MAGMATISM ALONG THE NORUMBEGA FAULT SYSTEM CORRIDOR IN MAINE: A REVIEW IN HONOR OF SHEILA SEAMAN
Prior to the initiation dextral shear deformation associated with the NFS, rocks of the Middle to Late Ordovician Liberty-Orrington belt formed outboard of Laurentia in an evolving volcanic arc- to back-arc tectonic setting. Initial accretion of these rocks occurred in the mid-Silurian and this signaled the beginning of a nearly 75 million year period of widespread felsic to intermediate plutonism along the length of the NFS corridor. This stage of plutonism includes a curious period of Late Silurian-Early Devonian ultra-potassic magmatism along much of the length of the currently mapped NFS, although this also likely pre-dated initiation of NFS-related dextral shear deformation. Magmatism later in the Devonian (i.e., post 400 Ma) and extending into the Early Carboniferous was common along the length of the NFS and there has been speculation it may have been related to the initiation and prolonged evolution of the NFS (e.g., Kuiper, 2016). Permian felsic magmatism (peraluminous granites and pegmatites) in the Casco Bay restraining bend of the NFS are locally abundant. Finally, significant Mesozoic magmatism, both in the form of Late Triassic-Early Jurassic basaltic dikes and small Cretaceous alkalic mafic plutons are found along the NFS corridor in southern Maine.
Although episodic in both time and space, the area currently occupied by the NFS contains a wide variety of igneous rocks spanning over 350 million years of time (480 to 120 Ma). Pre-Devonian magmatism clearly pre-dated the initiation of the NFS; connections between the later magmatism and deformation associated with the NFS remain key themes of current research.