structural geology & mountain building

structural geology&mountain building

key termsDeformation – change in the original form, size or

shape of a rock unitStress – amount of force applied to a given areaDifferential stress – stress applied unequally in

different directionsStrain – causes irreversible changes in the shape,

size or orientation of a rock unitElastic 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 stressCompress 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

Terminologyof folds

Limb – sides of a foldAxial plane – imaginary plane that divides a fold as

symmetrically as possibleAxis – intersection of axial plane with the surfaceSymmetrical – limbs are mirror images of each otherAsymmetrical – limbs are not mirror images of each

otherOverturned – a fold in which one limb is tilted

beyond the verticalRecumbent – a fold folded over on itselfPlunge – a fold in which the axis is tilted from the

horizontal

types of foldsSyncline –

downwarping of strata, older strata are found on limbs, younger strata in the axial region

types of foldsAnticline – upwarping

of strata, older strata are found in axial region, younger strata on the limbs

types of foldsAsymmetrical – either

an anticline or syncline in which one limb is tilted at a different angle than the other

types of foldsRecumbent – 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 faultFootwall – rock surface below the faultFault scarp – exposed fault surface

types of faultsNormal fault – a fault

in which the hanging wall moves down relative to the footwall

types of faultsReverse fault – a fault in

which the hanging wall moves up relative to the footwall

types of faultsStrike-slip fault – a

fault in which the motion is in the horizontal plane, also a transform fault

types of faultsThrust 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

strataHighly resistant

sedimentary layers remain exposed along the edge of an uplifted block

orogenesis

Orogenesis – processes by which mountain belts are createdOros – 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 accretionSmall 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