Evidence for Grenville‐Age Seismicity and Thick‐Skinned Deformation in Northern Wisconsin, U.S.A.

Abstract

Abstract One of the great continental rifts in all of Earth’s history, the ca. 1.1‐Ga Midcontinent Rift of the Lake Superior region, was closed and inverted less than 100 m.yr. after its initiation, when rift‐bounding normal faults were reactivated as reverse faults. This was a major crustal event involving “thick‐skinned” deformation, but its tectonic importance has not been fully appreciated, owing in part to the lack of good outcrop along the traces of the rift‐closing faults. A rare exposure of one of these faults occurs near the falls of the Marengo River in southeastern Bayfield County, Wisconsin. The Atkins Lake–Marenisco fault is a crustal‐scale structure that juxtaposes Archean granitic gneisses with Paleoproterozoic and Neoproterozoic metasedimentary and metavolcanic rocks. Pseudotachylytes, cataclasites, and microbreccias in the fault zone record large ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont...