structural geology

what is structural geology?

• focuses on how rocks are deformed

• folds and faults are the main structures that result when rocks are deformed or changed

• strike and dip are common geologic measurements (focuses on the angle/direction of rock beds)

strike

can be thought of as compass direction of a planar (flat) geologic feature, when it intersects with the horizontal

• givn in degrees or direction (ex: a strike facing exactly north would give a strike of 0 degrees or 360)

dip

the angle at which a rock layer/bed it inclined, or tilted, from the horizontal

geologic compasses

used to make field measurements of strike and dip

strike and dip symbol

T

principle of superposition

generally, rocks at the bottom are the oldest, those on top are the youngest

folds

• are geologic structures generated by a compressive stress

• fold axis

• axial plane

• limbs

anticlines

• an upturning fold (makes an A like shape)

• youngest rocks are on the edges

• oldest rocks closest to axial plane (middle) of fold

syncline

• downturning fold (makes a U shape)

• youngest rocks will be in the middle of fold near axial plane

• oldest rocks are on the outside

plunge

• when the fold axis of an anticline/syncline is not completely horizontal

• folds are not flat (planar)

normal faults

• rocks break (brittle deformation) which results in faulting

• occur in rifting zones, form from extension (vertical displacement)

• hanging wall moves down relative to foot wall

reverse faults

• form from compression

• hanging wall moves up relative to foot wall

thrust faults

• type of reverse faults

• low dip of faulting, think mountain building zones

strike slip

formed by shear stress, side to side displacement rather than vertical

geologic map symbols

“strata” refers to rock beds

fold axis

where the bend of the fold is sharpest

axial plane

divides the fold along its axis

limbs

flanks of the fold

horizontal strata