Polyphase deformation and metamorphism in the Penobscot Bay Area, Coastal Maine

Abstract

Metasediments from portions of two of the six lithologic terranes in the Penobscot Bay area of coastal Maine, the Late Precambrian (?) to Ordovician(?) Copeland Formation and the Rider Bluff member of the Copeland Formation at the eastern margin of the Passagassawakeag (PGW) terrane and the Silurian (?) to Devonian (?) Bucksport Formation of the adjacent Bucksport terrane, shared a common history of four phases of ductile deformation (D₁, D₂, D₃, and D₄) and interdeformational metamorphism and granitic plutonism. D₁ (early Acadian ?) produced tight to isoclinal folds and an axial plane foliation that deform all three metasedimentary units and contacts between these units. These units and therefore the PGW and Bucksport terranes must have been together by the time of D₁ at the latest. No observed structural or metamorphic gradient coincides with this terrane boundary in the Penobscot Bay area, but in southeast Maine it is mapped as a pre-metamorphic thrust fault. Intrusion of the 412± 14 Ma Stricklen Ridge granite and amphibolite facies metamorphism occurred between D₁ and D₂ . D₂ (Acadian) produced open, upright folds and a strong, hinge-parallel mineral elongation lineation under upper greenschist to lower amphibolite conditions. D₃ ductile strike-slip shear zones formed under upper greenschist to lower amphibolite conditions, and D₄ open, steeply-reclined folds formed under greenschist facies conditions. D₃ and D₄ are probably related to the same stress system as the dextral strike-slip Norumbega Fault Zone (Alleghanian) and probably represent a continuum of deformation. Brittle faults exploit D₃ shear zones and may also be related to the same stress system.