DEXTRAL MOTION ON THE KUGRUK SHEAR ZONE, SEWARD PENINSULA, DURING OPENING OF THE CANADA BASIN

Seward Peninsula is often correlated to the southern Brooks Range because rocks of similar age, composition, and metamorphic history occur in both areas. In particular, Lower Paleozoic fossiliferous carbonates provide strong stratigraphic ties that support the inclusion of both areas in the Arctic Alaska microplate. Tectonic models explain the southward offset of the Seward Peninsula from the Brooks Range by a left-lateral strike-slip displacement. The N-S-striking Kugruk shear zone, located in eastern Seward Peninsula, has been proposed as a candidate to accommodate part of this sinistral strike-slip motion, although no direct field evidence has been used to support this interpretation.

The 6-km-wide Kugruk shear zone separates unmetamorphosed calcareous sandstone of likely Cretaceous age (according to our detrital zircon data) on the east from the 98 Ma Darby pluton and its high grade, Devonian and older, country rock on the west. Shear zone rocks include steeply-dipping bands of marble, dolostone, crossite-bearing metabasites, serpentinite, and mylonitic calcareous white mica-albite schists. Contrary to expectations, the kinematics of fabrics in the mylonitic schists, which have subhorizontal lineations, are clearly right-lateral. Four samples of schist yielded biotite and muscovite ⁴⁰Ar/³⁹Ar ages of 130 ± 2 Ma. We interpret these ages as dating syn-kinematic crystallization of the schists.

The strain recorded by the shear zone rocks, their diversity of lithologies, and the sharp difference of metamorphic grade across it, demonstrate that the Kugruk shear zone is a major tectonic feature. However, it cannot be the easternmost boundary of Arctic Alaska because Nome Group-equivalent rocks are found to the east. Early Cretaceous dextral strike-slip motion along a shear zone in this area is consistent with the counter-clockwise rotation of Arctic Alaska during the opening of the Canada Basin as illustrated in the GPlates model presented here.