Interpretation of magnetic anomalies over southern Idaho using generalized multibody models

Abstract

Data from1 several aeromagnetic surveys over southern Idaho, flown at different times, elevations, and line spacings, have been integrated into a consistent digital data set at a fixed elevation of 3.8 km above sea level* The integrated magnetic field then was continued upward to the levels of 7.6 km and 11.4 km above sea level. The dominant feature in the magnetic maps of southern Idaho is a zone of high intensity over the Snake River Plain. Over the western part of the plain, the magnetic anomaly pattern is relatively simple, with a high along the south edge and a low along the north edge of the plain. The eastern part of the plain is characterized by a complex anomaly pattern having numerous highs and lows. The Idaho batholith lies in an area of low-amplitude anomalies. Large magnetic anomalies are produced by Tertiary intrusive and extrusive rocks east and west of the Idaho batholith. Anomalies in some areas southeast of the Snake River Plain are apparently associated with the Precambrian basement. A method using generalized multibody models has been applied to the inversion of magnetic anomalies over the horizontal surface at a height of 7.6 km above sea level. A three-layer model is selected for the analysis. The direction of magnetization is constrained to lie within +15 degrees of the normal or reversed direction of the present geomagnetic field vector. The results of analysis provide generally satisfactory interpretation of the magnetic anomalies. Reasonable estimates have been obtained for the depth to the top and the thickness of an upper layer of magnetic material consisting mainly of basalt and silicic volcanic rocks. This material is often overlain by nonmagnetic rocks. The thickness of both magnetic and nonmagnetic near-surface materials ranges from a few hundred meters to a few kilometers. The magnetized bodies in the middle layer are responsible for the generation of a majority of the larger observed anomalies. Major intrusive bodies, at least 6 km thick, are detected in the magnetic data over many areas, particularly in the eastern Snake River Plain and in the areas to the north. At Craters of the Moon and the northeastern end of the plain, elongate highly magnetized zones underlie major volcanic rifts. The Idaho batholith seems to consist of uniform, weakly magnetized rocks from the surface to a depth greater than 6 km. The bottom of the magnetic crust ranges in depth from 7 to 26 km below sea level, agreeing well with depths to the top of the highly conductive crust interpreted from magnetotelluric soundings in eastern Idaho.