Structure and Deformation: Stress and Strain: Strain is deformation. Structure and Deformation: Folds and Faults. Stress is an applied force

Transcription

1 Structure and Deformation: Folds and Faults Deformation Terms Stress Strain Compression stress Tension stress Shear stress Elastic strain Plastic strain Ductile strain Brittle strain Anticline Syncline Monocline Strike Dip Joint Fault Dip-slip fault Strike-slip fault Oblique-slip fault Normal fault Reverse fault Thrust fault Left-lateral Right-lateral Structure and Deformation: The driving force for geologic deformation is Earth s internal heat engine: Radioactive decay of U, Th, K. Stress and Strain: Stress is an applied force. Strain is the resultant deformation. The rocks get deformed because the continents move. 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 are said to be ductile. Materials that fracture are said to be brittle. 1

2 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 concavedownward fold. (arch) A syncline is a concaveupward fold. (smile) A monocline is a flexure. Folds in Sediments Anticline, Split Mountain Monocline Syncline 2

3 Monocline Recumbent Fold, Yukon 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. Strike and Dip 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. Joints in basalt: Giant s Causeway, Co. Antrim, Ireland 3

4 Joints in basalt: Grand Canyon Folds and Faults Dip-Slip Faults 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.) 4

5 San Andreas Fault (Right- Lateral) 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. Angular Unconformity, GCNP Deformation Terms Stress Strain Compression stress Tension stress Shear stress Elastic strain Plastic strain Ductile strain Brittle strain Anticline Syncline Monocline Strike Dip Joint Fault Dip-slip fault Strike-slip fault Oblique-slip fault Normal fault Reverse fault Thrust fault Left-lateral Right-lateral 5

6 1 2 3 D. Right-lateral Strike-slip Fault E. Left-lateral Strike-slip Fault A. Angular unconfomity B. Disconformity D. Right-lateral Strike-slip Fault E. Left-lateral Strike-slip Fault D. Dome 6

7 1 2 D. Reverse fault D. Reverse fault 3 D. Reverse fault 7