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1B ESA > Sedimentary Basins (L15-20) > Flashcards

Sedimentary Basins (L15-20) Flashcards

1
Q

What have most sedimentary basins formed by?

What % of basins is this?

A

Thinning of continental lithosphere

~80%

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2
Q

What are the other ways to form sedimentary basins besides lithospheric thinning?

A

Flexure by loading

Pull-apart/strike-slip

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3
Q

What’s the best way to classify basins?

A 
Extensional: formed by thinning crust
Flexural/foreland basins
Basins associated with subduction zones
Impact basins
Mystery basins
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4
Q

What are the main basin subsidence mechanisms?

A

Thermal (cooling lithosphere i.e. at mid-ocean ridge)
Thinning lithosphere = upwelled asthenosphere to maintain isostasy
End loading of broken plate
Line load on unbroken plate

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5
Q

What is backstripping?

What does it correct for?

A

Converting sedimentary loads into uniform water loads

Compaction and isostatic loading by sediments

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6
Q

What causes compaction in sediment layers?

A

Weight of upper layers presses pore fluid out of the deeply buried layers

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7
Q

How is loading corrected for in backstripping?

A

S_w = 0.36 x S_s

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8
Q

How does initial crustal thickness affect the result of lithosphere stretching?

A 
Thin crust -> subsidence
Thin mantle lithosphere -> uplift
Effects balanced at t_c ~ 15km
Oceans: t_c < 15km so uplift wins
Continents: t_c > 15km so subsidence wins
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9
Q

In subsidence calculations, what is β?

A

Stretching factor (of crust)

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10
Q

What are the two phases of continental lithosphere stretching?

A

Syn-rift phase = active extension

Post-rift phase = sag phase due to cooling of upwelled asthenosphere

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11
Q

What is a reasonable value of β for extensional basins?

At what value of β is oceanic lithosphere generated?

A

1.5-2

3-4

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12
Q

How can the upper crust be extended by faulting?

A

Acts as tilted blocks

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13
Q

What happens if stretching continues beyond rifting?

A

Crust thinned enough for decompression melting to occur
Mid-ocean ridge will form
Basin is rifted and either side is a passive margin

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14
Q

Where does flexural loading occur in the oceans and continents?

A

Oceans: hotspots like Hawaii, trenches
Continents: foreland basins in continent-continent collision

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15
Q

What are strike-slip basins?

A

Sub-category of extensional basins associated with major transcurrent and transform faults

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16
Q

How does the lithosphere respond to loading?

A

Partly supported by elastic stresses

17
Q

What happends to foreland basin loads over time?

A

They advance

Basin infill becomes the load

18
Q

What are the settings of foreland basins?

A

Continent-continent collision: peripheral (on lower plate) and retro (on upper plate) basin
Ocean-continent margin: moving in from ocean trench -> magmatic arc -> backarc compression -> retro-foreland basin
Subduction zone rollback: peripheral foreland basin-trench

19
Q

What occurs in the overriding plate during slab rollback?

A

Backarc extension

20
Q

What happens to a foreland basin when the load is eroded?

A

Basin is exhumed

Flexural unloading

21
Q

How can underwater foreland basins be imaged?

A

Reflection seismology

22
Q

Describe the subsidence associated with strike-slip basins

A

Rapid

Localised

23
Q

What are the two types of strike-slip offsets?

A

Restraining bend - compression

Releasing bend - extension

24
Q

How does slab rollback work?

A

Weight of subducting slab provides a downward force
Component parallel to slab causes slab to move down into mantle
Component perpendicular to slab is rollback, if it overcomes pressure field in mantle then slab rolls back

25
Q

When is slab rollback possible?

Why?

A

Short subduction zones

Mantle can flow around the slab

26
Q

What are mystery basins?

A

Crustal extension and long-duration subsidence records but occur in very old regions of thick lithosphere

27
Q

What is suevite?

A

A rock consisting partly of melted material formed during an impact event

28
Q

How is coesite associated with impact sites?

A

Polymorph of quartz that only forms at very high pressures achieved during impact events

29
Q

What is the process for an impact crater’s development?

A

Post impact excavation and beginning of uplift: impact melt in crater, ejection of material occurs
Central uplift becomes unstable
Uplift collapses to form peak ring

30
Q

Why are foreland basins poorly represented in the geological record?

A

Once thrusting ceases and thrust sheets erode

Foreland basin rapidly uplifts as the lithosphere regains isostatic eqm

31
Q

Why are basins formed by lithospheric extension well represented in the geological record?

A

Isostatic equilibrium is maintained throughout their development

32
Q

What is uplift and erosion of sedimentary basins called?

How is this term commonly applied?

A

Structural inversion

Compressed extensional basins, where normal faults reactivated as thrust faults

33
Q

What is epeirogenic uplift?

A

Regional uplift of 100s of metres not caused by lithospheric shortening
Can be transient or permanent

34
Q

What causes transient epeirogenic uplift?

A

Mantle convection
Hot upwelling = less dense than normal = surface uplift
Cool downwelling = more dense than normal = surface depression

35
Q

What causes permanent epeirogenic uplift?

A

Underplating: subsurface igneous thickening of the crust

Mixture of deep intrusion and ponding mantle melt fractions

36
Q

What is the equation for uplift generated by underplating?

A 
U = X(ρ_a - ρ_x)/ρ_a
X = underplate thickness
ρ_a = asthenosphere density
ρ_x = underplate density
37
Q

What happens as the elevated sediment brought about by underplating gets eroded?

A 
Mass of crustal column decreases
Compensated by a rising of the mantle
Further elevation of sediment
More erosion
Cycle continues until equilibrium
38
Q

Roughly half a million cubic km of rock has disappeared from the British Isles due to denudation, where has it gone?

A

Offshore: North Sea and Atlantic Ocean

39
Q

What is the cause of the uplift and denudation in the British Isles?

A

Underplating associated with the proto-Iceland plume and breakup of the North Atlantic Ocean

1B ESA (10 decks)

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