We resolved the architecture of the early Proterozoic Penokean orogen suture and late middle Proterozoic (Keweenawan) Midcontinent rift system magmatic overprint in east-central Minnesota and western Wisconsin through recovery and analysis of a legacy magnetotelluric (MT) data set. We digitized printed plots of off-diagonal MT and controlled-source audio (CSA) MT responses, including error intervals, to provide 22 soundings along a profile of ∼225 km length extending from north of Lake Mille Lacs, Minnesota, southeastward to Flambeau Ridge, Wisconsin. The MT data were inverted to a smoothed electrical resistivity structure using a two-dimensional finite-element, regularized Gauss-Newton algorithm emphasizing the transverse magnetic (TM) mode data subset. Our model reveals a major electrically conductive zone dipping moderately to the southeast for >50 km in the 5–35 km depth range, which marks the probable Penokean suture in easternmost Minnesota. We interpret the conductor to reflect a package of graphitized metasediments of the former Archean continental margin and near foreland zone, underthrust as the Penokean terrane collided with the Superior Province. The large-scale conductor is now hidden beneath mainly Yavapai-aged plutonic rocks of the East-Central Minnesota batholith. Below the axis of the later Midcontinent rift (St. Croix horst, subsequently), a compact resistive body ranging from 5 to 20 km deep overlies the large conductor. We interpret this resistor to be mafic volcanic and intrusive rocks of the Midcontinent rift event, which correlate spatially with a high Bouguer gravity anomaly similarly modeled. The rift here coincides with the lower-crustal reaches of the suture, but the specific influence of the suture on rift emplacement is unclear.
Architecture of the hidden Penokean terrane suture and Midcontinent rift system overprint in eastern Minnesota and western Wisconsin from magnetotelluric profiling
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R.L. Wunderman, P.E. Wannamaker, C.T. Young; Architecture of the hidden Penokean terrane suture and Midcontinent rift system overprint in eastern Minnesota and western Wisconsin from magnetotelluric profiling. Lithosphere doi: https://doi.org/10.1130/L716.1
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