Direction and shear sense during suturing of the Seven Devils-Wallowa terrane against North America in western Idaho

Abstract

A northeast-dipping 1.5-km-thick mylonite near Dworshak Dam marks the suture zone between Precambrian North America and the Seven Devils-Wallowa terrane in western Idaho. The mylonite formed under amphibolite facies conditions from quartz diorite containing apparently synplutonic mafic and synkinematic pegmatite dikes of the Kamiah plutonic complex. Mylonitic lineations and fold axes have a mean plunge of 48° toward 056°, nearly down the dip of the mylonitic foliation. Shear sense, given by offset of late-stage crosscutting pegmatites, is consistently top-to-the-southwest, reverse-slip, parallel to the mylonitic lineation. Folds that formed by progressive folding of the mylonitic foliation approach sheath-fold geometry. Axial planes and fold limbs are nearly parallel to the mylonitic foliation. Mafic dikes that are apparently synplutonic in the undeformed quartz diorite immediately south of the mylonite zone and north of Kamiah have variable dips and azimuths. In the shear zone, however, these dikes lie nearly in the mylonitic foliation. Transposition of the dikes into near concordance with the foliation by simple shear requires high values of shear strain and suggests that cumulative top-to-the-southwest, reverse-slip displacement across the mylonite zone is at least 27 km, and likely more than 80 km. This displacement involves underthrusting of the Kamiah plutonic complex, emplaced within the Seven Devils-Wallowa terrane, beneath North America during Late Cretaceous docking with continental North America.