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Framework for Structural Analysis / What is deformation?
Ch. 1: p.15-‐19; Ch. 2: p. 21-‐
1. Geophysical Imaging: Observations of structures in the subsurface (vital for building a 3D picture of deformation) must rely on geophysical imaging techniques such as seismic reflection, seismic refraction, and ground penetrating radar.
Most seismic data has been collected in the pursuit of oil and gas reserves. Both 2D and 3D datasets exist. [Box 1.1. Offshore seismic reflection data acquisition]
2. Geophysical Imaging: [Fig. 1.6. Offshore Brazil 2D seismic reflection line clearly showing important structures like faults and salt diapirs. Vertical axis scale is in two-‐way travel time] 3. Geophysical Imaging: e.g., Below: faults interpreted from seismic reflection data in the Wytch Farm oilfield, English Channel. Right: time slice data shows fault locations. Caveat: cannot see offsets smaller than ~12 m. 4. Geophysical Imaging: Subsurface imaging can also be achieved using gravimetric data and magnetic data. These techniques rely on the different density and magnetic properties of different rock types. They are good for identifying structures buried below recent surface deposits. [Fig. 1.9. Gravimetric (left) and magnetic (right) data for the state of Minnesota, revealing bed rock structures covered by glacial deposits] 5. A Stepwise Approach: Let us return to how we began this lecture with our stepwise approach:
Nomenclature (DESCRIPTION) – based on field observations and remote sensing -‐ it is important to know the types of features, but this is not enough!
Processes (THEORY/EXPERIMENT) – based on theoretical and laboratory experiments -‐ we need to understand what processes control the different types of deformation we observe.
Models (INTERPRETATION) – based on analytical and numerical models -‐ ultimately, we need to place what we observe in the context of how we think things work.
6. Laboratory Experiments: Structural geologists have a long history of trying to replicate deformation using laboratory experiments. Although they help us understand processes, problems exist with scaling, strain rates (much higher than in nature), and deformation conditions (depth and temperature).
e.g., A. Sandbox models of extensional faulting (courtesy, NAGT). B. Sandbox models of strike-‐slip fault deformation (McClay and Bonora, 2001).
7. A Stepwise Approach: Our final technique we may utilize in structural geology is modeling:
Nomenclature (DESCRIPTION) – based on field observations and remote sensing -‐ it is important to know the types of features, but this is not enough!
Processes (THEORY/EXPERIMENT) – based on theoretical and laboratory experiments -‐ we need to understand what processes control the different types of deformation we observe.
Models (INTERPRETATION) – based on analytical and numerical models -‐ ultimately, we need to place what we observe in the context of how we think things work.
8. Numerical Modeling: Numerical modeling utilizes the underlying physical laws by which deformation proceeds to develop predictive models that may explain observed deformation.
e.g., A. 3D model of faults in the Los Angeles Basin for use in a numerical model of fault slip (courtesy, Michele Cooke). B. Numerical models of surface deformation caused by dike intrusion (left) and slip on a fault (right).
9. A Stepwise Approach: The stepwise approach allows us to differentiate three different hierarchies of structural analysis:
GEOMETRY – type of structure, location, shape, size, orientation, relationship to other features, age sequence.
KINEMATICS – having to do with motions that occurred to produce observed structures. e.g., fault slip; opening across a dike.
MECHANICS – the nature of forces or stresses that produced the observed motions and features (a.k.a. dynamic analysis), and dependence on rheology.
Note: not all structural analyses utilize all three of these approaches. They help us to develop conceptual models to explain how features formed.
10. Geometric Analysis: The descriptive approach requires us to present geometries in some way, such as with maps, cross sections, rose diagrams, 3D models, or stereographic projection. [Fig. 1.10. Map representation of orientation data and stereographic projection] [Fig. 1.12. Lineation data plotted as rose diagrams and streographic projections (stereonets)] 11. Geologic Maps: Spatial distributions and geometric information for geologic features are represented on geologic maps. 12. What is Deformation?: In structural geology, we study deformed rocks. Usually, these rocks have experienced forces that caused them to undergo a change in shape (strain), and we are interested in the process by which that occurred.
However, some deformation occurs through rotation or translation of rigid blocks, allowing us to distinguish between rigid body deformation and non-‐rigid body deformation (strain or distortion).
This leads us to a very general definition of deformation: Deformation is the transformation from an initial to a final geometry by means of rigid body translation or rotation, strain (distortion), and/or volume change.
- OUTER CORE liquid -‐ undergoes convection
- INNER CORE solid (Fe, Ni) 19. Earth Interior Context for Deformation: The Earth’s interior is a giant heat engine, through radioactive decay, latent heat of crystallization, and tidal heating. The thermal gradient is ~25°C/km in the lithosphere, but is less deeper down. Heat flow drives internal convection in the liquid outer core and solid mantle. Conduction of heat occurs through the lithosphere, plus magmatic heat loss and asthenospheric upwelling at mid-‐ocean ridges. Plate boundaries and motions strongly correlate to the mantle convection system. 20. Earth Interior Context for Deformation: The Earth has 7 major tectonic plates and several minor ones. They are approximated as undergoing rigid body motion with most deformation occurring in 10s-‐100s km wide belts at their boundaries. The style of deformation varies with the type of plate boundary: divergent, convergent, and transform. 21. Age of Oceanic Crust: The oldest oceanic crust is <200 m.y. (last 4% of Earth history) so the ancient deformation history of the planet is only recorded in continental rocks (up to 3.96 Ga). 22. Divergent Plate Boundaries: Divergent plate boundaries include the mid-‐ocean ridge spreading system and rift zones. Extension is the dominant process, causing new lithosphere to be created at some critical threshold of thinning. The dominant features are normal faults, dikes, and volcanic rocks. 23. Convergent Plate Boundaries: Convergent plate boundaries include subduction zones and fold-‐and-‐thrust belts. Contraction is the dominant process. The dominant features are thrust faults, folds, linear mountain belts, volcanism, plutonism, and metamorphism. 24. Transform Plate Boundaries: Transform plate boundaries include transform faults (often between spreading ridge segments) and transcurrent faults (within continental lithosphere). Lateral sliding is the dominant process. The dominant features are strike-‐slip faults.
conduction
