Page 427 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 13 Tectonics, Earthquakes, and Volcanism 391
Animation
Folds, Anticlines, and Synclines
Overturned anticline
Anticline
Hinge
Syncline
Hinge
Axial plane of overturned anticline
Thrust fault
Synclinal ridge
Anticlinal valley
Axial plane of syncline
Axial plane of anticline
Fault plane
(a) Types of folds and their features.
(c) Folded strata along San Andreas fault.
(b) Synclinal ridge exposed in a highway cutting.
▲Figure 13.8 Folded landscapes. [(b) Mike Boroff. (c) gunter Marx Photography/Encyclopedia/Corbis.]
the resulting configuration is an overturned anticline, in which folds have been compressed so much that they overturn upon their own strata. Further stress eventually fractures the rock strata along distinct lines, a process that forms a thrust fault (see Figure 13.8; faults are dis- cussed ahead); some overturned folds are thrust upward, causing a considerable shortening of the original strata. Areas of intense folding are visible in Figure 13.8c.
Knowledge of folding and stratigraphy are important for the petroleum industry. For example, in addition to other locations, petroleum geologists know that oil and natural gas collect in the upper portions of anticlinal folds in permeable rock layers such as sandstone.
Over time, folded structures can erode to produce interesting landforms (Figure 13.9). An example is a dome, which is an area of uplifted rock strata resembling an anticline that has been heavily eroded over time (Figure 13.9a). Since an anticline is a fold that is convex in an upward direction, erosion exposes the oldest rocks
in the centre of a dome, which often have a circular pat- tern that resembles a bull’s eye when viewed from the air—the Richat Structure in Mauritania is an example, and the Isachen Dome on Ellef Rignes Island in Nuna- vut is another. A basin forms when an area resembling a syncline is uplifted and then erodes over time; in this structure, the oldest rock strata are at the outside of the circular structure (Figure 13.9b). Since a syncline is con- cave when considered from above, erosion exposes the youngest rocks in the centre of the structure.
Mountain ranges in North America, such as the Canadian Rockies and the Appalachian Mountains, and in the Middle East exhibit the complexity that folding can produce. The area north of the Persian Gulf in the Zagros Mountains of Iran, for example, was a dispersed terrane that separated from the Eurasian plate. However, the northward push of the Arabian plate is now forcing this terrane back into Eurasia and forming an active plate margin known as the Zagros crush zone, a continuing