STRAIN ANALYSIS OF SYNTECTONIC GRANITE INTRUSIONS EAST OF THE NORUMBEGA FAULT ZONE AT PEMAQUID POINT, MID-COAST MAINE

The Pemaquid Point area of mid-coast Maine is 30 km SE of the main zone of the dextral Norumbega fault system. Regional strain accommodation has resulted in initially orthogonal granites that were rotated CCW to oblique asymmetric boudin strings due to sinistral as opposed to dextral shear. Other granites were intruded along the metamorphic layering and were then elongated parallel to a regional near-horizontal stretching lineation forming symmetric layer parallel boudin strings. Seven sites within the Pemaquid exposures were mapped using total stations, survey grade RTK GPS, and handheld mapping grade GPS to study this boudinage. The data were displayed and edited using ArcGIS and ArcView software to better analyze the regional deformation. Surface area reconstruction was used to estimate elongation reflected in both symmetric and asymmetric granite boudinage. Geometric analysis of boudin strings relative to the fold limb layering (beta angle) was used to determine minimum apparent gamma values as a measure of shear strain, assuming a simple shear model. The string method was used in the field while asymmetric boudin reconstruction was used in the lab to estimate layer normal shortening during shear. The shear strain calculations determined using beta angles yielded an average gamma value of 3.02 and ranged from 0.64 to 10.07. Elongation values for the oblique asymmetric granite boudins averaged 209% and varied from 35-520%. The layer parallel symmetric boudin elongations averaged 178% and ranged from 14-416%. Asymmetric boudin reconstruction yielded an average layer-normal shortening of 50%, ranging from 32%-81% compared to the outcrop string method, which yielded a 60-65% layer-normal shortening. Dextral shear strain estimates from last year's work in the Casco Bay region in closer proximity to the main shear zone, were higher compared to the sinistral shear strain estimates from the Pemaquid study area. The layer normal shortening values for both study areas are in a similar range. The change in shear sense can be explained by either the oblique fold rotation model (Swanson, 1999) or the lateral (southerly) escape model for a midcoast block between the dextral Phippsburg and sinistral Pemaquid area shear zones during regional strain accommodation associated with dextral Norumbega shearing.