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!

Himalaya: Critical Wedge

 A while ago a friend commented that she had learned in school geography class that Himalayan faults are the youngest. Now, earthquakes in Bhuj, Gujarat, and Latur,Maharashtra, in recent times tell us that there is active faulting going on elsewhere in India too. But I could understand what she was trying to say, that the Himalaya is a growing mountain chain with active faulting. 

I think a lot of non-geology folks appreciate this point. What is not that well known is that within the Himalaya the locus of active faulting has shifted southwards over geologic time. The Himalaya is the northern margin of the Indian continental crust which has been broken up into blocks or litho-tectonic units and stacked by major faults. From north to south these faults are, South Tibetan Detachment, Main Central Thrust, Main Boundary Thrust, and Main Frontal Thrust. There are plenty of minor faults between these major breaks. 

As the Indian continent collided and underthrust Asia, slices of its crust were pushed up in the following order; The Tethyan Himalaya along the South Tibetan Detachment (45-35 million years ago), the Greater Himalaya along the Main Central Thrust (24-15 million years ago), the Lesser Himalaya by the Main Boundary Thrust (and many subsidiary faults, 11-5 million years ago), and the Siwaliks by the Main Frontal Thrust (and some subsidiary faults, 1 million years ago to recent). All these faults merge at depth with a north dipping (sloping) master fault known as the Main Himalaya Thrust along which the India plate is underthrusting or sliding underneath Tibet. The Himalaya is deformed Indian crust riding atop the MHT.

Why did faulting activity migrate southwards in this growing orogenic (fold and thrust) mountain belt? Geologists give a mechanical explanation of this style of mountain growth using the Critical Wedge Model. 

It will be worth pausing this post to watch a video made by Middlebury Plate Tectonics on Critical Wedge Theory. Email subscribers who cannot see the embedded video may watch it at this link- Critical Wedge Theory- Himalaya.

 

To summarize, the Himalaya may be abstracted as a wedge of crust which is thicker in the north and thinner towards south. The ratio of normal stress to shear stress controls whether a fault can slip. As crust thickens beyond a threshold value of the ratio, increased normal stress can pin down and lock a fault. Subsequently, the locus of active fault slip migrates towards the region with a more favorable stress ratio, which in the case of the evolving Himalaya orogen has been progressively southwards.

This is a very informative video but I know of many geologists who would protest. Their objection will not be that the Critical Wedge model is wrong but that it doesn't explain all of the Himalaya. They argue that the upper structural levels of the Greater Himalaya were extruded by a different mechanism. The growth of compressional mountain belts involves crustal thickening due to folding and thrusting. The Critical Wedge model explains this as taking place by the brittle breakage of slices of underthrusting crust along faults and their continuous accretion to a growing wedge. Rocks of the Greater Himalaya though show signs of ductile deformation. High grade gneisses were partially melted to form migmatites. Pods, lenses and sheets of granite magma was injected along fractures and planar rock fabric (schistosity).

The picture above shows leucogranite sills (white layers) intruding high grade gneisses near the village of Naagling in the Kumaon Himalaya, Uttarakhand. This partial melting and magma injection was contemporaneous with the extrusion of rock from deeper to shallower levels of the crust. 

All this took place beginning about 24 million years ago. Geologists have termed the movement of this hot mushy ductile rock mass as 'channel flow', literally to mean a channel of semi-solid rock that is being squeezed upwards like toothpaste from its container. In this case, the container were two bounding fault systems, the South Tibetan Detachment as the roof, and the Main Central Thrust as the floor. Supporters of channel flow say that the pervasive ductile deformation observed in the Greater Himalaya doesn't support the Critical Wedge mechanism of orogen growth. Instead, they propose that 'channel flow' was a unique phase in Himalaya development, restricted to the Miocene when deeply buried hot crust was being extruded. Over time  shallower levels of the crust were incorporated into the growing orogen where colder temperatures permitted brittle breakage of the crust and critical wedge growth. The Lesser Himalaya and the Siwalik ranges can be more satisfactorily explained by this mechanism of southwards fold and thrust propogation. 

'Critical Wedge' and 'Channel Flow' are statements on how crust with contrasting mechanical properties responds to compressional forces of tectonic origin and/or surface directed pressure gradients generated due to removal of overburden by erosion.

One final point. The video mentions 'out of sequence' thrusting referring to rejuvenation of extinct fault zones in the rear of the wedge. In case of the Himalaya this means renewed faulting in locales much to the north of the Main Frontal Thrust. This out of sequence thrusting manifest by low level earthquakes is taking place near and just southwards of the Main Central Thrust zone and seems to be driven by enhanced erosion stripping away rock, thereby reducing crustal thickness and normal stress.

Interestingly, I came across a paper by Paramjit Singh and colleagues which has used Apatite Fission Track (AFT) to reveal a pattern to this exhumation. Fisson Tracks is a kind of radiation damage in uranium bearing crystals. It is an ongoing process, but the tracks get preserved only below a critical temperature. The density of tracks is correlated to the time since the rock cooled below the healing temperature. In mineral apatite, fission tracks records the time when the rock cooled below 120 deg C. A young AFT date means that the rock was at around 4 km depth at a more recent time and has been exhumed to the surface much rapidly than a rock recording an older AFT date.

AFT dates taken along a north south profile in the Kumaon-Gharwal Himalaya from the Vaikrita Thrust to the Berinag Thrust  show a southward younging of dates, indicating sequential uplift and exhumation from north to south since Pliocene times (<5 million years ago). The 'out of sequence' faulting regime seems to be a second cycle of an 'in-sequence' pattern developing in the footwall (structurally underneath) of the Main Central Thrust zone. Similiar studies done by this team of scientists across nearby transects in the same climatic zone in the High Himalaya show that rocks are following different exhumation patterns. Contrary to what the video depicted, that does not sound like a climate controlled phenomenon. Rather variations in local tectonics may be dictating this style of exhumation. 

Himalaya never cease to be a mystery and a wonder.

Mountains Of Saint Francis - Walter Alvarez

A certain type of travel book or TV show on Italy features the adventurer driving through sun dappled rolling hills and winding narrow roads to a picturesque village in search of that one undiscovered Trattoria not featured in similar other books or TV shows. Walter Alvarez though refreshingly keeps driving past these rustic eating places to an old quarry just beyond the village. There, he begins poking around in the rocks in an attempt to unravel their secrets.

Walter Alvarez is quite a famous geologist. He was one of the proponents of the theory that a meteorite impact precipitated a mass extinction 65 million years ago, an idea that is now amply supported by evidence.  He has written a story about that discovery in T Rex And The Crater Of Doom (dinosaurs were the most famous casualty of this event). He has had a long professional relationship with Italian geologists and he uses the Italian rock record to explain the methods and basic principals used by geologists in this enjoyable book The Mountains Of Saint Francis: Discovering The Geologic Events That Shaped Our Earth. 

The Mountains of Saint Francis (after Francis of Assisi) is Alvarez's name for the Apennine mountains of the Tuscany and Umbria region and this books explains step by step how they came to be. Throughout the Mesozoic until mid Cenozoic, what is now Italy, was a promontory of the African continent sticking out like a north pointing thumb into the sea of Tethys that separated Africa and Europe. An enormous pile of mostly limestone accumulated on this submerged promontory. These Jurassic to mid Cenozoic limestones form the building block of the Apennine mountains. They stand spectacularly exposed in road cuts and cliffs and have attracted the attention of geologists from all over the world.  As a result, the Apennine rock exposures along with younger Pleistocene deposits have become some of the best studied strata in the world. They not only tell us about local geological evolution, but have provided key insights to answer some broad geological questions.

Going Hiking In Pangaean India

I have been thinking unhappy thoughts ever since I came upon this map of Pangaea with today's political boundaries overlaid on it.

Where would I have going hiking in Pangaean India?

1) The Himalayas, crown jewel of hikers arose begining early Cenozoic and reached their bewitching heights in the mid Miocene -Pliocene.

2) The Western Ghats, the poor man's Himalayas, arose in the Cenozoic too after the breakup of India from Madagascar (88 mya) and Seychelles (66 mya). They represent heights reached due to an initial high rift flank, amplified by denudational isostacy and crustal upwarp due to intraplate stresses propagated southwards from the Himalayan collisional zone.

3) The Eastern Ghats, a line of mountains parallel to the east coast of India also arose much later than Pangaea forming during and after the breakup of India with Antarctica about 130 mya.

4) The Vindhyan and Satpura mountains in central India are composed of Proterozoic and late Paleozoic -Mesozoic rocks resp. but much of today's relief also represents topography rejuvenated since mid Cenozoic, ultimately related to stresses from the Himalayan collision.

5) The Aravalli mountains in Rajasthan is a Proterozoic orogenic belt but probably didn't have much topography during Pangaean times.

For most of the time period from Cambrian to Carboniferous the Indian shield was a tectonically stable area. Pangaean India was a place where a vast peneplain had developed over most of the Indian shield in response to long lasting denudation. This cycle of deep weathering and erosion lasting tens to a hundred million years would have stripped and ultimately flattened the Aravalli and south Indian orogenic mountains, exposing mountain roots and lower crustal rocks like granulites and charnokites. The result would have been a subdued topography with a flat horizon as far as the eye can see, occasionally interrupted by gentle rolling hillocks made up of more resistant lithologies like quartzites and charnokites. 

Subsequent episodes of uplift and erosion has destroyed this flat topographic surface from all over the Indian peninsular region but some remnants of this peneplain termed the Gondwana surface can be observed at around 2400 m mantling the granulites of south India around the popular hill stations of Ooty and Kodaikanal in the Nilgiri mountains.  It has been lifted to these heights during Cenozoic uplift of the Western Ghats.

The only places of considerable relief would have been the emerging Permo-Triassic rift basins of the Satpura, Pranhita Godavari and Mahanadi belts in the central and eastern part of  country. A horst graben structure would have resulting in a ridge and flat valley type topography. Not particularly attractive for a challenging hike. Plus it was really swampy and hot in those rift basins.

I wouldn't have liked to live in Pangaean India. I am too spoilt by views like this one, which appeared only in the Miocene.

Photo: Nanda Devi and Namik Glacier in the Kumaon Himalayas, November 2012.

Review Article On The Origin Of Mountains

Simon Lamb and Anthony Watts have published a review article on the Origin of Mountains in a recent issue of Current Science. This volume has a special section: Perspectives on Earth Sciences 2010.

The article discusses at length the basic principles of mountain building.. isostacy, crust mantle density and thickness contrasts, horizontal forces,  lithosphere and asthenosphere strength and flow characteristics along with examples from the Andes and Himalayas and plenty of neatly annotated figures.

There is a nice symmetry to the way it ends, suggesting that the nuclei of stable continental crust are forged in the weak interior of great mountains:

An intriguing final insight of all this is that the central highly deformed parts of mountain belts, by being such weak and mobile parts of the Earth, may be the places where the strong cratonic cores of the continents were first formed, comprising what are today the most stable parts of the dry land we live on. This is because the process of mountain building, by squeezing both the crust and mantle parts of the lithosphere, creates a thick lithosphere.

Over time, as geotherms relax and the crust heats up as a result of the increased radiogenic heat generation in the thickened crust, granulite grade metamorphism will occur, eventually dehydrating and further strengthening the crust. If, at some later stage, the crust in this thick lithosphere is eroded back down to its original thickness of around 30–40 km, as isostasy would predict, the land surface will return to around sea level, but with the deep crustal levels of granulite grade metamorphic basement now exposed at the surface. So, as has been long suspected by geologists, mountain building, although occurring in only a small fraction of the surface area of the continents at any one time, might have shaped most of the Earth’s continental crust.

Graduate students and educators should find this a very good resource to brush up on the fundamentals.