Transcript Forces In Mountain Building

Forces In Mountain Building
Chapter 11
Tectonic Structures
• Tectonics is the study of crustal deformation and structural behavior.
Plate Tectonics is the deformation and structural behavior of crustal
plates
http://www.data.scec.org/larse/images/Figure12.gif
Stress
• Stress is any force which acts to
deform rocks.
Compression - a stress that acts to
press or squeeze rocks together.
Tension - a stress that acts to stretch
a rock, or pull a rock apart.
Shear - a stress which acts tangential
to a plane through a body, causing
two contiguous parts to slide past
each other
Structural Behavior
• As a general rule:
1) Rocks tend to have a
relatively high compressive
strength
2) Rocks tend to have a
relatively low tensile and shear
strength
Strain
• When a stress is applied,
deformation may occur,
depending on the amount of
stress
Strain is the change in shape or
volume of a body as a result of
stress; deformation.
There are Brittle and Ductile
deformation.
Ductile Deformation
• During ductile deformation rocks
bend or flow.
Folding or bending of material
without breaking.
Specifically defined as a rock that
is able to sustain, under a given
set of conditions, 5-10%
deformation before fracturing.
Folds can be microscopic in size
or kilometers in extent.
Ductile Deformation
http://facweb.bhc.edu/academics/science/harwoodr/GEOL101/Study/Images/DetachmentFault.gif
Anticlines
• Folds which arch up
Anticlines
http://www.es.ucsc.edu/~es10/classnotes/images/W.R.Olig.bedsoverMeso.d.jpg
Synclines
• Folds which sink down
Syncline Fold
http://facweb.bhc.edu/academics/science/harwoodr/GEOL101/Study/Images/DetachmentFault.gif
Folded Mountains
http://www.es.ucsc.edu/~es10/classnotes/images/W.R.Olig.bedsoverMeso.d.jpg
Monoclines
• Folds in which rock layers on both sides of
the fold are horizontal but at different
levels.
Domes
• Folds which are equivalant to anticlines,
but are comprised of layers which are
shaped like an inverted bowl.
http://www.es.ucsc.edu/~es10/classnotes/images/W.R.Olig.bedsoverMeso.d.jpg
Basins
• Folds which are equivalant to synclines, but are
comprised of layers which are shaped like a bowl.
http://facweb.bhc.edu/academics/science/harwoodr/GEOL101/Study/Images/DetachmentFault.gif
Brittle Deformation
• During brittle deformation rocks break or
fracture.
Two main styles of fracture: Joints and
Faults.
Both are the result of relatively rapid
stress.
For example: modeling clay will break if
stress is applied rapidly, but will bend if
stress is applied slowly.
http://ic.ucsc.edu/~casey/eart150/Lectures/BrittleDef/CorinthHrstMod.jpg
Joints
• Joints are fracture
surfaces along which
there has been no
displacement.
Joints can form from
compressional, tensional
and shear stress, and can
range in size from
microscopic to kilometers
in length.
Joint sets and jointing has
a major influence on
landform development.
Erosion is able to occur at
a faster rate along joints.
http
://facweb.bhc.edu/academics/science/harwoodr/GEOL101/Study/Images/DetachmentFault.gif
http://facweb.bhc.edu/academics/science/harwoodr/GEOL1
01/Study/Images/DetachmentFault.gif
Faults
• Faults are fractures along which there has
been displacement of the material on
either side of the fault.
Faults are classified based on:
1) the sense of movement (the direction in
which the blocks on either side of the fault
move) - this is controlled by the type of
stress that is applied.
2) the orientation of the fault surface (the
angle of the plane of fracture)
Fault Terminology
• Fault Plane - the plane along which the
rock or crustal material has fractured.
Hanging Wall Block - the rock material
which lies above the fault plane.
Footwall Block - the rock material which
lies below the fault plane.
Strike-Slip Faults
• Fault plane is generally vertical.
Movement is horizontal due to shear stress.
1) Left-Lateral Strike-Slip - displacement is such
that the material on the other side of the fault
appears to be displaced to the left.
2) Right-Lateral Strike-Slip - displacement is
such that the material on the other side of the
fault appears to be displaced to the right.
• Strike-Slip Faults
Normal Faults
• Fault plane is oriented between 30 and 90 degrees
(measured from horizontal)
Movement has both a horizontal and vertical component.
Normal faults result from tensional stress and results in
the hanging wall moving down relative to the footwall.
Normal Faults
http://facweb.bhc.edu/academics/science/harwoodr/GEOL101/Study/Images/DetachmentFault.gif
Reverse Faults
• Fault plane is oriented between 30 and 90 degrees
(measured from horizontal)
Movement has both a horizontal and vertical component.
Reverse faults result from compressional stress and
results in the hanging wall moving up relative to the
footwall.
Reverse Faults
http://facweb.bhc.edu/academics/science/harwoodr/GEOL101/Study/Images/DetachmentFault.gif
Thrust Faults
• Fault plane is at less than 30 degrees
Movement is more horizontal than vertical
due to the low angle of the fault plane.
Develop due to compressional stress.
http://facweb.bhc.edu/academics/science/harwoodr/GEOL101/Study/Images/DetachmentFault.gif
Horsts and Grabens
• Horsts are up thrown blocks bounded on
either side by parallel normal faults.
Grabens are downthrown blocks bounded
on either side by parallel normal faults.
Measurement of Orientation
• Strike - compass direction of the outcrop
- the line formed by the intersection of a horizontal plane with the
structure.
Dip - the angle between the horizontal plane and the planar surface
being measured.
- Dip is always perpendicular to Strike
Fault Map Symbols
References
• http://facweb.bhc.edu/academics/science/
harwoodr/Geog102/study/tecton1.htm