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
• 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
•Youngest rock found in the
center of the basin, older on
the flanks
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