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

Abstract

Was the Yukon-Tanana Terrane (YTT), a California-sized part of south-central Yukon, an autochthonous or para-autochthonous part of northern British Columbia in the Early Cretaceous or was it part of a proposed allochthonous 'Baja B.C.' continent offshore of southern California? To answer this fundamental question, a paleomagnetic study has been completed on 347 specimens from 24 sites in the 114.7 ± 1.1 Ma Quiet Lake batholith. This 1300 km 2 pluton is composed mostly of massive medium-to-coarse grained biotite quartz monzonite that exhibits no evidence of either deformation or metamorphism, and that intrudes metamorphosed pre-Cretaceous basement rocks of the YTT in southern Yukon. The paleomagnetic analysis utilized thermal and alternating field step demagnetization, and saturation isothermal remanence methods. A well-defined characteristic remanent magnetization (ChRM) direction was isolated throughout the 500-585 • C temperature range at Decl. = 340.6 • , Incl. = 77.4 • (N = 14 sites, k = 51.2, A 95 = 5.6 •). The ChRM resides in magnetite with a low titanium content and is interpreted to be a primary thermoremanent magnetization. After correction for 490 km of geologically demonstrable dextral displacement on the inboard Tintina fault zone, the Quiet Lake batholith's paleopole is not significantly different at 95 per cent confidence from the co-eval 115 Ma reference paleopole for North America, giving non-significant translation and rotation estimates of 1.4 • ± 5.1 • (1σ) northwestwards and 10 • ± 13 • (1σ) clockwise, respectively. Thus, this is the first Early Cretaceous paleopole to show clearly that the YTT in Yukon is a para-autochthon that was part of North America's continental margin at that time. Further, after correction for Tintina fault displacement, the eight available Mesozoic YTT paleopoles agree closely with the North American apparent polar wander path (APWP). In contrast, the 22 paleopoles from the Intermontane Belt show the expected behaviour of an allochthonous thin-skin of terranes (i.e. 'Baja B.C.' behaviour) and record ∼8 • of northward translation and ∼50 • of clockwise rotation relative to both the YTT and North American APWPs during the Mesozoic and Paleogene.

The 3D Geophysical Investigation of a Middle Cretaceous to Paleocene Regional Décollement in the Cordillera of Northern Canada and Alaska

“…Shortening in the Selwyn Mountains (Figure ) is estimated to be in excess of 150 km, but constraint on the timing is bracketed by deformed Late Triassic (Norian) sedimentary rocks intruded by undeformed middle Cretaceous plutonic rocks (~93–98 Ma) (Cecile, , ; Cecile et al, ). These estimates are lower than those from the paleomagnetism of Cretaceous‐ and Eocene‐aged plutonic rocks, which suggest that less than ~800 km of northward displacement of the Intermontane terranes (e.g., 650 ± 450 km, Symons et al, , ; 780 ± 730 km, McCausland et al, ) was accompanied by 305 ± 125 km of ENE‐WSW shortening across Selwyn basin and the Mackenzie Mountains (Symons et al, , , ). Approximately two thirds of the related terrane rotations are interpreted to have occurred between the middle Cretaceous and Early Eocene, with the remainder post‐54 Ma (Symons et al, , , ).…”

“…The outliers are commonly mapped to be block and internally faulted (e.g., Yukon Geological Survey, ), suggesting that they would also have been tilted. In addition, geobarometry (Harris et al, , , ; McCausland et al, , , ; Symons et al, , , ; Symons & McCausland, ) and fluid inclusion studies (Baker & Lang, ) of middle to Late Cretaceous plutonic rocks suggest regionally variable (~4–13 km) and locally inconsistent estimates of exhumation (Figure a and Table ). For example, results from geobarometry of parts of the Dawson Range batholith suggest it was exhumed in the order of circa 11–13 km (McCausland et al, ).…”

“…The degree of tilting of plutons and their host rocks within the Intermontane terranes and Selwyn basin during displacement on the décollement, especially near to where it shallows and deformation is focused in the Mackenzie Mountains fold and thrust belt (Figure ), and also during subsequent displacement on the Tintina and related faults is difficult to constrain in an exact way. However, it is commonly inferred on the basis of hornblende geobarometry and/or local geological relationships that many of the region's middle to Late Cretaceous plutons have not been significantly tilted since emplacement (e.g., Harris et al, , ; McCausland et al, ; Symons et al, , ; Symons & McCausland, ). For example, the Prospector Mountain stock (PM, Figure a) is inferred by Symons et al () to have not been tilted, but the intrusion is shown to be faulted along with the Late Cretaceous Carmacks Group volcanic rocks it intrudes (Ryan et al, ).…”

U-Pb isotopic ages of euhedral zircons in the Rhaetian of British Columbia: Implications for Cordilleran tectonics during the Late Triassic

“…It has been postulated that the Yukon-Tanana terrane had already been accreted onto the North American margin by the Early or Middle Triassic, an interpretation supported by the cessation of arc activity on the terrane, the formation of regional unconformities, the deposition of a middle to upper Triassic overlap assemblage, and the presence of a Permian stitching plu-ton (Nelson et al, 2006;Nelson and Colpron, 2007;Beranek and Mortensen, 2011). Paleomagnetic evidence suggests that the terrane accreted at a position on the margin similar to that which it occupies today (Symons et al, 2015), although paleontological data suggest that the terrane must have accreted farther south (Orchard, 2006). The Quesnel terrane is thought to have accreted to the margin in the Early Jurassic (186-180 Ma;Nixon et al, 1993), and paleontological evidence places it far to the south of its current position relative to the North American margin during the latest Triassic (Stanley and Nelson, 1996;Stanley and Senowbari-Daryan, 1999).…”

“…The Stikine terrane was outboard of the margin during the Late Triassic, but retained ties to the Yukon-Tanana and Quesnel terranes (Nelson and Mihalynuk, 1993;Mihalynuk et al, 1994;Nelson and Colpron, 2007). However, paleontological evidence suggests a more southern position for the Stikine terrane during the Triassic (Reid, 1985;Reid and Tempelman-Kluit, 1987), and paleomagnetic data imply that the terrane was far south of the latitude of the Yukon-Tanana terrane until after the Cretaceous (Symons et al, 2015). Conflicting data and uncertainty over the relationships between these three terranes during the Triassic limits our current understanding of the mechanisms and timing of the formation of the Canadian Cordillera.…”