ADVANCES IN UNDERSTANDING THE COMPLEX AND PROTRACTED TECTONIC AND DEFORMATIONAL HISTORY OF THE MUNSUNGUN–WINTERVILLE BELT IN THE MAINE APPALACHIANS

The Munsungun–Winterville Belt (MWB) is a major lithotectonic belt in the Northern Appalachians. It is mostly an ensialic Ordovician volcanic arc developed on the leading edge of the Ganderia. Detailed and reconnaissance mapping show that the MWB is wider than previously thought and includes 11 inliers, including the Munsungun and Winterville inliers. Several small inliers in the headwater of the East Branch Penobscot River and Aroostook River also belong to the MWB. The MWB correlates to Caucomgomoc inlier, Chesuncook “dome”, and Lobster Mountain “anticlinorium”, as part of the Bronson Hill–Popelogan arc. Its SE boundary is in hidden tectonic contact with the Weeksboro–Lunksoos Lake Belt, a Cambrian–Early Ordovician belt characterized by Early Ordovician (481 – 485 Ma) bimodal volcanics formed in ensialic rift setting. The MWB is largely a complex of imbricated stack of multiple NE-striking reverse-thrust faults resulted from a prolonged faulting history from the earliest Salinic Orogeny to the Neoacadian–Alleghanian orogenies, with the NE-striking, SE-directed post-Acadian reverse faulting dominating the MWB. Several small inliers even occur entirely as fault blocks. The MWB is dominated by several petrographically and geochemically distinct volcanic units derived from different sources. Most of them exhibit calc-alkaline or tholeiitic arc signatures, with the remaining lesser ones having tholeiitic non-arc affinity. Zircon U-Pb ages of the calc-alkaline and tholeiitic arc volcanics show a generally NW-younging trend across the MWB, from 471 to 451 Ma, indicative of northwestward trench-arc migration, probably associated with a prolonged SE-dipping, retreating subduction system between Laurentia and Ganderia. The tholeiitic non-arc volcanics were produced in extensional setting (back-arc or intra-arc rifts) and probably associated with the NW-directed Brunswick Subduction System from Late Ordovician to Early Silurian. Detrital zircon age spectra and sedimentary features of several Upper Ordovician clastic successor basin formations deposited on the Munsungun–Winterville arc point to a Laurentian provenance, suggesting that, after accretion of the Munsungun–Winterville ensialic arc to the Laurentia margin at a Late Ordovician time, the area began receiving sediments from Laurentia.