Tectonic analysis of the Nisling, northern Stikine and northern Cache Creek terranes, Yukon and British Columbia.

Abstract

Lower Mesozoic strata in the northern Cache Creek terrane range in age from Ladinian to Pliensbachian as shown by conodont and radiolarian collections from chert. Chert beds are interlayered with argillite that has ε(Nd)(t) values of -8.8 to -7.4, indicating detritus eroded from Precambrian source areas. These ε(Nd)(t) values are similar to those of fine-grained Middle Triassic sedimentary strata of the miogeocline (-10.5 to -6.7) and to sediments of the modern Pacific Ocean floor. Volcanic-lithic sandstone interbedded with the chert and argillite is petrographically similar to coeval sandstone from the northern Stikine terrane. ε(Nd)(t) values for northern Cache Creek sandstone are -1.1 to +5.8, similar to most coeval northern Stikine strata (-0.4 to +4.7). These observations, coupled with limited paleocurrent indicators, suggest that northern Cache Creek sandstone was deposited in the distal parts of clastic fans derived from the Late Triassic northern Stikine arc. Quartzofeldspathic sandstone layers in northern Stikine have ε(Nd)(t) values of -8.1 to -1.8 and are associated with conglomerate containing metasedimentary clasts similar to rocks of the Nisling terrane. Nisling rocks have ε(Nd)(t) values of -4.6 to +0.5 (Boundary Ranges suite) and -20.6 to -15.6 (Florence Range suite). These data and existing sedimentologic evidence corroborate interpretations that northern Stikine clastic rocks were derived in part from the Nisling terrane in Late Triassic time. Stacks of thrust sheets containing Cache Creek strata are floored by the Nahlin and King Salmon faults, and are bound on the north by the Squanga-Crag Lake tear fault system which has at least 90 km of right-lateral slip and 2.2 km of south-side-up slip. These strata were thrust southwestward over northern Stikine between Middle Jurassic and mid-Cretaceous time. North and west of this nappe, Stikine and Cache Creek strata are shortened by open, upright folds and only minor thrust faults. Structural, stratigraphic and isotopic data are consistent with a minimum-displacement model for development of the western Canadian Cordillera, in which terranes located east of the Coast Mountains batholith developed and remained in the eastern Pacific Ocean throughout their histories.