INTRODUCTION TO "THE LITHOSPHERE OF WESTERN NORTH AMERICA"

Recent results from geochemistry and geophysics have shown that the upper mantle beneath Western North America is highly heterogeneous. This session provides an opportunity to review the latest data and interpretations with important constraints coming from mantle xenoliths, heat flow data, geoelectrical data, seismological data, and modeling results. When considered together, these data indicate density variation in excess of 3% with similar variations in compressional wave velocity and even greater variations in shear wave velocity. These and other observations indicate that strong lateral thermal and compositional variations exist and can explain a large part of the observed topographic variations. Pronounced anomalies, both high and low density, exist locally such as beneath the Sierra Nevada of California, the Basin and Range Province, the Canadian Cordillera, and the Rocky Mountains. The upper mantle was once considered to be significantly more homogeneous than the overlying crust. However, modern techniques provide evidence of mantle upwelling, stalled slabs, and delaminated lower crust beneath Western North America. This talk provides an overview of these results and highlights directions for future research. Many of these directions will be addressed through the NSF funded EarthScope Program, but only if multiple data sets are jointly considered.