THE MCGRATH GNEISS DOME AND MALMO STRUCTURAL DISCONTINUITY IN E-C MINNESOTA: LINKED EXHUMATION STRUCTURES FORMED DURING COLLAPSE OF THE WESTERN PENOKEAN OROGEN

The Paleoproterozoic gneiss dome – plutonic terrane in east-central Minnesota is separated from the Penokean fold-thrust belt by the south-dipping Malmo structural discontinuity (MSD). West of the McGrath gneiss dome, the MSD is clearly marked by the sharp truncation of linear aeromagnetic anomalies associated with thin iron-formations in the Cuyuna range to the north and in an unnamed terrane to the southwest that is interpreted to correlate with the Cuyuna range. The western end of the MSD wraps to the south, probably due to effects of the large Minnesota River Valley promontory. The geophysical evidence, reinforced by scattered outcrop and drill hole control points, indicates juxtaposition of post-Penokean plutons and medium-grade metamorphic rocks to the south and east against widespread lower-grade metamorphic rocks to the north and west. Total-Pb metamorphic age data on monazite reveal that the MSD separates rocks of different metamorphic age (1830 Ma to the north from 1770 Ma age to the south) indicating that the MSD is a geon 17, possibly Penokean reactivated, structure that exhumed the plutonic terrane of east-central Minnesota.

In the region of the gneiss dome, a lack of contrast in geophysical data makes the location of the MSD obscure. Historically, the eastern end of the MSD has been placed at the northern margin of the McGrath gneiss. However, mapping by Boerboom and others (1999) indicates that the basement/cover contact is nonconformable and not faulted. It is possible that the MSD may lie a considerable distance north of the McGrath gneiss, but the presence of continuous geon 18 isograds mapped north of the gneiss argues against this.

The existence of a large structure (the gneiss dome) only where another major structure (the MSD) becomes obscure is perhaps not coincidental. We suggest that vertical displacement along the MSD in the west dies out eastward in the vicinity of the McGrath gneiss dome, and that the McGrath gneiss is a domed exhumation structure formed above a blind fault at depth. Also, in the east the displacement may have had more of a strike-slip component that to the west. Geon 17 differential exhumation along the MSD (greater in the west) can explain the apparent geophysical, structural, and metamorphic disparities across this region.