Term
| What is a geologic structure? |
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Definition
| A geologic structure is a geometric configuration of rocks |
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Term
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Definition
| Shiny, flat cleavage planes along which a rock will easily break |
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Term
| What is a primary structure? |
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Definition
| An original structure that is formed at the same time as the rock |
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Term
| What is a secondary structure? |
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Definition
| A structure formed after the original rock has formed; usually the result of metamorphism |
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Term
| What are some primary structures? |
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Definition
- Foliations: Bedding, igneous layering
- Lineations: current lineation, ripples |
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Term
| How is igneous layering and cross-bedding defined? |
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Definition
- Common in mafic rocks
- Usually defined by variations in feldspar/pyroxene ratios or grain/gradient size; modal layering |
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Term
| What are two criteria by which primary structures can be defined? |
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Definition
| Disharmonic folds and folds confined between flat layers; sediments are still soft when deformation occurs |
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Term
| What are four tectonic structures? |
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Definition
- Foliation: slatey cleavage, schistosity, gneissosity (related to metamorphic grade)
- Lineation: mineral lineations, intersection lineations
- Folds: anticlines, synclines, recumbent folds
- Faults: cataclasites and mylonites |
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Term
| What two processes control the nature of planar tectonic fabrics? |
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Definition
| Metamorphic grade (temperature) and rock composition (mineralogy) |
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Term
| What is the difference between a recumbent fold and a reclined fold? |
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Definition
- Recumbent fold: flat-lying axial surface
- Reclined fold: plunges down the dip of its axial surface |
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Term
| What is the difference between strike-slip and dip-slip faults? |
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Definition
- Strike-slip faults: faulting is parallel to strike plane
- Dip-slip faults: faulting is parallel to dip plane (ex. horst and graben structures) |
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Term
| What is the difference between cataclasites and mylonites? |
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Definition
- Cataclasite: a cohesive and fine-grained fault rock
- Mylonite: well-foliated tectonic rock formed by intense plastic deformation; characterised by grain size reduction |
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Term
| What are three types of geological contacts? |
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Definition
- Base-cover relationships: unconformities
- Intrusive contacts
- Faulted contacts |
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Term
| What do geological maps show, and what must you be able to read from them? |
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Definition
- Show the distribution of rocks and the orientation of structures in these rocks
- Allows rock identification, distribution, contact relations, structures and their orientations |
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Term
| What are the definitions of strike, dip, plunge, and pitch? |
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Definition
- Strike: orientation (azimuth) of the line of intersection between a horizontal plane and the inclined plane of interest
- Dip: angle between a horizontal plane and the inclined plane of interest as measured in a vertical plane PERPENDICULAR to strike
- Plunge: angle between the inclined linear structure and a horizontal plane, as measured in a vertical plane oriented PARALLEL to strike
- Pitch (rake): the acute angle between a horizontal line and the inclined lineation measured within the plane on which the lineation lies |
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Term
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Definition
| Surface expression of subsurface inclined foliation planes at different depths projected onto the map from the structural profile |
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Term
| How do you determine the thickness of bedding? |
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Definition
- Must be measured PERPENDICULAR to bedding planes
- Draw a construction line perpendicular to the strike of the bed of interest
- Mark top/bottom of bed on this line
- Transpose this line onto a work sheet
- Use a protractor to extend the top/bottom of bed in profile, using the known dip angle
- Measure thickness using map scale |
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Term
| What is apparent dip and how is it measured? |
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Definition
- Dip measured in any plane other than the vertical perpendicular plane to strike will give a value less than the true dip
- Draw a structural profile and strike lines for the foliation
- Draw a line oriented parallel to the plane of interest across the strike lines
- Transpose this line to a work sheet, and sketch in the map scale perpendicular to this
- Draw a profile for this line by plotting a sequence of points corresponding to the strike lines
- Connect the dots and measure the apparent dip |
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Term
| What are the differences between stereographic projections and orthographic projections? |
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Definition
- Stereographic projections: do not preserve spatial relations; easier to perform but more difficult to visualise than orthographic projections; use equal-angle and equal-area nets; planes project as curved lines; lines project as points
- Preserve spatial relationships; involve drawing cross-sectional profiles at right angles to the strike of foliations; can involve drawing structural contours; can be used to determine thickness of beds, apparent dip of inclined foliations, orientation of intersection lineations |
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Term
| What are equal area and equal angle nets used for? |
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Definition
- Equal area (Schmidt): used to determine the distribution of foliation and lineation measurements, and their statistical densities
- Equal angle (Wulff): used to solve geometrical problems |
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Term
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Definition
| The transformation from an initial to a final geometry via distortion, rotation, volume change (dilation), and/or translation |
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Term
| What structures easily show signs of dilation? |
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Definition
- Volume gain: extensional cracks filled with calcite
- Volume loss: stylolites |
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Term
| What two forms of deformation does strain produce? |
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Definition
| Distortion (change in shape) and dilation (change in size) |
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Term
| What happens to a structure that has undergone homogeneous deformation? |
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Definition
- Straight lines remain straight and parallel lines remain parallel
- Circles become ellipses, spheres become ellipsoids |
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Term
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Definition
- A smooth closed curve which is symmetric about its centre
- Corresponds to the shape given when a plane intersects a cone |
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Term
| What are the foci of an ellipse? |
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Definition
| Two points on an ellipses major axis such that the sum of the distances from any point of the ellipse to those two points is constant and equal to the major diameter |
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Term
| What is a strain ellipse? |
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Definition
| Represents the distortion accommodated by a rock during homogeneous deformation |
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Term
| What is the difference between an incremental strain ellipse and a finite strain ellipse? |
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Definition
- ISE: shows how an initial circle is affected by a tiny increment of deformation
- FSE: shows the total strain experienced by the rock; sum of all ISEs |
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Term
| What do the axes of the FSE and ISE indicate? |
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Definition
| If the axes are parallel, the strain is irrotational (coaxial). If not, the strain is rotational (non-coaxial). |
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Term
| What is the difference between pure and simple shear? What do they both have in common? |
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Definition
- Simple shear: non-coaxial strain (rotational)
- Pure shear: coaxial strain (irrotational)
- Both refer to strain histories |
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Term
| What symbol is used to describe the degree of rotational strain? |
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Definition
| Wk = w/[(2(sx^2 + sy^2 +sz^2)]^-2; where Wk is the kinematic vorticity number, w is the angular velocity vector, and s are principle strain rates parallel to incremental stretching axes |
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Term
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Definition
- Measures the relationship between internal rotation (vorticity) and how fast strain accumulates during deformation.
- Coaxial: Wk = 0
- Non-coaxial: Wk = 1
- Subsimple shear: 0 < Wk < 1
- Continuous spinning: Wk > 1 |
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Term
| What is the difference between 1D, 2D, and 3D strain? |
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Definition
- 2D: no strain along intermediate strain axis (Y); lengthening along X is compensated by shortening along Z; changes in angles between lines that are initially perpendicular
- 1D: changes in lengths of lines; extension, stretch, and quadratic elongation
- 3D: change in lengths of axes X, Y, Z |
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