Abstract

Some basic questions about Archean plate tectonics can be addressed by examining accretionary Archean margins, in particular fault zones with significant strike-slip components on the Canadian Shield. (1) Were the oceanic plates typically rigid like modern plates? Yes. Significant lateral viscosity contrasts in the lithosphere between plates and plate boundaries are required for major strike-slip faults to exist. Conversely, strike-slip faults are a kinematic consequence of rigid plates. (2) Did large oceanic plates exist in the Archean? Probably. First, the length and offset of the longest preserved segments of Archean faults are similar to modern examples such as in Alaska. Less directly, the duration of a period with a consistent sense of strike slip at a point on the continental side of an accretionary margin should be related to the time that a typical oceanic plate remains outboard of the margin. This time varies proportionally with size of typical ocean plates and inversely with their velocity. The duration of an example of persistent strike slip on the Canadian Shield is comparable to that of Cenozoic examples. (3) Did old oceanic crust and hence moderate plate velocities occur in the Archean? Perhaps. Paleomagnetic poles are the most direct line of evidence, but they usually relate to continental blocks. The duration of consistent strike-slip motion, preserved alkalic seamounts which record eruption on old oceanic crust, and the duration of ocean basins are potential indirect indications. Overall, the hotter mantle does not appear to have had a great effect on Archean plate motions. Thus, the geometry and rate of plate tectonics are strongly influenced by the lithosphere.

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