HR: 09:30h
AN: GP31A-07    [Abstracts]
TI: A new method for mapping depth to the Curie-temperature isotherm in the Great Basin from aeromagnetic anomalies
AU: * Bouligand, C
EM: cbouligand@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., MS989, Menlo Park, CA 94025, United States
AU: Glen, J
EM: jglen@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., MS989, Menlo Park, CA 94025, United States
AU: Blakely, R
EM: blakely@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., MS989, Menlo Park, CA 94025, United States
AB: We have revisited the problem of using aeromagnetic data to map depth to the Curie-temperature isotherm and tested our new methodology in an attempt to provide an independent estimate of heat flow in the Great Basin. Such methods typically assume that the depth-extent of crustal magnetic sources corresponds to the temperature at which rocks lose their spontaneous magnetization (e.g., 580°C for magnetite). They usually operate in the Fourier domain by analyzing the shape of the power-density spectrum calculated from aeromagnetic anomalies and critically depend on assumptions about the distribution of crustal magnetization. Early methods assumed that crustal magnetization is a completely random function of position characterized by a flat power- density spectrum. In this study, we attempted to incorporate more realistic geologic models for crustal magnetization and applied the method to newly released aeromagnetic compilations for Nevada and North America. We assume that crustal magnetization has fractal properties, as suggested previously by others, so that the power-density spectrum of the magnetization is proportional to the wavenumber raised to a power -β, where β is related to the geologic terrane. In this case, the theoretical power spectrum, as derived by Maus et al. (Geophys. J. Int., 129, 163-168, 1997), depends on three independent parameters: the depths to the top and bottom of the magnetic source layer and the fractal exponent β. We estimate these parameters by first calculating a three-dimensional matrix representing the misfit between the power spectrum computed from observed data and a variety of theoretical spectra calculated from a range of realistic parameter values. We then search the matrix for the set of parameters that leads to the minimum misfit. This operation was performed on overlapping sliding windows that were swept across the entire magnetic map. A matrix was developed for each window, thereby providing lateral variations in the depth to the bottom of magnetic sources. We tested this methodology on synthetic aeromagnetic data and applied it to aeromagnetic compilations from the Great Basin. Preliminary results obtained by assuming β is constant throughout the Great Basin show spatial variations in the depth to the bottom of magnetic sources that, in general, do not depend on the assumed value of β or on the size of the window. However, our observed variations also do not correlate to large extent with observed surface heat-flow anomalies. They may reflect real variations in crustal magnetic thickness, due either to undulations of the depth to the Curie-temperature isotherm not reflected in surface heat-flow measurements, or to lateral variations of shallower magnetic interfaces. Alternatively, they may be artifacts caused by variations in geologic terrane (i.e., variations in β). Future studies will attempt to include β explicitly, using mapped geology as a guide to help distinguish which of the observed patterns reflect real variations in depth to Curie-temperature isotherm.
DE: 1517 Magnetic anomalies: modeling and interpretation
DE: 1545 Spatial variations: all harmonics and anomalies
DE: 5418 Heat flow
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting