Geophysical framework of the continental United States: Progress, problems, and opportunities for research

Abstract

Significant progress has been made over the past five decades in determining the geophysical framework of the continental United States. Highlights include detailed maps of gravity and aeromagnetic anomalies, heat flow, crustal thickness, seismicity, state of stress, and paleomagnetic pole positions. Important tectonic insights have come from earthquake studies, and from knowledge of lithospheric structure derived from seismic reflection, refraction/wide-angle reflection, surface-wave, and teleseismic data. Additional major advances in lithospheric geophysics will depend on four key factors: the reduction of uncertainties in the measurement and interpretation of geophysical data, the widespread application of coincident geophysical methods in concert with geological investigations, the collection of a more uniform continent-scale data base for all geophysical measurements, and the investigation of topical geophysical questions regarding the physical state and properties of the lithosphere. The impracticality of repeating most geophysical field measurements introduces poorly known, but likely large, uncertainties. Since most measurements are not repeated, high priority must be given to the reduction and quantification of uncertainties in measurements and interpretations. The most productive future investigations, in terms of resolution and minimum uncertainties in interpretation, will be those that apply different geophysical methods along identical profiles or areas, and that include geological investigations as a vital ingredient. Important gaps remain in our knowledge of the geophysical framework of the United States on a continent-wide scale, including the deep conductivity structure, the nature of the Moho discontinuity, the structure of the subcrustal lithosphere, and the depth of the lithosphere/asthenosphere boundary. Most transition zones separating geologic or physiographic provinces are poorly studied, yet these zones are likely to be the locations of the most profound changes in the physical properties of the lithosphere. Application of coincident geophysical techniques is needed to study these transition zones. Several topical geophysical questions warrant special emphasis in the future. These questions include the rheology of the crust and subcrustal lithosphere; the distribution, composition, and abundance of fluids in the crust; the genesis and evolution of the Moho; the origin of crustal conductivity zones and deep crustal reflections; the evidence for seismic anisotropy; and the short-term prediction of earthquakes.