STRUCTURAL EVIDENCE FOR SOUTHWEST-DIRECTED SUBDUCTION IN THE EARLY ORDOVICIAN ROCKS OF THE CAUCOMGOMOC LAKE INLIER, NORTHERN MAINE

Preliminary structural analysis of the deformed rocks of the Caucomgomoc Lake inlier of northern Maine indicates west-southwest-directed subduction during the early Ordovician. The inlier exposes Ordovician and Devonian metasedimentary and metavolcanic rocks correlated with pre-Silurian subduction mélange (Hurricane Mountain, Chase Brook, and Grand Pitch formations) and associated rocks of the Lobster Mountain, Weeksboro-Lunksoos, and Munsungun anticlinoria. Two broad deformational events are exposed at the mesoscopic scale. Based on the tectono-stratigraphic correlations with other northern Maine inliers, the first event is assigned an Early Ordovician age and resulted in an early, steeply-dipping, pervasive foliation sub-parallel to relict bedding, rare isoclinal fold axial planes, and ductile shears displaying asymmetric shear-sense indicators and mineral lineations that plunge moderately to the west-southwest. The early structures are deformed by younger (Acadian?) chevron folds with axial planes marked by a subtle spaced cleavage and fold hinges that plunge moderately to the southwest. Retrodeforming the Acadian(?) chevron folds results in a southwest-dipping foliation plane that shows southwest-over-northeast shear sense. The Caucomgomoc inlier lies nearly along-strike to the northeast of the Chain Lakes Massif, but similarities with the Lobster Mountain Anticlinorium suggest an outboard (eastward) paleogeographic position, relative to the strike of the massif. The presence of metamorphosed Cambrian-Ordovician sandstones, wackes and conglomerates in the inlier (Caucomgomoc Lake and Hurd Mountain formations), coupled with the tectonic transport direction are consistent with a southwest-dipping, Early Ordovician subduction zone near the eastern margin of the Chain Lakes Massif.

Along the western side of the inlier, the Avery Brook Formation is more highly deformed, metamorphosed and marked by normal-sense shear indicators (down to the southwest) in the plane of the early Ordovician foliation, suggesting syn- or late subduction-related exhumation.