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

Abstract

The Upper Cretaceous Carmacks Group (70.4 ± 2.4 Ma) comprises gently dipping basaltic and andesitic lava flows overlying volcaniclastic deposits of the Intermontane Belt in the Whitehorse Trough. The sampling area is in southern Yukon and northern British Columbia; it lies west of the Tintina – Northern Rocky Mountain Trench fault and Teslin Suture Zone and east of the Denali – Shakwak fault. Volcanic sections were sampled in three regions spread over 300 km, providing the first paleomagnetic data from pre-Tertiary volcanic rocks in the northern Canadian Cordillera. Alternating-field and thermal demagnetization revealed stable magnetization for 18 of the 27 sites collected. The overall mean direction (D = 166.7°, I = −71.4°, k = 53, α95 = 4.8°, N = 18 sites) is pre-folding and is most probably primary (latest Cretaceous). This gives a paleopole at 109.4°E, 82.1°N, K = 21, A95 = 7.8°. A critical evaluation of North American cratonic data yields a reference paleopole for the latest Cretaceous at 185.8°E, 77.7°N, A95 = 7.7°, implying 13.4 ± 8.5 °(1500 ± 950 km) northward displacement and 10.2 ± 20.7 °(not significant) clockwise rotation of the Whitehorse Trough. The displacement occurred between 70.4 ± 2.4 and 59 – 54 Ma, the "docking" time suggested by other paleomagnetic studies in Alaska. The amount and timing of this displacement are consistent with Gabrielse's estimate of cumulative dextral displacements along the Northern Rocky Mountain Trench fault and associated lineaments. As expected, it is intermediate between the low paleolatitudes observed from middle Cretaceous rocks and the concordant paleolatitudes observed in Middle Eocene rocks of the Western Cordillera farther south, in British Columbia and in northern Washington. The estimated displacement is consistent with those predicted by kinematic models of Engebretson and Umhoefer based on North Pacific Basin plate motions.