Abstract

The ~75 Ma Mount Lorne monzodiorite stock has been studied paleomagnetically to estimate the tectonic motion of the northern Canadian Cordillera since the Late Cretaceous. The stock is one of several paleomagnetic studies currently underway at the University of Windsor Paleomagnetic Laboratory which are funded by the Lithoprobe - Slave northern Cordillera Lithospheric Evolution (SNORCLE) Project. Al-in-hornblende geobarometry and plagioclase-amphibole geothermometry data suggest that the stock has not been tilted since emplacement but does provide evidence that the stock has a normal fault trending north-south through its centre with the east side uplifted relatively by ~1000 m. Paleomagnetic measurements from 12 of 19 granitoid sites yield a well-defined characteristic remanent magnetization direction that is south-southwest and up, and a further six sites yield a direction that is north-northeast and down. Data from a mafic dike yield a negative contact test, suggesting that the dike is coeval with the stock. Combining the 18 granitic site mean characteristic remanent magnetization directions yields a paleopole at 69.1°W, 78.3°N (dp = 4.1°, dm = 4.5°) which suggests that the host Stikine Terrane has been translated poleward by 10.5 ± 3.5° (1170 ± 390 km) and rotated clockwise by 57 ± 11° relative to the North American craton between 75 and 50 Ma. Except for the estimate from the nearby coeval Carmacks Group volcanic rocks, the Mount Lorne estimate is consistent with all other paleomagnetic results within the Stikine Terrane. These latter estimates are also consistent with plate-tectonic models that suggest the Pacific oceanic plates had an increased velocity northwards during the Late Cretaceous.

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