midterm next monday, october 14 midterm review. what is structural geology? - study of rock...
TRANSCRIPT
![Page 1: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/1.jpg)
Midterm next Monday, October 14
Midterm Review
![Page 2: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/2.jpg)
What is structural geology?
- Study of rock deformation, “the study of the architecture of the Earth’s crust”
- “forensic science”
![Page 3: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/3.jpg)
What are structures?
Two main types:
(1) Primary structures: Develop during formation of a rock body; e.g.,cross-bedding, ripple marks, mudcracks, pillows (in basalt)
(2) Secondary structures: Form in rocks as a result of deformation- the structures this class are focused on!
![Page 4: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/4.jpg)
Goals of Structural Analysis
• Geometry: mapping, measurements
• Kinematics: movements related to deformation– Translation: change in position– Rotation: change in orientation
– Distortion: change in shape– Dilation: change in size
Dynamics/Mechanics: relating deformation to stresses
![Page 5: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/5.jpg)
Structural measurements
• Planar structures • Strike: compass direction of trace of
horizontal line on a plane; bearing (quadrant, E or W of north) or azimuth (degrees clockwise from N)
• Dip: inclination of plane from horizontal, perpendicular to strike
![Page 6: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/6.jpg)
Linear structures• Trend: direction of a vertical plane that
contains the linear feature in the direction of plunge.
• Plunge: angle between line and horizontal
![Page 7: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/7.jpg)
Oceanic Crust
- forms at mid-ocean ridges by partial melting of mantle
- basaltic (mafic) in composition
- igneous extrusion and intrusion
- 5 to 10 km-thick
- Oceanic crust is no older than ~200 Ma. Why??
![Page 8: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/8.jpg)
Continental Crust
- 5 to 10 times thicker than oceanic crust- 40 km avg.
- This is a simplified sketch! Continental crust is very heterogeneous
- Long and complex deformation history. Majority of continental crust formed during the Precambrian (before ~570 Ma) The oldest known rock is ~4 Ga!
Why so much older than oceanic crust??
![Page 9: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/9.jpg)
Rheology (behavior during deformation) of the Earth
Lithosphere: "lithos" = rock, implying strength. It exhibits a component of elastic strength over geological time scales. Includes crust + uppermost mantle! Varies in thickness. Moves as a plate- exceptions are orogens.
Asthenosphere: Weak. It is solid, but behaves like a viscous fluid (convective flow) over geological time scales.
![Page 10: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/10.jpg)
How do we know that plates move?- Earthquakes localized along plate boundaries
![Page 11: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/11.jpg)
Transform faults: more evidence for plate motion
Not observed!
Observed!
![Page 12: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/12.jpg)
Continents also break- to form new oceans
![Page 13: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/13.jpg)
Ocean-Continent convergent margins
![Page 14: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/14.jpg)
An example of continent-continent collision:
The Indo-Asian Collision
![Page 15: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/15.jpg)
Transform plate boundaries
An example: San Andreas
![Page 16: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/16.jpg)
Pangea supercontinent and Tethys Ocean
http://www.scotese.com/earth.htm
![Page 17: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/17.jpg)
Joint: A natural fracture that forms by tensile loading- walls of fracture move apart slightly as joint develops
![Page 18: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/18.jpg)
Plumose structure: A subtle roughness on surface of some joints; resembles imprint of a feather. Due to inhomogeneity of rock.
![Page 19: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/19.jpg)
Joints/Fractures: Kinematics
ribs are arrest lines- opening is not instantaneous, but rhythmic,
like splitting wood
![Page 20: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/20.jpg)
Cooling joints: form by thermal contraction
![Page 21: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/21.jpg)
Exfoliation joints: Form by unloading of bedrock through erosion.They form parallel to topography
![Page 22: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/22.jpg)
Tectonic joints: Form by tectonic stresses as opposed to stresses induced by topography.
![Page 23: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/23.jpg)
Strike-slip faults: Accommodate horizontal slip between adjacent blocks
left lateral vs. right lateral: sense-of-slip relative to a chosen block
left lateral (sinistral)
right lateral (dextral)
![Page 24: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/24.jpg)
Hanging wall: The block toward which the fault dips.
Footwall: The block on the underside of the fault.
![Page 25: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/25.jpg)
http://earth.leeds.ac.uk/learnstructure/index.htm
![Page 26: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/26.jpg)
Slip vs. Separation
Slip: actual relative displacementSeparation: apparent relative displacement
![Page 27: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/27.jpg)
The key to describing slip along a fault lies in measuring
(1) Direction of displacement
(2) Sense of displacement
(3) Magnitude of displacement
![Page 28: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/28.jpg)
Main types of folds
Anticline: fold that is convex in the direction of the youngest beds
Syncline: Fold that is convex in the direction of the oldest beds
*requires that you know facing direction (direction of youngest beds); know stratigraphy!
Antiform: convex up
Synform: convex down*simply describes geometry
anticline
syncline
synformal anticline
antiformal syncline
![Page 29: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/29.jpg)
Geometric analysis
inflection point: point of opposing convexity
median surface: imaginary surface connecting inflection points
fold width, fold height
symmetrical vs. asymmetrical
concept of vergence
![Page 30: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/30.jpg)
hinge zone – hinge line: zone of max. curvature
fold axis: imaginary line, which when moved parallel to itself can define the form of a fold
Geometric analysis cont.
![Page 31: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/31.jpg)
axial surface: surface that passes through successive hinge lines
axial trace: line of intersection of axial surface and ground surface
Geometric analysis cont.
![Page 32: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/32.jpg)
![Page 33: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/33.jpg)
parallel/concentric folds: layer thickness does not change (lower T)
similar folds: layer thickness changes; thickening in hinge and thinning along limbs (higher T)
![Page 34: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/34.jpg)
Fold mechanisms for "free folds", where fold shapes depend on layer properties
(1) Flexural-slip folding- accommodates buckling by layer-parallel slip
-direction of relative slip is perpendicular to hinge
-individual displacement small, but sum is enough to accommodate bending of rock
-marked by strong stiff layers with contacts of low cohesive strength
-occurs in uppermost levels of crust
![Page 35: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/35.jpg)
minor structures related to flexural-slip folding
![Page 36: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/36.jpg)
minor structures related to flexural-flow folding
occur at higher temperature
![Page 37: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/37.jpg)
plunging folds and map patterns, cont.
![Page 38: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/38.jpg)
Introduction to geologic maps
Geologic maps show traces of contacts between different rock units, commonly superimposed on topography
First step: Every time you see a contact, ask yourself the following questions:
(1) Is it a depositional contact?(2) Is it an intrusive contact?(3) Is it a fault contact?
So far, we have talked quite a bit about faults, but not the other types of contacts. To fill you in...
![Page 39: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/39.jpg)
ophiolitic melange
Permian limestone
Which way does the fault dip?
Second step: Study how the trace of the contact interacts with topography- It will tell you about orientation!
![Page 40: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/40.jpg)
Stereographic projection
plotting 3D structural data on a hemisphere (usually the lower), which is projected onto a horizontal plane
![Page 41: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/41.jpg)
Rake = The acute angle between the horizontal (strike line) and a line in the plane, MEASURED IN THE PLANE
![Page 42: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/42.jpg)
Determining the true thickness of a bed
For a dipping bed, the map-view thickness is an "apparent" as opposed to "true" thickness!
1. Draw a structural profile (X-section) perpendicular to strike
2. Plot the true dip of the beds and project them to depth
3. Use trigonometry to calculate the true thickness
![Page 43: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/43.jpg)
Determining strike and dip from geologic maps(revisited)
75 m
![Page 44: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/44.jpg)
What is strain?
Strain is dilation (change in size) and/or distortion (change in shape).
The Goal of strain analysis is to explain how every line in a body changes in length and angle during deformation.
How is this attempted?
![Page 45: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/45.jpg)
Some important quantities for describing strain
Extension (e): (Lf-Lo)/Lo, where Lf is the final length of a line and Lo is the initial length of a line
Stretch (S): Lf/Lo, where 0 = severe shortening, 1 = no shortening, and infinity = severe stretching
Quadratic elongation ((1+e)2 = (Lf/Lo)2 = S2
![Page 46: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/46.jpg)
So far- we have only talked about changes in lengths of lines- what about angles?
Angular shear (, psi): degree to which 2 initially perpendicular lines are deflected from 90 degrees
Shear strain (, gamma): = tan ()
![Page 47: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/47.jpg)
What does 'finite' mean? It is total strain, the final result of deformation that we see as geologists
Instantaneous or infinitesimal strain describes a tiny increment of deformation
As will become apparent when studying how fabrics form in rocks, the orientation of finite strain may be very different than that of instantaneous strain
Finite vs. Instantaneous strain
![Page 48: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/48.jpg)
Strain ellipse and ellipsoid for homogeneous deformation:
Shows how circular reference object is deformed
2-D3-D
Vs=4/3r3
Ve=4/3abc
![Page 49: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/49.jpg)
2 end-member types of plane strain
Simple shear: Rock is sheared like a deck of cards. A square becomes a parallelogram. **The finite stretching axes rotate during deformation. Distortion by simple shear is the most important process in shaping shear-zone structures!
![Page 50: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/50.jpg)
Pure shear: Rock is shortened in one direction and extended in the perpendicular direction. A square becomes a rectangle. **The finite stretching axes do not rotate.
![Page 51: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/51.jpg)
Strain Ratestrain rate = extension (e) divided by time (t) = e/t
The rate at which a rock is strained has important implications for the manner in which it deforms.
"Lab" Strain Rates
During 1 hour experiment, an initially 2.297 cm-long sample is shortened to 2.28 cm. What is the average strain rate during this experiment?
![Page 52: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/52.jpg)
Force vs. Stress
Force: That which changes, or tends to change, body motion
Newton's first law of motion: F=mamass in kg; acceleration in m/s2
1 Newton (1N) = 1kg m/s2
Forces are vector quantities; they have magnitude and direction.
![Page 53: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/53.jpg)
Stress may be thought of as a description of force concentration
Stress on a plane (traction), = F/A
1N/m2 = 1 Pa
what about units of stress?
100 MPa = 1 kbar
![Page 54: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/54.jpg)
lithostatic stress
vertical force = Vg = L3g
vertical stress = L3g/L2 = gL
gL = (2700 kg/m3)(10m/s2)(1500m) = 40500000 Pa
= 40.5 MPa = .405 kbar
![Page 55: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/55.jpg)
A complete definition of Stress = a description of tractions at a given point on all possible surfaces going through the point
1: axis of greatest principal stress3: axis of least principal stress
1
1
33
1 and 3 always perpendicularand always perpendicular to planes of no shear stress
![Page 56: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/56.jpg)
Geometric approach: Mohr Stress Diagram a plot of s vs. n
first step: plot 1 and 3 recalling that they are in directions of no shear stress; draw Mohr circle
![Page 57: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/57.jpg)
second step: Draw a line representing the plane at 2, measured from 3.
![Page 58: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/58.jpg)
mean stress: (1+3)/2 center of circlecauses dilation
differential stress: (1-3) diameter of circlecauses distortion
![Page 59: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/59.jpg)
1
13
3
instantaneous strain ellipse
![Page 60: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/60.jpg)
Common types of deformation experiments
![Page 61: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/61.jpg)
Compressive strength tests: The Approach
![Page 62: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/62.jpg)
c = critical shear stress required for failure0 = cohesive strengthtan = coefficient of internal friction () N = normal stress
Coulomb's Law of Failure
c = 0 + tan(n)
![Page 63: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/63.jpg)
Tensile strength tests with no confining pressureApproach: Similar to compressive strength testsResults: (1) Rocks are much weaker in tension than in compression (2) Fracture oriented parallel to 1 (= 0)
![Page 64: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/64.jpg)
Failure envelopes for different rocks: note that slope of envelope is similar for most rocks
c = 0 + tan(n)c = critical shear stress required for failure
0 = cohesive strength
tan = coefficient of internal friction
N = normal stress
![Page 65: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/65.jpg)
Byerlee's Law
Question: How much shear stress is needed to cause movement along a preexisting fracture surface, subjected to a certain normal stress?
Answer: Similar to Coulomb law without cohesionFrictional sliding envelope: c = tan(N), where tan is the coefficient of sliding friction
![Page 66: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/66.jpg)
Preexisting fractures of suitable orientation may fail before a new fracture is formed
![Page 67: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/67.jpg)
Increasing pore fluid pressure favors failure by counteracting confining pressure!
Effective stress = n – fluid pressure
What about fluid pressure?
![Page 68: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/68.jpg)
What happens at higher confining pressures?
Von Mises failure envelope- Failure occurs at 45 degrees from 1
![Page 69: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/69.jpg)
Anderson's Theory of Faulting
The Earth's surface is a free surface (contact between rock and atmosphere), and cannot be subject to shear stress. As the principal stress directions are directions of zero shear stress, they must be parallel (2 of them) and perpendicular (1 of them) to the Earth's surface. Combined with an angle of failure of 30 degrees from 1, this gives:
![Page 70: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/70.jpg)
![Page 71: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/71.jpg)
An isotropic, homogeneous elastic material follows Hooke's Law
Hooke's Law: = Ee
E (Young's Modulus): measure of material "stiffness"; determined by experiment
![Page 72: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/72.jpg)
Elastic limit: no longer a linear relationship between stress and strain- rock behaves in a different manner
Yield strength: The differential stress at which the rock is no longer behaving in an elastic fashion
![Page 73: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/73.jpg)
What happens at higher confining pressure and higher differential stress?
Plastic behavior produces an irreversible change in shape as a result of rearranging chemical bonds in the crystal lattice- without failure!
Ductile rocks are rocks that undergo a lot of plastic deformation
E.g., Soda can rings!
![Page 74: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/74.jpg)
![Page 75: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/75.jpg)
Viscous (fluid) behavior
Rocks can flow like fluids!
![Page 76: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/76.jpg)
For an ideal Newtonian fluid:differential stress = viscosity X strain rateviscosity: measure of resistance to flow
![Page 77: Midterm next Monday, October 14 Midterm Review. What is structural geology? - Study of rock deformation, “the study of the architecture of the Earth’s](https://reader036.vdocuments.us/reader036/viewer/2022062804/56649d1b5503460f949f0c14/html5/thumbnails/77.jpg)
The brittle-ductile transition