Wisconsin gravity minimum: Solution of a geologic and geophysical puzzle and implications for cratonic evolution

Abstract

An intense Bouguer gravity anomaly minimum extending across much of Wisconsin cannot be explained by the surface Phanerozoic sedimentary strata, the basement Precambrian geology, or the topography of the region. The most intense ({minus}100 mgal) part of the minimum coincides with the 1.47 Ga anorogenic granitic Wolf River batholith of northeastern Wisconsin. In southern Wisconsin, however, the densities of the Precambrian basement rocks, which are older than the batholith, provide no clue to the origin of the anomaly. The gradients of the minimum indicate that the source of the anomaly is in the upper crust. Furthermore, nearby deep seismic reflection data indicate that lower crustal structures do not significantly contribute to the gravity minimum. Thus, the minimum is appropriately interpreted as originating from the low-density Wolf River batholith that crops out only in northeastern Wisconsin but is buried beneath a veneer of older rocks in the southern and central parts of the state. Gravity modeling suggests that the batholith is at least 10 km thick and encompasses an area of {approximately}50,000 km{sup 2}. This interpretation provides an important clue to the origin of similar negative gravity anomalies of the Phanerozoic strata-covered craton. Also, the presence of this massive granitic bodymore » appears to have influenced the evolution of the craton - e.g., by controlling the location of the 1.1 Ga Midcontinent rift system and the Paleozoic Wisconsin arch. The fact that the Wolf River batholith is mostly buried suggests that central Wisconsin has been tectonically stable for the past 1.47 b.y. and that the Precambrian basement has been minimally eroded.« less