THE RECORD OF CHANGE FROM COMPRESSIONAL TO TRANSPRESSIONAL PLATE CONVERGENCE IN THE SOUTH-CENTRAL MAINE ACADIAN HINTERLAND

The Siluro-Devonian Acadian orogeny in the northern Appalachians resulted from the collision of the microcontinent Avalonia with the Laurentian margin. Although the foreland portion of the orogen experienced a relatively simple progressive northwestward migration of the deformation front, the hinterland recorded a long-lasting and complex tectonothermal history. New U-Pb ages for metamorphic monazite and zircon from the hinterland portion of this orogen in south-central Maine, coupled with new and previously published igneous and metamorphic ages, reveal the presence of three distinct tectonometamorphic zones. All three zones experienced an early phase(s) of contractional deformation associated with initial microcontinent collision in Late Silurian to Early Devonian time, but only in the Eastern and Central Zones was this contractional deformation synchronous with low-pressure amphibolite facies metamorphism and, during the waning stages of this orogenic activity, accompanied by plutonism. 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 at this time. 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, and we infer a significant increase in plate convergence obliquity between 420 and 380 Ma.