EARLY TECTONIC EVOLUTION OF THE NORTH AMERICA CONTINENT--A MODEL INVOKING SUBCONTINENTAL MANTLE DEFORMATION

The early tectonic evolution of the North American craton has been poorly understood and controversial. Recent teleseismic images of the upper mantle beneath the continent provide a means for reinterpreting the early stages of Earth history. Closely following consolidation and cooling of the craton (~2.7 Ga), the lithosphere was subjected to continent-scale transpression that involved oblique shortening and progressed from ductile to brittle. The deformation segmented the lithosphere into fault-bounded blocks of varying dimensions. The principal shears developed during transpression are northwest-trending strike-slip faults. Subsequent deformation was focused on these shears, causing periodic, but enduring dextral and sinistral displacement. Field studies indicate that the primary stress directions through time have been orthogonal, involving predominant northwest-southeast shortening during late Archean time, as indicated by kinematics in the southern Superior province, and both southeast- and southwest-directed shortening during Proterozoic (>1.35 Ga) time. The orthogonal stress field interpreted from the solid lithosphere coincides spatially with the two directions of seismic anisotropy of the upper mantle in the North American plate. The proposed model motivated by seismic anisotropy, accords with new geodynamic models for North and South American plate motions that invoke mechanical coupling and subsequent shear between the lithosphere and asthenosphere such that a major driving force for plate movement is deep-mantle flow.