Structural Geology

Structural geology examines how

rocks deform and how structures such as faults, folds, fractures, joints, shear zones, foliations, and lineations form. It links small-scale mineral deformation to crustal- scale tectonic systems and helps reconstruct both the geometry and history of Earth deformation.

Scale ranges from a few km to micrometer

-Range in time scale -

Range in length -

10^13

10^10

Geometric Models analyze

size, shape, orientation, structural associations (first-order structures and related second-order structure)

Kinematic Models -

Dynamic Models -

Show direction of deformation with arrows

Show direction of deformation & stress with arrows

Structural geology works across scales

Thin section - microstructures, crystal fabrics

Outcrops - folds, faults, fractures

Map Scale - terrane patterns, lineaments

Crustal Scale - orogens, rifts, plate motion

Scale -

Micro - need magnification (foliation)

Meso - hand specimens, outcrops (foliation, folds, faults)

Macro - mountainside to map scale (basins, domes, core)

Field data - take _____ samples

oriented

Integrated structural science
The strongest interpretations combine independent lines of evidence

Goal: connect grain-scale deformation mechanisms to fault zones, mountain
belts, basins, and plate motion.

Fault mechanics, fluids, and induced seismicity
Fault behavior depends on stress, pore pressure, permeability, and chemistry

Fluids can reduce effective normal stress
• Faults may act as conduits or seals
• Injection/withdrawal can alter stress
conditions
• Critical for geothermal, wastewater, carbon
storage, and subsurface energy

Structural geology for the energy transition
Faults, folds, fractures, and shear zones influence where fluids and minerals move

Critical minerals

Geothermal energy

Carbon storage

Groundwater + engineering

Homogeneous and Heterogeneous

Homogeneous geology means that a rock body or subsurface layer has uniform physical, chemical, and structural properties throughout

Heterogeneous geology means its properties vary spatially across different locations

Continuous and Isotropic

A continuous medium assumes that the geological material completely fills the space it occupies, with no empty voids, major cracks, or distinct fractures

An isotropic material possesses the same physical properties in all directions at any given point.

READING THE CRUST

UPPER CRUST 0–10/15 km
Brittle faulting localizes strain; fault rocks evolve from incohesive
to cohesive cataclasites

MIDDLE CRUST transition zone
The brittle–plastic transition varies by mineral: quartz weakens
near 250–350°C, feldspar at higher T.

LOWER CRUST ~23 to ~30–40 km
Crystal-plastic flow produces distributed deformation, mylonitic
rocks, and a broad ductile shear zone

Brittle deformation mechanisms

Particulate or granular flow or frictional sliding

Cataclasis and cataclastic flow

Grains translate and rotate to accommodate frictional grain boundary slip.
• This process without the generation of fractures typically occurs in poorly
consolidated porous rocks and sediments (soil).

• Cataclasis includes fracture and crushing of
grains, frictional sliding and grain rotation

Intergranular & Intragranular fractures

Intragranular fractures in cataclastically
deformed porous sandstone

Intergranular fractures in
metamorphic rock

Three Types of fracture or cracks

What is a fracture?
• Any planar or subplanar discontinuity that is very narrow that form
from external (tectonic) or internal (residual or thermal) stress.

Shear, Joint, Fissure

Modes of Fracture - Know diagram (pg 26)

1. Mode I (Opening or extension)
2. Mode II (Sliding)
3. Mode III (Tearing)
4. Mode IV (Closing)

Stylolite or Anticrack

(contraction or closing fracture)

Fracture termination and interaction (KNOW DIAGRAM) PG. 28
- Fracture-tip patterns reveal fracture behavior

Wingcrack

Horsetailing

Splaying

Antithetic shear fractures

Deformation bands and fractures in porous rocks

• Deformation bands are restricted to highly porous granular media, notably porous sandstones.

• Deformation bands occur as single structures, as clusters, or in zones associated with slip surfaces (faulted deformation bands).

Types of deformation bands based on deformation mechanisms (KNOW DIAGRAM) pg 30

The most important mechanisms are:
1. Granular flow (grain boundary sliding and grain rotation)
2. Cataclasis (grain fracturing)-promotes strain hardening
3. Phyllosilicate smearing
4. Dissolution and cementation

What is the difference between joints, fissures and veins?

Joint: is a fracture with minute opening mode displacement and little or no
displacement along its walls.
Fissure: fissures have clearly visible displacement and are open (centimeter to
meter scale)
Veins: are mineral-filled fissures (so they also have clearly visible displacements)
where the mineral may have filled the fissure after or during opening

Joint set provide

permeability, good connectivity and fluid flow

How and why joints form in the Crust (KNOW DIAGRAM) pg 35

• Jointing in response to burial of sedimentary rock layers
• Jointing in response to tectonic stress
• Jointing during exhumation
• Jointing due to removal of overburden or unloading
• Jointing due to temperature changes
• Jointing due to overburden

Jointing in response to tectonic stress
in areas of contractional regime

The tectonic compression associated with
shortening elevated the over pressure in
the sedimentary rocks to the point where
joints formed. Such joints are called cross-
fold joints

Also, release joints can form during
unloading of foliated rocks, which are
subparallel to fold hinges

Joint Stress Shadow & Joint Distribution (KNOW DIAGRAM) pg38

(Stress shadow/drop model)

Joint spacing is regulated by
• Bed thickness
• Bed strength
• Stress shadow

Growth and Morphology of joints-Plumose joint DIAGRAM PG 40

Plumose joints indicates heterogeneity in the rock and propagation direction of joints

Vein is a

fracture filled with secondary
minerals deposited from a water-rich
fluid entering the fracture

A fault is any

surface or plane or narrow
zone with visible shear displacement.
• A fault is a discontinuity in layers with wall-
parallel displacement dominated by brittle
deformation mechanisms.

Normal

Strike Slip

Reverse faults

Horst

Graben

Rollover reverse drag

Fault displacement, slip and separation pg 50

• Heave
• Throw

Net slip (true
displacement)

Anatomy of faults - p$ssy model

• Fault core (mm
to several meters
wide)
• Fault damage
zone (restricted
to brittle
deformation)
• Drag zone (an
expression of
ductility in fault-
related strain)

Fault rocks - Pseudotachylite

Cataclasite produced
by crushing
limestone in the
footwall

Fault damage zone

contains small-scale structures like deformation bands,
shear fractures, tensile fractures, and stylolites.

Fault displacement - Curved graph -

Largest part is greatest displacement

soft link faults

hard link faults

connect

no connect