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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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slide2

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
slide3

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
slide6

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
slide9

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)
nature and style of rifting
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
slide13

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
slide14

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
slide17

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