Abstract The Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE) was undertaken to provide a snapshot of lithospheric break-up above a mantle upwelling at the transition between continental and oceanic rifting. The focus of the project was the northern Main Ethiopian Rift (NMER) cutting across the uplifted Ethiopian plateau comprising the Eocene–Oligocene Afar flood basalt province. A major component of EAGLE was a controlled-source seismic survey involving one rift-axial and one cross-rift c . 400 km profile, and a c . 100 km diameter 2D array to provide a 3D subsurface image beneath the profiles' intersection. The resulting seismic data are interpreted in terms of a crustal and sub-Moho P-wave seismic velocity model. We identify four main results: (1) the velocity within the mid- and upper crust varies from 6.1 km s −1 beneath the rift flanks to 6.6 km s −1 beneath overlying Quaternary axial magmatic segments, interpreted in terms of the presence of cooled gabbroic bodies arranged en echelon along the axis of the rift; (2) the existence of a high-velocity body ( V p 7.4 km s- 1 ) in the lower crust beneath the northwestern rift flank, interpreted in terms of about 15 km-thiek, mafic under-plated/intruded layer at the base of the crust (we suggest this was emplaced during the eruption of Oligocene flood basalts and modified by more recent mafic melt during rifting); (3) the variation in crustal thickness along the NMER axis from c . 40 km in the SW to c . 26 km in the NE beneath Afar. This variation is interpreted in terms of the transition from near-continental rifting in the south to a crust in the north that could be almost entirely composed of mantle-derived mafic melt; and (4) the presence of a possibly continuous mantle reflector at a depth of about 15–25 km below the base of the crust beneath both linear profiles. We suggest this results from a compositional or structural boundary, its depth apparently correlated with the amount of extension.

Gravity and magnetic studies have been published in the GSA Bulletin during most of its history. As demonstrated in the pages of the Bulletin, these techniques have evolved from crude measurements of only regional significance into mature research tools useful at any scale. Gravity and magnetic data have contributed to our understanding of geologic problems and features around the world. The equipment, data processing, and interpretation techniques, and, in many cases, the actual data needed to employ gravity and magnetic data in integrated studies are readily available. Thus, it is appropriate to anticipate that these techniques will find even greater use in the future. It is disappointing, however, to note that papers in the Bulletin involving gravity and magnetic data have decreased in number in recent years.

Abstract The Ouachita system is a major orogenic belt whose importance can be overlooked because of its limited surface exposures. Since it is mostly buried, we must rely to a considerable degree on a few deep drill holes and geophysical data to infer its nature and extent. These data provide much valuable information, but as other papers in this volume document, many important questions will remain unanswered for many years.

Abstract West Texas is weli suited to a regional gravity and magnetic study, because such data have potential for resolving existing questions regarding the structural relations in this area as well as its tectonic history. In this study, gravity data have been compiled from many sources and analyzed along with aeromagnetic data from the National Uranium Resource Evaluation (NURE) program of the U.S. Department of Energy in an integrated regional study of west Texas. Complete Bouguer gravity-anomaly and total-magnetic-intensity maps and a variety of filtered maps were employed in this analysis. Several basins, such as the Hueco bol-son, Salt basin graben, Marfa basin, Presidio graben, and Valentine basin, can be delineated from the maps, as can interesting positive features associated with the Diablo Plateau, Davis Mountains, and Chalk Draw fault. Deep-seated anomalies were found to be associated with the Delaware basin, Central basin platform, and Ouachita orogenic belt.

Abstract The compilation of regional Bouguer gravity-anomaly and magnetic-anomaly maps, the gridding of those data, and the development of numerous filtered maps, together with detailed petrographic analysis of basement-rock drillhole samples, have provided significant insight into the Precambrian basement. The geophysical data have yielded important clues to the tectonic framework and the regional distribution of basement-rock types. In parts of the east-central Midcontinent the basement drillhole data have been extremely useful in the evaluation of geophysical anomalies and in the interpretation of the Precambrian geology. However, in other parts of the region, correlation of the drillhole data with the geophysical anomalies has been poor. This poor correlation has led to a consideration of factors that can produce ambiguities in correlating geophysical and geologic data. This investigation has shown the value of an integrated geophysical and geologic approach to studying the tectonic framework of basement rocks in the east-central Midcontinent. As a result of this study, four principal basement zones are recognized: an anorogenic granite-rhyolite terrane, several basement rift zones underlain primarily by mafic volcanic rock, the southern continuation of the Grenville province, and the New Madrid rift complex.