NEW AEROMAGNETIC MAP OF WISCONSIN: A WINDOW TO PRECAMBRIAN BASEMENT

A new aeromagnetic map of Wisconsin has been completed, the result of digitally blending 22 surveys flown between 1948 and 1999. The most recent survey (46,000 line-miles), acquired by the U.S. Geological Survey, covers much of southern Wisconsin. The flightline data for four surveys acquired by the USGS during the past 12 years have been released on CD-ROMs. These data were added to earlier USGS surveys and 4 surveys acquired by WGNHS. Flight lines are ½-mile apart or less for 95% of the state, giving the aeromagnetic map nearly uniform specifications. All surveys were either flown at or continued to an elevation of 1000 ft (305m) above mean terrain prior to assembling into a state grid. The data interval of the grid is 250 m. The map will be presented at a scale of 1:500,000. The resulting map strongly reflects the structure of the Precambrian rocks that are exposed in the north-central part of the state and covered by glacial deposits and gently dipping Paleozoic sedimentary rocks for most of the remainder. Some notable features of the aeromagnetic map include 1) narrow anomalies trace Keweenawan basaltic lava flows folded into an open syncline-anticline-syncline structure along the axis of the Midcontinent rift, 2) circular to oval positive and negative anomalies mark cross-cutting mafic and felsic plutons in the basement, 3) linear anomalies indicate probable Keweenawan dikes cutting the basement, 4) a marked change in the trend of basement structures in west-central Wisconsin is implied by a block of distinct NW-trending anomalies, and 5) linear anomaly discontinuities mark terrane boundaries and local faults.