DETAILED BASIN STRUCTURE AND TECTONIC EVOLUTION OF THE MIDCONTINENT RIFT SYSTEM IN EASTERN LAKE-SUPERIOR FROM REPROCESSING OF GLIMPCE DEEP REFLECTION SEISMIC DATA

Line F of the GLIMPCE deep marine reflection seismic survey has been r eprocessed according to a data-dependent strategy aimed at enhancing t he fine structural features of the Midcontinent Rift System in eastern Lake Superior. The processing sequence was specially designed to atte nuate first-order water reverberations and to reduce the excessive osc illatory character of the basic wavelet. A detailed examination of the final migrated stacked section reveals that, beneath line F, the Midc ontinent Rift System is an almost perfectly symmetric syncline. The st ructure appears to have formed in the beginning by the extrusion of la vas 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 c orrespond to the boundary between reverse- and normal-polarity volcani cs in eastern Lake Superior. Integrating the results of several recent investigations, a five-stage evolutionary scenario is proposed for th e Midcontinent Rift System in eastern Lake Superior: (1) onset of extr usive volcanism, (2) platform subsidence, (3) local crustal sagging, ( 4) deposition of postrift sediments, and (5) tectonic inversion.