Abstract

Apatite fission-track ages from opposite sides of the Norumbega fault zone in southern Maine reveal a significant Early Cretaceous time-temperature discontinuity across the structure. Ages west of the fault zone range from 113 to 89 Ma, while those to the east are significantly older, ranging from 159 to 140 Ma. Modeling of track length distributions reveals that at 120 Ma, an ∼55 °C difference in temperature existed between opposite sides of the fault zone currently exposed at the surface. This discontinuity is best explained by an episode of significant (to 2 km of net vertical separation) east-side-down vertical movement along the Norumbega fault zone in Late Cretaceous time. The modeling suggests contemporaneous cooling and that final movement along the fault zone occurred ca. 80 Ma. These results, when combined with earlier structural and thermochronological studies, suggest that faults mapped within the Norumbega fault zone were intermittently active from Middle Devonian to Late Cretaceous time, and that the structure is a fundamental zone of crustal weakness. The prolonged period of activity likely reflects stresses associated with late Paleozoic dextral transpression, early Mesozoic rifting associated with the initial opening of the North Atlantic Ocean basin, and processes associated with the late Mesozoic uplift and exhumation of the northern Appalachian orogen.

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