Term 2 PPQ knowledge

Question 1

Extension processes in continental collision

Answer 1

• Orogenic wedge destabilisation
• Gravitational orogenic collapse
• Channel flow
• Orogenic root collapse
• Post-orogenic basal thrust reversal

Question 2

Orogenic wedge destabilisation

Answer 2

Wedge thickens excessively
Becomes unstable
In response normal faults and shear zones form

Question 3

Gravitational orogenic collapse

Answer 3

Crustal material is subducted and heated
Weakens the crust
Collapses under its own weight along extensional faults

Question 4

Channel flow

Answer 4

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

Question 5

Basal thrust reversal

Answer 5

After orogeny, when thrust towards foreland has ceased
Basal thrust is reversed as strain directions change
Starts prograding towards hinterland
Forms metamorphic core complexes

Question 6

How is it known that earthquakes associated with geological faults are closely associated with major episodes of fluid flow in the crust?

Answer 6

• 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

Question 7

Principle mechanisms proposed linking fluid flow to episodic fault movement

Answer 7

Fluid valve model

Fault pump model

Question 8

The control exerted by tectonic regime in fluid flow in fault movement

Answer 8

Load strengthening vs load weakening faults

Question 9

Fluid valve model

Answer 9

• 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

Question 10

Fault pump model

Answer 10

• 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

Question 11

Load strengthening vs load weakening faults

Answer 11

• 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

Question 12

Give THREE different mechanisms by which magma may be emplaced in the Earth’s crust, and provide an example of each

Answer 12

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.

Question 13

Critical wedge basics

Answer 13

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

Question 14

Shape of critical wedge

Answer 14

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

Question 15

Variation and examples of critical wedge

Answer 15

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.

Question 16

Foreland

Answer 16

• Thin skinned deformation
• Occurs at the marginal regions of the orogeny – further into the continent
• Characterised by classic imbrication structures and duplex structures
• Structures found in the sedimentary sequence above the underlying basement

Imbrication zone
• A series of similarly orientated reverse faults connected through a low-angle floor thrust

Question 17

Hinterland

Answer 17

• More complexly strained
• Thick skinned deformation
• Occurs at collision zone
• Structures involve the basement
• Thrust nappes are therefore thicker
• Mostly metamorphic and magmatic rocks
• Alpine collision zone – whole lower crust is imbricated
• Characteristics include the occurrence of island-arc or outboard terranes, thrust onto the continental margin
• Also get fold nappes

Question 18

Orogenic wedge in Hinterland:

Answer 18

• Thrust nappes in orogenic zone form a thickening wedge above the basement
• Fragments of down-going slab are sliced off and incorporated

Question 19

Depths of structures:

Answer 19

• Hinterland structures form at greater depth than foreland structures
• Hinterland structures are favoured by plastic shear zones and plastic strain in general
• Some brittle deformation going on in both at shallow depths – influenced by extensional faulting

Question 20

Orogenic wedge model

Answer 20

Shape of the wedge factors at shallow depths:
• Force applied and gravity
• Friction along the basal thrust
• Internal strength or frictional coefficient of the material in the wedge
• Any erosion of the surface of the wedge

Will grow in length by frontal imbrication to maintain a stable shape

Critical taper model
• Wherever the stress get higher, the material will immediately deform until equilibrium is regained

Question 21

Kinematic role of strike slip faults

Answer 21

Transfer faults
• Transfer displacement between two extensional or contractional faults by strike-slip motion
• Bounded and can’t grow freely
• Can connect extension filled fractures, vein, dykes and normal faults of the same or different dip
• Can connect oblique and reverse faults

Transform faults
• Classic San Andreas fault
• Large scale strike-slip faults
• Segment plates or form plate boundaries
• Can form fault zones rather than simple faults

Pull-apart basins
Escape tectonics
Push-up structures - flower structures
Shortening array

Question 22

Transpression and Transtension

Answer 22

Transpression:
• Combined strike-slip and coaxial strain involving shortening perpendicular to the zone
• Horizontal shortening and vertical extension – squish the rock
• Produces oblate ellipsoid (flattening)
• At depth strain accumulates by plastic deformation processes (ductile strain)

Transtension:
• Combined strike slip and extension
• At depth strain accumulates by plastic deformation processes (ductile strain)
• Produces prolate ellipsoids