40 YEARS OF MAPPING AND STILL WE DON'T KNOW

Most tectonic models for the Northern Appalachians view the orogen through lenses focused on its western or eastern flanks. In the 40+ years since I first met Father Jim on an NEIGC trip, we have unraveled the stratigraphies of most terranes in Maine – in the core of the orogen -- but important questions from 1965 remain unanswered. 1. Relationships among the coeval Central Maine (CM), Aroostook-Matapedia (AM), and Fredericton (F) basins: Proximal/distal facies relationships are well-defined only for the western part of the CM belt. There is little control of basin geometry for AM and F despite local indications that the Miramichi terrane may have contributed to the AM (westward) and F (eastward). This permits antithetic single and multiple ocean tectonic models. Widespread sedimentation in CM continued while Acadian deformation and plutonism in F took place, leading this geologist to favor a multiple basin model, but any conclusion at this time is premature, partly because of 2. Possible disconnects between surface and basement geology: The USGS/GSC transect, other seismic data, and local structures suggest that some terrane boundaries are large scale thrust contacts and hint at mid-crustal blind thrusts. In addition, granite chemistry suggests that terrane boundaries do not coincide with those between the basement blocks that the plutons sample. How good is our understanding of the amount and nature of crustal shortening in the orogen? And in view of 3. The problematic role of orogen-parallel faults like the Norumbega fault system: The NFS is as long as the San Andreas and was sporadically active over 4x the span of the San Andreas, yet only records a fraction of the San Andreas offset. Is the NFS a reactivated pre-Silurian terrane boundary? To what extent are terrane boundaries obliterated/deformed by orogen-parallel fault systems? How well have we built late-stage dip-slip reactivations into our tectonic modeling?