Sedimentary Basins (L15-20)

Question 1

What have most sedimentary basins formed by?

What % of basins is this?

Answer 1

Thinning of continental lithosphere

~80%

Question 2

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

Answer 2

Flexure by loading

Pull-apart/strike-slip

Question 3

What’s the best way to classify basins?

Answer 3

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

Question 4

What are the main basin subsidence mechanisms?

Answer 4

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

Question 5

What is backstripping?

What does it correct for?

Answer 5

Converting sedimentary loads into uniform water loads

Compaction and isostatic loading by sediments

Question 6

What causes compaction in sediment layers?

Answer 6

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

Question 7

How is loading corrected for in backstripping?

Answer 7

S_w = 0.36 x S_s

Question 8

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

Answer 8

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

Question 9

In subsidence calculations, what is β?

Answer 9

Stretching factor (of crust)

Question 10

What are the two phases of continental lithosphere stretching?

Answer 10

Syn-rift phase = active extension

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

Question 11

What is a reasonable value of β for extensional basins?

At what value of β is oceanic lithosphere generated?

Answer 11

1.5-2

3-4

Question 12

How can the upper crust be extended by faulting?

Answer 12

Acts as tilted blocks

Question 13

What happens if stretching continues beyond rifting?

Answer 13

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

Question 14

Where does flexural loading occur in the oceans and continents?

Answer 14

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

Question 15

What are strike-slip basins?

Answer 15

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

Question 16

How does the lithosphere respond to loading?

Answer 16

Partly supported by elastic stresses

Question 17

What happends to foreland basin loads over time?

Answer 17

They advance

Basin infill becomes the load

Question 18

What are the settings of foreland basins?

Answer 18

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

Question 19

What occurs in the overriding plate during slab rollback?

Answer 19

Backarc extension

Question 20

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

Answer 20

Basin is exhumed

Flexural unloading

Question 21

How can underwater foreland basins be imaged?

Answer 21

Reflection seismology

Question 22

Describe the subsidence associated with strike-slip basins

Answer 22

Rapid

Localised

Question 23

What are the two types of strike-slip offsets?

Answer 23

Restraining bend - compression

Releasing bend - extension

Question 24

How does slab rollback work?

Answer 24

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

Question 25

When is slab rollback possible? | Why?

Answer 25

Short subduction zones | Mantle can flow around the slab

Question 26

What are mystery basins?

Answer 26

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

Question 27

What is suevite?

Answer 27

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

Question 28

How is coesite associated with impact sites?

Answer 28

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

Question 29

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

Answer 29

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

Question 30

Why are foreland basins poorly represented in the geological record?

Answer 30

Once thrusting ceases and thrust sheets erode | Foreland basin rapidly uplifts as the lithosphere regains isostatic eqm

Question 31

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

Answer 31

Isostatic equilibrium is maintained throughout their development

Question 32

What is uplift and erosion of sedimentary basins called? | How is this term commonly applied?

Answer 32

Structural inversion | Compressed extensional basins, where normal faults reactivated as thrust faults

Question 33

What is epeirogenic uplift?

Answer 33

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

Question 34

What causes transient epeirogenic uplift?

Answer 34

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

Question 35

What causes permanent epeirogenic uplift?

Answer 35

Underplating: subsurface igneous thickening of the crust | Mixture of deep intrusion and ponding mantle melt fractions

Question 36

What is the equation for uplift generated by underplating?

Answer 36

``` U = X(ρ_a - ρ_x)/ρ_a X = underplate thickness ρ_a = asthenosphere density ρ_x = underplate density ```

Question 37

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

Answer 37

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

Question 38

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

Answer 38

Offshore: North Sea and Atlantic Ocean

Question 39

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

Answer 39

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