A compaction correction for the paleomagnetism of the Nanaimo Group sedimentary rocks: Implications for the Baja British Columbia hypothesis

Abstract

A paleomagnetic and magnetic anisotropy study of the Late Cretaceous Northumberland formation on Hornby Island, British Columbia, was conducted to determine if burial compaction could have caused its anomalously shallow inclinations. The shallow Nanaimo Group inclinations have been used to support the Baja British Columbia (Baja BC) model of continental dynamics in which superterranes were transported thousands of kilometers northward along the active North American continental margin in the Cretaceous. A mean of the site means from the Northumberland formation is D = 5.8°, I = 50.9°, α95 = 7.9° (N = 8). Anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanence (AAR) were measured to identify and correct for any inclination shallowing caused by depositional/compactional processes. Both concretion and fine‐grained rock samples had typical depositional/compactional AMS and AAR fabrics with bedding perpendicular minimum principal axes. A revised correction equation was used to account for the effects of either triaxial magnetic fabrics or small angular deviations of principal axes from the bedding plane. The average inclination was increased by 9.6° after the compaction correction (D = 6.1°, I = 60.5°, α95 = 7.1°, N = 8), and the mean inclination of concretion and fine‐grained rock sites was significantly steeper than Ward et al.'s [1997] result (I = 42.5°) placing Hornby Island and the Insular superterrane at a paleolatitude of 41°N in the Late Cretaceous. This paleolatitude indicates that Baja BC did translate northward since the Late Cretaceous by about 1600 km but not by the 3500 km previously indicated.

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“…75 Ma (as shown by its apparent polar wander path; Torsvik et al, 2001); this and the smaller migration of the arc in southern Mexico (Henry et al, 2003) suggest that accelerated westward motion was higher to the north. The arc also migrated to the east in Canada (van der Heyden, 1992;Gehrels et al, 2009), but as the position of these rocks with respect to North America in the Late Cretaceous is disputed (e.g., Krijgsman and Tauxe, 2006;Cowan et al, 1997;Butler et al, 2001;Kim and Kodama, 2004), it is not entirely clear that this refl ects accelerated motion of North America. Less directly, the marine transgression in the Cretaceous that led to the Western Interior Seaway connecting the Gulf of Mexico to the Arctic Ocean is likely to have been the product of subsidence due to changes in fl ow in the upper mantle induced by shallowing subduction (Burgess et al, 1997;Mitrovica et al, 1989).…”

Evolution of the Jura-Cretaceous North American Cordilleran margin: Insights from detrital-zircon U-Pb and Hf isotopes of sedimentary units of the North Cascades Range, Washington

“…55 Ma (Garver and Brandon, 1994;Enkin et al, 2002). More recent paleomagnetic data from the Nanaimo basin that have been corrected for depositional flattening of magnetic inclinations suggest the strata originated significant distances (~1600-2000 km) to the south and were adjacent to the southern Sierra Nevada in the Late Cretaceous (Kim and Kodama, 2004;Krijgsman and Tauxe, 2006). Furthermore, the presence of Archean detrital zircons (older than ca.…”

“…72 Ma and were assembled in Washington by the early Tertiary. Strata with similar detritalzircon signatures currently located at northern latitudes (e.g., the Nanaimo Group and Yakutat Group of the Chugach terrane) are also interpreted to have been deposited at the latitude of the southern Sierra Nevada based on paleomagnetic data and detrital zircons, respectively (i.e., Kim and Kodama, 2004;Garver and Davidson, 2015;Matthews et al, 2017). These results support moderate terrane translation (~1600-2000 km) and place these units at the breach in the southern Sierra Nevada ca.…”

“…A combination of data from known offsets on identified faults, lithologies, and reconstructions based on paleomagnetic data has been used to argue that the rocks of the North Cascades were translated from more southerly latitudes to their present position. However, the various models suggest a wide range of points of origin and thus amounts of translation: (1) ~700 km from the latitude of the Klamath Mountains and northern Sierra Nevada (Brandon et al, 1988;Garver, 1988;Wyld et al, 2006); (2) ~1600 km from the latitude of the southern end of the Sierra Nevada batholith (Umhoefer, 2003;Kim and Kodama, 2004;Umhoefer and Blakey, 2006); or (3) ~2300-2500 km at the latitude of northern Mexico during the mid-Cretaceous ("Model B" of Cowan et al, 1997; "Baja-BC" hypothesis; Beck and Noson, 1972;Enkin et al, 2001).…”

“…90-55 Ma (the "Baja-BC" hypothesis; Beck et al, 1981;Umhoefer, 1987;Ague and Brandon, 1996;Enkin et al, 2001). More recent paleomagnetic studies interpret moderate (~1600-2000 km) translation of these same rocks from reconstructions based on a combination of corrections to the paleomagnetic data and known fault offsets (Kim and Kodama, 2004;Krijgsman and Tauxe, 2006;Umhoefer and Blakey, 2006;Rusmore et al, 2013). Conversely, evidence from measured lateral offsets on faults as well as stratigraphic correlations between similar packages of rocks indicate significantly less (<1000 km) northward translation (Price and Carmichael, 1986;Monger, 1997;Wyld et al, 2006).…”