History of Geology: Recognizing overthrusts

Middle section of the 38-inch cross-section

Cover

This Tectonic Tuesday is also an example of the History of Geology.

“The thrust producing these crustal movements and dislocations came from the west, and must have been highly energetic in its action, as some of the breaks are of huge proportions, and are accompanied by displacements of many thousands of feet. The faulted region is now about twenty-five miles wide, but a rough estimate places its original width at over fifty miles, the difference indicating the amount of compression is has suffered.”

With these words, R. G. McConnell was the first geologist to report the interpretation of low-angle thrust faults with significant throw in the western Cordillera of North America. McConnell’s study, entitled “Report on the Geological Structure of a portion of the Rocky Mountains,” was published in 1887 by the Geological and Natural History Survey of Canada. These cross-sections, near the 51st Parallel, accompanied the report. They document work during the field season of 1886, which was primarily spent in compiling a general geologic section along the line of the Canadian Pacific Railway.

The section begins on the right at Devil’s Gap, east of Devil’s Lake (now Lake Minnewanka) and extends westward toward Banff. Cascade Mountain, labeled on the section, is just north of Banff and rises to 9836 feet above sea level. The profile then approximately follows the railway up the valley of the Bow River, past Laggan (now the community of Lake Louise), and over the Continental Divide at Kicking Horse Pass. Cathedral Mountain (10,464 ft) and Mount Stephen, shown on the section, are just west of Kicking Horse Pass. The Burgess Shale, classic locality for soft-bodied Cambrian fossils, was discovered in 1909 by Chares D. Wolcott about three miles north of the summit of Mt. Stephen.

Frontispiece

When later geologists defined the continuity of individual thrust faults in the Canadian Rockies, one of the major thrusts was named for McConnell. The photo of the Geological Survey Camp, used as the frontispiece of this report, was actually taken during Dawson’s surveys a few years later. Dawson’s work was generally in country to the south of that described by McConnell.

When I found this report in a used book store, it was falling apart – covers off, pages torn, and the edges of the loose, folded cross-section were severely damaged. Unfolded, the plate with the cross-sections is 10”x38” and it is hand-colored. The plate is shown below.

Entire plate

April 28. Fault types

There are three basic types of faults, which you probably know are breaks in rocks in the earth – breaks with offset and movement along them. The three types represent the three ways earth materials can move relative to each other. They can pull apart, they can collide, or they can slide past each other. 

When rocks in the earth on scales of miles or many miles are pulled apart, eventually something has to give. If the rocks were plastic, like silly putty or caramel candy, they might stretch and thin, but in the upper crust of the earth, most rocks are not plastic. They’re brittle, and when you pull them apart, eventually they will snap. One piece will probably drop down – or appear to have dropped down –  relative to the other along a break, a fault. The part that slides down is above the other part, and above the inclined fault between them. Faults like that, where one block slides down relative to the other, are called normal faults.
 
Normal faults aren’t really any more “normal” than other kinds of faults, but the name comes from the way miners referred to fault blocks. In a mine, when an ore body was cut by a fault, miners called the two blocks the footwall – the block below the fault, which they were standing on – and the hanging wall, above the fault and hanging over their heads. That was the “normal” way of things, with the footwall down and the hanging wall up. When the miners encountered faults with the opposite sense of relative offset, with the footwall up and the hanging wall down, they called them reverse faults.

So, normal faults usually come from extension, pulling apart. A good example in the United States is the Wasatch Fault in Utah at Salt Lake City. The mountains, to the east, are uplifted relative to the Salt Lake Basin to the west, which has been faulted down. Most of the western half of Utah and most of Nevada are undergoing extension, which has caused many blocks to go up and down relative to each other.

When we talked about the Caledonian Orogeny a few days ago, we talked about colliding continents and other terranes, and it’s easy to see how collisions would push a mountain range up. It’s like two big pieces of furniture on a carpet and you’re pushing the furniture – the continents – together. The carpet between them is likely to be squeezed from a flat carpet to a rippled surface, and eventually the carpet ripples, which are like folds in rocks, might even get pushed up and over each other. If the carpet was something more brittle, it would break.

The faults that result from compression, squeezing the earth between two blocks, are usually the up-and-over type, which the miners called reverse faults. They could be at any angle, and today geologists have two general terms for them. High angle reverse faults are just reverse faults, but if one package of rocks was pushed up and over older rocks at a low angle, we usually call those thrust faults. It’s a little subjective as to when you say reverse fault or thrust fault, but they are basically the same thing, caused by the same mechanism – compression.

The third kind of fault occurs where two separate blocks of the earth slide past each other, and we call that a strike-slip fault. Strike is the orientation of anything in the earth with respect to the geography of the surface, so you might have a bed or a fault or something else that is oriented north-south, or east-west, or any other angle relative to geographic coordinates. And the two blocks slide along the strike of the fault.

Probably the most famous strike-slip fault in the United States is the San Andreas fault, which moved to cause the great earthquake we mentioned on its anniversary on April 18. But there are many strike-slip faults around the world.

That’s your lesson in geological jargon for today.

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Today is Eugene Shoemaker’s birthday. He was born in 1928 in Los Angeles, and he pioneered the field of planetary geology. He was involved in many American space missions, and co-discovered the comet Shoemaker-Levy in 1993, which gave scientists the first look at a planetary impact when the comet collided with Jupiter in 1994.

—Richard I. Gibson

Drawings by Richard Gibson
Fault types from USGS
Animated normal and reverse faults (USGS)