Today’s list____________Today’s list____________

Ch15: Rock Deformation

1)Mapping geologic structures

2)How rocks get deformed

3) Folds

Study questions____________Study questions____________Ch15: Rock Deformation·        What type of tectonic forces makes a normal fault? A reverse fault? A strike slip fault?

What is the difference between brittle and ductile deformation?

What are the strike and dip angle of a deformation?

What is the shape of an anticline, syncline, and what is the relative age of rocks exposed?

What type of deformation is the San Andreas Fault? The mid-ocean ridge?

1) Mapping geologic structures

Deformational processes create faults, folds, joints

rock structures

Visible when rock beds are exposed at surface

out crops

1) Mapping geologic structures

Strike and dip

TSP 15.8

1) Mapping geologic structures

Strike and dip

Strike

Dip

Compass bearing of a rock layer relative to North

Angle of tilt of a bed from a horizontal plane

Fig.11.4

1) Mapping geologic structures

N 75 E

2) How rocks become deformed

Confining pressure: force equal in all directions

Differential stress: force not equal in all directions

2. Brittle deformation rocks crack/fracture

3. Ductile deformationrocks flow

Shallow in crust Deeper down

2) How rocks become deformed

Response to differential stress

1. Elastic deformation – the rock returns to nearly its original size and shape when the stress is removed

2) How rocks become deformed

Fault definition: a fracture where displacement has occurred: rocks on either side of fault have moved relative to each other.

Fault definition: a fracture where displacement has occurred: rocks on either side of fault have moved relative to each other.

2) How rocks become deformed

JointsJoints

Fractures where no slip has occurredFractures where no slip has occurred

Not same as a fault! Not same as a fault!

Why?Why?

Slip happens on faults!Slip happens on faults!

2) How rocks become deformed- brittle materials

Compressive forces:shortening

Differential stress: force not equal in all directions

2) How rocks become deformed- brittle materials

Tensional forces:stretching

Differential stress: force not equal in all directions

Shearing forces:shearing

2) How rocks become deformed- brittle materials

Differential stress: force not equal in all directions

2) How rocks become deformed - brittle materials

2) How rocks become deformed - brittle materials

Rift valleys are the result of tensional forces in the crust, creating a series of normal faults.

The downfaulted blocks result in a Rift Valley.

Fig. 11.13

2) How rocks become deformed - brittle materials

St. Andreas fault is an example for a strike-slip fault

Because it is a plate boundary, it is also a transform fault

Fig. 11.10

Fault System Example : San Andreas

2) How rocks become deformed- ductile materials

Compressive forces:shortening

Differential stress: force not equal in all directions

Shearing forces:Shearing/bending

Tensional forces:stretching

3) How rocks fold

series of wavelike undulations of once-flat rock layers

many sizes: microscopic to 100’s of meters

compressive stresses: shorten+thicken crust

3) Folds

Anticlines fold upward… Synclines fold downward…

Fig. Story 11.16

horizontal and plunging anticline

3) Folds

Plunging fold:

axis of fold penetrates intothe ground

Fig. 11.16

Plunging anticlines and synclines

See Fig. 11.17

3) Folds

Types of folds

Other deformations

Symmetrical, asymmetrical, overturning, plunging…..

dome - circular upwarping (anticlinal structure)basin - circular downwarping (synclinal structure)

Fig 11.19