Crustal architecture of SW Yukon, northern Cordillera: Implications for crustal growth in a convergent margin orogen

Abstract

A structural analysis of southwest Yukon based on mapping and the compilation of structural data is used to determine if convergent margin orogenesis has contributed to westward growth of North America. The crust of SW Yukon is tilted regionally to the ESE, exposing a >20 km section of crust. A down‐plunge profile is used to determine the geometry and evolution of the crust. Four lithotectonic packages are recognized. These are, from east to west, (1) the Triassic‐Jurassic Whitehorse Trough assemblage of arc and arc‐derived volcanic and sedimentary rocks, (2) the Devonian‐Mississippian continental Aishihik metamorphic assemblage, (3) the Jurassic‐Cretaceous metapelitic Kluane metamorphic assemblage, and (4) the Jurassic‐Cretaceous Dezadeash Group of arc‐marginal graywackes. The Early Jurassic Aishihik and Tertiary Coast plutonic suites are major synkinematic tabular intrusive complexes that facilitated crustal growth by subcretion of the Aishihik metamorphic assemblage beneath the Whitehorse Trough and the Kluane metamorphic assemblage and Dezadeash Group beneath Aishihik metamorphic assemblage and overlying strata, respectively. Early Jurassic crustal growth gave rise to a crust that became older and more radiogenic at depth; Tertiary growth gave rise to a crust that became younger and less radiogeneic at depth. Crustal growth by subcretion necessarily involved the removal of the lower crust and mantle of the upper plate. The lithotectonic assemblages and the tabular intrusive complexes along which they were assembled are identifiable in the seismic reflection data along Slave Northern Cordillera Lithosphere Evolution Project (SNORCLE) line 3 and are broadly imaged by a regional electromagnetic (EM) survey. The ACCRETE program to the south has documented a comparable and directly correlative crustal geometry across the Great Tonalite Sill. Our findings are inconsistent with crustal growth models of tectonic flakes being emplaced onto an autochthonous lower crust and mantle. Instead our findings support models (Lithoprobe Vancouver Island, and ACCRETE) of crustal growth at convergent margins as resulting from synmagmatic subcretion.

Upper-crustal cooling of the Wrangellia composite terrane in the northern St. Elias Mountains, western Canada

“…Rocks of the Aishihik metamorphic assemblages (late Proterozoic-Mississippian) and Aishihik plutonic complex (ca. 186 Ma) are exposed within the study area as well (Erdmer and Mortensen, 1993;Johnston and Erdmer, 1995;Johnston et al, 1996;Johnston and Canil, 2007;Israel et al, 2011). Along the entire North American Cordillera, the southern margin of the Yukon composite terrane and the northern margin of the Wrangellia composite terrane are characterized by regional, mid-Cretaceous folding, thrusting, and metamorphic overprint, which is interpreted to reflect the final accretion of the Wrangellia composite terrane (e.g., Rubin et al, 1990;Dusel-Bacon et al, 1993;Rusmore et al, 2000;Gehrels, 2001).…”

Crustal influx, indentation, ductile thinning and gravity redistribution in a continental wedge: Building a Moldanubian mantled gneiss dome with underthrust Saxothuringian material (European Variscan belt)

“…It highlights the fact that neither the Stronie–Młynowiec metasedimentary rocks nor the augen orthogneiss experienced widespread HP metamorphic conditions. The contrasting metamorphic gradients between the MP–MT Stronie metasedimentary rocks and HP–LT eclogite and orthogneiss are explained by the continuous underplating of felsic material; this kept the metasedimentary units at a relatively moderate crustal level while the granulitized and eclogitized units underwent deep subduction and subsequent subcretion at the bottom of the continental wedge [e.g., Johnston and Canil , 2007].…”

Protolith of ultramafic rocks in the Kluane Schist, Yukon, and implications for arc collisions in the northern Cordillera

“…The Cordillera in Southwest Yukon exposes a crustal section from east to west 75 (5) Dezedeash Group -a Jurassic-Cretaceous sequence of arc-derived greywackes 80 (Johnston and Canil, 2007). The latter two units are divided by the Denali fault, a major 81 dextral strike slip fault of the region (Figs.…”

“…interpreted by Johnston and Canil (2007) to be a crustal section on the order of 40 km 50 thick that youngs and becomes more juvenile with depth (Johnston and Canil, 2007) In this study, we undertake a detailed examination of the petrology and 61 geochemistry of ultramafic rocks that occur throughout the Kluane Schist. The general 62 occurrence of these rocks was first described by Templeman-Kluit (1974).…”

“…Mesozoic magmatism and 637 metallogeny of the Hotailuh batholith, northwestern British Columbia, Ministry of 638 Energy, Mines and Natural Gas, British Columbia Geological Survey, Open File 639 report 2012-10, 77 pp. Regional geology of the western Canada showing allochtonous assemblages of 644 the northern Cordillera (afterJohnston and Canil, 2007). Dashed box shows location ofFigure 2.…”