Theme: How do geologists use information obtained at the surface to infer what is going on at depth?

Structural geology is the study of the structural features of rock, of the geographic distribution of the features and their causes.
Major questions asked by structural geologists:
What is the structure?
When did it develop?
Under what physical condition did it form?

Forces Acting in Crust
-Compression --><-- -
Tension <-- -->
-Bending _______________________
-Couple (shear) --><--
-torsion

Stress = Force/Area over which Force is applied.
Strain=deformation caused by stress.
Elastic Stages of Deformation
Elastic limit
Plastic
Rupture
Brittle vs. ductile rx
Folding and faulting are the most common forms of deformation.
Strike and Dip: are measurements made to determine the orientation of rock units in the crust.

Structural Geology II
Folds
oldest beds exposed in center
Anticline V's in direction of plunge
dome is a special case

youngest beds exposed in center
Syncline "opens" in direction of plunge
basin is a special case

Faults
-fault planes have a strike and dip
Normal fault hanging wall down caused by tension
Reverse fault hanging wall up caused by compression
Thrust fault = low angle reverse fault
Strike-slip (= transform) fault
-right and left lateral

some "rules" for constructing X-sections:
-dip is in direction of younger bed
-beds "out-of-sequence" are a consequence of faulting
-block with oldest bed moves up with respect to block with younger bed

Review Questions
1. Learn to complete a typical cross section of a folded and faulted area as illustrated in class.

2. Draw a geologic cross-section that depicts the following story: A series of marine sediments are deposited and subsequently deformed into folds and thrust faults. These events are followed by erosion. Volcanic activity ensues and lava flows over the eroded surface. A final stage of high angle faulting breaks the crust into several upheaved and down-throw blocks.

3. Draw cross-sections illustrating a thrust fault and a normal fault. Label the hanging and footwalls, indicate relative motion along the faults and describe the stresses that produced each fault.

4. Identify and briefly illustrate or describe four different forces which act in the crust.

5. Identify and briefly describe the three stages of deformation that a body subjected to directed forces typically passes through.