Knowing when to hold them and when to fold them

It’s not a big gamble to bet that when two continent-sized pieces of the earth’s crust collide into each other, there’s bound to be some dramatic results. Layers of rock several kilometres thick are crushed, fractured and folded. Living where we do in the cordillera region of western North America, we are able to see the power of plate tectonics up close. How do all these rocks get all bent out of shape?

You can most easily see folds in sedimentary or volcanic rocks that have layering visible in them. If you see any of these rocks and the layers are not horizontal, then you know they have been folded. 

There is a lot of terminology regarding the components of a fold, but you only need to know the basics. The simplest fold is a monocline, a horizontal band of rocks with an isolated flex or bend in it. With more bending you get synclines shaped like the letter “U”. Anticlines are the opposite of synclines, opening downwards. The point where the folds change direction is called the hinge, the two arms coming out of the hinge are called limbs. A recumbent fold is exactly what it sounds like, rocks that have been bent so far, they are lying over on themselves. 

A series of anticlines and synclines has been compared to the pattern of sound waves. If the folds are all the same size (amplitude) and wavelength they are called harmonic. If the amplitude and wavelength are constantly changing they are disharmonic. 

Folds are formed mainly by compression. The most dramatic would be the result of tectonic plate collisions. Another form of compression is from gravity. In situations where heavier rocks are on top of lighter ones, the lower rocks will be compressed and they will bend. 

If you can find a phone book these days (the bigger, the better), squeeze it and see how the pages form anticlines and synclines. If you want to get real funky, use layers of different-coloured plasticine. Squeeze them and see what landscapes you can produce.

Folds can also result when a rock is just forming. These are called “synsedimentary folds.” Silts and clays underwater on a slope can slump downwards and bunch up due to gravity. This can form centimeter scale fold structures that are preserved when the sediments harden into rock. 

It is hard to imagine that actual rocks can behave like plasticine or paper, but they can. When rock compression first begins they start getting thicker, especially if it is all the same type of rock. More often, the rocks being compressed are quite different, some strong, some weak. These differences trigger the rocks to bend and fold. The wavelength of folds can be controlled by the thickness and difference in strength of the various layers.

Rocks that are a few kilometres below the surface will also be hotter, making them easier to bend. The rock temperature increases on average 25oC per kilometre depth. They would be even hotter near colliding plates. 

If the pressure gets too strong, the rocks will bend, then fracture along weak points, forming thrust and fold belts. Thrust faults occur at a shallow angle, usually less than 10 degrees to the horizontal. The process repeats as compression continues, resulting in several slices of rock thousands of metres thick stacked up high on top of each another. The results are the spectacular mountains we see today.

There are several thrust and fold belts in the cordillera between Alaska and Mexico. When you drive south along the Alaska Highway and get south of Coal River you are entering the northern Rocky Mountain fold and thrust belt. It is most obvious when you reach the Muncho/Stone Mountain Park area where you can see extensive folding in the limestone layers.

The southern Rocky Mountain fold and thrust belt is one of the most accessible and studied in the world. The Trans-Canada Highway cuts a perfect path through this belt. The belt ranges from 100 to 200 kilometres wide. It has been estimated that the original surface has been shortened to almost half by the mountain building process. 

To the east of the Rocky Mountains beyond Sikanni Mountain in the north, and Canmore in the south the effects of the compression move beyond the fold and thrust belt to what is called the foreland basin area. The massive weight of the stacked rocks in the mountains compresses the rocks to the east forming large basins that have filled up over millions of years with sediment and vegetation from the eroding mountains.

These basins were ideal for the formation of hydrocarbon resources like coal, oil and gas. The basins are deepest in the west right up against the mountain range. That is why you get into the gas fields around Fort Nelson as you pass east of Sikanni Mountain. Anticlines can act as traps for oil and gas pools. If there is younger less permeable rock above, hydrocarbon deposits and an anticline forms the oil, and gas will be trapped and accumulate in the dome of the anticline.

Folding can play a large part in forming mineral deposits. The hinge area in tight folds can be quite fractured and broken due to the extreme stress during folding. These form ideal conduits for mineral bearing fluids to pass through and form deposits. 

An ore body itself can be folded after it has been deposited. The lead-zinc-silver deposits around Faro have been subjected to five different phases of deformation. Mine geologists had to carefully measure the orientations of the folding around the original deposit to help point the way to new ones.

Today computer programs help geologists visualize complex folding in 3D. For more than 100 years, geologists have used a special graph paper called a stereonet or Wulff net, after its Russian inventor George Wulff. It provides a 2D representation of 3D data. Stereonets are still used today and provide valuable information on the orientation of folds and other structural features.

It is not difficult to see examples of folding in the Yukon. One of the best spots in Whitehorse is the east side through Rabbit’s Foot Canyon. The Kluane area has interesting folding visible in the Front Ranges along the highway. 

Driving north through the Ogilvie Mountains on the Dempster Highway you are travelling through a fold and thrust belt with related foreland basin in the Eagle Plains area. Folding in metamorphic rocks can be more difficult to see. Driving into Dawson City, both sides of the Klondike River valley are thrust faults, as is the north side of the Indian River valley. 

The American poet, Gary Snyder wrote a book of poems and essays titled Turtle Island that had strong environmental values while delivering a political message. It won the Pulitzer Prize for poetry in 1975. In that book his poem “By Frazier Creek Falls” begins on a fold.

Rabbit’s Foot Canyon folding

Standing up on lifted, folded rock

looking out and down—

The creek falls to a far valley 

hills beyond that

facing, half-forested, dry 

–clear sky

strong wind in the stiff needle clusters

of the pine —their brown 

round trunk bodies

straight, still; 

rustling trembling limbs and twigs

listen.

This living flowing land

is all there is, forever

We are it

It sings through us –

We could live on this Earth

Without clothes or tools!”

 

Centimetre sized synsedimentary features, Clinton Creek, YT

This is Part 1 of a 4 Part series