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A matter of minutes; breccia dike paleomagnetism provides evidence for rapid crater modification

Luke M. Fairchild, Nicholas L. Swanson-Hysell and Sonia M. Tikoo
A matter of minutes; breccia dike paleomagnetism provides evidence for rapid crater modification
Geology (Boulder) (September 2016) 44 (9): 723-726


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 approximately 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 degrees 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.

ISSN: 0091-7613
EISSN: 1943-2682
Serial Title: Geology (Boulder)
Serial Volume: 44
Serial Issue: 9
Title: A matter of minutes; breccia dike paleomagnetism provides evidence for rapid crater modification
Affiliation: University of California at Berkeley, Department of Earth and Planetary Science, Berkeley, CA, United States
Pages: 723-726
Published: 201609
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 24
Accession Number: 2016-083467
Categories: GeomorphologyIgneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: GSA Data Repository item 2016233
Illustration Description: illus. incl. 1 table
N48°38'60" - N48°38'60", W87°00'00" - W87°00'00"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201640
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