THE REFLECTION MOHO BENEATH THE SOUTHERN CANADIAN CORDILLERA

The transition from the crust to the mantle beneath the Canadian porti on of the North American Cordillera varies in depth, geometry, and tec tonic age across the orogen. These variations are rarely spatially rel ated to the positions of morphologic or tectonic belts based on surfac e geology, nor to nearly 25 km of structural relief identified in outc rop and on seismic reflection data. The Moho in this region is thus in terpreted to be a long-lived feature, perhaps as old as Proterozoic in the eastern part of the Cordillera, that probably has been active as a structural boundary during periods of crustal contraction and subseq uent crustal stretching. Recognition of the Moho and lower crust as a zone of localized tectonic activity provides a partial explanation for the problem of where regional detachments that underlie the foreland thrust and fold belt go as they project westward to deep structural le vels beneath the interior of the orogen: they likely project to the ba se of the crust, where they flatten and cause imbrication of crustal r ocks.