THE KLINKIT BATHOLITH AND THE KLINKIT FAULT OF NORTHERN BRITISH COLUMBIA: MELT AND DISPLACEMENT

Quaternary fill in the broad valley of the Jennings River in northernmost British Columbia obscures the contact relationship between Mesozoic volcanic rocks of Quesnellia terrane (QN) to the south and Paleozoic greenschist to amphibolite facies metasedimentary and meta-igneous rocks of Yukon-Tanana terrane (YT) to the north. Nevertheless, it has been presumed that a thrust fault, the Klinkit fault (KF), underlies the Jennings River and places QN over YT. While initially QN may have been accreted to and thrust over YT, or built unconformably on a basement of YT, new fabric analysis demonstrates the present contact in this area is a sinistral strike slip fault.

The late Early Cretaceous, biotite-monzogranitic Klinkit batholith (KB) intrudes the southern margin of YT rocks adjacent to the Jennings River. In some places there is no fabric in the batholith; elsewhere two generations of fabric occur: 1) a restricted, moderately NW-dipping foliation with NW-plunging lineation, which is locally overprinted by 2) a dominant, near-vertical WNW-striking foliation with a subhorizontal lineation. The later fabric can occur either as a primary magmatic alignment of biotite and elongated quartz grains or be protomylonitic with grain size reduction of quartz and feldspar in discrete C-folia and aligned biotite in S-folia. The S and C fabrics indicate a sinistral sense of displacement across the batholith.

Because the WNW-trending KB lineation is parallel to the linear Jennings River valley, the KB fabric is interpreted to reflect displacement on the KF. Because there are both magmatic and mylonitic fabrics in the KB that are aligned, and because the lineation is also parallel to the long axis of the elliptical KB, the intrusion is considered early synkinematic, and sinistral offset on the KF can thereby be dated as mid-Cretaceous. This coincides with the earliest phase of dextral displacement on a well-documented network of NW-trending faults to the east and west of the KF. These seemingly antithetic displacements can be reconciled if, together, the sinistral Klinkit and dextral Kutcho-Cassiar faults accommodated modest northward escape of an intervening wedge-shaped block of YT as the allochthonous Intermontane terranes converged with ancestral North America in the culminating phase of Cordilleran orogenesis.