Fracturing and the emplacement of minor intrusions

Question 1

Define a fracture

Answer 1

A surface across which there is a discontinuit in displacement and mechanical properties

Question 2

What are the four classifications of fractures and what are they based on?

Answer 2

Based on displacement across the surface during their formation
Mode I: Extensional
Mode II,III: Shear
Mode IV: Contractional
Mixed mode: hybrid

Question 3

Describe extensional fractures (mode I)

Answer 3

Displacement is normal to the fracture walls in an opening sense

Question 4

Describe shear fractures (mode II,III)

Answer 4

Displacement is parallel to the fracture walls
Two modes (II is sliding, III is tearing) distinguis two orthongonal slip directions

Question 5

Describe contraction fractures (mode IV)

Answer 5

Displacement is normal to the fracture walls in a closing sense (anticracks)

Question 6

What are the four types of extensional fractures?

Answer 6

Fissures (filled with air/fluid), veins (filled with minerals, dykes (filled with magmas), joints (very small opening displacement and no observable displacement)

Question 7

What can be a character of a hydrid fracture?

Answer 7

Growth of mineral fibres and small fracture-parallel offsets (in thin section)

Question 8

Give a characteristic of contraction fractures

Answer 8

Stylolites (normal to the surface), slickolites (oblique to the surface)

Question 9

In which direction do extension fractures open?

Answer 9

Parallel to the direction of minimum compressive stress (σ3)
They propogate within the σ1 - σ2

Question 10

In which direction do contraction fractures form?

Answer 10

Perpendicular to the direction of maximum compressive stress (σ1)

Question 11

At which angle do shear fractures form?

Answer 11

At an angle of less than 45deg to σ1 and ideally contain σ2

Question 12

What is the result of small shear displacements?

Answer 12

En echelon arrays of tension gashes and pinnate fractures, they are often in conjugate pairs at 40-60deg to eachother

Question 13

What is a failure envelop on a Mohr diagram?

Answer 13

Plotting normal stress (σn) and shear stress (τ) on the fracture plane at failure for samples deformed at different confining pressures

Question 14

Explain the difference between extension fractures and dykes/veins

Answer 14

Extension fractures are restricted to shallow depths (small confining pressure), dykes and veins can form at greater depths (into the mantle)
Caused by pore pressure (acts against normal stresses, does not affect shear stress)

Question 15

When does increasing pore fluid pressure lead to a shear fracture?

Answer 15

If the failure envelope is touched while the effective σ3 (=σ3-ρfluid) is still compressive or to hybrid/extensional if ti is touched when the effective σ3 is negative

Question 16

Give five processes that increase pore fluid pressure relative to the confinding pressure and can induce fracturing

Answer 16

Compaction and cementation, erosion, osmosis, production of fluid by diagenetic/metamorphic reactions, differences in thermal expansion

Question 17

What surface feature can indicate fracture propagation direction of extensional fractures

Answer 17

Hackles (ridges and grooves) diverging from a central

Question 18

Describe a hackle fringe

Answer 18

En echelon arrays at the edge of the fracture face that slightly twist from the main face
Found on extensional fractures

Question 19

Where are rib amrks and ripple marks found?

Answer 19

Crossing the lines of the hackle on extensional fractures

Question 20

Describe rib marks

Answer 20

Smoothly curved ramps that are perpendicuar to the hackle lines
Arrest lines formed when fracture proagation temporarily stopped

Question 21

Describe ripple marks

Answer 21

Rounded and oblique to the hacckle lines
Form during very fast fracture propagation

Question 22

What does the analysis of hackle and rib marks indicate?

Answer 22

That fractures are approximately elliptical

Question 23

What is the difference between the propagation of extensional fractures and shear fractures?

Answer 23

Extensional fractures propagate in their own plane and shear fractures cannot, the propagate either by generating new extension cracks at their tips or linking up already existing extensional cracks

Question 24

What type of smalle fractures can be seen at the end of shear fractures?

Answer 24

Splaying, horsetailing, antithetic shear fractures

Question 25

Describe what happens to the stress field around fractures

Answer 25

The stress field is perturbed around fractures, new fractures growing towards it encounter to local perturbation and curves to maintain a 90deg angle to the local σ3 Sense and magnitude of curvature depends on rel. magnitude and orientation of principal stresses (can be used for information)

Question 26

How are fracture spacing and intensity estimated?

Answer 26

Counting number of fractures intersecting a line Topology of fracture can be characterised when number is combined with counts of fracture tips and intersections

Question 27

Describe fracture spacing

Answer 27

Changes with bed thickness and integrity, influenced the the nautre of the interfaces between beds (changes during diagenesis)

Question 28

Define fracture set

Answer 28

Group of fractures of similar orientation (strike/dip) Near vertical sets are shown on a rose diagram and non-vertical sets are shown on a stereogram Separate by lithology before plotting as it may affect their orientation

Question 29

What type of rock would you expect to find near vertical fracture sets in?

Answer 29

Horizontally bedded sedimentary successions that have been subjected to little or no deformation

Question 30

Name another feature associated with faults

Answer 30

Closely spaced set of fault-parallel fractures and a more weakly developed set of shear fractures in the conjugate orientation (~60deg to the fault)

Question 31

Give the three orientations that fractures develop in in association with folds

Answer 31

Longitudinal (bc), cross (ac), two conjugate sets of oblique joints

Question 32

Describe longitudinal (bc) joints

Answer 32

Parallel to fold axis and orthogonal to bedding

Question 33

Describe cross (ac) joints

Answer 33

Parallel to the profile plane

Question 34

Describe two conjugate sets of oblique joints

Answer 34

One conjugate pair bisected by bc joints (found in hinges) an the other bisected by ac joints (found in fold limbs)

Question 35

Describe exfoliation joints

Answer 35

Extension fractures that are smoothly curved over 100s metres and sub-parallel to topography

Question 36

What does a fracture surface with a thin mineral deposit show?

Answer 36

Fluid was important during the formation (assuming they didn't come later and use the fracture as a conduit)

Question 37

When do veins form?

Answer 37

When fluid pressure builds up enough to generate extension fractures, this is hydraulic fracturing, minerals then precipitate out of solution Solubility decreases as fluid pressure decreases when fracture opens

Question 38

When can tectonic brecciation occur?

Answer 38

During hydraulic fracturing in areas of very high fluid pressure

Question 39

How can magma intrude as planar sheets?

Answer 39

Flowing into an open fissure or being forcefully injected

Question 40

Why do planar sheets of magma imply hydraulic fracturing?

Answer 40

Open fissures cannot exist at depth, therefore there was likely an element of forceful injection

Question 41

Define dykes

Answer 41

Sub-vertical sheets

Question 42

Define sills

Answer 42

Sub-horizontal sheets

Question 43

What force drives hydraulic fracturing during the emplacment of intrusions?

Answer 43

The buoyancy force of the magma

Question 44

Define buoyancy of magma

Answer 44

Magma pressure - vertical stress in the surrounding rock Arises from the lower density of the magma compared with its host rock

Question 45

What does the intrusion of a vertical dyke require?

Answer 45

The buoyancy of the magma to be greater than the tensile strength of the rock in the horizontal direction (Th)

Question 46

What does the intrusion of a horizontal dyke require?

Answer 46

The buoyancy of the magma too be greater than the tensile strenfth of the rock in the vertical direction (Tv)

Question 47

What can happen when a propagating hydraulic fracture meets a layer with local stresses that are unfavourable for continued propagation?

Answer 47

Arrest, penetrate the contact, or defect along the contact in one or two directions These responses change reflect the change in local orientation of principal stresses

Question 48

Describe delamination of hydraulic fractures

Answer 48

The tensile stresses generated normal to the tip of a propagating hydraulic fracture induces tensil stresses parallel to the fracture If tensile strength is small in this direction, a crack will open up and the intrusion will propagate sideways

Question 49

Describe material property mismatch of hydraulic fractures

Answer 49

When a propagating fracture meets an interface with a layer that is elastically stiffer than the one it has been propagating through then it is energetically favourable for the crack to deflect along the interface

Question 50

What leads to the formation of en echelon dyke arrays?

Answer 50

The rotation of the stress field as the fracture propagates towards to surface (same way that forms hackle fringes) Difficult to distinguish from genuine transtension

Question 51

What happens when en echelon fractures enter another fracture's stress field?

Answer 51

Curve The nature of the curve is senstive to whether the fracture propagation is controlled by the remote stresses or magma pressure

Question 52

Describe the formation of orthogonal and polygonal fractures

Answer 52

Contraction due to cooling or to dessication

Question 53

Describe the evolution of polygonal fractures

Answer 53

Cracks nucleate at flaws, propagate towards earlier formed cracks, curve to meet them orthgonally In opening/closign cycles, previously opened cracks define lines of weakness that lead to the readjustment of the network to a polygonal shape

Question 54

Describe columnar jointing

Answer 54

Extensional fractures that form perpendicular to surfaces of equal temperature (and often perpendicular to the interface with the country rock)

Question 55

What leads to the polygonal fracture patterns of columar joints

Answer 55

In homogeneous media containing few flaws, rapid nucleation and propatation of multiple cracks from a few points of nucleation, polygonal fracture patterns are formed This geometry releases maximum strain energy per unit crack area