Abstract

The Long Range Inlier, a steep-sided plateau underlain mainly by Grenvillian gneisses, is the most prominent topographic feature of western Newfoundland. Apatite fission-track analysis of 31 samples from the Long Range Inlier and its surroundings yielded measured apparent ages of 343–152 Ma. Age versus elevation plots, track-length distributions, and model thermal histories indicate that the region experienced slow cooling in the late Paleozoic, with apparent exhumation rates of 7–9 m∙Ma−1 and cooling rates of 0.08–0.28 °C∙Ma−1. Model thermal histories suggest that the present upper surface of the Long Range plateau cooled below ~120 °C in Ordovician times. The thermal histories are compatible with, but do not require, some exhumation of the Long Range Inlier along Acadian thrust faults. Results from Early Carboniferous sedimentary rocks of the Deer Lake Basin are similar to Long Range Inlier data from similar elevations, implying that at some time between ~350 and 300 Ma, the entire region was buried to depths sufficient to induce total annealing (T > 120 °C) in these samples. Closure ages determined from model thermal histories indicate that regional cooling to temperatures below ~120 °C began before 300 Ma. The Carboniferous sedimentary cover was largely removed by Jurassic time, perhaps in response to lowering of regional base level by rifting associated with the opening of the Atlantic Ocean.

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