2Review session for final exam Thursday afternoon?Thursday evening? 7:30 -???Friday morning?
3Kinematics of folding- outline Active folds:Flexural foldsNeutral surface foldsPassive folds:Shear foldsInterpreting folds:
4Kinematics of folding Inner workings of folds and strain patterns: Three main models:-Flexural folding-Neutral-surface folding-Shear folding (passive)
5Kinematics of folding Two end-member ways to produce active folds: Both produce parallel folds
6Kinematics of folding- Flexural slip Flexural folding occurs where there is significant mechanical influence of layering in the rock.Flexural slip folding accommodates folding by layer-parallel slip between layers.The layers actively participate in folding by bending and flexing.
7Kinematics of folding- Flexural slip slip between layers.No strain (shear) at fold hingeStrain (shear) occurs on fold limbs
8(individual layers do not thin or thicken) Kinematics of folding- Flexural slipInner workings of folds and strain patterns:Parallel fold(individual layers do not thin or thicken)Are flexural slip folds Cylindrical?What class of fold are flexural slip folds?What kind of rocks do you think make flexural slip folds?
9Kinematics of folding- Flexural slip If all of the strain is in the limbs, what would strain pattern look like?Kink and Chevron folds form via flexural slip
10Kinematics of folding- Neutral Surface folding Neutral surface folds:Strain within hinge zone-extension in outer arccontraction in inner arcneutral surface in betweenNote: Neutral surface does not have to be in the middle of the layerAre Neutral surface folds Cylindrical?What class of fold are neutral surface folds?What kind of rocks do you think make neutral surface folds?
11Kinematics of folding- Neutral Surface folding Inner workings of folds and strain patterns:What type of structures would you expect to see on the outer arc?On the inner arc?
12Inner-arc compression Kinematics of folding- Neutral Surface foldingOuter-arc extension&Inner-arc compression
13Kinematics of folding: Outer- arc extension Possible structures w/in outer-arc extension.Boudinage: Pinch-n-swell structureFolding of layers of different stiffness or competency, the competent layers will responding differently to the soft layers. Stiff layers resist deflection.Layer parallel stretching
14Kinematics of folding: Outer- arc extension Boudinaged stiff-layer that is subsequently rotated & extended.Common stiff-soft pairssandstone-shalelimestone-shalechert-shalesandstone-limestonedolomite-limestonegneiss-schistcalcsilicate-marble
17Kinematics of folding- outline Active folds: What do we mean by active folds?-What are the two kinematic styles of active folds?-Where is the strain (shear) accomodated in each style?Passive folds:Shear foldsInterpreting folds:
18Kinematics of folding-Shear folding (Passive folds) Layer has no mechanical significance=> passive fold
19Kinematics of folding-Shear folding (Passive folds) Common in rocks at high temperaturesCan yield useful kinematic information (sense of shear, flow direction, etc)
20Kinematics of folding-Shear folding (Passive folds) Passive shear folding is favored when the mechanical influence of layering in a rock is very weak.Little to no mechanical contrast between the layers.The layer is passively carried along.
21Kinematics of folding-Shear folding (Passive folds) Found in igneous and metamorphic rocksRequires slip (or flow) along planes parallel to axial planeLike pushing your finger into a deck of cardsProduce similar folds
22Kinematics of folding-Shear folding (Passive folds) Produce similar folds (thinning and thickening of individual layers)
23Combinations of fold kinematics What is the evidence of flexural folding?What is the evidence of neutral surface folding?
24Combinations of fold kinematics Fractures open outward at fold hingeExtensional fractures end at sliding surface on fold limb
25Interpreting foldsLayer thickness and rheology: both define fold wavelengthWhat color strata are weakest? Light or Dark?
26Interpreting folds Parasitic fold: Felsic dike within diorite. Which is more “competent” (viscous)How many deformation “events” occurred?
27Interpreting folds Chevron fold: Where did slip occur? (hinge or limb?)
28Interpreting foldsFlexural-slip folding is enhanced by low cohesion between bedding layers – e.g., well bedded strata such as cherts & limestones.
29Interpreting folds If there is no fault on this mountain: How was shortening accomodated-in rocks on the left side of the mountain?- In rocks on the right side of the mountain?
30Interpreting folds If there is no fault on this mountain, What can you say about-the relative competence of the cliff-forming layers on the left?-the relative competence of the thinly bedded units on the right