Geochronology, meet GIS: reconstruction of detrital zircon data using Euler poles and implications for Appalachian terrane interactions.

Detrital zircon data for the Appalachian-Caledonide Orogen comprise tens of thousands of analyses from several hundred samples that span the Neoproterozoic and Paleozoic eras. These data provide extraordinary insights into the history of terranes in the orogen, because the candidate source continents Laurentia, Gondwana, and Baltica display contrasting histories. For example, the varied arrival of Ganderia-derived terranes at the Laurentian margin is constrained by the influx of abundant ~1 Ga zircon from the Grenville Orogen of Laurentia. These large data sets can be managed by handling the data in a geographic information system (GIS). An initial spreadsheet compilation of data incorporated locations and estimated depositional ages (Ma, with errors) for each sample, and used standardized concordancy cutoffs and isotopic system selections. Samples were assigned to terranes based on published information, or by spatially joining them with a terrane map. Data were binned into time intervals corresponding to known potential source orogens. Colours were assigned to each bin so as to be distinguishable by those with diverse colour vision. The resulting tables were displayed as pie charts sized in proportion to the number of dated grains using free GIS software (QGIS). An overall trend from Laurentia-to Gondwana-derived is immediately apparent across the orogen from NW to SE. The entire data set was saved in shapefile format. The free software GPlates allows import of shapefiles. Usually, these are polygons describing continents and terranes in paleocontinental reconstructions. GPlates transforms these using Euler poles, according to plate rotation models. It was found that GPlates would also import the detrital zircon point data and transform it using the same rotations. The resulting paleocontinental models, exported back to QGIS, were used to display detrital zircon data sets in their reconstructed positions at their depositional ages, providing a powerful test of proposed reconstructions. [ABSTRACT FROM AUTHOR]