USING MAGNETIC FIELD DATA TO DELINEATE BASINAL DEPRESSIONS AND OTHER VARIATIONS IN BASEMENT ROCKS OF THE EASTERN U.S

The depth to crystalline basement, especially for sediment-filled basins, is an important component of geologic characterization and key to assessments of mineral or energy resource potential. As part of a prototype USGS mineral assessment, we examined approaches to basement depth characterization over the eastern U.S. Depth estimates are often conducted by using well and/or seismic constraints locally and then extrapolating to scales of several to tens of kilometers using gravity data and measurements of density contrasts between crystalline rock and sediment fill. For an area as large as the Eastern U.S., seismic and well data can be relatively sparse so we are more dependent on gravity and magnetic data. However, features associated with the region’s complex geologic history such as terrane boundaries, igneous intrusions, and carbonate rock distributions can mask gravity signatures associated with basement depth variations, and few basins are clearly delineated by measured gravity anomalies.

We find instead that the magnetic field total-gradient amplitude provides an excellent tool for basin delineation and basement depth characterization in the eastern U.S. This approach is effective because crystalline rocks generally have stronger magnetic properties than overlying sediments, making them the primary source of magnetic anomalies, and these anomalies become wider and lower in amplitude as the depth of the associated source increases. The result is that shallower sources are usually associated with more rugged magnetic fabric and thus higher gradient values. The magnetic field gradient amplitude map clearly delineates the Appalachian and Michigan basins with low values. It also exhibits prominent lows that directly correlate with the numerous Mesozoic rift basins within the Atlantic coastal plain. Lows appearing over the Delmarva Peninsula and southern New Jersey may be associated with variations in sutured African crust. A wedge-shaped area in southern Georgia also exhibits a low, and may be associated with basement rocks that differ from surrounding regions. To further constrain these results, we also consider magnetic field depth-to-source techniques in combination with well and/or seismic data in select areas.