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40Ar/39Ar incremental-heating measurements of Grenville metamorphic rocks from the Adirondack Lowlands, New York, constrain a ca. 1100–900 Ma history of uplift, exhumation, and cooling in the eastern Frontenac terrane following the ca. 1200–1170 Ma Elzevirian orogeny and the ca. 1080–1070 Ma peak of the subsequent Ottawan orogeny. Four hornblende-biotite pairs yield a time-integrated post-Ottawan cooling rate of 1.6 ± 0.2 °C/m.y. and an inferred exhumation rate of 0.06 ± 0.02 km/m.y. between ca. 1100 Ma (oldest hornblende, northwest Lowlands) and ca. 900 Ma (youngest biotite, southeast Lowlands), based upon closure temperatures of 500 and 300 °C and a 30 ± 10 °C/km geotherm. Moreover, these and ∼20 additional cooling ages for hornblende and biotite define parallel northwest-to-southeast younging trends of ∼3 m.y./km across ∼45 km between the St. Lawrence River and the Carthage-Colton shear zone, which separates the Lowlands and the Adirondack Highlands domains. Two end-member models potentially explain the age trends: (1) ca. 1100–900 Ma cooling of the Lowlands through subhorizontal isotherms, followed by ca. 900–520 Ma regional tilting (∼9° ± 3°NW); or (2) ca. 1100–900 Ma cooling of the Lowlands against a warm Highlands footwall, through gently northwest-dipping inclined isotherms that flattened out by ca. 900 Ma. On balance, the regional tilting model is the favored explanation of observed trends in the hornblende and biotite ages. Differential uplift during regional extension most likely tilted the Lowlands, and the ca. 900–520 Ma time frame suggests that breakup of the supercontinent Rodinia was ultimately responsible for these tectonic events and the resultant trends in cooling age.

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