A matter of minutes: Breccia dike paleomagnetism provides evidence for rapid crater modification

Abstract

During an impact event, a crater's transient structure adjusts gravitationally. Within medium-sized complex craters, a central uplift rises and collapses resulting in large-scale rotations of the target rock. Estimated crater modification rates from numerical models indicate that complex impact craters modify to a structurally stable state within tens of seconds to several minutes after excavation. However, there is little direct geologic evidence constraining these rates. We show how paleomagnetic measurements of lithic breccia dikes emplaced during crater excavation can be used to constrain the rate of crater modification within the central uplift of the ~34-km-diameter Slate Islands impact structure, Ontario, Canada. The uniformity and linearity of paleomagnetic directions among the clasts and matrix of breccia dikes throughout the impact structure indicate that breccia dikes were frictionally heated above the magnetite Curie temperature (580 °C) during their emplacement and subsequently cooled in situ through magnetic blocking temperatures. The tight grouping of these paleomagnetic directions implies that these breccia dikes cooled and locked in magnetic remanence over a time interval in which the impact structure was not experiencing structural rotations and had already reached a stable state. Conductive cooling of the thinnest sampled breccia dike would have led to the recording of magnetic remanence approximately six minutes after emplacement. This constraint necessitates a stable crater structure only minutes after impact and presents a rare case in which a geological process can be resolved on such a short time scale.

A Paleozoic age for the Tunnunik impact structure

“…Such a timing of impact acquisition has been shown for clastic breccia dikes within the Slate Island impact structure (Fairchild et al. ). However, because frictional heating in the breccia dikes is very localized (Fairchild et al.…”

“…It has been shown that impact‐related tilting occurs quite rapidly, taking place over a time scale shorter than several minutes in midsized impact craters (Fairchild et al. ). Therefore, the impact breccia is expected to carry a thermoremanent magnetization acquired when the breccia cooled following impact‐related tilting.…”

“…However, because frictional heating in the breccia dikes is very localized (Fairchild et al. ), the target sedimentary rocks and the diabase dikes may or may not have been remagnetized by the impact.…”

“…The breccia dikes were formed during the impact event and their magnetic remanence would have been acquired both by the clasts and the fine-grained matrix as they cooled following emplacement and associated frictional heating. It has been shown that impact-related tilting occurs quite rapidly, taking place over a time scale shorter than several minutes in midsized impact craters (Fairchild et al 2016). Therefore, the impact breccia is expected to carry a thermoremanent magnetization acquired when the breccia cooled following impact-related tilting.…”

“…The purpose of this study was to provide a refined estimate for the age of this impact structure. In the absence of suitable material for radiometric dating, we utilize paleomagnetic dating of target rocks that were remagnetized by the impact (Deutsch and Sch€ arer 1994;Pilkington and Grieve 1992) and of the lithic breccia that formed during the impact event (Fairchild et al 2016).…”