Backus and Gilbert inversion of two and one-half-dimensional gravity and magnetic anomalies and crustal structure studies in western Arizona and the eastern Mojave Desert, California

An inversion method that solves for a parameter as a continuous function (Backus-Gilbert) is used to determine the lower boundary of a two and one-half dimensional body which causes either a gravity or a magnetic anomaly and then assesses the vertical accuracy (variance) and the lateral resolution (spread). Also the gravity and magnetic data are simultaneously inverted and this procedure improves the resolution of the model over that obtained by inverting either data set individually. The usefulness of the Backus-Gilbert approach is evaluated by a study of the inversion of both synthetic gravity and magnetic data. The convergence characteristics, the amount of data and the distribution of data, the role of incorrect or correct parameters and the integration technique was considered. Gravity and magnetic data from the Sanford Basin, North Carolina are used to illustrate the inversion procedure.

The second and third part of this study concerns the interpretation of regional geophysical data to obtain an idea of the crustal structure of the eastern Mojave Desert in California and western Arizona. The analysis of gravity and magnetic data in the Mojave Desert included the construction of Bouguer gravity, magnetic intensity, low and high band pass, and horizontal gradient maps. Based on the 100-250 km band pass gravity filtered map, four terranes were described: (1) Colorado River extensional corridor; (2) eastern Mojave gravity low; (3) Granite Mountain fault and (4) Colorado Plateau. Gravity modeling, geologic mapping and seismic refraction data indicate that the eastern Mojave gravity low is caused by a combination of low density Mesozoic intrusive rocks emplaced at depths of 8-9 km and a thickening of the crust. The Colorado River extensional corridor is underlain by a high density mid-crustal mylonitic zone based on seismic reflection and refraction data and gravity modeling.

Analysis of gravity, magnetic and magnetotelluric data along a profile in western Arizona included the construction of Bouguer gravity and magnetic intensity maps, gravity and magnetotelluric crustal models, and the depth to the Curie isotherm. Low Bouguer gravity values, shallow Curie point depths and low resistivity values in the Castle Dome and Aquarius-Mohon Mountains indicate the potential of geothermal resources. The gravity cross-section also suggests the presence of a mid-crustal mylonitic zone underneath the Buckskin and Rawhide Mountains.