Week 10 Flashcards
In order to initiate a tensile fracture, the PFP needs to overcome…
1) ROCK TENSILE STRENGTH
- varies for different minerals (due to microdefects/flaws)
2) σ3
σ3’
value of 2θ
= -ve = tensile stress
= σ3 - PFP
= -T
2θ = 90’
PFP required for tensile failure =
σ3 + T
Paper experiment results
Flaws cause stress concentrations
- greatest when flaw perpendicular to applied tensile stress
- i.e. 45’ to slit
σr =
applied remote tensile stress
σt =
crack tip stress
Influence of crack shape on σr and σt
Circular; σt = 3σr
Elliptical; 3:1 ; σt = 7σr
Realistic; 100:1 ; σt > 200σr
Stress intensity factor Nic =
fracture toughness/critical stress intensity factor
Griffith’s Crack Theory
Influence of crack orientation relative to principle stresses (θ)
Cracks are elliptical in cross section (a»c) and generate large tensile stress in crack tip (σt)
Griffith’s Crack Theory; in a tensile stress field
Open cracks
θ = 90
σt // to σ3
σt = σtmax
Unstable tensile (mode I) fractures propagate rapidly
I.E. MATERIALS ARE WEAK IN TENSION
Griffith’s Crack Theory; in a compressional stress field
Closed cracks
Cracks experience shear stress
Tensile ‘wing cracks’ when θ>45’
Propagate slowly due to compressional σ3’
I.E. MATERIALS ARE STRONGER IN COMPRESSION
Link and grow = through-going shear fractures i.e. FAULTS
Microcracks in elliptical process zones…
Leave behind damage zone surrounding fracture
Associated with widespread dilatancy
= fracturing/fluid flow processes interactions (+IMPLICATIONS)
Using Griffith’s Crack Theory as a failure criterion
Not a straight line
S = 2T
-t
Griffith’s vs Coulomb-Naiver
GRIFFITHS
- good for low and -ve σn’
- slope too shallow in compression
COULOMB-NAVIER
- good for shear failure
= can form a COMPOSITE ENVELOPE
Strain partitioning
Hybrid between shear and tensile
Shear failure (dry/drained rocks) values for:
- σ3-σ1
- σ3’
- θ
- > 8T
- > 0
- 60’
Shear hydraulic failure values for:
- σ3-σ1
- σ3’
- θ
- 8T
- > =0
- 60’
Hybrid hydraulic failure values for:
- σ3-σ1
- σ3’
- θ
- 4T < X < 8T
- -T < X < 0
- 60-90’
Tensile hydraulic failure values for:
- σ3-σ1
- σ3’
- θ
- =<4T
- -T
- 90’
What happens to θ as you +fluid and +open?
Gets higher
Fault refraction varies due to…
Different rock types
e.g. more steep in competent carbonate and sandstone than weak shales
(preferential mineralisation)
Fluid transport properties vary in…
TIME and SPACE
Wing cracks
Focus fluid flow and mineralisation
Dilation logs
Form when pre-existing structures can be re-activated by normal faults
Termination and interaction features
Splays
Horsetail splays
Antithetic splays
Jogs and relays
What kind of major hydrological changes follow modern earthquakes?
Liquefaction
Formation of new springs
Increased stream discharge
Change in groundwater levels
N.B. Mineralisation is widely associated with ancient fault zones
Is fluid flow driven by active faulting or are faults driven by fluid pressures?
Or both?
Earthquakes and cyclicity
Stick-slip behaviour = cyclic
Therefore PFP and fluid flow events also likely to be cyclic and correlated in time/space
Fault valve model; how to periodic build ups in pressure trigger earthquakes?
REQUIRES:
- high PFP gradients (>10MPa/km)
- focussed fluid source
- local/regional impermeable barrier
- once breached fault must be an effective fluid channelway
Fall in PFP leads to resealing
Example of fault valve seismicity
1997 Colfiorito (Umbria Marche) earthquake sequence, Apennines
Normal fault sequence lasting 30 days after two Mw5-6 earthquakes (several small and also thousands of aftershocks)
Regions on high CO2 flux
Deep evaporite seal
Modlled rupture event and fluid migration = matched to aftershock sequence observed
Complex evolution with multiple over pressuring events
Fault pump model
Brittle fracture = dilatancy = suction pumping
Only likely significant near surface (0-2km) where regional fluid pressures close to hydrostatic and known to be significant dilatancy associated with fracture systems
Importance of tectonic regime during seismogenic loading cycles
REVERSE FAULTING
- mean normal stress σm gets bigger
- influences evolution of τf = stronger?
NORMAL FAULTING
- mean normal stress σm gets smaller
- influences evolution of τf = weaker?
Load strengthening vs load weakening faults; influence on fluid transport
Normal faults = long suck, short blow
Reverse faults = long blow, short suck
(Strike-slip can show both)
Most likely significant in near surface where much more dilatancy, especially in crystalline rocks
Near surface normal faulting (0-2km depth)
Especially if competent host rocks e.g. crystalline (basement/granite/basalt/limestone)
e.g. Iceland/N Africa
In ancient settings e.g. Devon = common below unconformities, often filled with sedimentary material +/ hydrothermal minerals
How do fissures develop?
Decreasing differential stress with depth and development of near surface tensile stresses
Preservation favoured in strong host rocks - often see collapse features/infills e.g. Zechstein, NE England
Value of θ for tensile failure?
90’
i.e. on Mohr diagram 2θ = 180’
Why are features like dykes and joints geometrically quite simple?
Rapid propagation
Long and planar mode I fractures
