Cretaceous remagnetization in the Sylvester Allochthon: limits to post-105 Ma northward displacement of north-central British Columbia

Abstract

The Sylvester Allochthon of the Slide Mountain Terrane in northern British Columbia is a structurally interleaved assemblage of ocean-floor lithologies ranging in age from Late Devonian to Late Triassic. It is the most inboard of oceanic suspect terranes and rests as a vast klippe on miogeoclinal strata of the Cassiar Terrane. The Sylvester Allochton and the Cassiar Terrane lie west of the Northern Rocky Mountain Trench Fault. Both the Sylvester Allochthon and the Cassiar Terrane are intruded by mid-Cretaceous (105 Ma) granite of the Cassiar Batholith. Six oriented cores were collected at each of 12 sites in Guadalupian Parafusulina-bearing limestone of the Sylvester Allochthon at a location 4 km from the batholith. Isothermal remanent magnetization (IRM) acquisition and subsequent thermal demagnetization indicate that pyrrhotite is the dominant ferrimagnetic mineral. Least-squares line fitting to four thermal demagnetization steps between 150 and 310 °C was used to determine the characteristic natural remanent magnetization (NRM) directions that fail the fold test at the 99.5% confidence level. We interpret these observations as indicating that the NRM is a thermoremanent or thermochemical remanent magnetism associated with intrusion of the Cassiar Batholith. The resulting paleomagnetic pole location is latitude = 75.7°N, longitude = 171.7°E, α95 = 8.5°. When compared with the mid-Cretaceous pole for cratonic North America, a small but significant clockwise rotation (R ± ΔR = 23.9 ± 18.1 °) is evident, but poleward translation (p ± Δp = 5.3 ± 9.2°) is not significant at the 95% confidence level. The paleomagnetic results are consistent with geological evidence for moderate (700 km) northward transport of the Cassiar Terrane (along with the previously emplaced overlying Sylvester Allochthon) during mid-Cretaceous to Tertiary dextral transcurrent faulting.