EXHUMATION OF THE TURNER MOUNTAIN SYENITE FAULT SLIVER IN THE NORUMBEGA FAULT ZONE OF MAINE: IMPLICATIONS FOR KINEMATICS OF OROGEN-PARALLEL FAULT REACTIVATION

Along the length of the Norumbega Fault Zone, there exist two unusual igneous bodies, the Lincoln Syenite exposed in south-central Maine and the Turner Mountain Syenite at Turner Mountain (east of Old Town). Both syenites are characterized by dark-colored alkali feldspar megacrysts set in a finer-grained matrix dominated by alkali feldspar ± pyroxene ± hornblende + biotite + oxides. Both syenites are geochemically similar - they are intermediate in terms of SiO₂ content (55-59 wt. % for Lincoln Syenite and 58-65 wt. % for Turner Mountain Syenite), yet are ultrapotassic (6-7 wt. % K₂O for Lincoln Syenite and 6.4-7.9 wt. % for Turner Mountain Syenite). A LA-ICP-MS zircon dating of the Turner Mountain Syenite yields a U-Pb age of 411±1.6 Ma, close to the age of Lincoln Syenite (zircon U-Pb age of 418 ± 1 Ma). However, while the Lincoln Syenite is considerably ductily deformed as a result of early Norumbega ductile shearing, the Turner Mountain Syenite which is immediately adjacent to a 3-km-wide mylonite zone derived from Lucerne pluton, shows no evidence of ductile deformation but cataclasis along its margins. Recent mapping confirms that the Turner Mountain Syenite is a fault sliver sandwiched between the granitic mylonite zone and the tilted young redbeds. The redbeds were deposited in a narrow pull-apart basin produced by strike-slip faulting. The fault contacts between the syenite and the redbeds most likely facilitated certain degree of hydrothermal alteration in the adjacent redbeds so that some small biotite flakes were produced, which was mistaken as “contact metamorphism” in our preliminary study of the syenite. Detailed mapping and observations convince us that a large-scale vertical displacement took place between the syenite and the redbeds, and between the syenite and the granitic mylonite zone. The syenite sliver was exhumed by a late-stage, high-angle, southeast-over-northwest reverse faulting from a deeper level of the crust where it was probably not affected by the early Norumbega ductile shearing. It implies that this considerable, southeast-over-northwest reverse displacement would have taken place along the entire length of the Norumbega Fault Zone, thus helping explain the discrepancy in deformation style and metamorphic grade across the fault zone in central and southwestern Maine.