Oceans only grow
Plate tectonics recognises only three types of plate margin: (1) rifts and ridges that created 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 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 continuous. In particular it must not go into reverse - implying that crust is 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 the amount of shortening will be small (less than 10 km?) on the
scale of a global map. For the rest, once started, a rift turning into an ocean will show an increase in area with time. We can demonstrate this by drawing a median line down each ocean at regular time intervals and attaching it to each of the two conjugate fragments. After a time, these two features will have separated
and a new median can be drawn.
The animation illustrates the result of such an exercise for our Gondwana dispersal model. From 185 until 145 Ma we have drawn a grey median line every 5 myr to show the slow
ocean growth during Jurassic times. The 140 Ma median line, the first in Cretaceous times, is red and marks the onset of new types of dispersal activity. Spectral colours
follow at intervals of 5 myr until 105 Ma (magenta). From the 100 Ma median line we revert to grey lines and stop at 85 Ma, by which time India has taken off from Madagascar.
We show thick purple lines to represent the gravity margins we interpret from satellite gravity data around most continental areas. They serve as unequivocal proxies for
continental margins at this scale.
Note in the animation that in a relatively short time interval, 135 to 125 Ma approximately (orange-yellow-green median lines), most 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. A new ridge system becomes continuous all the way from West Africa to Australia while, with a short-lived
deviation (sidestep) around 130 Ma, the old-established spreading between Africa and Antarctica carries on undiminshed with the Bouvet plume centrally between Africa and Antarctica. Note particularly that the rapid growth of the Weddell Sea after 142.3 Ma slows markedly at about M0 time (121.4 Ma) with consequences for the subsequent consolidation of the South America plate in its present outline.