Contrasting thermal histories across the Flying Point fault, southwestern Maine: Evidence for Mesozoic displacement

Abstract

40 Ar/ 39 Ar analysis of hornblende, muscovite, biotite, and K-feldspar across the Flying Point fault in southwestern Maine reveals important information on the timing of thermal events and subsequent cooling history of the region. The Flying Point fault is the most significant structure associated with the Norumbega fault zone in southwestern Maine. It forms the boundary between two different lithotectonic sequences, offsets metamorphic isograds, and marks a remarkable time-temperature discontinuity. Southeast of the Flying Point fault in the Casco Bay Group, early Carboniferous hornblende ages and late Carboniferous muscovite and biotite ages reflect slow cooling following Middle to late Devonian (Acadian) metamorphism and deformation. No significant (>300 °C) late Paleozoic thermal event affected these rocks. In contrast, rocks northwest of the Flying Point fault were affected by a significant (>500 °C) late Paleozoic (Alleghanian) thermal event as indicated by latest Carboniferous-Permian hornblende cooling ages. This represents the northernmost occurrence of Alleghanian high-grade metamorphism reported in the Appalachian orogen. Cooling below muscovite and biotite closure temperatures northwest of the Flying Point fault did not occur until the Early Triassic. The thermochronologic data indicate that the rocks currently juxtaposed across the Flying Point fault underwent drastically different thermal histories during Permian and Triassic time. Approximately 4 km of post-Paleozoic west-side-up displacement along the Flying Point fault can account for the presently observed time-temperature discontinuity.