CONTINENTAL CRUSTAL MAGNETIZATION—FROM THE PRESERVATION OF GEOLOGICALLY ANCIENT MAGNETIZATIONS AT FINE CRUSTAL SCALE TO THE RECORDING OF MIDDLE AND DEEP CRUSTAL PROCESSES THROUGH GRADIENTS IN MAGNETIC PROPERTIES

When thinking of one synonymous with applying crustal geophysics to address important problems in the geosciences, G. Randy Keller, Jr. certainly is on a very short list. In 1976, he, with D.H. Shurbet and J.P. Freiss, published in Geophysics (v. 84, p. 107-114) “Remanent magnetization from comparison of gravity and magnetic anomalies”. That paper was early on in his very distinguished career of utilizing seismic reflection and refraction, gravity, and ground magnetic and aeromagnetic data to elucidate the crustal and mantle structure of the Kenya and Baikal rifts, the crustal structure and contemporary deformation of the Mississippi Embayment, the origin of the Colorado Plateau and surrounding tectonic features (e.g., Rio Grande rift), and the tectonic evolution of Eastern Europe, among numerous other contributions. His efforts to compile geophysics data in published map form cannot go unrecognized. In celebrating his science, and the importance of his 1976 paper, We will highlight the magnetization of continental crust, a far-reaching topic ranging from the retention of geologically ancient (stable) magnetizations in many geologic materials, hence the fields of paleomagnetism and rock magnetism, to the variations in magnetic properties of rocks at deeper levels in the crust where the retention of any form of early acquired (primary) remanence is less and less likely. Nevertheless, such variations form the basis for important magnetic anomaly studies carried out at a broad range of scales using ground, air, and satellite based data sets to understand the interior of continental lithosphere. Reflecting the first author's interests in the magnetic properties of igneous rocks, data from the Eocene shonkinitic Shonkin Sag and Square Butte laccoliths in north-central Montana, the lower mafic sill complex of the Early Jurassic Karoo Large Igneous Province, southern Africa; Proterozoic mafic dike swarms in the western US, a regionally extensive Keweenawan age mafic sill in west Texas, and mid-Cenozoic dikes and related rocks of the Spanish Peaks intrusive complex, Colorado, all of which retain primary magnetizations, will be examined. Using these data, we discuss plausible origins of crustal magnetization patterns from deeper crustal rocks, bringing the discussion back to Randy Keller’s 1976 contribution.