Geoscience Reference
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only in the Mediterranean-Black Sea region. The opening
of the Drake Passage and the Scotia arc between South
America and the Antarctic peninsula, and the closure of
the Panama seaway linking the Americas, are two further
Cenozoic stages in ocean evolution. The former opened
up the circumpolar southern ocean and the latter closed
the tropical Atlantic-Pacific strait. Both events had major
influences on Quaternary global glaciation. The next
supercontinent will take shape as Atlantic widening
continues and the Pacific subducts beneath the Americas,
whilst Africa rifts apart to potentially create a new ocean.
The birth and death of oceans
KEY PROCESSES
Formation of the Atlantic Ocean holds the key to the modern global ocean and related continental tectonic
architecture. Early
continental rifting commenced c. 220 Ma in what is now the northern Atlantic region, with rift
basalts extruded in a subtropical, arid continental environment of red sands and evaporiterocks. Rift graben formed
around the 'British Isles', including the Midland Valley of Scotland and what would become the North Sea in the later
Triassic period. Extension led eventually to sea-floor spreading in the New York-Liberia (West Africa) zone at around
165 Ma (Jurassic). Full continental separation was under way and the modern north central Atlantic Ocean was open
by 142 Ma (early Cretaceous), followed by the South and then the North Atlantic. Final separation of northern Europe
from northern North America commenced in the Labrador-Nordic-North Sea areas after 50 Ma (early Cenozoic era)
and the Tethys Ocean finally closed c. 37 Ma.
Sea-floor spreads progressively, like a slow-motion view of a chick hatching from its egg. Stronger individual cracks
developed but weaker cracks ceased to propagate, often surviving as aulacogens or failed rifts, whilst active rifts
developed mid-ocean ridges. There is no question of a single, linear rift opening uniformly over the 16,000 km of the
Atlantic, still less over the more than 50,000 km global extent of modern mid-ocean ridges. Alignment is driven by
random locations of hot spots, or controlled by structural
lineaments
from earlier events. The North Atlantic opened
between Florida to Britain, parallel to Caledonian/Hercynian orogens and the Iapetus suture. Spreading applies
extensional stress to adjacent areas, propagating through initially intact crust. Transform faults and
triple junctions
develop as crust adjusts to movements elsewhere across Earth's curved surface. Several developed in the Atlantic,
especially during the final separation of Canada, Greenland and northern Europe. Some arms became failed rifts,
now guiding major continental river basins.
We are, of course, in a never-ending story and as the Atlantic widens further, so it approaches its eventual nemesis
in a future Earth. Other currently active regions, such as the East African rift valley with its northern extensional rifts
into the Red and Dead Seas and the Gulf of Aden, the progressive slide of south-west California northwards and
continuing Mediterranean subduction, tempt us to ask where they might lead. Applications of twenty-first-century
understanding of tectonic processes and computer modelling are enabling projections of the death of existing oceans
and birth of new ones, foremost among which is the PALEOMAP Project (Scotese et al.) which, in effect, reviews
stages of a full supercontinental cycle from Pangaea (- 250 Ma) to Pangaea Ultima (+ 250 Ma). The projection for
+ 50 Ma (see
Figure 28.4
)
suggests the following:
•
Atlantic widening to the extent that its western oceanic crust is subducting beneath the eastern Americas seaboard.
•
Southern California has accreted to Alaska.
•
Africa has completed its collision with Europe and Asia, replacing the Mediterranean Sea with a mountain range.
•
This in turn has ended the East African rift system and closed the seaways between east Africa, Arabia and the
Indian subcontinent.
•
Extension of western Pacific island arcs and subduction around Australia, beginning to close the Indian Ocean.
The projection for + 250 Ma (
Figure 11.5
)
takes this much further, to the next supercontinent, which will have seen
the death of the Atlantic Ocean, shrinkage of the Indian Ocean to an inland sea and the restoration of a grand Pacific
Ocean to its former Panthalassic size!
