T74 The Archean of North America: The Core of a Continent

Monday, 10 October 2011
8:00 AM - 12:00 PM
Room 200H-J

VERTICAL TECTONISM SYNCHRONOUS WITH HORIZONTAL TECTONISM IN THE NEOARCHEAN: STRUCTURAL AND GEOCHRONOLOGICAL EVIDENCE FROM THE NORTHWESTERN SUPERIOR PROVINCE, CANADA

Paper number: T74-6
Program: Technical Sessions
Time: 9:15 AM – 9:30 AM

Authors

Abstract

“Vertical tectonism” and “horizontal tectonism” are two contrasting processes that have been proposed for Archean tectonics. Vertical tectonism is due to density inversion and is characterized by buoyant rising of granitoids (diapirism) and sinking of greenstones (sagduction). Horizontal tectonism is similar (but probably not identical) to the present-day plate tectonics and is characterized by regional scale horizontal motion (drift) of “plates” or “microplates” and the resulting interactions (e.g. collision) among them. The two processes need not to be mutually exclusive. Recent results show that both processes played an important role in Archean tectonic evolution. Furthermore, in the Superior Province, there is convincing evidence that the two processes occurred synchronously (and potentially interactively) at the late stages of Archean cratonization, and horizontal shearing (a result of horizontal tectonism) is concentrated in synclinal keels (a result of vertical tectonism).

“Timiskaming-type” groups are the stratigraphically youngest supracrustal rocks in many Archean greenstone belts. Traditionally these groups are interpreted to have been deposited in strike-slip basins opened by horizontal tectonic processes. More recent studies have suggested that they were deposited in inter-diapiric basins formed by vertical tectonic processes during synchronous vertical and horizontal tectonism.

The Island Lake Group is such a group in the northwestern Superior province. Ages of detrital zircons in the group match the known ages of volcanism and plutonism in the surrounding area. They change from the bottom to the top of the group and indicate a scenario that involves erosion down through a supracrustal pile in the early stage of basin formation and sedimentation, and unroofing of plutons in the later stages. This supports the interpretation that the sediments were deposited in synclinal keels between granitoid domes during diapirism and sagduction as a result of vertical tectonism.

It is suggested that synchronous vertical and horizontal tectonism was a common process in the Neoarchean and represents a transition from dominant vertical tectonism in the Mesoarchean (and Paleoarchean?) to dominant horizontal tectonism in the Proterozoic and Phanerozoic.