Eighty-eight percent of the Cambrian Sept-Îles layered mafic intrusion, possibly the second largest on Earth, lies under the estuary of the St. Lawrence River. A marine geophysical cruise in 1986 tied together all earlier gravity surveys and added gravity measurements along 1100 km of new track lines.The following results were based on the 1986 survey and other geophysical data. The Sept-Îles layered mafic intrusion is nearly circular in plan and about 80 km in diameter. It is funnel shaped and some 5.6 km thick at its centre. It has a volume of about 20 000 km3 and is composed of gabbro and allied rocks in a layered series (nearly graphic of the total bulk), ultramafic rocks in a lower series (about graphic), and gabbro, anorthosite, and potassium-feldspar-bearing rocks in a transitional and upper series (about graphic). Roof rocks on the intrusion may have been restricted to a consanguineous volcanic cap.The Sept-Îles intrusion is located at the northwestern end of a reentrant of the North American continent where Appalachian tectonostratigraphic zones are offset dextrally by 400 km. Emplacement of the intrusion may be related to the initial opening of Iapetus. The southeastern border of the intrusion is downwarped and overlain by a rocky cover 8 km thick. The cover may be composed of Taconian nappes thrust northwestward at the reclosing of Iapetus. If this is so, Logan's line crosses the Sept-Îles layered mafic intrusion about 10 km southeast of its centre.

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