A PROMINENT ZONE OF EARLY DEVONIAN CRUSTAL WEAKNESS AND MIGMATIZATION IN SOUTH CENTRAL MAINE

Recent 1:24,000-scale bedrock mapping in the Augusta 1:100,000 quadrangle in south-central Maine has delineated a high strain zone at least 40 km long and up to 3 km wide. This important zone of crustal weakness separates the Fredericton lithotectonic belt on the southeast from Casco Bay Group formations on the northwest. As remapped the high strain zone is subdivided into three map units, a large portion of which is migmatized and sheared sillimanite + K-feldspar grade metamorphic rocks. The zone locally includes metasedimentary rocks from both juxtaposed lithotectonic belts, as well as elongated orthogneisses (Haskell Hill and Mixer Pond), and the Lincoln Syenite, and younger unsheared granitoids.

In the north, rocks previously assigned to the Passagassawaukeag Formation, are now recognized to be sheared stromatic migmatites with local mylonite and striped gneiss. Surviving protoliths suggest the rocks were melted from a protolith similar in composition to the Cape Elizabeth Formation through dehydration melting of muscovite with melt assisted strain localization. The central portion of the zone narrows to a width of about 1.5 km, and for a distance of nearly 30 kilometers is characterized by more plutonic rock involvement. In particular, the southeastern margin of the 418 Ma Lincoln syenite is strongly deformed and recrystallized, and immediately adjacent Fredericton belt rocks (Bucksport Fm.) are strongly sheared and contain a greater proportion of pegmatite sills. At the southern end of the belt, the Dyer Long Pond complex partially consists of migmatite which largely resembles the Passagassawaukeag, but also includes the 408 Ma Haskell Hill granite gneiss, as well as rocks derived from Bucksport Formation of the Fredericton sequence.

The coincident abundance of migmatite, high strain, and strike parallel sillimanite + K-feldspar grade metamorphism in a narrow zone of thermally weakened crust is important in reconstructing the tectonic history of the region and contributes to understanding the preserved boundary between two important lithotectonic belts. Based upon previously dated plutons, migmatization and shearing of the belt is younger than Haskell Hill (408 Ma) and Lincoln Syenite, but pre-dates a 400 Ma unnamed, undeformed pluton, as well as the North Searsmont (389 Ma) and Mt. Waldo (371 Ma) plutons.