Vol. 84, No. 20, 20 May 2003

Global Paleomagnetic Visual Data Base Developed into Its Visual Form

Sergei A. Pisarevsky and Michael W. McElhinny The University of Western Australia, Crawley

The Global Paleomagnetic Database (GPMDB) created and developed by McElhinny and Lock [1993, 1996] is used by researchers all over the world. The well-known version of this data base is in Microsoft Access format, a user-friendly interface that requires no programming skills. Use of the GPMDB has accelerated a number of paleomagnetic, tectonic, and geomagnetic studies (for overviews see, e.g., Van der Voo [1993]; McElhinny and McFadden [2000]).

The next step in developing these data bases lies in the visualization of data and the integration of the paleomagnetic data with Geographical Information Systems (GIS). One of the most popular GIS software among Earth scientists is ArcView, which was developed by the Environmental Systems Research Institute (ESRI). This paper describes an ArcView 3.1 application that visualizes the GPMDB and provides a range of new possibilities for interactive search, analysis, and plate tectonic applications of the paleomagnetic data. We used the latest edition of GPMDB (version 4.3), which contains all published data through March 2002.

Data Organization

The GPMDB consists of six data tables (e.g., McElhinny and McFadden [2000]). However, in most cases, only the three main tables are required; namely, PMAGRESULT, ROCKUNIT, and REFERENCE (for their structures, see McElhinny and Lock [1996]). ArcView allows use of data from the Microsoft Access data base using the corresponding extension, or through SQL connection. However, we used another approach, transferring the three main tables into a single table in .DBF format directly acceptable by ArcView to simplify the process of writing supplementary Avenue (ArcView programming language) scripts. This approach makes the visual data base more user-friendly and reduces software costs, since users only need to have ArcView on their computers, not MS Access. The total number of entries in the GPMDB is about 9000 with an annual increase of several hundred. This means that such simplification of the data base structure will not cause significant consequences in the near future.

The POLES2002.DBF file contains almost all fields (Figure 1c) of the MS Access tables (PMAGRESULT, ROCKUNIT, and REFERENCE) plus two additional numerical fields (ROT and PALLAT) used for visualization (see below). The corresponding point theme (layer) POLES2002.SHP was created using latitude and longitude of the location, as Y and X fields (unprojected units). The usage of unprojected coordinates enables the application of a variety of map projections to the view. Figure 1a shows this theme overlapped with the world map in unprojected view. Each point represents a paleomagnetic result.

Fig. 1. The appearance of the Visual Palaeomagnetic Data base is shown: (a) a global view; (b) a regional view overlapping with a geochronological map; (c) a dialog window activated by the new identify tool.

Data Presentation

The wide variety of ArcView options enables the use of graduated colors, labels, and different symbols to emphasize ages, paleomagnetic directions, or other data features. For example, we use graduated colors in the theme legend to distinguish between the ages of magnetization, but users may change them as desired. The enlarged area in Figure 1b shows data points as arrows directed according to the paleomagnetic declinations (additional table field ROT was created for this). The transformation from dots to arrows occurs when the scale of the view becomes 1:25000000. For this purpose, the theme POLES2002.SHP is opened twice (Figure 1) with different Display properties.

The user should keep one of these themes active during the operations described below. The user may also label the data points with paleolatitudes, rocktypes, etc. A new identify tool (Figure 1b) was created using Dialog Designer (ArcView extension). The identify window (Figure 1c) contains all necessary information; however, the user can easily modify this window using Dialog Designer. Apart from the obvious advantages of the visual presentation of the GPMDB, some mistakes in the location coordinates for old paleomagnetic results are easily identified.

Data Manipulation

The ArcView Query Builder Tool provides all "traditional" opportunities for data queries similar to those used in MS Access and other systems. The user can make these queries both in SQL and interactively. In addition, the ArcView Menu Item Theme/Select by theme allows more complicated, "geographically-oriented" queries, such as the selection of all paleomagnetic results located inside a polygon. This would be quite a complicated task in the MS Access data base.

In addition to the basic ArcView items, we wrote several Avenue scripts to provide more opportunities. These utilities are available through a new menu item, Palaeo (Figure 2). Most of them can also be activated through additional buttons (Figure 2).

Fig. 2. Visualization of paleomagnetic poles is shown: (a) how to obtain poles; (b) how to show the ovals of confidence; (c) a dialog window activated by applying the identify tool to the Poles.shp theme.

Paleomagnetic data bases contain a lot of angular values, so they can be visualized in ArcView. One of the first things the user may want to look at is the geographical distribution of paleopoles for the selected data. For this, the user should make the selection of data either by hand or using one of the methods described previously. If no selection is made, the following procedure will be applied to the whole theme; this is also true for all of the operations described below. For this type of operation, choose the Palaeo/Display Poles menu item, or just click on the P button (Figure 2a). A new point theme, Poles.shp, will appear in the View (Figure 2a). The main information about these poles (Resultno, Rockname, Place, MinAge, MaxAge, Plat, Plong, Dp, Dm, Author, and Year) can be immediately printed or stored in the clipboard for use in spreadsheet---for example, Excel---format using the menu item Palaeo/Print List of Poles. It is also possible to see the ovals of confidence for some of these paleopoles. For this, the user needs to select some poles and choose the Palaeo/Put Ovals for Selected Poles menu item (Figure 2b). A new polyline theme, Ovals.shp, and a point theme, Sel-pol.shp, will appear (Figure 2b). The whole data base information is available for those new point themes (Poles.shp and Sel-pol.shp) through the new identify tool (Figure 2c). It is recommended that users delete themes Poles.shp, Sel-pol.shp, and Ovals.shp each time after finishing using them.

Any data base user wants to make quick calculations and diagrams from the results of a search. Figure 3 illustrates such possibilities. Suppose that the user is interested in the paleomagnetic results from the Cape Verde Islands. He selects their locations with the ArcView Selection Tool and chooses the Palaeo/Stereoplot menu item, or clicks the S button (Figure 3). A new view Stereoplot appears on the screen. It is a projected view in the Stereographic Polar Projection representing the distribution of paleomagnetic directions. It contains two point themes, Dirsn.shp ("normal" directions with downward inclinations) and Dirsr.shp ("reverse" directions with upward inclinations), painted in red and green, correspondingly (the user can easily change those colors). The polyline theme ster.shp represents the stereonet. The user must activate one of these themes. Then the menu item Palaeo/Mean-direction and mean pole or the A button should be used to calculate the mean values for any selected sub-array (Figure 3). Each point in the Dirsn.shp and Dirsr.shp themes maintains all data base information about the particular paleomagnetic result.

Fig. 3. Stereoplot and mean direction for selected data are provided.

This information may be retrieved by the new identify tool (Figure 3). In the illustrated case, the data are uni-polar, so the user may want to calculate an overall mean direction. For this, the user should make both the Dirsn.shp and Dirsr.shp themes active and merge them into the new theme (a request for a name will be prompted) using either the menu item Palaeo/Merge vector distributions or the M button. Then the user needs to apply the menu item Palaeo/Mean direction and mean pole or the A button to this new theme (or any selected sub-array). If both polarities are present, the user may apply the Palaeo/Invert or the I button to Dirsr.shp, creating a new theme in the Stereoplot view, merge this theme with the Dirsn.shp theme, as described above, and then calculate the mean direction. Very complicated situations may occur, but almost all of them can be resolved by using proper selection, inversion, and merging. We recommend deleting all of the themes in Stereoplot view except Dirsn.shp, Dirsr.shp, and ster.shp after using them.

Tectonic Applications

One of the advantages of using ArcView is the possibility of combining paleomagnetic data with digital geologic, tectonic, paleo-environmental, and other maps. One such example is shown in Figure 1b.

Two additional scripts for the spherical rotations of polygons, polylines, and points are also available in this application. These utilities enable the user to make plate tectonic reconstructions in the frame of this application with or without paleomagnetic data. Torsvik and Smethurst [1999] described basic rules for such reconstructions. Our utilities provide the possibility of applying these rules to ArcView shapes. If the Euler parameters of the spherical rotation are known, the user can apply them to any theme (polygon, polyline, or multipoint), or to the selected features in this theme. For this, the user needs to activate this theme, select features (as usual, the whole theme will be rotated if no selection is made), and choose the Palaeo/Rotate shape menu item, or click the R button (Figure 2). The latitude, longitude, and angle of rotation (counterclockwise is positive) should be entered into three consequent dialog windows. If only the paleomagnetic pole is known, the Palaeo/Palaeomagnetic Rotation menu item or corresponding button (Figure 2) must be used. Two dialog windows will appear, one after another, for the interactive entering of the latitude and longitude of the paleopole. For this case, we used the same algorithm as Torsvik and Smethurst [1999] in their GMAP software (Virtual Geomagnetic Pole Reconstruction). It places the studied theme into its paleo-position determined by the chosen paleomagnetic pole, with the "longitudinal uncertainity." Both methods may be applied to location points, poles, ovals of confidence, geological polygons, lines, etc.

The conversion of the GPMDB into its visual form in the frame of an ArcView application accelerates the search of the relevant paleomagnetic data, their quick analysis, and enables geological and tectonic applications. ArcView, with its incorporated paleomagnetic data base and additional Avenue scripts, enables the parallel use of paleomagnetic data and geological digital information for plate tectonic reconstructions. The visual GPMDB and utilities are available in the form of ArcView project by request at spisarev@tsrc.uwa.edu.au. It must be used with ArcView 3.1, 3.2, or 3.3. The ArcView Dialog Designer extension is necessary.

Acknowledgments

This research was supported by the Australian Research Council (ARC) through its Research Centres program and by U. S. National Science Foundation grant EAR-0003374. This is Tectonics Special Research Centre publication no. 217, and a contribution to International Geological Correlation Project 440.

References

McElhinny, M. W., and J. Lock, Global paleomagnetic data base supplement number one, update to 1992, Surv. Geophys., 14, 303-329, 1993.

McElhinny, M. W., and J. Lock, IAGA paleomagnetic data bases with Access, Surv. Geophys., 17, 575-591, 1996.

McElhinny, M. W., and P. L. McFadden, Paleomagnetism: Continents and Oceans, Academic Press, San Diego, California, 386 pp., 2000.

Torsvik, T. H., and M. A. Smethurst, Plate tectonic modelling: Virtual reality with GMAP, Computers and Geosciences, 25, 395-402, 1999.

Van der Voo, R., Paleomagnetism of the Atlantic, Tethys and Iapetus Oceans, Cambridge University Press, Cambridge, 411 pp., 1993.