Structure and Deformation Terms • Stress • Strike Deformation: • Strain • Dip • Compression stress • Joint • Tension stress • Fault • Shear stress • Dip-slip fault • Elastic strain • Strike-slip fault • Plastic strain • Oblique-slip fault Folds and Faults • Ductile strain • Normal fault • Brittle strain • Reverse fault • Anticline • Thrust fault • Syncline • Left-lateral • Monocline • Right-lateral Structure and Stress and Strain: Deformation: • The driving force for geologic deformation is Earth’s internal •Stress is an applied force. heat engine: •Strain is the resultant • Radioactive decay of U, Th, K. deformation. • The rocks get deformed because the continents move. Strain is deformation Stress is an applied force • Elastic deformation is temporary. •Compressional (pushed – Like an elastic band together) • Plastic deformation is permanent. – Materials that undergo continuous •Tensional (pulled apart) plastic deformation are said to be •Shear (tearing) ductile. – Materials that fracture are said to •Isostatic (hydrostatic) be brittle. 1 Folds and Faults • Stress applied to rocks can Folds result in ductile behavior • An anticline is a concave- (folding) or brittle behavior (faulting). downward fold. (arch) • Even though rocks are • A syncline is a concave- usually brittle, small upward fold. (smile) stresses applied over long • A monocline is a flexure. times can result in ductile behavior. Anticline, Split Mountain Folds in Sediments Monocline Syncline 2 Monocline Recumbent Fold, Yukon Strike and Dip 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. Joints in basalt: Giant’s Rock Fractures Causeway, Co. Antrim, Ireland • 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. 3 Joints in basalt: Grand Canyon Folds and Faults Faults 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 Strike-Slip Faults Movement • A strike-slip fault is the •Normal faults are tensional. horizontal movement of the •Reverse faults and thrust blocks relative to each other. faults are compressional. –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 Unconformities San • An unconformity is any break in the sedimentary geologic record. Andreas –A disconformity is a break with Fault no deformation. (Right- –An angular unconformity is a Lateral) break in which the lower beds have been tilted and eroded before sedimentation resumed. Angular Unconformity, GCNP Deformation Terms Clicker • Stress • Strike • Strain • Dip • Compression stress • Joint • Tension stress • Fault • Shear stress • Dip-slip fault • Elastic strain • Strike-slip fault • Plastic strain • Oblique-slip fault • Ductile strain • Normal fault • Brittle strain • Reverse fault • A. Normal Fault • Thrust fault • Anticline • B. Reverse Fault • Syncline • Left-lateral • C. Thrust Fault • Monocline • Right-lateral • D. Strike-slip fault 5 Clicker Clicker 1 2 • A. Normal Fault • A. Normal Fault • B. Reverse Fault • B. Reverse Fault • C. Thrust Fault • C. Thrust Fault • D. Strike-slip fault • D. Strike-slip fault Clicker 3 • A. Normal Fault • A. Normal Fault • B. Reverse Fault • B. Reverse Fault • C. Thrust Fault • C. Thrust Fault • D. Right-lateral Strike-slip Fault • D. Strike-slip fault • E. Left-lateral Strike-slip Fault Clicker • A. Angular unconfomity • B. Disconformity • C. Thrust Fault • D. Right-lateral Strike-slip Fault • E. Left-lateral Strike-slip Fault • A. Anticline • B. Syncline • C. Monocline • D. Dome 6 Clicker Clicker 1 2 • A. Anticline • A. Anticline • B. Syncline • B. Syncline • C. Monocline • C. Monocline • D. Reverse fault • D. Reverse fault Clicker 3 • A. Anticline • B. Syncline • C. Monocline • D. Reverse fault 7.