Lecture 14: Wrenched asunder: a supercontinent splits, an ocean forms Flashcards
The setting…
Pangaea – V-shaped
Landmass wrapped around the Tethys Ocean
Panthalassic Ocean (‘proto-Pacific’)
A Triassic global fauna
Worldwide distribution of various reptiles (incl. mammal ancestors)
Only global terrestrial vertebrate fauna
Bivalves too…
The weather in the Triassic will be..
very similar to the Permian
strongly seasonal
interior aridity
super monsoons
Meanwhile, elsewhere….
Late Triassic
central Pangaea: extensional tectonics
resulted in separation of northern landmass (Laurasia) from…
…southern landmass (Gondwanaland)
How do you break apart a supercontinent?
Extensional tectonics (perhaps driven by rising mantle ‘plumes’) thin the crust
Rifting (normal faulting) occurs
Surface volcanic activity
Rift widens, sea invades,
Eventually a new mid-ocean ridge may form
How does rifting begin?
New rift systems result from upwelling mantle material
under stationary plates up-doming creates three rifts at 120º to each other
Triple junctions
Start as rift-rift-rift junctions (rrr)
Separation begins, new ocean crust may form
Eventually become mid-ocean ridge systems
Ridge-ridge-ridge (RRR) or any other combination (e.g., R-r-R)
A modern analogue
East African Rift triple junction continental break-up Nubian, Arabian and Somalian tectonic plates Red Sea-Gulf of Aden-African Rift Valley Afar Triangle volcanism marine flooding – salt lakes
Triassic-Early Jurassic rifting
Rifting spreads W. from Tethys
‘North America’/’Africa’ separate
Rifting takes place in arid regions
Tethys spills west evaporates
Triassic salt deposits from ‘Nova Scotia’ to ‘Morocco’
Panthalassic Ocean spills east into ‘Gulf of Mexico’
The Louann Salt
Mid-late Jurassic salt
Known in great detail – oil exploration
Central Atlantic opens
Crusty old oceans
Proof of spreading: oceanic crust
Oldest is Jurassic in age
Central Atlantic had opened by the Late Jurassic-Early Cretaceous
The rifting moves south
Early Cretaceous
South America and Africa begin to rift apart (‘un-zip’)
Early Cretaceous evaporites – Brazil & central Africa
Start as non-marine sedimentary basins later become marine as sea invades
Many ‘failed’ arms of triple rift junctions
Later become locations of major river systems
South Atlantic opens
Rifting proceeds from south to north – ‘un-zips’
South Atlantic and Central Atlantic connected together by the Late Cretaceous
Rifting shifts northwards
~40 Ma: N. Atlantic opens
creates Laurentia and Eurasia
Laurentia breaks up into Greenland & N. America
‘modern aspect’
Case study: North Sea oil province
Atlantic rifting in NW Europe
‘North Sea’ area rifts in mid Jurassic
Failed rift – ocean crust not erupted
Viking Graben
Getting a basin-full?
extension rifting normal faulting half-grabens ‘sag’ basin forms sediment fills depression
