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Oceans only grow

Oceans Only Grow
Plate tectonics recognises only three types of plate margin: (1) rifts and ridges that create new crust by extension, (2) transforms where two plates slide past each other and (3) subduction zones where crustal material gets consumed back into the mantle. The threshold for starting a subduction zone is high and such features hardly appear in the dispersal story of central Gondwana. It follows that every plate margin we model in Gondwana dispersion must be either type (1) or type (2). This leads to a further test that any acceptable model must pass: the growth of an ocean must be more-or-less smoothly continuous unless there is good evidence to the contrary. In particular 'growth' must not go into reverse, implying that new crust was being consumed. There may be instances, particularly in young or proto-oceans, where local evidence of compression may be found (e.g. flower structures on seismic setcions) but even here the amount of shortening will be small on the scale of a global map. For the rest, once started, a rift turning into an ocean will show an increase in oceanic area with time.
The animation above starts with the present-day ocean-ages portrayed on the Geological Map of the World (CGMW, digitised with permission). Going backwards in time we see how the creation of the oceans separating Gondwana is reversed. The animation stops at 150 Ma (Late Jurassic). The model then runs forward in time. From 140 Ma, as new ocean growth sets in rapidly, a median line is drawn down the middle of each incipient ocean and copied to each of the two conjugate fragments. After a time (e.g. 5 myr), these two features will have separated somewhat and a new median line is drawn. The spacing between consecutive lines should not vary greatly for steady ocean growth. The 140 Ma median line, the first in Cretaceous times, is red and marks the onset of many new margins of dispersal activity within Gondwana. Spectral colours follow at intervals of 5 myr until 105 Ma (magenta). From the 100 Ma median line we use grey median lines and stop at 85 Ma, by which time India has taken off from Madagascar and the Cretaceous Quitet Zone (KQZ) is almost over.

Rapid progress in the Early Cretaceous

Note in the animation that, in the relatively short time interval 135 to 120 Ma approximately (orange-yellow-green-pale blue median lines), many of the outlines of the southern continents are established and (slow) rifting turns into (normal) ocean growth rates that continue, in many cases to the present day. By about 100 Ma and the end of the Early Cretaceous the new ridge system has become continuous all the way from West Africa to Australia. The old-established spreading between Africa and Antarctica carries on undiminished after a new location of the Bouvet triple junction is established at about 129 Ma. The Bouvet plume remains centrally situated between Africa and Antarctica throughout. Note particularly that the rapid growth of the Weddell Sea after 142.3 Ma (start of the Cretaceous) slows markedly at about M0 time (121.4 Ma). The movement of the South America plate is such that, even then, ocean growth in the Weddell Sea remains slightly positive.
Last update: 2024 February 1