Geologic Structures Folds and Faults Stress and Strain Stress is an applied force Strain is the resultant deformation Stress is an applied force Compressional pushed together Tensional pulled apart Shear tearing Isostatic hydrostatic Strain is deformation Elastic deformation is temporary Like an elastic band Plastic deformation is permanent Materials that undergo continuous plastic deformation a re said to be ductile Materials that fracture are said to be brittle Folds and Faults Stress applied to rocks can result in ductile behavior folding or brittle behavior faulting Even though rocks are usually brittle small stresses applied over long times can result in ductile behavior Folds An anticline is a concave downward fold A syncline is a concave upward fold A monocline is a flexure Strike and Dip Strike and dip are used to describe the orientation of any planar feature such as bedding in sedimentary rocks Strike is the compass direction of the intersection of the plane with the horizontal Dip is the measured angle between the plane and the horizontal Rock Fractures A joint is a fracture in rock on which there has been no movement A fault is a fracture in rock on which there has been movement Dip slip is movement in the direction of dip Strike slip is movement in the direction of strike Oblique slip is movement in both directions Dip Slip Dip slip is movement of one block up or down the plane of the fault Normal fault upper block moves down Reverse fault upper block moves up Thrust fault low angle reverse fault Stress and Fault Movement Normal faults are tensional Reverse faults and thrust faults are compressional Strike Slip Faults A strike slip fault is the horizontal movement of the blocks relative to each other Left lateral the opposite block has moved to the left Right lateral The opposite block has moved to the right It doesn t matter which block you re on Unconformities An unconformity is any break in the sedimentary geologic record A disconformity is a break with no deformation An angular unconformity is a break in which the lower beds have been tilted and eroded before sedimentation resumed
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