STRUCTURAL GEOLOGY
Structural geology is the study of:
- the distribution, geometry, and orientation of rock bodies (formations)
in the Earth's crust;
- the deformation of rock bodies by folds, faults, and
other types of geologic structures; and
- the origins and relative ages of rock bodies and their geologic structures.
GEOLOGIC MAPS
Geologic maps illustrate the outcrop patterns, spatial orientations
(or attitudes), and structures of rock formations.
A typical geologic map illustrates outcrop patterns of different rock
formations with different colors and alphanumeric characters. The map
key shows the color and characters that represent each formation.
The attitudes of rock bodies and structures are shown with strike-and-dip
symbols of various types.
STRIKE AND DIP
Strike:
- the compass bearing of the line formed by the intersection of an inclined
layer of rock and a horizontal plane
- the trend of a rock body's outcrop
Dip:
- the angle of inclination of a rock layer
- always perpendicular to strike
FOLDS
Folds:
- upward and downward bends in rocks
- most apparent in stratified rocks (sedimentary, volcanic, and metamorphic)
- consist of the following components:
- 1. two limbs
- 2. axis: line of maximum curvature of a fold
- 3. axial plane: connects all the axes in a fold
There are four types of folds:
- anticlines: upward elongate folds with limbs that dip away
from their axes
- syncline: downward elongate folds with limbs that dip toward
their axes
- domes: circular to elliptical upward-arching folds with limbs
that dip away from their centers
- basins: circular to elliptical downward-arching folds with limbs
that dip toward their centers
Anticlines and synclines can be:
- symmetric: limbs dip in opposite directions at equal angles,
and axial plane is vertical
- asymmetric: limbs dip in opposite directions at different angles,
and axial plane is inclined
- overturned: limbs dip in same directions, and axial plane is
inclined
- recumbent: overturned fold with horizontal limbs and axial plane
- plunging: axis is inclined downward, rather than horizontal
OUTCROP PATTERNS OF FOLDS
1. Outcrop patterns of nonplunging anticlines and synclines consist
of parallel belts elongate in the strike direction.
2. Outcrop patterns of plunging anticlines and synclines consist of
V-shaped belts that converge on axial trace:
- a. "V" of a plunging anticline points in the direction of
axial plunge
- b. "V" of a plunging syncline points away from the direction
of axial plunge
3. The "V"s of adjacent plunging folds form zig-zagging outcrop
patterns.
4. Outcrop patterns of domes and basins consist of concentric belts
around their centers.
FAULTS
Faults are fracture surfaces along which there has been some
movement or displacement of rocks.
Three types of faults can be defined on the basis of the principal
direction of rock displacement:
- dip-slip
- strike-slip
- oblique
Dip-slip faults:
- characterized by vertical rock displacement
- a dip-slip fault separates a hanging wall (which lies above
it) from a footwall (which lies below)
- relative movement of hanging wall and footwall defines three types
of dip-slip faults:
- normal: hanging wall moves down (relative to the footwall)
- reverse: hanging wall moves up (relative to the footwall)
- thrust: low angle reverse thrust
Strike-slip faults:
- characterized by horizontal rock displacement
- can be either left-lateral or right-lateral
Oblique faults:
- characterized by rock displacement in both the strike (horizontal)
and dip (vertical) directions
OUTCROP PATTERNS OF FAULTS
1. The surface traces of faults form abrupt lines and scarps that
separate rocks of different lithology, age or structure.
2. The rocks on the downthrown blocks of normal faults are younger
than the rocks of the footwall.
3. The rocks on the upthrown blocks of reverse faults are older than
the rocks of the footwall.
4. Reverse faults can superimpose older rocks over younger ones.
5. Outcrop belts, fault traces, streams, roads, etc. are laterally
offset by strike-slip faults.
UNCONFORMITIES
Unconformities: planar and wavy contacts between rocks which
represent gaps in the geologic record
There are three types of unconformities:
- disconformities
- angular unconformities
- nonconformities
Disconformity:
- unconformable surface that separates parallel layers of sedimentary
rocks
- record a period of nondeposition or erosion between deposition of lower
and upper rocks
Angular unconformity:
- unconformable surface separating nonparallel layers of sedimentary
rocks
- record a period of deformation and exposure between deposition of lower
and upper rocks
Nonconformities:
- unconformable surface separating sedimentary from nonsedimentary rocks
- formed by exposure and erosion of igneous and metamorphic rocks, followed
by their burial by sedimentary rocks
Outcrop patterns of unconformities:
- appear as abrupt lines separating rocks of different lithology, age,
or structure.
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