Avalonian proximity of the Ordovician Miramichi Terrane, northern New Brunswick, northern Appalachians: Paleomagnetic evidence for rifting and back-arc basin formation at the southern margin of Iapetus

摘要

A paleomagnetic investigation of the Middle Ordovician Tetagouche Group in northern New Brunswick was undertaken to determine the paleogeographic position of the Miramichi Terrane. Stepwise thermal demagnetization of pillow basalts reveals a high-temperature characteristic magnetization with a mean direction of D = 060[deg], I = +69[deg], k = 22, [alpha]95 = 13[deg] (tilt-corrected, N = 7 sites; 73 samples). A positive fold test and the presence of antipodal normal and reversed polarity directions indicate that this ancient direction is Ordovician in age, with a paleopole position of 52[deg]N, 352[deg]E. The corresponding paleolatitude of 53[deg]S places these volcanic rocks near the southern margin of the lapetus Ocean, at paleolatitudes similar to those revealed by Avalon for the Middle to Late Ordovician. The mafic and felsic volcanic rocks and marine sedimentary rocks of the Tetagouche and Fournier groups have been interpreted to be remnants of a rifted continental margin and a Middle Ordovician back-arc basin. Our results show that the process of rifting and back-arc basin formation occurred at the Avalonian margin of lapetus, which implies that Ordovician subduction was not restricted to the Laurentian margin, but also marks the southern margin of lapetus.