MICROSTRUCTURES AND KINEMATICS OF DUCTILE FABRICS IN L-S TECTONITES FROM QUARTZOFELDSPATHIC GNEISS ALONG THE SOUTHWESTERN MARGIN OF THE PEARYA TERRANE, ELLESMERE ISLAND, CANADA

Pearya is a composite terrane that occupies the northernmost part of North America on Ellesmere Island. The NE-striking Petersen Bay fault zone marks the southwestern boundary of the Pearya terrane and separates Tonian orthogneiss of Pearya from a mixed package of Paleozoic subduction-related serpentinite, marble, metasedimentary and metavolcanic rocks of the Petersen Bay Assemblage. The ductile history of this fault zone is enigmatic in part due to an intense brittle, strike-slip overprint (≤ 300 m of cataclasite) with an unknown amount of displacement that interrupts the continuity of the ductile fabrics. Regional structural and tectonic arguments call for sinistral translation to emplace Pearya; however, the down-dip lineation on steep foliation planes has led most workers to conclude that Pearya was thrust southeastwards over North America. We observed the fabric geometry, microstructure and kinematics of Pearya basement along a 10 km-long segment located within 2 km of the Petersen Bay fault zone. The rocks are L>S and L-S tectonites. Foliations are steep; they are parallel to the fault zone in the west versus at high angles in the central and eastern outcrops. Lineations plunge moderately to the SW in the west to more southerly (down-dip) in the east. The foliation is defined by the shape-preferred orientation of micas and compositional layering, while the lineation consists of quartz or feldspar aggregates. Most of the samples are protomylonites that preserve abundant evidence for high-temperature grain boundary migration, consistent with deformation at amphibolite facies given by the assemblage quartz + feldspar + muscovite + biotite ± garnet ± amphibole ± titanite. Shear sense indicators are uncommon in the protomylonites, but mylonitic samples show asymmetry in planes both parallel and perpendicular to L. Asymmetry in both these planes suggests a complex trishear deformation. Down-dip stretching lineations can be produced by a component of contraction (pure shear) perpendicular to the foliation in transpression, and do not preclude early ductile translation of the Pearya terrane along the northern margin of North America.