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structural geology & mountain building. 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 - PowerPoint PPT Presentation
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structural geology&mountain building
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
Stress,strain &
deformation
• When does failure occur?
• How is deformation in a stick like deformation of a rock unit?
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
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
Terminology
of 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
types of folds
• Syncline – downwarping of strata, older strata are found on limbs, younger strata in the axial region
types of folds• Anticline – upwarping
of strata, older strata are found in axial region, younger strata on the limbs
types of folds
• Asymmetrical – either an anticline or syncline in which one limb is tilted at a different angle than the other
types of folds
• Recumbent – a fold which is folded over on itself
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
types of faults• Normal fault – a fault
in which the hanging wall moves down relative to the footwall
types of faults• Reverse fault – a fault
in which the hanging wall moves up relative to the footwall
types of faults• Strike-slip fault – a
fault in which the motion is in the horizontal plane, also a transform fault
types of faults• Thrust fault – hanging
wall moves up and thrusts over the footwall, the dip is less than 45 degrees, low angle fault
domes & basins
• 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
•Basin – circular or elongated structure caused by downwarping of sedimentary rock
•Youngest rock found in the center of the basin, older on the flanks
Hogbacks • Ridges of steeply dipping
strata• Highly resistant
sedimentary layers remain exposed along the edge of an uplifted block
orogenesis
• Orogenesis – processes by which mountain belts are created• Oros – mountain, genesis – to come into being
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
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
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