Chapter 9: Geologic Structures & Mountain Building Flashcards
Orogens
Mountain belts (from the greek words oros, meaning mountain, and genisis meaning formation).
Orogeny
A mountain building event which may last for tens of millions of years.
Mountain Building
the process of forming a mountain belt
uplift
the vertical rise of the land surface and the rock beneath which also causes deformation
deformation
rocks that bend, break, or flow as a result of mountain building/uplift.
geologic structures
features produced by deformation including
- joints (cracks)
- faults (fractures where rock slide past each other)
- folds (bends, curves, or wrinkles of rock layers)
- foliation (a fabric or layering in rock)
changes caused by deformation
- change in location (displacement)
- change in orientation (rotation)
- change in shape (distortion/strain)
types of strain
- stretching (layer becomes longer)
- shortening
- shear strain (movement of one part of a rock body past another so that angles between features in the rock change)
types of strain
- brittle: many bonds break and stay broken, leading to the formation of a permanent fracture across which material no longer connects.
- plastic/ductile: some bonds break, but new ones quickly form. this way the atoms within grains rearrange and the grains change shape without permanent cracks forming.
- elastic
conditions where plastic deformation takes place
- warm rock
- great pressure
- slow rate of change
- variety of rock composition
conditions where brittle deformation takes place
- cooler temperatures
- closer to the earth’s surface (less pressure)
- sudden change in shape (marble bench example)
- rock composition also contributes to whether the rock is more brittle/plastic
stress
the force applied per unit area of the plane; by using the word stress, we emphasize that the consequences of applying a force depend not only on the amount of force but also on the area over which the force acts. Three types
- compression
- tension
- shear
compression
takes place when a rock is squeezed together
tension
occurs when the opposite ends of an object are pulled in opposite directions
shear stress
when one surface of an object slides relative to another
pressure
occurs when an object is subjected to the same stress on all sides
joints
natural cracks along which the rock bed broke and separated into separate pieces during brittle deformation, not the same as faults (faults are larger scale and moved or are active)
vein
a mineral filled crack where groundwater or hydrothermal fluids seep through cracks in rocks and precipitate out of the earth/groundwater
faults
a fracture on which sliding occurs and slip events, or faulting, can generate earthquakes.
strike line
deviation (in degrees) from north
- perpendicular to the dip
dip
a deviation (in degrees) from a flat, horizontal plane; slope of the plane
- perpendicular to the strike
- 0 degrees of dip mean that the bed is horizontal, 90 degrees of dip mean that the bed is vertical
fault displacement
the amount of movement that takes place across a fault
dip faults
Angled
- movement parallels the dip line
- hanging wall block slides up or down relative to the footwall block
types of dip faults
- reverse: hanging wall block slides up the fault (compression)
- thrust: a reverse fault with a gentle dip (less than 30 degrees, happens at continental collision)
- normal: hanging wall block slides down (accommodates stretching)
strike-slip faults
Straight down into the crust; movement parallel to the fault
If you are facing the fault and looking across it:
- if the block on the far side slipped to your left, the fault is a left-lateral
strike-slip fault.
- if the block slipped to the right, the fault is a right-lateral strike-slip fault
oblique slip faults
Also at an angle, but movement occurs diagonally across the fault plane (scrubby)
fault scarp
the “step” like structure created from dip-slip and oblique-slip faults. In other words: a planar feature formed by off-set at the surface by fault movement
fault breccia
shattered, angular fragments of rock from brittle conditions at faults
fold
a curve in the shape of rock layer; ductile deformation structures
parts of a fold
- limbs; the sides of the fold that have less curvature
- hinge: a line along which the curvature of the fold is greatest
- axial surface: an imaginary plane that contains the hinge lines of successive layers and effectively divides the fold into two halves
types of folds
- anticlines, syncline, monoclines
- non-plunging and plunging folds
- domes and basins
anticlines
folds with an arch-like shape: limbs dip away from the hinge
synclines
folds with a trough-like shape; limbs dip “toward” the hinge
monoclines
looks like a carpet draped over a stair-step
plunging vs. non-plunging folds
plunging: folds that do not have a horizontal hinge; the hinge is tilted/plunges into the surface of the earth
non-plunging: hinge is horizontal/flat
domes vs basins
both ciruclar
dome: a fold with the shape of an overturned bowl; oldest layer at the center
basin: a bowl shaped (right-side up) fold; youngest layer at the center
Stress causes _____, _______ results in ________.
Stress causes strain, strain results in structures.
mylonite
cohesive foliated rock; produced by tectonic reduction of grain size through crystal plastic processes in restricted areas of intense deformation.
triangular facets
occur at normal faults (extension) and at mountain fronts; mountains move up, valley move down
what are salt “tectonics”?
- any type of deformation involving salt
- salt beds become buried by other rock beds
- salt is less dense, behaves buoyantly, rises toward the surface
- intrudes overlying rocks creating faults and folds
diapir
a domed rock formation in which a core of rock has moved upward to pierce the overlying strata
