Dual polarity magnetizations from the Upper Devonian McAras Brook Formation, Nova Scotia, and their implications for the North American apparent polar wander path

Stearns, C. and Van der Voo, R., 1988. Dual polarity magnetizations from the Upper Devonian McAras Brook Formation, Nova Scotia, and their implications for the North American apparent polar wander path. Tectonophysics, 156: 179-191. Thermal demagnetization of 57 samples from four lava flows of the Upper Devonian McAras Brook Formation, sampled along the Northumberland Strait in northern Nova Scotia, has yielded complex paleomagnetic results. Samples from three of the four lava flows revealed two superimposed sets of magnetic directions (D/Z = 33,’ 2 and D/I = 7/23, both tilt-corrected) that both appear to be Paleozoic in age. The corresponding paleopole for the northeasterly direction is similar to previously published Devonian poles for North America. Although the age of this component cannot be constrained by a field stability test, the presence of stratigraphically separated dual polarities supports a primary age for the magnetization. Application of the structural correction (strike and dip of the beds = 214/36 NW) does not significantly change the direction (32/ 1, in-situ vs. 33/2, tilt-corrected). The palcopole for this direction is located at 35 o N, 76 o E. The additional and more northerly direction can be seen in samples from two lava flows. In this set of directions, both polarities occur without apparent stratigraphic separation. The corresponding paleopoles for this direction (46 o N, 116” E, in situ, 32ON, 109”E, tilt corrected) are similar to published Permian and Devonian paleopoles for North America, respectively. Again, the age cannot be constrained by a field test. Although the in-situ pole plots near the Permian paleopoles, Permian magnetizations are not expected to show dual polarities because of the long Riaman reversed interval. We therefore interpret it to be pre-Kiaman. Because the tilt-corrected paleopole is near Devonian paleopoles, we tentatively interpret the northerly component as secondary (Late Devonian), but acquired before tilting of the rocks. Our results show the same longitudinal distribution as that of the previously published Devonian paleopoles for North America. Although some authors have suggested that this spread is due to local tectonic rotation, the dispersion in our data cannot be explained by tectonic rotation. Instead, we conclude that the magnetizations of the McAras Brook Formation were acquired over a long period of time during the post-Acadian infilling of a rift basin and that the longitudinal distribution of the paleopoles was caused by the clockwise rotation of the North American plate as a whole during the Devonian. Magnetization acquisition over a long time interval is supported by the presence of dual polarities seen within individual samples. Finally, samples from the second highest lava flow of the McAras Brook Formation revealed univectorial magnetic directions (D/I = 269/34, in situ, 240/ 58, tilt-corrected) that are difficult to explain because the corresponding paleopole is dissimilar to any existing Phanerozoic pole for North America. Because these directions cannot be explained by remagnetization or by tectonic rotation, we tentatively ascribe them to a transitional field direction recorded during a reversal. 0040-1951/88/$03.50