Relict subduction zones in Canada

Abstract

Relict subduction zones that are interpreted on 10 Lithoprobe deep seismic reflection profiles in Canada provide key observational evidence that lithosphere plate interactions have operated with essentially the same resultant geometry for at least 2.7 Gyr. Subduction zone reflections are generally found beneath known collisional belts, and their orientations are often consistent with the locations of exposed arcs, sutures, and subduction complexes. They project from the lower crust into the upper mantle and provide key geometric relationships between the crust, Moho, and upper mantle. The interpretation of subduction zone reflections as being syntectonic establishes a minimum age for the overlying lower crust and mantle wedge and precludes inferred formation of the Moho and upper mantle by postorogenic delamination in these areas. The orientation of subduction zone reflections may be synthetic or antithetic to exposed sutures, indicating juxtaposition by subcretion or by delamination, respectively. The mantle wedge above the subduction zone reflections may exhibit sparse reflectivity. Subduction zone reflections commonly dip toward the craton, indicating a preferred dip of subduction during the last stages of convergence within an orogen, or else a higher likelihood of preservation beneath older domains. Interpreted ages of the Canadian examples range from Recent to Neoarchean (∼2.7 Ga), providing observational evidence that plate tectonics or similar processes have been active since at least the Neoarchean. The locations and geometries of these relict subduction zones provide a snapshot of accretion boundaries in the mantle and establish a framework for the Neoarchean-Recent accretionary architecture of the North American lithosphere. Copyright 2005 by the American Geophysical Union.