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Exposed evaporite diapirs and minibasins above a canopy in central Sverdrup Basin, Axel Heiberg Island, Arctic Canada



Axel Heiberg Island (Arctic Archipelago, northern Nunavut, Canada) contains the thickest Mesozoic section in Sverdrup Basin (11 km). The ca. 370-km-long island is second only to Iran in its concentration of exposed evaporite diapirs. Forty-six diapirs of Carboniferous evaporites and associated minibasins are excellently exposed on the island. Regional anticlines, which formed during Paleogene Eurekan orogeny, trend roughly north on a regular ca. 20-km wavelength and probably detach on autochthonous Carboniferous Otto Fiord Formation evaporites comprising halite overlain by thick anhydrite. In contrast, a 60-km-wide area, known as the wall-and-basin structure (WABS) province, has bimodal fold trends and irregular (<10 km) wavelengths. Here, crooked, narrow diapirs of superficially gypsified anhydrite crop out in tight anticline cores, which are separated by wider synclinal minibasins. We interpret the WABS province to detach on a shallow, partly exposed canopy of coalesced allochthonous evaporite sheets. Surrounding strata record a salt-tectonic history spanning the Late Triassic (Norian) to the Paleogene. Stratigraphic thinning against diapirs and spectacular angular unconformities indicate mild regional shortening in which diapiric roof strata were bulged up and flanking strata steepened. This bulging culminated in the Hauterivian, when diapiric evaporites broke out and coalesced to form a canopy. As the inferred canopy was buried, it yielded second-generation diapirs, which rose between minibasins subsiding into the canopy. Consistent high level emplacement suggests that all exposed diapirs inside the WABS area rose from the canopy. In contrast, diapirs along the WABS margins were sourced in autochthonous salt as first-generation diapirs. Apart from the large diapir-flanking unconformities, Jurassic-Cretaceous depositional evidence of salt tectonics also includes submarine debris flows and boulder conglomerates shed from at least three emergent diapirs. Extreme local relief, tectonic slide blocks, steep talus fans and subaerial debris flows suggest that many WABS diapirs continue to rise today. The Axel Heiberg canopy is one of only three known exposed evaporite canopies, each inferred or known at a different structural level: above the canopy (Axel Heiberg), through the canopy (Great Kavir) and beneath a possible canopy (Sivas).