Line F of the GLIMPCE deep marine reflection seismic survey has been reprocessed according to a data-dependent strategy aimed at enhancing the fine structural features of the Midcontinent Rift System in eastern Lake Superior. The processing sequence was specially designed to attenuate first-order water reverberations and to reduce the excessive oscillatory character of the basic wavelet. A detailed examination of the final migrated stacked section reveals that, beneath line F, the Midcontinent Rift System is an almost perfectly symmetric syncline. The structure appears to have formed in the beginning by the extrusion of lavas on a horizontal platform subsiding without major deformation. The initial phase was followed by local crustal sagging in the centre. The transition is marked by a major reflector, which is hypothesized to correspond to the boundary between reverse- and normal-polarity volcanics in eastern Lake Superior. Integrating the results of several recent investigations, a five-stage evolutionary scenario is proposed for the Midcontinent Rift System in eastern Lake Superior: (1) onset of extrusive volcanism, (2) platform subsidence, (3) local crustal sagging, (4) deposition of postrift sediments, and (5) tectonic inversion.

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