A deep seismic-reflection profile in northern Lake Michigan, midcontinent North America, provides a cross section of the crust across the 1850 Ma Penokean orogen, in which an Early Proterozoic island-arc complex was deformed between two converging Archean continental masses. The island-arc crust is about 40 km thick and has a few kilometres of intensely reflective rocks near its base, above which it is variably reflective to transparent. The Archean terranes have thicker crust, as much as 50 km, the lower 20-25 km of which is strongly reflective. Abrupt offsets of Moho near terrane boundaries may have been preserved since accretion during the Penokean orogeny.

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