Publication Summary
Issue ID: P2017-01-09
Title: Geology of the Sinwa Creek area (104K/14)
Author(s): Mihalynuk, M.G., Zagorevski, A., English, J.M., Orchard, M.J., Bidgood, A.K., Joyce, N., Friedman, R.M.
Series Name: Paper
Publication Year: 2017
Larger Work Citation: in Geological Fieldwork 2016, B.C. Ministry of Energy and Mines, Paper 2017-01
NTS Map Sheet(s): 104K/14
Place Keyword(s): British Columbia, Sinwa Creek, northwest British Columbia
Lat/Long (NSWE): 59, 58.5, -133.5, -133
Theme Keyword(s): Sinwa Formation, Stuhini Group, Lewes River Group, Whitehorse Trough, Cache Creek terrane
Stikine terrane is one of the largest crustal blocks in the Cordillera, measuring more than 1100 km long and 250 km wide. In northern British Columbia it is well known for its large porphyry Cu-Au ħMo-Ag deposits. Stikine terrane tapers northward, replaced by a broadening wedge of oceanic crustal rocks of the Cache Creek terrane, interpreted to have overthrust Stikine terrane reducing its exposed width to ~10 km at 60°N. This overthrust region is where Triassic-Jurassic magmatic belts with known porphyry deposits disappear, and it is mostly underlain by Triassic-Jurassic arc-derived clastic rocks of the Whitehorse trough. Overthrusting has traditionally been attributed to the northwest-trending King Salmon fault, which carries conspicuous Late Norian Sinwa Formation limestone in its hanging wall. On many terrane maps, the Sinwa Formation marks the western margin of the oceanic Cache Creek terrane. However, clast provenance, biochronology, conodont fossil fauna, and sedimentary facies carried by the King Salmon fault are inconsistent with this interpretation. Instead, these data suggest that rocks in the hangingwall of the King Salmon fault were deposited in the Triassic forearc of the Stikine terrane, isolated from the subducting Cache Creek oceanic lithosphere by an intervening trench. Thus, the King Salmon fault is not a terrane boundary, and although regionally important, it is but one of several faults that carry Sinwa Formation limestone. Complicating this simple tectonic picture are detrital zircons from one sample collected in the footwall of the King Salmon fault. They form a nearly unimodal population with a main peak at 242 Ma, an age unknown in Stikinia but common within volcanic and plutonic rocks of the Kutcho-Sitlika-Venables arc, which have historically been included in the Cache Creek terrane. If this provenance link is correct, it supports the Kutcho-Sitlika-Venables arc as a separate terrane, distinct from the Cache Creek, and juxtaposed with the Stikine forearc before the Bajocian (~173 Ma) juxtaposition of Cache Creek terrane. King Salmon and adjacent fault panels carry steep northeast plunging folds having southeast-dipping axial surfaces, consistent with a top to the north component of motion (or sinistral if originally steep) that may be related to a phase of deformation during latest Triassic Kutcho-Sitlika-Venables arc collision.