STRAIN VARIATIONS IN MARBLES ACROSS THE PROTEROZOIC GRENVILLE OROGEN, USA

The Grenville orogeny involved the accretion of numerous terranes, likely with reversals of slab polarity, adding a width of ~1000 km of new crust to Laurentia as part of the supercontinent Rodinia. Top-to-the-northwest thrusting is the common mechanism for accretion but there is a component of post-orogenic collapse documented by top-to-the-southeast kinematic indicators.

We report the results of a strain-measuring traverse across the Grenville orogen from Parry Sound, Ontario (NW) to Ft. Ann, New York (SE), including the younger, adjacent Taconic allochthon. Fifty six carbonates were collected resulting in 70 strain analyses on mechanically twinned calcite with the following distribution: Ottawan orogenic lid (11), Parry Sound domain (3), Bancroft domain (20), Harvey-Cardiff domain (1), Belmont domain (4), Marzinaw domain (1), Frontenac domain (1), Adirondack Lowlands (5), Adirondack Highlands (8), and the Taconic allochthon (2). From northwest to southeast, the twinning strain fabric is dominantly a layer-parallel shortening fabric oriented N-S (Parry Sound), then becomes parallel to the Grenville thrust direction (NW-SE) across the Composite Arc Belt (Central Metasedimentary Belt) to the Adirondack Highlands where the sub-horizontal shortening strain becomes margin-parallel (SW-NE). Marbles from the Bancroft shear zone preserve a secondary SW-NE horizontal strain fabric and a vertical shortening overprint. Vertical shortening could be related to collapse of the Ottawan orogenic lid, a terrane with N-S horizontal shortening strains. Across the orogen, differential stresses are uniform (-32 MPa) while shortening strains and NEVs are highest in the Bancroft shear zone and lowest in the Adirondack Mountains.