Geologic Structures:Folds and FaultsStress 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 plasticdeformation a re said to be ductile.– Materials that fracture are said to bebrittle.Folds and Faults• Stress applied to rocks can result inductile behavior (folding) or brittlebehavior (faulting).• Even though rocks are usuallybrittle, small stresses applied overlong times can result in ductilebehavior.Folds•An anticline is a concave-downwardfold.•A syncline is a concave-upward fold.•A monocline is a flexure.Strike and Dip• Strike and dip are used to describethe orientation of any planarfeature such as bedding insedimentary rocks.–Strike is the compass direction of theintersection of the plane with the horizontal.–Dip is the measured angle between the planeand the horizontal.Rock Fractures•A joint is a fracture in rock on which therehas been no movement.•A fault is a fracture in rock on which therehas been movement.–Dip-slip is movement in the direction of dip.–Strike-slip is movement in the direction ofstrike.–Oblique-slip is movement in both directions.Dip-Slip• Dip-slip is movement of one blockup 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 faultsare compressional.Strike-Slip Faults• A strike-slip fault is the horizontalmovement of the blocks relative to eachother.–Left-lateral: the opposite block has movedto the left.–Right-lateral: The opposite block hasmoved to the right.• (It doesn’t matter which block you’re on.)UnconformitiesUnconformitiesUnconformitiesUnconformities•An unconformity is any break in thesedimentary geologic record.–A disconformity is a break with nodeformation.–An angular unconformity is a breakin which the lower beds have been tiltedand eroded before
View Full Document
Unlocking...