Structural geology
Geology 101, Fall 2012
Structural geology
• The study of the deformation and fabric
of rocks in order to understand the
tectonic forces
• Rheology is the study of the effect of
stress on materials
Stress-strain diagram
• Stress is measured as
a force applied to a
material
• Strain is the resulting
change in volume of the
material
• Elastic means that the
material returns to its
normal volume once the
stress is removed;
plastic (or ductile)
means that it does not
Earth’s interior structure
• In fact, using
seismic studies, we
can measure the
depth at which the
Earth’s materials
reach the
elastic/ductile point
• This is where the
material becomes
“gooey”
The stress-strain diagram is misleading
• It shows only
one type of
tectonic
stress -compression
• Two other
types -extension
and shearing
“Classic” plate tectonic settings
and mountain building
• Divergent boundary – fault-block
mountains (horsts and grabens)
Extensional stress
• Normal faults arise
from extensional
stress
• Called “normal”
because of age
relationship of rocks
across the fault
• Detachment faults
are low-angle
normal faults
Orogeny - horst and graben
“Classic” plate tectonic settings
and mountain building
• Divergent boundary – fault-block
mountains (horsts and grabens)
• Collision (convergent) boundary – “foldand-thrust” belts
“Classic” plate tectonic settings
and mountain building
• Divergent boundary – fault-block
mountains (horsts and grabens)
• Collision (convergent) boundary – “foldand-thrust” belts
• Subduction (convergent) boundary –
volcanic arc mountains
Compression leads to certain
structures
• Specifically, ductile
structures called
folds
• Sedimentary rocks
can be deformed
this way, but some
metamorphism may
also occur
Rock fabric
Terms associated with folds
How folds look on the surface
• The combination of
folding and
differential erosion
creates interesting
rock outcrop
patterns, which help
explain human
history.
Plunging folds
• Because the whole
fold may be tilted
perpendicular to the
axial plane, folds
may plunge
• The plunge is
measured as an
orientation and an
angle off of
horizontal
But even the toughest rocks break,
and the break is called a fault
Faults
• Faults are a break in
a rock along which
offset has clearly
occurred
• Breaks where there
is no evidence of
motion are called
joints
• All types of tectonic
stress may lead to
faults
Compressive stress
• Reverse faults
result from
compression
• Called “reverse”
because of age
relationship of rocks
across the fault
• Thrust faults are
reverse faults with a
fault dip angle <
45°
Orogeny - fold and thrust belt
“Classic” plate tectonic settings
and mountain building
• Divergent boundary – fault-block
mountains (horsts and grabens)
• Collision (convergent) boundary – “foldand-thrust” belts
• Subduction (convergent) boundary –
volcanic arc mountains
• Transform boundaries are not
associated with mountain-building
Shearing stress
• Strike-slip faults result from shearing stress
• Called “strike-slip” because motion is along
strike (horizontal orientation)
• Types: left-lateral, right-lateral
Orogeny - transpressional
• Mountain building
occurs in strike-slip
fault areas with
some compression
• Called “obliqueslip”
Origin of the Transverse
Ranges
“Transpressional”
Origin of the Sierra Nevada
Relict subduction zone