Chapter 10: Deformation and Mountain Building

Fig. 10.1

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.

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.

• Deformation: change in position, shape, or volume

• Major process in mountain building

• Evidence for stress, tectonics

• Stress: force per unit area (cause of strain)

• Strain: change in shape or volume (effect of stress)

Deformation

Fig. 10.2

StressTwo types of 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

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

• 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

Orientation of Geologic Structures

Fig. 10.6

• Brittle deformation• No significant motion/displacement

• Columnar joins• Exfoliation joints• Tectonic joints

Fractures: Joints

Fig. 10.8

• Brittle deformation• Significant motion/displacement

• Strike-slip faults• Dip-slip faults

• Normal Faults• Reverse Faults

Fractures: Faults

Figs. 10.11, 10.15

Figs. 10.11, 10.15

• Ductile deformation• Result of compressional

stress

Folds

Fig. 10.17

Fold Types

Figs. 10.18, 10.19

Fold Types

Figs. 10.20, 10.22

• 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

Foliation

• 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

Unconformities

Fig. 10.23

• Orogeny: mountain building

• Mountains form• Along all types of plate

boundaries• At hot spots• Within plate interiors

Plate Tectonics and Mountain Building

Fig. 10.33

• Mountain building through faulting, volcanism

• Mid-ocean ridge is most continuous mountain system

• Large volcanic mountains and mountain chains formed at hot spots

Mountain Building: Rifting and Upwelling

Fig. 9.12

Mountain Building: Subduction Zones

Mountain building through

• Volcanism• Intrusion• Folding• Faulting• Accretion

Fig. 10.27

Mountain Building: Collision Zones

Mountain building through

• Volcanism• Intrusion• Accretion• Folding• Faulting

Fig. 10.28

Terrane accretion: important in the building of continents

Fig. 10.29

Formation of the Himalayas• Began about 50 million

years ago• Movement of India 2,000

km into Eurasia

Figs. 10.35, 10.36

Wilson Cycle: opening and closing of the oceans

a. Continental riftingb. Formation of ocean

basinc. Accumulation of

sedimentsd. Subduction and

formation of island arcs

e. Closing of the ocean basin

f. Continental collision

The Wilson Cycle

Fig. 10.38

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 beginsf. Continents come back together

into a new supercontinent

The Supercontinent Cycle

Fig. 10.39

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.

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.