THE ARCHEAN-PALEOPROTEROZIC BOUNDARY IN THE UPPER GREAT LAKES REGION: ITS FORMATION AND INFLUENCE ON CRUSTAL ARCHITECTURE

The north-central U.S. straddles several geologic terranes and their boundaries including the transition from Archean tectosphere (north) to juvenile Paleoproterozoic lithosphere (south). This region preserves a record of southward tectonic growth following breakup of the Neoarchean Superior Province at ca. 2200 Ma. A compilation of aeromagnetic and isotopic data, interpreted in conjunction with geologic bedrock ‘ground-truth’ reveals fundamental tectonic, structural, and thermochronologic boundaries that reflect important differences between Archean and accreted Proterozoic lithospheric rheologies. The locus of deformation and metamorphism during Paleoproterozoic crustal growth was significantly influenced by an embayment(east)-promontory(west) geometry of the rifted Archean continental margin along the southern ‘core’ of Laurentia. The promontory consists of high-grade Mesoarchean gneisses sutured to the Superior Province at the end of the Archean. The Minnesota River Valley promontory acted as a persistent stronghold that has remained largely unaffected by orogenic events since its formation. Significant Proterozoic accretionary deformation and amphibolite metamorphism ends abruptly at the A-P boundary in southern MN. Hornblende K-Ar ages in the promontory are a billion years older than hornblende Ar-Ar ages just south of the promontory. In the embayment to the east (northern WI/MI), Archean rocks north of the A-P boundary were deformed into gneiss domes and metamorphosed during subsequent Paleoproterozoic accretion. Here, Paleoproterozoic hornblende Ar-Ar ages are concordant with hornblende ages south of the A-P boundary. Proterozoic deformation of Archean crust was restricted to a narrow east-west oriented corridor south of the Great Lakes tectonic zone, an Archean suture that subparallels the A-P boundary. Interestingly, a 1650 Ma deformational/low-grade thermal front approximately coincides with the A-P boundary in northern WI/MI. The cumulative deformational and metamorphic effects of Paleoproterozoic accretionary tectonics in the continental interior is in part a reflection of the geometry of the southern rifted margin of the Archean Superior Province and the differing response of both Archean and Proterozoic lithosphere during southward growth of Laurentia.