Abstract

This paper considers the amalgamation of arc and oceanic terranes to be the main mechanism of ca. 1.8–1.6 Ga continental crustal growth in southwestern Laurentia. On the basis of geologic and seismic reflection data and teleseismic images, we propose the Farwell Mountain–Lester Mountain suture zone as the northernmost Paleoproterozoic arc-arc suture. North-dipping (Farwell Mountain) seismic reflections project from 18 km depths to the surface and are interpreted to represent conjugate thrusting as the 1.79–1.77 Ga Green Mountain arc was partially underthrust beneath the Archean craton. We speculate that a north-dipping high-velocity mantle “slab” in the teleseismic image is a continuation of this thrust zone. South-dipping (Lester Mountain) reflections project from 22 km depths to the surface and are interpreted to be a thrust zone between the Green Mountain arc and the 1.76–1.72 Ga Rawah block. Surface features of the Farwell Mountain–Lester Mountain suture zone are (1) marble, chert, rock with sillimanite pods, ultramafic rocks, sulfide deposits, and pillow basalts, which we interpret to be a dismembered accretionary complex; (2) an axial-planar fabric to north-verging isoclinal folds (F2), which we interpret to be part of a north-vergent fold-and-thrust system; and (3) a metamorphic break between ∼500 °C rocks to the north and ∼610 °C rocks to the south, which we attribute to reactivation of the zone. Movement on the Farwell Mountain backthrust is inferred to relate to Cheyenne belt suturing at 1.78–1.75 Ga. We correlate suturing in the Farwell Mountain–Lester Mountain suture zone to S1/D1 (1.746–1.74 Ga) in the Soda Creek–Fish Creek shear zone. We attribute the complexity of this broad suture zone to initial conjugate thrusting, plus overprinting and steepening of accretionary structures by subsequent tectonism.

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