Paper No. 8
Presentation Time: 3:30 PM
PROTEROZOIC EVOLUTION OF THE GREAT LAKES REGION (USA & CANADA) FROM A NEW PERSPECTIVE: AN EXAMPLE OF TECTONIC SWITCHING?
New detailed geochronology allows a re-evaluation of the geologic history of the Proterozoic basement in the Superior and Huron region that supports a tectonic switching model that can be applied to Laurentide crustal growth. Tectonic switching is the oscillation between extension and compression forces that provides a mechanism for episodic, short-lived orogenic contraction of an otherwise continuously extending upper plate formed by ongoing slab retreat. A long-lived convergent margin along southern Laurentia is now well accepted; but lithospheric extension and basin development can be interrupted by intermittent subduction of buoyant oceanic plateaus while the cyclicity depends on the frequency of plateau arrivals. Convergence with a protracted extension mode in the Great Lakes region of North America generated basins into which continental margin sediments were accumulated, associated with bimodal magmatism, illustrated by the 1) 2.4-2.2 Ga Huronian Supergroup including mafic and silicic igneous rocks, 2) Marquette Range Supergroup and contemporaneous ca. 1.87 Ga magmatic suite of Wisconsin, 3) ca. 1.7 Ga Baraboo Interval quartzites and abundant Yavapai-age magmatism, and 4) 1.5-1.4 Ga Belt Supergroup-related sediments and regional "anorogenic" magmatic suite. Short-lived compression-dominated forces temporarily interrupted extension, which are now recognized as the 1) ca. 2100 Ma Blezardian orogeny, 2) 1850 Ma Penokean orogeny, 3) 1650 Ma Mazatzal orogeny. In the case of the Penokean, contraction mode inverted basins and buried crustal rocks, which became the main source for granites generated during the Yavapai extensional phase when basaltic underplating and consequent melting of the thickened crust occurred, moreover explaining why this is the strongest signature of the switching intervals. Granite ascent and emplacement refocuses advective heat in the middle-upper crust, producing temporarily weak crust that is readily deformed. Blezardian and Mazatzal modes involved considerable shortening with only weak thermal perturbations, but nonetheless recognizable. This evolution is consistent with north-directed subduction that retreated to the south, which underwent tectonic switching and progressively converted the extending crust into a series of outboard migrating orogenic belts.