ALTERNATE Z-VALUE SURFACE ANALYSIS OF FABRIC ORIENTATION IN REGIONAL TRANSPRESSION RELATED TO DEXTRAL NORUMBEGA SHEARING, MID-COAST MAINE

Field measurement sites showing strike and dip symbols for metamorphic layering on a scanned and georeferenced Bath 1:100,000 geologic map were digitized as points in ArcGIS to create a data table of strike, dip and strike-angle relative to the NE trend of the main Norumbega shear zone. Regional strain accommodation adjacent to the Norumbega is reflected in tight upright folds oblique to the trace of the shear zone where continued strain led to clockwise rotation of these folds and layer-parallel shear concentrated in the steep planar limbs. The orientation of this regional layered fabric was studied to predict the likely location of discrete shear zones within the coastal block that flanks the Norumbega on the SE side. Selection of "alternate z-values" from an attribute database allowed surface modeling thru the generation of rastered images to illustrate the spatial distributions of the particular orientation characteristics. Color and interval selections help to highlight the observed spatial relationships. Applying map algebra using the raster calculator allowed the combination of two orientation characteristics to show the areas throughout the flanking coastal block with the most favorable orientation for shear according to the oblique fold model for regional strain accommodation during Norumbega shearing. This optimal orientation function selected the steepest dips 70-90⁰ and strike alignments closest to the 35-42⁰ oblique angle to the main shear. The optimal areas for shear appear as elongate zones that flank a wide mid-coast block of less favorable orientations. Fabric orientations with a high potential for shear on the west are coincident with the previously proposed Phippsburg dextral shear zone and on the east with sinistral shearing observed in the Pemaquid Point-Muscongus Bay area. This pattern of favorable orientation for shear coupled with field observations of kinematic indicators support a lateral (southerly) escape model for the mid-coast block during regional strain accommodation adjacent to the Norumbega fault zone.