AEROMAGNETIC AND GRAVITY DATA: KEYS TO CRUSTAL STRUCTURE AND SEISMIC SOURCE ZONES, CENTRAL AND EASTERN U.S. AND ADJACENT CANADA

Aeromagnetic data for most of the central and eastern U.S. were acquired during the 1970s and 1980s through the USGS efforts of Isidore Zietz and others in cooperation with the state surveys. These data are now digital, providing new opportunities for interpretation, processing, and modeling. Regional aeromagnetic data, along with gravity, other geophysical, and drill data, provide opportunities to understand the crustal structure and tectonics of the central and eastern U.S., and adjacent Canada (CEUS/C). This is particularly true where largely nonmagnetic sedimentary cover prevents direct observation of buried crustal rocks (e.g., mid-continent, and Atlantic and Gulf Coastal Plains). Structure of major crustal provinces, e.g., Wyoming, Trans-Hudson and Penokean orogens; Midcontinent and Mesozoic rifts; the Yavapai, Mazatzal, and granite-rhyolite provinces; and Grenville and Appalachian orogens are better understood because of these data.

Intraplate earthquakes are widespread throughout the CEUS/C, but are concentrated in several zones of frequent small to moderate-size earthquakes (New Madrid, Charleston, Central Virginia, New England, East Tennessee, and Charlevoix-Saint Lawrence), where many have produced large historic earthquakes (M_w > 6.5). The recent EPRI (2012) evaluation of CEUS/C seismicity compiled historic earthquakes and divided the CEUS/C into major source zones. These source zones and their boundaries can be refined using state-of-the-art processed aeromagnetic and gravity data, crustal seismic reflection/refraction, and drill data. For example, the Mesozoic extended crust source zone in the CEUS/C occurs beneath the entire Atlantic and Gulf Coastal Plains, marking the beginning of a new Wilson cycle. Moreover, the zone of Paleozoic extended crust can be refined based on new crustal thickness data, indicating that 50 km-thick Paleozoic crust occurs over much of the western southern and central Appalachians, and was likely not extended during the Paleozoic.