Modification of Rocks by Folding and Fracturing Grotzinger • Jordan Understanding Earth Seventh Edition Chapter 7: DEFORMATION Modification of Rocks by Folding and Fracturing © 2014 by W. H. Freeman and Company

Modification of Rocks by Folding and Fracturing Chapter 7: Deformation: Modification of Rocks by Folding and Fracturing

About Deformation Deformation mainly occurs near plate boundaries. Field observations of deformation show us how to reconstruct geologic history. Deformation includes faulting of rigid rocks and folding of rocks that can be bent.

Lecture Outline Plate tectonic forces 2. Mapping geologic structure 3. How rocks deform 4. Basic deformation structures 5. Styles of continental deformation 6. Unraveling geologic history

● Deformation ● tensional forces ● compressive forces 1. Plate Tectonic Forces ● Deformation ● tensional forces ● compressive forces ● shearing forces

● Outcrop – basic source of geologic information in the field 2. Mapping Geologic Structure ● Outcrop – basic source of geologic information in the field

2. Mapping Geologic Structure

2. Mapping Geologic Structure

2. Mapping Geologic Structure

● Measuring strike and dip 2. Mapping Geologic Structure ● Measuring strike and dip ● strike is the compass direction of a rock layer as it intersects a horizontal surface ● dip is the amount of tilting of the layer and is measured at right angles to strike

2. Mapping Geologic Structure

2. Mapping Geologic Structure

● a common scale for geologic maps is 1:24,000 2. Mapping Geologic Structure ● Geologic maps ● geologic maps represent the rock formations exposed at Earth’s surface ● a common scale for geologic maps is 1:24,000

● Geologic cross sections 2. Mapping Geologic Structure ● Geologic cross sections ● geologic cross sections – diagrams showing the features that would be visible if vertical slices were made through part of the crust

Thought questions for this chapter In what sense is a geologic map a scientific model of the surface geology? Is it fair to say that geologic cross sections in combination with a geologic map describe a scientific model of a three-dimensional geologic structure? Why is it correct to say that “large-scale geologic structures should be represented on small-scale geologic maps”? How large a piece of paper would be required to make a map of the entire U.S. Rocky Mountains at 1:24,000 scale? Can you explain the geologic story in Exercise 6 (p. 188) in terms of plate tectonics?

Thought questions for this chapter Can you explain the geologic story in Exercise 6 (see textbook, p. 193) in terms of plate tectonics? Where in the U.S. do geologists think this sequence of events has taken place?

● Rock behavior in the laboratory ● brittle ● ductile 3. How Rocks Deform ● Rock behavior in the laboratory ● brittle ● ductile

3. How Rocks Deform

● Rock behavior in Earth’s crust ● depths affect brittle v. ductile 3. How Rocks Deform ● Rock behavior in Earth’s crust ● depths affect brittle v. ductile ● rock type affects way rocks deform ● rate of deformation is a factor

● dip-slip – normal, reverse, and thrust 4. Basic Deformation Structures ● Types of faults ● dip-slip – normal, reverse, and thrust ● strike-slip – right- and left- lateral ● oblique-slip

4. Basic Deformation Structures

Example of a strike-slip fault

● symmetrical folds - anticlines and synclines ● asymmetrical folds 4. Basic Deformation Structures ● Types of folds ● symmetrical folds - anticlines and synclines ● asymmetrical folds ● overturned folds ● plunging folds

4. Basic Deformation Structures

4. Basic Deformation Structures

4. Basic Deformation Structures ● Circular structures ● dome ● basin

Example of a dome

Example of a basin

● Other features ● joints 4. Basic Deformation Structures ● deformation (cataclastic) textures ● fault breccia ● mylonite

joints

fault breccia

mylonite

● Compressive tectonics ● Shearing tectonics 5. Styles of Continental Deformation ● Tensional tectonics ● Compressive tectonics ● Shearing tectonics

5. Styles of Continental Deformation

5. Styles of Continental Deformation

5. Styles of Continental Deformation

Example of tensional tectonics

Example of compressive tectonics

Example of shearing tectonics

Thought questions for this chapter The submerged margin of a continent has a thick layer of sediments overlying metamorphic basement rocks. That continental margin collides with another continental mass, and the compressive forces deform it into a fold and thrust belt. During the deformation, which of the following geologic formations would be likely to behave as brittle materials and which as ductile materials? (a) sedimentary formations in the upper few km (b) metamorphic basement rocks at depths of 5-15 km (c) lower crustal rocks below 20 km In which of these layers would you expect earthquakes?

● Can be described in time steps 1, 2, 3, … 6. Unraveling Geologic History ● Geologic history is a succession of episodes of deformation and other geologic processes. ● Can be described in time steps 1, 2, 3, …

TIME 1 Sediments are deposited on the seafloor.

Compressive forces cause folding and faulting. TIME 2 Compressive forces cause folding and faulting. compressive forces faults

TIME 3 Uplift is followed by erosion, which creates new horizontal surface.

Volcanic eruptions cover the surface with lava flows. TIME 4 Volcanic eruptions cover the surface with lava flows. lava flows

Tensional forces cause normal faults, creating down-dropped blocks and TIME 5 Tensional forces cause normal faults, creating down-dropped blocks and breaking up earlier features. tensional forces normal faults

Thought questions for this chapter In his epic narrative about a geologic traverse across North America, Annals of a Former World, writer John McPhee called geologic maps “textbooks on a piece of paper” (p. 378). Can you locate a passage in this textbook that describes a geologic structure and sketch a geologic map consistent with McPhee’s description?

Key terms and concepts Basin Brittle Compressive force Deformation Dip Anticline Basin Brittle Compressive force Deformation Dip Dip-slip fault Dome Ductile Fault Fold Foot wall Formation Geologic cross section Geologic map

Key terms and concepts Joint Oblique slip fault Shearing force Strike Hanging wall Joint Normal fault Oblique slip fault Shearing force Strike Strike-slip fault Tensional force Thrust fault