EARLY PROTEROZOIC DEFORMATION OF WESTERN LAURENTIA: RACKLAN AND FORWARD OROGENIES, NORTHERN CANADA

Contractional deformation termed Racklan Orogeny affected northwestern Laurentia in Early Proterozoic time. The orogeny was identified from an angular unconformity between the Wernecke Supergroup and the Pinguicula Group in Yukon, Canada. Subsequent investigations have shown that the main pulses of deformation occurred prior to 1.60 Ga., when the Wernecke Supergroup (>1.71 Ga; >13 km thick) was crosscut by hydrothermal breccias. The Racklan Orogeny is best restricted to events affecting the Wernecke Supergroup prior to the 1.60 Ga brecciation. Three phases of Racklan deformation have been recognized through the identification of (1) a penetrative foliation followed by (2) crenulations and (3) kink bands. The foliation was synchronous with regional metamorphism characterized by chloritoid and garnet porphyroblasts in muscovite-chlorite-quartz schist, representing peak temperatures near 550^oC. Related folds tend to be north-trending and east-verging, although probable tightening, rotation, and refolding of Racklan structures during subsequent Proterozoic and Phanerozoic deformational events have obscurred the structural details. The Racklan Orogeny has been correlated with the Forward Orogeny to the east, which encompasses deformation observed in the subsurface by seismic reflection, and recognized on published geological maps of the northwestern Canadian shield in the Northwest Territories, Canada. At least two phases of Forward deformation affected the Paleoproterozoic Hornby Bay Group including an early phase dated at 1.667 Ga. Forward deformation occurred as thick-skinned contraction of variable vergence involving crystalline basement. Racklan deformation to the west may have been largely thin-skinned according to deep seismic imagery acquired by Lithoprobe. Together, Racklan-Forward orogenesis records moderate compressional tectonism of western Laurentia following the Wopmay and Trans-Hudson orogenies. Racklan-Forward deformation is broadly time-equivalent to the Yavapai-Mazatzal arc-continent collisions in southwestern Laurentia. All of these orogenies may be tectonically related, and are plausibly linked to events in Australia, notably the late stages of the Kimban Orogeny and/or the early stages of the Isan Orogeny.