An Overview of the Paleozoic Tectonic Evolution of Mid-Coastal Maine
David West, Department of Geology, Middlebury College
Emily Peterman, Department of Earth and Oceanographic Science, Bowdoin College
Abstract
Metamorphic rocks in the mid-coastal Maine region between Casco Bay and Muscongus Bay are dominated by two tectonostratigraphic sequences: (1) Ordovician (~460-475 Ma) peri-Gondwanan meta-volcanic and meta-sedimentary rocks that initially formed in an evolving volcanic arc to back-arc environment built upon converging Ganderian microcontinental crust (Falmouth-Brunswick, Casco Bay, and East Harpswell groups), and (2) Silurian meta-sedimentary rocks of the Fredericton trough (Bucksport Formation). The Fredericton trough is interpreted to represent a syn-accretionary, turbidite-dominated, foredeep associated with the closure of intervening oceanic lithosphere that separated the Laurentian margin from the converging off-shore Ordovician arc sequences. Initial accretion of these peri-Gondwanan rocks to the Laurentian margin, and the cessation of sedimentation in the Fredericton trough, occurred during the Late Silurian Salinic orogeny. In this region, the two important tectonostratigraphic sequences are now juxtaposed along the Boothbay thrust, which is correlated with the regionally extensive Dog Bay Line in Atlantic Canada.
Following the accretion of the aforementioned terranes (~415 Ma), break-off of the descending Salinic oceanic slab superimposed extension on a localized region of previously metasomatized lithospheric mantle and magma of ultra-potassic composition was generated (Lincoln syenite and Edgecomb Gneiss). This was followed by the outboard accretion of the Avalonian microcontinent (these rocks are not present here) which led to a complex sequence of Devonian polyphase deformation, amphibolite facies metamorphism, and granitic plutonism (i.e., the Acadian orogeny). The deformation associated with this tectonic episode is dominated by map-scale, upright, shallow-plunging, tight to isoclinal folds that control the strong north-northeast structural grain of the region. Although the initial stages of this deformation may have been the product of orthogonal convergence in Early Devonian time, by the Middle Devonian the setting had evolved into a strongly dextral transpressive regime. Mark Swanson, in his detailed studies of structures across the region, has attributed this phase of deformation to a tens of kilometer wide zone of overall dextral transpression associated with a regional-scale restraining bend along the Norumbega fault system. Deformation partitioning, at a variety of scales, is a hallmark of this episode of deformation and includes zones dominated by pure shear deformation flanked by zones of simple shear strike-slip deformation. Finally, late Triassic-Early Jurassic mafic dikes across the region and significant thermochronological discontinuities across late faults along the main trace of the Norumbega fault system record post-Paleozoic extensional tectonic activity.
Session
Session 3: Teaching Innovations in SG&T