Hot rocks, big rivers and the world’s tallest mountain face

Posted on 16 May, 2012 by Metageologist

In areas of active mountain-building the middle crust can get hot and weak, like a soft jam/jelly filling in a sandwich.  These squishy rocks are hidden from us by the cold rigid upper crust, so we wouldn’t expect to see them reach the surface, would we? Well, what happens if you overfill a sandwich and there’s a break in the upper layer? Nanga Parbat in the Karakoram Himalaya tells us what.

Nanga Parbat is one of only 14 mountains over 8,000m in height. It’s summit is a mere 25km from the Indus, a major river flowing off the Tibetan Plateau and then along the length of Pakistan. The mountain face between the two (the Rakhiot Face, above) is often called the world’s highest as there is a drop of 7,000m, from peak to river. Nanga Parbat sits at the Western syntaxis of the Himalaya, this is the point where the structural grain of the Himalaya changes from East-West to North-South. It corresponds roughly to the corner of the Indian plate (buried somewhere below), where the plate boundary swings around.

Nanga Parbat contains granites at the surface that cooled only a million years ago. Not lava but granite that was intruded at depth and yet is now at the surface. A active major thrust puts gneisses on top of river gravels. It’s estimated that 25km of crust has been eroded away in the last 10 million years (or 12-15km over the last 3Ma, take your pick).

A hard rain falling

In order to remove 25km of crust, you need a lot of erosion. That sort of erosion requires high rainfall and a major river to take all the bits away. Both of these things are present at Nanga Parbat. What is mind-expanding is the idea that the erosion and the tectonics are linked. 

The “tectonic aneurysm” model (defined in Zeitler et. al 2001) suggests that the tectonics of the area are caused by the high rainfall and presence of the Indus river valley. Consider a river valley cutting deep into a mountainous area – it’s made a kilometres deep hole in the crust and this weakens the rocks below and either side of it.

To state the obvious, remove rock from the surface and the material below moves closer to the surface. This does two interesting things – firstly the reduction in pressure makes brittle rocks weaker. Secondly, the valley will change the thermal structure of the crust. Cutting a hole into the crust will bring hotter rocks closer to the surface (because the surface is lower). Then if the rocks beneath the valley start moving up (a process called advection) then they may move up so fast that they can’t cool down on the way. There’s more. Releasing pressure in very hot rocks (decompression) can trigger melting and granite production. The presence of melt reduces the strength of the crust by an order of magnitude. This means that the crustal flow will become easier as the rocks become weaker.

Erosion causes uplift which causes crustal flow which causes more uplift which causes more erosion which….. This positive feedback loop turns the initial weakness into a much bigger structure that affects the entire crust. The term ‘tectonic aneurysm’ refers to a medical condition where a weakness in an artery wall can cause serious medical problems as it gets bigger. My analogy of the overfilled sandwich with a cut in the top is more cheerful, but ‘tectonic aneurysm’ sounds much more sciency.

Egg or chicken?

The tectonic aneurysm model has also been applied to the Eastern syntaxis, at Namche Barwa in Tibet, where the Tsangpo river forms a deep gorge. These syntaxes are tectonic features, related to the corners of the Indian indentor, yet the aneurysm model regards a river gorge as thing that initiates extrusion. Why is there a connection between the two?

For me this is the most satisfying part of the model - the rivers flow across the syntaxes because of the geometry of the mountains – tectonics controls the location of the rivers which in turn influences the tectonics. Processes that affect the surface and those involving the entire crust are intertwined in a dance that lasts millions of years.

Consider the growth of the Himalayas. Rivers that used to drain off proto-Tibet into the Tethys ocean are now blocked by a mountain range, where India is pushing into Asia. These rivers start to flow parallel to and behind the mountains. Over time syntaxes develop and news rivers start to cut into them. Eventually these new rivers reach the old mountain-parallel ones and ‘capture’ them. The precipitation that has been stuck to the North of the Himalayas can now get through and the major river systems of the Indus and the Tsangpo-Bhramaputra can finally reach the Indian Ocean. At the ‘knick-point’ where these rivers drop sharply in altitude major gorges are formed. These ‘cut the sandwich’ and allow the weak hot middle crust to extrude out to the surface, forming some of the most fantastic scenery on earth.

The idea that patterns of erosion affect tectonics is a lovely illustration of the interconnectedness of the geosciences and is a current topic. A  very recent paper in GSA Today by Paul Kapp and co-workers looks at wind erosion in the North of Tibet. They show significant rates of erosion and speculate a link with the (small-scale) tectonics of the area. It seems you don’t need big rivers (or indeed water) for the atmosphere to influence the way rocks deform.

The processes of crustal thickening that created the hot middle crust now extruding to the surface at Nanga Parbat affects all of Tibet. What happens to this soft material where it can’t reach the surface? Also are the syntaxes the only places the sandwich has leaked? Interesting questions. Someone should write a blog-post about them…

This post is part of my journey into the geology of mountains.

Further reading

ResearchBlogging.orgAll of the papers listed here are publicly available right now. Click on the links and you get the entire paper no matter who you are. There are links in the text above, but here’s a list of the good stuff.

The good folk of the Geological Society of America make GSA Today available to all.

The GSA Today paper Zeitler et. al (Erosion, Himalayan Geodynamics, and the Geomorphology of Metamorphism) is the place to start for the tectonic aneurysm model.

Peter K. Zeitler, Anne S. Meltzer, Peter O. Koons, David Craw, Bernard Hallet, C. Page Chamberlain, William S.F. Kidd, Stephen K. Park, Leonardo Seeber, Michael Bishop, & John Shroder (2001). Erosion, Himalayan Geodynamics, and the Geomorphology of Metamorphism GSA Today

There is also a great set of papers on the Lehigh University website.

The  Kapp et. al paper is in GSA Today, link to html version if you didn’t like PDF link above.  DOI: 10.1130/GSATG99A.1

Kapp, P., Pelletier, J., Rohrmann, A., Heermance, R., Russell, J., & Ding, L. (2011). Wind erosion in the Qaidam basin, central Asia: Implications for tectonics, paleoclimate, and the source of the Loess Plateau GSA Today, 21 (4), 4-10 DOI: 10.1130/GSATG99A.1

For more information on Nanga Parbat geology, the 2006 paper from Jones et al.  gives a good overview, plus a taster of what I’m writing about next.

Picture of Rakhiot Face from sunbeer on Flickr under Creative Commons
Diagrams from Lehigh University ‘indentor corners’ project pages, with permission.
Categories: mountains, open access, tectonics

Lovely Ladakhi landslides

Posted on 9 May, 2012 by Metageologist

Time for a post with an emphasis on photos. There’s more context here, but let’s get on with admiring the view.

That’s me, standing in front of chorten (Tibetan Buddhist religious structure). The rather pretty cliffs are Tethyan sediments, now a long way from the sea. Note that the red layers are above the green, yet there are big red blocks sitting quite below the green cliffs. The blocks suggest there has been a big landslide at some point, but no doubt fluvial processes also move material down.

A Google Earth image shows that this is a common pattern along the river valley. Most of the big ‘streams’ of material that reach the valley floor are red, even though the green outcrops are nearer.

Here’s one last image.

Here, my glamorous assistant points out what is unmistakeably a landslide. The link between the scar on the red cliffs and the big pile of boulders is very clear indeed.

Categories: mountains

Building models about building mountains

Posted on 2 May, 2012 by Metageologist

How do mountains form? It’s just thrusts, right? Compression causes thrust faulting which piles up layers and layers of rock. This causes Barrovian metamorphism and makes mountains. Simple, no?

No.

Thrusting is important, of course, but as always real rocks are more complicated. For example, across the whole of the Himalayan chain, coincident with the highest peaks, is a structure called the South Tibetan Detachment system. It has a normal sense of shear, so it moves in the opposite sense to a thrust. Normal faults are associated with extension, not compression. To find something like this in the heart of a mountain belt caused by two plates colliding is extremely counter-intuitive.

So, a simple explanation (“its all thrusting”) is woefully inadequate. What more sophisticated models are there and how do they explain the South Tibetan Detachment?

Critical taper theory is a model that was originally developed for accretionary wedges.

Image from Wikipedia

I like to think of it in terms of a bulldozer pushing a pile of sand up a slope. The bulldozer (e.g. Japan) is scraping sand off the floor (Pacific plate). The sand deforms into a wedge, and reaches a stable shape with a particular angle between the slope and the surface of the wedge – the critical taper. This angle is controlled by the balance between movement of the sand on the floor (the ‘basal décollement’ at the top of the oceanic plate) and deformation within the wedge itself.

The wedge is a dynamic system. If something changes (like material being eroded off the top) then the wedge will deform internally until the critical taper is reached.

"Call this a pun? Fool" - a critical tapir.

Critical taper theory works well for accretionary wedges and has been has been modified and applied to mountains, usually using the name of orogenic wedge. Talking about the Himalayas once more, there is a structure that acts like a basal décollement called the Main Frontal Thrust. This is a feature found in many mountains belts so the concept of an orogenic wedge is a useful one.

There are many papers on this subject, but a classic of the genre is John Platt’s 1986 paper “Dynamics of wedges and uplift of high-pressure rocks” (available to us all via the man himself). It extends critical taper concepts to mountain belts such as the Alps or the Franciscan complex in California and addresses the more complex rheology of orogenic wedges (they are made up of viscous metamorphic rocks, not just cold rigid sediments). Its purpose is to explain how high-pressure rocks come to be at the surface within these wedges.

An orogenic wedge will thicken by thrusting, until it reaches its a stable geometry. If new material is added at the front of the wedge, then the wedge will shorten internally to regain a stable geometry. If new material is added underneath, to thicken the wedge, then it will extend internally. This extension within the wedge provides a mechanism to exhume high pressure rocks, to bring them to the surface via extension, even within a compressional orogen. Look at the John Platt paper for more explanation and some nice diagrams.

This is a neat trick and gives us a way to explain the South Tibetan Detachment system. The Himalayas are made-up of weak hot rocks piled up in a wedge between Tibet and the Main Frontal Thrust. They want to collapse down again, but they are kept up by the continuous movement of the Indian plate. This delicate balance has been disturbed in the past, meaning that there has been extension within the wedge, forming our normal faults. Problem solved?

For the sake of this post, perhaps yes, but I’ll leave you with a cliff-hanger. Extensional movement on the South Tibetan Detachment can in places be shown to the happening at the same time as thrusting structurally below. Wedge dynamics can’t explain this, it can only explain  thrusting and extension at different times (when the wedge is too thin or too thick). Also think of the geometry of this. If you have an extensional fault above a thrust then the material in between is being squeezed out, like a soft filling in squashed sandwich. What fresh madness is this?

This post is part of my journey into the geology of mountains.

Image of tapir from http://www.northrup.org/photos/bairds-tapir/
Categories: mountains

Geological pilgrimage – Assynt, Scotland

Posted on 25 April, 2012 by Metageologist

In Accretionary Wege #45 Denise Tang asked for “Geological Pilgrimage – the sacred geological place that you must visit at least once in your lifetime “.

For me, and dare I say it for any British educated hard-rock geologist the answer has to the Assynt district, in Sutherland, Scotland.

Denise asks for somewhere relatively remote and hard to get to. In British terms, Assynt is  as far away from ‘civilisation’ as it gets. It is almost at the very northerly point of Britain. From the bits of Scotland where most people live (the Midland Valley) it takes most of a day to drive there. The final part of the journey is on single-track roads that weave their way through an amazing undulating landscape. It is in the bit of Scotland that is more Scandinavian than British – Sutherland means ‘south-land’ in the language of the Vikings, but it is far enough North that I’ve seen the aurora borealis there.

To the Geology. This photo encapsulates the main geological features of the western half of Assynt.

This is the a view of the amazing mountain of Suilven. You’ll have spotted that it is made up of sedimentary rocks – Proterozoic rocks called the Torridonian. These sit on Lewisian Gneiss, Archaean and Proterozoic basement gneisses, visible in the foreground hillock. If you home sits on the the Canadian or Scandinavian shields then these are familiar rocks, but for the British Isles these are exotically old and exotically high-grade.

Side view of a sheath fold, Lewisian Gneiss

Intense glacial scouring makes this an amazing landscape. The gneisses form a rugged craggy ‘cnoc and lochan’ landform with lots of little lakes and rock hillocks. The Torridonian forms spectacular mountains (well, hills really) that rise above. In Assynt the Torridonian also includes a layer of suevite, recording a major meteorite impact somewhere nearby.

Why should you care about this area? Well, it is a UNESCO Geopark, for one and perhaps the main reason for this is that it contains the Moine Thrust Zone. This is a major thrust structure, marking the edge of the Ordovician/Silurian Caledonian orogeny. It puts the Moine Schists (lateral equivalents of the Torridonian, now strongly deformed and metamorphosed) over undeformed ‘foreland’ rocks. The foreland contains Cambro-Ordovician sediments which sit unconformably over both the Lewisian and the Torridonian. Assynt is a particularly interesting part of the Moine thrust zone as it is a culmination – there is a big package of thrust slices between the undisturbed foreland and the Moine schists. The Cambro-Ordovician sediments are an important part of the picture as they have a regular stratigraphy made up of varied distinctive rock types. This makes the wild structure of this ‘zone of complexity’ much easier to map.

In the early 20th Century, Peach and Horne of the (state-funded) British Geological Survey produced a classic report on the area. This proved without any doubt the reality of thrust faulting and is a significant event in the history of geology as well as a classic of fieldwork. The rock type mylonite (characteristically formed at thrust contacts) was first identified and named (by others) in the Moine thrust zone just north of Assynt.

As well as showing the complex geometry of thrusting in three dimensions via detailed mapping and copious cross-sections, Peach and Horne also had influential thoughts on the broader tectonic implications of what they found.

In Assynt the ‘zone of complication’ is awesomely complicated. The structure is truly three-dimensional. Well four-dimensional really, as structures cross-cut and debate rages over the sequence of thrusting over time.

Here’s an picture of a relatively simple area (the Glencoul thrust), to give you a taste.

Starting from the base moving up, first you see a hummocky area of Lewisian Gneiss. Then there is a crag of slightly-pinkish layered Cambrian quartzites. There is an unconformity at the base of the crag – it was once a rocky seafloor that became covered in sand. There is a step in the slope and the final rock unit appears – hummocky Lewisian Gneiss again!

The step in the slope marks the position of the Glencoul Thrust, one of the thrust planes that make up the Moine thrust zone. Think about this – all of the upper part of the hillside has been pushed on top of the lower part. If you trace the fault you realise that this would mean 10s of kilometres of horizontal movement. This is an amazing thing. Peach and Horne’s work is important as it settled the matter, proving the reality of thrust faulting, for the first time.

When geologists first starting debating the Moine Thrust, the way Gneiss formed wasn’t understood. Geologists argued that the picture above showed a conformable set of sedimentary rocks. Imagine, if you didn’t know that gneisses aren’t formed on the sea-floor then this makes sense, more sense than the idea of hillsides climbing on top of each other, anyway. Charles Lapworth, who first named mylonite and identified thrusting in Assynt, at one point had nightmares of being bodily caught up in the Moine Thrust, being crushed under what he called the great Earth engine.

You should visit Assynt, if you get the chance, it will give you good dreams, not nightmares.  The site of the North West Highlands Geopark will help you plan your visit. While you are waiting, here are some ways to visit virtually. You can get to geological maps of the area via the BGS geology of Britain viewer.  There’s an introduction to the Geology on Leeds University’s Assynt Geology website.

If you are as obsessed with Assynt as me, you should buy the centenary Special Publication from the London Geol Soc. It’s not cheap, but it is very large and very good. Portions of it are available on the Internet via the BGS open access repository. Other books include Exploring the landscape of Assynt from the BGS which is an informal guide for walkers and ”A geological Excursion guide to the North-West Highlands of Scotland” for a more geology focused guide.

To get a sense of the structural complexity, the paper on the Traligill Transverse Zone by Maarten Krabbendam and Graham Leslie is hard to beat. Here’s a taster of some modern BGS cross-sections to close out with.

Image sources

Photo of Suilven from Neil Roger (neil1877) on Flickr. http://www.flickr.com/photos/neil_roger/4129901847/sizes/l/in/photostream/
Sheath fold picture by the author.
Image of Peach and Horne from Wikipedia
Glencoul thrust picture from Shandchem on http://www.flickr.com/photos/14508691@N08/4818337130/sizes/o/in/photostream/
Diagram from Krabbendam and Leslie with kind permission of primary author. Also thanks to Maarten for information of guide books.
Categories: Accretionary Wedge