Term 2: Continental Extensional systems

Question 1

A single normal fault

Answer 1

• Detailed observations in coalfields show single normal faults which are ‘blind’ with ‘tip lines’
• In order to maintain compatibility - strata bend up in FW (drag) down in HW (rollover)
• Flexural isostatic effects uplift the footwall
• Sediment source
• Topographic high
• Degree of uplift depends on type of rock
• Examples
• Basin and Range
• Baikal Rift, Siberia
• The degree of footwall topography depends strongly on the rock type involved.
• e.g. limestones = high scarps, schists or unconsolidated sediments = low scarps

Question 2

Normal faults in 3D

Answer 2

• Like thrust faults - normal faults are not continuous along strike and display ‘hard’ and ‘soft’ linkage –
• Hard linkage implies one or more minor faults linking the main segments
• Soft linkage involves ‘relay ramp’ structures

Question 3

Slip rates on faults:

Answer 3

To work out throw (slip) rate – divide throw (15 m) by time period 15 ka (15/15,000 yrs = a throw rate of 1 mm per year)

Question 4

Non - rotational planar faults

Answer 4

• ‘Classic’ Andersonian view of normal faults
• Forms horst and graben structures,
• e.g. Rhine graben

Question 5

Problems with classical Andersonian normal faults

Answer 5

• Large scale crustal extension cannot be accommodated due to compatibility problems
• Conjugate faults must move sequentially - not energetically efficient for large faulting (works for small faulting)
• Central graben ‘falls in’ under large extension
• No rotation of bedding allowed - but it clearly does happen

Question 6

Rotational – planar faults

Answer 6

Domino’ fault block model - Domino or bookshelf normal faulting
• Think about the base of each fault: space problem?
• This seems to be the main style of extension in the continental crust
• Large extension possible (e.g. 200%); commonly asymmetric half graben
• Large strain

Question 7

Detachments

Answer 7

• Extensional detachments are rotated high angle faults that have stopped moving
• Basin & Range is classic detachment country
• but active normal faults are steep

Some detachment faults could be the result of exhuming low angle ductile shear zones at the base of the seismogenic layer?

Question 8

Lister and Davies model:

Answer 8

• The idea is that detachments originate as low-angle ductile shear zones, with aseismic movement.
• They are then exhumed & reactivated in a brittle manner.
• The important property of these structures is that deeper levels are exhumed – metamorphic rocks at depth are brought to surface and juxtaposed with unmetamorphosed sediment.
• These structures are called ‘Metamorphic core complexes

Question 9

Metamorphic core complexes

Answer 9

• Mid-crustal metamorphic rocks supposedly exhumed by low angle, extensional
detachment faults”, underneath unmetamorphosed upper crustal rocks.
• Problem: scarcity of neotectonic (active) examples, especially evidence for low angle seismogenic normal faults.
• Alternative model - Detachments formed when lower crust flows?

Question 10

Rotational - non-planar faults

Answer 10

Stair-step - ramps and flats
or
Listric - smooth concave - up form to maintain compatibility the hangingwall must undergo gravity driven deformation - generates roll-over structures (passive folds)

Question 11

Gravity driven deformation zones

Answer 11

• Listric faults are thought to form in gravity driven deformation zones
• Either - Collapse of sediment in regions where sediment piles become unstable and collapse downhill, e.g., landslips - extension is balanced by compression at the toe
- and/or -
• Thick salt has acted as weak detachment horizon that facilitated collapse – Gulf of Mexico
• Passive continental margins provide both possibilities

Question 12

Extensional basin formation

Answer 12

• 2 end-member conceptual models
o ‘McKenzie’ pure shear model
o ‘Wernicke’ simple shear model

Question 13

McKenzie model (1978)

Answer 13

“pure shear”
• The deep crust thins by ductile deformation; the upper crust is extended by faults that deform the strong, seismogenic layer. This is the rift phase.
• A sedimentary basin fills up with ‘syn-rift’ sediments – this happens over a ‘short’ time scale, up to 20 Myrs
• Lithospheric mantle is thinned and replaced by hot asthenosphere mantle
• Following extension, the elevated part of the asthenosphere cools, becomes denser, and so subsides.
• It effectively becomes part of the mantle lithosphere. This subsidence takes place over 50-100 Myrs. This is post-rift subsidence.

Question 14

McKenzie and Beta Factor

Answer 14

• The model quantifies subsidence occurring due to crustal and lithospheric thinning.
• The beta (b) factor is the ratio of initial to final lithospheric thickness. So, if the lithosphere thins from 100 to 50 km, b = 2. If this happens, magmatism is generated.
• The second half of McKenzie’s model simulates the thermal subsidence phase of basin evolution, as asthenosphere cools and is added to the base of the lithosphere.

Calculaing Beta-Factor:

1. Sum of all heaves (horizontal offset)
2. Change in thickness

Question 15

Steers Head

Answer 15

Some, but not all, rift basins show a “Steer’s Head” geometry where the post-rift phase occurs over a wider area than the syn-rift phase.
This may be because mantle lithosphere stretches over a wider area than the crust.

Question 16

Wernicke model (1981)

Answer 16

• Based on a ‘simple shear’ regime where the crust is stretched asymmetrically by a large-scale detachment fault.
• The thermal subsidence region is offset from the area of rifting
• Low angle extensional “detachment” faults are a key element of this model – strong reliance on fieldwork data from the western USA.
• Metamorphic core complexes are supposedly exhumed by extension of this type.
• The Wernicke model originates in the Basin and Range, but is not applicable to the active normal faults in the area, which are relatively steeply-dipping.
• Core complexes and detachment faults now
• Proposed for sites on the ‘inside’ corners at mid ocean ridges.

Question 17

Fault-controlled sediments

Answer 17

• Extensional faulting controls
o Basin architecture
o Sediment distribution, facies types, etc.
• Faults active at same time as sedimentation are ‘growth faults’ - thickening of sequences into HW of faults
• Faulting affects drainage patterns through the evolution of fault-related topography
• All the elements necessary to produce and trap oil and gas can be contained within one half-graben: source, reservoir, seal and trap – and enough subsidence to mature the source.

Question 18

Elastic interaction between faults

Answer 18

• Assume random fault distribution at start of rifting
• Positive feedback during slip on en-echelon faults
• Gives rise to linkage & larger stress enhancement/ reduction
• Faults in stress shadows less likely to slip & eventually die

Question 19

Cowie’s model of strain localisation

Answer 19

• Numerical models suggest that strain localisation is driven by elastic interaction
• Key point: fault patterns observed in North Sea arise due to elastic interactions between seismogenic faults – no need to invoke large-scale ductile flow in upper mantle!
• Could ultimately lead to continental breakup?

Question 20

Inversion Tectonics

Answer 20

• Process whereby pre-existing normal faults are reactivated as a thrust fault by a later phase of shortening.
• Very common in extensional basins later affected by compressional deformation, e.g. the North Sea
• Inversion is very important for trapping hydrocarbons, e.g. in SE Asian Tertiary rifts.

Question 21

3 Types of normal fault

Answer 21

Non - rotational planar faults
‘Classic’ Andersonian view of normal faults
Forms horst and graben structures, e.g. Rhine graben
σ1 = max, σ2 - int, σ=med

Rotational - planar
Domino Model
This seems to be the main style of extension in the continental crust

Rotational - non-planar faults
Listric - Ramp shaped fault with ‘roll-over’ anticline