Approximately 150 Midwestern geophysicists attended the third annual AGU Midwest Meeting, which was held at Purdue University, September 26–28, 1977. The majority of the 75 papers presented were related to topical special sessions arranged by seven sections of the Union to be of particular, current importance to Midwestern geophysics. The interest shown in the meeting indicates not only the vigor of current geophysical research in the Midwest but also the desirability of continuing these meetings.
The majority of the papers presented were in the area of solid-earth geophysics, and these primarily were concerned with the paleogeodynamics and contemporary geodynamics of the Midwest. They reflect the growing interest in deciphering the complex evolutionary history of the continental interior by utilizing a wide range of methodologies and the concepts of plate tectonics which have generally emphasized oceanic and continental margin features. The papers in the session on Geology and Geophysics of the Precambrian Basement Rocks of the Midcontinent Region demonstrated that geophysical data can be used to clarify and amplify our understanding of the basement geology in poorly exposed regions. By extension of the data base to include buried portions of the basement, it soon may be possible to determine the major structural, lithologic, and chronologic provinces of the basement in the Midwest fairly accurately. The session dealing with Geodynamics of Plate Interiors demonstrated that the structures present in the midcontinent reflect a long and complex midplate tectonism which is not completely understood. The 15 papers of the sessions on the New Madrid Seismic Zone, the 38th Parallel Lineament, and the Seismicity of the Midwest illustrate the rapidly broadening interest in and knowledge of the tectonic framework of the seismically active New Madrid area and environs. It was emphasized that this area is an excellent example of reactivation of ancient structures during the Phanerozoic and locally into the present. Interpretation of surface, air, and satellite geophysical and geological data suggests the presence of an ancient easterly striking rift south of the 38th Parallel Lineament and a northeast-trending graben lying along the axis of the New Madrid Seismic Zone. Microearthquakes together with historical seismicity are being used to delineate active faults of this area and estimate earthquake recurrence rates and the local stress state.