CHARACTERISTICS OF THE NORUMBEGA FAULT ZONE, CENTRAL MAINE

The primary trace of the Norumbega fault zone in central Maine is characterized as a narrow structure which separates the Silurian(?) Central Maine Sequence on the northwest from the Coastal Lithotectonic belt on the southeast. A restraining bend of approximately 10 to 12 degrees is recognized in the area. The orientation changes from approximately N 55 E in the southwest to N 45 E in the northeast. In the southwestern area rocks of the Passagassawaukeag Gneiss are faulted against rocks previously assigned to the Vassalboro Formation on the northwest. In the vicinity of the bend the Bucksport Formation is faulted against rocks previously assigned to the Vassalboro Formation. Faults associated with the Norumbega in the Central Maine Sequence are of two types. The first is a brittle slate – rich fault gouge whose surfaces lack clear slip - sense indicators. These gouges are contained within slate –rich portions of the Central Maine sequence. The second is a narrow (3 – 4 meter wide) shear zone consisting of metasiltstone phacoids exhibiting a dextral shear sense, and rare to uncommon pseudotachylyte. Fault surfaces of the second type locally exhibit sub horizontal lineations. Within well bedded sequences, metasandstone beds are relatively undisturbed. However, within the slate beds there are discontinuous, thin quartz veins or quartz boudins obliquely oriented to bedding. These may exhibit a pronounced asymmetry suggesting dextral shear. Faults within the Coastal Lithotectonic Belt are primarily seen as both sharp boundaries and disrupted zones. Bedding within the Copeland Formation is commonly disrupted and accompanied by numerous, thin pseudotachylyte injections and rare inclusions of medium – grained biotite garnet schist; garnet – bearing quartzo - feldspathic gneiss and amphibolite. Within the Copeland Formation indicators of both dextral and sinistral shear are observed at separate localities.