Exotic terranes in the New England Appalachians—limits, candidates, and ages: A speculative essay

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Abstract
Recent discoveries in the North American Cordillera of composite exotic terranes that had become accreted to the Cordillera during its evolution require reexamination of the older Appalachian mountain systems for evidence of possibly similar history. In the New England segment of the Appalachian orogen, the three Paleozoic orogenies (Taconian, Acadian, Alleghanian) must be separately examined. Evidence for Taconian orogeny supplies the best support for subduction processes at the margin of a continent-ocean plate junction. Definition of ancestral North America prior to the completion of that subduction process is the starting point for a search of Taconian exotic terranes. On the basis of such criteria as age of basement, occurrence of in-place ophiolite, melange, blueschist, continental-margin facies, and island-arc rocks, this margin is proposed to be best preserved in northern Maine, where it runs from the Jim Pond-Boil Mountain ophiolite south of the Chain Lakes massif northeast to the Elmtree ophiolite in New Brunswick. Rocks of the Weeksboro-Lunksoos Lake and Miramichi anticlinoria are southeast of this boundary. In Maine, this boundary, which was the trace of a subduction zone, was marked by a residual marine basin in Late Ordovician and Early Silurian time. No Taconian accreted terrane has been detected on the North American craton side except for the Chain Lakes massif, which is suggested to be an obducted allochthon derived from the opposite side of Iapetus Ocean; this opposite side is labeled “Craton X” and is otherwise largely unknown. The Merrimack synclinorium is interpreted to have formed on Craton X. Acadian orogeny probably resulted from a continent-continent collision. The nature and extent of the Silurian and Devonian flysh sequences demand basins of deposition much larger than present geologic relations allow; these sequences may or may not be in mutual sedimentary contact, and may not have been even before their deformation and metamorphism. This fact and the anomalous paleomagnetic pole position for the Merrimack synclinorium suggest possible large-scale tectonic transport during the Acadian orogeny. In that sense, the terrane now occupied by the synclinorium may be exotic, both because its basement was originally Craton X and because the Taconian suture may have been disrupted by younger longitudinal transport of unknown extent. The coastal belt of Rhode Island, Massachusetts, and Maine contains rocks in distinct lithotectonic blocks. These blocks are best defined in northeast Massachusetts and around Penobscot Bay in Maine, where they are mutually separated and also separated from the Acadian version of North America by large faults. These blocks appear to be exotic; they may have arrived at their present locations since the peak of the Acadian orogeny and thus have been largely unaffected by it. This coastal belt includes the Avalonian terrane; it may have been emplaced during latest Acadian to early Alleghanian deformations. If the Avalonian terrane did arrive late, then it could not have constituted Craton X during the Taconian event. The three Paleozoic orogenies led to three types of accreted terranes: (1) Taconian, thrust allochthons directly attributable to subduction-induced collision during the closing of Iapetus Ocean; (2) Acadian, continent-continent collision and possible large concomitant transcurrent displacement; (3) Alleghanian, oblique-slip high-angle faulting, the concomitant formation of a sedimentary basin having no immediately identifiable sediment source, and the formation of a microplate collage. For ancient mountain belts, the detection of microplate accretion is at best difficult. The use of a combination of geological, geochemical, and geophysical methods is necessary. Sedimentologic analysis may furnish the best clue to the arrival of new terranes; criteria to detect root zones of transcurrent faults are needed. Geochemical study may lead to definition of discrete blocks and the nature of sutures between them. Geophysical data are generally corroborative rather than definitive; even paleomagnetic data need geologic confirmation and are best used to sniff out suspect land and eventually to define the extent of motion. The hard middle part of establishing an exotic terrane must remain a geologic task.

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