Origins of Outliers of the Huronian Supergroup within the Sudbury Structure

Abstract

The spatial relations between outliers of Huronian metasediment and the current outline of the Sudbury Igneous Complex (SIC) have been used previously to constrain the original size of the meteorite impact crater, despite uncertainty regarding their structure and origin. The outliers are tight to isoclinal, gently plunging synclines with northeast‐southwest‐trending axes, slightly overturned to the northwest. Major deformation of the Huronian Supergroup preceded the 1849‐Ma Sudbury meteorite impact event by sufficient time to permit peneplanation of the region. Impact occurred in a foredeep related to the subduction of the Laurentia plate under the Wisconson Magmatic Terranes. Faults truncate the northwest margins of most of the outliers. Where examined, at several localities, the northwest margin–bounding faults are invariably occupied by pseudotachylite. Structural considerations provide evidence for thousands of meters of slip along these surfaces, each of which was apparently accomplished in a single event, providing strong support for the contention that motion along these faults coincided with the collapse of the transient cavity after bolide impact to form the Sudbury Structure. The outliers, thus, owe their overall forms to preimpact folding long before the Penokean Orogeny. They owe their continued existence to large‐scale normal fault motions related to collapse of the transient cavity during the Sudbury impact event. The current map‐scale structural relations between the outliers and the outcrop area of the Sudbury Igneous Complex are largely due to the deep exhumation of a series of regional open folds that were formed during postimpact deformation, possibly at the termination of the Penokean Orogeny ca. 1840 Ma or during a later event ca. 1450 Ma.