Week 10 Flashcards by Elle Kinsman

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

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σ3’

value of 2θ

= -ve = tensile stress

= σ3 - PFP

= -T

2θ = 90’

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PFP required for tensile failure =

σ3 + T

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Paper experiment results

Flaws cause stress concentrations

greatest when flaw perpendicular to applied tensile stress
i.e. 45’ to slit

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σr =

applied remote tensile stress

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σt =

crack tip stress

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Influence of crack shape on σr and σt

Circular; σt = 3σr

Elliptical; 3:1 ; σt = 7σr

Realistic; 100:1 ; σt > 200σr

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Stress intensity factor Nic =

fracture toughness/critical stress intensity factor

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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)

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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

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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

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Microcracks in elliptical process zones…

Leave behind damage zone surrounding fracture

Associated with widespread dilatancy

= fracturing/fluid flow processes interactions (+IMPLICATIONS)

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Using Griffith’s Crack Theory as a failure criterion

Not a straight line

S = 2T
-t

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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

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Strain partitioning

Hybrid between shear and tensile

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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