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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.
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. Ftyty four carbonates (marbles, calcite veins , Ordovician limestone) were collected resulting in 68 strain analyzes on mechanically twinned calcite (n = 2337 grains) across the Central Gneiss Belt (CGB; 3 samples), the Central Metasedimentary Belt ; Adirondack’s; 13 samples) and the Ottawan Orogenic Lid (OOL; 11 samples). Twinning in the greenschist-grade OOL marbles preserve NS 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. North north to southeast, the Ottawan (1095-1020 Ma) twinning strain is dominantly a layer parallel shortening fabric oriented NS (Parry Sound), then becomes parallel to the Grenville thrust direction (NW-SE) across the CMB to the Adirondac k 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 while the younger e_2 lamellae (n = 146) preserve a margin-parallel (SW-NE) horizontal fabric.Both the e_1 and e_2 records 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.