Subsidence and Rift Basins Flashcards
Rift valley =
Low lying region that forms when the Earth’s tectonic plates move away from each other
Rift initiation
Initiated by “swell push”
Hot mantle upwells
- decompression melting
Exerts pressure on the overlying lithosphere
Isotherm decreases in depth = thinned lithosphere
Instability = eventually leads to separation
Seismicity
Mostly extensional focal mechanisms
Shallow (crustal) hypo centres at edges and deeper (upper mantle) in centre
Holocene volcanism along the rift axis
Shallow vs deep structure
SHALLOW
Linear topographic depressions bounded by segmented normal faults
DEEP
Crustal thinning and rise of hot mantle beneath rift axis
What factors affect a narrow vs wide rift?
Competition between
- LITHOSPHERIC HEATING
- due to raised isotherm during stretching - COOLING
- due to heat conduction
Narrow rifts
Fast stretching rate:
Lithospheric heating > cooling
- deformation is focussed
e. g. East African Rift
Wide rifts
Slow stretching rate:
Cooling > lithospheric heating
- new rifts form in originally undeformed material either side
- i.e. deformation less focussed§
Syn-rift subsidence
Stretch lithosphere = isotherm raised
Pressure in that ‘column’ dominated by the asthenosphere which is of greater density than the lithosphere
Compensated by increasing the rift basin height i.e. subsiding
(Applies Airy’s theory of isostasy)
Post-rift subsidence
Cools and isotherm falls
Older, colder, more dense = subsides
Rate of cooling and therefore subsidence decreases with time
Steer head geometry
Opposed Airy’s theory of isostasy
= flexural isostasy
The lithosphere has a finite strength = flexes
Thicker = distributed subsidence
Thinner = focussed
Over time = lithosphere thickens = over broader area
POST RIFT SEDIMENTS EXTEND BEYOND ORIGINAL RIFT MARGINS
