structural geology & mountain building

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