Architecture of the hidden Penokean terrane suture and Midcontinent rift system overprint in eastern Minnesota and western Wisconsin from magnetotelluric profiling

R.L Wunderman,C.T Young,P.E Wannamaker

doi:10.1130/l716.1

Abstract

Architecture of the hidden Penokean terrane suture and Midcontinent rift system overprint in eastern Minnesota and western Wisconsin from magnetotelluric profiling

Highlights

The northern midwestern region of the United States, and in particular the Lake Superior region, contains unique elements in the construction and evolution of North America
The MT/controlled-source audio magnetotelluric (CSAMT) results we interpreted here are somewhat reconnaissance in their coverage, but first-order features are strongly suggestive of convergent and rift structures
The recovered MT data in east-central Minnesota and western Wisconsin confirm an extension of Penokean orogen deep structure into eastern Minnesota from the comparable terranes in Wisconsin and upper Michigan

Summary

INTRODUCTION

The northern midwestern region of the United States, and in particular the Lake Superior region, contains unique elements in the construction and evolution of North America. Close to the southeast portion, over the Snake River area, the westernmost four MT sites show both ρxy and ρyx values plunging toward long periods with high values of φxy and φyx (Fig. 2) These lie mainly upon high-grade Archean-cored gneisses of the McGrath–Little Falls domain and post-Penokean plutons such as the Isle and Warman granitic panel of the East-Central Minnesota batholith (Chandler et al, 2008; see Fig. 1). Given the elevated ρa values of most CSAMT stations, which imply via skin depth that resistivities are high to ~5 km depth, the low values of φyx, the rising ρyx for T > 10 s at the northwesternmost MT site N03, and the tight data fit of the inversion model, some pinchout of the conductor under the McGrath gneisses seems likely. The 13C/12C isotope ratios of all graphitic samples studied strongly imply a biogenic origin (McSwiggen and Morey, 1989). Bedrosian (2016) proposed that graphite of the Mille Lacs and North Range schists could be responsible for the conductor in this general area resolved in the EarthScope MT TA data

80 Bouguer Gravity

CONCLUSIONS