LATE TRIASSIC TO MIDDLE JURASSIC BASIN DEVELOPMENT IN THE NORTHERN CANADIAN CORDILLERA: NEW CONSTRAINTS ON THE TIMING OF REGIONAL EXHUMATION FROM LASER ABLATION SPLIT-STREAM (LASS) DETRITAL ZIRCON U-PB GEOCHRONOLOGY AND HF ISOTOPE GEOCHEMISTRY

Upper Triassic to Middle Jurassic clastic strata in the northern Canadian Cordillera constrain the timing of mid-Mesozoic arc collision of the northern Intermontane terranes (Yukon-Tanana, Stikinia, Quesnellia) and subsequent exhumation along the western North American margin. Arc collision resulted in crustal thickening and widespread emplacement of Late Triassic-Early Jurassic (205-195 Ma) plutonic rocks at mid-crustal levels (26-23 km). The timing and spatial extent of Early to Middle Jurassic exhumation of this plutonic belt in central Yukon is documented by synorogenic strata of the Whitehorse trough; however, the stratigraphic evidence for Triassic-Jurassic tectonics farther north is uncertain. Enigmatic conglomerate and sandstone units assigned to the >560 m-thick Faro Peak formation were deposited near the Alaska-Yukon border and provide a key link to understanding the timing and rates of exhumation in the northern Intermontane terranes. New detrital zircon U-Pb-Hf results acquired by the laser ablation split-stream (LASS) technique constrain Rhaetian to Sinemurian maximum depositional ages (MDAs) for synorogenic strata of the Faro Peak formation.

LASS detrital zircon U-Pb and Hf isotope data show that Faro Peak formation units yield Late Triassic-Early Jurassic (~205-196 Ma) age peaks with mostly superchondritic εHf (-1 to +10) values. Early to Middle Jurassic exhumation of mid-crustal plutons via extensional faulting likely accommodated the removal of ~20 km of rock to explain Late Triassic-Early Jurassic granitoid clasts in the Faro Peak formation and Sinemurian MDAs. Whitehorse trough MDAs are consistent with the Faro Peak formation units with the addition of some younger (Aalenian) ages. The Faro Peak formation becomes distinct from rocks in the Whitehorse trough because in addition to plutonic clasts, the Faro Peak formation contains abundant clasts of Yukon-Tanana basement schist and quartzite. The data presented here suggests that following arc collision, crustal thickening, and emplacement of plutons at depth, a regional scale extensional period persisted into the Middle Jurassic and rapidly exhumed a high volume of crust exposing dissected plutons and the basement rocks to Yukon-Tanana.