Paleomagnetism of the Upper Cretaceous strata of Mount Tatlow: Evidence for 3000 km of northward displacement of the eastern Coast Belt, British Columbia

Abstract

This study contributes the first palcomagnetic data from dated Cretaceous units of the Coast Belt (51.3øN, 123.8øW) in which palcohorizontal bedding surfaces can be identified with certainty. We sampled 38 sites through a 1.5-km stratigraphic interval of Cenomanian Silverquick sedimentary strata and Cenomanian to Campanian Powell Creek volcanics in the Tatlow syncline in the Tyaughton-Methow Basin. Magnetizations at 21 sites have high unblocking temperatures (TUB) and pass the tilt test (precision k = 5 before untilting, maximum k = 48 after 100% untilting). Magnetic minerals of volcanics show original textures, mean declinations in volcanic and sedimentary rocks are statisticall•lelltlCal, and---'•: ........ ' •-• : ^,: •,.• • •o •.4 o pmanty is consistent + , 1101111•1,1. 111• mean lil•11natlon ..• _ •, probability P -0.05) of this pretilting magnetization is 20.1 ø _+ 4.0 ø shallower than that expected if the rocks were deposited in their present position relative to cratonic North America. Field and laboratory observations rule out other causes for these anomalous shallow inclinations. These rocks therefore have been displaced northward by 3000 +_ 500 km relative to cratonic North America since mid-Cretaceous time. This displacement is almost 3 times greater than that estimated from mid-Cretaceous volcanics in the nowadjacent Intermontane Belt. Evidently the two belts were not amalgamated until latest Cretaceous or Early Tertiary time. These results are inconsistent with all previous geologically based tectonic reconstructions of the Cordillera. Magnetizations at eight sites in Silverquick and Powell Creek beds have low Tus and directional characteristics similar to those in three younger diorite intrusions. Reversed and normal polarities occur, the rocks are commonly highly altered, and maximum precision occurs after 40% untilting. The magnetizations therefore are interpreted as overprints of latest Cretaceous age acquired during folding. magnetization direction to be corrected to the palcohorizontal plane, and (2) by obtaining observations from Cretaceous stratified rocks in which accurate bedding attitudes can be determined. The first test has already been carried out for the 96-93 Ma Mount Stuart batholith (Figure 1); Ague and Brandon [1992], on the basis of hornblende geobarometry, have corrected the palcomagnetic observations [Beck et al., 1981] for tilt and obtained an estimate of motion from the south of 2900 + 700 km. We describe the first palcomagnetic results from Cretaceous bedded rocks of the Coast Belt, which allow us to carry out the second test. The sedimentary and volcanic sequence studied is particularly suitable for six reasons. They contain thinly bedded, planar-laminated siltstones and fine-grained sandstones of fluvial overbank origin from which paleohorizontal surfaces can be determined with precision. The nøAr/39Ar data provide the age of the Powell Creek volcanics. The strata occur in a syncline, the Tatlow syncline (Figure 2), with an interlimb 6073 6074 WYNNE ET AL.' MOUNT TATLOW PA...

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

“…[4] Discordant paleomagnetic directions have also been observed in Cretaceous sedimentary and volcanic rocks of the northern North American Cordillera [Wynne et al, 1995;Ward et al, 1997]. Some workers have concluded that the consistently shallow paleomagnetic inclinations confirm the transport interpretation of discordant paleomagnetic directions from the plutonic rocks.…”

East-derived strata in the Methow basin record rapid mid-Cretaceous uplift of the southern Coast Mountains batholith

“…The extent of Cretaceous to early Paleogene displacement on the Pasayten fault is more controversial (e.g., Lawrence 1978;Butler et al 1989;Hurlow 1993;Wynne et al 1995;Wyld et al 2006). The Pasayten fault has been proposed as the locus for large-scale dextral translation; values of displacement on the Pasayten fault range from <1000 km (e.g., Butler et al 1989;Kodama and Ward 2001) to >3000 km of displacement (e.g., Wynne et al 1995;Irving et al 1996;Ward et al 1997).…”

Lithofacies control in detrital zircon provenance studies: Insights from the Cretaceous Methow basin, southern Canadian Cordillera

“…Controversial paleomagnetic data suggest that the basin (and the associated Insular superterrane) may have undergone large-scale northward latitudinal translation (e.g., Beck et al, 1981;Wynne et al, 1995;Irving et al, 1996;Enkin et al, 2003), potentially originating as far south as central Mexico (ϳ25ЊN). Conversely, structural data, basin reconstructions, timing constraints, and alternate interpretations of paleomagnetic data indicate that the basin developed at or near its current latitude and perhaps was trapped east of the Insular superterrane during sinistral (i.e., southward) translation in Cretaceous time (e.g., Butler et al, 1989;Miller et al, 1993;Umhoefer and Miller, 1996;Kodama and Ward, 2001).…”

“…Paleomagnetic data from the Intermontane and Insular superterranes have been interpreted to suggest that the modern Pasayten fault is coincident with the principal locus of (1) as much as 3000 km of dextral translation that occurred between 85 and 50 Ma (e.g., Wynne et al, 1995;Irving et al, 1996), or (2) less than 1500 km of translation (e.g., Butler et al, 1989;Kodama and Ward, 2001). Therefore, rocks on opposite sides of the modern Pasayten fault may or may not have been in geographic proximity in the Late Cretaceous, and strata of the Methow/Methow-Tyaughton basin may be cut off from their eastern source.…”