Essentials of Geology, 8e

1. Essentials of Geology, 8e Frederick K. Lutgens & Edward J. Tarbuck

2. Mountain Building Chapter 17 Essentials of Geology, 8e Stan Hatfield and Ken Pinzke Southwestern Illinois College

3. Deformation Deformation is a general term that refers to all changes in the original form and/or size of a rock body Most crustal deformation occurs along plate margins How rocks deform Rocks subjected to stresses greater than their own strength begin to deform usu-ally by folding, flowing, or fracturing

4. Deformation How rocks deform General characteristics of rock deform-ation Elastic deformation � the rock returns to nearly its original size and shape when the stress is removed Once the elastic limit (strength) of a rock is surpassed, it either flows (ductile deform-ation) or fractures (brittle deformation)

5. Deformation How rocks deform General characteristics of rock deform-ation Factors that influence the strength of a rock and how it will deform Temperature Confining pressure Rock type Time

6. Folds During crustal deformation rocks are often bent into a series of wave-like undulations called folds Characteristics of folds Most folds result from compressional stresses which shorten and thicken the crust

7. Folds Characteristics of folds Parts of a fold Limbs � refers to the two sides of a fold Axis � a line drawn down the points of maximum curvature of each layer Axial plane � an imaginary surface that divides a fold symmetrically

8. Folds Common types of folds Anticline � upfolded or arched rock layers Syncline � downfolds or troughs of rock layers Depending on their orientation, anticlines and synclines can be described as Symmetrical, asymmetrical, recumbent (an overturned fold), or plunging

9. A series of anticlines and synclines

10. Outcrop patterns of plunging folds

11. Folds Common types of folds Monoclines � large, step-like folds in otherwise horizontal sedimentary strata Other types of folds Dome Upwarped displacement of rocks Circular or slightly elongated structure Oldest rocks in center, younger rocks on the flanks

12. Monoclines are often the result of movement along buried faults

13. The Black Hills of South Dakota are a large domal structure

14. Folds Other types of folds Basin Circular or slightly elongated structure Downwarped displacement of rocks Youngest rocks are found near the center, oldest rocks on the flanks

15. The bedrock geology of the Michigan Basin

16. Faults Faults are fractures in rocks along which appreciable displacement has taken place Sudden movements along faults are the cause of most earthquakes Classified by their relative movement which can be Horizontal, vertical, or oblique

17. Faults Types of faults Dip-slip faults Movement is mainly parallel to the dip of the fault surface May produce long, low cliffs called fault scarps Parts of a dip-slip fault include the hanging wall (rock surface above the fault) and the footwall (rock surface below the fault)

18. Concept of hanging wall and footwall along a fault

19. Faults Types of dip-slip faults Normal fault Hanging wall block moves down relative to the footwall block Accommodate lengthening or extension of the crust Most are small with displacements of a meter or so Larger scale normal faults are associated with structures called fault-block moun-tains

20. A normal fault

21. Faults Types of dip-slip faults Reverse and thrust faults Hanging wall block moves up relative to the footwall block Reverse faults have dips greater than 45o and thrust faults have dips less then 45o Accommodate shortening of the crust Strong compressional forces

22. A reverse fault

23. A thrust fault

24. Faults Strike-slip fault Dominant displacement is horizontal and parallel to the strike of the fault Types of strike-slip faults Right-lateral � as you face the fault, the block on the opposite side of the fault moves to the right Left-lateral � as you face the fault, the block on the opposite side of the fault moves to the left

25. A strike-slip fault

26. Fault Strike-slip fault Transform fault Large strike-slip fault that cuts through the lithosphere Accommodates motion between two large crustal plates

27. The San Andreas fault system is a major transform fault

28. Joints Joints are among the most common rock structure Technically, a joint is a fracture with no movement Most occur in roughly parallel groups Significance of joints Chemical weathering tends to be con-centrated along joints

29. Joints Significance of joints Groundwater movement is often con-trolled by joint systems A joint system can influence the direction that a stream course follows

30. Mountain belts Orogenesis � the processes that col-lectively produce a mountain belt Includes folding, thrust faulting, meta-morphism, and igneous activity Mountain building has occurred during the recent geologic past Alpine-Himalayan chain American Cordillera Mountainous terrains of the western Pacific

31. Earth�s major mountain belts

32. Mountain belts Older Paleozoic- and Precambrian-age mountains Appalachians Urals in Russia Several hypotheses have been proposed for the formations of Earth�s mountain belts

33. Mountain building at convergent boundaries Plate tectonics provides a model for orogenesis Mountain building occurs at convergent plate boundaries Of particular interest are active sub-duction zones Volcanic arcs are typified by the Aleutian Islands and the Andean arc of western South America

34. Mountain building at convergent boundaries Aleutian-type mountain building Where two ocean plates converge and one is subducted beneath the other Volcanic island arcs result from the steady subduction of oceanic lithosphere Most are found in the Pacific Active island arcs include the Mariana, New Hebrides, Tonga, and Aleutian arcs

35. Mountain building at convergent boundaries Aleutian-type mountain building Volcanic island arcs Continued development can result in the formation of mountainous topography consisting of igneous and metamorphic rocks

36. Formation of a volcanic island arc

37. Mountain building at convergent boundaries Andean-type mountain building Mountain building along continental margins Involves the convergence of an oceanic plate and a plate whose leading edge contains continental crust Exemplified by the Andes Mountains

38. Mountain building at convergent boundaries Andean-type mountain building Stages of development - passive margin First stage Continental margin is part of the same plate as the adjoining oceanic crust Deposition of sediment on the continental shelf is producing a thick wedge of shallow-water sediments Turbidity currents are depositing sediment on the continental rise and slope

39. Mountain building at convergent boundaries Andean-type mountain building Stages of development � active continental margins Subduction zone forms Deformation process begins Convergence of the continental block and the subducting oceanic plate leads to deformation and metamorphism of the continental margin Continental volcanic arc develops

40. Mountain building at convergent boundaries Andean-type mountain building Stages of development � active continental margins Accretionary wedge may form Chaotic accumulation of sedimentary rocks and metamorphic rocks with occasional scraps of ocean crust Can become large enough to stand above sea level

41. Mountain building at convergent boundaries Andean-type mountain building Composed of roughly two parallel zones Volcanic arc Develops on the continental block Consists of large intrusive bodies intermixed with high-temperature metamorphic rocks

42. Mountain building at convergent boundaries Andean-type mountain building Composed of roughly two parallel zones Accretionary wedge Seaward segment Consists of folded, faulted, and meta-morphosed sediments and volcanic debris

43. Orogenesis along an Andean-type subduction zone

44. Orogenesis along an Andean-type subduction zone

45. Orogenesis along an Andean-type subduction zone

46. Mountain building at convergent boundaries Andean-type mountain building Sierra Nevada and Coast Ranges One of the best examples of an active Andean-type orogenic belt Subduction of the Pacific Basin under the western edge of the North American plate Sierra Nevada batholith is a remnant of a portion of the continental volcanic arc

47. Mountain building at convergent boundaries Continental collisions Two lithospheric plates, both carrying continental crust The Himalayan Mountains are a youthful mountain range formed from the collision of India with the Eurasian plate about 45 million years ago

48. Mountain building at convergent boundaries Continental collisions The Himalayan Mountains Spreading center that propelled India northward is still active Similar but older collision occurred when the European continent collided with the Asian continent to produce the Ural mountains

49. Plate relationships prior to the collision of India with Eurasia

50. Position of India in relation to Eurasia at various times

51. Formation of the Himalayas

52. Mountain building at convergent boundaries Continental accretion and mountain building A third mechanism of orogenesis Small crustal fragments collide and merge with continental margins Responsible for many of the mountainous regions rimming the Pacific Accreted crustal blocks are called terranes

53. Mountain building at convergent boundaries Continental accretion and mountain building Terranes consist of any crustal fragments whose geologic history is distinct from that of the adjoining terranes As oceanic plates move, they carry embedded oceanic plateaus, volcanic island arcs and microcontinents to an Andean-type subduction zone

54. Vertical movements of the crust In addition to the horizontal movements of lithospheric plates, vertical movement also occurs along plate margins as well as the interiors of continents far from plate boundaries

55. Vertical movements of the crust Isostatic adjustment Less dense crust floats on top of the denser and deformable rocks of the mantle Concept of floating crust in gravitational balance is called isostasy If weight is added or removed from the crust, isostatic adjustment will take place as the crust subsides or rebounds

56. The principle of isostasy

57. End of Chapter 17