Chapter 10: Deformation and Mountain Building. Fig. 10.1

Transcription

1 Chapter 10: Deformation and Mountain Building Fig. 10.1

2 OBJECTIVES Describe the processes of rock deformation and compare and contrast ductile and brittle behavior in rocks. Explain how strike and dip are used to measure the orientation of geologic structures. Compare and contrast joints and faults and discuss how each type of fracture forms and the geologic structures produced as a result. Identify types of fold structures and summarize how folds are describe based on the orientation of their axial plane and fold hinge.

3 OBJECTIVES Compare and contrast different types of unconformities, and assess their relationship to deformation. Discuss the plate tectonic causes of mountain building above subduction zones and at zones of continental collision. Explain how the geologic record helps us to explore the role of plate tectonics in the evolution of ancient mountain belts.

4 Deformation: change in position, shape, or volume Major process in mountain building Evidence for stress, tectonics Stress: force per unit area (cause of strain) Deformation Strain: change in shape or volume (effect of stress) Fig. 10.2

5 Two types of stress: Stress Confining Pressure: stress same in all directions Differential Stress: stress greater in one direction than another Tensional stress Compressional stress Shear stress Fig. 10.3

6 Strain Types of deformation: Elastic : rock returns to its original shape Plastic: strain is permanent Brittle: breaking Ductile: bending, stretching Deformation affected by Rock type Temperature (depth) Pressure (depth) Time (strain rate) Fig. 10.4

7 Orientation of Geologic Structures Major structures: Beds Folds Faults Fractures Foliation Structures described by their orientation Strike: orientation of a horizontal line in the plane Dip: maximum slope of the plane Fig. 10.6

8 Fractures: Joints Brittle deformation No significant motion/displacement Columnar joins Exfoliation joints Tectonic joints Fig. 10.8

9 Fractures: Faults Brittle deformation Significant motion/displacement Strike-slip faults Dip-slip faults Normal Faults Reverse Faults Figs , 10.15

10 Figs , 10.15

11 Folds Ductile deformation Result of compressional stress Fig

12 Fold Types Figs , 10.19

13 Fold Types Figs , 10.22

14 Foliation Planar alignment of mineral crystals Forms perpendicular to compressional stress Metamorphic fabric Parallel to axial plane of a fold Can be used to infer direction of stress

15 Unconformities Mark gap in time Commonly associated with deformation Can be used to infer timing of deformation Types: Angular unconformity: layers tilted prior to erosion and deposition above Nonconformity: layers uplifted and eroded prior to deposition above Disconformity: layers uplifted and/or eroded (without tilting) prior to deposition above Fig

16 Plate Tectonics and Mountain Building Orogeny: mountain building Mountains form Along all types of plate boundaries At hot spots Within plate interiors Fig

17 Mountain Building: Rifting and Upwelling Mountain building through faulting, volcanism Mid-ocean ridge is most continuous mountain system Large volcanic mountains and mountain chains formed at hot spots Fig. 9.12

18 Mountain Building: Subduction Zones Mountain building through Volcanism Intrusion Folding Faulting Accretion Fig

19 Mountain Building: Collision Zones Mountain building through Volcanism Intrusion Accretion Folding Faulting Fig

20 Fig Terrane accretion: important in the building of continents

21 Formation of the Himalayas Began about 50 million years ago Movement of India 2,000 km into Eurasia Figs , 10.36

22 The Wilson Cycle Wilson Cycle: opening and closing of the oceans a. Continental rifting b. Formation of ocean basin c. Accumulation of sediments d. Subduction and formation of island arcs e. Closing of the ocean basin f. Continental collision Fig

23 The Supercontinent Cycle Supercontinent Cycle: expansion of the Wilson Cycle idea a. Supercontinent traps heat beneath it b. Supercontinent eventually begins to break apart c. New ocean basins form and supercontinent divides into separate continents d. Ocean basins become thicker and denser with age e. Subduction begins f. Continents come back together into a new supercontinent Fig

24 SUMMARY Deformation is the result of stresses (compressional, extensional, shear). Stress causes strain: change in location, shape, or volume. Types of strain include brittle, ductile, elastic, and plastic. Strain is influenced by rock type, temperature, pressure, and strain rate. Geologic structures are described by their orientation: strike and dip of planar structures. Fractures are a manifestation of brittle deformation. Joints are fractures along which no significant motion has occurred. Faults are fractures along which significant motion has occurred.

25 SUMMARY Folds are a manifestation of ductile deformation. Unconformities are commonly associated with deformation. Mountain systems are formed through plate tectonics: through extensional, compressional, or shear stress and associated volcanism, faulting, and folding. The Wilson Cycle is the idea that over time, ocean basins open and close. The Supercontinent Cycle is the idea that over time, a supercontinent breaks up to form separate continents, which then come back together to form a new supercontinent.