[Tutorial] Using GPlates to model continental drift

[Charerg]

Right, so now we have our continents moving, oceanic crust forming, and we can even check their velocity! However, like I mentioned, the velocity can vary at different points of a plate. So, it can be useful to actually generate a velocity field to see how the velocities differ. A velocity field in GPlates looks like this:

VField Example.png

Let’s fire up our model then. I’ve created a new continent for the purpose of this tutorial, but you could also use one of the old ones (they’re just real slow moving in my version, as you noticed in the previous section). Anyway, for velocity fields to work, we need to first create some actual plates rather than continents. Since plates deform and change shape over time and we’re only interested in creating a velocity field for the present day, we only need to do so for 0 Mya though.

So, go under the Digitise tool and choose “Digitise new polyline geometry (hotkey L)”. We will use this to draw the different boundaries. To start off, draw the mid-oceanic ridge, Create feature and choose MidOceanicRidge as the feature type. Save the line in its own feature collection called “Plate Boundaries” (and remember to give it the right Plate ID!). Here are the settings I used (note that I only set this to exist at 0 Mya):

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I also gave the “Plate Boundaries” feature collection a unique colour to make it easy to see the lines. Next, if you envision your plate has a subduction boundary like mine has here, draw another line, Create feature again, and choose SubductionZone as the feature type. Again, save the feature in “Plate Boundaries”.

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Then we need to define the rest of the plate boundaries. Draw another line to connect the ridge and the subduction zone. Make sure that this line intersects both of the previous lines! I decided that this boundary is a transform fault, and chose Fault as the feature type (depending on the tectonics, could be a different type as well).

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And finally draw the 4th line to finish the plate, again making sure it intersects. I picked MidOceanicRidge as the feature type, but it could be something else as well. Your boundaries should now look something like this:

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Note that we could have also drawn just two or three lines (or five or nine, you get the idea). The important thing is that the lines intersect each other, not the number of lines drawn.

Now we can actually define the plate itself. Go under the “Topology“ tools (hotkey 4) and choose “Build new boundary topology (hotkey B)”. Remember to check you don’t have any feature selected, otherwise the option will be greyed out. After choosing “Build new boundary topology”, click one of your lines. The “Topology Tools” window to the right of your screen should now have some options:

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Now click “Add”. Notice how the “Topology boundary selections” window at the bottom of your screen shows your newly added feature:

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Now go ahead and add the rest of the lines into your topology, going around the plate in either clockwise or counter clockwise order. Once done, your screen should look something like this:

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Then click Create and now you can choose ClosedPlateBoundary as the feature type:

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Then give your plate the right Plate ID and the appropriate time of appearance. I used the following settings:

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Save your newly created plate in a new feature collection called “Plates”. Congratulations, you’ve now defined your plate! I chose to give the “Plates” feature collection a unique colour, infill at 0.5 opacity, and hid the previously created “Plate Boundaries”. So, my plate looks like this:

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Now that we have our plate, we can generate the velocity field. I should warn you that doing so will clutter our “Layers” with something like 20+ extra layers. And as far as I’m aware, there’s no way to organise the layers into several layer groups in GPlates. So, you might want to either create a duplicate model for the sake of generating velocity fields, or only generate them after your tectonics model is finished. But, to generate the velocity field Features->Generate Velocity Domain Points->CitcomS. The following window will pop up:

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Set the node density at 9x9 and I also recommend making a separate folder (name it Velocity Field or something) for the output directory, since this will generate quite a few files. Once done, click OK, and the velocity field should now appear.

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You’ll note that the “Layers” are now very cluttered up. You can define the rest of the plates in exactly the same way, and their movement directions will also be shown on the field (no need to recreate the field). I also recommend hiding the nodes (the “mesh” layers). I created the plates A and B as well, and here’s how my planet looks like in Mollweide:

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As you can see, the arrows denote absolute movement direction (movement relative to the planet’s spin axis), and the length of the arrows display the velocity. We can see here that Plate B is moving very slowly (0.7 cm/year), whereas Plate C is moving very swiftly relative to the others (about 9.3 cm/year). Generating a velocity field can be a useful tool if you’re using your GPlates model as a basis for creating a map of your present-day tectonic plates.

So, this is the last of the sections planned so far. You’ve essentially now learned all of my tricks! So, I’ll have to learn some new ones before the next update, which might be a while off. Best of luck with building up your models and happy (continental) drifting!