THE PORCUPINE SHEAR ZONE: A FUNDAMENTAL LINK BETWEEN THE ARCTIC AND CORDILLERAN MARGINS OF LAURENTIA

The boundary between the North Slope subterrane of northern Yukon and northeastern Alaska and the northwestern margin of Laurentia lies in a region of poor exposure and limited study along the Porcupine River corridor. Nevertheless, this region plays a key role in establishing the tectonic evolution of the Arctic and northern Cordilleran margins of Laurentia. Tectonic models for Paleozoic transfer of terranes from the circum-Arctic region to the Cordilleran margin are at odds with commonly accepted stratigraphic correlations that link Neoproterozoic–Silurian strata in the Yukon block and Selwyn basin with similar-age strata exposed in the British and Barn Mountains of the North Slope subterrane of Yukon. The commonly accepted rotation model for opening of the Canada Basin about a pivot point in the Mackenzie delta region also supports close correlations between the North Slope subterrane and the northwestern Laurentian margin.

Recent field studies along the Porcupine River southwest of Old Crow, Yukon, documented the presence of a structural boundary between deformed Proterozoic siliciclastic and carbonate strata broadly correlative with northwestern Laurentian deposits, to the south, and metaclastic rocks of the North Slope subterrane, to the north. The northeast-striking deformation zone is characterized by extensive brecciation, folding, and extension within blocks bound by anastomosing, steeply dipping brittle faults, with predominantly sinistral strike-slip shear sense. Fabrics within the blocks and bounding fault zones are commonly composite, recording a protracted history of deformation. Clear evidence for a major strike-slip fault and the absence of any demonstrable correlations across the Porcupine shear zone, lead us to infer that this boundary accommodated Paleozoic transfer of terranes from the Arctic to Cordilleran margins of Laurentia and facilitated Mesozoic opening of the Canada Basin. Tectonic models highlighting no pre-Paleogene displacement on the northwestern Laurentia margin and rotational opening of the Canada Basin must both incorporate the reality that a major deformation zone, the Porcupine shear zone, exists between the North Slope subterrane and northwestern Laurentia.