GEOL30350 Geological Structures

Academic Year 2023/2024

Geological structures provide an essential framework for defining the Earth’s deep-time development, present-day hazards and future economic resources. This module explores how geological structures result from the deformation of rocks. The scope of the module spans ductile deformation characteristic of the deeper Earth through to brittle deformation in the shallow Earth. The module covers the essential theoretical concepts –such as stress, strain, rheology, progressive deformation, failure criteria etc. – underpinning our understanding of the generation and expression of the main geological structures. The module also emphasizes the practical recognition and description of these geological structures in the field, combined with their quantitative analysis in the laboratory. Lecture and practical classes are complemented by one-day field trips to excellent exposures of geological structures in the greater Dublin area.

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Curricular information is subject to change

Learning Outcomes:

On completion of this module students should be able to:
1. Understand the fundamental concepts of stress and strain in two and three-dimensions and explain their relationships to a range of geological structures.
2. Identify the main ductile or brittle structures and articulate the differing physical conditions required for their development (and hence typical locations of development within the Earth).
3. Obtain and analyze geometric data for a range of geological structures.
4. Describe geological folds in three dimensions, quantitatively classify them and relate them to one or more folding mechanisms.
5. Understand the relationships of folding to other structures such as cleavage, foliation, and boudinage.
6. Decipher the large-scale structure of complexly folded rocks through analysis of smaller scale observations of minor folds, cleavage bedding intersection, facing on foliation, etc.
7. Analyze the kinematics of ductile, brittle-ductile and brittle shear zones.
8. Articulate the principals of, and differences between, progressive pure and simple shear.
9. Explain how progressive deformation (e.g. pure shear or simple shear) may account for the geometry, orientation and distribution of geological structures.
10. Illustrate the state of stress in rock by using Mohr’s circle and explain how this can be used determine the conditions for rock stability or failure.
11. Explain the how failure criteria and failure envelopes relate to the generation of structures such as joints, hydro-fractures, faults and shear zones.
12. Describe the 3D geometry of thrust and normal faults, and explain how such faults act as a geometrically coherent system.
13. Construct and read stereonet projections of structural data.
14. Construct and restore geological cross-sections.

Indicative Module Content:

LECTURES:
Lecture 1: Strain and geological structures.
Homogeneous and heterogeneous strain. The 2-D strain ellipse and Ramsay plot. Finite and incremental strain. The 3-D strain ellipsoid and Flinn plot. Relationship of strain ellipse/ellipsoid to observations of geological structures.
Lecture 2: Determination of strain in rocks.
Overview of techniques for 2-D and 3-D strain determination in rocks, such as by measuring eccentricity of initially spherical objects or the geometry of deformed fossils, and using the centre-to centre or Rf/Phi methods for non-spherical objects.
Lecture 3: The relationship of strain to stress.
Review of laboratory-based studies of rock deformation and recap of fundamental concepts of stress, strain and rheology. Relationship of brittle and ductile deformation to physical conditions of deformation and composition of rocks. Overview of main brittle and ductile structures.
Lecture 4: Fold geometry and classification.
Fundamentals of fold geometry and description. Fold vergence, harmony and parasitic folding. Non-cylindrical folds. Quantitative classification of folds by using orthogonal thickness and dip isogons. Relationship of folds to the 3D strain ellipsoid.
Lecture 5: Foliations.
Nature, geometry and origin of continuous vs non-continuous (disjunctive) foliations. Mechanisms of foliation development including flattening, mechanical rotation, dissolution/precipitation, recrystallization. Relationship of foliations to folding and to the 3D strain ellipsoid.
Lecture 6: Lineations and L/S fabrics
Nature and origin of structural and mineral lineations. Generation of L/S fabrics and relationships to folds and the 3D strain ellipsoid.
Lecture 7: Folding and Boudinage of Rocks - Mechanisms and Controls.
Folding mechanisms, including buckle, flexural slip, flexural flow folding, as well as shear, chevron and kink folding. Nature and origin of cylindrical and chocolate tablet boudinage. Consider also associated small-scale structures, including fractures, lineations and foliations. Controls from pressure, temperature, viscosity/competence contrast, layer thickness and layer spacing.
Lecture 8: Fold interference and facing.
Geometrical classification of 3-D fold interference patterns arising from the superimposition of shear folds on pre-existing fold forms. Recognition and analysis of related outcrop and map patterns. The use of cleavage/bedding, fold facing, vergence and facing on cleavage in the analysis of folded regions. Importance of way-up indicators. Impact of fold interference on fold facing and on parasitic folds. Use of facing directions, parasitic folds and folded lineations to unravel complex deformation.
Lecture 9: Progressive deformation 1 –Pure shear. .
The 2D finite and infinitesimal strain ellipses and their relationships in progressive pure shear.
Lecture 10: Progressive deformation 2 –Pure shear.
Implications of progressive pure shear for the nature, distribution and evolution of ductile structures, including non-cylindrical folds, cleavage/foliations, lineations, crenulation cleavage and boudins.
Lecture 11: Progressive deformation 3 – Simple shear.
The 2D finite and infinitesimal strain ellipses and their relationships in progressive simple shear. Implications of progressive simple shear for the nature, distribution and evolution of structures formed in ductile and brittle-ductile shear zones.
Lecture 12: Kinematic indicators in ductile shear zones.
Development of shear sense indicators, such as fabric rotation, S/C fabrics , sheath folds, porphyroblast rotation, porphyroclast tails, and sheared porphyroclasts, and their relationships to progressive simple shear.
Lecture 13: Geological analysis of a ductile shear zone.
The case study of the Needle Falls shear zone, Canada, illustrating typical structural observations of ductile shear sense, together with and kinematically-consistent late-stage brittle shear overprint.
Lecture 14: Transpression.
Review of principal structures formed in transpression and transtension, e.g. positive and negative flower structures. Overview of Sanderson and Marchini’s (1984) kinematic model for transpression.
Lecture 15: The fracturing of rocks.
A closer look at brittle rock failure. Principal, normal and shear stresses, and their relationships to rock fracture and the strain ellipse. The Mohr diagram for stress and the derivation of failure envelopes. Coulomb Failure criterion, Byerlee’s law, Griffith failure criterion. Link to von Mises’s ductile failure criterion.
Lecture 16: Geometry and growth of thrust fault systems.
Scaling properties and mechanical development of thrusts. Internal geometry of thrust sheets. Review of ramp-flat geometries, duplexes, antiformal stacks and imbricate systems. Side wall ramps and tear faults.
Lecture 17: Geometry and growth of normal fault systems.
Geometric and kinematic coherence of normal fault systems. Scaling properties and growth models of normal faults. Kinematic analysis of faults by using displacement back-stripping methods.
Lecture 18: Fault rocks and fault zones.
Geometry of fault zones and nature of kinematic indicators. Nature and origin of fault rocks. Geometric models for fault zone development. Effects of fault zone structure and fault rock properties on fluid flow.
Lecture 19: Joints, veins and hydrofractures.
Mechanics of the formation of joints, veins and their relationships to folding and regional uplift. Controls on joint spacing in layered rocks. Use of veins in kinematic analysis. Characteristics and mechanical principals of natural or man-made hydraulic fracturing.
Lecture 20: Structures related to salt movement and igneous activity
Overview of forms and mechanisms of salt and magma intrusion. Ductile structures related to igneous emplacement in mid/lower crust. Brittle structures, such as sills, dykes, ring faults and cone sheets, related to high level emplacement. Influences of physical conditions, rock properties and stress regime. Structural effects of salt on extension/compression.

Student Effort Hours: 
Student Effort Type Hours
Autonomous Student Learning

52

Lectures

24

Practical

24

Total

100

Approaches to Teaching and Learning:
Teaching and learning on this module comprises a set of practical exercises and activities that encompass: active/task-based learning; peer and group work; enquiry & problem-based learning; case-based learning; and student presentations. 
Requirements, Exclusions and Recommendations

Not applicable to this module.


Module Requisites and Incompatibles
Pre-requisite:
GEOL20190 - Sedimentology & Palaeobiology, GEOL20210 - Field Geology and Mapwork, GEOL20250 - Crystals to Sedimentary Rocks

Incompatibles:
GEOL30100 - Structural & Petroleum Geology


 
Assessment Strategy  
Description Timing Open Book Exam Component Scale Must Pass Component % of Final Grade In Module Component Repeat Offered
Continuous Assessment: Assessment of responses to task/questions posed in practical classes. Responses to be recorded as discursive text, annotations, diagrams, etc. in practical notebook. Throughout the Trimester n/a Standard conversion grade scale 40% No

45

No
Examination: Essay-style responses to three questions about theoretical topics covered on the course. Unspecified No Standard conversion grade scale 40% No

35

No
Practical Examination: Test of ability to recognise and analyse geological structures. Unspecified n/a Standard conversion grade scale 40% No

20

No

Carry forward of passed components
Yes
 
Resit In Terminal Exam
Spring Yes - 2 Hour
Please see Student Jargon Buster for more information about remediation types and timing. 
Feedback Strategy/Strategies

• Feedback individually to students, on an activity or draft prior to summative assessment
• Feedback individually to students, post-assessment
• Group/class feedback, post-assessment
• Peer review activities

How will my Feedback be Delivered?

Feedback will be provided on a weekly basis and will take the form of: (1) Oral feedback on activities prior to assessment (2) Oral and written post-assessment feedback individually to students and to the class as a whole; (3) Self-assessment and peer-review activities related to projects and presentations.

Name Role
Dr Conrad Childs Lecturer / Co-Lecturer
Professor John Walsh Lecturer / Co-Lecturer