The Slide Mountain Terrane and the structural evolution of the Finlayson Lake Fault Zone, southeastern Yukon

Abstract

The Finlayson Lake Fault Zone forms a fundamental, but little studied, tectonic boundary between strata of autochthonous North America and the accreted Slide Mountain and Yukon–Tanana terranes in southeastern Yukon. A structural and petrologic study was undertaken to examine the depositional environment of the Slide Mountain Terrane, its tectono-thermal evolution in the fault zone, and its relationship with the Yukon–Tanana Terrane. The Slide Mountain and Yukon–Tanana terranes are divisible into units dominated by metavolcanic and metasedimentary rocks. Field observations and whole-rock geochemistry indicate that Slide Mountain greenstone is ocean-floor basalt deposited in a deep submarine basin with a proximal terrigenous sediment influx. Either a marginal- or ocean-basin setting is supported by the data. Slide Mountain greenstone is thrust northeastward over metasedimentary rocks of Slide Mountain Terrane and southwestward over rocks of the Yukon–Tanana Terrane. Regional metamorphic grade ranges from subgreenschist to greenschist facies. Pressure–temperature estimates for the subgreenschist–greenschist facies transition are 270–310 °C and 2.1–3.6 kbar (1 kbar = 100 MPa), based on assumed geothermal gradients and the reaction isograd Pmp + Chl = Act + Ep + H2O. Metamorphic peak postdates motion along the westernmost reverse fault that juxtaposes the Slide Mountain and Yukon–Tanana terranes. We interpret the Finlayson Lake Fault Zone as a northeasterly directed thrust sequence disrupted by synmetamorphic back thrusts. The back thrusting may be the consequence of shortening in the upper crust, or larger scale processes such as "tectonic wedging" of Yukon–Tanana Terrane under Slide Mountain Terrane.