structural geology & mountain building

1. structural geology&mountain building

2. key terms • Deformation – change in the original form, size or shape of a rock unit • Stress – amount of force applied to a given area • Differential stress – stress applied unequally in different directions • Strain – causes irreversible changes in the shape, size or orientation of a rock unit • Elastic deformation – recoverable stress, when stress is removed rock returns to its original shape & size • Ductile deformation – elastic limit of rock is exceeded, the rock flows into a new shape, size or orientation • Brittle deformation – elastic limit and ductile limit of rock is surpassed, failure occurs, the rock breaks • Joint – a crack along which there is no motion

3. Stress,strain &deformation • When does failure occur? • How is deformation in a stick like deformation of a rock unit?

4. effects of directional stress • Compress ional stress – causes rock units to shorten horizontally & thicken vertically • Tensional stress – causes rock units to lengthen horizontally & thin vertically • Shear stress – causes horizontal displacement along fault zones

5. strike & dip • Strike – compass direction of a line where a rock layer intersects the horizontal • Dip – angle of inclination or tilt of a rock layer from the horizontal

6. Terminologyof folds • Limb – sides of a fold • Axial plane – imaginary plane that divides a fold as symmetrically as possible • Axis – intersection of axial plane with the surface • Symmetrical – limbs are mirror images of each other • Asymmetrical – limbs are not mirror images of each other • Overturned – a fold in which one limb is tilted beyond the vertical • Recumbent – a fold folded over on itself • Plunge – a fold in which the axis is tilted from the horizontal

7. types of folds • Syncline – downwarping of strata, older strata are found on limbs, younger strata in the axial region

8. types of folds • Anticline – upwarping of strata, older strata are found in axial region, younger strata on the limbs

9. types of folds • Asymmetrical – either an anticline or syncline in which one limb is tilted at a different angle than the other

10. types of folds • Recumbent – a fold which is folded over on itself

11. fault terminology • Fault – fracture in rock along which there has been displacement • Hanging wall – rock surface above the fault • Footwall – rock surface below the fault • Fault scarp – exposed fault surface

12. types of faults • Normal fault – a fault in which the hanging wall moves down relative to the footwall

13. types of faults • Reverse fault – a fault in which the hanging wall moves up relative to the footwall

14. types of faults • Strike-slip fault – a fault in which the motion is in the horizontal plane, also a transform fault

15. types of faults • Thrust fault – hanging wall moves up and thrusts over the footwall, the dip is less than 45 degrees, low angle fault

16. domes & basins • Basin – circular or elongated structure caused by downwarping of sedimentary rock • Youngest rock found in the center of the basin, older on the flanks • Dome – circular or elongated structure caused by upwarping of sedimentary rock, usually overlying igneous or metamorphic rock • Youngest rock found on the flanks of the dome, oldest in the center

17. Hogbacks • Ridges of steeply dipping strata • Highly resistant sedimentary layers remain exposed along the edge of an uplifted block

18. orogenesis • Orogenesis – processes by which mountain belts are created • Oros – mountain, genesis – to come into being

19. Types of mountains • Fault block mountains – mountain belts formed by extensional forces • Mountains are associated with high angle normal faulting • Examples include: Sierra Nevada, Grand Tetons, “Basin & Range province

20. Types of mountains • Folded mountains – result from large scale compressional forces • Rock units are shortened and thickened as a series of synclines and anticlines forms a mountain belt • Examples include: the Appalachians Valley and Ridge Province

21. Terrane accretion • Small crustal fragments collide and merge with larger continental masses • Accreted crustal blocks are called terranes • Accretion of larger fragments may result in formation of a mountain range • Examples include: the western coast of North America