Geology of Quiet Lake and Finlayson Lake map areas, south-central Yukon - an early interpretation of bedrock stratigraphy and structure

The Pelly Mountains in central Yukon expose a cross-section through part of the collision orogen that is the Canadian Cordillera. Elements of this cross-section are the pre-tectonic autochthonous succession, klippen of sheared and metamorphosed allochthonous rocks, thrusts that repeat the autochthonous succession, late tectonic regional metamorphic rocks and associated granite batholiths, and the Tintina fault, a late tectonic dextral strike-slip fault. Late Proterozoic through Triassic shelf strata, six to eight kilometres thick, comprise the autochthonous sequence. They are divided into four main divisions bounded by unconformities or a depositional hiatus. Oldest are Eocambrian and Lower Cambrian shale and siltstone, capped by an argillaceous limestone, which together form the Ketza group. Unconformably above it are Upper Cambrian and Ordovician slate, phyllite and alkaline basalt of the Kechika Group, and Silurian and Devonian siltstone, dolostone and quartz sandstone, of the Askin group. Profound and rapid facies variation characterizes this second division. Also part of this division is the shale-dominated Harvey group, which is time-equivalent to the Kechika and Askin groups. Unconformably above are Upper Devonian and Mississippian black slate and marine felsic volcanic rocks - the Seagull group. The fourth division is thin and restricted in distribution. It includes a Permian and an Upper Triassic siltstone separated by a disconformity. The autochthonous succession extends to, and is like that in, the Cassiar Mountains; it has equivalent units in the northern Rocky Mountains. Broadly it is equivalent to the outer shelf of the North American miogeoclinal wedge. Three assemblages of ductile deformed rocks are thrust over the "in place" succession from the southwest. Although each assemblage is dismembered so that its internal stratigraphy is unknown, the three are not mixed, and exhibit a consistent structural order. Lowest is a mylonite schist, that represents Triassic (and older?) immature clastic rocks and intermediate volcanics. Next is an assemblage of Late Paleozoic amphibolite and serpentinite with flaser limestone. Highest are granodiorite gneiss and schist, the sheared and metamorphosed remnant of Paleozoic intrusive rocks. K-Ar ages show that all three assemblages were sheared and metamorphosed during the Late Triassic and Early Jurassic. Four northeast directed thrusts repeat the "in place" beds beneath, and northeast of, the transported strata. Cumulative shortening is on the order of 100 kilometres. Each thrust carries a slice two or three kilometres thick and is itself deformed by newer thrusts, folds and tear faults. The thrusts, interpreted as splays of one basal detachment, formed in sequence from southwest to northeast during the Late Jurassic and Early Cretaceous. On the southwest the structurally imbricated "in place" strata and the transported rocks above them, were regionally metamorphosed about the late Early Cretaceous to form the Big Salmon and Mink complexes. At the same time partial melting produced granites like the Quiet Lake, Nisutlin and Big Salmon batholiths. The metamorphic rocks are exposed in large structural culminations centred roughly on the batholiths; these fold early thrusts, giving a structural relief near eight kilometres. Some granites may be cut by thrusts below and the metamorphic culminations may be severed by the basal detachment. Tintina fault is the locus of 450 kilometres of dextral slip as shown by offset of the autochthonous succession. Movement followed intrusion of most or all of the late tectonic granites. Movement may have been ceased by Early Eocene when fluvial conglomerate was laid down along the fault. Because its slip coincides with shortening in the Selwyn and Mackenzie mountains the Tintina is considered to be a listric branch of the same detachment surface on which shortening was accomplished. It is viewed as a giant tear fault in the detached slab. A model in which the Pelly Mountains are the product of Middle Jurassic oblique collision of a volcanic arc with the then-western margin of ancient North America fits these data. Sheared metamorphic rocks, which are interpreted as the arc?s subduction complex, were obducted in the Late Jurassic. Cretaceous imbrication and dextral translation of the autochthonous rocks are responses to obduction, while metamorphism and plutonism reflect heating of the overridden and imbricated "in place" slab.