Lecture index: Term/concept list. / Motivation for learning about Structural Geology. / Three basic questions. / Kinematics. / Mechanics and dynamics. / Some important structural themes.

Readings to extract information from:

• Chapt. 1 Introduction from van der Pluijm and Marshak, Earth Structure, p. 2-11.
• Chamberlin, T., 1965, The method of multiple working hypotheses, Science, 148, 754-759.
• Chapt. 3 Fractures and Joints from Twiss & Moores, Structural geology, p. 37-50
• Pollard, D. P. & Aydin, A., 1988, Progress in understanding jointing over the last century, Geological Society of America Spec. Paper, 253, 313-336.

• multiple working hypotheses
• elastic vs. inelastic
• brittle vs. ductile
• scale and penetrative vs. spaced structures
• kinematics
• deformation path
• types of younging criteria
• dynamics and continuum mechanics
• tractions, stress states, and stress fields
• sources of forces
• factors influencing deformation
• strike and dip
• trend and plunge
• structure contours
• common structural map symbols
• joints and joint sets
• plumose structures
• veins
• dikes
• crevices and crevasses

Utilitarian motivations for learning about Structural Geology:

• slope stability.
• foundation and tunnel stability.
• fluid flow pathways, especially in fractured and faulted rocks.
• aquifer geometry.
• structural integrity of waste sites.
• seismic hazard risk assessment (e.g. realized in last decade that fold formation can produce earthquakes).
• hydrocarbon structural traps.
• exploration models for mineralization - mineralization is often localized in specific structural sites.

Intellectual motivation for learning about Structural Geology. Continents move around, mountains grow and die and can be reborn, hard rocks deform like putty. These are initially counter intuitive ideas, and as such drive curiosity. Understanding these processes also helps one to understand basic physics much better. There can be a feedback loop of understanding between the disciplines. Pattern abounds in rocks and sediments, and humans gravitate toward pattern. Additionally so much else of the earth is connected to earth dynamics and structures that it is only natural to be curious about these processes that shape so much. Think of all that in the real world that is related to simple topography, and then how is topography related to structural processes. If you are curious about the planet you live on, how can you not be curious about structural geology?

Disciplines associated with structural geology: Structural geology is embedded within the larger discipline of geology, and overlaps with neighboring subdisciplines.

• Tectonics - focus on large scale crustal movements and structural associations.
• Geophysics - often the eyes of structural geology, critical for looking into the subsurface.
• Rock mechanics - the theory behind rock deformation.
• Basin analysis - a system science looking at behavior in sedimentary basins.

Structural geology - three basic questions are often asked. They are typically asked in sequence, because the answer to one helps develop the answer to the other.

1) What is the geologic architecture of the earth's crust? This can be considered a descriptive endeavor, and often is done in 3-D. The descriptive endeavor in structure is a crucial one. It is the fact base, the foundation, that the rest of sits on. It some circumstances it may not be given the credit it should be given, but one should obtain real satisfaction for contributing to the fact base that the rest sits on.
2) What was the history that produced that architecture? The term for this is kinematics, which is the structural history. Think of it as the 3-d through time. So by necessity structural geology is a historical science.
3) What are the forces or processes that shaped that history? The term often used for this is dynamics. It involves physics, modeling, and experimentation.

In class discussion question: Metaphors are common and important in science. Usually when we think of architecture we think initially of human constructs. Davis introduces a metaphor of architecture for structural geology, that we are attempting to understand the architecture of the crust. This metaphor can be explored in many ways in ways that do and don't work. For buildings blueprints of floor plans are used to depict the architecture. For this discussion explore what types of "blueprints" are used to depict geologic structures, and what types of architectural elements are being depicted. A more thoughtful answer will consider both traditional types and elements and more novel types and element

Kinematics = history of earth movements.

• Law of Original Horizontality: This is a very important principle in the history of geology, convincing because of its simplicity. It might be better thought of as - utility of recognizing initial conditions. In a deformed conglomerate or fossil we recognize deformation has occurred simply because we perceive (or even measure) the difference between a reasonable initial form and its present form.
• Younging criteria - features in sedimentary rocks that indicate which way was up (indicate superposition direction). Note that this is an example of the utility of recognizing initial conditions.
• Deformation path = the change in form with time of a geologic body.
• Example of a growth fault: This is a schematic cross section diagram of layers offset along a growth fault. Such faults are particularly common in association with crustal rifting, and with large scale slump and related salt tectonics, such as can be found in the Gulf Coast area. The important thing to note is the the different numbered layers are offset different amounts, and that the stratigraphic units on one side of the fault are thicker than on the other. This is due to a history of fault movement during sedimentation. Note that by 'removing' the amount of offset on layer 8, the amount of offset during the time span between deposition of layer 7 and 8 can be calculated. This can then be repeated working down through the layers and a history of offset with time can be generated. In such a case you end up knowing a lot more about the deformation path. Growth folds, where sedimentation is occurring over a growing fold, also exist.
• In class growth fault exercise. You will be give a copy of this cross section. Using a ruler measure and plot x, the vertical height above bed 1 in the footwall (right side), versus y, the amount of offset on the bed. What can you conclude from this plot? What additional information would make this plot much more useful? Any idea what other plots may be useful to make?
• Strain analysis = quantifying how much deformation has occurred.

What are geologic forces that cause deformation? In class exercise - take 5 minutes and write down your thoughts in your notebook.

How do we describe those forces inside the earth?

• traction -> stress state at a point -> stress fields. Distinguishing between these three basic entities is very useful. A traction is a component of a stress state, and how the stress state varies through space provides the stress field. The stress field changes with time.

Continuum mechanics: stress-strain relationships.

How to distinguish between brittle vs. ductile, with distributed slip as the link.

The importance of time in deformation behavior.

The need for complex systems approaches.

Recapitulation exercise.

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