Extensional faulting

Geometrical Problem

Steep normal faults have limited capacity to accomodate horizontal extension

∴ significant vertical displacement instigates limited horizontal displacement

Thus limits total extension possible

Anderson's
Law

  1. Earth surface is free of shear stress

No surface on top

No drag exerted by atmosphere

∴ Plots on shear stress axis in Mohr space & only conveys normal stress

  1. One of principal stresses always ⊥ Earth's surface

= consequence of inability to convey shear stress

High angle normal fault

∴ Ideal shear fracture orientation = 30° off vertical

Extensional settings, σ1= vertical
i.e. perpendicular to Earth surface

Hence high fault angle results

At depth, steep normal faults...?

High angle normal faults

--> Rotate & extend as low angle normal faults

Detachment fault = low angle normal faults

∴ Any slip @ depth extends extension

Detachment faults

i.e. other half missing

~ decollements / low angle normal faults

in GBRP - Rotated from initially steep angles

Initiate at depth

Cannot be explained with Anderson's Fault mechanics

Possible causes low angle normal faults

  1. Lowered fluid pressure
  1. Mineral/rock weaknesses
  1. Reactivation fault surfaces
  1. Pre-existing anisotropy (bedding/weak layer/foliation)

B&R Faults

GBR = underlain by high angle normal faults

which join low angle normal faults @ depth

most extension accommodated by low angle detachments

Detachment fault questions

  1. Do they move in brittle regime?
  1. Do they form with shallow dips or rotate to attain shallow dip?
  1. Why are they absent from seismic record?

Detachments
= Anomalous

σ1 should be vertical in extensional settings

∴ Faults should dip ~65-70° where Byerlee's law applies

Detachments do not adhere to this condition

How do shallow detachments exist?

1. Fault rocks = weaker than surrounding rock ?

i.e. μ < 0.6

Faults can slip @ smaller dip angles

However there are limits...dictated by horizontal effective stress

2. Horizontal effective stress determines faulting response?

Minimum fault dip defined by (σ1-p) / (σ3-p)

where p = pore fluid pressure

Extensional regions

σ1 determined by overburden thus is fixed

∴ (σ 3 - p) defines the response

(σ 3 - p) defines minimum fault dip for a given μ

Faults only slip where
(σ 3 - p) > tensile strength of overlying rock

Otherwise hydrofracture forms (water in system since Pf)

∴ p is such that

Not so much that it exceeds tensile strength (doesn't move left of origin)

Sufficient p to push Mohr circle to cross failure envelope

Problem so far

Shown slip = possible along low angle faults

Does not explain

whether detachment faults form at these angles

why they show no seismicity