The Mesoproterozoic Midcontinent Rift System, Lake Superior Region, USA

Exposures in the Lake Superior region, and associated geophysical evidence, show that a 2000 km-long rift system developed within the North American c raton similar to 1109-1087 Ma, the age span of most of the volcanic rocks. This system is characterized by immense volumes of mafic igneous rocks, mos tly subaerial plateau basalts, generated in two major pulses largely by a h ot mantle plume. A new ocean basin was nearly formed before rifting ceased, perhaps due to the remote effect of the Grenville continental collision to the east. Broad sagging/subsidence, combined with a system of axial half-g rabens separated along the length of the rift by accommodation zones, provi ded conditions for the accumulation of as much as 20 km of volcanic rocks a nd as much as 10 km of post-rift elastic sediments, both along the rift axi s and in basins flanking a central, post-volcanic horst. Pre-rift mature, q uartzose sandstones imply little or no uplift prior to the onset of rift vo lcanism. Early post-rift red-bed sediments consist almost entirely of intra basinally derived volcanic sediment deposited in alluvial fan to fluvial se ttings; the exception is one gray to black carbon-bearing lacustrine(?) uni t. This early sedimentation phase was followed by broad crustal sagging and deposition of progressively more mature red-bed, fluvial sediments with an extra-basinal provenance. (C) 2001 Elsevier Science B.V. All rights reserv ed.