Announcements

- Field trip in lab this week. - Field trip this Saturday- 7:30 AM.- Field books for sale! ($$ to SESS club)

Bound book = $15Spiral book = $5.50

- Thurs. 4 PM Haury Building, Rm 216, Charles Prewitt, "Deep Earth Mineralogy: Searching for Answers"

- SESS presentation Fri. 1 pm Rm 209: "Iceland- Land of Fire and Ice" by Peter Kresan

Today's Lecture

(1) Fold mechanisms

(2) Visualizing and mapping structures

Fold mechanisms for "free folds", where fold shapes depend on layer properties

(1) Flexural-slip folding- accommodates buckling by layer-parallel slip

-direction of relative slip is perpendicular to hinge

-individual displacement small, but sum is enough to accommodate bending of rock

-marked by strong stiff layers with contacts of low cohesive strength

-occurs in uppermost levels of crust

minor structures related to flexural-slip folding

minor structures related to flexural-flow folding

occurs at higher temperature

What is it?

(2) Kink folding-Occurs when there is strong cohesion between layers-Marked by sharp hinge, straight limbs (one short, two long)

Another mechanism: (3) Buckling

-Instability develops when layers of different mechanical properties are subjected to layer-parallel stresses

-Wavelength depends on layer thickness and stiffness- thinner layers yield shorter wavelengths

Visualizing & Mapping Structures

Goals

(1) To practice visualizing in 3-D:

Key to geologic mapping and structural analysis

- Topography + Geology

(2) To practice interpreting topographic and geologic maps

(3) Learn how to approach mapping

Topographic maps

hill on left

contours: lines of equal elevation

C:\Documents and Settings\Paul Kapp\My Documents\MyDocuments2\TEACHING\structure_movies\SteveReynolds\topowat_poland.mov

Introduction to geologic maps

Geologic maps show traces of contacts between different rock units, commonly superimposed on topography

First step: Every time you see a contact, ask yourself the following questions:

(1) Is it a depositional contact?(2) Is it an intrusive contact?(3) Is it a fault contact?

So far, we have talked quite a bit about faults, but not the other types of contacts. To fill you in...

Types of unconformities (surfaces characterized by

no deposition, and commonly erosion, over an

interval of geologic time)

angular

Types of intrusive contacts

Introduction to geologic maps, cont.

First step: Every time you see a contact, ask yourself whether it is depositional, intrusive, or a fault

Second step: Study how the trace of the contact interacts with topography- It will tell you about orientation!

intersection of planes and topography

horizontal 20 towards

45 away vertical

"The Rule of V's"

The rule states that the "V" of the trace of a plane in a valley points in the direction in which the plane dips.

Don't spend time memorizing this, just spend time visualizing topography- it will become obvious!

What is it?

plunging folds and map patterns, cont.

Plunging folds

"simple" structures + topography can result in complex map patterns

dipping planes overturned anticline-syncline pair

ophiolitic melange

Permian limestone

Which way does the fault dip?

Is the fault steeply, moderately, or shallow dipping?

How well you can "read" topographic and geologic maps hinges on how well you can visualize in 3D...

It takes practice, practice, practice at

http://www-glg.la.asu.edu/~sreynolds/home.htm

Next: Techniques for projecting and plotting structural data

Read D&R, p. 626-654

Contours- definition of

Three main types of contacts

Disconformity

Nonconformity

Angular unconformity

Buttress unconformity

Rule of V's

Map patterns of plunging folds

Determining surface orientations from maps

Contact traces

Important terminology/concepts

Remember

• The 3 spaces you deal with: topography, primary structures and deformational (secondary) structures

• Rule of v’s, intersections between topography and geologic surfaces

• A structure looks very different depending from where you look at it!