MAGMATISM ALONG THE NORUMBEGA FAULT SYSTEM CORRIDOR IN MAINE: A REVIEW IN HONOR OF SHEILA SEAMAN

The orogen-parallel Norumbega fault system (NFS) extends the length of Maine from New Brunswick to New Hampshire and includes a swath of dextral shear deformation tens of kilometers wide. Since its recognition nearly 50 years ago, controversy has surrounded its tectonic significance, including whether it played a role in the generation and/or ascent of magma during its long history of displacement (Middle Paleozoic-Mesozoic). In this contribution, we review the basics of igneous activity along the length and width of the structure, some of which may be related to the NFS; attempts to correlate magmatism with deformational processes await further studies.

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.