Northward motion of the Whitehorse Trough: paleomagnetic evidence from the Upper Cretaceous Carmacks Group: Discussion

Abstract

Discordant paleomagnetic directions in mid-Cretaceous plutons of the Coast Plutonic Complex and North Cascades have been interpreted as indicating mid-Cretaceous paleolatitudes 2000-2500 km south relative to North America (Beck et al. 1981; Irving et al. 1985; Umhoefer 1987). But this magnitude of northward transport contradicts interpreted pre-midCretaceous links between the North American miogeocline and westerly parts of the Cordillera (Price and Charmichael 1986). Because paleohorizontal cannot be directly determined in plutonic rocks, the discordant paleomagnetic directions may also be explained by tilting of the plutons about a horizontal axis. Primarily because of the consistent nature of the paleomagnetic discordance over a large region, Beck et al. (1981) and Irving et al. (1985) have preferred the interpretation of northwards transport. However, Butler et al. (1989) recently pointed out geological evidence indicating that widespread northeast-side-up tilting may have affected the western portion of the Coast Plutonic Complex and the North Cascades. Resolution of this debate is of obvious importance to understanding the tectonic evolution of the Canadian Cordillera. Marquis and Globerman reported results of a paleomagnetic study of the Upper Cretaceous Carmacks Group in the Whitehorse Trough. The Carmacks Group is composed of gently dipping basaltic and andesitic lava flows that have been dated at 70.4 -+ 2.4 Ma. These rocks lie west of the Tintina Northern Rocky Mountain Trench fault and the Teslin suture zone. Because paleohorizontal can be unambiguously determined for these volcanic rocks, the paleomagnetic data presented by Marquis and Globerman are potentially very important in resolving the tilt versus translation question. Marquis and Globerman collected paleomagnetic samples in three regions spread over 300 km. Based on detailed laboratory studies, they successfully isolated characteristic remanent magnetization (ChRM) from 18 of 27 paleomagnetic sites (= flows). The acquired paleomagnetic data pass a fold test of stability and contain sites of both normal and reversed polarity; and magnetic properties indicate that the ChRM is a primary thermoremanent magnetization. The observed angular dispersion of virtual geomagnetic poles (VGP's) indicates some degree of required sampling of geomagnetic secular variation. Marquis and Globerman determined a paleomagnetic pole from the Carmacks Group at 82N, 109E, with 95% confidence region (Ag5) of radius 7.8. Using a 70 Ma North American reference paleomagnetic pole of 77.7N, 185.8E (Ag5 = 7.7) and the methods of Beck (1980) as modified by Demarest