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The topography of tectonics

Geologists make erratic drivers. There are just too many interesting things to watch out for along the side of the road: faults, folds, and the details of sedimentary layers, to name just a few. But most geologists are also in the habit of stopping their car from time to time at a roadside scenic view in order to take in the sweep of the landscape, particularly its topography. A lot of geological information is embedded in topography, and this is especially true of tectonics.

Tectonics can be loosely defined as the set of processes that deform the rocks of the earth’s crust. For example, these can include the motion of the large lithospheric plates to which our continents belong. It is an easy thing to imagine how a slow collision between India and Asia caused enormous sheets of rocks to be thrust over or under one another. The Himalaya mountain chain is our textbook example of tectonic topography.

That’s not all to the story, of course. There is a tremendous, ongoing argument over the relative importance of tectonics versus climate and erosion in the Himalaya. One camp is adamant that once the mountains became sufficiently high they forced prevailing winds to flow over them. By cooling the air the mountains caused the annual monsoon rains, which in turn created increased erosion and more isostatic uplift to compensate for all the material washed out to sea. This is certainly a good point, but the opposing camp has an instant rejoinder. ‘Yes,’ says the tectonicist, ‘but without tectonic forces to have raised the earth’s surface, no mountains would have existed to make the monsoon in the first place!’

Mountains such as the Himalaya, the Alps, and the Andes are spectacular. But the earth’s topographic envelope also records the effects of less grand but equally important tectonic processes. A photograph that appears in many geology textbooks shows a concrete sidewalk curb in Hollister, California which has been broken and offset by the slow, steady, sideways creep of the Calaveras fault. It is easy to take a step back and imagine how active faulting can rearrange an entire landscape. Just like the sidewalk, a creek or a river that crosses an active fault will have to adjust itself after a large earthquake. Surface rupture can cause a displacement of many metres, which means its effect on a landscape can be rather substantial. One great earthquake that occurred in Owens Valley, California in 1872 created a fault scarp almost 10 metres high. California’s Sierra Nevada is on the side that went up. Repeated over and over for millions of years, this kind of faulting will make mountains.

No fault can run forever: all faults must have beginning and end points, which we call the fault tips. Offset will be greatest near the middle of the fault, and there the fault scarp will be at its highest. The scarp height will decay parallel to the fault plane in both directions, becoming zero at each fault tip. The scarp face will be steep and slope towards the fault, while the backside will slope gently away from the fault. This is called asymmetric escarpment topography and is quite common in regions that are undergoing active extension. It is also common on the continental scale at what are (probably incorrectly) called passive margins.

Folds in the earth’s crust can also influence topography. Imagine a set of sedimentary layers, one on top of the other and some better compacted and more able to resist erosion. Next, imagine the earth squeezing them, not enough to make thrust or reverse faults but sufficient to warp them upward into an elongated dome. Finally, imagine this structure — called an antiform, or sometimes an anticline — is still being squeezed, and is thus actively growing. In becoming higher its footprint gets larger: as the nose of the antiform migrates horizontally it can push rivers out of the way like a bulldozer making a dam. But should the softer core of our antiform become breached by erosion, the inner layers may be washed away very rapidly. The resulting landscape may end up looking like an amphitheater with some very diagnostic river patterns around and inside it.

With the proper geological toolbox a lot of tectonic history can be recovered from topography. These few patterns we’ve discussed constitute only a partial list of what causes the tectonic topographer to behave unpredictably behind the wheel. More formally, we should sub-categorize our traffic hazard as a tectonic geomorphologist. Tectonic geomorphology infuses the study of landforms with knowledge from sedimentology, geochronology, seismology, structural geology, and many other types of geology. Specialists in these and other fields commonly work together, one waxing poetic and waving his arms in the air whilst the other ignores him and tries to safely navigate the road ahead.  

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