THE NORUMBEGA FAULT SYSTEM: FORTY YEARS OF STUDY DIRECTED AT UNDERSTANDING 250 MILLION YEARS OF TECTONIC ACTIVITY

The Norumbega fault system (NFS) of the northern Appalachians is one of the most extensive and longest-lived structures in the Appalachian orogen, yet its tectonic significance has been debated for decades. Isolated faults had been mapped previously in southern, south-central, and eastern Maine but Stewart and Wones were the first to use the term “Norumbega” (in a 1974 NEIGC field guide) for a fault zone south of Bangor. Within ten years, the Maine bedrock map showed a continuous zone of northeast trending faults extending the entire length of the state. Fueled by Cordilleran exotic terrane models, the controversy surrounding the NFS at that time was whether it represented the elusive suture between ancestral North America and exotic terranes (the answer is “No” at the present erosional surface).

Controversy in the mid-80s to late 90s shifted to what structural features should be included in the NFS. This had implications for defining the width of the system, the amount and timing of displacement attributed to the system, and ultimately the role of the NFS in northern Appalachian tectonics. That debate continues today. The minimalist NFS is <10 km wide comprising a few narrow (< l km) zones of post-Devonian faulting and mylonitization that accommodated relatively small amounts (< 30 km) of predominantly dextral displacement. The broader view is of a much wider zone (up to 40 km) of distributed ductile deformation that was associated with an earlier (mid-Devonian) and more long-lived period of regional dextral transpression. Additionally, thermochronology reveals time-temperature discontinuities across parts of the system indicating significant late Mesozoic dip-slip reactivation.

Detailed mapping continues to reveal new zones of deformation and more work is needed to determine the details of timing and displacement. Additionally, the NFS remains an important venue for process-related work. Structures preserved at the present erosional level along the NFS are the result of superimposed deformational processes that formed at a variety of depths over tens of millions of years and thus provide a window into processes occurring deep beneath modern seismically active zones. While the tectonic role of the NFS will likely remain controversial, the natural laboratory it provides for fault studies may be its more lasting legacy.