TECTONIC SETTING OF THE ACADIAN OROGENY REVIEWED IN THE CONTEXT OF SILURO-DEVONIAN STRATIGRAPHIC AND VOLCANIC ROCKS OF THE NORTHERN MAINE INLIERS

First-order questions regarding the Acadian Orogeny in the northern Appalachians are controversial and competing tectonic models for Acadian orogenesis have been presented in the geologic literature in recent years. Stratigraphic analysis, whole rock geochemistry, and Nd isotopic ratios of basalts and gabbros from the Siluro-Devonian inliers of northern Maine are examined to place constraints on the possible tectonic settings that have been proposed.

The late Silurian and early Devonian stratigraphic sequences of the northern Maine inliers (Ripogenus and The Forks formations and East Branch Group) indicate that quiet, shallow water sedimentation changed to deposition in a rapidly subsiding basin that was accompanied by mafic to intermediate magmatism (West Branch, Spider Lake, Fish Pond volcanics, and diorites of the Flagstaff Lake Complex) shortly before and during the onset of Acadian deformation in this area. Whole rock geochemistry of mafic volcanic and intrusive rocks and moderately positive Nd isotopic ratios (+2.3 to +3.8) can be, in part, assigned to within-plate arc, or back-arc environments. The magmas were likely derived either from an uncontaminated, moderately enriched mantle source, or from a depleted mantle source and were subsequently contaminated by continental crust.

Four tectonic models for the Acadian Orogeny are evaluated in the context of a subsiding basin and associated magmatism. These include 1) slab detachment during southeast-directed subduction of the Laurentian continental margin; 2) “Laramide-style” thrust basins above a shallow, northwest-dipping subduction zone; 3) back-arc extension followed by thin- skinned shortening above a northwest-dipping subduction zone; and 4) “Moluccan- style” dual-dipping subduction zones.