Fracturing and the emplacement of minor intrusions Flashcards
Define a fracture
A surface across which there is a discontinuit in displacement and mechanical properties
What are the four classifications of fractures and what are they based on?
Based on displacement across the surface during their formation
Mode I: Extensional
Mode II,III: Shear
Mode IV: Contractional
Mixed mode: hybrid
Describe extensional fractures (mode I)
Displacement is normal to the fracture walls in an opening sense
Describe shear fractures (mode II,III)
Displacement is parallel to the fracture walls
Two modes (II is sliding, III is tearing) distinguis two orthongonal slip directions
Describe contraction fractures (mode IV)
Displacement is normal to the fracture walls in a closing sense (anticracks)
What are the four types of extensional fractures?
Fissures (filled with air/fluid), veins (filled with minerals, dykes (filled with magmas), joints (very small opening displacement and no observable displacement)
What can be a character of a hydrid fracture?
Growth of mineral fibres and small fracture-parallel offsets (in thin section)
Give a characteristic of contraction fractures
Stylolites (normal to the surface), slickolites (oblique to the surface)
In which direction do extension fractures open?
Parallel to the direction of minimum compressive stress (σ3)
They propogate within the σ1 - σ2
In which direction do contraction fractures form?
Perpendicular to the direction of maximum compressive stress (σ1)
At which angle do shear fractures form?
At an angle of less than 45deg to σ1 and ideally contain σ2
What is the result of small shear displacements?
En echelon arrays of tension gashes and pinnate fractures, they are often in conjugate pairs at 40-60deg to eachother
What is a failure envelop on a Mohr diagram?
Plotting normal stress (σn) and shear stress (τ) on the fracture plane at failure for samples deformed at different confining pressures
Explain the difference between extension fractures and dykes/veins
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)
When does increasing pore fluid pressure lead to a shear fracture?
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
Give five processes that increase pore fluid pressure relative to the confinding pressure and can induce fracturing
Compaction and cementation, erosion, osmosis, production of fluid by diagenetic/metamorphic reactions, differences in thermal expansion
What surface feature can indicate fracture propagation direction of extensional fractures
Hackles (ridges and grooves) diverging from a central
Describe a hackle fringe
En echelon arrays at the edge of the fracture face that slightly twist from the main face
Found on extensional fractures
Where are rib amrks and ripple marks found?
Crossing the lines of the hackle on extensional fractures
Describe rib marks
Smoothly curved ramps that are perpendicuar to the hackle lines
Arrest lines formed when fracture proagation temporarily stopped
Describe ripple marks
Rounded and oblique to the hacckle lines
Form during very fast fracture propagation
What does the analysis of hackle and rib marks indicate?
That fractures are approximately elliptical
What is the difference between the propagation of extensional fractures and shear fractures?
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
What type of smalle fractures can be seen at the end of shear fractures?
Splaying, horsetailing, antithetic shear fractures
Describe what happens to the stress field around fractures
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)
How are fracture spacing and intensity estimated?
Counting number of fractures intersecting a line
Topology of fracture can be characterised when number is combined with counts of fracture tips and intersections
Describe fracture spacing
Changes with bed thickness and integrity, influenced the the nautre of the interfaces between beds (changes during diagenesis)
Define fracture set
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
What type of rock would you expect to find near vertical fracture sets in?
Horizontally bedded sedimentary successions that have been subjected to little or no deformation
Name another feature associated with faults
Closely spaced set of fault-parallel fractures and a more weakly developed set of shear fractures in the conjugate orientation (~60deg to the fault)
Give the three orientations that fractures develop in in association with folds
Longitudinal (bc), cross (ac), two conjugate sets of oblique joints
Describe longitudinal (bc) joints
Parallel to fold axis and orthogonal to bedding
Describe cross (ac) joints
Parallel to the profile plane
Describe two conjugate sets of oblique joints
One conjugate pair bisected by bc joints (found in hinges) an the other bisected by ac joints (found in fold limbs)
Describe exfoliation joints
Extension fractures that are smoothly curved over 100s metres and sub-parallel to topography
What does a fracture surface with a thin mineral deposit show?
Fluid was important during the formation (assuming they didn’t come later and use the fracture as a conduit)
When do veins form?
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
When can tectonic brecciation occur?
During hydraulic fracturing in areas of very high fluid pressure
How can magma intrude as planar sheets?
Flowing into an open fissure or being forcefully injected
Why do planar sheets of magma imply hydraulic fracturing?
Open fissures cannot exist at depth, therefore there was likely an element of forceful injection
Define dykes
Sub-vertical sheets
Define sills
Sub-horizontal sheets
What force drives hydraulic fracturing during the emplacment of intrusions?
The buoyancy force of the magma
Define buoyancy of magma
Magma pressure - vertical stress in the surrounding rock
Arises from the lower density of the magma compared with its host rock
What does the intrusion of a vertical dyke require?
The buoyancy of the magma to be greater than the tensile strength of the rock in the horizontal direction (Th)
What does the intrusion of a horizontal dyke require?
The buoyancy of the magma too be greater than the tensile strenfth of the rock in the vertical direction (Tv)
What can happen when a propagating hydraulic fracture meets a layer with local stresses that are unfavourable for continued propagation?
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
Describe delamination of hydraulic fractures
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
Describe material property mismatch of hydraulic fractures
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
What leads to the formation of en echelon dyke arrays?
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
What happens when en echelon fractures enter another fracture’s stress field?
Curve
The nature of the curve is senstive to whether the fracture propagation is controlled by the remote stresses or magma pressure
Describe the formation of orthogonal and polygonal fractures
Contraction due to cooling or to dessication
Describe the evolution of polygonal fractures
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
Describe columnar jointing
Extensional fractures that form perpendicular to surfaces of equal temperature (and often perpendicular to the interface with the country rock)
What leads to the polygonal fracture patterns of columar joints
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
