The pygplates documentation has a guide for creating isochrons from ridges, which looks really useful. What happens if a new boundary (ridge, subduction, or transform) forms that bisects one or both of the plates after the isochrons have been created?
Example 1: Plates A and B are diverging at 200 Ma, and then at 150 Ma plate B splits into B1 and B2. The rift between B1 and B2 would extend to a triple junction with A, and any existing isochrons associated with B would also need to be split. In the simplest case, I guess the initial rift between A and B could be represented as two features, one between A and B1 and one between A and B2.
Example 2: Plates D and E are diverging for a few hundred Ma, and then a subduction zone appears along the margin of E which eventually pulls a microcontinent away from D after subducting the original ridge. Two plates become at least four: the original D and E, the oceanic crust that formed as part of E but subducts under E, and the microcontinent D1. I can draw the boundary features (and continental crust fragments) manually, but I don’t know how to create the isochrons so they’re attached to the correct plate.
Example 3: Plates F and G are diverging, and then the original ridge becomes inactive and a new ridge between them forms somewhere else. The new ridge bisects through oceanic crust that was part of both plates, so some crust that was originally part of F is now part of G, and vice-versa.
All of these (plus other, more complex examples) can be observed in Merdith et al (2020).
I’ve been doing this manually, but this is somewhat time-consuming and error-prone, so suggestions of a better way of creating complex isochrons would be greatly appreciated.