Week 1: Introduction to structural geology and tensional brittle structures.

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

Readings you can extract further information from:

Term/concept list for first week (note - plenty of room to add notes to the right. as you study):

Motivation for learning about Structural Geology

Utilitarian motivations:

Image to right is of a landslip in the Seattle Washington area that the USGS is monitoring. Note the curved main scarp, which is basically a slip surface. Note also the cracks within the sliding hill side. The same basic mechanics that influence tectonic fault movement also influence mass wasting features such as this one. Clearly, understanding whether a slope is stable or not is of use to those living on or beneath the slope. Source http://landslides.usgs.gov/monitoring/seattle/ .

Intellectual motivation: Continents move around, mountains grow and die and can be reborn, hard rocks deform like putty. These can be 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 different 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 overlapping with structural geology: Structural geology is embedded within the larger discipline of geology, and overlaps with neighboring sub-disciplines. Some examples are given below.

Three basic questions

Structural geology - three basic questions are often asked when trying to understand the structural geology of an area. 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 focuses on describing the 3-D geometry of associated and/or linked structures of different types. At its core is the recognition of the different types of structural features, an ability that comes with training and experience. Follow this link to a training the eye exercise to help practice developing this recognition skill. In many cases it involves using a visualization tool. The descriptive endeavor in structure is a crucial one. It is the fact base, the foundation for understanding. In 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 (creating maps, cross sections, databases) that the rest depends 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.

Image of Carboniferous crinoids from SEDL lineament in western Spitsbergen (a complex deformation zone with a strike-slip component). Crinoid cross sections are typically circular, but these are distinctly oval. This is due to distributed ductile deformation. By measuring the long and short axes of the crinoids one can say something about the magnitude of deformation, which would be an example of strain analysis on a small scale. Knife blade for scale.

Precision GPS has allowed us to understand modern kinematics and neotectonics in ways not possible before. Depicted above are GPS detected motions in Southern California associated with the San Andreas fault. Each vector represents the amount of movement relative to the more interior North American continent given the scale in the lower left. The circle at the head of the vector represents the error. For those vectors where the error circle includes the vector tail you can not be sure it has moved. Look for sharp breaks in the pattern across faults - this indicates the fault actively moved (at some depth) during the period of GPS monitoring. One can calculate larger scale crustal strain with such information. Image source: http://pasadena.wr.usgs.gov/office/hudnut/scec/97_SCEC_E_summary.html .

Example for USGS site on Appalachian mountains showing serial cross sections through time in order to document a large scale kinematic history (http://3dparks.wr.usgs.gov/nyc/valleyandridge/valleyandridge.htm). This is more of a tectonics perspective. Later on in the semester we will explore basic idea of restoring a cross section back in time through various stages in order to understand the magnitude and timing of deformation.

Mechanics and dynamics

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? In some fashion, even in your intro courses, you were probably introduced into the concept of stresses, and the basic idea that the forces inside the earth differ by direction. You will learn additional details in this course.

Continuum mechanics: stress-strain relationships, and material properties such as elasticity and viscosity.

Image of how the stress field is changed at the tips of a fault section that moved during an earthquake. Source: http://quake.usgs.gov/research/deformation/modeling/stress_trig/ourrecentpapers.html

Some important structural themes

Scale is an important consideration. How to distinguish between brittle vs. ductile (?), with distributed brittle slip as an intermediary, provides one example. The first image and caption above shows an example of linked distributed brittle structures at the microscopic scale.

In the above three cases the brown layer has the same vertical offset (shown by green and black line). In the case to the left, at this scale the brown line is continuous and this would be classified as ductile deformation (a fold). In the example to the right the vertical offset has been accomplished by a single fault, and it is clearly brittle deformation. The case in the middle is distributed slip which has attributes of both brittle and ductile deformation. If the slip is distributed at a small enough scale at larger scales it can appear to be ductile.

The importance of time in deformation behavior.

Exercise: classification of different types of deformational behavior.

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

On to joints and other brittle tensional phenomena.

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