Calcite twinning strain variations across the Proterozoic Grenville orogen and Keweenaw-Kapuskasing inverted foreland,USA and Canada

We report the calcite twinning strain results of a traverse across the Grenville orogen from Parry Sound,Ontario(NW)to Ft.Ann,New York(SE),including the younger,adjacent Ordovician Taconic allochthon.Fifty four carbonates(marbles,calcite veins,Ordovician limestone)were collected resulting in 68 strain analyses on mechanically twinned calcite(n=2337 grains)across the Central Gneiss Belt(CGB;3 samples),the Central Metasedimentary Belt(CMB;27 samples),the Central Granulite Terrane(CGT;Adirondack's;13 samples)and the Ottawan Orogenic Lid(OOL;11 samples).Twinning strains in the greenschist-grade OOL marbles preserve N-S shortening and U-Pb titanite ages(~1150 Ma;n=4)document these marbles formed during the Shawinigan(1190-1140 Ma)part of the Grenville orogen.From northwest to southeast,the Ottawan(1095-1020 Ma)twinning strain is dominantly a layerparallel shortening fabric oriented N-S(Parry Sound),then becomes parallel to the Grenville thrust direction(NW-SE)across the CMB to the Adirondack Highlands where the sub-horizontal shortening strain becomes margin-parallel(SW-NE).Within the regional sample suite there are two areas studied in detail,the Bancroft shear zone(n=11)and a roadcut on the southeast side of the Adirondack Mountains(Ft.Ann,NY;n=8).Marbles from the Bancroft shear zone contain calcite grains with 2 sets of twin lamellae(e_1 and e_2).The better-developed e_1 sets(n=406)record a horizontal fabric oriented NW-SE whereas the younger e_2 lamellae(n=146)preserve a margin-parallel(SW-NE)horizontal fabric.Both the e_1 and e_2 strains record an overprint vertical shortening strain(NEV),perhaps related to extensional orogenic collapse.We also report an Ottawan orogen-aged granoblastic mylonite(1093 Ma,U-Pb zircon;1102 Ma Ar-Ar biotite)in the Keweenaw thrust hanging wall 500 km inboard of the Grenville front and interpret the relations of Grenville-Keweenaw far-field dynamics.