Authors: Mark Turner & Mike Gurnis
Division of Geological and Planetary Sciences, California Institute of Technology
Editing the Sections Table
This tutorial is designed to teach the user how to create and use topological closed plate polygons.
Screen shots have been included to illustrate how to complete new steps within each exercise.
The data bundle for this tutorial, Topologies, can be located https://sites.google.com/site/gplatestutorials/.
Create a folder named Topology, unarchive the datasets into this folder.
See http://www.earthbyte.org/Resources/earthbyte_gplates.html for EarthByte data sets.
GPlates enables the user to create traditional geophysical features like hot spots, mid ocean ridges, and large igneous provinces, with static geometries such as points, lines, and polygons, respectivly. GPlates also has the ability to create specialized topological features with dynamic geometries.
A topology is specialized feature type whose geometry is determined not by a static list of vertices, but rather from the composition of geometries from other, regular features. A tpology does not contain any vertices itself, but instead contains a list of references to features. During the reconstruction process this boundary list is used to gather up the geometry of the boundary features, and form a complete closed polygon for the topology.
Using topology features, we have developed the concept of a "Continuously Closed Plate" (CCP), such that, as each margin moves independently, the plate polygon remains closed geometrically as a function of time. This methodology is a new improvement for combining kinematic plate models with geodynamic models.
Topologies are primarily used to model the large scale tectonic plates. Future versions of GPlates will use these topologies to model deforming regions of the Earth’s crust.
Figure 1: Schematic of the generic features that are incorporated into the Continuously Closed Plate (CCP) algorithm. Red dots represent dyanically computed intersections between plate boundaries.
The following diagrams show how GPlates creates a dynamic geometry for a simple hypothetical polygon, for two reconstruction times, 0 Ma, and 10 Ma.
A: 0 Ma
Figure 2: Hypothetical plate polygon from four intersecting lines at two times. Base features are drawn in blue. Plate polygon is outlined in gray, with gray dots showing polygon’s vertices. Red crosses have been added in the figures to highlight dynamically computed intersection points, relative to base features' vertices. First line on the boundary list is hightlighted white (North Line); last line is highlighted in black (East Line)
Figure A. Reconstruction at O Ma, the initial configuration of lines, intersection points, and resolved polygon vertices.
Figure B. Reconstruction at 1O Ma, the lines have been reconstructed, new intersection points computed, and the polygon resolved. Some vertices from the North Line are no longer used as it moved relative to the West Line.
In both reconstructions the East Line contributes none of its original vertices, and only the intersection points are used (no vertices exist on the East Line between the intersection points).
To demonstrate the CCP algorithms in action we present an example of using GPlates and its Topology Tools to create and edit a plate polygon. Using the data bundle we will create an instance of the Nazca Plate.
The following sub-sections first present an overview of the controls, displays, and drawing conventions in GPlates that are used to create plate polygons. Then we give a step by step guide to creating an example instance of the Nazca plate.
In addition to the Globe, the Topology Tools make use of a few other displays and controls on the Main Window. The Topology Tools Task Panel (to the right of the globe) and The Topology Sections Table (below the globe) work together to manipulate the list of features that form a topology’s boundary. As you edit the list of boundary features, you will work back and forth between the globe, the Task Panel, and the Sections Table.
The Topology Tools Task Panel has two sub-tabs, one for the topology itself, and the other for a feature intended to become a Section.
The Topology Sections Table holds an ordered list of boundary features. Upon each reconstruction, it is these features, and their relationships, that will be processed into the resolved polygon geometry for the topology.
While a topology feature is under edit GPlates will draw the topology, and its referenced features, with certain highlights and colors.
Figure 3: Topology Tools and Drawing Conventions. Topology properties, and Section controls will appear in Task Panels. List of boundary features will appear in Sections Table.
The Build Topology Tool is used to create new topology features from existing regular non-topological features. You will select a series of features, in an ordered fashion, moving around the intended boundary of the topology.
Figure 4: Build the Nazca Plate Step 0: Coloring and Orienting the Globe
Now we will select features by clicking on them on the Globe. Please Note: you do not have to switch to the Choose Feature Tool. The Build Topology Tool is a specialized version of choosing features.
At this point there is only one element on the boundary list, so the plate polygon will simply close using only that feature and its vertices.
Figure 5: Build the Nazca Plate Step 1: Adding the first section
We recommended creating a new topology by cycling around the boundary systematically (either a clockwise or counter-clockwise) and appending features to the end of the Topology Sections Table. In this example we will close Nazca in the counter clockwise direction.
Notice how now the south west corner of the plate is formed from the intersection of the two line data features, but it "wraps back upon it self"
With only two features on the list, the internal CCP algorithm chooses the shortest path to close the polygon and it results in a little wedge. This will be corrected as more features are added.
Figure 6: Build the Nazca Plate Step 2: Adding the second section results in a green and blue line being highlighted (the start and end of the topology so far); this will be corrected as more sections are added
Notice how the collection of vertices forming the work-in-progress plate polygon changes: the polygon uses most of the western margin, an intersection point with the southern margin, the central portion of the southern margin, an intersection point with the trench, and most of the trench.
Notice that the north margin has yet been defined, so the GPlates simply closes the gap by connecting the two features.
Figure 7: Build the Nazca Plate Step 3: Adding the third section results in good intersections in the south, and an automatic closing in the north.
Zoom in on the north east corner of the plate area and we will show some of the subtleties and detail work involved with plate closure. This region has a few different options available to us.
Figure 8: Build the Nazca Plate Steps 4, 5 & 6: Adding the fourth section gives the desired detailed topology in the north east.
You can check the outline of the final topology by turning off all line features that are loaded.
View > Geometry visibility > and untick the Show Line Geometries option.
Editing the Sections Table
If you have discovered that you forgot a feature in the middle of the boundary list list, or realize you want to modify the ordering of the Sections in the Table, then you may change the insert point via the Actions column of the table in the bottom panel, and continue with the same steps as above: Focus a feature on the Globe, then click the Add Focused Feature button.
Adding a feature does not change the insertion point, so be aware if you perform an Insert operation out of sequence.
Once you are satisfied with the entries in the Topology Sections Table, and the resulting geometry of the Closed Plate Polygon, then you may create the new Topology via the "Apply/Create …" button on the right Topology Tab of the Topology Tools Panel. This action will lead you through a series of dialogs to finalize the creation process.
The other basic feature properties must be set as well: Plate ID, Begin Time, End Time, Name.
Figure 9: Build the Nazca Plate Steps 7: Adding basic propeties to the Plate Polygon Feature.
In the final step of feature creation, the feature is assigned to a feature collection. All data files that are currently loaded in GPlates will be listed here, as well as the < New Feature Collection > option. Choosing any one of the existing feature collections and clicking Create will add the newly created topology feature to that collection and return the user to the GPlates main window.
If the < New Feature Collection > option is selected, a new feature collection will be created to hold the new feature. This feature collection will not yet have a name, and is not associated with a file on disk. Like all other feature collections, the new one will be found in the Manage Feature Collections dialog.
The convenience button "Create and Save" adds the feature to the selected feature collection, and opens the Manage Feature Collections dialog, so that you may immediately save or rename the collection.
The Edit Topology Tool is the tool used to edit existing topology features. Under this tool you will load an exiting topology into the Topology Sections Table, and make edits to that ordered list of features.
Notice that the topology will be drawn with the highlighting described above. The feature’s basic data is placed into the Topology tab of the Task Panel, and the Topology Sections Table will be filled with the list of boundary features.
Once you are satisfied with the changes, then click on the Apply/Create button and the Topology’s boundary list will be changed to the current contents of the Topology Section Table.
Gurnis, M., Turner, M., Zahirovic, S., DiCaprio, L., Spasojevic, S., Müller, R., Boyden, J., Seton, M., Manea, V., and Bower, D., 2012, Plate Tectonic Reconstructions with Continuously Closing Plates, Computers & Geosciences, 38(1): 35-42, doi:10.1016/j.cageo.2011.04.014.