Acadian transpression and the exhumation of the sillimanite-bearing Passagassawakeag terrane along the Liberty-Orrington Fault, coastal Maine

New detailed mapping in central coastal Maine shows the existence of a dext ral strike-slip mylonitic shear zone, 250-500m thick, separating the sillim anite-grade Passagassawakeag terrane and the chlorite-grade Bucksport Forma tion of the Fredericton Trough. Foliation within the shear tent: is near-ve rtical, with near-horizontal stretching lineations and pervasive dextral se nse-of-shear indicators. Followed eastward, this northeast-striking terrane boundary turns north. A thin unit of alternating layers of quartz and garn et+biotite+magnetite, previously interpreted as the stratigraphic Rider Blu ff unit, lies along this north-south striking (eastern) part of the Passaga ssawakeag-Bucksport boundary. Petrographic analysis demonstrates that the l ayering in this unit is a tectonic fabric, and the entire unit is interpret ed as consisting of mylonites formed along a transpressional thrust, now ov erturned, along which the Passagassawakeag terrane was exhumed. If this is the case. the Liberty-Orrington shear zone may represent part of an orogen- scale dextral shear zone, continuous with the Penobscot Bay and Norumbega f ault zones, through the Acadian orogeny.