GRAVITY AND MAGNETIC FRAMEWORK OF WESTERN NORTH AMERICA AT LITHOSPHERIC SCALE

Western North America (including Alaska, the Canadian Cordillera, and western conterminous United States) runs the tectonic gamut, from Archean/Precambrian craton formation to post-cratonic continental accretion to modern-day interactions between the North American, Pacific, Juan de Fuca, and several other plates. A number of significant, first-order challenges separate us from a complete understanding of the full lithospheric and tectonic setting of this complex region. For example: What are the significant tectonic basement domains within the outboard “suspect” terrains and what is the nature of the boundaries between these domains? Continental-scale potential-field data, including gravity, geoid, and magnetic measurements and their derivative combinations and constituents, provide fundamental constraints on structural and dynamic models of the lithosphere.

We present and discuss lithospheric-scale features of gravity and magnetic compilations for western North America. In Alaska and the Canadian Cordillera integrated modeling studies of very large amplitude magnetic anomalies strongly suggest a relationship between these features and significant zones of lithospheric strength that limit the craton-ward extent of backarc tectonic effects. Combined gravity and magnetic studies over the Pacific Northwest forearc reveal lower crust and upper mantle petrologic effects that contribute to our understanding of enigmatic deep earthquakes. Subtle long-wavelength characteristics of magnetic anomalies and scattered heat-flow data help explain the long-debated high elevations of the highly extended northern Basin and Range. Detailed study of magnetic anomaly gradients and trends within the Precambrian craton maps important, lithospheric-scale zones of weakness that have repeatedly localized tectonic strain within the mid continent. Challenges for the better utilization of potential-field data for lithospheric studies include better integration with regional seismic tomography, deep drill holes to provide control for deep modeling, and improved integrated regional datasets.