Pavement Geology: Ophicarbonate

You must have come across polished slabs of this dark green rock with white veins, used on stairways and as wall panels. It is an ophicarbonate.

The term encapsulates a range of geological processes, often separated by tens of millions of years. Oceanic lithosphere that is being generated by magmatism at mid ocean ridges, where plates diverge, is made up of igneous rocks arranged in a sequence. The top layer is basalt lava erupting and forming oceanic crust. Much lower down are ultramafic rocks like peridotites, composed mainly of the mineral olivine. In this lithospheric rock sequence, the boundary between the mantle and the crust is taken to be the transition from denser ultramafic rocks like peridotite to less dense feldspar containing rocks like a gabbro. Peridotites are rocks from the earth's mantle.   

As this newly formed lithosphere (tectonic plate) moves away from the active ridge, faulting can exhume these peridotites to shallower levels. The lithosphere is still warm and magmatic fluids and heated sea water hydrate and alter this peridotite to the mineral serpentine. Calcite can also form during these reactions, if these fluids are alkaline and bicarbonate rich. Such processes of serpentization have been observed, for example, at the Lost City hydrothermal vents in the Atlantic Ocean, where highly alkaline fluids are altering peridotite and precipitating large quantities of carbonate on the sea floor.

Being a part of a drifting tectonic plate, such serpentinized and carbonated peridotites eventually arrive at a subduction zone, where oceanic lithosphere is sliding underneath another plate. Slices of oceanic lithosphere including this altered peridotite gets scraped off along thrust faults and emplaced in a growing mountain chain. These fragments of oceanic lithosphere preserved on land along zones of plate convergence are called ophiolites. And the serpentine and calcite bearing altered peridotite is called an ophicarbonate.  

Alteration of the peridotite to serpentine and calcite can also occur during subduction by reaction with hydrothermal fluids expelled during metamorphism of buried sediments. During this alteration process the peridotite is crushed and acquires a broken fragmented appearance with veins of calcite surrounding blocks of serpentine. 

This example shows very clearly the brecciated nature of an ophicarbonate. You can observe fragments of green serpentine floating in and surrounded by large veins of calcite. 

Ophiolites and pockets of ophicarbonate are found all across the northern margin of the Indian tectonic plate from Ladakh and tracing the convergence zone southeastwards and south to the Naga Hills and the Andaman Island chain. 

Such deep sea processes are of interest to geologists who study the long term cycling of chemical elements on Earth. Alteration of  peridotites via carbonation reactions traps dissolved inorganic carbon in carbonate minerals like calcite and dolomite. The vast bulk of such altered rocks sink into the mantle along subduction zones, sequestering that carbon in the earth's interior for tens to hundreds of millions of years. Eventually that carbon may return to the surface as a gas via volcanic eruptions or geometrically bound as diamonds!

The next time you are climbing a stairway paved by this rock you can ponder on its fantastical journey from deep ocean to mountain front. And don't forget, you are stepping on a piece of the earth's mantle!

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.
 

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. 

Before The Himalaya: Story Of A Late Cretaceous Subduction Zone

My article on the geology of the upper catchment of the Brahmaputra River in southern Tibet has been published in The Wire Sciences. This region is technically called a suture zone. It contains the remnants of the Tethys Ocean floor made up of basaltic oceanic crust and overlying deep sea sediments. These rocks were deformed and uplifted during the India Asia collision.

I focus on a paper on the Jiachala Formation by Hanpu Fu and colleagues published in a recent issue of Science China Earth Sciences. They use a technique known as detrital zircon geochronology to resolve a long standing problem about the age of that sedimentary deposit. In my article I explain how this technique works. I also elaborate on the broader story these deposits tell us about plate tectonics and the beginnings of the Himalaya.

An excerpt:

In the Cretaceous Period, the Indian plate, which had been moving northwards since the breakup of Gondwanaland, was approaching the Asian continent. The southern edge of the Asian continent was lighter continental crust, whereas the leading part of the Indian plate was denser oceanic crust. As a result, in the zone where the the two plates converged, the denser Indian plate slid below the Asian plate, forming a subduction zone.

As the Indian lithosphere sunk deeper into the mantle, it heated up and released water trapped in sediments and hydrated oceanic crust. This water penetrated the overlying Asian plate, lowering the melting point of its rocks and triggering magma generation. This buoyant magma rose through the Asian continental crust. Some of it reached the surface, resulting in extensive volcanism. The rest solidified in the subsurface, forming giant bodies of granite known as batholiths.

Such terrains have the grandiose name of magmatic arcs. The town of Leh and the surrounding settlements in the Indian region of Ladakh are situated partly on a magmatic arc.

Two sedimentary basins since developed south of this arc. Immediately adjacent to the magmatic arc was the forearc basin. A deeper depression, known as the trench, formed further away on the Indian oceanic lithosphere, at the junction where the Indian plate had slid under the Asian plate. Both were receiving sediments derived from the erosion of the Asian continent.

During subduction, slices of the Indian plate were scraped off and thrust to the surface. Such fault-bounded piles of sediment and oceanic crust are called accretionary wedges, and they, along with a chain of oceanic volcanoes  that formed to the west in the region between Ladakh and Kohistan, would have been the first island ranges formed in the Tethys Ocean between India and Asia.

 Read the complete article here.

Tsunami History Preserved In Indonesian Cave Deposits

How would you know if a coastline had been inundated by a tsunami say 5000 years ago? Well, a tsunami carries sediment stripped from the ocean bed and deposits this material over the flooded coastline, beyond the range of what a regular storm would. The problem is that such deposits have poor preservation potential and over time get eroded away. There are however some environments where such tsunami deposits may get preserved inland. These are estuaries, coastal marshes and lakes. Here, interlayered with normal estuarine, marsh or lacustrine sediment, one may find layers of sand of a distinctly different composition and texture and containing remains of organisms which live in an open marine setting. This implies a sudden incursion of marine waters into these inland coastal settings. The other coastal setting with a good preservation potential are caves. These too get flooded by storm surges and tsunamis and may preserve a record of such events in the form of sand deposits. The picture to the left (Source: Rubin et.al. 2017)  shows sand layers deposited by the 2004 tsunami.

In one such cave on the coast of Aceh, Indonesia a record of the 2004 tsunami along with sand layers deposited by 11 older tsunamis going back to 7400 years ago have been preserved.

Highly variable recurrence of tsunamis in the 7,400 years before the 2004 Indian Ocean tsunami-
Charles M. Rubin, Benjamin P. Horton, Kerry Sieh, Jessica E. Pilarczyk, Patrick Daly, Nazli Ismail & Andrew C. Parnell

Extract:

 We identify coastal caves as a new depositional environment for reconstructing tsunami records and present a 5,000 year record of continuous tsunami deposits from a coastal cave in Sumatra, Indonesia (Fig. 1), which shows the irregular recurrence of 11 tsunamis between 7,400 and 2,900 years BP. The sedimentary record in the cave shows that ruptures of the Sunda megathrust vary between large (which generated the 2004 Indian Ocean tsunami) and smaller slip failures. The chronology of events suggests the recurrence of multiple smaller tsunamis within relatively short time periods, interrupted by long periods of strain accumulation followed by giant tsunamis. The data demonstrates that the 2004 tsunami was just the latest in a sequence of devastating tsunamis stretching back to at least the early Holocene and suggests a high likelihood for future tsunamis in the Indian Ocean. The sediments preserved in the costal cave provide a unique opportunity to refine our understanding of the behaviour of the Sunda megathrust, as well as study in detail the sedimentology and hydrological characteristics of tsunami deposits.

There is one point that cannot be over stressed. The average recurrence time for earthquakes and tsunamis has been estimated to be on the order of several hundred years. However, there is a great variation in the actual occurrence, with several smaller tsunamis occurring just decades apart. While our understanding of earthquake mechanisms and tsunami generation will go on improving, ultimately what will save lives is better preparedness. This includes adherence to structurally appropriate building codes, functioning tsunami warning systems and well drilled and practiced disaster management plans. South East Asia has long neglected these issues and there needs to be a renewed focus on them.

Which Are Older? Lakshadweep Islands Or Andaman Nicobar Islands?

A friend asked me this question:

Which formed first, Andamans or Lakshadweep?

My answer was-

Lakshadweep islands, as a system of living coral reefs, lagoons and sparkling shell sand beaches, is Holocene in age (past 12 thousand  years). These coral communities rest on earlier Pleistocene reefs. So, the history of exposed reefs and atolls is a Quaternary Period phenomenon going back several hundred thousand years. Periodic polar ice cap growth and melting drove sea level fluctuations, resulting in  episodic shallow seas and vertical coral growth and reef building. Below these Pleistocene and Holocene corals lie earlier Cenozoic carbonate sediments (Source 1, 2 ) . These sediments were deposited in a subtidal marine setting, with reefs and sand shoal type environments prevailing from time to time.

We are not sure whether vertical coral growth during deposition of these earlier carbonate sequences created coral islands. It is possible that during this long Cenozoic history, there may have been episodic appearance of islands. Coral island systems and small sand shoals, environments lasting for thousands of years, would have developed due to vigorous coral growth and a static sea level, before being submerged again as sea level rose and drowned them.

And what lies below? All this Eocene to Pleistocene  (56 million to 2.5 million years) sediment sequence has been deposited on top of a Palaeocene-Eocene  (66 million to 56 million years) volcanic basement. This basement is the northern part of the Chagos-Laccadive ridge, formed when the Indian plate rode over a hot area of the mantle known as the Reunion hotspot. Below the lava is Indian Precambrian continental crust. The foundation of the Chagos-Laccadive ridge is therefore a rifted sliver of continental crust separated from the west coast shelf margin during India's separation from Africa.

The map below summarizes the setting of the Chagos-Laccadive ridge with respect to the Indian shelf margin. 

Source: Deepwater West Coast India - Pre-Basalt and Other Mesozoic Petroleum Plays: Glyn Roberts et al. 2010

Regarding Andamans.. This island chain are the central part of the Burma-Sunda-Java subduction complex in which an accretionary prism and deep sea turbidite deposits are exposed. This means the islands are made up of marine sediment and oceanic crust of a subducting slab (oceanic Indian plate) which got scraped off and plastered on to the overriding plate (oceanic South East Asian plate).

A tectonic cross section of the Andaman subduction complex is shown below.

 Source:  Mud volcanoes show gas hydrates potential in India's Andaman Islands-  Vignesh Ayyadurai et. al. 2015

Sediment and volcanic material and mafic igneous oceanic crust making up the Andaman chain may have started appearing above sea level from Eocene times (~50 million years ago).  Eocene sediment of the Mithakari Group contains detritus derived from earlier Late Cretaceous -Early Eocene ophiolites (slices of oceanic crust). This indicates that ophiolite blocks were thrust up and were exposed above sea level and were being eroded.  Such accretionary prism settings and forearc basins are cannibalistic, in that, the older deposits are emplaced above sea level and become a source of sediment for younger sequences. As tectonic plates continue to push against each other, these younger sequences in turn are moved upwards along thrust faults and become exposed above sea level. Certainly, by Pliocene times (5 million years to 2.5 million years ago), there would have been a large enough island chain.

I guess to the best of my knowledge the answer is that, although in the Lakshasdweep area, coral reef and atoll environments may be emerged above sea level episodically over the past tens of millions of years, as permanent land the Andamans are older.

One misconception I have encountered regarding Lakshadweep is that the Chagos-Laccadive ridge is a southerly extension of the Aravalli mountain chain.

This is not correct.

As I mentioned above, the basement of the ridge is likely Precambrain continental crust  which rifted apart from the southerly west coast margin of India. So, the continental crust making up the ridge would have been part of the Southern Granulite Terrain and western Dharwar craton (craton- earliest formed pieces of continental crust going back more than 3 billion years ago) of south India. The Aravalli craton and the Southern Granulite Terrain / Dharwar craton were two distinct cratonic blocks which collided and sutured by early -mid Proterozoic times (2.5 billion to 1 billion years ago). The Chagos-Laccadive ridge is oriented NNW-SSE parallel to the Indian west coast shelf margin and the Dharwar structural trends.  Post rifting, as the Indian western margin moved over the Renunion hot spot, volcanism covered this basement with lava, enhancing the ridge structure. The Chagos-Laccadive-Maldive ridge is a hotspot trail which marks the movement of the Indian plate above the Reunion hotspot.

One can imagine extending in an arcuate line the Aravalli mountain trend south to connect with the Chagos-Laccadive ridge.

Source:  The Central India Tectonic Zone: A geophysical perspective on continental amalgamation along a Mesoproterozoic suture-  K. Naganjaneyulu and M. Santosh 2010

But these were two different pieces of continental crust in the Archean. In the above figure the Dharwar and the Bhandara Cratons form a South Indian crustal block, while the Bundelkhand and Aravalli Fold Belt form the North Indian crustal block.  The Aravalli mountains terminate north of the Central Indian Tectonic Zone (shown by roughly east-west trending fault lines). This is the suture zone between the North Indian and South Indian crustal blocks.