Cretaceous Coast Belt paleomagnetic data from the Spetch Creek pluton, British Columbia: evidence for the "tilt and moderate displacement" model

Abstract

Well-grouped stable magnetizations have been isolated at 14 of 20 sites sampled from the Spetch Creek pluton. The single-polarity primary magnetization directed at D = 026.2°, I = 73.5° (α95 = 4.4°, paleolatitude 59 ± 7°N, paleopole 73°N, 078°W, A95 = 7°) was acquired around 88 Ma during the Cretaceous normal polarity superchron (118–84 Ma). This direction is discordant from the expected mid-Cretaceous direction (D = 332.5°, I = 75.1°) for North America. The difference could be caused by one of two end member models: the all-tilt model requires a 15° east-side-up tilt about a horizontal axis striking 353°, and the displacement–rotation model requires 330 ± 770 km of northward displacement combined with 54 ± 14° of clockwise rotation. Regardless, this result provides a negative test of the Baja British Columbia model, which requires ~ 2400 km of northward displacement.A review of previously observed mid-Cretaceous magnetizations from the Coast Belt, which are also discordant, indicates that they exhibit common characteristics, although their discordance is not uniform. Assuming that present horizontal approximates paleohorizontal, post-mid-Cretaceous latitudinal displacements inferred from individual results vary between 330 and 3500 km northwards. Relative rotations about vertical axes vary between 17 and 57° clockwise. Such variation cannot be accounted for by the displacement and rotation of a superterrane as a whole. Recent studies emphasize that many of the intrusions have probably been locally tilted. Consistent with known geology, these discordant poles are best explained by the "tilt and moderate displacement" model. This model invokes moderate (500–1000 km) post-mid-Cretaceous northward displacement of the amalgamated Insular, Coast, and Intermontane terranes west of the major dextral Canadian Cordilleran fault systems, combined with variable local block tilting east- and northeast-side-up. Northward displacement was driven by Kula and (or) Farallon – North American plate interactions from 90 to 56 Ma. Tilting is most likely due to a combination of northeast–southwest compression, differential uplift, and extension.

Large (1000 to 4000 km) northward movements of tectonic domains in the northern Cordillera, 83 to 45 Ma

“…Displacements, with one exception, increase systematically from the eraton outward. The exception (ST) is from the Spetch Creek pluton whose tilt is unknown [Vandall, 1993].…”

“…of very large, post mid-Cretaceous, northward motion focussed on the absence of information on tilt of plutohie bodies from which most paleomagnetic observations had at that time been obtained. They argued that undetected southwesterly tilts could be the primary cause of discordant inclinations, and there was no need to invoke large-scale translations [Symons, 1974[Symons, , 1977Butler et al, 1989;Brown and Burroester, 1991;Vandall, 1993].…”

Paleomagnetism and geochronology of the Ecstall pluton in the Coast Mountains of British Columbia: Evidence for local deformation rather than large‐scale transport

“…There are no direct lithologic recordings of paleohorizontal as with stratified rocks. As a result, a ''tilt versus translation'' controversy has ensued regarding the discordant paleomagnetic directions from plutonic rocks of the North Cascades and Coast Mountains batholith [Butler et al, 1989;Brown and Burmester, 1991;Vandall, 1993;Brandon, 1996, 1997;Anderson, 1997].…”

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

“…Umhoefer (2003; see also Umhoefer and Blakey 2006) has also sought terrane correlations that support a microplate tectonic model with a moderate northward translation estimate of *1600-1800 km that better fits the less extreme paleomagnetic estimates for northward translation. On the paleomagnetic side, Vandall (1993) and Butler et al (2001a) recognized that the two hypotheses need not be mutually exclusive. In a study of the Spetch Creek pluton, Vandall (1993) found that the discordances in the then available CPC Cretaceous paleopoles could be well explained by a combination of 500 or 1000 km of northward translation plus 178 to 578 of clockwise rotation with minimal ENE-side up tilts of only *28.…”

“…On the paleomagnetic side, Vandall (1993) and Butler et al (2001a) recognized that the two hypotheses need not be mutually exclusive. In a study of the Spetch Creek pluton, Vandall (1993) found that the discordances in the then available CPC Cretaceous paleopoles could be well explained by a combination of 500 or 1000 km of northward translation plus 178 to 578 of clockwise rotation with minimal ENE-side up tilts of only *28. Building on an analysis by Beck (1992), paleomagnetists started to consider more carefully the temporal consequences on a pluton's ChRM from tilting and uplift by using Alin-hornblende geothermobarometry and combinations of radiometric age dating systems (Harris et al 1997;Butler et al 2002;Housen et al 2003).…”