Daniel Holm, L. Gordon Medaris, Kalin T. McDannell , David A. Schneider, Klaus Schulz, Brad Singer, Brian R. Jicha

New geochronologic data in the southern Lake Superior region provide key information on the timing and nature of tectonic activity that pre-and post-date initial Paleoproterozoic growth of Laurentia during the geon 18 Penokean orogeny. The obducted Pembine ophiolite formed along the edge of a Paleoproterozoic ocean basin at least 30 m.y. prior to Penokean island arc/microcontinent accretion beginning at 1860 Ma. Following Penokean orogenesis, intrusion of mafic dikes at 1817 ± 2 Ma indicate a period of extension that coincided with a 30 m.y. gap in orogenic felsic magmatism at 1835-1805 Ma (between the Penokean and Yavapai orogenies) and likely represents relaxation of Penokean compression and a tectonic switch to intra-arc extension related to initiation of Yavapai subduction. Subsequent Yavapai arc accretion (1750-1720 Ma) resulted in pervasive ductile deformation of the dikes and host rocks at temperatures of ~700 °C, previously attributed to Penokean deformation. Geon 16 Mazatzal overprinting of the accreted Penokean and Yavapai provinces was widespread but of lower grade overall (greenschist facies), and the thermal effects of the 1476-1470 Ma shallow level Wolf River batholith was limited to a 10-15 km contact zone surrounding the intrusion. In contrast to the Archean Superior Province to the north, Paleoproterozoic terranes in the southern Lake Superior area experienced widespread low-temperature reheating and cooling of shallow crustal levels at ca. 1.1-1.0 Ga attributed primarily to magmatic underplating with little subsequent Neoproterozoic exhumation. In the southern Lake Superior region widespread magmatic underplating likely thickened, strengthened, and stabilized Proterozoic lithosphere but destabilized Archean cratonized Superior Province lithosphere to the north.