GREAT DIVIDE MEGASHEAR, MONTANA AND IDAHO: AN INTRAPLATE LITHOSPHERIC SHEAR ZONE AND ITS IMPACT ON MESOPROTEROZOIC DEPOSITIONAL BASINS

The Great Divide megashear is a continental-scale shear zone that can be traced for a distance of more than 500 km. It is a mappable, mylonitic shear zone as much as 3 km wide that extends northwestward from the Snake River Plain across extreme southwest Montana along and parallel to the Montana–Idaho state line, thence west-northwestward, coincident with the Clearwater zone of Sims et al (2005)across central Idaho and connects with the Trans-Idaho Discontinuity of Yates (1968)into east-central Washington. The megashear shows left-lateral offset of the Laurentian continental margin in Idaho and Washington on the order several hundred kilometers; the left-lateral offset of the Discontinuity is in the same sense as but greater than the 50 km minimum offset of piercing points within the Great Falls tectonic zone that is offset by the megashear adjacent to the Continental Divide in extreme southwest Montana. The megashear first formed in Mesoproterozoic time and is interpreted to have influenced Mesoproterozoic sedimentation in Montana and Idaho. Mesoproterozoic Belt Supergroup and Lemhi Group strata are restricted to areas northeast and southwest of the megashear, respectively. Correlations between the two are difficult and subject to multiple interpretaations; correlations that have been made are based on the assumption that the two were deposited in the same depositional basin. Workers in the region do agree that there are unique sedimentary facies of quartzite that shares similar sedimentary environments of deposition with both the uppermost Belt and Lemhi strata; our work has shown that these unique facies are restricted to small extensional, left-stepping pull-apart basins that formed along the strike of the Great Divide megashear. The difficulties in correlation between the two Groups, with the exception the youngest unique faces of Mesoproterozoic strata deposited along the trend of the megashear, suggests that the Belt and Lemhi basins were separate during the majority of basin infilling and that direct correlations between the two are not possible. The youngest rocks, the Mount Shields Formation of the Missoula Group and the Swauger and Lawson Creek Formations overlying the Lemhi Group, are interpreted to have coalesced across the megashear during the later stages of deposition and basin infilling.