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Crustal Thickness, variation

Crustal Thickness, variation. 36-46 km (~42 km avg) paleothickness > by 6-20 km local Proterozoic (1.9 Ga) thickening (Kapuskasing) local Proterozoic thinning (1.1 Ga) no correlation with surface tectonic elements generally flat Moho local evidence for subduction scars at Moho.

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Crustal Thickness, variation

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  1. Crustal Thickness, variation • 36-46 km (~42 km avg) • paleothickness > by 6-20 km • local Proterozoic (1.9 Ga) thickening (Kapuskasing) • local Proterozoic thinning (1.1 Ga) • no correlation with surface tectonic elements • generally flat Moho • local evidence for subduction scars at Moho

  2. Crustal structure, velocity, reflectivity • Layered crust 6.9-7.5 km/s at base; >5.2 km/s at top • low-dipping reflectivity below 10 km • consistent gentle N dips • steep faults sole into mid-crustal subhorizontal reflectors • Moho imbrication

  3. Reflection Interpretation: Western Superior Transect

  4. Mantle lithosphere structure, composition • ca. 300 km-thick lithosphere • high velocity (<8.8 km/s), anisotropic shallow mantle • depleted composition • mantle domains recognized through differences in seismic, electrical anisotropy • NW Superior vs SW Superior

  5. Magnetotelluric

  6. Nature and age of microcontinental fragments • At least 7 independent Mesoarchean fragments recognized • variably reworked by Neoarchean (2.75-2.70 Ga) magmatism • Nd isotopes (700 analyses), inherited zircons (hundreds of observations)

  7. Nature and style of Rifting • thin quartz arenite, conglomerate, shale, carbonate, BIF, volcanics (komatiite & tholeiite) • local provenance • tidal, platform, submarine fan • some = rifting, ocean opening • others = small platforms

  8. Nature and Age of Accreted Oceanic Rocks • Plateau (rare MORB) basement • calc-alkaline edifices • arc-plume interactions • isotopically juvenile (εNd +5 to +2) • up to 25 km of section • juvenile tonalites • 2745-2700 Ma

  9. Nature and Age of Continental Magmatic arcs • N. Quebec: 2785-2700 Ma ton-gdi +/- charnockitic vertical sheets ca. 5 kb crystallization P • W. Superior: multiple ca. 2745-2710 Ma plutons, horizontal sheets ca. 4 kb crystallization P • prelude to continent-continent; ocean-continent collisions

  10. Nature and Style of Post-collisional processes • sanukitoid, alkaline, carbonatite magmatism • dextral transpression • granite “bloom” 2680-2660 Ma • deep crustal extension, metamorphism • peneplaned by 2480 Ma • Proterozoic dykes • local Laramide-style intracratonic uplifts • Keweenawan rift; alkaline magmatism

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