Eocene rotation of the Coast Plutonic Complex and Intermontane Belt: paleomagnetism of Eocene plutons along the Klondike Highway

Abstract

Paleomagnetic results are reported for the *59 Ma Skagway, *54 Ma Fraser, *53 Ma Summit Lake, and *48 Ma Clifton felsic plutons of the eastern Coast Plutonic Complex (CPC) that outcrop along the south Klondike Highway in Alaska and British Columbia. Thermal and alternating field step demagnetizing methods yielded stable characteristic remanent magnetization (ChRM) directions for all 29 sites of normal, reversed, and mixed polarity. The ChRM resides in single or pseudosingle domain magnetite and (or) pyrrhotite that is shown to be primary by contact tests with the *47 Ma vertical White Pass mafic dikes. Paleopoles from six 56 to 50 Ma (mean 52 ± 2 Ma) Intermontane Belt -Yukon-Tanana terrane (IMB-YTT) units that cannot be explained by tectonic tilt are compared with nine clustered 59 to 46 Ma (mean 52 ± 4 Ma) eastern CPC paleopoles. Both paleopole populations show nonsignificant poleward (northward) translation relative to North America (IMB-YTT, 3.78 ± 5.38N; CPC, 4.38 ± 6.48S; overall, 1.28 ± 4.98S), indicating that northward translation of the accreted terranes ended by *58 Ma. Conversely, both populations show clockwise (CW) rotation that is either highly significant or substantial (IMB-YTT, 19.38 ± 10.5 8CW; CPC, 7.18 ± 16.1 8CW; overall 12.88 ± 10.9 8CW). The results are best explained by tectonic rotation from *50 to *45 Ma of the IMB-YTT as a thin-skin on top of North America during emplacement and co-incident rotation of the massive Eocene plutons of the eastern CPC along the North American margin.Résumé : Des résultats de paléomagnétisme sont rapportés pour les plutons Skaway (*59 Ma), Fraser (*54 Ma), Summit Lake (*53 Ma) et Clifton (*48 Ma) de la partie est du complexe plutonique côtier (CPC) qui affleure le long du sud de l'autoroute du Klondike en Alaska et en Colombie-Britannique. Des méthodes de démagnétisation thermique et par étapes dans un champ alternatif ont donné des directions d'aimantation rémanente caractéristiques (ChRM) stables pour tous les 29 sites de polarité normale, inverse et mixte. La ChRM réside dans une magnétite à domaine unique ou pseudo-unique et (ou) une pyrrhotite qui a été démontée être primaire par des tests de contact avec les dykes mafiques verticaux de White Pass (*47 Ma). Les paléopôles de six unités de la ceinture du Plateau intérieur -terrane Yukon-Tanana (IMB-YTT), datant de 56 à 50 Ma (moyenne de 52 ± 2 Ma), qui ne peuvent être expliqués par un basculement tectonique, sont comparés à neuf paléopôles du CPC oriental regroupés autour de 59 à 46 Ma (moyenne de 52 ± 4 Ma). Les deux populations des paléopôles montrent une translation peu importante vers le pôle (vers le nord) par rapport à l'Amérique du Nord (IMB-YTT, 3,78± 5,3 8N; CPC, 4,38± 6,4 8S; de manière générale, 1,28± 4,9 8S) indiquant que la translation des terranes accrétés vers le nord s'est terminée vers 58 Ma. Réciproquement, les deux populations montrent une rotation, dans le sens horaire, hautement significative ou substantielle (IMB-YTT, 19,38± 10,58sens horaire; CPC, 7,18± 16,18sens horaire; de manière...

Post-Triassic para-autochthoneity of the Yukon-Tanana Terrane: paleomagnetism of the Early Cretaceous Quiet Lake batholith

“…Thus, Symons et al (2005) concluded that the YTT was either autochthonous or para-autochthonous by ∼100 Ma and was simply displaced northwestwards on the Tintina fault zone by ∼5 • (550 km) whereas the IMB was an allochthonous thin-skin belt of terranes on the YTT that underwent 8.3 • ± 7.0 • of northwestward translation (920 ± 775 km) and 35 • ± 14 • of clockwise rotation between ∼102 and ∼54 Ma as well as a further 16 • ± 6 • of clockwise rotation since ∼54 Ma. Subsequently, Symons et al (2009Symons et al ( , 2011 have shown that the YTT and IMB have generated very different incompatible APWPs for the past 200 Ma with the YTT path falling close to the North American path and the IMB path being widely divergent Table 2 • from the North American path from Early Jurassic at least to the Neogene. Three more recent paleomagnetic studies relate to the SAYBIA model of Johnston (2001Johnston ( , 2008.…”

“…These Figure 8. Schematic showing restored locations for the exposed Yukon-Tanana Terrane and Intermontane Belt terranes in Yukon as proposed by (a) Johnston (2001,2008) at 85 and 45 Ma in the SAYBIA tectonic model, and (b) Symons et al (2005Symons et al ( , 2009Symons et al ( , 2011. Option (a) proposes that the YTT and IMB were a coherent part of a 'ribbon' continent that was translated northwards off North America's western margin by ∼18 • of paleolatitude between ∼85 and ∼45 Ma to accrete in its present location.…”

“…From geologic evidence the batholith is known to have been displaced northwestwards relative to North America by ∼490 km during latest Cretaceous to Eocene time by dextral strike-slip displacement along the inboard Tintina fault (Gabrielse 1985;Gabrielse et al 2006). Conversely, recent paleomagnetic estimates for the total post-mid-Cretaceous northwestward displacement of the YTT have ranged from autochthonous or para-autochthonous at ≤1000 km (Symons et al 2005(Symons et al , 2009(Symons et al , 2011McCausland et al 2006) to allochthonous at ≥1500 to ∼4000 km (Johnston 2001(Johnston , 2008Enkin et al 2006). Thus, this paleomagnetic Hart et al (2004), Dusel-Bacon et al (2006), Gabrielse et al (2006) and Symons et al (2009).…”