CRUSTAL-SCALE GRAVITY AND MAGNETIC ANOMALIES OF SOUTHERN ALASKA

A well-known, theoretical ambiguity applies to the interpretation of long-wavelength potential field (gravity and magnetic) anomalies – it is not possible to distinguish mathematically between a broadly distributed shallow source and a more concentrated, but deeper, source. However, this theoretical ambiguity can generally be overcome in crustal-scale interpretations if we incorporate a modest set of assumptions (e.g. geologically reasonable physical property distribution) and additional geoscience information. In particular, it is important to note that lateral density and magnetic susceptibility variations (the source of gravity and magnetic anomalies, respectively) are not, a priori, limited to any specific level of the crust, and, depending on the tectonic or geologic setting, are expected to be distributed throughout the crust.

Southern Alaska, with a long history of continental margin convergence, strike-slip faulting, and accretion, is a setting where lateral density and magnetic susceptibility variations can develop at all levels of the crust. Thus, regional-scale gravity and aeromagnetic data contain distinctive anomalies at a range of wavelengths. We have identified, at the broadest scale, a series of aeromagnetic anomalies that reflect fundamental, crustal-scale, variations in magnetic character. The southern Alaska magnetic high (SAMH), a central, through-going magnetic element, spans wavelengths consistent with a thick source of highly magnetic rocks in the middle and lower crust. Discontinuously present on the northern side of the SAMH, the southern Alaska magnetic trough (SAMT) results from a large region of upper to middle crust with anomalously low magnetic susceptibility. In our analysis of these anomalies we assume that lateral physical property variation occurs at all levels of the crust, so that we can reasonably use mathematical methods, such as matched filtering and upward continuation, to derive geophysical depth slices. The validity of these depth slices can be tested against other geophysical data and viewed in terms of geological scenarios.