Evolution of the Late Cretaceous Nanaimo Basin, British Columbia, Canada: Definitive provenance links to northern latitudes

Abstract

Accurate reconstruction of the Late Cretaceous paleogeography and tectonic evolution of the western North American Cordilleran margin is required to resolve the long-standing debate over proposed large-scale, orogen-parallel terrane translation. The Nanaimo Basin (British Columbia, Canada) contains a high-fidelity record of orogenic exhumation and basin subsidence in the southwestern Canadian Cordillera that constrains the tectonic evolution of the region. Integration of detrital zircon U-Pb geochronology, conglomerate clast U-Pb geochronology, detrital muscovite 40Ar/39Ar thermochronology, and Lu-Hf isotopic analysis of detrital zircon defines a multidisciplinary provenance signature that provides a definitive linkage with sediment source regions north of the Sierra Nevada arc system (western United States). Analysis of spatial and temporal provenance variations within Nanaimo Group strata documents a bimodal sediment supply with a local source derived from the adjacent magmatic arc in the southern Coast Mountains batholith and an extra-regional source from the Mesoproterozoic Belt Supergroup and the Late Cretaceous Atlanta lobe of the Idaho batholith. Particularly robust linkages include: (1) juvenile (εHf >+10) Late Cretaceous zircon derived from the southern Coast Mountains batholith; (2) a bimodal Proterozoic detrital zircon signature consistent with derivation from Belt Supergroup (1700–1720 Ma) and ca. 1380 Ma plutonic rocks intruding the Lemhi subbasin of central Idaho (northwestern United States); (3) quartzite clasts that are statistical matches for Mesoproterozoic and Cambrian strata in Montana and Idaho (northwestern United States) and southern British Columbia; and (4) syndepositional evolved (εHf >−10) Late Cretaceous zircon and muscovite derived from the Atlanta lobe of the Idaho batholith. These provenance constraints support a tectonic restoration of the Nanaimo Basin, the southern Coast Mountains batholith, and Wrangellia to a position outboard of the Idaho batholith in Late Cretaceous time, consistent with proposed minimal- fault- offset models (<~1000 km).

Improved Provenance Constraints for Nanaimo Group Sediments, British Columbia, Canada, Through Zircon LA‐ICP‐MS Depth‐Profiling

“…The earliest sediments in the Nanaimo Basin (mostly detrital zircon Facies 1), the Comox Formation, were supplied by local sediment sources in Wrangellian basement and short transverse drainages reaching east into the Coast Mountains Batholith (Coutts et al., 2020; Huang et al., 2019; Mahoney et al., 2021; Matthews, Guest, Coutts, et al., 2017; Mustard et al., 1995) (Figure 12). Detrital zircon populations were dominated by Facies 1 distributions containing mostly Mesozoic zircon.…”

“…Despite the abundance of evidence supporting large magnitude northward displacement of the Insular Superterrane, two very different Late Cretaceous paleogeographic reconstructions have been proposed. The first paleogeography (Model 1; see Mahoney et al., 2021) places the Insular Superterrane entirely north of the Sierra Nevada Batholith at 100 Ma. This interpretation is largely consistent with the paleogeographic reconstruction of Wyld et al.…”

“…Facies 1 zircon populations are characterized by modes at 150 and 90 Ma and are interpreted to have been derived from the adjacent Coast Mountains Batholith and Cascade Crystalline Core (Alberts et al., 2021; Cecil et al., 2018; Coutts et al., 2020; Englert et al., 2018; Huang et al., 2019; Mahoney et al., 1999, 2021; Matthews, Guest, Coutts, et al., 2017; Mustard et al., 1995) where plutonic rocks of these ages are common (Cecil et al., 2018; Friedman & Armstrong, 1995; R. B. Miller et al., 2009) (Figure 1). Minor Paleozoic populations in some samples are interpreted to have been derived from rocks of the Sicker Group of the Wrangellia terrane (Coutts et al., 2020; Mahoney et al., 2021), consistent with interpreted local sediment sources for similar populations in Comox Formation samples from the base of the Nanaimo Group (Alberts et al., 2021; Huang et al., 2019, 2022; Mahoney et al., 2021) (Figure 2).…”

“…Proponents of Model 1 paleogeographies interpret Jurassic and Cretaceous zircon populations to derive from the Coast Mountains Batholith, while some Late Cretaceous grains derive from the Idaho Batholith and the Colorado Mineral Belt. Paleoproterozoic zircon populations (1,800–1,600 Ma) were interpreted to have been recycled from Mesoproterozoic metasedimentary rocks of the Lemhi subbasin in central Idaho while the Mesoproterozoic zircon populations (∼1,380 Ma) derive from metaigneous rocks that intruded the Lemhi subbasin (Evans & Zartman, 1990; Mahoney et al., 2021) (Figure 1). Proponents of Model 2 paleogeographies interpret Mesozoic zircon populations to derive from the Coast Mountains Batholith, the Cascade Crystalline Core, and at least partly from abundant <80 Ma plutons that intruded the Mojave region (Anderson & Cullers, 1990; Armstrong & Suppe, 1973; Barth et al., 2013; Matthews, Guest, Coutts, et al., 2017; Sauer et al., 2019; Wells et al., 2005).…”

“…Contrasting estimates for the displacement of these rocks from known fault displacements (<1,000 km) (e.g., Wyld et al., 2006) and paleomagnetic constraints (>1,500 km) (e.g., Bogue & Grommé, 2004; Enkin et al., 2003; Housen et al., 2003; Rusmore et al., 2013) make very different provenance predictions for Late Cretaceous sedimentary rocks of the western Cordillera (Cowan et al., 1997). So far, detrital zircon results from these rocks have proved equivocal and have been interpreted to support either small‐magnitude (Mahoney et al., 1999, 2021; Mustard et al., 1995) or large‐magnitude northward displacement relative to North America (Garver & Davidson, 2015; Housen & Beck, 1999; Matthews, Guest, Coutts, et al., 2017; Sauer et al., 2019). To advance our understanding of Cordilleran orogenesis, new constraints on the Late Cretaceous paleolatitude of accreted crustal fragments that comprise the orogen are required.…”