STACKING OF DISPARATE VOLCANIC AND SEDIMENTARY UNITS BY THRUSTING IN THE ARCHEAN FAVORABLE LAKE GREENSTONE-BELT, CENTRAL CANADA

In the Favourable Lake greenstone belt, older volcanic-sedimentary units have been stacked concordantly on younger units by three thrust faults. Units above and below thrusts differ in age and/or lithology, and unlike many thrust terranes elsewhere, there is no stratigraphic repetition; the lack of repetition may reflect rapid lateral facies changes. The lowest thrust sheet is at least 6 km thick and includes rock units of two distinct ages: 2925 Ma ultramafic to intermediate flows and intermediate fragmental rocks in the lower part and 2725 Ma mafic flows and a conglomerate-greywacke fan in the upper part. This sheet was thrust northward over a < 2734 Ma felsic to intermediate stratovolcano at least 4 km thick; this implies at least 7 km of vertical movement of the thrust sheet. The stratovolcano is the facies equivalent of, and probable provenance for, the greywacke fan in the upper part of the thrust sheet. The two upper thrust sheets are relatively thin and comprise rock units of 2870-2858 Ma age; rocks of this age were not found in underlying units. The thrust faults apparently developed in a relatively undeformed volcanic-sedimentary sequence; they predated regional isoclinal folding, many discordant faults, and emplacement of bordering granitoid batholiths. The root zone for the thrust sheets is hidden in an area now occupied by granitoid batholiths, but thrusts could have been generated by either horizontal compression at colliding plate margins and/or gravity sliding triggered by vertical uplift associated with emplacement of granitoid plutons.