Time‐Transgressive Archean Unconformities Underlying Molasse Basin‐Fill Successions of Dissected Oceanic Arcs, Superior Province, Canada

Late Archean diachronous unconformities underlying tectonically controlled coarse clastic successions are located throughout the Superior Province of Canada. Examples studied in detail include hiatuses at the base of the Timmins and Kirkland basins in the Abitibi greenstone belt and the Crowduck and Stormy basins in the Wabigoon Subprovince. The Timmins molasse basin‐fill succession overlies a high‐angle subaerial unconformity separating deepwater turbiditic deposits from fluvial conglomerates. A subaerial erosional unconformity defines the contact between mafic flows and overlying felsic volcaniclastic deposits and fluvial conglomerates of the Kirkland Basin. The Crowduck and Stormy successions are characterized by shallow‐water and subaerial unconformities between quartz‐feldspar porphyry stocks and clastic deposits. The pronounced characteristics of the molasse basins, which include (1) basal unconformities with older sedimentary, volcanic, or plutonic rocks; (2) bounding fault systems; (3) basin margin porphyry stocks; (4) coarse clastic facies; and (5) locally interstratified tholeiitic, calc‐alkaline, and ultrapotassic volcanic rocks, can be compared with several examples from other subprovinces within the Superior craton. The basal unconformities preserved along basin margins are time transgressive throughout the Superior Province, as indicated by a successive age decrease in molasse successions from north to south. These hiatuses mark a significant change in tectonic regime from compression, during which deepwater turbidite and pillowed flow deposits were uplifted, to extension and transcurrent (strike‐slip) movement, during which the molasse basins formed. This major change in deformation was possibly associated with the development of a forebulge during oblique convergence and accretion of multiple oceanic arcs.