Abstract

Evidence for both vertical and horizontal movement is well preserved in the Cross Lake greenstone belt in the northwestern Superior Province. The vertical movement components are concentrated in high-strain zones along pluton–greenstone contacts and are characterized by pluton-side-up or greenstone-side-down movement, and the geometry, kinematics, and strain distribution are consistent with a vertical tectonic model involving diapirism and sagduction. The horizontal components are concentrated in major east-southeast-trending dextral high-strain zones and in subordinate northeast-trending sinistral, antithetic high-strain zones and can be readily explained by a horizontal tectonic model involving dextral transpression. Results of a detailed structural analysis indicate that the vertical and horizontal tectonism were more or less synchronous, and there was a transition from dominantly vertical tectonism at the early stages to dominantly horizontal tectonism at the late stages. The Cross Lake Group, consisting of Timiskaming-type sedimentary rocks, was deposited in a synclinal keel between granitoid domes associated with vertical tectonism. It is suggested that synchronous vertical and horizontal tectonism was a common process in the Neoarchean and might represent a transition from dominant vertical tectonism in the Mesoarchean (and Paleoarchean?) to dominant horizontal tectonism in the Proterozoic and Phanerozoic.

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