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Two new techniques are employed in analysis of aeromagnetic data from the San Juan basin, New Mexico, to enhance the expression of buried basement structure and lithology: (1) the data are analytically continued downward onto the irregular basement surface in order to reduce the effect of variable depth to basement, and (2) magnetization boundaries are delineated by a linear filter based on the gradient of pseudogravity. Reduction to the basement involves three procedures that, together, provide a practical method for continuation of potential fields between general surfaces. Data are continued from the nonlevel flight-elevation surface onto a level surface (drape-to-level continuation) by means of a system of successive approximations based on expansion of a Taylor series. Data are continued from the new level surface downward to another level surface at the mean elevation of the basement (level-to-level continuation) by analytical downward continuation based on the fast Fourier transform, incorporating a high-cut filter. Data are continued from this level onto the nonlevel basement surface (level-to-drape continuation) by direct evaluation of a truncated Taylor series expansion, in the vertical dimension, of the field represented on the level surface.

Magnetization boundaries are determined by evaluating the magnitude of the horizontal gradient of the pseudogravity transform of the magnetic data. Lines drawn along ridges of the horizontal gradient mark inferred basement-magnetization boundaries.

After application of these techniques, the aeromagnetic data from the San Juan basin, New Mexico, revealed features of the Precambrian basement not evident, or only vaguely so, in the original data. Together with scanty drill data, the aeromagnetic data indicate a 70-km-wide belt of predominantly metasu-pracrustal rocks trending east-northeast across the center of the basin. Predominantly granitic terranes border this belt on the northwest and southeast. Numerous magnetization boundaries tens of kilometers in length are identified. Structural grain is strongly east-northeast; an area of prominent northerly trends occurs in the north-central part of the basin and locally elsewhere. Cenozoic intrusive rocks are unquestionably aligned with basement structural grain. Laramide and Neogene structures in general are not so aligned. There is no evidence of large strike-slip displacement of basement structures within the area surveyed.

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