Presentation on theme: "Folds con’t. Review session for final exam Thursday afternoon? Thursday evening? 7:30 -??? Friday morning?"— Presentation transcript:
Review session for final exam Thursday afternoon? Thursday evening? 7:30 -??? Friday morning?
Kinematics of folding- outline Active folds: Flexural folds Neutral surface folds Passive folds: Shear folds Interpreting folds:
Kinematics of folding Inner workings of folds and strain patterns: Three main models: -Flexural folding -Neutral-surface folding -Shear folding (passive)
Kinematics of folding Two end-member ways to produce active folds: Both produce parallel folds
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. Kinematics of folding- Flexural slip
slip between layers. No strain (shear) at fold hinge Strain (shear) occurs on fold limbs
Kinematics of folding- Flexural slip Inner workings of folds and strain patterns: Are flexural slip folds Cylindrical? What class of fold are flexural slip folds? What kind of rocks do you think make flexural slip folds? Parallel fold (individual layers do not thin or thicken)
Kinematics 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
Kinematics of folding- Neutral Surface folding Neutral surface folds: Strain within hinge zone- extension in outer arc contraction in inner arc neutral surface in between Are Neutral surface folds Cylindrical? What class of fold are neutral surface folds? What kind of rocks do you think make neutral surface folds? Note: Neutral surface does not have to be in the middle of the layer
Kinematics 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?
Possible structures w/in outer-arc extension. Boudinage: Pinch-n-swell structure 1) Folding of layers of different stiffness or competency, the competent layers will responding differently to the soft layers. Stiff layers resist deflection. 2) Layer parallel stretching Kinematics of folding: Outer- arc extension
Boudinaged stiff-layer that is subsequently rotated & extended. Common stiff-soft pairs sandstone-shale limestone-shale chert-shale sandstone-limestone dolomite-limestone gneiss-schist calcsilicate-marble Kinematics of folding: Outer- arc extension
Kinematics 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 folds Interpreting folds:
Kinematics of folding-Shear folding ( Passive folds) Shear folding: Layer has no mechanical significance=> passive fold
Common in rocks at high temperatures Can yield useful kinematic information (sense of shear, flow direction, etc) Kinematics 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. Kinematics of folding-Shear folding ( Passive folds)
Found in igneous and metamorphic rocks Requires slip (or flow) along planes parallel to axial plane Like pushing your finger into a deck of cards Produce similar folds Kinematics of folding-Shear folding ( Passive folds)
Produce similar folds (thinning and thickening of individual layers) Kinematics of folding-Shear folding ( Passive folds)
Combinations of fold kinematics What is the evidence of flexural folding? What is the evidence of neutral surface folding?
Extensional fractures end at sliding surface on fold limb Fractures open outward at fold hinge Combinations of fold kinematics
Layer thickness and rheology: both define fold wavelength What color strata are weakest? Light or Dark? Interpreting folds
Parasitic fold: Felsic dike within diorite. Which is more “competent” (viscous) How many deformation “events” occurred?
Interpreting folds Chevron fold: Where did slip occur? (hinge or limb?)
Interpreting folds Flexural-slip folding is enhanced by low cohesion between bedding layers – e.g., well bedded strata such as cherts & limestones.
Interpreting 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?
Interpreting 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