fed by streams running transverse to the ranges, also became an archive of material removed from the mountains. 

By carefully identifying the sedimentary grain types deposited in these basins and how their proportions change through the oldest to the youngest layers, geologists have been able to piece together the evolution of new topography and geologic provenance through time. 

I am presenting this reconstruction through a series of time slices which show topography formation in each of the geologic terrains and the resulting stream networks transporting the derived sediment from source to sink. The representation only shows the central foreland basin from Himachal Pradesh to Nepal. I have not shown the sedimentary history of the western and eastern basins. The trends at the Himalaya extremities are similar with some difference due to variations in the dominant geology of the contributing catchments. 

I have also not covered a short phase of basin formation in the Indus Tsangpo suture zone between 30 to 20 million years ago. This basin received sediments from both the Gangdese Arc and the Indian plate and was then uplifted to form the Indus Group ranges which include the famous Kailash mountain. For more details of this episode of Himalaya mountain building do refer to my post - Is Mount Kailash the Oldest Mountain in the Himalaya?

In the diagrams, the legend "foreland sandstone diagnostic grains" refers to the arrival of a suite of distinct grain types in the foreland basin. This signals the uplift and erosion of new rock types in the growing mountain range. Earlier formed ranges will in most cases continue to contribute sand, but it is the first appearance of a new grain type in successive strata that is the indicator of tectonic and topographic changes. 

The inspiration for these diagrams is from P.G. DeCelles and coworkers paper on the timing of the India Asia collision, published in the May 2014 issue of Tectonics.

A) Initial Continental Collision- 58-54 million years ago: What was the timing of the collision?

As long as Indian oceanic lithosphere was subducting under Asia, a deep trench and forearc basin separated the two continents. Sediments eroded from Asia were trapped in these depressions and could not travel on to the Indian continent. This situation changed on continental impact. By then the intervening basins were shallower, the Tethyan Ocean had started retreating and rivers originating on the Asian tectonic plate could now flow across the zone of collision and deposit sediments on the Indian continent. These Asia derived delta sediments rich in volcanic rock fragments eroded from the Gangdese Arc began to be deposited on the Indian continental shelf between 58-54  million years ago. TH, GH, and LH are the future Tethys Himalaya, Greater Himalaya, and Lesser Himalaya.

B) Proto Himalaya Stage- 45-30 million years ago. 

Preceding and during collision, slices of the Indian oceanic plate were thrust up and sandwiched between the two continental plates. Igneous rocks of this "suture zone" are made up of magnesium rich silicate minerals (olivine, pyroxene) and magnesium aluminum oxides (spinel). Only a remnant arm of the Tethys persists, but rivers can now flow across the Indian continent and deliver sediments to the foreland basin. This basin is located roughly over what is now the southern Lesser Himalaya and the Siwalik ranges. Gangdese Arc and suture zone derived sediments first appear in the foreland by 45 million years ago.

C) Early Himalaya Stage- 35-20 million years ago. 

A pulse of low grade metamorphic rock fragments start appearing in the early Oligocene to Miocene age foreland basin sediments. These are derived from the newly forming Tethyan fold and thrust belt. Volcanic rock fragments are now absent in the central foreland, suggesting that the growing Tethyan ranges are a barrier to rivers originating in the Gangdese Arc. The Indus and the Yarlung Tsangpo, initiated in the furrow of the suture zone and flowing parallel to the northern ranges before cutting across the rising mountain chain, continue to transport arc derived sediment to the basins at the western and eastern extremities. 

D) Main Himalaya Stage- From 20 million years ago.

Three pulses of mountain building are recognized during the Main Himalaya Stage. The early phase from 20 -11 million years ago records the uplift of the Greater Himalaya which are made up of high grade metamorphic rocks containing minerals like mica, feldspar, and garnet. Deformation continued southwards between 11-5 million years ago resulting in the formation of the Lesser Himalaya ranges. These shed low grade metamorphic and dolostone (magnesium calcium carbonate) detritus. 

Eventually the foreland basin got caught up in the progressing orogeny. New faults propagated southwards. Slices of the oldest foreland basin deposits were broken and accreted to the mountain front. These freshly exhumed rocks became the major source of sediments feeding the youngest depositional phase in the foreland. Pliocene-Pleistocene  (5 -0.5 million years ago) layers of the Siwalik hills are made up of rock fragments cannibalized from Eocene and Oligocene foreland strata. 

How is the age of the foreland basin sediments determined? The entire exercise of unraveling the topographic history of the Himalaya depends on that! 

These sediments have been dated using a variety of methods. Fossils provide age ranges for packages of sediment. Magnetic signals preserved in iron rich mineral grains are measured and pegged to an absolute date by comparing the magnetic pattern to a global magnetic chronology. Techniques such as fission track dating of zirconium silicate sand (zircon) tells geologists when that zircon in its source was being uplifted and cooled thus giving a fair idea of the time of its erosion, transport, and deposition as a sedimentary particle. 

Gathering all this age information on the sediments, the timing of the arrival of distinctive rock fragments and minerals in the foreland agrees well with the exhumation history of their provenance as deduced from bedrock geochronology. 

The growth of mountain chains is a long and complex process. The word "collision" may invoke ideas of near instant crustal response, but deformation and surface uplift moves rather slowly across strong and rigid plates. Surface and deep crustal process are linked. For example, the formation of the Tethyan fold and thrust belt resulted in crustal thickening and the deep burial and metamorphism of rocks that eventually became the Greater Himalaya. There was a long time lag between the initiation of collision and the main Himalaya pulse of uplift. The rise of  the Greater Himalaya took place a good 35 million years after the India Asia impact. 

Foreland basins give us valuable insights into the relationship between sedimentation and tectonics, but they too need careful evaluation. They are not static entities. Compare the proto Himalaya stage with the main Himalaya stage and you will notice that as mountain building moved southwards the location of the foreland shifted too in response to the migrating load of the thickened crust. This process continues to this day. If the region of the Siwalik hills was the foreland a few million years ago, today it is the Ganga alluvial plains. 

Orogeny and drainage impact the sedimentation patterns in the foreland. Different rivers breaking through along the mountain front may not be bringing uniform sediment mixtures at the same time. Hinterland differences in geology of the catchment results in variable sand composition along the length of the foreland. Take the example of river Ravi. It drains mostly the Lesser Himalaya and hence its brings with it low grade metamorphic sand grains. On the other hand, the Sutlej flows through a significant portion of the Greater Himalaya. High grade metamorphic grains make up a large proportion of its sand. 

Further, chemical reactions taking place in the subsurface may dissolve some types of minerals. This may create an apparent trend in sand composition through time, which may not accurately reflect the actual history of the provenance. 

Researchers need to be cognizant of these issues when they construct their answers. 

Understanding the timing of Himalaya uplift provides valuable insights into the geodynamic forces at play during continental collisions. But the interest in this question goes beyond geology. Climate scientists want to know more about the linkage between Himalaya evolution and the advent and shifts in the Asian monsoon. And the value of studying the Himalaya spills into many other areas. Orogeny exposed enormous volumes of fresh rock to chemical weathering, mobilizing nutrients and organic carbon which would then be sequestered in fluvial and marine environments. These elemental cycling and budgets are keenly studied by surface systems specialists. 

Visit the picturesque Kangra region in Himachal Pradesh. Climb the thick sandstone layers leading up to the historic Kangra fort. Go to nearby Jwalamukhi, where natural gas emanating from the deep keeps alight an eternal flame. Ascend the hills towards the famous town of Dharamshala. You will be traveling through the Miocene foreland basin. Sedimentary petrologists for decades have worked on these and other sites along the Himalaya frontal ranges amassing data on sandstone composition. Even as million dollar instrumentation keep revealing new facets of the earth, their main tool has remained the humble petrologic microscope mounted with a grain counting stage. From this labor of love has emerged the story of how and when the Himalaya came to be.

Oldest Himalaya Rocks

In Peninsular India, the most significant change in rock type occur across what is known as the Archean Proterozoic boundary.

Archean rocks, older than 2.5 billion years, are typically varieties of granite and granite gneiss. They formed when the earth was much hotter and silica rich continental crust was growing by injections of magma from the uppermost mantle and by partial melting of older mafic (silica poor) crust. At places the crust subsided by vertical movements, and lava and sediment filled the narrow depressions. These volcano-sedimentary rocks were deformed and metamorphosed to form linear schist enclaves within the granitic crust. The term granite greenstone terrain describes this rock association.

This phase of continental crust building petered out by around 2. 5 billion years ago. The thick crustal blocks or cratons became the nuclei for future continent growth. By 2 billion years ago or so in the Paleoproterozoic (the Proterozoic Eon spans from 2.5 billion years ago to 538 million years ago) , the Archean crust became the floor for several sedimentary basins. Erosion of the Archean rocks provided sediments that accumulated in these basins over the next 1 billion years, with long hiatuses punctuating pulses of sediment deposition. The names of these sites of deposition will be familiar to many readers and travelers.  Aravallis, Vindhyans, and Cuddapah, to name a few, represent this younger Proterozoic phase of crustal recycling. 

There was limited development in Peninsular India of younger sedimentary basins and as a result Archean and Proterozoic crustal sections are widely exposed all across the country.

The satellite imagery posted below shows one classic locality of the Archean Proterozoic boundary. This is from the Cuddapah Basin of South India.  

The Archean granitic terrain has a rough texture due to the bouldery nature of the landscape formed by weathering of fractured granite. Towards the east north east, the layering of sedimentary strata of the Cuddapah Basin is prominent and unmistakable.  The following graphic is a geologic log prepared to describe the succession of rock types from the Cuddapah Basin.

Source: Vivek S Kale and coworkers; Proc. Indian. Nat. Sci. Academy 2020.

Archean 'basement' and Proterozoic 'cover sequence' is a common stratigraphic motif of the Precambrian geology of India.

A few weeks ago I found an old paper from the 1970's on the sedimentology and stratigraphy of Tethyan sequences from the Kali valley area, near the Kumaon Nepal border. These are, as the name suggests, sediments deposited in the Tethyan Ocean along the  northern margin of the Indian continent. They range in age from the Proterozoic to the Mesozoic.

Here is the stratigraphic column from the paper. I have shown only the Precambrian (Archean and Proterozoic collectively make up the Precambrian) section of the column. The Central Crystallines are assigned an Archean age, while the Tethyan Sequence Martoli Formation is Early Precambrian. The name Proterozoic was not in use in the 1970's when this paper was written.

 Source: S. Kumar and coworkers; Journal of Paleontological Society of India 1977.

'Crystalline' in this context refers to the rock texture made up of large interlocking minerals formed during slow cooling of a magma or during high temperature metamorphism of a sedimentary rock.

The geologic sequence I described earlier took place along the northern margin of India too where the future Himalaya would form. At first glance the Himalaya sequence seems a replica of the geology of Precambrian Peninsular India. It records an Archean  'crystalline'  basement, succeeded by variably deformed and metamorphosed Precambrian (Proterozoic) sediments.

Except that it is wrong. There are no Archean age rocks exposed anywhere in the Himalaya. And the rock units immediately in contact with the Archean are not Early Precambrian.

When this paper was published there was precious little geochronology work done in the Himalaya. Geologists knew from sporadic absolute dating of Peninsular rocks that the granite and granite gneiss terrains are older than 2.5 billion years  (Archean).  The thick sedimentary sequences overlying the granite basement also were Precambrian as ascertained by dating intrusive granites and interbedded lava layers. The lack of any shelly fossils in them was another indicator of the Precambrian age of the cover sedimentary sequences. 

Given this familiarity with Peninsular geology, it would have been natural to assign the same chronology to a Himalaya rock sequence of high grade gneiss in contact with unfossiliferous sedimentary rocks. 

The Central Crystallines are now known as the Greater Himalaya Sequence and they are not Archean but Neoproterozoic (Late Precambrian) in age. Detailed work has shown that they represent sediments deposited roughly between 1 billion and 600 million years ago along the northern continental shelf of India. A paleogeographic reconstruction of the Himachal Himalaya by Alexander Webb and coworkers shows the original disposition of the different Himalaya rock divisions. Observe (A) that the Greater Himalaya (GHC) and the lower part of the Tethyan Himalaya (Haimanta/Martoli Formation) were deposited synchronously in adjacent areas of the continental shelf.  

 Source: Alexander Webb and coworkers; Geosphere 2011.

They look very different from each other today because they experienced different conditions during Himalaya mountain building. The Central Crystallines which began their life as marine sediments became crystalline gneisses and schists during high grade Cenozoic metamorphism 35 to 20 million years ago, while the Tethyan Sequence escaped being buried deep in the crust and retained much of their original sedimentary character. 

What about the oldest Himalaya rocks? These are Paleoproterozoic in age, dated to be about 1.9 to 1. 8 billion years old. The units Damtha, Berinag, Wangtu, Jeori and Baragaon in the Himachal Himalaya cross section are the Paleoproterozoic age rocks. They are remnants of a magmatic arc and associated basins which formed along the northern margin of India when continental blocks were colliding and suturing into an early supercontinent named Colombia.

Underneath these Paleoproterozoic rocks would have been the Archean 'basement'. But where is it now? 

We can take a step back and understand how the Himalaya are constructed. As the Indian continental crust collided and pressed into Asia, a crack or a fault initiated in the collision zone started propagating southwards, slowly splitting the Indian crust.  As India kept getting pushed under Asia along this master fault, slices of Indian crust get scraped off  and thrust upwards along subsidiary faults to form a growing mountain range. This tectonic evolution is depicted as stages B, C, D, E, F. You can also read my post Himalaya: A Critical Wedge for more details on the mechanisms of mountain building.

If as shown in the cross section, the faults that break and transport crustal sheets are located entirely within the Proterozoic and younger layers then the Archean rocks won't get incorporated into the Himalayan orogen. They lie below the basal detachment/master fault. Alternatively, this model may not be applicable everywhere in the Himalaya and there may be slivers of Archean rocks buried deep under the thrust pile, but erosion hasn't exposed them yet. 

In the geologic future, the plate tectonic engine that made the Himalaya will change track. The mountain ranges will stop growing. Erosion will wear down the Himalaya and eventually lay bare its roots. The Archean 'crystalline basement' so familiar all over Peninsular India will also be visible at the base of the gentle rolling hills that were once the mighty Himalaya.

Lithospheric Dehumidifier

A few years ago a friend decided to demolish his old house due to extensive and expensive water damage to the ground floor. 

Could this be the explanation for the damage?

xkcd comics

A spanking new apartment building now stands at the spot of the old bungalow. There are no signs of any water damage so far. A giant lithospheric dehumidifier may have been used during construction.

There is no end to the add on and perks developers promise these days.

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!

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. 

Tethyan Himalaya And Trans Himalaya

What is the difference between Tethyan Himalaya and Trans Himalaya?

I've seen the two terms being used interchangeably, but geologists recognize them as geologically distinct terrains. Their geographic locations and geologic context has been annotated in the satellite imagery below. 

The Indus Suture is the zone of collision between the Indian and Asian tectonic plates. It contains broken pieces of  oceanic crust and deep sea sediments which were uplifted and jammed between the colliding continents, forming a sort of a geologic no-man's land. The Tethyan Himalaya are the ranges immediately south of the Indus Suture. They are the deformed rocks of the Indian plate. The Trans Himalaya are the ranges north of the Indus Suture made up of a variety of rocks of the Asian plate.

The Tethyan Himalaya is a pile of Paleozoic and Mesozoic sedimentary rocks which was deformed into a fold and thrust belt during the early stages (45-35 million years ago) of the India Asia collision. At places, the sedimentary cover has been stripped away by erosion and high grade metamorphic rocks formed deep in the crust have been exposed. These 'windows' are known as gneiss domes since the sedimentary cover rocks have been arched up during uplift and exhumation of the high grade rocks. The area around the famous Tso Moriri lake is one of the best examples of a gneiss dome.

If your are traveling in Zanskar, Spiti, Lahaul, upper reaches of Kinnaur, and near about Milam and Panchachuli Glaciers,  you are in the Tethyan Himalaya. 

In the late Cretaceous (100 million  years ago), the leading edge of the Indian plate began subducting underneath Asia. As the plate dove deeper it heated up and released water, which triggered the formation of magma in the upper mantle of the Asian plate. This magma rose and assimilated rocks from the Asian lower crust. It then intruded older sedimentary and metamorphic rocks of the Asian crust and solidified as giant bodies of granites and granodiorites (containing calcium rich feldspars). These large granitic intrusions or 'batholiths' range in age from 100 million years to about 50 million years ago. One example is the Ladakh Batholith on which the town of Leh sits. Some of this magma also erupted on the surface through volcanoes. The rocks of Khardungla Pass are remnants of this ancient volcanic terrain. 

A similar situation today is along the western South American margin. There, subduction of the Nazca Plate underneath South America has triggered large scale magmatism and formation of giant batholiths of the Andes Mountains. 

Another impressive geologic feature of the Trans Himalaya is the Karakoram Fault Zone. It is a NW-SE aligned right lateral strike slip fault where crustal blocks have been sliding past each other since about 18 million years ago, resulting in a 150 km of offset of rocks. There has been some vertical movement also along this fault and this uplift has resulted in the formation of the Pangong Ranges where high grade metamorphic rocks have been exhumed from a deeper crustal level. The Pangong Lake is a drowned river valley formed by the damming of the river on its western end due to fault uplift. 

Strike slip faults have been in the news recently. The devastating earthquake in Turkey and Syria was caused by movement along the left lateral strike slip East Anatolian Fault. 

The India Asia collision resulted in the partial melting of deeply buried rocks of the Asian crust in the Miocene (21-16 million years ago) and the resulting granitic magmas have intruded the upper levels of the crust as dikes and sills. These melt channels also coalesce to form plutons and batholiths. Granitic intrusions of Miocene age which formed as crustal melts differ in their composition from the older Ladakh batholith which has a mixed mantle and crustal origin. There is a lot of interesting and complicated geology in the Trans Himalaya too! 

There are lots of technical papers on this topic. For good popular style book I recommend Mike Searle's Colliding Continents: A Geological Exploration of the Himalaya, Karakoram and Tibet. 

If you want a short answer to the question I posed, it is this: The Himalaya (including the Tethyan Himalaya) is the deformed northern edge of the India Plate. The Trans Himalaya is the deformed southern edge of the Asian Plate.
 

Links: Fire Use, Deep Water, Europa Geology

Sharing some interesting readings:

1) The Discovery of Fire by Humans. Jungle Book's primate king Louie was certainly aware of the transformative power of fire. As J.A.J Gowlett writes in a very informative review, many animals engage in fire foraging, opportunistically increasing their access to resources made available by natural fires. Early hominins too would have interacted with natural fires. The archeological record informs us that human engagement with and ultimately our control over fire was a long and convoluted process with evidence for early fire use going back to 1.5 million years ago. And would you believe it if I told you that the earliest preserved human fingerprint may be 80,000 years old and documents fire use? It was imprinted on a lump of pitch which is made by prolonged heating of tree bark. Pitch was used as a fixative in hafting. Fasinating stuff.  

2) The Deep Cycle of Water: Every schoolkid is taught about the hydrologic cycle wherein water moves between the atmosphere and shallow surface reservoirs. But water is present much deeper inside the earth, in fact it is present thousands of kilometers deep. It occurs not as free flowing H2O, but is incorporated inside the atomic structure of minerals as OH anions. It can escape this prison when minerals dehydrate during metamorphic reactions. The released water then rises and is expelled at the surface via volcanoes. In an alternate pathway, carried by sinking pieces of tectonic plates, water can reach even deeper in the earth, affecting the properties of the lower mantle and even the core. A short summary in Nature Geoscience on the state of our knowledge about this topic. 

3)  Plate Tectonics on Europa. The earth's outer silicate shell is broken up into tectonic plates which move around and jostle driving geologic activity and transforming the surface through geologic time. Scientists are looking to Jupiter's moon Europa and finding that its icy shell shows features indicative of intermittent plate motions, although the driving mechanisms will be different.  In Phys.Org, by Morgan Rehnburg. 

Mid Oceanic Ridges: Geodiversity And Biodiversity

Mid Oceanic Ridges. Unlike continental mountain chains, these undersea mountain ranges are invisible to our day to day gaze. 

Source: Gretchen L. Früh-Green et.al. Diversity of magmatism,hydrothermal processes and microbial interactions at mid-ocean ridges.

Yet, they are among the most dynamic of geologic features. They form where tectonic plates split and move apart, and new ocean floor is generated by upwelling magma. Sea water percolates through the cracks in this new crust, heats up in the subsurface and then rises carrying with it gases and metals. These vigorous hydrothermal circulation systems provide a link to exchange chemicals between the mantle and the crust. Varied microbial and macrofaunal communities colonize these environments depending on proximity to magma, the strength and chemistry of hydrothermal systems, and the nature of bedrock composition and fault controlled topography. 

Some specific geologic settings, those with serpentinite rocks,  have recently attracted great interest because they are thought to have provided the right combination of heat and chemicals to stimulate pre-biotic chemistry and the origin of life.

Gretchen L. Früh-Green and colleagues review this fascinating underworld, bringing out, both, the diversity of geologic processes at work and the resulting biodiversity that depends on this varied geology and energy supply. The introductory paragraph shared below gives an idea of the importance of this geological environment.

"Mid-ocean ridge (MOR) systems extend approximately 60,000 km around the globe and are the most dynamic and continuous tectonic feature on the planet (Fig. 1). On average, about 3.3 km2 yr−1 of new oceanic crust is generated at global spreading centres, which account for >70% of the total volcanism, and where about 60–70% of the Earth’s surface has been produced over the past 160–180 Myr (ref.2). Mantle melting, volcanism and faulting at MORs drive hydrothermal circulation that allows the transfer of heat, chemical compounds, metals and volatiles from the asthenosphere to the hydrosphere and biosphere. Approximately 75–80% of the Earth’s total heat flux occurs as the oceanic crust ages, and it is most pronounced at ridge flanks, where low-temperature fluid flow continues off axis for millions of years and contributes to global biogeochemical cycles. It is estimated that the volume of the ocean circulates through the oceanic ridge system in much less than 1 Myr (ref.6).

Spreading centres are one of the most extreme environments on Earth that can support oases of life at high temperatures and thriving in perpetual darkness. Microorganisms obtain energy from magmatic gases and chemical compounds of altered oceanic crust, rather than from light, through a process called chemosynthesis. In turn, many of these microorganisms symbiotically sustain macrofaunal communities that populate hydrothermal vent environments. The microorganisms with the highest known growth temperature on Earth are found within MOR hydrothermal systems and investigation of their genetic diversity has changed the current view of the tree of life".

I have put the last sentence of the first paragraph in bold to highlight the scale of this geological system. This is rich and rewarding reading. The paper is open access.

Gretchen L. Früh-Green, Deborah S. Kelley, Marvin D. Lilley, Mathilde Cannat, Valérie Chavagnac & John A. Baross: Nature Reviews Earth & Environment- Diversity of magmatism, hydrothermal processes and microbial interactions at mid-ocean ridges.

Cyanobacteria And Mountain Building In The Paleoproterozoic

By 2.3 billion years ago, cyanobacteria employing oxygenic photosynthesis began proliferating in the world's oceans. This resulted, several hundreds of millions of years later, in triggering the formation of fold mountain belts in places where tectonic plates converge and collide. 

This big idea linking geological processes and biological evolution is explored in an interesting paper by John Parnell and Conner Brolly published in Communications Earth and Environment.  

Source: Examining the tectono-stratigraphic architecture, structural geometry, and kinematic evolution of the Himalayan fold-thrust belt, Kumaun, northwest India- S Mandal et.al. 2019.

Consider how fold mountain grow using this example from the Lesser Himalaya. The different colored layers are rocks formed on the northern margin of the Indian plate. They range from 1.8 billion years to around 50 million years in age. As the Indian continent collided with Asia around 50 million years ago, this pile of metamorphic and sedimentary rocks was squeezed by compressional forces. They were folded and broken up by thrust faults into a series of panels or sheets. Deformation of the Indian plate spread from north to south. As you gaze from the top to the bottom of the graphic you will notice that folding and faulting first began at the zone of collision (extreme right of the graphic). Over time the locus of deformation migrated further and further away from the site of continental contact. New faults grew and moved sheets of rocks southwards, stacking them and building mountains by thickening the crust.

Now look at the more detailed cross section below from Mandal's paper. The fold and thrust architecture of Lesser Himalaya is clearly revealed. The thin pink lines are the thrust faults and they have partitioned the rock pile into a series of panels (A-G), causing 'stratigraphic repetition' across the mountain belt. Internally, each panel has the same sequence of layers. I am ignoring some of the complexities of the formation of the Greater Himalaya for the sake of brevity.

Parnell and Brolly argue that the increased biomass in oceans starting 2.3 billion years ago produced organic rich sediment, which transformed on burial into carbonaceous shale and graphite bearing metamorphic rocks. The rates at which tectonic plates move meant that basins that filled up with such sediments began arriving at collisional zones some 200 million years since their initiation. 

Carbon rich and graphite bearing layers reduce friction along faults making the contractional deformation I outlined in the Himalaya example easier. The widespread mountain building activity observed in the geological record from 2 to 1. 8 billion years ago was helped along by faults preferentially splitting the rock pile along weaker graphite rich layers with the carbonaceous material acting as a lubricant, allowing easier movement and stacking of the faulted blocks. The nice graphic below compiles instances of mountain building that took place during that time frame from across the globe.  

Source: Increased biomass and carbon burial 2 billion years ago triggered mountain building- Parnell and Brolly 2021.

The authors give several examples of Paleoproterozoic orogenic belts in which major fault zones are associated with graphite rich layers. They also point out that the atomic arrangement of carbon in graphite that has been disturbed by faulting is different from the background graphite in sediment. Many fault zones in their examples contain this 'disordered' graphite bolstering their case that graphite was mobilized during fault movement.

This connection between carbon rich sediment and faulting appears as a recurring theme in geologic history. Another example comes from the Cretaceous when there was abundant deposition of organic rich sediment during times of globally high sea levels and oxygen depleted ocean depths. The resulting black shales subsequently provided the weak zones for faults to split and move crustal sheets during the building of the Rockies, the Andes and other mountain ranges spread widely across continents.

While it makes sense that carbonaceous material will reduce frictional resistance and enhance fault movement, one must be careful not to overemphasize the causal link between carbon rich sediment and mountain building. After building a case for the close association of carbonaceous sediments and major faults, the authors write, "Given the link between organic matter and deformation evident in younger rocks, the scene was set for collisional orogenesis at ~2 Ga (Fig. 1) by the exceptional accumulation of organic carbon during the mid-Palaeoproterozoic.

I would think that 'the scene was set for collisional orogenesis at ~2 Ga' not because of the presence of carbon rich sediment, but because by that time a global plate tectonic regime was operating and plate forces were maneuvering continents in directions that resulted in them converging at multiple places. The pulse of mountain building during that time period occurred because several continents crashed into each other. 

That the presence of carbonaceous sediment may aid faulting but is not a necessary condition for thrust fault regimes to develop in convergent settings is supported by looking at what occurred earlier in geologic history before the exceptional bloom in cyanobacterial growth and carbon burial. Yatin Zong and colleagues in a recent paper published in Nature Communications describe the formation of the Central Orogenic Belt of Northern China which formed by the collision of a chain of oceanic volcanoes with the North China continent in Neoarchean times (2.8-2.5 billion years ago). They demonstrate the development of fold and thrust structures and large distance movement along thrust faults that occurred in this convergent plate setting along the North China continental margin between 2.6 and 2.5 billion years ago. Here, thrust faulting detached sheets of the crust made up of lava erupted from oceanic volcanoes and also sediments deposited in adjacent basins and moved them hundreds of kilometers.  The fault surface was lubricated by copious amounts of the shiny mineral mica. 

This is several hundred million years before the exceptional accumulation of carbonaceous sediment in Paleoproterozoic times. It appears that the lithosphere deforms in a mechanically consistent style under the horizontal compressive forces prevalent in plate tectonic settings, with hydrated oceanic crust, water saturated clays, micas, and carbon being alternative candidates for reducing friction.

How did the outer shell of the earth behave before plate tectonics? Early earth was a much hotter place. Warm rocks are weak. Plate motion is driven by a pull force that is imparted as a tectonic plate subducts or sinks underneath another tectonic plate. So, for plate tectonics to evolve, the lithosphere or the outer shell of the earth has to become strong enough not to disintegrate as it is being pulled by its subducting leading edge. Such conditions, geologists think, likely became more and more common as the earth cooled sufficiently by about 3 billion years ago. 

Before this time tectonics played out as gravity driven vertical movements of the crust . This inference is based on the structures observed in many Archean age terrains which show large granitic bodies encircled by steeply tilted and metamorphosed volcanic and sedimentary rocks. On a hotter earth, buoyant granitic magmas rose and intruded the near-surface volcanic and sedimentary layers pushing them aside and thermally transforming them into low grade metamorphic rocks.

This ensemble of metamorphosed volcanic and sedimentary rocks are called greenstones. The typical structure is a granitic dome encircled by vertically oriented layers of greenstone having a flattened or schistose fabric formed due to the parallel orientation of platy minerals like micas and chlorite. The satellite image below shows one of the classic examples of this 'dome and keel' structure from the Archean age terrain of Pilbara from northwest Australia. Grey green narrow schist belts are wedged between light colored oblate shaped granitic domes.  The Pilbara terrain ranges in age from 3. 5 billion to 2. 9 billion years.

The Indian continental crust has its share of these Archean granite-greenstone associations. Some prominent ones are in Karnataka, exposed near Dharwar, Shimoga and Kolar. Most of these belts are older than 3 billion years in age.

Contrast this structural style with that of a classic fold belt from younger Proterozoic times. These are the famous Nallamala ranges in south India formed by horizontal compressive forces.

The earth changed considerably between 3 and 2 billion years ago. Chemical differentiation of magmas separated heavier elements from the lighter ones, forming continents made up of lighter buoyant granitic rocks and oceanic depressions floored by denser basalt lava. The growing mechanical strength of its outer shell formed coherent slabs or plates ultimately resulting in the more familiar plate tectonic cycles of super-continent formation and breakups. 

Another striking change appeared on the surface by 2 billion years; the formation of high topography. In the Archean, growth of new crust by constant injections of magma kept the outer shell warm and weak, unable to support the weight of tall mountains.  As the earth cooled the lithosphere gained strength and became rigid. High mountains built by tectonic thickening of the crust at continent collision zones could now stand on a strong foundation.

Folds, faults, crushed rocks, and mountain belts. All these dramatic contortions of the crust readily invite us to probe the physical evolution of the lithosphere as an explanation for the geological dynamism of our planet. Parnell and Brolly in their paper remind us that unique biological events have played a role too in shaping earth's physical features, even those as immense as the Himalaya.  

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Wishing readers a very Happy and Safe New Year!

Articles: Trace Fossils, Supercontinents, Harappan Hydrology

 Some interesting geology rich readings from the past few weeks:

1) Ichnology is a branch of palaeontology that studies the traces made by organisms in soft sediment. These could be tracks and trails as animals move around on a substrate, or burrows constructed as escape structures or as dwellings, or bite marks on shells and bones. All these are indicative of behavior, which otherwise would be hard to discern from just the fossilized remains of body parts. Science writer Jeanne Timmons has written this lovely article on Ichnofossils and what they tell us about past ecology and animal behavior.

Trace fossils, the most inconspicuous bite-sized window into ancient worlds.

2) The earth has seen over its long geological history episodes of continents coming together to form a supercontinent, then breaking up and drifting apart forwhat seems an eternity, but eventually coalescing to form another giant landmass. When did this supercontinent cycle begin on earth. What are the forces that initiated and subsequently has maintained this mode of surface reconfiguration, and what are its consequences on tectonics, and the physical and chemical evolution of earth. A great review article by Ross N. Mitchell and colleagues.

The Supercontinent Cycle.

3) The rivers that sustained the Bronze Age Harappan Civilization have been the subject of lively research in recent years. Ajit Singh and colleagues have worked on the Markanda river catchment in the Sub-Himalaya dun region. Markanda joins the Ghaggar-Hakra river flowing through present day Harayana, Punjab and Rajasthan. They find that during the Mature Harappan Period (2600 B.C. to 1900 B.C.), large floods in the Himalaya foothill rivers sustained flow in downstream reaches, making  agricultural viable, even as northwestern parts of India experienced a reduction in summer monsoon strength.

Larger floods of Himalayan foothill rivers sustained flows in the Ghaggar–Hakra channel during Harappan age (behind paywall).

Readings: Myanmar Geology, Holocene Human Populations, Indian Archaeology

Some interesting readings over the past few weeks:

1) Myanmar Geology- Oblique convergence, where plates converge or collide at an angle, has produced some stunning geological features in Myanmar. Lon Abbot and Terri Cook sail down the Irrawaddy River describing vestiges of volcanic arcs, strike slip faults, en echelon sedimentary basins, and fold mountains, with a fair bit thrown in about the architecture and cultural history of the country.

Sailing Through A Subduction Zone.

2) Genetics And Human Evolution- Razib Khan compiles a nice list of the many aspects of human evolution and especially Holocene population history that has been brought out by recent work in genomics and ancient DNA.

What I'm Thankful To Know About Genetics And History In 2020.

3) Indian Archaeology- A sort of historiography of the field of Indian archaeology from Colonial times to today. Dilip Menon writes about the push and pull of ideas of conquest, politics, and nationalism that influence Indian archaeology research and narratives.

How Archaeology Has Shaped India’s Imagination Of Itself. 

Map: The Deep Geological Cycle of Carbon

When I was a kid not so long ago in geologic time, my understanding of how diamonds are created went something like this.

In forests and swamps, large trees grew and died. The wood got buried under more wood and layers of sand and mud. The wood in the bottom layers under the influence of great pressure and higher temperatures got converted first to coal. As burial to greater depths continued, this coal turned first to graphite and then finally to diamond. The story of woody material turning eventually to coal and then to graphite was roughly correct, but this geological path doesn't lead to diamonds.

Most diamonds form at depths of about 150- 200 kilometers. Rarer varieties known as sublithospheric diamonds form even deeper down. The carbon required to make a diamond is transported from the earth's surface to those depths by a subducting plate. Subduction is the process whereby an oceanic plate made up of dense Mg and Fe rich rocks sinks into the mantle. The Marianna trench for example marks the place where the Pacific plate is sinking underneath the Japan Plate. Why can't coal then move into the mantle this way? It doesn't because coal deposits occur in continental settings where the crust is made up of much lighter rocks richer in Si, Al, Na and Ca. This continental crust is buoyant and does not subduct into the denser mantle. So there is no way for coal that began its journey in ancient river floodplain, bogs, and swamps to get directly transformed into diamonds.

The denser oceanic plate contains carbon from a variety of sources. The lower layers of the oceanic plate is made up of a Mg rich rock known as peridotite. Occasionally, faulting may bring this peridotite to shallower levels where it interacts with sea water and gets transformed into a rock known as serpentinite with calcium carbonate minerals also forming alongside. Carbon is trapped in minerals like calcite and dolomite. The upper layers of the ocean plate is made up of the volcanic rock basalt. It too interacts with sea water, with calcite precipitating in rock cavities. This becomes another source of carbon.

Then there is carbon which is part of the shells and skeletons of planktonic marine creatures. These tiny photosynthesizing organisms which live in the sunlight zones of the ocean precipitate a calcium carbonate skeleton. When the organism dies, these carbon containing skeletons sink and blanket the sea floor. This source of carbon is a relatively late addition in geologic history. Calcareous nanoplankton first appeared in the early Mesozoic, some 225 million years ago. Organic tissue of marine creatures can also get buried, making this another carbon source. And finally, carbon coated sediment washed into the ocean by rivers and then transported into abyssal depths by deep sea currents contribute some carbon to the oceanic plate.

Each oceanic plate has its own selection from this carbon menu, depending on its unique geologic history. For example, calcium carbonate starts dissolving below a depth known as the calcite compensation depth. Sea floor below this depth doesn't retain much skeletal debris. Or, oceanic crust that formed in the Cretaceous contains abundant calcite in veins and vugs, likely because the warmer Mesozoic oceans promoted calcite precipitation on the sea floor.

This beautiful map shows several subduction zones. In the map, SedCarb refers to skeletal carbonate, SedOrgC to organic carbon, AOC Carb to carbonate in altered oceanic crust, SerpCarb to carbonate in serpentinite rocks. The major source of carbon is identified by a particular geochemical signature. Mineral carbonate for example has higher amounts of the heavier isotope of carbon (C13), while carbon that makes up organic tissue is much richer in the lighter isotope (C12).

  Source: Terry Plank & Craig E. Manning: Subducting Carbon

As the oceanic plate subducts carbon begins to get removed from the plate. Some carbon is removed when the sediment and altered oceanic igneous rocks are scraped off and plastered on to the sea floor. Such deposits made from scraped off sea floor are called accretionary prisms. They often poke out above sea level to form island chains. The Andaman Islands is an example of an accretionary prism that is made up of mechanically removed slices of the subducting Indian oceanic plate.

At greater depths, sediments and oceanic crust begins to be metamorphosed under higher temperatures and pressures resulting in the loss of carbon dioxide and water. This carbon dioxide makes its way into the overlying mantle and gets incorporated into magma. The spectacular volcanic eruptions along Japan, Indonesia, Caribbean and the Western North American coastlines are a result of the rising and depressurization of such volatile bearing magma. Some of the carbon in the belched out carbon dioxide has come from the burning of skeletons of marine organisms in the deeply buried plate underneath these volcanic systems.

A quantity of carbon does remain in the downgoing plate and reaches depths of 150-200 kilometer or more. The igneous rocks of the subducted plate gets altered to a dense rock known as eclogite. And at these temperatures and pressures, diamonds may form within these eclogites along carbon dioxide or methane rich domains. The subducting slab is also releasing some trapped carbon along with other volatiles which infiltrate the surrounding mantle. Diamonds can form in such metasomatized or fertile regions of the mantle as well. The main host rock here is peridotite. Subducted carbon is one source of carbon for diamonds.

Geologists think that primordial carbon retained in the mantle from when the earth formed may also be finding its way into diamonds.

Multiple sources perhaps, diamond forming chemical reactions can be summarized simply as driven by the reduction of carbon sourced from either carbon dioxide or methane.

CO2 = C + O2

CH4 + O2 = C + H2O.

From eclogite and peridotite parent rocks, diamonds are transported to the surface by an unusual magma type known as kimberlite and even less commonly by lamproites. These magmas are rich in volatiles like water, carbon dioxide, fluorine and chlorine and are also rich in magnesium. They are generated at the base of thick continental plates generally during episodes of continental fracturing. The volatile rich magma physically disaggregates diamonds from their parent rocks and carry them as they ascend through deep continent penetrating cracks with amazing speed, traveling 200 kilometers in a matter of hours, bringing to the surface its tiny but dazzling prize. The block diagram below summarizes the geological environments of diamond formation and their ascent.

Source: Steven B. Shirey et. al., Diamonds and Geology of Mantle Carbon; modified from Tappert R and Tappert M 2011.

The famous Panna diamonds from Bundelkhand in Central India came to the surface in a kimberlite magma eruption around 1 billion years ago. It is possible that its source carbon was transported from the surface to diamond forming depths hundreds of millions of years earlier, perhaps during the convergence and assembly of an earlier supercontinent.

Diamonds are often older than their host kimberlites by hundreds of millions to billions of years. During diamond growth, other minerals get trapped inside them as micro-inclusions. Their composition is therefore a record of the fluid chemistry of the mantle and the carbon cycle as it existed in deep time, billions of years before present.

Diamonds are one component of the deep geological cycle of carbon. We are familiar with the exchange of carbon between the atmosphere and the biosphere. Carbon is transferred to and fro in this system on a timescale of days to years to hundreds of years, but not much more. Longer geological sequestration of carbon occurs at shallower levels of the crust too. Soil can store carbon for thousands of years. Carbon can get trapped for millions of years in carbonate minerals that make up limestone and also in coal and oil. It is this shallow crustal carbon cycle that we are breaking by burning limestone and fossil fuels.

The deep geological cycle can take carbon from the surface and keep it in the mantle for hundreds of millions of years. The mantle releases it through sustained volcanism thus modulating earth's climate on long time scales. And occasionally as a return gift it throws up a few diamonds as well.