TWOPETE FAULT: A MULTIPLY REACTIVATED STRUCTURE THAT LOCALLY INFLUENCED THE DEVELOPMENT OF THE SOUTHERN SELWYN BASIN IN CENTRAL YUKON

The western edge of Laurentia in the Lower Paleozoic is a passive margin similar to passive margins found throughout time around the world. The northwestern part of the margin transitions from carbonate platform to basin (Selwyn basin) to carbonate platform (Cassiar/McEvoy platform) from northeast to southwest. In the Late Devonian and Early Mississippian, the Laurentian margin underwent a period of extension that disrupted passive margin sedimentation and resulted in deposition of primarily quartz-rich clastic rocks.

New mapping in central Yukon in the Selwyn basin has delineated a major structure, the Twopete fault, which is a reactivated structure that may be as old as the Cambro-Ordovician. The Twopete fault separates two distinct rock panels, Devonian to Triassic sedimentary strata in its footwall against Cambrian to Ordovician, variably metamorphosed sedimentary and volcanic rocks in its hangingwall. A succession of Ordovician siliclastic rocks deposited in a shallow water environment are locally exposed in the hangingwall of the fault.

We propose a margin parallel, crustal scale structure active in the Cambro-Ordovician and reactivated periodically throughout the Lower Paleozoic, to explain the geological pattern along the Twopete fault. For example, the eruption of Ordovician volcanic rocks that occur in the hangingwall. Shallow water siliclastic rocks in the Twopete hangingwall may represent the basement to a carbonate platform that developed during the Silurian and Devonian southwest of the Twopete fault equivalent to the Cassiar/McEvoy platform exposed in Yukon to the southeast. Detailed mapping along the Twopete fault provides evidence that it was a syn-sedimentary fault in the Late Devonian that controlled deposition of Upper Devonian sedimentary rocks and emplacement of coeval intrusive rocks. Lastly, the concentration and linear nature of Cretaceous intrusions near the Twopete fault suggest pre-existing structural features have influenced their emplacement.

The Twopete fault is a significant structure in central Yukon that likely influenced the eruption and deposition of Cambro-Ordovician, Late Devonian and Cretaceous igneous rocks, the formation of a Silurian to Devonian carbonate platform and deposition of Upper Devonian clastic rocks.