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/

Crème brûlée or jelly sandwich?

Posted on 18 April, 2012 by Metageologist

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

Rocks can behave rigidly, or under some conditions, they are ductile and can flow. This is a remarkable thing, which helps explain why the theory of Continental Drift was not more widely accepted (prior to the plate tectonic revolution of the 1950s). Seismic evidence from the deep earth shows that the crust and mantle rocks behave elastically (on short timescales). This was taken as evidence that they could never flow, meaning the continents couldn’t drift. We now know that hot rocks in the mantle can flow (over long timescales) and that this is a necessary part of plate tectonics.

What about other geological implications? What other things can be explained by the ‘Jekyll and Hyde’ way in which rocks deal with stress?

Small scale structures: Most rocks under surface conditions behave rigidly – they only change their shape by fracturing or by being dissolved. Rocks deformed near the surface are therefore typically fractured or faulted. Slightly hotter rocks may be deformed by dissolution, where material is dissolved into solution and redistributed into veins. Stylolites and cleavage are structures formed in this way that are common in deformed sediments. Salt and mud and are two substances that can flow in cold shallow rocks. Salt diapirs and mud volcanoes are structures where these light substances flow towards the surface.

Stylolite from Ron Schott on Flickr (http://www.flickr.com/photos/22644739@N00/4468019786)

To deform typical rocks in a ductile way, they need to be under conditions associated with metamorphism – below depths of about 15-25 km and above temperatures of ca. 300 ^oC.

Under these conditions, various forms of sold-state creep begin to work and rocks can ‘flow’. Typical structures here are fabrics, such as schistosity or gneissosity – entire rocks are deformed as shown by the flattened minerals within them. Often there are areas of very high strain called shear zones, typically containing mylonites. A process called strain softening, plus perhaps a focussing of water or heat flow makes these zones softer than the surrounding rocks, meaning that they get more and more deformed.

Shear zones are analogous to faults, indeed a major structure may be a fault near the surface but a shear-zone at depth. A shear-zone is likely to be wider than the equivalent fault – the deformation is distributed through a large volume of rock.

Shear zone in amphibolite from Ian Stimpson @hypocentre (http://www.flickr.com/photos/17907935@N00/6723121739)

What controls the brittle-ductile transition? Major controls on the rheology of rocks are composition, temperature, pressure, the presence of liquid, and the presence of melt. Composition is quite hard to change, but if you increase any of the other things, the rock is more likely to behave in a ductile fashion.

Large scale features, large scale evidence. There are some big features that can be explained in terms of their rheology, which means they can be used to directly measure the rheological properties of the crust.

Foreland basins are sedimentary basins where the accommodation space (the hole in the ground filling up with sediment) is created by the flexure of the rigid upper crust. Think of the Indian continent being pushed under the Himalayas and Tibet. The weight of Tibet is pushing down on the Indian plate and this bends it downwards. The space created fills with sediment being eroded from the Himalayas. The Ganges basin is formed.

The Indian crust is cold and rigid. Clever folk can do the maths on the shape of the crust as it bends down. This confirms that the pattern matches the model for rigid, elastic deformation. It also allows then to calculate the plate’s flexural rigidity, which is a measure of its strength. This means quantifying the rheology of real bits of the earth, which is a very useful trick.

If you read up more on foreland basins, the picture gets more complicated as, over time the crust will further deform in a viscous way. I want to take you away to a different example of viscous behaviour.

There are raised beaches all over high-latitude areas of the Northern hemisphere. These are fossil beach deposits now found metres above sea-level. They can’t be explained by global sea-level changes as equivalent features are not found further south.

Raised beach from Scotland (in foreground). From SAGT @http://www.flickr.com/photos/sagt/4304338029/

These raised beaches formed when global sea-level was similar to today’s level: they are raised because the earth’s surface has risen up since they were formed. This isn’t due to tectonics – these areas are far from any plate boundary. What these areas have in common is that they were covered with kilometres of ice during the last ice age. This ice weighed a lot and this force pushed the crust down. The upper crust bended rigidly, pushing underlying viscous rocks (probably in the mantle) out of the way. The ice melted relatively quickly, removing the downward force. The viscous mantle rocks are slowly recovering, flowing back into glaciated areas and lifting the earth’s surface, leaving fossil beaches high and dry. The overlying rocks are being passively pushed up, with occasional post-glacial earthquakes occurring in otherwise tectonically quiescent areas. Once again, these natural phenomena are an excellent opportunity to directly measure the physical properties of the earth’s interior: the rate of rise lets you measure the viscosity.

Crème brûlée versus jelly sandwich

So the earth is strong near the top and weaker at deeper levels? If so, it would be like a crème brûlée. A giant spoon digging into the earth would crack through an upper layer (like burnt sugar in a crème brûlée) and then scoop gently through the soft mantle beneath (creamy custard).

If the earth were boringly simple, this would be accurate, but as you know, life is more interesting than that. The earth gets hotter with depth, so rock is more likely to be weaker with depth, but the composition of the earth is not constant. It’s long been known that the surface layer that moves as part of plate tectonics (the lithosphere) is not just the crust. There is a layer of mantle material (lithospheric mantle) stuck to the bottom of the crust meaning that within the lithosphere, there is a dramatic compositional change which complicates matters.

An interesting paper by E.B. Burov and Tony Watts (available here, thanks GSA!) discusses the crème brûlée model and compares it with a ‘jelly sandwich’ model. A jelly (jam) sandwich has a soft layer between two stronger layers. The sandwich model supposes a strong rigid upper crust, a weak hot lower crust and then a strong layer of lithospheric mantle.

You’d have thought that, knowing the composition of average crust and mantle and the conditions with depth, we could resolve this debate easily. As it turns out, although we know that olivine-rich mantle is much stronger than quartz-rich crust we don’t know by how much. It is very hard to run experiments with low strain rates and high temperatures comparable to real-life conditions so we can’t measure directly.

Burov and Watts combine numerical modelling and indirect observations to settle the issue. Think back to the Indian crust subducting under Tibet. One of the things you can infer from geophysical studies of these situations is the effective elastic thickness of the lithosphere. This is the cumulative thickness of rocks that are behaving elastically. For India, this is 70km, which is greater than the thickness of the crust. This is hard to explain, unless the lithospheric mantle is strong, suggesting you have a thick sandwich, not a thin layer of burnt sugar. They also show numerical modelling that suggests that without a strong lithospheric mantle, subduction and mountain building wouldn’t be possible.

Note that for this last section I’ve switched to talking about weak/strong rather than rigid/ductile. If you a simple sort like me, you can think of the two as synonymous, but they are not really which is why Watts and Burov talk about long-term strength. Complications arise because most rocks are viscoelastic – they transmit earthquake waves elastically (over short timescales) but behave viscously over longer timescales. The paper gives you a taste of this, but I’m not the man to take you any further into that world.

Rocks behave in complicated ways when placed under stress. I hope I’ve given a taste of the complications that arise from this in the wider world. I leave you with one thought: if you squeeze an over-filled sandwich – what happens to the filling? I will return to this and how it relates to Tibet and the Himalayas, in another post.

Granitic gorgeosity in the high Himalaya

Posted on 11 April, 2012 by Metageologist

I’ve been writing a lot of rather ambitious wordy posts about Himalayan geology (perhaps too many, too ambitious) so I thought I’d give you a post with visual impact for a change. I can’t resist some words of explanation but I hope the pictures would speak for themselves, if I let them.

I’ve some pictures from the Indian Himalaya. I’ve put a satellite image of the area up in WoGE #322 and talked about growing a beard while here already.

Trekking in the Himalaya, you get used to amazing views. But this was a taste of something special – Gumbaranjon peak, approached from the south.

The main buff-coloured cliff face is granite, with dark layers of xenolith. The green/purple rocks on the right skyline are behind, on the other side of a major extensional detachment (in the heart of a compressional orogeny).

Walking closer and around the base, we get a better view. The dark xenoliths and light granite make it a bit like looking at a negative photo. The eye tries to trace the pattern of granite intrusion by putting patterns into the places where the xenoliths aren’t. There are hints here that the intrusion process wasn’t simple.

That’s a fold of granite in a xenolith! It looks like there must be deformation active during the intrusion process. An early dyke has been folded and now sits in a xenolith surrounded by granite. It is very unwise to infer strain from a fold (if you don’t know the initial orientation), but its interesting that the nearly extensional structure is top to the north, which corresponds to a top to the left sense of shear in this photo.

Looking at the rocks at the base on the cliff gives more evidence of syntectonic intrusion.

The central block shows undeformed tourmaline muscovite granite sitting in a triangular space within metamorphic rocks. The shape looks like a ‘strain shadow’ around porphyroclasts in a mylonite.

All good things come to an end, but as we walked away from Gumbarajon it showed us its most majestic profile.

Click for more information on the Geology of this area.

Rheology re: geology

Posted on 5 March, 2012 by Metageologist

“Wibble wobble, wibble wobble, jelly on a plate” Childrens’ rhyme.

To understand the geology of mountains, you need to know how they are built, which means you need to know how rocks deform. I have a tendency to throw phrases around, so I want to define some of them properly. Also I’ve bought a new toy I’d like to play with.

Rheology is the study of the flow of matter. A understanding of a rock’s rheological properties tells us how it behaves when a force is applied to it.

The trouble with rocks is we don’t have much direct experience of deforming them. So let’s start with jelly (or “jello” as most of you know it). If you put a big pile of jelly (not jam, jelly) on a plate and push it, it moves (wibble) and then moves back (wobble). It may then take a while to settle back to exactly where it started (wibble wobble). Once you get tired of poking, stick a spoon in it and eat some. Notice that the pieces break off with a clean sharp edge.

The properties I’ve described are those associated with elastic materials. If you apply a force to the material, it changes shape but returns to its original shape when the force is removed. If you apply a larger force it will permanently change its shape. Apply a large enough force and it will fracture.

Rocks as we know them behave like this. Earthquake waves can be thought of as little wibble-wobbles moving through elastic rock. Apply enough force to rock and it will fracture – if they are big we call these fractures faults.

But what happens if you leave jelly out in the sun, or make it with too much water? It starts to collapse under its own weight. It doesn’t fracture but flows, like a liquid. Materials that behave in this way exhibit viscous behaviour. Don’t just think of water, which has low viscosity, think of high viscosity liquids like tar or honey or felsic magma.

So, we have two broad ways in which materials can deform, elastically or viscously. In practice a material can exhibit type of behaviour, under different conditions or different rates of deformation. This hybrid behaviour is know as visco-elastic.

My new toy. I’ve bought some magic putty. It is a material with complicated properties, but for our purposes it shows how a material can have both elastic and viscous rheological properties.

First the elastic behaviours: silly putty is very bouncy. It will fracture too, but this is harder to do. I tried hitting it with a hammer; a word of advice – don’t. Matt Kutcha persisted and got some great video.

The viscous side of things is best expressed in photos. Here’s a nice round piece of silly putty, perched on a slab of Himalayan granite.

Here’s the same blob the morning after. It’s takes a while, but if definitely flows viscously.

How does this apply to rocks? Rocks are (usually) made of crystals which are formed by mineral lattices. The atoms within these crystals are joined together by atomic bonds. Apply a force to them and you can increase the gap between the bonds. Remove the force and the bonds return to their original size. Apply enough force and the bonds break. This explains elastic behaviour.

How to explain viscous behaviour in rocks? Just like jelly, if you heat rocks, or add water they are more likely to flow like a liquid, only very very slowly. Luckily geology has lots of time available. A crystal lattice can deform by crystal plastic mechanisms know as creep (diffusion or dislocation). These are where atoms in the lattice shift one by one (helped by little imperfections). In time, the mineral lattice becomes a different shape, and writ large, the rock has been deformed. Individual grains remain whole so conceptually the rock has flowed like a liquid. This type of deformation in rocks is referred to as ductile. The terms ductile and viscous are often used as synonyms*.

In summary, rocks can behave in an elastic manner, but also in a ductile/viscous way. The question of which rocks behave in which ways, and how this is relevant to the geology of mountains is a matter for another post.

* Note on terminology Viscous/ductile/plastic are terms that seem in practice to be used synonymously as being things different from elastic behaviour. It’s not clear to me which is correct, if any. Viscosity is a measure of the resistance of a fluid to stress whereas plasticity and ductility relate to changing the shape of solids. It seems that ductile deformation is a description of what happens to rocks whereas viscosity comes in where numerical modelling of rock flow is required. Plastic is deformation of materials without fracturing. Or so it seems to me. Anyone who knows more care to comment?

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