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 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

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