Geology Lecture Outline

Crustal Deformation (Ch 10)

 

I. Lecture Content

Introduction - Makings of High Places

Deformation - Stress and Strain

Structural Geology - Folding, Faulting, and Shearing

II. Introduction

A. Deformation and Mountain Building

1. Spectacular crustal upheavals are the results of:

            Same forces that power plate tectonics

            Interplay between crustal rocks and the global

and regional-scale deviatoric stresses created

by inter-plate motions

2. Deformation events are typically part of much larger

regional mountain building events called orogenies.

 

III. Deformation

A. Stress and Strain - Defined

1. STRESS Defined:

       Force applied to a given area of rock

       Stress = Force/unit area

 

              Stress causes strain (deformation) in rocks

 

              Stress applied to a rock can be exerted:

 

       Equally in all directions = Isostatic stress

 

       Unequally from 2 directions = Deviatoric stress

              Deviatoric Stress can be exerted in several ways

 

       Compressional

       Tensional

    Shearing

 

2. STRAIN

              Defined:

       Changes in the shape and/or volume of a rock

 

              Strain means the same thing as "deformation"

 

              Strain is the result of stress

       Stress is the cause

       Strain is the effect

 

              Three types of strain are recognized -

       Elastic

       Plastic

       Fracture

 

              The type of strain is dependent upon the response of

rocks to a particular stress

 

              Factors that control the type of strain in a rock

       Kind of applied stress

       Amount of pressure (force)

       Temperature

       Rock type

       Length of time under stress

 

3. Stress and Strain Illustrated

              See Figures

 

B. Three Types of Deviatoric Stress

1. Compressional Stress

              Defined:

       External forces applied toward one another

 

              Rock under compressional stress results in:

       Shortening in the direction of stress

 

       Thickening perpendicular to the stress

 

       Deformation in the form of folding and faulting

 

2. Tensional Stress

              Defined:

       External forces acting in opposite direction along

the same line

 

              Rock under tensional stress results in:

       Lengthening/extension in the direction of stress

 

       Thinning perpendicular to the stress

 

       Deformation in the form of faulting

 

3. Shear Stress

              Defined:

      External forces acting parallel to one another

but in opposite directions

 

              Rock under shearing stress results in:

       Displacement of adjacent layers along closely

spaced planes in the direction of stress

 

      Deformation in the form of folding, faulting, and

shear zones

 

C. Three Types of Strain

1. Elastic Strain

              Rocks behave in an elastic fashion

 

       Stressed rock returns to its original shape and

volume when the stress field is removed

 

              Elastic behavior in rock common in the upper crust

 

2. Plastic Strain

              Rocks behave in a plastic fashion

 

       Stressed rock remains deformed after the stress

field is removed

 

              Amount of plastic deformation in rock depends on its

degree of ductility

 

       Rocks that are said to be "ductile" are able undergo

large amounts of plastic strain

 

       Rocks that are said to be "brittle" are not able to

undergo much plastic strain

 

              Plastic behavior common in the lower crust

 

3. Fracture Strain

              Rocks behave in a brittle fashion

 

       Stressed rock breaks into two or more pieces

before the stress field is removed

 

       Rock remains deformed after stress is removed

 

            Brittle-Fracture behavior common in the upper crust.

IV. Structural Geology

A. Geologic Structures

1. Bodies of rock that are distinguished by their geometric

arrangement of both rock types and structural elements

such as size, layering, shape, and orientation.

 

2. Many types of geologic structures are the result of

deformation events such as uplift, folding and faulting

 

B. Orientation of Structural Layering

1. Strike - Definition

 

2. Dip - Definition

 

3. Determining Strike and Dip

 

C. Graphic Illustrations of Geologic Structures

1. Geology Maps

 

2. Block Diagrams

 

D. Fold Structures

1. Fold defined:

              Originally planar feature that is now bent

       Planer features like bedding

 

              Bending process is plastic deformation

 

2. Anatomy of a Fold

              Fold limb = "cline"

 

              Fold axis = Line separating limbs

 

              Axial plane = Plane equally separating limbs

 

3.Three Basic Types of Fold Structures

              Monoclines

       Simple bend with only one bent limb

 

       Illustrated in Figure

 

              Anticlines

       Convex-upward fold

 

       Limbs dip away from each other

 

       Oldest layers exposed in the core part of fold

 

       Illustrated in Figure

 

              Synclines

       Concave-upward fold

 

       Limbs dip towards each other

 

       Youngest layers exposed in the core part of fold

 

       Illustrated in Figure

 

4. Fold structures can be oriented in several different ways

              Upright fold

       Fold axis is horizontal

 

       Axial plane is vertical

      Illustrated in Figure

 

              Inclined fold

       Fold axis is horizontal

       Axial plane is inclined

       Fold limbs dip at different angles

       Illustrated in Figure

 

              Overturned fold

       Fold axis is horizontal

       Axial plane is inclined

       Both fold limbs dip in the same direction

       Illustrated in Figure 13.13(b)

 

              Recumbent fold

       Fold axis is horizontal

       Axial plane is inclined to near horizontal

       Illustrated in Figure

 

              Plunging fold

       Fold axis is inclined (plunging)

       Axial plane is vertical

       Illustrated in Figure

 

5. Two special types of fold structures

              Domes

       Circular to oval-shaped anticline

       All rock layers dip away from center of dome

       Oldest rocks exposed in center of structure

 

              Basins

       Circular to oval-shaped syncline

       All rock layers dip towards the center of basin

       Youngest rocks exposed in center of structure

 

6. Folds are typically the result of compressional stresses

acting on plastic-behaving layered rock.

E. Fault Structures

1. Fault defined:

A fracture along which opposing crustal blocks move

parallel to the fracture surface (plane of the fault)

 

2. Anatomy of a Fault - (See Figure )

              Hanging wall block

 

              Footwall block

 

              Fault plane (dip and strike)

 

              Fault scarp

3. Three Basic Types of Fault Movement

              Pure Dip-slip

       Relative fault motion parallel to dip direction

 

       Relative fault motion can be either hanging wall

up or hanging wall down

 

              Pure Strike-slip

       Relative fault motion parallel to strike direction

 

       Relative fault motion can be either right lateral or

left lateral

             Oblique-slip

       Relative fault motion has both dip and strike

directional components

 

      Relative fault motion can be any which way

 

4. Three Basic Types of Faults

              Normal faults

       Hanging wall moves down relative to footwall

 

       Caused by tensional stress

 

       Associated with divergent tectonics

 

              Reverse faults

       Hanging wall moves down relative to footwall

 

       Called a thrust fault if fault plane angle < 45

 

       Caused by tensional stress

 

      Associated with convergent tectonics

 

             Strike-slip faults

       Opposing blocks move laterally to each other -

either right-laterally or left-laterally

 

       Caused by shearing stress

 

       Associated with transform tectonics

 

5. Faults are the result of deviatoric stresses acting on

brittle-behaving rock

 

F. Joint Structures

1. Joint defined:

A fracture along which no movement has occurred, or

movement occurred perpendicular to joint surface

 

             Lack of movement along fracture plane is difference

between joints and faults

 

2. Commonest strain-related structures in rocks

 

3. Formation of joint fractures

              Caused by either compression, tension, or shear stress

 

              Typically caused by a release of pressure

 

              Also caused by the rapid cooling of magma or rock

 

              Joint-forming process a shallow crustal phenomenon

 

              Typically form either columnar or sheet-like patterns

 

 

V. Vocabulary Crustal Deformation and Structural Geology - CH10

 

Anticline

Basin

Compression

Continental accretion

Craton

Deformation

Dip

Dip-slip fault

Dome

Elastic strain

Fault

Fault plane

Footwall block

Fracture strain

Hanging wall block

Joint

Monocline

Normal fault

Oblique-slip fault

Orogeny

Plastic strain

Plunging fold

Reverse fault

Shear stress

Shield

Strain

Stress

Strike

Strike-slip fault

Suture zone

Syncline

Tension

Terrane

Thrust fault