THE MONROE HIGH STRAIN ZONE: A NEWLY RECOGNIZED FEATURE IN THE NORUMBEGA FAULT SYSTEM OF SOUTH – CENTRAL AND CENTRAL MAINE

Fault generated rocks near Monroe, ME represent a newly recognized major strand of the Norumbega Fault System. The fault zone is at least 55 km long and averages 1 km wide. Approximately 3 km north of the village of Monroe, seven distinct strike – perpendicular subdivisions are recognized in a large gravel pit. From southwest to northeast these are: 1) mylonite with feldspar, quartz and granitic porphyroclasts; 2) greenish black and dark greenish gray ultramylonite; 3) greenish black and dark greenish gray mylonite with feldspar and quartz porphyroclasts and sheared granite boudins and dikes; 4) cohesive quartz vein breccia; 5) rusty weathering, ultramylonite and fine – grained phyllonite; 6) ultramylonite with abundant pseudotachylyte veins; and 7) ultramylonite lacking pseudotachylyte. Protoliths for the mylonites and ultramylonites in this outcrop area are interpreted to have been a) Passagassawaukeag migmatite; b) mafic and/or calcsilicate rocks of the Scarboro Formation(?) and c) granofels and schists of the Cape Elizabeth Formation.

The termination of the zone is complex. At the southwestern end, the zone lies within rocks currently assigned to the Cape Elizabeth Formation. Here the zone thins to between 300 and 400 meters which then divides into two or three narrow mylonites, some of which have well developed pseudotachylyte. The northeastern termination cuts the Winterport granite. In this area the Winterport is sheared and extended several kilometers into an approximate 300 meter wide zone of granite mylonite. Further northeast the fault zone continues to thin and is recognized as a narrow (<100 meters?) boundary between the Cape Elizabeth and Copeland formations.

Pseudotachylyte veins are the youngest structure; they occur along the entire length of the zone; and are most common in the mid regions. They formed after peak metamorphism, within in the chlorite zone. The pseudotachylytes exhibit a variety of geometries and represent multiple paleoseismic events. The abundances and cross cutting vein relationships demonstrate the significant paleoseismic history for this zone.