Accretionary orogenesis in the Lake Superior region, USA: modern-like tectonics during the Paleoproterozoic

<p>Terrane accretion and tectonothermal activity associated with the Penokean and Yavapai Orogenies are recorded in various geologic elements of the Lake Superior region, USA, including: (1) mafic–ultramafic terranes comprising tholeiitic basalts and gabbros, boninites and calc-alkaline volcanics and intrusives (e.g., the Pembine–Wausau Terrane), and (2) multiple and distinct, short-length-scale (5–15 km) chlorite–biotite–garnet–staurolite–(kyanite–)sillimanite regional metamorphic isograd sequences. These geologic associations reflect development of a suprasubduction zone system (subduction initiation?) within a Paleoproterozoic ocean in the Orosirian Period, followed by episodes of short-duration (limited-length-scale) tectonometamorphism during accretionary orogenesis in the Statherian Period.</p><p>The geologic processes recorded in the Paleoproterozoic terranes of the Lake Superior region are very common in the Phanerozoic. We suggest that Paleoproterozoic tectonism in the Lake Superior region may reflect a West Pacific-type setting, involving distinct, short-lived tectonothermal events marking periods of subduction rollback and lithospheric extension, punctuated by episodes of arc/microcontinent collision, terrane accretion and lithospheric shortening.</p><p>The apparent operation of modern-like plate tectonics—accretionary tectonics involving rapid switching between lithospheric extension and shortening—in the Paleoproterozoic requires that a scenario of temporally-varying buoyancy forces at the subduction zone (spatially-varying density of the subducting slab?) be reconciled with the thicker (slower-densifying) oceanic lithosphere expected for a hotter Earth. Such a scenario may be explained by: (1) an anomalously cool mantle (producing anomalously thin oceanic crust) beneath the ocean basin whose closure led to the accretionary orogenesis recorded in the Lake Superior region, or (2) an incredibly long-lived (>> 100 Myr) ocean basin that allowed widespread development of critically-overdense lithosphere prior to subduction initiation and onset of accretionary orogenesis associated with the Penokean and Yavapai Orogenies.</p><p>We are currently investigating geologic associations in the Lake Superior region and their potential tectonic origins, using whole-rock geochemistry to test for the tectonic origins of the Pembine–Wausau Terrane, and <sup>40</sup>Ar/<sup>39</sup>Ar geochronology/geospeedometry to constrain time scales for the tectonometamorphism that produced the metamorphic isograd sequence in the region of Republic, Michigan. Results will provide new insights into accretionary tectonics during the Paleoproterozoic, and processes controlling the emergence and evolution of plate tectonics on Earth.</p>