Authors: Sabin Zahirovic, Serena Yeung, and James Egan
Updated for GPlates 2.2 and the reconstructions of Müller et al. (2019) by Behnam Sadeghi and Christopher Alfonso
EarthByte Research Group, School of Geosciences, The University of Sydney, Australia
Working with Mid-Ocean Ridge Features
Exercise 1 – Loading data and creating a MOR feature
Exercise 2 – Incorporating a new MOR feature into an existing plate polygon dataset
Extending a new MOR to intersect with existing plate boundaries
The aim of this tutorial is to teach the user how to (1) interactively create a new mid-ocean ridge (MOR) feature and (2) link it to existing adjacent plates in order for the MOR to reconstruct correctly through time.
GPlates allows you to interactively create features (see Tutorial 1.5 Creating Features) such as subduction zones or volcanoes. When creating a MOR feature, its motion relative to the two adjacent plates is able to be automatically calculated by GPlates using half stage rotations (version3). Previously, GPlates could not easily handle generating MOR motions on the fly – and so the original continuously closing polygons have half stage rotations (version3) that were manually calculated.
We will go through the steps of how to interactively create a MOR and attach it to two adjacent plates so that it reconstructs correctly through time. We will also demonstrate how to incorporate a newly created MOR feature into an existing plate polygon dataset. Note that this is only a generalised tutorial designed to teach the user the basics of creating and working with MOR features.
Click here to download the data bundle for this tutorial.
The tutorial dataset includes the following files:
This tutorial dataset is compatible with GPlates 2.2.
Figure 1. GPlates with the Müller et al (2019) datasets loaded.
Figure 2. Click and drag the Resolved Topological Geometries layer (coloured purple) to the top of the list in the Layers window. Hide the Resolved Topological Networks layer.
It is suggested that the colouring for this dataset is changed to black (or any other easily-visible colour) to help identify these plate boundaries from the isochron file.
Figure 3. Recolour the plate boundary topologies layer black in the Manage Colouring window.
We will now interactively construct a new MOR feature. For this example, we will create a new MOR in the Atlantic Ocean, between South America and Africa, which exists between 60 and 20 Ma. For example’s sake, we will disregard the existing South America-Africa MOR, visible in black, and draw our new MOR on top of it.
Figure 4. The Atlantic Ocean, Africa and South America reconstructed at 60 Ma.
In this example we will plot a fairly random MOR for the purposes of showing how to reconstruct it correctly through time. It does not have to intersect any existing plate boundaries, but this idea will come into play in Exercise 2 when we have to incorporate a MOR into an existing plate polygon dataset.
GPlates will automatically connect your series of plotted points in a ‘join-the-dots’ fashion to form a complete line coloured white (Figure 5). Notice that the coordinates of your points will appear in the ‘New Geometry’ window to the right of the GPlates main window. If you make a mistake in the location of your plotted point, use the keyboard command Ctrl-Z to undo the action.
Figure 5. A digitised line (shown in white) plotted using the ‘Digitise new polyline feature’ tool. Click the highlighted ‘Create Feature’ button to specify the properties of the newly created line.
Figure 6. The first ‘Create Feature’ window with the feature type ‘MidOceanRidge’ selected.
For this example, these are 201 and 701 representing the South American (SAM) and African plates, respectively. Since our MOR feature doesn’t intersect any plate boundaries, the choice of conjugate Plate IDs is crucial in order for GPlates to correctly calculate the motion of the MOR through time. However, generally it does not matter which you decide is the left or right Plate ID as GPlates figures out the rest on its own.
Figure 7. The second ‘Create Feature’ window where the reconstruction method, left and right Plate IDs, begin/end time and filename of the new MOR feature can be specified.
This opens up the third ‘Create Feature’ window which summarises the existing properties you have specified and any extra properties that are available to you.
Figure 8. Select the plate boundaries dataset and click Create.
You will then be taken back to the main GPlates window.
Figure 9. Use the ‘Choose Feature’ tool to inspect our newly created MOR and view the properties specified earlier.
Additionally, if you reconstruct forward in time, you will see your new MOR reconstructing correctly. Notice that after 20 Ma, the MOR ridge will disappear as we have specified. You have now learnt how to create a mid-ocean ridge feature in GPlates!
We will now interactively construct a MOR feature with the intention of replacing an existing MOR. This requires the user to ensure the new MOR geometry intersects with pre-existing plate boundaries so that it can replace an old MOR. The user then has to delete the old MOR, and manually edit existing topologies to insert the new plate boundary. When deleting a MOR, the user must fix at least two polygons at any one time. That is because the old MOR formed the mutual boundary between two polygons.
Users must also take care to correct any other polygon artefacts they may have introduced by deleting the old redundant MOR. GPlates does not automatically detect polygon artefacts, so a careful interactive reconstruction in GPlates through time is required to ensure polygon closure without gaps, overlaps or “rubber-banding” artefacts.
For this example, we will again create a new MOR in the Atlantic Ocean adjacent to the existing South America-Africa MOR, which exists between 60 and 20 Ma.
Figure 10. The Atlantic Ocean, Africa and South America reconstructed at 60 Ma, centred on the South America-Africa MOR.
Figure 11. Digitise the new MOR to intersect the plate boundaries of the SAM or African plate.
Figure 12. Specify all the properties of the new MOR feature in the ‘Create Feature’ window.
Ideally, the endpoints of your MOR should continue to intersect the adjacent plate boundaries but in the situation in which they do not (Figure 13), refer to the Appendix before continuing.
Figure 13. In this case, the southern end of the new MOR feature reconstructed at 20 Ma (circled) does not intersect with the pre-existing plate boundaries, and therefore must be modified.
Return to 60 Ma for the next steps.
Figure 14. Under the ‘Topology’ icon, select the SAM Plate using the ‘Choose Feature’ tool and then select the ‘Edit Topology Sections’ tool.
At this stage, the boundary topology should look like Figure 15. Notice that a “rubber-banding” artefact will appear until you insert the new MOR boundary.
Figure 15. The SAM plate after removing the three pre-existing plate boundary segments.
Figure 16. The SAM plate boundary topology after adding the new MOR boundary section.
In the Topology Tools window on the right-hand side of the main window, notice that the ‘Valid time’ for which the topology can exist for is limited from 60.1 to 58.1 (Figure 17). This is because the plate topology you are modifying will cease at 58.1 Ma (and transform into a ‘new’ one). The new MOR topology you have just incorporated into the polygon will similarly cease to exist at this point in time. If you select any feature, you will notice that the right-hand-side window will display the Valid time for which the feature will exist.
Figure 17. The ‘Valid time’ for which the newly edited topology can exist for is limited by the existing ‘Valid time’ of the old topology.
You have now successfully fixed one plate boundary topology.
Once you have done this you will want to make sure that you have not created any artefacts or discontinuities. The best way to see gaps, overlaps and rubber-banding artefacts is by turning off the lines and just displaying the resolved topologies.
You can help to minimise these sorts of artefacts by carefully extending the new MORs to intersect at the same place as the old boundary (see Appendix). Nonetheless, there is usually a lot of manual work involved in introducing new MORs into existing plate boundary datasets.
Müller, R. D., Zahirovic, S., Williams, S. E., Cannon, J., Seton, M., Bower, D. J., Tetley, M. G., Heine, C., Le Breton, E., Liu, S., Russell, S. H. J., Yang, T., Leonard, J., and Gurnis, M., 2019, A Global Plate Model Including Lithospheric Deformation Along Major Rifts and Orogens Since the Triassic: Tectonics, v. 38, no. 6, p. 1884-1907. doi: 10.1029/2018tc005462
If intending to replace an existing MOR with a new MOR feature, it is crucial that the new MOR geometry intersects with pre-existing plate boundaries in order for the user to edit the new topology in. If the MOR fails to intersect these plate boundaries, which may be the case after reconstructing back through time, it is possible to interactively extend the MOR to intersect with the required plate boundaries.
Using the ‘Choose Feature’ tool, select the problematic MOR and click the ‘Insert Vertex’ tool (Figure 18).
Figure 18: Select the MOR feature and click the ‘Insert Vertex’ tool.
Add points to make the sure MOR intersects with neighbouring boundaries (Figure 19).
Figure 19: Plot points interactively to extend the MOR until it intersects with adjacent plate boundaries.
Once you are satisfied, select any other tool on the main interface (such as the ‘Choose Feature’ tool) to halt the process. You have now successfully extended your MOR feature!