Use of U-Pb geochronology to identify successive, spatially overlapping tectonic episodes during Silurian-Devonian orogenesis in south-central Maine, USA

Silurian-Devonian orogenesis formed the dominant metamorphic and deformational features recorded in the south-central Maine portion of the northern Appalachians. New U-Pb ages for metamorphic monazite and zircon from this high-grade segment of the orogen, coupled with new and previously published igneous and metamorphic ages, reveal the presence of three distinct tectono-metamorphic zones. All three zones experienced an early phase or phases of contractional deformation in the Late Silurian to Early Devonian. In the Eastern and Central Zones, this contractional deformation was synchronous with low-pressure, amphibolite-facies metamorphism and accompanied by plutonism in the Central Zone during the waning stages of this phase. Beginning in Middle Devonian time, the kinematics of deformation in the Central and Western Zones became strongly transpressive, accompanied by low-pressure, amphibolite-facies metamorphism and plutonism. The Eastern Zone underwent no dextral transpression or amphibolite-facies metamorphism during this later event. Contemporaneous, spatially partitioned contractional and transcurrent strain is a feature of many transpressional orogens, but in south-central Maine, more than 30 m.y. passed between the peaks of the older contractional and younger dextral deformational episodes. Therefore, the system does not reflect contemporaneous strain partitioning as in a transpressional regime. Rather, our results, along with regional records of Late Silurian-Early Devonian sinistral and Middle Devonian dextral deformation features, are consistent with a significant change in plate kinematics between 430 and 370 Ma.