Structural and Metamorphic History of the Leech River Shear Zone, Vancouver Island, British Columbia

Abstract

Subduction zone dynamics are strongly influenced by the mechanical properties of subduction interface shear zones (Behr et al., 2022). However, the relevant properties of subduction interfaces, such as their widths, rock types, and internal structure, cannot be determined via geophysical observation. Instead, field-and microscale observations of exhumed shear zones that were active during subduction are needed to fill this knowledge gap. Subduction dynamics and thermal gradients evolve as they mature (e.g., Holt & Condit, 2021), and subduction zones can encompass a broad range of environments (Chelle-Michou et al., 2022), for example, when influenced by atypical subduction geometries like ridge subduction (e.g., Brown, 1998;DeLong et al., 1979;Sakaguchi, 1996). A more complete inventory capturing the variability of subduction interfaces is therefore needed to better understand their behavior.The Canadian Cordillera potentially exposes numerous subduction-related structures as it was assembled through a series of accretionary orogenies. One candidate structure is the Leech River Fault (Figure 1), a terrane-bounding fault located on southern Vancouver Island in British Columbia that juxtaposes mafic volcanic and igneous rocks of the Metchosin Igneous Complex (MIC) to the south with the predominantly metapelitic Leech River Complex (LRC). For clarity, we will term this structure the "Leech River Shear Zone" (LRSZ) to differentiate it from the recently active "Leech River Fault," which is locally coincident (

Pattern and source of unusually high‐temperature metamorphism in an Eocene forearc recorded by the Pacific Rim Terrane, British Columbia, Canada

“…The on‐strike extension of this body to the west is a strained rock with an appearance atypical of a felsic igneous rock (Figure 4c). The presence of an extensive population of detrital zircons with ages from 61 Ma to >2 Ga in this rock confirms that the previously named granodiorite sill at Valentine Mountain is instead a psammite (Seyler et al, 2022), as originally proposed by Fairchild and Cowan (1982).…”

“…The Leech River Complex comprises predominantly pelites and psammites, with subsidiary interfoliated tuffaceous, felsic and basic meta‐igneous units (Fairchild & Cowan, 1982; Groome et al, 2003). Metamorphic grade varies from lower greenschist facies in the north and east, to amphibolite facies in the south near the Leech River shear zone (Seyler et al, 2022). Pelites dominate, including graphitic phyllites and schist in which nearly all depositional textures are overprinted by penetrative deformation, thermal annealing and porphyroblast growth.…”

“…There is no increase in the abundance or size of any porphyroblasts in the metasedimentary units adjacent to these sills. The tonalite sills near the Leech River shear zone have U‐Pb zircon ages of 51 to 47 Ma (Groome et al, 2003; Seyler et al, 2022) and in places contain <10 cm long fragments of foliated rock (Figure 4a,b). Groome et al (2003) identified these fragments as xenoliths of the surrounding schist, but the mineral assemblage comprises nearly only biotite + quartz, a rock not observed elsewhere in the terrane.…”

“…However, this model has not fully assessed the detailed distribution of metamorphism and whether the extent of observed plutonic rock could provide sufficient heat to metamorphose the Leech River Complex. Furthermore, the inferred felsic pluton that served as a heat source for the mid‐Cretaceous phase of metamorphism in the region (Groome et al, 2003) was interpreted by earlier workers as a large metasandstone body (Fairchild & Cowan, 1982) and has been shown to contain a clear population of detrital zircons with multiple ages spanning 61 Ma to >2 Ga (Seyler et al, 2022).…”

“…This study provides detailed thermobarometric constraints for the Pacific Rim Terrane over a broader area than previous work, using Raman spectroscopy of carbonaceous material (RSCM) thermometry and garnet‐biotite geothermobarometry, and is a complement to the structure and metamorphism documented along a shear zone at its southern boundary (Seyler et al, 2022). Additionally, pseudosection results for a set of typical metasediment compositions are used to further assess the metamorphic P–T history.…”