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Announcements - Field trip in lab this week. - Field trip this Saturday- 7:30 AM. - Field books for sale! ($$ to SESS club) Bound book = $15 Spiral 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 flexuralslip 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 stiffnessthinner 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 contours: lines of equal elevation hill on left 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... angular Types of unconformities (surfaces characterized by no deposition, and commonly erosion, over an interval of geologic time) 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 45 away 20 towards 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 anticlinesyncline pair Which way does the fault dip? Permian limestone ophiolitic melange 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 Important terminology/concepts 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 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!