Silurian-Devonian tectonic evolution of mid-coastal Maine, U.S.A.: Details of polyphase orogenic processes

Detailed bedrock mapping, structural geology, meta-igneous whole rock geochemistry, and U-Pb geochronology from rocks sampled along a portion of a complexly deformed tectonic boundary between the Ordovician peri-Gondwanan Liberty-Orrington belt and Silurian syn-orogenic strata of the Fredericton trough (a.k.a. the Dog Bay Line) in mid-coastal Maine aid in deciphering the Silurian-Devonian tectonic evolution of the region. The new results provide constraints on several key events. First, initial terrane juxtapositioning occurred along the east-verging Boothbay thrust fault (D1). This tectonism occurred prior to 423 Ma and is associated with the accretion of the Ganderian microcontinent to the Laurentian margin (that is, the Salinic orogeny). Subsequently, intrusion of an ultra-potassic magma, the protolith of the Edgecomb Gneiss, occurred at ca. 413 Ma. Its distinctive whole rock geochemical signature allows for correlation with rocks of similar composition and age along a relatively narrow 140 kilometer long distance on the northwestern margin of the Fredericton trough. This restricted area of ultra-potassic magma generation is attributed to the breakoff of the descending Salinic oceanic slab that triggered decompression melting of a previously metasomatized mantle wedge region beneath the accreted Ganderian microcontinent. Early thrust faults (D1) and the ca. 413 Edgecomb Gneiss igneous protolith were overprinted by an episode of upright folding (D2) and low-pressure amphibolite facies metamorphism associated with the Early to Middle Devonian Acadian orogeny. Zircon overgrowths in the Edgecomb Gneiss dated at ca. 399 Ma grew during this tectonic episode. Comparisons with previous geochronological studies across the region suggest this dominant phase of Acadian deformation and metamorphism was long-lived (ca. 40 m.y.) and associated with the outboard accretion of the Avalonian microcontinent. Dextral shear structures represent the final phase of deformation (D3) superimposed on this terrane boundary and are associated with the Norumbega fault and shear zone system that was active in Middle Devonian-Carboniferous time.