Abstract

As a part of a regional effort to determine the extent of low-temperature thermochronological discontinuities across major orogen-parallel faults in northern New England, 41 apatite fission track (AFT) ages and 11 (U–Th)/He ages are used to constrain the ∼65 to 100 °C cooling history of rocks flanking a 160 km long segment of the Norumbega fault system in southern and south-central Maine. These data are used to evaluate the role of this structure in the late Mesozoic and younger exhumation history of the northern Appalachians. AFT ages flanking the fault system range from 159 to 95 Ma and record cooling below ∼100 °C in the late Mesozoic. (U–Th)/He ages from the same region range from 126 to 100 Ma and record cooling below ∼65 °C. Previously published AFT ages from an ∼40 km long segment of the fault system just north of Casco Bay reveal a dramatic time–temperature discontinuity across the structure and suggest kilometre-scale late Mesozoic displacement in this region. However, new AFT and (U–Th)/He ages along the strike of the Norumbega fault system to the northeast and southwest of this discontinuity show no significant differences in late Mesozoic cooling and suggest no significant displacements occurred along these portions of the fault system during this time. Collectively the data suggest differential late Mesozoic reactivation of the Norumbega fault system with the reactivation localized in areas that had previously experienced episodes of vertical displacement in the late Paleozoic (i.e., the “Casco Bay restraining bend”).

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