NEW AEROMAGNETIC SURVEY REVEALS WIDESPREAD QUATERNARY AND NEOGENE VOLCANIC ROCKS AND FOOTWALL STRUCTURE IN NORTHWEST UTAH

New detailed aeromagnetic data covering the Grouse Creek 30 by 60 minute quadrangle highlight the extent of concealed Cenozoic volcanic rocks beneath Curlew and Grouse Creek valleys, reveal structural grain within weakly magnetic Paleozoic rocks, and delineate magnetic sources within the footwall rocks of the Raft River and Grouse Creek detachment faults. The new data (flown along lines spaced 800-m apart) replace older data flown in the 1970’s along widely spaced (5 to 10 km) lines that showed only very broad magnetic anomalies across the region. For example, older data showed only three to four, 3-4 km wide, positive anomalies over basalt exposed in the Wildcat Hills, Cedar Hill, and Middle Shield in Curlew Valley; new data show a block of high-amplitude, short-wavelength positive and negative anomalies that extend 25-30 km west from the Hansel Mountains to Curlew Junction and as much as 50 km to the south. The pattern associated with the volcanic rocks appears to pass west of the Hogup Mountains and into the northern Great Salt Lake Desert. This pattern contrasts with the smooth, flat magnetic field over Paleozoic outcrops in the Hogup, Matlin, and Hansel Mountains. The linear, north-northeast-trending margin of the magnetic pattern associated with Quaternary to Miocene volcanic rocks argues for a fault-controlled margin on the east side of Curlew Valley. Although Paleozoic sedimentary rocks are weakly magnetic, filtering to enhance shallow sources northwest of the town of Grouse Creek displays north- to northwest-trending anomalies with amplitudes of 5 nT or less that may reflect structural grain.

One of the surprises revealed by the new aeromagnetic data is a large, semicircular magnetic high located at the eastern end of the Raft River Mountains. The wavelength of the anomaly suggests that the source resides in the footwall of the Raft River detachment fault. Another north-trending, broad magnetic anomaly underlies Grouse Creek Valley and Mountains. Superposed on the broad magnetic high are smaller highs that coincide with Tertiary plutons. Magnetic susceptibility measurements indicate that Archean granites that form much of the footwall of the detachment are not magnetic. The magnetic highs may reflect extensive Tertiary magmatism within the footwall and/or the mafic roots of the Archean granites.