LEFT-LATERAL MEGA KINK BAND STRUCTURES OF CASCO BAY, MAINE

Lower Goose and Little Whaleboat Islands, located approximately 4.5 km SE of the Flying Point Fault inside the Norumbega Fault Zone in Casco Bay, Maine, are part of the Early Ordovician Cushing Formation, characterized by steeply-dipping, strongly-lineated grey gneisses. Island exposures reveal large-scale left-lateral strike-slip kink bands with an average orientation of N74E 67SE, a width ranging from 70 to 100 m, and an estimated length of ~1 km. Metamorphic layers, inside and outside the kink bands, and the kink band boundaries were mapped using Total Station and RTK GPS survey equipment, and orientation measurements of gneissic layers, lineation, kink band boundaries, and kink band fold axes were made with Brunton compasses and handheld GPS. Low elevation aerial photos of the outcrop surfaces were taken using a 14 m camera pole with a 9 mpx digital camera and mosaicked for expanded visual representation. Survey data were plotted using GIS, and stereonets were used to analyze the geometry and orientation of the mega kink bands. The unkinked host rock has an average orientation N30E 67SE with lineation 15 S29W. Dextral shear parallel to these gneissic layers is evident by the clockwise rotation of initially orthogonal-to-lineation quartz veins. The quartz veins were deformed and rotated under simple shear about a reconstructed rotation axis at 73 S60E. Deformed layers within kink bands indicate counter-clockwise rotation about a reconstructed fold axis of 67 S62E and a measured mean fold axis of 76 S63E. The deformed layers also exhibit fault striations at 10 S84W along the flanking kink band boundaries supporting the strike-slip interpretation. The strong correlation between the quartz vein rotation axis and the measured and reconstructed kink band fold axes suggests a contemporaneous development of left-lateral kink bands and regional dextral shearing. The left-lateral mega kink band structures present within the predominantly dextral shear zone exhibit strike orientations at high angles to the gneissic layers similar to antithetic R′ faults in simple shear. The geometry of these Late Paleozoic mega kink bands also appears to have a direct structural control on the emplacement of the Mesozoic Christmas Cove Dike as it cuts across Casco Bay with a NE-SW strike parallel to these left-lateral mega kink structures.