Structural Geology and Tectonics

Course outline:

Web-page for the course http://courses.eas.ualberta.ca/eas421

What is the course about?

The objective of structural geology is to understand the deformation of the lithosphere - in other words, to find out how the Earth moves.

Structures are the observable products of deformation. Most introductory structural geology courses cover structures by type - brittle structures like faults are typically covered in one section, ductile structures like folds in another, etc. In this course we assume that you have a basic grounding in what types of structures exist in the Earth's Crust. This 'second look' at the deformation of the Earth will initially focus on measurement - we will try to quantify things that have previously been described qualitatively - and on the study of tectonic environments.

Principles

We will group these ideas under the fundamental concepts of geometry, kinematics, and dynamics.

• Geometry
  • Geometric or descriptive analysis (in structural geology) is concerned with accurately describing the shapes of bodies of rock as they are at the present day. Geometry is paramount in exploration for hydrocarbons and mineral resources - we want to know where exactly in the subsurface there are the geometries to create reservoirs, or mineral deposits.
• Kinematics
  • Kinematics is the study of the movement of the lithosphere - this includes measurements of the rates of plate movement, amounts of fault slip, and the distortion of rocks that have undergone ductile deformation. Kinematic analysis involves four basic types of change:
    • Translation - chages in position
    • Rotation - changes in orientation
    • Dilation - changes in size
    • Distortion - changes in shape
  • The last two of these - Dilation and distortion - are together grouped as strain
• Dynamics
  • Dynamic analysis involves measurement or estimation of the force or stress that has affected rocks. Dynamic analysis is the most difficult to do, and in fact, there are relatively few circumstances in structural geology where we can accurately say what stress was applied in the geologic past. Almost always, we have to measure deformation (a kinematic concept) and then make an assumption about the rheology - the relationship between strain and stress - in order to say anything about dynamics.

The first part of the course will look at all these methods of structural analysis individually.

Tectonic environments

In the second half of the course, we will apply these principles to the deformation of the Earth's lithosphere in the three principal types of tectonic environment:

• Crustal extension: Extensional tectonics is ultimately responsible for the formation of the world's ocean basins and for the development of rifts and passive continental margins which contain a major part of the Earth's fossil fuel
• Crustal shortening: Shortening produces an enormous range of geologic structures including ocean trenches and orogens (mountain belts). This will be the biggest section. It will include investigations of deformation in subduction zones, fold and thrust belts (like the Candian Rockies), and belts of regional metamorphism.
• Strike-slip motion occurs along transform faults (such as the San Andreas of California) at present day, and has left a record in may parts of the Earth's crust. Strike-slip motion may be combined with extension to produce transtension. Combined with shortening it produces transpression.

• Impact structures: If time permits at the end of the course we will have a look at deformation associated with meteorite impacts