SEISMIC DEFORMATION OF PALEOPROTEROZOIC AND ARCHEAN ROCKS IN NORTHERN MICHIGAN CAUSED BY THE 1850 MA SUDBURY IMPACT

A layer of ejecta-bearing breccia caused by the 1850 Ma Sudbury impact has recently been traced over a 100,000 km² area in the Lake Superior region of North America. Previously unreported effects of the impact are: 1) a zone of seismic liquefaction in pre-impact Paleoproterozoic sediments immediately below the ejecta layer and 2) fracturing and brecciation of underlying Archean granitic basement rocks. These features occur in a stratigraphic interval as much as 40 m thick immediately below the ejecta layer but have not been recognized in the ejecta layer itself or in overlying sediments, suggesting that the seismic event was coeval with and caused by the Sudbury impact. These seismogenic features have been recognized in a 4,000 km² area centered 500 km west of the impact site at Sudbury, Ontario, but probably are more widespread. Within the sediments, clastic dikes consisting of both quartz and carbonate sand are abundant. Some show drag of adjacent beds indicating that they have been injected upward. Much of the disturbed strata consist of interbedded sandstone and shale. Sandstone beds have been liquified whereas shale beds have been dismembered and extended by flow of liquefied sand. Archean granitic basement rocks have been intensely fractured and in places thoroughly brecciated. Overlying Paleoproterozoic sediments have been injected downward into fractures as much as 7 m into the Archean basement indicating transient inversion of fluid pressure gradients. Extensional fractures and their contained sediment dikes are as much as 0.75 m wide. Brecciation of Archean rocks, from outcrop- to mineral grain-scale, and injection of overlying sediments into even the thinnest of an intricate fracture network record a short event of intense dilational deformation, apparently produced by passage of a wave couplet consisting of strong compression followed by dilation. The stratigraphic thickness of seismically disturbed rocks, the intensity of brecciation of Archean rocks and the width of dilational fractures that were opened in them, along with widespread liquefaction more than 500 km from the point of impact, all indicate an exceptionally powerful earthquake, probably with a magnitude well in excess of 9 on the Richter scale