Week 5- DYNAMIC PLANET: Structural Geology and Vertical Crustal Movements

Question 1

what is structural geology?

Answer 1

the study of the deformation in the lithosphere generated by stresses

Question 2

why are rocks stressed?

Answer 2

1. Load pressure (lithostatic stress/confining stress)

-results from the weight of overlying rock
-stress acts uniformly in EVERY direction
-analogous to the hydrostatic pressure beneath a head of water

2. Deviatoric stress (differential/triaxial)

-stresses do not act uniformly on the rock
-caused y lateral transmission of horizontal tectonic forces through the solid Earth’s crust (contraction or extension)
-in 3D world will ALWAYS be 3 principal, perpendicular stresses acting on an object
-the maximum principal stress
-the intermediate principal stress
-the minimum principal stress
=TRIAXIAL STATE OF STRESS

Question 3

What categories do faults fall into? How are they classified?

Answer 3

1. Normal
2. reverse/ “thrust”
3. strike-slip

According to the orientation of principal stresses in the triaxial state (Anderson’s classification of tectonic stress)
-aka based on the direction of slip

Question 3

What are faults?

Answer 3

Fault are fracture surfaces, along which displacement has taken place.

Question 4

What are dip slip faults?

Answer 4

where movement has occurred along a dipping surface, and the two blocks have moved with a vertical component relative to the surface of the earth

Question 5

What are the characteristics of normal faults?

Answer 5

-hanging wall is downthrown
-Steep (~60 degrees from horizontal)
-when б1 is vertical, б2 and б3 are horizontal (extension)
-fault surface dips parallell to б3
-Thins the crust

Question 6

What are characteristics of thrust/reverse fault?

Answer 6

-Hangingwall is upthrown
-Low angle (~30o from horizontal_
-when б3 is vertical, б2 and б1 are horizontal (compression)
-fault surface dips parallel to б1
-Thickens crust
-thrusts occurring in contractional zones often occur with folds= “fold thrust belts”

Question 7

What are characteristic of strike-slip faults?

Answer 7

Movement has occurred along a largely vertical surface, and the two blocks have moved with a horizontal component relative to the surface of the earth
-No hangingwall-footwall
-Vertical fault
-when б2 is vertical, б1 and б3 are horizontal (shearing)
-No net change in crustal thickness

Question 8

What are folds? What are they caused by? How are they classified?

Answer 8

The majority of folding is caused by buckling: compression resulting in plastic deformation and shortening.
-Compression zones include continental-continental
collisions (orogens).

Classified:
1. according to the orientation of axial plane (=the plane that exactly bisects the interlimb angle)
2.the tightness of the fold (= the interlimb angle)
The classification works at all scales