KRIGING AS A TOOL FOR INTERPRETING STRUCTURAL DATA: EXPLORING SPATIAL ANALYSIS OF COMPLEX FOLDING ON SEGUIN ISLAND, MAINE

Seguin Island, located ~12 km SSW of Georgetown, Maine and SE of the Norumbega fault zone consists of Devonian upright F2 anticlines and synclines, with complex parasitic fold structures. Ordovician Cape Elizabeth amphibolite gneiss and syntectonic granite dikes are exposed over the entire coast of the island. Structural data was collected in the NE zone of the western lobe where the folds are best exposed to assess the results of the spatial analysis of collected field data. Orientations of gneissic layering, axial planes, and fold axes were taken with a Brunton compass and recorded with handheld GPS. Gneissic layer lines were traced 2-3 m apart using RTK GPS and Total Stations to delineate the exact fold geometry, and interpreted extensions were digitized in the lab. The fold structures were found to be tight, SW-plunging with NNE striking axial planes, a wavelength of ~10m and amplitude of ~5m but can be highly variable with smaller scale parasitic folds abundant. Kriging, a spatial analysis technique that interpolates values between measured points was used to analyze the structural data. The results of kriging are based on a selected density of data points, or kernel, which best represents the entire set. Effective selection of these points requires knowledge of variations in the dataset. Strike and trend structural measurements were normalized to an 180^o scale and divided into nine classes to best display the data. After kriging, the data was converted into a histogram with breaks that were manually shifted to reclassify the data. These breaks were adjusted to better represent the variance of data from the mode in each set. Strong correlations were found in the strike of gneissic layers, and axial planes, and the trend of fold axes. Strike of gneissic layers alternated NNE to NE orientations, suggesting differences in limb orientation across anticline and syncline fold axes. All dip and plunge measurements also became less steep from N to S, clearly visible in the kriging results. Kriging produced interpolated results that clearly reflected the structure of the local area, providing a useful means of visualization. Kriging can provide valuable insight into prevailing structural patterns, correlating features across a given area, and results in a representation of geologic data that is interpretable on an entirely new level.