Lecture 10: Structure Stress and Strain Stress uses units of pressure -- force/unit Area If stress is uniform (=confining pressure), then no deformation will take place. Differential stress is needed in order for deformation to take place. Tensional, compressional, shear. [do the face thing]. Strain measures the amount of deformation (the change in size +/- shape) that results from stress. Only differential stress causes a change in shape. Mesured in (change in size)/original size. Ex: Lo-L1/L1. [draw stress-strain diagram] Elastic deformation is reversable strain. Relationship with stress always linear, up until the elastic limit. Ductile deformation isirreversable. When stress is removed, elastic part of the deformation will rebound, but not ductile. Fracture occurs when the limits of ductile deformation is exceeded. [Draw stress-strain diagram to show difference between brittle rock and ductile rock] Ductile vs Brittle Deformation 1) Temperature makes things more ductile, so that deeper within the Earth, rocks are more ductile 2) Confining Stress reduces brittle properties also, because it hinders the ability of fractures (and faults) to form. 3) Time and Strain Rate slow strain rate favors ductile rather than brittle deformation. [silly putty experiment]. 4) Composition -- micas, clays, calcite, gypsum, shale, slate, phyllite, limestone, marble are more ductile; qtz, sandstone, granites, gneiss are more brittle. In folds, shale layers will pinch and swell. Deformation in the Field Outcrop Strike Dip Rule of Vs Ductile Deformation -- Folding -- compressional and shear stresses Anticline [draw] Syncline [draw] Monocline [draw] Fold Geometry [Use piece of paper] limbs axis axial plane axial plane cleavage plunge open symetrical isoclinal vergence (not in book) indicates direction of shear crenulations Folds and topography -- resistant beds stick out, like in Pennsylvania. Folds and Faults -- donŐt go on forever faults die out into folds, folds get ever smaller like wrinkles on a bed sheet. Brittle Deformation -- fracture and faulting Joints -- fractures in the rocks that do not show offset. -- The first brittle response to stress. --ÓJoints relieve stressÓ. -- Covered in chapter 7 (fig 7.3; weatherng) Fault Classification Normal Fault -- tensional stresses grabens, or rifts, -- horsts hanging wall, footwall younger rocks on top of older rocks, but out of sequence. Thrust Fault -- and Reverse Faults -- compressional stresses 15 degrees younger rocks on top of older rocks Strike-slip Fault -- shear stresses right and left lateral transform faults -- a special case of strike-slip faults