Structure, petrology, and tectonic history of the Leech River complex northwest of Victoria, Vancouver Island

Abstract

The Leech River complex 45 km northwest of Victoria consists of metamorphosed pelitic rocks, sandstone, and minor volcanic rocks, chert, and conglomerate of probable Late Jurassic to Cretaceous age. The assemblage experienced two similar deformational events during which regional shortening induced macroscopic east-plunging folds and related coaxial, mesoscopic linear structures, parasitic folds, and axial-plane cleavages. Fragmentation along the developing cleavages disrupted layering and eventually led to transposition during both events. Regional, progressive, low-pressure greenschist-to amphibolite-facies (andalusite-staurolite-biotite) metamorphism began during the first deformation and extended into the waning stages of the second. Intrusion of composite felsic sills was synchronous with deformation and metamorphism, which concluded about 39-41 Ma, according to K-Ar data. The Leech River fault, which forms the southern boundary of the complex, is a zone of two to four subparallel faults. All are relatively straight, narrow faults that appear to dip steeply. This structure is interpreted to be a left-lateral strike-slip fault, active exclusively after the 39-41 Ma conclusion of metamorphism and deformation.The Leech River complex originally may have accumulated somewhere along a late Mesozoic convergent margin, but there is no evidence that it either constitutes a subduction complex per se or was metamorphosed in such a setting in early Tertiary time. The Leech River complex is interpreted to be allochthonous with respect to the bulk of Vancouver Island, since neither older rocks of the Insular Belt (Wrangellia) to the north nor coeval rocks in northwestern Washington record the early Tertiary deformations and synkinematic low-pressure metamorphism. The complex apparently was derived from a cryptic terrane to the west and emplaced against Vancouver Island by left-lateral slip on the San Juan fault after 39-41 Ma.Le complexe Leech River, A 45 km au nord-ouest de Victoria, est composC de roches p6litiques mCtamorphostes, de grks, et en importance secondaire de roches volcaniques, de chert et conglomCrat probablement d'lge Jurassique sup6rieur i CdtacC.L'assemblage A subi deux phases analogues de dkfomation entrainant un rCtrCcissement dgional produisant des plis macroscopiques de plongCe vers I'est avec des axes parallkles, des structures IinCaires mCsoscopiques, des plis parasites et des clivages ardoisiers axiaux. La fragmentation causCe par les clivages a rompu la stratification et a provoquC Cventuellement uQe transposition durant les deux phases de dkformation. I1 est apparu pendant la premikre phase de deformation un faciks mktamorphique regional, progressif et de basse-pression, allant des schistes verts A celui de I'amphibolite (andalousite-staurotide-biotite) dont le developpement s'est poursuivi jusqu'i la @riode de dicroissance de la deuxikme dCfomation. Des filons-couches composites de roches felsitiques ont Ctt inject& lors de la dkformation et pendant le mCtamorphisme, et dont la fin des activitC...

Quaternary Rupture of a Crustal Fault beneath Victoria, British Columbia, Canada

“…The topographic scarps we identified are roughly parallel to the previously mapped location of the Leech River fault (Fairchild and Cowan, 1982;Massey et al, 2005), but none of the identified fault scarps coincide exactly with the fault contact between the Leech River Complex and the Metchosin Formation (Fig. 2).…”

“…Several previous authors suggest that this fault, which places Jurassic-Cretaceous schists of the Leech River Complex to the north against Eocene basalts of the Metchosin Formation to the south ( Fig. 2A) (Fairchild and Cowan, 1982;Rusmore and Cowan, 1985), was last active in the Eocene (MacLeod et al, 1977;Johnston and Acton, 2003). Yet, trenching, coring, and geophysical studies indicate multiple Quaternary ruptures of adjacent fault systems in Washington state, USA, including the Southern Whidbey Island fault, the Utsalady Point fault, and the DarringtonDevil's Mountain fault ( Fig.…”

“…Here, we use a combination of techniques to delineate Quaternary fault-related features along the Leech River fault, including (1) mapping of fault scarps from hillshade and local slope images generated from a high resolution (~2 m horizontal by ~10 cm vertical) LiDAR digital elevation model (DEM) collected by Natural Resources Canada (James et al, 2010); (2) first-order bedrock and surficial field mapping; (3) collection of detailed structural and geomorphic data at key sites; and (4) compilation of our observations with data from previous studies (e.g., Fairchild and Cowan, 1982 (Massey et al, 2005), surficial deposits (Blyth and Rutter, 1993), and local ice flow indicators (blue arrows, this study). Foliation measurements from this study are shown in bold and those from Muller (1983) are shown in italics.…”

Magmatic and tectonic history of the Leech River Complex, Vancouver Island, British Columbia: Evidence for ridge-trench intersection and accretion of the Crescent Terrane

“…Within a northern segment that extends from southeastern Alaska to southern Vancouver Island, forearc igneous rocks are hosted in accretionary complexes, including the Pacifi c Rim and Chugach terranes (Hill et al, 1981;Fairchild and Cowan, 1982;Rusmore and Cowan, 1985;Brandon, 1989;Barker et al, 1992;Sisson and Pavlis, 1993;Haeussler et al, 1995;Harris et al, 1996;Groome, 2000). These igneous rocks consist mainly of intermediate to felsic intrusions associated with high-temperature metamorphic assemblages (Hill et al, 1981;Fairchild and Cowan, 1982;Rusmore and Cowan, 1985;Brandon, 1989;Barker et al, 1992;Sisson and Pavlis, 1993;Groome, 2000). Subordinate felsic volcanics and mafi c intrusions are present locally (Hill et al, 1981;Groome, 2000).…”

Reduced granitic magmas in an arc setting: The Catface porphyry Cu–Mo deposit of the Paleogene Cascade Arc

“…On Vancouver Island, the Pacific Rim Terrane lies structurally below Wrangellia and contains a considerable section of graphitic metasediments (Brandon, 1989;Fairchild and Cowan, 1982). Paleogene Catface magmas may have ascended through and interacted with this potentially reducing country rock.…”