Paleomagnetism of the Pennington Mountain terrane: A near‐Laurentian back arc basin in the Maine Appalachians

Abstract

Paleomagnetic studies of volcanic terranes in the Appalachians provide quantitative data on the Ordovician paleogeography of the Iapetus ocean. New paleomagnetic results from submarine volcanics of the Middle to Late Ordovician Winterville Formation of Maine further constrain the evolution of Iapetus. Ten sites yield a tilt corrected direction of D/I = 3277/−21 (α95 = 9.3, k = 27.9); the corresponding paleomagnetic pole calculated for the average site location (46.8°N, 291.3°E) is located at 26°N, 148°E (dp = 5°, dm = 10°, A95 = 7.7). A Silurian conglomerate (Frenchville Formation) test is inconclusive due to a strong present‐day field overprint which obscures any primary remanence. The presence of a positive tilt test, however, supports the conclusion that the characteristic magnetization of the Winterville Formation is a prefolding, primary magnetization. An Ordovician paleolatitude of 11±5° for the Pennington Mountain terrane of northern Maine is indistinguishable from that of the Laurentian margin (15–20°) during the Middle to Late Ordovician. The paleolatitude of the Pennington Mountain terrane is also similar to, but slightly more equatorial than that of the previously studied Bluffer Pond Formation of the nearby Munsungun terrane (18±9°). We conclude that the Ordovician Pennington Mountain and Munsungun volcanic terranes of Maine were formed and acquired their characteristic magnetization near the Laurentian margin. The paleolatitudes of these terranes are also similar to that obtained from the Stacyville volcanics of the more outboard Lunksoos terrane (20±8°), but they contrast strongly with the Ordovician Miramichi terrane of northern New Brunswick that yielded a paleolatitude of 51° (+21, −16°). These results from Maine support the presence of one or more backarc basins adjacent to the Laurentian margin during the Middle to Late Ordovician.