RELOCATING THE BOUNDARY BETWEEN YUKON-TANANA AND CASSIAR TERRANES IN SOUTH-CENTRAL YUKON

The allochthonous, pericratonic Yukon-Tanana terrane (YTT) is a NW-trending belt of polydeformed metasedimentary, metavolcanic and meta-igneous rocks that lie within the central portion of the northern Cordillera. It records the evolution of a system of rifted continental fragments, island arcs and back-arc basins offshore of the western Laurentian margin between mid-Paleozoic and early Mesozoic time. Accretion of YTT to western Laurentia is interpreted to have begun in Permo-Triassic time. It was subsequently affected by several episodes of regional deformation and metamorphism in Jurassic and Cretaceous time that have obscured internal relationships within YTT and complicated the nature of its boundary with adjacent terranes.

In south-central Yukon, the boundary between YTT and the Cassiar terrane (CT) was originally interpreted to coincide with the d'Abbadie fault zone – a system of Late Cretaceous dextral strike-slip faults – and YTT west of the fault was considered part of a subduction complex. Subsequent work demonstrated that penetrative deformation in this region was superposed on older, syn-accretion structures and not the product of deformation in a subduction zone. We further suggest that rocks of YTT occur east of the d'Abbadie fault and that its boundary with CT is located ~20 km east of this Cretaceous fault zone. Identification of YTT east of the d'Abbadie fault is based on a sequence of metaclastic rocks containing amphibolite and minor marble, intruded by numerous sills and plutons of Late Devonian-Early Mississippian granitoids similar in age and composition to first cycle arc magmatism in YTT. In addition, Late Devonian detrital zircon grains in quartzite occupying a high structural and stratigraphic level indicate input from a juvenile arc source only known in YTT. In contrast, rocks of CT in the hanging wall of a west-verging, brittle-ductile thrust fault are characterized by massive horizons of dolomitic marble and metaclastic rocks with rare chloritic schist and the absence of granitoids. Mafic rocks of the Slide Mountain terrane (SMT) typically mark this terrane boundary and are generally interpreted to have been emplaced along east-verging thrust faults. In our study area, the SMT was probably omitted during subsequent reworking of the terrane boundary by west-verging thrust faults.