Paleogeography of the Insular and Intermontane terranes reconsidered: Evidence from the southern Coast Mountains Batholith, British Columbia

Abstract

New geologic and paleomagnetic data from Knight Inlet in the southwestern Coast Mountains Batholith, British Columbia, support significant revision to the paleogeography of the Insular and Intermontane terranes. Recompilation of radiometric ages confi rms that after 100 Ma, a magmatic arc migrated northeastward across the Coast Mountains Batholith at ~2 km/m.y. Magmatic age patterns suggest that plutons older than 100 Ma intruded the Intermontane terrane, not the expected Insular terrane. The distribution of brittle faults along Knight Inlet defi nes a structurally intact central domain, ~45 km wide, fl anked to the SW and NE by faulted domains, with no evidence of the widespread Tertiary extension affecting the batholith farther north. Al-in-hornblende geobarometry yields emplacement depths of ~2.5-4 kbar and does not reveal systematic postemplacement tilting. Plutons in the central structural domain yield a consistently oriented paleomagnetic remanence presumably acquired as the Late Cretaceous arc cooled from ca. 110 to 85 Ma. In the absence of recognizable tilting, this result indicates ~1700 km of northward translation since ca. 85 Ma, which is signifi cantly less than predicted for the Insular terrane in the "Baja British Columbia" model but similar to results from the Intermontane terrane. The pluton ages and the paleomagnetic results suggest that the Intermontane terrane, not the Insular terrane, underlies the southwestern fl ank of the Coast Mountains Batholith. This conclusion is compatible with a paleogeographic model in which the Vancouver Island fragment of Wrangellia was juxtaposed against the Intermontane terrane prior to ca. 120-100 Ma and emplaced in southern British Columbia after ca. 75 Ma.

Logan Medallist 1. Seeking the Suture: The Coast-Cascade Conundrum

“…This amount has been questioned by Kodama and Ward (2001) who prefer ≤1500 km of northward displacement, based on compaction corrections and paleobiogeography. Results from the Late Cretaceous (~80 Ma) McColl Ridge Formation in southern Alaska, deposited in a tectonic setting similar to that of the Nanaimo strata, indicate northward translation of only ~1650 km (Stamatakos et al 2001), an amount (~1700 km) similar to that determined from plutons in the central Coast Mountains that arguably were not tilted (Rusmore et al 2013).…”

Along‐Strike Variation in the Magmatic Tempo of the Coast Mountains Batholith, British Columbia, and Implications for Processes Controlling Episodicity in Arcs

“…Our study area crosses the CMB at its widest, where the absence of significant Tertiary extension (Rusmore et al, 2013) preserves the record of batholith growth. Wrangellia of the Insular terrane underlies Vancouver Island west of our study area, and Triassic-Cretaceous strata of the Intermontane terrane lie east of the batholith.…”

“…For this study, we interpret the post-100 Ma magmatism as a single arc because, as discussed herein, magmatism migrates steadily eastward as expected in a single east-dipping arc and lacks disruptions that would signal significant translation. Post-100 Ma translations suggested by paleomagnetic results (Rusmore et al, 2013) likely took place on faults east of the batholith (Monger, 2014) and/or intrabatholithic faults parallel to the arc, leaving the magmatic pattern undisturbed. Better delineation of the distribution of the pre-100 Ma arc or arcs and the boundaries of proposed far-traveled blocks is needed to resolve these paleogeographic uncertainties.…”

“…The early Tertiary fabrics may mask a Late Cretaceous, possibly transcurrent, history locally preserved in SE Alaska (Gehrels, 2000). Similarly, Paleogene dextral and dextral-transpressive faulting (e.g., Crawford et al, 1999) found within the central batholith has not been recognized in the southern CMB (e.g., Journeay & Friedman, 1993;Rusmore et al, 2013). Subsequent rapid cooling and crustal extension from 52 to 48 Ma (e.g., Andronicos et al, 2003;Rusmore et al, 2005) present in the central batholith did not affect the southern batholith (e. g. Brown et al, 2000;Journeay & Friedman, 1993;Rusmore & Woodsworth, 1994;Rusmore et al, 2013).…”

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

“…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).…”