Sedimentary Basins (L15-20) Flashcards
What have most sedimentary basins formed by?
What % of basins is this?
Thinning of continental lithosphere
~80%
What are the other ways to form sedimentary basins besides lithospheric thinning?
Flexure by loading
Pull-apart/strike-slip
What’s the best way to classify basins?
Extensional: formed by thinning crust Flexural/foreland basins Basins associated with subduction zones Impact basins Mystery basins
What are the main basin subsidence mechanisms?
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
What is backstripping?
What does it correct for?
Converting sedimentary loads into uniform water loads
Compaction and isostatic loading by sediments
What causes compaction in sediment layers?
Weight of upper layers presses pore fluid out of the deeply buried layers
How is loading corrected for in backstripping?
S_w = 0.36 x S_s
How does initial crustal thickness affect the result of lithosphere stretching?
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
In subsidence calculations, what is β?
Stretching factor (of crust)
What are the two phases of continental lithosphere stretching?
Syn-rift phase = active extension
Post-rift phase = sag phase due to cooling of upwelled asthenosphere
What is a reasonable value of β for extensional basins?
At what value of β is oceanic lithosphere generated?
1.5-2
3-4
How can the upper crust be extended by faulting?
Acts as tilted blocks
What happens if stretching continues beyond rifting?
Crust thinned enough for decompression melting to occur
Mid-ocean ridge will form
Basin is rifted and either side is a passive margin
Where does flexural loading occur in the oceans and continents?
Oceans: hotspots like Hawaii, trenches
Continents: foreland basins in continent-continent collision
What are strike-slip basins?
Sub-category of extensional basins associated with major transcurrent and transform faults
How does the lithosphere respond to loading?
Partly supported by elastic stresses
What happends to foreland basin loads over time?
They advance
Basin infill becomes the load
What are the settings of foreland basins?
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
What occurs in the overriding plate during slab rollback?
Backarc extension
What happens to a foreland basin when the load is eroded?
Basin is exhumed
Flexural unloading
How can underwater foreland basins be imaged?
Reflection seismology
Describe the subsidence associated with strike-slip basins
Rapid
Localised
What are the two types of strike-slip offsets?
Restraining bend - compression
Releasing bend - extension
How does slab rollback work?
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
When is slab rollback possible?
Why?
Short subduction zones
Mantle can flow around the slab
What are mystery basins?
Crustal extension and long-duration subsidence records but occur in very old regions of thick lithosphere
What is suevite?
A rock consisting partly of melted material formed during an impact event
How is coesite associated with impact sites?
Polymorph of quartz that only forms at very high pressures achieved during impact events
What is the process for an impact crater’s development?
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
Why are foreland basins poorly represented in the geological record?
Once thrusting ceases and thrust sheets erode
Foreland basin rapidly uplifts as the lithosphere regains isostatic eqm
Why are basins formed by lithospheric extension well represented in the geological record?
Isostatic equilibrium is maintained throughout their development
What is uplift and erosion of sedimentary basins called?
How is this term commonly applied?
Structural inversion
Compressed extensional basins, where normal faults reactivated as thrust faults
What is epeirogenic uplift?
Regional uplift of 100s of metres not caused by lithospheric shortening
Can be transient or permanent
What causes transient epeirogenic uplift?
Mantle convection
Hot upwelling = less dense than normal = surface uplift
Cool downwelling = more dense than normal = surface depression
What causes permanent epeirogenic uplift?
Underplating: subsurface igneous thickening of the crust
Mixture of deep intrusion and ponding mantle melt fractions
What is the equation for uplift generated by underplating?
U = X(ρ_a - ρ_x)/ρ_a X = underplate thickness ρ_a = asthenosphere density ρ_x = underplate density
What happens as the elevated sediment brought about by underplating gets eroded?
Mass of crustal column decreases Compensated by a rising of the mantle Further elevation of sediment More erosion Cycle continues until equilibrium
Roughly half a million cubic km of rock has disappeared from the British Isles due to denudation, where has it gone?
Offshore: North Sea and Atlantic Ocean
What is the cause of the uplift and denudation in the British Isles?
Underplating associated with the proto-Iceland plume and breakup of the North Atlantic Ocean
