Term 2 PPQ knowledge Flashcards
Extension processes in continental collision
- Orogenic wedge destabilisation
- Gravitational orogenic collapse
- Channel flow
- Orogenic root collapse
- Post-orogenic basal thrust reversal
Orogenic wedge destabilisation
Wedge thickens excessively
Becomes unstable
In response normal faults and shear zones form
Gravitational orogenic collapse
Crustal material is subducted and heated
Weakens the crust
Collapses under its own weight along extensional faults
Channel flow
Detached and heated basement slice can ascend with an increased buoyancy
Has a thrust fault on its lower side and a normal fault on its upper side
Extrusion of hot basement material in the hinterland
Basal thrust reversal
After orogeny, when thrust towards foreland has ceased
Basal thrust is reversed as strain directions change
Starts prograding towards hinterland
Forms metamorphic core complexes
How is it known that earthquakes associated with geological faults are closely associated with major episodes of fluid flow in the crust?
- It is well known that major hydrological changes follow modern earthquakes
- Associated with a whole range of large-scale geological phenomena such as:
- Liquefaction
- Formation of new springs
- Increased stream discharge
- Change in groundwater levels
- And we know that mineralization is widely associated with ancient fault zones
- Earthquake sequences are cyclic – stick-slip behaviour
- Therefore likely that pore fluid pressure and fluid flow events will also be cyclic – and correlated in time/space
Principle mechanisms proposed linking fluid flow to episodic fault movement
Fluid valve model
Fault pump model
The control exerted by tectonic regime in fluid flow in fault movement
Load strengthening vs load weakening faults
Fluid valve model
- Periodic build-ups in fluid pressure trigger earthquakes
- Requires
- High pfp gradients (>10MPa/km)
- Focussed fluid source
- Local or regional impermeable barrier
- Once breached, fault must be an effective fluid channelway
- Fall in pfp leads to resealing
Fault pump model
- Dilatancy associated with brittle fracture prior to, during or following an earthquake leads to fluids being drawn into and expelled from fracture systems: suction pumping
- Only likely to be a significant near to the surface (0-2km depth) where regional fluid pressures are close to hydrostatic & there is known to be significant dilatancy associated with fracture systems
Load strengthening vs load weakening faults
- The amount of associated dilatant fracturing during interseismic & co-seismic deformation will differ
- ‘Long suck, short blow’ (normal faults) vs ‘long blow vs short suck’ (reverse faults)
- Strike-slip faults can show both types of behaviour
- Most likely significant in near surface where there will be much more dilatancy, especially in crystalline rocks
Give THREE different mechanisms by which magma may be emplaced in the Earth’s crust, and provide an example of each
Dykes
• Fracturing – more likely to be the principal mechanism
– Hydraulic fractures in source region can become conduits
– E.g. dykes
Batholith
• during melting the source region becomes porous and dilates
• Fluid absent melting can cause up to 15% volume increase that produces a hydraulic overpressure in the source
• this is relaxed in the form of doming of the source and ultimately uplift of the Earth’s surface
• the doming sets up a radial stress field with vertical tensile cracks/dykes/conduits forming
• Buoyancy forces drive magma up the conduit
Sills
• Initial dyke feeds a sill thus releasing EMV (excess magma volume) in source
• However, once the initial sill forms the EMV is relaxed and the system would quickly stop however….
• The sill now supports its roof but is the floor is decoupled and then will subside back into its source
• This will expel magma from its source, increase sill size, dilate conduit, provide more material for melting – a feedback loop is set up.
Critical wedge basics
Critically tapered wedges: the idea that fold and thrust belts deform to keep the critical angle, a + b, constant
- The “toe” of the fold-and-thrust belt maintains a critical angle.
- The angle of critical taper is a function of the rock properties in the deforming wedge, pore fluid pressure and the strength of the basal detachment thrust (“decollement” horizon).
- Important parameters are the shear stress along the basal thrust and the gravitational force related to the height of the wedge – these govern angles, a + b
Shape of critical wedge
The shape of the wedge is controlled by basal friction, the strength of the wedge material and erosion.
• Basal friction – low friction = low angle long wedge
• Erosion – erosion and deposition at the surface where material is removed added or redistributed will lower the surface slope and make the wedge unstable. The result is that material in the wedge rises vertically by internal redistribution of rocks and sediment; this may mean reverse faulting or folding so equilibrium is achieved and the surface slope stabilises. During this process rocks move vertically so that metamorphic rocks are brought closer to the surface
Variation and examples of critical wedge
Angle a may vary across a wedge, and over time, leading to different behaviour, including thrusting and extensional faulting at the same time
Here a basement slice is ripped off and incorporated into the wedge which locally thickens. This thickening creates a slope of instability that is compensated for by means of extensional deformation.
- The Himalayas may fit the critical taper model. Not only do cross-sections based on fieldwork and seismic sections suggest a northward-thickening wedge, but the earthquakes are thrusts, dipping gently north.
- But the basal thrust in the wedge must lie within the basement, given the earthquake depths and basement thrust sheets.
