Download Lecture 6 Structural Geology, Gettysburg NMP, Chickamauga and Chattanooga NMP

Structural geology
Geology 115
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
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
Gettysburg NMP
(1895) in 1863
All of the folding in the
Gettysburg area is due to
the collision of African and
North American plates and
the closing of the Iapetus
Ocean 325 – 280 million
years ago
“Thin-skinned” tectonics = fold-and-thrust belt = a
region where there has been a shortening of the
crust due to tectonic compression, and a series of
folds and thrust faults have been created, with the
thrust faults all connecting at a relatively shallow
depth at a décollement
Gettysburg basin,
and other basins
aligned with it,
developed as a result
of the rifting of the
supercontinent
Pangaea and the
opening of the
Atlantic Ocean about
220 mya
Chickamauga and Chattanooga NMP (1890)
Moccasin Bend
Railro
ad Mountain
Lookput
Long linear ridge
Digital elevation model of eastern Tennessee – note
the eroded anticline and the thrust faults