Geologia

Methods of Structural Geology 1. Cross sections
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What is “Structural Geology”?
• The description and interpretation of structures in rocks • that result from deformation
– Changes in shape of rock volumes
• (µm-km scale)

• Why?
– Fundamental understanding of our planet(s) – Exploration (hydrocarbons, ore, etc.) – Nuclear waste disposal & CO2-sequestration
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Structural geologicalresearch
• Field-based research
– Reconstruct the history and architecture of deformed rocks
• Mountain belts, e.g. the Alps • Basin tectonics, e.g. for finding oil

Methods in structural geology
• Mapping structures in the field
– Recording folds, cleavages, lineations, etc
• Shape and character • Location • Orientation

• Theory-based research
– How do structures form? – How do rocksdeform?
• Laboratory experiments of folding or faulting • Numerical modelling

• Thin-section analysis (!Kristallingeologie) • Analysis
– Developing 2 or 3D models (GIS) – Plotting data in graphs and stereonets – Analyse strain, stress, etc.
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This course
• Constructing cross-sections • Strain analysis
– Fry method, Centre-to-centre method, … – Using stretch and rotation of lines &planes
• Mohr circle for strain!

Practical info
• During this course you will need
– – – – – Drawing materials: pencils, etc. Paper (plain, mm-grid, transparent) Ruler & compass Calculator Stereonet + thumb tacks

• PDF's of the lecture will be posted on: • Brittle deformation
– Analysis of fault & slip measurements – http://www.structural-geology.info

• There will be one written exam:February 10, 2009
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Lecture 1. Cross sections
• To make a cross section through a structure we need field data (& drill core data, seismic, etc.)
– Orientation data
• Sedimentary bedding • Faults • Fold axes and axial planes

This lecture
• Some basic techniques • Using a geological map with structure contours (lecture K&P) • Using data along a surface line ± drill core
– Assuminglayers have constant width – Dip domain method
• Chevron folds (Knickfalten) with straight hinges

– Stratigraphical (lithological) column – Type of structures
• Parallel folds or similar folds? • Thin- or thick-skinned tectonics?
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– Circular arcs method
• Cylindrical folds with curved hinges

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300 400 500 600

500 400 300

700 600

31° tan(")=300/500 -> " = 31° 090/31

300 400500

31° tan(")=300/500 -> " = 31° 270/31

600

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500 400 300 200

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Profiles from a line section
• Often data are available along a section only
– River bed or gorge – Cleared strip

. .. . . .

..

?
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?

?

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Example of field data to determine fold type
•

A profile line with some data
We know – Orientations – Stratigraphy – Fold type? Assumption –Chevron fold – Fold panels or domains – Axial plane bisects limbs in middle
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A: 20° B: 80°

C: 40°

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Very low grade rocks: ductile deformation absent ! Folding by flexural slip ! Sharp hinge: Kink or chevron folds

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A profile line with some data
• Construct axial planes Between domain A and B : 40° to E Between domain B and C: 70° to E • A: 20° B: 80° C: 40° • • •

A profileline with some data
Now fill in unit boundaries with constant dip in each domain With stratigraphic column we can extend further down But how to deal with crossing axial planes?

•

A: 20°

B: 80°

C: 40°

?
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A profile line with some data
• Where two axial planes meet, a fold panel disappears (B) The two adjacent panels now become neighbours:
– One fold

Exercise
• AnEW-profile • Draw dip domains • Draw profile with all units

•

A: 20°

B: 80°

C: 40°

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Determine axial plane between A & C: 80° to W

A: 20°

C: 40°

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Dip domains

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Parallel folds with curved hinges
• Folds do not always have sharp hinges and straight limbs (chevron folds) • The may have smoothly curving hinges

Circular arc method
• Dip domain...