Kinematics of Acadian Deformation in the Northern and Newfoundland Appalachians

Acadian tectonothermal effects in the northern and Newfoundland Appalacians are heterogeneous with respect to distribution, intensity, and timing. This complex orogenic imprint has been difficult to explain with tectonic models involving a single kinematic regime. In north-central Maine a two-phase kinematic scheme can account for the complicated orogenic pattern. Early phase Silurian structures related to an ENE-trending sinistral shear zone are interpreted to reflect ~N-S-directed shortening oblique to the orogen. Later phase Early-Middle Devonian "classical" Acadian NE-trending upright folds with penetrative axial planar cleavage are interpreted to indicate ~NW-SE-directed shortening and thus record a significant Early Devonian rotation of kinematic axes. Early phase, dominantly Silurian, structures related to ~N-S shortening are also documented in many other areas of the northern and Newfoundland Appalachians, and these are consistently overprinted by a later, Devonian NW-SE shortening event; thus a two-phase kinematic pattern is recorded throughout this portion of the orogen. The two Acadian deformation phases can be related to plate-scale interactions between Laurentia and Avalon involving Silurian sinistral convergence followed by Devonian NW-SE convergence. The rotation of Acadian kinematic axes may reflect significant reorganization of mid-Paleozoic plates, involving a triple junction between Laurentia, Baltica, and Avalon.