LITHODEMES, PSEUDOTACHYLYTE, ULTRAMYLONITE, MYLONITE AND STRIPED GNEISS WITHIN THE NORUMBEGA FAULT SYSTEM, SOUTH–CENTRAL AND CENTRAL MAINE

The Norumbega Fault System in portions of south – central and central Maine consists of complexly deformed, dextrally sheared units assigned to, or correlated with formations of the Casco Bay Group. In portions of south – central Maine the system is approximately 14 km wide. The fault system separates metasandstones of the Central Maine sequence on the northwest from rocks of the coastal lithotectonic belt on the southeast. Stratigraphic integrity of several Casco Bay formations within the system has been lost due to deformation. Several of these compositionally distinctive, but intensively deformed, units had previously been mapped and interpreted as stratigraphic assemblages. These were variously assigned to the Nehumkeag Pond, Cape Elizabeth, Scarboro and other formations. In south – central Maine at least two stratigraphic units are disrupted whereby small remnant metasandstones with probable relict bedding are enclosed within metapelites. A Casco Bay Group correlative in central Maine is commonly disrupted and contains rare inclusions of medium – grained biotite – garnet schist; garnet – bearing quartzo - feldspathic gneiss and amphibolite.

Additionally, within the system there are migmatite or migmatite – like rocks with an along strike length of 50 to 80 km that are one of the more significant regional units within this area. Within the sheared migmatized outcrop, the mylonites are coarser grained with porphyroclasts up to 1 cm. Granitoid boudins of varying size commonly exhibit a sigmoidal shape consistent with dextral shear. Striped gneiss is uncommon with the exception of a 160 to 240 meter – wide zone which forms the southeastern boundary of the Norumbega Fault System with the coastal lithotectonic belt. The disrupted units and migmatite qualify as lithodemes following the definition of the North American Stratigraphic Code.

Casco Bay Group formations in south – central Maine exhibit a variety of fault generated structures such as pseudotachylyte, phyllonite, mylonite and ultramylonite. Casco Bay Group correlatives in central Maine are locally characterized by numerous, thin pseudotachylyte injection veins. Ultramylonite in south – central Maine ranges in width from a < 15 cm to ~ 40 m. Cross cutting relationships and other features strongly suggest that some of the ultramylonite represents deformed pseudotachylyte.