UPPER-MANTLE STRUCTURE IN THE SOUTHEASTERN CANADIAN CORDILLERA

A seismic broadside wide-angle experiment in Canada's southeastern Cor dillera has revealed an anomalous and unexpected seismic arrival. Anal ysis of the data shows that the arrival is unlikely to be a crustal or shallow mantle phase or multiple or converted waves. The kinematic ch aracteristics of this arrival dictate that it is a reflection from wit hin, or from the base of, a low velocity zone (< 7.8-8.0 km s(-1)) sit uated at a depth of 59 to 62 km that we identify as the top of the ast henosphere. This leads to the conclusion that the base of the lithosph ere is at a depth of 50 +/- 2 km in the region. The interpreted shallo w asthenosphere bolsters earlier conclusions of a heat source at depth in the southeastern Canadian Cordillera. The interpretation also show s that stratification in the upper mantle is resolvable by current wid e-angle seismic experiments.