Faults are fractures along which there is relative sliding movement of the blocks in opposite directions on either side. We recognize them by various criteria:

1) layers of different types and ages of rock units sit side-by-side

2) abrupt topographic discontinuities of landforms

3) depressions along the fault trace (broken rock is more easily eroded)

4) scarps or cliffs

5) sudden shifts of drainage courses.

The scene below of the Kuruk Tagh fault in part of the Tian Shan mountains of westernmost China shows a sharp fault trace running east-west through the Kuruk Tagh hills. It's composed of folded sedimentary strata, metamorphic rocks, and igneous intrusions. The block of crust on the north side has shifted sub-horizontally to the west (left) at least 60 km relative to the block containing the corresponding segment of mountains to the south. This type is a left-lateral wrench fault (also called a strike-slip fault). It's a type similar to the San Andreas fault, which is a right-lateral fault, with the Pacific plate moving northward against the North American plate the famed earthquake-maker running from the Gulf of California through the California Coastal Ranges north of San Francisco (see the Los Angeles image in Section 4 and the San Francisco image in Section 6). The Kuruk Tagh fault is easy to identify because of topographic offset (as well as equivalent parts of the strata and metamorphosed rock units), fault scarps, and displaced drainage. This and similar major wrench faults in south-central Asia represent crustal adjustments to the stresses induced by the collision of India against Asia (see mosaic in Section 7).

2-13: Can you find a second major fault in this scene? ANSWER

Wrench faults have nearly vertical fault planes (contact surfaces between blocks). A second fault type is the thrust fault, in which the fault plane is at low angles relative to Earth's surface and the usual direction of movement carries the upper (near-surface) block over the lower block, causing rocks of different ages to be juxtaposed. Thus, a shallow, tabular slice of crust slides (thrusts) over the fault plane and on top of the surface ahead of it. Several such thrust slices (sheets) may stack one on top of the other in a staggered pattern, leading to a sequence of thrust block bands that outcrop in mountainous terrains. If each block consists of rock types that are different in composition and erosive response, these will appear at the surface as intervals of rock with contrasting topography. This topography is superbly displayed in the scene below of the Pindus Mountains of western Greece and Albania.

We can differentiate five tectonic zones, named in this generalized map, by sight because of distinct topographic variations and tonal differences related to contrasting rock types. The direction of tectonic transport is from east to west (right to left) causing the sheets to partially overlap below the surface, but each front edge occupies a different geographic position relative to the one it overrode and the one overriding it. This zone of thrust belts is part of the Balkan Alpine system, an offshoot of the European Alps that runs sub-parallel to the Apennine Mountains of Italy (see Alps mosaic in Section 7). As part of the tectonic adjustments caused by the African Plate shoving northward against the European Plate, a small tectonic plate underlying the Tyrrhenian Sea (off Sardinia) is squeezed against the Adriatic plate. The Adriatic plate then pushes it eastward against the Aegian plate, underthrusting it and causing the thrust slicing shown here.

2-14: Which two tectonic zones are hardest to recognize and separate in the above image? ANSWER

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