ACADIAN SLAB DETACHMENT RECORDED IN THE SILURO-DEVONIAN GEOLOGY OF NORTHERN MAINE

A compilation of new and existing data shows that the stratigraphy and magmatic rocks exposed in the Siluro-Devonian section of northern Maine record a series of events that include: 1) proximal, shallow water deposition and sub-aerial exposure, followed by 2) rapid foundering, normal faulting, and within-plate volcanism, and 3) subsequent flysch deposition, Acadian deformation and bimodal magmatism. The first two events are recorded by the Ripogenus Fm, West Branch Group, and Frost Pond Shale in the Chesuncook Dome, by The Forks, Carrabassett, and Capens formations in the Lobster Mountain Anticlinorium, by unnamed Silurian pebble conglomerates and limestones, and Siluro-Devonian siltstones overlain by intermediate-to-mafic volcanics on the northwest flank of the Weeksboro-Lunksoos Anticlinorium, and by the East Branch Group and Spider Lake Volcanics in the Munsungun Anticlinorium. These isolated inliers were all subsequently overlain by the Seboomook Flysch and experienced bimodal magmatism of the mafic Moxie Pluton and felsic Katahdin-Traveler Suite that occurred during the initial onset of Acadian deformation in northern Maine.

Our geochemical study of the West Branch Volcanics shows enrichment in LREE and thorium, and a negative Ta-Nb anomaly. These findings are consistent with earlier geochemical studies here and in Quebec, and indicate derivation from a previously subduction-modified upper mantle source contaminated by thorium-rich continental crust, emplaced in a continental within-plate setting.

We interpret this to be the result of slab detachment during the Acadian Orogeny, where the southeast-dipping margin of Taconic-modified Laurentia failed during the early stages of its subduction. Early, shallow-water sedimentation and sub-aerial exposure occurred along the margin during its approach to, and passage through the peripheral bulge. Rapid deepening occurred as the margin entered the outer trench slope. Concurrent failure of the margin resulted in decompression melting of the sub-continental lithosphere and extrusion of the within-plate volcanics and subsequent magmatism of the Moxie-Katahdin-Traveler suite and related igneous rocks as the Acadian deformation front crossed the region.