Foliated intrusions of the Early Jurassic Aishihik Plutonic Suite (APS), including the Aishihik Batholith, have been included in Stikinia and interpreted as allochthonous with respect to adjacent terranes, including the Nisling and Yukon-Tanana Terranes. The Nisling Terrane was thought to lack Early Jurassic igneous rocks. However, the Aishihik Batholith, a single plutonic body that crystallized at ca. 187 Ma, forms a west-tapering lopolith or sheet-like body that intrudes deformed strata of the Nisling Terrane. The batholith displays a margin-parallel foliation, defined by primary magmatic grains including feldspar and hornblende, that is considered to be magmatic. A parallel solid-state fabric overprints the magmatic foliation and fabric in wall rocks within 100 m of the batholith along the west (lower) margin of the batholith. This fabric is defined by gneissic banding, annealed mylonite, and by discrete shear bands. Shearing occurred at high temperatures, probably close to the granite solvus, as indicated by the breakdown of hornblende to biotite, the recrystallization of plagioclase feldspar, and by associated migmatite. Shear indicators are consistent with top-to-the-west displacement. The solid-state fabric postdates peak regional deformation of the Nisling Terrane and is inferred to have developed during late stage ballooning of the intrusion. A model of intrusion of the Nisling Terrane by the Aishihik Batholith, with subsequent ballooning of the batholith, is consistent with the lopolithic shape of the batholith, the distribution of solid-state fabrics, the shear sense and near-solvus temperatures during solid-state deformation, the presence of xenoliths similar to that of the Nisling Terrane in the batholith, and the development, in the Nisling Terrane, of a hot-side-up aureole beneath the batholith. Because the Aishihik Batholith intrudes the Nisling Terrane, (1) the APS cannot be considered diagnostic of Stikinia, and (2) the Nisling Terrane cannot be considered as lacking Early Jurassic igneous rocks. The APS may represent part of an igneous overlap assemblage that links together terranes of the Intermontane belt. Alternatively, Early Jurassic intrusions may have developed in response to the subduction of oceanic crust separating some of the Intermontane terranes.

Abstract We present the results of geological mapping and geochronological studies of the Tally Ho shear zone (THSZ) and adjacent rocks. The shear zone crops out near the west margin of Stikinia, an oceanic arc and the largest of the accreted terranes within the Cordilleran orogen of western North America. The hanging wall of the largely flat-lying shear zone consists of coarsely crystalline leucogabbro and cumulate pyroxenite interpreted as the lower crustal and possibly lithospheric mantle roots of a magmatic arc. Rocks in the footwall consist of volcanic and volcano-sedimentary sequences of the Lewes River Arc, a Late Triassic magmatic arc characteristic of Stikinia. Because the shear zone places lower crustal plutonic rocks over a supracrustal sequence, we interpret it as a crustal-scale thrust fault. Kinematic indicators imply top-to-the-east displacement across the shear zone. The geometry of folds of the shear zone is consistent with deformation in response to displacement over ramps in deeper-seated thrust faults kinematically linked to the THSZ. Crystallization of the hanging-wall leucogabbro at 208±4.3 Ma provides a maximum age constraint for deformation, whereas a post-kinematic granitoid pluton that plugs the shear zone and that crystallized at about 173 Ma provides a lower age limit. The THSZ is, therefore, coeval with: (1) a series of latest Triassic–Early Jurassic shear and fault zones that characterize the length of the west margin of Stikinia; (2) the termination of isotopically juvenile arc magmatism of the Lewes River Arc; (3) crustal loading of Stikinia giving rise to a foreland basin that rapidly filled with westerly derived orogenic molasse that includes clasts of ultrahigh-pressure metamorphic rocks; and (4) juxtaposition of Stikinia against continental crust of the Nisling Assemblage of the Yukon–Tanana terrane to the west. These constraints are consistent with a model of deformation in response to the entry of the continental Nisling Assemblage into the trench of the west-facing Lewes River Arc, terminating subduction and imbricating the arc along a series of east-verging thrust faults, including the THSZ.