Terrane assembly and structural relationships in the eastern Prince Rupert quadrangle, British Columbia

The eastern part of the Prince Rupert quadrangle, British Columbia, is subdivided into two regions: the western metamorphic belt and the central belt. The western metamorphic belt is underlain by five distinct mappable rock sequences. From west to east these are the Digby, Venn, Delusion Bay, Kaien, and Tsimpsean sequences. Rocks of the Digby sequence seem to correlate with Triassic and upper Paleozoic rocks of the Alexander terrane; the overlying Venn sequence contains rocks of the same age as parts of the Gravina belt. The Tsimpsean sequence is considered to be part of the Yukon-Tanana terrane, a continental margin assemblage. Correlation of the two intervening sequences, Kaien and Delusion Bay, is uncertain; they may belong either to the Taku terrane or to the Yukon-Tanana terrane. We tend to favor the latter interpretation. North- to northwest-striking thrust faults approximately bisect the belt and separate the Digby and Venn sequences on the west from the structurally overlying units to the east. Near Prince Rupert, west-directed, probably mid- Cretaceous, thrusting accompanied emplacement of tonalite and basalt dikes and sills. These thrusts and others to the north juxtapose upper amphibolite facies rocks over greenschist facies schist. The northern thrusts appear to postdate the regional metamorphism. Above and below the thrusts the rocks show intense ductile deformation. The Prince Rupert shear zone, east of the thrusts, records a strong flattening deformation, possibly due to overthrusting by the 91 Ma Ecstall pluton and associated high-grade gneiss. This tectonic emplacement of hot rocks and the associated igneous activity represented by syntectonic tonalite sills emplaced in the shear zone are inferred to be the cause of the 90 Ma amphibolite facies metamorphism that underlies the Prince Rupert shear zone. The western metamorphic belt is bounded on the east by the Coast shear zone, which separates the western belt from the central belt. This shear zone evolved during emplacement of 65-52 Ma plutons of the Paleogene Coast Mountains batholith that underlie much of the central belt. As the Coast Mountains batholith was emplaced, strain within the arc changed from dominantly contractional normal to the batholith and to the orogen, to extensional parallel to the length of the batholith. The latter stages of batholith emplacement apparently accompanied batholith exhumation. The youngest part of the Coast shear zone is a 1-2-km-wide zone of vertical fabric along the western side of the shear zone and is at the westernmost extent of 85-50 Ma plutons. This Work-Behm shear zone formed during the late stages of uplift of the western metamorphic belt by westward tilting about a northwest-trending hinge. After a hiatus with no identified tectonic activity other than gradual exhumation, Miocene and younger felsic and mafic dikes and young brittle faults cut the rocks throughout the eastern part of the quadrangle.