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CHAPTER 11 Crustal Deformation and Building

1. Whether rock deformation is ductile or brittle depends on four variables: • temperature. If the rock is warm, it is more likely to flow than break. • confining pressure. If high, the rock is more likely to flow than break. • rock type. At shallow depth, crystalline igneous and metamorphic rocks deform () in a brittle manner whereas sedimentary strata deform () in a ductile manner. • length of time over which is applied. A rapid increase of strain is likely to end in breakage whereas a slow increase of strain can be continued by ductile flow.

2. Which of the following statements about rock deformation is false? a. deep crustal rocks are more likely to deform ductily than shallow crustal rocks; b. hotter rocks are more likely to deform ductily than cooler rocks; c. most sedimentary rocks are more deformable than igneous rocks; d. under low confining pressure rocks are more likely to deform ductily than under high confining pressure.

3. Structures formed by brittle deformation are faults and joints.

4. What types of tectonic forces cause faulting? a) compressive forces; b) tensional forces; c) shearing forces; d) all of these.

5. Sketch and briefly describe the relative motion of rock bodies located on opposite sides of normal, reverse, and thrust faults as well as both types of strike-slip faults.

6. The offset on a fault may be determined by comparing the blocks of rock on either side of the fault surface. If the movement is in the direction of the fault's dip (or inclination), then the rock above the fault plane is the hanging wall block, and the rock below the fault is the footwall block. If the hanging wall moves: • down relative to the footwall, the fault is a normal fault. • up relative to the footwall, the fault is a reverse fault.

7. What type of fault is characterized by the rocks above the fault plane moving downward relative to the rocks below the fault plane? a) normal faults; b) reverse faults; c) strike-slip; d) all of these.

8. Faults that intersect Earth's surface may produce a "step" in the land known as a . Areas of tectonic extension, such as the Basin and Range Province, produce fault-block —horsts separated by neighboring or half-grabens.

9. Infer the differential stresses and the type of plate boundary that were responsible for the Valley of East Africa.

10. Large normal faults with low dip angles are detachment faults. Large reverse faults with low dip angles are thrust faults.

11. Areas of tectonic compression, such as mountain belts, are dominated by reverse faults that shorten the crust horizontally while thickening it vertically. Page 42

12. Strike-slip faults have most of their movement in a horizontal direction along the trend of the . Transform faults are strike-slip faults that serve as tectonic boundaries between lithospheric plates.

13. Which of the following is an example of a fault where the motion is primarily horizontal? a) a strike-slip fault; b) a right-lateral fault; c) a ; d) all of these.

14. Joints form in the shallow crust when rocks are stressed under brittle conditions. They facilitate groundwater movement and mineralization of economic resources, and they may result in human hazards.

15. Structures formed by ductile deformation are folds.

16. Folds are wavelike undulations in layered rocks that develop through ductile deformation in rocks undergoing compressional stress.

17. Which of the following conditions would favor folding rather than faulting? a) low temperatures and low confining pressures; b) low temperatures and high confining pressures; c) high temperatures and low confining pressures; d) high temperatures and high confining pressures.

18. Folds may be described in terms of their geometric configuration: If the limbs of a fold dip away from the hinge, the fold has an overall arch-like structure and is called an . If the limbs of a fold dip upward, the fold has an overall trough-like structure and is called a . and may be symmetrical, asymmetrical, overturned, or recumbent.

19. The two sides of a fold are called its ______. a) anticlines; b) synclines; c) limbs; d) axial planes.

20. An overturned fold is characterized by ______. a) two limbs at right angles to one another; b) two limbs dipping in the same direction - with one tilted beyond vertical; c) two limbs dipping in opposite directions; d) two limbs not parallel to each other.

21. The shape of a fold does not necessarily correlate to the shape of the landscape above it. Rather, surface topography usually reflects patterns of differential weathering.

22. Explain why the shape of the landscape can be opposite to the shape of the folds below.

23. A fold is said to plunge when its axis penetrates the ground at an angle. This results in a V-shaped outcrop pattern of the folded layers.

24. If the sedimentary rocks on a geologic map form a zigzag pattern, the underlying structure probably consists of ______. a) horizontal anticlines and synclines; b) plunging anticlines and synclines; c) domes and basin; d) strike-slip faults. Page 43

25. Domes and basins are large folds that produce roughly circular-shaped outcrop patterns. The overall shape of a or basin is a saucer or a bowl, either right-side-up (basin) or inverted (dome),

26. What do domes and anticlines have in common? What sets them apart from one another?

27. are large step-like folds in otherwise horizontal strata that result from subsurface faulting. Imagine a carpet draped over a staircase to envision how the strata can go from horizontal to tilted and back to horizontal again.

28. Orogenesis is the process of , especially by a folding and faulting of Earth's crust.

29. Orogenesis most often occurs along convergent plate boundaries, where compressional forces cause folding and faulting of the rock, thickening the crust vertically and shortening it horizontally. Some mountain belts are very old, while others are actively forming today.

30. Infer the differential stresses and the type of plate boundary that were responsible for the Canadian Rockies

31. Sediment scraped off the subducting plate builds up in an . The thickness of the wedge varies in accordance with the amount of sediment available. Between the accretionary wedge and the volcanic arc is a relatively calm site of sedimentary deposition, the forearc basin.

32. If a subducted plate is overridden by oceanic lithosphere, island arc-type mountain building results, with a thick accumulation of erupted volcanic rocks mixed with sediment scraped off the subducted plate. Andean-type mountain building occurs where subduction takes place beneath continental lithosphere.

33. Explain why the Andes Mountains are located on the western margin of South America rather than the eastern margin.

34. Release of water from a subducted slab triggers melting in the wedge of mantle above the slab. These primary magmas have a basaltic (mafic) composition. They rise to the base of the continental crust and pond. Fractional crystallization then separates ferromagnesian minerals from a melt that grows increasingly granitic (felsic) in composition.

35. The evolved magma rises further and may either cool and crystallize below the surface to form a batholith or erupt at the surface and form a continental volcanic arc.

36. Sketch a cross section of an Andean-type mountain belt and describe how its major features are generated. Page 44

37. are relatively small crustal fragments (microcontinents, volcanic island arcs, or oceanic plateaus). Terranes may be accreted to continents when subduction brings them to a trench, but they cannot subduct due to their relatively low density. The terranes are "peeled off" the subducted slab and thrust onto the leading edge of the continent.

38. The geography of central California preserves an accretionary wedge (Coast Ranges), a forearc basin (Great Valley), and the roots of a continental volcanic arc (Sierra Nevada).

39. are geologically young fold mountains formed by the collision of India and Eurasia starting around 50 million years ago. They are still rising today.

40. Isostasy is the principle that the elevation of the crust can change in accordance with gravitational balance. If additional weight is placed on the crust (a flood basalt, a thick layer of sediments, or an ice sheet), the crust will sink into the mantle until it comes to equilibrium with the extra load. If the new mass is removed, the crust will rebound upward.

41. When mountains form and rock mass is pushed up to the height of a young compressional mountain belt such as the Himalayas, the rocks at the base of the mountains become warmer and weaker. As the overlying rock mass pushes down on them through gravity, these deeper rocks flow out of the way. This causes the mountain belt to collapse outward into a wider, lower-elevation region.

42. Describe how isostasy will affect elevation of the highest peaks in a if rivers and glaciers eroded deep valleys through the mountains and removed large amounts of rock.

accretionary wedge half- anticline hanging wall block basin brittle deformation isostasy compressional stress isostatic adjustment compressional mountain deformation microcontinent differential stress normal fault dip-slip fault orogenesis dome reverse fault ductile deformation rock structure (geologic structure) elastic deformation fault-block mountain strain fault scarp stress, confining pressure fault strike-slip fault fold footwall block syncline forearc basin tensional stress graben gravitational collapse transform fault