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):
- multiple working hypotheses
- elastic vs. inelastic
- brittle vs. ductile
- scale and penetrative vs. spaced structures:
- The image to the right is a look at a calcite vein in a fault located in the Castle Rock area of west-central Kansas. The horizontal scale of the image is about 3 mm and the slide is from a cut across the fault in the direction of movement. The red arrow highlights one of several discrete slip surfaces with the motion shown by the red arrow pair. The blue arrow marks very thin septa of country rock material (Niobrara Chalk - actually a marl), that demarcate the walls of microveins that opened during slip. The small blue double arrow line shows the direction of opening. Thus, at this scale the brittle fault deformation consisted of distributed arrays of discrete but linked slip surface and microveins. A crudely similar pattern can be seen along some spreading ridges in the Pacific ocean basin. The pastel color is a result of the crossed nichols and shows most of the image to be one intact calcite crystal, with these inclusions inside, which suggests recrytallization of the calcite after fault movement or replacement of another material by the calcite after fault movement. This course is designed in part to train your eye so that you can recognize these types of patterns and features.
- 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
- crevices and crevasses
for learning about Structural Geology
- slope stability.
- foundation and tunnel stability (especially important for critical structures such as dams).
- fluid flow pathways, especially in fractured
and faulted rocks.
- aquifer geometry (e.g. faults as barriers versus conduits).
- structural integrity of waste sites.
- seismic hazard risk assessment (e.g. realized
in last decade that fold formation can produce earthquakes).
- finding and understanding hydrocarbon structural traps.
- exploration models for mineralization - mineralization
is often localized in specific structural sites.
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.
- Tectonics -
focus on large scale crustal movements and structural associations.
- 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, and tectonics play a crucial role in sedimentation patterns.
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
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
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 knowing 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
- Younging criteria
- features in tilted and deformed sedimentary rocks that indicate which way was up
(indicate superposition direction). Note that this is an example
of the utility of knowing initial conditions.
- Photo to right: Image of quartzites from basement rocks in Wedel Jarlsberg Land, Spitsbergen. Bedding here is subvertical, and in such a case the question arises as to which way do the rocks get younger. The cross beds here have a geometry with a tangential approach to the underlying layer and a steeper angle truncation on the top, from which you can ascertain the younging direction. The geometry of the cross beds doesn't appear quite right - as the dip of portions of the cross bed are at too high of an angle to the truncating top layer. That is because the rock has been ductilely deformed and the originally geometry has been distorted.
- other features that can indicate which way was up include:
- oscillation ripples.
- mud cracks.
- vertical burrows or trace fossils
- geopetal fabrics.
- Deformation path
= the change in form and position with time of a geologic body.
- Example of a growth fault and growth basins:
- To the left is 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 gravity 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.
The faulting produces an accomodation space on one side where a greater thickness of sediment can accumulate. 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. Sedimentation that occurs on top of a growing fold also exis, and the variable dips and thicknesses of the strata record the growth of the fold.
- View of cliff side exposing Triassic strata (from Kvalpynten, Edgeoya in Svalbard). The upper strata are undeformed, but a close look shows strata in the lower third to be both rotated and faulted. More to the point, the lighter colored sandstones clearly thicken as they approach the bounding fault to the left (or north). The faulting has made room (accommodation space) for the sediments to accumulate. These are growth faults with their associated basins, and deformation and sedimentation were coeval here. We will address the reason they are tilted later on in the course.
- In class growth fault exercise (optional). You will be give a copy of a 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 the geometry and magnitude of deformation that has occurred.
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.
- 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 acts on a specific plane, and how the stress state varies
through space describesthe stress field. The stress field changes
with time, which can also be thought of as a loading path.
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.
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.
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.
Copyright Harmon D. Maher Jr., This may be
used for non-profit educational purposes as long as proper attribution
is given. Otherwise, please contact me. Thank you.