SPATIAL ANALYSIS OF COMPLEX FOLD STRUCTURES ON SEGUIN ISLAND, MAINE

Seguin Island lies ~12 km SSW of Georgetown, in mid-coast Maine, SE of the Norumbega shear zone. Regional geology is dominated by several major Devonian-age F-2 phase anticlines and synclines. Seguin Island outcrops on the eastern flank of the south-plunging Robinhood Cove syncline and the Ordovician Cape Elizabeth Formation amphibolite (Hussey 1978), which extends throughout the region, is the dominant lithology. Orientations of beds, fold axes, and axial planes were measured using Bruntons, and positioned with Trimble GeoXT GPS. Bed trace form lines were mapped in the field using Trimble 5700 RTK GPS at the southern end of the island and digitized from aerial imagery for the rest of the island. Folds outcropping on NE Seguin show the upright, tight to isoclinal structure that is indicative of the regional F2 folding. Beds in this area are oriented at ~N14°E/62°NW, with fold axes and axial planes averaging S43°W/48° and N24E°/83°SE respectively. Detailed form lines collected from SW Seguin depict a complex fold structure that varies significantly from the regional pattern and includes a ~10 m wide SW-plunging synform adjacent to a ~15 m amplitude, Z fold structure, both with complex and highly-variable axial plane orientations. Attributes of structural measurement data points were modified to include these variations for use in IDW interpolation analysis. This analysis shows that: 1) bed orientations vary as expected around a fold nose; 2) SW-plunging fold axes vary little in orientation suggesting all folds are related, and; 3) the maximum variation in axial plane orientation is localized along the SE shore. Possible explanations for these relationships include: a) polyclinal folding, as illustrated by the range in axial plane orientations of these folds with a consistent fold axis attributed to room problems and layer-parallel shortening within a plunging fold nose, and b) distortion related to a hypothesized fault that divides the island into two lobes. An island-wide form line map incorporates the regional structure and illustrates the overall parasitic antiform/synform features of Seguin Island. However, a more detailed island data survey and spatial analysis will be needed to assess whether differences in fold characteristics between the two lobes of the island may be related to such an inferred fault zone.